jingsong wei laser heat-mode lithography · opment in lithography techniques. however, progresses...

Springer Series in Materials Science 291

Jingsong Wei

Laser Heat-Mode LithographyPrinciple and Methods

Springer Series in Materials Science

Volume 291

Series Editors

Robert Hull, Center for Materials, Devices, and Integrated Systems,Rensselaer Polytechnic Institute, Troy, NY, USAChennupati Jagadish, Research School of Physical, Australian National University,Canberra, ACT, AustraliaYoshiyuki Kawazoe, Center for Computational Materials, Tohoku University,Sendai, JapanJamie Kruzic, School of Mechanical & Manufacturing Engineering, UNSWSydney, Sydney, NSW, AustraliaRichard M. Osgood, Department of Electrical Engineering, Columbia University,New York, USAJürgen Parisi, Universität Oldenburg, Oldenburg, GermanyUdo W. Pohl, Institute of Solid State Physics, Technical University of Berlin,Berlin, GermanyTae-Yeon Seong, Department of Materials Science & Engineering,Korea University, Seoul, Korea (Republic of)Shin-ichi Uchida, Electronics and Manufacturing, National Institute of AdvancedIndustrial Science and Technology, Tsukuba, Ibaraki, JapanZhiming M. Wang, Institute of Fundamental and Frontier Sciences - Electronic,University of Electronic Science and Technology of China, Chengdu, China

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Jingsong Wei

Laser Heat-ModeLithographyPrinciple and Methods

123

Jingsong WeiShanghai Institute of Optics and FineMechanicsChinese Academy of SciencesShanghai, China

ISSN 0933-033X ISSN 2196-2812 (electronic)Springer Series in Materials ScienceISBN 978-981-15-0942-1 ISBN 978-981-15-0943-8 (eBook)https://doi.org/10.1007/978-981-15-0943-8

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https://doi.org/10.1007/978-981-15-0943-8

Preface

In recent years, the demands for powerful integrated circuits andmicro/nanostructure-based optical/electronic elements have promoted great devel-opment in lithography techniques. However, progresses toward higher resolutionand individual customer demands have proven to be increasingly difficult as featuresize decreases.

Various patterning and lithographic methods have been proposed. Among thesemethods, vacuum-based high-energy beam (such as X-ray and ion/electron beam)lithography has been extensively utilized to fabricate small area micro/nanostructuresbecause of their high resolution, however, the high-vacuum environment is requiredin the process of micro/nanofabrication, and this technique also suffers from lowspeed, high cost, and low throughput. Probe-based lithography is able to fabricatenanoscale arbitrary patterns in atmospheric environment, but the lithography remainsof low speed and fits only for small area. Template-based lithography (such asnanoimprint, projection lithography, and self-assembly) is good method for massproduction, while it is not good choice to meet individual demands due to somedifficulties in fabricating large-area stampers with nanoscale feature size.

Direct laser writing lithography based on light-mode exposure is a good methodto fabricate arbitrary structures for applications in microelectronics, integratedoptics, and diffractive optics, etc. Compared with other lithography methods, itsoperation is facile, and conducted in air. Moreover, its cost is relatively low.Unfortunately, the feature size is dominated by the laser spot size, and it is difficultto surpass the diffraction limit determined by 1.22k/NA, where k is the laserwavelength and NA is the numerical aperture of the converging lens. In order toobtain small feature size, one needs to reduce the laser wavelength of lithographysystem, which is required to develop new organic photoresists corresponding toevery new laser wavelength, accordingly, due to each photoresist being only sen-sitive to specific wavelengths. This often needs a very long research/developmentperiod and a high investment.

Laser heat-mode lithography uses a laser spot to irradiate onto the resist thinfilms. The light energy is absorbed and transferred into thermal energy. The ther-mally induced structural transformation takes place when the resist thin film is

v

heated to certain phase-change threshold temperature such as crystallization point.The phase-change region is further etched in acid/alkali solution due to the etchingselectivity between laser irradiated and non-irradiated regions.

