GB2491151A

GB2491151A - Method and apparatus for measuring the thickness distribution of a deposited layer of coating material

Info

Publication number: GB2491151A
Application number: GB1108755.8A
Authority: GB
Inventors: Paul Jones
Original assignee: Qioptiq Ltd
Current assignee: Qioptiq Ltd
Priority date: 2011-05-24
Filing date: 2011-05-24
Publication date: 2012-11-28
Anticipated expiration: 2031-05-24
Also published as: GB2491151B; GB201108755D0; WO2012160321A1

Abstract

A method of measuring the thickness distribution of a deposited layer of coating material, the method including: adapting the shape of a flexible test sheet so that the flexible test sheet has a curved test surface representative of a curved surface of a component; depositing vaporised coating material on the curved test surface of the flexible test sheet to form a layer of coating material on the curved test surface of the flexible test sheet; and measuring a parameter representative of the thickness of the deposited layer of coating material at two or more locations on the test surface of the flexible test sheet. The drawing shows a holder 30 including holding members or channels 32 for holding a flexible test sheet of, for example aluminium foil,. Curved support surface 36 of the holder is representative of the curved surface of the component.

Description

METHODS AND APPARATUSES FQRJIEASURING THE THICJçN$ DISTRIBUTION OFA DEPOSITED LAYERMF COATINGjIATERIAL This invention relates to a method of measuring the S thickness distribution of a deposited layer of coating material.

Methods of depositing vaporised coating material on a surface of a component are well known. Some of these depositing methods are referred to by the term physical vapour deposition (or "PVD") . Some of these depositing methods are referred to by the tern chemical vapour deposition (or "CVD").

In PVD, a coating material is vaporised by a physical process and is deposited on to the surface of the component by condensation (or precipitation) of the vaporised coating material on a surface of the component. For example, the coating material may be vaporised by high temperature evaporation (e.g. using an electron gun) or by sputtering.

Generally, PVD is carried out in an evacuated atmosphere, e.g. in a vacuum chamber. Generally, PVD does not involve a chemical reaction occurring at the surface of the component other than when a reactive gas such as oxygen is introduced, or dissociation of the evaporant occurs with subsequent recombination at the surface.

In CVD, the coating material is deposited on the surface of the component such that the coating material chemically reacts and/or decomposes on the surface.

In order to measure the thickness distribution of a layer of coating material deposited on a surface, it is necessary to measure a parameter representative of the thickness of the deposited layer of coating material at two or more locations on the surface. For example, the parameter representative of thickness of the deposited layer of coating material may be reflectance, which can be measured e.g. using a spectrophotometer.

In practice, it has been found to be difficult to measure the thickness distribution of a deposited layer of coating material on a curved surface of a component, particularly if that curved surface is steeply curved. For example, in the case of measuring reflectance, conventional spectrophotometers generally require flat substrates, e.g. flat glass test pieces. Although it is possible to position such flat test pieces at locations around a curved surface, those flat test pieces will not accurately represent the curved surface, particularly if that curved surface is steeply curved.

The present invention seeks to address difficulties associated with measuring the thickness distribution of a deposited layer of coating material on a curved surface of a component.

At its most general, the invention relates to the idea of depositing vaporised coating material on a flexible test sheet whose shape has been adapted so that the flexible test sheet has a curved test surface representative of a curved surface of a component. Because the curved test surface of the flexible test sheet is representative of the curved surface of the component, the layer of coating material deposited on the test surface of the flexible test sheet is representative of a layer of coating material that would have been deposited on the curved surface of the component, had the component been used in place of the flexible test sheet. In this way, the thickness distribution of a layer of coating material deposited on the curved surface of a component can be predicted or inferred using measurements obtained from the flexible test sheet.

Advantageously, because the test sheet is flexible, a parameter representative of the thickness of the deposited layer of coating material at two or more locations on the test surface of the flexible test sheet can more easily be measured by flattening the flexible test sheet. For example, the flexible test sheet may be flattened and the reflectance of the flexible test sheet measured, e.g. using a spectrophotometer.

Accordingly, a first aspect of the invention provides a method according to claim 1.

The parameter representative of the thickness of the deposited layer of coating material is preferably reflectance, which can easily be measured, for example, using a spectrophotometer. The method may further include determining the relative (e.g. percentage) difference or actual difference, if any, in thickness of the deposited layer of coating material between two of the locations based on the parameter representative of the thickness of the deposited layer of coating material (e.g. reflectance) as measured at those locations.

