Abstract:
Liner elements to protect the ion source housing and also increase the power efficiency of the ion source are disclosed. Two liner elements, preferably constructed from tungsten, are inserted into the ion source chamber, one placed against each of the two sidewalls. These inserts are electrically biased so as to induce an electrical field that is perpendicular to the applied magnetic field. Such an arrangement has been unexpectedly found to increase the life of not only the ion chamber housing, but also the indirectly heated cathode (IHC) and the repeller. In addition, the use of these biased liner elements also improved the power efficiency of the ion source; allowing more ions to be generated at a given power level, or an equal number of ions to be generated at a lower power level.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Ion implanters are commonly used in the production of semiconductor wafers. An ion source is used to create an ion beam, which is then directed toward the wafer. As the ions strike the wafer, they dope a particular region of the wafer. The configuration of doped regions defines their functionality, and through the use of conductive interconnects, these wafers can be transformed into complex circuits. 
         [0002]    A block diagram of a representative ion implanter  100  is shown in  FIG. 1 . An ion source  110  generates ions of a desired species. In some embodiments, these species are atomic ions, which are best suited for high implant energies. In other embodiments, these species are molecular ions, which are better suited for low implant energies. These ions are formed into a beam, which then passes through a source filter  120 . The source filter is preferably located near the ion source. The ions within the beam are accelerated/decelerated in column  130  to the desired energy level. A mass analyzer magnet  140 , having an aperture  145 , is used to remove unwanted components from the ion beam, resulting in an ion beam  150  having the desired energy and mass characteristics passing through resolving aperture  145 . 
         [0003]    In certain embodiments, the ion beam  150  is a spot beam. In this scenario, the ion beam passes through a scanner  160 , which can be either an electrostatic or magnetic scanner, which deflects the ion beam  150  to produce a scanned beam  155 - 157 . In certain embodiments, the scanner  160  comprises separated scan plates in communication with a scan generator. The scan generator creates a scan voltage waveform, such as a sine, sawtooth or triangle waveform having amplitude and frequency components, which is applied to the scan plates. In a preferred embodiment, the scanning waveform is typically very close to being a triangle wave (constant slope), so as to leave the scanned beam at every position for nearly the same amount of time. Deviations from the triangle are used to make the beam uniform. The resultant electric field causes the ion beam to diverge as shown in  FIG. 1 . 
         [0004]    In an alternate embodiment, the ion beam  150  is a ribbon beam. In such an embodiment, there is no need for a scanner, so the ribbon beam is already properly shaped. 
         [0005]    An angle corrector  170  is adapted to deflect the divergent ion beamlets  155 - 157  into a set of beamlets having substantially parallel trajectories. Preferably, the angle corrector  170  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. 
         [0006]    Following the angle corrector  170 , the scanned beam is targeted toward the workpiece  175 . The workpiece is attached to a workpiece support. The workpiece support provides a variety of degrees of movement. 
         [0007]    A traditional ion source is shown in  FIG. 2 . A chamber housing  10  defines an ion chamber  14 . One side of the chamber housing  10  has an extraction aperture  12  through which the ions pass. 
         [0008]    A cathode  20  is located on one end of the ion 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 chamber  14 . 
         [0009]    The filament  30  is energized by filament power supply  54 . The current passing through the filament  30  heats it sufficiently (i.e. above 2000° C.) so as to produce thermo-electrons. A bias power supply  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 chamber  14 . 
         [0010]    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 . 
         [0011]    A magnetic field is preferably created in the direction  62 , typically by using magnetic poles located outside the chamber. The effect of the magnetic field is to confine the emitted electrons within magnetic field lines. A second effect is to cause the electrons to move from the cathode toward the opposite end of the chamber in a spiraling fashion (as shown in  FIG. 3 ). 
         [0012]    Vapor or gas source  40  is used to provide atoms or molecules into the chamber  14 . The molecules can be of a variety of species, including but not limited to inert gases (such as argon), hydrogen-containing gases (such as PH 3  and AsH 3 ) and other dopant-containing gases (such as BF 3 ). The temperature of the chamber, coupled with the emitted electrons traveling in the chamber  14  serve to transform this injected gas into a plasma. 
