Figure 1. Selected bonding technologies overview, minor variants resulting from different surface pre-treatments is not included ... 21 Figure 2. Bimorph system of two bonded materials, E = Young’s Modulus, α = linear thermal expansion coefficient, 2l = lengths, b = thickness ... 22 Figure 3. Bimorph system with temperature change, y(l) = deformation ... 22 Figure 4. Setup used for electroplating of gallium ... 28 Figure 5. Current density vs. electro potential of the selected chloride-based gallium electrolyte at room temperature, red arrows indicate the time sequence of measurement.
... 29 Figure 6. Gallium electroplated forming small droplets due to surface tension showing its liquid structure ... 30 Figure 7. SEM cross section of sample after gallium electroplating showing severe
underplating (right side if picture) of OMR83 resist due to the non-optimised condition of resist coating. ... 34 Figure 8. Gallium structure (50µm wide bond frame with electrical contacts) electroplated inside negative resist without underplating after resist deposition parameter optimisation.
... 35 Figure 9. SEM top view of structure after gallium electroplating and resist stripping at room temperature without ultrasonic support. The resist is only partly removed. ... 36 Figure 10. SEM top view of structure after gallium electroplating and resist stripping at room temperature with ultrasonic support. The resist is entirely removed. ... 36 Figure 11. Optical microscope top view of gallium structure after successful seed layer removal. ... 38 Figure 12. Optical microscope top view of removed gallium structure with remaining chromium seed layer after unsuccessful etching trial of seed layer due to too bad
selectivity. ... 38
Figure 13. Mounting of shear test sample at 45° ... 40 Figure 14. Example for displacement/force measurement results of shear test of a bonded sample ... 41 Figure 15. Schematic cross section of a typical 3D integrated MEMS structure showing the need for vertical interconnect. ... 41 Figure 16. Schematic image of the test structure to observe the vertical resistance of bonded contacts ... 42 Figure 17. Schematic figure of 4-wire resistance measurement (Keithley Instruments, Inc., 2002) ... 43 Figure 18. Typical design scheme of a resonant MEMS structure used for hermeticity testing ... 46 Figure 19: Binary alloy phase diagram of Au-Ga (Massalski, et al., 2006) ... 54 Figure 20. Blue gold layer of AuGa2 on fine gold after dip-coating for jewellery (Klotz, 2010) ... 54 Figure 21. Interdiffusion coefficient D of different gold-based alloy formation (Marinković, et al., 1988) ... 56 Figure 22. Setup and layer thicknesses for Au-Ga bonding experiment ... 57 Figure 23. Cross section of gallium-gold bond interface achieved by breaking with visible alloy and partly remaining gold layer ... 58 Figure 24. Cross section of gallium-gold bond interface achieved by focused ion beam (FIB) cutting to make several distinct different (A, B, C and D) sections visible ... 58 Figure 25. Top view of gallium-gold interface after separation by shear force (section D) . 58 Figure 26. Clearly visible grain structure of cross section of gallium-gold interface (section D) ... 58 Figure 27. EDX spectrum at section A: 100wt.% Au ... 59 Figure 28. EDX spectrum at section B: 14wt.% Ga ... 59
Figure 29. EDX spectrum at section C: 25.2wt.% Ga ... 59 Figure 30. EDX spectrum at section D: 40wt.% Ga ... 59 Figure 31. Layer setup for Au-Ga bonding experiment to investigate interface composition and stability ... 60 Figure 32. SEM picture of cross section of Au/Ga bond at 50°C without annealing ... 61 Figure 33. SEM picture of cross section of Au/Ga bond at 50°C without annealing, alloy composition highlighted by different colour ... 61 Figure 34. SEM picture of cross section of Au/Ga bond at 50°C and annealing at 90°C for 80h ... 61 Figure 35. SEM picture of cross section of Au/Ga bond at 50°C and annealing at 90°C for 80h, alloy composition highlighted by different colour ... 61 Figure 36. SEM picture of cross section of Au/Ga bond at 50°C and annealing at 145°C for 80h ... 62 Figure 37. SEM picture of cross section of Au/Ga bond at 50°C and annealing at 1450°C for 80h, alloy composition ... 62 Figure 38. EDX line scan of the interface of Au/Ga bonded sample after 145°C annealing showing the complete homogenisation of the interface composition. ... 63 Figure 39. Results of shear strength measurement of bonded Au/Ga samples in
dependence of annealing temperature ... 64 Figure 40. Contact resistivity of vertical bonded contacts between two wafers with gold and gallium after annealing at different temperatures ... 65 Figure 41. Schematic cross section of test structure for hermeticity testing; cavity wafer with structured gallium on gold/titanium seed layer, and wafer with unstructured gold and adhesion layer chromium. ... 66 Figure 42. Mask design for the hermeticity bond test, bond frame layer. The outer large frame is only for rough mask alignment. The lower left corner structure is for electrical contact during electroplating. Bond frame with is 200µm. ... 66
