Role Of Energetic Ions Research Articles

Destabilization of low-frequency modes (LFMs) driven by a thermal pressure gradient in EAST plasmas with q min ⩽ 2

Mode structures and excitation conditions for the low-frequency modes (LFMs) have been investigated in experimental advanced superconducting tokamak (EAST) plasmas with q min ⩽ 2. Two different stages/categories of the LFM instabilities are observed during the oscillation of annular/central collapse events: (I) the upward sweeping frequency of LFMs; (II) the upward frequency jumpsof LFMs. The annular/central events are triggered by the m/n = 2/1 double tearing modes with different q-profiles, while the LFMs are characterized by higher mode numbers m/n = 4/2, 6/3, …, where m and n are the poloidal and toroidal mode numbers, respectively. The maximum radial coverage of the LFMs is located in the annular region of 1.97 2.07 m with the normalized minor radius 0.2 0.4, while the higher-frequency (or upward sweeping frequency) branch is more localized to the radial position of 2 2.02 m (q min). The frequency characteristics of upward sweeps or upward jumps of the LFMs are mainly attributed to the change in the q-profile, e.g. the upward sweeping frequency in stage I is caused by q min decreasing. Accordingly, the linear wave properties of LFMs in EAST with weak/reversed magnetic shear are studied numerically and analytically based on a general fishbone-like dispersion relation. Without considering the contribution of energetic ions, it is shown that the LFM with Alfvénic polarization is an MHD-unstable kinetic ballooning mode with frequency of the order of the ion diamagnetic drift frequency. Several important factors for the excitation of LFM instability are analyzed: (1) the role of energetic ions is unimportant, and the LFMs can be excited under the two conditions of with/without energetic ions; (2) the higher τ = T e/T i with larger η i = L ni /L Ti are required, namely the normalized pressure gradient α ∝ (1 + τ)(1 + η i) should be large enough to overcome the stability effect of finite field line bending; (3) the weak/reversed shear q-profile with q min ⩽ 2 and suitable S ≡ (r/q)(q″)1/2 are required.

Role of energetic ions in the growth of fcc and ω crystalline phases in Ti films deposited by HiPIMS

In this work, we show how to control the nucleation of fcc and hexagonal (ω) crystalline phases in Ti films by adding a proper Ti ion flux to the film-forming species. To this aim, films with different thicknesses are grown by High Power Impulse Magnetron Sputtering (HiPIMS) and conventional Direct Current Magnetron Sputtering (DCMS) as a reference. HiPIMS depositions with different substrate bias voltage US (0 V, −300 V and −500 V) are performed to investigate different ion energy ranges. Microstructure, morphology and residual stress of the deposited films, as well as the DCMS and HiPIMS plasma composition, are analysed with different characterization techniques. The DCMS samples, where the species are mostly atoms, exhibit the Ti α-phase only. As far as HiPIMS samples are concerned, films deposited in low energy ion conditions (US = 0 V) show the presence of the Ti fcc phase up to a maximum thickness of about 370 nm. Differently, films deposited under high energy conditions (US = −300 V and −500 V) show the nucleation of the Ti ω-phase for thicknesses greater than 260 and 330 nm, respectively. The formation of these unusual Ti phases is discussed considering the different deposition conditions.

Role of Energetic Ions in the Growth of fcc and ω Crystalline Phases in Ti Films Deposited by HiPIMS

