Abstract:
The present invention includes several nanotube structures which can be made using catalyst islands disposed on a substrate (e.g. silicon, alumina, or quartz) or on the free end of an atomic force microscope cantilever. The catalyst islands are capable of catalyzing the growth of carbon nanotubes from carbon containing gases (e.g. methane). The present invention includes an island of catalyst material (such as Fe 2 O 3 ) disposed on the substrate with a carbon nanotube extending from the island. Also included in the present invention is a pair of islands with a nanotube extending between the islands, electrically connecting them. Conductive metal lines connected to the islands (which may be a few microns on a side) allows for external circuitry to connect to the nanotube. Such a structure can be used in many different electronic and microelectromechanical devices. For example, a nanotube connected between two islands can function as a resonator if the substrate beneath the nanotube is etched away. Also, the present invention includes a catalyst particle disposed on the free end of an AFM cantilever and having a nanotube extending from the particle. The nanotube can be used as the scanning tip of the AFM as is know in the art.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to the fabrication of nanotubes, and in particular to methods of fabricating nanotube structures from an array of catalyst islands on a semiconductor surface. 
     BACKGROUND OF THE INVENTION 
     Carbon nanotubes are recently discovered, hollow graphite tubules. When isolated, individual nanotubes are useful for making microscopic electrical, mechanical, or electromechanical devices. Obtaining individual, high quality, single-walled nanotubes has proven to be a difficult task, however. 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. 
     One conventional method for producing carbon nanotubes is disclosed in U.S. Pat. No. 5,482,601 issued to Oshima et al. on Jan. 9, 1996. The nanotubes are produced by successively repositioning a rod-like, carbon anode relative to a cathode surface such that a tip of the anode successively faces different portions of the cathode surface. A direct current voltage is impressed between the tip of the anode and the cathode surface so that an arc discharge occurs with the simultaneous formation of carbonaceous deposits containing carbon nanotubes on the cathode surface. The carbonaceous deposits are scraped and collected. 
     U.S. Pat. No. 5,500,200 issued to Mandeville et al. on Mar. 19, 1996 discloses a method for the bulk production of multi-walled tubes. According to the method, a catalyst is prepared using particles of fumed alumina with an average particle size of about 100 Å. Iron acetylacetonate is deposited on the alumina particles, and the resultant catalyst particles are heated in a hydrogen/ethylene atmosphere. The catalyst particles are preferably reacted with the hydrogen/ethylene mixture for about 0.5 hours in a reactor tube, after which the reactor tube is allowed to cool to room temperature under a flow of argon. Harvesting of the carbon tubes so produced showed a yield greater than 30 times the weight of the iron in the catalyst particles. 
     Although the methods described by Oshima and Mandeville are effective for producing bulk amounts of carbon tubes or carbon fibrils, the resulting bulk materials are “hairballs” containing tangled and kinked tubes which one collects into vials or containers. These bulk materials are useful to put into polymers or metals to make composites that exhibit improved properties of the polymers or metals. For making functional microscopic devices, however, these bulk materials are nearly useless because it is nearly impossible to isolate one individual tube from the tangled material, manipulate the tube, and construct a functional device using that one tube. Also, many of the tubes have molecular-level structural defects which results in weaker tubes with poor electrical characteristics. 
     Atomic force microscopes (AFMs) sometimes employ nanotubes as the scanning tip because nanotubes are resilient and have an atomically sharp tip. However, the manufacturing of nanotube-tipped AFM devices is problematic because the nanotubes must be painstakingly separated from disorganized bundles of nanotubes and attached to the AFM cantilever. It would be an advance in the art of atomic force microscopy to provide a nanotube-tipped AFM device that is simpler to manufacture. 
     OBJECTS AND ADVANTAGES OF THE INVENTION 
     In view of the above, it is an object of the present invention to provide a method for the large scale synthesis of individual distinct single-walled nanotubes. In particular, it is an object of the present invention to provide such a method which allows nanotube growth to be confined to desired locations so that the nanotubes can be easily addressed and integrated into structures to obtain functional microscopic devices. It is a further object of the invention to provide a method for integrating the nanotubes into semiconductor microstructures to obtain a variety of nanotube devices. Further, it is an object of the present invention to provide a nanotube-tipped atomic force microscope device which is simple to manufacture. 
