GB2125830A - Vacuum deposition apparatus and method - Google Patents

Abstract

In a vacuum coating apparatus, the vacuum deposition chamber is of a diameter at least twice its axial length, and the source of material to be applied to a substrate is provided as a coating on a flat metal sheet. By use of the metal sheet, a minimum of material is wasted and a maximum quantity is transferred to the substrate. The use of a comparatively small-volume vacuum chamber enables short cycle time, a cold finger nitrogen trap can be used and a vacuum of 10<-7> mm Hg can be achieved easily.

Description

SPECIFICATION Improved vacuum deposition apparatus and method This invention relates to a coating process in which a substrate in a vacuum chamber is coated with a material. One such process is vacuum deposition in which, in the prior art, the material which is to form the coating is supplied in a crucible in molten form, and is applied to a substrate which can be protected or exposed to deposition by a movable cover. Such an apparatus is described in the specification of UK Patent No. 1,452,720. The apparatus is generally circular in cross section and the vacuum chamber enclosing the molten source and the substrate is of such dimensions that it is longer in the axial direction than its diameter, so that a substantial volume must be evacuated, and high vacuums are difficult or costly to achieve.The chamber is similar in dimensions to the conventional bell-jar used in other prior art arrangements.

In a different arrangement in the specification of UK Patent No. 1,325,933, a large-area crucible is provided, and a long strip of substrate is moved over the crucible by supply and take-up rollers. The arrangement is suitable only for flexible, elongated substrates, and the roliers occupy a considerable volume so that again a high vacuum is difficult or costly to achieve.

It is a further disadvantage of prior art arrangements that a high proportion of the coating material is deposited on surfaces other than the substrate; since a coating material may be expensive, this is undesirable.

It is an object of the present invention to overcome the disadvantages of the prior art.

According to the invention, a vacuum coating apparatus comprises a circular vacuum deposition chamber of diameter at least twice its axial length; means for evacuating the chamber; a substrate support; spaced from the substrate support in the axial direction a source of material consisting of a flat metal plate parallel to the substrate and coated with a layer of material to be applied to the substrate; and means to heat the flat metal plate.

Typically the flat metal plate will be made of tungsten, and typical materials supplied as a coating on the tungsten plate are aluminium, copper, or gold, which may be applied to the tungsten plate by an evaporation process. Alternatively, the material may be coated by a process such as painting from a solution or suspension; for example magnesium fluoride mixed with cellulose nitrate may be painted onto the tungsten sheet which is then heated to remove the cellulose nitrate. Silicon monoxide in powder form can also be applied in this way. Further alternative processes are flame spraying and aerosol coating.

Once the source material has been coated onto the tungsten plate, the plate is positioned in the vacuum apparatus according to the invention, a vacuum is established, the tungsten plate is heated, and the substrate is coated by a conventional vapour deposition process. The coating thickness may be monitored by any conventional method, such as a measurement of coefficient reflection.

It is an advantage of the use of a comparatively small-volume vacuum chamber that the cycle time is short, that a cold finger nitrogen trap can be used to improve the vacuum, and that a vacuum of 10-7 millimetres Hg can be achieved easily, in contrast to the 10-5 or 10-6 achievable in prior art apparatus.

The vacuum chamber may be constructed of a circular steel tube with two flat steel plates acting as end closures, a seal being provided by circular rubber seals of "L" shaped cross section. This construction is simple, cheap and robust, and by use of steel tubes of different lengths, the volume and axial length/diameter ratio of the chamber can be varied. The end closures carry, for example, the vacuum connection, electrical connections for the substrate support and the support itself.

The invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 illustrates schematically in vertical cross section a vacuum deposition chamber according to the invention; Figure 2 is a plan view of a part of the Figure 1 apparatus; Figure 3 shows an alternative to the part shown in Figure 2.

In Figure 1, a vacuum chamber consists of a circular steel tube 10 closed by upper and lower flat steel plates 12,14 and sealed by rubber gaskets 16 of "L" shaped cross section. Below the vacuum chamber is an enclosure 18 which contains ancillary equipment. The lower plate 14 carries a connection 20 to a vacuum pump, and a "cold finger" 22 connected to a source of liquid nitrogen which with the pump is situated in chamber 18, which also contains electrical connections 33 to allow heating of plate 32 and other source plates. The vacuum enclosure 10, 12, 14 contains a substrate support 24 which carries on its lower surface a substrate to be coated 26. Below the substrate 26 is a shutter 28 having a segment-shaped aperture 30, and below the shutter 28 is a source holder 32 which carries at least one source plate 34.Conventional filmthickness monitoring apparatus, such as an optical reflectivity monitor, is also present but is not illustrated.

