Abstract:
A low energy ion gun for ion beam processing. The ion gun includes a plasma chamber having an open ended, conductive, non-magnetic body, a first end of which is closed by a flat or minimally dished dielectric member and with electrodes at a second end thereof opposite the first end. The ion gun also has primary magnets arranged around the body for trapping electrons adjacent the wall of the plasma chamber in use of the ion gun and an r.f. induction device. The electrodes include multi-aperture grids arranged for connection to respective positive potential sources and positioned to contact the plasma in the plasma chamber. The apertures of the grids are aligned so that particles emerging from an aperture of a first one of the grids are accelerated through corresponding apertures of the other grids in the form of a beamlet. A plurality of beamlets forms a beam.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to an apparatus for generating a beam of charged particles, particularly to an ion gun for use in an ion beam processing apparatus and to an ion beam processing apparatus incorporating same. 
     Ion beams have been used for many years in the production of components in the micro-electronics industry and magnetic thin film devices in the storage media industry. Typically, an ion beam, such as an argon ion beam, is required to have a large area, a high current and an energy of between 100 eV and 2 keV. The beam can be used in a number of ways to modify the surface of a substrate, for example by sputter deposition, sputter etching, milling, or implantation. 
     In a typical ion beam source (or ion gun) 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 gun by random thermal motion. The plasma will thus exhibit positive plasma potential which 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 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. 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, milling or implantation is to be practised the substrate is placed in the path of the ion beam. 
     Hence, in a typical ion gun an ion arriving at a multiaperture 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 an 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 potential of the first, positive, grid plus the sheath potential. 
     Hence, a conventional ion gun comprises a source of charged particles which are accelerated through an externally applied electric field created between a pair of grids. Conventionally, for low energy ion beam production, three grids are used, the first being held at a positive potential, the second being held at a negative potential adjusted to give the best divergence, and the third, if present, at earth potential, i.e. the potential of the chamber in which the beam is produced. Beams of this nature are well described in the open literature going back over 25 years. 
     In some applications it is desirable to obtain an ion beam of maximum current. However, in other applications it is the divergence of the ions comprising the beam, or rather the relative lack of it, that is critical to achieving a suitable process performance. 
     U.S. Pat. No. 4,447,773 discloses an ion beam accelerator system for extracting and accelerating ions from a source. The system includes a pair of spaced, parallel extraction grids, 60 mil (1.524 mm) apart, having aligned pairs of holes for extracting ion beamlets. The pairs of holes are positioned so that the beamlets converge and the merged beamlets are accelerated by an accelerator electrode which is 0.6 inch (15.24 mm) downstream of the extraction grid pair. The extraction grids are formed with numerous small holes through which beamlets of ions can pass and are maintained a potential difference of a few hundred volts. The accelerator electrode has a single hole, which is slightly greater in height than the height of the matrix of holes in the extraction grids, and is maintained at a much lower potential for accelerating the converged ion beam emerging from the extraction grid pair. 
     An extensive introduction to and prior art review of ion beam technology is provided in EP-B-0462165. EP-B-0462165 itself describes an ion gun in which the plasma from which the ions are accelerated by the accelerator grid is at a low potential of not more than about 500 V and is of uniform density so as to permit high current densities of the order 2 to 5 mA/cm 2  in the ion beam to be achieved at low potential (i.e. less than about 500 V), and with minimum risk of damage to the accelerator grid, in operation. This system provides an ion gun in which the plasma can be efficiently generated using a commercially acceptable high radio frequency, such as 13.56 MHz or a multiple thereof, and in which the resulting plasma has the desirable properties of high density, good uniformity and a relatively low plasma potential. 
     However, a perennial problem with the ion beam sources described in the prior art is the high magnitude of divergence to which the beam is susceptible. Whilst the system of EP-B-0462165 solves many of the problems associated with other prior art ion beam sources, it would still be highly desirable to improve this system to provide a reduced degree of ion beam divergence. There is a growing demand for ultra low divergence at low to medium beam energy. In this role the beam is typically described as an ion mill selectively etching deep trenches of perhaps 1×10 μm scale length. To do so requires a beam with a divergence of no more than approximately 1° at an energy dictated by the constraints of maximum rate at a processing energy of perhaps as low as 500 eV. Conventional ion guns operating at this energy cannot meet the divergence requirement at a high enough current to meet the process rate. 
     SUMMARY OF THE INVENTION 
     The present invention accordingly seeks to provide an ion gun which is capable of operation in a manner such that the above aims are substantially achieved. 
     According to the present invention there is provided apparatus for the production of low energy charged particle beams comprising: a plasma chamber; means for generating in the plasma chamber a plasma comprising particles of a first polarity and oppositely charged particles of a second polarity; means for restraining particles of the first polarity in the plasma chamber; a first multi-aperture electrode grid contacting the plasma, wherein the first electrode grid is arranged for connection to a first potential source so as to impart to the first electrode grid a first potential of the second polarity; a second multi-aperture electrode grid arranged for connection to a second potential source so as to impart to the second electrode grid a second potential, the second potential being less than in the sense of being either less positive than or less negative than the first potential so as to produce between the first and second electrode grids a first acceleration field for accelerating charged particles of the second polarity towards and through the second grid; and a third multi-aperture electrode grid arranged for connection to a third potential source so as to impart to the third electrode grid a third potential of the first polarity and to produce between the second and third electrode grids a second acceleration field for accelerating charged particles of the second polarity towards and through the third electrode grid, the grid spacing between the first and second grids being greater at the periphery of the grids than at the centre thereof, the apertures of the first, second and third grids being aligned so that particles emerging from an aperture of the first grid are accelerated through a corresponding aperture of the second grid and then through a corresponding aperture of the third grid in the form of a beamlet, a plurality of beamlets from the third grid forming a beam downstream of the third grid. 
