Stresses in Thin Films Final Presentation

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Stresses in Thin FilmsFinal Presentation

• Cynthia Macht, Nick Svencer, Heather Stern
• Tufts University, TAMPL
• July 31, 1998

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Overview

• Initial Goals
• Setup Modifications
• Samples
• Data
• Conclusions

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Initial Goals

• Design and Construct Apparatus
• Fabricate Samples
• Test Thin Film Samples for Stress and Relaxation
  at High Temperatures
• Polymers
• Silicon Wafers
• Analyze Data

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Final Apparatus

• Split-Beam Laser Technique
• Heater Raised Around Sample
• Stainless Steel Sample Holder
• Quartz Plate
• Two Fans
• Thermocouples

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Final Apparatus Continued
New Sample Holder -Stainless Steel Higher
Temperatures -58 Open Space
Fans in Place on Heater -Reduces Convection
Currents Above Quartz Plate
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Laser Schematic
Large Mirror
Wall
x
laser
Beam Splitter
Mirror
S
Sample
R
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Obstacles to Dot Measurement

• Extremely Slight Bending of Silicon Wafers
• Difficult to Detect Change in Dot Distance
• Dots Change Shape During Experiment
• Loss of Reference Point

Human Error
CCD Camera
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Polystyrene Samples

• Brass Substrate
• Thickness .010-.031
• Polystyrene Film
• Thickness .006-.007 /- .001
• Polymer Sold by Polaroid

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Formulas to Determine Stress of Thin Films
Curvature
Where, Kcurvature, Rradius of
curvature, xdistance between laser dots on
wall, Sdistance between dots on sample Ltotal
length traveled by laser beam
Stoney Formula
Where stress of the film, Youngs
modulus of the substrate thickness of
substrate Poissons Ratio of the
substrate thickness of the film
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Polystyrene Relaxation curve, average temp
344.45K, time constant 1/.01324475.51s
Time vs. Change in Curvature of
.010Brass/.006.001Polystyrene
Change in Curvature (1/m)
Time (s)
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Polystyrene Relaxation Curve, average temperature
330.75K time constant 1/.0319931.26s
Time vs. Change in Curvature of
.010Brass/.007.001Polystyrene
Change in Curvature (1/m)
Time (s)
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Polystyrene Relaxation Curve, average temperature
351.19K time constant 1/.0173957.50s
Time vs. Change in Curvature of
.031Brass/.006.001Polystyrene
Change in Curvature (1/m)
Time (s)
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Polystyrene Conclusions

• Relaxation time constant range of polystyrene
  31-76 s

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Silicon WafersFabricated by Northeastern
University

• Silicon Nitride on Silicon Wafer (SiNx)
• Silicon Nitride Thickness 840 Å
• Silicon Substrate Thickness .37 mm
• Silicon Dioxide on Silicon Wafer (SiO2)
• Silicon Dioxide Thickness 1970 Å
• Silicon Substrate Thickness .37 mm

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Formulas for Thin Film Curvature1
1Townsend, Barnett, Brunner, 1987.
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Silicon Coefficient of Thermal Expansion
3.58E-6/ºC Youngs Modulus
1.30E11 Pa
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Silicon Oxide Conclusions

• Coefficient of thermal expansion of Silicon Oxide
  is slightly smaller than that of Silicon which is
  3.58E-6

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Silicon Coefficient of Thermal Expansion
3.58E-6/ºC Youngs Modulus 1.30E11 Pa
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Silicon Nitride Wafer at Youngs Modulus 2.00E12
Pa
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Silicon Nitride Conclusions

• Coefficient of thermal expansion of SiNx is
  slightly smaller than that of Silicon which is
  3.58E-6
• SiNx relaxes at 773ºK

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Final Conclusions

• Developed an instrument to measure curvature in a
  variety of thin film materials, from polymers to
  semiconductors

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