Apparatus and method for metal contamination control in ion implantation systems using a charge stripping mechanism

Abstract

A method for implanting highly charged ions into a workpiece while mitigating trace metal contamination includes generating desired ions in a first charge state from a desired substance in an ion source, and generating trace metal ions of a contaminating substance in a first ion beam. The desired ions and the trace metal ions have an equal mass-to-charge ratio. The desired ions and the trace metal ions are extracted from the ion source. At least one electron is stripped from the desired ions to define a second ion beam of desired ions and trace metal ions in a second charge state. Desired ions only from the second ion beam are selectively passed through a charge selector to define a final ion beam of desired ions in a second charge state and without trace metal ions, and the desired ions in the second charge state are implanted into the workpiece.

Description

[0001] Citation of Related Applications

[0002] This application claims priority to U.S. Provisional Application No. 62 / 971,473, filed on February 7, 2020, the entire contents of which are incorporated herein by reference in their entirety. Technical Field

[0003] The present invention relates generally to ion implantation systems and, more particularly, to a system and method for controlling contamination and increasing ion beam current at high energies to achieve a desired charge state. Background Art

[0004] In the manufacture of semiconductor devices, ion implantation is used to dope semiconductors with impurities. Ion implantation systems are typically used to dope workpieces such as semiconductor wafers with ions from an ion beam to produce n-type or p-type material doping or to form a passivation layer during the manufacture of integrated circuits. This beam processing is typically used to selectively implant wafers with impurities of specific dopant materials at predetermined energy levels and controlled concentrations to produce semiconductor materials during the manufacture of integrated circuits. When used to dope semiconductor wafers, the ion implantation system implants selected ion species into the workpiece to produce the desired extrinsic material. For example, implanting ions produced from source materials such as antimony, arsenic, or phosphorus will produce an "n-type" extrinsic material wafer, while a "p-type" extrinsic material wafer is typically produced by ions produced from source materials such as boron, gallium, or indium.

[0005] A typical ion implanter includes an ion source, an ion extraction device, a mass analysis device, with or without a post-acceleration section, a beam transmission device, and a wafer processing device. The ion source produces ions of the desired atomic or molecular dopant material. These ions are extracted from the ion source by an ion extraction device, which is usually a set of electrodes that excite and guide the ion flow from the ion source to form an ion beam. In the mass analysis device, the desired ions are separated from the ion beam, and the mass analysis device is usually a magnetic dipole that mass disperses or separates the extracted ion beam. The beam transmission device is usually a vacuum system containing a series of focusing devices, which transmits the analyzed ion beam to the wafer processing device while maintaining the desired properties of the ion beam. Finally, the semiconductor wafer is transmitted to and from the wafer processing device by the wafer processing system, and the wafer processing system may include one or more robotic arms for placing the wafer to be processed in front of the analyzed ion beam and removing the processed wafer from the ion implanter.

[0006] Both RF-based accelerators and DC-based accelerators can be used in ion implanters, whereby ions can be repeatedly accelerated through multiple acceleration stages of the accelerator. For example, an RF-based accelerator can have a voltage-driven acceleration gap. Due to the time-varying characteristics of the RF acceleration field and multiple acceleration gaps, there are a large number of parameters that can affect the final beam energy. Because the charge state distribution of the ion beam can change, a lot of effort is spent to keep the charge value in the ion beam at a single value that was originally expected. Summary of the invention

[0007] The present invention recognizes that the significant demand for high-level implant recipes (e.g., ion beam energy, quality, charge value, beam purity, beam current and / or total dose level of implant) requires providing higher beam current and beam purity without unnecessarily damaging the ion source. Therefore, a system or method for increasing beam current with high purity is provided herein.

[0008] Accordingly, a brief description of the present invention is presented below to provide a basic understanding of some aspects of the present invention. This brief description is not a broad overview of the present invention. It is neither intended to indicate the key or important elements of the present invention nor to describe the scope of the present invention. Its purpose is to present some concepts of the present invention in a simplified form as a preface to the specific embodiments presented later.

[0009] Aspects of the present invention facilitate an ion implantation process for implanting ions (e.g., arsenic ions) into a workpiece. According to one exemplary aspect, an ion implantation system is provided having an ion source configured to form an ion beam, a beamline assembly configured to selectively transmit the ion beam, and an end station configured to receive the ion beam for implanting the ions into the workpiece.

[0010] According to an exemplary aspect, the ion source is configured to produce the desired ions of the desired substance and the trace metal ions of the contaminating substance. For example, the desired ions of the desired substance are in a first charge state, wherein the charge-to-mass ratio of the desired ions and the trace metal ions is equal. The extraction device is configured to extract the desired ions and the trace metal ions from the ion source to define a first ion beam, and the mass analyzer is configured to select the desired ions and the trace metal ions from the first ion beam, thereby defining the mass-analyzed ion beam. The accelerator is configured to accelerate the mass-analyzed ion beam from a first energy to a second energy, and the charge stripping device is configured to strip at least one electron from the desired ions, thereby defining a second ion beam containing the desired ions and the trace metal ions in the second charge state. In addition, a charge selector is located downstream of the charge stripping device, wherein the charge selector is configured to selectively pass only the desired ions in the second charge state. Therefore, a final ion beam is defined, wherein the final ion beam includes the desired ions in the second charge state and does not include the trace metal ions. The terminal station can be further configured to receive the final ion beam to implant the desired ions into the workpiece.

[0011] According to one example, the charge selector comprises an electromagnetic mass analyzer located near the charge stripping device. In another example, the charge stripping device is located within the accelerator.

[0012] For example, the accelerator may include a plurality of accelerator stages, wherein the charge stripping device is located between at least two of the plurality of accelerator stages. For example, the accelerator may include an RF accelerator having one or more resonators configured to generate an accelerating RF field. For example, in an RF accelerator, the plurality of accelerator stages each include one or more resonators configured to generate an accelerating field.

[0013] In another example, the accelerator includes a DC accelerator configured to accelerate the desired ions by a constant DC high voltage. In yet another example, the charge selector includes an energy filter. In addition, a scanner can be provided and configured to scan the final ion beam back and forth to define the scanned ion beam. For example, the scanner can include an electrostatic scanner or an electromagnetic scanner, which is configured to scan the final ion beam electrostatically or electromagnetically, respectively, to define the scanned ion beam. In another example, an angle correction lens is provided and configured to parallelize and offset the scanned ion beam for injection into the workpiece.

