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`
`U8006805779152
`
`(12) United States Patent
`(:10) Patent N0.:
`US 6,805,779 B2
`Chistyakov
`(45) Date of Patent:
`Oct. 19, 2004
`
`
`(54) PLASMA GENERATION USING MULTI-STEP
`IONIZATiON
`
`('75)
`‘
`
`Inventor: Roman Chistyakov Andover MA
`(US)
`’
`’
`
`(73) Assignee: Zond, Inc., Mansfield, MA (US)
`
`11/1995 Coombe --------------------- 427/596
`5,382,457 A
`4/1996 Yoshida et :11.
`............. 250/251
`5,506,405 A
`3/1998 Hershcovitch er al.
`. 204,192.11.
`5,733,418 A
`10/1998 Hinchliffe .............. 250/49221
`5,821,548 A
`5/2000 Hausmann et al.
`4381706
`6,057,244 A
`(List continued on next page.)
`FOREIGN PATENT DOCUMENTS
`
`( * ) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`W0
`W0
`
`WO 98/40532
`WO 01198553 A1
`
`9/1998
`12/2001
`
`OTHER PUBLICATIONS
`
`(21) Appl.No.:10/249,202
`(22) Filed:
`Mar. 21, 2003
`,
`.
`.
`P110“ Publication Data
`US 2004/0182702 A1 56!, 23; 2004
`
`(65)
`
`(51)
`
`Int. Ct.7 ......................... C23C 14,400; C23C 16/00;
`H01] 734
`(52) U-S- 0- ............................ 20439836; 204298137;
`204/298.38; 118/723 VE; 118/723 R; 315/111.81;
`BIS/111.91
`315111181, 111.91.,
`(58) Field of Search
`315/111.71, 111.41, 111.21; 2U4/298.37,
`298.38, 298.36; 118/723 VE, 723 R, 723 EB,
`723 E; 250/28, 283, 377, 423, 435, 489
`
`(56)
`
`References Cited
`‘
`.
`,
`U.S. PATENF DOCUMENTS
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`____
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`al., The Transition From Symmetric To
`et
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`Asymemtric Discharges In Pulsed 13.56 MHZ Capacitively
`Coupled P1asrnas,.l . Appl. Phys, Jul. 15, 1997, pp. 552~560,
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`Bunshah, et al., Deposition Technologies For Films And
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`
`(List continued on next page.)
`
`Primmiy Examiner—Tuyet T, V0
`(74) Attorney, Agent,
`or Firn'z—Kurt Rauschenbach;
`Rauschenbach Patent Law Group, LLC
`
`(57)
`
`ABSTRACT
`
`The Preset“ inventi‘?“ ”latest" a .9121:st generator 31.1“
`generates a plasma With a multi-step ionization process. [he
`plasma generator includes an excited atom source that
`generates excited atoms from ground state atoms supplied
`by a. feed gas source. A plasma chamber confines a volume
`of excited atoms generated by the excited atom source. Ari
`energy source is coupled to the volume of excited atoms
`confined by the plasma chamber. The energy source raises an
`energy of excited atoms in the volume of excited atoms so
`that at least a portion of the exc1ted atoms in the volumeof
`exeited atoms IS. ionized, thereby generating a plasma With
`8 mum-Step “Imam” Process-
`\
`
`46 Claims, 13 Drawing Sheets—
`
`200
`
`.—-..-.-.———.
`
`\ £IZ’;{/illllJill/”07””
`
`GILLETTE 1401
`
`GILLETTE 1401
`
`

`

`US 6,805,779 B2
`
`Page 2
`
`U.S. PATENT DOCUMENTS
`
`9/2000 Bertrand ct a1.
`6,124,675 A
`........ 315/11191
`3/2001 Yamauchi et al.
`6,207,951 Bl
`.
`...... 250/251
`10/2001 Kouznetsov
`6,296,742 B1
`204/192.12
`..
