Abstract:
A method of fabricating a nanotube structure which includes providing a substrate, providing a mask region positioned on the substrate, patterning and etching through the mask region to form at least one trench, depositing a conductive material layer within the at least one trench, depositing a solvent based nanoparticle catalyst onto the conductive material layer within the at least one trench, removing the mask region and subsequent layers grown thereon using a lift-off process, and forming at least one nanotube electrically connected to the conductive material layer using chemical vapor deposition with a methane precursor.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates generally to the fabrication of nanotube based electronic devices, and, more particularly, to the fabrication of electronic devices using nanotubes grown in a reaction chamber using a solvent based catalyst.  
         BACKGROUND OF THE INVENTION  
         [0002]    Carbon is one of the most important known elements and can be combined with oxygen, hydrogen, nitrogen and the like. Carbon has four known unique crystalline structures including diamond, graphite, fullerene and carbon nanotubes. In particular, carbon nanotubes refer to a helical tubular structure grown with a single wall or multi-wall, which can be obtained by rolling a sheet formed of a plurality of hexagons. The sheet is formed by combining each carbon atom thereof with three neighboring carbon atoms to form a helical tube. Carbon nanotubes typically have a diameter in the order of a fraction of a nanometer to a few hundred nanometers.  
           [0003]    Carbon nanotubes can function as either a conductor, like metals, or a semiconductor, according to the rolled shape and the diameter of the helical tubes. With metallic nanotubes, it has been found that 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. Introducing a defect into a metallic tube can result in a single electron charging effect. The single electron charging effect can be used to make a single electron transistor. When semiconductor nanotubes are connected to two metal electrodes, the structure can function as a field effect transistor wherein the nanotubes can be switched from a conducting to an insulating state by applying a voltage to a gate electrode. Therefore, carbon nanotubes are potential building blocks for nanoelectronic devices because of their unique structural, physical, and chemical properties.  
           [0004]    Existing methods for the production of nanotubes, including arc-discharge and laser ablation techniques, yield bulk materials with tangled nanotubes. The nanotubes in the bulk materials are mostly in bundled forms. These tangled nanotubes are extremely difficult to purify, isolate, manipulate, and use as discrete elements for making functional devices. Originally, carbon nanotubes produced by an arc discharge between two graphite rods was discovered and reported in an article entitled “Helical Microtubules of Graphitic Carbon” (Nature, Vol. 354, Nov. 7, 1991, pp. 56-58) by Sumio Iijima. This technique is commonly used to produce carbon nanotubes, however, yield of pure carbon nanotubes with respect to the end product is only about 15%. Thus, a complicated purification process must be carried out for particular device applications.  
           [0005]    Another conventional approach to produce carbon nanotubes, which was described in an article entitled “Epitaxial Carbon Nanotube Film Self-organized by Sublimation Decomposition of Silicon Carbide” (Appl. Phys. Lett. Vol. 71, pp. 2620, 1977), by Michiko Kusunoki, is to produce carbon nanotubes at high temperatures by irradiating a laser onto graphite or silicon carbide. In this case, the carbon nanotubes are produced from graphite at about 1200° C. or more and from silicon carbide at about 1600° C. to 1700° C. However, this method also requires multiple stages of purification which increases the cost. In addition, this method has difficulties for large-device applications.  
           [0006]    Some of the drawbacks of these two methods are that the tubes are formed under an extremely high temperature environment and are usually produced as bundles, embedded with catalyst particles which are covered with amorphous carbon. To fabricate devices using nanotubes produced from these methods, various cleaning and debundling steps are required. The debundled nanotubes are then suspended in a solution, which can then be positioned on a substrate with patterned electrodes or other circuitry. However, it is extremely difficult to control the placement and orientation of the nanotubes when using these methods. It is therefore very inefficient to fabricate electronic devices using nanotubes formed either by arc discharge or laser ablation.  
