JP2023172993A - Bulk elastic wave device and manufacturing method thereof - Google Patents

Abstract

To provide a bulk elastic wave device which can be made thin by a simple processing process, and a manufacturing method thereof.SOLUTION: A bulk elastic wave device 100 comprises: a placoid piezoelectric body 10; a support substrate 20 which supports the piezoelectric body 10; a pair of excitation electrodes 31 and 32 which is opposite to the piezoelectric body 10 with the support substrate 20 interposed between them, and to which voltage for exciting a bulk elastic wave to the piezoelectric body 10 is applied; and a floating conductor 40 which is opposite to the pair of excitation electrodes 31 and 32 with the piezoelectric body 10 and the support substrate 20 interposed between them. The support substrate 20 includes: a first surface 21 to which the piezoelectric body 10 is fixed; and a second surface 22 which is positioned opposite to the first surface 21. The pair of excitation electrodes 31 and 32 are spaced along the second surface 22.SELECTED DRAWING: Figure 1

Description

The present invention relates to a bulk acoustic wave device and a method for manufacturing the same.

Acoustic wave devices that excite elastic waves in piezoelectric bodies are known. For example, Patent Document 1 describes a bulk acoustic wave (BAW) device that uses a first electrode and a second electrode facing each other with a piezoelectric body in between as excitation electrodes. The device includes a support substrate supporting a piezoelectric material and a multi-gradient ridge frame structure provided on the opposite side of the piezoelectric material from the support substrate, the structure reducing lateral energy leakage. is trying.

Further, Patent Document 2 describes a surface acoustic wave (SAW) device having a structure in which a piezoelectric body is bonded to a support substrate.

Japanese Patent Application Publication No. 2022-51527 Patent No. 4657002

Devices such as those described above are used, for example, as frequency control elements, sensor materials, etc., and as the importance of high frequency technology has increased in recent years, there has been a demand for thinner devices. As for SAW devices, as described in Patent Document 2, there is a known technology that allows the support substrate to be made thinner while maintaining mechanical strength, but it is difficult to increase the frequency of SAW devices compared to BAW devices. However, as described in Patent Document 1, a BAW device in which excitation electrodes are formed on both sides of a piezoelectric body and a multi-gradient ridge frame structure is formed may complicate the processing process.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a bulk acoustic wave device that can be made thinner through a simple processing process, and a method for manufacturing the same.

In order to achieve the above object, a bulk acoustic wave device according to a first aspect of the present invention includes:
A plate-shaped piezoelectric material,
a support substrate that supports the piezoelectric material and has a first surface to which the piezoelectric material is fixed, and a second surface located on the opposite side of the first surface;
a pair of excitation electrodes that face the piezoelectric body with the supporting substrate in between, and to which a voltage for exciting bulk acoustic waves is applied to the piezoelectric body;
a conductor facing the pair of excitation electrodes with the piezoelectric body and the support substrate in between,
The pair of excitation electrodes are spaced apart from each other along the second surface.

The bulk acoustic wave device may further include an excitation substrate to which the support substrate is fixed and on which the pair of excitation electrodes are formed.

In order to achieve the above object, a bulk acoustic wave device according to a second aspect of the present invention includes:
A plate-shaped piezoelectric material,
a support substrate that supports the piezoelectric material and has a first surface to which the piezoelectric material is fixed, and a second surface located on the opposite side of the first surface;
a pair of excitation electrodes located in the direction in which the first surface faces and facing the piezoelectric body, to which a voltage for exciting a bulk acoustic wave is applied to the piezoelectric body;
a conductor facing the pair of excitation electrodes with the piezoelectric body and the support substrate in between,
The pair of excitation electrodes are spaced apart from each other along the first surface.

The piezoelectric body is a crystal substrate,
The support substrate may be a glass substrate or a crystal substrate.

Each of the piezoelectric body and the support substrate may be an AT-cut crystal substrate.

The angle between the electrical axis of the piezoelectric body and the electrical axis of the support substrate may be 0°, 90°, or 180°.

In order to achieve the above object, a method for manufacturing a bulk acoustic wave device according to a third aspect of the present invention includes:
producing a composite substrate by fixing the piezoelectric body to the support substrate;
polishing the manufactured composite substrate to make it thin;
forming the conductor on the composite substrate after polishing the composite substrate.

According to the present invention, it is possible to provide a bulk acoustic wave device that can be made thinner through a simple processing process and a method for manufacturing the same.

1 is a cross-sectional view of a bulk acoustic wave (BAW) device according to an embodiment of the invention. FIG. 3 is a plan view of the BAW device according to the embodiment. It is a diagram for explaining the electric axis direction according to an example in which an AT-cut crystal substrate is used as the support substrate, and (a) is a case where the angle between the electric axis of the piezoelectric body and the electric axis of the support substrate is 0°. , (b) shows the case of 90°, and (c) shows the case of 180°. (a) shows an impedance spectrum of a comparative example, and (b) and (c) are diagrams showing impedance spectra of each example. (a) to (c) are diagrams showing impedance spectra of each example following FIGS. 4(b) and (c). FIG. 6 is a diagram based on the impedance spectra of FIGS. 4 and 5, in which (a) shows the Q value and (b) shows the impedance at resonance. FIG. 3 is a diagram showing that the supporting substrate, which is an AT-cut crystal substrate, oscillates. FIG. 6 is a diagram showing frequency temperature characteristics of a comparative example and each example. A diagram showing the temperature characteristics of an ideal AT-cut crystal resonator.

An embodiment of the present invention will be described with reference to the drawings.

