Study on nano imprint lithography by the pre-exposure process (PEP)

Abstract

Numerous methods are available for lithography below the 100 nm node scale, including F2, 193 nm immersion, EB, EUV, and imprint lithography. Among these methods, imprint lithography has attracted significant attention because it does not require expensive exposure equipment. Imprint lithography can be performed by one of two primary methods: the thermal method or the UV curing method. In thermal imprinting, the resin is softened above Tg before being formed by a mold. In UV imprinting, a transparent mold is applied to a liquid resin, which is then exposed to UV light for curing. Thermal imprinting requires a pressure of 10 MPa and consumes throughput (to increase and reduce the temperature) time ["requires time for throughput (i.e., time required to increase and reduce temperatures)"]. In contrast, UV imprinting does not require high pressure, since the resin is basically a viscous liquid and soft enough to be deformed. However, since the resin is in liquid form, the UV imprinting process is sensitive to the flatness of the substrate and mold. Problems of non-uniformity (i.e., interference patterns) have been noted in residual film distribution. In response, we developed what we call the PEP method, which combines the advantages of both thermal and UV imprinting. We have performed various experiments to examine the consequences of the PEP approach. The Pre-Exposure Process method essentially consists of a type of UV imprinting, but one in which the resin is subject to extremely weak exposed prior to the pressing ["exposed to very weak UV radiation before pressing"], which slightly hardens the resist and increases rigidity. The mold is then pressed to shape the resin, followed by the primary exposure. This process allows the resin to maintain softness equivalent to that at or above Tg in thermal imprinting, while allowing processing, as in UV imprinting. We also examined the relationship between exposure and crosslinking ratios, using FT-IR equipment with an exposure function, to determine the optimal crosslinking ratio for the PEP method. The results of these examinations are also reported.