WO2009017265A1

WO2009017265A1 - Gas sensor and method for manufacturing the same

Info

Publication number: WO2009017265A1
Application number: PCT/KR2007/003656
Authority: WO
Inventors: Young Geun Park; Se Hee Cho
Original assignee: M2N Inc.
Priority date: 2007-07-27
Filing date: 2007-07-30
Publication date: 2009-02-05
Also published as: KR20090011631A; KR100906496B1

Abstract

The present disclosure relates to a gas sensor, and more particularly, to a gas sensor using carbon nanotube and a method of manufacturing the same. The present disclosure provides a method of manufacturing a gas sensor, including: (a) forming a first insulating film on upper and lower surfaces of a silicon substrate; (b) forming a first metal film having a predetermined pattern on the first insulating film formed on the upper surface of the silicon substrate; (c) forming a second insulating film on the first metal film; (d) forming a second metal film having a predetermined pattern on the second insulating film; (e) coating a carbon nanotube mixed solution mixed with carbon nanotube(CNT), conductive polymer and a deionized water on the second metal film; and (f) heating the carbon nanotube mixed solution to form a carbon nanotube film.

Description

GAS SENSOR AND METHOD FOR MANUFACTURING THE

SAME

Technical Field

[1] The present disclosure relates to a gas sensor, and more particularly, to a gas sensor using carbon nanotube and a method of manufacturing the same. Background Art

[2] In general, a gas sensor serves to detect a specific gas. If the gas sensor is exposed to a specific gas, the conductivity of a semiconductor serving as a detecting material used in the gas sensor is changed, or an electromotive force is generated. The gas sensor detects the specific gas by measuring the changed conductivity of the semiconductor or the generated force.

[3] Fig. 1 is a view illustrating a conventional gas sensor.

[4] The conventional gas sensor includes a heating portion 10, an electrode portion 20 and a gas detecting portion 30.

[5] The heating portion 10 serves to heat the gas detecting portion up to a temperature at which the gas detecting portion operates with its optimum performance. The heating portion 10 is generally made of a metal resistor having a predetermined resistance value.

[6] The electrode portion 20 serves to transmit to an external circuit the resistance value variation resulting from a gas absorption of the gas detecting portion 30. The electrode portion 20 may be made of platinum (Pt), gold (Au), silver (Ag), aluminum (Al) or the like. In particular, since the gas detecting portion 30 is heated up to a high temperature of 300⁰C or more during the operation of the gas sensor, the electrode portion 20 is mainly made of Pt having an excellent high-temperature stability.

[7] The gas detecting portion 30 serves to detect the change of electric resistance by directly contacting with the gas to be detected and absorbing the gas. The gas detecting portion 30 may be made of a metal oxide having semiconductor characteristics. If a chemical reaction is generated by contacting the gas with the gas detecting portion 30, electron exchange occurs between the gas and the gas detecting portion 30, and thus the resistance value of the gas detecting portion 30 is changed. Therefore, it is necessarily required that the gas detecting portion 30 have semiconductor characteristics.

[8] For example, in case an n-type semiconductor such as SnO comes in contact with a reductive gas such as a CO gas, the CO gas is reacted with the O existing on a surface of the semiconductor, and thus is converted into CO . Since electron exchange occurs between the SnO and the CO gas, the conductivity of a gas detecting film is changed.

[9] More specifically, the SnO is the n-type semiconductor, and the conductivity thereof is affected by oxygen supply, partial pressure of oxygen and excess Sn in a lattice thereof. If the SnO crystal is heated in the atmosphere, oxygen in the atmosphere receives a donor electron from the SnO crystal, so that the oxygen is adhered on surfaces of the SnO crystal. As a result, a potential barrier is formed in an interior of the SnO crystal by the adhered anion. At this time, when the reductive gas such as a CO gas is introduced on the SnO crystals, surface concentration of the anion is decreased and the potential barrier is lowered, which causes the conductivity of the SnO crystal to change. In order to accelerate the reaction between the gas detecting film and the gas to be detected, the gas sensor may be provided with a catalyst such as Pt, Pd or the like. Accordingly, a range of the resistance change resulted from the gas absorption is expanded to improve the sensitivity of the gas sensor or lower the operation temperature of the gas detecting film.

