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FABRICATION OF ARRAY WAVEGUIDE GRATINGS (AWGS) FOR MULTIPLEXERS/DEMULTIPLEXERS

(MUX/DEMUX) APPLICATION

PUA CHANG HONG

UNIVERSITY OF MALAYA 2009

FABRICATION OF ARRAY WAVEGUIDE GRATINGS (AWGS) FOR MULTIPLEXERS/DEMULTIPLEXERS

(MUX/DEMUX) APPLICATION

By

PUA CHANG HONG Department of Physics

Faculty of Science University of Malaya

Dissertation Presented For The Degree of Master of Science

UNIVERSITY OF MALAYA 2009

ii

UNIVERSITY OF MALAYA

ORIGINAL LITERARY WORK DECLARATION

Name of Candidate : Pua Chang Hong (IC No. : 821025-14-5491)

Registration/ Matric No : SGR060042

Name of Degree : Degree of Master of Science

Title of Project Paper/ Research Report/ Dissertation/ Thesis (“this work”):

Fabrication of Array Waveguide Gratings (AWGs) for

Multiplexers/Demultiplexers (MUX/DeMUX) Application

Field of Study: I do solemnly and sincerely declare that:

(1) I am the sole author/ writer of this work; (2) This work is original; (3) Any use of any work in which copyright exists was done by way of fair dealing

and for permitted purposes and any excerpt or extract from, or reference to or reproduction of any copyright work has been disclosed expressly and sufficiently and the title of this work and its authorship have been acknowledged in this work;

(4) I do not have any actual knowledge nor I do ought reasonably to know that the making of this work constitute an infringement of any copyright work;

(5) I hereby assign all and every rights in the copyright to this work to the University of Malaya (“UM”), who henceforth shall be owner of the copyright in this work and that any reproduction or use in any form or by any means whatsoever is prohibited without the written consent of UM having been first had and obtained;

(6) I am fully aware that if in the course of making this work I have infringed any copyright whether intentionally or otherwise, I may be subject to legal action or any other action as may be determined by UM.

Candidate’s Signature Date: Subscribed and solemnly declared before, Witness’s Signature Date: Name : Prof. Dr. Harith Bin Ahmad Designation : Professor

iii

ABSTRACT

In this dissertation, the fabrication process of Arrayed Waveguide Gratings

(AWGs) for Passive Optical Network (PON) implementation is presented. The study is

important to the fabrication of AWGs as the fabrication tools are unique and need to be

optimized. The study is also focuses on the metal masking process for the fabrication of

a quality AWGs patterning process.

Throughout this project, optimization has been carried out for direct-current (DC)

planar magnetron sputtering of Cr, photolithography, and wet etching of Cr. The

optimizations for the Cr metal mask are masking thickness, critical dimension (CD),

side wall angle (SWA), and surface roughness. After the optimization process, we

succeed to increase the patterning yield from 40% up to 100%. The critical dimension

was also being reduced to the range of 0.4 – 0.8μm for 7μm width and 350nm thick Cr-

pattern. The SWA of the photoresist (PR) was improved from 35° to 19°. Due to the

wet etching properties, the surface roughness of the Cr pattern did not seem to be

improved in this project.

iv

ABSTRAK

Dalam disertasi ini, proses fabrikasi “Arrayed Waveguide Gratings” (AWGs)

untuk implementasi dalam rangkaian optik pasif telah dikaji. Kajian ini adalah penting

untuk mengoptimisasikan setiap mesin fabrikasi yang unik supaya dapat menghasilkan

AWGs yang berkualiti. Kajian ini tertumpu pada penghasilan topeng logam yang

berkualiti dalam proses fabrikasi bagi penghasilan corak AWGs.

Sepanjang disertasi ini, kajian dan optimisasi telah dilakukan dalam proses

kromium “DC magnetron sputtering”, fotolitografi, dan pembuangan kromium secara

kimia. Optimisasi ini bertujuan untuk memperbaiki empat parameter utama dalam

penghasilan topeng logam kromium iaitu, ketebalan topeng, dimensi kritikal, sudut

dinding, dan kekasaran permukaan. Selepas kajian dan proses optimisasi, kami berjaya

meningkatkan pengeluaran corak dari 40% sehingga 100%. Bagi dimensi kritikal pula,

telah dikurangkan di antara 0.4 – 0.8μm bagi corak kromium pada kelebaran 7μm dan

ketinggian 350nm. Sudut dinding untuk photo resin telah ditingkatkan dari 35° ke 19°.

Tetapi, kekasaran permukaan dilihat tidak dapat diperbaiki didalam disertasi ini

disebabkan oleh sifat pembuangan kromium secara kimia.

v

ACKNOWLEDGEMENT

I would like to thanks my supervisor, Professor Harith bin Ahmad, for his ideas,

guidance, patience, and all the opportunities he gave. Thanks also to both of my co-

supervisors Associate Professor Sulaiman Wadi Harun and Dr. Fiasal Rafiq Mahamd

Adikan for their guidance, support, and encouraging.

Thousand thanks to my helpful colleagues especially Mr. Chong Wu Yi who

gave all the guidance and kind enough to share his experience and knowledge

throughout my dissertation, Mr. Nizam Tamchek who is not worked under planar

waveguide project but still willing to spend times for giving technical supports and

advices when necessary. Thanks also to other colleagues like Mr. Chuah Koon Seah, Mr.

Alvin Law Wen Pin, Mr. Zamani, Mr. Tan Chin Chong, Mr. Sua Yong Meng, Mr. Lim

Weng Hong, Ms. Yap Yen, Ms. Kow Siew Ting and other that I do not mention for all

the cheers and joys that we share in the laboratory.

I also feel grateful to my family for supporting me to further study and being

understanding. Finally, my gratitude goes to my sponsor – Ministry of Science,

Technology, and Innovation (MOSTI), government of Malaysia for giving me full

financial support, and also University of Malaya that offer me a great place to complete

my thesis.

vi

LIST OF PUBLICATION

1) Pua, C.H., Sulaiman, W.H., Chong, W.Y., Kanesh, J.K., & Harith, A. (2007).

Dynamic Dispersing Technique for PR Coating Process in Planar Lightwave Circuit

Fabrication. Microwave and Optical Technology Letters, 49 (8), 1993-1995.

LIST OF CONFERENCE

1) Pua, C.H., Chong, W.Y., Sulaiman, W.H., & Harith, A. (2007). Influence of Time

and Current on Chromium Thin Film Sputter Deposition by Direct Current (DC)

Planar Magnetron Sputtering System. 3RD Mathematics and Physical Sciences

Graduate Congress (3RD MPSGC) 2007, University of Malaya, Malaysia. 12th to 14th

Dec 2007.

2) Pua, C.H., Chong, W.Y., Sulaiman, W.H., & Harith, A. (2007). Dependence of Cr

Etching Rate on Cr Deposition Thickness by Direct Current (DC) Planar Magnetron

Sputtering System. National Physics Conference (PERFIK) 2007. Kuala

Terengganu, Malaysia. 26-28 Dec 2007.

vii

CONTENTS

PAGE

FRONT PAGE i

DECLARATION ii

ABSTRACT iii

ABSTRAK iv

ACKNOWLEDGEMENT v

LIST OF PUBLICATION vi

CONTENTS vii

LIST OF FIGURES xi

LIST OF TABLES xiv

LIST OF SYMBOLS AND ABBREVIATIONS xv

CHAPTER 1

INTRODUCTION

1.1 WHAT IS A PLANAR LIGHTWAVE CIRCUIT (PLC)? 1

1.1.1 EVOLUTION OF PLC 1

1.1.2 ADVANTAGES OF PLC 3

1.2 WHAT IS ARRAYED WAVEGUIDE GRATINGS (AWGS)? 5

1.2.1 EVOLUTION AND APPLICATION OF AWGS 5

1.2.2 ADVANTAGES OF AWGS IN WDM NETWORK 6

1.3 FABRICATION OF AWGS 6

1.4 MOTIVATION AND OBJECTIVES 8

REFERENCES 9

CHAPTER 2

ARRAYED WAVEGUIDE GRATINGS (AWGS)

2.1 THEORY OF OPTICAL WAVEGUIDE 12

2.1.1 TOTAL INTERNAL REFLECTION (TIR) 12

2.1.2 PRINCIPLE OF OPTICAL WAVEGUIDE 15

2.1.3 OPTICAL WAVEGUIDE ATTENUATION 17

viii

2.2 THEORY OF ARRAYED WAVEGUIDE GRATINGS (AWGs) 18

2.2.1 ATHERMAL AWGs 20

2.2.2 ATHERMAL AWG IN UNIVERSITY OF MALAYA 22

2.2.3 ISSUES AFFECTING THE PERFORMANCE OF AWG 23

REFERENCES 25

CHAPTER 3

FABRICATION OF ARRAYED WAVEGUIDE GRATINGS

3.1 FABRICATION PROCESSES 28

3.2 GLASS FABRICATION 29

3.3 METAL MASKING 31

3.4 GLASS ETCHING 32

3.5 SUMMARY 33

REFERENCES 34

CHAPTER 4

DC MAGNETRON SPUTTER DEPOSITION

4.1 BACKGROUND 35

4.2 THEORY OF SPUTTERING 36

4.2.1 SPUTTERING AND SPUTTER DEPOSITION PROCESSES 36

4.2.2 DC MAGNETRON SPUTTER DEPOSITION 38

4.3 DC MAGNETRON SPUTTERING SYSTEM AND PROCESS

METHODOLOGY IN PHOTONIC RESEARCH CENTER (PRC) 41

4.3.1 WAFER LOADING AND UNLOADING IN LOAD-LOCK

SYSTEM 41

4.3.2 PRC DC PLANAR MAGNETRON SPUTTERING SYSTEM 42

4.3.3 CHROMIUM SPUTTER DEPOSITION PROCESS

METHODOLOGY 45

4.3.4 PUMP PURGE CYCLES AFTER DEPOSITION 47

4.4 EXPERIMENTAL RESULTS AND DISCUSSIONS 47

4.4.1 STUDY OF DEPOSITION TIME 48

4.4.2 DC CURRENT EFFECT ON CHROMIUM DEPOSITION 51

4.4.3 PROCESS PRESSURE EFFECT ON CHROMIUM

DEPOSITION 56

ix

4.5 SUMMARY 59

REFERENCES 60

CHAPTER 5

PHOTOLITHOGRAPHY

5.1 BACKGROUND 62

5.2 THEORY 63

5.2.1 PHOTORESIST (PR) 63

5.2.2 PHOTOLITHOGRAPHY EXPOSURE METHOD 65

5.3 PHOTOLITHOGRAPHY PROCESS METHODOLOGY 68

5.3.1 PR COATING 69

5.3.2 PREBAKE 70

5.3.3 MASK ALIGNER 70

5.3.4 DEVELOPMENT 71

5.3.5 POSTBAKE 71


5.4.1 PHOTORESIST (PR) COATING 72

5.4.2 PREBAKE 76

5.4.3 UV EXPOSE 79

5.4.4 POSTBAKE 85

5.5 SUMMARY 87

REFERENCES 88

CHAPTER 6

WET ETCHING

6.1 INTRODUCTION 89

6.2 OVERVIEW 89

6.3 CR WET ETCHING PROCESS METHODOLOWY 91


6.5 SUMMARY 99

REFERENCES 100

x

CHAPTER 7

CONCLUSIONS & FUTURE WORK

7.1 CONCLUSIONS 101

7.2 FUTURE WORK 104

APPENDIX A 106

APPENDIX B 111

xi

LIST OF FIGURES Page

Chapter 1 Introduction 1.1: 1st Generation Optical Fibre Communication–Point to Point Single

Wavelength System 2 1.2: 4th Generation Optical Fibre Communication–DWDM on point to point

Network System 3 Chapter 2 Arrayed Waveguide Gratings (AWGs) 2.1: Light refraction from one material to another material with different

refractive index where nr > ni 13 2.2: Light refraction on the surface with nr < ni, and θr = 90° 14 2.3: Total Internal Reflection (TIR) where θ1 = θ2 15 2.4: Light ray travel in fiber with different angle, (a) blue colour ray with

incident angle θ1 < θc; (b) black colour ray with incident angle, θc; (c) green ray with incident angle θo > θc 16

2.5: Structure of AWG 18 2.6: Divergent of multiplexed wavelength to arrayed waveguides in first FPZ 19 2.7: Optical ray path of different wavelength at second FPZ 19 2.8: An athermal AWGs packaging design 21 2.9: Athermal AWGs photomask in University of Malaya 22 Chapter 3 Fabrication of Arrayed Waveguide Gratings (AWGs) 3.1: Process flow for the AWGs fabrication 29 3.2: Process flow Metal masking 31 Chapter 4 DC Magnetron Sputter Deposition 4.1: Elastic collision between two hard spheres 36 4.2: Collision of two particles 37 4.3: Schematic of a physical sputtering process 38 4.4: Various types of magnetron cathode 39 4.5: Magnetic field configuration for a circular planar magnetron cathode 40 4.6: DC magnetron sputtering system chamber 42 4.7: Grow discharge at target surface 46 4.8: Cr thickness for different sputtering time 48 4.9: Increment of Cr grains height according to thin film thickness 49 4.10: AFM images of Cr thin film for a) (70±5) nm and b) (330±5) nm

thickness 50 4.11: Etching time for different Cr thickness 51 4.12: The voltage and current as a function of total current drawn by DC glow

discharge 52 4.13: Cr film thickness with DC power supply current, Idc 53 4.14: Cr thickness for different deposition times for different Idc 54 4.15: Cr deposition rate on different DC current supply, Idc 55 4.16: Cr etching rate for different Cr thickness coated with different current 55 4.17: DC power supply for different process pressure 57 4.18: Cr film thickness with different pressure deposition 58 4.19: Etching time difference between the wafer center and edge 59

xii

Chapter 5 Photolithography 5.1: Schematic diagram showing the effect of using positive or negative PR

on silica glass fabrication 63 5.2: Chemical reaction of negative PR under UV radiation to form

crosslinked polymer matrix 64 5.3: Reaction of PAC prior to the exposure of the UV light 65 5.4: Various UV exposure methods used in photolithography process 66 5.5: PR thickness (μm) versus spin coating speed (rpm) for SDT, 1st DDT

and 2nd DDT 73 5.6: Illustration of PR thickness non-uniformity with various PR dispensing

techniques 74 5.7: Thickness variation of PR applied using the 1st DDT approach 75 5.8: PR thickness dependence on various prebake temperatures 76 5.9: PR residual that fail to remove from wafer after PR removal process 77 5.10: PR thickness for prebaking temperature of 60oC, 80oC, and 100oC, for

various prebake duration 78 5.11: Change of PR thickness with various prebaking duration 78 5.12:PR pattern obtained with (a) soft contact, (b) hard contact, and (c) and (d)

for vacuum contact 80 5.13: Error analysis scale on AWG photomask, (a) for CD error measurement

while (b) for resolution limit measurement 80 5.14: Resolution measurements of samples under (a) 0.2MPa and (b) 0.16MPa

contact pressure 81 5.15: Cross-section of PR pattern produced under (a) 0.2MPa and (b) 0.16MPa

contact pressure 82 5.16: Fringe pattern when the photomask is in contact with the PR coated

wafer 82 5.17: PR pattern with different exposure dosage. (a) 1860 mJ/cm2, (b) 1890

mJ/cm2, (c) 1920 mJ/cm2, and (d) 2100 mJ/cm2 84 5.18: Different exposure dosages for varying PR thickness 84 5.19: Resulting PR thickness for different postbake period 85 5.20: Etched Cr patterns with and without postbake condition of 120°C for 2

minutes and without postbake 86 5.21: Cross-section of PR pattern before and after postbake 86 Chapter 6 Wet Etching 6.1: Schematics of undercutting caused by the isotropic nature of wet etching 90 6.2: Cr wet etching time for different Cr deposition periods 92 6.3: Difference in etching time between the edge and center of a wafer in

various chromium deposition times 93 6.4: CD error for Cr pattern after static wet etching 94 6.5: Etching time needed for flipping and static wet etching over various Cr

deposition duration 95 6.6: The significant reduction of EC period for flipping etching as compared

to static etching 95 6.7: CD error for different Cr deposition times 96 6.8: Cr etching time by using static and ultrasonic etching method 97 6.9: CD error for Cr pattern with ultrasonic etching 97 6.10: Cr etching rate at different temperatures 98

xiii

Appendix A Theory and Design of Arrayed Waveguide Gratings (AWGs) A.1: Illustrative diagram of an N x N AWG 106 A.2: Geometry of the FPZ 108 A.3: Focusing of beam in two different wavelengths 109

xiv

LIST OF TABLES Page

Chapter 1 Introduction 1.1: Expansion of Leading Edge Transmission System Achieved with AWGs 6 Chapter 2 Arrayed Waveguide Gratings (AWGs) 2.1: Configuration of athermal AWGs 21 Chapter 3 Fabrication of Arrayed Waveguide Gratings (AWGs) 3.1: Hydrolysis and oxidation reaction of metal chloride materials 30 3.2: Important parameters in different processes 33 Chapter 5 Photolithography 5.1: PR usage in SDT, 1st DDT and 2nd DDT 75 Chapter 6 Wet Etching 6.1: Summary of different Cr wet etching techniques for 6 minutes Cr

deposition period 99

xv

LIST OF SYMBOLS AND ABBREVIATIONS AFM Atomic Force Microscopy Ar Argon Ar+ Argon ion AWGs Arrayed Waveguide Gratings B Boron B2O3 Boric Oxide BCl3 Boron Trichloride CD Critical Dimension CF4 Tetrafluoromethane/ Carbon Tetrafluoride CF3

+ Fluorocarbon ion III Co. Company CO2 Carbon Dioxide Cr Chromium Cr(NO3)3 Chromium (III) Nitrate CVD Chemical Vapour Deposition DC Direct Current DDT Dynamic Dispensing Technique DeMux De-multiplexing DI De-Ionized DOF Depth of Focus DWDM Dense Wavelength Division Multiplexing e Electron EC Edge to centre F Fluorine FBG Fibre Bragg Grating FHD Flame Hydrolysis Deposition FPZ Free Propagation Zone FSR Free Spectral Range g Gas Ge Germanium GeCl4 Germanium Tetrachloride GeO2 Germanium Dioxide H2O Water HCl Hydrogen Chloride HClO4 Perchloric Acid He Helium HF Hydrofluoric Acid IC Integrated Circuit ICP Inductive Couple Plasma InP Indium Phosphide ISEE Ion-Induced Secondary Electron Emmision ITU International Telecommunication Unit K Potassium Kr Krypton LiNbO3 Lithium Niobate Ltd. Limited MFC Mass Flow Controller Mux Multiplexing MZ Mach-Zehnder

xvi

N2 Nitrogen gas NA Numerical Aperture (NH4)2Ce(NO3)6 Ceric Ammonium Nitrate O2 Oxygen OC Over-Clad OPF Optical Path Function P Phosphorus P2O3 Diphosphorus Trioxide PAC Photoactive Compound PECVD Plasma Enhanced Chemical Vapour Deposition PHASARs Phased-Arrayed Gratings PIC Photonic Integrated Circuit PLC Planar Lightwave Circuit POCl3 Phosphorus Oxide Trichloride PON Passive Optical Network PR Photoresist RI Refractive Index RIE Reactive Ion Etching s Solid sccm Standard Cubic Centimeters per Minute SDT Static Dispensing Technique SGD Sol-Gel Deposition Si Silicon SiCl4 Silicon Tetrachloride SiF4 Tetrafluorosilane SiO2 Silica SoI Silica-on-Insulator SoS Silica-on-Silicon SWA Side Wall Angle TE Transverse Electric TFF Thin-Film Filter TI Titanium TIR Total Internal Reflection TM Transverse Magnetic TMP Turbo Molecular Pump UC Under-Clad UM University of Malaya UV Ultra-Violet v Vapour WDM Wavelength Division Multiplexing WDM-PON Wavelength Division Multiplexing of Passive Optical Network WGR Waveguide Gratings Router

Chapter 1: Introduction

1

CHAPTER 1

INTRODUCTION

1.1 WHAT IS A PLANAR LIGHTWAVE CIRCUIT (PLC)?

Planar Lightwave Circuit (PLC) is a waveguide circuit that is fabricated on a flat

substrate such as silicon wafer. PLC is a type of optical waveguide. An optical

waveguide is a physical structure that guides electromagnetic waves in the optical

spectrum. The light is guided within the core layer which has a different refractive

index compared to the surrounding material called cladding. The principle for optical

waveguide is based on the phenomenon of total internal reflection (TIR).

