Vibration sensor and sensor module

A vibration sensor according to an embodiment includes a laminated body. The laminated body includes a support layer a first end of which is fixed; a piezoelectric layer on the support layer; an insulating layer disposed between the support layer and the piezoelectric layer; a common electrode disposed on a first principal surface of the piezoelectric layer; a first sensing electrode disposed in a first area on a second principal surface of the piezoelectric layer on the side opposite to the first principal surface; and a drive electrode disposed in a second area different from the first area on the second principal surface of the piezoelectric layer. The first area is located near the first end of the support layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2018-100554, filed on May 25, 2018; the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to vibration sensors and a sensor module.

BACKGROUND

Vibration sensors have conventionally been around as sensors to detect minute vibrations of structures, electronic devices, or the like, and grasp the status and state thereof. Vibration sensors are very important in securing safety and reliability of structures, electronic devices, or the like, and in determining whether maintenance needs to be performed, for example.

DETAILED DESCRIPTION

A vibration sensor according to an embodiment includes a laminated body. The laminated body includes a support layer a first end of which is fixed; a piezoelectric layer on the support layer; an insulating layer disposed between the support layer and the piezoelectric layer; a common electrode disposed on a first principal surface of the piezoelectric layer; a first sensing electrode disposed in a first area on a second principal surface of the piezoelectric layer on the side opposite to the first principal surface; and a drive electrode disposed in a second area different from the first area on the second principal surface of the piezoelectric layer. The first area is located near the first end of the support layer.

Vibration sensors and a sensor module according to exemplary embodiments are described in detail below with reference to the accompanying drawings.

Examples of vibration sensors include piezoresistive sensors that utilize a piezoresistance effect, capacitive sensors that utilize a change in capacitance, and piezoelectric sensors that utilize a piezoelectric effect. Of the foregoing, piezoelectric vibration sensors need no external power supply, having a high degree of flexibility, and are also capable of generating enough power on their own to transmit detected data by radio waves, having the advantage of consuming less power. Moreover, piezoelectric vibration sensors, in which excitation can be achieved by applying a voltage, have an advantage in that calibration is possible not only for sensing but also for the sensors themselves.

Piezoelectric vibration sensors of micro electro mechanical systems (MEMS) are of great advantage to cost and size reductions. At the same time, MEMS-type piezoelectric vibration sensors have difficulty in enhancing their sensitivity to minute vibrations and sometimes have difficulty in achieving adequate sensitivity.

It is therefore an object of the embodiments described herein to provide vibration sensors and a sensor module that are capable of enhancing their sensitivity to minute vibrations.

First Embodiment

A vibration sensor according to a first embodiment is first described in detail with reference to the drawings.FIG. 1AandFIG. 1Bare schematic diagrams illustrating an example structure of a vibration sensor according to the first embodiment.FIG. 1Ais a top view of a vibration sensor10, andFIG. 1Bis a sectional view taken along line A-A inFIG. 1A.

As illustrated inFIG. 1AandFIG. 1B, the vibration sensor10according to the present embodiment is a MEMS-type piezoelectric vibration sensor including a cantilever structure (hereinafter referred to as a cantilever structure for convenience of description), and includes a support layer101, an insulating layer102, a common electrode103, a piezoelectric layer104, a sensing electrode105, and a drive electrode106. The support layer101, the insulating layer102, the common electrode103, the piezoelectric layer104, the sensing electrode105, and the drive electrode106constitute the cantilever structure jutting from a support100.

The support layer101is a plate-like structure jutting from the support100. The support layer101may jut from the support100in such a manner that at least one principal surface is substantially normal to the support100. The support layer101may be a member integral with the support100, or may be a member different from that of the support100. Various materials, such as bulk silicon, can be used as the material of the support layer101. In the description, a principal surface is a surface that has the largest area in the structure. The support layer101may be worked into a shape in which the weight concentrates on the tip of the cantilever structure, for example.

The plate-like support layer101includes two principal surfaces made up of the front and back sides thereof. Of these two principal surfaces, one principal surface (this is also referred to as a surface or top surface) has the piezoelectric layer104arranged thereabove that has the same size as the principal surface, for example. Various piezoelectric materials, such as aluminum nitride (AlN), aluminum scandium nitride (AlScN), (K,Na) NbO3(KNN), zinc oxide (ZnO), lead zirconate titanate (PZT), lead zinc niobate (Pb(Zn1/3Nb2/3)O3: PZN-PT), lead magnesium niobate (Pb(Mg1/3Nb2/3)O3: PMN-PT), can be used for the piezoelectric layer104.

Of two principal surfaces of the piezoelectric layer104, one principal surface on the support layer101side (this is referred to as a first principal surface) has the common electrode103disposed thereon. The common electrode103may have a size to cover the whole of the first principal surface, or may have a size to cover a part of the first principal surface. Metals, such as platinum (Pt), molybdenum (Mo), aluminum (Al), and gold (Au), or other conductive materials, for example, can be used for the common electrode103.

The insulating layer102is disposed between the common electrode103and the support layer101. This structure keeps the common electrode103and the support layer101electrically separated. Various insulating materials, such as silicon oxide (SiO2), can be used for the insulating layer102. The support layer101and the insulating layer102can be composed of the same material (for example, an insulating material such as silicon oxide).

