JP4867858B2 - SAW sensor - Google Patents

Description

  The present invention relates to a SAW sensor and a SAW sensor element that can detect vibration, acceleration, and the like using surface acoustic waves.

Conventionally, SAW sensors that detect various physical quantities using surface acoustic waves (SAW) have been proposed.
In Patent Document 1, a surface acoustic wave element (SAW element) that bends in the thickness direction due to vibration is used, and according to the expansion / contraction variation between the electrodes in the input side electrode pair and the output side electrode pair formed on the surface of the SAW element due to this bending A SAW sensor that causes a frequency modulation operation and detects vibration from the modulation frequency is disclosed.
Further, in Patent Document 2, a bimorph type acceleration detecting element is configured by facing the back surfaces of SAW resonators face to face, and the acceleration detecting element is deflected by application of acceleration to differentially change the frequency change or impedance change of the SAW resonator. A SAW sensor that can detect an acceleration that is not affected by a temperature change or the like is disclosed.

JP 2000-234594 A JP 2002-122614 A

However, the change in the frequency of the SAW element corresponding to the bending of the planar SAW element is very small. For this reason, the SAW sensor of Patent Document 1 has a problem that the SAW element is not bent with a small vibration and the vibration cannot be detected with high sensitivity.
Similarly, in Patent Document 2, since the bimorph type acceleration detection element is configured by facing the back surfaces of the SAW resonator face to face, the element may be thick and bend the element in the plate thickness direction. difficult. For this reason, there is a problem that the change in the frequency of the SAW resonator is small at a small acceleration, and the acceleration cannot be detected with high sensitivity.

  SUMMARY An advantage of some aspects of the invention is to solve at least a part of the problems described above, and the invention can be implemented as the following forms or application examples.

[Application Example 1] In the SAW sensor of this application example, an SH wave in which a first SAW resonator and a second SAW resonator are arranged in parallel on one quartz substrate with a vibration coupling region interposed therebetween. A laterally coupled dual-mode SAW resonator to be used, and on the quartz substrate in the coupling region, in a direction substantially orthogonal to the electrode fingers of the first SAW resonator and the second SAW resonator A SAW sensor element having a groove formed therein, the SAW sensor element being supported at an end portion having a side substantially parallel to the groove of the SAW sensor element, and in the fundamental wave symmetric mode S0 of the dual mode SAW resonator. A SAW sensor that detects an external force applied to the SAW sensor element by detecting a difference between a resonance frequency and a resonance frequency in a fundamental wave oblique symmetry mode A0. In another aspect, a laterally coupled double-mode SAW resonator in which a first SAW resonator and a second SAW resonator are arranged in parallel in a first direction with a vibration coupling region interposed therebetween. And a SAW sensor element provided with a groove in the coupling region, wherein the SAW sensor element is supported on at least one end of the substrate in the first direction, and the dual mode SAW resonator A difference between the resonance frequency in the fundamental wave symmetric mode S0 and the resonance frequency in the fundamental wave oblique symmetric mode A0 is detected.

As described above, the SAW sensor element has a laterally coupled double mode SAW resonator in which two SAW resonators are arranged in parallel on a quartz substrate, and the quartz substrate in a vibration coupling region between the SAW resonators. A groove is formed in a direction substantially perpendicular to the electrode fingers of the SAW resonator. The SAW sensor element is supported at an end portion having a side substantially parallel to the groove portion of the SAW sensor element.
According to this configuration, when an external force such as vibration or acceleration is applied in the plate thickness direction of the SAW sensor element, stress is concentrated in the groove portion thinned in the plate thickness direction of the SAW sensor element, and the SAW sensor element is greatly bent. It can be detected as a large frequency change of the SAW resonator.
Specifically, since the groove is formed in the vibration coupling region of the two SAW resonators, the fundamental wave symmetric mode S0 and the fundamental wave oblique symmetric mode A0 excited in the laterally coupled double mode SAW resonator. In FIG. 5, the frequency of the fundamental wave symmetric mode S0 changes greatly, while the frequency change of the fundamental wave symmetric mode A0 is small.
Thus, by detecting the difference in resonance frequency between the fundamental wave symmetric mode S0 and the fundamental wave oblique symmetry mode A0, it is possible to detect external forces such as vibration and acceleration applied to the SAW sensor element with high sensitivity.

