Optical Coating

This invention relates to a process for coating optical components, to apparatus for coating optical components in a glow discharge plasma and to coatings for optical components.

There are a number of known techniques for coating optical components but hitherto it has not proved practical to provide a coating which is multilayer, very hard, wear-resistant, substantially transparent to infrared radiation and substantially anti-reflective to infrared radiation. Primarily this arises because in a multilayer coating the thickness of each layer is of critical importance and existing methods of measuring thickness have proved expensive, complex and inadequate.

The present invention comprises a process for coating optical components comprising the steps of providing a vacuum chamber containing a cathode, arranging a substrate to be coated on the cathode, providing in the chamber a glow discharge plasma containing carbon and another element to be deposited as a coating, feeding gases containing carbon and said other element to the chamber during growth of the coating, and controlling the mass flow rate of said gases to be substantially constant at predetermined levels during respective predetermined time intervals, whereby the substrate is provided with a multilayer coating having predetermined characteristics.

Preferably the mass flow rate is maintained constant to within .+-.10% at each predetermined level.

Preferably the vacuum pressure within the chamber during growth of the coating is controlled to be substantially constant at predetermined levels during said respective predetermined time intervals. Conveniently the vacuum pressure is maintained constant to within .+-.25% at each predetermined level.

Preferably the electrical bias voltage applied to the cathode during growth of the coating is controlled to be substantially constant at predetermined levels during said respective predetermined time intervals. Conveniently the bias voltage is maintained constant to within .+-.15% at each predetermined level.

Preferably the temperature of the surface of the optical component being coated is controlled to be substantially constant at predetermined levels during said respective predetermined time intervals. Conveniently the temperature is maintained constant to within .+-.20.degree. C. at each predetermined level.

By virtue of the present invention where mass flow rate is controlled it has proved possible to dispense with layer thickness measuring devices whilst achieving accurate layer thicknesses.

The gases, the mass flow rates of which are controlled, may for example be Germane (so that the said other element is Ge) and a hydrocarbon gas such as butane. Silane may be used in place of Germane in suitable cases. The optical component may be made of Germanium or Silicon, for example.

The present invention further comprises apparatus for coating optical components, comprising a vacuum chamber containing a cathode, means for delivering a flow of feedstock gases to the chamber, a mass flow rate control arrangement for said means, and means for establishing a glow discharge plasma in said chamber.

The present invention further provides a coating for an optical component, said coating comprising a multiplicity of layers including at least one first layer which is amorphous hydrogenated germanium carbide, at least one second layer which is amorphous hydrogenated germanium, and one third layer which is amorphous hydrogenated carbon, the layers of said multiplicity being arranged such that each second layer is bounded on both sides by a first layer, and the third layer is bounded on one side by a first layer, its other side forming the exposed surface of the coating.

Preferably each first layer has a refractive index of about 2.8, each second layer has a refractive index of about 4.1, and the third layer has a refractive index of about 2.0, the refractive indices being at said predetermined wavelength.

In a first example there is only one second layer, two first layers and the aforesaid third layer. In a second example there are two second layers, three first layers and the aforesaid third layer.

The present invention further comprises an optical component made of germanium and having the aforesaid coating adherent to at least one optical surface of the germanium component.

Preferably the layers of the coating are formed by plasma enhanced chemical vapour deposition (i.e. a glow discharge plasma) from germane (GeH.sub.4) and Hydrocarbon (such as Butane (C.sub.4 H.sub.10)) feed gases.

Preferably the temperature of the surface of the optical component being coated is maintained at a first predetermined level during deposition of each of the first and second layers and at a second predetermined level during deposition of the third layer, the second predetermined level being substantially less than the first predetermined level. Conveniently the first predetermined level is 350.degree. C. and the second predetermined level is 200.degree. C.

Preferably an electrical bias voltage is applied to the optical component during deposition and the bias voltage is maintained at a first predetermined level during deposition of each of the first and second layers and at a second predetermined level during deposition of the third layer, the second predetermined level being substantially greater than the first predetermined level. Conveniently the first predetermined level is 500 volts and the second predetermined level is 1000 volts.

