Bonded body and surface acoustic wave device

Abstract

A bonded structure including a spinel polycrystalline substrate and having excellent frequency-temperature characteristics with little variation, and a surface acoustic wave device including the bonded structure are provided. [Solution] The bonded body (2) includes a piezoelectric substrate (5) and a spinel polycrystalline substrate (1) provided on a first main surface (5a) of the piezoelectric substrate (5), and when the average thickness of the piezoelectric substrate (5) is T1 and the average thickness of the spinel polycrystalline substrate (1) is T2, T1 / T2 is 0.1 or less. The spinel polycrystalline substrate (1) has a total thickness variation (TTV) of 1.5 μm or less on the opposing surface that contacts the piezoelectric substrate (5) and faces the back surface (1a).

Description

[Technical field]

[0001] The present disclosure relates to a bonded body and a surface acoustic wave device. [Background technology]

[0002] Mobile phones incorporate an electronic component called a SAW (Surface Acoustic Wave) filter, which is used to cut out noise in electrical signals and transmit and receive only electrical signals of the desired frequency. SAW filters use a piezoelectric substrate made of a material that exhibits the piezoelectric effect.

[0003] A comb-shaped electrode with a pitch corresponding to the wavelength of the transmission frequency band is formed on one surface of the piezoelectric substrate. An electrical signal input to the comb-shaped electrode causes the piezoelectric substrate to deform under stress, generating an elastic wave corresponding to the pitch.

[0004] The transmission frequency of a SAW filter is determined by the pitch of the comb electrodes. The pitch of the comb electrodes changes due to the expansion and contraction of the piezoelectric substrate caused by changes in the ambient temperature. To suppress changes due to thermal expansion, a support substrate with high strength and low thermal expansion is attached to the surface of the piezoelectric substrate opposite to the surface on which the comb electrodes are formed.

[0005] In Patent Document 1 (JP Patent Publication 2006-304206 A), a silicon substrate is used as the support substrate. The thermal expansion coefficient of silicon is much smaller than that of materials such as lithium tantalate that form the piezoelectric substrate. Therefore, when the piezoelectric substrate expands due to heat, the silicon may crack.

[0006] A technology using sapphire as the support substrate has also been proposed. However, sapphire is a single crystal and has high hardness, making it difficult to mold it into the desired shape for miniaturization. In addition, single crystal substrates of silicon and sapphire are generally expensive, and there has been a demand for lower-cost substrates.

[0007] In view of this, Patent Document 2 (JP Patent Publication No. 2011-66818 A) discloses a technique of using spinel, which has a lower cost and an appropriate strength, as a supporting substrate. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2006-304206 A [Patent Document 2] JP 2011-66818 A Summary of the Invention

[0009] [1] A bonded structure according to one embodiment of the present disclosure includes a piezoelectric substrate and a spinel polycrystalline substrate provided on one main surface of the piezoelectric substrate, the ratio T1 / T2 of the average thickness T1 of the piezoelectric substrate to the average thickness T2 of the spinel polycrystalline substrate is 0.1 or less; The spinel polycrystalline substrate has a total thickness variation (TTV) of 1.5 μm or less on a main surface in contact with the piezoelectric substrate.

[0010] [2] A surface acoustic wave device according to another embodiment of the present disclosure includes the above-described bonded body and and an electrode provided on a main surface of the piezoelectric substrate opposite to the surface on which the spinel polycrystalline substrate is provided. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic cross-sectional view of a bonded body according to an embodiment of the present disclosure. [Diagram 2] FIG. 2 is a perspective view showing an example of a bonded substrate in which electrodes are formed on the main surface of the bonded body of FIG. [Diagram 3] FIG. 3 is a cross-sectional view of the bonded substrate shown in FIG. 2 taken along line XX. [Figure 4]FIG. 4 is a schematic cross-sectional view of a surface acoustic wave device according to an embodiment of the present disclosure. [Diagram 5] FIG. 5 is a diagram for explaining the TTV. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] [Problem that this disclosure aims to solve] The inventors of the present invention fabricated a surface acoustic wave device using a substrate made of spinel as described in Patent Document 1 and evaluated its performance. It was found that although the substrate had sufficient strength, there were cases in which the temperature coefficient of frequency (TCF) varied.

[0013] Therefore, an object of the present invention is to provide a bonded body including a spinel polycrystalline substrate and having excellent frequency-temperature characteristics with small variations, and a surface acoustic wave device including the bonded body.

[0014] [Effects of this disclosure] According to the above aspect, it is possible to provide a bonded body including a spinel polycrystalline substrate and having excellent frequency-temperature characteristics with small variations, and a surface acoustic wave device including the bonded body.

