Patent Grant
7312579
1. Field of the Invention
This invention relates in general to the technology of ion and plasma sources, and more particularly to a Hall-current ion source designed for producing broad ion beams of various energies utilized in thin film technology and material processing.
2. Description of the Prior Art
A Hall-current ion source, in some cases also called end-Hall ion source, was described in U.S. Pat. No. 4,862,032 by Kaufman et al. Later it was modified in U.S. Pat. No. 6,608,431 by Kaufman as an ion source of a modular design for easy assembly/disassembly. A very similar concept of a Hall-current ion source was described in a U.S. Pat. No. 6,645,301 by Sainty. Varieties of another Hall-current ion source type called as closed drift ion sources in a form of electric propulsion thrusters and ion sources are known and described by Zhurin et al in article “Physics of Closed Drift Thrusters” in Plasma Sources Science & Technology, Vol. 8 (1999), beginning on page R1. A hybrid ion source of end-Hall type and closed drift ion source is presented in U.S. Pat. No. 7,116,054 by Zhurin.
This ion source utilizes features of both types of ion sources and provides more efficient ionization and acceleration of ion beam than regular end-Hall ion source. In this ion source, in comparison with Kaufman et al. and Sainty's end-Hall ion sources having magnetic field with reducing its strength from a gas distributing system to a discharge channel exit, there is utilized a positive gradient of magnetic field in a discharge channel for efficient acceleration of ions and for suppression of high amplitude discharge current and voltage oscillations. These publications are incorporated herein by reference. A Hall-current ion source belongs to a family of gridless ion sources and was introduced together with a gridded ion source for industrial applications. Description of gridded ion source for technological applications was given by Kaufman in article “Technology of Ion Beam Sources Used in Sputtering”, in Journal of Vacuum Science & Technology, Vol. 15, pp 272-276, March/April 1978. This publication is also incorporated herein by reference. All these ion sources are spin-offs from electric propulsion thrusters utilized with space satellites for producing thrust to move satellite with a certain momentum to a designated position in space.
If gridded ion sources can be considered as electrostatic ion sources, then gridless ion sources can be called electromagnetic ion sources. Operation of all Hall-current ion sources is based on electrical discharge in gas in magnetic field at pressures of about 10−5-10−3 Torr with reduced mobility of electrons in direction across to magnetic field lines. Because of this, it becomes possible in a direction of magnetic field lines to develop quite strong electric field strength that provides acceleration of ions.
In order to maintain electrical discharge in gas it is necessary to utilize conditions, which include presence of charged particles that depend on magnetic field value and its geometry, shape of electrodes and other factors influencing charge transportation in plasma. It can lead to a separation of charge particles caused by different trajectories and velocities of ions and electrons. Such separation generates a Hall current, which is directed along a normal to vectors of discharge current, I and magnetic field, B.
Hall current plays a major role in ion acceleration in plasma when a characteristic time of a process is an order of a period of charged particles rotation along a Larmour radius τ≧1/ωi with a condition that an electron component becomes “magnetized”. “Magnetized” plasma means that both electron and ion components of plasma experience many revolutions around a magnetic field line before they move due to a collision with a neighbor particle. However, it is not necessary that both plasma components will be magnetized. In the case of ion sources, only electrons are usually “magnetized” and ions are not magnetized.
It is important that for a Hall-current ion source there will be performed the following condition: rLe<<l<<rLi, where l is a characteristic dimension of acceleration zone of a discharge region (length or width of a discharge channel), rLe and rLi are Larmour radii, correspondingly for electrons and ions.
Electrons drift along equipotential surfaces, which are presented by magnetic field lines, and ions are accelerated in a direction of electrical field practically without any influence of magnetic field, because they are not magnetized as electrons. Electrons in existing Hall-current ion sources invented by Kaufman et al. and Sainty have strong axial magnetic field component, Bz in area of a gas distributing system and a hollow anode bottom part, and only close to exit from a discharge chamber magnetic field lines acquire a substantial value of radial magnetic field component, Br. The features of low value of radial component of magnetic field in a gas distributing system and anode area of Hall-current ion sources and influence of axial component on an ion beam current value were discussed in detail by Zhurin et al. in the above mentioned article “Physics of Closed Drift Thrusters” in Plasma Sources Science & Technology, Vol. 8 (1999), beginning on page R1 and by Zhurin in US 2005/0237000 A1.
