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

2. Summary 1 – Context & objectives 2 – Literature review 3 – Experimental setup from CSEM 4 – Single-Edge V-Notch Beam (SEVNB) tests 5 – Channel geometry beam specimens under bending stress 6 – Conclusion 2

3. 1 – Context Aim : reduce material budget Minimize of thickness of membranes Insure sufficient resistance to internal pressures Main goal : provide fracture data for the implementation of a fracture prediction tool Requirements : simple specimen & test process to enable the implementation of reliable models 3 1 – Context & Objectives

4. 1 – Objectives 2 testing rounds : Fracture toughness test => to set parameters of FEA model Test with slightly more complex geometry to validate those parameters 4 1 – Context & Objectives

5. 2 – Fracture Toughness Fracture toughness = critical stress intensity factor 5 Conservative assumptions  Test weakest mode & crystalline plane 2 – Literature review

6. 2 – Fracture toughness test To perform a fracture toughness determination test, one need : Sample with pre-existing crack of known geometry Tool able to apply and measure load & displacement The load at fracture to allow for fracture toughness calculation 6 2 – Literature review

7. 2 – Fracture toughness characterization methods 7 DCB CT 3-/4-PB CL-DCB Indentation 2 – Literature review Simple manufacturing process No need of complex machinery  Possible to perform FEA

8. 8 Tools available 3 – Experimental setup

9. 9 Tool : Custom test setup from CSEM 1 actuator 1 load cell Switchable chucks  Possible to perform any tensile/compressive test 3 – Experimental setup

10. 4 – SEVNB Specimens 10 General presentation 4 – 1 st test batch Top view Cross section

11. 4 – SEVNB Specimens 11 Mask 4 – 1 st test batch

12. 4 – SEVNB Specimens 12 Dimensions of specimens Standard sizes are too large (e.g. W=3mm) ASTM standards gives two main ratios to respect : W/B = 0.75 0.35 < a/W < 0.6 Fabricated specimens : 4 – 1 st test batch

13. 4 – SEVNB Results 13 *T. Ando, et al. “Effect of crystal orientation on fracture strength and fracture toughness of single crystal silicon”, Micro Electro Mechanical Systems, 17 th IEEE International Conference on MEMS, pp. 177-180 (2004) 4 – 1 st test batch K Ic(110) [MPa.√m -1 ] C [µm.N -1 ] Theory1.23 ± 0.18 *1.55 Experiments1.50 ± 0.091.27 ± 0.39

14. 4 – SEVNB Results 14 Scatter in results No correlation between fracture load and compliance Higher scatter for compliance than fracture toughness 4 – 1 st test batch

15. 4 – SEVNB Results 15 Sources of scatter Notch sharpness Notch depth Geometrical imprecisions Thickness / width of specimen Misalignment of sample Misalignment inside tool Experimental imprecisions Compliance of tool too high 4 – 1 st test batch

16. 4 – SEVNB Conclusions 16 Data gathered acceptable to set FEA parameters Specimens sent in Genova for testing (no results yet) How to reduce the scatter : Accurate measurements of sample’s geometry before testing Auto-alignment system implemented on the tool  Design & fabrication of custom testing tool at CERN 4 – 1 st test batch

17. 5 – 2 nd test session 17 Sample with channel-like notch Goals : 1 – validate simulation parameters established during SEVNB tests 2 – study influence of manufacturing method on overall strength of sample 5 – 2 nd test batch

18. 5 – Specimens 18 Obtained specimens 4 types of specimen : DRIE90° KOH DRIE80° DRIE60° 5 – 2 nd test batch

19. 5 – Experiments 19 Experimental process 3 point bending : Chuck just under channel Precise alignment 5 – 2 nd test batch  Tests both in three– and four point bending

20. 5 – Results 20 Fracture loads 5 – 2 nd test batch

21. 5 – Results 21 Compliance 5 – 2 nd test batch

22. 6 – Conclusion Gathering of fracture data with SEVNB specimens to allow setting parameters of fracture prediction tool Validation of parameters through second testing batch Observation of sensitive aspects in fracture tests (scatter sources) Observation of impact of manufacturing methods on fracture strength 22

23. 7 – Self-evaluation Too optimistic in the beginning (3 test batches) Not enough time on the report => possibly not as good as I hoped Data gathered & insights on how to go on with fracture tests => encouraging for the future 23

24. Acknowledgments Diego Alvarez for the FEA models, and his constant light and guidance throughout the project CMi staff for their advices and help for specimens manufacturing CSEM and Tobias Bandi for providing the testing setup their support during the experimental phase EO & Microsystems group at CERN You, for listening to my (long?) presentation ! 24

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26. Backup slides 26

27. Energy based approach Elastic energy released = surface energy created Energy release rate (G = πσ 2 a/E) : speed at which the energy is released by growth of the crack G Ic = K Ic 2 *(1-ν 2 )/E [hkl] (E : Young’s modulus, a : half crack length, σ : stress ; ν : Poisson’s coefficient ; [hkl] : Miller indexes) 27

28. Possible test methods 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) 28

29. A – Micro-indentation Easy to implement Possible to test several crystalline orientations Machinery quite common Measuring the crack length Residual stress after indentation Hard to simulate with FEA 29

30. B – Double Cantilever Beam Theoretically possible to measure propagation values Plastic deformation in arms Direction of crack growth Tensile load applied 30

