WO2011131921A1

WO2011131921A1 - High density plasma source

Info

Publication number: WO2011131921A1
Application number: PCT/GB2011/000519
Authority: WO
Inventors: Michael John Thwaites; Peter John Hockley
Original assignee: Plasma Quest Limited
Priority date: 2010-04-20
Filing date: 2011-04-05
Publication date: 2011-10-27
Also published as: TW201145349A; GB201006567D0

Abstract

An elongate gas plasma source (3) particularly configured for use within a sputter coating apparatus, comprising a sputter target assembly (4) positioned and supported within a suitably furnished vacuum chamber (1 ) such that an essentially uniform plasma of density 10¹¹ cm^-3 or more separately generated by the plasma source is magnetically guided and confined to the vicinity of the target material (19). Negative electrical bias applied to the target material causes sputtering, thereby coating surfaces and substrates (21) within the system with a film of the target material. The elongate gas plasma source supports the use of very large dimensioned targets for large area coating applications together with the capability for plasma densities of 10¹² cm^-3 or more over the full target surface. At least in preferred embodiments, the gas plasma source may yield very high coating rates over large areas and may be especially suited to high throughput, large substrate and web coating processes.

Description

HIGH DENSITY PLASMA SOURCE

FIELD OF THE INVENTION

The present invention relates to a gas plasma source for producing high density plasma, primarily but not uniquely for use in apparatus suitable for the sputter deposition of thin film coatings onto surfaces and materials.

BACKGROUND

Sputtering processes are widely used for the deposition of thin films of materials (coatings) onto various substrates. In general, the sputter process takes place within a vacuum chamber in which a small quantity of ionisable process gas, for example argon, is present. At appropriate process gas pressures, gas plasma may be produced through ionisation of the gas by well known means, for example the application of a high voltage between two electrodes within the chamber. A target material, which may itself form part of the plasma generating system, is bombarded by positive gas plasma ions and if the ion bombardment is of sufficient energy target atoms are ejected from the target surface into the vacuum. A substrate placed within the vacuum system, usually with line of sight to and in proximity to the target surface being bombarded, may then be coated by the released target material.

An example of a simple plasma sputtering system is the commonly named "diode" sputter system that essentially comprises of two metal plates, the anode and cathode, separated at an appropriate distance within a vacuum system and with a suitable DC voltage between them. The anode plate is usually (but not necessarily) connected to ground potential and a negative DC voltage, typically 600V or more, is then applied to the cathode, which is made of or has a surface made of the target material that it is desired to sputter. Any electrons emitted from the cathode will be accelerated away from the cathode and may cause ionisation of the sputter gas introduced into the vacuum system, producing positive gas ions which are then attracted to the cathode and cause sputtering (and further electron emission). It is usual (though not necessary) for the substrate that is to be coated to be placed on the anode electrode.

Whilst simple, the diode sputter system is however limited in its application as it is only efficient or useful under a narrow range of process conditions and generally does not generate high plasma intensity (density) and therefore sputters at lower rates than are desirable for many production purposes. It is however still used where other techniques are in some way unsuitable.

The evolution of sputtering technology has greatly improved upon the diode system in the drive to achieve higher sputter rates, better uniformity and properties of deposited films, and wider ranges of materials that can be sputtered. Thus it is well known that the cathode (target) can be powered with AC voltages, for example at radio frequency (RF) between 1 MHz and 1 GHz, and typically 13.56 MHz, to additionally allow or improve the sputtering of insulating or semi-insulating materials, and that magnetic fields can be used to confine or direct the electrons that originate from the cathode, for example to locally greatly increase the plasma intensity at the target to enhance sputtering rates. It should be noted that, in general, the achievement of higher sputter rates and hence higher thin film deposition rates is a primary commercial goal for sputter systems.

As an example of an improved system, a circular "magnetron" sputter target assembly has a torus shaped magnetic field penetrating the target material surface to confine emitted electrons and induce a locally far higher local ionisation level (or 'plasma density') than would otherwise be possible. This allows high sputtering rates to be achieved at low gas pressures, typically 1x10³ to 7x10^'3 torr, resulting in high material deposition rates onto substrates and good quality of the deposited thin films. As a result, sputter deposition apparatus using magnetron based designs are extensively used in industrial processes, e.g. in the

semiconductor and opto-electronic industries.

To further increase deposition rates and system capability, and overcome some of the limitations imposed by diode and magnetron sputtering systems, it is known that a plasma density of 10¹¹ cm^'3 or more, hereinafter "high density plasma", can be produced remotely from and independently of the target and directed to its vicinity by electric and / or magnetic fields.

The major change resulting from using remotely generated plasma is that the sputter target assembly is not required to produce, sustain or contribute energy to the high density plasma i.e. no 'cathode' plate is required for plasma generation. This permits the elimination of the toroidal magnetic field used in the magnetron designs with the result that with the remotely generated plasma guided to the target surface sputtering takes place over the whole target surface, not just the ring of material within the torus.

Effectively the process advantages of the magnetron design are retained whilst a major disadvantage is eliminated. For an appropriately designed and operated system, the plasma density delivered to the target surface is comparable to or greater than that which would be generated in the localised torus of the magnetron design. Since all areas of the target therefore I sputter material at the same high rate as is achieved only locally in the magnetron design, the overall deposition rate that may be achieved from the target is greatly increased, typically by a factor of 3 to 5 over the magnetron based system, substantially more when compared to the simple DC or AC powered diode based sputter systems that typically generate plasma densities 100 times less than that achievable with the remote plasma source systems,

i An additional advantage of using remotely generated plasma is that the target is able to be operated at process conditions that would be inappropriate to plasma generation from a diode or magnetron sputter system (e.g. low sputter gas pressure, low target voltage). This allows considerable scope for optimisation of the coating process and the coating's physical properties and it is widely reported that many thin film coatings produced by remote plasma sputter systems can be made with superior properties to those made with other thin film coating systems.

A variation of the sputter process is reactive sputtering, wherein a process gas or a component of a process gas mixture reacts with the sputtered target material or the deposited thin film to produce a compound material on the substrate. As an example, an aluminium target may be sputtered under appropriate conditions in plasma struck in a gas mixture of argon and oxygen to deposit an aluminium oxide film. It is usually advantageous to do this in preference to directly sputtering the compound target, for example aluminium oxide, as the compound target sputters far more slowly than the elemental target and deposition rates are correspondingly lower. However, there are often difficulties in achieving a stable process by reactive sputtering and it is not therefore as widely used as might be expected. A further advantage of using the remotely generated plasma is that the reactive sputter process becomes more stable and efficient due to the high rate sputtering of the full target surface that can be achieved.

A variety of techniques are known that may be used to generate remote, high density plasmas, as summarised by Popov in 'High Density Plasma Sources' (1995) and Chen in 'Lecture Notes on Principles of Plasma Processing' (2003). For example the electron cyclotron resonance (ECR) phenomena may be used to produce plasma by coupling a microwave source with a strong magnetic field in vacuum.

As a further example high density plasma waves may be generated by the use of an external antenna powered with 13.56 MHz radio frequency signal, as shown in original papers by Boswell and subsequently Chen. These have the advantage of using lower magnetic field strengths compared with ECR, but require careful antenna and magnetic field design to ensure the efficient production and propagation of the 'helicon wave' electrons which are used to generate the high plasma densities.

