JP2005300490A - Mechanical detection element and detector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a mechanical detection element and a detector high in sensitivity. <P>SOLUTION: The mechanical detection element, wherein the minute force impressed on a cantilever 1A is detected by the change of resistance caused by the piezoelectric resistance effect, is constituted of: the micro-fabricated cantilever 1A; electrodes 2, 3 capable of measuring the resistance value of a resistance body composed of the cantilever 1A, and a carbon nanotube 4 for measuring the objective resistance through the electrodes 2, and 3. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a mechanical detection element and a detector for detecting a small force or other physical quantity applied to a unit with high sensitivity by detecting elastic deformation of a solid unit processed into a beam shape. .

  The mechanical detection element is used as an element for consumer equipment. An example of this element is made as follows. Using a semiconductor microfabrication technique represented by lithography, a solid thin film such as silicon is processed into a beam shape to form a beam of a mechanical detection element. According to this detection element, a microscopic force or other physical quantity applied to the beam is detected by detecting a change in physical properties of the semiconductor due to elastic displacement of the beam electrically or optically.

Such a mechanical detection element is widely used as an element for consumer equipment such as a cantilever or an acceleration sensor of a scanning probe microscope. In particular, the following three types of electromechanical detection elements are widely used as self-detecting sensors that do not require external components such as an optical system.
1. 1. Piezoresistive effect detection element 2. Capacitance type detection element Tunnel current type detector

  The piezoresistive effect detecting element has an electrode capable of measuring the resistance value of all or a part of the beam, and detects a minute force applied to the beam as a change in the resistance value. The capacitance type detection element has one or two conductors whose capacitance can be measured, and detects a minute force applied to the beam as a change in the capacitance of the conductor (non-patent document). 1, 2). The tunnel current type detection element has two conductors close to the same wavelength as an electron, that is, a tunnel junction, and an electrode that can measure the tunnel current flowing through the tunnel junction. The force is detected as a change in the tunnel current (Non-patent Document 3).

  These electromechanical force detection elements are important as basic elements of all high-sensitivity sensors that use elastic deformation of beams such as acceleration, electric charge, magnetism, and displacement sensors as well as detection of minute forces.

  FIG. 4 shows an example of a piezoresistive effect detection element that is a self-sensing type force detection element. The piezoresistive effect detecting element of FIG. 4 includes an insulating Si thin film 101, a conductive Si thin film 102, and electrodes 103 and 104. In this detection element, a conductive Si thin film (conductive Si thin film 102) is formed on a highly insulating Si semiconductor (insulating Si thin film 101) using a doping technique such as impurity diffusion. .

  The insulating Si thin film 101 is an Si thin film layer having insulating properties. The conductive Si thin film 102 is a conductive Si thin film layer formed by doping on the insulating Si thin film 101. The electrodes 103 and 104 are ohmic electrodes used for injecting a current into the conductive Si thin film 102 which is a conductive layer. Then, the semiconductor thin film is processed into a beam shape by using a semiconductor microfabrication technique, and the cantilever 111 is formed.

  In this detection element, when a force is applied to the tip of the cantilever 111 and the cantilever 111 is bent, the conductive Si thin film 102 is subjected to in-plane strain. For example, when the cantilever beam 111 bends downward, the conductive Si thin film 102 receives tensile stress and expands its volume. With respect to this volume expansion, the resistance value of the conductive Si thin film 102 changes due to two effects (piezoresistive effect) of the deformation potential effect and the piezoelectric effect that are noticeable in the semiconductor. Therefore, the deflection of the cantilever 111 can be detected by detecting a change in the resistance value between the electrode 103 and the electrode 104 by passing a current through the wirings 105 and 106. As a result, the force applied to the cantilever 111 can be detected.

  Next, as another example of the prior art, examples of capacitance type and tunnel current type acceleration detecting elements are shown in FIG. The acceleration detection element of FIG. 5 includes conductive Si thin films 201 and 202, electrodes 203 and 204, a cantilever beam 211, and a gap 212. The air gap 212 has an opposing plane inclined with respect to the longitudinal direction of the cantilever beam 211. The cantilever beam 211 and the gap 212 are produced by anisotropic etching or integrated ion beam (FIB) machining of a cantilever beam produced by microfabrication.

  In this acceleration detection element, when a force is applied to the entire element and the cantilever beam 211 is bent by the acceleration generated by the force, the gap 212 changes. As a result, the capacitance between the conductive Si thin film 201 and the conductive Si thin film 202 changes. Therefore, the deflection of the cantilever 211 can be detected by detecting the change in the capacitance between the electrode 203 and the electrode 204 by passing an alternating current through the wirings 205 and 206. As a result, it is possible to detect the acceleration of the cantilever 211, that is, the force applied to the element. That is, this element operates as a capacitive acceleration detecting element.

