Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02495416 2005-01-28
WO 2005/008066 PCT/US2003/018956
1
Description
Modular Gridless Ion Source
Technical Field
This invention relates generally to ion and plasma sources, and more
particularly
it pertains to gridless or Hall-current ion sources.
Background Art
Industrial ion sources are used for etching, deposition and property
modification,
as described by Kaufman, et al., in the brochure entitled Characteristics,
Capabilities,
and Applications of Broad-Beam Sources, Commonwealth Scientific Corporation,
Alexandria, Virginia (1987).
Both gridded and gridless ion sources are used in these industrial
applications.
The ions generated in gridded ion sources are accelerated electrostatically by
the electric
field between the grids. Only ions are present in the region between the grids
and the
magnitude of the ion current accelerated is limited by space-charge effects in
this region.
Gridded ion sources are described in an article by Kaufman, et al., in the
AIAA Journal,
Vol. 20 (1982), beginning on page 745. The particular sources described in
this article
use a direct-current discharge to generate ions. It is also possible to use
electrostatic ion
acceleration with a radio-frequency discharge.
In gridless ion sources the ions are accelerated by the electric field
generated by
an electron current interacting with a substantial magnetic field in the
discharge region.
The overall size and weight of a gridless source is primarily determined by
the magnetic
circuit to generate this magnetic field. A substantial fraction of the overall
cost of a
gridless ion source is also associated with the magnetic circuit. In contrast,
when a
magnetic field is used in a gridded ion source, it is only to contain the 50
eV, or less
ionizing electrons. The magnetic circuit in a gridded ion source thus plays a
secondary
role to the ion optics in determining ion-source size and cost.
Because the ion acceleration takes place in a quasineutral plasma, there is no
space-charge limitation on the ion current that can be accelerated in a
gridless ion source.
CA 02495416 2009-11-23
'70953-54
2
The lack of a space-charge limitation is most important at low ion energies,
where a
gridded ion source is severely limited in ion-current capacity.
The closed-drift ion source is one type of gridless ion source and is
described by
Zhurin, et al., in an article in Plasma Sources Science & Technology, Vol. 8,
beginning
on page R1, while the end-Hall ion source is another type of gridless ion
source and is
described in U.S. Patent 4,862,032 - Kaufman, et al.
A Hall current of electrons is generated normal to both the applied magnetic
field
and the electric field generated therein, so that these ion sources have also
been called
Hall-current sources. Because the neutralized ion beams generated by these ion
sources
are also quasineutral plasmas, i.e., the electron density is approximately
equal to the ion
density, they have also been called plasma sources.
Gridless ion sources used in industrial applications need routine maintenance.
This
maintenance can result from the limited lifetimes of certain parts, such as
cathodes. The
need for maintenance can also result from the contamination of ion-source
parts due to
sputter deposition within the ion source, or from the contamination with
materials present
in the particular application in which the ion source is used. The
contamination can be
in the form of conducting layers on insulators, insulating layers on
conducting parts, or
deposited films that can peel off to cause electrical shorts or flake off in
smaller particles
to generate unwanted particulates.
Performing the routine maintenance typically involves replacing cathodes and
some
other parts with limited lifetimes, cleaning the remaining metal parts, and
replacing
insulators. The ion sources must be substantially disassembled to carry out
this
maintenance.
The expense of performing maintenance on gridless ion sources is not limited
to
the direct time and materials involved. The downtime for the vacuum chamber
and
associated hardware often constitutes a major expense. This latter expense can
be reduced
by purchasing two ion sources, so that maintenance can be performed on one ion
source
while the other is being used. However, the purchase of an additional ion
source is an
additional expense that must be balanced against the reduction in downtime
expense.
CA 02495416 2009-11-23
70953-54
3
Disclosure of Invention
In light of the foregoing, it is a general object of some embodiments
of the invention to provide a gridless ion source with a detachable anode
module
that facilitates rapid and economical maintenance.
A specific object of some embodiments of the invention is to provide
a gridless ion source with a detachable anode module in which the cost of that
module is substantially less than the expense of the entire ion source.
Another specific object of some embodiments of the invention is to
provide a gridless ion source with a detachable anode module in which the size
and weight of that module is substantially less than the size and weight of
the
entire ion source.
A further specific object of some embodiments of the invention is to
provide a gridless ion source with a detachable anode module in which the
contamination of ion-source parts due to sputter deposition within the ion
source,
and the associated maintenance, is essentially confined to that module.
