GB2593863A - High Density vacuum plasma source - Google Patents

Abstract

A vacuum gas plasma generation system comprising: a metal and dielectric tubular plasma source 8; a radio-frequency antenna 7 to form an inductively coupled plasma; plasma source magnet 4 and attached coil electromagnet 14 that provide an axial field through the plasma source 3 and chamber 1 to generate the high density plasma; and a multiple magnet assembly 11 that causes a local anisotropic distortion of the magnet linkage between the plasma source 3 and attached vacuum process chamber 1 to produce a large area, essentially planar high density plasma for use in a wide range of plasma processes. The multiple magnet assembly 11 locally compresses the initial tubular shape of the plasma in one direction, yielding a much thinner plasma zone 19, whilst spreading the plasma generating zone in the orthogonal direction thereby extending the area available for plasma processes.

Description

High Density Vacuum Plasma Source This invention relates to a novel means of producing high density vacuum gas plasma, primarily but not uniquely for use in apparatus that sputter deposit thin film coatings onto other surfaces and materials.

Vacuum gas plasmas are extensively used in a diverse range of research and production processes, especially within the semiconductor and large area flat panel electronics industries, e.g. for solar panel and flat panel display production. In general, whether for research, development or production, it is highly desirable to generate such plasmas uniformly over as large an area as possible whilst simultaneously achieving plasma densities in excess of 1012 cm-3 to support fastest processing; this is a challenging objective.

One class of plasma sources commonly used in industry inductively couples radio frequency (RE) energy into the vacuum process environment to produce the plasma: this is generically referred to as an Inductively Coupled Plasma (ICP). The simplest versions of these produce plasma densities of about le cm', more advanced system can achieve greater than 10' cm-Large area ICP systems can be constructed to produce large areas of plasma, the RF energy being transmitted via an equivalently large shaped 'serpentine' or large diameter 'coil' antenna placed within the vacuum process chamber and defining the extent of the plasma area. However, these tend to be restricted to lower than desirable plasma density levels, less than about 10' 'cm', partly due to issues arising from the direct electrical contact of the antenna to the plasma. More complex designs, utilising local magnetic enhancement and segregation of the (metal) antenna from the plasma are disclosed in the literature and can improve on this but remain limited to less than 10'-' cm" plasma density. Additionally, the plasma antenna itself overlays the process region, limiting its application.

There also exist a class of inductive plasma sources capable of generating plasma densities of 1013cm' or greater, though without large area capability unless used as a multi-source array, a potentially very expensive and/or complex arrangement. Examples of these are summarised by Popov in 'High Density Plasma Sources' (1995) and Chen in 'Lecture Notes on Principles of Plasma Processing' (2003). A further, highly efficient 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 ICP antenna in conjunction with particular magnetic field arrangements to both produce a high density plasma within a plasma chamber attached to the sputter process chamber and to extend this plasma to a sputter target surface out of line of sight of the plasma source.

The applicants have worked extensively with the Thwaites source to develop its capabilities and apply it to a range of applications. The Thwaites source has proven to be highly effective, efficient and robust, allowing the production of coating systems with enhanced capability compared to conventional coating systems: these are extensively covered in the open literature. For example, the separation of the plasma generation task (by the plasma source) from the sputter coating target material source provides more process options with commensurate advantages when optimising materials properties, for example as reported by Vospariou et al, IEEE Transactions on Magnetics, Vol.40, No.4, (July 2004), whilst full target material surface sputtering at high plasma currents delivers the very high coating rates needed for high volume manufacturing and, as reported for example by Hewitt et al, Semicond, Sci, Technol. 24 (2009), delivers highly stable reactive sputter deposition processes, further greatly increasing the coating rates achievable for dielectric materials when compared to conventional sputtering techniques.

In considering the manner in which the Thwaites source can be used or adapted, it is important to realise that the description of the observed plasma form applies to the visibly bright "plasma glow" region. For the Thwaites source, this is the region in which the RF powered energetic processes produce the plasma, but the plasma then extends less visibly or even invisibly from this region by diffusion processes. In the further descriptions the plasma glow region will be referred to as the "plasma generation" region to distinguish it from the overall process chamber plasma environment.

There are however disadvantages inherent with the Thwaites source, most especially if it is wished to use it for large area sputter coating applications. The diameter of the plasma tube of the Thwaites design largely determines both the maximum sputter target area that can be used and the minimum distance the substrate to be coated must be placed from that target, the latter as the plasma generation region needs to pass between the two. For a typical Thwaites plasma source of internal diameter about 75mm, a sputter target size of about 100mm and target to coating surface spacing of about 150mm -250mm are normally used: for reasons of coating uniformity, the effective coating region is about 150mm in diameter. This combination of factors also leads to inefficient transfer of sputtered target material (typically less than 10%) and slower deposition rates than the very fast target sputter rates achievable with the Thwaites source would otherwise deliver. Above about 150mm plasma source internal diameter, as required for large substrate sizes, the Thwaites plasma source becomes impractical, requiring very large process chambers and vacuum plant.

