Abstract:
A dual unbalanced indirectly heated cathode (IHC) ion chamber is disclosed. The cathodes have different surface areas, thereby affecting the amount of heat radiated by each. In the preferred embodiment, one cathode is of the size and dimension typically used for IHC ionization, as traditionally used for hot mode operation. The second cathode, preferably located on the opposite wall of the chamber, is of a smaller size. This smaller cathode is still indirectly heated by a filament, but due to its smaller size, radiates less heat into the source chamber, allowing the ion source to operate in cold mode, thereby preserving the molecular structure of the target molecules. In both modes, the unused cathode is preferably biased so as to be at the same potential as the IHC, thus allowing it to act as a repeller.

Description:
BACKGROUND OF THE INVENTION 
     Ion implanters are commonly used in the production of semiconductor wafers. An ion source is used to create a beam of charged ions, which is then directed toward the wafer. As the ions strike the wafer, they impart a charge in the area of impact. This charge allows that particular region of the wafer to be properly “doped”. The configuration of doped regions defines their functionality, and through the use of conductive interconnects, these wafers can be transformed into complex circuits. 
     A block diagram of a representative ion implanter  100  is shown in  FIG. 1 . Power supply  101  supplies the required energy to the ion source  102  to enable the generation of ions. An ion source  102  generates ions of a desired species. In some embodiments, these species are mono-atoms, which are best suited for high-energy implant applications. In other embodiments, these species are molecules, which are better suited for low-energy implant applications. The ion source  102  has an aperture through which ions can pass. These ions are attracted to and through the aperture by electrodes  104 . These exiting ions are formed into a beam  95 , which then passes through a mass analyzer  106 . The mass analyzer, having a resolving aperture, is used to remove unwanted components from the ion beam, resulting in an ion beam having the desired energy and mass characteristics passing through resolving aperture. Ions of the desired species then pass through a deceleration stage  108 , which may include one or more electrodes. The output of the deceleration stage is a diverging ion beam. 
     A corrector magnet  110  is adapted to deflect the divergent ion beam into a set of beamlets having substantially parallel trajectories. Preferably, the corrector magnet  110  comprises a magnet coil and magnetic pole pieces that are spaced apart to form a gap, through which the ion beamlets pass. The coil is energized so as to create a magnetic field within the gap, which deflects the ion beamlets in accordance with the strength and direction of the applied magnetic field. The magnetic field is adjusted by varying the current through the magnet coil. Alternatively, other structures, such as parallelizing lenses, can also be utilized to perform this function. 
     Following the angle corrector  110 , the ribbon beam is targeted toward the workpiece. In some embodiments, a second deceleration stage  112  may be added. The workpiece is attached to a workpiece support  114 . The workpiece support  114  provides a variety of degrees of movement for various implant applications. 
     Referring to  FIG. 2 , a traditional ion source that may be incorporated into the ion implanter  100  is shown. The ion source  102  may include a chamber housing  10  that defines an ion source chamber  14 . One side of the chamber housing  10  has an extraction aperture  12  through which the ions pass. In some embodiments, this aperture is a hole, while in other applications, such as high current implantation, this aperture is a slot. 
     A cathode  20  is located on one end of the ion source chamber  14 . A filament  30  is positioned in close proximity to the cathode  20 , outside of the ion chamber. A repeller  60  is located on the opposite end of the ion source chamber  14 . 
     The filament  30  is energized by filament supply voltage  54 . The current passing through the filament  30  heats it sufficiently (i.e. above 2000° C.) so as to produce thermo-electrons. A bias supply voltage  52  is used to bias the cathode  20  at a substantially more positive voltage than the filament  30 . The effect of this large difference in voltage is to cause the thermo-electrons emitted from the filament to be accelerated toward the cathode. As these electrons bombard the cathode, the cathode heats significantly, often to temperatures over 2000° C. The cathode, which is referred to as an indirectly heated cathode (IHC), then emits thermo-electrons into the ion source chamber  14 . 
     The arc supply  50  is used to bias the ion chamber housing  10  positively as compared to the cathode. The arc supply typically biases the housing  10  to a voltage about 50-100 Volts more positive than the cathode  20 . This difference in voltage causes the electrons emitted from the cathode  20  to be accelerated toward the housing  10 . 
