Effect of liquid dispensing on flow field for immersion lithography

Abstract

Immersion lithography has been proposed as a method for improving optical lithography resolution to 50nm and below. The premise behind the concept is to increase the refraction index in the space between the lens and the wafer by insertion of a high refractive index liquid in place of the low refractive index air that currently fills the gap. During the scanning and exposure process, immersion liquid is injected into the space between the wafer and the lens with certain inlet pressure and angle. Because the liquid will act as a lens component during the lithographic process, it must maintain high uniform optical quality. One source of optical degradation may be due to liquid contamination by chemical products or impurities during exposure period. Immersion liquid renovation is probably the best solution. Refreshing cycle time and flow field velocity distribution were investigated by experimental and simulation methods. An empirical formula of velocity distribution was proposed in this article. The other source of optical degradation may be due to lens distortion caused by the pressure distribution nonuniformity in the flow field of the immersion liquid. Consequently, any deviations of pressure distribution on the flow field boundary in direct contact with lens may damage the uniform optical path. Three-dimensional computational fluid dynamics models were built to assess the pressure distribution characteristics relevant to flow rates and injecting angles of the immersion liquid. Compared with experimental results, the numerical simulation results were presented and discussed, featuring flow field stream patterns, lens normal and shear pressure, and injection flow, considering fluid injecting velocity, dispense ports quantity, and direction angles.