[0014] According to yet another example, the charge stripping device includes a pump and a gas source, wherein the pump is configured to pump gas from the gas source and control the flow of the gas into the accelerator, wherein the flow of the gas is configured to strip electrons from ions of the desired species. For example, the gas includes sulfur hexafluoride.

[0015] For example, the desired ions may include arsenic ions, while the trace metal ions may include titanium or iron. For example, the desired ions in the first charge state may include 75 As 3+ ions, while trace metal ions include 50 In another example, the desired ions in the first charge state may include 75 As 4+ ions, while trace metal ions include 56 Fe 3+ ions.

[0016] According to another exemplary aspect, an ion implantation system is provided, comprising an ion source configured to generate an ion beam along a beam line, wherein the ion beam comprises ions of a first charge state and trace metal ions. A mass analyzer is provided for mass analyzing the ion beam along the beam line, and an accelerator is disposed along the beam line and configured to accelerate the ion beam from a first energy to a second energy.

[0017] For example, a charge stripping device is provided in the system and is configured to strip at least one electron from the first charge state ions along the beam line, thereby defining a stripped ion beam including second charge state ions and trace metal ions. For example, a charge selector device is located downstream of the charge stripping device along the beam line and is configured to select the second charge state ions in the stripped ion beam to define a final ion beam along the beam line, wherein the final ion beam includes the second charge state ions and approximately zero trace metal ions. In addition, an end station can be provided and positioned along the beam line and configured to support a workpiece to be implanted with the second charge state ions.

[0018] In one example, the accelerator includes a plurality of accelerator stages, wherein the charge stripping device is located downstream of at least one of the plurality of accelerator stages. For example, the plurality of accelerator stages may include a plurality of acceleration stages defined within a separate accelerator device. Alternatively, the plurality of accelerator stages may be defined by or within a plurality of separate accelerator devices. For example, the charge selector device may be further located between at least two of the plurality of acceleration stages. For example, the ion implantation system may further include an energy filter located downstream of the accelerator, wherein the energy filter is further configured to filter the final ion beam.

[0019] In another example, a scanner is provided in the system and configured to scan the final ion beam back and forth to define a scanned ion beam. The scanner may include an electrostatic scanner or an electromagnetic scanner configured to scan the final ion beam electrostatically or electromagnetically, respectively, to define a scanned ion beam.

[0020] According to another aspect, a method for implanting high charge state ions into a workpiece while mitigating trace metal contamination is provided. For example, the method includes generating desired ions of a desired substance in an ion source, wherein the desired ions are in a first charge state, and wherein generating the desired ions further generates trace metal ions of a contaminating substance, wherein the charge-to-mass ratio of the desired substance and the contaminating substance is equal. The desired ions and trace metal ions are extracted from the ion source, thereby defining a first ion beam. In one example, the first ion beam is mass analyzed, thereby defining a mass-analyzed ion beam. In one example, the mass-analyzed ion beam is accelerated. For example, the mass-analyzed ion beam passes through a charge stripping element, wherein the charge stripping element strips at least one electron from the desired ions, thereby defining a second ion beam including the desired ions and trace metal ions in a second charge state. The second ion beam passes through a charge selector element, thereby defining a final ion beam including the desired ions in the second charge state and not including trace metal ions. In addition, the desired ions in the second charge state are implanted into the workpiece. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1A is a simplified top view showing an ion implantation system according to one aspect of the present invention;

[0022] Figure 1B is a simplified top view showing an ion implantation system with a charge stripper after an accelerator according to one aspect of the present invention;

[0023] Figure 2 is part of an ion implantation system according to at least one aspect of the present invention;

[0024] Figure 3 shows the charge state distribution of an arsenic beam passing through argon and sulfur hexafluoride;

[0025] Figure 4 Various media used in charge stripping are shown;

[0026] Figure 5 is a flow chart showing a method for reducing metal contamination according to another example of the present invention;

[0027] Figure 6 is a flow chart showing a method for reducing metal contamination according to yet another example of the present invention;

[0028] Figure 7 is an exemplary layout diagram of an ion implantation system according to yet another example of the present invention;

[0029] Figure 8 is a simplified schematic diagram of an ion implantation system according to another example of the present invention. DETAILED DESCRIPTION

[0030] In contrast to diffusion, which is a chemical process, ion implantation is a physical process that is employed in semiconductor device manufacturing to selectively implant dopants into semiconductor workpiece and / or wafer materials. Therefore, the implantation action does not rely on chemical interactions between the dopant and the semiconductor material. For ion implantation, dopant atoms / molecules are ionized and isolated, sometimes accelerated or decelerated, formed into a beam, and then swept across the workpiece or wafer. The dopant ions physically bombard the workpiece, enter the surface, and typically reside in the lattice structure below the workpiece surface. U.S. Patent 8,035,080, commonly owned by Satoh, describes various systems and methods for increasing beam current, the contents of which are incorporated herein by reference in their entirety.

[0031] The present invention recognizes that high energy implant applications (e.g., for forming image sensors) can be very sensitive to metal contamination. High energy applications often require the use of higher charge state ions (e.g., arsenic ions), such as triply charged ions (e.g., As3+), quadruple charged ions (e.g., As4+), or even higher charge ions to achieve higher final energy implants. However, the generation and extraction of such higher charge state ions increases the risk of forming trace metal ions in the extracted ion beam.

[0032] For example, ion implantation is used to manufacture integrated circuits (ICs) for certain devices, such as CMOS image sensors (CIS), which are very sensitive to metal contamination, which can affect device performance, such as so-called white pixels and dark current performance issues. High-energy ion implantation for CIS device manufacturing increasingly requires lower levels of metal contamination, such as surface metal contamination and high-energy metal contamination. In high-energy ion implantation systems, for example, multiple charge state ions are used to obtain higher final implantation energies through acceleration stages. However, extracting higher charge state ions from the ion source for implantation results in a potential "high-energy metal contamination risk", where a small but traceable amount of one or more contamination ions generated from the ion source can gain energy and pass through the analysis magnet, electrical filter, and velocity filter, and ultimately be implanted into the workpiece.

