Multipole Resonance Probe Research Articles

The multipole resonance probe: simultaneous determination of electron density and electron temperature using spectral kinetic simulation

The multipole resonance probe (MRP) is an innovative sensor suitable for monitoring and control of industrial plasma processes. It is a realization of ‘active plasma resonance spectroscopy’, a class of electromagnetic diagnostic methods which employ the ability of plasmas to resonate on or near the plasma frequency. A signal in the GHz range is fed to the plasma via an electrical probe; the spectral response S(ω) is recorded, and then evaluated with a mathematical model to obtain information on the internal plasma parameters. In this study, a spectral kinetic model of the MRP is discussed. It is superior to previous analyses based on the Drude model, as it allows to determine not only the electron density n e but also the electron temperature T e from S(ω). Good agreement with independent measurements shows the suitability of the model.

Kinetic investigation of the planar multipole resonance probe under arbitrary pressure

A new plasma diagnostic tool called planar multipole resonance probe (pMRP) has been proposed in the past decade. It has a minimally invasive structure and allows simultaneous measurement of electron density, temperature, and collision frequency. Previous work has investigated the behavior of the pRMP by the Drude model, which misses kinetic effects, and the collisionless kinetic model, which misses collisional damping. To further study the pMRP at arbitrary pressure, a collisional kinetic model is proposed in this paper. The electron dynamics is described by the kinetic equation, which considers the electron-neutral elastic collision. Under the electrostatic approximation, the kinetic equation is coupled to the Poisson equation. The real part of the general complex admittance is calculated to describe the spectral response of the probe–plasma system. The calculated spectra of the idealized pMRP demonstrate that this collisional kinetic model can capture both collisionless kinetic damping and collisional damping. This model overcomes the limitations of the Drude model and the collisionless kinetic model and allows discussion of the validity of simpler models.

Kinetic simulation of the ideal multipole resonance probe

Active plasma resonance spectroscopy (APRS) is a process-compatible plasma diagnostic method, which utilizes the natural ability of plasmas to resonate on or near the electron plasma frequency. The multipole resonance probe (MRP) is a particular design of APRS that has a high degree of geometric and electric symmetry. The principle of the MRP can be described on the basis of an idealized geometry that is particularly suited for theoretical investigations. In a pressure regime of a few Pa or lower, kinetic effects become important, which cannot be predicted by the Drude model. Therefore, in this paper, a dynamic model of the interaction of the idealized MRP with a plasma is established. The proposed scheme reveals the kinetic behavior of the plasma that is able to explain the influence of kinetic effects on the resonance structure. Similar to particle-in-cell, the spectral kinetic method iteratively determines the electric field at each particle position, however, without employing any numerical grids. The optimized analytical model ensures the high efficiency of the simulation. Eventually, the presented work is expected to cover the limitation of the Drude model, especially for the determination of the pure collisionless damping caused by kinetic effects. A formula to determine the electron temperature from the half-width Δω is proposed.

Measurement of inverted n-hexane fireball properties with a Multipole Resonance Probe

In this paper we present for the first time the measurement of inverted fireballs in a low-pressure Ar/n-hexane plasma by using a Multipole Resonance Probe. The basic plasma parameters such as electron density, electron temperature and Debye length as well as the particle collision frequency have been obtained for different experimental settings. These measurements show that inverted fireballs that are created in chemically active plasma offer advantageous properties regarding deposition techniques. Furthermore, the influence of the plasma parameters on grid bias and changes in neutral gas pressure has been studied within this work. It is observed that the plasma density within an inverted fireball configuration can be raised by more than an order of magnitude compared to the background plasma or without biasing the grid electrode, respectively.

