Plasma Physics Reports, Vol. 21, No. 5, 1995, pp. 400 - 409. Translatedfrom Fizika Plazmy. V01. 21, No. 5, 1995, pp. 422 - 433.
`Original Russian Text Copyright © 1995 by Mozgrin. Felisov, Klzodachenlw.
`
`
`
`
`
`LOW-TEMPERATURE PLASMA
`
`High-Current Low-Pressure Quasi-Stationary Discharge
`in a Magnetic Field: Experimental Research
`
`D. V. Mozgrin, I. K. Fetisov, and G. V. Khodachenko
`Moscow Engineering Physics Institute, Kashirskoe sh. 31, Moscow, 115409 Russia
`Received October 22, 1993; in final form, July 12, 1994
`
`Abstract — The possibility of realizing several types of high-power quasi—stationary low-pressure discharge in
`a magnetic field was shown. Two noncontracted discharge regimes in crossed E and H fields were studied.
`These discharges had much higher cathode current densities than those of other known discharge types. Their
`parameter ranges were determined, and their operating regimes were investigated. The voltage for the high-volt—
`age form discharge ranged from 450 to 1000 V; the discharge current amounted to 250 A, and cathode current
`density reached 25 A/cmz. A low-voltage discharge form was first observed: voltage ranged from 75 to 120 V;
`discharge current amounted to 1800 A, and cathode current density reached 75 A/cmz; lifetime was about 1.5 ms.
`The ion density was 1.5 X 1015 cm”3 in argon discharges and amounted to 5 X 1014 cm“3 in He—Hz-mixture dis—
`charges, while the electron temperature was about 3 — 8 eV. The properties of both discharge types are expected
`to open up new fields of application in technology.
`
`1. lNTRODUCTION
`
`Low—pressure discharges in a magnetic field attract
`much attention due to their wide use in technological
`magnetron devices, closed—electron-drift plasma accel-
`erators, and, as plasma emitters in electron or ion injec—
`tors.
`
`Stationary regimes of the discharges in planar mag-
`netrons of technological use are characterized by p =
`10‘4 — 5 X 10‘2 torr operating pressure and 300 - 1000 G
`magnetic field at the cathode surface [1, 2]. Their AV
`characteristic is described by‘the formula 1,, = kUZ,
`where 1,, is the discharge current, and Ud is the dis-
`charge voltage. The quantities k and n depend on the
`device geometry, working gas type and pressure, and
`magnetic field strength. The condition n > 1 holds, if the
`cathode current density jL. does not exceed 0.03 A/cmz.
`In this case the discharge voltage amounts to 400 — 600 V,
`the plasma density n, ranges from 108 to 10“ cm‘3, and
`electron temperature T, reaches 20 eV. If the current
`density is higher, a transition of the discharge into the
`are regime is observed.
`Because of the need for greater discharge power and
`plasma density, pulse or quasi-stationary regimes
`appear to be of interest. Some experiments on magne-
`tron systems of various geometry showed that dis-
`charge regimes which do not transit to arcs can be
`obtained even at high currents. For example, a super—
`dense glow discharge, realized in a device of “reversed—
`magnetron” type of coaxial geometry, exhibited the fol—
`lowing parameters: about 70 A discharge current Id,
`400 V discharge voltage U,1, 60 us pulse duration, and
`1012 cm‘3 plasma density n,- [3]. A pulse duration decrease
`down to 100 us, which was performed in a planar magne-
`tron discharge in Ar, N2, or H2 at 10‘3 - 5 x 10‘2 torr
`
`pressures and 1.0 - 3.0 kG magnetic field strength, per—
`mitted a 1000 A current value to be obtained in the non—
`contracted regime, at 300 - 500 V discharge voltage,
`with about 50 J of total energy deposition [4]. In both
`examples, the discharge current—voltage characteristic
`increased and then became constant with the increase
`in the discharge currents. A further increase in the dis—
`charge currents caused the discharges to transit to the
`arc regimes, with voltage not higher than 50 V under
`those conditions.
`
`Our previous experiments demonstrated the possi—
`bility of realizing several stable discharge regimes in
`devices with closed electron drift [5 ~ 7]. Among these
`regimes which differed from the arcs, was an interme-
`diate low—voltage regime (Ud z 100 V, 1,, S 1.5 kA)
`of longer than l—ms pulse duration (hereafter called
`a “high—current diffuse regime”).
`The main purpose of this work was to study experi—
`mentally a high—power noncontracted quasi~stati0nary
`discharge in crossed fields of various geometry and to
`determine their parameter ranges. We investigated the dis—
`charge regimes in various gas mixtures at 10*3 — 10 torr,
`BO S 1000 G, and pulse durations exceeding 1 ms.
`Such regimes can be useful in generating large-volume
`dense plasmas and intense flows of charged particles.
`Furthermore, we consider qualitatively the mechanism
`of low—voltage high~current discharge formation.
`
`2. EXPERIMENT
`
`To study the high-current forms of the discharge,
`we used two types of devices: a planar magnetron and
`a system with specifically shaped hollow electrodes.
`The planar magnetron (Fig. 1)
`involved a plane
`cathode 120 mm in diameter and a ring—shaped anode
`
`l063-780X/95/2105~0400$10.00 © 1995 MAI/1K Hayxa /Interperiodica Publishing
`
`GILLETTE 1102
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`GILLETTE 1102
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`