[0033] The present invention recognizes that such trace metal ions in the ion beam may be undesirable in such implants because the trace metal ions are implanted into the workpiece along with the desired higher charge ions, thereby causing potential problems in the resulting IC. Heretofore, some of these trace metal contaminants could not be filtered out of the desired high charge state ion beam. For example, in an arsenic ion beam extracted from an ion source, the arsenic triple charge ( 75 As 3+) ions and titanium double charge ( 50Ti 2+) ions are usually not distinguishable by the downstream analysis magnet, electrofilter, and velocity filter because they all have the same magnetic stiffness, energy-to-charge ratio, and velocity. Therefore, both ions reach the workpiece, resulting in the injection of high-energy metal ions (e.g., titanium contamination) into the workpiece. Another similar example is when the downstream analysis magnet is configured to select quadruple arsenic ( 75 As 4+) ions, it is usually impossible to 56 Fe3+) iron ions separation.

[0034] The present invention provides a solution to trace metal contamination from an extracted ion beam (e.g. 75 As 3+), and then stripping off, for example, an electron during ion beam transport to produce the desired higher charge state ion beam (e.g., 75 As 4+). For example, by starting with lower charge state ions and selecting the desired higher charge state ions after the charge stripping process, the trace metal contamination ions that initially accompany the selected lower charge state ions can be separated from the desired higher charge state ions, while the higher charge state ions can be further accelerated as needed to reach the final energy of the implant. By doing so, the present invention can, for example, reduce the amount of trace metal ions initially formed in the ion source due to the lower arc power available to produce the lower charge state ions, while also providing a filtering mechanism to remove trace metal ions from the desired higher charge state ions before implantation into the workpiece, thereby reducing trace metal contamination in the workpiece.

[0035] To mitigate such contamination, the present invention provides a charge stripper configured to obtain the charge state of desired ions so that contaminant ions (e.g., the above-mentioned Ti or Fe ions) can be separated by selecting the desired ions downstream along the beamline after charge stripping.

[0036] Now refer to the accompanying drawings to better understand the present invention. Figure 1A-1B An exemplary ion implantation system 100 is shown in accordance with various exemplary aspects of the present invention. For example, the ion implantation system 100 may sometimes be referred to as a post-acceleration implanter, as described below.

[0037] For example, Figure 1AThe ion implantation system 100 includes a source chamber assembly 102, which includes an ion source 104 and an extraction electrode 106, which is used to extract and accelerate ions to intermediate energies and generally form an ion beam 108. For example, a mass analyzer 110 removes unwanted ion mass and charge species from the ion beam 108 to define an analyzed ion beam 112, whereby an accelerator 114 is configured to accelerate the analyzed ion beam to define an accelerated ion beam 116. According to one example of the present invention, for example, the accelerator 114 includes an RF linear particle accelerator (LINAC), in which ions are repeatedly accelerated by an RF field. Alternatively, the accelerator 114 includes a DC accelerator (e.g., a tandem electrostatic accelerator), in which ions are accelerated by a fixed DC high voltage.

[0038] For example, the ion implantation system 100 further includes an energy filter 118 located downstream of the accelerator 114, wherein the energy filter is configured to remove unwanted energy spectra from the accelerated ion beam 116 emitted from the output end of the accelerator 114 to define a final energy ion beam 120. For example, the beam scanner 122 is configured to scan the final energy ion beam 120 emitted from the energy filter 118, thereby scanning the final energy ion beam back and forth at a rapid frequency to define a scanned ion beam 124. For example, the beam scanner 122 is configured to scan the final energy ion beam 120 electrostatically or electromagnetically to define the scanned ion beam 124.

[0039] The scanned ion beam 124 further passes through an angle correction lens 126, wherein the angle correction lens is configured to convert the fan-shaped scanned beam 124 into a final ion beam 128. For example, the angle correction lens 126 can be configured to parallelize and shift the scanned ion beam 124 to define the final ion beam 128. For example, the angle correction lens 126 can include an electromagnetic or electrostatic device configured to define the final ion beam 128.

[0040] For example, the final ion beam 128 is then implanted into a workpiece 130 (e.g., a semiconductor wafer), which may be selectively positioned in a processing chamber or an end station 132. For example, in a hybrid scanning scheme, the workpiece 130 may be moved orthogonally to the final ion beam 128 (e.g., in and out of the paper) to uniformly irradiate the entire surface of the workpiece 130. It should be noted that the present invention recognizes various other mechanisms and methods for scanning the final ion beam 128 over the workpiece 130, and all such mechanisms and methods are considered to fall within the scope of the present invention.

[0041] For example, Figure 1A-1BThe ion implantation system 100 can be configured as a hybrid parallel scanning single workpiece ion implantation system. For example, the implantation system 100 can also be referred to as a post-acceleration implanter 134 because the main accelerator 114 is located downstream of the mass analyzer 110 and upstream of the energy filter 118. For example, this type of ion implanter provides an energy filter 118 after the accelerator 114 to remove unwanted energy spectra from the accelerator output. However, it should be noted that the present invention recognizes that various aspects of the present invention can be implemented in conjunction with any type of ion implantation system, including but not limited to Figure 1A-1B An exemplary system 100 of .

[0042] In one example, the final kinetic energy of the ion particles passing through the accelerator 114 can be increased by increasing the ion charge value (q). Figure 1A The accelerator 114 shown or as Figure 1B A charge stripper 136 is provided between the two accelerators 114A and 114B shown to increase the ion charge state (q). For example, the accelerator 114 may take a variety of forms and may include any number of accelerator stages defined by or within a single accelerator device, such as in Figure 1A As shown in the example shown in , or defined by or within multiple accelerator devices, such as in Figure 1B For example, in the accelerator 114 as an RF accelerator, as shown in the example shown in FIG. Figure 1A As shown, multiple accelerator stages (e.g., six or more) may include resonators (not shown) for generating accelerating fields, and at least one accelerator stage may include a charge stripper 136 in place of the resonator at that accelerator stage.

[0043] In another example, acceleration of the ion particles can occur before the charge stripper 136 located within the accelerator 114, such as through a first plurality of acceleration stages within the accelerator. Acceleration can also occur after the charge stripper 136, such as through a second plurality of acceleration stages within the accelerator 114. Alternatively, the first plurality of acceleration stages can be external to the accelerator 114. For example, although not shown, the first plurality of acceleration stages can be associated with the mass analyzer 110, so that the ion beam 108 is both accelerated and mass analyzed before entering the charge stripper 136.

