JP2010190587A - Method of evaluating electrical characteristicss of carbon nanotube - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of simply manufacturing a switching element containing a carbon nanotube, and to provide a method of measuring the electrical characteristics of the switching element. <P>SOLUTION: A method of evaluating the electrical characteristics of the carbon nanotube includes the step of sequentially forming a membrane containing the carbon nanotube and an upper electrode membrane divided into two or above independent regions, the step of applying potential difference across the lower and upper electrode membranes of a substrate to evaluate the electrical characteristics of the substrate, and the step of simultaneously analyzing the distribution of the average long diameter of the carbon nanotube, the distribution of the average diameter thereof, the ratio of SWCNT/MWCNT thereof, the ratio of metallic CNT and semiconductive CNT thereof and the content of impurities thereof on the basis of the evaluation results of the electrical characteristics of the substrate. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for manufacturing a simple switching element including carbon nanotubes and a method for measuring electrical characteristics of the element.

  Carbon nanotubes (hereinafter also referred to as CNTs) are being studied as a potential material for nanotechnology in a wide range of fields. For example, a method using a single CNT wire such as a transistor or a probe for a microscope, or a method using many CNT films such as an electron emission electrode, a fuel cell electrode, or a conductive composite in which CNTs are dispersed Etc. (See Patent Documents 1 and 2) As described above, there are many uses and application fields that make use of the specific electrical characteristics of carbon nanotubes, and there is a simple and efficient method for measuring electrical characteristics when a CNT film is formed Needed.

Its structure is a sheet in which hexagons of benzene ring units are arranged in a cylindrical shape, and becomes a single layer (SWCNT) or a plurality of stacked cylindrical shapes (MWCNT) by the synthesis method. Furthermore, the band structure changes depending on the arrangement of the six-membered ring and the diameter thereof, and the electrical conductivity changes, so that it is distinguished from metallic CNT and semiconducting CNT.
Further, impurities such as metals, metal oxides, and organic compounds derived from the catalyst or the purification process are mixed in or remain depending on the manufacturing method, which affects characteristics such as electrical conductivity.

  It is considered that the electrical properties of the carbon nanotube film are greatly influenced by these physical properties and impurities. It is currently very difficult to selectively synthesize CNTs having different properties or to separate and purify a mixture, and those obtained by ordinary synthesis methods often contain the above-mentioned ones.

JP 2008-166591 A US Patent No. 2007/236325

  The present invention creates a switching element including carbon nanotubes, and based on the electrical property evaluation results, the length and thickness distribution of carbon nanotubes, the ratio of SWCNT / MWCNT, the ratio of metallic CNT to semiconducting CNT, and the impurity content Including a method of analyzing the CNTs simultaneously and quality-controlling the CNTs.

  The present invention includes a step of sequentially forming a film containing carbon nanotubes and an upper electrode film divided into two or more independent regions, and applying a potential difference to the lower electrode film and the upper electrode film of the substrate to improve electrical characteristics. Based on the evaluation step and the electrical property evaluation result of the substrate, the average long diameter distribution, the average diameter distribution, the SWCNT / MWCNT ratio, the metallic CNT and semiconducting CNT ratio, and the impurity content are simultaneously determined. It is characterized by including a method of analysis.

A method for evaluating electrical characteristics of CNT, wherein the electrode is tungsten (W), aluminum (Al), copper (Cu), or titanium nitride (TiN _x ).

The method for evaluating electrical characteristics of CNTs, wherein the upper electrode is square, rectangular, circular or elliptical.

The electrical property evaluation method of CNT whose shortest distance of the said upper electrode and the said lower electrode edge part is 1-100 nm.

The area of the upper electrode is ^{1.0 × 10 -8 m 2 ~1.0 ×} 10 -4 m 2, electrical characterization method of CNT.

