Bulk acoustic wave device and manufacturing method thereof

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

[Technical Field]

[0001] The present invention relates to a bulk acoustic wave device and a method for manufacturing the same. [Background technology]

[0002] Acoustic wave devices that excite acoustic waves in a piezoelectric body are known. For example, Patent Document 1 describes a bulk acoustic wave (BAW) device that uses a first electrode and a second electrode that face each other across a piezoelectric body as excitation electrodes. This device includes a support substrate that supports the piezoelectric body and a multi-gradient raised frame structure provided on the side of the piezoelectric body opposite the support substrate, and attempts to reduce lateral energy leakage using this structure.

[0003] Furthermore, Patent Document 2 describes a surface acoustic wave (SAW) device having a structure in which a piezoelectric body is bonded to a support substrate. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2022-51527 [Patent Document 2] Patent No. 4657002 Summary of the Invention [Problem to be solved by the invention]

[0005] Such devices are used, for example, as frequency control elements, sensor materials, etc., and in recent years, as the importance of high-frequency technology has increased, there has been a demand for thinner devices. Regarding SAW devices, as described in Patent Document 2, a technology is known that allows for thinning while maintaining mechanical strength using a support substrate, but it is more difficult to achieve higher frequencies with SAW devices than with BAW devices. However, as described in Patent Document 1, BAW devices in which excitation electrodes are formed on both sides of the piezoelectric body and a multiple gradient raised frame structure is also formed may lead to complex fabrication processes.

[0006] The present invention has been made in view of the above circumstances, and has as its object to provide a bulk acoustic wave device that can be thinned using a simple processing process, and a method for manufacturing the same. [Means for solving the problem]

[0007] In order to achieve the above object, a bulk acoustic wave device according to a first aspect of the present invention comprises: a plate-shaped piezoelectric body; a support substrate having a first surface to which the piezoelectric element is fixed and a second surface located opposite to the first surface, and supporting the piezoelectric element; a pair of excitation electrodes that face the piezoelectric body across the support substrate and to which a voltage is applied to excite bulk acoustic waves in the piezoelectric body; The piezoelectric body and the support substrate are sandwiched between the pair of excitation electrodes. Floating a conductor; The pair of excitation electrodes are spaced apart from each other along the second surface.

[0008] 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.

[0009] In order to achieve the above object, a bulk acoustic wave device according to a second aspect of the present invention comprises: a plate-shaped piezoelectric body; a support substrate having a first surface to which the piezoelectric element is fixed and a second surface located opposite to the first surface, and supporting the piezoelectric element; a pair of excitation electrodes positioned in a direction in which the first surface faces and facing the piezoelectric body, to which a voltage for exciting a bulk acoustic wave is applied in the piezoelectric body; The piezoelectric body and the support substrate are sandwiched between the pair of excitation electrodes. Floating a conductor; The pair of excitation electrodes are spaced apart from each other along the first surface.

[0010] the piezoelectric body is a quartz crystal substrate, The support substrate may be a glass substrate or a quartz substrate.

[0011] The piezoelectric The body Alternatively, the substrate may be an AT-cut quartz crystal substrate. In order to achieve the above object, a bulk acoustic wave device according to a third aspect of the present invention comprises: a plate-shaped piezoelectric body; a support substrate having a first surface to which the piezoelectric element is fixed and a second surface located opposite to the first surface, and supporting the piezoelectric element; a pair of excitation electrodes that face the piezoelectric body across the support substrate and to which a voltage is applied to excite bulk acoustic waves in the piezoelectric body; a conductor facing the pair of excitation electrodes with the piezoelectric body and the support substrate interposed therebetween, the pair of excitation electrodes are spaced apart from each other along the second surface; The piezoelectric body and the support substrate are each an AT-cut quartz crystal substrate. In order to achieve the above object, a bulk acoustic wave device according to a fourth aspect of the present invention comprises: a plate-shaped piezoelectric body; a support substrate having a first surface to which the piezoelectric element is fixed and a second surface located opposite to the first surface, and supporting the piezoelectric element; a pair of excitation electrodes positioned in a direction in which the first surface faces and facing the piezoelectric body, to which a voltage for exciting a bulk acoustic wave is applied in the piezoelectric body; a conductor facing the pair of excitation electrodes with the piezoelectric body and the support substrate interposed therebetween, the pair of excitation electrodes are spaced apart from each other along the first surface; The piezoelectric body and the support substrate are each an AT-cut quartz crystal substrate.

[0012] The angle formed between the electric axis of the piezoelectric body and the electric axis of the support substrate may be 0°, 90°, or 180°.

