Downstream Plasma Research Articles

Plasma potential and ion energy characteristics in BP-HiPIMS discharge with double layer

Abstract As an emerging ion acceleration plasma source, the bipolar-pulse high power impulse magnetron sputtering (BP-HiPIMS) discharge has been widely studied by academia and industry due to its ability to adjust the ion kinetic energy. Formation of the double layer (DL) potential structure during the BP-HiPIMS positive pulse is vital for accelerating ions, but its structural characteristics are still unclear. In this work, to understand the DL characteristics affected by various discharge parameters, the evolution of plasma potential V p and ion energy in BP-HiPIMS discharge with copper target has been investigated systematically using an emissive probe and mass spectrometer together. Spatial plasma potential measurements show that the DL is established in front of the target during the positive pulse, whose boundary potential drop U DL to accelerate ions can be increased to ∼60 V at a lower operating gas pressure (p= 0.6 Pa) and a higher applied positive pulse voltage (U + = 200 V). The ignition onset time of DL after applying the positive pulse can be shortened to ∼25 μs by decreasing the gas pressure and increasing the positive pulse voltage or negative pulse duration. After DL ignition, a group of high-energy copper ions with energy higher than the surrounding plasma potential can be recognized in the ion energy distribution function curves in the downstream plasma. This result illustrates that the copper ions can be ionized in the high-potential plasma region and be accelerated by the DL boundary potential drop. In addition, a global current balance model of the DL in BP-HiPIMS is developed, which suggests that the U DL can be well adjusted by increasing the positive pulse voltage U + especially for U + > 200 V as verified by the experimental potential measurements. All results suggest that the copper particles play an important role in the formation of DL and the DL plays an important role in accelerating copper ions.

Disordered GPR43/NLRP3 expression in peripheral leukocytes of patients with atrial fibrillation is associated with intestinal short chain fatty acids levels

BackgroundAtrial fibrillation (AF) is associated with circulating inflammation. Short-chain fatty acids (SCFAs) derived from gut microbiota (GM) regulate leukocyte function and inhibit the release of inflammatory cytokines, which are partly mediated by the G-protein-coupled receptor 43 (GPR43) signaling. This study aimed to investigate the expression of GPR43/NOD-like receptors family pyrin domain containing 3 (NLRP3) in leukocytes and the interaction with intestinal SCFAs levels in AF patients.MethodsExpressions of GPR43 and NLRP3 mRNA in peripheral blood leukocytes from 23 AF patients and 25 non-AF controls were detected by quantitative reverse transcription-polymerase chain reaction (qRT-PCR). Expressions of leukocyte GPR43 and NLRP3 protein were evaluated by western blot analysis. The levels of plasma IL-1β were measured by enzyme-linked immunosorbent assay (ELISA). The fecal SCFAs levels based on GC/MS metabolome of corresponding 21 controls and 14 AF patients were acquired from our published dataset. To evaluate the expression of NLRP3 and GPR43 and the release of IL-1β, human THP-1 cells were stimulated with or without SCFAs (acetate, propionate, and butyrate), lipopolysaccharide (LPS), and nigericin in vitro, respectively.ResultsCompared to the controls, the mRNA expression in peripheral leukocytes was significantly reduced in AF patients (P = 0.011) coupled with the increase in downstream leukocyte NLRP3 mRNA expression (P = 0.007) and plasma IL-1β levels (P < 0.001), consistent with changes in GPR43 and NLRP3 protein expression. Furthermore, leukocyte GPR43 mRNA levels were positively correlated with fecal GM-derived acetic acid (P = 0.046) and negatively correlated with NLRP3 mRNA expression (P = 0.024). In contrast to the negative correlation between left atrial diameter (LAD) and GPR43 (P = 0.008), LAD was positively correlated with the leukocyte NLRP3 mRNA levels (P = 0.024). Subsequent mediation analysis showed that 68.88% of the total effect of intestinal acetic acid on AF might be mediated by leukocyte GPR43/NLRP3. The constructed GPR43–NLRP3 score might have a predictive potential for AF detection (AUC = 0.81, P < 0.001). Moreover, SCFAs treatment increased GPR43 expression and remarkably reduced LPS/nigericin-induced NLRP3 expression and IL-1β release in human THP-1 cells in vitro.ConclusionsDisrupted interactions between GPR43 and NLRP3 expression in peripheral blood leukocytes, associated with reduced intestinal GM-derived SCFAs, especially acetic acid, may be involved in AF development and left atrial enlargement by enhancing circulating inflammation.

