Submicron optical CD metrology on photomasks

As the wafer industry has driven the minimum feature size on the photomask to the submicron range, an increased focus has been placed on the metrology used to control such features. The most common method of metrology for photomask linewidth measurements is the optical CD metrology tool. However, now that the submicron range of photomask linewidths and features are aggressively pursued, the optical resolution limit of the optical CD measurement tool is limiting the ability to accurately perform photomask linewidth measurements. It is therefore essential to look beyond this limit, to either pursue new technology CD metrology tools, or to develop practical techniques to measure submicron photomask features approaching or extending beyond the optical resolution limit of the metrology tool. This paper investigates the later approach with discussion and evaluation of some techniques used as an attempt to enhance the current optical CD metrology capability in order to measure photomask features well into the submicron range.