Ion implanter scanning mechanism

Abstract: An ion beam scanning method and apparatus produce a parallel, scanned ion beam with a magnetic deflector having, in one instance, wedge-shaped pole pieces that develop a uniform magnetic field. A beam accelerator for the scanned beam has a slot-shaped passage which the scanned beam traverses. The beam scan and the beam traverse over a target object are controlled to attain a selected beam current and corresponding ion dose on a target object. Methods and apparatus are disclosed for increasing ion beam utilization efficiency without adversely effecting dose accuracy.

Description: This invention relates to the scanning of an ion beam over a target with high dose accuracy. US-A-4,276,477 discloses two magnetic deflectors to produce a parallel scanning beam in one dimension. Two-dimensional scanning of an ion beam with electrostatic deflectors has been achieved but the beam intensity is subject to uncontrolled fluctuation. The problem of uniformity of dosage has been addressed in three types of prior art ion implanters. One employs an ion beam which is swept into dimensions across a stationary semiconductor wafer or other target being irradiated with the ion beam. A second employs an ion beam which remaining stationary and in which the wafer or other target is moved in two dimensions, either rectangularly (x, y) or polarly (r, ϑ). The third type employs a hybrid system in which the wafer is moved along one coordinate direction while the ion beam is moved along the other direction. In each of these types, one scan speed is sufficiently faster than the other so that a non-overlying pattern of quasi-parallel lines is laid down on the wafer, i.e., a pattern in which each trace of the beam on the target is offset from another. The first type of implanter is embodied in a typical medium current instrument. One wafer at a time is implanted by electrostatically scanning the ion beam by two orthogonal pairs of sweep plates or other beam deflectors. The (x) and (y) scan frequencies are usually random, or if stabilized, are arranged so that the ion beam pattern is not routinely repeated, e.g. a Lissajous figure is not formed. The position of the ion beam at any time during the ion implantation operation is largely unknown. The second type of implanter is embodied in a typically high current instrument employing an apertured spinning scanning disk. US-A-3,778,626 describes how uniformity is obtained with this apparatus by varying the radial velocity of the disk to compensate for the geometrical change in angular velocity, which is proportional to (1/R), where (R) is the distance between the disk axis and the beam intercept. US-A-3,234,727 discloses positioning a precision scanning slot radially in the scanning disk, to avoid measuring (R). In other prior mechanisms, the scanning slot is required to be in the slow scan direction. By using one or more scanning slots to act as a pseudo-wafer, beam pulses measured in a Faraday cup mounted behind the disk are used to continuously adjust the radial velocity through the slot at all values of (R). Again, beam position at any time is largely unknown. Examples of the third type of implanter utilize an ion beam scanned electrostatically in one direction and impinging on target wafers mounted on the inside or outside of a spinning drum. Alternatively, a slow changing magnetic field scans the beam radially across an apertured spinning disk. EP-A-263 876 discloses a scanning ion beam source which scans across a target wafer and a detector and a uniformity control computer responds to the detector output to control the scanning ion beam source. The scan extends to the detector during each of the translation scans of the target object and this means that the time occupied in scanning the target is appreciably smaller than the total scanning time. This problem is also known from WO 88/10498. The present invention sets out to enable the proportion of scanning time occupied by scanning the target to be increased, and various aspects of the invention are set out in claims 1 to 3. In the drawings, Fig. 1 is a combined block pictorial representation of an ion implanting system of the prior art, according to Fig. 9 of WO 87/06391 (= EP-A-263876), Fig. 2 is a schematic diagram illustrating a first embodiment of ion beam scan compression according to the invention, and Fig. 3 is a schematic diagram illustrating a second embodiment of ion beam scan compression according to the invention, Fig. 4 is a schematic diagram illustrating a third embodiment of ion beam scan compression according to the invention, Fig. 5 is a schematic diagram illustrating compression of a mechanical scan to improve beam utilization efficiency, Fig. 6 is a graphic representation of scan triggering during mechanical acceleration and deceleration, Fig. 7 is a graphic representation of waveforms during mechanical deceleration, and Fig. 8 is a flow chart illustrating high efficiency scanning of a workpiece. Fig. 1 shows a combined block-pictorial representation of an ion beam scanning system 90 with a dose controller which employs a uniformity control computer 120 together with a computer memory 122 and a display terminal or like output element 124. Fig. 1 illustrates an ion source 92 and a deflection element 96 as a single scanning ion beam source 92, 96 connected with a separate deflection voltage waveform generator 96a. A position sensor 116 produces an electrical signal representative of the y