Patent Publication Number: US-2005132949-A1

Title: Forming carbon nanotubes by iterating nanotube growth and post-treatment steps

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
      This application is a continuation-in-part of U.S. application Ser. No. 10/302,126, filed Nov. 22, 2002, now U.S. Pat. No. 6,841,002, and is a continuation-in-part of U.S. application Ser. No. 10/302,206, filed Nov. 22, 2002, now U.S. Pat. No. 6,841,003, both of which are incorporated by reference in their entirety. This application is also related to U.S. application Ser. No. 10/226,873, filed Aug. 22, 2002, which is incorporated by reference in its entirety. 
    
    
     BACKGROUND  
      1. Field of the Invention  
      This invention relates generally to forming carbon nanotubes, and in particular to forming purified carbon nanotubes for an electron-emitting device.  
      2. Background of the Invention  
      Carbon has four crystalline states, including diamond, graphite, fullerene and carbon nanotubes. Among these states, carbon nanotubes exhibit a number of remarkable electrical and mechanical properties. Their properties make nanotubes very desirable for use in modem electronic devices, such as field emissive displays. Carbon nanotubes were originally created by means of an electric arc discharge between two graphite rods. However, this technique for forming nanotubes is not efficient and requires a complicated post-treatment and/or purification procedures.  
      Carbon nanotubes can be also grown on a substrate using plasma enhanced chemical vapor deposition (PECVD), as described for example in U.S. Pat. No. 6,331,209, which is incorporated by reference in its entirety. According to the conventional method disclosed in this patent, carbon nanotubes are grown on a substrate using PECVD at a high plasma density. Specifically, the conventional technique includes: growing a carbon nanotube layer on a substrate to have a predetermined thickness by plasma deposition; purifying the carbon nanotube layer by plasma etching; and repeating the growth and the purification of the carbon nanotube layer. For the purifying, a halogen-containing gas (e.g., a carbon tetrafluoride gas), fluorine, or an oxygen-containing gas is used as a source gas.  
      It should be noted that according to the conventional method for growing carbon nanotubes, after each nanotube growth step and before the nanotube purification step, the plasma in the process chamber has to be turned off and the process chamber has to be purged and evacuated. Subsequently, the pressure of the purifying gas needs to be stabilized and the plasma needs to be turned back on. These multiple steps, which must be completed after the nanotubes have been grown and before they are purified, make the conventional process for forming carbon nanotubes unduly expensive and time consuming.  
      Accordingly, what is needed is a technique for forming carbon nanotubes utilizing a fewer number of process steps.  
     SUMMARY OF THE INVENTION  
      Embodiments of the present invention are therefore directed to methods and systems that substantially obviate one or more of the above and other problems associated with conventional techniques for forming carbon nanotubes. Consistent with exemplary embodiments of the present invention, methods for forming carbon nanotubes are provided.  
      In one embodiment, a method for forming carbon nanotubes on a substrate in a process chamber includes growing a plurality of carbon nanotubes by plasma enhanced chemical vapor deposition using a source gas and a plasma. The grown carbon nanotubes are then purified by plasma etching using a purification gas, after which the growing and purifying steps can be repeated. Beneficially, the process chamber is not evacuated and the plasma is not turned off between the growing and purifying steps. Once the carbon nanotubes are grown and purified through the successive steps described, the nanotubes are post-treated by etching using the plasma. Again, the post-treatment is performed without turning off the plasma after the carbon nanotubes are grown.  
      A number of process variables may be chosen to improve the result of the grown carbon nanotubes. For example, the substrate surface may be coated with a catalytic material. In addition, a buffer layer may be provided between the substrate and the catalytic layer. During the growing, a hydrocarbon gas may be used as a source gas for the plasma chemical deposition, and a hydrogen-containing carbon gas may be used as an additive gas for enhancing the purification process. Alternatively, during the purification process, the additive gas (e.g., a hydrogen-containing gas) that is used during the growing of the carbon nanotubes is also used continuously as a source gas for the plasma etching process. Additionally, a plasma source gas may be added as an additive to the source gas for the plasma chemical deposition.  
