PROBE DIAGNOSTICS OF RF PLASMAS

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PROBE DIAGNOSTICS OF RF PLASMAS FOR MATERIAL
PROCESSING V. A. Godyak RF Plasma
Consulting, Brookline, MA 02446,
USA egodyak_at_comcast.net Invited talk at
38th International Conference on Plasma Science
and 24th Symposium on Fusion Engineering.
Chicago, Illinois, 26-30 June, 2011 Work
was supported by the Department of Energy, Office
of Fusion Energy Science, Contract
DE-SC0001939 and by the National Science
Foundation under Grant No CBET-0903635
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• Langmuir probe diagnostics
• Langmuir probe is a powerful diagnostic tool for
  low pressure gas discharge plasmas
• Practically all today knowledge on gas discharge
  was obtained with plasma spectrometry and
  Langmuir probes (both imply a Maxwellian EEDF)
• The ability of measurement of local plasma
  parameters and the electron energy distribution
  function (EEDF) makes it unique among other
  diagnostics
• EEDF is the most informative, universal and
  complete characteristic of the plasma electrons
  in any plasmas
• Basic plasma parameters (N and Te) and rates of
  plasma-chemical processes can be found as
  appropriate integrals of the measured EEDF

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Non Equilibrium Gas Discharge Plasmas
Diagnostics Gas pressure, p - between
fraction and hundred mTorr Mean
electron energy, ltegt between fraction and tens
eV Plasma density, N between 109-1013
cm-3 In gas discharge electrons are not in
equilibrium with RF field, nor with ions, nor
within its own ensemble, i.e. have a
non-Maxwellian EEDF with Te gtgt Ti, Tg
Classical Langmuir probe diagnostics (based on
electron and ion current), as
well as many other diagnostics, imply a
Maxwellian EEDF Found this way plasma
parameters and especially rates of inelastic
processes can be in dramatic disagreement
with those found from the measured EEDF
Probe measurements in processing RF plasma are
usually distorted. The problem is not
recognized when one just measures the probe I/V
characteristic, since distorted and
undistorted probe characteristics look very
similar. But the problem becomes apparent
after differentiation the I/V characteristic to
get the EEDF
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Langmuir formula and Druyvesteyn method
I. Langmuir, Gen. Electr. Rev. 25, 1924
 

Druyvesteyn formula
 
M. J. Druyvesteyn, Z. Phys. 64, 781, 1930
Plasma density and effective electron temperature
are
 
Similarly, all plasma parameters (Tesk, ?D, JB)
and rates of plasma-chemical processes (?ea,
?ee, ?, ?i, .) can be found as appropriate
integrals of the measured EEPF.
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• EEDF Measuring with Langmuir probe
• Variety of analog and digital techniques exists
  for differentiation of the probe characteristic
  to infer EEDF (see review by V. Godyak in NATO
  ASI Series, E. Appl. Sci., V. 176, Plasma Surface
  Interaction and Processing of Materials, pp.
  95-134, Kluwer, Acad. Publisher, 1990)
• The analog techniques are based on the
  measurement of non-linearity feature of the
  probe I/V characteristics (demodulation, second
  harmonic and biting frequencies)
• There are variety of algorithms for digital
  differentiation and deconvolution of the probe
  characteristics to infer EEDF
• The both, differentiation and deconvolution
  techniques are prone to error augmentation.
  Therefore, the problems of noise, distortion and
  measurement accuracy have to be properly
  addressed to obtain undistorted EEDF in a wide
  dynamic range (3-4 orders of magnitude)

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• What makes a good EEDF measurement?
• We want to have more than the classical Langmuir
  method can gives us
• EEDF has to resolve the tail electrons (e gt e)
  responsible for excitation, ionization and
  electron escape to the wall, as well as the low
  energy electrons (e lt 2Te) accounting for
  the majority of electrons
• Due to error augmentation inherent to
  differentiation procedure, small (invisible)
  inaccuracy in Ip(V) can bring enormous distortion
  in the inferred EEDF
• It is important to realize the source of the
  possible errors and to be able to mitigate them
• The sources of the errors are well elucidated in
  the literature, but are insistently ignored in
  the majority of published papers on EEDF
  measurement.
• The constrains for the Druyvesteyn method
  applicability coincide with those for Langmuir
  method
• There are techniques for EEDF measurement in
  collisional, magnetized and anisotropic plasmas
  (not considered here)

