Abstract:
Methods and apparatus for implanting ions in a workpiece, such as a semiconductor wafer, include generating an ion beam, measuring parallelism of the ion beam, adjusting the ion beam for a desired parallelism based on the measured parallelism, measuring a beam direction of the adjusted ion beam, orienting a workpiece at an implant angle referenced to the measured beam direction and performing an implant with the workpiece oriented at the implant angle referenced to the measured beam direction. The implant may be performed with a high degree of beam parallelism.

Description:
FIELD OF THE INVENTION 
     This invention relates to systems and methods for ion implantation of semiconductor wafers or other workpieces and, more particularly, to methods and apparatus for adjusting beam parallelism in ion implanters. 
     BACKGROUND OF THE INVENTION 
     Ion implantation is a standard technique for introducing conductivity-altering impurities into semiconductor wafers. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material to form a region of desired conductivity. 
     Ion implantation systems usually include an ion source for converting a gas or a solid material into a well-defined ion beam. The ion beam is mass analyzed to eliminate undesired ion species, is accelerated to a desired energy and is directed onto a target plane. The beam is distributed over the target area by beam scanning, by target movement or by a combination of beam scanning and target movement. An ion implanter which utilizes a combination of beam scanning and target movement is disclosed in U.S. Pat. No. 4,922,106 issued May 1, 1990 to Berrian et al. 
     The delivery of a parallel ion beam to the semiconductor wafer is an important requirement in many applications. A parallel ion beam is one which has parallel ion trajectories over the surface of the semiconductor wafer. In cases where the ion beam is scanned, the scanned beam is required to maintain parallelism over the wafer surface. The parallel ion beam prevents channeling of incident ions in the crystal structure of the semiconductor wafer or permits uniform channeling in cases where channeling is desired. Typically, a serial ion implanter is utilized when a high degree of beam parallelism is required. 
     In one approach, the beam is scanned in one dimension so that it appears to diverge from a point, referred to as the scan origin. The scanned beam then is passed through an ion optical element which performs focusing. The ion optical element converts the diverging ion trajectories to parallel ion trajectories for delivery to the semiconductor wafer. Focusing can be performed with an angle corrector magnet or with an electrostatic lens. The angle correction magnet produces both bending and focusing of the scanned ion beam. Parallelism may be achieved with an electrostatic lens, but energy contamination can be a drawback. 
     The output ion beam from the angle corrector magnet or other focusing element may be parallel or may be converging or diverging, depending on the parameters of the ion beam and the parameters of the focusing element. When an angle corrector magnet is utilized, parallelism can be adjusted by varying the magnetic field of the angle corrector magnet. The angle corrector magnet typically has a single magnetic field adjustment which varies both parallelism and bend angle, or beam direction. It will be understood that the ion implanter is often required to run a variety of different ion species and ion energies. When the beam parameters are changed, readjustment of the angle corrector magnet is required to restore beam parallelism. 
     In prior art ion implanters, the angle corrector magnet is typically adjusted so that the ion beam has normal incidence on a wafer plane of the ion implanter end station. However, the angle corrector adjustment which achieves normal incidence on the wafer plane may result in less than optimum parallelism. In particular, an ion beam that is adjusted for normal incidence on the wafer plane may be somewhat diverging or converging. As shown in FIG. 8, the angle corrector magnet is adjusted such that the center ray of ion beam  200  is normal to wafer plane  202 . However, when the beam  200  is adjusted to be normal to wafer plane  202 , the parallelism of beam  200  may be degraded such that the beam converges or diverges. The lack of parallelism is unacceptable in highly critical applications. 
     In another approach, the angle corrector magnet is designed for best parallelism under typical conditions, and the ion implanter end station is positioned for normal incidence of the ion beam on the wafer. However, beam parallelism and normal incidence are not maintained over a wide range of beam parameters, and changing the position of the end station is very difficult. 
     Accordingly, there is a need for improved methods and apparatus for adjusting beam parallelism in ion implanters. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention, a method is provided for implanting ions into a workpiece. The method comprises the steps of generating an ion beam, adjusting the ion beam for a desired measure of parallelism, measuring a beam direction of the adjusted ion beam, orienting a workpiece at an implant angle referenced to the measured beam direction, and performing an implant with the workpiece oriented at the implant angle. 
     The step of adjusting the ion beam may comprise adjusting the ion beam for substantially parallel ion trajectories. In general, the beam direction may differ from the beam axis of the ion implanter. The implant angle may be zero degrees, in which case the workpiece is oriented normal to the measured beam direction. 
