Apparatus

Abstract

An inductively coupled plasma apparatus 110 for producing an ion beam comprises a plasma chamber having an open ended body 118, an end plate 120 having an aperture, a control grid 114 for extracting i

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, and to a sealing element for 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 IILV 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., 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. 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. 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. 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. 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 i s around 0.013 3 Pa to around 0.13 3 3 Pa (around 1 x 1 O'4 torr to around 1 x 1 O'3 torr). For context, atmospheric pressure (i.e. 1 atm) is 101,325 Pa (or 760 Torr). 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 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. 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. 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. 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. 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. The inventors have recognised that it would be desirable to mitigate potential damage to the pump (that serves to evacuate the plasma containment space) in situations where the dielectric element fails and the plasma containment space becomes open to atmospheric pressure. 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. The apparatus further comprises a dielectric element disposed to close the aperture; wherein the open ended body, end plate, dielectric element and control grid serve to define a plasma containment space there between and wherein the plasma containment space is evacuable by a pump to lower the pressure in 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 sealing element coupled to an outer side of the end plate and arranged to cover the aperture, the dielectric element and at least antenna coils of the RF induction device. A space is defined between the sealing element, or cover, and the dielectric element. In the event of a fracture of the dielectric element, only a volume of air contained within the space within the cover, i.e. between the sealing element and the dielectric element, would be admitted to the plasma containment chamber. This is in contrast to the situation in prior art arrangements, i.e. without a sealing element, or cover, where if the dielectric element of the apparatus were to break, air from a space outside the apparatus (e.g. at ambient / atmospheric pressure) would rapidly rush into the plasma containment space. In such a prior art situation, this could cause catastrophic failure of the pump. In the apparatus of the present invention, on the other hand, a much smaller volume of air would be admitted to the plasma containment space, in the event of fracture of the dielectric element. While this still may have the potential to overburden the pump, since there is a much smaller volume within the space within the cover (compared to the volume of space outside the apparatus) then fracture of the dielectric element in this instance may be far less catastrophic than air at atmospheric pressure from the surroundings of the apparatus venting the plasma containment space (as in the prior art situation). Optionally, the sealing element may comprise a planar element. Optionally, the sealing element may comprise a housing, open at one end thereof, with walls that define the housing being configured at, or toward, the open end of the housing for attachment to the end plate. Further optionally, the housing may comprise a cylinder, open at one end thereof, with an annular flange at, or toward, the open end. Optionally, surfaces of the sealing element and the dielectric element that face each other may define there between a space. Further optionally, said antenna coils of the RF induction device may be moveable within the space. Optionally, the dielectric element may comprise 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. Further optionally, the dielectric element may be of a cylindrical shape, closed at a first end in a dome. Yet further optionally, a cross-sectional profile of the dielectric element may be a bell-shape. Still further optionally, a cross-sectional profile of the dielectric element may be a U-shape. Optionally, the dielectric element may comprise a planar element of dielectric material. 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. According to a further aspect of the present invention, there is provide a sealing element 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 sealing element is configured to be coupled to an outer side of an end plate of a plasma chamber and arranged to cover an aperture in the end plate, a dielectric element disposed to cover the aperture and at least antenna coils of an RF induction device. Optionally, the sealing element may comprise a planar element. Optionally, the sealing element may comprise an open ended housing, with walls that define the housing being configured at, or toward, an open end of the housing for attachment to the end plate. Further optionally, the open ended housing may comprise an open ended cylinder with an annular flange at, or toward, the open end. Brief Description of the Drawings Fig. 1 illustrates a cut-away schematic side view of a known apparatus for producing 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. 