Patent Publication Number: US-6992308-B2

Title: Modulating ion beam current

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to ion implantation systems, and more particularly to modulating ion beam current in such systems to attain more uniform ion implantations. 
     BACKGROUND OF THE INVENTION 
     Ion implantation systems are used to impart impurities, known as dopant elements, into semiconductor substrates or wafers, commonly referred to as workpieces. In such systems, an ion source ionizes a desired dopant element, and the ionized impurity is extracted from the ion source as a beam of ions. The ion beam is directed (e.g., swept) across respective workpieces to implant ionized dopants within the workpieces. The dopant ions alter the composition of the workpieces causing them to possess desired electrical characteristics, such a may be useful for fashioning particular semiconductor devices, such as transistors, upon the substrates. 
     The continuing trend toward smaller electronic devices has presented an incentive to “pack” a greater number of smaller, more powerful and more energy efficient semiconductor devices onto individual wafers. This necessitates careful control over semiconductor fabrication processes, including ion implantation and more particularly the uniformity of ions implanted into the wafers. Moreover, semiconductor devices are being fabricated upon larger workpieces to increase product yield. For example, wafers having a diameter of 300 mm or more are being utilized so that more devices can be produced on a single wafer. Such wafers are expensive and, thus, make it very desirable to mitigate waste, such as having to scrap an entire wafer due to non-uniform ion implantation. Larger wafers make uniform ion implantation challenging, however, since ion beams have to be scanned across larger angles and distances to reach the perimeters of the wafers. Scanning a beam over such larger angles and distances can cause variations in the flux of the beam that can lead to non-uniform implantation. 
     SUMMARY OF THE INVENTION 
     The following presents a simplified summary in order to provide a basic understanding of one or more aspects of the invention. This summary is not an extensive overview of the invention, and is neither intended to identify key or critical elements of the invention, nor to delineate the scope thereof. Rather, the primary purpose of the summary is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later. 
     The present invention is directed to modulating ion beam current in an ion implantation system to mitigate non-uniform ion implantations, for example. Multiple arrangements are disclosed for modulating the intensity of the ion beam. For example, the volume or number of ions within the beam is altered by biasing one or more different elements downstream of the ion source. Similarly, the dosage of ions within the ion beam can also be manipulated by controlling elements more closely associated with the ion source. In this manner, the implantation process can be regulated so that the wafer is coated with a more uniform concentration of ions. 
     According to one or more aspects of the present invention, an ion implantation system suitable for use in implanting ions into one or more workpieces is disclosed. The system includes an ion source for producing a quantity of ions that can be extracted in the form of an ion beam having a beam current. The system also includes a beamline assembly downstream of the ion source to receive and direct the beam of ions. An end station downstream of the beamline assembly is also included to hold the one or more workpieces toward which the ion beam is directed. Finally, the system includes a component associated with or downstream of the ion source for modulating the ion beam current. 
     In accordance with one or more other aspects of the present invention, an acceleration system suitable for use in implanting ions into a workpiece is disclosed. The system includes an ion source for producing a quantity of ions that can be extracted in the form of an ion beam, the ion beam having a beam current. The system also includes a beamline assembly downstream of the ion source to receive and direct the beam of ions. An end station downstream of the beamline assembly is also included to hold one or more workpieces onto which the ion beam is directed. Finally, the system includes a first modulating component associated with the ion source for modulating the beam current. 
     To the accomplishment of the foregoing and related ends, the following description and annexed drawings set forth in detail certain illustrative aspects and implementations of the invention. These are indicative of but a few of the various ways in which the principles of the invention may be employed. Other aspects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic block diagram illustrating components of an ion implantation system according to one or more aspects of the present invention to modulate ion beam current. 
         FIG. 2  is a cross sectional side view illustrating an exemplary ion implantation system in accordance with one or more aspects of the present invention. 
