Abstract:
A carbon nanotube magnetometer including a system and method of making the same that includes a single-walled carbon nanotube network formed on a substrate. Electrodes are deposited on opposite ends of the network and a magnetic needle is deposited on the network between the electrodes. A trench is formed under the network in the substrate to facilitate movement of the needle.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims the benefit of priority of U.S. provisional patent application Ser. No. 60/815,554 filed on Jun. 15, 2006, which is hereby incorporated by reference. 
     
    
     ORIGIN OF INVENTION 
       [0002]    The invention described herein was made by an employee of the United States Government, and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    A magnetometer is a device for measuring magnetic fields. Many spaced-based mission objectives will rely on the ability to make in situ magnetic field measurements. Magnetometers are of significant utility in attitude control of spacecraft in earth orbit, and in the study of magnetosphere and planetary sciences. 
         [0004]    A popular form of magnetometer is the fluxgate magnetometer. The fluxgate magnetometer utilizes a drive coil wound around a toroidal magnetic core, in conjunction with a sense coil. The existing fluxgate design is bulky, relatively massive and consumes precious power. 
         [0005]    It is an object of the present invention to provide a magnetometer that is up to five orders of magnitude less in mass than conventional fluxgate magnetometers and consumes up to two to three orders of magnitude less operating power. In addition, the magnetometer of the present invention will enable field measurements in the range of microTeslas, and optimization will enable nanoTesla measurements. 
       SUMMARY OF THE INVENTION 
       [0006]    In an embodiment of the present invention, a strain-based carbon nanotube magnetometer is presented. In this embodiment, the invention comprises a substrate having a trench therein extending down from a top surface; a carbon nanotube network disposed on the top surface and positioned over the trench; a first electrode connected to one end of the network; a second electrode connected to an opposite end of the network; and a magnetic needle positioned on the network between the electrodes and operable to twist into and out of the trench in response to a magnetic field. 
         [0007]    In another embodiment of the present invention, a strain-based carbon nanotube system is presented. In this embodiment, the invention comprises a substrate having a plurality of trenches each extending down from a top surface of the substrate; a plurality of carbon nanotube networks each covering a respective one of the trenches; a plurality of first and second electrodes, the first and second electrodes connected to opposite ends of a respective network; and a plurality of magnetic needles each positioned on a respective one of the networks, each needle being operable to twist into and out of a respective one of the trenches. 
         [0008]    In still another embodiment of the present invention, a method of making a carbon nanotube magnetometer is presented. In this embodiment, the invention comprises the steps of providing a substrate; growing a network of carbon nanotubes on the substrate; depositing first and second electrodes on ends of the network; depositing a needle on the substrate between the electrodes; and etching away the substrate below the network. 
         [0009]    The invention will be better understood, and features and advantages thereof will become apparent from the following detailed description taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    In the drawings, like or corresponding parts are denoted by like or corresponding reference numerals. 
           [0011]      FIG. 1  is a plan view of a carbon nanotube magnetometer in accordance with one embodiment of the present invention. 
           [0012]      FIG. 2  is a side view of the magnetometer of  FIG. 1 . 
           [0013]      FIG. 3  illustrates an apparatus for forming carbon nanotubes. 
           [0014]      FIGS. 4A to 4E  illustrate the fabrication of the device of  FIG. 1 . 
           [0015]      FIG. 5  is a graph illustrating resistance versus temperature. 
           [0016]      FIG. 6  is a graph illustrating current versus voltage for various values of applied magnetic field. 
           [0017]      FIG. 7  illustrates an array of magnetometers. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    Referring now to  FIG. 1 , one embodiment is illustrated, showing a strain-based carbon nanotube magnetometer  10  formed on a substrate  12 . The magnetometer  10  includes a network  14  of carbon nanotubes connected between electrodes  16  and  18 . Electrodes  16  and  18  may comprise gold. A large aspect ratio magnetic needle  20  overlies the carbon nanotubes  14  between electrodes  16  and  18  and may be made of iron, although other metals such as cobalt and nickel, by way of example, may be used. The aspect ratio for the needle  20  may be 1:25 to 1:500, by way of example. Needle  20  may be located midway between the electrodes  16  and  18 . 
         [0019]    The needle  20  may not withstand some fabrication steps and is subject to oxidation. To protect needle  20 , a protective coatings  20   a  and  20   b  may be formed on the top and bottom respectively of the needle  20 . Protective coatings  20   a  and  20   b  may consist of chromium or other suitable coating materials. In operation, in the presence of a magnetic field, the needle  20  will deflect in proportion to the strength of the field. This deflection results in a torque or strain on the network  14 . The torque reduces the electrical conductance of the network  14  and the reduction in conductance can be sensed. The network  14  does not have strong magnetic field dependence, thus the magnetic response is dominated by the response of the needle  20 . 
         [0020]    The sensing of the reduction in conductance is accomplished with the provision of a voltage source  22  connected to electrode  16 , in conjunction with a current transconductance amplifier  24  connected to the other electrode  18 . An analog-to-digital converter  26  converts the analog output of the current transconductance amplifier  24  to a digital signal for use by a computer  28 . By utilizing three devices of the type illustrated in  FIG. 1 , a vector magnetic field measurement may be made. The voltage source, current amplifier, analog-to-digital converter, and computer could, in principle, be replaced by miniaturized application-specific integrated circuitry (ASIC) components for a system-level spaceflight ready instrument. 
