Patent Publication Number: US-2015082898-A1

Title: Strain sensor

Description:
This application is a continuation of International Application PCT/JP2013/003717, filed on Jun. 13, 2013, claiming the foreign priority of Japanese Patent Application No. 2012-141570, filed on Jun. 25, 2012, the contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a strain sensor that detects a mechanical strain generated in an object due to a load applied to the object. 
     BACKGROUND ART 
     A microfabrication technique, such as Micro Electro Mechanical Systems (MEMS) technique, can provide a mechanical vibrator that is extremely small and thin. This technique realizes a small mass of the vibrator itself, and a high precision vibrator in which frequency and impedance greatly change even when a load to be applied is small. This micromechanical vibrator does not require making stress concentration points in the strain generating body itself. Therefore, the micromechanical vibrator attached to the strain generating body can provide a strain sensor that can easily measure load and strain applied to the strain generating body. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Patent Laid-Open Publication No. 03-103735 
     SUMMARY 
     A strain sensor includes a package to be connected to a strain generating body, a detector converting a mechanical strain of the strain generating body into an electric signal and output the electric signal, and a processor chip connected to the upper surface of the package and separated from the detector. A recess is provided in the upper surface of the package. The detector is accommodated in the recess and joined to the recess. 
     This strain sensor can have a small size. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an exploded perspective view of a strain sensor according to an exemplary embodiment of the present invention. 
         FIG. 2  is a cross-sectional view of the strain sensor according to the embodiment. 
         FIG. 3A  is a bottom view of a detector of the strain sensor according to the embodiment. 
         FIG. 3B  is a cross-sectional view of the detector at line  3 B- 3 B shown in  FIG. 3A . 
         FIG. 3C  is an enlarged cross-sectional view of the detector shown in  FIG. 3B . 
         FIG. 3D  is an enlarged cross-sectional view of another strain sensor according to the embodiment. 
         FIG. 3E  is an enlarged cross-sectional view of a still another strain sensor according to the embodiment. 
         FIG. 3F  is a bottom view of a detector of a further strain sensor according to the embodiment. 
     
    
    
     DETAIL DESCRIPTION OF PREFERRED EMBODIMENT 
       FIGS. 1 and 2  are an exploded perspective view and a cross-sectional view of strain sensor  20  according to an exemplary embodiment of the present invention, respectively. Strain sensor  20  is to detect a mechanical strain of strain generating body  120 , and includes package  21 , detector  30 , and processor chip  50 . According to the embodiment, processor chip  50  is an integrated circuit (IC) chip. Package  21  includes a wiring board, such as a multilayer printed wiring board or a multilayer ceramic substrate. Upper surface  21   c  of package  21  has recess  21   a  therein at substantially at a center of the upper surface. Recess  21   a  opens upward and has bottom surface  21   b  facing upward. Electrode pad  22  is provided on bottom surface  21   b  of recess  21   a.  Electrode pad  23  plated with gold is provided on upper surface  21   c  of package  21 . Inner electrodes of package  21  electrically connect between electrode pads  22  and  23 , or between an external electrode provided on lower surface  21   d  of package  21  and each of electrode pads  22  and  23 . Detector  30  is made of a silicon material-based substrate, such as a Silicon-On-Insulator (SOI) substrate, and converts a physical amount, such as a tensile force and a strain, into an electric signal. Processor chip  50  processes the electric signal output from detector  30 , and provides an electric signal corresponding to the physical amount, such as the tensile force and the load, applied to strain sensor  20 . Electrode pad  51  is provided on lower surface  50   b  of processor chip  50  at a position opposed to electrode pad  23 . Bump  52  containing metal, such as gold, is formed on electrode pad  51 . Lower surface  21   d  of package  21  is connected and fixed to strain generating body  120  with bonding member  61  containing metal-based material, such as Au—Au joining, or a material, such as an epoxy resin, having stiffness. 