      In another embodiment, a method of forming carbon nanotubes includes growing a plurality of carbon nanotubes by chemical vapor deposition using a plasma and then, without shutting off the plasma after the carbon nanotubes are grown, post-treating the grown carbon nanotubes by etching. The etching of the post-treatment step results in the shortening of at least some of the carbon nanotubes. The growing and post-treating steps are then repeated. In one embodiment, a first iteration of the post-treating step etches more carbon nanotube length than a second and/or subsequent iterations of the post-treating step.  
      It is to be understood that both the foregoing and the following descriptions are exemplary and explanatory only, and they are therefore not intended to limit the claimed invention in any manner except to the extent they are included as limitations in the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a cross-sectional view of a substrate for use in the formation of carbon nanotubes, in accordance with an embodiment of the invention.  
       FIG. 2  is a cross-sectional view of a substrate during a growth stage of a carbon nanotube formation process, in accordance with an embodiment of the invention.  
       FIG. 3  is a cross-sectional view of a purification stage of the carbon nanotube formation process, in accordance with an embodiment of the invention.  
       FIG. 4  is a cross-sectional view of a substrate having a purified carbon nanotube layer formed thereon, in accordance with an embodiment of the invention.  
       FIG. 5  is a cross-sectional view of a post-treatment stage of the carbon nanotube formation process, in accordance with an embodiment of the invention.  
       FIG. 6  is a cross-sectional view of a substrate having a purified and post-treated carbon nanotube layer formed thereon, in accordance with an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      In various embodiments of the invention, carbon nanotubes are formed on a surface of a substrate using a plasma chemical deposition process, such as PECVD. The material of the substrate may be chosen to provide for desired mechanical and electrical properties, such as conductivity and rigidity. In one embodiment of the invention, the substrate is made of an electrically insulating material, such as a glass, quartz, or ceramic plate. Alternatively, the substrate may comprise a metal or metal alloy. Persons of skill in the art will appreciate that any of a variety of materials may be selected for the substrate without departing from the scope of the invention.  
      Reference will now be made to  FIG. 1 , which illustrates a cross-sectional view of a substrate  101  for use in the formation of carbon nanotubes in accordance with an embodiment of the invention. To facilitate the formation of the carbon nanotubes thereon, the upper surface of the substrate  101  may be coated with a catalytic metal layer  103  of a predetermined thickness, as shown in  FIG. 1 . This catalytic layer  103  may comprise one or more of the transition group metals, including, without limitation, nickel, cobalt, and iron. Alternatively, the catalytic material  103  may comprise an alloy of one or more of those metals. Various methods for coating substrate with catalytic layers of predetermined thickness are well known to persons of skill in the art. One such widely used method is a sputtering deposition process. In one embodiment, the thickness of the catalytic layer  103  is within the range of about 1 nm to about 100 nm.  
      In one embodiment, an additional buffer layer  102  is disposed between the substrate  101  and the catalytic layer  103 . The buffer layer  102  prevents diffusion between the catalytic layer  103  and the substrate  101 . In one embodiment of the invention, the buffer layer  102  is formed of a metal, such as molybdenum, titanium, titanium tungsten, titanium nitride, an alloy of titanium, an alloy of titanium tungsten, or an alloy of titanium nitride. Once the catalytic layer  103  and the buffer layer  103  are formed on the substrate  101 , the substrate  101  is placed into a plasma process chamber, where the nanotube layer growth is performed.  
      In one embodiment, before the carbon nanotube layer  201  is grown, the catalyst layer  103  is granularized into nano-sized particles to facilitate the growth of nanotubes  201  on the catalyst layer  103 . This granularization step is performed before the nanotubes  201  are grown. To granularize the catalyst layer  103 , the substrate is exposed to a granulation gas, which causes patterning of the catalyst layer into nano-sized particles. In this phase, the catalyst layer  103  is granularized into multiple round shapes and randomly spread over the buffer layer  102 . Having round shaped nano-sized particles enhances the density of carbon nanotube formed on each catalyst particle.  
