Method and apparatus for enhancing the depth of focus in projection lithography

The invention provides a technique which enables projection lithography to extend to the sub-half micron range by compensating the Depth of Focus (DOF) budget lost in substrate topography with a projection of a non-planar image which is conformal to the substrate (10). The method of achieving a non-planar image field includes the formation of a mask reticle (20) which is a replica of the surface of the semiconductor (10) to be exposed, thus, eliminating substrate topography from the optical DOF budget.