       FIG. 3A  is a bottom view of detector  30  of strain sensor  20 .  FIG. 3B  is a cross-sectional view of detector  30  at line  3 B- 3 B shown in  FIG. 3A . Detector  30  includes substrate  31  that is made of a semiconductor material, such as silicon, and has a rectangular shape. Substrate  31  has upper surface  31   a  and lower surface  31   b.  Substrate  31  includes base  37 , and vibrators  32   a  and  32   b  that are connected to base  37 . Base  37  and vibrators  32   a  and  32   b  are formed by etching upper surface  31   a  and lower surface  31   b  of substrate  31 . Vibrators  32   a  and  32   b  are arranged along lower surface  31   b  of substrate  31 . Vibrator  32   a  has a beam shape having both ends connected to base  37 , and extends slenderly along longitudinal direction  132   a  that connects both ends. Vibrator  32   b  has a beam shape having both ends connected to base  37 , and extends slenderly along longitudinal direction  132   b  that connects both ends. Vibrator  32   a  has a length of 0.55 mm in longitudinal direction  132   a  of vibrator  32   a,  a width of 0.15 mm, and a thickness of 0.01 mm. Vibrator  32   b  has a length of 0.60 mm in longitudinal direction  132   b  of vibrator  32   b,  a width of 0.15 mm, and a thickness of 0.01 mm. According to the embodiment, vibrators  32   a  and  32   b  are arranged, such that lower surface  31   b  of substrate  31  has a rectangular shape, longitudinal direction  132   a  of vibrator  32   a  is parallel to one side of the rectangular shape of substrate  31 , and longitudinal direction  132   a  of vibrator  32   a  is perpendicular to longitudinal direction  132   b  of vibrator  32   b.    
     Detector  30  further includes drive element  33   a  provided at a center part of the beam shape of vibrator  32   a,  and sensing elements  34   a  and  35   a  provided respectively close to both ends of the beam shape. Each of drive element  33   a,  sensing element  34   a,  and sensing element  35   a  includes a grounded electrode provided on a surface of vibrator  32   a,  a piezoelectric body layer made of a piezoelectric material, such as PZT, provided on the grounded electrode, and an upper electrode provided on the piezoelectric body layer. Six electrodes: the upper electrode and the ground electrode of drive element  33   a;  the upper electrodes of sensing elements  34   a  and  35   a;  and the ground electrodes of sensing elements  34   a  and  35   a  are electrically connected to land  36  by a wiring pattern. Bump  40  is provided on the lower surface of land  36 . 
       FIG. 3C  is an enlarged cross-sectional view of detector  30  shown in  FIG. 3B , and illustrates a periphery of bump  40 . Bump  40  includes core  40   a  and solder  40   b  that covers the surface of core  40   a.  According to embodiment, core  40   a  is made of metal, such as gold, and has a spherical shape. 
     Detector  30  further includes drive element  33   b  provided at a center part of the beam shape of vibrator  32   b,  and sensing elements  34   b  and  35   b  provided respectively close to both ends of the beam shape. Each of drive element  33   b,  sensing element  34   b,  and sensing element  35   b  includes a grounded electrode provided on the surface of vibrator  32   b,  a piezoelectric body layer made of a piezoelectric material, such as PZT, provided on the grounded electrode, and an upper electrode provided on the piezoelectric body layer. Six electrodes: the upper electrode and the ground electrode of drive element  33   b;  the upper electrodes of sensing elements  34   b  and  35   b;  and the grounded electrodes of sensing elements  34   b  and  35   b  are electrically connected to land  36  by a wiring pattern. 
     As illustrated in  FIGS. 1 and 2 , lower surface  31   b  which has vibrators  32   a  and  32   b  of detector  30  and land  36  formed thereon faces bottom surface  21   b  of recess  21   a  that is formed on the upper surface  21   c  of package  21 . Electrode pad  22  is provided at the bottom surface  21   b  of recess  21   a.  Land  36  and electrode pad  22  are electrically and mechanically joined to each other by melting and solidifying solder  40   b  on the surface of bump  40 . Electrode pad  51  of processor chip  50  is electrically and mechanically joined to electrode pad  23  provided on upper surface  21   c  of package  21  by applying ultrasound while interposing bump  52  between electrode pads  23  and  51  and applying pressure. 
     An operation of strain sensor  20  will be described below. When an alternating-current (AC) voltage having a frequency around natural frequency f1 of vibrator  32   a  (200 kHz according to the embodiment) is applied from processor chip  50  to drive element  33   a,  drive element  33  causes a mechanical vibration. The mechanical vibration causes vibrator  32   a  to start a string vibration in vertical direction D 32  at natural frequency f1. The string vibration is sensed by sensing elements  34   a  and  35   a,  and an AC signal having a frequency equal to natural frequency f1 is fed back from sensing elements  34   a  and  35   a  to processor chip  50 . This configuration allows vibrator  32   a  to continue the string vibration at a frequency equal to natural frequency f1. Similarly, when an AC voltage having a frequency around natural frequency f2 of vibrator  32   b  (165 kHz according to the embodiment) is applied from processor chip  50  to drive element  33   b,  drive element  33   b  causes a mechanical vibration. The mechanical vibration causes vibrator  32   b  to start a string vibration in vertical direction D 32  at natural frequency f2. The string vibration is sensed by sensing elements  34   b  and  35   b,  and an AC signal having a frequency equal to natural frequency f2 is fed back from sensing elements  34   b  and  35   b  to processor chip  50 . This configuration allows vibrator  32   b  to continue the string vibration at a frequency equal to natural frequency f2. 
