Patent Publication Number: US-11662328-B2

Title: Capacitive sensor and manufacturing method of capacitive sensor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the Japanese Patent Application No. 2020-117659, filed Jul. 8, 2020, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The present embodiment relates to a capacitive sensor and a manufacturing method of the capacitive sensor. 
     BACKGROUND 
     The capacitive sensor disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2001-168133 is configured to detect various states of a detection target from a signal corresponding to a capacitance between the detection target and a sensing electrode when bringing the sensing electrode in close proximity to the detection target. The capacitive sensor of this type is employed, for example for determination of defectiveness or otherwise of a conductive pattern formed on a circuit board. 
     In principle, the sensitivity of a capacitive sensor increases as the distance to a detection target is shortened. In this aspect, it is preferable to design a capacitive sensor that can both reduce a distance to the detection target and be protected from breakage even if the sensor is accidentally brought into contact with the detection target. 
     SUMMARY 
     The purpose of the present embodiment is to offer a capacitive sensor that can be placed as close as possible to a detection target and is less prone to breakage, and a manufacturing method of such a sensor. 
     A capacitive sensor according to the first aspect includes a sensing electrode, a first electrode pad, a substrate, and a second electrode pad. The sensing electrode outputs a signal corresponding to a capacitance between the sensing electrode and a detection target. The first electrode pad is coupled to the sensing electrode. The substrate includes a substrate surface portion and a step portion. On the substrate surface portion are the sensing electrode and the first electrode pad mounted. The step portion is provided at a position in the substrate lower than the substrate surface portion. The second electrode pad is mounted on the step portion and coupled to an external line. 
     A manufacturing method of a capacitive sensor according to the second aspect includes forming a step portion by etching an end portion of a substrate that includes a sensing electrode and a substrate surface portion, the sensing electrode being configured to output a signal corresponding to a capacitance between the sensing electrode and a detection target, a first electrode pad being coupled to the sensing electrode and mounted on the substrate surface portion; forming a line extending from the first electrode pad to the step portion; and forming a second electrode pad on the step portion to be coupled to the line and an external line. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram showing a capacitive sensor according to an embodiment. 
         FIG.  2 A  is a frontal view of an exemplary structure of the capacitive sensor. 
         FIG.  2 B  is a cross section of an exemplary structure of the capacitive sensor. 
         FIG.  3 A  is a process diagram explaining a step in a manufacturing method of the capacitive sensor. 
         FIG.  3 B  is a process diagram explaining a step in the manufacturing method of the capacitive sensor. 
         FIG.  3 C  is a process diagram explaining a step in the manufacturing method of the capacitive sensor. 
         FIG.  3 D  is a process diagram explaining a step in the manufacturing method of the capacitive sensor. 
         FIG.  3 E  is a process diagram explaining a step in the manufacturing method of the capacitive sensor. 
         FIG.  3 F  is a process diagram explaining a step in the manufacturing method of the capacitive sensor. 
         FIG.  3 G  is a process diagram explaining a step in the manufacturing method of the capacitive sensor. 
         FIG.  3 H  is a process diagram explaining a step in the manufacturing method of the capacitive sensor. 
         FIG.  3 I  is a process diagram explaining a step in the manufacturing method of the capacitive sensor. 
         FIG.  3 J  is a process diagram explaining a step in the manufacturing method of the capacitive sensor. 
         FIG.  3 K  is a process diagram explaining a step in the manufacturing method of the capacitive sensor. 
         FIG.  3 L  is a process diagram explaining a step in the manufacturing method of the capacitive sensor. 
         FIG.  3 M  is a process diagram explaining a step in the manufacturing method of the capacitive sensor. 
         FIG.  3 N  is a process diagram explaining a step in the manufacturing method of the capacitive sensor. 
         FIG.  4    is a cross section of an exemplary structure of a capacitive sensor according to a modification example. 
     
    
    
     DETAILED DESCRIPTION 
     The embodiments will be explained with reference to the drawings.  FIG.  1    is a diagram showing a capacitive sensor according to an embodiment. A capacitive sensor  1  is a sensor configured to output a signal corresponding to a capacitance between the capacitive sensor  1  and a detection target  3  when being brought in close proximity to the detection target  3 . The capacitive sensor  1  is coupled to a control circuit  2 . The control circuit  2  inputs a signal for driving the capacitive sensor  1 , and receives a signal from the capacitive sensor  1  to perform necessary processing thereon. For instance, if the detection target  3  is a circuit board, the control circuit  2  detects a defective portion in the circuit board from a capacitance signal. 
