Abstract:
An ion source is provided that is constructed for use with a magnet that produces magnetic flux lines extending in a predetermined direction and a source of ionizable material for creating ion. The ion source includes a chamber, defined by walls, and a relatively narrow outlet aperture for ions produced in the chamber to leave the chamber. The chamber encloses a cathode and an anode spaced from the cathode and from the walls of the chamber. The anode is positioned with respect to the aperture, the cathode and the predetermined direction of the magnetic flux to cause ions produced in the chamber to drift in crossed magnetic and electric fields so as to concentrate near the aperture.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of application Ser. No. 08/980,513, filed Dec. 1, 1997 now abandoned. 
     This application is related to the commonly assigned applications “Space Neutralization of an Ion Beam”, filed herewith today, Ser. No. 09/083,706, “Ion Implantation with Charge Neutralization”, filed herewith today, Ser. No. 09/083,707, and “Transmitting a Signal Using Duty Cycle Modulation”, filed Dec. 1, 1997, Ser. No. 08/982,210, each of which is incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     This invention relates to an ion source, specifically an ion source for use in an ion implanter for implanting ions in a substrate. 
     In manufacturing semiconductors through ion implantation several types of ion sources are typically used. Ion implantation requires ion sources with long operational life and high ion source efficiency. One ion source used in ion implantation is the Bernas type ion source which has been widely accepted in ion implantation. 
     FIG. 1 shows a top view of a single filament Bernas type ion source  1  with its top plate removed. Ion source  1  has a cathode  12  connects to a power source that drives cathode  12  to therminiocally emit electrons. Walls  14  of ion source  14  are biased relative cathode  12  so as to act as an anode. A repeller plate  18  is positioned behind cathode  12  and another repeller plate  16  is positioned across from cathode  12 . The ion source is placed in a uni-directional magnetic field, as shown in FIG.  1 . 
     During operation, a gas to be ionized is discharged into the chamber and is ionized by electrons emitted from cathode  12 . Repeller plates  16  and  18  reflect primary fast electrons emitted from cathode  12  and generate an oscillatory electron movement along the axis of the magnetic field. In this manner, a plasma is generated in the ion source between cathode  12  and walls  14  for extraction by an extraction electrode outside ion source  1 . 
     When the ion source operates, material such as vaporized metal from cathode  12  are deposited and sputtered on walls  14  and create a film on walls  14 . Because this material is usually adhered weakly to walls  12 , it can generate particles and file-flakes which in turn can short out the cathode and anode, for example, by resting acrose insulations  18 . 
     SUMMARY 
     In one general aspect, the invention features an ion source constructed for use with a magnet that produces magnetic flux lines extending in a predetermined direction and a source of ionizable material for creating ion. The ion source includes a chamber, defined by walls, and a relatively narrow outlet aperture for ions produced in the chamber to leave the chamber. The chamber encloses a cathode and an anode spaced from the cathode and from the walls of the chamber. The anode is positioned with respect to the aperture, the cathode and the predetermined direction of the magnetic flux to cause ions produced in the chamber to concentrate near the aperture. 
     In another general aspect, the invention features an ion source constructed for use with a magnet that produces magnetic flux lines extending in a predetermined direction. The ion source includes a chamber defined by walls, and a relatively narrow, elongated outlet slit for ions produced in the chamber to leave the chamber. The chamber encloses a cathode and an anode spaced from the cathode and from the walls of the chamber. The anode is elongated and positioned adjacent to and generally parallel to the slit. The ion source and magnet being relatively positioned such that the magnetic flux lines are generally parallel to the anode and at an angle to an electrical field produced between the anode and the cathode. 
     In yet another aspect, the invention features an ion implanter for implanting ions in a work piece. The ion implanter includes an ion source, a plurality of magnets to focus and scan the ion beam in a first direction, and a workpiece holder to hold the workpiece and to move perpendicular to the first direction. The ion source is constructed for use with a magnet that produces magnetic flux lines extending in a predetermined direction. The ion source includes a chamber defined by walls, and a relatively narrow, elongated outlet slit for ions produced in the chamber to leave the chamber. The chamber encloses a cathode and an anode spaced from the cathode and from the walls of the chamber. The anode is elongated and positioned adjacent to and generally parallel to the slit. The ion source and magnet being relatively positioned such that the magnetic flux lines are generally parallel to the anode and at an angle to an electrical field produced between the anode and the cathode. 