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• Problems in probe measurements and their
  mitigations.
• Probe size aln(pl/4a), b, ?D ltlt le and
  Ip ltlt Id, Ir, Iz
• Ir IB ScheNsuB, is the current emission of
  the counter electrode uB (Te/M)1/2, Iz
  eGe is the current corresponding to generation
  rate of electrons with energy e in the volume
  defined by the chamber characteristic size ?, or
  by the electron energy relaxation length ?e
• To neglect the voltage drop across the wall
  (counter electrode) sheath, the following
  requirement has to be satisfied
• (SpN0/SchNs)(M/2pm)1/2 ltlt 1 V. Godyak
  V. Demidov, J. Phys. D
• Appl. Phys. 44, 233001,
  2011

a 38 µm b 175 µm
V. Godyak et al, PSST 1, 179, 1992
a b
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Probe constructions
V. Godyak et al, PSST 11, 525, 2002
V. Godyak et al, PSST 1, 179, 1992
P1 P2
c
b
a 0.05 mm, b 1 mm, c 4-6 mm
b
a
VGPS Probe System www.plasmasensors.com
The most popular bad probe design
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RF plasma potential
Criterion for undistorted by the probe rf sheath
voltage EEPF measurement is known for over 30
years V. Godyak and S. Oks, Sov.
Phys. -Tech. Phys. 24,
784, 1979 Vshrf (0.3-0.5)Te/eVplrf
A presence of a filter in the probe circuit
does not guarantee undistorted EEPF measurement.
To do the job, the filter has to satisfy the
following condition for all relevant
harmonics Zf (2-3)ZpreVplrf
/Te For filter design one needs to know
(measure, calculate) Vplrf and minimize Zpr Zpr
is the impedance between the probe and plasma
(Zpr is defined by its sheath capacitance at
floating potential, Zf is the filter impedance,
Vplrf is the rf plasma potential reference to
ground, Vshrf is the rf Voltage across the probe
sheath, and T is electron temperature
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Filter design procedure
CCP at 13.65 MHz, V 100 V
The filter has to be designed after the
measurements of the rf plasma potential spectrum!
V. Godyak et al, PSST 1, 179, 1992
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Probe measurement circuit for EEPF measurement in
Ar CCP, incorporating, dc voltage and low
frequency noise suppressions, rf compensation and
rf filter dc resistance compensation with I/V
-converter having a negative input resistance
(gyrator)
2- undistorted
1- distorted w/o filter
V. Godyak et al, PSST 1, 179, 1992
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Probe circuit resistance Rc (the most common
problem) Rc Rext Rpsh Rint
The voltage V applied to the probe is distributed
along the probe circuit elements (Rext, Rpsh,
Rint), thus, Vpshlt V !
V. Godyak et al, PSST 11, 525, 2002
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EEPF Druyvesteynization due to circuit resistance
Rc d Rc/Rpmin Rpmin Te/eIesat
Rc and LF noise compensation
Maxwellian
Druyvesteyn
V. Godyak et al, PSST 11, 525, 1002
Error in EEPF less than 3 requires Rc/Rpmin lt
0.01 !
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• Probe surface effects
• Probe surface work function (changes during
  probe scan)
• Non conductive layer of reaction product
  increases Rc
• Sputtering of an electrode and a probe and
  deposition conductive layer on the probe holder
  changes Sp
• Strong temperature dependence of work function
  and Rc
• Probe circuit resistance, probe surface
  contamination together with rich spectrum of high
  voltage rf plasma potential are the main
  obstacles to make meaningful EEDF measurement in
  plasma reactors
• REMEDIES
• Feedback with reference probe to compensate Rc
  and LF noise
• Continuous probe cleaning with ion bombardment,
  electron heating and rf biasing, together with
  fast probe scan (mS)
• Adequate RF filtering for all relevant RF
  plasma potentials

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• VGPS Probe System
• The aforementioned problems has been addressed
  in VGPS instrument and its prototypes
• Combined Langmuir probe and ion current
  diagnostics
•  
• Robust high resolution EEDF measurements in both
  
