High Precision Pressure Control Solution for PVT Method SIC Crystal Growth Process and Localized Replacement of Its Supporting Equipment

High Precision Pressure Control Solution for PVT Method SIC Crystal Growth Process and Localized Replacement of Its Supporting Equipment

1. What is Silicon carbide single crystal material?
Silicon carbide single crystal material, which is also known as a wide band gap semiconductor material. It has excellent physical and electrical properties, especially suitable for manufacturing high temperature, high frequency, high power, radiation resistance, short wavelength light emitting and optoelectronic integrated devices, so it is widely used in aviation, aerospace, radar, communications and other fields. At present, the growth of silicon carbide single crystal generally adopts the PVT method.

Since the ultimate purpose of growing silicon carbide single crystal is to obtain large-sized, low-defect silicon carbide single crystal, as the size of silicon carbide single crystal increases, the vacuum pressure control in the single crystal furnace is extremely demanding, and the process gas Pressure changes have a great influence on the growth rate and crystal quality of SiC crystals. Figure 1 shows the time-dependent curves of pressure, temperature and process gas in a typical SiC single crystal growth process.

It can be seen from the process curve shown in Figure 1 that the pressure control in the crystal growth furnace is a precise change process in the whole vacuum degree range, and the whole vacuum degree change range spans between low vacuum and high vacuum (-4 5 -1 510 Pa ~10 Pa), especially in the low vacuum range of 10 Pa~10 Pa, which requires precise control. At present, when using PVT method to prepare SiC single crystal, there are still the following problems:

(1) The downstream mode (adjusting the gas outlet rate) is generally used to control the change of vacuum degree in the whole process. The control accuracy is extremely poor in higher vacuum range of 0.1~1000Pa, and the pressure fluctuation in the crystal growth vessel is large (about ±10%).
(2) The regulating valve and PID controller used in vacuum control device basically use Upstream Flow Control Valves, Downstream Exhaust Throttle Valves and its PID Valve Controllers. Although there are various downstream exhaust throttle valves integrated with a PID controller in an integrated structure in order to reduce costs, the overall cost is still relatively high.
(3) Alternative products for vacuum pressure in Taiwan are also gradually emerging, but there are still problems such as large valve leakage rate, long valve adjustment response time, and inability to automatically switch vacuum gauges of different ranges, which makes it impossible to use both upstream and downstream control modes to reach high-precision control of vacuum pressure within the range.

This article will conduct a detailed technical analysis on the problems existing in vacuum pressure control of the above-mentioned PVT SiC single crystal growth process, and propose corresponding solutions. The core of solution is to adopt the upstream and downstream simultaneous control method to greatly improve the control accuracy and stability in the full pressure range, and introduce the corresponding low leakage rate and high response speed of KaoLu’s Proportional Pressure Regulator and ultra-high High-precision industrial PID controllers, thus reaching the replacement of corresponding imported products.

2. Analysis of Stress Change Process in Silicon Carbide Single Crystal Growth
Figure 1 shows the change curves of pressure, temperature and gas flow during the current PVT method for the growth of third-generation silicon carbide single crystal, in which red line represents a very typical vacuum pressure change process. By analyzing the change process of vacuum pressure at each stage, in order to deeply understand the requirements of vacuum pressure change during growth of SiC single crystal by PVT method.
As shown in Figure 1, the variation of vacuum pressure during the growth of SiC single crystal is divided into the following stages:
(1) High vacuum stage: In the high vacuum level, a high vacuum (1xPa - 1x Pa) to remove air and moisture from containers and materials. This high vacuum level requires that the air pressure needs to be depressurized at a slow and constant rate, thereby avoiding the formation of dust from the silicon carbide powder.
(2) Pre-growth stage: Similarly, in the pre-growth stage, with the filling of process gas and the gradual increase of temperature, the air pressure in container is also required to gradually increase to normal pressure or slightly positive pressure at a constant rate and gas displacement to further remove air and moisture.
(3) Growth stage: In the crystal growth stage, the pressure of container is required to be gradually reduced to a certain set value (growth pressure) at a constant speed and kept constant for a long time. Different growth equipment and processes generally use different growth pressures.
(4) Cooling stage: In the cooling stage, as the temperature gradually decreases, the air pressure in container is required to gradually increase to normal pressure or slightly positive pressure at a constant rate.

It can be seen from the several stages of pressure change during the above single crystal growth process that vacuum pressure control device must meet the following main technical indicators, and these are basically the technical indicators that imported products have reached:
(1) Leak rate: less than 1×Pa./s
(2) Control accuracy and long-term stability: Under any vacuum pressure, the control accuracy is better than 1% (or even 0.5%), and the long-term stability is better than 1% (or even 0.1%).
(3) Response speed: less than 1sec. The response speed also determines the control accuracy and long-term stability. Especially under the joint influence of temperature and flow, vacuum pressure will fluctuate rapidly. Fast response speed is the key to ensuring precise control.
(4) Connecting vacuum gauges with different ranges: 2 capacitance vacuum gauges with different ranges can be connected to cover the entire vacuum pressure measurement control range, and the sensor can be automatically switched and controlled according to the corresponding vacuum degree.
(5) Programmable control: The setting of any pressure control curve can be programmed, and multiple control curves can be stored for the calling of different process control.
(6) PID parameters: can be self-tuning, and can store and call multiple groups of PID parameters.
(7) Communication with the host computer: communicate with the host computer (such as PLC and computer), and has a standard communication protocol.

