JP2012002679A - Inertial sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inertial sensor which can improve mechanical characteristics such as impact, reduces the influence of temperature characteristics and can accurately detect acceleration or the like.SOLUTION: An acceleration sensor 1 has a beam-shaped extended piezoelectric body substrate 10. The piezoelectric body substrate 10 has first and second groove parts 141 and 142. A portion between the first and second groove parts 141 and 142 is made to be a weight 12. Both of end parts 131 and 132 of the piezoelectric body substrate 10 are supported. A surface 11a of the piezoelectric body substrate 10 has first and second SAW elements 21 and 22 for oscillating a surface acoustic wave formed thereon. The first and second SAW elements 21 and 22 have the same structure, and are formed so as to oscillate a surface acoustic wave having the same resonance frequency in a state in which the acceleration is not applied to the weight 12. A detection circuit 61 for detecting the differential value of the respective frequencies of the respective surface acoustic waves oscillated by oscillation circuits 31 and 32 is provided.

Description

  The present invention relates to an inertial sensor that detects acceleration, angular velocity, and the like using a surface acoustic wave (SAW) excited by a SAW element formed on the surface of a piezoelectric substrate.

  Conventionally, an acceleration sensor that detects acceleration and an angular velocity sensor that detects angular velocity are known. These inertial sensors such as an acceleration sensor and an angular velocity sensor are used for various purposes, for example, for car navigation devices, automobiles, robot posture control, pipe line measurement, camera shake correction, and the like. Applications of this inertial sensor have been expanded due to progress in cost reduction, etc., and with the expansion of the market, needs for low cost and high accuracy have been advanced.

  Here, the principle of detecting the acceleration in the acceleration sensor will be described using a model with a weight attached to a spring. That is, a weight is attached to the spring and the spring is vibrated at a resonance frequency. In this state, when acceleration is applied to the weight, the resonance frequency of the spring changes. The acceleration can be detected by detecting the change in frequency. This principle is also used in devices that accurately measure the weight of objects such as electronic balances. In this case, since the stress applied to the spring changes depending on the weight of the object placed on the balance, the frequency changes. Therefore, the weight of the object can be measured by detecting the frequency change.

  As an acceleration sensor for detecting acceleration using the same principle as that for detecting acceleration by changing the resonance frequency of the spring, an acceleration sensor for detecting acceleration using surface acoustic waves is known ( For example, see Patent Document 1). For example, in the acceleration sensor of Patent Document 1, a piezoelectric substrate made of a piezoelectric material such as quartz is formed in a beam shape. A SAW element composed of interdigital electrodes for oscillating surface acoustic waves (SAW) is formed on the surface of the piezoelectric substrate. Further, one end portion of the piezoelectric substrate is supported, and a weight is provided at the other end portion. An oscillation circuit is connected to the SAW element, and surface acoustic waves corresponding to the physical properties of the piezoelectric substrate are oscillated by the oscillation circuit. When acceleration is applied in this state, a force acts on the weight provided on the piezoelectric substrate, causing the piezoelectric substrate to be distorted. When the piezoelectric substrate is distorted, the physical properties of the piezoelectric substrate change, so that the resonance frequency of the oscillated surface acoustic wave changes. And the applied acceleration is detected by detecting the change of the resonance frequency.

JP 2009-243983 A

  However, in the acceleration sensor disclosed in Patent Document 1, since the piezoelectric substrate has a cantilever structure in which one end is a support end and the other end is a free end, there is a difficulty in mechanical characteristics such as impact. . In the conventional acceleration sensor, if a material having a large resonance frequency temperature characteristic is used for the piezoelectric substrate, the resonance frequency may change due to the influence of the temperature characteristic. In this case, even if the same acceleration is applied, the resonance frequency varies depending on the temperature, so that accurate acceleration cannot be detected.

  The present invention has been made in view of the above problems, and provides an inertial sensor that can improve mechanical characteristics such as impact and can detect accurate acceleration and the like by reducing the influence of temperature characteristics. Let it be an issue.

In order to solve the above problems, an inertial sensor of the present invention is a piezoelectric substrate extended in a beam shape, and both ends of the piezoelectric substrate are supported,
A weight provided between the both ends of the piezoelectric substrate;
A first driving electrode formed between the weight on the surface of the piezoelectric substrate and the first support portion, which is one of the end portions, is composed of interdigital drive electrodes for oscillating surface acoustic waves (SAW). A SAW element;
It is formed between the weight on the surface of the piezoelectric substrate and a second support portion which is the opposite end of the first support portion, and is composed of interdigital drive electrodes for oscillating surface acoustic waves. A second SAW element,
An oscillation circuit that oscillates a surface acoustic wave in each of the first and second SAW elements;
A detection circuit that detects a difference in frequency of the surface acoustic wave oscillated by each of the first and second SAW elements as a physical quantity corresponding to a force applied to the weight. To do.

  According to this, since the piezoelectric substrate has a doubly-supported beam structure in which both ends are respectively supported, mechanical characteristics such as impact can be improved as compared with the cantilever structure. In addition, a weight is provided between both end portions (first support portion, second support portion), and a first SAW element is formed between the weight and the first support portion. A second SAW element is formed between the first support portion and the second support portion. A surface acoustic wave is oscillated in each of the first and second SAW elements by the oscillation circuit. When a force such as acceleration is applied to the weight in this state, the resonance frequency of the surface acoustic wave oscillated by each of the first and second SAW elements changes. Here, considering the force applied to the weight in the direction in which the piezoelectric substrate is extended, the compressive force is applied to one support portion side of the piezoelectric substrate by the force, while the other support portion is applied. A pulling force is applied to the side. Accordingly, a surface acoustic wave having a resonance frequency corresponding to the compressive strain (compression force) is oscillated in one of the first and second SAW elements, and the elastic surface having a resonance frequency corresponding to the tensile strain (tensile force) in the other. A wave is oscillated. Therefore, based on the frequency of these surface acoustic waves, physical quantities such as acceleration and inclination angle corresponding to the force applied to the weight can be detected. At this time, in the present invention, the detection circuit detects the physical quantity based on the difference between the frequencies of the surface acoustic waves, so that the influence of the temperature characteristics among the frequencies of the surface acoustic waves is canceled out. Can do. Therefore, it is possible to detect accurate acceleration and the like by reducing the influence of the temperature characteristics.

