What problems should be paid attention to in the helium mass spectrometry leak detection of vacuum coating equipment?
When the PVD vacuum coating equipment uses a helium mass spectrometer leak detector for leak detection, all or part of the tested equipment components are in a vacuum or pressurized state, and the leak detection device is always on the low pressure side of the tested equipment components. Before the leaking gas enters the leak detection device through the leak holes on the tested equipment parts, a part of it is drawn away by the auxiliary vacuum pumping device, and only a part of the leak detection device enters the leak detection device, which reduces the sensitivity of the leak detection device. Therefore, how to make the leak detector's ability to be fully utilized, in order to obtain the leak detection with higher sensitivity, less operating cost, shorter leak detection time, reliability and stability, etc., should be paid attention to. This article will take the vacuum leak detection method as an example to briefly describe the physical process of the helium mass spectrometry leak detection method.
There are two ways to connect the leak detection device with the components of the equipment to be inspected and the auxiliary air extraction device. After the entire system is connected, the leak detection system operates normally, and the leak detection of the coating equipment can be started. In the case that helium is not sprayed on the leak, the leak detection device can be considered as a zero (or compensated to zero) background value. After helium is sprayed, helium will enter the system through the leak hole, and the partial pressure of helium in the leak detection system will start to increase until the amount of helium entering is equal to the amount drawn by the vacuum system, that is, the state is stable. At this time, if the helium injection is stopped, the pump will gradually reduce the partial pressure of helium in the system due to the effect of pumping, and finally tend to the initial zero (or compensate to zero) background value.
The clearing time and reaction time have been explained separately. These two parameters are indicators of the dynamic characteristics of the leak detection system, and they should be carefully measured and estimated before leak detection. When the response time is large, the signal rises slowly, and the response time is small, the signal rises quickly, so the leak detection speed must be able to match it. If the leak detection speed is too fast, the sensitivity will be reduced accordingly. The clearing time indicates the rate of signal decline after helium is removed, and determines the time required for the leak detection device to return to the state before leak detection, that is, the size of the time interval between two leak detections.
The shortening of response time has always been a concern. For example, people use carrier gas in helium mass spectrometry leak detection to shorten the reaction time. For example, in the suction nozzle method, CO₂ is fed into the vacuum chamber to change the gas flow through the leak from molecular flow to viscous. The stagnant flow speeds up the flow and greatly reduces the reaction time. CO₂ can be removed by the liquid nitrogen cold trap of the leak detection device; in addition, some people have proposed scanning by adding nitrogen to achieve the effect of shortening the reaction time. In the vacuum leak detection of large vessels such as vacuum coating equipment, a concave flange can be installed on the vacuum chamber to perform partial leak detection, so that the volume of the vacuum chamber to be inspected is reduced, thereby reducing the overall Response time.
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