Patent Publication Number: US-6703784-B2

Title: Electrode design for stable micro-scale plasma discharges

Description:
FIELD OF THE INVENTION 
     This invention relates generally to micro-scale cavity discharge devices, and, more particularly, to generating a plasma for light emitter or chemical processing applications. 
     BACKGROUND OF THE INVENTION 
     Micro-scale cavity discharge (hereinafter referred to as “MSCD”) devices have received considerable attention over the past years for potential application as ultraviolet (hereinafter referred to as “UV”) light sources and in-situ chemical processing tools. One application for UV light sources is in plasma displays wherein the UV light is used to excite a phosphor. Among the large, flat, full color, high-definition television screens or panels currently available, the gas-plasma type panel has achieved considerable success. While these panels are quite light and thin, they can produce extremely sharp pictures. Another application of micro-scale UV light emitters is in chemical and biochemical sensing by fluorescence or absorption spectroscopy. 
     One application for in-situ chemical processing tools is in analyzing and synthesizing a gaseous atmosphere, such as an engine exhaust or the like where it is desirable to detect and alter a chemical species. For example, a screen which includes an array of MSCD devices could be used to filter harmful chemicals from a gas flow and convert the harmful chemicals into more inert and less toxic species. 
     Low temperature co-fired ceramics (hereinafter referred to as “LTCC”) are particularly useful in the fabrication of MSCD devices because of the possibility to integrate micro-discharge devices with micro-fluidic devices and RF electronic components, which opens the door for numerous attractive applications. 
     Turn now to FIG. 1 which illustrates an example of a MSCD device  5  used in the prior art. In fabricating MSCD device  5 , a ceramic material layer  10  is screen printed with a conductive material layer  12  and a ceramic material layer  14  is screen printed with a conductive material layer  16 . Ceramic material layer  14  is positioned on conductive material layer  12  and a ceramic material layer  18  is positioned on conductive material layer  16  to form a ceramic material region  22 . Region  22  is typically held together by applying a force or pressure to ceramic material region  22  so that layers  10 ,  12 ,  14 ,  16 , and  18  are bonded together. A trench  20  is then punched through ceramic material region  22  wherein trench  20  typically has a cylindrical shape. Region  22  is then fired through a process well known to those skilled in the art. 
     Conductive material layers  12  and  16  function as two electrodes separated by a distance  19  which determines a breakdown voltage of MSCD device  5  wherein layers  12  and  16  are generally screen printed using a metal paste. The breakdown voltage is the voltage at which a plasma starts forming between layers  12  and  16 . The area of exposed conductive regions  12  and  16  is substantially determined by a region  25  and a region  23 , respectively. However, when trench  20  is punched, some of the metal paste used to screen print conductive material layers  12  and  16  is smeared in a region  21  on ceramic material layer  14  adjacent to trench  20 . The smearing of the metal paste effectively changes distance  19  between conductive material layers  12  and  16  so that the breakdown voltage changes. Since the smearing is not a controllable process this leads to a lack of reproducibility of the MSCD device (i.e. the operating conditions are significantly different from one MSCD device to the next) in the discharge. Further, the metal paste used in the prior art to form layers  12  and  16  is susceptible to sputtering from electron and ion bombardment when MSCD device  5  is generating a plasma. The metal paste used in the prior art is also susceptible to oxidation which increases the breakdown voltage. Thus, the device structure and materials used in the prior art leads to unreliable performance (i.e. reproducibility and instability of the discharge and occasional failure of the plasma device to function at all) due to the smearing of the metal paste and to poor lifetime due to the sputtering of the electrodes. 
     Accordingly, it is an object of the present invention to provide a new and improved micro-cavity plasma discharge device with improved performance and longer lifetime. 
     SUMMARY OF THE INVENTION 
     To achieve the objects and advantages specified above and others, a micro-scale cavity device is disclosed. The MSCD device includes a micro-cavity device structure with N dielectric material structures wherein N is a whole number greater than or equal to one. Each N dielectric material structure includes a dielectric spacer region with a first opening wherein the dielectric spacer region is sandwiched between a first dielectric material region with a second opening and a second dielectric material region with a third opening. The second opening and the third opening are aligned with the first opening to form a trench with a width. 
     In the preferred embodiment, at least one of the first dielectric material region and the dielectric spacer region includes a first conductive layer with a surface positioned adjacent to the dielectric spacer region and an opposed surface adjacent to the first dielectric material region wherein the first conductive layer is sandwiched between the first dielectric material region and the dielectric spacer region. 
     In the preferred embodiment, at least one of the second dielectric material region and the dielectric spacer region includes a second conductive layer with a surface adjacent to the dielectric spacer region and an opposed surface adjacent to the second dielectric material region wherein the second conductive layer is sandwiched between the second dielectric material region and the dielectric spacer region. 
