Reactive plasma cleaning of metals

Abstract

Reactive plasmas formed in an r.f. discharge are effective in removing contaminants and barrier layers from a variety of metals. Reactive d.c. discharges are ineffective. A mixture of argon (90–99%) with a reactive gas or a gas which will decompose to a reactive species in the discharge was used in the r.f. reactive plasmas. A soft X-ray appearance potential spectrometer was used for in situ surface analysis to evaluate the surface cleaning. Carbon-containing gases, such as those used in reactive plasma etching, left a carbon residue on the surface. Oxygen additions to the reactive plasma, such as are used in reactive etching, prevented cleaning of the metal surfaces. Reactive plasmas containing hydrochloric acid were the most effective. Reactive plasma cleaning in Ar-HCl was as effective as r.f. sputter cleaning and could be accomplished at much lower power levels. For example, reactive plasma cleaning of titanium was accomplished in 90 min using 500 V r.f. and 30 W in an Ar-5% HCl plasma. Similar r.f. sputter cleaning required 90 min at 2 kV r.f. and 600 W in Ar. It is proposed that the reactive species reacts with the surface, forming compounds which are more easily volatilized or sputtered.