Abstract:
A method of depositing elongated nanostructures that allows accurate positioning and orientation is described. The method involves printing or otherwise depositing elongated nanostructures in a carrier solution. The deposited droplets are also elongated, usually by patterning the surface upon which the droplets are deposited. As the droplet evaporates, the fluid flow within the droplets is controlled such that the nanostructures are deposited either at the edge of the elongated droplet or the center of the elongated droplet. The described deposition technique has particular application in forming the active region of a transistor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The current application is related to U.S. patent application Ser. No. 11/644,070, filed Dec. 22, 2006, entitled “Method For Aligning Elongated Nanostructures” which was filed on the same day and is hereby incorporated by reference. 
     BACKGROUND 
     Elongated nanostructures such as nanocrystals, nanotubes and nanowires have become increasingly important due to their interesting electrical and optical characteristics. Nanostructures comprise both inorganic materials, such as silicon, germanium, and gallium nitride, and organic materials, such as carbon nanotubes and semiconducting polymers. In particular, single-crystal nanowires and carbon nanotubes have proven useful for high-mobility transistors on a variety of substrates. 
     Typically, these devices are made by spin-coating liquids containing elongated nanostructures onto a substrate. Electron-beam lithography may be used to pattern the circuits using standard methods and thereby remove elongated nanostructures from select regions. The remaining elongated nanostructures typically have random orientation which may be undesirable for some applications. 
     In order to control orientation, U.S. Pat. No. 6,872,645 entitled Methods of Positioning and/or Orienting Nanostructures by Duan et al. describes using microfluidic channels to control nanowire orientation. Although the microfluidic channels achieve some level of patterning, it has been difficult experimentally to achieve arbitrary patterns with good registration of the nanowires. Furthermore, building three dimensional microfluidic systems substantially increases the complexity of device fabrication. 
     Another disadvantage of current deposition systems is that substantial nanostructure material is wasted. Both spin coating and microfluidic channels use substantially more nanostructure material than is incorporated into the final device, especially when areas that need nanostructures are highly localized. Nanostructure material is expensive. Substantial wasted nanostructure material makes the cost of forming large area devices, such as displays, using the described techniques prohibitively expensive. 
     Thus an improved method of depositing and orienting elongated nanostructures is needed. 
     SUMMARY 
     An improved method for forming a transistor is described. In the method, a dielectric substrate is formed, the dielectric substrate having a first surface that includes a fluid accumulation region between an area for a source electrode and an area for a drain electrode. A droplet of fluid is deposited on the dielectric substrate, the droplet including semiconductor nanowires suspended in a carrier fluid. The droplet is elongated such that upon evaporation of the carrier fluid, the nanowires are aligned to form the active region of the transistor between the area for a source electrode and the area for a drain electrode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a printhead ejecting a fluid droplet including elongated nanostructures. 
         FIG. 2  shows an elongated droplet including elongated nanostructures on a patterned substrate after printed deposition. 
         FIG. 3  shows an evaporating elongated droplet aligning elongated nanostructures at the droplet center. 
         FIG. 4  shows an elongated droplet including elongated nanostructures on a patterned substrate where the nanostructures agglomerate near the droplet edge. 
         FIG. 5  shows the deposition of elongated nanostructures which agglomerated near the droplet edge. 
         FIG. 6-12  show the fabrication of a transistor by printing nanowires in a carrier solution. 
         FIG. 13-14  show a variation of the method used in  FIG. 6-12  where the droplets agglomerate near the center of the transistor. 
         FIG. 16-18  show the process of forming an array of transistors using by printing droplets including semiconductor nanowires to form the transistor active region. 
         FIG. 18  shows the contact angle that results from one example of an expanding droplet. 
         FIG. 19  shows the contact angle that results from one example of a contracting droplet. 
         FIGS. 20-22  show evaporation of a droplet with a small contact angle hysteresis that results in deposition of suspended nanoparticles at the droplet center. 
         FIGS. 23-25  show evaporation of a pinned droplet with a small receding contact angle that results in edge deposition of suspended nanoparticles. 
         FIG. 26  shows combining two carrier fluids to induce a Marangoni effect. 