Yet another specific object of some embodiments of the invention is
to provide a gridless ion source with a detachable anode module in which the
deposition on ion-source parts due to contamination sources external to the
ion-
source are largely confined to that module.
Still another specific object of some embodiments of the invention is
to provide a gridless ion source with a detachable anode module in which the
loss
of working gas is minimized by a gas enclosure surrounding the anode in that
module.
In accordance with one embodiment of the present invention, the
ion-beam apparatus takes the form of an end-Hall ion source in which the
detachable anode module incorporates the outer pole piece and includes an
enclosure around the anode that both minimizes the loss of working gas and
confines sputter contamination to the interior of this enclosure. This
detachable
anode module is substantially smaller than the entire end-Hall ion source,
weighs
CA 02495416 2009-11-23
70953-54
3a
substantially less, and can be duplicated for significantly less cost than the
duplication of the entire ion source. In general, the components of the
magnetic
circuit determine the overall size, weight, and much of the cost of a gridless
ion
source. The reduced size, weight, and cost of the detachable anode module
compared to the entire ion source is due to most of the magnetic circuit being
excluded from the detachable module.
Another aspect of the invention provides a gridless ion-source
apparatus comprising: (a) an electron-emitting cathode means; (b) anode module
means comprising: (i) an anode; (ii) enclosure means surrounding said anode,
wherein said enclosure means includes wall means, an internal end, and an open
external end; (c) means for introducing an ionizable working gas into said
enclosure; (d) magnetic-circuit module means for generating a magnetic field
between said anode and said cathode means; wherein said anode module means
is supported by, and is detachable from, said magnetic-circuit module means.
There is also provided a gridless ion-source apparatus comprising:
(a) an electron-emitting cathode means; (b) anode module means comprising:
(i) an anode; (ii) non-magnetic enclosure means surrounding said anode,
wherein
said enclosure means includes wall means, an internal end, and an open
external
end; (c) means for introducing an ionizable working gas into said enclosure;
(d) magnetic-circuit module means for generating a magnetic field between said
anode and said cathode means; wherein said anode module means is supported
by, and is detachable from, said magnetic-circuit module means.
CA 02495416 2005-01-28
WO 2005/008066 PCT/US2003/018956
4
Brief Description of Drawings
Features of the present invention which are believed to be patentable are set
forth
with particularity in the appended claims. The organization and manner of
operation of
the invention, together with further objectives and advantages thereof, may be
understood
by reference to the following descriptions of specific embodiments thereof
taken in
connection with the accompanying drawings, in the several figures of which
like reference
numerals identify like elements and in which:
FIG. 1 is a prior-art gridless ion source of the end-Hall type;
FIG. 2 shows the prior-art ion source of FIG. 1 with the hot-filament cathode
assembly separated from the rest of the ion source;
FIG. 3 shows the prior-art ion source of FIGS. 1 and 2, without the hot-
filament
cathode assembly, but with the ion-source assembly separated from the socket
assembly;
FIG. 4 shows a cross section of the ion-source assembly of the ion source
shown
in FIGS. 1, 2, and 3;
FIG. 5 is an embodiment of the present invention wherein the gridless ion
source
is of the end-Hall type;
FIG. 6 shows the ion source of FIG. 5 with the hot-filament cathode assembly
separated from the rest of the ion source;
FIG. 7 shows the ion source of FIGS. 5 and 6, without the hot-filament cathode
assembly, but with the detachable anode module separated from the magnetic-
circuit
module;
FIG. 8a shows a cross section of the detachable anode module of the ion source
of FIGS. 5, 6, and 7;
FIG. 8b shows a cross section of the magnetic-circuit module of the ion source
of
FIGS. 5, 6, and 7;
FIG. 9a shows a partial cross section of the detachable anode module of the
ion
source of FIGS. 5, 6, and 7, showing additional features not shown in FIG. 8a;
FIG. 9b shows a partial cross section of the magnetic-circuit module of the
ion
source of FIGS. 5, 6, and 7 showing additional features not shown in FIG. 8b;
FIG. 10a is a simplified cross section of another embodiment of the present
invention wherein the gridless ion source is also of the end-Hall type;
CA 02495416 2005-01-28
WO 2005/008066 PCT/US2003/018956
FIG. 10b is a simplified cross section of the embodiment shown in FIG. 10a
wherein the anode module is separated from the magnetic-circuit module;
FIG. 11a is a simplified cross section of yet another embodiment of the
present
invention wherein the gridless ion source is of the closed-drift type; and
FIG. llb is a simplified cross section of the embodiment shown in FIG. 11a
wherein the anode module is separated from the magnetic-circuit module.
