Thermocapillary Effects in Laser Direct-Write Metallization
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THERMOCAPILLARY EFFECTS IN LASER DIRECT-WRITE METALLIZATION Peter E. Price Jr.*, Michael Stuke**, and Klavs F. Jensen"'* Department of Chemical Engineering & Materials Science, University of Minnesota, Minneapolis, MN 55455 Max-Planck-Institut fUr biophysikalische Chemie, Dept. Laserphysik, P. 0. 2841, D-3400 Gottingen, F. R. Germany Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139 ABSTRACT A model describing melting and thermocapillary (or Marangoni) flow in laser irradiated thin films is presented. The free surface formulation leads to predictions of surface profiles that are in qualitative agreement with experimental observations. Additional experimental results for directwriting of palladium from palladium acetate films demonstrate the influence of thermocapillary effects on the dynamics of the process. INTRODUCTION In one of the earliest papers on laser direct-writing from thin films, Fisanick et al. [1] observed that melting occured prior to decomposition and suggested that surface tension effects could play an important role in the process dynamics. For the organo-gold screen inks they were studying, however, the interesting dynamics (i. e., the development of periodic structures) appear to be the result of the strongly exothermic decomposition of the film [ 1,21. More recently, studies of laser direct-writing of palladium from palladium acetate films [3,4] indicate that the nonlinear changes in the optical properties of the film as it decomposes can lead not only to periodic structures, but also to multiple steady states. Our efforts to understand these periodic structures has led us to consider the case of a scanning laser slit, rather than the usual scanning beam. This has allowed us to derive a one dimensional model [2,4] that appears to capture much of the experimentally observed behavior. A schematic diagram of the scanning slit process is shown in Figure 1. By focusing the laser through a cylindrical lense, we have carried out experiments in the model geometry. During deposition of palladium from palladium acetate films using a 514 nm Ar' laser slit, we have observed the development of a molten region ahead of the scanning slit. The molten film can be up to several times the original thickness of the film. Figure 2 shows the thickness profile of a film in the direction perpendicular to the laser slit. The profile was obtained with a Dektak profilometer after the laser was switched off mid-scan, and the film allowed to solidify. The existence of this thicker molten region appears to have a significant impact on the development of periodic structures in the palladium acetate system. The thicker film absorbs more of the laser light. Under conditions where periodic structures were obtained, the reaction was found to ignite when the laser slit scanned into this thicker region. Thus, we are motivated to examine more closely the effects of melting and flow in laser heated films. THE LASER MELTING MODEL Laser direct-writing from thin films is an extremely complex
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