Regular Hall-current ion sources, in some cases called end-Hall ion sources, fabricated by Veeco Instruments through a license from Kaufman & Robinson Inc., by Kaufman & Robinson Inc. and Saintech Ltd operate with working reactive gases such as Oxygen, Nitrogen, and noble gases such as Argon, Xenon and other gases at discharge voltages, of about Vd=100-300 V (O2, N2), and Vd=80-300 V (Ar, Xe), and discharge currents from about Id=1 A up to 10-15 A. For most thin film deposition processes including a so-called ion assisted deposition and a sputtering deposition, it is necessary to have energies of ions in the range of 10-30 eV for ion assisted deposition, and in the range of 100-500 eV for a sputtering deposition. An ion beam mean energy of end-Hall ion sources in general is about 60% of discharge voltage, Vd. In other words, in order to have a range of mean ion energies from 15 to 500 eV an ion source should operate at discharge voltages from about 25 V to 830 V.
In a publication “Low-Energy End-Hall Ion Source Characterization at Millitorr Pressures” by Kahn et al., SVC (Society of Vacuum Coaters) 48th Annual Technical Conference Proceedings, p 445, 2005, there is presented an end-Hall an ion source EH-1000 with a modified anode that is designed to sustain oxidizing effects on anode performance. This publication is incorporated here by reference. This end-Hall ion source generates ion beams of low energy of about 25 eV with Argon as working gas, where discharge current is 10 A and discharge voltage is 41-43 V. In this case, the operation of ion source with an ion beam of low energies is realized by a high mass flow of working gas, so a background pressure in a vacuum chamber is about 1-2 mTorr. At such high gas pressures in a vacuum chamber a role of charge-exchange mechanisms becomes very important, in the way that ion beam particles-ions exchange energy and momentum with neutrals that become neutrals of high energy and a target receives a flow of energetic neutrals instead of ions. The charge-exchange mechanism plays especially important role at low energies of ions. Resonance charge exchange (on atoms of the same gas) cross section in such conditions is quite high: σch-ex≈0.5×10−14-10−15 cm2. During charge exchange a fast ion becomes a fast atom, and slow atom becomes a slow ion, the whole process makes good quantified reliable results for sputtering quite difficult.
Comparatively recent new technique called a biased target deposition was introduced by Zhurin et al. in article “Biased Target Deposition” in Journal of Vacuum Science & Technology, A 18(1), January/February 2000, p 37. In this article, a compact end-Hall ion source Mark-1 was used with low energy ions applied to a target at ion energy lower than a sputtering energy threshold. This publication is incorporated here by reference. Low energy ion beam interacts with a negatively biased target of several hundreds of electron volts, and ions are accelerated in a short Debye layer (typically 0.1-1 mm, depending on pressure in vacuum chamber). Such a technique is useful for obtaining very fine thin film depositions that are not contaminated by interaction of a low energy ion beam with a target surrounding, because an ion beam energy is not sufficient to sputter unwanted particles, which are not at a negative high potential. In this case, an ion beam interacts only with target having a negative potential. A Hall-current ion source of low energy ions is good candidate for a biased target deposition technique.
An ion assisted deposition is utilized as additional flow of low energy ions applied on a substrate deposited thin film that provided by a high-energy beam sputtering of a target. Low-energy ions from a secondary ion source help to improve adhesion of applied film, to control its stress, to increase hardness, density and structure, making possible to obtain a preferred crystal orientation of a deposited thin film. Recent trend in ion assisted deposition technique is a utilization of low-energy ion beam under 50 eV, so that an ion assist beam modifies a thin film deposition without its sputtering.
The range of discharge voltages of existing regular end-Hall ion sources at pressures in vacuum chamber of 10−5-104 Torr, Vd=80-300 V, is not satisfactory for obtaining low energies for certain thin film processes (lower than 50 eV, and about 15-20 eV) with ion assisted tasks and also is not sufficient to provide high sputtering or etching rates with required optimum energies (300-500 eV).
The most usable in thin film industry end-Hall ion sources such as Mark-2 (at present time, produced by Veeco Instruments) and EH-1000 (at present time, produced by Kaufman & Robinson Inc.) utilize as magnetic means permanent magnets with a magnet's strength of about 1.3-1.5 kG at a magnet's North Pole side. There are no any available publications about a role of magnetic field value and its influence on a range of existing operating discharge voltages of 80-300 V with mean ion energies of about 50-180 eV. If Mark-2 and EH-1000 operate up to 300 V, then Sainty's ST3000 operates up to 225 V.
