Amorphous Silicon Direct Bonding (a-SDB) with Improved Surface Roughness

Silicon direct bonding (SDB) is a bonding technique of two silicon wafers together with homogeneous or heterogeneous layers without the use of any intermediate adhesives. The SDB technique simplifies the process and cost by reducing mask level, and its necessity has increased in terms of its fields of applications such as power devices, SOI, sensors and actuators. Factors which can affect direct bonding between silicon wafers are wafer curvature, flatness, contamination and roughness of the surface among which roughness is very important. For effective wafer bonding, surface roughness must be less than 5 Å [1–3]. When the polysilicon layer used for most of MEMS structures on wafer has high roughness, conventional treatment yields so weak a bond that direct bonding between two wafers with polysilicon layer is rarely formed. The same phenomena has been reported for CVD oxide [1]. For reason that amorphous silicon is able to make polysilicon by annealing process, we propose amorphous silicon direct bonding (a-SDB) using a reduced roughness of surface. Amorphous silicon has less than 5 Å surface roughness and the a-SDB is completely compatible with subsequent high temperature process operations such as oxidation and diffusion because of the well matched thermal expansion coefficients of the bonded layers and very low thermal stress. This technique will be adapted for integrated microlens with multi-electrode focusing lens, amorphous silicon TFT and isolation of power device. In this paper, we will also investigate roughness criterion of bonding interface