Convective Flow Zones in FilamentDischarge Plasma Sources

1
Convective Flow Zones in Filament-Discharge
Plasma Sources

• R. McWilliams, M. Brown, D. Edrich, D.
  Zimmerman
• Dept. of Physics Astronomy
• University of California
• Irvine, CA 92697 USA

Supported by NSF Grant INT-9981978 and DoE Grant
DE-FG03-99ER54542 mcw_at_uci.edu
Slides at http//HAL9000.ps.uci.edu
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Light comes out of many plasmas. A solar flare,
viewed through a blue filter, on our most
important plasma, the Sun. Photo NASA
3
Collisionally-induced fluorescenceTemporal
evolution of emitted photons can be observed.
This is collisionally-induced fluorescence from
electrons raining down upon neutrals.
The Northern Lights (Aurora Borealis). Photo J.
Curtis
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ABSTRACT Convective flow zones were found to
surround the discharge current in an
electron-beam generated argon plasma. Flow zones
were created by the electric field associated
with the injected electron beam. A sheet-beam was
injected along a confining magnetic field. The
discharge current and voltage were varied, and
the ion flow near the beam was studied via
laser-induced fluorescence. It was observed that
the drift velocity, and thus the inferred
electric field, was nearly constant even with
increasing current, yet the electron density of
the beam did increase.
5
MOTIVATION Discharge plasma sources are in wide
use industrially from high intensity mercury lamp
discharges to metal halide lamp sources and many
others. The lamp sources come in several
configurations, often relying on the flow of
electrons due to an electric field in the source
for production of the plasma and associated light
output. Laboratory plasma sources frequently use
filament discharge sources for producing argon,
xenon, neon, and other plasmas for basic and
applied plasma physics research. These discharges
may be produced from an electron beam made via
thermionic emission from a filament biased below
the surrounding vacuum vessel. Such
configurations also are used in ion beam sources
for plasma etching and deposition processes.
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Experiment Geometry

• Horizontal electron current sheet creates local
  vertical E

• Vertical shear in horizontal plasma velocity (v)
  due to E x Bo

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E x B Induced Convection Shear

• Electron beam produces vertical potential profile
• Vertical E field combined with background
  horizontal Bo produces horizontal plasma drift
• Vertical Shear in horizontal plasma velocity due
  to varying E x Bo

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Irvine Torus

• R0.56m, a0.035m
• 0 lt Bo lt 0.25 T
• 1015 lt nionlt 1019 m-3
• 5 lt Te lt 15, 0.04lt Ti lt2 eV
• Argon

• wpe (1016m-3)5.6 x 109s-1
• wpi 2.1 x 107 s-1
• wce(1kG) 1.76 x 1010 s-1
• wci 2.4 x 105 s-1
• Vth,e(10 eV)1.87 x 106 m/s
• Vth,i (0.1 eV)6.9x102 m/s
• ?D 3.3x10-4 m
• ?i2.9x10-3m,?e1x10-4m

