The performance of an actinic full-field EUVL mask blank inspection system

We have developed an actinic full-field inspection system to detect multilayer phase defects with dark field imaging capabilities. With this inspection system programmed phase defects on a mask blank were observed, and their image quality was compared with that from a proof of concept (POC) tool. The maximum intensity of normalized defect signal with the inspection system is 3.1 times larger and the full width at the half maximum (FWHM) of the defect signal is 13 % narrower than those with the POC tool. And therefore, the image quality of this inspection system is considered to be superior to that of the POC tool. In the case where the defect image is captured with stage scanning using time delay integration (TDI) method, degradation of defect signals caused by stage scanning is evaluated. With a scan velocity of 1mm/s, the defect signal indicates a 19 % decrease in the maximum intensity and a 13 % extension to the FWHM as compared with those from the static mode. We believe that this degradation of defect signal is caused by the variation of scan velocity and yawing of the scan stage. The relation between defect sensitivity and scan velocity is analyzed with signal to noise ratio (SNR). As a result, a scan velocity of 1mm/s plays a role that is barely sufficient to detect a 1.5 nm-high 60 nm-wide defect with no false defect detection. Noise reduction of CCD or improvement of inspection algorithm will be required.