Abstract:
A planar substrate for electrochemical experimentation provides multiple isolated electrical conductors sandwiched between insulating layers of ultrananocrystalline diamond. The isolated electrical conductors may attach to conductive pads at the periphery of the substrate and exposed at apertures in the central region of the substrate for a variety of experimental purposes.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. provisional application 61/088,415 filed Aug. 13, 2008 and hereby incorporated by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to grids for transmission electron microscopes. 
     Transmission electron microscopy (TEM) uses a beam of electrons that is transmitted through a thin specimen to produce an image revealing the interaction of the electrons with the material of the specimen. The specimen may be held on a transmission electron microscope grid typically being a thin, electron transparent disk having a diameter of approximately 3 mm and a thickness on the order of 20-300 μm. 
     SUMMARY OF THE INVENTION 
     The present invention provides a TEM grid that can be used as a substrate for research and development requiring electrical interaction with a specimen under continuous or periodic in-situ microscopic imaging. One embodiment provides an extremely thin and electron transmissive multilayer specimen support. The support includes multiple internal conductors that permit electrical signals to be brought in from the periphery of the grid to a central experimentation area where they are exposed in central apertures. Selective stimulation of the conductors permits electrical “addressing” of the apertures. 
     Specifically, the present invention may provide a transmission electron microscope grid comprising: a substantially planar substrate adapted to fit within a specimen holder of a transmission electron microscope. The substrate may in turn provide a lower insulating layer; a coplanar conductive layer patterned to provide multiple electrically isolated conductors leading from apertures of a central experimentation region to peripherally located contact pads and a non-conductive uppermost layer with the conductive pads passing through the upper most non-conductive layer to contact the electrically isolated conductors. The layered structure has a plurality of centrally located apertures with each aperture being contained within one of the electrically conductive isolated layers. These central apertures pass through all three layers to selectively expose the multiple electrically isolated conductors at the central experimentation region. 
     It is thus a feature of at least one embodiment of the invention to provide a TEM grid presenting TEM imagable and electrically addressable central regions for electrochemical experimentation. 
     The conductive pads may be a metal. 
     It is thus a feature of at least one embodiment of the invention to provide a TEM grid allowing easy electrical connection using relatively large pads displaced from the experimentation region and connectable with conventional techniques such as soldering and welding. 
     The grid may further include a second insulating layer attached to a rear surface of the coplanar conductive layer. 
     It is thus a feature of at least one embodiment of the invention to provide a convenient, freestanding substrate that is physically robust and electrically isolated on its broad surfaces. 
     The first insulating layer and coplanar conductive layer may be fabricated of a common material with different doping. 
     It is thus a feature of at least one embodiment of the invention to provide an extremely thin yet dimensionally stable multilayer device by using the same basis material for each of the layers. 
     The first insulating layer and coplanar conductive layer may be ultrananocrystaline diamond. 
     It is thus an object of the invention to provide a chemically inert, low adhesive material suitable for a wide range of electrical experimentations. 
     The central apertures may pass through both the first insulating layer and coplanar conductive layer to expose one or more multiple electrically isolated conductors at sidewalls of at least one aperture. 
     It is thus a feature of at least one embodiment of the invention to permit electrochemical experiments taking advantage of multiple dimensions presented by the apertures and the surface of the substrate. 
     The thickness of the planar substrate along the path of the electrons maybe substantially less than 1000 nm. 
     It is thus a feature of at least one embodiment of the invention to provide a substrate that provides low interference in oblique imaging. 
     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 DRAWINGS 
         FIG. 1  is a perspective view of a TEM grid for the present invention as held in a TEM stage and connected with standard electrical conductors; 
         FIG. 2  is a top plan view of grid of  FIG. 1  in phantom showing multiple conductive segments presented by the grid together with a detail showing the termination of the segments at central apertures; and 
         FIG. 3  is a perspective, exploded view of the grid of  FIGS. 1 and 2  in cross-section showing use of the grid in a first experiment for electrochemical growth of materials in multiple dimensions. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIG. 1 , a TEM grid  10  of the present invention may be held within a stage  12  of transmission electron microscope  14  in the path of electron beam  16 . In one embodiment, the TEM grid  10  is a thin planar disk having a broad surface normal to an axis of the beam  16 . Electrical conductors such as wires  18  may lead from controlled voltage sources  20  to peripheral conductive pads  22  on the TEM grid  10 . The connections to the conductive pads  22  may be subsequently covered with insulation to allow the TEM grid  10  to be immersed in a liquid or the like. 
