Downstream Plasma Research Articles

Study of selective isotropic etching Si1−xGex in process of nanowire transistors

On approach towards the end of technology roadmap, a revolutionary approach towards the nanowire transistors is favorable due to the full control of carrier transport. The transistor design moves toward vertically or laterally stacked Gate-All-Around (GAA) where Si or SiGe can be used as channel material. This study presents a novel isotropic inductively coupled plasma (ICP) dry etching of Si1−xGex (0.10 ≤ x ≤ 0.28) in SiGe/Si multilayer structures (MLSs) with high selectivity to Si, SiO2, Si3N4 and SiON which can be applied in advanced 3D transistors and Micro-Electro-Mechanical System (MEMS) in future. The profile of SiGe etching for different thicknesses, compositions and locations in MLSs using dry or wet etch have been studied. A special care has been spent for layer quality of Si, strain relaxation of SiGe layers as well as residual contamination during the etching. In difference with dry etching methods (downstream remote plasma), the conventional ICP source in situ is used where CF4/O2/He gas mixture was used as the etching gas to obtain higher selectivity. Based on the reliability of ICP technique a range of etching rate 25–50 nm/min can be obtained for accurate isotropic etching of Si1−xGex, to form cavity in advanced 3D transistor processes in future.

Rapid thermal-cyclic atomic-layer etching of titanium nitride in CHF3/O2 downstream plasma

Isotropic atomic-layer etching (ALE) of TiN—by using a plasma-assisted thermal-cyclic process with a 300-mm tool—was demonstrated. The process consists of exposure to a downstream CHF3/O2 plasma (for surface modification) followed by infrared irradiation for thermal desorption of the modified surface. Etched amount of TiN per cycle saturated at around 0.6 nm/cycle with respect to both radical-exposure time and infrared-irradiation time. To examine the reaction mechanism of the cyclic etching, the surfaces of TiN samples after CHF3/O2 plasma exposure were analyzed by in situ x-ray photoelectron spectroscopy (XPS). Self-limiting formation of the surface-modified layer, which was tentatively identified as an ammonium salt such as (NH4)xTiFy , was observed after radical exposure. It was confirmed by in situ XPS that the surface-modified layer was removed after thermal annealing at 110 °C. Isotropic ALE of TiN was thus demonstrated by using formation and desorption of ammonium salt in CHF3/O2 downstream plasma, which was also used for isotropic ALE of Si3N4.

Self-limiting reactions of ammonium salt in CHF3/O2 downstream plasma for thermal-cyclic atomic layer etching of silicon nitride

Self-limiting reactions of ammonium salt in CHF3/O2 downstream plasma were demonstrated for thermal-cyclic atomic layer etching (ALE) of Si3N4. In situ x-ray photoelectron spectroscopy analysis shows that an (NH4)2SiF6 by-product of the same thickness forms on Si3N4 in a wide gas composition range. The (NH4)2SiF6 layer prevents etching of Si3N4 during continuous plasma exposure in that wide range. The (NH4)2SiF6 layer was sublimated by heating, which was consistent with the result of the thermodynamic calculation. The reactions of the (NH4)2SiF6 layer in CHF3/O2 downstream plasma are used for thermal-cyclic ALE of Si3N4 with a newly developed 300-mm tool equipped with an in situ ellipsometer. It was confirmed that the amount etched per cycle saturates with respect to both plasma exposure time and infrared irradiation time.

Enhancement of downstream plasma density by a stepped-diameter radiofrequency plasma source under a static magnetic field for a compact sputtering reactor

A stepped-diameter insulator source cavity is employed to a radiofrequency (RF) plasma source for development of a sputtering reactor, where an inner diameter near the open source exit is enlarged compared with the upstream diameter and convergent-divergent magnetic field is applied near the source exit to inhibit the plasma loss to the wall by separating the plasma from the wall. Furthermore argon gas is introduced at the interface between the different diameter regions; a high plasma density of ∼5 × 1017 m−3 can be obtained at ∼50 mm downstream of the source exit for an RF power of about 100 W. To enhance the sputtering performance, a permanent magnet is installed behind the target located downstream of the source and a target current can be increased. The results on the plasma density and a deposition rate of Cu film on a substrate are compared for different source cavities and gas injection locations, which confirm the validity of the stepped-diameter source cavity on the performance improvement of the sputtering reactor.

