JP2011237281A - Method and system for measuring piezoelectric characteristic - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for measuring a piezoelectric characteristic (d31 constant) with high accuracy.SOLUTION: The method related to the present invention is a method for measuring a piezoelectric characteristic of a measuring object having a substrate and a piezoelectric element formed thereon and comprises: holding the measuring object by fixing one end thereof; putting the measuring object into vibration state by applying a voltage signal to a piezoelectric layer of the piezoelectric element; receiving reflected light from the measuring object by illuminating the measuring object in the vibration state with laser light from a light source; obtaining secular change in displacement of the measuring object from the reflected light and reference light from the light source using a laser Doppler vibrometer; and calculating the piezoelectric characteristic of the piezoelectric layer of the measuring object from the displacement. In addition, in the process of obtaining the reflected light, the laser light is oriented so as to hit a node the secondary vibration of the measuring object.

Description

  The present invention relates to a piezoelectric characteristic measuring method and a piezoelectric characteristic measuring system.

  2. Description of the Related Art There are known piezoelectric actuators and ink jet recording heads having a thin film type piezoelectric element manufactured by using a thin film technology capable of forming a piezoelectric layer thinner than a bulk type piezoelectric layer. In such a thin film type piezoelectric element, when measuring piezoelectric characteristics such as d constant, the displacement amount is smaller than that of the bulk type piezoelectric element, and the reason is that it is in contact with the substrate. Therefore, it is known that measurement of piezoelectric characteristics is very difficult (Patent Document 1).

  For example, Patent Document 1 describes a technique for irradiating a measurement object with laser light and measuring a piezoelectric characteristic by a change in a reflection position accompanying a change in the state of warping of the measurement object.

  However, when designing a piezoelectric actuator or an ink jet recording head using a thin film type piezoelectric element, not only the d33 constant of the piezoelectric element but also the d31 constant of the piezoelectric element provided on the substrate is useful. Therefore, there is a demand for a method and system for measuring piezoelectric characteristics that can determine the d31 constant of a piezoelectric element with high accuracy.

Japanese Patent Laid-Open No. 9-325165

  According to some aspects of the present invention, it is possible to provide a piezoelectric characteristic measuring method and a piezoelectric characteristic measuring system capable of obtaining the d31 constant of a piezoelectric element with high accuracy.

(1) A piezoelectric property measuring method which is one aspect of the present invention is:
A method for measuring piezoelectric characteristics of an object to be measured, comprising: a substrate; and a piezoelectric element provided on the substrate,
Fixing the one end of the measured object to hold the measured object;
Applying a voltage signal to the piezoelectric layer of the piezoelectric element to bring the object to be measured into a vibrating state;
Irradiating a laser beam from a light source to the object to be measured in a vibrating state to obtain reflected light reflected from the object to be measured;
Using a laser Doppler vibrometer to obtain a change over time in the amount of displacement of the measured object from the reflected light and the reference light from the light source;
Calculating the piezoelectric characteristics of the piezoelectric layer of the measured object from the displacement amount;
Including
In the step of obtaining the reflected light, the laser light is applied to a secondary vibration node of the measured object.

  ADVANTAGE OF THE INVENTION According to this invention, the piezoelectric characteristic measuring method which can obtain | require the d31 constant of a piezoelectric element with high precision can be provided.

(2) In the piezoelectric characteristic measuring method which is one aspect of the present invention,
The voltage signal applied to the piezoelectric layer is a single sawtooth wave,
The frequency of the voltage signal may be 1/10 or less of the resonance frequency of the measured object.

  According to this, since the measured object can be easily brought into a stable resonance state and the signal / noise ratio (SN ratio) of the speed signal obtained by measurement can be increased, the d31 constant of the piezoelectric element can be further increased. It can be obtained with high accuracy.

(3) In the piezoelectric characteristic measuring method which is one aspect of the present invention,
When the applied voltage value of the voltage signal is X [V] and the resonance frequency of the object to be measured is Y [Hz], the voltage increase rate (dV / dt) when the voltage signal is applied to the piezoelectric element May be X / 5Y [V / sec].

  According to this, since the measured object can be easily brought into a stable resonance state and the signal / noise ratio (SN ratio) of the speed signal obtained by measurement can be increased, the d31 constant of the piezoelectric element can be further increased. It can be obtained with high accuracy.

(4) In the piezoelectric characteristic measuring method which is one aspect of the present invention,
The vibration state may be a resonance state.

(5) In the piezoelectric characteristic measuring method which is one aspect of the present invention,
The step of calculating the piezoelectric characteristics may use finite element simulation.

