Abstract:
A method provides a simple yet reliable technique to assemble one-dimensional nanostructures selectively in a desired pattern for device applications. The method comprises forming a plurality of spaced apart conductive elements ( 12, 20 ) in a sequential pattern ( 26 ) on a substrate ( 17 ) and immersing the plurality of spaced apart conductive elements ( 12, 20 ) in a solution ( 23 ) comprising a plurality of one-dimensional nanostructures ( 22 ). A voltage is applied to one of the plurality of spaced apart conductive elements ( 12, 20 ) formed in the sequential pattern ( 26 ), thereby causing portions of the plurality of one-dimensional nanostructures ( 22 ) to migrate between adjacent conductive elements ( 12, 20 ) in sequence beginning with the one of the plurality of spaced apart conductive elements ( 12, 20 ) to which the voltage is applied.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to one-dimensional nanostructures and more particularly to a method of forming one-dimensional nanostructures in a desired pattern. 
     BACKGROUND OF THE INVENTION 
     One-dimensional nanostructures, such as belts, rods, tubes and wires, have become the latest focus of intensive research with their own unique applications. One-dimensional-nanostructures are model systems to investigate the dependence of electrical and thermal transport or mechanical properties as a function of size reduction. In contrast with zero-dimensional, e.g., quantum dots, and two-dimensional nanostructures, e.g., GaAs/AlGaAs heterojunctions and superlattices, direct synthesis and growth of one-dimensional nanostructures has been relatively slow due to difficulties associated with controlling the chemical composition, dimensions, and morphology. Alternatively, various one-dimensional nanostructures have been fabricated using a number of advanced nanolithographic techniques, such as electron-beam (e-beam), focused-ion-beam (FIB) writing, and scanning probe. 
     Carbon nanotubes are one of the most important species of one-dimensional nanostructures. Carbon nanotubes are one of four unique crystalline structures for carbon, the other three being diamond, graphite, and fullerene. In particular, carbon nanotubes refer to a helical tubular structure grown with a single wall (single-walled nanotubes) or multiple walls (multi-walled nanotubes). These types of structures are obtained by rolling single layers of graphene sheets into cylinders forming a plurality of hexagons on the tubes&#39; surface. The sheet is a close packed array of carbon atoms having no dangling bonds. Carbon nanotubes typically have a diameter on the order of a fraction of a nanometer to a few hundred nanometers. As used herein, a “carbon nanotube” is any elongated carbon structure. 
     Carbon nanotubes can function as either a conductor (metallic) or a semiconductor, according to the rolled shape (chirality) and the diameter of the helical tubes. With metallic nanotubes, a one-dimensional carbon-based structure can conduct a current at room temperature with essentially no resistance. Further, electrons can be considered as moving freely through the structure, so that metallic nanotubes can be used as ideal interconnects. 
     Another class of one-dimensional nanostructures is nanowires. Nanowires of inorganic materials have been grown from metal (Ag, Au), elemental semiconductors (e.g., Si, and Ge), III-V semiconductors (e.g., GaAs, GaN, GaP, InAs, and InP), II-VI semiconductors (e.g., CdS, CdSe, ZnS, and ZnSe) and oxides (e.g., SiO 2  and ZnO). Similar to carbon nanotubes, inorganic nanowires can be synthesized with various diameters and length, depending on the synthesis technique and/or desired application needs. 
     Both carbon nanotubes and inorganic nanowires have been demonstrated as field effect transistors (FETs) and other basic components in nanoscale electronic such as p-n junctions, bipolar junction transistors, inverters, etc. The motivation behind the development of such nanoscale components is that “bottom-up” approach to nanoelectronics has the potential to go beyond the limits of the traditional “top-down” manufacturing techniques. A carbon nanotube is also known to be useful for providing electron emission in a vacuum device, such as a field emission display. The use of a carbon nanotube as an electron emitter has reduced the cost of vacuum devices, including the cost of a field emission display. The reduction in cost of the field emission display has been obtained with the carbon nanotube replacing other electron emitters (e.g., a Spindt tip), which generally have higher fabrication costs as compared to a carbon nanotube based electron emitter. 
     One well known way of growing one-dimensional nanostructures is by CVD, however, this is a high temperature process that may prevent simple integration of carbon nanotubes with other device fabrication processes. 