Al2O3/Si0.7Ge0.3(001) & HfO2/Si0.7Ge0.3(001) interface trap state reduction via in-situ N2/H2 RF downstream plasma passivation

A novel method for passivating the interface between Si0.7Ge0.3 using an in-situ downstream RF plasma consisting of a nitrogen-rich mixture of H2 and N2 gases at 250 °C prior to atomic layer deposition (ALD) of Al2O3 and HfO2 was demonstrated. XPS spectra of the interface with Al2O3 indicated the presence of a nitride layer enriched in SiONx and depleted in Ge relative to the substrate. The electrical properties of this interface were characterized using I-V and variable frequency C-V measurements of MOS capacitors. The N2/H2 plasma passivation process produced a reduced density of interface trap states (Dit) and lower gate leakage compared with ex-situ HF clean and sulfur passivation for Al2O3 gate oxides. The lowered leakage current and Dit observed compared with HF(aq) or sulfur-passivated surfaces were consistent with enhanced oxide nucleation due to N2/H2 plasma passivation lowering carbon surface contamination and dangling bonds. TEM/EELS analysis of the interface was consistent with the presence of a thin interfacial nitride layer suppressing the formation of GeO bonds at the interface to form an SiOx-rich interlayer (IL).

Direct investigation of near-surface plasma acceleration in a pulsed plasma thruster using a segmented anode

The ablative pulsed plasma thruster (APPT) has been widely used on satellites due to its low electrical power requirements and structural simplicity. Recently, a segmented anode schematic has been proposed to increase the main discharge arc current density and the propellant mass ablation of APPTs while using the same input energy. In this work, a structurally similar multi-segmented anode was instead used for plasma measurements. The incentive behind the use of the multi-segmented anode was to measure the plasma propagation process and downstream current flow inside the discharge channel of the APPTs (volume between the electrodes). The multi-segmented anode divides the anode into multiple parts in order to directly measure and analyze the current flow for each anode segment as the APPT plasma moved downstream between the electrodes. This provided new information regarding the plasma propagation process between the electrodes. Local magnetic probe measurements were also conducted inside the APPT discharge channel. From the experimental results, distinct plasma progression towards the downstream direction was identified. The plasma reached a velocity of up to 25 km s−1 at only 5 mm from the propellant surface. In contrast, the downstream portion of the electrodes was found to contribute relatively little to the final velocity (an increase of ~16%). Furthermore, the base (1st) element of the multi-segmented anode (where the discharge arc is located) was found to carry over 5 times more current than downstream segments (which carry the maximum downstream plasma current). These results directly indicate that the downstream plasma carries relatively little current, thus resulting in a lower Lorentz force contribution and downstream acceleration. Instead, most of the plasma acceleration process occurs very near to the propellant surface.

Selective radial release of hot, magnetised electrons downstream of a low-pressure expanding plasma

A thorough understanding of particle transport in expanding plasma devices under different geometric and magnetic configurations is necessary for future thruster and reactor design. Recently, we have shown that the development of high density conics via hot electron transport along the most radial field lines escaping the source region of an expanding plasma device is crucial for the development of a high velocity ion beam on axis. In this study, a gap between the source region and a movable extension tube inserted in the downstream of an expanding plasma device is used to create a geometric window of variable size. Only those electron populations situated on field lines that pass through this window can be transported to the region where high density conics normally form. Critical field lines for the plasma behaviour are identified by increasing the size of the geometric window while concurrently measuring the conics density and ion beam characteristics. A step change in the beam velocity is observed when high density conics are able to fully reform. The field lines responsible for full conics formation are found to be those that pass within ∼2 skin depths of the source chamber wall. As such, the downstream plasma behaviour is dominated by electrons situated on field lines that are most effectively heated by the radio-frequency antenna and escape the source.

Quantification of Stable Isotope Traces Close to Natural Enrichment in Human Plasma Metabolites Using Gas Chromatography-Mass Spectrometry.

Currently, changes in metabolic fluxes following consumption of stable isotope-enriched foods are usually limited to the analysis of postprandial kinetics of glucose. Kinetic information on a larger diversity of metabolites is often lacking, mainly due to the marginal percentage of fully isotopically enriched plant material in the administered food product, and hence, an even weaker 13C enrichment in downstream plasma metabolites. Therefore, we developed an analytical workflow to determine weak 13C enrichments of diverse plasma metabolites with conventional gas chromatography-mass spectrometry (GC-MS). The limit of quantification was increased by optimizing (1) the metabolite extraction from plasma, (2) the GC-MS measurement, and (3) most importantly, the computational data processing. We applied our workflow to study the catabolic dynamics of 13C-enriched wheat bread in three human subjects. For that purpose, we collected time-resolved human plasma samples at 16 timepoints after the consumption of 13C-labeled bread and quantified 13C enrichment of 12 metabolites (glucose, lactate, alanine, glycine, serine, citrate, glutamate, glutamine, valine, isoleucine, tyrosine, and threonine). Based on isotopomer specific analysis, we were able to distinguish catabolic profiles of starch and protein hydrolysis. More generally, our study highlights that conventional GC-MS equipment is sufficient to detect isotope traces below 1% if an appropriate data processing is integrated.

