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Claims  |
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We claim:
1. A radio frequency ion beam source for producing an ion beam of a desired
energy and current density, said ion beam source comprising a metallic
conductive ioniser vessel having an interior, means for feeding a working
gas to be ionised into said ioniser vessel; a radio frequency coil
disposed coaxially within the interior of said ioniser vessel; a radio
frequency source connected to said radio frequency coil for generating a
plasma in said ioniser vessel by an inductively generated discharge, said
plasma being a non-isothermal plasma of ions, electrons and neutral gas
particles; and a beam forming system comprising a combination of a
plurality of multi-hole extraction grids and an ion focussing unit which
directly follows them; and means for selectively switching on said ion
focussing unit.
2. A radio frequency ion beam source in accordance with claim 1 wherein
said ionizer vessel comprises chemically resistant stainless and
non-magnetic metal, and said source further comprises means for placing
said ioniser vessel at a positive beam potential capable of being varied
within a range from approximately +10 V to +3000 V and means for supplying
a cooling fluid to said ioniser vessel.
3. A radio frequency ion beam source in accordance with claim 1, wherein
said radio frequency coil consists of a copper tube through which a
cooling medium can flow, said copper tube being coated with an insulating
protective layer.
4. A radio frequency ion beam source in accordance with claim 1 wherein
said ioniser vessel comprises a metal cylinder with a cover formed in one
part, a connection flange provided at an end of said cylinder remote from
said cover, and feed connections for said radio frequency coil, said feed
connections passing through said cover; wherein insulators for said feed
connections are provided on or in said cover, there being inwardly
disposed protective screens to shade said insulators against the
deposition of metal thereon; wherein a gas inlet is provided in said cover
and is provided with a distributor inlet inside said ioniser vessel; and
wherein both said beam forming system and a mounting flange associated
with said beam forming system are fixed to said connection flange, with
insulating intermediate construction parts being provided between said
mounting flange and said connection flange, said mounting flange
permitting said ioniser vessel to be flanged onto an associated vacuum
chamber from an outside of said vacuum chamber.
5. A radio frequency ion beam source in accordance with claim 1 wherein
said ioniser vessel has a metallic jacket and wherein a ring of permanent
magnets of alternating polarity is mounted at said metallic jacket of said
ioniser vessel for concentration of said plasma and for reduction of
discharge losses at wall means of said ioniser vessel.
6. A radio frequency ion beam source in accordance with claim 1, wherein
said plurality of multi-hole extraction grids comprises a three grid
extraction system having first, second and third grid electrodes connected
to said ioniser vessel; wherein said grid electrodes are mutually
insulated, with said first and second grid electrodes having a respective
thickness dimension and a mutual spacing smaller than 1 mm; and wherein a
central support holder is provided to fix said mutual spacing of the first
and second grid electrodes.
7. A radio frequency ion beam source in accordance with claim 6, wherein
said first and second grid electrodes have a thickness of approximately
0.5 mm or less and said mutual spacing is of corresponding size over an
ion beam extraction area; and wherein said third grid electrode has a
thickness over its entire cross-section which is somewhat larger than said
thickness of said first and second grid electrodes, said third grid
electrode serving as a support for said central support holder.
8. A radio frequency ion beam source in accordance with claim 7 wherein
said grid electrodes are of thermally stable shape and are temperature
resistant, wherein said grid electrodes each have a plurality of
extraction bores for said ion beam, the extraction bores in each of said
grid electrodes having a respective diameter, and wherein the respective
diameter of said extraction bores in each of said grid electrodes differs
in size from the respective diameter of the extraction bores in the other
grid electrodes.
9. A radio frequency ion beam source in accordance with claim 1 wherein
said radio frequency coil has a diameter and said multi-hole extraction
grids forming part of said beam forming system has an ion beam extraction
area having a diameter, said diameter of said radio frequency coil being
substantially equal to said diameter of said ion beam extraction area.
