Abstract:
An ion source  10  for producing a beam of ions from a plasma is disclosed. A plasma is created at the center of an anode  12  by collisions between energetic electrons and molecules of an ionizable gas. The electrons are sourced from a cathode filament  11  and are accelerated to the anode  12  by an applied electric potential. A projection of the anode and a magnetic field having an axis aligned with the axis of the anode act together to concentrate the flow of electrons to the center of the anode  12 . The ionizable gas is introduced into an ionization region  13  of the ion source  10  at the point of concentrated electron flow. Ions created in the ionization region are expelled from the ion source as an ion beam centred on the axis of the magnetic field. The surfaces of the anode are coated with an electrically conductive non-oxidising layer of Titanium Nitride to prevent a build up of an insulating layer on the anode.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
   This application is a continuation-in-part of Application Ser. No. 09/744,205, filed 18 Jan. 2001 now U.S. Pat. No. 6,734,434, the entire contents of which are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   This invention relates to ion sources for producing an ion beam. The invention was developed through use with end-Hall effect ion sources and is, at times, described with particular reference thereto. It will be apparent to the skilled reader however, that the scope of the invention will encompass other types of ion sources. 
   Ion sources had their origins in space propulsion but more recently have found use in more industrial processes such as Ion Assisted Deposition (IAD) of thin film coatings. In an IAD process, an ion beam from an ion source is directed onto a target substrate to cause densification of the coating material as it is deposited. The process occurs within an evacuated chamber of pressure of the order 10 −2  Pa. 
   In a typical ion source, electrons are drawn from a cathode filament toward an anode through an ionizable gas. Collisions between the gas molecules and energetic electrons create a source of positive ions by inducing a plasma. In one type of ion source known as a gridless ion source, a magnetic field is applied across the plasma to shape the ions accelerated from the ion source into an ion beam. In a specific type of gridless ion source, known as an end-Hail effect ion source, the axis of the magnetic field is aligned with the electric potential between the cathode and the anode. The interaction of the magnetic and electric fields causes the charged particles to approximately follow the magnetic field lines. The anode in these devices is typically annular having an outwardly inclined inner diameter with the bulk of the plasma forming within the confines of the anode walls. 
   An example of an end-Hall effect ion source in common use, in particular in IAD techniques, is described in U.S. Pat. No. 4,862,032 to Kaufman el al. In this device, herein referred to as the Kaufman device, the ionizable gas is distributed uniformly across the plasma region. Magnetic field shaping disperses the electrons across the gas to ensure a large plasma capable of producing a high ion beam current. The result is that a relatively high gas flow (typically up to 50 sccm) is required maintain a sufficient pressure in the plasma region to achieve ionization of the gas. The resultant high background pressure within the interelectrode space creates electrical instability leading to the generation of cathode spots within the ion source and extending to the extremities of the vacuum environment. In addition, large vacuum pumps are required to maintain a sufficiently low pressure within the rest of the evacuated chamber to be compatible with the operation of other equipment used in IAD and other processes. In operation the pressure can only be increased to the point where the ion beam current is approximately 1 Amp before further instabilities are introduce. 
   A further problem with present ion sources is that their performance can decrease over the life of the ion source. Symptoms include difficulty in establishing the plasma and a reduced stability of the plasma. Investigations by the present inventor have found that the reduced performance capabilities are created, at least in part, by a decrease in the electron flux entering the ionization region due to a reduction in the effective surface potential of the anode. Further investigation into the cause of the reduced potential by the present inventor found that a dielectric oxide layer built up on the surface of the anode exposed to the plasma. It was previously believed that the observed build up of electrically insulating coatings on the anode were produced by scattering and sputtering from the thin film deposition processes for which these ion sources were commonly used. The inventor has found that the dielectric layer actually arises from a small percentage of negative ions produced in an oxygen plasma interacting with the surface of the anode and that this has the effect of shielding the anode from the cathode, dispersing the electron flow from the cathode and thus reducing the electron flux into the ionization region. The reduced electron flux into the ionization region firstly creates instability in the performance of the ion source and, secondly, causes an imbalance in the change neutrality of the resultant ion beam. 
