Plasma Physics Reports, Val. 21. No. 5, 1995, pp. 400 - 409. Translatedfrom Fizika Plazmy, Vol. 21, No. 5, 1995, pp. 422 - 433.
`Original Russian Text Copyright © 1995 by Mozgrin, Ferirov, Khodachenko.
`
`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 10” cm‘3 in argon discharges and amounted to 5 X 10” cm‘3 in He-H2-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. INTRODUCTION
`
`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 -—-
`l0“" - 5 X lO"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,, = kUf,‘,
`where 1,, is the discharge current, and U, 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 jc does not exceed 0.03 A/cm2.
`In this case the discharge voltage amounts to 400 - 600 V,
`the plasma density n,- ranges from 103 to 10“ cm'3, and
`electron temperature T, reaches 20 eV. If the current
`density is higher, a transition of the discharge into the
`arc 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 U4, 60 its 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 104 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 (U, = 100 V, 1,, S 1.5 kA)
`of longer than 1-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-stationary
`discharge in crossed fields of various geometry and to
`determine their parameter ranges. We investigated the dis-
`charge regimes in various gas mixtures at 104 - 10 torr,
`B0 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
`
`1063-78OX/95/2105-0400$l0.00 © 1995 MAI/IK Hayxa /Interperiodica Publishing
`
`GILLETTE 1203
`
`GILLETTE 1203
`
`

`
`HIGH-CURRENT LOW-PRESSURE QUASI-STATIONARY DISCHARGE
`
`401
`
`(6)
`
`
`
` %,!§
`
`
`
`
`
`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 lO'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 its 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'?‘ - 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 10° - 10" 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.
`
`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 z (Fig. l) and used
`two types of permanent magnets made of SmCo5 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 SmCo5 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
`B,,,,x(0, r) was about 3. The values of Bmax 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" 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 l<J 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
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`1995
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`MOZGRIN et al.
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`supply unit
`
`
`High-voltage
`
`
`
`supply unit
`
`Fig. 2. Discharge supply unit.
`
`3. QUASI-STATIONARY DISCHARGE REGIMES
`
`We studied the high—current discharge in wide
`ranges of discharge current (from 5 A to 1.8 kA) and
`operating pressure (from l0‘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 its per div., 180 A per div.,
`180 V per div.).
`
`U, V
`500
`
` 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.
`
`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" 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 RA 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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`HIGH-CURRENT LOW-PRESSURE QUASI-STATIONARY DISCHARGE
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`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/cm2.
`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 = 1 : 1 mixtures at l0‘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 BL, sort of the
`gas, cathode material, electrode configuration and dis-
`charge size. The discharge voltage increased monotoni-
`
`cally with current up to a maximum U?“ = 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 17 was less than 20 ms, the current of
`transition amounted to 250 A, which corresponded to
`25 A/cmz cathode current density j. A decrease in mag-
`netic field strength resulted in an increase in the dis-
`charge voltage UZ‘"*(BQ up to some value U3 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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`No. 5
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`1995
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`U, V
`1000
`
`
`100
`
`200
`
`0
`
`10
`
`100
`I, A
`
`1000
`
`Fig. 5. (a) High-current magnetron discharge: (1) planar
`magnetron, Cu, p = 5 X lO'3 torr, Ar; (2) planar magnetron,
`Ti, p = 5 X l0‘3 torr, Ar : N2 = 4 2 1; (3) planar magnetron,
`T1, p = 10"2 torr, N2; (4 and 5) shaped-electrode system, Cu,
`p = 5 x 10'2torr, He : H2: 1 : 1, and Cu,p =10‘1torr,Ar.
`(b) High-current diffuse regime: (I and 2) shaped—electrode
`system, Cu, p = 1 torr, He: H2 = l : l and Cu, p = 10" torr,
`He : H2 = 1 : 1; (3) planar magnetron, Cu, p = 1O“1 torr, Ar;
`(4) planar magnetron, Cu, p = 10‘! torr, Ar: SF6 = 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 B,; 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 were j = 4 A/cm2 (Ud = 540 V, I,,, = 225 A) and
`j = 25 A/cm2 (U,, = 500 V, I4 = 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—H2 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 um/min in the argon discharge, 1,, = 65 A,
`U4 = 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 um/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 10” cm‘3 in the regime of
`1,, =6OAand U,,=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 RG in var-
`ious gases. The pressure ranged from 10“ to 1 tom‘; 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., T, 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 10” cm‘3 at 360 - 540 A current up to
`(1 - 1.5) X 10” cm’3 at 1100 - 1400 A current. The max-
`imal plasma density of high-current diffuse discharge
`in argon was measured to be n, = 1.5 X 1015 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 jm 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 10” cm‘3 at the con-
`ditions similar to those mentioned above: p = 1.5 torr,
`B = 0.8 kG, I, =1100 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 1 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" 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
`
`