(6) A piezoelectric property measuring system which is one of the aspects of the present invention is:
A holding unit for holding one end of a measurement object including a piezoelectric element;
A signal generator for generating a voltage signal applied to the object to be measured;
An optical system including a light source of laser light that irradiates the measurement object, a detection unit that detects reflected light obtained by irradiating the measurement object with the laser light and reference light obtained from the light source, and the detection A laser Doppler vibrometer including a signal processing unit capable of obtaining a change over time in the amount of displacement of the measured object from a detection result in the unit;
Have
The laser light from the light source of the laser Doppler vibrometer is applied to a secondary vibration node of the measured object.

  ADVANTAGE OF THE INVENTION According to this invention, the piezoelectric characteristic measuring system which can obtain | require the d31 constant of a piezoelectric element with high precision can be provided.

The figure which shows typically the piezoelectric characteristic measuring system of this embodiment. The perspective view which shows an example of a to-be-measured object typically. The figure which illustrates the waveform of a voltage signal typically. The figure for demonstrating the piezoelectric characteristic measuring method of this embodiment. The figure explaining the measurement result which concerns on a present Example. The figure explaining the measurement result which concerns on a present Example. The figure explaining the measurement result which concerns on a present Example. The figure explaining the measurement result which concerns on this comparative example.

  DESCRIPTION OF EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. The embodiments described below do not unduly limit the contents of the present invention described in the claims. Also, not all of the configurations described below are essential constituent requirements of the present invention.

1. Piezoelectric Characteristic Measurement System FIG. 1 is a diagram schematically illustrating an example of a piezoelectric characteristic measurement system according to this embodiment. FIG. 2 is a diagram schematically illustrating an example of a measurement object used for piezoelectric characteristic measurement with the piezoelectric characteristic measurement system of the present embodiment. FIG. 2 is a perspective view of the DUT 10. FIG. 3 is a diagram schematically showing the waveform of the voltage signal generated by the signal generator.

  As shown in FIG. 1, a piezoelectric characteristic measuring system 100 that is a system for carrying out a piezoelectric characteristic measuring method according to the present embodiment applies a holding unit 20 that holds a measured object 10 and a measured object 10. It includes a signal generator 30 for generating a voltage signal and a laser Doppler vibrometer 40 (optical systems (41, 42, 43), detector 45, signal processor 46).

  In the present invention, the term “piezoelectric characteristics” is used as a term meaning, for example, the d constant of a piezoelectric body used in a piezoelectric element. In particular, in the present invention, the term “piezoelectric characteristics” means the piezoelectric constant of d31 of the piezoelectric layer.

  As shown in FIG. 1, the DUT 10 includes a substrate 5 and a piezoelectric element 4 provided on the substrate 5. The piezoelectric element 4 has a piezoelectric layer 2 sandwiched between the first electrode 1 and the second electrode 3. As shown in FIG. 1, the device under test 10 is held by fixing (cantilevered) at least one end of the device under test 10 by the holding unit 20. The external shape and configuration of the measurement object 10 are not particularly limited as long as they can be deformed / vibrated by applying a voltage to the piezoelectric element 4. Hereinafter, an example of the measurement target 10 will be described with reference to FIG. 2, but the configuration of the measurement target 10 is not limited to the following description.

  For example, as shown in FIG. 2, the DUT 10 may be a rectangular parallelepiped extending in one direction. The outer shape of the measurement object 10 may be, for example, an aspect ratio of the long side to the short side of 5 or more. Here, the measurement object 10 is the most distorted direction, the longitudinal direction of the outer shape is the X-axis direction, the short direction of the measurement object 10 (the width direction of the measurement object) is the Y-axis direction, and the XY-axis surface The normal direction with respect to (the thickness direction of the measured object) is taken as the Z-axis direction.

  As shown in FIG. 2, the measured object 10 includes a measured part 11 and a holding part 12. The part to be measured 11 is a part that can substantially be displaced / vibrated as the piezoelectric element 4 when a voltage is applied. The holding portion 12 is a portion that can be held by a holding portion 20 described later, and is a portion where displacement / vibration can be substantially ignored. Therefore, the measured part 11 may be a part longer than the holding part 12 in the X-axis direction. Here, as shown in FIG. 2, the end portion of the portion to be measured 11 located on the side opposite to the holding portion 12 is referred to as a tip portion 15.

