Plasma jets are an atmospheric pressure plasma source that projects streamers outside the bounds of the plasma device. The electric field produced by these streamers can generate a current in the substrate before the streamer contacts the substrate. This displacement current is a strong function of the proximity of the streamer to the surface. We develop a basic model of the streamer traveling towards the substrate that can determine the streamer location and velocity from the measured displacement current. This simple approach shows good agreement with optical imaging equipment and can provide a means to rapidly quantify the streamer velocity.

This study investigates the interaction between two counterpropagating atmospheric pressure plasma jets when their respective streamer ignition times are varied by introducing a phase difference between the AC waveforms used to generate them. When the plasma jets are driven in phase, the streamers form at approximately the same time, resulting in a dark region between the two jets. As the phase difference increases, this dark region shifts toward one of the electrodes. With a sufficiently large phase difference, this region vanishes, giving rise to a uniform plasma channel spanning the distance between the electrodes. High-speed imaging reveals that the interaction between the streamers within the channel reduces the streamer propagation length at intermediate phase differences. At large phase differences, the propagation distance of each streamer is enhanced due to the absence of the opposing streamer. Increasing the phase from 0° to 160° reduced the power consumption of the two jets by about 10%, while there was no significant change in the electron density or the N2 vibrational/rotational temperature. Finally, we show how phase-shifting enhances the interaction with three-dimensional objects located between the opposing jets, enabling the treatment of 3D substrates.

Atmospheric pressure plasma jets operate by projecting a streamer discharge toward remote substrates. The formation and propagation of streamers is an inherently stochastic process, which leads to cycle-to-cycle variations in streamer properties. The present work quantifies these variations through electrical measurements of a plasma jet striking a conductive substrate. It was found that streamer ignition and magnitude can change substantially from cycle to cycle and thus differ from time-averaged measurements taken over many cycles. For example, the energy required to drive the plasma jet and the amount of charge delivered to the substrate during a given cycle can vary from their respective averages by a factor of 2 or more. While some of this can be due to power supply stability, several other operating characteristics of the plasma jet were found to influence repeatability. It was found that helium and argon produced plasma jets with different levels of repeatability, with the repeatability of argon strongly dependent on the feed gas flow rate. Jets driven with pulsed-DC were notably more repeatable than those driven by a high-voltage sinewave. This is attributed to the difference in voltage rise time, which promotes more consistent ignition times. Increasing the voltage and frequency of the pulsed-DC plasma jet can further improve repeatability, where voltages above 2 kV and driving frequencies above 200 Hz significantly enhance the repeatability of the plasma jet.

A one-dimensional, hybrid hydrodynamics-Boltzmann model for electron beam (e-beam) generated plasmas in argon at low pressure and a constant magnetic field is developed. The e-beam separates the plasma into two distinct regions: an ionization zone irradiated by the e-beam that creates plasma and reactive species, and a diffusive zone between the e-beam and the chamber wall, where the species diffuse toward the wall. The Boltzmann equation is solved in the ionization zone and the electron energy distribution function is used to calculate rates participating in the species and power balance equations. Comparison of model and experimental data for the radial profiles of the electron density and temperature shows a favorable agreement. All plasma parameters are sensitive to their radial location.

Plasma enhanced atomic layer deposition (PEALD) is a cyclic atomic layer deposition (ALD) process that incorporates plasma-generated species into one of the cycle substeps. The addition of plasma is advantageous as it generally provides unique reactants and a substantially reduced growth temperature compared to thermal approaches. However, the inclusion of plasma, coupled with the increasing variety of plasma sources used in PEALD, can make these systems challenging to understand and control. This work focuses on the use of plasma diagnostics to examine the plasma characteristics of a remote inductively coupled plasma (ICP) source, a type of plasma source that is commonly used for PEALD. Ultraviolet to near-infrared spectroscopy and spatially resolved Langmuir probe measurements are employed to characterize a remote ICP system using nitrogen-based gas chemistries typical for III-nitride growth processes. Spectroscopy is used to characterize the relative concentrations of important reactive and energetic neutral species generated in the remote ICP as a function of gas flow rate, Ar/N2 flow fraction, and gas pressure. In addition, the plasma potential and plasma density for the same process parameters are examined using an RF compensated Langmuir probe downstream from the ICP source. The results are also discussed in terms of their impact on materials growth.