  Application Example 2 The SAW sensor according to Application Example 1, wherein one end of the SAW sensor element is a free end.

  Thus, the support structure of the SAW sensor element is a cantilever support structure in which one end is a free end and the other end is fixed. By supporting the SAW sensor element in a cantilevered manner, when an external force such as vibration or acceleration is applied in the thickness direction of the SAW sensor element, the amount of displacement of the SAW sensor element can be increased, and the amount of change in frequency increases. To do. Furthermore, since the SAW sensor element is cantilevered, the stress applied to the SAW resonator due to adhesion of an adhesive or the like can be suppressed as much as possible, and the frequency change of the SAW sensor due to the secular change of the adhesion portion can be reduced. Can do.

Application Example 3 In the SAW sensor of application example 1 or 2, the difference between the resonance frequency of the fundamental wave anti-symmetric mode A0 and the resonance frequency in the fundamental symmetric mode S0 of the double mode SAW resonator A SAW sensor, wherein a two-mode oscillation circuit and an up-and-down counter are used for detection of noise.

  According to this configuration, the SAW resonator is oscillated by switching one oscillator, so that the circuit configuration is small and the current consumption can be reduced.

  Application Example 4 In the SAW sensor of Application Examples 1 to 3, the crystal substrate is Euler angle display (φ, θ, ψ), and φ is first counterclockwise around the optical axis Z axis. It is in the range of 0 ° ± 1 °. Next, θ is in the range of 29.2 ° or more and 40.7 ° or less in the counterclockwise direction around the X axis which is an electric axis. In the quartz crystal substrate whose main surface is a plane composed of the generated Z ′ axis, the surface rotates in the counterclockwise direction around the Z ′ axis, and ψ is in the range of 90 ° ± 2 °. A SAW sensor characterized in that a direction is a phase propagation direction of the surface acoustic wave.

  According to this configuration, a quartz substrate (SH cut substrate) of Euler angle display excellent in frequency temperature characteristics (0 ° ± 1 °, 29.2 ° ≦ θ ≦ 40.7 °, 90 ° ± 2 °) can be obtained. By using it, a small and highly accurate SAW sensor can be realized.

  Application Example 5 A laterally coupled double mode SAW using SH waves in which a first SAW resonator and a second SAW resonator are arranged in parallel across a vibration coupling region on one quartz substrate. And a groove portion is formed in the quartz substrate in the coupling region in a direction substantially orthogonal to the electrode fingers of the first SAW resonator and the second SAW resonator. SAW sensor element.

According to this configuration, when an external force such as vibration or acceleration is applied in the plate thickness direction of the SAW sensor element, stress is concentrated in the groove portion thinned in the plate thickness direction of the SAW sensor element, and the SAW sensor element is greatly bent. It can be detected as a large frequency change of the SAW resonator.
Specifically, since the groove is formed in the coupling region of the two SAW resonators, in the fundamental wave symmetric mode S0 and the fundamental wave oblique symmetric mode A0 excited in the laterally coupled double mode SAW resonator, The frequency of the fundamental wave symmetric mode S0 changes greatly while the frequency change of the fundamental wave symmetric mode A0 is small.
Thus, by detecting the difference in resonance frequency between the fundamental wave symmetric mode S0 and the fundamental wave oblique symmetric mode A0, vibration, acceleration, and the like can be detected with high sensitivity.
In addition, by using the SH wave as the surface acoustic wave, it is possible to utilize the characteristic that the electromechanical coupling coefficient and the reflection coefficient are large compared to the Rayleigh wave, and the SAW sensor element can be downsized.

