Abstract:
An amorphous carbon layer sticking on a carbon nanotube surface is remarkably reduced when a carbon nanotube is joined to a conductive substrate by bringing a single fibrous carbonaceous material in contact with the tip of the conductive substrate and covering at least a part of the contact portion with a conductive material while at lest either of the fibrous carbonaceous material or the conductive substrate is heated in a vacuum.

Description:
CLAIM OF PRIORITY 
       [0001]    The present application claims priority from Japanese application serial No. 2006-111499, filed on Apr. 14, 2006, the content of which is hereby incorporated by reference into this application. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to: a method and an apparatus for producing a conductive probe having a fibrous carbonaceous material at the tip thereof; and further a conductive probe suitable for use in an electron beam application device including an electron microscope. 
       BACKGROUND OF THE INVENTION 
       [0003]    In recent years, in the field of scanning probe microscopy, a probe is required to be sharpened from the viewpoint of the trend of a higher resolution. In response to the trend, Japanese Unexamined Patent Publication No. 276720/2005 proposes to form the tip of a probe with a single carbon nanotube or bundled plural carbon nanotubes (for example, about one to several tens nanometers in diameter). 
         [0004]    When a conductive probe using carbon nanotubes (CNT) is used as the electron source of an electron microscope, some conductive probes have developed the problem in that the pattern of the electron beam is unstable and the emission current varies widely. 
       SUMMARY OF THE INVENTION 
       [0005]    An object of the present invention is to provide: an electron source that yields a stable emission current having a high intensity and a narrow energy width; and also an electron beam application device on which the electron source is mounted. 
         [0006]    A method for producing a conductive probe according to the present invention is characterized by fixing a fibrous carbonaceous material to the tip of a conductive substrate while at lest either of the fibrous carbonaceous material or the conductive substrate is heated in a vacuum. Then in the fixation, it is preferable to: bring the single fibrous carbonaceous material in contact with the tip of the conductive substrate; and coat at least a part of the contact portion with a conductive material. 
         [0007]    An apparatus to produce a conductive probe according to the present invention that solves the above challenge of the present invention is characterized in that the apparatus has a heating means for heating at least either of the fibrous carbonaceous material or the conductive substrate. It is particularly convenient to have: a first retainer to retain the fibrous carbonaceous material; a second retainer to retain the conductive substrate; and a means for heating at least one of the retainers. 
         [0008]    A conductive probe produced by a method according to the present invention comprises a fibrous carbonaceous material and a conductive substrate to support the fibrous carbonaceous material and the thickness of an amorphous carbon layer with which the fibrous carbonaceous material is covered is 2 nm or less. Further, in the conductive probe, a coat comprising a conductive material is formed at the portion where the fibrous carbonaceous material touches the conductive substrate to support the fibrous carbonaceous material. When a conductive probe according to the present invention is used as a field-emission electron source in one of the various kinds of electron beam application devices such as a field-emission electron gun, an electron microscope, a length-measuring SEM, an electron beam drawing device, and the like, it is possible to obtain a highly stable emission current having a high intensity and a narrow energy width. 
         [0009]    As a result of the study on the causes of the above problem, it has been clarified that an amorphous carbon layer existing at the tip of a carbon nanotube exerts a harmful influence on the field emission characteristics.  FIG. 1  is an SEM photograph of the tip of a carbon nanotube in the case of a field-emission electron source formed by joining a single carbon nanotube to a conductive substrate. From the photograph, it is understood that the amorphous carbon layer  1  (the part of the dotted lines in the photograph) exists at the outermost layer of the graphite layer  2  of the carbon nanotube. The carbon nanotube is joined to the conductive substrate in an electron microscope. A trifle amount of hydrocarbon exists in a specimen chamber of an electron microscope and, when a carbon nanotube is irradiated with an electron beam, the hydrocarbon is decomposed and amorphous carbon forms on the surface of the carbon nanotube. 
         [0010]    In view of the above situation, the present inventors provide a joining method capable of largely reducing an amorphous carbon layer that sticks on the surface of a carbon nanotube when the carbon nanotube is joined to a conductive substrate. 
         [0011]    The present invention makes it possible to provide a joining method capable of largely reducing an amorphous carbon layer that sticks on the surface of a carbon nanotube when the carbon nanotube is joined to a conductive substrate. Further, the present invention makes it possible to provide: an electron source that yields a highly stable emission current having a high intensity and a narrow energy width; and an electron beam application device on which the electron source is mounted. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is an SEM photograph of a carbon nanotube tip of a field-emission electron source formed by joining a single carbon nanotube to a conductive substrate by a conventional method. 
