Abstract:
Methods and apparatus are provided for measuring a profile of an ion beam. The apparatus includes an array of beam current sensors, each producing a sensor signal in response to incident ions of the ion beam, a translation mechanism configured to translate the array of beam current sensors along a translation path with respect to the ion beam, and a controller configured to acquire the sensor signals produced by the beam current sensors at a plurality of positions along the translation path, wherein the acquired sensor signals are representative of a two-dimensional profile of the ion beam.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to systems and methods for ion implantation and, more particularly, to methods and apparatus for measuring a two-dimensional profile of an ion beam.  
       BACKGROUND OF THE INVENTION  
       [0002]     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.  
         [0003]     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 may be distributed over the target area by beam scanning, by target movement or by a combination of beam scanning and target movement.  
         [0004]     In one prior art approach, a high current, broad ion beam implanter employs a high current density ion source, an analyzing magnet to direct a desired species through a resolving slit and an angle corrector magnet to deflect the resulting beam, while rendering the beam parallel and uniform along its width dimension. A ribbon-shaped ion beam is delivered to a target, and the target is moved perpendicular to the long dimension of the ribbon beam to distribute the ion beam over the target.  
         [0005]     Uniform implantation of ions over the surface of the semiconductor wafer is an important requirement in most applications. As semiconductor device geometries decrease in size and wafer diameters increase, device manufacturers demand minimal dose variation over large surface areas. Uniformity is determined, in part, by the profile of the ion beam used for ion implantation. The beam profile is a map of ion beam intensity in a plane orthogonal to the direction of beam transport. The beam current may vary over the cross-sectional area of the ion beam, particularly in the case of large area beams such as ribbon ion beams. Furthermore, the beam profile may vary with implant conditions, such as dopant species, energy and current, and with time. Accordingly, it is desirable to measure and, if necessary, adjust the beam profile in order to enhance ion implanter performance.  
         [0006]     A dose measurement and uniformity monitoring system for ion implantation, including a mask plate with sensing apertures and an annular Faraday cup aligned with the apertures, is disclosed in U.S. Pat. No. 4,751,393 issued Jun. 14, 1988 to Corey, Jr. et al. A beam scanning control device for ion implantation, including a plurality of fixed ion beam detectors, is disclosed in U.S. Pat. No. 4,494,005 issued Jan. 15, 1985 to Shibata et al. An ion beam profile monitor, including a two-dimensional array of sample points placed in the beam, is disclosed by E. P. EerNisse et al. in Rev. Sci. Instrum., Vol. 46, No. 3, (March 1975), pp. 266-268. A method and apparatus for high efficiency scanning in an ion implanter, including a single, slowly-translating Faraday detector, is disclosed in U.S. Pat. No. 4,980,562 issued Dec. 25, 1999 to Berrian et al. All of the prior art beam measuring techniques have had one or more drawbacks, including, but not limited to, low resolution, inaccuracy and slow operation.  
         [0007]     Accordingly, there is a need for improved methods and apparatus for ion beam profiling.  
       SUMMARY OF THE INVENTION  
       [0008]     According to a first aspect of the invention, apparatus is provided for measuring a profile of an ion beam. The apparatus comprises an array of beam current sensors, each producing a sensor signal in response to incident ions of the ion beam, a translation mechanism configured to translate the array of beam current sensors along a translation path with respect to the ion beam, and a controller configured to acquire the sensor signals produced by the beam current sensors at a plurality of positions along the translation path. The acquired sensor signals are representative of a two-dimensional profile of the ion beam.  
         [0009]     According to a second aspect of the invention, an ion implanter comprises an ion beam generator configured to generate an ion beam, a target site for supporting a target for ion implantation, and a system for measuring the ion beam. The system for measuring the ion beam comprises an array of beam current sensors, a translation mechanism configured to translate the array along a translation path with respect to the ion beam, and a controller configured to acquire the sensor signals produced by the beam current sensors at a plurality of positions along the translation path.  
