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

[+] Author Affiliations
Holly B. Burnett, Alex C. Wei, Mohamed S. El-Morsi, Timothy A. Shedd, Gregory F. Nellis

University of Wisconsin, Computational Mechanics Center, 1513 University Avenue, Madison, Wisconsin 53706

Chris Van Peski

International SEMATECH, 2706 Montopolis Drive, Austin, Texas 78741

Andrew Grenville

International SEMATECH, 2706 Montopolis Drive, Austin, Texas 78741 and Intel Corporation, 2200 Mission College Boulevard, Santa Clara, California 95052

J. Micro/Nanolith. MEMS MOEMS. 5(1), 013008 (January 24, 2006). doi:10.1117/1.2158818
History: Received April 26, 2005; Revised August 11, 2005; Accepted August 21, 2005; Published January 24, 2006
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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 studied 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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© 2006 Society of Photo-Optical Instrumentation Engineers

Citation

Holly B. Burnett ; Alex C. Wei ; Mohamed S. El-Morsi ; Timothy A. Shedd ; Gregory F. Nellis, et al.
"Modeling and experimental investigation of bubble entrapment for flow over topography during immersion lithography", J. Micro/Nanolith. MEMS MOEMS. 5(1), 013008 (January 24, 2006). ; http://dx.doi.org/10.1117/1.2158818


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