The present invention relates to apparatus and methods for use in producing ions by gaseous discharge. The invention relates to apparatus for providing a feed material for supply to an electric discharge ion source, and also to apparatus for producing ions of a required species by means of an electric discharge in gas, and to related methods.

Ions are commonly produced in an arc chamber by an electric discharge in a gas or vapour. In this specification where reference is made to a feed gas for a discharge, the term is to be taken also to include other gaseous material such as vapour. 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 and anti-cathode. 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. 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 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) ion implantation.

The success of the ion implantation technique depends crucially upon the high purity of the beam of ions which is directed at the target. This is normally achieved by mass analysis of the ions using an analysing magnet to select the required ion. Thus it is possible to remove unwanted ions from the beam being produced, provided the element to be removed has an isotope mass which is different from the element which it is desired to implant by the beam. If it happens that a required ion species is available only in an isotope having a mass which is the same as contaminant gases, then it is not possible to remove unwanted ions from the beam, and it will be usual to use ions of a different isotope where the mass is different from the mass of the contaminant ions.

Turning now to a specific case, there is a significant commercial requirement for high dose silicon (Si+) ion implantation in gallium arsenide (GaAs) technology and flat panel display (FPD) technology. Silicon has three isotopes:-

Mass 28 - abundance.92.23%

Mass 29 - abundance 4.67%

Mass 30 - abundance 3.10%

Clearly the mass 28 isotope would be the obvious ion species to use for ion implantation. Unfortunately mass 28 is also the mass of molecular nitrogen and carbon monoxide. The success of ion implantation in semiconductor technology results largely from the high purity of the doping species, which is possible as the result of mass analysis using an analysing magnet to select the required ion. Such analysis would not distinguish between the mass 28 isotope of silicon, and the unwanted nitrogen and carbon monoxide.

In the case of Si+ implantation in GaAs technology the problem has been overcome by using either the mass 29 or 39 isotopes of silicon. This is satisfactory when small doses are required as the low abundance of these species does not have any significant commercial impact. When high doses are required the situation changes dramatically with regard to cost.

Thus, it has previously been considered that pure Sin- beams can only be obtained from the 29 and 30 isotopes of silicon. The problems which have previously prevented use of the mass 28 isotope are as follows.

(i) Air leaks into the gas supply system, particularly in the part of the system at a low pressure, result in the introduction of both nitrogen and oxygen.

(ii) The supply gas for silicon has invariably been silicon tetrachloride or silicon tetrafluoride, and there is no way of being sure that there is no air in the supply gas.

(iii) Ion sources often use boron nitride as an insulator and this can introduce nitrogen into the discharge.

(iv) Arc chambers are usually made of graphite and this can introduce carbon into the discharge which can combine with oxygen to form a carbon monoxide ion.

It is an object of the present invention to provide apparatus, and methods, for use in the production of ions by a gaseous discharge, where there is particular difficulty in removing unwanted ions from an implantation beam.

According to the present invention in a first aspect there is provided apparatus for providing a feed material for supply to an electric discharge ion source comprising: a vessel assembly comprising a vessel for the feed material,- a feed conduit leading from the vessel for passing feed gas to an electric discharge ion source, and a jacket surrounding the feed conduit; and a purifying assembly comprising a flow chamber adapted for connection to the jacket to communicate with the jacket and the feed conduit, means for supplying into the flow chamber a feed gas, means for cooling the feed vessel to condense into the

vessel the feed material, means for evacuating the chamber and vessel to remove contaminating gases, means for supplying into the chamber and vessel an inert gas, and means for closing the feed conduit; whereby when the condensed feed material is allowed to return to ambient temperature, the feed material may be stored in the vessel at a pressure above atmospheric to ensure that any leaks are outward leaks.

In accordance with the invention in a second aspect, there is provided apparatus for producing ions of a required species by means of an electric discharge, comprising: a feed vessel assembly comprising a vessel for the feed material, a feed conduit leading from the vessel, and a jacket surrounding the flow conduit; and an ion source assembly comprising an arc chamber, means for producing an electric discharge in the chamber to produce ions of a required species, a further feed conduit for supplying a feed gas to the discharge, the further feed conduit being adapted' for connection to the feed conduit of the feed vessel assembly, and a further jacket adapted for connection to the jacket of the feed vessel assembly, means for evacuating the communicating jackets, and means for subsequently supplying to the communicating jackets an inert gas above atmospheric pressure; whereby feed gas may be supplied from the feed vessel to the ion source through the feed conduits while protected against inward air leaks by the jacket containing inert gas above atmospheric pressure. Preferably, the vessel assembly includes a feed valve in the feed conduit and the jacket of the vessel assembly surrounds the feed conduit and the feed valve.