         FIG. 3  is a schematic block diagram illustrating particular components of an ion implantation system wherein one or more aspects of the present invention are implemented; 
         FIG. 4  is a top view of an ion implantation system wherein one or more aspects of the present invention are implemented to modulate ion beam current; 
         FIG. 5  is a graphical representation depicting a relationship between changes in an extraction suppression voltage and changes in ion beam current; and 
         FIG. 6  is another graphical representation illustrating a relationship between changes in an extraction suppression voltage and changes in ion beam current. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will now be described with reference to the drawings wherein like reference numerals are used to refer to like elements throughout. The illustrations and following descriptions are exemplary in nature, and not limiting. Thus, it will be appreciated that variants of the illustrated systems and methods and other such implementations apart from those illustrated herein are deemed as falling within the scope of the present invention and the appended claims. 
     The present invention relates to modulating ion beam current in an ion implantation system to mitigate non-uniform ion implantations, for example. Multiple arrangements are revealed for modulating the intensity of the ion beam. For example, the volume or number of ions within the beam is altered by biasing one or more different elements downstream of the ion source. Alternatively, the dosage of ions within the ion beam can also be manipulated by controlling elements more closely associated with the ion source. In this manner, the implantation process can be regulated so that the concentration of ions implanted into the wafer is substantially uniform across the wafer. 
     Referring initially to  FIG. 1 , an ion implantation system  100  suitable for implementing one or more aspects of the present invention is depicted in block diagram form. The system  100  includes an ion source  102  for producing a quantity of ions that can be extracted in the form of an ion beam  104 . The ion source  102  generally includes a gas source  106  from which the ions are generated, and a power source  108  that facilitates the production of the ions from the gas. 
     A beamline assembly  110  is provided downstream of the ion source  102  to receive the ion beam  104 . The beamline assembly  110  in one example includes, among other things, a mass analyzer  112 . The beamline assembly  110  is situated along the path to receive the beam  104 . The mass analyzer  112  includes a field generating component, such as a magnet  114 , and operates to provide a field across the beam path  104  so as to deflect ions from the ion beam  104  at varying trajectories according to mass (e.g., charge to mass ratio). Ions traveling through the magnetic field experience a force that directs individual ions of a desired mass along the beam path  104  and deflects ions of undesired mass away from the beam path. 
     The ion implantation system  100  further includes an end station  116  to receive the mass analyzed ion beam  104  from the beamline assembly  110 . The end station  116  supports one or more workpieces such as semiconductor wafers (not shown) along the beam path for implantation using the mass analyzed ion beam  104 . The end station  116  includes a target scanning system  118  for translating or scanning one or more target workpieces and the ion beam  104  relative to one another. The target scanning system  118  may provide for batch or serial implantation, for example, as may be desired under given circumstances, operating parameters and/or objectives. 
     Additionally, a measurement component  120  is operatively coupled to the end station  116 . The measurement component may include, for example, a Faraday cup (not shown) that is operable to detect the intensity of the ion beam as the beam is scanned across the wafer. According to one or more aspects of the present invention, the detected current density is fed back (e.g., via a processor—not shown) to control one or more beam current modulation components  122 ,  124 . In accordance with one or more aspects of the present invention, one or more of the components  122 ,  124  can be closely associated with the ion source  102  and/or be situated downstream from the ion source  102  (e.g., on the beamline assembly  110  or end station  116 ). 
     The ion implantation system  100  may also include a mechanism  126  in front of the end station  116  to scan the ion beam  104  relative to the one or more workpieces. Such mechanisms may, for example, be electric and/or magnetic. For example, the mechanism may include one or more electrically conductive plates that can be biased to have an electromagnetic field controllably developed there-across. The field lines influence the direction of the ion beam  104  passing there-through, and can be selectively controlled to cause the beam  104  to scan across the workpieces in a desired manner. 
     Referring now to  FIG. 2 , an exemplary ion implantation system  200  suitable for implementing one or more aspects of the present invention is depicted in somewhat greater detail. Note that although the system in  FIG. 2  illustrates a low energy system, it should be appreciated that the system  200  is provided as an example, and that the present invention finds utility in various types of ion implantation systems, and such variations are contemplated as falling within the scope of the present invention. The system  200  includes an ion source  202 , a beamline assembly  204 , and a target or end station  206 . An expansible stainless steel bellows assembly  208 , which permits movement of the end station  206  with respect to the beamline assembly  204 , connects the end station  206  and the beamline assembly  204 . 
