APPARRTUS FOR AND METHOD OF PRODUCING IONS

The present invention relates to apparatus for producing ions by a discharge in a vapour or gas, and a method of producing such ions.

Three ion source types in common use are particularly relevant to this invention. In a Penning ion source (and the similar Bernas ion source) a generally cylindrical arc chamber has a heated filament cathode at one end for producing electrons to form the electrical discharge and has at the other end an anti-cathode for reflecting the electrons repeatedly along the arc chamber, thus producing a discharge between the cathode, the anti-cathode and the arc chamber body, which acts as the anode for the discharge. The discharge is collimated magnetically to an elongate column discharge, and the ions are extracted through a slit along one side of the arc chamber, by extraction electrodes. In a Freeman ion source, the ion producing cathode consists of an elongate heated filament passing along the length of the arc chamber parallel to the extraction slit for ions. Electrons are produced by the filament and travel repeatedly along the length of the arc chamber between opposed electron reflectors at opposite ends of the chamber. In a cold cathode ion source, two opposed cathodes are positioned at opposite ends of the arc chamber. The discharge is started by applying across the electrode a much higher potential difference than the normal operating potential. Once the discharge has started, it is maintained between the electrodes at a much lower voltage, but at a much higher voltage than would be necessary in a hot cathode discharge. Again the discharge is collimated parallel to the extraction slit. In all cases, the discharge is maintained by feeding into the arc chamber a vapour or gas which consists of or includes the element from which the required species of ion can be

produced. The entry point of the vapour or gas feed is positioned arbitrarily on the arc chamber, at some convenient structural position.

The principal use of ions produced from such ion sources is for implanting ions of a preselected species into a target element, for example the controlled introduction of a species into the surface layer of another material. The technique is important in semi-conductor technology where it is used in the manufacture of integrated circuits or other devices, particularly by modifying the conductivity of semi-conductor material by introducing chemical impurities into the material. Ion implantation is used particularly in the manufacture of large scale integrated circuit chips, and may be used in flat panel display (FPD) manufacture.

There commonly occurs in ion source technology a requirement to improve the efficiency of ion production, in order to give a higher output of a required species of ion from the feed material fed as a vapour gas into the arc chamber. For example in the case of boron and silicon ions, it is common to use as the feed material a halide of the element, for example BF ₃ , BC1 ₃ , SiF ₄ , or SiCl ₄ . In these cases the gas or vapour in which the discharge is produced contains only a minority of atoms of the required element compared with the halogen atoms.

The invention is concerned with supplying the feed material to the plasma of the ion-producing discharge in an ion source in a more efficient manner.

According to the present invention in one aspect there is provided apparatus for producing ions by a discharge in a vapour or gas, comprising an arc chamber, a first electrode constituting a cathode for providing electrons to

produce a discharge in a vapour or gas in the chamber whereby ions of a required species are produced from the vapour or gas, and supply means for supplying the vapour or gas into the plasma of the ion-producing discharge to maintain the discharge, in which the supply means provides at least one further electrode which is at a potential more negative than the said first electrode.

Preferably the apparatus includes means for collimating the ion-producing discharge in the arc chamber, the supply means being positioned so that the collimation is directed to, or close to, the region at which the supply means supplies the vapour and/or gas to the plasma of the ion producing discharge. Ideally the supply means should be on the axis of the collimated discharge but in the case of the Freeman source, where the filament is on the axis, the supply means is put as close to the axis as is practically possible.

Preferably the said further electrode is arranged to provide a secondary discharge in a position such that the vapour or gas supplied to the plasma of the ion producing discharge is exposed to the secondary discharge before reaching the main ion-producing discharge. For example the first electrode may be maintained at a negative potential in the range -40 to -80 volts, conveniently at -60 volts, and the further electrode may be maintained at a voltage in the range -300 to -700 volts, conveniently at -500 volts. Preferably the further electrode is at a substantial negative potential to provide an intense secondary discharge at the region of high pressure supply of vapour or gas to the primary discharge, by a process similar to that found in cold cathode discharges, thus exposing the feed vapour or gas to an intense discharge before reaching the primary discharge. Conveniently the further electrode comprises a hollow cathode structure such as to form a

localised secondary discharge in a position such that the vapour or gas supplied to the plasma of the main ion- producing discharge passes through the secondary discharge before reaching the main ion producing discharge.

It is to be appreciated that where features of the invention have been set out with regard to an apparatus, these features are also provided in accordance with a method of the invention, and vice versa.

