Abstract:
A micro heater includes a substrate and a heater electrode formed on the substrate. The substrate includes a plurality of pores formed to vertically extend through the substrate. A micro sensor includes a substrate, a sensor electrode formed on the substrate, and a heater electrode formed on the substrate. A protective layer may be used to protect any of the electrodes against oxidation. Any of the electrodes may be formed on a barrier layer positioned on the porous layer.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present invention relates to a micro heater and a micro sensor. More particularly, the present invention pertains to a micro heater and a micro sensor in which multiple pores are formed to vertically extend through a substrate. 
         [0003]    2. Description of Related Art 
         [0004]    As an interest on an environment gradually increases in recent years, a demand has existed for the development of a small-size sensor capable of accurately obtaining different kinds of information within a short period of time. Particularly, for the purpose of making a residential space pleasant, coping with a harmful industrial environment and managing a production process of beverage and foodstuff, efforts have been made to achieve the size reduction, precision enhancement and price reduction of a micro sensor such as a gas sensor for measuring a gas concentration or the like. 
         [0005]    The currently available gas sensor gradually evolves from a ceramic-sintered gas sensor or a thick-film-type gas sensor to a micro gas sensor having the form of a micro electro mechanical system (MEMS) due to the application of a semiconductor process technique. From the viewpoint of a measurement method, a method of measuring a change in the electric characteristics of a sensing material of a sensor when a gas is adsorbed to the sensing material is most frequently used in the currently available gas sensor. Typically, a metal oxide such as SnO 2  or the like is used as the sensing material to measure a change in the electrical conductivity depending on the concentration of a measurement target gas. This measurement method has an advantage in that it is relatively easy to use the method. A change in the measurement value becomes conspicuous when the metal oxide sensing material is heated to and operated at a high temperature. Accordingly, accurate temperature control is essential in order to rapidly and accurately measure a gas concentration. Furthermore, the gas concentration is measured after the sensing material is reset or restored to an initial state by forcibly removing gas species or moisture already adsorbed to the sensing material through high-temperature heating. Thus, the temperature characteristics in the gas sensor directly affect major measurement factors such as the sensor measurement sensitivity, the restoration time, the reaction time and the like. 
         [0006]    Accordingly, a micro heater configured to locally and uniformly heat only the region of a sensing material is effective for efficient heating. However, if a large amount of electric power is consumed in controlling a temperature when measurement is performed by a micro gas sensor, it is necessary to use a large battery or a large power supply source although the volume of a sensor and a measurement circuit remains small. This may decide the overall size of a measurement system. Thus, in order to realize a micro gas sensor, a structure having small power consumption need to be preferentially taken into account. 
         [0007]    Thus far, a silicon substrate having extremely large heat conductivity has been predominantly used in manufacturing most of micro gas sensors. Therefore, in order to reduce a heat loss, an etched pit or groove is formed in a sensor structure through a bulk macro-machining, thereby forming a suspended structure separated from a substrate. Thereafter, a micro heater, an insulation film and a sensing material are sequentially formed on the suspended structure. This makes it possible to partially reduce a heat transfer loss. However, this method is a manufacturing method primarily focused on wet etching that makes use of the crystal directivity of the substrate. Thus, this method has a limit in reducing the size of a sensor element. Furthermore, there is a problem in that the physical property of an etchant such as KOH (potassium hydroxide) or the like used in this method lacks compatibility with a standard CMOS semiconductor process. 
         [0008]      FIG. 1  is a perspective view of a humidity sensor, one of conventional micro sensors. The humidity sensor  10  includes a substrate  11 , a porous anodic aluminum oxide (AAO) layer  13  formed on the substrate  11 , and an electrode  15  formed on the porous anodic aluminum oxide layer  13 . 
         [0009]    The substrate  11  is made of aluminum and is formed in a substantially rectangular plate shape. The porous anodic aluminum oxide layer  13  is formed by oxidizing the substrate  11 . If aluminum is oxidized, it is possible to form the porous anodic aluminum oxide layer  13  having a plurality of holes  13   a  formed on the surface thereof. A barrier layer is formed between the porous anodic aluminum oxide layer  13  and the substrate  11 . 
         [0010]    In this case, the holes  13   a  are formed to have a diameter of 60 nm or less. By forming the holes  13   a  to have a diameter of 60 nm or less, it is possible to prevent the holes  13   a  from being damaged by an etching solution. The electrode  15  is made of metal such as platinum, aluminum, copper or the like. The electrode  15  may be formed by different methods such as a vapor deposition method or the like. 
         [0011]    The electrode  15  includes a first electrode  16  and a second electrode  17  disposed adjacent to the first electrode  16 . The first electrode  16  has a plurality of electrode projections  16   a  protruding toward the second electrode  17 . The second electrode  17  has a plurality of electrode projections  17   a  protruding toward the first electrode  16 . 
         [0012]    However, the micro sensor described above has a problem in that a thermal insulation property is reduced and a heat loss is generated. 
         [0013]    In the meantime, when forming electrode patterns, a liquid photoresist is often used in order to secure a pattern resolution. However, if the liquid photoresist is applied to a conventional micro sensor having a porous layer, there is posed a problem in that the liquid photoresist flows into pores and the patterns are not smoothly formed. Another problem is posed in that the electrode is oxidized or damaged. 