A further, more efficient remote plasma source is used in a sputter deposition system invented by Thwaites (UK patent GB2343992, US Patent No.6463873). This utilises a helically wound multi-turn coil antenna in conjunction with non-linear magnetic fields to both produce a high density plasma and to direct this to a target surface out of line of sight of the plasma source. The Thwaites plasma source has the advantages of a simpler, more robust antenna and magnetic field design than the 'helicon wave' based systems and is found to be more efficient and easier to operate in practice.

It is important to realise that the above remote plasma sources should be more properly regarded as plasma generation sources. They do not direct the plasma produced within the source into a process chamber, but rather act as the source of a plasma generation zone that extends from within the source and into the chamber. Because this plasma generation region is produced by energetic but low mass electrons, it can be guided and shaped with magnetic fields of strength of as little as 50 Oersteds, allowing it to extend for substantial distances from the plasma source itself, and to even be bent through, for example, 90 degrees as shown by Thwaites. The above plasma sources are therefore distinct from the well known class of apparatus commonly known as ion sources. An ion source, for example the "high frequency plasma source" disclosed by Weiler and Dahl in US Patent No. 6936144, generates high density plasma locally within the source, but does not propagate this plasma beyond the source itself. Instead, the ion component of the plasma is extracted through the use of electrically biased accelerating grids or electrodes and emerges from the source as an "ion beam", usually with an ion energy distribution that can be controlled by the source operating parameters. As a result of the much greater ion mass compared to electron mass, the ions are not confined or controlled by the moderate external magnetic fields used in the helicon and Thwaites plasma sources and it is necessary therefore to direct the emergent ion beam at the surface to be treated by the source. This major difference in output results in significant differences in potential application of the ion source compared to the plasma source; although ion sources can be used with sputter systems in a similar manner to plasma sources, they are generally far less efficient and hence of lesser utility due in part to the much lower ion densities that are achievable.

A limitation of the sputter systems discussed so far is the limited dimensions of the high intensity sputter region that thereby restrict the deposition rate that can be achieved and / or limit the number and size of substrates that may be coated. For example circular magnetron sources sputter from only the magnetic torus, typically representing less than 20% of the target area; a 200mm diameter target therefore delivers no more sputtered material than a 90mm diameter target would if uniformly sputtered. The remote plasma systems can overcome this and sputter from the entire target surface, but require a plasma source size of comparable dimension to the target to be sputtered. This generally limits the maximum commercially realistic target size to less than 200mm diameter. Additionally, geometric and layout requirements for successful implementation of the remote plasma systems result in larger target to substrate separations being needed, typically 1.5 times the target size. As the coating rate decreases proportional to the square of the separation distance, the deposition rate gains anticipated from the larger targets may not be realised in practice.

To overcome this, Hockley and Thwaites (2005 AIMCAL Fall Technical Conference, "A Remote Plasma Sputter Process for High Rate Web Coating ....") have disclosed details of a system that instead uses a cylindrical target and arranges for the plasma produced by a remote plasma source of the Thwaites design to be guided around and along the length of the full cylindrical target. Essentially, the plasma source, electromagnets and target are engineered to be co-axial such that the plasma forms a tube around the target, allowing sputtering of the full target surface. This has allowed targets of 500mm length to be used for coating using a far smaller diameter remote plasma source (150mm in their example); in addition the substrate to target spacing may be kept small, typically less than 200mm, allowing high coating rates to be realised despite the large target size.

The use of a remote plasma source according to the invention of Thwaites is critical to the above "Linear" coating system. Hockley and Thwaites disclosed that they had discovered that the Thwaites plasma source comprised a tubular plasma generation region (of dimension at the source exit similar to that of the remote plasma source tube diameter) and that, so long as this was not interrupted along its length, the plasma could therefore be propagated about the full length of the cylindrical target with good plasma density uniformity. A helicon type remote plasma source, e.g. of the Boswell design, could not achieve this as the high density plasma generation occurs along the source axis and would therefore be intercepted and stopped by the co-axial target.

It is obvious that if the plasma generation "tube" originating from the plasma source can be passed around the cylindrical target as disclosed by Hockley and Thwaites and still allow highly efficient sputtering of the entire target surface without the need to direct the plasma generating region onto the target surface (e.g. as required by the invention of Thwaites), then the target could also be placed external to a plasma generated by a similar source, or indeed the helicon wave sources (e.g. of Boswell) and still be sputtered. Essentially it is obvious that directing and guiding the plasma from the disclosed plasma sources over and in proximity to the target material is sufficient to allow for target sputtering to be implemented through the usual arrangements for applying suitable negative electrical bias to the target.

In such a situation, unlike the system disclosed by Hockley and Thwaites, the target can be of larger cross sectional dimension than the plasma source and only the side facing the plasma would be significantly sputtered. This configuration would allow the use of rectangular targets, as used in conventional large area diode or magnetron sputter systems, with advantage for example in the case where the target material was of high cost and it was undesirable to sputter in all directions, the latter being a particular disadvantage of the Linear system of Hockley and Thwaites. For that reason experimentation was undertaken by the inventors.

However, it was determined that the coating performance of such systems is poor when compared to other sputter techniques, primarily due to the large separation of target surface to substrate mandated by the need for the full plasma generation region, that can be twice or more the diameter of the plasma source in practice, to pass between the two. Accordingly, the inventors investigated the possibility of overcoming this through experimentation with alternate plasma source designs.

In the course of their experiments, the inventors unexpectedly discovered that the multiloop antennas defined as necessary for the helicon based sources and the multi-turn helical antennas as required by Thwaites are not after all essential to the production of high density plasma. It was determined that a plasma source based on that disclosed by Thwaites can be built and operated with a single turn antenna, or even a 'U' shaped antenna, and when powered via a suitable RF impedance matching circuit based on widely commercially available units, can produce efficient high density plasma generation, for example 10¹³ cm^"3, equivalent to that of the Thwaites source operating at the same process conditions. The inventors' studies show that although multi-turn antennae initially appear best for use in plasma sources of the Thwaites design, this is in reality a result of ease of impedance matching using commercially and readily available RF impedance matching units. The inventors have determined that single or even less than single turn antennae can also be used with appropriate antenna sizing and design, and/or improvement of the RF impedance matching units. Whilst this discovery of itself does not greatly change the performance or application of the plasma source, the realisation that unlike the case with the helicon wave based systems there is no fundamental need for multiple antenna loops when using the Thwaites source led the inventors to experiment with a wider range of antenna configurations and plasma source designs than the prior art would suggest possible. Indeed, some of the resulting discoveries are in direct conflict with the basic theories expounded in such prior art.

SUMMARY

Viewed from a first aspect, the present invention relates to a high density gas plasma source capable of producing high density gas plasma of density greater than 10¹¹ cm ³, the plasma source comprising: a plasma chamber having a sidewall, at least a portion of the sidewall of the plasma chamber permitting the transmission of radio frequency (RF) energy; an aperture being provided in said plasma chamber and at least one magnet being provided proximal said aperture; and a radio frequency (RF) antenna positioned externally of the plasma chamber; wherein the antenna has a non-circular plan form. At least in preferred embodiments, the antenna can be formed with an elongated plan form (such that its length is greater than its width). Preferably, the antenna is extended in a transverse direction. This arrangement is particularly advantageous since the resulting plasma can have an increased width, rendering it suitable for a variety of manufacturing applications.

