Abstract:
A high-frequency metal-insulator-metal (MIM) type diode is constructed as a bridge suspended above a substrate to significantly reduce parasitic capacitances affecting the operation frequency of the diode thereby permitting improved high-frequency rectification, demodulation, or the like.

Description:
[0001]    This invention was made with United States government support awarded by the following agencies:
       NAVY N66001-07-1-2046   USAF/AFOSR FA955-08-0337       
 
         [0004]    The United States government has certain rights in this invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0005]    The present invention relates to electrical diodes and in particular to high-speed tunnel diodes having low capacitance. 
         [0006]    Diodes are two-terminal electrical devices that block electrical current in one direction and allow it to flow in the opposite direction. This rectification property is often used in electrical circuits to extract a direct current from an alternating current source either for power generation or demodulation. 
         [0007]    Typical solid-state diodes (to be distinguished from vacuum tube diodes, for example) employ a junction of different materials that provides a nonlinear and asymmetric IV-curve (current plotted as a function of voltage) across the diode. Common pn-diodes employ a junction between specially doped semiconductors and provide a forward voltage drop, in the conducting direction, of between 0.7-1.7 V. Schottky diodes provide a lower forward voltage drop of approximately 0.15-0.45 V using a junction between a metal and a doped semiconductor. Metal-insulator-metal (MIM) tunnel diodes, in contrast, provide two metal terminals separated by an insulating layer, the latter is traversed by electrons through quantum tunneling. MIM diodes potentially can operate in the terahertz region because of the higher speed of tunneling in contrast to carrier transport through a depletion region in normal junction diodes. 
         [0008]    In part because of their high operating frequency, MIM diodes are potential candidates for rectennas, that is, antennas that provide for power rectification as the receiving device for wireless transmission of power through microwave transmission. MIM diodes have also been considered for use with so-called “nantennas” which are intended to employ similar principles to that of the rectenna for the conversion of light to electrical energy. 
       SUMMARY OF INVENTION 
       [0009]    The present invention provides a novel MIM-type diode design having extremely low parasitic capacitance to potentially provide high frequency operation necessary for rectenna, nantenna and other applications. 
         [0010]    Generally, the invention employs a three-dimensional fabrication technique in which the diode junction is suspended in a bridge-type structure over the substrate, removing it from the substrate and thus possible capacitive coupling with the substrate elements. By reducing capacitive coupling, improved junction speed can be obtained. 
         [0011]    Specifically, the present invention provides an electrical solid-state diode having a substrate supporting on a surface a first and second electrical conductor having corresponding first and second ends approaching each other across a gap, with an insulating material positioned within the gap to permit electron tunneling between the first and second conductors through the insulating material for at least some voltages less than 5 V applied to the first and second electrical conductors. An undercut is positioned beneath the insulating material and the first and second ends separating the insulating material and the first and second ends from the substrate. 
         [0012]    It is thus a feature of at least one embodiment of the invention to adopt a suspended diode topology to reduce parasitic capacitances. 
         [0013]    The first and second electrical ends may be metals. 
         [0014]    It is thus a feature of at least one embodiment of the invention to provide an improved method of fabricating MIM-type diodes. 
         [0015]    The gap may further include at least one conductive island and the insulating material may be positioned to separate the conductive island from both the first and second ends to permit electron tunneling between the first conductor and the island through a first portion of the insulating material and between the second conductor and the island through a second portion of the insulating material for at least some voltages less than 5 V applied to the first and second electrical conductors. 
         [0016]    It is thus a feature of at least one embodiment of the invention to permit the fabrication of more complex tunneling structures that can introduce asymmetry geometrically. 
         [0017]    The gap may further include at least two conductive islands and the insulating material may be positioned to separate the conductive island from both the first and second ends and from each other to permit electron tunneling between the first conductor and the first island through the first portion of the insulating material and between the first island and the second island through a third portion of the insulating material and between the second island and the second conductor through the second portion of the insulating material for at least some voltages less than 5 V applied to the first and second electrical conductors. 
         [0018]    It is thus a feature of at least one embodiment of the invention to permit the fabrication of more complex tunneling structures including, for example, resonant tunneling diodes having multiple islands. 
         [0019]    The distance between the first end and the island may be different from the distance between the second end and the island. 
         [0020]    It is thus a feature of at least one embodiment of the invention to provide a simple method of controlling diode asymmetry. 
         [0021]    The metals of the first and second electrical ends may have different work functions. 
         [0022]    It is thus a feature of at least one embodiment of the invention to provide an alternative or additional method to increase asymmetry useful for rectification. 
         [0023]    The insulator may be a halogenated carbon polymer. 
