Abstract:
An atomic force microscope (AFM) tipped with a single-wall conductive nanotube is operated to write bits onto a metal substrate by oxidizing the surface. The oxidized microregions project above an otherwise flat surface, and can therefore be detected—that is, the written bits can be read—using the same AFM arrangement.

Description:
PRIOR APPLICATION 
     This application stems from U.S. Provisional application Ser. No. 60/165,276, filed on Nov. 12, 1999. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to storage of information, and in particular to high-density computer storage. 
     BACKGROUND OF THE INVENTION 
     At present, the magnetic hard disk is the predominant device for mass data storage in microelectronics applications. The ever-growing demand for storage capacity has engendered dramatic increases in bit density and read/write speed, even as the price per stored megabyte has fallen. Whereas in 1990 the areal density of state-of-the-art hard disks was less than 0.1 Gbit/in 2 , currently available disks may have areal densities in excess of 5 Gbits/in 2 . It is expected that design scaling and the move toward giant magnetoresistive heads will push areal densities into the upper tens of Gbits/in 2 . This growth rate cannot be sustained indefinitely, however, and conventional scaling is expected to peak in 2006. Of course, this technological limitation will not reduce the demand for greater storage capacity in less space. 
     Numerous alternatives to magnetic storage have been proposed. Nanoimprintation, for example, has been used to fabricate 400 Gbit/in 2  read-only (compact disc) devices and 45 Gbit/in 2  read-write devices (see Krauss et al.,  Appl. Phys. Lett . 71:3174 (1997); Wu et al.,  J. Vac. Sci. Technol. B  16:3825 (1998); and Cui et al.,  J. Appl. Phys . 85:5534 (1999)). Read-write heads based on scanning probes have achieved areal densities of 400 Gbit/in 2  (see Binnig et al.,  Appl. Phys. Lett . 74:1329 (1999); Mamin et al.,  Appl. Phys. Lett . 69:433 (1996); Chui et al.,  Appl. Phys. Lett . 69:2767 (1996)). Efforts have also been made to utilize scanning probe microscopes to store data by surface modification (see Betzig et al.,  Science  251:1486 (1991); Barret et al.,  J. Appl. Phys. Lett . 70:2725 (1991); and Terris et al.,  Appl. Phys. Lett . 68:141 (1996)). None of these techniques, however, has approached the current goal of a terabit per square inch. 
     While devices operating on the atomic or molecular scale surpass this threshold, they are generally not suited for commercial data storage due to stringent low-temperature requirements or the need to operate under vacuum conditions. For example, the cryogenic scanning tunneling microscope (STM) has been used to move single atoms (see Stroscio et al.,  Science  254:319 (1991)), and the vacuum STM to align C 60  molecules on copper lattices (see Cuveres et al.,  Appl Phys. A  66:S669 (1998)). 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention utilizes an atomic force microscope (AFM) tipped with a single-wall conductive nanotube, and preferably operated in the “tapping” mode, to write bits onto a metal substrate by oxidizing the surface. The oxidized microregions project above an otherwise flat surface, and can therefore be detected—that is, the written bits can be read—using the same AFM arrangement. 
     In a preferred embodiment, the AFM tip is provided with a single-walled carbon nanotube, and is operated to oxidize an atomically flat titanium surface. Using this arrangement, bit densities of 1.6 Tbits/in 2  have been achieved. Moreover, the extreme hardness and cylindrical shape of the SWNT element avoids significant tip wear, thereby preventing bit degradation during the write process and minimizing tip convolution during read operations. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing discussion will be understood more readily from the following detailed description of the invention, when taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 schematically illustrates the approach of the present invention; 
     FIG. 2 is a detailed side view of a writing tip in accordance herewith; and 
     FIG. 3 graphically illustrates measurement of SWNT tip length. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Refer first to FIG. 1, which illustrates a system  100  in accordance with the invention. A controller  105  operates an AFM head  110  in three dimensions over the surface of a substrate S. As illustrated, the surface of substrate S extends in the x,y plane, while movement toward and away from the surface occurs along the z axis. A typical scanner configuration may execute movement along the three axes using a series of independently operable piezo elements, which are united into a single tube. AFM head  110  comprises a cantilever  115 , which terminates in a tip  120 . A piezoelectric oscillator  125 , itself operated by a frequency-synthesizer module  127  of controller  105 , deflects cantilever  115  as indicated by the arrow as tip  120  passes over the substrate S. The instanteous degree of cantilever deflection is monitored by an optical arrangement comprising a laser  130 , a split-photodiode detector  135 , and a detector circuit  140 . The output of detector  140  is fed back to controller  105 . 