     Referring now also to  FIG. 2 , generally each of the peripheral conductive pads  22  (here also labeled  1 - 7 ) may be in electrical communication with one of multiple internal conductors  24  as will be described below. Each of these conductors  24  leads to a central experimentation area  26 . As depicted, the conductors  24  may be approximately segments of a circle having their large arcuate edges attached to the pads  22  and their central vertices in the central experimentation area  26 . Each conductor  24  is separated by an insulating gutter  28  between the conductors  24  and extending along lines of radius from the central experimentation area  26 . A series of apertures  30  may be cut through the TEM grid  10  in the central experimentation area  26  along an axis generally parallel to the electron beam  16 . In one embodiment, the apertures have a diameter of approximately 10 μm. Each of these apertures  30  may pass through a single one of the conductors  24  to expose edges of the conductor  24  as will be described or may pass through multiple of the conductors  24  (not shown). The conductors  24  nevertheless remained isolated from each other within the central experimentation area  26 . 
     Referring now to  FIG. 3 , an upper planar layer  32  of the TEM grid  10 , may be fabricated of ultrananocrystaline diamond. The ultrananocrystaline diamond of layer  32  is un-doped and thus an insulator. In one embodiment, this layer  32  may be 75 nm thick. 
     This layer  32  may be on top of and attached to a layer  34  forming the conductors  24 . This layer  34  may be 50 nm thick and preferably is constructed of N-doped (nitrogen doped) ultrananocrystaline diamond so it is electrically conductive. A third layer  36  may be attached to the rear surface of the layer  34  and consists of ultrananocrystaline diamond preferably of approximately 500 nm thickness and undoped to be insulating. 
     Peripheral apertures  21  are cut only in the layer  32  to allow the introduction of the conductive pads  22  passing through layer  32  to contact the conductors  24 . The apertures  30  may be cut through all three layers  32 ,  34 , and  36  to expose the conductors  24  on the sidewalls  38  of the apertures  30 . 
     The TEM grid  10  may be constructed by first depositing layer  36  on a silicon substrate  40  having a surface layer of tungsten  42 . The layers  34  and  32  may be then successively overlaid on layer  36  using techniques understood in the art while providing the doping necessary for conductors  24 . Reactive ion etching can be used to create peripheral apertures for the introduction of the pads  22  and the central apertures  30 . The layers  32 ,  34 , and  36  may then be removed from the substrate  40  by etching away of the tungsten  42 . This can be done by aggressive treatments such as “piranha rinse” or may be possible through the use of selective copper etchants. Alternatively the silicon substrate  40  may be removed using a KOH etch or the tungsten  42  can be placed over a sacrificial copper layer (not shown) that is etched away. The tungsten  42  can then be removed from layer  36  as a separate step. 
     Referring still to  FIG. 3 , the TEM grid  10  may be used, for example, by placing an electrical voltage on one of the pads  22   a  to be conducted by conductor  24  to aperture  30   a  where it may be used for example to grow and electrochemically induced product  44  such as a nano dimension metal wire toroid deposited from an ionic solution or the like (not shown). In this case, the voltage at the pad  22  is referenced to the electrochemical equilibria established between the metal of the product  44  and a second surface such as in aperture  30   b . Through the use of ionic liquids, this deposition process can be conducted in the vacuum of the TEM  14 . 
     Different materials  46  and  48  may be grown in this fashion to mushroom to the upper surface of the layer  32  and connect together on that surface to permit for the study of such interfaces and junctions both through the use of the TEM  14  and by electrical measurements made through the conductors  24 . For example, a material  48  may be grown in aperture  30   b  by contact with pad  22   b  and a material  46  may be grown in aperture  30   c  by contact with pad  22   c . The time of growth, possibly observed by the TEM  14  can be accurately controlled to control the junction so formed. 
     The growing of nano wires in the aperture  30   a  can be used to study the mechanical properties of those wires (for example, tensile strength) and the adhesion to the material of the TEM grid  10  through the use of small force measuring transducers known in the art that may fit within the TEM  14 . Such transducers are commercially available from Hysitron, Inc. of Minneapolis, Minn. USA. 
     The TEM grid  10  provides a low-cost substrate for making electric devices such as diodes, transistors, LEDs, solar cells, and batteries at the nano scale without the need for expensive equipment. The TEM grid  10  may also be used for biological studies with the conductors used for electrical measurements or stimulation of biological tissue grown on the TEM grid  10 . The optical transparency of the ultrananocrystaline film makes the grid design suitable for optical observation while the small apertures  30  permit immobilization of individual cells. 
     The present invention has been described in terms of the preferred embodiment, and it is recognized that equivalents, alternatives, and modifications, aside from those expressly stated, are possible and within the scope of the appending claims.