1. 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.

2. Overview Introduction Thin film properties Heat transfer model
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

3. 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

4. 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)

5. 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-MEMS
Top view by optical microscope

6. Design & Modeling Behavior of T-MEMS depend on thin film properties
Young's modulus, E(T)
thermal expansion coefficient, a(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

7. Approach
New technique for determining thin film properties of poly-Si and SiO2
use T-MEMS as test structures to find a(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

8. Study of Thin Film Properties
T-MEMS design
Experimental setup
Numerical model
Results

9. T-MEMS Design Bending T-MEMS
array of multilayered cantilevers over Si substrate 6 mm gap by design, ~23 mm in actual sample
deflect down at high temperature due to difference in thermal expansion coefficients of layers
adhere to substrate at contact
LPCVD SiO2
LPCVD poly-Si
thermal SiO2
Si substrate
0.19 mm SiO2
0.54 mm poly-Si
1.03 mm SiO2

10. T-MEMS Design beams are initially curved up due to residual stress
3” wafer
changing widths
decreasing lengths
. . .
100 mm
99 mm .
50 mm
die size ~ 4 mm × 4 mm
beams are initially curved
up due to residual stress

11. Microscale Curvature Measurement
lamp housing
CCD camera with
telescopic lens
fiber optic
bundle
collimating lens
cube beamsplitter
quartz plate with
Al foil reflector
Al reflector
thermocouple
embedded
in Si wafer
W-halogen
lamp
T-MEMS sample
quartz rods (support)
Si wafer (support)
output to
computer
light source

12. 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

13. Thermally Induced Curvature
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
S Force = 0
S Moment = 0
SiO2
poly-Si t z ti i n .

14. Curvature Equation Curvature: Thermal strain: Neutral plane:
-1 for j < i
bij = 0 for j = i
1 for j > i
(Townsend, 1987)

15. Reduction of Variables
Curvature at temperture T* is dependent on 4 variables:
ESi,ESiO2  at T*
aSi, aSiO2  variation from initial temperature to T*
E and a 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 a(T)
other film structures can be designed to isolate the effects of E

16. Piecewise-Linear Approximation of a(T)
Low temperature range (< 300 °C)
aSiO2 is constant  in general,a of silica glass materials do not vary significantly at temperatures below 300 °C
aSi increases linearly up to 300 °C
High temperature range (300 ~ 1000 °C)
aSi is proportional to specific heat of Si  based on physics principle, verified for bulk crystalline Si
aSiO2 increases linearly up to 1000 °C
aSi (10-6 °C-1)
temperature (°C)
a for bulk, crystalline Si

17. Strategy for Low Temperature Range
a25°C
a300°C
linear fit 1 2 3 4 5
aSi
(°C-1)
temperature (°C)

18. Results: Curvature Measurements
temperature (°C)
K (mm-1)
tip deflection for 100 mm beam (mm)
initial upward curvature
due to residual stress
minimum curvature
limit of system

19. Results: aSi(T) at Low Temperatures
aSi(T) approximated to be linear up to 300 °C
temperature (°C)
a Si (10-6 °C-1)

20. Results: a(T) at High Temperatures
aSi(T) assumed to be proportional to specific heat
aSiO2(T) approximated as linear between 300 ~ 1000 °C
temperature (°C)
a (10-6 °C-1)
poly-Si film
bulk crystalline Si
SiO2 film

21. Results: Numerical Fit
temperature (°C)
D K (mm-1)

22. Heat Transfer Model Thermal requirements
Radiative properties of T-MEMS
Steady-state heat transfer model
Wafer temperature distributions

23. 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

24. 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
raverage = SFiri
Fi = Ai / Atotal
where area fill factor is:
(Abramson, 1998)

25. 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

26. T-MEMS Radiative Properties
Five T-MEMS Regions
Si partial
transparency
thin film
interference
incoherent
effects
Si substrate 
3-films  
1-film  
3-films & air   
1-film & air    2 1 3 4 5
Find net property of T-MEMS die by averaging

27. Total Radiative Properties of T-MEMS
total normal absorptivity
total normal emissivity
temperature (°C)
temperature (°C)
Si substrate 3-films 3-films & air
T-MEMS average 1-film 1-film & air

28. Steady-State Heat Transfer Model
Simulates a patterned wafer heated radiatively
Heat transfer terms:
conduction through wafer
radiation from lamp
radiative heat loss from wafer
steady state: Sq = 0
Parameters:
heat source: 2200 °C, e = 0.3
flampwafer = 0.1; constant
use a and e of wafer at 800 °C
kwafer = 30 W/mK
1/4 of wafer modeled due to symmetry
no convective term: assumes vacuum
3” wafer
4 mm
1 mm
1.5" (38 mm)
temperature
profile location
thickness 0.35 mm
die size 4 mm
die spacing 1 mm
element size 0.25 mm

