Drag-a-drop: a characterization tool for immersion lithography Derek W. Bassett a* , J. Chris Taylor a , Will Conley b , C. Grant Willson a & Roger T. Bonnecaze a** a Department of Chemical Engineering, The University of Texas at Austin - Austin, TX 78712 b Freescale Semiconductor, Inc. - Austin, TX 78729 ABSTRACT In order to quickly and cheaply test candidate fluids and coatings for immersion lithography, we have devised a fluid handling scheme that we call drag-a-drop. We have constructed a prototype tool in order to test materials using this fluid scheme, and conducted several experiments with it. From these tests, we have determined that a hydrophobic topcoat with low contact angle hysteresis on the substrate increases the maximum stable scanning velocity by at least a factor of 2 over a standard 193 nm photoresist. We observed that instabilities on the receding contact line are unaffected by height, but the onset of instability on the advancing contact line occurs when the height of the lens is low. We also examined the drag-a-drop technique for possible use in laser mask writing, and found that by means of a hydrophobic topcoat, the lens can be completely removed from the substrate while keeping the immersion droplet affixed to the lens. Keywords: immersion lithography, fluid management, contact angle, stability limits, laser mask writing 1. INTRODUCTION Immersion lithography has become one of the primary methods of maintaining current trends for feature size reduction in microprocessor fabrication. As happened before in the transitions from G-line to I-line, I-line to 248 nm, and 248 nm to 193 nm, the change in scanning technology also necessitated a change in photoresist and topcoat systems. The current change from dry 193 nm to immersion 193 nm is no different. As such there is a need for suitable resists and topcoats that are compatible with the immersion scanning process. Although there are many different factors that need to be tested and accounted for in the immersion scanning process, such as immersion fluid purity, leaching from the substrate, deposition on the lens, etc., one of the factors that is currently being correlated with scanning defects is the stability of the immersion fluid. On the advancing edge, micro- bubbles can be entrained into the immersion droplet which can cause scanning defects. More importantly, it has been shown that instabilities of the receding edge that cause micro-droplets to be left behind on the substrate are responsible for a large percentage of scanning defects. 1 Because of this it is important to be able to test possible immersion fluids, resists, topcoats, and coatings for the lens and lens housing in an environment that closely simulates the scanner in order to determine if they are suitable before testing them on the scanner itself. By using such a test apparatus, one could determine the maximum scanning velocity that would keep the contact lines stable before scanning, and thus greatly minimize scanning defects. Because current immersion systems are still in the prototype and testing phase, the design details and implementation of the “showerhead” fluid handling system is proprietary. However, with a simple fluid handling scheme, it is possible to understand the basic fluid mechanics that control the stability of the system, and there would be no danger of compromising any company’s copyrighted information. To do this, we have developed such a fluid handling scheme that we call drag-a-drop. * derek@che.utexas.edu; phone 1-512-471-7393. ** rtb@che.utexas.edu; phone 1-512-471-1497; fax 1-512-471-7060.