Abstract:
Nanotube on gate FET structures and applications of such, including n 2  crossbars requiring only 2n control lines. A non-volatile transistor device includes a source region and a drain region of a first semiconductor type of material and a channel region of a second semiconductor type of material disposed between the source and drain region. A gate structure is made of at least one of semiconductive or conductive material and is disposed over an insulator over the channel region. A control gate is made of at least one of semiconductive or conductive material. An electromechanically-deflectable nanotube switching element is in fixed contact with one of the gate structure and the control gate structure and is not in fixed contact with the other of the gate structure and the control gate structure. The device has a network of inherent capacitances, including an inherent capacitance of an undeflected nanotube switching element in relation to the gate structure. The network is such that the nanotube switching element is deflectable into contact with the other of the gate structure and the control gate structure in response to signals being applied to the control gate and one of the source region and drain region. Certain embodiments of the device have an area of about 4 F 2 . Other embodiments include a release line is positioned in spaced relation to the nanotube switching element, and having a horizontal orientation that is parallel to the orientation of the source and drain diffusions. Other embodiments provide an n 2  crossbar array having n 2  non-volatile transistor devices, but require only 2n control lines.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority under 35 U.S.C. § 119(e) to U.S. Provisional patent application Ser. No. 60/459,224, filed on Mar. 28, 2003, entitled Nanotube-On-Gate FET Structures and Applications, which is incorporated herein by reference in its entirety.  
         [0002]     This application is related to the following U.S. applications, the contents of which are incorporated herein in their entirety by reference: 
        U.S. Provisional application Ser. No. 60/459,223 filed on Mar. 28, 2003, entitled NRAM Bit Selectable Two-Device Nanotube Array; and U.S. patent application Ser. No. (inserted upon USPTO designation), filed on the same day as the present application, entitled NRAM Bit Selectable Two-Device Nanotube Array.     U.S. Provisional Patent Application No. 60/459,253, filed on Mar. 28, 2003, entitled Single Transistor with Integrated Nanotube (NT-FET), and U.S. patent application Ser. No. (inserted upon USPTO designation), filed on the same day as the present application, entitled A Four Terminal Non-Volatile Transistor Device.        
 
         [0005]     U.S. Provisional Patent Application No. 60/459,222, filed on Mar. 28, 2003, entitled Non-Volatile RAM Cell and Array using Nanotube Switch Position for Information State, and U.S. patent application Ser. No. (inserted upon USPTO designation), filed on the same day as the present application, entitled Non-Volatile Ram Cell and Array Using Nanotube Switch Position for Information State. 
     
    
     BACKGROUND  
       [0006]     1. Technical Field  
         [0007]     The invention relates to non-volatile transistors structures and more specifically to non-volatile transistors using nanotube (NT) components.  
         [0008]     2. Discussion of Related Art  
         [0009]     Important characteristics for a memory cell in electronic device are low cost, nonvolatility, high density, low power, and high speed. Conventional memory solutions include Read Only Memory (ROM), Programmable Read only Memory (PROM), Electrically Programmable Memory (EPROM), Electrically Erasable Programmable Read Only Memory (EEPROM), Dynamic Random Access Memory (DRAM) and Static Random Access Memory (SRAM).  
         [0010]     ROM is relatively low cost but cannot be rewritten. PROM can be electrically programmed but with only a single write cycle. EPROM has read cycles that are fast relative to ROM and PROM read cycles, but has relatively long erase times and reliability only over a few iterative read/write cycles. EEPROM (or “Flash”) is inexpensive, and has low power consumption but has long write cycles (ms) and low relative speed in comparison to DRAM or SRAM. Flash also has a finite number of read/write cycles leading to low long-term reliability. ROM, PROM, EPROM and EEPROM are all non-volatile, meaning that if power to the memory is interrupted the memory will retain the information stored in the memory cells.  
         [0011]     DRAM stores charge on transistor gates that act as capacitors but must be electrically refreshed every few milliseconds complicating system design by requiring separate circuitry to “refresh” the memory contents before the capacitors discharge. SRAM does not need to be refreshed and is fast relative to DRAM, but has lower density and is more expensive relative to DRAM. Both SRAM and DRAM are volatile, meaning that if power to the memory is interrupted the memory will lose the information stored in the memory cells.  
         [0012]     Consequently, existing technologies are either non-volatile but are not randomly accessible and have low density, high cost, and limited ability to allow multiples writes with high reliability of the circuit&#39;s function, or they are volatile and complicate system design or have low density. Some emerging technologies have attempted to address these shortcomings.  
         [0013]     For example, magnetic RAM (MRAM) or ferromagnetic RAM (FRAM) utilizes the orientation of magnetization or a ferromagnetic region to generate a nonvolatile memory cell. MRAM utilizes a magnetoresistive memory element involving the anisotropic magnetoresistance or giant magnetoresistance of ferromagnetic materials yielding nonvolatility. Both of these types of memory cells have relatively high resistance and low-density. A different memory cell based upon magnetic tunnel junctions has also been examined but has not led to large-scale commercialized MRAM devices. FRAM uses a circuit architecture similar to DRAM but which uses a thin film ferroelectric capacitor. This capacitor is purported to retain its electrical polarization after an externally applied electric field is removed yielding a nonvolatile memory. FRAM suffers from a large memory cell size, and it is difficult to manufacture as a large-scale integrated component. See U.S. Pat. Nos. 4,853,893; 4,888,630; 5,198,994  
         [0014]     Another technology having non-volatile memory is phase change memory. This technology stores information via a structural phase change in thin-film alloys incorporating elements such as selenium or tellurium. These alloys are purported to remain stable in both crystalline and amorphous states allowing the formation of a bi-stable switch. While the nonvolatility condition is met, this technology appears to suffer from slow operations, difficulty of manufacture and reliability and has not reached a state of commercialization. See U.S. Pat. Nos. 3,448,302; 4,845,533; 4,876,667; 6,044,008.  
         [0015]     Wire crossbar memory (MWCM) has also been proposed. See U.S. Pat. Nos. 6,128,214; 6,159,620; 6,198,655. These memory proposals envision molecules as bi-stable switches. Two wires (either a metal or semiconducting type) have a layer of molecules or molecule compounds sandwiched in between. Chemical assembly and electrochemical oxidation or reduction are used to generate an “on” or “off” state. This form of memory requires highly specialized wire junctions and may not retain non-volatility owing to the inherent instability found in redox processes.  