Of the two principal surfaces of the piezoelectric layer104, a second principal surface on the side opposite to the first principal surface has the sensing electrode105and the drive electrode106disposed thereon. Metals, such as gold (Au), molybdenum (Mo), aluminum (Al), and platinum (Pt), or other conductive materials, for example, can be used for the sensing electrode105and the drive electrode106. The sensing electrode105and the drive electrode106are both electrodes that confront the common electrode103through the piezoelectric layer104. Of these electrodes, the sensing electrode105serves as an electrode for extracting, as an electric signal, polarization generated on the surface of the piezoelectric layer104by vibrations. Meanwhile, the drive electrode106serves as an electrode for calibrating the vibration sensor10by utilizing an inverse piezoelectric effect obtained by applying an electric field to the piezoelectric layer104.

The sensing electrode105and the drive electrode106are both designed to have optimum sizes and arrangements. For example, the sensing electrode105may be disposed in an area that is on the second principal surface of the piezoelectric layer104and near the base of the vibration sensor10, which is a structure jutting from the support100. Meanwhile, the drive electrode106may be disposed in the remaining area on the second principal surface of the piezoelectric layer104so as to surround the sensing electrode105(except the base side. the same applies hereinafter) while spacing a few micrometers to a few tens of micrometers from the sensing electrode105.

When the vibration sensor10including the structure as described above undergoes a vibration from the outside, a stress is generated near the base of the vibration sensor10. As a result, an electric charge generated on the surface of the piezoelectric layer104by a piezoelectric effect builds up on the common electrode103and the sensing electrode105between which the piezoelectric layer104is sandwiched. A potential difference arisen between the sensing electrode105and the common electrode103is then detected, whereby the vibration can be detected.

With a structure in which resonance of the cantilever structure is utilized, the sensitivity to vibrations can be enhanced and frequencies of the vibrations can also be identified depending on whether the structure is in a resonance state.

Furthermore, in order to check the state of the vibration sensor10itself, an alternating voltage can be applied to the drive electrode106and the common electrode103to excite vibrations, thereby enabling calibration to be performed. At that time, a potential difference between the sensing electrode105and the common electrode103is utilized to monitor the amplitude of the vibration sensor10during calibration, so that the state of the vibration sensor10itself can be grasped.

Because arranging the sensing electrode105near the base of the cantilever structure increases the output voltage of the vibration sensor10during sensing, the sensitivity of the vibration sensor10to vibrations can be enhanced. Also, arranging the drive electrode106so as to surround the sensing electrode105can cause the vibration sensor10to vibrate efficiently. As a result, driving performance and sensing sensitivity during calibration can be mutually compatible.

The size of the sensing electrode105is described next. In order to determine the size of the sensing electrode105, the dependence on sensitivity when the length and width of the sensing electrode105were changed with respect to a length L and a width W of the second principal surface of the piezoelectric layer104has been studied in the present embodiment.

FIG. 2is a diagram for illustrating the vibration sensor used for a study to determine the size of a sensing electrode according to the present embodiment. As illustrated inFIG. 2, the contraction ratio of the length of the sensing electrode105with respect to the length of the cantilever structure (corresponding to the length of the support layer101) L is assumed to be Lc, the contraction ratio of the width of the sensing electrode105with respect to the width of the cantilever structure (corresponding to the width of the support layer101) W be Wc, the size of the piezoelectric layer104be the same as the size of the support layer101, the length L of the piezoelectric layer104be 400 μm, and the width W thereof be 50 μm in the study. The support layer101is assumed to be a Si layer having a film thickness of 3 μm, the insulating layer102be a SiO2layer having a film thickness of 200 nm, the common electrode103be a Pt layer having a film thickness of 100 nm, the piezoelectric layer104be a PZT layer having a film thickness of 3 μm, and the sensing electrode105be a Au layer having a film thickness of 100 nm. Furthermore, as illustrated inFIG. 2, the drive electrode106is omitted, and the size of the common electrode103is assumed to be the same as the size of the first principal surface of the piezoelectric layer104for convenience of description in the study.

FIG. 3is a graph illustrating sensitivity characteristics with respect to the contraction ratio of the electrode in the length direction of the vibration sensor according to the present embodiment.FIG. 4is a graph illustrating sensitivity characteristics with respect to the contraction ratio of the electrode in the width direction of the vibration sensor according to the present embodiment.FIG. 3andFIG. 4illustrate sensitivity characteristics as absolute values (corresponding to the output voltages) of the potential difference on the surfaces of upper and lower electrodes (corresponding to the common electrode103and the sensing electrode105) with respect to the frequency of the vibration.

FIG. 3illustrates the sensitivity characteristics when the contraction ratio Lc of the sensing electrode105in the length direction is changed by 0.1 from 0.1 to 0.8 in a case in which the width of the sensing electrode105is the same as the width W of the piezoelectric layer104. As illustrated inFIG. 3, when the contraction ratio Lc is 0.5, in other words, when the length of the sensing electrode105is half the length L of the cantilever structure, the sensitivity characteristics of the vibration sensor10are most favorable. When the contraction ratio Lc is within a range of 0.3 to 0.7, the sensitivity characteristics of the vibration sensor10are kept in a favorable range. Even when the contraction ratio Lc is within a range of 0.1 to 0.3 or 0.7 to 0.9, it is evident that adequate sensitivity characteristics of the vibration sensor10are achieved.