[0013] In order to achieve the above object, the present invention 5 The method for manufacturing a bulk acoustic wave device according to the present invention comprises the steps of: a step of fixing the piezoelectric body to the support substrate to fabricate a composite substrate; a step of polishing the fabricated composite substrate to thin it; After polishing the composite substrate, Floating and forming a conductor. [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a bulk acoustic wave device that can be thinned using a simple processing process, and a method for manufacturing the same. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a cross-sectional view of a bulk acoustic wave (BAW) device according to one embodiment of the present invention. [Figure 2] FIG. 2 is a plan view of the BAW device according to the embodiment. [Figure 3] 1A and 1B are diagrams for explaining the electric axis direction in an embodiment in which an AT-cut quartz crystal substrate is used as the support substrate, and show the cases in which the angle between the electric axis of the piezoelectric body and the electric axis of the support substrate is 0° in (a), 90° in (b), and 180° in (c). [Figure 4] FIG. 1A shows the impedance spectrum of a comparative example, and FIGS. 1B and 1C show the impedance spectra of each example. [Figure 5] 4(a) to 4(c) are diagrams showing the impedance spectra of the respective examples following FIGS. 4(b) and 4(c). [Figure 6] 6A and 6B are diagrams based on the impedance spectra of FIGS. 4 and 5, where (a) shows the Q value and (b) shows the impedance at resonance. [Figure 7] FIG. 1 shows that a support substrate, which is an AT-cut quartz crystal substrate, oscillates. [Figure 8] FIG. 10 is a graph showing frequency temperature characteristics of a comparative example and each example. [Figure 9] A diagram showing the temperature characteristics of an ideal AT-cut quartz crystal unit. DETAILED DESCRIPTION OF THE INVENTION

[0016] An embodiment of the present invention will be described with reference to the drawings.

[0017] As shown in FIGS. 1 and 2, a 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.

[0018] Fig. 1 is a cross-sectional view of a BAW device taken along line AA shown in Fig. 2. For ease of viewing, hatching indicating cross sections of components other than excitation electrodes 31 and 32 and the floating conductor 40 has been omitted in Fig. 1. In addition, in the plan view of Fig. 2, components located between the support substrate 20 and the excitation substrate 30 are indicated by dashed lines.

[0019] The piezoelectric body 10 is plate-shaped and formed with a thickness on the order of μm (for example, several tens of μm to several hundreds of μm). The piezoelectric body 10 is made of, for example, an AT-cut quartz crystal substrate, and is formed in a substantially circular shape in plan view, as shown in FIG.

[0020] When an AC voltage is applied to an AT-cut quartz substrate via a pair of excitation electrodes 31 and 32, a thickness shear mode (TSM) vibration, which vibrates parallel to the substrate surface, is excited. When an electric field is applied to the quartz substrate, distortion occurs due to the inverse piezoelectric effect, and the distortion is eliminated by returning the electric field. By repeatedly turning the electric field ON and OFF in this way, thickness shear vibration can be excited in the quartz substrate. A certain amount of time is required from the application of the electric field until the crystal distortion is complete. This time is primarily determined by the thickness of the quartz substrate, and the relationship between the fundamental frequency F0 (MHz) and thickness t (mm) of the AT-cut quartz substrate is expressed as follows: [Equation 1]. By synchronizing the applied voltage as an oscillating electric field with the period of the thickness shear vibration, the quartz substrate can be used as a vibrator. In other words, the BAW device 100 is a bulk acoustic resonator (BAR) that utilizes the piezoelectric effect of the piezoelectric element 10, which significantly changes the impedance seen from the electrical terminals near the resonance point.

[0021]

number

[0022] The relationship between the change in the resonant frequency of the quartz crystal oscillator and the change in mass of the adsorbed substance is expressed by the following equation (2), known as the Sauerbrey equation: where ΔF is the frequency change (Hz) of the quartz crystal oscillator, F0 is the fundamental frequency (Hz) of the quartz crystal oscillator, and A is the area (cm) of the thin-film electrode placed on the quartz crystal substrate. 2 ), μ q is the elastic modulus of quartz (=2.947×10 11 gcm -1 s -2 ), ρ q is the density of quartz (=2.648gcm -3 ), Δm * is the mass (g) of the substance adsorbed on the electrode.

[0023]

number

[0024] Here, in equation 2, Δm * If / A is the mass of material adsorbed per unit area, Δm, and the remaining constant term is K, then this can be simplified to the following equation (3).

[0025]

number

[0026] In other words, by detecting the frequency change of the quartz oscillator, it is possible to measure the change in mass of the adsorbed substance on the electrode. From equation (2), it can be seen that the higher the fundamental frequency F0 of the quartz oscillator, the larger the frequency change ΔF, resulting in a sensor known as a quartz crystal microbalance (QCM) with higher sensitivity. Furthermore, from equation (1), it can be seen that the smaller the thickness t of the quartz substrate, the higher the fundamental frequency F0. In other words, a thinner quartz substrate is required to increase the frequency of the quartz oscillator and the sensitivity of the QCM.

[0027] There is a limit to how thinly a quartz substrate can be polished alone due to its mechanical strength. Therefore, a method has been proposed in which the quartz substrate is polished even thinner by being attached to a separate support substrate for reinforcement. However, this method requires that the quartz substrate be attached to a glass substrate, polished, and then peeled off from the glass substrate after polishing. This method complicates the processing process, requiring steps such as removing the quartz substrate and re-fixing it when mounting it on a device. As will be described below, the BAW device 100 is a device that can excite a quartz substrate (piezoelectric element 10) that has been thinned by polishing while it remains fixed to the support substrate 20.

[0028] The support substrate 20 is a substrate that supports the piezoelectric body 10, and is made of, for example, an AT-cut quartz crystal substrate, a Z-cut quartz crystal substrate, a glass substrate, or the like. The support substrate 20 is formed to a thickness on the order of μm (for example, several tens of μm to several hundreds of μm). As an example, the support substrate 20 is formed in a rectangular shape that is larger than the piezoelectric body 10 in a plan view, as shown in FIG.