     The ion source  202  comprises a plasma chamber  210  and an ion extraction assembly  212 . Energy is imparted to an ionizable dopant gas to generate ions within the plasma chamber  210 . Generally, positive ions are generated, although the present invention is applicable to systems wherein negative ions are generated by the source  202 . The positive ions are extracted through a slit in the plasma chamber  210  by the ion extraction assembly  212 , which comprises a plurality of electrodes  214 . Accordingly, the ion extraction assembly  212  functions to extract a beam  216  of positive ions from the plasma chamber  210  and to accelerate the extracted ions into the beamline assembly  204 , and more particularly into a mass analysis magnet  218  within the beamline assembly  204 . 
     The mass analysis magnet  218  includes a curved beam path  220  within a passageway  222  defined by a metal (e.g., aluminum) beam guide having side-walls  224 , evacuation of which is provided by a vacuum pump  226 . The ion beam  216  that propagates along this path  220  is affected by the magnetic field generated by the mass analysis magnet  218 , to reject ions of an inappropriate charge-to-mass ratio. Control electronics  228  are included to adjust the strength and orientation of this dipole magnetic field, among other things. The magnetic field is controlled by the electrical current through the field windings of the magnet  218  through a magnet connector  230 . It will be appreciated that control electronics  228  may include a processor and/or computer system for overall control of the system  200  (e.g., by an operator). 
     The dipole magnetic field causes the ion beam  216  to move along the curved beam path  220  from a first or entrance trajectory  232  near the ion source  202  to a second or exit trajectory  234  near an exiting end of the passageway  222 . Portions  236  and  238  of the beam  216 , comprised of ions having an inappropriate charge-to-mass ratio, are deflected away from the curved trajectory and into the beam guide side walls  224 . In this manner, the magnet  218  only allows those ions in the beam  220  which have the desired charge-to-mass ratio to traverse entirely through the passageway  222 . 
     The beamline assembly  204  can also be said to include an accelerator  240 . The accelerator  240  includes a plurality of electrodes  242  arranged and biased to accelerate and/or decelerate ions, as well as to focus, bend and decontaminate the ion beam. A dosimetry indicator such as a Faraday cup  244  may also be included to detect a sample current of the ion beam. A source of plasma  246  may also be included to provide a plasma shower  248  for neutralizing a (positive) charge that would otherwise accumulate on a target workpiece as a result of being implanted by the (positively) charged ion beam  216 . A vacuum pump  250  may further be included to evacuate the accelerator  240 . 
     Downstream of the accelerator  240  is the end station  206 , which includes a support  252  upon which one or more wafers  254  or other workpieces to be treated are mounted. The wafer support  252  resides in a target plane that is generally oriented relatively perpendicularly to the direction of the implant beam, although the wafer support may also be oriented at angles substantially different from that shown and described. Wafer support may also, for example, take the form of a mechanical arm capable of moving a wafer through the beam or a rotating disk.  FIG. 2  illustrates a disc shaped wafer support  252  that is rotated by a motor  256  at the end station  206 . The ion beam thus strikes wafers mounted to the support as they move in a circular path. The end station  206  pivots about point  258 , which is the intersection of the path  260  of the ion beam and the wafer  254 , so that the target plane is adjustable about this point  258 . 
     It will be appreciated that the Faraday cup  244  can be utilized in mapping the ion implantation on one or more wafers. For example, the cup  244  can be effectively substituted for the one or more wafers during a test run. The ion beam and Faraday cup  244  can then be moved relative to one another while the beam current is held constant. In this manner, variations in ion dosage (e.g., that may occur at the perimeters of the wafers) can be detected. A waveform or map of beam current intensity versus scan position can thus be identified (e.g., by feeding the readings taken by the cup back to the control electronics  228 ). The detected waveform(s) can then be utilized to adjust the beam current during actual implantation. For example, the beam current can be adjusted upward as the beam approaches the perimeters of the wafers and/or may correspondingly be reduced as the beam implants into more centralized locations on the wafers. 
     Additionally, during actual implantation, the Faraday cup  244  can be utilized to take readings of beam intensity just as the beam finishes respective scans (e.g., so that the cup  244  does not interfere with the implantations). These readings can then be compared to the waveforms to determine just how much the beam current should be ramped up (or down) to comport with the waveforms. It will be appreciated that such end of scan measurements can be done continuously or intermittently to mitigate non-uniform ion implantations. It will be further appreciated that a biasing voltage applied to the extraction assembly  212  can, for example, be varied to adjust the beam current density. Similarly, the source of plasma  246  can be adjusted to vary (e.g., the intensity) a plasma shower  248  in accordance with one or more aspects of the present invention. 
