Abstract:
Methods for making non-volatile switches include depositing gate material in a recess of a substrate; depositing drain metal in a recess of the gate material; planarizing the gate material, drain metal, and substrate; forming sidewalls by depositing material on the substrate around the gate material; forming a flexible conductive element between the sidewalls to establish a gap between the flexible conductive element and the gate material, such that the gap separating the flexible conductive element and the gate material is sized to create a negative threshold voltage at the gate material for opening a circuit; and forming a source terminal in electrical contact with the flexible conductive element.

Description:
RELATED APPLICATION INFORMATION 
       [0001]    This application is a Continuation application of co-pending U.S. patent application Ser. No. 13/693,285 filed on Dec. 4, 2012, incorporated herein by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present invention relates to nano-scale switching and, more particularly, to non-volatile nanomechanical switches that employ graphene ribbon conductors. 
         [0004]    2. Description of the Related Art 
         [0005]    As manufacturing methods improve, increasingly small-scale electromechanical devices can be formed. Present manufacturing methods allow for the creation of nanometer-scale electromechanical systems (NEMS). One material that may be used in such systems is graphene, an atom-thick lattice of carbon. Graphene has good electrical and mechanical properties, making it useful in NEMS switches. However, existing switches need ongoing application of energy to maintain their state. 
       SUMMARY 
       [0006]    A non-volatile switch is shown that includes a gate material formed in a recess of a substrate; a flexible conductive element disposed above the gate material, separated from the gate material by a gap, where the flexible conductive element is supported on at least two points across the gap, wherein a voltage above a gate threshold voltage causes a deformation in the flexible conductive element such that the flexible conductive element comes into contact with a drain in the substrate, thereby closing a circuit between the drain and a source terminal, wherein the gap separating the flexible conductive element and the gate material is sized to create a negative threshold voltage at the gate material for opening the circuit. 
         [0007]    A non-volatile switch is shown that includes a gate material formed in a recess of a substrate; a flexible conductive element disposed above the gate material, separated from the gate material by a gap, where the flexible conductive element is supported on at least two points across the gap, where the flexible conductive element is supported on at least two points across the gap, wherein a voltage above a gate threshold voltage causes a deformation in the flexible conductive element such that the flexible conductive element comes into contact with a drain formed in a recess centered in the substrate, thereby closing a circuit between the drain and a source terminal; and a top gate that applies a restorative force to the flexible conductive element to open the circuit when a top gate threshold is applied, wherein the gap separating the flexible conductive element and the gate material is sized such that a gate voltage of zero will not open the circuit. 
         [0008]    A method for forming a non-volatile switch is shown that includes depositing gate material in a recess of a substrate; depositing drain metal in a recess of the gate material; planarizing the gate material, drain metal, and substrate; forming sidewalls by depositing material on the substrate around the gate material; forming a flexible conductive element between the sidewalls to establish a gap between the flexible conductive element and the gate material, wherein the gap separating the flexible conductive element and the gate material is sized to create a negative threshold voltage at the gate material for opening a circuit; and forming a source terminal in electrical contact with the flexible conductive element. 
         [0009]    These and other features and advantages will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0010]    The disclosure will provide details in the following description of preferred embodiments with reference to the following figures wherein: 
           [0011]      FIG. 1  is a cross-sectional view of a non-volatile graphene switch in an “open” state in accordance with the present principles; 
           [0012]      FIG. 2  is a cross-sectional view of a non-volatile graphene switch in a “closed” state in accordance with the present principles; 
           [0013]      FIG. 3  is a graph illustrating threshold voltages of a graphene switch according to the present principles; 
           [0014]      FIG. 4  is a graph illustrating threshold voltages of a non-volatile graphene switch according to the present principles; 
           [0015]      FIG. 5  is a diagram of a step of forming a non-volatile graphene switch according to the present principles; 
           [0016]      FIG. 6  is a diagram of a step of forming a non-volatile graphene switch according to the present principles; 
           [0017]      FIG. 7  is a diagram of a step of forming a non-volatile graphene switch according to the present principles; 
           [0018]      FIG. 8  is a diagram of a step of forming a non-volatile graphene switch according to the present principles; 
           [0019]      FIG. 9  is a diagram of a step of forming a non-volatile graphene switch according to the present principles; 
           [0020]      FIG. 10  is a diagram of a step of forming a non-volatile graphene switch according to the present principles; 
           [0021]      FIG. 11  is a diagram of a step of forming a non-volatile graphene switch according to the present principles; 
           [0022]      FIG. 12  is a block/flow diagram of fabricating a non-volatile graphene switch according to the present principles; and 
           [0023]      FIG. 13  is a diagram of an alternative embodiment of a non-volatile graphene switch according to the present principles. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0024]    The present principles provide a non-volatile logic switch. Graphene&#39;s elastic properties make it a suitable substance for creating an electromechanical switch. Furthermore, graphene has a high carrier density capability, making it suitable for use as a channel material, and it can be transferred to almost any substrate in, e.g., a top-down process flow. Charging a gate region causes a graphene ribbon that is connected to an electrical source to make contact with a drain region, closing the circuit. Due to the small scale of the device, the elastic restorative force within the graphene can be overcome by Van der Waals forces, creating a hysteresis effect that creates a non-volatile switch that maintains its state even if the charge in the gate region is allowed to dissipate. The overall hysteresis effect is determined by the balance between graphene elastic restorative force and the Van der Waals force between the graphene and the drain. The hysteresis effect and can be adjusted by changing the distance between graphene ribbon and the drain electrode, where a smaller distance causes larger hysteresis due to, e.g., additional contact surface area between the deformed graphene layer and the drain contact causing a stronger Van der Waals force. 
