U.S. patent number 3,913,320 [Application Number 05/523,483] was granted by the patent office on 1975-10-21 for electron-bombardment ion sources.
This patent grant is currently assigned to Ion Tech, Inc.. Invention is credited to Harold R. Kaufman, Paul D. Reader.
United States Patent |
3,913,320 |
Reader , et al. |
October 21, 1975 |
Electron-bombardment ion sources
Abstract
An electron-bombardment ion source includes a chamber into which
a propellant is introduced. The propellant is ionized by means of
electrons drawn toward an anode from a cathode. At one end of the
chamber is an apertured screen followed by an aligned apertured
grid. The grid is maintained at a potential that accelerates the
ions out of the chamber through the screen and the grid. A surface
within the chamber, such as the screen, the wall of the chamber or
both, and other than the anode and cathode, is maintained at a
potential approaching the anode during the initiation of ion
production, while being maintained at a potential approaching that
on the cathode during steady-state ion production. Conveniently,
this is achieved by interposing a resistance, of appropriate value,
between that surface and a potential source.
Inventors: |
Reader; Paul D. (Fort Collins,
CO), Kaufman; Harold R. (Fort Collins, CO) |
Assignee: |
Ion Tech, Inc. (Fort Collins,
CO)
|
Family
ID: |
24085223 |
Appl.
No.: |
05/523,483 |
Filed: |
November 13, 1974 |
Current U.S.
Class: |
60/202;
313/362.1 |
Current CPC
Class: |
F03H
1/0043 (20130101); H01J 27/205 (20130101) |
Current International
Class: |
F03H
1/00 (20060101); F02K 009/00 (); F03H 001/00 ();
H05H 005/02 () |
Field of
Search: |
;60/202 ;313/359-363
;315/111.3,111.6,111.8,111.9 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Husar; C. J.
Assistant Examiner: Garrett; Robert E.
Attorney, Agent or Firm: Drake; Hugh H.
Claims
We claim:
1. An electron-bombardment ion source comprising:
means defining a chamber for containing an ionizable
propellant;
means for introducing said propellant within said chamber;
an anode disposed within said chamber;
an electron-emissive cathode disposed within said chamber;
means for impressing a potential between said anode and cathode to
effect electron emission at a sufficient velocity to ionize said
propellant;
an apertured screen disposed in the vicinity of one end of said
chamber;
an apertured grid spaced from said screen in a direction away from
said chamber with the apertures in said grid being alined relative
to the apertures in said screen so that said screen shields said
grid from ionic bombardment;
means for impressing a potential between said grid and both said
anode and cathode for accelerating ions out of said chamber through
said screen and said grid;
and passive means for maintaining a surface, within said chamber
and exclusive of said anode and said cathode, at a potential at
least approaching the potential on said cathode during steady-state
production of ions within said chamber and at a potential at least
approaching the potential on said anode during the initiation of
ion production prior to the existence in said chamber of a
discharge plasma.
2. An ion source as defined in claim 1 in which said surface
includes said screen.
3. An ion source as defined in claim 1 in which said surface
includes the inner wall of said chamber.
4. An ion source as defined in claim 1 in which said surface
includes both said screen and the inner wall of said chamber.
5. An ion source as defined in claim 1 in which said maintaining
means includes a source of potential difference, means for coupling
the more negative end of said source to said cathode, and means for
variably coupling the more positive end of said source to said
surface.
6. An ion source as defined in claim 5 in which said source of
potential is in common with said means for impressing a potential
between said anode and cathode.
7. An ion source as defined in claim 1 in which said maintaining
means includes impedance means connected at one end to said surface
and maintained at its other end at a potential positive with
respect to said cathode, said impedance means exhibiting a
resistance sufficiently large that, during said steady-state
production, current flow through said impedance means is but a
small fraction of the current flow delivered by said means for
impressing a potential between said anode and cathode, with said
resistance yet being sufficiently low that, during said initiation
of ion production, the potential drop across said impedance means
is very substantially less than said potential difference between
said anode and cathode.