  According to the present invention, the distribution of the average major axis and the average diameter of the CNT carbon nanotubes can be analyzed by evaluating the electrical characteristics of the CNT-containing film by a simple method. In addition, it is possible to quantitatively analyze the SWCNT / MWCNT ratio, the ratio of metallic CNT and semiconducting CNT, the amount of mixed organic impurities and metallic impurities in the materials that are conventionally difficult to quantitatively measure. Furthermore, by comparing the measurement points on the wafer surface and the production lots of the CNT solution, it is possible to analyze the variation in the measurement values and apply it as a quality assurance item for the material.

It is the conceptual diagram which showed the evaluation method of this invention. It is a pattern conceptual diagram of the lower electrode of this invention. It is a pattern conceptual diagram of the CNT film | membrane of this invention. It is a pattern conceptual diagram of the upper electrode of the present invention.

  Hereinafter, the method for measuring the electrical characteristics of CNT according to the present invention and the creation of an evaluation element for measuring electrical characteristics (hereinafter also referred to as an evaluation element) will be described in order with reference to the drawings.

1. Method for Analyzing Switching Element Containing CNT Hereinafter, a method for producing a switching element containing CNT, a method for measuring its electrical characteristics, and a method for analyzing a measurement result will be described in detail.

1.1 Step of Forming Electrode Film and Film Containing CNT In the method of the present invention, a lower electrode film, a film containing CNT, and an upper electrode film divided into two or more independent regions are sequentially formed on a substrate. (FIG. 1). As the substrate, although not limited, a silicon wafer, silicon carbide, carbide, glass, or a resin substrate can be preferably used.

  As a first step in creating the evaluation element, a lower electrode is formed on the entire surface by a vapor deposition method using a hard mask or the like on the substrate, or a pattern as shown in FIG. By forming an electrode on the entire surface or forming a pattern as shown in FIG. 2, it is sufficient to connect to only one point on the substrate to which the external voltage terminal is connected, and by forming external terminal junctions at multiple points. Measurement errors due to short circuits or differences in electrode resistance between a plurality of measurement points, or complicated electrical circuits can be omitted.

Any material can be used for the lower electrode and the upper electrode as long as they are conductive metals, and the upper electrode and the lower electrode may be the same material or different materials. A material that does not react with CNT is preferred. For example, tungsten (W), aluminum (Al), copper (Cu), and titanium nitride (TiN _x ) can be preferably used. The electrode film can be formed by a method suitable for each material, such as a vapor deposition method or a sputtering method, but a method that does not cause damage by altering the film containing CNT is preferable.

  Next, the CNT layer is formed on the lower electrode over the entire substrate or a pattern as shown in FIG. It is preferable that the deposited film thickness is uniform within the wafer surface, specifically, the error of the film thickness between multiple evaluation elements is preferably within 5%, and the measurement error between multiple evaluation elements can be minimized. . At this time, coating film formation using CNTs dispersed in water or an organic solvent is preferable. In some cases, solid CNTs may be deposited to form a film. As other film forming methods, general methods such as a casting method, a spray coating method, an ink jet method, a blade coating method, a dip method, a bar coater method, and a dropping method can be used.

  After the CNT layer is formed, the upper electrode is formed by coating or vapor deposition to form a pattern as shown in FIG. At this time, the upper electrode is preferably circular or elliptical, and ashing / etching is performed using a hard mask having a circular or elliptical pattern. Alternatively, a photosensitive resist may be used for pattern formation.

The upper electrode is preferably circular or elliptical. Due to the circular or elliptical shape, there are no corners, and no electric field concentration occurs when a potential difference is applied, and stable measurement can be performed. The shortest distance between the end portions of the upper electrode is preferably 1 to 100 nm. When the shortest distance is 1 to 100 nm, the electric field leakage between adjacent electrodes is reduced, and a stable measurement result can be obtained. Furthermore, the area of the electrode is preferably 1.0 × 10 ⁻⁸ m ^{2 to} 1.0 × 10 ⁻⁴ m ² . Since the electrode area is 1.0 × 10 ⁻⁸ m ^{2 to} 1.0 × 10 ⁻⁴ m ² , the influence of the film defects of the electrode film and the film containing CNT is suppressed to the minimum, and stable measurement can be performed. It can be carried out.

