Patent Publication Number: US-2006006787-A1

Title: Electronic device having a plurality of conductive beams

Description:
BACKGROUND  
      Description of the Art  
      The demand for cheaper and higher performance electronic devices has led to a growing need to manufacture electronic devices having lower power consumption as well as improved efficiency. Photonic crystals, micro-vacuum devices, and, in general, in the emerging area of Micro Electro Mechanical Systems (MEMS), which are being developed as smaller alternative systems, to conventional electromechanical devices such as relays, actuators, sensors, valves, and other transducers are all good examples of the ever-increasing demands on power consumption and the need to better handle thermal transfer. The integration of electromechanical devices incorporated in a MEMS device with integrated circuits provides improved performance over conventional systems; however it also requires improved thermal isolation.  
      Although incandescent lamps are inexpensive and the most widely utilized lighting technology in use today, they are also the most inefficient lighting source in regards to the amount of light generated per unit of energy consumed. An incandescent lamp works by heating a filament, typically tungsten, to a very high temperature so that it radiates in the visible portion of the electromagnetic spectrum. Unfortunately, at such high temperatures the filament radiates a considerable amount of energy in the non-visible infrared region of the electromagnetic spectrum. Photonic crystals, typically, are spatially periodic structures having useful electromagnetic wave properties, such as photonic band gaps. Photonic crystals, having the proper lattice spacing, offer the potential of improving the luminous efficacy of an incandescent lamp by modifying the emissivity of the tungsten filament. Such a filament, incorporated into a photonic crystal, would emit most if not all light in the visible portion of the spectrum and little or no light in the non-visible infrared portion. However, such a filament still must be heated to a temperature in excess of 1500° K.  
      Micro-vacuum devices used as amplifying and switching devices are more radiation resistant than semiconductor devices. Such devices also typically utilize a filament heated to a sufficiently high temperature to emit electrons. There is a need for mitigation of the thermal problems such as heat loss and overheating of nearby devices in such micro-fabricated devices. In addition, micro fuel cells, chemical reactors, and sensors all face similar problems.  
      If these problems persist, the continued growth and advancements in the use of electronic devices, especially in the area of photonic crystals and MEMS devices, in various electronic products, will be reduced. In areas like consumer electronics, the demand for cheaper, smaller, more reliable, and higher performance electronics constantly puts pressure on improving and optimizing performance of ever more complex and integrated devices. The ability to optimize thermal performance will open up a wide variety of applications that are currently either impractical, or are not cost effective. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is an isometric of a plurality of conductive beams according to an embodiment of the present invention.  
       FIG. 2   a  is a plan view of a plurality of conductive beams according to an alternate embodiment of the present invention.  
       FIG. 2   b  is a plan view of a portion of a conductive beam according to an alternate embodiment of the present invention.  
       FIG. 2   c  is a plan view of a portion of a conductive beam according to an alternate embodiment of the present invention.  
       FIG. 3  is a cross-sectional view of a conductive beam according to an alternate embodiment of the present invention.  
       FIG. 4  is an isometric view of a photonic band gap crystal according to an embodiment of the present invention.  
       FIG. 5   a  is a plan view of a photonic band gap crystal according to an alternate embodiment of the present invention.  
       FIG. 5   b  is a cross-sectional view along  5   b - 5   b  illustrating the photonic band gap crystal shown in  FIG. 5   a.    
       FIG. 6   a  is a plan view of a vacuum device having multiple thermionic electron emitters according to an embodiment of the present invention.  
       FIG. 6   b  is a cross-sectional view along  6   b - 6   b  showing the relationship between the cathode beams disposed directly under the anodes shown in  FIG. 6   a.    
       FIG. 6   c  is a cross-sectional view along  6   c - 6   c  showing a cathode beam and an anode shown in  FIG. 6   a.    
       FIG. 6   d  is a cross-sectional view along  6   d - 6   d  showing a shield disposed between two cathode beams shown in  FIG. 6   a.   
