Simulation of fluid flow in the step and flash imprint lithography process

Abstract

Step and flash imprint lithography (SFIL) is a photolithography process in which the photoresist is dispensed onto the wafer in its liquid monomer form and then imprinted and cured into a desired pattern instead of using traditional optical systems. The mask used in the SFIL process is a template of the desired features that is made using electron beam writing. Several variable sized drops of monomer are dispensed onto the wafer for imprinting. The base layer thickness at the end of the imprinting process is typically about 50 nm, with an approximate imprint area of 1 in 2. This disparate length scale allows simulation of the fluid movement between the template and wafer by solving governing equations of fluid mechanics simplified by lubrication theory. Capillary forces are also an important factor governing fluid movement; a dimensionless capillary number describes the relative importance of these forces to the viscous forces in the fluid. This paper presents a simulation to model the flow and coalescence of the multiple fluid drops and the effect the number of drops dispensed has on imprint time. The imprint time is shown to decrease with increasing numbers of drops or with an applied force on the template. Appropriate filling of features on the template is an important issue in SFIL, which is presented in this study by simulating the interface movement into and around the feature by a modified boundary condition on the governing equations. It is found that above a critical aspect ratio, features do not fill and fluid does not spread outside the mask edge. The simulation provides a predictive tool for understanding and optimizing fluid management in SFIL.