These disadvantages can be eliminated if the form of the plasma generation region is a thin flat sheet. The Thwaites plasma source itself has therefore been adapted to new forms by the applicants to produce derivative systems that permit the formation of large area plasma generation regions of "slab" form, i.e. one dimension (usually regarded as the thickness) being significantly less than the other two. These deliver an acceptably uniform, high plasma density (1013 cm' or greater) over a large area from a single, usually elongate, plasma source. An early example of these is described by the invention of Hockley and Thwaites in Patent Application GB1006567.0. However, these derivatives are intended for very large area coating, in excess of 300x300mm area, and their form, complexity and resultant cost is unsuited to their use for a wide range of applications, especially thin film R&D, where a planar plasma version of the Thwaites source would provide an ideal, cost effective solution.

The applicants have therefore experimented extensively with minor modifications of and additions to the Thwaites plasma source design in attempting to achieve the "planar plasma" form using means well known in the art, but with only partial success. For example, it has been shown that alternate RF antenna configurations, e.g. in proximity to only part of the tube, can reduce the plasma generation form thickness, allowing some reduction of target to substrate spacing, though insufficient to provide the required advantages without other disadvantages arising. It has also been shown that placing physical "shaping" or "extraction" apertures in the plasma path, as usually used in ion beam based systems, are not viable due to severe and highly destructive plasma heating issues.

In the course of these experiments the inventor has found that the Thwaites plasma source responds unusually to modification of the magnetic field arrangements, with experimental findings contradicting the behaviour that would be expected from the literature. In further research the inventor has discovered a novel and unexpected means for achieving the planar plasma form using the Thwaites plasma source in its tubular, i.e. usual, form.

Accordingly, the inventor has discovered a means of controllably changing the form of the plasma produced within the process chamber from the essentially tubular form within a Thwaites plasma source to a wider spread and very much thinner planar form, without detriment to the other properties of the plasma, thereby greatly increasing the utility of the system.

Therefore the present invention relates to a high density gas plasma source capable of being used with a vacuum chamber and associated vacuum systems to produce within said vacuum chamber a vacuum gas plasma of density greater than 1012 cm' the plasma generation region being of planar form, that is with one dimension substantially smaller than the other two orthogonal dimensions, the plasma source being based upon the Thwaites plasma source and comprising: An essentially tubular construct that is at least in part transparent to radio frequency (RF) radiation, connected and vacuum sealed at one open end to an appropriately sized aperture in said vacuum chamber, the opposite tube end being either sealed or further connected to vacuum systems so that said tubular construct is operating at or about the same vacuum pressure as the process chamber: a radio frequency (RF) antenna positioned external to the chamber and in proximity to at least part of said RF transparent part of the tubular construct, with a connected RF power source and impedance matching system to permit transmission of power to said RF antenna, thereby allowing the formation of a plasma within the plasma source tubular construct at appropriate connected chamber vacuum conditions: at least one magnet, hereinafter "plasma source magnet", ideally a large diameter electromagnet coil, within or external to the vacuum chamber, positioned at or close to the plasma source connection to the chamber and of strength and alignment sufficient to interact with said plasma to permit its amplification to a high density plasma and spatial extension of said high density plasma into the vacuum chamber: a further magnet construct, hereinafter "magnet assembly", at or near said first plasma source magnet of form and alignment to modify the magnetic field in an anisotropic manner and thereby alter the spatial form of the plasma generation region to planar form at least within the chamber.

In a preferred embodiment of the invention, a second magnet is placed within or external to the process chamber, orientated or operated such that it is in attract mode with the plasma source magnet, so as to extend the plasma generation region further into and across the processing space from the plasma source, in the same manner as the original Thwaites source For example, a magnet placed on the opposite chamber face to the attached plasma source can result in an essentially flat planar plasma being generated over the full distance between the magnets as a flat planar plasma, or the second magnet can be positioned such that the planar plasma follows a curved path.

Experimentation shows that there are several key requirements and some useful general variations of the above configuration for plasma "reshaping" to occur: these are: the magnetic field produced by the plasma source magnet must be of a strength sufficient to dominate the magnet assembly field, this being more readily achieved with the plasma source magnet and the magnet assembly in proximity: the positioning of the magnetic assembly can, subject to engineering constraints, be mounted either side of or even within the plasma source magnet: a further magnet assembly, similar to the first, may be placed with or near the above second magnet (even outside the process chamber) to further improve the planar plasma form: the magnet assembly may comprise either permanent magnets or electromagnets or a mixture of both, so long as an anisotropic distortion of the plasma source magnetic field shape is achieved.

