Patent Publication Number: US-2022229019-A1

Title: Sensor device

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     The present application claims priority of Japanese Patent Application No. 2019-103238 filed in Japan on May 31, 2019, the entire disclosure of which is hereby incorporated by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a sensor device. 
     BACKGROUND ART 
     An elastic wave sensor including a sensitive film having adsorptivity to a measurement target substance on a propagation path of a surface acoustic wave is known (for example, see PTL 1). 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Unexamined Patent Application Publication No. 2008-122105 
     SUMMARY OF INVENTION 
     A sensor device according to an embodiment includes a substrate having a substrate surface, a first IDT electrode, a second IDT electrode, and a waveguide. The first IDT electrode and the second IDT electrode are positioned on the substrate surface. The waveguide is positioned on the substrate surface and between the first IDT electrode and the second IDT electrode. At least one of the first IDT electrode and the second IDT electrode includes a reference electrode and a signal electrode each including a plurality of electrode fingers, the plurality of electrode fingers being arranged in a juxtaposed manner in one direction. A distance between the at least one of the first IDT electrode and the second IDT electrode and the waveguide is shorter than an interval between the reference electrode and the signal electrode in the one direction. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram of a sensor device according to an embodiment. 
         FIG. 2  is a plan view of the sensor device according to an embodiment. 
         FIG. 3  is a sectional view taken along A-A of  FIG. 2 . 
         FIG. 4  is a schematic diagram illustrating an example of a composite wave of a SAW and a bulk wave. 
         FIG. 5A  illustrates a simulation result indicating a propagation state of the SAW in a case where a distance between a first IDT electrode and a waveguide is longer than ½ of the wavelength of the SAW. 
         FIG. 5B  illustrates a simulation result indicating a propagation state of the SAW in a case where the distance between the first IDT electrode and the waveguide is equal to ½ of the wavelength of the SAW. 
         FIG. 5C  illustrates a simulation result indicating a propagation state of the SAW in a case where the distance between the first IDT electrode and the waveguide is shorter than ½ of the wavelength of the SAW. 
         FIG. 6A  illustrates a simulation result indicating a propagation state of the SAW in a case where a distance between a second IDT electrode and the waveguide is longer than ½ of the wavelength of the SAW. 
         FIG. 6B  illustrates a simulation result indicating a propagation state of the SAW in a case where the distance between the second IDT electrode and the waveguide is equal to ½ of the wavelength of the SAW. 
         FIG. 6C  illustrates a simulation result indicating a propagation state of the SAW in a case where the distance between the second IDT electrode and the waveguide is shorter than ½ of the wavelength of the SAW. 
         FIG. 7  is a graph illustrating an example of measured data of a relationship between a distance between the IDT electrodes and the waveguide and a coefficient of variation of a sensitivity of the sensor device. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     &lt;Functions of SAW Sensor&gt; 
       FIG. 1  is a schematic diagram illustrating the configuration of a sensor device  1  according to an embodiment. The sensor device  1  according to an embodiment includes a substrate  10 , a first IDT (interdigital transducer) electrode  11 , a second IDT electrode  12 , and a waveguide  20 . The sensor device  1  functions as a SAW sensor that can detect a detection target on the basis of a change in the propagation characteristics of a surface acoustic wave (SAW)  70 . 
     The sensor device  1  inputs an electric signal to the first IDT electrode  11 . The first IDT electrode  11  can transmit the surface acoustic wave  70  that propagates along the substrate  10  on the basis of the input electric signal. The second IDT electrode can receive the surface acoustic wave  70  and can convert the surface acoustic wave  70  into an electric signal. 
     In the sensor device  1  according to an embodiment, the first IDT electrode  11 , the second IDT electrode  12 , and the waveguide  20  are positioned on the substrate  10 . In addition, the waveguide  20  is positioned between the first IDT electrode  11  and the second IDT electrode  12 . Thus, the propagation path of the surface acoustic wave  70  includes a surface of the substrate  10  and the waveguide  20  positioned on the surface of the substrate  10 . That is, the SAW  70  propagates from the first IDT electrode  11  to the second IDT electrode  12  through the waveguide  20 . In other words, the waveguide  20  is positioned in part of the propagation path of the SAW  70 . The electric signal herein is an electric signal for allowing the sensor device  1  to function. The electric signal may include, for example, a voltage signal, a current signal, and the like. 
     Note that, although omitted from the illustration, the sensor device  1  includes a control unit that can control the entire sensor device  1 , such as the input/output of an electric signal and various calculations based on the electric signal. The control unit may be configured by a known method of the related art. 
     The SAW  70  propagates with predetermined propagation characteristics. The propagation characteristics of the SAW  70  are determined on the basis of the state of the propagation path. The sensor device  1  can detect a change in the state of the propagation path by measuring a change in the propagation characteristics of the SAW  70 . The propagation characteristics include, for example, the propagation velocity, phase, amplitude, period, wavelength, and the like of the surface acoustic wave  70 . 
     The sensor device  1  according to an embodiment can detect a detection target that is present in the propagation path on th basis of the measured phase of the SAW  70 . 
     The phase of the transmitted SAW  70  changes, for example, depending on the state of the propagation path. Specifically, for example, if a substance is present in the propagation path, depending on a change in the mass, viscosity, density, or the like of the substance, the propagation velocity of the SAW  70  changes. In this case, in the surface acoustic wave  70  received by the second IDT electrode  12 , a difference is generated in the phase in accordance with a difference in the propagation velocity from the SAW  70  transmitted by the first IDT electrode  11 . Thus, on the basis of the magnitude of the phase difference of the SAW  70 , the sensor device  1  can measure the mass, viscosity, density, or the like of the substance that is the detection target present in the propagation path. 
     In this case, a calibration curve that specifies a relationship between the phase difference and the mass or the like of the substance that is the detection target may be prepared in advance. Specifically, by measuring a known substance having a known mass or the like and by measuring the phase difference in this case, the sensor device  1  acquires the relationship between the known mass or the like and the measured phase difference as the calibration curve. On the basis of the calibration curve, the sensor device  1  can convert the phase difference into the mass or the like of the substance that is the detection target present in the propagation path. 
     The waveguide  20  is a region that comes into contact with a specimen  60  in the propagation path of the SAW  70 . In the waveguide  20 , a substance (reactive substance) that can react with detection target in the specimen  60  is positioned. Detection targets herein are, for example, antibodies  51 , antigens  61 , and substrates. In this case, respective reactive substances are, for example, antigens  61 , antibodies  51 , and enzymes. In addition, for example, if the detection target is the antigens  61 , the reactive substance may be the same antigens  61  as the detection target or may be an analogue having an epitope that is similar to that of the detection target. In this case, the detection target and antibodies  51  are made to react with each other in advance, and then unreacted antibodies  51  are made to react with the antigens  61  that are the reactive substance or with an analogue, and thereby, the detection target can be detected indirectly. Note that combinations of the detection target and the reactive substance are not limited to these examples as long as the propagation velocity of the SAW  70  can be changed in the waveguide  20  to generate a phase difference. For example, if a specific molecule is wished to be a detection target, an aptamer that is designed to be bound to the specific molecule may be used as the reactive substance. The detection target of the sensor device  1  according to an embodiment is the antigens  61 , and the reactive substance is the antibodies  51 . 
     The reactive substance may be immobilized on the waveguide  20 . In this case, the waveguide  20  may be formed of, for example, Au, Pt, Ti, or the like. Note that the waveguide  20  is not limited to these examples as long as it is a metal that can immobilize the reactive substance on the substrate  10 . The waveguide  20  may also be formed of any given material that has oxidation resistance and corrosion resistance against contact with the specimen  60 . In addition, the reactive substance may be directly immobilized on the surface of the substrate  10 . In the sensor device  1  according to an embodiment, the waveguide  20  is formed of Au. 
     The sensor device  1  according to an embodiment includes a first channel including a pair of a first IDT electrode  11 - 1  and a second IDT electrode  12 - 1  and a waveguide  20 - 1 . The sensor device  1  includes a second channel including a pair of a first IDT electrode  11 - 2  and a second IDT electrode  12 - 2  and a waveguide  20 - 2 . Note that the number of channels of the sensor device  1  is not limited to two and may be one, or three or more. 
     The sensor device  1  detects the phase difference of the SAW  70  in each of the first channel and the second channel. The phase differences detected in the first channel and the second channel will be called a first phase difference and a second phase difference, respectively. 
