Apparatus for producing an ion beam

Abstract

The present invention provides an apparatus for producing an ion beam. The apparatus comprises a plasma chamber. The plasma chamber comprises: 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 and wherein the plasma containment space is evacuable by a pump to lower the pressure in the plasma containment space. The dielectric element is of a cylindrical shape, closed at a first end in a dome, and 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. The dielectric element is mounted relative to the end plate so that the trough of the inner region extends into the plasma containment space. The apparatus further comprises an RF induction device for inductively generating a plasma in the plasma chamber, the RF induction device located on an opposite side of the dielectric element from the plasma containment space. The apparatus further comprises a protective cover, which is formed of a dielectric material, located inside the plasma containment space around the dielectric element, so as to block material from inside the plasma containment space from being deposited on the dielectric element, wherein the protective cover is configured to conform with the profile of the dielectric element, wherein the protective cover comprises a first, cylindrical section, and a second, domed section, wherein the first section is configured to surround a respective cylindrical section of the dielectric element and wherein the second section is configured to surround a respective domed section of the dielectric element, and further wherein the first section and second section of the protective cover comprise discrete, separate parts and include at least one fixing element to couple the sections together.

Description

[0001] APPARATUS FOR PRODUCING AN ION BEAM

[0002] Technical Field

[0003] The present invention relates to an apparatus for producing an ion beam, to an apparatus for ion beam etching a substrate incorporating same, and to a protective cover for protecting a dielectric element of the apparatus for producing an ion beam.

[0004] Background

[0005] 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.

[0006] 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.

[0007] 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.

[0008] 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.

[0009] 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.

[0010] 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.

[0011] 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.

[0012] 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.

[0013] 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).

[0014] 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.

[0015] 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.

[0016] 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.

[0017] 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.

[0018] 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.

[0019] 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., it can be the walls of a jar or tube, it can be a flat plate or it can be the walls of a pipe.

[0020] In the known ion beam sources discussed above, a chamber of the plasma source, i.e., where the plasma is formed, is evacuated by a pump to create a low pressure in the chamber. A suitable type of pump is a turbomolecular pump. Such a turbomolecular pump may have a turbine of over 300 mm in diameter operative to rotate at around 27,000 rpm. Another suitable type of pump is a cryogenic pump. Where Oxygen is used as a process gas, either a turbomolecular pump, or a combination of a turbomolecular and a single stage cryogenic pump are employed, for safety reasons. Cryogenic pumps routinely have two stages; one that pumps non-permanent gases (set between 70 and 110 K) and a second stage (set between 10 and ~20 K). A single stage pump cools to ~ 100 K. The latter will not condense Ozone, eliminating the risk of detonation upon regeneration where a dual stage version is used.

[0021] A known apparatus for producing an ion beam is illustrated in Fig. 1. 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.

[0022] 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.

[0023] The apparatus 10 comprises a pump (not shown) that is operative to evacuate the plasma containment space 16. This is so as to achieve a low pressure condition within the plasma containment space 16. Atypical operating pressure range within the plasma containment space is around 0.0133 Pa to around 0.1333 Pa (around 1x1 O'4torr to around lxl0'3torr). For context, atmospheric pressure (i.e. 1 atm) is 101,325 Pa (or 760 Torr).

[0024] An ion beam 22 issues from the lower end of the apparatus 10 and comprises a plurality of beamlets 240 that coalesce downstream of the apparatus to form the broad ion beam 22.

[0025] The end plate 20 comprises a 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.

[0026] 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.

[0027] 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.

[0028] Optional 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.

[0029] 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.

[0030] 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 240 from each of the holes in the accelerator assembly 15. These beamlets 240 coalesce into a broad ion beam 22 downstream.

[0031] 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.

[0032] 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.

