EP1797579B1

EP1797579B1 - Microdischarge devices with encapsulated electrodes and its method of fabrication

Info

Publication number: EP1797579B1
Application number: EP05858440.0A
Authority: EP
Inventors: J. Gary Eden; Sung-Jin Park
Original assignee: University of Illinois
Current assignee: University of Illinois
Priority date: 2004-10-04
Filing date: 2005-10-04
Publication date: 2015-09-02
Anticipated expiration: 2025-10-04
Also published as: EP1797579A4; WO2007011388A2; WO2007011388A3; JP5435868B2; KR20070060151A; JP2008516380A; EP1797579A2

Description

TECHNICAL FIELD

The present invention relates to microdischarge devices.

BACKGROUND ART

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. US 2003/0080688 A1 to Eden et al. discloses microdischarge devices having electrodes exposed to plasma. The electrodes can be formed by deposition or plating. According to US2003/122466 A1 , field emitter devices can be fabricated by forming silicon rods on a silicon substrate, forming an insulating layer between the rods, forming a gate electrode on the insulating layer and then a nano hole in the insulating layer by removing the rods to expose the electrode. EP 0931859 A1 describes a method for forming a porous alumina film by anodizing an aluminum plate having an array of recesses in its surface. WO2004/079056 A2 describes a process for making a nanostructured component by producing an alumina layer by anodization of deposited aluminum film, etching for removal of alumina to generate open pores in the alumina and depositing a metal film thereon, electro-depositing a component having a plurality of reliefs onto the metal film and the residual alumina layer, the component filling the pores, and removing the residual alumina layer and the metal film to form reliefs of the component filling the pores. US2002/0153828 A1 relates to an electron emitter comprising on a substrate a stack of a lower electrode, an insulating layer having pores and an upper electrode, wherein the insulating layer is an anodic oxide layer with an electrically conductive carbon deposit formed in the pores and a small gap is provided between the carbon deposit and the upper electrode. JP 2004-178863 A relates to an electron source having conductive layers on both sides of a porous alumina layer. US 5808408 A describes a plasma display in which a discharge electrode having a sharp distal end portion is disposed in each discharge cell, wherein a counter electrode layer is arranged on the discharge electrode with an intermediate insulating layer covering the discharge electrode. The counter electrode is covered by another insulating layer.

SUMMARY OF THE INVENTION

The invention provides a method for manufacturing a microdischarge device according to claim 1 and a microdischarge device according to claim 6. An embodiment of the invention is a microdischarge device including a first electrode encapsulated in a dielectric, which may be a nanoporous dielectric film. A second electrode is provided which may also be encapsulated with a dielectric. The electrodes are configured to ignite a discharge in a microcavity when a time-varying (an AC, RF, bipolar or a pulsed DC, etc.) potential is applied between the electrodes. In specific embodiments of the invention, the second electrode may be a screen covering the microcavity opening and the microcavity may be closed at one end. In some embodiments of the invention, the second electrode may be in direct contact with the first electrode. In other embodiments, a gap separates the electrodes.
In a the method of manufacturing microdischarge devices with encapsulated electrodes, a metal substrate is used to form a nanoporous dielectric encapsulated electrode and dissolve 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.

BRIEF DESCRIPTION OF THE DRAWINGS

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:

Figs. 1A-1F show a diagram of a process for fabricating nanoporous encapsulated metal microplasma electrodes according to an embodiment of the present invention;
Figs. 1G and 1H are diagrams for further processing steps in the process shown in fig. 1;
Fig. 1I shows a flow chart for the process illustrated in figs. 1A-1F and 1G and 1H;
Fig. 2A shows a microdischarge device with an encapsulated electrode in cross-section according to an embodiment of the present invention;
Fig. 2B shows a top view of the device of fig. 2A;
Fig. 3A shows a microdischarge device in cross-section with an encapsulated electrode and an encapsulated metal screen for the other electrode, according to an embodiment of the present invention;
Fig. 3B shows a top view of the device of fig. 3A;
Fig. 4 shows a microdischarge device in cross-section where the microcavity is closed at one end, according to an embodiment of the present invention;
Fig. 5 shows a device similar to the device of fig. 2 where both electrodes are encapsulated;
Fig. 6 shows a stacked version of the device of fig. 5 where the two electrodes are not in direct physical contact;
Fig. 7 shows a stacked version of the device of fig. 5 forming a linear array in which the electrode pairs are in direct physical contact, according to an embodiment of the invention;
Fig. 8 shows a microdischarge structure where microcavities form a planar array according to an embodiment of the invention;
Fig. 9 shows a microdischarge device array for display applications in which the pixels are individually addressable, according to an embodiment of the invention; and
Fig. 10 shows a microdischarge device array formed by a plurality of dielectric-encapsulated microcavities on a cylinder and a center electrode, according to another embodiment of the invention;
Fig. 11 shows a two stage version of the device of fig. 10;
Fig. 12 shows voltage-current characteristics for 100 µm diameter Al/Al₂O₃ devices in neon at several values of the ac excitation frequency; and
Fig. 13 shows voltage-current characteristics for 100 µm diameter Al/Al₂O₃ devices in an Ar:N₂ (2%) mixture for two values of pressure.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