         [0025]    Van der Waals forces, and specifically the London dispersion force, is an intermolecular force generated by spontaneous polarizations within the molecules. In isolation, a spontaneous polarization in a molecule will be transient and disappear quickly. If other molecules are present nearby, however, the spontaneous polarization in a first molecule may induce a corresponding polarization in a second molecule. This generates an attractive electrostatic dipole force between the two molecules, such that the spontaneously generated dipole does not vanish (or does so less quickly). The result is an electrostatic attraction between materials that are otherwise electrically neutral and unpolarized. 
         [0026]    The Van der Waals forces are generally very weak, but increase in strength very quickly as the proximity increases. This is due to the inverse-distance-cubed relationship of dipole interactions. As a result, irregularities in two facing surfaces may increase the average intermolecular distance in the surfaces, reducing the overall attractive force. On the other hand, irregularities may increase the surface area, allowing a flexible material (such as graphene) will conform to the contours and experience an even higher attractive force. For the purpose of simplifying the present embodiments, it is specifically contemplated that the surfaces will be made as smooth as possible, though it should be understood that this is not limiting. Those having ordinary skill in the art would be able to select a surface configuration that best suits their purposes. 
         [0027]    Referring now to the drawings in which like numerals represent the same or similar elements and initially to  FIG. 1 , a graphene-based electromechanical switch  100  is shown. A substrate  102  is formed from, e.g., a silicon dioxide layer or other suitable dielectric. A gate material  104  is formed from, e.g., polysilicon and has a drain contact  110  formed within it with an exemplary width of about 10 μm. The drain contact  110  may be made from any suitable metal or other conductive substance, such as, e.g., a metal or silicide, and may be formed from a single electrode or an array of electrodes. The drain contact  110  may have a width smaller than that of the gate material  104  and an exemplary length of about 3 μm. The drain contact  110  is separated from the gate material  104  by a dielectric layer  112  that may be formed from any appropriate dielectric, such as an oxide, nitride, or high-k material having a thickness of several nanometers to micron-scale. 
         [0028]    A graphene ribbon or sheet  106  is set across the gap in the substrate  102  that is occupied by the gate  104  and drain  110 . The graphene layer  106  may be suspended over the gate  104  and drain  110  at an exemplary height of about 0.5 μm. A source contact  108  is set on the graphene  106 , providing an electrical voltage. In the state shown in  FIG. 1 , the switch is in an open state. There is no voltage applied to the gate  104  and hence no deformation in the graphene layer. The physical properties of graphene are particularly advantageous, because it allows for substantial deformation while maintaining electrical conductivity. Furthermore, being only a single atom thick, the surface of graphene layer  106  is as smooth as is physically possible, such that the strength of the Van der Waals force is maximized. Although the use of graphene for layer  106  is specifically contemplated for these reasons, it is also contemplated that other materials may be used if they have suitable physical and electrical characteristics. 
         [0029]    Referring now to  FIG. 2 , the switch  100  is shown in its closed state. A voltage is applied to the gate  104 , causing an electrostatic attraction between the graphene channel  202  and the gate  104 . The threshold voltage in this exemplary embodiment may be, for example, about 2.5V. As a result, the graphene channel  202  stretches in the direction of the gate  104 . The switch  100  is symmetrically constructed, such that the graphene channel  202  will touch in the center of the gap, making contact with the drain  110 . This completes the circuit, allowing current to flow from the source  108  to the drain  110  through the graphene channel  202 . The drain  110  should be sized such that the deformed graphene channel  202  touches only the drain  110 , without making contact with the gate  104 . 