  In the present invention, the first and second SAW elements are formed such that surface acoustic waves having the same resonance frequency are oscillated in a state where no force is applied to the weight. .

  As a result, the surface acoustic wave affected by the temperature characteristic is oscillated by the same amount, so that when the difference is taken, the influence of the temperature characteristic can be surely canceled.

  In the present invention, each of the first and second SAW elements is formed such that a surface acoustic wave is oscillated in a direction in which the piezoelectric substrate is extended.

  As a result, the physical quantity in the direction in which the piezoelectric substrate is extended can be detected. In this direction, the influence of the temperature characteristic can be canceled as described above, so that an accurate physical quantity can be detected. .

  In the present invention, the weight is provided in a central portion of the piezoelectric substrate in a direction in which the piezoelectric substrate is extended.

  As described above, by providing the weight at the center of the piezoelectric substrate, it is easy to form the first and second SAW elements in which surface acoustic waves having the same resonance frequency are oscillated in a state where no force is applied to the weight. it can.

In the present invention, the piezoelectric substrate may be formed on the back surface opposite to the front surface on which the first and second SAW elements are formed, between the first support portion and the central portion of the piezoelectric substrate. A first groove is formed between the second support and the central portion of the piezoelectric substrate, and a second groove is formed between the second support and the piezoelectric substrate.
A central portion of the piezoelectric substrate that is a portion between the first and second groove portions is the weight.

  As described above, the first and second groove portions are formed between the support portion and the central portion of the piezoelectric substrate, thereby making the central portion thicker than the first and second groove portions. Therefore, the central part can be a weight. Therefore, the weight and the piezoelectric substrate can be integrated.

In the present invention, the first SAW element is formed in a first thin portion that is a portion of the piezoelectric substrate in which the first groove portion is formed,
The second SAW element is formed in a second thin portion that is a portion of the piezoelectric substrate in which the second groove portion is formed.

  According to this, since the first and second thin-walled portions are thin, when the force is applied to the weight, the first and second SAW elements are located where the amount of distortion is large. Since the first and second thin films are formed, it is possible to oscillate a surface acoustic wave having a resonance frequency corresponding to the amount of strain with high sensitivity.

  In the present invention, the first and second groove portions are grooves having the same groove width and groove depth.

  According to this, since the first and second groove portions are grooves having the same groove width and groove depth, the shapes of the first and second thin portions can be made the same. As a result, the characteristics of the surface acoustic waves oscillated by the first and second SAW elements formed in the first and second thin portions can be made the same.

In the present invention, the piezoelectric substrate is a substrate made of lithium niobate crystal (LiNbO ₃ ). When the crystal axes of LiNbO ₃ are the X axis, the Y axis, and the Z axis, the piezoelectric substrate is The extended direction is the X-axis, and the Y-axis is a substrate cut by a 128 ° Y-X plane rotated 128 degrees around the X-axis.

Thus, by using a piezoelectric substrate of lithium niobate crystal (LiNbO ₃ ) cut at a 128 ° YX plane, surface acoustic waves can be efficiently oscillated. Moreover, since LiNbO ₃ has a larger electromechanical coupling coefficient than quartz, the inertial sensor can be miniaturized.

  In the present invention, a decoupling groove for separating a region where the first SAW element is formed and a region where the second SAW element is formed is formed on the surface of the piezoelectric substrate. It is characterized by.

  As described above, since the region where the first SAW element is formed and the region where the second SAW element is formed are separated by the decoupling groove, the surface acoustic wave oscillated by one SAW element is the other. The influence on the SAW element can be reduced. Therefore, a physical quantity such as acceleration can be accurately detected.

  Further, in the present invention, the interdigital drive electrodes constituting the first and second SAW elements are apodized electrodes in which the overlapping range of the cross electrodes constituting the drive electrodes is increased toward the center. Features.

  According to this apodized electrode, the weight is increased toward the center of the drive electrode, and the weight is decreased toward both sides of the drive electrode. Therefore, the generation of the third harmonic can be suppressed, and the oscillated surface acoustic wave is generated. Leakage outside the drive electrode can be suppressed.

  Further, in the present invention, the first and second SAW elements each have an interdigital reflection that reflects the surface acoustic wave oscillated by each drive electrode toward the drive electrode on both sides of the interdigital drive electrode. An electrode is provided.

  As a result, the surface acoustic wave oscillated by the drive electrode can be prevented from leaking out of the drive electrode.

  In the present invention, the piezoelectric substrate is a substrate having a length of each side constituting the piezoelectric substrate of less than 10 mm. Thus, since the piezoelectric substrate (inertial sensor) is miniaturized, it can be provided in various apparatuses without taking up space.

