The main component of the sublimation pump is a filament made of active metal titanium (Ti). By heating the filament with a large current, Ti sublimates and adsorbs on the inner wall of the cavity. Because of the high chemical reactivity of Ti, it immediately captures the surrounding gas molecules and reacts to generate stable compounds, thereby achieving the purpose of obtaining vacuum. After the Ti reaction layer on the inner wall reacts with the gas molecules to reach saturation, the sublimation pump can be started again to form a new Ti reaction layer, thereby continuously venting. If the sublimated Ti is adsorbed on the inner wall of a large area, a large exhaust velocity occurs. For example, Ti atoms are adsorbed on an area of ​​1 m 2 at 20° C., and for nitrogen, an exhaust rate of 24,000 L/s can be achieved. However, at higher pressures (>10-3 Pa), the exhaust speed will be greatly reduced, so sublimation pumps need to be used in combination with other exhaust pumps.
After the sublimation pump is repeatedly used for a long time, the thickness of the Ti film adsorbed on the inner wall of the cavity increases. When the chamber is exposed to the atmosphere, such a Ti film will adsorb a large amount of gas. These gases cannot be removed even after baking at 250°C, and they become the release source of hydrogen molecules. It is only possible to remove these adsorbed gases by maintaining the high temperature above 310°C for a long time.
Ti has a strong activity and can be chemically adsorbed with hydrogen, oxygen, nitrogen, carbon monoxide, and other reactive gases, and the exhaust function can be achieved by capturing and adsorbing these gases. The disadvantage of the sublimation pump is that it cannot exhaust the inert gas such as helium, and the life of the filament is limited.
Sublimation pumps are usually used in combination with ion pumps or molecular pumps. The capacity of the sublimation pump depends on the sublimation rate of Ti and the surface area of ​​the inner wall of the chamber. The sublimation pump has a larger displacement than the ion pump. In an ultra-high vacuum film growth apparatus, it is usually used before the film is grown, so that the film growth proceeds in a higher vacuum environment.
The preferred sublimation pump setting method is to separately set a small cavity of the sublimation pump outside the vacuum chamber (Figure 1); the small cavity is provided with a water-cooled (or liquid nitrogen cooling) structure at the periphery to avoid heat radiation of the high temperature Ti wire. The small cavity is heated to emit gas; the front end of the Ti wire is provided with a shielding plate to reduce the contamination of the vacuum chamber when Ti sublimates.
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