Influence of Electron Incident Angle Distribution on CD-SEM Linewidth Measurements

摘要

The linewidth measurement ability of the Model-Based Library (MBL) matching technique is evaluated by a simulation study, and an improvement in the technique is proposed. In this study, a focused electron beam model is introduced in the MONSEL Monte Carlo simulator to estimate the effects of the electron incident angle distribution on linewidth measurements. By using the focused electron beam model, the images that will be obtained by an actual critical-dimension scanning electron microscope (CD-SEM) were simulated. Measurements were carried out on the images which would be taken with the SEM focus conditions in a range maintained by the auto-focus system. As a result of measurements of simulated images with various sample geometries, it was confirmed that the current MBL matching with a simple Gaussian electron beam model could cause a measurement error of more than 3 nm for the linewidth and 2° for the sidewall angle. Since the incident angle distribution distorts the effective beam shape and image profile at the edge of a pattern, conventional MBL matching with a simple Gaussian beam model cannot give a proper measurement of sample geometry for the image profile formed by the focused electron beam, and this results in measurement errors. To eliminate these measurement errors, another library produced by the focused electron beam model, is employed for the MBL matching. The new library consists of simulated profiles at only the best focus, and it enables the MBL to use a better model and to achieve accurate measurements without increased computational costs. By using the new library, measurement errors are reduced to 0.6nm for the linewidth and to 0.2° for the sidewall angle.