Characterization of Carbon-Contaminated B4C-Coated Optics after Chemically Selective Cleaning with Low-Pressure RF Plasma

Boron carbide (B4C) is one of the few materials that is expected to be most resilient with respect to the extremely high brilliance of the photon beam generated by free electron lasers (FELs) and is thus of considerable interest for optical applications in this field. However, as in the case of many other optics operated at light source facilities, B4C-coated optics are subject to ubiquitous carbon contaminations. Carbon contaminations represent a serious issue for the operation of FEL beamlines due to severe reduction of photon flux, beam coherence, creation of destructive interference, and scattering losses. A variety of B4C cleaning technologies were developed at different laboratories with varying success. We present a study regarding the low-pressure RF plasma cleaning of carbon contaminated B4C test samples via inductively coupled O2/Ar, H2/Ar, and pure O2 RF plasma produced following previous studies using the same ibss GV10x downstream plasma source. Results regarding the chemistry, morphology as well as other aspects of the B4C optical coating before and after the plasma cleaning are reported. We conclude that among the above plasma processes only plasma based on pure O2 feedstock gas exhibits the required chemical selectivity for maintaining the integrity of the B4C optical coatings.

Rankine‐Hugoniot Relations in Multispecies Plasma With Gyrotropic Anisotropic Downstream Ion Distributions

AbstractIn the observed heliospheric shocks downstream distributions are usually measured in the regions where the kinematic relaxation has already happened but isotropy is not achieved. Rankine‐Hugoniot relations, connecting the upstream and downstream plasma parameters, should be able to take into account the downstream ion anisotropy. In a multispecies plasma different species behave differently at the ramp crossing and when collisionlessly gyrating in the downstream region. The downstream density and pressure of each species can be expressed in terms of the magnetic compression and the cross‐shock potential, which allows us to derive new Rankine‐Hugoniot relations.

Nonlinear Monte Carlo model of superdiffusive shock acceleration with magnetic field amplification.

Fast collisionless shocks in cosmic plasmas convert their kinetic energy flow into the hot downstream thermal plasma with a substantial fraction of energy going into a broad spectrum of superthermal charged particles and magnetic fluctuations. The superthermal particles can penetrate into the shock upstream region producing an extended shock precursor. The cold upstream plasma flow is decelerated by the force provided by the superthermal particle pressure gradient. In high Mach number collisionless shocks, efficient particle acceleration is likely coupled with turbulent magnetic field amplification (MFA) generated by the anisotropic distribution of accelerated particles. This anisotropy is determined by fast particle transport, making the problem strongly nonlinear and multiscale. Here, we present a nonlinear Monte Carlo model of collisionless shock structure with superdiffusive propagation of high-energy Fermi accelerated particles coupled to particle acceleration and MFA, which affords a consistent description of strong shocks. A distinctive feature of the Monte Carlo technique is that it includes the full angular anisotropy of the particle distribution at all precursor positions. The model reveals that the superdiffusive transport of energetic particles (i.e., Lévy-walk propagation) generates a strong quadruple anisotropy in the precursor particle distribution. The resultant pressure anisotropy of the high-energy particles produces a nonresonant mirror-type instability that amplifies compressible wave modes with wavelengths longer than the gyroradii of the highest-energy protons produced by the shock.

Sensitivity analysis of upstream plasma condition for SST-1 X-Divertor configuration with SOLPS

Extensive power exhausts and target heat loads are anticipated in reactor grade fusion devices. Prototyping of an X-Divertor based power exhaust scheme is being attempted by means of simulations of Scrape-off Layer plasma transport in the diverted plasma equilibria of SST-1 tokamak using SOLPS5.1. Evaluation of the relative advantages of an X-Divertor configuration involves simulating the SST-1 standard divertor scheme plasma transport for the reference and then achieving equivalent upstream plasma conditions in the X-divertor equilibrium to ensure equivalent core plasma in both the cases. The first optimization is to be achieved by simulating effects of an external gas puff in the SOL region for controlling separatrix density in the X-divertor configuration with visible modifications in the downstream plasma conditions. The present work analyzes sensitivity of the upstream SOL plasma conditions to the gas puff intensity and its effect on the plasma neutral transport in the divertor region