For the avoidance of doubt, the parameter that is measured could be the physical thickness of the deposited layer of coating material. In other words, the method could include measuring the physical thickness of the deposited layer of coating material at two or more locations on the test surface of the flexible test sheet. The physical thickness could be measured, for example, using a profilometer such as the Talystep profilometer.

Preferably, the curved test surface is representative of the curved surface of the component such that vaporised coating material deposited on the curved test surface corresponds to the layer of coating material that would have been deposited on the curved surface of the component, had the component been used in place of the flexible test sheet.

However, the curved test surface need not be representative of the curved surface of the component such that the curved surface of the flexible test sheet is substantially identical to the curved surface of the component, as it may be difficult to replicate the curved surface of the component using a flexible sheet, particularly if the curved surface of the component is curved in more than one direction, i.e. curved such that normals to the curved surface at each point on the surface do not lie in the same plane. By way of example, it may be difficult to replicate the curved surface of a dome using a flexible test sheet.

Preferably, the curved test surface of the flexible test sheet is curved in only one direction, i.e. curved such that the normal to the curved test surface at each point on the curved test surface lies in substantially the same plane. In this way, the flexible test sheet may be easier to flatten after the vaporised coating material has been deposited thereon, which may make it easier to measure the parameter representative of the thickness of the layer of coating material deposited thereon, e.g. using a spectrophotometer.

The curved test surface curved in only one direction may be representative of the curved surface of the component, even if the curved surface of the component is curved in more than one direction. Thus, for example, the curved test surface curved in one direction may be representative of a cross-section through the curved surface of the component, even if it is not representative of the curved surface of the component in other respects.

The method may further include depositing vaporised coating material on the curved surface of the component, wherein the vaporised coating material deposited on the curved surface of the component is deposited under substantially the same conditions as the vaporised coating material deposited on the curved test surface of the flexible test sheet. In this case, the parameter representative of the thickness of the deposited layer of coating material on the test surface as measured at the two or more locations would be representative of the same parameter as measured at the corresponding locations on the curved surface of the component. Accordingly, the method may include using the parameter representative of the thickness of the deposited layer of coating material on the test surface as measured at the two or more locations on the test surface of the flexible test sheet to infer the thickness distribution of a layer of coating material which has actually been deposited on the curved surface of the component.

However, the method does not require vaporised coating material to actually be deposited on the curved surface of the component. Instead, the parameter representative of the thickness of the deposited layer of coating material on the test surface as measured at the two or more locations on the test surface of the flexible test sheet may be used to predict or infer the thickness distribution of a layer of coating material that would have been deposited on the curved surface of the component, had the component been used in place of the flexible test sheet (during the depositing of vaporised coating material). In other words, the flexible test sheet can S be used to predict the thickness distribution of a layer of coating material deposited on a curved surface of a component, without having to actually deposit the layer of coating material on a component. This may be of particular benefit if adjustments are being made to a method of depositing vaporised coating material on a curved surface of the component, since the method can be optimised before the layer of coating is deposited on the component.

The method may further include flattening the flexible test sheet before measuring the parameter representative of the thickness of the deposited layer of coating material at the two or more locations on the test surface of the flexible test sheet. This may make it easier to make the measurements, e.g. using a spectrophotometer.

The method may be repeated for one or more further flexible test sheets, with each of the one or more further flexible test sheets being oriented differently (e.g. rotated) with respect to the other flexible test sheet(s) for the depositing of vaporised coating material. This may be particularly useful in inferring or predicting the thickness distribution of a layer of coating material deposited on the surface of a component curved in more than one direction using flexible sheets whose test surfaces are curved in only one direction.

The component could in theory be any component having a curved surface on which vaporised coating material might usefully be deposited. The invention has been found to have particular applicability with optical components having a curved (e.g. concave or convex) surface, e.g. on which it might be desirable to deposit an anti-reflection coating.

Accordingly, the component may be an optical component having a curved surface, e.g. a mirror having a curved surface or a lens having a curved surface.

The component may have a concave, i.e. inwardly curving, or convex, i.e. outwardly curving, surface. The curvature of the concave/convex surface may be regular, e.g. have spherical or elliptical curvature, or may be irregular. In some embodiments, the concave surface is hemispherical, i.e. corresponds in shape to the curved outer surface of a hemisphere.

The concave/convex surface may have a small radius of curvature, (e.g. 200mm or less, 150mm or less, 100mm or less) At such small radii of curvature, it has previously been found to be particularly difficult to measure the thickness of a deposited layer of coating material using conventional substrates, which are typically flat and inflexible.