As shown in FIGS. 1 and 2, the bulk acoustic wave (BAW) device 100 includes a piezoelectric body 10, a support substrate 20, an excitation substrate 30 on which a pair of excitation electrodes 31 and 32 are formed, and a floating conductor 40. , is provided.

FIG. 1 is a cross-sectional view of the BAW device taken along line AA shown in FIG. In FIG. 1, hatching showing cross sections of components other than the excitation electrodes 31 and 32 and the floating conductor 40 is omitted for ease of viewing. Moreover, in the plan view of FIG. 2, the structure located between the support substrate 20 and the excitation substrate 30 is shown by a broken line.

The piezoelectric body 10 is plate-shaped and has a thickness on the μm order (for example, several tens of μm to several hundred μm). The piezoelectric body 10 is made of, for example, an AT-cut crystal substrate, and is formed into a substantially circular shape in a plan view, as shown in FIG. 2, for example.

When an AC voltage is applied to the AT-cut crystal substrate through the pair of excitation electrodes 31 and 32, a thickness shear mode (TSM) is excited that vibrates in a direction parallel to the substrate surface. A crystal substrate to which an electric field is applied is distorted due to the inverse piezoelectric effect, and the distortion is eliminated by returning the electric field. By repeating ON and OFF of the electric field in this way, thickness shear vibration can be excited in the crystal substrate. A predetermined time is required from the application of the electric field until the distortion of the crystal is completed. This time is mainly determined by the thickness of the crystal substrate, and the relational expression between the fundamental frequency F ₀ (MHz) and the thickness t (mm) of the AT-cut crystal substrate is expressed as the following equation [Equation 1]. By synchronizing the applied voltage at this time with the period of the thickness-shear vibration as an oscillating electric field, the crystal substrate can be used as a vibrator. That is, the BAW device 100 is a BAR (Bulk Acoustic Resonator) that utilizes the fact that the impedance seen from the electrical terminal changes significantly near the resonance point due to the piezoelectric effect of the piezoelectric body 10.

The relationship between the change in the resonant frequency of the crystal oscillator and the change in the mass of the adsorbed substance is shown by the following equation [Math. 2], which is called the Sauerbrey equation. In the formula, ΔF is the amount of frequency change (Hz) of the crystal resonator, F ₀ is the fundamental frequency (Hz) of the crystal resonator, A is the area (cm ² ) of the thin film electrode placed on the crystal substrate, and μ _q is the amount of change in the frequency of the crystal resonator (Hz). The elastic modulus (=2.947×10 ¹¹ gcm ⁻¹ s ⁻² ), ρ _q is the density of the crystal (=2.648 gcm ⁻³ ), and Δm ^* is the mass (g) of the substance adsorbed on the electrode.

Here, in equation [2], if Δm ^* /A is the mass Δm of the substance adsorbed per unit area, and the remaining constant term is K, it can be simplified as shown in equation 3 below. .

That is, by detecting the frequency change of the crystal oscillator, it is possible to measure the change in mass of the adsorbed substance on the electrode. From the formula [Equation 2], if the fundamental frequency F ₀ of the crystal resonator is high, the frequency change amount ΔF becomes large, and a sensor called a quartz crystal microbalance (QCM) with higher sensitivity can be obtained. Furthermore, from the formula [Equation 1], it can be seen that the smaller the thickness t of the crystal substrate, the higher the fundamental frequency F ₀ becomes. In other words, a thinner crystal substrate is required for increasing the frequency of a crystal resonator and increasing the sensitivity of QCM.

There is a limit to polishing only a crystal substrate thinly due to its mechanical strength. Therefore, a method was proposed to polish the crystal thinner by attaching the crystal substrate to another support substrate for reinforcement. However, in this method, after a quartz substrate is attached to a glass substrate and polished, it is necessary to peel the polished quartz substrate from the glass substrate. This method requires steps such as removing the crystal substrate and re-fixing it when mounting it on a device, which complicates the fabrication process. As described below, the BAW device 100 is a device that can excite a crystal substrate (piezoelectric material 10) whose thickness has been reduced by polishing while it is fixed to the support substrate 20.

The support substrate 20 is a substrate that supports the piezoelectric body 10, and is composed of, for example, an AT-cut crystal substrate, a Z-cut crystal substrate, a glass substrate, or the like. The support substrate 20 is formed to have a thickness on the μm order (for example, several tens of μm to several hundred μm). For example, the support substrate 20 is formed into a rectangular shape that is larger than the piezoelectric body 10 in plan view, as shown in FIG.

The support substrate 20 has a first surface 21 (top surface in FIG. 1) to which the piezoelectric body 10 is fixed, and a second surface 22 (bottom surface in FIG. 1) located on the opposite side of the first surface 21. have

The piezoelectric body 10 is fixed to the first surface 21 by at least one of bonding and bonding. For example, an optical contact can be used as the bond. For example, the piezoelectric body 10 and the support substrate 20 can be bonded together via silicone such as polydimethylsiloxane (PDMS). Note that other known techniques may be used as appropriate to fix the piezoelectric body 10 to the support substrate 20.

A pair of excitation electrodes 31 and 32 face piezoelectric body 10 with support substrate 20 in between. A voltage for exciting bulk elastic waves in the piezoelectric body 10 is applied to the pair of excitation electrodes 31 and 32. As shown in FIG. 1, the pair of excitation electrodes 31 and 32 are spaced apart from each other along the second surface 22. The excitation electrodes 31 and 32 thus divided are capacitively coupled with the floating conductor 40 in opposite phases, and by applying an AC voltage to the two excitation electrodes 31 and 32, the piezoelectric material It is possible to excite 10 vibrations. Specifically, an AC voltage is applied to the excitation electrodes 31 and 32 via a wiring 31a connected to the excitation electrode 31 and a wiring 32a connected to the excitation electrode 32. An alternating current voltage is applied to the excitation electrodes 31 and 32 by a well-known oscillation circuit. The operation of the oscillation circuit can be controlled by a control unit including a microcontroller.

