Abstract:
An indirectly heated button cathode for use in the ion source of an ion implanter has a button member formed of a slug piece mounted in a collar piece. The slug piece is thermally insulated from the collar piece to enable it to operate at a higher temperature so that electron emission is enhanced and concentrated over the surface of the slug piece. The slug piece and collar piece can be both of tungsten. Instead the slug piece may be of tantalum to provide a lower thermionic work function. The resultant concentrated plasma in the ion source is effective to enhance the production of higher charge state ions, particularly P +++  for subsequent acceleration for high energy implantation.

Description:
This application claims priority under 35 U.S.C. §119(a) on patent application Ser. No(s). 10/091,351 filed in the United States on Mar. 6, 2002, the entire contents of which are hereby incorporated by reference. 

   FIELD OF THE INVENTION 
   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. 
   BACKGROUND OF THE INVENTION 
   It is known to use indirectly heated cathodes in the arc chambers of ion sources. In such arrangements, 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. With this construction, 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 U.S. Pat. No. 5,497,006. 
   Certain processes in the manufacture of semiconductor devices require the implantation of atomic species at relatively high energies, so that the species are implanted at greater depths in the semiconductor substrate. High energy ion implanters are disclosed in U.S. Pat. Nos. 4,667,111 and in 6,423,976. 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. 
   The operation of 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. 
   Attempts have been made to improve the performance of ion sources in order to maximise cathode life, while operating the cathode to generate relatively high currents of desired multiply charged species. For example, “ELS2: Extended Life Source With Dual Cathode”, I. Jonoshita et al, Ion Implantation Technology—98 pp.239-241, 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. Reference may also be made to U.S. Pat. No. 5,703,372. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide an improved cathode structure for the ion source of an ion implanter to enable the ion source to be optimised for the production of multiple charge state ions with a satisfactory cathode life time. 
   Accordingly, 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. 
   Because of the thermal barrier between the central slug piece of the button member and the collar piece, 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. In a particular embodiment, 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. As a result, 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. 
   Also, 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. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An example of the invention will now be described with reference to the accompanying drawings in which: 
       FIG. 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; 
       FIG. 2  is a front view of the modified button cathode embodying the invention; 
       FIG. 3  is a cross-sectional view of the button cathode, taken along line A—A of  FIG. 2 ; and 
       FIG. 4  is a perspective view of the button cathode. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   In the description and claims that follow, relative terms such as upper and lower, rear and front have been used for simplicity of description. Upper and lower have been used only with reference to the orientation of the example illustrated in  FIG. 1  of the drawings, and, in an actual installation of the embodiment, any orientation relative to vertical is feasible. Rear and front have been used such that the distinction should be apparent from the context. For example, the terms are used with reference to the arc chamber in accordance with the convention that the ion beam is emitted from the front. Similarly, the term front is used with reference to the face of the button member of the cathode to denote the face which is directed away from the neighbouring arc chamber wall and into the interior of the arc chamber. 
   In  FIG. 1 , 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 . 
   In the described embodiment, 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  FIG. 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 FIG.  1 . 
   In operation, a filament current from a filament supply (not shown) 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 energetic primary thermionic electrons from the cathode button member  21  ionise molecules of the process gas fed into the arc chamber by the feed tube  36 , to produce a plasma in the arc chamber in the space between the cathode button member  21  and the head  33  of the electron reflector. As is known to those skilled in this art, 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. Furthermore, it is known that 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. 
   As can be seen in  FIG. 1 , the button member  21  of the cathode structure  19  comprises a central stud or slug piece  40  fitted in an outer collar piece  41 . This structure will be described in greater detail with reference to  FIGS. 2  to  4 . 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 FIG.  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 . In assembly, 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  40  may be substantially the same as the diameter of the outer portion  50  of the bore  48 . On assembling the parts, 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 . 
   In the illustrated example, 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 . For example, 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 . 
   Also, instead of the collar piece  41  having the reduced diameter inner portion  49 , 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 . 
   As can be seen in  FIG. 3 , 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 . Also, the front face  52  of the slug piece  40  is formed to be spherically concave. In one example, 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. In this example, 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. 
   Because the slug piece  40  protrudes rearwardly relative to the collar piece  41  as shown in  FIG. 3 , the electric field between the filament  27  ( FIG. 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 . As a result, heating of the button member by the electron flux from the filament  27  is concentrated over the rear face of the slug piece. 
   Because of the thermal break provided by the method of securing the slug piece  40  within the collar piece  41 , 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. In practice 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. 
   Importantly, 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. 
   These features identified above serve to enhance the concentration of plasma generated in the arc chamber, enabling an increased plasma density, whilst maintaining a satisfactory cathode life time. This permits the arc chamber to be operated to optimise the production of ion species at higher charge states. In particular the ion source can be optimised for the production of triply charged phosphorus ions. 
   For an arc voltage of 100 Volts and an arc current of about 2.7 Amps, 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. By comparison 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. 
   In another embodiment, the slug piece  40  is made of tantalum. Tantalum has a lower thermionic work function (4.25 eV) than tungsten (4.55 eV). As a result, when the button member comprising the tantalum slug piece  40  and tungsten collar piece  41  is heated, 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. Instead of tungsten and tantalum, other materials having appropriate work functions may be employed for the collar piece  41  and slug piece  40  For example the collar could be made of Rhenium (work function 4.96 eV), in combination with a slug of Ta or W. 
   It has also been observed that 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. 
   In other examples of the invention, 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. In further examples 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. In a still further example, 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. 
   Other arrangements are also within the scope of the invention as defined by the following claims.