Stepped indirectly heated cathode with improved shielding.

Abstract

An ion source for forming a plasma includes a cathode having a cavity and a cathode surface defining a cathode step. A filament is disposed within the cavity, and a cathode shield has a cathode shield surface at least partially surrounding the cathode surface. A cathode gap is defined between the cathode surface and the cathode shield surface, and the cathode gap defines a tortuous path for restricting movement of the plasma through the gap. The cathode surface may have a stepped cylindrical surface defined by a first cathode diameter and a second cathode diameter. The first cathode diameter and the second cathode diameter 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 offset.

Description

Detailed Description of the Invention

[0001] [Reference to Related Applications] This application claims the benefit of U.S. Provisional Application No. 63 / 040,724, filed June 18, 2020, the entire contents of which are incorporated herein by reference.

[0002] [Technical field] The present invention relates generally to ion implantation systems, and more particularly to improved ion source and beamline components that improve the lifetime, stability, and operation of various aspects of ion implantation systems.

[0003] [Background technology] In the fabrication of semiconductor devices, ion implantation is used to dope semiconductors with impurities. Ion implantation systems are often utilized to dope workpieces, such as semiconductor wafers, with ions from an ion beam. This can result in n-type or p-type material doping or passivation during integrated circuit fabrication. Such beam processes are often used to selectively implant wafers with impurities of a specific dopant material at predetermined energy levels and controlled densities to create semiconductor materials during integrated circuit fabrication. When used to dope semiconductor wafers, ion implantation systems implant selected ion species into the workpiece to create the desired extrinsic material. For example, implanting ions generated from a source material such as antimony, arsenic, or phosphorus results in an "n-type" extrinsic material wafer. On the other hand, "p-type" extrinsic material wafers often result from ions generated using a source material such as boron, gallium, or indium.

[0004] 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 produces ions of the desired atomic or molecular dopant species. These ions are extracted from the ion source by an extraction system, typically a set of electrodes. The extraction system energizes and directs the ion stream from the ion source to form an ion beam. The desired ions are separated from the ion beam within a mass analysis device, typically a magnetic dipole that provides mass dispersion or separation to the extracted ion beam. A beam transport device, typically a vacuum system including a series of focusing devices, transports the ion beam to the wafer processing device while maintaining the desired ion beam characteristics. Finally, semiconductor wafers are transferred in and out of the wafer processing equipment via a wafer handling system. The wafer handling system may have one or more robotic arms for positioning the wafer to be processed in front of the ion beam and removing the processed wafer from the ion implanter.

[0005] Ion sources (commonly referred to as arc discharge ion sources) generate ion beams used in implanters and may include a heated filament cathode for generating ions that are shaped into an ion beam suitable for wafer processing. For example, 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-sealed chamber for emitting ionizing electrons into the gas-sealed chamber. The cathode in the Sferlazzo et al. patent is a tubular conductor with end caps that extend partially within the gas-sealed chamber.

[0006] FIG. 1 shows a cross section of a conventional ion source 10 used in conventional ion implantation systems. A filament 12 is resistively heated to a temperature at which thermionic emission of electrons occurs. A voltage between the filament 12 and a cathode 14 (the so-called "cathode voltage") accelerates electrons emitted from the filament toward the cathode until the cathode itself thermally emits electrons. This emission scheme is referred to in the industry as an indirectly heated cathode (IHC). The cathode 14, for example, serves two purposes: (i) it protects the filament 12 from bombardment by plasma ions, and (ii) it provides electrons for subsequent ionization.

[0007] The cathode 14 is negatively biased relative to the arc chamber 16. The cathode 14 exists within the arc chamber 16 at what is known as an "arc voltage." The 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. This forms a plasma (not shown), from which ions can be extracted through an extraction slit 20 in the arc chamber. For example, a repeller 22 can further charge the plasma to its negative floating potential and return electrons to the plasma, thereby enhancing ionization and increasing plasma density. A magnetic field (not shown) parallel to the central axis 24 defined by the cathode 14 and the repeller 22 generally confines the emitted and returned electrons, defining a so-called "plasma column." This further improves ionization and plasma density.