On the Multipole Resonance Probe: Current Status of Research and Development

During the last decade a new probe design for active plasma resonance spectroscopy, the multipole resonance probe (MRP), was proposed, analyzed, developed, and characterized in two different designs: the spherical MRP (sMRP) and the planar MRP (pMRP). The advantage of the latter is that it can be integrated into the chamber wall and can minimize the perturbation of the plasma. Both designs can be applied for monitoring and control purposes of plasma processes for industrial applications. As usual for this measurement technique, a mathematical model is required to determine plasma parameter (electron density, electron temperature, and collision frequency of electrons with neutral atoms) from the measured resonances. Based on the cold plasma model a simple relationship between the resonance frequency and the electron density can be derived and leads to excellent measurement results. However, a simultaneous measurement of the electron temperature in low-pressure plasmas requires a kinetic model, because the half-width of the resonance peak is broadened by kinetic effects. Such a model has been derived and first results show the broadening of the spectra as expected. Deriving a relation between the half-width and the electron temperature will allow the simultaneous measurement and an improvement of monitoring and control concepts.

Kinetic investigation of the planar multipole resonance probe in the low-pressure plasma

Active Plasma Resonance Spectroscopy (APRS) is a well-established plasma diagnostic method: a radio frequency signal is coupled into the plasma via a probe or antenna, excites it to oscillate, and the response is evaluated through a mathematical model. The majority of APRS probes are invasive and perturb the plasma by their physical presence. The planar multipole resonance probe (pMRP) solves this problem: it can be integrated into the chamber wall and minimize the perturbation. Previous work has studied the pMRP in the frame of the Drude model, but it misses important kinetic effects like collision-less damping. In this work, a collision-less kinetic model is developed to further investigate the behavior of the pMRP. This model consists of the Vlasov equation, which is coupled with the Poisson equation under electrostatic approximation. The spectral response of the probe–plasma system is found by calculating the complex admittance. This model covers the kinetic effects and overcomes the limitations of the Drude model.

Multipole resonance probe measurement in plasma

In the study of low-temperature plasma, the plasma parameters such as plasma density and plasma temperature are required. Recently, a multipole resonance probe (MRP) which utilized an active plasma resonance concept was proposed. The probe consists of two hemispheres, insulated by a thin dielectric sheet. The advantages of the probe include simple setup and localized measurements similar to those of the conventional Langmuir probe. The probe does not cause plasma contamination, one of the Langmuir probe disadvantages since the probe conductors are covered in the dielectric. However, it can only determine plasma density. The MRP is usually connected with a Network Analyzer for measuring the reflection coefficient spectrum which can be analyzed to obtain the plasma density. In this experiment, the constructed MRP is used to measure the plasma density comparing to the Langmuir probe. The results of the experiment showed that the plasma density result of the MRP gives a similar tendency as that of the Langmuir probe. However, the plasma density determined by the MRP was deviated when compared with the Langmuir probe. In order to explore the idea that MRP does not need to be constructed in hemisphere shape, MRP in different shapes have been assembled and verified.

The Planar Multipole Resonance Probe: A Minimally Invasive Monitoring Concept for Plasma-Assisted Dielectric Deposition Processes

In this article, a novel minimally invasive approach to plasma monitoring in the challenging environment of dielectric deposition processes based on the planar multipole resonance probe (pMRP) is presented. By placing the sensor on the plasma-remote side of a dielectric substrate to be coated, perturbations of the process due to its presence can be significantly reduced. Since the electric field of the sensor is able to penetrate dielectric layers, a plasma supervision through the substrate is enabled. To investigate the effect of increasing coating thicknesses on the measurement performance for a broad spectrum of materials and plasma conditions, the results of extensive 3-D full-wave simulations performed with CST Microwave Studio are evaluated. Finally, real-time monitoring results of an argon-oxygen plasma during a sputter deposition with aluminum oxide on a polyethylene terephthalate (PET) film substrate together with a comparison to external process parameters are presented. The results demonstrate both the applicability of the proposed concept and its insensitivity to additional dielectric coatings.

