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`1 of 29
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`US 6,805,779 B2
`Page 2
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`Lymberopoulos, et 31., Fluid Simulations 01' Glow Dis-
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`Fabrikant, et al., Electron Impact Formation Of Metastable
`Atoms, pp. 3, 31, 34-37, Amsterdam.
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`. 118/723 MW
`..
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`6,395,641 132
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`Surface and Coatings "Technology, 1999, pp. 290-293, vol.
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`cone Dioxide With A Dual Frequency 27/2 MHz Capacitive
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`Characterization Of A Novel High Plasma Density Pulsed
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`Ionized Sputter Deposition Using An
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`Sheet 12 of 13
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`Oct. 19, 2004
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`Sheet 13 of 13
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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
`
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`ATOMS
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`REMOVE ELECTRONS AND IONS
`FROM VOLUME OF METASTABLE
`ATOMS
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`806
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`RA|SE ENERGY OF METASTABLE
`ATOMS TO IONIZE A PORTION
`OF METASTABLE ATOMS,
`THEREBY GENERAT!NG PLASMA
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`TSMC-1401 I 15 of 29
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`» 1
`
`PLASMA GENERATION USING MULTI-STEP
`IONIZATION
`
`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 (c.g., argon) at a pressure
`that is between about .l0“1 and lU“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 lo 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. ‘lire drawings are
`not necessarily to scale, emphasis instead being placed upon
`illustrating the principles of the invention.
`
`U)C)
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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.
`excited
`FIG. 5 iiiustra es it cross-sectional view of
`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.
`
`DI;"I'AILI3.D 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” 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
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`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 sputtcring 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 lield
`increases the density of electrons and, therefore, increases
`the plasma density iii 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 tirst 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 sulficient
`to cause the resulting electric field to ionize the [eed 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‘~9Ar*+2e'
`
`where Ar represents a neutral argon atom in the feed gas
`110 and e‘ represents an ionizing electron generated in
`response to the voltage applied between the cathode assem-
`bly ll4 and the anode 124. The collision between the neutral
`argon atom and the ionizing electron results in an argon ion
`(Ar*) 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 dislodges 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. Poo_r 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,
`
`‘J:
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`
`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 elec-
`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/or 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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`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 ernbodirnerrt,
`the metastable atom source 204 includes an inductively
`coupled discharge chamber that receives the Volume of
`ground state atoms 208 from the gas llow control system 210
`and that generates a discharge that excites a portion of the
`volume of ground state atoms 208 to a metastable state.
`Atlange 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 diilerential 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” 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 linc 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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`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.
`Ar+e’”->Ar*+e‘
`
`‘Jr
`
`Ar*+e'——>Ar‘“+2e."
`
`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 the feed gas source to the metastable atom
`source 204. The power supply 222 applies a voltage to the
`volume of ground state atoms 208. The voltage excites at
`least a portion of the volume of the ground state atoms 208
`to creates a volume of metastable atoms 218. In one embodi-
`ment. the power supply 222 applies a voltage to the volume
`of ground state atoms 208. In one embodiment, the duration
`of the voltage pulse is chosen to optimize a process
`parameter, such as the rate of metastable atom generation or
`the efficiency of metastable atom generation.
`The term “metastable atoms” is defined herein to mean
`excited atoms having energy levels from which dipole
`radiation is theoretically forbidden. Metastable atoms have
`relatively long lifetimes compared with other excited atoms.
`Metastable atoms are created because, in theory, the selec-
`tion rules forbid relaxation of these excited atoms to the
`ground state and the emission of dipole radiation. However,
`the selection rules were determined using certain approxi-
`mations. Consequently, in practice, there is a finite prob-
`ability that the metastable atoms relax to the ground state
`and emit dipole radiation. The actual lifetime of metastable
`atoms is on the order of milliseconds to minutes. For
`example, lifetimes for argon metastables are 44.9 seconds
`and 55.9 seconds for metastable energies of 11.723 eV and
`11.548 eV, respectively.
`All noble gases have metastable states. For example,
`argon metastable atoms can be generated by a two-step
`ionization process. In the first step, ionizing electrons e‘ are
`generated by applying, a sufficient voltage between the
`cathode assembly 114 and the anode 124. When an ionizing
`electron e‘ collides with a ground state argon (Ar) atom, a
`metastable argon atom and an electron are generated. Argon
`has two metastable states, see Fabrikant, I. 1., Shpenik, O.
`B., Snegursky, A. V., and Zavilopulo,A. N., Electrorz Impact
`Formation of Metasmble Atoms, North-Holland, Amster-
`dam. The first metastable state is represented in j1—coupling
`notation as follows:
`
`4s[3:2],,°
`
`and is represented in the LS-coupling configuration as
`follows:
`
`31-’5(:P:~x2DJ455P2
`
`The energy and lifetime of the first metastable state are
`11548 eV and 55.9 seconds, respectively.
`The second metastable state is represented in jl-coupling
`notation as follows:
`
`4s*[1/2],,“
`
`and is represented in the LS-coupling configuration as
`follows:
`
`3175 (Z?3;2O)453P0
`
`The energy and lifetime of the second metastable state are
`11.723 eV and 44.9 seconds, respectively.
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`Plasma generation using multi-step