Patent Publication Number: US-8541946-B2

Title: Variable electric field strength metal and metal oxide microplasma lamps and fabrication

Description:
STATEMENT OF GOVERNMENT INTEREST 
     This invention was made with Government assistance under U.S. Air Force Office of Scientific Research grant Nos. F49620-03-1-0391 and AF FA9550-07-1-0003. The Government has certain rights in this invention. 
    
    
     FIELD OF THE INVENTION 
     The invention is in the field of microcavity plasma devices, also known as microdischarge devices or microplasma devices. 
     BACKGROUND 
     Microcavity plasma devices produce a nonequilibrium, low temperature plasma within, and essentially confined to, a cavity having a characteristic dimension d below approximately 500 μm. This new class of plasma devices exhibits several properties that differ substantially from those of conventional, macroscopic plasma sources. Because of their small physical dimensions, microcavity plasmas normally operate at gas (or vapor) pressures considerably higher than those accessible to macroscopic devices. For example, microplasma devices with a cylindrical microcavity having a diameter of 200-300 μm (or less) are capable of operation at rare gas (as well as N 2  and other gases tested to date) pressures up to and beyond one atmosphere. 
     Such high pressure operation is advantageous. An example advantage is that, at these higher pressures, plasma chemistry favors the formation of several families of electronically-excited molecules, including the rare gas dimers (Xe 2 , Kr 2 , Ar 2 , . . . ) and the rare gas-halides (such as XeCl, ArF, and Kr 2 F) that are known to be efficient emitters of ultraviolet (UV), vacuum ultraviolet (VUV), and visible radiation. This characteristic, in combination with the ability of microplasma devices to operate in a wide range of gases or vapors (and combinations thereof), offers emission wavelengths extending over a broad spectral range. Furthermore, operation of the plasma in the vicinity of atmospheric pressure minimizes the pressure differential across the packaging material when a microplasma device or array is sealed. 
     Research by the present inventors and colleagues at the University of Illinois has resulted in new microcavity plasma device structures as well as applications. As an example, semiconductor fabrication processes have been adapted to produce large arrays of microplasma devices in silicon wafers with the microcavities having the form of an inverted pyramid. Arrays with 250,000 devices, each device having an emitting aperture of 50×50 μm 2 , have been demonstrated with a device packing density and array filling factor of 10 4  cm −2  and 25%, respectively. Other microplasma devices have been fabricated in ceramic multilayer structures, photodefinable glass, and Al/Al 2 O 3  structures. 
     Microcavity plasma devices developed over the past decade have a wide variety of applications. An exemplary application for a microcavity plasma device array is to a display. Since the diameter of single cylindrical microcavity plasma devices, for example, is typically less than 200-300 μm, devices or groups of devices offer a spatial resolution that is desirable for a pixel in a display. In addition, the efficiency of a microcavity plasma device can exceed that characteristic of conventional plasma display panels, such as those in high definition televisions. 
     Early microcavity plasma devices exhibited short lifetimes because of exposure of the electrodes to the plasma and the ensuing damage caused by sputtering. Polycrystalline silicon and tungsten electrodes extend lifetime but are more costly materials and difficult to fabricate. 
     Large-scale manufacturing of microcavity plasma device arrays benefits from structures and fabrication methods that reduce cost and increase reliability. Of particular interest in this regard are the electrical interconnections between devices in a large array. If the interconnect technology is difficult to implement or if the interconnect pattern is not easily reconfigurable, then manufacturing costs are increased and potential commercial applications may be restricted. Such considerations are of increasing importance as the demand rises for displays or light-emitting panels of larger area. 
