Patent Abstract:
An apparatus is provided for growing high aspect ratio emitters ( 26 ) on a substrate ( 13 ). The apparatus comprises a housing ( 10 ) defining a chamber and includes a substrate holder ( 12 ) attached to the housing and positioned within the chamber for holding a substrate having a surface for growing the high aspect ratio emitters ( 26 ) thereon. A heating element ( 17 ) is positioned near the substrate and being at least one material selected from the group consisting of carbon, conductive cermets, and conductive ceramics. The housing defines an opening ( 15 ) into the chamber for receiving a gas into the chamber for forming the high aspect ratio emitters ( 26 ).

Full Description:
FIELD OF THE INVENTION  
       [0001]     The present invention generally relates to an apparatus and process for selective manufacturing of high aspect emitters and more particularly to an apparatus and process for manufacturing carbon nanotubes over a large surface area.  
       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, 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.  
         [0003]     Existing methods for the production of carbon 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. The selection of the desired production process should consider carbon nanotube purity, growth uniformity, and structural control. Arc-discharge and laser techniques do not provide the high purity and limited defectivity that may be obtained by the CVD process. The arc-discharge and laser ablation techniques are not direct growth methods, but require purification, placement and post treatment of the grown carbon nanotube. In contrast to the conventional plasma-enhanced CVD (PECVD) methode, a known hot filament chemical vapor deposition (HF-CVD) technique allows one to prepare high quality carbon nanotubes without damage to the carbon nanotubes structure. Because of the lack of a need for plasma generation, a HF-CVD system apparatus is usually of simple design and low cost. As compared to thermal CVD, HF-CVD demonstrates high carbon nanotube growth rate, high gas utilization efficiency and good process stabilization over large area substrate at relatively low temperature suitable with the glass substrate transformation point (typically between 480° C. to 620° C.).  
         [0004]     The hot filaments array is the thermal activation source of the HF-CVD apparatus. Its main functions are to heat the process gas, to dissociate the hydrocarbon precursors into reactive species and fragment molecular hydrogen into active atomic Hydrogen. These active species then diffuse to the heated substrate (typically a glass panel) where catalytic carbon nanotube growth takes place. In prior art HF-CVD systems, the heated surface of thin metal filaments are converted info carbide, or carburizes, in the presence of hydrocarbon gases. The formation of carbides is known to promote filament fragility and consequently filament lifetime issues. Furthermore, the filament brittleness outcome is intensified by the hydrogen that is present in the process gas mixture. Generally the diameter of hot filaments used in conventional HF-CVD processes is small (i.e. on the order of few hundred micro meters to about 1 milimeter) and the filaments are physically supported at their extremities by a rigid grid frame, so that the filaments are stretched in a horizontal direction. During filament resistive heating, due to thermal re-crystallization, these small diameter filaments tend to expand in the linear direction. As a result, the hot and thin filaments tend to physically sag toward the substrate due to gravity; thereby producing deformed filaments and uneven filament grid gap over the planar substrate surface. As the substrate to filament distance is thus distorted by this filament sagging, the non regular shape of the hot filament grid promotes localized temperature variation and consequently growth non uniformity over large substrate area.  
         [0005]     Field emission devices that generate electron beams from electron emitters such as carbon nanotubes at an anode plate for creating an image or text on a display screen are well known in the art. 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. Each of the electron beams are received at a spot on the anode plate, referred to as a pixel on the display screen. The display screen may be small, or very large such as for computers, big screen televisions, or larger devices. However, integration of carbon nanotube field emitters over very large display requires one to address many fabrication and process quality issues that have proven difficult to overcome. These issues include uneven heating of the substrate, limited temperature range of the glass substrate during carbon nanotube growth, poor control of thermal gas dissociation, contamination of the carbon nanotube, and inconsistent process reliability due to the drift of the filament resistivity at process temperature.  
