JP2005181292A - Pressure sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a pressure sensor with high reliability that increases sensitivity by increasing variations in surface stress at the time of receiving pressure and can reduce a size. <P>SOLUTION: A second piezoelectric substrate 2 with a thickness thinner than a first piezoelectric substrate 1 having a surface acoustic element 7a on a lower surface is mounted on the first piezoelectric substrate 1 having a reference surface acoustic element 4a on the upper surface thereof; and a surface acoustic element 7a for detecting a pressure and the reference surface acoustic element 4a are surrounded with a sealant intervened between the first piezoelectric substrate 1 and the second piezoelectric substrate 2. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a pressure sensor module used for detecting pressure fluctuations such as gas and liquid, such as monitoring of air pressure in a tire.

  2. Description of the Related Art Conventionally, pressure sensors that detect fluctuations in pressure applied to a sensor unit as changes in oscillation frequency have been used as pressure sensors that detect fluctuations in pressure of gas or liquid.

  Such a conventional pressure sensor has a surface acoustic wave element 54 and a surface acoustic wave element 57 formed of comb-like electrodes on a piezoelectric substrate 51, and a region where the surface acoustic wave element 54 is formed is elastic. A structure in which the thickness is set to be thinner than the region where the surface wave element 57 is formed is known (for example, see Patent Document 1).

In the above-described pressure sensor, in the surface acoustic wave element 54 formed in the region where the thickness is reduced, the surface stress changes and the sound velocity of the surface acoustic wave changes due to the application of pressure, and the surface acoustic wave element 54 also changes. By changing the distance between the electrodes, the resonance frequency of the surface acoustic wave element 54 is changed, and the pressure sensor has a function of detecting the pressure by the change of the resonance frequency. Further, it also has a function of correcting the temperature in accordance with the change in the resonance frequency of the surface acoustic wave element 57 formed on the same piezoelectric substrate.
Japanese Patent Publication No. 5-82537

  However, in the conventional pressure sensor described above, a thin portion is formed in a part of the piezoelectric substrate 51. For example, when it is necessary to reduce the size, a large area of the thin portion cannot be secured. However, there is a problem that even when pressure is applied, the amount of deformation is small and high sensitivity cannot be obtained as a pressure sensor. Further, since the surface acoustic wave element 57 and the surface acoustic wave element 54 are formed in the same plane, there is a problem that the area of the piezoelectric substrate is increased and it is difficult to reduce the size.

  The present invention has been devised in view of the above-described drawbacks, and its purpose is to increase the deformation of the surface acoustic wave element when subjected to pressure to improve sensitivity, and to enable downsizing and high reliability. It is to provide a pressure sensor.

  The pressure sensor of the present invention is a second piezoelectric substrate having a surface acoustic wave element for pressure detection on a lower surface of a first piezoelectric substrate having a surface acoustic wave element for reference on the upper surface, which is thinner than the first piezoelectric substrate. A piezoelectric substrate is mounted, and the surface acoustic wave element for pressure detection and the surface acoustic wave element for reference are surrounded by a sealing material interposed between the first piezoelectric substrate and the second piezoelectric substrate. It is characterized by.

  The pressure sensor according to the present invention is characterized in that a pressure fluctuation is detected by deformation of the surface acoustic wave element for pressure detection while temperature correction is performed in accordance with a frequency change of the surface acoustic wave element for reference. .

  Furthermore, in the pressure sensor of the present invention, the first piezoelectric substrate and the second piezoelectric substrate are made of the same piezoelectric single crystal and are arranged so that the directions of the corresponding crystal axes of the piezoelectric single crystal are substantially parallel. It is characterized by that.

  Furthermore, in the pressure sensor of the present invention, the first piezoelectric substrate and the second piezoelectric substrate are made of the same piezoelectric single crystal, and the cut angle of both piezoelectric substrates and the surface acoustic wave with respect to the crystal axis of the piezoelectric single crystal. The propagation directions are substantially the same or crystallographically equivalent.

  Furthermore, in the pressure sensor of the present invention, the first piezoelectric substrate and the second piezoelectric substrate have a rectangular shape and are made of the same piezoelectric polycrystal, and the linear expansion coefficient in the longitudinal direction of both piezoelectric substrates is the same. It is characterized by being substantially identical.