   SUMMARY OF THE INVENTION 
   In a first form, the present invention resides in an ion source comprising an electron producing cathode, an anode, an ionization region between said cathode and said anode, a gas supply path for introducing an ionizable gas into said ionization region, means for creating a potential difference between said cathode and said anode to produce a flow of electrons from said cathode toward said anode, said electron flow passing substantially through said ionization region and causing ionization of said gas, said potential difference also acting to expel ions created in said ionization region from said ion source, means for concentrating said electron flow to crate a region within said ionization region where the electron flux is a maximum, wherein said gas supply path terminates in at least one aperture disposed in proximity to said region of maximum electron flux. 
   Preferably the ion source includes a magnet. More preferably, the axis of the magnetic field lies substantially parallel to the direction of the electric potential between the anode and the cathode. With the magnetic and electric fields aligned in this way, the maximum electron flux occurs at the maximum magnetic field intensity. 
   Preferably the anode includes a projection extending into the ionization region for concentrating the electron flow. 
   The invention also provides an ion source comprising an electron producing cathode, an anode, an ionization region between said cathode and said anode, a gas supply path for introducing an ionizable gas into said ionization region, means for creating a potential difference between said cathode and said anode to produce a flow of electrons from said cathode toward said anode, said electron flow passing substantially through said ionization region and causing ionization of said gas; said potential difference also acting to expel ions created in said ionization region from said ion source, wherein said anode has at least one surface exposed to said ionization region, at least a portion of said at least one surface being of an electrically conductive non-oxidizing material. 
   Preferably the anode is annular having an axis lying in the same direction as the electric field between the anode and the cathode. The exposed surfaces of the anode are preferably a coating of Titanium Nitride (TiN). 
   BRIEF DESCRIPTION OF THE DRAWINGS 
   Further features and advantages of the invention will become apparent to the skilled reader from the following description of preferred embodiments made with reference to the accompanying Figures in which: 
     FIG. 1  is a partial cross-sectional elevation of the ion source according to the invention; 
     FIG. 2  is a plan view of the ion source in  FIG. 1 ; 
     FIG. 3  is a cross-sectional view of a preferred form of the invention; 
     FIG. 4  shows a schematic of an alternative gas supply for an ion source; 
     FIG. 5  shows a schematic of an anode having a heat sink; and 
     FIG. 6  shows a schematic of a further alternative gas supply system for an ion source. 

   DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIGS. 1 and 2  show an ion source generally at 10 having a cathode wire  11  and an anode  12 . The anode  12  has an inner surface  35  sloping outwards in the direction of the cathode. Between the cathode  11  and the anode  12  is an ionization region  13 . The cathode wire  11  is suspended above the anode by two mounting pins  20  that are held by, and in electric isolation from a shield plate  30 . The shield plate  30  substantially surrounds the anode, cathode and ionization region by extending from a point lower than the anode  12  to a point above the cathode  11  and is preferably maintained at earth potential to shield the anode and the cathode from external fields. 
   A magnet  14  is disposed outside the ionization region  13  but adjacent the anode  12 . The magnet  14  creates a magnetic field, the longitudinal axis of which is aligned with the axis of the anode  12 . The magnet may be a permanent magnet or an electromagnet. Preferably the magnet is a high flux rare earth magnet such as a NdFeB magnet. As an alternative, magnet  14  may be a ring magnet disposed around the anode  12  and ionization region  13 . 
   The alignment of the magnetic field with the electric field causes electrons emitted by the cathode to approximately follow the magnetic field lines as they move towards the anode. This has the effect of concentrating the flow of electrons toward the axis of the magnetic field. Therefore the region where the magnetic field intensity is a maximum will also be a region of maximum electron flux. 