The compound semiconductors (InAs, GaSb, AlSb) having nearly matched lattice constants (approximate to 6.1 angstrom) are of great interest in fabrication of infrared micro/optoelectronics, but are hampered significantly by a high density of interface states that lead to degenerately doped surface layers. Native oxides are a cause of interface states, as well as a barrier to the effective passivation of dangling bonds, and must be removed. Re-oxidation must then be prevented prior to deposition of a passivant. We demonstrate effective removal of the native oxide of InAs through utilization of an amorphous silicon plasma enhanced chemical vapor deposition process. A hydrogendiluted silane plasma provides a reducing environment to react and remove indium and arsenic oxide species while a thin layer of amorphous silicon is grown to passivate and prevent re-oxidation of the InAs surface. The reduction of native oxide species via hydrogen-diluted argon plasma as a pretreatment prior to amorphous silicon was also explored. The surface chemistry is verified via depth profiling X-ray photoelectron spectroscopy, and the impact of a hydrogen-argon plasma pretreatment investigated to further reduce oxygen concentration at the InAs/amorphous silicon interface.

Electron beam (e-beam) generated plasmas are useful for material processing applications such as deposition and etching because the plasmas deliver a large fluence of very low energy of ions to surfaces. Metastable species produced in the beam-region can also transport significant energy to the plasma periphery and surfaces. In this work, we have investigated the spatially resolved density of metastable Ar 1s 5 species produced in an Ar and Ar/N 2 e-beam generated plasma at pressures of 60–67 mTorr using laser-induced fluorescence (LIF). The experiments provide the first direct measure of absolute density and reduction of Ar 1s 5 in an e-beam generated plasma when argon is diluted with nitrogen. These results are consistent with previous predictions of numerical modeling and measurements using optical emission spectroscopy. The present spatially resolved LIF measurements directly quantify the reduction of Ar 1s 5 in the e-beam generated plasma by nitrogen admixing. This reduction was observed in the region of the electron beam and in the plasma periphery, where substrates are usually placed. For example, up to a threefold reduction of the density of Ar 1s 5 was measured when the argon background was diluted with 15.5% nitrogen at pressure of 60 mTorr. Ar 1s 5 reduction is attributed to excitation exchange with nitrogen molecules as well as the cooling of plasma electrons via inelastic collisions with nitrogen molecules.

Plasmas are an indispensable materials engineering tool due to their unique ability to deliver a flux of species and energy to a surface. This energy flux serves to heat the surface out of thermal equilibrium with bulk material, thus enabling local physicochemical processes that can be harnessed for material manipulation. However, to-date, there have been no reports on the direct measurement of the localized, transient thermal response of a material surface exposed to a plasma. Here, we use time-resolved optical thermometry in-situ to show that the energy flux from a pulsed plasma serves to both heat and transiently cool the material surface. To identify potential mechanisms for this ‘plasma cooling,’ we employ time-resolved plasma diagnostics to correlate the photon and charged particle flux with the thermal response of the material. The results indicate photon-stimulated desorption of adsorbates from the surface is the most likely mechanism responsible for this plasma cooling.