         [0024]    It is thus a feature of at least one embodiment of the invention to create the insulator using deposited materials from common etchants used to fabricate the device. 
         [0025]    The substrate may be a silicon wafer. 
         [0026]    It is thus a feature of at least one embodiment of the invention to provide a diode that can be fabricated using conventional integrated circuit processing techniques and materials. 
         [0027]    The first and second ends may taper to become narrower toward the gap. 
         [0028]    It is thus a feature of at least one embodiment of the invention to permit connection of the diode to an arbitrarily sized conductor. 
         [0029]    These particular features and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0030]      FIG. 1  is a perspective view of the diode structure of the present invention as connected across an alternating current electrical signal for rectification or demodulation, the diode having a conductive island suspended on insulators between terminals; 
           [0031]      FIG. 2  is a cross-sectional view along line  2 - 2  of  FIG. 1  showing the first stages of fabrication of the diode of  FIG. 1 ; 
           [0032]      FIG. 3  is a view similar to that of  FIG. 4 , showing the final stages in fabrication of the diode of  FIG. 1 ; 
           [0033]      FIG. 4  is a cross-sectional view along line  4 - 4  of  FIG. 1  showing the position of insulation between the island of the diode and the location of parasitic capacitances; 
           [0034]      FIG. 5  is a room temperature plot of the IV curve of the diode of  FIG. 1  above 300° K, the diagram showing the effective width of the insulation and metal zones caused by asymmetrical positioning of the island; 
           [0035]      FIG. 6  is a figure similar to that of  FIG. 5  showing the IV curve at 77° K; 
           [0036]      FIG. 7  is a top plan view of an alternative embodiment of the invention eliminating the island; 
           [0037]      FIG. 8  is a block diagram showing use of the diode in a rectenna or nantenna; and 
           [0038]      FIG. 9  is a figure of similar to that of  FIG. 7  showing a diode having multiple islands and optional gate structures. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0039]    Referring now to  FIG. 1 , a diode  10  of the present invention may provide for first and second substantially planar conductor ends  12   a  and  12   b  attached to a substrate  18  and opposed across a gap  14  above a cavity  16  in a substrate  18 . 
         [0040]    Positioned between the first end  12   a  and second end  12   b  within the gap  14  is a conductive island  20  suspended between the first end  12   a  and second end  12   b  by insulating material  22  on either side of the island  20 . 
         [0041]    The conductor ends  12   a  and  12   b  and island  20  may, for example, be constructed of identical metals such as gold, or metals with different work functions having a few tens of nanometers thickness. The substrate  18  may, for example, be a silicon-on-insulator (SOI) substrate having an exposed upper silicon surface. The gap  14  measured between the conductor ends  12   a  end  12   b  may be in one embodiment approximately 150 nm. The insulating material may, for example, be a halogenated carbon polymer, for example, formed mainly of CF 2  bonds similar to Teflon. 
         [0042]    The structure of the diode  10  thus formed is that of a metal-insulation-metal-insulation-metal junction (MIMIM) being effectively two series connected metal-insulation-metal (MIM) diodes in which current passes from a first terminal of the diode  10  connecting with end  12   a  and passes through a first insulating material portion  22  to the island  20  and then through a second insulating material portion  22  to a second terminal of the diode  10  formed by end  12   b . Passage of electrons through the insulating material portion  22  is by electron tunneling, a mechanism defined by quantum mechanics requiring a narrow gap  14  on the order of the few tens of nanometers for practical current flow at standard voltages. 
         [0043]    Referring now to  FIG. 2 , the diode  10  of the present invention may be fabricated on an SOI substrate  24  having an upper silicon layer  26  separated by a thin oxide layer  28  from a lower silicon layer  30 . Such substrates are commercially available from a variety of vendors. 
         [0044]    A first layer of photoresist  32  may be spin coated on top of the upper silicon layer  26 , for example a poly methyl-methacrylate (PMMA) with a molecular weight of 150 k and with a thickness of approximately 100 nm. On top of this photoresist  32 , a second over layer of photoresist  34  may be placed using 500 k PMMA and having a thickness of approximately 100 nm. 
         [0045]    Cavities  36  in the photoresist  34  and  32  to the substrate  24  may be formed by electron beam lithography using, for example, an electron beam writer with a 30 kV accelerating potential and a dose of 160 micro-coulombs per square centimeter. 
         [0046]    Metallization may then occur in the exposed cavity  36  by thermal process to define an adhesion layer  38  of chromium followed by a 60 nm layer  40  of gold to provide a bridge structure  41  and conductive traces leading up to the bridge structure  41 . Removal of the photoresist layers  32  and  34  may then be accomplished with acetone. 