[10] In the conventional gas sensor, however, since the gas detecting portion is made of the metal oxide, the sensitivity of the conventional gas sensor is lowered, so that it cannot properly correspond to recent environmental standards. Also, there is another problem in that it is not suitable to an air pollution measuring device to be downsized since the size of the gas sensor becomes larger.

[11] Further, since the conventional gas sensor operates at a high temperature of 30O⁰C or more, a large amount of power is consumed to raise the temperature of the gas detecting portion above 300⁰C.

[12] Accordingly, there has been a demand for a further advanced technique development of the gas sensor capable of operating with a lower power, the gas sensor having a compact size and high sensitivity. Disclosure of Invention Technical Problem

[13] In view of the forgoing, it is an object of the present disclosure to provide a gas sensor capable of operating at a lower temperature, and having a compact size and high sensitivity, and to provide a method of manufacturing the same by using a carbon nanotube as a gas detecting portion. Technical Solution

[14] In order to realize the above object, in accordance with an aspect of the present invention, there is provided a method of manufacturing a gas sensor, including: (a) forming a first insulating film on upper and lower surfaces of a silicon substrate; (b) forming a first metal film having a predetermined pattern on the first insulating film formed on the upper surface of the silicon substrate; (c) forming a second insulating film on the first metal film; (d) forming a second metal film having a predetermined pattern on the second insulating film; (e) coating a carbon nanotube mixed solution mixed with carbon nanotube (CNT), conductive polymer and a deionized water on the second metal film; and (f) heating the carbon nanotube mixed solution to form a carbon nanotube film.

[15] Further, the method may include (g) etching a portion of the first insulating film, the second insulating film and the lower surface of the silicon substrate to be formed on the lower surface of the silicon substrate.

[16] Further, an adhesion film may be formed before forming the first and second metal films.

[17] Further, the first insulating film may have a silicon oxide film and a silicon nitride film, and the second insulating film may have a silicon oxide film.

[18] Further, the first metal film and the second metal film may be made of platinum

(Pt).

[19] Further, the adhesion film may be made of titanium (Ti).

[20] Further, the step (d) may be executed by using an electron beam (E-beam) deposition method.

[21] In accordance with another aspect of the present invention, there is provided a gas sensor including: a gas detecting portion of which electric resistance is varied if the gas detecting portion is contacted with a gas to be detected; a heating portion for heating the gas detecting portion; and an electrode for transmitting the variation of the electric resistance of the gas detecting portion to an exterior, wherein the gas detecting portion has carbon nanotube and conductive polymer.

[22] Further, the heating portion and the electrode are made of platinum (Pt).

Brief Description of the Drawings

[23] Fig. 1 is a view illustrating a conventional gas sensor;

[24] Figs. 2 to 13 are drawings illustrating processes of manufacturing a gas sensor in accordance with an embodiment of the present invention; and

[25] Fig. 14 is a view illustrating the gas sensor in accordance with the embodiment of the present invention. Mode for the Invention

[26] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings so that the present invention may be readily implemented by those skilled in the art. However, it is to be noted that the present invention is not limited to the embodiments but can be realized in various other ways. In the drawings, parts irrelevant to the description are omitted for the simplicity of explanation, and like reference numerals denote like parts through the whole document. [27] Through the whole document, the term "connected to" or "coupled to" that is used to designate a connection or coupling of one element to another element includes both a case that an element is "directly connected or coupled to" another element and a case that an element is "electronically connected or coupled to" another element via still another element. Further, the term "comprises or includes" and/or "comprising or including" used in the document means that one or more other components, steps, operation and/or existence or addition of elements are not excluded in addition to the described components, steps, operation and/or elements.

[28] Referring to Figs. 2 to 13, processes of manufacturing a gas sensor in accordance with an embodiment of the present invention will be described in detail. Figs. 2 to 13 are drawings illustrating processes of manufacturing the gas sensor in accordance with the embodiment of the present invention

[29] As shown in Fig. 2, an SOI (Silicon On Insulator) silicon wafer 210 is prepared.

Preferably, the silicon wafer 210 may be a DSP (Double-Side Polished) (100) silicon wafer.

