GB2178228A - An arrangement for the treating of workpieces - Google Patents

Abstract

In order, in the case of the treatment of workpieces (3) by means of an electrical gas discharge, to be able to subject the individual groups of workpieces or individual workpieces as exactly as possible to a predetermined intensity of the treatment, there is provided with each of the individual groups of workpieces its own device for the setting of the electrical power transmitted to them through the gas discharge. For this are particularly suitable separate current supply devices (6) for the individual branches of the gas discharge. The invention is suitable above all for the heating of workpieces by means of electrical low-voltage arcs. <IMAGE>

Description

SPECIFICATION An arrangement for the treating of workpieces For many vacuum processes it is necessary to bring the workpieces which are to be treated in the vacuum treatment chamber up to a fairly high temperature. For example, through so-called baking-out surfaces which are to be coated may be freed of absorbed gases which otherwise would very seriously impair the adhesion of the layers to be applied subsequently. Many methods of coating need an increased temperature of the substrates in order to be able to perform chemical processes on the surfaces of the substrate. This is frequently the case, in particular with methods of so-called Chemical Vapour Deposition, if starting substances introduced into the vacuum chamber in the form of gas or vapour are to be thermally decomposed on the areas which are to be coated, and a layer is thereby to be deposited on the surfaces.

In doing so there arises in many cases the problem that for adequate degassing or other thermal pretreatement the temperature should be chosen to be as high as possible, but that at the same time the materials in question must not be heated above an admissible maximum temperature in order not to damage them. In particular plastics must not be heated above a predetermined temperature limit but even for steels the admissible treatment temperature is limited by the so-called tempering temperature to about 200 to 700"C. But to choose the treatment temperature to be as high as possible but at the same time not to exceed an admissible maximum temperature is a difficult problem.

In vacuum installations various methods of heating have hitherto been applied. For example, melting crucibles were provided with resistance heating or with the possibility of inductive heating and substrates to be coated were frequently heated by connecting their mounting to a resistance heating device.

For the thermal treatment of surfaces of workpieces, in particular those of more complicated shape, heating by means of an electrical glow discharge has often been used, in which case the workpiece connected as the cathode forms with an anode a glow discharge gap. Heating by electric arc has also become known, either, by the workpiece be ing connected likewise as the one electrode of the discharge gap, or by pure radiant heating by the arc. More recently a so-called low-voltage arc has frequently been employed for the heating, that is, an arc having a working voltage of below 200 V. The thermal power of such a low voltage arc arises above all at the electrodes, in which case the anode gets heated through intensive electron bombard ment out of the plasma. Heating devices of this kind have been utlized, e.g. for the melting of metals in crucibles.A particularly advantageous possibility of the heating of workpieces by means of low voltage arcs has been described in DE-OS 33 30 144. In that case the arc burns between a hot cathode arranged in a separate chamber which is connected to the treatment chamber via a opening, and a main anode arranged in the treatment chamber, in which case the workpieces which are to be heated are arranged round the axis of the arc like a jacket and are likewise connected as the anode. The heating is effected through electron bombardment the intensity of which may be controlled by the positive bias of the workpieces.

In order to achieve, in the case of heating by means of an electrical gas discharge, a uniform temperature of all of the areas which are to be treated, it has been proposed in the CH-PS 417 790 to subdivide the workpieces which are to be treated into a number of groups and to connect each of these groups to one phase respectively of a corresponding polyphase supply network and to interchange the connections cyclically to the individual phases at predetermined equal intervals of time. But this measure can obviously only equalize possibly differing power outputs from the different phases because of different voltages from them, but not differences in temperature which are brought about through the arrangement, mass and shape of the workpieces in the gas discharge chamber.Consequently in the known case it was then also necessary by careful arrangement of the workpieces in the three groups of workpieces inside the discharge vessel, to see to it that the heat losses through heat radiation were equal for all of the workpieces to be treated. Furthermore at the same time the arrangement with respect to the counterelectrodes had to be made such that no workpiece was placed at a disadvantage with respect to the others in the discharge. The simultaneous fulfilment of these two prescriptions was extremely difficult and hence an effectively uniform heating was scarcely achievable in spite of the aforesaid cyclical interchange.