  Furthermore, in the pressure sensor of the present invention, the sealing material is made of a conductive material, and the sealing material is electrically connected to a ground terminal provided on the lower surface of the first piezoelectric substrate. Is.

  Furthermore, in the pressure sensor of the present invention, an electrode pad electrically connected to the surface acoustic wave element for pressure detection is provided on the lower surface of the second piezoelectric substrate and inside the sealing material. A connection pad that is electrically connected to the electrode pad via a conductive bonding material is provided on the upper surface of the piezoelectric substrate and inside the sealing material.

  According to the pressure sensor of the present invention, the thickness of the second piezoelectric substrate provided with the pressure detecting surface acoustic wave element is generally thinner than the thickness of the first piezoelectric substrate including the reference surface acoustic wave element. As a result, deformation of the surface acoustic wave element for pressure detection when receiving pressure is increased, and high sensitivity can be obtained as a pressure sensor.

  On the other hand, since the surface acoustic wave element for reference and the surface acoustic wave element for pressure detection are provided on different overlapping piezoelectric substrates, fewer electrodes are formed on the same plane, and the surface area of the surface acoustic wave element is reduced. Since the size can be reduced, the size can be reduced.

  According to the pressure sensor of the present invention, the pressure detection accuracy is improved by detecting the pressure fluctuation by the deformation of the pressure detecting surface acoustic wave element while correcting the temperature according to the frequency change of the reference surface acoustic wave element. be able to.

  Still further, according to the pressure sensor of the present invention, the first piezoelectric substrate and the second piezoelectric substrate are made of the same piezoelectric single crystal, and the directions of the corresponding crystal axes of the piezoelectric single crystal are arranged substantially parallel to each other. As a result, the thermal expansion coefficients in the same direction in both piezoelectric substrates are equal, so that when the second piezoelectric substrate is mounted on the first piezoelectric substrate, there is a problem such as the occurrence of cracks due to the addition of thermal history. Can be reduced.

  Furthermore, according to the pressure sensor of the present invention, the first piezoelectric substrate and the second piezoelectric substrate are made of the same piezoelectric single crystal, and the cut angle of both piezoelectric substrates and the surface acoustic wave with respect to the crystal axis of the piezoelectric single crystal. By making the propagation directions substantially the same or crystallographically equivalent, it is possible to substantially match the temperature characteristics of the surface acoustic wave element for reference and the surface acoustic wave element for pressure detection. The used temperature correction can be easily performed.

  Furthermore, according to the pressure sensor of the present invention, the first piezoelectric substrate and the front two piezoelectric substrates have a rectangular shape and are formed of the same piezoelectric polycrystal, and the linear expansion coefficient in the longitudinal direction of both piezoelectric substrates. By making them substantially the same, when the second piezoelectric substrate is mounted on the first piezoelectric substrate, it is possible to reduce inconveniences such as generation of cracks due to application of thermal history.

  Furthermore, according to the pressure sensor of the present invention, the sealing material is made of a conductive material, and electrically connected to the ground terminal provided on the lower surface of the first piezoelectric substrate, so that the surface acoustic wave element for reference and the pressure detection are provided. It is also possible to improve the electromagnetic shielding state of the surface acoustic wave element for use.

  Furthermore, according to the pressure sensor of the present invention, an electrode pad electrically connected to the surface acoustic wave element for pressure detection is provided inside the sealing material on the lower surface of the second piezoelectric substrate, By providing a connection pad electrically connected to the electrode pad via a conductive bonding material inside the sealing material on the upper surface, the surface acoustic wave element for reference and the surface acoustic wave element for pressure detection are provided. In addition, the electrical connection between both elements can be better protected than the external environment.

  Hereinafter, the pressure sensor of the present invention will be described in detail with reference to the drawings.

  1 is a cross-sectional view of a pressure sensor according to an embodiment of the present invention, FIG. 2 is an external perspective view of a main part of the pressure sensor according to an embodiment of the present invention, and FIG. 3 is a pressure according to an embodiment of the present invention. 1 is an external perspective view of a first piezoelectric substrate used for a sensor. The pressure sensor shown in FIG. 1 is roughly composed of a first piezoelectric substrate 1, a second piezoelectric substrate 2, a sealing material 5, and a conductive bonding material 6. It is configured.