           [0013]      FIG. 2  is a sectional view showing the substantial parts of an apparatus to produce a conductive probe, according to the present invention. 
           [0014]      FIG. 3  is a perspective view of a conductive probe produced by forming a carbon nanotube attachment guide with a conductive coating material according to an example of the present invention and joining a carbon nanotube to a conductive substrate. 
           [0015]      FIG. 4(   a ) is an SEM photograph of a conductive probe tip according to the present invention, and  FIG. 4(   b ) is a TEM photograph of a carbon nanotube tip. 
           [0016]      FIG. 5(   a ) is an FEM (Field Emission Microscope) image of a carbon nanotube electron source produced by a conventional production method, and  FIG. 5(   b ) is an FEM (Field Emission Microscope) image of a carbon nanotube electron source produced by a method according to the present invention. 
           [0017]      FIG. 6  is a graph showing (a) variation on time of field emission current of a carbon nanotube electron source produced by a conventional production method and (b) variation on time of field emission current of a carbon nanotube electron source produced by a method according to the present invention. 
           [0018]      FIG. 7  is a general configuration view of a scanning electron microscope (SEM) wherein an electron source according to the present invention is used. 
           [0019]      FIG. 8  is a general configuration view of an electron beam drawing device wherein an electron source according to the present example is used. 
           [0020]      FIG. 9  is a view showing the electro-optic system configuration of an electron beam drawing device wherein an electron source according to the present example and an electron gun according to the present example are used. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0021]    The embodiments of the present invention are hereunder explained in reference to drawings. 
       Embodiment 1 
       [0022]      FIG. 2  is a sectional view showing the substantial part of an apparatus to produce a conductive probe according to the present invention. The conductive probe is produced in a scanning electron microscope (SEM). A carbon nanotube cartridge  4  wherein carbon nanotubes  3  are dispersed and oriented at the edge of a blade-shaped metal plate and a conductive substrate  5  (for example, 0.15 mm in diameter) having an acicular end  5   a  obtained by forming an end of the conductive substrate into a conical shape are attached onto independently movable stages  6  and  7  respectively in a three dimensional manner. 
         [0023]    One of the features of a method for producing a conductive probe according to the present invention is to heat at least a carbon nanotube cartridge or a conductive substrate where the carbon nanotube to be bonded is placed or supported in order to prevent formation of an amorphous carbon layer that sticks to the carbon nanotube when the carbon nanotube is joined to the conductive substrate while they are observed in an SEM. 
         [0024]    As a heating method, indirect heating with a heater or direct heating by electrical heating or the like may be adopted. In the case of the present embodiment, as shown in  FIG. 2 , a sheet heater  8  was disposed between the carbon nanotube cartridge  4  and the movable stage  7 , those were fixed with a metal plate  9 , and the carbon nanotube cartridge  4  was heated to 180° C. and maintained at the temperature. The conductive substrate  5  was heated to 150° C. through radiant heat with a ring-shaped heater  10  and maintained at the temperature. 
         [0025]    As a result of heating, it was observed that amorphous carbon came to hardly stick to a CNT by heating the CNT at 80° C. to 300° C. in the case of any carbon nanotubes. Here, when the heating temperature was too high, the problem was that a SEM image drifted at the time of joining or the Vander Waals&#39; forces did not function between a CNT and a substrate, and thereby the CNT came to hardly stick to the conductive substrate. 
         [0026]    Further, the CNT tip is a field emission site and hence to prevent amorphous carbon from sticking to the site is most important. Therefore, there was no major problem even when only the cartridge side was heated. On the other hand, when the substrate was heated, the CNT could be joined without sticking amorphous carbon to the joint face and resultantly a layer of a high electric resistance was not formed and the contact resistance could be kept low. 
         [0027]    Here, a planar face  5   b  was formed at the acicular end  5   a  of the conductive substrate  5 . The carbon nanotube  3  was brought in contact with and fixed to the planar face  5   b  in parallel with the center axis of the conductive substrate  5 . The carbon nanotube  3  was brought in contact with the planar face  5   b  of the acicular end  5   a , a coat comprising a conductive material was formed at an appropriate interval or continuously at the portion where the carbon nanotube  3  touched the acicular end  5   a , and thereby the carbon nanotube  3  was firmly fixed to the planar face  5   b  of the acicular end  5   a . The carbon nanotube may widely be covered with the conductive coating layer as shown in  FIG. 3  or several parts of the carbon nanotube may be covered with the conductive coating layer  12  as shown in  FIG. 4(   a ). 
