Non-destructive evaluation of the strength of direct bonded silicon wafers by acoustic measurements

The quality of direct bonded silicon wafers in terms of strength and defects was characterised non-destructively by a novel acoustic method. This method is based on the transmission of high amplitude longitudinal acoustic waves through the samples. The paper gives an introduction to the method and describes the measurement tool. For direct bonded wafers the strength relation matches results obtained by the razor blade test. Together with the strength results, information on defects at the interface can be extracted from the measured data. Results of strengthening in subsequent annealing steps and of strength distribution ate presented. Technical difficulties and questions of the result interpretation are discussed.