8. An ion source as defined in claim 1 in which said surface
includes said screen and in which said maintaining means includes a
resistor connected between said screen and a source of potential
positive with respect to said cathode.
9. An ion source as defined in claim 1 in which said surface
includes the inner wall of said chamber and in which said
maintaining means includes a resistor connected between said inner
wall and a source of potential positive with respect to said
cathode.
10. An ion source as defined in claim 1 in which said surface
includes both said screen and the inner wall of said chamber, and
in which said maintaining means includes a resistor connected at
one end to both said screen and said inner wall and connected at
its other end to a source of potential positive with respect to
said cathode.
11. An ion source as defined in claim 1 in which said surface
includes both said screen and the inner wall of said chamber, and
in which said maintaining means includes a first resistor connected
at one end to said screen and a second resistor connected at one
end to said inner wall with the other ends of both said resistors
being connected to a source of potential positive with respect to
said cathode.
12. An ion source as defined in claim 1 in which said maintaining
means includes an impedance connected at one end to said surface
and maintained at its other end at the potential on said anode.
13. An ion source as defined in claim 1 which further includes
neutralization means located beyond said apertured grid from said
chamber for neutralizing the electric charge in ions flowing
through said grid.
14. An ion source as defined in claim 1 which further includes
means for developing a magnetic field within said chamber to effect
gyration of electrons emitted from said cathode.
Description
The present invention pertains generally to electron-bombardment
ion sources. More particularly, it relates to circuitry for
enhancing the process of initiating the production of ions within
such sources.
Electron-bombardment ion sources were originally developed as a
means of propulsion in outer space. As compared with conventional
chemical rockets, the high exhaust velocities available from such
ion sources permit a reduction in propellant mass needed to meet
the same propulsion requirement. An earlier version of such an ion
source, as developed specifically for space propulsion, is
disclosed in U.S. Pat. No. 3,156,090. Various modifications and
improvements on such an ion source are disclosed in U.S. Pat. Nos.
3,238,715, 3,262,262 and 3,552,125.
More recently, electron-bombardment ion sources have found use in
the field of sputter machining. In that field, the ion beam
produced by the source is directed against a target, so as to
result in the removal of material from the target. This effect is
termed sputter erosion. By protecting chosen portions of the target
from the oncoming ions, material may be selectively removed from
the other portions of the target. That is, those other portions of
the target are thereby selectively machined.
Alternatively, essentially the same apparatus can be used for what
is called sputter deposition. In this case, a surface to be coated
is disposed so as to face the target in order to receive material
eroded from the target. Selected portions of the surface under
treatment may be masked so that the sputter material is deposited
in accordance with a chosen pattern. Moreover, several different
target materials may be ionically bombarded simultaneously so as to
result in a controlled disposition of alloys of the different
materials. In some cases, sputter deposition represents the only
way in which the formation and deposit of such alloys may be
achieved.
Still another use of the described ion sources is in the
implantation or doping of ions into a semiconductor material.
Basically, this usage differs from sputter machining only in that
higher ion energies are required in order to obtain a useful
distance of penetration into the semiconductor material.
Whatever the specific manner of utilization, such ion sources are
especially attractive for sophisticated tasks like those of forming
integrated circuit patterns. For example, conductive lines may be
deposited on a substrate in thicknesses measured in Angstroms and
with widths measuring but tenths or hundredths of a micron. Defects
in linearity may be held to less than a few hundredths of a
micron.
Electron-bombardment ion sources of the kind under discussion
include a chamber into which an ionizable propellant, such as
argon, is introduced. Within the chamber is an anode that attracts
high-velocity electrons from a cathode. Impingement of the
electrons upon the propellant atoms results in ionization of the
propellant. At one end of the chamber is an apertured screen
followed by an apertured grid. A potential impressed upon the grid
accelerates the ions out of the chamber through the apertures in
both the screen and the grid, while the apertures in the screen are
aligned with those in the grid so as to shield the latter from
direct ionic bombardment. At least usually, another
electron-emissive cathode is disposed beyond the grid for the
purpose of effecting neutralization of the electric space charge
otherwise exhibited by the accelerated ion beam. Preferably, the
interior of the chamber is subjected to a magnetic field which
causes the electrons emitted from the cathode to gyrate in their
travel toward the anode. This greatly increases the chance of an
ionizing collision between any given electron and one of the
propellant atoms, thus resulting in substantially increased
efficiency of ionization.
