APPARATUS FOR MAGNETICALLY SCANNING AND/OR SWITCHING A CHARGED-PARTICLE BEAM

Abstract: An apparatus is provided for deflecting a charged particle beam of a time-varying magnetic field emitted by a magnet with a laminated magnetic core and a non-conductive vacuum chamber placed in a gap of the laminated magnetic core. The charged particle beam travels in the vacuum through the time varying magnetic field to switch between beams of different ion species or to scan a single ion species with a controlled velocity across a target. The vacuum chamber includes multiple thin annular sheets of metal or of another conductive material placed within the vacuum chamber to dispose the charged particle beam that passes through holes in each annular sheet without striking the non-conductive vacuum chamber walls. Each annular sheet is electrically connected to each other in a manner that suppresses eddy currents that would otherwise be induced by the time-varying magnetic field. The apparatus can be extended to two dimensions at the cost of added complexity.

Description: APPARATUS FOR MAGNETICALLY SCANNING AND/OR SWITCHING A CHARGED-PARTICLE BEAM Related Application The current application claims priority from Provisional Patent Application S/N 60/231,190 entitled APPARATUS FOR MAGNETICALLY SCANNING AND/OR SWITCHING A CHARGED-PARTICLE BEAM which was filed on September 7, 2000, all naming the same inventors and the same assignee as this application, which is incorporated by reference herein. Background of the Invention This invention concerns apparatus and methods for scanning and switching charged particle beams with a periodic or quasi-periodic waveform, using a magnetic deflector. The use of magnetic deflectors and electrostatic deflectors for ion beams has been known since the Aston Mass Spectrometer. Both magnetic and electrostatic scanners have been used for scanning electron beams in cathode ray tubes, and methods for controlling non-linearities have been developed. The same scanning methods have been used in ion implanters for scanning ion beams across semiconductor wafers using both two-dimensional electrostatic scanning and one-dimensional magnetic scanning in combination with a one-dimensional "slow" translation. Fast switching and scanning magnets must be made of laminated steel, as in a transformer, to minimize losses from eddy currents. In the Proceedings of 1 st conference on radiocarbon dating with accelerators, held at the University of Rochester in 1978, a system was disclosed in which beams of three isotopes of carbon were sequentially switched through an accelerator by switching a potential through which the ions were accelerated upon entering an analyzing magnet, the ions being retarded by an equal amount on leaving the magnet, this is illustrated in Figure la. Figure lb illustrates a known apparatus designed by the General Ionex Corporation in Massachusetts. Figure lb illustrates an apparatus in which beams of three isotopes of carbon were sequentially switched through an accelerator by rapidly changing the field of an analyzing magnet. The magnet was manufactured from laminated steel, which for convenience was cut from commercial transformer laminations, to permit the field to be changed rapidly without incurring eddy current losses. The Thesis of, D. Phil White, Oxford University, England, 1981, disclosed a functional apparatus for implementing this scheme with a slew rate of 33.75 degrees in 25 microseconds, sequentially switching the isotopes 14 C, 13 C and 12 C through an accelerator with steady periods in between, at repetition frequencies of 10 to 50 Hz. However, it was recognized that the ion-optical properties of this system, although similar, were not identical for the three isotopes. Figure 2 illustrates the prior art laminated magnet of US Patent No. 4,276,477 of Enge. Enge discloses ah ion implanter in which an ion beam was slowly scanned across the path of a plurality of targets, which were moved through the beam on a spinning disk. The scanning speed was very slow, around 0.5 Hz. The angles of the scanned trajectories were rendered parallel by a constant shaped-field corrector magnet. The beam passed through an analyzer magnet prior to scanning, and the analyzer magnet was oriented so its dispersive plane was orthogonal to the scanning plane. The resolving aperture was located at a position downstream of the scanner magnet. Because the intended doses were high, and a batch of wafers was implanted (rather than single wafers), the slow scan frequency was acceptable. Figures 3a, 3b and 3c illustrate the prior art system disclosed in US Patent No. 4,922,106 of Berrian et al. Berrian et al discloses an ion implanter for implanting single wafers in which an analyzed beam is scanned electrostatically at a frequency close to 1 kHz is disclosed. Again the angles in the scanned beam are corrected by a constant shaped-field corrector magnet. The scanning waveform was tailored to achieve a uniform implant. Summary of the Invention The present invention addresses the above-identified limitations of conventional systems that magnetically switch or scan or both, a charged particle beam. The present invention provides an approach to enable an ion beam system for use as an ion implanter or a mass spectrometer that quickly and accurately switches between beams of different ion species. The present invention also provides an approach to scan a single ion species quickly at a precisely controlled velocity across a target. In one embodiment of the present invention, an apparatus is provided for deflecting a charged particle beam by means of a time-varying varying magnetic field emitted by a magnet with a laminated magnetic core and a non-conductive vacuum chamber placed in a gap of the laminated magnetic core. The charged particle beam travels in the vacuum through the time varying magnetic field to switch between beams of different ion species or to scan a single ion species with