Abstract:
An apparatus used to control a workpiece inside a vacuum chamber. The workpiece is supported on a workpiece holder in the vacuum chamber. The workpiece is isolated from the atmosphere outside of the vacuum chamber by differentially pumped vacuum seals and an integral air bearing support. The differentially pumped vacuum seals and integral air bearing support allow for multiple independent motions to be transmitted to the workpiece supported by the workpiece holder. The workpiece holder motions provided are (1) rotation about the X axis, (2) translation back and forth along the Y direction of an X-Y plane on the surface of the workpiece holder, and (3) rotation of the workpiece in the X-Y plane about its Z axis. Concentric seals, oval for the translation motion and circular for the rotational motion, are differentially pumped through common ports to provide successively decreasing pressure and gas flow in order to reduce the gas load into the vacuum vessel to a negligible rate.

Description:
This application is a Div. of 09/272,981 filed Mar. 19, 1999 now U.S. Pat. No. 6,163,033 which is a continuation of Ser. No. 08/926,650 filed Sep. 10, 1997 now U.S. Pat. No. 5,898,179. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to devices for providing individual workpieces such as silicon wafers or flat panel displays with a pre-selected orientation relative to a treatment beam. 
     BACKGROUND OF THE INVENTION 
     The manufacture of semiconductors during the front end stages includes a number of process steps whereby a silicon wafer is presented to an incoming ion beam, plasma, molecular beam, or other irradiating elements. In some cases, the irradiating element is scanned across the surface of the silicon wafer to provide a uniform spatial irradiation and the time spent determines the doping level. In others, the wafer is moved across a stationary beam of irradiating elements. High current ion implanters with purely mechanically scanned workpiece holders are examples of systems that scan the wafers through a stationary beam and provide on average uniform spatial doping. Doping uniformity is sero-controlled using the measured doping rate to vary the speed and duration of one mechanical axis while the other is controlled at a constant speed. Doping level is controlled by adjusting the number of completed scan passes in the servocontrolled direction such that the total dose is equally divisible by the number of scan passes. This technique is well known to those knowledgeable in the art and needs no further explanation. 
     The semiconductor industry is now migrating to 300 mm wafer diameters that cause the vacuum chambers and extent of mechanical motion to increase beyond practical limits for two direction mechanical scan systems. Furthermore, the cost of a single 300 mm wafer is currently very expensive which makes it desirable to process wafers individually rather than in batches because of the cost and wafer handling risks. Finally, the recent requirement of increasing the wafer tilt angles from the current 7 degrees to as much as 60 degrees precludes the use of mechanically scanned batch systems due to the variation in implant angle and twist across the wafer. 
     SUMMARY OF THE INVENTION 
     The present invention provides high angle tilt ion implants for silicon wafers with fast servo-controlled mechanical scanning in one direction and fast magnetic scanning in the orthogonal direction. Some of the features of this invention are: 
     (1) a differentially pumped integral air bearing vacuum seal for linear motion in the Y direction for the mechanical scan structure; 
     (2) a differentially pumped integral air bearing vacuum seal for rotary motion about the X-axis; 
     (3) air bearings for supporting the mechanical scan structure, centering and supporting the rotary seal, and centering and supporting the Y-scan linear seal; and 
     (4) synchronous gating of the ion beam during transitions between implant states. 
     In other words, the ion beam is held off the wafer whenever a loss of beam is detected or other requirements dictate that the system go from an implant in progress to an implant hold state. This can occur while a flag Faraday is inserted into the beam path for set-up or tuning purposes. 
     For purposes of describing the geometry of the system, the mechanical scanning system uses Cartesian coordinates X, Y, and Z while the magnetic scanned beam uses Cartesian coordinates X′, Y′, and Z′. In all cases X and X′ are identical. The ion beam is perpendicular to the X′Y′ plane and is magnetically scanned in the X′ direction. 