     These and other objects and advantages will become more apparent after consideration of the ensuing description and the accompanying drawings. 
     SUMMARY 
     These objects and advantages are provided by an apparatus including a substrate and a catalyst island disposed on the substrate. The catalyst island includes a catalyst particle that is capable of growing carbon nanotubes when exposed to a hydrocarbon gas at elevated temperatures. A carbon nanotube extends from the catalyst particle. The nanotube may be in contact with a top surface of the substrate. 
     The substrate may be made of silicon, alumina, quartz, silicon oxide or silicon nitride. The nanotube may be a single-walled nanotube. The catalyst may include Fe 2 O 3  or other catalyst materials including molybdenum, cobalt, nickel, or zinc and oxides thereof (iron molybdenum, and ruthenium oxides are preferred). The catalyst island is preferably about 1-5 microns in size. 
     The present invention also includes an apparatus having a substrate with two catalyst islands and a nanotube extending between the islands. The nanotube provides an electrical connection between the islands, which are electrically conductive. Conductive lines can provide electrical connections to the islands and nanotube. The nanotube may be freestanding above the substrate. A freestanding nanotube can be used as a high frequency, high-Q resonator. 
     Alternatively, one of the islands is replaced with a metal pad that does not have catalytic properties. 
     The present invention also includes an atomic force microscopy apparatus that has a catalyst particle disposed on a free end of a cantilever. A nanotube extends from the catalyst particle. The nanotube can be used as the scanning tip of the atomic force microscope apparatus. 
     The present invention also includes a method of making individually distinct nanotubes on a substrate surface. The method begins with disposing catalyst islands on the surface of a substrate. Then, the catalyst islands are contacted with methane gas at elevated temperature. The nanotubes grown are separate and extend over the surface of the substrate. The separate and distinct nanotubes can be incorporated into microelectronic or microelectromechanical devices. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a first step in making nanotubes according to the present invention. 
     FIG. 2 shows a second step in making nanotubes according to the present invention. 
     FIG. 3 shows a third step in making nanotubes according to the present invention. 
     FIG. 4 shows a top view of a substrate with three catalyst islands. 
     FIG. 5 shows a closeup top view of a single catalyst island which has been used to grow nanotubes. 
     FIG. 6 shows an apparatus according to the present invention which has a nanotube connected between a catalyst island and a metal pad. 
     FIG. 7 shows a preferred embodiment of the present invention in which metal covers are disposed on top of the catalyst islands and portions of the nanotubes. 
     FIG. 8A-8C illustrate how the metal covers of FIG. 7 can be made. 
     FIG. 9 shows a side view of a resonator according to the present invention made from a freestanding nanotube supported by the ends of the nanotube. 
     FIG. 10 shows a top view illustrating how the apparatus of FIG. 9 can be made. 
     FIGS. 11A and 11B illustrate an alternative method of making the apparatus of FIG.  9 . 
     FIG. 12 shows an atomic force microscope tip made according to the present invention. 
     FIGS. 13A-13D illustrate a method of producing a carbon nanotube on a tip of an atomic force microscope cantilever according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a first step in a method of the present invention for making individual carbon nanotubes which are individually separable and distinct. A layer of resist  20  is disposed and patterned on a top surface of a substrate  22 . Patterning can be performed by e-beam lithography. The substrate  22  can be made of silicon, alumina, quartz, silicon oxide or silicon nitride for example. The substrate can also have a metal film on top. 
     The patterned resist  20  has holes  24  which expose the underlying substrate  22 . The holes  24  are about 3-5 microns in size and spaced apart by a distance  26  of about 10 microns. The resist may have a single hole or many holes  24 . 
     Next, in FIG. 2, a solution of Fe(NO 3 ) 3  in methanol, mixed with alumina nanoparticles (about 15-30 nanometers in size, for example) is deposited on the surfaces of the resist  20  and substrate  22 . In a specific example, catalyst preparation includes mixing 4.0 grams of alumina nanoparticles with 1.0 gram of Fe(NO 3 ) 3 *9H 2 O in 30 mL methanol for 24 hours. After applying the mixture to the substrate, the solvent (i.e. methanol) is evaporated, leaving alumina nanoparticles coated with metal salt (i.e. Fe(NO 3 ) 3 )  28  adhering to the resist and in the holes  24 . Next, in FIG. 3, a lift-off process is performed, leaving isolated (nonconnected) islands  29  of Fe(NO 3 ) 3 -coated nanoparticles adhering in regions where holes  24  existed. FIG. 4 shows a top view of the islands  29 . 
