US3890535A

US3890535A - Ion sources

Info

Publication number: US3890535A
Application number: US296594A
Authority: US
Inventors: Guy Gautherin; Jean Aubert
Original assignee: Agence National de Valorisation de la Recherche ANVAR
Current assignee: Bpifrance Financement SA
Priority date: 1971-10-13
Filing date: 1972-10-11
Publication date: 1975-06-17
Anticipated expiration: 1992-06-17
Also published as: DE2249999A1; GB1411428A; NL7213791A; FR2156978A5

Abstract

The ion source comprises a primary plasma generator constituted by a hot cathode and an anode, between which an arc is produced, and by an intermediate electrode enclosing the arc in proximity to an extraction orifice pierced in the anode. Magnetic field creating means are provided between the intermediate electrode and the extraction orifice, which has a relative small crosssection so that the pressure of the gases occurring inside the primary plasma generator is independent of the pressure existing downstream of the extraction orifice. This pressure downstream of extraction orifice is less than the pressure inside the primary plasma generator. On the downstream side there is an extraction electrode. A fifth electrode is arranged downstream of the extraction orifice of the anode and upstream of the extraction electrode and means are provided for applying to this fifth electrode a positive potential with respect to that of the anode and of adjustable value, which is small in absolute value with respect to the potential of the extraction electrode.

Description

United States Patent Gautherin et a1.

1 1 June 17, 1975 1 1 [ON SOURCES [75] Inventors: Guy Gautherin, Orsay; Jean Aubert,

Vaucresson, both of France [73] Assignee: Agence National de Valorisalion de la Recherche (ANVAR), Paris, France [22] Filed: Oct. 11, 1972 [21] Appl. No.: 296,594

[30] Foreign Application Priority Data Oct, 13, 1971 France 71.36701 [52] US. Cl. 315/111; 250/423; 313/63 [51] Int. Cl H01] 37/08; HOSh l/OO [58] Field of Search 250/423-424, 250/427; 313/63; 315/111 [56] References Cited UNITED STATES PATENTS 3,238,414 3/1966 Kelly et al 315/111 3,265,918 8/1966 Wittkower 313/63 3,631,283 12/1971 Gautherin et al 313/63 Beams," Rev. of Scientific Instruments, Vol. 38, 4-1967, pp. 467, 470-473, 476-477.

Primary Examiner-James W. Lawrence Assistant Examiner-Wm. H. Punter Attorney, Agent. or F irm- Flynn & Frishauf s7 1 ABSTRACT The ion source comprises a primary plasma generator constituted by a hot cathode and an anode, between which an arc is produced, and by an intermediate electrode enclosing the arc in proximity to an extraction orifice pierced in the anode. Magnetic field creating means are provided between the intermediate electrode and the extraction orifice, which has a relative small cross-section so that the pressure of the gases occurring inside the primary plasma generator is independent of the pressure existing downstream of the extraction orifice. This pressure downstream of extraction orifice is less than the pressure inside the primary plasma generator. On the downstream side there is an extraction electrode. A fifth electrode is arranged downstream of the extraction orifice of the anode and upstream of the extraction electrode and means are provided for applying to this fifth electrode a positive potential with respect to that of the anode and of adjustable value, which is small in absolute value with respect to the potential of the extraction electrode.

10 Claims, 3 Drawing Figures ION SOURCES The present invention relates to improvements in or to ion sources.

There is already known an ion source called a duoplasmatron" which is constituted by a hot cathode, an intermediate electrode and an anode pierced by an extraction orifice.

In this known device, an arc is produced between the cathode and the anode and this arc is pinched in the vicinity of said extraction orifice by means of, on one hand, an electrostatic effect caused by the intermediate electrode and, on the other hand, by the effect of a magnetic lens whose poles are constituted by the anode and the intermediate electrode, means for the production of a magnetic field being provided to create such a field between the intermediate electrode and the anode, this magnetic field having the axial direction, that is to say the direction of a line joining the hot cathode and the extraction orifice. There is obtained, at the outlet of this device, a plasma formed by a mixture of electrons and positive ions. In addition, in this duoplasmatron source, the extraction orifice has relatively small dimensions.