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      An embodiment of electronic device  100  of the present invention is shown, in an isometric view, in  FIG. 1 . In this embodiment, electronic device  100  includes multiple conductive beams electrically connected to first conductive pad  121  and second conductive pad  122 . Each conductive beam  131  includes central or mesial region  132 , which has a beam resistance. In alternate embodiments, the beam resistance is substantially uniform over the length of the mesial region. In addition, each conductive beam  131  also includes a first end region  134  that extends from mesial region  132  to first connection point  138  of first conductive pad  121 , and a second end region  136  that extends from mesial region  132  to second connection point  139  of second conductive pad  122 . Although in  FIG. 1 , mesial region  132  is illustrated as having a length greater than either first or second end regions  134  and  136 , it should be understood that this is for illustrative purposes only. Mesial region  132  may have a length that is shorter, longer, or equal to the length of the end regions. For example, the mesial region may have a very short length compared to the length of the end regions. In this embodiment, first end region  134  and second end region  136  each has a resistance that increases from first and second connection points  138  and  139  respectively to mesial region  132 . In this embodiment, conductive beam  131  may be formed utilizing any metal, alloy, semiconductor, cermet, conductive ceramic, or combinations thereof. In one embodiment, conductive beam  131  is formed from a refractory material such as tungsten, titanium nitride, tantalum, rhenium, molybdenum, iridium, niobium, and rhodium. In addition, first and second conductive pads  121  and  122  are generally formed from the same material utilized to form conductive beam  131 ; however, in alternate embodiments, the conductive pads may be formed utilizing a material that is different from that used to form conductive beam  131 . In this embodiment, various techniques may be utilized to generate the increase in resistance between the connection points and the mesial region. For example, the resistance may be varied by forming a gradient in the composition of the beam utilizing techniques such as varying the concentration of an alloying element or varying the concentration in a compounding element. Examples of materials that may be utilized include; tungsten alloyed with niobium, molybdenum, thorium, or tantalum; molybdenum alloyed with titanium and zirconium; iron alloys, chromium alloys, aluminum alloys; nickel alloys, tantalum alloys, rhenium alloys, silicon alloys; platinum alloyed with ruthenium, titanium nitride with a varying nitrogen concentration, molybdenum silicide with a varying silicon concentration, silicon carbide with a varying silicon to carbon ratio, zirconium carbide, rhodium, tantalum nitride, tantalum carbide, hafnium carbide, hafnium nitride, and ruthernium. In addition, first and second end regions also may be formed from a material that is different from mesial region  132 . For example mesial region  132  may be formed from tungsten or a tungsten alloy and first and second end regions  134  and  136  may be formed from titanium nitride with a varying nitrogen concentration. In another example a uniform composition may be utilized and the change in resistance may be formed by a change in thickness or a change in width or some combination thereof.  
      In one embodiment, the multiple conductive beams may form a portion of a photonic band gap crystal where the crystal materials and structure may be chosen to selectively emit electromagnetic radiation in a particular portion of the electromagnetic spectrum. The particular band gap location desired will depend on various factors such as the dielectric constant of the materials utilized to form the photonic crystal, the lattice constant of the crystal, the particular crystal structure utilized, as well as the particular fill fraction utilized. The width of the band gap will also depend on the uniformity and quality of the crystal structure as well as the number of layers utilized to form the crystal. In a second embodiment, the multiple conductive beams may form an array of electron emitters in, for example, an array of vacuum micro-diodes or triodes. In still other embodiments, the multiple conductive beams of the present invention may form an array of heaters in a MEMs device, micro-chemical reactor, fuel cell, or sensor.  
