Abstract:
In one embodiment of the present invention, the ion optics for use with an ion source have first and second electrically conductive grids having mutually aligned respective pluralities of apertures through which ions may be accelerated and wherein each has an integral peripheral portion. There is also a support member. There are first and second series of seats around the respective peripheral portions of the first and second grids. A plurality of first spherical insulators are distributed between seats of the first and second series, thereby establishing a predetermined distance between the grids while still enabling radial movement between their peripheral portions. There are third and fourth series of seats around the support member and the peripheral portion of the second grid, respectively, with seats of the fourth series displaced from those of the second series in the same grid. A plurality of second spherical insulators are distributed between seats of the third and fourth series, thereby establishing a predetermined distance between the support member and the second grid while still enabling motion in at least the radial direction between the support member and the peripheral portion of the second grid. A clamping force between the support member and the peripheral portion of the first grid maintains contact between the insulators and their seats.

Description:
FIELD OF INVENTION 
     This invention relates generally to gridded ion sources, and more particularly to the design of ion optics for such ion sources. 
     This invention can find application in a variety of thin film applications such as etching, sputter deposition, or the property modification of deposited films. It can also find application in space propulsion. 
     BACKGROUND ART 
     Gridded ion sources are described in an article by Kaufman, et al., in the AIAA Journal, Vol. 20 (1982), beginning on page 745, which is incorporated herein by reference. The ion sources described therein use a direct-current discharge to generate ions. It is also possible to use a radiofrequency discharge to generate ions, as shown by U.S. Pat. No. 5,274,306—Kaufman et al. 
     Typical ion optics for gridded ion sources are also described in the aforesaid article by Kaufman, et al. An improved ion optics design is described in U.S. Pat. No. 4,873,467—Kaufman, et al., which as incorporated herein by reference. The problems addressed in this patent are basic to ion optics: need to maintain the apertures in different grids in alignment while the grids and supporting members can vary in temperature, reach different equilibrium temperatures, and, at least for the grids, can have significant temperature variations within a part at equilibrium conditions. 
     Some specific grid temperatures are given in a chapter by Kaufman in a chapter beginning on page 265 of  Advances in Electronics and Electron Physics , Vol. 36 (L. Marton, ed.), Academic Press, New York, 1974. The center of the screen grid is typically at 400 to 500° C. during operation, while the center of the accelerator grid is 50 to 100° C. cooler. The edges of the grids operate at 100 to 300° C. cooler than the centers of the grids. Starting operation from ambient temperatures thus involves large temperature differences and gradients. 
     The temperature differences and variations are aggravated by the poor heat transfer in a vacuum environment, i.e., the heat transfer between parts bolted or riveted together is usually close to the heat transfer that would occur due to radiation alone. For industrial applications of ion sources, it is particularly important that routine assembly not depend on careful hand-eye coordination or the use of expensive and complicated instrumentation. 
     While the use of a design described in the aforesaid U.S. Pat. No. 4,873,467 is a considerable improvement over prior art in regard to maintaining alignment with varying temperatures, there are still serious problems. Using supporting members of normal flatness tolerances, large clamping forces are required to assure proper contact of parts. These forces are sufficient to plastically deform grids in the contact regions upon which the alignment depends, thereby degrading the precision of that alignment. 
     In some cases, positive contact of the insulator with adjacent parts is lost at some point in the startup-cooldown thermal cycle, resulting in rotation of that insulator. With a sufficient number of such cycles, a portion of the insulator that is coated with sputtered material can be rotated enough to cause electrical shorting of the ion optics. 
     SUMMARY OF INVENTION 
     In light of the foregoing, it is an overall general object of the invention to provide an improved ion optics design that greatly reduces the forces on insulator seats incorporated into ion optics grids and thereby reduces the associated plastic deformation that degrades the alignment precision of apertures through which the ions are accelerated. 
     Another object of the present invention is to provide a design in which the elastic motion of parts is sufficient to maintain the positive contact of insulators with adjacent parts and thereby prevent the gradual rotation of insulators during repeated thermal cycles and the eventual shorting of the ion optics due to that rotation. 
     A further object of the present invention is to provide a design that is more adaptable to ion optics configurations having more than two grids. 