[0029] The support substrate 20 has a first surface 21 (upper surface in FIG. 1) to which the piezoelectric body 10 is fixed, and a second surface 22 (lower surface in FIG. 1) located on the opposite side of the first surface 21.

[0030] The piezoelectric body 10 is fixed to the first surface 21 by at least one of bonding and lamination. For example, optical contact can be used as the bonding method. 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 also be used as appropriate to fix the piezoelectric body 10 to the support substrate 20.

[0031] The pair of excitation electrodes 31 and 32 face the piezoelectric body 10 across the support substrate 20. A voltage is applied to the pair of excitation electrodes 31 and 32 to excite bulk acoustic waves in the piezoelectric body 10. As shown in FIG. 1 , the pair of excitation electrodes 31 and 32 are spaced apart from each other along the second surface 22. Each of the separated excitation electrodes 31 and 32 is capacitively coupled in opposite phases to the floating conductor 40, and vibrations can be excited in the piezoelectric body 10 by applying an AC voltage to each of the two excitation electrodes 31 and 32. Specifically, the 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. The AC 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.

[0032] As an example, the pair of excitation electrodes 31, 32 are a pair of electrodes divided into one and the other by cutting out a strip shape along the vertical direction of Fig. 2 from a circle. The vertical direction is set, for example, along the crystal axis of the quartz crystal that constitutes the piezoelectric body 10.

[0033] The excitation electrodes 31, 32 and the wirings 31a, 32a are formed of a conductor such as Au (gold) or Cu (copper) and are patterned on the surface of the excitation substrate 30 facing the support substrate 20. 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.

[0034] The excitation electrodes 31 and 32 and the wirings 31 a and 32 a may be patterned on the second surface 22 of the support substrate 20.

[0035] The floating conductor 40 faces the pair of excitation electrodes 31, 32 across the piezoelectric body 10 and the support substrate 20. The floating conductor 40 is formed of a conductor such as Au (gold) or Cu (copper) and is patterned on the upper surface of the piezoelectric body 10 (the surface of the piezoelectric body 10 opposite to the surface facing the support substrate 20). 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, 32.

[0036] The floating conductor 40 is provided to control the electric field generated from the excitation electrodes 31 and 32. Without the floating conductor 40, the electric field would spread too much. However, the floating conductor 40 allows the electric field to be concentrated between the excitation electrodes 31 and 32. Considering this electric field, the inventors' research has shown that it is preferable for the BAW device 100 to satisfy the following (i) and (ii). The total thickness of the piezoelectric body 10 and the supporting substrate 20 is T. (i) The distance between the excitation electrodes 31 and 32 (the distance in the horizontal direction in FIGS. 1 and 2 ) is approximately 0.5 to 1.5 times the total thickness T. (ii) If the diameter of the circle that defines the outer shape of the floating conductor 40 is D and the diameter of the circle that follows the outer edges of the pair of excitation electrodes 31 and 32 is d, for example, the centers of the two circles are aligned, and d≦D≦d+T holds.

[0037] This completes the configuration of the BAW device 100. Next, as an example, an experiment will be described in which a piezoelectric body 10 is fixed to a support substrate 20 and an experiment is performed to demonstrate that it is possible to excite the piezoelectric body 10 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 those in the above embodiment.

[0038] (Example) First, the configuration of the BAW device 100 used in the experiment will be described.

[0039] (1. Process for producing the piezoelectric body 10 and the excitation substrate 30) The piezoelectric body 10 and the excitation substrate 30 were fabricated using semiconductor micromachining technology based on photolithography. 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 wiring 31a, 32a on the excitation substrate 30 are similar, so they will be described together below. The terms electrode material and electrode pattern used below include not only the excitation electrodes 31, 32, but also the materials and patterns for forming the wiring 31a, 32a and the floating conductor 40.

[0040] The piezoelectric element 10 was an AT-cut quartz crystal substrate (Seiko E&G Corporation) with a diameter of 12 mm and a thickness of 100 μm. The excitation substrate 30 was a glass substrate with dimensions of 22 mm length, 22 mm width, and 1.2 mm thickness. After ultrasonic cleaning for 3 minutes using acetone (Junsei Chemical Co., Ltd.) and Etacol (99% denatured alcohol, Imazu Pharmaceutical Co., Ltd.), approximately 100 nm of gold (gold wire 99.95%, Nilaco Corporation) was deposited on the electrode material by vacuum deposition (VPC-260, ULVAC Techno Co., Ltd.). A 30 nm thick Cr (99.9%, Nilaco Corporation) adhesive layer was then deposited (Step 1). Next, a resist pattern was formed on each substrate with the gold deposited using photolithography. First, OAP (Tokyo Ohka Kogyo Co., Ltd.) was applied to improve adhesion between the photoresist and the substrate, followed by a photosensitive resin positive photoresist (OFPR-800 LB 54 cp, Tokyo Ohka Kogyo Co., Ltd.). The OAP and photoresist were applied using a spin coater (MS-A100, Mikasa Co., Ltd.) at 3000 rpm for 20 seconds. This was followed by pre-baking at 90°C for approximately 15 minutes in a high-temperature oven (Step 2). Next, each substrate was exposed to UV light for 90 seconds using a mask alignment system (M-1S, Mikasa Co., Ltd.) through a photomask with the electrode pattern transferred onto it, transferring the electrode pattern to the resist on the substrate (Step 3). The resist in the exposed areas was removed using a developer (NMD-3, Tokyo Ohka Kogyo Co., Ltd.), and the substrate was then post-baked at 100°C for 30 minutes in a high-temperature oven (Step 4). Then, Au and Cr are removed in that order using an Au etching solution and a Cr etching solution (Step 5). Finally, ultrasonic cleaning is performed using acetone and then ethacol to remove the resist remaining on the pattern, and the electrode shape is obtained (Step 6).

