Stepped indirect heating cathode with improved shielding

Abstract

An ion source for forming a plasma has a cathode having a cavity and a cathode surface defining a cathode step. A filament is disposed within the cavity. A cathode shield has a cathode shield surface that at least partially surrounds the cathode surface. A cathode gap is formed between the cathode surface and the cathode shield surface, defining a tortuous path for restricting the plasma from passing through the gap. The cathode surface may have a stepped cylindrical surface defined by a first cathode diameter and a second cathode diameter, which are different from each other to define the cathode step. The stepped cylindrical surface may be an outer surface or an inner surface. The first cathode diameter and the second cathode diameter may be concentric or axially eccentric.

Description

[0001] Citation of relevant applications

[0002] This application claims the benefit of U.S. Provisional Application No. 63 / 040,724, filed on June 18, 2020, the entire contents of which are incorporated herein by reference in their entirety. Technical Field

[0003] The present invention relates to ion implantation systems, and in particular to improved ion sources and beamline assemblies for improving the lifespan, stability, and various aspects of operation of ion implantation systems. Background Technology

[0004] In the manufacturing 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 using ions from an ion beam to form passivation layers or generate n-type or p-type material dopant during integrated circuit manufacturing. This beam treatment is typically used during integrated circuit manufacturing to selectively implant impurities from specific dopant materials into wafers at predetermined energy levels and controlled concentrations to generate semiconductor materials. When used to dope semiconductor wafers, ion implantation systems implant a selected ion material into the workpiece to generate the desired intrinsic material. For example, implanting ions from source materials such as antimony, arsenic, or phosphorus produces “n-type” intrinsic material wafers, while “p-type” intrinsic material wafers are typically produced by ions 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, a beam delivery device, and a wafer processing device. The ion source generates ions of the desired atomic or molecular dopant material. These ions are extracted from the source by an extraction system, typically a set of electrodes that excite and guide the ion stream from the source, forming an ion beam. In the mass analysis device, the desired ions are separated from the ion beam; this device is typically a magnetic dipole that performs mass dispersion or separation of the extracted ion beam. The beam delivery device is typically a vacuum system containing a series of focusing devices that delivers the ion beam to the wafer processing device while maintaining the desired properties of the ion beam. Finally, the semiconductor wafer is transferred to and from the wafer processing device by a wafer processing system, which may include one or more robotic arms for placing the wafer to be processed in front of the ion beam and removing the processed wafer from the ion implanter.

[0006] Ion sources (often called arc discharge ion sources) generate ion beams for use in implanters. These may include a heatable filament cathode for generating ions shaped to suitably process wafers. U.S. Patent No. 5,497,006 to Sferlazzo et al. discloses an ion source having a cathode supported by a base and positioned relative to a gas confinement chamber for injecting electrons for ionization into the gas confinement chamber. The cathode disclosed in Sferlazzo et al.'s patent is a tubular conductor with end caps extending partially into the gas confinement chamber.

[0007] Figure 1 The image shows a cross-section of a conventional ion source 10 used in a conventional ion implantation system. A filament 12 is resistively heated to a temperature at which thermionic emission of electrons occurs. The voltage between the filament 12 and the cathode 14 (the so-called "cathode voltage") causes the emitted electrons to be accelerated from the filament toward the cathode until the cathode itself emits electrons thermoelectrically. This emission mechanism is known in the industry as an indirectly heated cathode (IHC). The cathode 14 serves two purposes: protecting the filament 12 from bombardment by plasma (also known as "plasma") ions and providing electrons for subsequent ionization.

[0008] The cathode 14 is negatively polarized relative to the arc chamber 16, in which a so-called "arc voltage" exists, and the emitted electrons are accelerated toward the center 18 of the arc chamber. A feed gas (not shown) flows into the arc chamber 16, and the emitted electrons subsequently ionize the feed gas to form a plasma (not shown), in which ions can be extracted through an extraction slit 20 in the arc chamber. In one example, the repulsion electrode 22 can be charged to the negative plasma drift potential, repelling electrons back into the plasma, thus resulting in enhanced ionization and a higher plasma concentration. A magnetic field (not shown) parallel to the central axis 24 defined by the cathode 14 and the repulsion electrode 22 generally confines the emitted and repelled electrons to define a so-called "plasma column," further enhancing ionization and plasma concentration.