         [0006]     As mentioned above, known manufacturing methods of carbon nanotube display devices require a high temperature. These methods typically require a substrate heater and a gas dissociation source made of an array that encompasses a plurality of resistively heated metallic filaments overlying the nanotube growth region. However, for the HF-CVD of carbon nanotubes over larger display panels, equal distribution of heat required for uniform carbon nanotube growth has not been obtained due to the metallic heater filament bending, or sagging, towards the substrate due to gravity. This creates hotter localized areas where the metallic heater filament sags. The resistively heated metallic filament also provides for thermal dissociation of the process gases; however, the variation of the electrical properties of the metallic filament due to resistance drift leads to variation in the gas dissociation, radical species, and consequently in non uniformity and non reproducibility of the carbon nanotube growth process.  
         [0007]     Accordingly, it is desirable to provide an apparatus for manufacturing large scale carbon nanotube display devices. 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  
       [0008]     An apparatus is provided for growing high aspect ratio emitters on a substrate. The apparatus comprises a housing defining a chamber, and a substrate holder attached to the housing and positioned within the chamber for holding a substrate having a surface for growing the high aspect ratio emitters thereon. A heating element is positioned near the substrate and being at least one material selected from the group consisting of carbon, conductive cermets, and conductive ceramics. The housing defines an opening into the chamber for receiving a gas into the chamber for forming the high aspect ratio emitters. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and  
         [0010]      FIG. 1  is an isometric schematic of a growth chamber in accordance with an embodiment of the present invention;  
         [0011]      FIG. 2  is a side schematic view of the growth chamber of  FIG. 1 ;  
         [0012]      FIG. 3  is an isometric view of a heater element shown in  FIG. 1 ;  
         [0013]      FIG. 4  is a schematic showing the spacing of the heater element shown in  FIG. 3 ;  
         [0014]      FIG. 5  is an isometric view of another embodiment of the heater element;  
         [0015]      FIG. 6  is an isometric view of yet another embodiment of the heater element;  
         [0016]      FIG. 7  is a schematic side view of the substrate and heater element showing direct radiation from the heater element;  
         [0017]      FIG. 8  is a schematic side view of another embodiment of the substrate and heater element showing direct radiation from the heater element.  
         [0018]      FIG. 9  is a schematic side view of the substrate showing electron movement during growth;  
         [0019]      FIG. 10  is a schematic side view of a first biasing scheme in accordance with an embodiment of the present invention;  
         [0020]      FIG. 11  is a schematic side view of a second biasing scheme in accordance with an embodiment of the present invention; and  
         [0021]      FIG. 12  is a schematic side view of a third biasing scheme in accordance with an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]     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.  
         [0023]     A hot filament chemical vapor deposition apparatus is described in detail below that comprises a plurality of heated filaments having a high melting temperature, a non-metal, electric conductiveness, chemical and thermal inertness, and stability to the process gas (e.g., hydrogen and a hydrocarbon gas mixture, or other reactive gases such as O 2 , N 2 , and NH 3 ) used for carbon nanotube growth.  
         [0024]     Referring to  FIGS. 1 and 2 , a simplified schematic view of a growth chamber includes a substrate holder  11  attached to a housing  10 . The growth chamber  20  may be used to grow high aspect ratio emitters  26 , e.g., carbon nanotubes, on the substrate. A substrate heater  12  is generally positioned below the substrate holder  11  for heating a substrate  13  which is positioned on the substrate holder  11  during growth. Although the substrate heater  12  is typical in most applications (such as the fabrication of integrated circuits), applications are envisioned where it is not required and can be replaced by a water cooled substrate holder (e.g., growth of carbon nanotubes on a low melting point substrate of less than 150° C. such as polymer or plastic). An optional gas showerhead  14  receives reactive feed gas via the gas inlet  15  and is positioned above the hot filament array  17  for distributing gas evenly over the substrate  13 . The shower head  14  may not be necessary if the gas transmitted into the chamber  20  is sufficiently pressurized. A substrate for a large glass display is heated by placing it above a substrate heater  12 , which typically comprises electrical resistance wire embedded in and electrically insulated from the substrate holder  11  which provides radiative and conductive heat to the substrate holder  11  (a graphite material is the preferred embodiment use for substrate heater to minimize the reactive interaction of the substrate heater element with the reactive gases process). Because the substrate holder  11  has a large thermal mass (compared to the substrate  13 ), its temperature varies very slowly. This permits better temperature control and uniformity for a large area substrate. The substrate  13  (e.g., glass panels) is placed on the substrate holder  12 , and is heated by radiation, conduction, and/or convection. As compared to direct heating by the hot filaments, one of the key advantages of heating with the use of an additional substrate heater is that narrow glass temperature uniformity of the glass panel can be achieved while the water-cooled HF-CVD reactor walls are kept at room temperature. The substrate heater  12  allows better control for adjusting the substrate  13  temperature with the glass substrate in close contact to the substrate heater  12 , the temperatures of the two elements are quite close at all times. This offers a practical way to monitor the glass panel average temperature using thermocouples (not shown) embedded in the substrate holder.  