Referring to FIG. 1, there is shown a prior-art gridless ion source 10 of the
end-
Hall type. Ion source 10 is generally of the type described in U.S. Patent
4,862,032 -
Kaufman, et al. More specifically, it is a Mark II ion source marketed first
by
Commonwealth Scientific Corporation, Alexandria, Virginia, and more recently
by Veeco
Instruments Inc., Plainview, New York. Differences of the Mark II ion source
from the
aforementioned U.S. Patent 4,862,032 include the use of a plug-and-socket
design to
facilitate removal for maintenance and the use of a permanent magnet in place
of the
electromagnet to generate the magnetic field. The plug-and-socket concept is
generally
similar to that shown in the earlier U.S. Patent 4,446,403 - Cuomo, et al.
Ion source 10 includes ion-source assembly 11, socket assembly 12, and cathode
assembly 13. The components of the ion-source assembly shown in FIG. 1 include
plug
body 14, outer shell 15, and outer pole piece 16, all of which are also parts
of the
magnetic circuit. Also included in ion-source assembly 11 and shown in FIG. I
are anode
17, external anode support 18, retaining nuts 19 that must be removed to
disassemble the
ion-source assembly, threaded retainer rods 20 to which nuts 19 attach, and
knobs 21 that
attach to plug-and-socket retaining rods 22. When knobs 21 are tightened, ion-
source
assembly 11 is clamped to socket assembly 12, establishing both the electrical
connections
and the gas connection necessary for operation. Cathode assembly 13 includes
cathode
supports 23, cathode 24, and cathode retaining nuts 25. To separate the
cathode assembly
from the rest of the ion source, the two cathode supports are grasped with the
fingers of
two hands and lifted, overcoming the friction with which the cathode supports
are
attached to the rest of the ion source.
Referring to FIG. 2, ion source 10 is shown with cathode assembly 13 separated
from the rest of the ion source. At the separated location, the lower ends of
cathode
supports 23 are exposed to show connectors 26 thereon, with each connector
comprised
CA 02495416 2009-11-23
70953-54
6
of elastic spring "fingers" to establish an electrical connection with a
complementary
cylindrical contact. The spring fingers of the connectors also generate the
friction that
must be overcome in removing the cathode assembly from the ion-source
assembly. Ion
source assembly 11 can be separated from socket assembly 12 by rotating knobs
21,
thereby removing the threaded ends of plug-and-socket retaining rods 22 from
socket
assembly 12.
It should be noted that the hot-filament cathode shown in FIGS. 1 and 2,
together
with its particular installation, is exemplar only. Different mounting
arrangements are
possible for hot-filament cathodes. Also, end-Hall ion sources have been
operated with
hot-filament, hollow-cathode, and plasma-bridge types of electron-emitting
cathodes.
These alternate cathodes are described in "Ion Beam Neutralization," anon.,
CSC
Technical Note, Commonwealth Scientific Corporation, Alexandria, Virginia
(1991).
Referring to FIG. 3, ion-source assembly 11 is shown separated from socket
assembly 12. The socket assembly is comprised of socket body 30, openings 31
with
socket connectors 32 therein to provide the electrical connections for cathode
24, opening
33 with socket connector 34 to provide the electrical connection for anode 17,
threaded
opening 35 with threaded gas fitting 36 to provide a flow path for the
ionizable working
gas used in the ion source, and threaded openings 37 for the threaded lower
ends of plug-
and-socket retaining rods 22 to be threaded into and thereby clamp ion-source
assembly
11 to socket assembly 12. The socket connectors in FIG. 3 are generally
similar in
function to connectors 26 shown at the lower ends of cathode supports 23.
Referring to FIG. 4, there is shown a cross section of ion-source assembly 11.