       FIG.  2 A  is a frontal view of an exemplary structure of the capacitive sensor  1 , and  FIG.  2 B  is a cross section of an exemplary structure of the capacitive sensor  1 . 
     As illustrated in  FIG.  2 A , the capacitive sensor  1  includes a substrate  11 , a sensing electrode  12 , first electrode pads  13   a  and  13   b , lines  14   a  and  14   b , and second electrode pads  15   a  and  15   b.    
     On the substrate  11  are various circuits of the capacitive sensor  1  mounted. The substrate  11  is, for example, a silicon substrate. According to the present embodiment, the substrate  11  includes a substrate surface portion  11   a  and a step portion  11   b . The substrate surface portion  11   a  is a surface of the substrate  11  closest to the detection target  3  during its movement toward the detection target  3 . On the substrate surface portion  11   a  are the sensing electrode  12  and first electrode pads  13   a  and  13   b  mounted. The step portion  11   b  is a portion of the substrate  11  containing a step. On the step portion  11   b  are the second electrode pads  15   a  and  15   b  formed. This means that, on account of the step portion  11   b , the second electrode pads  15   a  and  15   b  are provided at a lower position in comparison with the sensing electrode  12  and first electrode pads  13   a  and  13   b.    
     The sensing electrode  12  is configured to detect a capacitance. In particular, the sensing electrode  12  is configured to output a signal corresponding to the capacitance between the sensing electrode  12  and the detection target  3 . For instance, if the detection target  3  is a circuit board, a detection signal is applied to the circuit board. Under this condition, when the sensing electrode  12  is brought in close proximity to the circuit board, which is a detection target  3 , a capacitor is created between the sensing electrode  12  and detection target  3 , and the sensing electrode  12  outputs a signal corresponding to the capacitance between the sensing electrode  12  and the detection target  3 . 
     The sensing electrode  12  may be formed of a single sensing electrode, or by a plurality of linear or area sensing electrodes. For instance,  FIG.  2 A  shows the sensing electrode  12  formed of two sensing electrodes  12   a  and  12   b  laterally separated at the broken-line border. The sensing electrodes  12   a  and  12   b  are arranged a certain distance apart, and respectively output a signal corresponding to the capacitance with respect to the facing detection target  3 . 
     The first electrode pad  13   a  is arranged in the same layer as the sensing electrode  12 , for example on the right side of the sensing electrode  12 . The first electrode pad  13   a  is coupled to the right sensing electrode  12   a , and also to the second electrode pad  15   a  by way of a line  14   a . The first electrode pad  13   b  is arranged in the same layer as the sensing electrode  12 , for example on the left side of the sensing electrode  12 . The first electrode pad  13   b  is coupled to the left sensing electrode  12   b , and also to the second electrode pad  15   b  by way of a line  14   b.    
     First electrode pads are provided in equal number to sensing electrodes. For instance, when the sensing electrode  12  is formed of two sensing electrodes, the first electrode pad is formed of two first electrode pads  13   a  and  13   b . Only one sensing electrode may be provided, and in this case only one first electrode pad is provided. 
     The line  14   a  couples the first electrode pad  13   a  to the second electrode pad  15   a . The line  14   b  couples the first electrode pad  13   b  to the second electrode pad  15   b . As illustrated in  FIG.  2 B , the line  14   a  is formed along the right-side wall of the step portion  11   b . Similarly, as illustrated in  FIG.  2 B , the line  14   b  is formed along the left-side wall of the step portion  11   b.    
     The second electrode pad  15   a  is provided on the right-side step portion  11   b , and coupled to the first electrode pad  13   a  by way of the line  14   a . As illustrated in  FIG.  2 B , a flexible print circuit board (FPC)  18   a  with an external line  17   a  attached is flip-chip bonded to the second electrode pad  15   a  with a bump  16   a  interposed. The second electrode pad  15   b  is provided on the left-side step portion  11   b , and coupled to the first electrode pad  13   b  by way of the line  14   b . As illustrated in  FIG.  2 B , a flexible print circuit board (FPC)  18   b  with an external line  17   b  attached is flip-chip bonded to the second electrode pad  15   b  with a bump  16   b  interposed. The external lines  17   a  and  17   b  are coupled to the control circuit  2 . 