10. A radio frequency ion beam source in accordance with claim 1 wherein
said plurality of multi-hole extraction grids comprises a three grid
system having first, second and third grid electrodes and wherein said ion
focussing unit comprises an ion optical focussing lens formed by said
third grid electrode and first and second ring electrodes having radially
inner edges, said radially inner edges of said ring electrodes lying on an
imaginary conical surface having a central axis.
11. A radio frequency ion beam source in accordance with claim 10 wherein
said first and second ring electrodes are of thermally stable shape and
temperature resistant and have at least substantially the same spacing
from one another as said third grid electrode has from said first ring
electrode, said first ring electrode being adjacent to said third grid
electrode.
12. A radio frequency ion beam source in accordance with claim 1 and
further comprising a magnetic lens having a field strength matched to said
ion energy and arranged proximate an outlet of said source to assist the
bundling of said beam.
13. A radio frequency ion beam source in accordance with claim 1 wherein a
beam neutralizer comprising at least one glow filament is provided
adjacent an output of said source, or within said ion focussing unit, for
the injection of electrons into said ion beam.
14. A radio frequency ion beam source in accordance with claim 1 wherein
means is provided for directly or indirectly cooling said ioniser vessel
by a cooling medium.
15. A method of operating a radio frequency ion beam source for producing
an ion beam of a desired ion energy, and current density, said ion beam
source comprising a metallic conductive ioniser vessel having an interior,
means for feeding a working gas to be ionised into said ioniser vessel; a
radio frequency coil disposed coaxially within the interior of said
ioniser vessel; a radio frequency source connected to said radio frequency
coil for generating a plasma in said ioniser vessel by applying radio
frequency power to said radio frequency coil to produce an inductively
generated discharge, said plasma being a non-isothermal plasma of ions,
electrons and neutral gas particles; and a beam forming system comprising
a combination of a plurality of multi-hole extraction grids and an ion
focussing unit which directly follows them; and means for selectively
switching on said ion focussing unit, wherein said current density, which
is dependent on said radio frequency power supplied to said coil and on an
extraction voltage, i.e. the potential difference between a first and
second grid electrodes, and said ion energy, which is dependent on a
potential applied to the first one of said multi-hole extraction grids,
are varied independently from one another.
16. A method in accordance with claim 15 wherein said ion focussing unit is
switched on when said ion energy falls below approximately 1000 eV.
17. A method in accordance with claim 15 wherein a post focussing device
comprising at least one magnetic coil arranged between said ion focussing
unit and a substrate is switched on when said ion energy falls below about
500 eV.
18. A method in accordance with claim 15 and comprising the further step of
effecting a space charge compensation by means of a beam neutralizer when
operating with a substrate having at least one insulating surface.
19. A method in accordance with claim 15 and comprising the further step of
effecting a space charge compensation by means of a beam neutralizer when
said ion energy is below 500 eV.
20. A radio frequency ion beam source in accordance with claim 1 wherein,
for an ion energy above 3000 eV, an acceleration system is arranged
following said ion focussing unit at a predetermined space therefrom.
21. Method of using a radio frequency ion beam source for treatment of a
substrate, said ion beam source comprising a metallic conductive ioniser
vessel having an interior, means for feeding a working gas to be ionised
into said ioniser vessel; a radio frequency coil disposed coaxially within
the interior of said ioniser vessel; a radio frequency source connected to
said radio frequency coil for generating a plasma in said ioniser vessel
by an inductively generated discharge, said plasma being a non-isothermal
plasma of ions, electrons and neutral gas particles; and a beam forming
system comprising a combination of a plurality of multi-hole extraction
grids and an ion focussing unit which directly follows them; and means for
selectively switching on said ion focussing unit, the method comprising a
first step of surface preparation and a subsequent step of deposition of
at least one layer of material having a good adhesion to said substrate,
whereby both said surface preparation step and said subsequent deposition
step are done one after the other with a continuous transition from said
first step to said subsequent step, with an optimum ion energy and an
optimum ion current density being used for each said step.
