Influence of electron incident angle distribution on CD-SEM linewidth measurements

Abstract

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 2º 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 0.2º for the sidewall angle.Keywords: CD-SEM, model-based library, focused electron beam model, electron incident angle, measurement error

Application of model-based library approach to photoresist pattern shape measurement in advanced lithography

“…We previously began a study of the MBL approach through collaboration with NIST 6 . Most previous studies improved the measurement accuracy, the physics based mathematical model and the library [7][8][9][10][11] .…”

Application of model-based library approach to Si 3 N 4 hardmask measurements

“…On the other hand, if a measurement tool is stable and well controlled, the instrument parameters might not have to be estimated at every CD measurement. Our approach for improving MBL matching is to build a tool model reflecting the CD-SEM characteristics and set appropriate tool parameters for the model 8 important for achieving stable, reliable measurements for MBL matching. To fit the simulated material contrast to the actual SEM image, the residual energy loss rate parameter was determined empirically.…”

“…In the focused electron beam model, a Gaussian filter was introduced as a parameter for adjusting the beam width 8 . In this study, the beam width parameter was estimated beforehand, and the estimated parameter was used in the measurement process with MBL matching.…”

“…We previously began a study of the MBL approach through collaboration with NIST 7 . The effectiveness of the MBL matching technique has been demonstrated in various substantial studies, through both simulation and experiment 8,9 . In this study, we applied the MBL matching technique in hardmask linewidth metrology with a CD-SEM.…”

Monte Carlo Simulation of SEM and SAM Images

“…However, it has been pointed that some critical beam shape dependencies that are not correctly account for by the simple Gaussian model (Tanaka et al, 2005), and that the effect of electron incident angle is not negligible; the electron distribution far from focus is not consistent with a Gaussian model at the impact position of electrons on a line by the effect of aberrations of the beam shape. The requirement of the accuracy and stability of measurement in CD-SEM needs a better beam shape model (Tanaka et al, 2006). The feature of a modeled line can be characterized by several pattern parameters.…”