Patent Publication Number: US-2018045663-A1

Title: Micro sensor

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims priority to Korean Patent Application No. 10-2016-0102340, filed Aug. 11, 2016, the entire contents of which is incorporated herein for all purposes by this reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention generally relates to a micro sensor. More particularly, the present invention relates to a micro sensor including a first sensor electrode formed on a first side of a substrate and a second sensor electrode formed on a second side of the substrate provided with an etching hole penetrating from the first side of the substrate to the second side of the substrate. 
     Description of the Related Art 
     Recently, as concern about the environment is gradually increasing, there is a need for development of a micro sensor being capable of acquiring accurate and various pieces of information in a short period of time. Especially, for comfort of residential space, management of harmful industrial environment, management of foods and food production processes, etc., efforts have progressed in terms of realizing miniaturization, high-precision, and cost reduction of a micro multi-array sensor, such as a gas sensor, for easily measuring the relevant gas concentration. 
     Gas sensors are gradually evolving from conventional sintered ceramic or thick film structures into micro-electro-mechanical system (MEMS) gas sensors by applying semiconductor process technologies. 
     In terms of a measurement method, the most widely used method of a gas sensor today is measuring changes in electrical characteristics when the gas is adsorbed onto a sensing material of the sensor. Generally, metal oxides such as SnO 2  are used as a sensing material, and the change in the electrical conductivity thereof depending on the concentration of the target gas is measured. This measurement method is relatively simple. Here, the change in the measurement value is more significant when the metal oxide sensing material is operated at the high temperature. Therefore, precise temperature control is essential for fast and precise measurement of the gas concentration. Also, when measuring, the gas concentration is measured after resetting the sensing material to its initial state through high temperature heating, thereby forcibly removing gas species or moisture already adsorbed onto the sensing material. 
     However, a conventional sensor detects one kind of gas, and thus, several sensors are required to detect various kinds of gas, whereby the volume and power consumption increase. 
     Also, the sensing material is placed at the outside of the substrate and the measurement precision is low due to the moisture (humidity) in air. 
     The foregoing is intended merely to aid in the understanding of the background of the present invention, and is not intended to mean that the present invention falls within the purview of the related art that is already known to those skilled in the art. 
     DOCUMENT OF RELATED ART 
     
         
         (Patent Document 1) Korean Patent Application Publication No. 10-2009-0064693. 
       
    
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention has been made keeping in mind the above problems occurring in the related art, and an object of the present invention is to provide a micro sensor being capable of minimizing the influence of moisture in air. 
     In order to accomplish the above object, the present invention provides a micro sensor including: a substrate; and a sensor electrode unit provided on the substrate, wherein the sensor electrode unit includes a first sensor electrode and a second sensor electrode spaced apart from the first sensor electrode, the first sensor electrode is provided on a first side of the substrate, the second sensor electrode is provided on a second side of the substrate, and the substrate is provided with an etching hole penetrating from the first side to the second side of the substrate. 
     The etching hole may be provided at a circumference of the first sensor electrode. 
     The second sensor electrode may block at least a part of the etching hole on the second side. 
     The first sensor electrode may include a first sensor wire, the second sensor electrode may include a second sensor wire, and an area of the second sensor wire may be larger than an area of the first sensor wire. 
     The first sensor electrode may include a first sensor wire and a first sensor electrode pad connected to the first sensor wire, the second sensor electrode may include a second sensor wire and a second sensor electrode pad connected to the second sensor wire, the first side of the substrate may be provided with an assistant pad, and the assistant pad may be connected to the second sensor electrode pad. 
     The substrate may be provided with a pad hole penetrating from the first side of the substrate to the second side of the substrate so as to connect the assistant pad and the second sensor electrode pad to each other. 
     The micro sensor may include a heater electrode unit provided on the first side of the substrate. 
     The substrate may be an anodized film obtained by anodizing a metallic base material and removing the base material. 
     The substrate may be provided with an air gap surrounding the heater electrode unit. 
     At least a part of a surface of the heater electrode unit may be provided with a passivation layer. 
     According to another aspect, there is provided a micro sensor including: a substrate; a sensor electrode unit provided on the substrate; and a heater electrode unit provided on the substrate, wherein the sensor electrode unit includes a first sensor electrode unit and a second sensor electrode unit, the heater electrode unit includes a first heater electrode unit having a first heating wire and a second heater electrode unit having a second heating wire, the first sensor electrode unit is closer to the first heating wire than to the second heating wire, the second sensor electrode unit is closer to the second heating wire than to the first heating wire, at least one of the first sensor electrode unit and the second sensor electrode unit includes a first sensor electrode and a second sensor electrode spaced apart from the first sensor electrode, the first sensor electrode is provided on a first side of the substrate, the second sensor electrode is provided on a second side of the substrate, and the substrate is provided with an etching hole penetrating from the first side of the substrate to the second side of the substrate. 
     The first heating wire and the second heating wire may have different heating values. 
     According to the micro sensor of the present invention as described above, effects as follows may be obtained. 
     The first sensor electrode is formed on the first side of the substrate and the second sensor electrode is formed on the second side of the substrate. The substrate is provided with the etching hole penetrating from the first side of the substrate to the second side of the substrate. A sensing material provided between the first sensor electrode and the second sensor electrode is inserted in the etching hole. When detecting gas, influence of moisture in air can be minimized, thereby enhancing measurement precision. 
