KR100778164B1 - Cathode assembly for indirect heated cathode ion source - Google Patents

Abstract

In an indirectly heated cathode ion source, the cathode is supported by at least one rod or pin. The negative electrode is preferably in the form of a disc and the support rod is smaller in diameter than the disc to limit thermal conduction and radiation. In one embodiment, the cathode is supported by a single rod at or around the center. The support rod can be held by spring-actuated clamps for simple and reliable fastening and release. The disc-shaped negative electrode and the supporting rod can be made of a single part. A filament that emits electrons thermally can be disposed around the rod in proximity to the cathode.

Indirect heated cathode ion source, cathode, single rod, support rod, filament

Description

Cathode assembly for indirect heated cathode ion source {CATHODE ASSEMBLY FOR INDIRECTLY HEATED CATHODE ION SOURCE}

Cross Reference to Related Application

This application claims priority to provisional application serial number 60 / 204,936, filed May 17, 2000 and provisional application serial number 60 / 204,938, filed May 17, 2000.

FIELD OF THE INVENTION The present invention relates to ion sources suitable for use in ion implanters, and more particularly to ion sources having an indirectly heated cathode.

Ion sources are an important component of ion implanters. The ion source passes through the beam of the ion implanter to generate an ion beam and is transferred to the semiconductor wafer. Ion sources are required to generate stable and well defined beams for a variety of different ion types and for a variety of extraction voltages. In semiconductor production plants, ion implanters containing ion sources are required to operate for extended periods of time without the need for maintenance or repair.

Ion implanters have used an ion source with a direct heated cathode in which filaments emitting electrons are mounted in an arc chamber of the ion source and exposed to a highly corrosive plasma in the arc chamber. Such direct heated cathodes typically comprise relatively small diameter wire filaments that degrade or break in the corrosive environment of the arc chamber in a relatively short time. As a result, the lifetime of the direct heated cathode ion source is limited.

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Indirectly heated cathode ion sources have been developed to improve the lifetime of ion sources in ion implanters. Indirectly heated cathodes include relatively large cathodes that are heated by electron collisions from the filaments to release hot electrons. The filaments are isolated from the plasma in the arc chamber and have a long lifetime. Although the cathode is exposed to the corrosive environment of the arc chamber, the structure of the relatively large cathode ensures that it operates for an extended period of time.

The cathode in the indirectly heated cathode ion source must be electrically insulated from the surroundings, electrically connected to a power source, and thermally insulated from the surroundings of the cathode to prevent cooling that causes interruption of electron emission. Known conventional indirect heating cathode designs use a disk-shaped cathode whose outer periphery is supported by a thin wall tube of approximately the same diameter as the disk. The tube has a thin wall to reduce the cross-sectional area to reduce the conduction of heat radiated from the heated cathode. Thin tubes typically act as insulating breaks and have cutouts along their length to reduce the conduction of heat radiated from the cathode.

The tube used to support the cathode does not emit electrons, but has a large surface area, mostly hot. This area loses heat by radiation, which is the main way the cathode loses heat. The large diameter of the tube increases the size and complexity of the structure used to clamp and connect to the cathode. One known cathode support comprises three parts and requires screws to assemble.

Indirectly heated cathode ion sources typically include a filament power source, a bias power source and an arc power source and require a control system to regulate these power sources. Prior art control systems for indirect heating cathode ion sources regulate the sources so that the arc current is constant. The difficulty with using a constant arc current system is that when the beam line is regulated, the amount of current extracted from the regulation or source that causes the beam current measured at the downstream end of the beam line to increase the percentage of current transmitted through the beam line. Can increase because of Because beam current and transmission are affected by the same plurality of variables, it is difficult to adjust to maximum beam transmission.

The prior art approach used in ion sources with direct heated cathodes is to control the source for a constant extraction current rather than a constant arc current. In all cases where the source controls a constant extraction current, the control system drives a Bernas type ion source whose cathode is a direct heated filament.

