Effect of pre- and postepitaxial deposition annealing on oxygen precipitation in silicon
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J. H. Matlock SEHAmerica, Incorporated, 4111 Northeast, 112th Avenue, Vancouver, Washington 98662 (Received 31 March 1986; accepted 31 July 1986) Substrate material used for fabrication of P/P + epitaxial silicon wafers was preannealed at 650 °C in nitrogen ambient prior to the epitaxial deposition process for various times up to 300 min. The substrate material originated from a characterized crystal ingot. The results show that annealing before epitaxial deposition can preserve oxide precipitate nuclei from dissolution during the epitaxial deposition process. Additional postepitaxial annealing at 750 °C further enhances the growth of bulk defects.
I. INTRODUCTION In silicon technology internal gettering is a wellaccepted method to remove the heavy metal contamination from the front surface of the wafers during device fabrication. A well-controlled internal gettering process can, therefore, reduce surface defects caused by heavy metal precipitation and also improve the minority carrier lifetime.1'2 In as-received polished silicon wafers, various heat treatment cycles for internal gettering processes have been studied, and a two-step heat treatment cycle (low-high) seems to provide a minimum condition for internal gettering effect. Actually, wafers that are considered to be as-received have already been preheat treated. These heat treatments may include thermal donor annihilation and polysilicon backside processes. The temperatures of these processes are normally between 600° and 700 °C, the nucleation temperature range of oxygen precipitation.3 Unlike polished silicon wafers, more heat treatment cycles are involved in the fabrication of epitaxial silicon wafers. Specifically, high-temperature processes involved in epitaxial deposition can dissolve preexisting precipitate nuclei in the as-grown stage and therefore influence subsequent precipitation. For this reason the study of nucleation and growth processes of oxygen precipitation in as-received epitaxial silicon wafers is very difficult in both P/P + and N / N + . Another reason that makes the precipitation process in epitaxial silicon wafers different from that in polished silicon wafers is the bulk doping concentration. The doping concentration of epitaxial substrate wafers is approximately 1O20 atoms/cm 3 , much higher than that of comparable polished wafers. Dopants in high concentration are known to interact with point defects (vacancies and silicon self-interstitials) and influence 698
J. Mater. Res. 1 (5), Sep/Oct 1986
http://journals.cambridge.org
the precipitation process by either enhancing or retarding the growth of precipitate nuclei.4"6 The importance of the thermal history during crystal growth on the formation of the bulk defects and the precipitation processes has been shown by many researchers.7'8 During cooling after crystal growth, supersaturated oxygen atoms tend to cluster and form large precipitate nuclei of several hundred to a few thousand angstroms in size.9'10 This results in formation of preexisting precipitate nuclei with various s
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