As an example, the pair of excitation electrodes 31 and 32 are one pair and the other pair, which are divided by cutting out a band shape along the vertical direction in FIG. 2 from a circle. The vertical direction is set, for example, along the crystal axis of the crystal that constitutes the piezoelectric body 10.

The excitation electrodes 31 and 32 and the wirings 31a and 32a are formed of a conductor such as Au (gold) or Cu (copper), which is patterned on the surface of the excitation substrate 30 facing the support substrate 20, for example. The excitation substrate 30 has insulating properties and is made of, for example, a glass substrate. The excitation substrate 30 supports the support substrate 20 by a known fixing method.

Note that the excitation electrodes 31 and 32 and the wirings 31a and 32a may be patterned on the second surface 22 of the support substrate 20.

The floating conductor 40 faces the pair of excitation electrodes 31 and 32 with the piezoelectric body 10 and the support substrate 20 in between. The floating conductor 40 is made of, for example, a conductor such as Au (gold) or Cu (copper) that is patterned on the top surface of the piezoelectric body 10 (the surface located on the opposite side of the surface of the piezoelectric body 10 facing the support substrate 20). It is formed. As shown in FIG. 2, the floating conductor 40 is formed in a circular shape and is provided at a position overlapping the pair of excitation electrodes 31 and 32.

Floating conductor 40 is provided to control the electric field generated from excitation electrodes 31 and 32. If the floating conductor 40 is not provided, the electric field will spread too much, but the floating conductor 40 allows the electric field to be concentrated between the excitation electrodes 31 and 32. In consideration of this electric field, research by the inventors of the present application suggests that it is preferable for the BAW device 100 to satisfy (i) and (ii) below. Note that T is the total thickness of the piezoelectric body 10 and the support substrate 20. (i) The distance between the excitation electrode 31 and the excitation electrode 32 (the distance in the left-right direction in FIGS. 1 and 2) is about 0.5 to 1.5 times the total thickness T. (ii) If the diameter of the circle that is the outer shape of the floating conductor 40 is D, and the diameter of the circle along the outer edge of the pair of excitation electrodes 31 and 32 is d, then for example, the centers of both circles coincide, and d≦ D≦d+T.

The configuration of the BAW device 100 has been described above. Next, as an example, an experiment will be described in which the piezoelectric body 10 was fixed to the support substrate 20 and it was demonstrated that the piezoelectric body 10 could be excited through the support substrate 20. Note that components having the same functions as those in the above embodiment will be described using the same reference numerals as in the above embodiment.

(Example)
First, a description will be given of how the BAW device 100 used in the experiment was configured.

(1. Manufacturing process of piezoelectric body 10 and excitation substrate 30)
The piezoelectric body 10 and the excitation substrate 30 were manufactured using semiconductor micromachining technology based on photolithography. Hereinafter, since the process of forming the floating conductor 40 on the piezoelectric body 10 and the process of forming the excitation electrodes 31, 32 and the wirings 31a, 32a on the excitation substrate 30 are similar, they will be explained together. Terms such as electrode material and electrode pattern used below include materials and patterns for forming not only the excitation electrodes 31 and 32 but also the wirings 31a and 32a and the floating conductor 40.

The piezoelectric body 10 uses an AT-cut crystal substrate (Seiko E&G Co., Ltd.) with a diameter of 12 mm and a thickness of 100 μm, and the excitation substrate 30 uses a glass substrate with a length of 22 mm, a width of 22 mm, and a thickness of 1.2 mm. Using. These substrates were ultrasonically cleaned for 3 minutes using acetone (Junsei Kagaku Co., Ltd.) and ethachol (99% denatured alcohol, Imazu Pharmaceutical Co., Ltd.), and then Au (gold wire 99.95%, Nilaco Co., Ltd.) is deposited to a thickness of about 100 nm by vacuum evaporation (VPC-260, ULVAC Techno Co., Ltd.). At this time, Cr (99.9%, Nilaco Co., Ltd.) with a thickness of about 30 nm is deposited as an adhesive layer (Step 1). Next, using a photolithography method, a resist pattern is formed on each substrate on which Au is deposited. First, OAP (Tokyo Ohka Kogyo) is applied to increase the adhesion between the photoresist and the substrate, and then a positive photoresist (OFPR-800 LB 54 cp, Tokyo Ohka Kogyo), which is a photosensitive resin, is applied. A spin coater (MS-A100, Mikasa Co., Ltd.) was used to apply the OAP and photoresist, and spin coating was performed at 3000 rpm for 20 seconds. Thereafter, prebaking is performed at 90° C. for about 15 minutes in a high-temperature furnace (step 2). Next, each substrate is exposed to ultraviolet light for 90 seconds using a mask alignment device (M-1S type, Mikasa Co., Ltd.) through the photomask onto which the electrode pattern has been transferred, and the electrode pattern is transferred to the resist on the substrate. (Step 3). After removing the resist on the photosensitive area of this substrate using a developer (NMD-3, Tokyo Ohka Kogyo), post-baking is performed at 100° C. for 30 minutes in a high-temperature furnace (step 4). Thereafter, Au and Cr are removed in this order using an Au etching solution and a Cr etching solution (Step 5). Finally, in order to remove the resist remaining on the pattern, ultrasonic cleaning is performed in the order of acetone and ethanol to obtain an electrode shape (step 6).