a controlled velocity across a target. The vacuum chamber includes multiple thin annular sheets of metal or of another conductive material placed within the vacuum chamber to dispose the charged particle beam that passes through holes in each annular sheet without striking the non- conductive vacuum chamber walls. Each annular sheet is electrically connected to each other in a manner that suppresses eddy currents that would otherwise be induced by the time-varying magnetic field. Brief Description of the Drawings Figure la illustrates the magnet "bouncer" of General Ionex Corporation of Massachusetts. Figure lb illustrates the laminated magnet of General Ionex Corporation of Massachusetts. Figure 2 illustrates the system disclosed in US Patent No. 4,276,477 of Enge. Figures 3a, 3b and 3c illustrate the disclosed in US Patent No. 4,922,106 of Berrian et al. including a representative scanning waveform of the disclosed ion beam scanning system. Figure 4 illustrates an exemplary scanned implanter of an illustrative embodiment of the present invention. Figure 5 illustrates an exemplary switcher of an illustrative embodiment of the present invention. Figure 6a illustrates an exemplary scanning waveform of an illustrative embodiment of the present invention. Figure 6b illustrates an exemplary switching waveform of an illustrative embodiment of the present invention. Figure 7 illustrates an exemplary tube with apertures of an illustrative embodiment of the present invention. Figure 8a illustrates a first exemplary circuit of an illustrative embodiment of the present invention. Figure 8b illustrates a second exemplary circuit of an illustrative embodiment of the present invention. Figure 9a illustrates an exemplary charging waveform and a steady state waveform of an illustrative embodiment of the present invention. Figure 9b illustrates exemplary waveforms for an additional illustrative embodiment of the present invention. Figure 10 illustrates an exemplary magnet construction for a scanner of an illustrative embodiment of the present invention. Figure 11 illustrates an exemplary magnet construction for a switcher of an illustrative embodiment of the present invention. Detailed Description of the Invention This invention relates to one-dimensional magnetic scanning or switching of a charged particle beam. The invention can be extended to two dimensions at the cost of added complexity. The distinctions between scanning and switching are that a scanned beam is used while its position is changing, whereas a switched beam is used while its position is stationary, and in a scanning application, a single beam species is usually used, whereas in some switching applications it is desired to switch between different beam species, for example between different isotopes in a mass spectrometer. For a scanned beam the velocity with which the beam is translated is critical, whereas for a switched beam it is the accuracy and stability with which position can be maintained between changes in position that is important. The apparatus is essentially the same in each case, and only the waveform is different. The stored magnetic energy U in any ion beam-deflecting magnet is related to the magnetic rigidity of the ion beam, the cross section of the beam, the scan angle θ, and the length of the magnet as follows: r r 1 t n rrJ ^ , 2 . 2MqV d 2 4kMVr 2 & 2 , . 1 λ U = - \B.Hdv « 2kr 2 1. ^- 5 — 5 - = (equation 1) 2 J μ 0 q ~ 1 μ 0 ql where M is the highest ion mass, qV its highest ion energy, q the charge of the ions, r the radius of the ion beam (including clearance for vacuum chamber walls, etc), μ 0 the permittivity of vacuum, 1 the length of the scanner magnet, and k is a numerical factor from the integration covering fringe fields, whose value is 2 to 2.5. This relationship assumes the scan angle is small, and that l<r/θ but reveals the scaling law. The stored energy is related to the inductance, and to the reactive power required to scan the beam. If the scanning current waveform is an approximately triangular function of time, the reactive power required must exceed four times (4x) the peak-stored energy in the magnetic field. The stored energy may be 100 Joules in a compact system. At a frequency of 100 Hz this means a power of 40 kVA; at 2.5 kHz it means 1 MVA. Typically, ion beam switching or scanning magnets are placed within a vacuum, since the changing magnetic field is strongly attenuated by metal vacuum walls. Instead the illustrative embodiment of the present invention uses glass, quartz, or other insulating material as a vacuum wall within the poles, along with disks of conductive material grounded through a relatively high resistance to shield the insulating walls from charged particles. The method of construction keeps the magnetic losses low and the construction inexpensive. The innovation here is laminated beam baffles. In addition, a low inductance is desired, which is proportional to the square of the peak flux, while beam deflection is proportional only to the flux. Hence, a low flux density is economic. The construction of the magnet is optimized if laminated steel separated by laminated dielectric sheets are used. For a field of 0.3 Tesla, 0.014" laminations of steel separated by 0.040" of dielectric may be used. The power supply must deliver a voltage V=LdI/dt +IR. The reactive power requirement may be many tens of kVA. We address only those applications where dl/dt varies by only a few percent from a nominal value, or else is zero within a few percent. Under these conditions we can separate the power supplies required to meet the real (V=IR) and reactive (V+Ldl/dt) parts of the load voltage requirement. The real part is supplied by a power amplifier. The reactive part is provided by a capacitor charged to the required voltage (V = | dl/dt | ) which is either disconnected from the load or connected in either polarity in series with the load by a commutator. It may be necessary to