Mask inspection method using the electron-beam inspection system based on projection electron microscopy

Abstract

We developed the electron beam inspection system based on projection electron microscopy (EBI-PEM), and then applied this system to inspection of mask defects. Usually, inspection of mask defects (such as monitoring of growing defects) is carried out with resist pattern on Si wafer by using an optical inspection tool. In recent years, the shrinking of the design rule for LSI devices has fueled demand for mask inspection for small defects, which are hard to detect with the resolution of an optical inspection tool. Therefore, a high-resolution electron beam inspection tool is desired. However, conventional electron beam inspection systems based on scanning electron microscopy (EBI-SEM) require very long inspection time (10-100 times longer than in the case of optical inspection tool) and inspection costs are very high. In addition, it is difficult to inspect resist pattern by using an electron beam inspection tool, because of the charge- up problem. In order to solve the problem, we examined a new mask inspection method using an electron beam inspection system based on EBI-PEM. Although, EBI-PEM have an advantage in terms of inspection speed, it is more difficult to inspect resist pattern by EBI-PEM than by EBI-SEM, because EBI-PEM is very sensitive to charge-up of a sample surface. Therefore, we tried a method in which inspection is performed after transferring a pattern to SiO2 thin film formed on Si wafer. By optimizing the thickness of SiO2 thin film and the electron beam condition of EBI-PEM, we were able to minimize the influence of charge-up and obtained a higher contrast image. Using this method, EBI-PEM achieved inspection sensitivity of 35nm in the case of programmed defect wafer. We confirmed the probability of realizing high-speed and high-resolution mask inspection by using EBI-PEM.