Stepped indirect heating cathode with improved shielding

Abstract

An ion source for forming plasma has a cathode having a cavity and a cathode surface representing a cathode step. A filament is disposed within the cavity, and a cathode shield has a cathode shield surface that at least partially surrounds the cathode surface. A cathode gap appears between the cathode surface and the cathode shield surface, wherein the cathode gap represents a tortured path to restrict the movement of plasma through the gap. The cathode surface may have a stepped cylindrical surface represented 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 to represent a cathode step. The stepped cylindrical surface may be an outer surface or an inner surface. The first and second cathode diameters may be concentric or axially offset.

Description

Technology Field

[0001] Reference to related applications

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

[0003] Technology field

[0004] The present invention generally relates to ion implantation systems, and more specifically to improved ion source and beamline components that improve the lifespan, stability, and various aspects of operation of an ion implantation system. Background Technology

[0005] In the manufacture of semiconductor devices, ion implantation is used to dope semiconductors with impurities. Ion implantation systems are often used to dope workpieces, such as semiconductor wafers, with ions from an ion beam to create n-type or p-type material doping, or to form passivation layers during the manufacture of integrated circuits. This beam processing is frequently used to selectively implant impurities of specific dopant materials into the wafer at predetermined energy levels and controlled concentrations to create 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 create the desired exogenous material. For example, implanted ions generated from source materials such as antimony, arsenic, or phosphorus produce "n-type" exogenous material wafers, whereas "p-type" exogenous material wafers are often produced from ions generated from source materials such as boron, gallium, or indium.

[0006] A typical ion implanter includes an ion source, an ion extraction device, a mass analysis device, a beam transport device, and a wafer processing device. The ion source generates ions of the desired atomic or molecular dopant species. These ions are extracted from the source by an extraction system, typically a set of electrodes that activates and directs the flow of ions from the source to form an ion beam. The desired ions are separated from the ion beam by a mass analysis device, typically a magnetic dipole that performs mass dispersion or separation of the extracted ion beam. A vacuum system, typically including a series of focusing devices, transmits the ion beam to the wafer processing device while maintaining the desired characteristics of the ion beam. Finally, the semiconductor wafer is transported in and out of the wafer processing device via a wafer handling system, which may include one or more robotic arms for placing the wafer to be processed in front of the ion beam and for removing the processed wafer from the ion implanter.

[0007] An ion source (commonly referred to as an ‘arc discharge ion source’) may include a heated filament cathode for generating an ion beam used in an injector and for generating ions formed into an ion beam suitable for wafer processing. U.S. Patent No. 5,497,006 by Sferlazzo et al. discloses an ion source having a cathode supported by a base and positioned relative to a gas confinement chamber for emitting ionized electrons into a gas confinement chamber. The cathode of the patent by Sferlazzo et al. is a tubular conductive body having an endcap that partially extends into the gas confinement chamber.

[0008] FIG. 1 illustrates a cross-sectional view of a conventional ion source (10) used in a conventional ion implantation system. The filament (12) is resistively heated to a temperature at which thermionic emission of electrons occurs. A voltage between the filament (12) and the cathode (14) (so-called ‘cathode voltage’) accelerates electrons emitted from the filament toward the cathode until the cathode itself thermally emits electrons. This method of emission is referred to in the industry as an indirectly heated cathode (IHC). The cathode (14) serves, for example, two purposes: namely, to protect the filament (12) from being bombarded by plasma ions, and to provide electrons for subsequent ionization.

[0009] The cathode (14) is negatively biased toward the arc chamber (16), which exists as the so-called "arc voltage," and emitted electrons are accelerated toward the center (18) of the arc chamber. A supply gas (not shown) flows into the arc chamber (16), and the emitted electrons subsequently ionize the supply gas to form a plasma (not shown) from which ions can be extracted through an extraction slit (20) within the arc chamber. A repeller (22) further charges the plasma, for example, to a negative floating potential and repels the electrons back into the plasma, thereby inducing enhanced ionization and a denser plasma. A magnetic field (not shown) parallel to the central axis (24) formed by the cathode (14) and the repeller (22) generally confines the emitted and repelled electrons to form a so-called "plasma column," which further improves ionization and plasma density.