[0009] Typically, the cathode 14 itself will fail first due to sputtering and erosion by plasma ions, as well as eventual penetration or punch-through. This is especially true when tuning multi-charge ion beams, resulting in a significantly shorter ion source lifetime compared to tuning single-charge ion sources. Puncture-through typically occurs when the cathode wall 26 is thinnest. Figure 2The image shows a penetration 28 occurring at the wall 26 of the cathode 14, which causes the ion source to fail even when there is still sufficient material at the front 30 of the cathode. This type of failure is common when high-energy injection of multi-charged ions. Significant sputtering of the cathode 14 frequently occurs because elements such as arsenic (As) are extremely heavy. Furthermore, for multi-charged ions, the applied arc voltage is higher, further shortening the lifespan of conventional ion sources. Summary of the Invention

[0010] Therefore, this invention provides a system and apparatus for increasing the efficiency and lifetime of an ion source. To facilitate understanding of some aspects of the invention, a brief overview is provided below. This overview is not a comprehensive summary of the invention, nor is it intended to indicate key or essential elements of the invention, nor to describe its scope. Its purpose is to present some of the concepts of the invention in a simplified form as a prelude to the detailed embodiments presented later.

[0011] According to one aspect of the invention, an ion source for forming a plasma is provided, the ion source comprising a cathode and a cathode shield. In one example, the cathode comprises a cavity in which a filament is disposed. The cathode further comprises a cathode surface defining a cathode step. In one example, the cathode shield has a cathode shield surface at least partially surrounding the cathode surface, and a cathode gap is defined between the cathode surface and the cathode shield surface. In one example, the cathode gap defines a tortuous path for the plasma to extend the cathode lifetime.

[0012] In one example, the cathode surface comprises a stepped cylindrical surface defined by a first cathode diameter and a second cathode diameter. The first cathode diameter differs from the second cathode diameter, thus defining the cathode step.

[0013] In one example, the stepped cylindrical surface includes an outer surface defining the diameters of the first and second cathodes. The cathode shield surface may further define a cathode shield step, wherein one profile of the cathode shield step substantially matches the cathode step, thereby maintaining the cathode gap between the cathode surface and the cathode shield surface. In one example, the second cathode diameter is larger than the first cathode diameter, and the second cathode shield diameter is larger than the first cathode shield diameter. The first and second cathode diameters may be concentric or axially offset by a predetermined distance.

[0014] In another example, the stepped cylindrical surface of the cathode includes an inner surface defining the diameters of the first and second cathodes. In one example, the diameter of the second cathode is larger than the diameter of the first cathode, thereby forming a thickened wall of the cathode. In one example, a filament gap may be further defined between the filament and the thickened wall of the cathode, wherein the thickened wall of the cathode typically extends the life of the cathode.

[0015] In one example, the cathode includes a hollow cylindrical portion extending from a solid cylindrical portion at a transition region, within which a cavity is defined, and a cathode step is defined within the transition region. In another example, the cathode shield radially surrounds the cathode while axially exposing the front cathode surface of the cathode to the plasma.

[0016] According to another aspect of the invention, an ion source is provided, wherein a cathode has a cathode step defined therein. A cathode shield has a cathode shield step defined therein, wherein the cathode shield radially surrounds the cathode while maintaining a cathode gap between the cathode and the cathode shield. In one example, the cathode shield step substantially matches the cathode step while maintaining the cathode gap, thereby providing a curved path for plasma propagation.

[0017] In one example, the cathode includes a hollow cylindrical portion extending from a solid cylindrical portion adjacent to a transition region, wherein the cathode step is defined within the transition region. In one example, a filament may be disposed within the hollow cylindrical portion of the cathode. In one example, the cathode has a first cathode diameter and a second cathode diameter, thereby defining the cathode step, wherein the first cathode diameter and the second cathode diameter are concentric or axially eccentric by a predetermined distance.

[0018] According to another aspect of the invention, a cathode system (also referred to as a "cathode assembly" or cathode "combination") for an ion source is provided. In one example, the cathode system includes a stepped cathode and a stepped cathode shield radially surrounding the stepped cathode, with a gap maintained between the stepped cathode and the stepped cathode shield. In one example, the inner surface of the stepped cathode shield substantially matches the outer surface of the stepped cathode, thereby defining a curved path between the stepped cathode and the stepped cathode shield. In one example, the stepped cathode may include a plurality of cathode steps defined within the outer surface of the stepped cathode, and the stepped cathode shield further includes a plurality of shielding steps defined within the inner surface of the stepped cathode shield.

