GB2124259A - Apparatus for the ion-cladding of bulk material - Google Patents

Abstract

The apparatus comprises an earthed vacuum chamber 1, a rotatable drum 2 for holding the bulk material 6, and a vaporiser arrangement 7. In order to confine the glow-discharge to the spaces in which it is required, and simplify the electrical arrangements, substantially the same potential is applied to the drum 4 and to the vacuum chamber 1 which are connected as a cathode. Also provided are two anodes 8, 12, one of which (8) is arranged within the drum 4 and the other (12) between the vaporiser arrangement 7 and the drum 4. A potential which is positive with respect to earth is applied to the anodes. <IMAGE>

Description

SPECIFICATION Apparatus for the ion-cladding of bulk material The invention concerns apparatus for ioncladding of substrates in the form of bulk material.

Ion-cladding is concerned with producing, by means of a vaporiser arrangement, a stream or cloud of vapour which, upon condensation, forms the cladding layer on the substrates, and the system is also aimed at maintaining a glowdischarge between the vaporiser and the substrates by creating potential differences between the vaporiser and the substrates in conjunction with a certain pressure in the vacuum chamber. This glow-discharge causes the particles of vapour to become ionised and accelerated toward the substrates and to penetrate the surface of the substrates, so that a high rate of deposition in conjunction with very firm adhesion of the cladding layer is achieved.

Acceleration of the particles of vapour toward the substrates assumes a negative initial potential in the substrates, so that as a rule, a negative initial potential is applied to the substrate holder.

Such an apparatus is disclosed in US PS 3 926 147. The substrate holder is a drum to which negative potential is applied by way of a sliding contact, and a dark-space screen, which surrounds part of the periphery of the drum, is provided. However, this dark-space screen is not identical to the vacuum chamber, which is arranged outside the dark-space screen.

Located within the drum is a further screen in the form of a metal sheet bent to a substantially cylindrical shape, which lacks a segment only at the bottom zone, since the metal sheet has a flat horizontal face in this zone. The substrates to be coated are located in the space thus formed which is in the shape of the mentioned cylinder segment, and the patent specification states that the glow discharge is limited just to this space.

However, a fallacy is involved here, since the screens arranged over part of the periphery have no corresponding means at the end faces of the drum, and because of the vaporiser arranged below the drum, the outer dark-space screen, in particular, is open at the bottom. Starting from the parts to which cathode potential is applied and proceeeding in the direction of the chamber walls to which earth potential, i.e. relatively considered, anode potential is applied, a glowdischarge is unavoidably propagated, and this not only represents a source of electrical loss but also a source of contamination since parts of the equipment are reduced to dust under the effect of this undesired glow-discharge. A phenomenon of this kind is known from the field of cathode atomisation.The electrical losses are converted into an extremely undesirable heating-up of the components affected by the glow-discharge, so that additional cooling problems arise. The described problems are not so serious in the case of laboratory-type equipment where, mostly, the major part of the vacuum chamber is made of glass. The above-described disadvantages become intolerable, however, in the case of production installations wherein the vacuum chamber, because of its size, must of necessity be made of metal and must be kept at earth potential.

The object of the invention is therefore, to provide an improved apparatus wherein glow discharge is actually confined, on the one hand, to the space between the vaporiser arrangement and the drum and, on the other hand, to the space within the drum. In this apparatus it is also intended, in particular, to avoid the need for maintaining narrow gaps between rotatable and stationary parts, a requirement which can be met only with great difficulty in the case of drums for the coating of bulk material.

According to the invention apparatus for ioncladding of substrates in the form of bulk material, comprising a vacuum chamber connected to earth, a rotatable drum within the vacuum chamber and having apertured walls for accommodating and circulating the substrates, and a vaporiser arrangement, wherein the drum 4 and the vacuum chamber 1 are connected as a cathode so that substantially the same potential is applied thereto, and anodes for maintaining a glow-discharge and ionisation of the vapour, are located respectively within the drum and between the vaporiser arrangement and the drum, a potential that is positive with respect to earth being applied to the anodes.