[0015] [Overview of the embodiment] First, the embodiments of the present disclosure will be listed and described.

[0016] (1) A bonded structure according to one aspect of the present disclosure includes a piezoelectric substrate and a spinel polycrystalline substrate provided on one main surface of the piezoelectric substrate, the ratio T1 / T2 of the average thickness T1 of the piezoelectric substrate to the average thickness T2 of the spinel polycrystalline substrate is 0.1 or less; The spinel polycrystalline substrate is a bonded body having a TTV of 1.5 μm or less on the main surface in contact with the piezoelectric substrate.

[0017] According to the above embodiment, the bonded body includes a spinel polycrystalline substrate and can have an excellent frequency-temperature characteristic with small variation. Here, in this specification, the temperature coefficient of frequency (TCF) is a value (unit: ppm / °C) indicating the change in frequency due to the change in ambient temperature. The smaller the TCF value, the better the temperature characteristic. In addition, in this specification, the TTV is a value indicating the difference between the maximum and minimum values ​​of the distance from the back surface of the substrate to the surface facing the back surface. For example, in the spinel polycrystalline substrate 51 shown in FIG. 5, the difference L3 between the maximum value L1 and the minimum value L2 of the distance from the back surface 1a of the spinel polycrystalline substrate 51 to the surface (facing surface) 1b facing the back surface 1a corresponds to the TTV.

[0018] (2) It is preferable that the T1 / T2 ratio is equal to or greater than 0.0002 and equal to or less than 0.1, which can further reduce the variation in the frequency temperature characteristics.

[0019] (3) The T1 is 0.1 μm or more and 25 μm or less, The T2 is preferably 100 μm or more and 500 μm or less.

[0020] According to this, the piezoelectric substrate can effectively suppress loss of surface acoustic waves, and the spinel polycrystalline substrate can sufficiently suppress thermal expansion of the piezoelectric substrate.

[0021] (4) The piezoelectric substrate is preferably made of lithium tantalate or lithium niobate, which can increase the electromechanical coupling coefficient of the piezoelectric substrate and improve the frequency filter characteristics of the piezoelectric substrate.

[0022] (5) the total thickness of the T1 and T2 is 100.1 μm or more and 525 μm or less, the spinel polycrystalline substrate has a main surface that is circular and has a diameter of 100 mm or more and 200 mm or less, or has a circular shape with an orientation flat formed thereon; The bonded body preferably has an outer surface with a TTV of 10 μm or less.

[0023] This allows the piezoelectric substrate to effectively suppress loss of surface acoustic waves, and the bonded body to have excellent frequency-temperature characteristics with smaller variations.

[0024] (6) A surface acoustic wave device according to another embodiment of the present disclosure includes the above-described bonded body and and an electrode provided on a main surface of the piezoelectric substrate opposite to the surface on which the spinel polycrystalline substrate is provided.

[0025] According to the above aspect, the surface acoustic wave device can have excellent frequency-temperature characteristics with small variations.

[0026] [Details of the embodiment] A specific example of a bonded body and a surface acoustic wave device according to an embodiment of the present disclosure will be described below with reference to the drawings. In the drawings of the present disclosure, the same reference numerals represent the same or corresponding parts. Furthermore, dimensional relationships such as length, width, thickness, and depth are appropriately changed for clarity and simplification of the drawings, and do not necessarily represent actual dimensional relationships.

[0027] In this specification, the expression in the form "A to B" means the upper and lower limits of a range (i.e., greater than or equal to A and less than or equal to B), and when no unit is specified for A and only a unit is specified for B, the units of A and B are the same. Furthermore, when the upper limit of the range is C, it means that the upper limit of the range is less than or equal to C, and when the lower limit of the range is D, it means that the lower limit of the range is greater than or equal to D.

[0028] [Embodiment 1: Bonded body] <zygote> The bonded body according to this embodiment will be described with reference to Fig. 1. As shown in Fig. 1, the bonded body 2 according to this embodiment includes a piezoelectric substrate 5 and a spinel polycrystalline substrate 1 provided on one main surface (hereinafter also referred to as the "first main surface") 5a of the piezoelectric substrate 5. The ratio T1 / T2 of the average thickness T1 of the piezoelectric substrate to the average thickness T2 of the spinel polycrystalline substrate is 0.1 or less, and the spinel polycrystalline substrate has a TTV of 1.5 µm or less on the main surface in contact with the piezoelectric substrate.

