GB2058142A

GB2058142A - Sputtering electrodes

Info

Publication number: GB2058142A
Application number: GB8024998A
Authority: GB
Original assignee: Nordiko Ltd
Current assignee: Nordiko Ltd
Priority date: 1979-07-31
Filing date: 1980-07-31
Publication date: 1981-04-08
Also published as: GB2058142B

Abstract

A sputtering electrode for d.c., a.c. or r.f. sputtering apparatus has a face having a valley formed in a central portion thereof. The electrode may be rectangular, for example, or may be circular. The electrode is used in magnetron sputtering apparatus.

Description

SPECIFICATION Method of and apparatus for sputtering This invention relates to sputtering.

Sputtering is an erosion process in which the material to be eroded is caused to be ejected from the surface of a target by bombardment with atomic sized particles of sufficient energy. This specification is concerned with one form of sputtering known as glow discharge sputtering which is characterised by the removal of the target material by positive ion bombardment. This bombardment is brought about by establishing a negative potential difference between the target surface and a plasma which contains a population of suitable positive ions.

Sputtering processes are usually carried out in a vessel which is maintained at a reduced pressure where the pressure and the composition of the gas in the vessel is determined by the technique used to generate the plasma and the requirements of the process. The material that is sputtered from the target is transported as a low density gas phase and condenses on any surface. Where the conditions permit, the condensed material accumulates to form a thin film. The composition of this film will include some or all of the constituent elements of the target material and will also include some or all of the constituent elements of the gas contained in the vessel.The final composition of the thin film can often be closely controlled and depends on the target composition, the composition of the gas in the vessel, the conditions at the target and the conditions at the surface where the thin film is growing. The study of the various parameters that control the properties of thin films grown by the sputtering process continues and is reviewed in many books and journals including "The Handbook of Thin Film Technology"; edited by L. I.

Maissel and R. Glang, published by McGraw-Hill Book Company, New York. The origins of sputtering can be traced back to the early part of this century. Thus Gijntherschulze, Zeit. F. Phys., 24 (1924), pages 140-147, studied the effects on a low pressure glow discharge of a magnetic field with a transverse component with respect to the electric field direction and observed that the cathode dark space was reduced in depth under these conditions. J. J. and G. P. Thomson in "Conduction of Electricity Through Gases", Vol. 2, pages 332-335 (1933, Cambridge University Press) showed that diminution of the cathode dark space occurred most rapidly when the magnetic field became sufficient to deflect the cathode dark space electrons back towards the electrode.With a planar cathode the electrons were shown to make a series of loops in the cathode dark space.

Penning (U.S. Patent Specification No.

2146025) recognised the advantage of using crossed electric/magnetic fields to improve the performance of a sputtering plant. He proposed three forms of electrode, each of which exhibits magnetron behaviour. In the apparatus of Figures 1 and 2 there is an axial cathode rod, a cylindrical anode and an axial magnetic field so that the magnetic and electric field lines are everywhere perpendicular so that the electrons are made to circle around the cathode. In Figures 3 and 4 Penning showed that the cylindrical anode could be replaced by two annular discs one at each end of the cathode rod. The configuration of Figures 5 and 6 of Penning's U.S. Patent No.2146025 is now commonly employed in sputter-ion pumps.

Such pumps employ sputtering techniques to produce extremely low pressures; gas molecules are removed either by reaction with the material being sputtered or by being mechanically trapped in the sputtered layer.

A planar diode sputtering system derived from the Penning type is described in British Patent Specification No.736512.

W. D. Gill and E. Kay (Rev. Sci. Instrum., 36 (1965), pages 277-281) studied an inverted magnetron sputtering system with an axial anode and a cylindrical cathode (see also Kay's U.S.

Patent Specification No. 3282816).

An axial cathode magnetron sputtering system was studied by K. Wasa and S. Hayakawa (IEEE Trans. Parts Mat. Pack. PMP-3 (1967), pages 71-76; Proc, IEEE (Proc. Letters), Dec. 1967, pages 2179-2180). Reference should also be made to these workers' U.S. Patent Specification No. 3528902.

Planar sputtering electrodes have also been studied in magnetron configurations. E. Kay and his co-workers studied the effects of quadrupole fields on an inverted magnetron structure (see, for example, Figure 9 of U.S. Patent-Specification No.

3282815). In Kay's apparatus the magnetic field generating means was placed outside the sputtering vessel in order to avoid enlarging the vessel and aggravating the pump down problem and hence quadrupole fields were used.

Mullaly (Research/Development, February 1 971, pages 40 44) also used a quadrupole field generated by coils positioned outside the sputtering vessel. He used a hemispherical cathode and superimposed a cusp magnetic field on the electric field of the cathode. The lines of force of this magnetic field intersect the cathode.