`HIGH~CURRENT LOW~PRESSURE QUASI-STATIONARY DISCHARGE
`
`401
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`160 mm in diameter. The electrodes were immersed in
`a magnetic field of annular permanent magnets. Mag—
`netic circuits were used to vary the size of the region of
`the large magnetic-field radial component and mag—
`netic field inhomogeneity degree. To control the mag-
`netic field strength at the cathode surface, We displaced
`the magnetic system along the axis 2 (Fig. l) and used
`two types of permanent magnets made of SmC05 and
`NdFeB. The discharge had an annular shape and was
`adjacent to the cathode. The maximum of the magnetic
`field radial component B, at the cathode surface was
`800 G for the SmCos magnet or 1200 G for the NdFeB
`magnet. The cathodes we used were made of Cu, Mo,
`Ti, Al, or stainless steel. The cathode was placed on a
`cooled surface. The anodes were made of aluminum or
`stainless steel.
`
`The system with shaped electrodes involved two
`hollow axisymmetrical electrodes 120 mm in diameter,
`separated by about 10 mm, and immersed in a cusP—
`shaped magnetic field produced by oppositely directed
`multilayer coils. The discharge volume bounded by the
`electrodes was about 103 cm3. The ratio of the maximal
`magnetic field at the axis of symmetry Bmax (z, 0) to the
`maximal magnetic field at
`the plane of symmetry
`Bmax (0, r) was about 3. The values ofBmax were controlled
`by coil current variation to range from 0 to 1000 G.
`The electrode shapes followed the magnetic line pro—
`file, which enabled the electric field to be perpendicular
`to the magnetic field along the cathode surface. Such
`a field configuration allowed us to combine a high—cur—
`rent magnetron discharge with a hollow—cathode dis—
`charge.
`The gas from the discharge volume was pumped out;
`minimal residual gas pressure was about 8 X 10‘6 torr.
`It was possible to form the high-current quasi-sta-
`tionary regime by applying a square voltage pulse to the
`discharge gap which was filled up with either neutral or
`pre—ionized gas. Estimates Were made to determine
`both the quasi—stationary plasma density and its build—
`ing~up time [5, 7]. The necessary pre-ionized plasma
`density n, turned out to be 107 ~ 109 cm”3 for argon.
`In addition,
`the estimates determined the shape and
`parameters of the voltage pulse. The pre«ionization
`could be provided by RF discharge, anomalous glow or
`magnetron discharge, etc.
`Figure 2 presents a simplified scheme of the dis—
`charge supply system. The supply unit
`involved
`a pulsed discharge supply unit and a system for pre—
`ionization. The quasi-stationary discharge-supply unit
`consisted of a long line of W = 5.5 kl maximal energy
`content, a switch, and a matching unit. The pre-ioniza-
`tion system provided direct current up to 0.3 A and volt-
`age up to 3 kV.
`The frequency parameters of the pulsed supply unit
`were chosen in accordance with the increase in time of
`the quasi-stationary plasma density formation and the
`times of the ionization instability and ionization~over—
`heating instability development. Designing the unit, We
`
`PLASMA PHYSICS REPORTS
`
`Vol. 21
`
`No. 5
`
`1995
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`--"Illllllflllllilllllllllllfl
`
`VW
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`
`5‘ yr
`
`
`
`
`
`Fig. 1. Discharge device configurations: (a) planar magne-
`tron;
`(b)
`shaped-electrode configuration.
`(1) Cathode;
`(2) anode; (3) magnetic system.
`
`took into account the dependences which had been
`obtained in [8] of ionization relaxation on pre-ionization
`parameters, pressure, and pulse voltage amplitude.
`In addition, we allowed for the fact that the development
`time for the ionization—overheating instability was about
`10‘3 — 3 x 10”3 s in the pressure range up to 0.5 torr [9].
`Thus, the supply unit was made providing square volt-
`age and current pulses with raise times (leading edge)
`of 5 — 60 us and durations of as much as 1.5 ms. Short—
`circuit current amplitude was up to 3 kA; no—load volt—
`age was as much as 2.4 kV.
`
`For pre—ionization, we used a stationary magnetron
`discharge; the discharge current ranged up to 300 mA.
`We measured the discharge current—voltage characteris—
`tics (CVC) in a 10“3 — 10 torr pressure range and plasma
`parameters of the discharge at the symmetry center of
`the shaped—electrode system using a probe technique.
`We found out that only the regimes with magnetic field
`strength not
`lower than 400 G provided the initial
`plasma density in the 109 — 10H cm‘3 range. This initial
`density was sufficient for plasma density to grow when
`the square voltage pulse was applied to the gap. So we
`chose these regimes as pre—ionization regimes.
`
`

