Abstract:
A moisture-sensitive element with an interdigital capacitor (IDC) and fabrication thereof are disclosed, wherein the moisture-sensitive element with the interdigital capacitor investigates a moisture sensor for the determination of human skin moisture based on the interdigital capacitor. The moisture-sensitive element with an interdigital capacitor comprises a printed circuit board (PCB), an interdigital capacitor formed on the printed circuit board, and a moisture sensing layer formed on the interdigital capacitor.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention is generally related to a moisture-sensitive element, and more particularly to a moisture-sensitive element with an interdigital capacitor. 
         [0003]    2. Description of the Prior Art 
         [0004]    Research on moisture-sensitive elements has been developed for years. In the areas of automation development, living quality promotion, and environmental quality, sensing elements are necessary. Generally, moisture-sensitive elements are utilized in environmental monitoring and control. For example, the common seen moisture-sensitive element is the household electronic thermo-hygrometer, dehumidifier, hygrostat, etc. However, not only air contains moisture but also a general living body has water content. Especially, due to development of biotechnology and prosperousness in cosmetic industry, the technique of moisture-sensitive elements has been applied in determining skin moisture content. Applying the technique of moisture-sensitive elements in cosmetic medical field is an important subject to be solved. 
       SUMMARY OF THE INVENTION 
       [0005]    In light of the above mentioned background, the present invention provides a moisture-sensitive element with an interdigital capacitor to overcome the above disadvantages of a traditional moisture-sensitive element. 
         [0006]    Generally, sensing elements has two major categories, capacitance type and resistance type. The invention basically uses a printed circuit board to make interdigital electrodes and then uses the hot pressing method to fix polyimide film on the electrodes to thereby fabricate a capacitance type sensing element with two sensing functionalities, determining environment relative humidity and skin cuticle moisture content. In addition, the invention discusses the effect of the shape of interdigital electrodes on the basic capacitance of the sensing element and sensing characteristic and finds out the suitable interdigital electrodes for environmental and skin measurements. 
         [0007]    One object of the present invention is to provide a moisture-sensitive element with an interdigital capacitor, comprising a printed circuit board (PCB), an interdigital capacitor, and a sensing layer. The interdigital capacitor is formed on the printed circuit board. The sensing layer is formed on the interdigital capacitor. The interdigital capacitor comprises a first electrode and a second electrode. The first electrode comprises a plurality of first extending electrodes and the second electrode comprises a plurality of second extending electrodes. The plurality of second extending electrodes are provided interlaced with the plurality of first extending electrodes, i.e. the neighboring electrode of the first extending electrode is the second extending electrode and the neighboring electrode of the second extending electrode is the first extending electrode. Each first extending electrode together with its neighboring second extending electrode form a pair of extending electrodes. Furthermore, by electrically coupling the first electrode and the second electrode to a LCR meter (inductance-capacitance-resistance meter), a moisture-sensitive system with an interdigital capacitor is formed to measure environment moisture and skin moisture content. 
         [0008]    In addition, the method for fabricating the moisture-sensitive element with the interdigital capacitor comprises the following steps: providing the printed circuit board (PCB); forming the interdigital capacitor on the printed circuit board by etching via double-side printed circuit board technique; and mounting the sensing layer on the interdigital capacitor by hot pressing. 
         [0009]    Furthermore, the method for determining skin moisture by the moisture-sensitive system with the interdigital capacitor comprises the following steps: placing the moisture-sensitive element with the interdigital capacitor on the skin; absorbing the moisture on the skin by the sensing layer and thus changing an output capacitance value of the interdigital capacitor; and receiving and measuring the output capacitance value by the LCR meter to determine the moisture content of the skin. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  shows a schematic diagram illustrating the structure of a moisture-sensitive element with an interdigital capacitor; 
           [0011]      FIG. 2  shows a schematic diagram illustrating the structure of an interdigital capacitor; 
           [0012]      FIG. 3  shows a schematic diagram illustrating the structure of a moisture-sensitive system with an interdigital capacitor; and 
           [0013]      FIGS. 4 and 5  show schematic diagrams illustrating the experimental data of a moisture-sensitive system with an interdigital capacitor. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0014]    What is probed into the invention is a moisture-sensitive element with an interdigital capacitor. Detail descriptions of the structure and elements will be provided in the following in order to make the invention thoroughly understood. Obviously, the application of the invention is not confined to specific details familiar to those who are skilled in the art. On the other hand, the common structures and elements that are known to everyone are not described in details to avoid unnecessary limits of the invention. Some preferred embodiments of the present invention will now be described in greater detail in the following. However, it should be recognized that the present invention can be practiced in a wide range of other embodiments besides those explicitly described, that is, this invention can also be applied extensively to other embodiments, and the scope of the present invention is expressly not limited except as specified in the accompanying claims. 
