Dielectric Breakdown in Thin Evaporated Films of CaF2, MgF2, NaF, and LiF

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Abstract
Prebreakdown dc conduction and breakdown properties have been studied in thin‐film capacitors with dielectrics of CaF2, MgF2, NaF, and LiF. The results are compared mostly with theory developed for electronic breakdown in ionic crystals due to Fröhlich, Callen, and Forlani and Minnaja. Capacitors were formed on glass substrates with dielectric thicknesses from 700 to 30 000 Å and electrodes of Al. Between −200° and 100°C prebreakdown and breakdown conduction appear to be independent. The transition time between prebreakdown and breakdown conduction is 10−8–10−7 sec. Except for some cases where the breakdown field Fmax (≈106 V/cm) displays an initial rise, an increase in temperature from −200 to 100 C is accompanied by a decrease in Fmax. Only the initial rise can be interpreted in terms of theoretical developments by Fröhlich and Callen. An increase in Fmax is observed as the rate of voltage rise varies from 15 to 15 000 V/sec. The breakdown field also varies in most cases as w1/2, where w is dielectric thickness. This behavior is in agreement with the theory of Forlani and Minnaja, implying that impact ionization plays an important role in breakdown. A voltage threshold (≈15 V) for the cessation of breakdown which is independent of thickness and temperature is found for each material studied. This is consistent with the notion that Vmin is the cutoff voltage of a gaseous arc. The above facts attribute an electronic nature to breakdown where, at the onset, the magnitude of current density is not altogether the critical factor. Evidence implies a period devoid of current enhancement just prior to breakdown during which the dielectric accumulates energy necessary to change to a gaseous state. It is the gas which affords the high conductance during breakdown.

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