Electroless deposition of thin-film cobalt-tungsten-phosphorus layers using tungsten phosphoric acid (H-3[P(W3O10)(4)]) for ULSI and MEMS applications

This paper describes an electroless deposition method for the formation of thin metallic films that contain mainly cobalt with significant amount of t ungsten (up to similar to 11 atom % ) and phosphorus (in the range of 1-3 a tom % ). The Ca(W, P) films can be applied for microelectronics multilevel metallization or for ultralage-scale integrated (ULSI) and microelectromech anical systems (MEMS) applications. The film was deposited from an aqueous electrolyte that contained tungsten phosphoric acid, H-3[P(W3O10)(4)], as a source for tungsten. Cobalt sulfate was used as a source for the cobalt io ns and sodium hypophosphite as the reducing agent and a source for phosphor us. The Co(W, P) thin films from solutions with tungsten phosphoric acid we re studied and compared to Co(W, P) films that had been obtained from solut ions wherein the tungsten was supplied by tungstate ions. The use of tungst en phosphoric acid allowed higher tungsten content and lower deposition rat e when compared to solutions with Na2WO4 and (NH4)(2)WO4. The deposition wi th tungsten phosphoric acid was studied for 20-200 nm thick layers and was found to ire reproducible. The deposited layers were bright colored and the ir specific resistance was in the rang of 48-90 mu Ohm cm. The process para meters of typical solutions are presented and discussed. The film sheer res istance, composition, and surface morphology are presented for various solu tions and process conditions. The deposition process involves several react ions that occur simultaneously and are described in this work. Finally, var ious applications are proposed for copper metallization as a barrier layer, to confine the copper, or as a capping layer to prevent copper oxidation a nd improve adhesion of the inlaid copper to upper interlevel dielectric lay ers. (C) 2001 The Electrochemical Society. All rights reserved.