     Heating the substrate  22  and nanoparticles decomposes the Fe(NO 3 ) 3  to Fe 2 O 3 . This is performed by placing the substrate in a furnace with an Argon atmosphere and heating to about 100-400° Celsius. The Fe 2 O 3 /nanoparticle mixture is an active catalyst which will catalyze the formation of carbon nanotubes when exposed to methane gas at elevated temperature. 
     Growth of single-walled nanotubes is performed by heating the substrate with catalyst islands in the furnace at about 850-1000° C. and flowing 99.99% pure methane over the catalyst islands  29  at a velocity of about 2-20 centimeters per second (e.g., for a 1-inch diameter tube, flowing methane at a rate of about 600-6000 cm 3 /min). Use of these parameters results in nanotubes which are substantially perfect and straight, with no structural flaws (i.e. all the carbon rings in the nanotubes have 6 carbon atoms instead of 5 or 7 carbon atoms). Most of the nanotubes are single-walled, with diameters in the range of about 1-5 nanometers. When grown at 1000° C., 90% of the tubes were single-walled; when grown at 900° C., 99% of the tubes were single-walled. Most of the nanotubes have diameters in the range of 1-2 nanometers. The nanotubes have large aspect ratios (length/diameter) approaching about 10,000, and are very straight (a result of the absence of structural flaws). 
     It is noted that many different recipes for nanotube catalysts are known in the art. For example, Fe(SO 4 ) or other Iron salts can be substituted for the Fe(NO 3 ) 3 . The quality of the nanotubes depends upon the catalyst material used. Iron, molybdenum and zinc oxides are preferred for making high quality tubes. A particularly good catalyst is made with a mixture of iron, molybdenum and ruthenium oxides. Most generally, both elemental metals and their oxides can be used to grow nanotubes. 
     Also, the nanoparticles can be made of many ceramic materials besides alumina. Silica, for example, can also be used. Generally, refractory oxide ceramic materials can be used in place of the alumina nanoparticles. Still further, nanoparticles may not be used at all. Small quantities of Iron salts can be deposited on the substrate (for example, by dissolving in a solvent and evaporating the solvent) and heated to decomposition without being mixed with nanoparticles. 
     FIG. 5 shows a closeup top view of the island  29  and substrate after the growth of nanotubes has been performed. Carbon nanotubes  30  extend from the island  29  in random directions. The carbon nanotubes  30  are not freestanding, but are disposed in contact with the substrate surface. Also, the carbon nanotubes are firmly attached to the island  29 . The nanotubes generally grow in a ‘base-growth’ mode, where new carbon is added to the nanotubes  30  at the point where they are attached to the island  29 . The nanotubes are attached at one end to the island, and the opposite end is free. The nanotubes can be used as resonators by allowing the free end to vibrate. 
     The carbon nanotubes  30  are not tangled together, but are individually separable. This is due to the fact that a small number of nanotubes grow from each island. Also, the nanotubes are spaced apart by a substantial distance. Typically, about 10-50 nanotubes are grown from each island. If larger numbers of nanotubes are grown (e.g. by using a more effective catalyst), then the nanotubes may form bundles. This is undesirable for applications requiring single distinct nanotubes. However, bundles of nanotubes can also be useful for many electrical and mechanical devices such as interconnects, field effect transistors, single electron transistors, and resonators which have only one fixed end. 
     Individually separable nanotubes are useful for the manufacturing of electronic and micromechanical devices because individual nanotubes can be incorporated into the devices by appropriately locating islands  29 . Electrical and mechanical connections can be made to individual nanotubes if they are spatially separated and distinct. 
     FIG. 6 shows a top view of an electronic device made by locating the island  29  close to a patterned metal pad  32 . A single nanotube  30   a  extends from the island  29  to the metal pad  32 , thereby providing electrical contact between the island  29  and pad  32 . The island  29  and pad  32  are spaced apart by a distance in the range of 100 nanometers to about 5 microns. The island  29  and pad  32  are both electrically conductive, so patterned conductive lines  33  on the substrate surface can provide for macroscopic electrical connections to the nanotube  30   a . The nanotube  30   a  with a macroscopic electrical connection on each end can be used in many devices including field-effect transistors, single electron transistors, or low current value fuses. 