     The apparatus of the invention may be used to generate an electron beam. in which case the charged particles of the first polarity are ions and the charged particles of the second polarity are electrons, or to generate an ion beam, in which case the charged particles of the first polarity are electrons and the charged particles of the second polarity are ions. 
     Accordingly, the invention provides a low energy ion gun for use in ion beam processing comprising: a plasma chamber comprising an open ended, conductive, non-magnetic body, a first end of which is closed by a flat or minimally dished dielectric member and with electrodes at a second end thereof opposite the first end; primary magnet means arranged around the body for trapping electrons adjacent the wall of the plasma chamber in use of the ion gun; and an r.f. induction device including a substantially flat coil which lies adjacent to the dielectric member for inductively generating a plasma in the plasma chamber, characterised in that the electrodes include a first multi-aperture grid arranged for connection to a first positive potential source and positioned to contact the plasma in the plasma chamber; a second multi-aperture grid arranged for connection to a second potential source of lower potential than the first source so as to produce a first acceleration field for accelerating ions towards and through the second grid; and a third multi-aperture grid arranged for connection to a third potential source of lower potential than the second potential source so as to produce a second acceleration field for accelerating ions towards and through the third grid, the grid spacing between the first and second grids being greater at the periphery of the grids than at the centre thereof, the apertures of the first, second and third grids being aligned so that particles emerging from an aperture of the first grid are accelerated through a corresponding aperture of the second grid and then through a corresponding aperture of the third grid in the form of a beamlet, a plurality of beamlets from the third grid forming a beam downstream of the third grid. 
     The provision of a grid spacing between the first and second grids which is greater at the periphery of the grids than at the centre thereof is an important feature of these embodiments of the invention. Preferably this variation in grid spacing is achieved by contouring one or both of the neighbouring surfaces of the first and second grids. Thus, in one preferred embodiment, the second grid has a generally flat surface towards its periphery but in its central region bulges outwardly towards the first grid. The provision of this variation in grid spacing over the grids recognises that the plasma density of the beam approaching the first grid tends to diminish towards the periphery of the beam. The acceleration field to which individual beamlets are subject on passing through the first grid depends to some extent upon the grid spacing, which may therefore be selected to optimise the divergence of individual beamlets, whether from the periphery or the central region of the first grid. 
     In one embodiment of the invention, the third potential source may be arranged to impart a negative potential to the third grid. Alternatively, the third potential source may be arranged to earth the third grid. In this case, a fourth grid may be provided and arranged for connection to earth. 
     The invention further provides a low energy ion gun for use in ion beam processing comprising: a plasma chamber comprising an open ended, conductive, non-magnetic body, a first end of which is closed by a flat or minimally dished dielectric member and with electrodes at a second end thereof opposite the first end; primary magnet means arranged around the body for trapping electrons adjacent the wall of the plasma chamber in use of the ion gun; and an r.f. induction device including a substantially flat coil which lies adjacent to the dielectric member for inductively generating a plasma in the plasma chamber, characterised in that the electrodes include a first multi-aperture grid arranged for a connection to a first positive potential source and positioned to contact the plasma in the plasma chamber; a second multi-aperture grid arranged for connection to a second potential source of lower potential than the first source so as to produce a first acceleration field for accelerating ions towards and through the second grid; a third multi-aperture grid arranged for connection to a negative potential source so as to produce a second acceleration field for accelerating ions towards and through the third grid; and a fourth multi-aperture grid arranged for connection to earth, the apertures of the first, second, third and fourth grids being aligned so that ions emerging from an aperture of the first grid are accelerated through a corresponding aperture of the second grid and then through a corresponding aperture of the third grid before passing through a corresponding aperture of the fourth grid in the form of a beamlet, a plurality of beamlets from the fourth grid forming a beam downstream of the fourth grid. 
     Also provided in accordance with the invention is a low energy electron gun for use in electron beam processing comprising: a plasma chamber comprising an open ended, conductive, non-magnetic body, a first end of which is closed by a flat or minimally dished dielectric member and with electrodes at a second end thereof opposite the first end; primary magnet means arranged around the body for trapping ions adjacent the wall of the plasma chamber in use of the electron ion gun; and an r.f. induction device including a substantially flat coil which lies adjacent to the dielectric member for inductively generating a plasma in the plasma chamber, characterised in that the electrodes include a first multi-aperture grid arranged for connection to a first negative potential source and positioned to contact the plasma in the plasma chamber; a second multi-aperture grid arranged for connection to a second negative potential source of less negative potential than the first source so as to produce a first acceleration field for accelerating electrons towards and through the second grid; and a third multiaperture grid arranged for connection to a third potential source of higher potential than the second potential source so as to produce a second acceleration field for accelerating electrons towards and through the third grid, the apertures of the first, second and third grids being aligned so that electrons emerging from an aperture of the first grid are accelerated through a corresponding aperture of the second grid and then through a corresponding aperture of the third grid in the form of a beamlets, a plurality of beamlets from the third grid forming a beam downstream of the third grid. 