         [0013]    At the far end of the chamber, 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 repelled away from the repeller  60  and back toward the cathode  20 . The use of these like-biased structures at each end of the chamber  14  maximizes the interaction of the emitted electrons with the material (i.e. gas and plasma) that exists in the ion source chamber. The result of these interactions between the emitted electrons and the gas and plasma is the creation of ions. 
         [0014]      FIG. 3  shows a different view of the ion source of  FIG. 2 . The 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 repel the electrons away and toward the center of the chamber. When the gas interacts with the electrons, plasma  80  is created. An electrode  90  is biased to attract the ions through the extraction aperture  12 . These extracted ions form an ion beam  95  and are used as described above. 
         [0015]    The temperatures, corrosive gasses and emitted electrons create a harsh environment within the ion source. In fact, the conditions are such that tungsten parts that make up the housing are damaged within days. As a result, some ion source manufacturers have developed liners that can be inserted into the ion source. These liners cover the side and bottom surfaces of the ion source housing, thereby protecting the housing  10  from this harsh environment. These liners are typically made using tungsten or molybdenum and are simply slid into the housing. These liners are still damaged by the environment in the ion source; however, since these liners are typically much less costly than the ion source housing itself, it is cost effective to insert them into an ion source housing to prolong the useful life of the ion source housing. When the liners wear out, they are simply discarded and replaced with new ones. 
         [0016]    These liners perform an important function in prolonging the life of an ion source and reducing the annual operating cost. In addition, it would also be beneficial if the ion source liner were also able to improve the power efficiency of the ion source, such that either more ions can be produced at a given power level, or an equal number of ions can be produced at a lower power level. 
       SUMMARY OF THE INVENTION 
       [0017]    The problems of the prior art are addressed by the present disclosure, which describes liner elements to protect the ion source housing and also increases the efficiency of the ion source. 
         [0018]    Two liner elements, preferably constructed from tungsten, are inserted into the ion source chamber, one placed against each of the two side walls. These inserts are biased so as to induce an electrical field that is perpendicular to the applied magnetic field. Such an arrangement has been found to increase the life of not only the ion chamber housing, but also the indirectly heated cathode (IHC) and the repeller for a given ion output. 
         [0019]    In addition, the use of these biased liner elements also improved the power efficiency of the ion source; allowing more ions to be generated at a given power level, or an equal number of ions to be generated at a lower power level. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  illustrates a block diagram of a representative ion implanter; 
           [0021]      FIG. 2  illustrates a traditional ion source; 
           [0022]      FIG. 3  shows the major components of the traditional ion source of  FIG. 2 ; 
           [0023]      FIG. 4  illustrates a first embodiment; 
           [0024]      FIG. 5   a  shows an expanded view of one connection mechanism for the liner elements; 
           [0025]      FIG. 5   b  shows an expanded view of a second connection mechanism for the liner elements; 
           [0026]      FIG. 6   a  shows a first shape of the liner elements; 
           [0027]      FIG. 6   b  shows a second shape of the liner elements; and 
           [0028]      FIG. 6   c  shows a third shape of the liner elements. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0029]      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. Located on each side of ion source housing  10  is a liner element  200 ,  210 . These liner elements are positioned within the ion source housing  10 , such as by bolts or clamps. In some embodiments, the liner element is affixed directly to the housing, while in others, it is electrically isolated from the housing. A first bias voltage  270  is applied to first liner element  200 . In one embodiment, the power source is connected to the shaft  220  used to secure the liner element within the chamber  14 . A second bias voltage  275  is connected to second liner element  210 . In one embodiment, this bias voltage is the same as that applied to the source housing, as shown in  FIG. 2 . In a second embodiment, a bias voltage, different from that applied to the first liner element  200  and different from that applied to the source housing  10  is used. 