     The first sensor electrode includes the first sensor wire and the first sensor electrode pad connected to the first sensor wire. The second sensor electrode includes the second sensor wire and the second sensor electrode pad connected to the second sensor wire. The first side of the substrate is provided with the assistant pad, and the assistant pad is connected to the second sensor electrode pad. Soldering of the first sensor electrode and the second sensor electrode can be performed on the same surface, and thus a soldering process is easier. 
     The substrate is provided with the pad hole penetrating from the first side of the substrate to the second side of the substrate so as to connect the assistant pad and the second sensor electrode pad, whereby the structure of the sensor can be simple. 
     The substrate is the anodized film obtained by anodizing the metallic base material and removing the base material, thereby enhancing insulation effect. 
     The substrate is provided with the air gap surrounding the heater electrode unit, whereby a heat capacity is small and high temperature can be maintained by low power. 
     At least a part of the surface of the heater electrode unit is provided with the passivation layer. Thus, the heater electrode unit can be electrically insulated from the sensing material, and the heater electrode unit can be protected from oxidation. 
     The first heating wire and the second heating wire of the sensor have different heating values. When applied to a gas sensor, various kinds of gas can be simultaneously detected with a simple structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a plan view showing a micro sensor according to an exemplary embodiment of the present invention (except for a sensing material and a passivation layer); 
         FIG. 2  is an enlarged view of A portion of  FIG. 1 ; 
         FIG. 3  is an enlarged view of B portion of  FIG. 1 ; 
         FIG. 4  is a plan view showing a micro sensor according to an exemplary embodiment of the present invention; 
         FIG. 5  is a bottom view showing a micro sensor according to an exemplary embodiment of the present invention; and 
         FIG. 6  is a cross-sectional view of C-C portion of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinbelow, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
     For reference, in the following descriptions, the same configurations of the present invention as those of the related art will not be described in detail. Reference is made to the foregoing descriptions of the related art. 
     When it is said that any part is positioned “on” another part, it means the part is directly on the other part or above the other part with at least one intermediate part. In contrast, if any part is said to be positioned “directly on” another part, it means that there is no intermediate part between the two parts. 
     Technical terms used here are to only describe a specific exemplary embodiment and are not intended to limit the present invention. Singular forms used here include a plurality of forms unless phrases explicitly represent an opposite meaning. A meaning of “comprising” used in a specification embodies a specific characteristic, area, integer, step, operation, element and/or component and does not exclude presence or addition of another specific characteristic, area, integer, step, operation, element, component and/or group. 
     Terms representing relative space of “low” and “upper” may be used for more easily describing a relationship to another portion of a portion shown in the drawings. Such terms are intended to include other meanings or operations of a using apparatus together with a meaning that is intended in the drawings. For example, when an apparatus is inverted in the drawings, any portion described as disposed at a “low” portion of other portions is described as being disposed at an “upper” portion of other portions. Therefore, an illustrative term of “low” includes entire upper and lower directions. An apparatus may rotate by 90° or another angle, and a term representing relative space is accordingly analyzed. 
     As shown in  FIGS. 1 to 6 , according to the embodiment, a micro sensor includes a substrate  100 , a sensor electrode unit formed on the substrate  100 , and a heater electrode unit formed on the substrate  100 . The sensor electrode unit includes a first sensor electrode unit  1300  and a second sensor electrode unit  2300 . The heater electrode unit includes a first heater electrode unit  1200  having a first heating wire  1210  and a second heater electrode unit  2200  having a second heating wire  2210 . The first sensor electrode unit  1300  is closer to the first heating wire  1210  than to the second heating wire  2210 . The second sensor electrode unit  2300  is closer to the second heating wire  2210  than to the first heating wire  1210 . At least one of the first sensor electrode unit  1300  and the second sensor electrode unit  2300  includes a first sensor electrode  1301 ,  2301  and a second sensor electrode  1302 ,  2302  spaced apart from the first sensor electrode  1301 ,  2301 . The first sensor electrode  1301 ,  2301  is formed on a first side of the substrate  100 , and the second sensor electrode  1302 ,  2302  is formed on a second side of the substrate  100 . The substrate  100  is provided with an etching hole  103  penetrating from the first side of the substrate  100  to the second side of the substrate  100 . 
     When anodizing a metallic base material, an anodized film is formed. The anodized film is composed of a porous layer having several pores on the surface (upper surface) and a barrier layer existing at the lower portion of the porous layer. Here, the metallic base material may be aluminum (Al), titanium (Ti), tungsten (W), zinc (Zn), etc. It is desired that the metallic base material is made of aluminum or aluminum alloy material that is lightweight, is easy to process, is excellent in thermal conductivity, and obviates concern about heavy metal contamination. 
     For example, when anodizing the surface of aluminum, an alumina film is formed. The alumina film is composed of a porous alumina layer having several pores  102  penetrating the surface (upper surface) in a vertical direction and a barrier layer existing at the lower portion of the porous alumina layer. According to the embodiment of the present invention, the substrate  100  may be, for example, an anodized film obtained by removing aluminum. Accordingly, the porous alumina layer is formed at the upper portion of the substrate  100 , and the barrier layer is formed at the lower portion of the substrate  100 . Alternatively, the substrate  100  may be composed of only the porous alumina layer through which the pore  102  penetrates in a vertical direction by removing the barrier layer of the alumina film. 
