Abstract
Electroless or chemical vapor deposition of copper onto commercial samples of skived poly(tetrafluoroethylene) (PTFE) that have been chemically etched with sodium naphthalenide results in Cu films sufficiently adherent that attempts to remove the Cu cause near-cohesive failure in the PTFE. Such strong Cu adhesion forms the basis for several approaches to the production of high-resolution PTFE-based printed-circuit boards. Similarly strong adhesion does not occur to melt-processed fluorocarbon polymers (Teflon-AF, FEP, Teflon-PFA, etc.) nor to samples of PTFE annealed by hot pressing or mechanical polishing, nor to radiation-crosslinked PTFE. Adhesion to etched, skived PTFE is dominated by mechanical interlocking due to penetration of Cu into the highly-crazed, porous surface produced by etching of the stressed surface caused by skiving. Patterned irradiation (electrons, X-rays, etc.) at low dose levels results in crosslinking of virgin PTFE, preventing appreciable chemical etching and subsequent metal adhesion in the irradiated areas, and resulting in a three-step process (irradiation, chemical etching, and metal deposition) with a non-optimized lateral resolution of 35 μm. Optical power absorption by an etched PTFE layer is a factor of 300 greater than for underlying (virgin) PTFE, allowing a second three-step approach to patterned metallization: chemical etching, patterned excimer-laser ablation, and metal deposition. A third approach combines chemical etching and uniform thin-metal-film deposition with standard photolithographic processing techniques, resulting in a patterned metallization process where adhesion is controlled by the initial etching step and resolution is controlled by standard lithographic technology. A non-optimized feature resolution of 17 μm for 4-μm thick Au conductors has been demonstrated with this technique.
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