The gas plasma source according to the present invention can be an integral part of a vacuum system. For example the gas plasma source can be capable of being used within a vacuum system. Alternatively, the gas plasma source can be a separate piece of apparatus permanently or removably (albeit optionally attached to an exterior of the vacuum system) connected to the vacuum system. The present invention can also relate to a combination of the gas plasma source and a vacuum system.

It will be appreciated that the gas plasma source according to the present invention would typically be used in conjunction with appropriate supply means for introducing a process gas and/or one or more additional magnets to direct, augment or otherwise alter the plasma. For example, the additional magnet(s) could be provided remote from the aperture (e.g. at the other end of the plasma chamber) to further confine, shape, direct or augment the plasma produced by the plasma source. The process gas supply means could also form part of the plasma source. The present invention can also relate to a combination of the gas plasma source and a process gas feed and/or one or more additional magnets.

The antenna can be arranged proximate to only one side of the plasma chamber; or can at least substantially encircle the plasma chamber. The gas plasma source could comprise two or more antennae. The antennae could each be arranged proximate to a respective side of the plasma chamber. A shaped antenna or a series of antennae proximate to two or more sides of the plasma chamber are also possible.

The antenna(e) can be spaced apart from an outside of the plasma chamber. The antenna(e) can be shaped substantially to match the outer profile of the plasma chamber.

Viewed from a further aspect, the present invention relates to a gas plasma source capable of producing high density gas plasma of density greater than 10¹¹ cm^"3, the plasma source comprising: a plasma chamber; an aperture being provided in said plasma chamber and at least one magnet being provided proximal said aperture; and a radio frequency (RF) antenna provided within the plasma chamber; wherein the antenna has a non-circular plan form.

An enclosure or housing is preferably provided within the plasma chamber. The enclosure preferably houses at least a portion of the antenna. The antenna can be fully or partially enclosed within the enclosure and preferably at least a portion of the enclosure permits the transmission of radio frequency (RF) energy. More than one enclosure could be provided to accommodate a plurality of antennae, for example each enclosure could house a single antenna.

The antenna enclosure can be tubular and is preferably sealed at its interface(s) with the plasma chamber. The antenna preferably extends partially or fully across the width of the plasma chamber. The antenna enclosure preferably extends across the width of the plasma chamber.

In those arrangements of the gas plasma source comprising a plurality of antenna, the antennae can be independent of each other. Alternatively, the antennae can be joined together. Preferably, the antennae are joined together outside of the plasma chamber. For example, two antennae could extend through the plasma chamber and be joined together outside of the plasma chamber. The two antennae could, for example, extend through the plasma chamber parallel to each other and be joined to each other by a u-shaped section. Moreover, a single continuous length of electrically conductive material can be used to form multiple antennae. For example, the conductive material can define a serpentine path in which each intermediate length effectively forms a separate antenna. The bends joining the intermediate lengths of the serpentine path could be positioned externally of the plasma chamber.

The plasma chamber has a longitudinal axis extending in a longitudinal direction. The plasma chamber preferably comprises a tube extending in said longitudinal direction. The longitudinal axis of the plasma chamber is preferably coincident with a central longitudinal axis of the tube. At least one end of the tube can be open to form the aperture in the plasma chamber.

The antenna, magnet and aperture are preferably displaced along the longitudinal axis of the plasma chamber. The magnet and the aperture are preferably positioned proximate to each other with the antenna displaced from the pairing. The components can be arranged in the following orders: antenna(e)-magnet-aperture; or antenna(e)-aperture-magnet. The plasma chamber has a transverse cross section (arranged substantially orthogonal to the longitudinal axis of the plasma chamber) which, at least in preferred embodiments, is elongated. Alternatively, the plasma chamber can comprise a plurality of tubular members arranged in an array, the array having an elongate transverse cross section. Preferably the plasma chamber has a rectangular, rounded rectangular, oval or elliptical transverse cross section.

The plasma chamber and/or the enclosure may comprise one or more quartz tubes, or one or more quartz members. It will be appreciated that the gas plasma source could comprise a plurality of plasma chambers and/or enclosures.

Preferably, the plasma chamber and/or the antenna enclosure is/are capable of supporting a pressure differential across their respective inner and outer surfaces.

The RF supply can be matched to the impedance of the antenna. For example, a larger diameter loop helical antenna may have less turns than a smaller diameter one for a given RF impedance matching unit. Alternatively, the properties of a RF impedance matching unit can be adjusted to suit the antenna impedance.

In use, the antenna(e) can be supplied with power from a radio frequency power supply system operating at a frequency between 1MHz and 1 GHz; a frequency between 1 MHz and 100MHz; a frequency between 10 MHz and 40 MHz; or at a frequency of approximately 13.56 MHz or multiples thereof.

The at least one magnet is preferably positioned proximate to the plasma chamber aperture. The at least one magnet may at least substantially encircle the plasma chamber transverse cross section; or the at least one magnet may be positioned at least substantially proximate to one of the longer dimensioned sides of the plasma chamber. Preferably, the at least one magnet generates a magnetic field of strength greater than or equal to 50 Oersteds, and preferably between 50 and 500 Oersteds.

The at least one magnet can comprise: (i) one or more electromagnets; (ii) one or more permanent magnets; or (iii) a combination of one or more permanent magnets and one or more electromagnets. Preferably at least one of the electromagnets is controllable. The gas plasma source can further comprise a power supply means for supplying the electromagnets with direct current (DC), an alternating current (AC) or a pulsed source to induce changes to the plasma.

The at least one magnet, and any additional magnet(s) used with the plasma source, are to be placed, shaped and/or orientated such that at least a portion of the imaginary magnetic field lines produced by said magnets when the plasma source is in use pass through both the plasma chamber aperture and the plane(s) of the antenna(e) projected through said plasma chamber. The at least one magnet preferably does not obstruct or only partially obstructs the aperture formed in the plasma chamber.

In a preferred configuration, the plasma chamber aperture is encircled by the magnet(s). The aperture is preferably, but not necessarily, aligned orthogonal to the plasma chamber longitudinal axis. The axis or plane of symmetry of the imaginary magnetic field lines produced by said magnet(s) is preferably essentially parallel to said plasma chamber longitudinal axis.

The antenna(e) is preferably positioned remote from the aperture provided in the plasma chamber. The antenna(e) is preferably displaced from the aperture along the longitudinal axis of the plasma chamber.

The term "antenna" used herein is to be understood as referring to the apparatus operative to generate a plasma within the plasma chamber. The skilled person will understand that ancillary devices, such as leads, connections and mounting devices do not form part of the antenna within the meaning of the term in the present application. Moreover, unless specified otherwise, references herein to the "plan form" of the antenna are to the effective profile of the antenna when projected onto a plane arranged substantially perpendicular to the longitudinal axis of the plasma chamber. Thus, the "plan form" refers to the profile of the antenna when viewed along the longitudinal axis of the plasma chamber.