     As illustrated in  FIG.  2 B , a passivation layer  19  may be formed of polyimide (PI) and deposited as a protective film on the substrate surface portion  11   a  and step portion  11   b  of the substrate  11 . The passivation layer  19  may be formed of a plurality of films. The passivation layer  19  serves as a protective film that protects the substrate surface portion  11   a  from being soiled, and from directly touching the detection target  3  even if the capacitive sensor  1  is accidentally brought into contact with the brought detection target  3 . The passivation layer  19  protects the sensing electrode  12  and first electrode pads  13   a  and  13   b  provided on the substrate surface portion  11   a . Furthermore, the substrate  11  may be adhered with an adhesive  21  to a print circuit board (PCB)  20 , on which the control circuit and the like of the capacitive sensor  1  are mounted. Preferably, the passivation layer  19  is formed to be as thin as possible. The film thickness d 2  of the passivation layer  19  may be 5 μm. 
     The capacitive sensor  1  is brought to and used in the proximity of a detection target, meaning that the total height of the components mounted on the step portion  11   b  will not exceed the height of the passivation layer  19 . On the other hand, in order to increase the depth of the step portion  11   b , high-level processing technique is required leading to increased costs. High-level processing technique is also required, however, to form the step portion  11   b  with a little depth. It is therefore preferable that a suitable depth of the step portion  11   b  be determined by balancing the processing difficulties. 
     Particularly when the FPC is flip-chip bonded, the depth of the step portion  11   b  is determined in consideration of the thickness of FPC  18   a  and  18   b , the thickness of the bumps  16   a  and  16   b , and the thickness of the second electrode pads  15   a  and  15   b . For instance, when the thickness d 2  of the passivation layer  19  is 5 μm, the depth d 1  of the step portion  11   b  may be determined to be 100 μm in consideration of the level of processing difficulty and the thicknesses of the components. 
     Next, the manufacturing method of the capacitive sensor  1  according to the present embodiment will be explained.  FIGS.  3 A to  3 N  are process diagrams explaining the steps of the manufacturing method of the capacitive sensor  1 . 
     The following manufacturing method begins with the state of a sensing electrode  102  and a first electrode pad  103  formed on a silicon substrate  101 , which is the substrate  11 , as illustrated in  FIG.  3 A . A passivation layer  104  is formed on the portion of the silicon substrate  101  except for the first electrode pad  103 . The sensing electrode  102  and the first electrode pad  103  are formed in such a manner as to provide a step formation region  101   a  at the end of the silicon substrate  101  to form the step portion  11   b.    
       FIGS.  3 A to  3 N  show the manufacturing method of only the right-side portion of the substrate  11  in  FIG.  2   . The left-side portion of the substrate  11  in  FIG.  2    can be formed in the same manufacturing method of the right-side portion explained below. 
     The preparation of the sensing electrode  102  and first electrode pad  103  on the silicon substrate  101  may adopt any method. The passivation layer  104  may be an oxide film or nitride film of any kind, which can be formed by chemical vapor deposition (CVD). 
     First, as illustrated in  FIG.  3 B , etching is conducted on the passivation layer  104 . For this etching, the silicon substrate  101  is coated with a photoresist  105  in such a manner as to have only the step formation region  101   a  exposed. Thereafter, as illustrated in  FIG.  3 C , the portion of the passivation layer  104  in the step formation region  101   a  of the silicon substrate  101  is etched off. Then, as illustrated in  FIG.  3 D , the photoresist  105  is removed. In this manner, a step portion  106 , which will serve as the step portion  11   b , is formed in the silicon substrate  101 . Next, as illustrated in  FIG.  3 E , a passivation layer  107  is formed, for example through CVD, in the etched portion. 
     As illustrated in  FIG.  3 F , a PI layer  108  is deposited on the sensing electrode  102  serving as a passivation layer. Thereafter, a portion of the passivation layer  107  above the first electrode pad  103  is etched off, as illustrated in  FIG.  3 G . 
     As illustrated in  FIG.  3 H , a metal seed layer  110  is formed, for example through sputtering, on the insulative passivation layer  107  in order to prepare a metal line  14   a . Thereafter, as illustrated in  FIG.  3 I , a photoresist  111  is applied in order to form a line  14   a  from the seed layer  110  in the vicinity of the first electrode pad  103 . Then, through photolithography conducted upon the seed layer  110  in the vicinity of the first electrode pad  103 , the seed layer  110  is processed into the shape of the line  14   a . Thereafter, as illustrated in  FIG.  3 J , metal plating is conducted to form the line  14   a . The line  112  in  FIG.  3 J  will serve as the line  14   a.    