  Application Example 6 The SAW sensor element according to Application Example 5 in which the quartz crystal substrate is Euler angle display (φ, θ, ψ), and φ is zero counterclockwise around the optical axis Z. It is in the range of ± 1 °. Next, θ is in the range of 29.2 ° or more and 40.7 ° or less counterclockwise around the X axis, which is the electric axis. In the quartz crystal substrate whose main surface is a plane constituted by the Z ′ axis to be rotated in a plane counterclockwise around the Z ′ axis, and ψ is in a range of 90 ° ± 2 ° Is a phase propagation azimuth of the SH wave.

  According to this configuration, a quartz substrate (SH cut substrate) of Euler angle display excellent in frequency temperature characteristics (0 ° ± 1 °, 29.2 ° ≦ θ ≦ 40.7 °, 90 ° ± 2 °) can be obtained. By using it, a small and highly accurate SAW sensor element can be provided.

DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, embodiments of the invention will be described with reference to the drawings. In the following embodiments, an acceleration sensor that detects acceleration will be described as an example of a SAW sensor.
(Embodiment)

  FIG. 1 shows a configuration of an acceleration sensor as a SAW sensor of the present embodiment, FIG. 1 (a) is a schematic plan view, and FIG. 1 (b) is a schematic cross-sectional view taken along the line AA in FIG. is there.

The acceleration sensor 1 includes a package 20, a SAW sensor element 30 and an IC chip 60 housed in the package 20, and a lid 25 that hermetically seals the inside of the package 20.
The package 20 is formed by laminating a plurality of ceramic green sheets and firing them. A recess is formed in the package 20, and connection terminals 21 and 22 are formed in the recess, and the connection terminals 21 and 22 are connected to each other inside the package 20. An external connection terminal 23 is formed on the outer periphery of the package 20 and is configured to be connected to a part of the connection terminals 21 and 22. Further, a metal seam ring 24 such as Kovar is fixed to the periphery of the recess in the package 20.

The IC chip 60 is fixed to the bottom surface of the recess of the package 20 and connected to the connection terminal 22 by a metal wire 26 such as a gold wire. The IC chip 60 includes an oscillation circuit that excites the SAW sensor element 30 and an acceleration detection circuit that detects an acceleration signal.
The SAW sensor element 30 is disposed above the IC chip 60. The SAW sensor element 30 includes a laterally coupled double-mode SAW resonator 43 formed by arranging a first SAW resonator 41 and a second SAW resonator 42 in proximity to the quartz substrate 10. In addition, a groove 40 is formed in the quartz substrate 10 between the first SAW resonator 41 and the second SAW resonator 42. The groove 40 is formed on the surface of the quartz substrate 10 opposite to the surface on which the IDTs 31 and 33 (see FIG. 2) are formed. The SAW sensor element 30 is fixed to the stepped portion of the recess of the package 20 with an adhesive 27 at one end thereof. In this way, the SAW sensor element 30 is cantilevered with one end of the end being a free end. Further, the first SAW resonator 41 and the second SAW resonator 42 of the SAW sensor element 30 are connected to connection pads 35, 36, 37, and 38, and the connection pads 35, 36, 37, and 38 are gold wires. It is connected to the connection terminal 21 by a metal wire 28 such as.

  A lid 25 is disposed above the seam ring 24 formed at the periphery of the recess of the package 20, and the inside of the recess of the package 20 is hermetically sealed by seam welding the seam ring 24 and the lid 25. Has been.

