US3610985A

US3610985A - Ion source having two operative cathodes

Info

Publication number: US3610985A
Application number: US87671A
Authority: US
Inventors: Wayne P Fleming; Stephen A Thompson
Original assignee: Hughes Aircraft Co
Current assignee: Raytheon Co
Priority date: 1970-11-09
Filing date: 1970-11-09
Publication date: 1971-10-05
Anticipated expiration: 1988-10-05

Abstract

The ion source comprises a discharge chamber in which is located a tubular anode. A knife-edged cathode is mounted closely adjacent one edge of the anode to provide arc electrons for ionization of a fuel passing through the chamber. An axial magnetic field controls electron path length to provide optimum ionization. Alternatively, a hot wire cathode may be located interiorly of the discharge chamber to produce electrons by thermionic emission, thereby creating a Penning discharge. The electron source employed is chosen in accordance with the fuel to be ionized, the desired ion current level, and the desired lifetime of the ion source.

Description

United States Patent CATHODES l 1 Claims, 4 Drawing Figs.

U.S. Cl 313/63, 250/4l.9 SB, 313/230, 313/231 Int. Cl ..l-l01j 17/26, l-lOlj 27/00, H05h l/OO Field of Search 313/63,

230, 231, 351; 250/41.9 SB, 41.9 SE

Primary Examiner-Roy Lake Assistant ExaminerPalmer C. Demeo Attorneys-W. H. MacAllister, Jr. and Allen A. Dicke, Jr.

ABSTRACT: The ion source comprises a discharge chamber in which is located a tubular anode. A knife-edged cathode is mounted closely adjacent one edge of the anode to provide arc electrons for ionization of a fuel passing through the chamber. An axial magnetic field controls electron path length to provide optimum ionization. Alternatively, a hot wire cathode may be located interiorly of the discharge chamber to produce electrons by thermionic emission, thereby creating a Penning discharge. The electron source employed is chosen in accordance with the fuel to be ionized, the desired ion current level, and the desired lifetime of the ion source.

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PATENTEnnm SIB?! 3,610,985

sum 2 or 3 PATENTEU um slam SHEET 3 OF 3 ION SOURCE HAVING TWO OPERATIVE CATHODES CROSS REFERENCE This application is a continuation-in-part of patent application Serial No. 832,602, filed June 12, 1969, for Ion Source.

BACKGROUND This invention is directed ton an ion source, particularly to an ion source for producing selected ions from an inlet fuel.

Many types of ion sources have been devised in the past. Although ion extraction techniques vary, a widely accepted method of generating ions is in a discharge chamber wherein electrons are produced and are made to travel on a spiral path by means of an axial magnetic field. The electrons are produced by a cathode-anode arrangement and act upon the fuel to ionize it. The ions drift toward an outlet screen and, when they pass the screen, they are accelerated and employed for whatever purpose they were created.

More specifically, the most conventional electrode arrangement is the employment of facing cathodes with a tubular anode therebetween. The two cathodes with a tubular anode therebetween. The two cathodes generate a potential trough in which electrons oscillate inside the anode tube, and the magnetic field forces the electrons into helical paths to increase the electron path length before the electrons are captured by the anode.

Hot filament cathodes are limited to use with fuels which are chemically compatible with the filament, which is usually made of tungsten or tantalum. Boron is a fuel material with which such a cathode is unsatisfactory. A boron-bearing compound must be introduced into the discharge chamber, because elemental boron does not have a sufficient vapor pressure at practical operating temperatures. If a compound such as boron trichloride (BCI;,) is employed, boron becomes dissociated from the chlorine and much of it deposits on the hot filament. This changes the emission characteristics of the filament, as well as the power required to keep the ever en- 7 iarging filament at emitting temperature. Ultimately, the filament heater supply is unable to furnish enough power. Alternatively, some part of the (tungsten or tantalum) filament where boron does not deposit so readily (due to thermal gradient) becomes overheated and fails.

