Patent Publication Number: US-2016233047-A1

Title: Plasma-based material modification with neutral beam

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 14/952,624, filed Nov. 25, 2015, which is a continuation-in-part of U.S. patent application Ser. No. 14/201,747, filed Mar. 7, 2014, both of which are hereby incorporated by reference in their entirety for all purposes. 
    
    
     BACKGROUND 
     1. Field 
     The present disclosure relates generally to plasma-based material modification and, more specifically, to plasma-based material modification with a neutral beam. 
     2. Related Art 
     Ion-based material modification is an important process used in semiconductor manufacturing. For example, ion-based material modification may be used to amorphize crystalline materials, alloy metals, densify or mix layers of materials, facilitate removal of materials, or introduce impurities into materials. During ion-based material modification, ions are accelerated to bombard the surface of a work piece (e.g., a semiconductor substrate). The ions may be positive or negative ions comprising species or elements that are chemically reactive or inert with respect to the surface of the work piece. The ions may thus modify the physical, chemical, or electrical properties of the surface of the work piece. 
     Currently, ion-based material modification is performed predominantly using beam-line ion implantation systems. In beam-line ion implantation systems, an ion beam is extracted from an ion source and filtered by mass, charge, and energy through a magnetic analyzer before being accelerated towards a work piece. However, as dictated by Liouville&#39;s Theorem, the transport efficiencies of the ion beam decrease with decreasing ion energy. Thus, for low energy processes, beam-line ion implantation systems suffer from low beam currents and thus require long processing times to achieve the required doses. Further, the cross-section of the ion beam is significantly smaller than the area of the work piece where only a fraction of the surface of the work piece may be treated at any given moment. Thus, the ion beam or substrate must be scanned to uniformly treat the entire surface of the work piece. As a result, beam-line ion implantation systems suffer from low throughputs for high dose, low energy implant processes. 
     Plasma-based material modification systems are an alternative to beam-line ion implantation systems.  FIG. 1  depicts an exemplary plasma-based material modification system  100 . Plasma-based material modification system  100  comprises plasma source chamber  102  coupled to process chamber  104 . Plasma  106 , which contains ions, neutral species, and electrons, is generated in plasma source chamber  102 . Work piece  118  is supported by support structure  116  within process chamber  104 . In this example, plasma-based material modification system  100  has one or more biased grids  120  positioned between plasma  106  and work piece  118  to extract ion beam  112  from plasma  106  and accelerate ion beam  112  to work piece  118 . However, in other examples, plasma-based material modification system  100  may not include grids  120 . Instead, work piece  118  may be biased at a potential and immersed in plasma  106  by support structure  116 . Ions are thus accelerated from plasma  106  to work piece  118  across a plasma sheath formed between plasma  106  and work piece  118 . In some cases, work piece  118  may be treated with both ions and neutral species from plasma  106 . Currently, most conventional plasma-based material modification systems do not have grids. 
     Unlike beam-line ion implantation systems, plasma-based material modification systems do not utilize a magnetic analyzer to filter ions by mass or energy. Rather, the work piece is treated with ions directly from the plasma in close proximity. Thus, plasma-based material modification systems can treat a work piece at significantly higher ion currents than beam-line ion implantation systems. In addition, the plasma sources of plasma-based material modification systems may have cross-sectional areas that are larger than the area of the work piece. This enables a large portion of or the entire surface of the work piece to be treated simultaneously without scanning the work piece. Therefore, plasma-based material modification systems offer significantly higher throughputs for high dose, low current processes. 
     Convention plasma-based material modification systems, however, suffer from poor system reliability and process control. Due to the proximity of the plasma to the process chamber, neutral species from the plasma flow into the process chamber and encounter the work piece. The neutral species cause undesirable parasitic effects such as etching, oxidation, and film deposition on the walls of the process chamber as well as the surface of the work piece. In conventional plasma-based material modification systems, such parasitic effects are substantial and may result in frequent process excursions and low product yields. Further, ions from the plasma may cause the work piece to become excessively charged. This may damage devices being formed on the work piece. In addition, excessive charging of the work piece may repel ions of the plasma, thereby causing non-uniform treatment of the work piece. 
     BRIEF SUMMARY 
     Systems and processes for plasma-based material modification of a work piece are provided. In an example process, a first plasma in a plasma source chamber may be generated. A magnetic field may be generated using a plurality of magnets. The magnetic field may confine electrons of the first plasma having energy greater than 10 eV within the plasma source chamber. A second plasma may be generated in a process chamber coupled to the plasma source chamber. An ion beam may be generated in the process chamber by extracting ions from the first plasma through the plurality of magnets. The ion beam may travel through the second plasma and may be neutralized by the second plasma to form a neutral beam. The work piece may be positioned in the process chamber such that the neutral beam treats a surface of the work piece. 
     In an example system for plasma-based material modification of a work piece, a plasma source chamber may be configured to generate a plasma. A process chamber may be coupled to the plasma source chamber. A first plurality of magnets is disposed on an end wall of the plasma source chamber. A second plurality of magnets is disposed on a sidewall of the plasma source chamber. A third plurality of magnets is positioned between an interior region of the plasma source chamber and an interior region of the process chamber. The third plurality of magnets is configured to confine a majority of electrons of the plasma having energy greater than 10 eV within the interior region of the plasma source chamber. A fourth plurality of magnets is disposed on a sidewall of the process chamber. A fifth plurality of magnets is disposed on a base wall of the process chamber. A support structure is disposed within the process chamber, the support structure is configured to support a work piece. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an exemplary plasma-based material modification system. 
         FIG. 2  illustrates a cross-sectional view of an exemplary plasma-based material modification system. 
         FIG. 3  illustrates a cross-sectional view of an exemplary plasma source chamber. 
         FIGS. 4A and 4B  illustrate a perspective view and a cross-sectional perspective view of an exemplary plasma source chamber respectively. 
         FIGS. 5A and 5B  illustrate a perspective view and a cross-sectional perspective view of an exemplary plasma source chamber respectively. 
         FIG. 6  illustrates an exemplary absorber of a plasma-based material modification system. 
         FIG. 7  illustrates a cross-sectional view of an exemplary plasma source chamber. 
         FIG. 8  illustrates an exemplary process for plasma-based material modification. 
         FIG. 9  illustrates an exemplary process for plasma-based material modification. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is presented to enable a person of ordinary skill in the art to make and use the various embodiments. Descriptions of specific systems, devices, methods, and applications are provided only as examples. Various modifications to the examples described herein will be readily apparent to those of ordinary skill in the art, and the general principles defined herein may be applied to other examples and applications without departing from the spirit and scope of the various embodiments. Thus, the various embodiments are not intended to be limited to the examples described herein and shown, but are to be accorded the scope consistent with the claims. 
     It may be desirable to generate a uniform neutral beam to perform plasma-based material modification. The neutral beam can treat a work piece without causing the work piece to become excessively charged. As discussed above, an excessively charged work piece can damage devices (e.g., integrated circuits) being formed on the work piece. Additionally, even if the work piece becomes charged, the neutral beam would not be repelled by the charged work piece and thus the neutral beam may uniformly treat the work piece. In particular, the neutral beam may be effective for the treatment of high-aspect ratio structures on the work piece. For example, the neutral beam may be directed into high-aspect ratio trenches to uniformly treat (e.g., deposit or implant neutral species) the bottoms of the trenches without excessively treating the sidewalls of the trenches. In an exemplary process for plasma-based material modification with a neutral beam, a first plasma in a plasma source chamber may be generated. A magnetic field may be generated using a plurality of magnets. The magnetic field may confine electrons of the first plasma having energy greater than 10 eV within the plasma source chamber. A second plasma may be generated in a process chamber coupled to the plasma source chamber. The second plasma may be generated to contain a high concentration (e.g., greater than 1E12/cm 3 , greater than 1E13/cm 3 , or 1E12/cm 3  to 1E13/cm 3 ) of low energy electrons (e.g., less than 2 eV or 1 eV). An ion beam may be generated in the process chamber by extracting ions from the first plasma through the plurality of magnets. The ion beam may be directed through the second plasma and may be neutralized by the low energy electrons of the second plasma to generate a neutral beam. The work piece may be positioned in the process chamber such that the neutral beam treats a surface of the work piece. Because the ion beam has a large cross-sectional area, conventional electron showers would be unable to uniformly neutralize the ion beam to form a uniform neutral beam. The second plasma may provide a uniform distribution of low energy electrons across the process chamber, which enables the ion beam to be uniformly neutralized as the ion beam passes through the second plasma. A uniform neutral beam may therefore be generated to effectively treat the surface of the work piece. 
     1. Plasma-Based Material Modification System 
       FIG. 2  depicts an exemplary plasma-based material modification system  200 . As shown in  FIG. 2 , plasma-based material modification system  200  includes plasma source chamber  202  coupled to process chamber  204 . Plasma source chamber  202  is configured to generate plasma  220  containing ions within plasma generation region  232 . Support structure  208  is disposed within process chamber  204  and is configured to support work piece  206 . A series of optional grids  224  are positioned between plasma source chamber  202  and support structure  208  to extract ion beam  234  from plasma  220  and accelerate ion beam  234  towards work piece  206 , thereby causing material modification of work piece  206 . 
     In the present embodiment, plasma source chamber  202  includes end wall  216  disposed at one end  217  of plasma source chamber  202  and at least one sidewall  218  defining the interior of plasma source chamber  202  between end wall  216  and opposite end  222  of plasma source chamber  202 . In this example, sidewall  218  is cylindrical and has a circular cross-section. However, in other cases, sidewall  218  may have a rectangular cross-section. 
     As shown in  FIG. 2 , plasma source chamber  202  has an internal diameter  236 . Internal diameter  236  defines the cross-sectional area of plasma source chamber  202  and thus at least partially determines the cross-sectional area of plasma  220  and of ion beam  234 . Due to ion drift or diffusion losses to sidewall  218 , the current density of ions incident to grids  234  may be significantly lower at the outer regions further away from the center axis of ion beam  234  and more proximate to the chamber walls than at the center regions closer to the center axis of ion beam  234 . It is thus desirable to implant the entire area of work piece  206  using only the center regions of ion beam  234  nearer the center axis of ion beam  234  where the current density is more uniform. In the present example, internal diameter  236  of plasma source chamber  202  is larger than the diameter of work piece  206 . Additionally, the extraction area of grids  224  is larger than the area of work piece  206 . Thus, ion beam  234  is generated having a cross-sectional area larger than the area of work piece  206 . In one example, internal diameter  236  may be greater than 45 cm. In another example, internal diameter  236  may be between 45 and 60 cm. In a specific example, internal diameter  236  may be 50% to 100% larger than the diameter of work piece  206 . 
