Abstract:
An ion source includes an ion source chamber having a longitudinal axis, the ion source chamber operative to define a plasma therein. The ion source also includes a split solenoid assembly comprising a first solenoid and a second solenoid that are mutually disposed along opposite sides of the ion source chamber, where each of the first solenoid and second solenoid comprises a metal member having a long axis parallel to the longitudinal axis of the ion source chamber, and a main coil having a coil axis parallel to the long axis and comprising a plurality of windings that circumscribe the metal member. The main coil defines a coil footprint that is larger than an ion source chamber footprint of the ion source chamber.

Description:
FIELD 
       [0001]    This disclosure relates to ion implantation and semiconductor fabrication. More particularly, the present disclosure and in particular to improved ion sources. 
       BACKGROUND 
       [0002]    In high volume manufacturing processes such as semiconductor device fabrication and solar cell manufacturing, there is a continuing need to improve substrate throughput. This places a demand to improve throughput for processes including ion implantation. In one example, as the size of silicon wafers continues to scale upwardly, ion sources having a much larger current output are needed to meet required wafer throughput. 
         [0003]    Beamline ion implantation apparatus may employ indirectly heated cathode (IHC) ion sources or other sources in which an elongated aperture is used to extract an ion beam. One manner of achieving higher ion current for implantation is to employ an ion source having a longer extraction aperture for a given ion density so that a greater total current may be extracted from the ion source. Dipole magnets are used to generate magnetic fields to enhance plasma density in conventional ion sources such as IHC sources that have more compact extraction optics where the extraction aperture is typically less than about 100 mm in length. However, such dipole magnets do not generate desired beam uniformity in elongated ion sources where the extraction aperture is longer. In view of the above, it will be appreciated that there is a need to improve ion implantation apparatus, and in particular to develop ion source technology to increase the current generating capability in the ion source while maintaining acceptable ion beam properties. 
       SUMMARY 
       [0004]    This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended as an aid in determining the scope of the claimed subject matter. 
         [0005]    In one embodiment, an ion source may include an ion source chamber having a longitudinal axis, the ion source chamber operative to define a plasma therein. The ion source may also include a split solenoid assembly comprising a first solenoid and a second solenoid that are mutually disposed along opposite sides of the ion source chamber, where each of the first solenoid and second solenoid comprises a metal member having a long axis parallel to the longitudinal axis of the ion source chamber, and a main coil having a coil axis parallel to the long axis and comprising a plurality of windings that circumscribe the metal member. The main coil defines a coil footprint that is larger than an ion source chamber footprint of the ion source chamber. 
         [0006]    In a further embodiment, an ion implantation system for implanting a substrate includes an ion source chamber having a longitudinal axis, the ion source chamber operative to define a plasma therein. The ion implantation system also includes a split solenoid assembly comprising a first solenoid and a second solenoid that are mutually disposed along opposite sides of the ion source chamber. Each of the first solenoid and second solenoid may include a metal member having a long axis parallel to the longitudinal axis of the ion source chamber, a main coil having a coil axis parallel to the long axis and comprising a plurality of windings that circumscribe the metal member, the main coil defining a footprint that covers the ion source chamber; and beam components to direct a beam of ions extracted from the ion source chamber to the substrate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  depicts an ion implantation system consistent with various embodiments of the disclosure; 
           [0008]      FIG. 2A  depicts a side view of an ion source consistent with the present embodiments; 
           [0009]      FIG. 2B  depicts a top view of the ion source of  FIG. 3A ; 
           [0010]      FIG. 2C  depicts an end view of the ion source of  FIG. 3A ; 
           [0011]      FIG. 3  depicts a perspective view of another ion source consistent with additional embodiments; 
           [0012]      FIG. 4A  depicts a side view during operation of the ion source of  FIG. 3  consistent with the various embodiments; 
           [0013]      FIG. 4B  depicts a top view of the scenario of operation of the ion source shown in  FIG. 4A ; 
           [0014]      FIG. 4C  depicts an end view of the scenario of operation of the ion source shown in  FIG. 4A ; 
           [0015]      FIG. 5A  illustrates variation of magnetic field intensity in an ion source configured according to embodiments of the disclosure; 
           [0016]      FIG. 5B  illustrates a comparison of experimental and simulated variation of magnetic field intensity in an ion source configured according to embodiments of the disclosure; 
           [0017]      FIG. 6A  depicts magnetic field in an ion source in one exemplary configuration of ion source chamber and split solenoid assembly; 
           [0018]      FIG. 6B  depicts magnetic field in an ion source in another exemplary configuration of ion source chamber and split solenoid assembly; and 
           [0019]      FIG. 7  depicts an end view of another embodiment of an ion source. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention, however, may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, like numbers refer to like elements throughout. 
