Simulation of the velocity distribution for immersion lithography

Immersion lithography has been accepted as a method for improving optical lithography resolution to 45 nm, and allows improved resolution without a large shift in infrastructure. The premise behind the concept is to increase the refraction index in the space between the lens and resist-coated wafer by insertion of a high refractive index liquid in place of the low refractive index air that currently fills the gap, and the liquid must maintain a high uniform optical quality. However, the immersion liquid within the gap has to be updated as substances such as photo acid generators (PAGs) and bubbles, may affect the optical quality of the liquid. The efficiency of the contaminant elimination mainly depends on flow conditions, especially velocity distribution of flow field under lens region. Three-dimensional (3-D) computational fluid dynamics (CFD) models of the fluid between the lens and wafer are developed using the software Fluent and used to investigate the velocity distribution between the lens and wafer, considering fluid injection pressure, dispense ports quantity and wafer scanning velocities. Moreover, the numerical results are compared with experimental results.