     Plasma source chamber  202  includes first set of magnets  210  disposed on end wall  216 , second set of magnets  212  disposed on sidewall  218 , and third set of magnets  214  extending across the interior of chamber  202 . Each magnet of third set of magnets  214  may be housed within a protective tube. End wall  216 , sidewall  218 , and the third set of magnets  214  define plasma generation region  232  within the interior of plasma source chamber  202 . In this example, first set of magnets  210 , second set of magnets  212 , and third set of magnets  214  are configured to confine energetic electrons of plasma  220  within plasma generation region  232 . Energetic electrons may be defined as electrons having energy greater than 10 eV. Particularly, third set of magnets  214  is configured to confine a majority of electrons of plasma  220  having energy greater than 10 eV within plasma generation region  232  while allowing ions from plasma  220  to pass through third set of magnets  214  into process chamber  204  for material modification of work piece  206 . 
     As shown in  FIG. 2 , plasma-based material modification system  200  may optionally include a series of grids  224  positioned between third set of magnets  214  and support structure  208 . One or more grids of grids  224  may be coupled to one or more bias power sources  248  to apply a bias voltage to grids  224 . Bias power source  248  may be, for example, a DC power source, a pulsed DC power source, a radio frequency (RF) power source, or a combination thereof. In this example, grids  224  are configured to extract ion beam  234  from plasma  220  and accelerate ion beam  234  to a desired energy level towards work piece  206 . Additionally, grids  224  may be configured to focus ion beam  234  and thus collimate ion beam  234 . It should be recognized that grids  224  may be configured to extract multiple ion beamlets from plasma  220  and that ion beam  234  may thus comprise multiple ion beamlets. 
     The distance at which grids  224  are positioned from third set of magnets  214  affects the current density uniformity across ion beam  234  and thus the uniformity with which work piece  206  is treated with ions. Positioning grids  224  too close to third set of magnets  214  results in poor current density uniformity across ion beam  234  due to significant ion shadowing effects of third set of magnets  214 . However, positioning grids  224  too far from third set of magnets  214  also results in poor current density uniformity across ion beam  234  due to ion drift or diffusion losses to the chamber walls becoming more pronounced as the distance traveled by ions across drift region  226  increases. In the present example, grids  224  are positioned at an optimal distance  228  from third set of magnets  214  to minimize the net effects of ion shadowing by third set of magnets  214  and ion drift or diffusion losses to the chamber walls. In one example, distance  228  is between 0.10D and 0.33D, where D is the internal diameter  236  of plasma source chamber  202 . In another example, distance  228  is between 0.2D and 0.3D. In yet another example, distance  228  is between 6 cm and 18 cm. 
     As shown in  FIG. 2 , plasma-based material modification system  200  may optionally include absorber  250  for adjusting the current density profile of ion beam  234 . Absorber  250  is configured to absorb a fraction of ions flowing from plasma  220  to absorber  250  while allowing the non-absorbed ions to pass through towards support structure  208 . In particular, absorber  250  is configured such that the ion transparency of absorber  250  varies across absorber  250 . Ion transparency is defined as the percentage of ions incident to absorber  250  that are allowed to pass through absorber  250 . Thus, regions of absorber  250  having higher ion transparencies allow a higher percentage of ions to pass through compared to regions of absorber  250  having lower ion transparencies. Absorber  250  may be configured to have regions of lower ion transparency and regions of higher ion transparency. In the present example, the regions of lower ion transparency may be positioned in areas of drift region  226  having higher current densities while the regions of higher ion transparency may be positioned in areas of drift region  226  having lower current densities. Thus, absorber  250  may be configured such that the current density profile of ions exiting absorber  250  is more uniform than the current density profile of ions flowing from plasma  220  to absorber  250 . In one example, absorber  250  is configured to have increasing ion transparency from the center to the outer edge of absorber  250 . 
     Absorber  250  may, in some examples, be approximately parallel with respect to end wall  216  and concentric with sidewall  218 . In this example, the center of the cross-section of plasma  220  may be aligned with the center axis of plasma source chamber  202 . In another example, the center of absorber  250  may be approximately aligned with the center of the cross-section of plasma  220  and the center of work piece  206 . In this example, the diameter of absorber  250  may be less than or equal to internal diameter  236  of plasma source chamber  202 . For example, the diameter of absorber  250  may be between 0.3D and 1.0D, where D is internal diameter  236  of plasma source chamber  202 . In an example, the diameter of absorber  250  may be between 0.5D and 0.8D, where D is internal diameter  236  of plasma source chamber  202 . 
     Absorber  250  may be positioned between the center of plasma  220  and support structure  208 . In cases where plasma-based material modification system  200  includes grids  224 , absorber  250  may be positioned either between the center of plasma  220  and third set of magnets  214  or between third set of magnets  214  and grids  224 . In other cases where plasma-based material modification system  200  does not have grids  224 , absorber  250  may be positioned either between the center of plasma  220  and third set of magnets  214  or between third set of magnets  214  and support structure  208 . In some cases, absorber  250  may be positioned no closer than 5 cm from support structure  208 . It should be recognized that in some examples, plasma-based material modification system  200  may have more than one absorber. 
     In one example, absorber  250  may be coupled to a ground potential or to a bias voltage source (not shown). Bias voltage source may be, for example, a DC, pulsed DC, or RF power source. The bias voltage source may function to apply a bias potential to absorber  250  to attract or repel ions towards or away from absorber  250 . In another example, absorber  250  may be configured to have a floating potential. For example, absorber  250  may be electrically isolated from any power source or power sink and thus the potential of absorber  250  is determined predominately by charging from plasma  220 . In some cases, absorber  250  may comprise two or more regions and the two or more regions may be configured to be independently biased. Independently biasing multiple regions of absorber  250  may be advantageous in achieving a more uniform current density profile of ions exiting absorber  250 . 
     Support structure  208  in process chamber  204  is configured to position work piece  206  in the path of ion beam  234  for material modification. Work piece  206  may be a semiconductor substrate (e.g., silicon wafer) used in fabricating IC chips or solar cells. In other cases, work piece  206  may be a glass substrate with thin-film semiconductor layers used in fabricating flat panel displays or thin-film solar cells. Support structure  208  is configured to position work piece  206  at distance  242  from grids  224 . Positioning work piece  206  too close to grids  224  may result in poor current density uniformity of ion beam  234  due to the ion shadowing effects of grids  224 . Positioning work piece  206  too far from grids  224  may also result in poor current density uniformity of ion beam  234  due to the effects of ion divergence or scattering losses. In one example, distance  242  is between 10 cm and 100 cm. In another example, distance  242  is between 30 cm and 40 cm. 
     In some embodiments, support structure  208  may be configured to rotate work piece  206 . Rotating work piece  206  during plasma-based material modification may be advantageous in improving the uniformity with which work piece  206  is treated with ions. Additionally, support structure  208  may be configured to tilt work piece  206  to control the incidence angle of ion beam  234  with respect to the perpendicular of work piece  206 . It should be recognized that support structure  208  may be configured to rotate work piece  206  while tilting work piece  206  at a given angle. 
     Although in this example, plasma-based material modification system  200  is shown as having optional grids  224 , in other cases, plasma-based material modification system  200  may not include grids  224 . In such cases, support structure  208  may be configured to apply a bias voltage on work piece  206 . For example, support structure may be coupled to bias power source  254  to apply a bias voltage to work piece  206 . Biasing work piece  206  functions to accelerate ions from plasma  220  towards work piece  206 , thereby treating work piece  206  with ions. Additionally, support structure  208  may be configured to position work piece  206  at an optimal distance from third set of magnets  214  to minimize ion shadowing effects of third set of magnets  214  and ion losses to the chamber walls. In one example, support structure  208  may be configured to position work piece  206  at a distance of 0.10D to 0.33D from third set of magnets  214 , where D is internal diameter  236  of plasma source chamber  202 . In another example, support structure  208  may be configured to position work piece  206  at a distance of 0.25D to 0.30D from third set of magnets  214 . 
     As described above, first set of magnets  210 , second set of magnets  212 , and third set of magnets  214  are configured to confine energetic electrons of plasma  220  within plasma generation region  232 . Confining energetic electrons of plasma  220  is advantageous because it enables a higher ionization rate and thus lower operating pressures of plasma-based material modification system  200 . At lower operating pressures, there is less angular scattering of ion beam  234  due to collisions with background gases, which results in ion beam  234  having a tighter distribution of incidence angles. Additionally, at lower operating pressures, electron temperature is greater, causing ionization rates in plasma  220  to be higher, which reduces the concentration of neutral species relative to ions. Lower concentrations of neutral species generally result in less film deposition on the walls of plasma source chamber  202  and process chamber  204  and thus higher gas efficiency. Particle contamination from film deposits flaking off of the chamber walls is also reduced, which improves system reliability, system availability for production, and device yields. Further, lower concentrations of neutral species reduce parasitic etching, oxidation, and deposition on work piece  206  and thus result in less device damage and higher device yields. 
     In the present example, first set of magnets  210 , second set of magnets  212 , and third set of magnets  214  are configured to enable plasma-based material modification system  200  to operate at pressures below 0.1 Pa. Particularly, first set of magnets  210 , second set of magnets  212 , and third set of magnets  214  may be configured to enable plasma  220  to be stably generated and sustained at a pressure below 0.1 Pa. In another example, first set of magnets  210 , second set of magnets  212 , and third set of magnets  214  may be configured to enable plasma  220  to be stably generated and sustained at a pressure below 0.02 Pa. In yet another example, first set of magnets  210 , second set of magnets  212 , and third set of magnets  214  are configured to enable plasma  220  to be stably generated and sustained at a pressure of below 0.1 Pa without the use of an additive gas (e.g., hydrogen, argon, xenon) to help sustain the plasma. Conventional plasma-based material modification systems typically operate at pressures of about 1 Pa. At pressures below 0.1 Pa, conventional plasma-based material modification systems may be unable to generate and sustain a stable plasma and thus material modification cannot be reliably performed. A “stable plasma” or a “stably generated and sustained plasma” is defined as a plasma where the average current density does not vary more than ±5% and in some cases, ±3% during the material modification process. Additionally, the concentration of ions having an atomic or molecular mass greater than 20 AMU in a “stable plasma” or a “stably generated and sustained plasma” does not vary more than 10%. 
