Assessment for Identification of An Optimal Wafer Bonding Process for Industrial Production of Highly Integrated MEMS Applications

Abstract — Wafer bonding material systems are an important aspect of hermetically sealing the functional cavities of MEMS devices. Typically, wafer bonding processes are set up  as an integral part of the development of MEMS device  manufacturing technology. This results in an optimal bonding process for a specific application, but requires a lot of effort. In fact, this is one of the reasons why a high number of wafer bonding processes are now available and in use. However, for economic and flexibility reasons, wafer foundries are more interested in well-defined bonding processes. Therefore, in this study, known wafer bonding processes are briefly described and evaluated for use in at least two different MEMS foundry processes, considering a variety of requirements for the applications and the production processes, mainly, but not only, lowest possible bond frame width, vacuum sealing behavior, bonding temperature, material system and their CMOS compatibility along with their process complexity. Under these conditions, Au-Si eutectic bonding was found to be the most suitable material system, with other material system such as AlGe eutectic or Ti-Ti direct bonding as an alternative, which require more development and production effort, while other material systems are found not suitable for high volume manufacturing in foundry applications. Finally, the preferred Au-Si bonding system is discussed in more detail.