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.
In a first embodiment of the invention, a microdischarge device is provided that includes a first electrode encapsulated in a dielectric, which may be a nanoporous dielectric film. A second electrode is provided which may also be encapsulated with a dielectric. The electrodes are configured to ignite a discharge in a microcavity when a time-varying (an AC, RF, bipolar or a pulsed DC, etc.) potential is applied between the electrodes. In specific embodiments of the invention, the second electrode may be a screen covering the microcavity opening and the microcavity may be closed at one end. In some embodiments of the invention, the second electrode may be in direct contact with the first electrode. In other embodiments, a gap separates the electrodes.
In another embodiment of the invention, a microdischarge device array is provided. The array includes a plurality of electrode pairs. Each electrode pair includes a first electrode and a second electrode with each electrode comprising a metal encapsulated with a dielectric. Each pair of electrodes is configured to ignite a discharge in a corresponding microcavity when a time-varying potential is applied between the electrodes. In a specific embodiment of the invention, the electrode pairs are stacked, forming a linear array of microdischarge devices.
In a further embodiment of the invention, a microdischarge device array is provided that includes a planar electrode array including a plurality of metal electrodes encapsulated in a dielectric. The encapsulated electrode array forms a plurality of microcavities. A common electrode is configured to ignite a discharge in each microcavity when a potential is applied between the common electrode and the electrode array. In some embodiments, the common electrode is transparent to the light emitted by the array.
In another embodiment of the invention, a microdischarge device array for display applications is provided. The array includes a first electrode comprising a metal encapsulated with a first dielectric; a plurality of microcavities associated with the first electrode; a second electrode comprising a metal encapsulated with a second dielectric; and a plurality of microcavities associated with the second electrode. The first electrode and the second electrode are configured to ignite a microdischarge in a given microcavity when a potential is applied between the first and second electrode but only if the given microcavity is a member of both the first plurality of microcavities and the second plurality of microcavities.
In another embodiment of the invention, a cylindrical microdischarge device array is provided that includes a metal cylinder (tube). A plurality of microcavities is formed on the inner surface of the cylinder which is then encapsulated with a dielectric. An electrode is disposed along the center axis of the cylinder and the electrode is configured to ignite a discharge in each microcavity when a time-varying potential is applied between the electrode and the cylinder. Toxic gas remediation may be effected by introducing a flow of gas along the center electrode. A potential is applied between the center electrode and the cylinder to ignite a discharge in each microcavity. The discharges dissociate the impurities in the gas as the gas flows through the microcavities. In other embodiments of the invention, this structure may be used for photochemical treatment of gases flowing through the cylinder. It may also serve as a gain medium for a laser.
Embodiments of the invention introduce microdischarge device array geometries and structures for the purpose of scaling the active length and/or area that is required for applications in medicine and photopolymerization (photoprocessing of materials), for example.
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).
Figs. 1A-1F illustrate a process for growing a nanoporous dielectric on a metal, in this case aluminum, according to an embodiment of the invention. A nanoporous dielectric layer 20 of Al₂O₃ can be grown on an aluminum substrate 10 in any form including, but not limited to: thin films, foils, plates, rods or tubes. The aluminum substrate should first be thoroughly cleaned by, for example, electrochemical or other chemical polishing methods, such as by subjecting the substrate to a bath of an acidic etchant such as perchloric acid (fig. 1A). This process also serves to remove some irregularities from the surface, thereby making the surface flatter. The next step is to form microcavities of the desired cross-section and array pattern in the metal by one or more of a variety of techniques including microdrilling and chemical etching (fig. 1B). (A microcavity is a cavity that has a characteristic dimension (diameter, length of a rectangle, etc.) approximately 500 µm or less). The dielectric deposition process is then initiated by anodizing Al 10 which yields a nanoporous surface 20 of Al₂O₃ (fig. 1C) with columnar voids 25, but this surface has nanostructure that is irregular. The anodization can occur in an acidic solution with the metal substrate as the anode and a suitable material, such as graphite, copper, or platinum as the cathode. In one embodiment of the invention, the acidic solution is oxalic acid at a 0.3 -0.4 M concentration and a temperature preferably less than about 15 degrees Celsius. The selection of the solution temperature represents a trade-off: a higher solution temperature causes the dielectric to deposit faster, but the dielectric structure is less regular. In other embodiments of the invention, sulphuric acid, phosphoric acid, chromic acid, or mixtures of organic and inorganic acids may be used as the anodizing solution.
Next, removing the nanocolumns 20 by dissolution yields the structure shown in fig. 1D. The dissolution may be accomplished, for example, by applying a mixture of chromic acid and mercuric chloride (or other alumina etchant solution such as Transetch N™) to the deposited dielectric. Anodizing the remaining structure, which can be considered a template, a second time results in the very regular structure of columnar voids 45 between columns of dielectric 40 shown in fig. 1E. This second anodization may be accomplished in the same fashion as the first anodization, as described above. In specific embodiments of the invention, the thickness of this dielectric material 40 can be varied from hundreds of nanometers ("nm") to hundreds of microns. Furthermore, the diameter of the columnar voids 45 in the dielectric can be adjusted from tens to hundreds of nm by varying the solvent and anodization conditions (temperature and molar concentration).