     An AFM can operate in any of three modes: contact mode, non-contact mode, and “tapping” mode. In contact mode, cantilever  120  is not actively deflected. Instead, the tip  115  contacts substrate S (actually, a thin layer of fluid adsorbed thereon) as it is scanned over the surface. Controller  105  moves AFM  110  head along the z axis in response to the detector signal in order to maintain a constant cantilever deflection as tip  115  is scanned over the changing surface topography. By virtue of this feedback loop, the force between tip  115  and the surface remains constant. To obtain an image of the topography of a surface, the changing z-axis position is recorded as the head is scanned over the surface. 
     In non-contact mode, piezo oscillator  125  deflects cantilever  120  at a frequency slightly above its resonance frequency, and typically with an amplitude of a few (&lt;10) nm, to obtain an AC signal from cantilever  120 . Tip  115  does not contact the sample surface, but oscillates above an adsorbed fluid layer. The detector feedback loop maintains a constant oscillation amplitude by moving the AFM head  110  along the z axis until a “setpoint” amplitude or frequency is reached. 
     In “tapping” mode, the form of operation preferred herein, cantilever  120  is oscillated at or near its resonance frequency with an amplitude ranging, typically, from 20 nm to 100 nm. For example, cantilever  120  may be 250 μm in length and composed of silicon, with a resonance frequency of 60-80 kHz; such elements are available from Digital Instruments, Santa Barbara, CA. Tip  115  lightly “taps” on the surface of substrate S during scanning, contacting the surface at the bottom of its oscillation excursion. The feedback loop maintains a constant oscillation amplitude by keeping the root mean square (RMS) amplitude of the detector signal constant, raising or lowering AFM head  110  as necessary. In this mode of operation, the surface need not have an adsorbed liquid film. Tapping mode reduces lateral forces and is well suited to the SWNT tip. 
     In accordance with the invention, bits are written by application of a voltage, via tip  115 , to an oxidizable metal surface. For example, substrate S may be a thin film of titanium with a layer of water adsorbed on the surface thereof. Tip  115  is brought into close proximity to the substrate at the bottom of each excursion, and at points where data is to be written, a negative bias is applied to the tip. This produces a local oxidation of the surface, which is manifested as a small hump whose dimensions are determined chiefly by the tip diameter and the roughness of the substrate, but also by the field strength, the scanning rate, the tip-to-surface distance when voltage is applied, and the environment. The SWNT tips employed herein are capable of writing sub-10 nm features, and at the same time permit larger features to be written more reliably. 
     Such a tip, indicated generally at  200 , is illustrated in FIG.  2 . The tip  200  is based on a commercially available tip  115  as described above, but projecting from the terminus of tip  115  is a SWNT  210 . The SWNT is preferably &lt;5 nm in diameter, and suitable carbon nanotube tips may be synthesized by chemical vapor deposition (CVD) on a commercial silicon cantilever  120  and tip  115 . A catalyst solution to promote SWNT growth is prepared by hydrolyzing FeCl 3  in ethanol to produce iron oxide. A supporting gel matrix is formed of aluminum and silicon oxides, and molybdenum oxide used as a promoter. The tip is dipped in the catalyst solution and then exposed to CVD of CH 4  at 900° C. Since the catalyst coats the entire surface of the cantilever tip, nanotubes grow at many sites and follow the contour of the surface. The sharp discontinuity at the pyramidal terminus of tip  115  allows the nanotube to extend beyond the surface, sometimes by more than a micron. 
