Metal-to-metal bonding technology for MEMS application

Summary form only given. Accompanying of MEMS sensor using in consumer production widely, system in package (SiP) technology becomes popular to integrate CMOS device (such as ASIC or DSP) with MEMS sensor together. It is excellent to connect sensor to signal amplifier directly to deduce RC delay and board level noise. However, with CMOS technology evolution to 0.13um or 90nm, the thermal stability temperature of low k IMD material (such as black diamond or a-F:C) is around 400°C. That means for this CMOS/MEMS SiP technology, the first challenge is to develop low temperature process, which has to be compatible with the thermal budget of the CMOS devices. In this paper, the temperature target is set as 350 °C for this concern. Secondly, the next challenge is the manufacturing method for this CMOS/MEMS SiP technology. Three common manufacturing schemes are chip-to-chip (C2C), chip-to-wafer (C2W) and wafer-to-wafer (W2W), and the specifics of the product will determine which approach has to be taken. The related concerns are including wafer size, die size, alignment accuracy and individual yields issue. In this paper, C2W approach is employed for the constraints in different die size and know good die (KGD), especially in bonding yield rate below 98%. The third challenge is to provide interconnection and hermetic at same time. For certain MEMS device, such as RF MEMS, vacuum or atmosphere is a necessity and influence sensor performance directly. For traditional glass frit or AuSi eutectic bonding, the process temperature is always higher than 400 °C and can' t provide interconnection well. Therefore, metal-to-metal (M2M) technology is focused recently to provide not only high quality hermetic but also fine pitch interconnection. In this paper, two kinds M2M bonding systems are developed below 350 °C, which are AuAu diffusion bonding and AuCuSn eutectic bonding. FIB analysis and die shear test are used to understand the influence of material, thickness and process selection in bonding quality. Besides, those two different systems will be compared in considering of wafer topography tolerance and MEMS integration issue. Consequentially, we will demonstrate the application with MEMS package and MEMS device with those two kinds M2M bonding technology respectively. For MEMS sealing package, the general seal ring width used is 100um, which is better than glass frit (200um) when device size shrinking. For MEMS device, diaphragm-type device are used to stacking CMOS and MEMS to form multilayer device. Finally, we also will introduce some new bonding concepts in undergoing or future works, such as different material system or nano-technology.