Ion source with particular grid assembly

An ion source has the typical chamber wherein ions are produced and caused to be propelled outwardly through at least a pair of grids which have a mutually-aligned respective plurality of apertures. Thus, there are the usual cathode, anode, magnet assembly, ionizable gas inlet and supporting power supplies as well as neutralizing means. First and second grids each have an integrally-formed peripheral marginal portion. A support element has a shape which matches and overlies the marginal portion of one grid, while a clamp has a shape which matches that of and overlies the other marginal portion. The support element and clamp are secured together. First and second mutually-aligned seats are successively spaced around the respective marginal portions. A plurality of insulators, each having of circular cross-section, are individually seated between the two different marginal portions in a manner to cause general alignment while enabling radial movement due to thermal expansion of the marginal portions relative to the support element and the clamp and each other.

The present invention pertains to an ion source. More particularly, it 
relates to a grid assembly used therein. The source is intended to be 
useful in any of thrusting, etching, deposition or enhancement 
applications. 
Early uses of ion sources were developed in connection with propulsion in 
outer space, as described in U.S. Pat. No. 3,156,090 and discussed in an 
article entitled "Technology of Electron-Bombardment Ion Thrusters" by H. 
R. Kaufman, Advances in Electronics and Electron Physics", Vol. 36, L. 
Martin Ed., Academic Press, New York, pp. 265-373 (1974). Thereafter, such 
ion sources began to find use in industrial fields such as in sputter 
etching and deposition. For background, reference may be made to U.S. Pat. 
Nos. 3,913,320, 3,952,228, 3,956,666 and 3,969,646. 
Besides use either as a thruster or in depositing or removing material, 
such sources have now also found use in enhancement of the properties of a 
material being subjected to the ion beam. Actually, no limits to the 
possible utility have yet been defined. 
While several different gridless ion sources are known, most ion sources 
heretofore used or otherwise reported in the literature employ a plurality 
of apertured grids disposed across the outlet of a discharge chamber in 
which an ion-producing plasma is contained. There typically is first a 
screen grid having apertures through which ions are withdrawn from the 
chamber by the influence of an apertured accelerator grid. The two grids 
are to be mutually aligned in an effort to prohibit impingement of the 
ions upon the accelerator grid during passage on outwardly to where they 
are utilized. In some cases, a third grid, beyond the accelerator grid, 
has been advantageously employed; it may be called either a decelerator 
grid or a suppressor grid. 
A reading of the earlier literature, especially of the related patents, 
could make it appear that the field has matured. While most or all of that 
reported before did work, continued experience with the prior apparatus 
has revealed that many problems remain for solution before appropriate 
efficiencies, reliability, durability and the like are all obtained in 
this field. 
It is, therefore, a rather general objects of the present invention to 
provide a new and improved ion source which at least contributes to the 
solution of some of those problems. 
Problems that remain are well identified in connection with a very long 
project undertaken by the NASA Lewis Research Center. For background, 
reference should be made to "Design, Fabrication and Operation of Dished 
Accelerating Grid on a 30-CM Ion Thruster" by Rawlin et al, AIAA paper No. 
72-486 (1972); "Dished Accelerator Grids on a 30-CM Ion Thruster", Journal 
of Spacecraft and Rockets, Vol. 10, No. 1, (1973) by Rawlin et al; 
"Characteristics of LeRC/Hughes J -Series 30-CM Engineering Model 
Thruster" by Collett et at, AIAA paper No. 79-2077 (1979); "Results of 
Mission Profile Life Test", Bechtel et al, AIAA paper No. 82-1905 (1982) 
and "Low Specific Impulse Electric Thrusters", NASA Contract Report No. 
CR-174678, Kaufman et al, NASA Lewis Research Center, July (1984). 
All of those papers pertain to problems of maintaining alignment and 
spacing between the grids by reason of expansion and contraction due to 
induced temperature changes. It was suggested to slot the grid margins in 
order to enable movement or to mount the marginal portions by means of 
flexible supports which yielded for radial movement of the grids. The 
recognition that a dished shape to the grids could be of assistance gave 
rise to a need for accurate ways of accomplishing the dishing of 
respective plurality of grids. That subject, it itself, was addressed by 
Banks in his U.S. Pat. Nos. 3,864,797, 3,914,969 and 3,947,933. 
