Microplasmas from Diamond Arrays

Microplasmas are smaller scale versions of the hollow cathode discharges which have been widely used for almost 100 years as high electron density, low pressure discharge devices for a variety of applications. Hollow cathode discharges comprise two electrodes - an anode, and a cathode shaped like a hollow tube or cavity - separated by a small gap. When a high voltage is applied across these electrodes, a plasma is formed in the gap which extends inside the cavity. Normally such hollow cathode devices work at very low pressures, but as the dimensions of the electrodes and the cavity decrease to below a few 10s of um, the pressure at which the plasma can be maintained rises to ~1 atmosphere. Often referred to as 'microdischarges' or 'microplasmas', these atmospheric pressure discharges represent a new and fascinating realm of plasma science. Many thousands of microcavities can be fabricated as arrays onto a substrate, allowing large area flat panels plasma devices to be made. The number of applications for these devices is growing rapidly. Microplasmas have begun to find uses such as the destruction of volatile organic compounds (VOCs) which can be contaminants in air supplies or present in waste gases from industrial processes. This makes such devices candidates for advanced life support systems, such as in submarine, aircraft, and spacecraft, and for exhaust gas clean up from, say, the electronics industry. One application is the potential use of large arrays of microcavity plasma devices as flat panel displays for computers or TVs. Microplasmas can also be used as excimer light sources, particularly in the deep-UV, suggesting the possibility of large area flat panel monochromatic light sources. Microplasmas can also be used as micro-sized chemical-reactors. To date, the range of electrode materials employed in microplasma devices ranges from refractive metals to semiconductors, and different, but compatible materials are required for different parts of the device. For the electrodes, metals such as Mo, Ni, Pt, Ag and Cu have been used, whereas alumina and boron nitride are needed for other parts of the device.The aim of the proposed work is to fabricate microplasma arrays using diamond in the active components. The superlative properties of diamond, such as its chemical inertness, low wear rate, low sputter rate, negative electron affinity, high secondary electron yield, and compatibility with Si technology, give it a number of major advantages over conventional materials used for these devices. In particular, the fact that the electrical conductivity of diamond can be controllably varied from highly insulating through to metallic, simply by changing the concentration of the dopant, will allow essentially all the components of the microplasma array to be fabricated from this one material. The electrodes would be made from highly B-doped diamond (deposited in Bristol by chemical vapour deposition techniques), giving high conductivity with high electron emission efficiency. The insulating dielectric would be made from the oxidised undoped diamond surface. The entire device could be made on a Si wafer for compatibility with existing Si microfabrication techniques, or from a thick undoped CVD diamond substrate, thus giving all the benefits of high thermal conductivity and therefore the potential for device operation at high power levels. The alternative strategy is to use inkjet coating technology that developed at Bristol to direct-write thick layers of doped/undoped/doped nanodiamond powder sandwich structures onto a suitable substrate.The project is a collaboration between Bristol University, who will deposit the CVD/inkjet diamond layers, the Rutherford-Appleton lab, who will develop etching processes to pattern the diamond into microcavities, and the Open university, who will test the devices and arrays for performance and lifetime.