JP2006265079A - Apparatus for plasma enhanced chemical vapor deposition and method for manufacturing carbon nanotube - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an apparatus for microwave plasma enhanced chemical vapor deposition, with which carbon nanotubes containing impurities in a low concentration and oriented in a direction vertical to a substrate can be produced; and a method for manufacturing carbon nanotubes. <P>SOLUTION: The apparatus for chemical vapor deposition is equipped with a vacuum vessel capable of forming a plasma at its inside, a control electrode to which a direct current voltage or a high frequency voltage is applied, a substrate holder to which the direct current voltage or the high frequency voltage is applied and which holds the substrate, a gas introducing port for introducing a gas into the vacuum vessel and a gas discharge port for discharging the gas. The control electrode and the substrate holder are provided at such positions that the plasma generated at the inside of the vacuum vessel reaches the substrate holder through the control electrode. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

The present invention relates to an apparatus for producing carbon nanotubes grown by being oriented perpendicular to a substrate.

  Carbon nanotubes are various devices such as electron source materials for field emission display, capacitor electrode materials, semiconductors for nanoelectronic devices, and metal materials (hereinafter, these devices are collectively referred to as “ Abbreviated as "CNT device"). Known methods for producing carbon nanotubes include a method using arc discharge (arc discharge method) and a method using laser evaporation (laser evaporation method). When these methods are applied to the manufacture of CNT devices, the first step is to obtain powdery carbon nanotubes with low purity in the gas phase, then purify them, and finally attach or apply them on the device substrate. It is common to go through.

  By the way, in order to use the CNT device most effectively, it is important to align and fix the carbon nanotubes in the vertical direction as much as possible with respect to the substrate. As a result, the field electron emission display increases the field electron emission current density, the capacitor increases the capacity per unit area, and the nanoelectronic device increases the degree of integration. This is to bring great profits.

  However, when the powder obtained by the conventional arc discharge method or laser evaporation method is used, the carbon nanotubes are fixed in a shape lying on the substrate and cannot be oriented in the vertical direction. A method for improving the orientation in the vertical direction by post-processing has been proposed, but the actual situation is that a sufficient effect has not been obtained. Under such circumstances, the chemical vapor deposition method (hereinafter referred to as “CVD method”) is expected as a method capable of growing carbon nanotubes directly on a substrate. In particular, in the “plasma CVD method” in which a source gas is reacted in plasma, the growing carbon nanotubes are pulled up to the plasma side by a strong electric field in a sheath portion formed between the plasma and the substrate. Therefore, it is superior to other conventional methods in that highly oriented growth can be performed in the vertical direction (see Patent Documents 1 and 2).

JP 2000-057934 A Japanese Patent Laid-Open No. 11-157991

  The problem with the plasma CVD method is that there are a large number of extremely high energy ions in the plasma, so the vacuum wall material is sputtered by the high energy ions, and the constituent atoms of the vacuum wall material are mixed into the growing carbon nanotubes. It is to end up. The mixed atoms act as impurities and degrade the performance of the carbon nanotube. In particular, when carbon nanotubes are applied to a semiconductor element, there is a concern that even a very small amount of impurities (for example, even in the order of ppm) may greatly affect the electrical characteristics.

  The microwave plasma apparatus has a high plasma density and is a method suitable for producing highly aligned carbon nanotubes. However, in the conventional microwave plasma CVD apparatus, a vacuum chamber wall and a window for allowing microwaves to pass (this specification) Is called a “microwave window”), and the carbon nanotubes are easily mixed with impurities when exposed to plasma.

  The present invention has been made in view of the above, and provides a microwave plasma chemical vapor deposition apparatus and a carbon nanotube production method capable of producing carbon nanotubes having a low impurity concentration and oriented in a direction perpendicular to a substrate. The purpose is to provide.