31. C – Compact Tests Less plastic deformation in thick arms Short arms => crack growth direction more controllable Plastic deformation & parasitic crack growth at load pins Only initiation Tensile load applied 31

32. D – Compression-Loaded Double Cantilever Beam No tensile stress applied Side groove to help crack grow in the intended direction Stable crack growth Crack growth monitored with thin film resistance Wrong direction of crack growth Hard to test such specimens (need a frame to hold them) 32

33. E – Three/four point bending of notched specimen ASTM standard procedures available Test under bending conditions Easy modeling and manufacturing Standard not adapted to our needs (scale) No propagation measurements 33

34. F – Other On chip tensile test device : Complicated analysis Sharpness of notch Complicated fabrication process Easy to apply the load 34

35. Wet etching for vertical sidewalls 45° orientation of specimens relative to primary flat can lead to vertical sidewalls with KOH etch %wt of KOH and temperature have an influence on the obtained slope of the walls Ex : 60%wt & 60°C 35

36. Alignment to crystalline planes Anisotropic KOH etching step results in squares under the circle openings Squares which are the most alligned give the cristalline orientation 36

37. Support of the beams Beam should be free to rotate around the rollers Diameter : 0.5 – 1 mm Supports considered : Razor blades Steel wires 37

38. 3 points bending manufactured wafer 38

39. Reasons of the choice CL-DCB has been chosen as second specimen : Easy to manufacture Possible to perform FEA No tensile stress (except at crack tip) Eventually possible to measure both initiation & propagation values Stable crack growth Easy to monitor crack growth (metal gage) 39

40. Dimensions of specimens b defined by wafer thickness Other dimensions set to keep ratios of previous experiments with this specimen 40

41. Fabrication process 1 st step : Alignment to crystalline structure 2 nd step : DRIE with specimen shape 3 rd step : Smoothing (RCA cleaning, SiO 2 oxidation, etching of SiO 2 layer) 4 th step : Releasing of specimens by grinding of back side 41

42. Required machinery Possible to estimate fracture load P and displacement by assuming K Ic = 1Pa√m For B=1mm, W=525μm, L=10mm, a=300μm : F c = 450 mN y c = 23 μm =>Resolution required : ≈ 10mN & 1μm (Tensile test machine at CERN has 8mN and 1μm resolution) 42

43. 3 – Experimental setup 43 Compliance calibration Extremely stiff sample in place of specimen 1mm diameter steel rod 3 – Experimental setup

44. 44 Data analysis Initial slack correction by linear fit Tool compliance correction on measured deformation δ b : Deformation of the beam P : Load δ t (=P*C t ) : Deformation of the tool δ m : Measured deformationC t : Compliance of the tool, 3 – Experimental setup

45. 4 – SEVNB Specimens 45 Analytical formula for fracture toughness FEA Comparison between Sharp and V-Shaped crack 4 – 1 st test batch * G.V.Guinea et al. “Stress intensity factor, compliance and CMOD for a general three-point-bend beam”, International Journal of Fracture, Vol 89, pp.103-106 (1998) *

46. 4 – SEVNB Specimens 46 Analytical formula for fracture toughness Comparison between various analytical formulas for fracture toughness 4 – 1 st test batch * G.V.Guinea et al. “Stress intensity factor, compliance and CMOD for a general three-point-bend beam”, International Journal of Fracture, Vol 89, pp.103-106 (1998) ** ASTM C1421-10, “Standard Test Methods for Determination of Fracture Toughness of Advanced Ceramics at Ambient Temperature” (2010) * * **

47. 3.4 – Sources of scatter 47 Sharpness of notch Notch sharpness : 22.89nm radius for this measurement

48. 3.4 – Sources of scatter 48 Depth of notch (a) Notch depth conform to what was planned (143um for the measured sample, 2um bigger than expected) Make more measurements to see standard deviation

49. 3.4 – Sources of scatter 49 Misalignment of sample 500um misalignment => 5.8% compliance variation

50. 3.4 – Sources of scatter 50 Thickness of sample (B) 10um variation of B => 1.1% variation of compliance B fixed by dicing => low variability expected

51. 3.4 – Sources of scatter 51 Misalignment of tool Multiple types of possible misalignments (In plane, out of plane…) Not possible to estimate without changing boundary conditions of FEA model Source of global error ? (same misalignment for a batch)

52. 3.4 – Sources of scatter 52 Compliance of tool too high Tool compliance higher than sample compliance… Not ideal ! But in theory possible to correct

53. 3.4 – Sources of scatter 53 Conclusion Most sources did not have a big enough impact to explain all the scatter Misalignment inside the tool was not estimated, but is probably the main source of mismatch (not scatter) Brittle materials typically have a random behaviour

54. 4.2 – Specimens 54 Dimensions of specimens Optical measurements performed on 5 samples for each type

55. 5 – Results 55 3 point bending DRIE 60° DRIE80° DRIE 90° KOH 5 – 2 nd test batch

56. 5 – Results 56 4 point bending DRIE 60° DRIE80° DRIE 90° KOH 5 – 2 nd test batch

57. 5 – Parametric analysis 57 FEA estimation of influence of notch depth on compliance Estimation of measurement errors through FEA model 5 – 2 nd test batch

58. 5 – Parametric analysis 58  Measurement errors have more impact for 3 point bending tests than for 4 point ones Estimation of measurement errors through FEA model 5 – 2 nd test batch