`

`402
`
`MOZGRlN et al.
`
`
`
`High-voltage
`supply unit
`
`
`
`
`
`Stationary
`discharge
`
`supply unit
`
`
`Fig. 2. Discharge supply unit.
`
`3. QUASI-STATIONARY DISCHARGE REGDVIES
`
`We studied the highwcurrent discharge in wide
`ranges of discharge current (from 5 A to 1.8 kA) and
`operating pressure (from 10‘3 to 10 torr) using various
`
`(a)
`
`1
`
`2a 2b
`
`3
`
`(b)
`
`1
`
`2a 2b
`
`3
`
`Fig. 3. Oscillograms of (a) current and (b) voltage of the
`quasi—stationary discharge (50 us per div., 180 A per div.,
`180 V per div.).
`
`U, V
`500
`
`gases (Ar, N2, SF6, He, and H2) or their mixtures of vari-
`ous composition (argon percentage in Ar—N2 and Ar—SF6
`composition ranged from 10 to 90%; He : H2 = 1 : 1).
`We investigated the planar-magnetron and cusped—mir-
`ror configurations varying the magnetic field strength.
`We obtained current-voltage characteristics of the dis-
`charge, time-integrated photographs of the discharge
`glow, and probe characteristics of the discharge plasma.
`We detected the particle flux from the plasma and mea-
`sured their intensities. As a result, we found out that
`a variety of regimes differing in discharge voltage, cur-
`rent range, and discharge space structure occurred.
`
`Figure 3 shows typical voltage and current oscillo—
`grams of the quasi—stationary discharge. Part 1 in the
`voltage oscillogram represents the voltage of the sta—
`tionary discharge (pre—ionization stage). Part 2a dis—
`plays the square voltage pulse application to the gap.
`At this stage, the plasma density grows and reaches its
`quasi—stationary value (part 2b); the discharge current
`also grows, and then both the discharge current and
`voltage attain their quasi~stationary values (part 3).
`The time it takes for the plasma density to reach its
`quasi-stationary value corresponds to the ionization
`relaxation time. For example, for argon, discharge when
`pre-ionization plasma density is about 109 — 10“ cm“3
`this time is about 50 us. Each point of the discharge
`characteristic represents a pair of voltage and current
`oscillograms. We detected inhomogeneity of the dis—
`charge plasma or cathode spots visually, using filters,
`or by photographing the discharge.
`
`The current—voltage characteristic of the low-pres—
`sure quasi—stationary discharge in a magnetic field had
`four different parts corresponding to stable forms of the
`discharge. Figure 4 shows a typical CVC of the dis—
`charge in argon at 10“1 torr pressure and 0.4 kG mag-
`netic field. One can differentiate two parts: part I cor—
`responds to the magnetron discharge with current up to
`0.2 A and voltage range from 260 to 280 V; part 4 cor—
`responds to the high—current low-voltage arc discharge
`of current greater than 1 kA and 10 — 30 V voltage with
`a cathode spot. In addition, we found out experimen—
`tally that two other stable forms of quasi—stationary dis—
`charge could exist. Both the plasma and cathode layer
`had a diffuse character at cathode current density much
`higher than that of typical magnetron discharge. If the
`discharge current ranged from 0.2 to 15 A, a high—cur—
`rent magnetron discharge having initial discharge char-
`acteristics was observed (part 2 of the oscillogram).
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`PLASMA PHYSICS REPORTS
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`Vol. 21
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`No. 5
`
`1995
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`100
`
`400
`
`300
`
`200
`
`0
`
`0.1
`
`1
`
`10
`I, A
`
`100
`
`1000
`
`Fig. 4. Current-voltage characteristic of the quasi—stationary
`discharge with shaped electrodes in argon, p = 0.1 torr; B =
`0.4 kG.
`
`