       SUMMARY 
       [0014]    In view of the problems mentioned above, it is an object of the present invention to provide a micro heater and a micro sensor, which are capable of improving a heat insulation property and achieving a high temperature through the use of reduced electric power. 
         [0015]    According to one aspect of the present invention, there is provided a micro heater, including: a substrate; and a heater electrode formed on the substrate, wherein the substrate includes a plurality of pores formed to vertically extend through the substrate. 
         [0016]    The substrate may formed by anodizing an aluminum material and then removing the aluminum material and a barrier layer so that the pores vertically extend through the substrate, and the heater electrode may be formed on a surface of the substrate from which the aluminum material and the barrier layer are removed. 
         [0017]    The substrate may be formed by anodizing an aluminum material and then removing the aluminum material and a barrier layer so that the pores vertically extend through the substrate, and the heater electrode may be formed on a surface of the substrate opposite to a surface from which the aluminum material and the barrier layer are removed. 
         [0018]    The height of an upper surface of the substrate in a region where the heater electrode is formed may be smaller than the height of the upper surface of the substrate in the remaining region. 
         [0019]    According to another aspect of the present invention, there is provided a micro sensor, including: a substrate; a sensor electrode formed on the substrate; and a heater electrode formed on the substrate, wherein the substrate includes a plurality of pores formed to vertically extend through the substrate. 
         [0020]    According to a further aspect of the present invention, there is provided a micro heater, including: a substrate; and a heater electrode formed on the substrate, wherein a protective layer is formed on the heater electrode. 
         [0021]    The heater electrode may be made of at least one member selected from a group consisting of Pt, W, Co, Ni, Au, Cu and C and a mixture thereof, and the protective layer may be made of at least one member selected from a group consisting of tantalum oxide, titanium oxide, silicon oxide, aluminum oxide, indium tin oxide, indium oxide and tin oxide. 
         [0022]    The heater electrode may be made of at least one member selected from a group consisting of Pt, W, Co, Ni, Au, Cu and C and a mixture thereof, an intermediate layer may be disposed between the substrate and the heater electrode, and the intermediate layer may be made of at least one member selected from a group consisting of tantalum oxide, titanium oxide, silicon oxide, aluminum oxide, indium tin oxide, indium oxide and tin oxide. 
         [0023]    The substrate may include a plurality of pores formed to vertically extend through the substrate. 
         [0024]    The protective layer may be formed in a portion of the heater electrode, and a non-protective portion in which the protective layer is not formed may be formed in the heater electrode. 
         [0025]    The heater electrode may include a heater wiring line and heater electrode pads connected to the heater wiring line, and the non-protective portion may be formed on a portion of the heater electrode pads. 
         [0026]    The protective layer may be formed on an upper portion and a side portion of the heater electrode. 
         [0027]    The protective layer may be formed on the substrate. 
         [0028]    According to a still further aspect of the present invention, there is provided a micro sensor, including: a substrate; a sensor electrode formed on the substrate; and a heater electrode formed on the substrate, wherein a protective layer is formed on an upper portion of at least one of the heater electrode and the sensor electrode. 
         [0029]    According to a yet still further aspect of the present invention, there is provided a micro heater, including: a substrate including a porous layer having a plurality of vertically-extending pores and a barrier layer positioned on the porous layer; and a heater electrode formed on the barrier layer. 
         [0030]    The heater electrode may be formed using a liquid photoresist. 
         [0031]    The barrier layer may be partially removed so that the pores of the porous layer vertically extend through the porous layer. 
         [0032]    The heater electrode may be made of platinum, and a tantalum oxide layer may be disposed between the substrate and the heater electrode. 
         [0033]    According to an even yet still further aspect of the present invention, there is provided a micro sensor, including: a substrate including a porous layer having a plurality of vertically-extending pores and a barrier layer positioned on the porous layer; a sensor electrode formed on the substrate; and a heater electrode formed on the substrate, wherein at least one of the sensor electrode and the heater electrode is formed on the barrier layer. 
         [0034]    According to an additional even yet still further aspect of the present invention, there is provided a micro heater, including: a porous substrate having a plurality of pores; and a heater electrode formed on the substrate, wherein the pores disposed under the heater electrode have closed top ends. 
         [0035]    The micro heater and the micro sensor of the present invention mentioned above may provide the following effects. 
         [0036]    Since the pores are formed in the substrate so as to vertically extend through the substrate, it is possible to improve a heat insulation property and to achieve a high temperature through the use of reduced electric power. Furthermore, since the electrodes are stably supported by the porous layer, it is possible to maintain the mechanical durability. Moreover, it is possible to prevent the electrodes from being damaged by the organic substances remaining within the pores in a heat treatment process. In addition, it is possible to optimally adapt the micro heater and the micro sensor to a small device such as a mobile device or the like. 