The antenna could be arranged in a single plane, for example a plane arranged orthogonal to the longitudinal axis of the plasma chamber or a plane inclined at an angle between 0° and 90° relative to said longitudinal axis. Optionally, the antenna could extend in a direction parallel to the longitudinal axis of the plasma chamber, for example in the form of a helix or a spiral.

The antenna has a length and a width in plan form. The antenna is preferably configured such that its length is greater than its width. The ratio of the length to the width of the antenna is preferably greater than or equal to two (2), three (3), five (5), ten (10), fifteen (15) or twenty (20).

The antenna can consist of or comprise an elongate strip, a length, a partial turn, a single turn or multiple turns of an electrically conductive material. The antenna preferably has a plan form which is generally elongated, linear, rectangular, rounded rectangular, polygonal, elliptical, and/or U-shaped. The antenna in transverse cross section is preferably elongated (i.e. a first dimension of the antenna is greater than a second dimension).

The antenna can comprise a loop or a partial loop which defines an operating plane. The operating plane can be arranged substantially perpendicular to the longitudinal axis of the plasma chamber. Alternatively, the operating plane can be inclined at an angle of between 0° and 90° relative to the longitudinal axis of the plasma chamber. It will be appreciated that inclining the antenna in this manner can alter its plan form as defined herein. For example, an antenna having a circular loop could be inclined such that its plan form (i.e. its profile when viewed along the longitudinal axis of the plasma chamber) is elliptical. In one embodiment, the antenna can be configured so as not to have a circular profile when projected onto any plane.

Viewed from a further aspect, the present invention relates to a gas plasma source capable of producing high density gas plasma of density greater than 10¹¹ cm^"3, the plasma source comprising: a tubular assembly that permits the transmission of radio frequency (RF) energy; the tubular assembly being open at a first end and at least one magnet being provided proximal said first end; and a radio frequency (RF) antenna positioned externally of the tubular assembly; wherein the tubular assembly has an elongated transverse cross-section; or the tubular assembly comprises a plurality of tubular members arranged collectively to define an elongated transverse cross-section.

At least in preferred embodiments, the inventors have accordingly invented new forms of the Thwaites plasma source that are of especial utility when applied to sputter coating.

The first and simplest form is to use the usual cylindrical cross section plasma source geometry disclosed in the prior art with the antenna or antennae designed through shape and positioning to excite high density plasma to only a chosen side or sides of the plasma source. When used with a cylindrical target, this allows particular longitudinal sections of the target to be preferentially sputtered, overcoming one of the main disadvantages of the disclosed Hockley and Thwaites system.

However, a superior and preferred design is to elongate the transverse cross section of the plasma source, i.e. the cross section in a plane orthogonal to the plasma source longitudinal axis, to produce an essentially rectangular or rounded rectangular or oval or elliptical cross section and to place the antenna in proximity to at least one or both of the longer dimensioned sides.

This is a particularly advantageous development: the longer plasma source cross sectional dimension can be made comparable to the target length without any need to increase the plasma source short cross sectional dimension. Essentially a very wide yet thin plasma 'slab' may therefore be generated which may be passed across and in proximity to the much shorter target width, allowing a greatly improved and more uniform magnetic plasma guiding field to be realised than is the case with the large electromagnet separation required if the plasma source is placed at the end of the target as disclosed by Hockley and Thwaites. As will be obvious to those skilled in the art, the resulting improved plasma confinement in turn allows target to substrate spacing to be minimised and improves the plasma and hence sputter uniformity with consequent major benefits to the coating process. More importantly, there is now no fundamental limit to the target length that may be used, unlike the case of the Hockley and Thwaites system where it has been determined that increasing electromagnet separation can raise non-uniformity and plasma propagation issues for target lengths beyond about 500mm.

The ability to elongate the transverse cross section of a plasma source of the Thwaites design is an important and unexpected discovery. Ion sources that can generate an essentially rectangular ion beam have been disclosed, for example by Weiler and Dahl as referenced above, and are widely used for various large area vacuum processes. However, a plasma source able to produce equivalent dimensioned high density plasma has not been disclosed, primarily because the fundamental operating principles of the helicon type plasma sources require a cylindrical transverse cross section plasma generation tube and it has also been theorised that the Thwaites source operates on similar principles. As a result, no developments of elongate versions of these high density gas plasma sources have been attempted. The elongate gas plasma source has significant advantage over the disclosed rectangular ion sources, opening up new possible uses and making it an attractive alternative for many existing applications of the ion source. Unlike the ion sources, there are no parts internal to the source such as filaments, RF antennae or ion extraction grids and plates which can become detrimentally coated or eroded by the processes for which the plasma source is intended to be used. In addition, in common with the helicon type sources, the elongate plasma source is capable of achieving very much higher plasma (and hence ion) densities than have been reported for the ion sources and, as previously discussed, allows control and guidance of the plasma by magnetic means beyond the plasma source, enabling more versatile and improved plasma based processing systems, especially sputter systems, to be realised.

In isolation, the invented elongate gas plasma source, in common with other plasma and ion sources, is of limited utility. It is when it is used in conjunction with other apparatus to produce enhanced processes and capability that it becomes of value.

Therefore, a preferred embodiment of the invention is an elongate gas plasma source based upon the operational principles of the Thwaites source fitted to and configured for use in a sputter coating apparatus, essentially comprising of a sputter target assembly, positioned and supported within a vacuum chamber; means for vacuum pumping the apparatus and for controllably introducing one or more process gases into the apparatus; the elongate gas plasma source as means of remotely generating a plasma of essentially oblong or oval cross section and density 10¹¹ cm^'3 or more, hereinafter a "high density plasma", within the apparatus and independently of the target assembly; means for magnetically guiding and confining electrons from the above remotely generated plasma such as to continue the aforesaid high density plasma to the vicinity of the target material surface of the sputter target assembly; means for controllably applying negative polarity voltage either continuously or intermittently to the target material; and means for positioning or introducing into the vacuum chamber the surfaces or items to be coated by the sputter target material.

The plasma chamber can have a longitudinal axis and a transverse axis. The plasma chamber preferably extends further in a first transverse direction than in a second transverse direction. The first and second transverse directions are preferably substantially perpendicular to each other. Thus, the plasma chamber preferably has an elongated transverse cross- section. At least in preferred embodiments, the magnet and the antenna are offset from each other along the longitudinal axis of the plasma chamber.

An impedance matching circuit or device can be provided for matching the radio frequency (RF) energy supply to the antenna or antennae.

The plasma source according to the present invention has particular applications in sputter coating systems. The present invention may further extend to a sputter coating system comprising a vacuum chamber; means for generating a vacuum in the vacuum chamber; a gas feed system attached to the vacuum chamber; a plasma source of the type described herein operative to produce a high density gas plasma within the vacuum chamber; a system for confining and guiding a gas plasma within the vacuum chamber; a sputter target assembly located within the vacuum; means for applying a negative polarity voltage to the surface of a material to be sputtered in order to induce the sputtering process to occur; means for cooling the target assembly and materials to be sputtered; means for positioning within the vacuum chamber or introducing into the vacuum chamber one or more substrates to be coated, said means including any mechanisms for translating, rotating, shielding, application of electrical potential to or cooling or heating of said substrates as required to achieve the desired process; and optional means of preventing coating of the substrate when not desired through the use of moveable shutters, shielding or substrate relocation.

