Abstract:
A radio frequency (RF) driven plasma ion source has an external RF antenna, i.e. the RF antenna is positioned outside the plasma generating chamber rather than inside. The RF antenna is typically formed of a small diameter metal tube coated with an insulator. A flange is used to mount the external RF antenna to the ion source. The RF antenna tubing is wound around the flange to form a coil. The flange is formed of a material, e.g. quartz, that is essentially transparent to the RF waves. The flange is attached to and forms a part of the plasma source chamber so that the RF waves emitted by the RF antenna enter into the inside of the plasma chamber and ionize a gas contained therein. The plasma ion source is typically a multi-cusp ion source.

Description:
RELATED APPLICATIONS  
       [0001]    This application claims priority of Provisional Application Ser. No. 60/382,674 filed May 22, 2002, which is herein incorporated by reference. 
     
    
     GOVERNMENT RIGHTS  
       [0002] The United States Government has the rights in this invention pursuant to Contract No.DE-AC03-76SF00098 between the United States Department of Energy and the University of California. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0003]    The invention relates to radio frequency (RF) driven plasma ion sources, and more particularly to the RF antenna and the plasma chamber.  
           [0004]    A plasma ion source is a plasma generator from which beams of ions can be extracted. Multi-cusp ion sources have an arrangement of magnets that form magnetic cusp fields to contain the plasma in the plasma chamber. Plasma can be generated in a plasma ion source by DC discharge or RF induction discharge. An ion plasma is produced from a gas which is introduced into the chamber. The ion source also includes an extraction electrode system at its outlet to electrostatically control the passage of ions from the plasma out of the plasma chamber.  
           [0005]    Unlike the filament DC discharge where eroded filament material can contaminate the chamber, RF discharges generally have a longer lifetime and cleaner operation. In a RF driven source, an induction coil or antenna is placed inside the ion source chamber and used for the discharge. However, there are still problems with internal RF antennas for plasma ion source applications.  
           [0006]    The earliest RF antennas were made of bare conductors, but were subject to arcing and contamination. The bare antenna coils were then covered with sleeving material made of woven glass or quartz fibers or ceramic, but these were poor insulators. Glass or porcelain coated metal tubes were subject to differential thermal expansion between the coating and the conductor, which could lead to chipping and contamination. Glass tubes form good insulators for RF antennas, but in a design having a glass tube containing a wire or internal surface coating of a conductor, coolant flowing through the glass tube is subject to leakage upon breakage of the glass tube, thereby contaminating the entire apparatus in which the antenna is mounted with coolant. A metal tube disposed within a glass or quartz tube is difficult to fabricate and only has a few antenna turns.  
           [0007]    U.S. Pat. Nos. 4,725,449; 5,434,353; 5,587,226; 6,124,834; 6,376,978 describe various internal RF antennas for plasma ion sources, and are herein incorporated by reference.  
         SUMMARY OF THE INVENTION  
         [0008]    Accordingly, it is an object of the invention to provide an improved plasma ion source that eliminates the problems of an internal RF antenna.  
           [0009]    The invention is a radio frequency (RF) driven plasma ion source with an external RF antenna, i.e. the RF antenna is positioned outside the plasma generating chamber rather than inside. The RF antenna is typically formed of a small diameter metal tube coated with an insulator. A flange is used to mount the external RF antenna to the ion source. The RF antenna tubing is wound around the flange to form a coil. The flange is formed of a material, e.g. quartz, that is essentially transparent to the RF waves. The flange is attached to and forms a part of the plasma source chamber so that the RF waves emitted by the RF antenna enter into the inside of the plasma chamber and ionize a gas contained therein. The plasma ion source is typically a multi-cusp ion source. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    In the accompanying drawings:  
         [0011]    FIGS.  1 - 5  are side cross sectional views of various embodiments of a plasma ion source with an external RF antenna according to the invention.  
         [0012]    [0012]FIGS. 6A, B are end and side views of a flange for mounting an external antenna to a plasma ion source according to the invention.  