(2. Method for fixing piezoelectric body 10 to support substrate 20)
The piezoelectric body 10 was temporarily fixed to the support substrate 20 by optical contact using methanol. Optical contact is a bonding technology that does not use adhesives or the like. Bonding by optical contact is thought to be due to van der Waals forces on the surfaces of quartz and glass substrates, and hydrogen bonding forces of silanol groups (Si--OH) formed on the substrate surfaces due to moisture adsorption. After ultrasonically cleaning the piezoelectric body 10 and the supporting substrate 20 with acetone and ethanol, 0.5 μL of methanol (manufactured by Junsei Kagaku Co., Ltd.) was dropped onto the supporting substrate 20, and the piezoelectric body 10 was placed and fixed thereon.

In order to fix the piezoelectric body 10 to the support substrate 20, in addition to optical contact, adhesive fixation was performed using PDMS (SILPOT 184, Dow Coming Corp.). PDMS is prepared by mixing a base agent and a curing agent at a ratio of 10:1 (w/w) and further diluting it with hexane (Junsei Kagaku Co., Ltd.). The diluted PDMS is spin-coated onto the support substrate 20 at 3000 rpm for 15 seconds, and degassed for 15 minutes. After completing the degassing, the crystal resonator was placed on the support substrate 20 coated with PDMS, and was bonded and fixed by baking at 90° C. for 1 hour in a high temperature oven. Note that dilution with hexane was performed at a magnification of 100, 25, 15, 10, and 6 times (w/w), and each was evaluated after adhesion.

(3. Support substrate 20 used in the experiment)
As the supporting substrate 20, a glass substrate (Asahi Techno Glass), an AT-cut crystal substrate (Seiko E&G Co., Ltd.), and a Z-cut crystal substrate (Citizen Fine Device) each having a thickness of 100 μm was used.

Here, the chemical composition of the crystal is SiO2, belongs to the trigonal system, and the central axis of the hexagonal column is the Z axis. The axis that connects the diagonals of the cross-section (hexagonal shape) of the hexagonal prism perpendicular to the Z-axis is the X-axis, and is called the "electrical axis." Furthermore, the Y-axis perpendicular to the X-axis is called the "mechanical axis." A crystal substrate cut perpendicular to the X-axis is called an X-cut, a crystal substrate cut perpendicular to the Y-axis is called a Y-cut, and a crystal substrate cut perpendicular to the Z-axis is called a Z-cut. In particular, a crystal substrate in which the Y-cut is rotated around the X-axis to form an angle of 35° 15' with the Z-axis is called an AT-cut.

3(a) to (c) show the electrical axis 10a (X-axis) of the piezoelectric material 10 and the electrical axis 20a of the supporting substrate 20 when AT-cut crystal substrates are used for each of the piezoelectric material 10 and the supporting substrate 20. An example of the relationship between the angle and the (X axis) is shown. In FIGS. 3A to 3C, the direction in which the electric axis 10a of the piezoelectric body 10 faces is shown by a solid line arrow, and the direction in which the electric axis 20a of the support substrate 20 faces is shown by a broken line arrow.

Since the AT-cut crystal substrate has an inclination in the direction of the crystal axis, if the angle between the electric axes 10a and 20a is θ, it is expected that the characteristics will differ depending on θ. Therefore, when using an AT-cut crystal substrate as the support substrate 20, evaluation was performed using three patterns of θ=0°, 90°, and 180°. FIG. 3(a) shows the case when θ=0°, FIG. 3(b) shows the case when θ=90°, and FIG. 3(c) shows the case when θ=180°.

Furthermore, as a comparative example, measurements were also conducted without the support substrate 20. In order to match the distance between the pair of excitation electrodes 31 and 32 and the floating conductor 40 with the condition in which the support substrate 20 is present, in the comparative example without the support substrate 20, a 100 μm spacer is prepared, and the distance between the piezoelectric body 10 and the excitation substrate is A gas phase was established between 30 and 30 minutes.

(4. Configuration of experimental system)
The excitation electrodes 31 and 32 were connected to an oscillation circuit, and the frequency was measured using a frequency counter (Agilent 53131A 225 MHz universal counter, KEYSIGHT TECHNOLOGIES). At this time, a DC stabilized power supply (AND AD-8735D, A&D) was used to supply power to the circuit. In addition, the BAW device 100 to be measured was placed on a hot plate (Digital Hot Plate 722A-1, As One). The hot plate was used for temperature control during evaluation of frequency temperature characteristics. Furthermore, when impedance was measured, the excitation electrodes 31 and 32 were connected to an impedance analyzer (Network/Spectrum/Impedance Analyzer 4395A, KEYSIGHT TECHNOLOGIES).

(5. Evaluation method)
(resonant frequency stability)
BAW device 100 was used as an oscillation circuit, and the resonance frequency was measured using a frequency counter. The standard deviation σ was calculated from the variation in the resonance frequency for 60 seconds, and evaluation was performed using 3σ, which is three times the standard deviation, as an index of stability.

(Impedance analyzer)
The BAW device 100 was connected to an impedance analyzer via an impedance test kit (43961A, KEYSIGHT TECHNOLOGIES) and a test fixture (16192A, KEYSIGHT TECHNOLOGIES). An impedance spectrum near the resonance frequency was measured using an impedance analyzer, and thereby the impedance and Q value at resonance were obtained.

