Effects of lens aberrations on critical dimension control in optical lithography

For sub half-micron lithography, control of the Critical Dimension, represents the key to a robust and manufacturable process. The lithographer must understand and tune the exposure system for critical dimension performance using variables such as Numerical Aperture (NA), Partial Coherence (PC), position of the Focal Plane. Exposure tool must be characterized for its lens signature and the tools must be matched for their CD variation within the optical field In a previous study, we demonstrated that the phase-shift focal plane monitor, PSFM accurately measures focal plane variations when appropriate calibrations are employed. That paper also described the development of a model to determine lens classic aberrations such as Coma, Astigmatism and Field Curvature. The publication correlated the results to an approximation of the stepper´s Critical Dimension (CD) behavior for a matrix of NA and PC settings. The present study, continues to study CD uniformity through examination of the lens aberrations and field focal signature of optical steppers in a 0.35 um process. We describe a method of measuring the uniformity of numerical aperture (NA) and partial coherence (PC) across the exposure field this new tool is then applied as an aid in optical lens characterization and tool-to-tool matching. The information gathered is thereupon applied to measure lens aberrations and predict CD variation across the field under various settings of the lens focal plane, the predictions are validated by a comparison against CD uniformity as measured by an advanced critical dimension metrology system