     The invention also provides a method for generating a low energy ion beam, which method comprises; 
     (a) providing an ion gun according to the foregoing description; 
     (b) supplying to the plasma chamber a plasma forming gas; 
     (c) exciting the R.f. induction device to generate a plasma within the plasma chamber; 
     (d) supplying the plasma to an inlet end of the first grid so that the plasma passes through the first grid towards an outlet end thereof; 
     (e) accelerating the plasma between the outlet end of the first grid and an inlet end of the second grid so that the plasma passes through the second grid towards an outlet end thereof; 
     (f) further accelerating the plasma between the outlet end of the second positive grid and an inlet end of the third grid so that the plasma passes through the third grid towards an outlet end thereof; and 
     (g) recovering an ion beam from the outlet end of the third grid. 
     The invention further provides a low energy ion beam processing apparatus comprising 
     (1) a vacuum chamber; 
     (2) an ion gun arranged to project an ion beam into the vacuum chamber; 
     (3) an ion beam neutraliser for projecting electrons into the ion beam; and 
     (4) a support for a target or a substrate in the path of the ion beam; the ion gun comprising: 
     a plasma chamber comprising an open ended, conductive, non-magnetic body, a first end of which is closed by a flat or minimally dished dielectric member and with electrodes at a second end thereof opposite the first end; 
     primary magnet means arranged around the body for trapping electrons adjacent the wall of the plasma chamber in use of the ion gun; and 
     an r.f. induction device including a substantially flat coil which lies adjacent to the dielectric member for inductively generating a plasma in the plasma chamber, characterised in that the electrodes include a first multi-aperture grid arranged for connection to a first positive potential source and positioned to contact the plasma in the plasma chamber; 
     a second multi-aperture grid arranged for connection to a second potential source of lower potential than the first source so as to produce a first acceleration field for accelerating ions towards and through the second grid; and 
     a third multi-aperture grid arranged for connection to earth or to a negative potential source so as to produce a second acceleration field for accelerating ions towards and through the third grid, the apertures of the first, second and third grids being aligned so that particles emerging from an aperture of the first grid are accelerated through a corresponding aperture of the second grid and then through a corresponding aperture of the third grid in the form of a beamlet, a plurality of beamlets from the third grid forming a beam downstream of the third grid. 
     By “low energy” is meant up to about 10 kV, for example 5 kV or less. Usually, the ion beam generated by the apparatus of the invention will have an energy of 1 kV or less. 
     The ion gun of the invention is capable of generating an ion beam of significantly lower divergence than has conventionally been achievable. A 500 eV ion beam generated by a gun according to the invention may have a divergence of as little as 1°. This compares directly with values of between 3° and 5° for prior art ion beams utilising conventional three grid electrode grid structures. It has surprisingly been discovered that an underlying design rule for ultra-low divergence ion beams has not been recognised in the prior art. The basis of the prior art, as exemplified by EP-B-0462165, lies in the electrostatic lens principle underpinning the simple two/three grid conventional accelerator structures and its balance with the natural space charge repulsive force in the beam. This repulsive force leads to an irreducible divergence limit for such structures. The ion beams of the prior art are vigorously compressed by a strong accelerating force provided by a first, positively charged grid and a second, negatively charged grid. The potential difference between the first and second grids may be of the order of 1000 V. As the beam passes through the second grid, the space charge force reaches a maximum and acts upon the beam to cause it to diverge as it propagates beyond the second grid. The space charge force increases with increasing beam current, with reducing beam radius and with reducing beam energy. Empirically it has been found that the lower limit of divergence for a 500 eV beam for a three grid accelerator with a beam current viable for industrial processing lies between 3° and 5°. In contrast, the ion gun of the present invention is capable of yielding a divergence value of 1°. 
     One preferred way in which the angular divergence of the beam may be minimised in the present invention is by adopting a more gentle acceleration field between the first and second grids than between the second and third grids. This allows the beamlets to propagate at a larger net area, hence reducing the space charge repulsion inside the accelerator grid structure itself. 
     The ion gun of the invention may of course be provided with more than three grids. For example, three, four or more positive biassed grids, each of successively lower positive bias than its upstream neighbour, could be used. Alternatively, or in addition, a plurality of negatively biassed or earthed grids could be incorporated towards the downstream end of the grid structure. However, for most applications, it is envisaged that a three or four grid structure will be preferred. In the three grid structure, the first and second grids may be positively biassed, the first grid being in contact with the plasma in the plasma chamber, the second grid being of lower positive bias than the first grid, and the third grid may be negatively biassed or earthed. In the four grid structure the first and second grids may be positively biassed, as described above, while the third grid may be negatively biassed and the fourth grid is earthed or negatively biassed. Thus, in one preferred embodiment of the invention, the third grid of the electrodes is arranged for connection to a negative potential source and the electrodes include a fourth grid arranged for connection to earth. The fourth grid may be used to provide an extra degree of control over the rates of acceleration and divergence of the ion beam. 
     Preferably, the grid arrangement is rigid since the mechanical separation of the grids plays a large part in determining the divergence of the beam. For example, a variation of 10% or more in the distance between two grids can have a significant impact on the net divergence of a large area beam. Furtherrnore, the relationship between beam divergence and the magnitude of the gap between the grids is substantially non-linear. 
     However, beam divergence is also a function of the local ion current density in the beam. As the cross-sectional area of the beam increases, the current density in the beam cross-section may vary by up to about 10%. The current density is lower towards the periphery of the beam. 