     The diameter of the pore  102  is formed in nano-meters. Also, the diameter of the pore  102  is smaller than the minimum lateral width of the heater electrode unit or the sensor electrode unit formed on the substrate  100 . A part or all of the pores  102  may be blocked by the heater electrode unit or the sensor electrode unit. Due to the pores  102  placed at the lower portion of the heater electrode unit, the heat of the heater electrode unit is prevented from being transferred to the lower portion. 
     Hereinafter, as shown in  FIG. 6 , the description is based on a substrate  100  where only aluminum is removed. 
     By removing the aluminum from the anodized aluminum, the upper portion of the pore  102  of the substrate  100  is opened and the lower portion of the pore  102  is blocked by the barrier layer. As described above, the porous alumina layer is formed on the substrate  100 , and thus heat capacity of the micro sensor is reduced. 
     The substrate  100  includes: two first supporting portion  110  formed in cylindrical shapes on both sides of the substrate  100 ; a second supporting unit  120  formed outside of the first supporting portion  110  being spaced apart from the first supporting portion  110 ; and several bridge portions  130  connecting the first supporting portions  110  and the second supporting unit  120 . In the embodiment, two first supporting portions  110  are formed on one substrate  100 , but one or three or more first supporting portions  110  may be formed. The first supporting portions  110  are spaced apart from each other. 
     Also, the substrate  100  is provided with the etching hole  103  penetrating from the first side of the substrate  100  to the second side of the substrate  100 . In the embodiment, the etching hole  103  is formed by penetrating from the upper surface of the substrate  100  to the lower surface of the substrate  100 . The etching hole  103  described below is formed by penetrating both the surface on which the first sensor wire  1310   b,    2310   a  is formed and the surface on which the second sensor wire  1310   a,    2310   b  is formed. 
     The etching hole  103  is formed on the central portion of the first supporting portion  110 . 
     The maximum width of the etching hole  103  is wider than the maximum width of the pore  102 . As described above, the width of the etching hole  103  is formed in micrometers, and thus the etching hole is easily inserted in the sensing material. 
     The etching hole  103  is formed at the circumference of the first sensor electrode  1301 ,  2301  described below. Specifically, the etching hole  103  is formed to surround the first sensor wire  1310   b,    2310   a  of the first sensor electrode  1301 ,  2301 . 
     Accordingly, the etching hole  103  is placed at the side of the first sensor wire  1310   b,    2310   a.    
     The etching hole  103  is formed in the arc shape at an area except for the portion supporting the first sensor wire  1310   b,    2310   a.  Specifically, in the first supporting portion  110 , the etching holes  103  are placed between the central portions supporting the first sensor wires  1310   b  and  2310   a  and portions supporting the first and second heating wires  1210  and  2210 . The portions supporting the first sensor wires  1310   b  and  2310   a  and the portions supporting the first and second heating wires  1210  and  2210  are respectively connected to each other on the first side. 
     The etching hole  103  is formed between the first sensor wire  1310   b,    2310   a  and the second sensor wire  1310   a,    2310   b.  The etching hole  103  is formed to connect the first sensor wire  1310   b,    2310   a  and the second sensor wire  1310   a,    2310   b  via a sensing material described below. 
     A manufacturing method of the micro sensor where the etching hole  103  is formed will be described as follows. 
     A porous alumina layer (Anodic Aluminum Oxide, AAO) is formed on the substrate  100 . Next, the sensor electrode unit and the heater electrode unit are formed on the substrate  100 . Next, etching is performed on the substrate  100  by using etching liquid etching the porous alumina layer (Anodic Aluminum Oxide, AAO) of the substrate  100 , and the etching hole  103  is formed as penetrating in a vertical direction between the first sensor wires  1310   b  and  2310   a  and the first and second heating wires  1210  and  2210 . In this etching step, the sensor electrode unit and the heater electrode unit function as masks, whereby a manufacturing process is simple. 
     Several air gaps are formed near the first supporting portion  110 , namely, between the first supporting portion  110  and the second supporting unit  120 . 
     The air gap is placed at a side of the first supporting portion  110 . 
     The air gap includes a first air gap  101   a  surrounding the first supporting portion  110  placed at the left and a second air gap  101   b  surrounding the first supporting portion  110  placed at the right. 
     Also, several air gaps are formed at the outer circumference of each first supporting portion  110 . Several air gaps are discontinuously formed. Air gaps and bridge portions  130  are alternately placed around the first supporting portion  110 . The bridge portions  130  are formed by forming air gaps through etching the periphery of the first supporting portion  110 . Thus, a first end of each bridge portion  130  is connected to the first supporting portion  110  and a second end thereof is connected to the second supporting unit  120 . 
     Hereinafter, the sensor electrode unit, the heater electrode unit, and an anti-etching dam  500  that are formed on the substrate will be disclosed. 
     The sensor electrode unit is formed on the substrate  100 . 
     The sensor electrode unit detects gas by measuring the changes in the electrical characteristics when the gas is adsorbed onto the sensing material. 