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 Fig. 1F may be assembled, according to an embodiment of the invention. Simple, two layer devices are shown, the top one of which has two microcavity diameters to facilitate alignment of the two electrodes. In the lower structure, the microcavity cross-sectional dimensions are approximately the same for both electrode structures. After the desired device structure is completed, the device is evacuated by a vacuum system and may be heated under vacuum to de-gas the structure. Subsequently, the microcavity (or microcavities) in the device (or array of devices) is back-filled with the desired gas or vapor and it is then generally desirable to seal the device or array by one of a variety of well-known processes such as anodic bonding, lamination or sealing with glass frit or epoxy. All of the microdischarge devices are powered by a time varying voltage that may be AC, RF, bipolar or pulsed DC. Electrical contact is made directly to the metal within the dielectric layer. Finally, the discharge medium may be produced by introducing to the microcavity a small amount of a metal-halide salt which, when heated by the operation of the microdischarge in a background gas, produces the desired vapor.
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 Fig. 2B, the columnar pores 45 can be partially filled 60 with a material(s) such as magnesium oxide or other dielectric materials. This can be done by a variety of well-known processes such as sputtering, spin coating, chemical "dipping," and sol-gel processes. Thus, considerable flexibility may be achieved in tailoring the properties of the nanostructured dielectric. Properties that may be tailored in this manner include the dielectric constant of the dielectric and its electrical breakdown potential or optical properties.
Alternatively, as illustrated in Fig. 1G - 1H, the Al₂O₃ "barrier" at the base of the nanopores, formed naturally in the anodization process, can be removed by chemical etching. One can then backfill the nanopores with a conducting material 55. Metals can be deposited into the nanopores by electroplating, for example. Any metal deposited onto the surface of the array can be removed, if desired, by etching. Also, carbon nanotubes may be grown within the nanopores by chemical vapor deposition. The nanotubes may be used to produce electrons by field emission. The electrons can be extracted from the open end of the nanopores by an electric field.
Fig. 1I illustrates a process 80 for forming a nanoporous dielectric encapsulated electrode according to an embodiment of the invention. First a metal substrate is provided that may include microcavities 82 and cleaned 84 as described above (see fig. 1A). Next, the microcavity (or array of microcavities) is formed and, if necessary, debris removed by further cleaning (see fig. 1B). Then, the substrate is anodized 86 (see fig. 1C) and a nanoporous dielectric layer is deposited. Next, the deposited layer is partially dissolved 88 (see fig. 1D). The substrate with the remaining dielectric layer template is then anodized 90 a second time (see fig. 1E). If further processing is not required 92, the process ends 94. Alternatively, a third anodization may be performed 96 and the base of the columnar voids may be filled (see fig. 1G) or the columnar voids can be backfilled with a desired material, as described above (see fig. 1H). Microdischarge devices may be completed (not shown in fig. 1I) by filling the microcavity with the discharge medium and sealing the device.
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 fig. 1 has been measured to be higher than 2000 V whereas twice that thickness (40 µm) of bulk alumina has a breakdown voltage of only ∼ 1100 V. Also, thick barrier layers at the base of the nanopores and back-filling the pores with another dielectric are effective in increasing the breakdown voltage.
In various embodiments of the invention, microdischarge devices are provided that include one or more electrodes encapsulated in a nanoporous dielectric. The nanoporous dielectric may be formed, for example without limitation, by a wet chemical process, as described above. Thus, a variety of device structures may be fabricated economically. These devices include a first electrode encapsulated in the dielectric and a second electrode that may also be encapsulated with the dielectric of the first electrode or another dielectric. The electrodes are configured to ignite a microdischarge in a microcavity (i.e., a cavity having a characteristic dimension (diameter, length of a rectangle, etc.) approximately 500 µm or less) when a time-varying (AC, pulsed DC , etc.) excitation potential is applied between the first and second electrodes. The encapsulated electrodes are not exposed to the microplasma discharge, facilitating a longer electrode life.
A microdischarge device 200 is shown in cross-section in fig. 2A, according to a first embodiment of the invention. A first electrode 230 is formed from a metal 210, such as aluminum, encapsulated with a dielectric 220. The dielectric may be a nanoporous dielectric, such as Al₂O₃. A second electrode 240 is placed adjacent to the first electrode and a microcavity 250 of diameter "d" is formed by one of a variety of well-known processes such as microdrilling, laser machining, chemical etching, etc. The microcavity extends through electrode 240 but does not necessarily extend completely through electrode 230. The diameter d typically may be on the order of 1 to 500 microns. Furthermore, the cavity cross-section need not be circular, but can assume a variety of shapes. The second electrode can be any conducting material including metals, indium tin oxide ("ITO"), doped crystalline or polycrystalline semiconductors or even a polymer. An alternating-current ("AC") or other time-varying voltage 260 applied between the first electrode and the second electrode will ignite a microplasma in the microcavity 250 if a discharge gas or vapor of the proper pressure is present and the peak voltage is sufficient. Fig. 2B shows a top view of the device 200. While the microcavity 250 shown is a cylinder, such microcavities are not limited to cylinders and other shapes and aspect ratios are possible. The metal 210 in the first electrode advantageously does not come in contact with the microplasma, facilitating a longer electrode life.
