GB2255105A - Dual magnetron/cathodic arc vapour source - Google Patents

Abstract

A vapour source for use in any of the following sputtering modes: balanced magnetron, unbalanced magnetron, random cathodic arc and magnetically confined magnetic arc comprises an evaporation target plate 1 of material to be evaporated; a cathode plate 2 mounted to and in electrical connection with the evaporation target and to which electrical power is supplied; an arc containment ring 10 mounted on the evaporation target to confine a cathode arc within an evaporation area of the evaporation target; a magnet assembly, comprising a central magnet 13 located beneath the evaporation area and one or more outer magnets 11 surrounding the central magnet, and means whereby the field strengths at the evaporation area arising from the central magnet and the one or more outer magnets are variable relative to each other; and an electrically conducting shield 5 mounted above the evaporation target and electrically isolated therefrom. The magnet assembly may be designed to move automatically in such a manner so as to allow evaporation of different parts of the target and provide for the deposition of compound or multiple layer coatings. <IMAGE>

Description

IONISED VAPOUR SOURCE This invention relates to a vapour source which may be used in a variety of modes; namely the balanced magnetron mode, the unbalanced magnetron mode, the random cathodic arc mode, and the magnetically confined cathodic arc mode.

Apparatus for producing ionized vapour from solid, single or multiple part electrically conductive targets is used for the purposes of deposition on to, or heating or sputter cleaning of, substrates. Such apparatus may be used to apply wear resistant, corrosion resistant, decorative, optical or other coatings.

There are many methods of producing ionized vapour from solid material for coating purposes. Of those which do not involve large-scale thermal melting of the source material, two of the most well known are magnetron sputtering and cathodic arc evaporation.

Sputtering involves subjecting a target of the coating material to bombardment by ions and neutral atoms in a low pressure glow discharge plasma, usually of a inert gas. This bombardment, produced by holding the target at a high negative potential (usually several hundred volts) causes atoms and ions of the target material to be ejected from the target. The target also emits electrons, which then contribute to the ionisation of the plasma. The important feature of the magnetron is a magnetic field which causes the path of the emitted electrons to be constrained in particular paths, causing greater ionisation of the plasma close to the surface of the target and hence increasing the sputtering rate. In an important variation of the magnetron, the central or outer poles of the magnet assembly are weakened in such a way as to allow some of the electrons to escape.This allows controllable increase of ionization of the coating flux to the substrate. The magnetic field may be moved in the plane of the target in order to effect more uniform sputtering of the full target area.

Cathodic arc evaporation operates by the action of a high current (usually greater than 20A) on the target in a low voltage (typically 30V) glow discharge. In the arc discharge, the entire current is carried to the target through one or more small, highly luminous cathode spots.

At these spots the current density is sufficiently high that the material is instantaneously converted to vapour, a large fraction of which is ionized. The cathode spots move in a random fashion (random arc mode) unless constrained by a magnetic field (steered arc mode).

The major problem with current cathodic arc vapour sources is that they produce undesirable droplets of molten material in addition to the desired vapour. These droplets are emitted when the cathode spot lingers in the same region of the target, causing the local temperature to rise. An effective method of reducing this problem is to apply a magnetic field to the target. This causes the cathode spot to move in a direction perpendicular to the field, at a velocity dependent upon the field strength. By suitable arrangement of magnets, the cathode spot motion may be constrained to a closed path. This ensures that the spot is prevented from causing local overheating of the target and thus reduces droplet formation.Provision for movement of the magnetic fields allows the cathode spot to be guided around the target as required, for example for evaporation layers of different material from a multi-part cathode.

Known in the art are deposition apparatuses for random (unsteered) arc evaporation (see Snaper, UK Patent No. 1 263 830), steered arc deposition (see Ramalingam, world patent WO/85/03954), magnetron sputtering (see Thornton, Surface Engineering 2(4) (1986) p.283), magnetically steered magnetron sputtering (see Garrett, US Patent No. 4 552 639) and unbalanced magnetron sputtering (see Window and Savvides, J. Vac. Sci. Technol. A 4(3) (1986) p453).

It is an object of the present invention to provide in a single unit a source of ionized vapour which may be operated without modification in any of the above modes and which may be electrically switched between operating modes as desired without the necessity of breaking the seal of the vacuum chamber in which it is fitted. In a previous publication (Robinson and Matthews, Surface and Coatings Technology, 43/44 (1990) pp288-298), an earlier form of source was described. However, this source did not allow for change of operation between all modes without some disassembly.