[0041] (2. Method of Fixing Piezoelectric Body 10 to Support Substrate 20) The piezoelectric element 10 was temporarily fixed to the support substrate 20 by optical contact using methanol. Optical contact is a bonding technique that does not use adhesives. Bonding by optical contact is thought to be due to the van der Waals forces between the quartz crystal and glass substrate surfaces and the hydrogen bonding forces of silanol groups (Si-OH) formed on the substrate surface due to water adsorption. After ultrasonically cleaning the piezoelectric element 10 and support substrate 20 with acetone and ethacol, 0.5 μL of methanol (Junsei Chemical Co., Ltd.) was dropped onto the support substrate 20, and the piezoelectric element 10 was placed on top of it to fix it.

[0042] In addition to optical contact, the piezoelectric element 10 was fixed to the support substrate 20 by adhesive fixation using PDMS (SILPOT 184, Dow Corning Corp). PDMS was prepared by mixing a base agent and a curing agent in a 10:1 (w / w) ratio, which was then further diluted with hexane (Junsei Chemical Co., Ltd.). The diluted PDMS was spin-coated onto the support substrate 20 at 3000 rpm for 15 seconds and then degassed for 15 minutes. After degassing was complete, a quartz crystal oscillator was placed on the PDMS-coated support substrate 20 and baked in a high-temperature oven at 90°C for 1 hour to adhere and fix it. Dilution with hexane was performed at 100, 25, 15, 10, and 6 times (w / w), and each was evaluated after adhesion.

[0043] (3. Support substrate 20 used in the experiment) For the support substrate 20, a glass substrate (Asahi Techno Glass) with a thickness of 100 μm, an AT-cut quartz crystal substrate (Seiko E&G Corporation), and a Z-cut quartz crystal substrate (Citizen Fine Devices) were used.

[0044] Here, the chemical composition of quartz is SiO2, it belongs to the trigonal crystal system, and the central axis of the hexagonal prism is the Z axis. The axis connecting the diagonals of the cross section (hexagon) of the six-sided prism perpendicular to the Z axis is the X axis, and this is called the "electrical axis." Furthermore, the Y axis, which is perpendicular to the X axis, is called the "mechanical axis." A quartz substrate cut perpendicular to the X axis is called an X cut, a quartz substrate cut perpendicular to the Y axis is called a Y cut, and a quartz substrate cut perpendicular to the Z axis is called a Z cut. In particular, a quartz substrate that has been rotated around the X axis from a Y cut so that the angle it makes with the Z axis is 35°15' is called an AT cut.

[0045] 3(a) to 3(c) show examples of the relationship between the angle formed by the electric axis 10a (X-axis) of the piezoelectric body 10 and the electric axis 20a (X-axis) of the support substrate 20 when AT-cut quartz crystal substrates are used for both the piezoelectric body 10 and the support substrate 20. In Figs. 3(a) to 3(c), the direction in which the electric axis 10a of the piezoelectric body 10 faces is indicated by a solid arrow, and the direction in which the electric axis 20a of the support substrate 20 faces is indicated by a dashed arrow.

[0046] Because AT-cut quartz crystal substrates have a tilt in the direction of their crystal axes, it is expected that their characteristics will differ depending on the angle θ, where θ is the angle between electrical axis 10a and electrical axis 20a. Therefore, when using an AT-cut quartz crystal substrate as support substrate 20, evaluation was performed at three angles: θ = 0°, 90°, and 180°. Figure 3(a) shows the case where θ = 0°, Figure 3(b) shows the case where θ = 90°, and Figure 3(c) shows the case where θ = 180°.

[0047] As a comparative example, measurements were also made without the support substrate 20. In order to match the distance between the pair of excitation electrodes 31, 32 and the floating conductor 40 with the condition when the support substrate 20 is present, in the comparative example without the support substrate 20, a 100 μm spacer was prepared and a gas phase was provided between the piezoelectric body 10 and the excitation substrate 30.

[0048] (4. Experimental system configuration) The excitation electrodes 31 and 32 were connected to the oscillator circuit, and the frequency was measured using a frequency counter (Agilent 53131A 225 MHz Universal Counter, KEYSIGHT TECHNOLOGIES). A DC stabilized power supply (AND AD-8735D, A&D) was used to supply power to the circuit. 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 to control the temperature during the evaluation of frequency-temperature characteristics. The impedance was measured by connecting the excitation electrodes 31 and 32 to an impedance analyzer (Network / Spectrum / Impedance Analyzer 4395A, KEYSIGHT TECHNOLOGIES).

[0049] (5. Evaluation Method) (resonant frequency stability) The oscillator circuit was equipped with a BAW device 100, and the resonant frequency was measured using a frequency counter. The standard deviation σ was calculated from the variation in the resonant frequency over 60 seconds, and three times this value, 3σ, was used as an index of stability for evaluation.

[0050] (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). The impedance spectrum near the resonance frequency was measured by the impedance analyzer, and the impedance and Q value at resonance were obtained.

