Abstract:
A nano-electrode or nano-wire may be etched centrally to form a gap between nano-electrode portions. The portions may ultimately constitute a single electron transistor. The source and drain formed from the electrode portions are self-aligned with one another. Using spacer technology, the gap between the electrodes may be made very small.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. patent application Ser. No. 10/819,790, filed Apr. 7, 2004. 
     
    
     BACKGROUND 
       [0002]    This invention relates generally to nanotechnology and to the fabrication of very small electronic devices. 
         [0003]    In nanotechnology, very small electronic devices may be fabricated from physical parts. For example, a field effect transistor may be made of sources and drains fabricated from nano-wires such as carbon nanotubes. 
         [0004]    Carbon nanotubes are graphene cylinders whose ends are closed by caps, including pentagonal rings. The nanotube is an hexagonal network of carbon atoms forming a seamless cylinder. These cylinders can be as little as a nanometer in diameter with lengths of tens of microns, in some cases. Depending on how they are made, the tubes can be multiple walled or single walled. 
         [0005]    The nano-wires may be utilized to form the source and drain of a transistor. However, the source and drain must be aligned with one another and a channel must be defined between the carbon nanotubes. The channel or gap between the two nano-wires is very small. 
         [0006]    Thus, there is a need for better ways to make nano-devices using nano-wires having relatively small nano-gaps between the nano-wires. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0007]      FIG. 1  is an enlarged, cross-sectional view at an early stage of manufacture in accordance with one. embodiment of the present invention; 
           [0008]      FIG. 2  is an enlarged, cross-sectional view at a subsequent stage of manufacture in accordance with one embodiment of the present invention; 
           [0009]      FIG. 3  is an enlarged, cross-sectional view at a subsequent stage of manufacture in accordance with one embodiment of the present invention; 
           [0010]      FIG. 4  is an enlarged, cross-sectional view at a subsequent stage of manufacture in accordance with one embodiment of the present invention; 
           [0011]      FIG. 5  is an enlarged, cross-sectional view at a subsequent stage of manufacture in accordance with one embodiment of the present invention; 
           [0012]      FIG. 6  is an enlarged, cross-sectional view taken generally along the line  6 - 6  in  FIG. 7  in one embodiment of the present invention; 
           [0013]      FIG. 7  is a vertical, cross-sectional view taken generally along the line  7 - 7  in  FIG. 6  in accordance with one embodiment of the present invention; 
           [0014]      FIG. 8  is a cross-sectional view taken generally along the line  1 - 1  in  FIG. 9 ; and 
           [0015]      FIG. 9  is an enlarged, cross-sectional view taken vertically through  FIG. 8 . 
       
    
    
