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Tai D. Nguyen Center for X-ray Optics, Lawrence Berkeley Laboratory, and Department of Materials Science and Mineral Engineering, University of California, Berkeley, California 94720 (Received 11 January 1993; accepted 7 May 1993)

The interaction of cobalt (Co) and low-pressure chemical-vapor-deposited silicon nitride (LPCVD Si 3 N 4 ) during anneals from 200 "C-1000 °C in vacuum, Ar, and A r - H 2 ambient (95% Ar and 5% H 2 ) has been studied. After the anneals, reduction of Si3N4 by Co to form cobalt silicide and cobalt nitride phases has been observed. Reduction of Si 3 N 4 initially occurs at 600 °C; however, gross physical damage occurs at temperatures of ~900 °C in Ar. The addition of hydrogen to the ambient enhances the onset of physical damage to the nitride film by as much as 200 °C. Mechanisms governing the Co/Si 3 N 4 reaction have been proposed.

I. INTRODUCTION Silicon nitride (Si 3 N 4 ) and silicon dioxide (SiO2) are presently the two most widely used insulating materials in Very-Large Scale Integrated (VLSI) circuit technology. In both the Dynamic Random Access Memory (DRAM) and microprocessor technologies, silicides of Ti, and more recently of Co, are used in source, drain, and gate regions of Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFET's) to minimize contact resistance and increase device speed.1'2 Since these components must be fully isolated from one another, usually by sidewall insulating spacers, the possibility of metal-oxide and metal-nitride interdiffusion and reaction becomes important. Degradation of spacers will lead to short circuit paths between adjacent gate-source/drain regions. Metal penetration underneath the gate of the MOSFET will lead to the formation of carrier traps in the depletion region of the junction, thereby increasing junction leakage. Interfacial reactions of Ti/SiO 2 and Ti/Si 3 N 4 thin film composites have been studied recently, and results indicate that Ti may react with both types of insulating materials during silicidization to form the stable C54 TiSi2 phase. The interfacial reaction Ti with SiO 2 has been found to occur as low as 700 °C.3 Reaction between Ti and Si 3 N 4 has been observed after annealing in vacuum at 500 °C for 1 h.4 In the Ti/Si 3 N 4 study, the resulting phases after the anneal have been determined to be titanium nitride, as well as several titanium silicides, which indicate that the Si3N4 film has been reduced by the Ti. Currently, self-aligned CoSi2 is being considered as a replacement for TiSi2 for junction contacts. There are two principal reasons for using CoSi2 instead of TiSi2. First, lateral growth of TiSi2 over sidewall spacers 2354

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J. Mater. Res., Vol. 8, No. 9, Sep 1993

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has always been a concern as it is known to cause shorts between source and drain.5 A possible solution to the "bridging effect" is to use a two-step anneal process in which a low temperature anneal first forms the intermediate C49 TiSi2 phase. A selective wet etch is employed to remove the re