     The sensor electrode unit includes the first sensor electrode unit  1300  and the second sensor electrode unit  2300 . 
     At least one of the first sensor electrode unit  1300  and the second sensor electrode unit  2300  includes a first sensor electrode  1301 ,  2301  and a second sensor electrode  1302 ,  2302  spaced apart from the first sensor electrode  1301 ,  2301 . In the embodiment, the first sensor electrode unit  1300  and the second sensor electrode unit  2300  respectively includes the first sensor electrodes  1301  and  2301  and the second sensor electrodes  1302  and  2302  spaced apart from the first sensor electrodes  1301  and  2301 . 
     The first sensor electrodes  1301  and  2301  are formed on a first side of the substrate  100 , and the second sensor electrodes  1302  and  2302  are formed on a second side of the substrate  100 . In the embodiment, the first sensor electrodes  1301  and  2301  are formed on the upper surface of the substrate  100 , and the second sensor electrodes  1302  and  2302  are formed on the lower surface of the substrate  100 . That is, at the substrate  100 , the second sensor electrode  1302  and  2302  are formed on a surface opposite to a surface on which the first sensor electrodes  1301  and  2301  are formed. 
     The first sensor electrode  1301 ,  2301  includes the first sensor wire  1310   b,    2310   a  and the first sensor electrode pad  1320   b,    2320   a  connected to the first sensor wire  1310   b,    2310   a.    
     The first sensor wires  1310   b  and  2310   a  are respectively formed on the upper surfaces of the first supporting portions  110  at the left and the right. 
     The first sensor wire  1310   b,    2310   a  is formed in a straight line shape. 
     The first sensor electrode pad  1320   b,    2320   a  is formed on the upper surface of the bridge portion  130  and the second supporting unit  120 . 
     The second sensor electrode  1302 ,  2302  includes the second sensor wire  1310   a,    2310   b  and the second sensor electrode pad  1320   a,    2320   b  connected to the second sensor wire  1310   a,    2310   b.    
     The second sensor wires  1310   a  and  2310   b  are respectively formed on the lower surfaces of the first supporting portions  110  at the left and the right. The second sensor wires  1310   a  and  2310   b  are spaced apart from the first sensor wires  1310   b  and  2310   a  in a vertical direction. 
     When viewed in a plan view, an area of the second sensor wire  1310   a,    2310   b  is larger than an area of the first sensor wire  1310   b,    2310   a.    
     The second sensor wire  1310   a,    2310   b  of the second sensor electrode  1302 ,  2302  may block at least a part of the second side (lower portion) of the etching hole  103 . In the embodiment, the second sensor wire  1310   a,    2310   b  is formed in a circular plate shape and blocks the entire lower portion of the etching hole  103 . The diameter of the second sensor wire  1310   a,    2310   b  is larger than the external diameter of the etching hole  103 . 
     The second sensor electrode pad  1320   a,    2320   b  is formed on the lower surface of the bridge portion  130  and the second supporting unit  120 . 
     An assistant pad  1320   c,    2320   c  is formed on the first side, namely, the upper surface of the substrate  100 . The assistant pad  1320   c,    2320   c  is spaced from the first sensor electrode pad  1320   b,    2320   a.    
     The assistant pad  1320   c,    2320   c  is placed above the second sensor electrode pad  1320   a,    2320   b.    
     The assistant pad  1320   c,    2320   c  is connected to the second sensor electrode pad  1320   a,    2320   b.    
     Also, the second supporting unit  120  of the substrate  100  is provided with a pad hole  104  penetrating from the first side of the substrate  100  to the second side of the substrate  100  so as to connect the assistant pad  1320   c,    2320   c  and the second sensor electrode pad  1320   a,    2320   b.  The pad hole  104  is formed as penetrating in a vertical direction. The pad hole  104  is placed between the assistant pad  1320   c,    2320   c  and the second sensor electrode pad  1320   a,    2320   b.    
     The horizontal cross-sectional area of the pad hole  104  is smaller than the horizontal cross-sectional area of the assistant pad  1320   c,    2320   c  and of the second sensor electrode pad  1320   a,    2320   b.  Therefore, the upper and the lower portions of the pad hole  104  are blocked by the assistant pad  1320   c,    2320   c  and the second sensor electrode pad  1320   a,    2320   b.    
     A connecting bar  1320   d  is placed in the pad hole  104 , and thus the assistant pad  1320   c,    2320   c  and the second sensor electrode pad  1320   a,    2320   b  are electrically connected to each other. 
     The widths of the first and second sensor electrode pads  1320   b,    2320   a  and  1320   a,    2320   b  are larger than the widths of the first and second sensor wires  1310   b,    2310   a  and  1310   a,    2310   b.    
     The widths of the first and second sensor electrode pads  1320   b,    2320   a  and  1320   a,    2320   b  are wider towards the end portions. 
     The first sensor electrode unit  1300  and the second sensor electrode unit  2300  are formed of a mixture including one of or at least one of Pt, W, Co, Ni, Au, and Cu. 
     The assistant pad  1320   c  of the first sensor electrode unit  1300  is close to the end of the first sensor electrode pad  2320   a  of the second sensor electrode unit  2300 . In the embodiment, the assistant pad  1320   c  of the first sensor electrode unit  1300  is spaced apart from the first sensor electrode pad  2320   a  of the second sensor electrode unit  2300 . 
