GB2058143A

GB2058143A - Sputtering electrodes

Info

Publication number: GB2058143A
Application number: GB8024999A
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: GB2058143B

Abstract

A shuttering electrode comprises a number of lengths of elongate section material, typically rectangular or square, arranged to form a closed figure. Such an electrode can be incorporated in sputtering apparatus by clamping to a central cooling member. Magnet means are arranged in the apparatus so that the lines of magnetic flux extend in an arch from one part of the section material to another part thereof, thereby forming an endless tunnel around the periphery of the closed figure.

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 Güntherschulze, 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", Viol. 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 1971, 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.3216652). 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. 1453377 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.

The present invention seeks to provide an improved form of sputtering electrode, an improved apparatus incorporating such an electrode, and an improved method of sputtering.

According to the present invention there is provided a sputtering electrode comprising a number of lengths of elongate section material arranged to form a closed figure.

The invention further provides a sputtering apparatus comprising a sputtering electrode which comprises a number of lengths of elongate section material arranged to form a closed figure, and magnet means for producing a magnetic field whose magnetic lines of flux extend in an arch from one part of the elongate section material to another part thereof so as to form an endless tunnel extending around the periphery of the closed figure.

There is also provided in accordance with the present invention a method of forming a sputtered film on a substrate which comprises providing a sputtering electrode comprising a number of lengths of elongate section material arranged to form a closed figure, establishing a magnetic field whose lines of magnetic flux extend in an arch form one part of the elongate section material to another part thereof to form an endless tunnel extending around the periphery of the closed figure, 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.

Preferably the area extends from one face of the section material to another face thereof. If the section material is at least part arcuate in section, as for example in the case of circular section material, elliptical section material, or sector section material the tangents to the section material surface at the ends of the arch may make an angle one with another.

The elongate section material may be of any desired cross-section. Usually however it will be preferred to use as simple a cross section as possible, e.g. a circular, elliptical, rectangular or square section. Conveniently straight lengths are used to build up a polygonal figure, e.g. a triangular, square, rectangular, pentagonal, hexagonal or higher polygonal shape. The lengths may be joined together, as for example by welding. Usually however it will be convenient to form the electrode from separate lengths. In a preferred arrangement four lengths of elongate section material are arranged to form a rectangle.

It is also contemplated to use curved lengths of elongate section material. For example, two or more curved lengths can be assembled to form an electrode in the shape of a circle.

The lengths of elongate section material can be clamped to a central cooling member. Moreover, a shield member may be provided for shielding the exposed face of the central cooling member during sputtering. This shield member can be isolated from ground so as to enable its use as an anode for the glow discharge during sputtering. A further shield member or members may be provided for shielding the clamping means during sputtering.

In order that the invention may be clearly understood and readily carried into effect a preferred embodiment thereof will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, wherein: Figure 1 is a cross section of an improved electrode structure in accordance with the invention; and Figure 2 is a perspective view of the rectangular electrode of Figure 1 showing how it is made up from four lengths of elongate section material.

Referring to the drawings, in Figures 1 and 2, the lengths of rectangular electrode 1, ',1 1" and 1"' are clamped t6 a cooled central piece 2 by means of clamps 3 and 3' which are bolted to a piece 4 which is also cooled. The pieces 2 and 4 are cooled in conventional manner by means of a coolant, such as water, supplied through coolant passages (not shown). In the present example magnet means comprising permanent magnets 5 and 5' are used to provide a magnet field which forms an endless tunnel 9 and 9' over the electrode surfaces 10 and 11 and 10' and 11'.

The piece 4 is made of an appreciably nonmagnetic material. The pieces 2, 3 and 3' may be used as pole pieces for the permanent magnets 5 and 5'. These pieces may be shaped to provide an optimum magnetic field shape. Shields 6, 6' and 7 are provided to prevent unwanted sputtering of the electrode structure. The shield 7 may be isolated from ground and used as an anode for the glow discharge.

When a suitable voltage is applied to the electrode (in the range 100 to 10,000 volts negative) and the gas environment is at a suitable pressure (10-4 torr to 1 torr) a glow discharge will form and sputtering of the electrode will take place. Other components in contact with, or close to, the plasma are held at, or close to, ground potential although the surface receiving the film may be held at any suitable potential, and in particular at a negative potential of between -10 volts and -10,000 volts to perform bias sputtering or ion plating.

The 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 electrode and extending around its periphery. 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 erosion of the electrode by sputtering. Material sputtered from the electrode deposits on any surface in its path, e.g. on the substrate (not shown).

The ion bombardment of the electrode in addition to causing sputtering and the emission of secondary electrons, also causes heating of the electrode. The temperature rise of the electrode consequent on this heating is restricted by clamping the electrode to the cooled central piece 2.

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 sputtering in inert and reactive atmospheres and combinations of both; to include various well known electrode shapes where possibie 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 practice of the invention it is further contemplated to use, if desired, targets bonded or otherwise secured to the electrode. In this case sputtering of target material occurs rather than sputtering of the electrode itself. The target material may be the same as or different from that of the electrode and will usually be shaped to conform to the shape of the section material. For example, in the embodiment of Figures 1 and 2, lengths of L-section target material could be bonded to the exposed surfaces of the electrode parts 1, 1', 1" and 1"' prior to clamping.

Alternatively, separate L-section target pieces could be clamped in place.

Claims

1. A sputtering electrode comprising a number of lengths of elongate section material arranged to form a closed figure.

2. A sputtering electrode according to claim 1, in which the elongate section material is rectangular in section.

3. A sputtering electrode according to claim 1 or claim 2, in which the elongate section material is square in section.

4. A sputtering electrode according to any one of claims 1 to 3, in which four lengths of elongate section material are arranged to form a rectangle.

5. A sputtering electrode constructed and arranged substantially as herein described with particular reference to the drawings.

6. A sputtering apparatus comprising a sputtering electrode which comprises a number of lengths of elongate section material arranged to form a closed figure, and magnet means for producing a magnetic field whose magnetic lines of flux extend in an arch from one part of the elongate section material to another part thereof so as to form an endless tunnel extending around the periphery of the closed figure.

7. A sputtering apparatus according to claim 6, in which the elongate section material is rectangular in section.

8. A sputtering apparatus according to claim 6 or claim 7 in which the elongate section material is square in section.

9. A sputtering apparatus according to any one of claims 6 to 8, in which the electrode comprises four lengths of elongate section material arranged to form a rectangle.

10. A sputtering apparatus according to any one of claims 6 to 9, further including clamping means for clamping the lengths of elongate section material to a central cooling member.

11. A sputtering apparatus according to claim 10, in which a shield member is provided for shielding the exposed face of the central cooling member during sputtering.

12. A sputtering apparatus according to claim 11, in which the shield member is isolated from ground so as to enable its use as an anode for the glow discharge during sputtering.

13. A sputtering apparatus according to any one of claims 10 to 12, in which a further shield member or members is or are provided for shielding the clamping means during sputtering.

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

1 5. A method of forming a sputtered film on a substrate which comprises providing a sputtering electrode comprising a number of lengths of elongate section material arranged to form a closed figure, establishing a magnetic field whose lines of magnetic flux extend in an arch from one part of the elongate section material to another part thereof to form an endless tunnel extending around the periphery of the closed figure, 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.

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

17. A substrate having a sputtered film formed thereon using an electrode according to any one of claims 1 to 5, a sputtering apparatus according to any one of claims 6 to 14, or a method according to claim 1 5 or claim 1 6.