         [0016]     Recently, memory devices have been proposed which use nanoscopic wires, such as single-walled carbon nanotubes, to form crossbar junctions to serve as memory cells. See WO 01/03208, Nanoscopic Wire-Based Devices, Arrays, and Methods of Their Manufacture; and Thomas Rueckes et al., “Carbon Nanotube-Based Nonvolatile Random Access Memory for Molecular Computing,” Science, vol. 289, pp. 94-97, July 7, 2000. Hereinafter these devices are called nanotube wire crossbar memories (NTWCMs). Under these proposals, individual single-walled nanotube wires suspended over other wires define memory cells. Electrical signals are written to one or both wires to cause them to physically attract or repel relative to one another. Each physical state (i.e., attracted or repelled wires) corresponds to an electrical state. Repelled wires are an open circuit junction. Attracted wires are a closed state forming a rectified junction. When electrical power is removed from the junction, the wires retain their physical (and thus electrical) state thereby forming a non-volatile memory cell.  
         [0017]     The NTWCM proposals to date rely on directed growth or chemical self-assembly techniques to grow the individual nanotubes needed for the memory cells. These techniques are now believed to be difficult to employ at commercial scales using modern technology. Moreover, they may contain inherent limitations such as the length of the nanotubes that may be grown reliably using these techniques, and it may difficult to control the statistical variance of geometries of nanotube wires so grown.  
         [0018]     The ideal memory for at least some purposes is one which would offer low cost per bit, high density, fast random access, read/write cycle times of equal duration, low power consumption, operation over a wide temperature range, a single low-voltage power supply, with a high degree of radiation tolerance. The non-volatile described herein cell offers high speed read, but also high speed write (nanosecond) versus the slow (microsecond &amp; millisecond) write time of EEPROM and FLASH EEPROM type of memories. The memory is much denser than conventional SRAM because it has a two device, NT and 3 array line structure, and offers competitive performance. The density is less than that of DRAM cells, however, the product offers NDRO operation and non-volatility.  
         [0019]     U.S. patent Publication No. 2003-0021966 discloses, among other things, electromechanical circuits, such as memory cells, in which circuits include a structure having electrically conductive traces and supports extending from a surface of a substrate. Nanotube ribbons are suspended by the supports that cross the electrically conductive traces. Each ribbon comprises one or more nanotubes. The ribbons are formed from selectively removing material from a layer or matted fabric of nanotubes.  
         [0020]     For example, as disclosed in U.S. patent application Publication No. 2003-0021966, a nanofabric may be patterned into ribbons, and the ribbons can be used as a component to create non-volatile electromechanical memory cells. The ribbon is electromechanically-deflectable in response to electrical stimulus of control traces and/or the ribbon. The deflected, physical state of the ribbon may be made to represent a corresponding information state. The deflected, physical state has non-volatile properties, meaning the ribbon retains its physical (and therefore informational) state even if power to the memory cell is removed. As explained in U.S. patent application Publication No. 2003-0124325, three-trace architectures may be used for electromechanical memory cells, in which the two of the traces are electrodes to control the deflection of the ribbon.  
       SUMMARY  
       [0021]     The invention provides nanotube on gate FET structures and applications of such, including n 2  crossbars requiring only 2n control lines.  
         [0022]     According to one aspect of the invention, a non-volatile transistor device includes a source region and a drain region of a first semiconductor type of material and a channel region of a second semiconductor type of material disposed between the source and drain region. A gate structure is made of at least one of semiconductive or conductive material and is disposed over an insulator over the channel region. A control gate is made of at least one of semiconductive or conductive material. An electromechanically-deflectable nanotube switching element is in fixed contact with one of the gate structure and the control gate structure and is not in fixed contact with the other of the gate structure and the control gate structure. The device has a network of inherent capacitances, including an inherent capacitance of an undeflected nanotube switching element in relation to the gate structure. The network is such that the nanotube switching element is deflectable into contact with the other of the gate structure and the control gate structure in response to signals being applied to the control gate and one of the source region and drain region.  
         [0023]     According to another aspect of the invention, the nanotube switching element is an article formed from a porous fabric of nanotubes.  
         [0024]     According to another aspect of the invention, the fabric is substantially a monolayer of nanotubes.  
         [0025]     According to another aspect of the invention, the nanotubes are single-walled carbon nanotubes.  
         [0026]     According to another aspect of the invention, the source and drain regions are disposed on or in a horizontal substrate and the nanotube switching element is suspended horizontally in relation to the horizontal substrate.  
         [0027]     According to another aspect of the invention, the source and drain regions are formed as diffusions and one of the source and drain diffusions is a selection line for the device, and wherein the nanotube switching element is a second selection line for the device.  
         [0028]     According to another aspect of the invention, the source and drain regions are disposed orthogonal to the nanotube element.  
         [0029]     According to another aspect of the invention, the device has an area of about 4 F 2 .  
         [0030]     According to another aspect of the invention, the device includes a metallic layer disposed on the gate structure on a surface facing the nanotube switching element.  
         [0031]     According to another aspect of the invention, the nanotube switching element is supported by supports having a known dielectric characteristic and wherein the geometry of the supports is of a tailored size to create a known inherent capacitance of an undeflected nanotube switching element in relation to the gate structure and wherein the known inherent capacitance of the undeflected nanotube switching element has a known ratio relationship to an inherent capacitance characterizing the gate structure in relation to the channel region.  
         [0032]     According to another aspect of the invention, the ratio relationship is about 0.25.  
         [0033]     According to another aspect of the invention, a release line is positioned in spaced relation to the nanotube switching element, and having a horizontal orientation that is parallel to the orientation of the source and drain diffusions.  
         [0034]     According to another aspect of the invention, the device includes a metallic layer disposed on the release line on a surface facing the nanotube switching element.  
         [0035]     According to another aspect of the invention, the contact between the nanotube switching element and the one of the control gate and gate structure is a non-volatile state.  