     
    
    
     DETAILED DESCRIPTION 
     A method of depositing, positioning and orienting elongated nanostructures is described. As used herein, “elongated nanostructures” is broadly defined to mean nanostructures (structures with a smallest dimension below 100 nanometers) that have a length substantially greater than a width. Typically, the length of an elongated nanostructure is at least five times the width. Examples of elongated nanostructures include, but are not limited to, nanowires, nanotubes and nanocrystals. The method involves printing droplets of a fluid that includes the nanostructures. The droplet position and shape are controlled to define the location and orientation of the deposited nanostructures. 
       FIG. 1  shows a printhead  104  ejecting a fluid droplet  108  including an elongated nanostructure  112  onto a substrate  116 . Fluid droplet  108  is a droplet of printable fluid solution that includes a stable suspension of nanostructures in a carrier fluid. The printable fluid should have lower viscosities to avoid printhead clogging. Examples of carrier solutions include, but are not limited to, water, organic solvents and hot-melt wax. Chemical methods may also be used to prevent nanostructures aggregation. One method of preventing aggregation is to derivative the elongated nanostructure surface with reactive chemicals such as organotrichlorosilanes, or organothiols that chemically bond to the nanostructure. Alternately, added chemicals such as surfactants or organic polymers, may non-covalently attach to the nanostructure surface preventing nanostructure aggregation. 
     The nanostructure concentration in a droplet varies according to the nanostructure concentration to be deposited in a unit area. An example droplet size may be approximately 100 pL (pico-liters), thus if 100 silicon nanowires were deposited in an area covered by a droplet, the solution concentration would be 100 nanowires per 100 pL of solvent. Assuming 10 nm diameter nanowires that are approximately 2 microns long, the approximate mass concentration might be 1-10 parts per million. 
     Various technologies may be used to eject droplet  108 . In one embodiment, printhead  104  is an inkjet print head that uses piezoelectrics to controllably eject droplet  108 . Acoustic nozzleless printheads are especially useful when depositing nanostructures with a length that exceeds 2 microns. The lack of a nozzle avoids the problem of nozzle clogging. Jet printing techniques enable precise alignment of droplets with already fabricated features. However, it should be understood that other printing techniques such as flexographic printing and grauvre printing may also be used. 
     Elongated structure orientation or “directionality” may be controlled by controlling the droplet shape after deposition.  FIG. 2  shows a top view of droplet  108  after deposition on substrate  116 . Substrate  116  has been treated to create a “fluid accumulation region” that causes droplet elongation. In one embodiment, the fluid accumulation region has been prepatterned via changing surface energies or changing topographies to cause droplet  108  elongation. In the illustrated embodiment, substrate  116  has been treated to produce hydrophobic regions  204 ,  208 . Region  212 , the fluid accumulation region, may be hydrophilic or simply less hydrophobic then regions  204 ,  208 . Thus droplet  108  elongates to minimize contact with the hydrophobic regions. In an alternate embodiment, surface height variations of substrate  116  may be used to induce droplet elongation. For example, regions  204 ,  208  may be fabricated slightly elevated compared to fluid accumulation region  212  creating a “trench” in region  212 . Capillary forces elongate droplet  108  as it spreads between the walls of the trench. 
     As the carrier solution evaporates, surface tension keeps the nanostructures within droplet  108 . Thus, as the elongated droplet slowly decreases in size, the elongated nanostructures gradually align to the shape of the elongated droplet.  FIG. 3  shows the gradual alignment of the aligned nanoparticles  304 ,  308 ,  312  in the decreased shrinking droplet  108 . The elongated nanostructures eventually precipitate from the fluid in an aligned state. 
     It is noted that the precipitation from the fluid is carefully controlled. In an alternate embodiment, a “coffee stain” or “edge deposition” effect can also be achieved. This effect is shown in  FIGS. 4-6 . In  FIGS. 4-6 , the lower density wires are transported to the long side of the droplet by mass flow in the drop due to evaporation as shown in  FIG. 4 . The long side of elongated nanostructure  404  tends to align along the long side of the droplet maximizing surface contact with droplet perimeter  408 . Higher frequency vibrations may be used to move the elongated nanostructures to align as desired. 
       FIG. 5  shows the resulting structure after evaporation. As the droplet  504  evaporates, the elongated nanostructures  404  fall out of solution early leaving them oriented along the original droplet perimeter. Thus as the droplet shrinks, the remaining droplet  504  contains only carrier solution. 
     The chemistry for selecting between the “edge deposition” effect and the deposition of nanostructures at the droplet center is shown in  FIGS. 18-25 . Typically, the “edge deposition” effect shown in  FIG. 20-22  occurs when a single solvent is used. The solvent is selected such that the droplet edge is pinned to a position on the substrate. Droplet pinning typically occurs for a fluid that has significant contact angle hysteresis as illustrated in  FIGS. 18 and 19 . 
