Addtime： 2020-07-16 Browse times： 2388
Smart phones, tablet computers, game machines, navigation systems, flat screen displays, and many other high-tech products are available because of the semiconductor industry. Without this industry, the memory chips and processors used in modern media cannot be produced.
Their production requires extremely clean vacuum conditions and accurate monitoring. For example, after a semiconductor production facility (known as a "wafer plant") is put into operation, a leak test needs to be performed by measuring the pressure rise. The process flow can only be approved if the tightness of the system is approved. If the pressure rise is greater than the defined threshold, the fab is shut down and a leak detection is initiated to identify and repair the leak. With its comprehensive after-sales service system, PFA vacuum provides the best expert support (even for on-site leak detection).
A list of typical on-site services provided by one of our after-sales service engineers:
Helium leak detection in test production environment
"More than 5 milliTorr, I'll get the leak detector!" E. he provides vacuum equipment and on-site leak detection support to operators of a large semiconductor supplier. At present, polysilicon dry etching system must be restarted after standard maintenance.
"Pressure attenuation needs to be measured before restart." He explained. "Pressure rise includes many different sources of air in the system. This may be internal surface degassing or penetration (i.e. gas released through the seal) and one or more leaks. If the pressure rises above our threshold, there is too much leakage. This can interfere with the process. Then the leak must be detected and sealed. "
The measurement showed a pressure rise of 10 milliTorr. This is twice the allowable threshold. If the chamber volume is known, the display scale of the leak detector can be estimated.
The pressure rise and helium leakage rate must be converted into comparable units to feel the value displayed on the helium leak detector.
The leakage rate is mostly shown in mbar L / s. The measurement of pressure rise shows the change of pressure with time. For conversion, the chamber volume of the system must be known. The unit of pressure rise is then converted from "mTorr" to "mbar" and minutes to seconds. In an actual example, 10 mTorr is equivalent to 0.013 mbar or 1.3 · 10-2 mbar L / s. Five minutes is 300 seconds. The volume of the chamber is 5 liters. Therefore, the leakage rate is 2.2.10-4 mbar L / s.
Different air sources in the system
Leakage is not the only source of air in the system that causes pressure rise. In addition, special attention should be paid to the degassing of the inner surface and the penetration of the seals. Which source has the greatest impact depends on the particular system and must be determined separately.
E enters the system and connects with the oil-free, high-performance vacuum leak detector ASM 380. Therefore, he installed the leak detector on the shut-off valve between the turbine molecular pump and the process pump in the basement pipeline.
Observing his system, the operator was surprised: "there is a flange in the chamber of the system that can be easily accessed. This is where we usually connect the leak detector. Why don't you climb into the system to contact the internal flange instead? It means more effort and therefore time-consuming! "
"There may be some truth in this statement," the after-sales engineer replied, "but it's worth the effort. If the leak detector is connected between the turbo molecular pump and the dry pump, the leakage signal is higher and the reaction time is shorter. In addition, leaks can be detected and located more quickly. So in the end, you even save time! "
The flange at the vacuum line in front of the basement is indeed the best connection. Here, the leak detector does not have to compete with the running turbomolecular pump.
Compared with the pumping speed of turbo molecular pump, the effective pumping speed of leak detector is smaller. Therefore, it is much smaller than the leakage rate of 2.2.10-4 mbar L / s measured by pressure rise. When the pumping speed of the turbomolecular pump is about 1000 l / s, the leakage rate will drop below 10-6 mbar L / s.
If the leak detector is connected between the turbomolecular pump and the front stage pump, some airflow still runs into the front stage pump, but the helium pumping speed of the process pump is much lower than that of the turbomolecular pump. If the leak detector is connected to the front vacuum line, the leak detector can predict a leak signal of approximately 1.1.10-6 mbar L / s instead of 2.2.10-4 mbar L / s measured by pressure rise. Therefore, the leakage that causes the pressure rise must be within this scale.
In leak detection, the time constant depends on the volume of the process chamber and the effective pumping speed of helium. When the leakage rate signal reaches 63% of its maximum intensity, the time constant can be determined.
If the leak detector is only connected to the chamber, the time constant in the actual example is about 1.4 seconds. If the leak detector is connected behind the turbomolecular pump, the time constant is 4 ms, so the reaction time of the leak detector is much shorter than that of the leak detector. In our example, reaction time is not important. However, the larger the volume, the more important the pumping speed is to the reaction time.
Special requirements for semiconductor wafer plants
E open the pressure regulator at the helium bottle. He adjusted the helium spray gun so that he could feel the smallest flow of test gas through the gun head. This is absolutely enough to detect the smallest leak. In addition, it can prevent helium loading in a closed environment. Slowly, methodically and patiently, he blows suspected leaking system components from bottom to top.
"If you start at the bottom, the light gas helium will rise to the top. You see, the leak does not exist, it is a virtual leak! " An operator suddenly interrupted. "Yes, that's what we often talk about in leak detection training courses. However, this is true only if the air is absolutely calm and helium is evenly distributed to the environment. In such a clean room, there is always a laminar flow from the ceiling to the floor. It disrupts the test gas. In this case, the test must therefore be carried out against the laminar flow - from bottom to top. "
Suddenly, the leak detector responded. E injection chamber seal. But he is not satisfied. "The value displayed is too small to account for the pressure rise."
He marked the leak and continued. The measuring equipment reacts the second time. The leak seems to have been found. Nevertheless, he checked the rest of the system so that he would not ignore any leaks. It took only a few minutes, but if no leaks were found, another set of tests that required shutting down and restarting the system would have to be taken. Finally, the measured values are stored in the integrated SD card of ASM 380 leak detector. This allows the detection of leaks to be recorded in the report.
Two defective seals can be easily replaced. System restart. Ready for new process measurement
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