Use of Wavefront Coding in the Design of Sub-Wavelength Lithographic Imaging Systems

Off-axis illumination, optical proximity correction, phase shift masks, interferometric techniques, and pupil plane phase masks have been used to increase lithographic resolution. Further progress requires new techniques and methodologies. The optimalcombination of aspheric optics and signal processing which we term Wavefront Coding has been effective in dramatically increasing the imaging performance of other high quality imaging systems such as microscopes and endoscopes. Similar techniques can beused in lithography. Wavefront Coding can be used to increase the resolution and the robustness of the lithographic system. Examples include increased aberration tolerance to increase system z-alignment tolerance (depth of focus); increased instantaneousfield of view in proportion to the increase in the depth of focus; increased numerical aperture by 20% to 30%; and the design of better object amplitude and phase masks to reduce the adverse effects of diffraction. Phase masks that are designed especiallyfor lithography would require no signal processing, but the exposure level would be adjusted appropriately. The illumination, object mask and pupil mask could be jointly optimized, for a class of object masks. A successful project will demonstrate theusefulness of Wavefront Coding in increasing the resolution and throughput of lithographic systems. We anticipate a 20% increase in numerical aperture, an increase in the field that is proportional to the increase in the depth of focus (An increase indepth of field of more than 15 was obtained in microscopy), and a systematic methodology to jointly design the illumination and pupil function of lithographic systems optimized for a class of object masks. More effective object masks to reduce the adverseeffects of diffraction should be possible with systems designs where all of the components are jointly optimized. As a result of the new methodology, lithographic lens manufacturers and mask makers will be able to produce products that give much smallerfeatures with a higher throughput rate and an improved tolerance to system errors.