Abstract:
A method and device for providing structurally robust and commercially feasible nanotube-based nanomechanical devices is provided. Specifically, a method of fabricating a carbon nanotube that is securely attached to a substrate, or atomic force microscopy tip, is provided by a process that uses silicide and palladium to secure the carbon nanotube to a commercially produced AFM cantilever, as well as self-aligning thin film deposition techniques.

Description:
1. RELATED APPLICATION INFORMATION  
       [0001]     This application claims the benefit of priority to Provisional Patent Application No. 60/507,666, filed Sep. 30, 2003, which application is hereby incorporated by reference in its entirety.  
       2. FIELD OF INVENTION  
       [0002]     The present invention relates to a method and device used for the fabrication of structurally robust nanotube-based nanomechanical devices. Specifically, the present invention provides a method and device for fabricating nanotube-based nanomechanical devices that not only demonstrates an extraordinarily strong attachment between the nanotubes and the surfaces to which they are attached, but is also more capable of mass fabrication than techniques currently available.  
       BACKGROUND  
       [0003]     Carbon nanotubes display unique properties 1  making them ideal for various applications. They have high aspect ratios and a Young&#39;s modulus of approximately 1 Tpa, characterizing unique strength, ideal electrical properties, and extraordinary mechanical resilience. Moreover, these properties make them ideal for force measurements and for imaging steep-walled features. Applications using carbon nanotubes include cantilever beam flexural oscillators in the megahertz range 2  and atomic    1 Wong E., Sheehan P., Lieber.;  Science  1997, 277, 1971.      2 Poncharal P., Wang, Z., Ugarte D., de Heer W.;  Science  1999, 283, 1513    
         [0000]     force microscopy as probe tips 3 . In fact, carbon nanotube use in atomic force microscopy is increasingly prevalent because it enables optimal high-resolution imaging.  
         [0004]     Atomic force microscopy is a relatively new and rapidly developing technique for imaging the nanometer scale topography of surfaces. Since its development in 1986, it has found great utility in fields ranging from semiconductor fabrication to membrane biology. In biological studies, for example, this data aids understanding of the function of nanometer scale molecular structures, an understanding that ultimately contributes to medicine. The need for nanometer scale microscopy is also increasing in the semiconductor industry. Atomic force microscopy is now finding widespread use in this industry for critical dimension metrology, surface roughness measurements and particulate contamination detection.  
         [0005]     As background, atomic force microscopy uses a raster scanned probe to image surface topography. A typical atomic force microscopy cantilever is flexible and consists of a probe with a microscopic tip. The tip is brought very close to a surface, where it experiences inter-atomic forces. The force on the tip causes the cantilever to bend. A laser beam then reflects off the top surface of the cantilever and senses the end of the cantilever. As the probe is raster scanned across the surface, it follows the ups and downs of the surface. In other words, the surface topography is followed and recorded by a computer that monitors the bending of the cantilever. A computer controls the raster scanning of the probe and displays the recorded topographic image. Atomic force microscopy holds great promise as a powerful imaging tool at the molecular level. Its excellent vertical resolution allows it to detect height changes of less than 0.1    3 Hafner J., Cheung C., Oosterkamp T., Lieber C.,  J. Phys. Chem. B  2001, 105, 743    
         [0000]     nanometers. Moreover, it can be performed under fluid at close to physiologic conditions.  
         [0006]     Though atomic force microscopy is a powerful imaging tool with excellent vertical resolution that can be performed even under fluid conditions, it has limitations. Specifically, at this point in time, lateral resolution of commercial atomic force microscopy is significantly less than its vertical resolution. The major factor currently limiting atomic force microscopy lateral resolution is the radius of curvature of the end of the atomic force microscopy tip. Current commercial atomic force microscopy tips are made from silicon or silicon nitride and have a radius of curvature between 5 and 40 nm. The large discrepancy is due to uncontrolled fluctuations occurring in the manufacturing process.  
