METHOD AND APPARATUS FOR ELIMINATING DISPLACEMENT CURRENT FROM CURRENT MEASUREMENTS IN A PLASMA PROCESSING SYSTEM

Abstract: In a plasma processing system, displacement current is removed from a current measurement by providing a secondary capacitor connected in parallel with a primary capacitor defined by an anode and a cathode spaced from one another within the plasma doping chamber. The secondary capacitor is chosen or adjusted to have a same or nearly the same capacitance as the primary capacitor. When a voltage pulse is applied to both the primary and secondary capacitors, the respective currents through the capacitors are measured. In order to remove the effects of the displacement current that is present at the leading and falling edges of the voltage pulse, and in order to measure the ion current within the doping chamber, the secondary current through the secondary capacitor is subtracted from the primary current through the primary capacitor.

Description: METHOD AND APPARATUS FOR ELIMINATING DISPLACEMENT CURRENT FROM CURRENT MEASUREMENTS IN A PLASMA PROCESSING SYSTEM Field of the Invention This invention relates to plasma doping systems used for ion implantation of workpieces and, more particularly, to methods and apparatus for eliminating the displacement cunent from current measurements in plasma processing systems. Background of the Invention Ion implantation is a standard technique for introducing conductivity-altering impurities into semiconductor wafers. In a conventional ion implantation system, 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. Exacting requirements are placed on semiconductor fabrication processes involving ion implantation with respect to the cumulative ion dose implanted into the wafer, implant depth, dose uniformity across the wafer surface, surface damage and undesirable contamination. The implanted dose and depth determine the electrical activity of the implanted region, while dose uniformity is required to ensure that all devices on the semiconductor wafer have operating characteristics within specified limits. Excessive surface damage, particularly chemical etch, or contamination of the surface can destroy previously fabricated structures on the wafer. In some applications, it is necessary to form shallow junctions in a semiconductor wafer, where the impurity material is confined to a region near the surface of the wafer. In these applications, the high-energy acceleration and the related beam forming hardware of conventional ion implanters are unnecessary. Accordingly, it has been proposed to use Plasma Doping (PLAD) systems for forming shallow junctions in semiconductor wafers. In a plasma doping system, a semiconductor wafer is placed on a conductive platen located in a chamber, and the platen functions as a cathode. An ionizable gas containing the desired dopant material is introduced into the chamber, and a high voltage pulse is applied between the platen and the anode (or the chamber walls) causing the formation of a plasma having a plasma sheath in the vicinity of the wafer. The applied voltage causes ions in the plasma to cross the plasma sheath and to be implanted into the wafer. The depth of implantation is related to the voltage applied between the wafer and the anode. A plasma doping system is described in U.S. Patent No. 5,354,381 issued October 1 1, 1994 to Sheng. In the plasma doping system described above, the high voltage pulse generates the plasma and accelerates positive ions from the plasma toward the wafer. In other types of plasma implantation systems, known as Plasma-Source Ion Implantation, PSII, systems, a separate plasma source is used to provide a continuous plasma. (These implantation systems are also known by several other acronyms, the most common being Plasma-Immersion Ion Implantation, PHI.) In such systems, the platen and the wafer are immersed in this continuous plasma and. at intervals, a high voltage pulse is applied between the platen and the anode, causing positive ions in the plasma to be accelerated toward the wafer. Such a system is described in U.S. Patent No. 4,764,394, issued August 16. 1988 to Conrad. Both PLAD and PSII systems require accurate dose measurement to achieve high- quality semiconductor devices. One approach to dose measurement in plasma doping systems involves measurement of the cunent delivered to the workpiece by the high voltage pulses, as described in the aforementioned Patent No. 5,354,381. However, this approach is subject to inaccuracies. The measured current includes electrons generated during ion implantation and excludes neutral molecules that are implanted into the workpiece, even though these neutral molecules contribute to the total dose. Furthermore, since the measured cunent passes through the wafer being implanted, it is dependent on the characteristics of the wafer, which may produce enors in the measured current. Those characteristics include emissivity. local charging, gas emission from photoresist on the wafer, etc. Thus, different wafers may give different measured cunents for the same ion dose. A technique for dosimetry on a plasma based doping system is described by E. Jones et al. in IEEE Transactions on Plasma Science, Vol. 25, No. 1. Feb. 1997, pp. 42-52. Measurements of implant cunent and implant voltage are used to determine an implant profile for a single implant pulse. The implant profile for a single pulse is used to project the final implant profile and total implanted dose. This approach is also subject to inaccuracies, due in part to the fact that it depends on power supply and gas control stability - _> - to ensure repeatability. Furthermore, the empirical approach is time consuming and expensive. In comparison, typically in known beamline implantation systems, cumulative ion dose and uniformity are measured with Faraday cups, or a Faraday cage. The Faraday cage is typically a conductive enclosure, often with the wafer positioned at the downstream end of the enclosure and constituting part of the Faraday system. The ion beam passes through the Faraday cage to the wafer and produces an electrical cunent. The Faraday cunent is supplied to an electronic dose processor, which integrates the cunent with respect to time to determine the total ion dosage. The dose processor may be part of a feedback loop that is used to control the ion implanter. Dose and dose uniformity have also been measured in conventional beamline ion implantation systems using a corner cup arrangement as disclosed in U.S. Patent No. 4,751,393 issued June 14. 