[0010] Generally, the cathode (14) itself fails first due to sputtering and erosion by plasma ions, and final penetration or punch-through of the cathode. This significantly shortens the lifespan of the ion source when tuning a multi-charged ion beam compared to a single-charged ion. Punch-through typically occurs when the wall (26) of the cathode is at its thinnest. FIG. 2 illustrates such punch-through (28) in the wall (26) of the cathode (14), which causes the ion source to fail even if sufficient material remains in the front portion (30) of the cathode. Such failure commonly occurs in ion sources when multi-charged ions are injected for high energy. Since elements such as arsenic (As) are very heavy, significant sputtering of the cathode (14) occurs. In addition, for multi-charged ions, substantially higher arc voltage is achieved, further reducing the lifespan of conventional ion sources. The problem to be solved

[0011] Accordingly, the present invention provides a system and apparatus for improving the efficiency and lifespan of an ion source. Accordingly, a simplified summary of the present invention is provided below to provide a basic understanding of some aspects of the invention. The following summary is not a comprehensive overview of the invention. It is not intended to identify the core or important elements of the invention or to describe the scope of the invention. Its purpose is to present some concepts of the invention in a simplified form as an introduction to the more detailed description presented below. means of solving the problem

[0012] According to one aspect of the present invention, an ion source is provided to form a plasma, wherein the ion source comprises a cathode and a cathode shield. The cathode comprises, for example, a cavity, wherein a filament is disposed within the cavity. The cathode further comprises a cathode surface representing a cathode step. The cathode shield has, for example, a cathode shield surface that at least partially surrounds the cathode surface, wherein a cathode gap appears between the cathode surface and the cathode shield surface. The cathode gap represents, for example, a tortured path for the plasma to extend the lifetime of the cathode.

[0013] According to one embodiment, the cathode surface comprises a stepped cylindrical surface formed by a first cathode diameter and a second cathode diameter. The first cathode diameter and the second cathode diameter are different from each other and, for example, represent a cathode end therein.

[0014] In one embodiment, the stepped cylindrical surface includes an outer surface representing a first cathode diameter and a second cathode diameter. The cathode shielding surface may further represent a cathode shielding end, wherein the profile of the cathode shielding end generally coincides with the cathode end, thereby maintaining a cathode gap between the cathode surface and the cathode shielding surface. The second cathode diameter is, for example, larger than the first cathode diameter, and the second cathode shielding diameter is larger than the first cathode shielding diameter. The first cathode diameter and the second cathode diameter may be concentric or may be axially offset by a predetermined distance.

[0015] In another embodiment, the stepped cylindrical surface of the cathode includes an inner surface representing a first cathode diameter and a second cathode diameter. The second cathode diameter is, for example, larger than the first cathode diameter, thereby forming a thick wall of the cathode. A filament gap may additionally appear, for example, between the thick wall of the cathode and the filament, thereby generally increasing the life of the cathode.

[0016] The cathode comprises, for example, a hollow cylindrical portion extending from a solid cylindrical portion in a transition region, wherein the cavity appears within the hollow cylindrical portion and the cathode end appears in the transition region. In another embodiment, the cathode shield radially surrounds the cathode while axially exposing the front cathode surface of the cathode to the plasma.

[0017] According to another aspect of the embodiment, an ion source is provided, wherein the cathode has a cathode end appearing therein. A cathode shielding portion has a cathode shielding portion appearing therein, said cathode shielding portion radially surrounds the cathode while maintaining a cathode gap between the cathode and the cathode shielding portion. The cathode shielding portion generally coincides with the cathode end while maintaining the cathode gap, for example, and thus provides a meandering path for plasma to travel.

[0018] The cathode comprises, for example, a hollow cylindrical portion extending from a solid cylindrical portion adjacent to a transition region, wherein the cathode end appears in the transition region. The filament may be disposed, for example, within the hollow cylindrical portion of the cathode. The cathode comprises, for example, a first cathode diameter and a second cathode diameter, thereby representing the cathode end, wherein the first cathode diameter and the second cathode diameter are concentric or axially offset by a predetermined distance.

[0019] According to another aspect of the embodiment, a cathode assembly for an ion source is provided. The cathode assembly includes, for example, a stepped cathode shield that 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 generally coincides, for example, with the outer surface of the stepped cathode, whereby a meandering path is formed between the stepped cathode and the stepped cathode shield. The stepped cathode may include, for example, a plurality of cathode ends appearing on the outer surface of the stepped cathode, wherein the stepped cathode shield further includes a plurality of shield ends appearing on the inner surface of the stepped cathode shield.