         [0010]     According to a third aspect of the invention, a method is provided for measuring an ion beam. The method comprises providing an array of beam current sensors, each producing a sensor signal in response to incident ions of the ion beam, translating the array of beam current sensors along a translation path with respect to the ion beam, and acquiring the sensor signals produced by the beam current sensors at a plurality of positions along the translation path. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     For a better understanding of the present invention, reference is made to the accompanying drawings, which are incorporated herein by reference and in which:  
         [0012]      FIG. 1  is a simplified schematic diagram of an embodiment of an ion implanter;  
         [0013]      FIG. 2  is a schematic block diagram of apparatus for measuring a two-dimensional profile of an ion beam in accordance with an embodiment of the invention;  
         [0014]      FIG. 3  is a front view of a beam sensing assembly in accordance with an embodiment of the invention;  
         [0015]      FIG. 4  is a cross-sectional view of the beam sensing assembly, taken along the line  4 - 4  of  FIG. 3 ;  
         [0016]      FIG. 5  is a schematic cross-sectional view of a beam current sensor with suppression electrodes in accordance with another embodiment of the invention;  
         [0017]      FIG. 6  illustrates a cross section of a ribbon ion beam;  
         [0018]      FIG. 7  illustrates a cross section of a scanned ion beam; and  
         [0019]      FIG. 8  illustrates a cross section of a spot ion beam. 
     
    
     DETAILED DESCRIPTION  
       [0020]     A block diagram of an embodiment of an ion implanter is shown in  FIG. 1 . An ion source  10  generates ions and supplies an ion beam  12 . Ion source  10  may include an ion chamber and a gas box containing a gas to be ionized. The gas is supplied to the ion chamber where it is ionized. The ions thus formed are extracted from the ion chamber to form ion beam  12 . Ion beam  12  is directed between the poles of a resolving magnet  32 . A first power supply  14  is connected to an extraction electrode of ion source  10  and provides a positive first voltage V 0 . First voltage V 0  may be adjustable, for example, from about 0.2 to a 80 kV. Thus, ions from ion source  10  are accelerated to energies of about 0.2 to 80 KeV by the first voltage V 0 .  
         [0021]     Ion beam  12  passes through a suppression electrode  20  and a ground electrode  22  to a mass analyzer  30 . The mass analyzer  30  includes resolving magnet  32  and a masking electrode  34  having a resolving aperture  36 . Resolving magnet  32  deflects ions in ion beam  12  such that ions of a desired ion species pass through resolving aperture  36  and undesired ion species do not pass through resolving aperture  36  but are blocked by the masking electrode  34 . In one embodiment, resolving magnet  32  deflects ions of the desired species by 90°.  
         [0022]     Ions of the desired ion species pass through resolving aperture  36  to a first deceleration stage  50  positioned downstream of mass analyzer  30 . Deceleration stage  50  may include an upstream electrode  52 , a suppression electrode  54  and a downstream electrode  56 . Ions in the ion beam may be decelerated by deceleration stage  50  and then pass through an angle corrector magnet  60 . Angle corrector magnet  60  deflects ions of the desired ion species and converts the ion beam from a diverging ion beam to a ribbon ion beam  62  having substantially parallel ion trajectories. In one embodiment, angle corrector magnet  60  deflects ions of the desired ion species by 70°.  
         [0023]     An end station  70  supports one or more semiconductor wafers, such as wafer  72 , in the path of ribbon ion beam  62  such that ions of the desired species are implanted into the semiconductor wafer. The end station  70  may include a cooled electrostatic platen and a scanner (not shown) for moving wafer  72  perpendicular to the long dimension of the ribbon ion beam  62  cross-section, so as to distribute ions over the surface of wafer  72 . The ribbon ion beam may be at least as wide as wafer  72 .  
         [0024]     The ion implanter may include a second deceleration stage  80  positioned downstream of angle corrector magnet  60 . Deceleration stage  80  may include an upstream electrode  82 , a suppression electrode  84  and a downstream electrode  86 .  
