KR20050032821A - Carbon nanotubes based gas sensor on mems structure and method for fabricating thereof - Google Patents

Abstract

A carbon nano tube gas sensor and a method for manufacturing the same are provided to minimize the size of the carbon nano tube gas sensor by directly growing the carbon nano tube in an MEMS structure. A carbon nano tube gas sensor includes a substrate, an insulation layer formed at an upper portion of the substrate and a protective layer formed at a lower portion of the substrate. A heater(3-3) and a heater electrode are formed on the insulation layer. Another insulation layer is formed on the heater(3-3). A carbon nano tube electrode(3-6) and an electrode line are formed on the insulation layer. A catalyst metal(3-8) is formed on the carbon nano tube electrode(3-6). A lower portion of the substrate is anisotropically etched.

Description

Carbon nanotube gas sensor using MEMS structure and its manufacturing method {CARBON NANOTUBES BASED GAS SENSOR ON MEMS STRUCTURE AND METHOD FOR FABRICATING THEREOF}

The present invention relates to a carbon nanotube gas sensor using a MEMS structure and a method for fabricating the same, and more specifically, carbon nanotubes are directly grown on a structure manufactured by a microelectromechanical system (MEMS) technology. The present invention relates to a carbon nanotube gas sensor applied as a sensitive material of a sensor and a manufacturing method thereof.

In general, the gas sensor is operated by the principle of measuring the amount of harmful gas by using the characteristic that the electrical conductivity changes according to the adsorption of gas molecules.

Conventionally, as a sensitive material of a gas sensor, metal oxide semiconductors such as SnO ₂ , solid electrolyte materials, various organic materials, and composites of carbon black and organic materials have been widely used. However, there are many problems with the gas sensor made of such a sensitive material. For example, in the case of a metal oxide semiconductor or a solid electrolyte, since the sensitivity is very low at room temperature, the sensor operates normally when heated to a temperature of 200 ° C. to 600 ° C. or higher to improve the reaction between the gas and the sensitive material. In the case of organic materials, the electrical conductivity is very low, and the composite of carbon black and organic materials has a very low sensitivity.

On the other hand, carbon nanotubes, which have recently been spotlighted as new material devices, can operate at room temperature, have a very good sensitivity, and have a fast reaction speed. This advantage is due to the physical properties of carbon nanotubes. Carbon nanotubes are a tube-shaped molecule formed by rounding a graphite plate (sp ² ) composed of carbons connected by hexagonal rings, and their diameters are several tens of nm. Carbon nanotubes are strong and well bent and do not damage or wear out even after repeated use, and the electrical properties vary according to the dried form, structure and diameter. In addition, since carbon nanotubes have a very large surface area compared to their volume, surface reactivity is higher than that of conventional gas sensitive materials. In addition, carbon nanotubes have a characteristic of degassing the adsorbed gas after gas sensing and returning to the original electrical conductivity, so even if repeated gas measurement is performed, the performance is not degraded (Jimg Kong, "Nanotube molecular wires as chemical sensors ", Science, 2000; OK Varghese," Gas sensing characteristics of multi-wall carbon nanotubes ", Sensors and Autuators, 2001).

The gas sensor made according to the conventional manufacturing method includes a contact continuous gas sensor (Fig. 1) using a resistance change with respect to the temperature of the platinum wire 1-1, and a chemical interaction between the air component and the semiconductor surface 2-1. There was a micro gas sensor (Fig. 2) using the change of the density of the conduction electrons on the surface by the action, but recently, the manufacturing technology of the gas sensor has been developed with high sensitivity and miniaturization, using micromachining technology on the silicon wafer. Increasingly, technologies such as buffers, amplifiers, converters, and the like have been integrated into high integration technologies. In addition, the structurally applied method is a trend of applying a heater technology, a diaphragm structure and an air bridge method for thermal insulation from peripheral circuits (C. Hagleitner, "Smart single-chip gas sensor microsystem ", Nature, 2001; Philip CH Chan," An integrated gas sensor technology using surface micro-machining ", IEEE-MEME 2001 conference, 2001).