  When the lower electrode and the CNT layer are patterned by the above-described method, the pattern edge part is peeled off, the pattern is collapsed or damaged, and the measurement error between a plurality of evaluation elements is affected. It is preferable to form a film on the entire surface.

1.3 Step of Evaluating Electrical Characteristics by Providing Voltage Difference The method of the present invention includes a step of evaluating electrical properties by providing a voltage difference between the lower electrode film and the upper electrode film of the substrate.
A constant voltage is applied between the lower electrode and the upper electrode of the measurement substrate for a predetermined time. At this time, a time for switching the switch from OFF to ON (0 → 1) is recorded (response time is recorded). A voltage is sequentially applied to a plurality of evaluation elements existing on the wafer, and measurement is performed in the same manner. When the CNT average major axis is large, the number of junction points required for energization can be reduced, so that the response time is shortened. When the average CNT diameter is large or the MWCNT mixing rate is large, the conductivity is increased and the response time is shortened. If the amount of semiconducting CNT, organic impurities, and metal impurities in the material is large, these become factors that lower the conductivity, and the response time becomes long. By measuring these parameters in advance and analyzing the correlation with the response time, the influence of each factor on the response time is quantitatively correlated, and by setting a tolerance, quality control as a CNT with a specific response time Is possible. Further, by comparing the measurement points on the wafer surface and the production lots of the CNT solution, it is possible to analyze the variation of the measurement values and apply it as a material quality assurance item.

2.1 CNT
CNTs that can be used in the present invention are not particularly limited, but those obtained by an arc discharge method, a chemical vapor deposition method (hereinafter referred to as a CVD method), a laser ablation method, or the like are preferable. In addition, a single-walled CNT (hereinafter referred to as SWCNT) in which one carbon film (graphene sheet) is wound in a cylindrical shape, and a two-layer CNT in which two graphene sheets are wound in a concentric circle ( (Hereinafter referred to as DWCNT) and multi-layer CNT (hereinafter referred to as MWCNT) in which a plurality of graphene sheets are concentrically wound. . In particular, SWCNT and DWCNT can be preferably used because they have excellent properties in terms of conductivity and semiconductor properties. Among them, SWCNTs that have properties in terms of conductivity and semiconductor properties are more preferably used than DWCNT.

  When producing CNTs, fullerene, graphite, and amorphous carbon are produced as by-products at the same time, and catalyst metals such as nickel, iron, cobalt, yttrium, etc. remain, so these impurities are removed and purified. It is preferable. In order to remove impurities, sonication is effective together with acid treatment with nitric acid, sulfuric acid, hydrofluoric acid, etc., or base treatment with tetramethylammonium hydroxide (TMAH), ammonia, potassium hydroxide, etc. It is more preferable to use separation by centrifugation together in order to improve purity. Although the diameter of CNT used by this invention is not specifically limited, 0.8 nm or more and 100 nm or less are preferable, More preferably, it is 50 nm or less, More preferably, it is 10 nm or less.

  In the present invention, CNT can be used as it is after the above purification, but when the coating film is used as a semiconductor, in order to prevent a short circuit between the device electrodes, it is possible to use a CNT shorter than the distance between the device electrodes. desirable. However, since CNTs are generally produced in the form of strings, it is desirable to cut them for use in the form of short fibers. In order to cut into short fibers, ultrasonic treatment is effective together with acid treatment with nitric acid, sulfuric acid and the like, and it is more preferable to use separation with a filter in combination for improving purity. In addition, not only the cut CNT but also a CNT prepared in a short fiber shape in advance is preferably used according to the present invention. Such short fibrous CNTs form a catalytic metal such as iron or cobalt on a substrate, and thermally decompose carbon compounds at 700 to 900 ° C. by CVD on the surface thereof to vapor-phase grow the CNTs. It is obtained in a shape oriented in the vertical direction. The short fiber CNTs thus produced can be taken out by a method such as peeling off from the substrate. In addition, the short fibrous CNTs can be obtained by supporting a catalytic metal on a porous support such as porous silicon or an anodic oxide film of alumina and growing the CNTs on the surface by the CVD method. Using oriented molecules such as iron phthalocyanine containing a catalytic metal in the molecule as a raw material and producing CNTs on a substrate by performing CVD in an argon / hydrogen gas flow, producing oriented short fiber CNTs You can also. Furthermore, it is also possible to obtain short fiber CNTs oriented on the SiC single crystal surface by an epitaxial growth method. When the dispersion obtained by applying the solution is used as a semiconductor, the average length of CNT depends on the distance between the electrodes, but is preferably 2 μm or less, more preferably 0.5 μm or less.