     The workpiece may comprise a semiconductor wafer, and the step of orienting the workpiece may comprise tilting the semiconductor wafer at the implant angle referenced to the measured beam direction. 
     The method may further comprise the step of measuring an angle of non-parallelism of the ion beam. The step of adjusting the ion beam may be based on the measured angle of non-parallelism. The beam direction and the angle of non-parallelism of the ion beam may be measured with a movable beam profiler and one or more beam detectors. 
     According to another aspect of the invention, apparatus is provided for implanting ions into a workpiece. The apparatus comprises means for generating an ion beam, means for measuring parallelism of the ion beam, means for adjusting the ion beam for a desired parallelism based on the measured parallelism, means for measuring a beam direction of the adjusted ion beam, means for tilting a workpiece at an implant angle referenced to the measured beam direction, and means for performing an implant with the workpiece tilted at the implant angle referenced to the measured beam direction. 
     According to a further aspect of the invention, apparatus is provided for implanting ions into a workpiece. The apparatus comprises an ion beam generator, an ion optical element for adjusting the ion beam for a desired parallelism, a measuring system for measuring a beam direction of the adjusted ion beam, and a tilt mechanism for tilting a workpiece at an implant angle referenced to the measured beam direction. An implant is performed with the workpiece tilted at the implant angle referenced to the measured beam direction. 
     The ion optical element may comprise an angle corrector magnet for adjusting the ion beam for substantially parallel ion trajectories. The measuring system may comprise a movable beam profiler and one or more beam detectors. Where the implant angle is zero degrees, the workpiece is tilted normal to the measured beam direction. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the present invention, reference is made to the accompanying drawings, which are incorporated herein by reference and in which: 
     FIG. 1 is a schematic diagram of an ion implanter suitable for implementing the present invention; 
     FIG. 2 is a schematic diagram that illustrates the operation of an angle corrector magnet for the case of a relatively large bend angle and converging ion trajectories; 
     FIG. 3 is a schematic diagram that illustrates the operation of an angle corrector magnet for the case of a relatively small bend angle and diverging ion trajectories; 
     FIG. 4 is a flow chart of a process for adjusting an ion implanter in accordance with an embodiment of the invention; 
     FIG. 5 is a schematic diagram of a parallel ion beam incident on a tilted wafer in accordance with an embodiment of the invention; 
     FIGS. 6A-6C are schematic diagrams that illustrate operation of a device for measuring beam parallelism and beam direction; 
     FIGS. 7A-7C are graphs of beam detector output as a function of beam profiler position for the beam conditions illustrated in FIGS. 6A-6C, respectively; and 
     FIG. 8 is a schematic diagram that illustrates the prior art method of adjusting beam parallelism. 
    
    
     DETAILED DESCRIPTION 
     A simplified block diagram of an example of an ion implanter suitable for incorporating the present invention is shown in FIG.  1 . An ion beam generator  10  generates an ion beam of a desired species, accelerates ions in the ion beam to desired energies, performs mass/energy analysis of the ion beam to remove energy and mass contaminants and supplies an energetic ion beam  12  having low level of energy and mass contaminants. A scanning system  16 , which includes a scanner  20  and an angle corrector  24 , deflects the ion beam  12  to produce a scanned ion beam  30  having parallel or nearly parallel ion trajectories. An end station  32  includes a platen  36  that supports a semiconductor wafer  34  or other workpiece in the path of scanned ion beam  30  such that ions of the desired species are implanted into the semiconductor wafer  34 . The ion implanter may include additional components well known to those skilled in the art. For example, the end station  32  typically includes automated wafer handling equipment for introducing wafers into the ion implanter and for removing wafers after implantation, a dose measuring system, an electron flood gun, etc. It will be understood that the entire path traversed by the ion beam is evacuated during ion implantation. 
     The principal components of ion beam generator  10  include an ion beam source  40 , a source filter  42 , an acceleration/deceleration column  44  and a mass analyzer  50 . The source filter  42  is preferably positioned in close proximity to ion beam source  40 . The acceleration/deceleration column  44  is positioned between source filter  42  and mass analyzer  50 . The mass analyzer  50  includes a dipole analyzing magnet  52  and a mask  54  having a resolving aperture  56 . 
     The scanner  20 , which may be an electrostatic scanner, deflects ion beam  12  to produce a scanned ion beam having ion trajectories which diverge from a scan origin  60 . The scanner  20  may comprise spaced-apart scan plates connected to a scan generator. The scan generator applies a scan voltage waveform, such as a sawtooth waveform, for scanning the ion beam in accordance with the electric field between the scan plates. 