1.1.1 EVOLUTION OF PLC

In telecommunication technology, the demand on network bandwidth goes

beyond the limits of the copper based technology. The creation of optical fibres has

successfully brought the telecommunication technology from conventional copper

technique to lower price and higher capacity fiber technology. The migration from

copper technology to optical fibre communication started in the 1980s with the 1st

generation of optical fibre communication. Optical fibre communication at that time

consisted of simple point to point, single wavelength systems using optical fibers and

the light wavelength was around 0.8μm [1- 3], as depicted in Figure 1.1. The maximum

distance that the optical signal was transmitted is about 50km without amplifier.

The 2nd generation optical networks utilized 1.3μm operating wavelength

increased the capacity from 45Mb/s to 1.7Gb/s. This was followed by 3rd generation

networks which were available commercially in 1990. The laser wavelength for the 3rd

generation switched to 1.55μm by using dispersion-shifted fibers together with single-

longitudinal-mode lasers [3].


2

Figure 1.1: 1st Generation Optical Fibre Communication–Point to Point Single Wavelength System [5].

Although the speed and capacity from 1st to 3rd generation have increased, the

demand for higher speed networking is always increased. The 1st to 3rd generation

optical networks which do not have any branching, routing or multiplexing components

are difficult to be upgraded. The capacity expansion could only be achieved through

extensive deployment of additional fibers or use of higher speed laser source and

receiver [2].

PLC was introduced amongst other things to address some of the mentioned

challenges. The development of PLC has brought the optical network to the 4th

generation which we call optical Dense Wavelength Division Multiplexing (DWDM)

networking [4]. With PLC, the transmission capacity can be increased at far lower costs

by multiplexing several wavelengths on a single optical fiber without the need to deploy

any additional fibers, as shown in Figure 1.2. The optical DWDM network system is

able to increase the network links to 10Gbps and towards 40Gbps and faster

transmission speeds [2, 4] by increasing the channels.


3

Figure 1.2: 4th Generation Optical Fibre Communication–DWDM on point to point network system [6]

PLC itself has already been developed to the 4th generation. The 1st generation

includes simple Y-branches and directional couplers, which are used as optical splitters

and taps. The 2nd generation includes Mach-Zehnder (MZ) interferometers and ring

resonators, which are used as optical switches and filter. The 3rd generation includes N x

N star couplers and arrayed waveguide gratings (AWGs) multiplexers, which are used

to multiplex/de-multiplex (Mux/DeMux) signal wavelengths into a single fiber. The 4th

generation includes optical transversal filters and lattice filters, which can be

programmed to perform different lightwave functions, which is analogous to electronic

digital filters. For future research, the expected evolution of PLC will lead to the 5th

generation multi-functional PLC and optical signal processing PLC in the 6th generation

[5, 6].

1.1.2 ADVANTAGES OF PLC

The main advantage of PLC is its scalable semiconductor device manufacturing.

Processes like Plasma Enhanced Chemical Vapor Deposition (PECVD),

photolithography, Reactive Ion Etching (RIE) and more are also used to fabricate PLC

[6]. PLC also allows high component density where all needed function components

can be performed on a single chip. The end result is a much smaller device can be

fabricated. In PLC, not only optical components can be fabricated, but electronic


4

components can also be included. Hence, smaller space and less equipment are

necessary to fabricate all the different optical and electronic components as PLC needs

only the generic semiconductor manufacturing equipments to fabricate all those

different components. A single PLC might include waveguides, filters, gratings,

emitters (light sources), and detectors (photodiodes) [6- 8]. This is not the case in fibre-

based devices where single-function devices follow different fabrication processes.

The third advantage is the circuit integration. The ultimate strength of PLC is its

ability to combine hundreds of what are now discrete components into one device. This

makes PLC one of the key technologies for driving optical networking costs down, and

thus reduces cost per bandwidth. The integration of various components within a same

glass chip will provide higher optical stability and lower sensitivity to external

constraints compared to fiber-based components [9]. The forth advantage is lower

insertion loss. At low channel counts, DWDM Mux/DeMux which are made using thin-

film may have lower loss, but the loss in these serial components increases linearly with

the number of channels. The more the number of fiber-based components that need to

be coupled together, the more losses will there be. As an example, the insertion loss of a

planar Arrayed Waveguide Gratings (AWGs) scales sub-linearly with the increment of

channels count, which means that at higher channel counts it has relatively less loss.

This same phenomenon will be true as more and more components are integrated onto a

single PLC [6- 8].

Apart from applications in the telecommunication system, PLCs are also utilised

in other fields. There are researches on developing the PLC for sensing devices, private

data network, cable-TV, medical, military and also aerospace use. The application of

PLC will become wider with time due to the reasons mentioned earlier [4, 10].


5

1.2 WHAT IS ARRAYED WAVEGUIDE GRATINGS (AWGS)?

The Arrayed Waveguide Gratings (AWGs), also referred to as phased-array

gratings (PHASARs) and waveguide gratings router (WGR), is a well known

technology that is deployed in the wavelength-division-multiplexed WDM systems.

Basically, AWGs perform the task of multiplex and demultiplex the various signal

wavelengths in the network.

1.2.1 EVOLUTION AND APPLICATION OF AWGS

Starting from 1994 when the optical fibre communication first employ of

DWDM system, the MUX/DeMUX of the various channels was achieved via Thin-Film

Filter (TFF) and Fiber Bragg Grating (FBG). Both TFF and FBG based MUX/DeMUX

filters require manual integration of discrete components in direct proportion to the

number of optical channels [5]. Additional channels of such filter system in serial form

action produce more losses and additional cost that increase directly with the number of

channels. Unlike the TFF and FBG filters system, the number of channels in AWGs are

added in parallel form, and hence the lost is lower in large number of channels. The

high number of optical channels is achieved in a single process step with AWGs, which

is also much more cost effective than TFF and FBG filters system. The one step process

fabrication of AWGs has reduced the cost per channel as the numbers of optical

channels increase in single AWGs [6].

Table 1.1 shows the achievement in AWGs in DWDM system from 1997 to

2007. From Table 1.1, the number of channels has increased from 16 in 1997 to 80 in

2007. With the increment in the number of channels, the speed and the capacity of the

DWDM system also increases. Applications of AWGs are not only confined in

MUX/DeMUX application in DWDM network and routing but are also expanding to

other fields. Researchers are now looking at applications of AWGs in optical signal


6

processing field, generation of flat-topped femtosecond pulse trains, optical sensors,

wavelength interrogation and Photonic IC’s (PIC’s) [11- 16].

Table 1.1: Expansion of Leading Edge Transmission System Achieved with AWGs [6] Year System Mux/DeMux Fibre Speed Capacity 1990 1–λ point to point OC-48

(2.5Gbps) 2.5Gbps

1994 8–λ point to point DWDM TFF or FGB OC-48 (2.5Gbps)

20Gbps

1997 16–λ point to point DWDM AWG OC-48 (2.5Gbps)

40Gbps

1999 40–λ point to point DWDM AWG OC-192 (10Gbps)

400Gbps


640Gbps


3.2Tbps

1.2.2 ADVANTAGES OF AWGS IN WDM NETWORK

There are a lot of AWGs advantages in WDM system compared to TFF and

FBG filters. The main advantages are the lower loss, higher number of channels, and

lower cost per channel as described in Section 1.2.1. All these advantages carry

DWDM network to a much higher speed and capacity. Besides that, AWGs also carry

the characteristic of narrow and accurate channel spacing, polarization insensitivity,

high stability and reliability. Because the channels for AWGs are added in parallel form,

the size of AWGs device will not increase much although the number of channels

increases significantly [16, 17].

1.3 FABRICATION OF AWGS

Fabrication of AWGs is based on PLC fabrication technology. AWGs has been

fabricated on various optical material, such as silica-on-silicon (SiO2:Si) [19], silicon-

on-insulator (SoI) [20], silicon oxynitrides [21], indium phosphide (InP) [22], lithium

niobate (LiNbO3) [23], polymers [24], organic-inorganic hybrid materials [25], and


7

more. In this research, the optical material for the fabrication of AWGs is silica-on-

silicon. Silica-based materials are employed because of their excellent design flexibility,

stability, and mass-productivity. Besides that, the modal field matches well with that of

a fiber. Low propagation loss (<0.05 dB/cm) and high fiber-coupling efficiency (losses

in the order of 0.1 dB) also encourage the usage of SoS. Size is one of the disadvantages

of using SiO2:Si as the optical material. The fiber matched waveguide properties that

prohibit the use of short bends causes the AWGs size to be relatively large. But this

problem is presently being improved by using higher index contrasts in combination

with spot-size converters to keep fiber coupling losses low [15].

Silica-based optical waveguide are fabricated on silicon substrates by a

combination of silica deposition by Flame Hydrolysis Deposition (FHD), chromium (Cr)

coating by DC Magnetron Sputtering System, photolithography process, wet etching

process, and silica etching by Inductive Couple Plasma System (ICP). The waveguide

on silicon wafer will then be sent for dicing, lapping, polishing, testing, and packaging

to produce commercial product.

Pattern transfer is an important element in fabricating the PLC devices. This is

where the circuit designs of the PLC being replicated. Photolithography is a Latin word

which mean light-stone-writing [26]. Photolithography plays an important role in the

pattern transfer process as it is used to transfer pattern from a master copy of the

patterned mask onto the photoresist (PR) on the substrate surface. Photolithography is

the most complicated, expensive, and critical processes in mainstream integrate circuit

fabrication [27]. Photolithography accounts for about one-third of the total fabrication

cost, a percentage that is rising based on the requirement to be achieved.

Pattern transfer processes involve photolithography process, wet etching process

where the PR pattern is transferred to the Cr layer, and the dry etching process where

the Cr pattern is transferred to the silica core layer. Metal masking is considered as part


8

of the pattern transfer processes which start from the Cr coating to the Cr wet etching

process.

1.4 MOTIVATYION AND OBJECTIVES

The focus of this research of the fabrication process of AWGs which involve Cr

coating, photo-resist (PR) coating, ultra-violet (UV) light expose, PR develop, Cr wet

etching, and PR removal. The objective of metal masking control is to ‘print’ Cr pattern

profiles that meet the required specifications. The Cr profile is described by three

parameters: critical dimension (CD), sidewall angle (SWA), and the final Cr thickness.

CD is the most important parameter to control since it has the greatest impact on device

performance because CD is the parameter which determines the dimension of the core

and the core side wall surface of the waveguide. SWA and final Cr thickness are also

important parameters to control because these will have impact on subsequent processes

[28]. The main issue in CD control is the inexistence of satisfactory control steps or

parameters for CD variation. The CD controlling is done based on the operator

experience.

The aim of the metal masking processes is to create a layer of Cr AWGs pattern

on the core layer from the patterned mask. The Cr pattern CD, SWA and thickness are

very important parameters in determining the accuracy of the CD, SWA and thickness

of the core in RIE process. The core dimension, SWA and thickness in turn will create

losses in signal that will affect the performance of the fabricated devices.


9

REFERENCES

[1] Hecht, J. (2002). Understanding Fiber Optics (4th ed.). Upper Saddle River, N.J.,

USA: Prentice Hall.

[2] ANDevices, Inc. (2007, March). Planar Lightwave Circuits: A Vital Technology

for ROADM Networks of Today and Tomorrow. Retrieved September 21, 2007,

from http://www.andevices.com/PDF/PLCs_in_ROADM-white_paper-03-21-

2007.pdf

[3] Agrawal, G. P. (1997). Fiber-Optic Communication Systems (2nd ed.). New

York, U.S.A.: John Willey & Sons.

[4] ElectonicCast, Corp. (2001, December). Planar Waveguide Circuits Global

Market Forecast. Retrieved October 21, 2007, from

http://www.the-infoshop.com/pdf/el8685.pdf

[5] Okuno, M., (1999). Recent Progress on Silica-Based Planar Lightwave Circuit.

Laser and Electro-Optics, 3, 583 – 584.

[6] Suzuki, S., & Sugita, A. (2005, July). Recent Progress in Silica-Based Planar

Lightwave Circuits (PLCs). NTT Technical Review, 3 (7), 12-16.

[7] Pearson, M. (2007, June). FTTx Technologies: Planar Lightwave Circuits

Revolutionize photonics. Laser Focus World, 43 (6), Retrieved November 2,

2007, from

http://www.laserfocusworld.com/display_article/294655/12/ARCHI/none/Feat/F

TTx-TECHNOLOGIES:-Planar-lightwave-circuits-revolutionize-photonic

[8] Shani, Y., & Kopelovitz, B. Z. (2004, November 9). Constant Power Operation

Thermo-Optic Switch. US Patent 6816665. Retrieved October 29, 2007, from

http://www.freepatentsonline.com/6816665.html

[9] Kern, P., Berger, J. P., Haguenauer, P., Malbet, F., & Perraut, K. (2001,

January). Planar Integrated Optics and Astronomical Interferometry. Comptes

Rendus de l'Academie des Sciences Series IV Physics, 2 (1), 111-124.

[10] Shmulovich, J., & Stevens, R. (2005, January). Planar Waveguide Circuits will

be a Key Technology for Next-Generation Military Systems. Military &

Aerospace Electronics. Retrieved November 2, 2007, from

http://mae.pennnet.com/Articles/Article_Display.cfm?Section=Articles&Subsec

tion=Display&ARTICLE_ID=220669

[11] Sun, F. G. Xiao, G. Z., Zhang, Z. Y., & Lu, Z. G. (2007). Modeling of Arrayed

Waveguide Grating for Wavelength Interrogation Application. Optics

Communications, 271, 105-108.


10

[12] Komai, Y., Nagano, H., Kodate, K., Okamoto, K., & Kamiya, T. (2004).

Application of Arrayed Waveguide Grating to Compact Spectroscopic Sensors.

Japanese Journal of Applied Physics, 43 (8B), 5795-5799.

[13] Muralidharan, B., Balakrishnan, V., & Weiner, A. M. (2006). Design of Double-

Passed Arrayed-Waveguide Gratings for the Generation of Flat-Topped

Femtosecond Pulse Trains. Journal of Lightwave Technology, 24 (1), 586-597.

[14] Takenouchi, H., Tsuda, H., & Kurokawo, T. (2000). Analysis of Optical-Signal

Processing Using an Arrayed-Waveguide Grating. Optics Express, 6 (6), 124-

135.

[15] Smit, M. K. (2005, June 22-24). Progress in AWG Design and Technology.

Fibres and Optical Passive Components, Proceedings of 2005 IEEE/LEOS

Workshop, 26-31.

[16] Parker, M. C., Walker, S. D., Augustin Y. T., & Mears, R. J. (2000).

Applications of Active Arrayed-Waveguide Gratings in Dynamic WDM

Networking and Routing. Journal of Lightwave Technology, 18 (12), 1749-1756.

[17] Kamei, S., Ishii, M., Kitagawa, T., & Hibino, Y. (2002). 64-Channel Very Low

Crosstalk Arrayed-Waveguide Grating Multi/Demultiplexer Module Using a

Cascade Connection Technique. Optical Fiber Communication Conference and

Exhibit, 68-69.

[18] Apollo, Inc. (2003). Array Waveguide Grating (AWG) – Design, Simulation and

Layout. Retrieved September 24, 2007 from

http://www.apollophoton.com/apollo/APNT/APN-APSS-AWG.pdf

[19] Takada, K., & Abe, M. (2002). Determination of Leaky Modes in Planar

Multilayer Waveguides. Photonics Technology Letters, 14, 813.

[20] Liu, W. J., Lai, Y. C., Weng, M. H., Chen, C. M., & Lee, P. H. (2004).

Simulation and Fabrication of Silicon-Oxynitride Rib Structure Arrayed

Waveguide Grating Devise. Proceedings of SPIE, 5363, 164-175.

[21] Liu, W. J., Chen S., Cheng, H. Y., Lin, J. D., & Fu, S. L. (2007). Fabrication of

Amorphous Silicon Films for Arrayed Waveguide Grating Application. Surface

& Coatings Technology, 201, 6581-6584.