     One known approach to manufacture nanowires is a top-down approach which uses e-beam lithography. However, this e-beam process is not desirable for mass production due its throughput limitations. Nanowire devices have also been fabricated by post synthesis assembly techniques, such as dispersion on an insulating substrate followed by patterning of electrodes on a few selected nanowires using lithography. Furthermore, nanowire synthesis methods typically, whether chemical vapor deposition or solution based, produce nanowires with a range of dimension and a range of properties. Conventional nanowire fabrication approaches include forming the nanowire using, for example, chemical vapor deposition (for crystalline semiconducting nanowires) or porous alumina membrane as a template (for metallic nanowires). Once the nanowires are fabricated, they are assembled on a substrate using either a random assembly approach or an ordered approach using micro fluidic channels for potential application. 
     Accordingly, it is desirable to provide a simple yet reliable technique to assemble one-dimensional nanostructures selectively in a desired pattern for device applications. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention. 
     BRIEF SUMMARY OF THE INVENTION 
     A method provides a simple yet reliable technique to assemble one-dimensional nanostructures selectively in a desired pattern for device applications. The method comprises forming a plurality of spaced apart conductive elements in a sequential pattern on a substrate and immersing the plurality of spaced apart conductive elements in a solution comprising a plurality of one-dimensional nanostructures. A voltage is applied to one of the plurality of spaced apart conductive elements in the sequential pattern, thereby causing portions of the plurality of one-dimensional nanostructures to migrate between adjacent conductive elements in sequence beginning with the one of the plurality of spaced apart conductive elements to which the voltage is applied. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and 
         FIGS. 1 and 2  are a partial cross-sectional view and a top view of a first exemplary embodiment; 
         FIGS. 3-5  are a partial top view of the growth process of the first exemplary embodiment; and 
         FIG. 6  is a flow chart of the method of the exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention. 
     One-dimensional nanostructures show promise for the development of molecular-scale sensors, resonators, field emission displays, and logic/memory elements. One-dimensional nanostructures are herein defined as a material having a high aspect ratio of greater than 10 to 1 (length to diameter) and include, for example, belts, rods, tubes and wires, and more preferably carbon nanotubes. Furthermore, the positioning of individual carbon nanotubes at specific locations has previously been challenging and is not amenable to scale-up of a large number of devices. 
     The formation of one-dimensional nanostructures is disclosed for use in various applications wherever a pattern is desired, such as an on-chip inductor for electronic circuitry. The method comprises assembling one-dimensional nanostructures with macro/microscopic dimensions that are not confined to a uni-directional orientation. In the exemplary embodiment, small metallic structures at a floating potential, e.g., dots, are formed on the substrate, in a sequential trail or pattern from a starting structure, e.g., electrode. The size and spacing of the dots are dependent on the one-dimensional nanostructure material being used, its concentration, and parameters used in controlling the fabrication. The substrate, which may be a flexible substrate, is immersed in a solution of the one-dimensional nanostructures, and using the technique of dielectrophoresis, an alternating current bias is applied to the starting electrode. With the appropriate choice of amplitude and frequency, the use of an AC bias dramatically enhances the placement of desired one-dimensional nanostructures. The nearest dot to the starting electrode will generate the strongest electric field gradient and thus be the strongest attractive force for the one-dimensional nanostructures materials suspended in the solution. As the one-dimensional nanostructures accumulate from the electrode toward the first dot, they will eventually bridge the gap therebetween. The next dot closest to the first dot will then exhibit the strongest field gradient, allowing the growth process to continue. This process is repeated for as many dots as have been formed. The dots may be positioned on the substrate in a pattern by, for example, ink jet printing, to form circuit elements such as an inductor. When the one-dimensional nanostructures are grown vertically, they may form a patterned light emitting device (field emission display). 
     Referring now to  FIG. 1 , illustrated in simplified cross-sectional views, and in  FIG. 2  in a top view ( FIG. 1  is taken along line  1 - 1  of  FIG. 2 ), is an assembled structure utilized for selective growth of catalytic nanostructures according to an exemplary embodiment of the present invention. More specifically, illustrated in  FIG. 1  is an assembly  10  including an electrode  12 . Although the electrode  12  is shown as positioned on insulating layer  18 , it could be recessed or buried. Assembly  10  in this particular embodiment includes a substrate  17 , comprising a semiconductor material  16  which has been coated with an insulating material  18 . It should be understood that anticipated by this disclosure is an alternate embodiment in which substrate  17  is formed as a single layer of insulating material, such as glass, plastic, ceramic, or any dielectric material that would provide insulating properties. By forming substrate  17  of an insulating material, the need for a separate insulating layer formed on top of a semiconductive layer, or conductive layer, such as layer  18  of  FIG. 1 , is eliminated. 
