Abstract:
An electrode assembly for use with an ion source chamber or as part of an ion implanter processing system to provide a uniform ion beam profile. The electrode assembly includes an electrode having an extraction slot with length L aligned with an aperture of the ion source chamber for extracting an ion beam. The electrode includes a plurality of segments partitioned within the length of the extraction slot where each of the segments is configured to be displaced in at least one direction with respect to the ion beam. A plurality of actuators are connected to the plurality of electrode segments for displacing one or more of the segments. By displacing at least one of the plurality of electrode segments, the current density of a portion of the ion beam corresponding to the position of the segment within the extraction slot is modified to provide a uniform current density beam profile associated with the extracted ion beam.

Description:
BACKGROUND OF THE INVENTION  
       [0001]    1. Field of the Invention 
         [0002]    Embodiments of the invention relate to the field of semiconductor device fabrication. More particularly, the present invention relates to an apparatus and method for controlling beam current uniformity in the ion beam extraction region of an ion implanter. 
         [0003]    2. Discussion of Related Art 
         [0004]    Ion implantation is a process used to dope ions into a work piece. One type of ion implantation is used to implant impurity ions during the manufacture of semiconductor substrates to obtain desired electrical device characteristics. An ion implanter generally includes an ion source chamber which generates ions of a particular species, a series of beam line components to control the ion beam and a platen to secure the wafer that receives the ion beam. These components are housed in a vacuum environment to prevent contamination and dispersion of the ion beam. The beam line components may include a series of electrodes to extract the ions from the source chamber, a mass analyzer configured with a particular magnetic field such that only the ions with a desired mass-to-charge ratio are able to travel through the analyzer, and a corrector magnet to provide a ribbon beam which is directed to a wafer orthogonally with respect to the ion beam to implant the ions into the wafer substrate. The ions lose energy when they collide with electrons and nuclei in the substrate and come to rest at a desired depth within the substrate based on the acceleration energy. The depth of implantation into the substrate is based on the ion implant energy and the mass of the ions generated in the source chamber. Typically, arsenic or phosphorus may be doped to form n-type regions in the substrate and boron, gallium or indium are doped to create p-type regions in the substrate. 
         [0005]    The series of electrodes are regulated at particular voltages with respect to ground to extract ions from the ion source chamber. For ribbon beams each of the electrodes include a slot having a particular length to extract the ions into a beam for downstream wafer implantation. When an ion beam is extracted, variations in beam related characteristics, such as contaminants, pressure, temperature, beam drift etc. influence beam uniformity.  FIG. 1  illustrates a plot of beam position across the length of an elongated slot of an extraction electrode against beam current densities. As can be seen, the beam profile includes a plurality of variations in beam current densities within the range of ±2 mA/cm for positions −125 mm to approximately 75 mm. It is believed that these non-uniformities are the result of various electrode characteristics. Ideally, the current density of the beam components would be constant about point E. The disparity in uniformity along the length of the electrode slot compromises the uniformity of the beam at wafer implantation. Prior attempts to correct for these non-uniformities include providing a segmented electrode with conducting sections and insulating sections. The insulating sections are used to prevent interference between the electrode sections. Another attempt at correcting for these non-uniformities included providing a plurality of insulating rings surrounding a corresponding electrode segment. Actuators are used to displace only the insulating ring portions perpendicular to the electrode segments to control the power delivered to each electrode segment. However, these prior attempts have not sufficiently corrected the non-uniformities resulting from extraction of an ion beam from an ion source chamber. In addition, corrector magnet rods and poles may also be employed in ion implanters to provide beam uniformity tuning. However, this may require long tune times for certain desired beam profile features. Thus, there is a need to provide a beam profile pre-tuning method utilizing an electrode configuration which is capable of extracting an ion ribbon beam having a uniform beam profile for wafer implantation. 
       SUMMARY OF THE INVENTION  
       [0006]    Exemplary embodiments of the present invention are directed to a segmented electrode assembly. In an exemplary embodiment, a segmented electrode assembly is used with an ion source chamber. The electrode assembly comprises an electrode having a slot with length L aligned with an aperture of the ion source chamber for extracting an ion beam. The electrode is partitioned into a plurality of electrode segments defined at least within the length L of the extraction slot of the electrode. Each of the electrode segments is configured to be displaced in at least one direction with respect to the ion beam extracted from the ion source chamber. A plurality of actuators are connected to the plurality of electrode segments for displacing one or more of the segments. The ion beam extracted from the ion source chamber has a current density profile. The displacement of at least one of the electrode segments modifies the current density of a portion of the ion beam corresponding to the position of the segment within the extraction slot region to provide a uniform current density ion beam profile. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0007]      FIG. 1  is a graph illustrating ribbon beam profile generated from an ion source chamber. 
           [0008]      FIG. 2  is a top plan view of an exemplary ion source chamber together with a three (3) electrode configuration to extract an ion beam in accordance with an embodiment of the present invention. 
           [0009]      FIG. 3  is a perspective view of exemplary single mode ground electrode configured to correct current density non-uniformities in accordance with an embodiment of the present invention. 