Therefore, when the structure of the gas sensor is divided into the sensing unit, the electrode, and the heater, the high sensitivity is achieved through the improvement of the sensing unit materials (SnO ₂ , ZnO, Fe ₂ O ₃ , TiO _2, etc.) and the manufacturing process. Miniaturization is achieved by the comprehensive improvement of the sensing unit, the electrode and the heater. In addition, the design of the gas sensor is important in that it not only affects the sensing characteristics, but also provides great advantages in power consumption reduction and manufacturing economics. Recently, in order to improve characteristics such as high sensitivity, miniaturization, and fast response speed, packaging research for device protection is being conducted by integrating CMOS and the like in GaAs substrates and SOI substrates.

In line with these technical trends, the present inventors have also made efforts to fabricate new gas sensors that make up for the shortcomings of conventional gas sensors.As a result, by applying carbon nanotubes as sensitive materials to structures fabricated by micromachining technology, room temperature In order to reduce the time it takes for the carbon nanotubes to return to their original state at room temperature, a heater is provided which can operate at. Therefore, the present invention has been completed after structural design for reducing heat loss.

The gas sensor according to the present invention also has various types of gas that can be measured in terms of response characteristics, and the division thereof is also clear, and thus, the gas sensor has a high possibility as a gas sensor array.

In addition, since the structure of the carbon nanotubes in the form of a circular tube structure, the surface area per unit area is wider than that of a conventional gas sensor, which makes it possible to miniaturize and to enable the integration and the commercialization of chip scale using a micromachining structure. Further, by miniaturizing and growing carbon nanotubes directly on microstructures manufactured by micromachining technology for miniaturization and integration, further miniaturization and integration are possible.

Accordingly, an object of the present invention is to provide a carbon nanotube gas sensor capable of operating at high temperature with high sensitivity by using carbon nanotubes as sensitive materials.

Another object of the present invention is to provide a carbon nanotube gas sensor that can be miniaturized and integrated by directly growing carbon nanotubes on a microstructure manufactured by micromachining technology.

Another object of the present invention is to provide a carbon nanotube gas sensor that can be returned to its original state at room temperature by including a heater (heater) in the configuration of the gas sensor.

Another object of the present invention is to provide a method of manufacturing the gas sensor.

In order to achieve the object of the present invention, the manufacturing method of the carbon nanotube gas sensor according to the present invention,

Providing a substrate,

Forming an insulating layer and a protective layer on upper and lower portions of the substrate,

Forming a heater on the insulating layer formed on the substrate and forming a heater electrode connected to the heater,

Forming an insulating layer over the heater,

Forming an electrode line and a carbon nanotube electrode on the insulating layer;

Forming a catalyst metal on the carbon nanotube electrode;

Etching the lower part of the substrate to produce a diaphragm,

-Growing carbon nanotubes,

-Packaging with a cap in which the hole is processed.

Another carbon nanotube gas sensor manufacturing method according to the present invention,

Providing a substrate,

Etching the substrate to fabricate a channel,

Forming an insulating layer on the entire surface of the substrate,

Depositing heaters on both sides of the channel,

Depositing a heater electrode connected to the heater on the substrate;

Forming an insulating layer over the heater,

Forming a carbon nanotube electrode and a catalyst metal on the insulating layer;

Growing carbon nanotubes between both electrodes of the channel,

-Packaging with a cap in which the hole is processed.

Carbon nanotube gas sensor according to the present invention,

A protective layer formed under the substrate,

-A substrate made of a diaphragm structure,

An insulating layer formed on the substrate,

A heater and a heater electrode formed on the insulating layer,

An insulating layer formed on the heater,

A carbon nanotube electrode and an electrode line formed on the insulating layer;

A catalyst metal formed on the carbon nanotube electrode;

Carbon nanotubes formed on the catalytic metal,

-Characterized in that the hole is made of a cap processed.

Another carbon nanotube gas sensor according to the present invention,

A substrate made of a bulk structure comprising a channel,

An insulating layer formed on the entire surface of the substrate,

A heater electrode formed on both sides of the channel and a heater electrode formed on the substrate;

An insulating layer formed on the heater,

A carbon nanotube electrode and a catalyst metal formed on the insulating layer;

Carbon nanotubes formed between the carbon nanotube electrodes on both sides of the channel,

-Characterized in that the hole is made of a cap processed.