1 is a diagram illustrating a schematic configuration of an acceleration sensor 1. FIG. It is the figure which looked at the piezoelectric substrate 10 from the side surface direction. 2 is a diagram for explaining a crystal orientation of a piezoelectric substrate 10. FIG. 2 is a diagram showing a relationship between first and second SAW elements 21 and 22 and a crystal axis of a piezoelectric substrate 10. FIG. It is a figure explaining the structure of the 1st, 2nd SAW element 21 and 22. FIG. It is the figure which showed an example of the principle confirmation circuit for confirming that the acceleration (alpha) can be detected with the acceleration sensor 1 of FIG. It is the figure which showed the manufacturing procedure of the acceleration sensor. It is the figure which showed typically the condition of the process of S13 of FIG. It is the figure which showed typically the condition of the process of S17 of FIG. It is the figure which showed typically the condition of the process of S18 of FIG. It is the figure which showed typically the condition of the process of S19 of FIG. It is the figure which showed typically the condition of the process of S21 of FIG. It is the figure which showed typically the condition of the process of S22-S24 of FIG.

  Hereinafter, embodiments of the inertial sensor according to the present invention will be described. The inertial sensor of the present embodiment is an acceleration sensor that detects acceleration. The acceleration sensor has a so-called MEMS (Micro_Electro_Mechanical_Systems) structure. That is, the micro acceleration sensor is formed using a semiconductor processing technique such as etching.

Here, FIG. 1 is a diagram showing a schematic configuration of the acceleration sensor 1 of the present embodiment. As shown in FIG. 1, the acceleration sensor 1 includes a piezoelectric substrate 10, SAW elements 21 and 22, oscillation circuits 31 and 32, and a detection circuit 61. FIG. 1 is a perspective view of the piezoelectric substrate 10 and the SAW elements 21 and 22. The piezoelectric substrate 10 is obtained by processing a substrate of lithium niobate crystal (LiNbO ₃ ), which is a piezoelectric body, as shown in FIG. Specifically, the piezoelectric substrate 10 has a rectangular shape in a plan view (view of the piezoelectric substrate 10 in FIG. 1 as viewed from above) and extends in a beam shape. Here, the direction in which the piezoelectric substrate 10 is extended is the a-axis, and the width direction of the piezoelectric substrate 10 (the depth direction of the piezoelectric substrate 10 in FIG. 1) is the b-axis. The plate thickness direction of the piezoelectric substrate 10 is perpendicular to the axis and the b axis, and is the c axis. In this embodiment, the size of the piezoelectric substrate 10 is as follows: a-axis direction length W = 5.55 mm, b-axis direction length D = 2.0 mm, c-axis direction length H = 0. It is 35 mm. As described above, the piezoelectric substrate 10 is a minute substrate in which each side has a length of 10 mm or less.

  The piezoelectric substrate 10 has first and second groove portions 141 and 142 formed on the back surface 11b side. Each of these two groove portions 141 and 142 has a rectangular shape in a sectional view, and the rectangular shape is a rectangular parallelepiped shape extending in the c-axis direction of the piezoelectric substrate 10. The first groove 141 of the two grooves 141 and 142 is formed adjacent to one of the first ends 131 of both ends 131 and 132 of the piezoelectric substrate 10. The second groove 142 is formed adjacent to the other second end 132.

  Each of the first and second end portions 131 and 132 has a rectangular shape in a sectional view, and the rectangular shape extends in the c-axis direction of the piezoelectric substrate 10. And these 1st, 2nd edge parts 131 and 132 each function as a support part to be supported, and in this embodiment, the surface 131a on the back surface 11b side of the 1st and 2nd edge parts 131, 132. 132a (see FIG. 2) are supported. Hereinafter, the first end portion 131 is referred to as a first support portion 131, and the second end portion 132 is referred to as a second support portion 132.

  The portions of the piezoelectric substrate 10 on which the first and second groove portions 141 and 142 are formed are thin. And if these thin parts are called the 1st thin part 151 and the 2nd thin part 152, respectively, the 1st and 2nd SAW element 21 mentioned below in these 1st and 2nd thin parts 151 and 152, 22 will be formed.

  In addition, since the first and second groove portions 141 and 142 are formed, the portion 12 of the piezoelectric substrate 10 between the first and second groove portions 141 and 142 has the first and second thin wall portions. 151 and 152 are made thicker. This portion 12 becomes a weight to which force is applied when acceleration is applied. Thus, the weight 12 is integrated with the piezoelectric substrate 10 by forming the first and second groove portions 141 and 142. In addition, since the 1st, 2nd groove parts 141 and 142 are made into the rectangular parallelepiped shape, the weight 12 is also made into the rectangular parallelepiped shape.

  Further, a decoupling groove 16 is formed on the surface 11a of the piezoelectric substrate 10 in order to prevent the first and second SAW elements 21 and 22 described later from affecting each other. The decoupling groove 16 is formed to extend in the width direction (b-axis direction) of the piezoelectric substrate 10 so as to separate the piezoelectric substrate 10 left and right at the center in the a-axis direction of the piezoelectric substrate 10. It is a groove.

  Here, the position and size of each part of the piezoelectric substrate 10 will be further described with reference to FIG. FIG. 2 shows the piezoelectric substrate 10 as viewed from the side. As shown in FIG. 2, it can be seen that the first and second support portions 131 and 132, the first and second groove portions 141 and 142, and the weight 12 are each rectangular in cross-section. The width W31 of the first support portion 131 and the width W32 of the second support portion 132 are the same, specifically, for example, 0.5 mm to 0.6 mm. Further, the width W21 of the first groove portion 141 and the width W22 of the second groove portion 142 are the same, specifically, for example, 0.45 mm. The height H21 of the first groove 141 and the height H22 of the second groove 142 are the same, specifically, for example, 0.3 mm to 0.35 mm. The heights H21 and H22 of the first and second groove portions 141 and 142 are also the heights on both sides of the weight 12.