[22] Kohtoku, M., Sanjoh, H., Oku, S., Kadota, Y., Yoshikuni, Y., & Shibata, Y.

(1997). InP-Based 64-Channel Arrayed Waveguide Grating with 50 GHz

Channel Spacing and Up To -20dB Crosstalk. Electronic Letter, 33, 1786.

[23] Okayama, H., Kawahra, M., & Kamijoh, T. (1996). Reflective Waveguide Array

Demultiplexer in LiNbO3. Journal of Lightwave Technology, 14 (6), 985-990.


11

[24] Ma, C. S., Zhang, H. M., Zhang, D. M., Cui, Z. C., & Liu, S. Y. (2004). Effects

of Trapezoid Core Cross-Section on Transmission Characteristics of Polymer

Arrayed Waveguide Grating Multiplexers. Optics Communications, 241, 321-

331.

[25] Wang, B. L., & Hu, L. L. (2005). Effect of Water Content in Sol on Optical

Properties of Hybrid Sol-Gel Derived TiO2/SiO2/Ormosil Film. Material

Chemistry and Physics, 89, 417-422.

[26] Darling, R. B. Micro Fabrication – Photolithography. Retrieved October 29,

2007, from University of Washington website:

http://www.ee.washington.edu/research/microtech/cam/PROCESSES/PDF%20F

ILES/Photolithography.pdf

[27] Campbell, S. A. (2001). The Science and Engineering of Microelectronic

Fabrication (2nd ed.). USA: Oxford University Press.

[28] Chemali, C. E., Freudenberg, J., Hankinson, M., & Bendik, J. J. (2004). Run-to-

Run Critical Dimension and Sidewall Angle Lithography Control Using the

PROLITH Simulator. Transactions on Semiconductor Manufacturing (OPAC),

17 (3), 388-401.

Chapter 2: Arrayed Waveguide Gratings (AWGs)

12

CHAPTER 2

ARRAYED WAVEGUIDE GRATINGS (AWGS)

2.1 THEORY OF OPTICAL WAVEGUIDE

Optical waveguide is a transparent structure that can guide light. Most of optical

waveguides are made from glass material, especially very pure glass material. From a

chemical standpoint, the most efficient waveguide used in telecommunication is

essentially pure silicon dioxide, known as silica (SiO2). However, some of the

waveguides do exist in other types of materials beside glass such as plastic and fluoride

compounds for specific usage [1].

2.1.1 TOTAL INTERNAL REFLECTION (TIR)

The speed of light in vacuum (c) is often considered the universal speed limit.

Light always travels more slowly when it passes through any transparent material. The

slowing down effect depends on a parameter of the material call refractive index, n.

Refractive index, n is the ratio of the speed of light in vacuum to the speed of light in

the material:

mat

vac

ccn = (2.1)

Speed of light in vacuum is always faster than speed of light in other transparent

material, hence n is always greater than 1 [1].

From the characteristic of light, it is well known that light always travel in a

straight line. But this does not happen when it reaches a surface where refractive index,

n of both side of the surface is different. Light is bent at the surface of the material

when travelling from one material to the other material with different refractive index, n.


13

The angle of refraction can be calculated by using Snell’s law:

rrii nn θθ sinsin = (2.2)

where ni is the refractive index of incident medium, nr is the refractive index of the

material where the light travel into after the refraction, θi is the angle of the incident

light to the normal of the surface, and θr is the angle of the refracted light to the normal

of the surface as shown in Figure 2.1.

Figure 2.1: Light refraction from one material to another material with different refractive index where nr > ni

From equation (2.1) and (2.2),

r

i

i

r

r

i

cc

nn

==θθ

sinsin (2.3)

For light travelling from a higher refractive index material to a material with

lower refractive index where nr < ni, θr will eventually reach 90° with increase in θi.


14

When θr = 90°, θi = θc

i

rc

nn

=°90sin

sinθ

i

rc n

n=θsin

)arcsin( irc nn=θ (2.4)

According to Snell’s Law, refraction cannot take place for any incident angle

above θc, which is called the critical angle, due to the value of sin θ equalling 1. At this

point, light will travel on the surface of the two materials as shown in Figure 2.2.

Figure 2.2: Light refraction on the surface with nr < ni, and θr = 90°

For any incident light above the critical angle, θc light will experience reflection,

and this is called total internal reflection (TIR). TIR effect bounce the light back into the

first material, obeying the law that the angle of incidence equals to the angle of

reflection as shown in Figure 2.3 [1- 4].


15

Figure 2.3: Total Internal Reflection (TIR) where θ1 = θ2

2.1.2 PRINCIPLE OF OPTICAL WAVEGUIDE

There are two common types of optical waveguides, cylindrical waveguide and

rectangular waveguide. The application of cylindrical waveguide is fiber optic, while

the rectangular waveguide as planar waveguide. The basic structure of the optical

waveguide is quite simple. The light-guiding core of the waveguide of a transparent

material which has refractive index that is slightly higher than the surrounding cladding.

The difference in refractive index causes the total internal reflection that helps to guide

the light through the core [1].

The refractive index difference for core and cladding is very small. In practice, it

is only about 1% where n1/n2 ≈ 0.99 (n1 = core refractive index, n2 = cladding refractive

index). From Snell’s Law, the critical angle, θc is approximately 82°. Hence the incident

light that has angle smaller than 82° will lose part of its energy to the cladding every

time it bounces at the surface until it disappear as shown by the blue ray in Figure 2.4.

Incident light with critical angle, θc will travel along the surface between core and

cladding after the first bounce on the surface as indicated by the black ray in Figure 2.4.

While the incident light with incident angle larger than θc will travel along the fiber

without significant loss [1].


16

Figure 2.4: Light rays travelling in fiber with different angles, (a) blue colour ray with incident angle θ1 < θc; (b) black colour ray with incident angle, θc; (c) green ray with incident angle θo > θc

Another way to consider the propagation of the light in the waveguide is through

the acceptance angle, Ac from Figure 2.4. The acceptance angle is the angle where the

light ray entering the waveguide will be guided along its core inside the waveguide. It is

intended as a measurement of the light capturing ability of the fiber. The sine of

acceptance angle, sin Ac, known as numerical aperture (NA) [2], [3].

cAnnNA sin)( 22

21 =−= (2.5)

Looking at Figure 2.4, The NA is sin Ac. Another useful expression for NA is:

)90sin(1 cnNA θ−°= (2.6)

This relates the NA to the refractive index of the core and the maximum angle at

which a bound ray may propagate [2]. To summarize, NA carries the following

information:

1) The ability of the optical waveguide to gather light at the input end.

2) The contrast in refractive index between the core and the cladding which

will give an idea of the maximum bending radius for the optical waveguide

before the loss becomes a problem.

3) The number of modes and the dispersion of the signal in multimode fiber.

4) The level of dopant in the cladding, hence the attenuation due to the level of

dopant.


17

2.1.3 OPTICAL WAVEGUIDE ATTENUATION

Attenuation or transmission loss makes signal strength fade with distance in an

optical waveguide. There are three main types of attenuation in optical waveguide

which are absorption, scattering, and leakage of light from core. Normally, absorption is

caused by the core material. Every material absorbs some light energy. The percentage

of absorption depends on the wavelength and material. For pure silica, the absorption is

negligible in the entire 0.8-1.6 μm band that is used for optical communication. This

kind of absorption is called intrinsic absorption [1, 3]. The main loss in material

absorption is caused by the impurities of the core layer. The impurities of the core layer

will increase the absorption portion dramatically. The absorption caused by the

impurities is called extrinsic absorption [3]. Absorption is uniform and cumulative. If

the impurities of a material are uniform, the fraction of light being absorbed will be the

same per unit length. To calculate the fraction of light remaining after certain lengths of

waveguide:

Fraction of remaining light = (1 – α) D (2.7)

where α is the fraction of light absorbed per unit length and D is the total length [1].

Rayleigh scattering takes place when the light is scattered in another direction

from its original track. The scattering depends on the size of the particles relative to the

wavelength of light. The closer the wavelength to the particle size, the more scattering

will happen. The loss coefficient, αR of Rayleigh scattering at a wavelength λ can be

written as

4/λα AR = (2.8)

where A is called the Rayleigh scattering coefficient. Like absorption, Rayleigh

scattering is uniform and cumulative [1]. Hence the relationship for the fraction of

remaining light is the same for scattering as in equation 2.7.


18

The leakage loss or bending loss is mainly caused by the bending of the optical

waveguide, the smaller the bending radius the bigger the leakage loss from the core to

the cladding. There are two types of bending in optical waveguide, one is macro

bending and the other one is micro bending. Macro bending is the bending of the

waveguide while micro bending is caused by the roughness or imperfection of the core

surface. Macro bending loss can be controlled by controlling the bending of the optical

waveguide, but micro bending loss can only be improved through the fabrication

processes of the waveguide [2].

2.2 THEORY OF ARRAYED WAVEGUIDE GRATINGS (AWGs)

AWGs are formed by three main parts, two input/output waveguide, two slab

waveguide (or free propagation zones, FPZ) and one arrayed waveguide with equal

length difference between adjacent array waveguides as shown in Figure 2.5.

Figure 2.5: Structure of AWG

For a demultiplexer, multiplexed optical signals with different wavelength from

λ1 to λn are transmitted from the input waveguides to the first FPZ. When the input

signals enter the first FPZ they will diverge in the FPZ and be transmitted to the arrayed

waveguide as shown in Figure 2.6.


19

Figure 2.6: Divergence of multiplexed wavelengths to arrayed waveguides in first FPZ

The length of the arrayed waveguides is designed such that the optical path

length difference ∆L between adjacent waveguides is equal. The equal length difference

between adjacent array waveguides will create a phase difference, so that focusing

occurs at spatially separated points at the end of the second FPZ depending on the

wavelength as shown in Figure 2.7. Thus signal of differing wavelengths can be

coupled to separate output waveguides that will lead to the end of the AWGs. The basic

operation of the AWGs demultiplexer is the same as the basic operation of AWGs

multiplexer. Hence the AWGs demultiplexer can be used as a multiplexer in the reverse

direction because of the reciprocity [5, 6]. The theoretical design of AWGs can be

referred to Appendix A.

Figure 2.7: Optical ray path of different wavelengths at second FPZ


20

2.2.1 ATHERMAL AWGs

Thermal dependence of refractive index in silica based AWGs causes the

shifting of the output channels wavelength of AWGs [6- 8]. The output channels

wavelength will normally be shifted approximately 11pm/°C typical for conventional

SiO2 AWGs [9, 10]. There are two methods to eliminate the wavelength shift. One is by

using active temperature stabilization to keep the AWGs channel on the ITU grid

(International Telecommunication Unit). The AWGs chip temperature must be stable to

better than ±1°C to avoid affecting the AWGs performance. However this method is

normally not considered due to the AWGs multiplex/demultiplexer (Mux/Demux) in a

wavelength division multiplexer of passive optical network (WDM-PON) system is

designed to work passively. Thus, temperature insensitive AWGs which are named

athermal AWGs were introduced [6- 22]. Figure 2.8 shows one of the packaging

designs for athermal AWGs and Table 2.1 shows the operation of athermal AWGs. A

compensating plate is added to move the position of the output waveguide by the

contraction and expansion of the compensating plate. The compensating plate is

normally made of copper or aluminum [12, 22]. The creation of the athermal AWGs has

successfully reduced the shifting of the center wavelength in conventional AWGs

significantly. The temperature dependence of a center wavelength is reported to be as

low as ±15pm for all channels for operating temperature range of -30~70°C [22].


21

Figure 2.8: An athermal AWGs packaging design

Table 2.1: Configuration of athermal AWGs Conventional AWG Athermal AWG

Low

Temperature

Focus Point Shift

Waveguide is Moved by Thermal Contraction

Room

Temperature

High

Temperature

Focus Point Shift

Waveguide is Moved by Thermal Expansion


22

2.2.2 ATHERMAL AWG IN UNIVERSITY OF MALAYA

In the University of Malaya (UM), athermal AWGs are fabricated using a

master copy of athermal AWG photomask provided by Fira Photonics Co. Ltd. The

photomask contains six athermal AWG pattern as shown in Figure 2.9 (a). A clearer

picture of the athermal AWG design can be seen from Figure 2.9 (b). It is a 6 x 22

athermal AWG which means that the fabricated AWG will comprise of 6 input and 22

output ports. Conventional demultiplexer AWGs needed only single input where the

signal of multiple wavelengths couples in.

There are two cases where the multi input port function differently. The first

case is the coupling in of multichannel signals with constant channel spacing, entering

from any input ports. The signal will be split into individual channels, which will appear

at different output ports as the signal is coupled into the AWGs in single input. Thus,

the multiple input ports in AWGs act as the multiplexer. The second case is the

coupling of multiple WDM signals with the same channel wavelengths entering through

different input ports. The transmission spectrum from any output port will then be

periodic [23].

(a) (b) Figure 2.9: Athermal AWGs photomask in University of Malaya


23

The separation between adjacent channels for both input and output ports is

designed to be 250μm to match optical fibre diameter. This will allow optical fibres or

optical fibre ribbons to bond to the AWGs easily. The core of each rectangular

waveguide is 6 x 6μm2 in size to facilitate coupling of light to optical fibre.

2.2.3 ISSUES AFFECTING THE PERFORMANCE OF AWGs

There are a few issues affecting the performance of AWGs. The main issues

include crosstalk, insertion loss, polarization, and passband [24].

A) Crosstalk

Crosstalk is generally caused by a combination of six mechanisms which are

receiver crosstalk, truncation, mode conversion, coupling in the array and phase transfer

incoherence, and background radiation. The first four can be kept low by proper design,

but the other two are caused by the imperfections in the fabrication process. The most

obvious crosstalk will be the receiver crosstalk which is caused by the coupling between

the receivers through the exponential tails of the field distribution. Truncation of

propagation field by the finite width of the output array aperture will cause power to be

lost at the input aperture, and at the output aperture the sidelobe level of the focal field

will increase. Mode conversion is caused by the “ghost” image which exists due to the

multimode junction. The “ghost” image that occurs at different locations may couple

into an undesired receiver thus degrading the crosstalk performance. Coupling in the

array is the crosstalk incurred by phase distortion when coupling in the array input and

output. Phase transfer incoherence and background radiation is the crosstalk caused by

the imperfections in fabrication process which include the deviation of propagation

constant and rough waveguide edges [24-27].


24

B) Insertion loss

Insertion loss in AWGs is mainly caused by the inefficient coupling between the

Free Propagation Zone (FPZ) and the arrayed waveguide. First is the diffraction loss in

the first FPZ due to the finite number of arrayed waveguides. Second is the imperfect

focusing loss in the second FPZ due to the waveguide gap between arrayed-waveguides

at the slab-array interface that is determined by the mask process. Other reasons that

cause insertion loss include the fiber to waveguide coupling loss, bending loss at the

arrayed waveguide, material’s intrinsic loss, scattering loss due to fabrication errors and

waveguide roughness, and more [24-27].

C) Polarization

There appear two kinds of polarization in AWGs, one is the polarization

dependent dispersion and the other one is polarization rotation. In normal waveguide

boundary conditions, quasi-TE (Transverse Electric) and quasi-TM (Transverse

Magnetic) polarized modes will propagate at different speeds (birefringence) and results

in a shift of spectral responses with respect to each other which is known as polarization

dispersion. Polarization dispersion may cause the wrong coupling at the output

waveguide and causing crosstalk problems [24, 27]. Curve waveguide like arrayed

waveguide will exhibit a certain amount of polarization rotation by nature [28, 29].


25

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26

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27

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http://www.apollophoton.com/apollo/APNT/APN-APSS-AWG.pdf

[28] Van Dam, C. et al, (1996). Novel Compact Polarization Converters Based on

Ultra Short Bents. Photonics Technology Letters, 8, 1346-1348.

[29] Lui, W. W., Hirono, T., Yokoyama, K., & Huang, W. P. (1998). Polarization

Rotation in Semiconductor Bending Waveguides: A Coupled-Mode Theory

Formulation. Journal of Lightwave Technology, 16 (5), 929-936.

Chapter 3: Fabrication of Arrayed Waveguide Gratings (AWGs)

28

CHAPTER 3

FABRICATION OF ARRAYED WAVEGUIDE

GRATINGS (AWGS)

3.1 FABRICATION PROCESSES

The AWG fabrication process flow is shown schematically in Figure 3.1. The

AWGs fabrication process can be divided into three major steps which are silica grown

glass, metal masking, and silica etching. Silica growth is the process where the silica

(SiO2) layers with different Refractive Index (RI) are deposited on a silicon (Si) wafer.

At the initial stage, two different RI SiO2 layers are deposited on the Si wafer. The first

layer deposited is called under-clad (UC) layer while the second layer is called core.

The core layer has a slightly higher RI compared to the UC. A process called metal

masking is employed to create a thin layer of Chromium (Cr) metal mask with an AWG

pattern imprinted onto the core layer. The wafer will then go through the glass etching

process. The etching process will etch the area on the core layer which are not covered

by the Cr mask and leaves the protected core under the Cr mask. The Cr layer is then

removed, leaving the patterned core layer on the UC layer. The wafer will go through

the glass growing process again to deposit another SiO2 layer which is called over-clad

(OC) layer to cover the core pattern on the UC layer.


29

Figure 3.1: Process flow for the AWGs fabrication

3.2 GLASS FABRICATION

There are large numbers of deposition methods that can be adopted to deposit

the thick glass layers required for silica-on-silicon integrated optics. These include

thermal oxidation and nitridation, sputtering, chemical vapour deposition (CVD),

plasma-enhanced chemical vapour deposition (PECVD), flame hydrolysis depositions

(FHD), sol-gel deposition (SGD) [1, 2]. The most commonly used methods nowadays

are PECVD and FHD. In this research, FHD is the chosen method to fabricate the

AWGs. FHD is a promising method due to its high deposition rate at low production

cost to gain a thick silica glass layer which can be loaded with other dopants such as Ge,

B, P, and Ti [3, 4].

