PRODUCING A STABLE CATALYST FOR NANOTUBE GROWTH

FIELD OF THE INVENTION

The present invention generally relates to a catalytic process for growing carbon nanotubes and more particularly to a process for producing a stable and highly reactive catalyst for carbon nanotube growth.

BACKGROUND OF THE INVENTION

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, and commonly referred to as single-walled nanotubes (SWNTs), or multi-walled nanotubes (MWNTs), respectively. These types of structures are 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.

A carbon nanotube is 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.

The manufacturing costs for vacuum devices (e.g., a field emission display) that use a carbon nanotube can be further reduced if the carbon nanotube is grown on the

field emission substrate from a catalytic surface using chemical vapor deposition or other film deposition techniques. Nanotube growth can be conducted as a last deposition process preventing the degradation of the electron emitter properties by other device processing techniques or steps (e.g., wet processes).

Carbon nanotubes can also function as either a conductor, like metal, or a semiconductor, according to the rolled shape and the diameter of the helical tubes. With metallic-like 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-like nanotubes can be used as ideal interconnects. 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.

Existing methods for the production of nanotubes, include arc-discharge and laser ablation techniques. Unfortunately, these methods typically yield bulk materials with tangled nanotubes. Recently, reported by J.Kong, A.M. Cassell, and H Dai, in Chem. Phys. Lett. 292, 567 (1988) and J. Hafner, M. Bronikowski, B. Azamian, P. Nikoleav, D. Colbert, K. Smith, and R. Smalley, in Chem. Phys Lett. 296, 195 (1998) was the formation of high quality individual single-walled carbon nanotubes (SWNTs) demonstrated via thermal chemical vapor deposition (CVD) approach, using Fe/Mo or Fe nanoparticles as a catalyst. The CVD process has allowed selective growth of individual SWNTs, and simplified the process for making SWNT based devices. However, the choice of catalyst materials that can be used to promote SWNT growth in a CVD process has typically been limited to Fe/Mo nanoparticles. Furthermore, the catalytic nanoparticles were usually derived by wet chemical routes, which are time consuming and difficult to use for patterning small features.

Another approach for fabricating nanotubes is to deposit metal films using ion beam sputtering to form catalytic nanoparticles. In an article by L. Delzeit, B. Chen, A. Cassell, R. Stevens, C. Nguyen and M. Meyyappan in Chem. Phys. Lett. 348, 368 (2002), CVD growth of SWNTs at temperatures of 900°C and above was described using Fe or an Fe/Mo bi-layer thin film supported with a thin aluminum under layer. However, the required high growth temperature prevents integration of CNTs growth with other device fabrication processes.

Ni has been used as one of the catalytic materials for formation of SWNTs during laser ablation and arc discharge process as described by A. Thess, R. Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, C. Xu, Y. H. Lee, S. G. Kim, A. G. Rinzler, D. T. Colbert, G. E. Scuseria, D. Tomanet, J. E. Fischer, and R. E. Smalley in Science, 273, 483 (1996) and by D.S. Bethune, C. H. Kiang, M. S. de Vries, G. Gorman, R. Savory, J. Vazquez, and R. Beyers in Nature, 363, 605 (1993).

Regardless of how the nickel catalyst nanoparticles are formed, an oxidation layer forms on the catalyst nanoparticles in the ambient environment. Conventionally, hydrogen is used in the reduction phase of growth cycle, to remove the oxidation prior to growing the nanotubes. However, this must be done immediately prior to growing the nanotubes and depending on the CNT growth technique and process conditions employed, such as thermal versus plasma enhanced, active gas composition, gas temperature, not all of the oxidation is removed. This results in a decrease of catalyst activity and in a reduction of the active site density leading consequently to the prevention of carbon nanotubes from growing on the catalyst as desired. The embodiment of this disclosure involves the passivation of the catalyst with a diamond like carbon (DLC) layer prior growth process, it permits an increase of catalyst activity and selectivity resulting in better carbon nanotubes, as compared to known art catalyst.

Accordingly, it is desirable to provide a process for producing a stable catalyst for carbon nanotube growth. 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 process is provided for preparing a catalyst. A catalyst is formed over a substrate. A gas comprising hydrogen and carbon is applied to the catalyst, wherein a carbon seeding layer is formed on the catalyst. Carbon nanotubes may then be grown from the catalyst having the carbon seeding layer thereon.

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

FIG. 1 is a cross sectional view of a previously known catalyst structure;

FIG. 2 is a cross sectional view of the previously known catalyst structure being subjected to a gas in accordance with the preferred process of the present invention;

FIG. 3 is a cross sectional view of the preferred embodiment of the present invention;

FIG. 4 is a flow chart showing the steps in one embodiment of the present invention; and

FIG. 5 is as graph showing field emission performance of carbon nanotubes grown in accordance with the preferred embodiment of the present invention versus field emission performance of carbon nanotubes grown with prior art technology.

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.

