The present invention relates to a chemical etchant for etching metals in the presence of one or more metals not to be etched, the etchant comprising 10-25 gms EDTA, 15-35 gms K2 HPO4 and 25-45 gms oxalic acid in a liter of 30% H2 O2. More particularly, in the fabrication of interconnections for microchip structures, the present invention addresses the removal of intermediate adherent layers, e.g., Ti--W, without damaging other microchip structures made of other metals, such as Al or Al--Cu test pads; Cu and phased Cr--Cu layers; and Sn--Pb solder bumps. The use of potassium phosphate in the hydrogen peroxide+EDTA bath has been found to significantly reduce the attack on the metal not to be etched. Furthermore, the use of oxalic acid in the bath prevented the deposition of tin oxide on the substrate adherent layer metal, thus facilitating its complete removal.
J. Michael Cotte, Dr. Madhav Datta, Dinan, T. Edward, and Shenoy, R. Vaman, “Selective chemical etching in microelectronics fabrication”, U.S. Patent US5800726 A1998.