         [0037]    The substrate is formed of the porous anodic aluminum oxide layer. It is therefore easy to form the porous layer. The heater electrode is formed on the surface of the substrate from which the aluminum material and the barrier layer are removed. In this case, it is possible to form the heater electrode on the surface which is better in planarity than the surface of the porous layer. This makes it possible to enhance the bonding strength of the heater electrode. 
         [0038]    The heater electrode may be formed on the surface of the substrate opposite to the surface from which the aluminum material and the barrier layer are removed. In this case, the aluminum material and the barrier layer can be removed after the heater electrode is formed. Thus, when forming the heater electrode or the sensing material, the aluminum material and the barrier layer serve to support the porous layer, thereby maintaining the porous layer. 
         [0039]    Since the air gaps are formed so as to surround the heater wiring line, the micro heater has a reduced thermal capacity and can be heated to a high temperature through the use of reduced electric power. 
         [0040]    Since the height of the upper surface of the substrate in the region where the heater electrode is formed is smaller than the height of the upper surface of the substrate in the remaining region, it is possible to easily form the heater electrode. Furthermore, the pores are disposed around the heater electrode. An air exists within the pores. Thus, a plurality of air columns is disposed around the heater electrode. Since the air columns serving as heat insulation materials are disposed around the heater electrode in this way, it is possible to prevent the heat of the heater electrode from being transferred to the side surface. This makes it possible to further enhance the heat insulation efficiency of the micro heater. Since the protective layer is formed on the heater electrode, it is possible to prevent the heater electrode from being oxidized. This makes it possible to protect the heater electrode. 
         [0041]    The heater electrode is made of at least one member selected from a group consisting of Pt, W, Co, Ni, Au, Cu and C and a mixture thereof. The protective layer is made of at least one member selected from a group consisting of tantalum oxide, titanium oxide, silicon oxide, aluminum oxide, indium tin oxide, indium oxide and tin oxide. It is therefore possible to effectively protect the heater electrode and to effectively prevent oxidation of the heater electrode. 
         [0042]    The intermediate layer is disposed between the substrate and the heater electrode. This makes it possible to improve the bonding strength of the heater electrode. 
         [0043]    There is provided the non-protective portion in which the protective layer is not formed. The non-protective portion is formed in the region of the sensor wiring lines covered with the sensing material. It is therefore possible for the micro sensor to accurately perform a sensing operation. Furthermore, the non-protective portion is formed in a portion of the heater electrode pads and the sensor electrode pads. It is therefore possible to smoothly perform a soldering work. 
         [0044]    The protective layer is formed in the upper portion and the side portion of the heater electrode. The protective layer is also formed on the substrate. It is therefore possible to effectively protect the heater electrode and the substrate. 
         [0045]    The heater electrode is formed on the barrier layer of the substrate. Thus, when forming the heater electrode, the liquid photoresist is prevented from flowing into the pores. It is therefore possible to smoothly form the patterns of the heater electrode. It is also possible to accurately form the patterns of the heater electrode in a small size. Due to the existence of the porous layer formed in the substrate, it is possible to improve a heat insulation property and to achieve a high temperature through the use of reduced electric power. Furthermore, since the electrodes are stably supported by the porous layer, it is possible to maintain the mechanical durability. Moreover, it is possible to prevent the electrodes from being damaged by the organic substances remaining within the pores in a heat treatment process. In addition, it is possible to optimally adapt the micro heater and the micro sensor to a small device such as a mobile device or the like. When forming the electrodes or the sensing material, the barrier layer plays a role of supporting the porous layer of the substrate. Thus, the porous layer is maintained. 
         [0046]    Since the heater electrode is formed using the liquid photoresist, it is possible to reduce the size of a product and to finely form the electrode patterns. The barrier layer is formed only in a portion of the porous layer. The porous layer includes the pores vertically extending through the porous layer. It is therefore possible to further improve the heat insulation property. The heater electrode is made of platinum. The tantalum oxide layer is disposed between the substrate and the heater electrode. It is therefore possible to increase the bonding strength of the electrode. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0047]      FIG. 1  is a perspective view showing a conventional humidity sensor. 
           [0048]      FIG. 2  is a partially-cutaway perspective view showing a conventional porous aluminum oxide layer. 
           [0049]      FIG. 3  is a plane view of a micro sensor provided with a micro heater according to a first embodiment of the present invention (in which a sensing material is omitted). 
           [0050]      FIG. 4  is a sectional view taken along line A-A in  FIG. 3 . 
           [0051]      FIG. 5  is an enlarged view of the portion indicated by B in  FIG. 4 . 
           [0052]      FIG. 6  is a sectional view of a micro sensor provided with a micro heater according to a second embodiment of the present invention. 
           [0053]      FIG. 7  is an enlarged view of the portion indicated by C in  FIG. 6 . 
           [0054]      FIG. 8  is a sectional view of a micro sensor provided with a micro heater according to a third embodiment of the present invention. 
           [0055]      FIG. 9  is a sectional view of a micro sensor provided with a micro heater according to a fourth embodiment of the present invention. 
           [0056]      FIG. 10  is an enlarged view of the portion indicated by D in  FIG. 9 . 
       
    
    
     DETAILED DESCRIPTION 
       [0057]    Preferred embodiments of the present invention will now 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. 
