Abstract:
An indirectly heated button cathode for use in the ion source of an ion implanter has a button member formed of a slug of tantalum mounted in a collar of tungsten. The lower thermionic work function of tantalum causes electron emission to be concentrated over the surface of a tantalum slug. The tantalum slug may be mounted to enable it to operate at a higher temperature compared to the surrounding tungsten collar portion. 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:
FIELD OF THE INVENTION  
         [0001]    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  
         [0002]    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 the 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, due to 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.  
           [0003]    An indirectly heated button cathode for an ion source is disclosed in U.S. Pat. No. 5,497,006.  
           [0004]    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. No. 4,667,111 and in EP-A-1056113. 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.  
           [0005]    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.  
           [0006]    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.  
         SUMMARY OF THE INVENTION  
         [0007]    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.  
           [0008]    Accordingly, the invention 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.  
           [0009]    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. 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. 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.  
           [0010]    The slug of tantalum can be made thicker than the tungsten collar, so that the slug protrudes rearwards of the rear face of the button. As a result, the rear face of the tantalum slug receives the dominant part of the electron heating by the accelerated electrons from the filament located behind the cathode button member.  
           [0011]    Also, the front face of the tantalum slug, 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.  
           [0012]    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  
       [0013]    An example of the invention will now be described with reference to the accompanying drawings in which:  
         [0014]    [0014]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;  
         [0015]    [0015]FIG. 2 is a front view of the modified button cathode embodying the invention;  
         [0016]    [0016]FIG. 3 is a cross-sectional view of the button cathode, taken along line A-A of FIG. 2; and  
         [0017]    [0017]FIG. 4 is a perspective view of the button cathode. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0018]    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.  
         [0019]    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.  
         [0020]    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.  
         [0021]    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 .  
         [0022]    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 .  
         [0023]    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  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 .  
         [0024]    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.  
         [0025]    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.  
         [0026]    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 .  
         [0027]    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.  
         [0028]    As can be seen in FIG. 1, the button member  21  of the cathode structure  19  comprises a central stud or slug  40  fitted in an outer collar  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  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  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.  
         [0029]    The collar  41  carries the cylindrical slug  40  of the cathode button member. The slug  40  in this example is made of tantalum. The tantalum slug  40  is fitted in a cylindrical bore  48  in the collar  41 . The bore  48  has an inner end  49  which has a diameter slightly less than the diameter of the major part  50  of the rest of the bore  48 . The difference in diameter may be as small as about 0.2 mm. The outer diameter of the slug  40  may be substantially the same as the diameter of the larger portion  50  of the bore  48 . On assembling the parts, the slug  40  is shrink fitted into the bore  48 , by cooling the slug  40  in liquid nitrogen. Because the primary connection between the slug  40  and the collar  41  is only at the inner end  49  of the bore  48 , the thermal conductivity between the slug  40  and the collar  41  is reduced.  
         [0030]    As can be seen in FIG. 3, the slug  40  has a rear face  51  protruding rearwardly by a short distance, typically about 1 mm, beyond the rear face of the collar  41 . Also, the front face  52  of the slug  40  is formed to be spherically concave. In one example, the slug 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 may be about 10 mm. In this example, the outer diameter of the collar  41  may be about 16 mm and the axial thickness may be about 6 mm.  
         [0031]    The indirectly heated button cathode described above and illustrated in the drawings has a number of advantages over prior art cathodes.  
         [0032]    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  40  and tungsten collar  41  is heated, electrons are thermionically emitted preferentially from the front face  52  of the tantalum slug  40 . In operation within the ion source as illustrated in FIG. 1, this can result in the plasma produced in the arc chamber being more concentrated along the axis of the chamber. Although tungsten and tantalum are used respectively for the collar portion  41  and slug  40  in the described example, other materials having appropriate work functions may be employed. For example the collar could be made of Rhenium (work function 4.96 eV), in combination with a slug of Ta or W.  
         [0033]    Because the tantalum slug portion  40  protrudes rearwardly relative to the tungsten collar  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  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, so that the temperature of the front face  52  of the slug can be higher than the temperature of the surrounding collar  41 .  
         [0034]    Because of the method of securing the slug  40  within the collar  41 , thermal conduction from the slug  40  to the collar  41  is also reduced, again enhancing the temperature of the slug and thermionic emission from the front face  52 .  
         [0035]    The concave shape of the front face  52  of the slug  40  tends to concentrate thermionically emitted electrons towards the axis of the arc chamber.  
         [0036]    All 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.  
         [0037]    For an arc voltage of 120 Volts and an arc current of 2.0 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 mA. By comparison a prior art tungsten cathode 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.  
         [0038]    A contributing factor to the improved performance enabling the source to produce a higher P +++  beam current is that the Ta slug cathode produces less spurious arcing during operation, than a W cathode. This is believed to be because the Ta slug is operating near its melting point (about 2850° C.), and therefore recrystallises quickly, eliminating small grain structure. By comparison, W has a melting point of about 3300° C. and, at the operating temperature, recrystallises more slowly, maintaining a small grain structure for longer. As the W cathode is sputtered, the small grains can become dislodged and cause a spurious arc discharge.  
         [0039]    In prior art Bernas ion sources using an indirectly heated button cathode, the counter cathode or electron reflector is usually made of tungsten. In the embodiment of the present invention as illustrated in FIG. 1, the counter cathode  30  is also made of tantalum. This contributes to the improved performance by reducing spurious arcing as outlined above.  
         [0040]    The cathode structure described above is one example of the invention. In another example, the central portion of the button member, while having a lower thermionic work function than the collar portion, may have a flat electron emitting face and may have the same axial thickness as the collar portion. In a further example, the slug portion and the collar portion may both be made of the same material, e.g. tungsten or tantalum, and the slug portion is arranged to protrude rearwardly relative to the collar portion. In this example, the front face of the slug portion may be flat or concave. 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.  
         [0041]    Other arrangements are also within the scope of the invention as defined by the following claims.