Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
  • H01J37/02—Details
  • H01J37/04—Arrangements of electrodes and associated parts for generating or controlling the discharge, e.g. electron-optical arrangement or ion-optical arrangement
  • H01J37/08—Ion sources; Ion guns
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J27/00—Ion beam tubes
  • H01J27/02—Ion sources; Ion guns
  • H01J27/08—Ion sources; Ion guns using arc discharge
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J2237/00—Discharge tubes exposing object to beam, e.g. for analysis treatment, etching, imaging
  • H01J2237/30—Electron or ion beam tubes for processing objects
  • H01J2237/317—Processing objects on a microscale
  • H01J2237/31701—Ion implantation

Definitions

• This invention relates to an indirectly heated button cathode for an ion source, in particular for use in an ion implanter for implanting ions into target substrates such as semiconductor wafers.
• the cathode is formed as a button having a front thermionic electron emitting surface and a rear surface.
• the button is typically heated by electron impact on the rear surface, by electrons emitted and accelerated from a filament located behind the rear surface of the cathode button.
• the filament is protected from sputtering by energetic particles in the arc plasma formed in the arc chamber of the ion source.
• the heated cathode button emits thermionic electrons at its front surface, and these are accelerated by the arc potential in the arc chamber, to initiate and maintain the required arc.
• the cathode button can be made relatively thick and substantial, by comparison to directly heated filament cathodes, to give the cathode longer life in operation.
• An indirectly heated button cathode for an ion source is disclosed in US 5497006.
• High energy ion implanters are disclosed in US 4667111 and in US 6423976. These prior art high energy implanters use rf linear accelerators to accelerate the ions to the high energies required for implantation. Other forms of high energy accelerators are also known for use in ion implanters, including radio frequency quadrupole (RFQ) accelerators and tandetron accelerators. Such devices have been used to produce singly charged ions of species desired for implantation at energies up to between 500 KeV and 2 MeV. Fixed voltage electrostatic accelerators are also known which can provide singly charged ions at energies in excess of 200 KeV. However, for higher energies it is known to use ions of the desired species at higher charge states, typically doubly or triply charged. The energy delivered to a charged particle by an electric field is directly proportional to the number of charges on the particle.
• ion sources can be optimised to enhance the production of ions at higher charge states. However this usually involves operating the ion source with a more intense arc, so that the life of consumable elements within the ion source, particularly the cathode, is reduced. A compromise is usually made between cathode life and the beam current at the desired higher charge state.
• ELS2 Extended Life Source With Dual Cathode
• I. Jonoshita et al Ion Implantation Technology - 98 pp.239-241
• US 5703372 describes a scheme using a second button cathode in the arc chamber of an ion source to replace the usual electron reflector. A modest increase in life time is demonstrated.
• US 5703372 describes a scheme using a second button cathode in the arc chamber of an ion source to replace the usual electron reflector.
• the invention provides an indirectly heated cathode for an ion source comprising a button member having a front face for emitting thermionic electrons, when in use, to form a plasma and a rear face opposite to said front face for exposure to electron heating in use, the button member comprising a collar piece and a slug piece secured in the collar piece, the slug piece providing respective central portions of the front and rear faces of the button member and the collar piece providing respective peripheral portions of the front and rear faces surrounding said central portions, the button member having a thermal barrier between the slug piece and the collar piece so that the central portion of the front face of the button member is at a higher temperature than the peripheral portion thereof, when the central portion of the rear face of the button member is electron heated in use.
• the thermal mass to be heated is reduced and the central portion of the front face of the button member can be heated to a higher temperature to increase thermionic emission.
• the electron emission area of the cathode can therefore be reduced in size, resulting in a more concentrated plasma within the arc chamber.
• the more concentrated plasma tends to have a higher energy density resulting in more favourable production of ions at higher charge states.
• Both the slug piece and the collar piece of the button member may be of tungsten and can be made thicker to maximise cathode life.
• the invention also provides an indirectly heated button cathode for an ion source, comprising a button member having a front face for emitting thermionic electrons, when in use, to form a plasma, said face for emitting having a central portion provided by a first material having a first thermionic work function and a peripheral portion, around said central portion, provided by a second material having a second thermionic work function greater than said first work function. Because the central portion of the electron emitting face of the button cathode has a lower work function than the peripheral portion, electron emission is concentrated, in use, from the central portion.
