`(16) Patent N0.:
`US 6,853,142 B2
`
`Chistyakov
`(45) Date of Patent:
`Feb. 8, 2005
`
`U5006853142B2
`
`(54) METHODS AND APPARATUS FOR
`GENERATING HIGH-DENSITY PLASMA
`
`(75)
`
`Inventor: Roman Chistyakov, Andover, MA
`(US)
`
`.
`.
`(73) ASSlgnee‘ Z‘md’ Inc” ManSfield> MA (Us)
`.
`.
`.
`.
`.
`( * ) Notice:
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U‘S‘C‘ 154(b) by 0 days‘
`
`6,057,244 A
`6,238,537 B1
`6,296,742 B1
`
`......... 438/706
`5/2000 Hausmann et a1.
`5/2001 Kahn et a1.
`............ 204/598.04
`10/2001 Kouznetsov ........... 204/192.12
`
`6,361,667 B1
`6,413,382 B1
`6,413,383 B1
`6,432,260 B1
`6,436,251 B2
`6 451 703 B1
`’
`’
`
`204/298.11
`3/2002 Kobayashi et a1.
`7/2002 Wang et al.
`........... 204/192.12
`7/2002 Chiang et al.
`......... 204/192.13
`8/2002 Mahoney et a1.
`...... 156/345.35
`8/2002 Gopalraja et a1.
`..... 204/298.12
`9 2002 L'
`t
`l.
`.................... 438 710
`/
`m e a
`/
`(List continued on next page.)
`FOREIGN PATENT DOCUMENTS
`
`(21) Appl. No.: 10/065,629
`
`(22)
`
`Filed:
`
`Nov. 4, 2002
`
`EP
`W0
`W0
`
`0 650 183 A1
`WO 98/40532
`W0 01/98553 A1
`
`4/1995
`9/1998
`12/2001
`
`(65)
`
`Prior Publication Data
`
`OTHER PUBLICATIONS
`
`US 2004/0085023 A1 May 6, 2004
`7
`
`Int. Cl.
`(51)
`(52) US. Cl.
`
`.............................................. C23C 16/452
`............................. 315/111.41; 156/345.33;
`118/7231
`(58) Field of Search ........................ 315/111.01—111.91;
`156/34521, 345.29, 345.33, 345.42, 345.44,
`345, 204/298.06, 298.04, 298.08; 118/723 FE,
`7231’ 723 MP; 423/210> 246’ 248
`References Cited
`U.S. PATENT DOCUMENTS
`
`(56)
`
`5/1986 Cuomo et a1.
`.............. 204/298
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`8/1990 Eldridge et a1.
`....... 372/87
`4,953,174 A
`
`5/1991 Gwen ...................... 427/38
`5 015 493 A
`8/1991 Kojoe ................... 315/11141
`5:041:760 A *
`1/1992 Koshiishi et a1.
`....... 315/111.81
`5,083,061 A
`9/1993 Muller—Horshe ............. 372/38
`5,247,531 A
`2/1994 SZCYTbOWSki 6t 91-
`- 204/298-O8
`5,286,360 A
`7/1995 Barnes et a1.
`............ 156/643.1
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`12/1998 PDaSCh ”””d“t“‘l’ ””” 20319212
`39%;??? 2
`rummon e a.
`.
`,
`,
`3/1998 Okamura et a1.
`...... 204/298.11
`5,728,278 A
`3/1998 Hershcovitch et a1.
`. 204/19211
`5,733,418 A
`8/1998 Kinoshita et a1.
`...... 204/298.37
`5,795,452 A
`6/1999 Kumagai
`..................... 216/68
`5,916,455 A
`5,993,761 A * 11/1999 Czernichowski et a1.
`423/210
`
`US 5,863,392, 1/1999, Drummond et al. (Withdrawn)
`a
`a
`Booth et al. The Transition From Symmetric To Asymmet-
`ric Discharges In Pulsed 13.56 MHz Capacity Coupled
`Plasmas, J. Appl. Phys., Jul. 15, 1997’ Pp 552—560, V01.