Examples of experiments demonstrating the above will be given later.

At least in preferred embodiments, the inventor has accordingly produced a new form of a high density plasma source that is of especial utility when applied to large area plasma processing, including but not limited to sputter deposition from large, possibly multiple, target assemblies, in a configuration that greatly improves the achievable deposition rate and area compared to the original Thwaites plasma source on which it is based.

Detailed specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:-Figure 1 is a schematic cross section of a preferred embodiment of the invented gas plasma source shown in the gas plasma source longitudinal cross section as fitted to a vacuum system that permits its operation, and Figure 2 is the schematic cross section A-A' shown in figure 1 viewed from the right hand side of Figure 1; and Figure 3 is the schematic cross section B-B' shown in figure I viewed from the top of Figure 1, and Figure 4 is the schematic cross section of the preferred embodiment of the invention shown in Figure 1 as applied to its use as a sputter coating system, and Figure 5 is the schematic cross section C-C' shown in Figure 4 viewed from the right hand side of figure 4, showing a transverse cross section of the gas plasma sheet and its preferred positioning with respect to the sputter and substrate components.

In the preferred embodiment of the invention, a vacuum chamber I and controllable means of vacuum pumping the chamber by a pumping system 2, both well known in the art, are fitted with the invented vacuum gas plasma generation system 3, an electromagnet 4 and associated DC electrical power supply 5 capable of producing an axial magnetic field strength of up to 500 Gauss and a controllable process gas feed system 6.

The vacuum chamber I and associate vacuum pumping systems 2 can be of any shape, form or materials that both integrate the invented plasma source and other required apparatus for the intended application and provide the vacuum and magnetic environment, conditions, monitoring and control needed for the invented plasma source to operate, means of achieving this are well known in the art. It will also be obvious to those skilled in the art that the system orientation shown is not exclusive and that it may be inverted, rotated or tilted to suit the application.

The invented vacuum gas plasma generation system 3 in this embodiment comprises a helical coil RF antenna 7 placed around and spaced from direct contact with, typically at a distance of 5-10mm from, the walls of a quartz tube 8, typically of about 75 to 100mm external diameter and about 5mm wall thickness, mounted at one open end and vacuum sealed to an aperture 9 in the vacuum chamber 1 and vacuum sealed at the other tube end 10 by means well known in the art, a plasma source electromagnet 14 of large aperture annular design, shown in cross section in Figures 1, 3 and 4 and with its position indicated by annular dashed lines in Figures 2 and 5 to further elucidate its positioning, a magnet assembly 11 in this example comprising two permanent magnets 12 fixed to a non-magnetic frame 13 and mounted close to the plasma source electromagnet 14, in this example within the vacuum chamber 1 at the aperture 9, these magnets having their respective same poles facing each other, i.e. the magnets repel. Optionally, further magnets 15 may be used, these being mounted with opposite facing magnetic polarity to the magnets 12 but again with their same poles facing each other; pole orientation is clarified in Figure 2 where the North and South polarity of the four faces are indicated by proximity to N and S labels respectively.

The electromagnet 14 has a connected DC electrical power supply 16 designed and operated to be capable of producing an axial magnetic field strength of up to 500 Gauss within the electromagnet core, as for the chamber electromagnet 4. It is usually desirable for both electromagnets 4 and 14 to have a similar cross-sectional shape to the antenna 7. In this embodiment of the apparatus, this is an annular shape as indicated in Figure 2. Note that electromagnet 4 is not essential to the operation of the invention, but can greatly improve the plasma spatial extent, form, overall intensity and uniformity within the chamber 1.

The RF antenna 7 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.

By well-known means the RF antenna 7 is connected to and powered by a 13.56MHz RF power generator 17 and RF impedance matching network 18.

It will be understood by those skilled in the art that additional items, for example safety panels, safety interlocks, cooling fans and the inclusion of water cooling in some elements, will be required though these are not essential to the fundamental operation of the plasma source. The means of mounting and integrating the components are also well documented in the literature and no especial other considerations apply.

In this form, as shown in Figures 1 to 3 inclusive, the invented plasma source is able to be operated to produce the required planar form of high density plasma 19, but has limited utility without other apparatus being added, as required for the specific desired process.

An example application of the invented plasma source is its use to provide an improved sputter coating process; a preferred embodiment of this is shown in Figures 4 and 5. The prior Figure 1 embodiment of the plasma source is added to with the provision of multiple planar diode sputter target assemblies 20, electrical power supplies 21, a substrate carrier or chuck 22 and optional shutter assembly 23.