In laser heat-mode lithography, the resist thin films are essentially opto-thermalresponse mode to the laser spot with broad light spectrum features, thus, the laserwavelength can be in the broad range of light spectrum. The laser heat-modelithography greatly simplifies production procedures because they need neither aparticular light source nor a particular environment, there are no pre-baking andpost-baking steps required for inorganic resists. That is, the resist thin films are notsensitive to laser wavelength, thus different from light-mode exposure, the writingprocess does not need any darkroom or yellow light environment. The advantagescan be summarized as follows:

1. The laser heat-mode lithography usually uses a semiconductor diode as the lightsource, the laser wavelength is in the range of visible light, which does not needany vacuum environment. Thus, the lithography cost is very low.

2. The laser beam is focused on a far-field range, and the distance between sampleand writing head is far larger than light wavelength, thus the writing operation isconducted in far-field and atmospheric environments, the operation is easy andwriting speed is very fast, accordingly.

3. The lithographic feature size is determined by some main factors including laserspot, thermal phase-change threshold, thermal diffusion, and nonlinear respon-ses. The pattern feature size can be either larger or far smaller than laser spot bytuning the writing strategy. That is, the laser heat-mode lithography can breakthrough the diffraction limit and realize super-resolution nanolithography andtrans-scale lithography. One can also arbitrarily tune the lithographic feature sizefrom nanoscale to tens of micrometers without changing the laser spot size.

4. The laser heat-mode resists thin films are usually chalcogenide phase-changematerials. The etching process is a broken process of bonds among atoms. Thus,the edge of lithographic patterns is more smooth and clear than the smallmolecular photoresist materials. That is, the line edge roughness can be con-trolled at a very low value.

This book provides a systematical description and analysis of laser heat-modelithography, including principle, lithography system, and manipulation on featuresize, resist thin films, and some typical experimental results and applications. I hopethat this book can drive lithography to continue to advance, and provide someindividual demands.

The book is helpful for advanced undergraduates, graduate students, andresearchers and engineers working in related fields of nanofabrication, lithographytechniques and systems, and phase-change materials, etc. It is unavoidable thatsome errors may occur in this book, I hope that the readers can point them out.I also will further correct them and improve my work on future releases.

vi Preface

The work in this book is partially supported by the National Natural ScienceFoundation of China (Grant Nos. 61627826, 51672292). Here please allow me toexpress my appreciations to my family, colleagues, and students due to their sup-ports and helps in my work and life.

Shanghai, China Jingsong Wei

Preface vii

Contents

1 Current Status of Lithography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Lithography Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2.1 Template-Based Lithography . . . . . . . . . . . . . . . . . . . . . . 11.2.2 Maskless Lithography . . . . . . . . . . . . . . . . . . . . . . . . . . . 81.2.3 Probe-Based Lithography . . . . . . . . . . . . . . . . . . . . . . . . . 101.2.4 Comparison Among Lithography Techniques . . . . . . . . . . 16

1.3 Light-Mode Resist Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171.3.1 Organic Resist Thin Films . . . . . . . . . . . . . . . . . . . . . . . . 171.3.2 S/Se-Based Chalcogenide Thin Films . . . . . . . . . . . . . . . . 20

1.4 Remarks and Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

2 Principles of Laser Heat-Mode Lithography . . . . . . . . . . . . . . . . . . . 272.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272.2 Light-Mode Lithography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272.3 Heat-Mode Lithography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

2.3.1 Atomic-Scale Line Edge Roughness . . . . . . . . . . . . . . . . . 312.3.2 Non-diffraction-Limited Lithography . . . . . . . . . . . . . . . . . 332.3.3 Trans-Scale Lithography . . . . . . . . . . . . . . . . . . . . . . . . . 352.3.4 Broadband Lithography . . . . . . . . . . . . . . . . . . . . . . . . . . 372.3.5 Convertible Characteristics Between Positive

and Negative Resists . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382.4 Mechanism of Fast Phase Change of Te-Based Chalcogenides . . . 40

2.4.1 Umbrella-Flip Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . 412.4.2 Multi-Ring Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

2.5 Comparison Between Laser Heat-Mode and Light-ModeLithography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

2.6 Summary and Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

ix

3 High-Speed Rotation-Type Laser Heat-Mode LithographySystem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 493.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 493.2 Basic Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 503.3 Servo-Tracking Module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

3.3.1 Schematic of Optical Astigmatic Methodwith a Double Cylindrical Lens Group . . . . . . . . . . . . . . . 53

3.3.2 Theoretical Analysis of Focusing Error Signals . . . . . . . . . 533.3.3 Calculated and Simulated Results . . . . . . . . . . . . . . . . . . . 563.3.4 Testing of Servo-Tracking Module . . . . . . . . . . . . . . . . . . 583.3.5 Experimental Results of Servo-Tracking Module . . . . . . . . 61