(Frequency temperature characteristics)
While checking the temperature of the BAW device 100 using an infrared thermography camera (InfRec H2640, Nippon Avionics Co., Ltd.), heat the BAW device 100 with a hot plate from 20°C to 100°C in 5°C increments, and measure the resonance frequency at each temperature. was measured using a frequency counter. The measured values were plotted on a graph as a rate of change with respect to the resonance frequency at room temperature (11° C.), and differences due to the support substrate 20 were evaluated.

(6. Results and discussion)
Below, the experimental results of the BAW device 100 in which the piezoelectric body 10 is adhesively fixed to the support substrate 20 using PDMS will be described. A similar experiment was also conducted on the BAW device 100 in which the piezoelectric body 10 was fixed to the support substrate 20 with optical contacts, and the same results as in the case of PDMS were obtained, but due to variations in the measurement results, they will not be discussed here. do. The reason for this variation is thought to be that the reproducibility of the fixation state by the optical contact is poor.

(Study of adhesive fixing conditions using PDMS)
For adhesive fixation, PDMS diluted with hexane at a ratio of 100, 25, 15, 10, and 6 times (w/w) was used. When the glass substrate and the crystal resonator were fixed with adhesive, the crystal resonator came off immediately after baking under conditions of 100 to 15 times dilution. On the other hand, under the conditions of 10-fold and 6-fold dilution, the crystal resonator was completely adhesively fixed after baking. Regarding the possibility of oscillation, in the case of 10 times dilution, a clear resonance point was seen on the impedance spectrum, indicating that oscillation occurred. However, in the case of 6-fold dilution, no clear peak could be observed on the impedance spectrum, and it was determined that oscillation would be difficult. This is thought to be due to the fact that in the case of 6-fold dilution, the PDMS adhesive layer is thick and acts as an elastic body, damping vibration energy. On the other hand, it was found that if the material was diluted ten times, the thickness could be controlled to be sufficient for the crystal resonator to oscillate. The following are experimental results of a BAW device 100 in which a crystal resonator (piezoelectric material 10) was adhesively fixed to a support substrate 20 using PDMS diluted 10 times.

(resonant frequency stability)
[Table 1] shows the resonant frequency stability of each example in which the type of support substrate 20 was changed and a comparative example without a support substrate.

According to [Table 1], there is an overall decrease in resonance frequency stability compared to the condition without a support substrate. Although the PDMS used in each example is thin enough for oscillation, considering the decrease in stability, it is considered that the PDMS has an effect as an elastic body. Further, in each example, a decrease in stability was observed only when a Z-cut crystal substrate was used as the support substrate 20. This is considered to be caused by the Z-cut crystal substrate vibrating in the thickness direction.

(Impedance spectrum measurement)
FIG. 4(a) shows the impedance spectrum of a comparative example without a support substrate, and FIGS. 4(b), (c), and FIGS. 5(a) to (c) show the impedance spectra of each example. In each figure, the vertical axis represents impedance and phase, and the horizontal axis represents frequency normalized based on the resonance frequency. Moreover, the solid line in each figure shows the impedance spectrum, and the dotted line shows the phase. Looking at each figure, it can be seen that only in the example where θ=180° (FIG. 5(b)), the phase changes in the range of -90° to 90°. Here, when the impedance spectrum was measured using a commercially available QCM (Quartz Crystal Microbalance) for calibration, the phase changed in the range of -40° to 140°. This change in phase was similar to the measurement results under conditions other than θ=180°. From this, when an AT-cut crystal substrate is used as the support substrate 20, specific oscillation occurs under the condition of θ = 180° where the crystal axes are not aligned with the crystal resonator (piezoelectric body 10). It is possible that there are. When the crystal axis directions are aligned as in the condition of θ=0°, it is considered that when an electric field is applied, the upper and lower AT-cut crystal substrates (piezoelectric body 10 and supporting substrate 20) oscillate in the same direction. On the other hand, if the condition that the crystal axis direction is θ=180° is not met, the upper and lower AT-cut crystal substrates (piezoelectric body 10 and support substrate 20) vibrate in opposite phases. It is presumed that the unique oscillation is produced by synchronizing these antiphase oscillations.

Next, the Q value calculated based on the measured impedance spectrum is shown in FIG. 6(a), and the impedance at resonance is shown in FIG. 6(b).

In the condition without a support substrate (comparative example), an air layer of 100 μm is sandwiched, so the impedance is higher than in other conditions in which a dielectric glass or crystal substrate is used as the support substrate 20. You can check it.

Further, when comparing the Q values in each example other than the comparative example, it is found that the Q value is higher when the support substrate 20 is a crystal substrate than when the support substrate 20 is a glass substrate. . This is considered to be due to a difference depending on whether the crystal of the supporting substrate 20 is isotropic or anisotropic. Glass is isotropic, meaning there are variations in crystal orientation. Therefore, when the electric field generated from the excitation electrodes 31 and 32 passes through the glass substrate, polarization occurs in various directions and the electric field is dispersed. On the other hand, since the crystal substrate is anisotropic, the electric field generated from the excitation electrodes 31 and 32 is not dispersed within the support substrate 20 and can reach the upper crystal resonator (piezoelectric body 10). When a glass substrate, which is an isotropic material, is used as the support substrate 20, it is considered that the Q value becomes low due to a decrease in electric field strength due to electric field dispersion.

