PHASE-TRANSFORMATION OF MO AND W OVER CO OR ITS ALLOY IN CONTACT WITH SI

Abstract

Tungsten or molybdenum deposited on a cold silicon substrate (W/Si or Mo/Si) using an electron-beam gun at a base pressure of 10(-6) Torr, are unable to form a silicide, even when annealed at 900-degrees-C in either ambient N2 or H-2. We present an easy method by which Mo and W silicides can be formed under the same deposition and annealing conditions with the help of an intervened layer of cobalt or its alloy. Investigations were performed on various metallizations of Mo/Co/Si, W/Co/Si, W/Co-Mo/Si, Co-Mo/Si and Co/Mo/Si, annealed in a normal flowing-nitrogen furnace or in ambient H-2. In the Mo/Co/Si systems (or W/Co/Si), Mo (or W) silicide can be formed at 900-degrees-C over CoSi2 which is formed at a lower temperature and is in contact with Si. The residual silicon oxide on the silicon surface is thought to inhibit the silicide formation of W and Mo. The presence of cobalt, which is not impeded by the surface residual silicon oxide or other impurities for silicide formation, can induce silicidation of W and Mo. The reason for silicide formation at higher temperatures in preference to the metal oxide is based on the Ellingham diagram. In the W/Co-Mo/Si system, three silicides Of WSi2, MoSi2 and CoSi2 are formed at a high temperature of 900-degrees-C. A mechanism for phase array of the silicides is proposed. In the Co/Mo/Si system, where stable Co3Mo and Co7Mo6 compounds are formed in advance, no silicide can be formed even at 950-degrees-C.