Next, the SAW sensor element 30 will be described in detail. 2A and 2B show the configuration of the SAW sensor element of the present embodiment. FIG. 2A is a schematic plan view, and FIG. 2B is a schematic cross-sectional view taken along the line BB in FIG.
In the SAW sensor element 30, a first SAW resonator 41 and a second SAW resonator 42 are formed on the quartz substrate 10. The first SAW resonator 41 includes an IDT (Interdigital Transducer) 31 formed by alternately interposing electrode fingers 31a and 31b having opposite polarities, and a reflector 32 made of a metal conductor 32a on both sides of the IDT 31. I have. The electrode finger 31 a is connected to the connection pad 35, and the electrode finger 31 b is connected to the connection pad 36.
The second SAW resonator 42 includes an IDT 33 formed by alternately interposing electrode fingers 33a and 33b having opposite polarities, and a reflector 34 made of a metal conductor 34a on both sides of the IDT 33. . The electrode finger 33 a is connected to the connection pad 37, and the electrode finger 33 b is connected to the connection pad 38.
The connection pads 35, 36, 37 and 38 are collected and arranged near one end face of the SAW sensor element 30.
The IDTs 31, 33, the reflectors 32, 34, and the connection pads 35, 36, 37, 38 are made of aluminum (Al). Note that these may be formed of a metal film such as copper (Cu), gold (Au), tungsten (W), or tantalum (Ta).

The propagation directions of the surface acoustic waves (SH waves) of the first SAW resonator 41 and the second SAW resonator 42 are substantially orthogonal to the electrode fingers 31a, 31b, 33a, 33b, The first SAW resonator 41 and the second SAW resonator 42 are arranged close to each other in parallel so that the propagation directions are substantially parallel. A region sandwiched between the first SAW resonator 41 and the second SAW resonator 42 arranged in parallel is a coupling region 44 of surface acoustic wave vibration. In this proximity arrangement, for example, the distance between the tip of the electrode finger 31b and the electrode finger 33b facing each other is set to about 30 to 50 μm.
The first SAW resonator 41 and the second SAW resonator 42 constitute a laterally coupled double-mode SAW resonator 43. Thus, the first SAW resonator 41 and the second SAW resonator 42 are formed. By arranging them closely, acoustic coupling is generated between the two SAW resonators, and two different resonance frequencies can be obtained.

A groove portion 40 is formed in the crystal substrate 10 in the coupling region 44 in a direction substantially orthogonal to the electrode fingers 31a, 31b, 33a, and 33b. The groove 40 is formed on the surface of the quartz substrate 10 opposite to the surface on which the IDTs 31 and 33 are formed. The width W and the depth H of the groove 40 are the thickness of the quartz substrate 10, the magnitude of the detected acceleration, and the coupling It is determined appropriately in consideration of the size of the region 44 and the like.
Connection pads 35, 36, 37, 38 are arranged on one end 46 having a side 45 substantially parallel to the groove 40 of the SAW sensor element 30, and the surface on which the connection pads 35, 36, 37, 38 are formed. The SAW sensor element 30 is supported by fixing the back surface of the substrate to the package with an adhesive or the like. Thus, the SAW sensor element 30 has a cantilever support structure in which the other end 47 is a free end. Since it is such a structure, an adhesive agent is not arrange | positioned and fixed to the back surface of IDT31,33.
In the SAW sensor element 30, the quartz substrate 10 is extended from the second SAW resonator 42 to form the other end 47. This end portion 47 supports one end portion 46 of the SAW sensor element 30 and functions as a weight when stress is concentrated in the groove portion 40 when the acceleration is applied. Further, the effect of the weight can be increased by forming a film of metal or the like on the end portion 47.

Next, the cut surface of the quartz crystal substrate used in this embodiment will be described. FIG. 3 is a schematic diagram for explaining a cut surface of a quartz crystal substrate in the present embodiment.
A quartz crystal substrate 10 made of a quartz single crystal has an X axis (electrical axis), a Y axis (mechanical axis), and a Z axis (optical axis) as basic axes of the quartz crystal, and constitutes a right-handed orthogonal coordinate system. Yes.
The quartz substrate 10 is displayed with Euler angles (φ, θ, ψ). First, φ is in the range of 0 ° ± 1 ° counterclockwise around the Z axis, and then counterclockwise around the X axis. θ is in the range of 29.2 ° to 40.7 °, and the surface formed by the X axis and the newly generated Y ′ axis is the main surface. Then, in this quartz substrate 10, the in-plane rotation in the counterclockwise direction around the newly generated Z ′ axis, and the direction X in which ψ is in the range of 90 ° ± 2 ° from the X axis to the Y ′ axis direction 'Is the phase propagation direction of the SH wave. Such a quartz substrate 10 is referred to as SH cut. It is known that when an SH wave is excited using this SH-cut quartz substrate 10, a surface acoustic wave resonator having excellent frequency temperature characteristics can be formed.