Equally, the concave/convex surface may have a large radius of curvature, e.g. 500mm or more, 1000 mm or more, 2000 mm or more. At such large radii of curvature, it has previously been found difficult to measure the thickness of a deposited layer of coating material using conventional substrates, which are typically small such that a plurality of substrates are needed to measure the thickness of a layer of coating material deposited on a concave/convex surface having a large radius of curvature.

The coating material could in theory be any material that can be vaporised and deposited on a surface. Suitable coating materials include zinc sulphide, zinc selenide, titanium oxide, tantalum oxide, silicon dioxide and magnesium fluoride.

The flexible test sheet may include a metal foil, e.g. aluminium foil.

The vaporised coating material may be deposited in an evacuated atmosphere, e.g. in the vacuum chamber of a vapour deposition apparatus. This is typical, for example, in physical vapour deposition.

The vaporised coating material may be deposited by physical vapour deposition.

A second aspect of the invention provides an apparatus for, or configured to, carry out a method described herein.

The apparatus may include: a shape adapting means for S adapting the shape of a flexible test sheet so that the flexible test sheet has a curved test surface representative of a curved surface of a component; a vapour deposition apparatus for depositing vaporised coating material on the curved test surface of the flexible test sheet to form a layer of coating material on the curved test surface of the flexible test sheet; and/or a measuring device for measuring a parameter representative of the thickness of the deposited layer of coating material at two or more locations on the test surface of the flexible test sheet.

Preferably, the shape adapting means is a holder for releasably holding the flexible test sheet, the holder being configured to adapt the shape of the flexible test sheet so that the flexible test sheet has a curved surface representative of the curved surface of a component when the flexible test sheet is releasably held by the holder.

The holder may include one or more holding members which define a channel in which the flexible test sheet can be releasably held.

The holder may include a curved support surface for supporting the flexible test sheet in its adapted shape when the flexible test sheet is releasably held by the holder, the curved support surface of the holder being representative of the curved surface of a component.

The holder may be mountable, or mounted, in the vapour deposition apparatus. The holder may be rotatably mountable/mounted in the vapour deposition apparatus, e.g. such that a flexible sheet held by the holder can be rotated with respect to the vapour deposition apparatus.

The vapour deposition apparatus may include means for providing an evacuated atmosphere, i.e. a vacuum chamber. The vapour deposition may include a source of coating material, e.g. at which vaporised coating material is produced when the vacuum deposition apparatus is in use.

The vapour deposition apparatus may be a physical vapour deposition apparatus, i.e. a vapour deposition apparatus configured to perform physical vapour deposition.

The measuring device may be a spectrophotometer.

A third aspect of the invention provides a shape adapting means described herein. Accordingly, the third aspect of the invention may provide a holder as described above.

The invention also includes any combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided.

Embodiments of our proposals are discussed below, with reference to the accompanying drawings in which: Fig. 1 is a perspective view of a holder for releasably holding a flexible test sheet.

Fig. 2 is an example reflectance plot for a single layer of coating material deposited on a flexible sheet.

Fig. 3 shows a vapour deposition apparatus.

Figs. 4 and 5 are reflectance plots produced during experimental work.

Fig. 1 shows a holder 30 for releasably holding a flexible test sheet (not shown), e.g. of aluminium foil. The holder 30 is configured to adapt the shape of the flexible test sheet so that the flexible test sheet has a curved surface representative of a curved surface of a component (not shown) when the flexible test sheet is releasably held by the holder 30. The holder 30 may be mounted in a vapour deposition apparatus (not shown), e.g. the vapour deposition apparatus of Fig. 3.

In this example, the component is a dome having a concave surface which is hemispherical. In this example, the curved test surface of the flexible sheet is curved in only one direction and is representative of a cross-section through the hemispherical surface of the dome.

The holder 30 may include a pair of holding members 32 which define a channel in which the flexible test sheet can be releasably held. The flexible test sheet can be inserted into (and removed from) the channel via an insertion point 34. The holder may also include a curved support surface 36 for supporting the flexible test sheet in its adapted shape when the flexible test sheet is releasably held by the holder 30, the curved support surface 36 of the holder 30 being representative of the curved surface of the component.

An outer surface 38 of the holder 30 is shaped to allow the holder 30 to be mounted in a vapour deposition apparatus (not shown) . The holder 30 may be rotatably mountable in a vapour deposition apparatus via the outer surface 38 of the holder 30.