         [0036]     According to another aspect of the invention, a crossbar array has n input lines and n output lines and n 2  non-volatile transistor devices, each device including a source region and a drain region of a first semiconductor type of material, a channel region of a second semiconductor type of material disposed between the source and drain region, a gate structure made of at least one of semiconductive or conductive material and disposed over an insulator over the channel region, a control gate made of at least one of semiconductive or conductive material, an electromechanically-deflectable nanotube switching element wherein the element is in fixed contact with one of the gate structure and the control gate structure and wherein the element is not in fixed contact with the other of the gate structure and the control gate structure; the nanotube switching element being deflectable into non-volatile contact with the other of the gate structure and the control gate structure in response to signals being applied to the control gate and one of the source region and drain region, and a release line positioned in spaced relation to the nanotube switching element. The crossbar also includes a switch line decoder providing n select lines, each select line coupled to one of the source and drain regions of each device of a corresponding set of devices, and a release line decoder providing n select lines, each release line coupled to a release line of each device of a corresponding set of devices. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0037]     In the Drawing:  
         [0038]      FIG. 1A  is a nanotube (NT)-on-gate of FET schematic;  
         [0039]      FIG. 1B  is a nanotube (NT)-on-gate of FET schematic with a diode to discharge node;  
         [0040]      FIG. 1C  shows a NT on gate structure and a corresponding capacitance network that can be modulated;  
         [0041]      FIG. 2  is a cross section, conceptual schematic showing the structural location of the capacitance network of  FIG. 1C ;  
         [0042]      FIG. 3A  is a cross section, conceptual schematic showing the structural location of the capacitance network in a certain regime;  
         [0043]      FIG. 3B  is a cross section, conceptual schematic showing the structural location of the capacitance network in another regime;  
         [0044]      FIGS. 4A and 4B  illustrate the write operations of the NT switch in the OFF mode and ON mode respectively;  
         [0045]      FIG. 5  shows the top view of a non-volatile array cell according to certain embodiments of the invention;  
         [0046]      FIGS. 6A and 6B  show cross sections of the cell of  FIG. 5  in the OFF mode and ON mode respectively;  
         [0047]      FIGS. 7A and 7B  show cross sections of  FIG. 5  with a metallic layer between the polysilicon gate and the NT control gate in the OFF mode and ON mode respectively;  
         [0048]      FIGS. 8A and 8B  show cross sections of  FIG. 5  that illustrate vertical structures in which the FG is metallic line in the OFF mode and ON mode respectively;  
         [0049]      FIG. 9  shows intermediate structures used in building the NT support structure of  FIG. 7A  &amp; B;  
         [0050]      FIG. 9A  is the starting structure;  
         [0051]      FIG. 9B  shows the structure after planarization by CMP or other means;  
         [0052]      FIG. 9C  shows the metallic layer after being etched back;  
         [0053]      FIG. 9D  shows the structure after a sacrificial layer has been deposited; and planarized prior to the deposition of the NT ribbon;  
         [0054]      FIG. 10  illustrates a top view of a higher performance One Time Programmable (OTP) cell;  
         [0055]      FIG. 11  illustrates a cross section of  FIG. 10 ;  
         [0056]      FIG. 12  is a cross section of  FIG. 10  along the NT word line;  
         [0057]      FIG. 13  illustrates four bits of an OTP array;  
         [0058]      FIG. 14  illustrates the timings of the read and write-once operation;  
         [0059]      FIGS. 15A, 15B  and  15 C illustrate the Nanotube-on-Gate FET structures identical to  1 A,  1 B and  1 C respectively in read, release/write mode;  
         [0060]      FIG. 16  shows  FIG. 5  with a release line RL added above and orthogonal to the NT patterned ribbon;  
         [0061]      FIGS. 17A and 17B  illustrate cross sections of  FIG. 16  along the nanotube word line in the OFF mode and ON mode respectively;  
         [0062]      FIG. 18  is a schematic of four cells of an array formed using the cell described in  FIGS. 16 , and  17 A&amp;B;  
         [0063]      FIG. 19  illustrates the timing diagram of the memory array in  FIG. 18 ;  
         [0064]      FIG. 20  is an illustration of a conventional (prior art) crossbar switch;  
         [0065]      FIG. 21  illustrates the top view of a high performance NT-on-Gate FET device; and  
         [0066]      FIG. 22  illustrates a 4×4 crossbar switch using NT-on-Gate FET switches. 
     
    
     DETAILED DESCRIPTION  
       [0067]     The NT electromechanical switching components in the described embodiments incorporate a nanofabric component. More specifically, preferred embodiments provide transistor devices in which a nanotube switching element is integrated into a control element of the transistor, such as the gate of a FET device. (Nanotube switching elements are described in more detail in other applications and/or patents identified below, and are discussed herein to the extent relevant for understanding the preferred embodiments.)  
         [0068]      FIG. 1A  illustrates a nanotube (NT)-on-gate of FET approach. This structure utilizes a nanofabric-based switch NT 1  which controls the FET transistor gate which controls the transistor channel, and therefore the current through the transistor as shown. The gate voltage of FET 1  is not defined; it floats. Therefore if a voltage is applied to node  1  using SEL 1 , the voltage across nanotube NT 1  is not known.  
         [0069]      FIG. 1B  changes the arrangement by adding DIODE 1  between substrate diffusion  7  and discharging node  2 ′, allowing a leakage current to flow between diffusion  7  and discharging node  2 ′. Node  2 ′ will be kept at 0 volts. In operation, sufficient voltage applied to node  1  using SEL 1  will cause nanotube NT 1  to switch. Because this cell requires an additional connection in relation to the cell shown in  FIG. 1A , the cell of  FIG. 1B  is relatively large.  
         [0070]      FIG. 1C  illustrates a different cell that is smaller than that shown in  FIG. 1B , and that may be operated as a memory cell or non-volatile transistor device. The cell is shown in a capacitance network that can be modulated in such a way that the voltage at node  2  is defined, and in such a way that NT 1  will switch. The voltage at node  2  can be detected without the need for additional elements such as the diode of  FIG. 1B .  
