Evaluation of Titanium Vacuum Chamber as Getter Pump

Abstract

Methods for removing the surface oxide film have been investigated to make a titanium vacuum chamber itself act as a getter pump. The build-up test of the titanium chamber has been performed to investigate the effect of baking. The result showed that the higher the baking temperature the lower the pressure rise during the build-up. Especially the baking at 400°C, the pressure kept less than 10^-3 Pa after 2 h of build-up. The lower pressure rise in the build-up test of the vacuum-fired titanium chamber suggests that once the surface oxide has been removed by the high temperature such as 850°C in the high vacuum furnace, the surface oxide film can be removed more easily because the regenerated oxide film is thinner than before. The results of the X-ray photoelectron spectroscopy supported these facts; namely, it is shown by the photoelectron spectrum of Ti 2p near the surface that the titanium oxide peak intensity decreases while that of the metal titanium increases by the 400°C baking; the O 1s peak intensity becomes smaller near the surface while that in the bulk becomes larger by the 400°C baking, which implies the thermal diffusion of the oxygen to the bulk; the O 1s peak intensity is reduced near the surface by the vacuum firing. The effect of the removal of the surface oxide film by the sputtering has also been investigated by a throughput method. After sputtering the surface of the titanium chamber, the pressure in the titanium chamber has become lower than that in the chamber of the other side of the orifice, which is pumped by the turbomolecular pump. A large pumping speed was obtained for CO, O₂, and CO₂. Furthermore, the titanium chamber, which is non-evaporable getter (NEG) coated after sputtering the surface titanium oxide film, has been developed to prevent the pumping speed from decreasing by the repeated air exposure and activation process. The result showed the final pressure has not deteriorated even after more than 10 times air exposure and activation. This fact indicated that the surface oxide on the NEG coating would diffuse into the titanium bulk because there is no oxide barrier between the NEG coating and the titanium vacuum chamber.