Plasma-surface Interface Research Articles

In Situ Ambient Pressure Photoelectron Spectroscopy Study of the Plasma-Surface Interaction on Metal Foils.

The plasma-surface interface has sparked interest due to its potential of creating alternative reaction pathways not available in typical gas-surface reactions. Currently, there are a limited number of in situ studies investigating the plasma-surface interface, restricting the development of its application. Here, we report the use of in situ ambient pressure X-ray photoelectron spectroscopy in tandem with an optical spectrometer to characterize the hydrogen plasma's interaction with metal surfaces. Our results demonstrate the possibility to monitor changes on the metal foil surface in situ in a plasma environment. We observed an intermediate state from the metal oxide to an -OH species during the plasma environment, indicative of reactive hydrogen radicals at room temperature. Furthermore, the formation of metal-carbides in the hydrogen plasma environment was detected, a characteristic absent in gas and vacuum environments. These findings illustrate the significance of performing in situ investigations of the plasma-surface interface to better understand and utilize its ability to create reactive environments at low temperature.

In situ measurement of dynamic surface charge on dielectrics interacted with plasma jet arrays

As a flexible tool to generate large-scale non-thermal plasma in ambient air, plasma jet arrays have attracted academic attention from multiple fields of biomedicine and material surface sciences. Optimization of plasma processing that heavily relies on trial-and-error experiments requires quantitative in situ diagnostics of plasma–surface interaction. This study focuses on the fundamental surface charge evolution in quartz dielectric deposited by plasma jet arrays. The thin quartz plate is attached to an electro-optic BSO crystal that is conductive and grounded in the back. These jet arrays are driven by a positive microsecond pulsed voltage with kHz frequency. Special attention is paid to the properties and mechanisms of the intriguing convergence of multiple adjacent surface charge patterns. The measured 1D and 2D jet arrays verify the trend of homopolar charge fusion at the plasma–surface interface, which was initially thought to be repulsive under electrostatic forces. The fusion of surface charges is considerably enhanced when more charges are transported onto targeted dielectrics, as well as at smaller separations. The simulation results reveal the mechanism of charge fusion that occurs when surface ionization waves (IWs) are in close proximity, and the high electric field between their heads provides a cluster of high-energy electrons that forces two IWs to merge until they connect. In the case of extremely small separations, the close-in IWs directly merge in bulk before touching the target. Quantitative in situ measurements of surface charges and the corresponding simulations provide fresh insight into the electrical interaction in large-scale atmospheric-pressure plasma jets. The properties of charge fusion can help with manufacturing and optimizing large-area uniform plasma jet sources for various dielectric materials.

Advances in Magnum-PSI probe diagnosis in support of plasma–surface interaction studies

Magnum-PSI is a linear plasma generator equipped with a superconducting magnet, assuring fusion devices relevant conditions at plasma–surface interface. The plasma column was diagnosed using 64 probes embedded in the target. The cross-sectional distributions of plasma parameters (floating potential, ion saturation current and electron temperature) were measured for hydrogen and deuterium plasmas under various discharge conditions. The radial profile of the floating potential across the plasma column can be described by a reversed Mexican hat-like wavelet, having the most negative potential at the center of the plasma column. The negativity of the floating potential diminishes when the discharge current increases or the magnetic field decreases. The axial gradient of the floating potential is reduced by increasing the magnetic field. The ion saturation current is maximum at the center of the plasma column, increasing with both the discharge current and magnetic field. The ion flux to the target, estimated from Thomson scattering (TS) data, was confirmed by probe measurements. The electron temperature estimated from the ion branch of the probe current–voltage characteristic is few times larger than that obtained from TS. By increasing the gas pressure in the target chamber, the time-dependent ion saturation current measured by probes changes from a constant average current (when the plasma column is attached to the target) to a fluctuating average current with scattered peaks (in a partially detached regime) which vanishes completely in the fully detached regime. With respect to hydrogen, the plasma column is wider in deuterium and is characterized by less negative floating potential distributions.