           [0007]    U.S. Pat. No. 6,346,189 issued to Dai et al. on Feb. 12, 2002 discloses a method of selectively producing high quality single walled carbon nanotubes on a substrate using catalyst islands. The catalyst particles consisting of Fe 2 O 3  or other transition metal oxides are suspended in methanol. According to the method, a first lithography step is used to pattern a substrate with catalyst islands, wherein the first lithography step uses e-beam lithography. Nanotubes are then grown using a chemical vapor deposition process. Electrical contact to the nanotubes is made by performing a second lithography step to form electrodes. However, during the second lithography step, the nanotubes may be damaged and contaminated.  
           [0008]    Accordingly, it is an object of the present invention to provide a new and improved approach for fabricating nanotube based electronic devices.  
         SUMMARY OF THE INVENTION  
         [0009]    To achieve the objects and advantages specified above and others, a method of fabricating a nanotube structure is disclosed which includes providing a substrate, providing a mask region positioned on the substrate, and patterning and etching through the mask region to form at least one trench. A conductive material layer is deposited within the at least one trench and a nanoparticle catalyst is deposited onto the conductive material layer within the at least one trench. The mask region is removed using a conventional lift-off technique, such as a single step lift-off process, to form nanoparticle catalyst coated electrodes. The nanotubes are formed from the catalyst using a reaction chamber with a hydrocarbon gas atmosphere. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    The foregoing and further and more specific objects and advantages of the instant invention will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment thereof taken in conjunction with the following drawings:  
         [0011]    [0011]FIG. 1 is a cross sectional view of a step in a sequence of fabricating a nanotube structure;  
         [0012]    [0012]FIG. 2 is a cross sectional view of another step in the sequence of fabricating a nanotube structure;  
         [0013]    [0013]FIG. 3 is a cross sectional view of still yet another step in the sequence of fabricating a nanotube structure;  
         [0014]    [0014]FIG. 4 is a cross sectional view of a step in the sequence of fabricating a nanotube structure;  
         [0015]    [0015]FIG. 5 is a cross sectional view of another step in the sequence of fabricating a nanotube structure; and  
         [0016]    [0016]FIG. 6 is a cross sectional view of still yet another step in the sequence of fabricating a nanotube structure. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0017]    Turn now to FIG. 1 which illustrates a step in a method to fabricate a nanotube structure  5  in accordance with the present invention. In the preferred embodiment, nanotube structure  5  includes a substrate  10  wherein substrate  10  includes silicon. However, it will be understood that other substrate materials may be appropriate, such as a glass, a ceramic, a metal, or other semiconductor materials. Other semiconductor materials can include, for example, gallium arsenide (GaAs) or the like. Further, substrate  10  can include control electronics or other circuitry, which are not shown in this embodiment for simplicity. Also, substrate  10  can include an insulating layer, such as silicon oxide (SiO), silicon nitride (SiN), or the like.  
         [0018]    A mask region  13  is positioned on surface  11  of substrate  10 . In the preferred embodiment, mask region  13  includes a bi-layer resist of a photoresist layer  14  positioned on surface  11  and a photoresist layer  14  positioned on layer  12 . A bi-layer resist is used in the preferred embodiment to facilitate the lift-off process, as will be discussed separately.  
         [0019]    As illustrated in FIG. 2, mask region  13  is patterned and etched through layers  12  and  14  to form at least one trench. In the preferred embodiment, a trench  15  and a trench  17  are formed within mask region  13 , but it will be understood that it is anticipated that an array of trenches could be formed therewith. In this embodiment, two trenches are illustrated for simplicity and ease of discussion. Further, mask region  13  can be patterned using optical lithography, e-beam lithography, or other techniques well known to those skilled in the art.  
         [0020]    Turning now to FIG. 3, a conductive material layer  18  is deposited on surface  11  within trench  15  and a conductive material layer  20  is deposited on surface  11  within trench  17 . Further, it is anticipated that a conductive material layer  16  will be formed on mask region  13  as illustrated. In the preferred embodiment, layers  16 ,  18 , and  20  include gold (Au), but it will be understood that other conductive materials, such as aluminum (Al), platinum (Pt), silver (Ag), copper (Cu), or the like, may be used.  
