A new method for enhancing focus latitude in optical lithography: FLEX

A new concept for enhancing the focus latitude in optical lithography is proposed that makes it possible to overcome the Rayleigh optical limit in the depth of focus. The method, called focus-latitude enhancement exposure (FLEX), uses multiple exposures in several different focal planes to extend the image contrast of the mask pattern along the light axis. It is confirmed both in simulations and experiments that the focus latitude for patterns with a feature size of about a half-micrometer can be increased by more than two times using FLEX, compared with the conventional method. FLEX is especially effective when applied to small isolated transparent patterns like contact holes.