[0044] In another example, the charge stripper 136 may be located downstream or after the first accelerator 114A, such as Figure 1BAs shown. For example, the charge selector 138 is located after the charge stripper 136 so as to select the desired ions with a higher charge state after the stripping process. The selected ions with a higher charge state can therefore enter the second accelerator 114B to obtain a higher maximum energy than the ions with the original charge state. Figure 1A and Figure 1B When configuring, for example, Figure 1B The charge selector 138 shown in FIG. 1 can be configured to select only specific ion charge states of the desired ion species while preventing ions of other charge states from entering the second accelerator 114B. Figure 1A Compared to the configuration shown in Figure 1B The configuration of system 100 shown in FIG. 1 can significantly clean up the energy spectrum of desired ions after full acceleration.

[0045] In one example, Figure 1A The mass analyzed ion beam 112 includes positive ions having a first charge state (e.g., 75 As 3+), where the net charge of the ions can be positive. After entering the charge stripper 136, a portion of the positive ions in the first charge state can be converted to more positive positive ions in the second charge state (e.g., 75 As 6+). Thus, the accelerated ion beam 116 exiting the accelerator 114 includes a lower concentration of positive ions of the first charge state and a concentration of ions of the second charge state having an energy higher than the maximum kinetic energy level obtainable using the first charge state. For example, the concentration of the second charge state ions converted from the charge stripping process in the charge stripper 136 can be higher than that of such second charge state ions directly extracted from the ion source 104. For example, the ion beam 108 extracted from the ion source 104 can include any beam species, such as arsenic, boron, phosphorus, or other species.

[0046] Figure 2 An example of a portion of an example accelerator 200 of an ion implantation system according to an aspect of the present invention is illustrated. For example, the accelerator 200 may include an RF accelerator and may include any number of accelerator stages, shown in this example as accelerator stages 202, 204, 205, 206, 208, 210, and 212. Accelerator stages 202, 204, 205, 206, 208, 210, and 212 may each include at least one accelerator electrode 214 driven by, for example, an RF resonator for generating an RF accelerating field (not shown) on either side thereof. An inbound ion beam 216 of charged particles having a charge state (e.g., a net charge or valence) may sequentially pass through the aperture of the accelerator electrode. The principles of acceleration are well known in the art.

[0047] Beam focusing may be provided by a lens 217 (e.g., an electrostatic quadrupole) incorporated into the accelerator 200. In one embodiment, the accelerator 200 may accelerate singly charged ions to a maximum kinetic energy level of a first charge state. In one embodiment, ions having a higher second charge state may be used to reach energy levels higher than the maximum kinetic energy level of a lower first charge state. Thus, an inbound ion beam 216 including ions of a first charge state may enter the accelerator 200 as an incoming beam and be converted into ions of a second charge state having a higher or lower net charge valence. The inbound ion beam 216 may be converted into ions of a higher second charge state (e.g., 75 As 3+ converts to 75 As 6+), thereby increasing the beam energy to a maximum kinetic energy level exceeding the first charge state.

[0048] For example, once the inbound ion beam 216 is extracted and formed, the inbound ion beam can be accelerated by an accelerator 200 (e.g., a 13.56 MHz twelve-resonator RF linear accelerator). The present invention is not limited to a particular accelerator or linear accelerator (LINAC) type. In one embodiment, the accelerator 200 can include: a first plurality of accelerator stages 230 integrated into the accelerator 200 for accelerating the inbound ion beam 216 therein, and a second plurality of accelerator stages 232 integrated into the accelerator 200 for further accelerating the inbound ion beam 216 to exit the accelerator 200 as an outbound ion beam 218. Although in Figure 2 In the illustrated example, the first plurality of accelerator stages are integrated into the accelerator 200 and are upstream of the charge stripper 220, but the first plurality of accelerator stages 230 may alternatively be located before a mass analyzer (e.g., the mass analyzer 110 of FIG. 1 ). Thus, for example, the charge stripper 220 may be located at any of the acceleration stages of the accelerator 200, as long as the first plurality of accelerator stages provide sufficient energy to the ions of the first charge state, such that the energy is high enough to produce a high stripping efficiency for producing a concentration of ions of the second charge state greater than that available at the ion source.

[0049] For example, the resonator of the RF accelerator may be replaced with a charge stripper 220 at any acceleration stage. In one embodiment, for example, the charge stripper 220 may be disposed downstream of at least one of the first plurality of accelerator stages 230 of the accelerator along the direction of the inbound ion beam 216 and upstream of at least one of the second plurality of accelerator stages 232 of the accelerator 200. In other embodiments, for example, the first plurality of accelerator stages 230 of the accelerator 200 may include more or fewer accelerator stages than the second plurality of accelerator stages 232. Alternatively, the first plurality of accelerator stages 230 of the accelerator 200 may include the same number of accelerator stages as the second plurality of accelerator stages 232. The number of stages is not limited to Figure 2 Example shown.

[0050] In a further embodiment, an inbound ion beam 216 entering the accelerator 200 includes a positive ion beam of a first charge state, and an outbound ion beam 218 includes a positive ion beam of a second charge state, the second charge state including a charge state that is more positive than the first charge state. The inbound ion beam 216 can enter a charge stripper 220, which, for example, includes a stripper tube 234 (e.g., a thin tube filled with a heavy molecular weight gas). For example, the charge stripper 220 can also include a pump 236 (e.g., a differential turbo pump) for pumping gas from a gas source 238 to reduce or control the amount of gas flow flowing into adjacent accelerator sections. For example, the gas includes sulfur hexafluoride (SF6) or another high molecular weight gas for effectively stripping electrons from the inbound ion beam 216 and producing a higher concentration of ions including a higher positive charge state within the outbound ion beam 218. For example, the charge stripper 220 and / or the pump 236 may include a control device 240 configured to regulate the flow rate of gas from the gas source 238 into the charge stripper 220. The flow rate of the gas may be functionally based on at least one of the energy, current, and / or substance of the inbound ion beam 216. The charge stripper 220 may also include pumping baffles 242 (e.g., differential pumping baffles) on both sides of the charge stripper 220. For example, the pumping baffles 242 may be used with the differential pump 236 to minimize gas leakage into adjacent accelerator stages (e.g., accelerator stages 205 and 206).

[0051] For example, an ion beam accelerated by a first linear accelerator (LINAC) is directed to a gas layer configured to strip electrons around ions in a charge stripper 220 to increase the charge state of the ions to achieve a higher energy gain by a second LINAC. For example, for the highest energy range of arsenic (As), 3+ As ions accelerated by the first LINAC are stripped to 6+ As ions by the charge stripper 220. Thus, for example, approximately 8% of 7MeV 3+ As ions are converted to 6+ As ions. However, if the conversion efficiency is higher, more 6+ beam current can be achieved.