     In accordance with one specific embodiment of the present invention, the ion optics for use with an ion source have first and second electrically conductive grids having mutually aligned respective pluralities of apertures through which ions may be accelerated and wherein each has an integral peripheral portion. There is also a support member. There are first and second mutually aligned series of seats spaced around the respective peripheral portions of the first and second grids. A plurality of first spherical insulators are distributed between corresponding seats of the first and second series, thereby establishing a predetermined distance between the grids while still enabling radial movement between the peripheral portions of the grids relative to each other. There are third and fourth mutually aligned series of seats spaced around the support member and the peripheral portion of the second grid, respectively, with seats of the fourth series displaced from those of the second series in the same grid. A plurality of second spherical insulators are distributed between corresponding seats of the third and fourth series, thereby establishing a predetermined distance between the support member and the second grid while still enabling motion in at least the radial direction between the support member and the peripheral portion of the second grid. A clamping force between the support member and the peripheral portion of the first grid maintains contact between the first plurality of insulators and the first and second grids and between the second plurality of insulators and the support member and the second grid. 
    
    
     BRIEF DESCRIPTION OF FIGURES 
     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 which: 
     FIG. 1 is a schematic cross-sectional view of a prior-art gridded ion source; 
     FIG. 2 is an enlarged schematic cross-sectional view of a matching pair of ion optics apertures in the prior art ion source of FIG. 1 in which the effect of a longitudinal displacement (the X-direction in FIG. 1) of one grid on ion trajectories is shown; 
     FIG. 3 is an enlarged schematic cross-sectional view of a matching pair of ion optics apertures in the prior art ion source of FIG. 1 in which the effect of a transverse displacement (the Y-direction in FIG. 1) of one grid on ion trajectories is shown; 
     FIG. 4 is a front elevation view of a prior art ion optics constructed in accord with U.S. Pat. No. 4,873,467—Kaufman et al.; 
     FIG. 5 is an enlarged schematic cross-sectional view of the prior art ion optics of FIG. 4 along section A—A, which extends from the peripheral portions of the grids into the apertured regions through which ions are accelerated; 
     FIG. 6 is an enlarged schematic cross-sectional view of the prior art ion optics of FIG. 4 along section B—B in the peripheral portions of those ion optics and the grids therein; 
     FIG. 7 is a further enlarged schematic cross-sectional view of one embodiment of the prior art ion optics of FIG. 6; 
     FIG. 8 is a further enlarged schematic cross-sectional view of another embodiment of the prior art ion optics of FIG. 6; 
     FIG. 9 is a front elevation view of an ion optics constructed in accord with the present invention; 
     FIG. 10 is an enlarged schematic cross-sectional view of the ion optics of FIG. 9 along section A—A, which extends from the peripheral portions of the grids into the apertured regions through which ions are accelerated; 
     FIG. 11 is an enlarged schematic cross-sectional view of one embodiment of the ion optics of FIG. 9 along section B—B in the peripheral portions of those ion optics and the grids therein; 
     FIG. 12 is an enlarged schematic cross-sectional view of another embodiment of the ion optics of FIG. 9 along section B—B in the peripheral portions of those ion optics and the grids therein; 
     FIG. 13 is a front elevation view of a three-grid ion optics constructed in accord with the present invention; 
     FIG. 14 is an enlarged schematic cross-sectional view of the ion optics of FIG. 13 along section A—A, which extends from the peripheral portions of the grids into the apertured regions through which ions are accelerated; 
     FIG. 15 is an enlarged schematic cross-sectional view of the ion optics of FIG. 13 along section B—B in the peripheral portions of those ion optics and the grids therein; 
     FIG. 16 is front elevation view of a rectangular ion optics constructed in accord with the present invention; 
     FIG. 17 is an enlarged schematic cross-sectional view of the ion optics of FIG. 16 along either section A—A or section B—B in the peripheral portions of those ion optics and the grids therein; and 
     FIG. 18 is an enlarged schematic cross-sectional view of the ion optics of FIG. 16 along either section C—C or D—D also in the peripheral portions of those ion optics and the grids therein. 
    
    
     It may be noted that the aforesaid schematic cross-sectional views represent the surfaces in the plane of the section while avoiding the clutter which would result were there also a showing of the background edges and surfaces of the overall assemblies. 
