CD bias reduction in CD-SEM of very small line patterns: sidewall shape measurement using model-based library matching method

摘要

The purpose of this study is to reduce the critical-dimension (CD) bias (i.e., the difference between actual and measured CD values) for very small line patterns with line widths smaller than 15 nm. The model-based library (MBL) matching technique, which estimates the dimensions and shape of a target pattern by comparing a measured SEM image waveform with a library of simulated waveforms, was modified in two ways to enable it to accurately measure very small patterns. The first modification was the introduction of line-width variation into the library to overcome problems caused by significant changes in waveform due to changes in both sidewall shape and line width. This modification improved the measurement accuracy. The second modification was the fixation of MBL tool parameters that relate to signal-intensity conversion to overcome problems caused by the reduction in pattern shape information due to merging of right and left white bands. This modification reduced the solution space and improved the measurement stability. We confirmed the effectiveness of the modification by using simulated images. We then verified the effectiveness of the modified MBL matching by applying it to actual SEM images. Silicon line patterns with line widths in the range 10-30 nm were used in this experiment, and the CD bias was evaluated by one-to-one comparison with atomic force microscopy (AFM) measurements. The CD bias measured by MBL matching for three heights (20, 50, and 80%) was consistent with the AFM results. The CD biases at all heights were smaller than 0.5 nm and the slopes of the CD biases with respect to the CD were smaller than 3%.