US3930163A

US3930163A - Ion beam apparatus with separately replaceable elements

Info

Publication number: US3930163A
Application number: US453750A
Authority: US
Inventors: Robert L Gerlach; George Rossini
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1974-03-22
Filing date: 1974-03-22
Publication date: 1975-12-30
Anticipated expiration: 1992-12-30
Also published as: DE2512626A1; FR2265171A1; GB1494398A; JPS5845140B2; JPS50135500A; FR2265171B3

Abstract

In an ion beam apparatus wherein ions are generated by the collision of electrons from a filamentary emitter with gas atoms in the vicinity of an electron-attracting grid structure, with subsequent extraction of the ions so generated from the vicinity of the grid structure and the focusing of these ions into a beam by an electrostatic lens system, a new way of mounting the ion extractor and the electrostatic lens system is disclosed which facilitates the removal and replacement of the filament and the grid structure. A configuration of the grid structure is also disclosed which optimizes the profile and the current density of the ion beam.

Description

United States atent [1 1 Gerlach et a1.

[ Dec. 30, 1975 ION BEAM APPARATUS WITH SEPARATELY REPLACEABLE ELEMENTS [75] Inventors: Robert L. Gerlach, Los Altos;

George Rossini, Mountain View, both of Calif.

SUPPLY 3,369,148 2/1968 Hitchcock 250/423 3,547,074 12/ l 970 Hirschfeld .1 3,619,684 11/1971 Charlwood et al. 250/427 Primary Examiner-Archie R. Borchelt Assistant Examiner D. C. Nelms Attorney, Agent, or FirmStanley Z. Cole; Leon F. Herbert; John J. Morrissey [57] ABSTRACT In an ion beam apparatus wherein ions are generated by the collision of electrons from a filamentary emitter with gas atoms in the vicinity of an electron-attracting grid structure, with subsequent extraction of the ions so generated from the vicinity of the grid structure and the focusing of these ions into a beam by an electrostatic lens system, a new way of mounting the ion extractor and the electrostatic lens system is disclosed which facilitates the removal and replacement of the filament and the grid structure. A configuration of the grid structure is also disclosed which optimizes the profile and the current density of the ion beam.

18 Claims, 5 Drawing Figures US. Patent Dec. 30, 1975 Sheet10f2 3,930,163

FIG. I

FROM ARGON SUPPLY US. atent Dec. 30, 1975 Sheet20f2 3,930,163

W mm FW m m m 2 141m 12% m 0 nw 0 M23252; NE $5222 E 55x8 5% 22 E m 5513 25m 22 BEAM CROSS SECTION (mm) ION BEAM APPARATUS WITH SEPARATELY REPLACEABLE ELEMENTS BACKGROUND OF THE INVENTION l. Field of the Invention This invention is a further development in the art of generating ion beams.

2. Description of the Prior Art Among the many applications involving the use of ion beams, the sputter cleaning or etching of surfaces is of particular importance. With an Auger surface analysis instrument, for example, the chemical composition of the surface of a sample can be non-destructively analyzed by bombarding the surface with an electron beam. However, it is frequently desirable to subject such a surface to ion bombardment prior to electron bombardment in order to remove impurities and contaminants from the surface. If it is desired to conduct a continuing Auger analysis of successive layers within the sample, it becomes necessary to remove each successive layer by ion bombardment after having been analyzed so that fresh layers can be exposed to the electron beam for analysis.