In the overall, the aforesaid publications indicate that substantial 
improvements have been made. At the same time, they reveal that 
significant room remains for further improvements. Misalignments as 
between successive grids have continued to occur, attempts at a solution 
have, in turn, brought about new problems and, in short, nothing 
resembling an ultimate answer has yet been found. That has led applicants 
to seek further in the quest of better construction for multi-grid ion 
sources. 
The present invention pertains to an ion source of the type that has a 
chamber wherein ions are produced and propelled outwardly through at least 
a pair of grids having a mutually-aligned respective plurality of 
apertures. The grid assembly includes first and second grids each of 
conducting material and having integrally-formed peripheral marginal 
portions that have, inside of each marginal portion, an array of apertures 
distributed in a predetermined pattern. A support element has a shape 
which matches that of, and is mounted over the side of, the marginal 
portion of the first grid facing away from a second grid. A clamp has a 
shape which matches that of, and is mounted over the side of, the marginal 
portion of the second grid facing away from the first grid. Included are 
means for securing the clamp to the support element with the marginal 
portions sandwiched thereinbetween and respectively positioned to mutually 
align the respective ones of the apertures in the first and second grids. 
Defining a first and second mutually-aligned series of seats are means 
defined to be successively space-opposed around respective ones of the 
marginal portions. A plurality of insulators, each having a circular 
cross-section, or other cross-section suitable for self-alignment or 
positioning with the seats, are individually seated in and between 
corresponding ones of the first and second series of seats for enabling 
mutual radial movement of the marginal portions and movement relative to 
the support element and the clamp. 
The 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 objects 
and advantages thereof, may best be understood by reference of 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:

Referring now to FIG. 1, an ion source 10 includes an outer shell 12 which 
defines an interior chamber 14. An ionizable gas is introduced, as 
indicated by arrow 16, through a port into chamber 14. Within chamber 14 
is disposed a cathode 18 and an anode 20. Mounted across the outlet of 
chamber 14 is a generally planar screen grid 22 beyond which, downstream 
in the direction of the ion source flow, is an apertured accelerator grid 
24. Outwardly of accelerator grid 24 is a neutralizer cathode 26 that 
produces electrons to counter the positive charge of the ions and, 
therefore, assist in preventing the ion beam from spreading. 
This much represents a fundamental approach in the field of ion sources. 
Cathode 18 is energized from an alternating current supply 28 the 
potential center of which is returned to the negative terminal of a 
discharge supply 30. The positive of discharge supply 30 is connected to 
anode 20. A beam supply 32 applies a positive potential to anode 20. The 
negative terminal of beam supply 32 is paralleled with the positive 
terminal of accelerator supply 34, with the negative potential from the 
latter being applied to accelerator grid 24 in order to draw the positive 
ions through screen grid 22. The positive terminal of accelerator supply 
34 also is returned to system ground as indicated at 36. Neutralizer 26 is 
energized from supply 38. 
Meters are normally provided for the voltages and currents of the supplies 
shown (I.sub.c, V.sub.c, etc.). Meter 40 is normally required in addition 
to the power supply meters in order to monitor the electron emission from 
neutralizer 26. Moreover, sophisticated implementation of the overall 
system will justify computer-type control with processing, including 
algorithms, to make interacting adjustments of the different supply 
components as operation variables change through a long period of time. 
In FIG. 2, the viewer is looking at an ion source 10 from a downstream 
location. Presented is a mounting flange 50 around the forward part of 
cylindrical outer shell 12 and which flange includes apertures 52 by means 
of which chamber 14 is fastened into the bulk of a vacuum system in which 
is contained the substrate or other article to be bombarded by the ions. 
When assembled with that bulk of the vacuum system, outer shell 12 also 
forms part of the vacuum chamber wall in this particular embodiment. 
Also immediately present to view in FIG. 2 is accelerator grid 24 behind 
which is screen grid 22. A conductive lead 54 serves to connect grid 24 
back to supply 34. Spaced in front of accelerator grid 24, spanning the 
distance between support and connecting posts 56 and 58, is neutralizer 
filament 26. Similarly spanning the distance between support and 
connecting posts 56a and 58a is a second neutralizer element 26a. In 
normal operation, only one of the neutralizer filaments is in use, the 
other being a spare. 
Certain details of the mounting of one of the neutralizer support posts are 
shown in FIG. 3. Also to be seen in FIG. 3 are grids 22 and 24 secured 
around their edges between a clamp 60 and a support element 62. Overlying 
support element 62 is a sputter cover 64. 
