1. In-situ AFM Tensile Testing
ME 395
Professor Horacio D. Espinosa
Chris Biedrzycki, Matthieu Chardon, Travis Harper, Richard Neal

2. Micron Scale Mechanical Properties
Once structures enter microscale, laws of macro mechanics no longer dominate mechanical response
Samples approach size scale of grains and dislocations, also dominated by surface effects
Multimaterial interfaces, sharp corners in MEMS devices require in-situ characterization
Devices in market require a high amount of reliability, necessitating direct fatigue testing methods

3. Advantages of In-Situ Testing
Model entire range of behavior
Differentiate elastic and plastic regimes
Understand more than ultimate tensile strength (UTS)

4. Similar Works 1 3 1
=2 μm
No testing results 2
1) Mechanical Measurements at the Micron and Nanometer Scales, Knauss, Huang
2) Microtensile Tests with the Aid of Probe Microscopy for the Study of MEMS Materials, Knauss, Chasiotis
3) MEMS Fatigue Testing to Study Nanoscale Response, Fisher, Labiossiere

5. Testing stage Polysilicon specimen
Integrated during fabrication
Pulled by mass
Actuators move mass when voltage applied
Force measured by capacitive sensor
Strain measured by DIC
From Zhu et. al., 2003

6. ANSYS Modeling of System
The maximum load deliverable by the loading device is ~100 mN
From Zhu et. al., 2003

7. Polysilicon Sample Properties 4μm 10μm Young's Fracture Method Modulus
Strength
(Gpa)
(GPa)
MUMPs21
tensile
MUMPs19
132 -
Stress Concentrations
Size Effect
Size Effect-doped
Microcantilever
174±20
2.8±0.5
bending
AFM deflection
173±10
2.6±0.4
Bulk
indentation
Doped-Undoped
95-175
4μm
10μm
From Prorok, et al. Vol 5, 2004.

8. Integrated in Chip
Yong’s Chip
Chris’s Quarter

9. Testing Setup
Difficult to land the cantilever on the test specimen without repeatedly breaking tips

10. Atomic Force Microscopy (AFM)
Benefits
Very fine resolution (~0.02nm)
Variable scanning size/rate
Risks
Noise sensitivity, drift
Constraints on sample size/mass
From Espinosa, “Introduction to AFM and DPN”, ME395 Winter 2004

11. AFM Resolution Resolution
Dimension 3100: 512x512 pix
JEOL: 256x256 pix
Digital Image Correlation (DIC) can interpret change in position of a pixel by change in grayscale pixel strength (from 1=black to 255=white).
Ideally, DIC can understand a resolution of 1/255th of a pixel

12. AFM Resolution Scan size (say, 1.0 μm) corresponding to 512 pixels
DIC resolution: 1/255th of a pixel
Theoretical (maximum) displacement resolution (a)
If length of sample is 10m, theoretical resolution of strain is (b)
(a)
(b)

13. Polysilicon pad imaged using D3100 AFM
AFM Imaging Types
Amplitude
Topography
Polysilicon pad imaged using D3100 AFM

14. Polysilicon pad imaged using JEOL AFM
AFM Imaging Types
Amplitude
Topography
Polysilicon pad imaged using JEOL AFM

15. AFM Problems Adjustment of voltage, frequency for tip
Sensitivity to dust/scratches
Breakage of tip
Crash of tip into sample
Sensitivity of feedback control system

16. Digital Image Correlation
No automatic compensation for drift
Correlation on stationary samples
Drift strain is only strain
Used to generate correction factor

17. Digital Image Correlation (DIC)
Example Illustrates DIC Capabilities
Sample Prior to Deformation
Error
X-X Tensile Strain
Y-Y Tensile Strain
Shear Strain

18. DIC Problems Out-of-plane Deformation Displacement Gradients
Scanning Noise

19. DIC Resolution
Displacement resolution of DIC is 1/255pix, much smaller than the pix minimum displacement in pixels for the polysilicon at failure.
AFM/DIC should be able to display 0.512/(1/255), or 130 images for the range of displacement to failure based on ideal resolution/largest scan area.
*Note: actual resolution smaller than ideal, fewer images result.
Smaller scan areas will provide more images, and thus finer understanding of the change in strain over time.

20. Digital Image Correlation (DIC)
Sample DIC images from 10x10μm sample using D3100 AFM

21. Digital Image Correlation (DIC)
Sample DIC images from 10x10μm sample using JEOL 5200 AFM

22. The EPIC AFM, scanned in amplitude mode, yields the best results
AFM Comparison
The EPIC AFM, scanned in amplitude mode, yields the best results

23. Experimental Design Optimal Designs Computer generated
Best with respect to a particular criterion
D-Optimal minimizes l(X’X)-1l
Places points at regions of greatest standard error

24. Experimental Design Robust Design
All relevant information obtained using less than half the required experiments in a full factorial study
H0: μ1 = μ2
H1: μ1 ≠ μ2
α = P(type I error) = P(reject H0 l H0 true)
β = P(type II error) = P(fail to reject H0 l H0 is false)
Power = 1 - β

25. Experimental Design Standard Order #19 Removed (Above 3.5)
Outliers distort the analysis of variance
Outlier when residuals +/- 3σ

26. Significant Factors: A, B, AB
Experimental Results
Significant Factors: A, B, AB
R-Squared: .776

27. Experimental Results

28. In Conclusion… Optimal testing conditions for drift reduction:
JEOL 5200 AFM (Forced Feedback)
Amplitude Scan
Tapping Mode
High Scan Area
Sample choice and scan rate effect drift rate very little

29. In Conclusion…
Optimized drift condition remains too large to distinguish polysilicon strains from error
1% Strain at failure for polysilicon – minimized drift remains 100% of experimental strain
With current minimized strain values, brittle samples cannot be assessed

30. Future Work Drift Effects Reduction Opportunities
Appraiser Variation in DIC Analysis
Environmental Effects
Electromechanical Noise
Temperature, Pressure, Humidity Variation
Feedback Mechanism - Consider Image Repeatability
Sample Ductility

31. Thank You For Your Time Questions? In-Situ AFM Tensile Testing Group
Chris Biedrzycki, Matthieu Chardon, Travis Harper, Richard Neal