       FIG. 3  depicts a cross-sectional view of an exemplary plasma source chamber  202 . As shown in  FIG. 3 , first set of magnets  210 , second set of magnets  212 , and third set of magnets  214  are arranged with alternating polarities to produce multi-cusp magnetic fields (illustrated by magnetic field lines  302 ) that surround plasma generation region  232 . The multi-cusp magnetic fields confine a majority of energetic electrons of plasma  220  within plasma generation region  232  by repelling the energetic electrons from end wall  216 , sidewall  218 , and third set of magnets  214 . More specifically, the multi-cusp magnetic fields function to reflect energetic electrons of plasma  220  from end wall  216 , sidewall  218 , and third set of magnet, thereby enabling most energetic electrons to traverse at least several times across the length and/or diameter of plasma generation region  232  before finally being lost to end wall  216  or sidewall  218 . By increasing the path length travelled by energetic electrons within plasma generation region  232 , the probability of ionizing an atom or molecule increases. Thus, first set of magnets  210 , second set of magnets  212 , and third set of magnets enable higher ionization rates in plasma  220  compared to the plasmas generated by conventional plasma sources having no magnetic confinement or only partial magnetic confinement. 
     Although  FIG. 3  depicts magnetic field lines  310  between second set of magnets  212  and third set of magnets  214 , it should be recognized that the magnetic fields represented by magnetic field lines  310  may apply only in limited locations where the magnet of second set of magnets  212  adjacent to third set of magnets  214  is approximately parallel to the linear magnets of third set of magnets  214  adjacent to second set of magnets  212 . In other locations, the geometry of magnetic field lines between second set of magnets  212  and third set of magnets  214  may be more complex and three-dimensional. Therefore, in general, the magnetic fields near end wall  216  and sidewall  218  may be line cusps while the magnetic fields between second set of magnets  212  and third set of magnets  214  may have more complex geometries. 
     The strength of the magnetic fields produced by first set of magnets  210  and second set of magnets  212  affects the operation and reliability of plasma source chamber  202  and thus the productivity and cost of ownership of plasma-based material modification system  200 . A magnetic field strength that is too high (e.g., greater than 1 kG) at the inner surfaces of end wall  216  or sidewall  218  may cause excessive power densities of plasma  220  incident to the inner surfaces of end wall  216  or sidewall  218  at cusp regions  304  (i.e., regions directly in front of the magnetic pole faces). This may result in non-uniform film deposition on the inner surfaces of end wall  216  and sidewall  218 , which may cause film deposits to flake off and contaminate work piece  206 . In addition, excessive power densities of plasma  220  may cause material from end wall  216  and sidewall  218  to be sputtered off, which may also contaminate work piece  206 . Thus, in the present example, first set of magnets  210  and second set of magnets  212  are not configured to produce a magnetic field strength greater than 1 kG at the inner surfaces of end wall  216  and sidewall  218 . It should be recognized that magnets such as samarium cobalt, neodymium iron, or nickel iron boron may be undesirable because such magnets are more likely to produce a magnetic field strength greater than 1 kG at the inner surfaces of end wall  216  and sidewall  218 . In one example, first set of magnets  210  and second set of magnets  212  are configured such that the magnetic field strength at the inner surfaces of end wall  216  and sidewall  218  is between 0.1 kG and 1 kG. In another example, first set of magnets  210  and second set of magnets  212  are configured such that the magnetic field strength at the inner surfaces of end wall  216  and sidewall  218  is between 0.3 kG and 0.7 kG. In a specific example, first set of magnets  210  and second set of magnets  212  comprise ceramic permanent magnets (e.g., ferrite magnets) and are configured such that the magnetic field strength at the inner surfaces of end wall  216  and sidewall  218  is between 0.1 kG and 1 kG. 
     As shown in  FIG. 3 , each magnet of first set of magnets  210 , second set of magnets  212 , and third set of magnets  214  has a width  306 . In one example, width  306  is between 2 mm and 15 mm. In another example, width  306  may be between 4 mm and 8 mm. Magnets of first set of magnets  210 , second set of magnets  212 , and third set of magnets  214  may be evenly spaced apart at spacing  308 . In one example, spacing  308  between adjacent magnets is between 2 cm and 15 cm. In another example, spacing  308  is between 4 cm and 8 cm. 
     Third set of magnets  214  may have a magnetic field strength similar to that of first set of magnets  210  and second set of magnets  212 . For example, third set of magnets  214  may be configured such that the magnetic field strength is between 0.2 kG and 2 kG at the outer surfaces of the protective tubes housing third set of magnets  214 . The magnetic field strength of third set of magnets  214  may be at least partially determined by the width and the spacing of third set of magnets  214 . In some cases, third set of magnets  214  may have a smaller width (e.g., 2 to 6 mm) and a larger spacing (e.g., 7 to 15 cm) to reduce ion shadowing caused by third set of magnets  214 . In such cases, third set of magnets  214  may have a magnetic field strength greater than that of first set of magnets  210  and second set of magnets  212 . In one example, third set of magnets  214  may be configured to have a width of between 4 and 6 mm, a spacing of between 7 and 15 cm and configured such that the magnetic field strength is between 1 kG and 2 kG at the outer surfaces of the protective tubes housing third set of magnets  214 . 
     Although in the present example, first set of magnets  210 , second set of magnets  212 , and third set of magnets  214  may comprise permanent magnets, it should be recognized that in other cases, any one of or all of first set of magnets  210 , second set of magnets  212 , and third set of magnets  214  may comprise electromagnets configured to produce multi-cusp magnetic fields similar or identical to that described above in connection with  FIG. 3 . The electromagnets may include ferromagnetic structures that enable the electromagnets to have effective pole-faces similar to that of first set of magnets  210 , second set of magnets  212 , and third set of magnets  214  of  FIG. 3 . In one example, first set of magnets  210  and second set of magnets  212  may comprise electromagnets that are configured to produce a magnetic field strength of between 0.1 kG and 1 kG at the inner surfaces of end wall  216  and sidewall  218 . Third set of magnets  214  may comprise electromagnets that are configured to produce a magnetic field strength of between 0.2 kG and 3 kG at the outer surfaces of the protective tubes housing third set of magnets  214 . 
       FIGS. 4A and 4B  depict a perspective view and a cross-sectional perspective view of plasma source chamber  202  respectively. In the present embodiment, as shown in  FIGS. 4A and 4B , first set of magnets  210  and second set of magnets  212  have a circular configuration while third set of magnets  214  has a linear configuration. Referring to  FIG. 4A , first set of magnets  210  comprises concentric rings of permanent magnets distributed along end wall  216 . Second set of magnets  212  comprises rows of permanent magnets that extend around the circumference of sidewall  218 . Referring to  FIG. 4B , third set of magnets  214  comprises linear magnets extending across the interior of plasma source chamber  202  and distributed approximately evenly across the interior cross-sectional area of plasma source chamber  202 . The linear magnets of third set of magnets  214  may be aligned with respect to a plane that is approximately parallel to end wall  216 . Additionally, the linear magnets of third set of magnets  214  may or may not be aligned with respect the magnets of first set of magnets  210  and second set of magnets  212 . As described above, each magnet of third set of magnets  214  may be housed within a protective tube to prevent damage caused by direct exposure to plasma  220 . Additionally, plasma source chamber  202  may be configured to flow cooling fluid (e.g., water, ethylene glycol, etc.) through internal channels disposed between each magnet and the inner surface of the corresponding protective tube to keep third set of magnets  214  cool. 
     In the present example, as described above with reference to  FIG. 3 , the linear magnets of third set of magnets  214  are configured to have alternating polarities such that the pole-face field direction of each linear magnet is approximately perpendicular to end wall  216 . However, in other examples, the linear magnets of third set of magnets  214  may be configured to have alternating polarities such that the pole-face field direction of each linear magnet is approximately parallel to end wall  216 . 
     Although in the present example, first set of magnets  210  and second set of magnets  212  each have a circular configuration while third set of magnets  214  has a linear configuration, it should be recognized that first set of magnets  210 , second set of magnets  212 , and third set of magnets  214  may have alternative configurations. For example, in some cases first set of magnets  210  and/or second set of magnets  212  may have a linear configuration. Additionally, third set of magnets  214  may have a circular configuration. 
       FIGS. 5A and 5B  depict a perspective view and a cross-sectional perspective view of plasma source chamber  500  having an alternative configuration of first set of magnets, second set of magnets, and third set of magnets. As shown in  FIGS. 5A and 5B , first set of magnets  502  and second set of magnets  504  have linear configurations while third set of magnets  506  has a circular configuration. With reference to  FIG. 5A , first set of magnets  502  and second set of magnets  504  comprise linear magnets arranged with alternating polarities and distributed along end wall  508  and sidewall  510  respectively. The linear magnets of second set of magnets  504  may be positioned parallel to length  512  of plasma source chamber  500 . With reference to  FIG. 5B , third set of magnets  506  comprises concentric rings of permanent magnets arrange with alternating polarities. Similar to third set of magnets  214  of  FIG. 3 , the pole-face field direction of each magnet of third set of magnets  506  may be parallel or perpendicular to end wall  216 . Additionally, first set of magnets  502  and second set of magnets  504  may be configured such that the magnetic field strength at the inner surfaces of the end wall and the sidewall are similar or identical to that of first set of magnets  210  and second set of magnets  212  described above with reference to  FIG. 3 . Third set of magnets  506  may be configured such that the magnetic field strength at the outer surfaces of the protective tubes housing third set of magnets  506  are similar or identical to that of third set of magnets  214  described above with reference to  FIG. 3 . 
       FIG. 6  depicts a front view of an exemplary absorber  250  that may be used in plasma-based material modification system  200  of  FIG. 2  to adjust the current density profile of ion beam  234 . As shown in  FIG. 6 , absorber  250  comprises a pattern of ion-absorbing material. Openings  606  are disposed between the ion-absorbing material. The ion-absorbing material may be a conductive material, such as, a metal. In some cases, absorber  250  may include an outer coating (e.g., semiconductor material) to prevent impurities from sputtering off and contaminating work piece  206 . 
     In the present example, the pattern of ion-absorbing material comprises a pattern of concentric rings  602  attached to linear rods  604 . Linear rods  604  are arranged symmetrically with respect to the center of absorber  250 . Two of linear rods  604  form a cross pattern in the center ring of absorber  250 . Concentric rings  602  and linear rods  604  are configured to absorb ions that are incident to concentric rings  602  and linear rods  604  while allowing ions to pass through the openings  606  between concentric rings  602  and linear rods. It should be recognized that absorber  250  may include fewer or additional concentric rings  602  or linear rods  604  to either increase or decrease ion transparency. 