Improvements according to the present invention seek to increase to an appreciable extent, the ion current of the plasma obtained. The ion source according to the invention enables, in addition, variation of the energy of the electrons that it produces, to obtain not only positive ions but also negative ions and, the generation of a plasma composed of neutral particles.

The ion source according to the invention is characterised by the fact that it comprises, in addition to the three usual electrodes of the duoplasmatron (that is to say a hot cathode, an intermediate electrode and an anode pierced by an extraction orifice) constituting a primary plasma generator, and the usual downstream extraction electrode a fifth electrode arranged downstream of the extraction orifice of the anode but upstream of the extraction electrode and means for applying to the fifth electrode a positive potential with respect to that of the anode, these means being arranged so that said positive potential has an adjustable value, preferably higher than a critical valve, but much less in absolute value than the potential of the extraction electrode.

The fourth electrode has advantageously the shape of an expansion chamber; in this case, this electrode can also comprise means for creating magnetic induction in said chamber. The fifth electrode is advantageously located adjacent the anode without an intervening expansion space.

In a first embodiment of the invention, the expansion chamber comprises an orifice for the introduction of gas or vapor intended to be ionised by the primary plasma. In a second embodiment, said chamber comprises heating means to vaporise solid substances introduced inside this chamber, the vaporised substances being also intended to be ionised.

Other features or characteristics according to the invention will appear also in the course of the more detailed description which follows of preferred embodi ments according to the invention, with reference to the drawings in which:

FIG. 1 illustrates a first embodiment of the invention,

FIG. 2 is a diagram which shows the intensity of the currents of ions and of electrons obtained by the device according to the invention, as a function of the voltage between the fourth electrode and the anode, and

FIG. 3 illustrates a second embodiment of the invention.

The ion source according to the invention comprises, firstly, in known manner, a primary plasma generator 1 (FIGS. 1 and 3) of the duoplasmatron type. This primary plasma generator is composed of a hot cathode 2, of an intermediate electrode 3 brought to a potential greater than that of the cathode 2 and an anode 4 pierced by an extraction orifice 5 and brought to a potential, generally that of ground, greater than that of the electrode 3. Magnetic field-creating means 3a are provided to create a magnetic field between the intermediate electrode and the extraction orifice. There is introduced into such a device, which is shown diagramatically in FIGS. 1 and 3, a gas for example hydrogen and there is produced an arc between the cathode 2 and the anode 4. This generator 1 then emits a jet of plasma downstream of the extraction orifice 5, this jet of plasma being composed of electrons and primary ions. The extraction orifice 5 has relatively re duced dimensions its cross-section being at the most, equal to 5 mm. The latter feature has the object of enabling the pressure p, of the gases occurring upstream of the anode to be uninfluenced by the pressure p (less than p of the gases occurring downstream of the anode; in other words, the gases downstream of the anode must not reach the cathode 2.

The ion source shown in FIG. 1 includes an extraction electrode 18 and also comprises, in addition, according to the invention, a fifth electrode 6 and means 7 for bringing this electrode 6 to a positive potential with respect to the anode 4, whose value is adjustable. Preferably, this potential is greater than a critical value V The potential of the electrode 6 being positive with respect to that of the anode, there is hence obtained, at the outlet of the duoplasmatron, current of electrons.

Means 7 are composed, for example, as in the embodiment of the invention shown in FIG. I, of a direct current source 8 in series with an adjustable resistance 9. In the embodiment of the invention shown in FIG. 3, said means 7 are composed of a direct current source 10 whose voltage is adjustable.