1988 to Corey, Jr. et al. As discussed in the '393 patent, a mask having a central opening is positioned in the path of the ion beam. The beam is scanned over the area of the mask with the portion passing through the central opening impinging on the wafer. Small Faraday cups located at the edge of the mask measure the beam current at these locations. There is a need for accurate measurements of the ion current being delivered to a target in a PLAD or PSII system. The ion cunent must be properly measured, so that proper process conditions can be maintained. Regardless of the method of measuring cunent applied to the target, the measured current pulse includes capacitive. or displacement, cunent components that introduce enors in the measurement. This displacement cunent is the current observed when a capacitor, or any structure with capacitance, is subject to a rapid increase/decrease in the applied voltage. This cunent is required to provide the charge that maintains the relation V=Q/C, where V is the instantaneous applied voltage. C is the capacitance and Q is the instantaneous charge stored in the capacitor. In either a PLAD or PSII system, the cathode and anode together can be considered to be a capacitor C that is charged and discharged by the applied pulse. A Faraday cup. or similar cunent measurement tool, also has a capacitance and the measured cunent includes a displacement cunent. The cunent measurement signals generated by a Faraday cup, or similar measurement tool, are much smaller than other cunent measurements, since they represent only a portion of the current the workpiece is receiving. The capacitance of the Faraday cup is also smaller than the capacitance of the anode and the cathode, but not in the same proportion as the measured currents. As a result, the displacement cunent is a larger proportion of the total current measured by the Faraday cup. Also, since the signal-to-noise ratio is lower for Faraday cup measurements, the presence of displacement cunents is less desirable. Thus, it is more important that the displacement current in Faraday cup measurements be removed. Conventionally, the displacement current at the leading and trailing edges of the pulse is ignored. Unfortunately, in many applications, the voltage pulse lengths are in the range of twenty μs (microseconds). The typical rise time of the pulse is about three μs while the fall time is about seven μs. Since the displacement current is observed during the rise and fall times, it contributes to ion cunent measurement errors over a significant proportion of the voltage pulse. The displacement cunent contributes to an inaccurate measurement of ion cunent in a plasma processing system. Accurate measurement of the ion cunent is necessary to efficiently operate the system. Therefore, a way to reduce or eliminate the enors in ion cunent measurements which are caused by displacement current is needed. Summary of the Invention According to a first aspect of the invention, a method of measuring an ion cunent in a plasma processing system including first and second electrodes, wherein a voltage pulse is applied across the first and second electrodes is provided. The method comprises electrically coupling a capacitor between the first and second electrodes to receive the voltage pulse; measuring a first cunent supplied to the first and second electrodes during the voltage pulse and providing a first cunent signal representative of the first cunent; measuring a second cunent supplied to the capacitor during the voltage pulse and providing a second cunent signal representative of the second cunent; and subtracting the second cunent signal from the first cunent signal to provide an ion cunent signal representative of the ion cunent. The capacitor may have a same or nearly the same capacitance as a first capacitance between the first and second electrodes. Alternately, the capacitor may have a capacitance different from a first capacitance between the first and second electrodes; and the method includes adjusting at least one of the first and second cunent signals to compensate for the difference in capacitance. In another variation, a target is disposed on the first electrode and the step of providing the first cunent signal comprises placing an ion detecting device adjacent the target to generate the first cunent signal, selecting a capacitor having a capacitance different from a capacitance of the ion detecting device; and adjusting at least one of the first and second cunent signals to compensate for the difference between the two capacitances. Alternately, the method comprises selecting a capacitor having a capacitance substantially the same as a capacitance of the ion detecting device. According to another aspect of the present invention, an apparatus for measuring ion cunent in a plasma processing system wherein a voltage pulse is applied across first and second electrodes is provided. The apparatus comprises a capacitive device coupled to the first and second electrodes to receive the voltage pulse; first means for measuring a first cunent supplied to the first and second electrodes during the voltage pulse and for providing a first cunent signal representative of the first current; second means for measuring a second cunent supplied to the capacitive device during the voltage pulse and for providing a second cunent signal representative of the second current; and means for subtracting the second cunent signal from the first cunent signal to provide a measurement of the ion cunent in the plasma processing system. In another aspect of the present invention, a method of measuring an ion cunent in a plasma processing system including first and second electrodes in a processing chamber is provided. The method comprises measuring a pulsed current through the chamber and providing a pulsed cunent signal that is representative of the pulsed cunent; providing a compensation signal that is representative of a displacement cunent component of the pulsed cunent; and subtracting the compensation signal from the pulsed cunent signal to provide an ion cunent signal that is representative of ion cunent delivered to a target. In a variation of this aspect, the step of providing a compensation signal comprises simulating the displacement cunent component of the pulsed cunent. Alternately, the step of providing a compensation signal comprises: providing a capacitor having a capacitance that is the same or nearly the same as a capacitance between the first and second electrodes, applying a pulsed voltage to the capacitor in synchronism with the pulsed cunent, and measuring a second cunent supplied to the capacitor in response to the pulsed voltage and providing a second cunent signal that constitutes