     Angle corrector  24  is designed to deflect ions in the scanned ion beam to produce scanned ion beam  30  having parallel ion trajectories, thus focusing the scanned ion beam. In particular, angle corrector  24  may comprise magnetic pole pieces  26  which are spaced apart to define a gap and a magnet coil (not shown) which is coupled to a power supply  28 . The scanned ion beam passes through the gap between the pole pieces  26  and is deflected in accordance with the magnetic field in the gap. The magnetic field may be adjusted by varying the current through the magnet coil. Beam scanning and beam focusing are performed in a selected plane, such as a horizontal plane. 
     In the embodiment of FIG. 1, end station  32  includes a beam parallelism and direction measuring system  80 . System  80  measures beam parallelism and direction as described below. In addition, end station  32  includes a tilt mechanism  84  for tilting wafer support platen  36  with respect to the scanned ion beam  30 . In one embodiment, tilt mechanism  84  may tilt wafer support platen  36  with respect to two orthogonal axes. 
     Examples of operation of angle corrector  24  are shown in FIGS. 2 and 3. As shown, the pole pieces  26  of angle corrector  24  may be wedged shaped or similarly shaped so that different ion trajectories have different path lengths through the gap between the pole pieces. In FIG. 2, a relatively high intensity magnetic field is applied. The ion trajectories have a relatively large bend angle and may be converging as they exit from angle corrector  24 . In the example of FIG. 3, a relatively low intensity magnetic field is applied. The ion trajectories have a relatively small bend angle and may be diverging as they exit from angle corrector  24 . Thus, scanned ion beam  30  is incident on a wafer plane  70  at a positive angle  72  with respect to a normal to wafer plane  70  in the example of FIG.  2  and is incident on wafer plane  70  at a negative angle  74  with respect to a normal to wafer plane  70  in the example of FIG.  3 . It will be understood that parallel or nearly parallel ion trajectories can be produced by appropriate adjustment of the magnetic field in angle corrector  24 . However in general, the magnetic field that provides the best parallelism does not necessarily result in normal incidence of scanned ion beam  30  on wafer plane  70 . 
     A flow chart of a process for adjusting an ion implanter and performing ion implantation in accordance with an embodiment of the invention is shown in FIG.  4 . In step  100 , an ion beam is generated and is transported through the beamline of an ion implanter. As shown in FIG. 1, ion beam  12  is generated by ion beam generator  12  and is transported through scanner  20  and angle corrector  24  to end station  32 . 
     In step  102 , the parallelism of the ion beam is measured at or near the plane where the ion beam is incident on the semiconductor wafer or other workpiece. An example of a technique for measuring ion beam parallelism is described below in connection with FIGS. 6A-6C and  7 A- 7 C. The parallelism measurement typically provides an angle of non-parallelism of the ion beam and, in particular, provides a half angle of convergence or divergence of the ion beam. The measured angle of non-parallelism represents the maximum excursion of the ion beam trajectories from the center ray of the ion beam. 
     In step  104 , the ion beam is adjusted for a desired measure of parallelism, typically near zero divergence or convergence. As shown in FIG. 5, the parallelism of the ion beam may be changed by adjusting the current supplied by power supply  28  to the magnet coil. The adjusted current causes a change in the magnetic field of angle corrector  24 , which in turn changes the ion trajectories in the ion beam. The adjustment is made by monitoring the measured parallelism of scanned ion beam  30  as power supply  28  is adjusted. When the best parallelism is achieved, the adjustment process of step  104  is terminated. Typically, the ion beam may be adjusted within 0.1° half angle of divergence or convergence. 
     The magnetic field which provides the best parallelism is, in general, not the same magnetic field which directs scanned ion beam  30  normal to wafer plane  70  of the ion implanter end station. Instead, parallel ion beam  30  is incident on wafer plane  70  at an angle  120  relative to a normal to wafer plane  70 , as shown in FIG.  5 . It will be understood that the angle  120  is exaggerated in FIG. 5 for purposes of illustration. 
     In step  106 , the direction of the adjusted ion beam is measured. In particular, the angle  120  of the adjusted ion beam relative to the normal to wafer plane  70  is measured. An example of a technique for measuring ion beam direction is described below in connection with FIGS. 6A-6C and  7 A- 7 C. Beam parallelism and beam direction are measured in the plane of scanning and focusing of the ion beam. 
