Verification of Optics for the Die-to-Wafer-like Image Mask Inspection

Due to the feature size shrinking, the application of 193nm-ArF scanner systems with high numerical aperture (NA) and the use of resolution enhancement technologies (RET) have been essential for obtaining the desired pattern accuracy on a wafer. Thus the complexity and volume of data required for masks have been rapidly increasing. Moreover, the complexity of mask pattern makes mask inspection increasingly more difficult. The most annoying problem relating to the sensitivity of inspection system is the encountering of false signals arising from nuisance defects. Setting up thresholds in the defect detection algorithm is a difficult task between high sensitivity and less false defect detection. In addition, the effect of variations in defect printability which is strongly dependent on defect types and position must be considered in order to correctly evaluate mask defect inspection procedures. In order to overcome the problems we have previously proposed new algorithm for die-to-wafer-like image (D-to-WI) in real time. This inspection method compares the die, i.e. the wafer image calculated from CAD data, with the wafer-like image calculated from the mask images detected by the mask inspection system. This paper described optimum mask inspection optics for the D-to-WI mask inspection. We verify the optimum mask inspection optics with numerical simulation for various NA and partial coherence of illumination (σ) in the mask inspection optics. The simulated result shows that the optimum mask inspection optics has NA 0.9 and σ=1 for ArF-6%-PSM (Phase Shift Mask) 65/65 nm Line/Space pattern of 193nm-ArF scanner with NA 0.92. In this case the difference of the critical dimensions (CDs) found by D-to-WI and rigorous simulation results from CAD data was less than 1.5nm.