Patent Publication Number: US-2023162941-A1

Title: Shield For Filament In An Ion Source

Description:
FIELD 
     Embodiments of the present disclosure relate to shields for a filament, and more particularly a filament in a Bernas source. 
     BACKGROUND 
     Semiconductor devices are fabricated using a plurality of processes, some of which implant ions into the workpiece. Various ion sources may be used to create the ions. One such ion source is a Bernas ion source. A Bernas ion source comprises a filament disposed in a chamber. As current is passed through the filament, the filament emits thermionic electrons into the chamber of the ion source. The filament is disposed at one end of a chamber. An extraction aperture may be disposed on the end opposite the filament. 
     In certain embodiment, a particular configuration of a Bernas ion source, referred to as a multicusp ion source, may be utilized to generate negative ions, and more particularly negative hydrogen ions. 
     Because the filament is directly exposed to the plasma, it may be back heated by the energetic ions in the plasma. At high plasma densities, this may cause the filament to become hotter than desired, potentially causing thermal runaway. Thus, in certain embodiments, active control of the current passing through the filament is performed. However, this complicates the control of the ion source and may result in instabilities. 
     Therefore, it would be beneficial if there was a Bernas ion source in which the back heating of the filament could be controlled so as minimize the possibility of thermal runaway. It would be advantageous if this system could be readily incorporated in existing Bernas and multicusp ion sources. 
     SUMMARY 
     A Bernas ion source having a shield is disclosed. The shield is disposed between the distal portion of the filament and the first end of the chamber and serves to confine the plasma to the region between the shield and the second end of the chamber. The shield may be electrically connected to the negative leg of the filament so as to be the most negatively biased component in the chamber. In other embodiments, the shield may be electrically floating. In this embodiment, the shield may self-bias. The shield is typically made of a refractory metal. The use of the shield may reduce back heating of the filament by the plasma and reduce the possibility for thermal runaway. This may allow denser plasmas to be generated within the chamber. 
     According to one embodiment, an ion source is disclosed. The ion source comprises a chamber comprising a first end, a second end and walls connecting the first end and the second end, the second end having an extraction aperture; a filament disposed in the chamber, extending from the first end and having a negative leg, a second leg and a distal portion connected to the negative leg and the second leg; a filament power supply in communication with the negative leg and the second leg; and a shield disposed between the first end and the distal portion of the filament, wherein the shield is electrically connected to the negative leg. In some embodiments, the ion source comprises an electrode disposed outside the chamber and proximate the extraction aperture, having an electrode aperture; and an electrode power supply to provide a voltage to the electrode. In certain embodiments, the electrode is biased positively relative to the chamber so as to attract negative ions and electrodes through the extraction aperture. In some embodiments, the ion source is a multicusp ion source and comprises magnets disposed along the walls. In certain embodiments, the shield comprises a round plate. In certain embodiments, the shield comprises a metal mesh or a plate comprising a plurality of holes. In some embodiments, the filament comprises a plurality of filament elements, disposed at different heights, and wherein the shield comprises a multitier structure. In some embodiments, the multitier structure comprises a lower tier comprising a cylinder having a first height and a first diameter; and an upper tier comprising a concentric cylinder having a second height and a second diameter, smaller than the first diameter. 
     According to another embodiment, an ion implantation system is disclosed. The ion implantation system comprises the ion source described above; a mass analyzer to receive ions extracted from the ion source; and a tandem accelerator to accelerate ions exiting the mass analyzer. 
     According to another embodiment, an ion source is disclosed. The ion source comprises a chamber comprising a first end, a second end and walls connecting the first end and the second end, the second end having an extraction aperture; a filament disposed in the chamber, extending from the first end and having a negative leg, a second leg and a distal portion connected to the negative leg and the second leg; a filament power supply in communication with the negative leg and the second leg; and a shield disposed between the first end and the distal portion of the filament, wherein the shield is a refractory metal. In some embodiments, the shield is electrically connected to the negative leg. In some embodiments, the shield is electrically floating. In some embodiments, the ion source comprises an electrode disposed outside the chamber and proximate the extraction aperture, having an electrode aperture; and an electrode power supply to provide a voltage to the electrode. In certain embodiments, the electrode is biased positively relative to the chamber so as to attract negative ions and electrodes through the extraction aperture. In some embodiments, the ion source is a multicusp ion source and comprises magnets disposed along the walls. In certain embodiments, the shield comprises a round plate. In certain embodiments, the shield comprises a metal mesh or a plate comprising a plurality of holes. In some embodiments, the filament comprises a plurality of filament elements, disposed at different heights, and wherein the shield comprises a multitier structure. In some embodiments, the multitier structure comprises a lower tier comprising a cylinder having a first height and a first diameter; and an upper tier comprising a concentric cylinder having a second height and a second diameter, smaller than the first diameter. 
