Abstract:
Physical neural networks based nanotechnology include dendrite circuits that comprise non-volatile nanotube switches. A first terminal of the non-volatile nanotube switches is able to receive an electrical signal and a second terminal of the non-volatile nanotube switches is coupled to a common node that sums any electrical signals at the first terminals of the nanotube switches. The neural networks further includes transfer circuits to propagate the electrical signal, synapse circuits, and axon circuits.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. national phase application under 35 U.S.C. §371 of International Patent Application No. PCT/US2009/038265 filed on Mar. 25, 2009, entitled CARBON NANOTUBE-BASED NEURAL NETWORKS AND METHODS OF MAKING AND USING SAME, which claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application No. 61/039,204, filed on Mar. 25, 2008, entitled CARBON NANOTUBE-BASED NEURAL NETWORKS AND METHODS OF MAKING AND USING SAME, the contents of each are incorporated herein in their entirety by reference. 
     This application is related to the following applications, the entire contents of which are incorporated herein by reference in their entirety:
         U.S. patent application Ser. No. 11/280,786, filed 15 Nov. 2005, entitled TWO-TERMINAL NANOTUBE DEVICES AND SYSTEMS AND METHODS OF MAKING SAME;   U.S. patent application Ser. No. 11/835,583, filed 8 Aug. 2006, entitled LATCH CIRCUITS AND OPERATION CIRCUITS HAVING SCALABLE NONVOLATILE NANOTUBE SWITCHES AS ELECTRONIC FUSE REPLACEMENT ELEMENTS;   U.S. patent application Ser. No. 11/835,612, filed 8 Aug. 2006, entitled NONVOLATILE RESISTIVE MEMORIES HAVING SCALABLE TWO-TERMINAL NANOTUBE SWITCHES;   U.S. patent application Ser. No. 11/835,651, filed 8 Aug. 2006, entitled NONVOLATILE NANOTUBE DIODES AND NONVOLATILE NANOTUBE BLOCKS AND SYSTEMS USING SAME AND METHODS OF MAKING SAME;   U.S. patent application Ser. No. 11/835,759, filed 8 Aug. 2006, entitled NONVOLATILE NANOTUBE DIODES AND NONVOLATILE NANOTUBE BLOCKS AND SYSTEMS USING SAME AND METHODS OF MAKING SAME;   U.S. patent application Ser. No. 11/835,845, filed 8 Aug. 2006, entitled NONVOLATILE NANOTUBE DIODES AND NONVOLATILE NANOTUBE BLOCKS AND SYSTEMS USING SAME AND METHODS OF MAKING SAME;   U.S. patent application Ser. No. 11/835,852, filed 8 Aug. 2006, entitled NONVOLATILE NANOTUBE DIODES AND NONVOLATILE NANOTUBE BLOCKS AND SYSTEMS USING SAME AND METHODS OF MAKING SAME;   U.S. patent application Ser. No. 11/835,856, filed 8 Aug. 2006, entitled NONVOLATILE NANOTUBE DIODES AND NONVOLATILE NANOTUBE BLOCKS AND SYSTEMS USING SAME AND METHODS OF MAKING SAME;   U.S. patent application Ser. No. 11/835,865, filed 8 Aug. 2006, entitled NONVOLATILE NANOTUBE DIODES AND NONVOLATILE NANOTUBE BLOCKS AND SYSTEMS USING SAME AND METHODS OF MAKING SAME; and   U.S. patent application Ser. No. 11/835,613, filed 8 Aug. 2006, entitled MEMORY ELEMENTS AND CROSS POINT SWITCHES AND ARRAYS OF SAME USING NONVOLATILE NANOTUBE BLOCKS.       

     BACKGROUND 
     1. Technical Field 
     The present application relates generally to nanotube switches and methods of making same, and, more specifically, to carbon nanotube fabrics and methods of making same for use in information processing circuits and systems. 
     2. Discussion of Related Art 
     As CMOS technology is scaled to smaller dimensions with an ever increasing number of devices per chip (in the billions of transistors), the FET complexity is increasing, wiring complexity is increasing, and electronics is approaching quantum-mechanical boundaries. As a result, power dissipation is rapidly increasing. For example, at the 1 μm technology node, an Intel i486 microprocessor dissipated approximately 2 Watts/cm 2  but at the 0.18 μm technology node, the Intel Pentium III microprocessor dissipates approximately 70 Watts/cm 2 , a 35× increase. Further scaling results in still higher power dissipation. What is needed is a way of improving electronic system function while reducing power dissipation. 