[0051] (Frequency temperature characteristics) While checking the temperature of BAW device 100 using an infrared thermography camera (InfRec H2640, Nippon Avionics Co., Ltd.), BAW device 100 was heated on a hot plate from 20°C to 100°C in 5°C increments, and the resonant frequency at each temperature was measured using a frequency counter. The measured values ​​were plotted on a graph as the rate of change relative to the resonant frequency at room temperature (11°C), and the difference due to support substrate 20 was evaluated.

[0052] (6. Results and Discussion) Below, we will describe the experimental results of a BAW device 100 in which the piezoelectric body 10 is adhesively fixed to the support substrate 20 with PDMS. Similar experiments were also conducted on a BAW device 100 in which the piezoelectric body 10 is fixed to the support substrate 20 with optical contact, and similar results were obtained as in the case of PDMS. However, since there was variation in the measurement results, we will not discuss them here. This variation is thought to be due to the poor reproducibility of the fixed state achieved by optical contact.

[0053] (Examination of adhesive fixation conditions using PDMS) For adhesive fixation, PDMS diluted with hexane at 100, 25, 15, 10, and 6 times (w / w) dilutions was used. When a quartz crystal was bonded to a glass substrate, the quartz crystal came off immediately after baking under conditions of 100 to 15 times dilution. On the other hand, under conditions of 10 times and 6 times dilution, the quartz crystal was completely bonded after baking. Regarding the possibility of oscillation, in the case of 10 times dilution, a clear resonance point was observed in the impedance spectrum, indicating oscillation. However, in the case of 6 times dilution, no clear peak was observed in the impedance spectrum, and oscillation was deemed difficult. This is thought to be due to the fact that in the case of 6 times dilution, the PDMS adhesive layer was thick, acting as an elastic material and attenuating the vibration energy. On the other hand, in the case of 10 times dilution, it was found that the thickness could be controlled to be sufficient for the quartz crystal to oscillate. Below are the experimental results of a BAW device 100 in which a quartz crystal (piezoelectric element 10) was bonded to a support substrate 20 with 10 times diluted PDMS.

[0054] (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 in which no support substrate was used.

[0055] [Table 1]

[0056] According to Table 1, a decrease in resonant frequency stability is observed overall compared to the condition without a support substrate. The PDMS used in each example is thin enough to oscillate, but considering the decrease in stability, it is thought that it acts as an elastic body. Furthermore, in each example, a decrease in stability was observed only when a Z-cut quartz crystal substrate was used as the support substrate 20. This is thought to be due to the Z-cut quartz crystal substrate vibrating in the thickness direction.

[0057] (Impedance spectrum measurement) Figure 4(a) shows the impedance spectrum of a comparative example without a support substrate, while Figures 4(b), 4(c), and 5(a)-(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 with respect to the resonant frequency. The solid line in each figure represents the impedance spectrum, and the dotted line represents the phase. Looking at each figure, it can be seen that only in the example where θ = 180° (Figure 5(b)), the phase varies between -90° and 90°. When the impedance spectrum was measured using a commercially available QCM (Quartz Crystal Microbalance) for calibration, the phase varied between -40° and 140°. This phase variation was similar to the measurement results for conditions other than θ = 180°. This suggests that when an AT-cut quartz substrate is used as the support substrate 20, a unique oscillation occurs under the condition of θ = 180°, where the crystal axis direction is not aligned with that of the quartz crystal resonator (piezoelectric body 10). When the crystal axes are aligned, as in the condition of θ = 0°, it is believed that the upper and lower AT-cut quartz substrates (piezoelectric body 10 and support substrate 20) oscillate in the same direction when an electric field is applied. On the other hand, when the crystal axes are not aligned, as in the condition of θ = 180°, the upper and lower AT-cut quartz substrates (piezoelectric body 10 and support substrate 20) vibrate in opposite phases. It is believed that the synchronization of these opposite-phase vibrations produces the unique oscillation.

[0058] 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).

[0059] In the condition without a support substrate (comparison example), an air layer of 100 μm is sandwiched, and it can be confirmed that the impedance is higher than in other conditions in which a dielectric glass or quartz substrate is used as the support substrate 20.

[0060] Furthermore, when comparing the Q values ​​of each example other than the comparative example, a higher Q value is shown when the support substrate 20 is a quartz substrate than when the support substrate 20 is a glass substrate. This difference is thought to be due to whether the crystal of the support substrate 20 is isotropic or anisotropic. Glass is isotropic, meaning there is variation in the crystal direction. Therefore, when the electric field generated from the excitation electrodes 31 and 32 passes through the glass substrate, polarization occurs in various directions, causing the electric field to disperse. On the other hand, because the quartz substrate is anisotropic, the electric field generated from the excitation electrodes 31 and 32 does not disperse within the support substrate 20 and can reach the quartz oscillator (piezoelectric body 10) above. When a glass substrate, which is an isotropic material, is used as the support substrate 20, the Q value is thought to be lower due to the reduction in electric field strength caused by the dispersion of the electric field.