         [0025]     The ion implanter may include additional components known to those skilled in the art. For example, end station  70  typically includes automated wafer handling equipment for introducing wafers into the ion implanter and for removing wafers after ion implantation. End station  70  may also include a dose measuring system, an electron flood gun and other known components. It will be understood that the entire path traversed by the ion beam is evacuated during ion implantation.  
         [0026]     The ion implanter of  FIG. 1  may operate in one of several modes. In a first operating mode, known as the drift mode, deceleration stages  50  and  80  are connected to ground, and the ion beam  12  is transported through the beamline at the final beam energy established after extraction from ion source  10 . In a second operating mode, known as the enhanced drift mode, the ion beam  12  is accelerated to an intermediate energy at electrode  22  before passing through mass analyzer  30  and then is decelerated to the final beam energy by first deceleration stage  50 . In a third operating mode, known as the double deceleration mode, the ion beam is accelerated to a first intermediate energy at electrode  22  before passing through mass analyzer  30 , is decelerated by first deceleration stage  50  to a second intermediate energy as it passes through angle corrector  60  and then is decelerated to the final beam energy by second deceleration stage  80 . A fourth operating mode transports the beam at the intermediate energy through to the second deceleration stage  80 , and the gap at the first deceleration stage  50  is operated with a short circuit shunt. By transporting the ion beam through part of the beamline at higher energy, space charge expansion can be reduced in comparison with the drift mode for a given final beam energy.  
         [0027]     In accordance with a feature of the invention, end station  70  may include an ion beam profiler  100  as shown in  FIG. 1 . A block diagram of ion beam profiler  100  in accordance with an embodiment of the invention is shown in  FIG. 2 . Ion beam profiler  100  is configured to acquire a profile of a cross section of ion beam  62 , typically in a plane orthogonal to the direction of ion beam transport. The direction of beam transport is perpendicular to the plane of  FIG. 2 . Typically, the ion beam profile in or near the plane of wafer  72  is of interest. However, ion beam profiler  100  can acquire the ion beam profile in any desired plane.  
         [0028]     The beam profiler measures beam current in incremental areas over the cross-sectional area of the ion beam to obtain a beam profile in the form of a two-dimensional map of beam current density. The two-dimensional map may be configured as an X-Y array of pixels, each of which contains a measured value of beam current density in an incremental area of the beam cross section. The pixel values may be measured as described below. The two-dimensional map of beam current density may be utilized to confirm that the ion beam profile is within specification. If the beam profile is not within specification, the beam profile may be adjusted and an updated beam profile may be acquired to confirm the adjustment. Other uses of the two-dimensional beam profile may be contemplated within the scope of the invention.  
         [0029]     Referring to  FIG. 2 , ion beam profiler  100  may include a beam sensing assembly  110 , a translation mechanism  112  and controller  114 . Ion beam profiler  100  is configured for measuring the two-dimensional profile of ion beam  62 . As discussed below, the ion beam profiler may be configured for measuring an ion beam having an arbitrary cross-sectional shape and size.  
         [0030]     Beam sensing assembly  110  includes an array  122  of beam current sensors  124  mounted to a frame or housing  126 . Beam current sensors  124  may be Faraday cups which produce an electrical signal in response to an intercepted ion beam. As known in the art, the magnitude of the sensor signal is a function of the intercepted ion beam current. Each beam current sensor may be a cup-shaped conductor with an aperture that faces the ion beam. The size of the aperture determines the area of the ion beam that is sampled by the beam current sensor.  
         [0031]     In the embodiment of  FIG. 2 , the array  122  is a linear array of beam current sensors  124  which are equally spaced along a Y direction. As described below, other array configurations may be utilized within the scope of the invention.  