Next, the operation of the acceleration sensor having the above configuration will be described.
4A and 4B are explanatory diagrams for explaining the operation of the acceleration sensor. FIG. 4A is a schematic diagram showing a cross section of the SAW sensor element, and FIG. 4B is a diagram showing vibration displacement of the dual mode SAW resonator. FIG. 4C shows the displacement of the quartz substrate in the SAW sensor element.
4A, in the SAW sensor element 30, the first SAW resonator 41 and the second SAW resonator 42 are disposed close to the quartz substrate 10, and a laterally coupled double mode SAW resonator 43 is configured. ing. Then, a groove 40 is formed in the coupling region 44 between the first SAW resonator 41 and the second SAW resonator 42, and is fixed at one end 46 of the SAW sensor element 30, and is cantilevered. Yes.
In the fundamental wave excited by the laterally coupled double mode SAW resonator 43, two fundamental wave symmetric modes S0 and fundamental wave oblique symmetric mode A0 generated as transverse modes as shown in FIG. Has vibration mode.
In the fundamental wave symmetric mode S0, the first SAW resonator 41 and the second SAW resonator 42 are excited in the same phase and are in an oscillation state. In contrast, in the fundamental wave oblique symmetry mode A0, the first SAW resonator 41 and the second SAW resonator 42 are excited in opposite phases and are in an oscillation state.
Further, assuming that the resonance frequency in the fundamental wave symmetric mode S0 is F (S0) and the resonance frequency in the fundamental wave oblique symmetry mode A0 is F (A0), F (A0)> F (S0). The difference between F (A0) and F (S0) is approximately 700 to 1000 ppm.

Next, when the acceleration G is applied to the SAW sensor element 30 in FIG. 4A in the direction of the arrow, the quartz substrate 10 of the SAW sensor element 30 is displaced as shown in FIG.
The quartz substrate 10 has a displacement of 0 because the SAW sensor element 30 is fixed at one end 46 between J and K, and is slightly displaced to the minus side between K and L due to the external force of the acceleration G. In the groove portion 40 of the quartz substrate 10, the plate thickness of the quartz substrate 10 is thin, and stress concentrates on this portion, and the displacement between L and M increases greatly toward the minus side from the L point to the M point. To do. The displacement of the quartz substrate 10 increases as the acceleration G increases, and the bending stress generated in the groove 40 also increases.
As described above, when an external force such as acceleration is applied in the plate thickness direction, the SAW sensor device 30 concentrates the stress on the groove portion 40 thinned in the plate thickness direction of the SAW sensor device 30, thereby increasing the size of the SAW sensor device 30. It can be bent and can be detected as a large frequency change of the laterally coupled double mode SAW resonator 43.

In the deformed state of the quartz substrate 10, the resonance frequency F (S0) in the fundamental wave symmetry mode S0 and the resonance frequency F (A0) in the fundamental wave oblique symmetry mode A0 change.
When the change in the resonance frequency is represented by the relationship between the frequency change rate and acceleration, a graph as shown in FIG. The frequency change rate of the resonance frequency F (S0) in the fundamental wave symmetric mode S0 is larger than the resonance frequency F (A0) in the fundamental wave oblique symmetry mode A0, and the frequency change rate increases as the acceleration G increases. It has become.
The reason why the frequency change rate is different in each vibration mode is that in the fundamental wave symmetric mode S0, the coupling region 44 is a region where a large vibration energy is generated, and the elastic constant that determines the resonance frequency is nonlinear due to the bending stress. By changing, the resonance frequency F (S0) in the fundamental wave symmetric mode S0 changes greatly. On the other hand, in the fundamental wave oblique symmetry mode A0, it is considered that the change in the resonance frequency F (A0) is small because the coupling region 44 is a region having a small vibration amplitude.
The relationship between the stress generated in the deformation state of the piezoelectric substrate and the resonance frequency of the surface acoustic wave element is “perturbation theory for a linear equation in an elastic body subjected to minute deformation” (reference: F. Tiersten: “Perturbation theory for linear electroelastic equations for small fields superposed on a bias”, Journal of the Acoustical Society of America, Vol. 64, no. 3, pp. 832-837 (1978)) Yes.

