Lens wavefront compensation for 3D photomask effects in subwavelength optical lithography

As semiconductor optical lithography is pushed to smaller dimensions, resolution enhancement techniques have been required to maintain process yields. For some time, the customization of illumination coherence at the source plane has allowed for the control of diffraction order distribution across the projection lens pupil. Phase shifting at the photomask plane has allowed for some phase control as well. Geometries smaller than the imaging wavelength introduce complex wavefront effects that cannot be corrected at source or mask planes. Three-dimensional photomask topography effects can cause a loss of both focal depth and exposure latitude across geometry of varying density. Wavefront manipulation at the lens pupil plane becomes necessary to provide the degrees of freedom needed to correct for such effects. The focus of this research is the compensation of the wavefront phase error introduced by the topographical photomask structures of high resolution phase shift masking combined with off-axis illumination. The compensation is realized through phase manipulation of the lens pupil plane, specifically in the form of spherical aberration. Subwavelength resolution optimization and imaging is presented showing how phase pupil filtering can measurably improve the depth of focus for several photomask structures and types.