According to one aspect of the invention, a negative electrode assembly for use in an indirectly heated negative ion source is a negative electrode assembly comprising a negative electrode and a support rod fixedly mounted thereon, the outer side of the arc chamber being very adjacent to the support rod of the negative electrode subassembly Filaments emitting electrons, a negative electrode insulator electrically and thermally insulating the negative electrode from the arc chamber housing positioned around the negative electrode of the negative electrode subassembly.

The negative subassembly may comprise an indirectly heated negative electrode and a support rod fixedly attached to the indirectly heated negative electrode to support the negative electrode in the arc chamber of the ion source. In one embodiment, the support rod is attached to the surface of the cathode facing away from the arc chamber. The support rod supports the cathode and conducts electrical energy to the cathode. The negative electrode may be in the form of a disk and the support rod may be attached to or near the center of the negative electrode along its axis. The support rod can be cylindrical and the diameter of the cathode can be larger than the diameter of the cylindrical support rod. In one embodiment, the diameter of the cathode is at least four times larger than the diameter of the support rod. The negative subassembly may further comprise a spring loaded clamp to hold the support rod.

The filament is very adjacent to the cathode, can be disposed around the support rod and separated from the plasma in the arc chamber. The filament may be made of an electrically conductive material and includes an arc shaped pivot having an inner diameter greater than or equal to the diameter of the support rod. The cross-sectional area of the filament can vary along the length of the filament and is the smallest along the arc pivot.

A negative electrode insulator may be provided to electrically and thermally insulate the negative electrode from the housing of the arc chamber. In one embodiment, the negative electrode insulator includes an opening having a diameter greater than or equal to the diameter of the negative electrode. A vacuum gap may be provided between the negative electrode insulator and the negative electrode to define thermal conductivity. The cathode insulator generally has the form of a tube with sidewalls and includes a flange to isolate the sidewall of the cathode insulator from the plasma of the arc chamber. Such a flange may have a groove on the side of the flange facing away from the plasma to increase the path length between the cathode and the arc chamber housing.

According to another aspect of the invention, a method of supporting and heating a cathode of an ion source includes supporting the cathode by a rod fixedly attached to the cathode, and colliding the cathode with electrons. According to another aspect of the present invention, there is provided a negative electrode, a support rod fixedly attached to the negative electrode, a negative electrode insulator which electrically and thermally insulates the negative electrode from the arc chamber housing, and indirect heating means for indirectly heating the negative electrode.

In order to assist in understanding the present invention, it will be described with reference to the drawings incorporated herein by reference.

1 is a schematic block diagram of an indirectly heated cathode ion source in accordance with an embodiment of the present invention.

2A and 2B are, respectively, front and perspective views of an embodiment of a cathode in the ion source of FIG.

3A-3D are perspective, front, top, and side views, respectively, of one embodiment of the filament in the ion source of FIG.

4A-4C are respective, perspective, cross-sectional and partial cross-sectional views of one embodiment of a negative electrode insulator in the ion source of FIG.

5 schematically shows the feedback loop used to control the extraction flow for the ion source controller.

6 schematically illustrates the operation of the ion source controller of FIG. 1 in accordance with a first control algorithm.

7 schematically illustrates the operation of the ion source controller of FIG. 1 in accordance with a second control algorithm.

Indirectly heated cathode ion source according to an embodiment of the present invention is shown in FIG. The arc chamber housing 10 with the extraction hole 12 defines the arc chamber 14. The cathode 20 and the repeller electrode 22 are positioned in the arc chamber 14. The repeller electrode 22 is electrically insulated. The negative insulator 24 electrically and thermally insulates the negative electrode 20 from the arc chamber housing 10. Cathode 20 may be selectively separated from insulator 24 by a vacuum gap to prevent thermal conduction. The filament 30 positioned outside of the arc chamber 14 in close proximity to the cathode 20 generates heat of the cathode 20.

