Copper Metallization of Polyimide Film by Excimer Laser Irradiation and Electrodeposition
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KAPTON is DuPont's trademark for its polyimide film products. 571 Mat. Res. Soc. Symp. Proc. Vol. 354 01995 Materials Research Society
a few Hz over a range of pulse energies at 248 nm. They found a sharp onset of conductivity enhancement when total fluence exceeded about 20 J/cm and per-pulse fluence also exceeded about 20 mJ/cm. Enhancement saturated after about 1000 pulses. The saturation conductivity increased wi~h per-pulse fluence up to the onset of ablation (about 55 mJ/cm ), then remained constant in non-oxidizing atmospheres. TEM and ATR/IR indicated that material in the 50 nm-thick transformed zone is best described as graphitic clusters in a polymer matrix. However, the penetration depth of the ATR analysis compared to the transformation depth was not clear. Other researchers using 248 nm found in contrast that the maximum effect occurs for per-pulse fluences near the ablation threshold [9]. Still others rapidly scanned an Ar-ion laser (350 - 380 nm) to deliver about ten times the total fluence,. transforming the polyimide to a depth of 110 microns into a porous carbonaceous material [10]. These graphitic materials should be chemically and thermally robust. The mass loss accompanying the transformation must be accomodated somehow, perhaps adversely impacting mechanical durability. Further, the relatively small dimension of the conductive material might not provide enough current-carrying capacity for all applications. We therefore investigated plating of polyimide made surface conductive, seeking the advantages of metal conductors while exceeding the present system performance. We sought to achieve a plateable surface at lower cost and with better adhesion than possible by thin film techniques. EXPERIMENTS We studied 200 micron-thick, Kapton* 100 HN PMDA-ODA polyimide film (DuPont), using a Questek 2260 excimer laser at 248 nm (KrF, 23 ns FWHM) as shown in Fig.l. For surface chemistry studies, we expanded a static beam to give the desired per-pulse fluence onto 10 mm-wide pre-cleaned (as below) sample strips mounted in 35 m slide holders for contamination-free handling. We used 58 mJ/cm per-pulse fluence at 10 Hz to best compare with earlier work. We also used 200 Hz, since a viable industrial process will need high throughput. To make materials for plating studies, we placed a roll-to-roll transport at the sample location. We cut pieces about 10 X 15 mm from the 35 mm slide mounts and, gripping them with previously-washed tweezers, rinsed them successively in reagent-grade 2-propanol and fluorocarbon FC-113. We attached the washed specimens without using adhesives to similarly-washed specimen stubs and loaded them into a VG EscaLab II surface spectroscopy instrument. We also cut specimens from some of the continuously-treated film and Dandled them similarly. After pumping to a base pressure below 10- torr, we operated the Mg anode (1254 eV) at 300W to collect XPS data from a 1 X 3 mm area. For continuously treated film, we varied per-pulse fluence, pulse repetition rate, transverse (galvanometer) scan
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