The substrate support 24 is continuously rotatable about its centre by means not shown. The shutter 28 can be rotated (by means not shown) about its centre so that the segment-shaped aperture lies at a chosen azimuth within the apparatus.

The source holder 32 is shown in plan view in Figure 2; three source plates 34, 38,40 are now visible, each plate comprising a rectangular strip of tungsten with an upper surface carrying a layer of a material (e.g. aluminium or copper or gold) to be deposited on the substrate 26. Each strip is pivoted, as indicated by reference 42, at its radially outer end, and the strips are of such length as to be greater than the radial distance from the respective pivot points 42 to the centre of plate 32. The radially inner ends are linked to each other by pivoted linkages 44 and connected to a pivot point 46 near the plate circum ference by a pivot arm 48. Operation of the pivot arm causes the radially inner ends of the three tungsten strips 34,38,40 to move together so that any one of the three strips can lie along a radius of the plate 32.

For example, the strips can lie either in their positions illustrated byfull lines with plate 34 in a radial position or all three strips can be rotated by pivot arm about the pivots 42 so that the strip 40 lies along a radius, and the other two strips are displaced, as shown by the chain-dotted lines.

By this movement, a selected one, e.g. strip 40, of the three tungsten strips can be arranged to lie under the aperture 30 in shutter 28. When the strip 40 is heated, and a vacuum is established in the vacuum chamber, and the substrate 26 is rotated, then by a vacuum deposition process similar to that in a conventional vacuum chamber, the layer of material on the tungsten strip 40 is transferred to the substrate as an even coating.

It is an advantage of apparatus according to the invention that, by use of a material coated onto a metal plate as a source of deposition material, a minimum of material is wasted and a maximum quantity is transferred to the substrate; the tungsten plates are reusable and can be coated by any suitable process.

It is a further advantage that use of a vacuum chamber of smaller volume than in the prior art, allowing effective use of a liquid nitrogen "cold finger", a harder vacuum can be established. Typical dimensions of the chamber are a diameter-to-length ratio of 2:1 or more, often 3:1. The small volume makes feasible the use of an ion pump. The use of a high vacuum allows the application of high purity coatings with minimal oxidation and the possibility of improved adherence and higher reflectivity of the coating.

By use of tungsten strips in the Figure 2 arrange ment carrying surface layers of different materials, multilayer coatings can be applied to the substrate, or a very thick layer of a single material, without the need to break and re-establish the vacuum.

Typical dimensions will be a vacuum chamber 1 metre in diameter and 30 to 40 centimetres in axial length, using a tungsten sheet 0.10 millimetres thick, 36 cms long and 760 millimetres wide. A heating current of 400 to 500 amps is supplied. A quarterwavelength coating, i.e. about 6500A or 180 nanometres in thickness, can be applied in one pass.

Figure 3 shows an alternative arrangement. The source holder 32 carries a tungsten source plate 50 which carries segment-shaped deposits 52, 54, 56 of material to be transferred to a substrate. Rotation of either the shutter or the source holder to align the aperture and a selected deposit would then be necessary before deposition.

The type of source of material for deposition used in the present invention may also be used in an ion plating process in which material is removed from the tungsten sheet by a combination of heat and ionised gas, instead of by the combination of heat and low pressure.

Claims (9)

1. Avacuum coating apparatus comprises a circular vacuum deposition chamber of diameter at least twice its axial length; means for evacuating the chamber; a substrate support; spaced from the substrate support in the axial direction a source of material consisting of a flat metal plate parallel to the substrate and coated with a layer of material to be applied to the substrate; and means to heat the flat metal plate.

2. Apparatus according to Claim 1 in which the flat metal plate is a tungsten plate.

3. Apparatus according to Claim 2 in which the layer of material is applied to the tungsten plate by an evaporation process.

4. Apparatus according to Claim 3 in which the layer of material is aluminium or copper or gold.

5. Apparatus according to Claim 2 in which the layer of material is applied to the tungsten plate by the application of a solution or suspension of a solid material in a liquid, followed by removal of the liquid by heating.

6. Apparatus according to any preceding claim in which the vacuum deposition chamber consists of a circular steel tube having two flat steel plates as end closures.

7. Apparatus according to any preceding claim in which the diameter of the circular vacuum deposition chamber is three times its axial length.

8. A method of vacuum deposition comprises supplying a material to be deposited in the form of a layer on a flat metal plate; heating the plate; and exposing to the heated plate a substrate being in an evacuated chamber.

9. Vacuum coating apparatus substantially as hereinbefore described with reference to Figures 1 and 2 or to Figures 1 and 3 of the accompanying drawings.