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Proceedings Paper

Modeling and experimental investigation of bubble entrainment for flow over topography during immersion lithography
Author(s): 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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Paper 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.

Paper Details

Date Published: 6 December 2004
PDF: 12 pages
Proc. SPIE 5567, 24th Annual BACUS Symposium on Photomask Technology, (6 December 2004); doi: 10.1117/12.569295
Show Author Affiliations
Holly B. Burnett, Univ. of Wisconsin/Madison (United States)
Alexander C. Wei, Univ. of Wisconsin/Madison (United States)
Mohamed S. El-Morsi, Univ. of Wisconsin/Madison (United States)
Timothy A. Shedd, Univ. of Wisconsin/Madison (United States)
Gregory F. Nellis, Univ. of Wisconsin/Madison (United States)
Benjamin T. Spike, Univ. of Wisconsin/Madison (United States)
Chris Van Peski, International SEMATECH (United States)
Andrew Grenville, International SEMATECH (United States)
Intel Corp. (United States)
Roxann L. Engelstad, Univ. of Wisconsin/Madison (United States)

Published in SPIE Proceedings Vol. 5567:
24th Annual BACUS Symposium on Photomask Technology
Wolfgang Staud; J. Tracy Weed, Editor(s)

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