The holder 30 and all its parts may, for example, be made of stainless steel, although other materials could equally be used, e.g. aluminium.

In use, a flexible test sheet (e.g. of aluminium foil) may be inserted into the holder 30 via the insertion point 34, thereby causing the holder 30 to adapt the shape of the flexible test sheet so that the flexible test sheet has a curved test surface representative of a curved surface of a component. Next, the holder 30 may be mounted in a vapour deposition apparatus, e.g. the vapour deposition apparatus of Fig. 3. The vapour deposition apparatus may then be used to deposit vaporised coating material on the curved test surface of the flexible test dheet to form a layer of coating material on the curved test surface of the flexible test sheet. The flexible test sheet may then be removed from the holder 30 via insertion point 34, and then flattened. Finally, a parameter representative of the thickness of the deposited layer of coating material on the test surface may be measured at two or more locations on the flexible test surface. For example, reflectance of the deposited layer of coating material may be measured at the two or more locations, e.g. using a spectrophotometer. In this way, the thickness distribution of the deposited layer of coating material can be measured.

As explained above, in this example, the component is a dome having a concave surface which is hemispherical, yet the curved test surface of the flexible sheet is curved in only one direction and is representative of a cross-section through the hemispherical surface of the dome. Accordingly, the above described method may be repeated for one or more further flexible test sheets, with each of the one or more further flexible test sheets being oriented differently with respect to the other flexible test sheet(s) for the depositing of vaporised coating material. In this way, it becomes possible to infer or predict the thickness distribution of a layer of coating material deposited on many different positions around the dome using flexible test sheets curved in only one direction. However, owing to the symmetrical positioning of the holder 30 relative to the source 4, it is expected that in practice there would be little difference between different positions. In addition, any differences which did exist between different positions could be eliminated by rotating the holder 30 about an axis through the apex 20 as indicated in Fig. 3.

Fig. 2 is an example reflectance plot, produced by a spectrophotometer, for a single layer (film) of coating material (zinc sulphide) deposited on a flexible test sheet of a commercially available heavy-duty aluminium foil of 0.1mm thickness, part number 3D481 913T (Supplier: LEYBOLD OPTICS UK).

To produce the reflectance plot shown in Fig. 2, the holder 30 was used to adapt the shape of the flexible test sheet such that the flexible test sheet had a curved surface representative of the hemispherical concave surface of a dome having a radius of curvature of 75mm. The coating material was then deposited on the flexible test sheet using the apparatus shown in Fig. 3 and described below. The reflectance plot of Fig. 2 shows the reflectance of the layer of coating material as measured at the centre of the aluminium foil, this location on the aluminium foil corresponding to the apex of the dome.

Although the reflectance from the aluminium foil used to produce the reflectance plot of Fig. 2 does not conform to the theoretical reflectance for aluminium metal, the theoretical interference turning points for a thin dielectric layer deposited on the foil are clearly visible in the reflectance plot of Fig. 2.

S By measuring reflectance at two or more locations on the surface of the flexible test sheet to produce reflectance plots, such as the one shown in Fig. 2, it is possible to determine the relative (e.g. percentage) difference (if any) in optical thickness, and therefore to determine the relative (e.g. percentage) difference (if any) in physical thickness, between those locations on the surface of the flexible test sheet.

Alternatively, instead of measuring reflectance, the physical thickness of the deposited layer of coating material could be measured directly at two or more locations on the test surface of the flexible test sheet, for example, using a profilometer such as the Talystep profilometer. This could be achieved by masking half of the flexible test sheet along its length before depositing coating material thereon. In this case, a step in the coating would be produced at the boundary of the mask on the flexible test sheet. On removal of the mask, a physical thickness measurement could then be made of the step in the coating at various positions along the length of the flexible test sheet, e.g. using a profilometer.

Fig. 3 shows schematically a vapour deposition apparatus for depositing vaporised coating material on a component having a curved surface (e.g. a dome having a hemispherical concave surface) . The vapour deposition apparatus may also be used to deposit vaporised coating material on a flexible test sheet held by a holder such as the one shown in Fig. 1. The complete experimental vapour deposition apparatus is not shown in Fig. 3. Rather, Fig. 3 is a schematic diagram showing the vacuum chamber of a vapour deposition apparatus.