Although the diameter of CNT used by this invention is not specifically limited, 0.8 nm or more and 100 nm or less, More preferably, 50 nm or less is used favorably. Furthermore, the present invention can also be applied to CNTs with modified surfaces that have been studied in the prior art.
Although the CNT content contained in the CNT dispersion of the present invention is determined as necessary, 0.00001 to 10 parts is preferable and 0.0001 to 1 part is more preferably used in 100 parts by mass of the CNT dispersion.

2.2 Dispersion medium The composition containing CNTs for producing the CNT dispersion used in the present invention contains a dispersion medium. As the dispersion medium, at least one selected from the group of water, alcohol-based dispersion media, ketone-based dispersion media, amide-based dispersion media, ester-based dispersion media, and aprotic dispersion media can be used. Such a dispersion medium preferably has a boiling point of 50 to 300 degrees, more preferably 80 to 250 degrees. Further, the dispersion medium preferably contains water, and water, particularly deionized water is preferable from the viewpoint of safety.
Further, when forming a film, by using such a dispersion medium, an appropriate vapor pressure and evaporation rate, wettability to a substrate, and viscosity can be imparted in a film forming process such as spin coating.

2.3 Additive The composition containing CNTs for producing the CNT dispersion used in the present invention may contain an additive in order to adjust the properties of the CNT dispersion. As the additive, for example, one or more organic polymers selected from the group consisting of polyether, polyester, polycarbonate, polyanhydride, polystyrene polymer, and (meth) acrylic polymer can be used. Furthermore, an organic polymer having a weight reduction rate of 90% or more by differential thermothermal gravimetric analysis at 80 to 250 ° C. in a nitrogen atmosphere can be preferably used by appropriately selecting the monomer composition, molecular weight and the like. The organic polymer that can be used in the present invention is preferably one in which the residue after decomposition does not affect the properties of the CNT, and particularly an organic polymer that decomposes by depolymerization. Moreover, it is preferable to use 0.01-99 mass parts of (B) component with respect to 100 mass parts of (A) component, and it is still more preferable to use 0.1-80 mass parts.

Further, a surfactant may be added in order to improve the dispersibility of CNTs in the composition containing CNTs for preparing the CNT dispersion. As the surfactant, any of anionic, nonionic, and cationic can be used. For example, anionic surfactants include fatty acid salts, alkyl sulfate esters, alkylaryl sulfonates, etc., and nonionic surfactants include polyoxyethylene alkyl ethers, polyoxyethylene alkylaryl ethers, and other cationic interfaces. Examples of the activator include alkylamine salts and quaternary ammonium salts.

