Abstract:
An ion source and method of cleaning are disclosed. One or more heating units are placed in close proximity to the inner volume of the ion source, so as to affect the temperature within the ion source. In one embodiment, one or more walls of the ion source have recesses into which heating units are inserted. In another embodiment, one or more walls of the ion source are constructed of a conducting circuit and an insulating layer. By utilizing heating units near the ion source, it is possible to develop new methods of cleaning the ion source. Cleaning gas is flowed into the ion source, where it is ionized, either by the cathode, as in normal operating mode, or by the heat generated by the heating units. The cleaning gas is able to remove residue from the walls of the ion source more effectively due to the elevated temperature.

Description:
[0001]    This application is a divisional of U.S. patent application Ser. No. 12/533,318, filed Jul. 31, 2009, which claims priority of U.S. Provisional Patent Application No. 61/085,943, filed Aug. 4, 2008, the disclosures of which are incorporated herein by reference in their entireties. 
     
    
     FIELD 
       [0002]    Present disclosure relates to an apparatus for material processing, more particularly, to an ion source of an ion implantation system, and a method of cleaning the ion source. 
       BACKGROUND 
       [0003]    Ion implantation is a type of process that may be performed on, among others, a semiconductor to alter its mechanical, optical, and electrical properties. Among other tools, a beam-line ion implanter may be used. A block diagram of a conventional ion implanter is shown in  FIG. 1 . The conventional ion implanter may comprise an ion source  102  that may be biased by a power supply  104  and that generates ions. The ion source  102  is typically contained in a vacuum chamber known as a source housing (not shown). In the ion source  102 , a filament (not shown) for emitting electrons may be disposed. 
         [0004]    The ion implanter system  100  may also comprise a series of beam-line components, through which the ions  10  may pass. The series of beam-line components may include, for example, extraction electrodes  106 , a 90° magnet analyzer  108 , a first deceleration (D 1 ) stage  110 , a 70° magnet collimator  112 , and a second deceleration (D 2 ) stage  114 . Typically, the wafer  116  may be mounted on a platen  118  that can be moved in one or more dimensions (e.g., translate, rotate, and tilt) by an apparatus, sometimes referred to as a “roplat” (not shown). 
         [0005]    In operation, feed gas may be introduced into the ion source  102 . Depending on the type of desired ions species, different types of feed gas may be used. For generating dopants for p-type doping, boron trifluoride (“BF 3 ”) feed gas may be introduced to the ion source. For generating Hydrogen (“H”) and Helium (“He”) ions for cleaving process, H 2  and He feed gas may be introduced. For generating molecular p-type ions for low energy p-type doping, carborane (C x B y H z ) and decaborane (“B 10 H 14 ”) may be introduced. For generating dopants for n-type doping, phospine (“PH 3 ”) or arsine (“AsH 3 ”) feed gas may be introduced to the ion source. 
         [0006]    After the feed gas is introduced, the filament may be powered to emit electrons. The electrons may then excite the feed gas into plasma containing charged and neutral particles, the particles including desired ions  10 , unwanted ions, and neutrals. The desired ions  10  are extracted through an extraction aperture (not shown) of the ion source  102 , manipulated into a beam-like state, and directed toward the wafer  116  by the beam-line components. Unwanted ions may also be extracted, and may be separated from the desired ions  10  through the use of a mass analyzer magnet. 
         [0007]    Depending on the species contained in the feed gas, ions and neutrals in the ion source  102  may condense and coat the internal wall of the ion source  102 , the extraction aperture, and/or the extraction electrode  106 . If, for example, carborane or diborane feed gas is used, the internal wall of the ion source  102  may be coated with, among others, a film containing carbon or boron. The coating may change the electrical characteristics of the ion source  102  or even cause ion source  102  failure. In addition, the deposited layer may act as a source for contamination in subsequent implantation. 
         [0008]    To prevent excess coating, the ion source  102  is cleaned periodically. After performing several ion implantations, the ion source  102  may be removed from the implantation system  100 , taken apart, and cleaned. However, such a process does not adequately and efficiently clean the ion source  102 . In addition, the cost of the cleaning process may be high, placing additional financial burden on semiconductor device manufacturers and, ultimately, consumers. As such, a new ion source and ion source cleaning method are needed. 
