Evaluation of crystallinity and film stress in yttria-stabilized zirconia thin films

Yttria (3 mol %)-stabilized zirconia (YSZ) thin films were deposited using radio frequency (rf) magnetron sputtering. The YSZ thin films were deposited over a range of temperatures (22-300 °C), pressures (5-25 mTorr), and gas compositions (ArO ratio). Initial studies characterized a select set of properties in relation to deposition parameters including: refractive index, structure, and film stress. X-ray diffraction (XRD) showed that the films are comprised of mainly monoclinic and tetragonal crystal phases. The film refractive index determined by prism coupling, depends strongly on deposition conditions and ranged from 1.959 to 2.223. Wafer bow measurements indicate that the sputtered YSZ films can have initial stress ranging from 86 MPa tensile to 192 MPa compressive, depending on the deposition parameters. Exposure to ambient conditions (25 °C, 75% relative humidity) led to large increase (∼100 MPa) in the compressive stress of the films. Environmental aging suggests the change in compressive stress was related to water vapor absorption. These effects were then evaluated for films formed under different deposition parameters with varying density (calculated packing density) and crystal structure (XRD).