  The substrate 5 is a plate-like member made of an elastic conductor, semiconductor, or insulator. The substrate 5 is a member that can be deformed / vibrated by deformation / vibration of the piezoelectric element 4 described later. The material of the substrate 5 is not limited as long as the piezoelectric element 4 can be formed with good adhesion, but it is desirable to use a material having a low elastic constant in order to increase the amount of change. As the material of the substrate 5, for example, the same material as that of the assumed substrate of the piezoelectric device can be used. Examples of the material of the substrate 5 include inorganic oxides such as zirconium oxide, silicon nitride, and silicon oxide, semiconductors such as silicon and germanium, alloys such as stainless steel, and organic materials such as polypropylene, polyimide, silicon resin, and fluororesin. Molecular materials can be used. The substrate 5 may have a laminated structure of two or more kinds of the exemplified substances. The thickness of the substrate 5 is optimally selected according to the elastic modulus of the material used. The thickness of the board | substrate 5 can be 30 micrometers or more and 1000 mm or less, for example. According to this, the DUT 10 can be handled and the measured displacement can be read.

  As shown in FIG. 2, the piezoelectric element 4 is formed on the first electrode 1 formed on the substrate 5, the piezoelectric layer 2 formed on the first electrode 1, and the piezoelectric layer 2. Second electrode 3. Therefore, the first electrode 1, the piezoelectric layer 2, and the second electrode 3 have a structure that is laminated in the Z-axis direction. Here, the surface on the first electrode 1 side of the piezoelectric layer 2 is defined as a first surface 2a, and the surface on the second electrode side is defined as a second surface 2b.

  The first electrode 1 is formed on a substrate 5 as shown in FIG. The first electrode 1 is paired with the second electrode 3 and functions as one electrode sandwiching the piezoelectric layer 2. As shown in FIG. 2, the first electrode 1 is formed so as to cover the piezoelectric layer 2 on the first surface 2 a of the measured portion 11. Further, as shown in FIG. 2, the first electrode 1 is electrically connected to the lead portion 1a. The lead portion 1 a is electrically connected to the first electrode 1 of the portion to be measured 11 and is formed on the first surface 2 a of the holding portion 12. The lead portion 1 a may be formed so as to continue from the side surface of the piezoelectric layer 2 to the second surface 2 b of the piezoelectric layer 2. Accordingly, as shown in FIG. 2, the electrical connection portion 1 b of the first electrode 1 may be provided on the second surface 2 b of the holding portion 12.

  The material of the first electrode 1 is not particularly limited as long as it is a conductive material. Examples of the material of the first electrode 1 include various metals such as Ni, Ir, Au, Pt, W, Ti, Ta, Mo, and Cr, alloys of these metals, and conductive oxides thereof (for example, iridium oxide) ), A composite oxide of Sr and Ru, a composite oxide of La and Ni, or the like can be used. Further, the first electrode 1 may be a single layer of the exemplified materials or may be a structure in which a plurality of materials are stacked. The thickness of the first electrode 1 is optimally selected according to the elastic modulus and conductivity of the material used. When Pt is used as the material of the first electrode 1, the thickness of the first electrode 1 can be set to 30 nm or more and 200 nm or less, for example.

  The piezoelectric layer 2 is disposed between the first electrode 1 and the second electrode 3 as shown in FIG. As shown in FIG. 2, the piezoelectric layer 2 is provided so as to be sandwiched between the first electrode 1 and the second electrode 3 in the measured portion 11 of the measured object 10. As shown in FIG. 2, the piezoelectric layer 2 may be a plate-like member. The thickness of the piezoelectric layer 2 is not particularly limited as long as it can be substantially expanded and contracted when a voltage is applied. The thickness of the piezoelectric layer 2 can be, for example, 100 nm or more and 100,000 nm or less.

As the material of the piezoelectric layer 2, a perovskite oxide represented by the general formula ABO ₃ is preferably used. Specific examples of such materials include lead zirconate titanate (Pb (Zr, Ti) O ₃ ), lead zirconate titanate niobate (Pb (Zr, Ti, Nb) O ₃ ) (hereinafter, In this specification, it may be abbreviated as “PZTN”), barium titanate (BaTiO ₃ ), potassium sodium niobate ((K, Na) NbO ₃ ), bismuth sodium titanate (Bi, NaTiO ₃ , bismuth ferrite ( BiFeO ₃ ), and complex oxides of these.

  The second electrode 3 is formed on the piezoelectric layer 2 as shown in FIG. The second electrode 3 is paired with the first electrode 1 and functions as one electrode sandwiching the piezoelectric layer 2. As shown in FIG. 2, the second electrode 3 is formed on the second surface 2 b of the portion to be measured 11. As shown in FIG. 2, the second electrode 3 may be rectangular. Here, as shown in FIG. 2, the boundary between the measured portion 11 and the holding portion 12 may be defined by one side of the second electrode 3 on the holding portion 12 side. Further, as shown in FIG. 2, the second electrode 3 is electrically connected to the lead portion 3a. The lead part 3 a is electrically connected to the second electrode 3 of the part to be measured 11 and is formed on the second surface 2 b of the holding part 12. As shown in FIG. 2, an electrical connection portion 3 b of the second electrode 3 that is continuous with the lead portion 3 a may be provided on the second surface 2 b of the holding portion 12.

