Abstract:
A process of fabricating a molecular electronic device that preserves the integrity of the active molecular layer of the electronic device during processing is described. In one aspect, a barrier layer is provided to protect a molecular layer sandwiched between a bottom wire layer and a top wire layer from degradation during patterning of the top wire layer. A molecular electronic device structure and a memory system that are formed from this fabrication process are described.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is related to U.S. application Ser. No. 09/282,048, filed on Mar. 29, 1999, by James R. Heath et al., and entitled “Chemically Synthesized and Assembled Electronic Devices,” which is incorporated herein by reference. 
     
    
     
       TECHNICAL FIELD  
         [0002]    This invention relates to systems and methods for fabricating molecular electronic devices.  
         BACKGROUND  
         [0003]    Many different molecular electronic logic and memory devices have been proposed.  
           [0004]    For example, in one molecular electronic device structure, a molecular layer (e.g., a Langmuir-Blodgett film) is sandwiched between a pair of electrically conducting layers (e.g., a pair of metal layers, a metal layer and a doped semiconductor layer, or a pair of doped semiconductor layers). The molecular layer serves as a thin insulating film that may be used in a metal-insulator-metal (MIM) structure that may be configured as a tunnel junction device or a switching device, or a metal-insulator-semiconductor (MIS) structure that may be configured as an electroluminescent device.  
           [0005]    U.S. Pat. No. 6,128,214 describes another molecular electronic device structure that is configured as a molecular wire crossbar memory (MWCM) system formed from a two-dimensional array of nanometer-scale devices. Each MWCM device is formed at the crossing point (or junction) of a pair of crossed wires where at least one molecular connector species operates as a bi-stable molecular switch between the pair of crossed wires. The resulting device structure may be configured as a resistor, a diode or an asymmetric non-linear resistor. The state of each MWCM device may be altered by applying a relatively high, but non-destructive, state-changing voltage and may be sensed with a non-state-changing voltage.  
           [0006]    Still other molecular electronic devices have been proposed.  
         SUMMARY  
         [0007]    The invention features a novel process of fabricating a molecular electronic device that preserves the integrity of the active molecular layer of the electronic device during processing. In addition, the invention features a novel molecular electronic device structure and a novel memory system that are formed with this fabrication process.  
           [0008]    In one aspect, the invention features a method of fabricating a molecular electronic device in accordance with which a barrier layer is provided to protect a molecular layer, which is sandwiched between a bottom wire layer and a top wire layer, from degradation during patterning of the top wire layer.  
           [0009]    Embodiments of the invention may include one or more of the following features.  
           [0010]    The molecular layer and the top wire layer preferably have a combined thickness that is less than the barrier layer thickness in a region defining the molecular electronic device.  
           [0011]    The top wire layer preferably is patterned by disposing a lift-off layer over the barrier layer, disposing an electrically conductive layer over the molecular layer and the lift-off layer, and dissolving the lift-off layer. The lift-off layer preferably has a different solubility characteristic than the barrier layer. The lift-off layer preferably is dissolved with a solvent with respect to which the barrier layer is substantially insoluble. In some embodiments, the lift-off layer may comprise a polymer (e.g., PMMA) and the barrier layer may comprise a different polymer (e.g., PDMS). In these embodiments, the lift-off layer may be dissolved in acetone. In other embodiments, the lift-off layer may comprise a polymer and the barrier layer may comprise an inorganic electrical insulator.  
           [0012]    In another aspect of the invention, a molecular electronic device is fabricated as follows. A patterned bottom wire layer is disposed over a substrate. A composite layer, which includes a lift-off layer and an underlying barrier layer, is disposed over the patterned bottom wire layer. The composite layer is patterned to define a device region in which the bottom wire layer is exposed through the lift-off layer and the barrier layer. A molecular layer and an overlying top wire layer are disposed over the patterned composite layer and the exposed bottom wire layer. The molecular layer and the top wire layer have a combined thickness in the device region that is less than the thickness of the barrier layer defining the device region. The top wire layer is patterned by dissolving the lift-off layer with a solvent with respect to which the barrier layer is substantially insoluble.  