     DESCRIPTION OF PRIOR ART 
     Referring to FIG. 1, there is shown a schematic cross section of a prior art gridded ion source  20 . There is an outer enclosure  22  that encloses a volume  24 . Within this volume is an electron emitting cathode  26  and an annular anode  28 . An ionizable gas  30  is admitted to volume  24  through an opening  32 . Electrons emitted from cathode  26  are contained by magnetic field  34  and reach anode  28  only after having ionizing collisions with gas atoms or molecules. The electrically conductive gas of ions and electrons that fills most of the volume  24  constitutes a plasma. Some of the ions in this plasma reach the ion optics grids  36  and  38 . The ions are formed into beamlets by apertures  40  in the screen grid  36  and are extracted by the negative potential of the accelerator grid  38  and pass through matching apertures therein. The apertures in the screen and accelerator grids are usually circular. The ions continue after passing through the ion optics to form an ion beam  42 . The ion beam is charge- and current-neutralized by electrons emitted from the electron emitting neutralizer  44 . 
     The potential difference between the electron emitting cathode  26  and the anode  28  is typically 30 to 40 volts. The ions are formed at approximately the potential of the anode. The energy of the accelerated ions can be adjusted by varying the anode potential relative to ground. The screen grid  36  is either at cathode potential or allowed to electrically float. An enclosure that is exposed to the plasma, as shown in FIG. 1, will also be at either cathode potential or allowed to electrically float. The accelerator grid  38  is operated at a negative potential at least sufficient to keep the electrons from the neutralizer  44  from flowing backwards through the ion optics. The neutralizer is operated at or near ground potential. 
     Referring to FIG. 2, there is shown an enlarged schematic cross-sectional view of a matching pair of ion optics apertures in the prior art ion source of FIG.  1 . The boundary between the plasma filling volume  24  and the ion optics is the plasma sheath  46 . To the left of the plasma sheath is a quasineutral plasma with approximately equal densities of electrons and ions. The increasingly negative potentials to the right of this sheath reflect electrons and leave essentially only the ions that are accelerated. Ideally, the two apertures are aligned so that the ion beamlet formed by the aperture  40  in the screen grid  36  and indicated by the central and outer ion trajectories  48  passes through the aperture in the accelerator grid  38  without striking that grid. 
     When evaluating the alignment of a pair of apertures such as those shown in FIG. 2, departures from alignment can be resolved into longitudinal and transverse displacements, i.e., displacements parallel and transverse to the general direction of ion motion, shown respectively as the X and Y directions in FIG.  1 . In FIG. 2 the longitudinally displaced accelerator grid location  38 ′ and the displaced ion trajectories  48 ′ are indicated by dashed lines and the size of the longitudinal displacement is shown as ΔX. Depending on the operating condition at the initial location of the accelerator grid  38 , a displacement in the longitudinal direction can either enlarge or decrease the beamlet diameter. In general, small longitudinal displacements have little effect on the beamlet shape. This relative insensitivity to longitudinal grid displacement results in a typical ion optics production tolerance of ±0.1 mm for this type of displacement with circular apertures having a diameter of about 2 mm. 
     Referring to FIG. 3, there is shown another enlarged schematic cross-sectional view of a matching pair of ion optics apertures in the prior art ion source of FIG.  1 . In FIG. 3 the transversely displaced accelerator grid location  38 ″ and the displaced ion trajectories  48 ″ are indicated by dashed lines and the size of the longitudinal displacement is shown as ΔY. For a transversely displaced accelerator grid  38 ″ the ion beamlet  48 ″ is displaced in the direction opposite to the direction of the grid displacement ΔY. The-sensitivity to a transverse displacement is approximately one degree of angular displacement for the beamlet  48 ″ for a value of ΔY equal to 0.025 to 0.05 mm for aperture diameters of about 2 mm. This relative sensitivity to transverse grid displacement results in a typical ion optics production tolerance of ±0.025 to 0.05 mm for this type of displacement with circular apertures having a diameter of about 2 mm. In practice, machining parts to tolerances of ±0.025 mm is readily achieved, but the tolerance in the assembled grid is degraded from this value for reasons that are inherent in the prior art. 
     It should be noted that the apertures in grids  36  and  38  can be given a systematic and intentional transverse offset relative to each other to produce a desirable shaping to the overall ion beam. The “alignment” of apertures in two grids would then refer to the agreement with the desired relationship of the apertures, which may or may not include coincident axes for circular apertures. 