     In the preferred embodiment, the first conductive layer extends past one of the first dielectric material region and the dielectric spacer region into the trench to expose at least one of the surface and the opposed surface of the first conductive region. Further, the second conductive layer extends past one of the second dielectric material region and the dielectric spacer region into the trench to expose at least one of the surface and the opposed surface of the second conductive region. The first conductive layer behaves as a first electrode and the second conductive layer behaves as a second electrode. 
     In the preferred embodiment, the first dielectric material region, the second dielectric material region, and the dielectric spacer region include a low temperature co-fired ceramic. Further, the first and second conductive material layers include a platinum (Pt) paste. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and further and more specific objects and advantages of the instant invention will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment thereof taken in conjunction with the following drawings: 
     FIG. 1 is a sectional view of a prior art micro-scale cavity discharge device; 
     FIG. 2 is a sectional view of an embodiment of a simplified micro-scale cavity discharge device in accordance with the present invention; and 
     FIG. 3 is a sectional view of another embodiment of a simplified micro-scale cavity discharge device in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Turn now to FIG. 2, which illustrates a simplified sectional view of a micro-scale cavity discharge device  30  in accordance with the present invention. Device  30  includes N dielectric material structures  44  wherein N is a whole number greater than or equal to one. While MSCD device  30  may be formed in a variety of different embodiments, including the formation of a cavity in a single thick layer, in a preferred embodiment, the cavity is formed through the cooperation of a plurality of thin layers fixed into a unit (e.g. firing layers and lamination of ceramic tape, bonding layers of polymer, etc.). Electrical connections and circuitry can then easily be formed on various layers to make a complete device once the layers are assembled and fixed. 
     A stack of dielectric layers is used to form the device wherein the dielectric material can be any convenient material which is capable of withstanding a plasma discharge within a micro-scale cavity discharge to generate a plasma based electromagnetic emission (as will be explained in more detail presently). Typical materials that can be used include ceramic, various polymeric material (e.g. PDMS or poly dimethyl sulfoxane), PMMA plus hybrid system, some materials used in the semiconductor art, etc. In the following description, for convenience, the layers are formed of green or unfired ceramic which, as explained below, is assembled and fired to form a single unit. It will be understood, however, that many of the steps of formation and usage described herein can be incorporated with other materials (e.g. various polymers and some materials used in the semiconductor art) in a similar fashion. 
     Each N dielectric material structure  44  includes a dielectric spacer region  38  with a thickness  66  sandwiched therebetween a dielectric material region  34  and a dielectric material region  42  wherein spacer region  38  includes a LTCC. In the preferred embodiment, N is equal to one for simplicity and to illustrate the basic structure and operation of device  30 . However, it will be understood that N can be greater than one wherein device  30  will include a plurality of electrodes for each polarity of voltage, as will be discussed separately. 
     In the preferred embodiment, dielectric material region  34  includes an opening  54  and has a conductive material layer  36  positioned thereon region  34  wherein conductive material layer  36  is positioned adjacent to dielectric spacer region  38 . Generally, dielectric material region  34  includes a LTCC wherein conductive material layer  36  is typically screen printed on region  34  in a desired pattern, as will be discussed separately. 
     In the preferred embodiment, dielectric material region  42  includes an opening  58  and has a conductive material layer  40  positioned thereon region  42  wherein conductive material layer  40  is positioned adjacent to dielectric spacer region  38 . Generally, dielectric material region  42  includes a LTCC wherein conductive material layer  40  is typically screen printed on region  42  in a desired pattern, as will be discussed separately. It will be understood that regions  34 ,  38 , and  42  are illustrated as a single layer for simplicity, but each region can include multiple layers. 
     In the preferred embodiment, openings  54 ,  56 , and  58  are generally aligned to form a trench  46  with a width wherein device  30  forms a screen. Thus, a cavity (trench  46 ) has been formed without punching through region  44  so that the metal paste used to form layers  36  and  40  does not smear on a surface  39 . The width of trench  46  is typically less than approximately 500 μm. Further, openings  54 ,  56 , and  58  are generally circular in shape. However, it will be understood that openings  54 ,  56 , and  58  can have other shapes, such as square, elliptical, or the like. 
     Illustrated in MSCD device  30  are layers of green or unfired ceramic materials with portions thereof broken away. As understood in the art, unfired or green sheets or layers (e.g. layers  34 ,  38 , and  42 ) are formed of unfired or green ceramic material which usually includes aluminum oxide (AlO) particles, glass particles, and a binder, generally including organic material. Conductive layers  36  and  40  define electrodes and surround the openings. It will be understood that the electrodes can be electrically connected to additional electrical components, such as transistors, capacitors, inductors, resistors, or the like. The conductive layers can be formed by screen printing (or the like) a metal paste. 