         [0032]     The beam sensing assembly  110  is translated along a translation path  140  in an X direction by translation mechanism  112  to cover the entire cross-sectional area of ion beam  62 . Beam current measurements are acquired from each of beam current sensors  124  as beam sensing assembly  110  is translated along translation path  140 . The beam current measurements may be acquired when sensing assembly  110  is moving or, in the case of stepwise movement, each time sensing assembly  110  stops along translation path  140 . The sensor signals generated by beam current sensors  124  may be measured continuously or may be sampled at desired intervals. The current measurements are supplied to controller  114  for storage in a memory  130  and/or transmission to a host computer. The set of measurements as beam sensing assembly  110  is translated across ion beam  62  represents a two-dimensional map of beam current density of ion beam  62 . The set of current measurements may be used to generate a display or a printout of the ion beam profile.  
         [0033]     The parameters of ion beam profiler  100  depend on the characteristics of ion beam  62 , such as beam current and cross-sectional dimensions and shape, and on the desired resolution and measuring speed. In the embodiment of  FIG. 2  where array  122  includes beam current sensors  124  along the Y direction, the length of array  122  should be at least as large as the maximum expected height of ion beam  62 . The length of translation path  140  of beam sensing assembly  110  should be at least as large as the maximum expected width of ion beam  62 . The size of beam current sensors  124  in array  122  depends on the desired resolution of the beam profile and on the ability of the sensor to produce an acceptable signal level. Individual beam current sensors  124  may sense abutting areas so as to provide a contiguous profile of the entire ion beam.  
         [0034]     The translation of beam sensing assembly  110  along translation path  140  may be continuous or in discrete steps. In one embodiment, beam sensing assembly  110  is translated in steps equal to one half the width of the apertures in beam current sensors  124 . In that embodiment, the ion beam profiler  100  acquires a two-dimensional beam profile including rows and columns of pixels, each containing a measured current value. The array  122  of beam current sensors  124  defines a column of pixels, and the rows of pixels are defined by each beam current sensor  124  as it is translated along translation path  140 .  
         [0035]     The translation mechanism  112  may utilize a rack and pinion drive mechanism, for example. Other suitable translation mechanisms include a ball and screw assembly, a linear motor and an air piston.  
         [0036]     As shown in  FIG. 2 , controller  114  may provide position control signals to translation mechanism  112  to control translation of beam sensing assembly  110  along translation path  140 . For example, controller  114  may control translation mechanism  112  to translate beam sensing assembly  110  in steps across ion beam  62  and to record the beam current sensed by each of the beam current sensors  124  at each position. The measured current values and the corresponding positions form a data set which represents the two-dimensional ion beam profile. The data set may be stored in memory  130  and/or transmitted to a host computer.  
         [0037]     In the embodiment of  FIG. 2 , beam sensing assembly  110  includes beam current sensors  124  positioned along the Y direction, and the beam sensing assembly  110  is translated in the X direction. In other embodiments, beam current sensors  124  can be positioned along the X direction, and the beam sensing assembly can be translated along the Y direction. In addition, the beam sensing assembly  110  is not necessarily aligned with the X direction or the Y direction.  
         [0038]     An implementation of a beam sensing assembly in accordance with an embodiment of the invention is shown in  FIGS. 3 and 4 . In the embodiment of  FIGS. 3 and 4 , a beam sensing assembly  210  includes a housing  226 , an array  222  of beam current sensors  224  and a single, elongated beam current sensor  228 . Array  222  includes a first subarray  250  of beam current sensors  224  and a second subarray  252  of beam current sensors  224 . The first and second subarrays  250  and  252  each include a linear arrangement of beam current sensors, with the beam current sensors of subarray  252  being offset in the Y direction by an offset OY with respect to first subarray  250 . This array configuration permits measurement of the ion beam along a contiguous strip of ion beam  62 .  
         [0039]     As shown in  FIG. 4 , beam current sensors  224  and  228  are mounted in housing  226 . Housing  226  is enclosed by a cover  260  having apertures  262  and  264  which define the incremental areas of the ion beam  62  that are sensed by the respective beam current sensors. In particular, aperture  262  defines an area sensed by beam current sensor  224 , and aperture  264  is an elongated slot that defines the area sensed by beam current sensor  228 .  
