3. Increase the cold trap device to prevent the backflow and migration of pump oil

The phenomenon that the pump oil flows through the inlet of the pump to the pumped container is called backflow. The phenomenon of oil molecules moving along the wall into the pumped container is called back migration. Regardless of reflux or reverse migration, it affects the ultimate pressure of the vacuum system. Cold traps are widely used in ultra-high vacuum systems. It is installed between the pump inlet and the vacuum chamber. It can not only effectively capture the reflux vapor and part of the cracked product of the diffusion pump, but also extract the vapor from the vacuum chamber. Ultra-high vacuum systems equipped with various types of traps. After thorough high temperature baking and degassing, the ultra-high vacuum degree of 10-8Pa ~ 10-10Pa (or higher) can be obtained. The commonly used cold trap is the liquid nitrogen cold trap, because at the temperature of liquid nitrogen, the vapor pressure of the pump oil is much lower than the requirement of ultra-high vacuum. The design of the liquid nitrogen cold trap must ensure that the molecules passing through the trap will meet the cold surface, and the structure must be reasonable. After the liquid nitrogen cold trap is installed, the pumping speed of the oil diffusion pump is reduced by no more than 40%.


4. Reduce the back diffusion of the pumped gas

The back-diffusion of the pumped gas is one of the factors that limit the ultra-high vacuum obtained by the diffusion pump. Gas anti-diffusion is related to molecular mass. The smaller the mass, the easier the anti-diffusion. Diffusion pump is easy to reverse diffusion of hydrogen, and there are two methods to prevent hydrogen reverse diffusion: one is to reduce the exhaust pressure at the outlet of the diffusion pump, and two diffusion pumps are used in series to achieve; the other is to install it on the inlet pipeline of the diffusion pump Liquid nitrogen-cooled titanium sublimation traps to trap back-diffused hydrogen.