PROCEEDINGS
OF THE TOMSK ORDER OF THE RED BANNER OF LABOR
POLYTECHNIC INSTITUTE

Vol. 180　　　　　　　　　　　　　　　　　　　　　　　　　1971

INVESTIGATION OF BREAKDOWN IN SILICON MONOXIDE FILMS UNDER CONSTANT VOLTAGE

A. V. BARANOV, G. A. VOROB'EV, N. P. PERVUKHIN, A. A. KHOROMENKO

(Presented by the scientific seminar of the Research Institute of High Voltages)

The study of the operation of thin dielectric films is of practical and scientific interest.

We investigated the breakdown of silicon monoxide films under constant voltage. Figure 1 schematically shows the structure of the specimen. Five aluminum strips 1 mm wide were deposited on a glass substrate. Then a silicon monoxide film was deposited and, finally, five aluminum strips were deposited perpendicular to those applied earlier. Thus, 25 capacitors were obtained on one substrate.

Fig. 1. Specimen design (schematic): 1, 2, 3, 4, 5—numbers denote the upper electrodes; а, б, в, г, д—letters denote the lower electrodes; Д—dielectric

The aluminum and silicon monoxide were evaporated in a vacuum of \(2 \div 5 \cdot 10^{-5}\) mm Hg. The thickness of the silicon monoxide film in the specimens studied was \(300 \div 5000\) Å.

In the investigation, the electrical circuit shown in Fig. 2 was used; it made it possible to measure the voltage across the specimen and the current through the specimen. At voltages of tens of volts, the current through a single capacitor is \(10^{-8} \div 10^{-7}\) A (current density \(10^{-10} \div 10^{-9}\) A/cm\(^2\)) and, as the voltage increases, usually grows approximately according to a linear law if the current is plotted on a logarithmic scale (Fig. 3).

Fig. 2. Circuit for investigating breakdown and electrical conductivity of films

Fig. 3. Current–voltage characteristics of an SiO film, \(d = 2600\) Å

At voltages of tens of volts, the first flashes appear on the surface of the film; they are associated with breakdown of the weakest spot in the dielectric and melting of the metallic electrode around the site

breakdown. As the voltage is increased, flashes appear more and more often, and then again more rarely. Sometimes the flashes occur predominantly along the boundary of the electrodes, which is associated with the edge effect. The edge effect indicates the electrical character of the breakdown. Beginning at a certain voltage, burnout of the upper electrode around the punctured spot is observed. Evidently, during breakdown of the dielectric film silicon is released, as a result of which the surface conductivity increases greatly.

Fig. 5. Dependence of the electrical strength of films on thickness.

In this process the current through the breakdown site increases so much that the electrode overheats and burns through. The lower electrode is destroyed to a much lesser degree, evidently because of poorer access of oxygen.

As the voltage is increased, the burned-out area of the electrode increases. The process ends with the entire area of the electrode burning out, or with the electrode burning through at the edge (this occurs more often in an edge discharge), and the current ceasing. Flashes on the surface of the film appear up to the end of the tests. Some stages of breakdown and destruction of the film are shown in the micrographs of Fig. 4. The black spots in Fig. 4a indicate the sites of flashes; in Fig. 4b these spots have increased because of burnout of the upper electrode; in Fig. 4c strong burnout of the electrode is shown.

In Fig. 3 the arrows indicate the points corresponding to flashes. The first flashes correspond to the minimum breakdown voltages—

Fig. 4. Characteristic stages of film breakdown:
a—traces of flashes; b—onset of film burn-out; c—complete burn-out of the film.

… due to the coarsest defects in the capacitor. The maximum breakdown voltages \(U_{\max}\), which characterize a solid dielectric without defects, could not be determined in the present case because of electrode burn-out. However, it should be assumed that the value of the voltage near which the greatest number of flashes is observed corresponds to some mean probable value of the breakdown voltage.

In order to determine this value unambiguously for different films, further experiments were carried out at a constant rate of voltage rise equal to \(1\ \mathrm{V/sec}\).

From the graphs, the minimum \(U_{\min}\) and average \(U_{\mathrm{av}}\) breakdown voltages are determined.

The values of \(U_{\min}\), as well as \(U_{\mathrm{av}}\), for different capacitors with the same dielectric thickness were statistically processed, and the mean probable values of these quantities, \(U_{\min.\mathrm{s}}\) and \(U_{\mathrm{av.s}}\), were found.

The obtained dependences \(E_{\min.\mathrm{s}} = f(d)\) and \(E_{\mathrm{av.s}} = f(d)\) are presented in Fig. 5 \(\left(d\right.\) is the dielectric thickness, \(\left. E = \dfrac{U}{d}\right)\).

The value \(E_{\mathrm{av.s}}\) is more suitable for characterizing the breakdown process than the value \(E_{\min.\mathrm{s}}\), since the values of \(E_{\min.\mathrm{s}}\) may apparently vary within much wider limits than the values of \(E_{\mathrm{av.s}}\). However, in determining the operating voltage of a capacitor one should be guided by the value \(E_{\min}\). Taking into account the values of \(E_{\mathrm{av.s}}\), the following two conclusions can be drawn from the data of Fig. 5:

1. \(E_{\mathrm{br}}\) of the dielectric film increases as \(d\) decreases.
2. \(E_{\mathrm{br}}\) of the dielectric film is lower when the upper electrode is the cathode.

The latter can be given the following explanation. During deposition of the upper electrode, some aluminum particles penetrate into the dielectric film and form something like points. When the upper electrode is the cathode, a large field-emission current is apparently obtained from the embedded aluminum particles, which leads to a decrease in electric strength. The same effect is obtained for a continuous solid dielectric [1]. Thus, the observed polarity effect shows that breakdown of a silicon monoxide film is electrical. The increase of \(E_{\mathrm{br}}\) with decreasing \(d\) is associated with electrical strengthening of the dielectric film.

REFERENCES

1. G. A. Vorob’ev, L. G. Nekrasova. FTT, vol. 7, no. 4, 1965.