Simulation of secondary electron emission in helium ion microscope for overcut and undercut line-edge patterns

Abstract

In order to study the topographic contrast of line-edge patterns in a scanning ion microscope (SIM) using helium (He) beam, a Monte Carlo simulation of secondary electron (SE) emission from silicon (Si) by the impact of He ions in the energy range of tens of keV is performed. The edges with overcut and undercut profiles for different sidewall angles are modeled and the patterns are scanned by using 30 keV He ion beam, so that the line profiles of the SE intensity are calculated assuming zero-sized beams. The results are compared with those of 30 keV Ga ion and 1 keV electron beams. Furthermore, the pseudo-images of critical-dimension (CD) line patterns with different widths are constructed from the SE profiles. The calculated SE yields of Si for 10-40 keV He ions increase with increasing impact energy, which become larger than that for low-energy electrons (keV or less). When scanning the line edges formed on a Si surface, there appear both large and sharp peak and small dip of the SE yield. The height of the peak is much more for the He ion beam than the Ga ion and electron beams, whereas the width is less: the FWHMs are 3.8 nm for 30 keV Heion, 7.2 nm for 30 keV Ga-ion and 8.0 nm for 1 keV electrons. This indicates that the line edge is more clearly distinguished by He ions. The change in the sidewall angle causes the change in the shape of the hump in the SE profile at the sidewall of overcut edges due to the incident angle dependence of the SE yield, which is clearly seen for all beams. However, much less change in the line profiles of undercut edges is found for Ga ion and electron beams.