FORMATION OF COBALT SILICIDE UNDER A PASSIVATING FILM OF MOLYBDENUM OR TUNGSTEN

Abstract

A very simple and reproducible cobalt silicide process with Mo/Co or W/Co bilayer metallization to overcome the oxidizing liability of Co annealed in a normal flowing-nitrogen furnace has been developed. Cobalt is deposited on blank and patterned silicon wafers in an electron-beam evaporation system followed by Mo (or W) deposition without breaking the vacuum. The cobalt silicidation is carried out using a two-step annealing process. The first annealing is performed at a temperature ranging from 400 to 600-degrees-C, during which the role of the upper layer of Mo (or W) is to protect the underlying Co layer from being oxidized while not disturbing the cobalt's silicidation process. Perfect selective etching of Mo (or W) can be accomplished by a NH4OH + H2O2 + (2-3)H2O solution. The second annealing is performed at a higher temperature of 750-degrees-C to completely transform the CoSi obtained in the first annealing into CoSi2 and induce grain growth of CoSi2, thus lowering the resistivity. X-ray diffraction, Auger electron spectroscopy, scanning electron microscopy, and sheet resistance measurement are used to characterize the silicide phase and microstructure.