In particular, there may be provided in accordance with another aspect of the invention, a method of producing ions by a discharge in a vapour or gas, comprising the steps of: producing a discharge in a vapour or gas, extracting ions of a required species from the said discharge in the vapour or gas, supplying a vapour or gas into the plasma of the ion-producing discharge to maintain the discharge, and creating at least one secondary discharge in the region at which the vapour or gas is supplied into the plasma of the main ion-producing discharge.

In accordance with yet another aspect, there may be provided in accordance with the invention a method of producing ions by a discharge in a vapour or gas, comprising the steps of: producing a discharge in a vapour or gas by means of a first electrode which constitutes a cathode providing electrons to produce the discharge, extracting ions of a required species from the said discharge in the vapour or gas, and supplying a vapour or gas into the plasma of the discharge to maintain the discharge, in which the vapour or gas is supplied into the plasma through or in the region of at least one further electrode at a potential more negative than the first electrode.

Preferably the method includes the step of creating an intense, localised secondary discharge in the region of the further electrode in a position such that the vapour or gas supplied to the plasma of the ion-producing discharge is exposed to the secondary discharge before reaching the main ion-producing discharge.

In one preferred form the method includes collimating the main ion-producing discharge, and supplying the vapour or gas into the plasma of the ion-producing discharge at a position such that the ion-producing discharge is directed to, or close to, the entry position of the vapour or gas into the plasma of the main discharge.

Preferably the method includes providing in the region of the secondary discharge a body of material which ^' includes or consists of an element for forming the required ions produced in the ion-producing discharge. The said body may be provided close to the further electrode, or the said secondary discharge may established at the surface of the said body of material, whereby the said material constitutes the said further electrode.

The said supply means may comprise a container communicating with the interior of the chamber for containing the material which includes or consists of the element for forming the required ions produced by the ion- producing discharge. Preferably the container is arranged to present a free surface of the material to the interior of the chamber in a position such that a discharge occurs at the surface of the material. In one form, the container is electrically insulating and an electrical contact is provided inside the container to apply to the material the said negative voltage of the supply means, and in an alternative form the container is formed of electrically conducting material and includes an electrical contact to

apply the said negative voltage of the supply means to the container. This latter technique is essential when the feed material is an electrical insulator (e.g. phosphorus).

In a number of preferred embodiments, the supply means includes a conduit for feeding a gas or vapour into the arc chamber through the material in the container, or into the arc chamber at a position close to a surface of material in the container.

It is a particularly preferred feature of the method that the method may include the step of transferring into the plasma of the main discharge, material from the said body of material which includes the element from which the ions are to be produced. The transferring step may be carried out by sputtering of the material and/or by a chemical reaction between a vapour or gas and the body of material, and/or by plasma reactions at the surface of the body of material. Conveniently the supply means includes heating means for heating the said material so as to produce in combination with the heat of the discharge at the further electrode one or more of the series of interactions set out above.

There are a number of variations in the physical arrangements of embodiments of the inventions, but the following main preferred forms may be used. Where the element of the required ions takes the form of a refractory material having a high melting point, conveniently the element can be provided in the said body of material as a powder, and a suitable vapour or gas, also containing the element preferably, can be passed through the powder into the arc chamber towards the plasma of the main discharge. The powder can be made to constitute the second electrode and the secondary discharge can take place at the surface of the powder. The main ion producing discharge can be

collimated towards the surface of the powder, and the secondary discharge at the surface of the powder can constitute an electron reflector for the primary discharge. In this case the container would be made of an insulating material.

In another form, for example where the element from which the ions are to be formed has a relatively low melting point this may be provided in a container communicating with the arc chamber, but remote therefrom, conveniently in liquid form. The material may be heated and a separate further electrode may be provided by the container itself, which may be formed with a hollow cathode structure where the container communicates with the arc chamber, so as to produce an intense secondary discharge through which vaporized material passes from the container into the arc chamber.

Another situation where the container itself maybe made to form the second electrode may be where the body of material containing the chosen element may be non- conductive. In such a case it will be necessary for the secondary discharge to be formed by a negative voltage applied to the container, rather than to the material itself. It will be appreciated that a number of variations may be provided suitable to different forms of material containing the chosen element.