By means of the arrangement whereby the same potential is applied to the drum and the vacuum chamber, the creation of glow-discharges between the drum and the vacuum chamber is effectively prevented without the need for maintaining narrow gaps such as are necessary for example, in the case of what are termed dark space screens. This freedom of choice as regards the magnitude of the gaps is very advantageous particularly as regards the construction and mounting of the drum.

The statement to the effect that the drum and the vacuum chamber are connected as a cathode also includes the earth potential for these components, if this is sufficiently negative in relation to the anode potential.

By preventing a glow-discharge between the drum and the vacuum chamber, undesirable electrical losses are prevented as is the heatingup of components of the apparatus that is associated with these losses.

The arrangement of an anode not only in the drum but also between the vaporiser and the drum results in the glow-discharge penetrating as far as the vaporiser arrangement. The glowdischarge present outside the drum then burns between the outer electrode, which preferably takes the form of a grid constituted by a large number of anode wires, and the drum and the vaporiser.The potential distribution achieved by the measures proposed in the invention has a particularly advantageous effect in a preferred form of the apparatus which comprises a plurality of drums rotatably arranged on the periphery of a cage rotatable about a horizontal shaft, drive means for rotating the drums relative to the cage and about their own shafts, an anode in each drum the position of which remains unchanged in relation to the drum cavity, i.e. to the bulk material, and a fixed anode and vaporiser arrangement located beneath the path of rotation of the drums and their anodes.

If, in an arrangement of this kind the potential distribution described in US-PS 3 926 147 were to be used, a glow-discharge would also be propagated in the spaces between the various drums, and this would result in the abovedescribed disadvantageous consequences. Quite apart from this, the mounting of several drums in a rotatable cage would lead to considerable difficulties in achieving the known potential distribution.

Two embodiments of an apparatus in accordance with the invention will now be described in detail with reference to the accompanying generally schematic drawings, in which: Figure 1 shows an embodiment comprising only one drum, Figure 2 shows an embodiment comprising several drums which are secured to a rotatable cage, and Figure 3 is a longitudinal section through one of the drums shown in Figure 2.

The apparatus illustrated in Figure 1 comprises a vacuum chamber 1 which is earthed at 2 and is connected, by way of suction pipe 3, to a vacuum-pump unit not illustrated. Located in the upper part of the vacuum chamber 1 is a rotatable drum 4 which has apertured walls 5 and accommodates and circulates substrates 6 in the form of a bulk material which is to be provided with a layer of coating material. The apertured walls 5 take the form of grids or wire netting, though for the sake of simplicity this is not shown in the drawing.

Located in the lower part of the vacuum chamber 1 is a vaporiser arrangement 7 which consists of a plurality of single vaporisers which are arranged one behind the other in the direction in which the drawing is viewed, so that only the front vaporiser can be seen. The number of vaporisers is such that a substantially uniform stream of vapour can be produced over the length of the drum 4.

Located within the drum 4 is a plate-like anode 8 which extends along substantially the entire diameter of the internal cross-section of the drum 4 and to which is applied a voltage of +2 to 5kV with respect to earth. The anode 8 is provided, on its rear face 8a, with a dark-space screen 9 to which earth potential is likewise applied and which prevents the formation of a glow-discharge on the rear face 8a. The plate-like anode 8 lies in a horizontal plane and is held in this position by a weight 10 which prevents the anode 8 from rotating when the drum turns about its shaft 11.

Arranged between the vaporiser arrangement 7 and the drum 4 is a further anode 12 which consists of a frame 1 2a with a plurality of anode wires 1 2b that form a grid which, however, does not appreciably impede the passage of vapour emanating from the vaporiser arrangement 7. A positive potential of 2 to 5 kV with respect to earth 2 is likewise applied to the anode 12.

because of this potential distribution, a potential which is negative but substantially the same in relation to the two anodes 8 and 12 is applied to the vacuum chamber 1 and the drum 4 which considered relatively-are consequently connected as a cathode. As a result of this arrangement, glow-discharges occur only in the spaces 13 and 14, i.e. between the anode 12 and the drum 4, between the anode 12 and the vaporiser arrangement 7, and in the lower half of the drum, i.e., in the zone extending from the substrates 6 to the anode 8. Thus, three plasma zones are involved and they have a substantially complementary effect upon the coating process, i.e. the formation of the plasma is restricted to those spaces in which a plasma is required so that a good power rating is established.