The surface of the target assembly can comprise more than one type of material to be sputtered. The system for confining and guiding the plasma can comprise one or more electromagnets in addition to the at least one magnet in the plasma source. One or more of electromagnet can be controllable. Preferably, at least two electromagnets are operated with identical polarity.

The system can include further electromagnets positioned so as to influence the magnetic field profile produced by the interaction of the at least one magnet in said plasma source and said additional electromagnet. One or more of said electromagnets is preferably controllable.

The means for applying a negative polarity voltage is preferably adapted to supply during the coating process one of the following: (i) a continuous negative polarity; (ii) an intermittent negative polarity; (iii) an alternating voltage polarity using to produce a cyclic variation of the applied voltage polarity and magnitude; or (iv) a pulsed voltage to produce an asymmetric cyclic variation of the applied voltage polarity and magnitude.

The plasma produced by the plasma source preferably has a density in excess of 10¹¹ cm^"3 at one or more points within the vacuum chamber; or has a density in excess of 10¹² cm^"3 at one or more points within the vacuum chamber.

The target assembly can provide means for applying electrical bias independently to any or all of the differing target materials.

The sputter coating system can further comprise means for positioning within the vacuum chamber or introducing into the vacuum chamber more than one substrate, either simultaneously or sequentially, so as to receive the sputtered material.

The present invention also relates to the use of a gas plasma source of the type described herein in a vacuum system for the purpose of substrate cleaning, surface modification or other preparation processes through exposure of said substrate to the gas plasma generated by said source.

Equally, the present invention also relates to the use of a gas plasma source of the type described herein in a vacuum system for the purposes of substrate etching through application of a negative bias voltage, either intermittently or continuously, to said substrate and optionally with heating of the substrate and resulting sputtering of said substrate surface by the gas plasma generated by said source.

The present invention still further relates to the use of a gas plasma source of the type described herein in a vacuum system for the purposes of substrate etching through use of a chemically reactive gas or vapour, optionally with application of a negative bias voltage, either intermittently or continuously, to said substrate and optionally with heating of the substrate and resulting in etching of said substrate surface by the gas plasma generated by said source.

Moreover, the present invention further relates to the use of a gas plasma source of the type described herein in a vacuum system for the purposes of providing a plasma assist to any physical vapour deposition (PVD) coating process, with said plasma assist being provided by the gas plasma generated by said source, optionally with application of a bias voltage, either intermittently or continuous, to said substrate and optionally with heating of the substrate.

Furthermore, the present invention still further relates to the use of a gas plasma source of the type described herein in a vacuum system for the purposes of substrate coating by the process of Plasma Enhanced Chemical Vapour Deposition (PECVD) using an appropriate chemically reactive gas or vapour with said plasma enhancement being provided by the gas plasma generated by said source, optionally with application of a bias voltage, either intermittently or continuous, to said substrate and optionally with heating of the substrate.

Viewed from a yet further aspect, the present invention relates to a gas plasma source capable of producing high density gas plasma of density greater than 10¹¹ cm^'3, the plasma source comprising: a plasma chamber; an aperture being provided in said plasma chamber and at least one magnet being provided proximal said aperture; and a radio frequency (RF) antenna provided within the plasma chamber; wherein the antenna has an elongated transverse cross- section.

Viewed from a still further aspect, the present invention relates to a gas plasma source capable of producing high density gas plasma of density greater than 10¹¹ cm^'3, the plasma source comprising: a plasma chamber; an aperture being provided in said plasma chamber and at least one magnet being provided proximal said aperture; and a radio frequency (RF) antenna provided within the plasma chamber; wherein an enclosure is provided within the plasma chamber fully or partially to enclose said antenna, at least a portion of said enclosure permitting the transmission of radio frequency (RF) energy.

Viewed from a yet further aspect, the present invention relates to a high density gas plasma source capable of producing high density gas plasma of density greater than 10¹¹ cm^'3, the plasma source comprising: a plasma chamber having a sidewall, at least a portion of the sidewall of the plasma chamber permitting the transmission of radio frequency (RF) energy; an aperture being provided in said plasma chamber and at least one magnet being provided proximal said aperture; and a radio frequency (RF) antenna positioned externally of the plasma chamber; wherein the antenna consists of a single coil or a partial coil of electrically conductive material. The high density gas plasma source according to the present invention is capable of being used within, or being attached to a vacuum system to produce a high density gas plasma of density greater than 10¹¹ cm-³.

The terms "length", "height", "width" etc. are intended merely to differentiate between certain characteristics of a preferred arrangement of the gas plasma source described herein. These terms are not in themselves intended to limit the present invention to a particular orientation or configuration.

DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:-

Figure 1 is a schematic cross section of a preferred first embodiment of the elongate gas plasma source shown in the gas plasma source longitudinal cross section as applied to use in sputter apparatus; and

Figure 2 is the schematic cross section A-A' shown in figure 1 viewed from the left hand side of figure 1 , showing the elongate transverse cross section of the gas plasma source; and

Figure 3 is the schematic cross section B-B' shown in figure 1 viewed from the bottom of figure 1 ; and

Figure 4 is a schematic cross section of a second embodiment of the elongate gas plasma source shown in the gas plasma source longitudinal cross section as applied to a sputter apparatus; and

Figure 5 is a schematic of a third embodiment of the gas plasma source according to the present invention shown in elongate transverse cross section that may be used in place of the gas plasma source shown in the previous embodiments.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A first preferred embodiment of the present invention is shown in Figure 1. The apparatus comprises a vacuum chamber 1 and controllable means of vacuum pumping the chamber by a pumping system 2, both well known in the art, fitted with an elongate remote gas plasma generation system 3, a planar rectangular target assembly 4, a DC power supply 5, an electromagnet 6 and associated DC power supply 7 capable of producing an axial magnetic field strength of up to 500 Oersteds, substrate carrier or chuck 8, optional shutter assemblies 9 and a controllable process gas feed system 10.

The elongate remote plasma generation system 3, as further shown in figure 2, comprises an assembly mounting a RF antenna 1 1 in external proximity to at least the long sides of an essentially rounded oblong cross section plasma chamber 12, in this embodiment being a quartz tube, typically of wall thickness 2 to 3mm, that is mounted to an electromagnet 13 of comparable shape to and surrounding the quartz tube 12 at the end from which the high density plasma is to emerge. The assembly includes an enclosure 25 constructed to support and appropriately align and position said plasma chamber, electromagnet and RF antenna within the vacuum system chamber, the enclosure and plasma chamber being provided with suitable vacuum seals such that the inner volume of the plasma chamber is open to the vacuum system chamber at one or both ends, whilst the enclosure is or may be isolated from said vacuum system chamber, allowing it to be independently evacuated or pressurised if desired. The RF antenna 1 1 is constructed most simply from shaped copper tube, though alternate electrically conducting materials, for example brass or aluminium, could be used, as can differing cross sectional shapes, for example rod, strip or a combined assembly, for example of strip and tube. The use of tubular construction for the RF antenna has the advantage of allowing it to be water cooled, hence allowing the use of higher RF powers than would otherwise be the case.

