Abstract:
The present invention relates to ion sources comprising a cathode and a counter-cathode that are suitable for ion implanters. The present invention provides an ion source comprising a vacuum chamber; an arc chamber operable to generate and contain a plasma; a cathode operable to emit electrons into the arc chamber along an electron path; a counter-cathode disposed in the electron path; respective separate electrical connections from each of the cathode and the counter-cathode including respective vacuum feedthroughs to outside the vacuum chamber; and a voltage potential adjuster located outside the vacuum chamber that is connected at least to the counter-cathode via the vacuum feed-through and is operable to alter the potential of the counter-cathode relative to the cathode.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to ion sources comprising a cathode and a counter-cathode that are suitable for ion implanters.  
       BACKGROUND OF THE INVENTION  
       [0002]     A contemplated application of the present invention is in an ion implanter that may be used in the manufacture of semiconductor devices or other materials, although many other applications are possible. In such an application, semiconductor wafers are modified by implanting atoms of desired dopant species into the body of the wafer to form regions of varying conductivity. Examples of common dopants are boron, phosphorus, arsenic and antimony.  
         [0003]     Typically, an ion implanter contains an ion source held under vacuum within a vacuum chamber. The ion source produces ions using a plasma generated within an arc chamber. Plasma ions are extracted from the arc chamber and passed through a mass analysis stage such that ions of a desired mass are selected to travel onward to strike a semiconductor wafer. A more detailed description of an ion implanter can be found in U.S. Pat. No. 4,754,200.  
         [0004]     In a typical Bernas-type source, thermal electrons are emitted from a cathode and are constrained by a magnetic field to travel along electron paths towards a counter-cathode. Interactions with precursor gas molecules within the arc chamber produces the desired plasma.  
         [0005]     In one known arrangement, the counter-cathode is connected to the cathode such that they are at a common potential (U.S. Pat. Nos. 5,517,077 and 5,977,552). The counter-cathode repels electrons travelling from the cathode, increasing ionisation efficiency in the arc chamber.  
         [0006]     In another known arrangement, the counter-cathode is electrically isolated so that it floats to the potential of the plasma (U.S. Pat. No. 5,703,372).  
         [0007]     The mass analysis stage of the implanter is operated by control of a magnetic field to allow selection of ions of a desired mass (via their momentum to charge-state ratio) and rejection of unwanted ions (to the extent that they follow a different path in the magnetic field). In the case of boron doping for example, BF 3  is normally used as a precursor gas. Dissociation in the arc chamber results in a plasma typically containing B + , BF +  and BF 2   +  ions. This mixture of ions is extracted and enters the mass analysis stage which ensures that only the preferred B/BF x  species is delivered to the semiconductor wafer. Although many implant recipes require B +  ions to be implanted, others use BF 2   +  ions. Because the BF 2   +  ions dissociate on impact with a semiconductor wafer, the resulting boron atoms are implanted with reduced energy yielding shallower doping layers as is required in some applications.  
       SUMMARY OF THE INVENTION  
       [0008]     An object of this invention is to increase the flexibility of operation of an ion source, for example to optimise the source for implanting different species derivable from a common source material or to optimise the output of a specific ion species from a particular feed material.  
         [0009]     From a first aspect, the present invention resides in an ion source comprising a vacuum chamber; an arc chamber operable to generate and contain a plasma; a cathode operable to emit electrons into the arc chamber along an electron path; a counter-cathode disposed in the electron path; respective separate electrical connections from each of the cathode and the counter-cathode including respective vacuum feedthroughs to outside the vacuum chamber and a voltage potential adjuster located outside the vacuum chamber that is connected at least to the counter-cathode via the vacuum feed through and is operable to alter the potential of the counter-cathode relative to the cathode.  
         [0010]     The term voltage potential adjuster should be construed broadly to include any type of component that is operable to alter the potential of the counter-cathode relative to the cathode. For example, the voltage potential adjuster may comprise one or more of a switch, a variable resistor, a power supply or a potential divider.  
         [0011]     In this way, the potential of the counter-cathode can be varied such that its effectiveness in reflecting electrons can be adjusted. For example, if the counter-cathode is held at the same potential as the cathode, the lifetimes in the arc chamber of the electrons emitted by the cathode are increased to produce a more intense plasma, enhancing ionisation and cracking of the source gas molecules. Alternatively, the counter-cathode may be set to a different potential or may be allowed to float, whereupon the lifetime of electrons capable of causing ionization and molecular cracking are decreased within the arc chamber. This may be advantageous where a relatively low plasma intensity is required and it is desired to limit cracking of the source gas molecules. Hence, control of the ion source in this way allows the relative concentrations of ion species in the arc chamber, and delivered to the mass analysis stage in an ion implanter, to be controlled. This is particularly useful, for example, in boron implantation where operation of the ion source can be adapted to suit the use of B + , BF +  or BF 2   +  ions, as required.  