In ion beam bombardment apparatuses known to the prior art, the ion beam source and the target to be bombarded by the ion beam were mounted within an evacuable envelope, and an atmosphere of ionizable gas such as argon was maintained in the envelope at a pressure in the range from 10 to ltorr. The ion beam source comprised an electron emitter, an electron-attracting grid structure, an ion extractor and an electrostatic lens system. The electron emitter was typically a filamentary cathode. The grid structure was a helically wound cage-like wire structure typically made from 0.005-inch diameter tungsten wire wound to a pitch of turns per inch. The helical configuration of the grid structure for prior art apparatuses was apparently a carry-over from the design of the grid structure for vacuum-ionization gauges such as the gauge described in U.S. Pat. No. 3,435,334, issued on Mar. 1969 and assigned to Varian Associates. Electrons from the filament were attracted to the vicinity of the grid structure, whereupon inelastic collisions of these electrons with gas atoms in the vicinity of the grid structure would produce ions. The ion extractor was typically a cylindrical screen surrounding both the filament and the grid structure and was maintained at suitable electrical potential to extract the ions away from the vicinity of the grid structure and to accelerate the ions toward the region of influence of the electrostatic lens system. The electrostatic lens system served to focus the extracted ions into a beam and to accelerate the ions of the beam toward the target. The principles of electrostatic lens systems are well-known, and are treated in texts such as Electron Optics by B. Paszkowski, Iliffe Books Ltd., London, 1968 and Electron Optics by P. Grivet, Pergamon Press, London, 1965.

In the ion beam apparatuses of the prior art, the electron-emitting filament, the electron-attracting grid structure and the ion extractor were all mounted as a single assembly on a vacuum feed-through member, which provided electrical connections to the outside of the evacuable envelope. Sometimes the various components of the electrostatic lens system were also mounted as parts of the same assembly. To replace a filament or a grid structure, it was necessary to demount the ion extractor from its position surrounding the filament and the grid structure. Sometimes it was also necessary to demount the electrostatic lens system in order to replace the filament or the grid structure. The grid structure of the prior art was essentially a helical wire having rod-like supporting members at tached thereto as by welding. The grid structure supporting members were affixed to the vacuum feedthrough member by barrel connectors or by welding. Thus, to replace a grid structure in the prior art required the unscrewing of a number of barrel connector screws to remove the old grid structure and the subsequentrealignment of the new grid structure while reinserting and tightening the barrel connector screws, or else it required the breaking and subsequent reforming of a number of welds.

SUMMARY OF THE INVENTION The present invention provides an improved mounting arrangement for the filament, grid structure, ion extractor and electrostatic lens system of an ion beam apparatus. In addition, a new configuration for the grid structure of the ion beam apparatus is provided which optimizes the beam profile and the current density of the ion beam.

In particular, the electron-emitting filament and the electron-attracting grid structure are mounted on one assembly, while the ion extractor and the electrostatic lens system components are mounted on a different assembly. The filament-grid assembly is joined to the extractor-lens assembly by having a mating portion of one assembly plug into a corresponding mating portion of the other assembly. However, the two assemblies are readily separable, thereby providing easy access to the filament and grid structure without having to demount the ion extractor and the lens system components from their assembly. The grid structure is demountably attached to the filament-grid assembly by a single screw, rather than by a number of barrel connectors as in the prior art. Thus, with the mounting arrangement of the present invention, replacement of the grid structure is much easier than was the case with prior art apparatuses. With the present invention, the extractor-lens assembly as a whole can be unplugged from the filament-grid assembly, thereby providing access to the filament and the grid without having to demount the ion extractor and the components of the electrostatic lens system. The grid structure itself can be removed from the filament-grid assembly by removing a single screw, and a replacement grid structure can be substituted in proper alignment by reinserting the single screw. The tedious task inherent in the prior art mounting technique of adjusting a number of barrel connectors to achieve proper alignment of the grid structure is thereby obviated.

In addition, the grid structure of this invention is made from a refractory metal into a fine mesh, rather than into a helical structure as in the prior art. The mesh configuration of this invention provides several significant advantages over the helical configuration of the prior art. For example, it has been found that for a given filament-to-grid spacing, the space-charge limited current occurs at a lower voltage with the fine mesh grid structure than with the helical grid structure. This means that electrons are drawn off the filament more efficiently with the fine mesh grid structure. This greater efficiency is attributable to the greater electric field strength that can be obtained between the filament and the fine mesh grid of this invention than was possiblebetween the filament and the helical grid of the prior art. It has also been found that for a given filament-to-grid electron emission current, the resulting ion beam current density is significantly higher with the fine mesh grid of this invention than with the helical grid of the prior art. This increased ion beam current density is attributable to the fact that a more uniform electric field distribution can be maintained in the region of the fine mesh grid configuration than inside the helical grid configuration of the prior art, so that a more uniform electron density can be achieved in the vicinity of the fine mesh grid configuration of this invention. In addition, it has been found that the. beam profile of the ion beam generated by an apparatus hav ing a fine mesh grid configuration is superior to the profile of an ion beam generated by a prior art apparatus. Specifically, the ion beam profile generated by an apparatus according to this invention is narrower and more uniform than the ion beam profile obtainable in the prior art. This narrower and more uniform ion beam profile is likewise attributable to the fact that the electric field distribution and the electron density within the fine mesh grid structure are more uniform than in the helical type of grid structure used in the prior art.

