Electric lamp and discharge devices: systems – High energy particle accelerator tube
Reexamination Certificate
2003-04-01
2004-04-20
Philogene, Haissa (Department: 2821)
Electric lamp and discharge devices: systems
High energy particle accelerator tube
C315S501000, C315S111810, C315S116000, C313S037000
Reexamination Certificate
active
06724160
ABSTRACT:
FIELD OF INVENTION
This invention relates generally to ion and plasma sources, and more particularly it pertains to the neutralization of the ion beams from such sources with some or all of the electrons from hot-filament cathode-neutralizers.
BACKGROUND ART
Industrial ion sources are used for etching, deposition and property modification, as described by Kaufman, et al., in the brochure entitled
Characteristics, Capabilities, and Applications of Broad-Beam Sources
, Commonwealth Scientific Corporation, Alexandria, Va. (1987).
Both gridded and gridless ion sources are used in these industrial applications. The ions generated in gridded ion sources are accelerated electrostatically by the electric field between the grids. Only ions are present in the region between the grids and the magnitude of the ion current accelerated is limited by space-charge effects in this region. Gridded ion sources are described in an article by Kaufman, et al., in the
AIAA Journal
, Vol. 20 (1982), beginning on page 745. The particular sources described in this article use a direct-current discharge to generate ions. It is also possible to use electrostatic ion acceleration with a radio-frequency discharge, as described in U.S. Pat. No. 5,274,306—Kaufman, et al. These publications are incorporated herein by reference.
In gridless ion sources the ions are accelerated by the electric field generated by an electron current interacting with a magnetic field in the discharge region. Because the ion acceleration takes place in a quasineutral plasma, there is no space-charge limitation on the ion current that can be accelerated in this type of ion source. Because a Hall current of electrons is generated normal to both the applied magnetic field and the electric field generated therein, these ion sources have also been called Hall-current sources. The end-Hall ion source is one type of gridless ion source and is described in U.S. Pat. No. 4,862,032—Kaufman, et al., while the closed-drift ion source is another type of gridless ion source and is described by Zhurin, et al., in an article in
Plasma Sources Science & Technology
, Vol. 8, beginning on page R1. These publications are also incorporated herein by reference.
An end-Hall ion source has a discharge region with only an outside boundary, where the ions are generated and accelerated continuously over the cross section of the region enclosed by the boundary. The shape of this cross section can be circular, elongated, or some other shape as long as there is only an outer boundary to this region.
A closed-drift ion source has a discharge region with both inner and outer boundaries, where the ions are generated and accelerated only over the cross section between these two boundaries. The shape of this cross section is usually of an annular shape. It can also be of an elongated or “racetrack” shape, or some other shape as long as it has two separate and distinct boundaries—usually inner and outer boundaries.
Both gridded and gridless ion sources use electron-emitting cathodes to neutralize the ion beams that are generated, as well as to provide electrons to sustain the discharge. These electron-emitting cathodes are most often called “neutralizers” in publications describing gridded ion sources, and most often called “cathodes” in publications describing gridless ion sources. For consistency, all such electron-emitting cathodes will herein be called “cathode-neutralizers.” The most common cathode-neutralizers are the hot-filament, hollow-cathode, and plasma-bridge types, all of which are described in “Ion Beam Neutralization,” anon.,
CSC Technical Note
, Commonwealth Scientific Corporation, Alexandria, Va. (1991). This publication is also incorporated herein by reference. Because of their reliability, low cost, and simple maintenance, hot-filament cathode-neutralizers are widely used.
Because the neutralized ion beams are also quasineutral plasmas, i.e., the electron density is approximately equal to the ion density, ion sources have also been called plasma sources. It should be noted that the electrons emitted from the cathode-neutralizer do not recombine with the ions in the ion beam. Such recombination depends on three-body collisions that are negligible at the several millitorr or less background pressure in the space between the ion source and the surface struck by the ion beam. There are, however, charge-exchange collisions between energetic beam ions and background neutral atoms or molecules so that some energetic ions become energetic neutrals and some background neutrals become low-energy charge-exchange ions. The number of ions is conserved in the charge-exchange process, so that the number of ions requiring electrons to neutralize their current—whether beam ions or charge-exchange ions—is unchanged by the charge-exchange process.
The proper magnitude of electron emission from the cathode-neutralizer is required to reduce or eliminate electrostatic charging damage to the surfaces near or in the ion beam, particularly the surfaces of targets and deposition substrates. A prior-art method of doing this is to set the cathode-neutralizer emission in a gridded ion source at a magnitude equal to the ion beam current. This is defined as “current neutralization.” Current neutralization is obtained in a gridless ion source by setting the cathode-neutralizer emission at a magnitude equal to the discharge current to the anode.
In practice, the two currents are set equal to each other by comparing the readings on two meters and adjusting the emission of the cathode-neutralizer until the two readings are equal. In some cases automatic controls are used to maintain the two currents at the values at which they are set. Even though set equal, the currents can still be unequal due to errors in either reading or calibrating the meters. In addition, the dynamics of control circuits frequently results in departures from current neutralization when operating conditions are changed.
A deficiency in the magnitude of the electron emission from the cathode-neutralizer results in the elevation of the potential within the ion beam until the electron and ion currents at electrically isolated surfaces reach equal magnitudes. When the potential elevation is sufficient, the electron emission from the cathode-neutralizer is augmented by the generation of micro-arcs between the ion beam and the surrounding vacuum chamber, the work piece, or other nearby hardware. These micro-arcs are of very short duration. Depending on the degree of electron emission deficiency, they may be observed with a frequency of one or less per minute up to one or more a second. These micro-arcs result either in direct damage where the micro-arc takes place or indirect damage in the form of particulates generated by the micro-arc and deposited elsewhere.
When the magnitude of the electron emission from the cathode-neutralizer exceeds the ion beam current, the excess electrons are in many cases, but not all, able to flow to the grounded vacuum enclosure or other grounded hardware within that enclosure without generating damaging micro-arcs. The fairly common situation of the ion beam being able to dissipate excess neutralizing electrons without substantial electrostatic charging, together with variations in the accuracy of current measurements, is the justification for the common practice of setting the cathode-neutralizer electron emission somewhat greater than the value required for current neutralization.
Problems have been encountered with electrostatic charging during ion beam etching, as described in an article by Olson in the
EOS/ESD Symposium
, 98-332 (1998). These problems have been most serious when portions of the work piece at which the ion beam is directed are electrically isolated from each other. Differential charging of these isolated portions can result in an electrical breakdown between the two portions. Such a breakdown will damage the work piece.
As described in the aforesaid article by Olson, setting the cathode-neutralizer emission current equal to or greater than the ion beam curr
Kahn James R.
Kaufman Harold R.
Zhurin Viacheslav V.
Edmundson Dean P.
Kaufman & Robinson, Inc.
Philogene Haissa
Tran Thuy Vinh
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