Since the introduction of end-Hall ion source in 1989 there was not much done in improvement of operation characteristics such as broadening of discharge voltage range of end-Hall ion sources. The introduction of a water-cooled anode helped to use higher discharge currents up to 10-15 A, but with low discharge voltages, not over 150 V. In a patent U.S. Pat. No. 6,750,600 B2 “Hall-Current Ion Source” by Kaufman et al. there was introduced a grooved anode for better operation with reactive gases. This publication is incorporated herein by reference. Anode grooves help to reduce influence of oxidized substances on anode operation, because anode covered with oxide film loses its electrical conductivity and gradually increases designated discharge voltages to high undesirable values at constant discharge current, or gradually reduces discharge current with constant discharge voltage. All these conditions depend on a Power Supply, whether it operates with a constant current, or a constant voltage mode. Anode grooves have areas that will be not covered with oxide film, because these areas will be not “seeing” oxide particles that travel in straight lines from outside of an ion source.
There is nothing much changed in a working gas distributing system of existing Hall-current ion sources. Working gas is applied through holes in a gas distributor (often called reflector) under a hollow anode bottom part. The reflector is placed between anode and a permanent magnet and also serves as a shield between hot ionized plasma consisting of high energy ions and low energy electrons supplied by a cathode made of Hot Filament, or Hollow Cathode; this reflector-shield protects a permanent magnet from overheating and direct impact from plasma. An ion beam that is developed at a discharge channel is “supposed” to flow to an ion source exit, but quite a good part of an ion beam flows into opposite direction, into a reflector's surface. Reflector usually after about 20-25 hours of operation at moderate discharge currents (about 5 A and over) and regularly used “optimum discharge voltages”, Vd=100-150 V (end-Hall Mark-2 and EH-1000 have a maximum ion beam current value at about Vd=100-125 V, and Oxygen's ion beam current is higher than Argon's ion beam current by about 20% at the same discharge current and voltage) becomes sputtered in a center part and eventually eroded into quite a substantial hole, so it is necessary to substitute such reflector for a new one.
Some users of end-Hall ion sources that understand the problem of a reflector's sputtering are trying to reduce this damage with theirs own means, or to make reflector's substitution easier and convenient. One of recently approved U.S. Pat. No. 6,963,162 B1 “Gas Distributor for an Ion Source” by Centurioni describes substitution of a reflector's eroded part with an insert of about 1.8 cm in diameter that can be placed in a central part of a reflector. After a certain time of operation this insert is substituted for a new one through an ion source top with tweezers, or similar instrument. Such substitution certainly makes sense, especially if one wants to utilize a reflector's central eroded part with an expensive material such as Tantalum, or Molybdenum-Rhenium, etc. However, the manipulation presented in a Centurioni's patent does not reduce erosion of a reflector, and a reflector's erosion problem remains unsolved. A Centurioni's patent publication is incorporated herein by reference.
There are other problems with regular end-Hall ion sources such as a high level of discharge current and voltage oscillations at higher discharge voltages over, Vd≧250 V and a low efficiency of transformation of a discharge current into an ion beam current. This problem was discussed in U.S. Pat. No. 7,116,054 “High-Efficient Ion Source with Improved Magnetic Field” by Zhurin in a hybrid Hall-current ion source that has both features of end-Hall and closed drift ion sources.
There are no quantified values for magnetic field in end-Hall ion sources that could help to select the correct optimum magnetic field value for certain operation conditions and discharge voltage ranges (energies) utilized in technological processes requiring necessary values of ion beam energies and ion beam current densities.
In a light of foregoing, it is an object of the invention to introduce an ion source of a Hall-current type with improved features, in comparison with existing Hall-current ion sources. Such an ion source with selected ranges of ion beam mean energies provides a broad ion beam of high current. This Hall-current ion source utilizes various values of permanent magnets, or electromagnet's magnetic field for certain ranges of discharge voltages, such as low discharge voltages, which can provide low energies of working gases ions in a range of 15-20 eV, and for high discharge voltages, which can provide high energies of working gases ions in a range of 100-500 eV. Low ion beam ion energies of 15-25 eV are at or under sputtering threshold energies of most materials used in practical applications. In the case of one of the “most popular” working gas such as Argon, a referenced literature gives for various materials the following threshold sputtering energies: Aluminum—13 eV, Titanium—20 eV, Iron—20 eV, Copper—17 eV, Molybdenum—24 eV, Tungsten—33 eV, Tantalum—26 eV. These experimental results are taken from a monograph “Ion-Plasma Processing of Materials” by Ivanovsky et al., Publishing House “Radio and Communications”, Moscow, 1986, p. 29. The optimum energies for maximum sputtering by ion beams are 300-500 eV and also presented in Ivanovsky's et. al. monograph. This publication is incorporated herein by reference.
