Abstract
A survey of dielectrics which could be deposited as films by evaporation indicated that oxides with permittivity >20, such as those of titanium or tantalum, had high losses and low breakdown voltage, associated with defects in structure and stoichiometry. The low permittivity oxides, alumina, silica and magnesia, were evaporated by electron bombardment heating, which has the advantage of avoiding contamination by other materials. Evaporated alumina and magnesia films had permittivities of about 9, giving 8000 pF/cm 2 for films 1 μ thick. The corresponding figures for evaporated silica films were 5 and 4500 pF/cm 2, respectively. Temperature coefficients were about + 300 ppm/°C. Loss and leakage characteristics were associated with structural defects and moisture absorption, and varied widely. Wiping the films during evaporation moved dust particles and eliminated short circuit or low resistance paths. Electrochemical etching or anodising the base electrode, by conduction through defects in the oxide film, was even more effective in reducing leakage and loss tangent. Evaporated alumina films gave the highest resistance and stability. An insulance of 10 4 ωF was obtained with some anodised specimens, and up to 10 2 ωF without electrochemical treatment. Loss tangent was 0·005 for anodised films, or 0·005 to 0·03 for unanodised films protected from moisture by a varnish or impregnant. Working voltage was 20 to 50 V for a 1μ film. Evaporated silica films, when dry, gave the lowest values of tan δ, 0·002, but capacitance, leakage resistance and dielectric loss were unstable except with full hermetic sealing. Evaporated magnesia films had low breakdown voltage and poor leakage characteristics.
Published Version
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