In another related embodiment of the invention 300, as shown in cross-section in fig. 3A, the second electrode may be a metal screen 340 that covers, at least partially, the microcavity 250. The screen electrode may also be encapsulated with a nanoporous dielectric (as shown) if the metal is chosen properly (e.g., Al, W Zr, etc.). Fig. 3B shows a top-down (plan) view of the device.
In a further related embodiment 400 of the invention, as shown in cross-section in fig. 4, one end 480 of the microcavity discharge channel 450 is closed. The dielectric "cap" 480 can serve to reflect light of specified wavelengths by designing a photonic band gap structure into the dielectric 220 or the dielectric 220 at the base of the microcavity 450 can be coated with one or more reflective materials. If the dielectric is transparent in the spectral region of interest, the reflective layers 480 may be applied to the outside of the dielectric 220.
In other embodiments of the invention, both electrodes of the microdischarge device may be encapsulated with a dielectric. Fig. 5 shows a device 500 with a structure similar to the device of fig. 2, except that the second metal electrode 240 is encapsulated with a dielectric 510 forming a second encapsulated electrode 530. In fig. 5, electrode 230 and electrode 530 are in direct physical contact. In other embodiments of the invention, such as that shown in fig. 6, microdischarge devices 600 may be formed where the electrode pairs 230, 530 are stacked with a gap between the dielectric layers for adjacent electrodes. The number of electrode pairs that may be stacked is a matter of design choice and linear arrays 700 of microplasmas having an extended length may be achieved, as illustrated in fig. 7. Such stacked devices can advantageously provide increased intensity of light emission and are suitable for realizing a laser by placing mirrors at either end of the microchannel 750. Alternatively, the structure of fig. 7 may be used in other applications in which a plasma column of extended length is valuable.
In another embodiment of the invention, as shown in cross-section in fig. 8, a microplasma device array with a planar geometry 800 is formed. In this embodiment, a metal electrode array 810 defining the individual "pixel" size is encapsulated in a dielectric 820. The electrode array 810 can be economically fabricated by laser micromachining in a metal substrate or, alternatively, by wet or plasma etching. Once the electrode array is formed, the dielectric 820 can be deposited over the entire array by a wet chemical process. All of the pixels in the array may share a common transparent electrode 840, such as ITO on glass, quartz or sapphire. Applying a potential 830 between the electrodes ignites discharges in the microcavities 850. Light emitted from the microdischarges can escape through the common electrode 840 or out the other end of the microcavities 850. Alternatively, the common electrode 840 need not be transparent but can be a dielectric-encapsulated metal electrode as described earlier. Light can then be extracted out of the end of the microcavities away from the electrode 850.
In a further embodiment of the invention, as shown in fig. 9, a microdischarge array 900 can be formed that permits individual microcavities (pixels) to be selectively excited. Pixels 930 of the desired shape can be fabricated in a dielectric-encapsulated electrode 910 of extended length. Below (or above) this first electrode 910 is a second dielectric encapsulated electrode 920 that may also be of extended length. With the application of a voltage V₁ to the first electrode 910 and no voltage (V₂ = 0) to the second electrode 920, the pixel at the intersection of the first and second electrodes will not ignite. However, if the proper voltage V₂ is also applied to the second electrode, then only the pixel located at the intersection of both electrodes will ignite, emitting light 940. Other pixels in the array will remain dark. In this way, large arrays of pixels, each of which is individually addressable, can be constructed and applied to displays and biomedical diagnostics, for example.
The ability to produce nanoporous dielectrics on conducting (e.g., metal) surfaces in any configuration (geometry) may be used to advantage in plasma arrays and processing systems. Fig. 10, for example, illustrates a cylindrical array of microplasma devices 1000 each of which is fabricated on the inside wall of a tubular section 1010 of a metal (foil, film on another surface, aluminum tubing, etc.). After the microcavities have been fabricated in the wall of tube 1010, the array is completed by forming a nanoporous dielectric 1030 on the inner surface of the cylinder 1010 with the dielectric also coating the interior of each microcavity, as described above. Depending on the intended application, the microcavities may be of various shapes and size. For the embodiment of fig. 10, the microcavities extend through the wall of the cylinder 1010. Gas enters the system from the outside of the cylinder 1010 and passes through the microcavities. If the application of the system is to dissociate (fragment) a toxic or other environmentally-hazardous gas or vapor, passage of the gas through the microdischarges will dissociate some fraction of the undesirable species. If the degree of dissociation in a one stage arrangement is acceptable, the gaseous products can be removed from the system along its axis, as shown in fig. 10. If the degree of dissociation in one stage is insufficient, then a second stage, concentric with the first stage, may be added, as shown in fig. 11. In this case, the center electrode 1020 is tubular and an array of microcavities is fabricated in its wall that is similar to that in the tubular section 1010. The microcavities again extend through the wall. Along the axis of the electrode 1020 is a second electrode which may be a tube, rod or wire. Both the first and second electrode are encapsulated by the dielectric. With this two stage system, the gas or vapor of interest is now required to pass through two arrays of microdischarges prior to exiting the system.
