Imaging characteristics of reflective EUV masks

The transition from DUV to EUV has moved the industry to use reflective masks. The purpose of this paper is to examine image characteristics of EUV reflective masks, some of which are very different than those of the transmissive masks from DUV. One of these new characteristics is mask shadowing, which reduces the image intensity, thereby slowing throughput. We introduce the “mask shadow bias”  parameter which is proposed as an absolute measure of shadowing. Another characteristic parameter is pole-to-pole image shift, which causes image contrast loss from fading. Typically, the image shift is larger for thinner absorbers, which poses a dilemma for mask optimization. Thin absorbers with less shadowing are good for throughput, but thick absorbers can have better contrast from less image shift. The imaging parameters described in this paper are all measurable, and should be helpful to sort out the large space of possible mask structures. We are strongly driven towards high contrast images, so as to push down the stochastic errors that can limit pattern quality. But we also need bright images, where we can get more throughput to reduce patterning costs. Image shift and shadow bias are useful metrics for optimizing EUV contrast and throughput.