Two Atmospheric Pressure Plasma Jets Driven by Phase-Shifted Voltages: A Method to Control Plasma Properties at the Plasma–Surface Interface

Atmospheric pressure plasma jets have the distinctive ability to project a plasma away from its electrodes, making it an attractive plasma source for a variety of surface treatment applications. Unfortunately, the small effective area of plasma jets inhibits their more widespread use. To remedy this, multiple plasma jets can be used in tandem to enhance their effective area. When used together, the plasma jets will interact and alter one another’s properties. The goal of this work is to understand the interaction between plasma jets and use this interaction to influence their properties at the plasma–surface interface by tailoring the voltage waveforms driving two interacting plasma jets. In this work, two piezoelectric-driven plasma jets are positioned at a 130° angle from one another such that they will intersect on the surface of a substrate. The phase difference between the high-voltage waveforms generating the plasma jets was varied to manipulate the properties of the resultant plasma. It was found that increasing the phase difference between the plasma jets required less total power than when the two jets were operated independently and caused changes to the emission spectrum and an increase in the electron density at their intersection point. As such, changing the phase between two plasma jets can be used to alter the power consumption, the electron density, and the electron kinetics of the plasma at the surface of a substrate, which may be useful for a variety of applications.

Surface state change influence’s theoretical approach of pressed iron on hollow cathode discharge characteristics: first results of plasma heating reproducibility for sintering purposes

In this work first results of the plasma heating process reproducibility for sintering purposes of pressed iron powder sample was investigated analyzing changes on the current-voltage characteristics of pulsed direct current (dc) Ar + H2 hollow cathode discharges sequentially carried out. For this purpose, the temperature of a sample acting as central cathode was varied by changing the switched-on time (duty cycle) of the pulse, via plasma species bombardment (ions and fast neutrals), which typically occurs in both cathode cylindrical surfaces that constitute the annular (hollow cathode) glow discharge, and measured by a thermocouple inserted in the central cathode sample holder. After two sequential plasma heating experiments, the third one practically reproduced the measured plasma parameters evidenced in the second heating, and the respective heating curves as a function of the time have led to similar current-voltage characteristics suggesting the iron sample sintering in non-isothermal way. Principles comprising plasma-surface interface and metallurgical-physical-chemical reactions, powder surface aspects, thermodynamic properties, metallurgical transformation of the pressed sample, and hollow cathode discharge properties are presented and discussed. The main points considered in the present approach are related to plasma (oxygen-affected plasma ionization, high ionization and excitation rate, and the intense light radiation of the annular glow discharge), thermodynamics (oxidation-reduction reactions, electron work function, sintering driving force, and surface energy), and powder (particle size distribution, morphology, specific surface, and iron recrystallization). To the best of the author’s knowledge, it is the first time that it is brought to the light the changes on hollow cathode discharge characteristics while an iron sample acting as the central cathode of an annular glow discharge is subjected to distinct heating steps on the temperature range of 20 to ∼1250 °C, leading it to have its sintering initiated.

Atmospheric Pressure and Ambient Temperature Plasma Jet Sintering of Aerosol Jet Printed Silver Nanoparticles.

Atmospheric pressure nonthermal plasmas hold great promise for applications in environmental control, energy conversion, and material processing. Even at room temperature, nonthermal plasmas produce energetic and reactive species that can initiate surface modifications at a plasma-surface interface, including thin-film nanoparticle assemblies, in a nondestructive and effective way. Here, we present the plasma-activated sintering of aerosol jet printed silver thin films on substrates ranging from glass to delicate materials including blotting paper, fruits, and flexible plastic. We characterize the microstructural evolutions and electrical properties of printed films along with the electrical, thermal, and optical properties of an argon plasma jet. We demonstrate an electrical conductivity as high as 1.4 × 106 S/m for printed films sintered under atmospheric conditions in which the surface temperature stays below 50 °C. These results highlight a future direction where additive manufacturing of electronic devices can be achieved on flexible and low-melting-point materials under ambient conditions without requiring additional thermal processing by utilizing nonthermal plasmas.

Particle distribution functions at plasma–surface interface

Particle simulations of bounded plasmas require assumptions on the distribution function of the incoming particles at the plasma–surface interface. Two independent distributions are usually defined, one angular and one energy dependent, disregarding the unified distribution as a whole. In this paper, two types of angular distribution (isotropic and cosine-type) and three types of energy distribution (energy dependent, uniform, and mono-energetic) are compared in a correlated manner via the resulting velocity distribution functions. Examples of incorrectly defined angular distributions are included. Numerical algorithms are indicated for the generation of the discussed distributions. Calculations of the incoming average velocity of the particles, directed perpendicularly to the surface, show the effect of each distribution on subsequent calculations.