     In one embodiment of the invention, the grids are arranged in parallel alignment with each other. The gap between neighbouring grids is preferably between about 0.5 mm and about 3.0 mm, typically about 1.00 mm. 
     In a preferred embodiment, however, one grid of a neighbouring pair of grids is contoured so that the gap between the two grids of the pair is larger towards the periphery of the grids than towards the centre of the grids. For example, the gap at the centre of the pair may be about 1.00 mm while the gap at the periphery is about 1.3 mm. Preferably, the second grid is contoured. 
     The variation in current density across the beam is usually constant and repeatable and this may be exploited to obtain best average divergence across a large beam. Numerical simulation may be used to confirm this variation with respect to the magnitude of the in gap between neighbouring grids and the current density in the beam. 
     In one preferred embodiment of the invention, four grids are provided, the gap between each grid, at the centre thereof, being about 1.00 mm. Preferably, one or more of the grids is contoured as described above. Even more preferably, only the second grid is contoured. 
     The ion gun of the invention is of particular value for generating low energy beams of heavy ions, such as argon, which are commonly used in ion milling applications. Since the space charge force increases in inverse proportion to the ion velocity, the effect on the divergence of an argon ion beam at 500 eV is over 50 times larger than for a hydrogen ion beam at 50 KeV for a comparable beam current. Other heavy ions commonly used in ion milling applications include ions derived from krypton, xenon, H 2 , O 2 , Cl 2 , N 2 , CO 2 , SF 6 , C 2 F 6  or a C 2 F 6 /CHF 3  mixture. 
     In the ion gun of the present invention inductive r.f. coupling is used to generate a plasma in the plasma chamber. The resulting plasma typically exhibits a plasma potential that is no more than a few tens of volts above the potential of the plasma chamber or of the highest potential of the internal surface thereof. This is in contrast to many of the prior art designs of ion gun which utilise capacitative r.f. coupling to generate the plasma and which form a plasma with a plasma potential of some hundreds of volts. 
     The wall means may be constructed from an electrically conductive material. However, if it is desired, for example, to avoid any possibility of contamination of the ion beam by metallic ion contaminants, then the wall means may be constructed from a dielectric material. 
     The primary magnet means may comprise an array of magnets arranged to produce lines of magnetic flux within the plasma chamber which extend in a curve from the wall of the plasma chamber and return thereto so as to form an arch over a respective one of a plurality of wall regions of said plasma chamber, for example, wall regions which extend substantially longitudinally of the wall of the plasma chamber. Rare earth magnets are preferably used. Specific arrangements of the primary magnet means are described in EP-B-0462165 and are well understood by those skilled in the art. 
     It is preferred to use as near flat dielectric member as possible. Hence minimal dishing of the dielectric member is preferred. However, it may not be practical to avoid all dishing of the dielectric member as it must be ensured that the integrity of the vacuum equipment be preserved and that all risk of fracture of the dielectric member due to pressure differences exerted across it during operation is substantially obviated. 
     The r.f. emitter means associated with the dielectric member comprises a substantially flat spirally wound coil which preferably lies adjacent to, or is embedded within, the dielectric member. Hence the coil is preferably flat or as near flat as practicable. Such a coil may take the form of a tube of conductive material e.g. copper, through which a coolant, such as water, can be passed. This type of coil, and the advantages thereof, are also described in EP-B-0462165. 
     Typically the r.f. emitter means associated with the dielectric member is arranged to be connected to an r.f. power source which operates at a frequency in the range of from about 1 MHz up to about 45 MHz, e.g. at about 2 MHz or, more preferably, at one of the industrially allotted wavebands within this range of frequencies e.g. at 13.56 MHz or 27.12 MHz or 40.68 MHz. 
     By appropriate choice of geometry for the spiral driving coil and by modifying the magnetic field strength and/or distribution within the plasma chamber it is possible to tune the excitation of the discharge for a variety of gases, e.g. Ar, O 2  or N 2 . 
     In a preferred form an ion gun according to the invention further includes secondary magnet means associated with the r.f. emitter means for producing a magnetic dipole field that penetrates the r.f. energising coil or other form of r.f. emitter means. 
     It is also possible to provide a further magnet means, hereinafter called a tertiary magnet means, for superimposing a longer range axial field on top of the field produced by the multipole array of said primary magnet means. Such a tertiary magnet means can, for example, take the form of an electromagnet surrounding the plasma chamber whose axis is arranged to be substantially aligned with or parallel to that of the plasma chamber. 
     In an ion beam apparatus according to the invention it is preferred to utilise an ion beam neutraliser that is powered by an r.f. energy source to produce a beam of electrons that can be projected into the ion beam. Conveniently, such an r.f. energy source operates at the same frequency as that of the r.f. generator means of the ion gun. 