     Alternatively, the assistant pad  1320   c  of the first sensor electrode unit  1300  may be connected to the first sensor electrode pad  2320   a  of the second sensor electrode unit  2300 . In this case, when a middle portion between the assistant pad  1320   c  of the first sensor electrode unit  1300  and the first sensor electrode pad  2320   a  of the second sensor electrode unit  2300  is used as a common electrode, the first sensor electrode unit  1300  and the second sensor electrode unit  2300  are connected to each other in parallel. 
     The heater electrode unit is formed on the upper surface of the substrate  100 . 
     When the electrode is formed on the porous alumina layer of the alumina film, the pore  102  placed at the lower portion of the heater electrode unit and the sensor electrode unit has the upper portion and the lower portion thereof blocked by the heater electrode unit and the sensor electrode unit. As described above, the heater electrode unit is formed on the porous alumina layer, and thus the heat capacity of the micro sensor is small. 
     The heater electrode unit includes the first heater electrode unit  1200  and the second heater electrode unit  2200  spaced apart from the first heater electrode unit  1200 . 
     The first heater electrode unit  1200  includes: the first heating wire  1210  closer to the first sensor wire  1310   b  than to the first sensor electrode pad  1320   b;  the first heater electrode unit pad  1220  formed on the second supporting unit  120  and the bridge portion  130  by being connected to the first heating wire  1210 . 
     The first heating wire  1210  is formed on the first supporting portion  110  at the left, and surrounds at least a part of the first sensor wire  1310   b.  The first heater electrode unit pad  1220  includes a first heater electrode unit first pad  1220   a  and a first heater electrode unit second pad  1220   b  that are respectively connected to both ends of the first heating wire  1210 . The first heater electrode unit first pad  1220   a  and the first heater electrode unit second pad  1220   b  are spaced apart from each other. 
     When viewed in a plan view like  FIG. 2 , the first heating wire  1210  is formed to be symmetrical about the vertical center line of the first supporting portion  110 , and includes several arc portions formed in the arc shapes and several connecting portion connecting the arc portions. 
     The outermost side of the first heating wire  1210  is formed close to the edge of the first supporting portion  110 . 
     The first heating wire  1210  includes: a first arc portion  1211   a  formed in an arc shape close to the first air gap  101   a;  a first connecting portion  1212   a  extending from an end of the first arc portion  1211   a  and bent toward the inside of the first supporting portion  110 ; a second arc portion  1211   b  in an arc shape extending from an end of the first connecting portion  1212   a  and spaced apart from the first arc portion  1211   a  inwards; a second connecting portion  1212   b  extending from an end of the second arc portion  1211   b  toward the inside of the first supporting portion  110 ; and a third arc portion  1211   c.  In this manner, several arc portions and connecting portions are formed by being repeatedly connected to each other. 
     The first heating wire  1210  is connected from the first arc portion  1211   a  to the third arc portion  1211   c  to have an integral body, and is symmetrical about the vertical center line of the first supporting portion  110  at the left. 
     As shown in  FIG. 2 , several arc portions of the first heating wire  1210  are formed in about the half-arc shape, and are symmetrically formed. Thus, the first heating wire  1210  is overall in a circular shape. Accordingly, temperature uniformity of the first supporting portion  110  may be enhanced. 
     The central portion of the first heating wire  1210  is a point where the left and right arc portions meet, and the central portion is in a lower side opened circular shape by joining two arc portions in arc shapes together. An isolated space portion  1214  is formed inside of the central portion. The isolated space portion  1214  is formed by extending from the central portion of the first heating wire  1210  to the front of the first heating wire  1210 . That is, to form the isolated space portion  1214  from the central portion to the front of the first heating wire  1210 , the left and right arc portions are spaced apart from each other. The first sensor wire  1310   b  is placed at the isolated space portion  1214 . Accordingly, the first heating wire  1210  surrounds the rear and both sides of the first sensor wire  1310   b.    
     Also, a second end portion of the first arc portion  1211   a  is connected to the first heater electrode unit second pad  1220   b,  and a first end portion of the third arc portion  1211   c  is connected to the first heater electrode unit first pad  1220   a.    
     The first heater electrode unit  1200  is formed of a mixture including one or at least one of Pt, W, Co, Ni, Au, and Cu. 
     In the meantime, an anti-etching dam  500  is formed between both ends of the first heating wire  1210 , namely, the ends of the first arc portion  1211   a  and the third arc portion  1211   c  respectively connected to the first heater electrode unit first pad  1220   a  and the first heater electrode unit second pad  1220   b.    
     The anti-etching dam  500  at the left is formed in the arc shape between the first heating wire  1210  and the first air gap  101   a.  The anti-etching dam  500  is spaced apart from the first heating wire  1210  adjacent thereto. 
     The anti-etching dam  500  is close to the edge of the first supporting portion  110 . 
     It is desired that the anti-etching dam  500  is formed outside of the first heating wire  1210  and is a metal. The material of the anti-etching dam  500  may be the same as the electrode material, and the electrode material may be a metal such as platinum, aluminum, copper, etc. 