the compensation signal. Brief Description of the Drawings For a better understanding of the present invention, reference is made to the accompanying drawings, in which like reference numbers designate the same element and in which: FIG. 1 is a schematic block diagram of a plasma doping system in accordance with a first embodiment of the present invention; FIG. 2 is a simplified schematic diagram of the plasma doping system of FIG. 1 ; FIG. 3 is a schematic block diagram of the plasma doping system in accordance with the first embodiment of the present invention; FIG. 4 is a schematic block diagram of a plasma doping system in accordance with a second embodiment of the present invention; FIG. 5 is a schematic block diagram of the plasma doping system in accordance with the second embodiment of the present invention; FIG. 6 is a simplified schematic diagram of a third embodiment of the present invention; FIG. 7 is a simplified schematic diagram of a fourth embodiment of the present invention; FIGs. 8A and 8B are graphs showing a voltage pulse and cunent through a conventional plasma doping system with plasma as a function of time; FIGs. 9A and 9B are graphs showing the voltage pulse and the cunent as a function of time when tuning a system with plasma according to the present invention; FIG. 10 is a schematic block diagram of a plasma doping system in accordance with a fifth embodiment of the present invention; FIGs. 1 1 A, 1 IB and 1 1C are graphs showing the voltage pulse and the cunent as a function of time when tuning a system without plasma according to the present invention; FIG. 12 is a schematic block diagram of a plasma doping system in accordance with a sixth embodiment of the present invention; and FIG. 13 is a schematic block diagram of a plasma doping system in accordance with a seventh embodiment of the present invention. Detailed Description An example of a plasma doping system in accordance with a first embodiment of the present invention is shown schematically in FIG. 1. A plasma-doping chamber 10 defines an enclosed volume 12. A platen 14 positioned within chamber 10 provides a surface for holding a workpiece, such as a semiconductor wafer 20. Semiconductor wafers are only one example of possible target types. The invention is not limited to ion implantation in only semiconductor wafers. For example, implantation in metals for tools, automotive components, stamping dies, and plastics are also possible. The wafer 20 may, for example, be clamped at its periphery to a flat surface of platen 14. The platen 14 supports wafer 20 and provides an electrical connection to wafer 20. In one embodiment, the platen has an electrically-conductive surface for supporting wafer 20. In another embodiment, the platen includes conductive pins for electrical connection to wafer 20. An anode 24 is positioned within chamber 10 in spaced relation to the platen (cathode) 14. Anode 24 may be movable in a direction, indicated by anow 26, perpendicular to platen 14. The anode 24 is typically connected to electrically-conductive walls of the chamber 10. both of which may be connected to ground. The cathode 14 and. therefore, the wafer 20, are connected to a high voltage pulse generator 30. The pulse generator 30 typically provides pulses in a range of about 100 to 10,000 volts, about 1 to 100 microseconds in duration and at a pulse repetition rate of about 50 Hz to 5 KHz. It will be understood that these pulse parameter values are given by way of example only and that other values may be utilized within the scope of the invention. The enclosed volume 12 of chamber 10 is coupled through a controllable valve 32 to a vacuum pump 34. A gas source 36 is coupled through a mass flow controller 38 to chamber 10. A pressure sensor 44 located within chamber 10 provides a signal indicative of chamber pressure to a controller 46. The controller 46 compares the sensed chamber pressure with a desired pressure input and provides a control signal to valve 32. The control signal controls valve 32 so as to minimize the difference between the chamber pressure and the desired pressure. Vacuum pump 34, valve 32, pressure sensor 44 and controller 46 constitute a closed loop pressure control system. The pressure is typically controlled in a range of about one milliton (mton) to about 500 mton, but is not limited to this range. Gas source 36 supplies an ionizable gas containing a desired dopant for implantation into the workpiece. Examples of ionizable gases include BF-,, N 2 , Ar, PF, and B 2 H 6 . It will be understood that these gases are given by way of example only and that other gases may be utilized within the scope of the invention. Mass flow controller 38 regulates the rate at which gas is supplied to the chamber 10. The configuration shown in FIG. 1 provides a continuous flow of process gas at a constant gas flow rate and constant pressure. The pressure and gas flow rate are preferably regulated to provide repeatable results. In operation, wafer 20 is positioned on platen 14. Then the pressure control system, mass flow controller 38 and gas source 36 are set to produce the desired pressure and gas flow rate within chamber 10. By way of example, the chamber 10 may operate with BF, gas at a pressure of ten mtorr. The pulse generator 30 applies a series of high voltage pulses to wafer 20. causing formation of a plasma 40 between wafer 20 and anode 24. As is known in the art. the plasma 40 contains positive ions of the ionizable gas from gas source 36. The plasma 40 further includes a plasma sheath 42 in the vicinity of platen 14. The electric field that is present between anode 24 and platen 14 during the high voltage pulse accelerates positive ions from plasma 40 across plasma sheath 42 toward platen 14. Secondary electrons that are produced by the ion impact on the platen 14 and wafer 20 are accelerated back across the sheath 42 and into the plasma 40. The accelerated ions are implanted into wafer 20 to form regions of impurity material. The pulse voltage is selected to implant the positive ions to a desired depth in wafer 20. The number of pulses and the pulse duration are selected to provide a desired dose of impurity material (the positive ions) in wafer 20. The current per pulse is a function of pulse voltage, gas pressure and species and any variable position of the electrodes. For example, the cathode to anode spacing may be adjusted for different voltages. In the plasma doping system, the pulsed current is measured to provide a pulsed cunent signal that is indicative of ion cunent delivered to the wafer. The pulsed cunent through the plasma doping system is a sum of an ion