K. Wasa and S. Hayakawa (Review of Scientific Instruments, 40 (1969), No. 5, pages 693-697) proposed a "bell jar type" sputtering system with a magnetic coil positioned inside the sputtering vessel immediately behind the planar anode. The same apparatus and variants thereof are described in Japanese Patent Publication 46-34605. An ion pump with planar cathodes and magnet coils positioned behind them is disclosed by Knauer (U.S. Patent Specification No. 3126652). Such a structure can be described as a planar magnetron.

This type of structure has also been proposed by I. G. Kesaev and V. V. Pashkova (Sov. Phys.-Tech.

Phys. 4 (1959), pages 254-264) who proposed its use as a method of anchoring the cathode spot of a mercury arc. A planar magnetron structure has also been proposed in British Patent Specification No. 1 453377 as a means of achieving high rate sputtering. A planar electrode with a magnet coil behind it is also proposed in British Patent Specification No. 1338370. U.S.

Patent Specification No. 4060470 discloses a planar magnetron arrangement in Figures 10 and 11; it also discloses a device in Figures 1 to 9 incorporating an annular concave cathode with a frusto-conical sputtering surface. It is, however, relatively expensive to make such an annular concave cathode.

When using a planar target in a planar magnetron arrangement erosion of the target is greatest in the part of the target which is adjacent to that part of the target which is adjacent to that part of the magnetic field where the lines of magnetic flux are parallel to the target surface. As erosion proceeds the effective thickness of the target decreases at the areas of maximum erosion leaving a central area having the original thickness of the target. The ion bombardment of the target in addition to causing sputtering and the emission of secondary electrons, also causes heating of the target. The temperature rise of the target consequent on this heating is usually restricted either by bonding the target to a water cooled surface or, more usually with high power operation, by clamping the edges of the target to a water cooled surface.The effectiveness of the clamping technique in restricting the temperature of the target depends on the cross sectional area of the target normal to the direction of heat flow from the area of maximum erosion to the clamped edges of the target. Hence as erosion proceeds there is a danger of overheating of at least the central area of the target. This means that it becomes necessary to replace the target after a period of operation.

The present invention seeks to provide an improved form of sputtering electrode.

According to the present invention there is provided a sputtering electrode having a face which has a valley formed therein.

The invention further provides a sputtering apparatus comprising an electrode having a face which has a valley formed therein and magnet means positioned behind the electrode to form a magnetic field whose lines of magnetic flux extend from the sides of the valley in an arch extending over an area of the face adjacent the valley so as to form an endless tunnel extending around the periphery of the valley.

Preferably the valley is V-shaped.

The electrode may be of the rectangular, circular or sector type.

The sputtering apparatus of the invention may be arranged either for d.c. operation, or for a.c.

operation, or for r.f. operation. Hence the apparatus may further include an anode and means for establishing a d.c. electric potential between the anode and the first-mentioned electrode. An r.f. sputtering apparatus according to the invention may further include a second electrode and means for establishing an r.f.

electric potential between the first-mentioned and second electrodes.

It is preferred that in sputtering apparatus according to the invention the valley is so positioned in relation to the magnet means that the area or areas within the valley that is or are substantially parallel to the lines of magnetic flux are substantially equal to the area or areas of the face outside the valley that is or are substantially parallel to the lines of magnetic flux.

There is further provided in accordance with the invention a method of forming a sputtered film on a substrate which comprises providing a sputtering electrode having a face which has a valley formed therein, establishing a magnetic field whose lines of magnetic flux extend from the sides of the valley in an arch extending over an area of the face adjacent the valley so as to form an endless tunnel extending around the periphery of the valley, establishing a glow discharge in the vicinity of the electrode so as to cause sputtering, and allowing sputtered material to form a film on the substrate.

In order that the invention may be clearly understood and readily carried into effect, a, preferred embodiment and a modification thereof will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a cross section of a sputtering electrode according to the invention and shows also one form of means for providing a suitable magnetic field; Figure 2 is a perspective view of the electrode of Figure 1; Figure 3 is a cross section of a sputtered electrode showing erosion profiles; and Figure 4 is a cross section through an electrode with an alternative magnet configuration.

Referring to the drawings, the apparatus comprises an electrode 1 in a vacuum environment 2 which is maintained at a suitable pressure in the range 10-4 torr to 1 torr. A negative d.c. voltage of between100 volts and -10,000 volts is applied to electrode 1. Other components, such as the anode, in contact with, or close to, the plasma are held at, or close to, ground potential although the surface receiving the sputtered film may be held at any suitable potential, and in particular at a negative potential of between10 volts and -10,000 volts to perform bias sputtering or ion plating.