Still another object of the present invention is introduction of a Hall-current ion source with specific characteristics of gas discharge utilizing high electron emissions from a cathode that are higher than a discharge current. High electron emissions maintain and enhance gas discharge at low discharge voltages. These low discharge voltages are under 50 V, which at regular electron emissions are impossible to achieve, because discharge voltages lower than 50 V are not able to maintain gas discharge in an ion source, unless there are utilized quite high pressures in vacuum chamber of about 1-2 and more Millitor. Ion beam energies under 30 eV produced by this ion source are utilized for certain technological processes such as a biased target deposition and an ion assisted deposition. Selected low magnetic field values and high electron emission currents stimulate discharge at low discharge voltages, which are equivalent of low ion beam energies that are impossible to have at magnetic fields regularly used in Hall-current ion sources and electron emission currents that are equal to discharge currents, Iem≈Id. High electron emissions enhance discharge and help to obtain higher ion beam currents in comparison with approximately equal emission and discharge current, Iem≈Id.
Another object of the invention is to present certain quantitative relationship between a Hall-current ion source discharge voltage Vd, a magnetic field value, Bz provided by a permanent magnet or electromagnetic coil placed at a bottom of a conical anode, under a gas distributor-reflector, or in any other area of an ion source magnetic system, and a value of electron emission current applied into discharge, Iem, which is higher than discharge current value, or Iem>Id. Such relationship can help for ion source users in selection the correct magnetic field values, Bz and electron emission current values, Iem for obtaining necessary range of discharge voltages (ion beam energies). Magnetic fields for low ion beam energies and magnetic fields for high ion beam energies can be varied by simple substitution of permanent magnets of necessary magnetic field strength, or by applying various electrical currents into an electromagnetic coil. High electron emission currents require a cathode that can provide an emission current higher than a discharge current and this cathode also provides electrons for neutralization of ions.
A further object of the present invention is introduction of a gas distributing system with placement of working gas holes with direction of these holes tangentially to a cylindrical buffer area located under anode in a way that a flow of working gas coming at pressure substantially higher (several Torr) than in a vacuum chamber (10−5-10−3 Torr) and at this condition providing a vortex flow that helps to eliminate discharge pinching and reduces erosion of a gas distribution system (reflector) substantially. As a part of a such gas distributing system a placement of a circular enclosure (a ring) around a reflector's central part serves a purpose to divide working gas flow into separate areas under anode lower part in order to reduce discharge pinching in a reflector's central part.
Another object of the present invention is introduction of a certain safety device that can detect an ion beam penetration through reflector. A stainless steel cup under a floating potential is placed on a top of a permanent magnet for registering a possible ion beam penetration through reflector. This cup is separated from permanent magnet by a dielectric disk. Electrical potential from a floating cup shows a moment of ion beam penetration into a permanent magnet. This simple device allows stopping an ion source operation immediately at ion beam penetration through reflector.
Another object of the present invention is introduction of working gas through holes in anode, as alternative way in a working gas application into a discharge channel, and as a method for prevention from anode oxidation when an ion source operates at a close distance to a target and in oxygen or similar environment producing dielectric depositions on anode surface.
Yet another object of the present invention is to provide an ion source with a main magnetic system that is easy to open from an ion source outside shell for a substitution of a permanent magnet of a certain magnetic field strength for a required permanent magnet for providing an ion source with specific range of discharge voltages that are necessary for various technological processes.
Another object of the present invention is to introduce a Hall-current ion source with an ion beam utilizing a permanent magnet's, or electromagnetic coil magnetic field that provides a broad beam of high current with necessary mean energies and a with a filtering schematic between a Power Supply and an ion source's anode electrical circuit that helps to suppress high amplitude discharge current and voltage oscillations at high discharge voltages over 300 V, and provide ion source operating discharge voltages over 300 V and up to about 1000 V. Ion beams of high energies of such an ion source can be utilized for etching and sputtering in optimum range of bombarding ion energies of 300-500 eV.