The glass growing process is done by the combination of FHD porous silica

glass “soot” deposition and furnace consolidation. In FHD, the metal chlorides material

such as SiCl4, POCl3, BCl3, and GeCl4 (act as a dopant and not necessary in every

deposition) are vaporized under bubbling technique with He or O2 as carrier gases. A

mixture of raw gaseous metal chloride materials will experience hydrolysis and

oxidation in the high temperature oxy-hydrogen torch flame (1300-1500oC) which is

directed towards the samples. The deposition is done by thermophoretic mechanism

where the metal oxide particles are nucleated by the hydrolysis reaction, cohered by the


30

Brownian motion, and then synthesized in polydisperse particles [5]. The hydrolysis

and oxidation reaction of metal chloride materials to form the silica glass soot layer is

summarized in Table 3.1 [5, 6]:

Table 3.1: Hydrolysis and oxidation reaction of metal chloride materials Material Product and chemical reaction Effect

Silicon Pure silica glass (SiO2)

SiCl4(v)+2H2O(v)→SiO2(s)+4HCl(v)

None

Phosphorous Phosphosilicate glass (SiO2:P2O5)

2POCl3(v)+3H2O(v)→P2O5(s)+6HCl(v)

Thermal expansion: increase

Glass melting point: decrease

Refractive index: increase

Boron Borosilicate glass (SiO2:B2O3)

2BCl3(v)+3H2O(v)→B2O3(s)+6HCl(v)



Refractive index: decrease

Germanium Germanosilicate glass (SiO2:GeO2)

GeCl4(v)+2H2O(v)→GeO2(s)+4HCl(v)



Refractive index: increase

* H2O is the result of oxy-hydrogen flame where 2H2 (v) + O2 (v) → 2H2O (v)

The porous silica glass soot needs to be consolidated before it become hard and

transparent as a silica glass. The consolidation process includes a few temperature steps

for certain reason. First stage, the silica will be heated at 200oC for dehydration to

remove water or H2O. The removal of H2O will reduce the water absorption peak in

silica significantly. Then the temperature will be ramped up to 850oC and is held for

~30 minutes for bubble out-gassing. This step will prevent the formation of bubbles

(pores) at the silica glass. After that, the temperature will be ramped up to ~1300oC for

consolidation process. In consolidation process, the silica glass soot is melted into

viscous liquid form. Finally the temperature will start to ramp down as the glass molten

is slowly quenched into a final transparent glass coating.


31

3.3 METAL MASKING

Metal masking is a series of processes to create a metal mask pattern on the

surface of silica layer. The usage of the metal mask is to protect the needed core layer

during glass etching in later processes. Metal masking starts from chromium (Cr) thin

film deposition, followed by photolithography, and finally Cr wet etching. The Cr thin

film deposition is done by using Direct Current (DC) magnetron sputtering system to

deposit a layer of Cr thin film which is thick enough to protect the core layer in glass

etching later. Details of Cr deposition will be discussed later in Chapter 4.

Photolithography is the process where the pattern from the photomask (original copy

pattern) is transferred to the photoresist (PR) on the Cr thin film. The pattern transfer

processes include PR coating, Ultra-Violet (UV) exposure, and PR developing.

Photolithography will also be discussed in more detail in Chapter 5. The last part of the

metal masking is the Cr etching process. The etching method of choice is wet etching

which depends on the chemical reaction of the chemical on Cr thin film. The chemical

reaction will etch away the areas not covered by Cr thin film and leave those parts

which are covered by the PR. Details of Cr etching will also be discussed in Chapter 6.

The PR will then be removed by using acetone stripping, leaving the Cr mask behind.

The process flow of metal masking is shown in Figure 3.2.

Figure 3.2: Process flow for the metal masking


32

3.4 GLASS ETCHING

Glass etching is often divided into two classes, wet etching and dry etching. Wet

etching consists of chemical reaction between the etchant and the glass and it is

normally done by immersing or bath technique. Wet etching is an isotropic etching

technique which is difficult to control and prone to high defect level. Problems like

undercut, rough surface and high etch bias will always exist in wet etching. Dry etching

or normally called plasma etching is an anisotropic etching. Dry etching can be done by

chemical reaction between the gaseous etchant and the glass layer or by physical

etching using ion bombardment to erode the glass layer. Dry etching carries the

advantages of less sensitivity to temperature, high repeatability, no undercut, and less

chemical waste [7, 8].

In this research, Inductively Coupled Plasma (ICP), which is one of the plasma

etching method, is chosen for the silica core etching. ICP has the advantages of high

etch rate, cleaner, higher selectivity, and lower surface damage due to the lower

operating energy compared to other plasma etching techniques. CF4 that is used in the

ICP as an etchant gas will react with the silica layer as:

In plasma condition, CF4 + e → CF3+ + Fo +2e

SiO2 + 4F → SiF4 + O2

Generally, SiO2 (s) + CF4 (g) → SiF4 (g) + CO2 (g) (3.1)

The reaction shows above will remove the unwanted area of silica layer leaving

behind those areas which cover by the Cr mask.


33

3.5 SUMMARY

In this chapter, the main parameters related to sputter deposition,

photolithography, and etching were studied. These parameters are specific to the

equipments employed and are therefore, unique to the processes we use here. The

production yield and Critical Dimension (CD) of the fabrication process by using the

original fabrication recipe and method are unfortunately extremely low. The production

yield of the complete Cr hard mask pattern is below 40% and the CD of the Cr pattern is

totally out of the measurement scale printed on the AWG photomask. Optimization of

these important parameters is essential in order to produce commercialize grade AWGs.

Table 3.2 shows the important parameters in these three process mention above.

Table 3.2: Important parameters in different processes Process Parameters Important

Sputter

Deposition

- Thickness

- Uniformity

- Roughness

- Chemical stability

- To protect the core layer in glass etching.

- To increase the uniformity in wet etching.

- To reduce the pinhole defect in ICP

etching.

- To optimize the wet etching process in

order to improve the production yield.

Photolithography - PR Thickness

- PR Uniformity

- Critical Dimension

- To protect the Cr pattern area in wet

etching.

- To reduce the effect of depth of focus

when

UV expose.

- To maintain the original pattern

dimension.

Wet Etching - Critical Dimension

- Side Wall Angle

- Side Wall

Roughness

- To maintain the original pattern

dimension

- To create a rectangular core pattern.

- To create a smooth core side wall.


34

REFERENCES

[1] Wu, Y. D., Xing, H., Zhang, L. T., Li, A. W., Zheng, W., Liu, G. F., et al.

(2004). Fabrication and Properties of Vitreous Silica Films Prepared by Flame

Hydrolysis Deposition. Material Chemistry and Physics, 84, 234-237.

[2] García-Blanco, S., Glidle, A., Cooper, J. M., De La Rue, R. M., & Aitchison, J.

S. (2004). Characterization of Germanium-Doped Silica Layers Deposited by

Flame-Hydrolysis. Optical Material, 27, 365-371.

[3] Zhang, L., Xie, W., Xie, W. F., Hou, Y., Zheng, W., & Zhang, Y. (2004).

Characterization of Ge-Doped Silica Films with Low Optical Loss Grown by

Flame Hydrolysis Deposition. Materials Science and Engineering B, 107 (3),

317-320.

[4] Zhang, L., Xie, W., Wu, Y., Xing, H., Li, A., Zheng, W., & Zhang, Y. (2003).

Optical and Surface properties of SiO2 by Flame Hydrolysis Deposition for

Silica Waveguide. Optical materials, 22, 283-287.

[5] Choi, C. G., Jeong, M. Y., & Choy, T. G. (1999). Characterization of

Borophophosilicate Glass Soot Fabrication by Flame Hydrolysis Deposition for

Silica-On-Silicon Device Applications. Journal of Material Science, 34, 6035-

6040.

[6] Sonia, G. B., & Aitchison, J. S. (2005). Direct Electron Beam Writing of Optical

Devices on Ge-Doped Flame Hydrolysis Deposition Silica. Journal of Selected

Topics in Quantum Electronics, 11 (2), 528-538.

[7] Sami Franssila (2004). Introduction to Micro Fabrication. England: John Wiley

& Sons.


Fabrication (2nd ed.). New York: Oxford University Press.

Chapter 4: DC Magnetron Sputter Deposition

35

CHAPTER 4

DC MAGNETRON SPUTTER DEPOSITION

4.1 BACKGROUND

Sputtering or physical sputtering (pulvérization in French) involve the

vaporization of target by momentum transfer as a result of ions bombardment at the

target surface. It was first observed by Groove in 1852 and Pulker in 1858 using von

Guericke-type oil-sealed piston vacuum pumps [1]. Sputter deposition is just the

accumulation of sputtered atoms which are dislodged from the target (cathode) surface

onto a substrate. The sputter deposition is random where not only the substrate will be

deposit but the whole surrounding surface. Sputter deposition of films was first reported

by Wright in 1877 with a relatively poor vacuum. A sputter deposition process to

deposit silver onto wax photograph cylinder was patented by Edison in 1904 [1]. The

wide usage of sputter deposition in industry began when the needed of the deposition

process to be reproducible especially in semiconductor fabrication. The development in

sputter deposition process continued until today with a few different methods like diode

sputter deposition, reactive sputter deposition, bias sputter deposition, ion beam sputter

deposition, and more [2].

Magnetron sputter deposition is one of the variant from sputter deposition

sources which uses magnetic fields transverse to the electric fields at sputtering-target

surfaces. Planar magnetron sputtering was derived from the development of the

microwave klystron tube during world war two, the work of Kesaev and Pashkova

(1959) in confining arcs and Chapin (1974) in developing the planar magnetron

sputtering source [1]. From here the applications of magnetrons with plasma-based

sputtering application were developed. The most useful description available has been


36

the work of Thornton and Penfold, in Vossen and Kern’s book dating from the late ‘70s

[3].

4.2 THEORY OF SPUTTERING

4.2.1 SPUTTERING AND SPUTTER DEPOSITION PROCESSES

As mentioned in the introduction, sputtering is a process that involves the

vaporization of surface atoms by the bombardments of incident ions. The incident ions

are normally generated by plasma or glow discharge. The incident ions will then

accelerate towards the cathode surface electrically [2]. The bombardment between the

incident ion and the surface atom results in energy transfer, similar to the physical

collision between two hard spheres (considered as elastic collision) as shown in Figure

4.1.

Figure 4.1: Elastic collision between two hard spheres

The energy transfer process consider two important laws below,

Conservation of Momentum,

ttiittii vmvmumum +=+ (4.1)

Conservation of Energy,

2222

21

21

21

21

ttiittii vmvmumum +=+ (4.2)

where m is the mass, u is the initial velocity, v is the final velocity, i represents incident

ion, and t represent target ions.


37

By solving the equations 4.1 and 4.2, the energy of the incident ion is relate to

the energy transfer by,

2

2

)(cos4

ti

ti

i

t

mmmm

EE

+=

θ (4.3)

where E is energy, m is the mass, u is the initial velocity, v is the final velocity, i

represents incident ion, t represents target particles (surface atoms), and θ = angle of

incidence as measure from a line joining their centers of masses as shown in Figure 4.2.

Figure 4.2: Collision of two particles

The bombarding effect can physically penetrate into the surface region and

creates a series of collisions among the atoms near the surface. Surface atoms will break

the bonding and dislodge when they gain enough energy from the collisions. All the

removed atoms from the surface are considered as sputtered atoms [2]. This physical

sputtering process is shown in Figure 4.3. The number of sputtered atom for each

bombardment of an incident ion is called sputter yield, Y = (number of sputtered atoms/

number of incident ions). Sputter yield is dependent on the energy transfer as mentioned

earlier. Most of the transferred energy (>95%) appears as heat in the surface region and

near-surface region [1].


38

Figure 4.3: Schematic of a physical sputtering process

4.2.2 DC MAGNETRON SPUTTER DEPOSITION

Magnetrons are a class of cold cathode discharge devices used generally in

diode mode. In DC diode sputtering, secondary electrons are generated from the

cathode due to the bombardments by the surrounding ions. These electrons are

accelerated toward the anode and ionize the surrounding atoms by collision. However

these ionizations are not efficiently use for sustaining the discharge. Furthermore the

secondary electrons that reach the anode will bombard on the substrate that is held on

the anode. These bombardments will increase the substrate temperature and cause

radiation damage [2].

A magnetron sputtering is designed to overcome all the above issues. A

magnetron cathode which a static magnetic field configured at the cathode location is

used in magnetron sputtering system. There are a few types of magnetron in the market

as shown in Figure 4.4.


39

Figure 4.4: Various types of magnetron cathode (Picture taken from [2])

Among those, the planar magnetron is the most common magnetron where the

sputter-erosion path is a close circle or elongate circle (“race-track”) on the target or

cathode surface. To create the magnetic field that is parallel to the cathode surface, the

pole piece assembly is placed behind the cathode. The local polarity of the magnetic

field in the magnetron is oriented such that the ExB will drift the secondary electron in

the desired closed loop depending on the design. The ExB that drifts the secondary

electron can be explained by Hall Effect. The electric field direction, E is always

perpendicular to the cathode surface while the magnetic field direction, B depends on

the design. According to Hall Effect, if an electric current flows through a conductor in

a magnetic field, the magnetic field exerts a transverse force on the moving charge

carriers. The path that the electrons follow can be predicted using the right-hand rule.

Figure 4.5 shows the secondary electrons drift path under the influence of the circular

planar magnetron cathode.


40

Figure 4.5: Magnetic field configuration for a circular planar magnetron cathode (Picture taken from [2])

The secondary electrons that were accelerated inside the closed loop will

experience frequent collisions with atoms of the background gas and this creates a high

discharge density near to the cathode surface. Thus, high discharge density can be

sustained even with lower chamber pressure down to 1 - 2 mTorr compared to the

general diode sputtering which need at least 15 mTorr[3]. The low operation pressure

will significantly reduce the scattering of the sputtered atoms by the background gas on

the way to the sample. This results in the increased probability for the sputtered atoms

to reach the substrate and the increasing of the average kinetic energy of the sputtered

atom. The high density of discharge also means that a high sputtering rate can be

achieved, resulting in higher deposition rate. Besides that, the secondary electrons that

were trapped inside the circle trajectory will not contribute to the increase of the

substrate temperature and radiation damage as they do not bombard the substrate at the

anode [2].


41

4.3 DC MAGNETRON SPUTTERING SYSTEM AND PROCESS

METHODOLOGY IN PHOTONIC RESEARCH CENTRE (PRC)

4.3.1 WAFER LOADING AND UNLOADING IN LOAD-LOCK SYSTEM

PRC own dc magnetron sputtering system is designed to have two different

chambers, one is called process chamber and the other one is call load-lock chamber as

shown in Figure 4.6. Both chambers are connected to each other but the process

chamber is isolated from the load-lock chamber by a load-lock gate valve. The process

chamber is where the sputtering and the sputter deposition processes take place, while

the load-lock chamber is where the loading and unloading of samples is performed.

To load a wafer into the process chamber, the wafer must be cleaned beforehand

(using the cleaning procedures discussed in Appendix B) to reduce the contamination

on the wafer surface and the chamber environment. Argon (Ar) gas is supplied into the

load-lock chamber to increase the pressure in the load-lock chamber until it reaches

atmospheric pressure. The clean wafer is then placed onto a mechanical arm in the load-

lock chamber through the loading door. The load-lock chamber will then be pumped

down with a rotary pump until the pressure is below 1x10-2 Torr to match the process

chamber pressure which is always kept below 1x10-2 Torr. The chamber pressure is

measured using a Pirani gauge.

Pressure matching is very important because when the load-lock gate valve open,

a pressure difference between both chambers will create air flow that might displace the

wafer. The wafer is delivered into the process chamber by the mechanical arm and is

then unloaded onto a substrate stage in the process chamber after the load-lock gate

valve is opened. The load-lock gate valve is then closed to isolate the process chamber

from the load-lock chamber. Thus, with the load-lock system, the pressure of the

process chamber can be kept below 1x10-2 Torr all the time for wafer loading and

unloading.


42

Figure 4.6: DC magnetron sputtering system chamber

There are a few advantages of the load-lock system compared to loading the

wafer into the process chamber directly from ambient. The main advantage is to reduce

the contamination by the water vapor from ambient to the process chamber. Due to the

process chamber not directly exposed to the ambient, the chances for the water vapor

from ambient to be adsorbed by the system inner surface are highly reduced. Secondly,

this system reduces the time needed for the chamber to pump down to pressure below

1x10-2 Torr after exposure to the ambient due to the smaller volume of load-lock

chamber compared to the process chamber, thus reducing the process duration.

4.3.2 PRC DC PLANAR MAGNETRON SPUTTERING SYSTEM

The process chamber consists of a cylindrical chamber, a substrate holder

(anode) with shutter, and a planar magnetron cathode behind the Chromium (Cr) target

(sputtering or deposition material). Besides that, the process chamber is also connected

to an Ar gas inlet which is used as the background gas, a Turbo Molecular Pump (TMP)

that is supported by the rotary pump, and three pressure gauges (one pirani gauge, one

baratron gauge and one ion gauge).


43

a) Cr thin film as silica etching mask and Cr target design

Cr thin film or Cr mask plays an important role in glass etching process for

planar waveguide fabrication. The Cr layer is patterned according to the designed

waveguide circuitries on the silica layer to cover the wanted silica area in the silica

etching process. Silica etching process is normally done either by HF (isotropic etching)

or Reactive Ion Etching (RIE) (anisotropic etching). Metal mask is needed for high

etch depth (5-10μm) for silica RIE process in the planar waveguide fabrication. A metal

mask carry advantages like low temperature deposition process, easy patterning of the

layer by lithography and wet etching solutions and easy removal of the mask after the

glass etching. A Cr etching mask is selected since it adheres to silica film well and is

strong enough to resist the bombardment of the reactive gas and Ar+ ions in the

discharge during an etching process of long duration, typically 40 minutes [4, 5].

