Exploring the Threshold of Bondable Area Using SiNx-SiNx Direct Bonding

Direct bonding is an important tool for fabricating multilayer MEMS devices. Unlike silicon silicon or silicon-oxide bondings, SiNx-SiNx is quite under-reported [1, 2], especially for thicker silicon nitride layers (i.e. > 300 nm) [3]. However, there are multiple applications that can benefit from this bonding technology, from optoelectronics to high frequency devices [4], and most especially, microfluidic devices [5]. This application constitutes the primary motivation for the present work. Indeed, the ultimate goal is to fabricate hollow microfluidic resonators, which can then be used to measure the physical properties of a single particle, such as its mass or density. The quality of the SiNx-SiNx bonding directly affects the strength, leakage resistance, and long-term reliability of the final structure. One of the main challenges, and the goal of this project, is identifying the smallest area that remains bonded even after the surrounding material is etched away. However, standard bonding strength measuring methods such as the razor-blade test, wafer dicing, and SEM inspection, cannot be easily downscaled to the needed range for this application. Therefore, the project focuses on fabricating multiple bonded wafers samples, and studying the effects of different bonding parameters, such as force, annealing temperature, and annealing time, by using patterned test structures with different dimensions in the microscale. It is important to note that this is a purely qualitative study of the bonding strength.