         [0021]    Various embodiments involve apparatus and systems to produce high current ion sources. Referring to the drawings,  FIG. 1  is a block diagram of an ion implantation system  100  including an ion source  102 . A power supply  101  supplies the required energy to source  102  which is configured to generate ions of a particular species. The generated ions are extracted from the source through a series of electrodes  104  (extraction electrodes) and formed into a beam  95  which is directed and manipulated by various beam components  95 ,  106 ,  108 ,  110 ,  112  to a substrate. In particular, after extraction, the beam  95  passes through a mass analyzer magnet  106 . The mass analyzer is configured with a particular magnetic field such that only the ions with a desired mass-to-charge ratio are able to travel through the analyzer. Ions of the desired species pass through deceleration stage  108  to corrector magnet  110 . Corrector magnet  110  is energized to deflect ion beamlets in accordance with the strength and direction of the applied magnetic field to provide a ribbon beam targeted toward a work piece or substrate positioned on support (e.g. platen)  114 . In some cases, a second deceleration stage  112  may be disposed between corrector magnet  110  and support  114 . The ions lose energy when they collide with electrons and nuclei in the substrate and come to rest at a desired depth within the substrate based on the acceleration energy. 
         [0022]    The present embodiments may be implemented in ion implantation systems, such as ion implantation system  100 . In particular, the present embodiments may be implemented using a novel “split solenoid” ion source as described herein below.  FIGS. 2 to 4C  depict embodiments of split solenoid ion sources which may be used as the ion source  102  of the ion implantation system  100  in various embodiments. In other embodiments, the split solenoid ion sources as stand-alone devices or may be deployed in any other apparatus that employs ion sources. 
         [0023]    The terms “split solenoid” and “split solenoid assembly” refer to a configuration or magnetic assembly that includes two or more separate main coils having axes that are generally aligned parallel to one another, where each coil is wound round a metal piece or member. The two or more main coils impart solenoid like properties to a region or space between the two or more main coils, which space contains an ion source chamber. However, rather than circumferentially enclosing the ion source chamber as in an ideal cylindrical solenoid, the two or more “split solenoids” of a split solenoid assembly only bound the ion source along separate portions that are separated by open spaces. This facilitates convenient extraction of an ion beam from the ion source chamber that is bounded by the split solenoid assembly. 
         [0024]    In a given solenoid of a split solenoid assembly, each main coil surrounds a metal member that is relatively long in two dimensions and relatively short in a third dimension. Notably, a main coil, together with its metal member may be referred to herein as a “solenoid.” The solenoids may generally have a planar shape but may also be curved at least along one direction as shown in  FIG. 7 . As detailed below, this magnetic assembly is used to generate uniform magnetic fields in a space between the main coils. This allows the length of such split solenoid ion sources to be scaled up to a large size not achieved by conventional by conventional dipole magnetic structures that are used to generate magnetic fields in conventional ion sources. 