         [0012]     The voltage potential adjuster may be operable to make or break electrical contact between the cathode and the counter-cathode. Optionally, the ion source is arranged such that the voltage potential adjuster is operable to isolate electrically the counter-cathode when set to break electrical contact between the cathode and the counter-cathode. This is convenient as it allows the counter-cathode to float to a potential set by the plasma.  
         [0013]     Optionally, the voltage potential adjuster is operable to select the potential of the counter-cathode relative to the cathode. The voltage potential adjuster may comprise at least one of the group comprising a switch, a variable resistor, a power supply and a potential divider. Where a power supply is used, potentials on the counter-cathode not intermediate between floating and the cathode potential are possible.  
         [0014]     The present invention may be used with any ion source type containing both a cathode and a counter cathode reflector or repeller.  
         [0015]     Often the ion source further comprises a magnet assembly arranged to provide a magnetic field in the arc chamber to define the electron path, although such a magnet arrangement is by no means necessary. This provides a longer electron path length for the thermal electrons that may otherwise be attracted directly to the adjacent arc chamber walls. The magnetic field constrains the electrons to pass along the length of the arc chamber where, for example, cathode and counter-cathode are located at opposed ends of the arc chamber.  
         [0016]     From a second aspect, the present invention resides in an ion implanter comprising an ion source as described above, wherein the arc chamber further comprises an exit aperture and the ion implanter further comprises an extraction electrode operable to extract ions from the plasma contained within the arc chamber through the exit aperture, a mass analysis stage located to receive ions extracted from the arc chamber and operable to deliver ions of a selected mass and charge state, at a particular energy, for implanting into a target.  
         [0017]     A further aspect of the invention provides a method of operating an ion source as described above comprising the steps of setting potentials across the cathode and anode; setting the voltage potential adjuster to place a desired potential across the counter-cathode; filling the arc chamber with gas; and heating the cathode sufficiently to cause emission of electrons. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  is a schematic representation of an ion implanter;  
         [0019]      FIG. 2  is a side view of a first ion source;  
         [0020]      FIG. 3  is a side view of a second ion source that comprises an indirectly-heated cathode arrangement;  
         [0021]      FIG. 4  is a simplified representation of an ion source with an indirectly-heated cathode arrangement, showing a biasing arrangement according to a first embodiment of the present invention; and  
         [0022]      FIG. 5  is a simplified representation of an ion source with a simple filament arrangement showing a biasing arrangement according to a second embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]     In order to provide a context for the present invention, an exemplary application is shown in  FIG. 1 , although it will be appreciated that this is merely an example of an application of the present invention and is in no way limiting.  
         [0024]      FIG. 1  shows an ion implanter  10  for implanting ions in semiconductor wafers  12  including an ion source  14  according to the present invention. Ions are generated by the ion source  14  to be extracted and passed through a mass analysis stage  30 . Ions of a desired mass are selected to pass through a mass-resolving slit  32  and then to strike a semiconductor wafer  12 .  
         [0025]     The ion implanter  10  contains an ion source  14  for generating an ion beam of a desired species that is located within a vacuum chamber  15 . The ion source  14  generally comprises an arc chamber  16  containing a cathode  20  located at one end thereof and an anode that is provided by the walls  18  of the arc chamber  16 . The cathode  20  is heated sufficiently to generate thermal electrons.  
         [0026]     Thermal electrons emitted by the cathode  20  are of course attracted to the anode, i.e. the adjacent chamber walls  18 . The thermal electrons ionise gas molecules as they traverse the arc chamber  16 , thereby forming a plasma and generating the desired ions.  
         [0027]     The path followed by the thermal electrons is controlled to prevent the electrons merely following the shortest path to the chamber walls  18 . A magnet assembly  46  provides a magnetic field extending through the arc chamber  16  such that thermal electrons follow a spiral path along the length of the arc chamber  16  towards a counter-cathode  44  located at the opposite end of the arc chamber  16 .  
         [0028]     A gas feed  22  fills the arc chamber  16  with a precursor gas species, BF 3  in this case. The arc chamber  16  held at a reduced pressure by the vacuum chamber  15 . The thermal electrons travelling through the arc chamber  16  ionise the precursor BF 3  gas molecules and also crack the BF 3  molecules to form BF 2 , BF and B molecules and ions. The ions created in the plasma will also contain trace amounts of contaminant ions (e.g. generated from the material of the chamber walls).  
         [0029]     The ion source  14  (including the magnet assembly  46 ) is shown rotated by 90° in  FIG. 1  compared to our actual ion implanter  10 . In fact, the cathode  20  and counter-cathode  44  are aligned on an axis perpendicular to the plane of the page, but have been illustrated in a rotated arrangement for the sake of clarity.  
