`
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`Unlted States Patent
`Chistyaknv
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`l|||||||||||||||||||||||||||||||||l||||||||||||||||||||||||||||||||||||||||
`
`U3006853l 4232
`
`{10) Patent N0.:
`(45) Date of Patent:
`
`US 6,853,142 B2
`Feb. 3, 2005
`
`METHODS AND APPARATUS FOR
`GENERATING HIGH-DENSITY PLASMA
`
`Inventor: Roman Chistyakov, Andover,MA
`(US)
`L
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`Awgncc’ Lend, lnc., Mansmld’ MINUS)
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`Notice:
`SUbJCCl to any disclaimer, the term of this
`patent is extended or adjusted under 35
`UM" 154th) by 0 days”
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`FOREIGN PATENT DOCUMENTS
`
`Appl. No: 10{065,629
`
`Filed:
`
`Nov. 4, 2002
`
`El’
`W0
`W0
`
`0 650 133 Al
`W0 98.40532
`W0 (ilt98553 At
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`411995
`919998
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`Prior Publication Data
`
`OTHER PUBLICATIONS
`
`(54)
`
`(as)
`
`(73)
`
`(*)
`
`{21)
`
`(22}
`
`(65)
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`(51)
`(52)
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`US ZULH’UUBSDZC’: A1 May 6. 2004
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`345; 204it298$[|6, 293.04, 298.08; 118723 513,
`L31! 7‘3 MP; 423;210! 246' ~48
`References Cited
`U.S. l’A’l‘EN'l‘ DOCUMENTS
`
`2041298
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`
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`
`{”1991 Role:
`315,111‘41
`5,0419“; A =5
`1i1992 Koshmhi er al.
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`US 5,863,392, 111999, Drummond et al. (withdrawn)
`Booth, et al., The Transition From Symmetric To Asymmet-
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`Plasmas, J_ Appl- Phys, Jul. 15g 199’}:‘ pp. 552—560, vol.
`82(2), American Institute of Physics.
`_
`_
`(LiaI confirmed on next page)
`
`pmwr}. 5,,“th Wilson L6,,
`(74) Attorney, Agent, or FirmbKurt Rauschenbach;
`Rausehenbaeh Patent Law Group, LLC
`5'."
`ABSTRACT
`(
`J
`
`Methods and apparatus for generating a strongly—ionized
`plasma
`aaacribad- An apparatus for generating a
`strongly-ionized plasma according to the present invention
`Includes an anode and a cathode that Is posrtloned adjacent
`to the anode to form a gap there between. An ionization
`source generates a weakly-ionized plasma proximate to the
`cathodeApower supply produces an electric field in-lhe gap
`between the anode and the cathode. The electric field
`generates excited atoms in the weakly~ionized plasma and
`generates secondary electrons from the cathode. The sec-
`ondary electrons ionime the excited atoms, thereby creating
`“1° Stronglyflommd Plasma
`
`43 Claims, 13 Drawing Sheets
`
`200
`
`GILLETTE 1001
`
`W 2
`
`\
`
`2132
`
`PU LS ED
`
`
`
`3331??
`
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`
`
`
`‘16
`
`
`GILLETTE 1001
`
`
`
`US 6,853,142 B2
`Page 2
`
`US. PATEN'I‘ DOCUMENTS
`
`10;“2002 Kumar et al.
`6,471,833 132
`6,488,825 B1 "‘ 121'2002 Hilliard ...........
`
`2mzrm19139 Al
`23'2002 Zhang et a].
`2002-0114897 A1
`@2002 Sumiya et al.
`2m310000008 AI
`19003 l'lorioka clal.
`
`20411913?
`204998.06
`438nm
`427569
`155845.46
`
`0TH ER PU BLI CATI 0N5
`
`Bunshah, et al., Deposition Technologies For Films And
`Coatings, Materials Science Series, pp. 176—183, Noyes
`Publications, Park Ridge, New Jersey.
`Daugherty, ct al., Attachment—Dominated Electron—Bow
`m—Ionized Discharges, Applied Science Letters, May 15,
`1976, vol. 28, No. 10, American Institute of Physics.
`Goto, et al., Dual Excitation Reactive Ion Etcher for Low
`Energy Plasma Processing, J. Vac. Sci. Technol. A, SepJOct.