• laboratory and industrial reactive gas
  plasma
•  Accurate probe diagnostic measurements in the
  presence of rf plasma
  potentials, over a wide frequency range
•  
•  Work function and plasma drift distortion are
  eliminated using fast (0.5-5 ms) probe
  sweep
•  
• Local and global plasma perturbations are
  minimized using small diameter probe
  holder (1 mm) and probe supports a 4
  mm, 6.3mm)
•  
• Plasma noise suppression and probe circuit
  impedance compensation are
  effectuated by a feedback system with
• a reference probe
•  
•  Proprietary  signal tracking, auto ranging, and
  probe ion, electron and rf
  cleaning features

VGPS Probe System www.plasmasensors.com
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VGPS Display
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Examples of EEPF evolution measurements
in CCP at 13.56 MHz
Heating mode transition in Ar
Transition from the a to the ? - mode
V. Godyak et al, Phys. Rev. Lett. 68, 49, 1992
V. Godyak et al, Phys. Rev. Lett. 65, 996, 1990
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Example of EEPF measurement in argon ICP with a
low disturbance probe having Rc and noise
compensation
The maximal argon pressure, was limited by the
chamber surface when Iich gt Iesat
V. Godyak et al, PSST 11, 525, 2002
DC plasma potential
V. Godyak et al, PSST 11, 525, 2002
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Examples of EEPF measurements in Ar ICP
Frequency dependence Power
dependence
W
V. Godyak et al, PSST 11, 525, 1002
V. Godyak V. Kolobov, Phys. Rev. Lett., 81,
369, 1998
In high density plasmas, the EEPF at low energy
must be Maxwellian
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Time resolved EEPF measurements EEPF measured in
afterglow stage of ICP with internal ferrite core
inductor
Single pulse
Repetitive
Room temperature
V. Godyak B. Alexandrovich, XXVII ICPIG, vol.
1, p. 221, 2005
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EEPF and Te measured along the rf period in an
induction lamp operated at Ar _at_ 300 mTorr and Hg
_at_ 7 mTorr Time resolution dt 0.5 µS.
Note asymmetry in Te(t)
V. Godyak et al, 10th Intern. Symposium. on Sci.
Technol. of Light Sources, p. 283, Toulouse,
France, 2004
Probe sheath capacitance to plasma is the main
limiting factor in EEPF measurement speed. The
minimal time resolution limit dt gt (10-30)/?pi
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EEDF measurements in a commercial ICP reactors
Comparison of EEPF measured with different
commercial probe stations, Espion of Hiden and
VGPS of Plasma Sensors At maximal discharge power
of 2 kW, N 11012 cm-3, thus the EEPF _at_ e lt e
has to be a Maxwellian one Druyvesteynization
effect is found in many publications of EEDF
measurements made with home-made and commercial
probe systems
V. Godyak et al, GEC 2009, Saratoga Springs, NY,
USA
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EEPF measurements with VGPS in plasma
reactors Wide specter and large amplitude of rf
plasma potential and high rate of probe
contamination are the major problems making even
classic Langmuir probe diagnostic
impossible Microcrystalline silicon deposition
in Ar/SiF4 at H2/CF4 30
mTorr (ICP) with polymer 10 mTorr
(ECR). Ecole Polytechnique, France
deposition. University of Maryland,
USA

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Final Remark Today, plasma simulation codes are
practically main tool for study plasma in
industrial plasma sources. These codes applied
to complicated processing gas mixture are missing
many cross sections for variety of
plasma-chemical reactions. They also are
missing effects of nonlocal and nonlinear plasma
electrodynamics that has been proved are
important and even dominant in rf plasmas at low
gas pressure. In such situation, a reliable
measurement of EEDF and plasma parameters would
give a valuable experimental data for
understanding variety of electrodynamics,
transport and kinetic process in such plasmas and
validation of existing theoretical models and
numerical codes. For more complete
information see 1. V. Godyak, Measuring EEDF in
Gas Discharge Plasmas, review in NATO ASI Series,
E. Appl. Sci., V. 176, Plasma Surface Interaction
and Processing of Materials, pp. 95-134, Kluwer,
Acad. Publisher, 1990 2. V. Godyak and V.
Demidov, Probe Measuring of Electron Energy
Distribution in Plasmas What Can We Measure and
How Can We Achieve Reliable Results?, review to
be published in J. Phys. D Appl. Phys. 44,
233001, 2011 3. VGPS Probe System,
www.plasmasensors.com