22. Method in accordance with claim 21 in combination with a substrate
comprising crystals or crystallites, said crystals having a crystal
structure and crystal orientation and said crystallites having a texture
and a shape, wherein said first step comprises at least one of the
following substeps:
(a) a surface of said substrate is cleaned and partially eroded by ion beam
etching using an appropriately chosen ion energy and ion current density,
(b) a surface of said substrate is heated and degassed by means of an ion
beam of low ion energy and current density greater than 1 mA/cm2,
(c) a surface of said substrate is roughened by ion bombardment to produce
craters and grooves for mechanical anchoring of a layer of material to be
subsequently deposited thereon;
and wherein said subsequent step comprises at least one of the following
substeps:
(d) ions of energy greater than 50 keV but with an ion beam of low current
density, are implanted into said substrate to a depth of a plurality of
atomic planes into said crystallites of said substrate and into
interstitial spaces between crystal atoms of said substrate and optionally
into interspaces of said crystallites, whereby to initiate interdiffusion
of atoms;
(e) layers of metals, alloys or chemical compounds are built up on a
surface of said substrate using ions of said metals, alloys or chemical
compounds, with an optimum ion energy and an optimum ion beam density
being selected in dependence on said texture and shape of said
crystallites; and
(f) layers of metals, alloys or chemical compounds are built up on a
surface of said substrate using an optimum ion energy and an optimum ion
current density selected in dependence on said structure and orientation
of said crystals.
23. Method in accordance with claim 21, comprising using a plurality of
radio frequency ion beam sources to simultaneously deposit ions of
constituent elements of an alloy using suitably selected ion beam
densities and ion energies to deposit said alloy comprising said
constituent elements on said substrate.
24. Method in accordance with claim 21, comprising using a plurality of
radio frequency ion beam sources to simultaneously deposit ions of
constituent elements of a chemical compound using suitably selected ion
beam densities and ion energies to deposit said chemical compound
comprising said constituent elements on said substrate.
25. A Radio frequency ion beam source in accordance with claim 3 wherein
said insulating protective layer comprises a woven quartz fibre cover.
26. A Radio frequency ion beam source in accordance with claim 3 wherein
said insulating protective layer comprises glass coating.
27. A Radio frequency ion beam source in accordance with claim 4 wherein
said ioniser vessel has an axis and wherein said insulators for said feed
connections are uniformly displaced relative to one another and to said
vessel axis.
28. A Radio frequency ion beam source in accordance with claim 5 wherein
said ring of permanent magnets is coaxially arranged relative to said
ioniser vessel and to said radio frequency coil.
29. A Radio frequency ion beam source in accordance with claim 8 wherein
said grid electrodes comprise molybdenum.
30. A Radio frequency ion beam source in accordance with claim 8 wherein
said grid electrodes comprise stainless steel.
31. A Radio frequency ion beam source in accordance with claim 8 wherein
said extraction bores comprise conically tapering bores.
32. A Radio frequency ion beam source in accordance with claim 8 wherein
said respective diameters of said extraction bores in said first, second
and third grid electrodes are 3 mm, 2 mm and 3.2 mm respectively.
33. A Radio frequency ion beam source in accordance with claim 10 wherein
said conical surface diverges away from said ioniser vessel and has an
acute angle of about 15.degree. with respect to said central axis.
34. A Radio frequency ion beam source in accordance with claim 11 wherein
said third grid electrode and said ring electrodes comprise stainless
steel or molybdenum.
35. A Radio frequency ion beam source in accordance with claim 13 wherein
said source output is grounded.
36. A Radio frequency ion beam source in accordance with claim 10 wherein
said grid electrodes and said ring electrodes comprise a metal and are
directly or indirectly cooled by a cooling medium.
37. A Radio frequency ion beam source in accordance with claim 12 wherein
said magnetic lens is directly or indirectly cooled by a cooling medium.