By the way, in this acceleration detecting element, when the gap 212 is close to the wavelength of the electron, the tunnel between the conductive Si thin film 201 and the conductive Si thin film 202 is caused by the quantum mechanical tunnel effect. Current flows. Since this tunnel current is extremely sensitive to a change in the interval of the gap 212, the acceleration of the cantilever 211 can also be detected by detecting the change in the tunnel current by passing the current through the wirings 205 and 206. Is possible. That is, this element functions as a tunnel current type acceleration detecting element when the air gap 212 is sufficiently small.
Sensors and Actuators, A21-A23,1990 pp.297 Applied Physics Letter, 62, 1993, 834-836 (Applied Physics Letter, Volume 62, 1993 pp.834-836) Applied Physics Letter, 58, 1991, 100 (AppliedPhysics Letter, Volume 58, 1991 pp.100)

  The conventional techniques described above have the following problems. A commonly used piezoresistive effect detecting element uses a deformation potential effect of a conductive semiconductor thin film and a change in resistance value inherent to a bulk semiconductor due to a piezoelectric effect. For this reason, there is a problem that the change in the resistance value with respect to the displacement of the beam is small, and the sensitivity is remarkably lower than that of a detection element to which a displacement sensor using a laser interferometer or the like is applied.

  In addition, since the capacitance type detection element that is normally used measures the change ratio of the capacitance, in order to improve the sensitivity of the element, it is necessary to reduce the capacitance during non-operation. The gap for measuring the capacitance is produced by electron beam exposure or FIB processing. For this reason, the opposing joint surface has a size of about several tens of nanometers at the minimum, and there is a manufacturing limit for the purpose of reducing the capacitance. In particular, when an electrostatic junction is manufactured using a semiconductor such as silicon, the distance between the junction surfaces is not less than the depletion layer thickness due to the depletion layer of the semiconductor, and there is a limit to the improvement in sensitivity. A similar situation also occurs in the case of a tunnel current type detection element which is a tunnel junction type force detection element, and the sensitivity limit caused by the limit of processing accuracy is unavoidable.

  Therefore, in any of the detection elements, the common point that a bulk material having conductivity is manufactured using a fine processing technique such as lithography is that a sensitivity limit is generated due to processing accuracy and characteristics of the bulk material. Can be pointed out as the biggest problem.

  The present invention solves the above-described problems and provides a highly sensitive mechanical detection element and detector.

In order to solve the above-mentioned problem, the invention of claim 1 has a beam produced by microfabrication and an electrode capable of measuring a resistance value of a resistor composed of all or a part of the beam, and is added to the beam. In a mechanical detection element that detects a minute force as a change in the resistance value by a piezoresistance effect, mechanical detection is characterized in that all or a part of an object whose resistance value is measured through the electrode is composed of carbon nanotubes. It is an element.
The invention of claim 2 has a beam produced by microfabrication and one or two conductors whose capacitance can be measured, and a minute force applied to the beam is used to change the capacitance of the conductor. In the mechanical detection element to be detected, the mechanical detection element is characterized in that all or part of the conductor is composed of carbon nanotubes.
According to a third aspect of the present invention, there is provided a tunnel junction formed by a beam fabricated by microfabrication, two conductors close to the same wavelength as an electron, and an electrode capable of measuring a tunnel current flowing through the tunnel junction. And a mechanical detection element for detecting a minute force applied to the beam as a change in the tunnel current, wherein all or part of the tunnel junction is composed of carbon nanotubes. It is.
According to a fourth aspect of the present invention, in the mechanical detection element according to any one of the first to third aspects, an electrical characteristic of a structure or circuit in which two or more of the resistor, the conductor, and the tunnel junction are combined. In the mechanical detection element for detecting a minute force applied to the beam by measuring the change in the above, the structure or a part of the circuit is constituted by carbon nanotubes.
The invention according to claim 5 is a detector including the structure equivalent to the mechanical detection element according to any one of claims 1 to 4 as a whole or as a component thereof, and includes acceleration, magnetism, charge, displacement, mass, molecule, and atomic One of the detectors has generating means for generating elastic deformation of the beam.