     The waveguide  20 - 2  in the second channel has the antibodies  51  on a surface thereof. The antibodies  51  react with specific antigens  61  that are the detection target. By reacting with the antigens  61 , the antibodies  51  can be bound to the antigens  61  to form complexes  52 . That is, after the specimen  60  is supplied, on the surface of the waveguide  20 - 2  in the second channel, both the antibodies  51  and the complexes  52  may be present. The complexes  52  on the waveguide  20 - 2  in the second channel may be ones generated through the reaction between the antibodies  51  that the surface of the waveguide  20  has had and the antigens  61  included in the specimen  60  that has been supplied to the waveguide  20 . On the other hand, the waveguide  20 - 1  in the first channel has no antibodies  51  on a surface thereof. 
     The complexes  52  include the antigens  61 . That is, by the antibodies  51  bound to the antigens  61 , the mass of the antigens  61  is further added to the substrate  10 . In addition, by the antibodies  51  bound to the antigens  61 , the density in the vicinity of the surface of the waveguide  20 - 2  increases. Thus, as the proportion of the complexes  52  in the waveguide  20 - 2  increases, the change in the state of the propagation path in the second channel increases. That is, in accordance with the change in the proportion of the complexes  52 , the propagation velocity of the SAW  70  that propagates through the waveguide  20 - 2  decreases, and thus, the phase difference to be detected also increases. 
     The antibodies  51  may be replaced with aptamers. The aptamers include nucleic acid molecules, peptides, and the like that specifically bind to specific molecules that are the detection target. When the waveguide  20  has aptamers on the surface thereof, by the aptamers bound to the specific molecules, the mass of the specific molecules is further added to the substrate  10 . In addition, the density in the vicinity of the surface of the waveguide  20  increases. The antibodies  51  may also be replaced with enzymes. For example, when the enzymes form reactants by reacting with the substrate of the detection target, by the reactants deposited on the waveguide  20 , the mass of the reactants is further added to the substrate  10 . In addition, the density in the vicinity of the surface of the waveguide  20  increases. The antibodies  51  are not limited to these examples and may be replaced with another element that can react with a substance of the detection target or that can bind to a substance of the detection target. 
     The first channel of the sensor device  1  can allow the SAW  70  to propagate through the waveguide  20  and can detect the first phase difference. The second channel of the sensor device  1  can allow the SAW  70  to propagate through the waveguide  20  including the antibodies  51  or the complexes  52  and can detect the second phase difference. The second phase difference is a phase difference in accordance with the proportion of the complexes  52  included in the waveguide  20 . 
     The sensor device  1  can use the first phase difference as a reference value. That is, the sensor device  1  may correct the result of detection by subtracting the first phase difference from the second phase difference. Thus, the sensor device  1  can reduce effects of noise, such as variation of the phase differences based on the temperature characteristics of the substrate  10 . On the basis of the corrected result of detection of the second phase difference, the sensor device  1  may calculate the amount, concentration, density, or the like of the antigens  61  included in the specimen  60 . In this case, a calibration curve that specifies the relationship between the second phase difference and the amount, concentration, density, or the like of the antigens  61  may be prepared in advance. On the basis of the calibration curve, the sensor device  1  may covert the second phase difference into the amount, concentration, density, or the like of the antigens  61 . The specimen  60  may include, for example, human body fluids such as blood and urine. The specimen  60  is not limited to this and may include any appropriate chemical substance. The specimen  60  may be preprocessed before the specimen  60  is introduced into a channel of the sensor device  1 . 
     The reaction through which the antibodies  51  bind to the antigens  61  to become the complexes  52  progresses with a predetermined reaction velocity. Accordingly, in accordance with the elapsed time after introduction of the specimen  60  into the channel, the proportion of the complexes  52  included in the waveguide  20  increases. As a result, the phase difference that the sensor device  1  detects in the channel increases in accordance with the elapsed time. 
     The phase difference becomes constant when the reaction between all of the antigens  61  in the specimen  60  and the antibodies  51  finishes. Thus, for example, the sensor device  1  can calculate the phase difference after an elapse of a sufficiently long time since the introduction of the specimen  60  into the channel, the time being necessary to finish the reaction between the antibodies  51  and the antigens  61 . On the basis of the calculated phase difference, the sensor device  1  may calculate the amount of the antigens  61 . In addition, since the phase difference increases at a constant rate, the sensor device  1  may calculate the amount of the antigens  61  on the basis of a phase difference after an elapse of a certain time since the start of detection. In addition, when the change in the phase difference is regarded as a time function, the sensor device  1  may calculate the amount of the antigens  61  on the basis of the inclination of a function curve after an elapse of a certain time since the start of detection. 
     The sensor device  1  may detect a period of time from input of an electric signal to the first IDT electrode  11  to detection of the electric signal by the second IDT electrode  12 . The sensor device  1  may detect a change in the state of the vicinity of the surface of the waveguide  20  by detecting a change in the propagation velocity by calculating the propagation velocity on the basis of the period of time from input of the electric signal to detection of the electric signal and the distance between the electrodes. Note that the sensor device  1  may detect, as propagation characteristics, a change in the amplitude of the SAW  70  or a plurality of characteristics. 
     &lt;Configuration of SAW Sensor&gt; 
     Referring to  FIGS. 2 and 3 , each element of the sensor device  1  will be described in detail. As described above, the sensor device  1  includes the substrate  10 , the first IDT electrode  11 , the second IDT electrode  12 , and the waveguide  20 . The sensor device  1  further includes a protective film  30 . 
     The substrate  10  has a substrate surface  10   a . The substrate  10  is, for example, a plate-like member. The substrate  10  may be formed of, for example, quartz, lithium tantalate, lithium niobate, or the like. Note that the substrate  10  is not limited to these examples and may be formed of a material that causes a piezoelectric phenomenon. In an embodiment, the substrate  10  is formed of quartz. 
     The first IDT electrode  11  and the second IDT electrode  12  are positioned on the substrate surface  10   a . The first IDT electrode  11  and the second IDT electrode  12  are, for example, formed as a layer. The first IDT electrode  11  and the second IDT electrode  12  may be formed of, for example, a metal alone, such as gold (Au) or aluminum (Al). The first IDT electrode  11  and the second IDT electrode  12  may also be formed of two or more materials, such as an alloy (AlCu) of aluminum (Al) and copper (Cu). Note that the material for forming the first IDT electrode  11  and the second IDT electrode  12  are not limited to these examples as long as the material functions as an electrode. In an embodiment, the first IDT electrode  11  and the second IDT electrode  12  are formed of Au. 
     On a side surface of the I-th DT electrode  11 , the upper end of the side surface may be positioned in an inner direction of the first IDT electrode  11  than the lower end of the side surface. In addition, the side surface of the first IDT electrode  11  may be tilted. On a side surface of the second IDT electrode  12 , the upper end of the side surface may be positioned in an inner direction of the second IDT electrode  12  than the lower end of the side surface. In addition, the side surface of the second IDT electrode  12  may be tilted. The upper surface of the first IDT electrode  11  may be smaller than the lower surface of the first IDT electrode  11 . The upper surface of the second IDT electrode  12  may be smaller than the lower surface of the second IDT electrode  12 . In addition, when the side surface of the first IDT electrode  11  is tilted, a side-surface region may be smaller than an upper-surface region in a top view of the first IDT electrode  11 . In addition, when the side surface of the second IDT electrode  12  is tilted, a side-surface region may be smaller than an upper-surface region in a top view of the second IDT electrode  12 . 
     The first IDT electrode  11  may have a substrate-side close-contact layer  15  between the first IDT electrode  11  and the substrate surface  10   a , and the second IDT electrode  12  may have the substrate-side close-contact layer  15  between the second IDT electrode  12  and the substrate surface  10   a . The first IDT electrode  11  and the second IDT electrode  12  may have a protective-film-side close-contact layer  17  between the protective film  30  and surfaces thereof on a side opposite to a side facing the substrate surface  10   a . Each of the first IDT electrode  11  and the second IDT electrode  12  can be stably in close contact with the substrate  10  and the protective film  30  with the substrate-side close-contact layer  15  and the protective-film-side close-contact layer  17  interposed therebetween. The substrate-side close-contact layer  15  and the protective-film-side close-contact layer  17  may be formed of, for example, titanium (Ti), chromium (Cr), or the like. The substrate-side close-contact layer  15  and the protective-film-side close-contact layer  17  may also be formed of mutually different materials. Specifically, the substrate-side close-contact layer  15  may be Ti, and the protective-film-side close-contact layer  17  may be Cr. Note that the substrate-side close-contact layer  15  and the protective-film-side close-contact layer  17  are not limited to these examples as long as the first IDT electrode  11  and the second IDT electrode  12  can be in close contact with the substrate  10  and the protective film  30 . In an embodiment, the substrate-side close-contact layer  15  and the protective-film-side close-contact layer  17  are each formed of Ti. 