[0052] Tandem high energy accelerators typically rely on charge stripping to produce high energy ions, whereby such tandem high energy accelerators typically use argon gas for this charge stripping. On so-called "ultra-high energy" tandem accelerators, extremely thin carbon foils have also been used as charge strippers, but the short lifetime of carbon foils has limited their applicability to any industrial application in ion implantation, and they are currently known to be used only in academic research accelerators. For example, Figure 3 The graph 300 shown in provides charge stripping capabilities associated with passing a 10 MeV iodine ion beam through various gases and foils.

[0053] So far, the stripping of ions has generally been limited to e.g. Figure 4 400 in FIG. 4 and is primarily limited to the use of argon. For example, sulfur hexafluoride (SF6) gas can be advantageously used to strip arsenic ions in a gas stripper, whereby the charge state distribution tends to shift to higher charge states, so that the yield of 6+ ions, for example, is nearly twice that of conventional use of argon.

[0054] For example, Figure 3 The graph 300 shown in shows a comparison of the charge state distribution of a 3+ arsenic ion beam at 7200 KeV after passing through a gas stripper containing SF6 and after passing through a gas stripper containing argon according to one example of the present invention. As shown, the use of SF6 doubles the 5+ and 6+ ion yields, thereby increasing the final beam current of the 5+ and 6+ ion beams by a factor of two. This increase in efficiency is significant, with about 8% conversion being achieved when argon is used in the stripper to process 6+ ions, while the use of SF6 provides about 16% conversion or about twice the amount of 6+ ion beams.

[0055] For example, a gas stripper works by passing ions through a material, and if the ions pass through the material at a fast enough rate, interactions with background gas or solid film atoms in the stripper can cause the ion beam to lose electrons. Thus, depending on the rate at which the ions enter the stripper, the ion beam exits the stripper with a higher charge. While a gas stripper may be desirable in some cases, the present invention further contemplates the use of a thin film stripper using a very thin carbon film to provide a greater number of higher charge ions. However, a thin film stripper may have a shorter lifetime than a gas stripper, depending on the process requirements.

[0056] In an example gas stripper, generally speaking, a tube is provided within the gas stripper, whereby stripping gas is supplied to the center of the tube, wherein the gas has a higher gas density than the surrounding vacuum. At the end of the tube, a vacuum is provided (e.g., by a vacuum pump) so that a minimum amount of stripping gas propagates to the rest of the system. Thus, a high pressure region is provided within the stripper, whereby the accelerated arsenic ions pass through the high pressure region and interact with the stripping gas atoms, thereby stripping electrons from the ions and producing higher charged ions that are ejected from the stripper. Such higher charged ions are also advantageously used in tandem accelerators.

[0057] For example, argon has a molecular weight of about 40, while SF6 is much heavier, having a molecular weight of about 146. Therefore, one theory is that the heavier the gas used in the stripper, the more efficient it is at stripping electrons from the ion beam. SF6 is one of the heavier gas molecules that is more readily available for commercial use, and is therefore considered superior to other heavier gases. However, other heavy molecular weight gases (e.g., heavier than argon) can be similarly used for electron stripping. For example, SF6 can be advantageous because it is an effective gas for suppressing high voltage arcs.

[0058] To date, SF6 has not been considered an ideal gas for use in gas strippers or elsewhere in beamlines. For example, SF6 is toxic to the environment, which can be a problem if the gas is pumped out to the atmosphere, or otherwise escapes from within a gas stripper. Therefore, the present invention further contemplates decomposing SF6 into its less toxic and / or less volatile components. Alternatively, the SF6 can be recycled and reused.

[0059] It will be appreciated that the conventional use of SF6 is to suppress arcs outside of a beamline in a high voltage insulating tank, whereas the present invention uses SF6 within a beamline to strip electrons, wherein the beamline provides an environment and application that is significantly different from previous uses of SF6. Conventionally, one of ordinary skill would not use SF6 in the high voltage region of a beamline vacuum, as doing so would make it difficult to maintain voltage. For example, it will be appreciated that when SF6 is provided in a vacuum, sparks tend to occur, making the use of SF6 in a gas stripper counterintuitive, as one of ordinary skill would not want the presence of SF6. In a beamline, this is because SF6 is assumed to cause harmful arcs or sparks.

[0060] SF6 advantageously provides additional benefits in a gas stripper because SF6 is a heavier gas than the argon used in conventional gas strippers, thereby advantageously facilitating localization of high pressure regions due to lower conductivity through the tube, which is inversely proportional to the square root of the molecular weight of the gas. For example, SF6 can be fed into the middle of the tube of a gas stripper to create a localized high pressure region. If a lighter gas, such as hydrogen, is used in a gas stripper, it diffuses rapidly and is difficult to confine to a localized region.

[0061] According to one or more examples, various systems and methods are provided herein to increase the beam current available at the maximum kinetic energy of a charge state without requiring the use of a higher or different charge state at the ion source. For example, an ion source of an ion implantation system may include a particular charge state (e.g., 75 As 3+) ions (e.g., arsenic ions) are selected to produce an ion beam therefrom. Processing within an ion implantation system (e.g., within an accelerator located along the beam path) can cause the ions to change their initial charge values ​​(e.g., charge exchange reactions). For example, in one embodiment, arsenic ions containing three net positive charges can be selected into an accelerator and stripped of electrons by a charge stripper including a gas source and a turbo pump. According to the present disclosure, the gas source includes a high molecular weight gas, such as sulfur hexafluoride (SF6).

[0062] In one embodiment, the accelerator may include multiple accelerator stages and charge strippers therein. When a high-speed ion approaches another gas molecule or atom within the charge stripper, the ion may gain an electron from the molecule or atom (i.e., an electron capture reaction), or may lose an electron to the molecule or atom (i.e., a charge stripping reaction). The former reaction reduces the value of the ion charge by one; for example, a singly charged ion may become a neutral, i.e., electrically neutral atom. The latter reaction increases the value of the ion charge by one (e.g., a singly charged ion becomes a doubly charged ion).

[0063] In one embodiment, positive ions (e.g., arsenic ions) comprising a first positive charge state (e.g., +3 net positive charge or valence) are introduced into an accelerator comprising a charge stripper and multiple acceleration stages. Each acceleration stage may include an RF resonator for generating an RF acceleration field to accelerate the ions along a beam path. The charge stripper may include: a gas source for discharging a high molecular weight gas (e.g., SF6) into the accelerator, and a turbo pump for generating a vacuum to exhaust the gas and prevent the gas from flowing into the acceleration stage. The charge stripper may replace one of the acceleration stages within the accelerator to strip ions of electrons, thereby converting the positive ions entering the accelerator into more positive positive ions of a second charge state (e.g., +6 net positive charge or valence) exiting the accelerator.