         [0025]     In the growth of nanotubes  26 , a catalyst (not shown) typically is deposited on the substrate  13  prior to growing the nanotubes  26 . The catalyst may comprise Nickel, or any other catalyst made of transition metal known in the industry. Finally to cool the glass panel at the end of the CNT growth process, the glass panel can be removed from the substrate heater and transferred to another load lock chamber (not shown) to speed up the reduction of temperature.  
         [0026]     In accordance with the preferred embodiment of the present invention (also referring to  FIG. 3 ), the heating element  16  is a gas dissociation source comprising a plurality of equidistant filaments  17  positioned parallel above the substrate  13 . The heating element  16  is coupled between two parallel supports  18  made of conductive material (i.e. metal, graphite, conductive ceramic) and electrically insulated from each other. Each support  18  is connected to a DC voltage source or a low frequency AC voltage source  21  which supply current to resistively heat the filaments  17 . When the filaments  17  are heated, the substrate  13  temperature starts to increase up to a certain temperature. This upper limit temperature reached by the substrate  13  is the result of both the amount of heat transfer from the filament  17  and the substrate heater  12 , and the heat conductance between the substrate  13  and the substrate holder  11 . Therefore, to improve the controllability of the substrate temperature, both the reduction of the heat transfer from the filaments  17  and the increase of the heat conductance are required. A solution to improve the controllability of substrate temperature is to use a carbon mesh-shaped array  41  ( FIG. 4 ) instead of the filaments array  17  ( FIG. 3 ). This mesh shaped array permits a reduction in the amount of heat transfer from the filament and to reduce the difference in temperature between the substrate temperature and the temperature of the substrate holder  11 . A bias is provided between the substrate holder  11  and the heating element  16 . The parallel filament array  17  is the preferred embodiment for uniform carbon nanotube  26  growth on large substrate area. For a given substrate  13  area and optimized substrate-filament distance, the filament diameter, the minimum filament length, the number of parallel filaments, and the separation between them are considered when designing for efficiency.  
         [0027]     The heating element  16  comprises an electrically conducting, high melting temperature material consisting of at least one of carbon (including graphite), conductive cermet, and a conductive ceramics (e.g., B, Si, Ta, Hf, Zr, that form a carbide and/or nitride). According to the preferred embodiment, the filaments  17  are made of straight graphite wires 0.25 mm to 0.5 mm or larger in diameter, and heated by a DC or low frequency AC current. The filaments  17  are arranged to form an array of parallel linear filaments  17  that are parallel to the plane of the substrate  13 . They are electrically connected in parallel, each having a length varying from few cm to over 50 cm. must be positioned close enough to the substrate  13  wherein the radiation pattern  61  of each overlap to provide a uniform distribution of heat to the substrate  13 . For a given filament diameter, the number of filaments  17  and the distance D between the filaments  17  is determined with respect to an optimum distance H between the filaments  17  and the substrate  13  (see  FIG. 4 ). Generally, to obtain carbon nanotube  26  growth, uniformity apart from ensuring uniform substrate temperature, the filament array  17  is designed in such a way that the distance between the filaments  17  is less than half the distance between the filaments  17  and the substrate  13 .  
         [0028]     Referring again to  FIG. 1 , a DC or low frequency AC current source  21  supplies current through connectors  22  and  23  to the supports  18  and thus to the heating element  16  for generating a radiant heat. A resistor  24  is coupled between the gas distribution element  14  and the connector  23  for biasing the gas distribution element  14  so electrons from the heating element  16  are directed away from the gas distribution element  14 . A DC voltage source  25  is coupled between the substrate holder  11  and the low frequency AC current source  21 , preferably at the center point as shown, for attracting electrons from the heating element  16  towards the substrate  13 .  