     Trench  46  and layers  34 ,  36 ,  38 ,  40 , and  42 ) form MSCD device  30 . MSCD device  30  is capable of containing an environment for carrying a plasma discharge within trench  46  to generate a plasma based electromagnetic emission. Here it will be understood that the term “electromagnetic emission” includes UV to infrared emission, various particles (e.g. electrons, protons, ions, etc.), and any other emissions capable of being formed by the plasma discharge. 
     In the example, MSCD device  30  is formed as a ceramic module, but it will be understood that an array of MSCD devices could be included in a single ceramic module. 
     In the example of an array of MSCD devices, electrical traces can be included to connect the external connections for unique addressing (e.g. by row and by column). 
     While a single MSCD device is disclosed by FIG. 2, it will be understood by those skilled in the art that, for convenience in manufacturing, components of a plurality of modules are generally defined on each sheet. Also, laminated ceramic devices are generally formed using a plurality of the sheets (sometimes are many as fifty), which are stacked or positioned in overlying relationship. As is understood by those skilled in the art, the sheets are very thin (on the order of several tens to a few hundreds of microns) and, generally, the total number of sheets used depends upon the circuit or circuits being integrated. During the stacking process, the sheets are vertically aligned to form common modules sides and features (e.g. trench  46 ) through the entire stack (i.e. each module layer in a sheet overlies mating module layers in lower sheets). 
     After stacking and alignment of the sheets is accomplished, the stack is pressed under a uniaxial pressure (e.g. 0 psi to 5000 psi) at an elevated temperature (e.g. 500° C. to 1500° C.) to produce bonding between adjacent sheets. As understood by those skilled in the art, the pressure and temperature must be sufficient to produce some bonding between the binders of adjacent sheets. 
     Once the stack of unfired or green ceramic sheets has been bonded together, the stack can be cut or otherwise divided into individual modules. The cutting is easily accomplished since the sheets are still formed of unfired or green ceramic. As is understood in the art, the firing temperature is generally dictated by the composition of the green ceramic material. Generally, the green ceramic material includes aluminum oxide (AlO) particles, glass particles, and an organic binder. In most cases, the glass particles melt sufficiently to bind the aluminum particles together at a temperature of approximately 875° C. During the firing process, most of the organic binder is driven off to leave a ceramic comprising aluminum oxide (AlO) particles bound together by at the least partially melted and reformed glass. Also, the various sheets are bound into a virtually single structure by the firing process. In the firing process the individual modules contract or shrink approximately 13%, but the shrinkage is substantially uniform so that it does not affect the final module and the final size of features (e.g. trench  46 ) can easily be calculated. 
     MSCD device  30  is capable of containing an environment for carrying a plasma discharge within MSCD device  30  to generate a plasma based electromagnetic emission when a cathode discharge potential is applied to conductive layers  36  and  40 . In this embodiment, the cavity (trench  46 ) is open at both ends so that a variety of environments, cathode discharge potentials, and pressures can be applied through an encompassing assembly (e.g. a larger housing, interconnecting conduits, etc.) to “tune” the cavity to various electromagnetic emissions. For example, the cavity can be tuned to change the electromagnetic emission to any desired emission in a range from infrared to ultraviolet. 
     In the preferred embodiment, the width of trench  46  adjacent to conductive material layer  36  is greater than the width of trench  46  adjacent to dielectric material layer  34 . Further, the width of trench  46  adjacent to dielectric spacer region  38  is greater than the width of trench  46  adjacent to conductive material layer  36  so that a surface  52  with an area and a surface  62  with an area of conductive material layer  36  are exposed. In the preferred embodiment, the width of trench  46  adjacent to dielectric material layer  42  is less than the width of trench  46  adjacent to conductive material layer  40 . Further, the width of trench  46  adjacent to conductive material layer  40  is less than the width of trench  46  adjacent to dielectric material layer  38  so that a surface  60  with an area and a surface  64  with an area of conductive material layer  40  are exposed. The plasma discharge is substantially generated within the region between surfaces  52  and  60  and within the region between surfaces  62  and  64 . By forming trench  46  in this way, the alignment requirements of each successive layer ( 34 ,  36 ,  38 , etc.) are less restrictive and are easier to accomplish, as will be discussed presently. 
     In the preferred embodiment, any misalignment in layers  34 ,  36 ,  38 ,  40 , and  42  does not lead to variations in the total area of surfaces  52  and  62  nor in the total area of surfaces  60  and  64  as long as the width of trench  46  adjacent to conductive material layers  36  and  40  do not substantially overlap with the width of trench  46  adjacent to dielectric material regions  34  and  42 , respectively, and as long as the width of trench  46  adjacent to conductive material layers  40  and  36  do not substantially overlap with the width of trench  46  adjacent to dielectric material region  38 . 