     According to another embodiment, an ion implantation system is disclosed. The ion implantation system comprises the ion source described above; a mass analyzer to receive ions extracted from the ion source; and a tandem accelerator to accelerate ions exiting the mass analyzer. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       For a better understanding of the present disclosure, reference is made to the accompanying drawings, which are incorporated herein by reference and in which: 
         FIG.  1    shows a block diagram of a multicusp ion source according to one embodiment; 
         FIG.  2    shows a block diagram of a multicusp ion source according to a second embodiment; 
         FIG.  3    shows a top view of a filament according to one embodiment; and 
         FIG.  4    shows an ion implantation system that utilizes the multicusp ion source. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    shows a cross-sectional view of a Bernas ion source. A Bernas ion source is one in which electrons from the heated filament directly enter the chamber to create the plasma. Thus, unlike an indirectly heated cathode (IHC) ion source, the filament of a Bernas ion source is not shielded by a cathode, but rather is exposed directly to the plasma. One specific type of Bernas ion source is a multicusp ion source, where magnets are used to create a magnetic field within the chamber.  FIG.  1    illustrates a Bernas ion source  1  that may be utilized to extract ions according to one embodiment. In one particular embodiment, the extracted ions may be negatively charged. The Bernas ion source may be a multicusp ion source. The multicusp ion source includes a chamber  100 , comprising two opposite ends, and walls  104  connecting to these ends. In one embodiment, the walls  104  may be cylindrical. The walls  104  of the chamber  100  may be constructed of an electrically conductive material and may be in electrical communication with one another. In some embodiments, the walls  104  may be constructed from a refractory metal. 
     A filament  130  extends into the chamber  100  from one end, referred to as the first end  101 . The opposite end, or second end  102 , includes the extraction aperture  103 . 
     Ions are extracted through the extraction aperture  103 . An electrode  120  is disposed outside the chamber  100 , proximate the extraction aperture  103 . The electrode  120  includes an electrode aperture  121  which is aligned with the extraction aperture  103 . 
     The electrode  120  may be electrically biased to attract ions from the plasma  160  through the extraction aperture  103 . An electrode power supply  112  may be used to bias the electrode  120  relative to the chamber  100 . In certain embodiments, the electrode power supply  112  is configured such that the electrode  120  is more positive than the chamber  100 . In this way, negatively charged ions and electrons are drawn through the extraction aperture  103  and through the electrode aperture  121 . In some embodiments, filter elements  122 , such as magnets or electrodes, may be disposed on the electrode  120  or between the electrode  120  and the second end  102 . The filter elements  122  serve to repel or redirect the electrons that are extracted from the chamber  100  such that most of the electrons do not pass through the electrode aperture  121 . In this way, the beam  170  that is extracted through the electrode aperture  121  comprises mostly negative ions. In some embodiments, the beam current may be up to 100 mA. 
     The filament  130  is disposed near the first end  101  and has a negative leg  131 , a second leg  132  and a distal portion  133  that extends furthest into the chamber  100 . The negative leg  131  and the second leg  132  are each in communication with a filament power supply  110 . The negative leg  131  is more negative than the second leg  132 . The filament power supply  110  is configured to pass a current through the filament  130 , such that the distal portion  133  emits thermionic electrons. This current may be up to 100 A or more. 
     The second leg  132  is in communication with an arc voltage power supply  111 . The arc voltage power supply  111  supplies a voltage to the second leg  132  relative to the chamber  100 . This may be a negative voltage, such as between 40 and 200 V. This arc voltage accelerates the thermionic electrons emitted by the filament  130  into chamber  100  to ionize the neutral gas. The current drawn by this arc voltage power supply  111  may be a measurement of the amount of current being driven through the plasma  160 . In certain embodiments, the walls  104  provide the ground reference for the other power supplies. 
     In certain embodiments, magnets  105  are disposed around the outside of the walls  104  to create a multi-cusp magnetic field within the chamber  100 . For example, a plurality of magnetic bars may be used to create the cusp lines within the chamber  100 . The magnetic bars may be samarium cobalt, although other materials may be used. 