         [0040]     One of the difficulties in acquiring a two-dimensional beam profile, particularly when measuring low ion beam currents, is to insure that only the current due to the ion beam is measured. The profiler operates in a region of the ion implanter where electrons and low energy ions are also present, generated both through collisions of the ion beam with the background gas and by introduction from an electron flood gun used to neutralize charge on the wafer. The flood gun may be located immediately upstream of the profiler in the beamline. The beam profiler may be provided with suppression elements for suppressing entry of electrons and low energy ions into the beam current sensors.  
         [0041]     The beam sensing assembly may include magnets to prevent low energy electrons from entering the beam current sensors along with the beam ions or from leaving the beam current sensors once the ions have entered. Referring again to  FIG. 4 , beam sensing assembly  210  may include magnets  270 ,  272 ,  274  and  276  positioned on opposite sides of beam current sensors  224  and  228 . In the embodiment of  FIG. 4 , magnets  270 ,  272 ,  274  and  276  may be aligned with their respective north and south poles facing each other to produce a dipole field in the entrance to beam current sensors  224  and  228 . The magnets are selected to produce magnetic fields at the center of each beam current sensor which are orthogonal to the direction of beam transport and which have a magnitude of about 500-600 Gauss. It will be understood that these parameters are given by way of example only and are not limiting as to the scope of the invention.  
         [0042]     The beam sensing assembly  210  may also include a positively-biased plate  280  positioned in front of the beam current sensors to prevent low energy ions, particularly those generated by the flood gun, from entering the beam current sensors along with the beam ions and being measured as part of the ion beam current. By way of example, plate  280  may be biased at a voltage of about +20 volts.  
         [0043]     Another embodiment of a beam current sensor including suppression elements for electron and low energy ion suppression is shown in  FIG. 5 . A beam current sensor  284  may be configured as a Faraday cup. Electrons and low energy ions are suppressed by an arrangement including a ground plate  286 , a negative suppression plate  288 , a positive suppression plate  290  and a negative suppression plate  292 . In the embodiment of  FIG. 5 , ground plate  286  is connected to ground, negative suppression plates  288  and  292  are biased at −200 volts and positive suppression plate  290  is biased at +400 volts. Positive suppression plate  290 , which prevents low energy ions from entering the beam current sensor, is separated from the beamline and from the beam current sensor by negatively-biased plates which prevent electrons in the beamline from entering the Faraday cup and electrons from leaving the Faraday cup. In the absence of suppression magnets, the positively-biased plate alone would prevent low energy ions from entering the Faraday cup but would also distort the electron flow.  
         [0044]     In the embodiment of  FIGS. 3 and 4 , each of beam current sensors  224  has height H and a width W, and adjacent beam current sensors are spaced apart in the Y direction by a spacing SY that is equal to height H. Subarrays  250  and  252  are spaced apart in the X direction by a spacing SX. The arrangement of two spaced-apart subarrays of beam current sensors avoids any gap in measurement which would otherwise result from the wall thicknesses of individual beam current sensors arranged in a single linear array. Beam current measurement along a contiguous strip of ion beam  62  includes a first measurement with subarray  252  at a specified X position, translation of beam sensing assembly  210  in the X direction by a distance equal to the width W plus the spacing SX and a second measurement at the same X position with subarray  250 . The two sets of measurements are combined to provide a beam profile along a contiguous strip of the ion beam at a given X position. In practice, measurements are acquired simultaneously by subarrays  250  and  252  at different X positions, and the beam sensing assembly  210  is translated along the X direction to provide a complete data set. The acquired current values are appropriately processed to provide contiguous data values in X and Y directions. Thus, current values acquired by subarrays  250  and  252  at the same X position are combined to provide a single column of the profile data set. The beam sensing assembly may include a single array of beam current sensors or two or more subarrays of beam current sensors.  
