Mark J. Kushner

A model for the discharge kinetics and plasma chemistry during plasma enhanced chemical vapor deposition of amorphous silicon

A model for the plasma enhanced chemical vapor deposition of amorphous hydrogenated silicon (a‐Si:H) in rf and dc discharges is presented. The model deals primarily with the plasma chemistry of discharges sustained in gas mixtures containing silane (SiH4). The plasma chemistry model uses as input the electron impact rate coefficients generated in a separate simulation for the electron kinetics and therefore makes no a priori assumptions as to the manner of power deposition. Radical densities and contributions to film growth are discussed as a function of gas mixture, electrode separation, and locale of power deposition, and comparisons are made to experiment. A compendium of reactions and rate constants for silane neutral and ion chemistry is also presented.

The 2012 Plasma Roadmap

Low-temperature plasma physics and technology are diverse and interdisciplinary fields. The plasma parameters can span many orders of magnitude and applications are found in quite different areas of daily life and industrial production. As a consequence, the trends in research, science and technology are difficult to follow and it is not easy to identify the major challenges of the field and their many sub-fields. Even for experts the road to the future is sometimes lost in the mist. Journal of Physics D: Applied Physics is addressing this need for clarity and thus providing guidance to the field by this special Review article, The 2012 Plasma Roadmap.

The Gaseous Electronics Conference radio‐frequency reference cell: A defined parallel‐plate radio‐frequency system for experimental and theoretical studies of plasma‐processing discharges

A “reference cell” for generating radio-frequency (rf) glow discharges in gases at a frequency of 13.56 MHz is described. The reference cell provides an experimental platform for comparing plasma measurements carried out in a common reactor geometry by different experimental groups, thereby enhancing the transfer of knowledge and insight gained in rf discharge studies. The results of performing ostensibly identical measurements on six of these cells in five different laboratories are analyzed and discussed. Measurements were made of plasma voltage and current characteristics for discharges in pure argon at specified values of applied voltages, gas pressures, and gas flow rates. Data are presented on relevant electrical quantities derived from Fourier analysis of the voltage and current wave forms. Amplitudes, phase shifts, self-bias voltages, and power dissipation were measured. Each of the cells was characterized in terms of its measured internal reactive components. Comparing results from different cells provides an indication of the degree of precision needed to define the electrical configuration and operating parameters in order to achieve identical performance at various laboratories. The results show, for example, that the external circuit, including the reactive components of the rf power source, can significantly influence the discharge. Results obtained in reference cells with identical rf power sources demonstrate that considerable progress has been made in developing a phenomenological understanding of the conditions needed to obtain reproducible discharge conditions in independent reference cells.

A model for plasma modification of polypropylene using atmospheric pressure discharges

Atmospheric pressure plasmas are commonly used to improve the wetting and adhesion properties of polymers. In spite of their use, the mechanisms for achieving these properties are unclear. In this regard, we report on a computational investigation of the gas phase and surface kinetics during humid-air corona treatment of polypropylene (PP) and the resulting modification of its surface properties while varying energy deposition, relative humidity (RH), web speed, and gas temperature. Using results from a global plasma chemistry model validated against experiments, we found that increasing energy deposition increased the densities of alcohol, carbonyl, acid, and peroxy radicals on the PP surface. In doing so, significant amounts of gas phase O3 and NxOy are produced. Increasing the RH increased the production of peroxy and acid groups, while decreasing those of alcohol and carbonyl groups. Production of O3 decreased while that of HNO3 increased. Increasing the temperature decreased the concentrations of alcohol, carbonyl, and acid groups on PP while those of the peroxy radicals increased. For a given energy deposition, higher web speeds resulted in decreased concentrations of alcohols, peroxy radicals, carbonyl, and acid groups on PP.

Two‐dimensional modeling of high plasma density inductively coupled sources for materials processing

Inductively coupled plasma sources are being developed to address the need for high plasma density (1011–1012 cm−3), low pressure (a few to 10–20 mTorr) etching of semiconductor materials. One such device uses a flat spiral coil of rectangular cross section to generate radio‐frequency (rf) electric fields in a cylindrical plasma chamber, and capacitive rf biasing on the substrate to independently control ion energies incident on the wafer. To investigate these devices we have developed a two‐dimensional hybrid model consisting of electromagnetic, electron Monte Carlo, and hydrodynamic modules; and an off line plasma chemistry Monte Carlo simulation. The results from the model for plasma densities, plasma potentials, and ion fluxes for Ar, O2, Ar/CF4/O2 gas mixtures will be presented.