Other variations occur with regard to the supply of vapour gas into the plasma of the primary discharge. In most arrangements, it will be convenient to feed into the arc chamber a vapour or gas, of the normal kind which would be use to support the discharge, and then to enrich the vapour or gas by the various reactions taking place at the body of material including the chosen element. However, in some circumstances the entire supply of vapour or gas may

be provided from the said body of material, for example where the body of material is a material having a vapour pressure equal to the required discharge pressure at conveniently achieved temperatures. In such circumstances, the material may be provided in a heated container towards which the main discharge is collimated, and the required vapour or gas may be provided solely by vaporization from the body of liquid material.

Another variation is for a further electrode to be made of the required element itself; in this case no powder or liquid feed material is required. This means that two anti-cathode further electrodes can be used as in a cold cathode source, there being no problem of the feed material falling out of the "upside-down" electrode. The feed gas can be fed through the bore of both anti-cathodes or through just one.

The present invention has particular application in the production of ions of boron and silicon, particularly ions of boron. Boron is a very important implant species for semi-conductor and flat panel display (FPD) ion implantation» Compared with most other species in these technologies, the ion beam currents are relatively low. Nevertheless it is often required to provide high dose implants, such as the 2E15 ions/sq.cm. implant required for FPD implant, and in such instances the lack of beam current has a significant commercial cost impact.

Boron has a low vapour pressure and temperatures of the order of 2000°C are required if the element is to be used as the feed material. If boron were to be used as the feed material in conventional arc chambers, then the arc chamber would have to be at a similar temperature in order to prevent condensation on the walls of the arc chamber. Conventionally it has therefore been necessary to use

compounds with higher vapour pressures as the feed material. The most used feed gas is BF ₃ . Clearly there are four atoms in this molecule and only one is of interest. Therefore any technique which can increase the percentage of boron in the primary plasma is beneficial, and the present invention allows such an improvement. Conveniently boron powder can be used ^* as the said body of material, and BF ₃ can be used as an input gas passing through the boron powder which constitutes the said further electrode.

A number of advantages arise in at least preferred embodiments of the invention. For feed materials with low vapour pressures at high temperatures, and having high melting points, the arrangement of having the feed material surface exposed to the plasma allows the surface to get hot and to react with the reactive radicals present in the plasma discharge. The primary and/or secondary discharges enable reactions to occur which would normally need significantly higher temperatures. Positively charged ions and radicals in the plasma will hit the feed material surface with an energy determined by the negative bias on the feed electrode.

Having the feed material in the form of a powder, where convenient, with poor thermal conductivity in a low pressure environment (small contact area between the powder particles) causes the surface exposed to the plasma to reach a temperature significantly higher than the bulk of the feed electrode.

In known systems, melting a high melting point metallic element (for example silicon) or vaporizing a high melting point solid element (for example boron) by inducing high temperature reactions therein, is not an attractive proposition at temperatures much above 1200°C when the heat is supplied from an external heater, because the bulk being

heated becomes inconveniently large. The advantage in the present invention when used with such materials is that all the heating power can be directed directly onto the target of interest, i.e. the surface of the body of material. In this case the heat is transported by the bombarding positive ions from the plasma which also induces physical, chemical and other plasma reactions, for example caused by active radicals from the plasma, such a fluorine atoms from fluoride feed gases such as BF ₃ and SiF ₄ .

For easily vaporized or gaseous feeds, for example phosphorous, the main advantage of using a secondary discharge in accordance with the invention is to have an ionisation and molecular breakup stage before the feed enters the primary plasma. If a sufficiently high voltage is used (for example -500V), an intense, hollow cathode ^' discharge can be created at the end of the supply tube.

Important benefits which arise at least in preferred aspects, arise from the combination of substantial negative bias at the second electrode, and high pressure at the feed vapour/gas entry region. This results in an efficient means of creating a high temperature (the heating is directly onto the reaction surface making high temperatures of the order 1500 to 2000"C a far more practical proposition than would be the case for externally applied heat) and also results in an important plasma component to the reaction process (due to ions and active radicals in the arc discharge plasma) together with the physical (sputtering and evaporation) and chemical processes that transport the required species into the primary plasma.