     As shown in  FIG. 2 , the lengths of the first arc portion  1211   a  and the third arc portion  1211   c  are short, compared to the remaining arc portions located inside thereof. At the outer circumference of the first heating wire  1210 , a space  510  is defined between the ends of the first arc portion  1211   a  and the third arc portion  1211   c.  The anti-etching dam  500  is placed at the space  510 . The width of the anti-etching dam  500  is formed the same as or similar to the width of the first heating wire  1210 . 
     The space  510  at the outer circumference of the first heating wire  1210  is partially filled by the area of the anti-etching dam  500 . Thus, when viewed in a plan view, the outer circumference of the first heating wire  1210  and the anti-etching dam  500  is in a circular shape, whereby temperature uniformity of the first supporting portion  110  may enhance. 
     Also, by forming the anti-etching dam  500  at the space  510  between the ends of the first arc portion  1211   a  and the third arc portion  1211   c,  the bridge portion  130  may be designed to make the structure of the substrate  100  more stable. Also, it is easy to form the overall shape of the first supporting portion  110  in a circular shape. Accordingly, the position of the bridge portion  130  connecting the first supporting portion  110  and the second supporting unit  120  may be designed considering the stability of the entire structure of the micro sensor. 
     The anti-etching dam  500  prevents a space  510  of the first supporting portion  110  from being damaged by the etching liquid when forming the air gap  101  through etching. That is, the anti-etching dam  500  is formed close to the first heating wire  1210  formed on the first supporting portion  110 , and prevents a particular shape (for example, the circular shape) of the first supporting portion  110  supporting the first heating wire  1210  from being damaged. 
     The widths of the first heater electrode unit first pad  1220   a  and the first heater electrode unit second pad  1220   b  are wider towards the outside. That is, the width of the first heater electrode unit pad  1220  is narrower towards the first heating wire  1210 . The first heater electrode unit pad  1220  has a width wider than that of the first heating wire  1210 . 
     The distance between the first heater electrode unit first pad  1220   a  and the first supporting portion  110  at the right is shorter than the distance between the first heater electrode unit second pad  1220   b  and the first supporting portion  110  at the right. 
     The second heater electrode unit  2200  is similar to the first heater electrode unit  1200 . 
     The second heater electrode unit  2200  includes: the second heating wire  2210  closer to the first sensor wire  2310   a  than to the first sensor electrode pad  2320   a;  and the second heater electrode unit pad  2220  formed on the second supporting unit  120  and the bridge portion  130  be being connected to the second heating wire  2210 . 
     The second heating wire  2210  is formed on the upper surface of the first supporting portion  110  at the right. 
     Thus, the first sensor wire  1310   b  and the first heating wire  1210  are formed on the upper surface of the first supporting portion  110  at the left, the first sensor wire  2310   a  and the second heating wire  2210  are formed on the upper surface of the first supporting portion  110  at the right. 
     Accordingly, the first sensor wire  1310   b  of the first sensor electrode unit  1300  is closer to the first heating wire  1210  than to the second heating wire  2210 . The first sensor wire  2310   a  of the second sensor electrode unit  2300  is closer to the second heating wire  2210  than to the first heating wire  1210 . 
     Also, the first heating wire  1210  and the second heating wire  2210  have different heating values. 
     For different heating values of the first heating wire  1210  and the second heating wire  2210 , as shown in  FIG. 1 , the first heating wire  1210  and the second heating wire  2210  may have different lengths, or the first heating wire  1210  and the second heating wire  2210  may have different thicknesses. 
     In the embodiment, the length of the first heating wire  1210  is longer than the length of the second heating wire  2210 , and thus the first sensing material  400   a  formed on the upper portion of the first supporting portion  110  at the left may be heated is to a higher temperature than that of the second sensing material  400   b  formed on the upper portion of the first supporting portion  110  at the right. Accordingly, the first sensor electrode unit  1300  and the second sensor electrode unit  2300  may detect different kinds of gas. 
     The gaps of the first heating wire  1210  are narrower than the gaps of the second heating wire  2210  such that the first heating wire  1219  is more bent, whereby the first heating wire  1210  and the second heating wire  2210  may have different lengths in a limited space (first supporting portion). 
     Also, different from the embodiment, the both sides of the first heating wire and the second heating wire may be not symmetrical about the vertical center line or the horizontal center line of the first supporting portion  110 . That is, the first heating wire and/or the second heating wire may be formed by connecting two heating wires bent in different shapes in series. 
     The second heating wire  2210  is formed on the first supporting portion  110  at the right, and surrounds at least a part of the first sensor wire  2310   a  of the second sensor electrode unit  2300 . The second heater electrode unit pad  2220  includes a second heater electrode unit first pad  2220   a  and a second heater electrode unit second pad  2220   b  that are respectively connected to both ends of the second heating wire  2210 . The second heater electrode unit first pad  2220   a  and the second heater electrode unit second pad  2220   b  are spaced apart from each other. 
     When viewed in a plan view like  FIG. 3 , the second heating wire  2210  is formed to be symmetrical about the vertical center line of the first supporting portion  110  at the right. 
     The outer most side of the second heating wire  2210  is formed close to the edge of the first supporting portion  110 . 
