Actinic blank inspection for high-NA EUV lithography

Actinic blank inspection is an essential tool for EUV mask blank qualification. It can detect defects with lithographic impact by utilizing 13.5nm EUV light, which is the same wavelength used in EUV lithography. ABICS E120, developed by Lasertec, is an inspection and review system that aids in defect management and yield improvement in EUV mask blanks production. It is equipped with two optical configurations, low magnification mode to achieve high-throughput inspection and high magnification mode to realize high-resolution review for defect positioning. These functions meet the current requirements with its sensitivity for 1nm in height 40nm in width defect and coordinate accuracy of 20nm. ABICS E120 series are currently used for EUV mask blank qualification. With the introduction of high-NA EUV scanners and the ongoing progress of technology nodes, actinic blank inspection tools must be able to detect even smaller defects. For this reason, Lasertec is currently developing a next-generation ABICS for advanced nodes and planning to release at the end of 2024. Its target performance is a sensitivity for 1nm in height 30nm in width defect and coordinate accuracy of 10nm. The next-generation ABICS will enhance sensitivity by providing a higher Signal-to-Noise ratio (SNR). This performance will be achieved by increasing the magnification of its Schwarzschild objective to shrink a pixel size on the mask, thereby augmenting the relative scale of defect signals to background noise per pixel. To maintain the current model's throughput of 45 minutes per sample, a new CCD sensor with an expanded imaging area is going to be utilized. The next-generation model is also designed to have better coordinate accuracy by enabling the Schwarzschild objective to be adjusted inside the vacuum chamber to capture coordinates of defects using images with minimal aberrations. In addition, new model is expected to have a higher magnification than the current model for its high-magnification review to further enhance its coordinate accuracy.