Note that the cross section of FIG. 4 is not a particular cross section of the
Mark II ion
source, but instead is one that has been constructed to include the major
design features
of that ion source. That is, only one exemplar feature is shown when there are
typically
a plurality of such features. As an example, only one accommodation is shown
for a
cathode support, when two cathode supports are normally installed on opposite
sides of
the ion-source assembly, so that both would normally show in the same cross
section
through the center line. When the cathode assembly is installed on the ion-
source
assembly, sockets 26 of cathode assembly 13 (shown in FIG. 2) are electrically
connected
CA 02495416 2005-01-28
WO 2005/008066 PCT/US2003/018956
7
to cylindrical contacts 40, which are integral parts of cathode support rods
41. Cathode
support rods 41 are spaced from and located relative to main support plate 42
and plug
body 14 by ceramic insulators 43 held in place by nuts 44. The lower ends of
cathode
support rods 41 form contacts 45 which, when ion-source assembly 11 is clamped
to
socket assembly 12, provide electrical connections with complementary cathode
connectors 32 shown in FIG. 3. It should be noted that to provide an
insulative function
at high temperature without adverse outgassing, insulators 43 are typically
fabricated from
a refractory ceramic material such as alumina.
Anode 17 is held between external anode support 18 and internal anode support
46, with the external and internal anode supports in turn held together with
screws 47.
The assembly of anode and internal and external anode supports is spaced from
and
located relative to main support plate 42 by additional ceramic insulators 43
held in place
by screws 48. Reflector 49 is also spaced from and located relative to main
support plate
42 by additional ceramic insulators 43 held in place by screws 50 and
additional nuts 44.
Still referring to FIG. 4, the anode is connected by conducting wire covered
with
ceramic insulator beads 52 to anode rod 53 which is spaced from and located
relative to
plug body 14 by additional ceramic insulators 43 held in place by additional
nuts 44.
Contact 54 is an integral part of anode rod 53 and is electrically connected
to
complementary anode connector 34 in the socket assembly (FIG. 3) when the ion-
source
assembly is clamped to the socket assembly. Permanent magnet 55 magnetically
energizes the magnetically permeable parts of the magnetic circuit, which
include plug
body 14, outer shell 15, and outer pole piece 16. Parts other than those of
the magnetic
circuit are constructed of essentially nonmagnetic materials, i.e., parts with
a magnetic
permeability not significantly different from free space. Main support plate
42 is spaced
from and located relative to plug body 14 by threaded retainer rods 20.
The ionizable working gas is introduced through gas fitting 36 which is
attached
to a gas feed tube (not shown) and installed in threaded opening 35 (see FIG.
3).
Returning to FIG. 4, when ion-source assembly 11 is clamped to socket assembly
12, the
working gas flows from the socket assembly into volume 57, through first gas
fitting 59,
through tube 58, through second gas fitting 59, to circumferential manifold
61. From this
manifold, the working gas flows though apertures 62 in reflector 49 to reach
discharge
CA 02495416 2009-11-23
70953-54
8
volume 63, where collisions of energetic electrons emitted from cathode 24
(shown in
FIGS. 1 and 2) ionize the working gas. The ions formed by these collisions in
volume
63 are accelerated by electric fields in that volume to form an energetic ion
beam. A
more detailed description of the operation of an end-Hall ion source is
included in the
aforementioned U.S. Patent 4,862,032 - Kaufman, et al. A schematic diagram
showing
the required power supplies to operate an end-Hall ion source is also included
in
the aforementioned patent.
Those skilled in the art of ion sources will recognize that, similar to other
ion
sources used in industrial applications, ion source 10 is installed in a
vacuum chamber.
The vacuum chamber is normally assumed to be ground in the ion-source circuit,
and is
usually also at earth ground.
The magnetic circuit is comprised of those parts that are used to generate a
magnetic field between the anode and electron-emitting cathode, i.e., the
magnetic field
that electrons from the electron-emitting cathode must cross to reach the
anode. The
magnetic-circuit parts include a magnetic-field energizing means of one or
more
electromagnets or permanent, magnets. It also includes magnetically permeable
parts that
have a magnetic permeability that is significantly greater than that of free
space,
preferably greater than one or two orders of magnitude greater than that of
free space.
The preferred permanent magnet material would be one of the Alnico alloys,
which would
have a substantial advantage in maximum temperature compared to rare-earth
permanent-
magnet materials. It should be noted that the magnetic-circuit parts, plug
body 14, outer
shell 15, outer pole piece 16, and permanent magnet 55, constitute the largest
and
heaviest parts of the ion source. The magnetic circuit also accounts for a
major fraction
of the cost.
The need for maintenance can result from the limited lifetime of some parts,
usually the cathode and the reflector. Maintenance can also result from
insulative
coatings on anode 17. Such coatings can result from the formation of compounds
with
the working gas (e.g., the formation of oxides or nitrides with oxygen or
nitrogen as the
working gas). Such coatings can also result from the external sources, such as
when an
ion source is used -in an ion-assist function with the thermal deposition of a
dielectric
coating.