       FIG. 18  shows the advancing contact angle  1800  when droplet  1804  is expanding.  FIG. 19  shows the receding contact angle  1900  when droplet  1804  is receding. The difference between the receding contact angle and the advancing contact angle is the contact angle hysteresis. A free contact line where the elongated nanostructures move to the droplet center during droplet evaporation typically occurs when the receding contact angle and the advancing contact angle are similar, typically differing by less than 10 degrees. 
       FIGS. 20-22  show the effect of moving an evaporating droplet  2000  that has a free contact line.  FIG. 20  shows the droplet upon initial deposition. As the droplet contracts, the droplet retains an approximately similar overall shape and the contact line  2004 , defined as the point at which the droplet perimeter edge  2008  contacts the underlying substrate  2012  also moves inward as the droplet evaporates.  FIG. 21  illustrates the contact line  2004  inward movement as the droplet evaporates. Surface tension pushes nanoparticles  2016  suspended in the droplet toward the droplet center until in  FIG. 22 , the amount of carrier fluid no longer suspends the nanoparticles and the nanoparticles fall out of solution. 
       FIG. 23-25  shows an example case in which the nanoparticles are deposited near the droplet perimeter creating an “edge deposition” effect. Droplet pinning produces the edge deposition effect.  FIG. 23  shows the droplet  2300  upon initial deposition,  FIG. 24  shows droplet  2300  at a later stage in evaporation and  FIG. 25  shows the droplet near a final stage where nanoparticles  2304  suspended in droplet  2300  that have accumulated near the droplet outer edge  2308  fall out of solution. 
     Droplet pinning that produces the “edge deposition” effect typically occurs when the receding contact angle  2408  is small, typically approaching zero degrees. This is most common in solutions that exhibit a large contact angle hysteresis, typically exceeding 10 degrees. In pinning solutions, regions of high curvature undergo much higher evaporation rates then other droplet regions. During pinning, the contact line  2320  where the edge of the droplet meets the substrate stays approximately fixed. The pinned droplet  2300  thus becomes fairly flat with the region of high curvature and thus the high evaporation rates occurring near droplet edge  2312 . High edge evaporation rates produces bulk fluid flow towards the droplet edge. The bulk fluid flow carries particles such as colloids, or nanoparticles  2304  to the droplet edge where the colloids or nanoparticles fall out of solution. Such an effect is described in an article entitled “Capillary flow as the cause of ring stains from dried liquid drops” by Deegan et al. Nature, Oct. 23, 1997, pp827 to 829 which is hereby incorporated by reference. 
     One method of avoiding pinning and assuring that elongated nanoparticles are deposited in the center of the droplet is to take advantage of Marangoni flows. If a printing fluid comprises two solvents with different vapor pressures and different surface tensions, during evaporation the mass flow in the drop will be complex.  FIG. 26  shows combining two carrier fluids to achieve a Marangoni flow. If first fluid  2604  with the highest vapor pressure, that is the fastest to evaporate, also has the highest surface energy, then the second fluid will flow to the edge of the drop and some of the first fluid will flow back towards the center of the drop. Examples of pairs of fluids include water and ethylene glycol or ethylacetate and acetophenone. Such a flow profile will prevent suspended nanostructures from being deposited at the edge of the drying drop as they will follow the mass flows of the fluids. Thus the two fluid mixture will maintain a more homogeneous distribution of nanostructures during drying. 
     The methods of orienting and depositing elongated nanostructures may be used to form various devices.  FIGS. 6-12  show using the “edge deposition” effect to print semiconductor nanowires used to form a transistor active region.  FIG. 6  shows a top view and  FIG. 7  shows a side view of gate lines  604 ,  606  deposited over a substrate  608 . Gate lines are typically made of a conducting material such as a metal deposited over a nonconducting substrate. A dielectric material  704  is deposited over the gate lines. The dielectric material over the gate lines may be patterned to elongate a droplet positioned in a region between gate lines. Thus in one embodiment, top surface  708  is treated such that regions directly over the gate lines are hydrophobic relative to other surface  708  regions. In an alternate embodiment, top surface  708  regions directly over the gate lines are slightly elevated compared to regions between gate lines. 