     As shown in  FIG. 6 , the spacing between adjacent rings  602  and thus the size of the openings  606  increases with distance from the center of absorber  250 . Accordingly, the ion transparency of absorber  250  increases from the center of absorber  250  to the edge of absorber  250  where regions closer to the center of absorber  250  have a lower ion transparency than regions further from the center of absorber  250 . With reference to  FIG. 2 , absorber  250  may function to compensate for non-uniformities in the current density profile of ions flowing from plasma  220 . Due to ion losses to the chamber walls, ions flowing from plasma  220  may have higher current densities at the center regions closer to the center axis of plasma source chamber  202  than at the outer regions further from the center axis of plasma source chamber  202  and closer to the chamber walls. Absorber  250  may thus be used to reduce the current density at the center regions relative to the outer regions to achieve a more uniform current density profile. Thus, the current density profile of ions exiting absorber  250  may be more uniform than the current density profile of ions flowing from plasma  220  to absorber  250 . 
     It should be recognized that absorber  250  may have other configurations for adjusting the current density profile in various ways. In general, absorber  250  may be configured such that the ion transparency of one region of absorber  250  is different from the ion transparency of another region of absorber  250 . Ion transparency of a region is at least partially determined by the ratio of the area occupied by openings in the region to the area occupied by the pattern of ion-absorbing material in the region. Regions of absorber  250  having a higher ratio are thus more transparent to ions than regions of absorber  250  having a lower ratio. For example, the ion transparency of a region of absorber  250  may be increased by increasing the size and density of openings  606  in the region. 
     Unlike grids  224 , the ratio of total area occupied by openings in absorber  250  to total area occupied by the pattern of ion-absorbing material in absorber  250  is greater than 2:1. Having a ratio that is less than 2:1 would be undesirable because absorber  250  would absorb too large of a fraction of ions flowing from plasma  220 , thereby causing low ion current densities at work piece  206 . In one example, absorber  250  may have a ratio of total area occupied by openings to total area occupied by the patterned of ion-absorbing material that is between 2:1 and 20:1. In another example, the ratio may be between 5:1 and 15:1. 
     Although absorber  250  is described in conjunction with plasma-based material modification system  200 , it should be recognized that absorber  250  may be used to adjust the current density profile of any plasma-based material modification system. For example, absorber  250  may be implemented in a conventional plasma-based material modification system not having a plasma source with magnetic confinement. 
     In the present example, with reference back to  FIG. 2 , grids  224  comprise a series of five grids  224 . Each grid of grids  224  is positioned in parallel relation to each of the other grids. In this example, grids  224  are positioned approximately parallel to end wall  216 . However, in other cases, grids  224  may be tilted at an angle with respect to end wall  216 . Grids  224  may occupy the internal cross-sectional area of the region between plasma source chamber  202  and process chamber  204 . In this example, grids  224  have a diameter that is approximately equal to internal diameter  236  of plasma source chamber  202 . However, in other cases, grids  224  may have a diameter that differs from internal diameter  236  of plasma source chamber  202 . For example, the region between plasma source chamber  202  and process chamber  204  may have an internal cross-sectional area that is greater than that of plasma source chamber  202 . In such an example, the diameter of grids  224  may be greater than internal diameter  236 . Having a larger internal cross-sectional area in the region between plasma source chamber  202  and process chamber  204  may be advantageous in reducing ion losses to the sidewalls and thus improving the uniformity of the current density profile of ion beam  234  exiting grids  224 . 
     Each grid of grids  224  includes an array of apertures to allow ions to pass through. The apertures of each grid are substantially aligned with the apertures of each of the other grids. Ion beam  234  may thus pass through the aligned apertures of grids  224  in the form of multiple small diameter ion beams (i.e. beamlets). In some cases, the beamlets may diverge after exiting grids  224  and merge to form a single and uniform ion beam prior to encountering work piece  206 . The profile of ion beam  234  exiting grids  224  is at least partially determined by the profile of the beamlets exiting each grid of grids  224 . The profile of the beamlets exiting each grid is at least partially determined by the size and alignment of the apertures of each grid, the spacing and thickness of each grid, and the bias applied to each grid. It should be recognized that each of these variables may be adjusted to achieve the desired profile of ion beam  234 . In the present example, the apertures of each grid may have a diameter of between 1 mm and 10 mm, the spacing between adjacent grids  224  may be between 2 mm and 10 mm apart, and the thickness of each grid may be between 1 mm and 10 mm. 
     Although in this example, grids  224  includes five grids, it should be recognized that in other examples, grids  224  may include more or less grids to achieve the desired ion beam current, energy, and profile. For example, grids  224  may include between 2 and 6 grids. In some examples, grids  224  may include 3 or 4 grids. Having 4 or 5 grids may be advantageous over having 3 or fewer grids because it enables greater flexibility in focusing and adjusting the profile of ion beam  234 . 
     As previously described in connection with  FIG. 2 , plasma source chamber  202  is configured to generate plasma  220  having ions within plasma generation region  232 . Plasma  220  may be generated by supplying a process gas into plasma source chamber  202  and introducing power (e.g., electrical power or AC electric power) from a power source (e.g., electrical power source or AC electrical power source) into plasma source chamber  202  to ionize and dissociate the process gas. The process gas may contain one or more elements for modifying the physical, chemical, or electrical properties of work piece  206 . In this example, plasma source chamber  202  is coupled to gas source  244  to supply the process gas into plasma source chamber  202 . Power source  246  is coupled to one or more antennas  230  through an impedance matching network (not shown) to introduce low frequency (LF), radio frequency (RF), or very high frequency (VHF) power into plasma source chamber  202  via the one or more antennas  230 . The introduced LF, RF, or VHF power energizes electrons in plasma generation region  232 , which in turn ionize and dissociate the process gas, thereby forming plasma  220  in plasma generation region  232 . Antenna  230  is disposed within plasma source chamber  202  and is configured to enable plasma  220  to be stably generated and sustained at pressures below 0.1 Pa without the use of an additive gas (e.g., hydrogen, argon, etc.). 
     Although in this example, plasma source chamber  202  is configured to supply LF, RF, or VHF power through antenna  230  to form plasma  220 , it should be recognized that other configurations may be possible to supply power into plasma source chamber  202 . For example, in place of antenna  230 , induction coils may be disposed around the outside of plasma source chamber  202 . In such an example, power source  246  may be coupled to the induction coils to supply power (e.g., electrical power or AC electrical power) into plasma source chamber  202 . In another example, plasma source chamber  202  may be configured to supply ultra-high frequency (UHF) or microwave power into plasma source chamber  202  to form plasma  220 . In yet another example, plasma source chamber  202  may be configured to generate energetic thermionic electrons in plasma generation region  232  to form plasma  220 . For example, a tungsten filament may be heated in plasma generation region  232  to generate energetic thermionic electrons. 
     Process chamber  204  may be coupled, via throttle valve  238 , to high-speed vacuum pump  240 . For example, high-speed vacuum pump  240  may be configured to pump at a rate of at least several hundred liters per second. Throttle valve  238  and high-speed vacuum pump  240  may be configured to maintain an operating pressure of below 0.1 Pa (and in some cases below 0.02 Pa) in plasma source chamber  202  and process chamber  204 . Additionally, plasma-based material modification system may include one or more cryo-panels disposed within process chamber. The one or more cryo-panels may serve to capture residual gases or organic vapors to achieve ultra-low operating pressures. In one example, the one or more cryo-panels may be configured to maintaining a pressure of below 0.02 Pa in plasma source chamber  202  and process chamber  204 . 
     Additionally, electron source  252  may be coupled to process chamber  204  to supply low-energy electrons between grids  224  and work piece  206  to neutralize the space charge of ion beam  234 . In one example, electron source  252  is a plasma source for generating low energy electrons. In another example, electron source  252  may be an electron flood gun. Neutralizing the space charge of ion beam  234  is desirable to reduce the spread of ion beam  234  that is caused by space charge effects. In addition, electron source  252  may serve to prevent excessive localized charging (e.g., &gt;10 V) on work piece  206  which may cause undesirable damage such as threshold voltage shifts or gate dielectric damage to devices on work piece  206 . 
       FIG. 7  depicts another exemplary plasma-based material modification system  700 . System  700  may be similar to system  200  described above. In particular, system  700  may include plasma source chamber  702  coupled to process chamber  704 . Plasma source chamber  702  may be configured to generate plasma  720  within plasma source chamber  702 . Further, in this example, process chamber  704  may be configured to generate second plasma  721  within process chamber  704 . Plasma  720  may be separate from second plasma  721 . Further, as shown in  FIG. 7 , system  700  may include multiple arrays of magnets ( 710 ,  712 ,  762 , and  764 ) that surround each of plasma source chamber  702  and process chamber  704  to confine plasma  720  and second plasma  721 . A magnetic filter ( 714 ) may be disposed between plasma source chamber  702  and process chamber  704  to resist high energy electrons (e.g., greater than 10 eV) of plasma  720  from leaving plasma source chamber  702  into process chamber  704 . Support structure  708  may be disposed within process chamber  704  and may be configured to support work piece  706 . One or more bias voltage sources  770  may be configured to apply a bias voltage between plasma source chamber  702  and process chamber  704 . The applied bias voltage may cause ions to be extracted from plasma  720  to form ion beam  734  in process chamber  704 . Further, the applied bias voltage may cause ion beam  734  to be accelerated and directed through second plasma  721  where ion beam  734  is neutralized to form neutral beam  735  for treating work piece  706 . It should be recognized that system  700  may include or exclude any of the features of system  200  discussed above with respect to  FIGS. 2 through 6 . 
     Plasma source chamber  202  may be coupled to process chamber  704  to form a continuous passage from interior region  732  of plasma source chamber  702  to interior region  733  of process chamber  704 . Sidewall  718  of plasma source chamber  702  may be approximately parallel to sidewall  766  of process chamber  704 . End wall  717  of plasma source chamber  702  may be positioned opposite base wall  768  of process chamber  704 . Insulating layer  703  may be disposed between sidewall  718  of plasma source chamber  702  and sidewall  766  of process chamber  704  and may be configured to electrically isolate plasma source chamber  702  from process chamber  704 . 