The invention has particular application where the ion source is arranged to produce predominantly the mass 28 ion of silicon using a discharge with a trichlorofluorosilane

(SiCl ₃ F) feed. Preferably the apparatus includes means for

supplying argon as the said inert gas. Also preferably the apparatus includes means for testing, after an evacuation, for the presence of oxygen or nitrogen or compounds thereof in the assembly which has been evacuated.

It is to be appreciated that where features of the invention have been set .out with regard to an apparatus, these features may also be provided in accordance with a corresponding method, and vice versa. In particular there may be provided in accordance with the first aspect of the invention a method of providing a feed material for supply to an electric discharge ion source, comprising the steps of: evacuating a vessel for the feed material; supplying to the vessel at a pressure above atmospheric pressure a gaseous feed material suitable for the required discharge; cooling the vessel to condense the gaseous feed material to a liquid or solid in the vessel; evacuating the vessel sufficiently to remove contaminant gases; supplying into the jacket an inert gas; warming the condensed feed material to ambient temperature to provide gaseous feed material in the vessel above atmospheric pressure so as to avoid inward air leaks into the vessel.

In accordance with the second aspect of the invention there may be provided a method of supplying a gaseous feed material to an electric discharge ion source, comprising the steps of: providing feed material in a vessel in a condensed state at above atmospheric pressure; evacuating a jacket which surrounds a feed conduit leading from the vessel to the discharge ion source; filling the jacket with an inert gas above atmospheric

pressure; passing the feed material gas at a pressure below atmospheric pressure from the vessel to the discharge ion source through the feed conduit while protected by the inert gas shield above atmospheric pressure in the jacket.

Preferably the said step of providing the feed material in the said vessel, comprises: evacuating the vessel for the feed material; supplying to the vessel at a pressure above atmospheric pressure the gaseous feed material suitable for the required discharge; cooling the vessel to condense the gaseous feed material to a liquid or solid in the vessel; evacuating the vessel assembly sufficiently to remove contaminant gases; supplying into the jacket an inert gas; and warming the condensed feed material to ambient temperature to provide gaseous material in the vessel above atmospheric pressure so as to avoid air leaks into the vessel.

Preferably, the electric discharge produces predominantly the mass 28 ion of silicon, and preferably the feed material gas comprises trichlorofluorosilane (SiCl ₃ F). Preferably the said inert gas is argon, and preferably after the or each evacuating step the evacuated volume is tested for the presence of oxygen or nitrogen or compounds thereof. It is particularly preferred that the feed material is such as to react with water vapour to produce a non-volative compound containing any contaminant oxygen present, and the method includes removing such contaminant gas by evacuation.

There will now be described a number of preferred features which may be incorporated in the apparatus and method of the invention. In its preferred application, the invention may provide a technique for the production of substantially nitrogen and oxygen free feed material for

the production of the mass 28 ion of silicon (thus uncontaminated by nitrogen or carbon monoxide ions) by the use of two main features, namely:-

(i) by using a feed material having a storage pressure at ambient temperatures in excess of atmospheric pressure, and having a vapour pressure when at a conveniently achieved low temperature, which is low enough to allow removal of contaminating nitrogen and oxygen by pumping with a high vacuum pump; and

(ii) by arranging the delivery of the purified feed material at a relatively low pressure (below atmospheric), for example between a regulating valve and the arc chamber, by a system which is protected from air leaks by the use of a protective jacket containing a suitable non contaminating gas, or at vacuum.

In the preferred application of the invention with mass 28 silicon ions, features may be selected to deal with the two requirements of avoidance of nitrogen contamination, and avoidance of carbon monoxide contamination. Considering first the avoidance of nitrogen contamination, the entire gas supply system is jacketed so that the environment outside the gas supply system can be controlled so as to make air leaks impossible. This region can either be filled with a convenient gas at a pressure in excess of atmospheric pressure or a vacuum shield can be used. The first technique is preferred because air leaks are both prevented and leaks of the shielding gas can be detected if the species of the shielding gas is chosen to be one which would not naturally be found in the system. A second step is that the feed material is chosen to be a compound of silicon which has a vapour pressure in excess

of atmospheric pressure at ambient temperature (or at a controlled temperature) so that leaks are outwards, not into the containment vessel, but which has a vapour pressure at liquid nitrogen temperature which is so low that an extensive period of pumping the solid or liquid (at liquid nitrogen temperature) makes it possible to pump any nitrogen away.