3.4 Measurement Module of Sample Movement Fluctuation Error . . . 643.4.1 Optical Astigmatic Method with a Single Cylindrical

Lens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 643.4.2 Theoretical Analysis of Focus-Seeking with a Pinhole

Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 673.4.3 Measurement System for Sample Movement Fluctuation

Error . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 693.4.4 Sample Movement Fluctuation Error Measurement . . . . . . 71

3.5 Arbitrary Pattern Generation in the (r; h) Polar CoordinateSystem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

3.6 Typical Arbitrary Pattern Structures . . . . . . . . . . . . . . . . . . . . . . . 753.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

4 Manipulation of Thermal Diffusion Channels . . . . . . . . . . . . . . . . . . 814.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 814.2 Thermal Diffusion in Laser Heat-Mode Lithography . . . . . . . . . . . 814.3 Manipulation Through Changing Heat-Mode Resist Materials . . . . 83

4.3.1 Thermal Diffusion Coefficient . . . . . . . . . . . . . . . . . . . . . . 834.3.2 Thin-Film Thickness . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

4.4 Manipulation Through Thermal Conduction Layers . . . . . . . . . . . 874.4.1 Lower Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 874.4.2 Upper Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 904.4.3 Both Lower and Upper Layers . . . . . . . . . . . . . . . . . . . . . 93

4.5 Manipulation Through Writing Strategy . . . . . . . . . . . . . . . . . . . . 964.5.1 High-Speed Writing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 964.5.2 Short Laser Pulse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 994.5.3 Optimization of Laser Writing Strategy . . . . . . . . . . . . . . . 101

4.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

x Contents

5 High-Speed Laser Heat-Mode Lithography on ChalcogenideResists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1055.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1055.2 Te-Based Chalcogenides as Heat-Mode Resists . . . . . . . . . . . . . . 106

5.2.1 AgInSbTe Resists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1065.2.2 Ge–Sb–Te Resists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1095.2.3 TeOx Resists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

5.3 High-Speed Laser Heat-Mode Nanolithography . . . . . . . . . . . . . . 1125.3.1 Photothermal Localization Analysis . . . . . . . . . . . . . . . . . 1125.3.2 Short Irradiation Time Through High-Speed Rotation

Movement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1165.3.3 Laser Heat-Mode Nanolithography . . . . . . . . . . . . . . . . . . 118


6 Laser Heat-Mode Lithography Using Organic Thin Films . . . . . . . . 1236.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1236.2 Patterning Through Combination of Thermal Deformation

with Thermal Gasification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1246.2.1 Molecular Structure Analysis . . . . . . . . . . . . . . . . . . . . . . 1256.2.2 Thermal Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1266.2.3 Optical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1266.2.4 Theoretical Analysis of Photothermal Localization

Response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1276.2.5 Physical Picture of Patterning Process . . . . . . . . . . . . . . . . 1296.2.6 Patterning Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130

6.3 Patterning Through Direct Thermal Gasification . . . . . . . . . . . . . . 1326.4 Patterning Through the Assistance of Thermal Crosslinking

Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1356.4.1 Focused-Spot-Induced Local Post-exposure Baking . . . . . . 1356.4.2 Light-Mode Photoresist Transferring to Heat-Mode

Lithography Through Increasing Exposure Laser Power . . . 1376.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139

7 Laser Heat-Mode Lithography on Transparent Thin Films . . . . . . . 1417.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1417.2 Principle of Laser Heat-Mode Patterning of Transparent

Thin Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1417.3 Thermo-Optical Properties of ZnS–SiO2 Thin Films . . . . . . . . . . . 1437.4 Selective Wet Etching Mechanism of ZnS–SiO2 Thin Films . . . . . 144

7.4.1 Bonding Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1447.4.2 Cladding Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149

Contents xi

7.5 Micro/Nanolithography Assisted by Light Absorption Layer . . . . . 1507.5.1 AgInSbTe as Light Absorption Layer . . . . . . . . . . . . . . . . 1507.5.2 Ge as Light Absorption Layer . . . . . . . . . . . . . . . . . . . . . 1607.5.3 Amorphous Si as Light Absorption Layer . . . . . . . . . . . . . 1627.5.4 AlNiGd Metallic Glass as Light Absorption Layer . . . . . . . 163

