Targets For Radio Frequency Sputtering Apparatus

Abstract

A target arrangement for use in radio frequency sputtering techniques in which a metal electrode has clamped to it a dielectric cover which carries target material to be sputtered.

Description

This invention relates to target arrangements for use in radio frequency sputtering apparatus.

There are a number of methods of depositing films under vacuum, the most commonly used being various vacuum thermal evaporation and sputtering techniques. Of these techniques sputtering has a number of advantages over vacuum evaporation as a method of final deposition. These advantages include the ease of coating relatively large areas of a stationary substrate, the simultaneous coating of the front surface and the edges of a substrate, the ability to deposit certain substances in a crystalline form, the possibility of depositing compound substances using, for instance, multiple cathodes when such substances would decompose during evaporation, and the possibility of forging compounds with selected environmental gases. The advantages are common to all types of sputtering methods including in particular radio frequency sputtering techniques.

According to the present invention a target arrangement for use in radio frequency sputtering apparatus includes a metal electrode, a dielectric cover for at least one surface of the electrode and a layer of material carried by the cover, said layer being the target material to be sputtered.

Preferably the dielectric cover is silica or alumina whilst the material to be sputtered may comprise either a conductor, a semiconductor or an insulator as desired.

The layer of material to be sputtered may be deposited on the cover by a spraying process, such as plasma spraying. Alternatively it may be applied by vacuum evaporation.

In a preferred arrangement a metal alloy is interposed between the dielectric cover and the layer of material to be sputtered, the alloy having such thermal properties that its molecular structure takes up any differential expansion between the dielectric cover and the material to be sputtered. In such a case the layer of material to be sputtered may be relatively thick and is suitably vacuum brazed to the alloy which is itself brazed to the dielectric cover.

In carrying out the invention, the dielectric cover should preferably be resistant to thermal expansion effects throughout the range to which the target would be subjected in use. Thus, it should, typically, withstand a temperature of about 30.degree. C. on its surface in contact with the metal electrode while its outer surface is heated, nonuniformly, at heat dissipation levels between 0 and 20 watts cm..sup..sup.-2. It is also preferred that it be mechanically satisfactory for normal handling and not susceptible to accidental breakage from minor shocks. For instance, the cover must be sufficiently strong to withstand the forces involved if it is clamped at the edge so that the back surface is maintained in contact with the metal, normally copper, electrode. Silica and alumina, as mentioned above, fulfills these requirements and are relatively inexpensive.

As is usual in RF sputtering techniques, the target will have to be shielded in use against unwanted discharges. In other words there must be no low resistance path between, for instance, the plasma ion source and the metal electrode or, if the plasma is self generated between two target electrodes, between the metal electrodes and any other part of the apparatus. The dielectric cover could cover all the exposed surfaces of the metal electrode but this is not necessary as long as suitable shielding is provided. One shielding technique is described in the specification of the Assignees' U.S. Pat. No. 3,558,467.

The dielectric cover need not normally exceed one quarter inch in thickness and it may be much thinner if the target material which is to be deposited on it is thick and a good insulator. Silica fulfills all the requirements mentioned for a suitable dielectric cover. To ensure a good heat exchanging relationship with the electrode the silica cover may be provided with a thin surface layer of thermally conductive material, suitably of the same material as the electrode and vacuum evaporated onto the silica.

The exposed surface of the cover which is to face the bombarding ions is covered by a layer of the material to be sputtered. If the layer is to be a metal it may be suitably applied to the cover by metal spraying. The spraying of any suitable metal powder which does not decompose is preferred, aluminum being an example.

When the layer is applied by evaporation such substances as gold and silver are suitable.

A preferred embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing in which:

FIG. 1 is a side elevation of a target arrangement constructed in accordance with the invention; and

FIG. 2 is a plan view of the arrangement shown in FIG. 1.

Referring now to the drawing, the target arrangement is provided with a disc-shaped water cooled copper electrode 1, the water cooling being shown diagrammatically by the water inlet 2 and outlet 3. This electrode is one separate part of the arrangement, the remaining part 4 being a composite part which basically comprises a silica disc 5 (the dielectric cover) and a layer 6 of material to be sputtered, this material being a metal, a semiconductor or an insulator as required. The part 4 is clamped by three L-clamps 9, indicated only in FIG. 2, to the electrode 1.

In order to optimize the durability of the arrangement it is important that there should be a compatible temperature gradient during use between the upper face of the layer 6 of material to be sputtered and the electrode. It is, therefore, desirable to ensure good thermal contact between the electrode 1 and the silica disc 5, this being achieved by the provision of a vacuum evaporated copper layer 7 on the lower face of the silica disc. Any movement between the disc 5 and the electrode 1 due, for example, to differential thermal expansion effects is taken up without subjecting the silica to undue stresses by ensuring that the clamps 9 only maintain a slight pressure (finger pressure) between electrode 1 and part 4. However, there is a permanent bond between the silica disc 5 and the layer 6 of material to be sputtered and it has also to be ensured that there are no undue stresses on the silica due to differential expansion effects in this region. This is achieved by vacuum brazing a disc 8 of nickel iron alloy (kovar) to the upper surface of the silica disc 5 and then vacuum brazing the layer 6 to the kovar disc. This alloy has such a structure that it takes up any differential movement when subjected to thermal strains.

The electrode 1 will, of course, be provided with suitable earth shielding over the regions not covered by the part 4.

The arrangement described is particularly strong and by brazing the layer 6 to the part 4 a thicker layer can be used than when it is sprayed onto the silica disc. The arrangement is therefore suitable for continuous long life sputtering. Moreover, this arrangement is preferred because the layer 6 may be a pure piece of material whilst layers which are plasma sprayed tend to take up impurities, such as oxides, during deposition.

All target arrangements of the present invention may be used in any orientation and, when constructed as described from the preferred types of material, are relatively strong mechanically.

Claims

1. A target for use in radio frequency sputtering apparatus including in combination: a. a first member of silica, said member having a first surface and a second surface; b. a second member of a nickel iron alloy having a third surface and a fourth surface; and c. a third member of target material to be sputtered having a fifth surface, said third surface being bonded to said second surface and said fifth surface being bonded to said fourth surface; and in which said nickel iron alloy has the thermal property that its molecular structure takes up any differential expansion between said first member and said

2. A target as claimed in claim 1, including a thermally conductive copper

3. A target arrangement including a target as claimed in claim 1 and a water-cooled copper electrode to which said target is clamped.