arXiv subjects

Julian Schulze

Publications and source records attributed to Julian Schulze.

At least 19 recordsLinked to original sources

arXiv2026

Non-invasive monitoring of process-relevant plasma parameters by Fiber PROES in capacitively coupled Ar/CF$_4$ plasmas

Monitoring changes of process-relevant plasma parameters, such as the electron density and ion flux to the wafer, is essential for the development and control of plasma processes. However, invasive plasma diagnostics, such as probe measurements, typically cannot be applied to commercial reactors. At the example of a low pressure capacitive radio frequency discharge operated in different mixtures of CF$_4$ and Ar, we demonstrate that changes of such plasma parameters can be monitored non-invasively by phase resolved optical emission spectroscopy via optical fibers (Fiber PROES), for which the ports are usually available at industrial plasma sources. In this way, the spatio-temporally resolved dynamics of energetic electrons are tracked by observing a selected emission line. By measuring the electron density and ion flux directly via probe and retarding field energy analyzer diagnostics as a function of driving voltage and pressure, changes of these plasma parameters, including hysteresis effects, are found to be correlated with transitions of the electron power absorption mode revealed by Fiber PROES. Such mode transitions cause the electron energy distribution function (EEDF) to change and, thus, affect such plasma parameters. Based on these findings, Fiber PROES can be used as a non-invasive diagnostic for the monitoring and knowledge-based development of plasma processes.

physics.plasm-ph↗

arXiv2026

Control of Electron Energy Distribution Functions by Current Waveform Tailoring in Inductively Coupled Radio Frequency Plasmas

Based on two-dimensional particle-in-cell simulations a novel approach towards Electron Energy Probability Function (EEPF) and plasma chemistry control by Current Waveform Tailoring (CWT) in the coil of inductively coupled discharges is proposed. Varying the shape of this current waveform provides electrical control of the dynamics of the electric field in the plasma. Using sawtooth instead of sinusoidal waveforms allows breaking and controlling the temporal symmetry of the electric field dynamics. In this way CWT allows controlling the EEPF, the ionization-to-excitation rate ratio, and the plasma chemistry.

physics.plasm-ph↗

arXiv2025

Multi-diagnostic characterization of inductively coupled discharges with tailored waveform substrate bias for precise control of plasma etching

Precise control of ion energy distribution functions (IEDF) is crucial for selectivity as well as control over sputter rate and substrate damage in nanoscale plasma processes. In this work, a low frequency (100 kHz) tailored pulse-wave-shaped bias voltage waveform is applied to the substrate electrode of an inductively coupled plasma (ICP) and its effects on the IEDF, electron density, electron dynamics and the etch rates of silicon dioxide as well as amorphous silicon are investigated in a commercial 200 mm reactive ion etching (RIE) reactor. While the tailored waveform substrate bias hardly affects the electron density above the substrate and the spatio-temporally resolved electron power absorption dynamics, it is found to affect the ion flux to the substrate at high ICP source powers. Monoenergetic IEDFs with a full width at half maximum (FWHM) below 10 eV are realized with mean ion energies ranging from 20 eV to 100 eV in both argon and SF6. Such monoenergetic IEDFs are used to determine the Ar ion sputter threshold energies of amorphous silicon and silicon dioxide to be 23 eV and 37 eV, respectively, and to realize selective etching of these two materials by Ar ion sputtering based on tailoring the IEDF to ensure that all incident ions are within this narrow ion energy selectivity window.

physics.plasm-ph↗

arXiv2025

Two-dimensional spatially resolved measurements of helium metastable densities by tunable diode laser absorption spectroscopy in atmospheric pressure RF plasma jets