As compared with this, the present invention poses itself the problem of specifying a (circuit) arrangement by which at option the individual workpieces or groups of workpieces may be treated either uniformly or even deliberately differently.

The arrangement in accordance with the invention for the coating of workpieces by an evacuable chamber having a coating device ar ranged in it and a holding device for the workpieces which are to be coated, as well as having electrodes in order to be able to perform between the latter and the workpieces for the purpose of treating these an electrical discharge, is characterized in that with each of individual groups of workpieces is associated its own device for the setting of the electrical power transmitted to them.

In the special case where the individual workpieces are to be subjected to a deliberately different thermal treatment, it is proposed that with each individual workpiece there is associated a device for the setting of the heating power transmitted to it. As devices for the setting of the power transmitted to the individual workpieces or groups of workpieces are particularly suitable separate current supply devices for the gas discharge.

The arrangement in accordance with the invention also offers the advantage that it is possible to heat at high power and upon approaching the maximum temperature to regulate back the power so that in spite of a steep rise in temperature the maximum temperature of each individual group of workpieces or each individual workpiece does not get exceeded. Further, in the case of mixed loads of workpieces to be treated, having widely different shapes or of different materials one is no longer dependent upon the fortuitous current distribution but the heating powers may be set individually.The possibility of starting the heating at high heating power and thereby rapidly reaching an optimum treatment temperature and nevertheless remaining safely for each group below the admissible maximum temperature even if under certain circumstances one must come very close to it, yields an economically optimum heating process.

One embodiment of an arrangement according to the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawing which shows a vacuum plant in which the heating is performed by means of a lowvoltage arc.

The arrangement comprises a bell-shaped vacuum recipient 1 containing holding devices 2 by which the material 3 which is to be heated is carried. The holding devices 2 are fastened by means of electrical insulators 4 to the bottom plate 5 of the recipient 1 and connected electrially, via vacuum tight current leads, to the positive pole of two supply devices 6. At the upper part of the recipient 1 a hot cathode chamber 8 is provided and connected, via an opening 9, to the interior of the recipient 1. In this hot cathode chamber 8, carried by an insulting plate 11, is accommodated a hot cathode 12 which may be a wire heated by current passing through it, but it may also be constructed in the form of a heated or self-heating hollow cathode. A regulating valve 13 serves for the admission of gases into the hot cathode chamber 8.A magnet coil 14 generates a magnetic field coaxial with the recipient 1. For the purformance of a heating process the recipient 1 and the hot cathode chamber 8 connected to it are evacuated through a pump connection 7 by means of a high-vacuum pump until a pressure is reached which is less than about 0.01 Pa. With the pump running sufficient gas, e.g.

helium or neon, may now be admitted through the valve 13 for the pressure in the recipient 1 to be adjusted to between 0.1 Pa and 1 Pa.

The hot cathode 12 is then heated and the supply devices 6 are switched on. The latter generate an electrical voltage of e.g. 100 volts. (In order to ignite the low voltage arc it is advantageous to bring the insulated wall with the opening 9 briefly to anode potential or to connect it permanently, via an ohmic resistor, to the plus pole of the supply devices 6 so that the ignition is effected automatically). In the case of adequate magnetic flux density (e.g. 0.01 tesla) the electrons entering the recipient 1 through the opening 9 follow the lines of force along helical tracks of very small radii so that alorig the central axis a column of plasma arises, the diameter of which is determined by the diameter of the opening 9.At an arc current of 100 A and an arc voltage of 70 V, e.g. a heating power of about 4.2 kW (efficiency: 60%) might be transmitted to the holder 2 or to the material 3 which is to be heated.

The coil 14 generates essentially a magnetic filed parallel to the vertical axis of the recipient 1 and it may be seen that the magnetic lines of force running along near to the axis through the plasma stream do not intersect the material which is to be heated. In the space between the plasma stream and the areas which are to be heated a magnetic field prevails which is likewise essentially parallel to the axis, which has the effect that the electrons are distributed fairly uniformly in the axial direction before they strike the areas which are to be heated. A largely equal transmission of power to the individual areas is thereby achieved.