The electrode 6 has, in general, the shape of a reservoir or expansion chamber and is arranged downstream and facing the expansion orifice 5 so as to receive the jet of plasma produced by the generator 1. In a first embodiment of the invention, the reservoir formed by the electrode 6 is empty and, in another embodiment, said reservoir contains a gas or a vapor. This gas or this vapor can be such that, if it were in contact with the filament of the cathode 2, it would damage the latter.

The abovesaid value V, is a critical value for the voltage between the anode 4 and the electrode 6; when this voltage exceeds this value V there is produced a sudden increase in intensity, respectively, of the ion current and, to a lesser degree of the electron current, at the level of the electrode 6. The inventors have, in fact, noted that starting from a certain positive value V of the potential difference between the electrode 6 and the anode 4, there is produced a new ionisation or arc between the anode 4 and the electrode 6 manifested by the abovesaid sudden increases in intensity. This phenomenon is illustrated by FIG. 2 which is a diagram in which there is indicated, as abscissae, the potential V applied between the electrode 6 and the anode 4 and, as ordinates, the intensity I of the current of ions or of electrons at the level of the electrode 6. In this diagram, the curve 11, in full line, shows the variations in the intensity of the ion current (these ions being possibly. as will be seen below, positive or negative) and the curve 12, in interrupted line, shows the variations in the intensity of the current of electrons, these intensities being, of course, measured at the level of the electrode 6. There is seen in this Figure, besides the said sudden increases in the intensity from the value V that the intensity of the electron current obeys a linear law of variation as a function of the potential V This intensity of the electron current increases linearly when the potential V increases, increases suddenly when this potential passes from a value below to a value greater than V and then increases linearly when V has exceeded V,, with a growth rate substantially equal to that which appears when V is less than V,. It will also be noted that the intensity of the ion current tends appreciably to approach a limiting value when the potential V increases, above the value V It should be noted that there is obtained a discharge of the diode type between the anode 4 and the electrode 6.

As regards the values V there may be mentioned the two following examples, obtained in the course of ex periments carried out within the scope of the invention:

In the first experiment, the duoplasmatron or generator 1 contained argon, and the electrode 6, in the shape of a reservoir, also contained argon. In this case, the critical value V was found to be comprised, according to the conditions of the experiment, between 25 and 30 volts.

In the second experiment, the generator 1 contained argon whilst the electrode 6, in the form of a reservoir, contained oxygen, it was then noted that the critical voltage was around 50 volts and that this voltage V diminished when the pressure of oxygen in the electrode 6 increased.

In general, the inventors noted that the critical voltage V depends, firstly, on the nature of the gases present in the generator 1 and, possibly in the electrode 6, secondly, on the conditions of discharge in the generator l and on the pressure of the gas which is possibly present, in the electrode 6. It was found that V had a value comprised between 5 and 100 volts.

Referring now, more particularly, to the embodiment of the invention shown in FIG. 1, it is seen that the electrode 6 has substantially the shape of a cylinder of which the axis extends that of the duoplasmatron and provides an inlet aperture 13, facing the extraction orifree 5, and an outlet aperture 14 through which is injected the ion current and electrons formed inside this electrode 6.

The electrode 6 which forms, in this case, an expansion chamber, comprises, in addition, means 15 for enabling the introduction of a gas or of a vapour inside the said chamber, this gas or this vapour being intended to be ionised by the electrons and primary ions emerging from the duoplasmatron. The means 15 are, for example, constituted by an outer reservoir (not shown) and an intake pipe.

Moreover, there are provided means constituted by coils l6 and 17 arranged, for example, on the outer periphery of the electrode 6 to create a magnetic induction inside the said chamber formed by the electrode 6,

this induction having preferably a direction substantially parallel to the axis of the cylinder formed by the electrode 6. The inventors have, in fact, noted that the presence of a magnetic induction inside the expansion chamber increases the ionic and electronic intensities inside the latter as well as the yield of ionisation in this chamber.