       SUMMARY 
       [0009]    The problems of the prior art are overcome by the ion source system and method of cleaning the same disclosed herein. One or more heating units are placed in close proximity to the inner volume of the ion source, so as to affect the temperature within the ion source. In one embodiment, one or more walls of the ion source have recesses into which heating units are inserted. In another embodiment, one or more walls of the ion source are constructed of a conducting circuit and an insulating layer. In another embodiment, a heating unit replaces one or more walls of the ion source and is located near the liner. 
         [0010]    By utilizing heating units near the ion source, it is possible to develop new methods of cleaning the ion source. Cleaning gas is flowed into the ion source, where it is ionized and/or become reactive species, either by the cathode, as in normal operating mode, or by the heat generated by the heating units. The cleaning gas is able to remove residue from the walls of the ion source more effectively due to the elevated temperature. In some embodiments, the temperature of the ion source may be modifying through control of the heating units, allowing different operating temperatures for normal operation and cleaning processes. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    In order to facilitate a fuller understanding of the present disclosure, reference is now made to the accompanying drawings. These figures should not be construed as limiting the present disclosure, but are intended to be exemplary only. 
           [0012]      FIG. 1  is a block diagram illustrating a conventional ion implantation system. 
           [0013]      FIGS. 2A and 2B  are block diagrams illustrating an ion source according to one embodiment of the present disclosure. 
           [0014]      FIGS. 3A and 3B  are block diagrams illustrating another ion source according to another embodiment of the present disclosure. 
           [0015]      FIGS. 4A ,  4 B,  4 C,  4 D, and  4 E are block diagrams illustrating another ion source according to another embodiment of the present disclosure. 
           [0016]      FIGS. 5A ,  5 B,  5 C,  5 D, and  5 E are block diagrams illustrating another ion source according to another embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    In the present disclosure, several embodiments of an ion source and a method for cleaning thereof are disclosed with reference to the accompanying drawings. For purpose of clarity, the disclosure is made in context to a term “substrate.” Those of ordinary skill in the art will recognize that the substrate may be an electricity conducting, semiconducting, or insulating substrate, or a combination thereof. 
         [0018]    For illustrative purpose, the present disclosure is made in context to a beam-line ion implantation system. However, those in the art will recognize that the present disclosure may be equally applicable to other systems that generate and/or manipulate particles, whether the particles are charged or neutral, and whether atomic, subatomic, or molecular particles. For example, the present disclosure may be equally applicable to plasma based systems including plasma doping (“PLAD”) or Plasma Immersion Ion Implantation (“PIII”) systems, plasma etching systems, and plasma enhanced chemical vapor deposition (“PECVD”) systems. The present disclosure may also be equally applicable to systems including mass spectrometer system and molecular beam epitaxy (MBE) system. Further, the present disclosure may also be applicable to other chemical based process system including, for example, chemical vapor deposition (“CVD”) systems and its variants. 
         [0019]    Referring to  FIG. 2A , there is shown an ion source  200  according to one embodiment of the present disclosure.  FIG. 2B  shows a side view of the ion source  200 . As illustrated in  FIGS. 2A and 2B , the ion source of the present disclosure comprises a plurality of walls  202  defining a volume  204  where the ions  10  are generated. Although  FIG. 2B  illustrates that the ion source  200  may comprise a plurality of wall pieces, it is also contemplated that the ion source  200  may comprise a single, continuous piece defining the volume  204 . 
         [0020]    Near the internal surface of at least one wall  202 , the surface facing the volume  204 , a liner  205  may be disposed. Analogous to the wall  202  of the ion source  200 , the liner  205  may be a single piece or multiple pieces. In the present disclosure, the liner  205  may be made from an inert material. In addition, the liner  205  may be made from a material capable of withstanding corrosive and reactive conditions associated with plasma. In the present embodiment, the liner  205  may be a tungsten liner or a graphite liner. In another embodiment, the liner  205  may be a tantalum (“Ta”) liner. Yet in another embodiment, the liner  205  may be a molybdenum (“Mo”) liner. On at least one of the walls  202  of the ion source  200 , an extraction aperture  206  may be disposed. 
