Iron Precipitation and Dissolution in Float-Zone Silicon
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valuated. The thermal stability of bulk iron-silicide precipitates in silicon wafers is quantitatively analyzed. Dissolution of FeSi 2 over the temperature range of 700'C to II 00°C over different process durations have been experimentally evaluated. To analyze precipitation of iron in silicon, quantitative interstitial iron (Fe1) concentration measurements, were performed using the technique of surface photo voltage (SPV) minority carrier lifetime analysis. Interstitial Fe concentrations are measured before and after precipitation and dissolution anneals. Any decrease in Fe, is assumed to be due to formation of the FeSi 2 precipitate phase. Increase in Fe1 concentrations following an anneal indicates dissolution of the precipitate phase. This change in interstitial Fe (AFe,) is used to analyze the iron precipitation and dissolution quantitatively with respect to time and temperature. A characteristic relationship between the precipitating iron and annealing time over the process temperature range is deduced. EXPERIMENT The experimentation was performed using 3 inch, 1-2 f)-cm, P-type FZ grown wafers. Starting wafer background iron concentration was determined to be 4x10'0 atoms/cm 3 . The background level of iron contamination from the starting wafers, cleaning procedure and subsequent furnace processing was established to be about 6 x 10'0 atoms/cm 3 at 900°C. Thus the wafer processing procedure introduced about 2 x 1010 atoms/cm3 of Fe, into the samples. This is typical after a high temperature thermal process. Selected samples were intentionally doped with iron by immersion in a 100 ppm solution of ferric chloride (FeC13) for 10 minutes and then nitrogen blow-dried. In accordance with published iron solubility data' a drive-in temperature of 900'C was used to obtain a target Fei concentration of 2x 013 atoms/cm3 . The wafers were diffused for 40 minutes in a nitrogen ambient. After the in-diffusion heat treatment the samples were quench cooled with cool nitrogen vapors. Such rapid cooling was performed to prevent any unwanted precipitation and to ensure most of the iron remained in an interstitial state. The surface of the samples were chemically polished to remove any Fe rich areas on the surface. Interstitial iron concentration (Fe1) and the change in Fe, was monitored using a surface photo voltage (SPV) minority carrier lifetime analysis technique. SPV measurements were performed using a CMS system. The SPV method is an analytical technique for the measurement of minority carrier diffusion length and thus lifetime of carriers in semiconductors. Iron concentration was determined from the lifetime measurements using well known Fe-B pairing relationships outlined below and described in detail elsewhere.' Interstitial iron is a very effective minority carrier recombination center, with an energy level' E, +0.4eV (Fe1). In p-type silicon, interstitial iron electrostatically pairs with boron dopant atoms to form weakly bonded Fe-B pairs. The donor-acceptor pair Fe-B has an energy level E, +0.1eV and an electron capt
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