The construction of the planar diode target assemblies 20 will be obvious to those skilled in the art, comprising vacuum chamber feedthroughs 24 that feed cooling water and electrical power to a mounting assembly 25, the target being thereby water cooled and capable of having a voltage applied to it from the power supplies 21 external to the vacuum chamber. A target material 26 is fitted to the face of the mounting assembly 25 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 25 a shield 27 that is electrically grounded is provided around this item, allowing only the target material 26 to be directly exposed to the plasma; the requirements and design of this shield are also well known.

It should be noted that the plasma source allows use of a wide range of target power supply types, including DC, pulsed DC and RF classes, the choice being largely determined by the intended target sputter material as will be apparent to those skilled in the art. No magnets are required at or within the targets to enable, assist or augment the invented plasma source operation, but may be included if it is desired to alter the plasma path at or near the target, for example to pull the plasma in whole or part onto or into closer proximity to the target surface The substrate carrier 22 will usually include means to position and hold the substrates 28 that are to be coated within the vacuum chamber, though this will not be essential if the system is built in an inverted configuration: these means are not detailed in the Figures, being well known in the art. 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 toil 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 23 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 6 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 and vacuum gas diffusion processes and, optionally, directed pipework.

An example of the operation of the plasma source as applied to a sputter coating process will now be described with reference to Figures 1 to 5 inclusive.

The substrates 28 to be coated are loaded onto the substrate carrier 22 and the shutter 23 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. The process gas feed system 6 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' torr. The electromagnets 4 and 14 in conjunction with their respective power supplies 5 and 16 are then used to produce a magnetic field of strength approximately 100 to 300 Gauss 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 17 via the impedance matching network 18 to the antenna 7. In combination with the magnetic field produced as described above, these result in a visibly bright high density plasma glow discharge being produced across the chamber and over the multiple target assemblies 20, as approximately indicated by the region 19.

One or more of the power supplies 21 are then used to apply an overall negative polarity voltage to their connected target assembly 20. 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 I minute, the shutter assembly 23 is set to the open position to expose the surface of substrates 28 facing the target assembly to the sputtered material, thereby coating the substrate surfaces with a film of the target material(s) 26. After a time determined by the required film thickness and the deposition rate at the substrate surface, the shutter assembly 23 is set to the closed position and deposition onto the substrates ceases.

The various power supplies and gas flows can then be turned off 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.

It will be appreciated that the ability to utilise multiple sputter target units, each individually powered and potentially of different sputter materials, is a significant advantage compared to the single target that the Thwaites source would usually allow. Equally significantly, it is possible to place the target materials and substrate much closer than in the usual Thwaites plasma source embodiment, with consequent gains in material usage and speed of process. A further benefit is that with the visible plasma generation region (glow discharge) passing over and in proximity to the target assemblies, but not intercepting them, individual targets can also be shuttered to lessen the chance of cross-contamination between differing active and non-active targets.

The results achieved in experimenting with plasma sources based on the above example embodiments will now be described.

Most experiments used a standard high vacuum process chamber of approximate dimensions 400mm wide, 400mm depth and 500mm height, able to achieve a vacuum pressure below 10 5 torr. This chamber was fitted internally with a substrate carrier with attached substrates as required and a planar 100mm diameter diode sputter target of construction essentially as indicated in Figure 5. Both the substrate table and sputter target unit were repositionable and could be removed if required. A vacuum sealed door on one face of the chamber permitted access to these items.

A controllable argon gas feed was additionally provided to allow an argon partial pressure of about 2x10' torr to be maintained, this being controlled by the dynamic balance of inlet gas flow and vacuum pumping outflow.

A plasma source of the Thwaites design, constructed substantially as shown in Figure 1 and as described in the example embodiment, was attached via a vacuum sealed aperture to one side of the chamber, as indicated in the Figures. Two coil electromagnets, of construction well known in the art, of internal aperture dimension larger than of the antenna, approximately 265mm, or 185mm in some experiments for the second magnet 4, were installed either side of the vacuum chamber and plasma source, the 265mm plasma source electromagnet 14 essentially being a fixed integral part of the plasma source construct, the second electromagnet being removable and repositionable.

A removable magnet assembly 11 was made using a simple 30mm by 30mm box section aluminium support frame of internal rectangular form and dimension to mount a "magnet stack" of one or more identically magnetically orientated C5 ferrite permanent magnets, of dimension 100mm long, 20mm wide, lOmm thickness and pull strength 1.96kg on each of the one to four of the inner support frame faces without impeding the plasma source exit, as shown in Figure 2, and in a pole orientation orthogonal to the plasma source electromagnet poles. For example, with the polarity of the electromagnets 14 and 4 being set such that their right sides as shown in Figure 1 were of North polarity and hence the magnetic field direction across the chamber essentially horizontal, the magnet assembly 11 permanent magnets were orientated such as to face toward each other as shown in Figures 1 to 3, the first opposing pair 12 with their North poles facing, the second optional pair 15 with their South poles facing. This assembly could be placed in a variety of positions within the chamber to test theories of operation but was usually placed close to and surrounding the plasma source open end with the magnet centres on axis with the plasma source tube as shown in the Figures. The spacing between the different magnet pairs could be independently altered to alter the relative effect of the magnet pairs, or adjust magnet "off axis" spacing for each magnet, allowing imbalance effects to be tested.

