Shinji Togawa

Oxygen transport analysis in Czochralski silicon melt by considering the oxygen evaporation from the melt surface

Abstract Oxygen transport analysis of silicon melt in a silica crucible by considering the oxygen evaporation from the free surface was performed. The oxygen concentration stays in the region of 1015–1016 atoms/cm3 along the free surface and it strongly depends on the vertical flow under the free surface. It has been found that the oxygen distribution along the free surface affects the oxygen concentration in wafers obtained from grown crystals. The suction and the sweeping out flows at the peripheral region of the crystal are formed beneath the periphery of the crystal, and these oscillatory flows determine the oxygen concentration at the periphery of the crystal. Present results yielded good agreement with the experimental results of the radial oxygen distribution in grown crystals. Thus we established the method for prediction of the oxygen concentration in silicon crystal using the oxygen concentration conditions based on the experimental results for the free surface.

Surface tension of a Si melt : influence of oxygen partial pressure

Abstract Surface tension of molten Si was measured by the sessile drop method in Ar atmosphere with different oxygen partial pressures. It was found that the surface tension of molten Si decreased with increasing temperature, and it was 825 mN/m at the melting point (about 1415°C) in a pure Ar atmosphere. The surface tension also decreased with increasing oxygen partial pressure in the Ar atmosphere, and the temperature coefficient of the surface tension changed from −0.1 mN/(mK) to about −0.26 mN/(mK) with increasing oxygen partial pressure from 0 to 0.319 Torr.

Oxygen transport from a silica crucible in Czochralski silicon growth

In this work, Czochralski silicon crystals were grown from a limited oxygen source crucible to determine the dominant flow of oxygen transport in silicon melt. The oxygen concentration in silicon crystal was found mainly to be controlled by the upward flow below the growth interface, with the crucible bottom as the dominant oxygen source. In this phenomenon, the region under the growth interface is assumed to remain rich in oxygen. Numerical simulation in the same system as that used in the experiment showed the same tendency. It also indicates that the concentration of this region strongly depends on the free surface region, whose concentration is to a large extent determined by dissolution from the crucible corner, which is heated to the highest temperature during crystal growth.

Density anomaly effect upon silicon melt flow during Czochralski crystal growth I. Under the growth interface

Abstract Temperature fluctuations under the growth interface region were measured to evaluate the flow structure during the Czochralski silicon growth process. The impurities which influence the density anomaly of the silicon melt are doped as experimental parameters. The flow structure of this region was found to be basically a random one that is supposed to represent the “soft turbulence” induced by Rayleigh-Benard convection. The addition of gallium to the melt changes the flow structure to a laminar-like one when the melt depth is shallow while the flow structure is still similar to a soft turbulence in the cases of boron-doped and undoped at the same melt depth. These results suggest that the density anomaly of the silicon melt influences the melt flow structure of this region.

Oxygen Transport Mechanism in Czochralski Silicon Melt I . The Whole Bulk Melt

The mechanism of oxygen transport in the bulk silicon melt with and without crystal pulling were investigated experimentally and computationally. Crucible rotation rates were chosen as a parameter, and results suggest that rotation suppresses transport of oxygen from the crucible wall to the crystal growth interface, while calculations indicated the crucible bottom as the oxygen source when the crystal is pulled. The temperature distribution across the crucible bottom, possibly the impetus of the flow, was the most important term in control of the oxygen concentration in silicon single-crystal growth.

Oxygen Transport Mechanism in Czochralski Silicon Melt II . Vicinity of Growth Interface

The mechanism governing oxygen transport in the vicinity of the growth interface with varying rates of crystal rotation was investigated both experimentally and computationally. Rotation induces a sweeping flow out of high oxygen content melt contributing to uniform radial oxygen concentration. At low rates of rotation, low oxygen content melt flows into the growth interface region, rarefying the oxygen concentration at the boule periphery and affecting the longitudinal micro‐oxygen distribution. Calculation verified the correlation between radial velocity and oxygen fluctuation, that fluctuation within the crystal is attributable to the flow of low oxygen content melt from the free surface. At higher rotational speeds, radial outflow obstructs flow from free surface, resulting in a relatively uniform longitudinal micro‐oxygen distribution within the boule.

Influence of Surface Melt Flow on Oxygen Inhomogeneity in Czochralski-Grown Silicon Single Crystal: Studied by Double-Layered Czochralski (DLCZ) Melt Quenching Technique

The oxygen incorporation mechanism at the growth interface during Czochralski (CZ) silicon single crystal growth has been studied using melt quenching technique developed by the double-layered Czochralski (DLCZ) process. A slow crystal rotation rate of 0.5 rpm was chosen to investigate the influence of melt convection in relation to the source of oxygen on the oxygen inhomogeneity in the grown crystal. Micro-Fourier transform infrared spectroscopy (micro-FTIR) measurement and preferential etching revealed that the oxygen variation in the crystal was determined by the balance of the bulk melt with a high oxygen content and the oxygen-depleted melt from the free surface. The major cause of the periodic intake of the oxygen-depleted melt flow during one crystal rotation was probably the inhomogeneous radial temperature gradient. This suggests that thermal asymmetry in the melt is a fatal factor in oxygen inhomogeneity in the crystal. We have postulated that the equilibrium oxygen segregation coefficient is not much smaller than unity, but is rather close to it.

Density anomaly effect upon silicon melt flow during Czochralski crystal growth II. Time-topical flow structure under the growth interface

Abstract The purpose of this work is to investigate the effect of density anomaly upon the time-topical structure of the melt flow under the crystal growth interface by applying a wavelet transform to the temperature fluctuation in this region. The fluctuation components that have a period of about 44 seconds appear irregularly on the time axis and are considered to be caused by detachment of the boundary layer near the bottom of the crucible in the case of a relatively deep melt. This is the characteristic phenomenon of “soft turbulence”, as described in a previous paper [Togawa et al., J. Crystal Growth 160 (1996) 41]. These fluctuation components disappear when gallium is added to the shallow melt, whereas the flow structure remains turbulence-like in the cases of undoped and B-doped shallow melts. A detailed statistical analysis of this time-topical flow structure showed that this flow regime stays unsteady and the flow situation changes from moment to moment. When gallium is added, the flow structure becomes laminar-like and differs clearly from the other two cases. We can thus confirm the existence of a silicon melt density anomaly in the crystal growth system and its influence upon the flow structure.

Melt quenching technique for direct observation of oxygen transport in the Czochralski-grown Si process

Oxygen distribution in the Czochralski-grown (Cz) silicon single crystal is closely associated with melt convection occurring near the growth interface because oxygen atoms in the melt are transported by fluid motion. Using melt quenching developed utilizing the double-layered (DL) Cz process, we studied oxygen transport phenomena around the growth interface. Micro-Fourier transform infrared spectroscopy (micro-FTIR) measurements showed that the oxygen concentration distribution in the growing crystal was related to melt convections, such as the Cochran flow and fluid motion from the melt surface. This also agrees with characteristic oxygen striation in both the crystal and melt observed using micro-FTIR and Wright etching. We concluded that melt quenching using the DLCz process is effective in directly observing the oxygen concentration distribution in the melt during crystal growth.

Influence of the thermal history of melts on the formation of grown-in defects in silicon

We have investigated the influence of time-dependent changes of melt properties on the formation of grown-in defects in silicon single crystals grown by the Czochralski method. It is found that the density of grown-in defects increases if the crystal is pulled immediately after melting. The results indicate that the change in the state of melts can influence the formation of grown-in defects in silicon single crystals.