Dynamics of a coplanar-electrode plasma display panel cell. I. Basic operation

Plasma display panels (PDPs) are a technology for large-area high-brightness flat panel displays. There is considerable interest in improving PDP efficiency by optimizing the cell design, input voltage characteristics, operating conditions and gas mixture. In this article, we report on a two-dimensional computer model for PDPs which has been used to investigate the operation of a coplanar-electrode PDP cell sustained in He/Ne/Xe gas mixtures. The plasma transport equations are implicitly integrated in time to enable simulation of complex gas mixtures and PDP cell designs. To resolve the details of the electron dynamics, the electron temperature is computed by solving the electron energy equation. A Monte Carlo simulation for secondary electrons and a radiation transport model for visible light emission are also employed. The basic operation of the PDP cell is described in this article. The first pulse was usually found to initiate a discharge between the top electrodes and the bottom address electrode, wh...

Hybrid modelling of low temperature plasmas for fundamental investigations and equipment design

The modelling of low temperature plasmas for fundamental investigations and equipment design is challenged by conflicting goals—having detailed, specialized algorithms which address sometimes subtle physical phenomena while also being flexible enough to address a wide range of process conditions. Hybrid modelling (HM) is a technique which provides many opportunities to address both fundamental physics and practical matters of equipment design. HM is a hierarchical approach in which modules addressing different physical processes on vastly disparate timescales are iteratively combined using time-slicing techniques. By compartmentalizing the physics in each module to accept given inputs and produce required outputs, different algorithms can be used to represent the same physical processes. In this manner, the algorithms best suited for the conditions of interest can be used without affecting other modules. In this paper, the basis and implementation of HM are discussed using examples from simulations of inductively coupled plasmas.

Modelling of microdischarge devices: plasma and gas dynamics

Microdischarge devices (MDs) share many properties with their macroscopic counterparts while having unique features resulting from their ability to sustain large current densities and power depositions on a continuous basis at pressures approaching atmospheric. The dynamics of cylindrical, metal–dielectric–metal sandwich MDs sustained in Ar having characteristic sizes of a few hundred micrometres have been computationally investigated using a plasma-transport model which includes gas dynamics. We found that these devices closely resemble negative glow discharges, as they are sustained by, and particularly sensitive to, ionization resulting from secondary electron emission from the cathode. Since these MDs operate on a cw basis with large current densities and power deposition, gas heating and flow dynamics are important considerations in optimizing their electrical and kinetic properties. For example, the formation of excimer species is particularly sensitive to gas heating and rarefaction due to their dependence on three-body formation processes. Scalings of MDs with pressure, current and secondary emission coefficient are discussed.

Large‐bore copper‐vapor lasers: Kinetics and scaling issues

The scaling characteristics of large diameter (6≤d≤12 cm) high repetition‐rate copper‐vapor lasers are investigated using a radially dependent laser‐discharge model. Results of the model are verified with experimental data obtained on several devices including an 8‐cm‐diam 100‐W copper‐vapor laser. Two effects, penetration of the applied electric field into the plasma and gas heating, which we show are the primary considerations in volumetric scaling, are discussed in detail. Thermal constriction of the discharge resulting from gas heating is prevented by plasma skin effects that allow stable performance at large diameters. These same effects are shown to explain the sequential excitation of the laser levels, resulting in both temporally and spatially dependent behavior of the laser pulse. Projections of output power and light‐pulse characteristics are made for lasers up to 12 cm in diameter.

Reaction chemistry and optimization of plasma remediation of NxOy from gas streams

Increasing environmental awareness and regulatory pressure have motivated investigations into energy efficient methods to remove oxides of nitrogen (NxOy) from gas streams resulting from the combustion of fossil fuels. Plasma remediation of NxOy is potentially an efficient removal technique due to the relative ease of generating reactants by electron‐impact processes. Previous works have investigated the use of electron‐beam, corona, and dielectric barrier discharge (DBD) generated plasmas for this purpose. In those works, reduction (N+NO→N2+O) and oxidation (NO2+OH→HNO3) reactions were identified as major removal channels. A computational study of the plasma remediation of NxOy from humid air using repetitively pulsed DBDs is reported. The dominant reaction pathways are discussed and scaling laws are proposed to optimize the energy efficiency of removal. Three reaction periods are identified: the current pulse (during which electron‐impact processes generate radicals), the postpulse remediative period (d...