     Previous work conducted by some of the present inventors has resulted in thin, inexpensive metal/metal oxide arrays of microcavity plasma devices. Metal/metal oxide lamps are formed from thin sheets of oxidized electrodes, are simple to manufacture and can be conveniently formed by mass production techniques such as roll-to-roll processing. In some manufacturing techniques, the arrays are formed by oxidizing a metal screen, or other thin metal sheet having cavities formed in it, and then joining the screen to a common electrode. The metal/metal oxide lamps are light, thin, and can be flexible. While individual arrays can be joined with other arrays to form larger arrays, rapidly fabricating individual arrays having radiating areas that exceed approximately 100 cm 2  is challenging. As arrays become larger, avoiding stress that can reduce the flatness of the array is of increasing importance. 
     Eden et al. U.S. Pat. No. 7,385,350, entitled “Arrays of Microcavity Plasma Devices with Dielectric Encapsulated Electrodes,” which issued on Jun. 10, 2008, discloses arrays of microcavity plasma devices with dielectric encapsulated electrodes. A pattern of microcavities is produced in a metal foil, or the metal foil can be a pre-formed metal screen. Oxide is subsequently grown on the foil and within the microcavities (where plasma is to be produced) to protect the microcavity from the plasma and electrically isolate the foil. A second metal foil is also encapsulated with oxide and is bonded to the first encapsulated foil. A thin glass layer or vacuum packaging, for example, is able to seal the plasma medium into the array. The second electrode can be a solid sheet common electrode, which requires no particular alignment, or can be a patterned electrode, which requires alignment with the first electrode. 
     SUMMARY OF THE INVENTION 
     Preferred embodiments of the invention provide microcavity plasma lamps having a plurality of metal and metal oxide layers defining a plurality of arrays of microcavities and encapsulated thin metal electrodes. Packaging encloses the plurality of metal and metal oxide layers in plasma medium. The metal and metal oxide layers are configured and arranged to vary the electric field strength and total gas pressure (E/p) in the lamp. The invention also provides methods of manufacturing a microcavity plasma lamp that simultaneously evacuate the volume within the packaging and a volume surrounding the packaging to maintain an insignificant or zero pressure differential across the packaging. The packaging is backfilled with a plasma medium while also maintaining an insignificant or zero pressure differential across the packaging. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a schematic cross-sectional view of an exemplary embodiment array of microcavity plasma devices of the invention; 
         FIG. 1B  is a schematic cross-section of a portion of one of the electrodes of the array; 
         FIG. 2  is a schematic cross-sectional view of a portion of another exemplary embodiment array of microcavity plasma devices of the invention; 
         FIG. 3  is a schematic cross-sectional view of another exemplary embodiment array of microcavity plasma devices of the invention; 
         FIG. 4  is a schematic cross-sectional view of another exemplary embodiment array of microcavity plasma devices of the invention; 
         FIG. 5  is a schematic cross-sectional view of another exemplary embodiment array of microcavity plasma devices of the invention; 
         FIG. 6  is a schematic cross-sectional view of another exemplary embodiment array of microcavity plasma devices of the invention; 
         FIG. 7  is a schematic cross-sectional view of another exemplary embodiment array of microcavity plasma devices of the invention; 
         FIG. 8  is a schematic cross-sectional view of an exemplary embodiment array of microcavity plasma devices of the invention; 
         FIG. 9  presents luminous efficacy data obtained with a prior device that hand a single metal/metal oxide layer with microcavities and a continuous metal electrode surrounded by oxide; 
         FIGS. 10A and 10B  illustrate a preferred embodiment vacuum processing system for manufacturing arrays of microcavity plasma devices of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the invention provide microcavity plasma lamps having a plurality of metal and metal oxide layers defining a plurality of arrays of microcavities and encapsulated thin metal electrodes. Packaging encloses the plurality of metal and metal oxide layers in plasma medium. The metal and metal oxide layers are configured and arranged to vary the electric field strength and total gas pressure (E/p) in the lamp. The invention also provides methods of manufacturing a microcavity plasma lamp that simultaneously evacuate the volume within the packaging and a volume surrounding the packaging to maintain an insignificant or zero pressure differential across the packaging. The packaging is backfilled with a plasma medium while also maintaining an insignificant or zero pressure differential across the packaging. 