BRIEFDESCRIPTION OF THE DRAWING FIG. 1 shows a longitudinal cross-sectional view of an ion beam apparatus which embodies the present inventron.

FIG. 2,shows an alternative configuration for the grid structure of the present invention.

FIG. 3 shows a fragmentary longitudinal cross-sectional view of an alternative embodiment of the ion extractor of. an ion beam apparatus according to the present invention.

FIG. 4 shows curves indicative of the ion beam profile of prior art apparatuses.

FIG. 5 shows curves indicative of the ion beam profile of an apparatus according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 shows an ion beam apparatus wherein an ion beam source and a means for holding a sample 21 for bombardment by the ion beam 11 are mounted within a chamber which is surrounded by a vacuumtight enclosure 22. The sample holding means 20 may be an electrically grounded sample holder of any suitable mechanical design such as, for example, the carousel-type of sample holder disclosed in copending US. Pat. application Ser. No. 453,749, filed on Mar. 22, 1974 and assigned to Varian Associates. The chamber 25 is evacuable through port 23 in wall 22 by a suitable vacuum pumping means, and an inert ionizable gas such as argon can be introduced into the evacuated chamber 25 through port 24 in wall 22.

The ion beam source 10 comprises a filament-grid assembly designated by the reference number /40 and an extractor-lens assembly designated by the reference number 50/60. The principal components of the filament-grid assembly 30/40 are an electron-emitting filament 30 and a fine mesh grid structure 40. The principal components of the extractor-lens assembly 50/60 are an ion extractor 50 and an electrostatic lens system 60.

The filament 30 is preferably made of 0.007-inch diameter non-sag tungsten wire of the type sold by General Electric Company under the catalog designation 2l8-tungsten. The filament 30 is formed into for example a hair-pin configuration, and the prongs thereof are demountably attached by

barrel connectors

31 and 32 to rigid

electrical leads

33 and 34, respectively, which extend through an insulating ceramic plate 35 to electrical contact pins (not shown) outside the vacuum enclosure 22. The electrical contact pins provide connection with an electrical circuit for resistively heating the filament 30 to a temperature at which electrons will be emitted therefrom, and for maintaining the filament at a desired electrical potential as of 2.8 kilovolts.

The grid structure 40 comprises a generally cylindrically shaped fine mesh 41, which is made from a refractory metal such as tungsten, iridium, tantalum or molybdenum. The mesh 41 may be a net-like screen of interwoven wires as shown in FIG. 1, or alternatively may be formed from a perforated sheet of refractory metal as shown in FIG. 2. The perforations in the metal sheet may be circular as illustrated in FIG. 2, or of any other shape such as square or rectangular. The mesh 41 is characterized as a fine mesh, the measured fineness being in the range from 20-mesh to IOO-mesh. Thus, for either the net-like screen embodiment or the perforated sheet embodiment, the mesh 41 is equivalent in fineness to a screen as of for example 0.00l-inch diameter wires evenly spaced apart at a closeness in the range from 20 to 100 per inch. Preferably, a IOO-mesh fineness is used. The mesh 41 is inherently non-rigid. Therefore, in the case of the net-like screen embodiment, the ends of the non-rigid mesh cylinder are affixed as by welding to metal rings 42 and 43, one at either end of the cylinder. Two or more rigid support members 44 extending the length of the cylinder are joined to the ring 42 and to the ring 43 so as to provide a rigid frame for maintaining the non-rigid mesh 41 in a rigid cylindrical configuration. In the case of the net-like screen embodiment shown in FIG. 1, one end of each of several metal stilt-like supports 45 is affixed as by welding to ring 43. In the case of the perforated sheet embodiment shown in FIG. 2, a rigid cylindrical configuration can be maintained for the mesh 41 by extending each of the metal stilt-like supports 45 the length of the cylinder. In either case, the other end of each of the supporting members 45 is affixed as by welding to a cup-like metal base member 46. The base member 46 is demountably attached by means of a single screw 47 to a metal plate 48, which is affixed to ceramic plate 35 by