Abstract 7631: Protein Plus BCT: A novel blood collection tube for stabilizing plasma proteins of interest in ambient whole blood storage

Abstract Introduction: Many of the soluble plasma proteins (e.g., cytokines) that are crucial for cancer diagnosis, prognosis, and prediction of cancer immunotherapy outcomes are found in blood cells such as platelets and leukocytes. During blood collection and sample storage, ex vivo platelet activation, hemolysis, and blood cell degradation can occur, which causes the release of blood cell-secreted proteins into the plasma. The contamination of these proteins may confound true in vivo levels of plasma proteins of interest, resulting in inaccurate or inconsistent assay results. Protein Plus BCT is a new blood collection tube that maintains accurate plasma protein levels during ambient whole blood storage. Methods: Blood samples from self-proclaimed healthy donors were drawn into Protein Plus BCT, EDTA, and ACD-A tubes. All the samples were stored at ambient temperature for up to 5 days. Plasma was isolated using a general double-spin protocol and frozen at -80 °C until use. Plasma levels of protein biomarkers were measured by Ella Simple Plex Assays (Bio-Techne) and Luminex xMAP® Technology Assays (Bio-Techne, samples tested by LuminexPLORE Lab) following manufacturers’ dilution and processing recommendations. Results: Over 50 key plasma proteins like cytokines, cancer, and immuno-oncology checkpoint markers in samples collected into Protein Plus BCT were analyzed by immunoassays and compared side-by-side to those collected in EDTA and ACD-A tubes. EDTA and ACD-A samples exhibited a substantial elevation in the plasma concentration of many, such as IL-8, IL-1β, HGF, VEGF, MMP-9, CCL5/RANTES and TGF-β1 post draw, indicating platelet activation and blood cell degradation during ambient temperature storage. Samples collected into Protein Plus BCT displayed limited ex vivo platelet activation and minimal contamination by blood cell-secreted proteins, maintaining the draw-time plasma levels of proteins of interest for up to 5 days in whole blood at ambient temperature. Conclusions: The novel Protein Plus BCT maintains draw-time plasma protein levels during extended whole blood storage at room temperature by limiting ex vivo platelet activation, hemolysis, and blood cell degradation. Protein Plus BCT ensures sample integrity and minimizes preanalytical variables, providing cancer researchers and clinical assay developers greater flexibility in sample handling and improved confidence in downstream plasma protein analysis. Protein Plus BCT is for Research Use Only. Not for use in diagnostic procedures. Citation Format: Jing Li, Nicholas M. George, Sama Mehta, Jordan LaRue, Eunju Seong, Nursen Binbuga, Rachel Miller, Karl Bisselou. Protein Plus BCT: A novel blood collection tube for stabilizing plasma proteins of interest in ambient whole blood storage [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 7631.

Plasma Cleaning of Hydrocarbon and Carbon Contaminated Surfaces of Accelerator Components

To achieve the vacuum quality required for the operation of particle accelerators, the surface of the vacuum vessels must be clean from hydrocarbons. This is usually achieved by wet chemistry processes, e.g., degreasing chemical baths that, in case of radioactive vessels, must be disposed accordingly. An alternative way exploits the oxygen plasma produced by a downstream RF plasma source. This technique offers the possibility of operating in-situ, which is an advantageous option to avoid the handling of voluminous and/or fragile components and a more sustainable alternative to large volume disposable baths. In this work, we test a commercial plasma source in dedicated vacuum systems equipped with quartz crystal microbalances (QCMs). The evolution of the etching rates of amorphous carbon (a-C) thin films deposited on the QCMs to mimic contamination are studied as function of operating parameters. We present the results of the plasma cleaning process applied to the real case of a hydrocarbons-contaminated large vacuum vessel. The studies are complemented by transport simulations and surface contamination monitoring by X-ray photoelectron spectroscopy (XPS) analysis. The evaluation of the vessel cleanliness, which is performed via residual gas analysis (RGA) measurements, is based on CERN’s outgassing acceptance criteria and agrees with both simulations and XPS results.