38. A method in accordance with claim 16 wherein said ion focussing unit is
switched on when said ion energy fall below approximately 300 V. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
The invention relates to a radio frequency ion beam source comprising an
ioniser vessel which can be charged with the particular working gas to be
ionised, in particular, gaseous, condensable metal vapours and metal
compounds; a coil connected to a radio frequency source for generating a
plasma in the ioniser vessel, with the plasma arising through inductively
stimulated discharge; and also a beam forming system with several
extraction grids.
The technology of surface treatment and in particular the manufacture of
thin layers has become very significant in recent years, particularly with
regard to the industrial application of such processes. There are nowadays
numerous processes for the manufacture or preparation of thin layers and
for the treatment of material surfaces. They all require either reduced
pressure or vacuum in the processing chamber and are thus carried out in
vacuum systems.
Some very well known processes relate to vaporisation by Joule heating in
furnaces, boats, crucibles etc. either by electric heating or by
electronic or ion bombardment. Other processes use vaporisation produced
by means of an anodic or cathodic arc or also by eddy current heating of
conductive material in an induced AC field. Mention should also be made of
large area sublimation on cathodes using various cathode sputtering
arrangements with and without magnetic enhancement of the ionisation in
the DC or AC glow discharge.
The known and customary vaporisation sources deliver atoms and ions for the
condensation on the substrate which have a broad distribution of kinetic
energies of the particles. This is a problem because the highly energetic
particles cause damage rather than the generally desired uniform
condensation, which results in faultfree crystal growth. The damage caused
can take the form of crystal splitting, crystal destruction or
decomposition of the surface, in a similar form to cathode sputtering. On
the other hand the incident particles of low energy frequently do not
attain the required kinetic energy at the surface for an orderly
incorporation into a crystal grid.
These energies are frequently not sufficient in order to obtain the desired
bond strength of the coating in the boundary surface between the substrate
and the layer. In some cases a high energy of the particles is required in
the coating beam in order to increase the surface energy of the substrate,
when the substrate temperature has to be kept low for certain reasons, or
when the heat exchange between the substrate and the uppermost
condensation plane is deficient.
Radio frequency ion sources of the initially named kind are also known (see
EP-A No. 2 0 261 338 and DE-A No. 1 37 08 716) in which the r.f. coil is
arranged outside of the respective ioniser vessel and specially formed ion
extraction systems are used. Such ion beam sources make it possible to
generate only simply charged ions, i.e. only a single monoenergetic beam
peak, can also be used with reactive gases, have a robust and simple
construction and also a simple supply regulation unit, they are also able
to satisfy the practical requirement placed on operational reliability and
working life.
It is however not possible with the known radio frequency ion beam sources
to satisfy the requirements which arise in practice for intensive beams of
variable ion energy right down to extremely low values, and in particular
the requirement to also generate metal ions.
SUMMARY OF THE INVENTION
The object of the invention is thus in particular to develop a radio
frequency ion beam source of the initially named kind so that, with an
extensively monoenergetic energy distribution of the ions, both the ion
energy and also the ion flux density can be continuously and extremely
sensitively varied in a large range, and above all, so that intensive
metal ion beams of very low energy can be generated. Beam divergence
should be avoided, or at least substantially avoided.
This object is satisfied in accordance with the invention in that the radio
frequency coil which brings about an automatic ring discharge and serves
to generate a non-isothermal plasma of ions, electrons and neutral gas
particles is coaxially arranged in the interior of the ioniser vessel; in
that the ioniser vessel is formed as a metallic conductive vessel; and in
that the beam forming system comprises a combination of multi-hole
extraction grids and an ion focussing unit which directly follows them and
which can in particular be selectively switched on.
The arrangement of the r.f. coil in the interior of the ioniser vessel, and
the use of a metallic ioniser vessel, results not only in a very rigid
arrangement having an overall robust and price-worthy construction, but
above all also makes it possible to generate metal ions. This is possible
since the danger which arises with the known quartz vessels with outwardly
disposed r.f. coils is no longer present, namely that the dissociated
conducting substances deposit on the internal wall of the quartz vessel,
form a conductive coating and then screen the r.f. energy against
penetration into the discharge plasma.