Plasma enhanced atomic layer deposition (PEALD) is a cyclic atomic layer deposition process that incorporates plasma-generated species into one of the cycle substeps. The addition of plasma is advantageous as it generally provides unique gas-phase chemistries and a substantially reduced growth temperature compared to thermal approaches. However, the inclusion of plasma, coupled with the increasing variety of plasma sources used in PEALD, can make these systems challenging to understand and control. This work focuses on the use of plasma diagnostics to examine the plasma characteristics of a hollow cathode enhanced capacitively coupled plasma (HC-CCP) source, a type of plasma source that has seen increasing attention in recent years for PEALD. Ultraviolet to near-infrared spectroscopy as well as spatially resolved Langmuir probe and emissive probe measurements are employed to characterize an HC-CCP plasma source using nitrogen based gas chemistries typical of nitride PEALD processes. Spectroscopy is used to characterize the relative concentrations of important reactive and energetic neutral species generated in HC-CCP systems as a function of applied RF power, gas chemistry, and pressure. In addition, the electron energy distribution function, electron temperature, plasma potential, and plasma density for the same process parameters are examined using an RF compensated Langmuir probe and emissive probe. These measurements indicated that electron temperature ( Te), electron density ( ne), and plasma potential ( Vp) varied significantly over the operating conditions examined with Te varying from 1.5 to 8 eV, Vp varying from 30 to 90 V, and ne varying between 1015 and low 1016 m−3. This wide range of plasma conditions is mediated by a mode transition from a low Te, high ne mode of operation at low pressure (<100 mTorr) to a high Te, low ne mode at higher pressures (>100 mTorr). These operational modes appear analogous to the classical γ and α modes of traditional capacitively coupled plasmas. Atomic N and H densities also vary significantly over the operating conditions examined.

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.

Plasma-enhanced atomic layer deposition (PEALD) enables the epitaxial growth of ultrathin indium nitride (InN) films at significantly reduced process temperatures and with greater control of layer thickness compared to other growth methods. However, the reliance on plasma-surface interactions increases the complexity of the growth process. A detailed understanding of the relationship between the plasma properties and the growth kinetics is therefore required to guide the tuning of growth parameters. We present an in situ investigation of the early-stage PEALD growth kinetics of epitaxial InN within three different plasma regimes using grazing incidence small-angle x-ray scattering (GISAXS). The GISAXS data are supported by diagnostic studies of the plasma species generation in the inductively coupled plasma source as a function of the relative concentrations of the nitrogen/argon gas mixture used in the growth process. The growth mode is found to be correlated to the production of nitrogen species in the plasma, with high concentrations of the atomic N species promoting Volmer–Weber growth (i.e., island growth) and low concentrations promoting Stranski–Krastanov growth (i.e., layer-plus-island growth). The critical thickness for island formation, island center-to-center distance, and island radius are found to increase with ion flux. Furthermore, the island center-to-center distance and areal density are observed to change only during plasma exposure and to continue changing with exposure even after the methylindium adlayer is believed to have fully reacted with the plasma. Our results demonstrate the potential to control the growth kinetics during PEALD of epitaxial films by intentionally accessing specific regimes of plasma species generation.

Recently, there is a rapidly growing body of work studying plasma-based water treatment for applications within the medical, environmental, and agriculture sectors. Atmospheric pressure plasmas produced by high voltage pulses with 10’s of nanoseconds in duration are well suited for treatment of liquids 1 , 2 , 3 and surfaces. Modeling of nanosecond pulsed plasma is challenging due to plasma complexity, as well as different reaction time-scales; from nanoseconds to seconds. Moreover, the plasma is affected by sheath formation 4 and reactions of aqueous species with gas phase plasma species at the plasma/liquid interface 5 . We present a time-dependent global model to study the decay of various plasma species in helium carrier gas and obtain the trends with changing the pulse frequency and pulse duration. The model in conjunction of voltage measurements and OES diagnostics provide a useful tool for such analyses.

Atmospheric pressure plasmas generate a variety of chemically active species in open air, thus providing the unique ability to treat a variety of materials that do not require or are not compatible with vacuum systems. Producing the plasma-surface interaction that leads to a desired change in the substrate is complicated by the codependency between the plasma and the substrate: while the plasma will modify the surface, the surface will also influence the plasma properties. In this work, a pulsed-DC plasma jet produced in helium and impinging upon glass and metal substrates is studied over a range of applied voltage pulse widths extending from 1 to 10 mu s. Current-voltage measurements, high speed images, and time-resolved optical emission from three important He and nitrogen excited species are used to examine the evolution of the plasma and its interaction with the surface. At ignition, a streamer is ejected into the open air from the jet exit and eventually collides with the substrate. For a glass substrate, the streamer will hit the surface and form a short-lived plasma across it. This surface plasma is almost completely unaffected by changes in the voltage pulse width. In contrast, when the streamer hits a metal substrate, a surface discharge will form that will last the entirety of the voltage pulse. If the pulse is long enough, a 'reflected discharge' will slowly develop that extends from the substrate back towards the outlet of the plasma jet. The emission intensity of the surface discharge closely matches that of the initial streamer, but not the reflected discharge, which suggests different electron kinetics between the two features. The addition of capacitors or resistors between the metal substrate and ground show how differences in substrate electrical properties can account for some of these behaviors. Emission line ratios are used to examine the evolution of electron temperature and the relative importance of Penning processes during the different plasma phases.