screws

36 and 37. Electrical lead 38 forms an electrical contact with bolt 37 and extends to an electrical contact pin (not shown) extending outside the vacuum enclosure 22. The electrical contact pin provides connection with an electrical circuit for maintaining the grid structure 40 at a desired electrical potential as of 3.0 kilovolts.

The ceramic plate 35 is bolted by a number of bolts, which are represented in FIG. 1 by bolt 70, onto one side of a flanged annular metal fitting 39. The ceramic plate 35 covers the opening on one side of the fitting 39. The electrical leads 33 and 34 which pass through the ceramic plate 35, and the electrical lead 38 which is connected to the electrically conducting bolt 37 which passes through the ceramic plate 35, all extend into the opening through the fitting 39. Other electrical leads 84, and 86, whose function will be discussed hereinafter, also extend into the opening through the fitting 39. A ceramic plug 71 fits snugly into the opening at the other side of the fitting 39. The electrical leads 33, 34, 38, 84, 85 and 86 all pass via the opening through fitting 39, and thence through the ceramic plug 71, to electrical contact pins (not shown) which extend outside the vacuum envelope 22. The flanged annular fitting 39 is seated in an outer flange 72. Vaccum-tight seals between the ceramic plug 71 and the fitting 39, and between the fitting 39 and the outer flange 72, are effected by means (not shown) which are well-known to the vacuum art. Outer flange 72 is bolted by

bolts

74 and 75 onto a mating flange 73 which is formed integrally with the enclosure wall 22 around an opening 76 which is large enough to permit the ion beam source to project into the chamber 25. Vacuum tightness of the mating of the

flanges

72 and 73 is provided by the metal sealing gasket 77, as described more fully in US. Pat. No. 3,208,758, issued on Sept. 28, 1965 and assigned to Varian Associates.

The cylindrical axis of the grid structure 40 is aligned substantially parallel to the prongs of the hair-pin shaped filament 30. It is a feature of this invention that the grid structure 40 can easily be demounted, and a new grid structure can be substituted in its place in proper alignment with the prongs of the filament 30, simply by removing and subsequently reinserting the the single screw 47. In the grid structure mounting technique ofthe prior art, on the other hand, a number of supporting members affixed to a helical type of grid structure were joined by barrel connectors to a corresponding number of support members affixed to an insulating base. It was a tedious process with the mounting technique of the prior art to adjust the barrel connectors so that the grid structure could be properly aligned. With the mounting technique of the present invention, proper alignment of the grid structure can be attained by tightening the single screw 47.

Another difficulty experienced with prior art mounting techniques was that in prior art apparatuses the ion extractor and sometimes also the components of the electrostatic lens system were mounted as parts of the same assembly to which the grid structure and the filament were mounted. Thus, in the prior art it was necessary to demount the ion extractor and perhaps also the components of the electrostatic lens system in order to have access to the grid structure or the filament. In the present invention, on the other hand, the ion extractor 50 and the electrostatic lens system 60 are mounted as parts of an assembly 50/60 which is easily separable from the assembly 30/40 which comprises the filament 30 and the grid structure 40.

