Abstract:
A device for measuring an incidence angle of an ion beam impinging a planar substrate includes an aperture plate having an aperture for intercepting the ion beam and passing a beam portion therethrough, and a sensor located in the substrate plane or a plane parallel thereto behind the aperture plate and having a length along which the beam portion impinges on the sensor at a location which is a function of the incidence angle of the ion beam, the sensor configured to produce a sensor signal indicative of the location of impingement of the beam portion on the sensor and representative of incidence angle. A computing unit may be configured to compare the sensor signal to a predetermined function for determining the incidence angle of the ion beam. Spaced apart sensing devices may be used to determine beam divergence.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of provisional application Ser. No. 60/262,245, filed Jan. 17, 2001, which is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to systems and methods for treatment of workpieces, such as semiconductor wafers, with an ion beam and, more particularly, to methods and apparatus for measuring the incidence angle and divergence of the ion beam. 
     BACKGROUND OF THE INVENTION 
     In the manufacture of semiconductor devices using ion implantation, it is sometimes desirable to provide beam incidence angles which are other than perpendicular to the substrate plane. A description of ion implantation techniques using tilted implantation steps can be found in U.S. Pat. No. 5,696,382, issued Dec. 9, 1997 to Kwon, and U.S. Pat. No. 5,909,622, issued Jun. 1, 1999 to Kadosh et al. As device geometries are reduced, semiconductor manufacturers increasingly require improved accuracy in measuring and controlling beam incidence angle in ion implanters. 
     Further, semiconductor device manufacturers typically require the use of parallel scan techniques, wherein the ion beam has a constant angle of incidence on the semiconductor wafer. 
     Prior art techniques for measuring beam incidence angles typically involve measurement of the wafer chuck or supporting hardware at maintenance intervals. Such measurements can be correlated with implant angles determined by crystallographic examination of test wafers implanted at specified settings. Such techniques are tedious, have limited accuracies, do not provide real time measurements of the beam and do not provide any measurement of beam divergence. 
     U.S. Pat. No. 5,180,918, issued Jan. 19, 1993 to Isobe, describes a method and apparatus for measuring ion beam collimation, shaping the ion beam and controlling scanning thereof. The method utilizes a time-dependent change in the scanning position of the ion beam at an upstream location and a downstream location at mutually corresponding times. 
     It is desirable to provide a simple sensor system which allows ion beam incidence angle to be monitored in real time in order to comply with requirements of semiconductor manufacturers for reduced incidence angle tolerances. It is also desirable to determine deviation from a desired incidence angle, to permit accurate adjustment thereof, and to determine beam divergence (angular variation across the beam). 
     SUMMARY OF THE INVENTION 
     In one aspect, the invention relates to a sensing device which monitors the incidence angle of an ion beam in real time. The sensing device includes an aperture plate having an aperture for intercepting the ion beam and passing a beam portion therethrough, and a sensor located in the substrate plane or a plane parallel thereto behind the aperture and having a length along which the beam portion impinges on the sensor at a location which is a function of the incidence angle of the ion beam, the sensor being configured to produce a sensor signal indicative of the location of impingement of the beam portion on the sensor and representative of incidence angle. A computing unit may be configured to receive the sensor signal and compare it to a predetermined function for determining the incidence angle of the ion beam from the sensor signal. 
     The sensing device described above provides an incidence angle measurement with respect to one dimension of the substrate plane. Two dimensional measurement may be provided with an X-Y angle sensing apparatus which includes first and second sensing devices disposed orthogonal to each other in a plane parallel to the substrate plane. 
     The invention may be used while a semiconductor wafer is being processed, providing data on ion beam characteristics during implant. The sensing device may be mounted in an ion implanter to intercept unused areas of the ion beam to permit concurrent ion implantation and beam monitoring. Additionally, as beam properties are held stable, mechanical alignments may be checked for precision and accuracy, without the use of cumbersome measurement devices or procedures. Monitoring may be performed in vacuum, thereby eliminating possible discrepancies between measurements in vacuum and measurements in atmosphere. 
