Multicharged ion source with several electron cyclotron resonance zones

Multicharged ion source with several electron cyclotron resonance zones. The source comprises a sealed enclosure containing a gas for forming a plasma confined in said enclosure, means for producing a high frequency electromagnetic field within the enclosure, means for producing within the enclosure a group of radial and axial, local magnetic fields and defining equimagnetic surfaces permitting the confinement of the plasma on one of these surfaces, the electron cyclotron resonance condition being satisfied, the group having an axis of symmetry, means for extracting the ions through the orifice made in the wall of the enclosure and located on the axis of symmetry and means for reducing, outside the volume occupied by the plasma, the amplitude of the local axial magnetic fields in the vicinity of and slightly upstream of the extraction orifice in n zones located outside the axis, so that new ionizing electron cyclotron resonances appear in these zones.

BACKGROUND OF THE INVENTION 
The present invention relates to a multicharged ion source with a plurality 
of electron cyclotron resonance zones. It has numerous applications, as a 
function of the different kinetic energy values of the extracted ions, in 
the field of ion implantation, microetching and more particularly in 
particle accelerator equipment, used both in the scientific and medical 
fields. In electron cyclotron resonance sources, the ions are obtained by 
ionizing, within a closed enclosure of the ultra-high frequency cavity 
type, a gas e.g. constituted by metal vapours, by means of an electron 
plasma highly accelerated by electron cyclotron resonance. This resonance 
is obtained as a result of the combined action of a high frequency 
electromagnetic field injected into the enclosure containing the gas to be 
ionized and a magnetic field prevailing in said enclosure, whose amplitude 
B satisfies the following electron cyclotron resonance condition: 
B=f.2.pi.m/e, in which e represents the electron charge, m its mass and f 
the frequency of the electromagnetic field. 
In this type of source, the quantity of ions which can be produced results 
from the competition between two processes, on the one hand the formation 
of the ions by electron impact on neutral atoms constituted the gap to be 
ionized and on the other hand the destruction of the same ions by single 
or multiple recombination, during a collision of the latter with a neutral 
atom. The latter can come from a gas which is not yet ionized or can be 
produced on the enclosure walls by the impact of an ion thereon. 
The problem in this type of source is consequently to minimize the 
destruction of the ions formed, by preventing any collision thereof with a 
neutral atom. 
In order to obviate this disadvantage, consideration has been given to the 
confinement within the enclosure forming the source of the ions formed, 
together with the electrons used for their ionization. This is brought 
about by producing within the enclosure a group of radial and axial, local 
magnetic fields, defining a so-called equimagnetic closed surface having 
no contact with the walls of the enclosure and on which the electron 
cyclotron resonance condition is satisfied. This surface forms the 
location of the points, where the amplitude of the local magnetic field 
has the same value. Such a source is described in French Pat. No. 
2,485,798, filed on 13th Feb. 1980 by the present Applicant. 
The nearer this equimagnetic surface to the walls of the enclosure, the 
greater its effectiveness, because it makes it possible to limit the 
volume of the neutral atoms present and consequently the neutral atom - 
ion collision quantity. However, there is a serious risk of this surface 
touching the inner walls of the enclosure and it is then preferable to use 
a second equimagnetic surface, whose amplitude is tuned to a frequency 
differing from the electromagnetic field, which automatically imposes the 
use of a second ultra-high frequency generator. 
SUMMARY OF THE INVENTION 
The present invention relates to a multicharged ion source with electron 
cyclotron resonance making it possible to minimize the effects of 
recombination by the collision of ions with neutral atoms, whilst 
obviating the use of a second ultra-high frequency generator. 
More specifically, the present invention relates to a multicharged ion 
source comprising a sealed enclosure containing a gas for forming a plasma 
confined in said enclosure, means for producing within said enclosure a 
high frequency electromagnetic field, means for producing in said 
enclosure a group of radial and axial, local magnetic fields defining at 
least one equimagnetic surface permitting the confinement of the plasma 
produced by the electron cyclotron resonance whereof the condition on said 
surface has been satisfied, said group having an axis of symmetry, and 
means for extracting the ions through an orifice made in the walls of the 
enclosure and located on the axis of symmetry, wherein the source 
comprises means for reducing, outside the volume occupied by the 
confinement plasma, the amplitude of the local axial magnetic fields in 
the vicinity of and slightly upstream of the extraction orifice in n samll 
zones located outside the axis of symmetry on the one hand and on the 
other hand in a more total manner in the complete volume downstream of 
said orifice. This reduction makes it possible for new ionizing electron 
cyclotron resonances to appear in the n zones. 