The ion extractor 50 comprises a generally cylindrical net-like screen 51 which surrounds the mesh 41 of the grid structure 40 and the greater portion of the filament 30. The cylindrical axis of the extractor 50 is aligned substantially parallel to the cylindrical axis of the grid structure 40 and to the prongs of the filament 30. In the preferred embodiment, the netlike screen 51 is made of 0.003-inch diameter metal wire as of stainless steel of I00 mesh. A screen made to these specifications will remain rigid without crumpling at the temperatures usually encountered. The screen 51 is affixed as by welding to a flanged metal annular member 52 which covers the end of the extractor 50 adjacent the electrostatic lens system 60, except for an annular opening 55 therein which is concentric with the grid structure 40 but of a diameter which is smaller by approximately percent. Ions extracted from the vicinity of the grid structure are accelerated through the opening 55 toward the electrostatic lens system 60. The

6 annular member 52 provides the means whereby the extractor 50 is mounted within the ion beam source 10. A metallic cylindrical sleeve 53 having an electrically insulating ion extractor support member affixedly disposed therein, such as a ceramic support 54, surrounds but is separable from the filament-grid assembly 30/40. A flanged portion of the metal annular member 52 is received in and is fixedly attached to a mating portion of the ceramic support 54, whereby the ion extractor 50 is mechanically mounted on the sleeve 53 but is electrically insulated therefrom. It is anticipated that the net-like screen 51 described above could be replaced by a cylindrical structure made from a sheet of perforated metal of IOO-mesh. The ion extractor 50 is maintained at an electrical potential as of 2.7 kilovolts to attract positive ions away from the vicinity of the grid structure 40, which is maintained at a higher electrical potential as of 3.0 kilovolts. The ion extractor 50 thus serves to isolate the filament 30 and the grid structure 40 electrostatically from the metal sleeve 53, and to accelerate the ions toward the electrostatic lens system 60. The operation of the ion extractor 50 is well-known and is explained in more detail in texts such 3 as Ion Bombardment of Solids by G. Carter and J. S

Colligon American Elsevier Publishing Company, Ne w York, 1968, particularly at pages 423 et seq. Aninnovative alternative embodiment for the ion extractor 50 is shown in FIG. 3. In particular, the innovation provides for a screen 56 of IOO-mesh, 0.001-inch diameter tungsten wire to be mounted on a sleeve 57 projecting from the annular cover member 52 into the nearest lens element 61 of the electrostatic lens system 60. It has been found that the apertured end screen 56 in combination with the projecting sleeve 57 causes a sharper focusing of the ion beam at lower energies than is possible without the end screen 56 and the projecting sleeve 57. In the embodiment shown in FIG. 3, the projecting sleeve 57 is formed integrally with the cover member 52 and extends into the interior of a cylindrical electrostatic lens element 61. The apertured end screen 56 is affixed as by welding to the end of the sleeve 57 and covers the opening 55 in the cover member 52. Ions generated in the vicinity of the grid structure 40 pass through the screen 56'into the electrostatic lens system 60 wherein they are formed into an ion beam 11. An electrical lead 58, which terminates in an electrical contact plug 59, is affixed to the ion extractor 50 and provides the means for maintainingthe ion extractor 50 at the desired electrical potential. The plug 59 is insertable into an electrically conductive female contact (not shown) in an insulator which is fixedly attached to the metal sleeve 53. An electrical path is provided from plug 59 through the insulator 90 to a male contact (not shown) on the opposite side of the insulator- 90. This male contact of the insulator 90 is insertable into a metallic female receptor (not shown) in the ceramic plate 35, which provides an electrical path through the ceramic plate 35 to an electrical lead 86. The electrical lead 86 passes via the opening in fitting 39 through the ceramic plug 71 to an electrical contact pin (not shown) which extends outside the vacuum envelope 22. The ion extractor 50 is maintained at its proper electrical potential by means of this external electrical contact pin.

The electrostatic lens system 60 comprises, typically, a number of cylindrically configured lens elements, as indicated by

reference numbers

61, 62 and 63, which form the ions into an ion beam and which accelerate the ions of the beam toward the target 21. The

lens elements

61, 62 and 63 are mounted on the sleeve 53in a manner similar to the way in which the ion extractor 50 is mounted on the sleeve 53. As shown in FIG. 1, electrically insulating

support members

64, 65 and 66 are affixedly disposed within the metal sleeve 53 and serve as mechanical supports for the

lens elements

61, 62 and 63, respectively. The

lens elements

61, 62 and 63 are maintained at their required electrical potentials by electrical leads 67, 68 and 69, respectively. In the electrostatic lens system configuration shown in FIG. 1,

