Abstract: This article introduces the process of upgrading the vacuum control system of CVD tube furnaces according to customer requirements, and analyzes the problems existing in the vacuum control system of CVD tube furnaces used by customers. These problems are currently encountered in domestic CVD and PECVD tube furnaces. commonly found in . This article also introduces the working principle, structure and related component parameters of the modified vacuum pressure control system in detail. The accuracy of vacuum pressure control after the modification has been greatly improved.
1. Background introduction
The customer ordered a CVD tube furnace to produce small-size materials. The CVD tube furnace and its structure are shown in Figure 1. During use, the customer found that the tube furnace could not guarantee the quality and repeatability of the material during the CVD process, and the material performance fluctuated greatly. The analyzed reason was that the vacuum pressure control was inaccurate and unstable. In order to solve this problem, the customer proposed to upgrade the vacuum control system of this CVD tube furnace.
Figure 1 The CVD tube furnace purchased by the user and its structural content
By analyzing the overall structure of the CVD tube furnace shown in Figure 1, we found that the reason for the poor vacuum pressure control effect is mainly the vacuum control of this tube furnace The system has serious problems in the following aspects, and these problems are common in current domestic CVD and PECVD tube furnaces.
(1) Unreasonable selection of vacuum gauge: For most CVD and PECVD tube furnaces, the vacuum control range is generally 1Pa~0.1MPa ( absolute pressure ), and precise control of the vacuum is required. In the CVD tube furnaces purchased by customers (including products of other brands), in order to save costs, the tube furnace manufacturers are equipped with Pirani gauges and Pirani + capacitance vacuum gauges, but this combined capacitance vacuum gauge is The accuracy in the range of 10kPa~95kPa is only ±5%, and the accuracy in the range of 0.1Pa~10kPa becomes ±15%, which is much different than the full range accuracy of a simple film capacitor vacuum gauge of ±0.25%. A reasonable choice is to use a simple film capacitor vacuum gauge, and two vacuum gauges must be configured to cover the measurement and control of the entire vacuum range.
(2) Control method error: For vacuum degree control in the range of 1Pa~0.1MPa (absolute pressure), it is necessary to use upstream and downstream control modes respectively to achieve good control accuracy. For example, the upstream control mode is used in the range of 1Pa~1kPa, that is, the pumping speed of the vacuum pump is fixed and only the upstream inlet flow is adjusted; the downstream control mode is used in the range of 1kPa~0.1MPa, that is, the upstream inlet flow is fixed and only the downstream is adjusted. exhaust flow. The CVD tube furnace used by the customer only uses the upstream control mode to adjust the inlet air flow, which will inevitably cause great fluctuations in vacuum control within the range of 1kPa~0.1MPa, and also cause a huge waste of working gas.
(3) Multiple proportions of mixed gas control structure errors: In the CVD process, the reaction gas is a mixture of multiple working gases configured in proportion. Although four gas mass flow meters are used in the CVD tube furnace to configure the working gas, the mass flow meter can only ensure the accuracy of the gas mixing ratio but cannot accurately control the vacuum degree. Unless it is a single gas, it can pass A mass flow meter regulates the inlet air flow to achieve vacuum control.
In summary, the CVD tube furnace purchased by the customer has some problems that seriously affect the accuracy of vacuum control. This text will introduce in detail the specific methods to solve these problems and the details of the upgrade. The modified vacuum control system can achieve a control accuracy better than ±1% within the full range.
2. Technical indicators for upgrading
To upgrade and transform the vacuum control system of the customer's CVD tube furnace, the technical indicators that need to be achieved are as follows:
(1) Vacuum degree control range: 1Pa~0.1MPa (absolute pressure).
(2) Vacuum degree control accuracy: ±1% (full range).
(3) control form: fixed point control and curve control.
(4) Input form: programmatic or manual.
(5) PID parameters: self-tuning.