         [0015]      FIG. 1  shows a schematic diagram illustrating the structure of a moisture-sensitive element  100  with an interdigital capacitor. The moisture-sensitive element  100  comprises a printed circuit board (PCB)  110 , an interdigital capacitor  120 , and a sensing layer  130 . The interdigital capacitor  120  is formed on the printed circuit board  110 . The sensing layer  130  is formed on the interdigital capacitor  120 . 
         [0016]    The sensitivity of such moisture-sensitive element depends on not only the sensing layer  130  but also the structure of the electrodes. Therefore, the interdigital capacitor  120  according to the invention further promotes the sensitivity of the moisture-sensitive element  100 . As shown in  FIG. 2 , the interdigital capacitor  120  comprises a first electrode  122  and a second electrode  124 . The first electrode  122  comprises a plurality of first extending electrodes  1222  and the second electrode  124  comprises a plurality of second extending electrodes  1242 . The plurality of second extending electrodes  1242  are provided interlaced with the plurality of first extending electrodes  1222 , i.e. the neighboring electrode of the first extending electrode  1222  is the second extending electrode  1242  and the neighboring electrode of the second extending electrode  1242  is the first extending electrode  1222 . Each first extending electrode  1222  together with its neighboring second extending electrode  1242  form a pair of extending electrodes. 
         [0017]      FIG. 3  shows a schematic diagram illustrating the structure of a moisture-sensitive system  150  with an interdigital capacitor. The moisture-sensitive system  150  comprises the moisture-sensitive element  100  with the interdigital capacitor and a LCR meter  160 . The first electrode  122  is electrically coupled to the LCR meter  160  via a first terminal  126  and the second electrode  124  is electrically coupled to the LCR meter  160  via a second terminal  128 . 
         [0018]    As the moisture-sensitive element  100  with the interdigital capacitor is power-on, an electric field is produced between the first electrode  122  and the second electrode  124  of the interdigital capacitor  120  to have capacity effect. The dielectric constant of water is about  80 . Thus, if water enters the electric field, the total dielectric constant increases for the interdigital capacitor  120  due to water molecules and thus the output capacitance is increased. The more is the water content, the more is the change of the capacitance. Therefore, the amount of capacitance change measured by the LCR meter  160  can determine the amount of water content. 
         [0019]    According to the above, the invention provides a method for determining skin moisture by the moisture-sensitive system with the interdigital capacitor, comprising the following steps: at first placing the moisture-sensitive element  100  with the interdigital capacitor on the skin; absorbing the moisture on the skin by the sensing layer  130  to thus change an output capacitance value of the interdigital capacitor  120 ; and receiving and measuring the output capacitance value by the LCR meter  160  to determine the moisture content of the skin. 
         [0020]    However, the geometrical structure of the interdigital capacitor  120  has many variable parameters. For example, the width of the first extending electrode  1222  and the second extending electrode  1242  is a, the pitch between the first extending electrode  1222  and its neighboring second extending electrode  1242  is b, and the interlaced overlapping length of the first extending electrode  1222  and the second extending electrode  1242  is c, as shown in  FIG. 2 . 
         [0021]      FIG. 4  shows  27  different types of geometrical structures by taking three different values for each parameter described in the above. After experiments, the following better parameters are obtained and thus better measurement efficiency of the interdigital capacitor  120  is achieved. The width a of the first electrode  1222  and the second electrode is 0.2 mm. The pitch b between the first extending electrode  1222  and its neighboring second extending electrode  1242  is 0.2 mm. The interlaced overlapping length c of the first extending electrode  1222  and the second extending electrode  1242  is 7 mm. Besides, the interdigital capacitor  120  comprises eight pairs of extending electrodes for better capacity effect. 