position, i.e., the translate position along arrow 114, of the wafer surface 100. This signal is applied to the uniformity control computer and, either directly as indicated, or by way of the computer 120, to the computer memory 122. The illustrated display terminal 124, which is optional in the system, is connected with the computer 120 and with the memory 122 and provides a display of accumulated ion implant dose as a function of translation position (y). With further reference to Fig. 1, an ion beam detector 106 which typically includes a current integrator, produces a signal responsive to integrated beam current and applies it to the control computer 120. In the illustrated arrangement, the computer 120 also receives from the deflector generator 96a a signal representative of the scanning duration time t. The control computer 120 produces an accumulated dosage signal, i.e., corresponding to D = kit. The computer memory 122 can store this signal for each translate position, and the display unit 124 as noted can display it as a function of translate position. The uniformity control computer 120 also produces an adjusted duration signal (t') in accordance with the Equation t′ = t d₀ v dv₀ where v is the measured translate velocity of the target transport in the direction of arrow 114, v₀ is the desired constant translate velocity, d₀ a uniform dose per pass required over the whole target surface and d the actual implant dose. and it applies it to the deflector generator 96a for altering the scan rate to reduce the difference between the actual implant dose and the desired dose at that time in the implant operation. In this manner, the system of Fig. 1 performs dose control operations. With further reference to Fig. 1, the illustrated dose controller also includes a traveling beam detector 124, coupled with a drive and position sensing unit 126 that moves the traveling detector along the beam path 98 at a rate that is slow relative to the scan on the ion beam. The drive and sense unit 126 maintains the traveling detector 124 positioned out of the path 98 and removed from the target transport when the target transport is positioned to move a target object across the scanning path 98. The illustrated drive and position unit 126 moves the traveling detector 124 along the scan path only when the target transport is entirely clear of the scan path, for example, when the target transport is moved to a loading or transport station for the removal of a completed workpiece or for loading a fresh workpiece. Note that the system of FIG. 1 can employ a single ion beam detector both for the traveling detector 126 as well as for the end-of-scan detector 106. However, for clarity, FIG. 1 illustrates a system employing two beam detectors. The traveling detector 124, which typically is a Faraday type detector, detects a signal (i) responsive to the time integral of a number of current pulses, each produced when a scanning beam 102 sweeps across the detector 124 as it slowly moves across the target path 98. The drive and position unit 126, in addition to controlling the movement of the traveling detector 124, develops a signal (x) which identifies the position of the detector 124 along the scan path 98 as a function of time. This position signal thus identifies the location of the traveling detector 124 at essentially each instant, as it slowly moves along the scan path and the signal thus identifies where along the path the detector is measuring beam current, at any given time. The deflector voltage waveform generator 96a receives the current signal (i) from the detector 124 and receives the position signal (x) from the drive and position sensing unit 126. In response to these signals, and typically in conjunction with the uniformity control computer 124, the waveform generator 96a produces a deflection voltage waveform, in response to which the scanning ion beam source 92, 96 scans the ion beam. The deflection generator computes the waveform to produce a beam scan that delivers a uniform level of ion current to each point on the target surface, or another selected spatial distribution of current along the scan path 98, on each fast scan of the ion beam across the path. This computer deflection waveform can be represented by the equation dV/dt = dV/dt₀ * I x /I₀ where I₀ is the desired ideal beam current, I x is the beam current at the point (x) as measured with the traveling detector 124 when the gradient of the deflection waveform is dV/dt₀, and dV/dt is the computed gradient for the waveform required to correct for the difference between I x and I₀. In summary, the dose uniformity control system as illustrated in FIG. 1 can adjust and modify the deflection waveform to control the ion beam scan to attain a uniform or other selected beam current on the target path at each point along the fast scan. The system further can adjust the duration and correspondingly the rate of each fast scan, with adjustment of the duration time (t) to introduce a selected ion dose to a target object with each scan and further selected in accordance with the position of the beam scan on the target object, i.e., along the coordinate direction which is transverse to the ion beam scan direction. The system, moreover, can adjust the relative time (T) when each beam scan commences in correspondence with the transverse scan, i.e., translation, of the target object relative to the scan path. The system can also control the rate of transverse scan, i.e., of translation, and can