     Advantageously, the multicusp ion source also comprises a shield  150 . The shield  150  may be constructed from a refractory metal, such as tungsten or molybdenum. The shield  150  is disposed between the distal portion  133  of the filament  130  and the first end  101 . In certain embodiments, the shield  150  is electrically connected to the negative leg  131  of the filament  130 . In this way, the shield  150  is the most negatively biased component in the chamber  100 . The shield  150  may be dimensioned so as to extend radially further than the distal portion  133 . The shape of the shield  150  may depend on geometry of the chamber  100  and the filament  130 . In a cylindrical chamber with a circular or near circular filament, the shield  150  may be a round plate. A round plate may have small holes or be a mesh to reduce weight. In the embodiment shown in  FIG.  1   , the cross-section of the shield  150  may be planar on the two opposite surfaces. The shield  150  may be rectangular, square, circular or any other suitable shape. 
     A controller  180  may be in communication with one or more of the power supplies such that the voltage or current supplied by these power supplies may be monitored and/or modified. Additionally, the controller  180  may be in communication with the mass flow controller  142  of the gas container  140  so as to regulate a flow of gas into the chamber  100 . The controller  180  may include a processing unit, such as a microcontroller, a personal computer, a special purpose controller, or another suitable processing unit. The controller  180  may also include a non-transitory storage element, such as a semiconductor memory, a magnetic memory, or another suitable memory. This non-transitory storage element may contain instructions and other data that allows the controller  180  to perform the functions described herein. 
     For example, the controller  180  may be in communication with the filament power supply  110  to control the current passing through the filament  130 . Additionally, the controller  180  may be in communication with the arc voltage power supply  111  to allow the multicusp ion source to vary the voltage applied to the second leg  132  of the filament  130  relative to the chamber  100 . Further, the controller  180  may be able to monitor and adjust the voltage supplied by electrode power supply  112 . 
     In operation, gas from gas container  140  is introduced into the chamber  100  through gas inlet  141 . The gas may be any suitable species, such as hydrogen. The flow of gas may be regulated by a mass flow controller  142 . 
     The arc voltage power supply  111  is used to bias the second leg  132  of the filament  130  negatively relative to the chamber  100 . The filament power supply  110  is used to supply current through the filament  130 . The amount of current may be up to 350 A. In certain embodiments, the current may be at least 100 A. The current causes the filament  130  to heat, emitting thermionic electrons into the chamber  100 . Since the filament  130  is negatively biased relative to the chamber  100 , the electrons are accelerated away from the filament  130 . These emitted electrons interact with the gas to create a plasma  160 . The shape of the plasma  160  within the chamber  100  is controlled by the magnets  105 , the electric field between filament  130  and walls  104 , and optional additional electrodes that are immersed in the plasma. 
     Electrode power supply  112  is used to bias the electrode  120  positively relative to the chamber  100  and the plasma  160 . In this way, negative particles, such as electrons and negative ions, are drawn through the extraction aperture  103  and toward the electrode aperture  121 . In some embodiments, filter elements  122  are disposed between the electrode  120  and the second end  102  to redirect any electrons that are extracted. For example, a magnetic field may be created by the filter elements  122 . The field may be of such a strength that its effect on heavier negative ions is minimal, but electrons, which may have a mass that is at least 1000 times less than the negative ions, are deflected or repelled. In this way, the negative particles that pass through the electrode aperture  121  are mostly negative ions. 
       FIG.  2    shows the multicusp ion source according to another embodiment. Components that are the same as those in  FIG.  1    have been given identical reference designators. In this embodiment, as can be seen in  FIG.  3   , the filament  130  comprises four separate filament elements which are all configured in parallel. The distal portion of each filament element is a semicircle. Further, the inner distal portions  133   a  of two of these filament elements extend further into the chamber  100  than the outer distal portions  133   b  of the other two elements. Further, the diameter of the inner distal portions  133   a  may be smaller than the diameter of the outer distal portions  133   b.    
     Therefore, in this embodiment, the shield  150  may be multitiered. In one embodiment, the shield  150  may be shaped to have two tiers. The lower tier may be a cylinder having a first height and a first diameter. The upper tier may be a concentric cylinder having a second height and a second diameter, smaller than the first diameter. The first height may be larger, smaller or equal to the second height. The second height may be related to the distance between the inner distal portions  133   a  and the outer distal portions  133   b.    
     While the shield  150  is described in  FIGS.  1  and  2    as being connected to the negative leg  131  of the filament  130 , other embodiments are also possible. For example, in one embodiment, the shield  150  may be electrically floating. In this embodiment, the shield  150  may self-bias to a negative voltage that restricts the plasma  160  to the region in front of the shield  150 . 
     The above describes the ion source as being a multicusp ion source. However, other ion sources may also be used with the shield  150 . For example, any Bernas ion source may also utilize the shield described herein. 