Embodiments of the invention will now be described by way of example by reference to the accompanying drawings in which:-

Figure 1 is a diagrammatic representation of a previous known Penning/Bernas ion source;

Figure 2 is a diagrammatic representation of a known Freeman ion source;

Figure 3 is a diagrammatic representation of a previously known cold cathode ion source;

Figure 4 is a diagrammatic side section through apparatus for producing ions, embodying the invention, generally of the Penning/Bernas type, suitable for use with a high melting point, low vapour pressure, feed material;

Figure 5 is a diagrammatic side section of an apparatus for producing ions, embodying the present invention, generally of the Penning/Bernas type, and suitable for use with a material of relatively high vapour pressure;

Figure 6 is a diagrammatic side section of apparatus for producing ions, embodying the invention, generally of the Freeman type, and suitable for use with a material of high melting point and low vapour pressure;

Figure 7 is a diagrammatic side section of apparatus for producing ions, embodying the invention, generally of the Freeman type, and suitable for use with a material of relatively high vapour pressure;

Figure 8 is a diagrammatic side section of apparatus for producing ions, embodying the invention, generally of the Freeman type, and suitable for use with a material of high melting point and very low vapour pressure, showing an alternative configuration of electrodes;

Figure 9 is a diagrammatic side section of apparatus for producing ions, embodying the invention, generally of the Freeman type, and suitable for use with a material of relatively high vapour pressure, but lower than that shown in Figure 7;

Figure 10 is a diagrammatic side section of an apparatus for producing ions, embodying the present invention, generally of the Penning/Bernas type, and suitable for use with a material of relatively low melting point, but with the facility for the input of feed gas just above the surface of the molten feed material; and

Figure 11 is a diagrammatic side section of apparatus for producing ions, embodying the invention, generally of the Freeman type, and suitable for use with a material of relatively low melting point, and with the facility for extra input of feed gas above the molten feed material; and

Figure 12 is a diagrammatic side section of apparatus for producing ions, embodying the invention, generally of the Freeman type and suitable for use where the required feed material may form a supply means without a separate container.

Figure 1 shows a previously known ion source of the Penning type, also referred to as the Bernas type. An arc chamber 11 has a cathode 12 comprising a heated coil for generating electrons to produce the discharge. At the other end of the chamber 11, and anti-cathode 13 reflects the electrons back towards the cathode 12. An electron reflector 14 further reflects the electrons, so as to produce a discharge 15. Vapour or gas is supplied to the arc chamber 11 through a conduit 16 from a container 9 for a feed material 6 having a heater coil 8, and an input conduit 7 for feed gas. The discharge 15 is collimated by

electro-magnets (not shown). The arc chamber 11 has an extraction slit 17 parallel to the discharge 15 through which selected species of ion are extracted by extraction electrodes 18. The extracted ion beam is indicated diagrammatically at 19.

Figure 2 shows a previously known ion source of the Freeman type. This is generally similar to the ion source shown in Figure 1, except that the cathode comprises a single filament 20 extending through the arc chamber parallel to the extraction slit. Electron reflectors 14 are provided at each end of the chamber.

Figure 3 shows a previously known cold cathode source. Again the general arrangement is similar to that shown in Figure 1, except that in this form, two cold cathodes 21 and 22 are provided one at each end of the arc chamber. In normal use the cathodes 21 and 22 are maintained at a voltage of approximately -200 to -800 volts. To start the discharge a much higher voltage of perhaps -2000 volts is applied.

Figure 4 is a diagrammatic side section of a first embodiment of the invention, comprising an ion source generally of Penning/Bernas type. The apparatus comprises an arc chamber 11, heated cathode coil 12, extraction slit 17, and extraction electrodes 18, generally as described with reference to Figure 1. At the end of the arc chamber 11 remote from the cathode 12, there is positioned supply means 24 for supplying vapour or gas into the plasma of the ion-producing discharge 15. The supply means comprises a cylindrical container 25, set in an insulating plug 26 in the wall of the arc chamber 11, containing a body of material 27. At the base of the container 25 is a porous plug/electrical contact 28 connected by leads not shown to means for supplying a required negative voltage to the

interior of the container 25. At the base of the container 25 a conduit 29 is provided for passing a feed gas through the body of material 27 into the interior of the arc chamber 11. Around the outside of the container 25 is wound a heater coil 30.

The discharge 15 is collimated by an electro-magnetic field produced by means not shown, in the normal manner of an Penning/Bernas ion source. The discharge 15 is collimated so as to be directed towards the surface of the body of material 27, also referred to as the feed material. The feed material is chosen to be a material which includes or consists of an element for forming the required ions produced by the ion producing discharge 15. The embodiment of Figure 4 is suitable for producing Boron ions, and in such a case the material 27 may comprise Boron powder. The feed gas fed along the conduit 29 may conveniently be BF ₃ , and the container 25 may conveniently be constructed of an electrically insulating material, conveniently Boron nitride.