         [0029]     Referring to  FIG. 5 , a second embodiment of the graphite heating element  16  comprises a mesh  41 , positioned between the supports  18 . And a third embodiment of the heating element  16 , as shown in  FIG. 6 , comprises a hollow rod acting both as an heating source and a gas distributor  51 . The hollow rod  51  comprises an input  52  for receiving process gas and a plurality of orifices  53  for distributing the gas over the substrate  13  as indicated by the arrows  54 . As with the first embodiment, the mesh  41  and hollow rod  51  comprise an electrically conducting, high melting temperature material consisting of at least one of carbon (including graphite), conductive cermet, and a conductive ceramics (e.g., B, Si, Ta, Hf, Zr, that form a carbide and/or nitride).  
         [0030]     Referring to  FIGS. 7 and 8 , the filaments  17  radiation is exemplified as two components: one for the direct radiation from the filament  17  and another component for the indirect reflected radiation from the filament, respectively. As expected, approximately half of the radiation power is from direct radiation. The other half is from indirect radiation which is either partially reflected or absorbed by the gas distributor  14  located above the filaments  17 . The purpose of the reflector-like gas distributor  14  shape, represented in  FIG. 8 , is to reflect the radiation from the filament as much as possible downwards towards the substrate  13  and improved radiation uniformity distribution by the showerhead  14  surface facing each filament being shaped more of less like an ellipse. The filament  17  is perfectly centered with respect to this elliptic shape and this elliptic surface is very smooth and preferably coated with highly reflective material.  
         [0031]     The substrate  13  is heated by radiation from the heating element  16  and by hydrogen atom recombination. In known CVD processes, a mixture of CH 4  in H 2  flows through the chamber, and a hot filament or plasma is used to dissociate the gas precursors into CH y  and H radicals, where y=4, 3, 2, 1, 0. In the HF-CVD method of the preferred embodiment, CH y  and H are mainly generated at the surface of the hot filament  17 . These species are then transported by diffusion and convection to the substrate. Depending on the catalyst, the carbon nanotube  26  formation consumes the CH y  radicals causing their concentrations to decline to the level at which catalytic particle activation and consequently the carbon nanotube growth is reduced or stopped.  
         [0032]     One of the primary functions of the heating element  16  temperature is to set the upper limit of the gas process temperature. The heating element  16  temperature is large enough it produces a thermionic electron emission current whose intensity can be controlled by a positive bias voltage applied to the substrate  13 . The electrons interact with the process gases, because there are high densities at the surface of the heated heating element  16 . The reaction with CH 4  is well known i.e. e-+CH 4 -&gt;CH+3+H+2e. even without any acceleration voltage the electrons have an energy of 5 eV. Hence applying a bias increase or decrease the electron energy as shown in  FIG. 9 . In the absence of a substrate  13  bias, carbon nanotube  26  growth rates are slow. Thus, this thermionic electrons emission enhances the gas molecular fragmentation reactions which form the precursors necessary for the carbon nanotube  26  growth.  
         [0033]     The heating element  16  provides several advantages over known systems. First, the non-metalic material used is rigid and does not sag like known metallic filaments. During heating, the metallic filament expansion is a major cause of non-uniform carbon nanotube  26  growth. The known metallic filaments expand when heated to the operating temperatures ranging from 1500° C. to greater than 3000° C. The filament sagging induces hot spots on the glass substrate (where it sags) and relatively cold spots (where it doesn&#39;t sag). Therefore, by not sagging, the heating element  16  of the present invention provides a uniform distribution of heat over the substrate  13 . The use of carbide or nitride, which has no liquid state, avoids transformation of material characteristics due to temperature change. Secondly, during the carbon nanotube growth, the metallic filaments of the known art typically react with the hydrocarbon gases to form carbide. This carbide formation leads to more thermal-induced stress (more sagging), strong intrinsic resistivity variation and change in the work function. Therefore, one object of this invention is to provide an apparatus where the heated gas dissociation source is made of a non-metallic heating element  16  that is inert to the process reactive gases.  