FIG. 4illustrates the sensitivity characteristics when the contraction ratio Wc of the sensing electrode105in the width direction is changed by 0.1 from 0.1 to 0.9 in a case in which the length of the sensing electrode105is half the length L of the piezoelectric layer104. As illustrated inFIG. 4, the sensitivity characteristics of the vibration sensor10are stable toward the change of the sensing electrode105in the width direction, compared with the sensitivity characteristics of the vibration sensor10toward the change of the sensing electrode105in the length direction (seeFIG. 3). When the contraction ratio Wc is 0.5, in other words, when the width of the sensing electrode105is half the width W of the cantilever structure, the sensitivity characteristics of the vibration sensor10are most favorable. When the contraction ratio Wc is within a range of 0.3 to 0.7, the sensitivity characteristics of the vibration sensor10are kept in a favorable range. Even when the contraction ratio Wc is within a range of 0.1 to 0.3 or 0.7 to 0.9, it is evident that adequate sensitivity characteristics of the vibration sensor10are achieved. However, the sensing electrode105has lower limits of the length and width because of the constraints arising from its manufacturing process, the sensing electrode105preferably has a length and width that are equal to or greater than the lower length and width limits (for example, a few micrometers to 10 micrometers).

The findings suggest that the vibration sensor10has the highest sensitivity when both the width and the length of the sensing electrode105are half the width W and the length L of the cantilever structure.

As described above, the vibration sensor10according to the present embodiment is a MEMS-type piezoelectric vibration sensor including the cantilever structure, and includes the structure in which the sensing electrode105is disposed near the base of the cantilever structure. In this manner, the sensing electrode105for detecting vibrations is disposed near the base of the vibration sensor10on which the stress generated by vibrations concentrates, so that vibrations can be detected more effectively. Consequently, the sensitivity to vibrations can be enhanced.

The vibration sensor10according to the present embodiment can also utilize the resonance of the cantilever structure. Thus, the sensitivity to vibrations can be enhanced and frequencies of the vibrations can also be identified depending on whether the structure is in a resonance state.

Furthermore, the drive electrode106is disposed in the entire remaining area on the second principal surface of the piezoelectric layer104so as to surround the sensing electrode105while spacing a few micrometers to a few tens of micrometers from the sensing electrode105in the vibration sensor10according to the present embodiment. Consequently, the vibration sensor10can be achieved in which driving performance and sensing sensitivity are mutually compatible.

Second Embodiment

Subsequently, a vibration sensor according to a second embodiment is described in detail with reference to the drawings. In the first embodiment, the vibration sensor10has been described that has the cantilever structure in which one end is a fixed end and the other end is a free end. In contrast to this, a vibration sensor is described with an example that has a fixed-fixed beam structure in which both ends are fixed ends in the second embodiment. In the description below, components similar to those in the first embodiment are given the same reference numerals, and overlapping description thereof is omitted.

FIG. 5AandFIG. 5Bare schematic diagrams illustrating an example structure of a vibration sensor according to the second embodiment.FIG. 5Ais a top view of a vibration sensor20, andFIG. 5Bis a sectional view taken along line B-B inFIG. 5A.

As illustrated inFIG. 5AandFIG. 5B, the vibration sensor20according to the present embodiment includes a structure in which both ends of a laminated body that is made up of the support layer101, the insulating layer102, the common electrode103, and the piezoelectric layer104are fixed to supports100A and100B in a structure similar to that of the vibration sensor10according to the first embodiment. A sensing electrode105A is disposed near the base of the laminated body on the side of the support100A, and a sensing electrode105B is disposed near the base of thereof on the side of the support100B. In other words, stresses generated near the bases of two ends are utilized to sense vibrations in the vibration sensor20according to the present embodiment. This enables vibrations to be detected more effectively, as is the case with the first embodiment. Consequently, the sensitivity to vibrations can be enhanced.

A drive electrode206may be disposed in the remaining area on the second principal surface of the piezoelectric layer104so as to surround the sensing electrodes105A and105B while spacing a few micrometers to a few tens of micrometers from the sensing electrodes105A and105B.

At least one of the supports100A and100B does not need to be a fixed member, and may serve as a weight, for example. In this case, the amplitude of the output voltage from the vibration sensor20can be increased, so that the sensitivity can be enhanced.

The vibration sensor20has a sectional structure and other structures, operations, and effects similar to those of the first embodiment, and detailed description thereof is thus omitted.

Third Embodiment

Subsequently, a vibration sensor according to a third embodiment is described in detail with reference to the drawings. In the present embodiment, a case is described with an example in which a plurality of vibration sensors according to embodiments described above or below are connected to each other to constitute one vibration sensor. In the description below, components similar to those in the above embodiments are given the same reference numerals, and overlapping description thereof is omitted.