     The first IDT electrode  11  includes a first reference electrode  11 G and a first signal electrode  11 A to which voltage is applied. The first reference electrode  11 G and the first signal electrode  11 A are positioned to face each other on the substrate  10   a . The sensor device  1  applies a voltage signal between the first reference electrode  11 G and the first signal electrode  11 A to generate the SAW  70  in the first IDT electrode  11 . The first reference electrode  11 G may be grounded. The SAW  70  is generated between the first reference electrode  11 G and the first signal electrode  11 A. The interval between the first reference electrode  11 G and the first signal electrode  11 A is denoted by W 1 . In the range of the length denoted by W 1 , the SAW  70  has an energy that is larger than that in the other range. 
     The second IDT electrode  12  includes a second reference electrode  12 G and a second signal electrode  12 A to which voltage is applied. The second reference electrode  12 G and the second signal electrode  12 A are positioned to face each other on the substrate surface  10   a . The sensor device  1  detects an electric signal generated by the SAW  70  that has propagated, by using the second reference electrode  12 G and the second signal electrode  12 A. The second reference electrode  12 G may be grounded. The SAW  70  propagates between the second reference electrode  12 G and the second signal electrode  12 A. The interval between the second reference electrode  12 G and the second signal electrode  12 A is denoted by W 2 . The SAW  70  that has propagated in the range of the length denoted by W 2  generates a larger electric signal in the second IDT electrode  12  than the SAW  70  that has propagated in the other range. That is, the second IDT electrode  12  can detect the SAW  70  efficiently in the range denoted by W 2 . 
     The waveguide  20  is formed as a film, for example. The planar shape of the waveguide  20  may be, for example, a square, a rectangle, a parallelogram, a rhombate, or the like. Note that the planar shape of the waveguide  20  is not limited to these examples as long as a sufficient amount of the reactive substance can be immobilized. In an embodiment, the planar shape of the waveguide  20  is a square. 
     On a side surface of the waveguide  20 , the side surface facing the first IDT electrode  11 , the upper end of the side surface may be positioned in an inner direction of the waveguide  20  than the lower end of the side surface. In addition, the side surface of the waveguide  20 , the side surface facing the first IDT electrode  11 , may be tilted. On a side surface of the waveguide  20 , the side surface facing the second IDT electrode  12 , the upper end of the side surface may be positioned in an inner direction of the waveguide  20  than the lower end of the side surface. In addition, the side surface of the waveguide  20 , the side surface facing the second IDT electrode  12 , may be tilted. The upper surface of the waveguide  20  may be smaller than the lower surface of the waveguide  20 . In this case, all side surfaces of the waveguide  20  may be tilted. In addition, when a side surface is tilted, a side-surface region may be smaller than an upper-surface region in a top view of the waveguide  20 . 
     The waveguide  20  is positioned on the substrate surface  10   a  and between the first IDT electrode  11  and the second IDT electrode  12 . In an embodiment, the distance between a side of the waveguide  20  facing the first IDT electrode  11  and a side of the first IDT electrode  11  facing the waveguide  20  is constant. That is, the sides are in parallel to each other. In addition, in an embodiment, the distance between a side of the waveguide  20  facing the second IDT electrode  12  and a side of the second IDT electrode  12  facing the waveguide  20  is constant. That is, the sides are in parallel to each other. 
     Note that the distance between the side of the waveguide  20  facing the first IDT electrode  11  and the side of the first IDT electrode  11  facing the waveguide  20 , and the distance between the side of the waveguide  20  facing the second IDT electrode  12  and the side of the second IDT electrode  12  facing the waveguide  20  is may be different from each other. 
     The waveguide  20  and at least one of the first reference electrode  11 G and the second reference electrode  12 G may be formed as a single unit. That is, the waveguide  20  may be connected to at least one of the first reference electrode  11 G and the second reference electrode  12 G. The potential of the waveguide  20  may be equal to the potential of at least one of the first reference electrode  11 G and the second reference electrode  12 G. The potential of the waveguide  20  may be floating. 
     If the waveguide  20  and the first reference electrode  11 G and the second reference electrode  12 G are formed as a single unit, the waveguide  20  is formed of the same material as that of the first reference electrode  11 G and the second reference electrode  12 G. In addition, if the waveguide  20  is formed as a different unit from the first reference electrode  11 G and the second reference electrode  12 G, the waveguide  20  may be formed of the same material as or a different material from that of the first reference electrode  11 G and the second reference electrode  12 G. 
     The waveguide  20  has a surface  20   a  on a side opposite to a side facing the substrate surface  10   a . For example, the antibodies  51  may be immobilized on the surface  20   a . The waveguide  20  may include the substrate-side close-contact layer  15  between the waveguide  20  and the substrate surface  10   a . The substrate-side close-contact layer  15  of the waveguide  20  may be formed as a single unit with or a different unit from the substrate-side close-contact layer  15  of the first IDT electrode  11  and the second IDT electrode  12 . 
     The protective film  30  covers the substrate surface  10   a , the first IDT electrode  11 , and the second IDT electrode  12 . The protective film  30  may be formed of a TEOS oxide film. The TEOS oxide film is a silicon oxide film deposited by a plasma CVD (Chemical Vapor Deposition) using tetraethoxysilane as a material gas. Note that the protective film  30  is not limited to the above example as long as the material has an insulating property. 
     In an embodiment, a side wall  30   a  of the protective film  30  may intersect with the substrate surface  10   a  and may define the opening of the protective film  30 . That is, the substrate surface  10   a  may have a region that is not covered with the protective film  30 . In an embodiment, the waveguide  20  is positioned in the opening of the protective film  30 . That is, the protective film  30  does not cover the surface  20   a  of the waveguide  20 . 
     The sensor device  1  desirably detects a change in the state of the vicinity of the surface  20   a  of the waveguide  20  with high accuracy. The SAW  70  propagates through a region including the vicinity of the surface  20   a  of the waveguide  20 . The more the energy of the SAW  70  propagating through the waveguide  20  is distributed in the vicinity of the surface  20   a , the more influenced the SAW  70  is from the state of the vicinity of the surface  20   a  of the waveguide  20 . Thus, a correlation between the change in the propagation characteristics of the SAW  70  and the change in the state of the vicinity of the surface  20   a  of the waveguide  20  becomes strong. 
     The energy of the SAW  70  may concentrate to the vicinity of the surface of the waveguide  20 . The width of the waveguide  20  in the direction intersecting with the direction in which the SAW  70  propagates is denoted by W 3  in  FIG. 2 . W 3  may be longer than or equal to W 1  and W 2 . Thus, the surface acoustic wave  70  may efficiently propagate from the first IDT electrode  11  to the vicinity of the surface of the waveguide  20 . 
     If W 3  is shorter than W 1  and W 2 , in the energy of the SAW  70  that propagates from the first IDT electrode  11  to the second IDT electrode  12 , the proportion of the energy of the SAW  70  that propagates through the waveguide  20  decreases. Thus, the detection sensitivity of the sensor device  1  decreases. On the other hand, by W 3  being longer than W 1  and W 2 , the proportion of the energy of the SAW  70  that propagates through the waveguide  20  increases. Thus, the detection sensitivity of the sensor device  1  may be improved. 
     Each of the first reference electrode  11 G and the first signal electrode  11 A of the first IDT electrode  11  includes a first common electrode  11 Y and a plurality of first electrode fingers  11 X. In a plan view, the first common electrodes  11 Y extend in one direction (in the present disclosure, the direction in which the SAW  70  propagates or the direction in which the first IDT electrode and the second IDT electrode are arranged in a juxtaposed manner), and the plurality of first electrode fingers  11 X extend from the first common electrodes  11 Y in a direction intersecting with the one direction. The first common electrodes  11 Y are a pair of electrodes facing each other. In addition, the first electrode fingers  11 X are an electrode part that branches off and protrudes from the first common electrode  11 Y of the first reference electrode  11 G and the first common electrode  11 Y of the first signal electrode  11 A. The first electrode fingers  11 X of the first reference electrode  11 G protrude toward the first common electrode  11 Y of the first signal electrode  11 A, and the first electrode fingers  11 X of the first signal electrode  11 A protrude toward the first common electrode  11 Y of the first reference electrode  11 G. In addition, each of the first electrode fingers  11 X protrudes in a direction that goes straight to the direction in which the SAW  70  propagates. Furthermore, the first electrode fingers  11 X of the first reference electrode  11 G and the first electrode fingers  11 X of the first signal electrode  11 A are arranged in a juxtaposed manner in the one direction. 