provide a low-power high voltage supply to maintain the correct charge on this capacitor, as discussed in detail below. Fig. 4 illustrates a beam scanning apparatus 20, which happens to be bipolar (since this reduces the total energy), while Fig. 5 illustrates a beam switching apparatus 22, which is monopolar. The beam switching apparatus 22 is illustrated in Figure 11 in more detail. The corrector magnet 10 depicted in Fig. 4 can be shaped to control the relationship between deflection angle in a scanner magnet and beam position in an object plane, such as an implant location. It is possible to simultaneously correct the trajectories to be parallel and to achieve a linear relationship between scan angle and position. However, it is often necessary to minimize the need for a non-linear scan waveform by the correct design of the shape of the corrector magnet 10, but nonetheless to retain the capability of programming the scanner with a non-linear waveform so that correction of residual errors or unforeseen systematic errors is possible. For more detail involving the operation and construction of the corrector magnet 10 we hereby incorporate by reference US Patent No. 5,834,786 of White et al. and US Patent No. 5,350,926 of White et al. The scanning waveform is generated by a computer or sequencer through a digital-to-analog converter (DAC). The scanning waveform is related to the desired position of the beam as a function of time at a reference plane. Historically, in dosing applications, the average current at different locations has been measured, and the scan waveform his been iteratively corrected until this is uniform. Advantageously, the illustrative embodiment of the present invention provides a direct method of setting up the desired waveform, which is described below in more detail. First, we note that the relationship between deflection angle and beam position will not be perfectly linear. If no optics intervene between scanner and reference plane, then the beam position is related to tanθ, where θ is the deflection angle. Then we note that the deflection angle may not be perfectly linearly related to the field in the magnet. Finally we note that the placement of a "corrector" magnet 10, or other optics (not shown) between the scanner and the reference plane will tend to introduce even-ordered aberrations. In summary the relationship between beam position, x, in the reference plane and the current, I, in the scanner is of the form I = a υ + a x + a 2 x 2 + a 3 x 3 + a x A + a 5 x 5 +... (equation 2) where at least the coefficients through third-order should be non-zero. Note that the dose rate from a scanned beam is inversely proportional to dx/dt, hence to first-order is proportional to V/L, and that it is imperative that V be almost constant and smoothly varying during periods when uniform doping is desired. The commutator must not switch until the beam is scanned beyond the implanted target. As Figure 4 illustrates, an aperture plate 12 is placed in the path of the beam at the reference plane. Apertures are placed at known values of x. The scanner power supply is operated to determine the current I required to place the beam centroid over each of the apertures, by maximizing the transmitted current. This can be done using a scanned beam and fast automated data acquisition, or a dc beam and manual data acquisition. Provided the number of apertures equals or exceeds the number of coefficients to be determined, we can solve the equation for the coefficients. If we desire that the scan position varies uniformly in time, we can create a table of scan digital to analog converter (DAC) values at uniform increments in x by substituting in equation 2. The magnet is then scanned by sequentially sending each value in the table to the DAC at equal time intervals. If it is desired that the relationship between x and time is other than linear, we simply substitute the desired x-position at each linear time increment into equation 2 to calculate the DAC values. The required non-linear scan profile with then be generated. To generate a quasi-triangular waveform, the values in the scan table are programmed sequentially, and on reaching the end point, the sequence is reversed. In the implant plane, an aperture plate 12 can be placed in the path of the beam. The aperture plate 50 has a hole at a reference position at its center, and additional holes at 25.0mm increments on either side, in the scanning direction. Behind its aperture plate 12 is a Faraday cup 14, which is sized to accept 100% of the beam that is scanned with a +/- 110mm path. In operation, an ion beam is passed through the system 20 with the scanner magnet 74 programmed to zero, and the corrector magnet 10 is adjusted from zero until the beam is centered on the central aperture, as determined by maximizing the beam current on the fifth major peak encountered (the others having been caused by the beam traversing the first four holes). If the scanner magnet 74 is now energized by programming triangular waveform (whose amplitude in amps can be estimated from the setting of the corrector magnet in direct proportion), there is a brief period during which the capacitor 36 of the driver circuit 50 which is discussed below in more detail, charges to a quiescent value, after which the beam is stably scanned across all nine apertures in the aperture plate 12. The capacitor 36 may charge to a voltage, which is either too high or too low, if the phase relationship is incorrect. The "correct" voltage is one that causes the excursion of the amplifier 40 to be minimized, and it is therefore possible to create an error signal based upon the average amplifier voltage or to current or both which are used to adjust the phase of the commutator 36, either as a dynamic readback adjustment or as a preset adjustment. A shift of 1 degree in phase can be sufficient to prevent correct operation of the driver circuit 50. It is necessary to measure the