         [0021]    In the present embodiment, the ion source may be a single mode ion source. As illustrated in  FIGS. 2   a  and  2   b , the single mode ion source  200  may comprise an indirectly heated cathode (“IHC”) source  208  and a repeller  210  disposed on opposite ends of the ion source  200 . However, it is also contemplated that the ion source  200  of the present disclosure may be a dual or multi-mode ion source. In the dual or multi-mode ion source, one or more IHC sources may be disposed in the position of the repeller  210 . Accordingly, in the dual or multi-mode ion source, several IHC sources may be disposed on opposite ends of the ion source  200 . The size of the IHCs in the dual or multi-mode ion source may be the same or different. In the dual or multi-mode ion source, one IHC source may be used as the IHC source for one type of ion implantation process, whereas another, different IHC source may be used as the IHC source for another type of ion implantation process. Different types of implantation processes may be those involving different types of ions or different energies. Examples of the dual or multi-mode ion source and its operation may be found in U.S. patent application Ser. No. 12/079,978, which is incorporated in its entirety by reference. 
         [0022]    Although the ion source  200  of the present disclosure may preferably be a source including the IHC source  208 , it is also contemplated that the ion source  200  may be a Bernas type source, Freeman type source, or any other source capable of generating ions or other charged particles. 
         [0023]    The indirectly heated cathode source  208 , if present, may include a cathode  208   a  and a hollow skirt  208 B disposed on the periphery of the cathode  208   a . Optionally, the IHC  208  may include a support rod (not shown) that is positioned in a space defined by the cathode  208   a  and the hollow skirt  208   b  and that is coupled to the cathode  208   a . Examples of ion source containing the optional support rod can be found in U.S. Pat. No. 7,138,768 and U.S. Pat. No. 7,276,847, each of which is incorporated in its entirety by reference. As shown in  FIG. 2B , a filament  208   c  may be disposed in the space, and near the optional support rod, if the optional support rod is present. 
         [0024]    Proximate to at least one of the walls  202 , a heating unit  220  may be disposed. Any type of heating unit may be used. For example, the heating unit may be a heating unit that emits photons or phonons; a particle based heating unit that emits charged or neutral particles; a resistive based heating unit that emits heat converted from electrical current; a mechanical based heating unit that generates heat via friction; a chemical reaction based heating unit that generates heat via chemical reaction; other convective or conductive heating unit; or a combination thereof. In the present embodiment, the heating unit  220  may preferably be that which emits photons such as, for example, a halogen lamp or resistive heater. The heating unit  220  may be electrically coupled to a separate power supply or the power supply that powers the ion source, the IHC  208 , and/or the repeller  210 . 
         [0025]    In the present embodiment, the heating unit  220  may be disposed on the outer surface of one of the walls  202 . It is also contemplated that additional heat sources  220  may be provided near surfaces of other walls  202  of the ion source  200 . If necessary, the heating unit  220  may be disposed around the extraction aperture  206 , the IHC  208 , the repeller  210 , or a combination thereof. 
         [0026]    When powered, the heating unit  220  may provide heat to the ion source  200 . In addition, the heating unit  220  may maintain the temperature of the feed gas in the ion source  200  at high and uniform temperature level. High and uniform temperature level may be maintained whether the IHC source  208 , the repeller  210 , and/or the optional IHC is active or passive. High and uniform temperature level throughout the ion source  200  may enable more efficient and effective dissociation of the gas, as described below, to generate, for example, more reactive gas molecules for more effective source cleaning. 
         [0027]    Referring to  FIGS. 3A and 3B , there is shown another ion source  300  according to another embodiment of the present disclosure. Much like the ion source  200  of the earlier embodiment, the ion source  300  of the present embodiment may comprise a plurality of walls  302  defining a volume  304  where the ions  10  are generated; an extraction aperture  306 ; an IHC  308 ; and a repeller  310 . Alternatively, the ion source  300  may be a dual mode or multi-mode ion source comprising a second or additional IHC disposed opposite to the IHC  308 . In addition, a liner (not shown) may be disposed between the volume  304  and the internal surface of the wall  302  of the ion source  300 . 
         [0028]    The indirectly heated cathode  308  may be similar to the IHC  208  of the earlier embodiment. In particular, the IHC  308  of the present embodiment may comprise a cathode (not shown) and a hollow skirt  308   b  disposed on the periphery of the cathode. In the area defined by the cathode  308   a  and the hollow skirt  308   b , a filament (not shown) and an optional support rod (not shown) may be positioned. For purpose of clarity, a detailed description of similar features will be omitted. 
