Inspection of templates for imprint lithography

Abstract

Masks of any next generation lithography (NGL), such as imprint lithography, must eventually achieve and maintain the very low defect counts of current production masks. This requires typically fewer than 10 or even no defects over the entire field. We describe an inspection methodology and how it can be applied to the imprint template. Special test patterns etched onto the template enable both a die to die comparison, to find nuisance defects, and also calibration of sensitivity to different types of preprogrammed defects. A state of the art deep ultraviolet photomask inspection system (KLA-Tencor model 526) can detect these rare events with about 70nm threshold for imprint masks with reflection mode contrast. Initial scans are made at various stages of the imprint process: the newly processed mask, after dicing, and after several imprints. The scans show mostly isolated point defects at a density of ∼10–100permm2. This is an encouraging start for a new NGL, and reductions are expected from better processes, equipment, and handling. In the future viable mask inspection at the 45–22nm node will be very demanding for these 1× masks and will require sensitivities approaching 10–20nm defect size. To this end, special masks suitable for electron (e)-beam inspection are being made and show good contrast and immunity to e-beam charging.