By way of example, the invention may be used with the vapour deposition apparatus 1 shown in Fig. 3. The apparatus 1 shown in Fig. 3 includes a vacuum chamber 2, a source 4 of coating material and an electron gun 6, which acts as a means for vaporising coating material at the source 4 to produce streams 8 of vaporised coating material. A component, which may be in the form of a dome 10 or a holder 30 as shown in Fig. 1, is supported above the source 4 by a support tool 12.

As shown in Fig. 3, the component is a dome 10 having a S hemispherical concave inner surface.

In use, the apparatus 1 rotates the dome 10 about a first axis l4a which passes through the apex of the dome 10, and is also an axis of rotational symmetry for the dome 10. In use, the apparatus further rotates the dome 10 about a second axis 14b, which does not pass through the dome 10, and which is fixed with respect to the manufacturing plant in which the apparatus 1 is installed. In this way, the second axis 14b acts as a plant axis of rotation. Whilst the dome 10 is rotated about the second axis 14b, the first axis 14a undergoes precession with respect to the second axis 14b such that the concave surface always faces the source 4. As shown in Fig. 3, the source 4 is located in a position that is offset from the second axis 14b. In a single-rotation arrangement, this offset between the source 4 and the second axis 14b can be advantageous for improving the uniformity of the coating. In a planetary system there is little practical improvement in uniformity by using such an offset arrangement.

However, because two or more materials are usually deposited in a coating process, it is often convenient to use a separate gun for each material, in which case the sort of offset illustrated in Fig. 3 would be necessary to ensure that the guns are positioned appropriately. The distance between the source 4 and the dome 10 is relatively large, e.g. a vertical distance z between the source 4 and the apex of the dome 10 of 600 mm. In this way, the effects of non-uniformities in the spatial profile of vaporised coating material produced at the source 4 (e.g. by the electron gun 6) are reduced.

When used in this specification and claims, the terms "comprises" and "comprising" arid variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or integers.

The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure.

Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

APPENDIX -Experimental Work This experimental work was carried out using a diffusion pumped Balzers 1052 box coating machine (produced by Baizers), equipped with two Telemark electron guns and an Inf icon 1C5 quartz crystal process controller. All depositions were carried out at ambient temperature, and in-situ component plasma cleaning was applied with a customised glow discharge pole.

Single layers of coating material were deposited on a flexible test sheet of aluminium foil. To mount the flexible test sheet, the holder 30 shown in Fig. 1 and described above, was used.

The holder 30 was constructed to adapt the shape of the flexible test sheet so that the flexible test sheet held by the holder 30 had a curved test surface (curved in one direction) with a radius of curvature of 75mm. In this way, the curved test surface of the flexible sheet was representative of a dome having a hemispherical concave surface with a radius of curvature of 75mm.

Two coating materials were chosen, one having a low refractive index and the other having a relatively high refractive index.

In order to determine process and thickness distribution repeatability, a series of three coating cycles was undertaken for both materials to deposit a single layer of each coating material onto six flexible test sheets. Specifically, a single layer of the low refractive index material was deposited on each of three flexible test sheets and a single layer of the high refractive index material was deposited on each of three (further) flexible test sheets.

Some materials such as silicon are well behaved in terms of evaporation characteristics, since they melt in a consistent fashion and thereby produce a repeatable spatial profile. Other materials such as aluminium oxide however sublime, and their spatial profiles are more difficult to control. However, through careful selection of the form of coating material and electron beam parameters, adequate repeatability can be achieved.

Fig. 4 shows a reflectance plot produced for a single layer of a low refractive index material deposited on a flexible test sheet as measured at the centre of the flexible test sheet (which corresponds to the apex of the dome) and at the edge of the flexible test sheet (which corresponds to the edge of the dome) . Fig. 5 shows a corresponding reflectance plot produced for a high refractive index material.

Based on the reflectance measurements used to produce the reflectance plots shown in Figs. 4 and 5, a percentage difference in physical thickness of -3% between the apex and the edge of the dome was determined for the low refractive index material, and a percentage difference in physical thickness of -4% between the apex and the edge of the dome was determined for the high refractive index material. Thus, for both the low refractive index material and the high refractive index material, the thickness of the deposited coating material at the edge was measured to be less than the thickness of the deposited coating material at the apex.

Normally, the amount of coating material deposited is such that the desired (i.e the nominal or "perfect") coating thickness occurs at the apex of the dome. This means that the thickness error (i.e. the amount by which the thickness of deposited coating material deviates from the desired coating thickness) at the apex of the dome is approximately zero, but there will be a relatively large thickness error at the edge of the dome.

