High vacuum system with throttling
High vacuum system with throttling
The pressure range of sputtering and other plasma processes is higher than that of various high vacuum pumps. These pumps can be used in the range of 0.5 to 10 Pa by adding a throttle valve between the pump and the vacuum chamber. This kind of throttle valve with small flow conductivity allows the gas from the high pressure and strong vacuum chamber to flow into the pump, while keeping the pump inlet pressure below the maximum pump pressure or critical inlet pressure. The typical sputtering chamber with cathode diameter of 150 to 200 mm has a diameter of 500 mm and a height of 250 mm. The traditional pumping unit is equipped with a 6-inch diffusion pump, but there are also some similar low-temperature pump or turbine molecular pump. The maximum displacement of such a pump is 100 to 200 PA · L / s. The pump is larger and, although more expensive, it also removes gas faster.
The problem of residual gas in the sputtering system is much larger than that in the high vacuum evaporation system. On the one hand, the plasma desorption of impurities on the wall is increased; on the other hand, the conventional sputtering deposition rate is much lower than the typical evaporation rate. Even if the production rates of the two processes are in the same order of magnitude, the sputtering films are more likely to be exposed to the residual gas impurities than the films condensed by the evaporation source. Electron ion collision desorption is an effective way to release gas from the wall of vacuum chamber, which is even more effective than mild baking. In the plasma, the desorbed gases appear to exist in the atomic state, so they can easily react with the sputtered films.
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