It will be appreciated that the Boron material 27 constitutes a further electrode (also referred to as a second electrode) provided by the supply means 24, and the material 27 is maintained at a potential which is more negative than the potential of the cathode 12, preferably at a substantial negative voltage which is considerably more negative than the voltage of the cathode 12. By way of example the cathode 12 may be maintained at minus 50 volts, and the second electrode 27 may be maintained at -

500 volts. The effect of this is that the feed material 27 constitutes a negative electrode producing an intense localised secondary discharge at the surface of the material 27. This secondary discharge 30 acts as an electron reflector for the main primary discharge 15. The feed gas passing through the material 27 has to pass

through the secondary discharge before reaching the plasma of the primary discharge 15. A number of complex interactions take place at the surface of the material 27, which result in transfer of the material 27 into the plasma of the arc 15 together with the feed gas passing through the material 27.

Taking the example of Boron powder, this may be raised to a high temperature by the effect of the heater 30, and more importantly, by the bombarding of the surface of the material by the main discharge 15. There are a number of processes that can occur when the boron powder is raised to a high temperature for example 1500°C at a substantial negative voltage, for example 500 volts, and it is exposed to the primary and secondary plasma, and the feed gas. These interactions are:-

(a) Physical reactions, for example boron atoms can be sputtered from the boron powder surface by the incoming 500ev ions.

(b) Chemical reactions, for example the boron can react with BF ₃ at such temperatures. The large surface area of the boron powder exposed to the BF ₃ passing through the powder will facilitate reactions. Also the boron may react with fluorine molecules, formed after the breakup of the feed gas to form a number of fluorides.

(c) -Plasma reactions will take place at the boron surface due to the secondary and primary discharges, and it is thought that these reactions may be of most significance in carrying boron into the primary discharge 15.

(d) A boron fluoride ion with any number of fluorine atoms in the ion hits the boron surface at a high energy (say 500ev) and is therefore effectively at a much higher temperature than the general background temperature. It is therefore to be expect that the reactions will take place at a much more convenient temperature than would be the case if the boron powder 27 were heated uniformly only by an external heater.

(e) The plasma will produce fluorine radicals i.e. fluorine atoms which will rapidly react with the boron surface even when in the unionised state.

The arrangement of Figure 4 is also suitable for producing silicon ions. Silicon is an important ion for GaAs implant and FPD implant. Like boron the most favoured feed material is the halide, usually the fluoride or chloride. Unlike boron a useful degree of vaporization can be achieved at 1500 ^β C, but the element is liquid at this temperature. The arrangement of Figure 4 is suitable for use with silicon powder in the container 25, if kept below 1400°C. An arrangement for use with silicon above this temperature is described hereinafter with reference to Figures 10 and 11.

Figure 5 shows an alternative embodiment of the invention comprising an ion source generally of the Penning/Bernas type, but adapted for use with an easily vaporized or feed material, for example phosphorous. in the arrangement of Figure 5, there is provided at the end of the arc chamber 11 remote from the cathode 12, a container 25 having an elongated neck 33 communicating with the interior of the arc chamber 11 through an insulating plug 26. Between the neck portion 33 and the main body of the container 25, there is a restricted opening 34. In

the arrangement of Figure 5 there is no provision for feeding a feed gas or vapour through the body of material 27. In use, the material 27, for example phosphorous is heated by the heating coil 30 until it reaches the required vaporisation temperature. The primary discharge 15 is produced by electrons generated by the cathode 12, and is collimated towards the opening of the neck 33 to form a secondary hollow cathode discharge of the cold cathode type.

Where the material contained in the container 25 is not electrically conducting, for example phosphorous, the container 25 is made of electrically conducting material, and is held at the high voltage of for example 500 volts to produce the second electrode of the supply means. In such a case the neck 33 of the container 25 constitutes the second electrode, and produces an intense secondary discharge inside the neck 33, generally in the manner of a hollow cathode discharge. The primary discharge 15 extends down through the neck 33 into the main container 25, but its effect is reduced compared with that shown in Figure 4. This is necessary because otherwise the easily vaporized material 27 would be overheated and would disperse within the arc chamber 25.

In modifications, the container 25 may be made of electrically insulating material, and may contain an electrically conductive liquid material 27 or electrically conducting solid powder. In such a case an electrical contact is provided within the container 25 in the same way as was shown in Figure 4, and in the case of the powder this contact must be porous to allow the passage of feed gas through the powder.