`
`HIGH-CURRENT LOW-PRESSURE QUASI-STATIONARY DISCHARGE
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`405
`
`Fig. 6. High-current quasi-stationary discharge regimes. (a) planar magnetron: (1) high-current magnetron regime (p = 5 x l0'3 torr,
`Ar, 1,, = 70 A, u,, = 900 v); (2) high-current diffuse regime (p = 10*‘ torr, Ar, 1,, = 700 A, u,, = 30 V); (3) are regime (p = 10-‘ torr,
`Ar, Id = 1000 A, Ud = 45 V). (b) Shaped-electrode system: (1) high-current diffuse regime (p = 10" torr, Ar, Id = 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
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`No. 5
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`1995
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`

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`
`U, V
`500 — 1000
`
`MOZGRIN et al.
`
`
`
`4
`-----------------
`15 — 225
`1000-1800 13
`
`70 — 170
`15 — 45
`0
`
`Fig. 7. Generalized ampere-voltaic characteristic CVC of
`quasi-stationary discharge.
`
`We estimated the steady-state average density of the
`cathode material atoms nc 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 n,:
`
`1‘
`
`Bnc
`at +nc '— Sg—c gng
`
`V:
`
`V67
`1
`+ ZSc—cB¢n'c (V0):
`
`where Sg_c 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;
`(V6)
`is the angle-averaged
`emitted atom velocity-component perpendicular to the
`cathode surface; [33 and Be are ionization degrees of gas
`atoms and cathode material atoms, respectively.
`The steady-state solution of (1) is as follows:
`
`1
`V§
`n = ZS1:-0 gne<Vc)
`c
`1
`V:
`1—_s _c
`4 0 B-0/.>
`
`'
`
`As an example, we considered the copper-cathode
`argon discharge at 10" torr, Id = 65 A, and Ud = 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
`104 - 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]:
`
`(T, To)MgC,,vT = Pg,
`
`(2)
`
`where Pg is the power consumed to heat the gas; T3 is
`the established gas temperature; To is the electrode tem-
`perature; M3 is the mass of the gas in the discharge vol-
`ume; Cp is the heat capacity at constant pressure; v,- =
`
`Table 1. Existence conditions and regimes of high-current magnetron discharge: (I) shaped-electrode system; (11) planar magnetron
`
`
`
`Discharge de-
`vice type
`
`
`
`
`
`
`r al
`i
`
`u
`
`C
`
`Cu
`
`Cu
`
`I
`
`11
`
`
`
`Ar
`
`10-2 — 10-‘
`
`260 - 990
`
`5 x 10-2 — 1
`
`320 — 950
`
`5 x 10-2 — 1
`
`400 - 650
`
`
`
`0.2 — 36
`
`9 - 120
`
`He/H2
`50/50%
`
`He/H,
`50/50%
`
`
`
`
`
`Cu
`10-3 — 5 x 10-‘
`540 - 990
`Ti
`540 - 1100
`10-3 — 5 x 10-1
`Ti
`10-3 — 5 x 10-‘
`540 - 720
`Ti
`10-3 — 5 x 10-‘
`540 - 900
`
`
`0.2 — 72
`0.2 - 250
`0.2 — 180
`0.2 - 140
`
`
`
`
`
`
`
`
`
`PLASMA PHYSICS REPORTS
`
`Vol. 21 No.5
`
`1995
`
`