select the number of times a target object is transversely scanned, i.e., translated past the scan path 98. In this manner the system provides a selected accumulated ion dose at each selected transverse, i.e., translational, position on the target object. To attain these results, the dose controller includes, as described with reference to FIG. 1, a programmable digital computer together with a computer memory and a display terminal. The system also includes devices for sensing the ion beam current, as provided by the illustrated Faraday detectors 106 and 124, together with drive and position sensing elements as illustrated by the drive and position sensor 126 and the position sensor 116, and further includes a deflection waveform generator 96a which operates in conjunction with the scanning ion beam source. It is possible to increase ion beam utilization efficiency in an ion implanter. In the scanning techniques described hereinabove, the ion beam is scanned between a beam stop and Faraday detector 106 in the X direction. In the Y direction, the translate driver 108 translates target wafer 112 from a point completely below the ion beam 102 to a point completely above the ion beam. The direction of translation is then reversed and the target wafer 112 is translated to a position completely below the ion beam. The Y translation dimension is increased to provide sufficient time for the mechanical translate driver 108 to reach full speed before the target wafer 112 begins to intercept the ion beam 102. Thus, the target wafer 112 intercepts the ion beam 102 during only a portion of the scanning cycle. An ion beam utilization efficiency can be defined as the area of the workpiece or target wafer 112, typically a semiconductor wafer, divided by the effective area over which the beam is scanned. In the above described apparatus, the effective scanning area is approximately given by the electrostatic scanning width in the horizontal or X direction multiplied by the amplitude of the mechanical motion in the vertical or Y direction. Methods and apparatus will now be described for reducing the effective area of scanning by reducing the dimensions of the unused scan in both the horizontal and vertical directions, thereby improving beam utilization efficiency. A schematic representation of target wafer 112 and the slot of Faraday detector 106 is shown in FIG. 2. The slot of Faraday detector 106 is spaced from target wafer 112 in the X direction by an amount on the order of 12 to 25 mm due to mechanical constraints, including a clamping ring (not shown) for clamping target wafer 112 in its implant position and the mechanical structure of Faraday detector 106. In order to accurately monitor the ion beam current during implantation, it is necessary to scan the ion beam 102 over Faraday detector 106. In the above-described prior art scanning technique, the ion beam 102 is scanned over Faraday detector 106 once each scan in the X direction, as indicated by dashed waveform 202 in FIG. 2. It has been found that dose accuracy can be maintained and ion beam utilization efficiency can be increased by scanning the ion beam 102 over Faraday detector 106 less frequently than once per normal scanning cycle. In accordance with the scanning technique as shown in full in Fig. 2, the X direction scan includes a fast scan signal 204 which scans the ion beam across the target wafer 112 and the Faraday detector 106, and a slow scan signal 206 which scans the ion beam only across target wafer 112. The scan time for slow scan signal 206 is a known multiple of the scan time for fast scan signal 204. The slow scan signal 206 can be repeated one or more times. Then, fast scan signal 204 is repeated in order to measure the ion beam current. Thus, a larger fraction of a complete scan is spent with the ion beam 102 incident on the target wafer 112. Even though the ion beam current is measured by Faraday detector 106 less frequently, high dose accuracy is maintained. The fast scan is an uncompressed, or normal, scan, and the slow scan is compressed in width. It is necessary, even with the compressed slow scan 206 to scan the ion beam 102 beyond the edges of the target wafer 112 by a predetermined fraction of the ion beam width in order to avoid dose variations near the edges of the target wafer 112. Preferably, the ion beam 102 is scanned beyond the edges of target wafer 112 by approximately one-half the ion beam diameter. The ion beam current measured by Faraday detector 106 is less than the ion beam current applied to target wafer 112, since the ion beam current is measured only during the uncompressed scans. The ion beam current delivered to the target wafer 112 and to the Faraday detector 106 is shown by waveform 210 in FIG. 2. Since the ratio between the fast scan time and the slow scan time is known, the total ion dose delivered to target wafer 112 can be calculated from the current measured during each fast scan. An alternative technique for improving ion beam utilization efficiency is illustrated schematically in FIG. 3. In this embodiment, a first scan signal 214 scans the ion beam across the target wafer 112 and the Faraday detector 106. Then, multiple second scan signals 216 scan the ion beam across target wafer 112 and any required overscan. In this embodiment, the scan rates of first scan signal 214 and second scan signal 216 are the same, and the second scan signal 216 is repeated a predetermined number of times, typically five or more. Yet another