Another limitation of the hot filament cathode is that the cathode-to-anode voltage cannot be raised arbitrarily high for extended periods of time. Although increased voltage aids ionization, at voltages of above about 40 volts the ions in the plasma are sufficiently energetic to be above the sputtering threshold of the filament. These ions sputter the filament significantly upon impact. Ions arriving at the cathode are unavoidable so that the cathode-to-anode voltage must be maintained near the sputtering threshold, if a lifetime of many hours is to be achieved. Accordingly, a balance of factors must be considered in a proper ion source of long life.

Another electron source, the oxide-coated cathode, is a very satisfactory source for those few applications where cathode poisoning cannot occur. However, since they are easily poisoned in all but the cleanest environments, oxide-coated cathodes are unsatisfactory in an ion source such as contemplated in this invention, because of the diverse fuels which are to be ionized. Thus, the poisoning characteristics of oxidecoated cathodes make them unsatisfactory.

SUMMARY ion source which is capable of ionizing diverse fuels. It is another object to provide an ion source whichcomprises a tubular anode and a cathode arranged in association therewith, together with an axial magnetic field so that, as the gaseous fuel is fed through the anode, electrons cause ionization of the fuel. It is a further object to provide an ion source which has either or both a hot wire and an arc electron source so that the electron source can be chosen in accordance with the character of the particular fuel, and as fuels are changed, the proper electron source can be chosen. Other objects and advantages of this invention will become apparent from a study of the following portion of the specification, the claims and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of the ion source of this invention.

FIG. 2 is an enlarged section taken generally along the line 2-2 of FIG. 1.

FIG. 3 is a side-elevational view of the ionization chamber structure within its housing and electrode structure, shown in longitudinal section.

FIG. 4 is a schematic section through the ionization chamber, showing the magnetic field ions therethrough.

DESCRIPTION Referring to the drawings, the source 10 of this invention has an ionizer 11. It is mounted on flange 12 by means of insulator posts 14. Insulator posts 14 are identical to insulator posts 16, hereinafter described. In view of the fact that the ion source 10 is employed in substantial vacuum, flange I2 is preferably of such nature as to act as a flange on a vacuum chamber, to thus permit the mounting of ion source 10 in the wall 15 of the vacuum chamber. In this context, flange I2 preferably has incorporated therein electric throughputs of vacuum-type nature to permit the operation of ion source 10. Such throughputs include electrical throughputs, as well as a tube for the introduction of gaseous fuel to the discharge chamber of the ion source.

Mounted on top of insulator posts 14 is main support ring 18. Main support ring 18 is in the form of annular, tubular, cylindrical ring, having tube 20 therein. Tube 20 forms the wall of discharge chamber 22. The end of the discharge chamber away from flange I8 is closed by cover 24 having central opening 26 therein, which cover and the opening serve as a screen electrode for the discharge chamber.

Mounted within discharge chamber 22 is anode 28. Anode 28 is in the form of an open-ended, cylindrical tube which is supported from tube 20 at three points around its circumference, preferably midway along its length, by insulating supports 30. Insulating support 30 comprises cylindrical insulator tubes 32 and 34 which engage around a hole in tube 20 and embrace the tube 20 therebetween. The inner insulator tube 34 is located between tube 20 and anode 28, while insulator tube 32 is outside of tube 20. Washer 36 engages over the outer end of insulator tube 32, while clamp screw 38 engages the washer under its head and threadably engages anode 28 to clamp the anode and insulator tubes in place. The opening in chamber tube 20 is sufliciently large that screw 38 clears the metallic structure of the discharge chamber tube 20 to thus provide electrical isolation between the anode and the discharge chamber structure. The three insulating supports 30 provide rigid support for anode 28 so that it is properly and firmly located with respect to main support ring 18.