     Referring to FIG. 4, there is shown a prior art ion optics  50  constructed in accord with U.S. Pat. No. 4,873,467—Kaufman et al. In FIG. 5 is shown an enlarged schematic cross-sectional view of the prior art ion optics of FIG. 4 along section A—A therein. The ion optics include a first grid  52  (either the screen or accelerator grid), a second grid  54  (the remaining one of the two grids), a first support member  56 , a second support member  58 , screws  60 , nuts  62 , and ceramic insulators  64 . The screws, nuts, and insulators hold the ion optics together at several locations while, at the same time, keeping the first and second support members  56  and  58  electrically isolated from each other. 
     The portions of the grids  52  and  54  containing apertures for accelerating the ions are often formed into partial spherical shapes, which provide improved structural stability for those portions. The attachment of the ion optics to the rest of the ion source is not shown in FIGS. 4 and 5 but could be accomplished with screws and insulators to either of the first or second support members. An example of such attachment is shown in the aforementioned U.S. Pat. No. 4,873,467. 
     FIG. 6 shows an enlarged schematic cross-sectional view of the prior art ion optics of FIG. 4 along section B—B therein. In addition to the parts described above, there are shown spherical insulators  66 , typically made of high-strength alumina (Al 2 O 3 ), which hold the first and second grids  52  and  54  apart. The details of contact between the spherical insulators and the first and second grids are shown in FIG. 7 which is a further enlarged view of one part of FIG.  6 . The spherical insulators  66  extend through openings in the periphery of the first grid  52  and are seated on the edges  68  of that opening, as well as extending through openings in the periphery of the second grid  54  and being seated on the edges  70  of those openings. The spherical insulators  66  further extend into depressions  72  in the first support member  56  and are seated on the edges  74  of those depressions, as well as extend into depressions  76  in the second support member  58  and are seated on the edges  78  of those depressions. The seats in the first and second grids defined bad the edges  68  and  70  and the seats in the first and second support members as defined by the edges  74  and  78  extend both inwardly and outwardly beyond the contact region shown in FIG. 7 in the radial direction from the center of the ion optics shown in FIG.  4 . The thermal expansion in circular ion optics is approximately radially symmetric for each of the parts. The radial extensions of these seats therefore permit the relative radial motion of grids to accommodate the relative thermal expansion of the perpheral portions of the grids while keeping the centers of those grids in alignment, in accord with U.S. Pat. No. 4,873,467. Also in accord with that patent, the openings in the grids and the depressions in the support members can be sized so that contact of spherical insulators  66  with edges  68  and  70  is assured before contact takes place with edges  74  and  78 . 
     It should be noted that to properly perform their ion acceleration function the ion optics grids must be constructed of thin material—often only 0.2 to 0.5 mm thick. Grids that are sufficiently thin are also flexible and depart substantially from the required dimensional precision. As described in U.S. Pat. No. 4,873,467, a thick peripheral portion cannot be attached directly to a thin grid without a serious thermal expansion mismatch during startup and cooldown transients. In that patent, the required precision is obtained by pressing the peripheral region of each grid against a flat support member. 
     The surfaces of the support members  56  and  58  in which the depressions  72  and  76  are located ideally are flat, but have normal fabrication limits on this flatness. The tolerance typically increases with the size of the ion optics and is of the order of ±0.1 mm. Variations in temperature during ion source operation will tend to cause further departures from the ideal. In addition, to assure continuity of the flat surfaces of support members  56  and  58  between the screw, nut, and insulator assemblies shown in FIGS. 5 and 6, the support members  56  and  58  must be stiff structural members. 
     As the result of these tolerances, temperature variations, and stiffnesses, the experimental force to hold all these parts in contact is typically about 1000 newtons at each screw. This magnitude of force is sufficient to plastically deform the grid material in the region of contact with the alumina spherical insulators  66 . The edges  68  and  70  will be deformed until there is sufficient contact area with the spherical insulator to withstand a force of 1000 newtons at each screw, nut, and insulator assembly. In U.S. Pat. No. 4,873,467 there were two screws and one spherical insulator in each assembly. The force per spherical insulator, and therefore the amount of deformation, can be reduced by using one screw and two spherical insulators as shown in FIG.  6 . Even with one screw and two spherical insulators, the force sustained per insulator is about 500 newtons. Grids are often made of molybdenum, which has a yield strength of about 500 newtons/mm 2 . This means that each spherical insulator, made of high-strength alumina, will be pressed into the grids until the contact area between the insulator and each grid is approximately one square millimeter. The edges  68  and  70  of the openings in the grids can-be machined with a precision of ±0.02 mm or better. The deformation under a force of 500 newtons degrades the precision of the transverse grid alignment to ±0.04 mm or more. From the discussions of FIGS. 2 and 3, the transverse alignment (the Y-direction in FIG. 1) is more critical than the longitudinal alignment (the X-direction in FIG.  1 ), so that it is the transverse alignment that is of primary concern. 