technique for increasing beam utilization efficiency is illustrated schematically in FIG. 4. A scan signal 220 includes a slow scan portion 222 when the ion beam 102 is incident on target wafer 112. When the ion beam 102 reaches the edge of target wafer 112, the scan rate is increased to provide a rapid scan portion 224 in which the ion beam is scanned over Faraday detector 106. On the return scan, a slow scan portion 226 is utilized when the ion beam is incident on target wafer 112. Thus, the ion beam 102 spends more time over target wafer 112 and less time scanning over Faraday detector 106 in proportion to the scanning rates. Thus, the beam utilization efficiency is increased. In each of the above high efficiency scanning techniques, it will be understood that the ion beam 102 is scanned beyond the edges of target wafer 112 by a predetermined distance on the order of one half of the beam diameter in order to insure dose accuracy near the edges of target wafer 112. The ratio between the fast scan and slow scan speeds and/or the ratio between the number of uncompressed scans and the number of compressed scans is selected to balance the dose accuracy and the ion beam utilization efficiency. As indicated above, ratios of five or greater are preferred. The scan waveforms illustrated in FIGS. 2 to 4 are typically generated by a conventional integrator having a digital-to-analog converter connected to its input. The DAC supplies appropriate voltages to the integrator to control scan speed and direction. Both the compressed and uncompressed scan signals utilize a rest period between scans, between time, t, and time T. The rest period is required for dose uniformity control during implantation of the wafer 112. The vertical, or Y direction, mechanical scan described above insures that the target wafer 112 is mechanically overscanned above and below the ion beam 102. The mechanical overscan is illustrated in FIG. 5 by dashed lines 230 and 232. The purpose of the overscan is to ensure a constant mechanical speed while the ion beam 102 is scanning target wafer 112. A motor used to implement the translate driver 108 requires an appreciable time to decelerate, stop and then accelerate in the reverse direction. The uncompressed mechanical scan is illustrated by dashed line 234 in FIG. 5. The mechanical scan amplitude is sufficiently large to insure that the ion beam 102 is not applied to the target wafer 112 during acceleration and deceleration. It can be seen that the portions of the mechanical scan in which the beam is not applied to the wafer result in reduced ion beam utilization efficiency. The mechanical scan can be reduced in amplitude as indicated by line 236 in FIG. 5. With the reduced amplitude mechanical scan, the target wafer 112 is being accelerated or decelerated during a portion of the time that the ion beam 102 is being scanned over the target wafer 112. In the prior technique wherein X direction scans are triggered at predetermined time intervals, dose errors would occur in the crosshatched regions 238 and 246 at the top and bottom of the wafer with a reduced amplitude mechanical scan, since the wafer is not moving at a constant speed when these portions of the wafer are being scanned. Dose variations in regions 238 and 246 are prevented, as shown in FIGS. 6 and 7, by triggering the horizontal beam scan only after the wafer 112 has moved in the Y direction by a predetermined constant distance. In FIG. 6 , the mechanical scan speed near the end of a scan is illustrated generally at 248. The target wafer 112 is decelerated during portion 252 from constant speed during scan portion 250 to zero. The target wafer 112 is then accelerated during scan portion 254 in the reverse direction to constant speed during scan portion 256. During portions 250 and 256 when the target wafer is being moved at a constant speed, the horizontal scan is triggered at constant intervals by trigger pulses 260. For a 1 kilohertz uncompressed scan, the trigger pulses 260 occur at 1 millisecond intervals. When a compressed scan utilizing a slow scan 206 as shown in FIG. 2 is utilized, the trigger pulses 260 occur at intervals of 5 milliseconds or more. When the mechanical speed of the vertical motion of the target wafer 112 is varying due to acceleration or deceleration near the ends of the scan, the time between X direction scan trigger pulses is increased so that trigger pulses 262, 264, 266 and 268 occur only after the wafer 112 has moved a predetermined constant distance p = s x dt, where s is the instantaneous speed and dt is the time between trigger pulses. The distance p is the scan pitch, or the distance between adjacent scan lines on the target wafer 112. By adjusting the time dt between trigger pulses 262, 264 etc., the distance p is maintained constant independent of the mechanical speed of the wafer 112 in the vertical direction. During a time 270, when the mechanical speed of the translate driver 108 is near zero, the beam 102 is off the wafer 112, and no trigger pulses are generated. The vertical position of the target wafer 112 is determined by measuring the position of the translate driver 108 by means of position sensor 116 (FIG. 1). The trigger pulses 260, 262, 264, etc. are generated each time the position of the target wafer 112 is incremented by the distance p. Thus, the scan lines are evenly spaced over the target wafer 112, including regions 238 and 246 where the mechanical scan speed is varying, thereby ensuring a constant dose over