     Preferred embodiments of the invention may include one or more of the following features. 
     The aperture is a relatively narrow, elongated slit. The anode is elongated and positioned adjacent to and parallel to the aperture and may extend substantially the full length of the slit-form aperture. The anode is of generally rod form. The elongated anode is arranged to be substantially parallel with the predetermined direction of the magnetic flux. 
     The chamber is elongated in the direction of the elongated slit, two cathodes are located at each of the two ends. The cathodes are positioned symmetrically at either end of the chamber relative to the elongated slit and the anode. A negatively biased electrode can be used for sputtering material into the chamber for ionization. 
     The walls of the chamber can have a potential selected to deflect electrons. The walls of the chamber can have substantially the same potential as the cathode. The cathode can be a hot, indirectly heated, or cold cathode. The cathode can be a coil of tungsten wire, the coil having a generally circular form. 
     A magnet produces a magnetic field having flux lines in the above predetermined direction. The anode and chamber lie within the magnetic field. The magnet is arranged relative to the aperture and electrical field condition produced within the chamber to apply a force to the ions in the direction of the aperture. The lines of the magnetic field cross lines of an electrical field generated between the cathode and the anode. The anode is positioned with respect to the cathode to cause an electrical field between the anode and the cathode to concentrate the ions near the anode. The anode is positioned with respect to the aperture, the cathode, and the magnetic flux lines to cause ions near the anode to drift towards the aperture. 
     Embodiments of the invention may include one or more of these advantages. 
     Embodiments of ion source have efficient ion production because the anode being separated from the walls of the source allows the walls of the ion source to float relative to the anode and reach a potential close to that of the cathode potential. This results in the walls acting as an electron reflector rather than an anode. Therefore, the electrons can only be absorbed by an anode that is smaller than the walls. Therefore, the electrons trace an extended path in the source and increase the efficiency of the ion source. 
     In some embodiments, the material deposited on the walls strongly adhere to the walls, reducing the flaking of deposited material. This in turn reduces the possibility of the flakes short circuiting the source. 
     In other embodiments, because the cathode and the walls have the same potential, arcing in the source is reduced. 
     In some embodiments, the location of the anode relative to the magnetic field in which the ions source operates causes the plasma to drift towards the ion source emission slit and to concentrate near the emission slit. This increases the efficiency of extracting ions from the ion source and the current of the extracted beam. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a top view of a typical Bernas type ion source with its top plate removed. 
     FIG. 2 is a plan view of an implanter in which an ion source according to the present invention is used. 
     FIG. 3 is a perspective view of an embodiment of an ion source according to the present invention. 
     FIG. 4 is a top view of the ion source in FIG. 2 with its top plate removed. 
     FIG. 4A is a top view of the ion source showing the relationship between magnetic field and the electrical fields in the source. 
     FIG. 5 shows the electrical circuit in which the ion source is connected during use. 
     FIG. 6 shows results of an experiment conducted on the performance of an embodiment of an ion source according to the present invention. 
     FIG. 7 shows an alternative embodiment of a ion source according to the present invention. 
    
    
     DESCRIPTION 
     FIG. 2 shows an example of an ion implanter  200  in which embodiments of an ion source according to this invention may be used. General features of such an ion implanter is disclosed in e.g. U.S. Pat. No. 5,393,984, hereby incorporated by reference. 
     Ion implanter  200  is composed of an ion source  100 , an extractor electrode  214 , an analyzer magnet  216 , a scanner magnet  218 , a collimator magnet  220 , a plasma charge neutralizer  222  and a wafer  224 . Generally, ion implanter  200  produces a ribbon-shaped beam which in some embodiments has a range of energies from 1 keV to 100 keV. The beam is a high current, high perveance beam (in some embodiments the beam has a perveance in the order of or greater than 0.02 (Ma) (amu) ½  (KeV) 31{fraction (3/2)} ), as explained in the referenced patent. The beam is magnetically scanned over the wafer in one direction. The wafer may also be moved in another direction to enable scanning in a second direction. 
     Ion source  100  generates positively charged ions for implantation, including gases such as argon, nitrogen, disassociated boron (as in BF3), arsine, and phosphine. Solids may also be implanted after vaporization. Such solids include phosphorus, arsenic, and antimony. Other material may also be implanted. The ions emerge from an emission slit  10  (shown in FIG.  3 ), extracted by extraction electrode  214 , which has a negative potential compared to the source. The shape and position of extractor electrode  214  is such that a well-defined ion beam emerges from the electrode. 
