Patent Publication Number: US-2010117764-A1

Title: Assisted selective growth of highly dense and vertically aligned carbon nanotubes

Description:
TECHNICAL FIELD 
     The present invention relates in general to the growth of carbon nanotubes in a selective manner. 
     BACKGROUND INFORMATION 
     Carbon nanotubes (CNTs) have been proposed as building blocks for the future generation of computer chips due to their high thermal conductivity, large current-carrying capacity, and excellent physical and chemical stabilities. However, to integrate with conventional chips based on silicon technology, highly dense and ordered aligned CNTs are needed. Although CNTs have been produced by many different methods, most of such efforts to control CNT growth have been achieved by adjusting the precursor gases and their flow rates, synthesis pressure and temperature, external bias, and catalyst compositions and sizes. The quality of the CNTs in terms of yield, film coverage, density, alignment, uniformity and pattern formation have not been sufficient to meet the requirements of microelectronics applications. So far, the integration of CNT structures with devices on silicon chips has been very limited, and significant improvements are required. 
     Careful selection of catalysts and supporting materials are known to be critical in controlled growth of CNTs. Several groups have studied CNT growth on different catalysts and supporting metal layers. A supporting layer may be added below the catalyst layer to prevent the catalyst from reacting with or diffusing into the substrate, or to improve the adhesion between the catalyst layer and the substrate. However, in these studies, catalyst films thicker than 10 nm were used and only low-density CNTs with diameters larger than 50 nm or carbon fibers with diameters larger than 100 nm and with stacking cups or bamboo structures were obtained. In most cases, rough catalyst surfaces or large catalyst islands, formed prior to the growth, were provided as nucleation sites for the formation of large carbon nanotubes or fibers; the surface morphology of the supporting layer did not affect much of the CNT growth morphology. For growth of highly dense CNTs with small diameters, small catalyst particles or thin catalyst films need to be used, where the surface morphology and microstructures of the catalyst layer become important and have to be controlled. In a recent study, CNTs were grown on thin cobalt/titanium/tantalum/copper multi-layers for an ULSI interconnect application where a tantalum (Ta) layer was used as a barrier to prevent copper from diffusing into the substrate, and the cobalt/titanium bilayer was used to catalyze the growth of CNTs. The CNTs were found to be curly and not well aligned. This indicates that the use of a Ta layer without an appropriate match with the catalyst layer is not sufficient to achieve the growth of dense and aligned CNTs. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the foregoing needs by selective growth of dense CNT structures using a catalytic template layer. A template formed by depositing a thin iron (Fe) catalytic layer on a thin layer of tantalum (Ta) significantly enhances the growth of vertically aligned CNT arrays with densities exceeding 10 11  per cm 2 . 
     One advantage of the present invention is that it improves CNT yield, film coverage and uniformity. Another advantage of the present invention is that it produces patterned highly dense CNT films with a vertical alignment. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  shows a cross-sectional SEM image of vertically aligned highly dense CNTs grown on a Ta barrier layer on copper interconnect lines on a wafer; 
         FIG. 2  shows SEM images of CNTs grown on various supporting materials; 
         FIG. 3  shows SEM images of surface morphologies of annealed Fe layers on various supporting layers of Ta, SiO 2 , Cr and Pd, where inset images to (c) and (d) are SEM images of the surface of the Cr and Pd supporting layers after annealing without Fe deposition (scale bars are 200 nm for (a)-(d), and 1 μm for all insets, respectively); 
         FIG. 4  shows cross-sectional TEM images of Fe islands formed on Ta and SiO 2  supporting layers, where inset image (a) shows 9 nm thick Fe on Ta, inset image (b) shows 9 nm thick Fe on SiO 2 , inset image (c) shows a high resolution TEM image of a CNT grown on 3 nm Fe on Ta, and inset image (d) shows a schematic of a catalyst island formation under balance of the surface energies; 
         FIG. 5  shows SEM images of patterned vertically aligned CNTs with high densities where inset image (a) show 5, 10 and 20 μm wide highly dense vertical CNT columns grown on pre-defined patterns of 3 nm thick Fe on a Ta support, and inset image (b) show 4 μm wide highly dense vertical CNT films grown in via holes, on the bottom of which 9 nm thick Fe was deposited on Ta; 
         FIGS. 6A-6E  illustrate process steps in accordance with embodiments of the present invention; and 
         FIG. 7  illustrates an embodiment of an RF filter configured in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth such as specific device configurations, etc. to provide a thorough understanding of the present invention. However, it will be obvious to those skilled in the art that the present invention may be practiced without such specific details. 