     The second heating wire  2210  includes a first arc portion  2211   a  and a third arc portion  2211   c  formed in arc shapes and closed to the second air gap  101   b,  and a sensor wire surrounding portion  2212  formed between the first arc portion  2211   a  and the third arc portion  2211   c.    
     The first arc portion  2211   a  is connected to the second heater electrode unit first pad  2220   a,  and the third arc portion  2211   c  is connected to the second heater electrode unit second pad  2220   b.    
     The sensor wire surrounding portion  2212  is connected to the front ends of the first arc portion  2211   a  and of the third arc portion  2211   c,  and is bent to surround the first sensor wire  2310   a.  Accordingly, the sensor wire surrounding portion  2212  is provided with the isolated space portion  2214  having the opened front. 
     An anti-etching dam  500  is formed in an arc shape between both ends of the second heating wire  2210 , namely, the ends of the first arc portion  2211   a  and the third arc portion  2211   c  respectively connected to the second heater electrode unit first pad  2220   a  and the second heater electrode unit second pad  2220   b.    
     The anti-etching dam  500  is placed between the second heater electrode unit first pad  2220   a  and the second heater electrode unit second pad  2220   b.    
     The anti-etching dam  500  formed on the upper surface of the first supporting portion  110  at the right and the anti-etching dam  500  formed on the upper surface of the first supporting portion  110  at the left have the same or similar shapes and effects, and thus the detailed description thereof will be omitted. 
     As described above, the anti-etching dam  500  is formed on the first supporting portion  110  of the substrate  100  between the first air gap  101   a  and the first heating wire  1210 . The anti-etching dam  500  is formed on the first supporting portion  110  of the substrate  100  between the second air gap  101   b  and the second heating wire  2210 . 
     The widths of the second heater electrode unit first pad  2220   a  and the second heater electrode unit second pad  2220   b  are wider towards the outside. That is, the width of the second heater electrode unit pad  2220  is narrower towards the second heating wire  2210 , and the width of the second heater electrode unit pad  2220  is wider than the width of the second heating wire  2210 . 
     The distance between the second heater electrode unit first pad  2220   a  and the is shorter than the distance between the second heater electrode unit second pad  2220   b  and the first supporting portion  110  at the left. 
     The second heater electrode unit first pad  2220   a  is close to the first heater electrode unit first pad  1220   a.  In the embodiment, the second heater electrode unit first pad  2220   a  is spaced apart from the first heater electrode unit first pad  1220   a.    
     As described above, the second heater electrode unit first pad  2220   a  is separated from the first heater electrode unit first pad  1220   a.  The assistant pad  1320   c  of the first sensor electrode unit  1300  is separated from the first sensor electrode pad  2320   a  of the second sensor electrode unit  2300 . Accordingly, the left and right sensors can be individually controlled. Therefore, depending on the situation, only the left sensor is turned on to detect gas, or only the right sensor is turned on to detect gas. 
     Different from the above description, the second heater electrode unit first pad  2220   a  may be connected to the first heater electrode unit first pad  1220   a.  In this case, when a middle portion between the second heater electrode unit first pad  2220   a  and the first heater electrode unit first pad  1220   a  is used as a common electrode, the first heating wire  1210  and the second heating wire  2210  are connected to each other in parallel. Alternatively, when simply connecting the second heater electrode unit first pad  2220   a  and the first heater electrode unit first pad  1220   a  without using the middle portion therebetween as a common electrode and applying electricity to the first heater electrode unit  1200  or to the second heater electrode unit  2200 , the first heating wire  1210  and the second heating wire  2210  are connected to each other in series. As described above, depending on position in the heater electrode unit to which electricity is applied, the first heating wire  1210  and the second heating wire  2210  are connected to each other in parallel or in series. 
     A passivation layer  600  is formed on at least some surfaces (the upper surface and the side surface) of the first heating wire  1210  and the second heating wire  2210  of the heater electrode unit. The passivation layer  600  may be formed of oxide type material. Moreover, the passivation layer  600  may be formed of at least one of tantalum oxide (TaOx), titanium oxide (TiO 2 ), silicon oxide (SiO 2 ), and aluminum oxide (Al 2 O 3 ). 
     The passivation layer  600  is formed on the upper surface of the first supporting portion  110 . The passivation layer  600  is formed to cover the upper portions and the side portions of the first heating wire  1210  and the second heating wire  2210 . The passivation layer  600  surrounds the perimeter of the first and second sensing materials  400   a  and  400   b.  Accordingly, the passivation layer  600  is formed in a ring shape. The passivation layer  600  covers a part of the first sensor wire  1310   b,    2310   a.    
     Also, the first air gap  101   a  surrounds the first heating wire  1210 , and the second air gap  101   b  surrounds the second heating wire  2210 . As described above, at least one air gap is formed between the first heating wire  1210  and the second heating wire  2210 . 
     The first air gap  101   a  and the second air gap  101   b  are formed in the same shape. 
     The widths of the first and second air gaps  101   a  and  101   b  are wider than the maximum width of the pore  102 , and wider than the maximum widths of the first and second heating wires  1210  and  2210 . The first and second air gaps  101   a  and  101   b  is formed corresponding to the shape of the first and second heating wires  1210  and  2210  and the shape of the first supporting portion  110 . The first and second air gaps  101   a  and  101   b  are formed in arc shapes, and are formed in threes. The several first and second air gaps  101   a  and  101   b  are spaced apart from each other in a circumferential direction. That is, several first and second air gaps  101   a  and  101   b  are discontinuously formed. 