  In order to solve the above problem, a first chemical vapor deposition apparatus according to the present invention includes a vacuum container capable of generating plasma, a control electrode to which a DC voltage or a high-frequency voltage is applied, a DC voltage or A chemical vapor deposition apparatus comprising a substrate holder to which a high-frequency voltage is applied and holding a substrate, and a gas introduction port and a gas discharge port for introducing and discharging gas into the vacuum vessel, wherein the vacuum The control electrode and the substrate holder are installed at a position where the plasma generated inside the container reaches the substrate holder via the control electrode.

A second chemical vapor deposition apparatus according to the present invention includes a vacuum vessel capable of generating plasma therein, a control electrode to which a DC voltage or a high-frequency voltage is applied, a DC voltage or a high-frequency voltage being applied. A chemical vapor deposition apparatus comprising a substrate holder for holding a substrate, a gas inlet and a gas outlet for introducing and discharging a gas into the vacuum vessel, a microwave power source and a microwave waveguide. ,
The vacuum vessel is a cavity resonator, the control electrode is provided at the terminal position, and a TE-TM mode converter is provided so that plasma is generated on the microwave introduction side of the control electrode. .

  In the first and second chemical vapor deposition apparatuses according to the present invention, since the substrate holder is installed on the side opposite to the plasma via the control electrode, the plasma is stably generated continuously, and the plasma uses the control electrode. Via the substrate holder. In this apparatus, since the carbon nanotube grows normally with the substrate side (substrate holder) serving as the negative electrode and the control electrode applied with the positive electrode, a strong sheath electric field is formed on the substrate, resulting in a perpendicular to the substrate. Carbon nanotubes that are strongly oriented in the direction can be produced. Note that the control electrode is not necessarily the positive electrode and the substrate holder is always the negative electrode because the control electrode may be relatively higher in potential than the substrate holder. For example, the control electrode may be set to zero potential and the substrate holder may be set to negative potential.

A third chemical vapor deposition apparatus according to the present invention includes a vacuum vessel capable of generating plasma therein, a control electrode to which a DC voltage or a high-frequency voltage is applied, a DC voltage or a high-frequency voltage being applied. A chemical vapor deposition apparatus comprising a substrate holder for holding a substrate, a gas inlet and a gas outlet for introducing and discharging a gas into the vacuum vessel, a microwave power source and a microwave waveguide. ,
The vacuum vessel serves as a cavity resonator, the control electrode is provided at the terminal position, and a TE-TM mode converter is provided so that the first plasma is generated on the microwave introduction side of the control electrode, A second plasma is generated between the control electrode and the substrate holder.

  Thus, when the second plasma is generated between the control electrode and the substrate holder, the orientation is further enhanced. In order to generate the second plasma, the electric field between the control electrode and the substrate holder and the pressure in the vacuum vessel may be adjusted so that the conditions are equal to or higher than the discharge start voltage.

  In this case, it is preferable that at least a part of the shape of the control electrode in the first to third chemical vapor deposition apparatuses is a mesh shape. If it does in this way, a plasma will pass through the fine hole of a mesh-like control electrode, and it can be made to reach the plasma facing position (under the control electrode).

  Alternatively, the shape of the control electrode in the first to third chemical vapor deposition apparatuses may be a structure in which holes having a diameter less than ½ of the microwave wavelength are provided. This is because if the wavelength is less than ½ of the microwave wavelength, the microwave can be sufficiently reflected to the extent that it is almost the same as when no holes are provided, and the resonance mode is not changed.

  In addition, the vacuum vessel of the first to third chemical vapor deposition apparatuses according to the present invention is preferably made of a transparent material. This is because if it is transparent, the inside can be observed and the plasma can be easily controlled.

