Microwave Resonator Probe Research Articles

Diagnostic Capabilities of a Microwave Resonator Probe for Studies of the Parameters of a Nonstationary Magnetoplasma

Diagnostic Capabilities of a Microwave Resonator Probe for Studies of the Parameters of a Nonstationary Magnetoplasma

Density irregularities, currents, and magnetic fields generated by pulsed local rf heating of a magnetoplasma: Disturbances in rf antenna vicinity

The dynamics of narrow, field-aligned magnetoplasma irregularities is studied, which develop under the action of a short rf pulse. The laboratory experiment is aimed at demonstrating the rapid, so-called “unipolar” plasma transport mode, which is accompanied by excitation of eddy currents, in the case of localized rf heating of plasma electrons. The experimental parameters are chosen in a special way. The size of the heating spot, determined by the diameter of the loop antenna, exceeds the electron gyroradius significantly but is smaller than the ion gyroradius. The rf pulse duration encompasses several electron collision times but is shorter than the gyroperiod of ions. As a result, the electrons, which are strongly magnetized, acquire energy in rf antenna vicinity and can escape the heating region only along the magnetic field B0. In turn, collisionless ions can travel across B0 under the action of space-charge electric fields. For these conditions, redistribution of the plasma occurs with “unipolar” transport coefficients and is accompanied by excitation of electric currents. Weak plasma density disturbances, which are less than 5% of the background, are measured precisely with a microwave resonator probe. Parallel electron currents are obtained from magnetic probe measurements; the ion current across B0 is restored from the density profile modifications in their dynamics. It is shown that the ions traveling across B0 with a velocity about one third of the ion-acoustic velocity can easily close the current loop, which is driven by the parallel motion of heated electrons. This regime of plasma irregularities evolution is discussed in application to previous laboratory measurements, as well as to active ionospheric experiments.

Real-time curling probe monitoring of dielectric layer deposited on plasma chamber wall

A microwave resonator probe called a curling probe (CP) was applied to in situ monitoring of a dielectric layer deposited on a chamber wall during plasma processing. The resonance frequency of the CP was analytically found to shift in proportion to the dielectric layer thickness; the proportionality constant was determined from a comparison with the finite-difference time-domain (FDTD) simulation result. Amorphous carbon layers deposited in acetylene inductively coupled plasma (ICP) discharge were monitored using the CP. The measured resonance frequency shift dictated the carbon layer thickness, which agreed with the results from the surface profiler and ellipsometry.

Helicon double layer thruster operation in a low magnetic field mode

Direct thrust measurements are made of a helicon double layer thruster operating in a low magnetic field mode. The relationship between the imposed axial magnetic field and generated thrust is investigated for a radio frequency input power range 200–500 W for propellant flow rates of 16.5 and 20 sccm (0.46 and 0.55 mg s−1) of argon. The measured thrust shows a strong dependence on the magnetic field strength, increasing by up to a factor of 5 compared with the minimum thrust level recorded. A peak thrust of 0.4–1.1 mN depending on thruster operating conditions is obtained. This increase is observed to take place over a small range of peak magnetic field strengths in the region of 70–110 G. The magnitude of the thrust and the corresponding magnitude of the magnetic field at which the peak thrust occurs is shown to increase with increasing input power for a given propellant flow rate. The ion current determined using a retarding field energy analyser and the electron number density found using a microwave resonator probe both correlate with the observed trend in thrust as a function of applied magnetic field.

Modulation of microwave resonance probes

In this paper we describe further development of a type of microwave resonator probe known as a hairpin probe. The first section introduces the probe and explains how it can be used to measure electron density in plasmas by measuring plasma frequency via a change of microwave resonance of the probe. Later sections describe a new technique for easier and more accurate measurement of the resonance peak and, indirectly, electron density.

Application of floating microwave resonator probe to the measurement of electron density in electronegative capacitively coupled plasma

In electronegative or reactive plasmas, the problems such as negative ions floating near the sheath edge or deposition contamination cause more challenges for the diagnosis of conventional Langmiur probe. The electron density measured by microwave resonance probe is only a function of dielectric constant of plasma, there should be less or no influence of electronegative or reactive plasma. In this paper, a floating microwave resonator probe is proposed to measure electron density of capacitively coupled Ar plasma. A comparison with Langmuir double probe measurement shows that microwave resonance probe is applicable for measuring low electron density of plasma. The experimental results from the measurements of Ar/SF6 and SF6/O2 capacitively discharge driven by 40.68 MHz show that addition of SF6 into Ar plasma reduces the electron density significantly, with further increase of SF6 flow rate, electron density shows a gradual decrease. While for the addition ofO2 into SF6 discharge, the electron density continuously decreases with the increase ofO2 flow rate. Additionally, the electron density does not vary with lower frequency input power for SF6/O2 capacitively discharge driven by 40.68 MHz/13.56 MHz. The preliminary interpretations of the above experimental phenomena are presented.

