Improved Control of 15 Inches Diameter Sapphire Crystal Growth

The world's largest sapphire boules up to 340-mm diameter are produced by the Heat Exchanger Method (HEM). In order to meet all applications, the highest purity crackle is used so the product has impurity levels very near the detectability limit of Glow Discharge Mass Spectroscopy (GDMS). The charge size of production 340-mm diameter sapphire boules was increased from 55-kg to 70-kg, and larger 380-mm diameter, 84-kg boules were produced. These boules were used to produce 315-mm diameter, 132-mm high sapphire cylinders to meet customer requirements. Efforts have been taken to produce a nearly flat top surface of HEM-grown boules with minimal undulations along the sidewalls to allow fabrication of larger sapphire pieces for production boules.Up-scaling of the gradient furnace technique has resulted in the growth of large monocrystals of Al2O3 up to 15 cm in diameter, 6.8 cm in height, and 4.5 kg in weight.

The technique was developed to detect the crystallization of the upper-central surface of sapphire crystals grown in crucibles. This technique takes advantage of the large difference in optical absorption coefficients of molten and solid sapphire at the melting point, and is based on the recording of pyrometer readings throughout the course of growth. At the point of solidification of the observed sapphire surface, a knee occurs on the recorded graph. The accumulated data enable the optimization of growth rates at the end of the growth as well as the duration of the growth by proper timing of the beginning of crystal cool-down. It is also possible to detect and terminate unsuccessful growth runs at an early stage.


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