(Invited) Enabling High Quality Dielectric Passivation on Monolayer WS2 Using a Sacrificial Graphene Oxide Template

Two-dimensional transition metal dichalcogenides (2D TMDC’s) hold a wide variety of applications, among which microelectronic devices. 2D TMDC’s are promising alternatives for today’s silicon-based technology but suffer from various integration challenges. Among others, direct dielectric growth on the 2D surface occurs poorly due to their self-passivated surfaces resulting in island-like growth initiated at defect sites. This work focuses on enabling uniform high-k dielectric nucleation and growth on 2D TMDC’s via a sacrificial, graphene oxide-based buffer layer. Essentially, the role of the sacrificial graphene buffer layer is twofold: 1) serve as a passivation layer, protecting the underlying 2D TMDC (i.e., WS2) during processing and 2) act as a nucleation layer, enabling uniform atomic layer deposition (ALD) high-k dielectric (i.e,. HfO2) growth. A graphene layer is transferred on monolayer WS2, after which polymeric transfer residues are cleaned via a combination of wet treatments and a dry hydrogen downstream plasma exposure. The cleaned graphene is functionalized via a dry UV/O3 oxidative treatment, which enriches its basal plane with carbon-oxygen single bond functionalities by sacrificing its π-network. This way, the graphene carbon lattice itself remains intact and retains its passivation properties. Moreover, a substrate dependency is observed where graphene transferred on WS2 requires longer UV/O3 exposure to ensure functionalization compared to a SiO2 substrate and can be explained by UV-light induced, ultrafast charge transfer between the graphene and WS2 monolayer. The carbon-oxygen groups formed on graphene’s basal plane act as nucleation sites during a subsequent HfO2 ALD process. A constant roughness is noted for the HfO2 film grown on the graphene oxide capped WS2, indicating uniform material deposition even in the early stages of the ALD process. This is in sharp contrast when HfO2 is directly grown on the WS2 monolayer, whose roughness stabilizes only after many ALD cycles (up to 256), demonstrating the poor, island-like growth mechanism. In addition, a similar hafnium areal density is measured for the graphene oxide capped WS2 compared bare SiO2, which was selected to represent a well-functionalized surface and thus confirming the efficiency of the graphene oxide-based nucleation layer. For devices incorporating the graphene oxide buffer layer concept, a crucial question is to what extent the graphene oxide layer gives rise to leakage currents by providing a conductive path between source and drain. Electrical evaluation of a GFET by means of I-V measurements reveal a significant shift in Dirac point towards positive gate voltage for transferred graphene, which is typically associated with p-type doping originating from the polymeric transfer residues still present on the surface. The Dirac point shifts towards 0-point voltage after final hydrogen downstream plasma cleaning, confirming the effectiveness of the established cleaning protocol. Moreover, Graphene’s conductivity is suppressed after UV/O3 to the nA range, as caused by the formation of carbon-oxygen functionalities and associated sacrifice of its π-network. The residual current may be further suppressed by extending the UV/O3 treatment, but nevertheless demonstrates that the graphene oxide buffer layer will not give rise to significant leakage currents.

Change of Spectral Properties of Magnetic Field Fluctuations across Different Types of Interplanetary Shocks

The interaction between interplanetary (IP) shocks and the solar wind has been studied in the past for the understanding of energy dissipation mechanisms within collisionless plasmas. Compared to the study of fast shocks, other types of IP shocks, including slow mode shocks (i.e., fast forward, fast reverse, slow forward, and slow reverse shocks) remained largely unnoticed. We analyze magnetic field fluctuations observed by the Wind spacecraft from 1995 to 2021 upstream and downstream of the IP shocks using a continuous wavelet transform. The evolution of spectral indices in the ion inertial and transition ranges and the changes in distributions of characteristic ion length scales with respect to the spectral break and proton beta are presented. We found that spectral indices in both inertial and transition ranges and the characteristic length scale distributions are statistically conserved across three types of IP shocks, suggesting that mechanisms associated with the energy dissipation are unaffected by the shocks. The results obtained for the transition range of fast reverse shocks show a larger difference between upstream and downstream plasmas and this will be further studied.

