(Invited) Oxygen Precipitation in Highly Doped Silicon Substrates

Abstract

Heavily doped silicon substrates are widely used for the fabrication of discrete devices for power applications. The basic wafer template is generally composed of an epitaxial layer over a heavily doped silicon substrate whose dopant type and resistivity vary depending on the final device characteristics but, in either case of P- and N-type substrates, the demand is more and more oriented towards very low resistivity ranges, such as 0.5-5 mΩ·cm, in order to minimize the substrate contribution to the transistor drain-source resistance. Furthermore, the continuous effort to reduce manufacturing costs is moving discrete device fabrication from 100-150 mm in wafer diameter to 200 mm and even to 300 mm. Often a vertical device structure is chosen, where the electric current flows through the entire wafer thickness, therefore the properties of the wafer bulk affect the device performance and the bulk defectivity level have to be understood and controlled carefully. This paper focuses on the microdefect formation in heavily donor-doped silicon, and, in particular, on the behaviour of oxygen precipitation in heavily arsenic-doped silicon. A challenge when dealing with experimental investigations on the behaviour of oxygen in heavily donor-doped silicon is the difficulty of decoupling the oxygen level and the dopant concentration level during crystal growth. Differently from the case of lightly doped or even of heavily boron-doped crystals, during the growth of silicon crystals heavily doped with volatile donors, such as arsenic, antimony or red phosphorus, a strong interaction is unavoidably observed between oxygen and dopant evaporation, leading to a certain degree of correlation between the resulting oxygen level and the dopant concentration level. To overcome this problem, a set of lightly doped silicon samples with similar initial oxygen concentration and growth conditions are included in this study, to discriminate the impact of the oxygen concentration and the impact of the dopant concentration. A wide arsenic concentration range is explored, from 3 × 1018 cm-3 to 4 × 1019 cm-3. The oxygen precipitation is characterized after a thermal cycle of 4 h at 800°C for nuclei stabilization + 16 h at 1000°C for nuclei growth. Oxide precipitates (BMD) are counted under microscope on the cleaved sample surface after preferential etching. A precipitation retardation effect is observed in the arsenic doped samples when the dopant concentration is higher than 1.7x1019 cm-3 compared with lightly doped samples having the same initial oxygen content and grown under similar conditions. The BMD density in the arsenic-doped samples is uniform along the radius, with no rings or cores, contrary to what is commonly observed in lightly doped samples grown with similar V/G values. This finding is discussed by considering the role played by vacancies in the formation of oxygen precipitates and the impact of the arsenic concentration on the equilibrium concentration of point defects in silicon, deduced from the experimentally observed voids revealed as light-scattering surface defects in polished wafers. For arsenic-doped wafers, the voids count first increases but then – at [As] > 1.7x1019 cm-3 - sharply drops. This dependence of counts on concentration can be explained by assuming that the substitutional dopant Ms can trap vacancies by forming vacancy-impurity complexes VMs (trapped vacancies) while a small fraction of interstitial dopant Mi acts as trapped self-interstitials. The net amount of incorporated vacancy species includes the initial concentration difference [VMs] – [Mi]. The VMs species – assumed of the same positive charge state as Ms - provide a dominant contribution at lower [Ms] whereas the negatively charged Mi species dominate at higher [Ms]. Accordingly, the net vacancy concentration increases at first, but then decreases and tends toward zero [1], reducing the number of vacancies available for void formation as well as for assisting BMD nucleation. [1] V. V. Voronkov, R. Falster, M. Porrini, and J. Duchini, Phys. Status Solidi A 209, 1898 (2012)