  The material of the second electrode 3 is not particularly limited as long as it is a conductive material. The material and configuration of the second electrode 3 may be, for example, the same material and configuration as the first electrode 1.

As shown in FIG. 2, the measured object 10 has a laser irradiation portion 16 in the measured portion 11. The laser irradiation portion 16 is a portion above the second surface 2b of the measurement object 10 and irradiated with laser light. In other words, it is a portion where the optical axis of the laser beam is aligned during the laser beam irradiation. The position of the laser irradiation portion 16 is the position of a secondary vibration node when the measured portion 11 is in a resonance state. That is, the position of the secondary vibration node in the vibration when one end is the fixed end (holding portion 12) and the other end is the free end (tip portion 15) is the laser irradiation portion 16. Therefore, the length L ₁ in the X-axis direction from the holding portion 12 to the position of the laser irradiation portion 16 is expressed by the following equation when the length in the X-axis direction of the measured portion 11 is L _0. The

Further, the laser light irradiation position 16 has a region width because the laser light B may be irradiated so that the influence of the higher-order vibration mode higher than the secondary vibration does not substantially affect the measurement result. May be. X-axis direction length L ₁ to the position of the laser irradiation portion 16 from the holding portion 12, for example, may range in the following equation.

  If it is in said range, a measurement result will be substantially unaffected by the higher order vibration mode more than a secondary vibration, and highly accurate displacement amount data can be obtained as a measurement result.

  In the system for carrying out the piezoelectric characteristic measuring method according to the present embodiment, not only the measured object 10 having the above-described form but also a form in which the piezoelectric element can be deformed / vibrated by applying a voltage. It can be applied as appropriate. For example, although not shown, the outer shape of the DUT 10 viewed from the Z-axis direction may have a rhombus. Even in that case, the position of the secondary vibration node can be calculated as appropriate to determine the laser light irradiation position 16.

  As shown in FIG. 1, the holding unit 20 is a member that cantilever the measured object 10. Specifically, the holding unit 20 holds the device under test 10 by fixing one end of the device under test 10. As shown in FIG. 1, the holding unit 20 is arranged so that laser light from a laser Doppler vibrometer 40 described later can be irradiated to a predetermined position of the measurement object 10. Although not shown, the holding unit 20 includes a wiring electrically connected to a signal generation unit 30 described later. Accordingly, the holding unit 20 can apply a voltage to the piezoelectric element 4 of the measured object 10.

  The signal generator 30 can apply a voltage signal (drive signal) having a desired frequency to the piezoelectric element 4 of the measurement target 10 via the internal wiring of the holding unit 20. The signal generator 30 includes a known function generator. The signal generator 30 includes an amplifier circuit unit such as a power amplifier. In the signal generation unit 30, the voltage signal generated from the function generator may be sent to the holding unit 20 via an amplifier circuit. For example, the signal generator 30 can generate a voltage signal composed of a voltage having a desired frequency.

  The voltage signal generated by the signal generator 30 may be a single sawtooth wave as shown in FIG. 3, and the frequency of the voltage signal may be 1/10 or less of the resonance frequency of the DUT 10. . More specifically, when the applied voltage value of the voltage signal is X [V] and the resonance frequency of the DUT 10 is Y [Hz], the voltage increase rate when the voltage signal is applied to the piezoelectric element 4 ( dV / dt) may be X / 5Y [V / sec]. In this way, by increasing the voltage value of the voltage signal over a sufficient time (dt) with respect to the target voltage value (dV), the measured object can be easily brought into a stable resonance state and measured. Since the signal / noise ratio (SN ratio) of the speed signal obtained in this way can be increased, the d31 constant of the piezoelectric element can be determined with higher accuracy.

  As shown in FIG. 1, the laser Doppler vibrometer 40 is an apparatus including at least an optical system (a light source 41, a polarization beam splitter 42, a mirror 43), a detection unit 45, and a signal processing unit 46.