     The invention thus may utilise an ion beam neutraliser comprising an open ended plasma source chamber, means for admitting a plasma forming gas to the plasma source chamber, an r.f. generating coil surrounding the plasma source chamber for generating a plasma therein, and an extraction grid structure across the open end of the plasma source chamber including a first grid arranged for connection to a negative potential source and a second grid arranged for connection to a positive potential source so as to produce a first acceleration field for accelerating electrons towards and through the second grid of the extraction grid structure. Such an ion beam neutraliser may use an inert gas, a reactive gas or a mixture of an inert gas and a reactive gas, as plasma forming gas. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the invention may be clearly understood and readily carried into effect, some preferred forms of ion beam processing apparatus will now be described, by way of example only, with reference to the accompanying semi-diagrammatic drawings, in which: 
     FIG. 1 is a vertical section through an ion beam processing apparatus; 
     FIG. 2 is a plan view of the top of the ion gun of the apparatus of FIG. 1; 
     FIG. 3 is a partial horizontal section through the plasma chamber of the apparatus of FIGS. 1 and 2; 
     FIG. 4 is an enlarged view of part of FIG. 3; 
     FIG. 5 is a view of the primary magnet array of the apparatus of FIGS. 1 to  4 ; 
     FIG. 6 is a vertical section on an enlarged scale through the control grid structure of the apparatus of FIGS. 1 to  5 ; 
     FIG. 7 illustrates the magnetic field produced by the secondary magnets of the ion gun shown in FIGS. 1 and 2; 
     FIG. 8 is a vertical section through a second form of ion gun constructed according to the invention; 
     FIG. 9 is a top plan view of the ion gun of FIG. 8; 
     FIGS. 10 and 11 are sections on the lines A—A and B—B respectively of FIG. 9; 
     FIGS. 12 and 13 are a partly cut away side view and a top plan view respectively of the body of the ion gun of FIG. 8; 
     FIGS. 14 and 15 are sections on the lines C—C and D—D respectively of FIG. 12; 
     FIG. 16 is an enlarged section of part of the body of the ion gun of FIG. 8; 
     FIG. 17 is a schematic diagram of the axis of the tube from which the r.f. emitter coil is formed; 
     FIGS. 18 and 19 are respectively a section and a side view of the r.f. emitter coil; 
     FIG. 20 shows, for comparative purposes, a representation of ion beam divergence in a prior art ion gun having a conventional three grid electrode structure; 
     FIG. 21 shows a representation of ion beam divergence, directly comparable to the representation shown in FIG. 20, in a four grid electrode ion gun according to the invention; 
     FIG. 22 is a top plan view of an electrode grid for use in the ion gun of the invention; and 
     FIG. 23 is a vertical section through a third form of ion gun constructed according to the invention, showing an electrode grid arrangement in which the second grid is shaped. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1 to  7  of the drawings, an ion beam processing apparatus  1  comprises a vacuum chamber (indicated diagrammatically at  2 ) surmounted by an ion gun  3 . Ion gun  3  comprises a plasma generator  4  mounted on top of an open ended plasma chamber  5 , the lower end of which is closed by a control grid structure  6 . Control grid structure  6  is described in detail below. A plasma neutraliser  7  is mounted within vacuum chamber  2  for neutralising the ion beam  8  which issues from the lower end of ion gun  3 . A target  9  is placed in the path of the ion beam  8 . 
     Plasma generator  4  comprises a dielectric member  10  which closes the top open end of plasma chamber  5 . A number of gas inlet nozzles are provided, as indicated by arrows  11 , through which a plasma forming gas, such as argon, or a mixture of a plasma forming gas and a reactive gas, such as oxygen, can be admitted to the plasma chamber  5 . An r.f. coil  12  surmounts member  10  and is connected to a suitable r.f. power source operating at, for example 13.56 MHz. Magnets  13  and  14  are provided for a purpose which will be further described below. 
     Plasma chamber  5  comprises an open-ended metallic body  15 , made of aluminium or of an aluminium alloy or another conductive non-magnetic material, within which are mounted a plurality of primary bar magnets  16 . For ease of assembly body  15  is made in two parts, i.e. an inner part  17  and an outer part  18 , between which the primary magnets  16  are positioned. 
     As can be seen from FIG. 3, there are thirty-two primary bar magnets  16  secured longitudinally to the cylindrical outer face of inner part  17 . Preferably the strongest available magnets, e.g. rare earth magnets such as samarium-cobalt magnets, are used. Typically such magnets exhibit a field strength of the order of 1 to 2 kGauss. As illustrated in FIG. 3 there are thirty-two primary magnets  16 . However, a larger or smaller number of primary magnets, for example thirty or less (e.g. twenty-four) or up to forty or more (e.g. forty-eight), may be used, provided always that there is an even number of primary magnets  16 . Such primary magnets  16  are evenly spaced around the outer periphery of inner part  17  with their longest dimension arranged substantially parallel to the axis of the plasma chamber  5 . As indicated in FIG. 4, however, the magnetic axes of primary magnets  16  are arranged radially with respect to plasma chamber  5  so that their respective north and south poles (indicated as N and S respectively in FIG. 4) are separated in the direction of their shortest dimension, the primary magnets  16  being arranged with alternating magnetic polarity around the periphery of inner part  17 . 
     Above primary magnets  16  is an annular groove  19  and below them a corresponding annular groove  20 . Grooves  19  and  20  communicate one with another via spaces  21  between adjacent primary magnets  16 . The grooves  19  and and spaces  21  form channels for coolant fluid (e.g. water) by means of which the primary magnets  16  and body  15  can be cooled in use. Reference numerals  22  and  22   a  indicate coolant fluid supply and withdrawal conduits provided in annular member  23 . Baffles  24 ,  25  are provided in grooves  19 ,  20 , as can be seen in FIG. 5, in order to make the coolant fluid follow a predetermined path. 
     FIG. 4 indicates the lines of magnetic force  26  produced by primary magnets  16 . These lines of force extend from the inner surface of body  15  into cavity  27  in plasma chamber  5  and back into the wall of cavity  27  in an arch over regions  28  which extend parallel to the axis of body  5 . 