Features of the present invention, which believed to be patentable are set forth with particularity in the appended claims. The organization and operation manner of the invention, together with further objectives and advantages thereof, may be understood by reference to the following descriptions of specific embodiments taken in connection with accompanying drawings, in the several figures of which like reference numerals identify similar elements in which:
End-Hall ion sources presented in
Both ion sources utilize massive hollow conical anodes 13. Gas distributing system 24 plays very important role in a supply of a working gas into a discharge channel. How well working gas is applied into a discharge channel, how uniformly it is distributed, influences on stability and range of operation conditions. For example, in the Kaufman's ion source (
Before discussion of the invented Hall-current ion source with expanded discharge voltage parameters into low energy and into a high energy sides of discharge voltages it is necessary to present fundamental features and behavior of electrical discharge in regions close to discharge extinction. In
In article by Liapin et. al., “Modern State of investigations of Accelerators with Anode layer” in “Ion Injectors and Plasma Accelerators” edited by Morozov et. al., Moscow, Publishing House “Energoatomizdat”, 1990, pp 20-33, there are presented similar V-A characteristics for a Hall-current closed drift thruster/ion source with description of various areas of gas discharge behavior at a regular condition with Iem≈Id, working gas is Argon. This publication is incorporated herein by reference.
As one can see in
It is necessary to note that in order to have conditions of a low voltage discharge, in practice, it is advisable to start discharge at higher discharge voltage, like Vd=100 V with Iem≈Id, or at Vd≈60-70 V with Iem>Id, and then gradually decrease discharge voltage value by a Power Supply to a required lower value. A “nice” first ionization potential gas Xenon “allows” is starting with voltages of 50 V and lower with Iem≈Id, and at about 30 V at Iem>Id.
Discharge behavior with a discharge voltage over Vd≈370-380 V presents itself a self-sustained discharge when it is not necessary to supply electrons from a cathode-neutralizer for neutralization of ions. After discharge is initiated, a high voltage discharge produces sparks creating electrons in discharge channel and outside of an ion source, in vacuum chamber walls. Also, discharge at low discharge voltage and up to about Vd=180 V presents itself a distributed discharge over a discharge channel. A discharge at higher discharge voltage over 180 V presents itself another modification called a concentrated discharge. These modifications received such names because they are observed from outside of a vacuum chamber as distributed and concentrated forms of discharge.
Most Hall-current ion sources that are on a market for thin film technology have cathodes-neutralizers. The invented Hall-current ion source in area with a non-self-sustained discharge was studied with a cathode-neutralizer (both, Hot Filament and Hollow Cathode) that is placed at the exit of an ion source discharge chamber. Electrical discharge in an ion source discharge channel can be characterized by the following operating parameters: discharge current, Id, emission current, Iem provided by a cathode-neutralizer, discharge voltage, Vd, working gas (any gas that can be ionized, noble or reactive) mass flow supplied into anode area, {dot over (m)}a, magnetic field in a discharge channel, axial component, Bz and radial component, Br. If a cathode-neutralizer is a Hollow Cathode, or a Plasma Bridge, or similar device, a cathode mass flow value, {dot over (m)}HC should be also taken into account for a total mass flow applied into a vacuum chamber. In general, an anode mass flow is significantly higher than a Hollow Cathode mass flow, or {dot over (m)}a>>{dot over (m)}HC. Usually, in conditions of a non-self-sustained discharge, both discharge and emission currents are utilized approximately equal to each other, or Iem≈Id. It is assumed that emission current provides a number of electrons per unit time and area that is equal to a number of ions per unit time and area. There are no works in scientific literature justifying the right value of the emission current for the non-self-sustained discharge, especially for the low voltage discharge area. In practice, Hall-current ion sources operate with an emission current that is slightly higher than a discharge current by about 5%.
An approximate equality of discharge and emission currents works satisfactory (meaning that there are no observed electric sparks, or arcing in a vacuum chamber) at discharge voltages Vd≧100 V. Though, the right ratio of emission current to a discharge current should be determined by measuring a charging potential of target placed at working distance from an ion source in a specific technological process. Target placed at certain distance from an ion source exit should not be charged neither positively (insufficient neutralization caused by insufficient supply of emission of electrons, called sometime as under-neutralization), nor negatively (over-neutralization, overflow of electrons). Experiments with ion sources and targets show that for a target with a zero potential it is usually necessary to apply emission current higher than discharge current, and, in many cases, substantially more than 10% over a discharge current.