A common problem encountered with prior electron-bombardment ion
sources has been that of satisfactorily initiating the formation of
the ion plasma. In designing the ion sources, the anode shape and
the magnetic field configuration are chosen so as to result in good
performance in terms of a low discharge-energy loss per beam ion
formed with a high fraction of the propellant being ionized.
However, the meeting of that objective results in a comparatively
poor extraction of electrons from the cathode prior to the
formation of a discharge plasma. After initiation of the discharge,
the resulting plasma of electrons and ions exhibits a conductivity
that permits the anode to be much more effective in extracting
electrons from the cathode. In consequence of these considerations,
a relatively high potential difference between the anode and
cathode has been required in order to initiate the production of
ions. After initiation, however, the potential difference can be
significantly reduced without causing extinguishment of the plasma
discharge. Moreover, it has been necessary to effect such a
potential reduction in order to obtain an ion beam which includes a
desired high fraction of singly ionized atoms.
Several approaches have heretofore been utilized for the purpose
of, at least in effect, creating a high potential difference during
starting so as to promote the generation of a useful electron
current, while subsequently reducing that potential difference upon
the formation of the desired discharge plasma. In perhaps the most
direct approach, the ion source is started by impressing a high
direct-current potential difference between the anode and cathode.
Upon initiation of the plasma discharge, that potential is manually
or automatically lowered, as by switching. In another approach, a
high-voltage pulse or series of pulses is superimposed, during
initiation of discharge, upon the normal or steady-state potential
difference supplied between the anode and cathode. A different
technique involves control of the magnetic field to which the
electrons within the chamber are subjected. In this case, the
current supplied to the associated electromagnet is applied only
after plasma discharge has been initiated; in the absence of the
magnetic field, the coupling between the anode and cathode is
increased so as, in turn, to decrease the amount of required
additional potential difference necessary to effect the initiation
of the production of ions. In all of these various approaches, some
kind of active and additional switching or generating device has
been required.
It is, accordingly, a general object of the present invention to
provide an electron-bombardment ion source with a new and improved
starting circuit that overcomes disadvantages or deficiencies
present in previous systems.
A specific object of the present invention is to provide a new and
improved starting arrangement which eliminates or at least
minimizes the need to increase ion source discharge potential
difference above its normal steady-state operating value.
Another object of the present invention is to provide a new and
improved starting arrangement which employs only passive components
and yet which is reliable and economical
An electron-bombardment ion source constructed in accordance with
the present invention includes a chamber for containing and into
which an ionizable propellant is introduced. Disposed within the
chamber are an anode and an electron-emissive cathode, a potential
being impressed between the anode and cathode to effect electron
emission at a sufficient velocity to ionize the propellant.
Disposed in the vicinity of one end of the chamber is an apertured
screen. Spaced from that screen in a direction away from the
chamber is an apertured grid, the apertures in the grid being
aligned relative to the apertures in the screen so that the screen
shields the grid from ionic bombardment. A potential is impressed
between the grid and both the anode and the cathode so as to
accelerate ions out of the chamber through the screen and the grid.
Finally, the ion source includes passive means for maintaining a
surface, within the chamber but exclusive of the anode and the
cathode, at a potential which at least approaches the potential on
the cathode during steady-state production of ions within the
chamber, while at a potential at least approaching the potential on
the anode during the initiation of ion production prior to the
existence in the chamber of a discharge plasma.