According to a preferred embodiment of the invention, the local radial 
fields are produced by means of several magnetic bars arranged 
symmetrically around the enclosure and each constituted by several 
elementary magnets, the terminal elementary magnets of said bars level 
with the extraction orifice having the same polarity, so as to in part 
form the means for reducing the amplitude of the local axial magnetic 
fields. 
Preferably, the magnetic bars are made from SmCo.sub.5, said material 
having remarkable macroscopic anisotropy properties and a high magnetic 
rigidity. 
In order to increase or modify in space the effect of the reduction in the 
axial magnetic fields, an iron shield joined to the enclosure externally 
thereof and level with the extraction orifice can be advantageously 
provided. The axis of symmetry of this shield coincides with that of the 
group of magnetic fields.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 diagrammatically shows in longitudinal section an electron cyclotron 
resonance ion source. This source comprises a sealed confinement enclosure 
2 constituting a resonant cavity. Enclosure 2 is joined by means of a pipe 
4 to a vacuum pump 5 making it possible to produce a high vacuum in the 
enclosure. Enclosure 2 can be excited by an ultra-high frequency 
electromagnetic field produced by a generator 6, said field being 
introduced into the enclosure by means of a waveguide 8. An ionizable gas 
can be introduced into enclosure 2 by a pipe 10. 
Coils such as 12 arranged around enclosure 2 make it possible to produce 
therein local magnetic fields, which are symbolized by arrow 16 and which 
are parallel to an axis 18, which can e.g. be the axis of symmetry of 
enclosure 2. In the same way, magnetic bars 20 arranged around the said 
cavity make it possible to produce local magnetic fields, symbolized by 
the arrows 22 and positioned radially with respect to axis 18. As the 
group of axial and radial, local magnetic fields has as its axis of 
symmetry the axis 18, it is possible to define closed equimagnetic 
surfaces such as 23 (location of the points where the amplitude of the 
local magnetic fields has the same value) having no contact with the walls 
of enclosure 2. In the manner explained hereinbefore, the electron 
cyclotron resonance condition is satisfied on one of these inner surfaces. 
The existence of such a resonant surface (cf the aforementioned patent 
specification) makes it possible to strongly ionize the gas contained in 
enclosure 2, thus giving rise to a very high energy electron plasma. This 
surface also makes it possible to confine the ions and the electrons 
produced by the ionization of the gas. As a result of this confinement, 
the electrons produced have the time to bombard the same ions several 
times and to totally ionize it. 
It is of fundamental importance that this resonant surface also permits a 
very effective in situ ionic pumping, which ipso facto limits destructive 
neutral atom - ion charge exchange collisions within the volume defined by 
said resonant surface. 
The highly charged or multicharged ions formed in this way can then be 
extracted from the enclosure 2, which for this purpose has an extraction 
orifice 24 on the axis of symmetry 18, e.g. by means of an electrode 26 
raised to a negative potential with the aid of a power supply 28. The ions 
extracted from enclosure 2 in this way can then be selected as a function 
of the degree of ionization with the aid of any known means utilizing the 
magnetic field and/or an electric field. 
In order to minimize the destructive effects of the charge exchange 
collisions referred to hereinbefore, but on this occasion between the 
resonant surface and the extraction orifice, the invention proposes the 
reduction of the amplitude of the local axial magnetic fields in the 
vicinity of the extraction orifice 24 and more specifically downstream 
thereof and slightly upstream in the vicinity of the axis of symmetry 18. 
This reduction of the local axial magnetic fields can be effected outside 
the volume occupied by the electron plasma, confined within the 
equimagnetic surface 23 which is furthest to the outside and which is not 
intersected by the wall, so as to prevent any modification to the shape 
and location of said surface. 