     In step  108 , the implant angle is set relative to the direction of the adjusted ion beam and, in particular, referenced to angle  120 . The implant angle is set by tilting wafer support platen  36  relative to the wafer plane  70  of the implanter using tilt mechanism  84 . Where normal incidence of the parallel scanned ion beam  30  on the wafer  34  is desired, wafer support platen  36  is tilted by an angle  122  that is equal to angle  120 . Thus, the wafer support surface of platen  36  is normal to parallel scanned ion beam  30 . Where non-zero implant angles are desired, wafer support platen  36  is tilted relative to the measured beam direction. The measured beam direction is thus the reference for setting the implant angle. The non-zero implant angle may be set by tilting the wafer in a direction parallel to the plane of scanning and focusing or may be set by tilting the wafer in a direction orthogonal to the plane of scanning and focusing. In each case, the non-zero implant angle is referenced to the measured beam direction. 
     In step  110 , the implant is performed with the wafer support platen at the desired implant angle referenced to the measured beam direction and with the scanned ion beam  30  adjusted for best parallelism. Thus, the best parallelism is achieved at the desired implant angle. 
     An example of a technique for measuring ion beam parallelism and direction is described with reference to FIGS. 6A-6C and  7 A- 7 C. FIGS. 6A-6C are schematic diagrams which illustrate the measurement of different ion beams with a beam profiler and two beam detectors. FIGS. 7A-7C are graphs that illustrate the outputs of the beam detectors as a function of profiler position. 
     As shown in FIGS. 6A-6C, ion beam parallelism and direction are measured using a moving beam profiler  150  and spaced-apart beam detectors  152  and  154 , which correspond to beam parallelism and direction measuring system  80  (FIG.  1 ). Beam profiler  150  may be any element that partially blocks the ion beam and is laterally movable relative to the ion beam. Detectors  152  and  154 , for example, may be Faraday cups which produce an electrical output signal in response to an incident ion beam. As the profiler  150  is moved across the ion beam, it blocks a portion of the ion beam and produces an ion beam shadow. The beam shadow moves across detectors  152  and  154  and produces output signals in the form of negative-going output current pulses. 
     As shown in FIG. 6A, a parallel scanned ion beam  160  has normal incidence on a wafer plane  170 . Detectors  152  and  154  produce output pulses as shown in FIG. 7A when the profiler  150  is positioned in alignment with each detector. The profiler positions at which detector output pulses are generated can be used to determine that ion beam  160  has parallel trajectories and is normal to wafer plane  170 . 
     Referring to FIG. 6B, a diverging ion beam  162  has normal incidence on wafer plane  170 . In this case, detector  152  produces an output pulse as shown in FIG. 7B when profiler  150  is positioned to the right of detector  152 , and detector  154  produces an output pulse when profiler  150  is positioned to the left of detector  154 . The profiler positions at which detector output pulses are generated can be used to determine the angle of divergence of ion beam  162 . In response to a converging ion beam (not shown), detector  152  produces an output pulse when profiler  150  is positioned to the left of detector  152 , and detector  154  produces an output pulse when profiler  150  is positioned to the right of detector  154 . The profiler positions at which detector output pulses are generated can be used to determine the angle of convergence of the ion beam. 
     As shown in FIG. 6C, a parallel ion beam  164  is incident on wafer plane  170  at an angle  166 . In this case, detectors  152  and  154  produce output pulses as shown in FIG. 7C when the profiler  150  is positioned to the left of the respective detectors  152  and  154 . The profiler positions at which detector output pulses are generated can be used to determine the direction and parallelism of ion beam  164 . 
     In general, the ion beam may be converging or diverging and may have a non-zero beam angle relative to the wafer plane. The profiler positions when detector outputs pulses are generated can be analyzed to determine both the parallelism and direction of the ion beam. The parallelism may be specified as the half angle of divergence or convergence, and the beam direction may be specified relative to a normal to a wafer plane  170 . Additional details regarding techniques for measuring ion beam parallelism and direction are provided in U.S. application Ser. No. 09/588,419, filed Jun. 6, 2000, which is hereby incorporated by reference. 
     It will be understood that different techniques may be used for measuring beam parallelism and direction within the scope of the invention. In addition, the invention is not limited to use with a scanned ion beam. For example, the invention may be used with a ribbon ion beam as disclosed in U.S. Pat. No. 5,350,926, issued Sep. 27, 1994 to White et al. 
     While there have been shown and described what are at present considered the preferred embodiments of the present invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.