Band 74, Heft 3

An overview of the current literature on diagnostics and modeling of energy transfers by long-lived species in N2­CO, He­NO, He­N2­O2, N2­O2, and N2­H2O plasma is given. Energy transfers by vibrationally excited ground-state long-lived species and by electronic molecular and atomic metastables will be highlighted.

Thermal plasmas offer unique advantages for materials processing, such as high fluxes of heat and of reactant species. Recent developments have concentrated on improving control of these fluxes across the boundaries surrounding the thermal plasma. Secondary discharges (hybrid plasma generators) and pulse modulation of the plasma have been some of the approaches for this end. The use of such methods is described for selected applications. Plasma characterization through advanced models and diagnostics are concentrating on description of plasma instabilities and various nonequilibrium conditions. Understanding of these effects will allow their use for enhanced processing methods.

Diffuse glow discharges were produced in low temperature (<2000 K) atmospheric pressure air and nitrogen plasmas with electron number densities in excess of 1012 cm­3, more than six orders of magnitude higher than in thermally heated air at 2000 K. The measured discharge characteristics compare well with the predictions of a two-temperature kinetic model. Experimental and modeling results show that the steady-state electron number density exhibits an S-shaped dependence on the electron temperature, a behavior resulting from competition between ionization and charge-transfer reactions. Non-Maxwellian effects are shown to be unimportant for the prediction of steady-state electron number densities. The power requirements of DC discharges at atmospheric pressure can be reduced by several orders of magnitude using short repetitive high-voltage pulses. Between consecutive pulses, the plasma is sustained by the finite rate of electron recombination. Repetitive discharges with a 100-kHz, 12-kV, 10-ns pulse generator were demonstrated to produce over 1012 electrons/cm3 with an average power of 12 W/cm3, 250 times smaller than a DC discharge at 1012 cm­3.

Utilizing a plasma to achieve sterilization is a possible alternative to conventional sterilization means as far as sterilization of heat-sensitive materials and innocuity of sterilizing agents are concerned. A major issue of plasma sterilization is the respective roles of ultraviolet (UV) photons and reactive species such as atomic and molecular radicals. At reduced gas pressure (£10 torr) and in mixtures containing oxygen, the UV photons dominate the inactivation process, with a significant contribution of oxygen atoms as an erosion agent. Actually, as erosion of the spore progresses, the number of UV photons successfully interacting with the genetic material increases. The different physicochemical processes at play during plasma sterilization are identified and analyzed, based on the specific characteristics of the spore survival curves.

Powder formation in silane plasmas has been considered as a technology drawback because it might lead to the formation of macroscopic defects in the deposited layers. Here we summarize our recent efforts in controlling the formation of powder precursors, in particular, nanocrystalline silicon particles, aiming at their incorporation in the films. Indeed, the incorporation of clusters and crystallites along with SiHx radicals allows production of polymorphous silicon films with improved structure and transport properties with respect to standard amorphous silicon films.

Plasma deposition and plasma conversion can be characterized by five steps: production by ionization, transfer of chemistry to precursors, transport of radicals to the surface, surface interactions with deposition, recirculation and generation of new monomers. For very fast deposition, large flows of radicals are needed and a regime is reached, in which monolayer coverage is reached in a very short time. Such large flows of radicals can be obtained by ion-induced interactions, as the C2H radical from acetylene for a-C:H deposition, or by H atom abstraction as the SiH3 radical from SiH4 for a-Si:H deposition. These radicals with intermediate sticking coefficient are advantageous as they are mobile and have a finite dwelling time at the surface. By such a pure radical mechanism, good layers can be formed with very high growth rates, if large radical fluxes can be reached. This regime of high fluence is also interesting for conversion, of which ammonia formation from hydrogen and nitrogen atoms is given as an example. These new approaches offer new possibilities for further development of the field in close connection with surface science, catalysis, and materials science.

Conventional developments were conducted in a very empirical way, such as a trial and error with many speculations using qualitative data. This approach requires more and more resources and time for the development of future devices with a design rule below 100 nm in the system on a chip (SOC) era. It is necessary to establish a systematic methodology for process development and qualification. ASET Plasma Laboratory had been found to research a basis for the systematic development of the plasma etching technology. Fluorocarbon (CF) plasma for the etching of high-aspect-ratio contact holes in SiO2 was investigated intensively in the 5-year program that finished in March 2001. They introduced 5 plasma sources that can etch 0.1-mm contact holes on a 200-mm wafer in production, and state-of-the-art diagnostics tools for the plasma and etched surface. The SiO2 etch mechanism was revealed from the etch species generation to the reaction in a deep hole. The number of electron collisions to fluorocarbon gas molecule is proposed as an important parameter to control the gas dissociation and etch species flux to the surface. An etch reaction model was also proposed using the estimated-surface-reaction probability that is a function of ion energy and CF polymer thickness that reduces the net ion energy to the reaction layer. The CF polymer thickness was determined by a balance equation of generation term (radical fluxes) and loss terms (etching by ions, radicals, and out-flux oxygen from SiO2). A program was developed and successfully predicts the etch rates of Si-containing materials, including organic dielectrics. Requirements for the next-generation plasma etch tools are also discussed.