Lift Enhancement of an Airfoil and an Unmanned Aerial Vehicle by Plasma Gurney Flaps

A novel conception of the plasma Gurney flap is proposed, which consists of two dielectric barrier discharge plasma actuators attached to the airfoil pressure surface near the trailing edge. The upstream and downstream plasma actuators can be actuated individually, where it is found that the most significant control effect takes place when only the downstream plasma actuator is turned on. Such optimal conception is applied to a NACA 0012 airfoil and an unmanned aerial vehicle without and with flaps deflection. The present proposed plasma Gurney flap is found to have the similar lift-increment effect and the control mechanism with the conventional Gurney flap. A large-scale counterclockwise recirculation region forms over the airfoil pressure surface near the trailing edge with plasma control, acting as the virtual aeroshaping effect. Consequently, both the suction over the upper surface and the pressure over the lower surface are increased. Thus, the plasma Gurney flap can increase the lift coefficient before the stall angle, reduce the drag coefficient, and increase the lift-to-drag ratio at small angles of attack. In addition, the plasma Gurney flap has additional advantages, such as active control ability and small device drag, making it a very attractive lift-enhancement technique for aeronautical applications.

Hydrogen-plasma patterning of multilayer graphene: Mechanisms and modeling

We study the atomic-scale etching mechanisms of multilayer graphene, and the subsequent formation of nanopores, when exposed to downstream hydrogen plasma. Our molecular dynamics simulations based on reactive force-field potential reveal precise energy regimes for the transport of ions through, and selective etching of, individual graphene layers within the multilayer structure. Etching initiates with hydrogenation of the graphene basal plane, followed by localized CC bond breaking which leads to the formation of CH2, and subsequently, unstable CH3 bond configurations. We establish the basal plane and edge etching rates of the individual graphene layers as a function of ion energy, and introduce a micromechanics model to predict the 3D-patterned pore structure at experimental length- and time-scales. Our results demonstrate the development of columnar holes in multilayered graphene, which transition to stepped-edge holes at higher fluence due to cumulative effects of basal-plane etching. The contributions of thermal radicals and dehydrogenation effects on the hole growth process are discussed.

Functionalization of CVD Grown Graphene with Downstream Oxygen Plasma Treatment for Glucose Sensors

The development of graphene-based devices requires control over the functionalization and modification of the graphene surface. We studied the modification of the monolayer graphene surface with downstream oxygen plasma treatment to form graphene oxide (GO) for use in glucose biosensors. Amperometric and voltammetric methods were applied to GO for glucose oxidase (GOx) immobilization with a simultaneous reduction of GO to reduce graphene oxide. The fabricated glucose sensor shows a sensitivity of 0.118 μA mM−1cm−2 and a detection limit of 0.0526 mM of glucose. This method provides fast and simultaneous immobilization of GOx and reduction of GO, and can be used in the fabrication of other electrochemical biosensors.

Downstream plasma parameters in laminar shocks from ion kinetics

Ion dynamics in oblique shocks is governed by the macroscopic electric and magnetic fields of the shock front. In laminar shocks, these fields are time-independent and depend only on the coordinate along the shock normal. The shock ramp is narrow and the ion motion across the shock is manifestly non-adiabatic. The ion distribution just behind the ramp is significantly non-gyrotropic. Gyrotropy is achieved well behind the ramp mainly due to the gyrophase mixing. The asymptotic values of the ion density and temperature are determined by the eventual collisionless relaxation of the gyrating ion distribution. Given a distribution at the downstream edge of the ramp, the moments of the distribution after gyrophase mixing are derived using proper spatial averaging. The obtained expressions can be used for independent determination of the downstream plasma state and implementation in Rankine-Hugoniot relations.

Thin Layer Etching of Silicon Nitride: Comparison of Downstream Plasma, Liquid HF and Gaseous HF Processes for Selective Removal after Light Ion Implantation

For technology nodes beyond 14nm silicon nitride spacer etching has become a major challenge. Conventional plasma etching techniques based on CHF3/O2 cannot achieve thorough nitride removal on horizontal surfaces without inducing either CD loss or Si/SiGe source/drain recess. This leads to either gate leakage increase or poor raised source/drain epitaxy. To overcome atomic scale control issues faced with continuous plasma processes, several techniques aiming at achieving atomic layer etching or thin layer etching were recently described [1]. An original etching approach has been reported which consists in modifying the silicon nitride through H2 ion implantation by plasma (ICP or CCP) and then selectively removing the modified fraction of the layer thanks to chemical etching [2]. Layer modification depth is controlled thanks to plasma parameters (bias voltage and process time). This unconventional technique was demonstrated on 14nm FDSOI logic device and showed less than 1nm spacer CD loss, less than 0.6nm SiGe recess which enabled defect-free source/drain epitaxy [2]. Mechanisms for silicon nitride modifications and selective removal are discussed in this article by comparing downstream plasma, liquid-phase HF and gas-phase HF as removal techniques.

