Measurement of Work of Adhesion on Wafers for Direct Bonding

ABSTRACTThe displacement loaded double cantilever beam (DCB), often referred to as the blade-insertion test or crack-opening method by the wafer bonding community, has become a common method for evaluating the work of adhesion of bonded wafer pairs. The test, while easy to perform, often yields results with large scatter and questionable accuracy. The mechanics of the specimen are investigated in detail in the current work. Expressions that demonstrate how wafer bow may lead to residual stresses that result in large errors in the calculated work of adhesion are developed. A three-dimensional finite element model is used to show that due to the circular wafer geometry and silicon anisotropy there is a large variation of the strain energy release rate across a straight crack front. The model is used to predict the actual crack front shape and shows good agreement with experiments. The results of the finite element simulations are compared to the traditional expression used for data reduction and implications of the model highlighted.