Apparatus

Abstract

An inductively coupled plasma apparatus 110 for producing an ion beam comprises a plasma chamber having an open ended body 118 and an end plate 120, a control grid 114 for extracting ions from the plasma chamber, and a dielectric window 124. The dielectric window comprises an outer region 124a seated around a periphery of an aperture of the end plate, and an inner region 124b extending into a plasma containment space to form a trough, hollow or recess. A radiofrequency (RF) induction device 128 (e.g. RF coil) for inductively generating a plasma is located in a space defined by the trough. The spacing between the outer region of the dielectric window and the outer surface of the end plate, and the position of the RF induction device in the trough, may both be adjustable. The invention may result in a more homogenous distribution of ion densities across the ion beam produced. A dielectric element and a magnetic configured for location in the plasma confinement space is also disclosed.

Description

Technical Field The present invention relates to an apparatus for producing an ion beam, to an apparatus for ion beam etching a substrate incorporating same, to a dielectric element for the apparatus for producing an ion beam, and to a magnet array the apparatus for producing an ion beam. Background In a typical ion beam source a plasma is produced by admitting a gas or vapour to a low pressure discharge chamber containing a heated cathode and an anode which serves to remove electrons from the plasma and to give a surplus of positively charged ions which pass through a screen grid or grids into a target chamber, which is pumped to a lower pressure than the discharge chamber. Ions are formed in the discharge chamber by electron impact ionisation and move within the body of the ion beam source by random thermal motion. The plasma will thus exhibit positive plasma potential that is higher than the potential of any surface with which it comes into contact. Various arrangements of grids can be used, the potentials of which are individually controlled. In a multigrid system the first grid encountered by the ions is usually positively biased (for the extraction of positive ions) whilst the second grid is negatively biased. A further grid may be used to decelerate the ions emerging from the ion source so as to provide a collimated beam of ions having more or less uniform energy. Ion beam sources displaying high current operation and delivering ion energies in the range up to about 1500 volts, as a generic type find wide use in thin film technology. For ion sputtering a target is placed in the target chamber where this can be struck by the beam of ions, usually at an oblique angle, and the substrate on to which material is to be sputtered is placed in a position where sputtered material can impinge on it. When sputter etching or milling is to be practised the substrate is placed in the path of the ion beam. Hence, in a typical ion beam source an ion arriving at a multi-aperture extraction grid assembly first meets a positively biased grid. Associated with the grid is a plasma sheath. Across this sheath is dropped the potential difference between the plasma and the grid. This accelerating potential will attract ions in the sheath region to the first grid. Any ion moving through an aperture in this first grid, and entering the space between the first, positively biased grid, and the second, negatively biased, grid is strongly accelerated in the intense electrical field. As the ion passes through the aperture in the second grid and is in flight to the earthed target it is moving through a decelerating field. The ion then arrives at an earthed target with an energy equal to the voltage of the first, positive, grid plus the sheath potential. Plasma generation by means of radio frequency (RF) excitation relies upon the ability of electrons to respond to a high frequency field and the inability of ions to do so because of their relatively high inertia. As a result electrons are stripped off the gas molecules. The electrons then become trapped by magnetic confinement cusps formed by alternate north and south poles of magnets that face the plasma, leaving a positively charged plasma in the central part of the plasma generation chamber. Known types of ion beam source use an inductively coupled RF induction device to excite a gaseous material to a plasma state and to produce an ion beam. Low pressure gas discharges are used in a variety of applications. These include thin film deposition and etching, surface treatment systems, ion thrusters and nuclear applications. Broad ion beam sources that extract a monochromatic beam of ions from a glow discharge generator using a perforated accelerator (or grid set) were predominantly developed within National programs for Nuclear research. Such programmes flourished in the USA, Europe, Asia and the Far East. Applications such as fuelling and heating fusion reactors and more recently ion thrusters are responsible for much of the development. Sources designed for ion thrusters tend to be fairly compact, while those for injecting ions into a fusion reactor are typically more substantial. For an ion thruster, the homogeneity of the extracted beam is not necessarily that important. The emphasis for ion thruster applications centres around reliability and longevity. For non-terrestrial applications, the power efficiency is also an important consideration. For fuelling and heating a plasma within a nuclear fusion reactor very high energy ions are generated before they are neutralised and injected into the plasma. Early ion beam sources used DC arc discharges to produce the plasma from which ions are extracted. The need to improve reliability and in particular longevity drove the migration to filament-less discharge excitation. Of the three most likely candidates as excitation techniques, namely: hollow cold cathode; microwave; and RF (discussed above), the latter has emerged as the most popular. It is predominantly RF excitation that has been adopted in equipment using glow discharges for the processing of thin film devices, particularly for film removal i.e„ etching and PECVD (Plasma Enhanced Chemical Vapour Deposition). Over the last fifty years glow discharges have come to be used extensively in thin film processing for applications in semiconductor technologies. The scale of discharge sources has increased over this time with the size of the wafers used within the industry. 1” (25.4mm) diameter wafers were introduced in 1960. Further major milestones were 1972 for the introduction of 3” (76.2mm) wafers. Then a few years later, 1976 for the migration to 4” (101.6mm). 