Thermal analysis for step and flash imprint lithography during UV curing process Eui Kyoon Kim, C. Grant Willson * Department of Chemical Engineering, The University of Texas at Austin, Austin, TX 78712, United States Received 17 August 2005; accepted 17 August 2005 Available online 26 September 2005 Abstract Acrylate and vinyl ether monomers have previously been shown to be effective photocurable materials for step and flash imprint lithography. The photo polymerization of these monomer systems is exothermic. If the imprint process was adiabatic, the heat generated during polymerization could increase the temperature of the material to greater than 300 °C, possibly resulting in material degradation and image distortion. A finite element method was used to analyze the temperature profile during photopolymerization for non-adiabatic conditions. The heat from UV absorption is negligible because acrylate and vinyl ether imprint materials are transparent in the UV lamp exposure region and the loading of photo acid generator and photo radical initiator is low. This model indicates that the temperature increase from polymerization is very small (less than 0.05 °C) due to the rapid heat transfer from the curing material to the silicon wafer. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Step and flash imprint lithography; Thermal analysis; Finite element model; Acrylate and vinyl ether 1. Introduction Imprint lithography has gained much interest recently in the semiconductor industry because it can generate sub-100 nm features at a manageable cost [1–3]. The process can be accomplished via two different approaches. In one method called thermal imprint lithography, a thermoplastic mate- rial is imprinted above its glass transition temperature. Chou et al. [4] demonstrated printing of 10 nm size features using thermal imprint lithography. Step and flash imprint lithography (SFIL) is distin- guished from thermal imprint by the use of a low viscosity UV curable monomer during the imprint step [2]. High res- olution patterns, as small as 20 nm, can be generated by SFIL using a low imprint pressure at room temperature [5]. An acrylate based formulation is commonly used as the patterning material due to its high radical polymeriza- tion reactivity. Unfortunately, oxygen inhibits the radical polymerization, resulting in an inhibition period and an under-cured region on the periphery of the imprint due to oxygen diffusion [6]. A simple way to overcome oxygen inhibition is to implement a polymerization mechanism that is insensitive to oxygen. For example, vinyl ethers can be cured by the oxygen insensitive cationic mechanism [6]. The viscosity of vinyl ethers is very low, and the tensile strength of a cured vinyl ether formulation is higher than that of an acrylate formulation of similar viscosity [6]. In addition, certain vinyl ethers cure faster than the acrylate based formulation [6]. Highly reactive imprint materials, such as acrylates and vinyl ethers, are chosen for SFIL to enhance process throughput [6,7]. Since the polymerization of these sys- tems is exothermic, there is a possibility that the imprint material could decompose due to the rapid heat genera- tion from the reaction [7]. A large temperature increase in imprinted materials would result in several problems, including: (1) safety concerns associated with rapid heat generation; (2) thermal decomposition that can weaken 0167-9317/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2005.08.007 * Corresponding author. Tel.: +1 512 471 4342; fax: +1 512 417 7222. E-mail addresses: eui@che.utexas.edu (E.K. Kim), willson@che.utexas. edu (C.G. Willson). www.elsevier.com/locate/mee Microelectronic Engineering 83 (2006) 213–217