`82(2), American Institute of Physics.
`.
`.
`(Llst confirmed on next page)
`Primary Examiner_wflson Lee
`(74) Attorney, Agent,
`or Firm—Kurt Rauschenbach;
`R
`h
`b
`h P t
`t L
`G
`LLC
`ausc en ac
`aen
`aw roup,
`(57)
`ABSTRACT
`
`MethOds and WW?“ for generatmg a Strongly'lqmzed
`Plasma 3}“? descrlbed' A“ aPParatuS for gen8ratlng. a
`strongly-ionized plasma according to the present invention
`includes an anode and a cathode that is positioned adjacent
`to the anode to form a gap there between. An ionization
`source generates a weakly-ionized plasma proximate to the
`cathode. Apower supply produces an electric field in the gap
`between the anode and the cathode. The electric field
`generates excited atoms in the weakly-ionized plasma and
`_
`gegeratesl secondgryplecfimns fro? the catEOd‘; The 55C
`on ary e ectrons ionize t e exc1te
`atoms, t ere y creating
`the Strongly'lomzed Plasma
`
`43 Claims, 13 Drawing Sheets
`
`
`
`MATCHING
`UNIT
`
`20°
`/
`
`202
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`
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`
`
`
`INTEL 1206
`
`
`
`INTEL 1206
`
`
`
`US 6,853,142 132
`
`Page 2
`
`U.S. PATENT DOCUMENTS
`
`.......... 204/192.37
`10/2002 Kumar et al.
`6,471,833 B2
`6,488,825 B1 * 12/2002 Hilliard ..........
`204/298.06
`
`2002/0019139 A1
`2/2002 Zhang et al.
`..... 438/714
`
`..
`2002/0114897 A1
`8/2002 Sumiya et al.
`..... 427/569
`........ 156/345.46
`2003/0006008 A1
`1/2003 Horioka et al.
`
`OTHER PUBLICATIONS
`
`Bunshah, et al., Deposition Technologies For Films And
`Coatings, Materials Science Series, pp. 176—183, Noyes
`Publications, Park Ridge, New Jersey.
`Daugherty, et al., Attachment—Dominated Electron—Bea-
`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, Sep./Oct.
`1992, pp. 3048—3054, vol. 10, No. 5, American Vacuum
`Society.
`Kouznetsov, et al., A Novel Pulsed Magnetron Sputter
`Technique Utilizing Very High Target Power Densities,
`Surface & Coatings Technology, pp. 290—293, Elsevier
`Sciences S.A.
`
`Lindquist, et al., High Selectivity Plasma Etching Of Silicon
`Dioxide With A Dual Frequency 27/2 MHZ Capacitive RF
`Discharge.
`Macak, Reactive Sputter Deposition Process of A1203 and
`Characterization Of A Novel High Plasma Density Pulsed
`Magnetron Discharge, Linkoping Studies In Science And
`Technology, 1999, pp. 1—2, Sweden.
`
`Ionized Sputter Deposition Using An
`Macak, et al.,
`Extremely High Plasma Density Pulsed Magnetron Dis-
`charge,
`J. Vac. Sci. Technol. A., Jul/Aug. 2000, pp.
`1533—1537, vol. 18, No. 4, American Vacuum Society.
`Mozgrin, et al., High—Current Low—Pressure Quasi—Station-
`ary Discharge In A Magnetic Field: Experimental Research,
`Plasma Physics Reports, 1995, pp. 400—409, vol. 21, No. 5,
`Mozgrin, Feitsov, Khodachenko.
`Rossnagel, et al., Induced Drift Currents In Circular Planar
`Magnetrons, J. Vac. Sci. Technol. A., Jan/Feb. 1987, pp.
`88—91, vol. 5, No. 1, American Vacuum Society.