Through the beam formation system of the invention in the form of a
combination of multi-hole extraction grids and an ion focussing unit which
directly follows the latter, which can operate on an ion optical,
electrostatic, magnetostatic or electromagnetic basis, and which can in
particular be selectively switched in, it is ensured that continuously
selectable ion current densities can be ensured in a large range, in
particular to the range from less than 1 mA/cm.sup.2 to more than 10
mA/cm.sup.2 with approximately monoenergetic and continuously variable ion
energy, in particular between approximately 10 eV and 3 keV, and indeed
also with independently adjustable and high current densities. The radio
frequency ion beam source in accordance with the invention is usable for a
wide range of ions including metal ions from gaseous metal compounds and
ions of reactive gases. It is characterised by a long working life and
long intervals between services.
The ioniser vessel preferably consists of a chemically resistant, stainless
and non-magnetic steel and can be provided with liquid cooling, for
example water cooling.
The r.f. coil preferably consists of a non-magnetic electrically well
conducting metal tube coil through which coolant liquid flows, with the
copper tube coil being coated, in view of the high plasma conductivity and
the differential r.f. voltages along the coil, with an insulating layer,
for example in the form of a woven quartz fibre cover or a glass coating.
The ioniser vessel is provided at one end with a cover and at the other end
with a mounting flange. The ion source can be externally flanged via this
mounting flange onto the associated vacuum chamber in which there prevails
a pressure of 10.sup.-4 to 10.sup.-8 Torr for example.
Insulators for the passage of the coil connections through the cover are
provided in or on the cover and permit the supply of r.f. power to the
coil. The gas inlet is also present in the cover, which is preferably
formed in one piece with the container. Inwardly disposed screens are
preferably used in the ioniser vessel to prevent metal being deposited on
the insulators for the coil connections.
A ring of permanent magnets of alternating polarity is mounted on the
metallic outer jacket of the ioniser vessel centrally relative to the
ioniser vessel and the coil, to reduce discharge losses at the wall of the
ioniser vessel. For a given r.f. power the plasma density and the beam
density can be increased in this way since a "Cusp field arrangement" of
this kind greatly reduces the plasma current losses at the wall of the
ioniser, and with a suitable arrangement of these permanent magnets, the
plasma is moreover concentrated via the extraction openings.
The extraction grid provided in accordance with the invention preferably
forms a multi-hole three grid extraction system, the first grid electrode
of which is connected with the metallic ioniser vessel. The thickness
dimensions of the individual, mutually insulated, grid electrodes and
their mutual spacings are very small and preferably substantially smaller
than 1 mm. A central support is usefully provided to ensure precise
spacing of the grid electrodes and also to restrict thermal bending
deflections of the grids.
The ion focussing unit provided in accordance with the invention, which is
in particular switched on at low ion energies and conjointly at high
current densities, preferably consists of an ion optical focussing lens
which is formed by the third grid electrode and two ring electrodes. The
two ring electrodes preferably lie on an imaginary conical surface, in
particular a conical surface which diverges away from the ioniser vessel
and has an included angle of 30.degree..
In accordance with an embodiment of the invention a beam neutraliser having
a glow filament coil can also be provided in the region of the preferably
earthed source output, or inside the ion lens. The beam neutraliser is
advantageously used with insulating targets, or at least with targets
having an insulating surface to additionally reduce the space charge
broadening of the ion beam at low energies.
In addition, a magnetic lens with a field strength matched to the ion
energy can be provided in the region of the source outlet to assist the
bundling of the beam.
In connection with a method for operating the ion beam source in accordance
with the invention it is advantageously possible to vary the current
density, which depends on the r.f. power and/or on the extraction voltage,
and the ion energy independently from one another. In this way it is
possible to ideally satisfy the most diverse and also most difficult tasks
which arise in practice.
