Development of Nonuniform Residual Stresses during Anodic Bonding

Nonuniform residual stresses may develop during anodic bonding due to variations in wafer curvature prior to bonding or due to nanotopography interactions between the bonding surfaces. In this work, we discuss the significance of nonuniform residual stress in anodically bonded wafer pairs and present a method for measuring local stress variations even in cases when the bonded wafers are apparently defect free based on conventional inspection tools. These residual stresses can vary significantly between two different wafers processed identically, depending on the local interactions, but they are present in all standard bonding methodologies. In some instances, these local residual stresses can significantly alter the resulting wafer curvature in apparently defect free substrates. Currently, anodically bonded devices or substrate wafers are considered "defect free" if no large debonds are found after bonding; however the residual stress state of the wafers cannot be determined using the same conventional techniques.