`
`HIGH—CURRENT LOW-PRESSURE QUASI-STATIONARY DISCHARGE
`
`407
`
`Table 2. Existence conditions and regimes of high—current diffuse discharge: (I) shaped-electrode system; (H) planar magnetron
`
`
`
` Voltage, V
`
`15 - 1500
`
`7 -1200
`
`
`
`l
`
`0'2 ~ 5
`2 x10‘2 - 3
`
`
`
`rial
`
`Cu
`C
`
`u
`
`Cu
`
`Cu
`Mo
`Mo
`
`Ar
`
`Ar/SF5
`80/20%
`
`He/H2
`50/50%
`
`Ar
`Ar
`Ar/SF6
`80/20%
`
`Discharge de-
`vice type
`
`
`
`I
`
`11
`
`
`
`
`
`7 - 1300
`
`4 - 300
`4-1200
`18 - 1800
`
`4 - 1600
`13 - 1200
`
`
`5 x 10-2 - 3
`
`10-2 - 1
`5x10-2-1
`10“2 - 1
`
`
`
`
`
`10-2 — 5 x 10-‘
`10-2 - 1
`
`SF6
`Mo
`Ar/SF6
`stainless
`
`steel 80/20%
`
`
`
`x/ A; is the frequency characteristic of the heat evacu-
`ation; x = X/(MngC,,) 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 intere1ec-
`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/71:. The power consumed for gas heating can be
`evaluated from the energy balance equation:
`
`In
`P3 = IdU,,— E,
`
`(3)
`
`where I,,U,, 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,, = 15 A;
`U,, = 300 V; p = 1 torr; B = 0; argon was used as a work-
`ing gas; L = 1 cm; and S ~—~ 60 cm2. Under these condi-
`tions, the gas temperature could increase to T3 = 1.1 eV.
`It follows from (2) that
`
`n, > nfi‘. The value of n‘;' 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]:
`
`a
`(a—’:)
`
`dif
`
`= DAAn = —v,,n =
`
`_ -3",
`
`(5)
`
`where VB 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, =
`T + T.
`v +v .
`
`2
`
`(g) n, and at the lateral sur-
`
`9‘)
`ea
`I) (
`8
`(
`in
`la
`mcofi + (veg + V2,.) 1.1 v.
`
`face of the discharge volume VD: =
`
`(Te+
`R
`uiavia
`[12]. Here, R is the discharge radius; (0, is the electron
`cyclotron frequency; V“, v,,-, and V,-,, are the frequencies
`of electron—atom, electron—ion, and ion—atom colli-
`sions, respectively; p.,-,, is the reduced mass. One can
`compare the frequencies of diffusion, ionization, and
`recombination in argon discharge and, then, fmd out of
`establishing a detailed balance the possibility in the dis-
`charge plasma.
`
`T -—-
`‘
`
`Mg CPVT
`
`———,
`7»
`
`(4)
`
`The estimates of the plasma density, in the case T =
`T8 = T,- = 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:
`
`PLASMA PHYSICS REPORTS
`
`Vol. 21
`
`No. 5
`
`1995
`
`a2
`1—oc2=
`
`gi znm 3/2 (kn 3/2
`)
`
`g,,
`
`h2
`
`ng+n,.
`
`eU,.
`exp {—— (6)
`H.
`
`where g,- = 6 (Ar) and gg = 1 are the statistical weights
`of ions and atoms, respectively; U, is the argon ioniza-
`tion potential. The ionization degree or = n,/(ng + n,.)
`ranges from ct = 1 (p = 0.01 torr) to OL = 0.7 (p = 1 torr),
`
`

`
`408
`
`MOZGRIN et al.
`
`the plasma density in the discharge passing to
`i.e.,
`regime 3 exceeds 12,, 2 5 X 1013 cm'3.
`
`plasma. Then, the positive space charge layer thickness
`at the cathode surface is determined as follows [9]:
`
`Z: _
`
`(9)
`
`.
`
`m
`
`3/4
`
`1:
`
`kg)
`
`e
`
`(47%)
`
`(3
`
`l/2
`
`.
`
`<9)
`
`According to [9], the thermal ionization requires far
`less discharge voltage than electron impact ionization
`for current sustainment. This appears to cause the dis-
`charge to transfer to the low-voltage regime.
`
`One can estimate both the gas temperature and asso-
`ciated ionization degree from (2) - (4) and energy bal-
`ance change. The balance equation for the power con-
`sumed by neutral gas heating in regime 3 is as follows:
`
`I
`3
`3
`Pg = IdU,,—;d(‘£,-+§kT_,+§kT,.),
`
`(7)
`
`where 3- is the ionization energy, because, unlike the
`case of (2), the fraction of energy lost due to charged
`particle escape increases with discharge current. Simi-
`larly to the column of ar