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A detailed introduction to 10 methods for air tightness testing

For water and gas pipelines, vacuum chambers, etc., airtightness is an important indicator to ensure safe operation of equipment and achieve good process results, especially when there are flammable, explosive, corrosive, or toxic gases inside semiconductor equipment. There are different requirements for airtightness in different occasions, and the 10 common leak detection methods are as follows:

1. Soap soaking method

Fill the valve with a certain pressure of gas (such as nitrogen) and apply soapy water to the suspected leak area. If soap bubbles form, it indicates a leak. Professional standard testing can use gas leak detection sprays.

2. Ultrasonic method

The container or pipeline is filled with gas, and once there is a leak, the gas will rush out. When the size of the leak is small, the flow rate is high, and the Reynolds number is large (describing the relative magnitude of inertial and viscous forces), the gas that rushes out is prone to turbulence. Turbulence produces sound waves of a certain frequency near the leak, and the frequency of sound wave vibration is related to the size of the leak. When the leak is large, the human ear can hear the sound of gas leakage. When the leakage is small and the frequency of the sound waves is high, the human ear cannot hear them, but they can propagate in the air and are called airborne ultrasonic waves. Ultrasonic waves are high-frequency short wave signals, and their intensity rapidly decays with increasing propagation distance. Ultrasonic waves have directionality, which can be used to determine the correct leak location.

3. Thermal radiation detection

At the leakage point, gas escapes or enters the chamber, causing thermal expansion and contraction, resulting in local temperature changes. The location of leaks can be identified through high-precision thermal imaging equipment, which is commonly used for leak detection in heating or refrigerant pipelines.

4. Halogen method

Commonly used halogen gases include chloroform, carbon tetrachloride, trichloroethylene, and fluorocarbons. The halogen leak detector is connected to the exhaust pipeline, and the chamber is evacuated and sprayed with halogen gas. When the halogen gas enters the chamber through the leak hole, the leak detector can detect it. Alternatively, the chamber can be filled with halogen gas, and the leak detector can be connected to a suction gun and placed in a location where leakage may occur. If halogen gas leaks, it can be detected.

5. Sniffing leak detection

Similar to halogen gas positive pressure leak detection, helium gas is filled into the chamber or pipeline, and the leak detector is connected to the suction gun and placed in a possible leak location to detect if there is a helium gas leak.

6. Helium detection

Vacuumize the chamber or pipeline, connect the helium leak detector to the exhaust pipeline, and spray helium gas into the possible leakage area. When helium gas enters the chamber through the leak hole, the leak detector can detect it. If the volume of the chamber to be tested is small, a helium detector can be used to evacuate it directly. The helium detector has a front-end pump and a molecular pump inside. If the volume of the chamber to be tested is large, a bypass high-speed vacuum pump needs to be used to evacuate first, and then a helium detector is used for leak detection to reduce the vacuuming time and improve efficiency. If the leakage rate to be detected is accurate, when spraying helium gas, the test chamber and the high-speed vacuum pump need to be disconnected. Otherwise, a portion of the helium gas entering the chamber will be pumped away by the auxiliary vacuum pump, resulting in a lower measured leakage rate.

7. Bubble method

Fill the pipeline or chamber with air or other gas, and place the potential leak into water. If bubbles emerge, it indicates a leak.

8. Water pressure method

Mainly for water pipe sealing testing, filling the pipeline with water, closing the inlet and outlet valves, maintaining pressure for a certain period of time, and testing pressure drop.

9-10. Pressure reduction and rise method

Pressure drop: Fill the chamber or pipeline with gas and test how much pressure drops within a certain period of time. Pressure rise: Vacuum the chamber or pipeline and test how much pressure rises within a certain period of time. This method is often used to roughly test the airtightness of semiconductor equipment before processing.