7.6 Direct Patterning Through AgOx as Light Absorption Layer . . . . . 1637.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166

8 Laser Heat-Mode Grayscale Lithography . . . . . . . . . . . . . . . . . . . . . 1698.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1698.2 Grayscale Lithography Through Laser-Induced

Micro/Nano-Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1708.2.1 Micro/Nanopatterning Through Marangoni Effect . . . . . . . 1708.2.2 Bump Patterns Through Interior Vaporization

Expansion Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1728.2.3 Grayscale Lithography Through Laser-Induced

Micro/Nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1758.3 Grayscale Lithography Through Laser-Induced Crystallization

Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1798.3.1 Optical Reflectivity Change with Laser Irradiation

Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1798.3.2 Grayscale Image Lithography on the Ge2Sb2Te5

Thin Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1808.3.3 Application of Grayscale Lithography of Ge2Sb2Te5

Thin Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1818.4 Grayscale Lithography on TeOx Thin Films Through

Structural Evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1838.4.1 Structural Evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1838.4.2 Structural Evolution Induced Grayscale Patterns . . . . . . . . 185

8.5 Other Grayscale Lithography Methods . . . . . . . . . . . . . . . . . . . . . 1868.5.1 Surface Oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1868.5.2 Grain Refinement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187


9 Pattern Transfer for Laser Heat-Mode Lithography . . . . . . . . . . . . 1919.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1919.2 Pattern Transfer Through ICP/RIE Etching . . . . . . . . . . . . . . . . . 191

9.2.1 Inorganic Thin Films as Heat-Mode Resists . . . . . . . . . . . 1919.2.2 Transferring from Organic Heat-Mode Resists

to Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198

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9.3 Pattern Transfer to GaAs Substrates Through Wet Etching . . . . . . 2029.4 Patterns Transferring for Applications in Optical Storage . . . . . . . 203

9.4.1 Electroform Transferring . . . . . . . . . . . . . . . . . . . . . . . . . 2039.4.2 Direct Mastering of Stamper . . . . . . . . . . . . . . . . . . . . . . . 2049.4.3 RIE Transferring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207

Contents xiii

Chapter 1Current Status of Lithography

1.1 Introduction

Lithography is one of the critical processes used for the fabrication of microelec-tronic chips and micro/nanostructure-based electro-optical devices. The patternstructures are usually fabricated on the resist thin films and then transferred to thesilicon or fused quartz substrates through the exposure and etching techniques. Inthe current lithographic methods, the exposure is generally based on a photo-chemical reaction after the resist thin film absorbs the light energy, which is referredto as light-mode lithography. The resists are known as light-mode resist materials.There is a variety of lithography methods for complying with different require-ments, including template-based, vacuum-based, and probe-based methods.

1.2 Lithography Methods

1.2.1 Template-Based Lithography

In template-based lithography, the micro/nanostructures are first fabricated on thetemplates using maskless lithography methods, such as laser and electronic beamwriting, and then further transferred to the silicon wafers through projection ornanoimprint lithography [1, 2]. Template-based lithography is suitable for massproduction, such as for chips of very large-scale integrated circuits (ICs).

1.2.1.1 Projection Lithography (DUV/EUV)

Projection light lithography has been used in the mass production of verylarge-scale integrated circuits. According to the development of the technique and

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requirements, the projection lithography can be classified into two categories; one isrefraction-mode projection lithography and the other is reflection-mode projectionlithography.

(1) Refraction-Mode Projection Lithography

Contact optical lithography has been used since the early days of integrated circuitmanufacturing. In this method, an optical mask is in direct contact, or has aproximity gap, with a photoresist-coated substrate. The features in the mask aretransferred to the light-mode resist at 1:1 ratio without reduction [3]. In order toachieve high resolution and eliminate mask damages and contamination problems,projection optical lithography is developed, which can project a demagnified imageonto light-mode resist at the reduction ratio of 5:1 or 10:1 [4]. Thus far, projectionoptical lithography prevails for submicron patterning. Figure 1.1 shows the basicschematic of projection optical lithography, where the light is first adjusted into acollimated beam. The collimated beam passes through the mask and is then focusedonto the light-mode resist layer. The resolution limit (R) of projection lithography isexpressed as

R ¼ k1kNA

ð1:1Þ

where k is the wavelength of the light source, NA is the numerical aperture of theoptical system, and k1 is a processing factor related to a specific imaging process.