         [0034]     Another advantage of the heating element  16  is an enhanced disassociation of the gas used in the growth process. In accordance with the process of the present invention in the growth of the high aspect emitters  26 , e.g., carbon nanotubes, a gas comprising CH 4  and H is applied evenly across the heating element  16  at a temperature preferrably of 1500° C. to greater than 3000° C. and a pressure in the range of 10-100 Torr, cracking the gas, thereby forming various hydrocarbon radicals and hydrogen suitable for the growth process. Referring to  FIG. 9 , electrons coming out of the hot filaments  17  pass through the vacuum region between the heating element  16  and substrate  13  and hit the substrate, causing a current flow to ground. The heating element  16 , being negatively biased to the substrate  13 , causes the electrons to accelerate and reach the substrate  13 .  
         [0035]     One of the key parameters in a HFCVD process is the production rate of atomic hydrogen at the heating element  16 . Atomic hydrogen plays a key role in the growth of carbon nanotubes  26  for two reasons: it is crucial in the generation of the hydrocarbon radicals, and it plays an important role in the fragmentation and oxide reduction of catalyst particle as well as in the growth of carbon nanotubes  26 . The difference in the characteristics of the synthesized carbon nanotubes  26  in accordance with the present invention is caused by the difference in radical species desorbed from hot surfaces at different heating element  16  temperatures. Radicals generated by the thermal decomposition of hydrocarbon gases (i.e. CH 4 ) at the hot surface react with gas phase species to produce the precursor molecules for carbon nanotube  26  growth. Control of the gas species desorbed from the heating element  16  is essential for managing of chemical kinetics for the catalytic carbon nanotube  26  growth by HF-CVD processes.  
         [0036]     Referring to  FIG. 9 , electrons are also responsible for the generation of the reactive species which will form the carbon nanotubes  26  upon impact dissociation of the gas molecules, a relevant parameter in the deposition process is the electron current flowing to the substrate  13  in the region between the heating element  16  and the substrate holder  11 . If the electric field in this region is sufficient to accelerate the heating element  16  free electrons to energies large enough to produce ionization of the gas molecules, the current collected by the substrate  13  is composed of electrons thermionically generated by the heating element  16  and electrons detached from the gas molecules due to ionization.  
         [0037]     As compared to previous art HF-CVD techniques utilizing a metal filament, the electrical resistivity of carbon, a conductive cermet, and conductive ceramics, e.g., B, Si, Ta, Hf, Zr, that form a carbide and/or nitride is greater than the resistivity of pure metal. Thus, the heated heating element  16  can be constructed with a larger diameter. This favors the mechanical strength and rigidity of the heating element  16 . It minimizes even more the sagging effect, and improves the lifetime of the heating element  16 .  
         [0038]     Because graphite heating element  16  do not form carbide (do not carburize), do not melt, and have an extremely high solid to gas phase transition temperature (about 4000° C. for graphite), a broader range of temperatures can be used during the carbon nanotube  26  growth process and contamination of the substrate and subsequently of the carbon nanotubes  26  is less likely to occur. The non-carburization of the heating element  16  is an advantage leading to a reproducible, controllable and uniform carbon nanotube  26  HF-CVD process.  