     This multicusp ion source with the shield  150  may be part of an ion implantation system.  FIG.  4    shows such a system. The ion implantation system comprises a Bernas ion source  1 , which may be the multicusp ion source with the shield  150 . The multicusp ion source with the shield  150  is used to generate a negative ion beam  200 . A feedgas is supplied to the Bernas ion source  1 , which is then energized to generate ions. In certain embodiments, the feedgas may be hydrogen or another suitable species. Extraction optics are then used to extract these ions from the Bernas ion source  1 . 
     The negative ion beam  200  may be directed toward a mass analyzer  210 , which only allows the passage of certain species of ions. The negative ions that exit the mass analyzer  210  are directed toward a tandem accelerator  230 . 
     The tandem accelerator  230  has two pathways, which are separated by a stripper tube  233 . The input pathway  231  comprises a plurality of input electrodes. These input electrodes may be any suitable electrically conductive material, such as titanium or other metals. The outermost input electrode may be grounded. Each of the subsequent input electrodes may be biased at an increasingly more positive voltage moving closer to the stripper tube  233 . 
     The input pathway  231  leads to the stripper tube  233 . The stripper tube  233  is biased positively relative to the outermost input electrode. The stripper tube  233  includes an injection conduit where a stripper gas is injected. The stripper gas may comprise neutral molecules. These neutral molecules may be any suitable species such as, but not limited to argon and nitrogen. The stripper tube  233  has an inlet disposed on the same side as the input pathway  231 . The outlet of the stripper tube  233  is in communication with the output pathway  232 . 
     In other words, the stripper tube  233  is positively biased so as to attract the negative ion beam  200  through the input pathway  231 . The stripper tube  233  removes electrons from the incoming ions, transforming them from negative ions into positive ions. 
     The stripper tube  233  is more positive than the electrodes in the output pathway  232 . Each subsequent output electrode may be less positively biased moving away from the stripper tube  233 . For example, the outermost output electrode may be grounded. Thus, the positive ions in the stripper tube  233  are accelerated through the output pathway  232 . 
     In this way, the ions are accelerated two times. First, negative ions are accelerated through the input pathway  231  to the stripper tube  233 . This acceleration is based on the difference between the voltage of the outermost input electrode and the voltage of the stripper tube  233 . Next, positive ions are accelerated through the output pathway  232 . This acceleration is based on the difference between the voltage of the stripper tube  233  and the voltage of the outermost output electrode in the output pathway  232 . 
     An accelerator power supply  234  may be used to supply the voltages to the stripper tube  233 , as well as the electrodes in the input pathway  231  and the output pathway  232 . The accelerator power supply  234  may be capable of supply a voltage up to 2.5 MV, although other voltages, either higher or lower, are also possible. Thus, to modify the implant energy, the voltage applied by the accelerator power supply  234  is changed. 
     After exiting the tandem accelerator  230 , the positive ion beam  235  may enter a filter magnet  240 , which allows passage of ions of only a certain charge. In other embodiments, the filter magnet  240  may not be employed. 
     The output of the filter magnet, which may be a spot ion beam  255 , is then directed toward the workpiece  300 . 
     The present system has many advantages. In Bernas ion sources, the filament  130  is heated due to the current supplied by the filament power supply  110  and by the bombardment of the filament  130  with ions from the plasma  160 . In traditional Bernas ion sources, which do not include a shield  150 , the plasma  160  is disposed on both sides of the distal portions of the filament  130 . Consequently, there may be significant heating from the plasma  160 . At higher plasma densities, the heating from the plasma  160  may reach a level where the filament  130  becomes hotter than intended and therefore, generates more electrons. These additional electrons cause the plasma  160  to become more dense, and the denser plasma further heats the filament  130 . This situation may be referred to as thermal runaway. However, by including the shield  150 , the plasma  160  is confined to the region in front of the shield  150 , which is the region between the shield  150  and the second end  102 . This reduces the interaction between the plasma  160  and the filament  130 . Further, the shield  150  may serve as a sink for some of the positive ions that are attracted toward the filament  130 , reducing the number of ions that strike the filament  130 . In one test, it was found that, without the shield, the extracted beam  170  may be about 2 mA before active control of the filament power supply  110  is used to prevent thermal runaway. This active control may be a PID controller. When the shield of  FIG.  1    was added to the multicusp ion source, the extracted beam  170  may be more than more than 6 mA without the use of a PID controller. Thus, the use of the shield  150  may allow denser plasmas to be created without causing thermal runaway that currently exists. 
     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. Furthermore, 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.