Radio-frequency excited, low-pressure plasmas in halogen-containing gases are widely used to etch submicronic features in a range of materials during integrated circuit manufacture. Costly process-drift problems are often caused by the ubiquitous deposition of polymer layers on the reactor walls. Simple and robust sensors of the reactor performance are needed to monitor and manage these effects. This paper presents results obtained in industrial plasma-etching machines using a deposition-tolerant ion flux probe and broadband UV­vis absorption spectroscopy.

Good-quality silicon dioxide films have been deposited at low temperature (200 °C) using continuous and pulsed oxygen/silane plasmas coupled in a high-density, low-pressure helicon reactor. Although the total ion flux determines many of the structural properties of the deposited oxide, we have found that silicon-containing ions contribute to the film growth (up to 50 %) and appear to be responsible for the measured compressive stress.

In this work, we report on our last results concerning the polyethyleneterephtalate barrier performance optimization as a function of the chemistry of the SiOx coating, deposited by means of hexamethyldisiloxane/oxygen-fed plasmas ignited in an RF capacitively coupled parallel plates reactor. The influence of the gas feed composition and power on the plasma phase/coating chemistry and on the O2 gas transmission rate has been investigated. The plasma phase has been studied by means of Fourier transform infrared absorption (FT-IR) and optical emission spectroscopies (OES), and the results have been coupled to the film chemistry investigated by means of FT-IR spectroscopy. These techniques show excellent potentialities in the correlation of the plasma phase and film chemistries with the barrier properties of the SiOx-coated polymer, especially when a very good barrier level is required.

This paper is dedicated to the discussion of possible plasma applications for hydrogen-rich gas production from hydrocarbons. Different types of plasma, both thermal and nonequilibrium ones, are under consideration. A special attention is devoted to experimental and theoretical results on hydrocarbon conversion in nonequilibrium plasma of pulse microwave discharge. A comparison of plasma methods and conventional catalytic technology is presented as well.

A study of the interactions of energetic argon ions with silicon surfaces using molecular dynamics simulations is reported. A dynamic balance between ion-induced damage and recrystallization of the surface is detected. By manipulating ion energy, argon ions are able to both create disordered regions near the surface, and recrystallize these disordered regions.

Low-temperature plasma technologyboth glow discharge under reduced pressure as well as barrier discharge under normal pressureare well established in different industrial applications. Since recently, however, the plasma technology is being introduced in textile industry as well. Fields of application are desizing, functionalizing, and design of surface properties of textile fibers. Plasma technology is suitable to modify the chemical structure as well as the topography of the surface of the material. Examples of natural as well as man-made fibers prove the enormous potential of plasma treatment of textile materials. It has proven to be successful in shrink-resist treatment of wool with a simultaneously positive effect on the dyeing and printing. Not only the chemical structure of the surface is modified using different plasma gases but also the topography of the surface. A highly hydrophobic surface with a particular surface topography in contact with water is extremely dust- and dirt-repellent and hence should be also repellent to bacteria and fungi. Man-made fibers to be used under chemical stress are modified with diffusion-barrier layers on their surfaces without modifying the bulk properties; hence, the stability of those fibers is significantly improved.

The paper presents survey of principles and basic physical parameters of liquid and hybrid gas/liquid dc-arc plasma torches and discusses physical mechanisms that control plasma properties. Special attention is devoted to properties of generated plasma that are decisive for performance characteristics in plasma spraying. Characteristics of water and water-argon torches are compared with typical characteristics of commonly used gas torches.

Newly developed pulse-modulated high-power inductively coupled plasma (ICP) is expected to offer the unique physicochemical condition, such as the increased concentration of chemically reactive species, as well as the appropriate heat flux for materials processing. Two kinds of oxide materials, titanium and zinc oxide, were placed at the downstream of Ar­H2 ICP and irradiated in the plasma of continuous (CN) and pulse-modulated (PM) modes. The CN-ICP irradiation at the position close to the plasma tail gave rise to the thermal reduction of oxides. In the PM-ICP irradiation, the degree of thermal reduction depended on the lower power level during pulse-off time, as well as the total electric power. Irradiation in PM-ICP led to the increased formation of oxygen vacancies in titanium dioxide. In the case of zinc oxide, the UV emission efficiency was improved by PM-ICP irradiation, while the green emission became predominant by CN-ICP irradiation at the appropriate position. Induced effects in the two oxides by PM-ICP would be related to the high concentration of hydrogen radicals in the plasma.

The paper describes recent developments in modeling formation of plasma-spray coatings. This is a stochastic process in which particle-impact conditions, thermophysical properties, substrate topology, and temperature all play important roles in determining the structure of coating. Specific attention is paid to the simulation of droplet impact and solidification under plasma-spraying conditions. Results of 3-dimensional models show that on a flat surface solidification may cause splashing and break-up of the impacting particle. Undercooling effects result in faster solidification and, thus, smaller extent of spreading. More recent works investigate the effect of surrounding gas on dynamics of impact. It is shown that this could result in entrapment of a bubble under the droplet.