[0020] To achieve the aforementioned and related objectives, the present invention comprises features that are sufficiently described below and, in particular, set forth in the claims. The following description and the accompanying drawings describe specific exemplary embodiments of the present invention in detail. However, these embodiments represent some of the various ways in which the principles of the present invention may be employed. Other objectives, advantages, and novel features of the present invention will become apparent from the following detailed description of the present invention when considered together with the drawings. Brief explanation of the drawing

[0021] Figure 1 shows a cross-sectional view of a conventional ion source having a conventional indirect heating cathode. Figure 2 illustrates three conventional cathodes in a failed state. FIG. 3 is a block diagram of an exemplary vacuum system using an ion source according to some aspects of the present invention. FIG. 4 is a schematic diagram of an exemplary arc chamber according to some aspects of the present invention. FIG. 5 is a cross-sectional view of an exemplary stepped cathode and stepped cathode shielding part associated with an ion source chamber according to various aspects of the present invention. FIG. 6 is a cross-sectional view of an exemplary ion source chamber having an offset stepped cathode according to various aspects of the present invention. FIG. 7 is a plan view of an exemplary offset stepped cathode according to various aspects of the present invention. FIG. 8 is a cross-sectional view of an exemplary stepped cathode having an internal step according to various aspects of the present invention. Specific details for implementing the invention

[0022] The present invention generally relates to ion implantation systems and ion sources associated therewith. More specifically, the present invention provides the configuration and forming of a cathode and a cathode shield in a predetermined manner to prevent weaknesses as described above and to significantly delay cathode wall punch-through. Accordingly, in some cases, the present invention can double the lifetime of an ion source comprising the cathode of the present invention. Furthermore, the present invention improves the generation of multi-charged ions for subsequent acceleration for high-energy implantation. Thus, the present invention generally relates to ion implantation systems and ion sources associated therewith. More specifically, the present invention relates to components for said ion implantation systems that improve the lifetime, stability, and operation of said ion implantation systems.

[0023] Accordingly, the present invention will be described below with reference to the drawings, and the same reference numerals may be used throughout to refer to the same elements. It should be understood that the description of these aspects is for illustrative purposes only and should not be interpreted in a limiting sense. In the following description, for the purpose of explanation and to provide a complete understanding of the present invention, a number of specific details are presented. However, it will be apparent to those skilled in the art that the present invention can be practiced without these specific details. Furthermore, the scope of the present invention is not limited by the embodiments or examples described below with reference to the accompanying drawings, but is intended to be limited only by the appended claims and their equivalents.

[0024] In addition, it should be noted that the drawings are provided to provide examples of some aspects of embodiments of the present invention and are therefore to be regarded merely as schematic. In particular, elements depicted in the drawings are not necessarily proportional to one another, and the positions of various components in the drawings are selected to aid in a clear understanding of each embodiment and should not be interpreted as representing the actual relative positions of the various components according to the embodiments of the present invention. Furthermore, unless otherwise specifically stated, the features of the various embodiments and examples described herein may be combined with one another.

[0025] Additionally, in the following description, any direct connection or coupling between functional blocks, devices, components, elements, or other physical or functional units illustrated in the drawings or described herein should be understood as potentially being implemented by indirect connection or coupling. Furthermore, it should be understood that functional blocks or units illustrated in the drawings may be implemented as distinct features in one embodiment, or alternatively, fully or partially implemented as common features in another embodiment.

[0026] Ion implantation is a process employed in the manufacture of semiconductor devices in which ions of one or more elements are accelerated into a workpiece to alter its properties. For example, it is common for dopants such as boron, arsenic, and phosphorus to be implanted into silicon to modify their electrical properties. In an exemplary ion implantation process, an element or molecule of interest is ionized, extracted, and electrostatically accelerated to form a high-energy ion beam, which is filtered by its mass-to-charge ratio and directed to collide with the workpiece. The ions physically collide with the wafer, enter the surface, and are placed at a depth below the surface related to their energy.