         [0021]    Further, in the preferred embodiment, layers  16 ,  18 , and  20  are illustrated to include the same conductive material for simplicity, but it will be understood that they can include different conductive materials. For example, layer  18  can include gold (Au), layer  16  can include aluminum (Al), and layer  20  can include platinum (Pt) wherein it will be understood that the fabrication sequence would be, in general, different from the preferred embodiment. However, the differences are well known to those skilled in the art and will not be elaborated upon further here.  
         [0022]    Turning now to FIG. 4, a solution containing nanoparticle catalyst  22  is deposited on conductive material layer  18 , a nanoparticle catalyst  24  is deposited on conductive material layer  16 , and a nanoparticle catalyst  26  is deposited on conductive material layer  20 . Nanoparticle catalysts  22 ,  24 , and  26  include nanoparticles suspended within the solvent which is compatible with the material included in mask region  13 . In the preferred embodiment, the nanoparticles can include a transition metal, such as iron (Fe), nickel (Ni), cobalt (Co), or the like, or another suitable nanoparticle catalyst well known to those skilled in the art. Further, catalysts  22 ,  24 , and  26  can be deposited by several methods including spraying on, spinning on, or the like, which are well known to those skilled in the art.  
         [0023]    Turning now to FIG. 5, a lift-off process is performed to remove mask region  13  from substrate  10 . Further, conductive material layer  16  with catalyst particles  24  thereon is also removed during the liftoff.  
         [0024]    Turning now to FIG. 6, nanotube structure  5  is placed in a reaction chamber with a hydrocarbon gas atmosphere to form at least one nanotube  28 . The reaction chamber can include a chemical vapor deposition chamber, a chemical beam epitaxy chamber, a molecular beam epitaxy chamber, or the like. Further, in the preferred embodiment, the hydrocarbon gas atmosphere includes methane. However, it will be understood that the hydrocarbon gas atmosphere can include other gases, such as ethylene, acetylene, carbon monoxide, or the like. In the preferred embodiment, a single nanotube  28  is illustrated, but it will be understood that a plurality of nanotubes could be formed and electrically connect layers  18  and  20 . However, a single nanotube is illustrated in this embodiment for simplicity and ease of discussion.  
         [0025]    In the preferred embodiment, nanotube  28  is a carbon nanotube, but it will be understood that nanotube  28  can include other nanotube forming materials, such as boron nitride (BN), with the desired electrical and physical properties. In the preferred embodiment, an end  27  of nanotube  28  is electrically connected to conductive material layer  18  and an end  29  is electrically connected to conductive material layer  20 . During the formation of nanotube  28 , an electric field may be applied between layers  18  and  20  to align nanotube  28  in a preferred direction and facilitate the electrical connection therewith.  
         [0026]    Thus, a new and improved method of fabricating a nanotube structure has been disclosed. The method involves using a single step patterning process which simplifies the fabrication process. A bi-layer resist patterning process is used to facilitate the lift off process and reduce residual catalyst particles that may be present in undesired regions when using a single resist layer. The method also involves using a solvent, which in the preferred embodiment is water (H 2 O), to suspend the nanoparticle catalyst. Water is a solvent that is compatible with most resist material included in mask region  13 , and, therefore, eliminates pattern deformation caused by the reaction between an organic solvent and mask region  13 . Another important aspect of this method is that contamination of the nanotubes is minimized. Further, the nanotubes are less likely to be damaged. Contamination and damage typically occur during a post nanotube growth patterning process. Further, this method allows the alignment of the nanotubes with an electric field during chemical vapor deposition processing.  
         [0027]    Various changes and modifications to the embodiments herein chosen for purposes of illustration will readily occur to those skilled in the art. To the extent that such modifications and variations do not depart from the spirit of the invention, they are intended to be included within the scope thereof which is assessed only by a fair interpretation of the following claims.