           [0013]    The invention also features a molecular electronic device that includes a bottom wire layer, a molecular layer disposed over the bottom wire layer in a device region, a top wire layer disposed over the molecular layer in the device region, and a barrier layer. The barrier layer defines the device region and has a thickness that is greater than a combined thickness of the molecular layer and the top wire layer.  
           [0014]    In another aspect, the invention features a molecular memory system comprising an array of devices corresponding to the molecular electronic device described in the preceding paragraph.  
           [0015]    Other features and advantages of the invention will become apparent from the following description, including the drawings and the claims. 
       
    
    
     DESCRIPTION OF DRAWINGS  
       [0016]    [0016]FIG. 1 is a diagrammatic perspective representation of a molecular electronic device formed from at least one electrically addressable molecular species that is sandwiched between two crossed electrically conductive wires.  
         [0017]    [0017]FIG. 2 is a flow diagram of a process of fabricating the molecular electronic device of FIG. 1.  
         [0018]    [0018]FIGS. 3A and 3B are orthogonal diagrammatic cross-sectional side views of a patterned bottom wire layer disposed over a substrate.  
         [0019]    [0019]FIGS. 4A and 4B are orthogonal diagrammatic cross-sectional side views of a patterned composite layer disposed over the patterned bottom wire layer of FIGS. 3A and 3B.  
         [0020]    [0020]FIGS. 5A and 5B are orthogonal diagrammatic cross-sectional side views of a molecular layer disposed over the patterned composite layer of FIGS. 4A and 4B.  
         [0021]    [0021]FIGS. 6A and 6B are orthogonal diagrammatic cross-sectional side views of an electrically conductive layer disposed over the molecular layer of FIGS. 5A and 5B.  
         [0022]    [0022]FIGS. 7A and 7B are orthogonal diagrammatic cross-sectional side views of the electrically conductive layer of FIGS. 6A and 6B patterned to define a top wire layer of a molecular electronic device.  
         [0023]    [0023]FIG. 8 is a circuit diagram of a resistive crossbar memory structure that includes an array of devices corresponding to the molecular electronic device of FIGS. 7A and 7B. 
     
    
     DETAILED DESCRIPTION  
       [0024]    In the following description, like reference numbers are used to identify like elements. Furthermore, the drawings are intended to illustrate major features of exemplary embodiments in a diagrammatic manner. The drawings are not intended to depict every feature of actual embodiments nor relative dimensions of the depicted elements, and are not drawn to scale.  
         [0025]    Referring to FIG. 1, in one embodiment, a molecular electronic device  10  includes two electrically conducting wires  12 ,  14  that are crossed at a non-zero angle. Each wire  12 ,  14  may be formed from a metal or a doped semiconductor material. A layer of bi-stable molecules or molecular compounds  16  (denoted by the symbol R) is sandwiched between wires  12 ,  14 . The particular molecule or molecules  18  (denoted by the symbol R S ) that are located at the intersection (or junction) of wires  12 ,  14  act as switch molecules and correspond to the active portion of molecular electronic device  10 . In operation, the state of molecular electronic device  10  may be changed by applying a relatively high state-changing voltage across wires  12 ,  14 . The magnitude of the state-changing voltage is sufficient to oxidize or reduce switch molecules  18 . Switch molecules  18  may include a redox pair of molecular species that cooperate to balance charge such that when one of the molecular species is oxidized (or reduced), the other molecular species is reduced (or oxidized). In operation, in one example, one molecular species may be reduced and the associated molecular species (the other half of the redox pair) may be oxidized. In another example, one molecular species may be reduced and one of the wires  12 ,  14  may be oxidized. In a third example, one molecular species may be oxidized and one of the wires  12 ,  14  may be reduced. In a fourth example, one wire may be oxidized and an oxide associated with the other wire may be reduced. In each of these examples, oxidation or reduction affects the tunneling distance or the tunneling barrier height between the two wires, thereby exponentially altering the rate of charge transport across the wire junction. This electronic functionality serves as the basis for operating molecular electronic device  10  as an electrical switch.  