         [0007]     In an attempt to overcome the deficiencies of silicon-based atomic force microscopy tips, methods for producing and using carbon nanotube-based atomic force microscopy tips with a radius of curvature much smaller than commercial tips are increasingly utilized. In fact, atomic force microscopy probes based on carbon nanotubes results in resolutions ten times better than found in non-carbon nanotube-based probes. The improved resolution is due to the small single wall carbon nanotubes having diameters less than 0.5 nm that are ideal for imaging, because the tip should be as precise as the object under investigation. These ultra-sharp probes significantly impact atomic force microscopy, and various written articles detail their potential impact in the biology and semiconductor industry. 4      4  Cheung, C. L., Hafner, J. H., Lieber, C. M.;  PNAS,  97, 3809 (2000); Dai, H., Hafner, J. H., Rinzler, A. G., Colbert, D. T., Smalley, R. E.;  Nature,  384, 147 (1996)    
         [0008]     A recent advance in the development of nanotube-based probes involves the growth of carbon nanotubes directly on silicon atomic force microscopy probes by chemical vapor deposition. 5  This procedure, first published in October 1999, directly grows tubes on commercially produced silicon atomic force microscopy tips through chemical vapor deposition. The procedure entails dipping commercial probes into a suspension of colloidal catalyst, then placing them in a tube furnace heated to 750° C. under a flow of hydrogen and argon. When the furnace reaches the target temperature, a small amount of ethylene is added and nanotubes grow from the catalyst particles. Experiments show that in about 90% of cases, a tube protrudes from the silicon tip apex.  
         [0009]     Though a significant improvement above silicon-based tips, carbon nanotube-based tips are also not without limitations. A weak Van der Waals attraction holds the nanotube to the silicon tip, (or in non-atomic force microscopy applications, to a substrate), which renders it problematic for imaging in fluid where most biological atomic force microscopy imaging is performed.  
         [0010]     To overcome this lack of a rigid attachment between the nanotubes to atomic force microscopy probe tips, one method 6  uses an acrylic adhesive obtained from briefly sticking the probe tip to carbon tape before manually attaching the tube. Another method 7  involves welding a nanotube onto a silicon atomic force microscopy probe tip using a scanning electron microscope (SEM) beam. Though these methods produce rigid attachment, they are time consuming and yield nanotube probes with inconsistent and variable lengths and diameters.    5 Dai, H., Hafner, J. H., Rinzler, A. G., Colbert, D. T., Smalley, R. E.;  Nature,  384, 147 (1996) and Hafner, J. H., Cheung, C. L., Lieber, C. M.,  J. Am. Chem. Soc.,  121, 9750 (1999)      6 Dai H., Hafner J., Rinzler A., Colbert D., Smalley R.;  Nature  1996, 384, 147.      7 Akita S., Nishijima H., Nakayama Y., Tokumasu F., Takeyasu K.;  J. Phys. D: Appl. Phys.  1999, 32, 1044    
         [0011]     Also, since the above methods sometimes yield nanotubes protruding from the silicon tip apex that are too long, and therefore, too flexible for ideal imaging, they are individually shortened using an electrical cleaving process. This precludes the chemical vapor deposition process from being readily scalable to mass production, because the shortening process is slow and performed one probe at a time. Hence, current nanotube fabrication processes are unsuitable and not feasible for commercial probe manufacturing.  
         [0012]     Moreover, in nanomechanical device fabrication, there is difficulty securely attaching nanotubes to three-dimensional structures. Thus, there remains a need for nanomechanical fabrication techniques and devices that are capable of mass fabrication, produce consistent nanotube lengths, result in a rigid attachment between the nanotube and the surface to which it is attached, and that enable fluid imaging where rigid attachment to the tip is critical.  
       SUMMARY AND OBJECTS OF THE INVENTION  
       [0013]     Some embodiments of the present invention provide a method and device for nanotube-based nanomechanical devices that are: capable of mass fabrication; yield rigid attachments between the nanotubes and the substrates to which they are attached; result in rigid nanotube attachment on three-dimensional structures; produce nanotube probes with consistent lengths and diameters; and in the atomic force microscopy context, enable fluid imaging where rigid attachment of the nanotube to an atomic force microscopy tip is critical.  
