In order to accurately measure narrow-space patterns, we propose an improved secondary-electron extraction efficiency model for the model-based library method. In the conventional model, the same extraction efficiency is applied to all electrons, regardless of from where the electrons are emitted. This is a simplified model that assumes 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 (SEM) optics. We verify the effectiveness of the improved extraction model by applying this model to measurements of actual patterns with space widths in the (20 to 30) nm range. The measurement bias of the sidewall angle (SWA) is evaluated through comparison to cross-sectional SEM measurements. We demonstrate that the average SWA bias is improved from 0.8 deg for the conventional model to 0.04 deg for the improved model.