Helium metastable species play a critical role in sustaining radio-frequency (RF) driven micro atmospheric pressure plasma jets through Penning ionization and for the generation of reactive oxygen and nitrogen species (RONS). Their densities are typically measured using tunable diode laser absorption spectroscopy (TDLAS). Most spatially resolved TDLAS approaches rely on mechanical scanning of a narrow laser beam across the plasma, which is time-consuming and limits spatial resolution. In this work, we present an advanced two-dimensional (2D) TDLAS method that enables direct spatial mapping of helium metastable densities without the need for mechanical scanning. A rotating optical diffuser is employed to suppress speckle interference and generate uniform illumination across the plasma region. The absorption profile is captured using a short-wavelength infrared camera equipped with a telecentric lens, achieving high spatial resolution (approximately 10 μm) across the entire field of view. This approach significantly enhances both data quality and acquisition speed. The improved 2D TDLAS system is applied to measure helium metastable densities in plasma jets with structured electrodes driven by different tailored voltage waveforms. The results show very good qualitative agreement with fluid simulations and previously reported experimental data.

physics.plasm-ph↗

arXiv2025

Synergistic control of radical generation in a radio frequency atmospheric pressure plasma jet via voltage waveform tailoring and structured electrodes

The synergy between voltage waveform tailoring and structured electrodes is investigated in a radio-frequency (RF) atmospheric-pressure microplasma jet operated in helium with a 0.1% oxygen admixture. The device incorporates rectangular trenches in both electrodes and is driven by "Peaks" and "Valleys" waveforms synthesized from four harmonics (base frequency $f_{\rm b} = 13.56$~MHz, $V_{\rm pp} = 500$~V, $P=$1.2~W). Two-dimensional plasma fluid simulations, together with spatially and temporally resolved optical diagnostics (Phase-Resolved Optical Emission Spectroscopy and Tunable Diode Laser Absorption Spectroscopy), are used to demonstrate that the combination of asymmetric voltage waveforms with electrode structuring leads to strong spatial localization of electron power absorption and radical generation. This synergy results in a single pronounced maximum inside a trench at either the powered or grounded electrode, depending on the applied waveform, unlike a symmetric excitation, which produces a spatially symmetric enhancement at both electrodes. The effect is attributed to the interplay between waveform-induced sheath dynamics and geometric focusing provided by the trenches, enabling electrically reversible and selective enhancement of electron power absorption at a chosen location.

physics.plasm-ph↗

arXiv2025

Electron dynamics of three distinct discharge modes of a cross-field atmospheric pressure plasma jet

This paper investigates the electron dynamics in three distinct discharge modes of a cross-field atmospheric pressure plasma jet, the COST-Jet. Thereby, the discharge modes are the non-neutral, the quasi-neutral, and the constricted mode. Using a hybrid Particle-In-Cell/Monte-Carlo Collisions (PIC/MCC) simulation, the study systematically varies the applied voltage and driving frequency to explore the operation modes and their relations. The results reveal that at low input power, the COST-Jet operates in a non-neutral mode, characterized by a discharge close to extinction, analogous to the chaotic mode observed in other plasma devices. As power increases, the jet transitions to a quasi-neutral mode, which aligns with the well-known Ω- and Penning modes, comparable to the bullet mode in parallel-field jets. At the highest power levels, the COST-Jet enters a constricted mode, where the plasma significantly densifies and constricts towards the electrodes along the entire discharge channel. Experimental validation using phase-resolved optical emission spectroscopy (PROES) supports the simulation findings, particularly identifying the constricted mode as a distinct operational regime. These insights into the mode transitions of the COST-Jet under varying operational conditions help optimize plasma applications in various fields.

physics.plasm-ph↗

arXiv2024

Cs-O$_2$-Li as enhanced NEA surface layer with increased lifetime for GaAs photocathodes

GaAs-based photocathodes are the only viable source capable of providing spin-polarized electrons for accelerator applications. This type of photocathode requires a thin surface layer, in order to achieve negative electron affinity (NEA) for efficient photo-emission. However, this layer is vulnerable to environmental and operational effects, leading to a decay of the quantum efficiency $η$ characterized by a decay constant or lifetime $τ$. In order to increase $τ$, additional agents can be introduced during the activation procedure to improve the chemical robustness of the surface layer. This paper presents the results of recent research on Li as enhancement agent for photocathode activation using Cs and O$_2$, forming Cs-O$_2$-Li as enhanced NEA layer. Measurements yielded an increase in lifetime by a factor of up to 19 $\pm$ 2 and an increase in extracted charge by a factor of up to 16.5 $\pm$ 2.4, without significant reduction of $η$. This performance is equal to or better than that reported for other enhanced NEA layers so far.

