Novel Microelectromechanical Systems (MEMS) for the Study of Thin

Film Properties and Measurement of Temperatures During Thermal

Processing

Haruna Tada

M.S. Thesis Defense

July 21, 1999

Committee Members:

Peter Wong and Ioannis Miaoulis, Tufts University

Paul Zavracky, Northeastern Univ. / MicroOptical Corp.

Tufts University

Overview Introduction

• background & motivation• what are T-MEMS?

Thin film properties• experimental setup• numerical model• results

Heat transfer model• T-MEMS radiative properties• steady state temperature distribution

Evaluation• temperature range & resolution• proposed modifications• effects of high temperature & adhesion

Conclusions

Tufts University

Rapid Thermal Processing (RTP) RTP in Microelectronics Industry

• single wafer processing with radiant heat source• high temperatures (up to ~1000 °C)• high heating rates (100 °C/sec)• short processing times (~seconds)

Thermal requirement forecast for the year 2000• uniformity (± 2 °C) over 12" wafer• accuracy (± 3 °C)

Challenge• accurate temperature measurement techniques are

needed to meet the requirements

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Temperature Measurements in RTP

Thermocouples• highly intrusive• delicate & difficult to handle• contact resistance between

thermocouple and wafer Pyrometers

• non-intrusive, optical technique• unknown wafer emissivity;

changes with temperature and film deposition Alternative methods needed to meet thermal requirements

of the microelectronics industry

Thermocouple wafer(Sensarray)

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MEMS Temperature Sensors Microelectromechanical Temperature Sensors (T-MEMS)

• small temperature sensors based on MEMS technology• ex-situ measurement of maximum process temperature• based on differences in thermal expansion coefficients

SEM micrograph of T-MEMSTop view by optical microscope

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Design & Modeling Behavior of T-MEMS depend on thin film properties

• Young's modulus, E(T)• thermal expansion coefficient, (T)• functions of temperature

Previous study of thin film properties• Young's modulus of thin films

– resonance structures– tensile testing of micromachined specimen– mostly done at room temperature

• lack in information on thermal expansion coefficient at elevated temperatures

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Approach New technique for determining thin film properties of

poly-Si and SiO2

• use T-MEMS as test structures to find (T) Evaluate T-MEMS design

• effect on wafer temperature– numerical models for radiative property and

temperature distribution• performance

– temperature range & resolution Refine T-MEMS design

• model beam curvature based on properties found

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Study of Thin Film Properties

T-MEMS design Experimental setup Numerical model

Results

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T-MEMS Design Bending T-MEMS

• array of multilayered cantilevers over Si substrate6 m gap by design, ~23 m in actual sample

• deflect down at high temperature due to difference in thermal expansion coefficients of layers

• adhere to substrate at contact

0.19 m SiO2

0.54 m poly-Si1.03 m SiO2

LPCVD SiO2

LPCVD poly-Si

thermal SiO2

Si substrate

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T-MEMS Design

3” wafer

beams are initially curved up due to residual stress

changing widths

dec

reas

ing

len

gth

s

. . .1 2 3 . . . 14

100 m99 m

.

.

.

50 m

die size ~ 4 mm × 4 mm

..

.

..

.

..

.

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Microscale Curvature Measurement

lamp housing

CCD camera with telescopic lens

fiber opticbundle

collimating lens

cube beamsplitter

quartz plate withAl foil reflector

Al reflector

thermocoupleembeddedin Si wafer

W-halogenlamp T-MEMS sample

quartz rods (support)

Si wafer (support)

output to

computer

output to

computer

light source

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Microscale Curvature Measurement

Imaging System• collimated light source

illuminating curved sample only flat portion of beam is seen by the camera

Curvature Measurement• analyze CCD image to find

"apparent length"• curvature found through

geometric relation between beam curvature and apparent length

substrate beam

image of beam on camera

apparent length

Tufts University

Numerical model developed by Townsend (1987) Discretize beam layers into small sub-layers

• assume no stress gradient within each sub-layer

Solve for curvature:• constrain interface• Force = 0• Moment = 0

Thermally Induced Curvature

SiO2

poly-Si

SiO2

t

z

ti

i

n

.

.

.