         [0021]    A side view of the device of  FIG. 1  is illustrated in  FIG. 2 . As illustrated in  FIG. 2 , the substrate  12  is composed of two layers. The first layer  30  is an electrically conducting semiconductor such as silicon while the second layer  32  is an electrically insulating material such as silicon dioxide. The reason for the two different materials will be explained with respect to the manufacturing steps of  FIGS. 4A to 4E . The substrate is etched away underneath the network  14  to provide a void or trench  34 . The needle  20  has a persistent magnetization based on the fact that it is a ferromagnetic. That being the case, the magnetic dipole wants to align with the magnetic field. This causes a twisting movement of the needle  20  into and out of the trench  34 . 
         [0022]    The carbon nanotube network  14  may be fabricated by various methods. In one embodiment a CVD (chemical vapor deposition) is utilized. Initially, a sub-monolayer of thin film iron is evaporated onto the surface of substrate  12  in an evaporator to act as a catalyst. The next step in the process is the growing of the carbon nanotubes, as illustrated in  FIG. 3 .  FIG. 3  illustrates a tube furnace  36  through which extends a tube  38 . Elevated temperatures are attained within the tube  38  by means of a heater  40  surrounding the tube. Inside the tube  38  is a substrate  12  on which is grown a carbon nanotube network  14 . 
         [0023]    In an embodiment of the present invention, single-walled carbon nanotubes may be grown as opposed to double-walled carbon nanotubes. To grow the carbon nanotubes the substrate  12  is heated to 950° C. for 5 minutes in flowing feedstock gases that include methane and ethylene at 900 sccm (standard cubic centimeters per minute) units of air for the methane and 80 sccm (air) for the ethylene. At elevated temperatures, the thin film iron on the substrate  12  will liquefy and form hemispherical nanoparticles. Carbon from the feedstock gases dissociates from the hydrogen in the gases and becomes dissolved into the iron nanoparticles. When the iron catalyst becomes saturated with carbon, the carbon starts to form the nanotube end cap at the particle surface and additional dissolved carbon adds to the structure to lengthen the nanotube and eventually form the network  14 . 
         [0024]    During the growth process argon is introduced into the tube  38  to maintain an inert environment. Hydrogen is also introduced to reduce any iron catalyst that has been oxidized during air handling to its elemental form. That is, the hydrogen ensures that the catalyst is iron and not iron oxide. 
         [0025]      FIGS. 4A to 4E  illustrate an embodiment that shows the process for making a device as in  FIG. 1 .  FIG. 4A  shows the substrate  12  with the network  14 , as it would come from the tube furnace  36  of  FIG. 3 . The structure of  FIG. 4A  is suitably masked and the gold electrodes  16  and  18  are deposited, as shown in  FIG. 4B . With another masking, the needle  20  along with the chromium protective coatings are deposited as in  FIG. 4C , leaving a structure ready for etching The relatively thin silicon dioxide substrate layer  32  acts as an insulator to prevent current shunting through the silicon layer  30 . After suitable masking, and as illustrated in  FIG. 4D , the silicon dioxide is etched away under the network  14  by using an etchant such as hydrogen fluoride. 
         [0026]    Although the entire substrate may be made of silicon dioxide, the etching rate through silicon dioxide is relatively slow. Therefore, the silicon dioxide layer is very thin. The etching rate through the thicker silicon layer may be faster than the etching rate through the thinner silicon dioxide layer. The silicon is next etched away with, for example, potassium hydroxide, resulting in the trench  34 . As illustrated in  FIG. 4E , trench  34  allows needle  20  to twist into and out of the trench  34  thereby straining the network  14 . 
         [0027]    It may be desirable that a magnetometer intended for space-based science be insensitive to thermal fluctuations, such that changes in the incidence of solar radiation do not dramatically affect operation. The magnetometer of the present invention meets this requirement. Graphs  42  and  44  of  FIG. 5  illustrate the response of two different magnetometers of the type illustrated in  FIG. 1 . Resistance in ohms is plotted on the vertical axis and temperature in degrees Kelvin is plotted on the horizontal axis. The plots illustrate a fairly constant and level response for the two devices from room temperature (around 100° K.) out to 300° K. 
         [0028]      FIG. 6  is a plot illustrating the lack of magnetic field dependence. Current in milliamps is plotted on the vertical axis and voltage in millivolts is plotted on the horizontal axis. Each data point illustrated is actually the result of 10 different magnetic field values ranging from 0 T to 0.36 T in a test performed on a device as in  FIG. 1 . 
         [0029]      FIG. 7  illustrates yet another embodiment wherein an array  46  of individual carbon nanotube magnetometers on a large substrate  48 . Each magnetometer of the array  46  includes a carbon nanotube network  50 , first and second electrodes  52  and  54 , and a needle  56  disposed between the electrodes. The array  46  may be manipulated to form a plurality of individual magnetometers. The entire array as illustrated in  FIG. 7  may be used for various purposes. For example, the array  46  may find utility when the spatial variation of a magnetic field in a particular area is to be studied. There may also be applications in space and planetary science for studying various structures on the micron scale without the need for scanning and for studying highly varying magnetic fields in space. Other applications may include the reading of magnetic data without the need for swiping. The magnetometer can be used in applications where a small, compact low power magnetic field sensor is needed, such as a personal electronic compass in a cell phone or in military applications in a distributed sensor net to monitor movement of vehicles, by way of example. 
         [0030]    Various options are available for the growth of the carbon nanotubes on the substrate  48  or the substrate  12  of  FIGS. 4A to 4E . One option is to grow the carbon nanotubes over the entire substrate and then deposit the electrodes to define an operating magnetometer while leaving portions of unused network on the substrate. Another option includes trimming the network to size after growth of the carbon nanotubes. In a third option, the thin film iron is deposited only in selected masked areas on the substrate. 
         [0031]    Although a few embodiments of the present invention have been shown and described, it may be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention. Other embodiments of the invention may be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.