`1992, pp. 304843054, vol. 10, No. 5, American Vacuum
`Society.
`Kouznctsov, et al., A Novel Pulsed Magnetron Sputtcr
`Technique Utilizing Very High Target Power Densities,
`Surface 8:. Coatings Technology, pp. 290—293, Elsevier
`Sciences SA.
`
`Lindquist,et al., High Selectivity Plasma Etching Of Silicon
`Dioxide With A Dual Frequency 279 MHz Capacitive RF
`Discharge.
`Macak, Reactive Sputter Deposition Process of A1203 and
`Characterization Of A Novel I-Iigh Plasma Density Pulsed
`Magnetron Discharge, Linkoping Studies In Science And
`Technology, 1999, pp. 1—2, Sweden.
`
`Ionized Sputter Deposition Using An
`al.,
`et
`Macak,
`Extremely High Plasma Density Pulsed Magnetron Dis-
`charge,
`J. Vac. Sci. Technol. A., JulJAug. 2000, pp.
`1533—1537, vol. 18, N0. 4, American Vacuum Society.
`Mozgrin, et al., High—Current Low—Pressure Quasi—Station—
`ary Discharge lnAMagnetic Field: Experimental Research,
`Plasma Physics Reports, 1995, pp. 400—409, vol. 21, No. 5,
`Mozgrin, Fcitsov, Khodachenko.
`Rossnagel, et al., Induced Drift Oirrcnts In Circular Planar
`Magnetrons, J. Vac. Sci. ’l‘echnol. A, Janflieb. 1987, pp.
`88——91, vol. 5, No. 1, American Vacuum Society.
`Sheridan, et al., Electron Velocity Distribution Functions In
`A Sputtering Magnetron Discharge For The EXB Direction,
`J. Vac. Sci. Technol. A., JulJAug. l998, pp. 2173—2176, vol.
`16, No. 4, American Vacuum Society.
`
`Steinbruchel, A Simple Formula For Low—Energy Sputter-
`ing Yields, Applied Physics A, 1985, pp. 37—42, vol. 36,
`Springer‘Verlag.
`
`Chistyakov, Roman, High—Power Pulsed Magnetron Sput-
`tering, Application No.: 10;“065377, Filed: Sep. 30, 2.002.
`
`Chistyakov, Roman, High—Power Pulsed Magnetically
`Enhanced Plasma Processing, Application No.: 10!065,551,
`Filed: Oct. 30, 2002.
`
`Encyclopedia Of Low Temperature Plasma, p. 119, 123, vol.
`3.
`
`* cited by examiner
`
`
`
`US. Patent
`
`Feh. 8,2005
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`\
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`START
`
`5‘32
`
`460
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`PUMP DOWN CHAMBER
`
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`CHAMBER
`PRESSURE
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`END
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`
`526
`PLASMA
`PROCESS
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`
`FIG. 8
`
`
`
`US 6,853,142 B2
`
`1
`METHODS AND APPARATUS FOR
`GENERATING HIGH-DENSITY PLASMA
`
`BACKGROUND OF INVENTION
`
`Plasma is considered the fourth state of matter. A plasma
`is a collection of charged particles moving in random
`directions. A plasma is, on average, electrically neutral. One
`method of generating a plasma is to drive a current through
`a low-pressure gas between two parallel conducting elec-
`trodes. Once certain parameters are met.
`the gas "breaks
`down” to form the plasma. For example, a plasma can be
`generated by applying a potential of several kilovolts
`between two parallel conducting electrodes in an inert gas
`atmosphere (e.g., argon) at a pressure that is between about
`10‘] and 10‘: Torr.
`
`Plasma processes are widely used in many industries,
`such as the semiconductor manufacturing industry. For
`example, plasma etching is commonly used to etch substrate
`material and films deposited on substrates in the electronics
`industry. There are four basic types of plasma etching
`processes that are used to remove material from surfaces:
`sputter etching, pure chemical etching, ion energy driven
`etching, and ion inhibitor etching.