         [0025]    Turning to  FIG. 2A  there is shown a side view of a split solenoid ion source  200 .  FIG. 2B  shows a top view of the split solenoid ion source  200 , while  FIG. 2C  shows an end view of the split solenoid ion source  200 . As shown in the figures, the split solenoid ion source  200  includes an ion source chamber  202 , which is bounded by a split solenoid assembly  203  that includes a solenoid  204  located on one side of the ion source chamber  202  and another solenoid  204  located on an opposite side. The ion source chamber  202  may be generally constructed according to known technology. The ion source chamber  202  and solenoids may be affixed to other structures (not shown). In various embodiments the ion source chamber  202  may be constructed as Bernas type ion source, indirectly heated cathode (IHC) ion source, or other type of in source. The embodiments are not limited in this context. The ion source chamber  202  is characterized by a longitudinal axis  212  (also termed “long axis”), which is parallel to the X-direction in the Cartesian coordinate system used in the figures. The ion source chamber is generally elongated along this longitudinal axis  212 , and may extend up to one half meter or more in some embodiments. 
         [0026]    As illustrated, the solenoids  204  each have flat faces  214  that face one another and extend so as to create a footprint  216  that encompasses the ion source chamber  202 , as illustrated in  FIG. 2B . In particular, the footprint  216  represents a projected area of the main coils  205  in the X-Z plane. As detailed below, the solenoids  204  are configured to generate uniform magnetic fields in the ion source chamber  202 , which facilitates the production of more uniform ion beams, in addition to affording scalability of such ion sources to larger dimension. In this manner higher current ion beams having acceptable beam uniformity are achievable using the split solenoid ion source  200 . 
         [0027]    In the present embodiments, a solenoid may include a main coil and a set of optional trim coils. This is illustrated in particular in  FIGS. 2A to 2C , which depict a main coil  205  that is wound around a flat metal plate  208 . The flat metal plate  208  is elongated in a direction parallel to the longitudinal axis  212  of the ion source chamber  202 . The coil axis  207  of the main coil  205  is generally parallel to the longitudinal axis  212  of the ion source. The flat metal plate acts to block magnetic fields generated by outer portions of each coil from extending into the region containing the ion source chamber  202 . In various embodiments the flat metal plate  208  may be a steel or similar metal. As illustrated, the flat metal plate extends beyond outer ends of the coils of each solenoid  204  so as to screen out magnetic fields generated by outer portion  218  of each solenoid from penetrating into the region  220  between the respective solenoids  204 . Accordingly, when current is sent from the current source  224  to the main coils  205 , a magnetic field that is generated by opposing main coils  205  and penetrates the ion source chamber  202  is generated from inner portions  222  of each main coil  205 , as shown in  FIG. 2A . 
         [0028]    In various embodiments, in addition to the main coil  205 , a pair of trim coils  206  are included at opposite ends of each solenoid  204 . As shown in  FIG. 2A , the trim coil axis  209  of a trim coil  206  is aligned with the coil axis  207  of a main coil  205 . Each trim coil  206  is coupled to a current source  226  that is separate from the current source  224 . In this manner, the trim coils  206  are configured to receive, if desired, a different amount of current as compared to that sent to the main coils  205 . Although the current direction of current sent to the main coils  205  and trim coils  206  may generally be the same, the current in trim coils  206  may be generated in a direction opposite to that of the current in main coils  205 . As detailed below, the trim coils  206  may be used to adjust magnetic fields produced in the vicinity of the ion source chamber  202 . 
         [0029]    Notably, the split solenoid ion source  200  provides advantages over conventional ion sources that employ dipole source magnets. The split solenoid ion source  200  in particular embodies useful properties of an ideal solenoid. In an infinitely long ideal solenoid the magnetic field inside is homogeneous and magnetic field strength does not depend on distance from the solenoid axis. Thus, an ideal cylindrical solenoid magnet that encompasses an ion source chamber may produce uniform magnetic fields therein. However, extraction of ions from an ion source chamber within an ideal solenoid is not practical because of the complete envelopment by the solenoid of the ion source chamber except along its ends. 