     With the large forces that are involved, it is easy to damage the edges  68  and  70 . For example, these edges can be indented enough to prevent the relative radial motion between grids that is necessary to accommodate thermal expansion. 
     Referring to FIG. 8, there is shown a further enlarged view of an alternate embodiment of one part of FIG.  6 . In this alternate embodiment the edges  80  and  82  of the openings in the grids  52  and  54  are chamfered to better distribute the contact force between a spherical insulator and a grid. This practical improvement reduces but does not eliminate the plastic deformation in the contact region. 
     A related problem encountered with the prior art is the rotation of insulators. At some point in a startup, operation, and shutdown thermal cycle, positive contact can be lost between a spherical insulator and adjacent parts. The spherical insulator can then shift its contact points when contact with adjacent parts is re-established. After a large number of thermal cycles, the accumulated rotation can be of the order of 90 degrees. It is difficult to shield an insulator so that sputtered material from the grids and other hardware is completely excluded and, in practice, some accumulation is accepted as normal. However, when the spherical insulator rotates far enough, the sputter deposits on it can move from a relatively benign location to one that causes electrical shorting between the grids, thereby terminating normal operation. With the substantial relative thermal expansion that takes place and the stiffness required to assure flatness for the support members  56  and  58 , the rotation of insulators has been a recurring problem. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 9, there is shown ion optics  90  constructed in accordance with a specific embodiment of the present invention. In FIG. 10 is shown an enlarged schematic cross-sectional view of the ion optics of FIG. 9 along section A—A therein. This view shows the apertured regions of grids  92  and  94  through which the ions are accelerated as well as the surrounding peripheral regions where the grids are supported and held in alignment. Ion optics  90  includes a first grid  92 , a second grid  94 , a first support member  96 , a second support member  98  screws  100 , and nuts  102 . The screws and nuts hold the ion optics together. Grids  92  and  94  are separated from support members  96  and  98 , both by spaces  104  and  106  and by clearance holes  108  and  110  for screws  100  in grids  92  and  94 . This separation permits grids  92  and  94  to be electrically isolated from support members  96  and  98 , as well as from each other. 
     FIG. 11 shows an enlarged schematic cross-sectional view of the ion optics of FIG. 9 along section B—B therein which passes through grids  92  and  94  in the peripheral portions of those grids. In addition to the parts described above, there are shown spheres  112  which hold apart the first and second support members  96  and  98 . Note that the support members  96  and  98  are electrically connected by screws  100 , so that spheres  112  can be metallic. Spheres  112  extend into depressions in the first support member  96  and are seated on the edges  114  of these depressions. Spheres  112  also extend into depressions in the second support member  98  and are seated on the edges  116  of these depressions. There are clearance holes  118  and  120  in grids  92  and  94  to avoid contact of spheres  112  with said grids. 
     Continuing with FIG. 11, the first grid  92  is spaced from the first support member  96  and positioned relative thereto by spherical insulators  122  which penetrate into depressions  124  in said first support member and are seated on edges  126  in said depressions and also penetrate into openings in the first grid  92  and are seated on edges  128  of said openings. The edges  126  are recessed behind the surface  130  of the first support member  96  to provide protection from sputtered material. The separation  104  in FIG. 10 permits sputtered material to approach the spherical insulators  122 . If the edges  126  were coplanar with surface  130 , sputtered material could make a continuous coating on the insulators  122  from support member  96  to grid  92 . 
     Continuing on with FIG. 11, the second grid  94  is spaced from and located relative to the first grid  92  with spherical insulators  132  which fit into openings in said first and second grids and are seated on edges  134  and  136  of said openings. The second grid is held against insulators  132  with spherical insulators  138  which fit into openings in said second grid and are seated on edges  140  of said openings in addition to extending into depressions in the second support member  98  and being seated against the flat surfaces  142  of said depressions where said surfaces are displaced from and parallel with the surface  144  of said second support member. 