         [0026]    Additional details regarding the general features of molecular electronic device  10  may be obtained from U.S. Pat. No. 6,128,214, which is incorporated herein by reference.  
         [0027]    As mentioned above, molecular electronic device  10  may be fabricated in a way that preserves the integrity of the active molecular layer  18 . Referring to FIGS.  2 - 8 B, in one embodiment, molecular electronic device  10  may be fabricated as follows.  
         [0028]    Referring initially to FIGS. 2, 3A and  3 B, a patterned bottom wire layer  12  may be disposed over a substrate  20  (step  22 ). Bottom wire layer  12  may be formed from an electrically conducting metal or a doped semiconductor material, and may be deposited onto substrate  20  by a conventional thin film deposition process, including a physical film deposition process (e.g., magnetron sputtering or electron beam deposition) or a chemical film deposition process (e.g., chemical vapor deposition). Substrate  20  may be formed from an insulating material, for example, an oxide layer formed on a semiconductor substrate (e.g., a silicon dioxide (SiO 2 ) layer formed on a silicon substrate) or sapphire. After patterning (e.g., by lithography), bottom wire layer  12  may have a thickness dimension that ranges from 0.01-0.1 μm and a width dimension within a range that extends from on the order of 1 nm to several microns.  
         [0029]    Referring to FIGS. 4A and 4B, a composite layer  24 , which includes a lift-off layer  26  and an underlying barrier layer  28 , is disposed over bottom wire layer  12  and patterned to define a device region  30  (step  32 ; FIG. 2). Lift-off layer  26  and barrier layer  28  are formed from different materials. In particular, lift-off layer  26  and barrier layer  28  are formed from materials with different solubility characteristics such that lift-off layer  26  may be dissolved in a solvent with respect to which barrier layer  28  is substantially insoluble. Lift-off layer  26  may be formed from a polymer (e.g., PMMA (poly-methyl methacrylate)) and barrier layer may be formed from a different polymer (e.g., PDMS (polydimethylsiloxane)) or an inorganic insulator (e.g., an oxide, such as SiO 2 , Si 3 N 4  or AlO x ). Polymer layers, such as PMMA and PDMS, may be deposited and patterned by conventional lithographic techniques (e.g., optical lithography, ultraviolet lithography, electron beam lithography or imprinting lithography). Inorganic insulator layers may be deposited and patterned by conventional lithographic techniques (e.g., optical lithography, ultraviolet lithography, electron beam lithography or imprinting lithography) or etch patterning techniques.  
         [0030]    As shown in FIGS.  5 A- 6 B, a molecular layer  16  and an electrically conducting top wire layer  14  are disposed over the patterned composite layer  24  (step  34 ; FIG. 2). In the device region  30 , the thickness of barrier layer  28  is selected to be greater than the combined thickness of molecular layer  16  and top wire layer  14 . As explained in detail below, the thickness and solubility characteristics of barrier layer  28  protect molecular layer  16  from degradation during the subsequent patterning of top wire layer  14 .  
         [0031]    Molecular layer  16  may be formed from a variety of different bi-stable molecular species (e.g., one or more of the rotaxane molecules described in U.S. application Ser. No. 09/282,048, filed on Mar. 29, 1999, which is incorporated herein by reference). In some embodiments, the selected molecular species may be dissolved in a solvent (e.g., tetrahydrofuran), prepared as a Langmuir monolayer, and transferred as a Langmuir-Blodgett single molecular monolayer film  16  over composite layer  24  and the portion of bottom wire layer  12  exposed through composite layer  24  in device region  30 . Alternatively, the selected molecular thin film may be prepared by a self-assembled monolayer method or by a thermal deposition process. In other embodiments, a suitable molecular species may be deposited directly onto substrate  20 .  