         [0030]    By providing a split solenoid assembly that contains two solenoids the split solenoid ion source  200  combines the benefits of a relatively uniform magnetic field as in an ideal solenoid with an easily accessibly ion source chamber  202  from which a uniform ion beam may be readily extracted, as discussed further below. In particular variants of the split solenoid ion source  200  may provide an almost uniform magnetic field within the ion source chamber  202 , including a nearly parallel arrangement of magnetic field lines in the region of the ion source chamber  202  from which an ion beam is extracted. This enables the ability to scale the ion source chamber  202  size by simply extending the length of the split solenoid assembly that flanks such an ion source chamber. 
         [0031]    Consistent with the present embodiments, a length of the split solenoid assembly along the longitudinal axis  212  may range from 250 mm to 2000 mm, and the length L S  of the ion source chamber is about 100 mm to 500 mm, while the aperture length L A  of an aperture  211  of the ion source chamber  202  is less than or equal to L S . Moreover, for a given split solenoid ion source, such as split solenoid ion source  202 , the length of the split solenoid assembly  203  along the longitudinal axis  212  is generally greater than L S . 
         [0032]    Consistent with further embodiments,  FIG. 3  depicts a perspective view of another split solenoid ion source  250  in operation. The split solenoid ion source  250  contains an ion source chamber  252  and a split solenoid assembly  253  that includes a pair of solenoids  255  that extend on two opposite sides of the ion source chamber  252 . In this embodiment, the solenoids  255  each include a main coil  256  and a pair of trim coils  258  that are arranged similarly to the arrangement of a split solenoid shown in  FIGS. 2A-2C . In particular the main coils  256  and trim coils  258  are each wound around an elongated flat metal member  260  whose long direction extends parallel to the longitudinal axis  262  of the ion source chamber  252 . When current is drawn through the solenoids  255  a uniform magnetic field  272  is generated through the center of the ion source chamber  252 . As further illustrated in  FIG. 3 , when a plasma (not shown) is generated in the ion source chamber  252  an ion beam containing the ions  264  may be extracted from the ion source chamber. Due to the uniform magnetic field  272 , the ion beam may be uniform over its width when extracted. 
         [0033]      FIGS. 4A ,  4 B, and  4 C depict a side view, top view, and end view respectively of the split solenoid ion source  250  that highlight further advantages of the present embodiments. In  FIG. 4A , an aperture  254  in the ion source chamber  252  is also depicted. Referring also to  FIG. 4C , ions generated in a plasma  266  are extracted through the aperture  254  and may be accelerated by an extraction system (not shown) to direct the ions  264  as a beam of ions having a desired energy. The aperture  254  is characterized by an aperture length L A  along the longitudinal axis  262  of the ion source chamber  252 . The length of the aperture may be used to define the initial size or width of the ion beam formed by ions  264  as the ions  264  are extracted from the ion source chamber  252 . 
         [0034]    By scaling upwardly the length L S  of the ion source chamber  252 , the aperture length L A  can be concomitantly scaled upwardly to increase the size of a beam of ions  264 . For a given plasma density, this may lead to a proportional upward scaling of ion current with increased L S . Because such an ion source chamber in principle only needs an increase in length along the X-direction, scaling of ion sources constructed according to the present embodiments for larger current production is straightforward. In the example particularly illustrated in  FIG. 4A , the length L S  of the ion source chamber  252  is 325 mm. An experimental embodiment has produced an operational ion source chamber of similar dimensions having an aperture length L A  of 250 mm for an IHC ion source, thereby increasing the current capability over conventional IHC ion sources that are typically less than 100 mm in length. For example, a conventional apparatus based upon an IHC source having a 55 mm extraction aperture yields about 50 mA current, while an apparatus designed according to the present embodiments having a 225 mm aperture yields 120 mA or more of high quality beam current. 
         [0035]    In addition, the present embodiments provide for increased uniformity of magnetic fields within an ion source even when sources are scaled to large dimensions, such as dimensions greater than 100 mm in length.  FIG. 5A  compares magnetic uniformity of a split solenoid ion source constructed according to the present embodiments with that of a conventional dipole magnet ion source. The curve  502  represents the magnetic field strength (in Tesla) as a function of position for a dipole magnet source showing the calculated variation in magnetic field in a middle region of an ion source along the X-direction over a 350 mm range, which approximates the length of the aforementioned 325 mm IHC source. In this case the magnetic field strength is greatest at the extremities of the ion source (−175 mm and +175 mm) and decreases by about two thirds at the center region. 