     The multiple arrays of magnets that surround plasma source chamber  702  and process chamber  704  may include first set of magnets  710 , second set of magnets  712 , fourth set of magnets  762 , and fifth set of magnets  764 . First set of magnets  710  may be disposed on end wall  717  of plasma source chamber  702 , second set of magnets  712  may be disposed on sidewall  718  of plasma source chamber  702 , fourth set of magnets  762  may be disposed on sidewall  766  of process chamber  704 , and fifth set of magnets  764  may be disposed on base wall  768  of process chamber  704 . In some examples, base wall  768  may include an opening for an exhaust pump port. A high-speed vacuum pump (not shown) may be coupled to process chamber  704  via the opening. In these examples, fifth set of magnets  764  may be disposed on base wall  768  around the opening for the exhaust pump port. It should be recognized that in some examples, fifth set of magnets  764  may be optional. Specifically, base wall  768  may not include a set of magnets. In these examples, fourth set of magnets  762  may be sufficient to confine second plasma  721  such that the plasma density profile is uniform (e.g., less than ±5% or ±3% variation) across the inner diameter of process chamber  704 . First set of magnets  710  and fifth set of magnets  764  may each be similar or identical to first set of magnets  210  of system  200 . Second set of magnets  712  and fourth set of magnets  762  may each be similar or identical to second set of magnets  212  of system  200 . In particular, the magnets of first set of magnets  710 , second set of magnets  712 , fourth set of magnets  762 , and fifth set of magnets  764  may be ceramic permanent magnets (e.g., ferrite magnets) having similar or identical dimensions, spacing, and/or magnetic field strengths as first set of magnets  210  and second set of magnets  212 . Further, any one of first set of magnets  710 , second set of magnets  712 , fourth set of magnets  762 , and fifth set of magnets  764  may have a linear configuration (e.g., similar to first set of magnets  502  and second set of magnets  504  shown in  FIGS. 5A and 5B ) or a circular configuration (e.g., similar to first set of magnets  210  and second set of magnets  212  shown in  FIGS. 4A and 4B ). 
     Third set of magnets  714  may be positioned between interior region  732  of plasma source chamber  702  and interior region  733  of process chamber  704 , and may function as a magnetic filter. In particular, third set of magnets  714  may be configured to confine a majority of high energy electrons (e.g., greater than 10 eV) of plasma  720  within interior region  732  of plasma source chamber  702 . In particular, third set of magnets  714  may be configured to generate multi-cusp magnetic fields that extend continuously across interior region  732  of plasma source chamber  702  from one portion of sidewall  718  to an opposite portion of sidewall  718 . These multi-cusp magnetic fields may resist high energy electrons (e.g., greater than 10 eV) of plasma  720  from leaving plasma source chamber  702  into process chamber  704 . Further, the multi-cusp magnetic fields may resist high energy electrons of second plasma  721  in process chamber  704  from back-flowing into plasma source chamber  702 . 
     First set of magnets  710 , second set of magnets  712 , and third set of magnets  714  may be configured to confine plasma  720  by generating a first plurality of multi-cusp magnetic fields that surround plasma  720 . In particular, the first plurality of multi-cusp magnetic fields may approximately surround the entire plasma  720 . Similarly, third set of magnets  714 , fourth set of magnets  762 , and fifth set of magnets  764  may be configured to generate a second plurality of multi-cusp magnetic fields that surround second plasma  721 . The second plurality of multi-cusp magnetic fields may approximately surround the entire second plasma  721 . The first plurality of multi-cusp magnetic fields may resist high energy electrons (e.g., greater than 10 eV) of plasma  720  from colliding with and being absorbed by end wall  717  and sidewall  712 . The second plurality of multi-cusp magnetic fields may resist high energy electrons (e.g., greater than 10 eV) of second plasma  721  from colliding with and being absorbed by base wall  768  and sidewall  766 . By resisting high energy electrons from being absorbed by walls  717 ,  718 ,  766 , and  768 , the first and second plurality of multi-cusp magnetic fields may enable plasma  720  and second plasma  721  to be stably generated at lower pressures (e.g., less than 0.1 or 0.02 Pa) and with higher ionization rates (e.g., greater than 70%). This enables greater efficiency in generating ions in plasma  720  to form ion beam  724 , and in generating electrons in second plasma  721  to neutralize ion beam  724  to form neutral beam  735 . Further, the higher ionization rates may enhance the conductivity of plasma  720  and second plasma  721  and thus improve the uniformity of their plasma density profiles across the inner diameters of plasma source chamber  702  and process chamber  704 , respectively. As a result, ion beam  734  and neutral beam  735  may be generated uniformly. 
     Plasma source chamber  702  may be configured to introduce one or more process gases from one or more gas sources (not shown) into plasma source chamber  702 . One or more RF antennas  730  may be disposed within plasma source chamber  702  to generate plasma  720  in interior region  732  of plasma source chamber  702 . In particular, RF antenna(s)  730  may be configured to introduce LF, RF, or VHF power from RF power source  746  to ionize and dissociate the one or more process gases to form plasma  720 . In a specific example, RF power source  746  may be configured to introduce greater than 200 W of RF power at a frequency of between 100 kHz and 100 MHz via RF antenna(s)  730 . The diameter of RF antenna(s)  730  may be greater than a diameter of work piece  706 . In particular, RF antenna(s)  730  may be configured to generate plasma  720  that is sufficiently large such that the diameter of the cross-section of ion beam  734  is greater than the diameter of work piece  706 . 
     Process chamber  704  may be configured to introduce one or more second process gases from one or more gas sources (not shown) into process chamber  704 . One or more second RF antennas  731  may be disposed in process chamber  704  to generate second plasma  721  in interior region  733  of process chamber  704 . In particular, second RF antenna(s) may be configured to introduce LF, RF, or VHF power from second RF power source  747  to ionize and dissociate the one or more second process gases to form second plasma  721 . In a specific example, RF power source  746  may be configured to introduce greater than 50 W of RF power at a frequency greater than 30 MHz via second RF antenna(s)  731 . The diameter of second RF antenna(s)  731  may be greater than the diameter of work piece  706 . In particular, second RF antenna(s)  731  may be configured to generate second plasma  721  having a diameter greater than the diameter of work piece  706 . This enables neutral beam  735  to be generated such that the cross-section of neutral beam  735  has a diameter greater than the diameter of work piece  706 . Neutral beam  735  may thus simultaneously treat an entire surface of work piece  708 . 
     In some examples, second RF antenna(s)  731  may be positioned between third set of magnets  714  and support structure  708 . In these examples, second RF antenna(s)  731  may be positioned closer to third set of magnets  714  than support structure  708 . Positioning second RF antenna(s)  731  too close to support structure  708  may result in excessive dissociation of the neutral particles in neutral beam  725 . Positioning second RF antenna(s)  731  too close to third set of magnets  714  may cause low energy electrons generated in second plasma  721  to backstream into plasma source chamber  702 , which reduces the efficiency of second plasma  721 . Thus, to avoid these undesirable effects, second RF antenna(s)  731  may be positioned at a distance of between 0.1L and 0.3L from the end of process chamber  704  proximate to plasma source chamber  702 , where L is the distance from support structure  708  to the end of process chamber  704  proximate to plasma source chamber  702 . 
     Alternatively, in some examples (not depicted in  FIG. 7 ), second RF antenna(s)  731  may be positioned between support structure  708  (or work piece  706 ) and base wall  768 . In these examples, ion beam  734  does not pass through second RF antenna(s)  731 . Positioning second RF antenna(s)  731  in this manner may be advantageous to reduce the dissociation of neutral particles in ion beam  734  or neutral beam  735  prior to neutral beam  735  striking the surface of work piece  706 . Second RF antenna(s)  731  may be positioned closer to support structure  708  than base wall  768 . In a specific example, second RF antenna(s)  731  may be positioned at a distance of about 0.1L from work piece  706  on support structure  708 , where L is the distance from support structure  708  to the end of process chamber  704  proximate to plasma source chamber  702 . As discussed above, magnets  762  and  764  may enable second plasma  721  to be generated at lower pressures (e.g., less than 0.1 or 0.02 Pa). The lower operating pressures may result in a large mean free path of electrons in process chamber  704 . Thus, in these examples, low energy electrons generated by second RF antenna(s) may readily diffuse towards ion beam  734 . Further, the low energy electrons may be attracted to the oppositely charged ions in ion beam  734 , which further promotes the diffusion of low energy electrons towards ion beam  734 . A cloud of low energy electrons may thus form between third set of magnets  714  and support structure  708  and extend across the inner diameter of process chamber  704 . In these examples, ion beam  734  may thus be directed through the cloud of low energy electrons (e.g., in second plasma  731 ) and be neutralized by the cloud of low energy electrons to generate neutral beam  735 . 
     Although  FIG. 7  is shown to include RF antenna(s)  730  and second RF antenna(s)  731 , it should be recognized that other configurations may be possible to supply RF power into plasma source chamber  702  and/or process chamber  704 . For example, in place of RF antenna(s)  730  or second RF antenna(s)  731 , induction coils may be disposed around the outside of plasma source chamber  702  or process chamber  704 . In such an example, power sources  746  and/or second power sources  747  may be coupled to the induction coils to supply power (e.g., electrical power or AC electrical power) into plasma source chamber  702 . In another example, plasma source chamber  702  and/or process chamber  704  may be configured to supply UHF or microwave power into plasma source chamber  702  and/or process chamber  704  to generate plasma  720  and second plasma  721 , respectively. 
     As briefly discussed above, one or more bias voltage sources  770  may be configured to apply a bias voltage between plasma source chamber  702  and process chamber  704 . In some examples, the one or more bias voltage sources  770  may be configured to apply a bias voltage between the walls ( 717 ,  718 ) of plasma source chamber  702  and the walls ( 766 ,  768 ) of process chamber  704 . Specifically, walls  717 ,  718  of plasma source chamber  702  may be biased at a first potential whereas walls  766 ,  768  of process chamber  704  may be biased at a second potential that is lower than the first potential. In other examples, the one or more bias voltage sources  770  may be configured to apply a bias voltage between walls  717 ,  718  of plasma source chamber  702  and work piece  706  (via support structure  708 ). In particular, walls  717 ,  718  of plasma source chamber  702  may be biased at a first potential whereas support structure  708  may be biased at a second potential that is lower than the first potential. In these examples, the applied bias voltage may cause ion beam  734  to accelerate from third set of magnets  714  to work piece  706  on support structure  708 . 