lens elements

61 and 63 are maintained at the same electrical potential, so that electrical lead 69 can be connected to electrical lead 67 in the space within the sleeve 53. The electrical leads 67 and 68 then pass through the region within the sleeve 53 externally of the ion extractor 50, and terminate in electrical contact plugs 87 and 88, respectively. The

plugs

87 and 88 are insertable into electrically conductive

female contacts

92 and 93, respectively, in the insulator 90. An electrical path is provided from plug 87 through the insulator 90 to a male contact 94, and an electrical path is provided from plug 88 through the insulator 90 to a male contact 95, both contacts 94 and 95 being on the opposite side of the insulator 90 from the

female contacts

92 and 93. The male contacts 94 and 95 are insertable into metallic female receptors 82 and 83, respectively, in the ceramic plate 35. The ends of the receptors 82 and 83 facing the region between the ceramic plate 35 and the ceramic plug 71 are in electrical contact with electrical leads 84 and 85, respectively. The electrical leads 84 and 85 pass via the opening in fitting 39 through the ceramic plug 71 to electrical contact pins (not shown) which extend outside the vacuum envelope. The lens elements of the electrostatic lens system 60 are maintained at their proper electrical potentials by means of these external electrical contact pins.

The metal sleeve 53, together with the electrostatic lens system supporting members 64. 65 and 66 and the

lens elements

61, 62 and 63 so supported, as well as the electrical leads 67, 68 and 69 with their terminating

plugs

87 and 88, and also together with the ion extractor 50 and its supporting member 54 and the electrical lead 58 and its terminating plug 59, along with the plug-receiving insulator 90, collectively comprise the extractor-lens assembly 50/60. It can be appreciated from FIG. 1 that the entire extractor-lens assembly 50/60 can be mounted in proper position with respect to the filament-grid assembly 30/40 by placing the sleeve 53 over the ceramic plate 35 such that the filament 30 and the grid structure 40 are received into the cylindrical interior of ion extractor 50, and such that the male contacts 94 and 95 for the electrostatic lens system 60 and the male contact (not shown) for the ion extractor 50 are inserted into electrically conductive frictional contact with the corresponding female receptors 82, 82 and the other contact not shown in the ceramic plate 35. The sleeve 53 fits snugly over the ceramic plate 35 and also over a flanged portion of the fitting 39. The sleeve 53 is firmly but demountably attached to the fitting 39 by one or more screws represented by reference number 99 which pass through aligned threaded holes in both the sleeve 53 and the fitting 39. To demount the extractor-lens assembly 50/60, it is only necessary to remove screws 99 and to lift the sleeve 53 away from the ceramic plate 35, thereby easily exposing the filament 30 and the grid structure 40.

The advantages of an ion beam apparatus according to this invention over prior art apparatuses are clearly indicated by the curves shown in FIGS. 4 and 5. The curves in FIG. 4 represent plots of the ion beam current density, measured in microamperes per square centimeter, versus the cross-sectional width of the ion beam, measured in millimeters, for ion beams of two different energy levels, namely 1000 electron volts and 2,000 electron volts, generated by an apparatus of the prior art. The curves in FIG. 5 represent the same parameters for an apparatus according to the present invention. It can be seen by comparing the curves of FIGS. 4 and 5 that for a given ion beam energy, an apparatus according to the present invention provides a beam which is narrower and more symmetrical than could be obtained with prior art apparatuses.

Variations in particular details of the construction of an ion beam apparatus according to this invention may be made without departing from the scope of the invention as described herein. Accordingly, the invention is limited only by the following claims.

What is claimed is:

1. An ion beam source comprising an electron emitter, a grid structure maintainable at an electrical potential to attract electrons from said emitter toward the vicinity of said grid structure for collision with ionizable gas atoms in the vicinity of said grid structure thereby providing ions, said grid structure comprising a refractory metal mesh, means for extracting said ions from the vicinity of said grid structure, and means for forming said ions into an ion beam, said electron emitter and said grid structure being mounted on a first assembly member and forming therewith a first assembly unit, said ion extracting means and said ion beam forming means being mounted on a second assembly member and forming therewith a second assembly unit, said second assembly member being demountably secured relative to said first assembly member, and said electron emitter being demountably secured to said first assembly member, whereby said second assembly unit is insertable and removable separately from said first assembly unit and said electron emitter is insertable and removable as an individual unit.