         [0071]     More specifically, the device of  FIG. 1C  may be operated as follows. The voltage at node  2  is controlled by a combination of the voltages applied to node  1  and node  4 . The voltage at node  1  is applied to the capacitance network and channel region  3  is inverted. If the voltage at node  4  is set to zero, then electrons remain in the channel region, and capacitor C 36  between the channel region and the semiconductor substrate is bypassed (not in the capacitor network). In such a case, node  2  voltage swing is minimized, which in turn maximizes the voltage difference between node  1  and node  2 . If the difference in voltage between node 1  and node  2  exceeds the nanotube electromechanical switching threshold voltage (V NT-TH ), then nanotube switch NT 1  closes and a conductive path is formed between node  1  and node  2 . If, however, the voltage at node  4  is not zero (1 to 3 volts, for example), then capacitors C 23  and C 36  are in series, the node  2  voltage swing is maximized, which in turn minimizes the voltage difference between node  1  and node  2 . In such a case the nanotube electromechanical switching threshold voltage (V NT-TH ) is not exceeded, and switch NT 1  does not switch, and remains in the open position. Note that during the switch setting operation, node  5  may be left floating (open), but should be grounded for the read operation. It should be noted that  FIGS. 1A, 1B , and  1 C show the arrow as only closing the NT 1 ; these figures are thus depictive of one time programmable (OTP) devices; however, the invention itself and preferred embodiments described below include devices allowing multiple programming.  
         [0072]      FIG. 2  is a simplified cross section of the structural location of the capacitance network and nodes in the schematic of the  FIG. 1C  structure. A nanotube NT fabric element is used as the control gate  1  of the device and is capacitively coupled to a floating gate (FG)  2  by capacitor C 12 . A NT fabric element is used as the word line in array configurations, and as a switch that contacts floating gate  2  when activated. Other materials may be used as the word line under other embodiments of the present invention. Gate FG is referred to herein as a floating gate, but in reality it only floats when the NT is in the “OFF” state. In the “ON” state, NT contacts FG  2  with an ohmic contact, and FG is no longer floating. When NT contacts FG, then capacitor C 12  is shorted out. The capacitor between FG node  2  and the FET channel  3  is C 23 . Capacitor C 36  is also referred to as the depletion capacitor C DEP . Capacitor C 36  not part of the network when an inverted channel exists; capacitor C 36  is part of the network when there is no inverted channel.  
         [0073]     FIGS.  3 A-B illustrate channel modulation using voltage V D . During a write (program) operation, a channel region is formed by applying a control gate voltage that couples to the FG node by means of a capacitor network such that the transistor threshold voltage V TH  is exceeded. The voltage of a diffusion (either diffusion) is modulated by applied voltage. For example, if V D =0, electrons stay in the channel region, and the channel stays inverted. If V D  is positive, e.g., between 1-3 volts, electrons are removed from the channel region which becomes depleted. The diffusion voltage modulates the capacitance network controlling the voltage on the floating gate FG (node  2  in  FIGS. 1A &amp; 1C ).  
         [0074]      FIG. 4  illustrates the write operation of the NT switch. The nature in which the nanotube element may be caused to deflect from one state to the other and the manner in which such switches may be arranged and constructed are more fully described in incorporated references, identified below. However, for purposes of describing the preferred embodiments, one can envision an electromechanical switch. The NT begins in the “OFF” (open) state after fabrication and can make one “OFF”-to-“ON” transition for OTP embodiments. Alternatively, the NT switch may be switched to the “OFF” (open) state if released by a release node prior to write in the read, release/write mode of operation (discussed below). Exemplary relative capacitance ratios are shown in  FIG. 4 , and are as follow: C 12 =0.25, C 23 =1 and C 36 =0.2.  
         [0075]     Transistor gate capacitor C 23  is determined by the gate oxide thickness and dielectric constant (relative dielectric constant of gate oxide is typically 3.9), and the transistor geometries, width and length, for example, as described in Baker et. al., “CMOS Circuit Design, Layout, and Simulation”, IEEE Press Series, 1998 pages 84 and 85. For this example, capacitor C 12  is designed to be equal to 0.25 C 23 , by a choice of a control gate to FG overlap area, control gate to FG separation, and dielectric constant. When designing capacitor C 12 , the coupling area (region) between NT and FG includes the NT support area and the gap switching area (region) as illustrated below. The NT support area dielectric constant depends on the choice of insulator support dielectric material. For example, polyimide has a relative dielectric constant of 2.5; silicon dioxide has relative dielectric constant of 3.9; and alumina has a relative dielectric constant of 9.5. The relative dielectric constant in the gap region is 1. The relative plate area of capacitor C 12  may also be controlled by the choice of NT fabric clamping material (insulator or conductor) above the nanotube fabric in the NT support region in the perimeter of the NT fabric suspended region. For example, a metallic support may be used instead of an insulating support, filling-in a controlled portion of the porous NT fabric over the NT support region to increase control gate to FG coupling area as needed. The depletion capacitance is typically 0.2 C 23 . The NT switching (select) voltage, modulated by the capacitance network, results in a difference in voltage between NT and FG ranging from 1.0 and 2.0 volts. NT threshold voltage should be above 1.0 volts, and below 2.0 volts; 1.5 volts, for example. In the “ON” state, the NT control gate is in electrical contact with gate FG, and the transistor threshold voltage is low, 0.3 to 0.7 volts, for example. In the “OFF” state, the NT control gate is not in direct electrical contact with NT, and the control gate voltage is coupled to the NT gate through a capacitor voltage divider network. Because of the voltage reduction at transistor gate NT due to the capacitor voltage divider, the threshold voltage for control gate formation of a channel region between source and drain is increased to 1.2 to 1.5 volts, for example. The effective threshold voltage of the transistor has been increased to 1.2 to 1.5 volts.  
         [0076]     The stored state may be detected by applying a control gate voltage that is higher than the transistor “ON” state threshold voltage 0.3 to 0.7 volts, and below the transistor “OFF” state effective threshold voltage of 1.2 to 1.5 volts. A control voltage of 1 volts will cause a corresponding precharged bit line voltage to discharge for an “ON” state, and to remain charged for an “OFF” state. A sense amplifier/latch may be used to detect the state of the cell. The cell may be written (programmed) once (OTP) and read an unlimited number of cycles.  
