C. Gabry A. Mapelli , P. Renaud, G. Romagnoli , D. Alvarez

1. Investigation of the structural resistance of Silicon membranes for microfluidic applications in High Energy Physics C. Gabry A. Mapelli, P. Renaud, G. Romagnoli, D. Alvarez

2. Summary • 1 – Objectives of the project • 2 – Simple test geometry • 3 – Study of tool behavior • 4 – Next steps for the project

3. Context • 1st project at CERN studying Single Crystal Silicon as a structural material • Lots of data on fracture of micro-channels, but no reliable simulation

4. 1 – Objectives • Main goal : Perform simple test to calibrate FE simulation parameters • Selection of simple test method, fabrication & test of specimens • Design, fabrication and test of more complex specimens to validate the simulation parameters • Provide a basis on which upcoming student can start (FEA performed by CERN’s engineering office)

5. 2 – Simple test geometry

6. Possible test methods – fracture toughness characterization • A – Micro-indentation • B – Double Cantilever Beam • C – Compact Tests • D – Compression Loaded Double Cantilever Beam • E – Three/four point bending • F – Other (cantilever bending, On-chip tensile test device)

7. Chosen method • 3-point bending test have been chosen as first specimen : • Easy to manufacture • Existing standards (ASTM C1421-10) • Possible to perform FEA • Easy to test • No tensile stress (except at crack tip)

8. Fabrication process • 1st step : Alignment to crystalline structure • 2nd step : Notch etching (KOH) • 3rd step : Dicing

9. Testing setup (CSEM) • 1 actuator • 1 load cell • Mechanical support to link & align parts • Removable chuck to adapt to every test Taken from “In-situ MEMS Testing”, A. Schifferle, A. Dommann, A. Neels and E. Mazza Transportable & adaptable system

10. Results • Big error between theory (black) and experimental results • Possible sources of error : • A – Geometry of notch • B – Misalignment of tool • C – Misalignment of sample • D – Geometry of sample • E – Defects density due to dicing • F – Bad calibration / compliance of tool Fig 1 : Load-displacement curve of a B=1mm, So=5mm sample, frontside test

11. A – Geometry of notch • Sharpness • Radius of notch tip ≈ 23nm=> very sharp tip • The tip is sharp enough so that we can be sure the problem doesn’t come from this. Fig 2 : SEM picture of notch before testing

12. A – Geometry of notch • Depth • Depth of initial notch : 144um (planned : 140) • The difference between the measured depth of this sample (144um) and the value used to calculate the theoretical curve (140um) is too low to induce such a big error as the one observed Fig 3 : SEM picture of broken sample

13. B – Misalignment of tool • If the tool is misaligned, the load will be higher for a given deformation • This is not coherent with our error : the load we have is too low for the deformation we measure. Fig 1 : Load-displacement curve of a B=1mm, So=5mm sample, frontside test

14. C – Misalignment of sample • If the sample is misaligned, the load will be higher for a given deformation • This is not coherent with our error : the load we have is too low for the deformation we measure. Fig 1 : Load-displacement curve of a B=1mm, So=5mm sample, frontside test

15. D – Geometry of sample • Samples were measured with an optical micrometer • We have 3.4% of error for the thickness of the specimen • We have 1.7% of error for the width of the specimen Table 1 : Planned dimensions VS actual dimensions

16. D – Geometry of sample Table 1 : Planned dimensions VS actual dimensions Fig 4 : Load-displacement curves of samples with theoretical and actual dimensions Conclusion : Geometry of sample is not the main source of error

17. E – Defects density due to dicing • The depths at which defects are induced by dicing is small compared to the full width of the sample • It is unlikely that such a big error could be induced by defects due to dicing • A high defect density should increase the compliance of the sample, which is the opposite of what we have in the measurements Fig 3 : SEM picture of broken sample

18. F – Bad calibration • Calibration made without stiff sample for 1st testing round=> Compliance of upper part of chucks not taken into account=> error induced due to calibration Fig 5 : Chucks used for three points bending tests

19. F – Bad calibration • When calibration made with stiff sample between the chucks : Fig 6 : Load-displacement curves of un-notched samples with correct calibration

20. 3– Study of tool behavior

21. Frontside & Backside testing Fig 7 : Frontside and backside tests Backside tests are performed when sample is wider than 1mm and don’t fit between the horizontal pins

22. Frontside 3 points bending • With correct calibration : Fig 8 : Load-displacement curves of un-notched samples (left) and validation geometry (right) under 3 points bending loading, frontside testing

23. Frontside 3 points bending • Frontside testing problems : • Steps phenomena => 0.9mm wide specimens • Accurate geometry ? => characterization before testing • Misalignment effect ?=> check with FE

24. Backside 3 points bending • With correct calibration : Fig 9 : Load-displacement curves of un-notched samples (left) and validation geometry (right) under 3 points bending loading, backside testing

25. Backside 3 points bending • At a certain load, contact conditions change at the middle chuck : • Due to the rectangular shape of the backside of the chuck • The whole surface touches until ≈4N • Over 5N, only two point touch the specimen => Not reliable method for testing

26. Frontside 4 points bending • With correct calibration : Fig 10 : Load-displacement curves of un-notched samples (left) and validation geometry (right) under 4 points bending loading, frontside testing

27. Frontside 4 points bending • Steps phenomena • Doesn’t fit simulation at all… Suspicious ! • Un-explained behavior : something wrong occurred during the test ?

28. 4– Next steps for the project

29. Outline of the upcoming weeks • Testing of “Fracture toughness” specimens in 3 points bending with correct calibration • Testing of validation geometries with 3 & 4 points bending : • KOH, α= 90° • DRIE, α= 90° • DRIE, α= 90° + oxidation • DRIE, α= 80° • DRIE, α= 60° • Validation geometries will allow to validate the simulation parameters and to observe the influence of sidewall angle on overall strength of sample

30. Program Submission of 1st version of report

32. Backup slides

33. Problems of Frontside testing • When sample are wider than 1mm (B>1mm), they don’t fit between the horizontal pins • Wider samples allow a higher fracture load • A higher fracture load can help overcome accuracy problems (if load cell is not precise enough to get an accurate measurement of the fracture load) Fig 5 : Chucks used for three points bending tests

34. Problems of Backside testing • With backside tests, there is two points of contact instead of one at the middle chuck • As there is always misalignments in the system, only one of the corners of the middle chuck is in contact in the beginning Fig 7 : Frontside and backside tests

35. Problems of Backside testing • Once a high enough load is reached, the two points of the middle chuck come in contact with the specimen, modifying the overall compliance of the system Fig 7 : Frontside and backside tests

36. Conclusion • If possible (no precision problems), frontside testing is more reliable • If the load cell is not precise enough, tests can still be performed on the backside, provided a critical load is not reached(load at which two points come in contact instead of one) • For our tests, the load cell is precise enough so that we can test the specimens on the frontside • => Only front side testing !