Incorporation of Ambient Oxygen During Silicon Deposition and the Effects of Argon Ion Bombardment
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INCORPORATION OF AMBIENT OXYGEN DURING SILICON DEPOSITION AND THE EFFECTS OF ARGON ION BOMBARDMENT DAVID W. BROWN* AND GRAHAM K. HUBLER Naval Research Laboratory, Washington, D.C. 20375-5000 *NRC Research Associate at NRL
ABSTRACT The composition of SiOx films deposited by electron-beam evaporation of silicon in an oxygen atmosphere was measured as a function of silicon deposition rate and oxygen pressure. The compositions varied between X = 0.03 and 2.1 depending on the ratio of 02 molecules to Si atoms arriving at the substrate. Bombardment with 500 eV argon ions caused X to decrease at low arrival ratio and to increase at higher arrival ratio. The results are analyzed using an adsorption model in which chemisorption of 02 occurs after physisorption into one of two states: one on silicon and one on SiP 2. A fit to the data for the unbombarded samples is achieved if physisorption occurs only on silicon. From this fit a sticking coefficient of 0.05 is obtained for 02 chemisorption on silicon. The effects of ion bombardment are consistent with the model if bombardment allows 02 to also physisorb on Si0 2 and migrate to silicon where it chemisorbs. INTRODUCTION Ion beam assisted deposition (IBAD) has been shown to beneficially effect many properties of thin films [1]. The desired effect of the ion beam may result from atomic collisions or from the concentration of implanted ions. For thin films in which a specific concentration of the ion species is desired, incorporation of the species from the background ambient must be considered. For IBAD of silicon nitride [21 and titanium nitride 13.1, quantitative characterization of the nitrogen ion beamn allowed the fraction of nitrogen incorporated from tile ambient to be determined. In SiNx films, all the incorporated nitrogen was attributed to implantation and thus no ambient nitrogen was incorporated. This situation was independent of both the impingement ratio of ambient N 2 to silicon and the nitrogen beam flux. In contrast, a large fraction of the incorporated nitrogen in TiNx films carne From the ambient. Furthennore, for the samne impingement ratio of ambient N2 molecules to Ti atoms arriving at the surface, the amnount of ambient N2 incorporated was dependent on the beam flux. Using a gas adsorption model this beam effect was consistent with an increased sticking coefficient (from 0.1 to I) of N2 on pure titanium and a reduction in the number of active surface adsorption sites. Under the conditions of these two studies, surface science analyses were possible without ultrahigh vacuum because the sample surface was constantly being renewed, reducing the exposure to contaminants and allowing small exposures of desired species using high partial pressure. The goal of the present study, was to understand the effect of ion bombardment on the incorporation of ambient oxygen during silicon deposition. EXPERIMENTAL The substrates for deposition were either Poco graphite or carbon coated silicon fonned by carburization of photoresist on silicon wafers [4]. Carbon substrates
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