  Moreover, the width W1 of the weight 12 is set to, for example, 3.45 mm to 3.55 mm, and the heights H21 and H22 on both sides thereof are set to, for example, 0.3 mm to 0.35 mm as described above. As described above, since W31 = W32 and W21 = W22, one side of the weight 12 with respect to the center line CL in the direction (a-axis direction) in which the piezoelectric substrate 10 is extended. The width W11 up to and the width W12 up to the other side are the same. Thus, the weight 12 is located at the center of the piezoelectric substrate 10 in the a-axis direction.

  In addition, since the first and second thin portions 151 and 152 have the same heights H21 and H22 of the first and second groove portions 141 and 142, respectively, their heights H11 and H12 are also the same. Specifically, for example, it is smaller than 0.05 mm.

  Further, the decoupling groove 16 is formed on the surface 11a of the piezoelectric substrate 10 so as to extend over the center line CL, and its width W4 is set to be larger than 60 μm, for example.

  Thus, the piezoelectric substrate 10 has a symmetrical shape with respect to the center line CL, and the first and second support portions 131 and 132 are supported and the weight 12 is provided therebetween. Therefore, it can be said that it is a double-supported beam structure.

As described above, the piezoelectric substrate 10 is a substrate made of a lithium niobate crystal (LiNbO ₃ ) that is a piezoelectric body. Therefore, when a force is applied and distortion occurs, electric energy is generated. Conversely, when electric energy is applied, distortion occurs. Due to this property, the piezoelectric substrate 10 has a property of generating a surface acoustic wave (SAW). In order to efficiently generate a surface acoustic wave, in this embodiment, the piezoelectric substrate 10 in a predetermined crystal orientation is used. The body substrate 10 is used. Here, FIG. 3 is a diagram for explaining the crystal orientation of the piezoelectric substrate 10, and the crystal axes (X, Y, Z) of the lithium niobate crystal (LiNbO ₃ ) and the substrate 10 with respect to the piezoelectric substrate 10. The figure which piled up the axis | shaft (a, b, c) is shown. As shown in FIG. 3, the axis a in the direction in which the piezoelectric substrate 10 is extended coincides with the X axis of the lithium niobate crystal (LiNbO ₃ ). The axis obtained by rotating the Y axis of the lithium niobate crystal (LiNbO ₃ ) 128 degrees around the X axis is the axis b in the width direction of the piezoelectric substrate 10. That is, the piezoelectric substrate 10 is a substrate obtained by cutting lithium niobate crystal (LiNbO ₃ ) at a 128 ° YX plane (a substrate of Euler angle display (0 °, 38 °, 0 °)). . By using such a substrate, a surface acoustic wave can be efficiently generated in the direction in which the piezoelectric substrate 10 is extended (X-axis direction, a-axis direction). The surface acoustic wave at this time is a Rayleigh wave of LiNbO ₃ -128Y (38Z).

Returning to the description of FIG. 1, first and second SAW elements 21 and 22 are formed on the surface 11 a of the piezoelectric substrate 10. The first and second SAW elements 21 and 22 are IDTs configured of interdigital drive electrodes for oscillating surface acoustic waves (SAW) on the surface 11 a of the piezoelectric substrate 10. The first and second SAW elements 21 are made of, for example, gold Au. Of the first and second SAW elements 21 and 22, one first SAW element 21 is formed closer to the first support portion 131 (first thin portion 151) than the decoupling groove 16. The second SAW element 22 is formed closer to the second support part 132 (second thin part 152) than the decoupling groove 16. The first and second SAW elements 21 and 22 are formed so as to oscillate surface acoustic waves in the direction in which the piezoelectric substrate 10 is extended (X-axis direction, a-axis direction). Here, FIG. 4 is a diagram for explaining the relationship between the first and second SAW elements 21 and 22 and the crystal axes (X, Y, Z) of the lithium niobate crystal (LiNbO ₃ ). As shown in FIG. 4, the first and second SAW elements 21 and 22 are formed in the direction of the X axis, in other words, the interdigital electrodes are perpendicular to the X axis. As a result, the surface acoustic waves oscillated by the first and second SAW elements 21 and 22 propagate in the direction in which the piezoelectric substrate 10 is extended (X-axis direction).

  Here, FIG. 5 shows a plan view of the first and second SAW elements 21, 22. Specifically, FIG. 5A shows the first and second SAW elements 21, 22. The figure which showed the state in which it arranged, (b) of the figure has shown the enlarged view of the 1st, 2nd SAW element 21 and 22. FIG. As shown in FIG. 5A, the first and second SAW elements 21 and 22 are formed side by side on the surface 11a of the piezoelectric substrate 10 and have the same structure. As shown in FIG. 5B, the first and second SAW elements 21 and 22 have an element length D1 larger than 2.75 mm and an opening length (width) D4 of 1.6 mm. Each of the first and second SAW elements 21 and 22 includes an interdigital drive electrode 201 and interdigital reflective electrodes 202 and 203 formed on both sides of the drive electrode 201.

  As shown in FIG. 5B, the drive electrode 201 has two drive reference electrodes 201a and 201b having a length D2 = 345 μm and formed to face each other in parallel. Interdigital electrodes 201c and 201d are connected to the drive reference electrodes 201a and 201b, respectively. The electrodes 201c and 201d are formed so that the electrodes constituting the electrodes 201c and 201d intersect each other. Hereinafter, these electrodes 201c and 201d are referred to as crossed electrodes 201c and 201d. That is, the electrodes constituting one cross electrode 201c and the electrodes constituting the other cross electrode 201d are alternately positioned. In the present embodiment, there are 11 pairs of the crossing electrode 201c and the crossing electrode 201d that face each other.