     A magnetic field is preferably created in the direction  62 , typically by using magnetic poles  86  located outside the chamber. The effect of the magnetic field is to confine the emitted electrons within magnetic field lines. The emitted electrons, electro-statically confined between cathode and repeller, take the spiral motions along the source magnetic field lines, thus effectively ionize background gases, forming ions (as shown in  FIG. 3 ). 
     Vapor or gas source  40  is used to provide atoms or molecules into the ion source chamber  14 . The molecules can be of a variety of species, including but not limited to inert gases (such as argon or hydrogen), oxygen-containing gases (such as oxygen and carbon dioxide), nitrogen containing gases (such as nitrogen or nitrogen triflouride), and other dopant-containing gases (such as diborane, boron tri-fluoride, or arsenic penta-fluoride). These background gasses are ionized by electron impact, thus forming plasma  80 . 
     At the far end of the chamber  14 , opposite the cathode  20 , a repeller  60  is preferably biased to the same voltage as the cathode  20 . This causes the emitted electrons to be electro-statically confined between cathode  20  and repeller  60 . The use of these structures at each end of the ion source chamber  14  maximizes the interaction of the emitted electrons with the background gas, thus generating high-density plasmas. 
       FIG. 3  shows a different view of the ion source of  FIG. 2 . The source magnet  86  creates a magnetic field  62  across the ion chamber. The cathode  20  and repeller  60  are maintained at the same potential, so as to effectively confine the electrons, which collide with the background gas thus generate the plasma  80 . The electrode set  90  is biased so as to attract the ions to and through the extraction aperture  12 . These extracted ions are then formed into an ion beam  95  and are used as described above. 
     The above described technique of generating ions is highly effective for high-energy implant applications. Applications using high implant energies typically utilize mono-atoms, which are preferably created through the use of emitted electrons via an indirectly heated cathode. The indirectly heated cathode coupled with the magnetic fields, creates an environment where molecules are broken down into mono-atomic ion species. In these applications, source gas which breakdown into mono-atoms, such as H 2 , NF 3 , and B 2 H 6 , are supplied to the ion chamber. However, there are applications where such ions are not desirable. For example, there are applications that require ultra shallow junction formation, obtained with very low energy implants. Due to their inefficiency of beam transport, low energy applications preferably require the use of heavier charged molecules. These heavier molecules, such as decaborane, carborane and others, cannot be ionized using the above technique, since the high temperature environment would break apart the heavy molecules into smaller molecules or atoms. It is important for these applications that the molecules retain their molecular structure, losing only electrons before being extracted from the chamber. 
     Therefore, to create these heavier ions, alternative ion sources are typically used. In most cases, the ion source operates at much lower temperatures to preserve the molecular structure of the target species. In some embodiments, RF power is used to ionize the molecules. 
     Thus, there are two distinct modes of operation; one used for generating atomic ion species for high-energy applications, also known as hot mode, and a second for generating molecular ion species for low-energy applications, also known as cold mode. Because there are two distinct modes, there are typically separate ion sources, depending on the application and the source molecules. This complicates the ion implanter, and increases cost and complexity. A single ion source that can effectively generate ions for use in both modes, i.e. mono-atomic ions for high-energy implant applications and molecular ions for lower-energy implant applications, would be very beneficial. 
     SUMMARY OF THE INVENTION 
     The problems of the prior art are addressed by the present disclosure, which describes a dual-mode, unbalanced indirectly heated cathode (IHC) ion source chamber. The cathodes have different surface areas, thereby affecting the amount of heat radiated by each. In the preferred embodiment, one cathode is of the size and dimension typically used for an IHC ion source, as traditionally used for hot mode operation. The second cathode, preferably located on the opposite wall of the chamber, is of a smaller size. This smaller cathode is still indirectly heated by a filament, but due to its smaller size, radiates less heat into the source chamber, allowing the ion source to operate overall in cold mode, thereby preserving the molecular structure of the target molecules. 
