A Novel Defect Diagnosis Method for Kyropoulos Process-Based Sapphire Growth

When sapphire crystal is prepared with Kyropoulos method, the necking-down growth process is a key stage. Sapphire growth defect is a big problem in this stage. However, diagnosing growth defects is subject to the interference of workers subjectivity and accuracy always goes down. To address the problem, a novel defect diagnosis method is proposed for necking-down growth process in this paper. Industrial CCD sensors replace eyes of skilled workers to observe in this method. A new Defect-Diagnosing Siamese network (DDSN) is used in this method. We use Siamese architecture to learn similarity through pairs of images. We use the deep separable convolution (DSC) into the DDSN to optimize running speed and model size. In experiment, dataset is acquired by industrial CCD sensors in the necking-down growth process. The accuracy of defect diagnosis can reach up to 94.5%. The method significantly improves the traditional way.

I. Introduction

Sapphire single crystal is an important industrial raw material, which has excellent physical properties, such as physical strength, chemical stability and thermal stability [1]. It is widely used in various fields, especially in some harsh environments. Sapphire has been used in many fields. C-plane sapphire is the ideal material for making light-emitting diodes (LEDs) [2] and sapphire can be used for bearing surfaces [3]. Kyropoulos (Ky) method is one of common methods for preparing sapphire [4], which is widely used because of its relatively low production cost. The process of preparing sapphire with the Ky method is as follows: 1) The material in the sapphire crucible is heated to complete melting, and the temperature is controlled at 2050°C (workers estimate temperature by voltage value in the equipment); 2) The control device lowers the seed rod so that the seed rod is inserted into the seed point of the melt; 3) The power of the device and the rotation speed of the seed rod are adjusted. The temperature is controlled so that the free surface temperature of the melt is at the melting point, and the crystal grows slowly; 4) In the process of the crystal necking-down growth, the pulling seed rod slowly rises while the seed crystal rotates to adjust the crystal to growing uniformly in all directions. Using of one or a few necking steps to avoid seed defects [5]. 5) The crystal forms a crystal neck and begins equal diameter growth. Meanwhile, temperature gradually decreases in the furnace. The melt gradually solidifies from top to bottom to form a single crystal ingot. The rate of good sapphire crystal can arrive at 90% in the industry studied.