Vector diffraction analysis of phase-mask imaging in photoresist films

Abstract

A vector diffraction analysis is applied to methods of image manipulation like phase masks or off-axis illumination. When such techniques eliminate the DC order altogether, as with alternating phase masks, it becomes possible to print a given intensity pitch with only half as large an NA as with a chrome mask. Some non-scalar imaging effects are therefore reduced. For example, in present lithographic practice the depth of focus with standard masks is often little larger than the thickness of the resist film, so it is no longer accurate to neglect defocus arising during multiple reflections within the resist. The angle dependence of the film stack then becomes significant, and small film perturbations give rise to swing-curve oscillations in pupil apodization and image shape as well as exposure dose. Specific film thicknesses can, for example, have a similar effect to stopping down the lens. Other thicknesses can enhance resolution of certain patterns. We treat these film effects by representing each layer in the process stack with pupil transfer functions. Like defocus, these pupil functions posses a symmetry that eliminates relative dephasing or apodization in the two-beam fringe pattern produced by an alternating phase mask. However, at finer pitches such two-beam patterns exhibit a higher contrast-loss than the three-beam patterns produced by standard masks, in part because at high NA the interfering E components become significantly non-parallel, and hence incapable of producing complete constructive or destructive interference. (This vector effect adds to the scalar contrast difference.) An alternating phase mask can thus produce stronger vector diffraction anomalies than a standard mask, despite being able to print a given pitch with half as large a lens NA.