Characterization of Hysteresis of Surface Energy in Room-Temperature Direct Bonding Processes

Abstract

Room temperature direct wafer bonding is driven by surface forces and the formation of hydrogen bonds that provides a driving force to pull the wafers being bonded into intimate contact. The magnitude of these forces is often represented as a work of adhesion and is critical as it determines the level of non-flatness and patterning that may be tolerated in bonding. Traditionally, it has been assumed that the work of adhesion that drives the formation of the bond is equal to the work of separation that is measured through fracture tests, such as the familiar razor-blade test. In the present work, we use a novel test structure to measure the work of separation and work of adhesion of room temperature hydrophilic Si-Si bonds as function of relative humidity (~15 - 50% RH). The results show that across a range of values of RH the work of adhesion is significantly lower than the work of separation.