29. Uniform Wafers a e Si 0.664 0.669 3-films 0.603 0.525 Si wafer
3-film wafer
position (mm)
temperature (°C)
Si wafer
3-film wafer
a e Si
3-films

30. Effect of T-MEMS on 3-Film Wafer
position (mm)
temperature (°C)
T-MEMS wafer
uniform wafer
a e
uniform
T-MEMS

31. Effect of T-MEMS on Si Wafer
position (mm)
temperature (°C)
T-MEMS wafer
uniform wafer
a e
uniform
T-MEMS

32. Effect of T-MEMS: Other Cases
die spacing
= 10 mm
"packed" die
position (mm)
temperature (°C)
uniform wafer
10mm spacing
packed die
a e
uniform
T-MEMS
packed

33. Evaluation of T-MEMS Evaluation of original design
Proposed design modification
Effect of high temperature
Comment on adhesion

34. Performance of Original Design
beam length: 50 ~ 100 mm
width ratios: 0.2 ~ 0.85
6 mm 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 °C temperature range

35. 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 mm in length
width ratios
6 mm gap
total of 867 beams tested
selected 97 beams having contact temperature between 900 °C ~ 1100 °C

36. Evaluation of Modified Design
0.2 ~ 1.0 width ratios
62 ~ 73 mm in length
6 mm 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

37. 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

38. 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
K (mm-1)
tip deflection (mm)
total time (min)

39. 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

40. 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

41. 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 Ea
SEM micrograph of
SiNx-on-SiO2 beams

42. 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-mm gap
Model temperature gradient during transient state

43. 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

45. --- Extra Slides ---

46. T-MEMS Fabrication Process
~1 mm thermal SiO2, ~0.6 mm LPCVD poly-Si, ~0.2 mm LPCVD SiO2
deposited on single-sided 3” Si wafer
apply photoresist (PR) to pattern top layer
LPCVD low thermal SiO2
LPCVD poly-Si
thermal SiO2
Si substrate
photoresist
etch top layer (LTO)
etch bottom layer (poly-Si), remove PR

47. Fabrication Process (continued)
grow thin thermal SiO2 layer
to protect poly-Si layer during final etch
LPCVD low thermal SiO2
LPCVD poly-Si
thermal SiO2
Si substrate
photoresist
apply PR to pattern bottom layer
pattern bottom layer (thermal SiO2), remove PR
release structure
by etching Si substrate

48. E(T) of Poly-Silicon
From Kahn, et.al, 1998; using lateral resonance structures
Varies from ~168 GPa at room temp. to ~163 GPa at 500 °C
temperature (°C)
E (GPa)
Comparison: ~ 6 GPa higher than crystalline Si values;
similar temperature-dependence

49. Beam Curvature Geometry
q/2 R L q C A B
By geometry:
beam
Curvature:
R radius of curvature of beam
L apparent length of beam from CCD image
q cone angle of imaging system; found at room temperature

50. Reflectivity Measurement
reference
port
sample
port
focusing mirror
monochromator
Si or PbS
detector
(on top)
diffraction
gratings 8°
integrating
sphere
Order-sorting filters
Chopper
collimator
fiber optics
W-Hg lamp
SR510
lock-in amplifier
chopper
controller
RS-232
interface
focusing mirror PC
RS-232 interface

51. Reflectivity Measurement
high temperature modification
45° aluminum ramp
cooling systems
detector
light source
sample mount
cooling
system
heater
45° ramp
sphere wall
sample
transmitted light
reflected
light
incident

52. Spectral Reflectivity of 3-Film Region
temperature (°C) rl

53. Spectral Reflectivity of Siliconrl
500 °C
600 °C
1000 °C
temperature (°C)

54. Spectral Reflectivity of Stripes at 500 °C
temperature (°C) rl

55. 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)
incident
radiation
thin
films
substrate
coherent effects
incoherent
effects

56. Control Volume Sq = 0 qrad,in = awafer f elamp s Tlamp4 Az d
qrad,in
qcond,1
qcond,4
qcond,2
qcond,3
qrad,out
bottom
top
qrad,in = awafer f elamp s Tlamp4 A
qrad,out = ewafer Twafer4 A
qcond,i = kwafer Ac (Ti-T) / d
Sq = 0
at steady state
d = 0.25 mm