The features of the present invention which are believed to be
novel are set forth with particularity in the appended claims. The
invention, together with further objects and advantages thereof,
may best be understood by reference to the following description
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 schematic diagram of a known electron-bombardment ion
source with its associated electrical circuitry;
FIG. 2 is a schematic diagram of an electron-bombardment ion source
and its associated electrical circuitry constructed in accordance
with one embodiment of the present invention; and
FIG. 3 is a schematic diagram of an electron-bombardment ion source
and its associated electrical circuitry constructed in accordance
with an alternative embodiment of the present invention.
In order perhaps to gain a better understanding of the subject
matter, an explanation will first be given with respect to the
nature and operation of a typical known electron-bombardment ion
source as illustrated in FIG. 1. It will initially be observed that
FIG. 1, like FIGS. 2 and 3, is set forth in schematic form. The
actual physical structure of the apparatus may, of course, vary,
but a suitable and workable implementation is that disclosed in the
aforesaid U.S. Pat. No. 3,156,090, which patent, therefore, is
expressly incorporated herein by reference. Thus, a housing 10 is
in the form of a cylindrical metallic shell 12 that circumscribes
and defines a chamber 14 in which an ionizable propellant, such as
argon, is to be contained. As indicated by the arrow 16, the
propellant is introduced into one end of shell 12 through a
manifold 18. Disposed symmetrically within shell 12 is a
cylindrical anode 20. Centrally positioned within anode 20 is a
cathode 22.
In the vicinity of the end of shell 12, opposite that in which, in
this case, manifold 18 is located, is an apertured screen 24.
Spaced beyond screen 24 is an apertured grid 26. The apertures in
screen 24 are aligned with the apertures in grid 26 so that the
solid portions of grid 26 are shielded from bombardment of ions
that are withdrawn from chamber 14 through screen 24 and grid 26 so
as to proceed along a beam path indicated by the arrow 28. As
mentioned in the introduction, a magnetic field, indicated by arrow
H, preferably is established within chamber 14 as by inclusion of a
suitable electrogmagnet or permanent magnet structure surrounding
shell 12. The direction of the magnetic lines of force is such as
to cause electrons emitted from cathode 22 to gyrate or convolute
in their passage toward anode 20. Situated beyond grid 26 from
chamber 14 is a neutralization cathode 30.
As herein embodied, cathodes 22 and 30 are each formed of tungsten
wire the opposite ends of which are individually connected across
respective energizing sources 32 and 34. Sources 32 and 34 may
deliver either direct or alternating current. Other types of
cathodes, such as hollow cathode which, during normal, operation
requires no heating current, may be substituted. For creating and
sustaining electron emission from cathode 22, a direct-current
source 36 is connected with its negative terminal to cathode 22 and
its positive terminal to anode 20. Connected with its positive
terminal to anode 20 and its negative terminal returned to system
ground, as indicated, is a main power source 38 of direct current.
Another direct-current source 40 has its negative terminal
connected to accelerater grid 26 and its positive terminal returned
to system ground. Finally, one side of neutralizing cathode 30 also
is returned to ground. Completing the energization arrangements,
both screen 24 and the wall of shell 12 are connected to one side
of cathode 22 by leads 42 and 44.
In operation, the gaseous propellant introduced through manifold 18
is ionized by high-velocity electrons flowing from cathode 22
toward anode 20. The pressure within chamber 14 is sufficiently
low, of the order of 10.sup.-.sup.4 Torr, that the emitted
electrons tend to proceed to anode 6 with a rather low probability
of creating ionization of the propellant. However, the magnetic
field causes the electrons to gyrate so as very substantially to
increase the probability of collisions between the electrons and
the atoms in the propellant. Ions in the plasma which is thus
produced are attracted by accelerator grid 26 so as to be directed
along path 28. Screen 24 serves to focus the withdrawn ions so that
they escape through grid 26 without impinging upon its solid
portions. The resulting ion beam traveling along path 28 is then
neutralized in electric space charge by means of the electrons
emitted from neutralizing cathode 30. Power source 36 serves to
maintain the discharge current between cathode 22 and anode 20. The
energy in the ions which constitute the ion beam is maintained by
power source 38. Power source 40 supplies the negative potential on
grid 26 necessary to accelerate the ions out of chamber 14.
