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Microwave excitation of large volumes of plasma at electron cyclotron resonance in multipolar confinement
Volume 106A, number 7
PHYSICS LETTERS
17 December 1984
MICROWAVE EXCITATION OF LARGE VOLUMES OF PLASMA AT ELECTRON CYCLOTRON RESONANCE IN MULTIPOLAR CONFINEMENT
L. POMATHIOD, R. DEBRIE l, Laboratoire de Physique et Chimie de I’Environnement, CNRS, 3A, Avenue de h Recherche Scientifique, 45045 Orlkans Cedex, France
and Y. ARNAL and J. PELLETIER Laboratoire de Physique des Milieux ionis&, CNRS-ERA 993, CMGCNETBP 98, 38243 Mtylan Cedex, fiance Received 25 July 1984 Revised manuscript received 5 October 1984
Very large volume (>2 m3) homogeneous maxwellian plasmas in the lo9 -lot0 cmW3density range have been easily obtained by using a microwave electron cyclotron resonance source operating at 2.45 GHz. The magnetic multipole device has proved its efficiency in confining plasmas containing no primary electrons.
Numerous experimental studies require large volumes of uniform, quiescent plasmas. Most often, plasmas are produced in a multipole magnetic confmement device by means of electron emission from a spaced array of filaments [ 11. At moderate neutral pressure, this yields uniform plasmas in the lOlo cm -3 density range. In this ionisation mode, uniformity of the plasma is attributed to the localisation of ionisation by primary electrons trapped in the cusped magnetic field near the walls of the multipole device [2,3]. Confinement in multipole devices likewise proves efficient for lower density plasmas produced by a localized source, a Kaufman source [4] for instance. The production of plasmas by emission of primary electrons from hot filaments presents some disadvantages, however. Contamination resulting from filament sublimation or radiation of thermal energy occurs; moreover, one cannot generally produce plasmas of reactive gases.
1 UER de Sciences, Universite d’Orl&ans, 45046 Orleans Cedex, France.
0.375.9601/84/$03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Our purpose in this letter is to show that largevolume, high-density, uniform plasmas can be obtained in a multipole magnetic device by use of a localized microwave source operating at electron cyclotron resonance (ECR). Such electrodeless excitation leads to high-density plasmas over a wide neutral pressure range. We report here the first results obtained in a large vacuum chamber (5.5 m3). A schematic drawing of the experimental arrangement is shown in fig. 1. Experiments were performed in the large vacuum chamber of the Laboratoire de Physique et Chimie de 1’Enviromrement. The inner diameter of the multipole confinement device, containing about 6500 permanent ceramic magnets, is 1.2 m and it is 2 m long. The alternating rows of north and south poles (magnet size 4 cm X 2 cm X 1 cm) are spaced 2.5 cm apart. The magnetic field at the center of the pole face is 1100 G. An aperture in the multipole device situated in front of the plasma source allows the plasma to spread into the chamber. Plasmas have been created at the exit of a small (10 cm diameter, 10 cm long), non-magnetic, cylindrical vessel of stainless steel mounted on a side window 301
Volume 106A, number 7
a) alumina window
PHYSICS LETTERS
2.45 GHz Dower waveguide discharge vessel magnetic coil gas inlet chamber wall
-/’
V
multipole structure
B field =B75gaus:
Fig. 1. Experimental arrangement. (a) The ECR microwave plasma source. (b) the large vacuum chamber and the multipole confinement structure fitted with the plasma source.
17 December 1984
krypton. With these two gases, plasma homogeneity is achieved at pressures below 2 X 10m4 Torr for which the ion mean free path becomes of the order of magnitude of the chamber size. At higher pressures, the decrease in density is linked to the poor diffusion of the plasma and the simultaneous rise of density gradients. Fig. 2 shows a typical Langmuir characteristic curve as measured by the futed probe in an argon plasma at a pressure of 8 X 10e5 Torr and under 800 W of power input. The curve is typical of a purely maxwellian plasma. As seen in the logarithmic plot, it is characterized by the absence of primary electrons and of the hot electron population, resulting from primary electrons, usually found in the tail of the thermal elec-
15 ;i
-
E
_
J?
