The present invention relates to microdischarge devices and, in particular, to nanoporous dielectric-encapsulated electrodes for use in such devices.
Microplasma (microdischarge) devices have been under development for almost a decade and devices having microcavities as small as 10 μm have been fabricated. Arrays of microplasma devices as large as 4*104 pixels in ˜4 cm2 of chip area, for a packing density of 104 pixels per cm2, have been fabricated. Furthermore, applications of these devices in areas as diverse as photodetection in the visible and ultraviolet, environmental sensing, and plasma etching of semiconductors have been demonstrated and several are currently being explored for commercial potential. Many of the microplasma devices reported to date have been driven by DC voltages and have incorporated dielectric films of essentially homogeneous materials.
Regardless of the application envisioned for microplasma devices, the success of this technology will hinge on several factors, of which the most important are manufacturing cost, lifetime, and radiant efficiency. A method of device fabrication that addresses manufacturing cost and lifetime is, therefore, highly desirable.
In a first embodiment of the invention, a method for manufacturing microdischarge devices with encapsulated electrodes is provided. The method includes anodizing a metal substrate to form a nanoporous dielectric encapsulated electrode and dissolving a portion of the dielectric layer. The dielectric layer is then anodized a second time, resulting in a nanoporous dielectric encapsulated electrode with improved regularity of the nanoscale dielectric structures. In some embodiments of the invention, the columnar voids in the dielectric may be backfilled with one or more materials to further tailor the properties of the dielectric.
The foregoing features of the invention will be more readily understood by reference to the following detailed description, taken with reference to the accompanying drawings, in which:
The present application is related to U.S. patent application Ser. No. 10/958,175, entitled “Metal/Dielectric Multilayer Microdischarge Devices and Arrays”, filed on the same day as this application, which is incorporated herein by reference.
In certain embodiments of the invention, a columnar nanostructured dielectric is grown on a metal substrate to form a microdischarge electrode. The metal substrate may have any form such as, for example, thin films, foils, plates, rods or tubes. This method facilitates fabricating microdischarge device arrays that will accommodate the shape of any surface. The dielectric is grown by first anodizing the metal substrate, which may be aluminum. A portion of the resulting dielectric layer is then dissolved (dissolution) and a second anodization step is then performed. The resulting dielectric structure is highly regular and nanoporous, having cylindrical cavities of high uniformity and diameters from tens to hundreds of nanometers. In some embodiments of the invention, the nanoscale cavities may then be backfilled with a given material (dielectric or electrical conductor) to further adjust the properties of the structure. The resulting encapsulated metals can demonstrate superior properties, such as high breakdown potential, as compared to conventional dielectric materials such as bulk materials and thin films.
Note that as used in this description and in any appended claims, unless context indicates otherwise, “layers” may be formed in a single step or in multiple steps (e.g., depositions).
Next, removing the nanocolumns 20 by dissolution yields the structure shown in
The metal/nanostructured dielectric structure formed by this process may be used advantageously as electrodes in microplasma devices. The thickness of the nanoporous dielectric deposited on the various portions of an electrode can be tailored according to the properties desired in the device. For example, the thickness of the dielectric layer on portions of the electrode that will be adjacent to a microdischarge cavity may be set preferably in the range of 5 microns to 30 microns. A thicker dielectric layer increases the breakdown voltage of the dielectric and the lifetime of the dielectric against physical processes and chemical corrosion, but also increases the voltage required to ignite a discharge in the microcavity. Other portions of the electrode, not adjacent to the microcavity, may be advantageously covered with a thicker layer of dielectric, such as approximately 40 microns or more. This thicker layer of dielectric can extend the lifetime of the electrode, but also prevent electrical breakdown in regions outside the microcavities. The thickness of the dielectric layer formed on different portions of an electrode may be controlled by the use of a masking agent, such as a photoresist used in photolithography, or by other masking techniques as are known in the art. In some embodiments of the invention, the ratio of the thickness of the dielectric layer formed on the portions of an electrode that will contact a microdischarge cavity to the thickness of the dielectric layer on other portions of the electrode may be set to approximately 1:2 to 1:4.
Other materials may be substituted advantageously for aluminum in the preceding embodiment of the invention. For example, a variety of metals, such as titanium, tungsten, zirconium, and niobium may be used as a substrate on which to form a nanoporous dielectric by anodization. The process may be used to form a TiO2 dielectric layer on titanium substrates and a WO3 dielectric layer on tungsten substrates.
Once the fabrication of the electrode structures is completed, microplasma devices such as those illustrated in
In a further embodiment of the invention, the properties of the encapsulated electrode of the preceding embodiments can be modified substantially with further processing. For example, as illustrated in
The dielectric properties of the nanostructured dielectric are superior to those of dielectrics conventionally used in microplasma discharge devices. For example, the electrical breakdown voltage of a 20 μm thick layer of the Al/Al2O3 dielectric structure shown in
Voltage-current (“V-I”) characteristics for a small array of 100 μm Al2O3 devices are given in
In other embodiments of the invention, microdischarge electrodes according to any of the preceding embodiments of the invention may be incorporated in microdischarge devices and device arrays. Further, microdischarge electrodes comprising metal substrates on which nanoporous dielectrics have been formed by other processes may be employed advantageously in microplasma devices and arrays.
Similarly, it is of course apparent that the present invention is not limited to the aspects of the detailed description set forth above. For example, the dielectric encapsulated metal may be used in a variety of applications beyond microdischarge electrodes. Various changes and modifications of this invention as described will be apparent to those skilled in the art without departing from the spirit and scope of this invention as defined in the appended claims.
This invention was made with Government assistance under U.S. Air Force Office of Scientific Research grant Nos. F49620-00-1-0391 and F49620-03-1-0391. The Government has certain rights in this invention.
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