physics.acc-ph↗

arXiv2024

Electron dynamics and SiO2 etching profile evolution in capacitive Ar/CHF3 discharges driven by sawtooth-tailored voltage waveforms

The electron dynamics and SiO2 etching profile evolution in capacitively coupled Ar/CHF3 plasmas driven by sawtooth-waveforms are investigated based on a one-dimensional fluid/Monte-Carlo (MC) model coupled with an etching profile evolution model. The effects of the sawtooth-waveforms synthesized from different numbers of consecutive harmonics, N, of a fundamental frequency of 13.56 MHz on the electron dynamics, ion and neutral transport, as well as the etching profile evolution are revealed in different mixtures of Ar/CHF3. By increasing N, a reduction in electronegativity, a decrease of the DC self-bias voltage, and a transition of the discharge mode from the Drift-Ambipolar (DA) to an α-DA hybrid mode is observed accompanied by an enhanced plasma asymmetry. As the CHF3 gas admixture increases, the electronegativity initially increases and then decreases, following a similar trend as the absolute value of the DC self-bias voltage. This is mainly caused by the change in ionization, attachment and de-attachment reaction rates. The obtained results show that placing the substrate on the grounded electrode and using a higher number of harmonic frequencies (N) can achieve a faster etching rate, since higher ion fluxes can be obtained in these scenarios. Additionally, the Ar/CHF3 gas mixing ratio impacts the neutral surface coverage, which in turn affects the etching rate. Therefore, selecting an appropriate gas mixture is also essential for optimizing etching results.

physics.plasm-ph↗

arXiv2024

Electron dynamics and particle transport in capacitively coupled Ar/O2 discharges driven by sawtooth up voltage waveforms

One dimensional fluid/electron Monte Carlo simulations of capacitively coupled Ar/O2 discharges driven by sawtooth up voltage waveforms are performed as a function of the number of consecutive harmonics driving frequencies of 13.56 MHz, N (1-3), pressure (200-500 mTorr) and gas mixture (10-90 % admixture of O2 to Ar). The effects of these external parameters on the electron dynamics, and the transport of ions and neutrals are revealed at constant peak-to-peak driving voltage. The electronegativity is found to decline as the number of consecutive harmonics increases and the DC self-bias voltage decreases. Increasing the pressure also leads to a decrease in electronegativity. The combination of a decrease in the mean free path of electrons and the presence of the Electrical Asymmetry Effect (EAE) result in different spatio-temporal distributions of the ionization rate, which lead to a reduction in the amplitude of the DC self-bias at higher pressure. As the admixture of electronegative O2 increases, the electronegativity is enhanced, and the discharge mode changes from an α-Drift Ambipolar (DA) hybrid to DA mode. This work focuses on linking these fundamental changes of the plasma physics induced by changing external parameters to process relevant charged particle and neutral fluxes to the electrodes. Particular attention is paid to O(1D) flux, because it is a precursor of deposition. In discharges driven by sawtooth up voltage waveforms, placing the substrate on the grounded electrode and increasing the number of consecutive harmonics, N, can facilitate the deposition process, since the O(1D) flux to the substrate is higher in these scenarios. Moreover, at an O2 admixture of 20%, the O(1D) flux is nearly as high as that at an O2 admixture of 90%, indicating that a higher O(1D) flux can be achieved without excessively increasing the O2 admixture.

physics.plasm-ph↗

arXiv2023

Electrical Characteristics of the GEC Reference Cell with Impedance Matching: A Two-Dimensional PIC/MCC Modeling Study