Hereinafter, a gas sensor using a carbon nanotube according to an embodiment of the present invention and a manufacturing method thereof will be described in detail with reference to the accompanying drawings.

A gas sensor using carbon nanotubes according to an embodiment of the present invention will be described with reference to FIGS. 5 and 6.

As shown in FIG. 5, in the carbon nanotube gas sensor according to the present invention, an insulating layer 3-2 and a protective layer 3-2 are formed on upper and lower portions of the substrate 3-1, respectively. The heater 3-3 and the heater electrode 3-4 are formed on 3-2). Another insulating layer 3-5 is formed on the heater, and carbon nanotube electrodes 3-6 and electrode lines 3-7 are formed thereon. Catalyst metals 3-8 are formed on the carbon nanotube electrodes 3-6, and carbon nanotubes 3-9 are formed thereon. The lower part of the board | substrate 3-1 is anisotropically etched, the diaphragm 3-11 is manufactured, and is packaged with the cap 3-10 with which the hole is processed.

As shown in FIG. 6, another carbon nanotube gas sensor according to the present invention has a channel formed on the substrate 4-1, and is insulated on the entire upper surface (including the channel) of the substrate 4-1. A layer (oxide film) is formed. Heaters 4-2 are formed on both sides of the channel, and insulating layers 4-4 are formed thereon. A heater electrode 4-3 connected to the heater 4-2 is formed on the substrate 4-1. A carbon nanotube electrode 4-5 and a catalyst metal 4-5 are formed on the insulating layer 4-4, and the carbon nanotube 4 is disposed between the carbon nanotube electrodes 4-5 on both sides of the channel. -6) are formed and packaged with a cap 4-7 in which holes are machined.

Next, a method of manufacturing a carbon nanotube gas sensor having a diaphragm structure according to a first embodiment of the present invention will be described with reference to FIGS. 3A to 3C.

First, as shown in FIG. 3A, an insulating layer 3-2 and a protective layer 3-2 are formed on the upper and lower portions of the silicon substrate 3-1, respectively. The insulating layer 3-2 on the substrate serves to insulate the silicon substrate, the heater, and the devices on the upper substrate. The insulating layer may be formed of a silicon oxide film using SiO ₂ , or a material having other insulating properties may be used. In addition, the protective layer 3-2 under the substrate serves as a silicon protective layer to etch only a portion necessary for etching silicon to fabricate a diaphragm. The protective layer may be formed of a silicon oxide film / nitride film using SiO ₂ / Si ₃ N ₄ . The thickness of the insulating layer and the protective layer is preferably formed to be 500 nm or more for convenience of the process, and when thinner than this, the insulating and protective functions may not be sufficiently performed.

The heater 3-3 and the heater electrode 3-4 connected thereto are formed on the insulating layer 3-2 on the substrate. The heater serves as a heating system for degassing. In other words, in order to reuse the gas sensor, gas molecules adsorbed on the carbon nanotubes must be removed quickly, and one of the methods is to use a heater. Metal or polysilicon may be used as the material of the heater, and any one of Pt, Ti, and Cr may be used as the metal of the heater and the heater electrode. The pattern is fabricated by photolithography and the heater and the heater electrode are fabricated using a general thin film process such as sputter deposition, ion beam deposition, chemical vapor deposition (CVD), wet etching or dry etching. Can be formed. The heater pattern does not have a standardized form, and may be manufactured in a serpentine form as long as it is connected to one line. The thickness of the heater is preferably 0.5 to 1 μm, which is ideal for current transport.

Next, as shown in FIG. 3B, an insulating layer 3-5 is formed on the heater 3-3. The insulating layer serves to insulate between the heater and the carbon nanotubes, which are the sensor sensitive parts, and is formed on the metal or polysilicon of the heater part except the heater electrode. The insulating layer may be formed of any one of a silicon oxide film and a silicon nitride film, or a material having another insulating property (for example, ceramic) may be used. The insulating layer should be formed to have a thickness greater than or equal to the heater thickness. If the insulating layer is thinner than the heater thickness, a short will be generated between the heater and the sensor sensitive portion to be formed thereon, so that the insulating layer cannot function as an insulating layer. Because.