3. High precision vacuum pressure control solution
It can be seen from the above analysis that pressure required for different silicon carbide crystal growth processes is a wide range from 0.2Pa to 80kPa. At present, the downstream control mode is generally used in pressure process of crystal growth process around the world. A throttle valve is installed between growth vessels.

Vacuum pressure control is reached by constant upstream intake flow and adjusting downstream exhaust flow through throttle valve. For the high pressure range greater than 1kPa, this downstream control mode is very effective and can reach precise pressure control, but for low pressure range (0.1Pa~1kPa), the control effect of downstream mode is extremely poor, and it is necessary to adjust air intake flow and constant downstream. Upstream control mode is for pumping flow.

In addition, the upstream control mode has been widely used in the field of vacuum control, and we have also confirmed in the previous practical application and verification tests that upstream mode can reach precise control of low pressure below 1kPa.

In summary, to reach precise control of vacuum pressure in the full range of 0.2Pa to 80kPa, it is necessary to use upstream and downstream modes respectively. Therefore, we propose a high-precision control solution for vacuum pressure that can implement both upstream and downstream modes. The structure of vacuum pressure control system with simultaneous upstream and downstream control is shown in Figure 2.

In the solution shown in Figure 2, two capacitance vacuum gauges are used to cover the full vacuum range of 0.2Pa to 80kPa. The measurement signals of vacuum gauges are sent to PID controller, which drives upstream electronic proportional flow control valve respectively. And the downstream electric ball valve, the closed-loop control loop reaches the precise control of vacuum pressure in the full range. The specific control process of vacuum pressure is:
(1) When the pressure control setting value is in the high pressure range greater than 1kPa, the PID controller is in the downstream control mode, the PID controller adjusts upstream electronically controlled needle valve to a constant opening, and performs automatic PID control on downstream electronically controlled valve. The pressure measurement value in growth vessel can be quickly equal to the set value by quickly adjusting the opening degree change of electronically controlled ball valve.

(2) When the pressure control setting value is in the low pressure range of less than 1kPa, the PID controller is in upstream control mode, the PID controller adjusts downstream electronically controlled ball valve to a constant opening, and performs automatic PID control on upstream electronically controlled needle valve. The pressure measurement value in the growth vessel can be quickly equal to the set value by rapidly adjusting the opening of the electronically controlled needle valve.

4. Localized Substitution of Supporting Devices
The solution proposed in this article can reach high-precision vacuum pressure control on the premise that vacuum gauge, electronically controlled valve and PID controller meet the technical requirements. The highest accuracy, the stability can easily reach ±0.5% of the set value, and even in most vacuum pressure ranges, the stability can reach ±0.1% of set value.

In the process of vacuum degree control in the range of 0.1Pa~100kPa, foreign products are widely used in the current vacuum technology application field. With the development of localization technology, in addition to film capacitance vacuum gauges, the main supporting devices of other vacuum pressure control systems have completely reached breakthroughs in key technologies. Such as localization, low leak rate and fast response so that the overall technical indicators are similar to foreign products. The PID controller has higher measurement and control accuracy than foreign products, and also has a two-way mode control function that foreign products cannot temporarily reach.

The key supporting devices replaced by localization include electronically controlled needle valves and electronically controlled ball valves for high-speed and low-leakage vacuum, as well as multi-functional ultra-high-precision general-purpose PID controllers, as shown in Figure 3.

(Figure3 KaoLu’s Proportional Flow Control Valve)

KaoLu’s proportional flow control valve shown in Figure 3 all meet the technical requirements in Section 2, especially the high-precision industrial PID controller has excellent performance, among which 24-bit analog-to-digital conversion and 16-bit digital-to-analog conversion. The 0.01% minimum output percentage of double-precision floating-point operation is currently the top index of industrial PID controllers around the world, which can reach ultra-high-precision control of process parameters such as pressure, temperature and flow.

5. Conclusion
For the PVT single crystal growth process, the upstream and downstream bidirectional control solution proposed in this article can reach fast and high-precision control of vacuum pressure in the full range. This solution has been applied in many vacuum technology fields, and the corresponding electronic proportional flow control valve and electric ball valve have similar technical indicators of foreign products, and industrial ultra-high-precision PID controller has excellent performance. These supporting devices combine various vacuum pressure sensors and bidirectional control methods to achieve high-precision control of vacuum pressure.

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