         [0032]    Top wire layer  14  may be formed from an electrically conducting metal or a doped semiconductor material, and may be deposited onto molecular layer  16  by a conventional thin film deposition process, including a physical film deposition process (e.g., magnetron sputtering or electron beam deposition) or a chemical film deposition process (e.g., chemical vapor deposition).  
         [0033]    Referring to FIGS. 7A and 7B, top wire layer  14  is patterned by dissolving the lift-off layer with a solvent with respect to which the barrier layer is substantially insolvent (step  36 ; FIG. 2). Because barrier layer  28  is thicker than the combined thickness of molecular layer  16  and top wire layer  14 , barrier layer  28  seals and protects molecular layer  16  in device region  30  against intrusion of the lift-off solvent, an intrusion which otherwise would degrade or completely destroy molecular layer  16 . The resulting molecular electronic device  10  is characterized by a barrier layer  28  that defines the device region  30  and has a thickness that is greater than the combined thickness of molecular layer  16  and top wire layer  14 . Molecular electronic device  10  may have a thickness dimension that ranges from 0.01-0.1 μm and lateral dimensions that range from on the order of 10 nm to several microns.  
       EXAMPLE 1  
       [0034]    In one embodiment, bottom wire layer  12  is formed from an aluminum layer (˜0.01-0.1 μm thick) with a top AlO x  coating (˜1-2 nm thick) using conventional deposition and lithographic patterning techniques. Top wire layer  14  is formed from a titanium layer (˜1-5 nm thick) and a top aluminum layer (˜0.01-0.1 μm thick) that are deposited by electron beam deposition techniques. Bottom wire layer  12  and top wire layer  14  may have width dimensions ranging from about 10 nm to several microns.  
         [0035]    Molecular layer  16  may be formed from one of the rotaxane molecules described in U.S. application Ser. No. 09/282,048, filed on Mar. 29, 1999. The selected rotaxane molecule is dissolved in a solvent (e.g., tetrahydrofuran), prepared as a Langmuir monolayer with a surface pressure of 28 milli-Newtons/meter, and transferred as a Langmuir-Blodgett single molecular monolayer film  16  over composite layer  24  and the portion of bottom wire layer  12  exposed through composite layer  24  in device region  30 . The resulting molecular layer may have a surface coverage of 0.1-100 nm 2 /molecule with a thickness of about 5 Å to about 100 Å.  
         [0036]    Barrier layer  28  is formed from a PDMS layer (˜0.01-1 μm thick) and lift-off layer  26  is formed from an PMMA layer (˜0.01-1 μm thick). Barrier layer  28  and lift-off layer  26  may be patterned by a conventional optical lithography process or a conventional imprinting lithography process. Lift-off layer  26  is selectively removed during lift-off patterning of top wire layer  14  (step  36 ; FIG. 2) by dissolving lift-off layer  26  with acetone.  
       EXAMPLE 2  
       [0037]    In another embodiment, bottom wire layer  12  is formed from an aluminum layer (˜0.01-0.1 μm thick) with a top AlO x  coating (˜1-2 nm thick) using conventional deposition and lithographic patterning techniques. Top wire layer  14  is formed from a titanium layer (˜1-5 nm thick) and a top aluminum layer (˜0.01-0.1 μm thick) that are deposited by electron beam deposition techniques. Bottom wire layer  12  and top wire layer  14  may have width dimensions ranging from about 10 nm to several microns.  
         [0038]    Molecular layer  16  may be formed from one of the rotaxane molecules described in U.S. application Ser. No. 09/282,048, filed on Mar. 29, 1999. The selected rotaxane molecule is dissolved in a solvent (e.g., tetrahydrofuran), prepared as a Langmuir monolayer with a surface pressure of 28 milli-Newtons/meter, and transferred as a Langmuir-Blodgett single molecular monolayer film  16  over composite layer  24  and the portion of bottom wire layer  12  exposed through composite layer  24  in device region  30 . The resulting molecular layer may have a surface coverage of 0.1-100 nm 2 /molecule with a thickness of about 5 Å to about 50 Å.  
