In the realization of small devices which range from microscale to the nanoscale, the fabrication of nanostructures is one of the most important problems. Nanoimprint lithography (NIL), which was first proposed by Chou et al, is one of the most promising techniques for the fabrication of nanostructure.
In this study, a molecular dynamics simulations of nanoimprint lithography (NIL) and related adhesion phenomena are performed in order to investigate various phenomena in NIL process. At first, a molecular dynamics simulation model, similar to a real NIL process imprinting a quartz stamp with a rectangular stamp pattern into an amorphous poly-(methylmethacrylate) (PMMA) film, is proposed and, by using this model, NIL process is analyzed. The polymer deformation behavior and the adhesion and friction effects between the stamp and the polymer film are investigated and their dependency on the pattern aspect ratio is discussed. The deformation of the polymer film is observed for two stamps having different aspect ratio patterns. The distributions of density and stress in the polymer film are calculated for the detail analysis of deformation behavior. For a high aspect ratio pattern (aspect ratio = 2.5, imprint depth = 8.0nm), large amount of springback of the residual polymer film is observed, which is mainly due to the residual compressive stress left in the polymer film. However, for a low aspect ratio pattern (aspect ratio = 1.0, imprint depth = 3.0nm), the springback is not observed. In addition, adhesion and friction forces are obtained by dividing the polymer film into subregions and calculating the interacting force between each subregion and the stamp. While the adhesion force is nearly constant regardless of the pattern aspect ratio, the friction force increases as the pattern aspect ratio grows, so the friction force becomes larger than the adhesion force when the pattern aspect ratio increases.
Secondly, molecular dynamics simulations of NIL are performed in order to investigate effects of three critical process parameters in NIL: stamp shape, imprinting temperature and adhesive energy. The behavior of polymer deformation and effects of adhesion on the pattern transfer are investigated by observing deformation process, calculating imprint and separation forces, and analyzing density and stress distribution inside the polymer film. In addition, their dependency on the process parameters is also discussed by changing pattern shape, adhesive energy between the stamp and polymer atoms, and imprint temperature of the polymer film. During the imprint process, the rectangular pattern shows inferior cavity filling and higher stress concentration compared to trapezoidal and triangular patterns, because it requires much larger flow and deformation of polymer film. Low imprint temperature also produces high stress concentration and large imprint force due to the inferior fluidity of polymer film. In the separation process, rectangular pattern generates the largest separation force and causes the most serious defects of the transferred pattern and even the polymer film, while triangular pattern enables the most satisfactory pattern transfer. Additionally, adhesive energy between the stamp and the polymer film also greatly influences the adhesion between the stamp and the polymer film. Low adhesive energy reduces the separation force and defects of transferred pattern and, therefore, enhances the quality of pattern transfer.
Finally, adhesion characteristics between materials for NIL process, such as poly(methylmethacrylate) (PMMA), $SiO_2$, alkylsilane and fluoroalkylsilane self-assembled monolayers (SAMs), are investigated and their surface energies are estimated by using molecular dynamics simulations. The simulation models consist of PMMA film and amorphous $SiO_2$, $\alpha$-quartz, or three kinds of alkylsilane SAMs having different chains length and end-group on both $SiO_2$ substrates. A molecular dynamics model is also proposed in order to estimate the surface energy and work of adhesion of these materials. The calculated surface energy and work of adhesion are compared with various experimental results, some of which are obtained from current experiments on contact angle measurement and other previous researches. The BKS potential and OPLS force field are adopted in order to describe interacting forces between PMMA, $SiO_2$, and SAMs. In addition to these force fields, torsion potential parameters for dihedral angles of Si atom in alkylsilane head group are newly obtained based on ab initio calculations and applied to current simulations. Simulation results show that SAMs are effective to reduce adhesion force and prevent surface damages. In case of bare $SiO_2$, a PMMA film is damaged and even torn by adhesion, while all SAMs do not leaves any damages in the PMMA film. Adhesion force and energy between PMMA and SAMs are also reduced compared to those between PMMA and bare $SiO_2$. Maximum adhesion force is hardly related to the separation velocity, but force-displacement curve shape is highly related with the separation velocity because of relaxation time needed for rearrangement of SAM molecules. It is also found that SAM surface density has an influence on adhesion force and force-displacement curve shape because it affects the real contact areas and SAM stiffness. As SAM density increases, adhesion force tend to increase and force-displacement curve shows wider distribution. The estimated surface energy and work of adhesion show good agreement with current experimental results and other previous results. From these, it is shown that molecular dynamics methods can be utilized for the estimation of surface energy and work of adhesion.
최근 나노기술(nanotechnology)의 발달로 수십~수백 nm의 크기를 갖는 나노구조물이나 나노패턴을 대량으로 제조할 수 있는 기술이 요구되고 있다. 이러한 기술 중 최근 가장 주목 받는 기술로서 나노임프린트 리소그래피를 들 수 있다. 나노임프린트 리소그래피는 수십~수백nm 크기의 패턴이 각인된 스탬프를 폴리머 등의 비교적 변형이 쉬운 재질의 박막 표면에 눌러 박막을 소성 변형시킴으로써 패턴을 전사하는 방법이다.
첫 번째로 본 연구에서는 나노임프린트 리소그래피 공정에 대해 분자동역학 해석을 수행하였다. 본 연구에서 수행된 나노임프린트 리소그래피 공정에 대한 분자 동역학 시뮬레이션을 통해 공정 중에 일어나는 폴리머 박막의 변형 양상을 관찰하였고 박막 내부의 응력 분포 및 밀도 분포를 계산하였다. 또한 스탬프와 폴리머 재료 사이에서 발생하는 점착력과 마찰력을 계산하고 이들의 영향 및 특성에 관해 살펴보았다.
두 번째로 본 연구에서는 나노임프린트 공정에 대해 공정인자인 스탬프 형상, 표면 사이의 점착에너지 크기, 온도를 달리하여 분자동역학 해석을 수행하여 이들의 영향에 대해 연구하였다. 해석모델의 공정인자를 달리하며 폴리머 박막의 변형 양상을 관찰하고 박막 내부의 응력 분포 및 분리 중의 점착력을 계산하여 공정인자들이 박막의 변형 특성과 패턴 전사에 미치는 영향에 대해 알아보았다.
마지막으로, 나노임프린트용 소재인 PMMA 박막과 $SiO_2$ 및 SAM이 입혀진 $SiO_2$ 사이의 점착 거동을 알아보기 위해 분자동역학 해석을 수행하였다. 이를 위해 우선 SAM 분자의 헤드부 Si원자를 중심으로 하는 뒤틀림 포텐셜 함수를 양자역학 계산결과에 맞도록 피팅시켜 SAM의 구조 계산을 위한 보다 정확한 포텐셜 파라미터를 제시하였다. 제시된 파라미터를 적용한 분자동역학 해석을 통해 PMMA와 $SiO_2$ 및 SAM이 입혀진 $SiO_2$ 사이의 점착력을 계산하고 점착 현상을 관찰하여 SAM이 점착 특성에 미치는 영향을 알아보았으며, SAM의 종류 및 길이를 다르게 하여 해석함으로써 SAM의 종류가 점착 특성에 미치는 영향에 대하여 연구하였다. 또한 이들의 표면에너지 및 계면에너지를 예측하는 방법을 제안하고 이를 이용하여 표면 및 계면 에너지를 계산하였다. 그리고 이렇게 계산된 표면에너지 및 계면에너지 값을 접촉각 측정 및 점착력 측정 실험을 통해 계산한 값들과 비교 및 평가하였다.