     In one aspect of the present invention, there are two movable bearing plates spaced from a fixed plate using gas bearings with an integral differentially pumped vacuum seal to prevent physical contact between seal surfaces on each of the plates. The combination gas bearing and vacuum seal for the outermost plate provides friction free movement in the Y direction. The combination gas bearing and vacuum seal for the inner plate provides friction free rotation about the X axis. The combination of the two moveable bearing plates provides tilting of a workpiece holder at any angle between 0 and 60 degrees for ion implanting in a silicon wafer and 90 degrees for horizontal wafer handling. This is accomplished by rotating the two moveable bearing plates about the X axis creating an angle between the Z &amp; Z′ and Y &amp; Y′ directions. The Z′ direction is parallel with the incoming ion beam and Z is perpendicular to the surface of the workpiece holder. The tilting of the workpiece holder allows implants into the sides of deep trenches and gate structures located on the surface of the silicon wafer, a desirable feature for state of the art semiconductor manufacture. Horizontal wafer handling is a desirable feature in that it uses gravity to hold wafers while in motion obviating the need for edge clamping on the wafer that may result in damage to the wafer. Additional gas bearings center the rotating bearing plate about the X axis as well as prevent lateral motion of the outermost bearing plate along the Z direction. 
     In another aspect of the present invention, the ion beam intercepts each point on the surface of the workpiece (e.g., wafer) at the same distance along the Z′ axis as the workpiece is reciprocated in the Y direction. This is accomplished using only three axes of controlled motion. If one assigns a unit vector to the wafer surface orientated with respect to the crystal lattice and another unit vector to the incoming ion beam, the relationship between these two vectors is constant as the wafer is reciprocated in front of the ion beam throughout the implantation process. Furthermore, the distance along the Z′ axis to every point on the surface of the wafer as the wafer is reciprocated through the beam is the same such that each point on the wafer surface experiences exactly the same ion flux and trajectory. Thus enabling precise control over ion channeling through the crystal lattice during implantation leading to superior control over implant uniformity throughout the volume of the implanted surface. 
     In another aspect of the present invention, the magnetic scanner is used to hold the ion beam in the overscan region for a short duration while an upstream Faraday is inserted or retracted to prevent fine structure (i.e., non-uniformity) in the doping level across the wafer. To avoid non-uniformity in the doping, the ion beam is sampled when it is scanned off the edge of the wafer and both the magnetic and mechanical scanning controls are stopped if beam loss is detected. The implant is started in the same way, the beam is deflected off the wafer path before the Faraday is retracted and scanning starts precisely where it was interrupted. This method is also used to temporarily interrupt the implant for any reason deemed necessary, 
     In another aspect of the present invention, there is provided an apparatus having a vacuum chamber having a chamber wall, a workpiece holder disposed within the vacuum chamber and extending through the chamber wall, a reciprocating member receiving the workpiece holder, and a rotating member interposed between the reciprocating member and the chamber wall. 
     In yet another aspect of the present invention, there is provided a method for ion implantation of a workpiece, including the steps of generating an ion beam perpendicular to a first XY plane, tilting the workpiece to a second XY plane relative to the first XY plane, scanning the ion beam across the workpiece along the X axis of the first XY plane and translating the workpiece along the Y axis of the second XY plane with all points on a face of the workpiece being equidistance from the source of the ion beam. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic representation of an ion implantation device in accordance with the present invention. 
     FIGS. 2A-2D are detailed views of the translating and rotating seal assembly of the present invention. 
     FIGS. 3A-3C are detailed views of the rotating seal assembly. 
     FIG. 4 is a vacuum schematic. 
     FIG. 5 is a diagrammatic representation of a portion of the Faraday system in accordance with the present invention. 
     FIG. 6 is an illustration of the current integrator function. 
     FIG. 7 is a diagrammatic representation of the Faraday system in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention can be used to mechanically scan 200 or 300 mm silicon wafers through an ion beam at speeds sufficient to reduce wafer heating and charging effects. It is important to note that although the present invention is described herein with respect to ion implantation, the present apparatus can also be used for other scanning operations, such as for the treatment of flat panels for flat panel displays. The ion beam is either fanned (i.e., a large rectangular cross section) or scanned (i.e., a small beam swept back and forth to form a large rectangular scanned area) at high speeds (e.g., in the range of about 150 Hz) in a direction (e.g., the X direction) orthogonal to that of the mechanical scan direction (e.g., the Y direction). The term “scanning” as used herein encompasses either magnetic or electrostatic fanning and magnetic or electrostatic scanning. The mechanical scanning (i.e., reciprocating) in the Y direction moves the wafer back and forth at high speeds (e.g., in the range of about 0.5 to 1 Hz) through the ion beam at a speed that is proportional to the measured beam current. In this way, the doping level per mechanical scan pass is controlled and the total dose is proportional to the dose per scan pass times the number of scan passes. In order to achieve mechanical scanning within the vacuum chamber  24  at high speeds with frictionless operation for long wear life and no particle generation, the present invention uses a novel combination of a linear motion bearing with differentially pumped vacuum seal (for friction-free movement in the Y direction) mounted on a rotary motion bearing with differentially pumped vacuum seal (for friction-free rotation about the X axis) on the exterior of the vacuum chamber  24 . The linear motion bearing has a shaft  11  with a workpiece holder  10  at the distal end which extends through each of the vacuum seals into the vacuum chamber  24 . The shaft II and workpiece holder  10  are translated in the Y direction by reciprocating the bearing and seal member or plate  12 . The shaft  11  and workpiece holder  10  are tilted, along with bearing and seal member  12 , by rotating the bearing and seal assembly  17 . 