         [0013]    [0013]FIG. 7 is a graph of the relative amounts of various hydrogen ion species obtained with an external antenna source of the invention.  
         [0014]    [0014]FIG. 8 is a graph of hydrogen ion current density extracted from an external antenna source and from an internal antenna source, at the same extraction voltage.  
         [0015]    [0015]FIG. 9 is a graph of the electron current density produced by an external antenna source.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    The principles of plasma ion sources are well known in the art. Conventional multicusp plasma ion sources are illustrated by U.S. Pat. Nos. 4,793,961; 4,447,732; 5,198,677; 6,094,012, which are herein incorporated by reference.  
         [0017]    A plasma ion source  10 , which incorporates an external RF antenna  12 , is illustrated in FIG. 1. Plasma ion source  10  is preferably a multi-cusp ion source having a plurality of permanent magnets  14  arranged with alternating polarity around a source chamber  16 , which is typically cylindrical in shape. External antenna  12  is wound around flange  18  and electrically connected to a RF power source  20  (which includes suitable matching circuits), typically 2 MHz or 13.5 MHz. Flange  18  is made of a material such as quartz that easily transmits the RF waves. Flange  18  is mounted between two plasma chamber body sections  22   a ,  22   b , typically with O-rings  24  providing a seal. Chamber body sections  22   a ,  22   b  are typically made of metal or other material that does not transmit RF waves therethrough. The chamber body sections  22   a ,  22   b  and the flange  18  together define the plasma chamber  16  therein. Gas inlet  26  in (or near) one end of chamber  16  allows the gas to be ionized to be input into source chamber  16 .  
         [0018]    The opposed end of the ion source chamber  16  is closed by an extractor  28  which contain a central aperture  30  through which the ion beam can pass or be extracted by applying suitable voltages from an associated extraction power supply  32 . Extractor  28  is shown as a simple single electrode but may be a more complex system, e.g. formed of a plasma electrode and an extraction electrode, as is known in the art. Extractor  28  is also shown with a single extraction aperture  30  but may contain a plurality of apertures for multiple beamlet extraction.  
         [0019]    In operation, the RF driven plasma ion source  10  produces ions in source chamber  16  by inductively coupling RF power from external RF antenna  12  through flange  18  into the gas in chamber  16 . The ions are extracted along beam axis  34  through extractor  28 . The ions can be positive or negative; electrons can also be extracted.  
         [0020]    FIGS.  2 - 5  show variations of the plasma ion source shown in FIG. 1. Common elements are the same and are not described again or even shown again. Only the differences or additional elements are further described.  
         [0021]    Plasma ion source  40 , shown in FIG. 2, is similar to plasma ion source  10  of FIG. 1, except that flange  18  with external antenna  12  is mounted to one end of a single plasma chamber body section  22  instead of between two body sections  22   a ,  22   b . The chamber body section  22  and the flange  18  together define the plasma chamber  16  therein. The extractor  28  is mounted directly to the flange  18  in place of the second body section so that flange  18  is mounted between body section  22  and extractor  30 .  
         [0022]    Plasma ion source  42 , shown in FIG. 3, is similar to plasma ion source  40  of FIG. 2, with flange  18  with external antenna  12  mounted to the end of a single plasma chamber body section  22 . However, ion source  42  is much more compact than ion source  40  since the chamber body section  22  is much shorter, i.e. chamber  16  is much shorter. In FIG. 2, the length of chamber body section  22  is much longer than the length of flange  12  while in FIG. 3 it is much shorter. Such a short ion source is not easy to achieve with an internal antenna.  
         [0023]    Plasma ion source  44 , shown in FIG. 4, is similar to plasma ion source  42  of FIG. 3. A permanent magnet filter  46  formed of spaced magnets  48  is installed in the source chamber  16  of plasma ion source  44 , adjacent to the extractor  28  (in front of aperture  30 ). Magnetic filter  46  reduces the energy spread of the extracted ions and enhances extraction of atomic ions.  
