Titanium impurity effects in thin film oxide cathodes

This investigation concerns a relatively new form of oxide cathode which is fabricated on a sapphire wafer utilizing electron beam evaporated metallurgy and an alkaline-earth carbonate/photoresist mixture. The heater (Ti/Mo/Ti sandwich), cathode (Ba, Sr, Ca-oxide on W) and grid (Ti on W) structures are delineated using photolithography and wet-etching. During the initial thermal activation of the cathode, the photoresist and carbonates decompose to leave the electron emissive alkaline-earth oxides in well defined areas. The behavior of some prototype thin-film cathodes was found to depend upon the concentration of Ti impurities in the W base-metal film. Secondary ion mass spectroscopy (SIMS) was used to quantitatively determine the concentration of Ti in the W films. High temperature Auger electron spectroscopy (AES) showed rapid surface segregation of these Ti impurities upon heating to 850°C in ultra-high vacuum. The emission lifetime of cathodes fabricated with these W films was inversely proportional to the Ti impurity concentration. Moreover, AES analyses of the emitting oxide surfaces showed a correlation between the Ba/O ratio and the emission current density. It is proposed that the Ti impurities segregate to the oxide/W interface at 850°C where they can reduce the BaO and thereby effectively activate the cathode. However, segregation to the interface, and/or the subsequent BaO reduction, occurs rapidly in the case of Ti in W and leads to a shortened emission lifetime. It is likely that the Ti-oxide reaction products left behind at the interface increase the cathode resistance, and perhaps more importantly, create a chemico-physical barrier which limits any subsequent reaction of Ti, W or other activating agents with the BaO species.