Superconducting YBa2Cu3O7−xthin films were deposited on Si substrates at 500 °C by rf magnetron sputtering from a stoichiometric oxide target. Metallic oxide RuO2, sputtered by reactive dc magnetron, was used as a buffer layer to nucleate the superconducting film and minimize the reactions between Si and superconductor. The as‐deposited thin films, without further post high‐temperature annealing, were completely superconductive at 79 K. Very smooth surface morphology was demonstrated by scanning electron microscopy. X‐ray diffraction data indicated that the films had a randomly oriented polycrystalline structure. Auger electron spectroscopy did not reveal interdiffusion of elements in the three layers.