Dielectric behavior of SiO2films fabricated by microwave electron cyclotron resonance (ECR) plasmas under high‐electric stresses has been studied on the basis of the current–voltage (I–V) characteristics, the capacitance–voltage (C–V) characteristics, and the effects of the ramp rate as functions of film thickness. In the region from the current at the onset of Fowler–Norheim (FN) injection to the current at the onset of trapped space charge effects, theI–Vcharacteristics follow closely the FN relation, implying that at least in this region the motion of the electrons is controlled by their interaction with the lattice rather than by their interaction with traps in the forbidden gap. The width of this current region increases with increasing film thickness for a given ramp rate of applied field indicating that the injected electrons may travel quite a distance before being effectively trapped. This also implies that the effects of this trapped space charge on the reduction of the field at the cathode and hence the injecting current become important at a lower field for thinner films. There is no evidence of electron impact ionization taking place prior to dielectric breakdown. After the onset of FN injection,the current density in the leading filament could reach a value as high as 10 A/cm2at the breakdown field, which could therefore cause thermal instability in the filament. Dielectric breakdown may be initiated by thermal destruction and followed by impact ionization leading to a sharp increase in current at the breakdown field.