In particular, for variation between balanced and unbalanced magnetron modes, the inner magnet assembly needed to be removed. A similar problem would have existed in variation between steered and unsteered cathodic arc modes. The most significant change in apparatus was between magnetron and cathodic arc modes. The earthed shield required in the magnetron assembly to protect certain parts of the assembly from sputtering. This earthed shield needed to be located less than the cathode fall distance from the cathode to be effective for its function. However, this meant that in the cathodic arc mode, the arc had the possibility of escape through the shield. To prevent this, the shield was partly disassembled, with the rest of the shield protected by additional PTFE insulation.This vapour source could not solve the problem of the existing art in that it could not be used freely in any of the desired sputtering modes; disassembly and reconstruction were required to change between different modes.

Accordingly the invention provides vapour source for use in any of balanced magnetron sputtering, unbalanced ragnetron sputtering, random cathodic arc sputtering and magnetically confined cathodic arc sputtering, wherein said vapour source comprises: an evaporation target of material to be evaporated; a cathode plate mounted to and in electrical connection with the evaporation target; means whereby electrical power can be supplied to the cathode plate; an arc containment ring mounted on the evaporation target to confine a cathode arc within an evaporation area of the evaporation target; a magnet assembly, comprising a central magnet located beneath the evaporation area and one or more outer magnets surrounding the central magnet, and means whereby the field strengths at the evaporation area arising from the central magnet and the one or more outer magnets are variable relative to each other; an electrically conducting shield mounted above the evaporation target and electrically isolated therefrom.

An important aspect of the invention is the electrical disposition of the shield. As opposed to the earthed shield in the prior art described above, the shield of the invention is electrically isolated from the cathode; the shield floats. Such a shield does not cause problems of arcing in the cathodic arc mode, but it also proves still to be effective as a shield for the magnetron modes.

An important preferred feature of the source described here concerns the magnetic field used to cause the magnetron effect and contain the arc. This field can be provided by a combination of permanent magnets and variable electromagnets on the air side of the assembly.

Such a magnet assembly can be designed to be moved automatically in such a manner as to allow evaporation of different parts of the target, which may be made of segments of different materials in order to deposit compound or multiple layer coatings. The magnet assembly is preferably so designed as to create a particular shape of magnetic field which allows uniform evaporation of the target with only a simple linear scanning movement.

Alternatively, a more complex motion may be used allowing control over the position of the magnetic field in two axes, in the manner of an x-y plotter. Mounting the magnet assembly in air facilitates linear and two axis scanning.

A specific embodiment of the invention is described below, by way of example, with reference to the accompanying Figures, in which: Fig. 1. shows a perspective view of an embodiment of the invention, the front view being a vertical section Fig. 2 shows a comparison between the magnetic field shape and consequent erosion profiles of a conventional magnetic field and a magnetic field arising from a specific embodiment of the invention.

Figure 1 shows a schematic of an embodiment of such a multiple-mode vapour source, sectioned. Evaporation target plate 1, which is a simple rectangular plate of the material to be evaporated, is fixed to cathode plate 2 by means of clamping pieces 3. Clamping pieces 3 may be fastened to cathode plate 2 by means of screws, toggle clamps or any suitable means. Alternatively, the target plate may be bonded to the cathode plate by means of electrically conductive adhesive, soldering or brazing.

Cathode plate 2 is mounted by screws on electrically insulating spacer 4 which in turn is fixed to the inner side of the wall of vacuum chamber 6. O-ring seals 7 and 8 prevent air ingress into the vacuum chamber. Cathode plate 2 is grooved on the outer side so that backing plate 9 may be bonded to it, allowing cooling fluid (eg water) to flow between plates 2 and 9 to prevent undesirable heating of the target, insulator ring and O-ring seals. Shield 5 is mounted on insulating spacer 4 such that is electrically isolated from both cathode plate 2 and vacuum chamber 6.

This shield prevents sputtering of components other than the deposition target in both of the sputtering modes, and also serves to extinguish the arc should the cathode spot escape the magnetic confinement in the arc mode. Arc containment ring 10 is made of a material of low secondary electron emission ratio, such as boron nitride, and serves to contain the cathode spot motion to the target area in random arc mode. Alternatively, the containment ring may be made of magnetically soft material. This will also contain the cathode spot in the random arc mode, but may interfere with the magnetic field used in the steered arc mode.