     A printer ejection mechanism such as that shown in  FIG. 1  ejects a droplet of solution containing semiconductor nanowires into the area between the two gate lines  604 .  FIG. 8  shows a top view of the resulting structure and  FIG. 9  shows a side view. In  FIGS. 8 and 9 , droplet  804  center approximately aligns to the middle region  808  between gate lines  604 ,  606 . Droplet  804  is positioned such that the droplet outer edge is approximately in the center of gate lines  604 ,  606  as shown. Surface treatments of dielectric top surface  708  elongates droplet  804 . Initially, the nanowires  812  are somewhat randomly distributed through the droplet. With some agitation of the droplet, the nanowires gradually accumulate near the long edges  816 ,  820  of the droplet. 
       FIG. 10  shows a top view and  FIG. 11  shows a side view of nanowires  812  deposited using the “edge deposition” effect. Outline  1004  shows the approximate original outer perimeter of droplet  804  and outline  1008  shows the droplet perimeter at a later point in time after a substantial portion of the droplet has evaporated. Once droplet  804  is reduced to outline  1008 , no nanostructures remain in the droplet. 
     In the illustrated embodiment, the semiconductor nanowires form a transistor active region between a source and a drain electrode. Known processing techniques including wax resist printing may be used to form the source and drain electrode.  FIG. 12  shows a source  1204  and a drain  1208  electrode interconnected by an active region that includes semiconductor nanowires  1212  and  1216 . A voltage on a gate electrode underneath the nanowires controls current flow between the source and drain. 
     Although the structures of  FIGS. 6-12  were formed using an “edge deposition” effect, the same structures may be formed using a carrier that deposits the nanowires at approximately the droplet center as shown in  FIGS. 13-14 .  FIG. 13  shows a side view and  FIG. 14  shows a top view of a droplet deposited in the center of gate lines  1304 ,  1308 . In  FIG. 14 , hydrophobic regions  1404  is the area of dielectric  1312  top surface positioned between adjacent gate lines. The hydrophobicity results in elongated droplets positioned over gate lines  1304 ,  1308 . 
       FIGS. 15-17  show fabricating transistor arrays using the described techniques.  FIG. 15  shows one example of printing a nanowire  1504  array. A dielectric layer is patterned such that a hydrophilic, that is wettable by the printing fluid, or a depressed region  1508  runs approximately perpendicular to gate lines  1512 . The adjacent regions to region  1508  are less wettable by the printing fluid or are raised relative to region  1508 . Droplets containing nanowires  1504  are deposited on region  1508  of the patterned layer. The hydrophobic region edge approximately defines a long side of a deposited droplet as shown in  FIG. 16 . 
     The hydrophillic region edge runs approximately perpendicular to the gate lines  1624 . In  FIG. 16 , the nanowires  1604  align with the hydrophillic region  1609  edge and are thus also perpendicularly oriented with respect to the underlying array of gate lines  1624 . “Edge deposition” droplets may be used. When edge deposition is used, the droplet  1616  is centered with respect to the hydrophilic stripe  1609  and aligned to the gate line  1624 . During drying the nanowires deposit at the edge of the droplet  1616  near the border between hydrophilic region  1609  and the surrounding region  1610 . The nanowires from a single droplet can subsequently be used to form a transistor or nanowires from nearby drops can be used together such as shown in region  1612 . 
       FIG. 17  shows subsequent deposition of source lines  1704  and drain lines  1708  to form a transistor array. In one embodiment, the shortest distance between the source and drain lines is typically less than a nanowire length. When fabricating a display or sensor using the described technique, the linewidth of the printed nanowire solution approximates the pixel pitch. Thus the typical pixel pitch for such structures is between 100 and 500 microns. 
     From  FIG. 17 , it can also be seen that the nanowires should be positioned to avoid shorting a transistor source electrode to the drain electrode of an adjacent transistor. Thus the droplets containing the nanowires should be carefully positioned to avoid droplet centers at the areas between gate lines such as region  1620  that separates adjacent transistors. 
     Although various details have been provided in this description, including types of nanostructures, devices that can be made, various surface treatments and various printing techniques, the invention should not be limited to those details. Instead, the invention should be described by the claims, as originally presented and as they may be amended, and as they encompass variations, alternatives, modifications, improvements, equivalents, and substantial equivalents of the embodiments and teachings disclosed herein, including those that are presently unforeseen or unappreciated, and that, for example, may arise from applicants/patentees and others.