High-resolution hexagonal array patterns fabricated by dipole cross-exposure in deep ultraviolet lithography

Background: A persistent challenge in deep ultraviolet (UV) projection lithography is to develop processes that can print patterns with higher resolution while maintaining acceptable control of their critical dimensions. Aim: We have improved the resolution of hexagonal array patterns using a combination comprising dipole illumination, cross patterning, triple exposure, and assist lines. Approach: Off-axis dipole illumination, achieved by self-constructed dipole apertures, is used together with a triple cross-exposure technique to produce small pitch patterns of varying sizes. We use a 248-nm wavelength KrF DUV stepper to expose mask patterns comprising arrays of quadratic structures crossed by assisted lines to print patterns with a minimum pitch size of 320 nm. The pattern optimization was accompanied by lithographic simulation calculations optimizing both the aerial image and the resist pattern after development. Results: By varying the pattern dimensions on the mask, process control can be improved, which produces robust patterns with a minimum pillar size of ≈95 nm both for 320- and 340-nm pitch sizes. Changing the mask polarity delivers holes with similar dimensions, whereas off-axis dipole illumination along with double cross exposure can fabricate quasi-elliptical structures with different sizes and angles. In all cases, the size of pillars, holes, and quasi-elliptical structures can be altered by changing the size of the square and/or assist line on the mask. Conclusion: We have applied resolution enhancement techniques that apply to any kind of projection lithography system.