In this embodiment, the acceleration can be detected by detecting the frequency difference between the resonance frequency F (S0) in the fundamental wave symmetric mode S0 and the resonance frequency F (A0) in the fundamental wave symmetric mode A0.
Specifically, since the groove 40 is formed in the coupling region of the laterally coupled double mode SAW resonator 43, the fundamental wave symmetric mode S0 excited in the laterally coupled dual mode SAW resonator 43 and In the fundamental wave symmetric mode A0, the frequency of the fundamental wave symmetric mode S0 changes greatly while the frequency change of the fundamental wave symmetric mode A0 is small. By detecting the difference in resonance frequency between the fundamental wave symmetric mode S0 and the fundamental wave oblique symmetric mode A0, vibration, acceleration, and the like can be detected with high sensitivity.
FIG. 6 is a graph showing an example of the relationship between the acceleration G and the frequency difference between the resonance frequency F (A0) and the resonance frequency F (S0).
As described above, the resonance frequency F (A0) has a higher frequency than the resonance frequency F (S0), and the frequency change rate of the resonance frequency F (S0) in the fundamental wave symmetric mode S0 when acceleration is applied. Is larger than the frequency change rate of the resonance frequency F (A0) in the fundamental wave oblique symmetry mode A0, the difference between the resonance frequency F (A0) and the resonance frequency F (S0) becomes smaller as the acceleration increases. For example, when the frequency difference (F (A0) −F (S0)) is 300.00 kHz, the frequency difference is 299.46 kHz when the acceleration is 1 G.

Next, an oscillation circuit for exciting the dual mode SAW resonator of the SAW sensor element will be described.
FIG. 7 is an explanatory diagram of a two-mode oscillation circuit that excites a dual-mode SAW resonator.
The two-mode oscillation circuit uses a Colpitts oscillation circuit known in the art, and a SAW sensor element 30, a C-MOS inverter 51, and a feedback resistor 52 are arranged in parallel, and signals at both ends of the SAW sensor element 30 are connected to capacitors 53 and 54. It is comprised so that it may be divided by.
Further, this oscillation circuit is provided with a switch 55, and one electrode finger side of the IDT 33 and both electrode finger sides of the IDT 31 are connected. Similarly, a switch 56 is provided in the oscillation circuit, and the other electrode finger side of the IDT 33 and both electrode finger sides of the IDT 31 are connected. The switches 55 and 56 are connected to the input terminal 50.

In the above two-mode oscillation circuit, first, when a mode switching signal is input from the input terminal 50, when the mode switching signal is LOW, the switches 55 and 56 are connected to the L-side terminal. In this state, the terminals 57a and 58b have a positive potential and the terminals 57b and 58a have a negative potential, and the IDTs 31 and 33 are excited in the same phase to enter the fundamental wave symmetric mode S0. Then, a resonance frequency signal in the fundamental wave symmetric mode S0 is output from the output terminal 59.
On the other hand, when a HIGH mode switching signal is input from the input terminal 50, the switches 55 and 56 are connected to the H-side terminal. The IDTs 31 and 33 are excited in the opposite phase and enter the oscillation state of the fundamental wave oblique symmetry mode A0. Then, a resonance frequency signal in the fundamental wave oblique symmetry mode A0 is output from the output terminal 59.
Thus, by switching the mode switching signal, the resonance frequency signal in the fundamental wave symmetric mode S0 and the resonance frequency signal in the fundamental wave oblique symmetry mode A0 can be output.