         [0039]     All processes for the carbon nanotube  26  growth by conventional chemical vapor deposition involve the generation of the active species, the transport of the active species to catalyst, and activation of the growth species at the catalyst surface. However, to achieve a high growth rate, more power into the growth system is required to generate more active radicals and deliver them to the surface as fast as possible. A hot heating element  16  is known to be a perfect radiation heat source and a saturated source of electrons as seen in  FIG. 9 . Thus, the adjunction of negative bias voltage applied to the hot heating element  16  permits the extraction and acceleration of these saturated hot electrons. At a given heating element  16  temperature, electron flow is extracted and controlled by a positive bias  25  applied to the substrate  13 . At given pressure, the biased substrate  13  is sufficient to accelerate electrons to energies suitable for fragmentation and excitation of the process gas. Therefore, collision with accelerated electron becomes mainly responsible for gas dissociation and excitation, and permits to operate at lower heating element  16  temperature. This combination of electrical potential and HF-CVD favors a better thermal management between the substrate heater and the heating element  16 . It improves the temperature control and permits carbon nanotube  26  growth at lower temperatures. With respect to the heating element  16  temperature and the system pressure (mean free path of the electron) the extraction voltage can be tuned for optimizing the gas phase reaction and the carbon nanotube  26  growth rate. The reason HF-CVD methods can lead to high growth rates are its high working pressure as compared to plasma enhanced CVD (PECVD). In high pressure biased HFCVD, the mean free path for collisions between electrons and molecules is small and thus any excess energy absorbed by the electrons from the applied electric field is quickly redistributed to the larger gas molecules by electron and molecular collisions. Consequently the spacing between the hot heating element  16  and the substrate can be increased for better thermal management and better distribution uniformity of the carbon nanotubes  26 . The experimental results show that this combination has advantages in terms of growth rate of carbon nanotube  26  quality for field emission application, over conventional HF-CVD. Therefore, the temperature of the gas molecules and electrons equilibrate at a relatively high temperature. Generation of atomic hydrogen and molecular hydrocarbon radicals occur as the result of both high energy molecular and electron collisions. In addition, the convection and diffusion velocities are increased in this high gas temperature gradient region. Thus, the absolute concentration of atomic hydrogen and molecular radicals is increased in high pressure biased HF-CVD. This contributes to a high carbon nanotube  26  growth rate. In summary, the non-metallic material used for heating element  16  in the HF-CVD process in accordance with the present invention leads to filament  17  extended life time, reduced filament  17  evaporation, and reduced nanotube  26  and substrate  13  contamination, controlled stabilized carbon flux to the substrate  13  during carbon nanotube  26  growth, and reliable and reproducible process from run to run.  
         [0040]     Referring to  FIG. 10 , an intermediate electrode  81  having an alternating current or radio frequency signal  82  applied provides a means for imparting additional energy to the process to create additional disassociation of the gas with the subsequent creation of additional species. During the catalyst induction/or carbon nanotube  26  growth step, the HF CVD reactor could run in this hybrid configuration. First, an additional AC or RF bias voltage  82  is applied between the hot heating element  16  and a plasma-grid placed underneath in the space between the heating element  16  and the substrate  13 . Second, a DC or low frequency RF substrate bias  25  could be applied to the substrate  13  to impact its surface with electrons. The function of the AC or RF bias  82  is to generate conventional plasma between the heating element  16  and the intermediate grid  81  leading to gas process dissociation and activation enhancement in this filament-grid confined region. The function of the grid  81  and the DC bias  25  is to shield the effect of ion bombardment at the substrate  13  and to accelerate only the electrons and the reactive hydrocarbon radicals towards the substrate  13 . Independent control of the different voltages with respect to the heating element  16  temperature, permits a fine tuning of the gas dissociation and electrons flowing to the substrate  13 . In this hybrid mode arrangement, the HF-CVD reactor exhibits higher process flexibility and capability.  
         [0041]     Referring to  FIG. 11 , an alternating current or radio frequency signal is applied to the heating element  16  and gas showerhead  14 , or in absence of showerhead to a thermal shield located over the heating element  16 . This arrangement results in additional energy imparted to the precursor gas, causing more efficient disassociation of the gas species. A DC substrate bias is applied to the substrate  13  to extract the saturated electron from the heating element  16  and increase the electron impact of its surface. Both hybrid configuration of HF-CVD allow for independently control of the catalyst induction and carbon nanotube growth stages, to carry out homogenous and uniform carbon nonotube  26  growth, to enhance the substrate  13  bombardment by electrons and shift down the temperature to the range where only selective carbon nanotube  26  growth is still the dominant process. These hybrid HF CVD techniques in comparison to the standard HF CVD technique show significant advantage to control the carbon nanotube  26  growth kinetics over a broader range of substrate  13  materials.  
         [0042]     Referring to  FIG. 12 , yet another embodiment comprises the gas distribution element  14  including openings  101  formed as slits parallel and below the filaments  17  that are positioned within the gas distribution element  14  for distributing the gas as indicated by the arrows  104 . The slits ( 101 ) are biased with an additional power supply  102  which allows the element to act as a control grid. The addition of this control grid allows the control of the electron flux from the aperture of the slit, while at the same time the material of the gas distributor  14  surrounding the filament  17  rods reduces infrared radiation from the filaments  17 , and serves as a gas concentrator to allow more efficient disassociation of the gas species. Controlling the electron flux can be important in the growth and nucleation of certain types of nanotubes and nanowires and can also assist in the nucleation of the nanoparticle.  