The spherical multipole resonance probe: kinetic damping in its spectrum

The multipole resonance probe is one of the more recently developed measurement devices to measure plasma parameters like electron density and temperature based on the concept of active plasma resonance spectroscopy. The dynamical interaction between the probe and the plasma in an electrostatic, kinetic description can be modeled in an abstract notation based on functional analytic methods. These methods provide the opportunity to derive a general solution, which is given as the response function of the probe-plasma system. It is defined by the matrix elements of the resolvent of an appropriate dynamical operator. Based on the general solution a residual damping for vanishing pressure can be predicted and can only be explained by kinetic effects. Within this manuscript an explicit response function of the multipole resonance probe is derived. Therefore, the resolvent is determined by its algebraic representation based on an expansion in orthogonal basis functions. This allows us to compute an approximated response function and its corresponding spectra, which show additional damping due to kinetic effects.

Determination of atomic oxygen state densities in a double inductively coupled plasma using optical emission and absorption spectroscopy and probe measurements

A collisional radiative model for fast estimation and monitoring of atomic oxygen ground and excited state densities and fluxes in varying Ar:O2 mixtures is developed and applied in a double inductively coupled plasma source at a pressure of 5 Pa and incident power of 500 W. The model takes into account measured line intensities of 130.4 nm, 135.6 nm, 557.7 nm, and 777.5 nm, the electron densities and electron energy distribution functions determined using a Langmuir probe and multipole resonance probe as well as the state densities of the first four excited states of argon measured with the branching fraction method and compared to tunable diode laser absorption spectroscopy. The influence of cascading and self absorption is included and the validity of the used cross sections and reaction rates is discussed in detail. The determined atomic oxygen state densities are discussed for their plausibility, sources of error, and compared to other measurements. Furthermore, the results of the model are analyzed to identify the application regimes of much simpler models, which could be used more easily for process control, e.g. actinometry.

The Multipole Resonance Probe-based controller: a technology to investigate plasma-based deposition

With the objective to improve plasma-based deposition processes new sensors for the investigation of the plasma-surface interaction have been developed over the last years. However, the direct access and the reproducible operation of specific plasma conditions are yet unconsidered but required to receive related measurement results. Hence, desired unstable operation points have to be stabilized by feedback controllers, which are also able to automatically compensate process drifts. Existing control systems for plasma-based deposition rely on secondary process variables, but these variables cannot be correlated with the plasma variables as a general rule and, therefore, a direct study of the plasma is limited. To lift this restriction a novel electron-plasma-frequency-based control instrument is proposed, which uses the Multipole Resonance Probe, a feedback controller and a signal-based estimator. The control system directly measures the electron plasma frequency, online estimates the process mode and calculates a related control input to achieve and hold a prescribed working point. Experiments are shown as a proof of concept for the motivation of a plasma-based control system, the identification of a control-oriented model and the applicability of the instrument in an argon-oxygen mixture.

The planar Multipole Resonance Probe: a functional analytic approach

Active Plasma Resonance Spectroscopy (APRS) is a well known diagnostic method, where a radio frequency probe is immersed into a plasma and excites plasma oscillations. The response of the plasma is recorded as frequency dependent spectrum, in which resonance peaks occur. By means of a mathematical model plasma parameters like the electron density or the electron temperature can be determined from the detected resonances.The majority of all APRS probes have in common, that they are immersed into the plasma and perturb the plasma due to the physical presence of the probe. Thus, they are invasive and can at least influence the homogeneity of the plasma. To overcome this problem, the planar Multipole Resonance Probe (pMRP) was invented, which can be integrated into the chamber wall of a plasma reactor.Within this paper, the first analytic model of the pMRP is presented, which is based on a cold plasma description of the electrons. The general admittance of the probe-plasma system is derived by means of functional analytic methods and a complete orthonormal set of basis functions. Explicit spectra for an approximated admittance including a convergence study are shown. The determined resonance frequencies are in good agreement with former simulation results.