     DETAILED DESCRIPTION  
       [0016]    As shown in  FIG. 1 , nano-devices may be fabricated over a silicon substrate  22  with nano-electrodes  20  formed thereover. A nano-electrode is a conductive element having a diameter of less than 100 nanometers. The nano-electrodes  20  may be covered by an etch stop layer  44  and a dielectric layer  42 . The dielectric layer  42  may be patterned and etched to form the aperture  50  which extends through the dielectric layer  42  and stops at the etch stop layer  44 , as shown in  FIG. 2 . In one embodiment, the aperture  50  may be patterned using photoresist. 
         [0017]    Thereafter, as shown in  FIG. 3 , sidewall spacers  40  may be applied, in one embodiment, to the aperture  50 . The side wall spacers may be formed using conventional technology. The spacers  40  may be formed of any sufficiently etch resistant material. 
         [0018]    Referring to  FIG. 4 , using the spacers  40  as an etch mask, the gap  52  may be formed through the etch stop layer  44  and the nano-electrodes  20 . As a result, two nano-electrode portions are formed on either side of the gap  52 , forming self-aligned nano-electrodes  20 . Thereafter, the gap  52  may be cleaned to remove etch residues. 
         [0019]    As shown in  FIG. 5 , the gap  52  may be further narrowed by selective metal deposition, as indicated as  54 , over the nano-electrodes  20 . In one embodiment, electroless plating of gold, silver, platinum, lead with ruthenium, osmium, iridium, copper, cobalt, nickel, or iron alloys may be utilized. Narrowing of the gaps  52  may be self-terminated if absorbed layers of organics are used on the surface of metals during plating. The deposition  54  may be polyethylene glycol-type with chlorine, as well as disulfides on copper or thiol-based organics on gold in other embodiments. 
         [0020]    In one embodiment, the nano-electrodes  20  can be formed by a subtractive process, such as lithography, etching, and cleaning operations, or vapor deposition on patterned catalytic particles, such as gold. The nano-electrodes  20  may also be formed of metal nano-wires, such as copper, nickel, cobalt, gold, lead, aluminum, titanium, tungsten, tantalum, or ruthenium alloys in a damascene process. Metal nano-wires may also be formed by a subtractive process. 
         [0021]    The nano-electrodes  20  may also be nanotubes, such as carbon nanotubes, including single wall and multiple wall nanotubes, may be formed on the substrate surface by dispensing a solution containing the carbon nanotubes, followed by alignment or by chemical vapor deposition such as carbon monoxide decomposition on patterned catalytic particles, such as cobalt, nickel, iron, molybdenum, and alloys. 
         [0022]    Referring to  FIG. 6 , in accordance with another embodiment of the present invention, the nano-electrodes  20  may actually be separated into three parts  20   a ,  20   b , and  20   c , by any of the processes described above, forming a pair of nano-gaps  52  on either side of the portion  20   b . In one embodiment, a single mask with two exposures may be utilized. In another embodiment, a nano-gap  52  with a reentrant profile may be formed between two electrodes followed by a nano-dot deposition and lift off or selective etching. If an electron is trapped in a quantum nano-wire or nano-tube, current will not flow due to Coulomb blockage. 
         [0023]    The electron flow can be modulated by a gate  35 . While the gate  35  is shown below the nano-gaps  52 , other orientations may also be used. 
         [0024]    As a result of Coulomb blockage, electrons tunnel one by one through the nano-gaps  52 . The conductance versus gate voltage dependence is in the form of a series of sharp peaks. In effect, a single electron transistor is formed which, among other things, may be an extremely sensitive electrometer. 
         [0025]    The structure shown in  FIG. 6  may be covered with an optical layer  36  as shown in  FIG. 7 . Below the optical layer it may be a layer  34  in which a pair of contacts  12  are formed. The contacts  12  electrically connect to the nano-electrodes  20 , including the portions  20   a ,  20   b , and  20   c . The gate electrode  35  may be formed below the nano-gaps  52 . The gate electrode  35  may be coupled by a via  54  down to a conductive layer  28 . The contacts  12  may be coupled by vias  56  or  58  to a conductive layer  30  or a conductive layer  50  as the case may be. The conductive layers  50 ,  28 , and  30  may be electrically isolated from one another by insulators  28  and  52 . Thus, for example, the conductive via  56  extends through an insulator  31  and the conductive layer  50  down to a conductive layer  50 . The conductive layer  28  is isolated by the insulating layer  52  from the conductive layer  50 . The conductive layer  50  connects to the via  56  which connects, in turn, to one of the contacts  12  coupled to the nano-electrode  20   a . The contact  12  coupled to the nano-electrode  20   c  is coupled to the conductive layer  30  which is insulated from the via  56  by way of the insulator  31 . 
         [0026]    Thus, separate signals can be placed on each of the contacts  12  and on the gate  35  in order to control the flow of single electrons within the nano-gaps  52 . In this embodiment, the nano-gaps  52  may be basically filled in by the material forming the insulative layer  34 . The electrodes  20  may be self-aligned with one another and may be very tightly spaced by the nano-gaps  52 . Thus, in one embodiment, quantum nanotubes on the order of 0.8 millimeters in diameter with Coulomb blockage may be fabricated from the same nanotubes in a fashion described above. Single electron transistors may be used in memory sensor arrays and may be programmable with selective writing, selective access, and selective read-out. 
         [0027]    Referring to  FIG. 8 , a manifold  16  may be coupled to a pair of nano-gaps  52 . The manifold  16  may store an analyate which fills the nano-gaps  52 . The nano-gaps  52  separate a pair of spaced nano-electrodes  20 , which in one embodiment may be formed of carbon nanotubes. A series of contacts  12  are coupled to each electrode  20 . 
         [0028]    The nano-gap  52  forms a channel of a conductivity detector which can be used to flow chemicals and biological species, such as deoxyribonucleic acid (DNA), protein, and detect them through nano-electrodes  20 . 
         [0029]    Referring to  FIG. 9 , the contacts  12  are formed in a dielectric layer  34 . Each contact  12  may be coupled to a via  37  or  32 , which may be coupled to a conductive line  28  or  30 . In one embodiment, the lines  30  extend into the page and the lines  28  extend across the page to form a transverse array of rows and columns. The lines  30  and  28  may be separated by a nano-gap  52  and the individual lines  30  may be separated by a dielectric layer  31 . A dielectric layer  24  may be provided over a memory array  22  in one embodiment of the present invention. 
         [0030]    In one embodiment, an array of memory cells with two nano-wire electrodes  20  and a nano-gap  52  between them may be utilized. The manifold  16  connects the nano-gaps  52  and may also serve as a nano-fluidic and electrophoresis channel. 
         [0031]    As one example, the structure shown in  FIGS. 8 and 9  may be used as a memory sensor array for a bio chip. Deoxyribonucleic acid (DNA) may be extracted from the cell and purified. The DNA may be fragmented down to 20 to 30 bases, with a length of 0.3 nanometers per base, and a maximum 100 bases. An automated PCR may be used with a desktop computer or manual sample preparation. 
         [0032]    The manifold  16  and nano-gaps  52  may be filled with phosphate buffer solution by capillary force. A sample of target DNA fragments may be added to the manifold  16 . The target DNA may be stretched by shear strength and separated by size in the manifold  16  by using electrophoresis. For example, a net of 1 negative electron charge per base from the phosphate backgrounds and less than 1 micrometer per second velocity may be achieved. 
         [0033]    A target DNA may be attracted inside the nano-fluidic channel by applying potentials to channel electrodes  20 . Target DNA may be selectively immobilized on nano-electrodes  20  using a label and DNA probe by hybridization. The DNA probes may have labels and can be selectively attached by a nano-electrode  20  using selective access/charge, while protecting other electrodes by charge. As another example, steptavidin can form a SAS layer and biotin with a DNA probe attached selectively with an electrical signal to electrodes. Electrode materials may be metals, such as tantalum, gold, copper, aluminum, ruthenium, or titanium nitride that are coated with other metals and dielectrics. The dielectrics may be silicon dioxide or SiCN to avoid electrolysis. 
         [0034]    The reaction between the exposed nano-electrodes and the analyate in the nano-gaps  20  may be detected by a single electron switch acting as an electrometer. In other words, the chemical activity at the electrodes  20  may be secured. This arrangement may then function as a bio sensor that may be more accurate because of the precise control over the nano-gaps  52 . 
         [0035]    While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.