Avoidance of carbon monoxide contamination is an easier problem in that there are two elements required to produce the contaminating species, and if the two component species can be reduced to low levels then the concentration of the compound contaminant ion will be extremely low. The difficulty is that carbon and oxygen are common contaminants in vacuum systems, carbon from oils and greases, and oxygen from air, water vapour and metal oxides (as surface films or as ceramic construction materials for high temperatures or electrical isolation).

The solution provided, in preferred forms, by the invention is firstly to use the technique set out above for the elimination of air leaks and the removal of oxygen from the feed gas. Next, the choice is made of a feed gas that will react with water vapour to produce a non-volatile compound containing the oxygen. Use is made of a material such as molybdenum as the arc chamber construction material, so as to avoid the use of carbon extraction electrodes of molybdenum are a wise precaution. Next, use is made of clean vacuum pumps to avoid hydrocarbon contamination of the ion source vacuum, and an arc chamber design that avoids the use of exposed insulators containing nitrogen or oxygen.

Preferably, the protective jacket is pressurised to a few psi above atmospheric pressure with an inert gas, preferably neon. (Helium is an obvious candidate but it is

used for vacuum leak detection and therefore may be precluded. )

Possible feed gases or vapours which may be used are as follow.

a) Silicon tetrachloride (tetrachlorosilane) . This is not convenient because it would have to be heated above its boiling point of 56.8°C in order to have a storage pressure above atmospheric pressure.

b) Silicon tetrafluoride (tetrafluorosilane) . This is also not convenient because it would be stored as a high pressure gas. The pressure regulator necessary when using a high pressure gas acts as a poor vacuum conductance and makes the purification process difficult but not impossible. However, the vapour pressure at liquid nitrogen temperature is usefully low, approximately 2E-7 torr.

c) Trichorofluorosilane. This is the preferred feed vapour for this application of the invention. It may be stored at approximately

4 psi pressure at 20 ^C C, and has a boiling point of 12.2°C and vapour pressure of approx. 1E-11 torr at liquid nitrogen pressure.

d) Other possibilities are dichlorofluorobromo- silane and hexafluorodisilane; the latter has the advantage of a higher Si fraction in the molecule and has an extremely low vapour pressure at liquid nitrogen temperature (1E-17 torr) despite having a high vapour pressure at room temperature (230psi at 20°C).

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

Figure 1 is a diagrammatic representation of a feed containment vessel for use in a method embodying the invention for providing a feed material to an electric discharge ion source;

Figure 2 is a diagrammatic representation of apparatus embodying the invention for providing a feed material for supply to an electric discharge ion source, including the feed containment vessel of Figure 1, and a purification assembly for purifying the feed material;

Figure 3 is a diagrammatic representation of apparatus embodying the invention for producing ions of a required species by an electric discharge, and includes the feed containment vessel of Figure 1.

Referring to Figure 1, a vessel assembly 10 comprises a feed vessel 12 for containing a feed material for a gaseous discharge ion source, as will be described hereinafter. The feed material may for example be a silicon-containing material, preferably trichlorofluorosilane (SiCl ₃ F), and is shown diagrammatically in a condensed liquid state (as will be described hereafter) at reference numeral 11.

The vessel assembly 10 includes an upper housing 18 which acts as a protective jacket surrounding a conduit formed by a feed pipe 34 including a pipeline valve 17, also referred to as a feed valve. The feed valve 17 is operated by an extended shaft 37 which extends outside the jacket 18 through a vacuum seal 44 set in a demountable

joint 16. The feed pipe.34 exits from the feed vessel 12 through a further demountable joint 13, and the jacket 18 has two further demountable joints 14 and 15 which may be closed by blanking flanges, or may be used to join the jacket 18 to other components as will be described hereinafter. The demountable joints 13, 14, 15 and 16 are such that they can be protected from air leaks by evacuating or pressurising the upper vessel forming the jacket 18.