     According to another aspect of the invention, apparatus is provided for sensing an incidence angle of an ion beam on a substrate plane. The apparatus comprises an aperture plate having an aperture for passing a portion of the ion beam, and a position-sensitive sensor spaced from the aperture plate and located in or parallel to the substrate plane for intercepting the beam portion and producing a sensor signal that is representative of a location of impingement of the beam portion on the sensor and is thereby representative of the incidence angle of the ion beam on the substrate. 
     The position-sensitive sensor may comprise a resistor block having a length along which the beam portion impinges on the sensor and an output terminal which produces the sensor signal. The apparatus may further comprise a computing unit for determining the incidence angle of the ion beam in response to the sensor signal. The computing unit may determine the incidence angle of the ion beam from calibration data. 
     According to a further aspect of the invention, apparatus is provided for sensing divergence of an ion beam. The apparatus comprises a first incidence angle sensor for sensing a first incidence angle of the ion beam with respect to a substrate plane, a second incidence angle sensor, spaced apart from the first incidence angle sensor, for sensing a second incidence angle of the ion beam with respect to the substrate plane, and a computing device for determining divergence of the ion beam based on the first and second incidence angles. 
     The first and second incidence angle sensors may each comprise an aperture plate having an aperture for passing a portion of the ion beam, and a position-sensitive sensor spaced from the aperture plate and located in or parallel to the substrate plane for intercepting the beam portion and producing a sensor signal that is representative of a location of impingement of the beam portion on the sensor and is thereby representative of the incidence angle of the ion beam on the substrate plane. 
     The computing device may determine the first and second incidence angles based on calibration data and may determine divergence of the ion beam based on a difference between the first and second incidence angles. 
    
    
     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 simplified schematic block diagram of an ion implanter; 
     FIG. 2 is a schematic block diagram of an incidence angle sensor apparatus in accordance with an embodiment of the invention; 
     FIG. 3 is a schematic sectional view of a portion of the incidence angle sensor apparatus shown in FIG. 2; 
     FIG. 4 is a schematic view of a sensor apparatus in accordance with an embodiment of the invention, which is adapted to determine incidence angle in two dimensions, viewed from the upstream side relative to the ion beam direction; 
     FIG. 5 is a view of the sensor of FIG. 4 taken along line  5 — 5  of FIG. 4; 
     FIG. 6 is a schematic view of the area scanned by an ion beam in the target chamber of an ion implanter incorporating a pair of sensors as shown in FIG. 4, viewed from upstream side of the ion beam; 
     FIG. 7 is a schematic view of a region of the target chamber of an ion implanter, viewed from the side relative to the ion beam; 
     FIG. 8 is a block diagram of an ion beam incidence angle and beam divergence monitor in accordance with an embodiment of the invention; and 
     FIG. 9 is a schematic block diagram of an incidence angle sensor apparatus in accordance with another embodiment of the invention. 
    
    
     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 a 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 platen  36  may be scanned perpendicular to ion beam  30  to distribute the ion beam over the surface of 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 polepieces  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 incidence angle and beam divergence monitor  80 . Monitor  80  measures beam incidence angle and divergence as described below. In addition, end station  32  may include 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. 
     A variety of different ion implanter architectures are known to those skilled in the art. For example, the ion beam may be distributed over the wafer by beam scanning, by wafer movement or by a combination of beam scanning and wafer movement. Examples of ion implanter architectures are disclosed in U.S. Pat. No. 4,922,106 issued May 1, 1999 to Berrian et al, U.S. Pat. No. 4,899,059 issued Feb. 6, 1990 to Freytsis et al and U.S. Pat. No. 5,350,926 issued Sep. 27, 1994 to White et al. The present invention may be utilized in any ion implanter architecture where it is desired to measure beam incidence angle and/or beam divergence. 
     According to an aspect of the invention, an apparatus and method are provided for measuring an incidence angle of beam  30  with respect to wafer  34 . An incidence angle  27  may be defined as the angle between the direction of ion beam  30  and a normal to the plane  70  of wafer  34 , as shown in FIGS. 1-3. 
