Small random library method combined with local searching in optical critical dimension measurements

The critical dimensions (CDs) of gratings significantly influence their optical performances and require high-resolution measurements. To avoid damaging the gratings, a model-based optical critical dimension (OCD) measurement method utilizing ellipsometry or scatterometry was applied by matching the simulated and experimental values. However, online CD measurements during grating fabrication require a bulky presimulated library containing the condition points with various CDs, making it time consuming and resource intensive to build with large dimension ranges to account for grating fabrication errors. In this study, we proposed a smaller random library with an unevenly distributed resolution, offering finer resolution when the grating to be measured is close to the reference grating. This approach, validated using a home-constructed spectroscopic ellipsometer, resulted in better results. Finally, a local search algorithm based on a random library was applied to further improve the measurement accuracy. This approach extraordinarily reduced the preparation time for OCD measurements and achieved better performance, significantly improving the efficiency of grating development and fabrication inspection.