FIG. 4 depicts a view from the rear side of the unit, showing a cathode 
filament 18 in a partially removed condition. Filament 18 is mounted to a 
base 27 securable through an opening 29 formed through an end plate 31 
which is secured to flange 49 to form a portion of outer shell 12 for the 
assembled ion source. In principle, only one cathode filament is needed to 
serve as cathode 18 of FIG. 1 for operation. However, multiple cathode 
filaments operated in parallel serve to improve uniformity of the ion beam 
extracted from a large ion source. Multiple filaments also provide a 
redundancy to extend the lifetime in operation. In the prototype 
illustrated there are actually three additional filaments attached to 
cathode bases 27a, 27b and 27c. 
Also shown in FIG. 4 are neutralizer connection posts 66, 66a, 67 and 67a 
which inlet to respective opposite ends of redundant neutralizer filaments 
56 and 56a. Those connections could, of course, be made by way of many 
different routes, those shown simply being convenient. 
In FIG. 5, cylindrical outer shell 12 has been partially withdrawn away 
from end plate 31 so as to reveal an interior cage 70 which is composed of 
a series of longitudinally-spaced rings 72 between each of which is a 
circumferentially-spaced array of magnets 74. Cage 70 is insulatingly 
supported from plate 31. In succession from back to front, magnets 74 are 
reversed as between each successive pair of rings 72. As now well 
understood from the prior art mentioned in the introduction, the magnets 
create a magnetic field within the interior of chamber 14 that enhances 
ionization for the development of a plasma, In a known alternative, an 
energized electromagnet surrounds chamber 14. 
FIG. 6 reveals the primary portion of anode 20 as partially removed from 
its operative location just inside magnet cage 70. As further illustrated 
in FIG. 7, however, the total extent of the anode includes not only its 
cylindrical portion 20 but also rear end wall portions 20a, 20b and 20c 
which are all electrically connected to portion 20. Cathode openings as at 
80 are formed through portion 20b in alignment with the cathode filament 
openings as at 29. In order to represent both openings 80 and 29 in FIG. 
7, opening 80 is shown as being in a relatively much thicker anode wall 
segment than actually is the case as compared with plate 31. 
However actually fabricated, the whole purpose is to have a surrounding 
structure that cooperates with cathode 18 in order to produce an initial 
electron current which excites the formation of a plasma as well as to 
have all of that located inside a magnetic field which enhances the very 
same operation all to the end result of creating as intense a plasma as 
possible by way of utilization of the ionizable gas being introduced 
within chamber 14. With the exception of a portion of that shown in FIG. 
3, nothing that has been described this far is truly new in principle nor 
restricted as to manner of implementation. A particular prototype has been 
illustrated, for the reason that it has been found to work. When dealing 
with fields, forces and movements of small particles that cannot be seen, 
it is important to relate those things that cannot be seen to hardware 
elements that are visible. 
Turning now to FIG. 8, grids 22 and 24 are each formed of a conductive 
material such as molybdenum. Importantly, they have integrally formed 
peripheral marginal portions or rims 90 and 92, respectively, which have 
approximately the same thicknesses as those of the central portions inside 
the marginal portions or rims. Inside marginal portions 90 and 92 are in 
each case an array of apertures as at 94 in FIG. 8. Apertures 94 are 
distributed in a predetermined pattern. In this particular case, for an 
ion source beam diameter of thirty-eight centimeters, some 20,000 
apertures are contemplated within each of the two grids 22 and 24. 
While the drawings indicate what amounts to a circular structure, and hence 
a circular arrangement of the pattern of apertures 94 in the grids, this 
is not a necessary limitation. For providing a pattern of ion impingement, 
say, of an elongated rectangular formation, it may be necessary to 
rearrange the distribution of apertures in accordance with new dimensional 
requirements. With such a change, the term "radially" as used hereinafter 
would mean from the center of the screens in a direction across the 
respective rims. Should that happen, it will involve an adaptation, such 
as the change in cathode ray tubes from the original round to the 
rectangular format, a field wherein a large number of apertures had to be 
accurately aligned with an array of clusters of phosphor spots or triads. 
