Method of etching polyimides and resulting passivation structure

Abstract

A method of etching polyimide having metallization patterned thereon in which an epoxy resin system provides the etch mask for etching the polyimide and provides a resulting passivation structure overlying the metallization. The polyimide having a metallization pattern thereon is coated with the photoimageable material resists concentrated KOH etching when the epoxy is cured and adheres to the polyimide and the metallized pattern after the KOH etch providing passivation to the metallization. The process includes exposing the layer of photoimageable material to radiation to selectively pattern the material, developing the patterned material revealing the underlying polyimide to be etched, curing the remaining material and etching the revealed polyimide in concentrated KOH to remove the revealed polyimide. The remaining epoxy firmly adheres as a passivation layer for the metallization. Preferably the epoxy consists by weight essentially of from about 10% to about 80% of a polyol resin which is a condensation product of epichlorohydrin and bisphenol A having a molecular weight of between about 40,000 to 130,00, and between about 20% and 90% of an epoxidized octafunctional bisphenol A formaldehyde novolak resin, having a molecular weight of between 4,000 to 10,000 and about 0.1 to about 15 parts of a cationic photoinitiator capable of initiating polymerization of resin exposed to radiation. The resin may contain up to about 50% of epoxidized di-glycidal ether of tetrabromo bisphenol A having a molecular weight of between about 600 and 2,500, for flame retardancy.