[0019] To achieve the foregoing and related objectives, the present invention includes the features fully described below and particularly pointed out in the claims. Certain exemplary embodiments of the invention are illustrated in detail below with reference to the accompanying drawings. However, these embodiments are merely some of the various ways in which the principles of the invention can be employed. Other objects, advantages, and novel features of the invention will become clearer when the following detailed description of the invention is understood in conjunction with the accompanying drawings.

[0020] Brief description of the attached figures

[0021] Figure 1 A cross-sectional view of a known ion source with a known indirectly heated cathode is presented.

[0022] Figure 2 Three known cathodes in a failure state are shown.

[0023] Figure 3 This is a structural diagram of an example vacuum system utilizing an ion source according to several aspects of the present invention.

[0024] Figure 4 This is a schematic diagram of an example arc chamber according to several aspects of the present invention.

[0025] Figure 5 This is a cross-sectional view of an example stepped cathode and stepped cathode shield associated with an ion source chamber according to several aspects of the present invention.

[0026] Figure 6 This is a cross-sectional view of an example ion source chamber having an eccentric stepped cathode according to several aspects of the present invention.

[0027] Figure 7 This is a plan view of an example of an eccentric stepped cathode according to several aspects of the present invention.

[0028] Figure 8 This is a cross-sectional view of an example stepped cathode with internal steps according to several aspects of the present invention. Detailed Implementation

[0029] This invention generally relates to an ion implantation system and an associated ion source. More specifically, the invention provides a predetermined configuration and shape of the cathode and cathode shield to protect against the aforementioned weaknesses and significantly delay cathode wall penetration. In some cases, the invention can double the lifetime of an ion source incorporating the cathode of the invention. Furthermore, the invention improves the generation of multi-charged ions for subsequent accelerated high-energy implantation. Therefore, the invention generally relates to an ion implantation system and an associated ion source. More specifically, the invention relates to components for said ion implantation system that improve the lifetime, stability, and operation of the ion implantation system.

[0030] The invention is now described with reference to the accompanying drawings, wherein the same reference numerals may be used to refer to the same elements throughout. It should be understood that the description of these aspects is merely illustrative and should not be construed as limiting. In the following description, numerous specific details are set forth for illustrative purposes to provide a full understanding of the invention. It will be apparent to those skilled in the art that the invention can be practiced without these specific details. Furthermore, the scope of the invention is not limited to the embodiments or examples described below with reference to the accompanying drawings, but only to the appended claims and their equivalents.

[0031] It should also be noted that the accompanying drawings are provided to illustrate some aspects of embodiments of this disclosure and should therefore be considered illustrative only. In particular, the elements shown in the drawings are not necessarily drawn to scale, and the arrangement of the various elements in the drawings is intended to aid in a clear understanding of the respective embodiments and should not be construed as necessarily representing the actual relative positions of the various components in an embodiment according to the invention. Furthermore, unless otherwise specifically stated, features of the various embodiments and examples described herein can be combined with each other.

[0032] It should also be understood that, in the following description, any direct connection or coupling between functional blocks, devices, components, elements, or other physical units or functional units shown in the figures or described herein may also be achieved through indirect connection or coupling. Furthermore, it should be understood that the functional blocks or units shown in the figures may be implemented as independent features in one embodiment, while in other embodiments they may be implemented, either wholly or optionally, as combined features.

[0033] Ion implantation is a process used in the fabrication of semiconductor devices in which ions of one or more elements are accelerated and implanted into a workpiece to alter its properties. For example, dopants such as boron, arsenic, and phosphorus are typically implanted into silicon to change its electrical properties. In an exemplary ion implantation process, the element or molecule of interest is ionized, extracted, and electrostatically accelerated to form a high-energy ion beam, filtered by its mass-to-charge ratio, and guided to bombard the workpiece. The ions physically bombard the wafer, penetrate the surface, and remain below the surface at a depth associated with their energy.