The carbon nanotube electrodes 3-6 and the electrode lines 3-7 are formed on the insulating layer. The carbon nanotube electrode may be formed so that the electrodes at both ends face each other with a predetermined length. By making the length of the electrode constant, the resistance value is made constant, so that the measurement error can be reduced, and the area in contact with the carbon nanotube can be kept constant, which is advantageous. In addition, the carbon nanotube electrode may be formed in a form in which fine resistance is easily measured. In addition, considering that the growth length of the carbon nanotubes is about 2 to 5 µm, the carbon nanotube electrodes are appropriately formed to have a thickness of 1 µm or less.

During the formation of the heater electrode and the carbon nanotube electrode, a feedthrough pattern connected to the outside of the cap and the cap is previously defined through a lithography process. This is to facilitate the circuit configuration that is connected to the outside when the cap is put on.

Next, as shown in FIG. 3c, a catalyst metal 3-8 connected to the carbon nanotube electrode is deposited. The catalytic metal is formed using a lift-off method, and ion beam lithography may be used. The catalyst metal is a catalyst for growing carbon nanotubes and is formed of Co, Ni, Fe, or the like. The catalyst metal is preferably formed to be connected to both ends of the electrode, the thickness of which is less than 20 nm is a suitable thickness for forming carbon nanotubes.

Then, the lower portion of the silicon substrate is anisotropically etched to have a thickness of about 20 μm to produce a diaphragm 3-11. As such, by forming a structure in the form of a diaphragm to minimize the thickness of the substrate with the heater, it is possible to reduce the heat loss of the heater and consequently to improve the original function of the gas sensor. Subsequently, after the substrate is placed in the chamber and the vacuum is made, the gas is injected while the temperature of the chamber is raised to the carbon nanotube growth temperature. As shown in FIG. 3C, carbon nanotubes are vertically grown. At this time, it is preferable to increase the contact between the carbon nanotubes, it is good to grow the carbon nanotubes tangled with each other. In this case, as a method of growing carbon nanotubes, thermal chemical vapor deposition (thermal CVD) may be used at 1000 ° C. or lower, or plasma chemical vapor deposition (PECVD) at 500 ° C. or lower. It can also grow carbon nanotubes horizontally. In the horizontal growth of carbon nanotubes, it is preferable to grow the carbon nanotubes in a network state while depositing the catalyst as a whole and adjusting the temperature and gas uniformly. The longer the growth length of the carbon nanotubes, the larger the surface area where gas is adsorbed, so that the carbon nanotubes may be grown to have a length of 1 μm or more in order to increase sensitivity. If the length of the carbon nanotube is less than 1 ㎛ because of the low sensitivity, gas measurement is difficult.

And finally packaged with a glass cap (3-10) in which the hole is processed. In addition to glass, any material used for packaging may be used as long as it is a plastic and a non-conductive material. In order to secure the place where gas is inhaled, packaging is performed by manufacturing the cap which has many holes of 100 micrometers or less. The cap serves to protect the device and to secure a space for minimizing the amount of change after inflow of the gas. Bonding of the cap is performed on a wafer basis. Electrostatic heat bonding may be used for bonding the glass cap, and adhesive may be used for bonding the plastic cap.

A method of fabricating a carbon nanotube gas sensor having a bulk structure according to a second embodiment of the present invention will be described with reference to FIGS. 4A to 4D.

First, as shown in FIG. 4A, silicon is etched at a predetermined depth and width in consideration of various conditions including the length of carbon nanotube growth, and the like, and a channel is formed on the silicon substrate 4-1. Is oxidized to form an insulating layer. The channel can be formed in an ellipse or polygon when viewed from the top. The thickness of the insulating layer is preferably 1 μm or less. If the thickness of the insulating layer is 1 μm or more, the lower portion of the channel becomes narrower, so care should be taken.