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Comet Hale-Bopp. Photo D. Toohey
Ultraviolet-induced fluorescence Visible and
near-visible photons can provide non-intrusive
diagnosis of plasmas. Blue comet plasma tail is
visible due to fluorescence induced by
ultraviolet photons from the Sun. White tail is
sunlight scattered from comet dust tail.
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How to do LIF and optical tomography papers 1.)
Early LIF in plasmas - D.D. Burgess and C.H.
Skinner, J. Phys. B7, 297 (1974). 2.) Early LIF
in plasmas - R. Stern and J. Johnson, Phys. Rev.
Lett. 34, 1548 (1975). 3.) LIF dye lasers - D.
Hill, S. Fornaca, M. Wickham, Rev. Sci. Instrum.
54, 309 (1983) 4.) LIF diode lasers - G.D.
Severn, D.A. Edrich, R. McWilliams, Rev. Sci.
Instrum. 69, 10 (1998) 5.) Tomography - R.
Koslover and R. McWilliams, Rev. of Sci. Instrum.
57, 2441 (1986). 6.) Tomography M. Zintl and R.
McWilliams, Rev. Sci. Instrum. 65, 2574
(1994). 7.) Tomography D. Zimmerman, R.
McWilliams, D. Edrich, Plasma Sources Sci.
Technol. 14, 581 (2005).
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Laser-induced fluorescence (LIF) in plasmas
depends on having sufficient electrons populating
the state being interrogated by the laser photon.
Typically, visible light photons do not have
enough energy to interrogate ground states but
rely on sufficiently populated excited states. If
the plasma electron population does not have
enough high-energy electrons then visible LIF
will not work. On the other hand, ultraviolet LIF
can hit states requiring larger photon energy to
interrogate, including ground states. Examples of
ultraviolet LIF includeJ.P. Booth, G. Hancock,
N. Perry, Appl. Phys. Lett. 50, 318 (1987)J.P.
Booth, G. Hancock, N. Perry, M. Toogood, J. Appl.
Phys. 66, 5251 (1989)C. Suzuki, K. Kadota, Appl.
Phys. Lett., 67, 2569 (1995)C. Suzuki, K.
Sasaki, K. Kadota, J. Appl. Phys. 82, 5321
(1997)K. Sasaki, H. Furukawa, K. Kadota, C.
Suzuki, J. Appl. Phys. 88, 5585 (2000)G.
Hancock, J. Sucksmith, J. Vac Sci. Technol. A20,
270 (2002)
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Other examples of LIF in processing plasmaECR
-D. J. Trevor, N. Sadeghi, T. Nakano, J.
Derouard, R. Gottscho, P.D. Fow, and J. M. Cook,
Appl. Phys. Lett. 57, 1188 (1990)T. Nakano, N.
Sadeghi, and R.A. Gottscho, Appl. Phys. Lett. 58,
458 (1991)N. Sadeghi, T. Nakano, D.J. Trevor,
and R.A. Gottscho, J. Appl. Phys. 70, 2552
(1991)T. Nakano, N. Sadeghi, D.J. Trevor, R.A.
Gottscho, and R.W. Boswell, J. Appl. Phys. 72,
3384 (1992)R.A. Gottscho, J. Vac. Soc. Technol.
B11, 1884 (1993)Magnetron M.J. Goeckner, J.
Goree, and T.E. Sheridan, J. Vac. Sci. Technol.
A8, 3920 (1990)ICR M. Zintl, R. McWilliams, N.
Wolf, Phys. Plasmas 2, 4432 (1995)Glow
Discharges Review K. Muraoka, et al, Rev. Sci.
Instrum. 63, 4913 (1992)
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Helicon -K.P. Giapis, N. Sadeghi, J. Margot,
R.A. Gottscho, and T.C.J. Lee, J. Appl. Phys. 73,
7188 (1993).T. Nakano, K.P. Giapis, R.A.
Gottscho, T.C. Lee, and N. Sadeghi, J. Vac. Sci.
Technol. B11, 2046 (1993).R.F. Boivin and E.E.
Scime, Rev. Sci. Instrum. 74, 4352 (2003).X.
Sun, C. Biloiu, R. Hardin, and E. Scime, Plasma
Sources Sci. Technol. 13, 359 (2004).TCP and RIE
-M.V. Malyshev, N.C.M. Fuller, K.H.A. Bogart,
V.M. Donnelly, I.P Herman, Appl. Phys. Lett. 74,
1666 (1999).J.Y. Choe, N.C.M. Fuller, V.M.
Donnelly, I.P. Herman, J. Vac. Sci. Technol. A18,
2669 (2000).ICP -T. Chevolieau and W. Fukarek,
Plasma Sources Sci. Technol. 9, 568 (2000).S.
Jun, H.Y. Chang, and R. McWilliams, Phys. Plasmas
13, 052512 (2006).
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CVDT. Yasui, H. Kodera, H. Tahara, and T.
Yoshikawa, Bull. Am. Phys. Soc. (1998) Ion Beam
Etching Deposition A. Brockhaus, Y. Yuan, and
J. Engemann, J. Vac. Sci. Technol. A13, 400
(1995).R. McWilliams and D. Edrich, Thin Solid
Films 435, 1 (2003)H. Boehmer, D. Edrich, W.
Heidbrink, R. McWilliams, L. Zhao, and D.
Leneman, Rev. Sci. Instrum. 75, 1013 (2004).D.
Zimmerman, R. McWilliams, and D. Edrich, Plasma
Sources Sci. Technol., v.14, 581 (2005).H.J.
Woo, K.S. Chung, Y.S. Choi, D. Zimmerman, and R.
McWilliams, Contrib. Plasma Phys. 46, 451 (2006)
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Now on to laser-induced fluorescence (LIF)LIF
diagnosis of time and space varying events can be
studied up close. Here is an argon plasma made by
radio frequency waves in the laboratory. When LIF
is performed, the background light signal may be
subtracted.
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Plasma Test-Bed for Ion Beam Characterization
(chamber on left, background). LIF lasers 75 m
of fiber optics away in a remote room. Experiment
and lasers are run from a control room.
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A double LIF system Two pump ion lasers, in
background, drive two tunable dye lasers (one
with cover removed).
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Tunable Dye Laser
Pump Laser
Acousto-Optic Modulator
Square Wave Generator
Optical Fiber
Lock-In Amplifier
Lens
Digitizer
PMT
Boxcar Averager
Filter
Plasma
Laser-Induced Fluorescence Apparatus
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How does the tool work?

