Focus measurement using SEM image analysis of circuit pattern

摘要

We have developed a new focus measurement method based on analyzing Scanning Electron Microscope (SEM) images that can help to control a scanner. In advanced semiconductor fabrication, rigorous scanner focus control has been required because focus offset causes defects. Therefore, it is essential to ensure focus offset is detected at wafer fabrication. In the past, the focus has been measured using test patterns made outside the chip by optical metrology system. Thus, the present focus metrology system cannot measure the focus of an arbitrary point in the chip. The new method enables highly precise focus measurement of an arbitrary point of the chip based on a focus plane of a reference scanner. The method estimates the focus amount by analyzing the sidewall shapes of circuit patterns of SEM images. The sidewall shapes are quantified using multi-slice contours extracted from SEM images with high accuracy. By using this method, it is possible to measure the focus of an arbitrary circuit pattern area of the chip without a test pattern. We believe our method can contribute to controlling the scanner and detecting hot spots that appear due to focus offset.