Image reversal at nm-scales

Interferometric lithography (IL) techniques provide low-cost, large area nanoscale resolution. It is difficult to control positive resist process with narrow spaces (large line:space ratios) because of the large slope of the swing curve just as the resist clears. The sidewalls are inherently sloped with significant standing wave related distortions. Also, at nm-scales, resist pattern collapse becomes a serious limiting issue. We report on investigations of image reversal using positive photoresist AZ5206E. This photoresist is designed for reversal with a simple heating step followed by a blanket UV exposure. One-dimensional photoresist gratings were investigated with CDs varying from similar to 100-250 nm. The image reversal process significantly reduced standing wave effects, eliminated the collapse issue, and provided under-cut profiles suitable for lift-off. At periods < 300 nm, high aspect ratios with this photoresist could not be achieved probably due to a lack of resolution. Image reversal was also demonstrated in multiple exposure processes leading to sharp, square corners. Finally, the developed photoresist profiles with the image reversal process were found to be in good agreement with the Prolith((TM)) modeling.