Cr 4 inch target was used to match the circular planar magnetron cathode in the

system. The purity of the Cr target is up to 99.99% to avoid contamination from

unwanted materials. Besides that, the target was also designed to contain “racetrack”

depression on the surface to increase the deposition uniformity. In magnetron sputtering,

the erosion of the target is not uniform. The erosion is based on the concentration of the

magnetic field. Thus, the racetrack design not only increases the deposition uniformity,

it will also increase the utilization of the target.

b) Ar gas as background gas

The main consideration of taking Argon gas as the background gas is because it

is an inert gas which is stable and hardly reacts with other materials. Among the inert

gases, Ar is chosen based on its mass. From equation 4.3, notice that the energy transfer

is highly dependent on the mass of the incident particle. The higher the incident mass,

the higher the energy transfers (if the collision angle is neglected). Inert gases like He


44

and Ne are not considered as their atomic mass is too small compared to Cr. Inert gas

like Kr is also not considered due to its atomic mass being too heavy. Although heavier

ions are more efficient in transfer energy, they will also scattered away the sputter

atoms that are moving towards the wafer and hence affect the deposition process.

c) Magnetron cathode with water cooling circulation

The magnetron gun is attached to a water circulating system. As mentioned in

section 4.2.1, more than 95% of the energy transferred to the sputtering target will

appear as heat. The water circulating system keeps the Cr target cool during the

deposition process. The cold surface will minimize the amount of radiant heat in

sputtering systems which benefit the thermal evaporation in vacuum. Thus, even

thermally-sensitive surfaces can be placed near the sputtering target. Besides that,

cooling also prevents diffusion in the target which will lead to changes in the elemental

composition for alloy targets [1].

d) Turbo molecular pump (TMP) supported by a rotary pump

In the process chamber, pressure is maintained under “rough” vacuum

environment (10-2 Torr) by using a rotary pump. To reach a “good” vacuum (10-6 Torr)

environment which is clean enough from process contamination, TMP is used to pump

the chamber pressure further. Normally TMP is operated in the “rough” vacuum

environment as TMP cannot tolerate abrasive particles or large objects due to the close

tolerances in mechanical parts (especially between rotor blades in TMP). Thus, to create

a rough vacuum environment, TMP is always backed by a rotary pump. A disadvantage

of TMP is the poor pumping ability for water vapor since the water molecules will

undergo numerous adsorptions-desorption events before passing through the pump. This

is one of the reasons why the load-lock system is used to reduce the adsorption of the


45

water vapor from the ambient. The advantages of choosing TMP includes non

contamination of motive fluid, omission of high-vacuum valve, lower probability of

back streaming accidents, and it pumps all gases effectively[6].

4.3.3 CHROMIUM SPUTTER DEPOSITION PROCESS METHODOLOGY

After the wafer is loaded onto the substrate holder, the shutter above it is closed

to protect the wafer from deposition before the grow discharge is stable. Ar and N2 (N2

is needed for all the valve operation) pressure and supply is ensured to be sufficient for

the whole process and the water circulation is necessary for the magnetron gun. TMP is

backed by the rotary pump before it switches on. The process chamber is then pumped

by TMP after it reached the maximum spin speed (27000rpm).

After the process chamber reaches a pressure below 10-6 Torr, Ar gas is flown

into the process chamber forming the background gas for the sputtering process. The

flow rate of the gas Ar is controlled by MFCs (Mass Flow Controllers) in sccm

(Standard Cubic Centimeters per Minute) units. The process chamber pressure is set

using the pressure controller which obtains the feedback from a baratron gauge that

connects to the process chamber. Then the pressure controller will control the opening

percentage of the throttle valve to maintain the pressure inside the process chamber.

The process chamber pressure will be stable after a few minutes adjustment by

the pressure controller. DC power supply voltage is increase until the breakdown

voltage and the grow discharge is generated. The DC breakdown voltage can be

calculated by using the following equation

)]ln([

)(PdC

PdAV+

= (4.4)

where V is the DC voltage, A and C are constants depending on the gas used, P is the

chamber pressure, and d is the distance between the cathode and anode [7]. Figure 4.7

shows the glow discharge near the target.


46

Figure 4.7: Grow discharge at target surface

Figure 4.7 clearly shows that the glow discharge form a circular loop at the

target surface. The brightest ring tells the concentration of the magnetic field. After the

glow discharge has been generated, it will take a few minutes for the discharge to

become stable, and this can be observed from the reading of the power supply. Within

this period, the wafer is covered under the shutter to prevent from deposition, which is

referred to as pre-sputtering process. The purpose of pre-sputtering process is to remove

the oxide layer or contaminants which cover the target surface. When the DC power is

first applied to the Cr target, the power will be high and will eventually drop as the

discharge comes to equilibrium. The initial high power is due to the high secondary

emission of the metal oxide as compared to the clean metal and the high density of the

cold gas. As the oxide is removed from the surface and the gas heats up, the power will

drop [1]. After the pre-sputtering process, the deposition process is ready to commence.

After the deposition, the DC power supply voltage is reduced slowly to 0V. The

Ar gas supply is then stopped flowing in and the TMP gate valve is closed.


47

4.3.4 PUMP PURGE CYCLES AFTER DEPOSITION

After the deposition, pump purge cycles are applied to remove the remaining

particles in the process chamber before the wafer is taken out into the load-lock

chamber. The particles can be produced in the process chamber by many sources,

including when wafer loading, the impurities gas supply, and also process chamber

leakage. During the deposition process, these particles do not generally cause

processing problems as they are typically suspended above the wafer in the plasma due

to the electrostatic interaction. However, as the power reduces at the end of the

deposition process, the forces that suspend the particles dissipate allowing them to fall

and land on the wafer causing contamination. To avoid this problem, pump purge cycles

are applied to remove the remaining particles. During pump purge cycles, Ar gas is

supplied into the process chamber until the pressure increases to 10mTorr. After this

pressure is reached, Ar supply will be stopped and the TMP gate valve will be opened

to pump down the process chamber. This rapid removal of the Ar gas will create

turbulence which has a sweeping effect carrying particles with the escaping Ar gas

outside the process chamber [8]. This cycle will be repeated 3 times to make sure that

most of the particles are removed before the wafer is unloaded into the load-lock

chamber.

4.4 EXPERIMENTAL RESULTS AND DISCUSSIONS

Thickness, uniformity, surface roughness, and chemical stability of the Cr thin

film are the main issues to create a quality hard mask for glass etching process. Low

quality hard mask will creates problems like low production yield (<40% for original

process recipe) and poor Critical Dimension (CD). Studies are done on each process

parameters to determine their effect on those critical issues mentioned above. A process

recipe for better production result will be produced in this study.


48

4.4.1 STUDY OF DEPOSITION TIME

The Cr depositions are run under fix DC power (0.8A) (*voltage of power

supply is not considered as it is current control in plasma), substrate temperature (30°C),

Argon flow rate (5sccm), and process chamber pressure (10mTorr). Wafer is covered

with a simple mask when deposition for thickness measurement. Thickness of the Cr

layer was measure at multiple points on the wafer by using surface profiler (Alpha-Step

2000) and plotted in the graph shown in Figure 4.8.

Figure 4.7 shows a linear increment of the Cr thickness with the increase of time.

From the gradient of the graph, the deposition rate of Cr is 43nm/minutes. From the raw

data, it shows ±15% variants of Cr thickness from the mean value which is plotted in

Figure 4.8. The non-uniformity of Cr thickness is mainly cause by the target design. As

mention in the previous section, the target is design with “racetrack” depression to

increase the uniformity. Unfortunately, the uniformity of the deposition will still

degraded after a certain period due to the changes of the “racetrack” causes by

sputtering.

Figure 4.8: Cr thickness for different sputtering time


49

Figure 4.9 shows the change of the grains size due to the increment of the Cr

thin film thickness. The surface morphology of Cr thin film was inspected using AFM

(atomic force microscopy) to study the change of the grain height due to the Cr

thickness. The AFM images for Cr thin film with (70±5) nm and (330±5) nm thickness

are shown in Figure 4.10. For thinner Cr layer, the average grain size is smaller thus the

surface is smoother in Figure 4.10(a). When the Cr thickness increases, the grains grow

and agglomerated together, resulting in less number of bigger grains as shown in Figure

4.10(b). The grains growth can be explained by the surface energy minimization during

the growth process to achieve thermodynamic equilibrium. The growth directions of the

grains are toward a situation of low surface energy [9, 10].

Figure 4.9: Increment of Cr grains height according to thin film thickness

Chemical stability was studied by observing the etching rate of Cr thin film

under the reaction with Cr etchant (Cr-7S). The wafer is place in the Cr etchant until the

Cr thin film is totally removed (*details about Cr etching will be discussed in Chapter

6). During the etching process, we observe that the etching normally finish at the edge

of the wafer and proceed slowly to the center. The period for the etching process to be

start at the edge and period for the whole etching process were taken down and plotted

in the graph in Figure 4.11.


50

a)

b)

Figure 4.10: AFM images of Cr thin film for a) (70±5) nm and b) (330±5) nm thickness


51

The time for the Cr to be removed at the edge increases quite linearly with the

Cr thin film thickness. This is expected as more time is needed for etching a thicker Cr.

However the time needed for the Cr of the whole wafer to be removed increases non-

linearly with the Cr thickness. The higher exposure area and less inner bonding make

the etching process to be faster at the edge of the wafer compared to the center area [11].

This causes a time difference for the etching to finish at the edge of the wafer until the

etching process is done for the whole wafer as shown in Figure 4.11. The center area of

the wafer needed nearly double the time compare to the edge of the wafer to complete

the etching process.

Figure 4.11: Etching time for different Cr thickness

4.4.2 DC CURRENT EFFECT ON CHROMIUM DEPOSITION

In this section, the relation of voltage-current in DC power supply and the effect

of DC current supply is presented. The voltage-current relation in DC magnetron

sputtering system in University of Malaya is shown in Figure 4.12. From Figure 4.12, it

is noticed that the voltage increase quite significantly when the current increase. This is

quite different from a normal glow discharge where the voltage is independent of the


52

total current. In an abnormal glow discharge, the voltage increases significantly to force

the cathode current to increase above its natural value to produce enough secondary

electron for sputtering process [12]. The further increase of the current might causes the

transition of abnormal glow discharge to arc discharge (glow-to-arc transition) which

we do not want it to happen.

Figure 4.12: The voltage and current as a function of total current drawn by DC glow discharge

Study of DC current effect on Cr deposition is done by a series of deposition

with current, Idc from 0.5A to 1.2A. Figure 4.13 shows the Cr thin film thickness with

various DC power supply current, Ic, with deposition time of 5 minutes. The Cr thin

film thickness increases linearly with the Ic. This corresponds to a linear relation

between the Cr thin film deposition rate and Ic. This phenomenon can be explains from

Child’s law and the sputtering rate. The deposition rate is proportional to the sputtering

rate because the sputtering rate decides the number of sputtered Cr atoms from the Cr

target.


53

From the equation of sputtering rate, Rsput,

scmne

JR

Cr

isputsput /1γ= (4.5)

where γsput is the sputtering yield, Ji is the ion current density, and nCr is the atomic

density of Cr target [13]. Thus Rsput α Ji in a more simplify form.

Due to the demagnetized of the ions and the design of the magnetron sputtering

which operate in low pressure, non-collision Child’s law can be used to describe the

flow of ions from the surface to the ring of cathode. By assuming that w << R,

Rw

IJ dc

i π2= (4.6)

where Idc is the DC power supply current, R is the radius from the target center to the

magnetic ring, and w is the width of the magnetic field on the cathode [13]. Equation

(4.5) and (4.6) shows the relation that sputtering rate, Rsput is proportional to DC power

supply current, Idc which explain the experimental results. The graph in Figure 4.13

give a linear graph gradient of approximately 260nm/A for 5 minutes. In another word,

an increment of deposition rate of 52nm/minutes per Ampere (A) increase.

Figure 4.13: Cr film thickness with DC power supply current, Idc


54

A change in the current, Idc of the DC power supply after the plasma generation

will cause some fluctuations in the plasma and it will need some times to stabilize. For

more accurate result, a series of depositions with different time are done on 0.6A, 0.8A,

1.0A, and 1.2A. The results are plotted in Figure 4.14.

The deposition rate of each Idc is calculated and plotted in Figure 4.15. The

graph shows Cr deposition rate with increment of 58 nm/mins for each Ampere of

current which is slightly higher than the value above. The etching periods for samples

above are plotted in Figure 4.16. Although the samples are deposited with different DC

current supply but it does not change the chemical reactivity of the Cr thin film. This

can be seen from the graph in Figure 4.16 as the etching time for all samples are plotted

in the same trend line for different Cr thickness although the deposition DC current are

different.

Figure 4.14: Cr thickness for different deposition times for different Idc


55

Figure 4.15: Cr deposition rate on different DC current supply, Idc

Figure 4.16: Cr etching rate for different Cr thickness coated with different current


56

There are two lines in Figure 4.16, one show the time for the etching process

to be ended at the wafer edge while the other one shows the time for the etching

process to be ended at the wafer center. The etching time difference between the

wafer center and edge has create a huge issue in etching process. Due to the time

difference, pattern at the wafer edge was etched much longer compare to the time it

need. This is called over etch. Over etch will affect the pattern to experience series

undercut (more explanation in Chapter 6) which might destroy the Cr pattern and

cause low production yield (less than 40% in this case). Thus only pattern at the

center of the wafer can survive after the wet etching process.

4.4.3 PRESSURE EFFECT ON CHROMIUM DEPOSITION

In order to study the effect of process chamber pressure on the deposition

process, series of tests are run with different process pressure. Figure 4.17 shows the

DC power supply for different process pressure. It is clearly to observe that for lower

pressure, higher voltage need to be supply to maintain the current density in the glow

discharge condition. This can be explained by the equation of minimum discharge

voltage to sustain the magnetron discharge which proposed by Thornton [14]:

eiISEEei pEWW

Vεεγεγε )(

00min == (4.7)

where W0 is the effective ionization energy, εi is the ion collection efficiency, εe is the

fraction of the number of ions Vmin/W0 that can be made on average by primary electron

before it is lost form the system, and γ is the effective secondary electron emission yield

which is the product of ion-induced secondary electron emission (ISEE) coefficient

γISEE and the effective gas ionization probability E(p) [15].


57

Figure 4.17: DC power supply for different process pressure

The effective gas ionization probability, E(p) is depends on the pressure and

recapture of the electrons. At low pressure, the chances for the electron to collide with

particles reduce and the probability of electron recapture (capture of electron back to

the cathode due to the applied magnetic field) is increase. This is due to the path length

of the electron become much smaller than the mean free path. Hence, the effective

secondary electron emission coefficient becomes smaller than the material dependent

ISEE coefficient. From equation 4.7, Vmin α 1/γ and hence when γ decrease, Vmin

increase. For higher pressure, the probability for electron recaptures decrease due to the

probability of collision with other particles increase. Recapture is prevented if the

electron undergoes a collision which deflects its trajectory by a sufficiently large angle

(> 10°) or lost in energy [16]. Thus electrons will continue circulate and collide. This

phenomenon increases the effective secondary electron emission coefficient and hence

the discharge voltage lower at higher pressure.


58

Figure 4.18: Cr film thickness with different pressure deposition

Figure 4.18 shows the Cr thin film thickness that deposited with same DC

current supply but different pressure. The Cr thickness is quite consistence under such

condition with just a few nm difference between those samples. It shows that the

change in pressure did not give any significant effect on the Cr deposition rate. This is

due to the constant charge particles that maintain in the process chamber which already

explain in section 4.4.2.

However, the change of the process pressure did affect the properties of the

Cr thin film in the spec of chemical stability. Figure 4.19 shows the graph of etching

time difference between wafer center and wafer edge. Figure 4.19 shows that for

higher pressure, the time difference is higher compare to lower pressure. As we

know, the larger the difference time, the lower the pattern yield and poorer the

pattern CD. By changing the process pressure to 6mTorr, the pattern yield has

improve from around 40% to 100% where there do not have any part of the pattern is

totally etch out at the end of the etching process.


59

Figure 4.19: Etching time difference between the wafer center and edge

4.5 SUMMARY

Cr thin film thickness control and chemical stability is very important as it

will affect the end result of the wet etching process especially yield and CD.

Thickness control was studied by varying the deposition time and DC current. The

Cr thin film thickness was directly proportional to the deposition time. The

deposition rate was determined by the DC current supply where each increase in

Ampere will increase the deposition rate at around 58nm/minutes. The study on

deposition pressure give a surprisingly result where it reduce the Cr etching time

difference between the wafer center and wafer edge. The significant reduction in the

time difference at 6mTorr pressure has resulted in an increased in the Cr pattern

yield from 40% to 100%.


60

REFERENCES

[1] Mattox, D.M. (1998). Handbook of Physical Vapor Deposition (PVD)

Processing – Film Formation, Adhesion, Surface Preparation and Contamination

Control. Westwood, New Jersey, U.S.A.: Noyes Publications.

[2] Seshan, K. (2002). Handbook of Thin-Film Deposition Processes and

Technologies- Principles, Methods, Equipment and Applications (2nd ed.).

Norwich, New York, U.S.A.: Noyes Publications.

[3] Rossnagel, S.M., Cuomo, J.J., & Westwood, W.D. (1990). Handbook of Plasma

Processing Technology – Fundamentals, Etching, Deposition, and Surface

Interaction. Park Ridge, New Jersey, U.S.A.: Noyes Publications.

[4] Shin, D.W., & Eo, J.H. (2005). Plasma etching characteristics of Ge-B-P doped

SiO2 film for waveguide fabrication. Journal of Ceramic Processing Research, 6

(4), 345-350.

[5] Dutta, A.K. (1995). Side Wall Roughness Reduction in Deep Silicon Oxide

Etching Using C2F6 Based ECR-RIBE. Japan Journal Applied Physics, 34 (1),

365-369.

[6] Hablanian, M.H. (1997). High-Vacuum Technology: A Practical Guide (2nd ed.).

Marcel Dekker.

[7] Michael, A.L. & Allan, J.L. (1994). Principles of Plasma Discharges and

Materials Processing. New Jersey, U.S.A.: John Wiley & Sons.

[8] Mautz, K.E. (1999). Semiconductor Wafer Processing Chamber for Reducing

Particles Deposited onto the Semiconductor Wafer. United States Patent

5904800.

[9] Chan, K.Y. and Teo, B.S. (2006). Thickness dependence of the structural and

electrical properties of copper films deposited by dc magnetron sputtering

technique. Microelectronics Journal, 37, 608-612.

[10] Chan, K.Y. and Teo, B.S. (2006). Atomic force microscopy (AFM) and X-ray

diffraction (XRD) investigations of copper thin films prepared by dc magnetron

sputtering technique. Microelectronics Journal, 37, 1064-1071.

[11] Joseph A.M. (2003). Chemistry the Easy Way (4th ed.). Hauppauge, New York:

Barron’s Education Series.

[12] Roth, J.R. (1995). Industrial Plasma Engineering – Volume 1 (Principle).