  The first piezoelectric substrate 1 is made of a single crystal exhibiting piezoelectricity (hereinafter referred to as a piezoelectric single crystal) such as quartz, lithium niobate, or lithium tantalate, and is cut from an ingot at a predetermined cut angle. A main surface is formed, a reference surface acoustic wave element 4a and a connection pad 4b are attached to the upper surface, and an external terminal 9 is attached to the lower surface, for electrically connecting the upper surface and the lower surface. It has a structure in which a via-hole conductor 8 is formed.

  The surface acoustic wave element 4a for reference is formed on both sides of an interdigital transducer (hereinafter abbreviated as IDT) 4aa formed on the surface of the piezoelectric substrate 1 made of a piezoelectric single crystal and the surface acoustic wave propagation direction of the IDT 4aa. And a surface acoustic wave resonator that resonates at a predetermined frequency. The IDT 4aa and the reflector 4ab are formed by depositing a metal material such as aluminum or gold by a film forming method such as sputtering or vapor deposition, and patterning using a technique such as photolithography.

  The connection pad 4b and the external terminal 9 can be formed by the same material and method as the IDT 4aa and the reflector 4ab, but it is preferable to form a thick film for improving the adhesion strength. The via-hole conductor 8 is filled by forming a hole penetrating the first piezoelectric substrate 1 by a sandblast method or the like and then plating Ni, Cu, Au or the like.

The second piezoelectric substrate 2 is made of the same piezoelectric single crystal as the first piezoelectric substrate 1, and the cut angle and the propagation direction of the surface acoustic wave with respect to the crystal axis of the piezoelectric single crystal are substantially the same as the first piezoelectric substrate 1. Or it is made to become crystallographically equivalent. Here, “substantially identical” includes not only the case where they are completely the same, but also the case where they are shifted within a range of ± 0.5 °, and the “crystallographically equivalent” cut angle is This means that the main surface of the piezoelectric substrate cut out by the cut is a crystallographically equivalent surface, and the “crystallographically equivalent” surface is a surface that is equivalent by the symmetry of the crystal. It is. Similarly, the “crystallographically equivalent” direction is an equivalent direction due to the symmetry of the crystal. For example, in LBO (Li ₂ B ₄ O ₇ ) which is a tetragonal crystal, for example, the X plane and the Y plane are equivalent planes. For example, the X direction and the Y direction are equivalent directions.

  Further, when the second piezoelectric substrate 2 is mounted on the first piezoelectric substrate 1, the corresponding crystal axes of the piezoelectric single crystals on both the substrates are arranged so as to be substantially parallel. As a result, the thermal expansion coefficients in the same direction in both piezoelectric substrates are equal, so that when a large temperature change occurs, for example, when the second piezoelectric substrate is mounted on the first piezoelectric substrate, It is possible to effectively suppress the occurrence of problems such as generation of cracks and the like due to large stress at the joint. Here, “substantially parallel” includes not only the case of being completely parallel but also the case where the crystal axis is deviated within a range of ± 0.5 °.

  The second piezoelectric substrate 2 has a structure in which a surface acoustic wave element 7a for pressure detection and an electrode pad 7b are attached. The second piezoelectric substrate 2 is set to be thinner than the first piezoelectric substrate. For example, the thickness of the first piezoelectric substrate 1 is set to 200 to 300 μm, while the thickness of the second piezoelectric substrate is set to 50 to 75 μm.

  Similar to the reference surface acoustic wave element 4a, the pressure detecting surface acoustic wave element 7a includes a surface acoustic wave resonator including an IDT and a pair of reflectors formed on both sides in the propagation direction of the surface acoustic wave of the IDT. The resonance frequency is also matched with that of the reference surface acoustic wave element 4a.

  The electrode pad 7b is for electrical connection with the connection pad 4b described above via a conductive bonding material 6 described later, and it is preferable that the electrode pad 7b be formed thick like the connection pad 4b.

  As the conductive bonding material 6, solder or AuSn that is a high melting point brazing material can be used. However, in the process of mounting the pressure sensor 10 on a mother board or the like using AuSn that is a high melting point brazing material, heat is generated. Even when it is applied, it is preferable that the conductive bonding material 6 is not remelted to change its characteristics. In addition, AuSi, SnAgCu or the like can be used in addition to AuSn, and the same effect can be obtained.