[30] Then, as shown in Fig. 3, an oxide film 220 is deposited on upper and lower surfaces of the silicon wafer 210. The oxide film 220 is formed, for example, by growing a wet thermal oxide film at a temperature of 1000⁰C under H2O atmosphere.

Preferably, the oxide film 220 has a thickness of 5000A.

[31] Further, as shown in Fig. 4, a silicon nitride film 230 is deposited on the oxide film

220. The silicon nitride film 230 may be deposited by using a low pressure chemical vapor deposition (LPCVD) method, and preferably has a thickness of 3000A thinner than that of the oxide film 220.

[32] In accordance with the embodiment of the present invention, the silicon wafer 210 is insulated by depositing the oxide film 220 and the silicon nitride film 230 on the silicon wafer 210. Alternatively, the silicon wafer 210 may be insulated by using the oxide film 220 only, without depositing the silicon nitride film 230. Also, the oxide film may further be deposited on the nitride film/oxide film to form an oxide/ nitride/oxide (ONO) layer.

[33] Then, as shown in Fig. 5, an adhesion film (not shown) and a metal film 240 such as platinum are deposited on the silicon nitride film 230 in order to form a heater by using a DC or RF magnetron sputtering method. In case of depositing platinum, it is preferable that silver titanium is used as the adhesion film, and that platinum and the adhesion film have a thickness of 2000A and 200A, respectively.

[34] As shown in Fig. 6, the metal film 240 is patterned to form the heater. For example, after a photoresist film is coated on the metal film 240, the photoresist film is exposed and developed, and then the metal film 240 and the adhesion film are sequentially etched in order to expose the silicon nitride film 230, thereby forming the heater. Preferably, a width of each line in the heater ranges from 10 to 20 D.

[35] Next, as shown in Fig. 7, a silicon oxide film 250 having a thickness of 8000A is deposited on the patterned metal film 240 and the exposed silicon nitride film 230 by using a plasma enhanced chemical vapor deposition (PECVD) method. The silicon nitride film 250 serves to insulate the metal film 240 from an electrode layer to be deposited later.

[36] Referring to Fig. 8, an adhesion film (not shown) such as titanium and a metal film

260 such as platinum are deposited on the silicon oxide film 250 in order to form an electrode by using an electron beam. In case of depositing platinum, it is preferable that silver titanium is used as the adhesion film, and that platinum and the adhesion film have a thickness of 1000 A and 10OA, respectively.

[37] Then, as shown in Fig. 9, the metal film 260 is patterned to form the electrode. For example, after a photoresist film is coated on the metal film 260, the photoresist film is exposed and developed, and then the metal film 260 and the adhesion film are sequentially etched to expose the silicon oxide film 250, thereby forming the electrode.

[38] Referring to Fig. 10, the metal film 260 and the silicon oxide film 250 are etched by using a dry etching process, thereby exposing a heater pad.

[39] Next, as shown in Fig. 11, in order to minimize loss of thermal energy transmitted from the heater to the gas detecting film to be formed later, portions of the silicon nitride film 230 and the oxide film 220, which have been deposited on the lower surface of the silicon wafer 210, and a portion of the silicon wafer 210 are removed by using a dry etching process.

[40] As shown in Fig. 12, a carbon nanotube mixed solution mixed with carbon nanotube, conductive polymer and deionized water is coated on the metal film 260 for forming the electrode film by using an inkjet printing process or electrophoresis process. Preferably, the conductive polymer used in the embodiment of the present invention may be a material, e.g., PEDOT (polyethylenedioxythiophene), capable of be ing melted into a liquid.

[41] Then, as shown in Fig. 13, a gas detecting film 270 having the carbon nanotube is formed by carrying out a heat treatment on the coated carbon nanotube mixed solution at a temperature of 80 to 100⁰C.