     As illustrated in  FIG. 2 , when tensile force F in longitudinal direction  132   b  of vibrator  32   b  is applied to strain generating body  120 , vibrator  32   b  extends in longitudinal direction  132   b  and vibrator  32   a  contracts in longitudinal direction  132   a  of vibrator  32   a  by a length corresponding to a Poisson ratio of strain generating body  120 . This action increases the frequency of vibration of vibrator  32   a  from frequency f1 to frequency (f1+Δa), and decreases the frequency of vibration of vibrator  32   b  from frequency f2 to frequency (f2−Δb). When compressive force −F in longitudinal direction  132   b  is applied to strain generating body  120 , vibrator  32   b  contracts in longitudinal direction  132   b  and vibrator  32   a  extends in longitudinal direction  132   a  of vibrator  32   a  by a length corresponding to a Poisson ratio of strain generating body  120 . This action decreases the frequency of vibration of vibrator  32   a  from frequency f1 to frequency (f1−Δa), and increases the frequency of vibration of vibrator  32   b  from frequency f2 to frequency (f2+Δb). Processor chip  50  processes AC signals having the frequencies generated by drive element  33   a  of vibrator  32   a  and drive element  33   b  of vibrator  32   b,  and outputs a signal having difference δ between the frequencies of the AC signals. When tensile force F is applied to strain generating body  120 , difference δ between the frequencies is expressed as follows. 
       δ=( fa+Δa )−( fb−Δb )=( fa−fb )+(Δ a+Δb )
 
     Difference δ between the frequencies is larger than a change in frequency of vibration of a stand-alone vibrator. By measuring difference δ between the frequencies, strain sensor  20  can sensitively measure strain and load that are applied to strain generating body  120 . 
     A pair of bumps  40  out of plural bumps  40  are located on lower surface  31   b  of detector  30 , and arranged symmetrically to each other with respect to center line L 232  in which the beam shape of vibrator  32   b  extends. This configuration allows thermal stress due to a difference of thermal expansion coefficients of package  21  and detector  30  to be applied evenly to vibrator  32   b,  and suppresses fluctuations in temperature characteristics and sensitivity. Therefore, strain sensor  20  can sensitively detect a physical amount, such as a tensile force and a strain applied to strain generating body  120 . 
     For example, when an IC processor chip using silicon is used in a conventional strain sensor, detection accuracy may be degraded by expansion of signal errors or degradation of a signal during the signal process due to a piezoelectric effect or the like. 
     On the other hand, in strain sensor  20  according to the embodiment, vibrators  32   a  and  32   b  changing the frequency of vibration according to a tensile force or a strain are electrically and mechanically connected by the shortest distance via bump  40  to bottom surface  21   b  of recess  21   a  of package  21  that is connected to strain generating body  120 . Hence, the strain applied to strain generating body  120  is effectively transmitted to vibrators  32   a  and  32   b.  This configuration can secure a high S/N ratio even when the force applied to strain generating body  120  is small. Moreover, bump  40  including core  40   a  can secure a predetermined gap between bottom surface  21   b  of recess  21   a  of package  21  and each of vibrators  32   a  and  32   b,  hence not preventing the vibrations of vibrators  32   a  and  32   b.  By melting and solidifying solder  40   b  that covers the surface of core  40   a  of bump  40  with, e.g. a reflow furnace in a high temperature atmosphere, land  36  of detector  30  is electrically and mechanically joined to electrode pad  22  on bottom surface  21   b  of recess  21   a  of package  21 . Therefore, the above configuration provides smaller residual stress caused by a difference between thermal expansion coefficients of package  21  and detector  30  is applied more evenly to vibrators  32   a  and  32   b  than the connection with ultrasound adhesion using bump made of gold. Therefore, a variation of the frequency of vibration related to mounting can be almost zero. Strain sensor  20  according to the embodiment can secure a high S/N ratio even when a force applied to strain generating body  120  is small, and can sensitively detect the strain applied to strain generating body  120  by suppressing fluctuations in temperature characteristics and sensitivity. 
     Since processor chip  50  is disposed away from strain generating body  120  on upper surface  21   c  of package  21 , strain applied to strain generating body  120  can hardly be transmitted to processor chip  50 . Therefore, processor chip  50  can be connected to upper surface  21   c  of package  21  with bump  52  made of a general material, such as gold, instead of a material having high flexibility, and can increase connection reliability. Furthermore, since detector  30  and processor chip  50  are connected to package  21  with bump  40  and bump  52 , respectively, a bonding wire is not necessary. This configuration provides strain sensor  20  with a small size a thin profile, and allows strain sensor  20  to precisely detect the physical amount, such as a tensile force or a strain. 