         [0036]    In contrast to this extreme non-uniformity in magnetic field strength, the curves  504  and  506  present calculated magnetic field strength for a split solenoid ion source over the same range as for the dipole magnet case, showing that magnetic strength varies by less than 10% over the entire 350 mm range. Magnetic fields of about 200 Gauss (0.02 Tesla) are achievable in embodiments of a split solenoid ion source. In particular, the curve  504  represents magnetic field strength when no current is supplied to the trim coils while curve  506  represents magnetic field strength when a fixed amount of current is supplied to the trim coils. When no current is supplied to the trim coils, the magnetic field strength (curve  504 ) exhibits a “frown” shape in which magnetic field strength peaks in the center, while when a specific amount of current is supplied to the trim coils the magnetic field strength (curve  506 ) exhibits a “smile” shape in which the magnetic field strength reaches a minimum in the center. It is to be noted that the level of current supplied to trim coils may be used to further adjust the shape of magnetic field strength as a function of position so that the frown of smile can be minimized. 
         [0037]      FIG. 5B  provides further details showing a comparison of simulated and experimental data for magnetic field strength uniformity produced in a split solenoid ion source. The curve  512  illustrates simulated magnetic field strength along the X-direction with no trim coil current, while the curve  514  illustrates measured magnetic field strength with no trim coil current. The curve  516  illustrates simulated magnetic field strength with trim coil current, while the curve  518  illustrates measured magnetic field strength with trim coil current. In the case of curve  518 , when trim coil current is applied, the maximum variation in magnetic field strength is only about 5%. 
         [0038]    In addition to reducing variation in magnetic field strength along the long direction (parallel to the X-axis) of an ion source, the split solenoid ion source design of the present embodiments facilitates the ability to adjust the magnetic field direction in different portions of an ion source for optimal beam geometry.  FIGS. 6A and 6B  present the results of simulation of magnetic field shape in an ion source chamber  602  for a split solenoid ion source consistent with the present embodiments. The figures present a top view in the X-Z plane where the ion beam (not shown) exits toward the top of the page. In  FIG. 6A , a split solenoid ion source  600  includes the ion source chamber  602  whose center along the Z-direction is aligned with the longitudinal axis  606  of the split solenoid  604 . When the ion source chamber  602  has its center aligned with the center of the split solenoid  604 , the magnetic field lines  608  (shown in dashed form) near the faceplate (faceplate and extraction apparatus are not shown) edge  610  are substantially curved, showing an outward bulge toward the center of the faceplate edge  610 . Because the faceplate edge  610  is disposed toward the extraction side  612  where ions exit the split solenoid ion source  600 , magnetic field line curvature may affect the trajectories of exiting ions. 
         [0039]    In  FIG. 6B , a split solenoid ion source  620  includes the same ion source chamber  602  whose center is now shifted forward by 15 mm along the Z-direction with respect to the longitudinal axis  606  of the split solenoid  604 . In this case, the magnetic field lines  608  near the faceplate edge  610  are substantially straight. This latter embodiment may be useful where it is desired to generate parallel ion trajectories across a width of an ion beam extracted from the split solenoid ion source. 
         [0040]    In addition to the generally planar split solenoid ion sources disclosed hereinabove, the present embodiments include solenoid ion sources in which a pair of solenoids have a curved cross-section as illustrated in  FIG. 7 . The split solenoid ion source  700  of  FIG. 7  may have the general shape of the split solenoid ion source  200  except that the opposing pair of solenoids  702  are curved as viewed along the end view shown. In this case, each coil  704  surrounds a curved (though not planar) plate  706 . The solenoids  702  are generally curved inwardly so as to lie on portions of a curve that surround the ion source chamber  202 . 
         [0041]    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. 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.