         [0043]     The heating element  16  consisting of at least one of carbon (including graphite), conductive cermet, and a conductive ceramics (e.g., B, Si, Ta, Hf. Zr, that form a carbide and/or nitride), provides a more uniform distance to substrate  13  with an homogeneous radiation heating of the substrate  13 , and a controlled electro-thermal dissociation of the gases which leads to uniform growth of the high aspect ratio emitters  26  over a large area. The high melting temperature of these materials results in a broader range of temperature during emitter growth, a substantial increase in the electron current density flowing out of the heating element  16 , and consequently an increase of thermal gas dissociation and the formation of atomic hydrogen. Furthermore, the use of these materials for the heating element  16  eliminates the risk of catalyst and emitter contamination due to evaporation of heating element  16  material (hydrogen embrittlement), provides a constant resistance value of the heating element  16  due to chemical inertness and absence of carbide formation with the heating element  16 , and consequently a stable emission current for better gas dissociation reaction from one growth to the next, and longer heating element lifetime. An important consequence of the use of these materials for the heating element  16  is the increase of atomic hydrogen production rate at the heating element  16 . The generation of larger flux of electron modulated by an electric field permits more controlled gas dissociation and temperature uniformity, as well as a more mechanically robust and stable thermionic source. These improvements result in a practical reproducible production process and equipment for low temperature growth on a large area substrate.  
       Process Example  
       [0044]     During a batch HF-CVD process, the HF-CVD reactor is evacuated at a base vacuum pressure in the low 10E-6 Torr by using primary and a turbo-molecular pump package. Once the base pressure in the reactor is reached, the heating element  16 , comprising filaments  17  for example, is heated at a temperature preferrably greater than 1500 degree C. The substrate heater  12  is also switched on and allows the substrate  13  temperature to be controlled independently from the filament  17  temperature.  
         [0045]     When the substrate  13  reaches a temperature of 350 degree C., molecular high purity hydrogen gas is flowed through a mass flow controller (MFC—not shown) over the hot filament  17 . The pressure in the reactor  10  is controlled by adjusting the throttle valve between the deposition chamber (housing  10 ) and the vacuum pump (not shown), as well as by the MFC. The MFC provides a way to introduce fixed flow rates of process gases into the HF-CVD reactor. The first step of the carbon nanotube growth consists in the catalyst particle fragmentation and reduction in hydrogen at a partial pressure of 1E-1 Torr. The pressure in the HF CVD system is monitored by a MKS pressure manometer (not shown).  
         [0046]     When the substrate  13  temperature reaches 500° C., a hydrocarbon gas (e.g., CH 4 ) is flowed and mixed to the hydrogen gas in very specific hydrogen to hydrocarbon gases ratio, and the power input into the filament array  17  is increased. At the same time the pressure in the reactor is also increased to 10 Torr and then the incubation phase of the catalyst particles (nucleation of carbon nanotubes) is initiated for the time necessary, typically a few minutes, to reach the carbon nanotube growth temperature of 550 degree C.  
         [0047]     Once at temperature, the carbon nanotube  26  growth step is started by switching on the DC and/or RF power supply  21  biasing the filaments  17  and the substrate holder  11 . Depending on the previous process condition (i.e. pressure, gases ratio, bias current flowing to the substrate) and the carbon nanotubes  26  desired (e.g., length, diameter, distribution, density, etc.), the duration of the growth may vary from 2 minutes to 10 minutes.  
         [0048]     At the end of the growth, the filament array  17 , the substrate heater  12 , as well as the bias voltage  21  are turned off, the process gas flow is switched off and the substrate  13  is cooled down to room temperature. The long cooling down step in batch HF-CVD-reactor  20  can significantly be reduced by flowing a high pressure of neutral gas (e.g., He, Ar) that increases the thermal conduction exchange with the cold wall of the reactor.  
         [0049]     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.

Technology Classification (CPC): 3