`

`HIGH-CURRENT LOW—PRESSURE QUASI—STATIONARY DISCHARGE
`
`403
`
`In this case the discharge voltage was rather high,
`approximately 350 - 500 V. If the current was increased
`and ranged from 15 to 1000 A, a diffuse regime of high-
`current discharge was observed (part 3); its CVC was
`a straight line parallel to the current axis. The discharge
`voltage was about 90 V over
`the current
`range.
`The cathode current density was about 50 A/cmz.
`It should be noted that the boundaries of regimes
`could vary depending on the discharge conditions, e.g.,
`on pressure, magnetic field strength, etc. Then, we
`studied regimes 2 and 3 separately to determine the
`boundary parameters of their occurrence, such as cur—
`rent, voltage, pressure, and magnetic field.
`We studied the regimes both in the planar magne—
`tron and shaped—electrode system geometries and
`found out that both regimes could occur regardless of
`the type or particular parameters of the discharge con—
`figuration.
`Figure 5a exhibits representative CVC of the high-
`current magnetron discharge. They were measured in the
`discharge in Ar and N2, as well as in or Ar—N2 (10 — 90%
`of argon) or He : H2 = l : 1 mixtures at 10‘3 - 10 torr
`pressure range and 0.4 - 1.0 kG magnetic field. The
`cathodes we used were made of Cu, Ti, Al, M0, or
`stainless steel. To reduce the effect of cathode surface
`quality on the discharge parameters,
`the electrodes
`were preconditioned by multiple discharges or cleaned
`by glow discharge in argon. The dependence Ud(Id)
`remained qualitatively the same for all values of were
`the pressure p, transverse magnetic field BI, sort of the
`gas, cathode material, electrode configuration and dis-
`charge size. The discharge voltage increased monotoni-
`cally with current up to a maximum UT” = 500 — 1100 V
`depending on the magnetic field strength, sort of the
`gas, and cathode material. Then the discharge trans—
`ferred to regime 3 or to the arc regime. If the voltage
`pulse duration 1 was less than 20 ms, the current of
`transition amounted to 250 A, which corresponded to
`25 A/cm2 cathode current density j. A decrease in mag-
`netic field strength resulted in an increase in the dis—
`charge voltage UTYBL) up to some value UL1 depend-
`ing only on the cathode material and sort of the gas.
`A further decrease in B, caused the discharge to transit
`to a high—voltage regime which was characterized by
`a steep CVC and low discharge current (about 1 A).
`As the decreasing magnetic field approached the
`value of the discharge transit
`to the high-voltage
`regime, the discharge voltage increased smoothly, and
`the discharge current decreased.
`We measured the CVC of the high-current magne—
`tron discharge for two different discharge diameters.
`The CVC turned out to be independent of the diameter
`in the max B, region. It should be noted that, being
`transferred to the high-current regime, the discharge
`expands over a considerably larger area of the cathode
`surface than it occupied in the stationary pre-ionization
`regime. In the case of the planar magnetron, the dis—
`
`PLASMA PHYSICS REPORTS
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`Vol. 21
`
`No. 5
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`1995
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`U, V
`1000
`
`100
`
`200
`
`O
`
`10
`
`100
`I, A
`
`1000
`
`Fig. 5. (a) High-current magnetron discharge: (1) planar
`magnetron, Cu, p = 5 X 10'3 torr, Ar; (2) planar magnetron,
`Ti, p = 5 x 10‘3 torr, Ar : N2 = 4 : l; (3) planar magnetron,
`Ti, p = 10‘2 torr, N2; (4 and 5) shaped—electrode system, Cu,
`p = 5 x 10*2 torr, He : H2 =1 : 1, and Cu,p =10'1torr,Ar.
`(b) High-current diffuse regime: (I and 2) shaped—electrode
`system, Cu,p :1 ton, He : H2 :1 : land Cu,p = 10‘1 torr,
`He : H2 = 1 : l; (3) planar magnetron, Cu, p = 10‘1 torr, Ar;
`(4) planar magnetron, Cu, p = 10‘1 torr, Ar: SFG = 4 : 1.
`
`charge occupied either the ring area beyond the circular
`region of max B, or the disk area bounded by the circle
`of max 3,; the area depended on the magnetic field con—
`figuration. Because the discharge current was the same
`in both cases, the-current densities differed consider—
`ably, but the CVCs were similar. The current density
`values characteristic of these regimes for the argon dis-
`charges werej = 4 A/cm2 (Ud = 540 V, Id = 225 A) and
`j = 25 A/cm2 (Ud = 500 V, 1,, = 218 A).
`
`The roughness of the cathode surface was not
`important for the occurrence of regime 2, though the
`probability of discharge transit to the arc discharge was
`greater for the cathodes with rougher surfaces.
`A feature of the shaped-electrode discharges in the
`He—Hz mixture was a second form of high-current
`magnetron regime at a 400 ~ 650 V discharge voltage
`that was independent of discharge current until trans—
`ferring to regime 3 .
`Regime 2 was characterized by an intense cathode
`sputtering due to both high energy and density of ion
`flow. To study the sputtering, We used a probecollector
`placed 120 mm from the cathode. The pulsed deposition
`rate of cathode material (copper was used) turned out to
`be about 80 urn/min in the argon discharge, 1,, = 65 A,
`Ud = 900 V. The current pulse duration was 25 ms, and
`
`