Atmospheric pressure nonequilibrium plasmas have made a recent remarkable progress in formation techniques including atmospheric pressure glow discharge (APG), dielectric barrier discharge (DBD), corona discharge, surface discharge, ultrashort pulsed discharge, etc., and are expanding their applications into the field of energy and environment as well as material conversion processes. This paper will especially focus on a large improvement of DBD by combining it with squared ultrashort high voltage pulses and various applications.

In the current paper, the "simple" diagnostics for characterization of nonequilibrium chemically active plasmas are discussed. As to experimental methods, "simple" noninvasive spectroscopic methods of ionizing and recombining plasma characterization have been emphasized. The word "simple" in the title is presented in quotes, which means that the methods are assumed to be simple only conditionally. In order to be sure that the measurements indeed provide correct information on the plasma parameters, a detailed understanding of kinetics of each particular plasma source is required. Moreover, the validity of spectroscopic methods should be checked by independent diagnostic techniques.

Emission spectroscopy is used to investigate the effect of inert gas mixing in nitrogen plasmas generated in inductively coupled plasma (ICP) and electron cyclotron resonance (ECR) plasma sources. Vacuum ultraviolet (VUV) emission of resonance lines is used to determine concentration of atomic nitrogen while electron temperature is obtained from optical emission spectra. It is found that electron temperature can be either raised or reduced effectively by mixing helium or argon in a nitrogen discharge. Electron-electron collisions and superelastic collisions involving metastable species are key factors in electron temperature tuning.

Operating parameters of powerful UV and vacuum ultraviolet (VUV) excilamps with different discharge geometry excited by capacitive discharge, barrier discharge, glow discharge, and UV-preionized discharge were investigated. Intense radiation of Ar2*, Kr2*, Xe2*, ArF*, KrBr*, KrCl*, KrF*, XeI*, Cl2, XeBr*, XeCl*, XeF* molecules and I* atoms was obtained in rare gas or in rare gas­F2 (Br2, CH3Br, Cl2, Br2, HCl, I2, NF3) mixtures.

Plasma sputter deposition is introduced in the field of catalyst preparation. It is shown that growth kinetics and morphologies are determined by ion to neutral flux ratio and kinetic energies of sputtered atoms. Catalytic activity of such catalysts compares very well with classical catalysts.

A variety of superhard coatings with Vickers plastic hardness exceeding 40 GPa have been reported by several research groups during the last five years (for recent reviews see refs [1,2]). However, one has to distinguish between superhard nanocomposites, such as nc-TiN/a-Si3N4, nc-TiN/a-Si3N4/a- and nc-TiSi2, nc-(Ti1-xAlx)N/a-Si3N4, nc-TiN/TiB2, nc-TiN/BN, etc. where the high hardness originates from the nanostrucutre and, therefore, remains stable upon annealing to high temperatures [1], and coatings, such as CrN/Ni, ZrN/Ni, and others [2] in which the measured high hardness is due to a high compressive stress that is induced in the coatings due to energetic ion bombardment during their deposition (e.g., by magnetron sputtering). We also summarize the recent progress in the industrial applications of the superhard nanocomposite coatings on machining tools.

Clustering phenomena of particles below a few nm in size in low-pressure reactive plasmas have been studied in capacitively coupled high-frequency SiH4 discharges. Clusters nucleate around a size of 0.5 nm, Si4Hx and subsequently grow due to influxes of SinHx (1 < n < 4) and other molecular species. Because of long gas-residence time and small surface loss-probability of clusters, even neutral ones accumulate to nucleate in the reactor. Appearance of negatively charged clusters is considered to become noticeable at around Si4Hx. The growth of clusters is suppressed by using thermophoretic force, gas flow, periodical discharge modulation, and hydrogen dilution. The reactor has been newly developed to suppress the growth of clusters both by thermophoretic force and by gas flow and evacuation without stagnation. Using the reactor, a-Si:H films of extremely high quality have been successfully deposited.

A recently developed procedure is reviewed allowing thick (>1 mm), high-quality c-BN films (>80 % c-BN) to be grown. It is based on the observation that compressive stress inevitably present in such films can be released by medium-energy (some hundred keV) ion irradiation without destroying the cubic phase.

The document first gives definitions of basic terms related to liquid-crystalline and mesomorphic states of matter and then terms specific to the classification of liquid-crystal polymers. The terms have been restricted to those most commonly encountered in the structural description of the latter class of materials. The terms have been selected from the recently published comprehensive document "Definitions of basic terms relating to low-molar-mass and polymer liquid crystals" [ Pure and Applied Chemistry 73 (5), 845-895 (2001)] and are intended to form a readily usable guide for the reader interested in the structural description of polymer liquid crystals. The more comprehensive document should be used for terminology associated with types of mesophases and the optical and physical characteristics of liquid-crystalline materials. The advice given by representatives of the International Liquid Crystal Society for the preparation of this document is gratefully acknowledged.