         [0029]    Referring to  FIG. 3B , at least one wall  302  of the ion source  300  may comprise at least recess  303 . At least one heating unit  320  may be positioned within the recess  303 . The recess  303  may be a through hole in that it has holes on both ends of the wall and passes across the entirety of the wall  302 . Alternatively, the recess may only be in a portion of the wall  302  and may not have holes on both ends of wall  302 . The heating unit  320 , similar to the heating unit  220  of the earlier embodiment, may be, for example, a heating unit that emits photons or phonons; a particle based heating unit that emits charged or neutral particles; a resistive based heating unit that emits heat converted from electrical current; a mechanical based heating unit that generates heat via friction; chemical reaction based heating unit that generates heat via chemical reaction; other convective or conductive heating unit; or a combination thereof. In the present embodiment, a halogen lamp, a filament that emits photons, or a biased filament that also emits electrons may be preferred. 
         [0030]    By including at least one recess  303  in at least one wall  302  of the ion source  300 , the heating unit  320  of the present embodiment may efficiently and effectively apply heat to the ion source  300 . As known in the art, many of the heating units are multi-directional, emitting heat in multiple directions. By including the heating unit  320  into the walls  302  of the ion source  300 , at least a portion of the heat emitted away from the volume  304 , the volume  304  where ions  10  are generated, may be captured and redirected toward the volume  304 . In addition, by positioning the heating unit  320  within the walls  302  of the ion source  300 , the heating unit  320  may be shielded from conditions outside of the ion source  300  that may cool the unit  320 . As such, heat loss may be reduced. 
         [0031]    Referring to  FIG. 4A-4E , there is shown another ion source  400  according to another embodiment of the present disclosure. Much like the ion sources  200  and  300  of the earlier embodiments, the ion source  400  of the present embodiment may comprise an IHC  408  and a repeller  410 . In addition, the ion source  400  may be a dual mode or multi-mode ion source comprising a second or additional IHCs, instead of the repeller  410 , disposed opposite to the IHC  408 . 
         [0032]    Referring to  FIGS. 4A and 4B , the ion source  400  of the present embodiment may also comprise a plurality of walls  402  defining a volume  404  where the ions  10  are generated. In addition, an optional liner  405  may be positioned proximate to the volume  404 , between the walls  402  and the volume  404 . Further, an extraction aperture  406  may be disposed on one of the walls  402 . For purpose of clarity, a detailed description of similar features will be omitted. 
         [0033]    In the present embodiment, at least one of the walls  402  may also be a heating unit  420  that provides heat to the ion source  400 . The heating unit  420  of the present embodiment may comprise a circuit  422  surrounded by one or more electricity insulating layers  424  and  426 . In one embodiment, the circuit  422  may be a circuit made from graphite. However, in another embodiment, the circuit  422  may be a circuit made from another electricity conducting material such as tungsten. 
         [0034]    One or more insulating layers  424  and  426  surrounding the circuit  422  may be ceramic layers  424  and  426 . Although  FIG. 4B  illustrates multiple insulating layers  424  and  426 , it is within the scope of the present disclosure that the single insulating layers surrounds the circuit  422 . 
         [0035]    In the present embodiment, the insulating layers  424  and  426  may be pyrolitic boron nitride (“PBN”) layers  424  and  426 . However, it is contemplated that the insulating layers  424  and  426  may be made from some other insulating material. 
         [0036]    In choosing the materials for the circuit  422  and the insulating layers  424  and  426 , it may be prudent to consider the mechanical and thermal properties of the materials. The materials for the circuit  422  and the insulating layer  424  and  426  may preferably be chosen from those capable of maintaining their structural integrity at high temperature, for example, at or above 1000° C. 
         [0037]    Referring to  FIG. 4B , there is shown a detailed cross sectional view of the heating unit  420  of the present embodiment. The heating unit  420  may comprise the circuit  422 ; a first insulating layer  424  disposed on the circuit  422 ; a second insulating layer  426 , on which the circuit  422  is disposed; and a base  428 . Further, the heating unit  420  may optionally comprise a graphite layer  430  that surrounds the first insulating layer  424 . 
