Out- and in-diffusion of oxygen 16O in silicon

Oxygen was in-diffused into the float zone silicon either during oxidation or upon annealing in argon of previously oxidized Si wafers. The resultant in-depth profiles of the main isotope O-16 were measured by secondary ion mass spectrometry (SIMS) with detection limit below 10(16) cm(-3). The oxygen solubility C-s, as determined from the O-16 surface concentration, was found to be insensitive to the annealing ambient. This observation seems to contradict the effect of enhanced solubility under oxidizing conditions, reported for the in-diffused 180 isotope. Over a temperature range of 900-1250degreesC, the solubility of oxygen can be expressed by a relationship C-s = 9.1 x 10(22) cm(-3) exp(-1.57 eV/kT). The surface concentration of oxygen out-diffused from supersaturated CZ Si appears to be significantly higher than that of in-diffused O-16. This is because the out-diffusion process is accompanied by precipitation of oxygen atoms into immobile SiO2 particles. As a consequence, SIMS measurements of the out-diffusion profile cannot provide reliable information on the solid solubility of oxygen.