       First Embodiment of the Present Invention 
       [0058]    As illustrated in  FIGS. 3 to 5 , a method for manufacturing a micro sensor provided with a micro heater according to a first embodiment includes a step of forming a substrate  100 , and a step of forming a heater electrode  200  and a sensor electrode  300  on the substrate  100 . 
         [0059]    The substrate  100  as a porous layer having a plurality of vertically-extending pores  102  is formed. For the sake of convenience in description, the diameter and length of the pores  102  are slightly exaggerated in the drawings. 
         [0060]    At the step of forming the substrate  100  as a porous layer, a porous anodic aluminum oxide (AAO) layer is formed by anodizing an aluminum material. If the surface (upper surface) of the aluminum material is anodized, pores  102  having open top ends and closed bottom ends are formed. Thus, the aluminum substrate includes pores  102 , a barrier layer disposed under the pores  102 , and an aluminum material disposed under the barrier layer. 
         [0061]    Then, air gaps  101  are formed in the aluminum substrate by etching the aluminum substrate. The regions of the upper surface of the aluminum substrate, in which the heater electrode  200  and the sensor electrode  300  are to be formed, are etched so that the height of the upper surface becomes lower in the etched regions than the remaining regions. The heater electrode  200  and the sensor electrode  300  are formed in the etched regions of the aluminum substrate having a reduced height. The heater electrode  200  and the sensor electrode  300  may be formed by sputtering or the like. The heater electrode  200  and the sensor electrode  300  are formed on the opposite surface of the aluminum substrate from the aluminum material and the barrier layer. 
         [0062]    If the porous layer is supported by the aluminum material and the barrier layer when forming the heater electrode  200  and the sensor electrode  300  in this way, it is possible to maintain the porous layer even if the heater electrode  200  and the sensor electrode  300  are formed by sputtering or the like. 
         [0063]    The heater electrode  200  includes a heater wiring line  210  and heater electrode pads  220  connected to the heater wiring line  210 . The sensor electrode  300  includes sensor wiring lines  310  and sensor electrode pads  320  connected to the sensor wiring lines  310 . The heater wiring line  210  and the sensor wiring lines  310  are disposed in the central region of the aluminum substrate. The heater electrode pads  220  and the sensor electrode pads  320  are disposed outside the heater wiring line  210  and the sensor wiring lines  310 . 
         [0064]    Unlike the foregoing descriptions, the air gaps  101  may be formed after forming the heater electrode  200  and the sensor electrode  300 . 
         [0065]    The method for manufacturing the micro sensor provided with the micro heater according to the first embodiment further includes a step of forming a sensing material  400  on the upper surface of the aluminum substrate so as to cover the heater wiring line  210  and the sensor wiring lines  310 . 
         [0066]    The aluminum material existing under the aluminum substrate is removed and then the barrier layer is removed. Thus, the substrate  100  includes only the porous layer. 
         [0067]    If the aluminum material and the barrier are removed from the aluminum substrate in this way, the pores  102  are opened at the bottom ends thereof. As a result, the pores  102  having open top and bottom ends are formed in the substrate  100  so as to extend vertically. 
         [0068]    As described above, the heater electrode  200  and the sensor electrode  300  are formed on the surface of the aluminum substrate opposite to the surface from which the aluminum material and the barrier layer are removed. 
         [0069]    Unlike the foregoing descriptions, the heater electrode  200  and the sensor electrode  300  may be formed on the surface of the aluminum substrate from which the aluminum material and the barrier layer are removed. In this case, it is possible to form the heater electrode  200  and the sensor electrode  300  on the surface which is better in planarity than the surface of the porous layer. This makes it possible to enhance the bonding strength of the heater electrode  200  and the sensor electrode  300 . 
         [0070]    In this way, the heater electrode  200  and the sensor electrode  300  are formed on the substrate  100  in which the pores  102  having open top and bottom ends are formed. Thus, a vertical air layer is formed below and around the heater electrode  200  and the sensor electrode  300  due to the air existing within the pores  102 . Consequently, the micro sensor according to the present embodiment can enjoy an improved heat insulation property and can achieve a high temperature through the use of reduced electric power. 
         [0071]    The micro sensor manufactured by the aforementioned micro sensor manufacturing method includes a substrate  100 , a heater electrode  200  formed on the substrate  100 , and a sensor electrode  300  formed on the substrate  100 . A plurality of vertically-extending pores  102  is formed in the substrate  100 . 
         [0072]    The substrate  100  is made of aluminum and is formed in a substantially rectangular plate shape. The substrate  100  is formed as a porous layer. That is to say, the substrate  100  is made of a porous material. Thus, a plurality of pores  102  having open top and bottom ends is formed in the substrate  100  so as to vertically extend through the substrate  100 . 
         [0073]    The substrate  100  may be formed by anodizing an aluminum plate. Thus, the porous substrate mentioned above is a porous anodic aluminum oxide (AAO) layer. Since the aluminum material and the barrier layer are removed from the aluminum oxide substrate  100 , the pores  102  of the substrate  100  vertically extend through the substrate  100 . 