     Thus, MSCD device  30  is more likely to operate with more uniformity from one device to another in an array of MSCD devices since it is easier to control the area of the electrode rather than its thickness. The area of the electrode in the prior art is determined by regions  23  and  25  adjacent to trench  20  (See FIG. 1) which is difficult to control and varies substantially from one device to another. 
     As discussed previously, dielectric material layers  34 ,  38 , and  42  include a LTCC. However, it will be understood that layers  34 ,  38 , and  42  could include other suitable dielectric materials. Further, in the preferred embodiment, conductive material layers  36  and  40  include a platinum (Pt) paste. However, it will be understood that layers  36  and  40  can include other conductive materials, such as a silver (Ag) paste, or any other suitable conductive paste which can be screen printed or otherwise applied. The platinum (Pt) paste typically has more stable operation with a lower breakdown voltage and an improved lifetime than for silver (Ag) pastes wherein silver (Ag) paste oxidizes and is sputtered more readily. A metal paste that does not sputter will have a longer lifetime and a metal paste that does not oxidize will have a smaller breakdown voltage. Further, layers  36  and  40  can also include other metals, such as platinum (Pt), gold (Au), silver (Ag), molybdenum (Mo), tungsten (W) or the like, which can be deposited with conventional semiconductor deposition techniques well known to those skilled in the art. 
     Conductive material layer  36  behaves as an electrode with a polarity and conductive material layer  40  behaves as an electrode with an opposite polarity to that of layer  36  wherein layers  36  and  40  are capable of forming an electric field therebetween. It will be understood that in embodiments where N is greater than one that each conductive material layer is capable of behaving as an electrode wherein each conductive material layer has a polarity which is the opposite polarity to each adjacent conductive material layer. 
     As discussed previously, surfaces  60  and  64  of conductive material layer  40  and surfaces  52  and  62  of conductive material layer  36  are left exposed so that a plasma discharge of device  30  is more stable and reproducible. Also, the close proximity (thickness  66 ) of conductive material layer  36  to conductive material layer  40  determines the breakdown voltage needed to form the plasma discharge within trench  46 . 
     Turn now to FIG. 3, which illustrates a simplified sectional view of another embodiment of a micro-scale cavity discharge device  70  in accordance with the present invention. Device  70  includes N dielectric material structures  44  wherein N is a whole number greater than or equal to one. In this embodiment, N is equal to one for simplicity and to illustrate the basic structure of device  70 . 
     In this embodiment, the width of trench  46  adjacent to conductive material layer  36  is smaller than the width of trench  46  adjacent to dielectric material layer  34 . Further, the width of trench  46  adjacent to dielectric spacer region  38  is less than the width of trench  46  adjacent to conductive material layer  36  so that a surface  72  and a surface  74  of conductive material layer  36  are exposed. In this embodiment, the width of trench  46  adjacent to dielectric material region  42  is greater then the width of trench  46  adjacent to conductive material layer  40 . Further, the width of trench  46  adjacent to conductive material layer  40  is greater than the width of trench  46  adjacent to dielectric spacer region  38  so that a surface  78  and a surface  76  of conductive material layer  40  are exposed. 
     The plasma discharge is substantially generated within the region between surfaces  72  and  78  and within the region between surfaces  74  and  76 . By forming trench  46  in this way, the alignment requirements of each successive layer ( 34 ,  36 ,  38 , etc.) are less restrictive and are easier to accomplish and a plasma discharge of device  70  is more stable and reproducible. 
     Thus, a new and improved plasma discharge device has been disclosed which includes an electrode design that reduces the breakdown voltage needed to achieve electromagnetic emission. Further, the electrode design includes a platinum (Pt) paste which is substantially non-oxidizing and resistant to sputtering. An electrode that is resistant to sputtering improves the lifetime and stability of the plasma discharge device and an electrode that is non-oxidizing decreases the breakdown voltage needed to generate a plasma. 
     Applications of MSCD devices  30  and  70  include, but are not limited to, miniature UV light sources at wavelengths inaccessible to LED&#39;s where MSCD devices  30  and  70  are used to form a screen for a light emitting device wherein region  44  is sandwiched between a front glass plate and a rear glass plate and trench  46  is filled with an ionizing gas, such as argon (Ar), neon (Ne), or the like. MSCD devices  30  and  70  can be used in in-situ chemical processing applications such as deposition of catalyst, methanol reforming and surface modification for micro-fluidic applications. MSCD devices  30  and  70  can be integrated with microfluidic devices, detectors, and emitters to form compact microanalysis systems. The integration of these devices into a compact module greatly improves the operation of such systems. 
     Various changes and modifications to the embodiments herein chosen for purposes of illustration will readily occur to those skilled in the art. To the extent that such modifications and variations do not depart from the spirit of the invention, they are intended to be included within the scope thereof which is assessed only by a fair interpretation of the following claims.