         [0038]    In the present embodiment, the base  428  of the heating unit  420  may be a graphite base. However, it is also contemplated that the base  428  may be made from another type of conductor, semiconductor, or insulator, or a combination thereof. 
         [0039]    Referring to  FIG. 4C-E , there is shown plan views of several examples of the circuit  422 . As illustrated in each figure, the circuit  422  may preferably have a coil-like or serpentine-like shape with multiple windings. Preferably, one end of the circuit  422  may be connected to a first voltage potential, such as a power supply and another end may be connected to a second voltage potential, such as ground (not shown). 
         [0040]    As illustrated in  FIG. 4C-E , it may be preferable to minimize the spacing between the windings such that the circuit  422  covers substantially the entire surface of the second insulating layer  426 . In the process, the circuit  422  may provide heat to substantially the entire portion of the volume  404  of the ion source  400 . However, those of ordinary skill in the art should recognize that the spacing between windings may preferably be wide enough to prevent the current from, for example, tunneling through the spacing. While these figures show the insulating layer  426  as having the same general contour as the underlying circuit  422 , this is not required. The insulating layer  426  may be formed so as to cover the circuit  422  on its inner side, while being smooth on its outer side. 
         [0041]    When powered, current may flow from the first voltage potential, such as a power supply, to the second voltage potential, such as ground, via the windings of the circuit  422 . By disposing the first and second insulating layers  424  and  426  on the circuit  422 , and by sufficiently spacing the windings, the flow of the current may be limited to the circuit  422 . The circuit  422  may act as a resistive heating unit, converting the current to heat and emitting heat toward the volume  404  where ions  10  are generated. 
         [0042]    One advantage of the heating unit  420  of the present embodiment is that the circuit  422  may provide uniform heating to the ion source  400 , whether the IHC  408  or the repeller  410 , or the optional IHCs are powered. In addition, by disposing the circuit  422  toward the volume  404 , the heating unit  420  may emit heat in one direction. As such, heat loss may be minimized. 
         [0043]    Referring to  FIG. 5A-5E , there is shown another ion source  500  according to another embodiment of the present disclosure. Much like the ion sources  200 - 400  of the earlier embodiments, the ion source  500  of the present embodiment may comprise an IHC  508  and a repeller  510 . Similar to the ion source  400  of the earlier embodiment, the ion source  500  may be a dual mode ion source comprising a second IHC (not shown) disposed opposite to the IHC  508 . Referring to  FIG. 5A , the ion source  500  of the present embodiment also comprises a plurality of walls  502  defining a volume  504  where the ions  10  are generated. In addition, an optional liner  505  may be positioned between the walls  502  and the volume  504 . Further, an extraction aperture  506  may be disposed on one of the walls  502 . For purpose of clarity, a detailed description of similar features will be omitted. 
         [0044]    Referring to  FIG. 5B , a detailed cross sectional view of the heating unit  520  is provided. In the present embodiment, at least one of the walls  502  may be a heating unit  520  of the ion source  500 . The heating unit  520  of the present embodiment may comprise an electricity conducting circuit  522  disposed on a base  528 . In one embodiment, the circuit  522  may be a graphite circuit  522 . However, in another embodiment, the circuit  522  may be a circuit made from another electricity conducting material, such as tungsten. In some embodiments, the base  528  may be made from an electricity conducting material. In other embodiments, the base  528  of the heating unit  520  may be made from an electricity insulating material. If the base  528  is made from an electricity conducting material, it may be preferable to position a layer of electricity insulating material (not shown) between the circuit  522  and the base  528 . 
         [0045]    Similar to the liners of the earlier embodiments, the liner  505  may preferably be made from an inert material capable of withstanding reactive and corrosive conditions associated with plasma. If the liner  505  is made from electricity conducting material, the liner  505  may be, for example, graphite, tungsten, tantalum, or molybdenum. The liner  505  may be spaced apart from the circuit  522  such that the space between the liner  505  and the circuit  522  may act as an insulator, preventing current from flowing to the liner  505 . Alternatively, an electricity insulating layer (not shown) may be disposed between the liner  505  and the circuit  522 . In some embodiments, the liner  505  may be omitted. In such embodiments, the circuit  522  may be made from a liner material (e.g. graphite) and the circuit  522  may also act as the liner. 
