Sapphire Single Crystal Growth Research Articles

The OCS method of seeding point detection using visible vision for large-diameter sapphire single crystal growth via the Kyropoulos method

The seeding process is vital in the preparation of large-diameter sapphire single crystal. It is the key to detect the seeding point during the seeding process. The OCS method is proposed in the paper to detect the seeding point. The OCS method improves the detection method of spoke pattern center proposed by Churl Min Kim, based on this, and the convergence model of spoke pattern center is fitted, so that the real seeding point is detected. In the experiment, the OCS method is verified by comparing with the traditional manual seeding method (operated by skilled seeding technologists) and the method proposed by Churl Min Kim. The OCS method has the same effect as the traditional artificial seeding method and can reduce the number of attempts in a single seeding experiment. Compared with the method proposed by Churl Min Kim, the OCS method can meet the needs of actual industrial production in terms of the number of successful seeding, the number of seeding attempts and the average seeding time.

Numerical study on the effect of additional resistive heating and crystal rotation on sapphire single crystals (Al2O3) grown by the Kyropoulos method

Reduction of melt–crystal interface convexity during Kyropoulos growth of sapphire single crystals through rotation and crucible bottom heating.

Experimental research of optimization methodology for local, resistive - heating of thin molybdenum plates

Because of the unique mechanical and chemical behavior of molybdenum and its alloys, they become one of the most advanced materials for high-temperature heating elements, the most exposed areas of furnace, the aircraft industry and the components of the crystallization apparatus at sapphire single crystal growth. This paper presents unique design an experimental system, which shall be in future optimized through simulation model, for comprehensive analysis of important parameters of resistance heating of pure molybdenum sheet with the thickness of 0,5 mm. The specific details of the shape, size and spatial distribution of the heating electrodes based on copper and graphite are investigated. Technical solutions for generation of contact pressures as well as the mechanism for moving the sheet through the zone of thermal exposure is described in detail. Details of the control of power components of the experimental system are shown schematically in the paper. Numerical simulation of resistance heating are provided in Comsol SW, which gives optimal support for experimental analysis at different conditions of electric current and voltage exposure for resistive contacts made of copper, graphite, and combination of those two materials. Further optimization is done in the way of shape and location modification of the contact electrodes, modification of the specific contact pressure-lowering contact resistance, and modification of time of stress exposure and also the distribution of temperature fields in the exposed zones of molybdenum samples. Presented are also experimental thermal characteristics of the heated volumes of molybdenum metal after the end of the heating-response to thermal exposure using a thermal image technology.

TRIZ and HDC sapphire growth process

The application of Theory of Inventive Problem Solving (TRIZ) into sapphire single crystal growth process provided thorough analysis of a problem as well as introduced two conceptually new solutions. The innovation of the Bagdasarov - Horizontally Directed Crystallization (HDC) by resolving the pollution of the observation window improves the control over the process. The increased accuracy of the control leads to supreme quality of the resultant sapphire single crystal. The ultimate goal of betterment is the complete automation of the growth process.

Crystal front shape control by use of an additional heater in a Czochralski sapphire single crystal growth system

The quality of sapphire single crystals used as substrates for LED production is largely influenced by two defects: dislocation density and bubbles trapped in the crystal. In particular, the dislocation density has a higher value in sapphire grown by the Czochralski (CZ) method than by other methods. In the present study, we predict a decreased value for the convexity and thermal gradient at the crystal front (CF) through the use of an additional heater in an induction-heated CZ system. In addition, we develop a solute concentration model by which the location of bubble formation in CZ growth is calculated, and the results are compared with experimental results. We further calculate the location of bubble entrapment corresponding with the use of an additional heater. We find that sapphire crystal growth with an additional heater yields a decreased thermal gradient at the CF, together with decreased CF convexity, improved energy efficiency, and improvements in terms of bubble formation location.

Influence of temperature-dependent thermophysical properties of sapphire on the modeling of Kyropoulos cooling process

Sapphire is widely used as substrates in semiconductor industry. Kyropoulos (Ky) method is the most popular technique to grow large-size and substrate-quality sapphire single crystals. Computational modeling is a powerful tool to analyze the growth conditions for the improvement of this crystal growth. for simulation, thermophysical properties of sapphire are usually set constant, which may increase the deviation between experiments and modeling results. In this paper, temperature-dependent properties of sapphire, i.e., heat capacity, thermal conductivity, and thermal expansion coefficient, are summarized and adopted in the simulation studies. Their effects on the predicted results are discussed for the cooling process, during which large temperature change in the crystal leads to big differences of the material properties. Comparative analysis tells that temperature difference and thermal stress can be more than 200K and 4MPa for the cases with or without consideration of the temperature-dependence of the properties. Consequently, it is necessary to consider the impact of material properties on the accuracy of numerical simulation during the sapphire single crystal growth.

The influence of crucible and crystal rotation on the sapphire single crystal growth interface shape in a resistance heated Czochralski system

Sapphire single crystals growth by the Czochralski (CZ) method usually have a high dislocation density caused in large part by convexity of the crystal growth interface its thermal gradient. In order to address these issues, we focus on the influence of crucible and crystal rotation on the crystal growth interface shape and thermal gradient in a resistance heating CZ system. Compared to a configuration with no crystal or crucible rotation, rotating both the crystal and crucible in the same direction will result in lower thermal gradient variation with radial location, but a higher convexity. On other hand, rotating the crystal and crucible in opposite directions will result in both lower thermal gradient variation with radial location and a lower convexity.

