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Langmuir Probe diagnostics
Dr. N. K. Joshi
Dept. of Applies Physics
B.I.T., Mesra, Ranchi
e-mail: [email protected]
Autumn School on chemistry and physics of plasmas
SCAPP-2010, Oct. 27-30, 2010,
Ravenshaw University, Cuttack-753003
Contents Basics about diagnostics
Langmuir probe
V-I characteristics of single probe
Double Langmuir probe
Triple Langmuir probe
RF compensated probe
Practical Considerations
Conclusion
Various Plasma diagnostics techniques
Probes
Optical methods
Laser added diagnostics
Desirable Features
Non interference
Accuracy
Sensitivity
Resolution (spatial &temporal)
Automation
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Why diagnostics?
Plasma characterization
Process control
Relationship between external process variables to the plasma parameters and how
these are related to desired plasma processing
To understand the plasma process systematically and eventually for optimizing and
scaling of plasma reactors
Electrostatic Probes
Most widely used-- as old as plasma physics
Electrical probes are simple devices , consisting of one or more small metallic
electrodes immersed in to plasma
Can Measure Plasma density, electron temperature, Plasma potential, floating
potential &EEDF
Typical plasma parameters: Laboratory: ne = 107-1014 cm-3 , Te= 0.1-30 eV
Space : ne = 101-107 cm-3 , Te = 0 .1-10 eV
Single Probe Circuit
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V-I Characteristic of single Langmuir Probe
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Assumptions
Plasma is non-drifting
Electron energy distribution function (EDF) is isotropic and Maxwellian
Sheath thickness is smaller and electron- neutral collision mean free path is larger
than the probe dimension.
Probe surface is clean
Magnetic fields are absent
No secondary electron emission from probe
Single Probe Characteristics
Region I:
Probe is positive with respect to plasma space potential, near the probe, an electron sheath is
formed. When probe is at plasma space potential, there is no electric field between the
probe, both the negative and positive particles can reach the probe , since V e>> Vi , what is
predominately collected is electron current. In case of collision-less plasmas and if sheath is
thin
Ies = 0.25 Ap n e (8kTe / me)
Region II:
This region is called retarded field region and probe acts as an energy selector,
collecting only those electrons energetic enough to overcome the potential barrier. At
floating potential, the flux of electrons and ions reaching the probe is same and thus no
net current is drawn. In retarding potential region,
Ie = 0.25 Ap n e (8kTe / me) exp (evp / kTe)
= Ies exp (evp/kTe)
Te is determined from this region.
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Region III:
Because of large negative probe potentials, almost all electrons are repelled and we
have ion sheath and ion saturation current:
Iis = 0.605 Ap n e (k Te /mi)
Bohm sheath criteria, Plasma density is measured using this region
EEDF
Where fe ( ) is the electron energy distribution function, energy = e (Vpl V) given in eV, e
is the charge of an electron, A is the probe area and m e is mass of an electron.
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Kn < 10-3 Continuum region (high pressure)
In presence of magnetic field, an additional parameter, Larmor radius is to be considered
Double Langmuir Probe
Double Langmuir Probe Theory
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V-I Characteristic of Double Probe
V-I characteristic equation
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Triple Langmuir Probe
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Triple Probe analysis
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Experimental Results
RF generator -13.56 MHz, Power: 10-70 W, CCRF
Ar gas pressure 0.2-0.002 mbar
Cylindrical double probe made of tungsten wire, L-12mm, 1mm dia, probe
separation 15mm, bias from -80 V to 80 V
Tuning network consisted of a variable capacitance of 1-20 pf in parallel with 15 H
inductor. The filtering circuit was a low pass filter net work of 33 H inductor and
two 0.1f capacitors.
Variation of electron temperature with RF power
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Probe Construction
Material W.f. M. P.
Pt 5.36 eV 1769 C
W 4.53 eV 3395 C
Cleaning of the Probe by ion bombardment or by enough electron current to heat theprobe tip to in -cadence (1600 C)
Vacuum seal/ Wilson seal to permit the change in position of the probe
Conclusions
Basics about single, double and triple probes were discussed
Probe can determine electron temperature, plasma density, plasma potential and
floating potential.
EEDF can be determined
A tuned Langmuir probe can be used to eliminate rf interference
One must use proper theory for the interpretation of results
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General References
1. Plasma diagnostic techniques, Eds: R. H. Hudlestone and S. L. Leonard, p.113, A.P.,
New York, 1965
2. Plasma diagnostics, ed . Lochet-Holtgroven, Amesterdam: North- Holand, 1968
3. Reactions under plasma conditions, Vol.1, ed. M. Venugopalan, Wiley -Interscience,
New York, 1971
4. Principles of plasma diagnostics, I. H. Hutchinson, Cambridge University Press, 1987
5. Plasma diagnostic techniques, Eds: O. Auciello and D. L. Flamm, Academic Press,
New York, 1989
6. Electrical Probes for plasma diagnostics, J. D. Swift and M. J. Schwar, Elsevier
American, New York, 1969
7. Electrical probes in Stationary and Flowing plasmas, P. M. Chung, L. Talbot and
K. J. Touryan, Spring-Verlang, 1975
8. Understanding Langmuir probe current-voltage characteristics, Am. J. Phys., 75,
p.1078 (2007)
9. A tuned Langmuir probe for measurements in rf glow discharge, J. Appl. Phys.,67, p. 6718, 1990
10. Electron temperature measurements in UHV system by Spectroscopic and Langmuir
probe techniques, Vacuum, 46, p.223, 1995