     A one-dimensional incidence angle sensor in accordance with an embodiment of the invention is illustrated in FIGS. 2 and 3. An aperture plate  128  having a narrow aperture  130  of fixed width is placed in ion beam  30  to allow passage of a narrow width beam portion  132  of known cross section. Beam portion  132  impinges on a resistor block  134  which has a length L. The resistor block  134  is placed in the plane  70  of wafer  34  or in a plane parallel thereto. One end of resistor block  134 , along the length L, is connected to an amplifier  136 , the output of which is supplied to a computing unit  137  which determines incidence angle  27 . Computing unit  137  may include an analog-to-digital converter for converting the output of amplifier  136  to a digital value. The other end of the resistor block  134 , along the length L, can float electrically, or may be connected to a reference potential, such as ground, to limit noise. 
     As the ion beam impinges on resistor block  134 , a current flow change or voltage change to the amplifier  136  is generated. The distance which the current must travel through the resistor block  134  to the amplifier  136  varies depending on the location along length L where the beam portion  132  impinges on the resistor block  134 . The electrical resistance between the location where the ion beam  30  impinges on resistor block  134  and the location where resistor block  134  is connected to amplifier  36  varies according to the incidence angle  27  of the ion beam  30 . The variation in resistance results in a beam location-dependent variation in a sensor signal input to the amplifier  136 . The sensor signal input to the amplifier  136  represents the beam incidence angle and may be used to determine the beam incidence angle. A calibration can be performed to determine resistor value at a “known” or mechanically verified zero-degree or other known angle setting. Relative values may then be used to determine incidence angle. 
     The resistor block  134  may be any sufficiently resistive material. It may be as wide (in a direction perpendicular to the angular measurement) as necessary to collect enough ions of the ion beam  30  to provide a workable signal-to-noise ratio. The resistor block  134 , in a preferred embodiment, is a thin or thick-film deposited material, such as silicon or carbon, with homogenous electrical properties. 
     Referring to FIG. 3, the aperture  130 , in the preferred embodiment, is a slit in plate  128 , which is preferably made of graphite. The backside of the slit may have a beveled edge  138  to permit the plate  128  to be sufficiently thick to prevent heat distortion, while simultaneously preventing the beam from being blocked by aperture sides at large incident angles. The plate  128  may alternately be formed of a metal or ceramic material, and may also be formed of movable elements defining a slit of variable width. A metal plate or metal plate elements may be water cooled to prevent heat distortion, and may be coated with a material, such as silicon or a ceramic, to prevent sputtering into the implant chamber. The aperture plate  128  may be fastened to the resistor block  134  to maintain plate  128  at a fixed, accurate distance D from resistor block  134 . For any given incidence angle, a larger distance D produces a greater distance difference along the resistor block  134  between the calibration location and the measurement location. Consequently, the distance D can be selected to obtain a desired precision for incidence angle measurement. 
     The sensor signal from resistor block  134  is supplied through amplifier  136  to computing unit  137 . Computing unit  137  may be part of an ion implanter controller or may be a dedicated analysis circuit. Computing unit  137  may be implemented as a microprocessor or as a special purpose circuit. The computing unit  137  analyzes the sensor signal to determine a beam incidence angle. In one embodiment, the sensor signal is calibrated by applying to the apparatus an ion beam at a series of known incidence angles. The sensor signals are recorded for each known incidence angle to generate a calibration curve or table which contains sensor signal values and corresponding incidence angle values. When an ion beam having an unknown incidence angle is measured, the incidence angle is determined by comparing the sensor signal to the calibration curve or table. 
     The above-described sensor can measure the incidence angle along one axis or direction. In a preferred embodiment, an ion beam incidence angle and beam divergence monitor uses otherwise unused portions of an ion beam: 1) to determine the angle at which the ion beam is impinging on a semiconductor wafer, and, alternately or additionally, 2) to determine the amount, if any, of ion beam divergence or convergence (i.e. negative divergence). 