A case is first described in detail with reference to the drawings in which a plurality of vibration sensors10according to the first embodiment are connected to each other to constitute one vibration sensor.FIG. 6is a schematic diagram illustrating an example structure of a vibration sensor according to the third embodiment.FIG. 7is a diagram illustrating an example of an electric connection of a sensor portion in the vibration sensor illustrated in FIG.6.FIG. 8is a diagram illustrating an example of an electric connection of a drive portion in the vibration sensor illustrated inFIG. 6.

As illustrated inFIG. 6, a vibration sensor30A having a cantilever structure according to the present embodiment includes a structure in which a plurality of vibration sensors10A to10N each including a structure similar to the vibration sensor10according to the first embodiment, for example, are arrayed. A plurality of vibration sensors10A to10N may be fixed to the same support100, or at least a part thereof may be fixed to a different support.

A sensor portion of the vibration sensor30A having such a structure includes a structure in which the sensing electrode105of a vibration sensor10(for example, the vibration sensor10A) is connected to the common electrode103of a vibration sensor10(for example, the vibration sensor10B) that is arranged at the subsequent stage, as illustrated inFIG. 7. In other words, if attention is focused on the sensor portion, a plurality of vibration sensors10A to10N are connected in series. With this structure, the gain of the output voltage from the vibration sensor30A can be increased in accordance with the number of vibration sensors10combined.

Meanwhile, a drive portion of the vibration sensor30A includes a structure in which the drive electrodes106are connected in parallel and the common electrodes103are also connected in parallel across a plurality of vibration sensors10A to10N, as illustrated inFIG. 8. In other words, if attention is focused on the drive portion, a plurality of vibration sensors10A to10N are connected in parallel. With this structure, a plurality of vibration sensors10A to10N can be driven by batch during calibration, for example.

Subsequently, a case is described in detail with reference to the drawings in which a plurality of vibration sensors20having the fixed-fixed beam structure according to the second embodiment are connected to each other to constitute one vibration sensor.FIG. 9is a schematic diagram illustrating an example structure of another vibration sensor according to the third embodiment.FIG. 10is a diagram illustrating an example of an electric connection of a sensor portion in the vibration sensor illustrated inFIG. 9.FIG. 11is a diagram illustrating an example of an electric connection of a drive portion in the vibration sensor illustrated inFIG. 9.

As illustrated inFIG. 9, a vibration sensor30B having a cantilever structure according to the present embodiment includes a structure in which a plurality of vibration sensors20A to20N each including a structure similar to the vibration sensor20according to the second embodiment, for example, are arrayed. A plurality of vibration sensors20A to20N may be fixed to the same supports100A and100B, or at least a part thereof may be fixed to a different support.

As illustrated inFIG. 10, a sensor portion of the vibration sensor30B includes a structure in which a plurality of vibration sensors20A to20N are connected in series, as is the case with the vibration sensor30A illustrated inFIG. 6toFIG. 8. With this structure, the gain of the output voltage from the vibration sensor30B can be increased according to the number of vibration sensors20combined. A series of the respective sensing electrode105A sides of the vibration sensors20A to20N connected in series and a series of the respective sensing electrode105B sides thereof connected in series may be independent from each other, or both series may be further connected to each other in series. Alternatively, the respective sensing electrode105A sides of the vibration sensors20A to20N and the respective sensing electrode105B sides thereof may be configured to be connected alternately in series.

As illustrated inFIG. 11, a drive portion of the vibration sensor30B includes a structure in which a plurality of vibration sensors20A to20N are connected in parallel, as is the case with the vibration sensor30A illustrated inFIG. 6toFIG. 8. With this structure, a plurality of vibration sensors20A to20N can be driven by batch during calibration, for example.

As described above, the gain of the output voltage can be increased in accordance with the number of vibration sensors10combined according to the present embodiment. The vibration sensors30A/30B can be achieved in which a plurality of vibration sensors can be driven by batch during calibration, for example.

The vibration sensors10/20each has a sectional structure and other structures, operations, and effects similar to those of the above embodiment, and detailed description thereof is thus omitted.

Fourth Embodiment

Subsequently, a vibration sensor according to a fourth embodiment is described in detail with reference to the drawings. While the vibration sensors10/20having the cantilever structure or the fixed-fixed beam structure are illustrated by example in the above embodiments, vibration sensors are described with examples that each include a structure in which a drive portion is shared across a plurality of vibration sensors by tying respective one ends of the vibration sensors in a bundle in the present embodiment. In the description below, components similar to those in the above embodiments are given the same reference numerals, and overlapping description thereof is omitted.

First Example

FIG. 12AandFIG. 12Bare schematic diagrams illustrating an example structure of a vibration sensor according to a first example of the present embodiment.FIG. 12Ais a top view of a vibration sensor40A, andFIG. 12Bis a sectional view taken along line C-C inFIG. 12A. As illustrated inFIG. 12AandFIG. 12B, the vibration sensor40A according to the first example includes a plurality of (four in the present example) sensor parts41A to41D that each have a cantilever structure and that are arranged at predetermined spacings, and a U-shaped drive part43A that binds the free end sides of the sensor parts41A to41D. In other words, the vibration sensor40A according to the first example includes a structure in which the drive portion (corresponding to the drive part43A) is shared across a plurality of vibration sensors each having a cantilever structure, so that the respective tips of the sensor parts41A to41D are bound together.