      In one embodiment, the granule size of the catalyst particles ranges from about 1 nm to about 200 nm, and the granule density is in the range of about 10 8 /cm 2  to about 10 11 /cm 2 . In one embodiment, during the granulation phase, the reaction surface of the catalyst layer  103  is increased as round catalyst particles form from the originally flat catalyst layer  103 . The resulting three-dimensional surface of the catalyst particles enhances the growing of the carbon nanotubes and helps in the diffusion of the carbon radical or the plasma to the catalyst layer  103 , reducing the temperature at which the carbon nanotubes may be formed. After the granulation phase, the plasma chamber is purged with nitrogen (N 2 ) gas, argon (Ar) gas, Helium (He) gas, or any other suitable gas, and the chamber is then evacuated. The substrate is then placed in the chamber and heated to a temperature of about 400° C. to about 600° C.  
       FIG. 2  illustrates a cross-sectional view of a substrate during a growth stage of the carbon nanotube formation process. As illustrated, a deposition plasma  202  is produced by a plasma source in the chamber. This deposition plasma  202  results in the formation of a carbon nanotube layer  201 . To facilitate the growth process, the substrate  101  and the ambient gas in the plasma process chamber may be heated to a temperature within a range of about 400° C. to about 600° C. In one embodiment of the present invention, the plasma density for growing the carbon nanotubes is in the range of about 10 10 /cm 3  to about 10 12 /cm 3 .  
      In one embodiment of the invention, the deposition plasma  202  is produced by an inductively coupled plasma or a microwave plasma chanber, which is preferably capable of generating a high-density plasma. The source gas for the deposition plasma  202  may be a hydrocarbon containing gas, and it may have a hydrogen-containing gas as additive. The presence of the additive gas in the plasma process chamber facilitates the purification of the grown nanotube structures during a later purification process step without the need to purge the process chamber. In another embodiment, the deposition plasma  202  is produced by a capacitively coupled plasma device, which is preferably capable of generating a high-density plasma.  
      In one embodiment of the present invention, the plasma source gas for growing the carbon nanotubes may include CH 4  and/or C 2 H 2 . The temperature range of the substrate during the growing of the carbon nanotubes typically ranges between about 400° C. to about 600° C., and the plasma gas pressure ranges between about 500 mTorr to about 5000 mTorr. The carbon nanotube layer  201  is grown in the plasma process chamber to a predetermined thickness, although the nanotube layer  201  thickness is generally not linear with the time of growth.  
      To improve the vertical growth of the carbon nanotubes, a negative voltage bias may be applied to the substrate  101  during the growth stage of the carbon nanotubes. In one embodiment, the applied negative voltage is between about 50 and about 600 Volts. Preferably, the carbon nanotubes are grown perpendicular from the substrate&#39;s surface. The angle of the grown carbon nanotubes from the normal axis of the substrate is preferably less than 45 degrees. After the carbon nanotubes are grown, the granular particles from the catalyst layer may be on the bottom and/or top of the carbon nanotubes, and these particles are preferably removed.  
      U.S. application Ser. No. 10/889,807, filed Jul. 12, 2004, the contents of which are incorporated by reference in its entirety, describes a process for growing carbon nanotubes on a substrate using PECVD. In one embodiment, the carbon nanotube layer  201  is grown according to one of the processes described therein.  
      After the nanotubes have been grown, a purification step is performed on the newly formed nanotube structures in one embodiment. The purification step is not required, but the purification step may beneficially remove graphite and other carbon particles from the walls of the grown carbon nanotubes, as well as control the physical dimensions or physical characteristics of the carbon nanotubes.  FIG. 3  illustrates a cross-sectional view of a substrate during an intermediate purification stage of the carbon nanotube formation process. As illustrated, a purification plasma  301  is produced by a plasma source within the chamber. In one embodiment of the invention, the purification is performed with the plasma  301  at the same temperature as the substrate  101 .  
      In one embodiment, an additive hydrogen containing gas is used as the plasma source gas during the purification stage. The additive hydrogen containing gas may comprise H 2 , NH 3 , or a mixture of H 2  and NH 3 . Because the source gas for the purification plasma is added as an additive to the source gas for the chemical plasma deposition, the grown carbon nanotubes are purified by reacting with the continuous plasma, which is sustained in the plasma process chamber. This eliminates the need to purge and evacuate the plasma process chamber as well as to stabilize the pressure with the purification gas. After the carbon nanotube layer  201  is purified with the purification plasma  301 , the nanotube growth and purification steps may be repeated.  FIG. 4  illustrates a cross-sectional view of a substrate  101  having a carbon nanotube layer  201  grown and purified in accordance with an embodiment of the invention described herein.  