With low discharge voltages, less than 100 V, the situation is quite different. As one can see in
Since the introduction of a Hall-current ion source in 1989 by Kaufman et al. in U.S. Pat. No. 4,862,032 and later in patents by Kaufman and Sainty, there was no data presented in literature about a value of magnetic field that should be utilized in discharge channel of such ion sources. For larger ion sources, like Mark-2, EH-1000 and ST3000, as was discussed above, magnetic field at the side of a North pole magnet is about 1300-1500 G. In general, permanent magnets with high magnetic field and high Curie (about 500 C.) temperature are utilized. The higher is Curie temperature, the higher electrical power can be applied into a discharge channel. One of the main factors limiting utilization of high discharge currents and voltages, and correspondingly high powers, is possible overheating of a permanent magnet that can lose its magnetic strength being overheated.
Closed drift ion sources and thrusters utilize significantly lower magnetic fields in discharge channel than in end-Hall ion sources. Such data are presented in PA US 2005/0237000 by Zhurin and in article by Zhurin, et al., “Physics of Closed Drift Thrusters” in Plasma Science & Technology, Vol. 8 (1999), beginning on page R1. Experiments confirm that a kind of a working gas, its mass and “ionizing ability” (a first ionization potential value) influence on magnetic field that is applied into a discharge channel to provide conduction of electrons in anode area in a way that electrons would not go straight from cathode to anode area but with extended passage (closed loops) so it will be enough time for electrons to collide with neutrals providing theirs ionization. It was found that for Hall-current ion sources, at each value of a discharge voltage, Vd and working gas mass flow through anode, {dot over (m)}a there is an optimum value of magnetic field, Bopt, at which ratio of an ion beam current to a discharge current, Ii/Id reaches its maximum value, Ii,max. The optimum value of magnetic field in closed drift ion sources and thrusters increases with the increase of a discharge voltage, Vd and a working gas mass flow, ma. For closed drift thrusters/ion sources a following empirical dependence was obtained, Hopt˜Vd1/4{dot over (m)}a1/2. This relationship is given in a monograph “Stationary Plasma Thrusters” by Belan et al., published by Kharkov Aviation Institute, 1989, p 187. This publication is incorporated herein for reference. The dependence for Bopt is also influenced by specific working gas properties such as atomic mass and its first ionization potential. For working gases with high atomic mass Bopt, like Xenon an optimum magnetic field value for Vd=300 V is equal to about 600 G, and for Argon, at Vd=300 V an optimum magnetic field value is only 150-200 G. It is necessary to note that magnetic field values given for a closed drift ion thrusters-sources are at maximum at a discharge channel exit, because closed drift thrusters-sources as a rule use a positive gradient of magnetic field in a discharge channel and in an anode area magnetic field usually is about zero. In the case of Hall-current ion sources, such as end-Hall-type ion sources with a negative gradient, magnetic field decreases from a reflector, which is placed under an anode, from about 1000 G at reflector, and it becomes equal to about 50-70 G at a discharge channel exit.
From
From
As one can see from
A buffer chamber 41 plays important role in a Hall-current ion source operation. First of all, it helps to develop an optimum uniform gas distribution for ionization and acceleration, and, second, it gives a possibility, when necessary, for a preliminary ionization of applied working substances that in general are poorly ionized. For obtaining high ion beam currents a preliminary ionization helps increasing efficiency of utilization of a working material and provides regulation of an ion source in a broad range of major parameters such as discharge voltage and current, working gas flow.
For best performance with the invented Hall-current ion source, the buffer chamber dimensions are selected from the following relationships: Rbuf/Rch=1.3-1.5 and Lbuf/Rch=0.3-0.5, where Rbuf is an external radius of a buffer chamber, Rch is an external diameter of an ion source discharge channel in 19 (end of external magnetic pole), Lbuf is a buffer chamber length, Lch is a discharge channel length. These dimensions are shown in
In the invented design a stainless steel cup 42 is placed on a permanent magnet top 16 under a floating potential.