0

Tufts University

Thermal strain:Thermal strain:

Curvature Equation

-1 for j < iij = 0 for j = i

1 for j > i

Neutral plane:

(Townsend, 1987)

Curvature:

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Reduction of Variables Curvature at temperture T* is dependent on 4 variables:

• ESi,ESiO2 at T*

• Si, SiO2 variation from initial temperature to T*

• E and appear as a product• need to know three before finding the final property

Reduction of variables• parametric study to find the effect of each variable• for T-MEMS, E(T) found to have little influence on K

use literature values as approximation, then find (T)• other film structures can be designed to isolate the

effects of E

Tufts University

Piecewise-Linear Approximation of (T)

Low temperature range (< 300 °C)

• SiO2 is constant in general, of silica glass materials do not vary significantly at temperatures below 300 °C

• Si increases linearly up to 300 °C

High temperature range (300 ~ 1000 °C)

• Si is proportional to specific heat of Si based on physicsprinciple, verified for bulk crystalline Si

• SiO2 increases linearly up to 1000 °C

S

i (10

-6 °

C-1)

2

3

4

5

0 300 600 900 1200

temperature (°C)

for bulk, crystalline Si

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Strategy for Low Temperature Range

0 100 200 300

25°C

300°C

linear fit

Si

(°C-1)

1 2 3

4

5

temperature (°C)

Tufts University

Results: Curvature Measurements

-0.0015

-0.001

-0.0005

0

0.0005

0.001

0.0015

0 200 400 600 800 1000

-7.5

-5

-2.5

0

2.5

5

7.5

temperature (°C)

K (m

-1)

tip d

eflection

for 100

m b

eam (

m)

initial upward curvature due to residual stress

minimum curvature

limit of system

Tufts University

temperature (°C)

S

i (10

-6 °

C-1)

2

3

4

5

6

50 100 150 200 250 300

Results: Si(T) at Low Temperatures

Si(T) approximated to be linear up to 300 °C

Tufts University

01234567

0 200 400 600 800 1000

temperature (°C)

(1

0-6 °C

-1)

poly-Si film

bulk crystalline Si

SiO2 film

Results:(T) at High Temperatures

Si(T) assumed to be proportional to specific heat

SiO2(T) approximated as linear between 300 ~ 1000 °C

Tufts University

Results: Numerical Fit

temperature (°C)

K

(m

-1)

-0.003

-0.002

-0.001

0

0 200 400 600 800 1000

Tufts University

Heat Transfer Model

Thermal requirements Radiative properties of T-MEMS Steady-state heat transfer model Wafer temperature distributions

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Thermal Requirements of T-MEMS

Requirement of a non-intrusive temperature sensor:must not affect the heating of wafer• temperature of the wafer is same w/ or w/o the sensor

Requirement of an accurate temperature sensor:temperature indicated by the sensor is the same as actual wafer temperature• local temperature distribution surrounding the sensor is

uniform

Radiative effects on T-MEMS structures may affect the temperature of the wafer numerical model was developed to predict the effects

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Radiative Effects on a Wafer Properties of silicon wafer

• varies dramatically with temperature• partial transparency at low temperatures• wafer becomes opaque at temperatures above 700 °C

Thin films (< microns) • thin film interference effects at wafer surface

Thick films (> microns) • incoherent effects; analyzed by raytracing

Large 2-D surface patterns• averaging by area fill factors

average = Fii Fi= Ai / Atotalwhere area fill factor is:

(Abramson, 1998)

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Experimental Verification Si wafer at high temperatures

• partial transparency• increase in absorption at high temperatures

Single SiO2 films at high temperatures

• thin film interference Simple patterns (stripes) at high temperatures

• average area method for 2-D patterns Multilayered film at room temperature

• thin film interference for multilayered film• verify thickness measurement of T-MEMS films

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Five T-MEMS Regions

T-MEMS Radiative Properties

Find net property of T-MEMS die by averaging

Si partialtransparency

thin filminterference

incoherenteffects

Si substrate 3-films 1-film

3-films & air 1-film & air

2

1

3

4

5

Tufts University

Total Radiative Properties of T-MEMS

00.1

0.20.30.4

0.50.6

0.70.8

0 200 400 600 800 1000

00.10.20.30.4

0.50.60.70.8

0 200 400 600 800 1000temperature (°C) temperature (°C)

Si substrate 3-films 3-films & air

T-MEMS average 1-film 1-film & air

tota

l no

rmal

ab

sorp

tivi

ty

tota

l no

rmal

em

issi

vity

Tufts University

Simulates a patterned wafer heated radiatively Heat transfer terms:

• conduction through wafer• radiation from lamp• radiative heat loss from wafer

• steady state: q = 0 Parameters:

• heat source: 2200 °C, = 0.3

• flampwafer = 0.1; constant

• use and of wafer at 800 °C

• kwafer = 30 W/mK

• 1/4 of wafer modeled due to symmetry • no convective term: assumes vacuum

Steady-State Heat Transfer Model

4 mm

1 mm

1.5" (38 mm)

temperatureprofile location

3” wafer

thickness 0.35 mmdie size 4 mmdie spacing 1 mmelement size 0.25 mm

Tufts University

Uniform Wafers

position (mm)

tem

per

atu

re (

°C)

845846847

848849850851

852853854

0 10 20 30 40

Si wafer3-film wafer

Si 0.664 0.669

3-films 0.603 0.525

Si wafer

3-film wafer

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Effect of T-MEMS on 3-Film Wafer

position (mm)

tem

per

atu

re (

°C)

845846847

848849850851

852853854

0 10 20 30 40

T-MEMS waferuniform wafer

uniform 0.603 0.525

T-MEMS 0.623 0.580

T-MEMS on3-film wafer

3-film wafer

Tufts University

position (mm)

tem

per

atu

re (

°C)

Effect of T-MEMS on Si Wafer

845846847

848849850851

852853854

0 10 20 30 40

T-MEMS waferuniform wafer

uniform 0.664 0.669

T-MEMS 0.623 0.580

T-MEMS onSi wafer

Si wafer

Tufts University

Effect of T-MEMS: Other Cases

position (mm)

845846847

848849850851

852853854

0 10 20 30 40

tem

per

atu

re (

°C)

uniform wafer10mm spacingpacked die

uniform 0.603 0.525

T-MEMS 0.623 0.580packed 0.619 0.599

die spacing= 10 mm

"packed" die

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Evaluation of T-MEMS

Evaluation of original design Proposed design modification

Effect of high temperature Comment on adhesion

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Performance of Original Design Original Design

• beam length: 50 ~ 100 m• width ratios: 0.2 ~ 0.85• 6 m between Si and beam• total of 714 beams on a die

Theoretical temperature range• 460 to over 2000 °C• thermal processing rarely exceeds 1100 °C

large portion of beams will not be used Theoretical resolution

• varies between 0.1 °C and 9.7 °C in 900 - 1100 °C temperature range

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Modified Design Compile a "Wish List"

• temperature range: 900 ~ 1100 °C• resolution: < 0.5 °C• die size: as small as possible

Beam selection• 50 ~ 100 m in length• 0.2 - 1.0 width ratios• 6 m gap• total of 867 beams tested• selected 97 beams having contact

temperature between 900 °C ~ 1100 °C

Tufts University

Evaluation of Modified Design Modified design

• 0.2 ~ 1.0 width ratios• 62 ~ 73 m in length• 6 m gap depth• 97 beams, fits on ~1.3 mm square area

Resolution• vary between 0.1 °C to 9 °C

need to fill in "gaps" in temperature

900 950 1000 1050 1100

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Improving Resolution Customized beam designs with specific target temperature are

needed to fill in gaps in resolution

Proposed design: varying bottom layer length• adjusting the bottom layer length will give full control of

contact temperature• can be modeled by simple geometry

Tufts University

Effects of High Temperature Effect of long-time exposure to high temperatures (~850°C)

• room-temperature tip deflection decrease with time Possible reason: thermal oxide growth on top layer T-MEMS may be annealed to have zero initial curvature

0

0.0005

0.001

0.0015

0.002

0 15 30 45 60

-12

-10

-8

-6

-4

-2

0

K (m

-1)

tip d

eflection

(m

)

total time (min)

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Adhesion

Adhesion between bottom layer (SiO2) and substrate(Si) is a necessity for T-MEMS

Preliminary testing with loose beams on Si wafer• beams on plain Si wafer, heated to ~ 600 °C• test adhesion strength

– lightly rubbed by cotton swab after cooling• adhesion was confirmed under microscope

– adhesion stregth at room temperature is stronger than fracture strength of beams

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Conclusions Thin Film Properties

• T-MEMS used as testing structures for finding properties• developed experimental apparatus for measuring

microscale curvature at very high temperatures

• thermal expansion coefficient of poly-Si and SiO2 found for high temperatures

T-MEMS as Temperature Sensors• theoretical evaluation of original design• design modification to target specific temperature ranges• thermally non-intrusive when used on Si wafer • beam adhesion confirmed in preliminary study

Tufts University

Future Work: Thin Film Properties

Modify beam design to target other properties

Extend study to other materials

• SiNx (silicon nitride) on SiO2 beams

Modify experimental setup

• view larger curvatures

• reduce uncertainty

Verify results with alternative methods

• resonance method for E(T)

• wafer curvature measurement for the product E

SEM micrograph ofSiNx-on-SiO2 beams

Tufts University

Future Work: Temperature Sensors

Finalize design modifications

• define target temperature range

• temperature resolution

• optimize die size

Fabrication, testing & calibration of modified design

• experimental testing with thermocouples

Verify adhesion using 6-m gap

Model temperature gradient during transient state

Tufts University

Acknowledgements Committee Members:

• Professors Peter Wong & Ioannis Miaoulis, Tufts Univ.

