Background of the invention 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 cause ionisation, and in some circumstances create 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-Hall 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.

A gridless open ended ion source has been developed by the present applicant and described in granted Australian Patent No. 749256, the entire contents of which are herein incorporated by reference. In that patent, an ion source having greater stability and a greater range of operating parameters was described. The improved performance arose from introducing the ionisable gas at a localised region of high electron concentration. An electrically conductive projection, projecting from, and in

electrical contact with the anode, provided both the focal point for the electrons and a gas outlet member for introducing gas to the ionisation region.

One problem with the configuration taught in the abovementioned patent, was that because the apertures of the gas supply path were formed in the outlet member itself, the gas was directed away from the outlet member, and thus away from the electron concentration, when it was introduced into the ionisation region of the ion source. Thus the gas efficiency was not necessarily maximised.

A further problem is that because the outlet member concentrates the electric flux, it receives the majority of the heat load to the anode. However the gas lines pass through the outlet member, thereby making it difficult to provide adequate water cooling to the projection.

Summary of the invention In a first aspect, the 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 produced by 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 comprises an end wall, a side wall extending from the end wall in the direction of the cathode and sloping outwardly in the direction from the end wall toward the cathode such that the end wall and side wall together define a substantially conical ionization region with a closed end at the end wall and with an open end toward the cathode, and wherein said gas supply path comprises one or more channels extending through said anode side wall, each of said channels terminating at an aperture disposed substantially adjacent said end wall.

Preferably the ion source further comprises an electrically conducting projection extending from, and in electrical contact with, the end wall.

Preferably each channel extends through the side wall in a direction generally toward the end wall.

In a further aspect, the 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 produced by 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 comprises an end wall, a side wall extending from the end wall in the direction of the cathode and sloping outwardly in the direction from the end wall toward the cathode such that the end wall and side wall together define a substantially conical ionization region with a closed end at the end wall and with an open end toward the cathode, and wherein said gas supply path comprises one or more tubes extending into said ionization region, each tube terminating in an aperture disposed adjacent the end wall.

Preferably each tube extends into the ionization region from the open end of the ionization region. Preferably each tube is comprised of a non-conducting material.

In a further aspect, the invention resides in an anode for an ion source, the anode comprising an end wall, a side wall extending from the end wall and sloping outwardly in the direction away from the end wall such that the end wall and side wall together define a substantially conical region with a closed end at the end wall and with an open end at an end of the anode opposite the end wall.

Preferably the anode further comprises a projection extending from the end wall into the conical region, the projection being electrically conductive and in electrical contact with the end wall.

Preferably the conical region is substantially symmetrical about an axis of the anode, said projection being disposed on said axis.

Preferably the projection provides a sloping or curved surface having an apex disposed on said axis.

Preferably the anode further comprises a cavity adapted to receive a cooling fluid. Preferably a thickness of said end wall between said conical region and said cavity is less than 5mm.

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: Figure 1 is a partial cross-sectional elevation of the ion source according to the invention; Figure 2 is a plan view of the ion source in Figure 1; Figure 3 shows a schematic of an anode having a heat sink; Figure 4 shows a schematic of a further alternative gas supply system for an ion source; and Figure 5 shows an alternative construction of the ion source.

Detailed description of preferred embodiments Referring first to Figures 1 and 2, there is shown an ion source 100 in accordance with a preferred embodiment of the invention. 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, in particular where the anode is uncooled.

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 Figure 1 is curved having an apex located on an axis of the anode. In alternative embodiments, the projection may have angled faces 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 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 (Figure 2) 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 Figure 1) 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 ionisation efficiency is thereby increased because

the gas molecules are introduced in proximity to and in the direction of the region of highest electron concentration and electron energy.

As shown in Figure 1, 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 10mm 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 2mm.

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 Figure 3. 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 preset 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.

In operation an ionizable gas, for example oxygen, nitrogen or argon, is supplied to the gas chamber 140 through inlet 141 via a mass flow controller or like regulator as is well known in the art. The gas is conveyed from the chamber 140 to the ionisation region 113 through the anode channels 125. The anode is charged in the range 0-300 V, preferably 150 V relative to the cathode which is at or near earth potential. The shroud is maintained at earth potential. A DC or AC heating current of approximately 12A is passed through the cathode to stimulate electron emission.

Electrons generated at the cathode are influenced by the anode potential and are accelerated toward it. The alignment of the magnetic field with the electric field causes the electrons 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. 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 create positive ions which 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 111.

The anode 112 is preferably made of stainless steel but has a coating of a non- oxidising electrically conductive material, for example TiN, on the inner surfaces 120, 121,123 and any other surface that in use may be exposed to bombardment by electrons and/or negative ions produced in the ionisation region. 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 otherwise shield the anode potential from the cathode.

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. Therefore, the size of the inlet aperture 141 to the chamber 140 is made smaller than the combined sizes of the channel apertures 126 to prevent excess pressure in the chamber 140.

Referring now to Figure 4, there is shown a modification of the ion source shown in Figure 1. In the illustration of Figure 4 the filament mounting 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 like ceramic. The tubes 170 extend from a gas manifold 171 located outside of the ionization region. The outlet of the tube 172 is provided adjacent the projection 123 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.

In Figure 5, there is shown an alternative construction of the ion source.

Instead of the magnet being located between the base 101 and the lower sealing ring, in this alternative embodiment, the anode 112 has a collar 190 that defines a recess 191 for receiving the magnet 114 therein. In this embodiment, the lower sealing ring is replaced with a more substantial base insulator 192 that provides a lower seal for the chamber 140 as well as retaining the magnet 114 within the recess 191. The base insulator 192 is provided with a groove that receives an o-ring 193 that allows a degree of thermal expansion of the base insulator. The magnet is therefore disposed closer to the ionisation region, and therefore the field imparted by the magnet to the ionisation region can be made stronger, or alternatively, a cheaper magnet can be used. Importantly, the magnet is disposed in thermal contact with the back surface of

the anode 112, which is cooled by the fluid entering the cavity 127 of the anode.

Therefore, overheating of the magnet, which could reduce or destroy its magnetic properties, is prevented.

Throughout the specification, the word"comprising"and variations such as "comprise","comprises"etc, are used inclusively, that is, the features specified may be combined with further features in various embodiments of the invention.

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.