Intermittency at Earth's bow shock: Measures of turbulence in quasi-parallel and quasi-perpendicular shocks

Turbulent plasmas such as the solar wind and magnetosheath exhibit an energy cascade that is present across a broad range of scales, from the stirring scale at which energy is injected, down to the smallest scales where energy is dissipated through processes such as reconnection and wave–particle interactions. Recent observations of Earth's bow shock reveal a disordered or turbulent transition region exhibiting features of turbulent dissipation, like reconnecting current sheets. We used observations from magnetospheric multiscale (MMS) over four separate bow shock crossings of varying shock normal angle to characterize turbulence in the shock transition region and how it evolves toward the magnetosheath. These cases studies have been chosen to ensure validity of Taylor's hypothesis, which we discuss in depth. We observe the magnetic spectrum evolving by fitting power laws over many short intervals, finding that the power-law index in the shock transition region is separable from the upstream and downstream plasma, for both quasi-perpendicular and quasi-parallel shocks. Across the shock, we see a change in the breakpoint location between inertial and ion power-law slopes. We also observe the evolution of scale-independent kurtosis of magnetic fluctuations across the shock, finding a reduction of high kurtosis intervals downstream of the shock. Finally, we adapt a method for calculating correlation length to include a high-pass filter, allowing estimates for changes in correlation length across the shock. In a quasi-perpendicular shock, we find the correlation length to be significantly smaller in the magnetosheath than in solar wind; however, the opposite can occur for quasi-parallel shocks.

Gas-phase etching mechanism of silicon oxide by a mixture of hydrogen fluoride and ammonium fluoride: A density functional theory study

We report the selective etching mechanism of silicon oxide using a mixture of hydrogen fluoride (HF) and NH4F gases. A damage-free selective removal of native oxide has been used in semiconductor manufacturing by forming and removing the ammonium fluorosilicate [(NH4)2SiF6] salt layer. A downstream plasma of NF3/NH3 or a gas-phase mixture of HF and NH4F was used to form (NH4)2SiF6. We modeled and simulated the fluorination of silicon oxide and the salt formation by density functional theory calculation. First, we simulated the successive fluorination of silicon oxide using SiO2 slab models. The fluorination reactions of SiO2 surfaces by the mixture produced a volatile SiF4 molecule or a surface anion of –OSiF4−* with an NH4+ cation with low activation energies. Unlike HF, NH4F produced surface salt species consisting of a surface anion and an ammonium cation. Next, we simulated the (NH4)2SiF6 formation from the two reaction products on fluorinated SiO2 surfaces. (NH4)2SiF6 can be formed exothermally with low activation energies (0.27 or 0.30 eV). Finally, we compared silicon with SiO2 to demonstrate the inherently selective etching of silicon oxide. The fluorination reactions of silicon by the mixture showed the activation energies significantly higher than the SiO2 cases, 1.22–1.56 eV by HF and 1.94–2.46 eV by NH4F due to the less stable transition state geometries. Therefore, the selective salt formation on silicon oxide, not on silicon, is expected in near-room temperature processing, which enables selective etching of silicon oxide.

Rotating magnetic field configuration for controlled particle flux in material processing applications

Abstract Plasma technology has been an integral part of the semiconductor industries, especially to achieve the desired etch and selectivity of the outcome. These outcomes depend on various factors including the confinement of the charged particles of the plasma source. One of the widely employed confinement schemes is the multipole arrangement of magnetic fields, also known as a multicusp. Such arrangement provides minimum-B field value near the plasma axis and plays significant role in plasma-based ion sources for material processing and in plasma thrusters for spacecraft applications. In the present work, a novel rotating multicusp about its axis is studied to investigate its effect on the confinement of electrons present within it. The multicusp is allowed to rotate with a finite rotational speed, in the range of 0–107 rotation per second, thus inducing an axial electric field. It will lead to a directed axial flux of the electrons, determined by the rotational speed of the multicusp. The dynamics of the electrons enclosed within a rotating multicusp have been studied to explore its radial confinement. The results are of significance for semiconductor industries and others where downstream or afterglow plasmas are utilized for material applications.