The sensor parts41A to41D each include a structure in which a support layer401, an insulating layer402, a common electrode403, a piezoelectric layer404, and a sensing electrode405are laminated, as is the case with the sensor portions in the vibration sensors10/20illustrated by example in the above embodiments, for example. In the first example, however, the sensing electrode405has the same width as the width W of the piezoelectric layer404. The drive part43A also includes a structure in which the support layer401, the insulating layer402, the common electrode403, the piezoelectric layer404, and a drive electrode406are laminated, as is the case with the drive portions in the vibration sensors10/20.

In this manner, the structure in which the free end sides of the sensor parts41A to41D each having a cantilever structure are bound by the drive part43A can prevent sensitivity degradation of the sensor caused by drifts of the resonance frequencies of the sensor parts41A to41D.

Second Example

FIG. 13AandFIG. 13Bare schematic diagrams illustrating an example structure of a vibration sensor according to a second example of the present embodiment.FIG. 13Ais a top view of a vibration sensor40B, andFIG. 13Bis a sectional view taken along line D-D inFIG. 13A. As illustrated inFIG. 13AandFIG. 13B, the vibration sensor40B according to the second example includes a plurality of (four and four in the present example) sensor parts41A to41D and41E to41H that each have a cantilever structure and that are arranged at predetermined spacings, and an H-shaped drive part43B that binds the free end sides of the sensor parts41A to41D and41E to41H. A drive electrode416of the drive part43B is H-shaped. Of the sensor parts41A to41D and41E to41H each having a cantilever structure, sensor parts facing each other substantially constitute a vibration sensor having a fixed-fixed beam structure. The vibration sensor40B according to the second example therefore includes a structure in which a drive portion (corresponding to the drive part43B) is shared across a plurality of vibration sensors each having a fixed-fixed beam structure, so that the respective tips of the sensor parts41A to41D and41E to41H are bound together.

The sensor parts41A to41D and41E to41H and the drive part43B may include a laminar structure similar to the structure of the sensor parts41A to41D and the drive part43A described above.

In this manner, the structure in which the drive part43B is used as the common drive portion in the vibration sensor that has a fixed-fixed beam structure and that is made up of the sensor parts41A to41D and41E to41H, and the free end sides of the sensor parts41A to41D and41E to41H are bound can prevent sensitivity degradation of the sensor caused by drifts of the resonance frequencies of the sensor parts41A to41D and41E to41H, as is the case with the first example.

Third Example

FIG. 14is a top view illustrating an example structure of a vibration sensor according to a third example of the present embodiment. As illustrated inFIG. 14, a vibration sensor40C according to the third example includes a plurality of (four in the present example) sensor parts41A to41D,41E to41H,41I to41L, and41M to41P that each have a cantilever structure and that are arranged at predetermined spacings, and a rectangular drive part43C that binds the free end sides of the sensor parts41A to41P. In other words, the vibration sensor40C according to the third example includes a structure to hang the rectangular drive part43C located in the center so as to be surrounded by the sensor parts41A to41D,41E to41H,41I to41L, and41M to41P. The drive part43C binds together the tips of the sensor parts41A to41D,41E to41H,41I to41L, and41M to41P. The corner portions of the rectangular drive part43C are connected to support100A to100D. The rectangular corner portions of a drive electrode426extend to the support100A to100D.

The sensor parts41A to41P and the drive part43C may include a laminar structure similar to the structure of the sensor parts41A to41D and the drive part43A described above.

In this manner, a structure to hang the central rectangular drive part43C on every side with the sensor parts41A to41D,41E to41H,41I to41L, and41M to41P having a cantilever structure enables the number of sensor parts to be increased, so that the sensitivity of the vibration sensor40C can be further enhanced. The drive part43C is not limited to be rectangular, and may be changed into various shapes, such as a circle, oval, and a polygon with three or more sides.

Fourth Example, Fifth Example, and Sixth Example

FIG. 15is a top view illustrating an example structure of a vibration sensor according to a fourth example of the present embodiment.FIG. 16is a top view illustrating an example structure of a vibration sensor according to a fifth example of the present embodiment.FIG. 17is a top view illustrating an example structure of a vibration sensor according to a sixth example of the present embodiment.

In a vibration sensor40D according to the fourth example, which has a structure similar to the structure of the vibration sensor40A according to the first example illustrated inFIG. 12AandFIG. 12B, the respective sensing electrodes405in the sensor parts41A to41D are replaced with sensing electrodes415, as illustrated inFIG. 15. Likewise, in a vibration sensor40E according to the fifth example, which has a structure similar to the structure of the vibration sensor40B according to the second example illustrated inFIG. 13AandFIG. 13B, the respective sensing electrodes405in the sensor parts41A to41D and41E to41H are replaced with the sensing electrodes415, as illustrated inFIG. 16. Likewise, in a vibration sensor40F according to the sixth example, which has a structure similar to the structure of the vibration sensor40C according to the third example illustrated inFIG. 14, the respective sensing electrodes405in the sensor parts41A to41P are replaced with the sensing electrodes415, as illustrated inFIG. 17. The sensing electrode415has a width reduced by the predetermined contraction ratio Wc with respect to the width W of the piezoelectric layer404as is the case with the first embodiment, for example.