     In both modes, the unused cathode is preferably biased so as to be at the same potential as the IHC, thus allowing it to act as a repeller. In another embodiment, the smaller cathode (i.e. the cathode for cold mode operation) is surrounded by an electrically conductive ring, which is biased to the same potential as the smaller cathode in hot mode. However, the ring is thermally separated from the cathode (either using an insulating material or via an air gap). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a block diagram of a representative high-current ion implanter tool; 
         FIG. 2  illustrates a traditional ion source; 
         FIG. 3  shows the major components of the traditional ion source of  FIG. 2 ; 
         FIG. 4  illustrates a first embodiment; 
         FIG. 5  shows the embodiment of  FIG. 4  as used in hot mode; 
         FIG. 6  shows the embodiment of  FIG. 4  as used in cold mode; 
         FIG. 7  illustrates one embodiment of the cathode ring; 
         FIG. 8   a  illustrates a second embodiment of the cathode ring; 
         FIG. 8   b  illustrates a third embodiment of the cathode ring; 
         FIG. 8   c  illustrates a fourth embodiment of the cathode ring; and 
         FIG. 9  illustrates a second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 4  illustrates a first embodiment. Those elements that are common with those of a traditional ion source, as shown in  FIG. 2 , are given like reference designators. In place of the repeller, a second cathode  220 , having a surface area smaller than that of the first cathode  20 , is utilized. In certain embodiments, the second cathode may have a different geometry than the first cathode. For example, the second cathode may have a different shape or thickness than the first cathode. In other embodiments, the second cathode has the same shape as the first cathode with reduced dimensions, thereby reducing its surface area. This second cathode  220  is preferably connected to the bias supply  52 , so as to be at the same potential as first cathode  20 . Cathode ring  225  is also preferably connected to bias supply  52 , so as to be at the same potential as both cathodes. However, it is contemplated that each of the first cathode  20 , the second cathode  220 , and the cathode ring  225  may be biased at different potentials. Optionally, a switch  227  is utilized to selectively activate the bias potential, as will be explained below. In another embodiment, cathode ring  225  is not utilized, thus creating a much smaller repeller when the ion source is used in the hot mode. 
     In close proximity to cathode  220  is filament  230 . This filament is heated using filament supply  54 . Note that switches  250 , 251  have been inserted such that filaments  30 ,  230  can be turned on independently or simultaneously. Optionally, a second filament power supply can also be utilized to supply current to filament  230 , if desired. The position of filament  230  and the shape of cathode  220  are such as to maximize the percentage of emitted thermo-electrons that impact the cathode  220 , while minimizing the amount that impact the cathode ring  225 . Optionally, the bias supply  52  can be disconnected from the cathode ring  225 , leaving the cathode ring  225  electrically either floated or grounded to the source chamber  10  so that emitted electrons are not as attracted to the cathode ring  225 . This serves to minimize the overall thermal budget in the ion source chamber  14  and thermo-electron-containing plasma volume where most of molecular breakdown occurs, while providing enough electrons for generating molecular ion species. 
       FIG. 5  shows the operation of an ion source in hot mode. In this case, the cathode  220  and cathode ring  225  are similarly biased so as to act as the traditional repeller of the prior art. Switch  250  is closed allowing current to flow through filament  30 . However, switch  251  is open, preventing the operation of filament  230 . Thus, the ion source behaves exactly as that shown in  FIGS. 2 and 3 . 
       FIG. 6  shows the operation of an ion source in cold mode. In this case, the cathode  20  serves as the repeller and is biased to the same potential as cathode  220 . Switch  250  is open, preventing the electric current through filament  30 . However, switch  251  is closed, allowing current to flow through filament  230 . Cathode  220  is positively biased as compared to the filament, thus attracting the emitted thermo-electrons. Cathode ring  225  may be, for example, physically shielded from the filament  230 , or electrically biased so as not to attract emitted electrons from the filament. Thus, cathode ring  225  is not heated to the degree that the cathode  220  is heated. 
     Cathode  220  heats sufficiently so as to emit thermo-electrons. Since its surface area is much smaller than that of cathode  20 , it radiates far less heat into the ion source chamber  14 . Furthermore, cathode ring  225  is not heated by the filament, and thus does not add any heat to the ion source chamber  14 . Rather, the cathode ring  225  may serve as a heat sink absorbing heat from the nearby cathode  220 . Thus, the chamber  14  reaches a much lower internal temperature in this mode, than in hot mode. This lower temperature enables molecules to retain their structure during the electron bombardment. Ionized molecules then exit the ion source chamber  14  via the extraction aperture  12 . 
       FIG. 7  shows one embodiment of the cathode ring  225 . The ring, in this embodiment, is annular in shape and surrounds cathode  220 . The size of the cathode ring  225  is determined in part by its role as a repeller in hot mode. As stated above, cathode  20  will emit thermo-electrons in hot mode. Because of the effect of the applied magnetic field, these thermo-electrons tend to be confined along the magnetic field lines, whose shape and volume is roughly defined by the shape of the cathode. In other words, the electrons will appear to travel within a tube where the outer circumference roughly corresponds to the circumference of the cathode  20 . Since this tube extends beyond the circumference of cathode  220 , some of the emitted electrons may not be confined as desired, but lost to the source chamber housing  10 . Thus, a cathode ring  225 , having the same potential as cathode  220 , is used to create a repeller having the same size and shape as the cathode  20 . 
