A chemical etchant (and a method for forming the etchant) is disclosed for removing thin films of titanium-tungsten alloy in microelectronic chip fabrication. The alloy removed is preferably 10% Ti and 90% W, which is layered onto a substrate under chromium and copper seed layers for electrodeposition of C4 solder bumps. In this application the Ti--W etchant should not attack aluminum, chromium, copper, or lead-tin solders, and should dissolve Ti--W rapidly. The invention achieves this with a mixture of 30% by weight hydrogen peroxide and water, to which is added EDTA and potassium sulfate. The hydrogen peroxide etches Ti--W rapidly at temperatures between 40° C. and 60° C. EDTA forms a complex with tungsten to prevent plating of the Pb--Sn solder with W, and potassium sulfate forms a protective coating on the Pb--Sn solder to protect it from chemical attack.
Dr. Madhav Datta and Shenoy, R. V., “Selective etching of TiW for C4 fabrication”, U.S. Patent US 08/260,0491995.