• a slug of tantalum is fitted in a collar of tungsten in order to form the button member of the cathode.
• the slug piece can be made thicker than the collar piece, so that the slug protrudes rearwards of the rear face of the button.
• the rear face of the slug piece receives the dominant part of the electron heating by the accelerated electrons from the filament located behind the cathode button member.
• the front face of the slug piece, forming the central portion of the thermionic electron emitting face of the button member may be made concave. This produces a slight focus of the primary electrons emitted from the front face, so as further to increase the energy density of the plasma.
• the modified button cathode can be used to replace prior art button cathodes, typically having a solid tungsten button member, with minimum additional modification of the ion source.
• Figure 1 is a cross-sectional view of the arc chamber of an ion source for use in an ion implanter and including a modified button cathode embodying the present invention
• Figure 2 is a front view of the modified button cathode embodying the invention.
• Figure 3 is a cross-sectional view of the button cathode, taken along line A-A of Figure 2; and Figure 4 is a perspective view of the button cathode .
• the arc chamber of an ion source comprises an arc chamber body 10 having a rear wall 11 and upper and lower end walls 12 and 13.
• the rear wall 11 and upper and lower end walls 12 and 13 are protected by graphite liners 14, 15 and 16.
• the arc chamber has a front plate 17 which provides a slit opening 18 through which ions formed in the arc chamber can be extracted to provide the required ion beam.
• the upper end wall 12 of the arc chamber body 10 has an aperture 18 in which is mounted a cathode structure 19.
• the cathode structure 19 comprises a cylindrical body 20 bearing a button member 21 at its lower end.
• the cylindrical body is clamped in position by clamping members 22 which are in turn secured on electrically insulating mounts 23 to an ion source part 24 at the rear of the arc chamber body 10.
• the clamping members 22 hold the cylindrical body 20 of the cathode structure in spaced relation to the aperture 18 through the upper end wall 12 of the arc chamber body, and a corresponding aperture through the graphite liner 15, with the button member 21 penetrating a short distance into the interior of the arc chamber.
• a tungsten filament 25 is also clamped by additional clamping members 26 so that filament element 27 is positioned a short distance to the rear of button member 21 within the cylindrical cathode body 20.
• a bar 28 is secured across the outer end of the cylindrical cathode body 20, and one connecting lead to the filament element 27 extends on each side of the bar 28 out through the upper open end of the cylindrical cathode body 20 to be secured in the clamping member 26.
• the lower end wall 13 of the arc chamber body 10 has an aperture 29 communicating with the inside of the arc chamber through a corresponding aperture in the graphite liner 16.
• An anti-cathode or electron reflector 30, typically of tungsten, is mounted by means of a clamping arrangement 31 so as to extend through the aperture 29.
• the electron reflector 30 is formed with a shaft portion 32 having an outer end held in the clamp 31, extending in spaced relationship with the interior wall surface of the aperture 29.
• the shaft portion 32 is connected to a head portion 33 by means of a neck portion 34.
• the head portion 33 is circular about the axis of the shaft portion 32 and has a diameter substantially the same as or slightly greater than the diameter of the button member 21 of the cathode structure 19, and an axial thickness of about 6 mm.
• the clamping arrangement 31 supporting the electron reflector 30 is itself mounted by an insulating mount 35 on the ion source part 24.
• a feed tube 36 is fitted through the rear wall 11 of the arc chamber body 10, in order to provide a feed of a desired process gas to the arc chamber for generating desired ions for implantation.
• the ion source arc chamber described above and illustrated in Figure 1 is of the Bernas type and as will be known to the skilled person, a magnetic field extending axially between the cathode and the electron reflector is provided by magnet arrangements which are not shown in Figure 1.
• a filament current from a filament supply flows through the tungsten filament 27.
• the filament 27 is also biased negatively relative to the cathode structure 19. Thermionic electrons emitted by the filament 27 are thus accelerated to impact the rear face of the cathode button member 21, in order to heat the button member to a required thermionic electron emission temperature.