Nanoparticle forming reactive plasmas: a multidiagnostic approach

With an ever increasing interest in functional materials based on nanoparticles a large amount of research in this field is dedicated to the development of new production methods for nanoparticles. A promising class of methods for the production of nanoparticles is reactive plasmas. However, since the particle formation process and the interaction between the particles and the plasma are so far not completely understood, it remains difficult to control the particle formation. As the interaction between the nanoparticles and the plasma in which they are dispersed is complex the use of one or two diagnostics often provides only an incomplete understanding of the involved processes. Thus a multidiagnostic approach is needed. This contribution reviews the latest results from the study of nanoparticle formation in a hydrocarbon-based reactive plasma by such a multidiagnostic approach. It is shown that the use of various diagnostics like an IV-probe, optical emission spectroscopy, and a multipole resonance probe in conjunction with an investigation of the particle formation provides a much more detailed picture of these interesting, yet challenging, systems.

Comparison of Langmuir probe and multipole resonance probe measurements in argon, hydrogen, nitrogen, and oxygen mixtures in a double ICP discharge

The results of a Multipole Resonance Probe (MRP) are compared to a Langmuir probe in measuring the electron density in Ar, H2, N2, and O2 mixtures. The MRP was designed for measurements in industry processes, i.e., coating or etching. To evaluate a possible influence on the MRP measurement due to molecular gases, different plasmas with increasing molecular gas content in a double inductively coupled plasma at 5 Pa and 10 Pa at 500 W are used. The determined electron densities from the MRP and the Langmuir probe slightly differ in H2 and N2 diluted argon plasmas, but diverge significantly with oxygen. In pure molecular gas plasmas, electron densities measured with the MRP are always higher than those measured with the Langmuir Probe, in particular, in oxygen containing mixtures. The differences can be attributed to etching of the tungsten wire in the Ar:O2 mixtures and rf distortion in the pure molecular discharges. The influence of a non-Maxwellian electron energy distribution function, negative ions or secondary electron emission seems to be of no or only minor importance.

Implications of electron heating and non-uniformities in a VHF-CCP for sterilization of medical instruments

A capacitively coupled plasma driven at a frequency of 81.36 MHz from the VHF-band is investigated by means of optical emission spectroscopy (OES) and multipole resonance probe (MRP). The discharge is operated with hydrogen, yielding an electropositive discharge, as well as oxygen, yielding an electronegative discharge, and mixtures of both. Pressure is varied from Pa to Pa. Homogeneity of the discharge is investigated by CCD camera recordings as well as spatially resolved multipole resonance probe measurements. The results indicate the presence of electromagnetic edge effects as well as standing wave effects. Furthermore, a largely homogeneous discharge can be achieved with hydrogen as process gas at a pressure of –10 Pa. With increasing pressure as well as with increasing oxygen content, the discharge appears less homogeneously. The transition from an electropositive to an electronegative discharge leads to a change in electron heating mechanisms, with pronounced local maxima of electron density at the sheath edges. A comparison of OES and MRP results reveal a significant difference in electron density, which can be explained by a non-Maxwellian distribution function of electrons.

The Planar Multipole Resonance Probe: Challenges and Prospects of a Planar Plasma Sensor

A novel compact plasma sensor applicable for the supervision and control of industrial plasma processes is presented in this contribution. Based on the multipole resonance probe (MRP), the new planar MRP (pMRP) is introduced as a powerful and economical monitoring tool, flush-mounted into the reactor wall. Hence, it can be used for an effective suppression of disturbances of the plasma process itself. Using 3D electromagnetic field simulations with CST Microwave Studio, the pMRP is investigated and challenges as well as prospects of the new sensor design are discussed in detail. Three different sensor versions are presented and compared with the resonance behavior of the MRP. Furthermore, limitations concerning position tolerances are shown and the suitability of the pMRP is proven. Measurements in a double inductive coupled plasma, with argon as process gas and varying excitation powers, demonstrate the suitability of the pMRP for monitoring purposes.