`Plasma sputtering is a technique that is widely used for
`depositing films on substrates. Sputtering is the physical
`ejection of atoms from a target surface and is sometimes
`referred to as physical vapor deposition (PVD). Ions, such as
`argon ions, are generated and then are drawn out of the
`plasma, and are accelerated across a cathode dark space. The
`target has a loWer potential than the region in which the
`plasma is formed. Therefore, the target attracts positive ions.
`Positive ions move towards the target with a high velocity.
`Positive ions impact the target and cause atoms to physically
`dislodge or sputter from the target. The sputtered atoms then
`propagate to a substrate where they deposit
`a
`film of
`sputtered target material. The plasma is replenished by
`electron-ion pairs formed by the collision of neutral mol-
`ecules with secondary electrons generated at
`the target
`surface.
`
`Magnetron sputtering systems use magnetic fields that are
`shaped to trap and to concentrate secondary electrons, which
`are produced by ion bombardment of the target surface. The
`plasma discharge generated by a magnetron sputtering sys-
`tem is located proximate to the surface of the target and has
`a high density of electrons. The high density of electrons
`causes ionization of the sputtering gas in a region that is
`close to the target surface.
`BRIE!" DESCRIPTION OF [DRAWINGS
`
`'l'his invention is described with particularity in the
`detailed description. The above and further advantages of
`this invention may be better understood by referring to the
`following description in conjunction with the accompanying
`drawings,
`in which like numerals indicate like structural
`elements and features in various figures. The drawings are
`not necessarily to scale, emphasis instead being placed upon
`illustrating the principles of the invention.
`l-‘IG.
`1
`illustrates a cross-sectional view of a known
`
`plasma generating apparatus having a radio-frequency (RF)
`power supply.
`FIG, 2A through FIG. 2D illustrate cross-sectional views
`of a plasma generating apparatus having a pulsed power
`supply according to one embodiment of the invention.
`FIG. 3 illustrates a graphical representation of the pulse
`power as a function of time for periodic pulses applied to the
`plasma in the plasma generating apparatus of FIG. 2A.
`
`‘JI
`
`It)
`
`15
`
`3E]
`
`40
`
`45
`
`SD
`
`60
`
`2
`FIG. 4 illustrates graphical representations of the applied
`voltage, current, and power as a function oftime for periodic
`pulses applied to the plasma in the plasma generating
`apparatus of FIG. 2A.
`illustrate various simulated
`FIG. 5A through FIG. 5])
`magnetic field distributions proximate to the cathode for
`various electron 13x13 drift currents according to the present
`invention.
`
`FIG, 6A through FIG. 6D illustrate cross-sectional views
`of various embodiments of plasma generating systems
`according to the present invention.
`FIG. 7 illustrates a graphical representation of the pulse
`power as a function of time for periodic pulses applied to the
`plasma in the plasma generating system of FIG. 6A.
`FIG. 8 is a flowchart of an illustrative method of gener-
`ating a high-dc nsity plasma according to the present inven-
`tion.
`
`DETAILED DESCRIPTION
`
`illustrates a cross-sectional view of a known
`1
`FIG.
`plasma generating apparatus 100 having a radio—frequency
`(RF) power supply 102. The known plasma generating
`apparatus 100 includes a vacuum chamber 104 where the
`plasma 105 is generated. The vacuum chamber 104 is
`positioned in fluid communication with a vacuum pump 106
`via a conduit 108. The vacuum pump 106 is adapted to
`evacuate the vacuum chamber 104 to high vacuum. The
`pressure inside the vacuum chamber 104 is generally less
`than 10“3 Torr. A feed gas from a feed gas source 109, such
`as an argon gas source,
`is introduced into the vacuum
`chamber 104 through a gas inlet 110. The gas flow is
`controlled by a valve 112.
`The plasma generating apparatus 100 also includes a
`cathode 114. The cathode 114 is generally in the shape of a
`circular disk. The cathode 114 is electrically connected to a
`first
`terminal 118 of a blocking capacitor 120 with an
`electrical transmission line 122. Asecond terminal 124 of
`the blocking capacitor 120 iscoupled to a first output 126 of
`the RF power supply 102. An insulator 123 can be used to
`pass the electrical transmission line 122 through a wall of the
`vacuum chamber 104 in order to electrically isolate the
`cathode 114 from the vacuum chamber 104.