[0064] Now refer to Figure 5 and Figure 6, various methods 500, 600 are provided for controlling contamination and increasing ion beam current at relatively high energies to obtain a desired charge state. It should be noted that although the exemplary methods 500 and 600 are illustrated and described herein as a series of actions or events, it should be understood that the present disclosure is not limited by the illustrated order of these actions or events, because, according to the present disclosure, some steps can occur in a different order and / or occur simultaneously with other steps other than those shown and described herein. In addition, not all steps shown are required when implementing the methods according to the present disclosure. In addition, it should be understood that these methods can be implemented in association with the systems and devices shown and described herein, and can also be implemented in association with other systems not shown.

[0065] Figure 5 The method 500 begins at 502. In act 504, an ion source produces first charge state ions and trace metal ions, thereby extracting or otherwise forming a first ion beam including the first charge state ions and the trace metal ions in act 506. For example, the first ion beam formed in act 506 can be directed to a mass analyzer, thereby performing mass analysis on the first ion beam. The magnetic field strength of the mass analyzer can be selected based on the charge-to-mass ratio. In one example, the mass analysis can be downstream of the ion source.

[0066] In one example, first charge state ions and trace metal ions having the same magnetic rigidity may be selected (e.g., via a mass analyzer) to enter the accelerator. The selected first charge state ions are accelerated to an energy that produces a higher stripping efficiency for higher charge states than the energy available at the ion source. In action 508, after being accelerated to a higher energy, at least one electron is stripped from the first charge state ions via a charge stripper, thereby defining a second ion beam containing second charge state ions and trace metal ions. In action 510, second charge state ions are selected from the second ion beam to form a final ion beam mainly including second charge state ions and approximately zero trace metal ions. It should be noted that, compared to the amount of trace metal ions in the first or second ion beam, "approximately zero" trace metal ions represent a significant reduction in trace metal ions in the final ion beam. Therefore, "approximately zero" can be interpreted as zero trace metal ions or a relatively small number of trace metal ions, so that it can be considered insignificant to ion implantation. In one example, positive ions in the second positive charge state can be further accelerated to the final energy at action 512. Thus, the present invention provides that, in act 514, about 50% to 100% of the trace metal ions are removed from the second ion beam before the final ion beam implants the second charge state ions into the workpiece.

[0067] Reference Figure 6 , Figure 6Method 600 begins at 602. In act 604, an ion source generates an ion beam including ions in a first charge state and trace metal ions. For example, the ions in the first charge state may include 75 As 3+ ions, trace metal ions include 50 It should be noted that the present invention recognizes that various other ions of the first charge state may be produced together with various other trace metal ions, and all such combinations further fall within the scope of the present invention. For example, the ions of the first charge state may include 75 As 4+ ions, whereby trace metal ions include 56 Fe 3+ ions.

[0068] For example, the ion beam can be a variety of beam species (e.g., arsenic). In action 606, the ion beam can be accelerated and mass analyzed, with no particular order of operations. In action 608, ions of the first charge state can be selected into the accelerator simultaneously with trace metal ions in the ion beam. In action 610, the first charge state ions can be further accelerated to a final energy and stripped of electrons by passing the ion beam through a charge stripper to convert the first charge state ions to a second charge state (e.g., As 4+). In action 612, ions of the second charge state can be selected from the ion beam, thereby removing substantially all trace metal ions from the ion beam before the ions of the second charge state are implanted into the workpiece.

[0069] It should be understood that the exemplary contamination scenario discussed herein (eg, Fe contamination in an As ion beam) is merely an example, and the present invention further contemplates mitigating all other metal contaminations with similar principles.

[0070] In general, the desired ions extracted from the ion source can be written as m X a , the impurity ions embedded in the extracted ion beam are n y b , where "m" is the atomic mass of ion X, "a" is the charge state of ion X, "n" is the atomic mass of contaminant ion Y, and "b" is the charge state of ion Y. This principle applies to the following situations: when the energy of the desired ion is "a*E" and the energy of the contaminant ion is "b*E", and when the atomic mass ratio "m / n" is equal to the charge ratio "a / b", where the contaminant ion Y cannot be separated (e.g., distinguished) by magnetic filters, electrostatic filters, and velocity filters.

[0071] When a dopant gas material is introduced into an arc chamber of an ion source to generate ions, dopant species of various charge states, as well as possible trace metal ions, are generated during the plasma formation process. For example, when a dopant gas material containing AsH3 is fed into an arc chamber of an ion source for plasma ignition to obtain As ions, many dopant species of various charge states are generated. 75 As ions (e.g., As+, As 2+, As 3+, As 4+, etc.) and one or more such as 50 Ti 2+, 56 Trace metal ions such as Fe 3+ are generated in the ion source. For example, the arc chamber of the ion source may include various metal components composed of metals such as tungsten (W), molybdenum (Mo), and other metals. The arc chamber may also include other components that may contain trace metals such as titanium (Ti), iron (Fe), and / or other metals.

[0072] Typically, an extraction voltage V is applied to the extraction electrode to extract ions from the arc chamber of the ion source, thereby forming an ion beam and directing the ion beam to a mass analyzer. However, in some cases, different ion species may be selected to be output from the mass analyzer because their mass-to-charge ratios are substantially similar or identical. For example, when the mass analyzer is configured to select ions from the ion beam 75 For As 3+ ions, the mass analyzer can also be selected with 75 As 3+ has the same mass-to-charge ratio such as 50 ions of Ti 2+ (e.g., 50 / 2 for Ti equals a mass-to-charge ratio of 25, which is the same as 75 / 3 for As). Therefore, in this example, 50 Ti2+ and 75 The As3+ ions will all pass through the mass analyzer into the downstream beamline components for further acceleration. It should be noted that before this further acceleration, the example 75 As 3+ ions and 50 The energy ratio of Ti 2+ ions (e.g. E As / E Ti ) is 3:2, which is due to the applied extraction voltage and the charge state of the ions. In addition, 75 As 3+ ion (v As )and 50 Ti2+ ions (v Ti ) are also similar or identical in speed, because their mass ratio is also 3:2, which can be obtained by using the kinetic energy formula E = (1 / 2) mv 2 Conclude.