150 mm wafers were introduced in 1983, then 200 mm in 1992 and 300 mm in 2002. Larger wafers 450 mm have been proposed, but enthusiasm for the migration to such larger wafers has waned in recent years. Today, most advanced semiconductor devices are built on 300 mm wafer with a second tier for less advanced devices using 200 mm. Besides the processing of silicon wafers for integrated circuits and systems there are a number of affiliated applications. These include the use of group III-V semiconductors, for the fabrication of electronic processors, lasers and integrated optical components. The techniques developed for building silicon-based electronics are also used for building an abundance of sensors, MEMS (Micro Electro Mechanical Systems), thin film recording heads, micro-bolometers and others. For some fabrication processes that use a glow discharge as a source of ions, the homogeneity of the ions within the source are not especially important. A notable example is ion beam deposition. In this technique a target is illuminated by an ion beam extracted from a discharge source. The resulting flux of sputtered material is used to coat a substrate. The geometry and configuration of the apparatus is the major factor that impact the resulting nonuniformity of film deposition within the substrate. The beam extracted from the ion source is typically manipulated to be non-uniform in character. For applications including plasma etching, reactive ion etching (RIE), plasma assisted chemical vapour deposition (PECVD) and ion beam etching or milling, the homogeneity of the discharge become important. For RIE and PECVD the glow discharge that activates the process directly bathes the substrate to be processed. In the ion beam milling case the discharge bathes the ion extraction accelerator. The beam emitted from the accelerator bombards the substrate to be processed. Ion beam milling (or ion beam etching) is typically used for patterning layers where there is no simply utilised reaction that may be harnessed for chemically enhancing the rate, or the selectivity of one material over another. For many materials used in semiconductor device fabrication there are suitable chemistries that can be utilised. For other applications, e.g., those involving the fabrication of magnetic devices, chemical enhancements are typically not available for the metallurgies involved. In the case of noble gas ion beam milling there is no role for chemical activation of the process. The resulting non-uniformity of material removal is governed primarily by the homogeneity of the beam. RF glow discharges may be excited at a range of frequencies. There are a number of frequencies that are reserved for industrial, scientific and medical (IMS) applications. The most popular of these is 13.56 MHz. Radio frequency energy may be coupled into a low pressure discharge source by either capacitive or inductive coupling. For applications where the discharge bathes the substrate, capacitive coupling can be advantageous as it also serves to bias the substrate to encourage ion bombardment across the plasma sheath. Where the discharge is remote from the substrate and bathes an extraction accelerator (grid), capacitive coupling is far less attractive. There is a need to constrain the sheath potentials to minimise sputtering of the discharge enclosure. Inductive coupling is therefore adopted. The energy is coupled through a dielectric window. The window may take many forms, e.g., If can be the walls of a jar or tube, it can be a flat plate or it can be the walls of a pipe. A known apparatus for producing an ion beam is illustrated in Fig. 1 and an ion beam produced by the apparatus of Fig. 1 and an ion density of such an ion beam is illustrated in Fig. 2. In Fig. 1, there is shown an apparatus 10 for producing an ion beam, which comprises a plasma source 12 and an accelerator assembly 14. Plasma source 12 comprises a plasma containment space 16, which is defined by an open ended plasma vessel 18, an end plate 20 that closes a first end of the open ended plasma vessel 18 (the upper end in the figure), and the accelerator assembly 14 that closes a second end of the open ended plasma vessel 18 (the lower end in the figure). The plasma source 12 described here comprises a circular cross-section, having an internal diameter of the order of 500 mm and depth of 200 mm, but other known plasma sources comprise a rectangular crosssection. An ion beam 22 issues from the lower end of the apparatus 10 and comprises a plurality of beamlets 24 that coalesce downstream of the apparatus to form the broad ion beam 22. The end plate 20 comprises an open ended metallic body, made of, for example, aluminium, or of an aluminium alloy, or another conductive non-magnetic material (such as austenitic stainless steel). The end plate 20 comprises an aperture that is closed by a dielectric element 24, which engages an outer surface of the end plate 20 around a periphery of the aperture. A gas inlet nozzle 26 is provided, through which a plasma forming gas, such as Argon, can be admitted to the plasma containment space 16. An RF coil 28 surmounts dielectric element 24 and is connected to a suitable RF power source operating at, for example, 13.56 MHz. The RF power source could, in other arrangements, operate at other Industrial Scientific and Medical (ISM) permitted frequencies, such as, for example, 27 MHz or 40 Mhz. There are also known sources that operate in the region of 2 MHz. Plasma vessel 18 is made of aluminium, or of an aluminium alloy, or another conductive non-magnetic material(such as austenitic stainless steel), within the walls of which is mounted a magnet array 30. The magnet array 30 is also embedded within the walls of the end plate 20. Typically, the magnets of the magnet array 30 are permanent magnets, which may comprise, for example, neodymium magnets (i.e. NdFeB magnets). A configuration of the magnet array 30 can be in columns (i.e. substantially in the same direction as the axis of the vessel 18), or in rings (i.e. encircling the vessel 18) that may alternate in polarity in either one dimension or in two. With the one dimensional case there are columns, or lines of alternate N and S poles. For the two dimensional case, the pattern resembles a checker (or chess) board. The total number of N and S poles are the same and the physical bulk of N and S oriented magnets is the same. The arrangement is such that at a short distance (~ 30 to 40 mm) from the walls of the plasma vessel 18 in the plasma containment space 16, the field generated by the magnets should be close to zero. As stated above, the lower end of plasma vessel 18 is closed by accelerator assembly 14, which comprises three grids 14a, 14b, 14c, each formed with aligned holes. Grid 14a is positively biased, grid 14b is negatively biased, while grid 14c is at earth potential. Other accelerator configurations are possible, which may comprise two, or four electrically biased grids. Electromagnet coils 32 are provided in proximity to the dielectric element 24 and RF coil 28 and around the second end (accelerator end) of the vessel 18. They may be controlled to tune the plasma homogeneity / efficiency. A glow discharge, or plasma, is generated within the plasma containment space 16 of the plasma source 12 by the inductive coupling of RF power via RF coil 28. The discharge is generated close to the “window” (i.e., dielectric element 24) adjacent to which, exterior to the plasma containment space 16, the RF coil 28 resides. A skin depth of the excitation field only penetrates a few tens of millimetres into the vacuum in the plasma containment space 18. Ions generated in this region then diffuse through the volume of the plasma containment space 18. The ions generated have positive charge. Ions are lost when they strike any part of the plasma source 12, i.e. the inner surfaces of the walls of the plasma vessel 18, the inner surface of the end plate 20 and the “solid” regions of the accelerator assembly 14 (the parts without holes). Where an ion approaches one of the holes in the innermost grid 14a of the accelerator assembly 14 it firstly encounters the plasma sheath. The sheath forms a localised meniscus at the boundary between the glow discharge and the