  In the present invention, the term “laser Doppler vibrometer” is used, for example, as an apparatus that can measure the amount of displacement of a measurement object using the Doppler effect in an interference optical system of laser light. Specifically, a “laser Doppler vibrometer” is first given a frequency shift in advance with reflected light (Doppler shifted laser light) obtained by irradiating a laser beam to a measured object in a vibrating state from a light source. It is possible to obtain a beat frequency based on a difference in optical paths between the reference light and the reference light. Then, frequency analysis is performed in the signal processing unit 46 of the laser Doppler vibrometer, and only the Doppler-shifted frequency component is extracted, and an electric signal corresponding to the vibration speed is output by the FM demodulation circuit. Therefore, as the “laser Doppler vibrometer 40” according to the present invention, a known laser Doppler vibrometer can be appropriately applied. For example, the laser Doppler vibrometer 40 may be a reflective sensor using laser light. Specifically, the laser Doppler vibrometer 40 may be a heterodyne laser displacement meter. By using such a laser Doppler vibrometer 40, it becomes possible to measure a change in speed (displacement) of the measured object 10 in a resonance state over time.

  In addition, the structure (FIG. 1) of the laser Doppler vibrometer 40 demonstrated below is one form of the laser Doppler vibrometer applicable to this invention, Comprising: This invention is not limited to this.

  The optical system of the laser Doppler vibrometer 40 includes, for example, a light source 41 that generates laser light and irradiates the measurement object 10, and polarized light that polarizes the laser light A from the light source 41 into incident light B and reference light C. A beam splitter 42 and a plurality of mirrors 43 for causing the constant detection unit 45 to detect the optical path difference between the reference light C and the reflected light D may be included. The laser light A from the light source 41 may be generated, for example, by injecting a modulation current subjected to frequency modulation into a semiconductor laser that becomes the light source 41. Although not shown, the optical system of the laser Doppler vibrometer 40 appropriately includes optical elements such as a frequency shifter, an optical path length difference corrector, a wave plate, a collimator lens, a condenser lens, a light projecting lens, and a light receiving lens. Can do. According to the optical system of the laser Doppler vibrometer 40, by using the light source 41 to irradiate the measured object with laser light, to obtain the reflected light D and to cause the reflected light D and the reference light A to interfere with each other. The generated beat signal can be detected by the detection unit 45.

  The detection part 45 is comprised from a well-known photodetector. The detection unit 45 includes a known amplification circuit, and can send the detected beat signal to the signal processing unit 46 via the amplification circuit. The signal processing unit 46 performs frequency analysis from the beat signal detected by the detection unit 45 of the laser Doppler vibrometer 40, calculates the vibration speed of the DUT 10, and outputs a voltage signal corresponding thereto. it can. The voltage signal of the vibration speed calculated by the signal processing unit 46 can be displayed on the display unit 50. As the display unit 50, for example, an oscilloscope and / or an electronic computer can be used. Further, the display unit 50 can display the amount of displacement of the DUT 10 obtained by integrating the vibration speed. According to this, the piezoelectric characteristic d31 of the piezoelectric layer 2 of the measured object 10 can be easily calculated from the highly accurate displacement amount data obtained by the measurement.

  The piezoelectric characteristic measurement system 100 according to this embodiment is configured as described above. The piezoelectric characteristic measurement system 100 according to the present embodiment has the following features, for example.

  According to the piezoelectric characteristic measurement system 100 according to the present embodiment, when measuring the vibration speed of the measurement object 10 using the laser Doppler vibrometer 40, the laser irradiation portion 16 of the measurement object 10 is irradiated with laser light. Thus, reflected light that is Doppler shifted can be obtained. According to this, since it is possible to reduce the influence of the higher order vibration mode higher than the second order vibration on the reflected light, the d31 constant of the piezoelectric element can be obtained with high accuracy.

  Further, according to the piezoelectric characteristic measurement system 100 according to the present embodiment, the voltage signal generated by the signal generator 30 is, for example, a single saw wave, and the frequency of the voltage signal is the resonance of the measured object 10. The frequency is adjusted to be 1/10 or less of the frequency. Specifically, when the applied voltage value of the voltage signal is X [V] and the resonance frequency of the DUT 10 is Y [Hz], the voltage increase rate (dV when the voltage signal is applied to the piezoelectric element 4) / Dt) is adjusted to be X / 5Y [V / sec]. According to this, the DUT 10 can be easily brought into a resonance vibration state. Therefore, since the signal / noise ratio (SN ratio) of the speed signal obtained by measurement can be increased, the d31 constant of the piezoelectric element can be obtained with higher accuracy.

2. Piezoelectric Characteristic Measuring Method Hereinafter, a piezoelectric characteristic measuring method according to the present embodiment will be described with reference to the drawings.

  FIG. 4 is a flowchart showing the piezoelectric characteristic measuring method according to this embodiment.

