Improved secondary electron extraction efficiency model for accurate measurement of narrow-space patterns using model-based library matching

Abstract

In order to accurately measure narrow space patterns, we propose an improved secondary-electron extraction efficiency model for the model-based library (MBL) method. In the conventional model, the same extraction efficiency is applied to all electrons, regardless of where they are emitted from. This is a simplified model assuming a uniform extraction electric field strength. In the improved model, the extraction efficiency is calculated as a function of the pattern shape and the emission position of the electrons. The function is based on simulation results for the electric field strength of critical-dimension scanning electron microscopy (CD-SEM) optics. We verify the effectiveness of the improved extraction model by applying it to actual patterns with space widths in the range 20 to 30 nm. The measurement bias of the sidewall angle (SWA) is evaluated through comparison with cross-sectional SEM measurements. We show that the average SWA bias is improved from 0.8° for the conventional model to 0.04° for the improved model.