In Figure 2, the feed vessel assembly 10 is shown with a top blanking flange 33 closing off the joint 15, and with the joint 14 coupled to a purifying assembly indicated generally at 19, for purifying feed material to be stored in the feed vessel 12. The purifying assembly 19 comprises a flow chamber 8 having outlets leading to a residual gas analyser 22, to a high vacuum valve 20, and to a gas supply system through a supply pipe 9. The hi-vac valve 20 controls access to a high vacuum pump 21 powered by a rotary pump 30. The supply pipe 9 communicates with a

Pirani gauge 28, a high pressure gauge 26, and isolating valve 24, and two further supply valves 23 and 25 which control respectively supply of a silicon containing gas (to provide feed material for an arc discharge), and argon (to provide an inert gas for storage purposes in connection with the silicon containing feed gas).

The use of the purifying assembly 19 will now be described. Initially the system is arranged with the feed valve 17 open, the hi-vac valve 20 closed, the isolating valve 24 open, the silicon valve 23 closed, and the argon valve 25 open, so that argon at above atmospheric pressure is provided on both sides of the isolating valve 24 before the pump down begins. The isolating valve 24 is then closed, but the argon valve 25 is left open during the initial stages of the pump down.

Next, with the isolating valve 24 closed, the hi-vac valve 20 is opened, so that the pump 21 can evacuate the whole of the feed vessel assembly 10, and the purifying assembly 19, up to the isolating valve 24. The whole system is then pumped down to a high vacuum, IE-6 torr or better if required (1E-10 if UHV components are used) with the feed valve 17 open and the hi-vac valve 20 open. The partial pressures of the background gases in the system are monitored using the residual gas analyser 22. Masses 28 and 32 are monitored for evidence of air leaks. Water vapour at mass 18 is also a possible source of oxygen, but (at a later stage to be discussed) it will react with silicon halide feed materials and will be removed as a potential contaminant. The gas manifold valves (23 closed, 25 open) are set so that the silicon containing gas (with its mass 28 silicon isotope component) cannot reach the vacuum, by the effect of the argon at above atmospheric pressure.

When the background vacuum is at an acceptable level, there being no significant leaks, then the argon valve 25 can be closed and the isolating valve 24 opened so that the region between the valves 24 and 25 can be pumped and leak checked.

When a good leak free vacuum is achieved (the mass 32 peak is monitored to check for air leaks) the hi-vac valve 20 is closed and the silicon containing feed material, supply valve 23 opened. In the case of SiCl ₃ F, the pressure will rise to about 4 psi above atmospheric and is monitored on the high pressure gauge 26. Liquid nitrogen is then introduced into a Dewar flask 27 surrounding the feed material vessel 12, so that the feed material 11 will liquify and then solidify in the bottom of the vessel 12.

When all the feed material 11 has been transferred from the external source (not shown) to the feed vessel 12 the pressure will fall to a level determined by non- condensable background gases such as nitrogen and oxygen, the vapour pressure of the feed material 17 being extremely low (approx. 1E-11 torr). The pressure is monitored in the coarse vacuum range by the Pirani (or thermocouple) gauge 28 and in the hi-vac range by the residual gas analyser 22 or other gauges. The isolating valve 24 is then closed and valve 23 closed and valve 25 opened in order to have argon on the high pressure side of the valve 24; then the hi-vac valve 20 is opened so that the residual contaminant gases are pumped away.

The condensate feed material 11 will be a solid and there may be a small quantity of nitrogen and oxygen trapped in this solid material. It may therefore be desirable to close the feed valve 17, warm the condensate 11 until it becomes liquid, and then refreeze the feed material and open the feed valve 17 to complete the removal of contaminant gases.

Next, the feed valve 17 is closed, the hi-vac valve 20 closed and the system pressurised with argon (argon valve 25 open) or some other convenient gas, while the feed material 11 is warming to ambient temperature and consequently reaching a pressure in the feed material vessel 12 above atmospheric. It is then possible to turn off the argon supply at 25 and remove the feed vessel 12 from the system without any contamination hazard. The vessel 12 can be stored at a temperature above the boiling temperature of the feed material 11 or transferred to an ion implanter, as will be described.

