Chemistry: electrical and wave energy – Processes and products – Electrostatic field or electrical discharge
Patent
1987-08-10
1988-08-02
Andrews, R. L.
Chemistry: electrical and wave energy
Processes and products
Electrostatic field or electrical discharge
204 561, C25F 500
Patent
active
047612118
DESCRIPTION:
BRIEF SUMMARY
This invention relates to thin films. By "thin film" is meant a layer of approximately monomolecular thickness. In particular the invention relates to methods of improving the electrical characteristics of thin films formed on an electrically conductive substrate.
In recent years there has been a great deal of interest in the electrical characteristics of thin films of thicknesses in the order to nanometers formed on electrically conductive substrates, these films being either of insulating or semiconducting materials. As such thin insulating films can support extremely high electric fields, they find application in, for example, capacitor microphones whilst such thin semiconductor films find application in electroluminescent displays.
Such thin films suffer the disadvantage however that defects in the form of voids are frequently present in the films, such voids resulting in electrical short circuits. This is particularly the case where the thin films are formed on the substrate by means of, for example, vacuum evaporation, plasma deposition or spin coating. Furthermore whilst it has previously been thought that thin films formed by the Langmuir-Blodgett technique are defect free recent evidence has shown that this is not the case.
It is an object of the present invention to provide a method of improving the electrical characteristics of thin films formed on electrically conductive substrate.
According to the present invention a method of improving the electrical characteristics of a thin film formed on an electrically conductive substrate comprises immersing the film in a fluid and applying an electrical potential to the substrate such that a self limiting electrochemical reaction within the fluid causes any voids in the film to be preferentially filled.
The film is suitably formed on the substrate by a Langmuir-Blodgett process.
In one particular method in accordance with the invention the fluid is an aqueous liquid, and the potential causes the substrate under the voids to become anodised.
In another particular method in accordance with the invention, the fluid is an electrolyte, and the potential causes the voids to be filled by the product of the electrochemical reduction or oxidation of a substance present within the electrolyte.
Two methods of improving the electrical characteristics of thin films formed on an electrically conductive substrate will now be described by way of examples only, with reference to the accompanying figures which are respective illustrations of the methods in operation.
Referring firstly to FIG. 1 the first method to be described is designed to improve the insulating properties of a monolayer organic film 1 previously desposited on an aluminium substrate 3 using a Langmuir-Blodgett technique as described, for example, in the review article written by the inventors "Langmuir-Blodgett films" in Sci. Prog. Oxf. (1985) volume 69, pages 533-550. To reduce the number of voids in the film 1, the substrate 3 carrying the film 1 is immersed in an aqueous medium 5 such as diammonium hydrogen citrate solution and a positive potential applied to the substrate with respect to the potential of a negative electrode 7 within the aqueous medium 5. In the regions of the aluminium substrate 3 under any voids in the film anodisation of the aluminium will take place, whilst the remaining regions of the substrate 3 will be protected from the aqueous medium 5 by the overlying film 1. Thus the regions of the substrate 3 lying under the voids will be converted to a material which is less electrically conductive than aluminium, this process being self limiting in that the less conductive material will prevent further anodisation of the underlying substrate taking place, ultimately all the voids being sealed off.
It will be appreciate that many other substrate materials may similarly be anodised to a less conductive material to perform the required void sealing, examples of such other substrate matrials being silicon, germanium, iridium, titanium and tantalum.
Referring now to FIG. 2, the second meth
REFERENCES:
patent: 4087582 (1978-05-01), Shirahata et al.
patent: 4514254 (1985-04-01), Klepner
"New Technique for Pinhole Detection . . . ", by Kokkotakis et al. Extended Abstracts, J. Electrochem. Soc., vol. 80-1, 1980, pp. 141-143.
Girling Ian R.
Peterson Ian R.
Andrews R. L.
The General Electric Company p.l.c.
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