     Analyzer magnet  216  then analyzes the ion beam by removing undesired impurities according to the ion momentum to charge ratio (Mv/Q, where v is the velocity of the ion, Q is its charge, and M is its mass). Scanner magnet  218  then scans the ion beam in a direction perpendicular to the path of the beam. Following scanning, collimator magnet  220  reorients the ion beam such that the beam is parallel in the entire scan area. 
     Ion implanter  200  is sized to enable implantation on wafers that have a diameter of up to 300 millimeters. A wafer holder  226  holds wafer  224 , at a selected angle within a range of angles of incidence of the beam to the wafer, preferably from normal incidence to the ion beam to less than 10°. In this embodiment, the ion beam is a ribbon shaped beam having a beam height (i.e. the length of the beam along a cross section of the beam) of 90 mm the source and 60 mm at the wafer. 
     Referring to FIGS. 3 and 4, ion source  100  includes walls  20  defining a vapor discharge chamber and a front plate  30 . Front plate  30  includes an emission slit  10  which has an orientation parallel to the magnetic flux lines of a magnetic field  50  within which ion source  100  is placed during use. Emission slit  10  allows the plasma to be extracted in form of an ion beam from ion source  100 . Ion source  100  also includes a gas vapor delivery port  60 . 
     Ion source  100  has two spiral cathode filaments  40  wound such that the resulting magnetic field from flow of electricity through cathodes  40  has magnetic flux lines parallel to and in the same direction as the magnetic flux lines of magnetic field  50 . Cathodes  40  are insulated from walls  20  by filament insulators  52 . 
     Ion source  100  also includes an anode  70  that is spaced from and insulated from walls  20  by insulators  22 . The positioning of anode  70  relative to other components of ion source  100  will be discussed in detail below. However, briefly, anode  70  is located near the emission slit and parallel to magnetic field  50 . During use, an electrical field is generated between anode  70 , cathodes  40 , the plasma, and walls  20  (shown in FIG.  4 A). This electrical field crosses the magnetic field  50 . Anode  70  is positioned such that the crossed magnetic and electrical fields cause plasma generated in ion source  100  to drift towards emission slit  10  for better extraction of a high current ion beam. (Note that anode  70 , cathodes  40 , and emission slit are positioned symmetrically in ion source.) 
     Connectors  80  are used to connect cathodes  40  to power supplies during operation. Similar connectors (not shown) are provided for connecting anode  70  to power supplies during use. 
     Having described the structure of ion source  100 , we will now describe the operation of ion source  100 . 
     FIG.  4 . shows how ion source  100  is connected during use. Cathodes  40  are connected to a power supply  90  via the connectors  80 . Power supply  90  is a high current power supply which drives cathodes  40  so that cathodes  40  reach thermionic temperatures, e.g 2500° C. At these temperatures, cathodes  40  begin to emit electrons into the chamber of ion source  100 . Anode  70  and the plasma extract further electrons from cathodes  40 . 
     A biasing power supply  92  is connected to cathodes  40  and anode  70  to positively bias anode  70  relative to cathodes  40 , e.g in the order of hundreds or thousands of volts. Walls  20  are connected to the negative terminal of power supply  92  via a resistor  94  which keeps walls  20  at a floating potential having approximately the same voltage as cathodes  40 . In short, because anode  70  is separated and insulated from the walls, walls can be connected to float near the voltage of cathodes  40  as opposed to being at a voltage near that of anode  70 . 
     Because walls  20  have a voltage near that of cathodes  40 , the possibility of arcing between cathodes  40  and walls  20  across insulators  52  is reduced. Specifically, if walls  20  were at the same or near the voltage of anode  70 , arcing could have occurred across insulators  52 . This possibility could have increased as material, such as that evaporated from cathodes  40 , deposited on insulators  52 . Arcing across insulators  52  could then short circuit ion source  100 . Arcing could also cause the deposited material to separate and become foreign particles in the plasma and contaminate the plasma. However, because in ion source  100 , the wall can be kept near the voltage of cathodes  40 , the potential difference across insulators  52  can be kept to a minimum so that there is little possibility of arcing across insulators  52 . 
