In projection exposure of isolated pattern such as a contact hole, in order to increase the depth of focus a coherence reducing member is disposed on a Fourier transform plane in an image-forming optical path between a mask and a sensitized base, so that coherence is reduced between image-forming beams...http://www.google.de/patents/US6310679?utm_source=gb-gplus-sharePatent US6310679 - Projection exposure method and apparatus
