Computational Study on Polymer Filling Process in Nanoimprint Lithography for Bi-Layered Resist
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Computational Study on Polymer Filling Process in Nanoimprint Lithography for Bi-Layered Resist Kosei Araki, Masaaki Yasuda, Akira Horiba, Hiroaki Kawata and Yoshihiko Hirai Osaka Prefecture University, Sakai, Osaka 599-8531, Japan ABSTRACT A molecular dynamics simulation was performed to study the polymer filling process in nanoimprint lithography for a bi-layered resist. The bi-layered resist consisted of PMMA resins with different molecular weights. When the mold cavity size became smaller than the polymer size of the top layer resist, the required force to fill the cavity became large. The molecular weight of the top layer dominated the filling characteristics in the bi-layered resist process. INTRODUCTION Nanoimprint lithography (NIL) [1-3] is an important technology for fabricating nanostructures with high throughput and low cost. Single-nanometer resolution has been experimentally demonstrated in polymer NIL [4,5]. In the recent development of NIL, understanding polymer behavior on the atomic scale has taken on a growing importance. Molecular dynamics (MD) simulation became a powerful method for this subject [6,7]. In our previous studies, we performed MD simulation to analyze the polymer filling process in NIL [8,9]. We found that the force required to fill the mold cavity increased sharply, when the cavity size became less than the molecular size of the resist polymer [8]. We also found that the filling behavior of polymer molecules depended greatly on the initial orientation of the molecule and the mold pattern geometry [9]. In the present paper, we use MD simulation to study the polymer filling process for a bilayered resist [10]. We discuss the filling behavior of the bi-layered resist, which consists of polymers with high and low molecular weights (Mw), in terms of the relation between the molecular size of the polymer and the mold cavity size. SIMULATION Poly(methyl methacrylate) (PMMA) resins with two different Mw were selected as polymer materials to compose a bi-layered resist. To save calculation costs, a methyl and methylene groups were assumed to be unit giant atoms in the molecular model. In a previous study, we described the details of the PMMA resist model [8]. In MD simulation, the force field proposed by Okada et al. [11] was adopted for the PMMA polymer. It consisted of bond stretching, angular bending, torsion potentials, and nonbonding interaction including LennardJones and Coulomb potentials. On the other hand, the Lennard-Jones potential was adopted between the PMMA and the Si atoms. To save the calculation time, the mold and the substrate were assumed to be a rigid body. Figure 1 shows the configurations of the present simulation for bi-layered resists. It consisted of a Si mold, a PMMA bi-layered resist, and a Si substrate. In one case, a layer of high Mw (5000) PMMA resist was stacked on a layer of low Mw (500) resist, as shown in Fig. 1 (a).
In another case, the Mw of the two layers was reversed, as shown in Fig. 1 (b). Here, we refer to the bi-layered resists shown in Fig. 1 (a)
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