GB2165265A

GB2165265A - Infra-red coatings

Info

Publication number: GB2165265A
Application number: GB08524566A
Authority: GB
Inventors: James Smythe Orr; Richard Telford Corbett
Original assignee: Thales Optronics Ltd
Current assignee: Thales Optronics Ltd
Priority date: 1984-10-04
Filing date: 1985-10-04
Publication date: 1986-04-09
Also published as: GB2165265B; GB8524566D0

Abstract

An infrared waveband coating for a substrate in the form of a film predominantly comprising heavy atoms is formed within a vacuum chamber from a feedstock atmosphere comprising a gas or plasma containing the desired heavy atoms, eg carbon or silicon and deuterium atoms, any hydrogen atoms which may be present in the atmosphere being significantly less in number than the deuterium atoms. Coatings thus formed incorporate no more than trace quantities of hydrogen atoms and exhibit reduced absorption properties, a problem hitherto associated by the presence of hydrogen atoms. In examples a germanium substrate located on an RF cathode is coated using ionized gas atmospheres of C4H10 and SiH4, respectively with C6D6 and deuterium as the source of deuterium atoms.

Description

SPECIFICATION Infra-red coatings This invention relates to application of an infra-red waveband coating to a substrate, the coating being formed by a film predominately comprising heavy atoms such as carbon or silicon atoms.

At the present time the preferred method of applying such a coating to a substrate is by vacuum deposition, the film being derived from a feedstock atmosphere within the vacuum chamber which comprises a gas or plasma of the required heavy atoms in association with hydrogen atoms. By way of example carbon may be extracted from a hydrocarbon such as butane, and silicon may be extracted from silane. The resulting coating predominately comprises the required heavy atoms but the balance is formed by hydrogen atoms, for example of the order of 10% by number. It has been found that these H atoms whilst imparting desirable physical and optical properties to the coating also impart undesirable absorption properties.

It is an object of the present invention to provide an improved infra-red waveband coating formed by a film predominately comprising heavy atoms.

According to the present invention there is provided a method of applying an infra-red coating to a substrate, the coating being formed by a film predominately comprising heavy atoms, said method comprising deriving the film from a feedstock atmosphere within a vacuum chamber, said atmosphere being formed by a gas or plasma containing the said heavy atoms in association with deuterium atoms, significant numbers of hydrogen atoms relative to the number of deuterium atoms being absent from said atmosphere.

It will be appreciated that deuterium is an isotope of hydrogen and it has been found that coatings produced in accordance with the present invention retain the desirable physical and optical properties imparted previously by the presence of H atoms in the coatings but substantially eliminate the undersirable absorption properties hitherto associated therewith.

The method according to the present invention may be implemented by any one of a variety of production process, such as those described in U.K. Patent Specification No.

1582231 and U.S. Patent Specification No.

4444805, utilising a suitably deuterated atmosphere.

Although the present invention substantially eliminates H atoms from the coating trace quantities of H atoms may be present, e.g., of the order of 1%.

The present invention also provides a substrate having an infra-red coating, the coating being formed primarily of heavy atoms, the balance being deuterium atoms with no more than trace quantities of hydrogen atoms.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 illustrates the wavelength dependence of the absorptance of amorphous carbon-containing coatings prepared by the method of the present invention and by a prior art method; and Figure 2 illustrates the wavelength dependence of the absorption coefficient of amorphous silicon-containing coatings prepared by the method of the present invention and by a prior art method.

There is illustrated in Fig. 1 curves A and B depicting the optical properties in terms of percentage absorption of an amorphous carbon-containing coating carried on both surfaces by a thin substrate made of germanium, curve A being for a coating derived from a hydro-carbonatmosphere and curve B being for a coating derived from a deuterated-carbon atmosphere. It will be observed that in the 3-5 micron region curve B has noticeably less absorption than curve A and in the 8-12 micron region curve B has a lower mean level of absorption than curve A. In both cases each coating is quarter wavelength in thickness at the 10 micron wavelength. In both cases the coatings exhit substantially identical physical characteristics, i.e. adhesion tenacity, scratch resistance and hardness. The curve A coating was made by the process described in U.K.

Patent No. 1582231 utilising an atmosphere, within the vacuum chamber, of C4H,o (butane).

The curve B coating was made by the same process utilising an atmosphere of vapourised C6D6 (fully deuterated benzene). More particularly, the curve B coating was obtained by locating the germanium substrate on an RF cathode biassed at 500V and capacitively coupled to a 13.56 MHz generator, the substrate being maintained at a temperature of 250"C. During deposition the generator power was in the range 150-200W. The vacuum chamber atmosphere was established by a flow of 5 standard cubic centimeters per minute (5 sccm) fully deuterated benzene (C6D6) and the chamber pressure during deposition was 30,um.

The coating represented by curve A is primarily formed of carbon atoms the balance being H atoms whereas the coating represented by curve B is primarily formed of carbon atoms the balance being D atoms (deuterium atoms) there being no H atoms present in this case.

Fig. 2 illustrates curves E and F depicting the optical properties in terms of absorption coefficient of an amorphous silicon-containing coating carried on both surfaces of a thin substrate made of germanium. Curve E is for a coating derived from decomposition of silane (SiH4) whereas curve F is for a coating derived from decomposition of silane (SiH4) in an atmosphere of deuterium. It will again be observed that in the 3-5 micron region curve F has noticeably less absorption than curve E and the same applies in this case in the 8-12 micron region.

In both cases the coatings exhibit substantially identical physical characteristics, i.e. adhesion tenacity, scratch resistance and hardness. The curve E coating was made by locating the germanium substrate on an unheated RF cathode biassed at 500V and capacitively coupled to a 13.56 MHz generator. During deposition the generator power was held at about 80W. The vacuum atmosphere was established by a flow of 10 sccm silane (SiH4) and the chamber pressure during deposition was 20 item. The curve F coating was made by locating the germanium substrate on an unheated RF cathode biassed at 1 500V and capacitively coupled to a 13.56 MHz generator, the generator power being held at about 500W during deposition. The vacuum chamber atmosphere was established by a flow of 5 sccm silane (SiH4) and by a flow of 20 sccm deuterium (D2) and the chamber pressure during deposition was 20 jim.

The coating represented by curve E is primarily formed of silicon atoms the balance being H atoms whereas the coating represented by curve F is primarily formed of silicon atoms the balance being deuterium atoms with trace quantities of H atoms.

Claims

1. A method of applying an infra-red coating to a substrate, the coating being formed by a film predominately comprising heavy atoms, said method comprising deriving the film from a feedstock atmosphere within a vacuum chamber, said atmosphere being formed by a gas or plasma containing the said heavy atoms and deuterium atoms, significant numbers of hydrogen atoms relative to the number of deuterium atoms being absent from said atmosphere.

2. A method as claimed in claim 1, wherein said heavy atoms are associated in said feedstock atmosphere with said deuterium atoms.

3. A method as claimed in claim 1, wherein said heavy atoms are associated in said feedstock atmosphere with hydrogen atoms and said feedstock atmosphere further comprises deuterium gas, the number of deuterium atoms in said atmosphere being substantially in excess of the number of hydrogen atoms therein.

4. A method as claimed in any preceding claim, wherein said heavy atoms are carbon atoms or silicon atoms.

5. A method as claimed in claim 1, and substantially as hereinbefore described with reference to curve B of Fig. 1 of the accompanying drawing.