Laser-Assisted Seeding for Electroless Plating on Polyimide Surfaces

Abstract

Excimer laser pulses with wavelengths of 248 and 308 nm were used to selectively seed Pd on polyimide (PI) surfaces, making them suitable for electroless plating. This novel seeding process for insulating materials is accomplished with the sample immersed in the seeding solution, occurs only on the areas of the substrate that are illuminated (through the liquid) by the laser light, and does not require prior treatment of the surface. The seeding solution is transparent to the laser light and the metal deposition occurs as a consequence of the photoabsorption in the solid. This leads to electron transfer from the solid film into the solution and reduction of the Pd ions in contact with the surface. The Pd content of the seeded samples increased with the number of pulses, but was independent of repetition rate. The deposition rate of Pd did not exhibit a significant dependence on wavelength, in agreement with UV absorption spectra of PI and a single photon absorption process for electron excitation to allowed unoccupied states. As for the PD distribution, the deposits consisted of islands with distributions that depended on surface properties as well as on laser-material interactions. Sufficient PD seeds for uniform electroless plating of Cu and Co were attained after 3000 pulses at fluences ≃30 mJ/cm2. Although these fluences are much lower than those used for ablation of PI under water, distinct kinds of surface roughness were observed depending on the laser light and on the different types of PI. © 1995, The Electrochemical Society, Inc. All rights reserved.