3. Upgrade and transformation technical plan
For the customer's 4-channel air inlet CVD tube furnace, in order to achieve the above technical indicators of the vacuum control system, the technical plan adopted is shown in Figure 2.
Figure 2 Structural diagram of CVD tube furnace vacuum control system
As shown in Figure 2, the technical solution for the upgrade is mainly modified in the following aspects:
(1) also retains the Pirani vacuum gauge to control the vacuum degree. For rough measurements, the most important thing is to use a Pirani meter that can cover ultra-high vacuum monitoring of 0.001Pa~1Pa. However, in the vacuum range of 1Pa~0.1MPa, two film capacitor vacuum gauges are added to cover 1Pa~1kPa and 10kPa~0.1MPa respectively to improve the vacuum measurement accuracy during the CVD process.
(2) For vacuum degree control in the range of 1Pa~0.1MPa (absolute pressure), upstream and downstream control modes are used to control respectively to achieve higher control accuracy. For example, the upstream control mode is used in the range of 1Pa~1kPa, that is, the vacuum pump speed is fixed and only the upstream inlet flow is adjusted; the downstream control mode is used in the range of 1kPa~0.1MPa, that is, the upstream inlet flow is fixed and only the downstream exhaust is adjusted. air flow volume.
(3) For the CVD process that mixes working gases in various proportions, the 4-way gas mass flow controller continues to be retained to achieve accurate proportions of working gas mixing, but the precisely mixed gas enters an buffer tank . The flow of gas in the buffer tank into the CVD tube furnace is adjusted through an electric needle valve, which can not only ensure the accurate mixing ratio of the working gas, but also achieve precise adjustment of the upstream inlet flow.
(4) In order to realize the downstream control mode, an electric needle valve is added to the exhaust port of the CVD tube furnace. The function of this electric needle valve is to adjust the exhaust flow. The downstream control mode is very important in the CVD process. This mode can ensure precise control of the vacuum degree in the range of 1kPa~0.1MPa. If the upstream control mode is used in the range of 1kPa~0.1MPa, on the one hand, the vacuum degree control will fluctuate too much, and on the other hand, a large amount of working gas will be lost ineffectively. The control accuracy of
(5) vacuum degree is not only affected by the measurement accuracy of the vacuum gauge and the adjustment accuracy of the electric needle valve, but also is severely restricted by the accuracy of PID control . To this end, a high-precision PID controller with 24-bit AD and 16-bit DA is selected in the technical solution, which has fixed-point and programmable control functions. At the same time, the PID parameters can be self-tuned to accurately determine the control parameters.
(6) Since two high-precision capacitance vacuum gauges are used to measure the vacuum degree of the entire range, in the actual vacuum degree control process, it is necessary to select the corresponding capacitance vacuum gauge according to different ranges and perform vacuum degree control. Therefore, this requires the PID controller to have the automatic switching function between the two vacuum gauges.
(7) In the upgrade and transformation plan of the vacuum control system of CVD and PECVD tube furnaces, two control modes, upstream and downstream, are used. This requires the PID controller to have both forward and reverse operation functions, and 2 channels can also be used. PID controller that can work simultaneously, one channel corresponds to one electric needle valve.
4. Summary
In view of the serious instability of vacuum control in the CVD process of the customer's 4-channel air-inlet CVD tube furnace, the main reasons for the unstable vacuum control were analyzed: insufficient measurement accuracy of the vacuum gauge and the control method Incorrect, the mixing structure of multiple working gases is incorrect.
In order to solve the above problems, this article proposes a corresponding upgrade and transformation technical plan, replacing a higher-precision film capacitor vacuum gauge, adopting an upstream and downstream control method with higher control accuracy, and adding a buffer tank to the multi-gas mixing pipeline. , and uses an electric needle valve and a 2-channel PID controller with high adjustment and control accuracy. The upgraded vacuum control system can achieve control accuracy and stability of ±1% within the full range of vacuum degree range (1Pa~0.1MPa).