         [0014]     One embodiment provides a self-aligned thin-film deposition method, involving mechanical attachment of carbon nanotubes to surfaces that result in structurally robust nanotube-based nanomechanical devices. Also, this embodiment is capable of mass fabrication of nanotube-based atomic force microscopy probes and aids in making them available to a wider range of researchers. As an overview, in this embodiment, single-walled carbon nanotubes are grown by thermal chemical vapor deposition across 150 nm wide silicon dioxide trenches, where they are mechanically attached to the trench tops by selective silicon tetra acetate based silicon dioxide, or similarly suitable film, through chemical vapor deposition. Because the film deposited does not cover the portion of the nanotubes where they are suspended across the trenches, the diameter of the nanotube is not increased and its nanomechanical properties are preserved.  
         [0015]     The above embodiment of the present invention yields an improved nanomechanical fabrication process and device. Not only does it yield a rigid attachment between the nanotube and a surface and is capable of mass fabrication, but also, it is: more accurate than techniques used currently, such as lithography; more useful on three-dimensional structures where traditional lithography is difficult; and valuable for producing nanotube atomic fabrication microscopy tips for fluid imaging where rigid attachment to the atomic force microscopy is critical.  
         [0016]     In another embodiment of the present invention, a new class of carbon nanotube-based atomic force microscopy probes that overcomes the aforementioned limitations of existing nanotube-based probes is described. Specifically, in this embodiment, probes may be manufactured having very short (i.e., 3-8 nanometer) single-walled nanotubes protruding from the end of a silicon or silicon nitride atomic force microscopy tip, by using a palladium silicide thin film deposition technique, along with a subsequent silicide-etching step. A similarly suitable thin film other than palladium silicide may also be used. The probes are fabricated in a complete process capable of mass fabrication.  
         [0017]     Accordingly, it is an object of some embodiments of the present invention to use self-aligning thin-film deposition techniques to rigidly attach a nanotube to a surface that overcomes the difficulties of individual nanotube attachment.  
         [0018]     It is another object of some embodiments of the present invention to use self-aligning thin-film deposition techniques to rigidly attach a nanotube to a surface in order to define nanoscale features on atomic force microscopy probe tips.  
         [0019]     It is yet another object of some embodiments of the present invention to use self-aligning thin-film deposition techniques that produce pattern layers in registration with nanotube contacts.  
         [0020]     It is even another object of some embodiments of the present invention to use self-aligning thin-film deposition techniques that are useful on three-dimensional structures where traditional lithography is difficult.  
         [0021]     Another object of some embodiments of the present invention provides a process and device that yields a secure attachment between a nanotube and a surface to which the nanotube is attached, and is also capable of mass fabrication.  
         [0022]     It is a further object of some embodiments of the present invention to provide a process and device that makes mass fabrication of carbon nanotube-based atomic force microscopy probes possible.  
         [0023]     Yet another object of some embodiments of the present invention provides a process and device that does not require nanotube based atomic force microscopy probes to be individually shortened prior to imaging.  
         [0024]     An even further object of some embodiments of the present invention provides a carbon nanotube-based atomic force microscopy probe capable of fluid imaging.  
         [0025]     These and other objects of the present invention will become more fully apparent from the following description, drawings, and claims. Other objects will likewise become apparent from the practice of the invention as set forth hereafter.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]     The foregoing and other objects and features of the present invention will become more fully apparent from the accompanying drawings when considered in conjunction with the following description and appended claims. Although the drawings depict only typical embodiments of the invention and are thus not deemed limiting of the invention&#39;s scope, the accompanying drawings help explain the invention in added detail.  
         [0027]      FIG. 1  depicts one embodiment of the present invention portraying the self-aligning thin-film attachment method and device. Specifically,  FIG. 1 ( a ) depicts a 40 by 150 nm trench lithographically produced on a SiO 2  substrate.  FIG. 1 ( b ) depicts a carbon nanotube grown over the trench by thermal chemical vapor deposition.  FIG. 1 ( c ) depicts the substrate selectively coated with SiO 2  by a thermal chemical vapor deposition process, wherein the suspended nanotube is not coated. A similarly appropriate substance to SiO 2  may also be used as the selective coat.  
         [0028]      FIG. 2  depicts the same embodiment of  FIG. 1  and displays the heating apparatus for silicon dioxide (or similar substance) thermal chemical vapor deposition.  