cunent, a secondary electron cunent and a displacement cunent. In accordance with an aspect of the invention, a compensation signal is produced that is representative of the displacement cunent component of the pulsed cunent. The compensation signal is subtracted from the pulsed cunent signal to provide an ion cunent signal that is representative of the ion cunent delivered to the wafer. In one embodiment, the present invention makes use of a secondary capacitor that preferably has the same, or very similar, capacitance as the plasma system electrodes, i.e.. the anode, including the chamber walls, and the cathode, which constitute a primary capacitor. By applying the same or a similar voltage pulse to the secondary capacitor, a secondary displacement cunent, which is identical, or very similar, to a primary displacement cunent in the primary capacitor, is produced. As noted above, the primary current is a sum of an ion cunent. a secondary electron current and the primary displacement cunent. Thus, subtracting the secondary displacement current (which is substantially equal to the primary displacement current) from the primary cunent results in an accurate measurement of the ion current plus the secondary electron current. In one embodiment of the present invention, the secondary capacitor can be a variable capacitor that can be varied to match the capacitance of the plasma system. As shown in Fig. 1. the complete anode structure, consisting of anode 24 and chamber walls 10, and the platen (cathode) 14 of the plasma doping system can be considered as constituting a primary capacitor Cl . A secondary capacitor C2 is coupled between ground and the output of the pulse source 30. The secondary capacitor C2 is preferably selected to have the same or similar capacitance as that of the primary capacitor C 1. Effectively, the primary capacitor C 1 and the secondary capacitor C2 are connected in parallel. This circuit is shown in Fig. 2. As can be seen, the high voltage pulse from the pulse source 30 is applied to both capacitors Cl and C2. A first cunent measuring device 60 is provided to measure the current through the primary capacitor Cl and to output this measurement to a dose processor 70. A second cunent measuring device 62 is provided to measure the cunent through the secondary capacitor C2. The output of the current measuring device 62 is also provided to the dose processor 70. The current measuring devices 60, 62 can either be the same type of device or different devices. In one prefened embodiment, a Pearson coil is used to measure the cunent through each capacitor during the pulse. There are, however, many different types of cunent measurement devices cunently available. Any one of these devices can be used to measure the cunents. In operation, when the pulse is applied to the two capacitors, a primary cunent I C1 is produced in the primary capacitor Cl and a secondary displacement cunent I C2 , that is substantially identical, or at least very similar, to the primary displacement current component of primary cunent I C] is produced in the secondary capacitor C2. By subtracting the secondary displacement cunent from the primary current, a more accurate measurement is made of the ion cunent and secondary electron current being delivered to the target. The more accurate measurement of these cunents results in better process control and repeatability. In one embodiment of the present invention, as shown in Fig. 3. the cunent measuring device 60 is a first Pearson coil 64' and the current measuring device 62 is a second Pearson coil 64". An output of the first Pearson coil 64' is provided to a first input A of a cunent signal processor 66. An output of the second Pearson coil 64" is provided to a second input B of the cunent signal processor 66. An output 67 of the current signal processor 66 represents the measured current through the primary capacitor C 1 minus the measured current through the secondary capacitor C2. As a result, the displacement current has been removed from the primary cunent measurement. One of ordinary skill in the art can see that there are many devices and circuits that can function as the cunent signal processor 66. For example, a circuit using an operational amplifier may be implemented to provide an analog difference signal. Any necessary signal conditioning functions may also be utilized. Further, the cunent signal processor 66 may be a digital device where the signals I cι and I C2 are converted from analog to digital format, and the difference calculation is then performed in the digital domain. The function of the cunent signal processor 66. i.e., subtracting the current in the secondary capacitor C2 from the current in the primary capacitor Cl, can be performed either by the processor 66 or within the dose processor 70 (Fig. 1). In another embodiment of the present invention, as shown in Fig. 4, a single cunent measuring device 68 is utilized to measure the primary cunent I c , through the primary capacitor and the secondary cunent I C2 through the secondary capacitor and to output a signal to the dose processor 70 representative of the primary current minus the secondary cunent. As shown in Fig. 4. the device 68 is configured so that both currents I C1 . I C2 pass through it. The cunent measuring device 68 may comprise a single Pearson coil 64 as shown in Fig. 5. As shown, a wire canying the primary cunent I CI to the primary capacitor Cl passes through the Pearson coil 64 in a first direction (left to right). The wire canying the secondary current I C2 to the secondary capacitor C2 passes in a second direction (right to left) through the Pearson coil 64. such that the secondary current I C2 passes through the Pearson coil 64 in a direction opposite to that of the primary current I C1 . The output of the Pearson coil 64 is then the difference between the primary cunent and the secondary cunent (IcrW- As described above, the secondary capacitor C2 preferably is selected to have the same capacitance as the primary capacitor Cl . In order to provide more flexibility, the secondary capacitor C2 advantageously may be an adjustable capacitor. As shown in Fig. 6, a simplified schematic of the present invention, the secondary capacitor has been replaced with an adjustable capacitor C2'. The capacitance of the capacitor C2' may be adjusted to be the same as that of primary capacitor Cl. as described below. This provides flexibility as well as fine-tuning capabilities in the event that the capacitance of the primary capacitor Cl changes. In order to minimize "ringing" and noise on the line carrying the cunent to the secondary capacitor