Al-Ge eutectic bonding for wafer-level vacuum packaging of MEMS devices

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Bekir_Gurel_Dimez_Thesis.pdf

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2022-8

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Dimez, Bekir Gürel

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Packaging is one of the most critical processes regarding the commercialization of MEMS devices. Wafer-level packaging is both yield and cost-efficient compared to die-level packaging. Various MEMS-based devices require vacuum encapsulation, such as microbolometers and resonators. Among other bonding methods and alloy systems used for wafer-level packaging, the Al-Ge system is advantageous since it can bond wafers by coating all the metallic layers on a single wafer. This eliminates the need for additional process steps on the device wafer. On the other hand, the limitation of the system is that the eutectic melting point (425 ℃) is close to the CMOS process temperature limit (450 ℃), which restricts the use of Al-Ge in critical CMOS-based applications. In this regard, the applicability of Al-Ge eutectic alloys for vacuum encapsulation of microbolometers through wafer-level packaging has been studied in this thesis study. Throughout this study, the suitability of thermally evaporated and co-sputtered Al-Ge alloys has been examined in detail. Bonding performance after both methods appeared poorly. Results of annealing experiments indicated that the poor performance is due to sluggish diffusion between Al and Ge. Al-Ge layer stack has been coated on a single wafer using thermal evaporation and sputtering. The microstructural evolution of the layer stack has been examined in detail after annealing and bonding experiments to devise an optimum bonding procedure with the minimum eutectic bonding temperature. A eutectic bonding procedure with a solid-state bonding step at 400 ℃ has been compared with a traditional eutectic bonding procedure, the former of which has performed better. The vacuum level inside the packages has been characterized using cap deflection test. Vacuum packaging has been achieved using both thermal evaporation and sputtering. In pursuing the lowest possible eutectic bonding temperatures, a successful vacuum sealing with 27 MPa average shear strength was performed at 435 ℃, and 51 MPa average shear strength was obtained at 440 ℃.

Subject Keywords

Wafer-Level Packaging, Al-Ge alloy, Eutectic Bonding, MEMS Packaging, Vacuum Packaging

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https://hdl.handle.net/11511/98666

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Graduate School of Natural and Applied Sciences, Thesis

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Citation Formats

B. G. Dimez, “Al-Ge eutectic bonding for wafer-level vacuum packaging of MEMS devices,” M.S. - Master of Science, Middle East Technical University, 2022.