`

`404
`
`MOZGRIN et al.
`
`the repetition frequency was 10 Hz, which corre-
`sponded to =20 urn/min averaged deposition rate.
`We used a scanning electron microscope REM—101
`(Russian trade mark) to measure the thickness of
`deposited layers.
`
`We measured the plasma density n,» in the region
`near the collector by applying to the collector a pulse of
`biased voltage with respect to the anode. The density
`turned out to be about 3 x 1012 cm‘3 in the regime of
`Id =60AaIld Ud=9OOV.
`
`Figure 5b presents typical CVCs of high—current dif-
`fuse discharge measured at various pressures, gases,
`cathode materials, magnetic fields, and pre—ionization
`parameters. Analyzing the CVCs, we found out that the
`discharge voltage weakly depended on the magnetic
`field geometry and its strength, and on the cathode
`material; the constant voltage turned out to range from
`70 to 140 V as the current ranged from 5 to 1800 A.
`The voltage was slightly (within 50 V) changed from
`gas to gas. Transferring to regime 3, the discharge occu—
`pied a significantly larger cathode surface than in the
`stationary regime.
`
`The parameters of the shaped-electrode discharge,
`transit to regime 3, as well as the condition of its transit
`to are regime 4, could be well determined for every
`given set of the discharge parameters. The point of the
`planar—magnetron discharge transit to the arc regime
`was determined by discharge voltage and structure
`changes; the structure changes were recorded by opti-
`cal diagnostics. To study the structure of the discharge
`in regime 3, we photographed it using neutral light fil-
`ters of various attenuation factors. The filters and eXpo-
`sure times were chosen so that the pre-ionization dis—
`charge glow was not recorded. One can see from the
`photographs presented in Fig. 6 that the discharge was
`spatially uniform even at about 1 kA discharge cur-
`rents. If the current was raised above 1.8 kA or the
`pulse duration was increased to 2 — 10 ms, an instability
`development and discharge contraction was observed.
`The planar—magnetron discharge transfer to regime 3
`resulted in a smearing of the annular structure of the
`pre-ionization discharge: the discharge plasma and cur-
`rent area were seen to expand and cover the whole cath—
`ode surface (Fig. 6). If the discharge current or pulse
`duration were increased,
`the instability development
`accompanied by the plasma column contraction and the
`occurrence of one of several cathode spots were also
`observed in the planar magnetron.
`
`Chemical analysis of the collector surface layer was
`done;
`the cathode material was not detected there.
`Hence,
`there was no cathode sputtering in these
`regimes.
`
`We elaborated on a pulsed probe technique specially
`designed to measure the plasma parameters in regime 3.
`The technique provided probe characteristics to be
`measured in =10 us time intervals and allowed the
`probe current to amount to 50 A [10].
`
`We measured the parameters of pulsed high—current
`quasi—stationary discharge in a cusp magnetic—field
`configuration with B ranging from zero to 1 kG in var—
`ious gases. The pressure ranged from 10‘1 to 1 torr; the
`discharge current ranged up to 1500 A. The pulse volt—
`age applied to the probe was 100 ~ 500 us delayed with
`respect to the discharge current pulse, i.e., TL, and n,
`were measured after the establishment of the quasi—sta—
`tionary regime of the high-current discharge.
`The plasma parameters were determined from the
`probe measurements. Ion density measured at the sys—
`tem center in regime 3 in argon increased almost lin-
`early with the discharge current at various pressures
`and magnetic field strengths. The density ranged from
`(2 — 2.5) X 10M cm‘3 at 360 - 540 A current up to
`(l - 1.5) X 1015 cm’3 at 1100 — 1400 A current. The max—
`imal plasma density of high—current diffuse discharge
`in argon was measured to be n,- z 1.5 X 10‘5 cm’3, while
`the electron temperature T, was 4 - 6 eV, the discharge
`current was 1100 A, magnetic field strength B was
`0.8 kG, and the pressure p was about 0.2 torr. The ion
`saturation current of the probe jg,“ was about 11 A/cmz.
`Ion density increased with pressure;
`the density
`increase was accompanied by a decrease in the electron
`temperature.
`
`The plasma density in He—H2 discharge also
`increased with the discharge current. However, the max—
`imum of ion density was n, = 2.4 x 1014 cm‘3 at the con—
`ditions similar to those mentioned above: p = 1.5 torr,
`B = 0.8 kG, Id 21100 A.
`
`4. DISCUSSION
`
`We obtained a generalized CVC of the quasi-station-
`ary low—pressure discharge in a magnetic field (Fig. 7)
`based on a variety of measured AV discharge character—
`istics under various conditions. Parts I and 4 correspond
`to stationary magnetron and are discharges, respectively.
`They were inherent in the discharge throughout the pres—
`sure and magnetic field ranges. These two regimes were
`comprehensively described in [1, 11].
`Part 2 pertains to the high—current magnetron dis—
`charge regime occurring in the 0.2 - 250 A current
`range. The voltage increased with current up to some
`critical value of current and then became constant.
`The discharge voltage was rather high — up to 1.2 kV.
`The discharge had a greater probability of being real-
`ized if the pressure ranged from 2 X 10‘3 to 10‘1 torr.
`We suggested that this discharge was structurally
`very close to the high—current discharge described in [4].
`The reasons are the following: both the pressure and
`magnetic field ranges were almost the same, the dis—
`charge did not exhibit contraction, and their CVCs
`were very similar. However,
`the discharge we dealt
`with had a higher discharge voltage (500 - 1200 V) than
`the 300 ~ 500 V discharge described in [4]. Hence, one
`could expect
`the cathode sputtering to have more
`importance.
`
`PLASMA PHYSICS REPORTS
`
`Vol. 21
`
`No. 5
`
`1995
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`