exact programming voltage V pr0 g to the scanner power supply associated with maximum transmission through each of the nine apertures in the aperture plate 12. This measurement must be performed to a precision of about 0.1 amps, and can be accomplished automatically as the beam is scanned, or manually using a dc current to the scanner magnet 74. TABLE 1 As the illustrative table 1 indicates, one beam pass is designed to occupy 10 mS. The return pass would be a reverse of the table. The scanning frequency would thus be 50 Hz, and the total scan amplitude 250mm, which would cleanly scan a beam of up to 50mm diameter off the edge of a 200mm wafer. The actual table used to drive a DAC to drive the scanner requires many more points. If the sampling frequency is 20 kHz, for example, the table requires 201 points. These points must lie along a smoothly varying curve, and linear interpolation of the points in the measured table is insufficiently smooth, so the data should be fitted to a polynomial: Vpmg = a o + a ιt + a 2^ 2 + a 3 f3 + • (equation 3) from which the full table can be evaluated by substitution. Once the table is generated, the scan waveform is programmed by indexing up and down the table in equal time increments of 100 microseconds, and programming the scan amplifier 40 with the resulting values of V pr0 g. Implants are performed by traversing wafers at uniform velocity through the scanned beam. Typically five terms retained in equation 3 should be sufficient to cover known aberrations of sufficient magnitude to adversely affect the uniformity of the dose rate achieved by the scan system although fewer terms or additional terms can be utilized. To provide smooth scanning, a much larger table with finer time increments can be created. In practice a table of up to 500 points at time increments of 20 microseconds is considered sufficient. The waveform can be modified slightly at the turning points to splice in a smooth curve. This results in a waveform which can be followed with greater fidelity. For most applications, it is desired to hold the scanner current at a constant value for part of the time. This may be at asynchronous or synchronous moments. For example, in isotope switching applications, the desired scanned waveform can be as shown in Fig. 6a, whereas in ion implantation applications such as those described in US Patent No. 4,922,106, the desired switched waveform can be as shown in Fig. 6b. When the slew rate is desired to be high, the capacitor 36 of the driver circuit 50 or 60, which are described below in more detail, is switched in series with the scan amplifier 40 in the appropriate polarity. When it is desired to be low or zero, the capacitor is switched out and the terminals shorted (SI and S2 both in position A). In this mode, the "DC'mode, the magnet current can be changed slowly to any desired value. The driver circuit 50 illustrated in Fig. 8a operates in the following manner. The inductance L is the scanner magnet 30. The losses of the scanner magnet 30 are lumped with all other losses, including the sense resistor 32 (R S) , into a resistor 34 (R). If the laminated magnet is well-designed, most of the losses are ohmic, so R is a close representation of the DC resistance of the circuit. The amplifier 40 has an output V am p , which has a maximum value V ma χ and minimum value V mm . The storage capacitor 36 in the bridge 38 has a capacitance C that should exceed L/f", where f is the operating frequency. The amplifier 40 delivers a current I proportional to a programming signal Vp ro g unless there is insufficient voltage drive to overcome the reactance of the scanner magnet 30. To compensate for non-linearity in the behavior of the laminated steel core, i.e. between the current I and the total flux, Φ, a sense coil (not shown) could be wound on the inductor, and its output could be integrated and combined with the signal from the dc current sense resistor 32 for use as a feedback signal in the amplifier 40. In this manner, the amplifier 40 operates to control the rate of change of flux in the scanner magnet 30. However, the amplifier 40 is incapable of producing a sufficient output voltage to support the required dl/dt, as discussed above without going into saturation. The commutator 36 has three states, characterized by the symbol K ∞m , which can take the value +1, 0 or -1. K=+l represents the state where switch SI is in position B, and switch S2 is in position A. K=0 represents the state where SI and S2 are both in position A. K=-l represents the state where SI is in position A and S2 is in position B. The switches may be solid-state half-bridge elements utilizing insulated gate bipolar transistors (IGBT). The switches SI and S2 need to be break-before-make switches with overvoltage protection and freewheeling bypass diodes that operate in a time interval of a few microseconds maximum. Typically, switches SI and S2 are configured to be a pair of IGBT's or alternatively a pair of field effect transistors (FET). The electromagnetic field (EMF), E, in the driver circuit 50, is the algebraic sum of K com *Q*C-IR+V amp , and this can be equated to Ldl/dt. The programming signal for the current is electronically differentiated to give a signal V i t r- Assuming V dHf to run from -5 to +5V, the commutation signal can be generated from it by defining: K = +1 for V diff >0.5V, K=0 for -0.5<V diff <0.5V, and K=- 1 for V_i ff <-0.5V. This satisfactory provides V m aχ>0.1*L*(dI/dt) max in a system where the expected range of amplitude does not exceed a factor of ~5. The phase of the signal so derived needs to have a provision for fine adjustment by +/- 1 degree in order to ensure that the net current to the capacitor C is zero on average when it has the optimum voltage. Fig. 9a graphically depicts the results when the driver circuit 50 is started from a state where all voltages and currents are zero. With a frequency of about 50 Hz, the waveform, a pure triangular