Dielectric properties of films of silicon monoxide

Silicon monoxide is a material which is commonly used as a dielectric medium in thin film capacitors and as an insulating layer in various vacuum evaporated devices, e.g., thin film transistors. It has been shown in a previous publication¹ that the most important parameter in the preparation of this material is the ratio of the number of silicon monoxide molecules arriving at the substrate in unit time compared with the corresponding number of residual gas molecules. This ratio affects the degree of oxidation of the material during deposition and this in turn affects the physical properties, e.g., mechanical stress, porosity and susceptability to further oxidation. This is shown in Figure 1. The horizontal axis is calibrated in units of the arrival ratio N (i. e., the rate of arrival of SiO gas molecules/O2 in this case). The fraction of incident light transmitted gives a measure of the degree of film oxidation since at the wavelength used (3, 500 Å) the transmission of SiO is very much less than that of SiO2. It can be seen from the vacuum value curve that the film tends to SiO2 as N is decreased (either by decreasing the evaporation rate or by increasing the gas pressure). Typically, for a residual atmosphere of oxygen, N = 1 when the rate of deposition is 10 Å/ sec and the pressure is 8 × 10⁻⁶ torr. It can also be seen that exposing the film to the atmosphere causes an increase in the oxidation in every case. This increase is particularly strong for low N films which are very susceptible to the atmosphere, both the oxygen and the water vapor contents of the atmosphere having a strong effect. For this reason it was decided to perform all tests on the films in the original vacuum to ensure that the effects of oxidation were minimized.