In this manner, the width of each sensing electrode415is optimized in the fourth example, the fifth example, and the sixth example. Consequently, the sensitivity of the vibration sensors40D,40E, and40F can be further enhanced.

Other structures, operations, and effects are similar to those of the above embodiments, and detailed description thereof is thus omitted.

Fifth Embodiment

Subsequently, a vibration sensor according to a fifth embodiment is described in detail with reference to the drawings. In the above embodiments, one of the sensing electrodes105/405/415constitutes one sensor portion in the vibration sensors. In contrast to this, a case is described with an example in which the sensing electrodes105/405/415in the sensor portions are each split into a plurality in the fifth embodiment. In the description below, components similar to those in the above embodiments are given the same reference numerals, and overlapping description thereof is omitted. The description below is based on the vibration sensor10according to the first embodiment for convenience of description, but not limited thereto. The same applies to the vibration sensors according to the other embodiments.

FIGS. 18A and 18Bare schematic diagrams illustrating an example structure of a vibration sensor according to the present embodiment.FIG. 18Ais a top view of a vibration sensor50, andFIG. 18Bis a sectional view taken along line E-E inFIG. 18A.

As illustrated inFIG. 18A, the vibration sensor50according to the present embodiment includes a structure in which the sensing electrode105is replaced with a plurality of (three in the present example) split electrodes105ato105c, in a structure similar to the structure of the vibration sensor10according to the first embodiment.

A plurality of split electrodes105ato105care electrically connected in series, for example. This enables vibrations to be detected more effectively. Consequently, the sensitivity to vibrations can be enhanced.

The vibration sensor50has a sectional structure and other structures, operations, and effects similar to those of the above embodiments, and detailed description thereof is thus omitted.

Sixth Embodiment

Subsequently, a vibration sensor according to a sixth embodiment is described in detail with reference to the drawings. In the above embodiments, the size and shape of the common electrode103are assumed to be the same as the size and shape of the first principal surface of the piezoelectric layer104. In contrast to this, a case is described in detail with reference to the drawings in which the size and shape of the common electrode103are varied depending on the size and shape of the sensing electrode105, for example, in the present embodiment. In the description below, components similar to those in the above embodiments are given the same reference numerals, and overlapping description thereof is omitted. The description below is based on the vibration sensor10according to the first embodiment for convenience of description, but not limited thereto. The same applies to the vibration sensors according to the other embodiments.

FIGS. 19A and 19Bare schematic diagrams illustrating an example structure of a vibration sensor according to the present embodiment.FIG. 19Ais a top view of a vibration sensor60A, andFIG. 19Bis a sectional view taken along line F-F inFIG. 19A.

As illustrated inFIGS. 19A and 19B, the vibration sensor60A according to the present embodiment includes a structure in which the common electrode103is replaced with common electrodes103aand103b, in a structure similar to the structure of the vibration sensor10according to the first embodiment, for example. The common electrode103ahas the same size as the size of the sensing electrode105, for example, and the common electrode103bhas the same size as the size of the drive electrode106, for example.

As illustrated inFIGS. 19A and 19B, a trench61arising from a gap between the common electrodes103aand103bis present in a piezoelectric layer604, depending on a manufacturing process during which the vibration sensor60A is manufactured.

The sizes of the sensing electrode105and the common electrode103aare described next. In the present embodiment, the common electrode103a, which replaces the common electrode103in the vibration sensor illustrated with reference toFIG. 2, has been used for a study to determine the sizes of the sensing electrode105and the common electrode103a. Note that the material and the film thickness of the common electrode103aare the same as those of the common electrode103.

FIG. 20is a graph illustrating sensitivity characteristics with respect to the contraction ratio of the electrode in the length direction of the vibration sensor according to the present embodiment.FIG. 21is a graph illustrating sensitivity characteristics with respect to the contraction ratio of the electrode in the width direction of the vibration sensor according to the present embodiment.FIG. 20andFIG. 21, as is the case withFIG. 3andFIG. 4, illustrate sensitivity characteristics as absolute values (corresponding to the output voltages) of the potential difference on the surfaces of upper and lower electrodes (corresponding to the common electrode103aand the sensing electrode105) with respect to the frequency of the vibration.

FIG. 20illustrates the sensitivity characteristics when the contraction ratio Lc of the sensing electrode105and the common electrode103ain the length direction is changed from 0.05 to 0.9. As illustrated inFIG. 20, in a case in which the shape and size of the common electrode103aare matched to the shape and size of the sensing electrode105, the sensitivity characteristics of the vibration sensor60A are improved with decreased contraction ratio Lc, in other words, with decreased ratio with respect to the length L of the cantilever structure. The sensitivity characteristics are most favorable when the contraction ratio Lc is equal to or lower than 0.1. When the contraction ratio Lc is equal to or lower than 0.5, the sensitivity characteristics of the vibration sensor60A are kept in a favorable range. When the contraction ratio Lc is equal to or lower than 0.9, adequate sensitivity characteristics of the vibration sensor60A are achieved.