2. The ion beam source of claim 1 wherein said refractory metal mesh is made from a metal chosen from the group consisting of tungsten, iridium, tantalum and molybdenum.

3. The ion beam source of claim 1 wherein said mesh comprises a net of interwoven wires.

4. The ion beam source of claim 1 wherein said mesh comprises a perforated metal sheet.

5. The ion beam source of claim 1 wherein said first assembly member comprises a ceramic plate and said second assembly member comprises a metallic sleeve, said metallic sleeve being dimensioned to fit snugly over said ceramic plate.

6. The ion beam source of claim 1 wherein said electron emitter is a filamentary electrode.

7. The ion beam source of claim 1 wherein said mesh is formed into a generally cylindrical configuration.

8. The ion beam source of claim 1 wherein said ion extracting means has a generally cylindrical configura tion and surrounds the greater portions of said electron emitter and said grid structure.

9. The ion beam source of claim 8 wherein one end of said cylindrically configured ion extracting means is disposed adjacent said ion beam forming means, and wherein a sleeve member projects from said ion ex- 9 tracting means toward said ion beam forming means, and the end of said sleeve member toward said ion beam forming means is covered with an apertured screen.

10. The ion beam source of claim 1 wherein said ion beam forming means comprises an electrostatic lens system.

11. An ion beam bombardment apparatus for bombarding a sample, said apparatus comprising an envelope for containing an ionizable gas, means for holding said sample within said envelope, an electron emitter mounted within said envelope, a grid structure mounted within said envelope, said grid structure being maintainable at an electrical potential to attract electrons toward the vicinity of said grid structure whereby collisions of said electrons with atoms of said gas produce ions of said gas, said grid structure comprising a refractory metal mesh, means for extracting said ions from the vicinity of said grid structure, and means for forming said ions into an ion beam, whereby said samples may be subjected to ion sputter-etching, said electron-emitter and said grid structure being mounted on a first assembly member, and forming therewith a first assembly unit, said ion extracting means and said ion beam forming means being mounted on a second assembly member and forming therewith a second assembly unit, said second assembly member being demountably secured relative to said first assembly member, and said electron emitter being demountably secured to said first assembly member, whereby said second assembly unit is insertable and removable separately from said first assembly unit and said electron emitter is insertable and removable as an individual unit.

12. The apparatus of claim 11 wherein said ionizable gas is argon.

13. The apparatus of claim 11 wherein said refractory metal mesh comprises a net of interwoven wires.

14. The apparatus of claim 13 wherein said refractory metal mesh comprises a perforated metal sheet.

15. The apparatus of claim 11 wherein said envelope comprises a closure fitting for hermetically sealing an opening in the wall of said envelope, a plate mounted on the inner side of said fitting and forming said first assembly member, said ion extracting means and said ion beam forming means being mounted on the inward portion of a cylinder projecting into said envelope, the outward portion of said cylinder being positioned around said electron emitter and grid structure, and said cylinder forms said second assembly member.

16. The apparatus of claim 15 wherein the outward end portion of said cylinder fits snugly around said plate.

17. The apparatus of claim 16 wherein said fitting includes an inwardly extending flange on which said said flange.

Claims

8. The ion beam source of claim 1 wherein said ion extracting means has a generally cylindrical configuration and surrounds the greater portions of said electron emitter and said grid structure.

9. The ion beam source of claim 8 wherein one end of said cylindrically configured ion extracting means is disposed adjacent said ion beam forming means, and wherein a sleeve member projects from said ion extracting means toward said ion beam forming means, and the end of said sleeve member toward said ion beam forming means is covered with an apertured screen.

12. The apparatus of claim 11 wherein said ionizable gas is argon.

17. The apparatus of claim 16 wherein said fitting includes an inwardly extending flange on which said plate is mounted, and the outward end of said cylinder fits snugly around said flange.

18. The apparatus of claim 17 further comprising screw means securing said plate to said flange and screw means independently securing said cylinder to said flange.