     The novel combination of a linear motion bearing and seal member with a workpiece holder and shaft attached thereto mounted on a rotary motion bearing and seal member provides isocentric scanning of the workpiece using the least number (i.e., three) of axes of motion possible. Isocentric scanning means that every intersection point of the ion beam with the surface of the workpiece is the same distance from the collimator magnet  98  exit boundary and the angular orientation of the ion beam and the angular orientation of the workpiece remain constant during the implant. The only three axes of motion required are (1) magnetically scanning the ion beam back and forth in the X′ direction, (2) tilting the workpiece  18  and linear motion bearing and seal member  12  about the X axis, and (3) reciprocating the workpiece  18  and linear motion bearing and seal member  12  along the tilted Y axis, (i.e., reciprocating the workpiece  18  and linear motion bearing and seal plate  12  in the plane of the surface of the workpiece). 
     Referring to FIG. 1, the workpiece holder  10  is attached to a hollow shaft  11  connected to the linearly moveable bearing and seal member or plate  12 . Bearing plate  12  reciprocates in the Y direction providing mechanical scanning of the workpiece (e.g., a silicon wafer)  18  through the parallel magnetically scanned ion beam  13 . The ion beam  13  is directed along the Z′ direction and magnetically scanned back and forth in the X′ direction perpendicular to the X′Y′ plane creating a parallel scanned ion beam  13 . Hollow shaft  11  extends through bearing plate  12  and slot  32  (FIG. 3) in the raised portion  21  of wall of vacuum chamber  24  and rotating bearing assembly  17 . Portion  21  is described herein as a raised portion of the vacuum chamber wall but it should be understood that the portion  21  need not be raised. Likewise, it should be understood that the portion  21  can be a fixed plate attached to the wall of the vacuum chamber. 
     The combination of the moveable bearing members  12  and  17  provides tilting of the workpiece holder  10  (see FIG. 5) at any angle between 0 and 60 degrees from vertical for ion implanting in a silicon wafer and between 0 and 90 degrees for wafer handling. Tilting is accomplished by rotating the moveable bearing members  12  and  17  about the X axis creating an angle between the Z &amp; Z′ and Y &amp; Y′ axes. The Z′ direction is defined as being parallel with the incoming ion beam and Z is defined as being perpendicular to the surface of the workpiece holder  10 . The tilting of the workpiece holder  10  allows implants into the sides of deep trenches and gate structures located on the surface of the silicon wafer. Horizontal wafer handling (i.e., tilting the workpiece holder 90 degrees from vertical) in accordance with the present invention uses gravity to hold the wafer on the workpiece holder while in motion obviating the need for edge clamping on the wafer that may result in damage to the wafer. Gas bearings  28  (FIG. 2B) on the exterior of vacuum chamber wall  21  center the rotating bearing and seal assembly  17  about the X axis. Gas bearings  30  mounted on rectangular bearing plate  19  prevent lateral motion of the bearing member  12  along the Z direction. 
     The bearing member or plate  12  (FIG. 1) is connected to a drive motor  14  controlled by a computer  15 . The computer  15  in combination with a current integrator  73  monitors the ion flux arriving in a downstream Faraday  16 . The velocity in the Y direction imparted to the bearing plate  12  by the motor  14  is varied in proportion to the ion flux measured by the control computer  15  so as to create uniform average flux density across the surface of the workpiece  18 . With the laterally moveable bearing plate  12  connected to the rotating bearing member or assembly  17  by support arm  97  the intercept of the ion beam  13  with the workpiece  18  is maintained at a constant distance along the Z′ axis as the workpiece  18  is translated back and forth through the ion beam  13  by the linear drive motor  14 . The rotary motion of the rotating bearing assembly  17  is provided by a linear drive motor (not shown) and associated linkage (not shown) as known in the art. 