[0008] Typically, the cathode 14 itself fails, initially due to sputtering and erosion by plasma ions, and eventually due to cathode permeation or punch-through. This is particularly evident when preparing a multiply charged ion beam, significantly shortening the ion source's lifetime compared to single-charge ions. Punch-through typically occurs at the location where the cathode wall 26 is thinnest. Figure 2 shows such a punch-through 28 in the wall 26 of the cathode 14. In this example, punch-through resulted in source failure, even though sufficient material remained on the cathode's front surface 30. Such failures are common in ion sources implanted with multiply charged ions due to their high energy. Elements such as arsenic (As) are very heavy, resulting in significant sputtering of the cathode 14. Furthermore, multiply charged ions achieve substantially higher arc voltages, further reducing the lifetime of conventional ion sources.

[0009] [overview] Thus, the present disclosure provides systems and apparatus for improving the efficiency and lifetime of ion sources. Accordingly, the following presents a simplified summary of the disclosure in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is not intended to identify or delineate key or critical elements of the invention. Its purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.

[0010] According to one aspect of the present disclosure, an ion source is provided for forming a plasma. The ion source includes a cathode and a cathode shield. The cathode, for example, includes a cavity, with a filament disposed within the cavity. The cathode further includes a cathode surface defining a cathode step. The cathode shield, for example, includes a cathode shield surface at least partially surrounding the cathode surface. A cathode gap is defined between the cathode surface and the cathode shield surface. The cathode gap, for example, defines a tortuous path for the plasma to extend cathode life.

[0011] For example, the cathode surface may include a stepped cylindrical surface defined by a first cathode diameter and a second cathode diameter, which may be different from each other, thereby defining the cathode step.

[0012] For example, the stepped cylindrical surface may have an outer surface that defines the first cathode diameter and the second cathode diameter. The cathode shield surface may further define the cathode shield step. In this case, the contour of the cathode shield step generally matches the cathode step, thereby maintaining the cathode gap between the cathode surface and the cathode shield surface. The second cathode diameter may be larger than the first cathode diameter, and the second cathode shield diameter may be larger than the first cathode shield diameter. The first and second cathode diameters may be concentric or may be axially offset by a predetermined distance.

[0013] In another example, the stepped cylindrical surface of the cathode includes an inner surface that defines the first cathode diameter and the second cathode diameter. The second cathode diameter is, for example, larger than the first cathode diameter. This defines a thick cathode wall. A filament gap may be further defined, for example, between the filament and the thick cathode wall. The thick cathode wall generally increases the life of the cathode.

[0014] The cathode may, for example, comprise 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. In another example, the cathode shield axially exposes a front cathode surface of the cathode to the plasma and radially surrounds the cathode.

[0015] According to another exemplary aspect, an ion source is provided. The cathode has a cathode step defined therein. The cathode shield has a cathode shield step defined therein. The cathode shield maintains a cathode gap between the cathode and the cathode shield and radially surrounds the cathode. The cathode shield step, for example, maintains the cathode gap and generally coincides with the cathode step. This can provide a tortuous path for plasma travel.

[0016] The cathode may, for example, comprise a hollow cylindrical portion extending from a solid cylindrical portion proximate a transition region. The cathode step may be defined within the transition region. A filament may, for example, be disposed within the hollow cylindrical portion of the cathode. The cathode may, for example, define the cathode step by including a first cathode diameter and a second cathode diameter. The first and second cathode diameters may be concentric or offset by a predetermined distance.

[0017] According to yet another exemplary aspect, a cathode assembly for an ion source is provided. The cathode assembly includes, for example, a stepped cathode and a stepped cathode shield. The 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 generally coincides with, for example, the outer surface of the stepped cathode, thereby defining a tortuous path between the stepped cathode and the stepped cathode shield. The stepped cathode includes, for example, a plurality of cathode steps defined within the outer surface of the stepped cathode. The stepped cathode shield further includes a plurality of shield steps defined within the inner surface of the stepped cathode shield.