`6,311,638 B1 * 11/2001 Ishii et a1.
`. 118/723 MW
`
`6,395,641 132
`5/2002 Savas .........
`438/714
`. 204/192.12
`6,413,382 Bl
`7/2002 Wang et al.
`
`204092.13
`6,413,383 B1
`7/2002 Chiang etal.
`6,661,178 B1 * 12/2003 Bertrand et a1.
`..... 315/1119]
`2002/0153103 A1
`10/2002 Madocks ............... 156,645.46
`
`OTHER PUBLICATIONS
`
`Goto, et al., Dual Excitation Reactive Ion Etchcr For Low
`Energy Plasma Processing, J. Vac. Sci. Techno]. A, Sop/Oct
`1992, pp. 3048—3054, vol.
`'10, No. 5, American Vacuum
`Society.
`£11., A Novel Pulsed Magnetron Splitter
`Kouznetsov, of
`'I‘echnique Utilizing Very High Target Power Densities,
`Surface and Coatings Technology, 1999, pp. 290—293, vol.
`122, Elsevier Science SA.
`Lindquist, et 211., High Selectivity Plasma Etching Of Sili—
`cone 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, pp. 1—2.
`Macak, et al.,
`Ionized Sputter Deposition Using An
`Extremely High Plasma Density Pulsed Magnetron Dis-
`charge,
`J. Vac. Sci. Technol. A.,
`Jul/Aug 2000, pp.
`15334537, vol. 18, No. 4., American Vacuum Society.
`Mozgrin, et a1., 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.
`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 E x B Direction,
`J. Vac. Sci. ‘l‘echnol. A., Jul/Aug 1998, pp. 2173—2176, vol.
`16, No. 4, American Vacuum Society.
`
`Steinbruchel, A Simple Formula For Low—Energy Sputter-
`ing Yields, Appl. Phys. A., 1985, pp. 37—42, vol. 36,
`Sprigner Verlag.
`
`Encyclopedia Of Low Temperature Plasma, p. 119, vol. 3.
`
`Encyclopedia Of Low Temperature Plasma, p. 123, vol. 3.
`
`Lymberopoulos, et 31., Fluid Simulations OI' Glow Dis-
`charges: Etl'ect Of Metastable Atoms In Argon, J. Appl.
`Phys, Apr. 1993, pp. 3668—3679, vol. .73, No. 8, American.
`Institute of Physics.
`
`Burnham, ct al., Eflicient Electric Discharge Lasers In XeF
`and KrF, Applied Physics Letters, Jul. 1976, pp. 30—32, vol.
`29, No. 1, American Institute of Physics.
`
`Pabrikant, et a1, Electron Impact Formation Of Metastable
`Atoms, pp. 3, 31, 34—37, Amsterdam.
`
`Fahey, et 211., High Flux Beam Source Of Thermal Rare—Gas
`Metastable Atoms, 1980, J. Phys. E. Sci. Instrum, vol. 13,
`The Institute of Physics.
`
`Verheijen, et al., A Discharge Excited Supersonic Source Of
`Metastable Rare Gas Atoms, J.Phys. E. Sci. Instrum, 1984,
`vol. 17.
`
`Eletskii, Excimer Lasers, Sov. Phys. Usp., Jun. 1978, pp.
`502—521, vol. 21, N0. 6.
`
`Chistyakov, High Power Pulsed Magnetron Sputtering,
`Application No.: 10/065, 277, Filed: Sep. 30, 2002.
`
`Chistyakov, High—Power Pulsed Magnetically Enhanced
`Plasma Processing, Application No.2 10/065, 55], Filed:
`Oct. 29, 2002.
`
`(Ihistyakov, Method and Apparatus For Generating High—
`Density Plasma, Application No.: 10/065, 629, Filed: Nov.
`4, 2002.
`
`Chistyakov, High Deposition Rate Sputtering, Application
`No.2 10/065, 739, Filed: Nov. 14, 2002.
