Electrostatic probes

Cheng, Tung, and Soo [J. Eng. Power, 92, 135-149 (1970)] describe the use of an electrostatic probe for measurement of solids flow in a gas-solids mixture. 

Plasma analysis is essential in order to compare plasma parameters with simulated or calculated parameters. From the optical emission of the plasma one may infer pathways of chemical reactions in the plasma. Electrical measurements with electrostatic probes are able to verify the electrical properties of the plasma. Further, mass spectrometry on neutrals, radicals, and ions, either present in or coming out of the plasma, will elucidate even more of the chemistry involved, and will shed at least some light on the relation between plasma and material properties. Together with ellipsometry experiments, all these plasma analysis techniques provide a basis for the model of deposition. 

In order to relate material properties with plasma properties, several plasma diagnostic techniques are used. The main techniques for the characterization of silane-hydrogen deposition plasmas are optical spectroscopy, electrostatic probes, mass spectrometry, and ellipsometry [117, 286]. Optical emission spectroscopy (OES) is a noninvasive technique and has been developed for identification of Si, SiH, Si+, and species in the plasma. Active spectroscopy, such as laser induced fluorescence (LIF), also allows for the detection of radicals in the plasma. Mass spectrometry enables the study of ion and radical chemistry in the discharge, either ex situ or in situ. The Langmuir probe technique is simple and very suitable for measuring plasma characteristics in nonreactive plasmas. In case of silane plasma it can be used, but it is difficult. Ellipsometry is used to follow the deposition process in situ. 

Zhu, C. and Soo, S. L. (1992). A Modified Theory for Electrostatic Probe Measurements of Particle Mass Flows in Dense Gas-Solid Suspensions. J. Appl. Phys., 72, 2060. 

Langmuir probes. Electrostatic probe measurements give access in principle to ne, (Fp), Te, and to the electron energy distribution. If the implementation is easy (collection of the current using a biased conductor), it is much more difficult to obtain reliable measurements because the method is very intrusive. Most of the probes have a cylindrical geometry, but some probes are planar or spherical. The following conditions have to be fulfilled preferably. Ideally the probe dimension has to be smaller than XD, to limit perturbation of the plasma, also the sheath thickness around the probe has to stay smaller than XUn or Xehx in order to limit... 

The ionization processes have been studied extensively by microwave absorption to detect electrons by electrostatic probes which may be biased to detect either negative species (almost entirely electrons) or positive ions, and by mass spectrometry, which has shown that molecule ions such as SrOH" are common (6). 

Figure 4. Oscilloscope traces from electrostatic probes passing through flames...

The total positive-ion yield at slightly rich and very rich conditions agrees with earlier work by Lester et al. (7) and by Zallen et al. (13). Microwave and electrostatic probe observations by Schneider and Gronig (14) in noble gases have yielded self-consistent ion and electron concentrations and give confidence that the electrostatic probe results are in fact suflBciently accurate in the present context. In a very recent study by Bowser and Weinberg (12), ion concentrations of ions/cm ... 

Jensen (4) has obtained equilibrium constants for the reaction HBOg + e"+ H + BOg". This Involves several assumptions the most basic of which Is that boron added to Hg/Ng/Og flames Is converted completely to HBOg. The free electrons are produced by addition of potassium to the flame, and their concentration Is measured directly by a microwave cavity resonance method. The hydrogen atom concentration Is taken from previous studies on such flames. The BOg" concentration Is obtained from the difference between the concentration and the free electron measurement. The concentration Is measured by an electrostatic probe. 

Fig. 2. SPV curve obtained by Dresner et a/. (1981) on a sample of a-Si H (sample D-09-04-0 with substrate temperature 240°C and sample width 2.3 fim) using a Kelvin electrostatic probe pickup. [From Dresner et al. (1981).]...

Langmuir Probe. A water-cooled electrostatic probe was constructed from several telescoping brass tubes around a 0.124 cm i.d. stainless steel tube containing a Pt/10% Rh probe wire insulated by fine quartz and Teflon sleeves. The probe wire, usually 0.025 cm diam, protruded 0.1 to 1.0 cm from the end of the tubes exposed to the flame gases. This probe assembly is referred to as the "fixed probe." A similar assembly was also used in which the Pt/... 

The measurement of the positive ion concentration in flames is most effectively carried out by diffusion-limited negatively biased electrostatic probes, frequently known as Langmuir probes. ... 

Soundy and Williams, using their rotating electrostatic probe, made a very extensive study of recombination rates, showing that the actual ion current measured was the result of the charge transfer process already discussed, and the subsequent ion-electron recombination process (Table 3.4). 

At zero time delay, the two parts of the signal correlate exactly, but after some long delay there is no coherence at all, and the correlation coefficient reaches zero. Some recent results obtained by Roberts and Williams which illustrate this for an electrostatic probe following ionization in a turbulent diffusion flame of hydrogen containing 1 % of acetylene. The time delay required for the correlation coefficient to reach a value of is known as the e-folding distance and, transformed into... 

We may use an electrostatic probe (e.g.. a unit positive charge) to detect, which parts of the molecule like the approaching charge (energy lowering), and which do not (energy increasing). 

G. Maise and A. J. Sabadell, Electrostatic Probe Measurements in Solid Propellant Rocket Exhausts, AIAA J. 8, 895-901 (1970). 

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