Figure 43a-i. Schematic process sequence for fabrication of test structure for hermetic
testing of bonds ... 68
Figure 44. Cavity test structure directly after Ga/Au bonding (7MPa mechanical pressure and 2mm bond frame) ... 69
Figure 45. Membrane bending directly after bonding for test structure depicted in Figure 44 ... 69
Figure 46. Display of electric and mechanic parameters of Au/Ga bonds in relation to annealing temperature. ... 71
Figure 47. Cu/Ga binary phase diagram (Massalski, 1986) ... 72
Figure 48. Layer setup for bonding experiments with copper-gallium ... 73
Figure 49. SEM picture of cross section of Cu/Ga bond at 25°C without annealing ... 74
Figure 50. SEM picture of cross section of Cu/Ga bond at 25°C without annealing, alloy composition highlighted by different colour ... 74
Figure 51. SEM picture of cross section of Cu/Ga bond at 25°C and annealing at 50°C for 80h ... 75
Figure 52. SEM picture of cross section of Cu/Ga bond at 25°C and annealing at 50°C for 80h, alloy composition highlighted by different colour ... 75
Figure 53. SEM picture of cross section of Cu/Ga bond at 25°C and annealing at 90°C for 80h ... 75
Figure 54. SEM picture of cross section of Cu/Ga bond at 25°C and annealing at 90°C for 80h, alloy composition highlighted by different colour ... 75
Figure 55. SEM picture of cross section of Cu/Ga bond at 25°C and annealing at 145°C for 80h composition highlighted by different colour ... 76
Figure 56. SEM picture of cross section of Cu/Ga bond at 25°C and annealing at 145°C for 80h, alloy composition highlighted by different colour ... 76
Figure 57. SEM picture of cross section of Cu/Ga bond at 25°C and annealing at 200°C for 80h composition ... 77
Figure 58. SEM picture of cross section of Cu/Ga bond at 25°C and annealing at 200°C for 80h, alloy composition highlighted by different colour ... 77 Figure 59. Experimental setup to test the application of a gold cover layer to prevent the oxidation of copper ... 81 Figure 60. SEM picture of cross section of Cu/Ga bond at 25°C after annealing at 50°C with gold protective layer according to ... 81 Figure 61. SEM picture of cross section of Cu/Ga bond at 25°C after annealing at 50°C with gold protective layer according to ... 81 Figure 62. SEM picture of cross section of Cu/Ga bond at 25°C after annealing at 200°C with gold protective layer according to ... 82 Figure 63. SEM picture of cross section of Cu/Ga bond at 25°C after annealing at 200°C with gold protective layer according to Figure 59 (composition highlighted with colour and named) ... 82 Figure 64. Experimental setup to test the application of a gallium cover layer to prevent the oxidation of copper ... 83 Figure 65. SEM picture of cross section of Cu/Ga bond at 25°C with gallium cover on Cu and annealing at 50°C for 80h ... 84 Figure 66. SEM picture of cross section of Cu/Ga bond at 25°C with gallium cover on Cu and annealing at 50°C for 80h, alloy composition highlighted by different colour ... 84 Figure 67. Shear strength of Ga-Cu samples bonded at 25°C without Cu surface treatment and annealed at different temperatures ... 85 Figure 68. Shear strength of Ga-Cu samples bonded at 25°C with wet Cu surface treatment and annealed at different temperatures ... 86 Figure 69. Shear strength of Ga-Cu samples bonded at 25°C with Cu surface protected by electroplated gallium layer and annealed at different temperatures ... 87 Figure 70. Comparison of shear strength of Ga-Cu samples bonded at 25°C without Cu surface pre-treatment [Cu-Ga], with Cu surface pre-treatment [Cu(V)-Ga] and Cu protected by electroplated gallium layer [Cu(Ga)-Ga], annealed at different temperatures.
Furthermore, results from gold-gallium bonding [Au-Ga] are also included for reference. 88
Figure 71. Resistivity of bonded Cu/Ga interface with wet chemical pre-treatment
observed over time after bonding at 25°C ... 89 Figure 72. Resistivity of bonded Cu/Ga interface with wet chemical pre-treatment
observed over time after bonding at 25°C and annealed at different temperatures ... 90 Figure 73. Resistivity of bonded Cu/Ga interface with gallium cover on copper observed over time after bonding at 25°C ... 90 Figure 74. Resistivity of bonded Cu/Ga interface with gallium cover on copper observed over time after bonding at 25°C and annealed at different temperatures. ... 91 Figure 75. Membrane deflection of hermeticity test structure for gallium/copper bonds measured at different times after bonding. It shows the slow increase of pressure inside of the cavity due to diffusion of air. ... 92 Figure 76. Overview of mechanical and electrical parameters of Cu/Ga bonded interface depending on annealing temperature. ... 93 Figure 77. Diffusion streams DA and DB at the interface between elements A and B ... 95 Figure 78. Interfaces and diffusion on the example of Au/Ga ... 96 Figure 79. SEM cross section of bonded copper-gallium pair; dashed line shows interface with inhibited diffusion ... 97 Figure 80. SEM cross section of same sample and position like Figure 79, taken 88h later (no annealing, room temperature!);visible increase of CuGa2 phase and void formation in the gallium layer ... 97