Structure and Properties of Sputtered Ta–Al2O3 Cermet Thin Films

Ta–Al2O3 cermet thin films were produced by diode sputtering. Cathodes were fabricated by powder techniques, plasma spraying and as mechanical composites. Film compositions of 1–20 wt% Al2O3 produced resistivities of 250 to 25 000 μΩ·cm and TCR's of +100 to −450 ppm/°C, respectively. The films showed good thermal stability and could be anodized. Electron microscopy and x-ray analyses suggest a film structure consisting of long columnar grains of bcc Ta, approximately 100 Å in diameter, surrounded by Al2O3. A quantitative analysis is made of tunneling currents in an idealized film. Resistivity-composition and resistivity-temperature data are explained in terms of this analysis. The height of the Al2O3 barrier between grains is estimated to be 0.23 eV for an effective electron mass of 1/9. Barrier thicknesses are estimated at 15–30 Å.