[0018] To the accomplishment of the foregoing and related ends, the present disclosure comprises the structure hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail illustrative embodiments of the invention. These embodiments, however, are indicative of but a few of the various ways in which the principles of the invention are employed. Other objects, advantages, and novel features of the invention will become apparent from the following detailed description of the invention when read in conjunction with the drawings. [Brief explanation of the drawings]

[0019] [Figure 1] 1 is a cross-sectional view of a conventional ion source having a conventional indirectly heated cathode. [Figure 2] 1 shows three conventional cathodes in a fault condition. [Figure 3] FIG. 1 is a block diagram of an example of a vacuum system utilizing an ion source according to aspects of the present disclosure. [Figure 4] FIG. 1 is a schematic diagram of an example arc chamber according to aspects of the present disclosure. [Figure 5]FIG. 2 is a cross-sectional view of an example of a stepped cathode and stepped cathode shield associated with an ion source chamber according to various aspects of the present disclosure. [Figure 6] FIG. 2 illustrates a cross-sectional view of an example ion source chamber having an offset stepped cathode according to various aspects of the present disclosure. [Figure 7] FIG. 1 is a plan view of an example of an offset stepped cathode according to various aspects of the present disclosure. [Figure 8] FIG. 2 is a cross-sectional view of an example of a stepped cathode having an internal step according to various aspects of the present disclosure.

[0020] [Detailed explanation] The present disclosure is generally directed to ion implantation systems and associated ion sources. More specifically, the present disclosure describes a cathode and cathode shield configuration and shape in a predetermined manner to protect against the aforementioned weaknesses and significantly delay cathode wall punch-through. Accordingly, in some cases, the present disclosure can double the lifetime of an ion source incorporating a cathode of the present disclosure. Additionally, the present disclosure improves multi-charge ion generation for subsequent acceleration for high-energy implants. Accordingly, the present disclosure is generally directed to ion implantation systems and associated ion sources. More specifically, the present disclosure is directed to components for the ion implantation systems that improve the lifetime, stability, and operation of the ion implantation systems.

[0021] Accordingly, the present invention will now be described with reference to the drawings, wherein like reference numerals may be used to refer to like elements throughout. It should be understood that the description of these aspects is merely exemplary and should not be construed in a limiting sense. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced without these specific details. Furthermore, the scope of the present invention is not limited by the embodiments or examples described below, but is intended to be limited only by the appended claims and equivalents thereof.

[0022] It should also be noted that the drawings are presented to illustrate aspects of embodiments of the present invention and should therefore be considered merely schematic. In particular, the elements depicted in the drawings are not necessarily drawn to scale relative to one another. Furthermore, the arrangement of various elements in the drawings has been selected to facilitate a clear understanding of each embodiment. Furthermore, the arrangement of various elements in the drawings should not necessarily be construed as a representation of the actual relative positions of various components in an implementation according to one embodiment of the present invention. Furthermore, the configurations of the various embodiments and examples described herein can be combined with each other unless otherwise noted.

[0023] It should also be understood that in the following description, any direct connection or coupling between functional blocks, devices, components, elements, or other physical or functional units shown in the drawings may also be implemented by indirect connections or couplings. Furthermore, it should be understood that the functional blocks or units shown in the drawings may be implemented as separate structures in one embodiment. Also, the functional blocks or units shown in the drawings may be fully or partially implemented in a common structure or alternatively implemented in another embodiment.

[0024] Ion implantation is a process used in semiconductor device manufacturing in which ions of one or more elements are accelerated into a workpiece to modify its properties. For example, dopants such as boron, arsenic, and phosphorus are commonly implanted into silicon to modify its electrical properties. In an exemplary ion implantation process, predetermined elements or molecules are ionized, extracted, and electrostatically accelerated to form a high-energy ion beam, which is filtered by its mass-to-charge ratio and directed to impact the workpiece. The ions physically impact the wafer, penetrate the surface, and come to rest below the surface at a depth related to their energy.