As noted earlier, the center electrode 1020, which lies along the axis of the larger cylinder having the microplasma pixels, can be a solid conductor (such as a metal rod or tube) or can alternatively be a transparent conductor deposited onto an optically transparent cylinder (such as quartz tubing). The former design will be of interest for electrically exciting and dissociating gases to produce excited or ground state radicals - whereas the latter will be valuable for photo-exciting a gas or vapor flowing inside the inner (optically transparent) cylinder.
The array of fig. 10 can be used for photochemical processing such as toxic gas remediation, according to an embodiment of the invention. A time-varying potential is applied between the center electrode 1020 and the cylinder 1030. Another application is optical pumping for amplification of light in a gain medium disposed in the center 1020 of the cylinder.
Several of the devices and arrays described earlier, and those depicted in figs. 2, 3, and 5, in particular, have been constructed and tested. A typical microdischarge device fabricated to date consists of Al foil, typically 50-100 microns in thickness, which is first cleaned in an acid solution, and then a microcavity or array of microcavities is micromachined in the foil. The individual microdischarge cavities (i.e., microcavities) are cylindrical with diameters of 50 or 100 microns. After the microcavities are produced, nanoporous Al/ Al₂O₃ is grown over the entire electrode to a thickness of ∼10 microns on the microcavity walls and typically 30-40 microns elsewhere. After assembly of the devices, the devices are evacuated in a vacuum system, de-gassed if necessary, and backfilled with the desired gas or vapor. If desired, the entire device or an array of devices may be sealed in a lightweight package with at least one transparent window by anodic bonding, lamination, glass frit sealing or another process, as is known in the art.
A 2 x 2 array of Al/ Al₂O₃ microdischarge devices, each device having a cylindrical microcavity with a 100 micron diameter (device of fig. 5) has been operated in the rare gases and air. Typical AC operating voltages (values given are peak-to-peak) and RMS currents are 650 V and 2.3 mA for ∼700 Torr (0.93326 bar) of Ne, and 800-850 V and 6.25 mA for air. The AC driven frequency for these measurements was 20 kHz. It must be emphasized that stable, uniform discharges were produced in all of the pixels of the arrays without the need for electrical ballast. This result is especially significant for air which has long been known as one of the most challenging gases (or gas mixtures) in which to obtain stable discharges.
Much larger arrays may be constructed and the entire process may be automated. The low cost of the materials required, the ease of device assembly, and the stable well-behaved glow discharges produced in the areas tested to date, all indicate that the microdischarge devices and arrays of embodiments of the present invention can be of value wherever low cost, bright and flexible sources of visible and ultraviolet light are required.
It will, of course, be apparent to those skilled in the art that the present invention is not limited to the aspects of the detailed description set forth above. In any of the described embodiments, the dielectric used to encapsulate an electrode may be a nanoporous dielectric. While aluminum encapsulated with alumina (Al/Al₂O₃) has been used as an exemplary material in these devices, a wide variety of materials (e.g., W/WO₃) may also be used. Further, in any of the above described embodiments, the microcavities of the device may be filled with a gas at a desired pressure to facilitate microdischarges with particular characteristics. The microcavities may be filled with a discharge gas, such as the atomic rare gases, N₂, and the rare gas-halogen donor gas mixtures. Gas pressure and gas mixture composition may be chosen to maintain a favorable number density of the desired radiating species.
Experimental Al/Al₂O₃ microdischarge device arrays were fabricated in the manner described with respect to FIGs. 1A - 1I. Devices were formed in an array, and each had a microcavity diameter of 100 µm. The microcavities were produced in aluminum foil and extend through the foil. The Al₂O₃ dielectric lining the inner wall of each microcavity was visible in optical microgaphs as a black ring. The dielectric film is, in reality, transparent but appears dark only because of the manner in which the photographs were recorded. The Al₂O₃ film on top (and on the reverse side) of the Al substrate is transparent and observed speckling is the result of residual surface structure on the Al foil. Another experimental array of microdischarge devices was formed, also with cylindrical microcavities, but having diameters of 200 µm.
Voltage-current ("V-I") characteristics for a small array of experimental 100 µm Al₂O₃ devices are given in FIG. 12. The fill gas is Ne at a pressure of 700 Torr (0.93326 bar) and results are shown for AC-excitation of the array at one of several frequencies. The voltage values on the ordinate are peak-to-peak values. And, it should be noted that the operating voltage can be reduced below those shown in FIG. 12 by reducing the Al₂O₃ thickness in the microcavity V-I characteristics for a small array of Al₂O₃ microdischarge devices operating in Ar/2% N₂ mixtures are shown in FIG. 13 for two values of the total mixture pressure: 500 Torr (0.6666 bar) and 700 Torr (0.93326 bar). The operating voltages required are higher than those for Ne because of the attaching properties of N₂.
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.

