Investigation on the use of Al-Ge eutectic bonding in the structure part of a multilayer stacked MEMS device

Multiple wafer bonding is required to build up 3D integrated structures in complex Micro-Electromechanical Systems (MEMS) devices. Al-Ge eutectic bonding is investigated and shown to be a suitable technique for creating the functional MEMS device, particularly in the situation where subsequent high temperature is still required during the MEMS fabrication. A challenge in using this technique is to ensure sufficient bonding strength in sparsely distributed small area connections which is a common requirement in complex MEMS system. In this paper, we have successfully performed Al-Ge bonding to create a complex MEMS system on 200 mm wafers embedding CMOS and electromechanical components in it. A top wafer with sparely distributed 80 μm tall Si pillars coated with a Ge surface was bonded to a bottom wafer with a patterned Al surface. Due to the small connection area in this case, a dedicated characterization approach, consisting of grinding and plasma dicing, was successfully implemented to form small free-standing structures on which the bond strength can be assessed by measuring the shear force required to detach these structures. Bonding strength in the range of 30 MPa to 50 MPa is obtained by this technique. Moreover, Energy-Dispersive X-ray spectroscopy (EDX) in Transmission Electron Microscopy (TEM) confirmed Al interdiffusion into Ge, indicating the eutectic reaction happened at the bonding interface. A compliant layer was also introduced to improve the full wafer bonding quality on edge trimmed wafers, enhancing the stability of subsequent processing on bonded wafer to realize the MEMS device.