Depth perception using blurring and its application in VLSI wafer probing

In this paper, we present a technique for measuring the amount of blur of an edge and using this information to determine the distance of a micromanipulator probe from a wafer surface in very large scale integration (VLSI) wafer probing. In this application, a soft and reliable touch of the probe with a metal pad in the wafer is a sensitive operation. The wafer is focused and several images of the probe while approaching the wafer are analyzed. In our theory, the amount of blur is calculated from the height of the step edge and the slope of the intensity profile at the zero crossing. Hence, our formula is simple and easy to use. We estimate the distance of the probe from the surface of the wafer and obtain a robust measure, i.e., one which is valid even in the presence of significant noise in the images. In order to validate our methods, we have experimented with various VLSI patterns as backgrounds.