Low temperature bonding process for wafer-level MEMS packaging

In this paper, the development of low temperature (less than 300 degrees C) bonding techniques of Si-to-glass, glass-to-glass and Si-to-Si is reported. To improve the bond quality of Si-to-glass and make glass-to-glass bonding viable, an amorphous silicon layer of 20-100 nm thickness is deposited as an intermediate layer. For Si-to-glass bonding, without applying the amorphous film, both the bond efficiency and bond strength are low. With the assistance of the amorphous film, the bond quality is significantly improved. Glass-to-glass bonding has also been successfully achieved at low temperature with the amorphous film as intermediate layer. Direct Si-to-Si bonding can be realized at a bonding temperature of 400 degrees C with high bond strength and bond efficiency. Using a sol-gel intermediate layer, the bonding temperature can be reduced to less than 300 degrees C. High bond strength is still maintained, while the bond efficiency is becoming a concern. The bonding mechanisms are proposed.