Claims

A method for manufacturing a microdischarge device comprising:
a first electrode (230), the electrode comprising a conductor and a microcavity, the electrode encapsulated with a first dielectric; and

a second electrode (240), the electrodes configured to ignite a discharge in the microcavity when a time-varying potential is applied between the electrodes,

wherein the method for manufacturing the encapsulated electrode comprises:
providing a metal substrate, the metal substrate including at least one microcavity;

anodizing the substrate to form a first layer, the first layer including pores;

dissolving a portion of the first layer; and

performing a second anodization of the first layer when the portion of the first layer is dissolved, forming an encapsulating layer, thereby forming the encapsulated electrode.
The method according to claim 1, further including:
filling the pores of the encapsulating layer to a given depth with one of a metal, a dielectric and a nanotube.
The method according to claim 1, wherein the metal is aluminum and the encapsulating layer includes Al₂O₃.
The method according to claim 1, wherein the metal is titanium and the encapsulating layer includes TiO₂.
The method according to claim 1, wherein the thickness of the encapsulating layer differs between a first portion of the substrate and a second portion of the substrate.
A microdischarge device manufactured according to claim 1 comprising:
a first electrode (230), the electrode comprising a conductor and a microcavity, the electrode encapsulated with a first nanoporous dielectric; and

a second electrode (240), the electrodes configured to ignite a discharge in the microcavity when a time-varying potential is applied between the electrodes.
The device according to claim 6, wherein the second electrode is a screen.
The device according to claim 6, wherein the microcavity is closed at one end.
A device according to claim 6, wherein the second electrode comprises a conductor encapsulated with a second dielectric.
A microdischarge device array comprising a plurality of the microdischarge devices of claim 6.
A microdischarge device array according to claim 10 , wherein said first electrode comprises a planar electrode array (810) including a plurality of metal electrodes encapsulated in a dielectric, the encapsulated electrodes including a plurality of microcavities; and
said second electrode comprises a common electrode (840) configured to ignite a discharge in each of said plurality of microcavities when a potential is applied between the common electrode and the electrode array.
An array according to claim 11 , wherein said common electrode is transparent.
A microdischarge device array for display applications comprising an array according a to claim 10 , wherein said first electrode comprises an electrode array (910) including a plurality of metal electrodes encapsulated in a dielectric, the encapsulated electrodes including a plurality of microcavities; and
said second electrode comprises a plurality of intersecting electrodes (920) configured to ignite a discharge in individual ones of said plurality of microcavities.
The microdischarge device array for display applications of claim 13 , configured such that an individual one of said plurality of microcavities produces a microdischarge only when a time-varying potential above a threshold potential is applied between first electrode and an intersecting one of said plurality of intersecting electrodes.
An array according to claim 14 wherein at least one of the first
dielectric and the second dielectric is a nanoporous dielectric.
A cylindrical microdischarge device array comprising an array according a to claim 10, wherein
said first electrode comprises a metal cylinder, the cylinder characterized by a center axis, a plurality of microcavities being formed on an inner surface of the cylinder and encapsulated with said first dielectric; and
said second electrode is disposed along the center axis of the cylinder and configured to ignite a discharge in each of the plurality of microdischarge devices when a time-varying potential is applied between the center electrode and the cylinder.
An array according to claim 16 , wherein said second electrode comprises a transparent electrically-conducting tube.
An array according to claim 17 wherein said second electrode comprises a metal conductor.