wave, the inductance is 16 mH, the capacitance is 5,000μF, and V max is 75 V. For the first eight cycles the output current is way below the desired value, and the amplifier 40 saturates while trying to deliver the required voltage. But in the process, charge is injected into the capacitor 38, whose voltage increases so that by the end of the 10 cycle the amplifier 40 is substantially out of saturation and the error in current is small, recovering to be within 0.5% of the setpoint for 90% of the cycle. If the amplitude of the programming signal, or more particularly the value of dl/dt, should be reduced, a lower voltage is required on the capacitor 38 and the driver circuit 50 functions in an analogous fashion to reduce the voltage over ten or so cycles. The energy must be given up to the amplifier 40, which must be provided with energy- absorbing capabilities. For various reasons, this is likely to be a switching pulse width modulation (PWM) amplifier such as those manufactured for driving large DC motors, and having a useful bandwidth above 1kHz. Typically, these amplifiers are efficient, and have suitable energy-shedding capabilities. Reactive power levels are tens of kVA. When the apparatus is used to uniformly scan a beam across a workpiece in order to dope it, a more-or-less triangular wave is used. Commutation occurs at the turning points, but not while the beam is traversing the region where doping is to be uniform. Again, note that the dose rate from a scanned beam is inversely proportional to dx/dt, hence to first-order is inversely proportional to V/L, and that it is imperative that V be almost constant and smoothly varying during periods when uniform doping is desired. Note, however, that if the amplifier 40 is a pulse width modulated amplifier, or under other circumstances described blow, there may be switching noise superimposed on this waveform, requiring a high-order filter to be used to smooth V before judging its shape. The average voltage on the capacitor 36 remains constant at a desired value if the average current is zero. Because of the use of the commutator 36 this condition is not necessarily met if the average current in the inductor is zero. Also, it is possible to meet this condition even when the average current in the inductor is non-zero. This is determined by the timing of the commutation with respect to the state of the amplifier 46, and the precise timing with respect to the turning points when dl/dt reverses has a large effect on whether the capacitor 36 receives a net charging current or a net discharging current. It will be appreciated that the amplifier 40 must supply the power dissipated in the ohmic and other losses in the system. An alternative approach, dispenses with the amplifier 40 and utilizes a higher voltage DC power supply of equivalent power rating, which maintains the charge on the capacitor 36 directly. In this alternative approach, the DC voltage exceeds the highest voltage required to achieve the desired rate of current change. Since there is now no low- voltage amplifier 40 to correct errors in the current waveform, errors in its current waveform must be corrected in an alternative manner. The commutator 36 can be operated in a pulse- width-modulated mode at a high frequency, typically in the range of 10 to 30 kHz. Such systems are commercially available, and use a pulse-width-modulated amplifier switching at 20 kHz. A suitable system is manufactured by Glentek Incorporated of El Segundo, California. However, since these systems are primarily designed to deliver large amounts of real power to electric motors, they are not optimized for this application. One of the limitations of such a system is that the bandwidth is automatically limited. Under the Nyquist theorem, the bandwidth of a 20 kHz pwm amplifier cannot exceed 10 kHz. In practice it is limited to about 5 kHz by stability criteria. This limits the fidelity with which a 100 Hz waveform can be followed. A second limitation is that as the varying load current transitions through zero, the emf driving dl/dt falls by the forward voltage drop in the IGBTs that are performing the pwm switching. As a result, dl/dt also undergoes a change. The feedback loop controlling the pulse width modulator cannot respond within 200 microseconds, as mentioned above, resulting in a significant and asymmetric crossover distortion. As Figure 8b illustrates, the system need not operate at a fixed switching frequency. Instead, when the error between the desired and the actual current in the inductive load exceeds a certain threshold, the commutator 37 is switched on in the appropriate direction for an undetermined amount of time until the error is reduced below a lower threshold, such as zero. The error signal described above is used to switch the commutator 36 into the K COm = +1, 0 or -1 modes in such a manner as to reduce the error, whenever the error exceeds a certain preset threshold. Referring to Figure 8b, comparators 51 and 52 will change their output to a logic "1" level if the positive input value exceeds the negative input value by an amount DV, and will change from a logic "1" level to a logic "0" level if the negative input value exceeds the positive input value by an amount DV. Thus comparators 51 and 52 each have a total hysteresis of 2DV. Threshold Tl may be set to -DV, and threshold T2 may be set to + DV. The circuit 60 functions in the following manner. If the coil current through the coil 62 is less the desired value, the error signal from the current comparator 53 will be positive with respect to the ground mode 100. The ratio of the error signal to the error in coil current is determined by the values of resistors Rs, Rl and R2. If the error signal is positive by more than an amount 2DV, then comparator 51 switches the commutator 37 into state K COm = +1, by placing switch SI in state B. This applies the voltage across capacitor 35 so as to increase the current in the coil