Looking at FIG. 8, it will be observed that the integral outer rim of 
accelerator grid 24 has been deformed at 92 to be spaced more away from 
the outer rim 90 of grid 22. At the same time, the integrally formed outer 
rim of grid 22, again of molybdenum, has, in the sense of reference 
between the two grids, been spaced outwardly in the other direction as 
shown at 90 in order to define a space between the two grids. 
In rim 92 is a succession of holes 96 and 98 through which, as described 
later, fasteners are located. Between each pair of holes 96 and 98 is an 
opening 100. Similarly in rim 90 is a succession of holes 102 and 104 
individual pairs of which span another succession of corresponding 
openings 101. A ball-shaped insulator 106 is seated in a between each of 
those openings. 
When assembled, insulators 106 are sandwiched between rim 92 and rim 90 and 
seated between openings 100 in rim 92 and the corresponding openings 101 
in rim 90. Openings 100 and 101 are so sized that each ball protrudes 
through the rim partway into, and in contact with, corresponding slots 108 
and 109 in support 62 and clamp 60. Openings 100 and 101 are slightly 
elongated in the radial direction as to permit relative radial motion 
between rims 90 and 92 without insulators 106 becoming unseated. Thus, the 
balls ensure alignment of the two grids as deformation, flexing and 
whatever else may occur with heating and cooling. At the same time, 
buckling and other injury to the grids is prevented because rims 90 and 92 
are allowed to slide radially between clamp 60 and support elements 62. 
While the illustrated ball insulator 106 might be of any of several 
different materials, in the present embodiment it is formed of alumina, to 
have mechanical strength, as well as to work at the high operating 
temperatures therein. 
Preferably, slot 108 also is radially elongated as formed in the 
undersurface of support element 62 which faces rim 92 and is in alignment 
with that portion of ball 106 which protrudes through opening 100. Exactly 
the same, slot 109 is radially elongated as formed in the inner surface of 
clamp 60 upon which rim 90 slides. In turn, it receives the portion of 
ball 106 which protrudes through the opening 101 beneath that ball. 
In the vacuum environment in which these parts operate, heat transfer is 
almost entirely by radiation, and separate parts normally develop 
substantial temperature differences, even between those parts that are in 
nominal contact with each other. Without freedom to move, thermal 
expansion can easily develop forces that exceed the yield strength of the 
materials used. This is the reason why rims 90 and 92 are formed integral 
with grids 22 and 24. Further, differences in thickness will result in 
different rates of heating and cooling in different portions of the same 
part. The rims are therefore of approximately the same thickness as the 
inner portions of the grids where apertures 94 and 94a are located. In a 
large ion source with closely spaced grids, such as that illustrated, the 
grids must be thin enough so that rims 90 and 92 will require a separate 
supporting structure (support 62 and clamp 60) to provide the necessary 
stiffness. It should be further noted that the presence of support 62 and 
clamp 60 will reduce the radiation loss from rims 90 and 92, thereby 
reducing the radial temperature difference in grids 22 and 24 and the 
resulting thermal distortion of the grids. 
Thus, the two rims 90 and 92 are held so that the centers of the grids are 
maintained in alignment, spacing between the two grids is maintained, and 
the relative circumferential orientation between the two grids is 
maintained. At the same time, relative radial expansion is permitted 
between any of support 62, clamp 60 and rims 90 and 92 due to temperature 
differences that may exist between any of them. 
In principle, clamp 60 may be secured to support 62 in almost any manner. 
In the present embodiment, however, that is neatly accomplished by use of 
bolts 110 and 112 which extend from clamp 60, through respective openings 
102 and 104 in the offset portion of rim 90, and through openings 96 and 
98 in the offset portion of rim 92 and on through respective openings 114 
and 116 correspondingly spaced in succession around support element 62. 
Bolts 110 and 112 pass through respective insulative bushings 118 and 120, 
the lower portions of which are seated in openings 114 and 116 and which 
are then covered by respective sputter cups 122 and 124 with the bolts 
finally being secured between plate 62 and clamp 60 by respective nuts 126 
and 128. 
FIG. 9 details the mounting of sputter cover 64. Sputter cover 64 serves 
both to further protect the insulative bushings thereby covered from 
conductive coatings and to prevent discharges to the combination of 
fastener parts that includes nuts 126 and 128, bolts 110 and 112, and 
sputter cups 122 and 124 (all shown in FIG. 8). In the configuration 
shown, all those parts are at the potential of screen grid 22 and would 
draw large electron currents if exposed to the charge exchange plasma 
surrounding the ion beam. 