`
`* cited by examiner
`
`

`

`US. Patent
`
`Oct. 19, 2004
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`US. Patent
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`US. Patent
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`Oct. 19, 2004
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`Sheet 13 0f13
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`US 6,805,779 B2
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`800
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`
`SUPPLY VOLUME OF GROUND
`
`STATE ATOMS
`
`802
`
`GENERATE VOLUME OF
`
`
`
`METASTABLE ATOMS FROM
`
`
`VOLUME OF GROUND STATE
`
`ATOMS
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`
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`THEREBY GENERATING PLASMA
`
`FIG. 13
`
`

`

`,4
`1
`PLASMA GENERATION USING MUL’i‘l-STEP
`IONIZATION
`
`US 6,805,779 B2
`
`BACKGROUND OF THE INVENTION
`
`Plasma is considered the fourth state of matter. A plasma
`is a collection of charged particles that move 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 conducting electrodes that
`are positioned parallel to each other. Once certain param-
`eters 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 elec-
`trodes in an inert gas atmosphere (cg, argon) at a pressure
`that is between about .10“1 and 10‘2 Torr.
`
`10
`
`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 and other work pieces. Sput-
`tering 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 are then
`drawn out of the plasma and accelerated across a cathode
`dark space. The target surface has a lower potential than the
`region in which the plasma is formed. Therefore, the target
`surface attracts positive ions.
`Positive ions move towards the target with a high velocity
`and then impact the target and cause atoms to physically
`dislodge or sputter from the target surface. The sputtered
`atoms then propagate to a substrate or other work piece
`where they deposit a film of sputtered target material. The
`plasma is replenished by electron-ion pairs formed by the
`collision of neutral molecules with secondary electrons
`generated at the target surface.
`Reactive sputtering systems inject a reactive gas or mix—
`ture of reactive gases into the sputtering system. The reac-
`tive gases react with the target material either at the target
`surface or in the gas phase, resulting in the deposition of new
`compounds. The pressure of the reactive gas can be varied
`to control the stoichiometry of the film. Reactive sputtering
`is useful for forming some types of molecular thin films.
`Magnetron sputtering systems use magnetic fields that are
`shaped to trap and concentrate secondary electrons proxi-
`mate io the target surface The magnetic fields increase the
`density of electrons and,
`therefore,
`increase the plasma
`density in a region that is proximate to the target surface.
`The increased plasma density increases the sputter deposi—
`tion rate.
`
`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.
`
`mC)
`
`mL"
`
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`
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`
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`1 illustrates a cross-sectional View of a known
`FIG.
`plasma sputtering apparatus having a DC power supply.
`FIG. 2 illustrates a cross-sectional View of an embodiment
`of a plasma generator that generates a plasma with a
`multi-step ionization process according to the present inven—
`tion.
`FIG. 3 illustrates a cross-sectional View of another
`embodiment of a plasma generator that generates a plasma
`with a multi-step ionization process according to the present
`invention.
`HG. 4 illustrates a cross-sectional View of an embodiment
`of an excited atom generator that includes an excited atom
`source, such as a metastable atom source according to the
`present invention.
`FIG. 5 illustrates a cross—sectional view of an embodiment
`of a chamber of an excited atom source such as a metastable
`atom source according to the present invention.
`Jay!
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`FIG, 6
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`atom source such as a metastable atom source according to
`the invention.
`
`FIG. 7 is a perspective view of an excited atom source
`such as a metastable atom source according to one embodi-
`ment of the invention.
`FIG. 7A illustrates a cross-sectional View of the meta-
`stable atom source of FIG. 7 that illustrates the magnetic
`field.
`FIG. 8 illustrates a cross-sectional View of another
`embodiment of an excited atom source such as a metastable
`atom source according to the invention.
`FIG. 9 illustrates a cross-sectional View of another meta-
`stable atom source according to the invention.