  The reference surface acoustic wave element 4a, the pressure detecting surface acoustic wave element 7a, the connection pad 4b, and the electrode pad 7b are surrounded between the first piezoelectric substrate 1 and the second piezoelectric substrate 2. The sealing material 5 is interposed.

  A resin may be used for the sealing material 5. However, if the sealing pad 4 c and the sealing electrode 7 c are attached and formed in advance on the upper surface of the first piezoelectric substrate 1 and the lower surface of the second piezoelectric substrate 2, respectively. It is also possible to use a conductive material, and it is also possible to use the same material as the conductive bonding material 6. By using a conductive material excellent in heat conduction for the sealing material 5, in addition to the effect of the conductive bonding material 6, heat conduction between the pressure detecting surface acoustic wave element 7a and the reference surface acoustic wave element 4a. Therefore, the temperature of the surface acoustic wave element 7a for pressure detection and the surface acoustic wave element 4a for reference can be made substantially equal, and temperature correction can be performed simply as will be described later. Is possible.

  Further, when a conductive material is used for the sealing material 5 and is electrically connected to a ground terminal provided on the lower surface of the first piezoelectric substrate 1, the periphery of the reference surface acoustic wave element 4a and the pressure detecting surface acoustic wave element 7a Is surrounded by the sealing material 5 connected to the ground potential, and there is an effect of improving electromagnetic shielding. Specifically, the ground terminal described here means a terminal connected to the ground potential in the external terminal 9 described above, and the sealing material 5 is connected to the ground terminal via the sealing pad 4c and the via-hole conductor 8. Connected.

  The electrode 4d shown in the figure is a land that connects the surface acoustic wave element for reference 4a and the via-hole conductor 8, and is not electrically connected to the second piezoelectric substrate 2.

  In the pressure sensor 10 of the present embodiment, the surface acoustic wave element for pressure detection 7a formed on the lower surface of the second piezoelectric substrate 2 having a reduced thickness is deformed when subjected to pressure from the outside, and is a portion of the portion where the distortion has occurred. While the propagation speed of the surface acoustic wave changes, the distance between the electrode fingers of the IDT of the surface acoustic wave element for pressure detection 7a also changes, and the resonance frequency changes due to the action of both. Therefore, a change in pressure can be detected by a change in the resonance frequency of the surface acoustic wave element 7a.

  Here, the surface acoustic wave element 7a generally has a predetermined temperature characteristic, and its resonance frequency also changes due to a temperature change. Therefore, it is necessary to remove the influence due to the temperature change, and for this purpose, the reference surface acoustic wave element 4a is used. That is, since the surface acoustic wave element for reference 4a is thick, even if it receives pressure from the outside, the above-described change hardly occurs. For this reason, the resonance frequency changes only in accordance with a change in temperature. By using this, the change data of the resonance frequency of the surface acoustic wave element 7a for pressure detection is corrected, and the influence of the temperature change is almost eliminated. Can do.

  Here, if the temperature characteristics of the pressure detecting surface acoustic wave element 7a and the reference surface acoustic wave element 4a are different, the resonance frequency data and the temperature characteristic data of the reference surface acoustic wave element 4a are compared. The temperature is calculated, the temperature data is compared with the temperature characteristic data of the surface acoustic wave element for pressure detection 7a, and the amount of change in the resonance frequency due to the temperature of the surface acoustic wave element for pressure detection 7a is calculated. The effects of temperature changes must be removed.

  On the other hand, in the pressure sensor 10 of the present embodiment, the first piezoelectric substrate 1 and the second piezoelectric substrate 2 are made of the same piezoelectric single crystal, the cut angle of both piezoelectric substrates, and the crystal of the piezoelectric single crystal. By making the propagation directions of the surface acoustic waves with respect to the axes substantially the same or crystallographically equivalent, the temperature characteristics of the reference surface acoustic wave element 4a and the pressure detecting surface acoustic wave element 7a are matched. Therefore, by arranging the temperature of the surface acoustic wave element for pressure detection 7a and the temperature of the surface acoustic wave element for reference 4a to substantially coincide with each other, the resonance of both surface acoustic wave elements due to the temperature change generated in the pressure sensor 10 is achieved. The amount of change in the frequency becomes equal, and only the difference between the resonance frequency of the surface acoustic wave element 7a for pressure detection and the resonance frequency of the surface acoustic wave element 4a for reference can remove almost the influence of the temperature change. Thus, temperature correction can be realized with a simple configuration and method.