• Fluoresce ion

Emitted photon tells ion speed via Doppler shift
knowledge. Number of photons detected at set
laser frequency tells relative number of ions at
that speed. Scanning laser frequency gives
relative number of ions at all speeds, hence ion
distribution function is obtained. Average of
distribution gives convection.
Three fluorescence schemes. Laser in red, emitted
in blue.
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How does LIF work? Fluoresce ion.
Then scan laser frequency, or create test ion
1.5 GHz 1 km/s for argon
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Ion distribution for argon plasma emerging from
hollow cathode baffle.
Argon ions are near room temperature in hollow
cathode source. Here argon ion distribution is
compared to room temperature iodine calibration
scan.
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How much population is in the fluoresced
state?If metastable states are used which are 20
eV above the ground state, then the population
may be small and charge exchange may further, and
rapidly, diminish the population. Here we see how
increasing neutral pressure in an ion beam source
reduces the LIF signal.
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LIF in plasma sheaths and presheaths What do the
ion velocity distribution functions look like at
various distances from within the bulk plasma
into the presheath and the sheath regions near a
surface? What are the ranges of ion speeds
perpendicular to the surfaces and parallel to the
surface? Are there significant spreads in ion
velocities which may impede desired plasma
processing, such as aspect ratio issues and the
sharpness of surface features, especially as
critical dimensions reduce?
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Experiment layout
Vx
Ions
Conducting, bias-able surface plate
Bulk plasma
Vz
Presheath
Sheath
RF Coil to make plasma, 10-200 MHz
Ion mean free path
Debye length
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Plasma ions in the presheath drift towards the
surface with speeds around ion sound speed and
speeds pick up in the sheath.
Here we see ion distributions normal to the
surface, as reported by S.L. Gulick, et al, J.
Nucl. Materials 176 177, 1059 (1990).
28
Similarly, we see ion distributions normal to the
surface, as reported by G.D. Severn, X. Wang, E.
Ko, and N. Hershkowitz, Phys. Rev. Lett. 90,
145001 (2003).
What does the full multidimensional ion
distribution look like in the presheath and
sheath? Lets use optical tomography to get it.
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For present experiment, similar results are
obtained for ion drift speeds towards plate when
in the presheath and sheath.
30
Holding diagnosed volume constant while rotating
laser beam allows measure of full,
multi-dimensional ion velocity distributions. We
call this optical tomography, pioneered by
McWilliams, Koslover, and Zintl now joined by
Zimmerman.
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Optical Tomography Details
R. Koslover and R. McWilliams, Rev. Sci. Instrum.
57, 2441 (1986).R. McWilliams, R. Koslover,
Phys. Rev. Lett. 58, 37 (1987). M. Zintl and R.
McWilliams, Rev. Sci. Instrum. 65, 2574 (1994).
Next, reconstruct 2- or 3-D velocity-space image
via algorithms
First, take many 1-D distribution scans from many
angles
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Ion velocity distribution function in the bulk
plasma, showing ions heated in the perpendicular
direction by the rf coil electric fields.
33
Ion velocity distribution function in the sheath
plasma, showing ions accelerated in parallel
direction and slow ions also found in sheath.
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Sheath ions approach the substrate surface at
various angles. Will this prove important in
processing and need to be controlled? What
fraction of ions approach with an angle greater
than, say 30 degrees? Here we see LIF on sheath
ions which shows spread in one dimension, x, in
velocity angle away from the surface normal,
z-direction. 18.62 MHz ICP RFArgon, P 3.5x10-4
Torr
30
30
Ions at gt30
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On the right, we see the fraction of ions within
5 increments of the angle they present to the
substrate in the vx vz plane. In this
velocity-space integration all vy values are
included at each vx since the LIF has integrated
over all vy .
When considering the fraction of ions approaching
the substrate from greater than a specified
angle, the full 3-dimensional velocity
distribution must be measured or estimated. For
this example, the percent of ions with substrate
approach angle greater than 30 is about 18.
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Emissive Probe - Construction