`FIG. 10 illustrates a cross-sectional View of another
`metastable atom source according to the invention
`FIG. 11 illustrates a cross-sectional View of another
`metastable atom source according to the invention.
`FIG. 12A through FIG. 12C illustrate various embodi-
`ments of electron/ion absorbers according to the invention.
`FIG. 13 is a flowchart of an illustrative process of
`generating a plasma with a multi-step ionization process
`according to the present invention.
`
`DETAILED DESCRIPTION
`
`FIG. 1 illustrates a cross-sectional View of a known
`plasma sputtering apparatus 100 having a DC power supply
`102. The known plasma sputtering apparatus 100 includes a
`vacuum chamber 104 where a plasma 105 is generated. The
`vacuum chamber 104 can be coupled to ground. The vacuum
`chamber 104 is positioned in fluid communication with a
`vacuum pump 106 via a conduit 108 and a valve 109. 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 110 from a feed gas source 111, such as an argon gas
`source, is introduced into the vacuum chamber 104 through
`a gas inlet 112. The gas flow is controlled by a valve 113.
`The plasma sputtering apparatus 100 also includes a
`cathode assembly 114. The cathode assembly 114 is gener-
`ally in the shape of a circular disk. The cathode assembly
`114 can include a target 116. The cathode assembly 114 is
`electrically connected to a first terminal 118 of the DC power
`supply 102 with an electrical transmission line 120. An
`insulator 122 isolates the electrical transmission line 120
`from a wall of the vacuum chamber 104. An anode 124 is
`electrically connected to a second terminal 126 of the DC
`
`

`

`US 6,805,779 B2
`
`3
`
`power supply 102 with an electrical transmission line 127.
`An insulator 128 isolates the electrical transmission line 127
`from the wall of the vacuum chamber 104. The anode .124
`is positioned in the vacuum chamber 104 proximate to the
`cathode assembly 114. An insulator 129 isolates the anode
`124 from the cathode assembly 114. The anode 124 and the
`second output 126 of the DC power supply 102 are coupled
`to ground in some systems.
`The plasma sputtering apparatus 100 illustrates a magne-
`tron sputtering system that
`includes a magnet 130 that
`generates a magnetic field 132 proximate to the target 116.
`The magnetic field 132 is strongest at the poles of the
`magnet 130 and weakest in the region 134. The magnetic
`field 132 is shaped to trap and concentrate secondary elec-
`trons proximate to the target surface. The magnetic field
`increases the density of electrons and, therefore, increases
`the plasma density in a region that is proximate to the target
`surface.
`
`The plasma sputtering apparatus 100 also includes a
`substrate support 136 that holds a substrate 138 or other
`work piece. The substrate support 136 can be electrically
`connected to a first terminal 140 of a RF power supply 142
`with an electrical transmission line 144. An insulator 146
`isolates the RF power supply 142 from a wall of the vacuum
`chamber 104. Asecond terminal 148 of the RF power supply
`142 is coupled to ground.
`In operation, the feed gas 110 from the feed gas source 111
`is injected into the chamber 104. The DC power supply 102
`applies a DC voltage between the cathode assembly 114 and
`the anode 124 that causes an electric field 150 to develop
`between the cathode assembly 114 and the anode 124. The
`amplitude of the DC voltage is chosen so that it is sufficient
`to cause the resulting electric field to ionize the feed gas 110
`in the vacuum chamber 104 and to ignite the plasma 105.
`The ionization process in known plasma sputtering appa-
`ratus is generally referred to as direct ionization or atomic
`ionization by electron impact and can be described by the
`following equation:
`‘
`Ar+e‘~aAr*+2e'
`
`where Ar represents a neutral argon atom in the feed gas
`110 and 6‘ represents an ionizing electron generated in
`response to the voltage applied between the cathode assem-
`bly 114 and the anode 124. The collision between the neutral
`argon atom and the ionizing electron results in an argon ion
`(An) and two electrons.