[0027] Referring to the drawings, FIG. 3 illustrates a system (100) comprising an ion source (102) for generating an ion beam (104) along a beam path (106). A beamline assembly (110) is provided downstream of the ion source (102) to receive the beam therefrom. The beamline system (110) may include a mass spectrometer (not shown), an acceleration structure that may include, for example, one or more gaps, and an angular energy filter. The mass spectrometer includes a field-generating component, such as a magnet, and operates to provide a field along the beam path (106) to deflect ions from the ion beam (104) in trajectories that vary according to mass (e.g., mass-to-charge ratio). Ions traveling through the magnetic field experience a force that directs individual ions of the desired mass along the beam path (106) and deflects ions of the unwanted mass away from the beam path.

[0028] A process chamber (112) is provided within a system (100) that includes a target location for receiving an ion beam (104) from a beam line assembly (110) and supports one or more workpieces (114), such as semiconductor wafers, along a beam path (106) for injection using the final mass spectrometrically analyzed ion beam. The process chamber (112) then receives an ion beam (104) directed toward the workpiece (114). It should be recognized that different types of process chambers (112) may be employed within the system (100). For example, a “batch” type process chamber (112) may simultaneously support multiple workpieces (114) on a rotating support structure, wherein the workpieces (114) are rotated through the path of the ion beam (104) until all workpieces (114) are fully injected. On the other hand, a “serial” type process chamber (112) supports a single workpiece (114) along a beam path (106) for injection, wherein multiple workpieces (114) are injected one at a time in a serial manner, and each workpiece is fully injected before the injection of the next workpiece begins. The system (100) may also include a scanning device (not shown) for moving the ion beam (104) toward the workpiece (114), or the workpiece toward the ion beam.

[0029] The ion source (102) generates an ion beam (104) by ionizing a source gas containing, for example, a desired dopant element within the ion source. The ionized source gas is then extracted from the source chamber (102) in the form of an ion beam (104). The ionization process is influenced by a thermally heated filament, a filament heating the cathode (an indirectly heated cathode "IHC"), or an exciter that can take the form of a radio frequency (RF) antenna.

[0030] An IHC ion source (120) is schematically illustrated in FIG. 4, comprising, for example, a source chamber (122), one or more gas inlets (124), a filament (126), a cathode (128), a repeller (130) located opposite each other within the source chamber, and an aperture (132) (also called an arc slit). Additionally, a source magnet (not shown) may generally provide a magnetic field (134) along an axis between the cathode (128) and the repeller (130). During the operation of the IHC ion source (120), the filament (126) is resistively heated to a temperature high enough to emit electrons, which are then accelerated to collide with the cathode (128), which is maintained at a positive potential relative to the filament.

[0031] Electron bombardment heats the cathode (128) to a temperature high enough to thermally release electrons into the source chamber (122), which is maintained at a positive potential relative to the cathode (128) to accelerate electrons. A magnetic field (134) helps confine electrons along the field line between the cathode (128) and the repeller (130) along the plasma column (136) to reduce electron loss to the chamber wall (138) of the source chamber (122). Electron loss is further reduced by the repeller (130), which is typically at the potential of the cathode (128), to reflect electrons back toward the cathode. The excited electrons ionize the source gas supplied into the chamber through the gas inlet (124), thereby generating plasma. Ions are extracted through the opening (132) and electrostatically accelerated to form a high-energy ion beam by an electrode located outside the source chamber (122).

[0032] According to various exemplary aspects of the present invention, FIG. 5 illustrates an example of the present invention, wherein a stepped cathode (200) is provided. As illustrated in FIG. 5, the stepped cathode (200) generally comprises a cathode end (202) represented by a first cathode diameter (204) and a second cathode diameter (206) of the outer cylindrical wall (208) of the stepped cathode. For example, the cathode end (202) within the outer cylindrical wall (208) of the stepped cathode (200) is coupled with a shielding end (210) represented by the inner cylindrical wall (212) of the stepped cathode shielding part (214), thereby representing a first shielding diameter (216) and a second shielding diameter (218). As such, instead of simply surrounding the cathode directly with a shielding part as typically illustrated in FIG. 1, the stepped cathode shielding part (214) of the example illustrated in FIG. 5 generally corresponds to the shape of the stepped cathode (200), so that the cylindrical wall (208) of the stepped cathode corresponds closely to the inner cylindrical wall (212) of the stepped cathode shielding part. FIG. 5 illustrates a single cathode end (202) shown in the stepped cathode (200), but the present invention further considers various designs to provide a winding path (220) between the stepped cathode and the stepped cathode shielding part (214). For example, although not illustrated, a plurality of cathode sections (202) accompanied by a plurality of shielding sections (210) (e.g. similar to stairs) within a stepped cathode shielding section (214) may be provided in the stepped cathode (200), thus providing a winding path (220).