Further, the carbon nanotube manufacturing method according to the present invention introduces a reaction gas into the vacuum container in a state where the substrate is installed in a substrate holder provided in the vacuum container, and the first inside the vacuum container. A method of growing carbon nanotubes on the substrate by generating a plasma of 1 comprising:
A control electrode provided with one or a plurality of holes for allowing plasma to pass is installed in a position adjacent to the plasma in the vacuum vessel, and the substrate is located at a position facing the plasma through the control electrode. Install the holder,
Introducing the first plasma into the vicinity of the substrate surface through the holes and growing carbon nanotubes on the substrate while applying a DC voltage or a high-frequency voltage between the control electrode and the substrate holder; Features. Note that the first plasma is preferably a ball-shaped plasma from the viewpoint of preventing sputtering of the wall surface.

  According to this method, the carbon nanotubes can be aligned and grown while being pulled up vertically by a strong sheath electric field generated on the substrate holder. Further, by adjusting so that the ball-shaped plasma generated inside does not contact the wall surface of the surrounding vacuum vessel, the atoms constituting the wall surface are hardly sputtered, and the contamination of the carbon nanotube is reduced.

  At this time, the carbon nanotubes may be manufactured under such conditions that a second plasma is generated between the control electrode and the substrate holder. According to this method, since the sheath electric field is further increased by the second plasma, the orientation is further improved.

Hereinafter, the best embodiment will be described with reference to the drawings.
(Device configuration)
FIG. 1 shows an example of a configuration diagram of a microwave plasma chemical vapor deposition apparatus according to the present invention. For example, a quartz vacuum vessel (bell jar) 2 is provided inside an electromagnetic wave shield 1 made of punching metal or the like, a reactive gas is introduced into the vacuum vessel from a gas inlet 3, and exhausted from a gas outlet 4. It has a structure.

  A microwave oscillator 20 and a waveguide 21 are provided above the vacuum vessel, and an antenna 22 is provided at the center of the waveguide of the waveguide 21. The antenna 22 functions to convert a microwave mode from a TE wave to a TM wave and to guide the microwave into the vacuum chamber 2.

  When microwaves are introduced into the vacuum vessel through the microwave window, ball-shaped plasma is formed inside the vacuum vessel 2. The ball shape means a spherical shape, but it is not necessarily a perfect sphere, and means a closed curved surface such as an ellipse. Note that since the vacuum vessel 2 is transparent, plasma generated inside the vacuum vessel 2 when observed through the electromagnetic wave shield 1 can be observed. Although depending on the composition of the gas to be introduced, artificial diamond can be generated on the substrate by placing the substrate below the ball-shaped plasma.

  Now, when growing carbon nanotubes highly oriented in the vertical direction, which is the object of the present invention, this is not sufficient, and a strong electric field sheath must be formed on the substrate. Specifically, a substrate 10 for growing carbon nanotubes is placed on top of a metal substrate holder (susceptor) 11 to which a negative bias voltage is applied, electrodes are placed at slightly spaced positions, and a DC power supply 50 need to be connected.

  A control electrode 12 is provided at the end position of the cavity resonator above the substrate. The control electrode 12 is a positive electrode with respect to the substrate holder which is a negative electrode (provided that the potential of the control electrode is higher than the potential of the substrate holder, and the control electrode may be zero potential or negative potential). The shape is required to be provided with one or a plurality of holes, such as a lattice (mesh) shape, a so-called donut shape in which one circular hole is provided at the center. Since the holes provided in the control electrode allow the plasma to pass therethrough, the plasma can also be introduced below the control electrode. By taking such a positional relationship (that is, the order of plasma / control electrode / substrate holder), when the substrate holder is placed at a position facing the plasma via the control electrode, the control electrode is connected to the cavity resonator. It can be installed at the end position. For this reason, the plasma can be stably maintained and a sheath can be formed on the substrate, so that the vertical alignment of the carbon nanotubes is improved.