     Rotation of the bearing assembly  17  by 90 degrees from vertical about the X axis when the bearing plate  12  is in its uppermost position allows horizontal handling of the workpiece  18  during wafer load and unload from the wafer handler  99 . The surface of the workpiece holder  10  may be rotated about its Z axis to any rotation angle between 0 and 360 degrees through a drive system (not shown) connected through the hollow shaft  11 . This permits wafer flat or notch orientation prior to implantation and may be done while the workpiece holder  10  is in motion from the load position to the implant position eliminating time normally wasted for wafer flat orientation. The present invention uses a video camera and processing software for the purpose of locating the position and orientation of each wafer relative to the load/unload robot  99  and workpiece holder  10  while the handler is in motion. This allows precise loading of the wafer onto the workpiece holder  10  as well as correct flat or notch orientation. This video image may also be used to capture the part code or number scribed onto the wafer surface for material tracking purposes. Rotation about the Z axis of the surface of the workpiece holder is also an enabling function for implants into the sides of deep trenches and gate structures. 
     Referring to FIGS. 2A-3C, the details of the linear reciprocating bearing and seal plate  12  and the rotating bearing and seal assembly  17  are shown. Rotating bearing and seal assembly  17  is made up of bearing and seal plate  19  and a circular bearing and seal plate  20  attached on opposite sides of a center plate  31 . The gas bearings will be described first. Bearing and seal plate  20  of seal assembly  17  is separated from wall portion  21  by a gas bearing formed by an array of gas nozzles  25  (FIG. 3C) located on the surface  59  of the bearing plate  20 . A high pressure gas manifold  58  (FIG. 2D) is connected to each of the gas nozzles  25  to provide a steady supply of gas for the gas bearing. The pressure over surface  59  between the outer and inner gas nozzles  25  is maintained at a constant pressure by flow restrictors in the nozzles  25  and the spacing between the seal and bearing plate  20  and the wall portion  21 . The wall portion  21 , which is a circular seal plate, is fixed in position relative to the overall vacuum chamber  24 . Wall portion  21  contains a set of air bearings  28  (FIG. 2B) that center the rotating seal assembly  17  about the center of the fixed seal plate  21  by applying a gas force directed in the radial direction against the side of the center plate  31 . Bearing and seal plate  19  of seal assembly  17  is separated from bearing and seal plate  12  by a gas bearing formed by an array of gas nozzles  26  (FIGS. 2B and 3A) located on the surface  90  of the bearing plate  19 . A high pressure gas manifold (not shown) supplies a steady supply of gas for the gas bearing. The pressure over surface  90  between the outer and inner gas nozzles  26  is maintained at a constant pressure by flow restrictors in the nozzles  26  and the spacing between the seal and bearing plate  19  and the seal and bearing plate  12 . A set of gas bearings  30  attached to the bearing plate  19  prevent movement of the seal plate  12  in the Z direction by applying a gas force to the opposite sides of the seal plate  12 . 
     Having described the gas bearings, the vacuum seals will now be described. Pumping grooves  37 ,  40  and  41  (FIGS. 3B and 3C) in the surface of the bearing and seal plate  20  form a differentially pumped vacuum seal between bearing and seal plate  20  and wall portion  21  of the vacuum chamber  24 . Pumping grooves  33 .  38  and  39  (FIGS. 3A and 3B) in the surface of the bearing and seal plate  19  form a differentially pumped vacuum seal between bearing and seal plate  19  and bearing and seal plate  12 . Grooves  33 ,  38  and  39  have an oval shape to accommodate the rectangular shape of the reciprocating seal plate  12 . The bearings and seals are non-contact with respect to each other and the reciprocating shaft  11  thus providing a friction-free, non-particle generating, high speed rotation and linear motion vacuum feed-through. 