The gas to be ionized is provided from the gas source 32 to the arc chamber 14 via a gas inlet 34. Although not shown, in another configuration, the arc chamber 14 may be combined with an evaporator that vaporizes the material to be ionized in the arc chamber 14.

The arc power source 50 has a positive terminal connected to the arc chamber housing 10 and a negative terminal connected to the cathode 20. The arc power source 50 is rated at 100 volts at 10 amps and can operate at about 50 volts. The arc power source 50 accelerates the electrons emitted by the cathode 20 in the arc chamber 14 to the plasma. The bias power supply 52 has a positive terminal connected to the cathode 20 and a negative terminal connected to the filament 30. The bias power source 52 is rated at 600 volts at 4 amps and can operate at a voltage of about 400 volts and a current of about 2 amps. The bias power source 52 accelerates the electrons emitted by the filament 30 to the cathode 20 to produce heat in the cathode 20. The filament power supply 54 has an extraction terminal connected to the filament 30. The filament power source 54 is rated at 5 volts at 200 amps and can operate at a filament current of about 150 to 160 amps. The filament power source 54 produces heat in the filament 30, which in turn generates electrons accelerated to the cathode 20 to heat the cathode 20. The supply magnet 60 generates a magnetic field B in the arc chamber 14 in the direction indicated by the arrow 62. The direction of the magnetic field B can be reversed without affecting the operation of the ion source.

In the case of the ground electrode 71, the extraction electrode and the suppression electrode 72 are positioned in front of the extraction hole 12. Each ground electrode 70 and suppression electrode 72 has holes aligned with extraction holes 12 for extraction of well defined ion beams 74.

The extraction power source 80 has a positive terminal connected to the arc chamber housing 10 through a current sense resistor 110 and a negative terminal connected to the ground and the ground electrode 70. The extraction power source 80 is rated at 70 kilovolts (kV) at 25 milliamps to 200 milliamps. Extraction source 80 provides a voltage for extraction of ion beam 74 from arc chamber 14. The extraction voltage is adjustable in accordance with the desired energy of the ions in the ion beam 74.

The suppression power supply 82 has a negative terminal connected to the suppression electrode 72 and a positive terminal connected to the ground. Suppressed power supply 82 can be calculated in the range of -2 kV to -30 kV. The negatively biased suppression electrode 72 inhibits the movement of electrons in the ion beam 74. The voltage, current ratio, voltage operation and current of the power supply 50, 52, 54, 80 and 82 are represented by only one embodiment and are not limited to the scope of the present invention.

Ion source controller 100 provides control of the ion source. The ion source controller 100 may be a programmed controller or a controller for a specific purpose. In a preferred embodiment, the ion source controller 100 is integrated into the main control computer of the ion implanter.

The ion source controller 100 controls the arc power supply 50, the bias power supply 52, and the filament power supply 54 to produce a predetermined level of extraction ion current from the ion source. By fixing the current extracted from the ion source, because of the low beam, low contamination and improved retention of particles generated due to reduced abrasion from beam generation, the ion beam is controlled to transfer most beneficially to ion supply life and defect reduction. . An additional benefit is the faster beam control.

Ion source controller 100 may accept lines 102 and 104 of current sense signals representative of extraction current I _E supplied by extraction power source 80. The current sense resistor 110 may be connected in series with one of the sources derived from the extraction power source 80 to sense the extraction current I _E. In another arrangement, the extraction power source 80 may be configured to provide the extraction current I _E on the line 112 of the current sensing signal that it represents. The electrical extraction current I _E supplied by the extraction power source 80 corresponds to the beam current at the ion beam 74. The ion source controller 100 also receives a reference value signal I _E REF that represents a predetermined or reference value extraction current. The ion source controller 100 compares the detected extraction current I _E with the reference value extraction current I _E REF and determines an error value that may be positive, negative, or zero.