This reduction can be advantageously brought about by using magnetic bars 
20 formed from several joined elementary magnets 30, which are preferably 
made from SmCo.sub.5, the terminal elementary magnets 30a of the different 
bars 30 level with the extraction orifice 24 having the same polarity, 
which is in this case a north polarity (N), as shown in FIGS. 1 and 2. In 
the prior art ion sources, the polarities of the terminal elementary 
magnets 30a alternated between north and south. The uniform polarity of 
the elementary magnets 30a must have the same name or polarity (FIG. 1) as 
that of the face of the coils 12 located in the vicinity of said magnets, 
i.e. in the vicinity of the extraction orifice 24. 
This uniform polarity of elements 30a makes it possible to form several 
equimagnetic caps 32, on which the electron cyclotron resonance condition 
is satisfied. The dimensions and consequently the effectiveness of these 
caps 32 can be modified by slightly varying the amplitude of the local 
radial fields produced by coils 12. 
The number of equimagnetic caps 32 is dependent on the number of magnetic 
bars 20. The use of 2n magnetic bars permits the formation of n 
equimagnetic caps. In the case shown in FIG. 2, n is taken equal to 3. 
These caps 32 located in the ion extraction zone make it possible to 
minimize the recombination of the ions with the neutral atoms more 
particularly produced on the walls of the enclosure close to the 
extraction orifice. 
Thus, these caps obtained by a local decrease in the axial magnetic fields 
make it possible, as a result of the electron cyclotron resonance of their 
surface, to locally produce relatively high energy electron plasmas in 
order to ionize at least once the neutral atoms normally present at the 
extraction orifice 24. 
In FIG. 2, the regions 32 represent the zones in which is realised the 
reduction of the axial local magnetic fields in accordance with the 
invention. 
However, in said FIG. 2, the hatched zone 33 represents the zone for 
forming the neutral atoms responsible for the destructive ion charge 
exchange. 
The possibility of reducing the amplitude of the local axial magnetic field 
16 (FIG. 1) at the extraction orifice 24, by acting on the structure of 
the magnetic bars 20 producing the local radial magnetic fields 22 is 
based on the fact that said bars produce axial magnetic components at 
their end bearing in mind the inevitable magnetic leaks. 
FIG. 3 shows the magnetic lines of force of the axial leakage fields 
produced at the ends of magnetic bars 20. These magnetic lines of force 
are designated by the reference numeral 34. The use of terminal elementary 
magnets 30a of the same polarity (north) at the extraction orifice 24 
makes it possible, bearing in mind the flow direction of the axial leak 
field line 34a, to greatly locally reduce the axial magnetic fields mainly 
produced by coils 12, in the vicinity of and slightly upstream of the 
extraction orifice, outside the axis of symmetry on the one hand and on 
the other hand in a more total manner in the complete volume located 
downstream of said orifice. The weak axial magnetic field areas level with 
orifice 24 carry the reference numeral 35. 
In the same way, the use of these terminal elementary magnets 30a makes it 
possible, bearing in mind the direction of flow of the axial leak field 
line 34b, to increase the axial magnetic field produced by the coils 12, 
upstream of and relatively remote from the extraction orifice. This 
overall increase of the axial magnetic field makes it possible to move the 
resonant equimagnetic surface 23 away from the ion extraction zone and 
consequently to reduce the risk of having said surface touch the enclosure 
walls. 
In order to increase or modify in space the effect of the reduction in the 
axial magnetic fields already brought about by magnets 30a, an iron shield 
36, as shown in FIG. 4, can be joined to enclosure 2, externally thereof 
and level with the extraction orifice 24. The axis of symmetry of iron 
shield 36 coincides with axis 18. This shield 36 makes it possible to 
increase the reduction in the local axial magnetic fields downstream of 
orifice 24 and particularly the local axial magnetic field on axis 18. 
Thus, said shield 36 contributes to the formation and positioning of the 
resonant equimagnetic caps 32. However, it should be noted that the use of 
this shield 36 alone, i.e. without the terminal elementary magnets 30a of 
the same polarity, would not make it possible to form resonant 
equimagnetic caps. Curves a and b in FIG. 4 respectively illustrate the 
amplitude of the magnetic fields on axis 18 with and without shield 36.