Control systems for technological process of growing sapphire single crystal

During the last decades, three methods are mainly used for growing sapphire single crystals: horizontal directional crystallization (GDC), Kyropoulos and Stepanov. All methods are based on growing crystals from melts.

During the process, the following parameters are measured and monitored:

• temperature and water flow in each cooling circuit;
• state of the elements of gas-vacuum systems;
• pressure in the mains and the chamber;
• circuit energy parameters: phase currents and voltages;
• heater energy parameters: current, voltage, power;
• position and speed of rotation of actuators;
• crystallization rate according to the weight sensor;

Features of control systems architecture:

• division of the software for the levels in order to increase reliability: first level — programmable logic controller (PLC), second level — an industrial computer.
• interaction between devices based on Modbus RTU, Modbus TCP protocols.

Features of the control system work:

• implementation of a set of technology programs for each stage of the process;
• having a set of locks on each stage of the process, providing protection from «erroneous» action;
• realization of a script to automatically vacuum pumping system;
• control of emergency situations and the implementation of scenarios for abnormal situations;
• archiving of indications of technological parameters.

Features of the control system interface:

• multilanguage software interface;
• log of operator action and system messages;
• visibility of the state of the plant nodes is provided by a mnemonic scheme, which carries not only an informative, but also an interactive function (state management by clicking on the corresponding element of the mnemonic scheme);
• possibility of access control by entering a password to confirm the action;;
• mechanism of message prompts that explain the reason for the blocked actions of the operator;
• specialized software for viewing archives, allowing you to analyze data and compare event log entries and process parameter values.

Technical Vision in Control Systems

Technical vision using in control systems allows to automate the tasks, which, it would seems, can be solved only with “human factor”.

Determining the position of the crystallization front

One method of producing silicon rods involves founding silicon from a melt down onto a seed through a spinneret located between the melt zone and the inductor in an oxygen atmosphere. In the process of growing bars of polycrystalline silicon, an important factor in maintaining the diameter of the rods is to control the position of the crystallization front. The crystallization front is determined by the image from the video camera. Then the information about the position of the front are passed on the input of the automatic control system, which controls the distance between the water level in the cooling bath and the crystallization front. An important parameter ensuring the quality of determining the front is the indication of the frame area where the bar is located.

Determination of the gap between the seed and the plasma torch

Centrifugal spraying plants allow to obtain metal powders during rotation at high speed of the seed, the end of which is melted by a plasma torch. An important condition for obtaining the desired fraction of powder materials is to maintain a constant gap between the rotating seed (red line) and plasma torch (blue line). The gap is calculated from the frame received from the video camera. Next, the gap value is passed on the input of the automatic control system, which controls the feed rate of the seed to ensure a balance between the flow rate of the alloyed material and the feed rate of the seed.

Determining the position of the meniscus of the crystallization front

The production of sapphire single crystals by the method of stretching from the melt compared with other methods has the advantage that the crystal grows in free space without contact with the walls of the crucible. An important stage, which largely determines the quality of the crystal, is seeding. At this stage, the seed, mounted on a water-cooled rod, is lowered into the melt and a crystal begins to form on it. The temperature of the melt must be such that the rate of crystal growth in the radial direction is low, otherwise the impurities will be trapped and there will be a danger of the crystal sticking to the crucible walls. The melt column, which links the growing crystal with the melt, is supported by the surface tension force and forms a meniscus between the melt surface and the growing crystal. The position of the meniscus is calculated from the frames received from the video camera. Control of overgrowth consists in tracking the position of the meniscus during periodic rod raising (pulling the crystal out of the melt). If the growth rate is too high, the power supplied to the heater is changed, which leads to a change in the surface temperature of the melt.

Plasma spraying

Plasma spraying – the process of coating the surface of the product using a plasma jet.

Control system regulates:

• plasma torch parameters;
• gas distribution;
• powder feeding;
• environment of working chamber.