Electron Density Range Measurable by Microwave Resonator Probe with Higher Mode Resonance

Electron Density Range Measurable by Microwave Resonator Probe with Higher Mode Resonance

Electron Density Range Measurable by Microwave Resonator Probe with Higher Mode Resonance

A microwave resonator probe is a simple tool for measuring the electron density of 1010–1012 cm-3 based on the plasma-induced shift Δf in the resonance frequency of a U-shaped wire antenna. However, when the electron density is as low as 108–1010 cm-3, the measurement becomes difficult because of the small Δf (low-density limit), and partly because of the reduction in resonance signal amplitude (high-density limit). Here, the measurable electron density range for the given antenna length is elucidated by taking into account these limitations and the instrumental limit of the network analyzer system used in the measurement. To expand the measurable electron density range to measure lower densities, we propose the use of the second-harmonic resonance. In addition to the analysis of the measurable electron density range, the experiments using the harmonic resonance are presented in terms of the electron density dependences on the discharge power and pressure in a surface wave plasma at 2.45 GHz.

Plane-Type Microwave Resonator Probe for Electron Density Measurement and the Sheath Effects in Reactive Processing Plasmas

This paper reports plane-type microwave resonator (MWR) probe for electron density measurements in reactive processing plasmas. The sheath effects were investigated by electromagnetic field analysis with FDTD simulation, and the calculated characteristics were confirmed experimentally. The FDTD simulation revealed that the sheath formed around the probe leads to underestimation of electron density, in comparison to the real density given as an initial condition in the simulation, which is caused by a decrease in the effective dielectric constant. An increase in a slit width of the probe antenna suppressed the density underestimation. Actually, as the slit width increased, the electron density measured by the MWR probe approached that obtained by a reference surface wave probe. Therefore, it is useful to enlarge the slit width for improvement of accuracy of the density measurement with the MWR probe.

Simulation of resistive microwave resonator probe for high-pressure plasma diagnostics

A U-shape probe for microwave resonance in plasma was investigated in a range of high pressure (10–760 Torr). The electromagnetic field finite difference time domain-simulation of the probe revealed that, in the resistive resonance at high pressure, the resonant frequency fr approaches the dipole-antenna resonance frequency in vacuum, with almost no dependence on the electron density ne. However, the resonance width Δf is sensitive to the pressure, showing the maximum value at ∼20 Torr. The normalized resonance width (Δf/fr) corrected by the one of vacuum value (no plasma) is linearly proportional to ne, and inversely proportional to νm (electron–molecule collision frequency) for high-pressure plasmas. These simulation results are successfully explained in a model of two-conductor transmission line in cold collisional plasma. According to the resonance analysis, the methods for measuring the electron density and the electron collision frequency at high pressures (, ) were proposed.

The hairpin resonator: A plasma density measuring technique revisited

A microwave resonator probe is a resonant structure from which the relative permittivity of the surrounding medium can be determined. Two types of microwave resonator probes (referred to here as hairpin probes) have been designed and built to determine the electron density in a low-pressure gas discharge. One type, a transmission probe, is a functional equivalent of the original microwave resonator probe introduced by R. L. Stenzel [Rev. Sci. Instrum. 47, 603 (1976)], modified to increase coupling to the hairpin structure and to minimize plasma perturbation. The second type, a reflection probe, differs from the transmission probe in that it requires only one coaxial feeder cable. A sheath correction, based on the fluid equations for collisionless ions in a cylindrical electron-free sheath, is presented here to account for the sheath that naturally forms about the hairpin structure immersed in plasma. The sheath correction extends the range of electron density that can be accurately measured with a particular wire separation of the hairpin structure. Experimental measurements using the hairpin probe appear to be highly reproducible. Comparisons with Langmuir probes show that the Langmuir probe determines an electron density that is 20–30% lower than the hairpin. Further comparisons, with both an interferometer and a Langmuir probe, show hairpin measurements to be in good agreement with the interferometer while Langmuir probe measurements again result in a lower electron density.

Two-wire microwave resonator probe

Results are presented from theoretical and experimental studies of the influence of ponderomotive effects on the operation of a two-wire plasma microwave resonator probe. It is shown that the nonlinear regime of probe operation can be used to measure not only the plasma density, but also the plasma temperature.