     The invention provides high efficiency arrays of microcavity plasma devices including plural thin sheets of metal/metal oxide electrodes with associated microcavities. Arrays of the invention produce emissions from both the front and back sides of the array, and are especially well-suited for general lighting applications that can benefit greatly from high efficiency performance. 
     Preferred embodiments will now be discussed with respect to the drawings. The drawings include schematic figures that are not to scale, which will be fully understood by skilled artisans with reference to the accompanying description. Features may be exaggerated for purposes of illustration. From the preferred embodiments, artisans will recognize broader aspects of the invention. 
       FIG. 1A  is a cross-sectional diagram of an example embodiment of a microcavity plasma lamp  10  of the invention, and  FIG. 1B  is an expanded cross-section of a portion of one of the electrodes of the array  10 . An array of microcavities  12  are defined in first and second thin suspended and intentionally staggered metal oxide sheets  13   a  and  13   b . The metal oxide sheets  13   a  and  13   b  are suspended, meaning that they are separated from each other and/or from above and below with a small gaps  15   a  and  15   b . The metal oxide sheets  13   a  and  13   b  are staggered, such that microcavities  12  defined by the sheet  13   a  are offset from microcavities formed by the sheet  13   b . The thin metal oxide sheets  13   a  and  13   b  can be formed when a thin metal foil, mesh, or screen with pre-formed cavities is electrochemically anodized to convert the metal surface into metal oxide. Preferred metal and metal oxide is aluminum and aluminum oxide, respectively. Other metals and metal oxides, e.g., titanium and its oxide, can also be used. 
     Other methods of coating thin screens or foils having microcavities can also be used, but the anodization process is a simple and preferred method for manufacturing metal/metal oxide electrode layers  13   a  and  13   b.    
     Metal electrodes  16 ,  18  are encapsulated in metal oxide  17  as a result of the anodization, which protects the electrodes  16 ,  18  from plasma produced within the microcavities  12 , thereby promoting the lifetime of the array  10 , and electrically insulating the electrodes  16 ,  18 . The intentional staggering of the microcavities  12  in the lamp  10  provides an advantage in terms of efficiency by varying the ratio of the electric field strength and total gas pressure (E/p, where E is the electric field strength and p is the total gas pressure) in a periodic manner throughout the lamp. The suspension of the sheets  13   a  and  13   b  permits small microplasmas to form above and below the sheets  13   a  and  13   b  in addition to forming in the microcavities  12 . The small micro plasmas that form above and below the sheets  13   a  and  13   b  also have an E/p that is different from that in the microcavities. This permits design flexibility in varying E/p (accomplished by selecting gaps and offsets) which is a valuable mechanism (by which radiative efficiency of the lamp  10  can be optimized. Gaps  15   a  between electrodes  13   a  and  13   b  are capable of producing plasma that efficiently generates ultraviolet light. The gap between layers  13   a  and  13   b  can be maintained without spacers when the layers  13   a  and  13   b  have sufficient stiffness to be supported in a stable fashion when supported solely at one end. Alternatively, small dielectric, (e.g., glass) spacers (not shown in  FIG. 1A ) can be used between the layers  13   a  and  13   b . The staggering and separation of the layers  13   a  and  13   b  creates multiple distances between portions of the electrodes  16  and  18 . Various electric field strengths form around portions of the electrodes  16  and  18  to create varying E/p. 
       FIG. 1B  illustrates that an additional thin layer of dielectric  19  can be deposited onto the metal oxide dielectric  17 . A preferred additional layer of dielectric  19  is a thin glass layer, which can be formed by deposition of a thin layer of glass onto the metal oxide layer  17  by a sol-gel process. Dielectric layer  19  is desirable if the growth of metal oxide layer  17  along the surface of electrode  18  and into the cavities  12  results in excessive stress and micro-cracking in oxide layer  17 . The degree of micro-cracking is dependent upon processing conditions and the radius of curvature of the metal oxide layer  17  as it “turns the corner” into the microcavities. In this situation, dielectric layer  19  can be helpful in filling micro-cracks. 