[0025] Referring now to the drawings, FIG. 3 illustrates a system 100 including an ion source 102 for generating an ion beam 104 along a beam path 106. A beamline assembly 110 is disposed downstream of the ion source 102 and receives the beam from the ion source. The beamline system 110 may include a mass analyzer, an acceleration mechanism, and an angular energy filter (not shown). The acceleration mechanism may include, for example, one or more gaps. The mass analyzer includes magnetic field-generating components, such as magnets. The mass analyzer operates to provide a magnetic field across the beam path 106 to deflect ions from the ion beam 104 in various trajectories depending on their mass (e.g., mass-to-charge ratio). Ions traveling through the magnetic field are subjected to forces that direct individual ions of desired mass along the beam path 106 and deflect ions of undesired masses out of the beam path.

[0026] The system 100 includes a process chamber 112. The process chamber 112 has a target location that accepts the ion beam 104 from the beamline assembly 110. The process chamber 112 supports one or more workpieces 114, such as semiconductor wafers, along the beam path 106 for implantation using the final mass-analyzed ion beam. The process chamber 112 then accepts the ion beam 104, which is directed toward the workpieces 114. It will be understood that different types of process chambers 112 may be used in the system 100. As an example, a "batch" type process chamber 112 may simultaneously support multiple workpieces 114 on a rotating support structure. In this case, the workpieces 114 are rotated through the path of the ion beam 104 until all of the workpieces 114 are fully implanted. On the other hand, a "serial" type process chamber 112 supports a single workpiece 114 along the beam path 106 for implantation. In this case, multiple workpieces 114 are implanted sequentially, one at a time, with each workpiece being fully implanted before the implantation of the next workpiece begins. The system 100 may include a scanning device (not shown) for moving the ion beam 104 relative to the workpiece 114 or the workpiece relative to the ion beam.

[0027] The ion source 102 generates the ion beam 104, for example, by ionizing a source gas containing a desired dopant element within the ion source. The ionized source gas is then extracted from the source chamber 102 in the form of the ion beam 104. The ionization process is accomplished by an exciter, which may take the form of a thermally heated filament, a cathode heated filament (indirectly heated cathode, "IHC"), or a radio frequency (RF) antenna.

[0028] 4 shows a schematic diagram of an IHC ion source 120. For example, the IHC ion source may include a source chamber 122, one or more gas inlets 124, a filament 126, a cathode 128 and a repeller 130 disposed on opposite sides of the source chamber, and an aperture 132 (also referred to as an arc slit). Additionally, a source magnet (not shown) may provide a magnetic field 134 generally along an axis between the cathode 128 and the repeller 130. During operation of the IHC ion source 120, the filament 126 is resistively heated to a temperature high enough to emit electrons. The electrons are then accelerated to collide with the cathode 128, which is at a positive potential relative to the filament.

[0029] The electron bombardment heats the cathode 128 to a temperature high enough that it thermally emits electrons into the source chamber 122. The source chamber 122 is maintained at a positive potential relative to the cathode 128 to accelerate the electrons. A magnetic field 134 helps confine electrons along the magnetic field lines between the cathode 128 and the repeller 130 along the plasma column 136, reducing electron loss toward the chamber walls 138 of the source chamber 122. Electron loss is typically further reduced by the repeller 130, which is at the same potential as the cathode 128, reflecting the electrons back toward the cathode. The excited electrons ionize the source gas supplied into the chamber through the gas inlet 124, creating a plasma. The ions are extracted through an aperture 132 and electrostatically accelerated to form a high-energy ion beam by electrodes positioned outside the source chamber 122.