           [0010]      FIG. 4A  is a cut-away sectional view of the three electrode configuration taken along lines A-A of the ground electrode shown in  FIG. 3  in accordance with an embodiment of the present invention. 
           [0011]      FIG. 4B  is a side view in the z direction of the ground electrode shown in  FIG. 4A  in accordance with an embodiment of the present invention. 
           [0012]      FIG. 5  is a cut-away sectional view of the three electrode configuration taken along lines A-A of the ground electrode shown in  FIG. 3  in accordance with an embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS  
       [0013]    The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention, however, may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, like numbers refer to like elements throughout. 
         [0014]      FIG. 2  is a top plan view of an exemplary ion source chamber  10  together with a three (3) extraction electrode configuration (electrodes  15 ,  20  and  25 ) to extract an ion beam  5  for implantation into the surface of wafer  30 . It should be understood that a plurality of beam line components may be disposed between ground electrode  25  and wafer  50  to control the ion beam. Beam line components may include a mass analyzer configured with a particular magnetic field such that only the ions with a desired mass-to-charge ratio are able to travel through the analyzer, a corrector magnet to provide a ribbon beam which is scanned over the surface of wafer  50  orthogonally with respect to the ion beam to implant the ions into the wafer substrate, and a platen to secure the wafer that receives the ion beam. However, for illustrative purposes, these intermediate components have been omitted. 
         [0015]    Ion source chamber  10  typically includes a heated filament which ionizes a feed gas introduced into the chamber to form charged ions and electrons (plasma). The heating element may be, for example, an indirectly heated cathode (IHC). Different feed gases are supplied to the source chamber to generate ions having particular dopant characteristics. The ions are extracted from source chamber  10  via the standard three (3) electrode configuration comprising a plasma or arc slit electrode  15 , suppression electrode  20  downstream from plasma electrode  15  and ground electrode  25  downstream from suppression electrode  20 . These electrodes are used to create a desired electric field to focus ion beam  5  extracted from source chamber  10 . Plasma electrode  15  includes a slot  16  through which ions extracted from ion source chamber  10  pass. Plasma electrode  15  may be biased at the same large potential as the source chamber  10 . The slot  16  has a length that is significantly greater than its width to provide a high aspect ratio to form a ribbon ion beam. At high aspect ratios, the gas flow from the source is reduced which allows source  10  to function at higher plasma densities. Similarly, suppression electrode  20  includes slot  21  aligned with slot  16  which has a length significantly greater than its width. Suppression electrode  20  is connected to a power supply and is typically biased at a moderate negative value to prevent electrons from entering back into source chamber  10  and to assist in focusing ion beam  5 . Ground electrode  25  is positioned downstream from suppression electrode  20  and is at ground potential. Ground electrode  25  includes slot  26  aligned with slot  16  of plasma electrode  15  and slot  21  of suppression electrode  20  which also has a length significantly greater than its width. The strength of the electric field generated by the electrodes can be tuned to a desired beam current to extract a particular type of ion beam from ion source chamber  10 . 
         [0016]    Ion ribbon beam  5  is extracted from ion source  10  in the z direction with electrode slots  16 ,  21  and  26  having respective lengths in the x direction. Ion ribbon beam  5  includes beam components  5   1 - 5   N  which may have varying densities. For example, beam component  5   1  may have a different current density then beam component  5   N-3 . The energy of these beam components may be in the range of 1-100 keV with the ribbon beam having current densities in the range of 0.3 mA/cm 2  to 1.7 mA/cm 2  which corresponds to a beam current of 10 mA-60 mA. In order to scan the wafer surface with the ion beam sufficiently, the wafer and the ion beam must move relative to each other. In one embodiment, wafer  50  is scanned with beam  5  by moving wafer  50  in a particular direction with respect to ribbon beam  5 . Alternative embodiments in which beam  5  moves with respect to wafer  50  are also employed. If the ribbon beam is scanned on the surface of wafer  50  in direction y, variations in current density result in implantation non-uniformities in the wafer because each wafer region receives a different ion current dosage resulting in wafer “stripes.” These non-uniformities result in current density problems at the surface of wafer  50 . 
         [0017]      FIG. 3  is a perspective view of exemplary single mode ground electrode  25  configured to correct current density non-uniformities in beam  5 . Ground electrode  25  includes a centrally disposed slot  26  through which ion ribbon beam  5  is extracted. Again ground electrode  25  is at ground potential and functions with plasma electrode  15  and suppression electrode  20  to extract a beam of desired energy. Ground electrode  25  may also be configured as a dual mode electrode having a plurality of slots for high and low energy extraction. Slot  26  has a length of, for example, 70 mm and a height of 5.5 mm-7 mm that may vary depending on whether the ions to be extracted have high or low energies. 
