Threshold for Single Excimer Laser Pulse Backside Removal of Thin Metal Films from Optical Quartz
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THRESHOLD FOR SINGLE EXCIMER LASER PULSE BACKSIDE REMOVAL OF THIN METAL FILMS FROM OPTICAL QUARTZ
R.J. Baseman, J.C. Andreshak IBM T.J. Watson Research Center, Yorktown Heights, NY 10598 ABSTRACT The minimum energy in a 248 nm, 25 ns long excimer laser pulse required to remove thin Au and Cr films from optical quartz has been measured. Heating of the films by the laser has been modelled with a finite element calculation. Assuming that at threshold, all of the laser energy contributes to film removal, the calculations show that the gold films are removed when the heated gold surface reaches the atmospheric boiling point, and that temperatures well in excess of the atmospheric boiling point are required to remove the Cr films, with the required temperatures increasing with film thickness. INTRODUCTION The laser induced forward transfer technique of thin film deposition [1] and the laser blow off trace element injector [2] employ pulsed lasers to remove material, previously coated as a thin film on a nominally transparent support; from that support. The precoated film is irradiated through the transparent support (backside irradiation) and is removed with a single laser pulse. For trace element injection into tokomak accelerators, the removal process ideally results in short (100 usec), intense (10 1 /cm2 ) bursts of neutral metal atoms. These injected atoms can subsequently be monitored to characterize both transport and plasma properties in tokomaks [3]. As a thin film deposition process, the removal process results in the rapid, efficient transfer of the precoated material to a target substrate held in close proximity or in contact to the original film. Ideally, the material transfer results in the deposition of a thin film, similar to the original film, on the target substrate. It appears [4] that desirable film properties are more readily produced with laser fluences significantly lower (.5 to 10 J/cm2 compared to up to 100 J/cm2 ) than those commonly used for blow off applications. While technological applications are clear, the detailed mechanism of the film removal process is incompletely understood. Several mechanisms can be envisioned, including one suggested recently [51 (within the film deposition context) wherein the film is heated until the vapor pressure at the film-support interface becomes large enough to expel the (largely molten) film from the support. Within the context of the trace element injector, there have been several studies [6] of the pulsed beams formed by the blow off process, revealing that the blow off produces a plasma ball, including large numbers of neutral ground and excited state atoms, high densities of free electrons and ions, and in many cases, microscopic (as opposed to atomistic) clusters. However, it is difficult to extract detailed information about the removal process, especially at the lower laser fluences appropriate for film transfer, from these diagnostics. Here, we describe a probe of the film removal mechanism, measurements of the minimum energy required to remove th
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