The control algorithm is used to adjust the calculation of the power supply with respect to the error value. One embodiment of the control algorithm utilizes a Proportional-Integral-Derivative (PID) loop, shown in FIG. The goal of the PID loop is to maintain the extraction current I _E , which is used to generate an ion beam at the reference value extraction current I _E REF. The PID loop achieves this result by continuously adjusting the calculation of the PID calculation 224 as required to adjust the extracted current I _E sensed with the reference value extraction current I _E REF. The PID calculation 224 feeds back from the ion generator assembly 230 (FIG. 1) in the form of an error signal I _E ERROR generated by removing the sensed extraction current I _E and the reference value extraction current I _E REF. Accept it. The calculation of the PID loop is performed by the arc power supply 50, bias power supply 52 and filament power supply from the ion source controller 100 to maintain the extraction current I _E at or near the reference value extraction current I _E REF. 54 may be supplied.

According to the first control algorithm, the bias current I _B supplied by the bias power source 52 (FIG. 1) is such that the extraction current I _E controls the extraction current I _E at or near the reference value extraction current I _E REF. Changed for an error value (I _E ERROR). The bias current I _B represents the electron current between the filament 30 and the cathode 20. In particular, the bias current I _B is increased to increase the extraction current I _E and the bias current I _B is reduced to decrease the extraction current I _E. The bias voltage V _B is unregulated and varied to supply a predetermined bias current I _B. In addition, according to the first control algorithm, the filament current I _F supplied by the filament power supply 54 is maintained at a constant value along with the unregulated filament voltage V _F , and is applied to the arc power supply 50. The arc voltage V _A supplied by it is kept at a constant value with the unregulated arc current I _A. The first control algorithm can benefit from good performance, simplicity and low cost.

An embodiment of the operation of the ion source controller 100 according to the first control algorithm is shown schematically in FIG. 6. The inputs V ₁ , V ₂ and R shown in FIG. 1 are used to perform the extraction current calculation 220. The input voltages V ₁ and V ₂ are measured values and the input resistance R is based on the value of the resistor 110 (FIG. 1). The sensed extraction current I _E is calculated as follows:

I _E = (V ₁ -V ₂ ) / R

If the extraction power source 80 is configured to supply a current sense signal indicative of the extraction current I _E to the ion source controller 100, the calculation may be omitted. The sensed extraction current I _E and the reference value extraction current I _E REF are fed to the error calculation 222. The reference value extraction current I _E REF is a set value based on the predetermined extraction current. The extraction current error value I _E ERROR is calculated by subtracting the reference extraction current I _E REF from the detected extraction current I _E as follows:

I _E ERROR = I _E -I _E REF

The extraction current error value I _E ERROR and three control coefficients K _PB , K _IB and K _DB are fed into the PID calculation 224a. Three control coefficients are optimized to obtain the most effective control effect. In particular, K _PB , K _IB and K _DB are chosen to create a control system with a transient response with acceptable rise time, overshoot and steady-state error. The input signal of the PID calculation is judged as follows:

O _b (t) = K _PB e (t) + K _IB ∫e (t) dt + K _DB de (t) / dt

Here, e (t) is an instantaneous extraction current error value and O _b (t) is an instantaneous calculation control signal. The instantaneous output signal O _b (t) is provided at the bias power source 52 and provides information on how the extraction current I _B should be adjusted to minimize the extraction current error value. The intensity and polarity of the output control signal O _b (t) depends on the control requirements of the bias power source 52. In general, however, the output control signal (O _b (t)) are sensed extraction current (I _E) the reference value to extract current (I _E REF) than, and to increase the bias current (I _B) when the enemy sensed extraction current ( when the I _E) is greater than the reference extraction current (I _E REF) and to decrease the bias current (I _B).