  As shown in FIG. 4, the piezoelectric characteristic measuring method according to the present embodiment fixes the one end of the measured object 10 to hold the measured object 10 and the voltage applied to the piezoelectric layer 2 of the piezoelectric element 4. A step of applying a signal to bring the device under test 10 into a vibrating state, a step of irradiating the device under vibration 10 with a laser beam from a light source to obtain reflected light reflected from the device under test 10, and a laser Doppler Using a vibrometer, a step of obtaining a change over time in the amount of displacement of the measured object 10 from the reflected light and the reference light from the light source, and the piezoelectric characteristics of the piezoelectric layer 2 of the measured object 10 from the amount of displacement. Calculating.

2-1. Holding step (S1)
First, as shown in FIG. 4, the measurement object 10 is held by fixing (cantilevering) one end of the measurement object 10 by the holding unit 20 (S1). When fixing one end of the DUT 10, the holding unit 20 holds the holding portion 12 of the DUT 10. Here, for example, the measured object 10 has a second surface 2b on which the second electrode 3 is formed as an upper surface, and the laser light from the laser Doppler vibrometer 40 is incident on the second surface 2b. Retained. Here, for example, an alignment mark for holding by the holding unit 20 may be provided on the holding portion 12 of the DUT 10 and the DUT 10 may be held along the alignment mark (not shown). ). Here, the electrical connection portions 1b and 3b of the DUT 10 and the wiring in the holding portion 20 are electrically connected, whereby the piezoelectric element 4 and the signal generating portion 30 are electrically connected. The

2-2. Voltage signal application step (S2)
Next, as shown in FIG. 4, a voltage signal is applied to the piezoelectric layer 2 of the piezoelectric element 4 to bring the device under measurement 10 into a vibrating state (S2). Here, the voltage signal applied to the piezoelectric element 4 of the measured object 10 is a single sawtooth wave. The details of the voltage signal have been described above, and will be omitted. After applying the voltage signal, the DUT 10 can be in a vibration state in a resonance state having a displacement amount in the Z-axis direction.

2-3. Laser light irradiation process (S3)
Next, as shown in FIG. 4, the laser beam is irradiated onto the resonance object 10 to be measured, and the reflected light reflected from the object 10 is obtained (S3). As shown in FIG. 1, the incident light B is applied to the laser irradiation portion 16 of the measured portion 11 of the measured object 10 from the Z-axis direction. Thereby, the incident light B is reflected on the second surface 2b (or the second electrode 3), and the Doppler-shifted reflected light D can be obtained.

2-4. Step of obtaining displacement (S4)
Next, as shown in FIG. 4, using a laser Doppler vibrometer 40, the beat frequency of the measured object 10 obtained from the Doppler-shifted reflected light D and reference light C is subjected to frequency analysis and arithmetic processing is performed. Thus, displacement data is obtained (S4). Specifically, the reflected light D is captured by the laser Doppler vibrometer 40, and the reflected light D and the reference light C are optically interfered by an interference optical system that causes the reflected light D and the reference light C to interfere with each other. The detection unit 45 detects the interference intensity due to the interference between the two. Here, when the DUT 10 is in the vibration mode, a beat (swell) is generated in the interference intensity due to the time difference caused by the optical path difference length between the reflected light D and the reference light C. It can be continuously detected as a beat signal. The continuous beat signal detected by the detection unit 45 is an FM signal (Doppler shift amount) having the beat frequency generated by the optical path difference as the center frequency. Therefore, in the signal processing unit 46, the FM signal can be demodulated and the vibration speed of the DUT 10 can be obtained. That is, the displacement amount data of the measured object can be obtained by integrating the obtained vibration velocity data.

  The vibration speed data and displacement amount data of the object to be measured 10 obtained by the laser Doppler vibrometer 40 are output to a display unit 50 such as an oscilloscope and / or an electronic computer, for example, to display changes over time in the displacement amount. Can do.

  Here, since the members such as the first electrode 1 and the second electrode 3 are formed with a film thickness that does not substantially hinder the displacement of the piezoelectric layer 2, the displacement amount of the measured object 10 with respect to the applied voltage value. The data may be voltage-strain characteristic data of the piezoelectric layer 2 of the piezoelectric element 4. Further, correction may be applied in consideration of the film thickness of the substrate 5 and the rigidity of the material.

2-5. Step of calculating piezoelectric characteristics (S5)
Next, as shown in FIG. 4, the piezoelectric constant d31 of the piezoelectric layer 2 is calculated using finite element simulation based on the displacement amount data of the DUT 10. This will be specifically described below.

For example, when the Young's modulus of the substrate 5 and E _5, the relationship between the stress F necessary for the object to be measured is displaced amount deformation of D ₁ is expressed by the following equation.