`

`HIGH—CURRENT LOW—PRESSURE QUASI-STATIONARY DISCHARGE
`
`405
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`Fig. 6. High-current quasi-stationary discharge regimes. (a) planar magnetron: (1) high-current magnetron regime (p = 5 x 10’3 torr,
`Ar, Id = 70 A, Ud = 900 V); (2) high-current diffuse regime (p = 10“1 torr, Ar, Id = 700 A, Ud = 80 V); (3) are regime (p = 10“1 torr,
`Ar, Id = 1000 A, Ud = 45 V). (b) Shaped-electrode system: (I) high—current diffuse regime (p = 10”l tort, Ar, 1d = 1000 A, Ud = 90 V);
`(2) contracted arc regime (p = 10" torr, Ar, 1,, = 1500 A, Ud = 50 V).
`
`PLASMA PHYSICS REPORTS
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`Vol. 21
`
`No 5
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`1995
`
`

`

`MOZGRIN et al.
`
`406
`
`U, V
`500— 1000
`
` 0
`
`1000 —1800 1K
`
`Fig. 7. Generalized ampere-voltaic characteristic CVC of
`quasi-stationary discharge.
`
`We estimated the steady—state average density of the
`cathode material atoms 12, in the discharge plasma; the
`estimates were based on measured current density and
`initial working gas density ng. Using a one-dimensional
`continuity equation, it is possible to derive the follow—
`ing equation for no:
`
`t
`
`
`an,
`at
`
`+n~S
`c ‘—
`
`g—c
`
`
`VT
`n g+lS [in V:
`g K<Vc>
`4 c—c
`c c<Vc>i
`
`
`
`(1)
`
`where $8-, and SC_c are sputtering and self—sputtering
`factors of the cathode, respectively; V; and V: are ther—
`mal velocities of working gas atoms and cathode mate-
`rial atoms, respectively;
`(V0)
`is the angle—averaged
`emitted atom velocity—component perpendicular to the
`cathode surface; B8 and BC are ionization degrees of gas
`atoms and cathode material atoms, respectively.
`The steady—state solution of (1) is as follows:
`
`
`n V;
`1 S
`g V
`4 3—6
`n. = __1.._..<..‘_/T_
`
`1 —— ,1chch (V6)
`6
`
`g
`
`As an example, we considered the copper-cathode
`argon discharge at 10~2 torr, Id = 65 A, and U,1 = 900 V
`
`and estimated the copper fraction in its plasma. The
`fraction turned out to be about 30% and increased with
`
`ionization degree.
`
`Table 1 presents the parameter ranges correspond—
`ing to regime 2. The presented parameters are limiting
`values that could be independently realized.
`
`If the discharge current ranged from 10 to 1800 A,
`high-current diffuse regime 3 occurred. The voltage
`ranged from 70 to 140 V depending on the working
`gas sort. Regime 3 could be primarily realized in the
`10“2 — 5 torr pressure range no matter what the dis-
`charge electrode configuration, sort of working gas, or
`cathode material. Moreover, pre—ionization was not
`necessary; however, in this case, the probability of dis—
`charge transferring to are mode increased. The cathode
`current density of the high—current diffuse discharge
`amounted to 75 A/cmz.
`
`Table 2 presents the parameter ranges correspond-
`ing to regime 3 for both electrode configurations: vari—
`ous cathode materials, and various pressures and sorts
`of gases.
`
`It should be emphasized that the limiting values of
`regime 2 and regime 3 parameters were highly depen—
`dent on current pulse duration. The transit current val-
`ues increased with a decrease in the pulse duration.
`
`The process of the high—current magnetron dis-
`charge passing to a low—voltage regime seemed likely
`to resemble the anomalous glow discharge at moderate
`or high current densities [11]. In this case both the gas
`heating and electron density increase were important;
`these factors promoted both a partial equilibrium in the
`discharge plasma, and the transfer of the discharge to
`a regime of lower discharge voltage.
`
`The simplified quasi—stationary equation for gas
`temperature of regime 2 has the form [9]:
`(2)
`(Tg— To)MngvT = Pg,
`where P8 is the power consumed to heat the gas; Tg is
`the established gas temperature; To is the electrode tem—
`perature; Mg is the mass of the gas in the discharge vol—
`ume; CF is the heat capacity at constant pressure; VT =
`
`
`Table 1. Existence conditions and regimes of high-current magnetron discharge: (I) shaped-electrode system; (H) planar magnetron
`
` Discharge de—
`B, kG
` Pressure, torr
`Voltage, V
`
`
`
`
`
`10-2 — 10-1
`260 — 990
`Ar
`
`115/112
`5 x 10-2 — 1
`320 - 950
`0.2 — 36
`
`
`
`50/50%
`
`
`He/H2
`0.8 — 1.0
`5 x 10-2 — 1
`400 - 650
`
`
`50/50%
`
`0.2 - 72
`Ar
`0.3 — 0.7
`10—3 - 5 x 10"
`540 — 990
`
`
`
`
`0.2 — 250
`Ar
`03 — 0.7
`10—3 - 5 x 10-1
`540 — 1100
`
`
`
`
`0.3 - 0.7
`10-3 - 5 x 10-1
`540 - 720
`0.2 — 180
`N2
`
`
`
`
`Ar/N2
`0.3 — 0.7
`10-3 - 5 x 104
`540 - 900
`0.2 — 140
`
`
`
`
`90 — 10% Ar
`
`
`Cathode mate-
`rial
`
`Gas
`
`vice type
`
`11
`
`
`
`Current, A
`
`9 — 120
`
`PLASMA PHYSlCS REPORTS
`
`Vol. 21
`
`No. 5
`
`1995
`
`