     Tips prepared by this process often feature a single nanotube extending from the cantilever pyramid, although multiple nanotubes sometimes form small bundles. In general, SWNTs range in length from a few nanometers to over a micron. After synthesis, most tips must be trimmed in order to isolate a single nanotube less than 65 nm long that is suitable for reliable imaging and writing. A tip that is too long will either buckle during writing, producing excessively wide features, or will traverse the surface with a slip-stick motion, producing intermittent oxidation. 
     Nanotube tips may be shortened by applying a series of 500 μs voltage pulses ranging from 20 V to 60 V between the nanotube and a metal (e.g., titanium) surface. During the tip-shortening process, a commercial AFM is used to measure the (i) the RMS amplitude of the detection signal and (ii) the deflection of the cantilever as a function of tip-surface separation. The cantilever  120  is driven at resonance while it is also scanned over the substrate (in a direction orthogonal thereto). The scan range of the tip-surface separation is adjusted so that the tip only contacts the surface for the last ˜10 nm of the scan. 
     FIG. 3 shows the response from a nanotube that has been shortened to ˜45 nm. When the tip is far from the surface, the cantilever oscillates freely. As the tip is brought into contact with the surface (Z 1 ), the oscillation amplitude is reduced until the tip is in full contact with the surface (Z 2 ), at which point the RMS amplitude is zero. The tip is then lowered another 15 nm toward the surface until the SWNT buckles (Z 3 ). The length of the nanotube—i.e., its extension beyond the cantilever terminus—is given by the difference in distance between the point where contact is first made and the point where the cantilever terminus actually makes contact with the surface so the cantilever deflects linearly (that is, Z 2 -Z 4 ). 
     A preferred substrate is a 2 nm-thick conformal layer of titanium on an atomically flat α-Al 2 O 3  surface. Such a surface may be prepared by first cleaning an α-Al 2 O 3  surface with an organic solvent, followed by annealing in air at 1100° C. for 30 min. A 2-nm layer of titanium metal is then evaporated onto the α-Al 2 O 3  surface at a rate of 1 Å/s at˜10 −8  torr. 
     Our preferred substrate has a surface roughness of approximately 1 Å, which is important for two reasons. First, this degree of smoothness allows for consistent, repeatable surface patterning. Extreme discontinuities cause sticking and deformation of patterned features. Second, since the average height of the titanium oxide bits is approximately 1 nm, high surface roughness can cause ambiguity in reading features. 
     With renewed reference to FIG. 1, the system  100  includes a data-handling circuit  150  that orchestrates read and write operations and facilitates communication with standard or non-standard computer bus architectures. An interface module  155  sends commands to controller  105 , causing tip  115  to be brought adjacent a desired point on substrate S and to execute a read or write operation. When writing, tip  115  receives a 5 kHz square wave of +0.5 V and −9.5 V. Low voltages will not induce oxidation, while very high voltages will produce excessively large features. An average tip velocity of 100 μm/s may be used to produce 8-nm bits at a 20-nm pitch, which is equivalent to a bit density of 1.6 Tbits/in 2 . Using this approach, writing operations were performed at a rate of 5 kbits/s; although slow, this rate can be significantly improved through the use of multiple AFM heads. When reading, AFM head  110  is used to sense the presence or absence of a feature at each location of interest. 
     A data cache  160  directs the writing of data in write mode, and stores acquired data in read mode. The data, in turn, is received from or sent to a computer by means of an input/output module  170 . The computer&#39;s disk-operating system determines the physical locations at which data is to be written or read, and interface  155  causes the appropriate movement and operation of AFM head  110 . 
     Although the present invention has been described with reference to specific details, it is not intended that such details should be regarded as limitations upon the scope of the invention, except as and to the extent that they are included in the accompanying claims.