`The plasma 105 is maintained, at least in part, by sec-
`ondary electron emission from the cathode assembly 114.
`The magnetic field 132 that is generated proximate to the
`cathode assembly 114 confines the secondary electrons in
`the region 134 and,
`therefore, confines the plasma 105
`approximately in the region 134. The confinement of the
`plasma in the region 134 increases the plasma density in the
`region 134 for a given input power.
`Since the cathode assembly 114 is negatively biased, ions
`in the plasma 105 bombard the target 116. The impact
`caused by these ions bombarding the target 116 dislodge-s or
`sputters material from the target 116. A portion of the
`sputtered material forms a thin film of sputtered target
`material on the substrate 138.
`Known magnetron sputtering systems have relatively
`poor target utilization. The term “poor target utilization” is
`defined herein to mean undesirable non-uniform erosion of
`target material. Poor target utilization is caused by a rela-
`tively high concentration of positively charged ions in the
`region 134 that results in a non—uniform plasma. Similarly,
`
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`10
`
`[2c)
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`30
`
`4
`magnetron etching systems (not shown) typically have rela-
`tively non-uniform etching characteristics.
`Increasing the power applied to the plasma can increase
`the uniformity and density of the plasma. However, increas~
`ing the amount of power necessary to achieve even an
`incremental increase in uniformity and plasma density can
`significantly increase the probability of establishing an elec-
`trical breakdown condition leading to an undesirable elee~
`trical discharge (an electrical arc) in the chamber 104.
`Applying pulsed direct current (DC) to the plasma can be
`advantageous since the average discharge power can remain
`relatively low while relatively large power pulses are peri-
`odically 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.
`In one embodiment, an apparatus according to the present
`invention generates a plasma having a higher density of ions
`for a giving input power than a plasma generated by known
`plasma systems, such as the plasma sputtering apparatus 100
`of FIG. 1.
`FIG. 2 illustrates a cross-sectional view of an embodiment
`of a plasma generator 200 that generates a plasma 202 with
`a multi-step ionization process according to the present
`invention, In one embodiment, the plasma generator 200
`includes an exited atom source that generates excited atoms
`from ground state atoms from a feed gas source 206. In the
`embodiment shown, the excited atom source is a metastable
`atom source 204 that generates metastable atoms from the
`feed gas source 206. The feed gas source 206 provides a
`volume of ground state atoms 208 to the metastable atom
`source 204. The feed gas source 206 can provide any type of
`feed gas or mixture of feed gases, such as, noble gases,
`reactive gases, and mixtures of noble gases and reactive
`gases. In one embodiment, the feed gas source 206 com—
`prises a source of ground state noble gas atoms. For
`example,
`in one embodiment,
`the feed gas source 206
`comprises a source of ground state argon atoms.
`The feed gas source 206 is coupled to the metastable atom
`source 204 through a gas flow control system 210. In one
`embodiment, the gas flow control system 210 includes a first
`gas valve 212, a mass flow controller 214, and a second gas
`valve 216. The gas flow control system 210 can include any
`number of gas valves and/0r mass flow controllers. The gas
`flow control system 210 controls the volume and the flow
`rate of the ground state atoms 208 flowing into the meta-
`stable atom source 204. In one embodiment, the metastable
`atom source 204 includes a means of controlling the pres—
`sure of the feed gas inside the metastable atom source.
`The metastable atom source 204 receives the ground state
`/ atoms 208 from the gas flow control system 210 at an input
`217. The metastable atom source 204 generates a volume of
`metastable atoms 218 from the volume of ground state
`atoms 208. In one embodiment, the metastable atom source
`generates a Volume of ions that is relatively small compared
`with the volume of metastable atoms 218, A first output
`terminal 220 of a power supply 222 is coupled to an
`electrical input 224 of the metastable atom source 204. The
`type of power supply depends upon the type of metastable
`atom source. For example, the power supply 222 can be a
`pulsed power supply, a radio frequency (RF) power supply,
`an alternating current (AC) power supply, or a direct current
`(DC) power supply.