         [0030]      FIG. 5   a  shows an expanded view of the connection of the first liner element  200  to the ion source housing  10 . In one embodiment, holes are drilled through the sidewalls of the ion source housing  10 . An electrically conductive shaft  220 , having a diameter smaller than the drilled hole, is passed through this hole and into a receptacle in the first liner element  200 . In one embodiment, the shaft is a bolt, which can be threaded throughout its entire length or only threaded at its end. In an alternative embodiment, the shaft is an integral part of the liner element. In the preferred embodiment, two threaded shafts are used for each liner element to insure stability. A first insulator, such as an insulating ring,  230  is placed between the first liner element  200  and the sidewall of the housing  10 . A second insulator, such as insulating ring  235  is placed between the outside of the sidewall and the fastener  240  used to secure the shaft. In one embodiment, these insulators are made from aluminum oxide (Al 2 O 3 ) or boron nitride, capable of withstanding the temperatures within the ion source. It should be noted that the liner element  200  is sized so as not to contact the bottom or endwalls of the chamber housing, so as to be electrically isolated from the housing. Electrical conduit  250  is preferably attached to the shaft  220 , preferably between the second insulating ring  235  and the bolt  240 . Electrical conduit  250  is in communication with bias voltage  270 . The use of insulating rings  230 ,  235  allows the shaft  220  and the liner element  200  to be at a different potential than the housing  10 . The second liner element  210  can be attached to the housing  10  in a similar fashion. In an alternative embodiment, where the electrical potential of the second liner element  210  is the same as that of the housing  10 , the insulating rings  230 ,  235  can be eliminated. Alternatively, these rings can be conductive, thereby allowing the liner element and the housing to be at the same potential. Similarly, in this embodiment, there is no need for an electrical conduit, since the liner element is in direct contact with the housing  10 . In either scenario, the liner elements  200 ,  210  serve to block a portion of the sidewall from the ions generated within the chamber. 
         [0031]      FIG. 5   b  shows an expanded view of a second method of locating the liner elements within the ion source chamber. In this embodiment, holes are drilled through the sidewalls of the ion source housing  10 . An electrically conductive shaft  220 , having a diameter smaller than the drilled hole, is passed through this hole and into a receptacle in the first liner element  200 . In one embodiment, the shaft  220  is a bolt, which can be threaded throughout its entire length or only threaded at its end. In an alternative embodiment, the shaft  220  is an integral part of the liner element  200 . In the preferred embodiment, two shafts are used for each liner element to insure stability. Rather than utilize isolators to electrically separate the liner element  200  from the sidewall, an external positioning device  280 , such as a clamp, is used to hold the shaft in place. In this way, the shaft has no physical contact (either directly or through an isolator) with the sidewall. The liner element  200  is sufficiently spaced from the sidewall and the bottom of the chamber housing so as to remain electrically isolated from the chamber housing. Electrical conduit  250  is preferably attached to the shaft  220 , preferably outside of the source chamber housing. External positioning device  280  may be electrical isolated from the bolt  220 . Alternatively, the external positioning device  280  may be electrically connected to the bolt  220 . In this embodiment, the electrical conduit  250  can be connected directly to the external positioning device  280 . The use of an external positioning device  280  and enlarged holes in the sidewalls allows the shaft  220  and the liner element  200  to be at a different potential than the housing  10 . The second liner element  210  can be attached to the housing  10  in a similar fashion. In an alternative embodiment, where the potential of the second liner element  210  is the same as that of the housing  10 , the second liner  210  can be bolted directly to the housing. In this embodiment, there is no need for an electrical conduit, since the liner element is in direct contact with the housing  10 . In either scenario, the liner elements  200 ,  210  serve to block a portion of the sidewall from the ions generated within the chamber. 
         [0032]    Returning to  FIG. 4 , it can be seen that the liner elements  200 ,  210 , differ from the prior art in that they do not completely cover the housing  10 . Rather, the liner elements are positioned only on the two sides of the housing  10 , with no lining covering the bottom of the chamber. Furthermore, in the preferred embodiment, the liner elements do not cover the end walls of the chamber housing where the IHC and repeller are installed. Finally, the liner elements do not completely cover the side walls; rather the liner elements extended along the side wall only between the IHC and the repeller, as shown in  FIG. 4 . 
         [0033]    This simplifies the process needed to replace these elements. In the prior art, to replace the source liner, the IHC and repeller needed to be removed first, to allow the source liner to then be removed. This increased the amount of time required for preventative maintenance, and therefore increased the downtime for the ion source. In the present disclosure, the operator need only unfasten the bolts connected to the liner elements and replace the liner element with a new one. The repeller and IHC are untouched during this operation. 