     Reverting to FIG. 1, the lower end of plasma chamber  5  is closed by a control grid structure  6  which is shown in more detail in FIG. 6 on a greatly enlarged scale. Grid structure  6  comprises three grids  29 ,  29   a  and  30 , each formed with aligned holes  31 ,  31   a  and  32 . Grid  29  is positively biased while grid is negatively biased or earthed. Grid  29   a  is positively biased, but more weakly so than grid  29  so as to set up two acceleration fields between grids  29  and  29   a  and grids  29   a  and  30  respectively to accelerate ions towards and through grids  29   a  and  30 . Such grids can be manufactured from molybdenum or a molybdenum alloy or from carbon sheet or from a suitable aluminium alloy. Aluminium may offer specific benefits where heavy metal contamination is to be avoided. Typically grid  29  has a positive potential of up to about 1000 V applied to it, whilst a negative potential of from 0 to about 2000 V is applied to grid  30 . Grid  29   a  has a positive potential of up to about 850 V or more, for example 750 applied to it. The ion beam generated through this grid arrangement is discussed more fully below. 
     Turning now to ion beam neutraliser  7 , a gas, e.g. argon or oxygen, is supplied as indicated by arrow  33  through line  34  into a hollow insulated electrode assembly  35 . The open end of electrode assembly  35  is closed by a pair of grids  36  and  37 . An r.f. generator coil  38  surrounds electrode assembly  35 . Conveniently this is driven at the same frequency as r.f. generator coil  12 , e.g. 13.56 Mfz. Grid  36  is negatively biased, while grid  37  is positively biased so as to set up an acceleration field between grids  36  and  37  to accelerate electrons towards and through grid  37 . 
     FIG. 2 illustrates in plan view the positions of the optional secondary magnets  13  and  14  relative to the r.f. generator coil  12 . These secondary magnets produce a magnetic dipole field which penetrates the energizing coil  12 . The shape of this magnetic field is shown diagrammatically in FIG.  7 . As can be seen from FIG. 7 magnets  13  and  14  have their axes of magnetisation arranged so that either a north pole or a south pole faces the dielectric member  10  and so that the lines of force  47  penetrate the r.f. generator coil  12  and form an arch over the inner face of dielectric member  10 . 
     Reference numeral  48  indicates an r.f. power source connected to coil  12 ; it can also be connected to coil  38 . Alternatively coil  38  can have its own separate r.f. power source. Lines  49  and  50  indicate positive supply leads for electrode  17  and grid  29  respectively. Conveniently electrode  17  and grid  29  are at the same positive potential. Reference numeral  51  indicates a negative supply lead for providing the negative bias potential on grid  30 . 
     Vacuum chamber  2  can be evacuated by means of a suitable vacuum pumping system  52  connected via line  53  to vacuum chamber  2 . 
     In use of ion beam processing apparatus  1  vacuum chamber  2  is evacuated to a pressure of typically about 10 −5  millibar to about 10 −7  Pa (1 −7  millibar). A plasma forming gas, e.g. argon, a reactive gas, or a mixture of a plasma forming gas and a reactive gas, e.g. O 2 , CO 2 , Cl 2 , SF 6 , C 2 F 6  or a C 2 F 6 /CHF 3  mixture is admitted via inlets  11 . R.f. coil  12  is then excited to generate a plasma. This plasma has a plasma potential of at most a few tens of volts, for example about +10 V (volts) above the potential of the grid  29 , which is normally at the same potential as the electrode  17 . Electrons released are trapped within regions  28  by the magnetic lines of force  26 . Grid  29  is biased to a positive potential of about 900 V, while grid  29   a  is biased to a positive potential of 725 V and grid  30  is biased to a negative potential of about −100 V. The ions in the cavity  28  are accelerated towards and pass through grid  29  and are then gently accelerated by the electric field between grid  29  and grid  29   a  before being further accelerated by the electric field between grid  29   a  and grid  30  to emerge in the form of a collimated ion beam  8  of defined energy. After passage through grid  30  ions in flight to the target  9 , which is earthed, move in a decelerating field. The ions arrive at the earthed target  9  with an energy equal or approximately equal to the potential of the first, positive grid  29  plus the sheath potential. Thus, if a bias of +900 V is applied to grid  29 , which is immersed in the plasma, then the ions will arrive at the target  9  with a potential of about 910 V, irrespective of how high a negative potential is applied to the third grid  30 . Grid  29   a  is shaped to give a controlled variation of separation over the area of the grid. This enables changes in plasma density to be accommodated with minimal effect on the local beam divergence, as discussed below with reference to FIG.  23 . 
     A plasma forming gas, such as argon, a reactive gas, or a mixture of a plasma forming gas and a reactive gas, is admitted to neutraliser  7  through tube  34  at a rate of from about 1 cm 3  per minute to about 5 cm 3  per minute, usually at the higher end of this range. The r.f. generator coil  38  is turned on to initiate electron discharge. Once electron discharge has started it can be maintained by use of a keeper potential of from about 20 V to about 40 V following reduction of the gas flow rate in tube 33 to about 1 cm 3  per minute. 