[0030] According to various exemplary aspects of the present disclosure, FIG. 5 illustrates an example of the present disclosure showing a stepped cathode 200. As shown in FIG. 5, the stepped cathode 200 includes a cathode step 202 generally defined by a first cathode diameter 204 and a second cathode diameter 206 in an outer cylindrical wall 208 of the stepped cathode. The cathode step 202 in the outer cylindrical wall 208 of the stepped cathode 200 is associated with a shield step 210 defined in an inner cylindrical wall 212 of a stepped cathode shield 214, for example. This defines a first cathode shield diameter 216 and a second cathode shield diameter 218. Thus, instead of a conventional cathode simply surrounded by a straight shield, as shown in FIG. 1, the stepped cathode shield 214 in the example illustrated in FIG. 5 generally conforms to the shape of the stepped cathode 200. This causes the outer cylindrical wall 208 of the stepped cathode to closely conform to the inner cylindrical wall 212 of the stepped cathode shield. While Figure 5 shows a single cathode step 202 defined in the stepped cathode 200, the present disclosure further contemplates various designs for providing a tortuous path 220 between the stepped cathode and the stepped cathode shield 214. For example, although not shown, the tortuous path 220 can be provided by providing multiple cathode steps 202 in the stepped cathode 200, along with multiple shield steps 210 (e.g., similar to a staircase) in the stepped cathode shield 214.

[0031] Thus, the present disclosure represents various improvements over the prior art. For example, the stepped cathode 200 and stepped cathode shield 214 of the present invention generally prevent plasma in the source chamber 122 from reaching the thinned portion 222 of the stepped cathode's outer cylindrical wall 208 (e.g., used to limit heat transfer from the cathode) for extended periods of time. For example, such a design can result in up to a ten-fold improvement in cathode / source lifetime over previous lifetimes measured for As+++ and As++++ using conventional cathodes.

[0032] Furthermore, for multiply charged ion production, the plasma column 136 shown in FIG. 4 is radially smaller. This can result in improved ionization, or indeed a higher extracted beam current of multiply charged ions. For example, the stepped cathode 200 and stepped cathode shield 214 of FIG. 5 further confine electron emission to a smaller area 224 on the front cathode surface 226 compared to a conventional cathode. This results in a smaller radius 228 in the plasma column 136 of FIG. 4, resulting in a higher multiply charged beam current and throughput in the ion implanter 100 of FIG. 1.

[0033] 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 about the stepped cathode centerline 230. 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 about the centerline 230.

[0034] According to another example, Figures 6-7 illustrate another embodiment. In the example of Figures 6-7, an offset stepped cathode 250 and an offset stepped cathode shield 252 are shown, which have various similarities to the stepped cathode 200 and stepped cathode shield 214 of Figure 5. However, as shown in Figures 6-7, the centerlines 254 of the first cathode diameter 256 and first shield diameter 258 of the offset stepped cathode 250 and offset stepped cathode shield 252, respectively, are offset a predetermined distance 260 from the centerlines 262 of the second cathode diameter 264 and second cathode shield diameter 266 of the offset stepped cathode and offset stepped cathode shield. For example, the offset stepped cathode 250 of Figure 6 can provide a front cathode surface 226 closer to the aperture 132, which can advantageously further increase the ion beam current over conventional ion sources.

[0035] Thus, similar to the above example, the offset cathode step 268 defined in the offset stepped cathode 250 and the offset cathode shield step 270 defined in the offset stepped cathode shield 252 similarly substantially prevent plasma in the source chamber 272 (e.g., 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 extended periods of operation.

[0036] According to another example, FIG. 8 illustrates another stepped cathode 300. The stepped cathode 300 may additionally or alternatively include an internal step 302 at the inner diameter 304 (e.g., the inner surface). The punch-through described above can occur, for example, where a thin cathode wall joins the much thicker front portion of a conventional cathode. Therefore, in the example of the present disclosure shown in FIG. 8, a thick wall 306 is provided in close proximity to the filament 126. This delays punch-through in this region and extends cathode life beyond what has previously been realized. Providing such an internal step 302 on the inside of the cathode 300 can, for example, maintain a thin cathode wall 308 and high thermal resistance in a region 310 further from the plasma. This can promote superior ion source operation. For example, a gap 312 is provided between the filament 126 and the thick wall 306. The gap 312 may be, for example, approximately twice or more the back gap 314 between the filament and the rear (back) surface 316 of the cathode 300. The gap 312 may, for example, allow the front portion 318 of the cathode 300 to heat evenly without concerns about filament placement tolerances that may adversely affect operation. Additionally, the present disclosure contemplates sizing the curvature and / or bend radius of the filament 126 to coordinate the gap 312 and the back gap 314 associated with the inner step 302.

