Microplasmas from Diamond Arrays

微等离子体是空心阴极放电的较小规模版本，作为高电子密度、低压力的放电器件已被广泛使用近100年，应用广泛。中空阴极放电由两个电极组成--一个是阳极，一个是形状像中空管或空腔的阴极--由一个小间隙隔开。当在这些电极上施加高电压时，在腔内延伸的缝隙中形成等离子体。通常情况下，这种空心阴极器件工作在非常低的压力下，但随着电极和腔体的尺寸减小到几十微米以下，维持等离子体的压力上升到~1大气压。通常被称为“微放电”或“微等离子体”，这些大气压放电代表了等离子体科学的一个新的和迷人的领域。数以千计的微腔可以在基板上以阵列的形式制造出来，从而可以制造大面积的平板等离子体设备。这些设备的应用程序数量正在迅速增长。微等离子体已经开始被用于破坏挥发性有机化合物(VOCs)，这些挥发性有机化合物可能是空气供应中的污染物或存在于工业过程中的废气中。这使得这种设备成为先进的生命维持系统的候选设备，如潜艇、飞机和宇宙飞船，以及电子工业的废气净化。一种应用是将大型微腔等离子体设备阵列用作计算机或电视的平板显示器。微等离子体也可以用作准分子光源，特别是在深紫外光中，这意味着大面积平板单色光源的可能性。微等离子体也可以用作微型化学反应器。到目前为止，微等离子体设备中使用的电极材料范围从折射金属到半导体，并且设备的不同部分需要不同但兼容的材料。电极采用了钼、镍、铂、银和铜等金属，而器件的其他部分则需要氧化铝和氮化硼。本工作的目的是在有源部件中使用金刚石制作微等离子体阵列。金刚石的化学惰性、低磨损率、低溅射率、负电子亲和力、高二次电子产额以及与硅技术的兼容性等优良特性，使其与用于这些器件的传统材料相比具有许多重大优势。特别是，只要改变掺杂剂的浓度，钻石的导电性就可以从高度绝缘到金属可控地变化，这一事实将使微等离子体阵列的基本上所有组件都可以从这种材料制造出来。电极将由高度掺硼的钻石(通过化学气相沉积技术在布里斯托尔沉积)制成，具有高导电性和高电子发射效率。绝缘介质将由氧化后的未掺杂钻石表面制成。整个器件可以在硅片上制造，以与现有的硅微制造技术兼容，也可以由厚的非掺杂CVD金刚石衬底制成，从而提供高导热系数的所有好处，从而使器件有可能在高功率水平下运行。另一种策略是使用布里斯托尔开发的喷墨涂层技术，将厚厚的掺杂/非掺杂/掺杂纳米钻石粉末夹层结构直接写入合适的衬底上。该项目是布里斯托尔大学、卢瑟福-阿普尔顿实验室和开放大学的合作项目，布里斯托尔大学将沉积CVD/喷墨钻石层，卢瑟福-阿普尔顿实验室将开发蚀刻工艺，将钻石形成微腔，开放大学将测试设备和阵列的性能和寿命。

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.