CA 02495416 2005-01-28
WO 2005/008066 PCT/US2003/018956
9
Conductive coatings can be deposited on insulators 43 due to internal
sputtering
in the ion source from normal operation (from reflector 49 or outer pole piece
16).
Conductive coatings can also be deposited from occasional arcs that propagate
though gap
64 between the anode and main support plate 42 to reach volume 66 external to
the
anode. As is known to those skilled in the art, the proper use of shadow
shielding can
reduce the rate at which sputtered coatings are deposited on insulators 43
exposed to
volume 66, but it cannot completely eliminate such coatings.
Conductive coatings can also be deposited due to the decomposition of some
ionizable working gases, e.g. methane. Such coatings can be found on
insulators exposed
to the working gas, even if there is no exposure to either the discharge or
arcs propagated
outside of the discharge region, e.g., volumes 67. Because the decomposition
rate tends
to increase with increasing temperature, however, these coatings would be more
likely on
insulators in physical contact with warmer main support plate 42, rather than
cooler plug
body 14.
The deposition of conductive coatings on parts others than the insulators can
eventually be a problem because of the possible shorting due to loosened
flakes of
deposited layers. As described earlier in the Background Art section, the
deposited layers
can also come off as particulates that adversely affect the thin-film products
of the
industrial process.
Disassembly for maintenance of ion-source assembly 11 starts with the removal
of retainer nuts 19 from threaded retainer rods 20. The anode, together with
the external
anode support, can be removed for cleaning by removing screws 47. Removal of
screws
48 and 50 then permit removal of internal anode support 46 and reflector 49.
To
complete the maintenance, it is often necessary to replace all insulators 43
above main
support plate 42, as well as remove deposited films on all metal parts in the
same region.
If conducting deposits can come from the working gas, almost all insulators in
the entire
ion-source assembly may need to be replaced, as well as almost all metal parts
cleaned.
In addition to the extensive disassembly and maintenance procedures required
for
the prior-art ion source of FIGS. 1 through 4, there is also the reduced
utilization of the
working gas that is inherent to the design. The working gas can escape the
discharge
region through gap 64. From there the gas can escape through penetrations in
external
CA 02495416 2005-01-28
WO 2005/008066 PCT/US2003/018956
anode support 18 for threaded retainer rods 20 and cathode support rods 41, as
well as
through the gap between the external anode support and outer shell 15. Because
of the
large diameter of the outer shell compared to the diameter of the other parts
in the ion-
source assembly, the circumferential leakage area between the external anode
support and
the outer shell can be substantial. Better containment of the working gas
would reduce
both the loss of this gas, which results in a greater vacuum pumping
requirement, and the
deposition of conducting films on insulators when decomposition of the working
gas is
possible.
Best Mode for Carrying Out the Invention
Referring to FIG. 5, there is shown a gridless ion source 70 of the end-Hall
type
that is an embodiment of the present invention. Ion source 70 is also
generally of the
type described in U.S. Patent 4,862,032 - Kaufman, et al., although it
additionally
incorporates a detachable anode module that facilitates rapid and economical
maintenance.
Ion source 70 includes cathode assembly 13, detachable anode module 71, and
magnetic-circuit module 72. Cathode assembly 13 includes cathode supports 23,
cathode
24, and cathode retaining nuts 25. The components shown in FIG. 5 for the
magnetic-
circuit module include outer shell 15 and back plate 73, both of which are
also parts of
the magnetic circuit. Also parts of the magnetic-circuit module are threaded
retainer rods
74.
Retaining nuts 76 are used to clamp anode module 71 to magnetic-circuit module
72. Outer pole piece 16 is part of the anode module and also part of the
magnetic circuit.
Because outer shell 15 remains with the magnetic-circuit module 72, knobs 77
are
attached to outer pole piece 16 to facilitate removal of the anode module from
the
magnetic-circuit module when the latter is installed in a vacuum chamber.
Anode 17,
external anode support 18, and enclosure retainer screws 78 are also included
in the anode
module. To separate the cathode assembly from the rest of ion source 70, the
two
cathode supports are grasped with the fingers of two hands and lifted,
overcoming the
friction with which the cathode supports are attached to the rest of the ion
source.