  Furthermore, in this embodiment, the length of each cross electrode 201c, 201d differs depending on the position. That is, in each pair of the cross electrode 201c and the cross electrode 201d, a range (overlapping range) facing the other cross electrode is weighted differently depending on the position. Specifically, as shown in FIG. 5B, the drive electrode 201 is a so-called apodized electrode in which the overlapping range is increased toward the center of the drive electrode 201. Accordingly, generation of the third harmonic can be suppressed, and lateral coupling in which the surface acoustic wave oscillated by the drive electrode 201 affects other parts can be prevented.

  1 is connected to the drive reference electrodes 201a and 201b, and the oscillation circuits 31 and 32 generate surface acoustic waves having a resonance frequency corresponding to the physical properties of the piezoelectric substrate 10. It will oscillate. At this time, the length of each cross electrode 201c, 201d of the drive electrode 201 is set so that the surface acoustic wave oscillated has a predetermined resonance frequency f0 (predetermined wavelength λ) in a state where no acceleration is applied to the weight 12. , The interval is set. Specifically, the drive electrode 201 is formed so that a surface acoustic wave having a resonance frequency f0 = 66 MHz (wavelength λ = 60 μm) is oscillated in a state where no acceleration is applied to the weight 12. Therefore, the distance L / S between the crossing electrodes 201c and 201d is set to 15 μm (60 μm / 4).

  The drive electrode 201 is formed on the surfaces of the first and second thin portions 151 and 152 of the piezoelectric substrate 10. Since the first and second thin portions 151 and 152 are thinner than other portions, the first and second thin portions 151 and 152 are portions having a large amount of distortion when acceleration is applied to the weight 12. Therefore, by forming the drive electrode 201 in the first and second thin portions 151 and 152, the surface acoustic wave can be oscillated with high sensitivity.

  As described above, the interdigital reflective electrodes 202 and 203 are formed on both sides of the drive electrode 201. More specifically, each of the reflective electrodes 202 and 203 is formed with an interval of 3 / 8λ, that is, 22.5 μm (= 60 μm × 3/8) with respect to both sides of the drive electrode 201. These reflective electrodes 202 and 203 are for reflecting the surface acoustic wave oscillated by the drive electrode 201 to the drive electrode side, and for preventing the surface acoustic wave from leaking out of the drive electrode 201. The reflective electrodes 202 and 203 of the present embodiment have the same structure, and, like the drive electrode 201, reference electrodes 202a and 202b (in the case of the reflective electrode 202), 203a and 203b (in the reflective electrode 203) that face each other in parallel. Case). In the case of one reflective electrode 202, interdigital crossing electrodes 202c and 202d are connected to the respective reference electrodes 202a and 202b, and the crossing electrode 202c of one reference electrode 202a and the other reference electrode 202b are connected to each other. The cross electrodes 202d are alternately positioned. The same applies to the other reflective electrode 203. In addition, each of the reference electrodes 202a and 202b (in the case of the reflective electrode 203, the reference electrodes 203a and 203b) are connected to the ground GND. That is, each reference electrode 202a, 202b is short-circuited. Each reference electrode 202a, 202b may be open.

  The reflective electrodes 202 and 203 have the same lengths D31 and D32, respectively, specifically, greater than 20 times the wavelength λ (λ = 60 μm) of the surface acoustic wave, that is, D31 and D32> 20λ. Is done. By providing the reflective electrodes 202 and 203 having such a configuration, it is possible to suppress the leakage of the surface acoustic wave oscillated by the drive electrode 201. In this embodiment, the leakage rate is about 20%. In the present invention, since the two SAW elements 21 and 22 are provided side by side, it is particularly preferable to provide the reflective electrodes 202 and 203 in this way because it can prevent the other SAW element from being affected. In this embodiment, as described above, since the decoupling groove 16 is provided between the first and second SAW elements 21 and 22, it is considered that the leakage rate is further reduced.

  The drive electrode 201 is formed on the surfaces of the first and second thin portions 151 and 152, but the reflective electrodes 202 and 203 are not formed on the surfaces of the first and second thin portions 151 and 152. In this embodiment, it is formed on the surfaces of the support portions 131 and 132 and the weight 12.

  Returning to the description of FIG. 1, the first oscillation circuit 31 is connected to the first SAW element 21 (strictly, the drive reference electrodes 201 a and 201 b of the first SAW element 21), and the second oscillation circuit 32 is connected to the first oscillation circuit 32. This circuit is connected to the second SAW element 22 (strictly, the drive reference electrodes 201a and 201b of the second SAW element 22), and causes the first and second SAW elements 21 and 22 to oscillate surface acoustic waves, respectively. . In the present embodiment, the first and second oscillation circuits 31 and 32 are Colpitts oscillation circuits. The first and second oscillation circuits 31 and 32 do not need to be provided on the piezoelectric substrate 10.

  By these oscillation circuits 31 and 32, surface acoustic waves are oscillated in the first and second SAW elements 21 and 22, respectively. Hereinafter, a surface acoustic wave oscillated by the first SAW element 21 is referred to as SAW1, and a surface acoustic wave oscillated by the second SAW element 22 is referred to as SAW2. The resonance frequency f of the surface acoustic wave SAW1 is f1, and the resonance frequency f of the surface acoustic wave SAW2 is f2. As described above, in the state where no acceleration is applied to the weight 12, both the surface acoustic waves SAW1 and SAW2 are the waves having the resonance frequency f1 and f2 = f0 (= 66 MHz). On the other hand, when the acceleration α is applied to the weight 12, a force F (a force of M × α where M is the mass of the weight) acts on the weight 12, so that the piezoelectric substrate 10 is distorted. Therefore, both the surface acoustic waves SAW1 and SAW2 change the resonance frequencies f1 and f2. That is, the resonance frequencies f1 and f2 are functions of the acceleration α (hereinafter, the resonance frequencies f1 and f2 are referred to as f1 (α) and f2 (α), respectively).