[0033] In such an apparatus, as described above, it is known that, over time, deposits can buildup on the dielectric element 24. These deposits, which may arise from: (i) the plasma production process; and / or (ii) processing of a substrate, or process sample in a process chamber into which the ion beam is directed, can shield the plasma processing space (i.e., the plasma containment space) from the RF coil and may prevent the coupling of the RF energy into the plasma. Accumulation of the deposits can require the dielectric element to be cleaned on a frequent basis. This can result in additional interruptions in the productive use of the apparatus. This is undesirable in that it reduces productivity and increases the cost of maintaining the machine and of the products produced. Additionally, deposition onto the window may increase the thermal load on the dielectric element, which can reduce the life of the dielectric element. Failure of the dielectric element results in process interruption and part damage, requiring its replacement.

[0034] Deposits formed by way of the plasma production process tend to comprise material sputtered from within the plasma containment space. Deposits formed by way of the processing of a substrate, or process sample in the process chamber can arise from material that has been sputtered within the process chamber and that enters the plasma containment space through the apertures of the accelerator assembly 14. In the first instance the material may comprise substances from which the walls and accelerator are formed. In the second instance, the material that enters the source from the process chamber may comprise the materials sputtered from a substrate or process sample. These may be metals and / or dielectric materials and may include organic materials from a patterning mask. In addition, there may be deposits from materials that form the chamber shielding, substrate clamp(s) and shutter (where fitted). To mitigate the above-described issue, such apparatus may be provided with a protective cover, which is located inside the plasma containment space adjacent to the dielectric element. The protective cover is disposed relative to the dielectric element so as to intercept material from the plasma production process that would otherwise end up as deposits on the dielectric element. Thus, instead of being deposited on an inside surface of the dielectric element, i.e., the surface that faces the plasma containment space, the material is deposited on the protective cover. When deposits have built-up on the protective cover, it can simply be removed from the apparatus and replaced with a replacement protective cover. The removed replacement cover can be taken to a remote location for cleaning. Such a replacement process may be a lot quicker than that of cleaning an unprotected dielectric element.

[0035] Whilst apparatus that include such a protective cover for the dielectric element have been satisfactory, and may continue to be satisfactory for certain types of apparatus where the dielectric element comprises a flat (or planar) element, the inventors have recognised that it would be desirable to be able to employ similar dielectric element protective measures for types of apparatus where the dielectric element is not flat (or planar), e.g., in cases where the dielectric element includes a hemispherical, or domed portion.

[0036] The present invention has been devised with the foregoing considerations in mind.

[0037] Summary

[0038] 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. The apparatus further comprises 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 and wherein the plasma containment space is evacuable by a pump to lower the pressure in the plasma containment space; further wherein the dielectric element is of a cylindrical shape, closed at a first end in a dome, and 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. The apparatus further comprises an RF induction device for inductively generating a plasma in the plasma chamber, the RF induction device located on an opposite side of the dielectric element from the plasma containment space. The apparatus yet further comprises a protective cover, which is formed of a dielectric material, located inside the plasma containment space around the dielectric element, so as to block material from inside the plasma containment space from being deposited on the dielectric element, wherein the protective cover is configured to conform with the profile of the dielectric element. Further, the protective cover comprises a first, cylindrical section, and a second, domed section, wherein the first section is configured to surround a respective cylindrical section of the dielectric element and wherein the second section is configured to surround a respective domed section of the dielectric element, and further wherein the first section and second section of the protective cover comprise discrete, separate parts and include at least one fixing element to couple the sections together.

[0039] Positioning the protective cover relative to the dielectric element in this manner in the plasma containment space provides a means for intercepting material from a plasma production process taking place in the plasma containment space that would otherwise end up as deposits on the dielectric element. Thus, instead of being deposited on an inside surface of the dielectric element, i.e., the surface that faces the plasma containment space, the material is deposited on the protective cover.

[0040] The protective cover is designed to be removable. Therefore, when deposits have built- up on the protective cover, it can be removed from the apparatus and replaced with a replacement protective cover. The removed replacement cover can be cleaned for re-use, or disposed of, as appropriate. The removable nature of the protective cover may avoid the need for cleaning in-situ.

[0041] A possible benefit of a protective cover with a form that is configured to follow the form of the domed dielectric element is that it can be used in apparatuses having dielectric elements with such a domed form.