     The openings and the depressions against which spherical insulators are seated extend both inwardly and outwardly beyond the contact region shown in FIG. 11 in the radial direction from the center of the ion optics shown in FIG.  9 . The thermal expansion in circular ion optics is approximately radially symmetric for each of the parts. The extensions of these openings therefore permit relative radial motion to accommodate relative thermal expansion of the peripheral portions of the grids while keeping the centers of these grids in transverse alignment. The peripheral regions of grids  92  and  94  may be formed as shown in FIG. 10 so as to enhance their stiffness and thereby reduce the number of circumferential locations similar to that illustrated in FIG. 11 that are required to adequately support the periphery of a grid. 
     FIG. 11 is typical of the construction near the nut-bolt assemblies shown in cross sections in FIGS. 10 and 11 and in plan view in FIG.  9 . For the complete circumference of ion optics  90  shown in FIGS. 9,  10 , and  11 , the spherical insulators  122  constitute a plurality. Further these insulators are positioned between two series of seats, which are the edges  126  in the first support member  96  and the edges  128  in the peripheral portion of the first grid. In a similar manner a plurality of spherical insulators  132  are positioned between two series of seats, i.e. the edges  134  and  136 , in the peripheral portions of the first and second grids  92  and  94 , respectively. 
     In understanding the construction shown in FIG. 11 it is worth noting that a support function for one grid can be performed by another grid. In the same manner as the first support member  96  provides support for one side of the first grid  92  through spherical insulators  122 , the second grid  94  provides support for the other side of the first grid through spherical insulators  132 . Grids  92  and  94  are thus each supported from both sides. 
     There are several features shown in FIG. 11 that depart from prior art: 
     The transverse alignment of the second support member  98  with the first support member  96  is not critical, inasmuch as the insulators  138  are seated on the flat bottoms  142 . Some shift in transverse alignment of the second support member  98  relative to the first support member  96  due to the plastic deformation of edges  114  and  116  is therefore permissible. 
     The first and second support members  96  and  98  are at the same potential, so that there is no concern about electrical shorts between these two support members due to rotation of spheres  112  during repeated thermal cycles. Spheres  112  could be fabricated of alumina if the high strength of that material were desired, but the insulating capability of alumina is not needed. 
     The first and second support members  96  and  98  are also shown as having large flat surfaces  130  and  144 . While such construction may be convenient, it is not necessary. A variety of shapes could be used as long as the portions of the support members in contact with the spherical insulators  122  and  138  remain unchanged. 
     The grids  92  and  94  are typically held in location by forces between grids and spherical insulators ranging from about ten newtons to a few tens of newtons. Each grid is held in place by spherical insulators on both sides or surfaces of the grids—e.g. grid  92  is held in place on both sides by spherical insulators  122  and  132  and grid  94  is held in place on both sides by spherical insulators  132  and  138 . 
     The prior-art force of about  1000  newtons was required to assure that the support members in FIGS. 4 through 8 were held in a parallel-plane configuration. A force of about 1000 newtons can be used for each screw  100  in FIGS. 9 through 11, but that force is not. applied to the grids  92  and  94  because of the greater flexibility of the grids compared to that of the support members  96  and  98 . Overtightening screws  100  will therefore cause no damage to the edges  126 ,  134 , and  136  upon which the alignment depends. 
     The peripheral portions of the grids  92  and  94 , located between support members  96  and  98 , are more flexible than the support members. This means that spherical insulators that are larger than necessary for making contact with the grids can be used while still developing forces of a few tens of newtons. The oversize insulators will cause a slight longitudinal (X-direction in FIG. 1) waviness in the grid location around the grid periphery, but the longitudinal grid location is less critical than the transverse location and the variation around the rim is, to a large extent, averaged out over the portion of a grid containing the apertures for accelerating ions. The oversize insulators and the resultant waviness result in a spring retention of the spherical insulators that will prevent the loss of contact that causes rotation of spherical insulators. The degree of springiness in this retention can be predetermined by the displacement between spherical insulators  122  and  132  and the displacement between spherical insulators  132  and  138 . These displacements in FIG. 11 are in the circumferential direction, or angular direction about the center, in FIG. 9, but the displacements could also be in the radial direction. The sizes of these displacements are not critical. The thermal expansion in the length of screws  100  is of the order of 0.1 mm. A wide range of insulator displacements in grids that are only 0.2 to 0.5 mm thick will provide sufficient flexibility to accommodate this amount of thermal expansion. 
     The most fundamental difference from prior art, however, is that a grid is not supported directly by a support member, but indirectly by that member through insulators at several locations around the ion optics periphery. In addition to the advantages cited above, this permits multiple grids to be held in precise transverse alignment by one support member, e.g., support member  96  in FIG.  11 . 