         [0077]      FIG. 5  is a plan view of a non-volatile array cell. The bit lines BL are diffusion, the word lines are patterned NT ribbons, orthogonal to the diffused lines. The patterned nanotube ribbons may be formed and patterned by any appropriate method, such methods are not described herein for the sake of brevity but are described in “Methods Of Making Carbon Nanotube Films, Layers, Fabrics, Ribbons, Elements And Articles,” U.S. patent application Ser. No. 10/341005, filed Jan. 13, 2003. The control gate is the NT in the region above the FG region. As stated above, the NT does not contact FG in the “OFF” state, and therefore the FG in this state is truly floating. Length L and width W of a transistor device may be minimum dimensions and defined by the FG. As shown, the cell size is 4 F 2 ; there are no array contacts requiring via holes between layers of the structure.  
         [0078]     FIGS.  6 A-B show cross-sections of the cell taken along line A-A′ of  FIG. 5  and with the device in different states. The cell cross-section is shown with NT supports applied directly to the polysilicon gate. A release gap is shown between polysilicon gate and the NT film. Another release gap is above the film.  
         [0079]      FIG. 6A  shows a cell in the “OFF” (open) state, i.e., the initial state of the cells in the OTP memory.  FIG. 6B  shows the “ON” (closed or switched) state for selected bits.  
         [0080]      FIG. 7  shows a cross section of a cell like the cell shown in  FIG. 5  only having a metallic layer between the polysilicon gate and the NT control gate. The metal layer changes the electrode material and therefore it may be used in different applications and may by treated differently. Polysilicon can have asperities (spikes) on the top surface that can locally concentrate an electric field. Any potential asperities are covered by the metal layer. During metal layer formation, the metal layer electrode surface can be planarized using well known techniques such as chemical mechanical polishing (CMP) or reactive ion etch (RIE). Like the cells shown in  FIGS. 5 and 6 , the cell as shown in  FIG. 7  also has no contacts between layers. As shown, this cell operates in OTP mode.  
         [0081]      FIG. 8  illustrates yet another embodiment of an OTP device according to certain embodiments of the present invention. This cell is like the cell shown in  FIG. 5  but this cell has a FG that is metallic. As before, there are no contacts between layers.  
         [0082]      FIG. 9  shows intermediate structures used in building the NT support structure of  FIGS. 7 A  &amp; B. A goal that is achieved using this method is self alignment of the NT support structures using a sidewall spacer applied to the minimum dimensions D MIN  (FET channel length direction) prior to implantation of the self aligned source and drain regions of the FET.  FIG. 9A  is the starting structure having side spacers, a metallic layer over a sublayer. The structure is planarized by CMP or other appropriate means, creating the intermediate structure shown in  FIG. 9B . The metallic layer is etched back creating the intermediate structure shown in  FIG. 9C . A sacrificial layer is deposited and planarized ( FIG. 9D ) prior to the deposition of the NT ribbon. The NT fabric layer support illustrated in  FIG. 9D  is one example of a support structure adjacent to the sides of the NT gate. NT layer pinning structures are placed over the top and side-wall of the nanotube fabric overlapping the support structure of  FIG. 9D .  FIGS. 6 and 7  illustrate NT fabric support structure examples in which the NT fabric layer support is defined on the top layer of the FG structure. The sacrificial layer is removed later in the process. (Creation of such suspended nanotube fabric switches is described in incorporated references as stated above.)  
         [0083]     The foregoing process sequence can also used in the fabrication of the structure shown in  FIGS. 7A and 7B . The sidewall process approach integrates a support structure self aligned to the polysilicon gate structure, and it can be used to achieve a 4 F 2  cell area.  
         [0084]     Exemplary One-Time-Programmable cell as described and shown in  FIGS. 5, 6 ,  7 ,  8 , and  9  are very dense. The use of diffused bit lines will mean it is relatively slow in operation; however the use of salicided diffusions will reduce diffusion resistance.  
         [0085]      FIG. 10  illustrates a plan view of an OTP cell having higher performance than those described above. In the illustrated structure, metallic bit lines are used for cell selection in order to improve memory array performance. Metallic bit lines contact the drains of the FET devices. Source diffusions are common and may be stitched periodically in the array structure for higher array performance. Likewise, NT fabric array wiring may also be stitched periodically in the array structure to enhance array performance. (“Stitching” is a technique known in the art.) In one example of stitching, a conductor such as aluminum is patterned above a polysilicon word line. Via holes are provided at regular intervals interconnecting the conductor and polysilicon word line to reduce polysilicon word line resistance, which enhances the performance of DRAM, SRAM and other array structures. Because this structure uses contacts between layers, this cell requires additional room and the cell is larger than 4 F 2 . The layout of the cell changes the orientation of the NT support structures relative to the orientation in  FIG. 5 . The supports in this structure are along the width direction of the FET device and are shown in  FIG. 10 . Although the NT fabric structures of  FIGS. 5 and 10  are shown supported on two sides, the NT fabric structure may be supported on all four sides. As shown, the word lines are orthogonal to the bit lines. Such word lines may be formed from patterned nanotube-based fabrics.  
         [0086]      FIG. 11  illustrates a cross section of  FIG. 10  taken along the metal bit line B-B′ of  FIG. 10 . Bit line contact to drain diffusion is shown. Also in this figure is a cross section of the NT word line. The NT film is the control gate of the FET structure. In a preferred embodiment of the present invention, the structure is single poly; however other appropriate material may be used. The cell is shown in the “OFF” (open) state. The channel region and diffusions of the device are shown.  
         [0087]      FIG. 12  illustrates a cross section of  FIG. 10  along the NT word line C-C′ of  FIG. 10 . This illustration shows polysilicon FG along the width region of the FET device. The FG structure is situated over thin gate oxide in the channel region and the FG structure also overlays a thick field oxide region causing a step in the oxide layer. If used, a shallow trench isolation structure (not shown) can minimize the polysilicon step height.  
         [0088]     The cross section shown in  FIG. 12  illustrates how the cross section illustrated in  FIGS. 7A  and B can be planarized using the metallic layer/NT support films to form a controlled gap between NT film and metallic layer on the polysilicon gate.  
         [0089]     The cross section illustrated in  FIG. 12  further shows the NT film as an array wire (word line), and the control gate of the FET, and as a switch that can close and electrically connect NT with the metallic layer on the polysilicon gate.  