[0042] The inventors have recognised that it may be difficult to manufacture a single unit protective cover with walls that are both: (i) thin enough so as not to attenuate the RF signal from the RF coils to such a degree that it impacts the efficiency of the process; and (ii) thick enough so as not to be too fragile. To counter these competing requirements, the present invention comprises, as mentioned above, two separate parts. Such a two-part cover may be easier to manufacture than a single-piece unit and may be easier to maintain. Additionally, providing the cover in two parts may mean that a cap section can be removed (e.g., for cleaning, or replacement, etc.), but a separate cylindrical section can remain in-situ around the dielectric element. Additionally, build-up of deposits may be different on the different sections, e.g., deposits on the cap section may build-up much more quickly than on the cylindrical section. Therefore, the cap may require cleaning more frequently than the side walls (i.e., the cylindrical section). The two part configuration for the protective cover may allow for different cleaning cycles for the two different parts.

[0043] Optionally, a cross-sectional profile of the dielectric element may be a bell-shape.

[0044] Optionally, a cross-sectional profile of the dielectric element may be a U-shape.

[0045] Optionally, a diameter dimension of a space between inner surfaces of walls forming said first section is a close tolerance to a distance between external surfaces of walls forming the cylindrical section of the dielectric element to allow a push-fit connection between protective cover and the dielectric element.

[0046] Optionally, a diameter dimension of a space between inner surfaces of walls forming said first section may be greater than a distance between external surfaces of walls forming the cylindrical section of the dielectric element such that there is an annular space between the inner surface of the walls of the cylindrical section of the protective cover and the external surface of walls of the cylindrical section of the dielectric element.

[0047] Optionally, the at least one fixing element may comprise a nut and a bolt. Further optionally, the nut and bolt may be formed of alumina.

[0048] 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 an apparatus for producing an ion beam as described above and hereinafter.

[0049] According to a further aspect of the present invention, there is provided a protective cover for an apparatus for producing an ion beam as described above and hereinafter, or for an apparatus for ion beam etching a substrate as described above or hereinafter, wherein the protective cover is formed of a dielectric material, locatable inside the plasma containment space around the dielectric element, so as to block material from inside the plasma containment space from being deposited on the dielectric element, wherein the protective cover is configured to conform with a profile of the dielectric element, wherein the protective cover comprises a first, cylindrical section, and a second, domed section, wherein the first section is configured to surround a respective cylindrical section of the dielectric element and wherein the second section is configured to surround a respective domed section of the dielectric element, and further wherein the first section and second section of the protective cover comprise discrete, separate parts and include at least one fixing element to couple the sections together.

[0050] Optionally, a diameter dimension of a space between inner surfaces of walls forming said first section may be a close tolerance to a distance between external surfaces of walls forming the cylindrical section of the dielectric element to allow a push-fit connection between protective cover and the dielectric element.

[0051] Optionally, a diameter dimension of a space between inner surfaces of walls forming said first section may be greater than a distance between external surfaces of walls forming the cylindrical section of the dielectric element such that there is an annular space between the inner surface of the walls of the cylindrical section of the protective cover and the external surface of walls of the cylindrical section of the dielectric element.

[0052] Optionally, the at least one fixing element comprises a nut and a bolt. Further optionally, the nut and bolt may be formed of alumina.

[0053] Brief Description of the Drawings

[0054] Fig. 1 illustrates a cut-away schematic side view of a known apparatus for producing an ion beam.