       FIG. 3D  is an enlarged cross-sectional view of another strain sensor  420  according to the embodiment. In  FIG. 3D , components identical to those of strain sensor  20  shown in  FIG. 3C  are denoted by the same reference numerals. Strain sensor  420  includes bump  240  that connects land  36  of detector  30  to electrode pad  22  of package  21 , instead of bump  40  of strain sensor  20  shown in  FIG. 3C . Bump  240  includes core  240   a  and solder  240   b  that covers at least a part of the surface of core  240   a.  According to the embodiment, core  240   a  is made of metal, such as gold, and has a spherical shape. 
       FIG. 3E  is an enlarged cross-sectional view of still another strain sensor  320  according to the embodiment. In  FIG. 3E , components identical to those of strain sensor  20  shown in  FIG. 3C  are denoted by the same reference numerals. Strain sensor  320  includes bump  140  that connects land  36  of detector  30  to electrode pad  22  of package  21 , instead of bump  40  of strain sensor  10  shown in  FIG. 3C . Bump  140  includes thermosetting conductive adhesive  140   b  that contains spacer  140   a,  providing the same effect. 
     Bump  52  that joins processor chip  50  to upper surface  21   c  of package  21  may be made of a thermosetting conductive adhesive, providing the same effect. 
       FIG. 3F  is a bottom view of detector  530  of further strain sensor  520  according to the embodiment. In  FIG. 3F , components identical to those of detector  30  shown in  FIG. 3A  are denoted by the same reference numerals. Detector  530  of strain sensor  520  includes bumps  401  to  411  instead of bump  40  of detector  30  shown in  FIG. 3A . Each of bumps  401  to  411  of detector  530  of strain sensor  520  is the same as bump  40  of detector  30  shown in  FIG. 3A . The beam shapes of vibrators  32   a  and  32   b  extend slenderly along center lines L 132  and L 232 , respectively. Center lines L 132  and L 232  pass through center C 132  of the beam shape of vibrator  32   a  and center C 232  of the beam shape of vibrator  32   b,  respectively. Center line L 322  passes through center C 232  of vibrator  32   b  and crosses center line L 232  perpendicularly to center line L 232 . Center lines L 132  and L 232  cross at center C 132  of vibrator  32   a  perpendicularly to each other. Center lines L 132  and L 232  cross at center C 132  of vibrator  32   a  perpendicularly to each other. Bumps  401 ,  410 , and  409  are arranged symmetrically to bumps  402 ,  411 , and  408  with respect to center line L 132 , respectively. Bumps  401 ,  410 , and  409  are arranged symmetrically to bumps  408 ,  411 , and  402  with respect to center C 132 , respectively. Bumps  401  and  402  are arranged symmetrically to bumps  409  and  408  with respect to center line L 232 , respectively. Bumps  405 ,  410 , and  411  are arranged on center line L 232 . Bumps  403 ,  411 , and  407  are arranged symmetrically to bumps  404 ,  405 , and  406  with respect to center line L 332 , respectively. Bumps  403 ,  411 , and  407  are arranged symmetrically to bumps  406 ,  405 , and  404  with respect to center C 232  of vibrator  32   b,  respectively. Bumps  403  and  404  are arranged symmetrically to bumps  407  and  406  with respect to center line L 232 , respectively. This arrangement allows thermal stress due to a difference of thermal expansion coefficients of package  21  and detector  530  to be applied evenly to vibrator  32   b,  and suppresses fluctuations in temperature characteristics and sensitivity. Strain sensor  520  can precisely detect the physical amount, such as the tensile force and the strain, applied to strain generating body  120 . 
     In the embodiment, terms, such as “upper surface”, “lower surface”, and “upward”, indicating directions merely indicate relative directions that depend only on a relative positional relationship of structural components, such as package  21  and detector  30 , of strain sensor  20 , and do not indicate absolute directions, such as a vertical direction. 
     The strain sensors according to the embodiment are effective as a strain sensor having a small size and thin profile and precisely detecting a physical amount, such as a tensile force or a strain, and are useful for a strain sensor that detects a strain and load applied to an object. 
     REFERENCE MARKS IN THE DRAWINGS 
     
         
           20  strain sensor 
           21  package 
           30  detector 
           32   a,    32   b  vibrator 
           40  bump (first bump) 
           50  processor chip 
           52  bump (second bump)