Insulative posts 16, see FIG. 2, comprise

support tubes

40 and 42 mounted on the ends of insulative cylinder 44. Insulative tubes 32, 34, and insulative cylinder 44 are preferably made out of highly resistive ceramic material, such as A1 0 to thus provide a high degree of electrical separation. Support tube 40 ismo unted upon main support ring I8, and is retained by means of screw 46. The upper end support tube 42 retains decel electrode 48by means of screw 50. Shield 52 extends from support tube 42 downwards around insulative cylinder 44 to provide shielding of the external surface of the insulative cylinder to prevent the deposition thereon of conductive substances which would interfere with the insulative characteristics.

Two sets of three insulative posts 16 are provided. The first set of three supports decel electrode 48, as described, and the other three are somewhat shorter to support accel electrode 54 between the decel electrode and screen 24. Accel and

decel electrodes

54 and 48, respectively, have

central openings

56 and 58 therein in alignment with opening 26.

The decel electrode 48 is not essential to the ionizer 11, but forms part of the ion source by acting as part of the following lens, described herebelow. The screen and

electrodes

54 and 48 form a high perveance, high current, high current density optical system without the provision of an electrode which intercepts the beam. Thus, high beam current is achieved together with long life, because sputtering is avoided. Rear cover plate 60 closes the rear end of discharge chamber 22. Supply port 62, through rear cover 60, permits the supply of a fuel gas to the interior of the discharge chamber for the ionization thereof. Tube 64, see FIG. 1, is connected to supply port 62 and is sealed with respect to flange 12. It passes therethrough so that an external supply of the fuel from externally of the vacuum chamber can be managed.

Referring to FIG. 2, electrical leadthroughs 66 and 68 are mounted on flange 70 which is, in turn, mounted on rear cover 60. These leadthroughs electrically connect to and support filament cathode 72. As previously stated, hot filament cathodes are limited to use with fuels that are chemically compatible with the filament. In this case, the filament is made of tungsten or tantalum, and is suitable for emitting electrons in the presence of many fuels, such as nitrogen, argon, cadmium, xenon, sulfur, carbon, etc.

Magnet 76, see FIG. 3, surrounds the ionizer 11 to form a suitable generally axially directed magnetic field. As is seen in FIG. 3, the ionizer I1 is not positioned exactly midway up the magnet 76. This provides for a convergent field through the tube 20. The field lines are generally indicated at 78 in FIG. 4 and are convergent through the discharge chamber to provide a maximum field at central opening 26. The hot filament cathode 72, with establishment of a suitable potential difference between the hot filament cathode 72 and anode 28, emits electrons from the hot filament which spiral toward the anode, colliding with the fuel on the way causing ionization thereof. Screen 24 is at cathode potential and, thus, permits the ions to drift therethrough. Accel electrode 54 is held negative to accelerate the ions out of the opening in the screen. The extraction potential is the potential difference between the accel 54 and screen 24. Decel electrode 48 may be less negative than the accel electrode. When it is less negative, it decelerates the ion beam to the remainder of the optical system. Gas pressure and the extraction potential are the primary determinants of the initial optics. The shape of the magnetic field and the shape of the plasma surface from which the ions originate, and the electric fields involved, also affect the initial optics.

Referring to FIG. 3, tube 80 is mounted upon flange l2 and supports magnet 76. Magnet 76 may be either a permanent or an electromagnet, although the latter is preferable on account of the higher flux densities available and the variability to match different gas pressures within the discharge chamber. Focus electrode 82 is mounted on inlet and outlet tubes 84 and 86, so that focus electrode 82 is insulated from tube 80. Focus electrode 82 is formed of a planar front plate 88 having a focus tube 90 extending therepast. These two elements form two walls of annular chamber 92. Chamber 92 is supplied with coolant through tube 84, which is, in turn, exhausted through tube 86. In practice, the coolant is preferably liquid nitrogen. The disc and cylinder of focus electrode 82 affect the optics. Furthermore, liquid nitrogen coolant keeps the pressure through the optics sufficiently low that the optics work well. This is because the lower pressure means a reduction in the collisions between the ions in the beam and the ambient vacuum atmosphere. Such collisions result in charge exchange and scattering of ions form the created beam.

