Electrical Breakdown in Thin Dielectric Films

Abstract

In this review paper the basic mechanism of electrical breakdown in thin dielectric films is considered; this is still an argument of debate. Some critical points have been identified: it is, for instance, almost generally accepted that the formation of an electron avalanche is the prime agent of the electrical breakdown. Much evidence indicates the role of the cathode material and the increase of the electric strength as the thickness of the dielectric film decreases. It seems to be proved that a dependence upon the thickness according to an exponential law describes quite satisfactorily the results of breakdown measurements. In NaCl single-crystal layers the electric strength is a function of the thickness w proportional to w−1/2 in the range of the lower thicknesses, but a decrease as w−1/4 is observed in the range of higher thicknesses. These different exponents are justified by two mechanisms, namely the tunnel injection of electrons from the cathode and the electron-phonon scattering. The analysis of experimental results obtained in polycrystalline Al2O3 and SiO2 layers shows that the dependence as w.−1/4 holds in the range of lower thicknesses, but for higher thicknesses the electric strength decreases as w−1/2. This inversion of behaviour can be understood by considering the influence of charged defects in the dielectric film near the cathode. On the other hand it is not possible to conclude that the mechanism of breakdown in thin dielectric films is fully explained, at least when the structural defects play a predominant role.