Enhancing the •OH generation of atmospheric pressure plasma jets by mixing water aerosols at downstream region

AbstractWe established a downstream plasma–aerosol mixing system to increase the generation of •OH in an atmospheric pressure plasma jet (APPJ). Four downstream plasma–aerosol mixers were designed, and flow rates of water aerosol were adjusted. Methods to increase •OH in APPJ include increasing ambient humidity, adding water to working gas, treating on wet surfaces, or using water electro‐spray, but have disadvantages. Our method can increase the reaction area between water and plasma and prevent water from quenching the plasma. The ICCD, TA test, and sterilization results revealed that mixing aerosols in the downstream region effectively increased the amount of •OH generation and enhanced sterilization effect. The cone swirl mixer and two‐way swirl mixer generated the strongest and widest •OH emission region, respectively.

Particle acceleration at ultrarelativistic, perpendicular shock fronts

ABSTRACT Using an eigenfunction expansion to solve the transport equation, complemented by Monte Carlo simulations, we show that ultrarelativistic shocks can be effective particle accelerators even when they fail to produce large amplitude turbulence in the downstream plasma. This finding contradicts the widely held belief that a uniform downstream magnetic field perpendicular to the shock normal inhibits acceleration by the first-order Fermi process. In the ultrarelativistic limit, we find a stationary power-law particle spectrum of index s = 4.17 for these shocks, close to that predicted for a strictly parallel shock.

Long filamentary discharge produced in helium spiral vortex

We report the generation of long plasma filament confined in a helium spiral vortex at atmospheric pressure. The discharge is produced by a dielectric barrier discharge setup and confined in the center of a spiral vortex. The length of the discharge can be several centimeters with flow rate less than 1 SLM. We find that the long filamentary discharge is the trajectory of fast-traveling plasma bursts and plasma plumes, where the plasma bursts are similar to the plasma jet produced at atmospheric pressure. The speeds of the downstream and upstream plasma bursts are about 46 000 and 95 000 m/s, respectively, which are affected by the spirally upward helium gas flow. Based on the novel design of vortex-confined discharge, we show that a plasma filament with a length of 10 cm can be produced.

Combining Rankine–Hugoniot relations, ion dynamics in the shock front, and the cross-shock potential

Rankine–Hugoniot relations (RH) connect the upstream and downstream plasma states. They allow us to determine the magnetic compression, the density compression, and the plasma heating as functions of the Mach number, shock angle, and upstream temperature. RH are based on the conservation laws in the hydrodynamical form. In collisionless shocks, the ion distributions behind the shock transition are determined by ion dynamics in the macroscopic fields of the shock front. The ion parameters upon crossing the shock are directly related to the magnetic compression and the cross-shock potential. For given upstream parameters, RH provide the magnetic compression. If there is no substantial overshoot, an analytical estimate provides the cross-shock potential as a function of the magnetic compression and the Mach number. Numerical tracing of ions across a shock profile with the derived parameters provides the ion pressure, which is in good agreement with the combination of the two theoretical approaches.

Spectroscopic and electrical characterization of oxygen microwave discharge used for downstream plasma oxidation of Si

Actinometry optical emission spectroscopy (AOES), single cylindrical Langmuir, probe and electrostatic planar probe were used to characterize oxygen microwave plasma, at two distances from the plasma source (Z1 = 15 cm and Z2 = 25 cm), using three values of applied microwave power (800, 1000, and 1400 W) at a constant pressure of 70 Pa, involving different parameters, which are electron density (ne), effective electron temperature (Teff), plasma potential (Vp), floating potential (Vf), positive ion flux (Гi), negative ion ratio (α = n−/ne, n− is the negative ion density), and atomic oxygen density (AO: [O]). The electron density and the effective electron temperature were deduced from (I‐V) Langmuir probe characteristics. The atomic oxygen density was measured using actinometry method. Positive ion flux was evaluated from the positive ion saturation current (Isat) of the planar probe. The combination of both single Langmuir probe and the planar one enabled the calculation of electronegativity of the oxygen plasma. An application of this oxygen plasma is presented, which is the silicon surface oxidation. The formed SiO2 layer has been characterized by both scanning electron microscopy (SEM) and atomic force microscopy (AFM) analysis.

