Thermofluid simulations for immersion lithography

Immersion lithography has gone from a relatively unknown topic studied by only a handful of research labs and institutions into what many believe is the most promising technology for the manufacture of the next generations of microchips. The industry is actively pursuing the implementation of immersion lithography because it can simultaneously increase resolution and depth-of-focus. Investigation of a wide range of potential thermal-fluid issues that may be of concern to the industry as it moves forward with a full-scale implementation of immersion lithography were performed.;Initial filling of the gap was analyzed to understand the filling physics and identify fluid characteristics that encouraged faster fill times with reduced odds of bubble formation. Fluid induced stresses on the lens were also investigated to determine if induced shear and normal stresses would be significant enough to affect the integrity of the lens.;Since the fluid is now a lens element of the system, disturbances that can lead to unexpected changes in its optical characteristics were of concern. Heating of water can lead to index changes within the fluid. Models were developed to simulate the liquid heating due to exposure. Thermal profiles from these models were then input into optical models which predicted the optical aberrations that occur due to the increased temperatures.;Because immersion is concerned with increasing the index of the gap between the lens and the wafer, discontinuities, such as bubbles are undesirable. Concerns that flow over previously printed features would entrain air and cause imaging problems were addressed through models that analyzed flow over sharp-edged features, which represents a worst-case situation as well as the smoother features that are more likely to be found on wafers after they are coated with resist and polished.