An extraction electrode I8, known in itself, is arranged downstream of the electrode 6. This electrode 18 enables, when it is suitably polarised, the extraction, from the expansion chamber formed by the electrode 6, of positive or negative ions which have arrived or been formed in said electrode 6. The potential of this electrode 18 relative to the electrodes of the source has, in a general way, a very high value with respect to the relative potential of the electrodes of the source. If the potential of the electrode 18 is positive, it will be the negative ions and the electrons which will be extracted, whilst, if the potential of this electrode 18 is negative, it will be the positive ions which will be extracted through this electrode. Since the potential of the electrode 18 relative to the nearest electrode (the electrode 6) of the source is high but may be positive or negative, while the potential of the electrode 6 relative to that of the anode 4 is always positive but is at most of the order of magnitude of volts, a statement comparing these successive interelectrode potentials must take the form of saying that the potential of electrode 6 relative to electrode 4 is positive and small in absolute value compared to the potential of electrode 18 relative to electrode 6, the term absolute value being used in the common mathematical sense of the term, meaning magnitude regardless of sign. In other words, such a statement means that the difference in potential between electrodes 6 and 4 is small compared to the difference (regardless of sign) in potential between electrodes 18 and 6.

It is already known, from French Pat. No. 1,585,902 filed 9 Apr. I968 for Device for producing intense ion beams, especially metallic," that it is possible to create positive ions from a gas by means of a plasma jet emerging from the duoplasmatron. In the present case, the said gas is that which is contained in the chamber formed by the electrode 6. This ionisation has a double origin: on one hand, the electrons produced by the generator 1 ionised by shock, said gas by tearing off one or several electrons from each of its atoms A, which is expressed, in the case of simple ionisation, by the following relationship:

A e A 2e and, in general:

A-l-e A" +(n+ l)e,

or again, in the case of a molecular gas AB:

AB e A B'" (m+n+l )e;

on the other hand, positive ions produced by the generator 1 exchange their charge with neutral atoms from the expansion chamber. This phenomenon is expressed by the following relationship, in the simplest case:

A H A H;

of course, there could be a relationship of the type:

The inventors have noted that there could be created, by means of the beam emerging from the duoplasmatron, not only positive ions but also negative ions with a single or multiple charge. This ionisation is produced by the primary electrons generated by the generator l; in the case of a molecular gas AB affected by these electrons, the ionisation is produced according to one and/or other of the following equations:

In the case of a monatomic gas A affected by electrons generated by the beam produced by a duoplasmatron, the ionisation is effected according to the following relationship:

in which the expression hv expresses simply that the ionisation is radiative, that is to say that it produces an emission of light.

It will be noted that this production of negative ions can be achieved even if the source does not have means 7 for applying an adjustable potential to the chamber Of course, as has already been seen, according to the polarity of the extraction electrode 18, it will be positive or negative ions which will be extracted from the chamber formed by the electrode 6.

In the embodiment shown in FIG. 3, the electrode 6 also has the shape of an expansion chamber but it is formed in the shape of an oven provided with heating means 22 to vaporise a substance 19 which has been introduced into this oven. In this embodiment of the invention, the source comprises, in addition, downstream of the electrode 6, after its outlet aperture 14, magnetic separating means 20 adapted to separate the trajectory of primary ions from that of (positive or negative) ions produced in the expansion chamber. Moreover, immediately after the magnetic separating means 20, there is provided an electrode 21 brought to a positive potential with respect to that of the electrode 6 and which is adapted to trap the electrons of the plasma beam emerging from the source. There is hence obtained, by means of this modification of the source according to the invention, a beam of positive or negative ions which is purified of electrons and primary ions.

In another modification of the invention there are used negative and/or positive ions emitted by the source, to produce a neutral plasma. It suffices, for this, to send the beam of ions onto a gas target in which the ions, as the case may be, lose or gain one or several electrons and are neutralised whilst preserving their kinetic energy.