Comparing the conditions in which an AT-cut crystal substrate was used as the support substrate 20, it can be seen that a high Q value is exhibited under the conditions of θ=0° and 180°. Further, when comparing various conditions in which a quartz substrate was used as the support substrate 20, the Z-cut quartz substrate showed the highest Q value. As mentioned above, the AT-cut crystal substrate of the support substrate is considered to oscillate when an electric field is applied. The basis for this is shown in Figure 7. FIG. 7 is an impedance spectrum measured when the relationship between the X axes of the upper and lower AT-cut crystal substrates (piezoelectric body 10 and supporting substrate 20) is set to θ=0°. Two peaks can be confirmed on the spectrum. From this, it can be seen that the upper and lower AT-cut crystal substrates (piezoelectric body 10 and support substrate 20) oscillate simultaneously. If an AT-cut crystal substrate is used as the support substrate 20, the support substrate 20 will also oscillate at the same time, so there will be energy loss, and the Q value will be lower than when a Z-cut crystal substrate is used as the support substrate 20. It will be done.

(Frequency temperature characteristics)
FIG. 8 shows the frequency-temperature characteristics of the comparative example and each example described above. However, in the example in which a Z-cut crystal substrate was used as the support substrate 20, the frequency suddenly increased from around 95° C. and was unstable, so data exceeding 90° C. was omitted. In the figure, an enlarged view of frequency temperature characteristics from 10°C to 70°C is shown. Further, the thermal expansion coefficients of crystal and glass are shown in [Table 2].

When an AT-cut crystal substrate is used as the support substrate 20 and the conditions are θ=0° and 180°, the direction of thermal expansion is the same for the support substrate 20 and the crystal resonator (piezoelectric body 10), so the support substrate 20 is not used. Almost the same frequency-temperature characteristics as under the above conditions were obtained. On the other hand, when the condition was θ=90°, the frequency change rate with respect to temperature was smaller than under the condition without the support substrate 20, and almost no frequency change was observed up to 60°C. Here, the temperature characteristics of an ideal AT-cut crystal resonator are shown in FIG. Figure 9 shows the temperature characteristics of a crystal resonator (resonant frequency 2690 kHz) cut at an angle of around 54° around the It is thought that the rate of temperature change is the smallest. Comparing this ideal temperature characteristic, in the range of 10°C to 70°C, the results of the example with θ = 90° in the enlarged view in Fig. 8 almost match the curve 4 in Fig. 9. I understand. From this, by setting the relationship between the X axes of the upper and lower AT-cut crystal substrates (piezoelectric body 10 and support substrate 20) to θ = 90°, from the perspective of thermal expansion, it is possible to change from anisotropic to isotropic. It is thought that the temperature characteristics can be changed to similar to the theoretical values of AT-cut crystal.

Next, frequency-temperature characteristics when a glass substrate or a Z-cut crystal substrate is used as the support substrate 20 will be evaluated. Since the glass substrate and the Z-cut crystal substrate have different coefficients of thermal expansion from the AT-cut crystal resonator, the rate of frequency change with respect to temperature was greater than that without the support substrate 20. In particular, when a glass substrate, which is an isotropic material, was used as the support substrate 20, the difference was more noticeable. When used at room temperature, there is no problem in using a glass substrate or a Z-cut crystal substrate as the support substrate 20, but it is found that it is preferable to use an AT-cut crystal substrate as the support substrate 20 when used in a high-temperature environment.

(summary)
Based on the above, the support substrate 20 may be appropriately selected depending on the environment in which the BAW device 100 is used. Judging from the Q value, it is preferable to use an AT-cut crystal substrate or a Z-cut crystal substrate, which are anisotropic materials, as the support substrate 20, and if a higher Q value is desired, a Z-cut crystal substrate is more preferable. When used in a high-temperature environment, it is preferable to use an AT-cut crystal substrate as the support substrate 20. From a cost perspective, it is preferable that the support substrate 20 is a glass substrate. Overall, it is preferable that the support substrate 20 is an AT-cut crystal substrate if use in various environments is considered.

Crystal oscillation circuits are key devices in IoT, 5G (fifth generation mobile communication systems) and other technologies, and communication equipment including smartphones.According to the excitation technology of the BAW device 100 described above, crystal oscillation It is possible to oscillate without separating the support substrate 20 from the child. This is advantageous in terms of handling of the thinned crystal resonator in that it can oscillate while being fixed to the support substrate 20, and is expected to be applied to various ultra-thin crystal devices.

(1) The BAW device 100 described above includes a piezoelectric body 10, a support substrate 20 that supports the piezoelectric body 10, a pair of excitation electrodes 31 and 32 facing the piezoelectric body 10 with the support substrate 20 in between, and a piezoelectric body 10. A floating conductor 40 is provided which faces a pair of excitation electrodes 31 and 32 with the body 10 and support substrate 20 in between. The pair of excitation electrodes 31 and 32 are arranged at intervals along the second surface 22 of the support substrate 20.
According to this configuration, it is only necessary to provide the pair of excitation electrodes 31 and 32 on one side of the piezoelectric body 10 (lower side in FIG. 1), and moreover, the piezoelectric body 10 can be excited via the support substrate 20. Therefore, the BAW device 100 can be manufactured by polishing the piezoelectric body 10 while it is fixed to the support substrate 20 to reduce its thickness at the wafer level, and then patterning the pair of excitation electrodes 31 and 32 and the floating conductor 40. It is possible. In other words, the BAW device 100 can be made thinner through a simple processing process.

(2) The BAW device 100 may further include an excitation substrate 30 to which the support substrate 20 is fixed and on which a pair of excitation electrodes 31 and 32 are formed.
According to this configuration, the excitation substrate 30 can be easily changed depending on the purpose and use of the BAW device 100. Furthermore, the support substrate 20 fixed to the excitation substrate 30 can increase the electromechanical coupling coefficient of the vibration state of the piezoelectric body 10. Note that the BAW device 100 is not limited to the structure (2) above, and may have a structure in which the pair of excitation electrodes 31 and 32 are directly formed on the second surface 22 of the support substrate 20.