It will be apparent to those skilled in the art that, in the case of the plasma chamber comprising a single quartz tube, the elongate shape results in it being unlikely to be able to support a substantial pressure differential and that the enclosure space will also therefore need to be evacuated. Ideally therefore a high vacuum pumping system will be fitted or attached to the enclosure to evacuate the enclosure space in which the RF antenna resides to a vacuum sufficient to suppress plasma generation within the enclosure space that would be detrimental to the plasma source performance. The enclosure should also be capable through design and materials of construction of providing radio frequency shielding of the RF antenna to all but the enclosure volume and the internal region of the plasma chamber. In order that the RF antenna may produce gas plasma within the plasma chamber, it is essential that the plasma chamber be at least in part transparent to RF radiation, ideally along at least one of the longer cross sectional sides.

By well known means the RF antenna 11 is connected to and powered by a 13.56MHz RF generator 14 and impedance matching network 15 external to the vacuum chamber 1 and a DC power supply 16 is electrically connected to the electromagnet 13, the latter pair being designed to be capable of producing an axial magnetic field strength of up to 500 Oersteds.

The construction of the target assembly 4 will be obvious to those skilled in the art, comprising a vacuum chamber feedthrough 17 that feeds cooling water and electrical power to a mounting assembly 18, the target being thereby water cooled and capable of having a voltage applied to it from power sources external to the vacuum chamber. A target material 19 is fitted to the face of the mounting assembly 18 that faces the substrate, ensuring good electrical and thermal contact by well known means, for example bonding with silver loaded epoxy.

Additionally in order to prevent sputtering of the mounting assembly 18 a shield 20 that is electrically grounded is provided around this item, allowing only the target material 19 to be directly exposed to the plasma; the requirements and design of this shield are also well known. The substrate carrier 8 essentially provides a means to position and hold the substrates 21 that are to be coated within the vacuum chamber. By well known means, the carrier may be water cooled or include heaters to control the substrates temperature, be capable of having a voltage applied to it to assist control of deposited film properties, include means of rotating and / or tilting the substrates to improve coating thickness uniformity, and itself be capable of being moved and / or rotated within the vacuum chamber.

The optional moveable shutter assembly 9 is provided such that in the 'closed' position target sputtering can take place without coating the substrates. There are many means of achieving this, all well known to those skilled in the art.

The process gas feed system 10 comprises one or more gas inlets for one or more process gases or process gas mixtures, each gas flow being controllable for example using commercial mass flow controllers, and optionally including gas mixing manifolds and / or gas distribution systems within the vacuum chamber; the detailed design of such systems is well known. In the simplest embodiment of the invention, a single gas inlet is provided to the vacuum chamber, the process gas or gases then being distributed to all parts of the vacuum by normal low pressure diffusion processes or directed pipework.

An example of the operation of the above example system will now be described with reference to figure 1.

The substrates 21 to be coated are loaded onto the substrate carrier 8 and the shutter 9 set to the closed position. The vacuum chamber 1 is then pumped by the pumping system 2 to a vacuum pressure suitable for the process, for example less than 1x10^"5 torr. Initially the elongate gas plasma source enclosure is cross connected to the vacuum chamber during this pumping process to prevent any substantial pressure differential developing that might damage the plasma chamber. Once below about 1x10^"3 torr said enclosure is transferred to its own high vacuum pumping system so that it operates at a high vacuum independently of the vacuum chamber. The process gas feed system 10 is then used to flow at least one process gas, for example argon, into the vacuum chamber. The flow rate and optionally the rate of vacuum pumping are adjusted to provide a suitable operating pressure for the sputter process, for example 3x10^"3 torr. The electromagnets 6 and 13 in conjunction with their respective power supplies 7 and 16 are then used to produce a magnetic field of strength approximately 100 to 300 Oersteds between them and across the vacuum chamber. The magnetic 'polarity' of each electromagnet is identical (i.e. they attract).

The remote gas plasma is generated by applying RF power, for example 2kW, from the generator 14 via the matching network 15 to the antenna 11. In combination with the magnetic field produced as described above, these result in high density plasma being produced across the chamber and under the target assembly 4, as approximately indicated by the region 22 in figures 1 and 3.

The DC power supply 5 is then used to apply a negative polarity voltage to the target assembly 4. This results in ions from the plasma in the vicinity of the target being attracted to the target and, if the voltage is above the sputter threshold value for the target material (typically in excess of 65 volts), sputtering of the target material will occur. As the sputter rate for this example system is approximately proportional to the voltage above this threshold value, voltages of 400 volts or more will usually be applied; for very high rate applications higher voltages may be used, for example 1200 volts.

After an optional time delay to allow the target surface to clean and stabilise, for example 5 minutes, the shutter assembly 9 is set to the open position to expose the surface of substrates 21 facing the target assembly to the sputtered material, thereby coating the substrate surfaces with a film of the target material 19. After a time determined by the required film thickness and the deposition rate at the substrate surface, the shutter assembly 9 is set to the closed position and deposition onto the substrates ceases.

The various power supplies and gas flows can then be turned off as required, the elongate gas plasma source enclosure reconnected to the vacuum chamber and the vacuum system vented to atmospheric pressure using a suitable gas, for example nitrogen or air, to permit recovery and subsequent use of the coated substrates. All these procedures will be obvious and well known to those skilled in the art.

A number of other useful embodiments of the invention will now be described.

In an alternate embodiment of the elongate gas plasma source, the plasma chamber is constructed from an assembly of materials. The assembly may include multiple tubes, for example of 2 to 3mm thick quartz, placed side by side to produce the required elongate plasma cross section. The assembly may be constructed to be far more robust than a single elongate cross section quartz tube and thereby able to support atmospheric pressure differential. This has advantage in that the enclosure can now be constructed to operate at atmospheric pressure and there is no need for the complication and expense of a separate vacuum pumping system for the enclosure. In addition, the plasma chamber may be readily cooled using simple air flow, thereby allowing the plasma source to operate at higher RF powers than would otherwise be the case.

A transverse cross section of a third embodiment of the elongate plasma source according to the present invention is shown in figure 5. Like reference numerals have been used for like components in the description of this embodiment.

The elongate plasma source according to the third embodiment comprises an RF antenna 1 1 provided within a sealed vessel which forms the plasma chamber. The position of the RF antenna 11 with respect to the plasma chamber is fundamentally altered such that the antenna 11 lies within and is surrounded by the plasma chamber. The RF antenna is itself preferably enclosed within a suitably RF transparent enclosure fitted within the plasma chamber. At least in preferred arrangements, the RF antenna enclosure is tubular and constructed to support a pressure differential between the inside of the enclosure and the plasma chamber. Suitably constructed, for example by using a quartz tube vacuum sealed at it's interfaces to the plasma chamber, this enclosure allows the RF antenna to be operated at atmospheric pressure. This feature is particularly advantageous since it allows the antenna and the enclosure both to be cooled by air flow.

A number of further examples of useful alternate embodiments of the above described sputter system will now be described with reference to figure 4 which shows a schematic cross section of a sputter apparatus containing each of the embodiments.