[42] The carbon nanotube used in the embodiment of the present invention is allotrope made of carbon, in which one carbon atom is engaged with another carbon atom in a hexagonal honeycomb lattice to form a tube shape. Further, a diameter of the tube is in the order of a few nm(one billionth of a meter). Such carbon nanotube has physical characteristics that electric conductivity thereof is similar to that of copper, thermal conductivity thereof is similar to diamond, and strength thereof is 100 times stronger than that of steel. Although a carbon fiber is cut even at deformation of 1%, the carbon nanotube can withstand deformation of 15%. As described above, the carbon nanotube has good mechanical characteristics, a good electrical selectivity, excellent electric field emission characteristics and high-efficiency hydrogen storage medium characteristics. Because of such characteristics of the carbon nanotube, the detecting film in accordance with the embodiment of the present invention can operate at a relatively lower temperature, and have the sensitivity 1000 times higher than a metal oxide sensor.

[43] Accordingly, the gas sensor in accordance with the embodiment of the present invention is manufactured by executing the above-described processes.

[44] Fig. 14 is a plan view illustrating a gas sensor in accordance with the embodiment of the present invention.

[45] The gas sensor in accordance with the embodiment of the present invention includes a heater 310, an electrode 320 and a gas detecting portion 330. In particular, since the gas detecting portion 330 is made of a carbon nanotube film, the thermal conductivity thereof is excellent. Therefore, even though a heat discharge rate generated from the heater 310 is low, the gas sensor can operate. Accordingly, a gas sensor with reduced power consumption may be provided. Further, since the gas detecting portion 330 has excellent electric conductivity, a gas sensor of high sensitivity may be provided.

[46] Also, since the gas sensor in accordance with the embodiment of the present invention is manufactured by using a MEMS process through the whole processes, a compact gas sensor may be provided.

[47] In addition, since the gas sensor in accordance with the embodiment of the present invention is manufactured by using the MEMS process through the whole processes, the gas sensor is advantageous to a mass production, thereby reducing a manufacturing cost thereof.

[48] The above description of the present invention is provided for the purpose of illustration, and it would be understood by those skilled in the art that various changes and modifications may be made without changing technical conception and essential features of the present invention. Thus, it is clear that the above-described embodiments are illustrative in all aspects and do not limit the present invention.

[49] The scope of the present invention is defined by the following claims rather than by the detailed description of the embodiment. It shall be understood that all modifications and embodiments conceived from the meaning and scope of the claims and their equivalents are included in the scope of the present invention. Industrial Applicability

[50] In accordance with the embodiment of the present invention, a gas detecting portion of a gas sensor is manufactured by using the carbon nanotube film. Therefore, a gas sensor with reduced power consumption, and a gas sensor of high sensitivity may be provided

[51] Also, in accordance with the embodiment of the present invention, a size of a gas sensor may be compact and manufacturing cost thereof may be reduced.

Claims

[1] L A method of manufacturing a gas sensor, comprising:

(a) forming a first insulating film on upper and lower surfaces of a silicon substrate;

(b) forming a first metal film having a predetermined pattern on the first insulating film formed on the upper surface of the silicon substrate;

(c) forming a second insulating film on the first metal film;

(d) forming a second metal film having a predetermined pattern on the second insulating film;

(e) coating a carbon nanotube mixed solution mixed with carbon nanotube(CNT), conductive polymer and a deionized water on the second metal film; and

(f) heating the carbon nanotube mixed solution to form a carbon nanotube film.

[2] The method of claim 1, further comprising (g) etching a portion of the first insulating film, the second insulating film and the lower surface of the silicon substrate to be formed on the lower surface of the silicon substrate.

[3] The method of claim 1, further comprising (h) forming an adhesion film before forming the first and second metal films.

[4] The method of claim 3, wherein the first insulating film has a silicon oxide film and a silicon nitride film, and the second insulating film has a silicon oxide film.

[5] The method of claim 1, wherein the first metal film and the second metal film are made of platinum (Pt).

[6] The method of claim 3, wherein the adhesion film is made of titanium (Ti).

[7] The method of claim 1, wherein the step (d) is executed by using an electron beam (E-beam) deposition method. [8] A gas sensor comprising: a gas detecting portion of which electric resistance is changed if the gas detecting portion is contacted with a gas to be detected; a heating portion for heating the gas detecting portion; and an electrode for transmitting the change of the electric resistance of the gas detecting portion to an exterior, wherein the gas detecting portion has carbon nanotube and conductive polymer. [9] The gas sensor of claim 8, wherein the heating portion and the electrode are made of platinum (Pt).