Date

2015-09-02

Author

Yılmaz, Serkan
Temel, Oguzhan
Akın, Tayfun
Kalay, Yunus Eren

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Wafer level hermetic encapsulation of MEMS is crucial from both commercial and scientific point on view. Demands for the MEMS packaging can be listed as packaging with reasonable cost, low temperature and hermeticity, compensation of the wafer topology before packaging, easy and appropriate application [1]. There is still an excessive effort to create effective wafer level packaging method for MEMS devices. Each fabrication of MEMS devices requires special bonding path. Accordingly, there are different packaging techniques including fusion, anodic, polymer, thermo-compression, eutectic, and TLP (transient liquid phase) bonding. Each packaging method needs special parameters and shows different bonding characteristic. In terms of hermetic encapsulation, metallic bonding is still the best alternative. There are mainly two metallic bonding; eutectic and TLP (transient liquid phase bonding). In eutectic bonding, generally solution of two materials with high melting temperature melts at lower temperatures at special composition which is the eutectic composition. Whenever this temperature is reached after bonding, remelting will start again. In TLP bonding, bonding material has higher remelt temperature after bonding. Therefore, device can outstand high temperature during service or any other process like getter activation and bonding quality would not break down. TLP is the advanced type of soldering by the diffusion mechanism in liquid phase. To perform TLP bonding, one metal should have low melting point and the other metal should have high melting point. There are many systems such as Cu-Sn, Au-Sn, Au-In available for TLP bonding. In this study, Au-Sn system is chosen due to its high oxidation resistance, excellent mechanical property, and fluxless bonding capability. It is known that interdiffusion of Au and Sn at solid state is rapid. Therefore, obtaining pure Sn until reaching the above of the melting temperature of the Sn is almost impossible for low thickness values such as lower than 1 μm. Even if, electroplating method is commonly used usually with relatively high thickness bondings (around 10 μm), we used thermal evaporation process due to its unique advantages over electroplating such as the ability of creating uniform coating with non-electrical and dry process. By applying thermal evaporation process the aim is to obtain eutectic composition at the one pair of the Si wafers by applying proper heat treatment. The other pair of the Si wafers, contains pure gold which enables to get finally Au rich phase which has high melting temperature. Scanning transmission electron microscopy is used to analyze the interdiffusion area before and after the bonding process. The details Au-Sn eutectic formation by various heat-treatment processes will be presented and discussed in details.

Subject Keywords

MEMS, Electronic packaging, Electron microscopy

URI

https://hdl.handle.net/11511/76109

Collections

Department of Metallurgical and Materials Engineering, Conference / Seminar