In a first alternate embodiment of the invention, the elongate gas plasma source 3 and second electromagnet 13 are tilted from their in line positions so as to greatly reduce coating of the internal surfaces of the plasma chamber by material sputtered from the target which may be detrimental to the gas plasma source operation due to possible attenuation of the RF energy radiated into the plasma chamber space.

In a second alternate embodiment of the invention the target material 19 and mounting assembly 18 are constructed to be of circular or essentially circular, for example hexagonal, external cross section, ideally with means well known in the art for rotating the target material or assembly about the central longitudinal axis 24. This might be preferred over the original embodiment's planar geometry in order for example to maximise the target lifetime by essentially providing an increased surface area to be sputtered. The single target material 19 might also be replaced by two or more differing target materials, such that with appropriately fast rotation, for example 100rpm, a coating of material that is a composite mixture, alloy or compound of the differing individual materials is formed.

Alternatively the rotation might be used to allow the differing materials to be positioned sequentially and/or alternately in the position where they will be sputtered, thereby providing a basis for sequential deposition of different thin film materials onto the substrates. Partial and controlled rotational positioning of two or more differing target materials might also be used to vary the coating mixture during deposition to allow a variable composition thin film coating to be realised.

Additionally, the target assembly may be engineered to allow individual target materials to be separately electrically biased; this is of especial use in cases where one or more of the targets will be biased by RF power means and it is desired to prevent RF power induced low intensity plasma generation and sputtering of the other target materials that might contaminate the process.

In a third alternate embodiment of the invention, the target shield 20 is extended to cover the whole length of the target materials and mounting assembly and includes apertures that thereby only allow the plasma to interact with and sputter the target materials at those places, thereby limiting and defining the target regions to be sputtered. This embodiment is especially useful when combined with a target comprising several target materials and means of rotation as previously described as it is able to reduce cross-contamination of the materials at the substrates.

In a fourth alternate embodiment of the invention, the moveable shutter 9 may be replaced with a fixed set of shields 23 that define a coating aperture under which the substrate carrier 8 is translated so as to coat the substrates 21. For an appropriate substrate type and material the substrate carrier 8 may not be required.

There are many other minor changes to the invention that can be considered and which will be apparent to those skilled in the art. For example, the electromagnets may be

supplemented or even replaced by other magnetic means, for example additional permanent or electromagnets, in order to better control and guide the plasma. This may be required, for example, when a ferromagnetic target material is to be sputtered and additional field shaping is necessary to prevent the plasma being directed to the target assembly and thereby

extinguished. As a further example, although most RF power systems used for plasma processing operate at 13.56MHz, this being the frequency allocated for industrial use and thereby less prone to causing interference with other radio frequency users and so simpler to implement, alternate radio frequencies, for example 40MHz or harmonics of 3.56MHz, may be used to power the remote plasma source antenna or power the target assembly with appropriate RF shielding and suppression. As a further example, the process gas feed system 10 may be constructed so as to deliver one or more of the process gases direct to the inside of the plasma chamber of the elongate gas plasma source, this being particularly beneficial when a gas or vapour is required to be made more chemically active by the plasma.

EXAMPLES

The results achieved in experimenting with a sputter deposition system based on the above example embodiments will now be described.

A sputter deposition system was constructed substantially as shown in figure 1 and described above, omitting only the shutter assemblies. Both the elongate gas plasma source and second electromagnet were tilted to about 30 degrees from horizontal as described in an alternate embodiment and shown in Figure 4 and had a minimum separation of about 200mm. In addition, substrates were passed through the chamber for coating in the manner described in an alternate embodiment and shown in Figure 4.

The elongate gas plasma source produced a plasma region of approximately 460mm by 40mm cross section at the plasma source exit aperture; this was guided proximate to and across a planar target assembly fitted with an aluminium target surface that was exposed to the plasma in order to be sputtered. The dimensions of the exposed target surface were approximately 400mm long, thereby lying within the bounds of the plasma generated by the source, and 125mm wide.

The sputter deposition system was operated substantially according to the example description above, excepting that the deposition time was determined by the time for which the substrates were translated under the coating aperture. The following observations and results were obtained. The process conditions were set as follows: an argon gas flow of 180sccm, a resultant vacuum system pressure of about 4x10^'3 torr, 2.5kW RF power applied to the RF antenna and the electromagnet 13 axial magnetic field at approximately 100 Oersteds and the electromagnet 6 axial magnetic field at approximately 50 Oersteds. This produced intense argon- plasma of characteristic purple - blue colouration denoting the presence of a plasma density of between 10¹² and 10¹³ crrT³.

The plasma generation zone originating from the elongate gas plasma source could be guided and shaped using the electromagnets 6 and 13 to pass completely between them and thereby completely cover the whole target material surface, with no visible loss or non-uniformity of plasma density, i.e. the presence of the target material did not detrimentally affect the plasma, regardless of whether the target had negative bias applied to it or not. Furthermore, the target assembly did not substantially heat up, even in the absence of water cooling, despite being placed in proximity to the plasma. It was observed that the visible plasma profile followed the expected magnetic field profile between the electromagnets, expanding by about 60mm in both cross sectional dimensions at the mid point between the two electromagnets before narrowing again to the second electromagnet 6.

A negative polarity DC voltage of 500V was then applied to the target assembly (and thereby the target material) using the DC power supply 5. The current recorded by the DC power supply 5 was 5.4A and the plasma visibly changed to a more blue colouration characteristic of sputtering of aluminium occurring. The plasma density required to produce this current can be reliably calculated to be about 5.5x10¹² cm^'3, in agreement with the observation of plasma visual intensity above. It was confirmed that extinguishing the plasma resulted in the _§ zero target current even with negative voltages of up to 800V (the power supply limit) applied to the target assembly.

The plasma was restarted and a glass panel substrate was translated under the coating aperture defined by the shields 23. Following system shutdown and venting to atmosphere the substrate was observed to have been coated with a uniformly thick film of aluminium, consistent with uniform sputtering from the full target material surface of 400mm length.

Hence an elongate remote gas plasma source built according to the invention has produced a volume of high density plasma, greater than 10¹² cm^"3, of cross sectional long dimension in excess of 400mm and of uniformity at least adequate to allow uniform sputtering of a like dimensioned sputter target of width 125mm. This could not have been achieved with other remote high density plasma sources. It will be apparent to those skilled in the art that the inventive step of adopting a non-circular or non-helical RF antenna and gas plasma source geometry is essential to achievement of this capability and thereby distinguishes the invention from other disclosed remote plasma generation systems.

The invention can also be used in a reactive sputter process, that is a process in which a reactive gas or vapour is introduced via the gas feed system 10 to react with the sputtered target material or materials and thereby deposit a compound thin film on the substrate. For example, oxygen gas can be introduced into the sputter process with any of the embodiments previously described in order to deposit oxide thin films, for example to deposit alumina by sputtering of an aluminium target in the presence of oxygen gas or silica by sputtering of a silicon target in the presence of oxygen gas.