C2, C2'„ any number of different noise-reduction techniques, well known to those of ordinary skill in the art, can be applied. Components such as capacitors, inductors, high-pass, band-pass, low-pass filters and/or one or more resistors may be used. The parameters for selecting these components in order to improve the signal quality include choosing the resistance, capacitance, inductance, frequency range and frequency response characteristics, which can be fixed or variable. In one example, as shown in Fig. 7. a resistor R2 is connected in series with the secondary capacitor C2' and the series-coupled resistor R2 and secondary capacitor C2' are coupled in parallel with the primary capacitor Cl in series with a resistor R3. While the simplified schematic shown in Fig. 7 reflects the use of an adjustable capacitor C2'. the resistors can be provided in the embodiments discussed above, where a fixed capacitor C2 is used. In addition, a resistor Rl can be provided in series with the output of the high voltage pulse source 30. Further, the particular values of these resistors can be chosen by one of ordinary skill in the art to minimize ringing. Still further, variable resistors (not shown) can be used to adjust the resistance value similar to the ability to adjust the capacitance value of the adjustable capacitor C2'. One of ordinary skill in the art will understand that the resistors and capacitors C2. C2' must be selected to have the proper voltage and power ratings for the implementation. It should be noted that measures taken to improve the signal quality of I c , and I C2 may result in a voltage V C] on the primary capacitor C, being different from a voltage V c2 on the secondary capacitor C2. This may result in different displacement cunents in capacitor C l and capacitor C2. Accordingly, this voltage difference must be compensated for, as known to one of ordinary skill in the art, either within the cunent signal processor 66 or by other methods. In another embodiment of the present invention shown in Fig. 12. a second high voltage pulse source 30'. synchronized with source 30 by a synchronization device 120, is used to apply a pulse to capacitor C2 that is substantially the same, with respect to voltage amplitude, pulse width, rising edge timing and falling edge timing, as the pulse applied to the primary capacitor C l . High voltage pulse source 30 is connected to the primary capacitor Cl . Second high voltage pulse source 30' outputs a pulse, that is essentially the same and in synchronization with the pulse output of the high voltage pulse source 30, to a secondary capacitor C2 having substantially the same capacitance value as the primary capacitor C 1 . The measurement of the cunents in the two capacitors C 1 , C2 and the subtraction of the displacement cunent is then performed as described above. In addition, a variable capacitor and noise reducing techniques can be applied to this embodiment. In the foregoing description of the present invention, the capacitance of the secondary capacitor C2 is matched to the capacitance of the primary capacitor Cl . While it may be possible to measure the capacitance of the primary capacitor C l in a plasma doping system, there are so many variables inherent in the process that it may be difficult to measure this capacitance and to adjust the secondary capacitor to the measured capacitance. One method known to the inventors for setting the secondary capacitor C2 to substantially the same value as the primary capacitance Cl , is to "tune" the system. The tuning can occur with plasma present in the system or without plasma being present. An example of tuning of the plasma doping system with plasma present will now be described. The pressure within the chamber is set sufficient to support a plasma with a pulse frequency of 500 Hz and a pulse duration of twenty μs. As shown in Fig. 8A, waveform 80 represents the voltage pulse of approximately 2.5 kilovolts having a duration of approximately twenty μs. Without the secondary capacitor in place, the cunent measured through the system is shown by waveform 82 in FIG. 8B. As can be seen, the cunent waveform 82 includes displacement currents 82a and 82b at the leading and trailing edges of the pulse. In order to tune the system, the secondary capacitor C2 is connected in parallel with the primary capacitor Cl . Preferably, adjustable capacitor C2' is used. The system is then configured to display the primary cunent through the primary capacitor Cl minus the secondary cunent through the secondary capacitor C2. As shown in Fig. 9A, the voltage pulse 80. as before, is displayed. A current pulse waveform 84 which is representative of I -I C τ is shown in FIG. 9B. The secondary capacitor C2' is adjusted to reduce the displacement cunent observed in the waveform 84. When the observed displacement current in the waveform 84 has been reduced to or near zero, the system, according to the present invention, has been tuned such that the secondary capacitor C2' now has substantially the same capacitance value as the primary capacitor Cl . In a test run by the inventors, the required value was approximately sixty-eight picofarads for a laboratory plasma doping system. A similar procedure may be applied when tuning a plasma processing system without plasma present. Specifically, a voltage pulse represented by waveform 80, as shown in Fig. 1 1 A, is applied. As there is no plasma in this system, the measured cunent as shown by waveform 82' in Fig. 1 IB, includes displacement cunent but no ion cunent or secondary electron cunent, without the secondary capacitor in place. The secondary capacitor C2 is adjusted to reduce the displacement cunent observed in the waveform 84', which is representative of I cι - I C2 , as shown in Fig. 1 IC. When the observed displacement cunent, as indicated by waveform 84', has been reduced to or near zero, the system has been tuned by setting the secondary capacitor C2' to have substantially the same capacitance value as the primary capacitor C 1. The foregoing provides a method for tuning or calibrating the system to eliminate the displacement cunent from the cunent measurement. One of ordinary skill in the art will recognize that there are many different ways to tune the system in addition to using an adjustable capacitor. For example, multiple capacitors may be connected in a parallel and/or series combination in order to change the overall capacitance of the secondary capacitor. The inventors have tuned the secondary capacitor C2 by observing the displacement cunent waveform and manually adjusting the secondary capacitor ' s value until the displacement cunent has