In regular end-Hall ion sources a part of magnetic system 14 is made in a form of magnetically permeable material. It usually made of one piece as a hollow thin cylinder of about 1-1.2 mm thick and about 10-15 cm long. Since the invented ion source assumes utilization of various magnetic fields produced by permanent magnet 16, an external shell 14 of magnetically permeable material of
In a patent U.S. Pat. No. 6,750,600 B2 “Hall-Current Ion Source” by Kaufman et al. there is shown a modified end-Hall ion source that was described in a patent U.S. Pat. No. 4,862,032 “End-Hall Ion Source” by Kaufman et al. In its latest version of U.S. Pat. No. 6,750,600 there is presented a hollow anode of a grooved shape for operation with reactive gases such as Oxygen, Nitrogen and others and with a possibility to reduce influence of anode oxidation (or other dielectric depositions) on ion source performance. During operation with reactive gases anode becomes covered with, for example, oxide film, which deposition makes electron current difficult to penetrate into anode surface through a non-conducting film. In result, discharge voltage begins to rise continuously, which is difficult to control, leading to discharge extinguishing (with a Power Supply working in a constant current mode), or discharge current begins to decrease continuously (with a Power Supply working in a constant voltage mode) and discharge becomes extinguished after certain time. A grooved form of anode makes some parts of anode “invisible” for oxide particles (they travel at low pressures by straight lines) and helps to reduce influence of oxide film on ion source discharge parameters.
As it was above mentioned, in Hall-current, or end-Hall ion source a gas distributor called reflector 15, usually made of a stainless steel (in some cases, it made of Tantalum, Titanium, etc) for operation with reactive gases, or a graphite (for noble gases, such as Argon, Xenon, etc), which placed under a conical anode 13, suffers from damage produced by ion beam that takes place during regular ion source operation. An end-Hall ion source utilizesa magnetic field configuration with a substantial part of magnetic field having an axial direction instead of a radial direction that takes place in thrusters-ion sources with closed electron drift. A detailed investigation of differences between end-Hall type and closed drift type was presented in a U.S. Pat. No. 7,116,054 “High-Efficient Ion Source with Improved Magnetic Field” by Zhurin.
This reference was presented above. In end-Hall type ion sources, because of existence of a predominant axial magnetic field component, an ion beam is directed not only to exit from a discharge channel but also partially to opposite direction to a reflector's center part. In result, reflector becomes damaged and, in certain cases with utilized high discharge currents and voltages, it becomes completely eroded by an ion beam. Reflector 15, in such a case, gets a large hole of up to 5 mm in diameter. The damage is quite substantial because not only reflector is necessary to substitute, but permanent magnet 16 can be also damaged. In some cases, depending on an end-Hall ion source model, a small stainless steel cup 42 is placed on a top of permanent magnet, or, in order to prevent permanent magnet 16 damage the whole stainless steel plate containing an upper part of an ion source assembly is placed over a permanent magnet. The damage from a beam penetration through reflector can be very significant and plate placed over magnet and magnet must be replaced.
In
Present invention helps to reduce influence of film depositions from reactive gases on anode surface by utilizing another alternative version of a gas distribution system with working gas applied through holes in anode body as it is shown in
Due to the fact that in Hall-current ion sources magnetic field serves for keeping electrons from direct straight flight from cathode to anode area, so that electrons would have sufficient path and time for collisions with neutrals, the oscillations of current and voltage are playing very important role in such systems. In above mentioned article “Physics of Closed Drift Thrusters” by Zhurin et al in Plasma Sources Science & Technology, Vol. 8 (1999), beginning on page R1, and in a U.S. Pat. No. 7,116,054 “High-Efficient Ion Source with Improved Magnetic Field” by Zhurin there is presented quite a detailed information about various types of oscillations that take place during operation of Hall-current thrusters-ion sources.
It is necessary to note that some researchers consider oscillations as harmful feature of discharge that influence on behavior of ion beam, which is supposed to be just uniform current flow of ions of necessary energy (accompanied equal number of electrons) in certain area. Of course, it will be desirable to have such ion beams for interaction with targets, with thin film depositions as ion assist, for etching, etc. That is why all makers of ion sources have their ion sources with Power Supplies programmed in some specific modes, for example, providing Power Supplies with rigidly fixed constant output discharge voltage, or current value. Detailed studies show (see for example an above mentioned article “Physics of Closed Drift Thrusters” by Zhurin et al. in Plasma Sources Science & Technology, Vol. 8 (1999), beginning on page R1) that discharge in Hall-current ion sources with the increase of discharge voltages experiences high amplitude oscillations of both, discharge current and voltage parameters. Moreover, gas discharge at certain conditions can exist only with oscillations that actually help travel for electrons from cathode to anode area across magnetic field. In result, makers of Hall-current ion sources with Power Supplies do not provide the range of operating discharge voltages over 300 V. As it was discussed above, discharge voltage of 300 V is equivalent of only about 180 eV of mean ion beam energy in a Hall-current ion source. And for the optimum etching-deposition of most materials it is necessary to have ion beam energies in the range of 300-500 eV, or Vd=500-830 V.