FIG. 9 looks to be similar to FIG. 8. However, it is taken of a section of 
the perimeter circumferentially-spaced from that shown in FIG. 8. 
In FIG. 9, button-head screws 130 and 132 are threaded into support 62, 
thereby holding four insulative bushings 133 which in turn hold strap 134. 
Strap 134 is thereby held in location by, but electrically insulated from, 
screws 130 and 132. Four sputter cups 135 serve to protect insulative 
bushings 133 from the deposition of conductive films. Upstanding from 
strap 134 is a mounting bolt 136, over which cover 64 is held in place by 
a nut 138. The number of washers 140 can be adjusted to allow careful 
positioning of cover 64 relative to grids 22 and 24. 
The structure shown in FIG. 8 is repeated several times around rims 90 and 
92, and assures the relative placement of support 62, clamp 60 and grids 
22 and 24. The structure shown in FIG. 9 is also repeated several times 
around the rims, at different locations from the structure shown in FIG. 
8. It provides for the mechanical attachment of sputter cover 64 to 
support 62, while at the same time providing electrical insulation between 
the sputter cover and the support. In this manner sputter cover 64 can be 
supported by, and electrically connected to, outer shell 12 (see FIG. 2) 
which is at facility ground, without affecting the electrical potentials 
of grids 22 and 24. 
Numerous variations from that specifically shown are possible at least in 
some embodiments. For example, slots 108 and 109 as indicated in FIG. 8 
may be formed entirely through respective clamp 60 and support 62. That 
approach is implied in FIG. 10. Openings 100 and 101 may not be actual 
holes; they may be depressed areas which, in turn, are seated in 
respective slots 108 and 109 with the balls themselves seated only in the 
depressions. 
In the embodiments illustrated, however, the relative dimensions are such 
that, during the onset of clamping, the ball first engages the edges of 
openings 100 and 101. That forces circumferential alignment of the grids, 
while still allowing relative radial movement therebetween. Increasing 
clamping force then causes the balls to engage the edges of slots 108 and 
109. That engagement causes all of clamp 60, support 62, grid 22 and grid 
24 to be pulled into mutual overall alignment. 
That approach allows for compensation of minor manufacturing tolerance 
variations. While alignment of the grid apertures needs to be as perfect 
as possible, and here with less than 0.002 inch difference, the tolerances 
for alignment of clamp 60 and support 62 are not as tight. 
Of course, differences in overall size and in specific details of approach 
will necessitate changes in dimensions, that discussed being by way of a 
specific example of a thirty-eight centimeter source as illustrative. 
There are eight sets of the FIG. 8 assemblies and also an interspersed 
eight sets of insulative mounting assemblies for sputter cover 64 as in 
FIG. 9. 
In that specific case, balls 106 have a diameter of 0.28 inch. Openings 100 
and 101 each have an elongated shape with ends formed to have a radius of 
0.108 inch. Slots 108 and 109 each have such end radii of 0.090 inch. This 
allows balls 106, on clamping, to engage the edges of openings 100 and 101 
before engaging the edges of slots 108 and 109 when rims 90 and 92 have a 
thickness of 0.020 inch. 
FIG. 10 depicts a three-grid alternative to the two-grid structure shown in 
FIG. 8. Seated between a support element 144 and a clamp 146 are 
insulating elements 148 that are outwardly disposed as between an inner 
grid 150 and each of the two other grids 140 and 142. Flat insulators 152 
maintain the spacings between the grids and also define the openings in 
which insulators 148 are allowed to roll or slide. Note that flat 
insulators 152 are free to slide on the surfaces opposite insulators 148. 
Flat insulators 152 thus maintain the grid spacings, while insulators 148 
both maintain the grid spacings and maintain the grid alignments. 
While spherical insulators have been shown in FIGS. 8 and 10, it is 
possible to use cylindrical insulators instead, or any other shape which 
provides precision alignment. The straight portions of openings 100, 101, 
108 and 109 in FIG. 8, for example, may be extended sufficiently to permit 
use of a cylindrical insulator, with the axis of the insulator extending 
radially out from the center of the grids. With such cylindrical 
insulators, relative radial motion is accommodated by a sliding motion, 
rather than the rolling motion that may result when spherical insulator 
are used. 
While a particular embodiment of the invention has been shown and 
described, and certain alternatives have been mentioned, 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. 
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.