     In yet other examples, system  700  may include screen  772  disposed between third set of magnets  714  and second RF antenna(s)  731 . In some examples, screen  772  may be disposed between third set of magnets  714  and second plasma  721 . Screen  772  may extend across process chamber  704  from one portion of sidewall  766  to an opposite portion of sidewall  766 . The one or more bias voltage sources  770  may be configured to apply a bias voltage between walls  717 ,  718  of plasma source chamber  702  and screen  772 . Specifically, walls  717 ,  718  of plasma source chamber  702  may be biased at a first potential whereas screen  772  may be biased at a second potential that is lower than the first potential. In some examples, screen  772  may be configured such that substantially the entire screen  772  is biased at the second potential. In other examples, screen  772  may comprise multiple regions that may be independently biased at different potentials. Thus, one or more bias voltages may be applied between walls  717 ,  718  of plasma source chamber  702  and the multiple regions of screen  772 . The applied bias voltage(s) may cause ion beam  734  to accelerate from third set of magnets  714  to screen  772  on support structure  708 . Screen  772  may be a fine wire structure, such as a wire mesh. For example, screen  772  may comprise aluminum wire with titanium oxide coating. Screen  772  may include multiple openings for allowing ion beam  734  to pass through into second plasma  731 . The wire of screen  772  may be sufficiently fine and the openings may be sufficiently large such that shadowing of ion beam  734  is not created as ion beam  734  passes through screen  772 . For example, the wire of screen  772  may have a diameter of approximately 1-3 mm and the openings of screen  772  may have an area of at least 3 cm 2  or 4 cm 2 . Screen  772  may function to ensure a constant plasma potential of second plasma  721  across the inner diameter of process chamber  704 . In particular, when a high bias voltage (e.g., greater than 3 keV) is applied between plasma source chamber  702  and process chamber  704  to generate a high energy ion beam  734 , the high energy ion beam  734  entering process chamber  704  may cause second plasma  721  to become non-uniform across the inner diameter of process chamber  704 . Positioning screen  772  between third set of magnets  714  and second plasma  721  and applying a potential (e.g., ground potential) to screen  772  may serve to enforce the plasma potential uniformity across second plasma  721 . 
     In some examples, system  700  may not include any extraction grids (e.g., extraction grids  224  of system  200 ). For example, as shown in  FIG. 7 , no extraction grids are disposed between third set of magnets  714  and support structure  708 . It should be recognized that grids  224  are structurally different from screen  772 . Grids  224  may comprise smaller openings than screen  772 . Additionally or alternatively, the openings of grids  224  may be spaced further apart than screen  772 . Specifically, the openings of grids  224  may be configured such that a shadow is created downstream of grids  224 . As described above, grids  224  form a plurality of beamlets as the ion beam passes through grids  224 . In contrast, ion beam  734  may passes through screen  772  without forming beamlets. 
     2. Plasma-Based Material Modification Process 
       FIG. 8  depicts an exemplary plasma-based material modification process  800 . Process  800  may be performed using a plasma-based material modification system having a plasma source with magnetic confinement. In the present example, with reference to  FIG. 2 , process  800  is performed using plasma-based material modification system  200 . However, it should be recognized that process  800  may be performed using other plasma-based material modification systems (e.g., using plasma-based material modification system  700 ). Process  800  is described below with simultaneous reference to  FIG. 2  and  FIG. 8 . 
     Process  800  may be performed at a low pressure where the pressures in the plasma source chamber  202 , the drift region  226 , and the process chamber  204  are regulated to below 0.1 Pa or below 0.02 Pa. The pressure may be regulated by controlling throttle valve  238  and high-speed vacuum pump  240 . As described above, low operating pressures are desirable to achieve higher system reliability, superior process control, and higher device yields. 
     At block  802  of process  800 , work piece  206  is positioned on support structure  208 . In one example, work piece  206  may be a semiconductor substrate (e.g., silicon, germanium, gallium arsenide, etc.) with semiconductor devices at least partially formed thereon. In another example, work piece  206  may be a glass substrate with thin film semiconductor devices at least partial formed thereon. 
     At block  804  of process  800 , plasma  220  is generated in plasma generation region  232  of plasma source chamber  202 . Plasma  220  contains ions, neutral species, and electrons. In one example, the fraction of electrons of plasma  220  having energy greater than 10 eV may be greater than that of a plasma generated by a plasma source having no magnetic confinement or only partial magnetic confinement. Plasma  220  may be generated by supplying a process gas from gas source  244  into plasma source chamber  202  and introducing power from a power source into plasma source chamber  202  to ionize and dissociate the process gas. It should be recognized that multiple process gases may be supplied into plasma source chamber  202  to generate plasma  220 . 
     A process gas may be any pre-cursor gas containing one or more elements for modifying the physical, chemical, or electrical properties of work piece  206 . For example, the process gas may be a boron, phosphorous, or arsenic containing gas (e.g., arsine, phosphine, diborane, arsenic or phosphorus vapor, boron trifluoride, etc.) to introduce charge carriers (e.g., holes or electrons) into work piece  206 . Further, the process gas may include an inert gas such as helium or an additive gas such as hydrogen. In some examples, the process gas may contain elements such as carbon, nitrogen, noble gas or a halogen for modifying the intrinsic stress or other mechanical or chemical properties of the surface of work piece  206 . Such process gases may also be used for modifying the work function at layer interfaces of device structures on work piece  206 . In other examples, the process gas may contain elements such as silicon, germanium, aluminum, a chalcogen, or a lanthanide for modifying the Schottky barrier height at layer interfaces of device structures on work piece  206 . 
     The process gas may be ionized and dissociated by supplying power (e.g., electrical power or AC electrical power) from power source  246  (e.g., electrical power source or AC electrical power source) via antenna  230  into plasma source chamber  202 . In this example, LF, RF, or VHF power is supplied from power source  236  via antenna  230  into plasma source chamber  202  to generate high energy electrons in plasma generation region  232 . The high energy electrons ionize and dissociate the process gas to form plasma  220 . In one example, power source  246  may supply 200 W to 10 kW of RF power at a frequency between 100 kHz and 100 MHz via antenna  230  to ionize and dissociate the process gas in plasma source chamber  202 . It should be recognized that other forms of power may be supplied to ionize and dissociate the process gas. For example, as described above, UHF or microwave power may be supplied instead of LF, RF, or VHF power. In another example, a heated filament in the plasma generation region  232  may be used to ionize and dissociate the process gas. 
     In one example, plasma  220  may be generated in plasma source chamber  202  at a pressure below 0.1 Pa. In another example, plasma  220  may be generated at below 0.02 Pa. Generating plasma  220  at a lower pressure is advantageous because it increases the average energy of electrons (i.e., electron temperature) in plasma  220 , which, within a range of energies, exponentially increases the ionization rate per electron within plasma  220 . A greater ionization rate results in a higher concentration of ions and a lower concentration of neutral species within plasma  220 . For example, the ratio of neutral species to ions in plasma  220  may be at least an order of magnitude lower when plasma  220  is generated at the same power density at a pressure below 0.1 Pa than when plasma  220  is generate at a pressure of 1 Pa. Lower concentrations of neutral species in plasma  220  are advantageous in reducing the flux of neutral species to work piece  206 . Further, a greater ionization rate enables process  800  to be more gas efficient since less process gas is needed to generate ion beam  234  and treat work piece  206 . 
     Plasma  220  is generated within plasma generation region  232  of plasma source chamber  202  where a majority of electrons having energy greater than 10 eV are confined by first set of magnets  210 , second set of magnets  212 , and third set of magnets  214 . As described above in connection with  FIG. 3 , first set of magnets  210 , second set of magnets  212 , and third set of magnets  214  produce multi-cusp magnetic fields that surround the plasma generation region  232 . The multi-cusp magnetic fields repel energetic electrons from end wall  216 , sidewall  218 , and third set of magnets  214 , thereby increasing the efficiency at which plasma  220  is generated within plasma generation region  232  at distances greater than 5 cm from end wall  216  and sidewall  218 . By confining energetic electrons in plasma  220 , plasma  220  may be stably generated and sustained at pressures below 0.1 Pa or 0.02 Pa. In the absence of first set of magnets  210 , second set of magnets  212 , and third set of magnets  214 , plasma  220  may become unstable or unsustainable at pressures below 0.1 Pa and thus may be unsuitable for performing material modification for mass production. 
     Plasma  220  may be generated in plasma source chamber  202  having a cross-sectional area that is significantly greater than the area of work piece  206 . In one example, internal diameter  236  of plasma source chamber  202  may be greater than 45 cm. In another example, internal diameter  236  may be 50% to 100% larger than the diameter of work piece  206 . As previously described, a larger internal diameter  236  is advantageous in enabling work piece  206  to be treated with ions from only the center regions of ion beam  234  where the current density profile is more uniform. 
     At block  806  of process  800 , ions are accelerated from plasma  220  towards work piece  206  to treat work piece  206  with ions. In one example, block  806  may be performed by applying one or more bias voltages to one or more grids of grids  224  to accelerate ions from plasma  220  to work piece  206 . The one or more bias voltages may be a DC, pulsed DC, RF bias voltage, or a combination thereof. In such an example, plasma-based material modification system  200  includes grids  224  disposed between third set of magnets  214  and support structure  208 . As described above, grids  224  are positioned at an optimal distance  228  from third set of magnets  214  to achieve a more uniform current density profile to treat work piece  206 . In one example, distance  228  is between 0.10D and 0.33D, where D is internal diameter  236  of plasma source chamber  202 . In another example, distance  228  is between 0.2D and 0.30D. In yet another example, distance  228  is between 6 cm and 18 cm. 
     The one or more bias voltages may be applied to the one or more grids of grids  224  using one or more bias power sources  248 . Bias power source  248  may be a DC power source, a pulsed DC power source, or an RF power source. Applying the one or more bias voltages to the one or more grids of grids  224  extracts ion beam  234  from plasma  220  and accelerates ion beam  234  through grids  224  to work piece  206 . Additionally, grids  224  may focus or collimate ion beam  234 . For example, ion beam  234  may comprise multiple beamlets as it passes through grids  224 . Applying one or more bias voltages on grids  224  may focus and collimate the beamlets of ion beam  234 . 