`
`An anode 130 is positioned in the vacuum chamber 104
`proximate to the cathode 114. The anode 130 is typically
`coupled to ground using an electrical transmission line 132.
`A second output 134 of the RF power supply 102 is also
`typically coupled to ground. In order to isolate the anode 130
`from the vacuum chamber 104, an insulator 136 can be used
`to pass the electrical transmission line 132 through a wall 01‘
`the vacuum chamber 104. The vacuum chamber 104 can
`
`also be coupled to grou nd.
`In operation,
`the RF power supply 102 applies a RF
`voltage pulse between the cathode 114 and the anode 130
`that has a suli'tcient amplitude to ionize the argon feed gas
`in the vacuum chamber 104. Atypical RF driving voltage is
`between 100V and IUUOV, and the distance 138 between the
`cathode 114 and the anode is between about 2 cm and 10 cm.
`Typical pressures are in the range of 10 mTorr to 100 mTorr.
`Typical power densities are in the range of 0.1 Wi’cm2 to 1
`Wr’cmz. The driving frequency is typically 13.56 MHz.
`Typical plasma densities are in the range of 10‘3 ern'3 to 1011
`cm‘3, and the electron temperature is on the order of 3 eV.
`This typical ionization process is referred to as direct
`ionization or atomic ionization by electron impact and can
`be described as follows:
`
`
`
`US 6,853,142 B2
`
`Ar+e" —-i‘\r"+2e'
`
`3
`
`where Ar represents a neutral argon atom in the feed gas and
`e' represents an ionizing electron generated in response to
`the voltage applied between the cathode 114 and the anode
`130. The collision between the neutral argon atom and the
`ionizing electron results in an argon ion (AF) and two
`electrons.
`
`The plasma discharge is maintained, at least in part, by
`secondary electron emission from the cathode. However,
`typical operating pressures must be relatively high so that
`the secondary electrons are not lost to the anode 130 or the
`walls of the chamber 104. These pressures are not optimal
`for most plasma processes.
`It is desirable to operate a plasma discharge at higher
`current densities, lower voltages, and lower pressures than
`can be obtained in a conventional glow discharge. This has
`led to the use of a DC magnetic field near the cathode 114
`to confine the secondary electrons. Conlining the secondary
`electrons substantially confines the plasma at a location (not
`shown) in the chamber 104 thereby increasing the plasma
`density at
`that
`location for a given input power, while
`reducing the overall loss area.
`The magnetic confinement primarily occurs in a confine-
`ment region (not shown) where there is a relatively low
`magnetic field intensity. The shape and location of the
`confinement region depends on the design of the magnets.
`Generally, a higher concentration of positively charged ions
`in the plasma is present
`in the confinement region than
`elsewhere in the chamber 104. Consequently, the uniformity
`of the plasma can be severely diminished in magnetically
`enhanced systems.
`The non-uniformity of the plasma in magnetron sputtering
`systems can result
`in undesirable non-uniform erosion of
`target material and thus relatively poor target utilization. The
`non—uniformity of the plasma in magnetron etching systems
`can result in non—uniform etching of a substrate.
`Dramatically increasing the RF power applied to the
`plasma alone will not result
`in the formation of a more
`uniform and denser plasma. Furthermore, the amount of
`applied power necessary to achieve even an incremental
`increase in uniformity and density can increase the prob-
`ability of generating an electrical breakdown condition
`leading to an undesirable electrical discharge (an electrical
`arc) in the chamber 104.
`PuLsing direct current (DC) power applied to the plasma
`can be advantageous since the average discharge power can
`remain relatively low while relatively large power pulses are
`periodically applied. Additionally, the duration of these large
`voltage pulses can be preset so as to reduce the probability
`of establishing an electrical breakdown condition leading to
`an undesirable electrical discharge. An undesirable electrical
`discharge will cormpt the plasma process and can cause
`contamination in the vacuum chamber 104. However, very
`large power pulses can still result in undesirable electrical
`discharges regardless of their duration.
`FIG. 2Athrough FIG. 2D illustrate cross-sectional views
`of a plasma generating apparatus 200 having a pulsed power
`supply 202 according to one embodiment of the invention.