      An alternate embodiment of a plurality of conductive beams, of the present invention, is shown, in a plan view, in  FIG. 2   a . In this embodiment, each conductive beam includes first end region  234  and second end region  236  each having a beam width that increases, providing for a decrease in resistance in the direction from the central portion of the beam toward the conductive pad that electrically couples the multiple beams together. In this embodiment, conductive beam  231 , of electronic device  200 , has a uniform thickness (not shown) that is the same thickness as first and second conductive pads  221  and  222 . Conductive beam  231  includes mesial region  232 , which has uniform mesial beam width  240 . The uniform thickness and width provide a uniform resistance over the length of conductive beam  231  in the mesial region. Conductive beam  231  is connected to first conductive pad  221  at connection point  238  via first end region  234 , and is connected to second conductive pad  222  at connection point  239  via second end region  236 . In this embodiment, first end region  234  includes first end region beam width  242  that increases as one moves from mesial region  232  to connection point  238  of first conductive pad  221 . Second end region  236  includes second end region beam width  243  that also increases as one moves from mesial region  232  to connection point  239  of second conductive pad  222 . In this embodiment, as illustrated in  FIG. 2   a  end region beam widths  242  and  243  linearly increase from mesial region  232  to the respective connection point, however, as illustrated in  FIGS. 2   b  and  2   c  other variations also may be utilized. For example, as illustrated in  FIG. 2   b  second end region  236 ′ includes second end region beam width  243 ′ that non-linearly increases as one moves from the mesial region to connection point  239  of second conductive pad  222 . Another example is illustrated in  FIG. 2   c  where first end region beam width  242 ′ varies in a step-wise manner in the direction going from the mesial region to connection point  238  of first conductive pad  221 .  
      An alternate embodiment of a conductive beam, of the present invention, is shown, in a cross-sectional view, in  FIG. 3 . In this embodiment, conductive beam  331  has end regions whose thickness increases. Conductive beam  331  includes mesial region  332  having uniform mesial beam width  341  over the length of the beam in the mesial region. Conductive beam  331  is connected to first conductive pad  321  at connection point  338  via first end region  334 , and is connected to second conductive pad  322  at connection point  339  via second end region  336 . In this embodiment, first end region  334  includes first end region beam thickness  344  that increases in the direction from mesial region  332  to connection point  338  of first conductive pad  321 . Second end region  336  includes second end region beam thickness  345  that also increases in the direction from mesial region  332  to connection point  339  of second conductive pad  322 . In this embodiment, as illustrated in  FIG. 3  end region beam thicknesses  344  and  345  smoothly increase from mesial region  332  to the respective connection point. Such a smoothly varying thickness may be formed, for example, by utilizing a slope metal etch after the beam is formed. In addition, the smoothly varying thickness also may be formed via a shadow mask during formation of the beam. In still other embodiments, a stepwise variation in thickness also may be utilized.  
      An alternate embodiment of electronic device  400  is shown, in an isometric view, in  FIG. 4 . In this embodiment, electronic device  400  includes photonic band gap crystal  402  having a lattice spacing a and a face-centered-tetragonal lattice symmetry.  FIG. 4  illustrates only a portion of complete photonic band gap crystal. In this embodiment, photonic band gap crystal  402  has a lattice spacing of 0.7 micrometers and selectively emits in the visible and near-infrared wavelengths and provides for a more efficient incandescent emitter or light source compared to typical tungsten or refractory metal filaments. The particular region of the electromagnetic spectrum in which the photonic band gap crystal will emit depends on various factors such as the lattice spacing utilized, the dielectric constant of the materials utilized to form the crystal structure, and the crystal structure. Photonic band gap crystal  402  includes alternating layers  448  and  449 . Each layer  449  includes conductive beams  431  evenly spaced apart a distance a and substantially parallel to each other. In addition, each layer  449  is shifted relative to the other layer by 0.5a. Each conductive beam includes central region  432  having a substantially uniform resistance over the length of the mesial region. In alternate embodiments, the spacing between conductive beams  431  may be varied forming a disordered layer. In addition in still other embodiments, the shifting between each layer also may be varied forming a disordered crystal structure. Each conductive beam also includes first end region  434  that extends from central region  432  to conductive pad  421 , and has a resistance that increases in the direction from the conductive pad to the central region. In this embodiment, first end region  434  extends at least one lattice spacing into the photonic band gap crystal structure.  
