Modeling the Defect Inspection Sensitivity of a Confocal Microscope

The ability of a confocal microscope to inspect for defects on EUVL mask blanks has been investigated both experimentally and theoretically. A model was developed to predict the image contrast of a confocal microscope. Measurements were made on PSL spheres and programmed multilayer defects using a Lasertec M1350 operating with 488 nm light. The images obtained of PSL spheres on both fused silica and multilayer-coated blanks are accurately predicted with the model using no adjustable parameters. Good agreement is also demonstrated for the modeling of multilayer defects. Predictions are made for the expected increase in contrast at the shorter wavelength of 266 nm. Substrate roughness contributes to the “noise” which limits the sensitivity to small defects. The contrast fluctuations due to roughness have been modeled using a simple single surface approximation. The model has been validated with measurements on substrates with varying degrees of roughness. The contribution of mask roughness to the sensitivity of a 266 nm tool is estimated.