`

`I-HGH—CURRENT LOW—PRESSURE QUASI-STATIONARY DISCHARGE
`
`407
`
`Table 2. Existence conditions and regimes of high—current diffuse discharge: (1) shaped—electrode system; (H) planar magnetron
`
`
`
`Dlsfgftgggc- Cathogzlmate
`
`
`Cu
`
`
`
`
`
`
`
`
`
`
`4 - 1600
`90 — 135
`0.3 — 0.7
`SF6
`Mo
`
`
`18 ~ 1200
`90 — 135
`0.3 v 0.7
`Ar/SF6
`stainless
`
`
`80/20%
`steel
`
`
`10—2-5
`
`Voltage, V
`80—110
`
`80 ~ 120
`
`
`Current, A
`
`
`15 - 1500
`7 — 1200
`
`70 - 120
`
`7 - 1800
`
`65 - 90
`65 — 90
`
`90 — 135
`
`4 — 800
`4 ~ 1200
`
`18 - 1800
`
`II
`
`Cu
`
`Cu
`
`Cu
`Mo
`
`Mo
`
`Gas
`Ar
`
`Ar/SF6
`80/20%
`
`lie/Hz
`50/50%
`
`Ar
`Ar
`
`Ar/SF6
`80/20%
`
`B, kG
`04— 1.0
`
`0.4 — 1.0
`
`0.8 — 1.0
`
`03 - 0.7
`0.3 ~ 0.7
`
`0.3 — 0.7
`
`
`
`
`
`/A2 is the fre uenc characteristic of the heat evacu—
`X
`r
`q
`Y
`ation; x = k/(Mnng) is the temperature conductivity;
`AT is a characteristic heat evacuation length; 7» is the gas
`thermal conductivity at the established temperature;
`M is atomic mass; and ng is the gas density. The gas
`energy balance equation takes into account the dis—
`charge geometry, so the actual electrode profile was
`substituted for a plane one with area S and interelec-
`trode distance L corresponding to the operating area in
`the regimes involved. As far as the plane discharge
`layer of 2L thickness is concerned, the value of AT is
`about L/Tt. The power consumed for gas heating can be
`evaluated from the energy balance equation:
`
`Id
`(3)
`Pg = IdUd~;fi,
`where IdUd is the total power released in the discharge
`volume; Id/en is the power consumed for ionization;
`e is the electron charge; 1/1] is the Stoletov constant
`corresponding to the value of E/n characteristic of the
`cathode layer.
`The discharge conditions typical for the discharge tran-
`sit from regime 2 to regime 3 are as follows: 1,, z 15 A;
`Ud = 300 V; p = 1 torr; B = 0; argon was used as a work—
`ing gas; L = 1 cm; and S 2 60 cm2. Under these condi-
`tions, the gas temperature could increase to Tg = 1.1 eV.
`It follows from (2) that
`
`n, > ng'. The value of nfi’ can be determined experi—
`mentally; it depends on the excitation energy of the first
`excited level.
`
`If the cylindric layer is considered, the averaged rate
`of charged particle rate due to diffusion can be
`described as follows [9]:
`
`(
`
`DA
`_
`an
`3}- dif — DAAn = —-VDn z ——A—2n,
`
`(5)
`
`where VD is the diffusion frequency; A is a character—
`istic length of diffusion depending on the discharge