     Neurobiological systems reached a technology boundary long ago. The brain, for example, is far more efficient than any electronic device. The brain is based on water and electrolytes, is 3D, analog, complex, and dissipates very little power. Electronic circuits, made from sand, metal, and using 2D interconnections, have been shown to exhibit limited behavioral characteristics similar to neural network functions but none have made significant inroads in achieving efficient neural networks. 
     SUMMARY OF THE INVENTION 
     Nonvolatile nanotube switches enabling a new electronic implementation based on nanotube neural networks are disclosed. Systems of nanotube neural networks that use nanotube fabric switches and methods of making the same are disclosed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  illustrate a biological neural cell or “neuron”; 
         FIG. 2  illustrates a simple nanotube-based circuit, according to one embodiment; 
         FIG. 3  illustrates a more complex nanotube-based circuit, according to another embodiment; 
         FIG. 4  illustrates a circuit with NT Dendrite  400  or NT REF CKT  450  with five inputs (I 1 -I 5 ), according to another embodiment; 
         FIG. 5  illustrates a NT Neuron circuit according to another embodiment; 
         FIG. 6  illustrates a three-input NAND function used to form a NT Neuron circuit according to another embodiment; 
         FIG. 7  illustrates a three-input NOR function used to form a NT Neuron circuit according to another embodiment; 
         FIG. 8  illustrates a differential amplifier used to form a NT Neuron circuit according to another embodiment; 
         FIG. 9  illustrates a NT Synapse circuit with an Axon signal supplying an input, according to another embodiment; 
         FIG. 10  illustrates NT Axon bidirectional signal flow control circuit according to another embodiment; 
         FIG. 11  illustrates NT Axon bidirectional signal flow control circuit, according to another embodiment; 
         FIG. 12  is an illustration of various pulse options, according to another embodiment: 
         FIG. 13  illustrates pulse interference control at a NT Synapse circuit, according to another embodiment; and 
         FIG. 14  illustrates pulse interference control at NT Synapse circuit, according to another embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Nonvolatile nanotube switches enable new electronic implementations based on nanotube neural networks. One solution to the complexity and performance limitations of traditional electronic devices is the use of carbon nanotubes to fashion agile information processing circuits in analogy with biological neural networks. Nanotube-based circuit and network functions exhibit some of the desirable characteristics found in biological neural networks. For NT Neural Networks, these characteristics include:
         3D interconnections   High device density   Low power with nonvolatile nanotube switches   Low noise because of relatively slow operation   High fan-in compatibility via NT dendrite trees and high fan-out capability via NT axons   Lowered cost, since there is no immediate need for very small technology nodes   High reliability including tolerance to broad temperature ranges and high levels of radiation       

     Nanotube based circuits and networks using switches and memory elements comprising nanotube fabrics are described in detail in the incorporated references. Specifically, nonvolatile nanotube switches are disclosed in NAN-96 (U.S. patent application Ser. No. 11/280,786, filed 15 Nov. 2005), NAN-109 (U.S. patent application Ser. Nos. 11/835,583 and 11/835,612, each filed 8 Aug. 2006), NAN-116 (U.S. patent application Ser. Nos. 11/835,651, 11/835,759, 11/835,845, 11/835,852, 11/835,856, 11/835,865, each filed 8 Aug. 2006) and NAN-117 (U.S. patent application Ser. No. 11/835,613, filed 8 Aug. 2006. Also, nanotube neural networks use an extension of the disclosure “Nonvolatile Nanotube Select Circuits” by Claude Bertin dated Feb. 14, 2008 and concepts in two D-NT 62 disclosures submitted in mid-2006 on Nanotube Neurons. NAN-109 includes concepts of NV NT multi-resistance programmable values (multiple-ON states) that enable NV NT switches to exhibit analog behavior. Also, these switches exhibit digital operation switching between high-resistance (GΩ-range) OFF states and low-resistance (100 kΩ range) ON states. 
       FIGS. 1A and 1B  illustrate a biological neural cell or “neuron”  100  with key components labeled such as its dendrites axon, and synapses. Arrows indicate the direction of signal flow through the cell. Dendrites provide the neuron with its signal pickups. One or more received signals are processed in the cell body and communicated as a single signals along the axon where they are distributed to one or more synaptic terminals. Synapses are junctions between individual neurons where signal propagation is also modulated. Nanotube-based circuits playing the role of these various biological components are described below. 
       FIG. 2  is a simple nanotube-based circuit that may be used as a NT Dendrite  200  or as a NT Reference Circuit  250 . Switches SW 1  and SW 2  receive input I 1  and input  12  respectively, while the opposing end of each two terminal switch is connected to a common node O which is the output of the circuit. 