     A measurement device for measuring an incidence angle of an ion beam impinging on a planar substrate in two dimensions of the substrate plane can be provided in accordance with embodiments of the invention. Such a device includes a pair of sensors as described above, each member of the pair being located in the substrate plane or a plane parallel thereto and the members of the pair being disposed orthogonal to each other. FIGS. 4 and 5 illustrate an X-Y angle sensor  140 . X-Y sensor  140  includes an aperture plate  141  having apertures  142  and  144  orthogonal to each other. The apertures  142  and  144  are associated with resistor blocks  146  and  148 , respectively, of length L, mounted at a fixed distance D from plate  141 . Resistor blocks  146  and  148  are mounted on a substrate  149 , such as a printed circuit board. Outputs from the resistor blocks  146  and  148 , amplified as needed, provide signals which can be analyzed to provide a two-dimensional measurement of incidence angle (i.e. an X angle and a Y-angle) and/or a two-dimensional measurement of beam divergence. 
     FIG. 6 shows an area  150  in the substrate plane scanned by the ion beam of a typical ion implanter. The area  150  scanned by the beam is larger than a wafer area  152 . At least a part of the remaining area  154  is unused. One or more sensors, such X-Y sensors  156  and  158 , are positioned to intercept the unused area of the beam, thereby permitting real-time monitoring (i.e. concurrent with ion implantation or other wafer treatment) of beam incidence angle. The two X-Y sensors  156  and  158  are spaced apart and are located on opposite sides of wafer area  152 . Sensor  156  includes Y 1  sensor  156   a  and X 1  sensor  156   b.  Sensor  158  includes Y 2  sensor  158   a  and X 2  sensor  158   b.  Sensors  156  and  158  are the sensors of an ion beam incidence angle and beam divergence monitor, as described below. 
     Angle measurements taken by sensors  156  and  158  provide X and Y incidence angles at two locations. The difference between the incidence angle measurements along each axis or direction represents the beam divergence. Monitoring of divergence permits correction by manipulation of the ion beam profile until an acceptable divergence has been obtained. Such corrections may be performed automatically in real-time during ion implantation in response to sensing a deviation from a preset value or a range of values. 
     A block diagram of ion beam incidence angle and divergence monitor  80  is shown in FIG.  8 . Like elements in FIGS. 6 and 8 have the same reference numerals. Each of the sensors  158   a,    158   b,    156   a  and  156   b  may include an aperture plate having an aperture spaced from a position-sensitive sensor, such as a resistor block, as described above. The sensor signals from sensors  156   b,    158   b,    156   a  and  158   b  are supplied through amplifiers  200 ,  202 ,  204  and  206 , respectively, to a computing unit  210 . The computing unit  210  may include one or more analog-to-digital converters for converting the outputs of amplifiers  200 ,  202 ,  204 , and  206  to digital values. The computing unit  210  may analyze the sensor signal from each individual sensor by comparing the amplified sensor signal value with a calibration curve or table to determine an incidence angle. Sensors  156   b  and  158   b  provide incidence angle measurements in the X direction, and sensors  156   a  and  158   a  provide incidence angle measurements in the Y direction. The computing unit  210  may subtract the incidence angle measurements acquired by X direction sensors  156   b  and  158   b  to determine beam divergence in the X direction and may subtract the incidence angle measurements acquired by Y direction sensors  156   a  and  158   a  to determine beam divergence in the Y direction. As noted above, a negative value of beam divergence represents a converging beam. The measured values of incidence angle and beam divergence may be utilized to adjust the beamline and/or the tilt of the wafer via tilt mechanism  84  (FIG.  1 ), as necessary. The adjustment may be manual or automatic within the scope of the invention. 
     FIG. 7 illustrates an embodiment of the sensors mounted in an implanter. A substrate holder  160  includes a platen  162  holding wafer  34 . Platen  162  is mounted on a shaft  164  which extends rearwardly behind the substrate. X-Y sensors  156  and  158  are mounted to shaft  164  in a plane parallel to and behind the wafer and intercept beam portions passing through the unused beam area around the wafer  34 . 