The composition containing CNTs for producing the CNT dispersion according to the present invention may further contain a pH adjuster as necessary. For example, inorganic acids such as hydrochloric acid, nitric acid and sulfuric acid; alkali metal hydroxides such as sodium hydroxide, potassium hydroxide, rubidium hydroxide and cesium hydroxide; basic such as tetramethylammonium hydroxide (TMAH) and ammonia Substances. Particularly preferred are degradable or volatile basic substances, and the weight loss rate by differential thermothermal gravimetric analysis at 30 to 250 ° C. in a nitrogen atmosphere is preferably 90% or more, more preferably 95% or more. 99% or more is most preferable. When the weight reduction rate is less than 90%, a large amount of decomposition products (residues) remain in the CNT-containing film when the CNT-containing film is heated at a temperature of 250 ° C. or higher. I can't. The other weight reduction rates are as follows: a sample dried for 1 hour at 30 ° C. in a nitrogen atmosphere is subjected to conditions of 10 ° C./min from 30 ° C. to 500 ° C. by TG-DTA (simultaneous differential thermothermal weight measurement). The temperature is raised, the change in the weight of the sample is followed, and a value calculated by ((30 ° C. sample weight) − (250 ° C. sample weight)) / (30 ° C. sample weight) × 100.
Other than the above, one kind can be used alone, or two or more kinds can be mixed and used. Of these other components, potassium hydroxide, ammonia, and tetramethylammonium hydroxide (TMAH) are preferable, and tetramethylammonium hydroxide (TMAH) is more preferable.

  Hereinafter, the present invention will be described more specifically based on examples. However, the present invention is not limited to the following examples.

Example 1
Al is sputter-deposited as an electrode on the entire surface of a silicon wafer with a diameter of 10 cm, and 10 types (Sample 1 to 10) of CNT dispersions differing in average major axis, average diameter, SWCNT / MWCNT, metallic / semiconductor CNT, and the amount of impurities mixed Bodies (carbon nanotubes: manufactured by Nanocyl) were spin-coated on different bases, and subsequently heated on a hot plate at 100 ° C./60 sec. Drying at 200-300 ° C. for 1-10 min. The film was formed by heating. Finally, TiN is sputter-deposited as an electrode, and a pattern of several tens to 100 nm square is created using a metal hard mask or the like for the purpose of providing a plurality of measurement points on the wafer, and a test wiring is applied. It was.
Different types of CNT dispersions were measured in advance by SEM, TEM, and RAMAN analysis. As a result, Tables 1 and 2 show the results of measuring the average major axis, the average diameter, SWCNT / MWCNT, metallic / semiconductor CNT, and the amount of impurities mixed.

  When the correlation between these physical property values and the response time was analyzed, the average major axis, the average diameter, SWCNT / MWCNT, metallic / semiconductor CNT, and the amount of impurities mixed were -0.21, -0.32, -0.11, respectively. , −0.32, and 0.11 as a coefficient. When the response time as a CNT switching element is standardized to 30 ± 3 μsec, the samples satisfying this among 10 samples are Sample 1 (response time = 32.0 μsec), Sample 4 (response time = 29.4 μsec), Sample 10 There were only 3 samples (response time = 32.5 μsec).

Example 2
As described above, two different types of CNT dispersions (Sample 11, Sample 12) were formed on the switching element, and the response time for switching from OFF to ON was measured. As a result, Sample 12 that did not have a purification process to remove impurities was sampled. Compared to 11 (response time 31.6 μsec), the response time was 170 μsec. When the impurity concentration was actually measured, Sample 12 was 10000 ppb and about 80 times Sample 11. When the standard was set to a response time of 30 ± 3 μsec according to Example 1, Sample 11 was within the standard and Sample 12 was out of the standard.

Claims (5)

Sequentially forming a lower electrode film, a film containing carbon nanotubes, and an upper electrode film divided into two or more independent regions on a substrate;
Applying a potential difference between the lower electrode film and the upper electrode film of the substrate to evaluate electrical characteristics;
A method of simultaneously analyzing the length and thickness distribution of carbon nanotubes, the ratio of SWCNT / MWCNT, the ratio of metallic CNT to semiconducting CNT, and the impurity content based on the electrical property evaluation results of the substrate. The method according to claim 1, wherein the electrode is tungsten (W), aluminum (Al), copper (Cu), or titanium nitride (TiN _x ).   The method according to claim 1, wherein the upper electrode is square, rectangular, circular or elliptical.   The method according to claim 1, wherein the shortest distance between the upper electrode and the lower electrode end is 1 to 100 nm. The method according to claim 1, wherein an area of the upper electrode is 1.0 × 10 ⁻⁸ m ^{2 to} 1.0 × 10 ⁻⁴ m ² .

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