     The balance of force on each of the elements of the vacuum seal assembly is as follows. Atmospheric pressure working against the vacuum inside the vacuum chamber  24  applies an external force which balanced against the air cushion created by the gas being between the bearing and seal plate  12  and the bearing and seal plate  19  creates a slight separation between the plate  12  and plate  19  while preventing movement in the X direction of the seal plate  12 . The set of air bearings  30  located on opposite sides of the seal plate  12  and attached to the rotating seal assembly  17  apply equal forces in the positive Z and negative Z directions preventing contact and relative Z motion between the seal plate  12  and the bearing plate  19 . In this way seal plate  12  is prevented from moving in either the X or Z direction but allowed frictionless translation in the Y direction. Atmospheric pressure working against the vacuum inside the vacuum chamber  24  also applies an external force which balanced against the air cushion created by the gas bearing between the bearing and seal plate  20  and the wall portion  21  creates a slight separation between the plate  20  and wall portion  21  while preventing contact and relative X motion between the wall portion  21  and the bearing and seal plate  20 . The set of air bearings  28  attached to the wall portion  21  apply a uniform radial force against the center plate  31  preventing contact and relative radial motion between the seal assembly  17  and the wall portion plate  21 . The wall portion  21 , which is a seal plate, is attached to the vacuum chamber  24  fixing the position of the seal plate  21  which in turn fixes the position of the seal assembly  17  which in turn fixes the position of the seal plate  12  relative to the vacuum chamber  24 . The rotating seal assembly  17  is constrained in X, Y and Z but allowed frictionless rotary motion about the X axis. The pressure inside the air bearing regions  59  and  90  is self-regulated to some fraction of the pressure inside the high pressure manifold. This self-regulation occurs because the gap between the bearing and seal plates is constrained only by the atmospheric pressure applied to the outside of the seal plates, thereby controlling the leak rate of air out of the bearing regions. By adjusting the pressure in the high pressure manifold, one can vary the gap between the seal and bearing plates. The gap between the seal and bearing plates is, preferably, 0.001 inches or less. 
     Since the gas bearing gap is very small, the gas flow rates required to produce the gas bearing are also very small (e.g., 1 to 4 cubic feet per minute). For proper spacing, the opposing seal and bearing plate surfaces must be very flat across their entire width. A technique known as “lapping” performed by Form Centerless Co. in St. Medfield, Mass. can be used to achieve the desired flatness which should be within 0.0003 inches of true flatness. To prevent damage to the seal and bearing plate surfaces if they were to come into contact, an anodized surface such as polytetrafluoroethylene-penetrated hardcoat anodizing for aluminum alloys sold under the tradename NITUFF available from Nimet Industries, Inc. in South-Bend, Ind., nickel, or hard chrome can be applied to the surface of the seal surfaces and bearing plate. 
     Referring now to FIGS. 3 and 4, the differentially pumped vacuum seal system will now be described. An oval slot  32  extends through the plates  19 ,  20 , and  31  in direct communication with the high vacuum region of the vacuum chamber  24 . The slot  32  allows non-contact full translation of the workpiece holder  10  and shaft  11  in the Y direction. Adjacent to slot  32  is the oval pumping groove  33  (FIG. 3A) in the surface of the plate  19 . Ports  36  extending through the center plate  31  connect groove  33  to circular groove  37  in the surface of the bearing plate  20 . Oval grooves  38  and  39  are connected to circular grooves  40  and  41  through ports  42  and  43 , respectively. Each pair of oval and circular grooves are connected through ports (not shown) to the differential pumping vacuum system shown in FIG. 4 as follows. Grooves  33  and  37  and ports  36  are connected to the third stage  34  of the differential pumping system and nearly isolated from the high vacuum region  52  and the second stage  53  of the differential pumping system by the seal surfaces  29  and  35  and  46  and  49 , respectively. Grooves  38  and  40  and ports  42  are connected to second stage  53  of the differential pumping system and nearly isolated from the third stage  34  and the first stage  54  of the differential pumping system by the seal surfaces  46  and  49  and  47  and  50 , respectively. Grooves  39  and  40  and ports  43  are connected to the first stage  54  of the differential pumping system and nearly isolated from the second stage  53  of the differential pumping system and atmosphere  55  by the seal surfaces  47  and  50  and  48  and  51 , respectively. Grooves  56  and  57  in seal plates  19  and  20 , respectively, located at a greater diameter than the other grooves are ported to the atmosphere side of the vacuum seal assembly to exhaust the air that escapes the inside perimeter of the air bearing assembly. Each set of grooves are described as “nearly” isolated because there is some movement of gas over the seal surfaces toward the vacuum region. 
