Abstract:
An apparatus and method for ion implantation that include destabilizing the ion beam as it passes through magnetic field, preferably a dipole magnetic field is disclosed. By introducing a bias voltage at certain points within the magnetic field, electrons from the plasma are drawn toward the magnet, thereby causing the ion beam to expand due to space charge effects. The bias voltage can be introduced into the magnet in a region where the magnetic field has only one component. Alternatively, the bias voltage can be in a region wherein the magnetic field has two components.

Description:
BACKGROUND OF THE INVENTION 
     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. 
     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  10 , 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, or collimator, magnet  110  is adapted to deflect the divergent ion beam into a set of beamlets having substantially parallel trajectories. Preferably, the collimator 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. 
     The components between the ion source and the workpiece comprise the beamline. These components transform the ions generated by the ion source into a ribbon beam capable of implanting ions into a workpiece, such as a semiconductor wafer. 
     The workpiece support is used to both hold the wafer in position, and to orient the wafer so as to be properly implanted by the ion beam. To effectively hold the wafer in place, most workpiece supports typically use a circular surface on which the workpiece rests, known as a platen. Often, the platen uses electrostatic force to hold the workpiece in position. By creating a strong electrostatic force on the platen, also known as the electrostatic chuck, the workpiece or wafer can be held in place without any mechanical fastening devices. This minimizes contamination and also improves cycle time, since the wafer does not need to be unfastened after it has been implanted. These chucks typically use one of two types of force to hold the wafer in place: coulombic or Johnson-Rahbeck force. 
     The workpiece support typically is capable of moving the workpiece in one or more directions. For example, in ion implantation, the ion beam is typically a scanned or ribbon beam, having a width much greater than its height. Assume that the width of the beam is defined as the x axis, the height of the beam is defined as the y axis, and the path of travel of the beam is defined as the z axis. The width of the beam is typically wider than the workpiece, such that the workpiece does not have to be moved in the x direction. However, it is common to move the workpiece along the y axis to expose the entire workpiece to the beam. 
     The uniformity of the ion beam is critical to successful implantation. Low energy levels, such as those used for shallow implants, make it even more difficult to maintain ion beam uniformity. If the concentration of ions across the beam width is uneven, the implanted substrate will have non-uniform characteristics and parameters, which will affect its performance and usefulness. 
     SUMMARY OF THE INVENTION 
     The problems of the prior art are overcome by the ion implantation apparatus and method described in the present disclosure. The disclosure provides an apparatus and method for ion implantation that include destabilizing the ion beam as it passes through a magnetic field, preferably a dipole magnetic field. By introducing a bias voltage at certain points within that magnetic field, electrons from the plasma are drawn toward the electrodes, thereby causing the ion beam to expand due to space charge effects. The bias voltage can be introduced into the magnet in a region where the magnetic field has only one component. Alternatively, the bias voltage can be in a region wherein the magnetic field has two components. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
         FIG. 1  represents a traditional ion implanter; 
         FIG. 2  represents a cross-section of the collimator magnet in  FIG. 1 , taken in the x-y plane; 
         FIG. 3  represents a top view of a first embodiment of the electrodes shown in  FIG. 2 , taken in the x-z plane; 
         FIG. 4  represents a top view of a second embodiment of the electrodes shown in  FIG. 2 , taken in the x-z plane; 
         FIG. 5  represents a cross-section of a collimator magnet in the x-y plane and shows the effects of the electrodes as used in accordance with a first embodiment; 
         FIG. 6  represents a cross-section of a collimator magnet taken along the y-z plane and shows the magnetic field lines at the edge of the collimator magnet; 
         FIG. 7  represents a cross-section of a collimator magnet taken along the x-y plane and shows the effects of the electrodes as used in accordance with a second embodiment; 
         FIG. 8  represents a graph quantifiably showing the effects of the electodes as used in accordance with a second embodiment; 
         FIG. 9  represents a graph quantifiably showing the effects of a negative bias voltage when used with a boron ion beam; 
         FIG. 10  represents a cross-section of a collimator magnet taken along the x-y plane, showing the use of elements on both poles; and 
         FIG. 11  illustrated a representative ion density distribution for a low energy level ion beam. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As described above, the ion beam passes through various components before reaching the workpiece. One challenge in this process is to insure that the ion beam remain uniform across its width. In some cases, the ions tend to congregate close to the center, with fewer on either side of the centerline, as shown in  FIG. 11 . This results in an uneven ion implantation in the workpiece. 