  The reason why the displacement of the beam can be measured with high sensitivity is as follows. Carbon nanotubes are known to be ideal one-dimensional electrical conductors. The electric conductivity in the one-dimensional electric conductor is proportional to the number of quantized channels existing below the Fermi level. The correlation between the energy level and the Fermi level of this quantization channel is determined by the diameter and chirality of the carbon nanotube. When the shape of the nanotube is elastically deformed, the energy level structure of the conduction electrons changes greatly. Depending on the direction of elastic deformation, the one-dimensionality of the system itself disappears. Due to such an effect, the electrical conductivity of the carbon nanotube, that is, the resistance value which is the reciprocal thereof, changes extremely sensitively to the elastic deformation of the carbon nanotube.

In fact, as reported by Tombler et al. In Nature 405, page 769 in 2000, carbon nanotubes have a specific resistance change of 0.01 nm ⁻¹ with respect to the displacement of the central portion. This corresponds to a sensitivity that is 3 to 4 orders of magnitude higher than the typical displacement sensitivity of a semiconductor piezoresistive cantilever, 10 ⁻⁵ to 10 ⁻⁶ nm ⁻¹ . From this, if a carbon nanotube is used as a resistor of a piezoresistive effect detection element, it becomes possible to produce a detection element with higher sensitivity than in the past.

  On the other hand, the carbon nanotube has a very small capacitance because it has a fine diameter of nanoscale. Therefore, a capacitor having a very small capacitance can be manufactured by forming a junction using the nanotubes. By incorporating this capacitor into a capacitive mechanical detection element, it is possible to produce a detection element with high sensitivity.

  Similarly, by forming a tunnel junction between the carbon nanotube and the electrode or between the carbon nanotubes, it is possible to form a tunnel junction that is extremely sensitive to changes in the gap distance of the junction, improving the sensitivity of the detection element. Can be made.

  Therefore, by producing a beam containing carbon nanotubes in this way and applying force to them, elastic deformation of the carbon nanotubes or displacement of the carbon nanotubes is achieved, thereby realizing an order of magnitude higher sensitivity than conventional techniques. It is possible. In other words, the use of carbon nanotubes with extremely fine and characteristic electrical conduction characteristics overcomes limitations caused by processing accuracy and physical properties of bulk materials in conventional lithography technology, and provides a mechanical sensor with high sensitivity. Making is the essence of the present invention.

  According to the present invention, it is possible to obtain a detection element with extremely high sensitivity by using carbon nanotubes in a part of the portion constituting the detection element.

Next, embodiments of the present invention will be described in detail with reference to the drawings.
[Embodiment 1]
The mechanical detection element according to the present embodiment is a piezoresistive effect detection element, and this mechanical detection element uses insulating silicon as a material constituting the beam. The mechanical detection element according to the present embodiment is shown in FIG. The mechanical detection element includes an insulating silicon thin film 1, a cantilever 1 </ b> A, metal electrodes 2 and 3, and a carbon nanotube 4.

  The insulating silicon thin film 1 is a highly insulating Si semiconductor, and the cantilever 1 </ b> A is manufactured by microfabrication of a part of the insulating silicon thin film 1. The metal electrodes 2 and 3 are formed on the insulating silicon thin film 1. The metal electrodes 2 and 3 have L-shaped fixing portions 2A and 3A. The fixing portions 2A and 3A are arranged on the cantilever 1A so that the bent portions which are L-shaped tips face each other. The carbon nanotube 4 is fixed to the cantilever 1 </ b> A by the fixing portions 2 </ b> A and 3 </ b> A of the metal electrodes 2 and 3. The fixing portions 2 </ b> A and 3 </ b> A of the metal electrodes 2 and 3 form a good ohmic junction with the carbon nanotube 4. That is, the metal electrodes 2 and 3 act as electrodes of the carbon nanotube 4 and are also used to fix the carbon nanotube 4 on the cantilever 1A.

  When deflection is applied to the cantilever 1A of the mechanical detection element having such a configuration, the carbon nanotubes 4 fixed on the cantilever 1A are subjected to tensile strain or compression strain depending on the direction of deflection. As already described, the resistance value of the carbon nanotube 4 is extremely sensitive to elastic deformation due to these strains. Therefore, by measuring the change in the resistance value between the metal electrode 2 and the metal electrode 3 based on the current flowing through the wirings 5 and 6, the sensitivity is much higher than that of the conventional method. A minute force applied to the cantilever 1A can be detected.

[Embodiment 2]
Next, Embodiment 2 will be described. The mechanical detection element according to the present embodiment is a piezoresistive effect displacement detection element. In this embodiment, the carbon nanotube has a structure in which the carbon nanotube is attached to the distal end portion of the beam. The mechanical detection element according to the present embodiment is shown in FIG. This mechanical detection element includes insulating silicon thin films 11 and 14, a cantilever 11 A, metal electrodes 12 and 15, and a carbon nanotube 17.