         [0045]     In one example, each beam current sensor  224  has a height H of 6 mm (millimeters) and a width W of 6 mm, and the spacing SY between sensors in each column is 6 mm. The offset OY between subarrays  250  and  252  is 6 mm, and the spacing SX between subarrays  250  and  252  is 12 mm. Beam sensing assembly  110  may be translated along the X direction in increments of 3 mm, for example. Each of subarrays  250  and  252  may include 12 beam current sensors in this example for a total measurement height of 144 mm. The height H and width W of beam current sensors  224  is selected to collect an acceptable signal level in applications of interest and to provide a desired resolution. The length of translation path  140  may be equal to or greater than the width of ion beam  60  and in one example is 400 mm. In this example, a two-dimensional ion beam profile can be acquired in two seconds. It will be understood that these parameters are given by way of example only and are not limiting as to the scope of the present invention.  
         [0046]     Beam current sensor  228  may be configured as a single beam current sensor having an area equal to the total areas of the beam current sensors  224  in array  222 . More particularly, beam current sensor  228  may have a width V equal to the width W of beam current sensors  224  and may have a length that is equal to the total length of array  222  along the Y direction. Beam current sensor  228  may be used to confirm operation of sensors  224  in array  222 . The beam current measured by beam current sensor  228  at a particular X position should be equal to the total current measured by subarrays  250  and  252  at the same X position. Beam current sensor  228  averages variations in beam current density along the Y direction to produce a single measured current value for each X position. Thus, beam current sensor  228  provides a one-dimensional beam profile.  
         [0047]     The embodiment of  FIGS. 3 and 4  includes two subarrays that measure a contiguous strip of the ion beam. In other embodiments, the beam current sensors are spaced apart along the Y direction and interpolation is used to estimate beam current at positions between sensors. Array  122  shown in  FIG. 2  and described above is an example of spaced-apart beam current sensors  124 . In further embodiments, the beam current sensors have overlapping measurement areas.  
         [0048]     Different ion beam types can be profiled by the ion beam profiler shown and described herein. Referring to  FIG. 6 , a ribbon ion beam  300  typically has an elongated cross section characterized by a beam height BH and a beam width BW. The length of the array of beam sensors in the Y direction is selected to be equal to or greater than beam height BH, and the length of translation path  140  in the X direction is selected to be equal to or greater than the beam width BW. It is typically most practical to translate the beam sensing assembly along the long dimension of the ion beam cross section. However, the invention is not limited in this regard. Thus, for example, a horizontal array of beam current sensors can be translated in the vertical direction to acquire a profile of ribbon ion beam  300 .  
         [0049]     Referring to  FIG. 7 , the ion beam profiler of the present invention can be utilized to acquire the profile of a scanned ion beam. In  FIG. 7 , ion beam  310  is scanned along a scan direction  314  to provide a scan pattern  316 . The ion beam profiler can be utilized to acquire the profile of scan pattern  316  along scan direction  314 . In the example of  FIG. 7 , the beam sensing assembly is translated along scan direction  314 , and the translation path has a length equal to or greater than the width of scan pattern  316 . The speed of translation of the beam sensing assembly is slow in comparison with the beam scanning speed to ensure that ion beam  310  is measured at least once at each position along the translation path.  
         [0050]     A spot ion beam  320  is shown in  FIG. 8 . To acquire the profile of spot ion beam  320 , the height of the array of beam current sensors and the length of the translation path are both equal to or greater than the maximum expected diameter of ion beam  320 . It will be understood that the spot ion beam  320  does not necessarily have a circular cross section and in general has an irregular cross-sectional shape. For any of the beam types, the length of the beam current sensor array and the length of the translation paths are preferably somewhat larger than the maximum expected beam dimensions or scan pattern dimensions to accommodate abnormal ion beam conditions. The array of beam current sensors can have any desired configuration of individual beam current sensors and can be translated in the X direction, in the Y direction or in an arbitrary direction.  
         [0051]     Having thus described several aspects of at least one embodiment of this invention, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description and drawings are by way of example only.