XPS study on the thermal stability of oxygen-free Pd/Ti thin film, a new non-evaporable getter (NEG) coating

Non-evaporable getter (NEG) coating is a technique to coat the inner walls of vacuum chambers with NEG thin films that can be activated through baking under ultrahigh vacuum (UHV). The activated NEG films suppress outgassing from the chambers and evacuate residual gasses, and consequently maintain UHV in the range 10^–8 Pa with small ion pumps. The most widely-used NEG coating is deposited TiZrV films that can be fully activated after baking at 180 °C for 24 h. However, NEG coatings have rarely been applied to beamlines and endstations in synchrotron radiation facilities. To expand the scope of NEG coating applications, we developed a new NEG coating using oxygen-free Pd/Ti film deposition. The advantages of oxygen-free Pd/Ti coating are the following: 1) it can be activated by baking at 133–176 °C for 12 h; 2) it does not oxidize even after repeated venting-baking cycles; 3) it is potentially economical; 4) it does not require skilled technicians; 5) it can be applied to large chambers with a complex structure; and 6) it can be applied to narrow tubes where Pd and Ti filaments can be arranged. To investigate the thermal stability of oxygen-free Pd/Ti, we measured a series of X-ray photoelectron spectra of oxygen-free Pd(50 nm)/Ti(1 µm) film deposited on stainless steel 304L as a function of annealing temperature in the range 100–360 °C. Ti was found to segregate the surface and oxidizes forming Ti¹⁺ at 280–300 °C. The results indicate that the maximum allowable temperature of oxygen-free Pd/Ti films is approximately 260 °C.