     Gas(es), vapor(s) or a combination thereof are sealed in the microcavities  12  by packaging layers  20 , such as glass or plastic. The packaging layers are spaced apart and sealed by spacers  22 , by which the gaps  15   a  and  15   b  are set and by which the metal oxide layers  13   a  and  13   b  are suspended from their ends. A thickness T of the array in preferred embodiments is in the approximate range of 0.5-5.0 mm. Portions of the electrodes  16 ,  18  are illustrated as extending outside the spacers  22 , which can seal to the electrodes. The extension can provide for electrical connection to the electrodes. Application of an appropriate time-varying voltage between electrodes  16  and  18  will ignite plasmas in all of the microcavities  12 , and also in the gap regions  15   a ,  15   b . A voltage differential in the gaps  15   b  during positive half-cycles is the result of negative charge build on surfaces opposing the layers  13   a ,  13   b  (in this instance, a phosphor layer  26 ). The wavelength of the plasma emissions can be tailored using different gas(es) and/or vapor(s) in the array. A port  24  can be used to evacuate and fill the array  10 , and can be sealed once the array is filled with the desired gas/vapor mixture. 
     If the gas(es) introduced to array  10  of  FIG. 1A  emit ultraviolet light when excited in a microplasma, then visible light of the desired color(s) can be produced by coating the interior walls of the lamp structure with phosphor  26 . This can be done, for example, by screen printing. Although a phosphor layer is shown in  FIG. 1A  only on the interior structures of packaging layers  20 , the metal oxide sheets  13   a  and  13   b  may also be coated with phosphor. 
     In several preferred embodiments of the invention, the packaging layers  20  are glass sheets typically 50 μm-2 mm in thickness. Such sheets can be bonded to the spacers  22  by a sealant such as glass frit. Since the total pressure of the gas(es) in array  10  is on the order of one atmosphere, the pressure differential across the packaging layers  20  is small, and so the array  10  can also be sealed with packaging layers  20  that are thin sheets of plastic. In this embodiment, the spacers  22  may not be necessary and the plastic packaging layers can be sealed directly to one another by any of several methods well known in the art. 
     The array  10  of  FIGS. 1A and 1B  provides emissions from both its front and back sides, which is well-suited for many illumination applications. Because patterns comprising phosphors  26  emitting various colors can be screen printed onto the interior surface of packaging layers  20 , the lamp design of  FIG. 1A  is also well-suited for signage, or custom control of the color of the light emitted by the lamp. The particular gas(es) and/or (vapors) sealed in the microcavities  12  can also control the wavelength of emissions produced by the array  10 . 
       FIGS. 2 and 3  illustrate additional embodiments that use non-powered, additional thin metal/metal oxide layers  13   c ,  13   d ,  13   e  as spacers for the electrode metal/metal oxide layers  13   a ,  13   b  and with microcavities in the different layers aligned to form extended microcavities  12   a  that will form an extended microplasma during operation. In  FIG. 2 , which shows only the metal/metal oxide layers, a third metal oxide layer  13   c  with microcavities serves as a spacer. In  FIG. 3 , plasma  30  that is formed is longer than that in the  FIG. 1A  embodiment, and the use of spacer layers permits the length of the plasma  30  to be tailored. The length of the plasma  30  can impact favorably the UV/visible emission efficiency and use of spacing layers permits the plasma length to be adjusted from one array design to another. The primary benefit of the non-driven electrodes spacers  13   c  in  FIGS. 2 and 13   c ,  13   d , and  13   e  in  FIG. 3  is that they appear to assist in the dissipation of charge built up on the metal oxide dielectric. Experiments show these spacers to be beneficial with regard to light emission efficiency but their function appears to be much more than serving simply as a physical spacer. 