Plasma electron density generated by a semiconductor bridge as a function of input energy and land material

The plasma densities generated from a semiconductor bridge (SCB) device employing a capacitor discharge firing set have been measured using a microwave resonator probe in vacuum (/spl les/10/sup -5/ torr). The spatial resolution of the probe is comparable to the separation between the two wires of the transmission line (/spl ap/3 mm). This method is superior to Langmuir probes in this low density, small volume application because Langmuir probe measurements are affected by sheath effects, small bridge area, and unknown fraction of multiple ions. Measured electron densities are related to the land material and input energy, Although electron densities in the plasma generated by aluminum or tungsten-land SCB devices show a general tendency to increase steadily with input power, at the higher energies, the electron densities generated from the tungsten-land SCB devices are found to remain constant.

Two-photon ionization of trimethylamine using KrF laser radiation

The multiphoton ionization of a low-pressure (3×10−5 Torr) fill of trimethylamine using an electron-beam pumped KrF laser has been studied. A microwave resonator probe was used to measure the resultant free-electron density. The scaling of electron density with the square of the incident laser intensity was verified in the low flux limit. The electron production decreased significantly from the I2 scaling at laser fluxes exceeding 4 MW/cm2. These results are consistent with a simple two-photon model describing the interaction in the low flux limit and a saturation of the electron production in the high-flux limit.

Determination of the temporal plasma density above a semiconductor bridge

A new method of determining the temporal plasma density above a semiconductor bridge using a microwave resonator probe has been proposed. Experimental results indicate that the plasma density is observed to increase to a peak value of 4.2 × 1011 cm-3 and decay exponentially with time, which is consistent with diffusion dominating the plasma transport.

Measurement of Plasma Density Generated by a Semiconductor Bridge: Related Input Energy and Electrode Material

The plasma densities generated from a semiconductor bridge (SCB) device employing a capacitor discharge firing set have been measured by a novel diagnostic technique employing a microwave resonator probe. The spatial resolution of the probe is comparable to the separation between the two wires of the transmission lines (≈ 3 mm). This method is superior to Langmuir probes in this application because Langmuir probe measurements are affected by sheath effects, small bridge area, and unknown fraction of multiple ions. Measured electron densities are related to the land material and input energy. Although electron densities in the plasma generated by aluminum or tungsten-land SCB devices show a general tendency to increase steadily with power, at the higher energies, the electron densities generated from tungsten-land SCB devices are found to remain constant.

Temporal measurement of plasma density variations above a semiconductor bridge (SCB)

The plasma density variations above a semiconductor bridge (SCB) device employing a capacitor discharge firing set have been measured in vacuum (/spl les/10/sup -5/ torr). A novel diagnostic technique using a microwave resonator probe was used to measure the resultant plasma density variations as a function of time. This method is superior to Langmuir probes in this application because Langmuir probe measurements are affected by sheath effects, small bridge area, and unknown fraction of multiple ions. Our experimental results indicate that the plasma density is observed to increase to a peak value of 5.5/spl times/ 10/sup 11/ cm/sup -3/ and decay exponentially with time, which is consistent with diffusion dominating the plasma transport. The time needed for reaching the peak plasma density is about 1.6 /spl mu/s, which is the duration of the late time discharge of an SCB device.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Measurement of plasma electron density generated by a semiconductor bridge (SCB)

To gain better understanding of semiconductor bridge (SCB) discharge behaviour, we have measured the plasma electron density generated by SCBs using a microwave resonator probe technique in a vacuum (<or=10/sup -5/ torr). This method is superior to Langmuir probes in this application because of sheath effects, small bridge areas, and unknown fraction of multiple ions.

Microwave resonator probe for localized density measurements in weakly magnetized plasmas

A simple diagnostic tool for performing spatially resolved density measurements in a large, weakly magnetized, low-temperature plasma is described. The principle is based on a microwave technique to determine the cold plasma dielectric property ε=1−ωp2/ω2, where ωp is the electron plasma frequency. A parallel-wire quarter-wavelength resonator is immersed into the plasma, the resonance frequency (ωres≳ωp) is measured and the density derived from the simple relation ωp2=ω2res−ω2res(ε=1). In contrast to the familiar cavity resonance shift method, the present method is suited for localized density measurements. The spatial resolution is comparable to the size of the resonator (2-mm width×8 mm length, fres=7.45 GHz). The microwave technique is largely independent of sheath and thermal effects and is thus more reliable than probe measurements in nonuniformly rf-heated magnetoplasmas. A comparison with independent density diagnostic tools is presented and additional applications are pointed out.