         [0047]    Referring now to  FIG. 3 , the upper silicon layer  26  may then be removed beneath the bridge structure  41 , for example, by using a reactive ion etcher and an etchant stream  42  of CF 4  to produce cavity  16  beneath the bridge structure  41 . For this purpose, the etchant stream  42  may flow at 100 sccm at a chamber pressure of 10 mTorr with an RF power of 100 W. The etch rates of silicon in these conditions is about 25 nm per minute. 
         [0048]    Referring now to  FIG. 4 , during the etching process, the cavity  16  beneath the island  20  and the electrode ends  12   a  and  12   b  separates the bridge structure  41  from the upper silicon layer  26  reducing parasitic capacitance  44  between the island  20  and optionally between the ends  12   a  end  12   b  and the upper silicon layer  26  providing the substrate  24 . 
         [0049]    Further, during the etching process, the CF 4  gas forms an insulating halogenated Teflon-like polymer mainly composed of CF 2  bonds on the surface of the ends  12   a  and  12   b , islands  20 , and importantly as the insulating material  22  between the island  20  and each of the ends  12   a  and  12   b . This polymer material provides support of the island  20  in bridge fashion between the ends  12   a  and  12   b  before the cavity  16  is formed, completely undercutting these structures, and further provides a coating that passivates the ends  12   a ,  12   b , and island  20 . 
         [0050]    Referring now to  FIG. 5 , at room temperature, the diode  10  of  FIG. 1  provides an IV curve  50  exhibiting both substantial nonlinearity and asymmetry to provide current conduction in a forward direction and limited current conduction in reverse direction over the range of plus and minus 5 V. The asymmetry, useful for rectification, is believed to be caused by the positional offset of the island  20  between the ends  12   a  and  12   b , having a separation ratio in distance between the end  12   a  and the island  20 , and end  12   b  and island  20  of about 1:3. This offset produces an effective diode geometry  52  where the electron barrier between island  20  and end  12   b  dominates electron transport through the diode producing a greater defect density on the left side of the island  20  in comparison to the right side of the island  20 . 
         [0051]    This asymmetry disappears once the diode  10  is cooled to low temperatures as shown in  FIG. 6  producing an effective symmetric diode geometry  54  and symmetric IV curve  56 . 
         [0052]    Referring now to  FIG. 7 , the present invention raises the possibility of producing a simple MIM diode by placing end  12   a  closely adjacent to end  12   b  as separated by insulating material  22  above cavity  16  to provide tunneling directly therebetween. A portion of the ends  12   a  end  12   b  and the insulating material  22  may be positioned above cavity  16  to reduce parasitic capacitances. 
         [0053]    In all embodiments, the ends  12   a  and  12   b  may taper outward away from the insulating material  22  to join with larger conductors  60   a  and  60   b.    
         [0054]    Referring now to  FIG. 8 , a diode  10  of the present invention may be useful in the construction of a rectenna  70  having an antenna  72  for receiving electromagnetic radiation  75  (e.g. high-frequency microwaves or light). A current signal received by the antenna  72  may be joined by matching network  74 , for impedance matching purposes, to a diode  10  per the present invention. A filter  76  may receive rectified electrical signals from the antenna  72  to provide a smooth DC output, which may be applied to a load  78 . 
         [0055]    Referring now to  FIG. 9 , in an alternative embodiment, the diode  10  may include multiple islands  20   a  and  20   b  in the gap  14  between the ends  12   a  and  12   b  so that electrons may pass from ends  12   a  to island  20   a  through a first insulating material portion  22  and then from island  20   a  to  20   b  through second insulating material portion  22  and then from island  20   b  to end  12   b  through a third insulating material portion  22 , the insulating material portions  22  permitting tunneling at voltages less than approximately 5 V. These multiple islands  20  may be useful, for example, for constructing resonant tunneling diodes that provide for high degree of frequency transparency at particular frequencies or for higher voltage rectification. 
         [0056]    In addition or alternatively, for this embodiment or the embodiment described with respect to  FIG. 1 , one or more gate electrodes  80   a  and  80   b  may be positioned proximate to either or both of islands  20   a  and  20   b  as separated by insulating material  22  to provide for electrical biasing of the islands  20  by capacitive coupling or tunneling or the like. 
         [0057]    It will be understood that multiple diodes  10  of the present invention can be fabricated repeatedly in multiple locations on a given substrate  24  and connected in series or in parallel to provide for improved voltage breakdown and/or current carrying capacity. Further, the diodes  10  may be constructed both to extend horizontally, parallel to the broad surface of the substrate, but also vertically, in wells in the substrate, for greater density. The terms: “undercutting”, “beneath”, “above”, and the like. should therefore be given an interpretation relative to the diode structure. 
         [0058]    It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.