Here, the stress F required for displacement of ₁ minute D, is required for output by the piezoelectric layer 2 of the piezoelectric element 4, the d31 direction of displacement of the piezoelectric layer 2 and D _2, the piezoelectric element 4 when the Young's modulus and E _4, the stress F is expressed by the following equation.

In the above, the Young's modulus E ₄ Young's modulus E ₅ and the piezoelectric elements 4 of the substrate 5 are known values from conditions such as material and thickness. Therefore, the above D _2, by dividing by the applied voltage, it is possible to obtain the piezoelectric properties d31. Here, α and β are environment variables, but are calculated by the finite element method.

  The above is the description of the piezoelectric characteristic measuring method according to the present embodiment. The piezoelectric characteristic measuring method according to the present embodiment has the following features, for example.

  According to the piezoelectric characteristic measuring method according to the present embodiment, when measuring the vibration speed of the measurement object 10 using the laser Doppler vibrometer 40, the laser irradiation portion 16 of the measurement object 10 is irradiated with laser light. Thus, reflected light that is Doppler shifted can be obtained. According to this, since it is possible to reduce the influence of the higher order vibration mode higher than the second order vibration on the reflected light, the d31 constant of the piezoelectric element can be obtained with high accuracy.

  Further, according to the piezoelectric characteristic measuring method according to the present embodiment, the voltage signal generated by the signal generator 30 is, for example, a single saw wave, and the frequency of the voltage signal is the resonance frequency of the DUT 10. It is adjusted to be 1/10 or less of. Specifically, when the applied voltage value of the voltage signal is X [V] and the resonance frequency of the DUT 10 is Y [Hz], the voltage increase rate (dV when the voltage signal is applied to the piezoelectric element 4) / Dt) is adjusted to be X / 5Y [V / sec]. According to this, the DUT 10 can be easily brought into a resonance vibration state. Therefore, since the signal / noise ratio (SN ratio) of the speed signal obtained by measurement can be increased, the d31 constant of the piezoelectric element can be obtained with higher accuracy.

3. Examples of the piezoelectric characteristic measuring method and the piezoelectric characteristic measuring system according to the present invention will be described below with reference to the drawings.

3-1. Example In this example, a device to be measured 10 according to this embodiment is manufactured using a piezoelectric layer made of lead zirconate titanate (PZT), first and second electrodes made of platinum, and a silicon substrate, The piezoelectric characteristics were evaluated using the piezoelectric characteristic measuring method and the piezoelectric characteristic measuring system according to the present invention.

In the measurement object 10 according to the example, platinum (Pt) was deposited as a first electrode 1 on a 635 μm thick Ti / SiO ₂ / Si substrate by a sputtering method to a thickness of 100 nm. The lead zirconate titanate (Pb (Zr, Ti) O ₃ ), which is the piezoelectric material layer 2 of the material to be measured, is formed and fired to a thickness of 1.3 μm by a sol-gel method, On the body layer 2, platinum (Pt) was deposited as the second electrode 3 under the same conditions as the first electrode 1. The outer shape of the DUT 10 is rectangular when viewed from the Z-axis direction, which is the incident direction of the laser, and the length in the X-axis direction, which is the longitudinal direction, is 30 mm, and the length in the Y-axis direction, which is the short direction. The length in the X-axis direction of the measured portion 11 was 4 mm and 20 mm.

  As the laser Doppler vibrometer 40, LV-0180 manufactured by Ono Sokki Co., Ltd. was used. As the light source 41, a semiconductor laser having a wavelength of 632.8 nm and an output of 0.95 mW or less was used. The irradiation position of the laser beam was adjusted to the node portion of the secondary vibration in the measured portion 11, and the laser beam was irradiated to a position 15.2 mm from the fixed end held by the holding unit 20.

  The voltage signal applied to the piezoelectric element 4 of the DUT 10 was a single saw wave having a frequency range of 10 Hz to 100 Hz and a voltage value of 5 V (minimum value) to 80 V (maximum value).

  5 shows (1) voltage signal, (2) velocity data calculated from the detection result, and (3) displacement amount data calculated from the velocity data according to the present embodiment on an oscilloscope as the display unit 50. It is a figure which shows what was done. FIG. 6 is a diagram in which only velocity data obtained in the measurement according to the present embodiment and displacement amount data obtained from the velocity data are plotted. FIG. 7 is a graph plotting the piezoelectric characteristics calculated by the finite element simulation from the obtained displacement amount data. The measurement was performed twice under the same conditions. In FIG. 7, the results are plotted with triangles and squares.

  As shown in FIGS. 5 and 6, periodic voltage data having a very high SN ratio (signal-to-noise ratio) was obtained after the voltage signal was applied. That is, it was confirmed that the vibration velocity data of the measured object 10 can be obtained with high accuracy.

