Abstract
High aspect ratio imaging for immersion interference lithography in the ultrahigh numerical aperture regime, where evanescent fields are responsible for the exposure, is demonstrated experimentally using a resonant dielectric underlayer system consisting of HfO stacked upon SiO2. Improvements in producing these high aspect ratio grating structures compared with previous work [P. Mehrotra, C. A. Mack, and R. J. Blaikie, Opt. Express 21, 13710 (2013)] are shown, which has allowed subsequent lift-off pattern transfer for similar to 55 nm half-pitch gratings patterned using a 405 nm exposure wavelength, corresponding to better than lambda/7 resolution. Resist structures with aspect ratios close to 3: 1 (height to half-pitch) demonstrate the limitations of our lithography system and highlight necessary improvements for higher aspect ratio resist structures to be achieved. Model and preliminary experimental results are presented for a scheme to mitigate resist collapse with very high aspect ratio structures. (C) 2014 American Vacuum Society.