Optimizing the fluid dispensing process for immersion lithography

The concept behind immersion lithography is the insertion of a high refractive index liquid in the space between the final projection lens of an exposure system and the device wafer to improve the overall resolution of the exposure process. Computational fluid dynamics (CFD) simulations were performed in order to investigate the process of initially filling the lens-wafer gap with immersion fluid. The CFD models were used to investigate the effects of dispense velocity, gap height, and fluid dispense angle on the fill process; specifically on the possibility of air entrainment. The simulations revealed that there is an optimal region in the parameter space of gap height and dispense velocity for which the gap fills completely. Outside of this region, either excessive inertial or surface tension forces cause an undesirable, incomplete filling process. The optimal region was found to shift somewhat based on the fluid dispense angle. Finally, experiments were performed to verify the CFD models. The CFD simulations and the experimental results were in good agreement, both qualitatively with regard to the shape and evolution of the free surface and quantitatively with regard to the velocity of the contact line. © 2004 American Vacuum Society. [DOI: 10.1116/1.1824065]