     Referring now to FIG. 4, the vacuum schematic illustrates the differential pumping system which includes a high vacuum cryo-pump  60  to create a vacuum in the vicinity of the workpiece  18 , a turbomolecular mechanical pump  61  to maintain the pressure in the third stage differential pumping region  34 , a second turbomolecular pump  62  connected to the exhaust port  65  of the first turbomolecular pump  61  and to the second stage differential pumping region  53 , a dry mechanical pump  63  connected to the exhaust port  66  of the second turbomolecular pump and to the first stage differential pumping region  54  with its outlet exhausted to atmosphere  55 . The pressure in each of the successive differential pumping stages  54 ,  53  and  34 , drops by roughly an order of magnitude from atmosphere at  55  to less than a millibar in the third stage  34 . The conductance between the third stage  34  and the high vacuum region  52  is several orders of magnitude lower than the pumping speed of the high vacuum pump  60  reducing the pressure in the vicinity of the workpiece  18  to a level near the base pressure of the high vacuum pump  60 . 
     As bearing and seal member  12  reciprocates along the Y axis, the two outermost ends  92  and  94  (FIGS. 1 and 2B) are extended beyond the ends of the seal plate  19  thus exposing the ends  92  and  94  to the atmosphere where the ends pick up moisture. As the end exposed to the atmosphere is translated to a position where it is exposed to the vacuum seal grooves, moisture from the seal member  12  is drawn into the vacuum region creating a load causing the vacuum pumps to work harder. Therefore, in a preferred embodiment, a dry gas (e.g., nitrogen) blanket is applied using a shield or bag to each of the ends  92  and  94  as they travel past the ends of the seal plate  19  to prevent them from picking up moisture. 
     As conventionally known in the prior art, the workpiece holder  10  has an electrostatic chuck for holding silicon wafers onto a ceramic coated platen surface, a plurality of gas cooling ports to feed gas to the region between the back of the wafer and the surface of the platen, a plurality of water cooling passages to cool the backside of the electrostatic chuck, a rotary bearing, a differentially pumped rotary shaft seal assembly, a plurality of wafer lifting pins, and a drive assembly used to rotate the surface of the workpiece holder 0 to 360 degrees about an axis perpendicular to the workpiece. 
     Magnetic scanning is conducted with the present invention such that the ion beam trajectory is maintained perpendicular to the X′ Y′ plane at all times. As described previously, two magnetic deflection systems  95  and  98  (FIG. 7) located one after the other along the beam flight path are used. Referring to FIGS. 5 and 7, the implantation control system will be described. A Faraday assembly  16  is mounted to a linear actuator  68  that provides motion of the Faraday  16  along the X′ direction. The Faraday  16  is fitted with an aperture plate  69  positioned with its surface in the X′Y′ plane. A thin slit aperture  70  is located through the aperture plate  69  with its long dimension oriented in the Y′ direction. The Faraday  16  is moved by the linear actuator  68  such that the slit  70  may be positioned anywhere within the transverse range of the scanned beam  13  along the X′ direction. The aperture plate  69  and its slit  70  are longer in the Y′ direction than the Y′ height of the beam  13 . This allows for the beam  13  to be scanned across the surface of the aperture plate  69  admitting a fraction of the beam current into the Faraday cup  71  located behind the plate  69 . The current-time profile of the Faraday signal may be transformed into a one-dimensional beam intensity-position profile using suitable arithmetic in a computer controlled measurement system as known by those of ordinary skill in the art. This enables correlation between magnetic scan amplitude and beam position in the X′ direction. 
     In a preferred embodiment, there are two movable Faraday assemblies  16  and  72 . Downstream Faraday  16  is located in the beam scanning plane next to the workpiece holder  10  and the Faraday  72  is located upstream. Both Faraday assemblies have the same freedom of motion allowing identical measurements of the ion beam  13  both upstream and downstream inside the vacuum chamber  24 . The downstream Faraday  16  serves the dual purposes of beam setup and measuring and controlling implant dose. The upstream and dow am Faradays  16  and  72  are used to measure beam parallelism. Each of these Faradays is positioned in the beam path  13  at identical X′ positions but with different Z′ positions. Since the magnetic scan waveform (amplitude versus time) is repetitive, the amplitude versus beam position can be expressed in terms of the phase angle of the repeated wave form. The phase angle difference between measurements of beam position in the two Faradays  16  and  72  is used to calculate the deviation from parallel for the scanned rays of the ion beam  13 . These phase angle measurements are made when the workpiece holder  10  is moved out of the beam path. 
     The Faraday cup  72  is electrically connected to the vacuum chamber  24  through an electrometer circuit (not shown) to measure the total ion beam charge entering the Faraday cup  96  through slit  70 ′ in plate  69 ′. For each positive ion entering the field of the Faraday cup  72 , a negative charge is induced on the surface of the cup. These charges combine to maintain net neutrality. The flow of negative charge into the cup from the electrometer is a measure of ion beam flux entering the cup. When the ion beam consists of singlely charged ions, the number of negative charges equals the number of positive ions entering the Faraday cup  72  through the slit  70 ′. 