`
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`US 6,805,779 B2
`
`5
`The plasma generator of the present invention can use any
`type of metastable atom source 204. Skilled artisans will
`appreciate that there are many methods of exciting ground
`state atoms 208 to a metastable state, such as electron impact
`ionization, photo excitation, or thermal excitation. The
`operation of specific embodiments of metastable atom
`sources are discussed in more detail herein. For example, in
`one embodiment, the metastable atom source 204 includes a
`parallel plate discharge chamber (not shown) that receives
`the volume of ground state atoms 203 from the gas flow
`control system 210 and that generates a discharge that
`excites a portion of the volume of ground state atoms 208 to
`a metastable state.
`In another embodiment, the metastable atom source 204
`includes an electron gun (not shown) that receives the
`volume of ground state atoms 208 from the gas flow control
`system 210 and that generates and accelerates an electron
`beam that excites a portion of the volume of ground state
`atoms 208 to a metastable state. In yet another embodiment,
`the metastable atom source 204 includes an inductively
`coupled discharge chamber that receives the volume of
`ground state atoms 208 from the gas flow control system 210
`and that generates a discharge that excites a portion of the
`volume of ground state atoms 208 to a metastable state.
`Ailange 226 couples an output 227 of the metastable atom
`source 204 to an input port 228 of a plasma chamber 230.
`The metastable atom source 204 can be coupled to any type
`of process chamber, such as the chamber 104 of FIG. 1. In
`fact, a plasma generator according to the present invention
`can be constructed by coupling a metastable atom source to
`a commercially available plasma chamber. Thus, commer—
`cially available plasma generators can be modified to gen—
`erate a plasma using a multi—step ionization process accord-
`ing to the present invention.
`In one embodiment, a diameter of the input 217 of the
`metastable atom source 204 is dilferent than a diameter of
`the output 227 of. the metastable atom source 204. This
`difference in diameters creates a pressure differential
`between the input 217 and the output 227 of the metastable
`atom source 204. The rate of metastable generation in the
`metastable atom source 204 depends upon the pressure
`inside the source 204. In some embodiments, at least one of
`the diameter of the input 217 and the diameter of the output
`227 of the metastable atom source 204 is chosen so that a
`pressure differential is created that increases the generation
`rate of the metastable atoms 218 in the metastable atom
`source 204.
`The plasma chamber 230 confines the volume of meta-
`stable atoms 218. In one embodiment,
`the output of. the
`metastable atom source 204 is positioned so as to direct the
`volume of metastable atoms 218 towards the cathode assem-
`bly 114. In one embodiment, the geometry of the plasma
`chamber 230 and the cathode assembly 114 is chosen so that
`the metastable atoms reach the cathode assembly 114 at a
`time that is much less than an average transition time of the
`metastable atoms to ground state atoms.
`In some
`embodiments, ground state atoms from the metastable atom
`source 204 gain energy in the metastable atom source 204,
`but do not actually become metastable atoms until
`they
`reach the plasma chamber 230. Ground state atoms from the
`metastable atom source 204 can become metastable atoms at
`any place along the path from the metastable atom source
`204 to the cathode assembly 114. In some embodiments, the
`metastable atom source 204 generates some excited atoms
`that are in excited states other than a metastable state.
`The plasma chamber 230 is positioned in fluid commu—
`nication with the vacuum pump 106 via the conduit 108 and
`
`6
`the vacuum valve 109. The vacuum pump 106 evacuates the
`plasma chamber 230 to high vacuum. The pressure inside
`the plasma chamber 230 is generally maintained at less than
`10"1 Torr for plasma processing. In one embodiment, a feed
`gas (not shown) from a second feed gas source (not shown),
`such as an argon gas source, is introduced into the plasma
`chamber 230 through a gas inlet (not shown).