     The planar shape of the first common electrode  11 Y and the first electrode fingers  11 X of the first reference electrode  11 G, and of the first common electrode  11 Y and the first electrode fingers  11 X of the first signal electrode  11 A, may be, for example, a square, a rectangle, or the like. Note that the shape of the first common electrodes  11 Y and the first electrode fingers  11 X is not limited to this as long as the SAW  70  can be transmitted. In an embodiment, the planar shape of the first common electrode  11 Y and the first electrode fingers  11 X of the first reference electrode  11 G, and of the first common electrode  11 Y and the first electrode fingers  11 X of the first signal electrode  11 A, is a rectangle. 
     A sectional shape of the first common electrode  11 Y and the first electrode fingers  11 X of the first reference electrode  11 G, and of the first common electrode  11 Y and the first electrode fingers  11 X of the first signal electrode  11 A, may be, for example, a square, a rectangle, a trapezoid, or the like. Note that the shape of the first common electrodes  11 Y and the first electrode fingers  11 X is not limited to this as long as the SAW  70  can be transmitted. In an embodiment, the sectional shape of the first common electrode  11 Y and the first electrode fingers  11 X of the first reference electrode  11 G, and of the first common electrode  11 Y and the first electrode fingers  11 X of the first signal electrode  11 A, is a rectangle. Note that side surfaces of the pair of first common electrodes  11 Y, the side surfaces facing each other, may be tilted in an inner direction such that the distance between the pair of first common electrodes  11 Y increases. In addition, side surfaces of the plurality of first electrode fingers  11 X, the side surfaces facing each other, may be tilted in an inner direction such that the distance between the plurality of first electrode fingers  11 X increases. In addition, a side surface of a first electrode finger  11 X facing the waveguide  20 , the side surface facing the waveguide  20 , may be tilted in an inner direction such that the distance between the first electrode finger  11 X and the waveguide  20  increases. 
     In an embodiment, the first electrode fingers  11 X of the first reference electrode  11 G and the first electrode fingers  11 X of the first signal electrode  11 A are alternately positioned two by two in a direction from the waveguide  20  toward the first IDT electrode  11 . Thus, since multiple reflection between the electrodes is reduced, the sensor device  1  can reduce amplitude ripple and group delay ripple. That is, the sensor device  1  can improve the detection accuracy. Note that, alternatively, the first electrode fingers  11 X of the first reference electrode  11 G and the first electrode fingers  11 X of the first signal electrode  11 A may be alternately positioned one by one. In this case, each IDT electrode can be formed simply. 
     A pair (first pair) of two first electrode fingers  11 X of the first reference electrode  11 G and two first electrode fingers  11 X of the first signal electrode  11 A is arranged in a juxtaposed manner in the one direction at a first pitch P 1 . The first pitch P 1  corresponds to a repetition cycle of each of the two first electrode fingers  11 X of the first reference electrode  11 G and the two first electrode fingers  11 X of the first signal electrode  11 A. The number of first pairs is not limited to one and may be two or more. 
     A half pitch of the first pitch P 1  is referred to as a first half pitch HP 1 . The first half pitch HP 1  corresponds to an interval in the one direction from an end of the initial one of the first electrode fingers  11 X of the first reference electrode  11 G to an end of the initial one of the first electrode fingers  11 X of the first signal electrode  11 A in the first pair. The first half pitch HP 1  is also referred to as a predetermined interval. 
     As described above, the first IDT electrode  11  generates the SAW  70  on the surface of the substrate  10  on the basis of an electric signal that is input to the first reference electrode  11 G and the first signal electrode  11 A. The wavelength of the SAW  70  generated in the first IDT electrode  11  corresponds to the first pitch P 1 . 
     Each of the second reference electrode  12 G and the second signal electrode  12 A of the second IDT electrode  12  includes a second common electrode  12 Y and a plurality of second electrode fingers  12 X. In a plan view, the second common electrodes  12 Y extend in the one direction (in the present disclosure, the direction in which the SAW  70  propagates or the direction in which the first IDT electrode and the second IDT electrode are arranged in a juxtaposed manner), and the plurality of second electrode fingers  12 X extend from the second common electrodes  12 Y in the direction intersecting with the one direction. The second common electrodes  12 Y are a pair of electrodes facing each other. The second electrode fingers  12 X are an electrode part that branches off and protrudes from the second common electrode  12 Y of the second reference electrode  12 G and the second common electrode  12 Y of the second signal electrode  12 A. The second electrode fingers  12 X of the second reference electrode  12 G protrude toward the second common electrode  12 Y of the second signal electrode  12 A, and the second electrode fingers  12 X of the second signal electrode  12 A protrude toward the second common electrode  12 Y of the second reference electrode  12 G. In addition, each of the second electrode fingers  12 X protrudes in the direction that goes straight to the direction in which the SAW  70  propagates. Furthermore, the second electrode fingers  12 X of the second reference electrode  12 G and the second electrode fingers  12 X of the second signal electrode  12 A are arranged in the juxtaposed manner in the one direction. 
     The planar shape of the second common electrode  12 Y and the second electrode fingers  12 X of the second reference electrode  12 G, and of the second common electrode  12 Y and the second electrode fingers  12 X of the second signal electrode  12 A, may be, for example, a square, a rectangle, or the like. Note that the shape of the second electrode fingers  12 X is not limited to this as long as the SAW  70  can be received. In an embodiment, the planar shape of the second electrode fingers  12 X of the second reference electrode  12 G, and of the second electrode fingers  12 X of the second signal electrode  12 A, is a rectangle. 
     A sectional shape of the second common electrode  12 Y and the second electrode fingers  12 X of the second reference electrode  12 G, and of the second common electrode  12 Y and the second electrode fingers  12 X of the second signal electrode  12 A, may be, for example, a square, a rectangle, a trapezoid, or the like. Note that the shape of the second common electrodes  12 Y and the second electrode fingers  12 X is not limited to this as long as the SAW  70  can be transmitted. In an embodiment, the sectional shape of the second common electrode  12 Y and the second electrode fingers  12 X of the second reference electrode  12 G, and of the first common electrode  11 Y and the second electrode fingers  12 X of the second signal electrode  12 A, is a rectangle. Note that side surfaces of the pair of second common electrodes  12 Y, the side surfaces facing each other, may be tilted in an inner direction such that the distance between the pair of second common electrodes  12 Y increases. In addition, side surfaces of the plurality of second electrode fingers  12 X, the side surfaces facing each other, may be tilted in an inner direction such that the distance between the plurality of second electrode fingers  12 X increases. In addition, a side surface of a second electrode finger  12 X facing the waveguide  20 , the side surface facing the waveguide  20 , may be tilted in an inner direction such that the distance between the second electrode finger  12 X and the waveguide  20  increases. 
     In an embodiment, the second electrode fingers  12 X of the second reference electrode  12 G and the second electrode fingers  12 X of the second signal electrode  12 A are alternately positioned two by two in a direction from the waveguide  20  toward the second IDT electrode  12 . Note that, alternatively, the second electrode fingers  12 X of the second reference electrode  12 G and the second electrode fingers  12 X of the second signal electrode  12 A may be alternately positioned one by one. 
     A pair (second pair) of two second electrode fingers  12 X of the second reference electrode  12 G and two second electrode fingers  12 X of the second signal electrode  12 A is arranged in a juxtaposed manner in the one direction at a second pitch P 2 . The second pitch P 2  corresponds to a repetition cycle of each of the two second electrode fingers  12 X of the second reference electrode  12 G and the two second electrode fingers  12 X of the second signal electrode  12 A. The number of second pairs is not limited to one and may be two or more. 
     A half pitch of the second pitch P 2  is referred to as a second half pitch HP 2 . The second half pitch HP 2  corresponds to an interval in the one direction from an end of the initial one of the second electrode fingers  12 X of the second reference electrode  12 G to an end of the initial one of the second electrode fingers  12 X of the second signal electrode  12 A in the second pair. 
     As described above, the second IDT electrode  12  outputs the electric signal based on the SAW  70  that has propagated from the first IDT electrode  11  through the waveguide  20  to the second reference electrode  12 G and the second signal electrode  12 A. The closer the wavelength of the SAW  70  is to the second pitch P 2 , the higher the efficiency is at which the second IDT electrode  12  converts the SAW  70  into the electric signal. In other words, the smaller the difference between the first pitch P 1  and the second pitch P 2  is, the higher the efficiency is at which the second IDT electrode  12  converts the SAW  70  into the electric signal. In an embodiment, the sensor device  1  is configured such that the first pitch P 1  and the second pitch P 2  correspond to each other. 