aperture. In this region the ion experiences gentle acceleration. Once it enters the aperture then it is accelerated by the electric field generated by the electrodes forming the accelerator. The first grid (electrode) 14a encountered is positively biased, the second 14b is negatively charged and the third 14c is at earth potential. The accelerator assembly 14 serves to extract ions from the plasm source 12. It both accelerates and focuses the ions. The result is a small beamlet 24 from each of the holes in the accelerator assembly 15. These beamlets 24 coalesce into a broad ion beam 22 downstream. The focus achieved is not perfect and the resulting beam will display some divergence. The divergence arises predominantly from the space-charge interaction between the ions as they pass through the accelerator channel. The homogeneity of the discharge (that is the uniformity of the ion density) is a function of many parameters. The overriding driver of the ion density is the electron density. The mass of an electron is approximately 1 / 2000 that of a proton. The ions of interest are predominantly Ar+. Argon has an atomic weight of 40, so the ions are about 80,000 times heavier than the electrons. Electrons play a key role in sustaining the glow discharge. They react very quickly to the oscillating electric and magnetic fields established by the RF coil 28. They can also traverse the dimensions of the plasma source 12 more quickly that the ions. Therefore they are lost more quickly than the ions. As they are lost, the discharge becomes positively biased, a sheath is formed between the plasma and the containment walls and a balance is reached. To enhance the efficiency and homogeneity of the discharge, magnets are commonly used. These may be permanent of electromagnetic. Permanent magnets have the advantage of generating the high fields adjacent to the walls of the source. Ideally the bulk of the volume of the source should be free of a magnetic field. This places constraints on the design of the magnet array employed. Within these constraints there remain a multitude of configurations that can be assembled. A rare earth magnet (e.g., NeFeB) will have a field strength, measured at its pole in excess of 0.3 Tesla. Since the goal is to provide a central volume that is free of magnetic field, a multipolar array is beneficial, i.e. arranged in the manner discussed above. As a rule the design should provide a field strength of 0.06 Tesla perpendicular to the wall of the source (measured halfway between the magnet poles and 10 mm from the wall). Further this field should fall to 0.005 Tesla before the area of the accelerator apertures is encroached. Because of a drop-off in ion density of the beam towards the edges, only a portion of the beam width is useful for a particular processing application, e.g. etching. Therefore, where it is necessary to process substrate of a particular diameter, it must be ensured that the “useable” part of the ion beam that is used to etch the substrate is at least the same diameter as that of the substrate. Consequently, this means that the overall beam diameter must be greater than that of the substrate to be processed and this has an impact on the size of the ion beam source apparatus this is used to produce the ion beam. This issue is illustrated in Fig. 2, which shows ion beam 22, formed from beamlets 24, and illustrates the density distribution of ions across the beam. For an apparatus such as that illustrated in Fig. 1, the density distribution of ions across a beam produced by such an apparatus will resemble that illustrated in Fig. 2, in which the distribution has a so-called “bellcurve” distribution. The useable part of the ion beam is denoted by arrow A in the figure. As low pressure discharge sources become larger to accommodate the increasing sizes of substrates (wafers), discussed above, technical challenges can arise with respect to the physical configuration of the inductive energy coupling elements. These can impact design choices and result in compromises to maintain mechanical strength. A gridded accelerator structure used to extract and collimate ions from the discharge will extract whatever bathes it. It is possible to modify the accelerator to compensate for deficiencies in the source homogeneity (to address the ion density drop-off discussed above). This, however, has the disadvantage that such modifications can make it difficult to optimise performance cross a range of operational conditions. For example, different beam energies and levels of current extraction. It would be preferable to start with a homogeneous ion density. As indicated in Fig. 2, discussed above, ion density will naturally fall as the enclosure walls are approached, due to the loss of electrons from the discharge at its boundaries. This may be mitigated by the adoption of a magnet assembly, which, as illustrated in Fig. 1, takes the form of a electromagnet (often to provide an axial field) or a static permanent magnet array (usually employed at the walls). There are instances where a moving magnet array is used. The homogeneity of the discharge becomes a complex function of the RF antenna and electron confinement configurations. The inventor has recognised that it would be desirable to optimise and fine tune the ion density in an ion beam output by an ion beam source. The present invention has been devised with the foregoing considerations in mind. Summary According to an aspect of the present invention, there is provided an apparatus for producing an ion beam, comprising: a plasma chamber, the plasma chamber comprising: an open ended body; an end plate including an aperture therein, the end plate located to close a first end of the open ended body and the aperture for receiving a dielectric element; a control grid, the control grid located to close a second, opposite end of the open ended body, the control grid for extracting ions from plasma in the plasma chamber; a dielectric element disposed to close said aperture; wherein the open ended body, end plate, dielectric element and control grid serve to define a plasma containment space therebetween; further wherein the dielectric element comprises an outer region configured to be seated against a surface, around a periphery of the aperture, and an inner region where material that forms the dielectric element extends away from the material of the dielectric element at the outer region to form a trough, wherein the dielectric element is mounted relative to the end plate so that the trough of the inner region extends into the plasma containment space; and an RF induction device for inductively generating a plasma in the plasma chamber, the RF induction device located in a space defined by said trough on an opposite side of the dielectric element from the plasma containment space. Locating the RF induction device in the space defined by the trough may increase the useable part of the ion beam produced by the apparatus. Optionally, the dielectric element may be of a cylindrical shape, closed at a first end in a dome. Optionally, a cross-sectional profile of the dielectric element may be a bell-shape. Optionally, a cross-sectional profile of the dielectric element may be a U-shape. Optionally, a relative position of the RF induction device in the trough may be adjustable. Further optionally, the apparatus may comprise an actuator operative to control the relative position of the RF induction device in the trough responsive