Oxygen '96

Early Stages of Oxygen Precipitation in Silicon

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ON THE IMPACT OF GROWN-IN SILICON OXIDE PRECIPITATE NUCLEI ON SILICON GATE OXIDE INTEGRITY

J. Vanhellemont (a), G. Kissinger (b), K. Kenis (a), M. Depas (a) and D. Gräf (c)

(a) IMEC, Kapeldreef 75, B-3001 Leuven, BELGIUM
(b) Institute of Semiconductor Physics, D-15204 Frankfurt (Oder), GERMANY
(c) Wacker-Siltronic GmbH, PO Box 1140, D-84479 Burghausen, GERMANY

Keywords: gate oxide integrity, laser scattering tomography, flow pattern defects.

Small point defect clusters grown-in during crystal pulling in present day high quality silicon can have an important impact on gate oxide integrity (GOI). A pronounced effect of the Czochralski (Cz) crystal pulling conditions on GOI is indeed observed for gate oxide thicknesses between 10 and 100 nm. Due to the low density and small size of these grown-in defects, few tools are available to study their nature which is therefore not yet well understood. In the present paper the role of grown-in silicon oxide precipitate nuclei in reduced GOI is discussed as well as the impact of crystal cooling rate.

Both 150 and 200 mm p-type Cz substrates are studied obtained from crystals grown with different cooling ramps. In most cases the pulling rate was above the critical value for stacking fault ring formation, i.e. yielding crystals which are vacancy-rich. The crystal defect density before and after gate oxidation is studied with infrared laser scattering tomography (LST) using a MILSA IRHQ-2 instrument. This technique reveals in the as-grown wafers defect concentrations in the range of 1x10⁶ - 2x10⁷ cm^-3 in good agreement with the flow pattern defects (FPD's) observed after vertical immersion of the wafers in Secco etch for 30 min. The FPD's are often associated with large vacancy clusters as they are not observed in slowly pulled crystals which are known to be self-interstitial rich. Large vacancy clusters are also efficient scattering centres and therefore can not be differentiated from SiO_x particles by a single LST measurement. It can however be expected that the growth and anneal kinetics of both types of scattering particles are strongly different during thermal treatments.

In the present study the evolution of the LST defect density and size is studied as a function of the gate oxidation conditions and by performing low temperature thermal pretreatments. LST results are compared with known oxide precipitate growth kinetics and with the results of FPD etching before and after the different thermal treatments. The observed defect densities and sizes are correlated with gate oxide breakdown characteristics. The impact of crystal cooling rate and vacancy super-saturation on the defect distribution and type is discussed.

Last modified: Mon Feb 19 12:11:06 GMT 1996 JG