     The SAW  70  generated in the first IDT electrode  11  propagates to the waveguide  20 . The distance between an end of the first IDT electrode  11  on the waveguide  20  side and an end of the waveguide  20  on the first IDT electrode  11  side is denoted by D 1 . In an embodiment, the first IDT electrode  11  is configured such that the first electrode fingers  11 X of the first reference electrode  11 G are positioned on the waveguide  20  side. That is, a first electrode finger  11 X that faces or is adjacent to the waveguide  20  may be a first electrode finger  11 X of the first reference electrode  11 G. Thus, generation of direct waves serving as noise can be reduced. That is, the sensor device  1  can improve the detection accuracy. In this case, the end of the first IDT electrode  11  on the waveguide  20  side corresponds to the end of the first electrode fingers  11 X of the first reference electrode  11 G on the waveguide  20  side. If the first reference electrode  11 G and the waveguide  20  are connected to each other, D 1  is 0. 
     The first IDT electrode  11  may also be configured such that the first electrode fingers  11 X of the first signal electrode  11 A are positioned on the waveguide  20  side. That is, a first electrode finger  11 X that faces or is adjacent to the waveguide  20  may be a first electrode finger  11 X of the first signal electrode  11 A. Thus, since the SAW  70  is generated from a position close to the waveguide  20  compared with a case where the first electrode fingers  11 X of the first reference electrode  11 G are positioned on the waveguide  20  side, the propagation distance of the SAW  70  can be made shorter. That is, since attenuation of the SAW  70  can be reduced, the sensor device  1  can improve the detection accuracy. In this case, the end of the first IDT electrode  11  on the waveguide  20  side corresponds to the end of the first electrode fingers  11 X of the first signal electrode  11 A on the waveguide  20  side. If the first signal electrode  11 A and the waveguide  20  are connected to each other, D 1  is 0. 
     The SAW  70  propagates from the waveguide  20  to the second IDT electrode  12 . The distance between an end of the second IDT electrode  12  on the waveguide  20  side and an end of the waveguide  20  on the second IDT electrode  12  side is denoted by D 2 . In  FIG. 2 , the second IDT electrode  12  is configured such that the second reference electrode  12 G are positioned on the waveguide  20  side. That is, a second electrode finger  12 X that faces or is adjacent to the waveguide  20  may be a second electrode finger  12 X of the second reference electrode  12 G. In this case, the end of the second IDT electrode  12  on the waveguide  20  side corresponds to the end of the second reference electrode  12 G on the waveguide  20  side. If the second reference electrode  12 G and the waveguide  20  are connected to each other, D 2  is 0. 
     The second IDT electrode  12  may also be configured such that the second signal electrode  12 A is positioned on the waveguide  20  side. That is, a second electrode finger  12 X that faces or is adjacent to the waveguide  20  may be a second electrode finger  12 X of the second signal electrode  12 A. In this case, the end of the second IDT electrode  12  on the waveguide  20  side corresponds to the end of the second signal electrode  12 A on the waveguide  20  side. If the second signal electrode  12 A and the waveguide  20  are connected to each other, D 2  is 0. 
     The sensor device  1  according to an embodiment includes the substrate  10  having the substrate surface  10   a , the first IDT electrode, the second IDT electrode, and the waveguide  20 . The first IDT electrode and the second IDT electrode are positioned on the substrate surface  10   a . The waveguide  20  is positioned on the substrate surface  10   a  and between the first IDT electrode and the second IDT electrode. At least one of the first IDT electrode and the second IDT electrode includes a reference electrode and a signal electrode each including a plurality of electrode fingers, the plurality of electrode fingers being arranged in a juxtaposed manner in the one direction. The distance between the at least one of the first IDT electrode and the second IDT electrode and the waveguide  20  is shorter than an interval between the reference electrode and the signal electrode in the one direction. 
     Here, in an elastic wave sensor of the related art, part of the surface acoustic wave  70  diffuses inside the substrate  10  and becomes an unnecessary elastic wave whose propagation velocity does not change on the basis of the detection target. This unnecessary elastic wave is also referred to as a bulk wave. 
     The bulk wave propagates while receiving various effects inside the substrate  10 . For example, the bulk wave may receive effects of random phenomena such as thermal vibration of a crystal lattice and a stress inside the crystal. That is, the propagation characteristics of the bulk wave may change randomly. On the other hand, the surface acoustic wave  70  is unlikely to receive effects of random phenomena that occur inside the substrate  10 . Thus, in the sensor device  1  according to an embodiment, when the SAW  70  propagates through the waveguide  20  to the second IDT electrode  12 , the propagation velocity of the SAW  70  may change in accordance with the state of the surface  20   a  of the waveguide  20  without receiving effects of the state of the inside of the substrate  10 . 
     The SAW  70  and the bulk wave may each be indicated by a vector as illustrated in  FIG. 4 . A vector Vs indicates the phase and amplitude of the SAW  70 . A vector Vn indicates the phase and amplitude of the bulk wave. The circle illustrated by the broken line indicates that the phase of the bulk wave can be a value from −π to +π. A vector Vs+Vn is a composite vector of Vs and Vn and indicates the phase and amplitude of a composite wave of the SAW  70  and the bulk wave. The angle between the vector Vn and the vector Vs+Vn corresponds to a phase difference between the SAW  70  and the composite wave and is denoted by δ. As the vector Vn is longer, δ varies more greatly. The length of the vector Vn corresponds to the magnitude of the amplitude of the bulk wave, that is, the magnitude of the energy of the bulk wave. Thus, as the energy of the bulk wave increases, δ indicating the phase difference of the composite wave with respect to the SAW  70  varies more greatly. In the example in  FIG. 4 , when a possible maximum of 8 is denoted by δmax, sin(δmax)=|Vn|/|Vs| is satisfied. 
     The second IDT electrode  12  can detect the bulk wave that has propagated inside the substrate  10  along with the SAW  70  that has propagated on the surface  20   a  of the waveguide  20  and the substrate surface  10   a . That is, the second IDT electrode  12  can output an electric signal corresponding to the composite wave of the SAW  70  and the bulk wave. The bulk wave receives random effects while propagating inside the substrate  10  and does not reflect a change in the state of the surface  20   a  of the waveguide  20 . Thus, in the electric signal output by the second IDT electrode  12 , a component based on the bulk wave becomes a noise component. That is, the result of detection of the change in the state of the surface  20   a  of the waveguide  20  based on the phase of the electric signal output by the second IDT electrode  12  may include the noise component based on the bulk wave. Thus, as the energy to be converted into the bulk wave in the energy of the SAW  70  is larger, the detection error of the change in the state of the surface  20   a  of the waveguide  20  increases. In other words, as the energy to be converted into the bulk wave in the energy of the SAW  70  is smaller, the detection error of the change in the state of the surface  20   a  of the waveguide  20  decreases. 
     As described above, the sensor device  1  can increase the detection accuracy of the change in the state of the surface  20   a  of the waveguide  20  by reducing the proportion of the energy to be converted into the bulk wave. As the distance for which the SAW  70  propagates on the substrate surface  10   a  is shorter, the proportion of the energy to be converted into the energy of the bulk wave in the energy of the SAW  70  decreases. The distance for which the SAW  70  propagates on the substrate surface  10   a  corresponds to D 1  and D 2 . That is, as D 1  and D 2  are shorter, the energy of the SAW  70  is more unlikely to be converted into the bulk wave. That is, the sensor device  1  according to the present disclosure can improve the detection sensitivity. 
     For example, the sensor device  1  according to an embodiment may be configured such that at least one of D 1  and D 2  is 0. Thus, the energy of the SAW  70  is unlikely to be converted into the bulk wave. As a result, the detection accuracy of the change in the state of the surface  20   a  of the waveguide  20  can be improved. 
     In addition, in a case where the configuration is made such that D 1  is 0, alignment between the first IDT electrode  11  and the waveguide  20  is unnecessary. In a case where the configuration is made such that D 1  is 0, alignment between the second IDT electrode  12  and the waveguide  20  is unnecessary. In these cases, sensitivity variation due to the alignment are not generated. As a result, the detection accuracy of the sensor device  1  is improved. 
     For example, the sensor device  1  may be configured such that D 1  is shorter than HP 1  and is longer than 0. That is, the sensor device  1  may be configured such that the first IDT electrode  11  and the waveguide  20  are different units. In this case, as compared with a case where the first IDT electrode  11  and the waveguide  20  are configured as a single unit, the sensor device  1  can reduce the generation of the direct wave. 