     From the above description, those of ordinary skill in the art will recognize that the combination of the cathode  220  and the cathode ring  225  should be such that it presents a surface having an area that is comparable to that of the emitting cathode  20 , thereby allowing it to act as a repeller. However, while  FIG. 7  shows the cathode ring  225  being annular in shape and surrounding the cathode  220 , the present disclosure is not limited to this embodiment. 
     For some applications, it may be beneficial to have the cathode  220  positioned as close to the extraction aperture  12  as possible, to maximize the extraction of the molecular ions created and extracted in cold mode. In this case, the cathode ring  225  is not annular, rather it is shaped so that the combination of it and the cathode  220  results is a generally circular shape.  FIG. 8   a  shows a second embodiment of the cathode ring, configured to allow cathode  220  to be positioned close to the extraction aperture  12 . Alternatively, it may be desirous that the cathode  220  be placed as far from the extraction aperture  12  as possible. In this case, the cathode ring of  FIG. 8   b  is preferred. Other shapes for the cathode ring, such as that shown in  FIG. 8   c  are also contemplated and within the scope of the disclosure. 
     The size of cathode  220  may be beneficial in determining its effectiveness during cold mode. Since the electron emission density from a given cathode surface is a function of surface temperature, the approximate amount of heat radiated by the cathode is proportional to its surface area. Thus, if cathode  220  has a diameter that is ⅓ that of the hot mode cathode  20 , it will radiate only about 10% as much heat as hot mode cathode  20 . The disclosure is not limited to this dimension; other dimensions, having a surface area smaller than the cathode  20  for hot mode operation, are contemplated and within the scope of the disclosure. 
     While the above description recites the use of only one cathode at a time, the disclosure is not so limited. In certain applications, it may be beneficial to enable switches  250 ,  251  simultaneously or substantially simultaneously so that both cathodes  20 ,  220  are emitting electrodes. This has the added advantage of having heat generators at both ends of the ion source chamber  14 , while helps maintain a more uniform temperature throughout the ion source chamber. 
     Moreover, while the above description recites the use of two physically separate unbalanced IHC cathodes, the disclosure is not limited to only this embodiment. The cathode arrangement shown in  FIGS. 7 and 8  can be used to create appropriate cathodes for use in both hot and cold modes. In this embodiment, the cathode ring  225  can be used as either an IHC or can be disabled. Referring to  FIG. 9 , unitary cathode  20  is replaced by the combination of cathode  220  and cathode ring  225 , which together form a cathode unit. In one embodiment, two filaments are employed where one is directed toward cathode  220  and the second is directed toward cathode ring  225 . Switches  250 ,  251  determine which filaments are energized. 
     For cold mode, switch  251  is closed and switch  250  is open, allowing the filament directed toward the cathode  220  to be energized, while the filament directed toward cathode ring  225  is disabled. The cathode  220  is heated and emits electrons, as described above. A repeller  60  is located on the opposite side of the chamber and electrostatically confines the electrons, pushing them back toward the middle of the ion source chamber  14 . In hot mode, both switches are closed and the filaments heat both the cathode  220  and the cathode ring  225 . In this configuration, the resulting structure resembles in size and shape the traditional IHC used in hot mode operation. 
     In a second embodiment, a single filament is used, but differing electrical potentials are used to direct the emitted electrons either toward the cathode  220 , or toward both the cathode and the cathode ring  225 . By making the cathode  220  much more positively biased than the cathode ring  225 , electrons emitted from the filament will be accelerated toward the cathode  220 , thus operating the ion source in cold mode. If both the cathode and cathode ring are biased to the same potential, the electrons emitted from the filament will be equally attracted to both the cathode and the cathode ring, thereby creating hot mode operation. 
     While this disclosure describes specific embodiments disclosed above, those of ordinary skill in the art will recognize that many variations and modifications are possible. For example, while the description discloses a ribbon beam, the disclosure is not so limited and can also be employed with systems that utilize spot beams. 
     Accordingly, the embodiments presented in this disclosure are intended to be illustrative and not limiting. Various embodiments can be envisioned without departing from the spirit of the disclosure.