Evidence of Co/SiO 2 reaction during rapid thermal annealing
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Cobalt (Co)/silicon dioxide (SiO2) reactions during rapid thermal annealing (RTA) in an N2 ambient have been investigated. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) show that islands consisting of Co and the cobalt oxide phase, Co 3 O 4 , form during high-temperature anneals (30 s-10 min at 800 °C). After a selective wet etch to remove the islands, groove-like features are produced in the SiO2 layer. The morphologies of the islands and grooves are strikingly similar, which strongly suggest that Co and the underlying SiO2 layer have reacted, most likely to form CO3O4. We propose that small concentrations of O 2 are necessary to promote Co/SiO 2 interactions.
I. INTRODUCTION
II. EXPERIMENTAL
Integration of silicides (such as CoSi2) in VLSI device processing is primarily focused in two main areas of junction technology. In one process, CoSi2 may be utilized in MOS self-aligned technology (silicide) in which the polysilicon gate, source, and drain of an MOS transistor are simultaneously silicidized.1 A second and more recent utilization of CoSi2 is that of a diffusion source for dopants to form shallow junctions.2 The attraction of using CoSi2 as an outdiffusion source is its ability to stop and contain dopants (via implantation) entirely within the silicide film while conveniently providing a low-resistance contact. In both of these junction technologies, the silicidized junction must be effectively isolated by a dielectric such as SiO2 or Si 3 N 4 . Degradation of isolation materials can lead to excess junction leakage and possible shorting of devices.
For this study, thin Co films (12.5 nm) were deposited upon thermally oxidized Si wafers in an electron-gun evaporator at base pressures between 10"6 and 10"7 Torr. Thicknesses of SiO 2 used were 50 and 100 nm. Co/SiO 2 samples were annealed at temperatures of 750 "C-800 °C and for annealing times of 30 s-10 min under flowing N 2 in a rapid thermal annealing system (RTA). Co/SiO 2 samples were also annealed in forming gas (90% N 2 and 10% H 2 ) and in a 95% Ar and 5% H 2 gas to study the effects of other inert ambients on Co-SiO 2 interactions. After the anneals, some samples were treated in a metal etch consisting of HNO3 and DI water (6 vol. % HNO 3 ). Microstructural analysis was conducted on both wet-etched and asannealed samples utilizing TEM (bright-field imaging and electron diffraction) and SEM.
Co/SiO 2 interactions have been examined extensively and the studies indicate that Co does not react with SiO 2 . In an early work of Pretorius et al, in which they studied the interaction of Co and 11 other metals with thermally grown SiO 2 , it was found that Co does not react with SiO2 at 800 °C under vacuum-like conditions.3 Studies of Co/SiO 2 reactions by van den Hove et al. show that islands of Co tend to form on SiO2 during RTA at temperatures of 700 "C-900 °C for 30 s.4 Van den Hove et al. also report that CoO formed during the annealing studies, but it is concluded that oxide formation originated from concentrations of
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