         [0029]      FIG. 3 , also from the same embodiment of  FIG. 1 , depicts XPS data of native oxide ( 3 ( a )) and deposited oxide ( 3 ( b )), showing, among other things, that after deposition, an increase in strength of the SiO 2  peak is indicated and thus, indicating the deposited layer was SiO 2 . Alternatively, if using a film other than SiO 2 , analogous results would be produced.  
         [0030]      FIG. 4 , also from the same embodiment of  FIG. 1 , depicts an atomic force microscopy image of a carbon nanotube grown by chemical vapor deposition over the trenches in SiO 2 .  
         [0031]      FIG. 5 , also from the same embodiment of  FIG. 1 , depicts a nanotube buried in SiO 2    FIG. 5 ( a ) is an atomic force microscopy height image, scale bar 125 nm.  FIG. 5 ( b ) is a SEM image.  FIG. 5 ( c ) is a coaxial line scan of a nanotube buried in SiO 2  and suspended over a trench from the image taken in  5 ( a ).  FIG. 5 ( d ) depicts atomic force microscopy cross-sectional height measurements showing the distance from the top of the nanotube to the top of the trench as 8.2 nm.  
         [0032]      FIG. 6 , also from the same embodiment of  FIG. 1 , is a table displaying the deposition rate trials for times from 10-40 minutes.  
         [0033]      FIG. 7  is a typical arrangement of an atomic force microscopy probe imaging a surface.  
         [0034]     FIGS.  8 ( a ), ( b ), ( c ), ( d ), ( e ), and ( f ) depict another embodiment using a silicide process for attaching and shortening atomic force microscopy cantilevers.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]     It will be readily understood that the components of the present invention, as generally described and illustrated in the figures herein, could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of the system, device and method of the present invention, and represented in  FIGS. 1 through 8 , is not intended to limit the scope of the invention, as claimed, but is merely representative of the presently preferred embodiments of the invention.  
         [0036]     The presently preferred embodiments of the invention will be best understood by reference to the drawings wherein like parts are designated by like numerals throughout.  
         [0037]      FIG. 1  illustrates an overview of the method and device of one embodiment of the present invention. Specifically,  FIG. 1  depicts the method for attaching a carbon nanotube  12  to a surface or substrate  16  using the technique of self-aligned thin-film deposition. First, as background, self-alignment methods are currently a key technology in silicon device manufacturing 8  and benefit nanomechanical fabrication processes because they: produce patterned layers without additional lithography steps; provide more accurate alignment than lithography; and are useful on three-dimensional structures where traditional lithography is difficult. Consequently, self-aligning thin-film deposition techniques to secure rigid attachments to surfaces are especially useful for defining nanoscale features on atomic force microscopy probe tips.  
         [0038]     In the first step of the embodiment depicted in  FIG. 1 ( a ), a trench  10  is lithographically produced. Trenches  10  are produced by e-beam lithography in poly(methyl methacrylate) that is spun onto a SiO 2  substrate  16 . Dry etching is used to    8 Kerwin R., Klein D.; U.S. Pat. No. 3,475,234 1969    
         [0039]     transfer the pattern into the SiO 2,  resulting in trenches ( 10 ) 150 nm wide and 40 nm deep. Next, a nanotube  12  is grown over the trench  10  ( FIG. 1 ( b )) and a SiO 2  film  14 , or similarly suitable film, is selectively deposited over the trench  10  ( FIG. 1 ( c )) to rigidly attach the nanotube  12 . The SiO 2  film  14 , or similarly suitable film, is selectively deposited on the SiO 2  substrate  16 , so as to not cover the nanotube  12  where it is suspended  18  over the trench  10 . This results in a self-aligned attachment of the nanotube  12  where it contacts the SiO 2  substrate  16 . If this method were non-selective, the SiO 2  film  14 , or similarly suitable film, would coat the nanotube  12  in the suspended region  18 , increasing its diameter and altering its nanomechanical properties.  