Titanium (Ti), due to its excellent properties, is widely exploited in thin film technology that usually leads to the production of hcp (α-phase) Ti films. In this work, we show how to control the nucleation of fcc and hexagonal (ω-phase) crystalline phases in Ti films by adding a proper Ti ion flux to the film-forming species. To this aim, films with different thicknesses are grown by High Power Impulse Magnetron Sputtering (HiPIMS) and conventional Direct Current Magnetron Sputtering (DCMS) as a reference. Furthermore, HiPIMS depositions with different substrate bias voltage Us(0 V, -300 V and -500 V) are performed to investigate different ion energy ranges. Microstructure, morphology and residual stress of the deposited films, as well as the DCMS and HiPIMS plasma composition, are analysed with different characterization techniques. The DCMS samples, where the species are mostly atoms, exhibit the Ti α-phase only and show a tensile residual stress decreasing with thickness. As far as HiPIMS samples are concerned, films deposited in low energy ion conditions (Us=0 V) show the presence of the Ti fcc phase up to a maximum thickness of about 370 nm. Differently, films deposited under high energy conditions (Us= -300 V and -500 V) show the nucleation of the Ti ω-phase for thicknesses greater than 260 and 330 nm, respectively. A compressive-tensile-compressive (CTC) behavior is observed for residual stresses as thickness increases. The formation of these unusual Ti phases is discussed considering the different deposition conditions.

English

English

Effects of plasma and vacuum-ultraviolet exposure on the mechanical properties of low-k porous organosilicate glass

The effects of plasma exposure and vacuum-ultraviolet (VUV) irradiation on the mechanical properties of low-k porous organosilicate glass (SiCOH) dielectric films were investigated. Nanoindentation measurements were made on SiCOH films before and after exposure to an electron-cyclotron-resonance plasma or a monochromatic synchrotron VUV beam, to determine the changes of film hardness, elastic modulus, and crack threshold due to these exposures. This permits the effects of ion bombardment and photon bombardment to be analyzed separately. The role of energetic ions was examined with a variety of inert plasma-exposure conditions. The role of VUV photons was analyzed as a function of synchrotron photon energy. It was found that both energetic ions and VUV photons with energies larger than the bond energy of the Si-O bond cause a significant increase in film hardness along with a smaller increase in elastic modulus and crack threshold. Differential Fourier transform infrared spectra and x-ray photoemission spectroscopy results show that the energetic ions affect the SiCOH properties mainly through physical bombardment, during which the ions transfer their momentum to the Si-O-Si backbone and transform them into more energetically stable Si-O-Si network structures. This results in the Si-O-Si network structures becoming densified. VUV photons assist reaction that increase the number of bridging O3≡Si-O-Si≡O3 bonds and deplete nonbridging O3≡Si-O and C-Si≡O3 bonds. This increased degree of cross linking in porous organosilicate dielectrics can substantially enhance their hardness and elastic modulus while showing no significant film shrinkage or densification.

ChemInform Abstract: Surface Science Aspects of Etching Reactions

Abstract Basic studies of the surface science aspects of plasma-assisted etching were initiated over 10 years ago in laboratories throughout the world. Several approaches to this experimentally challenging problem have been taken: (1) simulate the reactive gas glow discharge environment with directed beams of energetic positive ions and thermal energy reactive molecules/radicals in a UHV environment; (2) simulate the reactive gas glow discharge by using a beam of reactive ions (reactive ion beam etching); and (3) carry out careful ex-situ analyses of surfaces etched in reactive gas glow discharges without air exposure. This report will be limited to a review of the work reported using the first of these approaches only and summarizes the status of this virtually unexplored field of surface chemistry. The report includes a discussion of the experimental aspects of this field and then focuses on the gas-solid system which has been studied most thoroughly; i.e., silicon-fluorine. A considerable quantity of new unpublished data is presented and a framework is proposed to explain the many observations associated with the spontaneous reaction of fluorine with silicon. The role of energetic ions in this reaction is then discussed in detail. Other materials combinations which are discussed are silicon-chlorine, silicon-bromine, silicon-hydrogen and SiO2-fluorine. The report concludes with a tabulation of the many other gas-solid systems which have been studied and also a discussion of the extent to which these basic studies relate to actual reactive gas processing environments.