     In operation, switches SW 1  and SW 2  respond to electrical signals that develop voltages across the switches allowing small currents to flow through them on the order of 100 nA to 10 uA for present generation switches. Present generation switches are typically fabricated at 0.18 μm to 0.25 μm technology nodes. For present generation switches, these developed voltages are less than 3 volts and typically do not modify the switch resistances. For voltages in, for instance, the 3 volt to 5 volt range, however, switches respond to signals (a) with relatively slow rise times (slower than 100 ns for example) and with (b) multiple excitations, by decreasing SW 1  and/or SW 2  resistances. Voltage pulses in the 5 volt to 8 volt range with faster rise time pulses (faster than 100 ns for example) result in increased resistance values for SW 1  and/or SW 2 . Such resistance changes remain in effect until another set of electrical signals meeting the criteria above is applied, therefore this behavior qualifies the device as non-volatile. Buy utilizing this characteristic response to multiple excitations, the nonvolatile resistance values of switches SW 1  and SW 2  can change over time depending on how the NT dendrite is used. 
       FIG. 3  is a more complex nanotube-based circuit that may be used as an NT Dendrite  300  or as an NT Reference Circuit  350 . In this case, however, the resistances of SW 1  and SW 2  are deliberately set by monitoring the behavior of selected nodes within the circuit and feeding that information back to a Neural Network Controller. 
     There may be one overall Neural Network Controller or multiple Neural Network Controllers distributed throughout the NT Neural Network, depending on the particular embodiment. If input signals I 1  and I 2  are of sufficiently low voltage values that SW 1  and SW 2  resistance values are unaffected by the input signals, then the SW 1  and SW 2  resistance values can be set using a feedback mechanism that involves the Neural Network Controller function—that is the stimulation of NT Dendrites via NT Synapses based on the behavior multiple feedback signals as is illustrated further below. 
     In feedback operation (learning mode), the values of SW 1  and SW 2  are set by the Neural Network Controller function based on inputs from, for example, axon  1 , axon n, and synapse k together with the Neural Network Controller algorithm and/or internal wiring configurations. Note that in this mode the output O of NT Dendrite  300  or NV REF CKT  350  is decoupled from switches SW 1  and SW 2  by transistor T 4 . Likewise, transistors T 1  and T 2  are turned OFF, decoupling SW 1  and SW 2  from inputs I 1  and I 2 , respectively. Transistor T 3  is turned ON connecting the commons SW 1  and SW 2  to a reference voltage such as ground. Transistors T 5  and T 6  are also turned ON providing the Neural Network Controller with access to sense and effect the SW 1  and SW 2  resistance values. This function employs the Weighting Factor Controller illustrated in  FIG. 3 . 
     The Weighting Factor Controller reads the value of SW 1  and SW 2  resistances using a drive/sense circuit, then converts the analog values to digital form using an A/D converter, and thus provides the values to the Neural Network Controller. The Neural Network Controller calculates new SW 1  and SW 2  resistance values, which can be considered as “weighting factors”, and supplies these new resistance values to the Weighting Factor Controller which translates them into analog signals using a D/A converter. The drive/sense circuit sets SW 1  and SW 2  resistances to the new resistance values (i.e. sets the new weights) using these analog signals. Note that these analog signals may take the form of multiple excitation signals as required. Methods of controlling resistance values of NV NT switches are described in NAN-109 (U.S. patent application Ser. Nos. 11/835,583 and 11/835,612). 
     In normal operation, transistors T 3 , T 5 , and T 6  are turned OFF and transistors T 1 , T 2 , and T 4  are turned ON enabling standard NT Dendrite  300  operation or NT REF CKT  350  operation. Although the circuit here is described in terms of transistors, FETs created in semiconductor substrates, and/or thin-film FETs not in semiconductor substrates, CNT-FETs (NAN-82, 86), NT electromechanical switches—either volatile (NAN-31) or nonvolatile (NAN-45)—may also be used. 
       FIG. 4  shows NT Dendrite  400  or NT REF CKT  450  with five inputs (I 1 -I 5 ). These inputs may be integrated on different physical wiring levels in a 3D wiring arrangement. A FET SWITCH is included in parallel with the output as well. SWITCH may be left OFF at all times resulting in a circuit operation similar to NT dendrite  200  or NT REF CKT  250 . Alternatively, SWITCH may be turned ON allowing the NV NT switch resistance values to be set in an approach similar to that used with respect to NT Dendrite  300  or NT REF CKT  350 . Returning SWITCH to the OFF position enables standard operation of the NT Dendrite  400  or NT REF CKT  450  according to the behavior of new NT NV Switch resistance values. The circuit is shown in schematic form with a corresponding symbolic representation. 