  The insulating silicon thin films 11 and 14 are highly insulating Si semiconductors. The cantilever 11 </ b> A is manufactured by microfabrication of a part of the insulating silicon thin film 11. Similarly, the protruding portion 14 </ b> A is formed on the insulating silicon thin film 14. The cantilever 11 </ b> A and the projecting portion 14 </ b> A are arranged such that their tip portions face each other through the gap 18. The metal electrode 12 is provided on the insulating silicon thin film 11, and the metal electrode 15 is provided on the insulating silicon thin film 14. The metal electrodes 12 and 14 have elongated fixed portions 12A and 15A. The carbon nanotube 17 is arranged at a position that bridges the gap 18 and is fixed to the cantilever 11A and the protruding portion 14A by the fixing portions 12A and 15A of the metal electrodes 12 and 15. The metal electrodes 12 and 15 function as good ohmic electrodes for the carbon nanotubes 17 and play a role of fixing the carbon nanotubes 17.

  When bending is applied to the cantilever 11A of the mechanical detection element having such a configuration, a large strain is applied to the carbon nanotubes 17 fixed to the cantilever 11A due to the displacement of the tip of the cantilever 11A. Similar to the first embodiment, the resistance value of the carbon nanotube 17 is extremely sensitive to elastic deformation due to this strain. Therefore, by measuring the change in the resistance value between the metal electrode 12 and the metal electrode 15 based on the current flowing through the wirings 13 and 16, the sensitivity is remarkably higher than that of the conventional method. It is possible to detect a minute displacement of the tip of the cantilever 11A.

[Embodiment 3]
The mechanical detection element according to the present embodiment is a tunnel junction type displacement detection element. In this mechanical detection element, a cut carbon nanotube is used as a tunnel junction. Note that the mechanical detection element according to the present embodiment can also be diverted to a capacitive detection element. The mechanical detection element according to the present embodiment is shown in FIG. This mechanical detection element includes insulating silicon thin films 11 and 14, a cantilever 11 </ b> A, metal electrodes 12 and 15, and a carbon nanotube 21. In FIG. 3, components that are the same as or the same as those in FIG. 2 described above are denoted by the same reference numerals, and the description thereof is omitted.

  The carbon nanotube 21 is cut into two by a focused ion beam or the like, one tube portion 21A is fixed to the cantilever 11A by the fixing portion 12A of the metal electrode 12, and the other tube portion 21B is fixed to the metal electrode 15. It is fixed to the protruding portion 14A by 15A. A tunnel junction 21 </ b> C is formed at the cut portion of the carbon nanotube 21.

  When bending is applied to the cantilever 11A of the mechanical detection element having such a configuration, the tube portion 21A of the carbon nanotube 21 fixed to the cantilever 11A is displaced up and down by the displacement of the tip of the cantilever 11A. On the other hand, since the tube portion 21B of the carbon nanotube 21 is independent of the cantilever 11A, no displacement occurs. Therefore, the distance between the tunnel junctions 21C changes by the displacement distance of the tip of the cantilever 11A. Compared to silicon, carbon nanotubes have good electrical conduction up to the cut tip, so the tunnel current changes very sensitively to this displacement. Therefore, by measuring the change in the tunnel current flowing between the metal electrode 12 and the metal electrode 15 using the wirings 13 and 16, the cantilever 11A has an extremely high sensitivity compared to the conventional method. It is possible to detect a minute displacement of the tip of the.

[Embodiment 4]
In each of the above-described embodiments, only the case where the object to be detected is a force applied to the mechanical detection element is shown. In this embodiment, a detector that performs various measurements using the mechanical detection element of each embodiment will be described. To do. In this embodiment, a magnetic material is provided near the tip of the cantilever beam of the mechanical detection element. According to this configuration, when magnetism is applied to the mechanical detection element, the cantilever beam including the magnetic material is displaced, so that magnetism can be detected. In this case, the generating means for generating elastic deformation is a magnetic material.

  Further, instead of the magnetic material, a conductor is provided near the tip of the cantilever and an electric field is applied to the mechanical detection element. According to this configuration, when the conductor of the cantilever beam is charged, the cantilever beam is formed by the electric field. As a result, it is possible to detect the electric charge charged on the conductor of the cantilever beam. In this case, the generating means is a conductor and an electric field.

  Further, the resonance frequency of the cantilever changes due to a change in mass near the tip of the cantilever of the mechanical detection element. That is, by electrically detecting a change in the resonance frequency of a cantilever beam vibrated from the outside, it is possible to detect masses of molecules, atoms, minute substances, minute particles, and the like. In this case, the generating means is a cantilever vibration structure.