[0034] Refer to the attached diagram. Figure 3A system 100 is shown, comprising an ion source 102 for generating an ion beam 104 along a beam path 106. A beamline system 110 is disposed downstream of the ion source 102 to receive the beam therefrom. The beamline system 110 may include (not shown) a mass analyzer, an acceleration structure that may include one or more gaps, and an angular energy filter. The mass analyzer includes a field-generating component (e.g., a magnet) that operates to provide a field across the beam path 106 to deflect ions from the ion beam 104 along different trajectories based on their mass (e.g., mass-to-charge ratio). Ions passing through the magnetic field are subjected to a force that guides ions with desired masses along the beam path 106 and deflects ions with undesired masses away from the beam path.

[0035] In system 100, a processing chamber 112 is provided. Processing chamber 112 includes a target location that receives an ion beam 104 from beamline system 110 and supports one or more workpieces 114 (e.g., semiconductor wafers) along beam path 106 for implantation using an ion beam with final quality analysis. Processing chamber 112 then receives the ion beam 104 directed toward the workpieces 114. It is understood that different types of processing chambers 112 can be employed in system 100. For example, a "batch" type processing chamber 112 can simultaneously support multiple workpieces 114 on a rotating support structure, wherein the workpieces 114 rotate through the path of ion beam 104 until all workpieces 114 have been implanted. On the other hand, a "serial" type processing chamber 112 supports a single workpiece 114 along beam path 106 for implantation, wherein multiple workpieces 114 are implanted one at a time in a serial manner, with the previous workpiece requiring implantation to begin implanting the next workpiece. System 100 may also include a scanning device (not shown) for moving the ion beam 104 relative to the workpiece 114 or for moving the workpiece relative to the ion beam.

[0036] In one example, ion source 102 generates ion beam 104 by ionizing a source gas containing the desired dopant element within the ion source. The ionized source gas is then extracted from the source chamber 102 in the form of ion beam 104. The ionization process is influenced by an exciter, which may take the form of a thermally heated filament, a filament for a heated cathode (indirectly heated cathode "IHC"), or a radio frequency (RF) antenna.

[0037] Figure 4A schematic diagram of an IHC ion source 120 is shown, comprising a source chamber 122, one or more gas inlets 124, a filament 126, a cathode 128, repulsion electrodes 130 positioned opposite each other within the source chamber, and an aperture 132 (also referred to as an "arc gap"). Additionally, a source magnet (not shown) provides a magnetic field 134 generally distributed axially between the cathode 128 and the repulsion electrodes 130. During operation of the IHC ion source 120, the filament 126 is resistively heated to a sufficiently high temperature to emit electrons, which are then accelerated to bombard the cathode 128, which is maintained at a positive potential relative to the filament.

[0038] Electron bombardment heats the cathode 128 to a sufficiently high temperature to thermally emit electrons into the source chamber 122, which is maintained at a positive potential relative to the cathode 128 to accelerate the electrons. A magnetic field 134 helps confine the electrons along the field lines between the cathode 128 and the repulsion electrode 130 to reduce electron loss to the chamber wall 138 of the source chamber 122. The repulsion electrode 130 is normally at the potential of the cathode 128 to reflect electrons back to the cathode, further reducing electron loss. The excited electrons ionize the source gas introduced into the chamber through the gas inlet 124, generating plasma. Ions are extracted through the aperture 132 and electrostatically accelerated by electrodes located outside the source chamber 122 to form a high-energy ion beam.

[0039] According to various exemplary aspects of the invention, Figure 5 An example of the invention is shown, in which a stepped cathode 200 is provided. For example... Figure 5 As shown, the stepped cathode 200 includes a cathode step 202 generally defined by a first cathode diameter 204 and a second cathode diameter 206 of the outer cylindrical wall 208 of the stepped cathode. In one example, the cathode step 202 in the outer cylindrical wall 208 of the stepped cathode 200 corresponds to the shielding step 210 in the inner cylindrical wall 212 of the stepped cathode shield 214, wherein the first shielding diameter 216 and the second shielding diameter 218 are defined. Thus, as shown... Figure 1 The structure shown differs from known structures that surround the cathode only by a vertical shield. Figure 5 The stepped cathode shield 214 of the example shown generally matches the shape of the stepped cathode 200, thereby the outer cylindrical wall 208 of the stepped cathode closely matches the inner cylindrical wall 212 of the stepped cathode shield. Figure 5Only one cathode step 202 defined in the stepped cathode 200 is shown, but the invention further encompasses various designs that provide a curved path 220 between the stepped cathode and the stepped cathode shield 214. For example, although not shown in the figures, multiple cathode steps 202 may be provided in the stepped cathode 200, corresponding to multiple shield steps 210 in the stepped cathode shield 214 (e.g., a staircase-like structure), thereby providing a curved path 220.

