Nanoconfinement Effect on the Mechanical Behavior of Polymer Thin Films
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Nanoconfinement Effect on the Mechanical Behavior of Polymer Thin Films
J. Zhou and K. Komvopoulos Department of Mechanical Engineering, University of California, Berkeley, CA 94720
ABSTRACT Nanoimprint is one of the most promising fabrication techniques for nanoelectronics. The main feature of this process is the compression of a thin film of a polymer resist by a rigid mold to produce a surface pattern. Hence, nanoimprint is essentially a mechanical forming process that depends greatly on the nanoscale mechanical behavior of the plastically deformed polymer film. Consequently, basic understanding of nanoimprint mechanics is imperative for improving pattern quality, reproducibility, and automation. The objective of this study was to elucidate the mechanical response of thin polymer films subjected to nanoindentation loading. Three deformation regimes were identified in the experiments performed with poly(methyl methacrylate) (PMMA) films of thickness in the range of 200-400 nm with a Berkovich tip of nominal radius of curvature equal to 100 nm. A three-layer model consisting of surface, intermediate, and interface layers was introduced to explain the mechanical response of the indented PMMA films. The spatial constraints imposed to the plastic flow of the interface layer by the rigid indenter and substrate surfaces produce a dynamic effect, demonstrated by the loading rate dependence of the deformation response. This phenomenon is of great importance to polymer plastic flow in nanoimprinting. INTRODUCTION Nanoimprint lithography is a low cost, high throughput nanofabrication technique with a line width resolution less than 25 nm.1 In this process, a film of a thermoplastic polymer resist with typical thickness of 50-200 nm is spin coated on a substrate. A hard mold is then pressed into the heated resist material to produce a desirable nanopattern. After removing the mold, the pattern imprinted onto the polymer is transferred to the substrate by reactive ion etching. Despite the high-resolution and high-throughput capabilities of the nanoimprint process, large-scale fabrication of high quality, uniform nanopatterns remains a major challenge. These characteristics are of prime importance in nanoelectronics. Because the nanoimprint process is essentially a forming process, which is controlled by the nanomechanical behavior of the compressed polymer film, basic understanding of the plastic flow behavior of the polymer film is imperative. Deformation of a polymer film compressed by a hard mold (Fig. 1a) results in pileup formation at the edges of the pattern produced (Fig. 1b). The increase of the pressure and the temperature in the polymer film enhance coalescence of the pile-up regions and filling of the mold cavities by polymer (Fig. 1c). Upon the retraction of the mold, the polymer recovers elastically, hence affecting the final dimensions of the imprinted nanopattern (Fig. 1d). The uniformity of the imprinted pattern depends on the flow of the polymer in the mold cavities and the elastic recovery at the
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