   The ionizable gas, for example oxygen, nitrogen or argon, is supplied to the ionization region through a gas flow path from gas feed line  22 . The gas flow path terminates at an outlet member  15 . The outlet member  15  has the form of a gas shower head, with a plurality of apertures  17 , that introduce the gas into the ionization region  13  in a substantially random direction. The gas shower head  15  is disposed on the axis of the anode and adjacent the ionization region  13  such that gas emanating from the apertures  17  enters the ionization region at a point of high electron flux. Because a large proportion of ionization occurs close to the outlet, the gas shower head is of a material such as stainless steel, that withstands the very high energy from the incoming electron flux 
   The anode  12  preferably has disposed within it a channel  53  in communication with a fluid conduit  55  that provides water to cool the anode. The channel  53  preferably extends into the body of the outlet member  15 . 
   The anode  12 , outlet member  15  and shield  30  are mounted on a non conductive mounting base  50  through which extends the gas flow path and fluid conduit  55 . A plurality of mounting screws  57  fix the anode  12  to the base  50 . The magnet  14  is housed within the base such that the external pole is exposed. The mounting base  50  has a conduit  58  that forms part of the gas flow path and connects the gas feed line  22  to the outlet member  15  such that no electrical connection can be made between the outlet member  15  and the gas feed line  22 . The mounting base  50  has a similar conduit for connecting the water feed line  55  to the channel  53 . The gas and water feed lines preferably screw into the mounting base  50 . A suitable material for the mounting base  50  is glass filled polytetrafluoroethylene. This arrangement reduces electrical hazards, simplifies mounting and installation and reduces risk of secondary plasmas forming within the gas feed line. 
   The size of the outlet is preferably half or less than the smallest inner diameter of the anode in order that a localised high pressure zone is created around the outlet, that decreases rapidly with distance. 
   In operation the anode is charged in the range 0-500 V, preferably 250 V relative to the cathode which is at or near earth potential. A DC current of approximately 12A is passed through the cathode to stimulate electron emission. An AC current may be used but the combination of an alternating current and the magnetic field has been found to cause vibrations in the cathode which reduces the cathode lifetime. Electrons generated at the cathode are influenced by the anode potential and are accelerated toward it. The magnetic field imparts a spiral motion on the electrons further increasing their potential to ionize gas molecules and focussing the electrons toward the longitudinal axis. Collisions between the energetic electrons with gas molecules emitted from the outlet member  15  cause ionization. If sufficient ionizing collisions occur then a plasma is formed. Positive ions created in the plasma experience the opposite effect to the electrons. The ions initially have a random velocity but are influenced by the electric potential gradient which accelerates them toward and past the cathode  11 . The magnetic field in this case acts to control the direction in which the ions are expelled from the ion source by focusing them into an ion beam centered on the longitudinal axis of the magnetic field. The dynamics of the interactions between the ions and the electric and magnetic fields for this configuration are known per se, for example from the above mentioned Kaufman patent. The current of the ion beam is effected by the size of the plasma which can be controlled by the gas flow rate. 
   The anode  12  is preferably made of stainless steel but has a coating of a non-oxidising electrically conductive material, for example TiN, on the inner surface  35  and any other surface that in use may be exposed to bombardment by electrons and/or negative ions from the plasma. The inner surface coating is unreactive with any negative ions produced in the plasma and therefore resists the build up of a dielectric layer on the anode surface. This provides a long term benefit in the performance of the ion source because a dielectric coating would shield the anode potential from the cathode. This would reduce the concentration of electrons flowing into the ionization region, thus reducing the size of the plasma and in turn the ion beam current. In addition, the concentration of electrons in peripheral regions of the ion source would increase, thereby increasing the frequency of arcing and sputtering in these regions. By coating the anode in a non-oxidising material, these problems can be eliminated as can the cleaning procedures previously required to maintain the anode in working order. 
   The ion source  10  can operate at a lower background pressure than prior art ion sources, allowing the anode and cathode to be in closer proximity than in previous devices.  FIG. 3  shows a preferred form of the invention where the inner edge  31  of the plasma shield  30  extends towards the anode  12 . Preferably the inner edge  31  of the shield  30  is disposed outside a projection of the inner surface  35  of the anode  12 . The extended edge  31  has a flange  32  that surrounds an upper portion of the anode  12 . The purpose of the flange  32  is to prevent gas entering the region  40  enclosed by the anode  12  and shield  30  where the gas could be ionized and cause electrical instability. A vent hole  41  is provided from the region  40  to outside the ion source to allow sufficient pumping of this region, thus ensuring a low pressure. To further prevent any instabilities an o-ring seal (not shown), preferably of an elastomer material, can be disposed between the flange  32  and an upper portion of the anode  12 . 