to a position control signal. Adjusting the depth at which the RF induction device is positioned in the trough may allow the ion beam produced by the apparatus to be “tuned”. That is, moving the RF induction device up or down in the trough may alter the ion density distribution of the beam, e.g. altering the shape of the density distribution curve. Optionally, the surface, around the periphery of the aperture, against which the outer region of the dielectric element is to be seated, comprises an inner surface of the end plate. Optionally, the surface, around the periphery of the aperture, against which the outer region of the dielectric element is to be seated, may comprise an outer surface of the end plate. Optionally, the apparatus may further comprise a spacing element, wherein the surface, around the periphery of the aperture, against which the outer region of the dielectric element is to be seated, comprises a surface of the spacing element that is located against an outer surface of the end plate. Providing a spacing element between the outer region of the dielectric element and the outer surface of the end plate allows for adjustment of the depth to which the dielectric element extends into the plasma chamber. Thus, the depth could be set to a first depth by employing a first spacing element having a first thickness. The depth could be set to a second, different depth by replacing the first spacing element with a second spacing element having a second thickness. The thickness of spacing element could be chosen dependent on a particular profile of the density distribution curve that is required. Optionally, the spacing element may be controllable to adjust a spacing between the outer region of the dielectric element and the outer surface of the end plate. Further optionally, the apparatus may comprise an actuator operative to control the spacing element to adjust the spacing between the outer region of the dielectric element and the outer surface of the end plate responsive to a dielectric element position control signal. Optionally, an outer region of dielectric element may comprise an annular flange, wherein a surface of the flange that faces the end plate comprises a mounting surface of the annular flange, further wherein the mounting surface is configured to engage the surface around a periphery of the aperture. Further optionally, the mounting surface may be machined to provided an 0.8 micrometer roughness average (RA) surface finish. Optionally, the apparatus may further comprise an annular retaining element to hold the mounting surface against the surface of the end plate around a periphery of the aperture. Optionally, the apparatus may further comprise an O-ring seal located between the mounting surface and the surface of the end plate around a periphery of the aperture. Optionally, the apparatus may further comprise an O-ring seal located between: (i) a surface of the outer region of the dielectric element that is on an opposite side of said annular flange to said mounting surface; and (ii) a surface of the annular retaining element that abuts the surface of the outer region of the dielectric element, as defined in (i). Optionally, the apparatus may further comprise a magnet array configured for location in the plasma containment space of the plasma chamber, the magnet array located toward, or against a surface of the control grid that faces the plasma containment space. Optionally, the magnet array may comprise an annular element comprising a plurality of annular sectors, each annular sector formed of a magnetic material and arranged so that a polarity thereof is opposite to neighbouring sectors on an alternate basis. Optionally, each annular sector may comprise a plurality of sub-sectors and further wherein each sub-sector may be of a same polarity as other sub-sectors in the same sector, but of an opposite polarity to sub-sectors in neighbouring annular sectors. Optionally, each annular sector may comprise a plurality of sub-sectors and further wherein each sub-sector may be of a polarity opposite to neighbouring sub-sectors in the same annular sector and to respective sub-sectors in neighbouring annular sectors. Optionally, a dimension of an aperture of the annular element may be sized so as be larger than an extraction area of the control grid. Optionally, the magnet array may comprise an array of rare earth magnets. Further optionally, the rare earth magnets may comprise samarium-cobalt (SmCo). According to another aspect of the present invention, there is provided an apparatus for ion beam etching a substrate, the apparatus comprising: a vacuum chamber comprising a gas evacuation system; a substrate holder, located in the vacuum chamber, the substrate holder configured to hold a substrate to be etched; and an ion beam source for directing ions into the vacuum chamber toward the substrate holder, the ion beam source comprising the apparatus as described above and hereinafter. According to a further aspect of the present invention, there is provided a dielectric element for the apparatus described above and hereinafter, or for an apparatus ion beam etching a substrate described above and hereinafter, wherein the dielectric element is configured to close an aperture in an end plate of a plasma chamber, the dielectric element comprising an outer region configured to be seated against a surface, around a periphery of the aperture, and an inner region where material that forms the dielectric element extends away from the material of the dielectric element at the outer region to form a trough, wherein the dielectric element is mountable relative to the end plate so that the trough of the inner region extends into a plasma containment space within the plasma chamber. Optionally, the outer region of the dielectric element may comprise an annular flange that forms a mounting surface configured to engage a surface around a periphery of the aperture. Further optionally, the mounting surface may be machined to provided an 0.8 micrometer roughness average (R.A) surface finish. According to a yet further aspect of the present invention, there is provided a magnet array for an apparatus as described above and hereinafter, wherein the magnet array is configured for location in the plasma containment space of the plasma chamber, the magnet array located toward, or against a surface of the control grid that faces the plasma containment space. Optionally, the magnet array may comprise an annular element comprising a plurality of annular sectors, each annular sector formed of a magnetic material and arranged so that a polarity thereof is opposite to neighbouring sectors on an alternate basis. Optionally, each annular sector may comprise a plurality of sub-sectors and further wherein each sub-sector may be of a same polarity as other sub-sectors in the same sector, but of an opposite polarity to sub-sectors in neighbouring annular sectors. Optionally, each annular sector may comprise a plurality of sub-sectors and further wherein each sub-sector may be of a polarity opposite to neighbouring sub-sectors in the same annular sector and to respective sub-sectors in neighbouring annular sectors. Optionally, a dimension of an aperture of the annular element may be sized so as be larger than an extraction area of the control grid. Optionally, the magnet array may comprise an array of rare earth magnets. Further optionally, the rare earth magnets may comprise samarium-cobalt (SmCo). Brief Description of the Drawings Fig. 1 illustrates a cut-away schematic side view of a known apparatus for producing an ion beam; and Fig. 2 schematically illustrates an ion beam produced by the apparatus of Fig. 1 and an ion density distribution of such an ion beam. One or more embodiments of the present invention are described further hereinafter, by way of example only, with reference to the accompanying drawings in which: Fig. 3a illustrates a cut-away side view of an apparatus for producing an ion beam according to one or more embodiments of the present invention; Fig. 3b illustrates a cut-away perspective view of the apparatus illustrated in Fig. 3a; Fig. 4 illustrates a representation of an ion density distribution curve of an ion beam produced by the apparatus illustrated in Figs. 3a and 3b; Fig. 5 illustrates a cut-away side view of the apparatus illustrated in Figs. 3a and 3b in another arrangement; Fig. 6 illustrates a cut-away side view of the apparatus illustrated in Figs. 3a and 3b in a further arrangement; Fig. 7a illustrates a cut-away side view of the apparatus illustrated in Figs. 3a and 3b in yet another arrangement; Fig. 7b illustrates a cut-away perspective view of the apparatus illustrated in Fig. 7a; Fig. 8a illustrates a cut-away perspective view of an “outer-mounted” dielectric element arrangement; Fig. 8b illustrates a magnified portion of Fig. 8a, to show the interface between dielectric element and end plate in more detail; Fig. 9a illustrates a cut-away perspective view of an “inner-mounted” dielectric element arrangement; and Fig. 9b illustrates a magnified portion of Fig. 9a, to show the interface between dielectric element and end plate in more detail. Detailed Description of the Invention Referring now to the drawings, Figs. 3a and 3b illustrate an apparatus 10 for producing an ion beam, which comprises a plasma source 12 and an accelerator assembly 14. Features similar to those illustrated in Figs. 1 and 2 are also illustrated in Figs. 3a and 3b. However, in Figs. 3a and 3b, the features common with those Figs. 1 and 2 are now designated with reference numerals of the type 1XX rather than XX. Thus, in Figs. 3a and 3b, the apparatus is denoted by reference number 110 (rather than 10), the plasma source, by the reference number 112 (rather than 12), the accelerator assembly, by the reference number 114 (rather than 14) and so on. The apparatus 110 differs from that described above in relation to Figs. 1 and 2 in that the dielectric element 24 is replaced with a different dielectric element 124. In the apparatus 110 of Figs. 3a and 3b, dielectric element 124 comprises a bell-shaped jar, which comprises an outer region 124a that extends from an outer surface of the bell-shaped jar at, or toward, an open end of the bell-shaped jar. The outer region 124a comprises an annular flange. The outer region 124a is configured to be seated against a surface, around a periphery of the aperture in end plate 120. The bell-shaped jar also comprises an inner region 124b where material that forms the dielectric element extends away from the material of the dielectric element at the outer region and terminates with a curved, or domed region to close-off the bell-shaped jar. The inner region 124b forms a “trough”. The dielectric element 124 is mounted relative to the end plate 120 so that the trough of the inner region 124b extends through the aperture of the end plate 120 into the plasma containment space 116. The surface of the annular flange (i.e. outer region 124a) that faces the end plate 120 comprises a mounting surface, i.e. a surface that faces the outer surface of end plate 120, which is configured to engage the outer surface of the end plate 120 around a periphery of the aperture. The mounting surface may be machined to provided an 0.8 micrometer roughness average (RA) surface finish, to aid the sealing fit with the surface of the end plate 120. Mounting of the outer region 124a of the dielectric element 124 is described in more detail in relation to Figs. 8a, 8b, 9a and 9b. In the apparatus 110 illustrated in Figs. 3a and 3b, the RF coil 128 is located so as to be within the trough defined between the walls of the inner region 124b of the dielectric element 124. That is, the RF coil 128 lies below the plane of end plate 120 so that it is, effectively in a same volume as the plasma containment space 116, albeit separated from the plasma containment space by the dielectric element 124. Locating the RF coil 128 in this manner affects the ion density distribution of an ion beam produced by the apparatus 110. That is, whilst the ion density distribution curve of an ion beam produced by the known apparatus 10, illustrated in Fig. 1 would look like that illustrated in Fig. 2, the ion density distribution curve of an ion beam produced by the apparatus 110, according to one or more embodiments of the present invention, would look different. A representation of an ion density distribution curve of an ion beam produced by the apparatus 110 is shown in Fig. 4. In Fig. 4, the ion density distribution curve of an ion beam produced by the apparatus 110 of one or more embodiments of the present invention is represented by reference number 400 (dashed line). The ion density distribution curve of an ion beam produced by a known apparatus, such as the apparatus 10 described above in relation to Fig. 1, is represented by reference number 402 (solid line). The respective “useable” parts of the ion beams produced by apparatus 110 of the present invention and the known apparatus 10 are denoted by arrows Al (apparatus 110) and A2 (known apparatus 10) in the figure. As can be seen, the “useable” part of the ion beam produced by apparatus 110 of the present invention is larger than that of the ion beam produced by known apparatus 10. As can be seen, the resulting ion beam has a more even distribution of ion densities across the beam, and is tending toward a beam that is more homogeneous than the beam produced by the known apparatus 10. One or more embodiments of the present invention utilise a helical excitation coil as RF coil 128. The RF coil 128 may comprise a single turn, or multiple turns (as illustrated in the figures). It could also take other forms such as a saddle shape, or dual saddle. For relatively high power applications (above 500 W) it is beneficial for the antenna to be water cooled. Optimisation of the uniformity of the plasma may require fine tuning of the depth of the penetration of the bell-shaped jar (i.e. the dielectric element 124) into the plasma containment space 116 and the position of the RF coil 128. Described below are one or more embodiments of the present invention in which the depth to which the bell-shaped jar extends into the plasma containment space 116 can be adjusted. Also described below are one or more embodiments of the present invention in which the position of the RF coil 128 can be adjusted. Fig. 5 schematically illustrates a cut-away side view of the apparatus 110 illustrated in Figs. 3a and 3b in an optional arrangement. Features similar to those illustrated in Figs. 3a and 3b are also illustrated in Fig. 5. The apparatus 110 illustrated in Fig. 5 is substantially the same as that illustrated in Figs. 3a and 3b, but now includes a spacing element 134. The spacing element 134 is located between an outer surface of the end plate 120 and the flange that forms the outer region 124a of the dielectric element. That is, an underside