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; Fig. 3 illustrates a cut-away perspective view of the apparatus illustrated in Fig. 2; Fig. 4 illustrates a cut-away side view of the apparatus illustrated in Fig. 2 in another arrangement; Fig. 5 illustrates a cut-away perspective view of the apparatus illustrated in Fig. 4; Fig. 6a illustrates a cut-away side view of the apparatus of Fig. 2 to show gas pressures in and around the apparatus; Fig. 6b illustrates a cut-away side view of the apparatus of Fig. 4 to show gas pressures in and around the apparatus. Detailed Description of the Invention Referring now to the drawings, Figs. 2 and 3 illustrate 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 Figs. 2 and 3. However, in Figs. 2 and 3, 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 and 3, 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. Fig. 3 illustrates the apparatus 110 with the accelerator assembly 114 removed, to aid understanding and to avoid overcomplicating the figure. The dielectric element 124 is mounted relative to the end plate 120 so as to cover an aperture in the end plate 120. A periphery of the outer surface of the dielectric element 124 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. In such an arrangement, the mounting surface of the dielectric element 124 is mounted against an inner surface of the end plate. An annular retaining element 133 is used to secure the dielectric element 124 against the inner surface of the end plate 120. As can be seen in Figs. 2 and 3, a portion of the end plate 120 around the aperture therein, across which aperture the dielectric element 124 extends, 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 dielectric element 124, i.e. the mounting surface, is 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 dielectric element 124. This serves to hold the dielectric element 124 in position in the annular seat of the end plate 120. O-ring seals (not shown) are provided between the outer facing surface of the dielectric element 124 and the inner surface of the end plate 120, and between the inner facing surface of the dielectric element 124 and the inner surface of the annular retaining element 133. That is, a first O-ring seal is provided at the interface between the outer facing surface of the dielectric element 124 and the inner surface of the end plate 120. A second O-ring seal is provided at the interface between the inner facing surface of the dielectric element 124 and the inner surface of the annular retaining element 133. These O-ring seals may serve to inhibit fluid movement around the edges of the dielectric element 124 at the interfaces between the dielectric element and the end plate 120 and / or the annular retaining element 120. The O-ring seals may also provide soft seats 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 apparatus 110 illustrated in Figs. 2 and 3, 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. The electromagnetic coils 132, which are provided in proximity to the dielectric element 24 and RF coil 28 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. The apparatus 110 differs from that described above in relation to Fig. 1 in that it further comprises a cover 134 located on an outer side of the end plate 120. The cover 134 is coupled to the outer side of the end plate 120 and is disposed so as to cover the aperture, the dielectric element 124 and at least antenna coils of the RF coil 128. The cover 134 may be coupled to the outer side of the end plate 120 by suitable fixing elements, e.g. screws, bolts, hinge arrangement, etc. The cover 134 comprises an open ended cylinder in the embodiment illustrated in Figs. 2 and 3. The space defined within the cylinder of the cover 134 should be such that the antenna coils of the RF coil 128 can be housed therein. However, the overall shape of the cover 134 is not important, merely that the space defined between its walls is suitable for housing the coils of the RF coil 128. In one arrangement, the cover 134 may comprises an open ended cylinder with an annular flange at, or toward, the open end thereof (e.g. a “top-hat” shape), with the annular flange being configured as a coupling surface to be coupled against the end plate 120. The cover 134 is configured such that, when located against the end plate 120, an enclosed space is defined between the walls of the cover 134 and the portions of the end plate 120 and dielectric element 124 that lie between the side walls of the cover 134. The cover 134 is further configured such that, when located against the end plate 120 in the manner described above (and as illustrated), a seal is formed at the interface between the side walls of the cover 134 and the outer surface of the end plate 120, against which those end walls are located. This serves to define a sealed space within the cover 134. With reference to Fig. 6a, when the apparatus 110 is in use, the plasma containment space 116 will be at a low pressure Pl (i.e. a pressure suitable to allow a plasma to form within the plasma containment space). Outside the apparatus 110, the pressure will be an ambient pressure P3 (e.g. atmospheric pressure). In the space defined between the cover 134 and the dielectric element 124, the pressure P2 therein will be a pressure that is somewhere between Pl and P3. In some arrangements, pressure P2 will be equal to, or around pressure P3, but in other arrangements the relative pressures may be, for example: • Pl<P2<P3;or • Pl«P2«P3;or • Pl<P2«P3;or • P1«P2<P3. Regardless of the relative pressures, the plasma containment space 116 of the apparatus 110 is evacuated by a pump (optionally a turbomolecular pump, further optionally a turbomolecular pump with a turbine over 300 mm in diameter and operative to rotate at 27,000 rpm). In a prior art arrangement, i.e. without the cover 134, if the dielectric element 124 of the apparatus 110 were to break, air from a space outside the apparatus 110 (e.g. at ambient / atmospheric pressure) would rapidly rush into the plasma