of the chamber. Sealing is achieved through an alumina window which allows transfer of the microwave power (2.45 GHz) delivered to the plasma by a continuously variable output generator (up to 1.5 kW) via a waveguide structure fitted with two mobile short circuit terminations for impedance matching. The argon or krypton gas is introduced into the vessel by means of an adjustable microleak. The plasma source is completed by a coil in order to obtain at the source mouth (see fig. la) the magnetic field of 875 G necessary to reach ECR. If the ion mean free path is long enough with respect to the chamber size, plasma diffusion occurs inside the entire multipole structure. Because of the small spacing between the magnets (2.5 cm) the plasma cannot cross the magnetic walls and escape from the confinement structure. Plasma densities and electron temperatures are measured by means of two cylindrical tungsten Langmuir probes. These are 0.6 mm in diameter and 5 mm long; one is fxed in front of the plasma source along the chamber axis, the other is movable and is used to check the homogeneity of the plasma. Experiments have been carried out with argon and 302
E g’o
;5-
Fig. 2. Typical Langmuir characteristic curve measured using the fixed probe. Argon, 8 X 10e5 Torr, 800 W. Plasma parameters: n = 1.1 X 10” cmw3, Te = 25000 K. Electron temperature isedetermined by plotting the decimal logarithm of the electron current collected by the probe (Ze) as a function of the probe biasing voltage. In the linear part of this curve, we have 7’e = 5040 A V, where Te is the electron temperature in Kelvins and A V the variation in probe voltages (volts) corresponding to a variation of one decade in the current. Plasma density is measured by plotting the square of the electron current as a function of the voltage, for voltage higher than the plasma potential. For a cylindrical probe, the curve obtained is linear, with a slope A$/AV. We haven, = 3.31 X 10’ X (AZ2 /A V) “*A, where rre is the electron density in cmm3, A the area of the probe in mm*, and AZ,‘/A V in (PA)* /V.
8.10W5Torr
0 .
d-_
fixed probe o movable probe
l
10-5
10-4 pressure
. fixed probe o movable probe
_
microwave power (watts)
(torr)
3. Krypton plasma parameters as a function of the neutral pressure. The movable probe is located 60 cm from the
Fig.
fixed one, along the chamber axis.
trons of classical multipolar
17 December 1984
PHYSICS LETTERS
Volume 106A, number 7
plasmas. plasma density, 70 cm from the plasma source and along its axis, is high (1 .l X lOlo cm-8). The electron temperature is 25000 K (2.2 eV). Fig. 3 shows the parameters of a krypton plasma for a constant input power of 300 W as a function of neutral pressure. The plasma density and electron temperature were measured by the two probes, the movable one being located 60 cm from the futed probe along the chamber axis. The two probes indicate nearly identical plasma characteristic parameters over the 10-5-10-4 Torr pressure range. The plasma, produced by a small source located on a side of the chamber, is well confined and homogeneized over large distances by the multipole structure. Using the movable probe, we haved checked that a significant gradient in density only occurs near the magnetic “walls”. The plasma density, in the 1Og-lO1o cme3 range, reaches a maximum for a neutral pressure of about 1O-4 Torr. Note that the density goes a high as log cmm3 over a large volume for a microwave power input of only 300 W at a low neutral pressure of lob5 Torr. The electron temperature is low (~2 eV) and does not vary significantly with pressure. ln plasmas
Fig. 4. Krypton plasma parameters as a function of the micro-
wave input power. The movable probe is in the same position as in fig. 3.
produced by hot filaments, on the other hand, the temperature rises as the pressure is lowered [I]. The last point investigated concerns the variation of the plasma parameters as a function of the microwave power input. The density and electron temperature of a krypton plasma for a constant neutral pressure of 8 X 10e5 Torr are shown in fig. 4. An increase in density and electron temperature is observed with the microwave input power. Let us now examine how these results can be correlated with the ECR plasma characteristics. The evolution of the density and the electron temperature of an ECR plasma shows two modes. If the source operates in a mode whereby the plasma density is lower than the cut-off density n,, - the latter being defined as the density at which the plasma permittivity, ep = 1 - (c~,~/w)~ where w is the microwave pulsation and ape the electronic plasma pulsation, goes to zero - the density is proportional to the input power and the temperature does not increase strong ly. At higher input powers, electron temperature would increase proportional with the microwave power while the density would saturate at n,,. At 2.45 GHz the calculated saturation density is nco = 7.5 X lOlo cme3. But it has been demonstrated [5-71 that 303
Volume 106A, number 7
PHYSICS LETTERS
saturation occurs for higher densities, i.e.: n, = 3 X 1011 cme3. Beyond saturation the plasma density still increases but much more weakly with input power. The same phenomena have been observed at 10 GHz
PI. Taking into account the characteristics of the plasma source above, it is of particular interest to examine if the density achieved in the multipolar structure is coherent from a physical point of view. Since the plasma is produced only in the ECR source, the total number of ions, of mass m+ which crosses per second the source mouth of area S and diffuses into the multipolar structure is at most iVs = nCS(kTs/m+)1/2 ,
(1)
where T, is the electron temperature in the plasma source. If Y is the volume and )2, the density of the multipolar plasma, the total number of ions N which are lossed per second is N= n,V/7,
(2)
(3)
,
where D is the plasma diameter and Te the electron temperature. Consequently, the maximum density which can be achieved in the confinement structure n, = n,(3DS/V)(T,/Te)112
.