`For example, FIG. 2Aillustrates a cross-sectional view of a
`plasma generating apparatus 200 having a pulsed power
`supply 202 at a time before the pulsed power supply 202 is
`activated. In one embodiment, the plasma generating appa-
`ratus 200 includes a chamber (not shown), such as a vacuum
`chamber that supports the plasma. The chamber can be
`coupler] to a vacuum system (not shown).
`The plasma generating apparatus 200 also includes a
`cathode 204. In one embodiment, the cathode 204 can be
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`composed of a metal material such as stainless steel or any
`other material that does not chemically react with reactive
`gases. In another embodiment, the cathode 204 includes a
`target that can be used for sputtering. The cathode 204 is
`coupled to an output 206 of a matching unit 208. An input
`210 of the matching unit 208 is coupled to a first output 212
`of the pulsed power supply 202. A second output 214 of the
`pulsed power supply 202 is coupled to an anode 216. An
`insulator 218 isolates the anode 216 from the cathode 204.
`
`In one embodiment, the first output 212 of the pulsed
`power supply 202 is directly coupled to the cathode 204 (not
`shown). In one embodiment (not shown), the second output
`214 of the pulsed power supply 202 is coupled to ground. In
`this embodiment, the anode 216 is also coupled to ground.
`In one embodiment (not shown), the first output 212 of the
`pulsed power supply 202 couples a negative voltage impulse
`to the cathode 204. In another embodiment (not shown), the
`second output 214 of the pulsed power supply 202 couples
`a positive voltage impulse to the anode 216.
`In one embodiment, the pulsed power supply 202 gener-
`ates peak voltage levels of up to about 30,000V. Typical
`operating voltages are generally between about 50V and 30
`kV.
`In one embodiment,
`the pulsed power supply 202
`generates peak current levels of less than 1 A to about 5,000
`Aor more depending on the volume of the plasma. Typical
`operating currents varying from less than one hundred
`amperes to more than a few thousand amperes depending on
`the volume of the plasma. In one embodiment, the pulses
`generated by the pulsed power supply 202 have a repetition
`rate that is below 1 kHz. In one embodiment, the pulse width
`of the pulses generated by the pulsed power supply 202 is
`substantially between about one microsecond and several
`seconds.
`
`The anode 216 is positioned so as to form a gap 220
`between the anode 216 and the cathode 204 that is sufficient
`
`to allow current to flow through a region 222 between the
`anode 216 and the cathode 204. In one embodiment, the
`width of the gap 220 is between approximately 0.3 cm and
`10 cm. The surface area of the cathode 204 determines the
`
`volume of the region 222. The gap 220 and the total volume
`of the region 222 are parameters in the ionization process as
`described herein.
`
`In one embodiment, the plasma generating apparatus 200
`includes a chamber (not shown), such as a vacuum chamber.
`The chamber is coupled in fluid communication to a vacuum
`pump (not shown) through a vacuum valve (not shown). In
`one embodiment, the chamber (not shown) is electrically
`coupled to ground potential. One or more gas lines 224
`provide feed gas 226 from a feed gas source (not shown) to
`the chamber. In one embodiment,
`the gas lines 224 are
`isolated from the chamber and other components by insu-
`lators 228. In other embodiments, the gas lines 224 can be
`isolated from the feed gas source using in-line insulating
`couplers (not shown). The gas source can be any feed gas
`226 including but not
`limited to argon.
`In some
`embodiments, the feed gas 226 can include a mixture of
`difierent gases, reactive gases, or pure reactive gas gases. In
`some embodiments, the feed gas 226 includes a noble gas or
`a mixture of gases.
`In operation, the feed gas 226 from the gas source is
`supplied to the chamber by a gas flow control system (not
`shown). Preferably, the feed gas 226 is supplied between the
`cathode 204 and the anode 216. Directly injecting the feed
`gas 226 between the cathode 204 and the anode 216 can
`increase the flow rate ofthe feed gas 226. This causes a rapid
`volume exchange in the region 222 between the cathode 204
`and the anode 216, which permits a high-power pulse having
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`a longer duration to be applied across the gap 220. The
`longer duration high-power pulse results in the formation of
`a higher density plasma. This volu me exchange is described
`herein in more detail.