     Refer now to the drawings wherein depicted elements are not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views. 
     Referring to  FIGS. 6A-6E , in embodiments of the present invention as described hereafter, carbon nanotubes may be grown using thermal catalytic chemical vapor deposition (CCVD) on a thick SiO 2  film (e.g., 300 nm) thermally grown on a Si wafer ( 601  in  FIG. 6A ). Substrate materials are not limited to SiO 2 . Other commonly used substrates may be used, such as silicon, aluminum oxide, quartz, glass, and various metal materials. As further described hereafter, a Fe/Ta bilayer provides a template for selective growth of vertically aligned, dense CNT films. Referring to  FIG. 6B , a film of Ta  602  is deposited on substrate  601 . Such a film may be ˜5-25 nm thick. However, the present invention is not limited to Ta. Other high surface energy materials, such as (but not limited to) tantalum nitride and tungsten may also be used. An iron (Fe) thick film  603  with a thickness of 3-9 nm is deposited by electron beam evaporation and used as a catalyst ( FIG. 6C ). The catalyst materials are not limited to iron. Other transition metals commonly used for CNTs may be used, e.g., nickel and cobalt. Annealing of Fe film  603  produces Fe islands  603  as illustrated in  FIG. 6D . The carbon nanotube  604  growth may be conducted in a quartz tube furnace (not shown). During the growth, the furnace may be ramped up from room temperature (RT) to 700° C. in hydrogen (H 2 ) with a flow rate of 11/min, and stabilized at 700° C. for 1 minute; then the growth is initiated by introducing acetylene (C 2 H 2 ) into a furnace with a flow rate of 100 ml/min. The growth is conducted at atmospheric pressure and the growth time varied from 1 to 6 min.  FIG. 1  shows a cross-section scanning electron microscopy (SEM) image of vertically aligned, highly dense CNTs grown on pre-patterned wafers in accordance with the present invention, with a CNT density of approximately 10 11 /cm 2 . 
     To demonstrate advantages of the present invention, a series of experiments were conducted to investigate the effect of the supporting material on CNT growth by thermal CCVD. First, an Fe (iron) catalyst with the same thicknesses of about 3 nm (nanometers) was deposited on different substrates, including a 300-nm-thick SiO 2  (silicon dioxide) film as well as a 20-nm-thick Ta (tantalum), Pd (palladium), and Cr (chromium) layers on a 300-nm-thick SiO2 (silicon dioxide) film. The Fe on Cr and Fe on SiO 2  produced random CNTs with low-density film coverage, where Fe on Pd resulted in the lowest growth yield, as shown in  FIGS. 2  ( a )-( c ). However, in the case of Fe on Ta, the growth was greatly enhanced yielding CNTs with high density and uniformity, as shown in  FIG. 2(   d ). Moreover, vertically aligned dense CNTs were always obtained within a range of Fe film thicknesses from 3 to 9 nm on the Ta support. In the thickness range of 3 to 9 nm, the CNT diameters were found to increase with increasing Fe thickness. On the contrary, random growth with poor coverage was always observed on the SiO 2.  Cr and Pd substrates. Additionally, different supporting layer thicknesses were investigated, including a 25 nm and a 50 nm thick Ta layer, with no apparent different effects on the CNT growth. 