is (4)
If one assumes that Ts and T, are of the same order of magnitude, a source mouth 15 cm in diameter, D = 1.2 m, V= 2 m3 and n, = 3 X 1011 cmP3, the plasma density reaches 10 lo cmm3, in good agreement with
304
the value obtained in the argon plasma at high input power. In conclusion, we have shown that very large volume, homogeneous, maxwellian plasmas in the loglOlo cmW3 density range, can be easily obtained by using a microwave ECR source operating at 2.45 GHz. The multipole magnetic confinement device, which has proved its efficiency in producing plasmas via electron emission, has also been shown highly suitable for confining plasmas from which primary electrons are absent. It should be mentioned that the optimisation of magnetic spacing has not been investigated. This will be the subject of further studies, as will be the means of achieving better microwave power-plasma coupling. References
where T is the lifetime of the plasma. In a multipolar magnetic structure, it has been found to be well fitted with the empirical formula [9] : r = 3D(kTe/m’)-1/2
17 December 1984
[l] R. Limpaecher and K.R. MacKenzie, Rev. Sci Instrum. 44 (1973) 726. [2] K.N. Leung, T.K. Samec and A. Lamm, Phys. Lett. 5 1A (1975) 490. 131 C. Gauthereau and G. Matthieussent, Phys. Lett. 102A (1984) 231. 141 Y. Amal, thbse Docteur Ingknieur, Universite d’OrlBans (1977). [51 P. Chabert, 3rd cycle thesis, Universite de Grenoble (1983). [61 K. Suzuki, S. Nishimatsu, K. Ninomiya and S. Okudaira, Proc. Intern. Ion Engineering Congress (Kyoto, 12- 16 September 1983) p. 1645. [71 Y. Suetsugu and Y. Kawai, Japan. J. AppL Phys. 23 (1984) 237. PI P. Sermet, These Docteur d’Etat, Universite de Grenoble (1978). PI J.M. Buzzi, J. Snow and J.L. Hi&field, Phys. Lett. 54A (1975) 344.
PHYSICS LETTERS
17 December 1984
MICROWAVE EXCITATION OF LARGE VOLUMES OF PLASMA AT ELECTRON CYCLOTRON RESONANCE IN MULTIPOLAR CONFINEMENT
L. POMATHIOD, R. DEBRIE l, Laboratoire de Physique et Chimie de I’Environnement, CNRS, 3A, Avenue de h Recherche Scientifique, 45045 Orlkans Cedex, France
and Y. ARNAL and J. PELLETIER Laboratoire de Physique des Milieux ionis&, CNRS-ERA 993, CMGCNETBP 98, 38243 Mtylan Cedex, fiance Received 25 July 1984 Revised manuscript received 5 October 1984
Very large volume (>2 m3) homogeneous maxwellian plasmas in the lo9 -lot0 cmW3density range have been easily obtained by using a microwave electron cyclotron resonance source operating at 2.45 GHz. The magnetic multipole device has proved its efficiency in confining plasmas containing no primary electrons.
Numerous experimental studies require large volumes of uniform, quiescent plasmas. Most often, plasmas are produced in a multipole magnetic confmement device by means of electron emission from a spaced array of filaments [ 11. At moderate neutral pressure, this yields uniform plasmas in the lOlo cm -3 density range. In this ionisation mode, uniformity of the plasma is attributed to the localisation of ionisation by primary electrons trapped in the cusped magnetic field near the walls of the multipole device [2,3]. Confinement in multipole devices likewise proves efficient for lower density plasmas produced by a localized source, a Kaufman source [4] for instance. The production of plasmas by emission of primary electrons from hot filaments presents some disadvantages, however. Contamination resulting from filament sublimation or radiation of thermal energy occurs; moreover, one cannot generally produce plasmas of reactive gases.