       FIG. 2  shows a first embodiment. The vertical lines  210  represent the magnetic (B) field within the collimator magnet  200 , while the ⊙ represents the path of the ion beam, coming out of the page. In this embodiment, electrical conductors, or electrodes  220 , preferably in the shape of strips, are inserted in the dipole magnetic field of the collimator magnet  200 . In one embodiment, the electrodes  220  are affixed directly to the magnet  200 , so as to remain out of the beam path. The electrodes  220  can be affixed to either the north or south poles, or both poles, if desired. Alternatively, the electrodes can be held in place using other methods. The only requirement is that the electrodes be electrically isolated from their environment. Each electrode  220  is preferably in communication with an independent power supply  250 , which is used to supply a bias voltage to the electrode. 
       FIG. 3  shows one possible shape of the electrodes. These electrodes  200  can be printed circuit strips. In certain embodiments, these strips are 1 inch wide, 2 inches long and 0.125 inches thick, although other dimensions are within the scope of the disclosure. In the preferred embodiment, each electrode is in communication with an independent power supply (as shown in  FIG. 2 ), such that the bias voltage applied to any electrode is independent of the bias voltage applied to any other electrode. 
       FIG. 4  shows a second embodiment. In this embodiment, rather than using elongated strips, electrically conductive elements, such as pins are used. These pins can be arranged as a single row of pins, as shown in  FIG. 4 . As described above, each of these pins is preferably in communication with a separate power supply so as to enable the creation of bias voltages, independent of the other pins. 
     In both embodiments, the electrodes  220  are preferably insulated from the magnet. The bias voltages applied to the electrodes  220  can be static (i.e. DC voltage), or more preferably, a pulsed D.C. voltage. A pulse of positive voltage will cause the electrons in the ion beam to move toward the electrode. This causes the ion beam to become de-neutralized, or “blow up”, in that region, due to space charge effects. As the ion beam continues to pass through the magnet, it regains its shape, although its ion concentration has been reduced. The magnitude and duration of the bias voltage pulse determines the amount of expansion experienced by the ion beam. In other embodiments, a negative bias voltage can be used. For example, low energy boron ion beams can be enhanced by the use of negative bias voltages. 
     In certain embodiments, the electrodes  220  are well within the boundaries of the magnet, such as more than 2-3 inches, so that the magnetic field has a significant vertical component (B y ), with little or no B x  or B z  component. Therefore, the existing magnetic field and the electric field created by the application of the bias voltage are in the same direction. Thus, the electrons are somewhat restricted in their motion, as charged particles are reluctant to cross magnetic field lines. Therefore, the ion beam expansion described above remains very localized. In other words, the electrode only controls that portion of the ion beam that exists in the volume defined by the magnetic field lines and the element itself.  FIG. 5  shows an example of this localized effect in creating a deneutralization region  230 . The deneutralization region appears as a rectangular column, defined by the electrode on one side and the height of the magnet on the other side. 
     Since this technique is primarily used to reduce the concentration of ions, it is used to reduce the intensity of the beam to its lowest level. In other words, this technique is not used to bring all portions of the ion beam to the average value. Rather, it is used to bring all portions of the ion beam to the lowest value. 
     In certain embodiments, as shown in  FIGS. 2 and 5 , the electrodes  200  are only placed on one pole of the magnet. In alternate embodiments, electrodes  200  are placed on both the north and south poles. 
     In a second embodiment, the electrodes are placed near the edge of the collimator magnet, or outside the edge of the magnet, but within the magnetic field. In these locations, the magnetic field has a z component, due to the irregularly shaped magnetic field lines, as seen in  FIG. 6 . In certain embodiments, the electrodes  220  are placed on the edge nearest the workpiece  175 , as the beam has greater parallelism at this location. 
     Since the magnetic field has a z component, the electrical field created by the electrodes crosses with this component of the magnetic field to create a force in the x direction.  FIG. 7  shows the effects of a bias voltage in this embodiment. The application of a positive D.C. voltage to an electrode causes several effects. Electrons within the column defined by the electrode are accelerated toward the electrode. However, the electrons are pushed in the x direction due to the force created by the crossed E and B fields. Thus, the region defined by the electrode is stripped of electrons, creating a deneutralization region  230 , where ions are not space charged neutralized. The region adjacent to this region receives a surplus of electrons, creating a region of improved space charge neutralization  240 . 