         [0074]    The sensor electrode  300  is formed on the upper surface of the substrate  100 . The sensor electrode  300  is configured to sense a gas. Unlike the foregoing descriptions, the sensor electrode  300  may be configured to sense humidity or the like. The sensor electrode  300  includes a first sensor electrode  300   a  and a second sensor electrode  300   b  disposed in a spaced-apart relationship with the first sensor electrode  300   a.  The first sensor electrode  300   a  and the second sensor electrode  300   b  are spaced apart from each other in a left-right direction and are symmetrical with respect to a centerline vertically extending on a plane. 
         [0075]    The first sensor electrode  300   a  includes a first sensor wiring line and a first sensor electrode pad connected to the first sensor wiring line. The second sensor electrode  300   b  includes a second sensor wiring line and a second sensor electrode pad connected to the second sensor wiring line. 
         [0076]    The sensor wiring lines  310  include the first sensor wiring line and the second sensor wiring line. The sensor electrode pads  320  include the first sensor electrode pad and the second sensor electrode pad. 
         [0077]    The sensor wiring lines  310  are disposed in the central region of the substrate  100 . The sensor wiring lines  310  are formed in a linear shape with a uniform width. The sensor electrode pads  320  are formed so as to have a width larger than the width of the sensor wiring lines  310 . Furthermore, the sensor electrode pads  320  are larger in area than the sensor wiring lines  310  when viewed in a plane view. 
         [0078]    The sensor electrode pads  320  of the first and second sensor electrodes  300   a  and  300   b  are respectively disposed at two adjoining corners of the substrate  100  having a rectangular shape and are formed so that the width thereof grows larger toward the free ends thereof. In other words, the sensor electrode pads  320  are formed so that the width thereof grows smaller toward the sensor wiring lines  310 . 
         [0079]    The heater electrode  200  is formed on the upper surface of the substrate  100 . Thus, the upper ends of the pores  102  disposed under the heater electrode  200  and the sensor electrode  300  are closed by the heater electrode  200  and the sensor electrode  300 . The lower ends of the pores  102  disposed under the heater electrode  200  and the sensor electrode  300  are opened. The upper end and the lower end of at least one of the pores  102  disposed in the region where the heater electrode  200  and the sensor electrode  300  are not formed (the region existing around the heater electrode  200  and the sensor electrode  300 ) are opened. 
         [0080]    In this way, the heater electrode  200  is formed on the porous layer. Due to the existence of the pores  102 , a heat insulation effect is enhanced. 
         [0081]    Furthermore, the heater electrode  200  and the sensor electrode  300  are formed in the regions of the substrate  100  where the height of the upper surface is smaller than the height of the upper surface of the remaining region. In other words, the heater electrode  200  and the sensor electrode  300  are formed in the depressed regions of the substrate  100 . Thus, the thickness of the substrate  100  in the regions where the heater electrode  200  and the sensor electrode  300  are formed is smaller than the thickness of the substrate  100  in the remaining region. 
         [0082]    The heater electrode  200  includes a heater wiring line  210  disposed closer to the sensor wiring lines  310  than the sensor electrode pads  320 , and heater electrode pads  220  connected to the heater wiring line  210 . 
         [0083]    The heater wiring line  210  is disposed in the central region of the substrate  100 . The heater wiring line  210  includes a first curved portion  211 , a second curved portion  213  spaced apart from the first curved portion  211 , and a curved connection portion  212  configured to interconnect the first curved portion  211  and the second curved portion  213 . When viewed in a plane view, the first curved portion  211  and the second curved portion  213  are curvilinearly formed in an inverted U-like shape. When viewed in a plane view, the curved connection portion  212  is curvilinearly formed in a U-like shape. Thus, an isolated space portion  214  is defined between the first curved portion  211  and the second curved portion  213 . The sensor wiring lines  310  are disposed in the isolated space portion  214 . In other words, the heater wiring line  210  is formed so as to surround at least some portions of the first sensor electrode  300   a  and the second sensor electrode  300   b.  This makes it possible to effectively heat a sensing material  400  which will be described later. 
         [0084]    The heater electrode pads  220  include first and second heater electrode pads  220   a  and  220   b  respectively connected to the opposite ends of the heater wiring line  210 . In this way, there are formed at least two heater electrode pads  220 . 
         [0085]    The heater electrode pads  220  are disposed at two corners of the substrate  100  other than the corners at which the sensor electrode pads  320  are disposed. The heater electrode pads  220  are formed so that the width thereof grows larger outward. In other words, the heater electrode pads  220  are formed so that the width thereof grows smaller toward the heater wiring line  210 . When viewed in a plane view, the heater electrode pads  220  are larger in area than the heater wiring line  210 . 
         [0086]    A discoloration-preventing protection layer (not shown) is formed over the entirety of the heater electrode  200  and the sensor electrode  300 . The discoloration-preventing protection layer may be made of an oxide-based material. Alternatively, the discoloration-preventing protection layer may be made of silicon dioxide or aluminum oxide. 
         [0087]    Furthermore, soldering metals are disposed in the end portions of the heater electrode pads  220  and the sensor electrode pads  320 . The soldering metals are disposed on the discoloration-preventing protection layer. The soldering metals may be at least one of gold, silver and tin. 