1 UER de Sciences, Universite d’Orl&ans, 45046 Orleans Cedex, France.
0.375.9601/84/$03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Our purpose in this letter is to show that largevolume, high-density, uniform plasmas can be obtained in a multipole magnetic device by use of a localized microwave source operating at electron cyclotron resonance (ECR). Such electrodeless excitation leads to high-density plasmas over a wide neutral pressure range. We report here the first results obtained in a large vacuum chamber (5.5 m3). A schematic drawing of the experimental arrangement is shown in fig. 1. Experiments were performed in the large vacuum chamber of the Laboratoire de Physique et Chimie de 1’Enviromrement. The inner diameter of the multipole confinement device, containing about 6500 permanent ceramic magnets, is 1.2 m and it is 2 m long. The alternating rows of north and south poles (magnet size 4 cm X 2 cm X 1 cm) are spaced 2.5 cm apart. The magnetic field at the center of the pole face is 1100 G. An aperture in the multipole device situated in front of the plasma source allows the plasma to spread into the chamber. Plasmas have been created at the exit of a small (10 cm diameter, 10 cm long), non-magnetic, cylindrical vessel of stainless steel mounted on a side window 301
Volume 106A, number 7
a) alumina window
PHYSICS LETTERS
2.45 GHz Dower waveguide discharge vessel magnetic coil gas inlet chamber wall
-/’
V
multipole structure
B field =B75gaus:
Fig. 1. Experimental arrangement. (a) The ECR microwave plasma source. (b) the large vacuum chamber and the multipole confinement structure fitted with the plasma source.
17 December 1984
krypton. With these two gases, plasma homogeneity is achieved at pressures below 2 X 10m4 Torr for which the ion mean free path becomes of the order of magnitude of the chamber size. At higher pressures, the decrease in density is linked to the poor diffusion of the plasma and the simultaneous rise of density gradients. Fig. 2 shows a typical Langmuir characteristic curve as measured by the futed probe in an argon plasma at a pressure of 8 X 10e5 Torr and under 800 W of power input. The curve is typical of a purely maxwellian plasma. As seen in the logarithmic plot, it is characterized by the absence of primary electrons and of the hot electron population, resulting from primary electrons, usually found in the tail of the thermal elec-
15 ;i
-
E
_
J?
of the chamber. Sealing is achieved through an alumina window which allows transfer of the microwave power (2.45 GHz) delivered to the plasma by a continuously variable output generator (up to 1.5 kW) via a waveguide structure fitted with two mobile short circuit terminations for impedance matching. The argon or krypton gas is introduced into the vessel by means of an adjustable microleak. The plasma source is completed by a coil in order to obtain at the source mouth (see fig. la) the magnetic field of 875 G necessary to reach ECR. If the ion mean free path is long enough with respect to the chamber size, plasma diffusion occurs inside the entire multipole structure. Because of the small spacing between the magnets (2.5 cm) the plasma cannot cross the magnetic walls and escape from the confinement structure. Plasma densities and electron temperatures are measured by means of two cylindrical tungsten Langmuir probes. These are 0.6 mm in diameter and 5 mm long; one is fxed in front of the plasma source along the chamber axis, the other is movable and is used to check the homogeneity of the plasma. Experiments have been carried out with argon and 302
E g’o
;5-
Fig. 2. Typical Langmuir characteristic curve measured using the fixed probe. Argon, 8 X 10e5 Torr, 800 W. Plasma parameters: n = 1.1 X 10” cmw3, Te = 25000 K. Electron temperature isedetermined by plotting the decimal logarithm of the electron current collected by the probe (Ze) as a function of the probe biasing voltage. In the linear part of this curve, we have 7’e = 5040 A V, where Te is the electron temperature in Kelvins and A V the variation in probe voltages (volts) corresponding to a variation of one decade in the current. Plasma density is measured by plotting the square of the electron current as a function of the voltage, for voltage higher than the plasma potential. For a cylindrical probe, the curve obtained is linear, with a slope A$/AV. We haven, = 3.31 X 10’ X (AZ2 /A V) “*A, where rre is the electron density in cmm3, A the area of the probe in mm*, and AZ,‘/A V in (PA)* /V.
8.10W5Torr
0 .
d-_
fixed probe o movable probe
l
10-5
10-4 pressure
. fixed probe o movable probe
_
microwave power (watts)
(torr)
3. Krypton plasma parameters as a function of the neutral pressure. The movable probe is located 60 cm from the
Fig.
fixed one, along the chamber axis.