Referring to FIG. 1, a previously known process, and one that may be used with the present invention, comprises depositing a metal 16 on a dielectric layer 14 such as silicon dioxide or silicon nitride grown or formed in ambient air on the substrate 12. The substrate 12, comprises silicon; however, alternate materials, for example, silicon, glass, ceramic, metal, a semiconductor material, or a organic material are

anticipated by this disclosure. Substrate 12 can include control electronics or other circuitry, which are not shown in this embodiment for simplicity. The metal 16 is molybdenum, but may comprise any metal. A layer of metal 18 is deposited on the metal 16 to support the catalyst 20 thereon. The metal 18 is aluminum, but may be any support material having inert interaction with the catalyst 20.

The catalyst 20 preferably comprises nickel, but could comprise any one of a number of other materials including cobalt, iron, and a transition metal or oxides and alloys thereof. The catalyst 20 may be formed in any number of ways known in the industry. One preferred method would be to form a relatively smooth film and subsequently etching the film to provide a rougher surface, or catalyst 40.

As used herein, carbon nanotubes includes any elongated carbon structure. The structure 10 having the catalyst 20 thereon may be exposed to ambient for some time prior to growing the carbon nanotubes 28. This exposure can allow ambient contaminants to be physi-absorbed or chemi-absorbed onto the catalyst 20, where one effect can be the formation of an oxide layer 22 to be formed on the catalyst 20. When the device 10 is placed into the nanotube growth chamber (not shown), hydrogen or a hydrogen-containing gas is introduced into the chamber to aid in chemically scrubbing the catalyst surface and promotes the reduction of surface contaminants including the oxidized catalyst layer. The chemical scrubbing efficiency of the catalyst scales with higher temperature, as such low temperature methods for growing carbon nanotubes 28 become less efficient in chemically scrubbing the catalyst, which can lead to poor catalytic reactivity resulting in fewer carbon nanotubes 28.

Referring to FIG. 2 and in accordance with the present invention, the structure 10 is placed in a chamber (not shown) and subjected to a gas 24. The gas 24 preferrably comprises methane (CH4), but may comprise any combination of hydrogen and carbon. The gas 24 is then excited to form a plasma which produces carbon and hydrogen radicals and ions. For a given condition, the gas 24 can form a film ranging from a hydrogen-rich amorphous carbon (polymer-like) to a more dense

amorphous carbon having lower hydrogen content and is classically referred to as a diamond-like carbon (DLC) film. Process temperatures for DLC films can range from 15°C to 600 ⁰ C and can encompass pressure ranges from a few milli-Torr to hundreds of Torr. For this invention, the gas 24 must comprise sufficient hydrogen to chemically reduce any oxides formed on the catalyst 22 while also depositing a dense amorphous carbon (DLC) 26 or a DLC matrix that consisting of DLC clusters supported in an amorphous carbon layer 26 having sufficient thickness to completely passivate the catalyst and is generally 5 nm or greater. The DLC passivation layer 26 subsequently becomes a catalyst seeding layer during the carbon nanotube growth process and significantly enhances catalytic reactivity.

Carbon nanotubes 28 are then grown from the catalyst 20 having the carbon layer 26 formed thereon in a manner known to those skilled in the art. Although only a few carbon 20 and carbon nanotubes 28 are shown, those skilled in the art understand that any number of carbon 20 and carbon nanotubes 28 could be formed.

The carbon nanotubes 28 may be grown, for example, as electron emitters for use in display devices or as conductive elements in sensors or electronic circuits. It should be understood that any nanotube having a height to radius ratio of greater than 100, for example, would function equally well with some embodiments of the present invention. Additionally, the catalyst 20 may be formed by any process known in the industry, e.g., co-evaporation, co-sputtering , co-precipitation, wet chemical impregnation, incipient wetness impregnation, adsorption, ion exchange in aqueous medium or solid state, before having the present invention applied thereto.

The process is further illustrated by the flow chart 40 in FIG. 4 wherein a conductive layer 16,18 is formed 42 over a substrate 12 and catalyst 20 are then formed 44 on the conductive layer 16,18. A gas 24 comprising carbon and hydrogen is applied 46 to the catalyst 20 to form a carbon seeding layer 26 on the catalyst 20. Carbon nanotubes 28 may then be grown 48 from the catalyst 20 having the carbon seeding layer 26 thereon.

Referring to FIG. 5, the graph illustrates the improved emission current density of the present invention versus the known art. The samples were created and tested alike except for data curves 52 and 54 the samples have seen pre-deposition of a DLC seeding layer over the catalyst 20 prior to HF-CVD processing. The samples associated with curves 56 and 58 were submitted to only hydrogen gas reduction step during HF-CVD to remove any oxide. The results reported in FIG. 5 show that the field emission current density extracted with catalyst 20 samples 52 and 54 processed using the present invention, are an order of magnitude better than the prior art catalyst samples 56 and 58. The "improvement" resulting from the present invention catalyst is mainly due to longer carbon nanotubes, thinner carbon nanotubes, higher density of carbon nanotubes, and less defective carbon nanotubes. Using the catalyst 20 and the carbon seed layer 26, the high current density measured for catalyst samples 52 and 54, reflect a higher density and more uniform carbon nanotube growth. Furthermore, the sharpness of the I-V characteristics exhibited by curves 52 and 54 and their low threshold of emission current, are an indication of carbon nanotube growth with better form factor (longer and thinner) due to better activity of carbon seed layer 26 passivated catalyst.

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.