Abstract:
The present invention generally relates to circuits on the nanotechnology scale. Specifically, it is directed to methods of fabricating carbon nanotube-based (i.e., CNT-based) circuits. The method involves providing a mixture of carbon nanotubes that is substantially disaggregated and patterning carbon nanotubes through the use of electrostatic forces. Carbon nanotubes in the mixture are typically disaggregated through the introduction of positive charge on the individual nanotubes. The patterning of the carbon nanotubes is typically accomplished using electrostatic attraction between pre-formed metal lines and the charged carbon nanotubes.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to circuits on the nanotechnology scale. Specifically, it is directed to methods of fabricating carbon nanotube-based (i.e., CNT-based) circuits. 
     BACKGROUND OF THE INVENTION 
     Nanotechnology involves the creation of materials by methodically organizing and manipulating matter on a scale of several to one hundred or more nanometers. Such materials, which include components, devices and systems, are oftentimes desirable and exploitable due to the novel properties that result from manipulation at that scale. 
     The focus of nanotechnology is fundamentally different from that of a related field—micro-technology. Micro-technology has been primarily directed to taking macro-scale devices and making them smaller. Examples of micro-technology efforts include the translation of vacuum tubes into solid state transistors and the conversion of electrical-mechanical systems into MEMS (i.e., Micro ElectroMechanical Systems). This direction is typically referred to as the “top-down” approach. Miniaturization in nanotechnology, in contrast, is a distinct but secondary, advantageous result. Nanotechnology is a “bottom-up” approach: Materials are tailored at the molecular or atomic level, which results in dramatic and systematic property changes; ones that have never been seen or even thought of at the macro scale. 
     One can also vary fundamental material properties (e.g., melting temperature, hardness and light dispersion) without changing chemical composition or molecular structure through patterning matter on the nanometer scale. This can allow the production of materials with completely novel specifications and characteristics. Such characteristics should provide for alternative approaches in the production of smaller, lighter, and cheaper devices with better functionality. 
     A variety of nanomaterials and nanotechnologies (e.g., nanocomposites, nanocrystals, nanoparticles, nanostructured materials, nanotubes, nanocatalysts and nanofilters) have been examined with a host of applications in mind. Envisioned applications include: giant magnetoresistance with nanocrystalline materials; nanolayers with selective optical barriers or hard coatings; chemical and biological sensors; advanced drug delivery systems; chemical-mechanical polishing with nanoparticle slurries; new generations of lasers; nanostructured catalysts; systems on a chip; thermal barriers; and, recording media. 
     In the area of microelectronics and computers, the fabrication of nanocircuits could significantly reduce routing length and dramatically increase computing capability and data speed. This is due to closer distances in both the planar dimension and layer to layer height. Shortening routing length always decreases electric resistance, as described by Ohm&#39;s Law. Unfortunately, decreasing wire diameter always increases electric resistance. This makes the construction of nanocircuits using ordinary wiring materials impractical. 
     There is accordingly a need for new materials making and novel methods for producing nanocircuits. That is an object of the present invention. 
     SUMMARY OF THE INVENTION 
     The present invention generally relates to circuits on the nanotechnology scale. Specifically, it is directed to methods of fabricating carbon nanotube-based (i.e., CNT-based) circuits. 
     The method involves providing a mixture of carbon nanotubes that is substantially disaggregated and patterning carbon nanotubes through the use of electrostatic forces. 
     The carbon nanotubes in the mixture are typically disaggregated through the introduction of positive charge on the individual nanotubes. Positive charge may be introduced in a variety of ways, but it is typically produced by exposing the nanotubes to an electrolyte solution having an suitably acidic pH. 
     The patterning of the carbon nanotubes is typically accomplished using electrostatic attraction between pre-formed metal lines and the charged carbon nanotubes. Pre-formed metal lines may either be continuous or discontinous (i.e., segmented). 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  shows an aggregation/disaggregation process for carbon nanotubes. 
         FIG. 2  shows and aggregation/disaggregation process for positively charged carbon nanotubes. 
         FIG. 3  shows the attraction between, and subsequent combination of, a charged carbon nanotube and a metal segment to which electric bias has been applied. 
         FIG. 4A  shows the patterning of charged carbon nanotubes along a continuous metal line. 
         FIGS. 4B and 4C  show the patterning of charged carbon nanotubes along metal segments using the application of a sweeping bias. 
         FIG. 5A  shows carbon nanotube/wire segment combinations containing a gap between them. 
         FIG. 5B  shows the gap of  FIG. 5A  which has been filed with a conducting material or bridge. 
         FIG. 5C  shows the structure of  FIG. 5B  which has been further filled with metal that is connecting the two charged nanotubes. 
         FIG. 6  shows a method of forming a nanocircuit using charged nanotubes. 
         FIG. 7  shows a method of forming a nanocircuit using charged nanotubes, where the method involves bridging the gaps between nanotube/wire segment combinations. 
     
    
    
     DETAILED DESCRIPTION 
     Carbon nanotubes (i.e., CNTs) are widely studied nanomaterials that exhibit excellent properties for various applications. In contrast to metal nanowires, CNTs can exhibit good electrical conductivity even in the nanoscale. This important phenomenon is illustrated if one views the CNTs as graphite laminates rolled into tubular shapes. The conduction of electric current is still in the planar dimension of the graphite laminates (i.e., the longitudinal direction of the carbon nanotubes). Shrinking the diameter of the nanotube to the nanoscale accordingly has little to no affect on conduction. Processing CNT connections using traditional methodologies, however, has proven difficult. 