         [0046]    Referring to  FIG. 5C-5E , plan views, the circuit  522  may preferably have a coil-like or serpentine-like shape with multiple windings. One end of the circuit  522  may preferably be connected to a first voltage potential, such as a power supply and another end may be connected to a second voltage potential, such as ground (not shown). As illustrated in  FIG. 5C-E , it may be preferable to minimize the spacing between the windings such that the circuit  522  may cover substantially the entire surface of the base  528 . 
         [0047]    When powered, current may flow from the power supply to the ground via the windings of the circuit  522 . The circuit  522  may act as a resistive heating unit, converting the current to heat and emitting heat toward the volume  504  where ions  10  are generated. 
         [0048]    One advantage of the heating unit  520  of the present embodiment is that the circuit  522  may provide uniform heating to the ion source, even if the IHC  508 , the repeller  510  or the optional IHC remains passive. In addition, by disposing the circuit  522  near the inner volume  504  or within the ion source  500 , the heat generated by the heating unit  520  may be directed toward the inner volume  504 . As such, heat loss may be minimized. 
         [0049]    Hereinafter, operation of the ion sources  200 - 500  of the present disclosure is disclosed with reference to  FIG. 2-5 . In operation, one or more types of feed gas are provided to the ion source  200 - 500 . Examples of feed gas may include H 2 , He, carborane, diborane, BF 4 , GeF 4 , SiF 4 , Si 2 F 6 , BF 3 , SF 6 , S 2 F 6 , SF 4 , AsH 3 , and PH 3 . 
         [0050]    The indirectly heated cathode  208 - 508  of the ion source  200 - 500  may then be powered to ionize the feed gas. In particular, the filament  208   c - 508   c  of the IHC  208 - 508  may be biased to excite the cathode  208   a - 508   a  into emitting heat and/or electrons via thermionic emission. Meanwhile, the repeller  210 - 510  may be biased and maintained at the same voltage as the cathode. Maintaining the repeller  210 - 510  and the cathode at the same voltage may electrostatically confine the electrons within the volume  204 - 504  of the ion source. If the ion source  200 - 500  is a dual mode or multi-mode ion source comprising additional IHCs, the additional IHC may also be biased at the same voltage to that of the cathode  208   a - 508   a.    
         [0051]    Electrons emitted by the IHC  208 - 508  and confined within the volume  204 - 504  of the ion source  200 - 500  may excite the feed gas into plasma containing the ions  10  of desired species. Depending on the type of applications, the ions  10  of the desired species may be atomic species or, alternatively, molecular species. The ions  10  generated are then extracted from the ion source  200 - 500  via the extraction aperture  206 - 506 . Thereafter, the extracted ions  10  may be directed toward a substrate to be processed, positioned downstream of the beam-line components. 
         [0052]    While ionizing the feed gas, the heating unit  220 - 520  may also be activated such that the heating unit  220 - 520 , along with the indirectly heated cathode  208 - 508 , may maintain the temperature of the ion source  200 - 500  uniformly. For hydride applications (e.g. PH 3 ), it may be desirable to maintain the temperature at about 800° C. or above. The heat may prevent the generated ions  10  from neutralizing or condensing to form deposits on the source chamber wall and/or the extraction electrodes. Formation of such deposits may be undesirable as the deposits may cause unstable source operation, glitching, uniformity degradation and source failure. 
         [0053]    In the application of heavier molecular species, including but not limited to carborane (such as C 2 B 10 H 12 ) and diborane (B 2 H 6 ), the cathode may operate at lower power so that these heavy molecular species do not break into smaller species. However, lower cathode power causes lower temperatures within the ion source, which may cause condensation and deposition. The heating unit  220 - 520  may be used to supplement or augment the heat provided by the cathode  208 - 508  so as to maintain the temperature above a first predetermined temperature to prevent the molecular ion species  10  in the ion source  200 - 500  from neutralizing and condensing. Other heavier molecular species may include carborane (C x B y H z ); any molecule containing boron, hydrogen and another atom or chain of atoms (e.g. X y B x H z ); decaborane, octadecaborane, any other borohydride; any molecule containing germanium and hydrogen (e.g. Ge x H y ); and carbon containing molecules such as ethane, propane, pyrene and bibenzyl. 