         [0090]      FIG. 13  illustrates a schematic representation of four bits of an OTP array. A corresponding array layout may use cells as illustrated in  FIG. 5  or  FIG. 10 . Cells  0 ,  1 , and  2  are shown in the “OFF” (open) state. And as stated earlier, all cells of an OTP memory product are open at fabrication. In this figure, cell  3  is shown with the NT switched to the “ON” position. This bit has been programmed once; as an OTP cell, it cannot be reprogrammed.  
         [0091]      FIG. 14  illustrates the timing and waveforms of the various lines to perform read and write-once operations. When reading the state of the device, a voltage of 1.25 volts is applied to the WL 0 , which is an NT ribbon; BL 0  is driven high, e.g., 1 volt, and released. If the device is in the “ON” state, then WL 0  (the NT 0  control gate CG 0 ) is in ohmic contact with floating gate FG 0 ; FG 0  is at 1.25 volts and turns on the FET that has a threshold voltage of 0.7 volts, for example; and BL 0  is discharged.  
         [0092]     For a device in the “OFF” state, the FGO is capacitively coupled to NT, and the FG 0  voltage is 0.25 volts; the FET remains “OFF” and BLO is not discharged. During read, adjacent cell disturb is avoided because the voltage between the NT and the electrode (voltage between CG 0 -FG 0 ) is no greater than 1 volt, and therefore cannot perform an unintended write operation.  
         [0093]     When writing-once, 2.5 volts is applied to the WL 0  (nanotube ribbon). If BL 0  is at 0V, then 2 volts appears between NT and electrode (between CG 0 -FG 0 ) and the device goes from “OFF” to “ON”. If BL 0  is at a positive voltage (1 to 2.5 volts, for example), then the voltage between CG 0  and FG 0  is 1 volt and does not switch the NT. Voltage control is as explained in reference to  FIGS. 4A and 4B , above.  
         [0094]     FIGS.  15 A-C illustrate the Nanotube-on-Gate FET structure in read, release/write mode. This structure (including the capacitance network) is a device analogous to the one shown in  FIG. 1 , except the arrow indicates switching from “OFF” to “ON” and “ON” to “OFF.” The cells described below have a release gate added to the NT structures. This allows the device to be rewritten multiple times.  
         [0095]      FIG. 16  illustrates a structure like the one shown in  FIG. 5  with a release line RL added above and orthogonal to the NT patterned ribbon. The cell illustrated in  FIG. 16  is a read, release/write cell. RL is shown as aligned over the portion of the NT film suspended between the NT supports. Even allowing for some misalignment of RL relative to the suspended NT region between NT supports, the cell is a read/write cell of minimum cell area 4 F 2.    
         [0096]     FIGS.  17 A-B illustrate a cross section of the structure shown in  FIG. 16  along the nanotube word line A-A′ of  FIG. 16 .  FIG. 17A  shows the cell in the “OFF” mode and  FIG. 17B  shows the cell in the “ON” mode. The cross section shows a release node (or release electrode) above the release gap above the NT film. Even allowing for some misalignment of the release node relative to the NT suspended region, the cell area is a read/write cell of minimum cell area 4 F 2 . In this case, the release line contacts the release node and alignment is not critical.  
         [0097]      FIG. 18  is a schematic of four cells of an array formed using the NT-on-Gate cell described in  FIGS. 16 , and  17 A and B. The memory array shown in  FIG. 18  is similar to the memory array of  FIG. 13 , except that a release node (electrode) coupled to a release line RL 0  has been added. The memory array of  FIG. 18  supports read, release/write mode operation, and stores the information in a non-volatile state, which means the information remains unchanged when power is turned off. Such an array may be referred to as a nanotube random access memory (NRAM) array. Cells  0 ,  1 , and  2  are illustrated in the “OFF” state, and cell  3  is illustrated in the “ON” state. Unlike the OTP cells described above, any cell can be turned (written/programmed) to the “ON” state, and be returned to the “OFF” state.  
         [0098]      FIG. 19  illustrates the timing diagram and waveforms of the memory array in  FIG. 18 . Read and write operation was illustrated in  FIG. 14  for the OTP array shown in  FIG. 13 , and such operation is the same for the array shown in  FIG. 18 . Release RL 0  timing is introduced with a release voltage pulse of 5.5 volts. Higher voltage may require some special layout and/or junction considerations for peripheral array drivers; however, ideally, no junction changes are needed.  
         [0099]     Under some embodiments of the present invention, triple-diffused junctions can be used analogously to their current use in high voltage circuits. Ideally, guard ring or other layout modifications may accommodate higher voltages if required in the peripheral driver region.  
         [0100]     As shown, the RL 0  lines are orthogonal to the nanotube ribbon word lines WL 0 ; however other layouts are possible. This specific layout accommodates single bit release, (byte release is described in U.S. Provisional Patent Application No. 60/459,222). The word line voltage is at zero for devices to be released. It is increased to 2.5 volts to avoid writing (disturbing) an adjacent bit. The release voltage is the difference in voltage between RL 0  and WL 0  (RL 0 -WL 0 ). For bit release level selectivity, release voltage needs to be above 3.0 volts and less than 5.5 volts for release operation as illustrated. (Other specific voltages are within the scope of the present invention; one skilled in the art will understand that the voltages given are merely exemplary). For byte release (8 bits simultaneously) or block release (typically greater than 8 bits such as 256, 512, 1024, or all bits simultaneously released), then release voltage is any voltage between 0 and 5.5 volts. Bit or block release arrays, typically include bit level read &amp; write.  
