Quantitative evaluation method for detectability of voids using ultrasound

We have developed a new method to quantitatively evaluate the detectability for voids in bonded wafers by using ultrasonic inspection. The test sample for evaluation consists of bonded two Si wafers and has artificial voids between the wafers. The depths of these artificial voids are 5, 10, 20, and 170 nm. In this study, the evaluation was made by obtaining the images of artificial voids by using transducers with various nominal frequencies and focal lengths. It was found that even the voids with a depth of 5 nm or more can be detected and that the minimum dimension of detectable void is independent of the void depth. Better resolution and detectability of small voids were achieved by transducers with higher frequency. Furthermore, it is demonstrated that a transducer having an appropriate focal length should be selected, in accordance with the thickness of the bonded wafer.