US Patent 7618250 - Nanoimprint lithography optical alignment

http://www.freepatentsonline.com/7618250.html

This patent from ASML Netherlands B.V. teaches diffraction gratings used to align nanoimprint
lithography and enable multilayer patterning. Claim 1 reads:

1. An imprint lithography apparatus comprising:

a template holder configured to hold an imprint template, a substrate table, and an optical encoder, the optical encoder having a radiation output, a first diffraction grating, and a detector, the radiation output arranged to illuminate a second diffraction grating and the detector positioned to detect radiation diffracted by the first and second diffraction gratings to provide an alignment signal.