Surface Activation of Polymers Assisted By Picosecond Laser Pulses for Selective Metallization Via Electroless Deposition

Abstract

The preparation of metallic tracks on non-conductive or semi-conductive substrates is attracting a strong scientific interest due to its useful applications in integrated circuit technology and its potential for miniaturization. The selective metallization on dielectric substrates can be carried out via laser-assisted direct writing techniques These involve processes such as thermally induced deposition, surface modification for subsequent electroless deposition as well as photocatalysis-mediated deposition among others [1]. Furthermore, a well-known methodology for producing metal tracks onto polymeric substrates is the so-called laser direct structuring (LDS) technology and is widely used for producing molded interconnect devices (MIDs). Conventional LDS uses polymeric materials loaded with a small amount of metallic compound (e.g. Pd particles) which after laser ablation can be released for catalyzing a further electroless plating process. Organometallic complexes based on palladium (Pd2+), copper (Cu2+) [2] as well as metal oxide composites of copper−chromium oxide (CuO·Cr2O3) or antimony-doped tin oxide (ATO) [3] have been used as catalyst sources for LDS technology in the last years. The active material is finely dispersed in the polymeric matrix and can be chemically modified by laser ablation in order to play a role as catalyst in a subsequent electroless deposition process. However, although the LDS method offers flexibility and freedom for 3D patterning design, the addition of those special additives (catalyst sources) is required in order to make the polymeric material suitable for LDS. This paper will discuss a simple methodology for patterning metal tracks onto the polymeric substrate polybutylene terephthalate (PBT). The selective metallization process consists of three main steps: (1) surface patterning and activation with picosecond-laser pulses in a dry processing environment; (2) treatment of the substrate in PdCl2 seeding solution, and (3) selective metallization of the substrate via electroless copper deposition. It was found that, besides a significant surface roughening, the use of ultra-short laser pulses promoted the chemical modification of the polymeric surface in a manner that the surface acquires the ability to reduce Pd-ions into metallic Pd clusters during a subsequent treatment in PdCl2 solution. The laser parameters as well as the concentration and the temperature of the PdCl2 solution were investigated and correlated with the surface morphology, the formation of Pd clusters as well as with the characteristics of the copper layers. The copper features were homogeneous and exhibited well defined geometries. Moreover, the adhesive properties of the copper structures were investigated. Tape tests showed that the copper structures exhibited good adhesion to the PBT substrate and this is ascribed to the high roughness obtained via laser patterning. The copper layers exhibit low electrical resistivity values and therefore show great potential for applications in electronic devices even with 3D complex geometries.