[0073] For example, when including ions of a first charge state (e.g. 75 As 3+ ions) and trace metal ions (e.g. 50When the ion beam containing Ti2+ trace metal ions) is further accelerated by a downstream RF linear accelerator (LINAC), the two ions will synchronize with the RF phase change at the accelerating electrode because they enter the RF LINAC at essentially the same speed. Therefore, the individual ions will leave the RF LINAC at essentially the same final velocity and energy ratio (e.g., 3:2). As well as providing acceleration, the RF LINAC can also act as a velocity filter; however, since the velocities are identical, the RF LINAC cannot distinguish between the two ions. 75 As 3+ ions and 50 Ti 2+ ions.

[0074] In the case where the ion beam is accelerated by a DC-based accelerator rather than the RF LINAC discussed above, the energy gain of the ions is based on their respective charge states and the voltage provided for acceleration. Thus, in a similar manner, 75 As 3+ ions and 50 The final energy ratio of the Ti 2+ ions will also be the same (eg, 3:2).

[0075] For example, downstream electromagnets cannot separate ions even after final acceleration by an RF or DC based accelerator, for reasons similar to those described above. On the other hand, electrostatic energy filters can be used in ion implantation systems to separate ions with different energy to charge ratio selections. However, in this case, electrostatic energy filters cannot separate them either; because after final acceleration, 75 As3+ ions and 50 The energy ratio of Ti 2+ is 3:2, and the charge ratio is also 3:2.

[0076] Therefore, when high charge state ions are required to achieve high energies for implantation into a workpiece, a filtering solution is needed for this particular trace metal contamination mechanism from the ion source without adding significant beamline length to accelerate low charge state ions.

[0077] Thus, the present invention describes an example apparatus including a charge stripper and a charge state selector, wherein the charge state selector is an electromagnetic mass analyzer located downstream of and immediately adjacent to the charge stripper. For example, when the charge state selector is set to 75 As 3+ ions undergo a charge stripping process and are selected from the ion beam 75 As 4+ ions (e.g. second charge state) rather than 75 As 3+ ions (e.g., the first charge state) are present, such a combined device can immediately remove from the above ion beam such as 50 Trace metal ions of Ti2+. 75The mass-to-charge ratio of As 4+ ion is 75 / 4 (i.e. 18.75), while 50 The mass-to-charge ratio of the Ti 2+ ion is still 50 / 2 (i.e. 25), so trace metal ions can be removed even though their velocities are similar.

[0078] Thus, for example, when the charge state selector after the charge stripper is set to select 75 As 4+ ions instead of 75 As 3+ ions, trace metal 50 The Ti 2+ ions will not be selected and therefore removed from the ion beam. In fact, after charge stripping there will be no other Ti charge state ions that can be selected to react with 75 As 4+ pass through the charge state selector magnet together. Therefore, the selected 75 As 4+ ions can be further accelerated by additional RF or DC based accelerators so that there are no trace metals in the ion beam 50 The required higher implant energy is achieved in the absence of Ti 2+ contamination, thereby providing the required implantation into the workpiece.

[0079] In another embodiment, the charge state selector may include an electrostatic filter, whereby the electrostatic filter selects the desired second charge state ions based on the energy-to-charge ratio after the charge stripping process. For example, as described above, 75 The first charge state of As 3+ ions is 50 The energy ratio of trace metal ions of Ti 2+ is 3:2, and their charge ratio is also 3:2. Therefore, since the energy-to-charge ratio of the ions is the same (i.e. 1), the electrostatic filter cannot separate these ions. After the charge stripping process, the electrostatic filter can be configured to select ions with an energy-to-charge ratio of 3:4. 75 The second charge state of As 4+ ions, and 50 The trace metal ions of Ti 2+ are filtered out due to their different energy to charge ratio of 2:2. Similarly, there will be no other possible 50 Ti charge state ions can be selected to 75 As 4+ pass through the electrostatic filter together. 75 As 4+ ions can be further accelerated by additional RF or DC based accelerators so that there are no trace metals in the ion beam 50 The required higher implant energy is achieved in the absence of Ti 2+ contamination, thereby providing the required implantation into the workpiece.

[0080] The present invention recognizes that when the mass analysis magnet is configured to select ions from the ion source 75 A similar mechanism to that described above for trace metal contamination may also occur when As 4+ ions are present, where56 Fe3+ trace metal ions become pollutants and can 75 The As 4+ ions pass through the mass analyzer together. Similarly, a filtering solution combining the above-mentioned charge stripper and a downstream charge state selector can also be used.

[0081] The present invention further provides for selecting charge states in combination with specific dopants and / or acceleration levels. Figure 7 A simplified system 700 according to another example of the present invention is further shown. For example, an ion beam 702 is formed at an ion source 704, whereby the ion beam is extracted from the ion source, mass analyzed by a mass analyzer 705, and passes through a first accelerator 706 (e.g., a linear accelerator or LINAC) and a charge stripper 708. As described above, the ion beam 702 at the exit of the first accelerator 706 may include ions of a first charge state (e.g., 75 As 3+) and trace metal ions (e.g. 56 Ti 2+). The charge stripper 708 in this example is located between the first accelerator 706 and the first magnet 710 and is configured to convert the first charge state ions into other charge state ions, including the desired second charge state ions (e.g. 75 As 4+). A second accelerator 712 (e.g., a so-called energy enhancer or E-enhancer) can be further disposed between the first magnet 710 and the second magnet 714 to accelerate the second charge state ions selected by the first magnet 710 to a higher energy. After passing through the second magnet 714, the ion beam 702 is directed to an end station 716 for implantation into a workpiece. For example, the first magnet 710 can be used as a charge state selector.

[0082] For example, the first magnet 710 may include a first 60 degree magnet disposed between the first accelerator 706 and the E-enhancer 712, and the second magnet 714 may include a 120 degree magnet disposed after the E-enhancer for energy filtering. It should be noted that although specific angles of the first magnet 710 and the second magnet 714 are described herein, various alternatives exist and are considered to fall within the scope of the present invention.

[0083] An advantageous aspect of the above example is that by providing a first magnet 710 downstream of the charge stripper 708 to select desired second charge state ions from the first charge state ions after undergoing the stripping process at the charge stripper 708, the first magnet 710 substantially removes all other charge states and trace metal ions.