These tasks can be explained in the following manner. The energies and
current densities of the ions which impinge on the substrates and must
namely be selected and optimised for the layer quality which is desired
for the particular material. This has the following reasons:
It is often necessary for the growth of a specific crystal orientation to
specify both the minimum and also the maximum energy of the arrival of the
ions. Many crystals are for example built up endothermically with their
build-up frequently being directionally dependent on the supply of energy
and thermal dissipation. Moreover, the crystallite form, which can appear
as round lumps, long needles, columns or slate-like platelets depends
during growth on the energy of the incident ions and atoms.
Furthermore, the ion current density or atom current density incident on
the substrate has a large influence on the overall crystallite texture and
thus also on the growth rate of the total layer. Both the ion current
density and the ion energy must be individually optimised for each kind of
material and layer quality, which is made possible by the ion beam source
of the invention.
In forming a well bonded boundary layer between the substrate and the
actual layer one often operates at high ion energies in order to obtain a
kind of ion implantation and thus a particular anchoring of the intended
layer in the substrate. The ion implantation results in atoms being
incorporated both between the crystallites and also within the
crystallites (interstitially) up to a depth of several atomic planes in
the crystals of the substrate. In so doing a diffusion exchange of the
arriving atoms with the atoms of the crystallite is simultaneously made
possible. This leads for example to solid solutions, mixed crystals and
for instance intermetallic compounds.
All the aforementioned is made possible by the use of the ion beam source
of the invention in sequential continuous method steps, without
interruption, in the same vacuum plant and at the same position. Thus
several method steps, such as surface cleaning by means of cathode
sputtering or ion beam etching through ions of high energy and then the
build-up of a well bonded boundary layer and finally the build-up of the
desired layer can be carried out as mentioned above without interruption
and with the same ion beam source.
By way of example the method steps can comprise combinations of at least
some of the following method steps:
(a) the substrate surface is cleaned and partially eroded by ion beam
etching with appropriately chosen ion energy and ion current density,
(b) the substrate surface is heated and degassed by means of an ion beam of
low ion energy and high current density (>1 mA/cm.sup.2),
(c) the substrate surface is made rough by means of craters and grooves for
the purpose of good mechanical anchoring of the desired layer,
(d) ions of higher energy (>50 keV), the beam being of low current density,
are shot into the substrate (are implanted) such that the ions penetrate
some atomic planes deep into crystallites (crystal grains), in particular
into the spaces between crystal atoms (interstitial) and even into the
interspace of the crystallites (inter grain), thus initiating of
interdiffusion of atoms, whereby the implantation of ions produces solid
solutions of the penetrating ions, mixed crystallisation and, for
instance, intermetallic compounds with the material of the substrate,
(e) layers of metals, alloys or compounds are built up on the surface of
the substrate with the optimum ion energy and optimum ion beam density
necessary for each crystallite texture and crystallite shape, and
(f) layers of metals, alloys or compounds are built up with the optimum ion
energy and optimum ion current density necessary for each crystal
structure and crystal orientation.
In order to explain this possibility further the abovementioned method
steps (a), (b), (d) and (f) can for example be used to apply hard metal or
ceramic layers onto an inorganic substrate. In contrast a combination of
the steps (a), (b), (c), (e) and (f) is suitable for the application of
metal layers onto modern plastics, for example a layer of gold on
polytetrafluoroethylene. The method of the invention is also entirely
suitable for the coating of semiconductors, for example for applying
contacts to semiconductors based on gallium arsenide. For this a
combination of the abovementioned method steps (a), (b), (e) and (f) is
suitable.
It is also particularly important that one can use ion sources with
suitable ion current densities and ion energies to produce or to deposit
on substrates, alloys and compounds which may for example also include
alloys of a complex nature.