   An ion source having an alternative construction and gas delivery method is described with reference to FIG.  4 . The ion source  100  includes a base plate  101  that screws or otherwise engages with a cylindrical shroud  102 . The shroud has an inner sloping surface  103  that defines an open end  116  of an ionization region  113  to be described below. The base plate  101  has a collar  105 , extending upward from which is a threaded section  106  for engagement with the shroud  102 . The base  101  has an upper annular face  107 . An inner circumferential flange  108  extends from the face  107  to locate a ring magnet  114  thereon. 
   Disposed on the magnet  114  is a spacer  117 , for example of aluminium, that provides a radiation shield to prevent the magnet  114  from overheating due to radiation from the anode  112 . 
   The anode  112  has an end wall  120  and an outwardly sloping side wall  121 . The side wall and end wall together define the ionization region  113 . A filament  111  is supported at the open end  116  of the ionization region  113  by filament support legs  130 . The filament legs  130  are connected to the shroud  102  through insulating mountings  131  to electrically isolate the filament legs  130  from the shroud  102 . The filament legs  130  are each electrically conducting and have an electrical connection point  132  for connecting into a filament supply circuit (not shown). 
   A projection  123  extends from the anode end wall  120  into the ionization region  113 . The projection  123  shown in  FIG. 4  is curved having an apex located on an axis of the anode. In alternative embodiments, the projection may have angled aces or the like. The projection provides a focal point for the electrons emitted by the cathode. The anode  112  is located within the shroud by upper and lower insulating rings  118 ,  119 . A gas chamber  140  is defined by the anode  112 , the insulating rings  118 ,  119  and the inside surface of the shroud  102 . The upper insulator  118  is a rigid insulator for holding and locating the anode  112  properly in place. The insulator  118  is also required to have a high temperature resistance and low thermal expansion in order that the insulator provides a seal for the gas chamber under operating conditions. Preferred materials for the upper insulator include glass, ceramic or some polymers such as PEEK (polyethylethylketone ). The lower insulator is preferably a high temperature elastomer ring that provides a resilient seal for the gas chamber  140  when the base  101  is screwed into the shroud. 
   An inlet  141  through the shroud is connectable to a gas line (not shown) that supplies gas to the gas chamber  140 . Control of the gas flow is governed by a mass flow controller or similar control mechanism disposed upstream of the ion source, as is well known in the art 
   Extending through the anode side walls  121  are a plurality of channels  125 , each terminating in the ionization region  113  at an aperture  126  disposed adjacent the end wall  120 . The channels  125  provide a conduit from the gas chamber  140  to the ionization region  113 . The channels  125  extend downwardly (as depicted in  FIG. 4 ) from the outer anode wall to the ionization region such that the channels are pointed at the projection  123 . This ensures that the incoming gas molecules are on average directed at the projection  123 . Gas ionization efficiency is thereby increased because the gas molecules are introduced in proximity to the region of highest electron concentration and electron energy. 
   As shown in  FIG. 4 , the projection  123  is integrally formed with the end wall  120 . Also shown within the anode  112  is a cavity  127  that receives a cooling fluid from an inlet conduit  150 . The cavity  127  extends to an underside surface  128  of the end wall and the projection  123 . The thickness of the end wall is preferably less than 10 mm in order that the cooling fluid can sufficiently cool the projection. The minimum thickness of the end wall and projection is determined only by the limits of the manufacturing processes used to fabricate the anode. In practice, the thickness of the end wall is approximately 2 mm. 
   The fluid conduit  150  is a coaxial conduit, having an inner conduit  151  for supplying fluid, eg water, to the cavity  127  and an outer conduit  152  for removing fluid from the cavity. The inner conduit  151  extends into the cavity so that the outlet end  153  of the conduit is disposed adjacent the underside surface  128  of the end wall. This ensures that the coolest water is directed at the end wall and projection, which receives the majority of the anode heat load. The outlet  153  of the inner conduit has a notch  154  so that in the event that the inner conduit is inserted into the cavity until the conduit abuts the underside surface of the end wall, the flow of water is not restricted. 
