Abstract:
The present invention discloses a method for assembling carbon nanotubes and microprobe, which employs the Electrophoresis or Dielectrophoresis principles to drive the carbon nanotubes self-assembling the microprobe under an electric field. The method comprises the steps of: forming at least one microprobe, the microprobe being covered by a conductive layer; exposing the microprobe to a solution having carbon nanotubes spreading therein, the solution being furnished with an electrode; applying a predetermined voltage between the conductive layer and the electrode, making at least one carbon nanotube to move and attach onto the top of the microprobe.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a method and an apparatus for assembling carbon nanotubes and microprobes, and more particularly, to a method and an apparatus for assembling carbon nanotubes and microprobes capable of employing the principles of Electrophoresis/Dielectrophoresis to drive the carbon nanotubes self-assembling the microprobe under an electric field.  
       BACKGROUND OF THE INVENTION  
       [0002]     Carbon nanotubes are single-layered/multilayered tubular carbon molecules that have properties that make them potentially useful in nanotechnology. They exhibit unusual strength and unique electrical properties, and are extremely efficient conductors of heat A nanotube is a structure similar to a fullerene, only the carbon atoms are rolled into a cylinder instead of a sphere; each end is capped with half a fullerene molecule. They are only several nanometers wide, and their length can be millions of times greater than their width, that is, several micrometers long. Carbon nanotubes have many structures, differing in length, thickness, type of spiral, and number of layers. Although they are formed from essentially the same graphite sheet, their electrical characteristics differ depending on these variations, acting either as metals or semiconductors. Carbon nanotubes are expected to become a key material in ultrafine devices of the future, because of their unique electrical characteristics, and their extraordinarily fine structure on a nanometer scale. Other merits offered by carbon nanotubes are light weight, extremely high mechanical strength (they have larger tensile strength than steel), their ability to withstand extreme heat of 2000° C. in the absence of oxygen, and the fact that they emit electrons efficiently when subjected to electrical field. Currently, research is being conducted throughout the world targeting the application of carbon nanotubes as materials for use in photoelectric elements, electronic elements, biochemistry medication, fuel cell, etc. Moreover, the high tensile strength, semi-conductivity and flexibility features thereof can be applied to a microprobe or a microelectrode of nanometer level. However, due to the tiny diameter of the carbon nanotubes, it is difficult to assembly a carbon nanotube to a microprobe.  
         [0003]     While assembling a carbon nanotube with a microprobe, the conventional method is by adopting a chemical vapor deposition (CVD) process along with a catalyst deposition technique for growing the carbon nanotube at the place adhered with the catalyst, such as: plasma enhanced chemical vapor deposition, normal atmospheric temperature chemical vapor deposition, arc discharge, etc. However, these processes must be performed in a vacuum environment (50˜400 Torr) or under a very high temperature. Even a low temperature CVD will require at least 450˜500° C. and can only be used for multi-walled carbon nanotube (MWNTs). On the other hand, a high temperature of 1000˜1200° C. is needed for single-walled carbon nanotubes (SWNTs). Such conventional techniques will have difficulties when depositing carbon nanotubes on a large-scale substrate under low temperature. Furthermore, the catalyst is usually made of materials such as iron, nickel, or molybdenum, etc., in nanometer-sized powder which are not only expensive, but also will generate by-products like crystal or non-crystal carbides and residual catalyst during the process of carbon nanotube deposition. Therefore, an additional purification processes will be needed and the difficulty of the whole process also increase. The present invention provides a method for driving the carbon nanotubes self-assembling the microprobe under an electric field for overcoming the obstacle of assembling a nano-sized matter with a much larger structure.  
       SUMMARY OF THE INVENTION  
       [0004]     The primary object of the invention is to provide a method for assembling a carbon nanotube and a microprobe under normal atmospheric temperature and pressure.  
         [0005]     Another object of the invention is to provide a method for assembling a carbon nanotube and a microprobe capable of employing the principles of Electrophoresis/Dielectrophoresis to drive the carbon nanotubes attaching itself on the tip of a microprobe under a high electric field.  
         [0006]     Yet another object of the invention is to provide a composite by assembling carbon nanotubes and a microprobe and the carbon nanotubes being attached to the tip of the microprobe in parallel to the direction of an electric field.  
