Adhesive Strength Characterization of CYTOP™: Low Temperature Wafer-Level Packaging

This study describes a wafer bonding technique using CYTOP™ inking method for the high volume packaging of micro-electro mechanical system (MEMS) devices. CYTOP™ is a class of perfluoro (alkenyl vinyl ether) polymer which is obtained by cyclopolymerization of perfluoro. The CYTOP™ adhesive bonding requires much lower temperature (150 to 200° C) compared to other bonding techniques such as soldering (> 250° C) or anodic (~350° C) bonding. The lower temperatures involved in the process reduce the risk of thermal damage to temperature sensitive devices during packaging. The described bonding process consists of a wet inking technique in which wet CYTOP™ ink is applied on soft cured CYTOP™ before bonding. In this study, CYTOP™ is characterized for its bonding strength and quality. The experiments are performed on silicon and glass wafer substrates. The bonded samples are pull-tested and tensile stress values are recorded at the instance of bond failure. About 90% of the samples failed at the bonding interface which indicates that the recorded stress values are the bond strength of CYTOP™. The bond strength of CYTOP™ depends upon the curing temperature and the curing time. The highest bond strength of 16. 46 MPa is recorded at 200° C and 45 min. of curing. The CYTOP™ bond strength at 200° C is comparable with bond strength of BCB at 250° C.