     Specifically, the first air gaps  101   a  are placed between the first sensor electrode pad  1320   b  and the first heater electrode unit second pad  1220   b,  between the first heater electrode unit second pad  1220   b  and the first heater electrode unit first pad  1220   a,  and between the first heater electrode unit first pad  1220   a  and the first sensor electrode pad  1320   b.    
     The second air gaps  101   b  are placed between the second heater electrode unit second pad  2220   b  and the second heater electrode unit first pad  2220   a,  between the second heater electrode unit first pad  2220   a  and the first sensor electrode pad  2320   a,  and between the first sensor electrode pad  2320   a  and the second heater electrode unit second pad  2220   b.    
     That is, the first and second air gaps  101   a  and  101   b  are formed at an area except for the portion supporting the first and second heater electrode units  1200  and  2200  and the first and second sensor electrode units  1300  and  2300 . 
     The first and second air gaps  101   a  and  101   b  are formed penetrating in vertical directions. That is, the first and second air gaps  101   a  and  101   b  are spaces formed by penetrating from the upper surface to the lower surface of the substrate  100 . 
     Due to the first and second air gaps  101   a  and  101   b,  on the substrate  100 , formed are the first supporting portion  110  at the left supporting the first heating wire  1210  and the first and second sensor wires  1310   b  and  1310   a  in common; the first supporting portion  110  at the right supporting the second heating wire  2210  and the first and second sensor wires  2310   a  and  2310   b  in common; and the second supporting unit  120  and the bridge portion  130  that support the first and second heater electrode unit pads  1220  and  2220 , the first and second sensor electrode pads  1320   b,    2320   a  and  1320   a,    2320   b,  and the assistant pads  1320   c  and  2320   c.    
     Each first supporting portion  110  is wider than the total area of the heating wire and the sensor wire formed on the first supporting portion  110 . 
     The first supporting portion  110  and the second supporting unit  120  are spaced apart from each other due to the air gaps at the portions other than the bridge portions  130 . Accordingly, the first supporting portion  110  and the second supporting unit  120  are connected to each other at three portions due to three bridge portions  130  as shown in  FIG. 1 . 
     The second supporting unit  120  is placed between the first air gap  101   a  and the second air gap  101   b  that are placed between the first supporting portions  110  at the left and the right. Accordingly, the first supporting portion  110  at the left, the first air gap  101   a,  the second supporting unit  120 , the second air gap  101   b,  and the first supporting portion  110  at the right are arranged from the left to the right direction in order. 
     Alternatively, the air gap may include the first air gap  101   a  and the third air gap coupled to the second air gap  101   b.  The third air gap may be placed between the first heating wire  1210  and the second heating wire  2210 . 
     At the central portions of the first supporting portions  110 , the first and second sensing materials  400   a  and  400   b  are formed. The first and second sensing materials  400   a  and  400   b  are formed at the upper portion and the side portion of the first sensor wire  1310   b,    2310   a,  at the inside of the etching hole  103 , and the upper portion of the second sensor wire  1310   a,    2310   b.  That is, the first and second sensing materials  400   a  and  400   b  are placed between the first sensor wires  1310   b  and  2310   a  and the second sensor wires  1310   a  and  2310   b.    
     As described above, the first and second sensing materials  400   a  and  400   b  are in contact with the surfaces of the first sensor wire  1310   b,    2310   a  and of the second sensor wire  1310   a,    2310   b,  and are placed at a space (gap) between the first sensor wire  1310   b,    2310   a  and the second sensor wire  1310   a,    2310   b.    
     The first and second sensing materials  400   a  and  400   b  are formed to cover the first sensor wire  1310   b,    2310   a  and the second sensor wire  1310   a,    2310   b.    
     The upper surfaces of the first and second sensing materials  400   a  and  400   b  are exposed to the outside. 
     The first sensing material  400   a  and the second sensing material  400   b  may be formed of the same material or different materials. Even with the same sensing material, adsorbed gas may differ depending on the heated temperature. 
     Hereinafter, the operation of the embodiment having the above-described configuration will be disclosed. 
     In order to measure the gas concentration, first, the same electric power is simultaneously applied to the first heater electrode unit pad  1220  and to the second heater electrode unit pad  2220  so as to heat the first heating wire  1210  and the second heating wire  2210 . The first heating wire  1210  is longer than the second heating wire  2210 , and thus the first sensing material  400   a  is heated to a higher temperature than that of the second sensing material  400   b.    
     Different kinds of gas are adsorbed onto or desorbed from the first and second sensing materials  400   a  and  400   b  heated to different temperatures. 
     Accordingly, electrical conductivity between the first sensor wire  1310   b,    2310   a  and the second sensor wire  1310   a,    2310   b  changes, and the changes in the electrical conductivity is measured to detect gas. 
     Through these processes, the micro sensor of the embodiment can simultaneously detect several kinds of gas. 
     As described above, although the exemplary embodiments of the present invention have been disclosed, those skilled in the art will appreciate that various modifications or changes are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.