  Further, considering the component force in the direction in which the piezoelectric substrate 10 is extended (X-axis direction), the compression force acts on one of the first and second thin portions 151 and 152. On the other hand, a pulling force acts. Therefore, one of the resonance frequencies f1 (α) and f2 (α) has a value corresponding to the compressive strain, and the other of the resonance frequencies f1 (α) and f2 (α) has a value corresponding to the tensile strain. It becomes. That is, the resonance frequency f1 (α) and the resonance frequency f2 (α) are different values (f1 (α) ≠ f2 (α)).

  However, since the piezoelectric substrate 10 has temperature characteristics, the resonance frequency of the surface acoustic wave can change depending on the temperature. Therefore, the resonance frequencies f1 ′ (α) and f2 ′ (α) of the surface acoustic waves SAW1 and SAW2 that are actually oscillated are equal to the resonance frequencies f1 (α) and f2 (α) that do not take temperature characteristics into consideration. This is a value obtained by adding the affected parts ft1 and ft2. That is, f1 ′ (α) = f1 (α) + ft1, and f2 ′ (α) = f2 (α) + ft2.

  Here, the first and second SAW elements 21 and 22 have the same structure, and a surface acoustic wave having the same resonance frequency f0 is oscillated when no acceleration is applied. The parts of the piezoelectric substrate 10 on which the first and second SAW elements 21 and 22 are formed have the same structure. Therefore, since the first and second SAW elements 21 and 22 are considered to be affected by the same temperature, the resonance frequencies ft1 and ft2 affected by the temperature characteristics are considered to be equivalent ( ft1 = ft2).

  Therefore, in the present invention, in order to remove the influence of temperature, the resonance frequency f1 ′ (α) (= f1 (α) + ft1) of the surface acoustic wave SAW1 and the resonance frequency f2 ′ (α) (= Based on the difference of f2 (α) + ft2), the acceleration α is detected. That is, as shown in FIG. 1, the acceleration sensor 1 includes a detection circuit 61, the detection circuit 61 and the first oscillation circuit 31 are signal lines 41, and the detection circuit 61 and the second oscillation circuit 32 are connected to each other. Are connected by a signal line 42. The signal of the resonance frequency f1 ′ (α) in the first oscillation circuit 31 and the signal of the resonance frequency f2 ′ (α) in the second oscillation circuit 32 are detected by the detection circuit 61 via the signal lines 41 and. To be input.

  The detection circuit 61 is configured by combining known logical operators, and is a circuit that detects the difference between the input resonance frequencies f1 ′ (α) and f2 ′ (α). That is, the detection circuit 61 is a circuit for obtaining the difference value Δf = f1 ′ (α) −f2 ′ (α). At this time, since the resonance frequencies ft1 and ft2 are set to ft1 = ft2, Δf = f1 (α) −f2 (α), that is, the difference value Δf from which the influence of the temperature is removed can be obtained. Further, since both f1 (α) and f2 (α) are functions of the acceleration α, the difference value Δf is also a function of the acceleration α. Therefore, since the difference value Δf can be said to correspond to the acceleration α, the acceleration α can be detected from the difference value Δf. In the state where the acceleration α is not applied, f1 (α) = f2 (α) = f0 (66 MHz), and therefore Δf = 0. Therefore, it can be detected that the acceleration α is not applied.

  Since the surface acoustic wave has a high frequency, each logical operator constituting the detection circuit 61 may not be able to follow the high frequency signal. Therefore, it is desirable to provide a frequency dividing circuit for reducing the frequencies of the surface acoustic waves SAW1 and SAW2 input to the detection circuit 61 in front of the detection circuit 61 (see frequency division circuits 51 and 52 in FIG. 6). As a result, the surface acoustic waves SAW1 and SAW2 of f0 = 66 MHz can be dropped to a fraction of a frequency (in the example of FIG. 6, 4.8 kHz that is one-fourth). The difference value Δf can be accurately detected.

  Here, FIG. 6 shows an example in which the acceleration sensor 1 is configured by a commercially available logic circuit in order to confirm that the acceleration α can be detected by the acceleration sensor 1 of FIG. In FIG. 6, parts having the same functions as those in FIG. At this time, the operation of the acceleration sensor 1 was confirmed by dividing it into a plurality of Phases. Specifically, first, as Phases 1 and 2, it is confirmed whether the first and second SAW elements 21 and 22 generate surface acoustic waves SAW1 and SAW2 having a resonance frequency f0 = 66 MHz. As Phase 3, it is confirmed whether the frequency of the surface acoustic waves SAW1 and SAW2 is changed from 66 MHz to 4.8 kHz by using commercially available frequency dividing circuits 51 and 52. As the last Phase 4, it is confirmed whether or not the commercially available phase comparison circuit 61 can determine the difference value Δf between the frequencies of the surface acoustic waves SAW1 and SAW2 dropped to 4.8 kHz. By confirming the operation step by step as described above, the cause can be determined when the acceleration sensor 1 does not perform a desired operation.

Next, the manufacturing method of the acceleration sensor 1 of this embodiment is demonstrated. As described above, the acceleration sensor 1 has a so-called MEMS structure and is manufactured by using semiconductor processing technology. Here, FIG. 7 is a diagram showing a manufacturing procedure of the acceleration sensor 1. First, as a premise, a 128 ° Y-cut LiNbO ₃ wafer is prepared. A LiNbO ₃ wafer having a 36 ° Y-cut surface may be used instead of LiNbO ₃ having a 128 ° Y-cut surface.

