Density variation of nanoscale patterns in thermal nanoimprint lithography was studied both by experiments and molecular dynamics simulations. A simple soft imprinting technique was used to fabricate various nanopatterns ͑70 nm and 600 nm lines and 150 nm dots͒ over a large area ͑2 ϫ 3 cm 2 ͒. Local density was measured by the relative magnitude of van der Waals interactions between a sharp tip and the patterned surface. In order to investigate the mechanism of density variation, molecular dynamic simulations were performed. Experimental and simulation results demonstrated that the density of the pressed region ͑valleys͒ was higher than that of the cavity region ͑hills͒ when a simple amorphous polymer is thermally imprinted with a patterned mold.

Molecular dynamic simulation of nano imprint lithography (NIL) in which nanoscale patterned stamp is pressed onto amorphous polyethylene (PE) surface is performed to study the deformation behavior of polymer resist. Force fields including bond, angle, torsion, and Lennard Jones potential are used to describe the intermolecular and intra-molecular forces of PE molecules and stamp. Periodic boundary condition is used in horizontal direction and canonical NVT ensemble is employed to control the system temperature. Using the simulation results, the behavior of polymer resist is investigated during the imprinting process. The mechanism of resist deformation is analyzed by considering various parameters including the surface geometry, atom distribution, and density. Especially, the density in the bottom (emboss) region is found to be larger than the top (cavity) region due to compression of polymer molecules. The result indicates that small scale patterning of polymer resist largely depends on compression rather than the flow of molecules. The numerical results are compared with the local density measurement data using the atomic force microscope (AFM). They exhibit a similar behavior at least in a qualitative sense. Additional simulations, where the stamp geometry and molecular mobility are varied and are performed to investigate the deformation characteristics of polymer resist. From these simulations, important elements for understanding of NIL process are obtained.

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