     In the example illustrated, a scanning mechanism is also included in the beamline assembly  204  to facilitate maneuvering the beam  216  relative to the one or more wafers  254 . In one arrangement, the mechanism may include, for example, two or more electrically conductive plates  272 ,  274  positioned so that the ion beam  216  passes there-through. The electrodes  272 ,  274  can be selectively biased with a potential to deflect the ion beam  216  in a prescribed manner. Similarly, the scanning mechanism may include a plurality of electromagnets (not shown) that may be energized to produce magnetic lines of flux there-between to selectively deflect the beam in a prescribed manner. 
       FIG. 3  illustrates in schematic block diagram form certain components of an ion implantation system  300  wherein one or more aspects of the present invention are implemented. The system  300  includes an ion source  302  for generating a source of ions. In the example shown, the ion source  302  includes a cathode  304 , an anode  306 , a mirror electrode  308 , a gas supply  310  and source magnet components  312   a ,  312   b . A power supply  314  and an arc power supply  316  are operatively coupled to the cathode  304 , and another supply  318  is also connected to the source magnet components  312   a ,  312   b  in the example shown. 
     In operation, the gas supply  310  provides one or more precursor gases (e.g., via a conduit  320 ) into an area  322  of the source  302  wherein the ions are generated. The cathode  304 , in one example, takes the form of a filament (e.g., a rod made of tungsten or tungsten alloy) is heated by the power supply  314  (e.g., to about 2000 degrees Kelvin) to excite electrons therein. The arc supply  316 , in turn, provides additional energy to the cathode  304  (e.g., to heat the cathode to about 2800 degrees Kelvin) to cause electrons to jump from the cathode  304  into the area  322  wherein the gas is situated. The anode  306  assists with drawing the electrons into area  322 , and may include sidewalls (not shown) of the ion source  302 , for example. Further, supply  314  may also be coupled to the anode  306  so that a bias can be set up between the cathode  304  and the anode  306  to facilitate drawing the electrons into area  322 . 
     The mirror electrode  308  assists with maintaining the electrons within area  322 . In particular, a bias imparted to the mirror electrode  308  serves to repel electrons emitted from the cathode  304  back into area  322 . Similarly, a magnetic field induced within the ion source  302  by the source magnet serves to maintain electrons within area  322  and off of sidewalls (not shown) of the source  302 . In the example shown, two components  312   a  and  312   b  of the source magnet are shown. These may be indicative, of a cross-sectional view of windings and/or a yoke of an electromagnet, for example. The electrons moving around within area  322  collide with the gaseous molecules within area  322  to create the ions. In particular, electrons that collide with gaseous molecules with sufficient force cause one or more electrons to become dislodged from the molecules, thus producing positively charged gaseous ions. It will be appreciated that the magnetic field applied by the source magnet may be parallel to the cathode to increase the electron path length and to assist with suspending a plasma of both ions and electrons within area  322 . 
     It will be further appreciated that the present invention contemplates and has application to negatively charged ions also. Additionally, it will also be appreciated that the beam current density or intensity is related to the number of ions produced in the ion source  302 . Thus, in accordance with one or more aspects of the present invention, any one or more of the components of the ion source can be selectively adjusted to modulate the beam current. By way of example only and not limitation, the magnetic field set up by the source magnet  312  can be altered by controlling the supply  318  to increase or retard the number of ions generated within the source  302  to correspondingly increase or decrease the beam current. Modulating the beam current via the source magnet may be more effective at higher energies (e.g., where the magnetic field is around 200 Gauss) since beam currents may have a heightened (e.g., non-linear) sensitivity to changes introduced by the supply  318  at lower energies. 
     It will be further appreciated that the present invention contemplates and has application to ion sources of types other than the arc discharge source described above. For example, an ion source may include a means of RF excitation to produce ions. Such a source is disclosed in U.S. Pat. No. 5,661,308, the entirety of which is hereby incorporated by reference. An additional example is an ion source that may include a means of excitation by electron beam injection to produce ions. This is sometimes referred to as a “soft ionization” type of source. An example of such a source is disclosed in U.S. Pat. No. 6,452,338, the entirety of which is also hereby incorporated by reference. An additional example of an ion source to which the present invention has application is an ion source that includes a means of microwave excitation to produce a plurality of ions. 