London, UK: IOP Publishing Ltd.

[13] Michael, A.L. and Allan J.L. (1994). Principles of Plasma Discharges and

Materials Processing, United States of America: John Wiley & Sons.


61

[14] Thornton, J.A. (1978). Magnetron sputtering: basic physics and application to

cylindrical magnetron. Journal of Vacuum Science and Technology, 15 (2), 171-

177.

[15] Depla, D., Buyle, G., Haemers, J., & Gryse, R.D. (2006). Discharge voltage

measurements during magnetron sputtering. Surface & Coating Technology, 200,

4329-4338.

[16] Gu, Q.C., Kunhardt, E.E., Levi, E., & Schaefer, G. (1989). The effect of

scattering on the recapture probability of secondary electrons by the cathode in

the presence of a magnetic field. Pulsed Power Conference, 798-800.

Chapter 5: Photolithography

62

CHAPTER 5

PHOTOLITHOGRAPHY

5.1 BACKGROUND

In this study, photolithography was used to create photoresist (PR) pattern on a

Chromium (Cr) layer after the layer deposition via sputtering. Photolithography is a

Latin word which means light-stone-writing [1]. It plays an important role in pattern

transfer processes. Photolithography was developed from lithography which was

invented by Alois Senefelder in Germany in 1798 [2]. First printing of lithography was

done by stone printing. Joseph Nicephore Niepce, in Chalon, France took the first

photograph in 1826 using bitumen of Judea on a pewter plate. The pattern was

developed using oil of lavender and mineral spirits. The first negative PR was

developed in 1935 by Louis Minsk of Eastman Kodak and followed by the development

of first diazoquinone-based positive PR in 1940 by Otto Suess of Kalle Div. of Hoechst

AG [1].

Photolithography was brought into semiconductor manufacturing sector for step

and repeat mask reduction devices since 1959 by using contact aligner. The contact

printing was the work-horse technology for exposing patterns onto Integrated Circuit

(IC) wafer well into the 1970 [3]. Since then, photolithography becomes widely used in

semiconductor manufacturing sector for mass production in fabrication of IC. This saw

the development of photolithography related devices such as stepper and mask aligner

in fulfilling the expanding IC market requirements.

The methodology used in photolithography and its processes determines the size,

weight, cost, reliability and capability of the fabricated device [4]. These variations are

mainly based on the PR used, coating method, and the UV exposure technique.


63

5.2 THEORY

As mentioned earlier, the major parameters determining the quality of the

finished samples are type of photoresist (PR), coating speed, prebake and postbake

temperature, Ultra-Violet (UV) exposure dose, temperature and humidity. As such, in

this section, a brief theoretical treatment of each parameter is provided.

5.2.1 PHOTORESIST (PR)

PR plays an important role in photolithography processes as PR contains the

photosensitive chemical which is sensitive to Ultra-Violet (UV) light. Generally there

are two types of PR, positive PR and negative PR. The type of PR is depending on

whether areas exposed to UV radiation are removed or retained. Positive PR on the area

exposed will be removed after the PR develops, while negative PR on the area exposed

will be retained after the PR develops, as shown in Figure 5.1.

Figure 5.1: Schematic diagram showing the effect of using positive or negative PR on silica glass fabrication


64

Negative PR is a PR with crosslinked polymer types. In these materials exposure

to UV radiation will cause crosslinking of polymer chains as shown in Figure 5.2. The

developer invades the matrix of crosslinked chains and removes any non-crosslinked

polymers. This invasion causes swelling and limits the dimensional control and hence

the resolution of such negative resists [5].

Figure 5.2: Chemical reaction of negative PR under UV radiation to form crosslinked polymer matrix [6]

Positive PR consists of base resin (novolak) which is an acidic polymer,

photoactive compound (PAC) which is the napthaquinone diazide, and organic solvent

[7]. The organic solvent is to maintain the PR in liquid form. Once the organic solvent

is removed, PR will harden and only compound of base resin and photoactive is left in

the PR. When exposed to the UV light, the photoactive compound will absorb radiation

in the exposed pattern areas and change in chemical structure to form carboxylic acid

which is soluble in the developer as shown in Figure 5.3.


65

Figure 5.3: Reaction of PAC prior to the exposure of the UV light [9].

The absorption of the UV radiation causes the nitrogen molecule (N2) to be free

from the carbon ring, leaving behind a highly reactive carbon site. The carbon that bond

to the oxygen atom will move out from the carbon rings to proceed to a more stable

structure which is called ketene. This process is known as a Wolff rearrangement [8].

With the presence of water, the double bond of the external carbon is replaced with the

single bond and an OH group creating a final product call carboxylic acid.

When the PR is in contact with the base solution, the PAC that was not exposed

to the UV radiation will not dissolve in the PR developer (base solution). On the other

hand, carboxylic acid which readily reacts with and dissolves in PR developer will

breakdown into water-soluble amines such as aniline and salts of K (or Na, depending

on the developer) [8]. The dissolution occurs rapidly for two reasons. The first is the

carboxylic acid mixture rapidly taking up water. Second, the nitrogen released in the

reaction will foam the resist, further assisting the dissolution [8].

5.2.2 PHOTOLITHOGRAPHY EXPOSURE METHOD

There are mainly three types of UV exposure method in photolithography

process. They are contact printing, proximity printing, and projection printing. Figure

5.4 reveals the basic structural difference of the methods by different mask alignment

systems. Contact printing is the simplest and most common method in photolithography

process as shown in Figure 5.4(a). In contact printing the photo-mask is held in the


66

direction where the chrome pattern is facing down and brought into intimate contact

with the PR cover wafer. The exposure of the UV light will copy the pattern of the

photo-mask to the PR in the ratio of 1:1. This capability of high resolution printing with

inexpensive equipment is the primary advantage of contact printing. The resolution of

contact printing is determined by mask dimensions and diffraction at mask edges.

Theoretically, submicron patterns can be produced by using contact printing but to

create a submicron mask will be prohibitively expensive. With contact printing method,

there does not exist any space between mask and PR thus, minimizing the effect of

diffraction [8-10]. The disadvantage of contact printing would be the high chances of

scratching on the mask surface. Permanent damage might appear on the mask surface

due to any particle scratch on the PR surface when the mask contact with the PR under

certain pressure.

(a) (b) (c) Figure 5.4: Various UV exposure methods used in photolithography process

Proximity printing is a modification of contact printing to avoid damage caused

by contact printing as shown in Figure 5.4(b). In proximity printing, the mask is

separated from the PR layer at 10 to 50μm [7]. Although it solves the mask damage


67

problem, the resolution also drops due to the diffraction effect. Light passing by the

edges of an opaque mask feature will form fringes and penetrates into the shadow

region. This effect can be explained by Fresnel diffraction, or more specifically,

Huygen’s wavelets [8]. The Fresnel diffraction limits the resolution to a minimum of

about 5μm.

Projection printing is designed to have a high resolution of contact printing

without the defect in contact printing as shown in Figure 5.4(c). In projection printing,

the aligner has been developed to project an image of the mask patterns onto a resist-

coated wafer many centimeters away from the mask. The light that passes through the

mask will be diffracted to a large angle. An objective lens called projector is placed in

between the mask and the wafer to reimage the pattern onto the PR. For the projector to

be able to collect the diffracted light, the acceptance angle (2α) of the projector must be

larger than the diffraction angle of the light. The acceptance angle of the projector is

related to the system numerical aperture, NA by the following equation:

αsinnNA = (5.1)

where α is one-half the angle of acceptance of the projector and n is the refractive index

of the media between the projector and the wafer. Normally the exposure is done in air

where n = 1.0. Typical values for NA range from 0.16 to 0.8 [8].

The resolution limitation of the projection printing is referred to as Rayleigh’s

criteria, Wmin and is given by

NAkW λ

≈min (5.2)

where k is a constant that again depends on the ability of the PR to distinguish between

small changes in intensity (typically k is of order 0.75) and λ is the wavelength of the

UV light source.


68

The reduction of the depth of focus (DOF), σ is the technical price that has to be

paid by projection printing to gain the high resolution with fewer defects. DOF is the

distance where the wafer can be moved while maintaining the projection image in focus

[8]. The DOF can be calculated by equation 5.3

2NAλσ = (5.3)

The degradation in DOF has made this approach to be difficult. In addition,

there are also issues such as wafer flatness, resist thickness, and stage level to overcome.

Thus a tradeoff between resolution and DOF limitations is necessary.

5.3 PHOTOLITHOGRAPHY PROCESS METHODOLOGY

In order to ensure minimal contamination of samples, both deposition and

photolithography are carried out in a controlled environment. The photolithography

process in University of Malaya (UM) is run in a class 10k clean room under yellow

light condition. The clean room is a necessary pre-requisite because dust particles in the

air can settle on the wafer or photomask and cause defects. The dust particles that lay on

the mask will behave as opaque patterns on the mask while the dust particles that lay on

the wafer surface might create permanent scratch or damage to the mask. Class 10k

(10,000) clean room means the clean room has maximum dust count of 10k particles/ft3

with particles diameters of 0.5μm or larger [7].

The 10k clean room is further divided into two sections. The larger section

houses fabrication instruments like Flame Hydrolysis Deposition (FHD), furnace,

Inductive Coupled Plasma (ICP), and Direc-Current (DC) sputtering system. Our

photolithography process is run in a separate room called yellow room. The yellow

color is the result of utilizing the UV filter of the fluorescent lamp. The Yellow room is

needed due to the application of PR in photolithography process. PR is photosensitive to


69

light from wavelength 310 - 440nm and completely transparent after wavelength 475nm.

Thus yellow light which contain wavelength from 570 – 580nm is use in

photolithography room.

5.3.1 PR COATING

Before starting the photolithography process, the wafer must be cleaned

(Appendix B provides the wafer cleaning procedure practiced by our lab) as

photolithography process require highly clean environment in order to work. In UM,

positive photoresist (AZ1518 or AZ1512) is applied on the top of the wafer by spin

coating method. Spin coating is one of the simplest and most common techniques of

applying thin PR onto wafers. PR coating is carried out by dispersing of the PR onto the

wafer, typically in the form of non-uniform stream by using pipette. The pipette is held

as near as possible to the wafer surface to reduce the chances of trapping air or dust

during the process. Air bubbles or dust trapped in the PR will cause defects in the

photolithography process. After the PR was dispensed onto the wafer, the wafer is

accelerated to the required speed slowly.

The rotation of the wafer will cause the PR to spread over the surface of wafer.

The increase of the rotation speed will then create an aggressive fluid expulsion from

the wafer surface. More and more PR on the top layer will be applied while the wafer

below rotates faster and faster. When the wafer reached the required speed, the PR is

thin enough that the viscous shear drag exactly balances the rotational accelerations. At

the constant speed, PR will experience the thinning process. The PR thinning is quite

uniform throughout this stage. This process becomes progressively slow as the coating

thickness is reduced. This phenomenon can be observed as the interference colors ring

stop expanding outward. When the PR become thin enough, solvent evaporation will

dominated the coating thinning behavior. At this point onwards, the PR will slowly


70

transform into “gels” as the solvents are removed. The wafer will then decelerate and

move to next process.

5.3.2 PREBAKE

The evaporation of solvent in the spin coating process involves only the top

surface layer. Further removal of the solvent is done by baking the wafer on a hotplate.

This process is prebaking, where the wafer is heated at temperatures not exceeding

100°C for less than 2 minutes. The baking process will remove most of the solvent.

Only a low percentage of the solvent remains in the dried resist due to the high boiling

point of solvent (>134°C). The residual solvent will influence the dissolution rate of

exposed resist. If the prebake temperature is increased above 100°C, the light sensitive

naphthoquinone diazide (PAC) is thermally decomposed and lithographic performance

is degraded. Thus, temperature and time are quite critical in prebaking.

5.3.3 MASK ALIGNER

Running UV exposure, the wafer is placed at the substrate holder and the

photomask is held above the wafer. A contact aligner is used in UM for UV exposure

purposes. The wafer on the substrate holder will then move up slowly until the wafer

and the photomask is nearly in contact with each other. Contact between wafer and

photomask can be observed when there are fringes appearing on at the mask. The

position of the substrate holder is adjusted so that the wafer is placed at the center of the

photomask. After that, the wafer is moved towards the photomask until they are in

contact. Vacuum is then applied to remove the air trapped in between wafer and

photomask. Once this is achieved, we can observe that fringes are distributed all over

the wafer. The vacuum pressure is increased slowly until the fringes seem to be static

throughout the whole wafer.


71

Once the wafer is properly positioned, it is UV exposed according to the preset

exposure dose. The UV light is generated from a mercury arc lamp source which emits

radiation wavelength from 275nm to 650nm. It has a very high radiant intensity in the

350 – 450nm wavelength range especially in 365nm (i-line), 405nm (h-line), and

436nm (g-line).

5.3.4 DEVELOPMENT

Those PR which expose to UV will become carboxylic acid and this can be

diluted in a PR developer. The developer used in UM is AZ 726 MIF, which is a metal

ion free aqueous alkaline solution of 2.38% tetra-methyl-ammonium-hydroxide with

surfactant to provide fast and uniform build-up of the puddle. The developing process is

run under room temperature for 20 to 50 seconds depends on the PR thickness. After

development, the wafer will be rinsed with DI (De-Ionized) water and dried with

nitrogen gas.

5.3.5 POSTBAKE

The developed wafer will need to be baked again before being sent for Cr wet

etching. This post baking process will further stabilize the PR, improving adhesion

during wet etching. The wet etching process must be carried out within 2 hours of post

baking to ensure good quality results.


72


5.4.1 PHOTORESIST (PR) COATING

In PR coating, PR dispending method and timing plays an important role to

produce a uniform PR layer. In this study, the PR was dispensed manually by using a

pipette onto the wafer in various stages. The first study is the static dispensing

technique (SDT) where the PR was dispensed onto the wafer surface before the wafer is

spin. This is followed by second study is the dynamic dispensing technique (DDT)

where the PR was dispensed onto the wafer surface while the wafer is spinning. In the

first DDT, the PR was dispensed while the wafer is being accelerated towards

maximum speed while in second DDT experiment, the PR was dispensed during the

wafer is in maximum spin speed.

After patterning on the PR layer, the PR thickness was measure using surface

profiler (Tencor Alpha Step 2000). For higher accuracy, thicknesses were measured at

various points across the wafer diameter. Figure 5.5 shows the PR thickness of the

patterned channel at different spin speeds for SDT and both DDTs. As shown in the

figure, the thickness of the PR coating reduces as the spin coater speed is increased.

This thinning behavior is due to the viscous fluid forces that spread the PR towards the

edge of the substrate. Both DDTs show a lower thickness compared to SDT. The second

DDT for instance shows a thickness reduction of 0.2 μm in comparison to the SDT at

the spinning speeds of above 8000 rpm. When comparing both DDTs, the second

technique shows a better thinning behavior especially for spinning speeds above 4000

rpm. At lower spinning speeds, the PR thinning behavior is only slightly improved due

to the small differences between the maximum spinning speed and acceleration speed.

The centrifugal forces that were created in both cases are also similar. Thus, thickness

differences between SDT and the two DDTs at lower speeds are seen to vary at around

tens of nanometers. When the maximum spinning speed is set higher, thickness


73

differences (due to the increasing centrifugal force) become more significant. At 9000

rpm, thicknesses different around (0.10±0.01) μm was observed.

Figure 5.5: PR thickness (μm) versus spin coating speed (rpm) for SDT, 1st DDT and 2nd DDT

Figure 5.6 shows the circular non-uniformity of the PR thickness measured from

the center of the substrate to its edge. As shown, the average non-uniformity of the PR

thickness is reduced from SDT (average= (0.28±0.01) μm) to 1st DDT (average=

(0.21±0.01) μm) and 2nd DDT (average= (0.14±0.01) μm). However, the variations in

the non-uniformity have increased from SDT to 1st DDT and 2nd DDT. This means that,

although the PR uniformity has averagely increased from SDT to the 2nd DDT, the

uniformity control is more difficult. In SDT, there is a delay between the PR dispensing

on the substrate and the spin cycle. The PR, when dispensed at the center of the

substrate will evaporate during this delay causing its distribution to be unevenly thicker

at the center. Using DDT on the other hand, the PR is spread out once it touches the

substrate surface. This application of PR without spin delay causes wider spreading of

the chemical layer and reduced central clumping. The PR layer uniformity and

distribution is thus, improved for DDT in comparison to SDT.


74

Figure 5.6: Illustration of PR thickness non-uniformity with various PR dispensing techniques

Since the increment of spinning speed increases the evaporation rate and

centrifugal force, the PR undergoes the highest evaporation rate and centrifugal force in

the 2nd DDT followed by the 1st DDT and SDT. Because of the high evaporation rate in

both DDTs, the PR dispensing rate will affect the PR uniformity on the wafer. The PR

dispensing rate should be fast and consistent to achieve a better uniformity control. If

the PR dispensing rate is too slow, more than one layer of PR will be formed on the

wafer surface.

Table 5.1 shows the amount of PR used for different dispensing techniques. As

shown, the minimum PR quantity required for the coating process is approximately

3.00ml in SDT. In both DDTs, the PR quantity has reduced to 0.25ml, which is about

8.33% of the quantity required by SDT. The excess quantities of PR required in SDT

are lost during the spin stage [11]. This means that the DDT methods are more

economical and environmentally friendly. Figure 5.7 shows the PR with spin speed of

6000rpm and various amount of PR. Notice from Figure 5.7 that the resultant PR

thickness is independent of the amount of PR applied. As such, less amount of PR can

be used without affecting the PR thickness.


75

Table 5.1: PR usage in SDT, 1st DDT and 2nd DDT

PR Usage (±0.05) ml

3.00 2.00 1.00 0.50 0.25 0.20

Static Dispensing √ X X X X X

1st Dynamic Dispensing √ √ √ √ √ X

2nd Dynamic Dispensing √ √ √ √ √ X

* √ = The PR amount used is sufficient for whole surface coverage of the substrate. X = The PR amount used failed to cover the whole surface of the substrate.