  In order to detect changes in the resonance frequency of the surface acoustic wave element 7a for pressure detection and the surface acoustic wave element 4a for reference, for example, an external amplifier circuit is connected to each of the surface acoustic wave elements, and the surface acoustic wave is connected. An oscillation circuit that oscillates at an oscillation frequency corresponding to the resonance frequency of the element is configured, and each oscillation frequency is detected. That is, the resonance frequency of the surface acoustic wave element changes according to changes in pressure and temperature, and the oscillation frequency of the oscillation circuit changes according to changes in the resonance frequency. Can be detected.

  Further, when performing the temperature correction, for example, if the output signal of the oscillation circuit including the pressure detecting surface acoustic wave element 7a and the output signal of the oscillation circuit including the reference surface acoustic wave element 4a are input to the mixer, Since it is possible to output a signal having a frequency corresponding to the difference in frequency, it is possible to easily remove the influence of the temperature change.

  As described above, in the pressure sensor 10 of the present embodiment, the thickness of the second piezoelectric substrate 2 provided with the pressure detecting surface acoustic wave element 7a is set to the thickness of the first piezoelectric substrate 1 including the reference surface acoustic wave element 4a. Compared to the overall thinning, the amount of deformation when pressure is applied increases, and high sensitivity is obtained as a pressure sensor.

  In addition, the surface acoustic wave element for reference 4a and the surface acoustic wave element for pressure detection 7a are provided on different piezoelectric substrates that overlap each other. As a result, the number of electrodes formed in the same plane is reduced, and the area of the piezoelectric substrate can be reduced, so that the pressure sensor can be miniaturized.

  In the pressure sensor 10 of the present embodiment, the first piezoelectric substrate 1 and the second piezoelectric substrate 2 are made of the same piezoelectric single crystal and are arranged so that the directions of the corresponding crystal axes of the piezoelectric single crystal are substantially parallel. As a result, since the thermal expansion coefficients in the same direction of the two piezoelectric substrates are the same, problems such as the occurrence of cracks due to the addition of thermal history are reduced.

  Furthermore, in the pressure sensor 10 of the present embodiment, the first piezoelectric substrate 1 and the second piezoelectric substrate 2 are made of the same piezoelectric single crystal, and the cut angle of both piezoelectric substrates and the crystal axis of the piezoelectric single crystal are relative to each other. Since the propagation directions of the surface acoustic waves are substantially the same or crystallographically equivalent, the temperature characteristics of the reference surface acoustic wave element 4a and the pressure detecting surface acoustic wave element 7a can be matched, By making the temperature of the surface acoustic wave element 7a for pressure detection and the temperature of the surface acoustic wave element 4a for reference substantially equal, temperature correction can be realized with a very simple configuration and method.

  Furthermore, in the pressure sensor 10 of the present embodiment, an electrode pad 7b that is electrically connected to the surface acoustic wave element 7a for pressure detection is provided on the lower surface of the second piezoelectric substrate 2 and inside the sealing material 5. Since the connection pad 4b that is electrically connected to the electrode pad 7b via the conductive bonding material 6 is provided on the upper surface of the first piezoelectric substrate 1 and inside the sealing material 5, the elasticity for reference is provided. Together with the surface acoustic wave element 4a and the pressure detecting surface acoustic wave element 7a, the electrical connection between both elements can be better protected from the external environment.

  Next, a method for connecting the first piezoelectric substrate 1 and the second piezoelectric substrate 2 described above will be described.

  First, a first wafer having a reference surface acoustic wave element 4a and connection pads 4b on the upper surface and a second wafer having a pressure detecting surface acoustic wave element 7a and electrode pads 7b on the lower surface are prepared. The first wafer used here is a collective substrate of the first piezoelectric substrate 1, the second wafer is a collective substrate of the second piezoelectric substrate 2, and the thickness thereof is set to 200 to 300 μm.

  Next, the connection pad 4b of the first wafer and the electrode pad 7b of the second wafer are temporarily connected to the sealing pad 4c of the first wafer and the sealing electrode 7c of the second wafer via solder paste, respectively. . In the present embodiment, a solder paste in which AuSn powder is dispersed in an organic vehicle is used. The solder paste was applied and formed on the connection pad 4b by a conventionally known screen printing method or the like. The electrode pads 7b and the sealing electrodes 7c of the second wafer face the corresponding connection pads 4b and the sealing pads 4c, respectively, and the first wafer and the second wafer have the corresponding crystal axis directions. Match each one.