         [0034]    The liner element can be formed in a variety of shapes. For example, the liner elements may be planar, as shown in  FIG. 6   a . In another embodiment, the liner elements are arcuate in shape, as shown in  FIG. 6   b . Preferably, the arcuate shape corresponds with the size and shape of the IHC and cathode. In another embodiment, shown in  FIG. 6   c  , the outer face of the liner element, which faces toward the chamber is arcuate in shape, while the opposite side, which faces the sidewall is planar. In designing the shape of the liner element, it is important that the liner elements do not obstruct the electron path between the IHC and the repeller. In most embodiments, the IHC and repeller are circular, thus an arcuate shape is suitable for the liner elements. Based on this requirement, the structure shown in  FIG. 6   c  allows the greatest amount of liner material, and thereby providing the greatest liner element useful life. However, other shapes are also possible and within the scope of the disclosure. 
         [0035]    As described above, the liner elements  200 ,  210  are biased so as to create an electrical field that is perpendicular to the applied magnetic field. As is known by those of ordinary skill in the art, when a magnetic field is crossed by an electrical field, charged particles will experience a force in a third direction, perpendicular to both the magnetic and electrical fields. By properly configuring the applied magnetic field and the electrical field between the liner elements, the ions created in the ion chamber will be pushed upward toward the aperture  12 . This force allows hotter ions to be pushed out of the ion source, thereby allowing the ion source to be more energy efficient than those of the prior art. Thus, the ion source can be powered such that at a given energy level, the ion source produces more ions than those of the prior art. Alternatively, the ion source can be powered so as to produce the same amount of ions at a lower power level. This is particularly advantageous in the production of multiply charged ions, i.e. ions with a charge of +2 or greater, such as P ++  and P +++ . 
         [0036]    Furthermore, the use of biased liner elements  200 ,  210  prolongs the life of the ion source. As described earlier, by covering the housing  10  with a liner, the life of the ion source housing can be prolonged. However, the source liners also increase the life of the IHC and the repeller, an effect not realized in the prior art. Because of the harsh conditions, the IHC and repeller are damaged during normal operation due to sputtering and tungsten deposition. To combat this, the thickness of the repeller can be increased to prolong its useful life. However, the same cannot be done for the IHC, since increased thickness would require more power to be consumed in allowing the IHC to reach the required temperatures. Thus, the IHC fails relatively quickly. By applying an electrical bias to one (or both) of the liner elements, sputtering and tungsten deposition is increased on the liner elements, and it is possible to operate the source with a reduced bias between the cathode and repeller and the walls of the arc chamber. This decreases wear on the cathode and repeller and serves to further decrease the cost of ownership associated with the ion source by prolonging the useful life of the IHC and the repeller. 
         [0037]    Because of these harsh conditions, preventative maintenance is typically performed on a regular basis on traditional ion sources. At certain intervals, the IHC and repeller must be replaced. At somewhat longer intervals, the source liners are also replaced. However, by using the liner elements, the preventative maintenance schedule is greatly reduced. Various studies have demonstrated that the liner elements described in the present disclosure allow the IHC to last up to three times as long as previously possible. For example, for the same ion output, the biased side plates have shown an increase of 300% on P ++  &amp; P +++  source life. 
         [0038]    To further increase the life of the liner elements, it is also envisioned that the electrical bias of the two liner elements  200 ,  210  can be reversed. It should be noted that the direction of the applied magnetic field must also be reversed at this time as well such that charged particles are still directed toward the aperture. 
         [0039]    The bias voltages applied to the liner elements  200 ,  210  can also be varied. For example, as described above, one element may be biased to the same potential as the source housing  10 , while the other is more positively biased. Alternatively, both can be biased at a potential higher than the housing  10 . Moreover, one liner element can be biased more negatively than the housing, while the other is more positively biased. An important characteristic is the difference in potential between the two liner elements. By varying the voltage potential between the liner elements, the rate of sputtering (and erosion) can be controlled. For example, in some applications, the liner elements can be excellent sputter sources. These sources tend to degrade over time, due to erosion. To reverse these effects, the bias potential between the liner elements can be increased, thereby maintaining the previous level of performance, and prolonging the life of the liner elements. 
         [0040]    While this disclosure has described specific embodiments disclosed above, it is obvious to one of ordinary skill in the art that many variations and modifications are possible. 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.