         [0024]    Plasma ion source  50 , shown in FIG. 5, is similar to plasma ion source  42  of FIG. 3, but is designed for negative ion production. An external antenna arrangement is ideal for surface conversion negative ion production. A negative ion converter  52  is placed in the chamber  16 . Antenna  12  is located between the converter  52  and aperture  30  of extractor  28 . A dense plasma can be produced in front of the converter surface. The thickness of the plasma layer can be optimized to reduce the negative ion loss due to stripping.  
         [0025]    [0025]FIGS. 6A, B illustrate the structure of a flange  18  of FIGS.  1 - 5  for housing and mounting an external antenna to a plasma ion source. Flange  18  is formed of an open inner cylinder  60  having a diameter D 1  and a pair of annular end pieces  62  attached to the ends of cylinder  62  and extending outward (from inner diameter D 1 ) to a greater outer diameter D 2 . Spaced around the outer perimeter of the annular pieces  62  are a plurality of support pins  64  extending between the pieces  62  to help maintain structural integrity. The inner cylinder  60  and extending end pieces  62  define a channel  66  in which an RF antenna coil can be wound. The channel  66  has a length T 1  and the flange has a total length T 2 .  
         [0026]    The antenna is typically made of small diameter copper tubing (or other metal). A layer of Teflon or other insulator is used on the tubing for electrical insulation between turns. Coolant can be flowed through the coil tubing. If cooling is not needed, insulated wires can be used for the antenna coils. Many turns can be included, depending on the length T 1  of the channel and the diameter of the tubing. Multilayered windings can also be used. Additional coils can be added over the antenna coils for other functions, such as applying a magnetic field.  
         [0027]    [0027]FIG. 7 is a graph of the relative amounts of various hydrogen ion species obtained with the source of FIG. 3. More than 75% of the atomic hydrogen ion H +  was obtained with an RF power of 1 kW. The current density is about 50 mA/cm 2  at 1 kW of RF input power. The source has been operated with RF input power higher than 1.75 kW.  
         [0028]    [0028]FIG. 8 is a comparison of hydrogen ion current density extracted from an external antenna source and from an internal antenna source, showing the extracted beam current density from an external antenna and internal antenna generated hydrogen plasma operating at the same extraction voltage. When operating at the same RF input power, the beam current density extracted from the external antenna source is higher than that of the internal antenna source.  
         [0029]    Simply by changing to negative extraction voltage, electrons can be extracted from the plasma generator using the same column. FIG. 9 shows the electron current density produced by an external antenna source. At an input power of 2500 W, electron current density of 2.5 A/cm 2  is achieved at 2500 V, which is about 25 times larger than ion production.  
         [0030]    The ion source of the invention with external antenna enables operation of the source with extremely long lifetime. There are several advantages to the external antenna. First, the antenna is located outside the source chamber, eliminating a source of contamination, even if the antenna fails. Any mechanical failure of the antenna can be easily fixed without opening the source chamber. Second, the number of turns in the antenna coil can be large (&gt;3). As a result the discharge can be easily operated in the inductive mode, which is much more efficient than the capacitive mode. The plasma can be operated at low source pressure. The plasma potential is low for the inductive mode. Therefore, sputtering of the metallic chamber wall is minimized. Third, plasma loss to the antenna structure is much reduced, enabling the design of compact ion sources. No ion bombardment of the external antenna occurs, also resulting in longer antenna lifetime.  
         [0031]    RF driven ion sources of the invention with external antenna can be used in many applications, including H −  ion production for high energy accelerators, H +  ion beams for ion beam lithography, D + /T +  ion beams for neutron generation, and boron or phosphorus beams for ion implantation. If electrons are extracted, the source can be used in electron projection lithography.  
         [0032]    A source with external antenna is easy to scale from sizes as small as about 1 cm in diameter to about 10 cm in diameter or greater. Therefore, it can be easily adopted as a source for either a single beam or a multibeam system.  
         [0033]    Changes and modifications in the specifically described embodiments can be carried out without departing from the scope of the invention which is intended to be limited only by the scope of the appended claims.