     The resistivity of the resistor block may change over time as dopants are implanted therein. However, the change in resistivity in most cases is small per wafer implant, and a recalibration procedure can be performed when necessary to adjust for these changes. The resistor block is cleanable and replaceable in a preferred embodiment. Other embodiments of the resistor block may include a resistive wire coil, as is commonly used in rheostats, or patterned metal traces on a printed circuit board. 
     Recalibration can easily be accomplished as the implanter and the sensor have a fixed operative range of angular variation, which defines the end points of a line or curve corresponding to the maximum and minimum sensor signals obtainable. The absolute values corresponding to these maxima and minima can be redefined at any time by moving the implanter beamline to the end points and monitoring the corresponding sensor signals obtained. In general, it is also usually possible to reorient the beam or the substrate to provide a zero angle measurement, so that resistance drift at this point can be periodically monitored and used for recalibration. 
     Periodic recalibration of the monitor device can be incorporated into the production cycle, for instance by programming a wafer handler to load a calibration blank rather than a production wafer from a prescribed storage location, or by including a calibration blank with the production wafers at fixed intervals, and running the recalibration scan while the calibration blank is in the implanter. The production cycle then can continue using the revised curve without breaking vacuum. Thus the sensor can continue to be operated within acceptable tolerances for extended times without significant disruption of the production cycle. Further, the resistors can be manufactured to be easily cleanable and/or replaceable, so that if sensitivity becomes too low over the length of the resistor, or if resistance drift due to implantation eventually becomes too localized for a reasonably accurate angular curve to be obtained, replacement of the resistor, and a calibration as described above permit a rapid resumption of the production cycle. 
     Another embodiment of a one-dimensional incidence angle sensor in accordance with the invention is illustrated in FIG.  9 . An aperture plate  300  having a narrow aperture  310  of fixed width is placed in ion beam  30  to allow passage of a narrow width beam portion  312  of known cross section. Beam portion  312  impinges on a second aperture plate  320  having a plurality of apertures  330 ,  332 ,  334 , etc. Aperture plate  320  is placed in the plane  70  of wafer  34  or in a parallel thereto and is spaced from aperture plate  300  by a known distance D. Beam sensors  340 ,  342 ,  344 , etc. are positioned behind apertures  330 ,  332 ,  334 , etc., respectively, for sensing beam portion  312 . The beam sensors  340 ,  342 ,  344 , etc. may for example be small Faraday cups. The beam sensors  340 ,  342 ,  344 , etc. are connected through appropriate signal processing circuitry to a computing unit (not shown). When beam portion  312  passes through one of the apertures in plate  320  and is sensed by one of the beam sensors, the sensor signal produced by that beam sensor indicates a known value of incidence angle  27 . 
     The embodiment of FIG. 9 provides measurement of a number of known fixed angles. The measurement resolution depends on the spacing between apertures  330 ,  332 ,  334 , etc. in aperture plate  320 , the sizes of the apertures in aperture plates  300  and  320  and the distance D between aperture plates  300  and  320 . It will be understood that FIG. 9 is not drawn to scale and that a practical implementation of the incidence angle sensor would have a larger number of apertures in aperture plate  320  and a smaller spacing between apertures. 
     In yet another embodiment, the second aperture plate  320  is omitted, and a plurality of beam sensors  340 ,  342 ,  344 , etc. is used to sense beam portion  312  at multiple discrete incidence angles. In this embodiment, the measurement resolution is determined by the size of the beam sensor aperture and the spacing between beam sensors, as well as the size of aperture  310  and the distance D. When the beam portion  312  is sensed by one of the beam sensors, the sensor signal produced by that beam sensor indicates a known value of incidence angle  27 . 
     The above description is intended to be illustrative and not exhaustive. The description will suggest many variations and alternatives to one of ordinary skill in this art. All these alternatives and variations are intended to be included within the scope of the attached claims. Those familiar with the art may recognize other equivalents to the specific embodiments described herein which equivalents are also intended to be encompassed by the claims attached hereto. Further, the particular features presented in the dependent claims below can be combined with each other in other manners within the scope of the invention such that the invention should be recognized as also specifically directed to other embodiments having any other possible combination of the features of the dependent claims.