Researchers working with closed drift thrusters that use regularly discharge voltages from 150 V (earlier models of SPT-50 and SPT-60) and up, in general use discharge voltages of 300-500 V (so-called thrusters with magnetic layer, such as SPT-100, which discussed in detail in above mentioned article “Physics of Closed Drift Thrusters” by Zhurin et. al.) or even higher 500-1000 V (anode layer and some magnetic layer thrusters of high power, also discussed in an article by Zhurin et al.) noticed that in order to have reliable operation of a thruster-ion source at high voltages it is necessary to provide a system: Power Supply—ion source with a connecting “filter” consisting of a capacitance-shunt in a discharge circuit that reduces amplitude of discharge voltage oscillations and stabilizes level of oscillations. Experiments show that this capacitance is necessary part for discharge voltage stabilization at significant values of discharge current oscillations. This value of a shunting capacitance can be estimated from the condition: τfilter=RC≧Td, where Td is a characteristic value of the oscillation period, R and C are resistance and capacitance, τ is an RC circuit time. Assuming that an ion source during developed oscillations preserves Ohmic character of electrical load, it is possible to estimate value of an ion source resistance during oscillations, which will be equal approximately: Rosc˜Ûd/Îd, where Ûd and Îd are discharge voltage and current amplitudes during developed oscillations.
For Hall-current ion sources with discharge currents of 5-10 A and with Vd≈300-500 V (with a shunting capacitance), one can have that Rd˜60-30 Ohm and 100-50 Ohm. For a typical period of current/voltage oscillations, which is about Td≈3×10−5 sec, (this value is typically (1-5)×10−5 sec and depends on ion source dimensions and a kind of working gas utilized) one can obtain necessary value for a shunting capacitance: C=Td/Rd≈(1-3) μF.
Together with a shunting capacitance it is advisable between capacitance and a Power Supply output voltage also to have an inductance that can limit current oscillations in a discharge circuit. In the case, if voltage oscillations are stabilized by a shunting capacitance with C≈1-3 μF with the voltage oscillation amplitude ΔVd≦kd·Vd, where the oscillation coefficient is selected as kd≈10−2, then an induction value can be estimated from the relationship: Lo(dI/dt)max≧kd·Vd, or LoΔId≧kdVd·Td, where ΔId is a permissible amplitude of current oscillations in a Power Supply. With ΔId=1 A and Td=3×10−5 sec, one can find an inductance for suppressing current oscillations: L≧kdVdTd/ΔId≈9×10−5 Hn. In practice selected values for the “filter” should be close to the calculated above and found experimentally for each particular setup of Power Supply and ion source. Such type of a circuit between a Power Supply and an ion source helps to have discharge voltages in the range of 300-700 V in a quite stable regime with discharge current and voltage oscillations that do not extinguish discharge.
While particular embodiments of the present invention have been shown and described, it will be evident to those skilled in the art that changes and modifications may be made without departing from the invention in its broadest aspects. Therefore, the aim in the appended claims is to cover all changes and modifications that are in the spirit and scope of what is patentable.
| Number | Name | Date | Kind |
|---|---|---|---|
| 4862032 | Kaufman et al. | Aug 1989 | A |
| 5274306 | Kaufman et al. | Dec 1993 | A |
| 5315121 | Kluge et al. | May 1994 | A |
| 6456011 | Bugrova et al. | Sep 2002 | B1 |
| 6645301 | Sainty | Nov 2003 | B2 |
| 6750600 | Kaufman et al. | Jun 2004 | B2 |
| 6870164 | Baldwin et al. | Mar 2005 | B1 |
| 6963162 | Centurioni | Nov 2005 | B1 |
| 7023128 | Madocks | Apr 2006 | B2 |
| 20030193295 | Kaufman et al. | Oct 2003 | A1 |
| 20040195521 | Alekseev et al. | Oct 2004 | A1 |
| 20040217713 | Madocks | Nov 2004 | A1 |
| 20050237000 | Zhurin | Oct 2005 | A1 |
| 20060152162 | Madocks | Jul 2006 | A1 |
| Number | Date | Country | |
|---|---|---|---|
| 20070241290 A1 | Oct 2007 | US |