     Moreover, we have observed that material deposited in ion source  100  during operation are strongly bonded to walls  20  and are less likely to flake off and produce flakes. This strong adhesion to the walls may be because walls  20  are kept at a voltage close to that of cathodes  40  and therefore cause an ion assisted deposition of material on walls  20 . Specifically, because of the biasing of walls  20  relative to anode  70 , positive ions in the source are attracted to walls  20 . The ions therefore bombard walls  20  and cause weakly bonded atoms that are deposited on walls  20  to separate. Therefore, only strongly bonded atoms remain on walls  20 . These atoms are much less likely to create flakes. 
     The voltage at which walls  20  are kept also assists in plasma production. As electrons that are emitted from cathodes  40  travel inside ion source  100 , magnetic field  50  deflects the electrons away from walls  20  and causes electrons to spin in the chamber of ion source  100 . Each cathode  40  and its reflector plate  54  also reflect the electrons away from themselves. Moreover, walls  20 , since they have a voltage near that of the cathode, also reflect the electrons. Since anode  70  has much smaller surface than walls  20 , electrons generally have a much smaller target to find for reabsorption and therefore have longer life in ion source  100  than if walls  20  were at the anode potential. Therefore, electrons generally trace an extended path in ion source  100  and have a prolonged period to ionize the gas in ion source  100  and generate the plasma. Moreover, because all electrons eventually move toward anode  70 , part of plasma production is concentrated near anode  70  which is also near emission slit  10 . 
     As described briefly above, referring to FIG. 4A, the potential difference between anode  70 , and cathodes  40  and walls  20  results in an electric field that crosses the lines of magnetic field  50 . The crossed electric and magnetic fields result in the plasma drifting towards the emission slit  10  and causing a high density of ions to gather near emission slit  10  for being extracted. 
     The position of anode  70  relative to magnetic field  50  determines the direction of the drift, because the position of anode  70  determines the direction of the electric field lines relative to the flux lines of magnetic field  50 . Generally, the electric field in ion source  100  applies a force on the positive ions in source  100  along the electric field lines. Magnetic field  50  in turn applies a deflecting force on the ions perpendicular to their plane of motion in the electric field. The direction of this deflecting force is determined by the so called “right-hand rule” (e.g. see Raymond A. Serway,  Physics: For Scientists and Engineers  (1982) 539, incorporated by reference). According to a version of the right hand rule, if one holds one&#39;s right hand such that one&#39;s thumb, index and middle fingers are all perpendicular to one another and the index finger represents the direction of the movement of the positive ion (or the electric field lines) and the middle finger represents the direction of the magnetic field, then the thumb represents the direction of the force exerted on the positive ion. In the case of ion source  100 , the anode is located such that the force on positive ions is upwards towards emission slit  10 . This results in plasma drifting toward emission slit  10  for more efficient extraction by the extraction electrode and a higher beam current. Moreover, anode  70  is located near emission slit  10  to further assist in concentrating the plasma near emission slit  10 . 
     FIG. 6 shows results of an experiment with an embodiment of an ion source constructed according to the principles disclosed herein. During the experiment, a 10 KeV 11B +  beam was generated. A Faraday cup was placed after the analyzer magnet. An oscilloscope recorded the current of the beam arriving at the Faraday cup as the arc current was varied. The arc current was varied by keeping constant the potential difference between the cathode and the anode while varying the filament heating. Graph  200  shows a relationship between the beam current and the arc current when the walls were used as the anode. Graph  202  shows the relationship when an anode similar to anode  70  was used and the wall was allowed to float at a potential near the cathode potential. As can easily be seen, for the same arc current, when the anode similar to anode  70  was used, the ion beam current was higher than when the walls were used as the anode. 
     Other embodiments are within the scope of the claims below. 
     For example, referring to FIG. 7, ion source  100  may include a sputtering electrode  110 . This electrode may be coated with a solid material that is to be implanted. Alternatively, electrode  110  may be made out of the material to be implanted. This electrode may be held at a negative potential relative to anode  70  so that it attracts positive ions in the chamber. These ions bombard the electrode and cause atoms of the material on electrode  110  to sputter into the chamber of the ion source. This material then forms a plasma which is then extracted for implantation. Typically, positive ions that bombard electrode  110  are positive ions in the plasma. An inert gas such as Argon may be used to create a plasma to begin the sputtering process or to assist with the sputtering process. 
     Other embodiments of the invention may include using the principle of the invention in other types of ion sources such as cold cathode, indirectly heated cathode or Freeman sources.