Thermal oxidation of silicides on silicon

Abstract

Thin layers of metal silicides on Si substrates can be thermally oxidized with the formation of SiO2. Under certain conditions the resulting oxide layer is free of metal impurities to a level which has been determined to be less than 1 at.% for light metals such as Ti, and less than 01 at.% for heavy metals such as W. The silicide layers remain apparently unaffected by this process; they appear to become buried under growing oxide films. In many cases the oxide growth is simply due to the diffusion of Si through the silicide layer, but the mechanism can be somewhat more complex. Some silicides are decomposed at the silicide-oxide interface; the integrity of the silicide layer is then maintained by the ‘reverse’ diffusion of the free metal atoms to the silicon-silicide interface where more silicide is formed. In some silicides with a relatively open crystalline structure this reverse motion of the metal atoms has been observed to be accompanied by a sizeable motion of Si atoms, also in the same reverse direction. This is reminiscent of the injection of interstitial Si during the oxidation of pure Si, resulting in such phenomena as oxidation-induced stacking faults and oxidation-enhanced diffusion. As with pure Si, the kinetics of oxidation of the silicides can be shown to follow the well-known linear-parabolic law. The parabolic term does not vary significantly from that obtained with pure Si under comparable conditions, which strongly implies that the rate-controlling factor remains the same, namely the diffusion of the oxidizing agent through the oxide film. The difference between Si and silicides becomes more marked when one examines the behaviour of the linear term in the oxidation law. With most silicides which exhibit low resistivities characteristic of metallic compounds, the linear rate of oxidation is much higher than with Si; however, in the case of two semiconducting silicides with a band gap comparable to that of Si, the linear term of oxidation kinetics is as small as that with Si itself. Surface-spectroscopy results obtained with silicides exposed to oxygen under ‘ultra-high vacuum’ conditions confirm the phenomenological observations based on the kinetics of oxidation. Whether one looks at the kinetics of oxide growth or at the reactivity of silicide surfaces with oxygen, the presence of free carriers at the Fermi surface of the material being oxidized enhances the charge-exchange processes which are necessary, and apparently rate-limiting for the formation of SiO2. The correlated effects of volume change, volume accommodation, stress relaxation, defect creation and injection, which are required by the formation of SiO2, play different roles in silicides with different crystalline structures and properties. Silicides provide investigators with a means of studying the growth of SiO2 in environments which differ electronically and metallurgically from Si; a better understanding of the behaviour of these materials should provide new insight into the old problem of SiO2 formation. © 1987 Taylor & Francis Ltd.