Development of a small-scale helical surface dielectric barrier discharge for characterizing plasma–surface interfaces

Understanding plasma–surface interactions is important in a variety of emerging research areas, including sustainable energy, environmental remediation, medicine, and high-value manufacturing. Plasma-based technologies in these applications utilize surface chemistry driven by species created in the plasma or at a plasma–surface interface. Here, we develop a helical dielectric barrier discharge (DBD) configuration to produce a small-scale plasma that can be implemented in a diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) cell and integrated with a commercial Fourier transform infrared (FTIR) spectrometer instrument to study plasma interactions with inert or catalytic solid media. The design utilizes the entire surface of a cylinder as its dielectric, enhancing the plasma contact area with a packed bed. In this study, we characterize the electrical and visual properties of the helical DBD design in an empty reaction cell and with added potassium bromide (KBr) powder packing material in both air and argon gas environments at ambient conditions. The new surface DBD configuration was integrated into a DRIFTS cell and the time evolution of water desorbing from the KBr packed bed was investigated. Measurements show that this configuration can be operated in filamentary or glow-like mode depending on the gas composition and the water content absorbed on KBr solid media. These results not only set the basis for the study of plasma–surface interactions using a commercial FTIR, but also show that controlling the gas environment and water content in a packed bed might be useful for studying different plasma regimes that are typically not possible at atmospheric pressure.

Optical isolation of spectral lines emitted by sputtered tungsten in a weakly magnetized plasma

Optical isolation of the clockwise or the counter-clockwise circular polarization of spectral lines was applied to emission of sputtered tungsten atoms. As a result one measures the weak magnetic field at plasma-surface interface resulting in splitting of spectral lines (W I at 4008.751 Å and 4982.593 Å) being negligibly small with respect to the Doppler broadening. One relies only on the phase rotation of a reflected circular polarized light propagating at the normal incidence to the mirror-like surface. The spectral shift of the signal from sputtered W atoms on the detector using high-resolution spectrometer provides the value of the magnetic field at a known dispersion, i.e. no further modeling or additional assumption on the distribution of atoms is required. The isolation of spectral line components is equivalent to isolation of the effect of the magnetic field from the measured line shape. It is complete for Zeeman triplet lines and partial for other lines. The results are found to be in a very good agreement of 3% with calculations and the Zeeman resolved laser absorption spectroscopy (10%) on metastable levels of Ar I atoms.

The Physics of Plasma Ion Chemistry: A Case Study of Plasma Polymerization of Ethyl Acetate.

Deposition chemistry from plasma is highly dependent on both the chemistry of the ions arriving at surfaces and the ion energy. Typically, when measuring the energy distribution of ions arriving at surfaces from plasma, it is assumed that the distributions are the same for all ionic species. Using ethyl acetate as a representative organic precursor molecule, we have measured the ion chemistry and ion energy as a function of pressure and power. We show that at low pressure (<2 Pa) this assumption is valid; however, at elevated pressures ion-molecule collisions close to the deposition surface affect both the energy and chemistry of these ions. Smaller ions are formed close to the surface and have lower energy than larger ionic species which are formed in the bulk of the plasma. The changes in plasma chemistry therefore are closely linked to the physics of the plasma-surface interface.

Machine learning plasma-surface interface for coupling sputtering and gas-phase transport simulations

A unified model of thin film deposition by means of sputter deposition requires a description which bridges the intrinsic time and length scales of both the solid and the gas-phase. As a proof of concept, in this work such a description is established based on machine learning techniques using artificial neural networks. Initially, sputtered particle distributions are obtained from transport of ions in matter based simulations for Ar projectiles bombarding a Ti–Al composite for a number of representative incident particle energy distribution functions. Subsequently, a multilayer perceptron network is trained and verified with this set of incident/outgoing distributions. An error analysis is carried out for the obtained training results and their quality is compared and discussed for two sets of hyperparameters. Therewith, it is demonstrated that the trained network is able to predict the sputtered particle distributions for unknown, arbitrarily shaped incident ion energy distributions. It is consequently argued that the trained network may be readily used as a machine learning based model interface, which is sufficiently accurate also in scenarios which have not been previously trained.