Referring to FIG. 6, ion source 70 is shown with cathode assembly 13 separated
from the rest of the ion source. At the separated location, the lower ends of
cathode
supports 23 are exposed to show connectors 26 thereon, with each connector
again
CA 02495416 2005-01-28
WO 2005/008066 PCT/US2003/018956
11
comprised of elastic spring "fingers" to establish an electrical connection
with a
cylindrical contact. To separate anode module 71 from magnetic-circuit module
72,
retaining nuts 76 are removed and the anode module lifted using knobs 77.
Referring to FIG. 7, there is shown the detachable anode module separated from
the magnetic-circuit module. Additional parts shown for anode module 71 are
enclosure
wall 79 and enclosure internal end 81. Note that the enclosure is closed on
the internal
end and open on the external end. Additional parts shown for the magnetic-
circuit
module are magnet 55, large support ring 82, and small support ring 83.
Referring to FIG. 8a, there is shown a cross section of anode module 71 of ion
source 70. Note that the cross section of FIG. 8a is again not a particular
cross section
of the ion source, but instead is one that has been constructed to include the
major design
features of that ion source. Parts not shown in FIG. 7, but shown in FIG. 8
include
internal anode support 46, screws 47 for holding the internal and external
anode supports
together, and reflector 49. The reflector again has apertures 62 therein.
Enclosure
internal end 81 has aperture 84 for introducing the ionizable gas into the
enclosure formed
by enclosure wall 79 and enclosure internal end 81. The gas flows from
aperture 84 to
circumferential manifold 86. The circumferential manifold has cover 87 with
apertures
88 therein to circumferentially distribute the gas to apertures 62 in
reflector 49, from
which the gas flows to discharge volume 63. Anode rod 89 electrically connects
with
anode 17, while being spaced from and located relative to reflector 49 and
enclosure
internal end 81 by ceramic insulators 43. The lower end of anode rod 89 forms
anode
cylindrical contact 91.
Referring to FIG. 8b, there is shown a cross section of magnetic-circuit
module
72 of ion source 70. Cathode contacts 40 are integral parts of cathode support
rods 92,
which are spaced from and located relative to large support ring 82 by
additional
insulators 43 held in place by nuts 44. Electrical connections of the cathode
contacts with
the cathode power supply (not shown) are provided by conducting wires covered
with
ceramic insulator beads 93. The ionizable working gas is provided through tube
94,
which connects to gas fixture 96 with nozzle 97. Anode connector 98 is
connected to the
anode supply (not shown) through conducting wire covered with ceramic
insulating beads
99. The anode connector is spaced from and located relative to small support
ring 83 by
CA 02495416 2005-01-28
WO 2005/008066 PCT/US2003/018956
12
additional insulators 43 held in place with nut 44. When the anode module is
clamped
to the magnetic-circuit module, nozzle 97 fits closely into aperture 84, so
that essentially
all of the working gas flows into the enclosure formed by enclosure wall 79
and enclosure
internal end 81. In addition, anode contact 91 is inserted into complementary
anode
connector 98 to electrically connect anode 17 to the anode power supply.
Referring to FIG. 9a, there is shown an additional partial cross section of
anode
module 71 of ion source 70. Internal anode support 46 and reflector 49 are
shown to be
spaced from and located relative to enclosure internal end 81 by screws 101
and additional
insulators 43 held in place by additional nuts 44. There is typically a
plurality of
screw/insulator/nut assemblies as shown in FIG. 9a and only one anode-
rod/insulator
assembly as shown in FIG. 8a, so that the clamping function of a nut is not
required on
the bottom of anode rod 89 in FIG. 8a.
Referring to FIG. 9b, there is shown an additional partial cross section of
magnetic-circuit module 72 of ion source 70. Threaded retainer rod 74 is
screwed into
back plate 73, while locating large support ring 82 relative thereto. Small
support ring
83 is located relative to back plate 73 by small ring support 102. When the
anode module
is inserted to the magnetic-circuit module, the ends of threaded retainer rods
74 fit though
apertures 103 in outer pole piece 16, so that nuts 76 (shown in FIGS. 5 and 6)
on the
ends of the threaded retainer rods can clamp the two modules together.
It should be apparent to one skilled in the art of ion-source design that
there are
many arbitrary design features in the embodiment shown in FIGS. 5 through 9b.