         [0039]    Barrier layer  28  is formed from a silicon dioxide layer (˜0.1-1 μm thick) and lift-off layer  26  is formed from a PMMA layer (˜0.01-1 μm thick). Barrier layer  28  and lift-off layer  26  may be patterned by conventional lithographic and etching techniques, respectively. Lift-off layer  26  is selectively removed during lift-off patterning of top wire layer  14  (step  36 ; FIG. 2) by dissolving lift-off layer  26  with acetone.  
         [0040]    Depending upon the molecules or materials selected for molecular layer  16 , molecular electronic device  10  may exhibit any one of a variety of different electrical switching functions that may be used to controllably connect or disconnect bottom wire layer  12  and top wire layer  14 . The molecular electronic device may be singly configurable or reconfigurable. In singly configurable embodiments, the initial state of molecular electronic device  10  may be open or closed. By electrically biasing molecular electronic device  10  beyond a particular threshold voltage, the active material or molecules  18  may be oxidized or reduced to permanently reverse the initial state. of the device and, thereby, irreversibly close or open the switching state of the device. In reconfigurable embodiments, the switching device may be opened and closed multiple times by cycling the polarity and the magnitude of the applied voltage beyond appropriate threshold values that are selected to reversibly oxidize and reduce the active material or molecules  18 .  
         [0041]    In general, the type of electrical connection formed between bottom wire layer  12  and top wire layer  14  depends upon the materials from which wire layers  12 ,  14  and molecular layer  16  are formed. Table 1 identifies the various types of electrical switching functions that may be obtained from different device material combinations.  
                                                 TABLE 1                                       Wire Layer Materials                            Semiconductor-   Semiconductor-       Device   Metal-Metal   Metal-Metal   Metal   Semiconductor   Semiconductor       Type   (same)   (different)   Semiconductor   (pn junction)   (heterojunction)               Resistor   X   X   X               Tunneling   X   X   X       Resistor       Resonant   X   X   X       Tunneling       Resistor       Diode       X   X   X   X       Tunneling       X   X   X       Diode       Resonant       X   X   X   X       Tunneling       Diode       Battery       X   X       X                  
 
         [0042]    Referring to FIG. 8, in one embodiment, molecular electronic device  10  may be implemented in a resistive molecular wire crossbar memory  40  that includes a plurality of memory cells  42  that are arranged in multiple rows and multiple columns. Each memory cell  42  includes a molecular electronic device  10  that is coupled between a respective bottom wire line  44 ,  46 ,  48 ,  50  and a respective top wire line  52 ,  54 ,  56 ,  58 . The voltage across a memory cell is determined by the voltages applied to the bottom wire line and the top wire line between which the memory cell is coupled. A control circuit  60  is configured to address (or select), program information into, and read information from one or more memory cells  42  within memory cell array  40 . Molecular electronic devices  10  are activated by electrochemical reduction or oxidation of the molecules  18  that are sandwiched between the bottom and top wire lines. In this embodiment, the molecules of molecular layer  16  are selected to have a large hysteresis in the voltammogram so that a switch may be oxidized at a relatively high voltage and its status may be read at a lower voltage. When a switch is (electrochemically) closed, the resistance between connecting wires is low, which may correspond to a logic level of “1”. When the switch is opened, the resistance is high, which may correspond to a logic level of “0”. Further details regarding the operation of a resistive molecular crossbar memory may be obtained from U.S. Pat. No. 6,128,214.  
         [0043]    Other embodiments are within the scope of the claims. For example, in addition to a resistive molecular wire crossbar memory, other molecular wire crossbar memory embodiments may include an array of molecular electronic devices that are configured to provide any one of the other switching functions identified in Table 1. In addition, the above-described molecular electronic devices may be implemented in a circuit designed to perform one or more logic (as opposed to memory) functions.  
         [0044]    Still other embodiments are within the scope of the claims.