Study of the reflectivity of silver films deposited by radio frequency and direct current coupled magnetron sputtering

Silver films are deposited on a SiO₂ surface and a cyclo-olefin polymer surface by radio frequency and direct current (RF-DC) coupled magnetron sputtering to realize light reflectors with high reflectivity in the visible light region. Because the reflectivity of silver is relatively low, especially in the short-wavelength region less than 500nm owing to a slight increase in the refractive index of silver, deposition conditions are studied with emphasis on maximizing the reflectivity in this region. Also, protection film depositions of Si₃N₄ on a silver surface by microwave-excited- plasmaenhanced chemical vapor deposition were carried out to improve the durability of silver films, which are easily oxidized and/or sulfurized, resulting in the degradation of the reflectivity of the films. It is found that a high reflectivity in the short-wavelength region can be obtained by Xe plasma sputtering and/or Ar plasma sputtering at a relatively high working pressure, i.e., the conditions with a low ion bombardment energy at the substrate surface. At the same time, a normalized ion flux supplied to the substrate surface at approximately 2 is optimum, where the normalized ion flux is defined as the ratio of plasma-ion flux to deposited-atom flux on the growing film surface. Under such optimized deposition conditions, a silver film with a low resistivity and a smooth surface is obtained. Furthermore, it is found that a Si3N4 film with a thickness of 8 nm on a silver surface serves as a good protection film without serious degradation of reflectivity, and the reflectivity can be maintained even after the immersion of the sample in the boiling water (100 °C) for 2 h.