Optical design of refractive imaging spectrometer for semiconductor metrology

Abstract

Critical dimension (CD) uniformity has recently become increasing essential with miniaturization in semiconductor production. For high-volume manufacturing, high-speed in-wafer uniformity inspection is inevitable. The conventional optical critical dimension (OCD) spectroscopy has low resolution issue because of the large spot size. Consequently, line-scan hyperspectral imaging (LSHI) is presented by Samsung Electronics in 2022. Compared with the OCD, the LSHI has advantages of high throughput, high spatial and spectral resolution, and large field of view. The LSHI can improve the throughput of OCD significantly and provide high-resolution spatial and spectral information. In this study, the optical design of the refractive imaging spectrometer based on the ruled reflection grating is presented. Most imaging spectrometers, such as Czerny-Turner or Ebert-Fastie setup, are off-axis reflective configuration. However, the performance is limited by serious astigmatism and smaller numerical aperture. The astigmatism leads to low spatial and spectral resolution, and the smaller numerical aperture requires more exposure time, which results in low throughput. Therefore, the refractive type using lens combinations for imaging spectrometers can eliminate aberrations effectively and increase numerical aperture significantly. In this study, the numerical aperture is designed as f/2.4, and the slit length is 14 mm for high throughput. The high spatial and spectral resolution is accomplished by optimizing lens combinations from 400 nm to 800 nm spectral region.