It should also be mentioned that a further magnetic post focussing can be
effected in the process chamber in order to counteract the tendency of the
ions to diverge, in particular at low ion energies. That is to say a
magnetic post focussing is effected, or a corresponding device is
arranged, in the process chamber between the ion radiation source and the
substrate. This post focussing device is particularly useful when the ion
energy falls below about 500 eV.
Further advantageous features and developments of the invention are set
forth in the subordinate claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be explained in more detail with reference to the
drawings, in which are shown:
FIG. 1 a schematic representation of a radio frequency ion beam source in
accordance with the invention for the production of coatings on substrates
and for the processing of material surfaces,
FIG. 2 a diagram of typical values of discharge and plasma data with regard
to their radial course in a radio frequency ion beam source in accordance
with FIG. 1,
FIG. 3 a schematic representation to explain the formation by extraction of
part of an ion beam from the plasma of a radio frequency ion beam source
in accordance with FIG. 1, by means of a three grid electrode system,
FIG. 4 a diagram of the extractable total ion current of the extraction
system of FIG. 3,
FIG. 5 a sectional representation of a practical embodiment of a radio
frequency ion beam source in accordance with the invention,
FIG. 6 a part illustration of an additional magnetic focussing device for
use with an ion beam source in accordance with FIG. 5 for very low ion
energies,
FIG. 7 a diagram of typical discharge characteristics of an ion beam source
in accordance with FIG. 5, and
FIG. 8 a diagram of the required extraction voltage of the grid system of
an ion beam source in accordance with FIG. 5 for argon as a function of
the current density and of the extracted ion current.
DESCRIPTION OF THE PREFERRED EMBODIMENT
As seen in FIG. 1 a high frequency ion beam source comprises a metallic
ioniser vessel 1 with an associated gas supply system 2, a radio frequency
coil 3 arranged inside the ioniser vessel 1 and having an associated radio
frequency generator 4, and also a beam formation system which comprises
three grid electrodes 6, 7, 8 and two ring electrodes 9, 10 which belong
to an ion lens.
The grid electrodes and/or ring electrodes are connected to high voltage
generators 11. The ioniser vessel is surrounded by permanent magnets in a
specific arrangement 5.
The working gases which flow in from the gas supply system 2 are ionised in
the ioniser vessel, i.e. split up into positive ions and electrons and
thus a plasma state is created.
The discharge power required for an inductive, radio frequency and
automatic ring discharge is generated by the r.f. generator which
preferably operates in the frequency range form 0.5 to 30 MHz and is
coupled in via the induction coil 3. This causes first of all a high
frequency axial magnetic field in the interior of the coil which
generates, by induction, an electrical eddy field with closed field lines
of the same frequency. In this eddy field discharge electrons are
accelerated on approximately circular paths until they are able to cause
new ionisation impacts. Through the cooperation of elastic central
collisions with direction reversal and the change of direction of the
radio frequency azimuthal electric field the electrons affected hereby can
rapidly accumulate energy. By optimum matching of the generator frequency,
the discharge pressure and the circumference of the ioniser vessel, this
accumulation process can be favourably influenced from a statistical
viewpoint. The length of the ioniser vessel can also be optimised in
dependence on the vessel radius and the ion mass in order to obtain
maximum ion yields.
As the ionising electrons are created from previous ionisation acts an
independent gas discharge is present which does not require any glowing
cathodes to feed it. This leads to the high degree of reliability and to
the long life of the radio frequency discharges, in particular also when
operating with reactive gases.
The ignition of the r.f. discharge takes place either automatically by a
high voltage pulse at an adequately high operating pressure or by a short
metered pressure surge at low gas pressure. The build-up time of the r.f.
discharge amounts to around only 30 .mu.s which is a further advantage
which is important for many applications. The r.f. discharge generates a
non-isothermal plasma comprising ions, electrons and neutral gas
particles.
As can be seen from FIG. 2 the electron temperature T.sub.e reaches some
10.sup.4 K while ion temperature and in particular the neutral particle
temperature, lies only a little above room temperature. This also
simplifies the cooling of the whole ioniser.