[0055] 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:

[0056] Fig. 2 illustrates a cut-away side view of an apparatus for producing an ion beam according to one or more embodiments of the present invention;

[0057] Fig. 3 illustrates a cut-away side view of a dielectric element and a protective cover of the apparatus illustrated in Fig. 2; Fig. 4a illustrate a cut-away side view of a dielectric element and a protective cover of the apparatus illustrated in Fig. 2 in an optional arrangement;

[0058] Fig. 4b illustrates a perspective view of the dielectric element and the protective cover illustrated in Fig. 4a;

[0059] Fig. 5a illustrates a cut-away side view of a domed section of the protective cover illustrated in Figs. 4a and 4b;

[0060] Fig. 5b illustrates a cross-sectional view of the domed section of the protective cover illustrated in Fig. 5a;

[0061] Fig. 5c illustrates a perspective view of the domed section of the protective cover illustrated in Figs. 5a and 5b; and

[0062] Fig. 6 illustrates a cut-away perspective view of the apparatus illustrated in Fig. 2 with the protective cover of the optional arrangement as illustrated in Figs. 4a, 4b and 5a to 5c.

[0063] Detailed Description of the Invention

[0064] Referring now to the drawings, Fig. 2 illustrates an apparatus 110 for producing an ion beam, which comprises a plasma source 112 and an accelerator assembly 114. Features similar to those illustrated in Fig. 1 are also illustrated in Fig. 2. However, in Fig. 2, the features common with those of Fig. 1 are now designated with reference numerals of the type 1XX rather than XX. Thus, in Figs. 2, 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.

[0065] The dielectric element 124 is mounted relative to the end plate 120 so as to cover an aperture in the end plate 120.

[0066] In the illustrated embodiment, 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 124a 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 into the plasma containment space 116.

[0067] 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 inner surface of end plate 120, which is configured to engage the inner surface of the end plate 120 around a periphery of the aperture. The mounting surface may be machined to provide an 0.8 micrometer roughness average (RA) surface finish, to aid the sealing fit with the surface of the end plate 120.

[0068] In such an arrangement, the mounting surface of the dielectric element 124 is mounted against an inner surface of the end plate 120. An annular retaining element 133 is used to secure the dielectric element 124 against the inner surface of the end plate 120.

[0069] As can be seen in Fig. 2, 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 124, 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 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.

[0070] O-ring seals (not shown) are provided between the inner facing surface of the outer region 124a and the outer facing surface of annular retaining element 133, 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 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 133. A second O-ring seal 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.

[0071] The O-ring seals 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.

[0072] In the apparatus 110 illustrated in Fig. 2, the RF coil 128 is located so as to be on an opposite side of the dielectric element 124 to the side that faces the plasma containment space 116. 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.

[0073] The electromagnetic coils 132, which are provided in proximity to the dielectric element 124 and RF coil 128 and around the second end (accelerator end) of the vessel 18, may be controlled to tune the plasma homogeneity / efficiency. These coils 132 are optional and may be excluded in some embodiments.

[0074] Referring still to Fig. 2, but also referring to Fig. 3, the apparatus 110 further includes a protective cover 134. The protective cover 134 is located in the plasma containment space 116 and is disposed relative to the dielectric element 124 so as to surround the dielectric element 124. In the illustrated embodiment, the protective cover 134 is of a similar form to that of the dielectric element 124, i.e., a bell-shaped, or U-shaped jar that has a cylindrical section 134a comprising straight, parallel side walls and a closed end section comprising a domed, or hemispherical cap 134b. In the illustrated embodiment, the cylindrical section and the cap section are a single, integral unit, i.e., the walls of the cylindrical section are continuous with the walls of the cap section.

[0075] The dimension of an inner diameter of the protective cover 134 is larger than the dimension of an outer diameter of the dielectric element 124. That is, the diameter dimension of a space between inner surfaces of walls forming the cylindrical section 134a is greater than a distance between external surfaces of walls forming the cylindrical section of the dielectric element 124. Configuring the respective dimensions of the protective cover 134 and of the dielectric element 124 in this manner may allow the protective cover 134 to form a sleeve around the dielectric element 124.

[0076] Positioning the protective cover 134 relative to the dielectric element 124 in this manner in the plasma containment space 116 provides a means for intercepting material from a plasma production process taking place in the plasma containment space 116 that would otherwise end up as deposits on the dielectric element 124. Thus, instead of being deposited on an inside surface of the dielectric element 124, i.e., the surface that faces the plasma containment space 116, the material is deposited on the protective cover 134.