What the present inventors have actually confirmed as the shape of the control electrode 12 is a metal having a mesh-like (lattice-like) metal electrode and a hole having a diameter less than a half wavelength (λ / 2) of a microwave. Electrode. In the case where the control electrode is a mesh electrode, it is possible to use one having as coarse an eye as possible (for example, a mesh metal electrode provided with about 60 to 100 lattices per inch).
Alternatively, a metal plate (a so-called donut on a concentric circle) having a hole (for example, a diameter of 2.6 cm) having a diameter less than ½ of the microwave wavelength (λ / 2 = about 6.1 cm in the case of 2.45 GHz). A thin metal plate) may be used as the control electrode. By interposing these control electrodes between the substrate and the plasma, the plasma can be maintained stably. In the case of the mesh shape, the metal atoms of the control electrode may be sputtered. However, since the plasma is designed to be generated under the condition that the electrode is provided at the position of the control electrode (end position of the cavity resonator), the control electrode itself cannot be omitted.

  In addition, as a material of the board | substrate 10, a catalyst metal (iron, nickel, cobalt etc.) can be used, for example. According to an experiment using the above experimental apparatus by the present inventors, as a result of using an iron thin plate having a size of about 1 cm square as the substrate 10, uniform carbon nanotubes that are well oriented in the vertical direction were obtained ( Figure 2).

  As described above, when microwaves are generated in the waveguide 21 from the microwave oscillator 20 and sent to the vacuum vessel 2, the vacuum vessel 2 becomes a cavity resonator, and the center inside the quartz tube a little away from the vacuum chamber. Ball-shaped plasma is generated near the portion. At this time, it is preferable that plasma is generated as far as possible from the wall of the vacuum chamber and the microwave window. This is because when the plasma contacts the vacuum chamber, quartz constituting the vacuum chamber or constituent atoms of a microwave window (not shown) provided in the vacuum chamber may be sputtered and mixed into the growing carbon nanotube as an impurity. Because.

  As described above, according to the chemical vapor deposition apparatus according to the present invention, the substrate holder is installed on the opposite side of the plasma generation unit with respect to the control electrode, and a direct current or high frequency is provided between the control electrode and the substrate holder. Since a strong sheath is generated on the substrate holder by applying a voltage, the carbon nanotubes can be oriented and grown while being pulled up vertically.

  Initially, the inventors of the present invention provided a substrate holder at a position where the control electrode in FIG. 1 is provided and made this a negative electrode (a negative bias is applied), and further above the control electrode (however, a mesh or a hole) Is not provided) and this was used as a positive electrode (positive bias applied) to try to grow carbon nanotubes. However, since the vacuum vessel 2 is a cavity resonator, the cavity resonator is terminated. Since an electrode was provided at the position and the inside was not made hollow, plasma was not generated or stably maintained. Therefore, when a fine hole is provided in a part of the control electrode and a substrate holder is provided on the opposite side of the plasma (below the control electrode 12 in FIG. 1), the ball-shaped plasma is stabilized in the vacuum vessel. Lasted. The carbon nanotubes that were strongly oriented in the vertical direction could be generated by the sheath between the control electrode and the substrate holder.

  Plasma is generated in the center of the vacuum chamber without contacting the vacuum chamber wall, the control electrode 12 is placed under the plasma, and the substrate holder 10 is further placed under the plasma. The substrate holder 10 is the negative electrode, and the control electrode 12 is the positive electrode. A DC voltage was applied to the substrate to grow carbon nanotubes on the substrate placed on the substrate holder.

As growth conditions, 10 hydrogen diluted methane was used as the reaction gas, the pressure was 2000 Pa, the microwave power was 500 W, the DC voltage between the control electrode and the substrate holder was about 450 V, and the iron thin plate was used as a substrate for 15 minutes. went.
FIG. 2 shows a scanning electron micrograph observing the surface of the carbon nanotube grown under the above conditions. From this observation result, growth of carbon nanotubes oriented perpendicular to the substrate surface was confirmed.