Equivalent remote plasma sputter coating systems were used for some experiments, but did not differ in any essential form, nor did they give inconsistent results.

In a first demonstration of the full use of the invention in accordance with its operation as part of a sputter coating system, the magnet assembly 11 was built to have a square internal form, 170mm wide, 170mm height, with equal permanent magnet stacks, each stack comprising two of the C5 ferrite magnets described above stacked face to face with the same magnetic polarity (i.e. in attract orientation), one stack being placed on each internal frame side as shown in the example embodiment, as close to the plasma source aperture as possible. A planar 6mm thick copper sputter target disc of approximately 100mm diameter, was installed in the process chamber, positioned such that the visible planar plasma passed over the surface at about 20mm separation. A substrate was mounted to the substrate stage at a vertical separation to the target of about 120mm. This was larger than necessary, but still much closer than the usual 200mm separation needed on our standard sputter coaters based on the original Thwaites plasma source. The 185mm internal diameter second electromagnet was used with this arrangement.

Following evacuation of the chamber and thereby the plasma source to a pressure of about 2 x 10' torr, an argon gas flow of 70 sccm was set to provide a base process pressure of about 3.5 x 10-3torr in the chamber. The electromagnets were then turned on, followed by the plasma source RF power which was set to input 2500W to the antenna This was seen to generate the expected bright blue glow discharge for a highly ionised argon plasma, with the usual cylindrical form within the plasma source tube, being reshaped as it passed through the magnet assembly to a thin, horizontal planar form, similar to that shown in Figures 1 to 3.

With the substrate shutter closed, a DC power input of 600W was provided to the sputter target, resulting in a change of plasma glow discharge colour local to the target surface typical of sputtered copper atoms entering and being excited by the plasma generation region. The shutter was then opened for 10 minutes, then closed and the power supplies turned off in reverse sequence to the start up process.

Following removal of the substrate, it was found that a layer of good quality copper had been deposited, of thickness 1000nm, giving a normalised system deposition rate of 167nm. min-1. kW-1. Compared to the usual 80nm. min-1, kW-I-typically achieved with our standard sputter coaters based on the original Thwaites plasma source, this was a substantial and useful increase despite not utilising the full available plasma extent, demonstrating that the plasma reshaping was not detrimental to the plasma formation process, a high density plasma still being generated in the process chamber. Subsequent modelling of the system assuming the use of multiple and larger targets has indicated that rates as much as 10 times that of our standard sputter coaters based on the Thwaites plasma source could be achieved. Our further development has confirmed this modelling.

In order to test this modelling and other expected improvements from experiments summarised below, a larger planar target assembly with an active sputter area about twice that of the above example was installed into the system. At the same time, revisions were made to the plasma source to make use of a new magnet assembly design expected to further optimise performance. This was built to have a rectangular internal form, 230mm wide, 170mm height, with unequal magnet stacks, being double magnet stacks on each of the faces with 230mm spacing, triple magnet stacks on each of the frame faces with 170mm separation. Again, this assembly was positioned as shown in the example embodiment, as close to the plasma source aperture as possible. A 265mm internal diameter second electromagnet was used with this arrangement, again to provide performance improvements.

The above coating procedure was repeated with an aluminium sputter target tile of dimensions 133mm x 119mm x Omm thick: a target power of IkW was therefore applied to replicate the power area density applied in the prior work. Target to substrate spacing was slightly closer at about 110mm.

Visually, the plasma form was similar to that previously achieved, though more symmetrical across the chamber as intended. Again, a good quality coating was deposited, measuring 286nm for a 2 minute deposition time. When normalised to account for the larger coating area of the increased target size, this is equivalent to 279nm. min4. kW' for comparison to the previous result, again a substantial and useful improvement on our standard sputter coaters based on the Thwaites plasma source.

A key advantage of the invention is that it provides a much greater usable area of plasma than the usual Thwaites source, permitting the use of far larger and / or multiple targets. In order to demonstrate this, we have experimented with determining the plasma uniformity of the planar plasma sheet to define the effective usable area, both through repositioning of the 100mm sputter target assembly and with a test array of small targets operated at low voltage, below the target material sputter threshold, as detectors only. This testing indicates that the plasma sheet is of sufficient uniformity to readily accommodate four 100-150mm diameter target assemblies as shown in Figures 4 and 5, or other combinations or single targets fitting within a 300mm x 300mm area.