Atomic layer etching of titanium nitride with surface modification by Cl radicals and rapid thermal annealing

Thermal atomic layer etching (ALE) is a promising method for isotropic etching with atomic level precision and high conformality over three-dimensional structures. In this study, a thermal ALE process for titanium nitride (TiN) films was developed using surface modification with a Cl2/Ar downstream plasma followed by infrared (IR) annealing of the films. The oxygen-free Cl2-based plasma was adopted to enable highly selective etching of TiN with regard to various materials. It was confirmed that spontaneous etching of TiN during radical exposure can be suppressed at a surface temperature of −10 °C. Measurements of etch per cycle (EPC) of TiN demonstrated that the EPC is self-limiting with respect to both the radical exposure and IR annealing times. With repeated steps of self-limiting radical exposure and IR annealing, TiN was etched at 2.0 nm/cycle, while no thickness change was observed for poly-Si, SiO2, Si3N4, W, and HfO2. The selectivity to amorphous carbon was higher than 4. X-ray photoelectron spectroscopy analysis revealed that during surface modification, NClx species sublimate spontaneously, while TiClx species remain in the surface-modified layer on TiN. This TiClx-based modified layer desorbs in the IR annealing step, and the TiN surface then returns to its original condition (pristine TiN) before surface modification.

Rarefied Flow Simulation of Conical Intake and Plasma Thruster for Very Low Earth Orbit Spaceflight

Air-breathing electric propulsion has the potential to enable space missions at very low altitudes. This study introduces to a 0D hybrid formulation for describing the coupled intake and thruster physics of an air-breathing electric propulsion prototype. Model derivation is then used to formally derive main system’s key performance indicators and estimate the figure of merit for the design of rarefied flow air intakes. Achievable performance by conical intake shapes are defined and evaluated by Monte Carlo simulations. Influence of inlet flow variation is assessed by dedicated sensitivity analyses. The set of requirements and optimality conditions derived for the downstream plasma thruster suggest concept feasibility within an achievable performance range.

Investigation of plasma-assisted functionalization of pristine single layer graphene

Investigations on the oxygen plasma treatment of graphene have been already reported in literature, but in this paper, as a novelty, we present a structural conversion of single layer graphene by O2 plasma treatment. Graphene Oxide is a chemically controlled graphene with hydroxyl, epoxy, carboxyl and carbonyl groups. Low O2 plasma treatment has been evidenced to be an efficient method to introduce defects and active groups in graphene structure with significantly contribution in biosensor area for detecting pathogens and biomolecules. Single Layer Graphene was grown on copper substrate by Chemical Vapor Deposition method and then transferred by wet chemical technique on gold substrate. Pristine Graphene was exposed to a downstream oxygen plasma and we have investigated the modification of the pristine graphene using Scanning Electron Microscopy (SEM), Raman Spectroscopy, Fourier Transform-Infrared Spectroscopy (FT-IR), X-Ray Diffraction (XRD), and X-Ray Photoelectron Spectroscopy (XPS) to examine the quality and structure modification of pristine graphene.

Magnetic Field Generation by Charge Exchange in a Supernova Remnant in the Early Universe

Abstract We present new-generation mechanisms of magnetic fields in supernova remnant shocks propagating to partially ionized plasmas in the early universe. Upstream plasmas are dissipated at the collisionless shock, but hydrogen atoms are not dissipated because they do not interact with electromagnetic fields. After the hydrogen atoms are ionized in the shock downstream region, they become cold proton beams that induce the electron return current. The injection of the beam protons can be interpreted as an external force acting on the downstream proton plasma. We show that the effective external force and the electron return current can generate magnetic fields without any seed magnetic fields. The magnetic field strength is estimated to be B ∼ 10 − 14 – 10 − 11 G , where the characteristic length scale is the mean free path of charge exchange, ∼ 10 15 cm . Since protons are marginally magnetized by the generated magnetic field in the downstream region, the magnetic field could be amplified to larger values and stretched to larger scales by turbulent dynamo and expansion.