         [0028]    It is preferable that the planar face  5   b  is a plane that passes through the tip of the acicular end  5   a  and is parallel with the axis of the cone. However, the planar face  5   b  is not limited to the case, but may be a planar face that passes through the vicinity of the tip of the acicular end  5   a  and has an angle to the axis of the cone within a certain range. The important point is that, in consideration of the allowance of the emission angle of an electron beam emitted from the tip of the carbon nanotube  3 , for example the angle between the center axis of the conductive substrate  5  and the tip of the carbon nanotube  3  is set at ±3° or less and thereby the adjustment of the optical axis of the electron beam can be facilitated. Here, the planar face can be formed by FIB machining or the like. 
         [0029]    Further, in order to control the angle of the attached carbon nanotube to the center axis of the conductive substrate, it is possible to form carbon nanotube attachment guides  11  with a conductive coating material  12  as shown in  FIG. 3 . 
         [0030]    A method for forming a coat of a conductive material is explained in detail. A gas containing a conductive element is introduced into a specimen chamber of an electron microscope and at least a part of the contact portion is irradiated with an electron beam. As the gas, an organic metal gas containing carbon and a metal such as platinum, gold, tungsten or the like, a fluorinated gas, or the like, those gases decomposing with an electron beam of 5 to 100 keV and vaporizing at 100° C. or lower, is preferably used. As the gas containing a conductive element, a gas that only decomposes with a high-energy heavy ion beam such as a gallium ion beam or the like that is generally used in FIB machining or the like cannot be used. The reason is that, if the carbon nanotube is irradiated with gallium ions, the carbon nanotube itself is instantaneously damaged and hence breakage and irradiation defects occur. Likewise, as a particle beam used for the decomposition of a gas, an electron beam of 5 to 100 keV that does not cause damages to the carbon nanotube is preferably used. By forming a conductive material by the above method, it is possible to form a coat of a sufficient thickness for a short period of time. 
         [0031]      FIGS. 4(   a ) and  4 ( b ) shows electron microscopic photographs of a conductive probe produced by the aforementioned method.  FIG. 4(   a ) is an SEM photograph of a conductive probe tip, and  FIG. 4(   b ) is a transparent electron microscopic (TEM) photograph of a carbon nanotube tip. As it is obvious from the comparison with  FIG. 1 , by heating the carbon nanotube cartridge and the conductive substrate, it is possible to considerably reduce the amount of the amorphous carbon sticking to the carbon nanotube tip. 
         [0032]    The field electron emission patterns were investigated with an FEM (Field Emission Microscope) when the thicknesses of the amorphous carbon layers of carbon nanotubes were varied. The results are shown in  FIGS. 5(   a ) and  5 ( b ). 
         [0033]      FIG. 5(   a ) represents the case where the carbon nanotube cartridge and the conductive substrate were not heated and the thickness of the amorphous carbon layer at the electron emission face of the carbon nanotube tip was 5 nm. A generally obscure FEM image as shown in  FIG. 5A  was obtained. The electron emission face was the outermost surface at the carbon nanotube tip and was the face comprising a five-membered ring that was the electron emission site and a six-membered ring. Here, a part of the six-membered ring may be replaced with a seven-membered ring in some carbon nanotube tip shapes. 
         [0034]      FIG. 5(   b ) represents the case where the carbon nanotube cartridge and the conductive substrate were heated and the thickness of the amorphous carbon layer at the electron emission face of the carbon nanotube tip was 2 nm or less. An FEM image comprising plural five-membered ring patterns each of which reflects a five-membered ring as the electron emission site was obtained as shown in  FIG. 5(   b ). A typical five-membered ring pattern shows nearly a pentagon having a dark center portion as shown in  FIG. 5(   b ) but sometimes an observed pentagon is somewhat distorted due to the arrangement of the five-membered ring. It was found that the electron emission face of a carbon nanotube was kept clean when a substrate or a cartridge was joined while being heated. 
         [0035]    Successively, influence of an amorphous carbon layer at a carbon nanotube tip on the stability of an emission current was investigated. The results are shown in  FIG. 6 . 
         [0036]    In the case of a sample having an amorphous carbon layer about 5 nm in thickness, electric current was unstable and fluctuated largely. On the other hand, in the case where the thickness of an amorphous carbon layer was about 2 nm, the electric current was remarkably stabilized in contrast to the case where the thickness of an amorphous carbon layer was 5 nm. For the reason, it is necessary to control the thickness of an amorphous carbon layer to less than 5 nm. 
         [0037]    From the above results, it has been found that a conductive probe that can stabilize emission current can be provided by forming an adhesion layer while the carbon nanotube is heated. 