This demonstrates a major advantage of the invention, as in addition to allowing the use of large sputter targets and consequent coating of large area substrates, it allows use of the sputter coating system for multiple independent target co-sputter applications. Both improvements can be achieved without any detriment to or compromising of the usual high density plasma process performance or coating material quality achievable with the more spatially limited Thwaites plasma source.

We have also experimented to further define the characteristics of the plasma form change and the key configuration options that might be used in practice. These experiments essentially use the systems and processes previously described and can be summarised as follows.

A first series of experiments demonstrates the critical requirement for the plasma source and optional second magnet combination to be of a strength sufficient to dominate the magnet assembly field. With the magnet assembly positioned distant, about 200mm, from the plasma source electromagnet, it could be shown that this condition was not achievable within the range of electromagnet power available, even with the second chamber magnet active and assisting. As expected, although the visible plasma generation region produced by the essentially separate Thwaites plasma source was of the usual tubular form up to the magnet assembly aperture, it was not noticeably present on the opposite side of the aperture. Effectively the magnet assembly magnetic field structure acted so as to block the plasma propagation beyond the magnet assembly aperture.

With the magnet assembly moved closer to the plasma source electromagnet 14, an electromagnet current and hence induced magnetic field threshold value was measurable above which a plasma of the required sheet form was generated on the non-plasma source side of the magnet assembly. The plasma on the plasma source side remained essentially of cylindrical form up to the magnet assembly itself Moving the magnet assembly closer to the electromagnet further reduced this threshold value, typically to about 30% of the usual current that would be applied for the source. For values down to about 10% of the usual operating current, the plasma was seen to distort to a flat form at the magnet assembly, but not propagate beyond it; lower values had no observable effect.

In further confirmation of the need to have the electromagnet magnetic field dominant at the magnet assembly, increasing the magnet strength of the magnet assembly resulted in increased electromagnet current threshold values.

Further experiments and modelling have shown that so long as the proximity and magnetic field requirement are met the magnet assembly can be placed either side or even within the plasma source electromagnet. The latter requires new engineering solutions to meet space constraints, for example a larger electromagnet aperture.

The factors influencing the magnet assembly effectiveness and impact on plasma form have also been investigated While the initial magnet assembly was a square aperture with magnet stacks of equal size and strength on all four internal faces as detailed above, studies have shown that improved planar form is obtained if the aperture is rectangular in form, with the magnets on the wider spaced section being of lower value than those on the closer faces. Other variations have been investigated as part of efficiency improvement studies and show that, although the rectangular four magnet stack assembly provides optimum capability for performance optimisation, the invention can also be realised with just the two closer spaced magnet stacks, or even a single one of these, though the form of the plasma obtained in both cases is not optimal for general use.

In further experimentation it has been discovered that the plasma form is further improved if a second magnet array, similar or the same as that used in the invented plasma source, is placed in proximity to and axially aligned with the chamber second electromagnet 4, the plasma form for example becoming more symmetrical about the chamber centreline than that shown in the Figure 3 plan view. It will be obvious that this assembly must have opposite magnetic polarity on the four faces compared to the plasma source magnet assembly as it is mounted to an electromagnet face of opposite polarity to that of the plasma source. An experiment where this change was not made showed no beneficial effect on plasma form.

Many different embodiments and applications of the invented plasma source will be apparent from the above descriptions.

As a first example, whether the magnets used in the invention are permanent magnets or electromagnets is determined only by the adjustment flexibility desired, which favours the use of electromagnets, or power efficiency and cost, which favours permanent magnets. In principle, the required magnetic fields can be provided by either or both.

As a further example, the symmetry of the proven effective electromagnet and magnet assembly layout where, based on the preferred embodiment, each electromagnet 4 and 14 has an associated magnet assembly as in the experiments described above, allows for the second magnet to be further equipped as a second plasma source facing the first plasma source. This has further potential benefit where highly uniform sputtering, a wider chamber or highest possible plasma power intensity for the process is desirable.

Use of other multiple invented plasma source configurations to provide plasma processes for even larger substrate areas will be obvious to those skilled in the art.

It should also be noted that the plasma source can be equipped with an electromagnet, magnetic assembly and further chamber, optionally with additional chamber vacuum and process furnishings, at the normally closed end 10 shown in the figures. Our experiments and experience of using the Thwaites plasma source have shown that there is no inherent directionality to the plasma propagation direction. This would allow the use of a single plasma source with two process chambers, either separately or simultaneously depending on process requirements.

The ability for the plasma sheet to be of curved form through second magnet repositioning is also of potential benefit where sputter targets positioned around a drum shaped substrate holder or carrier, for example as used in continuous "web" coating, are to be used. Additional magnets can also be added to the invented plasma source or chamber in order to alter the form, path or plasma generation efficiency as required by the intended process.

