The Auto-focus Method for Scanning Acoustic Microscopy by Sparse Representation

Abstract

As an efficient non-destructive checking technique, scanning acoustic microscopy has been rapidly developed and widely used in nondestructive detection for composites, chip manufacturing, bio-pharmaceuticals and so on. Scanning acoustic microscopy can obtain the two-dimensional image and reconstruct three-dimensional model of the internal structure inside the sample, compared with the optical microscope. Due to the geometrical features of high frequency transducer, the precision of focus has already become the key factor for the digital imaging and defect detecting. Especially for the current 3D packaging chips and multi-layer composite materials, thickness of layer has reached the level of nm. Different focus plane will provide a completely different digital image. However, the process of focusing has been relying on manual operation without great development until now. To improve the speed and accuracy of the focusing, we proposed a quick auto-focus method on scanning acoustic microscopy. This method can be divided into three steps. The first one is fast auto-focusing on upper surface, the second other one is auto-focusing on the interlayer base on V(z) curve, and the last one is to improve the A-Scan signal of points in the V(z) curve with spars signal representation. This method is integrated into Scanning Acoustic Microscopy imaging software and has been validated in the experiment with Olympus transducers at 30\\50 MHz. It can be seen that this method can effectively improve the accuracy of focus and shorten the focus time of Scanning Acoustic Microscopy through experiments.