Effect of Downstream Plasma Exposure on Schottky Diodes Fabricated on β-Ga2O3

The effects of downstream plasma exposure on Ga2O3 Schottky diode forward and reverse IV characteristics were investigated. Ultra-wide bandgap semiconductor β-Ga2O3 has attracted increasing attention because of the prospects for use in next generation high-power electronics. However, the Ga2O3 surface has been reported to be particularly sensitive to plasma-induced damage, which could lead to device performance degradation[1]. In our study, the plasma treatments were performed using a PIE Scientific Tergeo Plasma Cleaner with O2, N2 or CF4 discharges. The system can be operated either in immersion mode or downstream mode. The samples were exposed to fluxes of electrons, ions and neutral and an rf power of 50 W was used to generate the plasma at a pressure of 400 mTorr, with a fixed treatment time of 1 min. Ti/Au and Ni/Au metallization were employed as the Ohmic and Schottky contacts, respectively, on the test diodes. Prior to the Schottky metal deposition, the samples were treated to ozone plasma/dilute HCl solution/ozone treatments to remove surface contamination. A Schottky barrier height of 1.1 eV was obtained for the reference sample without plasma treatment, and an ideality factor of 1.001 was achieved. From the forward I-V characteristic, as illustrated in Figure 1, the diodes exposed to CF4 showed a 0.25V shift from the I-V of the reference sample due to a Schottky barrier height lowering around 13.9%, as show in Table 1. The diodes exposed with N2 and O2 plasmas showed a decrease of Schottky barrier height of 2.5 and 6.5 % with O2 or N2 treatments, respectively, as shown in Figure 2. The effect of plasma exposure on the ideality factor of diodes treated with these plasmas was minimal; 0.2% for O2 and N2, 0.3% for CF4, respectively, as shown in Figure 2. The reverse leakage currents were 0.0012, 0.0022 and 0.0048 mA/cm2 for the diodes treated with O2, and CF4, and N2 respectively, as shown in Figure 3. The effect of downstream plasma treatment on diode on-resistance and on-off ratio were also very minimal as shown in Figure 4 and 5.[1] Jiancheng Yang, Zachary Sparks, Fan Ren, Stephen J. Pearton, and Marko Tadjer, J. Vac. Sci. & Technol. B36, 061201-1-9 (2018). Figure 1

Atomic layer etching of Si3N4 with high selectivity to SiO2 and poly-Si

Atomic layer etching (ALE) is usually classified into ion-driven anisotropic etching or thermally driven isotropic etching. In this work, we present a thermal ALE process for Si3N4 with high selectivity to SiO2 and poly-Si. This ALE process consists of exposure to a CH2F2/O2/Ar downstream plasma to form an (NH4)2SiF6-based surface-modified layer, followed by infrared (IR) annealing to remove the modified layer. CH2F2-based chemistry was adopted to achieve high selectivity to SiO2 and poly-Si. This chemistry was expected to reduce the number density of F atoms (radicals), which contributes to decreasing the etching rate of SiO2 and poly-Si films. X-ray photoelectron spectroscopy analysis confirmed the formation of an (NH4)2SiF6-based modified layer on the surface of the Si3N4 after exposure to the plasma and subsequent removal of the modified layer using IR annealing. An in situ ellipsometry measurement revealed that the etch per cycle of the ALE process saturated with respect to the radical exposure time at 0.9 nm/cycle, demonstrating the self-limiting nature of this etching process. In addition, no etching was observed on SiO2 and poly-Si films, successfully demonstrating the high selectivity of this ALE process. This high selectivity to SiO2 and poly-Si is attributed to the fact that the spontaneous etching rates of these films are negligibly small and that there is no surface reaction to etch these films during the IR annealing step.