CFD analysis of the receding meniscus in immersion lithography

In an immersion lithography tool, a high refractive index liquid is introduced into the space between the last projection lens of the system and the wafer. The additional liquid increases the system's numerical aperture, thereby decreasing the theoretical limit of resolution. In order to achieve the levels of throughput that are demanded by the semiconductor industry, the wafer will be subjected to high velocities and accelerations which present challenges to the fluid management system. As the wafer velocity increases, the dynamic receding contact angle is reduced. At high velocities inertial forces can overcome surface tension forces that hold the fluid. If this occurs, the contact angle approaches zero and a very thin film of liquid is "pulled" from the receding meniscus, which is not desirable. A two-dimensional (2-D) computational fluid dynamics model has been developed to investigate the behavior of the receding meniscus under different operating conditions. The receding dynamic contact angle and film pulling velocity predicted by the model are compared with the same quantities measured experimentally. It is shown that a 2-D model provides predictions that are qualitatively accurate and therefore useful in the evaluation of alternative fluid management techniques. A parametric study of the effect of static receding contact angle and external pressurization on the film pulling velocity is described, as these quantities represent two design parameters that are currently being considered for immersion tool fluid management.