[0061] Comparing the conditions under which an AT-cut quartz substrate was used as the support substrate 20, it was found that a high Q value was observed under conditions of θ = 0° and 180°. Furthermore, comparing the conditions under which a quartz substrate was used as the support substrate 20, the Z-cut quartz substrate exhibited the highest Q value. As mentioned above, it is believed that the AT-cut quartz substrate used as the support substrate oscillates when an electric field is applied. The basis for this is shown in Figure 7. Figure 7 shows the impedance spectrum measured when the X-axis relationship between the upper and lower AT-cut quartz substrates (piezoelectric body 10 and support substrate 20) was set to θ = 0°. Two peaks can be seen in the spectrum. This indicates that the upper and lower AT-cut quartz substrates (piezoelectric body 10 and support substrate 20) oscillate simultaneously. When an AT-cut quartz substrate is used as the support substrate 20, the support substrate 20 also oscillates simultaneously, which is thought to result in energy loss and a lower Q value than when a Z-cut quartz substrate is used as the support substrate 20.

[0062] (Frequency temperature characteristics) The frequency-temperature characteristics of the comparative example and each example are shown in Figure 8. However, for the example using a Z-cut quartz substrate for the support substrate 20, the frequency rose sharply and became unstable at temperatures around 95°C, so data above 90°C is omitted. The figure also shows an enlarged view of the frequency-temperature characteristics from 10°C to 70°C. The thermal expansion coefficients of quartz and glass are also shown in Table 2.

[0063] [Table 2]

[0064] When an AT-cut quartz crystal substrate is used as the support substrate 20 and the conditions of θ = 0° and 180° are used, the direction of thermal expansion is the same for the support substrate 20 and the quartz crystal unit (piezoelectric element 10), resulting in frequency-temperature characteristics nearly identical to those obtained without the support substrate 20. On the other hand, when the condition of θ = 90° is used, the frequency change rate with temperature is smaller than when the support substrate 20 is not used, with almost no frequency change observed up to 60°C. Figure 9 shows the temperature characteristics of an ideal AT-cut quartz crystal unit. Figure 9 shows the temperature characteristics of a quartz crystal unit (resonant frequency 2690 kHz) cut at an angle of approximately 54° around the X-axis. While limited to quasi-static temperature changes, the temperature change rate is believed to be smallest near room temperature. Comparing this ideal temperature characteristic, it can be seen that the results for the example with θ = 90° in the enlarged view of Figure 8 and curve 4 in Figure 9 are nearly identical in the range from 10°C to 70°C. From this, it is thought that by setting the relationship between the X axes of the upper and lower AT-cut quartz substrates (piezoelectric body 10 and support substrate 20) at θ=90°, the properties change from anisotropic to isotropic in terms of thermal expansion, and temperature characteristics close to the theoretical values ​​of AT-cut quartz can be obtained.

[0065] Next, we evaluated the frequency-temperature characteristics when a glass substrate and a Z-cut quartz substrate were used as the support substrate 20. Because the thermal expansion coefficients of the glass substrate and Z-cut quartz substrate are different from those of the AT-cut quartz crystal unit, the frequency change rate with temperature was larger compared to the condition without the support substrate 20. This difference was particularly noticeable when a glass substrate, which is an isotropic material, was used for the support substrate 20. When used at room temperature, there is no problem with using a glass substrate or a Z-cut quartz crystal substrate as the support substrate 20, but it was found that an AT-cut quartz crystal substrate is preferable as the support substrate 20 when used in a high-temperature environment.

[0066] (summary) Taking the above into consideration, the support substrate 20 can be selected appropriately depending on the environment in which the BAW device 100 will be used. Judging from the Q value, it is preferable to use an AT-cut quartz substrate or a Z-cut quartz substrate, which are anisotropic materials, as the support substrate 20, and if a higher Q value is required, a Z-cut quartz substrate is more preferable. For use in high-temperature environments, it is preferable to use an AT-cut quartz substrate as the support substrate 20. From the perspective of cost, it is preferable that the support substrate 20 be a glass substrate. Overall, if use in a variety of environments is expected, it is preferable that the support substrate 20 be an AT-cut quartz substrate.

[0067] Crystal oscillator circuits are key devices for IoT, 5G (fifth generation mobile communication system), and future technologies, as well as for communication devices including smartphones, and the excitation technology for the BAW device 100 described above makes it possible to oscillate the crystal unit without peeling off the support substrate 20. This is advantageous in terms of ease of handling of a thinned crystal unit, as it can oscillate while fixed to the support substrate 20, and is expected to be applied to a variety of ultra-thin crystal devices.

[0068] (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 that face the piezoelectric body 10 across the support substrate 20, and a floating conductor 40 that faces the pair of excitation electrodes 31 and 32 across the piezoelectric body 10 and the support substrate 20. The pair of excitation electrodes 31 and 32 are arranged along the second surface 22 of the support substrate 20 at an interval. According to this configuration, it is sufficient to provide the pair of excitation electrodes 31, 32 on only one side (the lower side in FIG. 1) of the piezoelectric body 10, and furthermore, 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 at wafer level while it is fixed to the support substrate 20 to thin it, and then patterning the pair of excitation electrodes 31, 32 and the floating conductor 40. In other words, the BAW device 100 can be thinned using a simple processing process.

[0069] (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 application 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 mode of the piezoelectric body 10. Note that the BAW device 100 is not limited to the above structure (2), and may have a structure in which the pair of excitation electrodes 31, 32 are directly formed on the second surface 22 of the support substrate 20.

