G. J. Collins

Atmospheric-pressure plasma sources for biomedical applications

Atmospheric-pressure plasmas (APPs) have attracted great interest and have been widely applied in biomedical applications, as due to their non-thermal and reactive properties, they interact with living tissues, cells and bacteria. Various types of plasma sources generated at atmospheric pressure have been developed to achieve better performance in specific applications. This article presents an overview of the general characteristics of APPs and a brief summary of their biomedical applications, and reviews a wide range of these sources developed for biomedical applications. The plasma sources are classified according to their power sources and cover a wide frequency spectrum from dc to microwaves. The configurations and characteristics of plasma sources are outlined and their biomedical applications are presented.

Laser‐induced chemical vapor deposition of SiO2

We have demonstrated rapid (3000 A/min) photochemical deposition of silicon dioxide from gas phase donor molecules. An ArF (193 nm) laser was used to excite and dissociate gas phase SiH4 and N2O molecules in contrast to earlier work with incoherent mercury lamps. We have achieved 20 times the deposition rate, limited the dissociation volume to a localized region, and minimized the direct impingement of UV photons on the substrate. Although the SiO2 deposition rate was insensitive to substrate temperature from 20 to 600 °C, film quality noticeably improved above 200 °C. Metal‐oxide‐semiconductor capacitors were fabricated and characterized in order to measure SiO2 electrical properties. Film composition was investigated using Auger and infrared spectroscopy techniques and showed that the SiO2 is stoichiometric and contains less than 5% nitrogen.

Visible electroluminescence from porous silicon diodes with an electropolymerized contact

An electrochemical technique is proposed to form a solid‐state electrical contact of porous Si (PS) electroluminescence (EL) diodes. The PS layers were created by anodizing nondegenerate p‐type single‐crystal Si wafers in an HF solution, and then electrochemically polymerizing semitransparent conducting polypyrrole films into the PS layer. The current‐voltage characteristics and the voltage and current dependence of the EL intensity were significantly improved in comparison with our experimental PS‐EL diode with a thin Au film contact. Our result suggests that the electrode impregnation into PS is very useful for an efficient and stable EL operation.

Photodeposition of aluminum oxide and aluminum thin films

Uniform films of Al2O3 have been photodeposited using an excimer laser operating at 248 nm (KrF) or at 193 nm (ArF) and trimethylaluminum and N2O as the reactants. Deposition rates were typically 2000 A/min and the physical, chemical, and electrical properties of the photodeposited Al2O3 films are comparable to films deposited using conventional techniques. Properties of photodeposited aluminum films are also presented.

Low‐temperature refractory metal film deposition

We have deposited uniform films of Mo, W, and Cr over large areas (>5 cm2) using UV laser photodissociation of their respective hexacarbonyls. The depositions were made at room temperature over pyrex and quartz plates, as well as silicon wafers. We have examined the resistivity, reflectivity, stress, and step coverage of these films.

Metal‐vapor production by sputtering in a hollow‐cathode discharge: Theory and experiment

Laser action in singly ionized metal atoms has been obtained when a rare gas is excited in a metal hollow cathode. The required metal‐vapor density is produced by discharge sputtering from the cathode and the excitation of upper levels occurs via a charge‐transfer reaction of the type B++M → (M+)+B+ΔE. We present a unified discharge‐sputtering theory which describes the metal density created in the hollow cathode, including both the current and spatial dependence. The predictions of this model are then compared to the measured dependence of metal‐vapor density with current, spatial position, and buffer‐gas pressure. Discharge conditions which support laser oscillation are emphasized. Agreement between theory and experiment is good.

Glow‐discharge‐created electron beams: Cathode materials, electron gun designs, and technological applications

The operating characteristics of glow‐discharge‐created electron beams are discussed. Ten different cathode materials are compared with regard to maximum electron beam current achieved and the beam generation efficiency as measured calorimetrically. Specific electron gun designs are presented for a variety of applications that include: cw ion laser excitation; electron beam assisted chemical vapor deposition of microelectronic films; and wide area annealing of ion‐implantation damage to silicon substrates. The use of sintered metal‐ceramic (e.g., Mo‐Al2O3) cathodes to generate multikilowatt electron beams in a pure noble gas discharge is reported. Cathode materials with high secondary electron emission coefficients by ion bombardment allow for electron beam production in glow discharges at 50%–80% generation efficiency values.

Hollow cathode metal ion lasers

A new class of metal ion lasers with significant CW output power in the UV (220-320 nm) and near IR (800-2000 nm) spectral regions is described. In a hollow cathode discharge the upper laser levels are excited via charge transfer collisions between ground state buffer gas ions and ground state metal atoms. At the present stage of development, hollow cathode metal ion lasers are shown to be comparable in UV output power to rare-gas ion lasers but with lower threshold currents by a factor of more than twenty. Visible output powers are lower than rare-gas ion lasers. In the text we present device progress to date, measurements of important plasma parameters, and an outline of potential applications of hollow cathode metal ion lasers.

Laser induced deposition of zinc oxide

We have used an excimer laser to photodeposit uniform (±5%) films of zinc oxide from dimethylzinc and nitrogen dioxide. Deposition rates of 3000 A/min over 2 cm×5 cm areas were obtained. We have measured the stress, refractive index, etch rate, adhesion, pinhole density, and the stoichiometry of these photodeposited films.

Optical and RF electrical characteristics of atmospheric pressure open-air hollow slot microplasmas and application to bacterial inactivation

We report electrical properties of radio frequency (RF)-driven hollow slot microplasmas operating in open air but with uniform luminous discharges at RF current densities of the order of A cm −2 . We employ interelectrode separations of 100–600 µm to achieve this open-air operation but because the linear slot dimension of our electrode designs are of extended length, we can achieve, for example, open-air slot shaped plasmas 30 cm in length. This creates a linear plasma source for wide area plasma driven surface treatment applications. RF voltages at frequencies of 4–60 MHz are applied to an interior electrode to both ignite and sustain the plasma between electrodes. The outer slotted electrode is grounded. Illustrative absolute emission of optical spectra from this source is presented in the region from 100 to 400 nm as well as total oxygen radical fluxes from the source. We present both RF breakdown and sustaining voltage measurements as well as impedance values measured for the microplasmas, which use flowing rare gas in the interelectrode region exiting into open air. The requirement for rare gas flow is necessary to get uniform plasmas of dimensions over 30 cm, but is a practical disadvantage. In one mode of operation we create an out-flowing afterglow plasma plume, which extends 1–3 mm from the grounded open slot allowing for treatment of work pieces placed millimetres away from the grounded electrode. This afterglow configuration also allows for lower gas temperatures impinging on substrates, than the use of active plasmas. Work pieces are not required to be part of any electrical circuit, bringing additional practical advantages. We present a crude lumped parameter equivalent circuit model to analyse the effects of changing RF sheaths with frequency of excitation and applied RF current to better understand the relative roles of sheath and bulk plasma behaviour observed in electrical characteristics. Estimates of the bulk plasma densities are also provided. Finally, we present results of afterglow plasma based bacteria inactivation studies (Escherichia coli, Bacillus atrophaeus and B. atrophaeus spores) in which we employ the flowing afterglow plume from a hollow slot microplasma device rather than the active plasma itself, which is fully contained between electrodes. (Some figures in this article are in colour only in the electronic version)