Fig. 1.1 Schematic ofrefraction-mode projectionoptical lithography [4]

2 1 Current Status of Lithography

One can observe that a large NA value and a short wavelength will result inincreased lithography resolution.

In the early days, mercury lamps were used as the light sources. The spectral lineat the wavelength of 436 nm was used initially to illuminate the mask. With thereduction of the feature size of the masks, the short light source wavelength of365 nm became the common choice. However, additional demands imposed in theimaging of small mask features made the mercury lamp unsuitable as the illumi-nation source, owing to the insufficient photon energy at shorter wavelengths,which did not meet the mass production requirement of ICs.

Excimer laser has the advantages of possessing both high-photon energy and ashorter wavelength. Excimer laser results from a pulsed gas discharge, and canproduce the ultraviolet light [4]. In IC manufacturing, the mainstream wavelengthof the excimer laser is 193 nm, which is in the deep ultraviolet (DUV) region. Tothis date, the minimum feature size has reduced down to 10 nm. Further reductionof the feature size faces great challenges owing to the sophisticated processes andincreasing cost.

(2) Reflection-Mode Projection Lithography

Fortunately, extreme ultraviolet (EUV) lithography has been proposed to furtherreduce the feature size for node patterns from 7 to 3 nm and production costs [5].EUV is in a strict sense no longer optical irradiation. The wavelength of the EUVlithography is approximately 13.6 nm, and is often referred to as soft X-ray.Conventional refractive optics ceases to function at EUV because of the strongabsorption in almost all materials. Reflective optics has to be used, and the basicsystem is schematically shown in Fig. 1.2, where the EUV light from the EUVsource is collected by a set of reflective mirrors (illumination optics unit) andprojected onto a EUV mask. The mask image is then focused by another set ofreflective mirrors (projection optics unit) and projected onto a wafer to expose theEUV photoresist. EUV lithography must be performed in a near-vacuum conditionto reduce atmospheric absorption of EUV light [6].

According to EUV lithography, the feature sizes of the patterns can be furtherreduced below the 10 nm boundary to 7 nm nodes, or lower [8, 9]. Moreover, thecomplexity of the process can be significantly reduced compared to DUV lithog-raphy. In parallel, EUV lithography can increase the design flexibility, shorten thetime to yield, and lower the production cost.

Although the projection optical lithography has the advantages of mass pro-duction and high efficiency, its high-cost lithography system and masks imposeconstraints on individual and small-volume manufacturing.

1.2 Lithography Methods 3

1.2.1.2 Nanoimprint Lithography

There are different nanoimprint lithography methods. Four types of mainstreamoperation modes are introduced, including the thermal press mode, room temper-ature mode, UV-cured mode, and the reverse mode.

(1) Thermal Press Nanoimprint Mode

For the thermal press nanoimprint mode, the basic process is schematically illus-trated in Fig. 1.3 [10]. The process flow is as follows:

(1) Coating the polymer layer on the substrate: A thin polymer layer with athickness of 100–200 nm is coated on a flat substrate, which is first heated tothe glass transition temperature in the range of 50–100 °C and softened.

(2) Pressing the stamp: A stamp with surface relief structures is pressed onto thethin polymer layer with a pressure of approximately 50–100 bars, depending onthe viscosity of the polymer. The imprint depth is slightly smaller than thepolymer layer thickness so that the stamp surface does not have a hard contactwith the substrate to prevent any damage to the stamp.

Fig. 1.2 Schematic of reflection-mode EUVL system [6, 7]


(3) Removing the stamp: When the temperature is reduced to approximately 50 °C,the stamp and the polymer layer are separated. After the separation of thestamp, an impression of the stamp pattern is left in the polymer layer.

(4) Removing the residual polymer inside the imprinted pattern areas byreactive-ion etching (RIE) and baring the substrate surface.

(2) Room-Temperature Nanoimprint Lithography

Room-temperature nanoimprint lithography (RT-NIL) includes single RT-NIL andbilayer RT-NIL [3]. Figure 1.4 shows the schematics of RT-NIL where the HSQ/PMMA bilayer resists are used, and the imprinted patterns are further transferred tothe bottom layer by RIE. The sample structure can be designed as an “HSQ(40 nm)/PMMA (150 nm)/silicon substrate,” whereby the HSQ has similar prop-erties to SiO2 when heated to high temperature.