In this paper, the electrical characteristics of the Gaseous Electronics Conference (GEC) reference cell with impedance matching are investigated through a two-dimensional electrostatic implicit Particle-in-Cell/Monte Carlo Collision (PIC/MCC) model in an axisymmetric coordinate system. The coupling between the complex reactor geometry and the external circuit is included via an equivalent capacitance calculated from the electric energy density. The results of this model are compared with experimental measurements and other model calculations and show good agreement. This simulation obtains the plasma kinetics of the capacitively coupled discharge process at low pressure and detailed external circuit responses, including power transmission, reflection, and higher-order harmonics in the circuit, which provides important insights for impedance-matching design in semiconductor plasma processing.

physics.plasm-ph↗

arXiv2023

Experimental validation of particle-in-cell/Monte Carlo collisions simulations in low-pressure neon capacitively coupled plasmas

Plasma simulations are powerful tools for understanding fundamental plasma science phenomena and for process optimization in applications. To ensure their quantitative accuracy, they must be validated against experiments. In this work, such an experimental validation is performed for a 1d3v particle-in-cell simulation complemented with the Monte Carlo treatment of collision processes of a capacitively coupled radio frequency plasma driven at 13.56 MHz and operated in neon gas. In a geometrically symmetric reactor the electron density in the discharge center and the spatio-temporal distribution of the electron impact excitation rate from the ground into the Ne 2p$_1$ state are measured by a microwave cutoff probe and phase resolved optical emission spectroscopy, respectively. The measurements are conducted for electrode gaps between 50 mm and 90 mm, neutral gas pressures between 20 mTorr and 50 mTorr, and peak-to-peak values of the driving voltage waveform between 250 V and 650 V. Simulations are performed under identical discharge conditions. In the simulations, various combinations of surface coefficients characterising the interactions of electrons and heavy particles with the anodized aluminium electrode surfaces are adopted. We find, that the simulations using a constant effective heavy particle induced secondary electron emission coefficient of 0.3 and a realistic electron-surface interaction model (which considers energy-dependent and material specific elastic and inelastic electron reflection, as well as the emission of true secondary electrons from the surface) yield results which are in good quantitative agreement with the experimental data.

physics.plasm-ph↗

arXiv2023

Nonlocal dynamics of secondary electrons in capacitively coupled radio frequency discharges

In capacitively coupled radio frequency (CCRF) discharges, the interaction of the plasma and the surface boundaries is linked to a variety of highly relevant phenomena for technological processes. One possible plasma-surface interaction is the generation of secondary electrons (SEs), which significantly influence the discharge when accelerated in the sheath electric field. However, SEs, in particular electron-induced SEs ($\updelta$-electrons), are frequently neglected in theory and simulations. Due to the relatively high threshold energy for the effective generation of $\updelta$-electrons at surfaces, their dynamics are closely connected and entangled with the dynamics of the ion-induced SEs ($\upgamma$-electrons). Thus, a fundamental understanding of the electron dynamics has to be achieved on a nanosecond timescale, and the effects of the different electron groups have to be segregated. This work utilizes $1d3v$ Particle-in-Cell/Monte Carlo Collisions (PIC/MCC) simulations of a symmetric discharge in the low-pressure regime ($p\,=\, 1\,\rm{Pa}$) with the inclusion of realistic electron-surface interactions for silicon dioxide. A diagnostic framework is introduced that segregates the electrons into three groups ("bulk-electrons", "$\upgamma$-electrons", and "$\updelta$-electrons") in order to analyze and discuss their dynamics. A variation of the electrode gap size $L_\mathrm{gap}$ is then presented as a control tool to alter the dynamics of the discharge significantly. It is demonstrated that this control results in two different regimes of low and high plasma density, respectively. The fundamental electron dynamics of both regimes are explained, which requires a complete analysis starting at global parameters (e.g., densities) down to single electron trajectories.

physics.plasm-ph↗

arXiv2022

Electron dynamics in planar radio frequency magnetron plasmas: III. Comparison of experimental investigations of power absorption dynamics to simulation results