         [0101]     An alternative programming approach to the approach described in  FIG. 19 , is to set all array bit lines (BL 0 , BL 1 , and BL 2  in  FIG. 18 ) to zero volts. Then activate array word lines (WL 0  and WL 1  in  FIG. 18 ) to 2.5 volts, for example, and set all release line voltages to equal the word line voltage (such that the word line to release line electrostatic forces are zero), all bits are programmed to the “ON” state. Bits in “OFF” state switch to the “ON” state, and bits in the “ON” state remain in the “ON” state. Next, when selectively switching from the “ON” to the “OFF” state, all bit line voltages may be set at 1 to 3 volts, for example, to minimize the risk of switching unselected bits from the “ON” to “OFF” state. CELL  0  may then be selectively programmed to the “OFF” state by setting word line WL 0  to zero volts, and release line RL 0  to release voltage to 5.5 volts, for example. To avoid disturbing other array cells, word line voltage is increased to 3 volts, for example, for unselected bit locations. This operating mode is referred to as a partial select or half-select mode. For RL 0  set at 5.5 volts, and word line WL 1  set at 3 volts, CELL  2  remains in the “ON” state. For WL 0  at zero volts, and RL 1  at 3 volts, CELL  1  remains in the “ON” state and for WL 1  at 3 volts and RL 1  at 3 volts, CELL  3  remains in the “ON” state. The illustrative example assumes the same NT threshold voltages as illustrated in  FIG. 18 , and the capacitance network model described with respect to  FIGS. 2, 3 , and  4 .  
         [0102]      FIG. 20  illustrates a prior art, conventional 4-by-4 crossbar switch, having 16 switches, as described in Mead et. al., “Introduction to VLSI Systems”, Addison-Wesley Publishing Company, 1980, pages 157 and 158. The basic topology of a crossbar switch dictates that any input bus bit (for example, a bit from bus line I BUS1 ) must be available at any output bus position (O BUS2 , for example). Therefore, data paths must run at right angles to the normal bus data flow, as illustrated in  FIG. 20 . Note that information flow in the crossbar switch is bidirectional, so that input and output are interchangeable. For two buses, each n bits wide, the crossbar switch requires n 2  connections, one to each gate of the FETs illustrated in  FIG. 20 . For the n=4 bus illustrated in  FIG. 20 , 16 bits are required, with 16 lines from the crossbar switch controller (decoder), one output to each of the FET switches SW 00  to SW 33 . As bus width grows, the crossbar switch of  FIG. 20  becomes impractical to wire. For example, a 32 bit bus requires 1024 unique connections between crossbar switch controller and individual array switches. The FET switches of  FIG. 20  are conventional FETs used in a pass gate configuration.  
         [0103]     Carbon non-volatile nanotube switches use a NT-on-Gate device structure, and may also be used as pass gates; however, the state of the switch (“ON” or “OFF”) may be retained when power is turned off (or lost). Also, very significantly, NT switches may be decoded at each switch location. Therefore, each gate is set at the intersection of a switch (select) line and release line, and there are n+n (2n) array lines required to activate each location. For a 32 bit wide bus (n=32), the crossbar decoders require 64 control lines, and not 1024 lines as required for the conventional controller of  FIG. 20 . The FET sources and drains are interconnected by wires that may be used for programming the array switches, and also transmit signals between bus lines.  
         [0104]      FIG. 21  illustrates a plan view of a high performance NT-on-Gate FET device, designed as a logic switch, that forms the basis of a nanotube crossbar switch. The W/L ratio is typically in the 5:1 to 50:1 range. Advantages of this structure include high switching speed and low channel-on resistance. The presence of metallic terminals at the drain and source terminals of the wide device allow it to handle relatively large currents. The NT-on-Gate device includes the NT ribbon that also acts as FET control gate. The patterned NT fabric may be used as the word line array wiring as well as an electrostatic switching device. When used as array wiring, the NT fabric layer may be stitched (as is commonly done in the industry with polysilicon word lines, for example) with a conductor such as aluminum or copper for higher performance. The exemplary materials for construction of this device take advantage of single polysilicon technology. However, FET technologies with metal gates may also be used. As shown, the NT is suspended over the polysilicon gate electrode (metal gate electrode transistors may also be used). As drawn, the NT supports are along the width direction of the device (supports are shown in broken lines), although NT fabric supports may be used on all sides of the suspended NT fabric region. Since a wide device is used for performance reasons, relative switch spacing is non-critical. The release node RN (or release electrode) is situated above the NT ribbon, separated by a gap. The release line RL contacts RN, alignment, however is non-critical. Key device operational leverage is derived by the orthogonal intersection of the release line RL and the NT switch (select) line SL, and the capacitor network described with respect to  FIGS. 2, 3 , and  4 , that enable crossbar switch decoder to set the state of each non-volatile nanotube switch using 2n array lines in lieu of n 2  array lines as required for conventional FET switch implementations.  
         [0105]      FIG. 22  illustrates a nanotube crossbar switch with an array of 16 non-volatile nanotube switches, and associated controllers, decoders, bus interrupt devices, and buses. Crossbar switches interrupt bus interconnections long enough to change the interconnections by re-programming the “ON”/“OFF” state of the switches. The nanotube crossbar switch uses the interrupt time to reset the array switches using a release line (RL) decoder, a combined switch line (SL) and bit line decoder, and mode setting transistors in series with the bus lines.  