     CNTs are usually fabricated in the vapor phase using methods such as electric arc discharge, laser vaporization, and chemical vapor deposition (i.e., CVD). Like graphite, CNTs are intrinsically uncharged. This physical property, along with the other molecular features, causes CNTs to aggregate and tangle together into “ropes” due to strong van der Waals forces between the molecules.  FIG. 1  illustrates such aggregation. Individual CNTs ( 101 ) aggregate to form a CNT bundle ( 102 ). Bundle  102  is in equilibrium with a smaller CNT bundle ( 103 ) and individual CNT  101 . The equilibrium lies far toward bundle  102  due to the strong attractive forces between the molecules. 
     The present invention, at least in its first step, provides a method to substantially disaggregate CNTs in a mixture (e.g., solution or suspension). This process is illustrated by  FIG. 2 . By creating like-charged CNTs, the generated repulsive force overcomes the attractive van der Waal forces at play. For instance, aggregate  201  is in equilibrium with individual CNTs  202 . The aggregation equilibrium, represented by the arrows in  FIG. 2  is shifted to individual CNTs  202  because the mutually positively charged species repel one another. 
     Charged CNTs are typically generated by immersing CNTs in a solution that is sufficiently acidic such that charged CNTs result. The degree of charge will depend on the exact characteristics of the solution employed. Where the degree of charge affords a repulsive force that can overcome van der Waals attraction, then CNT disaggregation will occur. Should the repulsive force only partially overcome the existing molecular attraction, then a mixture of CNT bundles will be formed. Typically, either individual charged CNTs or relatively small CNT bundles can be used in the present invention. 
     The solutions in which the CNT immersion occurs include a solvent and at least one acidic element. Nonlimiting examples of solvents include organic solvents, inorganic solvents and water (e.g., electrolyte solution). Acidic elements present in the solution may be organic or inorganic acids that are compatible with the chosen solvent. 
     Once the CNTs are immersed in the appropriate solution, reaction to produce charged species is facilitated by agitating the mixture. Agitation may be produced in a variety of ways, including, for example, shaking a closed container, stirring through the using of mechanical means, or by forcing a gas through the mixture. 
     To construct a nanocircuit, the charged, disaggregated CNTs must be patterned. Patterning is achieved using electrostatic attraction between pre-formed metal lines and the charged CNTs. Advanced lithography may be used to produce metal lines on a substrate having a width in the tens of nanometers. Electric bias is applied to the metal lines, which causes the charged CNTs to align on top of them. This process is illustrated in  FIG. 3 . Disaggregated, charged CNT  301  is brought in proximity to metal line  302 , to which electric bias has been applied. The negatively charged wire ( 302 ) attracts CNT  301 . This attraction results in contact product  303 , a portion of the patterned circuit. 
     The extension of the process shown in  FIG. 3  is deposition of charged CNTs along a continous metal line. This is shown as  401  in  FIG. 4A . One may also employ a sweeping bias that is applied in a cyclic manner through wire segments, as shown in  FIGS. 4B and 4C . Individual CNTs ( 404 ) are sequentially attracted to wire segments  403  to produce combination products  402 . Voltage bias, cyclic time, and waveform are suitably controlled to achieve orderly placement of CNTs on metal traces. 
     When the metal segments are populated with CNTs, the patterning is done. The metal segments may stay underneath the CNTs. Alternatively, the patterned CNTs may be transferred to a second substrate where the actual circuits will be. In such a way, the metal segments on the first substrate can be used repeatedly as a template many times; the second substrate only contains CNTs, with no underlying metal segments. 
     There may be instances where gaps will occur between deposited CNTs. This is illustrated in  FIG. 5A . CNT/segment combinations  501  are in proximity, but they are not touching. Should the gap be wide enough, electrical conduction could be detrimentally affected. One approach to filling the gap involves inserting a conducting bridge ( 502 ,  FIG. 5B ) between the wire segments. Bridges may be of any conducting material, including, but not limited to, conductive polymers, conducting adhesive with metal filling, and deposited metals. Where bridge  502  is a metal, it is typically produced through electodeposition onto a pattern of nano-wire segments. Alternatively, bridge  502  may be a tiny metal dot placed between the segments using nanoscale lithography. 
     Where bridges  502  are employed, only the metal segments are biased during CNT deposition. Once CNT deposition is complete, the CNT suspension is replaced with an electrolyte containing ions (e.g., copper or nickel ions) for electrodeposition. Electrodeposition provides electroplated “bumps” ( 503 ) that serve to connect the gapped CNTS (see  FIG. 5C ). 
     The methods of the present invention are further described in reference to  FIGS. 6 and 7 . Scheme  600  illustrates one embodiment. Carbon nanotubes are immersed in an acidic solution, step  601 , which provides positively charged CNTs. The positively charged CNTs are brought into contact with a patterned substrate in step  602  to provide a patterned substrate coated with a CNT suspension. An electric bias is applied to the patterned substrate in step  603 , which causes the charged CNTs to align overtop the patterned nanowires. If the electric bias is simply removed, as in step  604 , a nanocircuit is provided. Alternatively, the electric bias may be removed to provide a template in step  605 , which produces a nanocircuit after transfer of the patterned CNTs (step  606 ). 
     Scheme  700  of  FIG. 7  illustrates a modified process according to the present invention. Carbon nanotubes are immersed in an acidic solution (step  601 ) to provide charged CNTs. The charged CNTs are brought into contact with a patterned substrate in step  702 . The substrates of the patterned substrate were previously filled with a conducting material (step  703 ). An electric bias is applied to the filled-patterned substrate in step  704 , which causes the charged CNTs to align overtop the patterned nanowires. The CNT suspension is replaced with an ion containing electrolyte solution and electrodeposition is performed (step  705 ). The electric bias is removed in step  706 , accordingly producing a nanocircuit.