Instead of this, it may be preferable to control the amount of coating material deposited such that the desired coating thickness occurs at some point that is between the apex and the edge of the dome. In this way, the thickness error can be shared between the apex and the edge of the dome, e.g. with the deposited coating material deposited at the apex of the dome being thicker than the desired coating thickness, and with the deposited coating material deposited at the edge of the dome being thinner than the desired coating thickness.

This may be preferable to having all of the error concentrated at the edge as it means that the maximum thickness error across the surface of the dome will be reduced.

Although in this experiment, for both the low refractive S index material and the high refractive index material, the thickness of the deposited coating material at the edge was measured to be less than the thickness of the deposited coating material at the apex, in other similar experiments, the thickness at the edge has been found to be greater than the thickness at the apex. Whether the thickness at the edge is greater or less than the thickness at the apex has been found to depend on a number of factors, including the vapour profile of the ccating material, the shape of the component (e.g. convex versus concave), the radius of curvature of the component, and the plant configuration used.

Claims

CLAIMS: 1. A method of measuring the thickness distribution of a deposited layer of coating material, the method including: adapting the shape of a flexible test sheet so that the flexible test sheet has a curved test surface representative of a curved surface of a component; depositing vaporised coating material on the curved test surface of the flexible test sheet to form a layer of coating material on the curved test surface of the flexible test sheet; and measuring a parameter representative of the thickness of the deposited layer of coating material at two or more locations on the test surface of the flexible test sheet.
2. A method according to claim 1 wherein the curved test surface of the flexible test sheet is curved in only one direction.
3. A method according to claim 1 or 2 wherein the curved test surface is representative of a cross-section through the curved surface of the component.
4. A method according to any one of claims 1 to 3 wherein the method includes depositing vaporised coating material on the curved surface of the component, wherein the vaporised coating material deposited on the curved surface of the component is deposited under substantially the same conditions as the vaporised coating material deposited on the curved test surface of the flexible test sheet.
5. A method according to any one of claims 1 to 4 wherein the method includes flattening the flexible test sheet before measuring the parameter representative of the thickness of the deposited layer of coating material at the two or more locations on the test surface of the flexible test sheet.
6. A method according to any one of claims 1 to 5 wherein the method is repeated for one or more further flexible test sheets, with each of the one or more further flexible test sheets being oriented differently with respect to the other flexible test sheet(s) for the depositing of vaporised coating material.
7. A method according to any one of claims 1 to 6 wherein S the depositing of vaporised coating material is performed in an evacuated atmosphere.
8. A method according to any one of claims 1 to 7 wherein the vaporised coating material is deposited by physical vapour deposition.
9. A method according to any one of claims 1 to 8 wherein the curved surface of the component is concave or convex.
10. A method according to claim 9 wherein the concave/convex surface has a radius of curvature of 200mm or less, 150mm or less, or 100mm or less.
11. A method according to claim 9 wherein the concave/convex surface has a radius of curvature of 500mm or more, 1000mm or more, or 2000mm or more.
12. An apparatus configured to carry out a method according to any one of claims 1 to 11, the apparatus including: a shape adapting means for adapting the shape of a flexible test sheet so that the flexible test sheet has a curved test surface representative of a curved surface of a component; a vapour deposition apparatus for depositing vaporised coating material on the curved test surface of the flexible test sheet to form a layer of coating material on the curved test surface of the flexible test sheet; and a measuring device for measuring a parameter representative of the thickness of the deposited layer of coating material at two or more locations on the test surface of the flexible test sheet.
13. An apparatus according to claim 12 wherein the shape adapting means is a holder for releasably holding the flexible test sheet, the holder being configured to adapt the shape of the flexible test sheet so that the flexible test sheet has a curved surface representative of the curved surface of a component when the flexible test sheet is releasably held by the holder.
14. An apparatus according to claim 13 wherein the holder S includes one or more holding members which define a channel in which the flexible test sheet can be releasably held
15. An apparatus according to claim 13 or 14 wherein the holder includes a curved support surface for supporting the flexible test sheet in its adapted shape when the flexible test sheet is releasably held by the holder, the curved support surface of the holder being representative of the curved surface of a component.
16. A shape adapting means as set out in any one of claims 12 to 15.
17. A method substantially as any one embodiment herein described with reference to and as shown in the accompanying drawings.
18. An apparatus substantially as any one embodiment herein described with reference to and as shown in the accompanying drawings.