Vacuum Measurement and Leak Detection

With the above introduction to the vacuum generation technologies, it is also important to accurately measure and to detect the level of the vacuum within a chamber. This section introduces the basic working principles of these devices covering the most common CVD systems. 

The mechanical phenomena gauges measure the actual force exerted by the gas. They include a U-tube, a capsule dial, a strain, a capacitance manometer, a McLeod gauge, etc. Vacuum is measured according to the displacement of an elastic material or by measuring the force required to compensate its displacement. The measurement ranges from atmospheric pressure to 102 Pa in rough vacuum conditions. 

CVD operation can still produce satisfactory products. It is therefore important and essential for the system to be able to detect the leakage occurring before and during a CVD operation. Once an acceptable level of leakage is maintained, a CVD process may commence. If there is an unacceptable level of leakage, it can cause a severe problem for the CVD process and the quality of the deposit. 

There are normally two types of leakage detection systems, namely halide leak detector and helium-specific mass spectrometer leak detector, based on the types of gases to be detected. As CVD systems usually use large quantities of halides as reactant precursors, there are normally some residual halides within the reaction chamber. This consequently influences the accuracy of the halide leak detector. In this case, helium-specific mass spectrometer leak detectors are normally used to avoid this problem. 

Under the influence of a homogeneous magnetic field, perpendicular to the path of the ions, they are deflected into circular orbits of a radius given by 

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