Novel Microelectromechanical Systems (MEMS) for the Study of Thin Film Properties and Measurement of Temperatures During Thermal Processing

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
• 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)
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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

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
  substrate6 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

10
T-MEMS Design
beams are initially curved up due to residual
stress
11
Microscale Curvature Measurement
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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

image of beam on camera
apparent length
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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

14
Curvature Equation
Curvature
Neutral plane
-1 for j lt i bij 0 for j i 1 for j gt i
(Townsend, 1987)
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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 (lt 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 physicsprinciple, verified for bulk
  crystalline Si
• aSiO2 increases linearly up to 1000 C

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Strategy for Low Temperature Range
1 2 3 4
5
aSi (C-1)
0 100 200 300
temperature (C)
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Results Curvature Measurements
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Results aSi(T) at Low Temperatures

• aSi(T) approximated to be linear up to 300 C

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

21
Results Numerical Fit
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
  sensormust not affect the heating of wafer
• temperature of the wafer is same w/ or w/o the
  sensor
• Requirement of an accurate temperature
  sensortemperature 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 (lt microns)
• thin film interference effects at wafer surface
• Thick films (gt microns)
• incoherent effects analyzed by raytracing
• Large 2-D surface patterns
• averaging by area fill factors

(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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T-MEMS Radiative Properties

• Five T-MEMS Regions

• Find net property of T-MEMS die by averaging

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Total Radiative Properties of T-MEMS
total normal absorptivity
total normal emissivity
temperature (C)
temperature (C)
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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

thickness 0.35 mm die size 4 mm die spacing 1
mm element size 0.25 mm
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Uniform Wafers
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Effect of T-MEMS on 3-Film Wafer
T-MEMS wafer uniform wafer
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Effect of T-MEMS on Si Wafer
T-MEMS wafer uniform wafer
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Effect of T-MEMS Other Cases
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Evaluation of T-MEMS

• Evaluation of original design
• Proposed design modification
• Effect of high temperature
• Comment on adhesion

34
Performance of Original Design

• 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 900 - 1100
  C temperature range

35
Modified Design

• Compile a "Wish List"
• temperature range 900 1100 C
• resolution lt 0.5 C
• die size as small as possible
• Beam selection
• 50 100 mm in length
• 0.2 - 1.0 width ratios
• 6 mm gap
• total of 867 beams tested
• selected 97 beams having contact temperature
  between 900 C 1100 C

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Evaluation of Modified Design

• 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

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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

38
Effects of High Temperature

• Effect of long-time exposure to high temperatures
  (850C)
• 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

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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

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-9612058

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--- 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
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
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

Comparison 6 GPa higher than crystalline Si
values similar temperature-dependence
49
Beam Curvature Geometry
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
W-Hg lamp
fiber optics
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

52
Spectral Reflectivity of 3-Film Region
53
Spectral Reflectivity of Silicon
300 C
20 C
rl
500 C
600 C
1000 C
temperature (C)
54
Spectral Reflectivity of Stripes at 500 C
55
Radiative Effects in a Wafer

• Radiative effects through a wafer
• coherent effects
• thin film interference
• scattering
• diffraction from smallpatterns (ltmicrons)
• incoherent effects
• partial transparency
• large patterns (gtmicrons)
• thick layers (gtmicrons)

56
Control Volume
d 0.25 mm