of the flange sits on a top surface of the spacing element 120. In a particular arrangement, the spacing element 124 comprises an annular element disposed around the periphery of the aperture of the end plate 120 against which the flange that forms the outer region 124a of the dielectric element 124 is to be seated. In this arrangement, the mounting surface of the annular flange (i.e. outer region 124a) engages an upper surface of the spacing element 134 around a periphery of the aperture. The effect of the spacing element 134 is to change the depth to which the bell-shaped jar (i.e. the dielectric element 124) extends into the plasma containment space 116. This change in depth has an effect on the shape of the ion density distribution curve of the ion beam produced by the apparatus 110. The depth (denoted by arrow “d”) to which the bell-shaped jar (i.e. the dielectric element 124) extends into the plasma containment space 116 can be changed by swapping a spacing element 134 of a first height, with a different spacing element 134 of a second, different height. Again, this will produce an ion density distribution curve with a different shape. In a further, optional, arrangement, the spacing element 134 may be configured to allow its height to be changed, i.e. so that the dielectric element 124 can be raised, or lowered, to adjust the depth to which the bell-shaped jar extends into the plasma containment space 116. In such an arrangement, the spacing element may be controllable to adjust a spacing between the outer region 124a of the dielectric element 124 and the outer surface of the end plate 120, which will adjust the depth which the bell-shaped jar extends into the plasma containment space 116. This may be by means of an actuator that operates to control the adjustable spacing element to adjust the spacing between the outer region of the dielectric element and the outer surface of the end plate. The actuator may be controlled by way of a dielectric element position control signal. Fig. 6 schematically illustrates a cut-away side view of the apparatus 110 illustrated in Figs. 3a and 3b in another optional arrangement. Features similar to those illustrated in Figs. 3a and 3b are also illustrated in Fig. 6. The apparatus 110 illustrated in Fig. 6 is substantially the same as that illustrated in Figs. 3a and 3b, but now includes a moveable RF coil 1280. In such an arrangement, a relative position of the RF coil 1280 in the trough is adjustable (in a direction indicated by arrow C). An actuator (not shown) is provided to move the coil up / down in the trough of the dielectric element 124 and the actuator is operative to control the relative position of the RF coil 1280 in the trough responsive to a position control signal. The actuator may comprise a servo-motor control to adjust the height of the RF coil 128 relative to the volume of the plasma containment space 116. This allows for fine tuning of the overall source performance and optimisation over a broader range of operational conditions than is achievable with a fixed position. The one or more embodiments of the present invention have been developed with an aim of optimising a process for 200 mm circular substrates. The outer diameter of the bellshaped jar 124 that is used is 170 mm. Alternative diameters, both smaller and larger could also be used. Since the skin depth of the RF at 13.56 MHz is between 20 and 25 mm using the 170 mm bell-shaped jar 124, a primary excitation annulus of around 170 to 220 mm diameter is generated. Optionally, the RF coil 128 / 1280 configuration is between two and three turns, as the low inductance has benefits with respect to keeping the plasma potential and the circulating current in the impedance matching transformer at modest levels. In a further optional arrangement, the apparatus 110 may comprise a combination of the features illustrated in Figs. 5 and 6, and as described above. Figs. 7a and 7b schematically illustrate cut-away side and perspective views of the apparatus 110 illustrated in Figs. 3a and 3b in a yet further optional arrangement. Features similar to those illustrated in Figs. 3a and 3b are also illustrated in Figs. 7a and 7b. The apparatus 110 illustrated in Figs. 7 and 7b is substantially the same as that illustrated in Figs. 3a and 3b, but now includes an internal magnet array 136. In this arrangement, the internal magnet array 136 is configured for location in the plasma containment space 116 of the plasma vessel 112 and is located toward, or against a surface of the accelerator assembly 114 that faces the plasma containment space The internal magnet array 136 comprises an array of rare earth magnets such as, for example, samarium-cobalt (SmCo) magnets. SmCo magnets may be advantageous, because they are stable at relatively high temperatures (-300 °C). Neodymium Iron Boron (NdFeB) magnets may be used in preference where the temperature is lower. The internal magnet array 136 comprises an annular element comprising a plurality of annular sectors (or truncated sectors). The magnetic pole of each sector is such so that a polarity thereof is opposite to those of neighbouring sectors on an alternate basis. In some arrangements, the annular / truncated sectors themselves may be each sub-divided into a plurality of subsectors where each sub-sector may be of a same polarity as other sub-sectors in the same sector, but of an opposite polarity to sub-sectors in neighbouring annular sectors. In other arrangements, each sub-sector may be of a polarity that is opposite to neighbouring sub-sectors in the same annular sector and to respective sub-sectors in neighbouring annular sectors (i.e. chessboard, or checkerboard arrangement). A dimension of an aperture of the annular element of the internal magnet array 136 is sized so as be larger than an extraction area of the control grid. That is, the diameter of the aperture is such that it does not cover the holes in the grid of the accelerator assembly 114. This is represented by arrow E in Fig. 7a, which indicates the useable width of the accelerator assembly, i.e. the part of the accelerator assembly 114 that contains the aligned holes in each of the grids. In a further optional arrangement, the apparatus 110 may comprise a combination of the features illustrated in Figs. 5, 6, 7a and 7b, and as described above. Although the dimensions of the plasma source in the described one or more embodiments may be the same as those stated in relation to the plasma source described in relation to Fig. 1, they may differ in other arrangements of the invention. Likewise, the cross-sectional shape of the plasma source in the described one or more embodiments may be the same as those stated in relation to the plasma source described in relation to Fig. 1, but may differ in other arrangements of the invention. In the described one or more embodiments, a preferred configuration for the arrangement of magnets in the magnet arrays 30 comprises alternating polarity “rings” in the side walls of the plasma source 12 (i.e. the walls of the plasma vessel 18) and a chessboard / checkerboard arrangement in the end plate 20. However, other combinations of the polarity arrangements are possible and may be employed in other arrangements of the present invention. In the one or more embodiments described above and illustrated in Figs. 3 to 7, a mounting surface of the outer region 124a of the dielectric element 124 is mounted against the outer surface of the end