`
`In one embodiment, the pulsed power supply 202 is a
`component in an ionization source that generates a weakly-
`ionized plasma 232. Referring to FIG. 2B, after the feed gas
`is supplied between the cathode 204 and the anode 216, the
`pulsed power supply 202 applies a voltage pulse between the
`cathode 204 and the anode 216. in one embodiment,
`the
`pulsed power supply 202 applies a negative voltage pulse to
`the cathode 204. The sire and shape of the voltage pulse are
`chosen such that an electric field 230 develops berveen the
`cathode 204 and the anode 216. The amplitude and shape of
`the electric field 230 are chosen such that a weaklynionized
`plasma 232 is generated in the region 222 between the anode
`216 and the cathode 204.
`
`The weakly-ionized plasma 232 is also referred to as a
`pre-ionized plasma. In one embodiment, the peak plasma
`density of the pre-ionized plasma is between about 10" and
`10'2 cm‘3 for argon feed gas. The pressure in the chamber
`can vary from about 10“3 to 10 Torr or higher. The pressure
`can vary depending on various system parameters, such as
`the presence of a magnetic field proximate to the cathode
`204. The peak plasma density of the weakly-ionized plasma
`232 depends on the properties of the specific plasma gen-
`erating system and is a
`function of the location of the
`measurement in the weakly—ionized plasma 232.
`In one embodiment.
`to generate the weakly—ionized
`plasma 232, the pulsed power supply 202 generates a low
`power pulse having an initial voltage of between about 100V
`and 5 kV with a discharge current of between about 0.1 A
`and 100 A. In some embodiments, the width of the pulse can
`be in on the order of 0.] microseconds up to one hundred
`seconds. Specific parameters of the pulse are discussed
`herein in more detail.
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`the DC power supply is adapted to generate and
`232,
`maintain an initial peak voltage between the cathode 204
`and the anode 216 before the introduction of the feed gas
`226.
`In another embodiment, an alternating current (AC)
`power supply (not shown) is used to generate and maintain
`the weakly—ionized or pre—ionized plasma 232. For example,
`the weakly-ionized or pre-iortized plasma 232 can be gen-
`erated and maintained using electron cyclotron resonance
`(ECR), capacitively coupled plasma discharge (CCP), or
`inductively coupled plasma (ICP) discharge.
`AC power supplies can require less power to generate and
`maintain a weakly-ionized plasma than a DC power supply.
`In addition, the pre-ionixed or weakly-ionized plasma 232
`can be generated by numerous other techniques, such as UV
`radiation techniques, X—ray techniques, electron beam
`techniques, ion beam techniques, or ionizing filament tech-
`niques. These techniques include components used in ion-
`ization sources according to the invention.
`In some
`embodiments, the weakly-ionized plasma is formed outside
`of the region 222 and then diffuses into the region 222.
`Forming the weakly—ionized or pre—ionized plasma 232
`substantially eliminates the probability of establishing a
`breakdown condition in the chamber when high-power
`pulses are applied between the cathode 204 and the anode
`216. The probability of establishing a breakdown condition
`is substantially eliminated because the weakly-ionized
`plasma 232 has a low-level of ionization that provides
`electrical conductivity through the plasma. This conductiv—
`ity substantially prevents the setup of a breakdown
`condition, even when high power is applied to the plasma.
`In one embodiment, as the feed gas 226 is pushed through
`the region 222,
`the weakly-ionized plasma 232 dilfuses
`somewhat homogeneously through the region 234. This
`homogeneous ditl‘usion tends to facilitate the creation of a
`highly uniform strongly—ionized plasma in the region 234.
`In one Embodiment
`(not show),
`the weakly—ionized
`plasma can be trapped proximate to the cathode 204 by a
`magnetic field. Specifically, electrons in the weakly-ionized
`plasma 232 can be trapped by a magnetic field generated
`proximate to the cathode 204.
`In one embodiment,
`the
`strength of the magnetic field is between about fifty and two
`thousand gauss.
`In one embodiment, a magnet assembly (not shown)
`generates the magnet field located proximate to the cathode
`204. The magnet assembly can include permanent magnets
`(not shown), or alternatively, electro-magnets (not shown).
