Corona-Discharge-Induced Stress Relaxation in Silicon Dioxide Films on Silicon
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CORONA-DISCHARGE-INDUCED STRESS RELAXATION IN SILICON DIOXIDE FILMS ON SILICON
L.M. LANDSBERGER* and W.A. TILLER** *Department of Electrical Engineering "**Departmentof Materials Science and Engineering Stanford University, Stanford, CA, 94305 ABSTRACT Variations in refractive index (density) with growth temperature are taken as a rough measure of residual compressive stress in dry Si0 2 films. This paper presents experimental data regarding the relaxation of these stresses by a low-temperature (600900'C) oxygen corona discharge process. For an initial oxide layer of 1100 A thickness grown at 800°C, relaxation is complete after a corona treatment of 10 minutes at -l1 iA. The dose of 0- ions required is found to be about 1-1.5% of the total oxygen atoms in the initial layer. The difference in dose between oxide on (111) and (100) Si is found to be proportional to the respective density changes from the initial oxide to fully relaxed oxide. INTRODUCTION In the last decade, the stress developed during the thermal oxidation of silicon has received increasing attention [1-15]. EerNisse [1] demonstrated the existence of stress in SiO 2 during growth at oxidation temperatures. Taft [21 demonstrated a relationship between growth temperature and refractive index of the Si0 2 film and noted that, for dry oxides, the refractive index decreases with increasing growth temperature until 11501200 0C, after which it is constant at the refractive index of pure fused quartz under no stress (Malitson [16]). Irene [5] related refractive index to density of the oxide, finding about a 2-3% density drop from an 800°C-grown oxide to an 1150°C-grown oxide, as depicted in Fig. 1. In this work, this density variation with growth temperature is taken as an indication of the existence of residual stresses and strains in the Si0 2 structure. In recent papers [14-15], a corona discharge technique was described
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T ('C) Figure 1: Density [5] and refractive index [2] vs. growth temperature for dry grown Si0 2 films on Si.
Mat. Res. Soc. Symp. Proc. Vol. 75. 1987 Materials Research Society
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which relaxes at low temperatures these built-in strains in a region about 15mm in diameter, the same as the lateral dimension of the corona ion beam. It was shown that the diffusivity of oxygen through such a relaxed layer in a subsequent oxidation step was enhanced, and was comparable to the diffusivity through a high-temperature-grown (unstrained) oxide. In the present paper, more basic data on the actual relaxation process is provided to show the progression of the relaxation with ion dose, q = tI (where t and I are corona treatment time and current, respectively), and to compare the dose required to relax oxides on Si of different orientations. This brings us one step closer to understanding the atomic mechanism involved in the relaxation of Si0 2.
EXPERIMENTAL PROCEDURE Initial dry oxide layers were grown to various thicknesses (900-2000 A) at various temperatures (800-1150°C) on lightly
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