Deposition of MgO/Au cermet films by triode sputtering and their characterization

Cermet films, which consist of a mixture of insulator and metal microcrystals, have been suggested as a way to achieve high secondary‐electron emission with good electrical conductivity in cathodes for high‐power microwave tubes. We have radio frequency (rf) sputtered 75% MgO and 25% Au (mol.%) films, 4000 Å thick, from composite targets in a turbopumped ultrahigh vacuum system. A triode sputtering source operated at 1.8 mTorr Ar pressure provided high deposition rates (60 Å min−1) over the surface of a cylindrical cathode that was rotated slowly to insure uniform deposition. The low pressure of the triode sputtering source enabled one to operate in a region of the Thornton structure‐zone diagram (J. Vac. Sci. Technol. 11, 666 (1974)) conductive to the formation of densely packed grains (zone T). The cathode substrate was sputter‐etched before film deposition to remove the surface oxide and contamination, and the deposited cermet was lightly sputtered afterwards with low‐energy Ar ions to enhance the surface MgO concentration by preferential sputtering. The resulting films were characterized by AES and SEM as to their bulk chemical composition, surface contamination, film morphology and thickness. The deposition rate in a geometry in which a small, relatively inexpensive, disk‐shaped target was used to coat a larger cylindrical substrate could be approximated by correcting an average planar deposition rate for the difference in areas. The films showed no evidence of a porous columnar structure but were finely grained and densely packed. Films deposited on Type 304 stainless‐steel substrates adhered well and passed simple adhesive tape tests. The films were the same composition as the sputtering target, with no bulk C contamination or impurity migration from the substrate.