Surface Processing and Micromachining of Polyimide Driven by a High Average Power Infra-Red Free Electron Laser
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SURFACE PROCESSING AND MICROMACHINING OF POLYIMIDE DRIVEN BY A HIGH AVERAGE POWER INFRA-RED FREE ELECTRON LASER MICHAEL J. KELLEY, Dept. of Applied Science, The College of William and Mary, Williamsburg VA 23187-7895, and Thomas Jefferson National Accelerator Facility, Applied Research Center, 12050 Jefferson Avenue, Newport News, VA 23606. ABSTRACT The long history and wide use of polyimide as a dielectric in the microelectronics industry has made it a favorite material for laser processing studies. The FEL used in the present work delivered picosecond-long 25 microjoule pulses at approximately 3.10 and 5.80 microns. The former is not associated with any strong absorption, while the latter is the strongest absorption band in the IR. This study explored hole drilling and surface transformation of as-made and aluminized DuPont Kapton* HN PMDA-ODA polyimde film. INTRODUCTION The opportunity for micromachining attracted the attention of the earliest investigators of laser ablation of polymers [1]. The need to cut small through-holes for vias in in printed wiring boards and for ink jet printer orifices focussed special attention on polyimide films. The research, development and deployment into production has been reviewed recently [2]. Polyimide's intense absorption throughout the UV [3] led to domination by excimer lasers. The discussion of the relative contributions of "true photochemistry" and thermal chemistry began at once [4]. The electronic structure of polyimides [5] is such that prospects are dim for exclusion of photochemical effects when UV irradiation is used. The success in manufacturing of ablative micromachining certainly questions the practical importance of the issue. Pulsed laser deposition (PLD) also depends on ablation, making use as it does of the ejected material to form a film. Though inorganic materials continue to attract the vast majority of research interest, some attention has always been paid to polymers. Interestingly, reports on
polymers having carbonyl groups (e.g, polyimides, polyamides, polyesters) are surprisingly sparse, despite their many important applications. It can be asked whether photochemistry leading to unwanted transformations (for PLD) is at least partly responsible. Previous results with polyamides [6] and polyimides [7] indicate that irradiation at strongly absorbed UV wavelengths results in elimination of the carbonyl leading to formation of polyamines in the first case and a carbonaceous network in the second. A potential ambiguity for polyimides is that purely thermal mechanisms, given sufficient time, can lead to a broadly similar result [8]. Polyimides thus are an appealing context in which to explore whether a suitable IR laser may both drive sufficiently rapid thermal processing to avoid pyrolysis on the one hand and avoid unwanted UV-driven photochemistry on the other. Absorption by polyimides in the infra-red is less intense than in the UV, but still significant. The strongest band of the familiar Kapton* Hseries PMDA-ODA polyimide films appears at about 5.80 mi
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