[0037] Thus, the present invention provides an improvement in ion source lifetime, for example, by a factor of up to 10. Furthermore, for multiply charged ions, the present disclosure provides a significantly narrower plasma column, which requires less power and therefore results in a more efficient ion source. The stepped cathode of the present disclosure, for example, reduces the diameter of the cathode, narrowing the plasma column. Furthermore, the stepped cathode shield is in close proximity to the stepped cathode and generally conforms to the stepped cathode, substantially limiting the diffusion of plasma formed within the ion source so that it is not exposed to the thin wall of the stepped cathode through the gap between the stepped cathode shield and the stepped cathode. The provision of the above-described stepped cathode and stepped cathode shield provides a tortuous path or labyrinth to substantially prevent such plasma diffusion. By providing such a tortuous path or labyrinth, substantially only the front, plasma-facing portion of the stepped cathode is heated, while also providing a thin cathode wall.

[0038] While the present invention has been illustrated and described above with respect to one or more specific embodiments, the above-described embodiments are merely examples of the implementation of multiple embodiments of the present invention, and the scope of the present invention is not limited to these embodiments. In particular, with respect to the various functions performed by the above-described components (assemblies, devices, circuits, etc.), the terms used to describe such components (including references to "means") are intended, unless otherwise indicated, to correspond to any component that performs the specified function of the described component (i.e., is functionally equivalent), but is not structurally equivalent to the disclosed structure that performs that function in the exemplary embodiment of the present invention illustrated herein. Furthermore, while a particular configuration of the present invention has been disclosed with respect to only one of multiple embodiments, such configuration may be combined with one or more configurations in other embodiments as may be desirable and advantageous for any given or particular application. Accordingly, the present invention is not intended to be limited to the above-described embodiments, but is intended to be limited only by the appended claims and equivalents thereof.

Claims

1. 1. An ion source for forming a plasma, comprising: The ion source comprises: a cathode; A filament; a cathode shield; and The cathode is a cathode surface defining a cathode step; It is equipped with a cavity and the filament is disposed within the cavity; the cathode shield has a cathode shield surface at least partially surrounding the cathode surface; a cathode gap is 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; the first cathode diameter and the second cathode diameter are different from each other to define the cathode step; the stepped cylindrical surface has an outer surface that defines the first cathode diameter and the second cathode diameter; the cathode shield surface defines a cathode shield step; the cathode shield step contour conforms to 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; the first cathode diameter and the second cathode diameter having respective centerlines offset by a predetermined distance; Ion source.

2. The ion source of claim 1 , wherein the cathode gap defines a tortuous path.

3. 2. The ion source of claim 1, wherein the stepped cylindrical surface includes an inner surface that defines the first cathode diameter and the second cathode diameter.

4. 4. The ion source of claim 3, wherein the second cathode diameter is greater than the first cathode diameter, thereby defining a thick wall of the cathode.

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

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

7. 7. The ion source of claim 6, wherein the cathode shield axially exposes a front cathode surface of the cathode to the plasma and radially surrounds the cathode.

8. A cathode assembly for an ion source according to any one of claims 1 to 7, comprising: The cathode assembly comprises: a stepped cathode; a stepped cathode shield; the stepped cathode shield radially surrounds the stepped cathode while maintaining a gap between the stepped cathode and the stepped cathode shield; an inner surface of the stepped cathode shield and an outer surface of the stepped cathode are coincident; defining a tortuous path between the stepped cathode and the stepped cathode shield; the stepped cathode includes a cathode step on the outer surface of the stepped cathode defined by a first cathode diameter and a second cathode diameter; The cathode assembly, wherein the first cathode diameter and the second cathode diameter have respective centerlines that are offset by a predetermined distance.

9. the stepped cathode includes a plurality of cathode steps defined on the outer surface of the stepped cathode; The cathode assembly of claim 8 , wherein the stepped cathode shield comprises a plurality of shield steps defined in the inner surface of the stepped cathode shield.

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