     The ion beam  326  is extracted from the ion source  302  by electrodes  330  which are generally biased negatively with respect to the ion source  302 . In addition to extracting ions from the ion source, electrodes  330  also serve the function of providing suppression of electrons which are attracted to the ion source  302  by its generally positive bias. Further on, the beam  326  encounters ground electrodes  332  and a subsequent aperture defined by plates  334  before entering the beamguide  336  and associated analyzer magnet  338 . After being separated according to a desired mass-to-charge ratio in the beamguide  336  and analyzer magnet  338 , the beam  326  then encounters another set of electrodes  340  before entering a resolving aperture defined by plates  342  that further separates out undesired species of ions. The beam  326  then encounters yet another set of electrodes  344  before being bathed in a plasma to neutralize space charge and neutralize charge build-up that would otherwise accumulate on a target workpiece. The beam  326  then impinges upon one or more workpieces (not shown) located within an end station  346 . 
     It will be appreciated that the beam current may be affected by many of the components of the system  300 . For example, respective biases on the extraction suppression electrodes  330 , ground electrodes  332 , plates  334 , electrodes  340 , plates  342  and electrodes  344  can affect the beam current. Accordingly, the beam current can be modulated by selectively controlling one or more of the extraction suppression supply  350 , floating ground supply  352 , supply  354 , supply  356 , supply  358  and supply  360  which control the respective voltages applied to these components. It will be appreciated that while a combined set of extraction suppression electrodes  330  are discussed herein, the present invention contemplates separate sets of extraction and suppression electrodes having respective supplies that can be independently varied to alter the respective voltages applied to those electrodes. It will be further appreciated that the ground electrodes  332  are generally modulated with a voltage different from, but approximately zero, on average. This distinguishes the ground electrodes  332  from a general case of an electrode which may have some non-zero bias. 
     The foregoing supplies can be controlled by a controller  364 , for example, that takes readings from a measurement system  368  (e.g., that includes a Faraday cup) indicative of end of scan beam current during ion implantation, for example. Similarly, the controller can be operatively coupled to the source of neutralization plasma  370  to modulate the beam current by selectively regulating the amount of active plasma to which the beam  326  is exposed. Modulating the beam current via the source of plasma  370  may be more effective at lower energies (e.g., less than about 10 keV) since active plasma neutralization is typically not required for efficient beam transport at higher energies. It will be appreciated that the controller may also assist (e.g., the measurement system  368 ) in developing implantation waveforms as previously discussed, and may make use of such waveforms in facilitating selective adjustments to the beam current. 
     It will be appreciated that the beam current can be modulated at one or more particular frequencies (e.g., in the range of about 1–1000 Hz) or over one or more dynamic ranges (e.g., of about 10–20% of the beam current). The modulation can also be done in an open-loop fashion, where initial measurements of beam current are performed prior to the implantation, instead of during. This would likely be based on some known non-uniformity in the system (e.g., where a particular beam current results in a predicable non-uniformity). It will also be appreciated that while one use of such modulation is to achieve a uniform dosage on a wafer, it could be used to achieve any predetermined dopant profile, where uniformity is a subset of the general case. 
     Additionally, the beam current can be modulated to maintain or otherwise regulate a relatively constant beam current where the source output fluctuates. Thus, the beam current can be “trimmed” to lower the current where the output increases, or to increase the current where the output decreases. The current can be increased, for example, by diverting, releasing or redirecting some rerouted or previously intercepted or stored beam current. 
     Referring now to  FIG. 4 , an ion implantation system  400  is illustrated wherein ion beam current may be modulated in accordance with one or more aspects of the present invention. The exemplary ion implantation system  400  is illustrated as having an ion source  402  for generating an ion beam  404 . An extraction power supply  406  is included to bias a pair of extraction suppression electrodes  408 ,  410  (e.g., to a potential of approximately 20 kV) to accelerate the ions from the source  402  along a trajectory leading to an ion mass analyzing magnet  412 . The magnet  412  bends the beam  404  at approximately a right angle and directs ions having an appropriate mass along a travel path through a shutter  414 . The shutter  414  rejects ions having an inappropriate mass from the ion beam  404 . 
