GB2240114A - Film nucleation process for growing diamond film - Google Patents

Abstract

A diamond film is provided on a substrate surface such as an infra-red element, e.g. a zinc sulphide window, for its surface protection, by a nucleation and growth process, in which a multiphase carbon film is deposited on the surface, etched with atomic hydrogen to remove preferentially non<diamond phases whereby to provide a surface nucleated with diamond nucleation centres, and a diamond film is grown on said nucleated surface. Layers of germanium, silicon or boron may be interspersed between the initially deposited carbon film and the diamond film. The carbon films may be deposited from methane or carbon monoxide.

Description

FILM NUCLEATION PROCESS This invention relates to a process for surface nucleation and growth of surface films, and in particular to the growth of diamond films, e.g. for the surface protection of infra-red optical elements or for the construction of infra-red windows.

Conventional long wavelength infra-red optical elements are generally manufactured from zinc sulphide (ZnS). This material has acceptable optical properties, but suffers from the disadvantage that its surface is subject to abrasive damage. It has been proposed to provide such devices with a protective diamond or diamond like surface film to provide a high degree of abrasion resistance. However, current processes for the formation of such films suffer from poor nucleation, high substrate temperatures and low growth rates.

Nucleation is a particular problem generally involving severe mechanical damage to the substrate surface and further requiring carbide forming substrate materials to achieve nucleation densities sufficiently high for the provision of coating with small grains and a smooth surface. A proposed solution to this problem is described in US Patent No. 4,767,517. This document describes a process in which sputtering of a graphite target produces particles having a diamond structure.

The films produced by that technique provide abrasion resistance, but are not of sufficient optical quality for use with infra-red elements.

The object of the present invention is to minimise or to overcome the above disadvantages.

According to the invention there is provided a process for growing a diamond structure film on a substrate surface, The process including depositing a multiphase carbon film on the surface, selectively etching the film with atomic hydrogen to remove preferentially non-diamond phases whereby to provide a surface nucleated with diamond nucleation centres, and growing a diamond film on said nucleated surface.

We have found that atomic hydrogen, e.g. from a radio frequency hydrogen plasma, preferentially etches graphitic and other non-diamond phases from a multiphase or glassy carbon film to provide a surface having diamond-like nucleation centres from which a diamond or diamond-like film can then be grown. Preferably the plasma is a pulsed radio frequency plasma.

An embodiment of the invention will now be described with reference to the accompanying drawings in which: Fig. 1 is a schematic diagram of a plasma deposition/etching apparatus; and Figs. 2 to 4 illustrate successive stages in the deposition of a diamond coating on a substrate surface.

Referring to Fig. 1, the plasma deposition/etching apparatus includes a reactor chamber 11 having an inlet 110 whereby gases are supplied to the chamber, and an outlet 111 whereby the chamber may be evacuated via a vacuum pump 12. Typically the reactor chamber comprises an insulating, e.g. quartz tube 112 sealed by end plates 113. A throttle valve 13 controls the evacuation rate and thus controls the gas or vapour pressure within the chamber 11. Deposition and/or etching are effected with an electrically conductive hollow cathode structure 14 within which a substrate 15 is mounted. Typically the hollow cathode structure comprises a cup-like member 141 partially closed by an annular plate member 142. A grounded plate electrode 16 is mounted adjacent the cathode structure 14.Pulsed radio frequency energy is applied to the reactor chamber from a generator 17 via a coupling coil 18.

Advantageously the hollow cathode is water cooled to prevent excessive heating of the substrate 15. A plasma generated in the reactor chamber extends into the hollow cathode. Typically we employ an energy density of about 75 watts/cc to achieve substantially complete dissociation of the plasma. In some applications an impedance matching transformer (not shown) may be employed to couple the generator 17 to the reactor chamber 11.

In use, an electrical bias appears between the grounded plate electrode 16 and the cathode 14. The plasma is substantially fully dissociated by each generator pulse and comprises a mixture of electrons and ions. Electrons have a high mobility, and those near to the grounded electrode 16 migrate to that electrode.

The result of this charge migration is to provide the cathode 14 with a positive electrical bias, typically of 300 to 1000 volts. This electrical bias of the cathode provides negative ion bombardment of the substrate 15. We have found that this results in an improvement of both deposition and etching.

Typically the vapour presence within the reactor is from about 200 torr to 50 torr (equivalent to 26.7 to 6667 neutrons/m2).

Referring now to Figs. 2 to 4, in a typical process a multiphase or glassy carbon film 21 (Fig. 2) is formed e.g. on a zinc sulphide substrate surface 22, by deposition from a pulsed radio frequency hydrocarbon plasma of a saturated hydrocarbon. Typically we employ methane for this purpose.

The multiphase carbon film is then exposed to a pulsed radio frequency hydrogen plasma whereby atomic hydrogen preferentially etches the non-diamond phases from the carbon film to leave areas of diamond nucleation 31 (Fig. 3). In some applications a small quantity of a halogen, e.g. bromine or iodine may be incorporated in the plasma to enhance the selectivity of the etching process.

The nucleated surface provides a diamond crystal seed layer or subsequent homo-epitaxial diamond growth to form a polycrystalline film 41 (Fig. 4). This may be effected by deposition from a pulsed radio frequency plasma of a saturated hydrocarbon such as methane, or of carbon monoxide. In order to minimise thermal expansion mismatch problems, the substrate temperature should not exceed 6000C during the growth process. Preferably the diamond growth should take place under conditions of high electron density and high negative ion concentration. Such conditions are provided by the hollow cathode technique described above.

We have also found that increased diamond growth rates are obtained by incorporation of a small quantity of water in the plasma. This is thought to increase the concentration of atomic hydrogen in the plasma.

In some applications, where thicker diamond films are required, the effects of columnar grain structure and grain growth may result in significant optical scattering loss in the film; This effect may be overcome by the use of a layer structure employing interspersed layers of germanium, silicon or boron.

Such interspersed layers provide a limitation of grain size during growth whilst maintaining a suitable growth surface.

Claims (9)

1. A process for growing a diamond structure film on a substrate surface, The process including depositing a multiphase carbon film on the surface, selectively etching the film with atomic hydrogen to remove preferentially non-diamond phases whereby to provide a surface nucleated with diamond nucleation centres, and growing a diamond film on said nucleated surface.

2. A process as claimed in claim 1, wherein the multiphase carbon film is deposited from a pulsed radio frequency plasma of a saturated hydrocarbon.

3. A process as claimed in claim 1 or 2, wherein said atomic hydrogen etching is performed in the presence of bromine, iodine or mixtures thereof.

4. A process as claimed in claim 1, 2 or 3, wherein a positive electrical bias is applied to the substrate whereby to effect negative ion bombardment of the substrate.

5. A process as claimed in claim 4, wherein said electrical bias is between 300 and 1000 volts.

6. A process as claimed in any one of claims 1 to 5, wherein the substrate is mounted within a hollow cathode structure.

7. A process as claimed in any one of claims 1 to 6, wherein said substrate comprises zinc sulphide.

8. A diamond film nucleation and growth process substantially as described herein with reference to the accompanying drawings.

9. An infra-red element provided with a diamond coating by a process as claimed in any one of claims 1 to 8.