Ta–Ru alloy films were studied for use as a heater material in thermal ink-jet printheads. Thin films with different Ta/Ru compositions were deposited on oxidized Si substrates by cosputtering. The Ru/Ta ratio in the alloy was found to strongly affect the structure, resistivity, stress, and thermal stability. From x-ray and transmission electron microscopy examinations, the Ta1Ru1 phase has formed and dominated in the compositions exceeding 54 at. % Ru. With increasing the Ru content above 13 at. %, the resistivities of the Ta–Ru thin films begin to rise and can reach a maximum of ∼320 μΩ cm in the composition range between 35 and 54 at. % Ru. It is found that the films with small Ru ratios (≤22%) exhibit poor thermal stability and the compressive stress increases after each thermal cycle (from 25 to 450 °C) in air, while the films with larger Ru atomic ratios show good stability. The measured stress-temperature behavior was investigated by Auger depth profiling analysis and x-ray photoelectron spectroscopy. The oxygen content of samples after repeated thermal cycling is largely related to the alloy composition. The observed preferential oxidation of Ta in the Ta–Ru samples can be further interpreted by thermodynamic calculations. The Ta-rich surface oxide is believed to be responsible for the oxidation resistance of the Ru atom at high temperatures. This results in the Ru of the metallic state though the oxidation of Ta occurs. Finally, an open pool test environment is arranged such that the liquid pool can be regarded as an infinite reservoir for the heater, from the standpoint of both momentum and energy transport. A lifetime over 1×107 driving pulses can be obtained for the Ta–Ru thin-film heater with 54 at. % Ru content. ©- - ; 1999 American Vacuum Society.