`The configuration of the magnet assembly can be varied
`depending on the desired shape and strength of the magnetic
`field. In alternate embodiments, the magnet assembly can
`have either a balanced or unbalanced configuration. In one
`embodiment,
`the magnet assembly includes switching
`electro-magnets, which generate a pulsed magnetic field
`proximate to the cathode 204. In some embodiments, addi-
`tional magnet assemblies (not shown) can be placed at
`various locations around and throughout the chamber (not
`shown).
`Referring to FIG. 2C. once the weakly—ionized plasma
`232 is formed,
`the pulsed power supply 202 generates
`high-poWer pulses betWeen the cathode 204 and the anode
`216 (FIG. 2C). The desired power level of the high-power
`pulses depends on several factors including the density of
`the weakly-ionized plasma 232, and the volume of the
`plasma, for example. In one embodiment, the power level of
`the high—power pulse is in the range of about 1 kW to about
`10 MW or higher.
`Each of the high-power pulses is maintained for a prede-
`ten'nined time that, in some embodiments, is approximately
`
`In one embodiment, prior to the generation of the weakly~
`ionized plasma 232, the pulsed power supply 202 generates
`a potential diiferenee between the cathode 204 and the anode
`216 before the feed gas 226 is supplied between the cathode
`204 and the anode 216. In another embodiment, the pulsed
`power supply 202 generates a current through the gap 220
`after the feed gas 226 is supplied between the cathode 204
`and the anode 216.
`In another embodiment, a direct current (DC) power
`supply (not shown) is used in an ionization source to
`generate and maintain the weakly-ionized or pie-ionized
`plasma 232. in this embodiment, the DC power supply is
`adapted to generate a voltage that is large enough to ignite
`the weakly—ionized plasma 232. In one embodiment, the DC
`power supply generates an initial voltage of several kilovolts
`that creates a plasma discharge voltage on the order of
`between about 100V and 1 kV with a discharge current in the
`range of about 0.1 A and 100 A between the cathode 204 and
`the anode 216 in order to generate and maintain the weakly- .
`ionized plasma 232. The value of the discharge current
`depends on the power level of the power supply and is a
`function of the volume of the weakly—ionizer] plasma 232.
`Furthermore, the presence of a magnetic field (not shown) in
`the region 222 can have a dramatic etl‘ect on the value of the
`applied voltage and current required to generate the weakly-
`ionized plasma 232.
`In some embodiments (not shown), the DC power supply
`generates a current that is between about 1 mA and 100 A
`depending on the size of the plasma generating system and
`the strength of a magnetic field in a region 234.
`In one
`embodiment, before generating the weakly-ionized plasma
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`one microsecond to ten seconds. The repetition frequency or
`repetition rate of the high-power pulses,
`in some
`embodiments, is in the range of between about 0.1 Hz to 1
`kHz. The average power generated by the pulsed power
`supply 202 can be less than one megawatt depending on the
`volume of the plasma.
`In one embodiment,
`the thermal
`energy in the cathode 204 andlor the anode 216 is conducted
`away or dissipated by liquid or gas cooling such as helium
`cooling (not shown).
`The high-power pulses generate a strong electric field 236
`between the cathode 204 and the anode 216. The strong
`electric field 236 is substantially located in the region 222
`between the cathode 204 and the anode 216.
`In on
`embodiment, the electric field 236 is a pulsed electric field.
`In another embodiment, the electric field 236 is a quasi—static
`electric field. By quasi-static electric field we mean an
`electric field that has a characteristic time of electric field
`
`variation that is much greater than the collision time for
`electrons with neutral gas particles. Such a time of electric
`field variation can be on the order of ten seconds. The
`strength and the position of the strong electric field 236 will
`be discussed in more detail herein.
`Referring to FIG. 21), the high-power pulses generate a
`highly-ionized or a strongly-ionized plasma 238 from the
`weakly-ionized plasma 232 (FIG. 2C). The strongly-ionized
`plasma 238 is also referred to as a high-density plasma. The
`discharge current that is formed from the strongly-ionized
`plasma 238 can be on the order of about 5 kA or more with
`a discharge voltage in the range of between about 50V and
`500V for a pressure that is on the order ofbetween about 100
`m'l'orr and 10 Torr.
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`enhances the formation of ions in the plasma. The multi-step
`or stepwise ionization process is described as follows.
`Apre-ionizing voltage is applied between the cathode 204
`and the anode 216 across the feed