been minimized. It is known that this tuning operation can be performed under control of a computer and a signal processing system for controlling the pulse source and the value of the secondary capacitor. The programming of such a computer and anangement of the components necessary is well within the capability of one of ordinary skill in the art and is included within the scope of the present invention. Further, a computer is not necessary, as an analog circuit may be configured to obtain the same results. As described above, an electrical signal representative of the ion cunent plus secondary electron cunent is supplied to the dose processor 70. In one embodiment, the electrical cunent from each cunent measuring device 60, 62 is supplied directly to the dose processor 70 located external to chamber 10. The dose processor then performs the subtraction operation. In another embodiment, the cunent subtraction is performed externally to the dose processor 70. The total ion dose delivered to wafer 20 is the instantaneous ion current integrated over the time of the implant. Dose processor configurations are known in connection with beamline ion implanters. While the previous embodiments measure the primary cunent externally to the doping chamber 10, the primary cunent may be measured within the chamber. In co-pending and commonly assigned U.S. patent application Serial No. 09/128,370, filed August 3. 1998, entitled "Dose Monitor for Plasma Doping System," which is hereby incorporated by reference in its entirety, one or more Faraday cups are positioned within the chamber 10 to measure the ion cunent delivered to the target. As shown in Fig. 10, Faraday cups 150, 152 are spaced around the periphery of wafer 20. Each Faraday cup comprises a conductive enclosure having an entrance 160 facing plasma 40. Each Faraday cup is preferably positioned as close as is practical to wafer 20 and intercepts a sample of the positive ions accelerated from plasma 40 toward platen 14. Each Faraday cup 150. 152 is coupled to the output of the high voltage pulse source 30. The positive ions entering each Faraday cup through entrance 160 produce a cunent in the electrical circuit connected to the Faraday cup. The electrical cunent is indicative of the number of positive ions received per unit time, or ion current. It is assumed that the ion cunents received by Faraday cups 150, 152 have a fixed relation to the number of ions implanted in wafer 20 per unit time. Depending on the uniformity of plasma 40 and the uniformity of ion acceleration toward platen 14, the ion cunent per unit area received by each Faraday cup may be substantially equal to or a fixed fraction of the ion cunent per unit area implanted in wafer 20. This electrical cunent is measured by cunent measuring devices 62-1, 62-2 which provide the cunent measurement to the dose processor 70. Since the electrical cunent output of each of the Faraday cups is representative of the ion cunent implanted in wafer 20, the Faraday cups 150, 152 and the cunent measurement devices 62- 1. 62-2, provide a measurement of the ion dose or cunent implanted in wafer 20. In the pulsed system of Fig. 10, the current output of Faraday cups 150, 152 includes a displacement cunent component as described above. The dose processor 70 receives the output from the current measurement devices 61 - 1. 62-along with the measurement of the secondary cunent in the secondary capacitor C2. The subtraction of the secondary cunent from the current measured by the Faraday cups may be performed within the dose processor 70. Alternatively, the cunent measurement devices 62-1, 62-2 may be incorporated into the dose processor where cunent measurement and subtraction is performed within the dose processor 70. In the previously described embodiments, secondary capacitor C2 was set to have the same capacitance as the primary capacitor Cl defined by the anode 24 and the cathode 14. In the embodiment of Fig. 10, the secondary capacitor C2 is set to have the same capacitance as the total capacitance of the Faraday cups. The secondary capacitor C2 is separately connected to pulse source 30 to ensure that the cunent measured represents only the current delivered to capacitor C2. The process for setting the value of the secondary capacitor the same as the capacitance of the Faraday cups is analogous to the above description of tuning the system. As above, this tuning can be performed with or without plasma being present. In addition, any necessary noise reduction techniques can be applied to the signals being measured. As the Faraday cups are relatively small devices, their capacitance is likely to be smaller than the capacitance of the anode and the cathode. It is feasible, however, that a secondary capacitor having a value k times as large as the capacitance of the Faraday cups can be used. One of ordinary skill in the art will understand that the removal of the displacement cunent from the cunent measured by the Faraday cup must account for this factor k. For example, if the secondary capacitor C2 is k times as large as the capacitance of the Faraday cups, the signal from capacitor C2 may be reduced by a factor k. This approach requires that the value of the capacitance of the Faraday cups be known. In yet another embodiment, the value of the primary capacitance can be determined and the displacement current through the primary capacitance having the voltage pulse applied to it can be simulated using circuit simulation programs known in the art. The simulated displacement cunent, synchronized with the voltage pulse, can be subtracted from an actual current measurement in an operating plasma processing system. Such a calculation may be easily performed in a computer-based signal processing system. In such an implementation, as shown in Fig. 13, measurements from device 64 and a pulse synchronization signal are provided to simulation system 130. System 130 includes an analog-to-digital converter (ADC) 132 to convert the cunent measurement signal and to provide a digital output to a signal processor 136. A displacement current simulator 134 is programmed to output to a signal processor 136 a digital representation of the displacement cunent component in synchronization with the pulse from pulse source 30. Signal processor 136 subtracts the output of displacement cunent simulator 134 from the output of ADC 132 to provide an accurate ion cunent measurement. Variations of the system shown in Fig. 13 are included within the scope of the present invention. An analog simulation of the displacement cunent can be subtracted from the analog measurement of the cunent. The simulated