FIG. 8 is a block diagram for explaining an example of a detection method for detecting acceleration as an acceleration detection circuit.
As shown in FIG. 8, a timing generator 70 and a two-mode oscillation circuit 71 are provided as an acceleration detection unit to obtain a resonance frequency signal in the fundamental wave symmetric mode S0 and a resonance frequency signal in the fundamental wave oblique symmetry mode A0. . In order to process these resonance frequency signals, an up-and-down counter 72, a latch circuit 73, a digital low-pass filter 74, a D / A converter 75, and a TCXO (temperature compensation crystal oscillator) 76 are provided.

  The timing generator 70 is connected to a two-mode oscillation circuit 71 and an up-and-down counter 72. The two-mode oscillation circuit 71 is connected to an up-and-down counter 72, and the up-and-down counter 72 is connected to a latch circuit 73. Further, the latch circuit 73 is connected to a digital low-pass filter 74, and the digital low-pass filter 74 is connected to a D / A converter 75. The TCXO 76 is connected to a timing generator 70, a latch circuit 73, a digital low-pass filter 74, and a D / A converter 75.

A mode switching signal is input from the timing generator 70 to the two-mode oscillation circuit 71, and the resonance frequency signal in the fundamental wave symmetric mode S0 and the resonance frequency signal in the fundamental wave oblique symmetry mode A0 are alternately switched from the two-mode oscillation circuit 71. It is output to the up-and-down counter 72.
The up-and-down counter 72 counts the resonance frequency signal alternately in the up and down states, and outputs count data to the latch circuit 73. In the latch circuit 73, a difference in resonance frequency (F (A0) −F (S0)) is taken in as count data.
The time sequence of this data is selected by the digital low-pass filter 74, the acceleration component frequency is selected as a low frequency, the D / A converter 75 converts the digital data into analog data, and the acceleration signal as an analog signal is converted. It has gained.
The TCXO 76 supplies a reference signal serving as a reference for operation to the timing generator 70, the latch circuit 73, the digital low-pass filter 74, and the D / A converter 75.

FIG. 9 is a time chart showing the operation timing of each signal in the detection of acceleration.
The timing generator 70 generates a mode switching signal that periodically repeats LOW and HIGH. Corresponding to the mode switching signal, the resonance frequency signal in the fundamental wave symmetry mode S0 and the resonance frequency signal in the fundamental wave oblique symmetry mode A0 are output from the two-mode oscillation circuit 71.
Count data is generated from the resonance frequency signal by the up-and-down counter 72, and latch data is generated from the count data and the gate signal in the latch circuit 73. Then, an output voltage corresponding to the latch data during the period T is obtained from the D / A converter 75.

As described above, the acceleration sensor 1 according to the present embodiment cantilever the SAW sensor element 30 so that when an external force such as acceleration is applied in the thickness direction of the SAW sensor element 30, the displacement amount of the SAW sensor element 30 is determined. The frequency can be increased and the amount of change in frequency increases. Furthermore, since the SAW sensor element 30 is cantilevered, the stress applied to the SAW sensor element 30 due to adhesion of an adhesive or the like can be suppressed as much as possible, and further, the frequency change of the acceleration sensor 1 due to the secular change of the adhesion portion can be suppressed. Can be reduced.
In particular, when an SH wave is used as the surface acoustic wave, the SAW sensor element 30 can be downsized because the SH wave has a larger electromechanical coupling coefficient and a larger reflection coefficient than the Rayleigh wave.