Electrode 1 is provided with means for generating a magnetic field 5 having lines of flux which extend in a curve from electrode 1 and return thereto to form an arch. The magnet means are commonly permanent magnets 3, 3' with poles of one polarity adjacent the electrode and a central permanent magnet 17 with a pole of opposite polarity adjacent to the electrode. Pole piece 4 is used to improve the magnetic field strength in front of electrode 1. Electrode 1 is shaped to provide a flat area 10 and 10' with a 'V' sectioned valley with sides 11,11 11" and 11"' in the centre. The bottom of the valley 12 is positioned centrally over the central permanent magnet 17 and along the line where the emerging lines of magnetic flux are essentially normal to the plane of the magnet structure.The angle that the walls 11,11 11" and 11"' make with the planar surface 10 and 10' is such that the areas 13 and 13', and the areas 14 and 14' that are substantially parallel to the lines of magnetic flux, are approximately equal. In this way the area of electrode surface that is essentially parallel to the adjacent lines of magnetic flux is improved.

Typically the slope of the valley sides ranges from about 100 up to about 450; in other words the angle between side 11 and the flat area 10 measured through the solid ranges from about 1700 to about 1350. Magnets 3,3' extend around the periphery of target 1 and magnet 17 extends the length of the valley so that the flux lines of the magnetic field define an endless "tunnel" extending over an approximately elliptical zone extending partly over the flat area 10, 10' and partly over the sides 11,11', 11" and 11 "' of the valley. The approximate extent of this zone is indicated diagrammatically at 30, 30' in Figure 2.

The action of the bias voltage on electrode 1 under suitable pressure conditions in the vacuum environment causes a glow discharge to be generated. This glow discharge has two characteristic regions of particular interest. There is a plasma region in which there are approximately equal numbers of positively charged ions and negatively charged particles (usually electrons). This plasma region is separated from the electrode surface by a region commonly known as the dark space across which most of the potential applied to the electrode is dropped. Thus, positive ions in the plasma can diffuse into the dark space and there be accelerated to the electrode surface. The bias voltage applied to the electrode is sufficient to accelerate the positive ions to energy levels where they cause appreciable amounts of sputtering when they hit the electrode.The positive ions in addition to causing the sputtering also cause secondary electrons to be emitted from the electrode surface. These electrons are accelerated away from the electrode by the potential across the dark space and are affected by the magnetic field. Those electrons which travel in a direction that is other than parallel with the direction of the lines of magnetic flux will have their motion greatly influenced by the magnetic field so that they are confined to travel for an appreciable time in an endless "tunnel" adjacent to the target over the zone whose limits are indicated at 30, 30' in Figure 2. The electrons emitted from the electrode and accelerated across the dark space have sufficient energy to cause ionisation of the gas environment. Thus the electrons that are confined to the endless "tunnel" region cause ionisation to form a dense plasma in this region.This plasma is a plentiful supply of ions for sputtering and, as a result, ions from this dense plasma region that are accelerated across the dark space cause preferential erosion of the electrode 1 at 9, 9' and 9"' as shown in Figure 3 adjacent to that part of the magnet field where the lines of magnetic flux are substantially parallel to the electrode surface.

The ion bombardment of electrode 1 in addition to causing sputtering and the emission of secondary electrons, also tends to cause heating of electrode 1. The temperature rise of electrode 1 consequent on this heating is restricted either by bonding electrode 1 to a water cooled surface or, more usually with high power operation, by clamping the edges 8 and 8' of electrode 1 to a water cooled surface.Since the effectiveness of the clamping technique in restricting the temperature of electrode 1 depends on the cross sectional area of the electrode normal to the direction of heat flow from the area of maximum erosion 9, 9' and 9"' to the clamped edges 8 and .8', it will be seen that the illustrated electrode provides an improvement over prior art planar electrodes in that there is no central non-eroded area of substantial thickness and hence there is little or no tendency for over-heating of the central area of the electrode to occur.

One advantage of the electrode of the invention compared with conventional electrodes with a planar surface is that electrons from the central region of the electrode on either side of the bottom 12 of the valley are accelerated across the dark space during sputtering in a direction that is not parallel to the adjacent lines of magnetic flux.

Thus, the electrons have a greater probability of being trapped in the endless "tunnel" of the magnetic field. This has two benefits. Firstly, the trapped electrons do not bombard the useful area of deposited film and secondly they can cause ionisation of the gas environment in the endless "tunnel" region.