plate. An annular retaining element 133 (see figures 3b and 7b) is mounted to the outer surface of end plate 120. The annular retaining element 133 serves to secure the outer region 124a of the dielectric element 124 against the outer surface of the end plate. In an alternative arrangement of the one or more embodiments described above and illustrated in Figs. 3 to 7, a mounting surface of the outer region 124a of the dielectric element may be an opposite surface of the flange to that previously described. In such an arrangement, the mounting surface of the outer region 124a of the dielectric element 124 is mounted against an inner surface of the end plate. Again, an annular retaining element is used to secure the outer region 124a of the dielectric element 124 against the inner surface of the end plate. The “outer-mounted” dielectric element arrangement is illustrated in more detail in Figs. 8a and 8b. The “inner-mounted” dielectric element arrangement is illustrated in more detail in Figs. 9a and 9b. Both arrangements are described further below. Fig. 8a illustrates a cut-away perspective view of an “outer-mounted” dielectric element arrangement that is compatible with the apparatus 110 described above (and illustrated in figures 3 to 7). As can be seen in Figs. 8a and 8b, a portion of the end plate 120 around the aperture therein, through which aperture the inner region of the dielectric element 124 extends, is configured to form an annular seat. This annular seat is on an outward facing surface of the end plate 120. The inner facing surface of the outer region 124a of dielectric element, i.e. the mounting surface, is then received by the annular seat in the end plate 120. Annular retaining element 133 is mounted to the outer surface of end plate 120 and is located such that a portion thereof overlaps with an outer facing surface of the outer region 124a of the dielectric element. This serves to hold the outer region 124a of the dielectric element 124 in position in the annular seat of the end plate 120. O-ring seals 138a, 138b (see Fig. 8b) are provided between the inner facing surface of the outer region 124a and the outer surface of the end plate 120, and between the outer facing surface of the outer region 124a and the inner surface of the annular retaining element 133. That is, a first O-ring seal 138a is provided at the interface between the inner facing surface of the outer region 124a of dielectric element 124 and the outer surface of the end plate 120. A second O-ring seal 138b is provided at the interface between the outer facing surface of the outer region 124a of dielectric element 124 and the inner surface of the annular retaining element 133. These O-ring seals may serve to inhibit fluid ingress from the ambient atmosphere outside the apparatus 110 to the plasma containment space 116. The O-ring seal 138b may provide a soft seat for the clamping force applied to the dielectric element. If a metal seating surface were to be used, instead of an O-ring seal, there may be a high risk of breaking the dielectric element. In the alternative arrangement, which was discussed briefly above, the surface of the outer region 124a that is mounted against end plate 120 is opposite to that in the arrangement described in relation to Figs. 8a and 8b. The alternative arrangement is illustrated in Figs. 9a and 9b and will be described further below. As can be seen in Figs. 9a and 9b, a portion of the end plate 120 around the aperture therein is configured to form an annular seat. This annular seat is on an inward facing surface of the end plate 120. The outer facing surface of the outer region 124a of dielectric element, i.e. the mounting surface, is then received by the annular seat in the end plate 120. Annular retaining element 1330 is mounted to the inner surface of end plate 120 (i.e. within the plasma containment space 116) and is located such that a portion thereof overlaps with an inner facing surface of the outer region 124a of the dielectric element. This serves to hold the outer region 124a of the dielectric element 124 in position in the annular seat of the end plate 120. O-ring seals 1380a, 1380b (see Fig. 9b) are provided between the inner facing surface of the outer region 124a and the outer facing surface of annular retaining element 1330, and between the outer facing surface of the outer region 124a and the inner surface of the end plate 120. That is, a first O-ring seal 1380a is provided at the interface between the inner facing surface of the outer region 124a of dielectric element 124 and the outer facing surface of the annular retaining element 1330. A second O-ring seal 1380b is provided at the interface between the outer facing surface of the outer region 124a of dielectric element 124 and the inner surface of the end plate 120. These O-ring seals may serve to inhibit fluid ingress from the ambient atmosphere outside the apparatus 110 to the plasma containment space 116. The O-ring seal 1380a may provide a soft seat for the clamping force applied to the dielectric element. If a metal seating surface were to be used, instead of an O-ring seal, there may be a high risk of breaking the dielectric element. Any references made herein to orientation (e.g. top, bottom, upper, lower, front, back, and rear) are made for the purposes of describing relative spatial arrangements of the features of the apparatus, and are not intended to be limiting in any sense. As used herein, the terms “comprises,” “comprising,” “includes,” “including,” “has,” “having” or any other variation thereof, are intended to cover a non-exelusive inclusion. For example, a process, method, article, or apparatus that comprises a list of elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Further, unless expressly stated to the contrary, “or” refers to an inclusive or and not to an exclusive or. For example, a condition A or B is satisfied by any one of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), and both A and B are true (or present). In addition, the terms “a” or “an” are employed to describe elements and components of the invention. This is done merely for convenience and to give a general sense of the invention. This description should be read to include one or at least one and the singular also includes the plural unless it is obvious that it is means otherwise. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention. For example, embodiments in accordance with the invention are not limited to any of the particular materials disclosed herein. Other materials suitable for performing the function described herein for a 5 particular material may also be utilized in embodiments of the invention. The scope of the present disclosure includes any novel feature or combination of features disclosed therein either explicitly or implicitly or any generalisation thereof irrespective of whether or not it relates to the claimed invention or mitigate against any or all of the problems addressed by the present invention. The applicant hereby gives notice that new 10 claims may be formulated to such features during prosecution of this application or of any such further application derived therefrom. In particular, with reference to the appended claims, features from dependent claims may be combined with those of the independent claims and features from respective independent claims may be combined in any appropriate manner and not merely in specific combinations enumerated in the claims.