[0077] The protective cover 134 is designed to be removable. Therefore, when deposits have built-up on the protective cover 134, it can be removed from the apparatus 110 and replaced with a replacement protective cover. The removed replacement cover can be cleaned for reuse, or disposed of, as appropriate. The removable nature of the protective cover 134 may avoid the need for cleaning in-situ.

[0078] A benefit of a protective cover 134 with the form as illustrated is that it can be used in apparatuses having dielectric elements with the form as illustrated. The benefits of an apparatus having a dielectric element with such are form are described in the applicant’s earlier application (UK patent application no. GB 2317557.3), namely, for example, increasing a “useable” part of an ion beam produced by the apparatus.

[0079] Figs. 4a, 4b and 5a to 5c illustrate an alternative embodiment of the protective cover. In these figures, the protective cover is denoted by reference numeral 1340.

[0080] The protective cover 1340 of this alternative embodiment differs from that described above (in relation to Figs. 2 and 3) in that it comprises two, separate parts, as opposed to being a single unit.

[0081] The inventors have recognised that it may be difficult to manufacture a single unit protective cover with walls that are both: (i) thin enough so as not to attenuate the RF signal from the RF coils to such a degree that it impacts the efficiency of the process; and (ii) thick enough so as not to be too fragile.

[0082] To counter these competing requirements, the arrangement comprises, as mentioned above, two separate parts. Such a two-part cover may be easier to manufacture than the singlepiece unit and may be easier to maintain. Additionally, providing the cover in two parts may mean that a cap section can be removed (e.g., for cleaning, or replacement, etc.), but a separate cylindrical section can remain in-situ around the dielectric element.

[0083] The two parts comprise a cylindrical section 1342 and a cap section 1344. As with the previously described embodiment, the cylindrical section 1342 is dimensioned so that an inner diameter of the cylindrical section 1342, i.e., the diameter between inner surfaces of the walls of the cylindrical section, is greater than an outer diameter of the walls forming the cylindrical section of the dielectric element 124. This allows the cylindrical section 1342 to surround the cylindrical section of the dielectric element 124 like a sleeve. Although there may, in some embodiments, be contact between the outer surface of the cylindrical section of the dielectric element 124 and the inner surface of the cylindrical section 1342, optionally there may be a small, annular space between the walls of the parts. In some arrangements, this space may be of the order of between, for example, 0.5 and 1 mm. The provision of the space between the dielectric element 124 and the protective cover 1340 may inhibit issues that arise from potentially different rates of thermal expansion / contraction of the protective cover 1340 relative to the dielectric element 124. Such thermal expansion / contraction may occur, because during plasma production the protective cover 1340 will be in contact with the plasma. During this time, it is likely that the protective cover 1340 will be hotter than the dielectric element 124. This may lead to there being a relaxed fit between the protective cover 1340 and the dielectric element 124. However, when the plasma production process is halted, the parts will cool. They may contract upon cooling, but possibly at different rates, which may lead to the protective cover 1340 putting stress on the dielectric element 124. Therefore, the space between the protective cover 1340 and dielectric element 124 may be helpful in mitigating against such an issue.

[0084] The cap section 1344 is formed to follow the contour of the domed, or hemispherical section of the dielectric element 124, but need not be a close fit, i.e., the curvature of the cap section 1344 may differ from that of the domed, or hemispherical section of the dielectric element 124 (and, indeed, does so in the illustrated embodiment in Figs. 4a, 4b, 5a to 5c and 6).

[0085] As can be seen, particularly in Figs. 4a and 6, end portions of the walls of the cap section 1344 overlap with end portions of the walls of the cylindrical section 1342.

[0086] The cap section 1344 can be coupled to the cylindrical section 1342 by at least one suitable non-metallic coupling element. In one example, the at least one coupling element may comprise a nut and bolt, which may, optionally, be formed from alumina. When the cap section 1344 is located over an open end of the cylindrical section 1342, to close the open end of the cylindrical section, respective end portions of the walls of the cap section 1344 and cylindrical section 1342 overlap. Apertures 1346 formed in the respective end portions of the walls of the cap section 1344 and cylindrical section 1342 are aligned to allow the bolt(s) to pass therethrough.