`In one embodiment, the power supply 201 is a pulsed
`power supply that is electrically coupled to the cathode
`assembly 114 with the electrical transmission line 120. In
`one embodiment,
`the duration of the pulse is chosen to
`optimize a process parameter. In other embodiments, the
`power supply 201 is a RF power supply, an AC power
`supply, or a DC power supply. The isolator 122 insulates the
`electrical transmission line 120 from the plasma Chamber
`230. The second output 126 of the power supply 102 is
`electrically coupled to the anode 124 with the electrical
`transmission line 127. The isolator 128 insulates the elec—
`trical transmission line 127 from the plasma chamber 230.
`Another isolator 129 insulates the anode 124 from the
`cathode assembly 114. Numerous other cathode and anode
`configurations known in the art can be used with the plasma
`generator of the present invention. In one embodiment, the
`plasma chamber 230 is coupled to ground potential.
`The cathode assembly 114 is formed of a metallic
`material, such as stainless steel or any other material that
`does not chemically react with reactive gases.
`In one
`embodiment (not shown),
`the cathode assembly 114
`includes a sputtering target 116 that is used for sputtering
`materials onto a substrate or other work piece. The sputter—
`ing target 116 can include any type of material. For example,
`the sputtering target 116 can be formed of magnetic, non-
`magnetic, dielectric, metals, and semiconductor materials.
`In one embodiment, a magnet (not shown) is disposed
`proximate to the cathode assembly 114. The magnet gener-
`ates a magnetic field that
`traps electrons in the plasma
`proximate to the cathode assembly 114 and,
`therefore,
`increases the plasma density In the region proximate to the
`cathode assembly 114.
`*
`The substrate support 136 is disposed in the plasma
`chamber 230. The substrate support 136 is designed to
`support a substrate 138 or other work piece.
`In one
`embodiment, a temperature controller 240 is positioned in
`thermal communication with the substrate support 136. The
`temperature controller 240 can increase or decrease the
`temperature of the substrate 138. In some embodiments, the
`temperature controller 240 is used to control the temperature
`of the substrate for various reasons including enhancing a
`chemical reaction, increasing a growth rate, and improving
`adhesion.
`In one embodiment, the power supply 142 is used to apply
`abias voltage to the substrate 138. The first output 140 of the
`power supply 142 is coupled to the substrate support 136
`with the transmission line 144. The isolator 146 insulates the
`
`transmission line 144 from a wall of the plasma chamber
`230. The second output 148 of the power supply 142 is
`coupled to ground. The power supply 142 can be any type
`of pulsed power supply such as a RF power supply, an AC
`power supply, or a DC power supply.
`The plasma generator 200 of FIG. 2 uses a multi-step or
`stepwise ionization process to generate the plasma 202. The
`term “multi-step ionization process” is defined herein to
`mean an ionization process whereby ions are ionized in at
`least
`two distinct steps. However,
`the term “multi-step
`ionization process” as defined herein may or may not
`include exciting ground state atoms to a metastable state. For
`example, one multi—step ionization process according to the
`
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`US 6,805,779 B2
`
`8
`Metastable atoms can be present in considerable densities
`in Weakly ionized discharges. In the second step, an ionizing
`electron e‘ collides with the metastable argon atom and the
`metastable argon atom is ionized and two electrons are
`generated, as shown below.
`A1‘+e’”-—>Ar*+e‘
`
`'J)
`
`Ar*+c'—>Ar"+2f
`
`7
`
`present invention includes a first step where atoms are
`excited from a ground state to a metastable state and a
`second step where atoms in the metastable state are ionized.
`Another multi-step ionization process according to the
`present
`invention includes a first step where atoms are
`excited from a ground state to an excited state and a second
`step where atoms in the excited state are ionized. The term
`“multi-step ionization process” also includes ionization pro-
`cesses with three or more steps.
`In operation, the plasma generator 200 operates as fol—
`lows. The gas flow control system 210 supplies ground state
`atoms 208 front