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 – Objectives of the project 2 – Literature review 3 – Single-Edge V-Notch Beam (SEVNB) tests 3.1 – SEVNB specimens 3.2 – Experiments 3.3 – Results 3.4 – Discussion 4 – Channel geometry beam specimens under bending stress 4.1 – Specimens 4.2 – Experiments 4.3 – Results 4.4 – Discussion 5 – Conclusion 2

3. Context Aim : reduce material budget of SCSi micro-devices Minimize of thickness of SCSi membranes Insure sufficient resistance to internal pressures => Need of fracture prediction tool to optimize structures 3

4. 1 – Objectives Main goal : provide fracture data for the implementation of a fracture prediction tool Requirements : simple situation to allow CERN’s Engineering Office to make reliable models 4

5. 1 – Objectives 2 testing rounds : Fracture toughness test => to set parameters of FEA model Test with slightly more complex geometry to validate those parameters 5

6. 2 – Literature review 6

7. Fracture Toughness Definition : Fracture toughness (K c ) is the ability of a material to resist against propagation of a pre-existing crack. Fracture toughness = critical stress intensity Unit : Pa√m High fracture toughness : Ductile fracture Low fracture toughness : Brittle fracture 7

8. Fracture modes 3 different fracture modes : tensile (mode I), -typically the weakest- shear (mode II), tear (mode III) 8

9. 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 9

10. 3 – Single-Edge V-Notched Beam (SEVNB) in three point bending tests 10

11. 3.1 – SEVNB Specimens Easy to manufacture Existing standards (ASTM C1421-10 for ex.) Possible to perform FEA Easy to test No tensile stress (except at crack tip) 11 Reasons of the choice

12. 3.1 – SEVNB Specimens 12 1 st step : Alignment to crystalline structure 2 nd step : Photolithography 3 rd step : Notch etching (KOH) 4 th step : Dicing Fabrication process

13. 3.1 – SEVNB Specimens 13 Mask for SEVNB specimens fabrication Fabrication process

14. 3.1 – SEVNB Specimens 14 Obtained specimens Obtained notches : Picture of full specimen ? Mask ?

15. 3.1 – SEVNB Specimens 15 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 :

16. 3.1 – SEVNB Specimens 16 Analytical formula for fracture toughness FEA Comparison between Sharp and V-Shaped crack

17. 3.1 – SEVNB Specimens 17 Analytical formula for fracture toughness Comparison between various analytical formulas for fracture toughness

18. 3.2 – SEVNB Experiments 18 Experimental setup – Overview 1 actuator 1 load cell Mechanical support to link & align parts Removable chuck to adapt to every test Transportable & adaptable system Taken from “In-situ MEMS Testing”, A. Schifferle, A. Dommann, A. Neels and E. Mazza

19. 3.2 – SEVNB Experiments 19 Experimental setup – Chucks Vertical pins (1mm spacing) Available chucks : S 0 =5mm S 0 =10mm

20. 3.2 – SEVNB Experiments 20 Compliance calibration Extremely stiff sample (1mm diameter steel rod) in place of specimen

21. 3.2 – SEVNB Experiments 21 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,

22. 3.3 – SEVNB Results 22 Compliance Analytical compliance : 1.55*10 -6 m.N -1 Mean experimental compliance : 1.27*10 -6 ± 0.39*10 -6 m.N -1

23. 3.3 – SEVNB Results 23 Fracture Load Fracture toughness from literature (for (110) plane)* : 1.23±0.18 MPa.√m -1 Experimental fracture toughness : 1.50±0.09 MPa.√m -1 *T. Ando, X. Li, S. Nakao, T. Kasai, M. Shikida and K. Sato, “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)

24. 3.3 – SEVNB Results 24 Scatter in results No correlation between fracture load and compliance Higher scatter for compliance than fracture toughness

25. 3.4 – Sources of scatter 25 Sharpness of notch Notch sharpness : 22.89nm radius for this measurement

26. 3.4 – Sources of scatter 26 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

27. 3.4 – Sources of scatter 27 Misalignment of sample 500um misalignment => 5.8% compliance variation

28. 3.4 – Sources of scatter 28 Thickness of sample (B) 10um variation of B => 1.1% variation of compliance B fixed by dicing => low variability expected Actual measurements of B with standard deviation !!

29. 3.4 – Sources of scatter 29 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)

30. 3.4 – Sources of scatter 30 Compliance of tool too high Tool compliance higher than sample compliance… Not ideal ! But in theory possible to correct

31. 3.4 – Sources of scatter 31 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

32. 4 – Channel geometry beam under bending stress 32

33. 4.1 – Introduction & goals 33 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

34. 4.2 – Specimens 34 1 st step : Alignment to crystalline structure 2 nd step : Photolithography 3 rd step : Notch etching (KOH or DRIE) 4 th step : Dicing Fabrication process

35. 4.2 – Specimens 35 Obtained specimens 4 types of specimen : DRIE90° KOH DRIE80° DRIE60°

36. 4.2 – Specimens 36 Dimensions of specimens Optical measurements performed on 5 samples for each type

37. 4.3 – Experiments 37 Experimental process 3 point bending : Chuck just under channel Precise alignment

38. 4.3 – Experiments 38 Experimental process  Tests both in three– and four point bending

39. 4.4 – Results 39 3 point bending DRIE 60° DRIE80° DRIE 90° KOH

40. 4.4 – Results 40 4 point bending DRIE 60° DRIE80° DRIE 90° KOH

41. 4.4 – Results 41 Fracture loads

42. 4.4 – Results 42 Compliance Much higher mismatch for 3 point bending than for 4 !

43. 4.4 – Results 43 Compliance – Summary  Much higher mismatch for 3 point bending than for 4 !

44. 4.4 – Parametric analysis 44 FEA estimation of influence of notch depth on compliance Estimation of measurement errors through FEA model

45. 4.4 – Parametric analysis 45  Measurement errors have more impact for 3 point bending tests than for 4 point ones Estimation of measurement errors through FEA model

46. 5 – Conclusion Gathering of fracture data with SEVNB specimens to allow setting parameters of fracture prediction tool Validation of parameters through second testing batch Essays on tensile-test machine for micro-scale specimens Observation of sensitive aspects in fracture tests (scatter sources) Observation of impact of manufacturing methods on fracture strength 46

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48. Backup slides 48

49. 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) 49

50. 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) 50

51. 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 51

52. B – Double Cantilever Beam Theoretically possible to measure propagation values Plastic deformation in arms Direction of crack growth Tensile load applied 52

53. 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 53

54. 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) 54

55. 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 55

56. F – Other On chip tensile test device : Complicated analysis Sharpness of notch Complicated fabrication process Easy to apply the load 56

57. 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 57

58. 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 58

59. Support of the beams Beam should be free to rotate around the rollers Diameter : 0.5 – 1 mm Supports considered : Razor blades Steel wires 59

60. 3 points bending manufactured wafer 60

61. 3.2 – Compression Loaded Double Cantilever Beam 61

62. 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) 62

63. Dimensions of specimens b defined by wafer thickness Other dimensions set to keep ratios of previous experiments with this specimen 63

64. 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 64

65. 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) 65

66. Tools available 66