     The beam  404  then passes through a pair of deflection electrodes  416 ,  418 . Control voltages applied to the electrodes  416 ,  418  by a control circuit  420  cause the ion beam  404  to pass through electric fields which deflect the ion beam  404  by a controlled amount. The magnitude of the voltage difference between the two plates  416 ,  418  controls the amount of deflection. A source of plasma  422  may also be included to bathe the beam  404  in neutralizing plasma to mitigate the number of positive charges that would otherwise accumulate on a target workpiece. 
     A beam accelerator  424  is also included to redirect ions along a travel path generally parallel to the trajectory they follow as they exit the analyzing magnet  412 . The beam accelerator  424  includes a curved metallic entrance electrode  426  and a plurality of spaced, parallel metallic plates  428 ,  430 ,  432 ,  434 . After passing through the accelerator  424  the ions in the beam  404  have been both redirected to a desired trajectory and accelerated to a desired implantation energy. The beam  404  generally travels in a vacuum maintained within a beamline assembly  436 . 
     Downstream from the beam accelerator  424 , an ion implantation station  440  includes one or more structures that support a semiconductor wafer  442  at a position to intercept ions that have been accelerated by the parallel plates  428 ,  430 ,  432 ,  434 . Ion beam collisions with other particles degrade beam integrity so that the entire beamline assembly  436  from the source  402  to the implantation station  440  is evacuated by one or more pumps (not shown). At a region of the ion implantation station  440 , a chamber  444  is similarly evacuated and the wafers are inserted into and withdrawn from load locks to avoid repeated pressurization and depressurization of the chamber  444 . 
     A mechanized arm  446  located outside the chamber  444  grasps a wafer supported on a shuttle  448  that has obtained the wafer from a cassette  450 . The arm  446  first positions each undoped wafer on a wafer orienter  456 . The orienter  456  rotates the undoped wafer to a particular orientation before ion implantation so that as ions strike the wafer they encounter a specific orientation of the wafer&#39;s crystal lattice structure. The wafer is then moved into a load lock  458  so that a second arm  460  can move the wafers to an implant position within the chamber  444 . At the implantation site a wafer support  462  orients the wafer  442  with respect to the ion beam  404  at a specific tilt angle that remains substantially constant. 
     The scanning electrodes  416 ,  418  produce side-to-side beam scanning of a controlled amount under the direction of the control circuitry  420 . The circuitry may include, for example, a programmable micro-controller for adjusting the scanning electrode voltages to achieve desired wafer scanning. It will be appreciated that drive mechanisms (not shown) may also be attached to support  462  to facilitate additional relative motion between the ion beam  404  and the surface of the wafer  442 . A measurement component  464  may be incorporated to monitor the beam current as the ion beam  404  is scanned across the wafer  442 . This may include, for example, a Faraday cup and/or terminal return current, and can be utilized to increase or decrease the ion beam concentration based upon a desired doping level for the silicon wafer  442  at the implantation station  440 . 
     Once the wafer  442  has been doped by treatment with the ion beam  404 , the wafer  442  is removed from the implantation station  440  by a third arm  466 . This arm  466  delivers the wafer  442  to a load lock  468  so that a fourth mechanized arm  470  can transfer the wafer  442  to a shuttle  472  that moves the doped wafer  442  to a cassette  474 . 
     It can be seen that the entrance electrode  426  of the accelerator  424  is constructed from an arcuate conductor. This electrode  426  is generally maintained at the same potential as a housing  476  which supports the source  402 , the analyzing magnet  412 , and the deflection electrodes  416 ,  418 . The second electrode  428  is positively biased by a high voltage power supply  478 . The voltage difference between the curved electrode  426  and the first generally planer electrode  428  is maintained by a lens power supply  480 . 
     Representative voltages of approximately 60 kilovolts for the lens power supply  484 , 20 kilovolts for the extraction power supply  406 , and 120 kilovolts for the high voltage power supply  482  accelerate the positively charged ions to a final energy of approximately 200 keV. These voltages are appropriate for the illustrated implantation system  400  where the entrance electrode  426  has a radius of curvature of approximately 13 inches and which is spaced approximately 24 inches from the scan vertex at the position of the deflection electrodes  416 ,  418 . An exemplary width of an entrance aperture defined within electrode  426  for this arrangement would be about 12 inches. 