   The fluid conduit  150  extends through the central aperture of the ring magnet  114  and the base plate  101  and can be used to provide an electrical connection to the anode with electrical breaks provided upstream of the connection. 
   An alternative system for cooling the anode is shown in FIG.  5 . In this embodiment a solid anode  160 , ie having no internal cavity, is provided with an aperture  162  extending to an underside surface of the anode end wall  120 . The anode is mounted on a shaft  161  that is received in the aperture and is of a material having a high thermal conductivity, such as copper, in order to provide a heat sink for the anode. Electrical connection to the anode can also be provided using the shaft. The copper shaft  161  can extend through a feedthrough of the vacuum chamber so that the heat sink is in direct communication with the atmosphere outside the chamber to provide enhanced cooling 
   To provide greater protection to the ion source, a thermal switch  163  may be placed on an underside surface of the anode, or on the copper shaft. Power to the ion source, for example the control signal to the mass flow controller or the anode signal, can be wired through the thermal switch. If a present temperature of the switch is exceeded, for example 100° C., the power to the ion source cuts out to prevent further heating. This protects components of the ion source such as the magnet and the projection, which can be destroyed by excessive temperatures. 
   Under some operating conditions, the voltage between the anode  112  and the shroud  102  can cause unwanted breakdown of the gas in the chamber  140 . To circumvent this, the size of the inlet aperture  141  to the chamber  140  is made smaller than the combined sizes of the channel apertures  126  thereby preventng excess pressure in the chamber  140 . 
   Referring now to  FIG. 6 , there is shown a modification of the ion source shown in  FIG. 4 , in which the filament mountings have been removed for clarity. In this embodiment, the gas supply is comprised of one or more tubes  170  extending into the ionization region through the open end  116 . Alternatively, the tubes may extend through channels in the anode side walls. In order that the tubes do not interfere with the establishment of the ion current, the tubes are non-conducting. The tubes also require a high thermal tolerance. Accordingly, a preferred material for the tubes is aluminium oxide or ceramic. The tube  170  extends from a gas manifold  171  located outside of the ionization region. The outlet of the tube  172  is provided adjacent the projection in order that the gas is provided into the ionization region at the point of highest electron concentration and electron energy, thereby increasing the ionization efficiency of the gas. 
   For each of the embodiments described herein, the ion beam can be maintained for a wider range of gas flow rates than for prior art ion sources because there is always at least a localised region of high pressure. The range of gas flows gives a corresponding range in the ion beam currents. A further advantage is that lower gas flow rates are required to achieve the equivalent or higher beam currents than for prior art devices. For example a gas flow rate of 4-5 sccm can achieve a beam current of 2 A in the present invention compared with 10-50 sccm required to produce 1 A of beam current in devices of the above mentioned Kaufman type. These lower gas flow rates assist in allowing a low background pressure to be maintained. 
   A further benefit of reduced flow rate is that the operational requirements of the vacuum pumping system used to evacuate the chamber in which the ion source is disposed can be reduced, while still maintaining lower background pressures than achieved in many prior art devices. This increases stability by reducing the chances of arcing and sputtering in the peripheral regions of the ion source. 
   Operating background pressures of the order 10 −3  Pa have been achieved with the present invention. At these pressures the mean free path of the ions is of the order of meters. This is important in many ion source applications because it is typically many times longer then the dimensions of the vacuum environment. For LAD processes, mean free paths of this order are longer than the typical distance between the ion source and the target substrates. The efficiency of the deposition process is therefore enhanced by these low background pressures because more primary ions impact the target substrates instead of undergoing secondary collisions with gas molecules. A further benefit of the reduced pressure is that contamination of the thin film coating, is considerably reduced. 
   While particular embodiments of this invention have been described, it will be evident to those skilled in the art that the present invention may be embodied in other specific forms without departing from the essential characteristics thereof. The present embodiments and examples are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.