[0070] (3) As a modified example, the vertical relationship between the excitation electrodes 31, 32 and the floating conductor 40 may be reversed from that in Fig. 1. That is, in the BAW device 100, the pair of excitation electrodes 31, 32 may be positioned in the direction in which the first surface 21 faces (upper side in Fig. 1) and face the piezoelectric body 10, and may be arranged at a distance from each other along the first surface 21. In this case, the floating conductor 40 is positioned in the direction in which the second surface 22 faces (lower side in Fig. 1) and faces the pair of excitation electrodes 31, 32 with the piezoelectric body 10 and the support substrate 20 sandwiched between them. In the above configuration (3), for example, a pair of excitation electrodes 31, 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. As with the above configuration (1), the above configuration (3) also makes it possible to carry out a procedure in which the piezoelectric body 10 is polished at wafer level while being fixed to the support substrate 20 to reduce its thickness, and then the pair of excitation electrodes 31, 32 and the floating conductor 40 are patterned, thereby enabling reduction in thickness through a simple processing process.

[0071] (4) In the BAW device 100, the piezoelectric body 10 may be a quartz substrate, and the support substrate 20 may be a glass substrate or a quartz 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 support substrate 20. Furthermore, if cost is a consideration, 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 environment in which the BAW device 100 is used.

[0072] (5) In the BAW device 100, the piezoelectric body 10 and the support substrate 20 may each be an AT-cut quartz crystal substrate. This configuration is preferable when use in various environments is considered, taking into consideration both the Q value and use in high-temperature environments.

[0073] (6) In the BAW device 100 described in (5) above, the angle θ formed between the electric axis 10a of the piezoelectric body 10 and the electric axis 20a of the support substrate 20 may be any of 0°, 90°, and 180°. A higher Q value can be obtained by setting θ = 0° or 180°. Also, by setting θ = 90°, it is possible to obtain temperature characteristics close to the theoretical values ​​of AT-cut quartz crystal in terms of thermal expansion, and spurious can be reduced (spurious can be effectively reduced to zero).

[0074] (7) The above BAW device 100 can be manufactured by a manufacturing method including the steps described in (7a) to (7c) below, since it is possible to excite the piezoelectric body 10 while it is fixed to the support substrate 20. This manufacturing method does not require a step of peeling the support substrate 20 from the piezoelectric body 10.

[0075] (7a) A composite substrate fabrication step in which the piezoelectric body 10 is fixed to the support substrate 20 to fabricate a composite substrate. The piezoelectric body 10 can be fixed to the support substrate 20 by at least one of lamination and bonding, as described above.

[0076] (7b) A polishing step of polishing and thinning the composite substrate produced in the composite substrate production step. For polishing, a known polishing method can be used, for example, a polishing machine using planetary rotation motion can be used to grind and polish the composite substrate to the desired thickness. In this polishing step, not only the piezoelectric body 10 side of the composite substrate but also the support substrate 20 side may be polished.

[0077] (7c) A conductor forming step of forming a floating conductor 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 element 10 in the composite substrate in the conductor formation step. Furthermore, in the conductor formation step, the excitation electrodes 31, 32 may also be formed on the second surface 22 of the support substrate 20 in the composite substrate. Furthermore, if the excitation electrodes 31, 32 are not provided on the support substrate 20, the excitation substrate 30 on which the excitation electrodes 31, 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 formation step. Furthermore, in the conductor formation 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) of the composite substrate.

[0078] 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.

[0079] The present invention is not limited to the above-described embodiments and drawings, and modifications (including the omission of components) can be made as appropriate within the scope of the present invention.

[0080] (Variation) The shape (including thickness and size) and material of each part constituting the BAW device 100 can be changed as desired as long as it can generate BAWs (bulk acoustic waves) as described above. For example, the shapes and materials of the excitation electrodes 31 and 32, the wirings 31 a and 32 a, and the floating conductor 40 are not limited to the above examples and can be changed as desired.

[0081] Furthermore, 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 quartz substrate or to ensure greater mechanical strength. Furthermore, a fixing substrate that further fixes the composite substrate consisting 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. This fixing substrate may be the excitation substrate 30 described above, or may be a substrate dedicated to fixing when the excitation electrodes 31 and 32 are formed on the support substrate 20. Furthermore, the support substrate 20 does not need to have a flat surface other than the region where the piezoelectric body 10 is fixed (hereinafter referred to as the fixing region). For example, resonance may be prevented by providing irregularities in any portion of the support substrate 20 other than the fixing region. Furthermore, the piezoelectric body 10, the support substrate 20, and the excitation substrate 30 are not limited to being flat plates (including thin films) but may be curved plates (including thin films) as long as they can generate BAW as described above.