         [0040]     The following describes the method in greater detail. A trenched wafer is dipped into a 150 μg/ml ferric nitrate nonhydrate in isopropyl alcohol catalyst solution 9 . Carbon nanotubes  12  are then grown on the trench  10  sample by chemical vapor deposition (CVD) at 700° C. The chemical vapor deposition is done at atmospheric pressure with flow rates of 150 sccm argon, 100 sccm hydrogen and 5.5 sccm ethylene for six minutes. The SiO 2  film  14 , or similarly suitable film, is thermally deposited from a silicon tetra acetate precursor in the reaction 10  Si(O(O)CCH 3 ) 4 (g)→SiO 2 (s)+2(CH 3 CO) 2 O(g), which occurs at 170° C. The (CH 3 CO) 2 O, or acetic anhydride (b.p. 138-140° C.), is volatile and not incorporated into the film. The silicon tetra acetate is heated to 100° C., just below its 111-114° C. melting point, while the nanotube-trench sample is held at 170° C. The pressure at the silicon tetra acetate source and the silicon sample surface are both 120 mTorr. Ambient air with relative humidity of 25% is flowed into the chamber at 10 scfm.    9 Hafner, J., Cheung C., Oosterkamp T., Lieber C.;  J. Phys. Chem. B  2001, 105, 743      10 Maruyama T., Shionoya J.;  Jpn. J. Appl. Phys.  1989, 28, L2253.    
         [0041]      FIG. 2  depicts a two stage heating process  20  specially designed for this chemical vapor deposition process. Each heater  22 ,  24  is connected to a variable voltage DC power supply and has a thermocouple to monitor temperature. The distance between the wafer substrate and the precursor sample is 12 mm. A shutter separates the wafer substrate and precursor sample and is removed during deposition. This allows precise timing of film  14  growth by blocking deposition until the heaters  22 ,  24  warm to operating temperature. The rate of SiO 2  film  14 , or similarly suitable film, growth is 0.2 nm/min, as determined by variable angle spectroscopic ellipsometry (M-2000, J.A. Woollam Co.) ( FIG. 6 ).  
         [0042]     Composition, chemical bonding and film thickness of the deposited SiO 2  film  14 , or similarly suitable film, may be studied using an SSX-100 x-ray photoelectron spectrometer with an aluminum kα monochromatic source and a hemispherical analyzer, as is partially illustrated in  FIG. 3 . The relative areas of the Si 2p peaks from bulk silicon and SiO 2  are used to calculate the film (or analogous film) thickness ( 14 ) using standard XPS theory 11 . Literature values for the mean free paths of Si 2p photoelectrons in silicon (1.6 nm) and SiO 2  (2.6 nm) are used in these calculations 12,13 .  
         [0043]     In order to obtain values for the carbon nanotubes  12  after SiO 2  film  14 , or similarly suitable film, deposition, the sample may be examined by a Digital Instruments Dimension 3100 atomic force microscopy and a Philips XL 30S FEG SEM. Nanotube  12  diameter is determined by measuring height in atomic force microscopy cross-sectional analysis ( FIG. 4 ).    11 Fadley C., Baird R., Siekhaus A W., Novakov T., Berstrom S.;  J. Electron Spectrosco.  1974, 4, 93.      12 Hochella M., Carim A.;  Surf. Sci.  1988, 197, L260      13 Suzuki M., Ando H., Higashi Y., Takenaka H., Shimada H., Matsubayashi N., Imamura M., Kurosawa S., Tanuma S., Powell C.;  Surf. Interface Anal.  2000 29, 330    
         [0044]     Specifically, in  FIG. 4 , an atomic force microscopy image of one of several carbon nanotubes  12  is displayed that was grown by chemical vapor deposition and spanned several trenches  10  in the SiO 2  substrate  16 . Landmarks near the nanotube  12  were recorded so that this specific tube could be located again and studied after SiO 2  film  14 , or similarly suitable film, deposition. The carbon nanotube  12  was measured by atomic force microscopy and found to be 1.9 nm in diameter. (Scale bar is 250 nm.) On top of the trench  10  and carbon nanotubes  12 , 11.8 nm of SiO 2  was deposited by chemical vapor deposition and left for 55 minutes at 172° C.  
         [0045]      FIG. 5  illustrates features and properties of a nanotube  12  buried in the SiO 2  substrate  16 .  FIG. 5 ( a ) shows the sample imaged with atomic force microscopy while  FIG. 5 ( b ) shows the sample imaged with SEM. Atomic force microscopy height measurements show a difference of 8.2 nm between the top of the nanotube  12  and the top of the trenches  10 . This is presented in  FIG. 5 ( c ). These atomic force microscopy height measurements are in agreement (20% lower) with ellipsometry thickness measurements.  