      Each layer  448  includes crystal or lattice beams  446  that are also evenly spaced apart a distance a, substantially parallel to each other, and mutually orthogonal to conductive beams  431 . Thus, photonic band gap crystal  402  has a stacking sequence that repeats itself every four layers with a repeat distance t. In this embodiment, conductive beams  431  and crystal beams  446  are formed from the same material and have a first dielectric constant; however, in alternate embodiments, lattice beams  446  also may be formed utilizing a material that is different from that used to form conductive beams  431 . Each layer  448  is also shifted relative to the other layer by 0.5a. As noted above for layers  449  the spacing between beams and between layers also may be varied forming disordered layers and a disordered crystal structure. Examples of materials that may be utilized to form lattice beams  446  include tungsten, titanium nitride, tungsten alloys, rhenium, carbon, titania, silicon carbide, and iridium. The volume between the beams, i.e. interstitial volume  418 , is filled by a material having a second dielectric constant. In this embodiment, interstitial volume  418  is a vacuum; however, in alternate embodiments other materials also may be utilized such as air or a ceramic material. The particular material utilized to form the interstitial volume will depend on various factors such as the desired energy band gap edge, the desired operating temperature and pressure as well as the environment in which the device will be utilized. In this embodiment, electronic device  400  also includes a cover and a device base or enclosure (not shown) that encloses photonic band gap crystal  402  forming a hermetic seal whereby the enclosure may be maintained at a pressure below atmosphere.  
      An alternate embodiment of a photonic band gap crystal is shown, in a plan view, in  FIG. 5   a . In this embodiment, photonic band gap crystal  502 , disposed within photonic device  500 , includes layer  548  and layer  549  disposed over cavity or recessed structure  526  as illustrated in a cross-sectional view in  FIG. 5   b . For illustrative purposes only  FIGS. 5   a  and  5   b  depict only two layers of photonic band gap crystal  502 ; however, it should be understood that the photonic crystal includes at least four layers as previously described, and, generally, it will include more than four layers. Layer  549  includes crystal beams  546  that are evenly spaced apart a distance a and are substantially parallel to each other with the majority of beams disposed over cavity  526  formed in substrate  524 . Interstitial volume  518 , in this embodiment, is a vacuum; however, in alternate embodiments other materials having a second dielectric constant different from that of the beams also may be utilized. In this embodiment, crystal beams  546  have a beam width of 425 nanometers and a spacing of 575 nanometers between the crystal beams; however, in alternate embodiments, other beam widths and beam spacings also may be utilized. The particular beam width and beam spacing utilized will depend on various factors such as the dielectric constant of the materials forming the crystal, and the temperature to which the crystal is heated.  
      Layer  548  includes conductive beams  531  that are evenly spaced apart a distance a, are substantially parallel to each other, and mutually orthogonal to crystal beams  546 . Each conductive beam includes central portion  532  having a substantially uniform resistance over the length of the central region. In addition, each conductive beam includes first and second end regions  534  and  536  that extend from central portion  532  to conductive pads  521  and  522  respectively. Each end region has a beam width  542  and  543  that increases in the direction from the central portion of the beam toward connection points  538  and  539  of their respective conductive pad, thereby providing for a decrease in resistance over this length. First and second end regions  534  and  536  extend at least one lattice spacing into the photonic band gap crystal structure. In addition, conductive beams  531  and crystal beams  546  have a uniform thickness, as is illustrated in  FIG. 5   b ; however, in alternate embodiments, first and second end regions  534  and  536  may also have a non-uniform thickness. In this embodiment, substrate  524  is a silicon substrate and cavity  526  is depicted as having sloping sidewalls; however, in alternate embodiments straight vertical sidewalls or other more complex structures also may be utilized. An anisotropic wet etch such as KOH or tetra methyl ammonium hydroxide (TMAH), may be utilized to etch a (100) oriented silicon wafer to produce various structures with sloped side walls generated by the slower etch rate of the (111) crystallographic planes. In alternate embodiments, combinations of wet and dry etch may also be utilized when more complex structures are desired. In still other embodiments, substrate  524  may be formed from a wide variety of materials including metals, ceramics and other semiconductors. In addition, a reflective layer (not shown) may be formed in cavity  526  to reflect light transmitted toward substrate  524 . The reflective layer may coat or line all of cavity  526  or it may be disposed only in portions of cavity  526 .  