Lens electrode 94 is positioned along the beam path beyond focus electrode 82. Lens electrode 94 is suitably secured in position so that it forms part of the optical system. Appropriate voltages are supplied to the various electrodes to perform the beam focus function. For example,

electrodes

48, 82, and 94 can be connected as an einzel lens, or can have different voltages, depending upon the desired focus effect. Beyond the optical system are suitable beam control devices, as required. Examples of devices downstream along the beam are accelerators, mass separators, and beam deflection devices for beam raster or beam pattern scanning. The present optics provide for a beam which can be conveniently directed.

With this configuration, the ion source 10 is capable of ionizing fuels compatible with the filament, including the fuels listed above. It is to be noted that the anode-to-cathode potential in the hot filament case is near the sputtering threshold and, therefore, the hot filament cathode 72 has a long life. With the fuels named, no deposition occurs on the hot filament cathode so that it is operative over a substantial lifetime.

For those cases of fuels, such as boron trichloride (BCl Boron trifloride (BF etc., which would deposit upon the hot filament cathode, ring cathode 74 is mounted upon rear cover 60. The ring cathode does not interfere with or exclude the hot cathode from the source. It is a sharp-edged, annular ring which is spaced a short distance from the end of circular, tubular anode 28. The spacing is preferably in the order of 0.0 l 0 inch to 0.020 inch. Thus, when the ring cathode 74 is held negative with respect to the anode to the extent of about 500 to 5,000 volts, in the presence of a sufiicient concentration of the fuels enumerated above in this paragraph, a discharge is established between the ring cathode and the anode. An arc is included in the definition of discharge. The free electrons are then influenced by the magnetic field, and their path length to the more positive anode is increased. The resulting collisions between the electrons and the gaseous fuel within the anode chamber create more ions and sustain the discharge. The ions created from this arc or discharge are then extracted in the manner previously described.

Using this mode of operation greatly increases the lifetime of the ion gun, mainly because the ionizer is not hot cathode dependent when operated in this mode, thereby eliminating fuel incompatibility with the hot cathode.

The ring cathode can also be used successfully with other fuels, such as PH Pcl AsCl Argon, Nitrogen, Neon, Xenon, etc. This capability provides a good backup source for the hot cathode operation.

In operation, the ion source 10 is employed to provide ions in various different ion equipment. In one use in particular, the ion source serves as the source for ions employed in ion implantation. In ion implantation, the substrate, often a semiconductor material, requires the implantation therein of an ion, in certain areas, so that certain areas are rich in holes or electrons. In modern implantation of semiconductor materials, more complex semiconductor units can be created by selection of the ion to be implanted. Thus, in a certain area of a semiconductor wafer, a particular ion may be required, while in another area, a different ion will be required. As previously pointed out, flexibility of supplying different ions from a single ion source has provided difficulties in the past, but with the present ion source, a long life is obtained, even though different fuels, which were previously considered incompatible in a single source, are employed. Accordingly, with the present ion source, any desired fuel may be employed to produce the desired ion stream, and the fuel can be changed to produce a different ion stream, without concern for the compatibility of the new fuel with the ion source.

This invention having been described in its preferred embodiment, it is clear that it is susceptible to numerous modifications and embodiments within the ability of those skilled in the art and without the exercise of the inventive faculty.

What is claimed is:

I. An ion source, said ion source comprising:

an ion chamber having a first end and an ion outlet end, ion

optic means at said ion outlet end of said chamber for extracting ions from said ion chamber;

an anode supported within said ion chamber, said anode being a cylindrical tube having first and second open ends;

a first cathode positioned within said anode, said first cathode being in the form of a wire adapted to be heated to emit electrons, a second cathode positioned adjacent said first end of said anode, said second cathode being ring-shaped and positioned in spaced edge-to-edge relationship with said first end of said anode, said second cathode having a sharp edge closely adjacent said first end of said anode so that, upon application of a voltage between said anode and said second cathode, an electric discharge occurs between said sharp ring edge of said second cathode and said first end of said anode and electrons are formed in the discharge.