Figure 6 shows a further alternative embodiment of the invention, comprising an ion source generally of the

Freeman type described previously with reference to Figure 2. The embodiment of Figure 6 has an arc chamber 11 containing a Freeman filament cathode 20, and electron reflectors 14, extraction slit 17 and extraction electrode 18, generally as previously described with reference to

Figure 2. At the base of the arc chamber 11 is positioned a container 25 generally ^" of the form of the container 25 shown in Figure 4, and containing a body of material 27, for example boron powder of silicon powder below 1400°C. A feed gas, for example BF ₃ in the case of boron, is fed upwardly through the conduit 29, and passes through the feed material 27. The operation of the device is generally the same as that described with reference to Figure 4, except the primary electrical discharge surrounding the Freeman filament 20 is not collimated directly at the surface of the feed material 27, but is directed generally to a position close to the filament and consequently close to the material 27. In the arrangement of Figure 6, the material of the container 25 is conveniently insulating material, and an electrical contact 28 is provided to apply a negative voltage to the material 27, which in this case acts as the second electrode.

In Figure 7 there is shown a modification of the arrangement of Figure 6, generally in the form of a Freeman ion source. Here the container 25 is generally of the form which has been described with reference to Figure 5, but is again set to one side of the Freeman filament 20. The arrangement of Figure 7 is again suitable for use with phosphorous as the material 27, heated for example to approximately 400°C by the effect of the heater 30 and the primary discharge from the filament 20. In Figure 7, the neck 33 of the container 25 is spaced somewhat from the Freeman filament 20, to avoid the container 25 becoming too hot in operation.

In Figure 7, two electron reflectors 14 are shown above the feed anti-cathode supply means 24 because it may be desirable to have one in contact with the filament to prevent electrons reaching the filament insulator and the other at anti-cathode potential to act as a high voltage reflector. This latter would favour multiple charged ion production. A similar set of reflectors 14 are found at the top of the arc chamber.

In Figure 8 is shown a further modification which corresponds generally to the arrangement of Figure 6, except that the container 25 is of annular form, surrounding the base of the Freeman filament 20. The container 25 is not coaxial ^' with the filament 20, but is offset in a direction away from the extraction slit 17, for structural convenience. In the arrangement of Figure 8, the container 25 is conveniently made of electrically conducting material or insulating material for a conducting powder and the negative voltage is applied to the container or directly to the powder material forming the second electrode. (Powder is a general term that could include lumps, turnings, wire etc.). This structure built around the filament will get hotter than the type in Figure 6 and will therefore be ideal for boron where high temperatures are desirable.

The arrangement of Figure 9 shows a further alternative embodiment, which again corresponds generally to the arrangement of Figure 7, except that in Figure 9 the container,25 has the lower end of the Freeman filament 20 set directly into the container 25. This has the effect of making the material 27 to be vaporized to be raised to a much higher temperature. The arrangement of Figure 9 is shown with the container 25 formed of electrically insulating material, and with an electrical contact plate 28 set into the bottom of the container 25.

Figure 10 shows a further alternative embodiment of the invention. This embodiment is an ion source generally of the Penning/Bernas type and corresponds to the embodiment shown in Figure 4 except that it is suitable where the material 27 is in the form a of liquid, for example molten material which has been raised to the liquid state by the heater 30, for example molten silicon. The silicon liquid can be raised to a temperature where the silicon vapour supports the discharge without a feed gas but in practice the high temperature required to prevent condensation of excessive quantities of silicon in the arc chamber make it inconvenient. In this case, it is not appropriate to pass the feed gas through the liquid material, so a separate conduit 16 is provided at the side of the container 25, with its inner end positioned close to the free surface of feed material 27. Suitably, the feed gas may be silicon fluoride.

Figure 11 shows a further alternative embodiment, comprising an ion source generally of the Freeman type and corresponding generally to the arrangement shown in Figure 6, but adapted again for use with a molten material 27 in the container 25. Again a side conduit 16 is provided for feeding a gas into the container 25 at a position close to the surface of the feed material 27.

Figure 12 shows a yet further alternative embodiment, comprising an ion source generally of the Freeman type and corresponding generally to the arrangement shown in Figures 6, 8 and 11. Figure 12 shows an arrangement where the feed anti-cathode supply means 24 is made from the required feed material and where such material 27 is conveniently machinable and no other powder or liquid is involved. The lack of a powder or liquid means that gravity is not an issue and feed supply means 24 can be used as shown in the drawing. This technique would be particularly suitable for

transition metals which in general have relatively volatile halides. The principle is shown in Figure 12 with a Freeman source but can equally well be applied to the Penning/Bernas source.