     In another aspect of the present invention, the magnetic scanner is used to hold the ion beam  13  in an overscan region off of the workpiece holder  10  for a short  13  duration while the flag Faraday  93  is inserted or retracted from in front of the ion beam to prevent fine structure (i.e., non-uniformity) in the doping level across the workpiece. To avoid non-uniformity in the doping, the ion beam  13  is sampled when it is scanned off the edge of the wafer with the present invention and both the magnetic and mechanical scanning controls are stopped if beam loss is detected. The magnetic scanner is capable of holding the ion beam off the edge of the wafer for approximately 200 milliseconds providing ample time to insert the flag Faraday  93  into the ion beam path. This method is also used to temporarily interrupt the implant for any reason deemed necessary. The implant state is started in a similar manner, the ion beam  13  is turned on before the flag Faraday  93  is retracted and scanning starts precisely where it was interrupted. 
     In other words, the ion beam is held off the wafer whenever a loss of beam is detected or other requirements dictate that the system go from an implant in progress to an implant hold state. This occurs within a few tens of milliseconds while a flag Faraday  93  is inserted into the beam path for set-up or tuning purposes. The process of starting an implant occurs in a similar way. First the scanning magnet is set to deflect the beam off of the wafer path while the flag. Faraday  93  is retracted. Then, the scanning starts with the beam off the wafer to prevent structure (i.e., non-uniformity) in the doping of the implanted wafer. 
     The ion beam  13  is scanned at a constant velocity V x  across both the Faraday cup  71  and the workpiece  18  such that the ion beam  13  moves completely off the workpiece  18  and past the slit  70  during ion implantation steps. The one dimensional dose D, is measured by integrating the flux of charge entering the Faraday cup  71 . This one dimensional dose D x  is simply the integral of the charge flux and is measured by the scan control computer  15 . The mechanical scan velocity V y  of the workpiece holder  10  in the Y direction is controlled by the scan control computer  15  in proportion to the one dimensional dose D x  measured during each back and forth pass of the ion beam  13  across the Faraday cup  71  and workpiece  18 . The dose D x  multiplied by a constant K determines the total dose per unit area that the workpiece  18  receives in a single back and forth pass of the workpiece  18  through the scanned ion beam  13 . The total dose per unit area received by the workpiece in a complete implant cycle is determined by the single pass dose times the number of passes N. Both the number of passes N and the constant K are predetermined such that after N passes the desired dose is received by the workpiece  18 . 
     During beam setup, the workpiece holder  10  is moved in the Y direction to a location clear of the Faraday  16  to allow for X′ motion of the Faraday  16  for purposes of measuring beam parallelism and scan uniformity. FIG. 6 illustrates the amplitude time wave form of the current integrator  73  (FIG. 1) associated with the scan control computer  15 . The wave form results from the ion beam  13  being scanned across the Faraday slit  70 . The current integrator  73  consists of a current-to-voltage converter section followed by an integrator section. The output wave form  74  of the current-to-voltage converter section is integrated to produce the integral wave form  75 . The flat regions  76  and  78  of the integrator output represent the periods when no part of the ion beam  13  is entering the Faraday cup  71 . The rising region  79  of the integrator output represents the period when the ion beam  13  passes over the Faraday slit  70  allowing a portion of the ion beam to enter the Faraday cup  71 . The sharp negative slope  80  of the wave form  75  represents the integrator-reset function. A fast sampling AID converter (not shown) is used to measure the amplitude of the integrator output during the periods  76 ,  79  and  78  to determine instrument offset, dark or stray current, and beam current reproduced by the current-to-voltage converter. Offset and dark current are determined by the slope of the amplitude during periods  76  and  78 . Beam current is measured during the period  79 . The slope of periods  76  and  78  are multiplied by the total integrator period  81  and then subtracted from the difference between the starting sample  82  and ending sample  83  to arrive at a corrected integral measurement. The time of the one-half height measurement  84  corresponds to the time when the beam is centered over the Faraday slit  70  which precisely defines the beam position in the X′ direction. Each of the Faradays  16  and  72  are stepped across the X′ positions and measure the beam arrival times  84  relative to the turn around points in the magnetic scan space X′. Although  20  the Faradays  16  and  72  cover the same X′ positions, they occupy separate but parallel X′Y′ planes during these measurement steps. 
