Abstract:
Provided is a humidity sensor whose electrode is set to have an island-like configuration and is enclosed by a humidity sensitive film. As a result, in the humidity sensor, adhesion property between the humidity sensitive films on the electrode is enhanced and thus high reliability is achieved.

Description:
[0001]     This application claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. JP 2006-009908 filed Jan. 18, 2006, the entire content of which is hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a semiconductor humidity sensor.  
         [0004]     2. Description of the Related Art  
         [0005]     As a conventional capacitive humidity sensor, a circuit as shown in  FIG. 5  has been known (see, for example, FIG. 1 of JP 2003-156464 A).  
         [0006]     In  FIG. 5 , comb-shaped electrodes are formed on a substrate. The electrodes are coated thereon with a humidity sensitive film whose dielectric constant varies depending on humidity. The humidity sensitive films are inserted between the comb-shaped electrodes. An approximate capacitance C between the electrodes  21  and  22  can be obtained by the following equation (1): 
 
 C=ε   0 ·ε H   ·S/d   H   (1) 
 
         [0007]     where d H  is an interval between electrodes  21  and  22 , S is an area where the electrodes  21  and  22  are opposing each other, ε H  is a dielectric constant of the humidity sensitive film between the electrodes  21  and  22 , and ε 0  is the permittivity of vacuum.  
         [0008]     Change in the dielectric constant ε H  of the humidity sensitive film between the electrodes on humidity causes change in the capacitance C between the electrodes  21  and  22 , thus measurement of the capacitance C enables the measurement of humidity.  
         [0009]     In addition, as a circuit similar to that of  FIG. 5 , a resistance type humidity sensor has been known (see, for example, FIG. 1 of JP 08-145932 A). In the resistance type humidity sensor, a humidity sensitive film whose resistivity varies depending on humidity is coated onto the electrodes  21  and  22  (between the electrodes  21  and  22 ), in place of the above-mentioned humidity sensitive film whose dielectric constant varies depending on the humidity. In this case, a resistance between the electrodes  21  and  22  varies depending on humidity, thus measurement of the resistance enables the measurement of humidity.  
         [0010]     Poor adhesion property between the humidity sensitive films on the electrode and between the electrodes and the substrate in the conventional humidity sensor raises a problem of easy peeling off of the humidity sensitive film. In addition, there is a problem in that a humidity distribution in a minute area cannot be determined.  
       SUMMARY OF THE INVENTION  
       [0011]     In order to solve the above-mentioned problems, the present invention has been made, and therefore, it is an object of the present invention to provide a humidity sensor in which adhesion property between humidity sensitive films on an electrode and between the electrodes and a substrate are enhanced, for exactly determining the humidity distribution in a small region.  
         [0012]     A humidity sensor according to the present invention adopts an electrode structure in which a plurality of island-like unit cells is arranged in a grid pattern.  
         [0013]     The humidity sensor according to the present invention exhibits enhanced adhesion property between the humidity sensitive films and has an enhanced reliability effectively 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     In the accompanying drawings:  
         [0015]      FIG. 1  shows electrodes of a humidity sensor according to a first embodiment of the present invention;  
         [0016]      FIGS. 2A and 2B  each show an electrode unit cell according to the first embodiment of the present invention;  
         [0017]      FIGS. 3A and 3B  each show an electrode unit cell according to another embodiment of the present invention;  
         [0018]      FIGS. 4A and 4B  each are a sectional view of the electrode after a thick nitride film is etched; and  
         [0019]      FIG. 5  shows electrodes of a conventional humidity sensor. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]     In order to solve the above-mentioned problems, the present invention provides a novel electrode structure of a humidity sensor. In the following embodiments, the structure is described in detail.  
       First Embodiment  
       [0021]     Description is given below on an embodiment of the present invention with reference to the drawings.  FIG. 1  shows an electrode structure of a humidity sensor according to a first embodiment of the present invention. In this structure a plurality of island-like separate electrodes  11  is respectively enclosed by an outer electrode. In  FIG. 1 , basic electrode unit cells are arranged in a 5×5 grid pattern.  
         [0022]      FIG. 2A  shows a top view of the unit cell for electrode structure and  FIG. 2B  shows a sectional view thereof (taken along the line A-B of  FIG. 2A ). In  FIG. 2A , a first electrode  11  and a second electrode  12  formed to enclose the electrode  11  are illustrated. A humidity sensitive film exists between the first electrode  11  and the second electrode  12 . Measurement of a capacitance or a resistance between the electrodes  11  and  12  enables the measurement of the humidity. In the sectional view of  FIG. 2B , a substrate  16  (e.g., semiconductor), an insulating film  14 A, a first metal wiring layer  10 , an insulating film  14 B, the electrodes  11  and  12  formed of a second metal wiring layer, a protective film  13  (e.g., nitride film) for protecting the electrodes, and a humidity sensitive film  15  are disposed in the stated order from the bottom. The protective film  13  is formed on surfaces of the electrodes  11  and  12 . The first electrode  11  formed of the second metal wiring layer is connected to the first metal wiring layer via a through-hole formed in the insulating film  14 B.  
         [0023]     If connection of all the electrodes  11  of the unit cells to the same first metal wiring layer is made, measurement of the capacitance or the resistance between the first metal wiring layer and the electrode  12  formed of the second metal wiring layer enables the measurement of the humidity.  
         [0024]     In the humidity sensor according to the present invention adoption of the electrode structure shown in  FIG. 1  results in the significantly larger adhesion property between the humidity sensitive film and the electrodes than that of the conventional humidity sensor. In the conventional structure shown in  FIG. 5 , formation of comb-shaped electrodes in a lateral direction caused easy peeling-off of the humidity sensitive film along that direction. On the other hand, in the present invention, as shown in  FIGS. 2A and 2B , formation of the unit from the island-like electrode  11  and the electrode  12  formed to enclose the electrode  11  enables the incorporation of the humidity sensitive film  15  to enclose the island-like electrode  11 , leading to the humidity sensor having high reliability, which can avoid poor adhesion property in a certain direction such as the tooth direction in the comb-shaped electrode structure.  
         [0025]     Further,  FIGS. 2A and 2B  show the square electrode  11  and the square electrode  12  enclosing the electrode  11  as a configuration of the unit cell. However, as shown in  FIGS. 3A and 3B , a polygonal electrode  31  having projections and depressions and a polygonal electrode  32  having projections and depressions and enclosing the electrode  31  can enhance adhesion properties between the electrodes and the humidity sensitive film as in the case of  FIGS. 2A and 2B .  
       Second Embodiment  
       [0026]     In the first embodiment, the electrode  11  of the unit cell is connected to the same first metal wiring layer as an example. As shown in  FIG. 1 , when the plurality of unit cells is arranged, measurement of a capacitance or a resistance for each unit cell enables the measurement of the humidity of a local small region where the unit cell is included. Thus, a humidity distribution of the humidity sensor can be measured.  
         [0027]     Using photolithography in a semiconductor process, the size (area) of the unit cell can be reduced to about several μm by several μm to several tens of μm by several tens of μm. On the other hand, a semiconductor wafer area limits the area of the maximum unit cell. A wafer having a diameter of 6 inches allows the unit cell to have the area of about 25 mm by 25 mm.  
       Third Embodiment  
       [0028]     Simultaneous integration of a signal processing circuit to the formation of a humidity sensor on the semiconductor substrate enables production of a semiconductor integrated circuit in which the humidity sensor is incorporated in 1 chip.  
         [0029]     In general, on the integrated circuit on the semiconductor substrate, a thick nitride film having a thickness of about 1 μm is diposed as a protective film. The nitride film is formed on a surface of the semiconductor substrate by a chemical vapor deposition (CVD) apparatus. When the integrated circuit and the electrode of the humidity sensor are integrated, the thick nitride film as a protective film having a thickness of about 1 μm is also coated onto the electrode of the humidity sensor. However, when the thick nitride film is coated, the capacitance C between the electrodes  11  and  12  shown in  FIGS. 2A and 2B  becomes equal to a series capacitance of CH and CN, where CH is a capacitance of the humidity sensitive film and CN is a capacitance of the nitride film, and the following equation (2) holds. 
 