First, the LiNbO ₃ wafer is cleaned to remove dust, dirt, and the like on the surface of the LiNbO ₃ wafer (S11). Next, a LiNbO ₃ wafer is placed in oxygen plasma, and O 2 ashing is performed to remove and remove impurities attached to the surface of the wafer (S12). Next, a protective film or the like is formed on the front and back surfaces of the wafer (S13). Here, FIG. 8 is a diagram schematically showing the state of the film forming step of S13. Since the LiNbO ₃ wafer is a member of the piezoelectric substrate 10 shown in FIG. 1, the reference numeral “10” is given to the LiNbO ₃ wafer in FIG. The surface of the wafer 10 on which the first and second SAW elements 21 and 22 are formed is denoted by “11a”, and the surface of the wafer 10 on which the first and second groove portions 141 and 142 are formed is denoted by “ 11b ". As shown in FIG. 8, in S13, about 500 mm of nickel chrome (NiCr) films 60 and 62 as protective films are formed on both the front surface 11a and the back surface 11b of the wafer 10. For example, this film formation is performed by sputtering or vacuum evaporation. Further, on the surface 11 a of the wafer 10, a gold Au film 63 that is the basis of the first and second SAW elements 21 and 22 is formed on the NiCr film 60 by about 500 mm. This film formation can also be performed by sputtering or vacuum deposition.

  Next, the wafer 10 is spin-coated at a high rotation speed so that each of the films 60, 62, 63 formed in S13 has a low film thickness and a uniform film thickness (S14). Next, contour patterns of the first and second SAW elements 21 and 22 or the first and second groove portions 141 and 142 are formed on the both surfaces 11a and 11b of the wafer 10 by exposure (S15). Next, the first and second SAW elements 21 and 22 are formed by development and etching (S16). Next, grooves are formed in portions corresponding to the first and second groove portions 141 and 142 of the back surface 11b of the wafer 10 to form first and second groove portions 141 and 142 (S17). Here, FIG. 9 is a diagram schematically showing the state of the process of S17. As shown in FIG. 9, in S <b> 17, the wafer 10 is grooved using a dicing blade 70. At this time, the outermost periphery of the wafer 10 is left for reinforcement.

  Next, in order to prevent the wafer 10 from being damaged in the subsequent operation, the formed first and second grooves 141 and 142 are filled with a sacrificial layer (S18). Here, FIG. 10 is a diagram schematically showing the state of the process of S18. Specifically, FIG. 10A is a perspective view of the wafer 10, and FIG. 10B is a cross-sectional view of the wafer 10. Is shown. As shown in FIG. 10B, for example, electron wax is filled as a sacrificial layer 64 from above the wafer 10. Next, the back surface 11b of the wafer 10 is ground to flatten the back surface 11b (S19). Here, FIG. 11 is a diagram schematically showing the state of the process of S19. As shown in FIG. 11, by grinding the back surface 11b, the portion above the broken line in FIG. 11 is removed, so that the sacrificial layer (electron wax) attached to the back surface 11b can be removed. FIG. 11 shows a state where the sacrificial layer 64 is filled in the first and second groove portions 141 and 142.

  Next, a dicing tape is affixed to the surface 11a of the wafer 10 (S20), diced and cut into the shape of the piezoelectric substrate 10 of FIG. 1 (full cut), and a decoupling groove 16 is formed (half-half). (S21). Here, FIG. 12 is a diagram schematically showing the state of the process of S21. As shown in FIG. 12, a plurality of substrates 10 are formed from one wafer 10 by full cutting with a dicing blade 91. Further, the decoupling groove 16 is formed in each substrate 10 by half-cutting with the dicing blade 91.

  Next, the formed piezoelectric substrate 10 is mounted on the package PKG (S22). Here, FIG. 13 is a diagram schematically showing the state of the process after the piezoelectric substrate 10 is mounted on the package PKG100. Specifically, FIG. 13A shows the state of the steps S22 and S23. FIG. 13B is a diagram showing a state of the process of S24 described later. As shown in FIG. 13A, in S22, the piezoelectric substrate 10 is mounted on the PKG 100, and the lower surfaces of the first and second support portions 131 and 132 are fixed by adhesives 81 and 82. Thereafter, the adhesives 81 and 82 are cured (S23). Next, the sacrificial layer 64 filled in the first and second groove portions 141 and 142 is removed (etched) with a cleaning liquid (S23, see FIG. 13B).

  Thereafter, wire bonding is performed (S25), the PKG 100 is sealed (LID sealing) (S26), circuit boards such as the oscillation circuits 31 and 32 and the detection circuit 61 are assembled (S27), and the acceleration sensor 1 is adjusted and corrected. Then, an operation confirmation test is performed (S29). Through the above process, the acceleration sensor 1 having a MEMS structure can be manufactured.

As described above, in the acceleration sensor 1 of the present embodiment, since both the support portions 131 and 132 of the piezoelectric substrate 10 are supported, the impact resistance can be improved. In addition, the temperature characteristics of the piezoelectric substrate 10 can be offset by using the first and second SAW elements 21 and 22. Therefore, it is possible to use a piezoelectric material such as LiNbO _{3 which} has a poor temperature characteristic but a good electromechanical coupling coefficient, and the sensor element can be miniaturized. Further, since it is not necessary to use analog circuit elements, a simple and inexpensive circuit configuration can be obtained.

  The inertial sensor of the present invention is not limited to the above embodiment, and can be modified without departing from the description of the scope of the claims. For example, in the above embodiment, an example of an acceleration sensor that detects the acceleration itself has been described. However, the present invention may be applied to an inclination angle sensor that detects a force applied to a weight as an inclination angle. In this case, the inclination angle can be detected by detecting a change in gravitational acceleration. That is, when the tilt angle changes, the component of the gravitational acceleration G acting in the X-axis direction of the piezoelectric substrate changes. Therefore, the inclination angle can be detected by detecting the change in the component of the gravitational acceleration G.