         [0088]    Air gaps  101  are formed in the substrate  100  so as to surround the heater wiring line  210  and the sensor wiring lines  310 . The air gaps  101  are disposed around the heater wiring line  210  and the sensor wiring lines  310 . The maximum width (left-right width) of the air gaps  101  is set larger than the maximum width of the pores  102 . In the present embodiment, three air gaps  101  are formed in an arc shape. The air gaps  101  are spaced apart along the circumferential direction. In other words, the air gaps  101  are discontinuously formed in a plural number. 
         [0089]    More specifically, the air gaps  101  are disposed between the sensor electrode pad  320  of the first sensor electrode  300   a  and the first heater electrode pad  220   a , between the first heater electrode pad  220   a  and the second heater electrode pad  220   b,  and between the second heater electrode pad  220   b  and the sensor electrode pads  320  of the second sensor electrode  300   b.  That is to say, the air gaps  101  are formed in the regions other than the regions which support the heater electrode  200  and the sensor electrode  300 . 
         [0090]    The air gaps  101  vertically extend through the substrate  100 . In other words, the air gaps  101  are formed to extend from the upper surface of the substrate  100  to the lower surface thereof. Unlike the foregoing descriptions, the air gaps  101  may be formed in a groove shape. 
         [0091]    Due to the existence of the air gaps  101 , a first support portion  110  which supports both the heater wiring line  210  and the sensor wiring lines  310  and a second support portion  120  which supports the heater electrode pads  220  and the sensor electrode pads  320  are formed in the substrate  100 . That is to say, the air gaps  101  are formed between the first support portion  110  and the second support portion  120 . A heat generation peak temperature becomes higher as the width of the air gaps  101  grows wider. 
         [0092]    The first support portion  110  is formed in a circular shape so as to support the heater wiring line  210  and the sensor wiring lines  310 . The first support portion  110  and the second support portion  120  are connected to each other in the regions where the wiring lines and the pads are connected to each other. In the remaining regions, the first support portion  110  and the second support portion  120  are spaced apart from each other. Accordingly, the first support portion  110  and the second support portion  120  are connected to each other at three points. 
         [0093]    The first support portion  110  is formed in a circular shape and is surrounded by the air gaps  101 . The first support portion  110  is larger in area than the heater wiring line  210  and the sensor wiring lines  310 . The air gaps  101  are formed so as to surround the first support portion  110 . Due to the existence of an air exists in the air gaps  101 , the heat insulation effect is enhanced, the heat conductivity is reduced, and the thermal capacity is made small. 
         [0094]    In addition, a sensing material  400  which covers the heater wiring line  210  and the sensor wiring lines  310  is formed on the first support portion  110 . That is to say, the sensing material  400  is formed in the position corresponding to the first support portion  110 . The sensing material  400  is formed by printing. If the sensing material  400  is formed by printing in this way, a mesh-like mark is left on the surface of the sensing material  400  after the sensing material  400  is formed. 
         [0095]    Next, descriptions will be made on the operation of the micro sensor provided with the micro heater according to the first embodiment of the present invention. 
         [0096]    In order to measure a gas concentration, electric power is first applied to the two heater electrode pads  220  of the heater electrode  200 , thereby heating the sensing material  400  formed in the central portion of the micro sensor to a predetermined temperature. 
         [0097]    In this state, a change in the characteristics of the sensing material  400  generated when a gas existing around the micro sensor is adsorbed to or desorbed from the sensing material  400  is measured by measuring a potential difference between the sensor electrode pads  320  electrically connected to the sensing material  400  via an external circuit and then quantifying the electrical conductivity of the sensing material  400 . 
         [0098]    In an effort to accurately perform the measurement, gas species or moisture already adsorbed to the sensing material  400  is forcibly removed by heating the same to a high temperature with the heater electrode  200 . After the sensing material  400  is restored to an initial state in this way, the concentration of a gas of interest is measured. 
       Second Embodiment of the Present Invention 
       [0099]    Next, descriptions will be made on a micro sensor provided with a micro heater according to a second embodiment of the present invention. In the following descriptions, the components identical with or similar to those of the micro sensor and the micro heater of the first embodiment will be designated by like reference numerals with the detailed description and illustration thereof omitted. 
         [0100]    As illustrated in  FIGS. 6 and 7 , the micro sensor provided with the micro heater according to the second embodiment includes a substrate  100 , a sensor electrode formed on the substrate  100 , and a heater electrode formed on the substrate  100 . A protective layer  600  is formed on at least one of the heater electrode and the sensor electrode 
         [0101]    The sensor electrode includes a first sensor electrode, and a second sensor electrode spaced apart from the first sensor electrode. Each of the first and second sensor electrodes includes sensor wiring lines  310  and sensor electrode pads connected to the sensor wiring lines  310 . The sensor electrode is formed on the upper surface of the substrate  100  from which the aluminum material and the barrier layer are removed. The sensor electrode may be made of one of Pt, W, Co, Ni, Au, Cu and C or a mixture thereof. 
         [0102]    The heater electrode includes a heater wiring line  210  disposed closer to the sensor wiring lines  310  than the sensor electrode pads, and heater electrode pads  220  connected to the heater wiring line  210 . 