         [0007]     Another object of the invention is to provide an apparatus for assembling a carbon nanotube and a microprobe capable of performing the process of driving the carbon nanotubes to attach itself on the tip of a microprobe under a high electric field.  
         [0008]     In order to achieve the aforementioned objectives, the method for assembling carbon nanotubes and microprobe of the invention comprises the following steps: forming at least one microprobe, the microprobe being covered by a conductive layer; exposing the microprobe to a solution having carbon nanotubes spreading therein, the solution being furnished with an electrode; applying a predetermined voltage between the conductive layer and the electrode for making at least one carbon nanotube to move and attach onto the top of the microprobe.  
         [0009]     Following drawings are cooperated to describe the detailed structure and its connective relationship according to the invention for facilitating your esteemed members of reviewing committee in understanding the characteristics and the objectives of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a schematic diagram showing a system for assembling carbon nanotubes and microprobes by electrophoresis (or dielectrophoresis) effect; and  
         [0011]      FIG. 2A  to  2 E are flow charts showing the process of making a substrate with microprobes.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0012]     The invention utilizing the electrophoresis (or dielectrophoresis) effect to assembling carbon nanotubes and a microprobe under normal atmospheric temperature and pressure.  
         [0013]     Please refer to  FIG. 1 , which is a schematic diagram showing an apparatus for assembling carbon nanotubes and microprobes by electrophoresis (or dielectrophoresis) effect.  
         [0014]     As shown in  FIG. 1 , a silicon substrate  11  with at least one microprobe  12  (four probes  12  are provided in  FIG. 1  as example) formed thereon by semiconductor processing is provided. A conductive layer  13  is formed and covers the surface of the substrate  11  and the microprobe  12 . The conductive layer  13  can be made of materials such as gold, copper, aluminum and other metals or alloys, and it is preferred to form such conductive layer by electroplating or film deposition. In addition, a non-conductive layer  14  is formed and covering the conductive layer  13 . Although a photoresist is selected as the non-conductive layer  14  for the present preferred embodiment, other non-conductive material can be used as the non-conductive layer  14  also. The non-conductive layer  14  covers a predetermined area of the conductive layer  13  such that the portion of the conductive layer covering the tip  121  of the microprobe  12  is not covered by the non-conductive material  14  and is exposed.  
         [0015]     The substrate  11  along with the microprobe  12 , the conductive layer  13  and the non-conductive layer  14  are placed in a container having a solution  20  therein, such as a reaction tank, and a plurality of carbon nanotubes  21  is suspended in the solution  20 . An electrode  31  is disposed in a position separated from the substrate  11  by a predetermined distance in the solution  20 . The conductive layer  13  and the electrode  31  are connected respectively to the positive and negative of a DC power supply  45  providing a preset DC voltage via conductive grease  41 ,  42 . Since only the portion of the conductive layer  12  covering the tip  121  of the microprobe  12  is exposed to the solution  20 , the electrode  31  has a much larger area exposed to the solution  20  than that of the microprobe  12 . Therefore, an electric field concentration will occur at the tip  121  of the microprobe  12 . Under the circumstance, most of the carbon nanotubes  21  in the solution  20  will move toward the tip  121  of the microprobe  12  by electrophoresis (or dielectrophoresis) effect, in addition, at the tip of the microprobe, the carbon nanotubes driven by the electric field are oriented longitudinally parallel to the electric field which is aligned with the extending direction of the microprobe  12 , and further are attached to the tip  121  of the microprobe  12  by Van der Waal&#39;s force.  
         [0016]     In a preferred embodiment, the solution  20  includes an anionic surfactant, such as Sodium dodecyl sulfate (SDS) or other surfactant, for attaching a layer of negative charges onto the surface of the carbon nanotubes  21 . The conductive layer  13  is connected to the positive of the DC power supply  45  and the electrode  31  is connected to the negative of the DC power supply  45  (as shown in  FIG. 1 ). Thus, the negative-charged carbon nanotubes  21  will move toward the tip  121  of the microprobe  12  (positive-charged) and attached to the tip  121  of the microprobe  12  by Van der Waal force. This is called electrophoresis (EP). The mobility of the carbon nanotubes  21  depends on the molecular weight thereof and is irrelevant to the charge born in the molecule.  