       FIG. 5  illustrates NT Neuron  500 . In this case three NT Dendrites, NT_D-A, NT_D-B, and NT_D-C, are shown. Each NT Dendrite may have any number of inputs (two are shown) and may also include a mode input M that determines whether the NT Dendrite is in standard operating mode or feedback mode (i.e. having NT NV Switch resistance values updated by a controller function similar to the  FIG. 3  circuit). An NT REF CKT controls the voltage applied to the gate G of the Transfer Device (Switch). 
     In standard operation, node voltage A is determined by the applied input signals IA 1 , IA 2 , IB 1 , IB 2 , IB 3 , IC 1 , and IC 2  along with NT NV Switch resistance values for each NT Dendrite. In  FIG. 4 , the NT NV Switches are inside the symbolic NT Dendrite blocks. If the NT REF CKT voltage applied to gate G is sufficiently high to activate the transfer device, then the signal voltage on node A is transmitted through the channel of the transfer device to the Driver and thus to output C which is connected to an NT Axon. 
       FIG. 6  illustrates a three-input NAND function used to form the NT Neuron  600 . NT Dendrites NT_D-A and NT_D-B provide signals to NAND gate inputs A and B, while the NT REF CKT provides a signal to NAND gate input R. If R is high, then the NAND gate output is the complement of the product of inputs A and B as indicated in the table NT Neuron Function. A Driver chain supplies this logic value to output C which is connected to an NT Axon. 
       FIG. 7  illustrates a three-input NOR function used to form the NT Neuron  700 . NT Dendrites NT_D-A and NT_D-B provide signals to NOR gate inputs A and B. The NT REF CKT provides a signal to NOR gate input R. If R is low, then the output is the complement of the sum of inputs A and B as indicated in the table NT Neuron Function. A Driver chain supplies this logic value to output C which is connected to an NT Axon. 
       FIG. 8  illustrates a differential amplifier used to form the NT Neuron  800 . In this case three NT Dendrites drive the input node D of the Differential Amplifier, while the NT REF CKT drives the Differential Amplifier reference node R. The difference in voltage between input node D and reference node R determines whether Differential Amplifier output node E is set to a high or low value. The voltage state of E is then transmitted by the Driver to node F which is connected to an NT Axon. 
       FIG. 9  illustrates NT Synapse  900  with an Axon signal supplying the input. NT Synapse  900  is formed using a Differential Amplifier as was done in forming NT Neuron  800 . NT Synapse  900  “fires” or does not “fire” depending on the difference in voltage between nodes D and R. Node E is set to a high or low voltage which is transmitted to output F by the Driver. Output F may be connected to an NT dendrite input and/or to a NT neuron input. Other NT Synapse circuits may be formed based on circuits similar to those used to form NT Neurons  500 ,  600 ,  700  as well. NT Axons may be formed using patterned Carbon Nanotube Fabrics or other conductors such as aluminum or copper for example. 
     Complex NT Neural Networks may be formed from the non-volatile analog and/or digital properties of combinations of the NT Dendrites, NT REF CKTs, NT Neurons, NT Axons, and NT Synapses described further above. Such networks may exhibit massive parallel processing capacity, learning behavior, etc. and thereby used to solve problems in fields such as pattern recognition, computing, etc. 
       FIG. 10  illustrates NT Axon bidirectional signal flow control circuit  1000  for controlling signal flow direction within the axon. A bidirectional buffer circuit was modified by the addition of a NT REF CKT. The output states of the NT REF CKT control the direction of signal flow in the NT Axon by preferentially selecting signal flow ( 1 ) or signal flow ( 2 ). Signal flow ( 1 ) illustrated by signal in ( 1 ) and signal out ( 1 ) occurs if transistor (switch) T 1  is ON and transistor (switch) T 2  is OFF. Signal flow ( 2 ) illustrated by signal in ( 2 ) and signal out ( 2 ) occurs if transistor (switch) T 1  is OFF and transistor (switch) T 2  is ON. NT Axon bi-directional signal flow control circuit  1000  also restores signal characteristics (for example, pulse amplitude, rise and fall time, etc.). Specifically, the amplitude of the restored signal levels is equal to power supply V. 
