Modeling and experimental investigation of bubble entrainment for flow over topography during immersion lithography

Holly B. Burnett; Alexander C. Wei; Mohamed S. El-Morsi; Timothy A. Shedd; Gregory F. Nellis; Benjamin T. Spike; Chris Van Peski; Andrew Grenville; Roxann L. Engelstad

doi:10.1117/12.569295

6 December 2004 Modeling and experimental investigation of bubble entrainment for flow over topography during immersion lithography

Holly B. Burnett, Alexander C. Wei, Mohamed S. El-Morsi, Timothy A. Shedd, Gregory F. Nellis, Benjamin T. Spike, Chris Van Peski, Andrew Grenville, Roxann L. Engelstad

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Proceedings Volume 5567, 24th Annual BACUS Symposium on Photomask Technology; (2004) https://doi.org/10.1117/12.569295
Event: Photomask Technology, 2004, Monterey, California, United States

Abstract

In immersion lithography, the air gap that currently exists between the last lens element of the exposure system and the wafer is filled with a liquid that more closely matches the refractive index of the lens. There is a possibility that air bubbles, which represent a refractive index discontinuity, may be present in the liquid within the active exposure region and cause errors in imaging. One potential source of bubble generation is related to the flow of liquid over previously patterned features, or topography, during scanning or filling. This microscale entrainment mechanism is investigated experimentally and analyzed using computational fluid dynamics (CFD) modeling. The contact angle is a critical parameter that governs the behavior of the contact line and therefore the entrainment of air due to topography; the same topography on a hydrophobic surface is more likely to trap air than on a hydrophilic one. The contact angle can be a strong function of the flow velocity; a hydrophilic surface can exhibit hydrophobic behavior when the velocity of the free surface becomes large. Therefore, the contact angle was experimentally measured under static and dynamic conditions for a number of different surfaces, including resist-coated wafers. Finally, the flow of liquid across 500-nm deep, straight-sidewall spaces of varying width was examined using both experimental visualization and CFD modeling. No air entrainment was observed or predicted over the velocity and contact angle conditions that are relevant to immersion lithography. The sharp-edged features studies here represent an extreme topography relative to the smoother features that are expected on a planarized wafer; therefore, it is not likely that the microscale entrainment of bubbles due to flow over wafer-level topography will be a serious problem in immersion lithography systems.

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Holly B. Burnett, Alexander C. Wei, Mohamed S. El-Morsi, Timothy A. Shedd, Gregory F. Nellis, Benjamin T. Spike, Chris Van Peski, Andrew Grenville, and Roxann L. Engelstad "Modeling and experimental investigation of bubble entrainment for flow over topography during immersion lithography", Proc. SPIE 5567, 24th Annual BACUS Symposium on Photomask Technology, (6 December 2004); https://doi.org/10.1117/12.569295

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