         [0046]     In  FIG. 5 ( c ), the atomic force microscopy line scan shows that the SiO 2 , or in other embodiments, a similarly suitable film, was deposited on the top of the trenches  10 , but not on the top of the suspended portion  18  of the nanotube  12 . This confirms that selective SiO 2  deposition is achieved and is achievable for deposition of similarly suitable films. Thus, silicon tetra acetate-based SiO 2  (or similarly suitable) chemical vapor deposition provides a self-aligned method to rigidly attach carbon nanotubes to SiO 2  structures. Other oxide structures are also compatible with this process, and the embodiments of the present invention are not limited as such. The self-aligned nature of the foregoing process allows rigid nanotube attachment on three-dimensional SiO 2  structures; such as atomic force microscopy probe tips. This process is also compatible with mass fabrication of nanotube atomic force microscopy probes, and aids in making them available to a wider range of researchers. This process is particularly valuable for producing nanotube atomic force microscopy tips for fluid imaging where rigid attachment to the tip is critical.  
         [0047]      FIG. 7  depicts another embodiment of the method and device used to develop commercially feasible carbon nanotube probes for high-resolution atomic force microscopy. Specifically,  FIG. 7  depicts typical atomic force microscopy probe  40  imaging a surface  42 . The probe  40  consists of a microscopic tip  44  attached to a flexible cantilever  46 . The tip  44  is brought very close to the surface  42 , where it experiences inter-atomic forces. The force on the tip  44  causes the cantilever  46  to bend. A laser beam is reflected off the top surface of the cantilever  46  and is used to sense the bending of the cantilever  46 . As the probe  40  is raster scanned across the surface, it follows the ups and downs of the surface  42 . These ups and downs (i.e., the surface  42  topography) are recorded by a computer that monitors the cantilever&#39;s  46  bending. A computer controls the raster scanning of the probe  40  and displays the recorded topographic image.  
         [0048]     As mentioned previously, atomic force microscopy&#39;s excellent vertical resolution allows detection of height changes of less than 0.1 nanometers and can be performed under fluid at close to physiologic conditions. However, typical atomic force microscopy tips, which are made from silicon or silicon nitride, have a relatively large radius of curvature. This results in poor lateral resolution.  
         [0049]     This embodiment of the present invention, as depicted in  FIG. 8 , provides a method and device overcoming the above-mentioned limitations of existing nanotube-based probes. Specifically, in this embodiment, small single-walled carbon nanotubes  12  may be produced having diameters less than 0.5 nm and are very short, 3-8 nm. These single-walled carbon nanotubes  12  protrude from the end of a silicon or silicon nitride atomic force microscopy tip  44 .  
         [0050]     Generally, as shown in  FIG. 8 , the method of this embodiment uses a palladium silicide  56  thin film deposition to rigidly attach the nanotube  12 . A similarly appropriate thin film may also be used. A subsequent silicide-etching step is then used to controllably expose a 3-8 nm length of nanotube  12 . Specifically, a carbon nanotube  12  ( FIG. 8 ( a )) is grown on a silicon atomic force microscopy tip  42 . A 5 nm palladium layer  54  ( FIG. 8 ( b )) is then deposited by thermal evaporation, where an interface is created between the palladium layer  54  and the silicon atomic force microscopy tip  42 . A high temperature step converts the palladium  54  and the silicon  52  from the silicon atomic force microscopy tip  42  at their interface into palladium-silicide  56  ( FIG. 8 ( c )). The unconverted palladium  54  is then removed with a palladium etch, leaving exposed carbon nanotube  12  ( FIG. 8 ( d )). The exposed carbon nanotube  12  is then removed with an oxygen plasma etcher ( FIG. 8 ( e )). Optical analysis (ellipsometry) may be used to very the expected thin films. Finally, a thin layer on the palladium-silicide  56  is electrochemically etched away to expose a short section of nanotube  12  ( FIG. 8 ( f )). The foregoing process enables mass fabrication of structurally strong nanotube-based atomic force microscopy probes that do not need individual shortening prior to imaging.