Next, as shown in FIG. 4B, the substrate is tilted through the lithography process to form the heater 4-2 on both channel surfaces, and the heater electrode 4-3 connected to the heater is deposited on the substrate. The heater may be formed by depositing an electrode material and a heater material by using a sputter or an E-beam on a pattern made through lithography. Metal may be used as a material of the heater and the heater electrode, and at this time, any one of Pt, Ti, and Cr may be used as the metal of the heater and the heater electrode. In addition to the metal, polysilicon may be used to fabricate the heater. Heaters using polysilicon are advantageous for integration and have excellent process applicability, and thus their use is increasing. It is good to form the thickness of a heater to 1 micrometer or less. Due to the characteristics of the bulk structure facing each other in the form of a channel, heat dissipation is concentrated inside the channel to prevent heat from escaping to the outside.

Next, as illustrated in FIG. 4C, an insulating layer 4-4 is formed on the heater. The insulating layer can be formed using a sputter or an E-beam. It is good to form the thickness of an insulating layer below 1 micrometer.

Next, as shown in FIG. 4D, a carbon nanotube electrode 4-5 is formed on the insulating layer, and the catalyst metal 4-5 is deposited thereon. The carbon nanotube electrode is formed of metal and connected to the catalytic metal on the insulating layer on the wall of the channel, and the thickness of the carbon nanotube electrode is preferably 1 μm or less. The catalyst metal is a catalyst for growing carbon nanotubes and is formed of Co, Ni, Fe, or the like. The catalyst metal is formed to be connected to the carbon nanotube electrode on the channel wall by a lift-off method, and the thickness thereof is preferably 20 nm or less.

Next, the carbon nanotubes 4-6 are grown between both electrodes 4-5. Carbon nanotubes can be grown directly using thermal chemical vapor deposition (CVD), plasma chemical vapor deposition (PECVD), or the like. In the channel structure, in order to increase the sensitivity, it is preferable to grow the carbon nanotubes in a direction perpendicular to the wall, that is, horizontally from the wall to the wall. At this time, it is preferable to increase the contact between the carbon nanotubes, and to grow the carbon nanotubes tangled with each other. The carbon nanotubes may be grown to have a length of 1 μm or more.

Next, as shown in Figure 4e it can be finally packaged into a glass cap or a plastic cap (4-7). Packaging materials can be used for all non-conductive materials, including glass and plastic. Packaging is carried out by producing a cap, preferably a cap having a large number of holes of 100 µm or less. Bonding of the cap is performed on a wafer basis. Electrostatic heat bonding may be used for bonding the glass cap, and adhesive may be used for bonding the plastic cap.

As described above, in the present invention, carbon nanotubes may be manufactured and applied to gas sensors.

In the present invention, by applying carbon nanotubes grown directly on microstructures produced by micromachining technology to a gas sensor, it is possible to provide a gas sensor with high sensitivity and room temperature operation, and also to reduce the size and integration size and power consumption. Can be reduced. In addition, there are various types of sensitive gas, so there is a high possibility of developing the gas sensor array. In addition, wafer-level semiconductor processes and chip-scale packaging can reduce production and production costs as commercial products.

1 shows a contact combustion gas sensor according to the prior art;

2 is a view showing a micro gas sensor according to the prior art,

3A to 3C are views illustrating a manufacturing process of a carbon nanotube gas sensor (diaphragm structure) according to a first embodiment of the present invention;

4A to 4D are views illustrating a manufacturing process of a carbon nanotube gas sensor (bulk structure having a channel) according to a second embodiment of the present invention;

5 and 6 are diagrams illustrating a carbon nanotube gas sensor 5 (diaphragm structure, bulk structure having 6: channels) according to an embodiment of the present invention, respectively.