• the cathode structure 19, including the button member 21, is itself negatively biased by an arc supply, so as to provide an arc potential between the button member 21 and the body 10 of the arc chamber.
• Thermionic electrons emitted from the front (lower) face of the button member 21 are confined by the magnetic field to travel substantially axially between the cathode button member 21 and the head 33 of the electron reflector 32.
• the electron reflector 32 is typically also biased relative to the cathode body 10, at the same potential as the cathode structure 19.
• the process feed gas is selected to include atoms of the species to be implanted and the plasma within the arc chamber may produce ions of various molecular and atomic species resulting from dissociation of the feed gas molecules.
• the gaseous species in the plasma in the arc chamber may be ionised to different charge states. Higher charge states are typically generated as a result of increased energy density within the plasma.
• the button member 21 of the cathode structure 19 comprises a central stud or slug piece 40 fitted in an outer collar piece 41.
• the cathode structure comprises a cylindrical body 20 which is typically made of tungsten.
• the body 20 has opposed cutouts 42 and 43, essentially dividing the cylindrical body into an inner end 44 carrying the collar piece 41 of the button member 21, and an outer end 45 by which the cathode structure is secured by the clamping arrangement 22 as shown in Figure 1.
• the collar piece 41 of the button member is formed with an external annular rebate 46 which forms a press fit with a slightly rebated inside edge 47 of the inner end 44 of the cathode body 20.
• the collar 41 is also formed of tungsten and is press fitted to engage with the inner end of the cathode body 20. The two parts are then electron beam welded together.
• the collar piece 41 carries the cylindrical slug piece 40 of the cathode button member.
• the slug piece 40 in this example is also made of tungsten.
• the slug piece 40 is fitted in a cylindrical bore 48 in the collar piece 41.
• the bore 48 has a first outer length portion 50 adjacent the front face of the button member and a second inner length portion 49 adjacent the rear face of the button member.
• the inner portion 49 has a diameter slightly less than the diameter of the outer portion 50 of the bore 48. The difference in diameter may be as small as about 0.2 mm.
• the outer diameter of the slug piece 4 , 0 may be substantially the same as the diameter of the outer portion 50 of the bore 48.
• the slug piece 40 is shrink fitted into the bore 48, by cooling the slug piece 40 in liquid nitrogen. Then, the primary connection between the slug piece 40 and the collar piece 41 is only at the inner portion 49 of the bore 48 and the slug piece 40 fits freely in the outer portion 50 of the bore 48. As a result, the thermal conduction between the slug piece 40 and the collar piece 41 is reduced and a thermal barrier is formed between the two parts.
• the axial length of the inner portion 49 of the bore 48 is preferably significantly less than (typically no more than 20% of) the length of the outer portion 50 so as to minimise the surface area of good thermal contact between the slug piece 40 and the collar piece 41.
• the bore 48 has a total axial length of about 6 mm and the inner portion 49 has a length of about 1 mm.
• the free fit between the slug piece 40 and the collar piece 41 over the greater part of the bore 48 provides a poor thermal contact over this region.
• the outer portion 50 of the bore 48 may be slightly larger in diameter than the slug piece 40 to provide a small annular gap between the two parts during operation. However any such gap should be as small as possible consistent with the requirement that heat loss by conduction from the slug piece 40 over the axial length of the outer portion 50 of the bore 48 is reduced.
• the thermal barrier between the slug piece 40 and the collar piece 41 may be produced by any technique which reduces the contact area between the two parts, while still permitting the collar piece 41 to have substantial axial thickness approaching that of the slug piece 40.
• the inner portion 49 of the bore 48 may be formed to have, in axial cross-section, a pyramidal or trapezoidal shape to reduce further the contact area with the slug piece 40.
• the bore 48 may have a uniform diameter sized to provide a free fit over most of the length of the slug piece 40, and the slug piece 40 may then have an enlarged annular rib at an axial position to grip the inner end of the bore 48.
• the slug piece 40 has a rear face 51 protruding rearwardly by a short distance, typically about 1 mm, beyond the rear face of the collar piece 41.
• the front face 52 of the slug piece 40 is formed to be spherically concave.
