An Investigation of Vacancy Concentrations in Bulk Silicon
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AN INVESTIGATION OF VACANCY CONCENTRATIONS IN BULK SILICON HORST ZIM•ERMANNa) AND H. RYSSEL Fraunhofer-Arbeitsgruppe for Integrierte W-8520 Erlangen, Germany
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ABSTRACT A method will be presented, which allows the quantitative determination of distributions of single vacancies in bulk silicon. The method uses deep level transient spectroscopy (DLTS) measurements of the platinum or gold concentration after diffusion at a low temperature. An analytical expression allows the calculation of the vacancy concentration from the measured platinum or gold concentration. Vacancy concentrations vary at least from 2.0x101 2 to 2.2x10 14 cm" 3 in float zone silicon.2 The vacancy concentrations in Czrochalski (CZ) silicon are in the range of 4x101 to 2X10 1 3 cm- 3 . Microwave photoconductive decay instead of DLTS allows much faster measurements of vacancy distributions on whole wafers. Furthermore, both methods allow the investigation of oxygen precipitation in CZ silicon. INTRODUCTION There is a substantial interest in the determination of self-interstitial and vacancy concentrations in silicon crystals and wafers. These intrinsic point defects play an important role for instance for the precipitation of oxygen in Czochralski silicon [1,2] and they take part in the formation of swirls, and other microdefects during crystal growth. Microdefects can deteriorate the yield of ultra large scale integrated (ULSI) circuits. So-called D-defects [3], which are believed to consist of vacancies [4], for instance reduce the breakdown voltage of metal oxide silicon capacitors [5]. In the past, rather indirect methods such as positron annihilation [6] and X-ray lattice parameter [7] measurements were used to detect vacancies or vacancy clusters in silicon. The purpose of this article is to show that now a more direct method with a much lower detection limit exists, which allows more accurate measurements of the concentration of single vacancies in silicon in the range of at least 1011 to 1015 cm- 3 . This method takes advantage of the dominance of the dissociative diffusion mechanism of platinum or gold at low temperatures and of the property of deep level transient spectroscopy (DLTS) to determine absolute concentrations. The use of photoconductive decay instead of DLTS, finally, allows the fast measurement of vacancy distributions. THEORY The diffusion of many elements in silicon proceeds via a substitutionalinterstitial exchange. One possibility to describe this exchange is the dissociative diffusion mechanism, also called Frank-Turnbull mechanism [8]. The corresponding reaction for an interstitial metal atom (Mi), which occupies a vacancy (V) and forms a substitutional metal atom (Ms), leads to k+ I Mi + V C-* Ms k-1 a) Present address: Duke University, School of Engineering, Department Mechanical Engineering and Materials Science, Durham, N.C. 27706, USA Mat. Res. Soc. Symp. Proc. Vol. 262. 01992 Materials Research Society
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Another possibility for the substitutional-interstitial exchang
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