Figure 5.7: Thickness variation of PR applied using the 1st DDT approach


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5.4.2 PREBAKE

As mentioned earlier, temperature and prebaking duration are two important

parameters that will determine solvent evaporation percentage and also the sensitivity of

the photoactive compound (PAC). Figure 5.8 shows the graph of PR thickness

dependence on various prebake temperatures. We notice that the thickness of the PR is

reduced when the wafer is prebaked at higher temperatures. For temperatures above or

equal to 140oC, the developing process became unsuccessful. In addition, prebaking at

high temperatures will leave behind residual PR that could not be removed from the

chromium surface. This is shown in the image in Figure 5.9. This will subsequently

cause contamination in the glass etching process. As such, temperatures within 60oC to

100oC are likely to produce the best results, along with shorter developing times (20s-

50s).

Figure 5.8: PR thickness dependence on various prebake temperatures


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Figure 5.9: PR residual that fail to remove from wafer after PR removal process

A comparison is made between prebake temperatures of 60oC, 80oC, and 100oC

for different baking period as shown in Figure 5.10. PR thickness for 60oC and 80oC

still show a decrease even after 5 minutes of prebaking on a hotplate while for the case

of 100oC, the thickness remains constant. Therefore, in light of these findings, we could

conclude that 100oC is the most suitable prebake condition..

Figure 5.11 shows the depletion of the PR layer while baking under 100oC. A

critical drop of PR thickness can be observed at during first minute of the prebake. The

depletion of the PR layer seems to be insignificant after 2 minutes when most of the

solvent is removed. Hence, prebake temperature is set at 100oC for 2 minutes since no

significant change to the PR thickness after 2 minutes baking. 2 minutes is chosen

because a longer exposure period will reduce the sensitivity of the PAC, and this would

result in the reduction of the pattern resolution.


78

Figure 5.10: PR thickness for prebaking temperature of 60oC, 80oC, and 100oC, for various prebake duration

Figure 5.11: Change of PR thickness with various prebaking duration


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5.4.3 UV EXPOSURE

In UV exposure, the contact method employed will have a significant impact on

the resultant transferred pattern. For this study, three different contact methods were

investigated – soft contact, hard contact, and vacuum contact. Figure 5.12 depicts the

resulting PR pattern using the respective approaches. Figure 5.12(a) shows the result for

soft contact printing where the resolution is lower compared to those presented in

5.12(b) and 5.12(c). The branching part was under exposed and subsequently causes

incomplete PR development. Figures 5.12(b) & (c) show comparatively better results on

PR patterning which are done under hard contact and vacuum contact. The hard contact

approach is quite consistent in generating repeatable results but it susceptible to mask

damage, creating permanent defects. Vacuum contact is also able to produce good PR

patterns, but the results repeatability is low. The success of the vacuum technique is

highly dependent on the uniformity of the PR layer using the vacuum technique on a

non-uniform PR layer will result in pattern with varying resolution, as depicted in

Figure 5.12(c) and 5.12(d).

The resolution and the CD of the patterning process can be measured using the

error analysis scale printed on the AWG mask used as shown in Figure 5.13. The

numbers under the pattern are all in micron unit. The scale in Figure 5.13(a) is used to

measure the CD error while that scale in Figure 5.13(b) is used to measure the

resolution limits of the patterning process. For a 7μm width pattern, the contact method

will display a typical CD error of ±0.4μm. However, contact plays an important role in

producing a high resolution pattern. When the contact between the mask and the wafer

is not close enough, the separation will cause Fresnel diffraction, and hence reduce the

resolution of the transferred pattern. Figure 5.14 depicts samples under different hard

contact pressure. Sample (a) under 0.2MPa pressure produced better resolution (1.2μm)

while sample (b) under 0.16MPa pressure displayed lower resolution (1.8μm).


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(a) (b)

(c) (d) Figure 5.12: PR pattern obtained with (a) soft contact, (b) hard contact, and (c) and (d) for vacuum contact

(a)

(b)

Figure 5.13: Error analysis scale on AWG photomask, (a) for CD error measurement while (b) for resolution limit measurement


81

(a) (b) Figure 5.14: Resolution measurement of samples under (a) 0.2MPa and (b) 0.16MPa contact pressure

Fresnel diffraction also affects the side wall angle of the pattern produced. When

UV radiation pass through the photomask, a portion of the UV radiation near the pattern

edges will experience diffraction, and gets diffracted into the pattern area. This

phenomenon causes the transferred pattern at the bottom of the wafer to be larger (as

the UV radiation dose is not enough to penetrate till the end of the PR to soften it) and

the pattern at the top to be smaller (due to the diffracted UV radiation that soften the

PR ). This is the reason why the side wall of the PR pattern produced is not vertical but

a slope with a certain angle. Figure 5.15(a) shows the cross-section of the PR pattern

observed under optical microscope. The pattern in Figure 5.15(b) was produced under a

lower contact pressure (0.16MPa), which experienced higher Fresnel diffraction effect,

and produced a larger angled side wall (35o) as compared to the pattern in Figure

5.15(a), which has produced under a higher contact pressure of 0.2MPa, with side wall

angle of 19°. The side wall angle was calculated using the following equation,

)2

(tan 1

HWWSWA TB −

= − (5.4)

where SWA is side wall angle, WB is channel bottom width, WT is channel top width,

and H is channel height.


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(a) (b) Figure 5.15: Cross-section of PR pattern produced under (a) 0.2MPa and (b) 0.16MPa contact pressure

The suitability of the contact method and the pressure applied can be observed

from the fringe patterns when the photomask is in contact with the PR coated wafer.

The fringe pattern shows the contact condition between the wafer and photomask. The

bigger the size of the fringes means the better the contact condition, and therefore,

better the pattern will be produced. The fringes should also be uniform throughout the

whole wafer. Figure 5.16 shows the fringe pattern when the photomask is in contact

with the PR coated wafer.

Figure 5.16: Fringe pattern when the photomask is in contact with the PR coated wafer


83

In UV exposure, the exposure dosage and development time are dependent on

each other, and both also depend on the prebake condition [12]. Since the prebake

condition is fixed at the previous stage, here we only consider exposure dosage and

developing time. In this study, the development time is best in 20 seconds to 30 seconds,

so any exposure dosage that need more than 30 seconds developing time will not be

considered. PR is developed for 30 seconds after expose with different dosage and the

results are observed under objective microscope. The dosages increase slowly until all

the PR residual successful to be removed within 30 seconds.

Figure 5.17(a) & (b) show the PR pattern that still contain PR residual which is

not being developed all in between the branches. Figure 5.17(c) shows the result of the

successful develop PR pattern while Figure 5.17(d) shows the results of the PR pattern

which exposed to over expose dosage. Figure 5.18 shows the optimized UV exposure

dosage for different thickness of PR layer. It shows a non-linear increase of exposure

dosage needed for different PR layer thickness.


84

(a) (b)

(c) (d) Figure 5.17: PR pattern with different exposure dosage. (a) 1860 mJ/cm2, (b) 1890 mJ/cm2, (c) 1920 mJ/cm2, and (d) 2100 mJ/cm2

Figure 5.18: Different exposure dosages for varying PR thickness


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5.4.4 POSTBAKE

The postbake process is recommended to improve the PR adhesion to the Cr

etchant. Figure 5.19 shows the independence of PR thickness on the postbake period by

using a hotplate set at 100°C. The PR thickness does not show any significant changes

as most of the solvent in the PR was already removed during prebake process. At the

same time, the Cr wet etching results which were done under postbake condition for 2

minutes and without postbake was observed under optical microscope. Figure 5.20

shows the overlapping image of the two results. The darker colour part is the etched Cr

pattern without postbake, while the brighter colour is the etched Cr pattern with 2

minutes of postbake. Both results do not show any significant difference in CD but the

pattern without postbake shows sharper edges at the corner.

Figure 5.19: Resulting PR thickness for different postbake period


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Figure 5.20: Etched Cr patterns with and without postbake condition of 120°C for 2 minutes

The successful etching of the Cr pattern means that the PR is adhesive enough to

proceed with Cr wet etching process without undergoing postbake. To study more about

the edges effect, the observation was made on the cross-section of PR pattern before

and after postbake as shown in Figure 5.21. The cross-section of PR pattern on the left

is the pattern before postbake while the right is the pattern after postbake. From the

image, we can notice that the pattern after postbake is slightly collapsed at the center

and the side wall angle of the pattern also increase due to the collapse of the channel.

Figure 5.21: Cross-section of PR patterns before and after postbake


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5.5 SUMMARY

In this chapter, the optimization of the photolithography process has been done

in every stage starting from PR coating to postbake. In PR coating, three different

dispensing method have been tested to obtain the best coating quality, and they were

SDT, 1st DDT, and 2nd DDT. Both DDT methods show improvement in PR layer

thinning, uniformity, and less chemical usage. However, DDT methods have lower

repeatability and higher percentage of creating defects during the coating process.

Prebake is critical as it will affect the UV exposure and developing processes.

Prebake temperature and period were optimized to remove most of the solvent in PR

while maintaining PAC sensitivity. Contact methods and UV exposure dosage were

presented in this chapter as well. It was found that by using the correct combination of

contact method and UV exposure, PR pattern of CD error less than 6% of the pattern

width, and side wall angle of less than 20o can be achieved.

The final section discussed the effect of postbake to the resulting PR pattern and

wet etching process. Although in most processes postbake is important to increase the

adhesiveness of the PR in wet etching process, however this step does not seems to be

necessary to our process. The postbake also causes the PR pattern to collapse at certain

areas and hence increase the CD error.


88

REFERENCES

[1] Darling, R. B. Micro Fabrication – Photolithography. Retrieved October 29,

2007, from University of Washington website:

http://www.ee.washington.edu/research/microtech/cam/PROCESSES/PDF%20F

ILES/Photolithography.pdf

[2] Snyder, I. (1996). Color printing in the nineteenth century. Retrieved January 24,

2008 from University of Delaware Library website:

http://www.lib.udel.edu/ud/spec/exhibits/color/lithogr.htm

[3] Bruning, J. H. (1997). Optical Lithography - Thirty years and three orders of

magnitude (The evolution of optical lithography tolls). SPIE, 3051, 14-27.

[4] Edgar, T. F., Butler, S. W., Campbel, W. J., Pfeiffer, C., Bode, C., Hwang, S. B.

et al. (2000). Automatic Control in Microelectronics Manufacturing: Practices,

Challenges, and Possibilities. Automatica, 36 (11), 1567-1603.

[5] Smith, H. I. Submicron and Nanometer Structures Technology. Retrieved from

Massachusetts Institute of Technology Lecturer Note.

[6] Wilson, C.G., Dammel, R.A., & Reiser, A. (1997). Photoresist Material: A

Historical Perspective. SPIE, 3051, 28-41.

[7] May, G. S., & Spanos, C. J. (2006). Fundamentals of Semiconductor

Manufacturing and Process Control. Hoboken, New Jersey: John Wiley & Sons.



[9] Franssila, S. (2004). Introduction to Microfabrication. Chichester, England: John

Wiley & Sons.

[10] Thompson, B. J. (2007). Microlithography - Science and Technology (2nd ed.).

New York: Taylor & Francis Group.

[11] Derksen, J., Han, S.J., & Chun, J.H. (2004). Extrusion Spin Coating: An

Efficient and Deterministic Photoresist Coating Method in Microlithography.

Semiconductor Manufacturing, 17 (1), 245-248.

[12] Hiroshi, M. & Mehran, M. (1995). High-Aspect-Ratio Photolithography for

MEMS Applications. Journal of Microelectronical Systems, 4 (4), 220-229.

Chapter 6: Wet Etching

89

CHAPTER 6

WET ETCHING

6.1 INTRODUCTION

Wet etching is a conventional, purely chemical, etching process where the wafer

is immersed in a solution that reacts with an earlier ultra-violet (UV) exposed thin film

to form soluble by-products [1]. Unfortunately, the limitations of wet etching due to the

isotropic etching profile, poor process control, large volumes of chemical waste,

unsuitability for small features, and excessive particle contamination, have reduced its

popularity [1, 2]. However, compared to dry or plasma etching, wet etching is a much

cheaper option as it does not rely on expensive equipment, more suitable for mass

production, highly selective and often does not damage the substrate. As a result, the

technique continues to be used for most semiconductor etching processes that are

“noncritical”. Tank (or bath), spray tool, and single-wafer processor are three general

methods for performing wet etching process.

6.2 OVERVIEW

Isotropic etching is a non-directional removal of material from a substrate via a

chemical process using an etchant substance. Most of the wet etching process

performed as an isotropic etching due to the random movement of the liquid molecular.

The isotropic etching nature of wet etching will create undercut issues. Undercut is the

lateral extent of etch under a photoresist (PR) mask as shown in Figure 6.1.

There are two ways to describing undercut. The first is by considering the

undercut distance per side. It can be calculated by dividing the difference between PR

patterned line and the etch chromium (Cr) pattern by 2. As shown in Figure 6.1, the side

wall of the Cr pattern may not be vertical. Thus the undercut value is based on how the


90

Cr pattern is measured. The second way is to quote the etch rate anisotropy. Anisotropy

is given by

V

L

RRA −= 1 (6.1)

where RL and RV are lateral and vertical etch rates. Thus for perfectly anisotropic (A =

1), lateral etch rate is zero while A = 0 for isotropic etching with same lateral and

vertical etch rate [1].

Figure 6.1: Schematics of undercutting caused by the isotropic nature of wet etching

For Cr wet etching, Cr etchant (Cr 7S) is used to remove the unwanted part of

the Cr which are not covered by PR. Cr etchant include composition of perchloric acid

(HClO4), ceric ammonium nitrate ((NH4)2Ce(NO3)6), and water (H2O) [3]. Perchloric

acid is a very strong acid and therefore almost completely dissociated in aqueous

solution (pKs < -8), and serves to chemically stabilize the ceric ammonium nitrate.

Ceric ammonium nitrate itself is a very strong oxidizer. Equation (6.2) below shows the

chemical reaction in the Cr etching process by using Cr etchant Cr-7S:

5324336324 )()(3)()()(3 NOCeNHNOCrCrNOCeNH +→+ (6.2)

From the equation, notice that cerium oxidation state is reduced from IV to III,

whereas the chromium oxidation state increases from II to III [4].


91

6.3 Cr WET ETCHING PROCESS METHODOLOGY

In this study, Cr was etched by using different wet etching methods to study the

advantages and disadvantages of each method. The methods that were used include:

a) Static immersion - wafer is immersed into beaker with 100ml of Cr etchant and

was left static until the etching process is done.

b) Flipping – wafer is immersed into beaker with 100ml of Cr etchant and the

wafer is flipped up and down until the etching process is done.

c) Spin stirrer – wafer is immersed into beaker with 100ml of Cr etchant and stirred

using magnetic bar until the etching process is done.

d) Ultrasonic – wafer is immersed into beaker with 100ml of Cr etchant and put

into an ultrasonic bath until the etching process is done.

e) Heating – wafer is immersed into beaker with 100ml of Cr etchant that was

already heated up to a predetermined temperature using a hotplate until the

etching process is done.

When the etching process starts, it can be observed that the wafer surface will

slowly darken. This is due to the chromium nitrates that were produced during the

etching forming a dark film on the chromium surface. Chromium nitrate is soluble and

it will dissolve in the etchant itself [4]. The etching process is completed when the

wafer surface reverts back into its shiny surface feature. The wafer will be left in the

etchant for an additional 5 seconds to make sure that all unwanted Cr are removed. The

etched wafer will then be rinsed using DI water for 5 minutes to remove residual

chemicals. Acetone is then used to remove the PR pattern on the wafer. The wafer will

then undergo a cleaning process, as described in Appendix B. The cleaned wafer will

then be sent for glass etching.


92


a) Static immersion

In static immersion wet etching, wafer was etched using Cr etchant in beaker

without any disturbance at room temperature. From observation, the etching processes

normally end faster at the edge of the wafer then only slowly proceeding towards the

center. The difference of the etching period between edge and core is caused by the

non-uniformity of the Cr thin film layer. Besides that, wafer edge which has higher

exposure area (wafer top area and side area) to the Cr etchant also increases the Cr

etching rate at the edge. Figure 6.2 shows the Cr etching time needed for the edge and

the center of the wafer with different Cr deposition time (*Cr deposition time is

proportional to Cr thin film thickness).

Figure 6.2: Cr wet etching time for different Cr deposition periods

Figure 6.3 shows the difference in etching time between the edges and center

(EC period). The difference in etching time between various locations on the wafer is a

critical issue that needs to be catered for. Uniform etching in this case could not be

achieved as some of the regions will etch quicker resulting in over-etching [5]. Over-


93

etching will cause undercutting in isotropic etching (wet etching). In more serious cases,

complete loss of pattern due to undercut can take place, reducing the yield of the

patterning process [2].

Figure 6.3: Difference in etching time between the edge and center of a wafer in various chromium deposition times

Figure 6.4 shows the critical dimension (CD) error of the AWG Cr pattern. CD

error is the difference of the transfer pattern width to the original pattern width. Positive

value means the dimension of the transfer pattern is smaller than the original pattern

while negative value means the opposite. Due to the increase of the EC period for

thicker Cr layer, and hence CD error also increase as the Cr deposition time increase.

CD errors for samples that deposited Cr for 8 and 10 minutes are out of the error

measuring scale which is larger than 2.4μm are not included in the graph.


94

Figure 6.4: CD error for Cr pattern after static wet etching

b) Flipping

In wet etching via flipping, the wafer is flipped up and down to create chemical

flow on the wafer surface. By flipping the wafer, chromium nitrate forming on the Cr

surface during the etching process [4] will be diluted and washed out more easily,

increasing the etching rate. Figure 6.5 shows a significant decrease of etching time via

flipping compared to static etching time. Besides that, by controlling the speed and

direction of the flipping movement, we can actually increase the Cr etching rate at

particular areas, especially at the center region of the wafer. With this, the EC period

was successfully reduced and better CD dimension can be created in the wet etching

process. Figure 6.6 shows the EC period of both static and flipping wet etching process

while Figure 6.7 shows the CD error for a successful transfer of Cr pattern.