The magnet assembly used to contain the plasma in the magnetron modes and to constrain the cathode spot motion in the steered arc mode consists of the following parts. Permanent magnets 11 are arranged around the periphery of the assembly. Central pole 13 is made of magnetically soft material and forms the core of a solenoid. The use of a solenoid as the central pole of the magnet assembly allows the magnetic field to be varied, allowing the use of the system in the balanced or unbalanced magnetron sputtering modes. Plate 14 is also made of magnetically soft material and forms part of the magnetic circuit. The magnet assembly is mounted on slider assembly 15, which in this example is driven in a reciprocating linear motion along rod 14 by an electric motor driving a toothed belt (not shown).In the particular case of linear motion of the magnet assembly, it is necessary to shape the magnetic field correctly to achieve maximum target utilisation. In the case of magnet motion controlled on two axes, this is less important.

Figure 2 shows the effect of the magnetic field shape on the way in which the target is eroded in the arc mode. The normal arc confinement configuration, when scanned in a single axis, produces highly uneven cathode erosion. The preferred cathode spot path is also shown. This gives a much more even erosion of the target material and hence longer target life. This is achieved by reducing the dwell time of the cathode spot in the lateral edge region of the target evaporation area.

The apparatus described is thus capable of being operated without modification in the balanced magnetron mode, the unbalanced magnetron mode, the random cathodic arc mode or the steered cathodic arc mode. It can be used for producing ionized vapour for sputter cleaning or heating of substrates prior to deposition of coatings.

Moreover, the shaping of the confining magnetic field provide an optimum path of arc motion in steered arc mode to allow maximum target utilisation.

Claims (15)

1. Vapour source for use in any of balanced magnetron sputtering, unbalanced magnetron sputtering, random cathodic arc sputtering and magnetically confined cathodic arc sputtering, wherein said vapour source comprises: an evaporation target of material to be evaporated; a cathode plate mounted to and in electrical connection with the evaporation target; means whereby electrical power can be supplied to the cathode plate; an arc containment ring mounted on the evaporation target to confine a cathode arc within an evaporation area of the evaporation target; a magnet assembly, comprising a central magnet located beneath the evaporation area and one or more outer magnets surrounding the central magnet, and means whereby the field strengths at the evaporation area arising from the central magnet and the one or more outer magnets are variable relative to each other; an electrically conducting shield mounted above the evaporation target and electrically isolated therefrom.

2. A vapour source as claimed in claim 1, wherein the central magnet is variable.

3. A vapour source as claimed in claim 2, wherein the central magnet comprises a solenoid with a core of magnetically soft material.

4. A vapour source as claimed in claim 2 or 3, wherein the one or more outer magnets are permanent magnets.

5. A vapour source as claimed in any preceding claim further comprising means for reciprocating the central magnet.

6. A vapour source as claimed in claim 5 wherein said means for reciprocating comprise a rod on which the central magnet is mounted, motor means for driving the central magnet along said rod.

7. A vapour source as claimed in any preceding claim, wherein at least a part of the magnet assembly is open to the atmosphere.

8. A vapour source as claimed in claim 7, wherein there are 0-ring seals between the magnet assembly and an upper surface of the evaporation target.

9. A vapour source as claimed in any of claims 5 to 8, wherein the magnet assembly comprises an arrangement of magnets such that in use, reciprocation of the central magnet provides even erosion of the target in magnetically confined cathodic arc sputtering by equalisation of a cathode spot dwell time over the evaporation area.

10. A vapour source as claimed in any preceding claim, wherein the cathode plate is bonded to the evaporation target.

11. A vapour source as claimed in any preceding claim, further comprising a conduit for coding water, said conduit being mounted adjacent to the cathode plate.

12. A vapour source as claimed in any preceding claim, wherein the arc containment ring is made of material having a low secondary electron emission ratio.

13. A vapour source as claimed in claim 12, wherein said material is boron nitride.

14. A vapour source as claimed in any of claims 1 to 11 wherein the arc containment ring is made of magnetically soft material.

15. A vapour source substantially as described herein, with reference to the accompanying drawings.