     The conductive lines  33  may be applied to the substrate  20  before the islands  29  are deposited. In this way, the islands rest on top of the conductive lines  33 . Also, the conductive lines  33  can be disposed on top of the islands (by applying the conductive lines on top of the islands. The conductive lines can be deposited before or after the growth of nanotubes. 
     The apparatus of FIG. 6 is made by simply locating the island and metal pad proximate to one another and catalytically growing nanotubes from the island. The closer the island  29  and pad  32 , the more likely that a nanotube will be grown that connects the island and pad. 
     Also, two or more nanotubes can simultaneously electrically connect the island  29  and metal pad  32 . If multiple nanotubes connect between the island and pad, then all but one of the nanotubes can be broken with an AFM tip. This is performed by dragging the AFM tip across the substrate surface so that it bends unwanted nanotubes until they break. 
     Further, a second catalyst island can be substituted for the metal pad  32 . In such a device, the nanotube  30   a  provides electrical contact between two catalyst islands  29  instead of between an island  29  and a metal pad  32 . Metal lines  33  can provide electrical connections to each catalyst island as in FIG.  6 . The same spacing distance can be used (100 nanometers to about 5 microns) if a catalyst island is substituted for the metal pad. 
     FIG. 7 shows a side view of a preferred embodiment of the present invention in which a metal cover  34  is deposited on top of each catalyst island  29 . The metal covers  34  can be made of platinum or titanium-gold alloy, for example. Each metal cover  34  covers a portion of each island  29  and covers an end portion  37  of the nanotube  30   a . The metal cover therefore serves to help hold the nanotube  30   a  rigidly in place. 
     The metal covers  34  help to provide Ohmic electrical connections to the ends of the nanotube  30   a . Ohmic electrical connections with the nanotube are assured by heating the apparatus after depositing the metal covers  34 . For example, heating the apparatus to about 300° C. in air will result in Ohmic electrical connections between the metal covers  34  and nanotube  30   a . Metal lines  33  as shown in FIG. 6 can be connected to the metal covers to provide macroscopic electrical connections with the nanotube  30   a . Electrical conduction through the catalyst island is therefore not necessary. 
     The metal covers  34  can be made by lithographically patterning the metal comprising the covers  34 . FIGS. 8A-8C illustrate how this can be done. First, a layer of spin-on resist  60  is deposited on top of the islands  29  and nanotube  30   a . Next, the resist  60  is etched in regions  61  where the metal cover  34  is to be located. The metal comprising the metal covers  34  is then deposited (by physical vapor deposition or CVD processes, for example), and the resist  60  is removed in a lift-off process which leaves only the metal covers  34 . 
     The present invention can provide freestanding nanotubes capable of acting as high-Q resonators. FIG. 9 shows a side view of a device including a freestanding nanotube  30   b . The freestanding nanotube  30   b  is suspended above the substrate  22  which is depressed in a trench region  35  between the islands  29 . The trench  35  can be formed by etching the substrate. The nanotube  30   b  therefore lies above a surface  36  of the etched substrate  22  and is supported only by nanotube ends  39 . The trench  35  and metal covers  34  can be combined in the same apparatus. 
     The nanotube  30   b  can be resonated by locating the nanotube  30   b  in a magnetic field (perpendicular to the length of the nanotube  30   b ) and passing an oscillating current through the nanotube. A conductive film  37  capacitively coupled with the nanotube  30   b  extracts a resonant signal from the nanotube. Alternatively, the conductive film  37  can be used to electrostatically excite mechanical vibrations in the nanotube  30   b.    
     FIG. 10 shows a top view of the substrate  22  and islands  29  illustrating how the apparatus of FIG. 9 can be made. First, the nanotube  30   b  which connects the islands  29  is grown. Other nanotubes will also be grown from both islands, but they are not shown for clarity. Then, all regions of the substrate except for a region defined by a box  38  are masked with resist. Spin-on resist can be used, for example. The act of spin-coating resist on the substrate will not damage the nanotube  30   b . Next, the region inside the box  38  is exposed to an etchant which removes substrate material, but does not affect the nanotube  30   b . Many different etchants can be used, depending upon the composition of the substrate (e.g. hydrofluoric acid can be used to etch SiO 2  or Si substrates). Etching the substrate  22  under the nanotube  30   b  results in the nanotube being supported only at its ends  39 . Metal lines  33  provide macroscopic electrical connections to the nanotube  30   b  through the catalyst islands  29 . Also, metal covers  34  can be deposited before or after etching the trench  35  to provide Ohmic electrical connections to the nanotube and improved mechanical stability for the nanotube ends  39 . 