In Figure 3 there is shown apparatus embodying the invention for producing ions of a required species by means of an electric discharge. The apparatus includes the feed vessel assembly 10, and an ion source assembly indicated generally at 31. The ion source assembly 31 comprises an ion source chamber 32 containing an ion source 31, which may for example be of the Freeman type consisting of an elongate heated filament 43 passing along the length of an arc chamber 44 situated within the ion source chamber 32. The filament 43 is arranged parallel to an extraction slit 45 for ions. Electrons are produced by the filament and travel repeatedly along the length of the arc chamber between opposed electron reflectors (not shown) at opposite ends of the chamber 44. Extraction electrodes 46 produce an ion beam which is directed out of the ion source chamber 32 at an exit 47. The discharge produced by the filament 43 is maintained by a gaseous feed material which is fed to the discharge along a further conduit comprising a feed pipe 35. The ion source assembly 31 also includes a residual gas analyser 39 communicating with the ion source chamber 32.

The arrangement of the connection of the feed vessel 12 to the ion source 31 is as follows. The top blanking flange 33 (shown in Figure 2) is removed, and the joint 15 of the feed vessel assembly 10 is bolted to a junction 50 of a second jacket 46, which in turn is bolted at a junction 51 to the ion source chamber 32. The feed pipe 34 of the vessel assembly 10 is joined to the feed tube 35 of the ion source by locating into an "0" ring sealed receptor tube indicated at 53. There is provided in the feed pipe 35 a further valve comprising a flow control valve 36.

The joint 14 of the feed vessel assembly 10 is joined to a supply tube 55 which leads to valves 41 and 40. The valve 40 controls supply of an inert gas such as neon into

the vessel assembly 10 from a source (not shown), and the valve 41 controls access to a further high vacuum pump (not shown) for evacuating the various assemblies.

The operation of the supply of feed gas to the ion source will now be described. After the feed vessel assembly 10 has been bolted at the joint 15 to the second jacket 46, and the feed tube 34 has been joined to the feed pipe 35, a shielding region 38 provided by the jackets 18 and 46, is evacuated by opening the vacuum valve 41, with the neon valve 40 closed. Next the vacuum valve 41 is closed and the neon valve 40 opened, so that the shielding region 38 in the jackets 18 and 46 is pressurised to a pressure above atmospheric pressure with the required inert gas. Neon is at this stage the preferred choice, because helium is already commonly used for leak detection and argon is commonly used as a support gas for the discharge and in order to provide argon ions. The feed valve 17 is then opened to make the feed material 11 available to the ion source 31 through the further feed pipe 35 with no danger of contamination by atmospheric gases. The supply system from the vessel 12 along the feed pipes 34 and 35 up to the entrance into the ion source assembly 31 is now protected by the neon shield in the region 38. Because this region is above atmospheric pressure, any leaks will be outward leaks, so that no air can leak into the shield gas. If at any time a leak occurs from the region 38 into the feed pipes 34 and 35, then the neon will be detected by the residual gas analyser 39 in the ion source chamber 32. The residual gas analyser 39 will also be used to ensure that there are no air leaks into the ion source chamber 32. Mass 32 is monitored to detect the oxygen component of an air leak.

The procedures described deal with the purity of the feed, the integrity of the feed supply system and the vacuum integrity of the ion source chamber. A number of other measures should preferably be taken, as follows.

a) Refractory materials such as boron nitride and silicon nitride cannot be used in the arc chamber either as electrical insulators or as vaporiser or reaction vessel construction materials.

b) Refractory oxides such as alumina are less of a problem but an ion source design is preferred which places all electrical insulators outside the arc chamber. This minimises potential oxygen contamination by not exposing these oxides to the source plasma.

c) Carbon cannot be used as an arc chamber material because of the need to avoid the formation of a carbon monoxide ion; molybdenum is probably the best material for semiconductor applications.

d) Clean vacuum pumps such as magnetically levitated turbopumps and cryopumps should be used to minimise carbon contamination due to hydrocarbon oils.

e) The ion source should be run on argon and the output spectrum (using the mass analyser of the implanter as a mass spectrometer) checked to make sure there is no significant oxygen peak. This ensures that the ion source chamber is free of air leaks and the source is run until any oxygen coming from water vapour or easily removed oxides is reduced to acceptable levels.

It is also be.desirable to check directly for nitrogen and this can be done if steps are taken to ensure that silicon containing feed material cannot leak through the flow control valve, by having an evacuable manifold on the high pressure side of this valve. For example, this can be filled with a suitable gas, and if this gas cannot be detected, then there is not a leak.