Langmuir probe RF plasma compensation using a simulation method

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Abstract

The problem of Langmuir probe data deformation due to RF pickup by the probe is treated through a computer simulation method. It is pointed out that proper RF compensations can be obtained by treatment of the Langmuir probe raw data through the use of computer software. It is demonstrated that correct, RF unaffected probe IV characteristics can be accurately reproduced from the RF contaminated data. This eliminates the need for the use of any filters or other hardware procedures. User friendly matlab based software is presented. The software automatically retrieves the correct RF IV characteristics for single Langmuir probe data which consequently allows for proper evaluation of plasma parameters such as the plasma electron temperature, electron number density and the electron energy distribution function (EEDF)

Program summary

Program title: RF Compensation

Catalogue identifier: AEQR_v1_0

Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEQR_v1_0.html

Program obtainable from: CPC Program Library, Queen’s University, Belfast, N. Ireland

Licensing provisions: Copyright (c) 2009, aasim Azooz

All rights reserved.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

  • Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.

  • Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS “AS IS” AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

No. of lines in distributed program, including test data, etc.: 1269

No. of bytes in distributed program, including test data, etc.: 179353

Distribution format: tar.gz

Programming language: MATLAB 6.5 or higher.

Computer: Any laptop or desktop.

Operating system: Windows XP.

RAM: Bytes 512 K

Classification: 19.

Nature of problem:

In RF plasma Langmuir probe diagnostics, the probe IV characteristics obtained experimentally does not represent the true IV. This is because the probe picks up some RF plasma voltage which modulates the applied bias voltage and causes a current flow that corresponds to the RF affected bias rather that the actual DC bias. This if untreated can lead to false results for the values of plasma parameters derived from the obtained IV. Several hardware based methods are used to perform such correction (compensation).

Solution method:

The suggested method is based on filtration of raw uncompensated IV data through software operation rather than hardware based filtrations which have their limitations.

Running time:

A few milliseconds.

References:

[1] A.A. Azooz, Review of Scientific Instruments 79 (2008) 103501.

Introduction

Since Langmuir introduced his famous method of plasma diagnostics using the single probe, this probe is still being regarded as a powerful tool for measuring plasma properties. Interest has grown in using this technique over the past two decades due to the adaptation of the Langmuir probe in conjunction with computer data acquisition systems allowing for fast, reliable, and relatively easy ways for obtaining and analyzing IV probe data. Such information is always necessary for any glow discharge plasma application such as material deposition and surface treatment. Langmuir probe data for DC glow discharge are only affected by the plasma electromagnetic noise which is often small and can usually be eliminated by some straightforward methods. The situation is somewhat different as far as RF glow discharge plasmas are concerned  [1], [2], [3]. In spite of the fact that RF discharges are becoming more important from an application point of view, diagnostic methods for such plasmas are still regarded as a subject for discussion. Langmuir probe IV characteristics obtained for RF plasma are customarily regarded as not being truly representative of the real IV characteristics  [4], [5], [6]. This is due to the fact that the external bias applied to the probe is not in fact the actual probe voltage which induces the current flow to the probe. Two important effects play a role in modifying the probe IV characteristics. The first is the oscillating plasma potential. The second is that the probe and the system wiring picks up some RF voltage from the plasma. Consequently, the instantaneous probe current is a result of these two effects rather than being due to the DC bias alone. Unfortunately, the amplitude and phase of the RF voltage picked up by the probe and the plasma potential oscillations are usually undeterminable. This results in situations where such probe data are considered to contain a certain degree of ambiguity if suitable RF compensations are not applied. Several techniques of RF compensation have been used. These fall into three main categories. The first is called active compensation where in principle, another RF voltage of a frequency equal to that of the main plasma RF is superimposed on the probe DC bias  [7], [8], [9]. The amplitude and phase of this superimposed RF voltage are adjusted such that it can act to neutralize any RF picked up by the probe from the plasma. Readjustments are necessary when the plasma power, probe position or probe dimensions are changed. The second type of RF compensation is called passive compensation. It usually involves the use of passive circuit elements, usually band stop or low pass filters to prevent any RF induced current from passing through the probe leaving only the DC probe current to be measured  [10], [11], [12], [13], [14]. A third technique has recently gained increased interest. It is based on applying mathematical convolution to the RF contaminated IV probe characteristics to obtain the actual uncontaminated one  [15].

It is the purpose here to present simulation analysis assessing the way RF contamination affects single probe characteristics in order to devise a simple, yet accurate method to eliminate such contamination.

Section snippets

Modeling and simulation

Let us assume a Langmuir probe is being subjected to an RF field. Let us further assume that the probe is biased with an arbitrary DC voltage V. For modeling purposes, let us consider the frequency of the RF field to be equal to 13.56 MHz which is customarily used in plasma RF discharges. The results are identical for other frequencies. The overall voltage affecting the probe will be VP=V+A1sin(27.12×106πt).

Let us further assume that the plasma potential is oscillating about the mean DC plasma

The software

In the following we present a MATLAB computer code that performs the above two tasks. The software is freely available from the program library. The software is user friendly and its use does not require proficiency in matlab programming apart from some basics. It can be used on matlab version 6.5 or higher. The software is called by entering the statement RF_compensation (V, I, Area, Mi) in matlab workspace. The input arguments are the probe bias voltage (V), and probe current (I) data arrays

Experimentation

In order to put the above simulation results to experimental test, the experimental setup shown in Fig. 5 is used to produce capacitively coupled Argon plasma (CCP).

The system consists of a Teflon based bell jar glass chamber 20 cm in height and 15 cm inner diameter. Two circular flat well polished and cleaned aluminum electrodes are installed inside the chamber. The upper electrode is 12 cm in diameter. The lower electrode is 5 cm in diameter. The cap between the two electrodes is 5 cm. The

Results and discussion

The experimental raw data as acquired by the computer are shown as red dots in Fig. 6. The data show scattering of points over a wide region. Data at other RF power values show similar behavior. These data are subjected to three filtration processes to remove the first, second, and third harmonics. The results of the first filtration are indicated as diamonds surrounding the unfiltered data on the same figure. Data points resulting from the second filtration are shown as circles surrounding the

Conclusion

A new method for treatment of the problem of RF compensation in Langmuir probe plasma diagnostics is suggested and tested. The method replaces active or passive compensation procedures using experimental hardware systems by a simple, yet effective software treatment of uncompensated experimental data. Special matlab based software is written to perform this task. The software can be downloaded from the program library.

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Cited by (5)

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    Citation Excerpt :

    In radio-frequency (RF) plasmas, Langmuir probe is often used to determine the plasma parameters either by active [6,7] or passive compensation [8–12] of the RF oscillation. In recent years, theoretical and computational approaches have been gained considerable interest to analyse the RF contaminated I–V characteristics to obtain the original uncontaminated one [13,14]. For collisionless plasmas, the probe theory has been incessantly developed since 1926 due to the wide application of such plasmas in industrial purposes.

This paper and its associated computer program are available via the Computer Physics Communication homepage on ScienceDirect (http://www.sciencedirect.com/science/journal/00104655).

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