[0082] The BAW device 100 can be used for any purpose, such as a BAR filter or a QCM. A grounded conductor (ground electrode) can be used instead of the floating conductor 40 (floating electrode). By using such a ground electrode, the BAW device 100 can be configured as, for example, a BAR filter. The piezoelectric element 10 can also be (i) a piezoelectric single crystal other than quartz, such as LiTaO3 (lithium tantalate), LiNbO3 (lithium niobate), or KNbO3 (potassium niobate); (ii) a ferroelectric ceramic such as Pb(Zr,Ti)O3 (lead zirconate titanate) or PbTiO3 (lead titanate); or (iii) a piezoelectric thin film material such as AlN (aluminum nitride) or ZnO (zinc oxide). The vibration mode used in the BAW device 100 can be changed depending on the frequency, and is therefore not limited to the thickness-shear mode (TSM). For example, in the low-frequency (LF) band, bending vibration of a plate, longitudinal vibration, extensional vibration of a rod, or torsional vibration are possible. For example, in the MF (medium frequency) band, spreading vibration of a circular plate or a square plate is possible. For example, in the HF (high frequency) band or higher, thickness-lifting vibration and thickness-shear vibration are possible. Furthermore, as long as the objective can be achieved, it is considered that the support substrate 20 can also be selected from these piezoelectric materials. Furthermore, in a composite substrate composed of the piezoelectric material 10 and the support substrate 20, whether or not to excite not only the piezoelectric material 10 but also the support substrate 20 by applying a voltage via the excitation electrodes 31 and 32 is optional, depending on the objective. For example, the support substrate 20 can also be made of a piezoelectric material, and the piezoelectric material 10 and the support substrate 20 can each be configured as vibrators with opposite phases.

[0083] The quartz 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 quartz crystal substrate, but may be a BT-cut quartz crystal substrate, an SC-cut quartz crystal substrate, etc. When a glass substrate is used as the support substrate 20, its composition may be any suitable material such as borosilicate glass, soda-lime glass, or quartz glass, as long as it contains a known glass material.

[0084] 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.

[0085] In the above description, in order to facilitate understanding of the present invention, descriptions of well-known technical matters have been omitted as appropriate.

[0086] This invention allows various embodiments and modifications without departing from the broad spirit and scope of this invention. Furthermore, the above-described embodiments are intended to explain this invention and do not limit the scope of this invention. That is, the scope of this invention is defined by the claims, not 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. [Explanation of symbols]

[0087] 100...Bulk Acoustic Wave (BAW) Devices 10...piezoelectric body, 10a...electric axis 20... supporting substrate, 20a... electrical axis, 21... first surface, 22... second surface 30... excitation substrate, 31, 32... excitation electrodes, 31a, 32a... wiring 40...Floating conductor (an example of a conductor)

Claims

1. a plate-shaped piezoelectric body; a support substrate having a first surface to which the piezoelectric element is fixed and a second surface located opposite to the first surface, the support substrate supporting the piezoelectric element; a pair of excitation electrodes that face the piezoelectric body across the support substrate and to which a voltage is applied to excite bulk acoustic waves in the piezoelectric body; a floating conductor facing the pair of excitation electrodes with the piezoelectric body and the support substrate interposed therebetween, the pair of excitation electrodes are spaced apart from each other along the second surface; Bulk acoustic wave devices.

2. The support substrate is fixed to an excitation substrate on which the pair of excitation electrodes are formed.

10. The bulk acoustic wave device of claim 1.

3. a plate-shaped piezoelectric body; a support substrate having a first surface to which the piezoelectric element is fixed and a second surface located opposite to the first surface, the support substrate supporting the piezoelectric element; a pair of excitation electrodes positioned in a direction in which the first surface faces and facing the piezoelectric body, to which a voltage for exciting a bulk acoustic wave is applied in the piezoelectric body; a floating conductor facing the pair of excitation electrodes with the piezoelectric body and the support substrate interposed therebetween, the pair of excitation electrodes are spaced apart from each other along the first surface; Bulk acoustic wave devices.

4. the piezoelectric body is a quartz crystal substrate, The support substrate is a glass substrate or a quartz substrate. The bulk acoustic wave device according to any one of claims 1 to 3.

5. The piezoelectric body is an AT-cut quartz crystal substrate. The bulk acoustic wave device according to any one of claims 1 to 3.

6. A plate-shaped piezoelectric body; a support substrate having a first surface to which the piezoelectric element is fixed and a second surface located opposite to the first surface, the support substrate supporting the piezoelectric element; a pair of excitation electrodes that face the piezoelectric body across the support substrate and to which a voltage is applied to excite bulk acoustic waves in the piezoelectric body; a conductor facing the pair of excitation electrodes with the piezoelectric body and the support substrate interposed therebetween, the pair of excitation electrodes are spaced apart from each other along the second surface, Each of the piezoelectric body and the support substrate is an AT-cut quartz crystal substrate. Bulk acoustic wave devices.

7. A plate-shaped piezoelectric body; a support substrate having a first surface to which the piezoelectric element is fixed and a second surface located opposite to the first surface, the support substrate supporting the piezoelectric element; a pair of excitation electrodes positioned in a direction in which the first surface faces and facing the piezoelectric body, to which a voltage for exciting a bulk acoustic wave is applied in the piezoelectric body; a conductor facing the pair of excitation electrodes with the piezoelectric body and the support substrate interposed therebetween, the pair of excitation electrodes are spaced apart from each other along the first surface, Each of the piezoelectric body and the support substrate is an AT-cut quartz crystal substrate. Bulk acoustic wave devices.

8. the angle formed between the electric axis of the piezoelectric body and the electric axis of the support substrate is 0°, 90°, or 180°; 8. The bulk acoustic wave device according to claim 6 or 7.

9. A method for manufacturing a bulk acoustic wave device according to any one of claims 1 to 3, comprising: a step of fixing the piezoelectric body to the support substrate to fabricate a composite substrate; a step of polishing the fabricated composite substrate to thin it; and forming the floating conductor on the composite substrate after polishing the composite substrate. Methods for fabricating bulk acoustic wave devices.

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