2. The ion source of claim 1 wherein said source comprises a main support ring, a cylindrical tube mounted within said main support ring, said tube forming the outer wall of said discharge chamber, a cover mounted on one end of said discharge chamber-defining tube, an opening in said cover, said cover and said opening forming a screen electrode for the extraction of ions from said chamber, said anode being supported by said chamber-defining tube.

3. The ion source of claim 2 wherein said anode support comprises first and second insulative tubes respectively positioned interiorly and exteriorly of said chamber-defining tube, a cap over said second insulative tube and securing means extending through said cap and said chamber-defining tube and secured to said anode to insulatively support said anode with respect to said chamber-defining tube.

4. The ion source of claim 2 wherein a rear cover is mounted on said main support ring, said rear cover carrying said second cathode thereon, a fuel supply port located in said rear cover, said fuel support port being connected to discharge fuel into the interior of said chamber through the interior of said second cathode.

5. The ion source of claim 4 wherein said first cathode is also secured to said rear cover.

6. The ion source of claim 5 wherein a first set of insulative posts is mounted upon said first set of support posts.

7. The ion source of claim 6 wherein a second set of insulative support posts is mounted upon said support ring, and a decel electrode is mounted upon said second set of insulative support posts.

8. The ion source of claim 7 wherein said insulative support posts comprise an insulative cylinder and a support tube mounted on each end of said insulative cylinder and secured thereto, said support tubes being respectively secured to said main support ring and to said electrode, a shield mounted upon one of said support tubes and extending part way along said insulative cylinder.

9. The ion source of claim 1 wherein said comprises:

a first electrode mounted at the outlet end of said ion chamber and having opening for passage of an ion beam;

second and third electrodes positioned away from said first electrode, each of said second and third electrodes having an opening therethrough, said second and third electrodes being positioned along the ion beam so that the beam passes through openings therein.

10. The ion source of claim 9 wherein one of said electrodes has a coolant chamber therein, connection means connected to said coolant chamber for the circulation of coolant through said coolant chamber for cooling of the electrode containing said coolant chamber.

11. The ion source of claim 10 wherein said second electrode has a cylindrical tube positioned axially along the beam path for defining the opening therethrough and has a front plate to which said tube is secured, said tube and said front plate forming walls of said coolant chamber, said second electrode being formed of material suitable for use at cryogenic temperatures so that liquid nitrogen can be employed as a coolant in said coolant chamber.

ion optic means 2 g UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION patent 3,610,985 Dated October 5, 1971 Inventor) Wayne 1?. Fleming, Stephen A. Thompson It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, lines 23 and 24, delete "The two cathodes with a tubular anode therebetween." (Page 2, line 1?) Column 6, line 5, after "mounted upon said," insert support ring, and an accel electrode is mounted upon said.

(Claim 6, lines 2 and 3) Signed and sealed this lLpth day of March 1972.

(SEAL) Attest:

EDWARD M.FLETGHER,JR. ROBERT GOTISCHALK Attesting Officer Commissioner of Patents

Claims

1. An ion source, said ion source comprising: an ion chamber having a first end and an ion outlet end, ion optic means at said ion outlet end of said chamber for extracting ions from said ion chamber; an anode supported within said ion chamber, said anode being a cylindrical tube having first and second open ends; a first cathode positioned within said anode, said first cathode being in the form of a wire adapted to be heated to emit electrons, a second cathode positioned adjacent said first end of said anode, said second cathode being ring-shaped and positioned in spaced edge-to-edge relationship with said first end of said anode, said second cathode having a sharp edge closely adjacent said first end of said anode so that, upon application of a voltage between said anode and said second cathode, an electric discharge occurs between said sharp ring edge of said second cathode and said first end of said anode and electrons are formed in the discharge.

9. The ion source of claim 1 wherein said ion optic means comprises: a first electrode mounted at the outlet end of said ion chamber and having opening for passage of an ion beam; second and third electrodes positioned away from said first electrode, each of said second and third electrodes having an opening therethrough, said second and third electrodes being positioned along the ion beam so that the beam passes through openings therein.