The filament current I _F and arc voltage V _A are kept constant by the filament and arc power source controller 225 shown in FIG. 6. Selected according to a given source operating situation, control variables are fed into the filament and arc power source controller 225. Control signals O _f (t) and O _a (t) are input by controller 225 and provided at filament power supply 54 and arc power supply 50, respectively.

According to the second control algorithm, the filament current I _F supplied by the filament power source 54 (FIG. 1) is such that the extraction current I _E controls the extraction current I _E at or near the reference value extraction current I _E REF. It is changed with respect to the error value I _E ERROR. In particular, the filament current I _F is reduced to increase the extraction current I _E , and the filament current I _F is increased to reduce the extraction current I _E. The filament voltage V _F is unregulated. In addition, according to the second control algorithm, the bias current I _B supplied by the bias power supply 52 is kept constant with the unregulated bias voltage V _B , and by the arc power supply 50. The supplied arc voltage V _A remains constant with the unregulated arc current I _A.

The operation of the ion source controller 100 according to the second control algorithm is shown schematically in FIG. Extraction current calculation 220 is performed with a first control algorithm based on inputs V ₁ , V ₂ and R to determine the sensed extraction current I _E. The sensed extraction current I _E and reference value extraction current I _E REF are fed to an error calculation 226. The extraction current error value I _E ERROR is calculated by subtracting the detected extraction current I _E from the reference extraction current I _E REF as follows:

I _E ERROR = I _E REF-I _E

This calculation differs from the error calculation of the first algorithm when the dimensions of the operand function are reversed. The operand function is inverted so that the control loop makes a direct relationship, as in the first algorithm, rather than the inversion relationship between the extraction current I _E and the controlled variable (in this case I _F ). The extracted current error value I _E ERROR and the three control coefficients are fed into the PID calculation 224b. The coefficients K _PF , K _IF and K _DF need not necessarily have the same value as the control coefficient of the first algorithm, as selected to optimize the performance of the ion source according to the second control algorithm. However, PID calculation 224b may be the same as follows:

O _F (t) = K _PF e (t) + K _IF ∫e (t) dt + K _DF de (t) / dt

The instantaneous output control signal O _F (t) is provided as a filament power supply and provides information on how the filament current I _F should be adjusted to minimize the extraction current error value. The brightness and polarity of the output control signal O _F (t) depends on the control requirements of the filament power supply 54. In general, however, the calculated control signals (O _F (t)) are sensed extraction current (I _E) the reference value to extract current (I _E REF) when less than and the filament current (I _F) so as to decrease the sensed extraction current ( I _E) is to be greater when the filament current (I _F) higher than the reference value to extract current (I _E REF).

The bias current I _B and the arc voltage V _A are kept constant by the bias and arc power supply controller 229 shown in FIG. Selected according to the desired source operating situation, control variables are fed into the bias and arc power source controller 229. Control signals O _B (t) and O _A (t) are input by the controller 229 and provided as a filament power supply 52 and an arc power supply 50, respectively.

While the first control algorithm and the second control algorithm are schematically represented separately, it can be understood that the ion source controller 100 can be configured to perform one or both algorithms. If the ion source controller 100 can perform both, a mechanism can be provided to select a particular algorithm to be implemented by the controller 100. It will be appreciated that different control algorithms may be used to control the extraction current of the indirectly heated cathode ion source. In a preferred embodiment, the control algorithm can be executed in software at the controller 100. However, a circuit-connected or microprogrammed controller can be used.