      After the nanotubes have been grown in the described manner, a post-treatment step is performed on the newly formed (and optionally purified) carbon nanotube layer  201 . Beneficially, the post-treatment removes graphite and other carbon particles from the walls of the grown nanotubes, and it controls the diameter of the carbon nanotubes.  FIG. 5  illustrates a cross-sectional view of the substrate during a post-treatment stage of the carbon nanotubes formation process. As illustrated, a post-treatment plasma  501  is produced by a plasma source in the process chamber. In one embodiment, the post-treatment is performed with the plasma  501  at the same temperature as the substrate  101 .  
      In one embodiment, an additive hydrogen containing gas is used as the plasma source gas during the post-treatment stage. The hydrogen containing gas may comprise H 2 , NH 3 , or a mixture of H 2  and NH 3 . During the transition from the nanotube growth step (and optional purification step) to the post-treatment step, the pressure in the plasma process chamber may be stabilized with the post-treatment gas without shutting off the plasma in the chamber. This eliminates the need to purge and evacuate the plasma process chamber.  FIG. 6  illustrates a cross-sectional view of a substrate  101  having a carbon nanotube layer  201  grown and post-treated in accordance with an embodiment of the invention described herein. The highly uniform carbon nanotubes grown on the structure shown in  FIG. 6  may be used in a number of applications, such as electron emitters in an electron emissive device (e.g., a field emissive display device).  
      In another embodiment, after the post-treatment of the carbon nanotube layer  201 , the pressure in the chamber may again be stabilized with the nanotube growing gas, and the nanotube growth may be repeated. By repeating the entire process of carbon nanotube growth (which may involve intermediate purification steps) and post-treatment, a more uniform carbon nanotube layer can be formed.  
      In one embodiment, once a first iteration of the growth is performed, the average value of the thickness of the carbon nanotube layer  201  (or length/height of the nanotubes) is assessed. Based on that assessment, the post-treatment step is designed so that the post-treatment step will “etch-off” the carbon nanotube layer  201  (i.e., remove carbon nanotube matter) to about half of that average value. This etching causes any carbon nanotubes in the layer  201  to be shortened to about that average length, whereas the nanotubes shorter than the average would remain intact. In this way, the variance in the length of the carbon nanotubes is reduced. In one embodiment, the time of the post-treatment etching step is adjusted so that the carbon nanotubes are reduced to the desired length.  
      Once the nanotube growth and post-treatment process is completed, another iteration may be run. For example, another growth step is performed (which may also include intermediate purification steps), causing the carbon nanotubes to lengthen. Once the nanotubes are lengthened through the additional growth process, another post-treatment step is performed. This subsequent post-treatment step preferably performs a lighter etching of the carbon nanotube layer  201 , removing less of the carbon nanotube matter. In one embodiment, an assessment is made to determine the new average length of the carbon nanotubes. The post-treatment step is then designed so that it etches off about 20% of the new average height. It can be appreciated that this iterative process results in a carbon nanotube layer  201  having more uniform height (i.e., nanotube length), which improves the operation of the nanotubes in an electrical device.  
      In another embodiment, a plurality of iterations as described above may be run to achieve a highly uniform carbon nanotube layer  201 . In each successive iteration, the amount of etching performed by the post-treatment step can be reduced.  
      It should be understood that processes and techniques described herein are not inherently related to any particular apparatus and may be implemented by any suitable combination of components. Moreover, various types of general purpose devices may be used in accordance with the teachings described herein. It may also prove advantageous to construct specialized apparatus to perform the method steps described herein. Accordingly, the foregoing description of the embodiments of the invention has been presented for the purpose of illustration; it is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Persons skilled in the relevant art can appreciate that many modifications and variations are possible in light of the above teachings. It is therefore intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.