While the various potentials involved will vary depending upon the
particular propellant utilized, a typical value for the potential
of source 36 is between 10 and 50 volts. The potential difference
exhibited by accelerating source 40 has an exemplary value of 500
volts in a sputtering application, 1,000 volts in usage of the ions
source for electric space propulsion and 50,000 volts or more for
ion implantation. The absolute potential magnitude of accelerating
source 40 is generally 0.1 to 1.0 times that of main power source
38. The current through accelerating source 40 is usually only a
small fraction of the ion beam current, often of the order of 0.01
or less. Consequently, the ions beam current is substantially equal
to the current delivered from main power source 38. For tungsten
filaments, cathode heating potentials are typically of the orderof
5 to 15 volts. The discharge power involved, the potential from
source 36 times the current delivered thereby, generally ranges
from about 200 to 1,000 watts per ampere of ions formed in the
ultimate ion beam.
For space propulsion, neutralizer 30 is always required. In other
applications, such as in sputtering, it may be possible to omit
neutralizer 30. For example, with the ion-impinged target connected
to the system ground, neutralizer 30 may not be required in cases
in which a comparatively low ion beam current is involved.
To initiate the production of ions within chamber 14, it is
necessary to impress a high potential difference between cathode 22
and anode 20. That starting potential may be either a steady direct
current or a pulse. Alternatively, or in combination, the applied
magnetic field strength may be decreased. In any event, the
effective initial high potential difference must usually be between
50 and 100% higher than the desired steady-state operating value as
discussed in the introduction.
A known alternative mode of operation involves omitting leads 42
and 44 so that shell 12 and screen 24 are isolated electrically
instead of being held at the potential of cathode 5. Because the
electrons existing within the plasma produced are considerably more
mobile than the ions, shell 12 and grid 24 quickly reach a
potential, during steady-state operation, such that most of the
electrons within chamber 14 are reflected from the shell and the
screen. Those elements thus reach an equilibrium floating potential
which is near the potential of cathode 22. Steady-state operation
of the ion source is, therefore, normally substantially the same as
in the actually illustrated case in which both the shell and the
screen are returned to the cathode potential. However, the starting
performance in such a modified arrangement, with leads 42 and 44
omitted, is likely to be erratic. This arises at least in part
because the various insulating supports which have to be included
tend to become high-value resistances after prolonged operation.
Consequently, the potentials which then exist on the inner walls of
shell 12 and screen 24 may range anywhere between the various
potentials that exist on adjacent elements prior to the initiation
of a discharge plasma.
Implementation of the improved features to which this application
is directed requires but very small physical change in the system
depicted in and described with respect to FIG. 1. For purpose of
illustration, therefore, the ion sources illustrated in FIGS. 2 and
3 are constructed in what is essentially the same manner as that of
FIG. 1. That is, the ion sources of FIGS. 2 and 3 again include a
shell 12 that defines a chamber 14 into which a propellant 16 is
introduced so as to be subjected to ionization by electrons
traveling from a cathode 22 toward an anode 20 under the influence
of a magnetic field H. Also included are screen 24, accelerator
grid 26 and neutralizer 30. Moreover, essentially the same power
sources 32, 34, 36, 38 and 40 are associated with the physical
structure and so connected as to create and cause the flow of a
beam of ions along path 28.
In addition, however, the ion sources shown in FIGS. 2 and 3
include means for maintaining a surface, within chamber 14 but
exclusive of anode 20 and cathode 22, at a potential which at least
approximately approaches the potential on the cathode during
steady-state production of ions within chamber 14 and at a
potential that at least approaches the potential on anode 20 during
the initiation of ion production prior to the existence in chamber
14 of a discharge plasma. While that surface may be a separately
incorporated conductive element located within chamber 14, it most
conveniently and simply is one or both of the inner wall of shell
12 and screen 24.