Cylindrical contacts and complementary connectors are used to make detachable
electrical
connections. The locations of these contacts and connectors can generally be
exchanged,
while still performing as a detachable electrical connection. Or a spring
contact and a flat
surface may be used instead to make a detachable electrical connection. The
locations of
a nozzle and an aperture for a detachable gas connection may, in a similar
manner, be
exchanged, while still performing as such a connection. Alternatively, two
flat surfaces
with matching apertures may be pressed together to perform as a detachable gas
connection. The magnetically energizing means is shown as a permanent magnet,
but
could have been an electromagnet. The magnetically energizing means could also
have
CA 02495416 2005-01-28
WO 2005/008066 PCT/US2003/018956
13
been a series of permanent magnets used in place of the outer shell, with the
central
permanent magnet replaced by a simple magnetically permeable path.
To review the maintenance advantages of the apparatus shown in FIGS. 5 through
9b, the enclosure formed by enclosure wall 79 and enclosure internal end 81
contains both
the electrons and ions that constitute the discharge plasma formed during
operation.
(Additional discussion of the constituents and properties of this discharge
plasma can be
found in the aforementioned U.S. Patent 4,862,032 - Kaufman, et al.) As is
known to
those skilled in the art of operating gridless ion sources in general and end-
Hall ion
sources in particular, sputtered particles are generated from parts exposed to
the discharge
and tend to flow outward in all directions from the sputtered surfaces of
these parts. The
enclosure contains these sputtered particles. The insulators and other parts
that are in
region 104, external to the enclosure but within the magnetic-circuit module
when the two
modules are clamped together, are thus protected from these sputtered
particles. As is
also known to those skilled in the plasma-physics art, the containment of the
plasma
electrons and ions by the enclosure greatly reduces the initiation of
discharges and arcs
in regions 104, further reducing the deposits on insulators and other parts in
regions 104.
Finally, if conductive deposits can result from the decomposition of the
ionizable working
gas, the containment of this gas within the enclosure also reduces the
deposits in regions
104. In summary the use of an enclosure surrounding the anode and discharge
region
limits the required maintenance to essentially the insulators and other parts
in the anode
module.
Compared to carrying out maintenance on the entire ion source, as required in
the
prior art, the use of modular construction with a removable anode module
permits the
maintenance to be carried out on the smaller and lighter anode module. In the
event that
downtime is to be reduced by purchasing a spare unit, only the less expensive
anode
module need be purchased. The use of modular construction also facilitates
maintenance
on parts less frequently replaced, e.g., ready access to the magnet in the
preferred
embodiment compared to essentially complete disassembly to reach the magnet in
the
prior art. The use of the invention described above thus results in the
general advantage
of more rapid and economical maintenance.
CA 02495416 2005-01-28
WO 2005/008066 PCT/US2003/018956
14
In addition to the maintenance advantages, the modular design of the invention
reduces the loss of working gas compared to the prior art. In the prior-art
design shown
in FIGS. 1 through 4, there is gas leakage between outer shell 15 and external
anode
support 18, as well as leakage through the penetrations through the external
anode support
18 for the cathode connections, the plug-and-socket retaining rods, and the
threaded
retainer rods that hold the ion-source assembly together. In the embodiment of
this
invention shown in FIGS. 5 through 9b, the smaller mean diameter of the gap
between
the enclosure wall and the external anode support reduces the circumferential
leakage
area, and there are no penetrations of the external anode support to add to
this leakage.
Comparing the invention to the prior art of FIG. 4, openings for the
attachment
of the cathode assembly in outer pole piece 16 are in the same enclosure
formed by the
parts of the magnetic circuit and therefore provide additional escape paths
for the
ionizable working gas. The use of a separate enclosure around the anode
(enclosure wall
79 and enclosure internal end 81) thus provides improved containment of the
working gas.
A simplified cross section of an alternate embodiment of the present invention
wherein the gridless ion source is also of the end-Hall type is shown in FIG.
10a. The
simplification is in the omission of the screws, nuts, insulators and other
common parts
that are required for most ion source hardware, but well understood by those
skilled in
the design art. For example, there are insulators, screws, and internal and
external anode
supports used to space the anode from the rest of the anode module, while
locating it
relative to that module - see FIG. 9a. As another example, insulators and
screws are
used to space the reflector from the rest of the anode module, while locating
it relative
to that module. In a similar manner, the cathode is not shown in FIG. 10a. Ion
source
110 in FIG. 10a is again generally of the type described in U.S. Patent
4,862,032 -
Kaufman, et al.
Ion source 110 is comprised of anode module 111 and magnetic-circuit module
112. The magnetic circuit is made up of permanent magnet 113, back plate 114,
outer
shell 116, and outer pole piece 117, all of which are in the magnetic-circuit
module.