To investigate interface shape and thermal stress during sapphire single crystal growth by the Cz method

Global modeling is performed to predict electromagnetic field, heat transfer, melt flow, solid/liquid (S/L) interface shape, and thermal stress during RF-heated Czochralski (Cz) single crystal growth of sapphire. The relations between the convexity of the S/L interface and growth parameters, i.e., crystal rotation rate, crystal size, furnace insulation, and RF coil, are established. Thermal stress is represented by the von Mises stress, and the stress status in the growing crystal is characterized by the maximum von Mises stress. The curves regarding growth parameters and the maximum von Mises stress are obtained. According to the analysis, a flat or slightly convex S/L interface could be achieved by modifying the growth parameters, and the crystal quality could be improved by reducing thermal stress and its related defects.

Numerical simulation of heat and fluid flows for sapphire single crystal growth by the Kyropoulos method

Numerical computation has been performed to investigate temperature and velocity distributions for different stages of the Kyropoulos sapphire single crystal-growth process. The finite-element method is employed to solve the governing equations with proper boundary conditions. In the power history considered here, a vortex appears in the melt during growth, and its strength decreases as the input power is reduced. Isotherms in the melt are distorted by flow motion. The crystal–melt interface is always convex towards the melt and in early stages the convexity increases as the input power decreases. When the crystal–melt interface is close to the bottom of the crucible, this interface is flat near the apex because of reduction in growth rate near the upper region caused by input heat from the bottom of the crucible. Therefore, convexity of the crystal–melt interface decreases the input power decreases. The crystal shape predicted by the present simulation is similar to that of crystals grown in the industry.

Influence of gas flow on thermal field and stress during growth of sapphire single crystal using Kyropoulos method

The professional modeling software package CrysVUn was employed to study the process of a large sapphire single crystal growth using Kyropoulos method. The influence of gas pressure on thermal field, solid-liquid interface shape, gas velocity field and von Mises stress were studied for the first time. It is found that the root of the seed melt when gas pressure equals to one atmosphere or more than one atmosphere, especially during the seeding period, this result is consistent with the experimental observation, and this paper presents three ways to solve this problem. The temperature gradient and stress decreases significantly as the gas pressure increases. The convexity of the solid-liquid interface slightly increases when the gas pressure increases. Numerical analysis was used to optimize the hot zone design.

The growth of sapphire single crystals

Sapphire (Al2O3) single crystals were grown by the Czochralski technique both in air and argon atmospheres. The conditions for growing sapphire single crystals were calculated by using a combination of Reynolds and Grash of numbers. Acritical crystal diameter dc = 20 mm and the critical rate of rotation c = 20 rpm were calculated from the hydrodynamics of the melt. The value of the rate of crystal growth was experimentally found to be 3.5 mm/h. According to our previous experiments, it was confirmed that three hours exposures to conc. H3PO4 at 593 K was suitable for chemical polishing. Also, three hours exposure to conc.H3PO4 at 523Kwas found to be a suitable etching solution. The lattice parameters a = 0.47573 nm and c = 1.29893 nm were determined by X-ray powder diffraction. The obtained results are discussed and compared with published data.

Growth of Sapphire Crystals of Complicated Shape

An original apparatus allows the growth of sapphire single crystals with complicated forms. Edge-defined Film-fed Growth (EFG) or Stepanov method, Growth from an Element of Shape (GES) method, as well as imaginative combined variants of those processes are available with this equipment. Related to the growth parameters, the quality of the crystals pulled from the melt is investigated in terms of minor gas bubbles distribution and misorientation. X-ray transmission, optical microscopy together with light transmittance measurements are the tools to make sure of optical applications for those products.

Interface shapes and thermal fields during the gradient solidification method growth of sapphire single crystals

We present a finite-element model describing the melt-growth of cylindrical sapphire single crystals via the gradient solidification method. The advantage of this model lies in its ability to accurately capture complex physical phenomena associated with heat transfer through the system, while remaining modest in its computational requirements. Internal radiative heat transport through the transparent crystalline phase is accounted for in our formulation, as are details of flow fields evolving in the melt during growth. Both buoyancy and surface-tension-gradient (Marangoni) driven convection effects are considered. Results show a strong dependence of the thermal field in the charge and of melt/crystal interface shapes on operating parameters such as crystal growth rate and furnace temperature gradient. Specifically, the large latent heat value associated with this system, coupled with enhanced radiative cooling through the crystalline phase, causes a dramatic reduction in interface curvature and position for relatively high growth rates and shallow furnace gradients. In addition, effects of fluid flow on the thermal field are shown to be unimportant in this system, even when considering growth in relatively large-diameter crucibles. Trends reported here are in general agreement with experimental observations.

The growth of dome-shaped sapphire crystals by the gradient solidification method (GSM)

We describe the growth of sapphire single crystals, in dome geometry, by the gradient solidification method (GSM). Double walled molybdenum crucibles are utilized to grow the near-net-shape geometry. This significantly shortens the fabrication process of the dome to final dimensions, thus reducing the costs. The process is used to grow domes with different shapes (hemispherical and others), sizes (up to 6 inches in diameter) and crystallographic orientations (including the optical axis). Control of the thermal fields and growth rates is very important for the prevention of grain boundaries and scattering centers, as well as for the prevention of geometric distortions arising from surface tension effects.

The EFG growth of sapphire single crystals

Sapphire crystals with plate, rod or tubular shapes have been grown by the EFG (Edge defined, Film-fed Growth) technique. The effect of growth parameters on the perfection of the crystals have been investigated. The crystalline perfection of the crystals grown by this technique is found to be comparable to that of crystals grown by flame fusion method. Some special applications of EFG sapphire are also described.