     In addition, for example, the sensor device  1  may be configured such that D 2  is shorter than HP 2  and is longer than 0. That is, the second IDT electrode  12  and the waveguide  20  may be formed as different units. In this case, as compared with a case where the second IDT electrode  12  and the waveguide  20  are configured as a single unit, the second IDT electrode  12  is likely to be excited, and thus, the sensor device  1  can improve the reception sensitivity of the SAW  70 . 
     As described above, the wavelength of the SAW  70  is determined on the basis of the pitch of the first IDT electrode  11  and the second IDT electrode  12 . The SAW  70  propagates as a transverse wave having an amplitude in the in-plane direction or the normal direction of the substrate surface  10   a . The SAW  70  as a transverse wave has crests and troughs that alternately proceed. The energy of the SAW  70  propagates in the traveling direction as the crests and the troughs proceed. The crests and the troughs of the SAW  70  store the energy of the SAW  70  as elastic energy. The energy stored as a crest is converted into energy for forming the coming trough. The energy stored as a trough is converted into energy for forming the coming crest. 
     The propagation state of the SAW  70  that propagates along the substrate  10  can be calculated by simulation. By simulation, the amplitude of the SAW  70  in a case where the SAW  70  generated in the first IDT electrode  11  travels toward the waveguide  20  is calculated. The simulation is executed by using D 1  and D 2  as variables. In the setting of the simulation, the substrate  10  is quartz, and the first IDT electrode  11 , the second IDT electrode  12 , and the waveguide  20  are Au. The simulation results are illustrated in  FIGS. 5A, 5B, and 5C  in which the D 1  side of the substrate  10  is enlarged and in  FIGS. 6A, 6B, and 6C  in which the D 2  side of the substrate  10  is enlarged. 
       FIGS. 5A, 5B, and 5C  illustrate the simulation results regarding the propagation of the SAW  70  on the D 1  side.  FIG. 5A  is a simulation result in a case (case 1) where D 1 &gt;λ/2 is satisfied.  FIG. 5B  is a simulation result in a case (case 2) where D 1 =λ/2 is satisfied.  FIG. 5C  is a simulation result in a case (case 3) where D 1 &lt;λ/2 is satisfied. 
       FIGS. 5A, 5B, and 5C  illustrate sections of the substrate  10  in the propagation direction of the SAW  70 . In addition, the amplitudes of the SAW  70  and the bulk wave are illustrated in grayscale. In each part of the sections, in a part where the color is closer to black (darker), the amplitude of the SAW  70  or the bulk wave is larger, and the elastic energy is larger. In a part where the color is closer to white or gray (lighter), the amplitude of the SAW  70  or the bulk wave is smaller, and the elastic energy is smaller. The first IDT electrode  11  and the waveguide  20  are positioned with the interval denoted by D 1  therebetween. The traveling direction of the SAW  70  is the direction from the first IDT electrode  11  toward the waveguide  20 . In the simulation, the wavelength (λ) of the SAW  70  is set to 11.52 μm, and D 2  is longer than λ/2. 
     In  FIG. 5A , D 1 &gt;λ/2 is satisfied. Specifically, D 1  is set to 18 μm. Part of the SAW  70  that propagates from the first IDT electrode  11  to the waveguide  20  changes to the bulk wave that propagates in the depth direction of the substrate  10  while propagating on the substrate surface  10   a  positioned between the first IDT electrode  11  and the waveguide  20 . In  FIG. 5A , the state where part of the SAW  70  changes to the bulk wave is obviously confirmed in at least the inside of the part surrounded by the broken line. After the SAW  70  has reached the waveguide  20 , the bulk wave still propagates inside the substrate  10 . 
     In  FIG. 5B , D 1 =λ/2 is satisfied. Specifically, D 1  is set to 5.76 μm. Part of the SAW  70  that propagates from the first IDT electrode  11  to the waveguide  20  changes to the bulk wave that propagates in the depth direction of the substrate  10  while propagating on the substrate surface  10   a  positioned between the first IDT electrode  11  and the waveguide  20 . In  FIG. 5B , the state where part of the SAW  70  changes to the bulk wave is confirmed in at least the inside of the part surrounded by the broken line. However, the bulk wave changed from the SAW  70  is reduced compared with a case where D 1 &gt;λ/2 is satisfied. The bulk wave radially propagates from the substrate surface  10   a  positioned between the first IDT electrode  11  and the waveguide  20  toward the inside of the substrate  10 . 
     In  FIG. 5C , D 1 &lt;λ/2 is satisfied. Specifically, D 1  is set to 2.2 μm. Almost no SAW  70  that propagates from the first IDT electrode  11  to the waveguide  20  diffuses in the depth direction of the substrate  10 . In  FIG. 5C , the state is confirmed where the bulk wave changed from the SAW  70  is further reduced compared with a case where D 1 =λ/2 is satisfied. That is, the generation of the bulk wave is further reduced in a case where D 1 &lt;λ/2 is satisfied. 
     On the basis of the distribution of the SAW  70  and the bulk wave illustrated in  FIGS. 5A, 5B, and 5C , the proportion of the energy of the SAW  70  propagating from the first IDT electrode  11  to the waveguide  20  converted into the bulk wave is the highest in case 1 and is the lowest in case 3. That is, the simulation results indicate that the proportion of the energy of the SAW  70  propagating from the first IDT electrode  11  to the waveguide  20  converted into the bulk wave decreases as D 1  is shorter. Thus, the sensor device  1  according to an embodiment can reduce the sensitivity variation when D 1 &lt;λ/2 is satisfied. 
       FIGS. 6A, 6B, and 6C  illustrate the simulation results regarding the propagation of the SAW  70  on the D 2  side.  FIG. 6A  is a simulation result in a case (case 4) where D 2 &gt;λ/2 is satisfied.  FIG. 6B  is a simulation result in a case (case 5) where D 2 =λ/2 is satisfied.  FIG. 6C  is a simulation result in a case (case 6) where D 2 &lt;λ/2 is satisfied. 
       FIGS. 6A, 6B, and 6C  illustrate sections of the substrate  10  in the propagation direction of the SAW  70 . In addition, the amplitudes of the SAW  70  and the bulk wave are illustrated in grayscale. The explanation regarding the display is substantially the same as that of  FIGS. 5A, 5B , and  5 C. The waveguide  20  and the second IDT electrode  12  are positioned with the interval denoted by D 2  therebetween. The traveling direction of the SAW  70  is the direction from the waveguide  20  toward the second IDT electrode  12 . In the simulation, the wavelength (λ) of the SAW  70  is set to 11.52 μm, and D 1  is longer than λ/2. 
     In  FIG. 6A , D 2 &gt;λ/2 is satisfied. Specifically, D 2  is set to 18 μm. Part of the SAW  70  that propagates from the waveguide  20  to the second IDT electrode  12  changes to the bulk wave that propagates in the depth direction of the substrate  10  while propagating on the substrate surface  10   a  positioned between the waveguide  20  and the second IDT electrode  12 . In  FIG. 6A , the state where part of the SAW  70  changes to the bulk wave is obviously confirmed in at least the inside of the part surrounded by the broken line. After the SAW  70  has reached the second IDT electrode  12 , the bulk wave still propagates inside the substrate  10 . 
     In  FIG. 6B , D 2 =λ/2 is satisfied. Specifically, D 2  is set to 5.76 μm. Part of the SAW  70  that propagates from the waveguide  20  to the second IDT electrode  12  changes to the bulk wave that propagates in the depth direction of the substrate  10  while propagating on the substrate surface  10   a  positioned between the waveguide  20  and the second IDT electrode  12 . In  FIG. 5B , the state where part of the SAW  70  changes to the bulk wave is confirmed in at least the inside of the part surrounded by the broken line. However, the bulk wave changed from the SAW  70  is reduced compared with a case where D 2 &gt;λ/2 is satisfied. The bulk wave radially propagates from the substrate surface  10   a  positioned between the first IDT electrode  11  and the waveguide  20  toward the inside of the substrate  10 . 
     In  FIG. 6C , D 2 &lt;λ/2 is satisfied. Specifically, D 2  is set to 2.2 μm. Almost no SAW  70  that propagates from the waveguide  20  to the second IDT electrode  12  diffuses in the depth direction of the substrate  10 . In  FIG. 6C , the state is confirmed where the bulk wave changed from the SAW  70  is further reduced compared with a case where D 1 =λ/2 is satisfied. That is, the generation of the bulk wave is further reduced in a case where D 2 &lt;λ/2 is satisfied. 
