Alternating PSM balancing characterization: a comparative study using AIMS and wafer print data

Abstract

Alternating Phase-Shifting masks (altPSM) are known to provide high contrast imaging combined with a low Mask Error Enhancement Factor (MEEF) at low k1. At feature sizes close to 60nm half-pitch and less the impact of mask topography effects increases. This applies in particular for altPSM. This is due to the quartz etch which is required for every second mask aperture to obtain the 180 degrees phase shift. It enlarges the mask profile height significantly. The influence of the quartz trench profile on the transmission and phase balancing performance has already been studied extensively. Basically it has been shown, that tighter quartz trench profile control, specifically for etch depth and width, is required with decreasing mask feature half pitch. The desired mask pattern geometry optimization is currently based on an evaluation of the printed resist pattern over defocus. However, a mask process engineer can use instead only AIMS measurements of the mask features. Therefore there is a mature interest to check, how good such measurements can replace resist pattern measurements. In the paper therefore it is evaluated how accurate AIMS measurements can describe the real printing performance of an alternating PSM in resist. Impact of differences of the image formation is investigated by use of analytical expressions. Furthermore, the influence of tool imperfections and the presence of resist are discussed. The theoretical results are compared to experimental data taken from AIMS measurements and wafer prints.