[0033] Accordingly, 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 generally prevent the plasma in the source chamber (122) from reaching the thin portion (222) of the outer cylindrical wall (208) of the stepped cathode for an extended period (e.g., used to limit heat transfer from the cathode). For example, this design can lead to an improvement of up to 10 times the cathode / source lifetime compared to previously measured as As+++ and As++++ using a conventional cathode.

[0034] Additionally, for multi-charge ion generation, the plasma column (136) shown in FIG. 4 is radially small, which can result in improved ionization or, in fact, a higher extracted beam current of multi-charge ions. For example, the stepped cathode (200) and stepped cathode shield (214) of FIG. 5 further restrict electron emission from the forward cathode surface (226) to a smaller area (224) compared to a conventional cathode, making the radius (228) of the plasma column (136) of FIG. 4 smaller, which in turn results in a higher multi-charge beam current and throughput of the ion implanter (100) of FIG. 1.

[0035] In the embodiment illustrated in FIG. 5, the first cathode diameter (204) and the second cathode diameter (206) of the outer cylindrical wall (208) of the stepped cathode (200) are concentric with respect to the centerline (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 with respect to the centerline (230).

[0036] According to another embodiment, FIGS. 6 and 7 illustrate other embodiments in which the offset stepped cathode (250) and the offset stepped cathode shield (252) have various similarities to the stepped cathode (200) and the stepped cathode shield (214) of FIG. 5. However, as illustrated in FIGS. 6 and 7, the centerline (254) of the first cathode diameter (256) and the first shield diameter (258) of each offset stepped cathode (250) and the offset stepped cathode shield (252) is offset by a predetermined distance (260) from the centerline (262) of the second cathode diameter (264) and the second cathode shield diameter (266) of the offset stepped cathode and the offset stepped cathode shield. The offset stepped cathode (250) of FIG. 6 can provide, for example, a forward cathode surface (226) close to the opening (132), which can further advantageously increase the ion beam current compared to a conventional ion source.

[0037] Accordingly, in a manner similar to that discussed above, the offset cathode end (268) appearing within the offset stepped cathode (250) and the offset cathode shield end (270) appearing within the offset stepped cathode shield (252) likewise substantially prevent the plasma within the source chamber (272) (e.g., the source chamber (122) of FIG. 4) from reaching the thin portion (274) of the outer cylindrical wall (276) shown in FIG. 6 of the offset stepped cathode over an extended operating period.

[0038] According to another embodiment, FIG. 8 illustrates a different stepped cathode (300), wherein the stepped cathode may additionally or alternatively include an internal step (302) on an internal diameter (304) (e.g., an internal surface). For example, the punch-through described above may occur when a thin cathode wall is connected to a very thick front portion of a conventional cathode. Thus, in the embodiment of the invention illustrated in FIG. 8, a thick wall (306) is provided close to the filament (126), thereby delaying punch-through in that region and extending the cathode lifespan more than conventionally shown. Providing such an internal step (302) on the interior of the cathode (300) allows for maintaining a thin cathode wall (308) and high thermal resistance, for example, in a region (310) further from the plasma, thereby facilitating the advantageous operation of the ion source. For example, a gap (312) is provided between the filament (126) and the thick wall (306), wherein the gap is, for example, more than twice the rear gap (314) between the filament and the rear (316) of the cathode (300). Such a gap (312) enables the heating of heat uniformly without tolerance issues in the filament placement, for example, where the front portion (318) of the cathode (300) may negatively affect operation. Additionally, the present invention considers sizing the curvature and / or bending radius of the filament (126) to accommodate the internal step (302) and the combined gap (312) and rear gap (314).

[0039] Accordingly, the present invention provides an improved ion source lifetime, for example, by up to 10 times. Additionally, for multi-charged ions, the present invention provides a significantly narrow plasma column, thereby allowing power to be contained within a smaller volume, which makes the ion source more efficient. The stepped cathode of the present invention, for example, provides a smaller diameter of the cathode, thereby narrowing the plasma column. Furthermore, the stepped cathode shield is positioned close to the stepped cathode and generally matches the shape of the stepped cathode, and generally limits the diffusion of the plasma formed within the ion source from exposure to the thin walls 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 winding path or labyrinth is generally provided to prevent the plasma diffusion. By providing the above-mentioned winding path or maze, and also by providing a thin wall of the cathode, only the forward plasma-facing portion of the stepped cathode is substantially heated.

