Ambient Gas Effects on Debris Formed During KrF Laser Ablation of Polyimide
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AMBIENT GAS EFFECTS ON DEBRIS FORMED I)URING KrF LASER ABLATION OF POLYIMIDE
Stephan Kiuper* .ud James Brannon IBM Alnaden Research Center, 650 Harry Rd., San Jose, CA 95120 *Present Address: Hoechst AG. Postfach 3540. D-6200 Wiesbaden, Gennmany
ABSTRACT The surface debris that results from KrF excimer laser ablation of polyimnide has been investigated as a function of (lie pressure. and atomic or molecular weight of several ambient gases: H, He. Ne, air, Ar, Kr, and Xe. A linear relation between the measured debris radius and the inverse third root of the ambient pressure was found to exist, consistent with the predictions of blast wave theory. No mneasureable debris could be observed using helium or hydrogen gases up to I attn, similar to previous reports on helium. The derived value of the blast energy. equal to about 5% of the incident pulse energy, was used to estimate a nascent blast pressure of approximately 150 attn. By making the assumption that surface debris will form if the ablation fragments are confined in a "small" volume for a "sufficient" time, then conclusions from blast wave theory suggest how to decrease the amount of generated debris.
INTROI)UCTION AND EXPERIMENTAL One significant drawback of excitner laser ablation of polyimide is the soot and debris that accumulate around the ablation site during the irradiation. Several studies of this debris have appeared in the literature, dealing with means for its removal 1. the effect of debris on ablation depth 2, how it effects surface morphology 3., 4 and the influence of various gases otn the chemical composition of the debris 5. Over the past two years. it has become known that ablation of polyimide in the presence of helium gas leaves a surface that is significantly free of debris relative to ablation in air 5. 6. In order to understand this phenomenon, we have studied the 248 ntn excimer laser ablation of polyilnide as a function of both gas atomic or molecular weight. and gas pressure. Our results indicate that blast wave theory can account semi-quantitatively for the role played by gas pressure and mass in debris formation. The experimental aspects of this study involved drilling I irnt diameter circular holes in KaptonTM 2 (DuPont ) polyitnide using 50WmJ/cm pulses (15 nsec) from a commercial KrF excimer laser. 30 pulses. fired at I Hz, were applied to a given irradiation site. At this fluence level and number of pulses. approximately 5 gtn of material is removed. The samples were housed in a scaled chtamber which could either be evacuated or filled with gas up to 760 torr (I atm) pressure. Only static fills of gas were used. with no device present to flow gas over the irradiation site oit the polymer surface. The chamber contained a silica window which permitted the laser light to enter. Several different gases were used in the study: H, (mass = 2). He (m = 4). a mixture of He and Ne (pressures adjusted to give in., = 10). Nc (m = 20), ,air (m,•r = 29), Ar (in = 40), Kr (in = 84), and Xe (m = 131). After laser drilling of the hole in the p