analog displacement cunent can be generated by a computer based simulation program driving an analog-to-digital converter, or the signal can be generated by a circuit configured to output a representation of the displacement cunent. All of these implementations are within the expertise of one of ordinary skill in the art. Using a simulated displacement cunent removes the need for the secondary capacitance C2 and thus simplifies the equipment, while providing an accurate cunent measurement. The invention has been described thus far in connection with a plasma doping system in which a plasma is formed upon the application of a high voltage pulse between platen 14 and anode 24. The present invention may also be utilized in a plasma immersion system wherein a plasma is present continuously in the region between the anode and the cathode, and ions are accelerated toward the cathode by application of a high voltage pulse. In the configuration of FIG. 1 , anode 24 is separate from but electrically connected to chamber 10. In other configurations, the conductive walls of the plasma doping chamber 10 may serve as an anode, and a separate anode is not utilized. While there have been shown and described what are at present considered the prefened embodiments of the present invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims. What is claimed is:

Claims: CLAIMS 1. A method of measuring an ion cunent in a plasma processing system including first and second electrodes, wherein a voltage pulse is applied across the first and second electrodes, the method comprising: (a) electrically coupling a capacitor between the first and second electrodes to receive the voltage pulse; (b) measuring a first cunent supplied to the first and second electrodes during the voltage pulse and providing a first cunent signal representative of the first cunent; (c) measuring a second cunent supplied to the capacitor during the voltage pulse and providing a second cunent signal representative of the second cunent; and (d) subtracting the second cunent signal from the first cunent signal to provide an ion cunent signal representative of the ion cunent. 2. The method of claim 1, further comprising: selecting the capacitor to have a same or nearly the same capacitance as a first capacitance between the first and second electrodes. 3. The method of claim 1, further comprising: selecting the capacitor to have a capacitance different from a first capacitance between the first and second electrodes; and adjusting at least one of the first and second current signals to compensate for the difference in capacitance. 4. The method of claim 1, further comprising: measuring a first voltage across the first and second electrodes present during the voltage pulse; measuring a second voltage across the capacitor present during the voltage pulse: and adjusting at least one of the first and second cunent signals to compensate for a difference between the first and second voltages. 5. The method of claim 1, further comprising: removing noise from at least one of the first and second cunent signals. 6. The method of claim 1 , wherein the plasma processing system is a plasma doping system and wherein the voltage pulse forms the plasma and accelerates ions in the plasma toward a target. 7. The method of claim 1 , wherein the plasma processing system is a continuous plasma system and wherein the voltage pulse accelerates ions in the plasma toward a target. 8. The method of claim 1 , further comprising: varying the capacitance value of the capacitor to minimize a displacement cunent component of the ion cunent signal. 9. The method of claim 1 , wherein a target is disposed on the first electrode and the step of providing the first cunent signal comprises placing an ion detecting device adjacent the target to generate the first cunent signal, the method further comprising: selecting a capacitor having a capacitance different from a capacitance of the ion detecting device; and adjusting at least one of the first and second current signals to compensate for the difference between the two capacitances. 10. The method of claim 9, wherein the plasma processing system is a plasma doping system and wherein the voltage pulse forms the plasma and accelerates ions in the plasma toward a target. 1 1. The method of claim 9, wherein the plasma processing system is a continuous plasma system and wherein the voltage pulse accelerates ions in the plasma toward a target. 12. The method of claim 9, wherein the ion detecting device comprises a Faraday cup. 13. The method of claim 1 , wherein the step of providing the first cunent signal comprises placing an ion detecting device adjacent the target to generate the first cunent signal, the method further comprising: selecting a capacitor having a capacitance substantially the same as a capacitance of the ion detecting device. 14. The method of claim 13, wherein the ion detecting device comprises a Faraday cup. 15. The method of claim 13. wherein the capacitor is a variable capacitor. 16. An apparatus for measuring ion cunent in a plasma processing system wherein a voltage pulse is applied across first and second electrodes, the apparatus comprising: a capacitive device coupled to the first and second electrodes to receive the voltage pulse; first means for measuring a first current supplied to the first and second electrodes during the voltage pulse and for providing a first cunent signal representative of the first cunent; second means for measuring a second cunent supplied to the capacitive device during the voltage pulse and for providing a second current signal representative of the second cunent: and means for subtracting the second current signal from the first cunent signal to provide a measurement of the ion current in the plasma processing system. 17. The apparatus of claim 16, wherein the capacitive device comprises a capacitor having a capacitance different from a capacitance between the first and second electrodes, the apparatus further comprising: means for adjusting at least one of the first and second current signals to compensate for the difference in capacitance. 18. The apparatus of claim 17, wherein the capacitor is a variable capacitor. 19. The apparatus of claim 16, wherein the first and second means each comprise a Pearson coil. 20. The apparatus of claim 16, wherein the plasma processing system is a plasma doping system and wherein the voltage pulse forms the plasma and accelerates ions in the plasma toward a target. 21. The apparatus of claim 16, wherein the plasma processing system is a continuous plasma system and wherein the voltage pulse accelerates ions in the plasma toward a target. 