Completing the improved arrangement, a source of potential
difference has its negative end coupled to cathode 22 and its
positive end variably coupled to the afrorementioned surface. To
this end, as shown in FIG. 2, the necessary added element is simply
a resistor 50 connected at one end to the positive terminal of
discharge potential source 36 and at its other end both to the wall
of shell 12 and the structure of grid 24. In the modified
arrangement of FIG. 3, a first resistor 52 is connected at one end
to the positive terminal of source 36 and at its other end to the
wall of shell 12, while a second resistor 54 is connected at one
end to that positive terminal of source 36 and at its other end to
screen 24. Although a separate source of potential, suitably
returned at its negative end to cathode 22, could be used for
connection to resistor 50 or resistors 52 and 54, it is at least
usually more economical to employ the already-present source 36 for
the purpose at hand. In any event, the lower end, as drawn, of the
resistance or impedance is connected to a source of potential which
at least is approximately the same as that applied to anode 20.
Resistor 50, or resistors 52 and 54, are selected to have a value
of resistance sufficiently large that, during normal steady-state
operation of the ion source, shell 12 and grid 24 are maintained at
a potential at least close to that of cathode 22. Consequently, the
resistor or resistors carry only a small fraction of the current
from discharge power source 36 with the result that steady-state
performance of the ion source is substantially the same as
described in connection with FIG. 1. On the other hand, the value
of resistance is sufficiently low that enclosure 12 and screen 24
are maintained at a potential close to that on anode 6 during
starting, prior to the full initiation of a discharge plasma, when
the level of electron emission from cathode 22 is comparatively
low. Thus, during the initial space-charge-flow condition that
exists before a discharge plasma is present, cathode 22 is
surrounded by a surface or surfaces at least substantially at the
potential of the anode. This situation promotes electron emission
from cathode 22 so as more quickly and efficiently to initiate the
actual production of ions. As the level of electron emission rises
during the starting process, the potentials on shell 12 and screen
24 rapidly approach their steady-state values which are at least
near the potential on cathode 22. Accordingly, the shell and screen
are first at a potential which is most conducive to starting the
plasma discharge in spite of very low electron emission, but such
potentials change to that which is consistent with efficient
operation as the electron emission rises toward the normal
operating level.
For any given physical embodiment of the overall ion source, of
course, the exact value of resistance required can best be
determined by simple trial measurements. For an exemplary electron
discharge of two amperes current at a discharge potential of 50
volts in a 10-centimeter-diameter chamber containing argon, a
resistance value of 5,000 ohms has been found to be sufficiently
low to permit proper initiation of ion production at the desired
steady-state discharge voltage of the value of 50 volts. Under such
starting conditions, the potential drop across the impedance of the
resistor is quite substantially less than the potential difference
which exists between anode 20 and cathode 22. Moreover, the
initiation of ion production was found to be independent of the
presence or absence of the potentials from source 38 and 40, which
potentials were respectively of typical values of 500 volts and 100
volts. This contrasted with the operation of the prior system of
FIG. 1 in which the initiation of the production of ions proves to
be more difficult under the application of the high voltage applied
to accelerator grid 26. At the same time, the 5,000-ohm resistance
value was found to cause no significant adverse effect on
steady-state performance when compared to the otherwise similar
operation of the system of FIG. 1. This lack of deterioration in
performance arises because less than 1% of the electron emission
from cathode 22 is conducted through the resistance.
It is evident that substantially improved starting performance of
the ion source is obtained in accordance with the disclosure. Yet,
the significant improvement in performance is attainable by means
of what may be only the addition of a single resistor, while
considerably more complex active components, such as a pulse source
or a switching mechanism, may be eliminated.
While particular embodiments of the present invention have been
shown and described, it will be obvious to those skilled in the art
that changes and modifications may be made without departing from
the invention in its broader aspects, and, therefore, the aim in
the appended claims is to cover all such changes and modifications
as fall within the true spirit and scope of the invention.
* * * * *