  As described above, TE-TM mode conversion is performed using a TE-TM converter such as an antenna, and plasma is generated between the control electrode and the substrate holder using an end launch type device in which the vacuum chamber itself is a cavity resonator. As a result, it was possible to produce uniform carbon nanotubes oriented in the vertical direction.

  Since the present invention is expected to be used as an elemental technology for manufacturing a CNT device that is expected to be put into practical use in the future, the industrial applicability is extremely large.

FIG. 1 shows an example of a configuration diagram of a microwave plasma chemical vapor deposition apparatus according to the present invention. FIG. 2 shows a scanning electron micrograph observing the surface of the grown carbon nanotube.

Explanation of symbols

1 Electromagnetic wave shield 2 Vacuum container (Bell jar)
3 Gas inlet 4 Gas outlet 10 Substrate 11 Substrate holder (negative electrode)
12 Control electrode (positive electrode, but may be zero potential)
20 Microwave Oscillator 21 Waveguide 22 TE-TM Mode Converter (Antenna)

Claims (8)

A vacuum vessel capable of generating plasma inside, a control electrode to which a DC voltage or a high frequency voltage is applied, a substrate holder to which a DC voltage or a high frequency voltage is applied and holding a substrate, and a gas inside the vacuum vessel A chemical vapor deposition apparatus having a gas inlet and a gas outlet for introducing and discharging gas, wherein plasma generated inside the vacuum vessel reaches a substrate holder via the control electrode A chemical vapor deposition apparatus in which a control electrode and a substrate holder are installed. A vacuum vessel capable of generating plasma inside, a control electrode to which a DC voltage or a high frequency voltage is applied, a substrate holder to which a DC voltage or a high frequency voltage is applied and holding a substrate, and a gas inside the vacuum vessel A chemical vapor deposition apparatus comprising a gas inlet and a gas outlet for introducing and discharging gas, a microwave power source and a microwave waveguide,
The vacuum vessel is a cavity resonator, the control electrode is provided at the terminal position, and a TE-TM mode converter is provided so that plasma is generated on the microwave introduction side of the control electrode. Chemical vapor deposition equipment. A vacuum vessel capable of generating plasma inside, a control electrode to which a DC voltage or a high frequency voltage is applied, a substrate holder to which a DC voltage or a high frequency voltage is applied and holding a substrate, and a gas inside the vacuum vessel A chemical vapor deposition apparatus comprising a gas inlet and a gas outlet for introducing and discharging gas, a microwave power source and a microwave waveguide,
The vacuum vessel serves as a cavity resonator, the control electrode is provided at the terminal position, and a TE-TM mode converter is provided so that the first plasma is generated on the microwave introduction side of the control electrode, A chemical vapor deposition apparatus for generating a second plasma between the control electrode and a substrate holder. The chemical vapor deposition apparatus according to claim 1, wherein at least a part of the control electrode has a mesh shape. The chemical vapor deposition according to any one of claims 1 to 3, wherein the control electrode has a structure in which holes having a diameter of less than ½ of the microwave wavelength are provided. apparatus. The microwave plasma chemical vapor deposition apparatus according to any one of claims 1 to 3, wherein the vacuum vessel is made of a transparent material. In a state where the substrate is set in a substrate holder provided inside the vacuum vessel, a reactive gas is introduced into the vacuum vessel, and a first plasma is generated inside the vacuum vessel to generate carbon on the substrate. A method for growing nanotubes, comprising:
A control electrode provided with one or a plurality of holes for allowing plasma to pass is installed in a position adjacent to the plasma in the vacuum vessel, and the substrate is located at a position facing the plasma through the control electrode. Install the holder,
Introducing the first plasma into the vicinity of the substrate surface through the holes and growing carbon nanotubes on the substrate while applying a DC voltage or a high-frequency voltage between the control electrode and the substrate holder; A method for producing a carbon nanotube, which is characterized. 8. The method for producing carbon nanotubes according to claim 7, wherein second plasma is generated between the control electrode and the substrate holder.

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