     In the present example, grids  224  include 5 grids. For convenience, the grids will be referred to in sequential order with the grid closest to plasma source chamber  202  being referred to as the “first grid” and the grid closest to process chamber  204  being referred to as the “fifth grid.” In one example, the first grid may function as an extraction grid and be biased at approximately ±100 V with respect to the potential of end wall  216  and sidewall  218  of plasma source chamber  202 . The second grid may be an acceleration grid that is biased at a negative extraction voltage of up to −20 kV with respect to the first grid to extract ion beam  234  from plasma  220 . It should be appreciated that the extraction voltage applied to the second grid with respect to the first grid must be approximately in accordance with the Child-Langmuir law, where the current density extracted is a function of the potential difference between the grids and the distance between the grids. The fifth grid may be biased at approximately ground while the fourth grid may be biased at a negative voltage (e.g., −200 V to 0 V) relative to the fifth grid to suppress electron back-acceleration into plasma source chamber  202 . The bias voltage applied to the third and the fourth grid may be selected to achieve the desired energy and profile of ion beam  234 . 
     It should be recognized that any number of grids may be used to extract, accelerate, and focus ion beam  234 . Additionally, it should be appreciated that using four or more grids offers greater flexibility in achieving the desire energy and profile of ion beam  234 . 
     In some examples, process  800  may be performed using plasma-based material modification system  200  without grids  224 . In such examples, block  806  may be performed by applying a bias voltage to work piece  206  to accelerate ions from plasma  220  to work piece  206 . The bias voltage may be applied to work piece  206  via support structure  208  using bias power source  254 . Bias power source  254  may be, for example, a DC power source, a pulsed DC power source, or an RF power source. Applying a bias voltage to work piece  206  accelerates ions from plasma  220  to work piece  206 . A plasma sheath may form between plasma  220  and work piece  206  where ions from plasma  220  accelerate across the plasma sheath to work piece  206 . Further, at low operating pressures, there is less charge exchange in the plasma sheath and thus the energy distribution of ions reaching work piece  206  is tighter. 
     To achieve uniform treatment of ions across work piece  206 , work piece  206  may be positioned by support structure  208  at an optimal distance from third set of magnets  214 . Positioning work piece  206  too close to third set of magnets  214  may result in non-uniform ion current density at work piece  206  due to ion shadowing from third set of magnets  214 . However, positioning work piece  206  too far from grids  224  may also result in non-uniform ion current density at work piece  206  due to ion losses to the sidewalls. In one example, work piece  206  may be positioned by support structure  208  at a distance between 0.10D and 0.33D from third set of magnets  214 , where D is internal diameter  236  of plasma source chamber  202 . In another example, work piece  206  may be positioned by support structure  208  at a distance between 0.2D and 0.3D from third set of magnets  214 . 
     As described above, energetic electrons of plasma  220  are confined by first set of magnets  210 , second set of magnets  212 , and third set of magnets  214 , which enables lower operating pressures and thus lower concentrations of neutral species reaching work piece  206 . Lower concentrations of neutral species reaching work piece  206  causes less parasitic etching, oxidation, or deposition on the surface of work piece  206  and thus results in higher device yields. In one example, the parasitic deposition or etching on work piece  206  may be less than 2 nm for an ion dose of 1 E14 cm −2  to 1 E17 cm −2  when process  800  is performed at an operating pressure of less than 0.1 Pa. In another example, an ion uniformity of less than 1% (one sigma variation from the mean) may be achieved using process  800  for an ion dose of 1 E14 cm −2  to 1 E17 cm 2 . 
     In some cases, process  800  may be performed using plasma-based material modification system  200  having absorber  250 . In such cases, absorber  250  may interact with ions flowing from plasma  220  towards support structure  208  and absorb a fraction of the ions. As described above, one region of absorber  250  may have an ion transparency that is different from that of another region of absorber  250 . In the present example, the ion transparency of absorber  250  increases from the center to the edge of absorber  250 . Thus, ions exiting absorber  250  may have a current density profile that is different from that of ions flowing from plasma  220  to absorber. In particular, ions exiting absorber  250  may have a more uniform current density profile than that of ions flowing from plasma  220  to absorber  250 . 
     Absorber  250  may be positioned between the center of plasma  220  and support structure  208 . In cases where plasma-based material modification system  200  includes grids  224 , absorber  250  may be positioned either between the center of plasma  220  and third set of magnets  214  or between third set of magnets  214  and grids  224 . In other cases where plasma-based material modification system  200  does not have grids  224 , absorber  250  may be positioned either between the center of plasma  220  and third set of magnets  214  or between third set of magnets  214  and support structure  208 . In some cases, absorber  250  may be positioned no closer than 5 cm from support structure  208 . 
     Unlike grids  224 , absorber  250  may be surrounded by plasma from plasma source chamber  202 . In cases where absorber  250  is positioned between third set of magnets  214  and support structure  208 , plasma from plasma source chamber  202  occupies drift region  226  and thus absorber  250  is surrounded by both ions and electrons of plasma from plasma source chamber  202 . In contrast, when grids  224  are biased to extract, accelerate, and focus ion beam  234 , regions between adjacent grids are denude of electrons. 
     Further, process  800  may include applying a bias potential on absorber  250  using a bias voltage source. In one example, absorber  250  may be biased at a DC or RF potential that is different from that of the local plasma potential or local floating potential adjacent to absorber  250 . In one such example, absorber  250  may be biased at a suitable potential to attract ions. This increases the rate at which absorber  250  absorbs ions and thus increases the extent at which the current density profile is adjusted through absorber  250 . In another such example, absorber  250  may be biased at a suitable potential to repel ions. This may decrease the rate at which absorber  250  absorbs ions and thus may decrease the extent at which the current density profile is adjust through absorber  250 . Additionally, the energy at which ions impact absorber  250  would be reduced, which may be advantageous in preventing impurities from being sputtered off absorber  250  and contaminating work piece  206 . In yet another example, absorber  250  may have a floating potential where it is electrically isolated from any power source or power sink and thus is allowed to absorb equal current of ions and electrons. 
     In some cases, absorber  250  may comprised more than one region that may be independently biased. In such cases, process  800  may include applying one or more bias voltages to one or more regions of absorber  250 . Each region may be biased dynamically to control the uniformity of the current density of ions treating work piece  206 . It should be recognized that absorber  250  may be biased at any time during process  800  to achieve a desired current density profile. 
     It should be recognized that applying a bias to absorber  250  does not substantially alter the energy of the ions passing through absorber  250 . This is due to the presence of plasma surrounding both sides of absorber  250 . Thus the average energy of ions exiting absorber  250  is approximately equal to the average energy of ions flowing from plasma  220  to absorber  250 . This is in contrast to grids  224  where ions are accelerated and thus the energy of ions change significantly as the ions pass through grids  224 . 
     Further, it should be recognized that absorber  250  may be used in various other plasma-based processing systems to improve the current density profile uniformity of ions treating the work piece. For example, absorber  250  may be used in conventional plasma-based material modification systems, plasma etchers, sputter systems, or plasma enhanced chemical vapor deposition system. Accordingly, process  800  may be performed using various other plasma-based processing systems having an absorber in a similar manner as described above. 
     Process  800  may include tilting and/or rotating work piece  206  using support structure  208  to improve the uniformity of ion treatment across work piece  206 . Tilting of work piece  206  enables ion beam  234  to impact work piece  206  at an angle with respect to the perpendicular of work piece  206  while rotating work piece  206  varies the azimuth to allow all sides of 3D-structures on work piece  206  to be treated with ions. Further, process  800  may include introducing low-energy electrons between grids  224  and work piece  206  using electron source  252  to neutralize the space charge of ion beam  234 . Neutralizing the space charge of ion beam  234  is desirable to reduce the spread of ion beam  234  caused by space charge effects. 
       FIG. 9  depicts another exemplary plasma-based material modification process  900 . Process  900  may be performed using a plasma-based material modification system that is configured to generate a first plasma in a plasma source chamber and a second plasma in a process chamber. For example, process  900  may be performed using plasma-based material modification system  700 , described above. Process  900  is described below with simultaneous reference to  FIG. 7  and  FIG. 9 . 
     At block  902 , plasma  720  may be generated in plasma source chamber  702 . Block  902  may be similar or identical to block  804 , described above. Plasma  720  may be generated by supplying one or more process gases into plasma source chamber  702  and by introducing RF power from RF power source  746  into plasma source chamber  702  (e.g., via RF antenna  730 ). The RF power may cause the one or more process gases to ionize and dissociate. Plasma  720  may thus contain ions, neutral species, and/or electrons derived from the one or more process gases. In some examples, plasma  720  may be generated without the use of inert additive gases (e.g., hydrogen, argon, xenon) to help sustain the plasma. Further, in some examples, RF power greater than 200 W may be supplied by RF power source  746  into plasma source chamber  702  (e.g., via RF antenna  730 ) to form a high density plasma. 
     In some examples, plasma  720  may be generated at a lower pressure (e.g., less than 0.1 or 0.02 Pa) in plasma source chamber  702  to improve plasma stability and increase ionization rates. Generating plasma  720  at a lower pressure may be possible with first set of magnets  710 , second set of magnets  712 , and third set of magnets  714 , which function to confine plasma  720 . In particular, a first plurality of multi-cusp magnetic fields that surround plasma  720  may be generated using first set of magnets  710 , second set of magnets  712 , and third set of magnets  714 . The first plurality of multi-cusp magnetic fields may form a continuous magnetic field barrier around approximately the entire plasma  720  to resist high energy (e.g., greater than 10 eV) electrons of plasma  720  from colliding into end wall  717  and sidewall  718  and escaping into process chamber  704 . The confinement of plasma  720  may enable plasma  720  to be generated stably at lower pressures (e.g., less than 0.1 or 0.02 Pa). Thus, plasma  720  may be stably sustained at a pressure below 0.1 Pa or 0.02 Pa while neutral beam  735  treats work piece  706 . 
     At block  904 , a magnetic field is generated using third set of magnets  714 . The magnetic field generated by third set of magnets  714  may comprise a plurality of multi-cusp magnetic fields that extend continuously across the interior of plasma source chamber  702  from one portion of sidewall  718  to an opposite portion of sidewall  718 . The generated magnetic field may confine high energy electrons (e.g., greater than 10 eV) of plasma  720  within the plasma source chamber  702 . In particular, the generated magnetic field may allow ions of plasma  720  to flow from plasma source chamber  702  into process chamber  704 , but may resist high energy electrons (e.g., greater than 10 eV) of plasma  720  from passing through third set of magnets  714  into process chamber  704 . This may increase ionization rates of plasma  720 , which would improve gas efficiency. In addition, resisting high energy electrons from passing into process chamber  704  can reduce undesirable dissociation, ionization, or neutralization of the ions of ion beam  734  in process chamber  704 . Further, the generated magnetic field may resist high energy electrons (e.g., greater than 10 eV) of second plasma  721  from back-flowing into plasma source chamber  702 . 
