Temperature-dependent evolution of Al-Ge microstructures for wafer-level vacuum packaging of MEMS devices

This paper reports the temperature dependency of eutectic aluminum-germanium (Al-Ge) microstructures that can be used for complementary metal-oxide semiconductor (CMOS)-compatible wafer-level vacuum packaging of microelectromechanical systems (MEMS). A significant advantage of the Al-Ge system is that the bonding material can be deposited only on the cap wafer side and bonded to the CMOS dielectric layers without the need to deposit another layer on the CMOS wafer. The Al-Ge system is also attractive for CMOS foundries, as both materials are CMOS compatible, and the required bonding temperatures to form the proper eutectic formation are within the post-CMOS process temperature limits, i.e., around 450 C. In pursuing the lowest bonding temperature with reasonable bond strength to prevent the deterioration of the MEMSlayers, the microstructure of the alloy is examined in detail after annealing and wafer bonding experiments at various temperatures, including 450, 440, 435, and 430 C. The experiments are conducted using sputtered Al-Ge layer stacks after observing that sputtered stacks yield better integrity and eutectic microstructure than thermal evaporation. The cap deflection of thinned cap wafers is monitored to validate the vacuum inside the sealed cavity, and the shear testing approach is used to measure the bond strength according to the MIL-STD-883 standard. The lowest bonding temperature with successful vacuum sealing is determined as 435 C, where an average shear strength of 27 MPa is measured. The average strength increases to 51 MPa when the bonding temperature rises to 440 C.