22. The apparatus of claim 16, wherein the capacitive device comprises a capacitor having a capacitance value the same or nearly the same as a capacitance between the first and second electrodes. 23. The apparatus of claim 22, wherein the capacitor is a variable capacitor. 24. The apparatus of claim 16, wherein the capacitive device comprises a capacitor having a capacitance value the same or nearly the same as a capacitance of the first means for measuring the first cunent. 25. The apparatus of claim 24, wherein the first means comprise a Faraday cup. 26. An apparatus for measuring ion current in a plasma processing system wherein a voltage pulse is applied across first and second electrodes, the apparatus comprising: a capacitive device coupled to the first and second electrodes to receive the voltage pulse; a first cunent measuring device to measure a first cunent provided to the first and second electrodes during the voltage pulse and to output a first cunent signal representative of the first cunent; a second cunent measuring device, coupled to the capacitive device, to measure a second cunent provided thereto during the voltage pulse and to output a second cunent signal representative of the second cunent; and a subtractor to receive the first and second current signals and to subtract the second cunent signal from the first cunent signal to provide a measurement of the ion cunent. 27. The apparatus of claim 26, wherein: the capacitive device comprises a capacitor having a capacitance value the same or nearly the same as a capacitance between the first and second electrodes . 28. The apparatus of claim 27, wherein the capacitor is a variable capacitor. 29. The apparatus of claim 26, wherein: the capacitive device comprises a capacitor having a capacitance value different from a capacitance of the first and second electrodes; and the subtractor comprises means for compensating for the difference in capacitance values. 30. The apparatus of claim 29. wherein the capacitor is a variable capacitor. 31. The apparatus of claim 26, wherein the plasma processing system is a plasma doping system and wherein the voltage pulse forms the plasma and accelerates ions in the plasma toward a target. 32. The apparatus of claim 26, wherein the plasma processing system is a continuous plasma system and wherein the voltage pulse accelerates ions in the plasma toward a target. 33. The apparatus of claim 26, wherein: the capacitive device has a capacitance different from a capacitance of the first cunent measuring device; and the subtractor comprises means for compensating for the difference in capacitance values. 34. The apparatus of claim 26, wherein: the capacitive device has a capacitance substantially the same as a capacitance of the first cunent measuring device. 35. The apparatus of claim 34, wherein the plasma processing system is a continuous plasma system and wherein the voltage pulse accelerates ions in the plasma toward a target. 36. The apparatus of claim 34, wherein: the first cunent measuring device comprises a Faraday cup; and the capacitance device has substantially a same capacitance as that of the Faraday cup. 37. The apparatus of claim 34. wherein the plasma processing system is a plasma doping system and wherein the voltage pulse forms the plasma and accelerates ions in the plasma toward a target. 38. A method of measuring an ion current in a plasma processing system including first and second electrodes in a processing chamber, the method comprising steps of: measuring a pulsed cunent through the chamber and providing a pulsed cunent signal that is representative of the pulsed current; providing a compensation signal that is representative of a displacement cunent component of the pulsed cunent and subtracting the compensation signal from the pulsed cunent signal to provide an ion current signal that is representative of ion current delivered to a target. 39. The method of claim 38, wherein the plasma processing system is a plasma doping system and wherein the voltage pulse forms the plasma and accelerates ions in the plasma toward a target. 40. The method of claim 38, wherein the plasma processing system is a continuous plasma system and wherein the voltage pulse accelerates ions in the plasma toward a target. 41. The method of claim 38, wherein the step of providing a compensation signal comprises simulating the displacement current component of the pulsed cunent. 42. The method of claim 38, wherein the step of providing a compensation signal 5 comprises: providing a capacitor having a capacitance that is the same or nearly the same as a capacitance between the first and second electrodes, applying a pulsed voltage to the capacitor in synchronism with the pulsed current, and o measuring a second cunent supplied to the capacitor in response to the pulsed voltage and providing a second cunent signal that constitutes the compensation signal. 43. The method of claim 38, wherein the pulsed cunent is responsive to a voltage pulse applied across the first and second electrodes, the method further comprising: 5 electrically coupling a capacitor between the first and second electrodes to receive the voltage pulse; and measuring a second cunent supplied to the capacitor during the voltage pulse, said second cunent signal constituting the compensation signal. 0 44. The method of claim 43, further comprising: selecting the capacitor to have a same or nearly the same capacitance as a first capacitance between the first and second electrodes. 45. The method of claim 43, further comprising: 5 selecting the capacitor to have a capacitance different from a first capacitance between the first and second electrodes; and adjusting at least one of the pulsed cunent signal and the second cunent signal to compensate for the difference in capacitance. 0 46. The method of claim 43, further comprising: measuring a first voltage across the first and second electrodes present during the voltage pulse; measuring a second voltage across the capacitor present during the voltage pulse; and adjusting at least one of the pulsed current signal and the second cunent signal to compensate for a difference between the first and second voltages. 47. The method of claim 43, further comprising: removing noise from at least one of the pulsed cunent signal and the second cunent signal.

Inventors: GOECKNER MATTHEW J, VAN WAGONER CHARLES E, GOECKNER, MATTHEW, J., VAN WAGONER, CHARLES, E.
Applicants: VARIAN SEMICONDUCTOR EQUIPMENT, VARIAN SEMICONDUCTOR EQUIPMENT ASSOCIATES, INC.
Application year: 2000
Main classification: H01J37
IPC codes: C23C14, G01R19, H01J37, H01L21