the entire surface of the wafer. The mechanical scan speed 248, including constant velocity portion 250 and deceleration portion 252, is illustrated in FIG. 7, along with trigger pulses 260, 262 and 264. The corresponding horizontal scan waveforms 270 are illustrated. It can be seen that the scan waveforms 270 are triggered less frequently as the mechanical scan speed decreases. The compressed scan waveforms in the horizontal direction have been omitted from FIG. 7 for simplicity. However, it will be understood that when horizontal scan compression is utilized, the scan waveforms 270 are modified as illustrated in one of FIG. 2, 3 and 4. Techniques for increasing ion beam utilization efficiency in horizontal and vertical directions, have been described hereinabove. It will be understood that these techniques can be utilized either separately or in combination. A flow diagram illustrating combined use of the techniques for increased beam utilization efficiency is illustrated in FIG. 8 in which one complete scan of target wafer 112 is illustrated. The movement in the vertical direction is checked in step 278 to determine if the target wafer 112 has been moved by a distance p. Until the wafer 112 is traversed a distance p, the procedure simply waits 280 until the distance p has been traversed. Then the required horizontal scan is triggered in step 282. When a fast scan and a slow scan are used in an alternating manner as shown in FIG. 2 and described hereinabove, the trigger at step 282 causes a fast scan followed immediately by a slow scan. When multiple compressed scans are utilized as shown in FIG. 3, each trigger produces a complete sequence of uncompressed and compressed scans. The beam current is measured by Faraday detector 106 in step 286 during the uncompressed part of the scan waveform. If the vertical translation is complete, as determined in step 290, one scan of target wafer 112 has been completed. If the scan is not completed, the procedure returns to step 278 and selects an uncompressed or compressed horizontal scan for the next scan line, and the procedure is repeated. The techniques described above for increasing beam utilization efficiency have been described in connection with an ion implanter which utilizes electrostatic scanning in the horizontal direction and mechanical scanning in the vertical direction. It will be understood that these techniques can be utilized in other types of ion implanter.

Claims: A method for ion implantation of a workpiece, comprising the steps of: generating an ion beam; scanning the ion beam across a workpiece in a first direction in response to a scan signal; scanning the workpiece in a second direction relative to the ion beam so that the ion beam is distributed over the workpiece; detecting the ion beam and providing a signal representative of the intensity of the ion beam, said step of detecting the ion beam being performed by a detection means located adjacent to said workpiece; and generating said scan signal including a first scan signal which scans said ion beam across said workpiece and said detection means during a first scan interval and a second scan signal which scans said ion beam across an area not substantially greater in dimension than the maximum dimension of said workpiece along said first direction, said second scan signal being generated during a second scan interval. Ion implantation apparatus comprising: means for generating an ion beam; first scanning means for scanning said ion beam relative to a workpiece in a first direction; second scanning means for scanning said ion beam relative to said workpiece in a second direction so that said ion beam is distributed over said workpiece by said first and second scanning means; and detection means located adjacent to said workpiece for detecting said ion beam and providing a signal representative of the intensity of said ion beam; said first scanning means including means for scanning said ion beam across said workpiece and said detection means during a first scan interval and means for scanning said ion beam across an area not substantially greater in dimension than the maximum dimension of said workpiece along said first direction during a second scan interval. Ion implantation apparatus comprising: means for generating an ion beam; first scanning means for scanning said ion beam relative to a workpiece in a first direction; second scanning means for scanning said ion beam relative to said workpiece in a second direction so that said ion beam is distributed over said workpiece by said first and second scanning means; and detection means located adjacent to said workpiece for detecting said ion beam and providing a signal representative of the intensity of said ion beam; said first scanning means including means for scanning said ion beam across said detection means during a first scan interval and means for scanning said ion beam across an area not substantially greater in dimension than the maximum dimension of said workpiece along said first direction during a second scan interval, said first scan interval being of a different period than said second interval or the ion beam scanning at a different rate in the first interval than in the second interval.

Inventors: BERRIAN DONALD W, KAIM ROBERT E, VANDERPOT JOHN W, BERRIAN, DONALD W., KAIM, ROBERT E., VANDERPOT, JOHN W.
Applicants: VARIAN ASSOCIATES, VARIAN ASSOCIATES, INC.
Application year: 1990
Main classification: H01J37
IPC codes: H01J37, H01L21, C23C14