     ALTERNATE EMBODIMENTS 
     A variety of alternate embodiments are evident to one skilled in the art. In FIG. 12 is shown an alternate arrangement of spherical insulators that is, at the same time, consistent with FIGS. 9 and 10. In this alternate interpretation of FIGS. 9 and 10, FIG. 12 shows an enlarged schematic cross-sectional view of ion optics  90  of FIG. 9 along section B—B therein. One difference from FIG. 11 is that second grid  94  is held in place by spherical insulators between it and the first support member  96  rather than the first grid  92 . This is accomplished by spherical insulators  146  which extend into depressions in the first support member  96  and are seated on the edges  148  of these depressions. The insulators  146  also extend into openings in the second grid  94  and are seated on the edges  150  of these openings, as well as pass through openings  152  in the first grid  92  without touching same. 
     Another difference of FIG. 12 from FIG. 11 is that the first grid  92  is held in place by spherical insulators between it and the second support member  98  rather than the second grid  94 . This is accomplished by spherical insulators  154  which are seated on the flat surfaces  144  of the second support member  98 . The insulators  154  also extend into openings in the first grid  92  and are seated on the edges  156  of the openings, as well as pass through openings  158  in the second grid  94  without touching same. 
     In summary, it is shown in the alternate embodiment of FIG. 12 that each grid can be supported directly from the support members without any insulator being seated simultaneously on the two grids. 
     Referring to FIG. 13, there is shown three-grid ion optics  160  constructed in accord with the present invention. It should be noted that while two-grid optics are most common in industrial ion sources, a greater number of grids may be used for particular applications. FIG. 14 is an enlarged schematic cross-sectional view of ion optics  160  of FIG. 13 along section A—A therein. Ion optics  160  includes a first grid  162 , a second grid  164 , a third grid  166 , a first support member  168 , a second support member  170 , screws  172 , nuts  174 , and spacers  176  between the first and second support members. The screws and nuts hold the ion optics together at several locations. 
     FIG. 15 is an enlarged schematic cross-sectional view of one embodiment of ion optics  160  of ° FIG. 13 along section B—B therein. In addition to the parts described above, there is shown spherical insulators  178  which penetrate into depressions  180  in first support member  168  and are seated on edges  182  in said depressions and also penetrate into openings in the first grid  162  and are seated on edges  184  of said openings. The edges  182  are recessed behind surface  186  of said first support member to provide shielding of spherical insulators  178  from sputtered particles in the manner described in connection with spherical insulators  122  in FIG.  11 . The first grid  162  is supported from the opposite side by spherical insulators  188  which fit into openings in said grid and are seated on edges  190  of said openings and also are seated against surfaces  192  of second support member  170 , as well as pass through openings  194  and  196  in the second and third grids  164  and  166  without touching same. 
     Continuing with FIG. 15, the second grid  164  is spaced from and located relative to the first support member  168  with spherical insulators  198  which fit into depressions in said support member and are seated on edges  200  of said depressions and also penetrate into openings in the second grid  164  and are seated on edges  202  of said openings, as well as pass through openings  204  in the first grid  162  without touching same. The second grid is held from the other side by spherical insulators  206  which fit into openings in said second grid and are seated on edges  208  of said openings and also are seated against surfaces  192  of second support member  170 , as well as pass through openings  210  in the third grid. 
     Continuing on with FIG. 15, the third grid  166  is spaced from and located relative to the first support member  168  with spherical insulators  212  which fit into depressions in said support member and are seated on edges  214  of said depressions and also penetrate into openings in the third grid  166  and are seated on edges  216  of said openings, as well as pass through openings  218  and  220  in the first and second grids  162  and  164  without touching same. The third grid is held from the other side by spherical insulators  222  which fit into openings in said third grid and are seated on edges  224  of said openings and also penetrate into depressions in second support member  170  and are seated on surfaces  226  of said depressions, where said surfaces are parallel to surface  192  of the second support member. 
     The openings and the depressions against which spherical insulators seat extend both inwardly and outwardly beyond the contact region shown in FIG. 15 in the radial direction from the center of ion optics  160  shown in FIG.  13 . These extensions permit relative radial motion to accommodate relative thermal expansion of the peripheral portions of the grids while keeping the centers of those grids in transverse alignment. 