     In the illustrated embodiments, each of the layers  13   a ,  13   b ,  13   c ,  13   d ,  13   e  can be formed from anodized screens where the holes in the screens constitute the microcavities. However, the spacer screen layers  13   c ,  13   d ,  13   e  that don&#39;t serve as electrodes are not necessarily anodized. The inclusion of a number of metal oxide layers as spacer layers has only a negligible effect on the overall thickness of the microcavity plasma device arrays, but the spacer layers are beneficial to efficiency for reasons that are not yet completely understood. The thickness T of the multiple layers  13   a ,  13   b ,  13   c ,  13   d ,  13   e  in  FIG. 3  can be, for example, in the approximate range of only 0.25-0.38 mm. In another preferred embodiment, the metal/metal oxide layers  13   a - 13   e  in  FIGS. 2 and 3  are staggered as in the  FIGS. 1A and 1B  embodiment for the purpose of tailoring the E/p ratio. 
     In the embodiment of  FIG. 4 , separate phosphors  26   a ,  26   b ,  26   c , and  26   d  are deposited onto the interior surfaces of packaging layers  20  in such a way that a different phosphor is associated with each separate extended microcavity formed by the alignment of microcavities in the multiple layers  13   a ,  13   b ,  13   c ,  13   d ,  13   e . In this embodiment, the microcavities of the individual layers are aligned and the choice of phosphors determines the color perceived by the eye of an observer viewing the array lamp  10  from either side. 
     The embodiment of  FIG. 5  achieves the generation of plasmas of differing lengths by varying the microcavity spacing among the metal/metal oxide layers  13   a - 13   e . In the specific instance of  FIG. 5 , the microcavity spacing is the same for the electrodes  13   a  and  13   b  but is significantly smaller than that for layers  13   c ,  13   d , and  13   e . A similar effect can be realized by varying the sizes of the microcavities in the individual metal/metal oxide layers  13   a - 13   e . As with the  FIGS. 1A and 1B  embodiment, microplasma formed in the gaps  15  between the layers  13   a  and  13   b  will have a different E/p ratio than microplasma formed in the extended microcavities  12   a  adjacent the gap areas. Extended microplasma is formed in the extended microcavities  12   a  in areas where the individual microcavities align, while shorter microplasmas form in the gaps  15  between sheets  12   a  and  13   b  adjacent to the extended microplasmas. Phosphors  26   a  and  26   b  are also incorporated into this embodiment. 
     In the embodiment of  FIG. 6  there is an additional common electrode  32  that is shared by the electrodes  16  and  18 . This is one alternative for electrically driving the array, and other possibilities include driving sets of electrodes out of phase with respect to another set. Generally, the multiple layer metal/metal oxide structure allows considerable flexibility in optimizing the optical efficiency for a given optical radiator (i.e., atom or molecule) while still offering a thin, lightweight structure and providing options for achieving a desired E/p ratio throughout the lamp or multiple E/p ratios in different regions of the lamp. 
       FIG. 7  is a cross-sectional diagram for an additional embodiment of the invention that achieves a periodic variable E/p ratio throughout the lamp. The lamp structure of  FIG. 7  has the encapsulated electrodes  16 ,  18  fabricated from mesh or screen that has been extruded, yielding a modulated contour for the electrodes  16 ,  18  that approximates a triangular wave, though extrusion can produce other modulated shapes. This creates an alternating pattern of extended microcavities  12   a  that generate microplasmas and smaller microplasmas formed in gaps  15   b  between the electrodes  16 ,  18  and the packaging windows  20 . Negative charge build up occurs on the inside of the windows  20 , and when the opposing electrode is in a positive half cycle this can generate a plasma in the gaps  15   b . Although no phosphor is shown in  FIG. 7  and it should be mentioned that, as described elsewhere, phosphor coatings can be applied to the interior surfaces of both windows  20  of  FIG. 7  and coated onto the electrodes  13   a  and  13   b  as well. The geometry of this system varies E/p spatially in the lamp but does so in a manner that optimizes the light transmission of the electrode stack. 