         [0103]    The heater electrode is formed on the upper surface of the substrate  100  from which the aluminum material and the barrier layer are removed. Unlike the foregoing descriptions, the heater electrode and the sensor electrode may be formed on the surface of the substrate  100  opposite to the surface from which the aluminum material and the barrier layer are removed. The heater electrode may be made of one of Pt, W, Co, Ni, Au, Cu and C or a mixture thereof. 
         [0104]    A protective layer  600  is formed over the entirety of the heater electrode and the sensor electrode. The protective layer  600  may be made of at least one of tantalum oxide (TaOx), titanium oxide (TiO 2 ), silicon oxide (SiO 2 ), aluminum oxide (Al 2 O 3 ), indium tin oxide (ITO), indium oxide (In 2 O 3 ) and tin oxide (SnO 2 ). The protective layer  600  prevents the heater electrode and the sensor electrode from being oxidized. Thus, the heater electrode and the sensor electrode are protected by the protective layer  600 . 
         [0105]    Furthermore, an intermediate layer  700  is disposed between the upper surface of the substrate  100  and the lower surfaces of the heater electrode and the sensor electrode. The bonding strength of the heater electrode and the sensor electrode is increased by the intermediate layer  700 . The intermediate layer  700  may be made of at least one of tantalum oxide (TaOx), titanium oxide (TiO 2 ), silicon oxide (SiO 2 ) , aluminum oxide (Al 2 O 3 ), indium tin oxide (ITO), indium oxide (In 2 O 3 ) and tin oxide (SnO 2 ). 
         [0106]    Air gaps  101  are formed in the substrate  100  so as to surround the heater wiring line  210  and the sensor wiring lines  310 . Due to the existence of the air gaps  101 , a first support portion  110  which supports both the heater wiring line  210  and the sensor wiring lines  310  and a second support portion  120  which supports the heater electrode pads  220  and the sensor electrode pads are formed in the substrate  100 . A sensing material  400  is formed so as to cover the upper surfaces and the side surfaces of the heater wiring line  210  and the sensor wiring lines  310  and so as to cover the upper surface of the first support portion  110 . 
         [0107]    A method of measuring a gas concentration through the use of the micro sensor according to the second embodiment is the same as the method of the first embodiment described above. Thus, the detail descriptions thereof will be omitted. 
       Third Embodiment of the Present Invention 
       [0108]    Next, descriptions will be made on a micro sensor provided with a micro heater according to a third embodiment of the present invention. In the following descriptions, the components identical with or similar to those of the micro sensor and the micro heater of the first and second embodiments will be designated by like reference numerals with the detailed description and illustration thereof omitted. 
         [0109]    As illustrated in  FIG. 8 , the micro sensor provided with the micro heater according to the third embodiment includes a protective layer  600 ′ formed only in a portion of the heater electrode or the sensor electrode. Thus, a non-protective portion  221  not covered with the protective layer is formed in the heater electrode or the sensor electrode. 
         [0110]    The protective layer  600 ′ is not formed in the regions of the sensor wiring lines  310  of the sensor electrode covered with the sensing material  400 . Thus, the upper surfaces and the side surfaces of the sensor wiring lines  310  make direct contact with the sensing material  400 . This enables a signal to be transmitted smoothly. 
         [0111]    Furthermore, the protective layer  600 ′ is not formed in some portions of the heater electrode pads  220  of the heater electrode and the sensor electrode pads of the sensor electrode. Thus, the protective layer  600 ′ is formed in the heater wiring line  210 , a portion of the heater electrode pads  220 , a connection portion of the heater electrode pads  220  and the heater wiring line  210 , a portion of the sensor electrode pads, and a connection portion of the sensor electrode pads and the sensor wiring lines  310 . In other words, the non-protective portion  221  is formed in the region of the sensor electrode covered with the sensing material  400  and in a portion of the sensor electrode pads and the heater electrode pads  220 . 
         [0112]    More specifically, the protective layer  600 ′ is not formed in the regions of the sensor electrode pads and the heater electrode pads  220 , which adjoin the respective corners of the substrate  100 . This makes it possible to smoothly perform soldering in the exposed regions. 
         [0113]    Furthermore, the protective layer  600 ′ is formed so as to cover the upper portions and the side portions of the heater electrode and the sensor electrode. The protective layer  600 ′ is also formed on the substrate  100 . 
         [0114]    The protective layer  600 ′ is formed in the first support portion  110  of the substrate  100  covered with the sensing material  400 . However, the protective layer  600 ′ may not be formed on the substrate  100  around the sensor wiring lines  310 . 
         [0115]    The protective layer  600 ′ is formed in the upper portion of the second support portion  120  of the substrate  100  which supports the sensor electrode pads and the heater electrode pads  220 . In the region of the substrate  100  where the protective layer  600 ′ is formed, the top ends of the pores are closed by the protective layer  600 ′. The protective layer  600 ′ is not formed in the regions of the substrate  100  where the air gaps  101  are formed. 