L in the positive direction. When the error falls to zero, the commutator 37 is again placed in state 0 by returning SI to A. Conversely, if the error signal from the current comparator 53 falls below the threshold T2, comparator 52 places switch S2 in state B, which places the commutator 37 instate K COm = -1, which tends to drive the current negative, again until the error is reduced in magnitude to zero. One skilled in the art will recognize that additional components may be added to ensure clean switching without oscillation, and that the selection of thresholds may be slightly different in each application. In this mode the frequency of switching is proportional to a) the error between the desired dl/dt and the instantaneous value obtained when the commutator 37 is conducting, and b) the reciprocal of the hysteresis in comparators 51 and 52. Nevertheless, those of ordinary skill in the art will recognize that issues of isolation and grounding need to be on a per application basis. To illustrate the above described operation, if DV = 0.5V, Rs = 0.01 ohms, and the desired current varies from +1 OOA to - 1 OOA at frequencies between 10 and 100 Hz, with a required precision of +/- 0.1 A, then comparator 53 requires a gain of -1000, which can be achieved by setting Rl = 100 ohms, and R2 = 100 kohms. The gain of the comparator 53 can be such that the maximum tolerable error in current generates an error signal of magnitude 0.5 V at the output of the comparator 53. The highest switching frequency is estimated to be 100/0.1 * 100 Hz = 100 kHz. However, the switching frequency is much lower if the waveform frequency or the peak current is reduced. Typically, the commercially available pwm amplifier is limited to a frequency response of ~ 5 kHz by the combination of theNyquist criterion and feedback loop stability, whereas the circuit 60 responds in a time interval on the order of five microseconds, which happen to be the minimum on and off times of the commutator switches 51 and 52. Under normal conditions, the circuit 60 switches at a far lower frequency because dl/dt is lower. However, the maximum error in I would not be exceeded. Furthermore there is no clock frequency, so no possibility of beat frequencies or alias frequencies can cause unexpected variations in dl/dt exists. A further refinement of the circuit 60 is to adjust the magnitude of both the hysteresis and the voltage on the capacitor 36 downward when the waveform frequency is reduced, so as to keep the commutation frequency relatively high and improve the fidelity of the output current waveform. Figure 9b graphically illustrates a comparison of the desired and resultant waveforms from the driver circuit 60. Preferred embodiment 1 for an ion implanter. The following description is for equipment capable of scanning a 1 MeV 123 Sb + beam across a 200 mm wafer, or any other beam of similar or lower magnetic rigidity. The beam is assumed to have a radius of 22mm at the scanner magnet 74. As Figure 7 illustrates, a set of annular disks 70 of tantalum surround the beam as it passes through a quartz tube 72 through the scanner magnet 74. The annular disks 70 prevent the beam from striking the interior walls of the quartz tube 72, thereby preventing forward sputtering by the ion beam of contaminants, and they provide a reference potential in , spite of the presence of quartz walls. The annular disks 70 are connected as shown in Figure 7 to prevent the creation of conductive loops, which would dissipate power as a result of the changing magnetic flux. The overall radius of the quartz tube 72 is about 35 mm. The scan angle desired is H-/-2.5 degrees. Substituting these values into equation 1 gives a stored energy of 45 J multiplied by a geometry factor k. Detailed modeling has shown that k has a value in this instance of approximately 2, and the stored energy is -85J. The scanner magnet 74 is constructed from a stack of 8" thick E-shaped transformer laminations 76 and 78, each 0.014" thick, from which a piece has been cut. The laminations 76 and 78 are 12" x 8" with 2" slots for the coil windings 80 and 82. Figure 10 illustrates a cross-section of the scanner magnet 74. The pole gap 84 is about 70 mm. The two coil windings 80 and 82 each consist of about seventy turns of square #4 wire. The coil windings 80 and 82 are water-cooled by means of a non-shorted 3/16" diameter copper tube passing cooling water that art built into the coil structure. Inductance is 16 mH and the operating maximum current is about over 100A. The power amplifier 40 consists of a bipolar amplifier sold commercially for the purpose of driving a dc electric motor, and rated at up to +/-75V and 100A. The capacitor 36 is switched by commercial half-bridge IGBT-based switches, with logic to prevent short-circuiting. The capacitor can charge to ~ 400V during operation. Overvoltage protection is provided built-in reverse-voltage clamps that keep the maximum IGBT voltage a few volts over the capacitor voltage by virtue of the bridge connections. Scan correction is accomplished by a 35 degree IT magnet with entrance and exit pole angles of 40 and -23 degrees, optimized by the computer programs TRANSPORT and TOSCA. The following coordinate definitions, typical of such applications, are used: x, y and z are beam-centered curvilinear coordinates, where the beam is traveling in the positive z-direction. The magnets deflect the beam in the x-z plane, i.e. the dominant magnetic field component is in the y-direction (positive or negative). The angles x' and y' are angles to the reference trajectory in the x-z and y-z planes. The scanner varies x' between -2.5 and +2.5 degrees. The suffixes 1 and 2 refer to input and output conditions, specifically to the centroid of the scanner field and the implant location respectively. The first-order condition is that x' /x'ι = 0 . for a parallel scanned output beam. The x' 2 /x'ι 2 aberration has been reduced to an insignificant level