Further, there is a method of detecting a difference in resonance frequency between the fundamental wave symmetric mode S0 and the fundamental wave oblique symmetric mode A0 of the laterally coupled double mode SAW resonator 43 using an up-and-down counter with a two-mode oscillation circuit. Adopted. From this, the SAW resonator is oscillated by switching one oscillator, so that the circuit configuration is small and the current consumption can be reduced.
Also, use a quartz substrate (SH cut substrate) 10 with Euler angle display (0 ° ± 1 °, 29.2 ° ≦ θ ≦ 40.7 °, 90 ° ± 2 °) with excellent frequency temperature characteristics. Thus, a small and highly accurate acceleration sensor 1 and SAW sensor element 30 can be realized.

As described above, in the present embodiment, the structure in which the SAW sensor element is cantilevered has been described, but the present invention can also be implemented in a structure in which both ends are supported.
In the present embodiment, the acceleration sensor has been described as an example. However, the acceleration sensor may be used as a vibration sensor that detects vibration.

The structure of the acceleration sensor of this embodiment is shown, (a) is a schematic plan view, (b) is a schematic sectional drawing in alignment with the AA disconnection of the same figure (a). The structure of the SAW sensor element of this embodiment is shown, (a) is a schematic plan view, (b) is a schematic sectional drawing in alignment with the BB disconnection of the same figure (a). The schematic diagram explaining the cut surface of the crystal substrate in this embodiment. It is explanatory drawing explaining operation | movement of an acceleration sensor, (a) is a schematic diagram which shows the cross section of a SAW sensor element, (b) is a figure which shows the vibration displacement of a dual mode SAW resonator, (c) is a SAW sensor element The figure which shows the displacement of the quartz substrate in. The graph which shows the relationship between the frequency change rate and acceleration in the acceleration sensor of this embodiment. The graph which shows the relationship between the frequency difference of the resonant frequency F (A0) and the resonant frequency F (S0), and the acceleration G in the acceleration sensor of this embodiment. Explanatory drawing of the 2 mode oscillation circuit which excites the dual mode SAW resonator in the acceleration sensor of this embodiment. The block diagram explaining an example of the detection method which detects an acceleration as an acceleration detection part in the acceleration sensor of this embodiment. The time chart which shows the operation timing of each signal in the detection of the acceleration of this embodiment.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Acceleration sensor, 10 ... Quartz substrate, 20 ... Package, 21 ... Connection terminal, 22 ... Connection terminal, 23 ... External connection terminal, 24 ... Seam ring, 25 ... Cover, 26 ... Metal wire, 27 ... Adhesive, 28 ... Metal wire, 30 ... SAW sensor element, 31 ... IDT, 31a, 31b ... Electrode finger, 32 ... Reflector, 32a ... Metal conductor, 33 ... IDT, 33a, 33b ... Electrode finger, 34 ... Reflector, 34a ... Metal conductor, 35, 36, 37, 38 ... connection pad, 40 ... groove, 41 ... first SAW resonator, 42 ... second SAW resonator, 43 ... laterally coupled dual mode SAW resonator, 45 ... Side 46. One end 47. Other end 50. IC chip 70. Timing generator 71. Two-mode oscillation circuit 72 72 Up-and-down counter 73 Latch circuit 74 Digital Low pass filter, 75 ... D / A converter.

Claims (3)

The substrate includes a laterally coupled double mode SAW resonator in which a first SAW resonator and a second SAW resonator are arranged in parallel in a first direction with a vibration coupling region interposed therebetween, and the coupling A SAW sensor element having a groove in the region;
The SAW sensor element is supported on at least one of the end portions in the first direction of the substrate ,
A SAW sensor that detects a difference between a resonance frequency in a fundamental wave symmetry mode S0 and a resonance frequency in a fundamental wave oblique symmetry mode A0 of the dual mode SAW resonator. The SAW sensor according to claim 1,
The SAW sensor, wherein one end of the SAW sensor element is a free end. The SAW sensor according to claim 1 or 2,
In the detection of the difference between the resonance frequency of the fundamental wave anti-symmetric mode A0 and the resonance frequency in the fundamental symmetric mode S0 of the double mode SAW resonator, that the two-mode oscillator and the up-and-down counter is used Characteristic SAW sensor.

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