EVIDENCE OF A LONG-RANGE DENSITY GRADIENT IN SIO2-FILMS ON SI FROM H-2-PERMEABILITY MEASUREMENTS

We describe a technique for the measurement of the diffusivity, solubi lity, and permeability of Hz through SiO2 thermally grown on Si substr ates at room temperature. We have applied this technique to determine the dependence of the H-2 permeability on oxide thickness and growth t emperature. Since permeability depends strongly on density, these meas urements provide a sensitive probe of the oxide density. Our results i ndicate that the H-2 permeability is considerably less in SiO2 films t hermally grown on Si : substrates than through bulk SiO2, in agreement with previous works which had indicated that thermally grown oxides a re denser than bulk SiO2. The permeability is found to decrease with o xide thickness, indicating that thinner oxides-have larger densities t han thicker oxides. Oxides grown at high temperatures are found to hav e higher permeabilities than oxides of similar thickness grown at lowe r temperatures. We present evidence that the dependence of permeabilit y on oxide thickness results from a density gradient throughout the ox ide rather than from a uniform reduction of the oxide density as the o xide becomes thicker.