[0040] It should be noted that although the present invention has been illustrated and described in relation to specific embodiment(s), the foregoing embodiments serve only as examples for carrying out some embodiments of the present invention, and the application of the present invention is not limited to these embodiments. In particular, regarding the various functions performed by the foregoing components (assemblies, devices, circuits, etc.), terms used to describe these components (including references to “means”) are intended to correspond to any component performing a specific function of the described component (i.e., functionally equivalent), even if it is not structurally equivalent to the disclosed structure performing the function in the exemplary embodiments of the present invention illustrated herein, unless otherwise indicated. Furthermore, while a specific feature of the present invention may be disclosed in relation to only one of some embodiments, such feature may be combined with one or more other features of other embodiments as may be desirable and advantageous for any given or specific application. Accordingly, the present invention is not limited to the foregoing embodiments, but is intended to be limited only by the appended claims and their equivalents.

Claims

Claim 1 As an ion source for forming plasma, the ion source comprises: a cathode surface representing a cathode step; and a cavity; a cathode comprising: a filament disposed within the cavity; and a cathode shield having a cathode shield surface that at least partially surrounds the cathode surface; wherein a cathode gap is present between the cathode surface and the cathode shield surface, and the cathode surface includes a stepped cylindrical surface formed 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 and thereby represent the cathode end, and the stepped cylindrical surface includes an outer surface representing the first cathode diameter and the second cathode diameter, and the cathode shield surface represents the cathode shield end, wherein the profile of the cathode shield end generally coincides with the cathode end, and thereby the cathode between the cathode surface and the cathode shield surface An ion source having a gap maintained, wherein the second cathode diameter is larger than the first cathode diameter, and the first cathode diameter and the second cathode diameter each have centerlines offset by a predetermined distance. Claim 2 In claim 1, the cathode gap is an ion source that exhibits a tortured path. Claim 3 An ion source according to claim 1, wherein the cathode comprises a hollow cylindrical portion extending from a solid cylindrical portion in a transition region, the cavity appearing within the hollow cylindrical portion and the cathode end appearing in the transition region. Claim 4 In claim 3, the cathode shielding portion is an ion source that radially surrounds the cathode while axially exposing the front cathode surface of the cathode to the plasma. Claim 5 An ion source comprising: a cathode having a cathode end shown internally; and a cathode shielding part having a cathode shielding part shown internally; wherein the cathode shielding part radially surrounds the cathode while maintaining a cathode gap between the cathode and the cathode shielding part, the cathode includes a first cathode diameter and a second cathode diameter, thereby indicating the cathode end, and the first cathode diameter and the second cathode diameter each have a centerline offset by a predetermined distance. Claim 6 In claim 5, the cathode shielding section generally corresponds to the cathode section, an ion source. Claim 7 In claim 5, the cathode comprises a hollow cylindrical portion extending from a solid cylindrical portion adjacent to a transition region, wherein the cathode end is an ion source appearing in the transition region. Claim 8 An ion source according to claim 7, further comprising a filament disposed within the hollow cylindrical portion of the cathode. Claim 9 A cathode assembly for an ion source, wherein the cathode assembly comprises: a stepped cathode; and a stepped cathode shield that radially surrounds the stepped cathode while maintaining a gap between the stepped cathode and the stepped cathode shield; wherein the inner surface of the stepped cathode shield generally coincides with the outer surface of the stepped cathode, thereby forming a meandering path between the stepped cathode and the stepped cathode shield, the stepped cathode includes a first cathode diameter and a second cathode diameter, thereby forming a cathode end on the outer surface of the stepped cathode, and the first cathode diameter and the second cathode diameter each have a centerline offset by a predetermined distance. Claim 10 A cathode assembly according to claim 9, wherein the stepped cathode comprises a plurality of cathode stages appearing on the outer surface of the stepped cathode, and the stepped cathode shielding comprises a plurality of shielding stages appearing on the inner surface of the stepped cathode shielding. Claim 11 delete Claim 12 delete Claim 13 delete Claim 14 delete Claim 15 delete Claim 16 delete Claim 17 delete Claim 18 delete Claim 19 delete Claim 20 delete Claim 21 delete

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