95

Figure 6.5: Etching time needed for flipping and static wet etching over various Cr deposition duration

Figure 6.6: The significant reduction of EC period for flipping etching as compared to static etching


96

Figure 6.7: CD error for different Cr deposition times

c) Spin stirrer

This technique failed to produce the desired results. The turbulence caused by

the technique resulted in a significantly higher etching rate at the edge of the samples to

that at the centre. This causes the outer patterns to be over-etched, and eventually

destroyed at the end of the etching process.

d) Ultrasonic

In ultrasonic etching, the etching beaker was put in the center of an ultrasonic

chamber filled with ultrasound conducting fluid. This method is to utilize the energy

carried by the vibration of ultrasound standing wave to increase the chemical reaction

[6]. From Figure 6.8, we can observe that the Cr etching rate is slightly higher

compared to the static etching method, due to the higher vibration energy transferred

from the ultrasound. In terms of CD error, this technique is comparable to static etching.

Figure 6.9 shows the CD errors for different Cr layer thickness.


97

Figure 6.8: Cr etching time by using static and ultrasonic etching method

Figure 6.9: CD error for Cr pattern with ultrasonic etching


98

e) Heating

In the heating wet etching process, the Cr etchant is heated to study the effect of

the etching temperature have on the chemical reaction. The process was carried on in

the fume hood as Cr etchant vapour is poisonous. Heating up the Cr etchant will provide

additional energy to the etchant molecules, which will then increase the chemical

reaction between the etchant and Cr. Figure 6.10 shows the dependence of the etching

rate on the temperature.

Although the etching rate was increase as the chemical temperature increases, it

does not seem to improve Cr pattern CD. The etching results look fine for low Cr

thickness (deposition period at 6 minutes and below) but over etch effect still appears

for high Cr thickness (deposition period at 8 minutes and above).

Figure 6.10: Cr etching rate at different temperatures


99

6.5 SUMMARY

In this chapter, various wet etching methods have been investigated to improve

the Cr pattern quality in the wet etching process. The flipping approach produced the

best results where the resulting Cr patterns displace the lowest CD error. Static and

ultrasonic etching produced almost similar results whereas heating increases the etching

rate, but does not produce a quality pattern as the chemical was too reactive. The spin

stirrer with hotplate technique was found to be unsuitable as the Cr patterns at the edge

of the samples were completely etched due to the large difference in etching rate caused

by the temperature gradient.

In this chapter we also observed that although a higher etching rate will shorten

the process duration, it does not lead itself to producing quality etching patterns. As

such, a compromise must be reached between etch rate and pattern quality. Finally,

from the results obtained, we could conclude that the flipping method is most suitable

for our process. A summary of the results is provided in table 6.1.

Table 6.1: Summary of different Cr wet etching techniques for 6 minutes Cr deposition period

Technique CD Error (±0.2μm) Pattern Yield (%) Cr Etch Rate (±5nm/minute)

Static immersion 1.0 - 2.0 100 40

Flipping 0.4 – 0.8 100 130

Spin Stirrer > 2.4 10 50

Ultrasonic 1.8 – 2.4 100 45

Heating * CD error and pattern yield are totally depend on technique used above. Heating only improve the etching rate with 0.1nm/minute for every Degree Celsius of temperature increase.


100

REFERENCES

[1] Campbell, S.A. (2001). The Science and Engineering of Microelectronic


[2] Franssila, S. (2004). Introduction to Microfabrication. Chichester, England: John

Wiley & Sons.

[3] MSDS (Material Safety Data Sheet) of Cr-7S from Cyantek Corporation.

Retrieved March 22, 2008, from http://www.cise.columbia.edu/clean/msds/CR-

7S.pdf

[4] Technical information of Cr etching from MicroChemicals. Retrieved March 22,

2008, from

http://www.microchemicals.com/technical_information/chromium_etching.pdf

[5] Peter, V.Z. (2000). Microchip Fabrication- A Practical Guide to Semiconductor

Processing (4th ed.). New York, US: The McGraw-Hill Companies, Inc.

[6] Crocker, M.J. (1998). Handbook of Acoustics. New York: J. Wiley.

Chapter 7: Conclusions & Future Work

101

CHAPTER 7

CONCLUSIONS & FUTURE WORK

7.1 CONCLUSIONS

During my master research, I have demonstrated the following

i) Characterization and optimization of the Chromium (Cr) thin film deposition

process by using Direct-Current (DC) planar magnetron sputtering deposition method.

Parameters like deposition period, DC power, argon (Ar) gas flow, substrate

temperature, and chamber pressure have been studied to understand the effects on the

thin film properties especially thickness, grain size and chemical stability. In this thesis,

discussion was concentrate in deposition period, DC power, and chamber pressure as

other parameters do not give any significant impact on those important parameters in

this thesis. Under the conditions of 0.8A DC current, 5sccm Ar flow rate, 30oC substrate

temperature, and 10mTorr process chamber pressure, the deposition process show a

linear increase in thickness when deposition period increase. The deposition rate is

43nm/minutes. The growth of the grain height from 3.5nm (for 2 minutes deposition) to

11.6nm (for 10 minutes deposition) when the Cr thickness is increase.

In DC current parameter study, the deposition rate is directly proportional to the

increase of DC current supply. The deposition rate shows increases of 52nm/minutes for

every ampere (A) of DC current supply. Besides the deposition rate, the change of the

deposition current do not shows any significant effect on the Cr thin film properties.

From these two parameters study, we notice that the chemical stability give a deep

impact in etching the thin film. The un-uniform etching rate at different area of the

wafer causes series over etching problem. This effect become more series when the Cr

thin film thickness increase until 60% of the Cr patterns are totally gone for etching Cr


102

film thickness around 500nm. However, this problem was improved in the study of

process pressure. By reducing the process pressure from 10mTorr to 6mTorr, the

different etching period in a same wafer was successfully reduced from ~200s to ~160s,

which is 20% improve from the original recipe. With this improvement, the Cr

patterning yield had improved from 40% to 100%.

ii) Photolithography process involved a series of processes from photoresist (PR)

spin coating till post baking. In PR spin coating, three different PR dispensing

techniques have been tested to study their advantages and reliability, which are the

conventional static dispensing technique (SDT), 1st dynamic dispensing technique (1st

DDT), and 2nd dynamic dispensing technique (2nd DDT). Three techniques show an

inversely proportional relation between coated PR thickness and spin coating speed.

However, there is a significant decrease in overall PR thickness from SDT to 1st DDT

and 2nd DDT. Besides that, PR uniformity also improved following the sequence above

from average 0.28μm to 0.21μm and to 0.14μm. Unfortunately, defects are easier to

create in 1st and 2nd DDT and the repeatability is also lower. Both DDT are more

economical and environmentally friendly as the PR usage needed is reduces more than

90% compare to SDT. The reduction of the PR usage does not affect the coated PR

thickness.

In prebake process, PR coated wafer was heated on the hotplate from 40oC to

160oC. PR thickness shows higher degradation for higher prebake temperature. Prebake

temperatures within 60oC to 100oC are likely to produce the best results with reasonable

developing period (20 to 50s). Temperature higher than 100oC will reduce the

sensitivity of the photoactive compound (PAC) and hence increase the developing

period. Besides that, it will also create PR residual which cannot be removed from the


103

Cr surface. The best prebaking period is also found at 2 minutes where most of the

solvents are being removed.

In UV expose, three different contact methods have been tested, which are soft

contact, hard contact, and vacuum contact. Soft contact produced lower resolution,

higher critical dimension (CD) error, and larger side wall angle (SWA) compare to

vacuum contact and hard contact. Vacuum contact can produce good quality PR pattern,

but the repeatability is low as it highly depends on the wafer surface condition. Hard

contact is easier to approach for good quality pattern but it has a very high risk to

damage the mask when it contact with the wafer. The best pattern we able to produce

are with resolution down to 1.2μm, CD down to 0.4μm, and 19° SWA for 7μm width

and 1.4μm thick PR pattern. The ultra-violet (UV) exposure doses are also been

optimized for PR thickness from 0.8μm to 2.1μm. The UV exposure dose shows

nonlinear increase when the PR thickness increases.

In postbake, the process does not give significant impact on the remaining PR

thickness and in the Cr wet etching process. It means that the PR pattern without

postbake is adhesive enough for the wet etching process. The results show that postbake

will cause the PR pattern to collapse and hence increase the SWA of the pattern.

Although it does not show significant effect on the CD error, but we still can observed

the degradation of the sharpness at the pattern corner.

iii) In Cr wet etching process, various wet etching methods have been study to

improve the etched pattern quality which included static immersion, flipping, spin

stirrer, ultrasonic, and heating. Among all the methods, spin stirrer gave the worse

results as this method create a huge different of etching rate between the edge and the

centre of the wafer. Thus, pattern at the outer side is totally etch out and left only

around 10% of pattern at the centre would be survived. Static immersion and ultrasonic


104

gave similar results where the Cr pattern yield are 100% for Cr low thickness (for 6

minutes of Cr deposition or lower). Ultrasonic has slightly higher etching rate,

45nm/minutes compare to static immersion which is 40nm/minutes. However, the CD

error of ultrasonic etching is higher, 1.8 - 2.4μm compare to static immersion which is

only 1.0 - 2.0μm. Flipping method shows encouraging results where the CD error of the

patterns produce are in the range of 0.4 -0.8μm. Besides that, the high etching rate at

130nm/minute also reduces the wet etching process period.

7.2 FUTURE WORK

In this section some recommendation for future undertaking the fabrication of

AWGs which have not been solved in this thesis.

(a) “Zero critical dimension error” pattern

In patterning process, errors are mainly causes by the CD error, surface

roughness and SWA. To minimize the patterning error, an improvement need to be

done in waveguide design, photolithography process, and etching process. The

easiest way to improve the pattern quality is by changing the wet etching method to

dry or plasma etching method as it produce smooth surface and vertical wall angle.

For existing wet etching process, undercutting will reduce the pattern CD and create

a rough side wall. Undercut effect on pattern CD can be reduced by creating larger

PR pattern in photolithography process. This can be done by creating the PR pattern

in under expose condition or design the photomask according to the instruments

fabrication error. With these methods, Cr pattern with almost zero CD error can be

created.


105

(b) Deposition of silica (SiO2) glass layer

Deposition of SiO2 glass layer plays an important role as well in planar

waveguide fabrication of AWGs. To fabricate a high performance and low loss

AWGs device, high quality pure fuse glass is needed to reduce the insertion loss to

minimum. The existing glass deposition method in our laboratory by FHD is not

optimized yet, and thus the quality of the glass is quite poor. The surface roughness

of the glass quality is roughly 200 to 300nm which will causes high propagation loss

and reduce the patterning quality. Thus an optimization should continue on the glass

deposition layer to produce a quality AWGs device.

(c) Glass etching

In glass etching process, the selectivity of the glass and the Cr mask layer is very

important. Low selectivity will require thick Cr mask to protect the core layer

throughout the whole etching process. Unfortunately, the patterning process

become harder to produce a good quality pattern when the require mask layer is too

thick. Besides that, optimization will also need to reduce defects like pinch holes,

crystallization and more which create in the glass etching process.

106

APPENDIX A

Theory and Design of Arrayed Waveguide Gratings (AWGs)

There are a few important properties of AWGs like optical path function,

focusing, dispersion, and free spectral range. The type of design adopted will affect the

AWG characteristic.

A. Optical Path Function (OPF)

Figure A.1: Illustrative diagram of an N x N AWG

The illustrative diagram of AWG is shown in Figure A.1. The AWG in figure

A.1 consist of N input waveguides, two N x M star couplers (FPZ or Free Propagation

Zone) jointed by M arrayed waveguides, and N output waveguides. The end face of

arrayed waveguides is called grating curve. The convex of the grating curve is defined

to be the origin and X axis is defined as the normal of the grating curve to setup the

rectangular coordinate systems of input, XOY and output, X’O’Y’. Due to both XOY and

X’O’Y being symmetrical, all of the optical path function (OPF) can be written in

coordinate XOY.

107

The grating curve can be expressed as a power series,

...!/)0(...2/)0('')0(')0()( )(2 +++++= nwuwuwuuwu nn (A.1)

Under the coordinate system, u(0) and u’(0) equals to 0. The coordinate of a common

point P along the grating curve is assumed to be (u, w) [1, 2].

The OPF for ray transmitted from A (xA, yA), coupled into the arrayed

waveguides at P (u, w), then through waveguide with length of L (w), and diffracted at

the grating curve at P’ (u’, w’), finally coupled into the output waveguide at B (xB, yB)

can be written as

)()()()()( wrNmwGwLNwrNwF BswAs +++= λ (A.2)

where

Ns and Nw effective refractive index of the FPZ and the arrayed waveguide;

m spectral order;

L (w) geometrical length of the waveguide between P and P’ in micron;

G (w) numbers of waveguides counted from the origin O to the point P,

...!/)0(...2/)0('')0(')( )(2 ++++= nwGwGwGwG nn (A.3)

rA(w), rB(w) geometrical length in the FPZ in micron.

22 )()()( AAA ywxuAPwr −+−>==< (A.4)

22 )()(')( BBB ywxuBPwr −+−>==< (A.5)

The OPF in (A.2) can be expanded into power series form as

...!/)0(...2/)0('')0(')0()( )(2 +++++= nwFwFwFFwF nn (A.6)

where λmGLNrrNF nnw

nB

nAs

n )0()0()]0()0([)0( )()()()()( +++= (A.7)

F(n) is the nth aberration coefficient. F(0) is a constant. The grating equation at O can be

obtained by letting F’(0) to be zero. F(2)(0), F(3)(0), and F(4)(0) correspond to defocus,

coma, and spherical aberration, respectively [1, 2].

108

B. Focusing

The input and output apertures of the phased array are typical examples of

Rowland-type mountings. The focal line of such a mounting, which defines the image

plane, follows a circle with radius Ra/2 as shown in Figure A.2. Transmitter and receiver

waveguides should be positioned on this line [3, 4].

Figure A.2: Geometry of the FPZ

The array is designed so that each successive waveguide has a different length

of ∆L. ∆L can be calculated by using

cgg

c

fNmc

NmL =⋅=Δ

λ (A.8)

where m is the order of the phased array, λc(fc) is the central wavelength (frequency) in

vacuum, and Ng is the effective index of the waveguide mode,

πλββ

20

==k

Ng (A.9)

where β is the propagation constant and wave-number, k0 = 2π/λ.

With this choice the array acts as a lens with image and object planes at a

distance Ra of the array apertures.

109

C. Dispersion

When wavelength shifts from λc to λc + ∆λ, there will be a linear phase shift in

each individual array waveguides and causing the phase front at the grating curve to be

slightly tilted. This tilting results in the beam focusing on a different position in the

image plane. The tilting angle θm is given as,

( )

a

g

f

g

a

g

f

gm d

mLNN

dmL

NN λλ

θ−Δ

≈⎥⎥⎦

⎤

⎢⎢⎣

⎡ −Δ= arcsin (A.10)

where Ng and Nf are the effective indices in the waveguide array and in the FPZs, da is

the pitch between the array waveguides and λg = λ/Ng. Thus different wavelength will

focus to a different output waveguide as shown in Figure A.3 [4].

Figure A.3: Focusing of beam in two different wavelengths

The dispersion D of the array is defined as the lateral displacement ds of the

focal spot on the image plane per unit wavelength change.

αλ

θλ Δ

−≈⎟⎟⎠

⎞⎜⎜⎝

⎛−=⋅==

mdm

NN

RddR

ddsD

af

ga

ga

g

(A.11)

where ∆α = da/Ra is the divergence angle of the array channels in the array aperture and

Ng/Nf ≈ 1 due to the insignificant difference of the effective indices between in array

waveguides and FPZs.

110

D. Free Spectral Range (FSR)

FSR is the distance in the wavelength domain which is found to be wavelength

shift when the phase shift equals to 2π. FSR can be found by comparing the optical path

length difference of orders (m-1) and m, from (A.8) [3, 4]:

( )g

FSR

g Nm

NmL

λλλ Δ+−==Δ 0

0,

0 1 (A.12)

rearranging (A.12):

1

/)(1 00,00 −

−−=Δ

mNNNm g

FSR λλ (A.13)

Lets define an effective index difference as ∆N=N0-Ng,0, (A.13) will become

1/1 0

0 −Δ−

=Δm

NNmFSR λλ (A.14)

If the comparison is did on the optical path length difference of the m-th order to that of

the (m+1)-th order [9], the FSR will be

1/1 0

0 +Δ+

=Δm

NNmFSR λλ (A.15)

REFERENCES

[1] Wang, D. Y., Jin, G. F., Yan, Y. B., & Wu, M. X. (2001). Aberration Theory of

Arrayed Waveguide Grating. Journal of Lightwave Technology, 19 (2), 279-284.

[2] Lu, S., Wong, W. H., Pun, E. Y. B., Yan, Y., Wang, D., Yi, D., et al. (2003).

Design of Flat-Field Arrayed Waveguide Grating with Three Stigmatic Points.

Optical and Quantum Electronic, 35, 783-790.

[3] Smit, M. K., & Dam, C. V. (1996). PHASAR-Based WDM-Devices: Principles,

Design and Applications. Journal of Selected Topic in Quantum Electronics, 2

(2), 236-250.

[4] Kok, A. A. M., Musa, S., Borreman, A., Diemeer, M. B. J., & Driessen, A.

(2003). Completely Multimode Arrayed Waveguide Grating-Based Wavelength

Demultiplexer. EUROCON 2003. Computer as a Tool. The IEEE Region 8, 2,

422-426.

111

APPENDIX B

Wafer cleaning process

The wafer is first immersed in the acetone solution (CH3COCH3), and followed

by methanol (CH3OH), IPA or isopropyl alcohol (C3H8O). The wafer will then be

rinsed with De-Ionized water (DI water). Acetone, methanol, and IPA are organic

solvents that are frequently used to remove different organic contaminants. Organic

contaminants usually originate from a variety of sources, such as fatty materials from

human handling, airborne particles, detergents, organic residues and more [1].

DI water used in cleaning process is ultra pure DI water with 18.4 Mega Ohms

(resistance to an electrical current). DI water is water that has had all of the minerals,

particulates, and dissolved ions removed by a filtration process. DI water is a good

cleaner due to the high capacity for ions as DI water does not contain ions (it will pull

the ions from the contaminants on the wafers). Continued rinsing in DI water will

remove contaminants effectively and leaves no spotting on wafers when drying.

REFERENCES

[1] Kern, W. (1993). Handbook of Semiconductor Wafer Cleaning Technology -

Science, Technology, and Applications. New Jersey, U.S.A.: Noyes Publications.

fabrication of array waveguide …repository.um.edu.my/1140/1/pua chang hong.pdfuntuk...

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