     An alternative method for making the apparatus of FIG. 9 is shown in the side views of FIGS. 11A and 11B. In FIG. 11A, the substrate  22  is etched to form the trench  35  where the nanotube  30   b  is suspended. Then islands  29  are disposed on opposite sides of the trench  35  and nanotubes are grown from the islands  29 . The nanotube  30   b  that connects the islands grows from one island to the other. Alternatively, one of the islands can be replaced with the metal pad  32 , in which case the nanotube grows from the island  29  to the pad  32 . Also, metal covers  34  can be deposited on top of the nanotube  30   b  and catalyst islands  29 . 
     The present invention includes an embodiment where the freestanding nanotube is only supported on one end by a catalyst island  29  (i.e. the freestanding nanotube does not extend all the way across the trench  35 ). The nanotube is therefore a cantilever, and can be used as a resonator. 
     It is noted that growing nanotubes between islands, or between an island and a metal pad is an uncertain endeavor. One cannot be sure that a particular arrangement of catalyst islands will result in a nanotube connection between a particular pair of islands, or how many nanotubes will connect. However, if a pair of islands are spaced less than about 10 microns apart, and are at least 1 micron wide, a nanotube is likely to connect the pair of islands. At least one bridging nanotube connection can be practically assured if a number of islands are disposed with various spacings in an array. 
     FIG. 12 shows another embodiment of the present invention in which a catalyst particle  45  is located on a tip  47  of an atomic force microscope (AFM) cantilever  42 . The cantilever  42  is supported by a base  49 , and has a free end  48  opposite the base  49 . The particle  45  may be made of Fe 2  O 3  (decomposed from Fe(NO 3 ) 3 ), for example. The catalyst particle  45  may or may not have supporting nanoparticles (i.e. silica or alumina particles). The catalyst particle is firmly attached to the tip  47 . Nanotubes  30  grown from the particle  45  are firmly attached to the cantilever and are atomically sharp. Nanotubes grown from the catalyst particle can be used as probe tips for AFM. Alternatively, the cantilever does not have a tip  47 , and the particle is disposed directly on the cantilever  42 . 
     FIGS. 13A and 13B illustrate how the apparatus of FIG. 12 can be made. First, in FIG. 13A, a substrate  50  is coated with a gold film  52 , and then droplets of Fe(NO 3 ) 3  dissolved in methanol are deposited on the gold surface. The methanol is then evaporated leaving only small particles  54  of Fe(NO 3 ) 3  on the gold film  52 . Next , as shown in FIG. 13B, the AFM tip  47  is brought into contact with a particle  54  of Fe(NO 3 ) 3 . An electric field is then applied between the tip  47  and the gold film  52 . The electric field causes the Fe(NO 3 ) 3  particle to adhere to the tip  47  and may cause the Fe(NO 3 ) 3  to decompose into Fe 2 O 3 . Then, in FIG. 13C, the cantilever  42  and tip  47  with the adhered Fe(NO 3 ) 3  particle  54  is removed from the gold film  52 . In FIG. 13D, the device is heated to fully decompose the Fe(NO 3 ) 3  particle  54  into Fe 2 O 3 . This transforms the Fe(NO 3 ) 3  particle  54  into a catalyst particle  45  (shown in FIG.  12 ). Then, nanotubes  30  are grown from the catalyst particle  45 . 
     An AFM cantilever with a catalytically grown nanotube tip has several advantages over an AFM cantilever with a nanotube bonded with other techniques. It is a relatively simple task to catalytically grow a nanotube from the catalyst particle on the cantilever. Also, the nanotube is firmly bonded to the cantilever. 
     It will be clear to one skilled in the art that the above embodiment may be altered in many ways without departing from the scope of the invention. Accordingly, the scope of the invention should be determined by the following claims and their legal equivalents.