Anode 118, reflector 119, and enclosure 121 are all in the anode module.
Enclosure 121
is in turn comprised of enclosure wall 121A and enclosure internal end 121B.
The
external end of the enclosure is again open. Other parts of the magnetic-
circuit module
CA 02495416 2005-01-28
WO 2005/008066 PCT/US2003/018956
are nozzle 122 to inject the working gas into enclosure 121 and anode
connector 123 to
establish the electrical connection to the anode.
Referring to FIG. 10b, there anode module 111 and magnetic-circuit module 112
are shown separated. Aperture 124 into which nozzle 122 fits and anode contact
125 that
electrically connects to complementary anode connector 123 are also shown in
FIG. 10b.
One difference between the embodiment of FIGS. 5 through 9b and that of FIGS.
10a and IN is that in the latter the outer pole piece is part of the magnetic-
circuit module
rather than the anode module. Both embodiments obtain substantial size,
weight, and cost
benefits from the present invention in that most of the large and heavy
magnetic circuit
is excluded from the anode module. As shown by the preferred embodiment of
FIGS.
5 through 9b, though, it is not necessary to exclude all of the magnetic-
circuit parts from
the anode module.
A related difference between the embodiment of FIGS. 5 through 9b and that of
FIGS. 10a and lOb is that in the latter the entire magnetic circuit is
external to enclosure
121. As shown by the preferred embodiment of FIGS. 5 through 9b, though, it is
not
necessary that all the magnetic circuit be external to the enclosure.
Referring to FIG. lla, there is shown a simplified cross section of an
alternate
embodiment of the present invention wherein the gridless ion source is of the
closed-drift
type. Ion source 130 is comprised of anode module 131 and magnetic-circuit
module
132.
The magnetic circuit includes inner pole piece 133, outer pole piece 134,
inner
magnetic path 135, back plate 136, outer permeable paths 137 (typically four),
inner
magnetically energizing coil 139, and outer magnetically energizing coils 141
(also
typically four), all of which are parts of the magnetic-circuit module.
Although both
permanent magnets and electromagnets have been used in closed-drift ion
sources, the use
of electromagnets is more common.
Closed-drift gridless ion source 130 is of the magnetic-layer type, which
generally
uses an insulating ceramic for discharge-chamber wall 142 - see the
aforementioned article
by Zhurin, et. al., in Plasma Sources Science & Technology, Vol. 8, beginning
on
page Rl. Anode 143 is of an annular shape with a plurality of apertures 144
for
distributing the working gas from internal manifold 145. Anode 143 connects to
gas
CA 02495416 2005-01-28
WO 2005/008066 PCT/US2003/018956
16
fitting 146 and electrical connector 147. Gas fitting 146 and connector 147
are protected
from external contamination by shield 148. A shield enclosing the outside
diameter of
the magnetic-circuit module would have provided the same protective function,
but would
also restrict thermal radiation from the outer electromagnets.
Referring to FIG. 11b, there is shown anode module 131 separated from magnetic-
circuit module, thereby exposing gas nozzle 149 and electrical contact 150,
with both
connected to anode 143.
From the above discussion and FIGS. 11a and 11b, it should be readily apparent
that the present invention can utilize a gridless ion source of the closed-
drift type. Note
that discharge-chamber wall 142 also serves as an enclosure with outer wall
142A, inner
wall 142B, internal end 142c, and an open external end.
The embodiments shown all implicitly use axially-symmetric configurations or,
in
the case of the closed-drift ion source with four outer magnetically permeable
paths, near-
axially-symmetric configurations. However, other shapes for the discharge
region such
as elongated or "racetrack" shapes. are well known to those skilled in the art
of gridless
ion sources. See for example the aforementioned U.S. Patent 4,862,032 -
Kaufman, et
al., or the aforementioned article by Zhurin, et. al., in Plasma Sources
Science &
Technology, Vol. 8, beginning on page R1. The present invention should
therefore
include embodiments in which the discharge chambers and the ion sources have
shapes
other than axisymmetric.
While particular embodiments of the present invention have been shown and
described, and various alternatives have been suggested, it will be obvious to
those of
ordinary skill in the art that changes and modifications may be made without
departing
from the invention in its broadest aspects. Therefore, the aim in the appended
claims is
to cover all such changes and modifications as fall within the true spirit and
scope of that
which is patentable.