According to the invention the transmission of power to the two holding devices 2 illustrated in the embodiment may be set separately, that is, by means of the current supply devices 6. These two devices 6 independently of one another allow the discharge current to each of the two groups of sbustrates to be suitably established so that each group receives from the gas discharge the power intended for it. Thus they may be adjusted either at option to exactly equal power transmission or equal temperature of the workpieces or, if desired, also to different power transmission or temperature. The parameters necessary to the achievement of an optimum heating of the individual workpieces or groups of workpieces may easily be determined beforehand either by calculation or else by prior tests. Obviously it is also possible to apply the invention not merely as in the example shown, to two groups of workpieces or separate workpieces in the same plant but also to a plurality of them. In the latter case, if each workpiece because of its different shape or because of the material of which it consists or for any other reasons must be heated in a different manner, there may be provided, as already mentioned, for each individually its own device for the setting of the heating power transmitted and thereby the temperature.

As devices for this purpose, beside the known current supply apparatus for gas discharges, naturally other current sources are also possible, under certain circumstances even batteries. Further, the individual setting may also be effected by a number or all of the workpieces or substrates being fed from one common current source, but setting or regulating devices being provided in the feed to the individual loads for the discharge current in these branches. In the simplest case it is sufficient in the case of a common current source to place in each brance of the circuit which is to be set individually a regulating resistor or a potentiometer in order to establish the discharge current strength precisely for each branch.

As is well known, instead of by electron bombardment the heating of surfaces may also be achieved by bombardment with ions.

That offers the advantage that the treated surface thereby gets etchsd at the same time and thus may be precleaned for a further treatment, e.g. for a coating. With the plant described such a bombardment by ions may easily be achieved by the workpiece holder being brought to a potential which is negative with respect to the plasma of the low-voltage arc discharge, in which case it is advantageous to arrange in addition an auxiliary electrode serving as the anode, preferably near to the bottom plate 5. Sufficient for this is a short metal rod kept at positive potential with respect to the hot cathode 12. The inner wall of the treatment chamber may also simply serve as the counterelectrode (anode), in which case the hot cathode must be insulated electrically with respect to earth (the treatment chamber) and brought to an appropriate negative potential.

If substrates which are at the same electrical potential are getting heated by electron or ion bombardment from a gas discharge, the distribution of power over the substrates depends upon their dimensions, their situation within the discharge and upon their mutual influence. The whole charge must be heated until the substrate having the highest heat capacity has reached the necessary temperature. In that case however, as already said, there existed according to the known technique the risk that the workpieces of lower heat capacity get overheated.

Although in the above examples the invention has been explained mainly with respect to the heating of substrates, to a person skilled in the art it is of course quite clear that the individual control of the electrical powers transmitted to the workpieces in the individual branches of a gas discharge may also be used for other methods of treatment in which such a gas discharge takes place. For example, a gas discharge may also be used for bombarding workpieces with ions in order to bring about in their surface an alteration in the crystal structure through implantation of ions. In each application the invention offers the advantage that the intensity of treatment for the individual workpieces or groups of workpieces may be so set that disadvantages which otherwise occur because of a different arrangement inside the gas discharge treatment chamber may be excluded.

Claims (5)

1. An arrangement for the treating of workpieces comprising an evacuable chamber containing a coating device and a holding device for the workpieces which are to be coated, as well as electrodes in order to be able to perform between the latter and the workpieces for the purpose of treating these an electrical discharge, wherein with each of individual groups of workpieces is associated its own device for the setting of the electrical power transmitted to them.

2. An arrangement according to Claim 1, wherein with each individual workpiece is associated a device for the setting of the power transmitted to it.

3. An arrangement according to Claim 1 or 2, wherein as devices for the setting of the transmitted power separate current supply devices for the gas discharge are provided.

4. An arrangement according to Claim 1, 2 or 3 wherein for the gas discharge a cathode chamber is provided, which is connected to the evacuable chamber via an opening and contains a hot cathode.

5. An arrangement according to Claim 1 constructed, arranged and adapted to operate substantially as herein described with reference to, and as shown in, the accompanying drawing.