Electroless deposition of thin-film cobalt-tungsten-phosphorus layers using tungsten phosphoric acid (H₃[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 tungsten (up to ∼11 atom % ) and phosphorus (in the range of 1-3 atom % ). The Co(W, P) films can be applied for microelectronics multilevel metallization or for ultralarge-scale integrated (ULSI) and microelectromechanical systems (MEMS) applications. The film was deposited from an aqueous electrolyte that contained tungsten phosphoric acid, H₃[P(W₃O₁₀)₄], as a source for tungsten. Cobalt sulfate was used as a source for the cobalt ions and sodium hypophosphite as the reducing agent and a source for phosphorus. The Co(W, P) thin films from solutions with tungsten phosphoric acid were studied and compared to Co(W, P) films that had been obtained from solutions wherein the tungsten was supplied by tungstate ions. The use of tungsten phosphoric acid allowed higher tungsten content and lower deposition rate when compared to solutions with Na₂WO₄ and (NH₄)₂WO₄. The deposition with tungsten phosphoric acid was studied for 20-200 nm thick layers and was found to be reproducible. The deposited layers were bright colored and their specific resistance was in the range of 48-90 μΩ cm. The process parameters of typical solutions are presented and discussed. The film sheet resistance, composition, and surface morphology are presented for various solutions and process conditions. The deposition process involves several reactions that occur simultaneously and are described in this work. Finally, various applications are proposed for copper metallization as a barrier layer, to confine the copper, or as a capping layer to prevent copper oxidation and improve adhesion of the inlaid copper to upper interlevel dielectric layers.