         [0030]     Ions from within the arc chamber  16  are extracted through an exit aperture  28  using a negatively-biased extraction electrode  26 . A potential difference is applied between the ion source  14  and the following mass analysis stage  30  by a power supply  21  to accelerate extracted ions, the ion source  14  and mass analysis stage  30  being electrically isolated from each other by an insulator (not shown). The mixture of extracted ions are then passed through the mass analysis stage  30  so that they pass around a curved path under the influence of a magnetic field. The radius of curvature travelled by any ion is determined by its mass, charge state and energy and the magnetic field is controlled so that, for a set beam energy, only those ions with a desired mass and charge state exit along a path coincident with the mass-resolving slit  32 . The emergent ion beam is then transported to the target, i.e. the substrate wafer  12  to be implanted or a beam stop  38  when there is no wafer  12  in the target position. In other modes, the beam may also be decelerated using a lens assembly positioned between the mass analysis stage  30  and the target position.  
         [0031]     The semiconductor wafer  12  will be one of many positioned on a carousel  36  that rotates to present the wafers  12  to the incident ion beam in turn. In addition, the rotating carousel  36  may be translated from side to side thereby allowing the incident ions to be scanned across each wafer  12 . As the wafers  12  are being rotated, there will be times when the ion beam will not be incident on a wafer  12  and so the ions will continue beyond the target position to strike a beam stop  38 . In an alternative arrangement, a single wafer  12  may be mounted and presented for implantation.  
         [0032]      FIGS. 2 and 3  show two ion sources  14  that may be used in the ion implanter  10  of  FIG. 1  in greater detail:  FIG. 2  corresponds to a filament arrangement and  FIG. 3  corresponds to an indirectly-heated cathode arrangement.  
         [0033]     Referring first to  FIG. 2 , a filament  40  that acts as a cathode is situated at one end of the arc chamber  16  to sit in front of an electron reflector  42 . The electron reflector  42  is held at the same negative potential as the filament  40  such that they both repel electrons. There is a small gap between the electron reflector  42  and a liner  56  that comprises the innermost part of the arc chamber  16 . This gap ensures that the electron reflector  42  is electrically isolated from the liner  56  that acts as an anode. The clearance is minimal to avoid loss of the precursor gas from the arc chamber  16 . A counter-cathode  44  is located at the far end of the arc chamber  16 , again with a small separation from the liner  56  to ensure electrical isolation and to minimise gas leakage. A magnet assembly  46  (shown only in  FIG. 1 ) is operable to provide a magnetic field that causes electrons emitted from the filament  40  to follow a spiral path  34  along the length of the arc chamber  16  towards the counter-cathode  44 . The arc chamber  16  is filled with the precursor gas species by a gas feed  22  or by one or more vaporisers  23  that may heat a solid or liquid.  
         [0034]     The filament  40  is held in place by two clamps  48  that are each connected to the body  50  of the ion source  14  using an insulating block  52 . The insulating block  52  is fitted with a shield  54  to prevent any gas molecules escaping from the arc chamber  16  from reaching the insulator block.  
         [0035]     As will be evident,  FIG. 3  corresponds largely to  FIG. 2  and so like parts will not be described again for the sake of brevity. In addition, like reference numerals are used for like parts.  
         [0036]     The difference between  FIG. 2  and  FIG. 3  lies in the top of the arc chamber  16  where  FIG. 3  shows an indirectly-heated cathode arrangement. A cathode is provided by an end cap  58  of a tube  60  that projects slightly into the arc chamber  16 , the tube  60  containing a heating filament  62 . The heating filament  62  and end cap  58  are kept at different potentials to ensure thermal electrons emitted by the filament  62  are accelerated into the end cap  58 , and a gap is left between the tube  60  and the liner  56  of the arc chamber  16  to maintain electrical isolation. Acceleration of electrons into the end cap  58  transfers energy to the end cap  58  such that it heats up sufficiently to emit thermal electrons into the arc chamber  16 .  
         [0037]     This arrangement is an improvement over the filament arrangement of  FIG. 3  because the filament  40  is corroded quickly by the plasma&#39;s reactive ions and through ion bombardment. In order to alleviate this problem, the heating filament  62  of the indirectly-heated cathode is housed within the enclosed tube  60  such that ions do not come into contact with the heating filament  62 .  
         [0038]     Turning now to  FIG. 4 , a simplified representation of the arc chamber  16  of  FIG. 3  alongside an electrical power supply  64  is shown. The dashed box  66  indicates the boundary between components that are housed within the vacuum chamber  15  and those components that are situated in atmosphere  70 . Clearly, components located in atmosphere  70  can be readily adjusted without the need to break vacuum  68 .  