     To investigate how the supporting materials affect the formation of the catalyst islands, SEM was used to examine the Fe thin films deposited on different supporting materials after the Fe films were annealed at 700° C. for 1 minute. The surface morphologies observed by SEM are shown in  FIGS. 3(   a )-( d ). For a 3 nm thick Fe layer deposited on Ta, the Fe islands formed after annealing show a narrow range of size distribution from about 15 to 30 nm, and the Fe islands are densely packed reaching a density of about  10   11 /cm 2  as shown in  FIG. 3(   a ). Similarly, the Fe islands formed on SiO 2  after annealing were 15-30 nm in size ( FIG. 3   b )  FIGS. 3(   c ) and ( d ) show the morphologies of 3 nm thick Fe layers deposited on the Cr layer and the Pd layer after annealing, respectively. The Fe layer on the Cr supporting layer was a continuous film with a very rough surface, and the annealed Fe layer on the Pd support exhibited isolated islands larger than 200 nm. 
     A 9 nm thick Fe layer was deposited on supporting layers of Ta, Cr, Pd and SiO 2 , respectively, and annealed under the same condition. It was found that the Fe island size, distribution, and density on Ta and SiO 2  were greatly influenced by the Fe film thickness, i.e., for 9 nm thick Fe on Ta, the islands were isolated with sizes ranging from about 20 to 90 nm. Similarly, annealed Fe islands on SiO 2  also showed increasing island sizes and a large size distribution. For the Cr or Pd supporting layer, the surface morphologies of the annealed Fe layer are similar to those shown in  FIGS. 3  ( c )-( d ), and there is no obvious dependence on the Fe film thickness. 
     Moreover, the different supporting layers were annealed without the Fe catalyst layer under the same conditions. The Ta support exhibits a smooth surface without pin holes after annealing, while both the Cr and Pd films became discontinuous with pin holes and large islands as shown in the insets of  FIGS. 3(   c ) and  3 ( d ). Thus, the Ta support layer shows a much better thermal stability in addition to better adhesion with the SiO 2  substrate than the Cr and Pd supporting layers. As the temperature is ramped up to 700° C. during the growth, the surface morphology of the Ta support layer remains smooth, thus providing a smooth and uniform template for the formation of uniform and fine Fe islands. In contrast, pinholes and large islands were found in both the Cr and Pd support layers after annealing, preventing the formation of uniform Fe islands. 
     Cross-sectional TEM was used to investigate the morphologies of the annealed Fe islands on the Ta and SiO 2  substrates in order to gain a better understanding of the effect of the supporting material on the formation and morphology of the Fe islands.  FIGS. 4(   a )-( b ) are TEM images of Fe islands formed after 9 nm thick Fe layers were deposited respectively on Ta and SiO 2  and annealed. The Fe islands on both supporting materials showed typical Vollmer-Weber mode of growth. The island shape, however, was distinctly different; on the Ta substrate, it had a hemispherical shape with small contact angles but on the SiO 2  substrate, a bead shape with much larger contact angles. High resolution TEM revealed that typical CNTs grown on a 3 nm thick Fe/Ta bilayer were hollow multi-wall carbon nanotubes with 5 walls and a diameter of about 10 nm, as shown in  FIG. 4(   c ). 
     The morphology and contact angle of the Fe islands can be accounted for by considering the balance of the surface energies for the catalyst island as shown in  FIG. 4(   d ) 
       cos θ=(γ sv −γ fs )/γ fv    
     where θ is the contact angle, and f, s, and v represent film, substrate and vacuum, respectively, and a pair of the subscripts refers to the interface between the indicated phases. For the case of the Fe island on Ta, 0&lt;cos θ&lt;1, implying that the surface energy of the Ta substrate exceeds that of the Fe/Ta interface. In contrast, for the Fe island on SiO 2 , 0&gt;cos θ&gt;−1, implying that the surface energy of SiO 2  is less than that of the Fe/SiO 2  interface. The observed morphologies of the Fe islands are consistent with the relative magnitudes of the reported surface energies, i.e. ˜2100-2200 ergs/cm 2  for Ta, 43-106 ergs/cm 2  for SiO 2 , and 1880-2150 ergs/cm 2  for Fe. These surface and interface energies control the island size formed on the substrate depending on the amount of Fe overlayer deposited. 