Determination of rotational and vibrational temperatures of CH in CH4 plasmas

Understanding fundamental plasma–surface interactions remains an important component of increasing the utility of plasma systems in a range of applications including plasma-assisted catalysis (PAC). A key element in realizing the potential of these applications is understanding the foundational data that control the overall process. Here, fundamental gas-phase and film chemistry trends in methane inductively coupled plasmas are studied to model PAC systems for decomposition of volatile organic compounds. Specifically, energy partitioning trends are presented in conjunction with surface characterization data to elucidate processes occurring at the plasma–surface interface. Optical emission spectroscopy yields rotational and vibrational distribution measurements that can be converted to temperatures (TR and TV) for CH in 100% CH4 and CH4/Ar plasmas as a function of pressure (50–200 mTorr) and applied rf power (25–150 W). Under these conditions, TV ranges from ∼3000 to ∼5000 K, whereas TR has values of 1500–2500 K. These are considerably above room temperature, even at pressures of 200 mTorr and Ar content as high as 25% of total precursor feed. Surface characterization of plasma-treated substrates yields minimal changes in chemical composition but more significant variations in film morphology as functions of plasma pressure and applied rf power. Collectively, these data aim to unravel the complex chemistry of plasma systems for PAC.

Review Article: Unraveling synergistic effects in plasma-surface processes by means of beam experiments.

The interaction of plasmas with surfaces is dominated by synergistic effects between incident ions and radicals. Film growth is accelerated by the ions, providing adsorption sites for incoming radicals. Chemical etching is accelerated by incident ions when chemical etching products are removed from the surface by ion sputtering. The latter is the essence of anisotropic etching in microelectronics, as elucidated by the seminal paper of Coburn and Winters [J. Appl. Phys. 50, 3189 (1979)]. However, ion-radical-synergisms play also an important role in a multitude of other systems, which are described in this article: (1) hydrocarbon thin film growth from methyl radicals and hydrogen atoms; (2) hydrocarbon thin film etching by ions and reactive neutrals; (3) plasma inactivation of bacteria; (4) plasma treatment of polymers; and (5) oxidation mechanisms during reactive magnetron sputtering of metal targets. All these mechanisms are unraveled by using a particle beam experiment to mimic the plasma–surface interface with the advantage of being able to control the species fluxes independently. It clearly shows that the mechanisms in action that had been described by Coburn and Winters [J. Appl. Phys. 50, 3189 (1979)] are ubiquitous.

Effects of assisted magnetic field to an atmospheric-pressure plasma jet on radical generation at the plasma-surface interface and bactericidal function

A configuration of magnetic-assisted-plasma (MAP) on helium-based atmospheric-pressure plasma jet (APPJ) with an axial magnetic-field of 0.587 T is proposed, which provides good ability for killing bacteria Escherichia coli on the agar surface. Optically, we confirmed that the MAP increased approximately 2.4 times in the electron density estimated by the Stark broadening of Hβ line emission, and approximately 1.5 times enhancement of atomic oxygen concentration measured by vacuum ultraviolet absorption spectroscopy (VUVAS). Moreover, the generation of hydroxyl radical in the water increased 1.5 times, confirmed by the spin-trapping electron spin-resonance technique. In addition, the bactericidal experiments demonstrated 2.4 times higher for E. coli by the MAP treatment. The MAP configuration is proposed to be highly useful for future bio-medical applications by enhancing the radical generation at the plasma/substrate interface region.

Influence of surface conditions on plasma dynamics and electron heating in a radio-frequency driven capacitively coupled oxygen plasma

The impact of changing surface condition on plasma dynamics and electron heating is investigated by means of numerical simulations, based on a semi-kinetic fluid model approach, and compared with measurements of the nanosecond electron dynamics in the plasma-surface interface region using phase resolved optical emission spectroscopy (PROES). The simulations are conducted in a one-dimensional domain and account for a geometrical asymmetry comparable to the experimental setup of a radio-frequency driven capacitively coupled plasma in a gaseous electronics conference reference cell. A simple reaction scheme is considered, including electrons, positive ions, negative ions and metastable singlet delta oxygen (SDO) as individual species. The role of surface loss and effective lifetime of SDO is discussed. To simulate different surface conditions, the SDO surface loss probability and the secondary electron emission coefficient were varied in the model. It is found that a change in surface condition significantly influences the metastable concentration, electronegativity, spatial particle distributions and densities as well as the ionization and electron heating dynamics. The excitation dynamics obtained from simulations are compared with PROES measurements. This allows to determine experimentally relevant SDO surface loss probabilities and secondary electron emission coefficient values in-situ and is demonstrated for two different surface materials, namely aluminum and Teflon.