       FIG. 11  illustrates NT Axon bidirectional signal flow control circuit  1100 , which is a modification of NT Axon bidirectional signal flow control circuit  1000  with the NT REF CKT replaced by a Neural Network Controller similar to the Neural Network Controller illustrated in  FIG. 3  and which also includes additional transistors (switches) T 3  and T 4  providing control over bias voltages V and V′. In this case the Neural Network Controller not only controls the direction of NT Axon signal flow by controlling the ON/OFF states of T 1  and T 2  as described in  FIG. 10 , but also controls the NT Axon signal polarity and amplitude as illustrated in  FIG. 12  by Pulse Control Examples  1200 . For example, if T 1  is ON, T 2  is OFF, and T 3  is OFF, the signal in ( 1 ) is not inverted at the signal out ( 1 ) terminal; however, if T 3  is ON bypassing driver DR 1  then the signal out ( 1 ) is inverted. Alternatively, if T 1  is OFF, T 2  is ON, and T 4  is OFF then the signal in ( 2 ) is not inverted at the signal out ( 2 ) terminal; however, if T 4  is ON bypassing driver DR 2 , then the signal at the signal out  2  terminal is inverted. The Neural Network Controller may also control other NT Axon bidirectional controllers, etc. The operation of the Neural Network Controller is similar to the description with respect to  FIG. 3  further above. 
       FIG. 12  is an illustration of various pulse options described with respect to  FIGS. 10 and 11 . A pulse temporal (timing) control function may be incorporated in  FIG. 11  for example. Bertin et al. U.S. Pat. No. 6,177,807 Jan. 23, 2001 incorporated by reference teaches precise pulse timing control. Round trip times on transmission lines of precise lengths result in precise timing control of high speed (or any speed) functions. In addition, transmission line length is modulated using fuses placed at various physical locations along the transmission line to precisely program various trip times. The number of precision controlled pulse delays depends on the number of fuses; however once a fuse is “blown” the timing cannot be changed. NAN- 109  incorporated by reference teaches substitution of NV NT switches for fuses (or antifuses). In this way timings can be changed without limit. In this manner pulse-to-pulse timing control may be incorporated (not shown) in the circuit illustrated in  FIG. 11 . In addition to pulse timing control, pulse rise and fall times can be automatically adjusted (or controlled by other circuits) as described in Bertin et al. U.S. Pat. No. 6,496,037 incorporated herein by reference in its entirety. 
       FIG. 13  illustrates pulse interference control at NT Synapse  1300 . In this case, NT AXON  1  BI-DI INPUT A and NT AXON  2  BI-DI INPUT B are fed by SW 1  and SW 2  to a common node A. NT Synapse  1300  then propagates or does not propagate a signal at node A depending on a combination of the timing of the arrival of the pulses, the pulse amplitudes, and the pulse polarities. In this example, signal propagation also depends on the output state of the NT REG CKT; however, NT Synapse  1300  may be fabricated with a two-input NAND gate similar to that of NT Neuron  600  in  FIG. 6  and thus respond only to inputs A and B. These conditions are set by Neural Network Controllers (or the same controller) for the two NT Axon inputs. In certain embodiments, signal propagation can be a function of pulse cancellation OR just control signal at gate G OR by embedded logic gate. 
     In other embodiments, the two NT axons may be connected directly to the NT synapse input node without going through the pair of NV NT switches illustrated in  FIG. 13 . NT AXON  1  BI-DI INPUT A and NT AXON  2  BI-DI INPUT B may be connected directly (not shown) to the NT synapse  1300  common node input without going through NV NT switches. Optionally, a termination may be used to minimize reflections in the NT axons. The termination (such as an impedance, for example) can be added (not shown) to the NT synapse  1300  common node input to minimize pulse reflections in NT AXON  1  BI-DI INPUT A and NT AXON  2  BI-DI INPUT B. 
       FIG. 14  illustrates pulse interference control at NT Synapse  1400 . In this case, NT AXON  1  BI-DI INPUT A and NT AXON  2  BI-DI INPUT B are two different inputs to a 3-input NAND gate at nodes A and B, respectively. NT Synapse  1400  then switches output state or does not switch output state depending on the timing of the arrival of the NT Axon signal characteristics (e.g. timing of arrival of pulses, pulse amplitudes, and pulse polarities). In this example, output state C also depends on the state of reference node R, set by the NT REG CKT; however, NT Synapse  1400  may be fabricated with a two input NAND gate similar to that of NT Neuron  600  in  FIG. 6  and thus respond only to inputs A and B. The condition suitable for this embodiment are set by Neural Network Controllers (or the same controller) for the two NT Axon. 
     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.