Wafer to Wafer Bonding Using Electroplated Co-Sn Solder Layer

Wafer to wafer bonding has many advantages in MEMS, optoelectronics, and other packaging applications where low temperature (<= 450 degrees C) bonding is essential to avoid thermal stress and defect generation. For these reasons, a number of wafer bonding techniques using Sn based low temperature solder were reported. The wafer bonding using Co-Sn solder layer was investigated in that context. Hence, two wafers were bonded with electroplated Co and Sn layer at varying temperature (200, 300, 350 degrees C) for varying times. During the bonding, electroplated Co and Sn layer transformed into the CoSn3, but voids were founded in the middle of the bonded layer at 250 and 300 degrees C and only the bonding 350 degrees C was void free. Subsequent die shear test showed superior shear strength of specimen bonded at 350 degrees C. The shear strength depended mostly on the bonding temperature and not on the reflow time.