         [0018]      FIG. 4A  is a cut-away sectional view of the three (3) electrode configuration taken along lines A-A of ground electrode  25  in the z direction parallel to the extracted beam path. The cut-away perspective view illustrates ground electrode  25 , suppression electrode  25  and plasma electrode  15  in alignment with respective extraction slots  26 ,  21  and  16 . In particular, slot  26  is aligned with slot  21  of suppression electrode  50  and slot  16  of plasma electrode  15 . Within the region of slot  26 , ground electrode  25  is partitioned into segments  26   1 - 26   8 . If, for example, extraction slot  26  of ground electrode  25  has a length of 70 mm, each segment  26   N  may have an equal width of 8.75 mm (70 mm/8). Alternatively, the segments may have differing widths or only certain portions of the slot region may be segmented depending on the degree of control of beam uniformity required. Each segment is distinctly partitioned from an adjacent segment such that each segment can be displaced in either the z or y direction on the order of μm. Electrode displacement is accomplished by using, for example, a piezo actuator  31   N  (or alternative mechanical device) shown in  FIG. 4A  as connected to electrode segment  26   8 . Additional actuators may also be employed for each of the electrode segments. A controller may also be used in combination with the piezo actuators as well as a user interface to control the magnitude of displacement of a particular segment. Exemplary electrode segments  26   4  and  26   6  are shown in  FIG. 4A  as being displaced in the y direction perpendicular to the ion beam extraction path. It has been found that displacement of one or more electrode segments locally increases beam current density in the beam profile. Various segments may be displaced individually or in combination with a plurality of segments to manipulate the current densities associated with a portion of an extracted ion beam. In this manner, beam current density within the beam profile may be tuned to correct for non-uniformities during wafer scanning to provide a flatter beam profile from that shown with reference to  FIG. 1 . Alternatively, segments  26   1 - 26   8  may be displaced locally in the z direction between suppression electrode  20  and ground electrode  25  parallel to the beam path again on the order of μm. However, displacement in the z direction is limited by the close proximity of suppression electrode  20  from ground electrode  25 . 
         [0019]      FIG. 4B  is a side view in the z direction of electrode  20  having extraction slot  26  with length L. Ground electrode  20  is individually partitioned into segments  26   1 - 26   N  where N=8 is this example. Again, each segment is distinctly partitioned from an adjacent segment such that one or more segments can be displaced in either the z or y direction on the order of μm. Segments  26   4  and  26   6  are shown, for example, as being displaced in the y direction perpendicular to the z direction beam path. The partitioned segments  26   1 - 26   8  are within the boundaries of slot  26  having length L. In particular, segment  26   1  is within the boundary of a first end  26   a  of slot  26  and segment  26   8  is within the boundary of second end  26   b  of slot  26 . Only electrode segments within the slot area through which beam  5  passes are displaced. In this manner, local control of the beam current density in the beam profile is provided at a particular position along extraction slot length L by displacing one or more segments  26   1 - 26   N . Each segment is distinctly partitioned from an adjacent segment such that each segment can be displaced in either the z or y direction on the order of μm using, for example, piezo actuators. 
         [0020]      FIG. 5  is a cut-away sectional view of the three (3) electrode configuration taken along lines A-A of ground electrode  25  in the z direction parallel to the extracted beam path. The cut-away perspective view illustrates ground electrode  25 , suppression electrode  25  and plasma electrode  15  in alignment with their respective extraction slots. In particular, slot  16  of plasma electrode  15  is aligned with extraction slot  21  of suppression electrode  20  which is aligned with extraction slot  26  of ground electrode  25 . Within the region of slot  21  of suppression electrode  20 , the electrode is partitioned into segments  21   1 - 21   8 . Similar to ground electrode slot  26 , if extraction slot  21  of suppression electrode  20  has a length of 70 mm, each segment  21   N  may have an equal width of 8.75 mm (70 mm/8). Alternatively, the segments may have differing widths or only certain portions of the slot region may be segmented depending on the degree of control of beam uniformity required. Again, each segment can be displaced in either the z or y direction on the order of μm using, for example, piezo actuators similar to that shown with reference to  FIG. 4A .  FIG. 5  illustrates exemplary segments  21   4  and  21   6  displaced in the y direction. Because suppression electrode  20  is usually at a high voltage, for example 4-15 kV, the beam is extremely sensitive to any geometric changes in suppression electrode  20 . In addition, displacement of suppression electrode segments are more complicated considering the high temperatures reached by both the plasma electrode  15  and suppression electrode  20 . Because of these high temperatures, shielded rods may be employed to connect the segments  21   1 - 21   8  to the piezo actuators to control displacement of the segments. By displacing individual segments of suppression electrode  20 , beam current density within the beam profile may be tuned to correct for non-uniformities during wafer scanning. Alternatively, segments  21   1 - 21   8  may be displaced locally in the z direction between suppression electrode  20  and plasma electrode  15  parallel to the beam path again on the order of μm. However, displacement in the z direction is limited by the close proximity of suppression electrode  20  from plasma electrode  25  as well as the high operating temperatures referenced above. 
         [0021]    While the present invention has been disclosed with reference to certain embodiments, numerous modifications, alterations and changes to the described embodiments are possible without departing from the sphere and scope of the present invention, as defined in the appended claims. Accordingly, it is intended that the present invention not be limited to the described embodiments, but that it has the full scope defined by the language of the following claims, and equivalents thereof.