When the ion source is operating, the filament 30 is resistively heated by the filament current I _F to a heat ion release temperature that can be dimensioned to 2200 ° C. Electrons emitted by the filament 30 are accelerated by the bias voltage V _B between the filament 30 and the cathode 20 and the impact machining and heating cathode 20. The cathode 20 is heated by electron impact processing with respect to the heat ion emission temperature. Electrons emitted by the cathode 20 ionize gas molecules from the gas source 32 in the arc chamber 14 to be accelerated by the arc voltage V _A and produce a plasma discharge. The electrons in the arc chamber 14 follow the spiral path by the magnetic field B. The repeller electrode 22 is sufficiently negatively charged to prevent electrons from returning through the arc chamber 14, which forms negative charges as a result of incident electrons and ultimately further ionizes to create a collision. Since the filament 30 is not exposed to plasma in the arc chamber 14 and the cathode 20 is heavier than a conventional direct heated cathode, the ion source of FIG. Indicates.

An embodiment of the indirectly heated cathode 20 is shown in FIGS. 2A and 2B. 2A is a side view and FIG. 2B is a perspective view of the cathode 20. The cathode 20 can be disc shaped and connected to the support rod 150. In one embodiment, the support rod 150 is attached to the center of the disc-shaped cathode 20 and has a substantially smaller diameter than the cathode 20 to limit thermal reduction and reflection. In one embodiment, multiple support rods are attached to the cathode 20. For example, a second support rod having a different size or shape than the first support rod may be attached to the cathode 20 to prevent incorrect installation of the cathode 20. The negative subassembly comprising the negative electrode 20 and the support rod 150 may be supported in the arc chamber 14 (FIG. 1) by a spring loaded clamp 152. The spring loaded clamp 152 properly holds the support rod 150 and is properly held by a support structure (not shown) relative to the arc chamber. The support rod 150 provides mechanical support for the cathode 20 and provides electrical connections to the arc power source 50 and the bias power source 52 as shown in FIG. 1. The support rod 150 has a relatively small diameter, so thermal reduction and reflection are limited.

In one example, the cathode 20 and the support rod 150 are made of tungsten and made in a single piece. In this example, cathode 20 has a diameter of 1.91 cm (0.75 inches) and a thickness of 0.508 cm (0.20 inches). In one embodiment, the support rod 150 has a length in the range of about 1.27 to 7.62 cm (0.5 to 3 inches). For example, in a preferred embodiment, the support rod 50 has a length of approximately 4.445 cm (1.75 inches) and a diameter in the range of about 0.102 to 0.635 cm (0.04 to 0.25 inches). In a preferred embodiment, the support rod 150 has a diameter of approximately good 0.318 cm (0.125 inch). In general, the support rod 150 has a diameter smaller than the diameter of the cathode 20. For example, the diameter of the cathode 20 may be at least four times larger than the diameter of the support rod 150. In a preferred embodiment, the diameter of the cathode 20 is approximately six times larger than the diameter of the support rod 150. These dimensions are not given by way of example only and are not limited to the scope of the present invention. In another example, the cathode 20 and the support rod 150 may be made from each component and attached together, such as by press fixing.

In general, the support rod 150 is a solid cylindrical structure and at least one support rod 150 is used to support the cathode 20 and conduct electrical energy to the cathode 20. In one embodiment, the diameter of the cylindrical support rod 150 is constant along the length of the support rod 150. In other embodiments, the support rod 150 may be a solid cylindrical structure having a diameter that varies with the function of positioning along the length of the support rod 150. For example, the diameter of the support rod 150 may be the smallest along the length of the support rod 150 at each end, thus facilitating thermal insulation between the support rod 150 and the cathode 20. The support rod 150 is attached to the surface of the cathode 20 facing away from the arc chamber 14. In a preferred embodiment, the support rod 150 is attached to the cathode 20 at or near the center of the cathode 20.