         [0030]    Referring now to  FIG. 3 , a first graph is shown that illustrates the hysteresis effect of the Van der Waals force. The vertical axis represents the current through the drain  110 , while the horizontal axis represents the voltage applied to gate  104 . At a threshold V t , the graphene  202  touches the drain  110 , causing a large increase in current. The curve produced by an increasing gate voltage is shown by the solid line in the graph. For comparison, the curve produced by a decreasing gate voltage is shown by the dashed line. For the decreasing voltage case, a different, lower threshold is evident. This is a result of the Van der Waals force causing the graphene  202  to remain in contact with the drain  110 , providing an additional force to counteract the restorative elastic force of the graphene  202 . 
         [0031]    Referring now to  FIG. 4 , a second graph is shown that illustrates the hysteresis effect. In this case, the gap between the gate  104  and the graphene  106  is smaller than in  FIG. 3 . The smaller gap moves both voltage thresholds down, with the decreasing threshold now being effectively below “zero,” where the gate voltage is turned off This effect provides non-volatility, such that the state of the switch  100  can be maintained without expending additional energy. 
         [0032]    Referring now to  FIG. 5 , a first step in forming a non-volatile graphene switch  100  is shown. A substrate  502  is formed from any suitable material. In the present embodiments the substrate  502  is specifically contemplated as being silicon dioxide, this is not intended to be limiting. A resist  504  is patterned onto the substrate  502  to delineate an area for the formation of a switch gap. Reactive ion etching (RIE) is applied to the substrate  502  to remove material that is not covered by the resist  504 . This etch should be anisotropic, such that material is removed vertically without undercutting the resist  504 . 
         [0033]    Referring now to  FIG. 6 , a poly-silicon gate material  602  is deposited in the gap of the substrate  502 . The resist  504  is removed and any irregularities in the surface of the substrate  502  and the gate material  602  are removed by chemical-mechanical planarization (CMP). The poly-silicon gate material  602  is then implanted with ions to make it suitable for triggering the switch  100 . This doping may use, e.g., a standard implant doping dose of As, P, B, BF 2 , etc., to make the poly-silicon gate material highly doped, with an exemplary concentration of about 1×10 19  cm −3  to about 1×10 21  cm −3 . 
         [0034]    Referring now to  FIG. 7 , the gate material  602  is etched to create a drain recess  702 . Any appropriate etch may be employed, though it is specifically contemplated that an anisotropic RIE may be used. It should be understood that the drain recess  702  is sized to accommodate a dielectric layer and the drain contact  110 . This size corresponds inversely to the gap between graphene layer  106  and gate  104 , as a smaller gap may increase the area of contact, necessitating a larger surface area for drain contact  110 . 
         [0035]    Referring now to  FIG. 8 , a dielectric is deposited on the surface of the substrate  502  and the gate material  602 . The dielectric  802  serves to create an insulation layer between the gate material  602  and the eventual drain  110 , and so many different dielectric materials will be appropriate for use. It is contemplated that those having ordinary skill in the art will be able to select an appropriate dielectric material according to the needs of the particular embodiment and the available materials and fabrication techniques. 
         [0036]    Referring now to  FIG. 9 , a drain metal  904  is deposited in the gap  702 . The drain metal  904  forms an electrical contact for drain  110  and may be formed from any suitable material such as, e.g., aluminum, copper, or gold. The surface is smoothed using, e.g., CMP, such that the surface of the gate material  602  is exposed and made even with the surface of the drain metal  904 . CMP provides a flat surface to maximize the Van der Waals forces produced by contact between the deformable layer  106  and the drain contact  110 . 
         [0037]    Referring now to  FIG. 10 , additional dielectric material is deposited onto substrate  502  to form extended substrate  1002 . It is specifically contemplated that the new material will be formed from the same material as the substrate  502 : in this case, silicon dioxide. The height of the added material is selected according to the desired threshold voltage and hysteresis effects of the switch  100 . As shown above in  FIGS. 3 and 4 , the depth of the cavity formed by extended substrate  1002  determines the electrical properties of the switch  100 . The depth of the cavity may range from, e.g., several nanometers to the micron scale and has an exemplary value of about 0.5 μm. The threshold voltage may range from, e.g., several tenths of a volt to tens of volts and has an exemplary value of about 2.5V. 
         [0038]    Referring now to  FIG. 11 , the switch  100  is completed by the addition of a graphene ribbon or sheet  1102  and source terminals  1104 . According to one exemplary transfer method, the graphene  1102  may be formed on a separate substrate, e.g., copper. A handling layer of, e.g., poly(methyl methacrylate) (PMMA) is formed on the graphene  1102  and the copper substrate is dissolved. The PMMA layer is then used to manipulate the graphene  1102  into position in contact with the extended substrate  1002 . The PMMA layer is removed by any appropriate process, leaving the graphene  1102  extending across a gap and supported on two sides by the extended substrate  1002 . Source terminals  1104  can then be added with a metal lift-off process and may be formed from any material suitable for electrical contacts. 
