Obtain and study of the Al O condensate with the rough surface during reactive ion deposition

Abstract

Properties of the surface determine its morphology. There are various methods for obtaining morphological structures on the surface. These methods can make the surface more hard, smooth, rough, slippery. Particular attention deserves the methods of coating in a vacuum environment. They provide the formation of film coatings of with a constant elemental composition, morphological structure and properties on the surface.

The paper presents the formation of coatings with rough surface, which can be used in various fields: thermal vacuum, solar energy, gas destroyers, optical coatings, electrolytic capacitors... These coatings were created by depositing aluminum on an aluminum capacitor foil in oxygen environment.

The condensate of the aluminum and oxygen compounds can form rough (porous, spongy) structures where there is no crystalline phase of Al₂O₃, but the amount of oxygen in such condensates corresponds to the oxygen content in Al₂O₃. The formation of such condensates occurred in a vacuum by means of the thermionic deposition set-up equipped with pre-vacuum and cryo-condensation pumps. The technological chamber of the set-up was pumped out to the pressure of the residual gases of 5×10^‑4 Pa. In the process of experiment oxygen was injected into the chamber. The oxygen pressure was supported constant within 10^‑1 ‑ 10^‑3 Pa. Evaporation of aluminum was carried out from a copper water-cooled crucible using a 10 kW thermocathode electronic gun with a direct incandescent cathode. The water-cooled high-frequency inductor was placed above the crucible to create plasma of vapor of aluminum and oxygen. It was supplied by a high-frequency generator with a pulse duration of 0.5 μs and a repetition rate of 880 kHz. To carry out thermionic deposition, a negative bias voltage of 500 V was applied to the substrate holder. The coatings were deposited on a smooth aluminum capacitor foil with a thickness of 50 μm and a silicon substrate КДБ -10 (100) in same technological cycle. Coatings with a thickness of 3 - 16 microns were obtained in the experiment.

The resulting samples were studied using scanning electron microscope, X-ray emission analysis with electronic excitation was carried out and the specific capacity of samples of aluminum foil coated with Al‑O condensate was measured by an electrolytic method.

So, coatings of Al‑O condensate were obtained in the work, whose structures were formed to a large extent under the influence of oxygen and depended on its concentration in the process chamber during condensation.

The condensate structures varied from the crystalline to large and small-globular with increasing oxygen pressure. The oxygen pressure is the main control parameter in the formation of Al‑O condensate microstructures. Only the diffraction lines of the aluminum phase are recorded on the X-ray diffraction patterns of the condensate films. A decrease in the intensity of the X-ray lines and their significant expansion are observed with increasing oxygen content in the condensate. The Al‑O condensate is a complex mix of the crystalline phase of aluminum and amorphous Al‑O, in which the diffractograms do not observe the oxide-phase lines of Al₂O₃. Probably, aluminum and oxygen form complex cluster formations on the condensation surface. The specific capacity of samples С₀ measured in an electrolyte at a bias voltage equal to zero according to the standard method had a maximum of the parameter P_O2/V_k. = 2,1×10^‑1 Pa×min./μm, related to the increase in the number of centers of chemical activity that can interact with the electrolyte. At thicknesses of a coating of 3 - 16 microns the specific capacity of a surface has increased in 2 - 10 times, and in some cases in 250, in comparison with a smooth surface of an aluminum capacitor foil.

Ref. 7, fig. 5.