The electrical resistivity and mechanical hardness of reactively sputtered tantalum nitride (TaN) thin films on ceramic substrates have been investigated. Depending on the nitrogen/argon gas flow rate ratio (defined as R), the resistivity of the tantalum nitride films varied unusually widely (10(7) orders) from metal to insulator. The big increase in the resistivity of the tantalum nitride films as the R value increased may be due to the theoretically predicted Ta vacancies and antisite defects (excess N atoms occupying Ta sites) or thermodynamically stable N-rich phase formation under N-rich conditions. N-rich TaN film in this study (R = 2) had a dramatically increased resistivity and seems to be a good candidate material as a seed layer and a capping layer of GMR sensor in data storage, while low-resistive stoichiometric TaN (R = 0.5) is a good candidate material as a barrier layer in semiconductors with its low contact resistance. The hardnesses of underlayer (10 mu m Al2O3), 100 nm Al2O3, and two TaN films (R = 1 and 2) were similar and between 600 and 1600 kg/mm(2). The hardness of the TaN film did not change much as the N content increased in this study, which seems to indicate that N-rich thermodynamically stable phases such as tetragonal Ta4N5 or orthorhombic Ta3N5 (rather than TaN film with anti-site defects) have been formed as the N content increased. (c) 2005 Elsevier B.V. All rights reserved.