• the slug piece may have a length of about 7 mm and a diameter of about 8 mm and the radius of curvature of the concave front surface 52 of the slug piece may be about 10 mm.
• the outer diameter of the collar piece 41 may be about 16 mm and the axial thickness may be about 6 mm.
• the indirectly heated button cathode described above and illustrated in the drawings has a number of advantages over prior art cathodes.
• the electric field between the filament 27 ( Figure 1) inside the cathode body 20, and the rear of the button member 21, is enhanced over the rear face 51 of the slug piece 40.
• heating of the button member by the electron flux from the filament 27 is concentrated over the rear face of the slug piece.
• thermal conduction from the slug piece 40 to the collar piece 41 is reduced.
• the thermal mass to be heated by electron impact on the rear face 51 is accordingly reduced, as are thermal losses from the slug piece 40 itself.
• the rear face 51 of the slug piece 40 may be heated close to the melting point of the material used, e.g. tungsten.
• the thermal break allows the front face 52 of the slug piece 40 to operate at a higher temperature, thereby enhancing thermionic emission of electrons from the front face 52, i.e. the central portion of the front face of the button member.
• the front face of the collar piece 41 is in substantially the same plane as the front face of the slug piece 40 and the slug and collar pieces have nearly the same axial length. This prevents premature failure of the cathode by erosion of the collar piece during operation. Also, because the mechanical connection between the slug piece 40 and the collar piece 41 is adjacent to the inner face of the collar piece, the cathode can tolerate erosion of nearly the full axial thickness of the collar piece 41 before failure.
• the design provides the benefit of a button member having substantial axial thickness over the full front face area of the button member and yet avoids the consequential problem of the high thermal mass of the whole button member by providing the thermal barrier.
• the concave shape of the front ' face 52 of the slug piece 40 tends to concentrate thermionically emitted electrons towards the axis of the arc chamber.
• the lifetime of the cathode embodying the invention and described above is more than 50 hours when continuously running a P +++ beam of 1.7 mA.
• a prior art cathode with a solid tungsten button member under the same arc voltage and arc current provides a P +++ beam current of only 0.5 mA and burns out in just 16 hours.
• the slug piece 40 is made of tantalum. Tantalum has a lower thermionic work function (4.25 eV) than tungsten (4.55 eV) .
• thermionic work function 4.25 eV
• tungsten 4.55 eV
• the button member comprising the tantalum slug piece 40 and tungsten collar piece 41
• electrons are thermionically emitted preferentially from the front face 52 of the tantalum slug piece 40, even if the front face portions of both the slug piece 40 and the surrounding collar piece 41 are at the same temperature.
• tungsten and tantalum other materials having appropriate work functions may be employed for the collar piece 41 and slug piece 40 .
• the collar could be made of Rhenium (work function 4.96 eV) , in combination with a slug of Ta or W.
• the Ta slug cathode produces less spurious arcing during operation. This may be because the Ta slug piece is operating near its melting point (about 2850°C) , and therefore recrystallises quickly, eliminating small grain structure. Sputtering of the cathode during operation can cause the small grains to become dislodged and cause a spurious arc discharge.
• the counter cathode 30 may also made of tantalum. This can contribute to improved performance by reducing spurious arcing as outlined above.
• the central portion of the button, member may have a lower thermionic work function than the collar portion but without the thermal break between the two portions.
• the central portion may have a flat electron emitting face or may have the same axial thickness as the rest of the button member portion.
• the button member may be made as a single disc of tungsten or tantalum with at least a central part of the front electron emitting face formed to be concave.

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• Analytical Chemistry (AREA)
• Engineering & Computer Science (AREA)
• Combustion & Propulsion (AREA)
• Electron Sources, Ion Sources (AREA)

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• 2003
  • 2003-02-20 EP EP03706715A patent/EP1481413A2/en not_active Withdrawn
  • 2003-02-20 JP JP2003573666A patent/JP4271584B2/ja not_active Expired - Fee Related
  • 2003-02-20 WO PCT/GB2003/000733 patent/WO2003075305A2/en not_active Application Discontinuation
  • 2003-02-20 AU AU2003208427A patent/AU2003208427A1/en not_active Abandoned

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