In magnetized capacitively coupled radio-frequency discharges operated at low pressure the influence of the magnetic flux density on discharge properties has been studied recently both by experimental investigations and in simulations. It was found that the Magnetic Asymmetry Effect allows for a control of the DC self-bias and the ion energy distribution by tuning the magnetic field strength. In this study, we focus on experimental investigations of the electron power absorption dynamics in the presence of a magnetron-like magnetic field configuration in a low pressure capacitive RF discharge operated in argon. Phase Resolved Optical Emission Spectroscopy measurements provide insights into the electron dynamics on a nanosecond-timescale. The magnetic flux density and the neutral gas pressure are found to strongly alter these dynamics. For specific conditions energetic electrons are efficiently trapped by the magnetic field in a region close to the powered electrode, serving as the target surface. Depending on the magnetic field strength an electric field reversal is observed that leads to a further acceleration of electrons during the sheath collapse. These findings are supported by 2-dimensional Particle in Cell simulations that yield deeper insights into the discharge dynamics.

physics.plasm-ph↗

arXiv2022

Electron dynamics in planar radio frequency magnetron plasmas: I. The mechanism of Hall heating and the μ-mode

The electron dynamics and the mechanisms of power absorption in radio-frequency (RF) driven, magnetically enhanced capacitively coupled plasmas (MECCPs) at low pressure are investigated. The device in focus is a geometrically asymmetric cylindrical magnetron with a radially nonuniform magnetic field in axial direction and an electric field in radial direction. The dynamics is studied analytically using the cold plasma model and a single-particle formalism, and numerically with the inhouse energy and charge conserving particle-in-cell/Monte Carlo collisions code ECCOPIC1S-M. It is found that the dynamics differs significantly from that of an unmagnetized reference discharge. In the magnetized region in front of the powered electrode, an enhanced electric field arises during sheath expansion and a reversed electric field during sheath collapse. Both fields are needed to ensure discharge sustaining electron transport against the confining effect of the magnetic field. The corresponding azimuthal ExB-drift can accelerate electrons into the inelastic energy range which gives rise to a new mechanism of RF power dissipation. It is related to the Hall current and is different in nature from Ohmic heating, as which it has been classified in previous literature. The new heating is expected to be dominant in many magnetized capacitively coupled discharges. It is proposed to term it the "μ-mode" to separate it from other heating modes.

physics.plasm-ph↗

arXiv2021

Electron power absorption in micro atmospheric pressure plasma jets driven by tailored voltage waveforms in He/N$_2$

In atmospheric pressure capacitively coupled microplasma jets, Voltage Waveform Tailoring (VWT) was demonstrated to provide ultimate control of the Electron Energy Distribution Function (EEDF), which allows to enhance and adjust the generation of selected neutral species by controlling the electron power absorption dynamics. However, at the fundamental level, the physical origin of these effects of VWT remained unclear. Therefore, in this work, the electron power absorption dynamics is investigated in a He/N$_2$ jet with a nitrogen concentration of 0.05\% driven by a valleys waveform at a base frequency of 13.56 MHz for different numbers of harmonics using a self-consistent Particle in Cell simulation coupled with a spatio-temporally resolved analysis of the electron power absorption based on the momentum balance equation. Due to the local nature of the transport at atmospheric pressure, ohmic power absorption is dominant. Increasing the number of harmonics, due to the peculiar shape of the excitation waveform the sheath collapse at the grounded electrode is shortened relative to the one at the powered electrode. As a consequence, and in order to ensure flux compensation of electrons and positive ions at this electrode, a high current is driven through the discharge at the time of this short sheath collapse. This current is driven by a high ohmic electric field. Close to the grounded electrode, where the electron density is low and the electric field is high, electrons are accelerated to high energies and strong ionization as well as the formation of a local electron density maximum are observed. This electron density maximum leads to a local ambipolar electric field that acts as an electric field reversal and accelerates electrons to even higher energies. These effects are understood in detail to fundamentally explain the unique potential of VWT for EEDF control in such plasmas.

physics.plasm-ph↗

arXiv2021

Zero-dimensional and pseudo-one-dimensional models of atmospheric-pressure plasma jet in binary and ternary mixtures of oxygen and nitrogen with helium background