It will be noted that the ion source according to the invention enables the obtaining of intense ion beams from various materials without there being any risk of contamination of the filament of the hot cathode by these materials.

Moreover, it will be noted that, when the electrode 6 has the shape of an expansion chamber, the pressure inside the latter can be varied on condition, of course, that this pressure remains less than that which exists inside the primary generator 1.

The ion source which has just been described with respect to FIGS. 1 to 3 can lend itself to numerous applications and there may be mentioned, by way of example, application of ions for doping semi-conductor materials. Thus, in the case of silicon, there have already been implanted positive ions (acceptors):l3 (boron), P (phosphorus) and 0* (oxygen), and negative ions (donors): Fe (iron), Si (silicon) and O. The ion source according to the invention can also be used for a mass spectrometer and a VAN-DE-GRAAFF type accelerator.

As is self-evident and as emerges already from the foregoing, the invention is in no way limited to those of its types of application, nor to those embodiments of its various parts, which have been more especially indicated; it encompasses, on the contrary, all modifications.

We claim:

1. Ion source comprising a primary plasma generator constituted by a hot cathode and an anode, between which an arc is produced, said anode defining an extraction orifice, and by an intermediate electrode pinching the arc in proximity to said extraction orifice in the anode, magnetic field producing means creating a magnetic field between the intermediate electrode and the extraction orifice, an extraction electrode downstream of said anode, said extraction orifice of the anode having a relatively small cross-section so that the pressure of the gases occurring inside said primary plasma generator is independent of the pressure existing downstream of said extraction orifice, said pressure downstream of the extraction orifice being less than said pressure inside the primary plasma generator, said generator comprising a fifth electrode arranged downstream of the. extraction orifice of the anode and upstream of the extraction electrode and means for applying to said fifth electrode a potential positive with respect to that of the anode and of adjustable value, said positive potential being small in absolute value compared to the potential of the extraction electrode with respect to that of said fifth electrode.

2. Ion source according to claim 1, wherein the crosssection of said extraction orifice is less than 5 mm*.

3. Ion source according to claim 2, wherein the fifth electrode is in the form of an enclosure including an inlet aperture, arranged adjacent the extraction orifice without intervening expansion space but leaving an insulation space, and an outlet aperture.

4. Ion source according to claim 1, wherein the fifth electrode is in the form of an enclosure including an inlet aperture, arranged adjacent the extraction orifice without an intervening expansion space but leaving an insulation space, and an outlet aperture.

5. Ion source according to claim 4, comprising means for creating magnetic induction inside said enclosure.

6. Ion source according to claim 4, comprising means arranged downstream of said outlet aperture for separating secondary ions formed in said enclosure from the primary ions formed in said primary plasma generator.

7. [on source according to claim 1, comprising means arranged downstream of the fifth electrode, for separating the electrons from the ions produced by said source.

8. Ion source comprising a primary plasma generator constituted by a hot cathode and an anode, between which an arc is produced, said anode defining an extraction orifice, and by an intermediate electrode pinching the arc in proximity to said extraction orifice in the anode, magnetic field producing means creating a magnetic field between the intermediate electrode and the extraction orifice, said extraction orifice having a relatively small cross-section so that the pressure of the gases occurring inside said primary plasma generator is independent of the pressure existing downstream of said extraction orifice, said pressure downstream of the extraction orifice being less than said pressure inside the primary plasma generator. said generator comprising an additional electrode arranged downstream of said extraction orifice of the anode. said additional electrode being in the form of an enclosure including an inlet aperture arranged adjacent the extraction orifice without an intervening expansion space but leaving an insulation space, and an outlet aperture, means for applying to said additional electrode a potential posi tive with respect to that of the anode and of adjustable value, and means for introducing a gas or a vapor into said enclosure.