to control scan linearity, and the coefficients x' 2 /x' \ and x' 2 /y ' have been reduced to a sufficiently low level to keep the parallelism of the beam within +/-0.5 degrees. These refinements are accomplished by means of the curvatures of the entrance and exit pole faces, and by controlling the non-uniformity of the field, as is well known to practitioners of the art. In the implant plane, an aperture 50 can be placed in the path of the beam. It has a hole at a reference position at its center, and additional holes at 25.0mm increments on either side, in the scanning direction. Behind this aperture array is a Faraday cup which is sized to accept 100% of the beam which is scanned with a +/-110mm path. The ion beam is passed through the system with the scanning magnet programmed to zero, and the corrector magnet is adjusted from zero until the beam is centered on the central aperture, as determined by maximizing the beam current on the fifth major peak encountered (the others having been caused by the beam traversing the first four holes). If the scanner is now energized by programming triangular waveform (whose amplitude in amps can be estimated from the setting of the corrector magnet in direct proportion), there will be a brief period during which the capacitor charges to a quiescent value, after which the beam is stable scanned across all nine apertures in the plate. The capacitor may charge to a voltage which is either too high or too low if the phase relationship is incorrect. The "correct" voltage is one which causes the excursion of the amplifier to be minimized, and it is therefore possible to create an error signal based upon the average amplifier voltage and/or current which is used to adjust the phase of the commutation, either as a dynamic readback adjustment or as a preset adjustment. A shift of 1 degree in phase can be sufficient to prevent correct operation of the circuit. It is necessary to measure the exact programming voltage V pr0 g to the scanner power supply associated with maximum transmission through each of the nine apertures in the plate. This measurement must be performed to a precision of <~0.1 amp, and can be accomplished automatically as the beam is scanned, or manually using a dc current to the scan magnet. A table, of which the following is illustrative is constructed: TABLE 2 According to Table 2, one beam pass is designed to occupy 10 mS. The return pass would be a reverse of the table. The scanning frequency would thus be 50 Hz, and the total scan amplitude 250mm, which would cleanly scan a beam of up to 50mm diameter off the edge of a 200mm wafer. The actual table used to drive a DAC to drive the scanner requires many more points. If the sampling frequency is 20 kHz, for example, the table requires 201 points. These points must lie along a smoothly varying curve, and linear interpolation of the points in the measured table is insufficiently smooth, so the data should be fitted to a polynomial: V prog = a 0 + "I + a 2 f2 + a + ■ (equation 3) from which the full table can be evaluated by substitution. Once the full table is generated, the scan waveform is programmed by indexing up and down the table in equal time increments of 100 microseconds, and programming the scan amplifier with the resulting values of V prog . Implants are performed by traversing wafers at uniform velocity through the scanned beam. Experience should determine the number of terms to be retained in equation 3, but 5 terms should be sufficient to cover known aberrations of sufficient magnitude to adversely affect the uniformity of the dose rate achieved by the scan system. To provide smooth scanning, a much larger table with finer time increments can be created. We have used a table of 500 points at time increments of 20 microseconds. The waveform can be modified slightly at the turning points to splice in a smooth curve. This results in a waveform which can be followed with greater fidelity. The commutator consists of a bridge of four FETs or four IGBTs. In general IGBTs handle higher power levels than FETs; other devices may be substituted as they become available. IGBTs are available in modules of two, in a package which contains the driving logic. Therefore Bridge 30 in figure 8a or 8b can comprise two such IGBT packages, one represented by switch SI, and one represented by switch S2. In a practical embodiment, ancillary components such as isolated power supplies, and opto- isolated signals, must be provided. Suitable electronic devices for switching high voltages and currents are constantly being invented, and any new embodiments of the bridge structure shown are intended to fall within the scope of this invention. Preferred embodiment 2 for an ion implanter The same beam and scanner apparatus is used as for the first embodiment, but an alternative power supply is used as illustration Figure 8b. The source of power in this embodiment is a high voltage power supply of 300 to 600V, which need not be well regulated but is attached to a large capacitance of 1000 microfarads. For convenience this is full- wave rectified three-phase power. The combination is placed within the commutator 37. A sense resistor Rs produces a signal proportional to the current in the scanner magnet 74. A comparator 53 compares this current with the desired current programming signal. Those skilled in the art will recognize that the comparison can also be performed in a digital manner. When the error exceeds a threshold for a certain time (e.g. 0.1 A for 2 microseconds) one of the commutator switches 51 or 52 is operated so as to apply the voltage on the capacitor 35 to the coil L in the direction which would reduce the error. The commutator 37 is switched off when the error falls below a second threshold for a certain time (e.g. 0.00A for 1 microsecond). There is a defined deadband and a minimum deadtime between changes in state of the commutator 37. The deadband and deadtime are related to the