Preferably the chamber vacuum pressure during deposition of the first and second layers is maintained at a first predetermined level and is maintained at a second predetermined level for the third layer the second predetermined level being substantially less (further from atmospheric) than the first predetermined level. Conveniently the first predetermined level is 50 m Torr and the second predetermined level is 8 m Torr.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which:

FIG. 1 illustrates apparatus for coating optical components in accordance with the present invention;

FIG. 2 illustrates the transmittance and absorptance characteristics of a thin germanium substrate coated on both faces with a coating in accordance with the present invention;

FIG. 3 illustrates the transmittance and reflectance characteristics of a thin germanium substrate coated on one face with a coating in accordance with the present invention and on its other face with a known `internal surface` coating;

FIG. 4 illustrates the transmittance characteristic of a thin germanium substrate coated on one face with a prior art coating and on its other face with the same known `internal surface` coating as in FIG. 3.

As is shown in FIG. 1 of the drawings apparatus 20 for coating optical components comprises a vacuum chamber 17 having a cathode 18 for supporting an optical component 16 to be coated on its exposed surface 15. The cathode 18 is heated by a unit 7 and controlled in temperature by a controller 12 so that surface 15 is maintained at a desired temperature. Additionally AC power at radio frequencies (RF) is supplied to the cathode 18 by a power supply unit 6 via an impedance matching unit 5 (for maximum efficiency). The required level of vacuum is established within the chamber 17 by a high vacuum pump 4, motorised throttle valve 3, capacitance manometer 1 and vacuum controller 2 connected in closed loop. Gases are delivered into the chamber 17 by three pipelines with respective closed-loop mass flow controllers 9, 10, 11 via a common ON/OFF valve 8.

In one example of a coating deposited on a substrate surface 15 the coating has four discrete layers and the parameters applicable to the apparatus 20 are set forth in Table I. Layers Nos. 1 and 3 are composed of amorphous hydrogenated germanium carbide: Layer No. 2 is composed of amorphous hydrogenated germanium: and Layer No. 4 is composed of amorphous hydrogenated carbon. Prior to the coating being deposited the surface 15 of the substrate 16 is cleaned by sputtering with Argon applied via mass flow controller 9. Layers Nos. 1 and 3 are deposited by feeding butane (C.sub.4 H.sub.10) via mass flow controller 10 and Germane (GeH.sub.4) via mass flow controller 11. Layer No. 4 is deposited by feeding only butane (C.sub.4 H.sub.10) via mass flow controller 10. The respective levels of flow rate, vacuum pressure, cathode bias voltage, time interval and temperature of surface 15 are identified in Table I together with the design optical thickness of each layer and the design refractive index each at the predetermined wavelength of 10 .mu.m.

In operation of the apparatus 20 the mass flow rates are controlled to within .+-.10%, the bias voltage is controlled to with .+-.15%, the vacuum pressure is controlled to within .+-.25% and the temperature is controlled to within .+-.20.degree. C. as a result of which it has been found that the characteristics achieved by the resultant coating is within 1% of its design values over the three portions of the infrared waveband of particular interest, namely 2.05 to 2.2 .mu.m, 3 to 5 .mu.m, and 8 to 11.5 .mu.m. The characteristics of the coated substrate are shown in FIG. 2 together with the design characteristics (both faces of the germanium disc having the same coating).

The coating referred to in Table I is designed to have good durability when exposed to environmental conditions, i.e. to be an `external coating` and when tested against the U.K. standard (TS 1888) provided the results tabulated in Table II. Further tests at more severe levels than TS 1888 produced the results shown in Table III. Exceptional durability is thereby demonstrated.

In practice the coating referred to in Table I would be applied only to one surface of an optical component (which might form a window having therefore only one surface exposed to environmental conditions), the other surface of the component being coated with a known `internal coating` of which various are known. FIG. 3 illustrates the optical characteristics of such an optical component in the form of a thin germanium disc and for the purposes of comparison FIG. 4 illustrates a characteristic of a similar disc coated with the same `internal coating` and a prior art `external coating`. It will be observed that the coating of the present invention (i.e. as per Table I) provides vastly superior characteristics.

The coating referred to in Table I is a four layer coating and, if expanded to a six layer coating as set forth in Table IV exhibits still further improved optical properties notably a 3% reduction in reflectance over the 8 to 11.5 .mu.m waveband. It will be noted that Table IV is relatively concise in the interests of simplicity, but layer Nos. 1, 3 and 5 are each identical in all respects to layers Nos. 1 and 3 of Table I, whilst layers Nos. 2 and 4 are each identical in all respects to layer No. 2 of Table I, and layer No. 6 is identical in all respects to layer No. 4 of Table I.