       FIG. 7  shows each electrical element  220  having a dedicated power supply  250 . This power supply  250  is capable of providing positive or negative voltage, and is capable of supplying a constant voltage or can generate voltage pulses. In other embodiments, these pulses can be of varying durations. In certain embodiments, each element is not in communication with a dedicated power supply, rather two or more elements share a common supply. 
     The effect of energizing a single electrode  220  is best seen in  FIG. 8 . Line  260  represents the percentage change in the ion beam moving along the X dimension (as compared to an average ion beam). Element  220  is located at its respective X position within the magnetic field. Electrode  220  is energized with a positive voltage. In the regions located remote from the electrode, no change in the ion beam is detected. However, in the column directly above the electrode, the ion beam is decreased by about 30%. This corresponds to region  230  in  FIG. 7 . Because of the force created by the orthogonal magnetic and electrical fields, the region adjacent to the electrode  220  is increased by nearly 40%, which corresponds to region  240  in  FIG. 7 . 
     Note that  FIG. 8  shows the region adjacent to the element  220  on its left is enhanced. This is due to the placement of the electrode with respect to the magnet. For example, by placing the electrode on the opposite pole, the force created would be in the opposite direction, and therefore the region adjacent to the right would be enhanced. Similarly, if the electrodes were placed on the opposite end of the magnet, the force created would be reversed. 
       FIG. 9  shows the effect of a negative bias voltage on a boron ion beam. Note that the ion density is greatly enhanced in the column where the energized electrode is located. 
     While these figures show the effects of energizing a single electrode, it is anticipated that a plurality of electrodes will be energized. In this scenario, the effects of each individual electrode add to the effects of the other energized electrodes, according to the laws of superposition and create an overall effect across the entire ion beam. 
     As described above, it is within the scope of the disclosure to affix electrodes  220  on both poles.  FIG. 10  shows one such embodiment. Note that in  FIGS. 7 and 8 , the beam was enhanced to the left of the energized element  220 . This was due to the direction of the electrical and magnetic fields. By placing electrodes  220  on both poles, and selectively activating them, it is possible to create forces that push away from the centerline and toward the outside of the ion beam. Dashed line  300  represents the uneven ion density of the ion beam. Positive voltages are applied to two elements in this example, as denoted by the “+++++” symbols. The electron depleted column bound by the electrode(s) has a higher space potential due to space charge dominated by residual ions. As a result, an electric field appears across the sheath that separates the column from its boundary (the biased electrode). This electric field, in conjunction with the z component of the magnetic field, causes a drift of charged particles (plasma) in the direction of E×B. The directions of E and B reverse at opposite ends of the column, leading to drifts in the same direction. It is this plasma migration from one column to the adjacent one that causes an enhancement of beam transport in the adjacent region, thus an increase in propagated beam current. To reverse the direction of the drift, the z component of the B field has to be reversed. This can be done, for example, by locating a set of controlling electrodes in the other fringe (in the direction of propagation) of the dipole field. 
     While the above disclosure describes the use of electrodes within the collimator magnet, the disclosure is not limited to this embodiment. In another embodiment, the electrodes are placed within the mass analyzer magnet  106 . In another embodiment, an additional dipole magnetic field, separate from those traditionally present in  FIG. 1 , is inserted into the beamline. One possible placement of such a magnet field is between the collimator magnet  110  and the second deceleration stage  112 . However, this additional magnetic field may be located anywhere in the beamline. In all cases, the dipole magnetic field is preferably created across the small dimension of the ribbon beam, as can be seen in  FIG. 2 . In these embodiments, the effect of the electrodes is as described above. 
     Measuring devices, such as Faraday cups, can be used to measure the resulting ion beam at a plurality of locations along the X dimension. These measurements then provide the feedback necessary to tune the bias voltages at the various elements  220 . This feedback is preferably provided to control logic, which then determines which elements to energize, and the appropriate bias voltages and durations to be used. In this manner, the uniformity of the ion beam can be improved.