CORRUGATION OF RELATIVISTIC MAGNETIZED SHOCK WAVES

ABSTRACT As a shock front interacts with turbulence it develops corrugation, which induces outgoing wave modes in the downstream plasma. For a fast shock wave, the incoming wave modes can either be fast magnetosonic waves originating downstream, outrunning the shock, or eigenmodes of the upstream plasma drifting through the shock. Using linear perturbation theory in relativistic MHD, this paper provides a general analysis of the corrugation of relativistic magnetized fast shock waves resulting from their interaction with small amplitude disturbances. Transfer functions characterizing the linear response for each of the outgoing modes are calculated as a function of the magnetization of the upstream medium and as a function of the nature of the incoming wave. Interestingly, if the latter is an eigenmode of the upstream plasma, we find that there exists a resonance at which the (linear) response of the shock becomes large or even diverges. This result may have profound consequences on the phenomenology of astrophysical relativistic magnetized shock waves.

SOFT X-RAY EXCESS FROM SHOCKED ACCRETING PLASMA IN ACTIVE GALACTIC NUCLEI

ABSTRACT We propose a novel theoretical model to describe the physical identity of the soft X-ray excess that is ubiquitously detected in many Seyfert galaxies, by considering a steady-state, axisymmetric plasma accretion within the innermost stable circular orbit around a black hole (BH) accretion disk. We extend our earlier theoretical investigations on general relativistic magnetohydrodynamic accretion, which implied that the accreting plasma can develop into a standing shock under suitable physical conditions, causing the downstream flow to be sufficiently hot due to shock compression. We perform numerical calculations to examine, for sets of fiducial plasma parameters, the physical nature of fast magnetohydrodynamic shocks under strong gravity for different BH spins. We show that thermal seed photons from the standard accretion disk can be effectively Compton up-scattered by the energized sub-relativistic electrons in the hot downstream plasma to produce the soft excess feature in X-rays. As a case study, we construct a three-parameter Comptonization model of inclination angle θ obs, disk photon temperature kT in, and downstream electron energy kT e to calculate the predicted spectra in comparison with a 60 ks XMM-Newton/EPIC-pn spectrum of a typical radio-quiet Seyfert 1 active galactic nucleus, Ark 120. Our χ 2-analyses demonstrate that the model is plausible for successfully describing data for both non-spinning and spinning BHs with derived ranges of 61.3 keV ≲ kT e ≲ 144.3 keV, 21.6 eV ≲ kT in ≲ 34.0 eV, and 17.°5 ≲ θ obs ≲ 42.°6, indicating a compact Comptonizing region of three to four gravitational radii that resembles the putative X-ray coronae.

(Invited) Rapid In-Situ Carbon and Oxygen Cleaning of In0.53Ga0.47As(001) and Si0.5Ge0.5(110) Surfaces via a H2 RF Downstream Plasma

The In0.53Ga0.47As(001) and Si0.5Ge0.5(110) surfaces were cleaned using a downstream RF plasma. On the air-exposed In0.53Ga0.47As(001) surface, a 2 second 100 millitorr H2 plasma dose fully removed carbon and oxygen. On the ex-situ wet cleaned Si0.5Ge0.5(110) surface, nearly all carbon and oxygen are removed via a 2 second exposure of 5% H2 in Ar plasma. To prevent oxygen deposition from the plasma tube while maximizing the atomic H flux, for Si0.5Ge0.5(110), the plasma power, pressure, and gas composition must be controlled. The Si0.5Ge0.5(110) surface is more sensitive than the In0.53Ga0.47As(001) surface to trace oxygen in the plasma stream consistent with the higher heat of formation per Si of SiO2 than the heat of formation per Ga of Ga2O3. The higher heat of formation of SiO2 is expected to both increase oxygen adsorption and prevent the atomic H from forming volatile products with SiO2 on Si0.5Ge0.5(110), in contrast to In0.53Ga0.47As(001).