Fig. 1.3 Schematic of the thermal press nanoimprint process. a Basic components, b pressingstamp, c removing stamp, and d removing residual by RIE [10]

Fig. 1.4 Schematic of the “HSQ/PMMA” bilayer of RT-NIL and O2 RIE [3]


The process flow is as follows:

(1) Coating PMMA on the silicon substrate.(2) Baking the PMMA layer in the oven at 180 °C for 1 h.(3) Coating the HSQ resist on the top of the PMMA layer.(4) Baking the sample on a hotplate at 150 °C for 2 min followed by oven baking

for 20 min at 180 °C.(5) Imprinting the pattern structures on the sample by a silicon stamp at 440 bars.(6) Transferring the HSQ pattern to the PMMA layer through oxygen RIE.(7) Removing the thin residual layer using RIE.

Compared with thermal press nanoimprint and single-layer RT-NIL, theadvantages of the bilayer RT-NIL are as follows:

(1) A high aspect ratio can be achieved through the RIE of the PMMA layer.(2) The HSQ layer of approximately 40 nm is needed owing to the high etching

resistance of HSQ to oxygen plasma.(3) PMMA can function as a buffer layer to provide a soft landing for the silicon

stamp and avoid damaging the stamp.(4) The stamp can be repeatedly used without additional cleaning.

(3) UV-Cured Nanoimprint Lithography

Nanoimprint can be performed at room temperature, however, it is still not alow-pressure process. A room temperature and low-pressure process corresponds tothe UV-cured NIL (UV-NIL) [3]. Figure 1.5 schematically depicts the process flow,which is almost the same as the thermal NIL. The key differences include the use oftransparent stamp (such as quartz glass) and UV-curable polymer. The UV-curablepolymers have viscosities ranging from 50 to 200 mPa.s in liquid form at roomtemperature. A very small pressure (<1 bar) is needed to press the stamp into theliquid polymer. The solidification of liquid polymer is conducted by UV illumi-nation through the transparent stamp.

(4) Reverse Nanoimprint

The reverse nanoimprint process is schematically shown in Fig. 1.6 [3], wherebythe polymer is spin-coated onto a structured stamp to fill up all the stamp cavities(Fig. 1.6a). Soft baking is carried out to evaporate the solvent in the polymer. The

Fig. 1.5 Schematic of UV-NIL. a Basic components, b UV exposure to cure the polymer,c demolding, d use of RIE to remove residual polymer [3]


molded polymer structures can be transferred to a flat substrate either throughthermal press (Fig. 1.6b1) if the polymer is polystyrene (PS) or polymethyl-methacrylate (PMMA), or through UV curing (Fig. 1.6b2) if the polymer is UVcurable, such as in the case of negative-tone light-mode resists or transparent molds.The UV curing can also be performed from the substrate side if the substrate istransparent and the mold is opaque. A pressure is needed to achieve intimatecontact between the polymer and substrate. After demolding, the polymer structuresare transferred onto the substrate (Fig. 1.6c), which is equivalent to that obtained byconventional imprinting.

Nanoimprint lithography can be used to fabricate high-resolution nanostructureswith low cost, high efficiency, and in large volumes. However, nanoimprint stampcan be polluted readily owing to the direct contact between the resist layer and themold. Additionally, the high-resolution stamp is generally fabricated byelectron-beam lithography. Thus, it is difficult to fabricate a stamp with a large area.Large size stamps are also very expensive if the feature size of the structures is atthe nanoscale.

1.2.1.3 Chemical Self-Assembly

Self-assembly is a process of spontaneous formation of pattern structures withouthuman intervention [11]. Self-assembly can occur at the molecular level throughnoncovalent or weak covalent interactions, such as van der Waals, electrostatic,hydrophobic interactions, or interfacial hydrogen bonding. Self-assembly can alsooccur at large-scale macroscopic levels based on gravitational, capillary, andexternal electromagnetic forces. In the self-assembly process, the individual partshave to be mobile in a particular order. Therefore, the self-assembly generallyoccurs in the liquid phase or on a smooth surface, such that the individual parts canmove around and interact with each other.