`geometry. In the presence of a magnetic field, A is
`dictated by the highest of the frequencies determin—
`ing the particle loss at
`the electrodes VD, =
`2
`(T,+ Ti) (vea+ve,.)
`7t
`——-2—————~———- (-) n, and at the lateral sur—
`mwe + (Vea + Vei) “'me L
`.
`(T. + T.)
`2.4 2
`face of the d1scharge volume vDs = ——u—v—— (7%—
`!“
`Ill
`[12]. Here, R is the discharge radius; (1),, is the electron
`cyclotron frequency; v”, vgi, and Via are the frequencies
`of electron—atom, electron—ion, and ion—atom colli-
`sions, respectively; um is the reduced mass. One can
`compare the frequencies of diffusion, ionization, and
`recombination in argon discharge and, then, find out of
`establishing a detailed balance the possibility in the dis-
`charge plasma.
`
`Id
`1 U — —
`213;“.
`x
`
`’
`
`(4)
`
`The estimates of the plasma density, in the case T =
`T3 = T, 2 1.1 eV, show that, according to {9], the follow—
`ing relation is valid:
`
`i.e., that the gas temperature does not depend on gas
`density. The action of the magnetic field serves only to
`limit the electron thermal conductivity and to provide
`collisions sufficient for efficient energy transfer from
`electrons to heavy particles.
`According to [11], one can evaluate the ionization
`degree from the Saha equation if ambipolar diffusion (A)
`is negligible, i.e., if plasma density is sufficiently high:
`
`
`a2
`gi
`21cm
`—
`1 — a2 —
`gg
`
`.h2
`
`eU.
`3/2 (kT) 3/2
`.
`) MCXP {____ .
`
`(6)
`
`where g, = 6 (Ar) and g, = 1 are the statistical weights
`of ions and atoms, respectively; U, is the argon ioniza-
`tion potential. The ionization degree or = 12,, /(ng + n,)
`ranges from 0L 2 1 (p = 0.01 torr) to 0!. = 0.7 (p = 1 torr),
`
`PLASMA PHYSICS REPORTS
`
`Vol. 21
`
`No. 5
`
`1995
`
`

`

`408
`
`MOZGRIN et a1.
`
`the plasma density in the discharge passing to
`i.e.,
`regime 3 exceeds n6 2 5 x 1013 cm’3.
`
`plasma. Then, the positive space charge layer thickness
`at the cathode surface is determined as follows [9]:
`
`8 1/2 M 1/4
`l = —
`—-
`.
`(9)
`(m)
`
`
`eUC 3/4
`kTe
`(kT,
`47:92",
`
`[
`
`>
`
`1/2
`
`.
`
`j
`
`(9)
`
`surfaces he =
`
`of electrons moving in
`
`We compared the value of 16, which corresponds to
`the electron density of the discharge transfer to regime 3,
`with electron free path lengths 7cm and he, (high pres—
`sures) and with the trajectory height over the cathode
`
`2m62 (EC)
`
`B?
`e
`crossed fields (low pressures) [13]. We determined that
`the above suggestion