A high-resolution spectroscopic characterization is conducted on the vibrational and rotational spectral emissions of an N2 plasma produced by an electron beam. The electron beam is initiated by the Naval Research Laboratory’s Febetron pulsed-power generator capable of 80 kV and 4 kA peak voltage and current pulses with a 100 ns pulse time width. The beam is directed into an N2 pressurized chamber, ranging in pressures from 10mTorr to 10 Torr, where the electron beam excites the N2 gas into both molecular and atomic emissions. A spectroscopic analysis is conducted on these emissions from the near-UV to visible (300-650 nm) by means of a sub-angstrom resolution 1.3 m focal length Czerny-Turner spectrometer. Multiple simultaneous spectral measurements are taken per pulse enabling measurements of spatial variations of the plasma spectra through a 7-fiber input array. Imaging of the spectra is conducted by a 2D intensified gated CCD over small time increments to analyze the time-resolved evolution of the emissions.

The plasma chemistry generated by an electron beam emitted from a Febetron pulsed-power generator operating in different background gases is being investigated spectroscopically. The generator located at the Naval Research Laboratory produces a peak voltage of 80 kV, peak current of 4 kA with a pulse width of 100 ns. Once formed, the electron beam passes through a thin anode followed by a thin pressure barrier into a volume filled with different background gases at pressures ranging from 1000 to 10 mtorr. The volume is filled with both molecular (nitrogen and air) and atomic gases (Ar) to study the behavior of the generated plasma in these different conditions. The present work focuses on analysis of the emission spectrum from the electron beam generated plasma; of particular interest are spectral lines from the near ultraviolet to the visible (300 nm to 600 nm). The temporal and spatial variation of excited species are studied to understand the characteristics of the plasma.

The volumetric photon emission from a pulsed, RF-driven atmospheric pressure plasma jet was monitored from ultraviolet to the near infrared part of the spectrum. When a small admixture of dry air, as low as ∼0.01%, is added to the helium flow, the helium line intensities decrease, while those of oxygen increase dramatically. To understand the emission trends, a non-equilibrium, collisional-radiative model is developed to estimate He and O excited state densities as a function of relative air concentration. The model is based on a numerical solution of the electron Boltzmann equation for the electron energy distribution function self-consistently coupled to the balance equations for helium and air species. The spectral analyses reveal a good agreement between experiments and modeling, with the latter indicating that a complex interplay between electron kinetics and plasma chemistry is responsible for the substantial changes in emission profiles when He is diluted with a small amount of air.

Atmospheric pressure plasma jets are a promising tool for a variety of applications. However, the small size of atmospheric pressure plasma jets limits their use to small scale processes and materials treatment. To address this issue, this work describes how to increase the plasma volume without additional power supplies or circuitry. Instead, additional noble gas streams are positioned orthogonal to the jet's direction of propagation which generates new plasma regions along these streams. This approach increases the plasma volume, which also increase the effective area available to interact with surfaces. The resulting augmented plasma is characterized with electrical, imaging, and time-integrated optical emission spectroscopy techniques. With these additional gas streams, the power required to drive the plasma jet increases sublinearly with the plasma volume, making this an attractive method to generate large area plasmas efficiently. The emission intensity of the He-air plasma lines along the plasma jet were compared to those in the added noble gas streams and were found to be similar in magnitude and follow similar trends as the main plasma jet. There does not appear to be a limit on the number of noble gas streams that can be added downstream of the plasma jet and thus volume expansion.