The role of energetic ions in the development of multilayered X-ray reflection optics: An overview

Multilayer systems at nanometre scale have been applied as spectroscopic elements for wavelengths >1 nm and optical reflection systems in the short-wavelength region (<100 nm). Main requirements are a high optical contrast for the multilayer components in the wavelength region of application and a density profile with sharp interfaces between the components. Two sources are responsible for interface roughness, the development of surface roughness during deposition of the multilayer components and intermixing of these components at the interfaces. Etching a freshly deposited layer by low-energy ions has demonstrated to be successful in reducing the surface roughness of W on C, Ni on C and Si on Mo. Implantation of low-energy N + ions into Si has been applied to reduce the chemical reactivity at the interface with Ni. Formation of a carbide interlayer by implantation of Si with C + ions successfully prevented intermixing of the Mo on Si interfaces, improving the thermal stability of Mo/Si multilayers. Finally, a combination of implantation and annealing was applied to form Si/SiC multilayers.

The role of energetic ions from plasma in the creation of nanostructured materials and stable polymer surface treatments

Plasma processes for the synthesis of new materials as thin films have enabled the production of a wide variety of new materials. These include meta-stable phases, which are not readily found in nature, and more recently, materials with structure on the nanoscale. Study of plasma synthesis processes at the fundamental level has revealed that ion energy, depositing flux and growth surface temperature are the critical parameters affecting the microstructure and the properties of the thin film materials formed.In this paper, we focus on the role of ion flux and impact energy in the creation of thin films with nanoscale structure in the form of multilayers. We describe three synthesis strategies, based on the extraction of ions from plasma sources and involving modulation of ion flux and ion energy. The microstructure, intrinsic stress and physical properties of the multilayered samples synthesized are studied and related back to the conditions at the growth surface during deposition.When energetic ions of a non-condensing species are used, it is possible to place active groups on the surfaces of materials such as polymers. These active groups can then be used as bonding sites in subsequent chemical attachment of proteins or other macromolecules. If the energy of the non-condensing ions is increased to a few keV then modified layers buried under the surface can be produced. Here we describe a method by which the aging effect, which is often observed in plasma surface modifications on polymers, can be reduced and even eliminated using high energy ion bombardment.

Analysis of sawtooth phenomena in Alcator C-Mod plasmas

The physics of sawtooth oscillations in plasmas produced by the Alcator C-Mod machine is discussed by identifying the regimes that characterize these oscillations, which occur in the central value of the electron temperature, as measured by X ray and/or ECE diagnostics. Sawteeth are generally present in ohmic as well as in RF heated plasmas. It is shown by numerical analysis that these plasmas are stable to ideal internal, pressure driven modes. Hence, the modes responsible for the crash phase of the sawteeth have to be resistive in nature. This is confirmed by numerical computations using a resistive code. The role of energetic ions, produced by ion cyclotron heating, is also discussed.

A role of energetic ions in RF-biased PECVD of TiO 2

Process plasmas generate energetic ions that participate in film growth through sputtering, atomic mixing, densification, enhanced migration of adatoms and field-enhanced diffusion via charging. RF-biased PECVD favors the anatase phase of TiO 2 due to film densification by energetic ion bombardment suppressing the rutile phase. With a given bias voltage, anatase dominates at low deposition temperatures, below 400°C, but transforms to rutile upon annealing at higher temperatures. The dielectric constant of 60 (±5) showed no variation with a film thickness in the range between 20 to 200 nm, due to a thin SiO 2 diffusion barrier intentionally grown at the Si/TiO 2 interface. This oxide barrier prevents TiSi x formation, resulting in a low leakage current below 10 −10 A/cm 2.

Low Energy Threshold in the Growth of Cubic Boron Nitride Films by ECR Plasma Assisted Magnetron Sputtering

ABSTRACTWe report the growth of cubic boron nitride (cBN) films by magnetron sputtering on Si (100) substrates. The films are grown in the presence of negative substrate bias voltages and a nitrogen plasma produced by an electron cyclotron resonance source. We find evidence for a sharp low-voltage threshold in the substrate bias (-105 V) beyond which the samples are predominantly cBN. The structural quality of the cBN films is optimized in a narrow range of voltages near this threshold. We discuss the important role of energetic ions in the formation of cBN in light of recent theoretical findings.