[0084] In an alternative embodiment, if Figure 8 As shown, it can be omitted Figure 7The second accelerator 712 and the second magnet 714 are shown. Figure 8 After charge stripping at the charge stripper 708, for example, the second charge state ion beam 702 in the second charge state selected by the first magnet 710 without trace metal ion contamination can be implanted into the workpiece in the end station 716 without further acceleration.

[0085] Although the present invention is shown and described with respect to certain applications and implementations, it should be understood that equivalent changes and modifications will occur to other skilled persons in the art after reading and understanding this specification and the accompanying drawings. In particular, for the various functions performed by the above-mentioned components (components, devices, circuits, systems, etc.), unless otherwise specified, the terms used to describe such components (including references to "devices") are intended to correspond to any component (i.e., a functionally equivalent component) that performs the specified functions of the above-mentioned components, even if the structure is not equivalent to the disclosed structure that performs the described functions in the exemplary embodiments of the present invention shown herein.

[0086] Furthermore, while particular features of the invention may be disclosed for only one of several embodiments, such features may be combined with one or more other features of other embodiments as may be desired and advantageous for any given or particular application. In addition, to the extent that the terms "including" and "having" are used in the specification or claims, these terms are intended to be inclusive in a manner similar to the term "comprising".

Claims

1. An ion implantation system comprising: an ion source configured to produce desired ions of a desired substance and trace metal ions of a contaminating substance, wherein the desired ions of the desired substance are in a first charge state, and wherein the charge-to-mass ratio of the desired ions and the trace metal ions is equal, wherein when the desired ions in the first charge state include 75 When As3+ ions are present, the trace metal ions include 50 Ti2+ ions, and when the desired ions in the first charge state include 75 When As4+ ions are present, the trace metal ions include 56 Fe3+ ions; an extraction device configured to extract the desired ions and trace metal ions from the ion source to define a first ion beam; a mass analyzer configured to select the desired ions and trace metal ions from the first ion beam, thereby defining a mass analyzed ion beam; an accelerator configured to accelerate the mass analyzed ion beam from a first energy to a second energy; a charge stripping device configured to strip at least one electron from the desired ions, thereby defining a second ion beam comprising the desired ions and the trace metal ions in a second charge state; and A charge selector is located downstream of the charge stripping device, wherein the charge selector is configured to selectively pass only the desired ions in the second charge state, thereby defining a final ion beam including the desired ions in the second charge state without trace metal ions.

2. The ion implantation system of claim 1, wherein the charge selector comprises an electromagnetic mass analyzer located proximate to the charge stripping device.

3. The ion implantation system of claim 1, wherein the charge stripping device is located within the accelerator.

4. The ion implantation system of claim 1, wherein the accelerator comprises a plurality of accelerator stages, and wherein the charge stripping device is located between at least two of the plurality of accelerator stages.

5. The ion implantation system of claim 4, wherein the accelerator comprises an RF accelerator, and wherein the plurality of accelerator stages each comprises one or more resonators configured to generate an accelerating field.

6. The ion implantation system of claim 1, wherein the accelerator comprises a DC accelerator configured to accelerate the desired ions by a constant DC high voltage.

7. The ion implantation system of claim 1, wherein the accelerator comprises an RF accelerator and includes one or more resonators configured to generate an accelerating RF field.

8. The ion implantation system of claim 1, wherein the charge selector comprises an energy filter.

9. The ion implantation system of claim 1, further comprising a scanner configured to reciprocally scan the final ion beam to define a scanned ion beam.

10. The ion implantation system of claim 9, wherein the scanner comprises an electrostatic scanner or an electromagnetic scanner configured to electrostatically or electromagnetically scan the final ion beam, respectively, to define a scanned ion beam.

11. The ion implantation system of claim 9, further comprising an angle correction lens configured to parallelize and deflect the scanned ion beam.

12. The ion implantation system of claim 1, wherein the charge stripping device comprises a pump and a gas source, wherein the pump is configured to pump gas from the gas source and control the flow of the gas into the accelerator, wherein the flow of the gas is configured to strip electrons from the ions of the desired species.

13. The ion implantation system of claim 12, wherein the gas comprises sulfur hexafluoride.

14. An ion implantation system comprising: An ion source configured to generate an ion beam along a beam line, wherein the ion beam includes ions of a first charge state and trace metal ions, wherein the trace metal ions include 50 Ti2+ ions, the first charge state ions include 75 As3+ ion; a mass analyzer for performing mass analysis on the ion beam along the beam line; an accelerator disposed along the beam line and configured to accelerate the ion beam from a first energy to a second energy; a charge stripping device configured to strip at least one electron from the first charge state ions along the beam line, thereby defining a stripped ion beam including second charge state ions and the trace metal ions, wherein the charge stripping device comprises a pump and a gas source, wherein the pump is configured to pump a gas from the gas source and control the flow of the gas into the accelerator, wherein the flow of the gas is configured to strip electrons from the first charge state ions, wherein the gas comprises sulfur hexafluoride; A charge selector device is located downstream of the charge stripping device along the beamline and is configured to select the second charge state ions in the stripped ion beam to define a final ion beam along the beamline, wherein the final ion beam includes the second charge state ions and approximately zero trace metal ions.

15. The ion implantation system of claim 14, wherein the accelerator comprises a plurality of accelerator stages, wherein the charge stripping device is located downstream of at least one of the plurality of accelerator stages.

16. The ion implantation system of claim 15, further comprising an energy filter located downstream of the accelerator, wherein the charge selector device is located between at least two of the plurality of accelerator stages, and wherein the energy filter is further configured to filter the final ion beam.

17. A method for implanting high charge state ions into a workpiece while mitigating trace metal contamination, the method comprising: generating desired ions of a desired substance in an ion source, wherein the desired ions are in a first charge state, and wherein generating the desired ions further generates trace metal ions of a contaminant substance, wherein the charge-to-mass ratio of the desired substance and the contaminant substance is equal, wherein when the desired ions in the first charge state include 75 When As3+ ions are present, the trace metal ions include 50 Ti2+ ions, and when the desired ions in the first charge state include 75 When As4+ ions are present, the trace metal ions include 56 Fe3+ ions; extracting the desired ions and trace metal ions from the ion source to define a first ion beam; mass analyzing the first ion beam to define a mass analyzed ion beam; passing the mass analyzed ion beam through a charge stripping element, wherein the charge stripping element strips at least one electron from the desired ions, thereby defining a second ion beam including the desired ions and the trace metal ions in a second charge state; as well as The desired ions in the second charge state are implanted into the workpiece.

18. The method of claim 17, further comprising accelerating the mass analyzed ion beam.

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