      An alternate embodiment of the present invention is shown, in plan view, in  FIG. 6   a . In this embodiment, vacuum device  600  includes an array of thermionic emitters. As illustrated in cross-sectional view in  FIGS. 6   b - 6   d  cavity  626  is formed in substrate  624 . Disposed over cavity  626  are multiple cathode beams  654  as shown in  FIGS. 6   b - 6   d . Each cathode beam includes filament region  632  having a substantially uniform resistance over the length of the filament region. In addition, each cathode beam includes first and second connecting portions  634  and  636  that extend from filament region  632  to conductive pads  621  and  622  respectively. Each connecting portion has a beam width  642  and  643  that increases in the direction from the central portion of the beam toward the respective conductive pad, thereby providing for a decrease in resistance. Vacuum device  600  also includes anode  650  disposed over cathode beams  654 . Disposed between each set of an anode and a cathode is shield structure  652  which may be biased to hinder cross talk between neighboring sets of anodes and cathodes. In addition, in this embodiment vacuum device  600  also includes a cover (not shown) disposed over the plurality of electrically conductive beams. The cover is attached either to substrate  624  or to a device base (not shown) on which substrate  424  is mounted to form a hermetic seal so that the vacuum device may operate at a pressure below atmospheric pressure.  
      In this embodiment, a voltage source (not shown) is connected between cathode beams  654  and anodes  650  where cathode beams  654  are negatively biased relative to anodes  650 . In addition, a current source (not shown) is applied across cathode beams  654  causing them to heat up to a sufficiently high temperature to thermionically emit electrons to anode  650 . When the polarity of the voltage source is reversed electrons current will no longer flow between the cathode and anode. Such a structure is a thermionic emission diode or vacuum diode, which can be utilized, for example, as a rectifier or as a sensor of external electric or magnetic fields. In addition, in alternate embodiments, anode  650  may be coated with a cathodoluminescent material, which will emit a well-defined spectrum of electromagnetic radiation when impacted by electrons emitted from cathode beam  654 . In still other embodiments grid electrodes may be disposed between cathode beam  654  and anode  650  to form triodes, tetrodes etc. In such devices the grid electrode is normally biased negative relative to cathode beam  654  so that as the grid voltage is reduced, the electric field at the cathode is decreased with a corresponding decrease in current flowing to anode  650 . Relatively small changes in the grid voltage cause relatively large changes in the anode current, thus the grid can be utilized as the input in an amplifying circuit.  
      Substrate  624 , in this embodiment, is silicon substrate; however, in alternate embodiments, a wide variety of substrate materials may be utilized including metal or semiconductive substrates having an insulating layer disposed between the cathode beams and the substrate surface. In addition, glass and ceramic substrates also may be utilized. In addition, a cover or vacuum enclosure is also utilized to provide for the low pressure environment in which the cathode beams are heated to emit electrons. Cathode beams  654 , anodes  650  and shields  652  are formed from tungsten; however, in alternate embodiments, other refractory materials such as tantalum, rhenium, platinum, iridium, zirconium, or molybdenum also may be utilized. In still other embodiments, various electron emitter materials such as lanthanumhexaboride, thorium oxide, or barium and strontium oxides may either be coated on or dispersed within cathode beams  650 . In one embodiment, anodes  650  are each coated with a cathodoluminescent material so that electrons emitted from filament regions  632  impact the cathodoluminescent material on the anodes thereby emitting radiation. In this embodiment, the cover may be transparent to the wavelengths emitted or the cover may include transparent portions.