  Next, as shown in FIG. 7, it is confirmed that the piezoelectric characteristic d31 of the piezoelectric layer 2 is arithmetically processed by a finite element simulation with the displacement amount data calculated based on the obtained vibration velocity data. It was done. In addition, since almost the same result was obtained in the two measurement results, it was confirmed that the reproducibility and reliability of the measurement result according to this example was high.

3-2. Comparative Example In this comparative example, using the measurement object 10 used in the example, the irradiation position of the laser beam is adjusted not to the laser irradiation part 16 but to a part close to the tip part 15 of the measurement part 11. Then, the measurement was performed. The laser beam was irradiated at a position 15.2 mm from the fixed end held by the holding unit 20. Other measurement conditions were adjusted in the same manner as the conditions according to the example.

  FIG. 8 is a diagram in which only velocity data obtained in the measurement according to this comparative example and displacement amount data obtained from the velocity data are plotted.

  As shown in FIG. 8, a plurality of vibration waveforms are affected by the obtained velocity data. That is, it has been clarified that the higher-order vibration mode higher than the secondary vibration affects the measurement result depending on the irradiation position of the laser light, and the accuracy of the measurement result is lower than the measurement result according to the example.

  The present invention is not limited to the above-described embodiments, and various modifications can be made. For example, the present invention includes substantially the same configuration (for example, a configuration having the same function, method and result, or a configuration having the same purpose and effect) as the configuration described in the embodiment. In addition, the invention includes a configuration in which a non-essential part of the configuration described in the embodiment is replaced. In addition, the present invention includes a configuration that achieves the same effect as the configuration described in the embodiment or a configuration that can achieve the same object. In addition, the invention includes a configuration in which a known technique is added to the configuration described in the embodiment.

DESCRIPTION OF SYMBOLS 1 1st electrode, 1a Lead part, 1b Electrical connection part, 2 Piezoelectric layer, 2a 1st surface,
2b 2nd surface, 3rd electrode, 3a lead part, 3b electrical connection part, 4 piezoelectric element,
5 Substrate, 10 Measured object, 11 Measured part, 12 Holding part, 15 Tip part,
16 laser irradiation part, 20 holding part,
30 signal generator, 40 laser Doppler vibrometer, 41 light source,
42 polarizing beam splitter, 43 mirror, 45 detector, 46 signal processor,
50 display unit, 100 piezoelectric characteristic measurement system

Claims (6)

A method for measuring piezoelectric characteristics of an object to be measured, comprising: a substrate; and a piezoelectric element provided on the substrate,
Fixing the one end of the measured object to hold the measured object;
Applying a voltage signal to the piezoelectric layer of the piezoelectric element to bring the object to be measured into a vibrating state;
Irradiating a laser beam from a light source to the object to be measured in a vibrating state to obtain reflected light reflected from the object to be measured;
Using a laser Doppler vibrometer to obtain a change over time in the amount of displacement of the measured object from the reflected light and the reference light from the light source;
Calculating the piezoelectric characteristics of the piezoelectric layer of the measured object from the displacement amount;
Including
The method for measuring piezoelectric characteristics, wherein, in the step of obtaining the reflected light, the laser light is applied to a secondary vibration node of the measured object. In claim 1,
The voltage signal applied to the piezoelectric layer is a single sawtooth wave,
The method for measuring piezoelectric characteristics, wherein the frequency of the voltage signal is 1/10 or less of the resonance frequency of the object to be measured. In claim 2,
When the applied voltage value of the voltage signal is X [V] and the resonance frequency of the object to be measured is Y [Hz], the voltage increase rate (dV / dt) when the voltage signal is applied to the piezoelectric element Is a method for measuring piezoelectric characteristics, which is X / 5Y [V / sec]. In any one of Claim 1 to 3,
The method for measuring piezoelectric characteristics, wherein the vibration state is a resonance state. In any one of Claims 1-4,
The step of calculating the piezoelectric characteristics is a piezoelectric characteristics measurement method using finite element simulation. A holding unit for holding one end of a measurement object including a piezoelectric element;
A signal generator for generating a voltage signal applied to the object to be measured;
An optical system including a light source of laser light that irradiates the measurement object, a detection unit that detects reflected light obtained by irradiating the measurement object with the laser light and reference light obtained from the light source, and the detection A laser Doppler vibrometer including a signal processing unit capable of obtaining a change over time in the amount of displacement of the measured object from a detection result in the unit;
Have
The piezoelectric characteristic measurement system, wherein the laser light from the light source of the laser Doppler vibrometer is irradiated to a secondary vibration node of the measured object.