       FIGS. 8 and 9  illustrate two additional embodiments that place an electrode for the array outside or within one of the packaging layers. In  FIG. 8 , transparent second electrodes  18   a ,  18   b  are disposed externally on the packaging layers  20 . In the  FIGS. 8 and 9  embodiments, the electrode  16  is suspended within the packaging as in  FIGS. 1A and 1B  to create areas of variable E/p ratio by virtue of the microcavities  12  and gaps  15   b . As one example, indium tin oxide (ITO) pads or strip electrodes can be produced by evaporative or deposition processes. If the microplasmas emit in the UV, phosphors layer  26  is again provided to produce the desired visible color(s). In  FIG. 9 , one part of the packaging layer includes encapsulated second electrodes  18   c , which are preferably metal electrodes encapsulated in metal oxide  17   a . Alternatively, electrodes  18  can be metal films deposited within slots or cavities etched into glass sheet  17   a . Rather than connecting all encapsulated electrodes  18   c  to the same terminal of the voltage source, one can drive each electrode with a different (independent) voltage source, thereby allowing for each microcavity to be addressed separately. Additionally, either of the  FIGS. 8 and 9  designs can be modified by using multiple metal/metal oxide layers to extend the plasma as in  FIGS. 2-6  and to provide different E/p ratio profiles that vary through the lamp. 
     During manufacture of the devices of the invention, it is important to maintain the flatness and alignment of the thin layers  13   a - 13   e  as the device assembly is finalized. In the case of the extruded embodiment of  FIG. 7 , the flatness can be considered to be the outermost surfaces (the surfaces opposite the packaging layers) of the layers  13   a  and  13   b . This can become difficult as the size of the array is extended to 1 ft 2  and beyond.  FIGS. 10A and 10B  illustrate a preferred embodiment vacuum processing system for manufacturing arrays of microcavity plasma devices of the invention. This system is a two-sided metal clamp designed to hold the array assembly as it is being evacuated and backfilled with the desired gases and/or vapors.  FIG. 10B  is a cross-sectional diagram of the clamp, which includes two stainless-steel plates  40 , each with an O-ring groove  42  machined into it.  FIG. 10A  shows one of the metal plates  40  of  FIG. 10B  in plan view. An O-ring  44  seated in each of the O-ring grooves makes a vacuum tight seal with the external surface of array structure  46 . The seals are made near the perimeter of array  46 , at or near the location of either the spacers  22  or where the packaging layers  20  are sealed. A vacuum/gas handling system  48  evacuates the interior and exterior of the lamp through least 3 ports  50   a ,  50   b , and  50   c . Port  50   c  accesses the interior of the lamp  46  and ports  50   a  and  50   b  provide access to the air immediately outside the lamp, so that the system of  FIGS. 10A and 10B  can evacuate the interior of the lamp while simultaneously evacuating the air outside the lamp. Therefore, an significant (or zero) pressure difference is maintained while the lamp is evacuated, outgassed, and (subsequently) back-filled with the desired gas or gas mixture. Once finished, the lamp will, as mentioned earlier, have gas at atmospheric pressure within the lamp and without, thus producing little stress on the packaging material. However, the device of  FIG. 10  is required to ensure that the lamp array is not damaged during processing. Specifically, the central vacuum ports  50   a .  50   b  in the plates  40  provide evacuation of the air outside of the array defined in a chamber that is sealed by the O-rings  44 . This pressure is then equalized to the pressure inside the array as the array is filled with gases or vapors used for plasma generation. This technique ensures that the packaging layers  20  and materials used to obtain a seal of the packaging layer are not stressed while the array is evacuated and then filled with the appropriate gas or gas mixture. 
     While various embodiments of the present invention have been shown and described, it should be understood that other modifications, substitutions and alternatives are apparent to one of ordinary skill in the art. Such modifications, substitutions and alternatives can be made without departing from the spirit and scope of the invention, which should be determined from the appended claims. 
     Various features of the invention are set forth in the following claims.