     At block  906 , second plasma  721  may be generated in process chamber  704 . Second plasma  721  may be generated by supplying one or more second process gases into process chamber  704  and introducing RF power from RF power source  747  into process chamber  704  (e.g., via second RF antenna  731 ). Second plasma  721  may serve as a source of low energy electrons (e.g., less than 2 eV, less than 1 eV, or 1-2 eV) to neutralize ion beam  734 . In some examples, RF power less than 50 W may be supplied at a frequency greater than 30 MHz by RF power source  747  into process chamber  704  (e.g., via second RF antenna  731 ) to generate second plasma  721 . The lower RF power enables a greater proportion of low energy electrons to be generated in second plasma  721 . For example, second plasma  721  may have a concentration of low energy electrons (e.g., less than 2 eV, less than 1 eV, or 1-2 eV) that is greater than 1E12/cm 3 , greater than 1E13/cm 3 , or between 1E12/cm 3  and 1E13/cm 3 ). Low energy electrons may be desirable to neutralize the ions in ion beam  734 . In contrast, high energy electrons (e.g., greater than 10 eV) may ionize and/or dissociate the ions in ion beam  734 , which may hinder the formation of a uniform neutral beam  735 . In some examples, the one or more second process gases used to generate second plasma  721  may include inert additive gases (e.g., hydrogen, argon, or xenon). The inert additive gases may facilitate with the generation of low energy electrons (e.g., less than 2 eV, less than 1 eV, or 1-2 eV) in second plasma  721  without generating additional species (e.g., ions or neutral species) that may cause undesirable material modification of work piece  706 . 
     In some examples, second plasma  721  may be generated at a lower pressure (e.g., less than 0.1 or 0.02 Pa) in process chamber  704  to improve plasma stability and increase ionization rates. Higher ionization rates may enable higher concentrations of electrons to be generated for neutralizing ion beam  734 . Generating second plasma  721  at a lower pressure may be possible with third set of magnets  714 , fourth set of magnets  762 , and fifth set of magnets  764 , which function to confine second plasma  721 . In particular, a second plurality of multi-cusp magnetic fields that surround second plasma  721  may be generated using third set of magnets  714 , fourth set of magnets  762 , and fifth set of magnets  764 . The second plurality of multi-cusp magnetic fields may form a continuous magnetic field barrier around approximately the entire second plasma  721  to resist high energy (e.g., greater than 10 eV) electrons of second plasma  721  from colliding into base wall  768  and sidewall  766 . The confinement of second plasma  721  by the second plurality of multi-cusp magnetic fields may enable second plasma  721  to be generated stably at lower pressures (e.g., less than 0.1 or 0.02 Pa). Thus, second plasma  721  may be stably sustained at a pressure below 0.1 Pa or 0.02 Pa while neutral beam  735  treats work piece  706 . 
     Second plasma  721  may include ions, neutral species, and/or electrons derived from the one or more second process gases (e.g., through dissociative ionization and electron impact ionization). In addition, second plasma  721  may include ions, neutral species, and/or low energy electrons (e.g., 10 eV or less) from plasma  720 . The ions, neutral species, and/or low energy electrons of plasma  720  may diffuse into process chamber  704  through third set of magnets  714  from plasma source chamber  702  and combine with second plasma  721 . Further, ion beam  734  (described in greater detail below) may transport ions of plasma  720  from plasma source chamber  702  into process chamber  704  and combine with second plasma  721  as ion beam  734  passed through second plasma  721 . 
     Second plasma  721  may extend across the interior of process chamber  704  such that the diameter of second plasma  721  is greater than the diameter of work piece  706 . In some examples, second plasma  721  may be generated such that the center portion of second plasma  721  is approximately uniform (e.g., less than ±5% or ±3% variation). The center portion of second plasma  721  may have a diameter that is approximately equal to the diameter of work piece  706  and may have a center axis aligned with the center axis of process chamber  704 . Additionally, the density of electrons across the center region of second plasma  721  may be approximately uniform (e.g., less than ±5% or ±3%). This may enable second plasma  721  to uniformly neutralize ion beam  734 , thereby generating a uniform neutral beam  735 . 
     Second plasma  721  may be generated using second RF antenna  731  having a diameter greater than the diameter of work piece  706 . As discussed above, in some examples, second RF antenna  731  may be positioned in process chamber  704  between third set of magnets  714  and work piece  706 . In these examples, ion beam  734  may pass through second RF antenna  731  in second plasma  721 . Second RF antenna  731  may be positioned closer to third set of magnets  714  than work piece  706 . For example, second RF antenna(s)  731  may be positioned at a distance of between 0.1L and 0.3L from the end of process chamber  704  proximate to plasma source chamber  702 , where L is the distance from work piece  706  to the end of process chamber  704  proximate to plasma source chamber  702 . 
     Alternatively, in some examples, second RF antenna  731  may be positioned between work piece  706  and base wall  768 . In these examples, ion beam  734  may not pass through second RF antenna  731 . Second RF antenna  731  may be positioned closer to support structure  708  than base wall  768 . In a specific example, second RF antenna  731  may be positioned at a distance of about 0.1L from work piece  706  on support structure  708 , where L is the distance from support structure  708  to the end of process chamber  704  proximate to plasma source chamber  702 . The low energy electrons generated by second RF antenna  731  may diffuse towards ion beam  734  and form a cloud of low energy electrons (e.g., in second plasma  721 ) between third set of magnets  714  and support structure  708 . The cloud of low energy electrons may extend across the inner diameter of process chamber  704  and may have a diameter greater than the diameter of work piece  706 . In these examples, ion beam  734  may be directed through the cloud of low energy electrons (e.g., in second plasma  731 ) and be neutralized by the cloud of low energy electrons to generate neutral beam  735 . 
     At block  908 , ion beam  734  may be generated in the process chamber by extracting ions from plasma  720  through third set of magnets  714 . In particular, ion beam  734  may be generated by applying a bias voltage from bias voltage source  770  between plasma source chamber  702  and process chamber  704 . For example, a first potential (e.g., ground potential) may be applied to sidewall  762  of process chamber  704 , and a second potential (e.g., greater than the first potential) may be applied to sidewall  718  of plasma source chamber  702 . Plasma source chamber  702  may be electrically isolated from process chamber  704 . The applied bias voltage may generate an electric field that causes the ions of plasma  720  to be extracted through third set of magnets  714  to generate ion beam  734 . In addition, the generated electric field may accelerate ion beam  734  towards work piece  706  from third set of magnets  714  to second plasma  821 . Ion beam  734  may be generated such that the cross-section of ion beam  734  has a diameter greater than the diameter of work piece  706 . In some examples, ion beam  734  may include greater than 95% or 99% ions as ion beam  734  enters process chamber  704 . 
     Although  FIG. 7  depicts the bias voltage being applied between sidewall  718  of plasma source chamber  702  and sidewall  766  of process chamber  704 , it should be recognized that the manner in which the bias voltage is applied at block  908  may vary. For instance, in some examples, the bias voltage may be applied between sidewall  718  of plasma source chamber  702  and work piece  706 . Specifically, work piece  706  may be disposed on support structure  708  within process chamber  704 . Work piece  706  may be biased at a first potential (e.g., ground potential) via support structure  708 , and sidewall  718  of plasma source chamber  702  may be biased at a second potential (e.g., greater than the first potential). In these examples, ion beam  734  may accelerate from third set of magnets  714  to work piece  706 . In other examples where system  700  includes screen  772 , the bias voltage may be applied between plasma source chamber  702  and screen  772 . In particular, screen  772  may be biased at a first potential (e.g., ground potential) and sidewall  718  of plasma source chamber  702  may be biased at a second potential (e.g., greater than the first potential). Screen  772  may be positioned between third set of magnets  714  and second plasma  721 . In these examples, ion beam  734  may accelerate from third set of magnets  714  to screen  772 . 
     As discussed above, second plasma  721  may include low energy electrons (e.g., less than 2 eV, less than 1 eV, or 1-2 eV). As ion beam  734  travels through second plasma  721 , the low energy electrons of second plasma  721  may neutralize the ions of ion beam  734  to generate neutral beam  735 . Neutral beam  735  thus emerges from second plasma  721  and travels towards work piece  706 . The cross-section of neutral beam  735  may be similar to the cross-section of ion beam  734 . In particular, the cross-section of neutral beam  735  may have a diameter that is greater than the diameter of work piece  706 . This enables neutral beam  735  to simultaneously treat approximately the entire surface of work piece  706 , which improves uniformity and throughput. Neutral beam  735  may comprise mostly of neutral species. In some examples, neutral beam  735  may comprise some percentage of ions. For example, neutral beam  735  may include no more than 10% or 20% ions. In some examples, neutral beam  735 , when incident on work piece  708 , may include more than 80% or 90% neutral species. 
     At block  910 , work piece  706  may be positioned in process chamber  704  such that neutral beam  735  treats a surface of work piece  706 . In particular, support structure  708  may position work piece  706  downstream of second plasma  721  such that neutral beam  735  is incident on the surface of work piece  706 . Because the diameter of the cross-section of neutral beam  735  is greater than the diameter of work piece  706 , work piece  706  may be positioned at block  910  such that approximately the entire surface of work piece  706  is simultaneously treated by neutral beam  735 . Neutral beam  735  may cause material modification of the surface of work piece  706 . In particular, neutral beam  735  may implant neutral species into work piece  706  to change the physical, chemical, or electrical properties of the surface of work piece  706 . In some examples, neutral beam  735  may deposit a layer of material on the surface of work piece  706 . Additionally or alternatively, in some examples, neutral beam  735  may etch a layer of material from the surface of work piece  706 . 
     While specific components, configurations, features, and functions are provided above, it will be appreciated by one of ordinary skill in the art that other variations may be used. Additionally, although a feature may appear to be described in connection with a particular embodiment, one skilled in the art would recognize that various features of the described embodiments may be combined. For example, any feature of system  200  may be combined with the features of system  700 , and vice versa. Similarly, any feature of process  800  may be combined with the features of process  900 , and vice versa. In addition, any feature described in connection with an embodiment may be omitted from the embodiment. Moreover, aspects described in connection with an embodiment may stand alone. 
     Although embodiments have been fully described with reference to the accompanying drawings, it should be noted that various changes and modifications will be apparent to those skilled in the art. Such changes and modifications are to be understood as being included within the scope of the various embodiments as defined by the appended claims.