(3) As a modification, the vertical relationship between the excitation electrodes 31 and 32 and the floating conductor 40 may be reversed to that in FIG. That is, in the BAW device 100, the pair of excitation electrodes 31 and 32 are located in the direction in which the first surface 21 faces (the upper part of FIG. 1), facing the piezoelectric body 10, and mutually extend along the first surface 21. They may be arranged at intervals. In this case, the floating conductor 40 is located in the direction in which the second surface 22 faces (downward in FIG. 1), and faces the pair of excitation electrodes 31 and 32 with the piezoelectric body 10 and the support substrate 20 in between.
In the configuration (3) above, for example, a pair of excitation electrodes 31 and 32 may be formed on the piezoelectric body 10, and a floating conductor 40 may be formed on the second surface 22 of the support substrate 20. According to the configuration (3) above, the piezoelectric body 10 is fixed to the support substrate 20 and polished at the wafer level to reduce the thickness, and then the pair of excitation electrodes 31, 32 are Since the procedure for patterning the floating conductor 40 can be realized, the thickness can be reduced by a simple processing process.

(4) In the BAW device 100, the piezoelectric body 10 may be a crystal substrate, and the support substrate 20 may be a glass substrate or a crystal substrate.
If a better Q value is desired, a quartz substrate such as an AT-cut quartz substrate or a Z-cut quartz substrate, which is an anisotropic material, can be used as the supporting substrate 20. Moreover, if cost is considered, a glass substrate can be used as the support substrate 20. In this way, the support substrate 20 can be appropriately selected depending on the usage environment of the BAW device 100.

(5) In the BAW device 100, each of the piezoelectric body 10 and the support substrate 20 may be an AT-cut crystal substrate.
This configuration is preferable in consideration of both the Q value and the use in a high temperature environment, when use in various environments is considered.

(6) In the BAW device 100 described in (5) above, the angle θ between the electrical axis 10a of the piezoelectric body 10 and the electrical axis 20a of the support substrate 20 is 0°, 90°, or 180°. It's okay.
If θ=0° or 180°, a higher Q value can be obtained. In addition, if θ = 90°, it is possible to obtain temperature characteristics close to the theoretical value of AT-cut crystal from the viewpoint of thermal expansion, and it is possible to reduce spurious (spurious can be substantially reduced to zero). can).

(7) The above BAW device 100 can be manufactured by a manufacturing method including the steps described in (7a) to (7c) below, since the piezoelectric body 10 can be excited while being fixed to the support substrate 20. . This manufacturing method does not require the step of peeling the support substrate 20 from the piezoelectric body 10.

(7a) A composite substrate manufacturing step of manufacturing a composite substrate by fixing the piezoelectric body 10 to the support substrate 20.
Note that, as described above, at least one of bonding and bonding can be used to fix the piezoelectric body 10 to the support substrate 20.

(7b) A polishing step of polishing and thinning the composite substrate manufactured in the composite substrate manufacturing step.
For polishing, a known polishing method can be used, and for example, a composite substrate having a desired thickness can be obtained by grinding and polishing using a polishing machine using planetary rotation motion. Note that in this polishing step, not only the piezoelectric body 10 side of the composite substrate but also the supporting substrate 20 side may be polished.

(7c) A conductor forming step of forming floating conductors 40 on the composite substrate after polishing the composite substrate in the polishing step.
When manufacturing the BAW device 100 having the configuration (1) above, the floating conductor 40 may be formed on the piezoelectric body 10 in the composite substrate in the conductor forming step. Further, in the conductor forming step, excitation electrodes 31 and 32 may also be formed on the second surface 22 of the support substrate 20 in the composite substrate. Furthermore, when the excitation electrodes 31 and 32 are not provided on the support substrate 20, an excitation substrate 30 on which the excitation electrodes 31 and 32 are formed may be prepared, and the composite substrate may be fixed to this excitation substrate 30.
When manufacturing the BAW device 100 having the configuration (3) above, the floating conductor 40 may be formed on the support substrate 20 of the composite substrate in the conductor forming step. Further, in the conductor forming step, excitation electrodes 31 and 32 may also be formed on the piezoelectric body 10 (the upper surface of the piezoelectric body 10 shown in FIG. 1) in the composite substrate.

After steps (7a) to (7c), a step of cutting the composite substrate into a desired shape is performed, and the BAW device 100 can be manufactured.

The present invention is not limited to the above embodiments and drawings. It is possible to make changes (including deletion of constituent elements) as appropriate without changing the gist of the present invention.

(Modified example)
The shape (the shape includes the thickness and size) and material of each part constituting the BAW device 100 can be arbitrarily changed as long as BAW (bulk acoustic waves) can be generated as described above. be. For example, the shapes and materials of the excitation electrodes 31, 32, the wirings 31a, 32a, and the floating conductor 40 are not limited to the above examples, and can be arbitrarily changed.

Also, for example, the thickness of the support substrate 20 may be set larger than that of the piezoelectric body 10 to prevent resonance with the piezoelectric body 10 when the support substrate 20 is a crystal substrate, or to further ensure mechanical strength. You may. Furthermore, the fixing substrate that further fixes the composite substrate composed of the piezoelectric body 10 and the support substrate 20 may have a thickness that is sufficiently larger than that of the piezoelectric body 10 and the support substrate 20. The fixing substrate may be the excitation substrate 30 described above, or if the excitation electrodes 31 and 32 are formed on the support substrate 20, it may be a substrate exclusively for fixation. Further, the area other than the area on the support substrate 20 where the piezoelectric body 10 is fixed (hereinafter referred to as a fixed area) does not need to be a flat surface. For example, resonance may be prevented by providing unevenness in any part of the support substrate 20 other than the fixed area. Furthermore, each of the piezoelectric body 10, the support substrate 20, and the excitation substrate 30 is not limited to a flat plate shape (including a thin film shape), but may be a curved plate as long as it can generate BAW as described above. It may be in the shape (including the shape of a thin film).