         [0054]    Generally, the conventional IHC ion sources rely only on the cathode for both ionization and heating. In such ion sources, the ions  10  positioned near cathode may be at a desirable temperature to prevent condensation and formation of deposits. The ions  10  or gas molecules located away from the cathode (e.g. near the extraction aperture or extraction electrode), however, may be at a temperature lower than the desired temperature, and may condense to form deposits. Accordingly, conventional IHC sources may have heavy deposit at regions away from the cathode (e.g. near the extraction aperture or extraction electrode). Simply raising the temperature of the cathode in conventional IHC ion sources, however, may lead to overheating near the cathode. Such overheating may be undesirable for the application of molecular species, as the overheating may promote breaking down of the molecular species into smaller species. 
         [0055]    In the present disclosure, the ion source  200 - 500  provides a heating unit  220 - 520  that is decoupled from the cathode and independently controlled. 
         [0056]    In some embodiments, current applied to the circuit  222 - 522  of the heating unit  220 - 520  may be varied. By varying the current, it is possible to control the heat emitted by the circuit and therefore the temperature of the ion source  200 - 500 . A variable power supply may be used to provide this type of temperature control. In some embodiments, a temperature sensor may be provided in the ion source  200 - 500  or at the wall  202 - 502 . In this way, the current provided by the power supply may be determined via a closed control loop. In another embodiment, the current supplied by the power supply is varied based on the type of species that is being ionized. Accordingly, temperature in different regions of the ion source  200 - 500  may be independently controlled. Thus, uniform temperature in the ion source  200 - 500 , if desired, may be provided. Overheating and/or underheating at different regions of the ion source  200 - 500  may be avoided. 
         [0057]    In addition to the heating unit  220 - 520 , the ion source  200 - 500  of the present disclosure may also efficiently clean and/or prevent deposition of unwanted coating. In particular, one or more types of cleaning gas may be introduced to the ion source  200 - 500 . The cleaning gas may be gas containing a reactive species, reactive in its ionized or neutral form, capable of preventing formation of the deposits or removing the formed deposits. For example, the cleaning gas, in its ionized or neutral form, may be an etchant capable of etching the coating formed on the ion source and preventing accumulation of the coating. In another example, the cleaning gas, in its ionized or neutralized form, may be capable of chemically reacting with the ions or neutrals of the feed gas to prevent formation or deposition of coating on the internal walls of the ion source  200 - 500 . Examples of the cleaning gas may include H 2 ; chlorine containing gas such as Cl 2 ; fluorine containing gas such as CF 4 , NF 3 , BF 3  and SF 6 ; nitrogen containing gas such as air, NF 3 , NO, N 2 O, NO 3 , N 2 O 3 , NO 3 F, NOBr, NOF, and NO 2 F; oxygen containing gas such as O 2 , O 3 . Those of ordinary skill in the art will recognize that other types of cleaning gas may be also used. In addition, those of ordinary skill in the art will recognize that several examples of the cleaning gas may also function as the feed gas or vice versa. 
         [0058]    In one embodiment of the present disclosure, the cleaning gas may be introduced to the ion source  200 - 500  before and/or after the introduction of the feed gas. In another embodiment, the cleaning gas and the feed gas may be introduced to the ion source  200 - 500  simultaneously. Yet in another embodiment, the cleaning gas may be introduced before, during, and after the introduction of the feed gas. If the cleaning gas is introduced more than once, the same type of the cleaning gas need not be introduced each time. Different types of the cleaning gas may be introduced. By introducing the cleaning gas into the ion source  200 - 500  prior to introducing the feed gas, the cleaning gas may remove coating from prior ion implantation processes. By introducing the cleaning gas into the ion source  200 - 500  after introducing the feed gas, the cleaning gas may remove coating formed by ionizing the feed gas. And, by introducing the cleaning gas and the feed gas simultaneously, the cleaning gas may prevent the formation of the coating and/or may remove the formed coating. 
         [0059]    In one embodiment of the present disclosure, the cleaning gas may be introduced to the ion source  200 - 500  as the substrate is being ion implanted. In other words, the cleaning process may occur simultaneously with the ion implantation process. In another embodiment, the cleaning process may occur before and/or after the implantation process, when the ion implantation process is not being performed. In the latter embodiment, the cleaning gas may be introduced as a maintenance process. Yet in another embodiment, the cleaning gas may be introduced before, after, and during ion implantation process. 