       Fourth Embodiment of the Present Invention 
       [0116]    Next, descriptions will be made on a micro sensor provided with a micro heater according to a fourth embodiment of the present invention. In the following descriptions, the components identical with or similar to those of the micro sensor and the micro heater of the first, second and third embodiments will be designated by like reference numerals with the detailed description and illustration thereof omitted. 
         [0117]    As illustrated in  FIGS. 9 and 10 , the micro sensor provided with the micro heater according to the fourth embodiment includes a substrate  100 ′ formed of a porous layer  103  having a plurality of vertically-extending pores  102  and a barrier layer  104  positioned on the porous layer  103 , a sensor electrode formed on the substrate  100 ′, and a heater electrode formed on the substrate  100 ′. At least one of the sensor electrode and the heater electrode is formed on the barrier layer  104 . 
         [0118]    The substrate  100 ′ includes a porous layer  103  and a barrier layer  104  formed on the porous layer  103 . That is to say, the substrate  100 ′ is formed of a porous material having a plurality of vertically-extending pores  102 . For the sake of convenience in description, the diameter and length of the pores  102  are slightly exaggerated in the drawings. 
         [0119]    The substrate  100 ′ may be formed by anodizing an aluminum plate. Thus, the porous layer is a porous anodic aluminum oxide (AAO) layer. Aluminum is removed from the anodized aluminum substrate  100 ′ so that only the porous layer  103  and the barrier layer  104  are left in the substrate  100 ′. The top ends of the pores  102  are closed by the barrier layer and the bottom ends of the pores  102  are opened. This enhances the heat insulation effect. 
         [0120]    Unlike the foregoing descriptions, the barrier layer may be partially removed so that the pores of the porous layer can vertically extend through the substrate  100 ′. In other words, the barrier layer may be formed only in a portion of the porous layer. The porous layer may include pores vertically extending through the substrate  100 ′. In other words, only the porous layer may be left by removing the aluminum material and the barrier layer from the regions of the substrate other than the heater electrode and the sensor electrode. 
         [0121]    The sensor electrode includes a first sensor electrode and a second sensor electrode spaced apart from the first sensor electrode. Each of the first and second sensor electrodes includes sensor wiring lines  310  and sensor electrode pads connected to the sensor wiring lines  310 . The sensor electrode is formed on the upper surface of the barrier layer  104  of the substrate  100 ′ from which the aluminum material is removed. The sensor electrode may be made of platinum (Pt). 
         [0122]    The heater electrode includes a heater wiring line  210  disposed closer to the sensor wiring lines  310  than the sensor electrode pads, and heater electrode pads  220  connected to the heater wiring line  210 . The heater electrode is formed on the upper surface of the barrier layer  104  of the substrate  100 ′ from which the aluminum material is removed. 
         [0123]    The sensor electrode and the heater electrode disposed on the barrier layer  104  are formed using a liquid photoresist (LPR). Due to the existence of the barrier layer  104 , the liquid photoresist does not flow into the pores  102 . Thus, the patterns of the sensor electrode and the heater electrode can be smoothly formed. It is also possible to accurately form the patterns of the sensor electrode and the heater electrode in a small size. The heater electrode and the sensor electrode may be formed by sputtering or the like. 
         [0124]    If the porous layer  103  is supported by the barrier layer  104  when forming the heater electrode and the sensor electrode in this way, the porous layer  103  can be maintained even if the heater electrode and the sensor electrode are formed by sputtering or the like. 
         [0125]    For example, the micro sensor may be manufactured by the following method. An aluminum substrate is subjected to an anodizing process. Thereafter, the aluminum is etched away. Then, a liquid photo resist is coated. Subsequently, a heater electrode and a sensor electrode are formed by sputtering. After the liquid photoresist is removed, air gaps  101  are formed by etching the substrate. The heater electrode is made of platinum (Pt). 
         [0126]    The heater electrode includes a heater wiring line  210  disposed closer to the sensor wiring lines  310  than the sensor electrode pads, and heater electrode pads  220  connected to the heater wiring line  210 . 
         [0127]    A protective layer (not shown) made of tantalum oxide (TaOx) may be formed over the entire upper portions of the heater electrode and the sensor electrode. The protective layer prevents the heater electrode and the sensor electrode from being oxidized. Thus, the heater electrode and the sensor electrode can be protected by the protective layer. 
         [0128]    Furthermore, a tantalum oxide layer  700  is disposed between the barrier layer  104  of the substrate  100 ′ and the heater electrode and the sensor electrode. The tantalum oxide layer  700  enhances the bonding strength of the heater electrode and the sensor electrode. 
         [0129]    Due to the existence of the air gaps  101 , a first support portion  110  which supports both the heater wiring line  210  and the sensor wiring lines  310  and a second support portion  120  which supports the heater electrode pads  220  and the sensor electrode pads are formed in the substrate  100 ′. A sensing material  400  is formed so as to cover the upper surfaces and the side surfaces of the heater wiring line  210  and the sensor wiring lines  310  and so as to cover the upper surface of the first support portion  110 . 
         [0130]    While preferred embodiments of the present invention have been described above, a person skilled in the relevant technical field will be able to differently change or modify the present invention without departing from the spirit and scope of the present invention defined in the claims.