It will be obvious to those skilled in the art that the ability of the elongate gas plasma source to operate independently of any sputter target allows further application to be realised. Thus the above described elongate gas plasma source may be used as a substrate cleaning, surface modification or etch tool with especial utility where large dimensioned substrates are to be processed at high throughput rates, for example in roll to roll ("web") coating. It is well known that merely running the substrate through or in proximity to high density plasma is sufficient to achieve great improvement in the adhesion of subsequent coatings, or to cause beneficial changes to the substrate surface. If substrate etching is required, then it is well known in the art that this may be achieved through the application of an electrical bias to the substrate, resulting in substrate surface sputtering, or by introducing a gas or vapour into the process which is then activated by the high density plasma to react with and etch the substrate surface, or by a combination of the two processes.

The elongate gas plasma source could also be used as a 'plasma assist' tool for other coating processes, as is typically used in evaporative coating process tools.

The elongate gas plasma source could also be applied to coating processes based on the technique of Plasma Enhanced Chemical Vapour Deposition (PECVD).

The disclosed elongate gas plasma source is of particular utility in all these processes due to the innate ability to generate uniform high density plasma over very long lengths, thereby allowing its use with large dimensioned substrates. The means for realising such applications will be readily apparent to those skilled in the art, essentially comprising vacuum systems of the form described in the preferred embodiments above, but with the omission of the sputter target assembly and, for the plasma assist tool, its replacement with another coating source.

For the sake of clarity, the present invention has been described with reference to a single antenna. However, it will be understood that either a single antenna or multiple antennae can be employed. The antennae may be independent or may be connected to each other.

It will be appreciated that various changes and modifications can be made to the gas plasma source according to the present invention without departing from the spirit and scope of the claimed invention.

Claims

1. A gas plasma source capable of producing high density gas plasma of density greater than 10¹¹ cm^"3, the plasma source comprising:

a plasma chamber having a sidewall, at least a portion of the sidewall of the plasma chamber permitting the transmission of radio frequency (RF) energy;

an aperture being provided in said plasma chamber and at least one magnet being provided proximal said aperture; and

a radio frequency (RF) antenna positioned externally of the plasma chamber;

wherein the antenna has a non-circular plan form.

2. A gas plasma source as claimed in claim 1, wherein the antenna is arranged proximate to only one side of the plasma chamber; or the antenna at least substantially encircles the plasma chamber.

3. A gas plasma source as claimed in claim 1 or claim 2 comprising two antennae, each antenna being proximate to a respective side of the plasma chamber.

4. A gas plasma source as claimed in any one of claims 1 , 2 or 3 comprising one or more of said antenna, wherein the antenna(e) is/are spaced apart from an outside of the plasma chamber; and/or the antenna(e) is/are shaped substantially to match at least a portion of the outer profile of the plasma chamber.

5. A gas plasma source capable of producing high density gas plasma of density greater than 10¹¹ cm^"3, the plasma source comprising:

a plasma chamber;

a radio frequency (RF) antenna provided within the plasma chamber;

wherein the antenna has a non-circular plan form.

6. A gas plasma source as claimed in claim 5, wherein an enclosure is provided within the plasma chamber fully or partially to enclose said antenna, wherein at least a portion of said enclosure permits the transmission of radio frequency (RF) energy.

7. A gas plasma source capable of producing high density gas plasma of density greater than 10¹¹ cm^"3, the plasma source comprising:

a plasma chamber;

a radio frequency (RF) antenna provided within the plasma chamber;

wherein an enclosure is provided within the plasma chamber fully or partially to enclose said antenna, at least a portion of said enclosure permitting the transmission of radio frequency (RF) energy.

8. A gas plasma source as claimed in claim 6 or claim 7, wherein said enclosure is tubular, the enclosure preferably being sealed at its interfaces with the plasma chamber.

9. A gas plasma source as claimed in any one of claims 5 to 8, wherein said at least one antenna extends partially or fully across the width of the plasma chamber.

10. A gas plasma source as claimed in any one of claims 5 to 9 comprising a plurality of said antenna; wherein said antennae are independent of each other, or are joined to each other outside of the plasma chamber.

11. A gas plasma source as claimed in any one of the preceding claims, wherein the plasma chamber comprises a tube, at least one end of the tube being open to form said aperture.

12. A gas plasma source as claimed in any one of the preceding claims, wherein the plasma chamber has an elongate transverse cross section; or the plasma chamber comprises a plurality of tubular members arranged in an array, the array having an elongate transverse cross section; preferably the plasma chamber having a rectangular, rounded rectangular, oval or elliptical transverse cross section.

13. A gas plasma source as claimed in any one of the preceding claims, wherein the plasma chamber or the enclosure comprises at least one quartz tube, or at least one quartz member.

14. A gas plasma source as claimed in any one of the preceding claims, wherein the plasma chamber and/or the enclosure is/are capable of supporting a pressure differential across an inner surface and an outer surface thereof.

15. A gas plasma source as claimed in any one of the preceding claims, wherein, in use, the antenna or antennae are supplied with power from a radio frequency power supply system operating at a frequency between 1MHz and 1 GHz; a frequency between 1 MHz and 100MHz; a frequency between 10 MHz and 40 MHz; or at a frequency of approximately 13.56 MHz or multiples thereof.

16. A gas plasma source as claimed in any one of the preceding claims, wherein said at least one magnet at least substantially encircles the plasma chamber transverse cross section proximate to the plasma chamber aperture; or wherein said at least one magnet is positioned at least substantially proximate to one of the longer dimensioned sides of the plasma chamber and proximate to the plasma chamber aperture, preferably said at least one magnet generating a magnetic field of strength greater than or equal to 50 Oersteds, and preferably between 50 and 500 Oersteds.

17. A gas plasma source as claimed in any one of the preceding claims, wherein said at least one magnet comprises: (i) one or more electromagnets; (ii) one or more permanent magnets; or (iii) a combination of one or more permanent magnets and one or more

electromagnets; preferably wherein at least one of said electromagnets is controllable.

18. A gas plasma source as claimed in any one of the preceding claims, wherein the antenna is positioned remote from said aperture provided in the plasma chamber, the antenna preferably being displaced from the aperture along a longitudinal axis of the plasma chamber.

19. A gas plasma source as claimed in any one of the preceding claims, wherein in plan form the antenna has a length and a width, the ratio of the length to the width of the antenna being greater than or equal to three (3), five (5), ten (10), or fifteen (15).

20. A gas plasma source as claimed in any one of the preceding claims, wherein the antenna has a plan form which is generally elongated, linear, rectangular, rounded rectangular, polygonal, elliptical, and/or U-shaped.

21. A gas plasma source as claimed in any one of the preceding claims, wherein the antenna consists of an elongate strip, a length, a partial turn, a single turn or multiple turns of an electrically conductive material.

22. A gas plasma source capable of producing high density gas plasma of density greater ^' than 10¹¹ cm^"3, the plasma source comprising:

a tubular assembly that permits the transmission of radio frequency (RF) energy;

the tubular assembly being open at a first end and at least one magnet being provided proximal said first end; and

a radio frequency (RF) antenna positioned externally of the tubular assembly;

wherein the tubular assembly has an elongated transverse cross-section; or-the tubular assembly comprises a plurality of tubular members arranged collectively to define an elongated transverse cross-section.