Abstract:
An embodiment of a system for forming carbon nanotubes (CNTs) using plasma enhanced chemical vapor deposition (PECVD) uses one or more of RF and DC power supplies coupled to electrodes in various configurations within a process chamber of the system. By application of a sufficient DC voltage to one or more electrodes, the system allows for growing CNTs that can be straighter and have improved electrical performance characteristics.

Description:
BACKGROUND  
     FIELD OF THE INVENTION  
       [0001]     This invention relates generally to systems for forming carbon nanotubes, and in particular to forming carbon nanotubes using plasma enhanced chemical vapor deposition (PEVCD).  
       BACKGROUND OF THE INVENTION  
       [0002]     Growing carbon nanotubes (CNTs), such as those suitable for use in electron emissive applications, generally involves two reactions: The first is disassociating, or decomposing, a hydrocarbon gas to generate carbon atoms and ions, and the second is causing the absorption of carbon atoms by a catalytic material to supersaturation to cause the precipitating effects that result in carbon nanotubular growth. Each of these reactions typically requires an energy source or other mechanism to deliver the energy required for the chemical reaction. Generally following these principles, there are distinct methods commonly used for growing CNTs, including arc discharging, laser ablation, and chemical vapor deposition (CVD).  
         [0003]     In arc discharging processes, two electrodes are used. One electrode, equipped with a power supply, is formed of or contains graphite. When the power supply is turned on, vaporized graphite in the form of composites of carbon are attracted to or are grown on the opposite electrode. Laser ablation is similar to arc discharging, but instead of an electrical arc to invoke or instigate the vaporization of the graphite material of the electrode, a laser is used to impinge upon a volume of feedstock gases to achieve the same results.  
         [0004]     Within CVD, there are various types of CVD processes, including thermal CVD and plasma enhanced chemical vapor deposition (PECVD). In thermal CVD processes, a single energy source produces high temperatures to cause both reactions involved in CNT growth, as described above. In typical PECVD systems, however, radio frequency (RF) energy is used to assist in the disassociation reaction with the source hydrocarbon gas to generate the necessary carbon ions or radicals. A heated electrode (generally opposite a RF power source linked to a top electrode) is used to cause the reaction of the carbon radicals with the catalytic particulates.  
         [0005]     Of these processes, the present invention relates to systems that use PECVD to grow CNTs. A significant limitation of existing PECVD-based systems is that they traditionally use only RF power supplies. Although some PECVD systems may have dual electrodes (e.g., top and bottom), and may even have RF supplies to both, none have DC power supplies coupled to them to apply DC power or a combination of RF and DC energy to synthesize CNTs. As a result, the CNTs grown with current methods are often “spaghetti”-like in appearance and have geometries and properties that are less controlled. Due to these deficiencies, existing methods produce lower quality CNTs, which have poor electron emission properties and resulting poor performance.  
       SUMMARY OF THE INVENTION  
       [0006]     To address the deficiencies of existing methods, a system and tool for forming CNTs using PECVD includes power supplies in addition to the RF power supplies used in traditional PECVD equipment. In one embodiment, in addition to supplying RF power, a PEVCD system includes a DC power supply to cause the growth of CNTs. In other embodiments, one or more each of RF and DC power supplies can be arranged in a number of different configurations (e.g., on bottom and top electrodes). These combinations and the variations for the different power supplies can produce superior CNTs compared with those formed using existing tools and equipment. For example, the CNTs grown with this method and equipment can be straighter and easier to control. Beneficially, straighter CNTs tend to have improved emission properties and characteristics. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  illustrates a system for forming carbon nanotubes using PECVD, in accordance with an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0008]      FIG. 1  illustrates a PEVCD system for forming carbon nanotubes using a combination of RF and DC power, in accordance with an embodiment of the invention. The PECVD system comprises a chamber  100  that houses many of the components of the system. In the chamber  100  is an electrode structure for delivering power for the PECVD process. In the embodiment shown in  FIG. 1 , the electrode structure includes a top electrode  105  and a bottom electrode  110 , implemented in this embodiment as a pair of opposing electrode plates. As described in more detail below, these electrodes  105  and  110  are coupled to various power supplies for applying energy to the reactants within the chamber  100  to cause CNT growth. Typically, the CNTs are growth on a target substrate  120 . Accordingly, the system further includes a pair of holding plates  115  to secure in place a target substrate  120  on which CNTs are to be grown. However, it can be appreciated that any of a number of mechanical and non-mechanical means can be used to secure a substrate  120  during the PECVD process.  
