Phase Transformation of Mo and W over Co OR Its Alloy in Contact with Si
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PHASE TRANSFORMATION OF Mo AND W OVER Co OR ITS ALLOY IN CONTACT WITH SI FANN-MEI YANG AND MAO-CHIEH CHEN Department of Electronics Engineering & the Institute of Electronics, National Chiao Tung University, 1001 University Rd., Hsinchu 300, Taiwan. ABSTRACT W or Mo directly deposited on Si cold substrate by electron-beam gun at a base pressure of 10-6 torr is not able to form silicide even annealed at 900 9C in either N2 or H2 ambient. We present an easy way that Mo and W silicides can be formed on the same depositing and annealing conditions with the help of an intervened layer of cobalt or its alloy. Investigation was made on various metallizations of Mo (or W)/Co/Si, W/Co-Mo/Si, and Co/Mo/Si in normal flowingnitrogen or in H2 ambient at various temperatures. In the systems of Mo (or W)/Co/Si and W/CoMo/Si, the overlying Mo (or W) can be transformed into silicide at 900 9C, while in the Co/Mo/Si system, where stable Co-Mo compounds are formed in advance, no silicide can be formed. Why silicide is formed in preference to metal oxide in N2 environment at higher temperature is based on Ellingham diagram. INTRODUCTION Metal silicides are necessary for the gates, interconnects and source/drain contacts in scaleddown very large scale integrated circuits (VLSIC) to reduce RC time delay and IR voltage drop, and relieve the problems of series resistance and contact resistance in the contact areas of devices However, silicidation of some refractory metals like Mo and W is not an easy process ecause it is very sensitive to surface condition of the Si substrate. Usually, these metals are deposited by sputter with the substrate preheated to 200 to 400 `C [2-4]. If e-beam evaporator is employed for the metal film deposition, the base pressure should be in the range of 10-8 torr [5]. In addition, the precleaning and annealing procedures are critical. In our previous study for cobalt silicide, Mo or W overlying Co was employed as a passivating layer [6]; both metals were deposited using an e-beam evaporator at a base pressure of 4.0 x 10-6 torr. It was found that in normal flowing-nitrogen (N2 furnace, the Co was transformed into silicide, while the Mo or W did not become silicide but was either inert or oxidized at 400-600 *C.However, both Co and Mo (or W) were well transformed into silicides at 900 *C.On the other hand, Mo or W cannot react with Si to form silicide if it is deposited directly on Si substrate on the same depositing conditions, followed by either N2 or H 2 furnace annealing. Thus the Co film is supposed to help formation of Mo or W silicide. In this work, in addition to the Mo (or W)/Co/Si system, phase transformation of W over Co alloy (like Co-Mo) in contact with Si is also studied. Annealing experiment was also made on the structure of Co/Mo/Si for comparison because, unlike the previous two systems, compounds of Co-Mo are formed in advance of any further phase transformation. Finally, how Co helps Mo (or W)-silicide formation and why high-temperature annealing prefers silicide formation to oxide formation i
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