`
`Sheridan, et al., Electron Velocity Distribution Functions In
`ASputtering Magnetron Discharge For The EXB Direction,
`J. Vac. Sci. Technol. A., Jul/Aug. 1998, 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.: 10/065,277, Filed: Sep. 30, 2002.
`
`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
`
`Feb. 8, 2005
`
`Sheet 1 0f 13
`
`US 6,853,142 B2
`
`T
`
`WIIIIIbIIIIIIA ‘30
`
`\ _______________________
`
`138
`
`[III/Ill’l'llllll 114
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`FIG. 1
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`PRIOR ART
`
`
`
`US. Patent
`
`Feb. 8, 2005
`
`Sheet 2 0f 13
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`US 6,853,142 B2
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`US 6,853,142 B2
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`Sheet 13 0f 13
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`US 6,853,142 B2
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`600
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`
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`
`PUMP DOWN CHAMBER
`
`
`
`
`CHAMBER
`PRESSURE
`
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`
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`WEAKLY~IONIZED PLASMA
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`WEAKLY-
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`
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`
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`
`END
`
`
`626
`PLASMA
`PROCESS
`
`COMPLETE
`
`
`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. Aplasma
`is a collection of charged particles moving in random
`directions. Aplasma 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
`10'1 and 10'2 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.
`BRIEF DESCRIPTION OF DRAWINGS
`
`This 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.
`FIG. 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.
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`2
`FIG. 4 illustrates graphical representations of the applied
`voltage, current, and power as a function of time for periodic
`pulses applied to the plasma in the plasma generating
`apparatus of FIG. 2A.
`FIG. 5A through FIG. 5D illustrate various simulated
`magnetic field distributions proximate to the cathode for
`various electron E><B 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-density plasma according to the present inven-
`tion.
`
`DETAILED DESCRIPTION
`
`FIG. 1 illustrates a cross-sectional view of a known
`
`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 10'1 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. A second terminal 124 of
`
`the blocking capacitor 120 is coupled to a first output 126 of
`the RF power supply 102. An insulator 128 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 of
`the vacuum chamber 104. The vacuum chamber 104 can
`
`also be coupled to ground.
`In operation,
`the RF power supply 102 applies a RF
`voltage pulse between the cathode 114 and the anode 130
`that has a sufficient amplitude to ionize the argon feed gas
`in the vacuum chamber 104. A typical RF driving voltage is
`between 100V and 1000V, 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 W/cm2 to 1
`W/cmz. The driving frequency is typically 13.56 MHZ.
`Typical plasma densities are in the range of 109 cm'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’—>Ar++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 (Ar+) 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. Confining 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 corrupt 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. 2A through 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. 2A illustrates 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
`coupled 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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`4
`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
`A or 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.
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`The anode 216 is positioned so as to form a gap 220
`between the anode 216 and the cathode 204 that is sufficient
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`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
`different 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 of the 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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`US 6,853,142 B2
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`5
`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 volume 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 size and shape of the voltage pulse are
`chosen such that an electric field 230 develops between the
`cathode 204 and the anode 216. The amplitude and shape of
`the electric field 230 are chosen such that a weakly-ionized
`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 106 and
`1012 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 0.1 microseconds up to one hundred
`seconds. Specific parameters of the pulse are discussed
`herein in more detail.
`
`In one embodiment, prior to the generation of the weakly-
`ionized plasma 232, the pulsed power supply 202 generates
`a potential difference 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 pre-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 Abetween 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-ionized plasma 232.
`Furthermore, the presence of a magnetic field (not shown) in
`the region 222 can have a dramatic effect 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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`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.
`
`(AC)
`In another embodiment, an alternating current
`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-ionized 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-ionized 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 diffuses
`somewhat homogeneously through the region 234. This
`homogeneous diffusion 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-
`termined time that, in some embodiments, is approximately
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`US 6,853,142 B2
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`7
`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 and/or 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
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`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
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`strength and the position of the strong electric field 236 will
`be discussed in more detail herein.
`
`Referring to FIG. 2D, 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-ion