• A 1 mm diameter semicircle is formed of 55 gauge
  tungsten wire
• The tungsten wire tip is inserted in a 2-hole
  cylinder of alumina
• Copper wires inside the cylinder make compression
  contact with the replaceable tungsten tip
• R infinity allows floating potential
  measurements

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Emissive Probe Operation Measuring Plasma
Potential

• Thermionic electron- emitting tip allows
  determination of plasma potential because
  thermionic electrons provide a high-mobility ion

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Electron BeamThe beam path is visible in photo
through a vacuum port window due to argon ion and
neutral excitation by the beam electrons
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2-D Mapping of the Plasma
Plasma Floating Potential
Z 4pRCL
Z 2pRCL
Z 0
40
Localized Electric Field Structure has been
created

• Red curve shows vertical (y-direction) electric
  field created by sheet e-beam at y0.

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Predict ExB Drift from Emissive Probe Measurements

• Ions just sampling electric field structure at
  edge of Larmor orbit see little ExB effect.

Electron beam sheet
E
?i
Ion orbit
B0
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Now look at an ion which samples electric field
towards center of gyro-orbit. Look first at
sampling just one electric field peak.
Net motion is VExB in x-direction at 2400 m/s.

• Ion receives 3 eV kick, expanding gyro-orbit,
  then receives 3 eV deceleration which reduces
  gyro-orbit. Motion then repeats at translated
  x-position.
• Net ExB motion is in x-direction as usual ExB,
  just not homogenous E.

Electron beam sheet
E
?i
B0
Ion orbit
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Finally, ions which get a kick from first
electric field peak pass over into opposite
electric field peak and receive opposing kick.

• Net ion drift is about 10 of single-peak kick.

Prediction Net motion is VExB in x-direction at
240 m/s.
Electron beam sheet
E
E
?i
Ion orbit
B0
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Sheared Ion Flow Field measured with LIF

• Convective flow, vx, due to local E shows zones
  of flow and velocity shear in ydirection.
• Peak flow value 280 m/s (compared to rough
  prediction of 240 m/s).

vx
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Convective flow of ions predicted in region of
sheared zonal flow by use of emissive probe for
plasma potential measurement and electric field
estimation agrees with laser-induced fluorescence
measurement of ion convective flow.
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Turbulence and Its Suppression

• Fluctuation spectrum as a function of vertical
  position moving through shear layer.
• Notice suppression of turbulence in shear layer

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• Low frequency (10 16 kHz) turbulence is
  suppressed in the shear region
• Turbulence suppression of 90 easily produced in
  shear layer

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Diffusion of Beam Electrons
Torus with axial, toroidal field of about 1 kG
has major radius R 56.5 cm. Injected electron
beam sheet has multiple passes with rising
vertical position due to vertical magnetic field
component. Vertical probe scan allows looking at
vertical sheet width (diffusive expansion) as a
function of axial travel distance. D is found
for electrons with v gtgt vth,e in sheared-flow
zone.
Probe for vertical scan
Z 2pR
R
Z 0
vbeam
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Measurement of Diffusion Coefficients
Z0
Z2pRCL
Z4pRCL
50
MagneticField Dependence of Electron Diffusion
Coefficients
3/16 is from empirical fit to these data.
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Beam-Energy Dependenceof Diffusion Coefficients
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Neutral Pressure Dependence of Diffusion
Coefficients
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Emission Current Dependence of Diffusion
Coefficients
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Data Collection

• Top mirror mobile such that the laser enters
  plasma at various heights.
• Photomultiplier filter bandpass 435 nm
  Fluorescence.
• Lock-In Amplifier process signal at chopper fan
  frequency.