A zero-dimensional (volume-averaged) and a pseudo-one-dimensional (plug-flow) model are developed to investigate atmospheric-pressure plasma jet devices operated with He, He/O$_2$, He/N$_2$ and He/N$_2$/O$_2$ mixtures. The models are coupled with the Boltzmann equation under the two-term approximation to self-consistently calculate the electron energy distribution function (EEDF). The simulation results are verified against spatially resolved model calculations and validated against a wide variety of measurement data. The nitric oxide (NO) concentration is thoroughly characterized for a variation of the gas mixture ratio, helium flow rate and absorbed power. The concentration measurements at low power are better captured by the simulation with a larger hypothetical "effective" rate coefficient value for the reactive quenching N$_2$(A$^3Σ$,B$^3Π$) + O($^3$P) $\to$ NO + N($^2$D). This suggests that the NO production at low power is also covered by the species N$_2$(A$^3Σ$,B$^3Π$;v>0) and multiple higher N$_2$ electronically excited states instead of only N$_2$(A$^3Σ$,B$^3Π$;v=0) in this quenching. Furthermore, the O($^3$P) density measurements under the same operation conditions are also better predicted by the simulations with a consideration of the aforementioned hypothetical rate coefficient value. It is found that the contribution of the vibrationally excited nitrogen molecules N$_2$(v$\geqslant$13) to the net NO formation rate gains more significance at higher power. The vibrational distribution functions (VDFs) of O$_2$(v<41) and N$_2$(v<58) are investigated. The sensitivity of the zero-dimensional model with respect to a variation of the VDF resolutions, wall reaction probabilities and synthetic air impurity levels is presented. The simulated plasma properties are sensitive to the variation especially for a feeding gas mixture containing nitrogen.

physics.plasm-ph↗

arXiv2021

Energy efficiency of voltage waveform tailoring for the generation of excited species in RF plasma jets operated in He/N$_2$ mixtures

Based on Tunable Diode Laser Absorption Spectroscopy (TDLAS) measurements of the spatially averaged and peak helium metastable atom densities in a capacitively coupled micro atmospheric pressure plasma jet operated in He/N$_2$ mixtures, the energy efficiency of metastable species (He-I 2$^3$S$_1$) generation is compared for three different scenarios: single frequency operation at (i) 13.56 MHz and (ii) 54.12 MHz, and Voltage Waveform Tailoring (VWT) at (iii) "valleys"-waveforms synthesized from four consecutive harmonics of 13.56 MHz. For each case, the dissipated power is measured based on a careful calibration procedure of voltage and current measurements. The results are compared to PIC/MCC simulation results and very good agreement is found. The computational results show that the choice of the surface coefficients in the simulation is important to reproduce the experimental data correctly. Due to the enhanced control of the spatio-temporal electron power absorption dynamics and, thus, of the electron energy distribution function by VWT, this approach does not only provide better control of the generation of excited and reactive species compared to single frequency excitation, but in case of helium metastables the energy efficiency is also shown to be significantly higher in case of VWT.

physics.plasm-ph↗

arXiv2021

Electron heating mode transitions in radio-frequency driven micro atmospheric pressure plasma jets in He/O$_{2}$: A fluid dynamics approach

A two-dimensional fluid model is used to investigate the electron heating dynamics and the production of neutral species in a capacitively coupled radio-frequency micro atmospheric pressure helium plasma jet -- specifically the COST jet -- with a small oxygen admixture. Electron heating mode transitions are found to be induced by varying the driving voltage amplitude and the O$_2$ concentration numerically and experimentally. The helium metastable density, and the charged species densities are highly relevant to the electron heating dynamics. By analyzing the creation and destruction mechanisms of the negative ions, we find that the generation of negative ions strongly depends on the O$_2$ concentration. The increase of the electronegativity with the increasing O$_2$ concentration leads to an enhancement of the bulk drift electric field. The distributions of the different neutral species densities along the direction of the gas flow inside the jet, as well as in the effluent differ a lot due to the relevant chemical reaction rates and the effect of the gas flow. The simulated results show that a fluid model can be an effective tool for qualitative investigations of micro atmospheric pressure plasma jets.

physics.plasm-ph↗