A projection exposure apparatus has an effective source, a mask, a projection lens and an optical filter. The mask is illuminated with the light from the effective source having a substantially annular illumination distribution. The optical filter is disposed in the approximate position of the pupil...http://www.google.com.au/patents/US5348837?utm_source=gb-gplus-sharePatent US5348837 - Projection exposure apparatus and pattern forming method for use therewith