Fig. 1.6 Reverse nanoimprint process. a Spin-coating stamp with polymer, b1 reverse imprint bythermal press, b2 reverse imprint by UV curing, and c demolding [3]


In real self-assembly process, numerous factors influence the formation of pat-tern structures, such as the environment, input of external energy, and confinedgeometrical boundaries. The final pattern structure is at the equilibrium state, wherethe individual parts are kept at equal distances with respect to each other, and not atrandom aggregation. The pattern structures are ordered in the long-range beyond afew molecules once the interaction reaches an equilibration state.

Figure 1.7 shows a schematic of the nanofabrication based on chemicalself-assembly [12]. Polyethylene terephthalate (PET) is chosen as the flexiblesubstrate owing to its optical transparency at visible wavelength. A thin Si layer isdeposited on PET substrate by electron-beam evaporation. Subsequently, amonolayer of polystyrene (PS) spheres is self-assembled on the surface of the Silayer, whereby the PS spheres form a compact hexagonal lattice array.A reactive-ion etching (RIE) is then utilized to transfer the close-packed latticepatterns to the Si layer. The polystyrene (PS) spheres act as a mask in the process ofpattern transfer. Finally, the sample is immersed in chloroform to remove all theremaining PS spheres with sonication, and Si nanostructures are fabricated onflexible substrates.

The self-assembly technique can only be utilized to fabricate periodic nanos-tructures such as nanoholes and nanodisks. Some complex and nonperiodic struc-tures are difficult to be fabricated through self-assembly.

1.2.2 Maskless Lithography

Maskless lithography is suitable for individuals and small-volume manufacturers ofmicro/nanostructure devices [13]. In order to meet the requirements, differentmaskless lithography methods have been proposed and developed in recent years.Based on the operation tools, the maskless lithography can be classified in three

Fig. 1.7 Schematic of the fabrication procedures of chemical self-assembly [12]


categories, including high-energy beam writing, probe-based lithography, and laserbeam writing.

1.2.2.1 High-Energy Beam Writing

The high-energy beam generally includes the electron beam, ion beam, and X-rays,etc. High-energy beam lithography has the merits of increased pattern resolution,and can be used for individual manufacturing as well as arbitrary nanofabrication. Itoperates in high-vacuum environments, thus the fabrication speed and cost are slowand high, respectively.

(1) Electron-Beam Lithography

Electron-beam lithography (EBL) emerged in the early 1960s at approximately thesame time as optical lithography [14]. EBL uses a Gaussian electron beam or beamswith variable shapes for high-resolution or high-throughput demands. The trade-offbetween resolution and throughput is determined by the sustainable beam currentand resist sensitivity. Generally speaking, both higher current and enhanced sen-sitivity lead to a reduction in resolution, thus limiting the throughput at highresolution.

Actually, EBL evolved from scanning electron microscopy (SEM) owing to thediscovery of an electron-sensitive polymer material known as polymethyl-methacrylate (PMMA) [15]. Because of the fine beam enabled by electron optics,EBL using PMMA resist can achieve a much higher resolution capability thanoptical lithography. Thus far, the EBL system combined with special e-beam resistsmaterials and process allows the formation of nanostructures with feature sizes of5 nm [16]. The state-of-the-art electron-beam lithography has been implementedextensively for patterning mesoscopic structures with the unique advantages ofincreased resolution in feature size, high reliability in processing, increased accu-racy in positioning/alignment, and increased flexibility in pattern replication.

(2) Focused Ion Beam Lithography

Compared to electrons, ions possess a larger mass. For example, a hydrogen ion is1840 times heavier than an electron. The heavy mass of ions makes them partic-ularly suitable for nanofabrication based on the direct structuring of the materialrather than through the exposure of an organic resist. Focused ion beam (FIB) reallybecame a nanofabrication tool following the introduction of liquid metal ionsources (LMISs) [17]. Gallium is a common and good liquid metal materialbecause of its low melting temperature (29.8 °C). An electrostatic force from a highvoltage is exerted on the liquid metal, thus pulling the liquid metal into an extre-mely small apex. The electrical field strength at the liquid apex can be as high as1010 V m−1. At this extremely high electric field, metal atoms at the liquid’s apexbecome ionized and escape from the liquid metal surface in the form of fieldevaporation, thus resulting in ion emission. Although the total emission current maybe only a few micro-amperes, the current density can be as high as *106 A cm−2


jingsong wei laser heat-mode lithography · opment in lithography techniques. however, progresses...

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