Focus monitoring utilizing an aperture in Cr film on backside of photo mask

A simple and high sensitive focus monitoring has been developed utilizing an aperture formed in Cr film on backside of photo mask. A special mask for focus monitoring is developed such that two mark patterns are irradiated by different illuminations which are generated by an aperture on the backside of the mask so as to make two complementally half parts of usually used illumination. Printing mark patterns with overlaying each other, focus deviation is detected as a mutual displacement of mark patterns. Very high focus sensitivity (/spl Delta/x//spl Delta/z) of /spl sim/0.9 is observed for NA=0.68 optics.