[0040] Therefore, the present invention provides various improvements over the prior art. For example, the stepped cathode 200 and stepped cathode shield 214 of the present invention can substantially prevent plasma in the source chamber 122 from reaching the thinner portion 222 of the outer cylindrical wall 208 of the stepped cathode for a longer period of time (e.g., to limit heat transfer from the cathode). Such a design can extend the cathode / source lifetime by ten times compared to the cathode / source lifetime of prior art cathodes measured using As+++ and As++++.

[0041] Furthermore, regarding the generation of multiply charged ions, Figure 4 The plasma column 136 shown has a smaller radial diameter, which allows for better ionization or, in practical applications, the extraction of higher multi-charged ion beam currents. Compared to known cathodes, in one example, Figure 5 The stepped cathode 200 and stepped cathode shield 214 shown can further restrict electron emission to a smaller region 224 of the front cathode surface 226, thus obtaining Figure 4 The smaller radius 228 of the plasma column 136 subsequently produces a larger... Figure 1 The ion implanter has 100% higher multi-charge beam current and penetration.

[0042] exist Figure 5 In the example shown, the first cathode diameter 204 and the second cathode diameter 206 of the outer cylindrical wall 208 of the stepped cathode 200 are concentric around the center line 230 of the stepped cathode. Similarly, the first cathode shield diameter 216 and the second cathode shield diameter 218 of the inner cylindrical wall 212 of the stepped cathode shield 214 are concentric around the center line 230.

[0043] Figures 6 to 7 Show another specific example, which provides the same as Figure 5 The stepped cathode 200 and stepped cathode shield 214 have various similarities to the eccentric stepped cathode 250 and eccentric stepped cathode shield 252. However, as... Figures 6 to 7As shown, the center line 254 of the first cathode diameter 256 and the first shield diameter 258 of the eccentric stepped cathode 250 and the eccentric stepped cathode shield 252 is offset from the center line 262 of the second cathode diameter 264 and the second shield diameter 266 of the eccentric stepped cathode and the eccentric stepped cathode shield by a predetermined distance 260. In one example, Figure 6 The eccentric stepped cathode 250 can provide a front cathode surface 226 near the gap 132, thereby further increasing the ion beam current compared with existing ion sources.

[0044] Therefore, in a similar manner as discussed above, the eccentric cathode step 268 defined in the eccentric stepped cathode 250 and the eccentric cathode shield step 270 defined in the eccentric stepped cathode shield 252 can similarly and substantially prevent contamination of the source chamber 272 (e.g., during extended operation) Figure 4 The plasma in the source chamber 122 shown reaches the eccentric stepped cathode as shown. Figure 6 The thinner portion 274 of the outer cylindrical wall 276 shown.

[0045] According to another example, Figure 8 Another stepped cathode 300 is shown, which may additionally or alternatively include internal steps 302 on its inner diameter 304 (such as an internal surface). In one example, the penetration described above may occur at the junction of the thin cathode wall and the extremely thick front portion of a known cathode. Therefore, in Figure 8 In the example of the invention shown, the thickened wall 306 is disposed near the filament 126, thus delaying penetration in this region and extending the cathode lifetime to a longer duration than that seen in the prior art. Providing this internal step 302 within the cathode 300 maintains high thermal resistance of the thin cathode wall 308 and the region 310 away from the plasma, thereby promoting favorable operation of the ion source. In one example, a gap 312 is disposed between the filament 126 and the thickened wall 306, which is larger than or approximately twice the rear gap 314 between the filament and the rear portion 316 of the cathode 300. This gap 312 allows for uniform heating of the front portion 318 of the cathode 300 and avoids tolerance issues in filament placement that could adversely affect operation. Furthermore, the invention encompasses setting the curvature and / or bending radius of the filament 126 to accommodate the internal step 302 and the associated gaps 312 and 314.