containment space 116. This could cause catastrophic failure of the pump. However, in the apparatus 110 of the present invention, in the event of a fracture of the dielectric element 124, only the volume of air contained within the space within the cover 134 would be admitted to the plasma containment chamber 116. While this still may have the potential to overburden the pump, since there is a much smaller volume within the space within the cover 134 (compared to the volume of space outside the apparatus 110) then fracture of the dielectric element 124 in this instance may be far less catastrophic than air at atmospheric pressure from the surroundings of the apparatus 110 venting the plasma containment space. Figs. 4 and 5 schematically illustrate a cut-away side view and a cut-away perspective view respectively of the apparatus illustrated in Figs. 2 and 3 in another arrangement. Features similar to those illustrated in Figs. 2 and 3 are also illustrated in Figs. 4 and 5. Fig. 5 illustrates the apparatus 1100 with the accelerator assembly 114 removed, to aid understanding and to avoid overcomplicating the figure. The apparatus 1100 illustrated in Figs. 4 and 5 is substantially the same as that illustrated in Figs. 2 and 3, but the dielectric element 124 is replaced with a different dielectric element 1240. In the apparatus 110 of Figs. 4 and 5, dielectric element 1240 comprises a bellshaped jar, which comprises an outer region 1240a 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 1240a comprises an annular flange. The outer region 1240a 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 1240b where material that forms the dielectric element extends away from the material of the dielectric element at the outer region 1240a and terminates with a curved, or domed region to close-off the bell-shaped jar. The inner region 1240b forms a “trough”. The dielectric element 1240 is mounted relative to the end plate 120 so that the trough of the inner region 1240b extends 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 provide an 0.8 micrometer roughness average (RA) surface finish, to aid the sealing fit with the surface of the end plate 120. As can be seen in Figs. 4 and 5, 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 1240a of dielectric element 1240, 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 1240a of the dielectric element. This serves to hold the outer region 1240a of the dielectric element 124 in position in the annular seat of the end plate 120. O-ring seals (not shown) a provided between the inner facing surface of the outer region 1240a and the outer facing surface of annular retaining element 1330, and between the outer facing surface of the outer region 1240a 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 1240a of dielectric element 1240 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 1240a of dielectric element 124 and the inner surface of the end plate 120. 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. Another difference of the apparatus 1100 illustrated in Figs. 4 and 5 from that of Figs. 2 and 3 is the cover. In the arrangement illustrated in Figs. 4 and 5, the cover 1340 comprises a planar, or flat element. In this arrangement, the volume in which the coils of the RF coil 128 are located is defined primarily by the walls of the dielectric element 1240, with the cover 1340 serving to close an open end of the dielectric element 1240. The cover 1340 in this arrangement, may be coupled to the end plate 120 in the same manner as the cover 134 described earlier (in relation to Figs. 2 and 3). The cover 1340 is configured such that, when located against the end plate 120, an enclosed space is defined between the cover 1340 and the dielectric element 1240. Therefore, in the apparatus 1100 illustrated in Figs. 4 and 5, the RF coil 128 is located so as to be within the trough defined between the walls of the inner region 1240b of the dielectric element 1240. 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 1240. Similar to the arrangement illustrated in Fig. 6a, Fig. 6b illustrates the apparatus 1100, of Figs. 4 and 5 in use. The plasma containment space 116 will be at a low pressure Pl (i.e. a pressure suitable to allow a plasma to form within the plasma containment space). Outside the apparatus 1100, the pressure will be an ambient pressure P3 (e.g. atmospheric pressure). In the space defined between the cover 1340 and the dielectric element 1240, the pressure P2 therein will be a pressure that is somewhere between Pl and P3. In some arrangements, pressure P2 will be equal to, or around pressure P3, but in other arrangements the relative pressures may be, for example: • Pl<P2<P3;or • P1<<P2<<P31 or • Pl <P2«P3; or • Pl«P2<p3. In some embodiments, values of Pl, P2 and P3 may be as follows: • Pl - between around 0.0133 Pa to around 0.1333 Pa (around lxl0‘4 torr to around 1x10‘3 torr); • P2 - between around 6700 Pa to around 67,000 Pa (around 50 torr to around 500 torr). Preferably around 26,664 Pa (around 200 torr); • P3 - around atmospheric pressure (-101.325 kPa (760 Torr)). 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. 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). 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. 5 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. 10 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 15 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 there between and wherein the plasma containment space is evacuable by a pump to lower the pressure in 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 sealing element coupled to an outer side of the end plate and arranged to cover the aperture, the dielectric element and at least antenna coils of the RF induction device.