9. Ion source comprising a primary plasma generator constituted by a hot cathode and an anode, between which an arc is produced, said anode defining an extraction orifice, and by an intermediate electrode pinching the arc in proximity to said extraction orifice in the anode, magnetic field producing means creating a magnetic field between the intermediate electrode and the extraction orifice, said extraction orifice having a relatively small cross-section so that the pressure of the gases occurring inside said primary plasma generator is independent of the pressure existing downstream of said extraction orifice, said pressure downstream of the extraction orifice being less than said pressure inside the primary plasma generator, said generator comprising an additional electrode arranged downstream of said extraction orifice of the anode, said additional electrode being in the form of an enclosure including an inlet aperture arranged adjacent the extraction orifice without an intervening expansion space but leaving an insulation space, and an outlet aperture, means for applying to said additional electrode a potential positive with respect to that of the anode and of adjustable value, and heating means for vaporizing a solid substance present inside said enclosure.

10. Ion source comprising a primary plasma generator constituted by a hot cathode and an anode, between which an arc is produced, said anode defining an extraction orifice, and by an intermediate electrode pinching the arc in proximity to said extraction orifice in the anode, magnetic field producing means creating a magnetic field between the intermediate electrode and the extraction orifice, said extraction orifice having a relatively small cross-section so that the pressure of the gases occurring inside said primary plasma generator is independent of the pressure existing downstream of said extraction orifice, said pressure downstream of the extraction orifice being less than said pressure inside the primary plasma generator, said generator comprising an additional electrode arranged downstream of said extraction orifice of the anode, said additional electrode being in the form of an enclosure including an inlet aperture arranged adjacent the extraction orifice without an intervening expansion space but leaving an insulation space, and an outlet aperture, means for applying to said additional electrode a potential positive with respect to that of the anode and of adjustable value, and means for creating magnetic induction inside said enclosure.

Claims

1. Ion source comprising a primary plasma generator constituted by a hot cathode and an anode, between which an arc is produced, said anode defining an extraction orifice, and by an intermediate electrode pinching the arc in proximity to said extraction orifice in the anode, magnetic field producing means creating a magnetic field between the intermediate electrode and the extraction orifice, an extraction Electrode downstream of said anode, said extraction orifice of the anode having a relatively small cross-section so that the pressure of the gases occurring inside said primary plasma generator is independent of the pressure existing downstream of said extraction orifice, said pressure downstream of the extraction orifice being less than said pressure inside the primary plasma generator, said generator comprising a fifth electrode arranged downstream of the extraction orifice of the anode and upstream of the extraction electrode and means for applying to said fifth electrode a potential positive with respect to that of the anode and of adjustable value, said positive potential being small in absolute value compared to the potential of the extraction electrode with respect to that of said fifth electrode.

2. Ion source according to claim 1, wherein the cross-section of said extraction orifice is less than 5 mm2.

7. Ion source according to claim 1, comprising means arranged downstream of the fifth electrode, for separating the electrons from the ions produced by said source.

8. Ion source comprising a primary plasma generator constituted by a hot cathode and an anode, between which an arc is produced, said anode defining an extraction orifice, and by an intermediate electrode pinching the arc in proximity to said extraction orifice in the anode, magnetic field producing means creating a magnetic field between the intermediate electrode and the extraction orifice, said extraction orifice having a relatively small cross-section so that the pressure of the gases occurring inside said primary plasma generator is independent of the pressure existing downstream of said extraction orifice, said pressure downstream of the extraction orifice being less than said pressure inside the primary plasma generator, said generator comprising an additional electrode arranged downstream of said extraction orifice of the anode, said additional electrode being in the form of an enclosure including an inlet aperture arranged adjacent the extraction orifice without an intervening expansion space but leaving an insulation space, and an outlet aperture, means for applying to said additional electrode a potential positive with respect to that of the anode and of adjustable value, and means for introducing a gas or a vapor into said enclosure.