If the space 13, for example, were free from plasma, particles of vapour would strike the substrates virtually without ionisation, so that the coating process would be a normal vapour deposition process.

The production of plasma outside the drum 4, which has the effect of a Faraday screen, is brought about by the anode 12. Without this anode, propagation of the plasma from the space 14 to the vaporiser arrangement 7 would not be possible in any case. The anode 1 2 is surrounded by a further dark-space screen 15, to which earth potential is likewise applied by means of a metallic connection. The anode 12 is supported relatively to the dark-space screen 1 5 by pin-type insulators.

Figure 2 shows an embodiment comprising several drums 4 having components identical to those of the drum of Figure 1. These drums are equidistantly distributed about the periphery of a rotatable cage 16 which is simply indicated by its pitch circle shown by a chain line. The cage 1 6 is rotatable about a horizontal shaft 1 7 close to which is fitted the vaporiser arrangement 7 over which the anode 12 is located. In this way, an upwardly directed ionised lobe of vapour is created, so that the required ion cladding is produced.

During the coating process, all of the drums 4 are moved, by means of the cage 1 6, through the space above the anode 12, and continuous rotation in the form of an involute movement about the shafts 11 is brought about by drive means, not illustrated. In its simplest form, the drive means may consist of toothed wheels which are fitted on the shafts 11 and around which a drive chain is caused to move. The number of drum rotations per rotation of the cage 1 6 can be set by the ratio of the speed of the chain to the speed of revolution of the cage. This method of providing the drive is, however, part of the prior art, so that no further explanation is required.

In Figure 3, parts similar to those already described carry the same reference symbols.

However, Figure 3 shows more clearly than Figure 2 that the cage 16 is in the form of two rings 1 6a which are connected to the central shaft 1 7 by means of spokes 1 6b. The shafts 11 are mounted in the cage 1 6 by means of roller bearings 1 6c and may consist of a coaxial arrangement of a metallic outer tube 18 and the actual shaft 11, electrically separated from each other by means of an insulating sleeve 19. The drum 4 and the dark-space screen 9 are electrically interconnected by the outer tube 18. In this way it is possible to apply earth potential to the drum 4 and the dark-space screen 9 by way of the shaft 11, the cage 1 6 and the outer tube 18, whereas a potential, positive with regard to earth, is applied to the anode 8 through the shaft 11 , this being achieved by, for example, sliding contacts not illustrated.

It can be seen, particularly from Figure 3, that the mechanical and electrical set-up of the entire equipment is simple and can be readily achieved.

The formation of undesirable glow-discharges outside the zone where the discharges are required does not take place.

Claims (5)

Claims

1. Apparatus for ion-cladding of substrates in the form of bulk material, comprising a vacuum chamber connected to earth, a rotatable drum within the vacuum chamber and having apertured walls for accommodating and circulating the substrates, and a vaporiser arrangement, wherein the drum 4 and the vacuum chamber 1 are connected as a cathode so that substantially the same potential is applied thereto, and anodes for maintaining a glow-discharge and ionisation of the vapour, are located respectively within the drum and between the vaporiser arrangement and the drum, a potential that is positive with respect to earth being applied to the anodes.

2. Apparatus according to Claim 1, wherein the anode located within the drum is a plate provided on its rear face with a dark-space screen to which an earth potential is applied.

3. Apparatus according to Claim 1, wherein the anode located between the drum and the vaporiser arrangement comprises a frame which, at the side toward the vaporiser arrangement, is provided with a dark-space screen to which an earth potential is applied.

4. Apparatus according to Claim 1, comprising a plurality of said drums rotatably arranged on the periphery of a cage rotatable about a horizontal shaft, drive means for rotating the drums relative to the cage and about their own shafts, a said anode in each drum the position of which anode remains unchanged in relation to the drum cavity, i.e. to the bulk material, the final anode and, the vaporiser arrangement being fixedly located beneath the path of rotation of the drums and their anodes.

5. Apparatus for ion-cladding of substrates in the form of bulk material, substantially as hereinbefore described with reference to the accompanying drawings.