frequency of switching, and also affect the amount of heating of the power devices in the commutator 37. The maximum required frequency is about 100 kHz. Preferred embodiment 3 for a beam switcher Fig. 11 illustrates the construction of an illustrative switching magnet 86. Construction of the switching magnet 86 is in the same E- core steel lamination 76 and 78 of the scanner magnet 74, but the center leg is trimmed at a slight angle, to create a slight gradient in the magnetic field. Each lamination 88 and 89 are clamped and glued to lie on an arc whose radius approximates the intended trajectory of a central ion of a selected species, at 300mm. The switching magnet 86 is designed to bend a selected species through 90 degrees, and to provide simultaneous focussing in x- and y- coordinates, as is inclined at 45 degrees to the beam axis, and this requires a field index n of approximately -0.29, where n is defined to be n=r/B dB/dr, where r=300mm is the radius of the trajectory, and B is the magnetic field strength seen by the reference trajectory. In operation, it is desired to switch rapidly between certain magnetic field settings, for example between those for 20 keV beams of 12 C " , 13 C " , and 14 C " , which would require fields of 0.23, 0.24, and 0.250 T respectively. Switching time is typically around 5 msec. Each beam is identically focussed when its respective field is produced. Systematic differences in the ion-optical conditions encountered by each beam consist only of variations in the stability of the field at each value. This depends on timing of the commutation to ensure that the current is within required limits at the moment the capacitor 36 is removed from the circuit 50. That is, the capacitor 36 is in use only during transitions from one deflection to the next. The power amplifier 40 is in this instance a standard commercial programmable DC switching power supply, to which over- and under- voltage protection devices have been added. Other variants of the invention should be apparent to those skilled in the art. For example, the optics of the magnets may be modified to achieve different beam position/divergence objectives, or obtain the same objectives by other pole profiles than those described herein. The invention may also be applied to higher and lower magnetic rigidity beams, and to applications requiring non-uniform beam scanning patterns, for example spinning disk batch processors.

Claims: Claims: 1. An apparatus to deflect a charged particle beam by means of a time-varying magnetic field said apparatus comprising: a magnet with a laminated magnetic core to emit said time-varying magnetic field; and a non-conductive vacuum chamber within said laminated magnetic core of said magnet, wherein said charged particle beam travels through said non-conductive vacuum chamber to deflect said charged particle beam by means of said time-varying magnetic field. 2. The apparatus of claim 1 , further comprising, a plurality of annular conductive sheets within said non-conductive vacuum chamber, wherein said plurality of annular conductive sheets are arranged to allow said charged particle beam to pass through one or more openings in said plurality of annular conductive sheets to prevent said charged particle beam from striking said non-conductive vacuum chamber. 3. The apparatus of claim 2, wherein said plurality of annular conductive sheets within said non-conductive vacuum chamber are electrically coupled in a manner to avoid eddy currents induced by the time-varying magnetic field. 4. An apparatus for generating a repetitive time- varying alternating current waveform to generate a time-varying magnetic field having a peak value in a magnetic core of a magnet, wherein said repetitive time-varying alternating current waveform alternates from a first constant value to a second constant value, the apparatus comprising: one or more coils coupled around said magnetic core, wherein each of said one or more coils have a first and second terminal for the application of an energizing power; a power amplifier capable of delivering excess electrical power of a desired polarity to each of said one or more coils to sustain a continuous magnetic field by overcoming ohmic losses in each of said one or more coils, said continuous magnetic field having a value equal to or exceeding the peak value in the desired alternating magnetic field waveform; and a capacitor having a positive and negative terminal which can be connected by switching means in series with said power amplifier and each of said one or more coils, the capacitor being large enough that the reactance of the capacitor is less than the inductive reactance of each of said one or more coils at a required frequency of operation, and the switching means being capable of being operated in three modes, named mode 1, mode 0, mode -1, where mode 1 is to connect the capacitor in series with its positive terminal connected to one terminal of each of said one or more coils and its negative terminal to one terminal of the power amplifier, mode 0 is to connect each of said one or more coils terminal directly to said power amplifier terminal omitting the capacitor, which is left open-circuit, and where mode —1 is to connect the capacitor in series with its negative terminal connected to said terminal of each of said one or more coils and its positive terminal to said terminal of the amplifier. 5. The apparatus of claim 4, wherein said amplifier is programmed by a voltage waveform proportional to the desired current in said coil. 6. The apparatus of claim 4, wherein said amplifier is programmed by a voltage waveform proportional to the desired magnetic flux in said magnetic core.

Inventors: BELL EDWARD, HARVEY PHILIP, WHITE NICHOLAS, BELL, EDWARD, HARVEY, PHILIP, WHITE, NICHOLAS
Applicants: DIAMOND SEMICONDUCTOR GROUP, DIAMOND SEMICONDUCTOR GROUP, INC.
Application year: 2001
Main classification: H01J37
IPC codes: H01J37, H01J49, H01L21