     The semiconductor material  16  comprises quartz, sapphire, or any semiconductor material well known in the art, for example, silicon (Si), gallium arsenide (GaAs), germanium (Ge), silicon carbide (SiC), indium arsenide (InAs), or the like. Insulating material  18  is disclosed as comprising any material that provides insulative properties such silicon oxide (SiO 2 ), silicon nitride (SiN), or the like. The insulating material  18  comprises a thickness of between 2 nanometers and 10 microns. Semiconductor material  16  and insulating material  18  form the substrate  17  as illustrated in  FIGS. 1 and 2 . In this specific example, assembly  10  includes the electrode  12  formed on an uppermost surface of insulating material  18 . Fabrication of the metal electrode  12  is carried out using by ink jet printing or any form of lithography, for example, photolithography, electron beam lithography, and imprint lithography on an oxidized silicon substrate  17 . In some embodiments, electrode  12  may comprise a highly doped semiconductor material. Electrode  12  comprises a thickness in the range of 1 nanometer to 100,000 nanometers. 
     A plurality of dots  20  are formed in a desired pattern on the uppermost surface of insulating material  18 . Fabrication of the dots  20  are carried out preferably by ink jet printing on an oxidized silicon substrate  17 . The dots  20  may comprise any conductive material, but preferably comprise a metal such as copper, and may comprise any form factor such as a circle, rectangle, or square. In some embodiments, the dots  20  may comprise a highly doped semiconductor material. The dots  20  comprise a thickness in the range of 1 nanometer to 100,000 nanometers. The electrode  12  and the dots  20  are formed to define between a gap  20  between the electrode  12  and a first dot  20  and between each dot  20  in the sequential pattern. The gap  20  may be between 1 nanometer and 100,000 nanometers. The pattern of dots  20  shown in  FIG. 2  is that of an inductor  26 , but it should be realized that any pattern of dots  20  could be formed, e.g., a passive element such as an inductor or an antenna, and an active element such as a transistor. 
     The solution  23  within container  21  is immiscible with one-dimensional nanostructures  22  in a solution such as an aqueous environment (water based), or non-aqueous based on, for example, methanol, ethanol, or acetone. The one-dimensional nanostructures  22  are grown in a manner known in the art and placed in the solution  23 . 
     During operation in accordance with an exemplary embodiment of the present invention as illustrated in  FIG. 3 , an AC field is applied to electrode  12  thereby causing an electric field  32  to form between the electrode  12  and the closest dot  28 . The dots  20 , including the closest dot  28 , are floating (without a potential). The voltage creating the AC field may range from 1.0 millivolt to 500 volts. This electric field  32  causes a migration, i.e., movement, of one-dimensional nanostructures  22  suspended within the aqueous environment  23  toward gap  24  between the electrode  12  and the closest dot  28  where the field and/or field gradient is the strongest. As the one-dimensional nanostructures attach to the electrode  12 , they will line up with the electric field  32 , thereby stretching out like a string from the electrode  12  to the first dot  28 . It should be understood that anticipated by this disclosure is the use of any environment, such as liquid or gaseous in which nanometer-scale components are contained.  FIG. 4  illustrates one-dimensional nanostructures  22  positioned between the electrode  12  and the dot  28 , and on the insulating material  18 . The AC field may be applied for a duration of up to several minutes depending on one-dimensional nanostructures  22  concentration in the solution  23 , to position a desired number of the one-dimensional nanostructures  22  in preferred locations. Optionally, a chemical functionalization step may be performed on the insulating layer  18  to immobilize, or attach, the one-dimensional nanostructures  22  in preferred locations. Similarly, for positioning the one-dimensional nanostructures  22  only in the desired positions, a chemical functionalization step may be performed on the insulating layer  18  to repel the one-dimensional nanostructures  22  from the insulating layer  18  ( FIG. 3 ). 