In the above embodiment, the example of the piezoelectric substrate 10 made of lithium niobate crystal (LiNbO ₃ ) has been described. However, a piezoelectric substrate made of other piezoelectric materials such as quartz may be used. Further, the size of the piezoelectric substrate is not limited to the size of the piezoelectric substrate 10 of the above-described embodiment, and may be a size as required.

  In the above embodiment, the first and second support portions 131 and 132 are thicker than the first and second thin portions 151 and 152. By making the first and second support portions 131 and 132 to have the same thickness as the weight 12, the support portions 131 and 132 can be stably supported, but the size and weight can be further reduced. For the purpose of illustration, the thickness of the first and second support portions 131 and 132 may be reduced. Even in this case, the central portion of the piezoelectric substrate can be made thicker than other portions to function as a weight, so that the effects of the present invention can be obtained.

  In the above-described embodiment, the acceleration sensor that detects the uniaxial acceleration (X-axis direction of the piezoelectric substrate 10) has been described. However, the number of SAW elements formed on the surface 11a of the piezoelectric substrate 10 is increased to increase the number of SAW elements. You may comprise so that an acceleration may be detected. In this case, for example, the third SAW element is formed on the weight on the surface of the piezoelectric substrate. In this case, the length is about 2.4 mm longer than that of the piezoelectric substrate 10 of FIG. 1, but the resonance frequency of the surface acoustic wave oscillated by the third SAW element and the first and second Based on the difference from the resonance frequency of the surface acoustic wave oscillated by the SAW element, it is also possible to detect the acceleration in the vertical direction of the piezoelectric substrate from which the influence of temperature has been removed.

1 Acceleration sensor (inertial sensor)
DESCRIPTION OF SYMBOLS 10 Piezoelectric substrate 12 Weight 131,132 1st, 2nd support part (1st, 2nd edge part)
141, 142 First and second groove portions 151, 152 First and second thin wall portions 16 Decoupling grooves 21, 22 First and second SAW elements 31, 32 First and second oscillation circuits 51, 52 Frequency divider 61 Detection circuit 201 Drive electrode 202, 203 Reflective electrode

Claims (12)

A piezoelectric substrate extended in a beam shape, the piezoelectric substrate on which both ends of the piezoelectric substrate are respectively supported;
A weight provided between the both ends of the piezoelectric substrate;
A first driving electrode formed between the weight on the surface of the piezoelectric substrate and the first support portion, which is one of the end portions, is composed of interdigital drive electrodes for oscillating surface acoustic waves (SAW). A SAW element;
It is formed between the weight on the surface of the piezoelectric substrate and a second support portion which is the opposite end of the first support portion, and is composed of interdigital drive electrodes for oscillating surface acoustic waves. A second SAW element,
An oscillation circuit that oscillates a surface acoustic wave in each of the first and second SAW elements;
A detection circuit that detects a difference in frequency of the surface acoustic wave oscillated by each of the first and second SAW elements as a physical quantity corresponding to a force applied to the weight. Inertia sensor.   The first and second SAW elements are formed such that surface acoustic waves having the same resonance frequency are oscillated in a state where no force is applied to the weight. Inertial sensor.   3. The inertia according to claim 1, wherein each of the first and second SAW elements is formed such that a surface acoustic wave oscillates in a direction in which the piezoelectric substrate is extended. Sensor.   4. The inertial sensor according to claim 1, wherein the weight is provided at a central portion of the piezoelectric substrate in a direction in which the piezoelectric substrate is extended. 5. The piezoelectric substrate has a first groove portion between the first support portion and the central portion of the piezoelectric substrate on the back side opposite to the front side where the first and second SAW elements are formed. Is formed, and a second groove is formed between the second support and the central portion of the piezoelectric substrate,
The inertial sensor according to any one of claims 1 to 4, wherein a central portion of the piezoelectric substrate, which is a portion between the first and second groove portions, is the weight. The first SAW element is formed in a first thin portion that is a portion of the piezoelectric substrate in which the first groove portion is formed,
The inertial sensor according to claim 5, wherein the second SAW element is formed in a second thin portion that is a portion of the piezoelectric substrate in which the second groove portion is formed.   The inertial sensor according to claim 5 or 6, wherein the first and second groove portions are grooves having the same groove width and groove depth. The piezoelectric substrate is a substrate composed of lithium niobate crystal (LiNbO ₃ ). When the crystal axes of LiNbO ₃ are the X axis, the Y axis, and the Z axis, the extending direction of the piezoelectric substrate is 8. The substrate according to claim 1, wherein the substrate is the X-axis, and the Y-axis is a substrate cut along a 128 ° Y-X plane rotated by 128 degrees around the X-axis. Inertial sensor.   The decoupling groove for separating the region where the first SAW element is formed from the region where the second SAW element is formed is formed on the surface of the piezoelectric substrate. The inertial sensor according to any one of 1 to 8.   2. The interdigital drive electrode constituting the first and second SAW elements is an apodized electrode in which the overlapping range of each cross electrode constituting the drive electrode is increased toward the center. The inertial sensor of any one of -9.   In each of the first and second SAW elements, interdigital reflectors for reflecting the surface acoustic waves oscillated by the respective drive electrodes toward the drive electrode are provided on both sides of the interdigital drive electrode. The inertial sensor according to any one of claims 1 to 10.   The inertial sensor according to any one of claims 1 to 11, wherein the piezoelectric substrate is a substrate having a length of each side constituting the piezoelectric substrate of less than 10 mm.