The equipment used to collect the data.
55
The Iodine and LIF peaks were observed to be
approximately Gaussian, and thus were fit with a
Gaussian approximation. A sample processed scan
is shown
56
Vertical scan of horizontal ion drift taken by
moving laser in y-direction (vertical) while
laser beam was in x-direction (horizontal).
Maximum ExB drift was about 300 m/s above and
below a horizontal baseline drift around -200 m/s
on this graph as observed away from the
convective zone.
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Once the vertical profile was obtained, the laser
was set to the position of maximum drift above
the beam, and the plasma parameters were varied
to determine effects on the ExB convective drift.
Varying neutral pressure and the energy of the
electrons in the beam had no effect on the ExB
drift. A factor of 7 increase in the injected
beam current produced no significant change in
drift speed
58
At first glance, it might be surprising to see
little change in ExB drift with discharge
emission current. However, one should note that
the beam energies are such that shielding within
a Debye length is not expected. Further, the
electron beam discharge, in argon here, has some
similarity with mercury discharge lamps. For
mercury discharges, Elenbaas performed
experiments and produced theories which showed
that the axial electric field sustained in a
mercury discharge goes as the discharge current
to the one-fifth power, a weak dependence on
discharge current (and within the error bars of
the present LIF observations). Elenbaas obtained
this result via power-balance considerations.
While this derivation is done for mercury, the
arguments carry over to argon filament
plasmas.W. Elenbaas, Temperatur Und Gradient Des
Quicksilberbogens, Physica 2 (1935) 757.W.
Elenbaas, The High Pressure Mercury Vapour
Discharge, North-Holland Publishing (1951) 29.
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A T probe was biased 150 volts below ground to
collect ions and beam electrons with an energy of
250 eV, but exclude the main plasma electrons.
These scans (next slide) showed that there was
very little broadening of the beam with increased
electron beam, and the ion density was
proportional to the emission current. The data
are shown, with a negative current corresponding
to ion density. Outside the inverted peaks, where
ion density is much lower than in the ionization
path of the electron beam, one may see the ion
density is proportional to beam current as
expected. Within the beam path, however, the
total collected probe current increases with
reduced rate as the discharge current is
increased. From 50 to 100 mA discharge current,
we observe only an 80 increase within the beam
path instead of an expected 100. From 100 to 150
mA discharge current, only a 10 increase in
probe current is seen instead of the expected
50. The beam did not broaden significantly over
this current range, and thus it is unlikely that
the increased density is spread over a greater
area.
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A vertical scan (next slide) of the floating
potential did not show increased vertical
electric field near the beam at higher emission
currents. Figure 7 shows the vertical scan of
floating potential at several different emission
currents. The slope in the region of the beam
appears nearly constant across the scans at
emission currents above 50 mA, which indicates
that the vertical electric field in this region
is not increasing with the increased discharge
current in the beam. The magnitude of the
vertical electric field (estimated via the ExB
drift observation or the floating potential
spatial variation) was consistent with earlier
work, with an electric field of approximately ?
25 V/cm 2. Only at emission currents less than
30 mA does the slope become shallower, showing
that the electric field decreases.
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Summary
These electric fields caused a convective zonal
plasma drift velocity, yet the the electric field
in the region of the beam does not increase
significantly with the beam current. It may be
that, with increasing plasma density, the plasma
shielding of the beam-produced electric field
increases with beam current. A change in
electric field was only observed at very low
plasma densities where the experimental setup was
no longer precise enough to produce a credible
measurement of the zonal flow. In other words,
some mechanism nearly clamps the zonal flow, the
zonal flow saturates as the beam current density
is raised.