     It is shown in FIGS. 13 through 15 that three grids can be supported with the same advantages shown for the preferred embodiment using two grids. Further, those skilled in the art should recognize that subject invention can be adapted to a larger number of grids, if desired. 
     In another departure from the configurations described, the different grids could be supported at different radii, instead of all insulators and all support being at essentially one radius from the ion optics center. 
     Noncircular ion optics could also employ this invention, preferably with locations close to the planes of symmetry for the insulators used for transverse alignment of the grids. In FIG. 16 is a rectangular ion optics constructed in accord with the present invention. FIG. 17 is an enlarged schematic cross-sectional view of ion optics  240  of FIG. 16 along either section A—A or section B—B therein. Ion optics  240  includes a first grid  242 , a second grid  244 , a first support member  246 , a plurality of second support members  248 , screws  250 , nuts  252 , and spacers  254 . The screws and nuts hold ion optics  240  together at several locations. There are openings  256  and  258  in grids  242  and  244  that are sized so that spacers  254  can pass through said grids without touching same. 
     Note that the plurality of support members constitutes a support means, rather than a support member. In addition, the construction shown in FIGS. 11,  12 ,  14 , and  15  has implied a fixed spacing between first and second support members, where that spacing has been selected to give adequate spring retention to the insulators in their seats while at the same time not causing excessive force that might damage the grids or the seats therein. In FIG. 17 the second support members  248  are indicated as being thin and therefore able to flex. In the construction shown in FIGS. 17, then, it would be appropriate to describe the support members  248  as providing a force sufficient to retain insulators in their seats. Providing a fixed spacing that results in an adequate force is considered functionally equivalent to providing a fixed force that results in an acceptable spacing. 
     Continuing with FIG. 17, the first grid  242  is spaced from the second grid  244  and positioned relative thereto by spherical insulators  260  which penetrate openings in the first grid and are seated on edges  262  therein and also penetrate into openings in the second grid  244  and are seated on edges  264  of said openings. The insulators  260  also penetrate into depressions in support member  246 , with said depressions having edges  266 . The depressions in the support member are sized so that the edges  262  in the openings in grid  242  are contacted by insulators  260  before the edges  266  of the depressions in support member  246  are contacted. This sequence of contact assures that contact of insulators  260  with the support member  246  will not degrade the transverse alignment of grids  242  and  244 . The second grid is held against insulators  260  with spherical insulators  268  which fit into openings in said second grid and are seated on edges  270  of said openings and also extend into depressions in the second support members  248  and press against surfaces  272  of said depressions where said surfaces are displaced from and approximately parallel with the first support member  246 . 
     The openings in grids  242  and  244  and the depressions in support member  246  against which spherical insulators  260  and  268  are seated extend both inwardly and outwardly beyond the contact region shown in FIG. 17 in the radial direction from the center of ion optics  240  shown in FIG.  16 . These extensions permit relative radial motion to accommodate relative thermal expansion of the grids while keeping the centers of these grids in transverse alignment. 
     Referring to FIG. 18, therein is shown an enlarged schematic cross-sectional view of ion optics  240  of FIG. 16 along either section C—C or D—D therein. FIG. 18 differs from FIG. 17 in that the first grid  242  is spaced from the second grid  244  by spherical insulators  274  which seat against surface  276  of first grid  242  and also penetrate into openings in the second grid  244  and seat on edges  278  of said openings. 
     In FIG. 17 both the transverse and longitudinal alignment of grids  242  and  244  is assured by the construction therein. In FIG. 18 only the longitudinal alignment is assured. This difference in construction is necessary to keep the centers of grids  242  and  244  in alignment while preventing the possible interference that could result from the non-axially symmetric thermal expansion of a rectangular shape together with trying to maintain transverse alignment from too many peripheral locations. Instead, transverse alignment is obtained only from locations near the two axes of symmetry. 
     In addition to the departure from a circular beam, the alternate embodiment shown in FIGS. 16 through 18 uses a number of separate parts to perform the function of what is a single second support member in the other embodiments. 
     Those skilled in the art will recognize that while spherical insulators are well suited for use in this invention, cylindrical insulators would work almost as well. In a similar manner, spherical insulators contact seats that are the edges of openings in grids, but indentations in grids could also have been used as the seats for these insulators. 
     Those skilled in the art will also recognize that while circular apertures are described herein for the acceleration of ions, it is possible and sometimes desirable to use noncircular apertures for this purpose. 
     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.