         [0009]     In the embodiment shown in  FIG. 1 , the top electrode  105  is coupled to a RF power supply  130  via a matching network  125 . In another embodiment, the bottom electrode  110  is coupled to a RF power supply  130  and to a DC power supply  140  via a matching network  135 . Preferably, the DC and RF power supplies  130  and  140  are coupled to their corresponding electrodes  105  and/or  110  through a plurality of electrical contact points, as shown in the figure. In other embodiments, additional or alternate RF and/or DC power supplies could be coupled to either or both of the top and bottom electrodes  105  and  110 , or such power supplies could be coupled to any other electrodes used in the system. Accordingly, a number of different configurations of electrodes and power supplies are encompassed by the scope of the invention.  
         [0010]     The matching networks  125  and  135  adjust the impedance between the corresponding RF power supply  130  and the plasma in the reactor chamber  100 . Due to possible variation of the impedance of the plasma within the chamber  100 , the effect of the applied RF power can be reduced as the process conditions change within the reactor chamber  100 . Accordingly, an unmatched impedance factor could have deleterious effects to the processing. The matching network  125  or  135  thus serves to optimize the RF power delivery and avoid power loss.  
         [0011]     In one embodiment of the PECVD system, the attached RF power supply  130  is designed to granularize a catalyst material on the target substrate  120 . CNTs are often formed on catalyst materials, typically a metal such as nickel, which have been laid down on the substrate  120  and then granularized (e.g., formed into small balls on the substrate  120 ). Accordingly, the RF power supply  130  is programmed to supply power to its corresponding electrode  105  or  110  during a pre-treatment stage of the CNT growth process, in which the catalyst material is granularized (an example of such a process described below). To perform this function, the RF power supply  130  can supply power sufficient to granularize a catalyst material on the substrate  120 . In one embodiment, the RF power supply  130  can supply power within a range of about 0.5 to about 1.5 Watts/cm 2  and within a frequency range of about 13.56 MHz to about 4 GHz.  
         [0012]     In this PECVD system, the DC power supply  140  is designed to cause the synthesis of CNTs on the granularized catalyst material on the target substrate  120 . Accordingly, the DC power supply  140  is programmed to supply DC power during a CNT growth phase. Preferably, this DC power, alone or in combination with another energy source, is sufficient to form carbon radicals or ions in a source gas in the chamber  100 , thereby causing the carbon to form CNTs on the substrate  120 . In one embodiment, the power supplied by the DC power supply  140  is sufficient by itself, without additional applied RF energy, to cause CNT growth under appropriate conditions, such as those described below. In one embodiment, the DC power supply  140  can supply a negative voltage within a range of about 100 to about 1000 V. Alternatively, the DC power supply  140  may have the opposite polarity depending on the configuration or orientation of the electrodes to which it is connected.  
         [0013]     The PECVD system further comprises a heating element  150  that can be thermally coupled to a target substrate  120  when the substrate  120  is placed in the chamber  100 . The heating element  150  is coupled to a heater power source  155 , which supplies a controllable amount of energy to the heating element  150 . In one embodiment, the heating element  150  is part of the electrode structure. As shown in  FIG. 1 , the heating element  150  is within the bottom electrode  110 . Because the bottom electrode  110  is configured to support the substrate  120  and is therefore in contact with the substrate  120 , heat energy from the heating element  150  is conducted through the bottom electrode  110  to the substrate  120 . Alternatively, heat conductive elements may be placed between the substrate  120  and the electrode  110 , allowing heat to be conducted from the heating element  150  to the substrate  120 . When thermally coupled to the substrate  120 , the heating element  150  is capable of raising the substrate  120  to an elevated temperature, which in one embodiment is within a range of about 300 to about 600° C.  