     To generate ions, it will be appreciated that the ion source assembly  402  can utilize a cathode to create an arc discharge, or RF or microwave excitation, or electron beam injection, for example, to excite free electrons in the interior of an ion generation chamber. The electrons collide with gas molecules injected into the chamber interior and ions are thereby generated. Ions may have an initial energy of between about 0.2 to 100 keV, for example, due in part to the effects of the extraction suppression electrodes  408 ,  410 . 
     It will also be appreciated that the ion source  402  may include a source magnet to assist with generating ions. In the example illustrated, the source magnet includes multiple components  482 ,  484  that may, for example, correspond to a yoke  482  and coil  484  of an electromagnet. A set of ground electrodes  486  and an additional set of electrodes  488  are also included close to the source  402  to help focus and shape the beam  404  before entering the analyzing magnet  412 . 
     Similarly, another set of electrodes  490 , a set of plates  492  defining a resolving aperture and a subsequent set of electrodes  494  can be included to block spurious ions and provide focusing to the beam  404 . A portion of the beam  404  can also be physically blocked to modulate beam current. For example, a mechanical structure (not shown) can be physically inserted (e.g., through a slit  496  in beamline assembly  436 ) to selectively block some of the ion beam  404 . Moving the mechanical structure in a prescribed manner to physically intercept some fraction of the beam current otherwise passing by would achieve the effect of modulating the beam current. 
     According to one or more aspects of the present invention, beam current can be modulated to achieve desired ion implantation by selectively controlling one or more components downstream of the ion source assembly  402 , as well as one or more components more closely associated with the ion source assembly  402 . For example, voltages applied to the deflection electrodes  416 ,  418 , electrodes  490 , plates  492  electrodes  494 , and plates  426 ,  428 ,  430 ,  432 ,  434  of the beam accelerator  424  can be selectively regulated to modulate beam current (e.g., via a controller, a measurement component and implantation waveforms as discussed above). Similarly, the extraction suppression electrodes  408 ,  410 , ground electrodes  486 , electrodes  488 , source magnet components  482 ,  484  and the neutralizing plasma source  422  can also be selectively adjusted to modulate the beam current. Additionally, these elements can be selectively adjusted alone or in combination to desirably modulate beam current. 
     Turning to  FIG. 5 , a graphical representation  500  illustrates changes in beam current  502  as a function of changes in extraction suppression voltage  504  over time. It can be seen that changes in the beam current  502  closely track those of the extraction suppression voltage  504  over the approximately 100 second time period. In particular, the beam current  502  modulates between about 4.0E-03 to about 6.0E-03 Amperes while it tracks the extraction suppression voltage  504  modulating between about 7000 to about 10000 Volts. 
     Similarly,  FIG. 6  is a graphical representation  600  illustrating a functional relationship between beam current (y-axis) and extraction suppression voltage (x-axis). The plot  602  reveals that changes in the beam current in response to changes in the extraction suppression voltage are a little more dynamic for voltages between about 6900 to about 8500 Volts where the beam current rather linearly goes from about 4.0E-03 to about 6.0E-03 Amperes. When the extraction suppression voltage goes from about 8500 to about 10000 Volts, however, the beam current hovers right around 6.0E-03 Amperes, indicating a saturation type condition. 
     It will be appreciated that the aspects described herein are equally applicable to primary electron beam current in “soft ionization” ion sources, RF or microwave power in RF or microwave ion sources, as well as to non-arc discharge sources. 
     Although the invention has been illustrated and described above with respect to a certain aspects and implementations, it will be appreciated that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described components (assemblies, devices, circuits, systems, etc.), the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component which performs the specified function of the described component (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure, which performs the function in the herein illustrated exemplary implementations of the invention. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms “includes”, “including”, “has”, “having”, “with” and variants thereof are used in either the detailed description or the claims, these terms are intended to be inclusive in a manner similar to the term “comprising”. Also, the term “exemplary” as utilized herein simply means example, rather than finest performer.