[0046] This invention thus extends the ion source lifetime, for example, by up to ten times. Furthermore, for multiply charged ions, this invention provides a significantly narrower and lower plasma column, thereby concentrating energy within a smaller volume and making the ion source more efficient. In one example, the stepped cathode of this invention provides a cathode with a smaller diameter, thereby narrowing the plasma column. Furthermore, by bringing the stepped cathode close to and matching its shape with the stepped cathode, plasma diffusion formed within the ion source is restricted, preventing it from reaching the thin wall of the stepped cathode through the gap between the stepped cathode shield and the stepped cathode. By providing the aforementioned stepped cathode and stepped cathode shield, a curved or meandering path is provided to substantially prevent this plasma diffusion. By providing such a curved or meandering path, and simultaneously providing a thin cathode wall, only the front portion of the stepped cathode facing the plasma receives significant heating.

[0047] Although the invention has been illustrated and described for specific examples above, it should be noted that these examples are merely illustrative of some embodiments of the invention, and the invention is not limited to these examples. In particular, with respect to the various functions performed by the components (systems, devices, circuits, etc.) described above, unless otherwise stated, the terminology used to describe such components (including "device / component") covers any component corresponding to performing the specified function (i.e., functionally equivalent) of the described component, even if it is not structurally equivalent to the structure described herein that performs the function of a specific example of the invention. Furthermore, while specific features of the invention may be described with respect to only one embodiment among several embodiments, such features may be combined with one or more other features of other embodiments where they may be desired and advantageous for any given or particular application. Therefore, the invention is not limited to the above embodiments, but only by the appended claims and their equivalents.

Claims

1. An ion source for forming plasma, the ion source comprising: The cathode comprises: The cathode surface, which defines the cathode steps, and Cavity; The filament, which is disposed within the cavity; and A cathode shield having a cathode shield surface that at least partially surrounds the cathode surface, and a cathode gap being defined between the cathode surface and the cathode shield surface; The cathode surface includes a stepped cylindrical surface defined by a first cathode diameter and a second cathode diameter, wherein the first cathode diameter and the second cathode diameter are different from each other, thereby defining the cathode step. The stepped cylindrical surface includes an outer surface that defines the first cathode diameter and the second cathode diameter, and the cathode shield surface defines a cathode shield step, the profile of which matches the cathode step to maintain the cathode gap between the cathode surface and the cathode shield surface. The second cathode diameter is larger than the first cathode diameter, wherein the centerline of the first cathode diameter and the centerline of the second cathode diameter are offset from each other by a predetermined distance.

2. The ion source according to claim 1, wherein the cathode gap defines a curved path.

3. The ion source according to claim 1, wherein the stepped cylindrical surface includes an inner surface defining the diameter of the first cathode and the diameter of the second cathode.

4. The ion source according to claim 3, wherein the diameter of the second cathode is larger than the diameter of the first cathode, thereby defining the thickened wall of the cathode.

5. The ion source according to claim 4, wherein a filament gap is defined between the filament and the thickened wall of the cathode.

6. The ion source of claim 1, wherein the cathode comprises a hollow cylindrical portion extending from a solid cylindrical portion at a transition region, the cavity being defined within the hollow cylindrical portion, and the cathode step being defined within the transition region.

7. The ion source according to claim 6, wherein the cathode shielding radially surrounds the cathode while axially exposing the front cathode surface of the cathode to the plasma.

8. An ion source comprising: A cathode having a cathode staircase defined therein; and A cathode shield having a cathode shield step defined therein, the cathode shield radially surrounding the cathode while maintaining a cathode gap between the cathode and the cathode shield; The cathode has a first cathode diameter and a second cathode diameter, thereby defining the cathode step, wherein the center lines of the first cathode diameter and the second cathode diameter are offset from each other by a predetermined distance.

9. The ion source according to claim 8, wherein the cathode shielding step matches the cathode step.

10. The ion source of claim 8, wherein the cathode comprises a hollow cylindrical portion extending from a solid cylindrical portion adjacent to a transition region, the cathode step being defined within the transition region.

11. The ion source according to claim 8, further comprising a filament disposed within the hollow cylindrical portion of the cathode.

12. A cathode system for an ion source according to any one of claims 1-7, the cathode system comprising: Stepped cathode; and A stepped cathode shield radially surrounds the stepped cathode while maintaining a gap between the stepped cathode and the stepped cathode shield, the inner surface of the stepped cathode shield matching the outer surface of the stepped cathode, thereby defining a curved path therebetween the stepped cathode and the stepped cathode shield.

13. The cathode system of claim 12, wherein the stepped cathode comprises a plurality of cathode steps defined on its outer surface, and the stepped cathode shield comprises a plurality of shield steps defined on its inner surface.

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