         [0017]     In another preferred embodiment, the solution  20  is un-charged, such as isopropyl alcohol or other organic solution. Since the carbon nanotube  21  itself is un-charged, it will not actively move toward any electrode. However, as the conductive layer  13  exposed to the solution  20  is only at the tip  121  of the microprobe  12 , the area thereof exposed to the solution  20  is far small than that of the electrode  31 . Therefore, the electric field will concentrate and intensify in the vicinity of the tip  121  of the microprobe  12  that enables the happening of a non-uniform electric intensity distribution. Under the influence of the inhomogeneous electric field, the un-charged carbon nanotubes  21  are polarized and thus induced to move sideway, that is, a dipole moment is being generated on the surface of a particle due to the polarization effect induced by an electric field. In this regard, even both the isopropyl alcohol and the carbon nanotubes  21  are uncharged, the carbon nanotubes  21  will be affected by the inhomogeneous electric field and be driven to move toward the position with higher electric field density, and eventually are attached to that position The above phenomenon is referred as dielectrophoresis.  
         [0018]     In this preferred embodiment, the apparatus of the present invention can further comprise an ultrasonic device  46  for providing ultrasonic oscillation to the solution  20  so as to prevent the carbon nanotubes  21  from gathering together and enable the carbon nanotubes  21  to be uniformly dispersed and suspended in the solution  21 .  
         [0019]      FIG. 2A  to  FIG. 2E  shows a processing step of the substrate  11  with microprobes  12  of  FIG. 1  according to a preferred embodiment of the present invention.  
         [0020]     First, as seen in  FIG. 2A , a silicon nitride layer  52  and a mask layer  53  (e.g. a photoresist) are formed successively on the silicon substrate  51  (e.g. a silicon wafer). A photolithography and development—is applied to form several openings  531  in a predetermined position of the mask layer  53  to expose the portion of the silicon nitride layer  52  defined by the openings  531 .  
         [0021]     Following, as shown in  FIG. 2B , the device of  FIG. 2A  is processed by reactive ion etching (RIE) and uses the substrate  51  as the end of the etching process, such that the exposed portion of the silicon nitride layer  52  defined by the openings  531  is development, and then the mask layer  53  is removed from the substrate to leave only several silicon nitride columns  521  disposed on the substrate.  
         [0022]     Next, as shown in  FIG. 2C , several tapered silicon nitride probe  522  are formed on the substrate  51  by applying anisotropic etching on the silicon nitride columns  521 .  
         [0023]     Next, as shown in  FIG. 2D , a conductive layer  54  (such as gold, copper, aluminum, nickel or other metals or alloys) is formed on the substrate  51  and probe  522  by electroplating, sputtering, physical vapor deposition, chemical vapor deposition or other methods. In the preferred embodiment, the conductive layer  54  covers the entire substrate  51  and probe  522 . However, in other embodiments, the conductive layer  54  covers at least the tip of the probe  522 .  
         [0024]     Finally, as shown in  FIG. 2E , a non-conductive layer  55  is formed on a predetermined area of the conductive layer  54  in a way that only the potion of the conductive layer  541  covering the tip of the probe  522  is exposed. In the preferred embodiment, the non-conductive layer  55  can be a photoresist, other non-conductive film or polymer material. The formation is, first, covering the entire conductive layer  54  with the photoresist, and then a predetermined thickness of the photoresist is etched using RIE to expose only the tip of the probe  522 . Thus, the substrate with probe of  FIG. 1  is accomplished.  
         [0025]     Although, the material of the microprobe used in  FIGS. 2A  to  2 E is silicon nitride (SiN4), other materials, such as silicon oxide, metal or polymer can also be used for forming the probe on the substrate.  
         [0026]     While the preferred embodiment of the invention has been set forth for the purpose of disclosure, modifications of the disclosed embodiment of the invention as well as other embodiments thereof may occur to those skilled in the art. Accordingly, the appended claims are intended to cover all embodiments which do not depart from the spirit and scope of the invention.