         [0022]    Moreover, the experiments are carried out in environment of 23±1° C. and 45±2%R.H. The LCR meter  160  can be HP4284A LCR meter, the frequency range of which is between 1 kHz and 1 MHz and the applied voltage of which is 3V. 
         [0023]    The initial capacitance value of the LCR meter  160  before measurement is 5.8 pF. When the moisture-sensitive element  100  with the interdigital capacitor is placed on skin, the capacitance measured by the LCR meter  160  starts to increase. According to the experimental result, the measured stable capacitance value is 11.3 pF. Thus, the difference in capacitance value is 5.5 pF and the sensing time is about 3 seconds, as shown in  FIG. 5 . 
         [0024]    Furthermore, the invention provides a method for fabricating a moisture-sensitive element with an interdigital capacitor, comprising the following steps. At first, the printed circuit board (PCB)  110  is provided. The PCB  110  is used to form complicate copper circuitry among electronic parts by etching an organized pattern thereon to provide support and installation for electronic parts and is a basic component for almost all electronic products. The PCB  110  is a plate made by insulating material. Generally, the plate is provides with holes for mounting chips and other electronic elements. The holes and the metallic paths pre-printed on the plate make electronic elements thereon easily electrically connected. For example, the pins of the electronic element go through the holes on the PCB  110  and then can be mounted on the PCB  110  by soldering to form circuitry. According to application fields, the PCB  110  can be a one-side plate, double-side plate, multi-layer plate more than four layers, and flexible plate. 
         [0025]    Then, the printed circuit board technique is used to form the interdigital capacitor  120  on the PCB  110  by etching. The first terminal  126  of the first electrode  122  and the second terminal  128  of the second electrode  124  are printed on two surfaces of the PCB  110  by etching. The first electrode  122  of the interdigital capacitor  120  is provided on one side of the PCB  110  and the second electrode  124  is provided on the other side of the PCB  110 , in order to maintain the flatness of the sensing area. The first electrode  122  and the second electrode  124  are electrically coupled to the LCR meter  160  via the first terminal  126  and the second terminal  128  on the back of the PCB  110 , respectively, to ensure the front smoothness of the PCB  110 . Thus, during measurement, the inaccuracy due to uneven contact area can be prevented. 
         [0026]    Finally, the sensing layer  130  is mounted on the interdigital capacitor  120  by hot pressing. The problem of uneven film thickness or difficulty in controlling film thickness by the dipping or spin coating method can be solved. Besides, the hot pressing method is relatively easy. The material of the interdigital capacitor  120  comprises copper and the material of the sensing layer  130  comprises polyimide. Polyimide has the polar group and thus is easy to absorb moisture to enhance the measurement of moisture content. In addition, polyimide does not fall off while contacting with skin and has the advantages of reusability, easy fabrication, and low cost compared to the flexible PCB. 
         [0027]    The moisture-sensitive element  100  with an interdigital capacitor can measure the environment moisture and the human skin moisture. It can be placed in common air environment to measure surrounding environment moisture and can measure the moisture content of the skin cuticle if contacting with skin. In addition, this moisture-sensitive element is not influenced by temperature within the temperature range around 10° C.˜30° C. Thus, the skin moisture measurement can be conducted at the common room temperature. 
         [0028]    In monitoring the environment moisture, the moisture-sensitive element  100  with an interdigital capacitor fabricated according to the invention can be utilized in the measure range of 50%R.H.˜95%R.H. In measuring the moisture content of the skin cuticle, it can measure common type and moist type skin. While measuring skin moisture, the moisture-sensitive element  100  with an interdigital capacitor can monitor the environment moisture at the same time to ensure the correctness of the skin moisture measurement. 
         [0029]    Obviously many modifications and variations are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims the present invention can be practiced otherwise than as specifically described herein. Although specific embodiments have been illustrated and described herein, it is obvious to those skilled in the art that many modifications of the present invention may be made without departing from what is intended to be limited solely by the appended claims.