The use of the BAW device 100 is arbitrary, and may be used as a BAR filter, QCM, etc. Moreover, it is also possible to use a grounded conductor (ground electrode) instead of the floating conductor 40 (floating electrode). By using such a ground electrode, the BAW device 100 may be configured as, for example, a BAR filter. The piezoelectric body 10 may also be made of (i) a piezoelectric single crystal other than crystal such as LiTaO3 (lithium tantalate), LiNbO3 (lithium niobate), KNbO3 (potassium niobate), or (ii) Pb(Zr,Ti)O3 ( (iii) A piezoelectric thin film material such as AlN (aluminum nitride) or ZnO (zinc oxide) may be used. Furthermore, the vibration mode used in the BAW device 100 is not limited to the thickness shear vibration mode (TSM) because it can be changed depending on the frequency. For example, in the LF (Low Frequency) band, bending vibration of a plate, longitudinal vibration, stretching vibration of a rod, and torsional vibration are possible. For example, in the MF (Medium Frequency) band, spreading vibration of a disk or a square plate is possible. For example, in the HF (High Frequency) band and above, thickness vertical vibration and thickness shear vibration are possible. Further, as long as the purpose can be achieved, it is considered that the support substrate 20 can also be selected from among these piezoelectric materials. Furthermore, in the composite substrate composed of the piezoelectric body 10 and the support substrate 20, whether or not not only the piezoelectric body 10 but also the support substrate 20 can be excited by applying a voltage through the excitation electrodes 31 and 32 is a matter of purpose. It is optional depending on the For example, the support substrate 20 may also be made of a piezoelectric material, and each of the piezoelectric material 10 and the support substrate 20 may be configured as a vibrator having an opposite phase.

The crystal substrate that can be used for at least one of the piezoelectric body 10 and the support substrate 20 is not limited to an AT-cut crystal substrate, but may be a BT-cut crystal substrate, an SC-cut crystal substrate, or the like. Further, when a glass substrate is used as the support substrate 20, the structure thereof may be arbitrary, such as borosilicate glass, soda lime glass, or quartz glass, as long as it is a substrate containing a known glass material.

The adhesive layer used when bonding the piezoelectric body 10 to the support substrate 20 is not limited to PDMS, and may be any adhesive such as spin-on glass or alumina.

In the above description, descriptions of known technical matters have been omitted as appropriate in order to facilitate understanding of the present invention.

This invention is capable of various embodiments and modifications without departing from the broad spirit and scope of this invention. Further, the embodiments described above are for explaining the present invention, and do not limit the scope of the present invention. That is, the scope of the invention is indicated by the claims rather than the embodiments. Various modifications made within the scope of the claims and the meaning of the invention equivalent thereto are considered to be within the scope of this invention.

DESCRIPTION OF SYMBOLS 100... Bulk acoustic wave (BAW) device 10... Piezoelectric body, 10a... Electrical axis 20... Support substrate, 20a... Electrical axis, 21... First surface, 22... Second surface 30... Excitation substrate, 31, 32... Excitation electrode, 31a, 32a... Wiring 40... Floating conductor (an example of a conductor)

Claims (7)

A plate-shaped piezoelectric material,
a support substrate that supports the piezoelectric material and has a first surface to which the piezoelectric material is fixed, and a second surface located on the opposite side of the first surface;
a pair of excitation electrodes that face the piezoelectric body with the supporting substrate in between, and to which a voltage for exciting bulk acoustic waves is applied to the piezoelectric body;
a conductor facing the pair of excitation electrodes with the piezoelectric body and the support substrate in between,
the pair of excitation electrodes are spaced apart from each other along the second surface;
Bulk elastic wave device. further comprising an excitation substrate to which the support substrate is fixed and on which the pair of excitation electrodes are formed;
The bulk acoustic wave device according to claim 1. A plate-shaped piezoelectric material,
a support substrate that supports the piezoelectric material and has a first surface to which the piezoelectric material is fixed, and a second surface located on the opposite side of the first surface;
a pair of excitation electrodes located in the direction in which the first surface faces and facing the piezoelectric body, to which a voltage for exciting a bulk acoustic wave is applied to the piezoelectric body;
a conductor facing the pair of excitation electrodes with the piezoelectric body and the support substrate in between,
The pair of excitation electrodes are spaced apart from each other along the first surface.
Bulk elastic wave device. The piezoelectric body is a crystal substrate,
The supporting substrate is a glass substrate or a crystal substrate,
The bulk acoustic wave device according to any one of claims 1 to 3. Each of the piezoelectric body and the supporting substrate is an AT-cut crystal substrate,
The bulk acoustic wave device according to any one of claims 1 to 3. The angle between the electrical axis of the piezoelectric body and the electrical axis of the support substrate is 0°, 90°, or 180°.
The bulk acoustic wave device according to claim 5. A method for manufacturing a bulk acoustic wave device according to any one of claims 1 to 3, comprising:
producing a composite substrate by fixing the piezoelectric body to the support substrate;
polishing the manufactured composite substrate to make it thin;
forming the conductor on the composite substrate after polishing the composite substrate;
A method for manufacturing bulk acoustic wave devices.

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