     During the CCVD process of the present invention, carbon (C) atoms are firstly driven by chemical potential gradient to the Fe island surface, then form sp2 carbon fragments. Subsequently, after the nucleation, more C atoms are incorporated into carbon fragment edges and sustain the growth. Both bulk (D b ) and surface diffusion (D s ) can transport C atoms to the growth edge. However, as catalyst particles are reduced to the smaller sizes, surface diffusion becomes the major mass transport mechanism due to large surface area volume ratios. (See, Wang, Y. Y., Gupta, S., Nemanich, R. J., Liu, Z. J. and Lu, C. Q.,  Hollow To Bamboolike Internal Structure Transition Observed In Carbon Nanotube Films,  J. Appl. Phys 98, 014312 (2005); Helveng S., López-Cartes C., Sehested J., Hansen P. L., Clausen B. S., Rostrup-Nielsen J. R., Abild-Pedersen F. and Nrskov J. K.,  Atomic - Scale Imaging Of Carbon Nanofibre Growth,  Nature 427, 426-429 (2004); and Raty, J., Gygi, F. and Calli, G.,  Growth of Carbon Nanotubes on Metal Nanoparticles: A Microscopic Mechanism From Ab Initio Molecular Dynamics Simulations,  Phys. Rev. Lett. 95, 096103 (2005)). Although surface diffusion is a fast process due to a lower activation energy compared to the bulk diffusion, it is greatly influenced by the shapes and curvatures of the surface structures, therefore resulting in distinctly different growth scenarios, i.e., uniform vs. random growth. Here surface diffusion is facilitated by an acute θ of less than 90° as for the case of Fe on Ta, but not for an oblique θ exceeding 90° as for the case of Fe on SiO 2 . This difference seems to have a significant effect in the aligned growth of the CNTs. Combined with the formation of uniform Fe islands with a narrow size distribution on the Ta support, the fast surface diffusion due to the acute θ and the high catalyst activity associated with small particle sizes may have facilitated the growth of dense CNTs that support each other to grow in the vertical direction. 
     To demonstrate that the Ta support allows for the growth of vertically aligned and dense CNTs on a pre-defined pattern, electron beam lithography (EBL) and a lift-off process was used to form 5, 10, and 20 μm wide square patterns of 9 nm thick Fe on 20 nm thick Ta support, along with the thermal CCVD method previously described to grow highly dense CNT columns on the patterns, as shown in  FIG. 5(   a ). 
     The method may also be employed with a Ta support to grow CNT films in patterned via holes. In the fabrication process, a 20 nm thick Ta layer is sputtered on a substrate, and a 500 nm thick SiO 2  film deposited on the Ta layer. About 260 nm thick polymethyl-methacrylate (PMMA) is spun on the SiO 2  film and patterned using EBL. Via holes are etched into the SiO 2  film with the PMMA pattern as an etching mask. Subsequently, a 9 nm thick Fe layer is deposited on the wafer. Only the Fe and Ta films deposited on the bottom of the via holes are left after the PMMA layer is stripped in acetone. As shown in  FIG. 5(   b ), highly dense CNTs may be grown from the Fe catalyst patterned at the bottom of the 4 μm wide via hole with the use of the thermal CCVD method. 
     This CNT growth method employing a Ta support can be readily used to grow 
     CNT via structures on ULSI copper interconnect structures since Ta is used as the barrier layer on copper interconnects. This method, which is simple without the need of complicated equipment such as a microwave plasma CVD tool, can also be used for selective growth of highly dense, vertically aligned, and high-quality CNTs directly on a metal electrode for other applications. 
       FIG. 7  illustrates a schematic of a waveguide-embedded nanotube array RF filter configured in accordance with embodiments of the present invention. Other devices, such as supporting structures, vias in microchips and field emitters in a flat panel display, may be constructed with dense groupings of aligned CNTs grown in accordance with the present invention. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.