Claims

1. An apparatus for producing an ion beam, comprising:a plasma chamber, the plasma chamber comprising:an open ended body;an end plate including an aperture therein, the end plate located to close a first end of the open ended body and the aperture for receiving a dielectric element;a control grid, the control grid located to close a second, opposite end of the open ended body, the control grid for extracting ions from plasma in the plasma chamber;a dielectric element disposed to close said aperture;wherein the open ended body, end plate, dielectric element and control grid serve to define a plasma containment space therebetween;further wherein the dielectric element comprises an outer region configured to be seated against a surface, around a periphery of the aperture, and an inner region where material that forms the dielectric element extends away from the material of the dielectric element at the outer region to form a trough, wherein the dielectric element is mounted relative to the end plate so that the trough of the inner region extends into the plasma containment space; andan RF induction device for inductively generating a plasma in the plasma chamber, the RF induction device located in a space defined by said trough on an opposite side of the dielectric element from the plasma containment space.

2. An apparatus according to claim 1, wherein the dielectric element is of a cylindrical shape, closed at a first end in a dome.

3. An apparatus according to claim 1 or 2, wherein a cross-sectional profile of the dielectric element is a bell-shape.

4. An apparatus according to any of the preceding claims, wherein a cross-sectional profile of the dielectric element is a U-shape.

5. An apparatus according to any of the preceding claims, wherein a relative position of the RF induction device in the trough is adjustable.

6. An apparatus according to claim 6, further comprising an actuator operative to control the relative position of the RF induction device in the trough responsive to a position control signal.

7. An apparatus according to any of the preceding claims, wherein the surface, around the periphery of the aperture, against which the outer region of the dielectric element is to be seated, comprises an inner surface of the end plate.

8. An apparatus according to any of claims 1 to 6, wherein the surface, around the periphery of the aperture, against which the outer region of the dielectric element is to be seated, comprises an outer surface of the end plate.

9. An apparatus according to any of claims 1 to 6, further comprising a spacing element, wherein the surface, around the periphery of the aperture, against which the outer region of the dielectric element is to be seated, comprises a surface of the spacing element that is located against an outer surface of the end plate.

10. An apparatus according to claim 9, wherein the spacing element is controllable to adjust a spacing between the outer region of the dielectric element and the outer surface of the end plate.

11. An apparatus according to claim 10, further comprising an actuator operative to control the spacing element to adjust the spacing between the outer region of the dielectric element and the outer surface of the end plate responsive to a dielectric element position control signal.

12. An apparatus according to any of the preceding claims, wherein an outer region of dielectric element comprises an annular flange, wherein a surface of the flange that faces the end plate comprises a mounting surface of the annular flange, further wherein the mounting surface is configured to engage the surface around a periphery of the aperture.

13. An apparatus according to claim 12, wherein the mounting surface is machined to provided an 0.8 micrometer roughness average (RA) surface finish.

14. An apparatus according to claim 12 or 13, further comprising an annular retaining element to hold the mounting surface against the surface of the end plate around a periphery of the aperture.15 . An apparatus according to any of claims 12 to 14, further comprising an O-ring seal located between the mounting surface and the surface of the end plate around a periphery of the aperture.

16. An apparatus according to claim 14, or claim 15, when dependent upon claim 14, further comprising an O-ring seal located between: (i) a surface of the outer region of the dielectric element that is on an opposite side of said annular flange to said mounting surface; and (ii) a surface of the annular retaining element that abuts the surface of the outer region of the dielectric element, as defined in (i).

17. An apparatus according to any of the preceding claims, further comprising a magnet array configured for location in the plasma containment space of the plasma chamber, the magnet array located toward, or against a surface of the control grid that faces the plasma containment space.

18. An apparatus according to claim 17, wherein the magnet array comprises an annular element comprising a plurality of annular sectors, each annular sector formed of a magnetic material and arranged so that a polarity thereof is opposite to neighbouring sectors on an alternate basis.

19. An apparatus according to claim 18, wherein each annular sector comprises a plurality of sub-sectors and further wherein each sub-sector is of a same polarity as other sub-sectors in the same sector, but of an opposite polarity to sub-sectors in neighbouring annular sectors.

20. An apparatus according to claim 18, wherein each annular sector comprises a plurality of sub-sectors and further wherein each sub-sector is of a polarity opposite to neighbouring sub-sectors in the same annular sector and to respective sub-sectors in neighbouring annular sectors.

21. An apparatus according to any of claims 18 to 20, wherein a dimension of an aperture of the annular element is sized so as be larger than an extraction area of the control grid.

22. An apparatus according to any of claims 17 to 21, wherein said magnet array comprises an array of rare earth magnets.

23. An apparatus according to claim 22, wherein said rare earth magnets comprise samarium-cobalt (SmCo).

24. An apparatus for ion beam etching a substrate, the apparatus comprising:a vacuum chamber comprising a gas evacuation system;a substrate holder, located in the vacuum chamber, the substrate holder configured to hold a substrate to be etched; andan ion beam source for directing ions into the vacuum chamber toward the substrate holder, the ion beam source comprising an apparatus according to any of claims 1 to 23.

25. A dielectric element for an apparatus according to any of claims 1 to 23, or for an apparatus according to claim 24, wherein the dielectric element is configured to close an aperture in an end plate of a plasma chamber, the dielectric element comprising an outer region configured to be seated against a surface, around a periphery of the aperture, and an inner region where material that forms the dielectric element extends away from the material of the dielectric element at the outer region to form a trough, wherein the dielectric element is mountable relative to the end plate so that the trough of the inner region extends into a plasma containment space within the plasma chamber.

26. A dielectric element according to 25, wherein the outer region of the dielectric element comprises an annular flange that forms a mounting surface configured to engage a surface around a periphery of the aperture.

27. A dielectric element according to claim 26, wherein the mounting surface is machined to provide an 0.8 micrometer roughness average (RA) surface finish.

28. A magnet array for an apparatus according to any of claims 1 to 16, wherein the magnet array is configured for location in the plasma containment space of the plasma chamber, themagnet array located toward, or against a surface of the control grid that faces the plasma containment space.

29. A magnet array according to claim 28, wherein the magnet array comprises an annular element comprising a plurality of annular sectors, each annular sector formed of a magnetic material and arranged so that a polarity thereof is opposite to neighbouring sectors on an alternate basis.

30. A magnet array according to claim 29, wherein each annular sector comprises a plurality of sub-sectors and further wherein each sub-sector is of a same polarity as other subsectors in the same sector, but of an opposite polarity to sub-sectors in neighbouring annular sectors.

31. A magnet array according to claim 29, wherein each annular sector comprises a plurality of sub-sectors and further wherein each sub-sector is of a polarity opposite to neighbouring sub-sectors in the same annular sector and to respective sub-sectors in neighbouring annular sectors.

32. A magnet array according to any of claims 29 to 31, wherein a dimension of an aperture of the annular element is sized so as be larger than an extraction area of the control grid.

33. A magnet array according to any of claims 28 to 32, wherein said magnet array comprises an array of rare earth magnets.

34. A magnet array according to claim 33, wherein said rare earth magnets comprise samarium-cobalt (SmCo).

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