         [0014]     To control the gases within the chamber  100 , the system further includes mass control and pressure control subsystems. A gas inlet  160  is used to introduce process gases into the reaction chamber  100 , such as in the example process described below. The gas inlet  160  can receive one or more different gases used in the PECVD process and control the flow of those gases into the camber  100 . Preferably, the gas inlet  160  spreads the process gases over a wide area to provide a gas shower that delivers the gases roughly evenly over a reaction area in the chamber  100 . The gas inlet  160  may be coupled to one or a plurality of sources of process gases, using mass controllers or any other suitable means to control the gases introduced into the chamber  100 . Moreover, the gas inlet  160  may be made of an electrically conducting material, such as a metal, so its structure can also be used as the top electrode  105 , as shown in  FIG. 1 .  
         [0015]     At another end of the chamber  100 , a pressure control subsystem  165  controls the pressure of the gases within the chamber  100 . The pressure control subsystem  165  may include one or more pumps, valves, flow meters, and other components commonly used in PECVD systems for controlling gas flow and chamber pressure. These pressure control subsystem  165  may be placed at corresponding ports in the chamber  100 . In one embodiment, the pressure control subsystem  165  is capable of reducing the pressure in the chamber  100  to a pressure within a range of about 0.1 to about 10 Torr.  
         [0016]     A hole plate  170 , also called a punching plate, is disposed between the reaction area in the chamber  100  and the pressure control subsystem  165 . The hole plate  170  includes a number of holes to allow the gases to escape the chamber  100 . One function of the hole plate  170  is to make uniform the flow of the gas during evacuation or exhausting of the chamber  100 , both when fully evacuating and during processing. The gas is evacuated or drawn from the chamber  100  by way of the pressure control subsystem  165 , using for example turbo pumps. The gas passes through the hole plate  170  and then through exhausting ports of the chamber  100 . The hole plate  170  typically comprises a metal cover perforated with many holes. In addition to the function noted above, these holes prevent larger particles from entering and damaging the pump/exhaust systems.  
         [0017]     A screen plate  175  at the top of the chamber  100  confines the electrical field from the top electrode  105  to the lower electrode  110 . In one embodiment, the screen plate  175  is adjustable to cover different areas of the top electrode  105 . Accordingly, the area exposed to or affected by the plasma can be changed or modified by varying the widths of the top electrode  105  that is exposed by the screen plate  175 .  
         [0018]     Also described herein is one embodiment of a process for forming carbon nanotubes on a substrate using embodiments of the PEVCD system, described above and shown in  FIG. 1 . It will be appreciated to those of skill in the art that various modifications can be made to this method and to the structure of the tool and system described herein without departing from the scope of the invention. The following patent applications describe processes and equipment for growing or forming carbon nanotubes: U.S. application Ser. No. 10/302,126, filed Nov. 22, 2002, entitled “Method for Forming Carbon Nanotubes”; U.S. application Ser. No. 10/302,206, filed Nov. 22, 2002, entitled “Method for Forming Carbon Nanotubes”; and U.S. application Ser. No. 10/600,226, filed Jun. 19, 2003, entitled “Forming Carbon Nanotubes at Lower Temperatures Suitable for Electron-Emitting Device, and Associated Fabrication Method.” Each of these applications is incorporated by reference in its entirety.  