FIG. 21illustrates the sensitivity characteristics when the contraction ratio Wc of the sensing electrode105and the common electrode103ain the width direction is changed by 0.1 from 0.1 to 0.9. As illustrated inFIG. 21, the sensitivity characteristics of the vibration sensor60A are stable toward the change of the sensing electrode105and the common electrode103ain the width direction, although the sensitivity characteristics of the vibration sensor60A are most favorable when the contraction ratio Wc is 0.5. When the contraction ratio Wc is within a range of 0.3 to 0.7, the sensitivity characteristics of the vibration sensor60A are kept in a favorable range. Even when the contraction ratio Wc is within a range of 0.1 to 0.3 or 0.7 to 0.9, it is evident that adequate sensitivity characteristics of the vibration sensor60A are achieved.

However, the sensing electrode105and the common electrode103ahave lower limits of the length and width because of the constraints arising from their manufacturing process, the sensing electrode105and the common electrode103apreferably have lengths and widths that are equal to or greater than the lower length and width limits (for example, a few micrometers to 10 micrometers).

A description is now given of which of the following two cases obtains better sensitivity characteristics: a case in which the common electrode103has a fixed size that is the same as the size of the piezoelectric layer104(for example, the first embodiment); and a case in which the shape and size of the common electrode103aare matched to the shape and size of the sensing electrode105(for example, the present embodiment). In the description, the length and the width of the sensing electrode105are assumed to be half the length L and the width W of the piezoelectric layer104, in other words, the contraction ratios Lc and Wc are both assumed to be 0.5.

FIG. 22illustrates sensitivity characteristics in the vicinity of the resonance frequency of the vibration sensor in a case in which the shape and size of the common electrode are matched to the shape and size of the sensing electrode.FIG. 23illustrates sensitivity characteristics in the vicinity of the resonance frequency of the vibration sensor in a case in which the common electrode has a fixed size that is the same as the size of the piezoelectric layer.FIG. 22andFIG. 23, as is the case withFIG. 20andFIG. 21, illustrate sensitivity characteristics as absolute values (corresponding to the output voltages) of the potential difference on the surfaces of upper and lower electrodes (corresponding to the common electrodes103/103aand the sensing electrodes105) with respect to the frequency of the vibration.

As is evident from comparison betweenFIG. 22andFIG. 23, the output voltage of the vibration sensor is increased more in the case in which the shape and size of the common electrode103aare matched to the shape and size of the sensing electrode105(for example, the present embodiment) than the case in which the common electrode103has a fixed size that is the same as the size of the piezoelectric layer (for example, the first embodiment). This shows that the sensitivity characteristics are enhanced more with the vibration sensor having a structure in which the shape and size of the common electrode103aare matched to the shape and size of the sensing electrode105(for example, the vibration sensor60A) than the vibration sensor having a structure in which the common electrode103has a fixed size that is the same as the size of the piezoelectric layer (for example, the vibration sensor10).

Thus, optimizing the sizes of the sensing electrode105and the common electrode103acan further improve the output voltage, thereby further enhancing the sensitivity to vibrations.

The findings as described above are not limited to the vibration sensor60A based on the vibration sensor10having the cantilever structure according to the first embodiment. The same can apply to a vibration sensor60B, as illustrated inFIG. 24, for example, based on the vibration sensor20having the fixed-fixed beam structure according to the second embodiment. In the vibration sensor60B illustrated inFIG. 24, the common electrode103is split into a common electrode103A corresponding to the sensing electrode105A, a common electrode103B corresponding to the sensing electrode105B, and a common electrode103C corresponding to a drive electrode216. Trenches61A and61B arising from gaps between the common electrodes103A to103C are present in a piezoelectric layer614.

AlthoughFIG. 19A,FIG. 19B, andFIG. 24illustrate by example the cases in which the common electrode103is separated to conform to the shapes of the sensing electrodes105/105A/105B and the drive electrodes106/216, the vibration sensors are not limited to these structures. Structures can be such that, in addition to the common electrode103, the piezoelectric layer104is also separated to conform to the shapes of the sensing electrodes105/105A/105B and the drive electrodes106/216, as illustrated inFIG. 25andFIG. 26, for example.

A vibration sensor60C illustrated inFIG. 25is based on the vibration sensor10according to the first embodiment, for example, and includes a structure in which the piezoelectric layer604is replaced with piezoelectric layers604aand604b, in a structure similar to the structure of the vibration sensor60A illustrated inFIG. 19AandFIG. 19B, for example. The piezoelectric layers604ahas the same size as the size of the sensing electrode105, for example, and the piezoelectric layers604bhas the same size as the size of the drive electrode106, for example. A trench62, on the underside thereof, exposes the insulating layer102.

A vibration sensor60D illustrated inFIG. 26is based on the vibration sensor20according to the second embodiment, for example, and includes a structure in which the piezoelectric layer614is replaced with piezoelectric layers614A,614B, and614C, in a structure similar to the structure of the vibration sensor60B illustrated inFIG. 24, for example. The piezoelectric layers614A has the same size as the size of the sensing electrode105A, for example, the piezoelectric layers614B has the same size as the size of the sensing electrode105B, for example, and the piezoelectric layers614C has the same size as the size of the drive electrode216, for example. Trenches62A and62B, on the underside thereof, expose the insulating layer102.