     A pulse integrator (not shown) in combination with a sampling AID converter (not shown) and the small movable Faraday cups  16  and  72  measure beam profiles, magnetic scan linearity, beam parallelism, dose rate, and instrumentation offset. This information is used in combination to compensate for offset or dark current, scan non-linearity, variations in beam current versus X′ position, and beam parallelism during set-up and during implant operations. The magnetic scan profile of magnet current versus time may be modified to produce a one-dimensional uniform doping profile across the target plane in the X′ direction. The method of measuring dark current (which is all unwanted constant currents including instrumentation offset) is accomplished by sampling the slope of the integrator output as the beam passes across the Faraday aperture, including a period before and a period after its passage. The pulse integrator is enabled for a precisely fixed period of time and produces an analog output that is the time integral of the beam current pulse and any stray current not related to the ion beam. The stray current may include instrument offset current, leakage current in the Faraday, electron current from wafer charged neutralizers, ion current from the background plasma surrounding the ion beam, or any other source of constant current summed together and included in the integral measurement. The characteristic wave form of the integrator output, when sampling a pulse or current with no contributing offsets has two periods of time one before and one after passage of the pulse when the slope of the integrator output versus time is zero. When an offset current is simultaneously summed with the true beam current pulse, the slope before and after the passage of the beam pulse is constant and is easily measured using a fast sampling A/D convertor. Since the slope is constant and measurable, the product of the slope and the integrator time period can easily be subtracted from the integral measurement to arrive at the true integrated beam current pulse. 
     The method of measuring beam parallelism and scan uniformity utilizes the two separate Faradays  16  and  72  in combination with the integrator  73  to measure the X′ position of the ion beam in two parallel X′Y′ planes. Each Faraday  16  and  72  is positioned using a stepper motor drive  91  in combination with a linear drive mechanism  68  and  68 ′ to provide accurate and repeatable X′ position in small discrete 0.001 inch steps (FIG.  5 ). Faraday  16  is positioned such that its slit  70  is located in the implant plane while the Faraday  72  is positioned upstream. The width of the beam  13  is larger than the slit width, however, this is of no consequence since the integrator output yields the total integrated current once the beam passes over the slits  70  and  70 ′. This integrated current is the one dimensional dose D x  at the X′ position of each of the Faradays. Varying the X′ position of each of the Faradays enables measurement of D x  at discrete locations across the magnetic scan space. The output wave form, after it has been corrected for offset or dark current will have three pieces of information critical for this control algorithm. The ending amplitude of the integrator output less the beginning amplitude is the integral of the beam current. The time at which the half amplitude of the integrator output is reached corresponds to the time when the beam center is coincident with the center of the Faraday slit. These two measurements, D x  and X′(t), provide the basis for calibrating parallelism and scan or dose uniformity of the system. Since the magnetic scan wave form is repetitive, the ion beam retraces the same space across the X′ direction in a continuous manner with each successive magnetic scan pass. It follows that by positioning a Faraday at discrete locations in the X′ scan space one can measure X′ and D x  at each of these locations. Assigning X′ i  and X′ j  to locations corresponding to the upstream and downstream Faradays  16  and  72 , respectively, with the i and j positions being identical in X′ but not Z′, then the angular deviation in the ion beam trajectory from the Z′ direction is the arc-tangent of Δx/Δz. Where Δx is equal to X′ i  minus X′ j . The collimator magnet  98  is adjusted under software control to assure a minimum angular variation across the scan space. 
     The next step is to calibrate the scanner magnet  95 . It is a requirement for uniform dose control in the implant plane that the discrete values of D xj  be equal. The scan velocity V x  must be constant to achieve a uniform dose when the beam current is constant. The scanner  95  is simply calibrated by measuring values of X j  versus B j  and finding a scan wave form that satisfies the requirement for constant scan velocity. Once the wave form is defined that produces a constant velocity V x , the doses checked, D x  against X is measured and variations are used to recalculate a function to modify the scan velocity. The final result is a polynomial in time T that defines the magnetic scan wave form that includes corrections for beam intensity variation as well as non-linearity in the scanner magnet  95 . 
     It will be appreciated by those of ordinary skill in the art that the invention can be embodied in other specific forms without departing from the spirit or essential character thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims rather than the foregoing description, and all changes which come within the meaning and range of equivalents thereof are intended to be embraced therein.