trons of classical multipolar
17 December 1984
PHYSICS LETTERS
Volume 106A, number 7
plasmas. plasma density, 70 cm from the plasma source and along its axis, is high (1 .l X lOlo cm-8). The electron temperature is 25000 K (2.2 eV). Fig. 3 shows the parameters of a krypton plasma for a constant input power of 300 W as a function of neutral pressure. The plasma density and electron temperature were measured by the two probes, the movable one being located 60 cm from the futed probe along the chamber axis. The two probes indicate nearly identical plasma characteristic parameters over the 10-5-10-4 Torr pressure range. The plasma, produced by a small source located on a side of the chamber, is well confined and homogeneized over large distances by the multipole structure. Using the movable probe, we haved checked that a significant gradient in density only occurs near the magnetic “walls”. The plasma density, in the 1Og-lO1o cme3 range, reaches a maximum for a neutral pressure of about 1O-4 Torr. Note that the density goes a high as log cmm3 over a large volume for a microwave power input of only 300 W at a low neutral pressure of lob5 Torr. The electron temperature is low (~2 eV) and does not vary significantly with pressure. ln plasmas
Fig. 4. Krypton plasma parameters as a function of the micro-
wave input power. The movable probe is in the same position as in fig. 3.
produced by hot filaments, on the other hand, the temperature rises as the pressure is lowered [I]. The last point investigated concerns the variation of the plasma parameters as a function of the microwave power input. The density and electron temperature of a krypton plasma for a constant neutral pressure of 8 X 10e5 Torr are shown in fig. 4. An increase in density and electron temperature is observed with the microwave input power. Let us now examine how these results can be correlated with the ECR plasma characteristics. The evolution of the density and the electron temperature of an ECR plasma shows two modes. If the source operates in a mode whereby the plasma density is lower than the cut-off density n,, - the latter being defined as the density at which the plasma permittivity, ep = 1 - (c~,~/w)~ where w is the microwave pulsation and ape the electronic plasma pulsation, goes to zero - the density is proportional to the input power and the temperature does not increase strong ly. At higher input powers, electron temperature would increase proportional with the microwave power while the density would saturate at n,,. At 2.45 GHz the calculated saturation density is nco = 7.5 X lOlo cme3. But it has been demonstrated [5-71 that 303
Volume 106A, number 7
PHYSICS LETTERS
saturation occurs for higher densities, i.e.: n, = 3 X 1011 cme3. Beyond saturation the plasma density still increases but much more weakly with input power. The same phenomena have been observed at 10 GHz
PI. Taking into account the characteristics of the plasma source above, it is of particular interest to examine if the density achieved in the multipolar structure is coherent from a physical point of view. Since the plasma is produced only in the ECR source, the total number of ions, of mass m+ which crosses per second the source mouth of area S and diffuses into the multipolar structure is at most iVs = nCS(kTs/m+)1/2 ,
(1)
where T, is the electron temperature in the plasma source. If Y is the volume and )2, the density of the multipolar plasma, the total number of ions N which are lossed per second is N= n,V/7,
(2)
(3)
,
where D is the plasma diameter and Te the electron temperature. Consequently, the maximum density which can be achieved in the confinement structure n, = n,(3DS/V)(T,/Te)112
.
is (4)
If one assumes that Ts and T, are of the same order of magnitude, a source mouth 15 cm in diameter, D = 1.2 m, V= 2 m3 and n, = 3 X 1011 cmP3, the plasma density reaches 10 lo cmm3, in good agreement with
304
the value obtained in the argon plasma at high input power. In conclusion, we have shown that very large volume, homogeneous, maxwellian plasmas in the loglOlo cmW3 density range, can be easily obtained by using a microwave ECR source operating at 2.45 GHz. The multipole magnetic confinement device, which has proved its efficiency in producing plasmas via electron emission, has also been shown highly suitable for confining plasmas from which primary electrons are absent. It should be mentioned that the optimisation of magnetic spacing has not been investigated. This will be the subject of further studies, as will be the means of achieving better microwave power-plasma coupling. References
where T is the lifetime of the plasma. In a multipolar magnetic structure, it has been found to be well fitted with the empirical formula [9] : r = 3D(kTe/m’)-1/2
17 December 1984
[l] R. Limpaecher and K.R. MacKenzie, Rev. Sci Instrum. 44 (1973) 726. [2] K.N. Leung, T.K. Samec and A. Lamm, Phys. Lett. 5 1A (1975) 490. 131 C. Gauthereau and G. Matthieussent, Phys. Lett. 102A (1984) 231. 141 Y. Amal, thbse Docteur Ingknieur, Universite d’OrlBans (1977). [51 P. Chabert, 3rd cycle thesis, Universite de Grenoble (1983). [61 K. Suzuki, S. Nishimatsu, K. Ninomiya and S. Okudaira, Proc. Intern. Ion Engineering Congress (Kyoto, 12- 16 September 1983) p. 1645. [71 Y. Suetsugu and Y. Kawai, Japan. J. AppL Phys. 23 (1984) 237. PI P. Sermet, These Docteur d’Etat, Universite de Grenoble (1978). PI J.M. Buzzi, J. Snow and J.L. Hi&field, Phys. Lett. 54A (1975) 344.