Limits of model-based CD-SEM metrology

Although the critical dimension (CD) is getting smaller following the ITRS roadmap, the scanning electron microscope (CD-SEM) is still the most general purpose tool used for non-destructive metrology in the semiconductor industry. However, we are now dealing with patterns whose dimensions are of the same order of magnitude as the electron interaction volume and therefore, the usual edge-based metrology methods fail. Like scatterometry has extended the resolution of optical imaging metrology through complex modeling of light-matter interaction, some electrons-matter simulation models have been proposed. They could be used to improve accuracy and precision of CD-SEM metrology. However, these model-based approaches also face to fundamental limits mainly due to probe size with respect to the considered structure and noise. This paper analyses these limits assuming the model is perfect and the microscope has no systematic defect. In this simulation study, we have used the model proposed by D. Nyyssonen, assuming to perfectly represent the SEM effects in the image. The feature of interest is limited to isolated trapezoidal lines with various CD, sidewall angles (SWA) and heights. We have carried out the study with several beam energies, tilts and probe sizes. Surprisingly enough, sensitivity analysis shows that with typical noise amplitude, sidewall angle can be determined with a reasonable precision using SEM images. Single tilted beam SEM images can also bring advantage to measure patterns height. Since these precision figures depend on the geometries, we provide useful graphs giving the ultimate precision for various dimensions (CD, height, SWA).