         [0038]    Further, the forming of an amorphous carbon layer having a high resistance on the joint surface of a conductive substrate was suppressed by heating the conductive substrate and maintaining the temperature, and thus the contact resistance between a carbon nanotube and the conductive substrate could be lowered. The contact resistance was 100 kO or more in the case of no heating and less than 10 kO in the case of heating, namely, the contact resistance in the case of heating lowered by about two digits from the case of not heating the conductive substrate. 
       EXAMPLE 1 
     Application to an Electron Beam Application Device 
       [0039]    A general configuration view of a scanning electron microscope (SEM) wherein a conductive probe according to the present invention is used as a field-emission electron source is shown in  FIG. 7 . A scanning electron microscope was provided with, in sequence along the flow of an electron beam emitted from an electron gun, an alignment coil  15 , a condenser lens  16 , an astigmatic correction coil  17 , a deflecting and scanning coil  19 , an objective lens  18 , and an objective lens aperture  22 . A specimen  20  was placed on a specimen stage  23  and irradiated with an electron beam. A secondary electron detector  21  was disposed on the side wall in a specimen chamber. Further, the specimen chamber was maintained at a high degree of vacuum through an exhaust system  24 . By the configuration, an electron beam emitted from an electron gun: was accelerated with an anode; converged with an electron lens; and was applied to a minute region on a specimen. The irradiation region was two-dimensionally scanned, secondary electrons, reflected electrons, and the like emitted from the specimen were detected with a secondary electron detector, and an enlarged image was formed on the basis of the difference of the detected signal volumes. 
         [0040]    By applying a conductive probe according to the present invention as a field-emission electron source to a scanning electron microscope, it is possible to realize the scanning electron microscope that can form a secondary electron image and a reflected electron image stably for a long period of time. Further, the resultantly obtained image has a high resolution and a high intensity. 
         [0041]    Here, the configuration of a scanning electron microscope is not limited to the example shown in  FIG. 7  and a conventional configuration can be adopted as long as the configuration can fully exhibit the characteristics of a field-emission electron source according to the present invention. 
         [0042]    Further, the basic configuration of the electro-optic system of a length-measuring SEM used for the observation and dimension measurement of a finely formed pattern in a semiconductor process is the same as that shown in  FIG. 7 . Therefore, similar results are obtained by applying a conductive probe according to the present invention to the field-emission electron source of a length-measuring SEM. 
       EXAMPLE 2 
     Application to an Electron Beam Application Device 
       [0043]      FIG. 8  is a general configuration view of an electron beam drawing device wherein a conductive probe according to the present invention is used as a field-emission electron source. The basic configuration of the electro-optic system is nearly the same as that of the aforementioned scanning electron microscope. The same reference numerals as those of  FIG. 7  denote the same elements or corresponding elements. Detailed explanation thereof is omitted to avoid superfluosness. 
         [0044]    An electron beam obtained by field emission from an electron gun  14  was stopped down with a condenser lens  16  and focused on a specimen with an objective lens  18 , and thus a beam spot of a nanometer order was obtained. On this occasion, it was preferable that the center of a blanking electrode  25  to control on/off of the electron beam irradiation onto the specimen coincided with the crossover point formed with the condenser lens. 
         [0045]    Electron beam drawing was carried out by irradiating a specimen while an electron beam was: turned on or off with a blanking electrode; and polarized and used for scanning on the specimen with a deflecting and scanning coil  19 . 
         [0046]    The function of an electron beam drawing device is to: irradiate a specimen substrate coated with a resist sensitive to an electron beam with an electron beam; and form various circuit patterns. Thus as higher definition of various circuit patterns advances, the miniaturization of an electron beam probe diameter is required. By applying a field-emission electron source according to the present invention, a very fine probe diameter causing an outstandingly high intensity is obtained in comparison with conventional devices and hence electron beam drawing of high efficiency and high definition can be obtained. 
         [0047]    Here, as shown in  FIG. 9 , by mounting a plurality of field-emission electron sources according to the present invention so as to be able to apply voltage to each of electron sources independently, it is possible to simultaneously irradiate a specimen with plural electron beams for collective drawing, and hence drawing efficiency can dramatically be improved. 
         [0048]    In  FIG. 9 , a plurality of field-emission electron sources each having a carbon nanotube  1  are supported on a electron source supporter  26 , and the electron sources are driven by a electron source drive circuit  27 . Electron beam  30  is emitted and focused on the object lens aperture  22 . Electron lens  28  is arranged to surround the electron beam  30 . Deflectors  29  are positioned in opposite relation with respect to the electron beam.