It will be obvious to those skilled in the art that the capability of the invented gas plasma source allows multiple applications to be realised Thus, in addition to the sputtering application used as an example above, the invented gas plasma source may be used for example as a substrate cleaning, surface modification or etch tool or source of plasma for plasma enhanced or assisted chemical vapour deposition applications.

The means of using the plasma generated by the invention are not in themselves unusual and will be obvious from the literature. For example, it is well known that merely running the substrate through or in proximity to medium 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 invented gas plasma source is of particular utility in all these processes due to the innate ability to generate uniform high density plasma over large process areas, thereby allowing its use with large dimensioned substrates. The means for realising such applications will be readily apparent to those skilled in the art.

Claims (25)

1. Claims I. A vacuum gas plasma generation system for use as part of a vacuum processing system to excite a magnetically shaped high density plasma, of density greater than 1011 cm', within one or more vacuum chambers of said vacuum processing system thereby providing a plasma environment for said processing system to utilise, the vacuum gas plasma generation system comprising: a vacuum gas plasma source of essentially tubular construction, largely of metals and dielectric materials, that connects at one open end to said vacuum chamber or chambers via an aperture through which the plasma generation region is intended to pass thereby sharing the vacuum gas environment of said vacuum chamber or chambers, being essentially closed in vacuum terms at the opposite tube end means of coupling energy into said vacuum gases within the plasma source to produce a vacuum gas plasma by means of an electrically conducting antenna or antennae placed external to said plasma source, in proximity to at least a portion of the dielectric elements of the plasma source constmct, such as to permit the transmission of radio frequency (RI) energy into said vacuum source by the process of inductive coupling to generate a glow discharge plasma a plasma source magnet that provides means of producing a magnetic field within the vacuum chamber(s) and attached plasma source using a coil electromagnet proximate to, within C\I 150mm of the plasma source aperture, the magnetic field being of magnitude alignment and shape that results in the generation of high density plasma within the plasma source and extension of said high density plasma generation from within said vacuum plasma source via the aperture to within the vacuum processing chamber or chambers an additional magnetic construct of two or more interacting magnets positioned within or C\I nearby to, within 100mm of, said vacuum plasma source aperture in a position and orientation that results in modification of the magnetic field shape to produce an anisotropic reshaping of the high-density plasma, the construction and positioning being such as to avoid blocking or obstructing the high density plasma generation region having the characteristic feature that the essentially cylindrical shape of the high density plasma generation region at the plasma source is modified by the magnetic construct such that, when viewed in cross section orthogonal to the plasma path axis from the plasma source across the chamber or chambers, it has a shape within the chamber or chambers which is elliptical, flattened elliptical, essentially rectangular, or curved equivalents of these as a result of the plasma region cross section being magnetically compressed in one direction and magnetically expanded in the orthogonal direction The vacuum processing system into which the invention is installed provides the means for controllably producing a vacuum of one or more ionisable gases in the vacuum chamber or chambers and attached or integrated plasma source and will also contain the facilities needed for a given desired vacuum process, including the power sources and other services that the vacuum gas plasma generation system requires for its operation.
2. 2. A vacuum gas plasma generation system as claimed in claim 1 in which the magnetic construct comprises a single pair of magnets with associated mechanical fixings and supports placed with their same magnetic poles facing each other and spaced apart such that the plasma generation region originating from the plasma source is able to pass between the two
3. 3. A vacuum gas plasma generation system as claimed in claim 1 in which the magnetic construct comprises two pairs of magnets with associated mechanical fixings and supports, each pair being placed with their same magnetic poles facing each other and spaced apart such that the plasma generation region originating from the plasma source is able to pass between the two, with the facing poles of a first pair being opposite to those of the second pair.
4. 4. A vacuum gas plasma generation system as claimed in claims 2 and 3 in which the magnet pairs are permanent magnets, each of the pair being an assembly of one or more magnets fixed and aligned to have identical pole facing.
5. 5. A vacuum gas plasma generation system as claimed in claims 2 and 3 in which one or both magnet pairs are electromagnets with associated power supplies and wiring, each one of each electromagnet pair being an assembly of one or more electromagnets and fixed, electrically connected and aligned to have identical pole facing.
6. 6. A vacuum gas plasma generation system as claimed in claims 2 and 3 in which one or both magnet pairs are a combination of permanent magnets and electromagnets with associated power supplies and wiring, each one of each magnet pair itself being an assembly of one or more permanent magnets or electromagnets or a mixture of both and fixed, electrically C\I connected and aligned to have identical pole facing.