2. An apparatus according to claim 1, wherein the sealing element comprises a planar element.

3. An apparatus according claim 1, wherein the sealing element comprises a housing, open at one end thereof, with walls that define the housing being configured at, or toward, the open end of the housing for attachment to the end plate.

4. An apparatus according to claim 3, wherein the housing comprises a cylinder, open at one end thereof, with an annular flange at, or toward, the open end.

5. An apparatus according to any preceding claim, wherein surfaces of the sealing element and the dielectric element that face each other define there between a space.

6. An apparatus according to claim 5, wherein said antenna coils of the RF induction device are moveable within the space.

7. An apparatus according to any preceding claim, 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.

8. An apparatus according to claim 7, wherein the dielectric element is of a cylindrical shape, closed at a first end in a dome.

9. An apparatus according to claim 7 or 8, wherein a cross-sectional profile of the dielectric element is a bell-shape.

10. An apparatus according to any of claims 7 to 9, wherein a cross-sectional profile of the dielectric element is a U-shape.

11. An apparatus according to any of claims 3 to 6, wherein the dielectric element comprises a planar element of dielectric material.

12. 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 11.

13. A sealing element for an apparatus according to any of claims 1 to 11, or for an apparatus according to claim 12, wherein the sealing element is configured to be coupled to an outer side of an end plate of a plasma chamber and arranged to cover an aperture in the end plate, a dielectric element disposed to cover the aperture and at least antenna coils of an RF induction device.

14. A sealing element according to claim 13, wherein the sealing element comprises a planar element.

15. A sealing element according claim 13, wherein the sealing element comprises an open ended housing, with walls that define the housing being configured at, or toward, an open end of the housing for attachment to the end plate.

16. A sealing element according to claim 15, wherein the open ended housing comprises an open ended cylinder with an annular flange at, or toward, the open end.Application No: GB2407160.7Examiner: Dr Joanna LeeClaims searched: 1-16Date of search: 10 October 2024Patents Act 1977: Search Report under Section 17Documents considered to be relevant:Category Relevant to claims Identity of document and passage or figure of particular relevance Y 1, 2 &5- 14 US 2014 / 0150975 Al (EBE et al.) See especially paragraphs 0035 &0045, and figures 1A &4A Y 1,2&5- 14 US 2015 / 0053553 Al (ANDO et al.) See especially paragraphs 0118-120 , and figures 4, 5, 18 & 23 Y 1, 2, 5, 6 &11-14 JP 2012049065 A (NISSIN ELECTRIC CO LTD) See especially paragraphs 0036-0040, 0044, 0049 &0052, and figures 2-7 1,3-13, 15 &16 US 6462483 Bl (JENG et al.) See especially column 4 lines 49-60, and column 5 lines figures 1-3 Y 1 at least US 2009 / 0189083 Al (GODYAK) See especially paragraphs 0002, 0046-0048, and figures 2 & 3 Y 1 at least US 2009 / 0250340 Al (SASAKI et al.) See especially paragraphs 0001-0002 &0088-0105, and figure 9Categories:X Document indicating lack of novelty or inventive step A Document indicating technological background and / or state of the art. Y Document indicating lack of inventive step if combined with one or more other documents of same category. P Document published on or after the declared priority date but before the filing date of this invention. & Member of the same patent family E Patent document published on or after, but with priority date earlier than, the filing date of this application.Field of Search:International Classification:Subclass Subgroup Valid From H01J 0037 / 32 01 / 01 / 2006 HOU 0037 / 08 01 / 01 / 2006 rlU jxl 0001 / 46 01 / 01 / 2006

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