Composition and Electrical Properties of R. F. Sputtered Stabilized Zirconia Thin Films

Abstract

Thin calcia stabilized zirconia films were obtained by R.F. sputtering (from 300 to 10,000 Å). The composition and relative distribution of the constituents of these films were analyzed by nuclear microanalysis and MeV ion back-scattering techniques. The stoichiometric composites are obtained in pure oxygen or in mixture of oxygen and krypton atmosphere (PO₂ ≥ 10^-5 Torr). The stoichiometric deposits are very stable and do not change during annealing at 525°C in vacuum or in air. The electrical properties of the deposits are measured by d.c. and a.c. techniques (amplitude of a.c. signal 10 mV) using following structures: (i) Al/CSZr/Si, (ii) Pt/CSZr/Pt/PtO_x/Si or (iii) Pt/CSZr/Ni/NiO/Si. The electrical properties i.e., a.c. conductivities are the same independently of the electrodes used. A conductance value of 10Ω⁻¹ cm^-2 at 350°C for films 1000 Å thick was deduced from the a.c. measurements. The activation energy of a.c. conductivity was measured between 200 and 350°C and was found to be equal to 1.18 eV, which is in good agreement with the literature data reporting the activation energy of oxygen movement in the thick, sintered CSZr electrolytes of the same composition. The structures (ii) and (iii) were tested as an oxygen monitoring solid cells, where Pt/PtO_x and Ni/NiO burried electrodes were used as a reference electrode for the potentiometric measurements of oxygen concentration. The Nernst law is found to be followed in the oxygen pressure range studied (10^-6 – 10^-2 atm. of oxygen) at temperatures as low as 320°C. Stability of the electrochemical measurements as a function of time, temperature and oxygen pressure, was found to be strongly related to the nature of reference, burried electrode; the physical meaning of this effect will be discussed. Some preliminary results concerning the improvement of speed of response of these thin solid oxygen sensors will be also reported.