7. 7. A vacuum gas plasma generation system as claimed in claims 4, 5 and 6 in which, when using an assembly of magnets for one or more sides of each magnet pair, some of the 1- magnets are deliberately placed or operated so as to oppose the overall magnet side required alignment, providing a means to optimise local field strengths for the purposes of optimising C\I the plasma generation region shape and uniformity of generated plasma.
8. 8. A vacuum gas plasma generation system as claimed in claims 4, 5, 6 and 7 in which the magnet pair(s) are instead replaced by a single multipolar magnet, either an electromagnet or permanent magnet, consisting of four poles, the poles aligned such that identical poles face each other.
9. 9. A vacuum gas plasma generation system as claimed in any one of the preceding claims in which the plasma source magnet proximate to the plasma source aperture 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; in which preferably at least one of said electromagnets is controllable to allow variation of the magnetic field strength and shape.
10. 10. A vacuum gas plasma generation system as claimed in any one of the preceding claims in which a second magnet, of similar size and operating capability to the plasma source magnet or alternatives defined in claim 9, is placed and operated with identical polarity to said plasma source magnet such as to extend the plasma generation zone between said magnets within the vacuum process chamber or chambers.
11. 11. A vacuum gas plasma generation system as claimed in claim 10 in which said second magnet instead 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; in which preferably at least one of said electromagnets is controllable to allow variation of the magnetic field strength and shape.
12. 12. A vacuum gas plasma generation system as claimed in claims 10 and 11 in which a second magnet construct made in accordance with the first magnet construct as claimed in preceding claims is placed proximate to, within 100mm of the second magnet of claim 10 or the equivalent magnet assemblies of claim 11, the second construct being aligned and of polarity facing to assist the anisotropic magnetic field reshaping introduced by the first magnet construct.
13. 13. A vacuum gas plasma generation system as claimed in any one of the preceding claims in which the plasma source dielectric elements through which the RE power is to be coupled to the plasma is of tubular form, the antenna being a helical coil shape of between one and six turns, dependent on RF impedance matching requirements, closely surrounding but not in contact with the dielectric, the whole in use therefore producing a tubular shaped plasma within the plasma source.
14. 14 A vacuum gas plasma generation system as claimed in claim H in which the dielectric tube has an internal diameter of between 50 and 200 mm C\I
15. 15. A vacuum gas plasma generation system as claimed in claim 13 in which the dielectric tube has an internal diameter of between 70 and 150 mm.
16. 16. A vacuum gas plasma generation system as claimed in claims 13 to 15 in which the dielectric tube is of length between 25 and 50mm, being only a minor part of the tubular element length forming the plasma source. C\I
17. 17. A vacuum gas plasma generation system as claimed in claims 13 to 15 in which the dielectric tube is of length greater than 50mm, being the major or sole part of the tubular element length forming the plasma source.
18. 18. A vacuum gas plasma generation system as claimed in any of the preceding claims in which the plasma source antenna or antennae essentially comprise a construct of electrically conducting materials in rod, tube, plate and sheet form or a mixture of rod, tube, plate and sheet forms having the property of being able to be powered with radio frequency (RE) electrical energy.
19. 19. A vacuum gas plasma generation system as claimed in any one of the preceding claims, wherein the plasma source antenna or antennae are supplied with power from one or more radio frequency power supply system(s) 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.
20. 20. A gas plasma source as claimed in any one of the preceding claims, wherein said plasma source magnet generates a magnetic field of strength greater than or equal to 5 Gauss, preferably between 50 and 500 Gauss, at a location proximate to, within 5mm distance of, any part of the dielectric elements of the plasma source construct
21. 21 A vacuum gas plasma generation system as claimed in any of the preceding claims in which a process gas or process gases are directly and controllably input to within the tubular construct of the plasma source either additionally to the process chamber gases or as the sole supply of process gases, passing into the process chamber via the plasma source aperture.
22. 22. A vacuum gas plasma generation system as claimed in any of the preceding claims in which the plasma source construct is itself furnished with vacuum pumping means to allow the plasma source pressure to be varied from that present in the process chamber or chambers.
23. 23. A vacuum gas plasma generation system as claimed in any of the preceding claims in which the normally closed end of the plasma source is instead also furnished with an aperture and connected to further regions of the process chamber or chambers, alternatively to a different process chamber or chambers, these optionally being themselves suitably equipped with magnets and other means in the same or similar manner to that described above to use the plasma source for similar or alternate plasma processing, either sequentially or simultaneously depending on the processes compatibility.
24. 24. A vacuum gas plasma generation system as claimed in any of the preceding claims in which the plasma source is constructed so at to attach to an outside surface of the processing chamber or chambers, or on a re-entrant construct, having the antenna or antennae operating outside of the vacuum process environment thereby preventing their direct electrical C\I connection to the process gases and plasma environment and maintaining the sole inductive coupling of RF power underpinning the vacuum gas plasma generation process.
25. 25. A vacuum gas plasma generation system substantially as herein described with reference to the accompanying Figures.