Another benefit is that the area of electrode 1 that is substantially parallel to the adjacent lines of magnetic flux is improved and that the area of electrode 1 that is eroded is correspondingly improved so that the percentage of electrode 1 that can be consumed is increased compared with a planar one. Hence the life of electrode 1 is extended compared with that of a planar electrode. Also, by suitable positioning of areas 13 and 14 and 13' and 14', the erosion patterns can be made to overlap as shown in Figure 3. This aspect of the invention can substantially prevent the deterioration of the distribution of deposited thin film material.

Another benefit is that when electrode 1 is substantially cooled by clamping at 8 and 8' to a heat sink, the cross-sectional area of the electrode available for heat flow is improved over that in a planar electrode.

A modification of the apparatus of Figures 1 to 3 is shown in Figure 4. In this case the electrode 22 may be substantially thicker than electrode 1.

In this case the outer permanent magnets 1 9 and 19', shown in Figure 4, are positioned not behind the electrode 22 but alongside and somewhat behind the front surface of the electrode.

Reference numeral 20 represents the central magnet and reference numeral 21 the pole piece.

The invention is intended to include various means of providing a magnetic field such as permanent magnets and current carrying coils; to include various forms of sputtering that are well known such as d.c., a.c. and r.f.; to include various well known electrode shapes such as rectangular, circular and sector and to include sputtering processes where the receiver of the deposited thin film is biased and heated or cooled.

In the case of a circular electrode the valley may be substantially conical or frusto-conical in shape.

If desired a target of the same material as the electrode 1 or 22 or of a different material can be bonded or otherwise secured to the face of the electrode 1 or 22 so that the material of the electrode itself is not sputtered. Such a target may be planar or may be contoured to fit in the valley; the surface of the target that is sputtered may be planar in this latter case or may have a valley in it.

Claims

1. A sputtering electrode having a face which has a valley formed therein.

2. An electrode according to claim 1, in which the valley is surrounded by a flat area.

3. An electrode according to claim 1 or claim 2, in which the valley is V-shaped in section.

4. An electrode according to any one of claims 1 to 3, in which the face has a substantially rectangular periphery.

5. An electrode according to any one of claims 1 to 4, in which the valley sides are planar.

6. An electrode according to any one of claims 1 to 5, in which the face has a substantially circular periphery.

7. An electrode according to claim 6, in which the valley is substantially conical or frusto-conical.

8. An electrode constructed and arranged substantially as herein described with particular reference to the drawings.

9. A sputtering apparatus comprising an electrode having a face which has a valley formed therein and magnet means positioned behind the electrode to form a magnetic field whose lines of magnetic flux extend from the sides of the valley in an arch extending over an area of the face adjacent the valley so as to form an endless tunnel extending around the periphery of the valley.

10. A sputtering apparatus according to claim 9, in which the valley is surrounded by a flat area.

11. A sputtering apparatus according to claim 9 or claim 10, in which the valley is V-shaped in section.

12. A sputtering apparatus according to any one of claims 9 to 11, in which the face has a substantially rectangular periphery.

13. A sputtering apparatus according to any one of claims 9 to 12, in which the valley sides are planar.

14. A sputtering apparatus according to any one of claims 9 to 11, in which the face has a substantially circular periphery.

1 5. A sputtering apparatus, according to claim 14, in which the valley is substantially conical or frusto-conical.

1 6. A sputtering apparatus according to any one of claims 9 to 13, further including an anode and means for establishing a d.c. electric potential between the anode and the first-mentioned electrode.

1 7. A sputtering apparatus according to any one of claims 9 to 13, further including a second electrode and means for establishing an r.f.

electric potential between the first-mentioned and second electrodes.

18. A sputtering apparatus according to any one of claims 9 to 17, in which the valley is so positioned in relation to the magnet means that the area or areas within the valley that is or are substantially parallel to the lines of magnetic flux are substantially equal to the area or areas of the face outside the valley that is or are substantially parallel to the lines of magnetic flux.

1 9. A sputtering apparatus constructed and arranged substantially as herein described with particular reference to the drawings.

20. A method of forming a sputtered film on a substrate which comprises providing a sputtering electrode having a face which has a valley formed therein, establishing a magnetic field whose lines of magnetic flux extend from the sides of the valley in an arch extending over an area of the face adjacent the valley so as to form an endless tunnel extending around the periphery of the valley, establishing a glow discharge in the vicinity of the electrode so as to cause sputtering, and allowing sputtered material to form a film on the substrate.

21. A method of forming a sputtered film on a substrate conducted substantially as herein described with particular reference to the drawings.

22. A substrate having a sputtered film formed thereon using a target according to any one of claims 1 to 8, a sputtering apparatus according to any one of claims 9 to 19, or a method according to claim 20 or claim 21.