         [0106]     In operation, the NT switches are set/reset as follows. Bus interconnection is interrupted, and all bus line voltages in the NT switch array are set to zero volts. Then activate all switch lines (SL 0 -SL 3  in  FIG. 22 ) to switching voltage 2.5 volts, for example, and set all release line voltages (RL 0 -RL 3 ) to equal the switch line voltage (such that the switch line to release line electrostatic forces are zero)—all NT switches are programmed to the “ON” state. NT switches in “OFF” state switch to the “ON” state, and NT switches in the “ON” state remain in the “ON” state. Next, when selectively switching from the “ON” to the “OFF” state, all bus line voltages in the NT switch array may be set at 1 to 3 volts, for example, to minimize the risk of switching unselected NT switches from the “ON” to “OFF” state. In this example, NT switches  1 ,  2 ,  3 , and  4  remain in the “ON” position. The other 12 NT switches in the nanotube crossbar switch in  FIG. 22  are selectively switched from the “ON” state to the “OFF” state. By way of example, the NT switch at the intersection of SL 3  and RL 2 , adjacent to NT switch  1 , is switched to “OFF” state as follows. Switch line SL 3  is set to zero volts, and release line RL 2  to release voltage to 5.5 volts, for example, releasing the NT switch to the “OFF” state. To avoid disturbing other array NT switches sharing release line RL 2 , the voltage on select lines SL 0 , SL 1 , and SL 2  s are increased to 3 volts, for example, for unselected NT switch locations. This operating mode is often referred to as a partial select or half-select mode. For RL 3  set at 5.5 volts, and select lines SL 0 -SL 2  set at 3 volts, all other NT switches remain in the “ON” state. Release lines RL 0 , RL 1 , and RL 3  are set at 3 volts, and all unselected NT switches remain undisturbed in the “ON” state. This half-select NT switch method is applied in turn, as illustrated, to all NT switches, except switches  1 ,  2 ,  3 , and  4  that remain in the “ON” state. The illustrative example assumes the NT switches having the same threshold voltages as illustrated in  FIG. 18 , and the capacitance network model described with respect to  FIGS. 2, 3 , and  4 . After all nanotube switches are programmed, switch line SL 3  is only in electrical contact with FG at NT switch  1 ; switch line SL 2  is only in electrical contact with FG of NT switch  2 ; switch line SL 1  is only in electrical contact with FG of NT switch  4 ; and switch line SL 0  is only in electrical contact with FG of NT switch  3  of  FIG. 22 . Next, SL 3  is set to 1.0 to 1.5, for example, and NT switch  1  forms a conductive channel between transistor source and drain, in order to electrically connect I BUS0  and O BUS3 ; SL 2  is set to 1.0 to 1.5, for example, and NT switch  2  forms a conductive channel between transistor source and drain, in order to electrically connect I BUS1  and O BUS2 ; SL 1  is set to 1.0 to 1.5, for example, and NT switch  4  forms a conductive channel between transistor source and drain, in order to electrically connect I BUS3  and O BUS1 ; SL 0  is set to 1.0 to 1.5, for example, and NT switch  3  forms a conductive channel between transistor source and drain, in order to electrically connect I BUS2  and O BUS0 . For all other NT switches, switch line SL is coupled to FG by a capacitor voltage divider network, and for SL voltages of 1.0 to 1.5 volts, for example, no conductive channel is formed between source and drain. Next, signals T 1  and T 5  restore bus interconnections. If I BUS  and O BUS  lines are in tristate mode during bus interrupt, then the transfer gates controlled by signals T 1  and T 5  are not needed and may be omitted.  
         [0107]     Table 1 compares the conventional crossbar switch requiring n 2  array connections to control the state of conventional FET switches, with the nanotube crossbar switch requiring 2n array connections to control the state of the NT switches, where n is the bus width. Therefore, the number of array lines scale linearly with the bus size such that two 32 bit buses require a total of 64 array lines and Nanotube Crossbar Switches can be scaled to any size bus. The conventional crossbar switch of  FIG. 20  requires 1024 individual switch array connections and is not scalable because of the geometric increases in the required array lines to control the switches.  
         [0108]     At least some of the advantages of the Nanotube Crossbar switch (scalability, wireability) are summarized in Table 1.  
                                                                                                                                     TABLE 1                                       4 × 4   16 × 16   32 × 32   64 × 64           CROSSBAR   CROSSBAR   CROSSBAR   CROSSBAR                CONVEN-       CONVEN-       CONVEN-       CONVEN-               TIONAL   NANOTUBE   TIONAL   NANOTUBE   TIONAL   NANOTUBE   TIONAL   NANOTUBE                        # CROSSBAR   16   16   256   256   1024   1024   4096   4096       SWITCHES            # CROSSBAR                                                               SWITCH       CONNECTIONS                  
 
         [0109]     In at least some cases, the technique chosen to create a nanofabric must result in a sufficient quantity of nanotubes in contact with other nanotubes which thereby matters as a result of the nanotubes&#39; adhesion characteristics. Certain embodiments (e.g., memory cells) benefit when the nanofabric is very thin (e.g., less than 2 nm); for example, when the nanofabric is primarily a monolayer of nanotubes with sporadic overlapping (sometimes fabric will have portions that are bi-layers or tri-layers), or a multilayer fabric with relatively small diameter nanotubes. Moreover, many of these embodiments benefit when the nanotubes are single-walled nanotubes (SWNTs). Other uses of patterned nanotube ribbons (e.g., conductive traces) may benefit from thicker fabrics or multi-walled nanotubes (MWNTs). Still other uses of patterned nanotube ribbons may benefit from combinations of single-walled nanotubes (SWNTs), and multi-walled nanotubes (MWNTs). For example, long distance wiring traces may be selected for lower wiring impedance using MWNTs, while local wiring and electromechanical switching regions may selected for planarity and electromechanical switching characteristics. The nanofabric is patterned using photolithographic techniques generating an electrically conductive trace of nanotubes.  
         [0110]     The following U.S. patent applications disclose various methods and techniques for making nanotube fabrics and nanotube switching elements that may be used as NT devices of preferred embodiments. The nanotube fabrics are porous and in some instances highly porous. The nanotube fabrics are substantially a monolayer of carbon nanotubes. In certain preferred embodiments, the carbon nanotubes are single-walled carbon nanotubes. Their disclosures are hereby incorporated by reference in their entirety: 
        “Electromechanical Memory Array Using Nanotube Ribbons and Method for Making Same,” U.S. patent application Ser. No. 09/915093, filed Jul. 25, 2001 (NAN-1);     “Electromechanical Three-Trace Junction Devices,” U.S. patent application Ser. No. 10/033323, filed Dec. 28, 2001 (NAN-4);     “Nanotube Films And Articles,” U.S. patent application Ser. No. 10/128118, filed Apr. 23, 2002 (NAN-6);     “Methods Of Making Carbon Nanotube Films, Layers, Fabrics, Ribbons, Elements And Articles,” U.S. patent application Ser. No. 10/341005, filed Jan. 13, 2003 (NAN-15);     “Devices Having Horizontally-Disposed Nanofabric Articles And Methods Of Making The Same,” U.S. patent application Ser. No. 10/776,059, filed Feb. 11, 2004 (NAN-19).     “Devices Having Vertically-Disposed Nanofabric Articles And Methods Of Making “The Same,” U.S. patent application Ser. No. 10/776,572, filed Feb. 11, 2004 (NAN-20).        
 
         [0117]     Other embodiments may employ double-walled nanotubes, including such nanotubes with metallic properties.  
         [0118]     The preferred embodiment uses electrostatic attractive forces to deflect the nanotube element, but other embodiments may use repulsive forces.  
         [0119]     The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of the equivalency of the claims are therefore intended to be embraced therein.