         [0019]     In a pre-formation step, a substrate  120  is loaded into the process chamber  100  of the system. This loading step may be performed manually or using robot handler and arm interface. In one embodiment, before the substrate  120  is placed inside the chamber  100 , a holding plate  115  comes up higher than the substrate  120 . Loading pins then lower the substrate  120  and the holding plates  115  to a secured position on the bottom electrode  110 . The holding plate  115  operates to secure the substrate  120  in position during the formation process. Thereafter, the chamber  100  door is closed, and the inside of the process chamber  100  is isolated from the other parts of the equipment. Although this step of loading is described, the substrate  120  on which CNTs are to be formed can be loaded in any appropriate technique.  
         [0020]     Once the substrate  120  is loaded into the chamber  100 , a pre-treatment step can be performed. In an example of a pre-treatment step, a catalyst material on the substrate  120  is granularized. The pressure in the process chamber  100  is lowered by the pressure control subsystem  165  to a desired pressure, approaching a vacuum pressure, and the distance between the top and bottom electrodes  105  and  110  is controlled to be a specified distance. In one embodiment, this distance is between about 1 to about 5 cm. The substrate  120  is then heated to process temperatures in the range of about 300 to about 600° C. When the pressure reaches the vacuum pressure level, process gases (e.g., gases comprising ammonia or ammonia mixed with hydrogen) are introduced into the chamber through mass flow controllers and the gas inlet  160 , producing a gas shower in the chamber  100 . After introducing process gasses into the chamber  100 , the pressure is controlled by the pressure control subsystem  165 . In one embodiment, this pressure is controlled to be within the range of about 0.1 to about 10 Torr. After the pressure stabilizes at the set value, RF power is turned on at an electrode  105  or  110 . In one embodiment, this RF power is in the range of about 0.5 to about 2.0 Watt/cm 2  and is applied for less than 10 minutes. When the processing time is complete, the applied RF power and gases are turned off, and the gases are pumped out of the process chamber  100 .  
         [0021]     To initiate synthesis of CNTs on the substrate  120 , process gasses are added to the chamber  100 . These process gases supply carbon for the CNT growth and may comprise hydrocarbon and ammonia, mixes of hydrogen, or other suitable gases. The pressure in the chamber  100  is controlled by the pressure control subsystem  165 , which in one embodiment maintains the pressure within a range of about 0.1 to about 10 Torr. After the pressure in the chamber  100  is stabilized, the substrate  120  is exposed to the process gases, which in one embodiment occurs for less than 10 minutes. The DC power supply  140 , coupled to supply DC power to the lower electrode  110 , is turned on. In one embodiment, the DC voltage applied to the electrode  110  is in the range of about −100 to about −1000 V and is applied for less than 20 minutes. After the growth is complete, the DC power is turned off and the process gases are pumped out of the chamber  100 .  
         [0022]     In a post-treatment step, process gasses are again pumped into the chamber. In one embodiment, these process gasses comprise hydrogen or hydrogen mixed with ammonia. The pressure within the chamber  100  is controlled by way of the pressure control subsystem  165 . The RF power supply  130  is then turned on. In one embodiment, the RF power supplied is within a range of about 0.5 to about 2.0 Watt/cm 2  and is applied for less than 20 minutes. After the post-processing is complete, the RF power supply  130  is turned off, and the gases are pumped out of the chamber  100 . Preferably, the hardware configuration of the PECVD system used in the post-treatment is consistent with the hardware configuration used in pre-treatment.  
         [0023]     Once the CNTs are grown on the substrate  120  and all processing is complete, the holding plates  115  are lifted to allow the substrate  120  to be removed from the chamber  100 . The lifting can be performed by loading pins and the substrate  120  retrieved by a robot arm.  
         [0024]     As described herein, a PECVD system with DC and RF power supplies allows for forming CNTs with improved properties. In one embodiment, the system comprises a standard PECVD tool that has been modified to have two electrodes (e.g., on the top and bottom) and two power supplies, where a RF power supply is coupled to the top or bottom electrodes, and a DC power supply is coupled to the bottom electrode. Alternative embodiments include any of these power supplies in other configurations (e.g., the DC power supply coupled to the top electrode) and in any combination with various known materials and devices associated with the process of growing CNTs using PECVD.  
         [0025]     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.