Specifying Optical Fabrication Tolerances for Soft X-ray Projection Lithography Systems

The rapidly emerging technology of soft X-ray projection microlithography requires very stringent tolerances upon the residual surface errors inherent with any optical fabrication process. The scattering effects of these optical fabrication errors can severely degrade system performance. These optical fabrication errors must therefore be controlled over the entire range of relevant spatial frequencies, including "mid" spatial frequency surface irregularities that bridge the gap between the traditional "figure" and "finish"errors. The surface power spectral density (PSD) function thus becomes the natural quantity to monitor during the optical fabrication process.