Each of the vibration sensors60A,60B,60C, and60D0has a sectional structure and other structures, operations, and effects similar to those of the above embodiments, and detailed description thereof is thus omitted.

Seventh Embodiment

Subsequently, a vibration sensor according to a seventh embodiment is described in detail with reference to the drawings. In the third embodiment described above, the vibration sensors10/20both having the same length L are arrayed and electrically connected. In contrast to this, a case is described with an example in which vibration sensors having a length L different from each other are arrayed and electrically connected in the present embodiment. In the description below, components similar to those in the above embodiments are given the same reference numerals, and overlapping description thereof is omitted.

First Example

FIG. 27is a top view illustrating an example structure of a vibration sensor according to a first example. As illustrated inFIG. 27, a vibration sensor70A according to the first example includes one each of vibration sensors71A to71N having a cantilever structure and having a length L different from each other. The vibration sensors71A to71N may be fixed to the same support100, or at least a part thereof may be fixed to a different support. The exemplary electric connection of the sensor part and the drive part as well as the sectional structure of the vibration sensors71A to71N may be the same as those of any of the vibration sensors having a cantilever structure in the above embodiments, for example.

Second Example

FIG. 28is a top view illustrating an example structure of a vibration sensor according to a second example. As illustrated inFIG. 28, a vibration sensor70B according to the second example includes a plurality of each (two each in the present example) of vibration sensors71A to71N each having the same length L, in a structure similar to the structure of the vibration sensor70A according to the first example.

Third Example

FIG. 29is a top view illustrating an example structure of a vibration sensor according to a third example. As illustrated inFIG. 29, a vibration sensor70C according to the third example has a structure in which the respective free end sides of the vibration sensors71A to71N are fixed to supports100B1to100Bn, in a structure similar to the structure of the vibration sensor70A according to the first example. The supports100B1to100Bn may be one support, or a support split into a plurality.

Fourth Example

FIG. 30is a top view illustrating an example structure of a vibration sensor according to a fourth example. As illustrated inFIG. 30, a vibration sensor70D according to the fourth example has a structure in which the respective free end sides of the vibration sensors71A to71N each having the same length L are fixed to respective common supports100B1to100Bn, in a structure similar to the structure of the vibration sensor70B according to the second example. The supports100B1to100Bn may be one support, or a support split into a plurality.

Fifth Example

FIG. 31is a top view illustrating an example structure of a vibration sensor according to a fifth example. As illustrated inFIG. 31, a vibration sensor70E according to the fifth example has a structure in which the vibration sensors71A to71N having a cantilever structure and having a length L different from each other is replaced with vibration sensors72A to72N having a fixed-fixed beam structure and having a length L different from each other, in a structure similar to the structure of the vibration sensor70C according to the third example. The exemplary electric connection of the sensor part and the drive part as well as the sectional structure of the vibration sensors72A to72N may be the same as those of any of the vibration sensors having a fixed-fixed beam structure in the above embodiments, for example.

Sixth Example

FIG. 32is a top view illustrating an example structure of a vibration sensor according to a sixth example. As illustrated inFIG. 32, a vibration sensor70F according to the sixth example includes a plurality of each (two each in the present example) of vibration sensors72A to72N each having the same length L, in a structure similar to the structure of the vibration sensor70E according to the fifth example.

The vibration sensors71A to71N and72A to72N each have a resonance frequency in accordance with the length L. As a result, the sensing band can be widened by arraying a plurality of vibration sensors having a length L different from each other, as in the first to the sixth examples.

At that time, the gain of the output voltage when vibrations at each resonance frequency is detected can be increased in accordance with the number of vibration sensors combined, by disposing a plurality of vibration sensors having the same length L, as in the second, the fourth, and the sixth examples.

Other structures, operations, and effects are similar to those of the above embodiments, and detailed description thereof is thus omitted.

Eighth Embodiment

Subsequently, a sensor module according to an eighth embodiment is described in detail with reference to the drawing. In the eighth embodiment, the sensor module is described with an example that includes the vibration sensors according to the above embodiments. In the description below, components similar to those in the above embodiments are given the same reference numerals, and overlapping description thereof is omitted. A case in which the vibration sensor10according to the first embodiment is used is described below for convenience of description, but the vibration sensor to be used is not limited thereto. The vibration sensors according to the other embodiments can also be used.

FIG. 33is a schematic diagram illustrating an example structure of the sensor module according to the present embodiment. As illustrated inFIG. 33, a sensor module80includes a controller81and a memory82, in addition to the vibration sensor10.

The controller81is made up of an information-processing device, such as a central processing unit (CPU), and detects vibrations input to the vibration sensor10on the basis of the potential difference arisen between the sensing electrode105and the common electrode103. The controller81applies a voltage signal of the frequency corresponding to the resonance frequency of the vibration sensor10to the drive electrode106and the common electrode103during calibration of the vibration sensor10, for example.

The memory82is a storage device, such as a dynamic random access memory (DRAM), and stores various computer programs and parameters to enable the operation of the controller81and data on vibrations detected by the vibration sensor10. Various computer programs and parameters include computer programs and parameters to be used for calibration of the vibration sensor10.

Other structures, operations, and effects are similar to those of the above embodiments, and detailed description thereof is thus omitted.