     Next, as illustrated in  FIG.  3 K , the photoresist  111  and seed layer  110  are removed. As illustrated in  FIG.  3 L , a PI layer  113 , which will serve as a passivation layer, is stacked on the portion except for the step portion  106 . 
     As illustrated in  FIG.  3 M , a backside metal  114  is formed on the back side of the silicon substrate  101 . Then, as illustrated in  FIG.  3 N , a second electrode pad  115  is formed, for example through flash plating, in the step portion  106 . Flash plating is short-time thin plating. 
     As discussed above, in the capacitive sensor according to the present embodiment, the sensing electrode and the first electrode pad coupled to this sensing electrode are formed in the substrate surface portion, while the second electrode pad coupled to both the first electrode pad and the external line is formed in the step portion, which is lower than the substrate surface portion. This means that the line between the second electrode pad and external line can be provided at a position lower than the substrate surface portion. For this reason, the substrate surface portion of the capacitive sensor, namely, the sensing electrode, can be placed sufficiently close to a detection target. This increases the sensitivity of the capacitive sensor. In addition, even if the substrate surface portion of the capacitive sensor accidentally touches the detection target, the FPC or the like will not be in contact with the detection target. Thus, the FPC or the like will not be broken under the stress from the detection target. 
     The line between the first electrode pad and second electrode pad is formed along the wall of the step portion. This enhances the strength of the line between the first electrode pad and second electrode pad. As a result, the reliability of the capacitive sensor can also be enhanced. 
     Furthermore, the present embodiment requires only the formation of a step portion in the silicon substrate. Thus, processing such as through-silicon via (TSV) is not required. If TSV is to be conducted, a through electrode needs to be created inside the silicon substrate, and the silicon substrate is therefore preferably thinned. In contrast, the present embodiment does not require a silicon substrate to be thinned. A thick silicon substrate can be adopted so that the strength of the capacitive sensor can be ensured. This will improve the reliability in the case of accidental contact with a detection target. Furthermore, TSV comes with considerable costs, whereas the present embodiment can reduce manufacturing costs for a capacitive sensor, in which only a step portion is formed through etching or the like. 
     Modified Examples 
     Modification examples of the present embodiment will be explained. According to the embodiment, the FPC with an external line is flip-chip bonded onto the second electrode pad with a bump interposed. An external line, however, need not always be provided by flip-chip bonding. For instance, as illustrated in  FIG.  4   , the second electrode pads  15   a  and  15   b  may be coupled to the external electrodes  23   a  and  23   b  through wire bonding with wires  22   a  and  22   b . In such an arrangement, the external electrodes  23   a  and  23   b  may be coupled to the external lines  25   a  and  25   b  on the back surface of the PCB  20  by way of the through holes  24   a  and  24   b.    
     In the case of  FIG.  4   , each of the wires  22   a  and  22   b  includes a bent portion. In order to ensure the strength of the wires  22   a  and  22   b , the vicinity of the wires  22   a  and  22   b  may be molded with a resin  26  or the like. 
     In the structure in which electrodes are mounted by wire bonding as in the modification example, the depth of the step portion  11   b  will be determined in consideration of both the height of the bent portions of the wires  22   a  and  22   b , and of the thickness of the mold in addition to the aforementioned processing difficulty of the step portion  11   b . With the currently available processing technique, it is difficult to design the height of the bent portions of the wires  22   a  and  22   b  to be less than 50 μm. If the depth d 1  of the step portion  11   b  is 100 μm, the same as the aforementioned dimension, the total height of components on the step portion  11   b  will not exceed that of the passivation layer  19  even if the height of the bent portions is 50 μm. 
     The present invention is not limited to the above embodiments, and can be modified in various manners without departing from the gist of the invention at the stage of executing the invention. The embodiments may be suitably combined, which will produce combined effects. Furthermore, various inventions are included in the above embodiments, and various inventions can be extracted from any combination of selected ones of the disclosed structural components. For instance, even if some of the structural components are omitted in the embodiments, the structure from which these components are omitted may be extracted as an invention as long as the targeted issue can be solved and effects can be achieved.