ANISOTROPIC PATTERNING OF COPPER-LAMINATED POLYIMIDE FOILS BY PLASMA-ETCHING

The aim of this study was to optimize a plasma etching process for the anisotropic patterning of copper-laminated polyimide foils. Vertical edges and minimal mask undercut should be achieved at an acceptable et ch rate. The experiments included reactive ion etching (RIE) in a RIE parallel plate reactor and plasma etching in a reactor with a slot ant enna microwave plasma source. The influence of process gas pressure, C F4-/O-2-ratio, gas-flow rates, bias voltage and supplied power was inv estigated on both reactors. Thermal stress of the foils during plasma treatment caused by ion bombardement, exothermic chemical surface reac tions and an insufficient thermal coupling of the polymer foils to the cooled substrate holder was also investigated. From the results, proc ess parameters were found for anisotropic patterning of copper-laminat ed polyimide foils by plasma etching. Vias with diameters down to 25 m u m in 50-mu m thick foils could be etched. The etch rate ratios obtai ned (ERvertical/ERlateral) were >5 and minimal mask underetching was a pproximate to 6.2 mu m. Therefore the achieved anisotropy a (a=1-ERlat /ERvert) was greater than or equal to 0.8. (C) 1997 Elsevier Science S .A.