• Professor Paul Zavracky, Northeastern Univ. / MicroOptical Corp.

Graduate Students:

• Seth Mann & Alexis Abramson, Tufts Univ.

• Patricia Nieva, Northeastern Univ.

Undergraduate Researchers:

• Amy Kumpel, Rich Lathrop, John Slanina (REU 99 T-MEMS Group)

• Emilie Nelson & Melissa Bargman

This work is supported by the National Science Foundation under grant

number DMI-9612058

--- Extra Slides ---

Tufts University

T-MEMS Fabrication Process~1 m thermal SiO2, ~0.6 m LPCVD poly-Si, ~0.2 m LPCVD SiO2

deposited on single-sided 3” Si wafer

apply photoresist (PR) to pattern top layer

etch top layer (LTO)

etch bottom layer (poly-Si), remove PR

LPCVD low thermal SiO2

LPCVD poly-Si

thermal SiO2

Si substrate

photoresist

Tufts University

Fabrication Process (continued)

grow thin thermal SiO2 layer to protect poly-Si layer during final etch

apply PR to pattern bottom layer

pattern bottom layer (thermal SiO2), remove PR

release structureby etching Si substrate

LPCVD low thermal SiO2

LPCVD poly-Si

thermal SiO2

Si substrate

photoresist

Tufts University

E(T) of Poly-Silicon

temperature (°C)

E (

GP

a)

y = -5.9816E-06x2 - 8.2225E-03x + 1.6806E+02162

163

164

165

166

167

168

169

0 100 200 300 400 500

From Kahn, et.al, 1998; using lateral resonance structures Varies from ~168 GPa at room temp. to ~163 GPa at 500 °C

Comparison: ~ 6 GPa higher than crystalline Si values;

similar temperature-dependence

Tufts University

Beam Curvature Geometry

R

L

C

A BR radius of curvature of beam

L apparent length of beam from CCD image

cone angle of imaging system; found at room temperature

2sin

RL

2sin

11 LR

K Curvature:

By geometry:

beam

Tufts University

Reflectivity Measurement

SR510lock-in amplifier

8°

Si or PbSdetector(on top)

collimator

fiber

opt

ics

W-Hg lamp

Order-sorting filtersChopper

focusing mirror

focusing mirror

diffraction gratings

RS

-232

inte

rfac

e

RS-232 interfacePC

choppercontroller

integratingsphere

monochromator

sampleport

referenceport

Tufts University

Reflectivity Measurement high temperature modification

• 45° aluminum ramp• cooling systems

heater

45° ramp

sphere wall

sample

transmitted light

reflectedlight

incidentlight

detector

light source

sample mount

coolingsystem

Tufts University

Spectral Reflectivity of 3-Film Region

temperature (°C)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

400 800 1200 1600 2000

Tufts University

Spectral Reflectivity of Silicon

0.3

0.35

0.4

0.45

0.5

0.55

0.6

400 800 1200 1600 2000temperature (°C)

20 °C

1000 °C

600 °C

500 °C

300 °C

Tufts University

Spectral Reflectivity of Stripes at 500 °C

temperature (°C)

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

400 800 1200 1600 2000

empirical10 micron pattern (f.38)3 micron pattern (f=0.37)numerical

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Radiative Effects in a Wafer Radiative effects through a wafer

• coherent effects: – thin film interference– scattering– diffraction from small

patterns (<microns)

• incoherent effects: – partial transparency– large patterns (>microns)– thick layers (>microns)

incidentradiation

thinfilms

substrate

coherent effects incoherenteffects

Tufts University

Control Volume

z

qrad,in

qcond,1

qcond,4 qcond,2

qcond,3

qrad,outbottom

qrad,outtop

qrad,in = wafer f lamp Tlamp4 A

qrad,out = wafer Twafer4 A

qcond,i = kwafer Ac (Ti-T) /

q = 0at steady state

= 0.25 mm