  Furthermore, since the displacement of the tip of the cantilever and the acceleration of the entire element also cause bending of the cantilever, a mechanical detection element can be used as a detection element for these displacement and acceleration. In this case, the generating means is a cantilever beam itself.

  In this way, it is possible to produce detectors for all kinds of physical quantities, such as magnetism, charge, mass, molecules, atoms, and minute substances / particles, using an equivalent structure without departing from the spirit of the present invention.

  As mentioned above, although Embodiment 1-4 of this invention was explained in full detail, even if there is a design change etc. of the range which does not deviate from the gist of the present invention, the concrete composition is not restricted to each embodiment, It is included in the present invention. For example, in Embodiment 3, the tunnel junction is formed by cutting the carbon nanotubes. However, it is also possible to use carbon nanotubes that function as tunnel junctions, including carbon nanotubes that function as single-electron transistors.

  Moreover, in each embodiment, although silicon was used as a material which comprises a beam, it cannot be overemphasized that not only a semiconductor but all kinds of solid materials can be used.

  In each embodiment, the carbon nanotubes are fixed to the surface of the beam, but a structure embedded in the beam can also be used.

  In each embodiment, a cantilever is used as the shape of the beam. However, a semiconductor or solid element of any shape that causes elastic deformation by an applied force, such as a coil spring, as well as a cantilever is used. It is clear that you can.

  In each embodiment, the electrode is described as directly forming an ohmic junction with the carbon nanotube. However, any type of metal electrode configured to measure the change in the characteristics of the carbon nanotube can be used. It is.

  In each embodiment, the case where “deflection” occurs as an elastic deformation of a thin film or a piece is used. However, other types of elastic deformation such as “torsion”, “compression” and “extension” are used. Even if a modification is used, a highly sensitive detection element and detector can be similarly produced.

  In each embodiment, a structure including only one resistor, tunnel junction, or the like has been described. However, it is also possible to detect a change in the electrical characteristics of a structure or circuit in which a plurality of these are combined.

It is a perspective view which shows the mechanical detection element by Embodiment 1 of this invention. It is a perspective view which shows the mechanical detection element by Embodiment 2 of this invention. It is a perspective view which shows the mechanical detection element by Embodiment 3 of this invention. It is a perspective view which shows the conventional semiconductor force detection element produced by forming a conductive layer thin film on an insulating semiconductor. It is a perspective view which shows the conventional electrostatic capacitance type / tunnel current type acceleration detection element.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Insulating silicon thin film 1A Cantilever 2, 3 Metal electrode 2A, 3A Fixed part 4 Carbon nanotube 5, 6 Wiring 11, 14 Insulating silicon thin film 11A Cantilever 14A Protrusion part 12, 15 Metal electrode 12A, 15A Fixed part 13, 16 Wiring 17 Carbon nanotube 18 Void 21 Carbon nanotube 21
21A, 21B Tube part 21C Tunnel junction

Claims (5)

A beam produced by microfabrication, and electrodes (2, 3) that can measure the resistance value of a resistor consisting of all or part of the beam, and the minute force applied to the beam is reduced by the piezoresistance effect. In the mechanical detection element that detects the change in resistance value,
A mechanical detection element characterized in that all or part of the object whose resistance value is measured through the electrodes (2, 3) is constituted by carbon nanotubes (4). In a mechanical detection element having a beam produced by microfabrication and one or two conductors whose capacitance can be measured, and detecting a minute force applied to the beam as a change in capacitance of the conductor ,
A mechanical detection element, wherein all or part of the conductor is composed of carbon nanotubes (21). A tunnel junction (21C) formed by a beam produced by microfabrication, two conductors close to the same wavelength as an electron, and an electrode that can measure the tunnel current flowing through the tunnel junction (21C) ( 12 and 15), and a mechanical detection element for detecting a minute force applied to the beam as a change in the tunnel current,
A mechanical detection element, wherein all or part of the tunnel junction (21C) is composed of carbon nanotubes (21). In a mechanical detection element that detects a minute force applied to the beam by measuring a change in electrical characteristics of a structure or circuit combining two or more of the resistor, the conductor, and the tunnel junction,
The mechanical detection element according to any one of claims 1 to 3, wherein a part of the structure or the circuit is constituted by carbon nanotubes. A detector including a structure equivalent to the mechanical detection element according to claim 1 as a whole or a component thereof,
A detector comprising generating means for generating elastic deformation of the beam by one of acceleration, magnetism, charge, displacement, mass, molecule, and atom.

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