LP Diagnostics Cuttack

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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

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