 C =1/(1 /C   H +2 /C   N )  (2) 
 
 Where 
 
 C   H =ε 0 ·ε H   ·S/d   H   , C   N =ε 0 ·εN ·S/d   N  
 
         [0030]     In this case, ε 0  is the permittivity of vacuum, ε H  is a dielectric constant of the humidity sensitive film, ε N  is a dielectric constant of a nitride film, S is an area where the electrodes  11  and  12  are opposing each other, d H  is a thickness of the humidity sensitive film incorporated between the electrodes  11  and  12 , and d N  is a thickness of the nitride film on the surface of the electrodes  11  and  12 . When the thickness d N  of the nitride film is sufficiently small, C H  is much smaller than C N , Equation (2) thus becomes substantially the same as Equation (1). As is apparent from Equation (2), the smaller the thickness d N  of the nitride film is, the larger the change in capacitance between the electrodes with respect to the humidity.  
         [0031]     Accordingly, it is preferable that the protective film of  FIGS. 2A and 2B  is thin. It is therefore necessary that the thick nitride film is deposited once, then the thick nitride film is etched for the electrode portion, and a thin nitride film is deposited again on the electrode portion.  
         [0032]     On the other hand, in the sectional view of  FIG. 2B  in the first embodiment, a barrier layer (hereinafter, referred to as barrier metal) is not illustrated under the metal wiring layer. In general, however, in order to enhance adhesion properties between the metal wiring layer and the insulating film thereunder, the barrier metal is coated in the semiconductor process. As the barrier metal, titanium nitride (TiN) and tantalum nitride (TaN) are known.  
         [0033]     When a metal containing a large amount of nitride is used for the barrier metal, a barrier metal  17  is etched at the same time as shown in  FIG. 4A  in removal of the thick nitride film. As a result, adhesion properties between the metal wiring layer  12  and the underlying insulating film  14 B deteriorates, which causes easy peeling-off of the metal wiring layer  12 . For example, in a case of TiN, the ratio of the content of titanium and nitrogen is 1:1 in terms of the atomic weight, and the barrier metal  17  is also etched at the time of etching the thick nitride film. On the other hand, in a case where a metal containing a small amount of nitrogen (containing 10% or lower of nitrogen in terms of the atomic weight) is used for the barrier metal, the barrier metal  17  is hardly etched when etching the thick nitride film as shown in  FIG. 4B . Therefore, adhesion properties between the metal wiring layer  12  and the insulating film  14 B thereunder is not impaired.  
         [0034]     In  FIG. 1 , the basic unit cells are arranged in the 5×5 grid pattern. However, the present invention can apparently provide the same effect irrespective of the number of the unit cells when a plurality of unit cells is arranged.