         [0060]    When introducing the cleaning gas, it may be preferable to introduce the cleaning gas at high pressure. In addition, maintaining the pressure level within the ion source  200 - 500  at high pressure level may be desirable. Maintaining high pressure level may be achieved by continuously introducing gas at high pressure level and compensating for a portion escaping through the extraction aperture  206 - 506 . Alternatively, the extraction aperture  206 - 506  may be blocked. One example of latter configuration may be found in a U.S. patent application Ser. No. 12/143,247, which is incorporated in its entirety by reference. 
         [0061]    In addition to the pressure level, it may also be preferable to maintain the temperature level in the ion source  200 - 500  at high temperature level. Hereinafter, operation to maintaining the temperature of the ion source  200 - 500  at high temperature level will be disclosed. First, the cleaning gas may be introduced to the ion source  200 - 500 . As noted earlier, the cleaning gas may be introduced with or without the feed gas. Thereafter, at least one of the IHC  208 - 508 , the repeller  210 - 310 , and the heating unit  220 - 520  may be powered. If the ion source  200 - 500  is a dual mode or multi-mode ion source comprising the additional IHCs, the additional IHCs may also be biased at the same voltage to the IHC  208 - 508  emitting electrons. At least one of the IHC  208 - 508 , the repeller  210 - 510  (or the additional IHC), and the heating unit  220 - 520  may raise the temperature of the cleaning gas contained in the ion source  200  and  300  and excite the cleaning gas. In one embodiment of the present disclosure, the cleaning gas may be excited without being ionized. In another embodiment, the cleaning gas may be excited and converted into plasma by the electrons emitted from the IHC  208 - 508 . 
         [0062]    If a variable power supply is utilized, the temperature of the heating unit  220 - 520  may be varied as a function of the operation being performed. For example, it may be desirable to maintain a certain temperature range while feed gas is being ionized. However, a second temperature range, such as a higher temperature range, may be beneficial during a cleaning process. 
         [0063]    The cleaning gas, in its excited state, may etch the coating deposited on the surface of the ion source  200 - 500  or may chemically react with the feed gas to prevent the formation of the coating. Thereafter, the cleaning gas, in its natural or chemically reactive form, may be extracted from the ion source  200 - 500 . The extracted cleaning gas, however, may be prevented from reaching the substrate by one of the beam-line components of the ion implantation system. Alternatively, a vacuum pump, for example, a turbo-molecular pump, (not shown) may be provided near to the extraction aperture  206 - 506 , and the vacuum pump may evacuate the cleaning gas from the ion implantation system. 
         [0064]    One specific method used to clean the ion source  200 - 500  will be described. After ion implantation has been completed, the flow of feed gas to the ion source  200 - 500  is ceased. The voltage applied to the extraction electrodes  106  may be disabled, so that ions are not drawn from the ion source  200 - 500 . A cleaning gas may then be introduced into the ion source  200 - 500 . In some embodiments, this cleaning gas is introduced at high pressure, as described above. The IHC  208 - 508  is then powered so as to produce plasma of the cleaning gas. The repeller  210 - 510  may also be powered in this embodiment. This action creates ions of the cleaning gas, which may be effective in removing film from the walls  202 - 502 . To further enhance the cleaning process, the heating units  220 - 520  may be powered to increase the temperature within the ion source. Such an increase in temperature may increase the reactivity of the cleaning gas. The temperature within the ion source  200 - 500  during the cleaning process may be the same or different than that used during the ion implantation process. 
         [0065]    Several embodiments of an ion source and a method for in situ cleaning the ion source are disclosed. Those of the art will recognize that the present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. For example, the heating unit described in the present disclosure may be activated along with the indirectly heated cathode. By activating the heating unit and the indirectly heated cathode at the same time, the heating unit may maintain the temperature of the ion source uniform and above which the feed gas may condense into deposits to prevent formation of deposits. In another example, the heating unit may be activated during cleaning stage, when the indirectly heated cathode is turned off. The heating unit may raise the temperature of the ion source and beak up any deposits formed on the wall of the ion source, thereby cleaning the ion source. Those of ordinary skill in the art will also recognize that the feed gas and the cleaning gas may be introduced to the ion source together or alone. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Further, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein.