Full wafer simulation of immersion fluid heating

• Published in: Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures 2005-11, Vol.23 (6), p.2596-2600
• Main Authors: El-Morsi, M., Nellis, G., Schuetter, S., Van Peski, C., Grenville, A.
• Format: Article
• Language: English
• ISSN: 0734-211X; 1071-1023; 1520-8567; 2327-9877
• DOI: 10.1116/1.2091091

In immersion lithography, fluid is introduced into the region between the lens and wafer. The benefit of the immersion fluid is that it has an index of refraction greater than air, which leads to a higher numerical aperture and improved depth of focus. Since the liquid becomes an optical element in the imaging system, it must maintain a uniform optical quality. However, the liquid’s index of refraction can depend on its temperature, and the temperature of the fluid will rise due to the exposure energy. This article describes a detailed two-dimensional computational fluid dynamics model of the hydrodynamic and thermal aspects of fluid flow. The model focuses on the successive scanning processes that occur as multiple chips are exposed on a wafer and accounts for the resulting oscillatory wafer motion and the thermal loads related to absorption of the incident energy in the fluid, indirect heating of the fluid due to its contact with the wafer as well as viscous dissipation. The global storage of energy in the fluid and the wafer as multiple chips are exposed is considered. The effect of the wafer acceleration, lens-to-wafer spacing, and the thermo-optic coefficient of the immersion fluid are studied parametrically and the impact of these quantities on the temperature distribution is described.