[0087] Fig. 6 illustrates the apparatus 110 with the protective cover 1340 of the optional arrangement. In Fig. 6, the accelerator assembly 114 is not shown (to aid understanding and to avoid overcomplicating the figure). Any references made herein to orientation (e.g. top, bottom, upper, lower, front, back, rear, inner and outer) 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.

[0088] As used herein, the terms “comprises,” “comprising,” “includes,” “including,” “has,” “having” or any other variation thereof, are intended to cover a non-exclusive 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).

[0089] 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.

[0090] 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 particular material may also be utilized in embodiments of the invention.

[0091] 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 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

CLAIMS1. 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 and wherein the plasma containment space is evacuable by a pump to lower the pressure in the plasma containment space; further wherein the dielectric element is of a cylindrical shape, closed at a first end in a dome, and 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; an RF induction device for inductively generating a plasma in the plasma chamber, the RF induction device located on an opposite side of the dielectric element from the plasma containment space; the apparatus further comprising a protective cover, which is formed of a dielectric material, located inside the plasma containment space around the dielectric element, so as to block material from inside the plasma containment space from being deposited on the dielectric element, wherein the protective cover is configured to conform with the profile of the dielectric element, wherein the protective cover comprises a first, cylindrical section, and a second, domed section, wherein the first section is configured to surround a respective cylindrical section of the dielectric element and wherein the second section is configured to surround a respective domed section of the dielectric element, and further wherein the first section and second section of the protective cover comprise discrete, separate parts and include at least one fixing element to couple the sections together.

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

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

4. An apparatus of any of the preceding claim , wherein a diameter dimension of a space between inner surfaces of walls forming said first section is a close tolerance to a distance between external surfaces of walls forming the cylindrical section of the dielectric element to allow a push-fit connection between protective cover and the dielectric element.

5. An apparatus of any of claims 1 to 3, wherein a diameter dimension of a space between inner surfaces of walls forming said first section is greater than a distance between external surfaces of walls forming the cylindrical section of the dielectric element such that there is an annular space between the inner surface of the walls of the cylindrical section of the protective cover and the external surface of walls of the cylindrical section of the dielectric element.

6. An apparatus according to claim any of the preceding claims, wherein the at least one fixing element comprises a nut and a bolt.

7. An apparatus according to claim 6, wherein the nut and bolt are formed of alumina.

8. 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 an apparatus according to any of claims 1 to 7.

9. A protective cover for an apparatus for producing an ion beam according to any of claims 1 to 7, or for an apparatus for ion beam etching a substrate according to claim 8, wherein the protective cover is formed of a dielectric material, locatable inside the plasma containmentspace around the dielectric element, so as to block material from inside the plasma containment space from being deposited on the dielectric element, wherein the protective cover is configured to conform with a profile of the dielectric element, wherein the protective cover comprises a first, cylindrical section, and a second, domed section, wherein the first section is configured to surround a respective cylindrical section of the dielectric element and wherein the second section is configured to surround a respective domed section of the dielectric element, and further wherein the first section and second section of the protective cover comprise discrete, separate parts and include at least one fixing element to couple the sections together.

10. A protective cover according to claim 9, wherein a diameter dimension of a space between inner surfaces of walls forming said first section is a close tolerance to a distance between external surfaces of walls forming the cylindrical section of the dielectric element to allow a push-fit connection between protective cover and the dielectric element.

11. A protective cover according to claim 9, wherein a diameter dimension of a space between inner surfaces of walls forming said first section is greater than a distance between external surfaces of walls forming the cylindrical section of the dielectric element such that there is an annular space between the inner surface of the walls of the cylindrical section of the protective cover and the external surface of walls of the cylindrical section of the dielectric element.

12. A protective cover according to any of claims 9 to 11, wherein the at least one fixing element comprises a nut and a bolt.

13. A protective cover according to claim 12, wherein the nut and bolt are formed of alumina.

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