     On the basis of the distribution of the SAW  70  and the bulk wave illustrated in  FIGS. 6A, 6B, and 6C , the proportion of the energy of the SAW  70  propagating from the waveguide  20  to the second IDT electrode  12  converted into the bulk wave is the highest in case 4 and is the lowest in case 6. That is, the simulation results indicate that the proportion of the energy of the SAW  70  propagating from the waveguide  20  to the second IDT electrode  12  converted into the bulk wave decreases as D 2  is shorter. Thus, the sensor device  1  according to an embodiment can reduce the sensitivity variation when D 2 &lt;λ/2 is satisfied. 
     As described above, the configuration in which the distance (D 1 ) between the first IDT electrode  11  and the waveguide  20  and the distance (D 2 ) between the waveguide  20  and the second IDT electrode  12  are made short has been described. According to this, it is indicated that the proportion of the energy of the SAW  70  propagating from the first IDT electrode  11  to the waveguide  20  converted into the bulk wave decreases as the D 1  is shorter. In addition, it is indicated that the proportion of the energy of the SAW  70  propagating from the waveguide  20  to the second IDT electrode  12  converted into the bulk wave decreases as the D 2  is shorter. Thus, the simulation results indicate that the proportion of the energy of the SAW  70  converted into the bulk wave decreases in the propagation path as D 1  and D 2  are shorter. Specifically, it is indicated that, when D 1 &lt;λ/2 and D 2 &lt;λ/2 are satisfied, the proportion of the energy of the SAW propagating from the first IDT electrode  11  to the waveguide  20  and from the waveguide  20  to the second IDT electrode  12  converted into the bulk wave decreases. Thus, the sensor device  1  according to an embodiment can reduce the sensitivity variation when D 1 &lt;λ/2 and D 2 &lt;λ/2 are satisfied. 
     Note that the simulation results illustrated in  FIGS. 5A, 5B, and 5C  and  FIGS. 6A, 6B, and 6C  illustrate sections of the substrate  10 . That is, the sensor device  1  according to an embodiment can reduce the sensitivity variation when D 1 &lt;λ/2 and D 2 &lt;λ/2 are satisfied between at least part of the waveguide  20  and at least part of the first IDT electrode  11  or at least part of the second IDT electrode  12 . 
     &lt;Example of Calculating Sensitivity Variation&gt; 
     An example of calculating the sensitivity variation of the sensor device  1  will be described. The sensitivity of the sensor device  1  indicates the sensitiveness of the change in the electric signal to the change in the phase of the SAW  70 . When a plurality of sensor devices  1  with different sensitivities measure the same specimen  60 , different detection results are obtained. Thus, a small sensitivity variation of the sensor device  1  means a small variation of the output obtained from a plurality of sensor devices  1  measuring the same specimen  60  and means high detection accuracy and stable detection results. 
     In a measurement example, the sensitivity variation is measured by using a difference in the viscosity of aqueous solutions. 
     As liquid to be dropped onto the surface  20   a  of the waveguide  20 , pure water and a 20 wt % aqueous glycerin solution are used. The viscosity of the aqueous glycerin solution is determined on the basis of the concentration of glycerin. The viscosity of the 20 wt % aqueous glycerin solution is higher than the viscosity of pure water. Experiments were performed in the following procedure. Conditions of the experiments described below may be changed as appropriate. 
     Step 1: A measurer substituted liquid on the surface  20   a  of the waveguide  20  for pure water and measured the phase of the SAW  70 . Measurement was performed after an elapse of two minutes from the substitution for pure water. The phase measured in Step 1 is denoted by θ1.
 
Step 2: The measurer substituted liquid on the surface  20   a  of the waveguide  20  for the 20 wt % aqueous glycerin solution and measured the phase of the SAW  70 . Measurement was performed after an elapse of two minutes from the substitution of pure water for the aqueous glycerin solution. The phase measured in Step 2 is denoted by θ2.
 
Step 3: A difference between θ2 and θ1 was calculated as a sensitivity. The sensitivity is denoted by Δθ. Δθ=θ2−θ1 is satisfied.
 
Step 4: The measurer performed Steps 1 to 3 on a plurality of sensor devices  1  and measures Δθ a plurality of times.
 
Step 5: The measurer calculated an average (Ave) and a standard deviation (σ) of the plurality of measured values of Δθ and calculated the result of dividing the standard deviation by the average as a coefficient of variation representing the sensitivity variation. The coefficient of variation is also referred to as a CV (Coefficient of Variation). CV=σ/Ave is satisfied.
 
     As the sensor device  1  on which the above procedure including Steps 1 to 5 was performed, the sensor device  1  in which some parameters for specifying the elements were varied was prepared. Among the parameters for specifying the elements of the sensor device  1 , the wavelength (λ) of the SAW  70  and the thickness (h) of the waveguide  20  were fixed. Among the parameters for specifying the elements of the sensor device  1 , the distance (D 1 ) between the first IDT electrode  11  and the waveguide  20  and the distance (D 2 ) between the second IDT electrode  12  and the waveguide  20  were varied. In the experiment of this measurement example, the sensor device  1  was configured such that D 1  and D 2  corresponded to each other. In this measurement example, D 1  and D 2  are collectively denoted as D. D is represented as a value standardized for the wavelength (λ) of the SAW  70 . A plurality of sensor devices  1  having different values as D in the range from 0 to λ were prepared. By performing the above procedure of Steps 1 to 5 on each of the sensor devices  1 , the sensitivity of the sensor device  1  was measured. 
       FIG. 7  illustrates an example of measured data of the coefficient of variation of the sensitivity of the sensor device  1 . In the graph in  FIG. 7 , the horizontal axis represents the value of D/λ. The vertical axis represents the coefficient of variation (CV). The measured data illustrated in  FIG. 7  was obtained under the following parameter setting. Note that h is the film thickness of the waveguide  20 . In addition, D indicates each of D 1  and D 2  unless they are distinguished from each other in the following description. 
     λ=11.5 μm (P1=11.5 μm), h=110 nm, D/λ=0 to 1 (D=0 to λ) 
     The value of CV in the range of D from λ/2 to λ (D/λ is from 0.5 to 1) decreases so as to have a relationship of a linear function with respect to D as D is reduced from λ to λ/2, as indicated by the approximate line of the solid line. On the other hand, the value of CV in the range of D less than λ/2 (D/λ is less than 0.5) suddenly decreases to a value of about 1% indicated by the approximate line of the dot-dash line, deviated from the relationship of the linear function with respect to D indicated by the broken line. This result indicates that causes for increasing the sensitivity variation are reduced as D becomes less than the length of ½ of the wavelength of the SAW  70 . That is, this result proves that the proportion of the energy of the SAW  70  converted into the bulk wave decreases as D becomes less than the length of ½ of the wavelength of the SAW  70 . 
     As described above, the sensor device  1  according to this embodiment can reduce the sensitivity variation by being configured such that at least one of D 1  and D 2  becomes less than λ/2. As a result, the detection accuracy of the change in the state of the surface  20   a  of the waveguide  20  by the sensor device  1  is improved. 
     In addition, in a case where the first IDT electrode  11  or the second IDT electrode  12  and the waveguide  20  are formed as different units, the first IDT electrode  11  or the second IDT electrode  12  and the waveguide  20  may be formed together in one step or may be formed independently in two or more steps. By the first IDT electrode  11  or the second IDT electrode  12  and the waveguide  20  the being formed as different units, the degree of freedom of the layout of the elements of the sensor device  1  increases. For example, at a position facing the waveguide  20 , either electrode of a reference electrode and a signal electrode may be provided. 
     &lt;Method of Manufacturing SAW Sensor&gt; 
     Hereinafter, a method of manufacturing the sensor device  1  will be described. 
     In a first step, the substrate-side close-contact layer  15 , a metal layer  16  (not illustrated), and the protective-film-side close-contact layer  17  are formed on the substrate surface  10   a  of the substrate  10 . In a second step, the first reference electrode  11 G and the first signal electrode  11 A, the second reference electrode  12 G and the second signal electrode  12 A, and the waveguide  20  are formed from the metal layer  16 . 
     The first reference electrode  11 G, the first signal electrode  11 A, the second reference electrode  12 G, the second signal electrode  12 A, and the waveguide  20  may be formed by using any appropriate processing technology. For example, etching based on a mask having a desirable pattern may be used. The mask may be formed, for example, by photolithography. As the mask, a resist resin or the like may be used. The etching may include wet etching or dry etching. The wet etching may include a step of dissolving a material in an acid solution, an alkaline solution, or the like. The dry etching may include a step of removing a material by using plasma, such as RIE (Reactive Ion Etching) or sputter etching. 