  Next, by heating the first wafer and the second wafer to melt the solder paste, the reference surface acoustic wave element 4a and the pressure detecting surface acoustic wave element 7a are surrounded by the sealing material 5, and The connection pad 4 b is electrically connected to each electrode pad 7 b through the conductive bonding material 6.

  Thus, the 2nd wafer fixed to the 1st wafer is ground by lapping etc. from the upper side, and thickness is set to 50-75 micrometers.

  Next, only the second wafer is cut by dicing, the second wafer is divided into a plurality of second piezoelectric substrates, and then a liquid resin is filled so as to fill the gaps between the adjacent second piezoelectric substrates. Apply and heat cure. In the present embodiment, when the liquid resin is applied, it is necessary to effectively fill the gap, so it is preferable to use vacuum printing.

  Then, by cutting the first wafer together with the above-described resin by dicing or the like, the first wafer is divided into the first piezoelectric substrate 1 and the pressure sensor 10 is manufactured.

  Here, the pressure sensor 10 of the present embodiment has the protective material 3 made of resin on the side surface of the second piezoelectric substrate 2 and the upper portion other than the region where the surface acoustic wave element 7a for pressure detection is formed. The end surface portion of the thin second piezoelectric substrate 2 is protected.

  The pressure sensor 10 thus configured, for example, is connected to an external amplifier in a circuit to form an oscillation circuit, and is combined with a power source and an antenna component, for example. It functions as a pressure sensor device that transmits and outputs wireless signals in response to changes.

  Next, a pressure sensor according to another embodiment of the present invention will be described with reference to FIG. In this embodiment, only differences from the above-described embodiment shown in FIGS. 1 to 3 will be described, and the same components will be denoted by the same reference numerals and redundant description will be omitted.

  FIG. 4 is an external perspective view showing the first substrate 1 used in the pressure sensor of this embodiment. The pressure sensor of this embodiment is different from the above-described pressure sensor of FIGS. 1 to 3 in that a reference surface acoustic wave element 14a is disposed on the surface of the sensor substrate 1 with a gap therebetween. 4 a and a surface acoustic wave propagation line 4 af between them, and a surface acoustic wave element 17 a for pressure detection (not shown) is similarly formed in the second piezoelectric substrate 2. The surface acoustic wave delay line is composed of a pair of IDTs arranged on the lower surface of the substrate with a predetermined interval and a propagation path of the surface acoustic wave therebetween. Further, on both sides of the surface acoustic wave propagation direction of both surface acoustic wave elements, in order to attenuate the surface acoustic waves and prevent the surface acoustic waves from being reflected at the ends of the piezoelectric substrate, etc. A damping material 4h is formed.

  Then, when external pressure is applied to the second piezoelectric substrate 2 from above and the second piezoelectric substrate 2 is deformed, the length of the propagation path of the surface acoustic wave changes in the surface acoustic wave element 17a for pressure detection. Since the propagation speed of the surface acoustic wave in the part where the distortion has occurred changes, and the delay time of the electric signal changes due to the action of both, by detecting the change in the delay time, the same as in the previous embodiment A change in pressure can be detected.

  In order to detect a change in the delay time, for example, an oscillation circuit that oscillates at a frequency corresponding to the delay time of the electric signal generated by the surface acoustic wave delay line by connecting an amplifier circuit to the surface acoustic wave element 17a for pressure detection. Configure. As a result, as in the previous embodiment, a change in pressure can be detected as a change in oscillation frequency, and the surface acoustic wave element 17a for pressure detection in this embodiment is also the elasticity for pressure detection in the previous embodiment. Like the surface wave element 7a, it functions as a pressure detection element.

  Similarly, the surface acoustic wave element 14a for reference is also connected to the amplifier circuit to constitute an oscillation circuit that oscillates at a frequency corresponding to the delay time of the electric signal generated by the surface acoustic wave element 14a for reference. As in the case of the previous embodiment, for example, the output signals of the two oscillation circuits are input to the mixer, and a signal having a frequency corresponding to the difference between the frequencies of the two signals is easily output. Can be almost removed.