* Explanation of symbols for the main parts of the drawings

3-1, 4-1: substrate

3-2, 3-5, 4-4: insulation or protective layer

3-3, 4-2: heater

3-4, 4-3: heater electrode

3-6, 4-5: carbon nanotube electrode

3-8, 4-5: catalytic metal

3-9, 4-6: carbon nanotubes

Claims (25)

Providing a substrate, Forming an insulating layer and a protective layer on upper and lower portions of the substrate, Forming a heater on the insulating layer formed on the substrate and forming a heater electrode connected to the heater; Forming an insulating layer on the heater; Forming an electrode line and a carbon nanotube electrode on the insulating layer; Forming a catalyst metal on the carbon nanotube electrode; Manufacturing an diaphragm by anisotropically etching the lower portion of the substrate, Growing carbon nanotubes, Method for manufacturing a carbon nanotube gas sensor comprising the step of packaging the hole is processed cap. Providing a substrate, Manufacturing a channel by etching the substrate; Forming an insulating layer on the entire surface of the substrate; Depositing heaters on both sides of the channel, Depositing a heater electrode connected to the heater on the substrate; Forming an insulating layer on the heater; Forming a carbon nanotube electrode and a catalyst metal on the insulating layer; Growing carbon nanotubes between both electrodes of the channel; Method for manufacturing a carbon nanotube gas sensor comprising the step of packaging the hole is processed cap. The method of manufacturing a carbon nanotube gas sensor according to claim 1 or 2, wherein the insulating layer is made of a silicon oxide film. The method of claim 1, wherein the protective layer is formed of a silicon nitride film. The method of claim 1, wherein the upper and lower portions of the insulating layer and the protective layer have a thickness of 500 nm or more. The method of claim 2, wherein the insulating layer has a thickness of 1 μm or less. The method of claim 1, wherein the heater is formed by sputter deposition, ion beam deposition, chemical vapor deposition (CVD), wet etching, or dry etching. The method of claim 2, wherein the heater is formed using a sputter or an E-beam process by tilting the substrate. The method of manufacturing a carbon nanotube gas sensor according to claim 1 or 2, wherein the heater is made of metal or polysilicon. The method of claim 9, wherein the metal is selected from Pt, Ti, or Cr. The method of manufacturing a carbon nanotube gas sensor according to claim 1 or 2, wherein the heater has a thickness of 0.5 to 1 µm. The method according to claim 1 or 2, wherein the heater electrode is made of any one metal selected from Pt, Ti, and Cr. The method of claim 1, wherein the thickness of the insulating layer on the heater is equal to or greater than the thickness of the heater. The method according to claim 1 or 2, wherein the carbon nanotube electrodes are formed such that electrodes at both ends thereof face each other with a predetermined length. The method of manufacturing a carbon nanotube gas sensor according to claim 1 or 2, wherein the carbon nanotube electrode has a thickness of 1 µm or less. The method of claim 1 or 2, wherein the catalytic metal is formed using a lift-off method or ion beam lithography. The method of claim 16, wherein the catalyst metal has a thickness of 20 nm or less. The method of claim 16, wherein the catalytic metal is selected from Co, Ni, or Fe. The method of claim 1, wherein the carbon nanotubes are directly grown using thermal CVD or plasma chemical vapor deposition (PECVD). The method of manufacturing a carbon nanotube gas sensor according to claim 1 or 2, wherein the carbon nanotube has a length of 1 µm or more. The method of claim 1, wherein the carbon nanotubes grow horizontally or vertically. The method of claim 2, wherein the carbon nanotubes are horizontally grown in a direction perpendicular to the wall surface. The method of manufacturing a carbon nanotube gas sensor according to claim 1 or 2, wherein the cap is made of glass or plastic. A protective layer formed under the substrate, A substrate made of a diaphragm structure, An insulating layer formed on the substrate, A heater and a heater electrode formed on the insulating layer, An insulating layer formed on the heater, A carbon nanotube electrode and an electrode line formed on the insulating layer; A catalyst metal formed on the carbon nanotube electrode; Carbon nanotubes formed on the catalyst metal, Carbon nanotube gas sensor with a capped hole. A substrate made of a bulk structure including a channel, An insulating layer formed on the entire surface of the substrate, Heaters formed on both sides of the channel and the heater electrodes formed on the substrate; An insulating layer formed on the heater, A carbon nanotube electrode and a catalyst metal formed on the insulating layer, Carbon nanotubes formed between the carbon nanotube electrodes on both sides of the channel; Carbon nanotube gas sensor with a capped hole.