The Multipole Resonance Probe: Progression and Evaluation of a Process Compatible Plasma Sensor

A robust and sensitive plasma sensor, the multipole resonance probe (MRP), and its process compatibility are presented and discussed in this paper. Based on its innovative concept and simple model describing the system “probe-plasma”, three steps of development are introduced. 3D electromagnetic field simulations are applied as an indispensable tool for an economical and efficient investigation and optimization of different sensor layouts. Independent of the chosen sensor design, a developed pulse-based measurement device yields an economical signal generation and evaluation. Electron density profiles, determined with the MRP and the pulse-based system utilized in a capacitive coupled plasma, confirm and demonstrate the simulation results and the measurement concept, respectively.

Active plasma resonance spectroscopy: eigenfunction solutions in spherical geometry

The term active plasma resonance spectroscopy denotes a class of related techniques which utilize, for diagnostic purposes, the natural ability of plasmas to resonate on or near the electron plasma frequency ωpe: a radio frequent signal (in the GHz range) is coupled into the plasma via an antenna or probe, the spectral response is recorded, and a mathematical model is used to determine plasma parameters like the electron density. The mathematical model of an arbitrarily shaped probe–plasma system can be written in an abstract but very compact equation. It contains an appropriate operator, which describes the dynamical behavior and can be split into a conservative and a dissipative part. Based on the cold plasma model, this manuscript provides a solution strategy to determine the electrical admittance of a specific probe–plasma system derived from the abstract dynamical equation. Focusing on probes with a spherical-shaped probe tip the general admittance can be derived analytically. Therefore, the matrix representation of the resolvent of the dynamical operator is determined. This matrix representation is derived by means of the eigenfunctions and eigenvalues of the conservative operator. It can be shown that these eigenvalues represent the resonance frequencies of the probe–plasma system which are simply connected to the electron density. As an example, the result is applied to established probe designs: the spherical impedance probe and the multipole resonance probe.

Study on electrostatic and electromagnetic probes operated in ceramic and metallic depositing plasmas

This paper discusses plasma probe diagnostics, namely the multipole resonance probe (MRP) and Langmuir probe (LP), operated in depositing plasmas. The aim of this work is to show that the combination of both probes provides stable and robust measurements and clear determination of plasma parameters for metallic and ceramic coating processes. The probes use different approaches to determine plasma parameters, e.g. electron density ne and electron temperature Te. The LP is a well-established plasma diagnostic, and its applicability in technological plasmas is well documented. The LP is a dc probe that performs a voltage sweep and analyses the measured current, which makes it insensitive against conductive metallic coating. However, once the LP is dielectrically coated with a ceramic film, its functionality is constricted. In contrast, the MRP was recently presented as a monitoring tool, which is insensitive to coating with dielectric ceramics. It is a new plasma diagnostic based on the concept of active plasma resonance spectroscopy, which uses the universal characteristic of all plasmas to resonate on or near the electron plasma frequency. The MRP emits a frequency sweep and the absorption of the signal, the |S11| parameter, is analysed. Since the MRP concept is based on electromagnetic waves, which are able to transmit dielectrics, it is insensitive to dielectric coatings. But once the MRP is metallized with a thin conductive film, no undisturbed RF-signal can be emitted into the plasma, which leads to falsified plasma parameter.In order to compare both systems, during metallic or dielectric coating, the probes are operated in a magnetron CCP, which is equipped with a titanium target. We present measurements in metallic and dielectric coating processes with both probes and elaborate advantages and problems of each probe operated in each coating environment.

On the electrical asymmetry effect in large area multiple frequency capacitively coupled plasmas

Recently, many publications have dealt with the electrical asymmetry effect in capacitively coupled radio frequency-discharges. The idea of this concept is the possibility of controlling the self-bias voltage by tuning the relative phase of harmonics in relation to the fundamental wave. In this work, we apply the electrical asymmetry effect on a large-area multiple frequency capacitively coupled plasma used for reactive sputtering by varying the relative phase of the 13.56 and 27.12 MHz excitation. The resulting voltage waveforms at the electrode are recorded using a high-voltage probe. The shape of the waveform is then analysed by Fourier analysis to study the influence of higher harmonics excited at the non-linearity of the plasma boundary sheath. To investigate the influence of the relative phase on the plasma itself, radially resolved multipole resonance probe measurements are performed.