The influence of surface properties on the plasma dynamics in radio-frequency driven oxygen plasmas: Measurements and simulations

Plasma parameters and dynamics in capacitively coupled oxygen plasmas are investigated for different surface conditions. Metastable species concentration,electronegativity, spatial distribution of particle densities as well as the ionization dynamics are significantly influenced by the surface loss probability of metastable singlet delta oxygen (SDO). Simulated surface conditions are compared to experiments in the plasma-surface interface region using phase resolved optical emission spectroscopy. It is demonstrated how in-situ measurements of excitation features can be used to determine SDO surface loss probabilities for different surface materials.

He + O 2 + H 2 O plasmas as a source of reactive oxygen species

The effect of water in the chemistry of atmospheric-pressure He+O2 plasmas is studied by means of a comprehensive global model. Water enables the generation of reactive oxygen species (ROS) cocktails that are rich not only in O, O2∗, and O3 but also in OH and H2O2. Due to its polar nature, water also leads to cluster formation, possibly affecting the plasma dynamics. Since the lifetime of many of the ROS is short, the plasma chemistry plays two roles: (i) direct interaction with superficial cells and (ii) triggering of a secondary chemistry that propagates the plasma treatment to regions away from the plasma-surface interface.

CN Surface Interactions and Temperature-Dependent Film Growth During Plasma Deposition of Amorphous, Hydrogenated Carbon Nitride

The properties of amorphous hydrogenated carbon nitride (a-C:N:H) materials create the potential for a variety of commercial applications. Plasma-enhanced chemical vapor deposition (PECVD) provides a convenient means of creating thin a-C:N:H films, but the chemistry that dominates the deposition remains unclear. The CN radical likely plays a role in the process; here, CH3CN was used as a single-source precursor for the generation of CN radicals and the PECVD of a-C:N:H films. Several plasma diagnostic techniques were used in an effort to clarify important deposition mechanisms, with special attention given to the role of CN radicals in the deposition process. Gas phase characterization methods, including laser-induced fluorescence (LIF) and optical emission spectroscopy (OES), provide insight into the formation and gas phase properties of CN(g). In addition, the behavior of CN at the plasma-surface interface was probed using the imaging of radicals interacting with surfaces (IRIS) technique. Results sugge...

Mechanisms for deposition and etching in fluorosilane plasma processing of silicon

A variety of analytical tools have been used to examine the gas-phase and surface chemistry of SiF4 and SiF4/H2 plasmas interacting with Si substrates. The effect of rf power (P) and source gas ratios on film composition, gas-phase species densities, and plasma–surface interactions of SiF and SiF2 have been studied. Film characterization was performed using Fourier transfer infrared, x-ray photoelectron spectroscopy, and spectroscopic ellipsometry. Using the imaging of radicals interacting with surfaces technique, spatially resolved laser-induced fluorescence images of SiFx radicals were collected and used to characterize both the plasma-surface interface and the gas phase. Additional gas-phase characterization was achieved using optical emission spectroscopy and mass spectrometry. From all of these data, three plasma types have been defined. In etching systems (type 1) (e.g., 100% SiF4 plasmas at P&amp;gt;20 W), no net deposition occurs, and SiF2 radicals are produced at the surface. When only F atom incorporation occurs with no net etching or deposition (type 2), SiF2 can exhibit either surface loss or surface production, depending on other plasma parameters. In a-Si:H,F film depositing systems (type 3), SiF2 is lost at the surface under most conditions. Gas phase and surface mechanisms to describe these three plasma types are proposed.

Substrate temperature effects on surface reactivity of SiFx (x=1, 2) radicals in fluorosilane plasmas

Control of substrate temperature (TS) during plasma etching and film deposition using fluorinated silicon gases has profound effects on etch and deposition rates as well as the overall composition of deposited materials. How TS directly affects individual species and reactions at the plasma–surface interface, however, is not fully understood. Using the imaging of radicals interacting with surfaces technique, we have measured the effect of TS on SiF and SiF2 surface reactivity in SiF4 and SiF4:H2 plasmas under a variety of plasma conditions. At TS=300 K, there is significantly more SiF2 than SiF emanating from the surface. This is expected as SiF2 is a known etch product. Interestingly, higher substrate temperatures result in significant increases in surface scatter for both molecules. These results are discussed with respect to the role that each molecule plays in etching and deposition mechanisms, as well as in comparison to results for plasma species in other plasma systems. In addition to surface interaction measurements, rotational temperatures (θR) for SiF and SiF2 were measured in a 170 W plasma as 450±50 and 752±100 K, respectively.