     The first step and the second step may be replaced with a step of forming the first IDT electrode  11 , the second IDT electrode  12 , and the waveguide  20  in a patterned state on the substrate surface  10   a . The step of forming these in a patterned state can be implemented, for example, by forming the substrate-side close-contact layer  15 , the metal layer  16 , and the protective-film-side close-contact layer  17  in a state of being covered with a metal hard mask, a resist resin mask, or the like. 
     The step of forming the waveguide  20  may be configured so as not to form the protective-film-side close-contact layer  17  on the surface  20   a  of the waveguide  20 . The step of forming the waveguide  20  may be configured such that the protective-film-side close-contact layer  17  is formed on the surface  20   a  of the waveguide  20  and then the protective-film-side close-contact layer  17  is removed. 
     In the first step and the second step, the first IDT electrode  11 , the second IDT electrode  12 , and the waveguide  20  are simultaneously formed. A step of forming the first IDT electrode  11  and the second IDT electrode  12  and a step of forming the waveguide  20  may be divided as independent steps. When the steps are divided, whichever of the steps may be performed first. When the first IDT electrode  11 , the second IDT electrode  12 , and the waveguide  20  are simultaneously formed, the position of the waveguide  20  relative to the first IDT electrode  11  and the second IDT electrode  12  can be controlled with high accuracy. As a result, the accuracy of D 1  and D 2  can be improved, and the measurement accuracy can be improved. 
     In a third step, the protective film  30  for covering elements formed on the substrate surface  10   a  is formed. In a fourth step, a part of the protective film  30  is removed. As a result of removing the part of the protective film  30 , an opening surrounded by the side wall  30   a  is formed in the protective film  30 . The protective film  30  may be removed, for example, by etching based on a mask having a pattern of the opening. The etching may include any appropriate method such as dry etching or wet etching. The protective film  30  may be removed such that the first IDT electrode  11  and the second IDT electrode  12  remain covered. The protective film  30  may be removed so as to expose at least part of the surface  20   a  of the waveguide  20 . It can be said that the surface  20   a  of the waveguide  20  is exposed in the opening of the protective film  30 . 
     In a fifth step, a substance, such as the antibodies  51 , aptamers, or enzymes, that reacts with a detection target is immobilized on the surface  20   a  of the waveguide  20 . In an embodiment, it is assumed that the antibodies  51  (see  FIG. 1 ) are immobilized on the surface  20   a . When the waveguide  20  is formed of gold (Au), the antibodies  51  may be formed on the surface  20   a  on the basis of, for example, a gold thiol bond, which is a bond between gold (Au) and divalent sulfur (S). In this case, a polymer film is formed on the surface  20   a , and the antibodies  51  may be bound to the polymer by amine coupling by using an appropriate condensation agent (such as EDC/NHS reagent) in the polymer. The antibodies  51  may be immobilized on the surface  20   a  by being bound to the polymer. The state of the surface  20   a  can exert an effect on immobilization of the antibodies  51 . For example, a surface state such as the composition and the surface roughness of the surface  20   a  can exert an effect as to whether or not the antibodies  51  can be easily immobilized on the surface  20   a . Note that it is possible to use, as the polymer, for example, polyethylene glycol, carboxymethyldextran, polyacrylic acid, polymethacrylic acid, polyhydroxyethyl methacrylic acid, polyacrylamide, polycarboxybetaine methacrylic acid, polybetain sulfonic acid, polymethacrylic acid ethyl phosphorylcholine, or polyhydroxypropyl acrylamide. 
     In a case where the protective-film-side close-contact layer  17  is formed on the surface  20   a  of the waveguide  20  and is removed, the protective-film-side close-contact layer  17  may be removed in the second step or may be removed in the fourth step. The surface  20   a  may be covered with another protective material other than the protective-film-side close-contact layer  17 . The surface  20   a  may be covered with a protective material until immediately before the fifth step. By the surface  20   a  being protected until the fifth step, it becomes easier to control the state of the surface  20   a . In a case where the waveguide  20  includes two or more layers, the layer constituting the surface  20   a  of the waveguide  20  may be formed after the opening of the protective film  30  is formed. As a result, it becomes easier to control the state of the surface  20   a.    
     The sensor device  1  according to the embodiment may be manufactured by performing each of the aforementioned steps. The aforementioned steps are each an example. Any appropriate step may be added. Some of the steps may be omitted. 
     The figures illustrating embodiments of the present disclosure are schematic. Not all of the figures are to scale, for example. 
     Heretofore, embodiments of the present disclosure have been described on the basis of the drawings and examples. Note that a person having ordinary skill in the art can easily perform various modifications and corrections on the basis of the present disclosure. Accordingly, note that these modifications and corrections are included in the scope of the present disclosure. 
     A case where detection is performed on the basis of the SAW  70  whose propagation velocity has changed by the weight added to the substrate  10  has been described in the above example. However, for example, the sensor device  1  may perform detection on the basis of the SAW  70  whose propagation velocity has changed by reduction in the weight added to the substrate  10 . 
     For example, first, an analogue that is similar to the antigens  61  are immobilized in advance on the substrate  10 . In this case, the analogue may have a binding affinity with the antibodies  51 , but the binding affinity with the antibodies  51  is lower than that with the antigens  61 , which is the detection target. Subsequently, a known amount of antibodies  51  is supplied to the substrate  10 , and the analogue and the antibodies  51  form the complexes  52 . Subsequently, the specimen  60  including the antigens  61 , which is the detection target, is supplied to the substrate  10 . In this case, since the antigens  61  have a higher binding affinity with the antibodies  51  than the analogue, the antibodies  51  are dissociated from the analogue and form the complexes  52  with the antigens  61 . Thus, the weight added to the substrate  10  is reduced by the dissociated antibodies  51 . The amount of the dissociated antibodies  51  corresponds to the amount of the detection target, and thus, the propagation velocity of the SAW  70  changes in accordance with the amount of the detection target. The sensor device  1  acquires the phase of the SAW  70  before and after supplement of the specimen  60  and calculates the phase difference. On the basis of the calculated phase difference, the detection target can be detected. 
     For example, it is possible to rearrange functions and the like included in each element without allowing logical contradiction, and it is possible to unite or divide a plurality of elements and the like. 
     A configuration in which the waveguide  20  is positioned between the first reference electrode  11 G and the second reference electrode  12 G has been described in the above example. However, the configuration of the sensor device  1  is not limited to this example. Specifically, the waveguide  20  may be positioned between electrodes of any of the following combinations: (1) the first reference electrode  11 G and the second signal electrode  12 A, (2) the first signal electrode  11 A and the second reference electrode  12 G, and (3) the first signal electrode  11 A and the second signal electrode  12 A. Note that in the combinations (1) to (3), the waveguide  20  may be a single unit with either one of the electrodes, but the waveguide  20  may not be a single unit with both of the electrodes. 
     In addition, a case where the waveguide  20  is formed of a single Au film has been described in the above example. However, the waveguide  20  may be formed of a plurality of metal films. For example, the waveguide  20  may be formed of two or more metal films including a plurality of metals, such as Au—Au, Au—Ti—Au. In this case, the end portion of the metal films contacting with the substrate surface  10   a  may be the end portion of each of D 1  and D 2  on the waveguide  20  side. 
     In the present disclosure, ordinal numbers such as “first” and “second” are identifiers for discriminating between the elements. In the present disclosure, regarding the elements that are discriminated by the ordinal numbers such as “first” and “second”, the ordinal numbers may be replaced with each other. For example, the identifiers “first” and “second” of the first IDT electrode and the second IDT electrode may be replaced with each other. Replacement of the identifiers is performed simultaneously. Even after the replacement of identifiers, the elements are discriminated. The identifiers may be omitted. Elements from which the identifiers are omitted are discriminated by reference numerals. In the present disclosure, only the identifiers “first”, “second”, and the like should not be used for the interpretation of the order of the elements and as the basis for the presence of an identifier with a smaller number. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1  sensor device 
               10  substrate 
               10   a  substrate surface 
               11  first IDT electrode ( 11 A: first signal electrode,  11 G: first reference electrode) 
               12  second IDT electrode ( 12 A: second signal electrode,  12 G: second reference electrode) 
               15  substrate-side close-contact layer 
               16  metal layer 
               17  protective-film-side close-contact layer 
               20  waveguide 
               20   a  surface 
               30  protective film 
               30   a  side wall 
               51  antibody 
               52  complex 
               60  specimen 
               61  antigen 
               70  SAW