A further advantage of the r.f. discharge is the approximately pure Maxwell
distribution of the plasma electrons and this signifies that on a
quantitative basis douple-ions hardly occur and thus that the desired
homogeneity of the energy is not disturbed by particles of double or
multiple energy. As a result of the induction law the electrical eddy
field strength Eind is zero at the axis of the ioniser and rises towards
the radius of the coil and this course is amplified further by the skin
effect. As a consequence of this the electron temperature T.sub.e rises
rapidly towards the outside as is shown in FIG. 2.
The plasma density n, however, decreases towards the edge and indeed in
consequence of the charge carrier movement with subsequent ion-electron
recombination.
As the extractable beam current density is proportional to n. T.sub.e both
effects hereby approximately cancel one another, so that the high
frequency ion beam source has the desired homogenous beam profile. As the
plasma density n, and thus also the extractable current density, increase
linearly with the power of the r.f. generator the achievable ion density
is only restricted by the extraction system and by the maximum operating
temperature of the ion beam source which is provided with a cooling means.
The system shown in FIG. 1 for the formation of the ion beam 12 has the
task of extracting the plasma ions from the ioniser, of accelerating them
and of focussing them into the beam 12. In order to also fulfil this task
for beam voltages or ion energies far below 100 V the radio frequency ion
beam source of the invention is provided with a combination of multi-hole
extraction grids and an ion optic focussing unit.
All the grid and ring electrodes 6, 7, 8, 9, 10 which belong to this beam
formation system, as shown schematically in FIG. 1, comprise preferably
molybdenum, stainless steel or the like as low thermal coefficient
expansion and high temperature stability is required of these electrodes.
The individual electrodes are connected to corresponding high voltage
sources.
The first grid electrode, which can also be termed an extraction anode lies
at a positive potential of approximately 10 to 3000 V, is electrically and
thermally conductingly connected to the ioniser vessel 1, which acts as an
anode in operation, and determines, together with the last electrode 10
which is held at earth potential, the beam voltage or the ion energy (10
to 3000 eV).
The second grid electrode 7, which can be termed an extraction cathode, is
negatively biased at a level which is just sufficiently high that the
potential difference from the first grid electrode 6 delivers the desired
current density which depends on the ideal extraction voltage. The spacing
of the plasma bounding surface 14 from the second grid electrode as
indicated in FIG. 3 hereby represents the acceleration path d of the ions
taking account of the bending of the equipotential lines (see FIG. 3). The
third grid electrode 8 which is to be termed a retarding electrode does
not have to be earthed. It simultaneously acts as the first electrode of
the ion lens which is inserted after the extraction path and has a freely
selectable potential.
The first ring electrode 9 represents the central electrode of the ion lens
and its potential must be ideally matched to the desired values of the
beam current and beam voltage.
The second ring electrode 10 lies at earth potential and terminates the ion
lens and also the entire beam forming system.
FIG. 3 shows the potential distribution and the ion tracks for a partial
beam (single hole arrangement) within the three grid electrodes 6, 7, 8.
The plasma ions are picked up by the extraction field between the first
two electrodes 6, 7 and accelerated towards the openings of the second
grid electrode 7 (extraction cathode) while the plasma electrons are held
back, so that a positive space charge arises within the extraction range.
The boundary between the neutral plasma of the r.f. discharge and the
positive space charge zone is termed the plasma boundary 14 and acts as an
ion emitter or "virtual anode".
When a specific ion energy and at the same time a defined current density
is required then this can substantially always be achieved by adjusting
the two first grid potentials U.sub.+ -U and U.sub.+ -U.sub.- -U.sub.ex.
The desired beam voltage U normally lies beneath the required dependent
extraction voltage which is dependent on the desired current density. In
this case one uses the so-called acceleration-deceleration technique and
slows the ions between the second grid electrode 7 and the third grid
electrode 8 down to the desired U-value. The total extracted ion current
results from the product of the current density in an individual
extraction bore, the number of the ex | | |