         [0039]     As can be seen from  FIG. 4 , a series of three power supplies located in atmosphere  70  provide electrical current to various components of the ion source  14  at different potentials. A filament supply  72  provides a relatively high current to the filament  62 . A bias supply  74  is used to set a potential on the end cap  58  that is positive with respect to the filament  62  such that thermal electrons emitted from the filament  62  are accelerated towards the end cap  58 . An arc supply  76  maintains the walls  18  (i.e. the liner  56 ) of the arc chamber  16  at a positive potential with respect to the end cap  58 .  
         [0040]     There is also an electrical connection provided to the counter-cathode  44  that passes through a vacuum feed through  80  at the vacuum/atmosphere boundary  66  to join the arc supply  76  via a control relay  78 . The control relay  78  allows electrical connection to be made and broken without the need to vent the vacuum chamber  15  to atmosphere  70 . When the control relay  78  is closed, the counter-cathode  44  is tied to the same potential as the end cap  58  thereby ensuring that electrons travelling toward the counter-cathode  44  are repelled to pass back through the arc chamber  16  and so have an increased chance of ionising pre-cursor gas molecules and cracking feed materials. When the control relay  78  is open, the counter-cathode  44  is free to float to the potential of the plasma within the arc chamber  16 . This means that electrons are no longer reflected as strongly by the counter-cathode  44 .  
         [0041]     When a tied potential arrangement is used, the chance of cracking BF 3  molecules in the arc chamber  16  is increased due to the higher electron density in the arc chamber  16 . Accordingly, the percentage of boron ions in the plasma relative to the total of other ion types increases (e.g. BF and BF 2  ions). When the counter-cathode  44  is isolated and allowed to float to a potential set by the plasma, cracking is reduced such that more molecular ions (e.g. BF +  and/or BF 2   + ) remain in the plasma. As described previously, either boron or BF 2   +  ions may be preferred for ion implantation of semiconductor wafers  12 . Switching the potential of the counter-cathode  44  maximises the number of preferred ions incident on the mass analysis stage  30  and hence available for onward transmission to the semiconductor wafer  12 . Therefore, the tied potential arrangement is better used for implantation using boron ions and the floating arrangement is better used for implantation using BF 2   +  ions.  
         [0042]      FIG. 5  corresponds broadly to  FIG. 4  and so like parts will not be described again for the sake of brevity. In addition, like parts are assigned like reference numerals.  
         [0043]      FIG. 5  shows an arrangement akin to  FIG. 4  but having a filament  40  rather than an indirectly-heated cathode. The ion source  14  of  FIGS. 2 and 5  comprises a filament  40  located in front of an electron reflector  42 . The filament  40  and electron reflector  42  are held at a common negative potential at all times via an electrical connection  82  that can be made within vacuum  68 . In addition, there is no need for a separate bias supply  74  as there is no potential difference between filament  40  and electron reflector  42 . Accordingly, a single arc supply  76  sets the potentials of the electron reflector  42  and the filament  40  with respect to the walls  18  (or liner  56 ).  
         [0044]     Otherwise, the embodiment of  FIG. 5  corresponds to the embodiment of  FIG. 4 . Accordingly, the counter-cathode  44  may be either tied to the common negative voltage of the filament  40  and electron reflector  42  or may float to a potential set by the plasma depending upon whether the control relay  78  is closed or open, respectively.  
         [0045]     The skilled person will appreciate that variations can be made to the above embodiments without departing from the spirit and scope of the present invention.  
         [0046]     Whilst the above embodiments use a control relay  78  as a switch to allow the counter-cathode  44  to be connected or disconnected from the arc supply  76 , other arrangements are possible. For example, a switch may be used to connect the counter-cathode  44  to either the cathode  20  or an alternative power supply. The alternative power supply may be one of those show in  FIGS. 4 and 5  or it may be a further power supply. A further alternative would be a potential divider connected to provide a divided voltage potential and a switch operable to connect the counter-cathode  44  to one of the cathode  20  or the divided voltage potential. Still further, a variable resistance or variable potentiometer may be used to supply a selected voltage to the counter cathode  44 .  
         [0047]     The example of a control relay  78  is but a preferred form of switching arrangement, and the switch can be implemented in any number of standard ways.  
         [0048]     Clearly the materials used in the construction of the ion source  14  and the particular arrangement of components can be chosen as required.  
         [0049]     Whilst the above embodiments present the invention in the context of an ion source  14  of an ion implanter  10 , the present invention can be used in many other applications such as an ion shower system, in which ions that are extracted from the ion source  14  are implanted into a target without undergoing mass analysis, or any other ion source  14  utilising a counter-cathode  44  in which selective ionization and/or molecular cracking are desirable.