Abstract
We have improved the well-known Czochralski single crystal silicon growth method by using two argon gas flows. One flow is the main one (15–20 nl/min) and is directed from top to bottom along the growing single crystal. This flow entrains reaction products of melt and quartz crucible (mainly SiO), removes them from the growth chamber through a port in the bottom of the chamber and provides for the growth of dislocation-free single crystals from large weight charge. Similar processes are well known and have been generally used since the 1970s world over. The second additional gas flow (1.5–2 nl/min) is directed at a 45 arc deg angle to the melt surface in the form of jets emitted from circularly arranged nozzles. This second gas flow initiates the formation of a turbulent melt flow region which separates the crystallization front from oxygen-rich convective flows and accelerates carbon evaporation from the melt. It has been confirmed that oxygen evaporated from the melt (in the form of SiO) acts as transport agent for nonvolatile carbon. Commercial process implementation has shown that carbon content in as-grown single crystals can be reduced to below the carbon content in the charge. Single crystals grown with two argon gas flows have also proven to have highly macro- and micro-homogeneous oxygen distributions, with much greater lengths of single crystal portions in which the oxygen concentration is constant and below the preset limit. Carbon contents of 5–10 times lower than carbon content in the charge can be achieved with low argon gas consumption per one growth process (15–20 nl/min vs 50–80 nl/min for conventional processes). The use of an additional argon gas flow with a 10 times lower flowrate than that of the main flow does not distort the pattern of main (axial) flow circumvention around single crystal surface, does not hamper the “dislocation-free growth” of crystals and does not increase the density of microdefects. This suggests that the new method does not change temperature gradients and does not produce thermal shocks that may generate thermal stresses in single crystals.
Highlights
Structural perfection of single crystal silicon and homogeneous distribution of doping impurities and background impurities are the main quality parameters of single crystals that determine their applicability for microelectronics, high-power electronics and solar engineering
Until recently the semiconductor industry was completely satisfied with Czochralski grown silicon single crystals (Cz-Si) having dislocation densities of less than 10 cm-2, a radial doping impurity distribution inhomogeneity of 7–15% and an oxygen content (NO) of within (5–9) × 1017 cm-3
For example the radial scatter of electrical resistivity should be within ±5% and the oxygen concentration should vary along the crystal length within (8 ± 1) × 1017 cm-3 or (7 ± 0.5) × 1017 cm-3
Summary
Structural perfection of single crystal silicon (absence of grain boundaries, dislocations and associations of vacancies or interstitial atoms and their low bulk distribution density) and homogeneous distribution of doping impurities and background impurities are the main quality parameters of single crystals that determine their applicability for microelectronics, high-power electronics and solar engineering. Homogeneity of doping impurity distributions is typically assessed from the difference of the electrical resistivity from the preset one at distances of within several fractions of a micrometer or within several decades of centimeters along or across single crystals. The distributions of background impurities (oxygen and carbon) are usually evaluated based on changes in background impurity concentrations as determined by IR absorption methods. Until recently the semiconductor industry was completely satisfied with Czochralski grown silicon single crystals (Cz-Si) having dislocation densities of less than 10 cm-2, a radial doping impurity distribution inhomogeneity of 7–15% and an oxygen content (NO) of within (5–9) × 1017 cm-3. The distribution and density of point microdefects depend largely on the distribution and concentration of impurities and the intensity of their mutual interactions during the growth and subsequent heat treatment of single crystals. Oxygen concentrations achieved using this approach may vary over a wide range, from (4–5) × 1017 to (9–18) × 1017 cm-3 [26,27,28,29,30]
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