An example of filament 30 is shown in FIGS. 3A-3D. In this example, the filament is 30 and made of conductive wire and includes a heating loop 170 and connecting leads 172 and 174. Connection leads 172 and 174 are provided with appropriate bending for attachment of filament 30 to the power supply, shown at filament power supply 54 in FIG. In the example of FIGS. 3A-3D, the heating loop 170 may be configured with a single arced rotation having an inner diameter equal to or greater than the diameter of the support rod 150 to accommodate the support loop 150. . In the example of FIGS. 3A-3D, the heating loop 170 has an inner diameter of 0.914 cm (0.36 inch) and an outer diameter of 1.37 cm (0.54 inch). The filament 30 may be made of tungsten wire having a diameter of 0.029 cm (0.090 inch). Preferably the wire along the length of the heating loop 170 is grounded or otherwise reduced to a smaller cross sectional area in the region adjacent to the cathode 20 (FIG. 1). For example, the diameter of the filament following arc-shaped rotation is 0.191 cm (0.075 inches) for increased resistance and increased heating very close to the cathode 20, and reduced heating of the connection leads 172 and 174. Can be reduced to a smaller diameter in dimensions. Preferably, the heating loop 170 is spaced apart from the cathode 20 by about 0.051 cm (0.020 inches).

Examples of the negative electrode insulator 24 are shown in Figs. 4A to 4C. As shown, the insulator 24 generally has a ring-shaped configuration with a central opening 200 for receiving the cathode 20. The insulator 24 is configured to thermally and electrically insulate the cathode 20 from the arc chamber housing 10 (FIG. 1). Preferably, the central opening 200 is dimensioned slightly larger than the cathode 20 to provide a vacuum gap to prevent thermal conduction between the insulator 24 and the cathode 20. The insulator 24 may be provided with a flange 202 that shields the side wall 204 of the insulator 24 from the plasma in the arc chamber 4 (FIG. 1). The flange 202 may be provided with a groove 206 on the side facing away from the plasma that increases the passage length between the cathode 20 and the arc chamber housing 10. This insulator design reduces the risk of deposits on the insulator which results in a short circuit between the cathode 20 and the arc chamber housing 10. In the preferred embodiment, the negative electrode insulator 24 is made of boron nitride.

While the presently preferred embodiments of the invention have been described and illustrated, various changes and modifications will be apparent to those skilled in the art without departing from the scope of the invention as defined by the appended claims. In addition, the features described herein may be used arbitrarily in combination or separately within the scope of the invention.

Claims (18)

A cathode assembly for use in an indirect heated cathode ion source comprising an arc chamber housing defining an arc chamber, A negative electrode subassembly comprising a negative electrode and a support rod rigidly mounted thereto; A filament positioned outside of the arc chamber proximate to the support rod of the cathode subassembly, for emitting electrons insulated from the plasma in the arc chamber; And a negative electrode insulator for electrically and thermally insulating the negative electrode from the arc chamber housing disposed around the negative electrode of the negative electrode subassembly. The negative electrode assembly of claim 1, wherein the filament is disposed around a support rod proximate to the negative electrode. The cathode of claim 1, wherein the filament is disposed around a support rod proximate to the cathode and is made of an electrically conductive material and comprises an arc-shaped pivot having an inner diameter equal to or greater than the diameter of the support rod. Assembly. The cross-section of the filament of claim 1, wherein the filament is disposed around a support rod proximate to the cathode and is made of an electrically conductive material and comprises an arc shaped pivot having an internal diameter equal to or greater than the diameter of the support rod. And a region varies along the length of the filament and least along the arced pivot. The negative electrode assembly of claim 1, wherein the negative electrode insulator comprises an opening having a diameter equal to or greater than the diameter of the negative electrode. 6. The negative electrode assembly of claim 5, wherein a vacuum gap is provided to limit thermal conduction between the negative electrode insulator and the negative electrode. 6. The negative electrode assembly of claim 5, wherein the negative electrode insulator generally takes the form of a tube with sidewalls and includes a flange to shield the sidewall of the negative electrode insulator from plasma in the arc chamber. 8. The negative electrode assembly of claim 7, wherein the flange has a groove on the side of the flange facing away from the plasma to increase the length of the passageway between the cathode and the arc chamber housing. delete delete delete delete delete delete delete delete delete delete

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