     Under the influence of the r.f. signal from coil  12  the gas supplied via inlets  11  is dissociated to form a plasma of ions and free electrons in plasma chamber  5 , the ions filling the central part of chamber  5  whilst the electrons are trapped adjacent the walls of chamber  5  by the magnetic lines of force  26 . Because of the geometrical separation of the r.f. generating coil  12  from the zones  28  of the magnetic confinement region in plasma chamber  5  the plasma in the centre part of chamber  5  is subsequently uniform and has a relatively low plasma potential. This means in turn that a relatively low acceleration potential only is needed to extract ions from this plasma and to accelerate them towards and through control grid structure  6 . Hence the risk of overheating of or damage to control grid structure  6 , and particularly grid  29 , is minimised. 
     Neutraliser  7  delivers a stream of electrons into the path of ion beam  8  and provides current neutralisation at the target  9 . 
     As a heated cathode is not used to generate the plasma the illustrated apparatus can be used with any type of inert or reactive gas. Typical gases that can be used include argon, krypton, xenon, H 2 , O 2 , Cl 2 , SF 6 , CO 2 , CF 4 , C 2 F 6 , CHF 3  and mixtures of two or more thereof. 
     As illustrated the ion beam processing apparatus  1  is set up for ion beam milling. It is a simple matter to modify the apparatus for sputter deposition; in this case a target replaces substrate  9  and is arranged so that it is struck by ion beam  8  at an oblique angle, while a target is placed in the path of the ensuing sputtered material but out of the path of the ion beam. 
     FIG. 8 is a vertical section through another form of ion gun  100  constructed in accordance with the invention. This comprises a body  101  made of austenitic stainless steel around which are mounted twenty bar magnets  102 , symmetrically disposed about the periphery of ion gun  100 . (There are few magnets in this embodiment than in that of FIGS. 1 to  7  because the diameter of body  101  is smaller than that of the ion gun of FIGS. 1 to  7 ). Typically the magnets  102  are rare earth magnets, such as samarium-cobalt magnets, with a field strength in the range of from about 0,1 T to about 0,2 T (1 kilogauss to about 2 kilogauss). As can be seen from FIG. 8, the magnetic axis of each magnet  102  is aligned so as to lie radially with respect to the axis of the ion gun  100  and to correspond to the shortest dimension of the magnet  102 . The magnets  102  are arranged with alternating polarity around the periphery of ion gun  100  so that the magnets adjacent to the magnet shown in FIG. 8 have their north poles (not their south poles) facing towards the axis of ion gun  100 , the next adjacent magnets to these have their south poles facing the axis of ion gun  100 , and so on. A pole piece  103  made of soft iron or of a soft magnetic material surrounds body  101  and magnets  102 . 
     Body  101  has an open upper end which is closed by dielectric end plate  104  made of alumina. Alternatively it can be made of another dielectric material such as silica. 
     Above end plate  104  is an r.f. generator coil  105  in the form of a spirally wound copper tube having four complete turns. (For the sake of clarity the coils of r.f. generator coil are omitted in FIG. 9; however, the construction of coil  105  is shown in more detail in FIGS. 17 to  19  as described (further below). Water can be pumped through coil  105  to cool it. The end portions of coil  105  are indicated by means of reference numerals  106  and  107 . 
     A top ring  108  holds end plate  104  in position and further provides support for a clamp  109  for coil  105 . Top ring  108  is held in place by means of socket headed bolts  110 . An O-ring  111  serves to provide a seal between end plate  104  and top ring  108 . 
     Ion gun  100  is mounted in a vacuum chamber, similar to vacuum chamber  2  of the apparatus of FIGS. 1 to  8 , which is designated by reference numeral  112  and is generally similar to vacuum chamber  2 . In particular it is provided with connections to a vacuum pump (not shown) and is fitted with an ion beam neutraliser (not shown) and with a target (also no shown). The gun  100  is mounted to the body of the vacuum chamber  112  by means of a spacer  113 , a clamp  114 , and bolts  115 . An O-ring  116  serves to provide a vacuum seal between pole piece  103  and spacer  113 . 
     FIGS. 12 to  15  show the construction of body  101  in more detail. This has twenty slots  117  formed in its outer surface, each adapted to receive a corresponding bar magnet  102 . At the lower end of body  101  there are machined a number of short grooves  118 , five in all, which are evenly spaced around the periphery of body  101 . There are also four grooves  119  at the upper end of body  101  but these are offset with respect to grooves  118 . Vertical bores  120  connect grooves  118  and  119 . Bores  121  and  122  provide an inlet and outlet passage to the tortuous path provided by grooves  118  and  119  and bores  120 . As can be seen from FIG. 8 grooves  118  are closed by means of split ring  123  which is welded on the lower end of body  101 , whilst insert plates  124  (shown in FIG. 13) are welded into the upper end of body  101  to close off grooves  119 . In this way a closed passage for a coolant, such as water, is formed through body  101  which follows a tortuous path passing between adjacent pairs of magnets  102 . Inlet and outlet connections  125  and  126  are provided to communicate with bores  121  and  122 . 
     Body  101  carries at its lower end a control grid structure which includes a first grid  127 , a second grid  128  and a third grid  129 . Grid  127  is bolted directly to body  101  and is in electrical contact therewith. Grids  128  and  129  are supported at three points around the periphery of body  101  by suitable insulator supports  129   a  (only one of which is shown). As is shown in FIG. 10, they are connected to terminals  130  and  130   a  by respective leads  131  and  131   a  which pass through respective insulating pillars  132  and  132   a  and then through respective insulating tubes  133  and  133   a  mounted in bores  134  and  134   a  and which are electrically connected to respective grids  128  and  129  by means of a respective feedthrough  135  or  135   a  which passes through a respective spacer  136  or  136   a  positioned in a hole or holes in grid  127 . Nut  137  or  137   a  and washer  138  or  138   a  complete the connection to respective grids  128  and  129 . With this arrangement grid  127  adopts the potential of body  101 , whilst grids  128  and  129  can be independently biased by applying a suitable potential to respective terminal  130  or  130   a.    
