Crystal growers have often encounterd mysterious behaviors of melt during their operations of growing crystals from melts. The mysteries include l) relaxation of melt properties, 2) aging effect of melts, 3) critical superheating temperature and critical holding time, 4) stored crystal structure in melt, and 5) priming melt with solid chips. Some of the observed facts are described as examples of these mysteries. As a rational basis for intepreting these mysteries, the concept of the relaxation of a melt from the non-equilibrium state to equilibrium one was introduced. Then possible models for the melt structures to cause relatively long relaxation time was pointed out and the importance of study on the influence of the slow-relaxation melt to crystal growth was emphasized.
BaB_2O_4 composition crystals have two crystal phases, alpha phase (high temperature phase) and beta phase (low temperature phase). They have been grown by the flux method or the flux pulling method, in order to obtain beta phase crystals. Recently, high quality β-BaB_2O_4 (BBO) was successfully grown from a BaB_2O_4 composition melt by D-CZ (direct Czochralski) method without flux elements. That is, metastable phase (beta phase) was crystallized from a BaB2O4 melt in advance of stable phase (alpha phase). Important points of this method are BaB_2O_4 melt structure consideration and growth temperature control. This report discusses the growth mechanism of the D-CZ from the viewpoint of thermodynamics.
Three types of LiNbO_3 single crystals were grown from melt doped with 1, 3 and 5 mol% of MgO. Homogeneities of these crystals without subgrain boundaries are useful enough for optical device substrates. No optical damage was observed in these crystals under irradiation of Ar laser with an incident power of 44 W/cm^2 and an exposure time of up to 10 min. However, optical damage was observed in the second harmonic generation of intracavity frequency doubling at room temperature with a diode-laser-pumped CW Nd: YAG laser and 1 mol% MgO doped LiNbO_3. On the other hand, the bulk laser damage threshold for MgO 1 mol% doped crystal is as high as 14 GW/cm^2 for pulse YLF laser (1.053,μm, 1 ns).
A numerical model has been made to determine the effect of various geometric and thermal variables. The model consists of the following three blocks: (1) the entire furnace system in which entire temperature distributions were calculated considering the effects of radiative heat exchange and gas flow; (2) a crystal and melt system in which temperature distributions and the crystal/melt interface were calculated; (3) thermal stress analysis by which resolved shear stress is obtained. Calculated results were in good agreement with experimental results; the temperature response to the change of heater power, the crystal/melt interface shape which was observed from growth striations and etch-pit densities on 4 inch diameter (100) oriented GaAs wafers.
Effects of applying magnetic field on GaAs crystal growth by LEG technique are reviewed. Temperature fluctuation in GaAs melt was continuously decreased as the vertical magnetic field strength increased from 0 Oe to 7000 Oe. Irregular striations with strong contrast in LEG crystal were reduced at 3000 Oe, but striations with strong contrast reappeared in the crystal grown at 6000 Oe. The shape of solid-liquid interface changed from convex to flat, and the length of slip-dislocation became shorter by applying magnetic field. Distribution of indium concentration along the axial direction was not remarkably changed even at 6000 Oe. The difference of indium concentrations between at the center region and at the peripheral region became larger as the magnetic field strength increased. The strations in IR topographic image which were generated by microscopic fluctuation of EL2 concentration were also reduced at 3000 Oe. It was found that the cellular growth of In-doped GaAs was suppressed, and that the yield was significantly improved by applying magnetic field.
There have been reported three types of magnetic fields to be applied to Czochralski (CZ) silicon crystal growths. This paper reviews the characterictics of impurity concentration and distribution in the crystals grown under the three magnetic fields, and summarizes, as follows: (1) A transverse magnetic field application produces a low or a high oxygen concentration which has not been obtained by the conventional CZ with no magnetic field. However, some serious problems often take place when we once want to obtain the same concentration as that by the conventional CZ which is available for LSI use. The problems are the degradations of oxygen and dopant distributions, and dislocation generations. (2) A vertical magnetic field application enables to obtain various oxygen concentration levels. However, the distributions are usually very inhomogeneous. (3) A cusp magnetic field application enables to obtain various oxygen concentration levels including that by the conventional CZ, and the obtained distributions of oxygen and dopant are always very homogeneous. These advantageous effects are originally caused by the axially symmetric two-dimensional distribution. This is very different from the substantially one-dimensional distribution of a transverse or a vertical magnetic field.
Convection on the silicon melt surface in a quartz crucible has great effect on the CZ silicon single crystallization. However it had been very difficult to visualize its motion. In this report, a silicon melt surface in a quartz crucibles was directly observed by a visicon camera in order to recongnize such a convective phenomena. Measured images were recorded on video tapes and their motion after enhancement of the image contrast by an image processor was analyzed. In the cases of applying cusp magnetic field or no magnetic field, irregularly shaped black band images were observed. When the crucible rotation rate was 0 rpm, those black bands seemed to flow towards only one direction on the melt surface in the case of no magnetic field. On the contrary, those black bands seemd to be almost stopped their motions in the case of applying cups magnetic field. When crucible rotation rates were raised from 2 rpm to 15 rpm, those black bands were splitted into many black bands and showed complicated network pat-terns in the case of applying cusp magnetic field or no magnetic field. From the above results, the convective phenomena on the silicon melt surface might be reflected on the observed images.
Precise in-situ multi-point measurement on the temperature of silicon melts has been carried out during crystal growth in CZ furnace of production size. Fourier analyses of the temperature data for 35 Kg melts show no specific frequency which has been predicted to exist due to a laminar flow by many computer simulation experiments. Two-body-correlation-functions of temperarture among several points in the melts indicate that the laminar flow, if exists, has small contribution to a heat transfer. Effective heat transfer from the crucible wall to the crystal-melt boundary layer in the melts changes drastically owing to a crucible rotation. In conclusion, a turbulent flow is proposed as the most effective factor for the heat transfer and dominant component of the flow.
The inhomogeneity of oxygen precipitates in Czochralski-grown silicon crystals was studied with the X-ray double crystal method. The characteristics of oxygen precipitate distribution in a wafer were clarified as follows: (1) concentric circle patterns were related to growth striations (2) the precipitation rate was dependent on the distance from the wafer center (3) anomalous precipitation occurred near the wafer periphery We discovered that the main factors for the in-homogeneity are the fluctuation in oxygen atom concentration in (1), and the thermal history of the crystal in (2). Although some reseachers explain the formation mechanism of (3) with point defects, this is not sufficiently understood. However we found that the cause of in-homogeneous precipitation in (3) is the cooling rate in high temperature regions (>1200℃). As a result, in order to eliminate inhomogenous precipitations and to obtain a uniform oxygen precipitation in a wafer, we need to decrease the fluctuation in oxygen atom concentration along the growth striations, and to control the crystal cooling process.
Interfacial phenomena of molten oxides were studied experimentally. Flow patterns on the interface depended on melt depth and the interfacial flows were suppressed under shallow melt conditions. Temperature distributions on the interface of molten LiNbO_3 and LiTaO_3 were stable, and fluctuations of flow patterns almost did not affect the temperature distributions. On the other hand, the temperature distributions of molten TiO_2 depended on the interfacial flows. In order to understand the relation between the interfaical flow and bulk flow, fluid flows in silicone oils were visualized under various heating conditions. A steady and stable spoke pattern was observed under a side heating and bottom insulating condition. The spoke pattern was broken when the temperature difference between the bottom and center of the interface was large. A stability diagram for flow in a vessel with a free interface was presented.