     Depending on the properties of the one-dimensional nanostructures being assembled, there may be some potential drop between connected dots  20 . The potential applied to the first conductor (dot)  12  may need to be increased as assembly between additional dots  20  occurs. Feedback circuitry may be used to monitor the voltage drop between dots  20  being connected in order to allow for real time modification of the voltage being applied to the first conductor  12 . The feedback circuitry may also be used to allow assembly only between intended dots  20 . 
     After the assembly process, the excess solution  23  may be evaporated, spin dried, critical freeze dried, for example, all of which may be preceded by a rinse in another solution. 
     Post assembly lithography may be used to eliminate, e.g., ash, etch, or laser trim, unwanted assembly between dots. 
     Immediately prior to the application of an AC field, substrate  17  is cleaned, followed by a 20 minute soak in ethanol to remove oxidized Au. It should be understood that the amplitude of the AC bias, frequency and trapping time may vary, dependent upon the nature, desired size, and concentration of the one-dimensional nanostructures  22  and the dielectric environment in which the one-dimensional nanostructures  22  are contained. Placement time in this particular example is typically between 5 and 30 seconds. In principle, one may use a direct current (DC) field to trap one-dimensional nanostructures  22  in the gap, but such DC field is not the field of choice herein as use of a DC field will result in a success rate that is much lower as compared to an AC field. Under the influence of an AC field, one-dimensional nanostructures  22  experience a dielectrophoretic force that pulls them in the direction of maximum field gradient found in gap  24 . 
     Although only a few one-dimensional nanostructures  22  are shown, those skilled in the art understand that any number of one-dimensional nanostructures  22  could be formed. Once the one-dimensional nanostructures  22  are positioned between the electrode  12  and the first dot  28 , an electric field  32  will then form between the first dot  28  and the second dot  34  ( FIG. 4 ). This electric field  32  causes a migration, i.e., movement, of one-dimensional nanostructures  22  suspended within the aqueous environment  23  toward gap  24  between the first dot  28  and the closest (second) dot  34  where the field  32  and/or field gradient is the strongest. As the one-dimensional nanostructures attach to the first dot  28 , they will line up with the electric field  32 , thereby stretching out like a string from the first dot  28  to the second dot  34 . This process of forming an electric field  32  and placement of the one-dimensional nanostructures  22  will continue ( FIG. 5 ) until the last dot  20  in the sequence is reached. 
     The spiral inductor  26  fabricated by the embodiment described above provides, in a first application, an electrical inductor coupled between conductive electrode  12  and the last dot  20  in the pattern for use in RF integrated circuits. The inductor  26  comprises a network of one-dimensional nanostructures  22  instead of traditional metals such as copper or aluminum. In the preferred implementation, this inductor has lower resistance than traditional metal inductors and thus has a higher Q factor. Additionally, the carbon nanotube, being an example of a one-dimensional nanostructure, has the added benefit of being immune to skin effect. In traditional metals, as the frequency of operation increases, the effective thickness of the metal is reduced as current is crowded to the outermost shell of the metal line. This results in an increase in the effective resistance of the metal and thus degrades the Q factor. In a carbon nanotube, however, the current transport is already confined to the outermost shell of the tube and this should be frequency independent. 
     In addition to the inductor  26 , the one-dimensional nanostructures  22  may be grown, for example, for use in sensors or electronic circuits, or as conductive elements, in which case a one-dimensional nanostructure  22  will be grown from one conductor to another one-dimensional nanostructures  28  to form a electrical connection between conductors as shown in  FIG. 5 . 
     The process is further illustrated by the flow chart  40  in  FIG. 6  wherein a material  16  is provided  42  to form a substrate  17 . The material  16  may be coated  44  with an insulating material  18 . An electrode  12  is fabricated  46  on the surface of the substrate  17 . A plurality of dots  20  is fabricated  48  in a pattern on the substrate  17 . A solution  23  comprising one-dimensional nanostructures  22  is applied  50  to the electrode  12  and the plurality of dots  20 . An alternating current is applied  52  to the electrode  12  causing the one-dimensional nanostructures  22  to migrate to a position contiguous to the electrode  12  and first dot  28 . The alternating current remains applied until one-dimensional nanostructures  22  have been positioned between each of the plurality of dots in sequence. 
     While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.