         [0039]    Referring now to  FIG. 12 , a method for forming a non-volatile graphene switch  100  is shown. Block  1202  etches a gate recess into a substrate  502 , as shown above in  FIG. 5 . This substrate  502  may be formed from any suitable substance, and it is specifically contemplated that silicon dioxide may be used. The etch may be an anisotropic RIE, though it is contemplated that any suitable etch may be used instead. 
         [0040]    Block  1204  fills the recess with gate material  602 , which is then doped at block  1206  by, e.g., ion implantation. The gate material is specifically contemplated as being poly-silicon, but it is contemplated that any suitable gate material can be used instead. Block  1208  etches a drain recess  702  into the gate material  602  and block  1210  lines the drain recess  702  with a dielectric  902 . Any suitable dielectric material may be used, as the dielectric&#39;s role is to insulate the drain  110 . Block  1212  fills the recess  702  with a gate metal  904 , such as aluminum. 
         [0041]    Block  1214  extends the walls of the gate recess by adding additional material to the substrate  502 . The height of these walls dictate the electromechanical properties of the switch, in particular determining the threshold voltages for changing the switch&#39;s state. This height controls the distance between graphene layer  106  and the drain contact  110 , where larger separations mean that a larger voltage threshold needs to be met to bring the graphene layer  106  into contact. Block  1216  transfers, e.g., a graphene layer  1102  onto the recess walls, e.g., as described above, by forming the graphene layer  1102  separately and then manipulating it into place. Any suitable technique for forming and transferring graphene may be used instead, and it is contemplated that those having ordinary skill in the art will be able to select a graphene transfer technique suitable for their particular application. Further, it should be understood that the use of graphene in the present embodiments is not intended to be limiting, and that any suitably flexible, conductive material may be used instead. Block  1218  forms a source metal  1104  at the graphene layer  1102 . The gate  104 , source  108 , and drain  110  of the completed switch  110  are then connected to electrodes that control the operation of the switch  110  in block  1220 . 
         [0042]    Referring now to  FIG. 13 , an alternative embodiment of a non-volatile graphene switch  1300  is shown. This switch  1300  has an additional top gate  1302  formed from any appropriate material and positioned above the gate  104 . The top gate  1302  may be formed from metal, poly-silicon, or any other convenient conductive material. The top gate  1302  operates to provide a restorative force on the graphene  106  when a voltage is applied. This contrasts to the embodiment of  FIG. 1 , where resetting the switch involves applying a reversed voltage to the gate material  104 , repelling the graphene  106 . Thus, the embodiment of  FIG. 13  using top gate  1302  may be simpler to wire in some circumstances, where reversing the polarity of the voltage applied to gate material  104  might be difficult. 
         [0043]    It is to be understood that the present invention will be described in terms of a given illustrative architecture having a wafer; however, other architectures, structures, substrate materials and process features and steps may be varied within the scope of the present invention. 
         [0044]    It will also be understood that when an element such as a layer, region or substrate is referred to as being “on” or “over” another element, it can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” or “directly over” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. 
         [0045]    A design for an integrated circuit chip of photovoltaic device may be created in a graphical computer programming language, and stored in a computer storage medium (such as a disk, tape, physical hard drive, or virtual hard drive such as in a storage access network). If the designer does not fabricate chips or the photolithographic masks used to fabricate chips, the designer may transmit the resulting design by physical means (e.g., by providing a copy of the storage medium storing the design) or electronically (e.g., through the Internet) to such entities, directly or indirectly. The stored design is then converted into the appropriate format (e.g., GDSII) for the fabrication of photolithographic masks, which typically include multiple copies of the chip design in question that are to be formed on a wafer. The photolithographic masks are utilized to define areas of the wafer (and/or the layers thereon) to be etched or otherwise processed. 
         [0046]    Methods as described herein may be used in the fabrication of integrated circuit chips. The resulting integrated circuit chips can be distributed by the fabricator in raw wafer form (that is, as a single wafer that has multiple unpackaged chips), as a bare die, or in a packaged form. In the latter case the chip is mounted in a single chip package (such as a plastic carrier, with leads that are affixed to a motherboard or other higher level carrier) or in a multichip package (such as a ceramic carrier that has either or both surface interconnections or buried interconnections). In any case the chip is then integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either (a) an intermediate product, such as a motherboard, or (b) an end product. The end product can be any product that includes integrated circuit chips, ranging from toys and other low-end applications to advanced computer products having a display, a keyboard or other input device, and a central processor. 
         [0000]    Having described preferred embodiments of a non-volatile graphene nanomechanical switch and method of fabrication (which are intended to be illustrative and not limiting), it is noted that modifications and variations can be made by persons skilled in the art in light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments disclosed which are within the scope of the invention as outlined by the appended claims. Having thus described aspects of the invention, with the details and particularity required by the patent laws, what is claimed and desired protected by Letters Patent is set forth in the appended claims.