  In addition, this invention is not limited to the above-mentioned embodiment, A various change and improvement are possible within the range which does not deviate from the summary of this invention.

  For example, in the above-described embodiment, the single crystal piezoelectric material is used for the two piezoelectric substrates, but a polycrystalline piezoelectric material may be used instead. In particular, in this case, the first piezoelectric substrate 1 and the second piezoelectric substrate 2 have a rectangular shape and are formed of the same piezoelectric polycrystal, and the linear expansion coefficients in the longitudinal direction of both piezoelectric substrates are substantially the same (identical and ± (Within a range within 10%), for example, when a large temperature change occurs when the second piezoelectric substrate 2 is mounted on the first piezoelectric substrate 1, a large stress is applied to the joint due to the difference in thermal expansion coefficient. It is possible to effectively suppress the occurrence of defects such as cracks.

  In this case, the first piezoelectric substrate 1 and the second piezoelectric substrate 2 are made of the same piezoelectric polycrystal, and the propagation direction of the surface acoustic wave with respect to the polarization direction is referred to the pressure detecting surface acoustic wave element 7a. By matching with the surface acoustic wave element 4a for use, the temperature characteristics of both surface acoustic wave elements can be matched, and temperature correction can be performed by a simple method as in the above-described embodiment.

It is sectional drawing of the pressure sensor which concerns on one Embodiment of this invention. It is an external appearance perspective view of the principal part of the pressure sensor which concerns on one Embodiment of this invention. It is an appearance perspective view of the 1st piezoelectric substrate of the pressure sensor concerning one embodiment of the present invention. It is an external appearance perspective view of the 1st piezoelectric substrate of the pressure sensor which concerns on other embodiment of this invention. It is an external appearance perspective view of the conventional pressure sensor. It is sectional drawing of the conventional pressure sensor.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... 1st piezoelectric substrate 2 ... 2nd piezoelectric substrate 3 ... Protective material 4a, 14a ... Surface acoustic wave element for reference 4b ... Connection pad 5 ... Sealing material 6 ... Conductive bonding material 7a, 17a ... surface acoustic wave element for pressure detection 7b ... electrode pad 8 ... via hole conductor 9 ... external terminal 10 ... pressure sensor

Claims (7)

On the first piezoelectric substrate having the surface acoustic wave element for reference on the upper surface, a second piezoelectric substrate having a thickness smaller than that of the first piezoelectric substrate and having the surface acoustic wave element for pressure detection on the lower surface is mounted. A pressure sensor in which the surface acoustic wave element for pressure detection and the surface acoustic wave element for reference are surrounded by a sealing material interposed between the first piezoelectric substrate and the second piezoelectric substrate. 2. The pressure sensor according to claim 1, wherein a pressure fluctuation is detected by deformation of the surface acoustic wave element for pressure detection while temperature correction is performed according to a frequency change of the surface acoustic wave element for reference. The first piezoelectric substrate and the second piezoelectric substrate are made of the same piezoelectric single crystal, and are arranged so that directions of crystal axes corresponding to the piezoelectric single crystal are substantially parallel to each other. The pressure sensor according to claim 1 or 2. The first piezoelectric substrate and the second piezoelectric substrate are made of the same piezoelectric single crystal, and the cut angle of both piezoelectric substrates and the propagation direction of the surface acoustic wave with respect to the crystal axis of the piezoelectric single crystal are substantially the same or crystallographic. The pressure sensor according to any one of claims 1 to 3, wherein the pressure sensors are equivalent to each other. The first piezoelectric substrate and the second piezoelectric substrate have a rectangular shape and are made of the same piezoelectric polycrystal, and the linear expansion coefficients in the longitudinal direction of both the piezoelectric substrates are substantially the same. The pressure sensor according to claim 1 or 2. The sealing material is made of a conductive material, and the sealing material is electrically connected to a ground terminal provided on the lower surface of the first piezoelectric substrate. A pressure sensor according to claim 1. On the lower surface of the second piezoelectric substrate, an electrode pad electrically connected to the surface acoustic wave element for pressure detection is provided inside the sealing material,
2. The connection pad that is electrically connected to the electrode pad via a conductive bonding material is provided on the upper surface of the first piezoelectric substrate inside the sealing material. The pressure sensor according to claim 6.

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