     As can be seen from FIGS. 8,  10 ,  11  and  12  body  101  has a groove  139  positioned under the edge of dielectric member  104 . This communicates by means of a transverse oblique bore  140  (see FIG. 16) with a further bore  141 . Reference numeral  142  denotes a plug closing the outer end of bore  140 . A connection  143  for a plasma gas supply (shown in FIG. 9) is screwed into bore  140 . Such plasma gas can enter the plasma chamber  144  within body  101  by leakage from groove  139  through a clearance gap  145  (which is more clearly seen in FIG. 16) under dielectric member  104 . 
     Reference numeral  146  in FIGS. 8 and 9 denotes a terminal by means of which a suitable potential, usually a positive potential, can be applied to body  101  and hence to grid  127 . 
     FIGS. 17 and 19 illustrate the construction of spiral r.f. coil  105  in greater details. Only the axis of the tube of coil  105  is depicted in FIG.  17 . The coil has  4  complete turns between portions  106  and  107 . 
     In use of the ion gun of FIGS. 8 to  19  water is passed through coil  105  and through the tortuous path in body  101  by means of inlet  125  and outlet  126  and vacuum chamber  112  is evacuated to a suitably low pressure, e.g. 10 −3  Pa to 10 −5  Pa (10 −1  millibar to 10 −7  millibar). Body  101  is biased to a suitable potential, e.g. +900 V, whilst grid  128  is biased to, for example +725V, and grid  129  to −100 V. Plasma forming gas, such as argon or a mixture of argon and a reactive gas (e.g. oxygen), is then bled into vacuum chamber  112  via inlet  43  whilst maintaining a pressure in the range of from about 10 −1  Pa to about 10 −2  Pa (from about 10 −3  millibar to about 10 −4  millibar). Upon application of a suitable r.f. frequency, e.g. 13.56 MHz, to coil  105 , a plasma is generated in plasma chamber  144 . This typically equilibrates at a plasma potential of about 10 volts above that of body  101  and grid  127 . Ions migrate by thermal diffusion, it is thought, to the vicinity of grid  127  and, upon passing through grid  127  are accelerated gently towards and through grid  128  by the electrical field caused by the potential difference between the two grids  127  and  128  (e.g. about 175 volts). After passage through grid  128  the ions are more rapidly accelerated (through a potential difference of about 825 volts) towards and through grid  129 . After passing through grid  129  the ions travel on in vacuum chamber  112  towards a target (similar to target  9  of FIG. 1, but not shown) which is typically earthed. An ion beam neutraliser (not shown) similar to neutraliser  7  of FIG. 1 may be located within vacuum chamber  112 . Grid  129  acts further to produce a deceleration field through which ions that have traversed grid  129  have to pass before hitting the target. In this way an ion beam with a suitable beam potential, which is normally approximately the same potential as that of the body  101  in the arrangement described, is produced. 
     Referring to FIG. 20, there can be seen the effect on ion beam divergence of a prior art three grid system in accordance with EP-B-0462165. The ion beam indicated by reference numeral  147  is accelerated from the plasma mass  148  towards grid  29 (p) which is held at a positive potential of around 500 V. Rapid acceleration then occurs through the electric field indicated by field lines  149  which focus the ion beam into a divergent beam through grid  30 (p) which is held at a potential of around −500 V. Grid  31 (p) is connected to earth. The colossal acceleration of the ion beam through the 1000 V potential difference between grids  29 (p) and  30 (p) gives rise to a very strong space charge repulsive force in the region of grid  30 (p). This repulsive force causes the strong divergence, seen at  150 , of around 79.3 mrad (i.e. about 4°). 
     In contrast, FIG. 21 shows the effect of the electrode grid arrangement according to the invention. First grid  29  is held at a positive potential of 900 V whilst second grid  29   a  is held at a lower positive potential of 725 V. Second grid  29   a  is contoured as shown in FIG. 23 so that the grid spacing between the first and second grids is greater towards the periphery of the grids than it is towards the centre thereof. The ion acceleration between grids  29  and  29   a  is gentler (a potential difference of just 175 V) than in the prior art grid of FIG.  20 . This allows formation of a more stably collimated beam which is less susceptible to space charge repulsive forces in the region of third grid  30  and hence shows a divergence of just 15.7 mrad (less than 1°). In the embodiment shown in FIG. 21, four electrode grids are used. Third grid  30  is held at a negative potential of −100 V whilst fourth grid  30   a  is connected to earth. 
     FIG. 22 shows a suitable grid in which holes  31 ,  31   a , or  32  can be seen. 
     FIG. 23 shows more clearly the preferred form of grid arrangement according to the invention, in which the second grid  29   a ;  128  is contoured. The ion beams passing through individual holes will repel each other to a certain extent. If the middle grid is arcuate in section it will tend to focus the ion beams to a point but then the repulsion will tend to make them straight and parallel to each other. 
     It will be appreciated by those skilled in the art that the illustrated ion guns can be used in inert gas ion beam etching; in reactive ion beam etching, or in chemically assisted ion beam etching by suitable choice of the gas or gases supplied to the plasma chamber and to the ion beam neutraliser.