[0029] As a result of intensive research, the present inventors have newly discovered that a bonded body comprising a piezoelectric substrate and a spinel polycrystalline substrate provided on one main surface of the piezoelectric substrate, in which the ratio T1 / T2 of the average thickness T1 of the piezoelectric substrate to the average thickness T2 of the spinel polycrystalline substrate is 0.1 or less, and the spinel polycrystalline substrate has a TTV of 1.5 μm or less on the main surface in contact with the piezoelectric substrate, can have excellent frequency-temperature characteristics with small variations. The reason for this is unclear, but is presumed to be as follows (i) and (ii).

[0030] (i) In the bonded structure according to this embodiment, T1 / T2 is 0.1 or less, and the average thickness T2 of the spinel polycrystalline substrate is sufficiently larger than the average thickness T1 of the piezoelectric substrate. Therefore, the spinel polycrystalline substrate can sufficiently suppress the thermal expansion of the piezoelectric substrate, and the bonded structure can have excellent frequency-temperature characteristics with little variation.

[0031] (ii) In the bonded structure according to this embodiment, the spinel polycrystalline substrate has a TTV of 1.5 μm or less on the main surface in contact with the piezoelectric substrate. This allows the thickness of the spinel polycrystalline substrate to be kept uniform, and the thermal expansion of the piezoelectric substrate can be sufficiently suppressed over the entire surface of the spinel polycrystalline substrate. This allows the bonded structure to have excellent frequency-temperature characteristics with little variation.

[0032] (shape) The average thickness of the bonded body 2 is preferably 100.1 μm or more and 525 μm or less, more preferably 200.1 μm or more and 510 μm or less, and even more preferably 101 μm or more and 510 μm or less. Here, the thickness of the bonded body is a value measured by a digital micrometer on a cross section parallel to the normal direction of the main surface of the bonded body. The measurement is performed at three points on one cross section, and the average value of the three points is regarded as the average thickness of the bonded body.

[0033] As far as the applicant has measured, it has been confirmed that, as long as the measurements are performed on the same sample, even if the average thickness of the bonded body is calculated multiple times by changing the selected location of the observation cross section, there is almost no variation in the calculation results, and that setting an arbitrary observation cross section will not be arbitrary.

[0034] It should be noted that the main surface of the bonded body includes the front surface 5b of the piezoelectric substrate and the back surface 1a of the spinel polycrystalline substrate, but in this specification, the main surface of the bonded body refers to the front surface 5b of the piezoelectric substrate.

[0035] The shape and size of the main surface of the bonded body 2 are not particularly limited and can be appropriately adjusted depending on the application of the surface acoustic wave device. Here, the main surface of the bonded body 2 includes both the main surface on the piezoelectric substrate 5 side and the main surface on the spinel polycrystalline substrate 1 side. The main surface of the bonded body 2 may be, for example, circular or rectangular. When the shape of the main surface of the bonded body 2 is circular, the diameter can be Φ100 mm or more and Φ200 mm or less. When the shape of the main surface of the bonded body 2 is rectangular, the length of one side can be 70 mm or more and 142 mm or less. An orientation flat may be formed on the main surface of the bonded body 2. It is preferable that the TTV of the outer surface of the bonded body 2 is 10 μm or less.

[0036] (Joining of Piezoelectric Substrate and Spinel Polycrystalline Substrate) The method for bonding the piezoelectric substrate 5 and the spinel polycrystalline substrate 1 is not particularly limited, and an adhesive may be used, or the bonding may be performed by van der Waals force. In order to bond the piezoelectric substrate and the spinel polycrystalline substrate with high accuracy, it is preferable that the piezoelectric substrate and the spinel polycrystalline substrate are bonded by van der Waals force. More specifically, it is preferable that the atoms of the material constituting the piezoelectric substrate and the atoms of the spinel constituting the spinel polycrystalline substrate are bonded by van der Waals force.

[0037] <Piezoelectric substrate> The piezoelectric substrate 5 is a substrate that exhibits the piezoelectric effect of converting an electric signal into a mechanical vibration. For example, lithium tantalate, lithium niobate, lithium borate, etc. can be used as the main component of the piezoelectric substrate 5. Among these, lithium tantalate or lithium niobate, which has an excellent electromechanical coupling coefficient, is preferable. The piezoelectric substrate 5 can be obtained by producing a single crystal rod of the above components by the Czochralski method, for example, and slicing it. The substrate orientation (cut angle) can be, for example, 36° to 50°, and can be appropriately selected depending on the application of the surface acoustic wave device.

[0038] The average thickness T1 of the piezoelectric substrate 5 is preferably 0.1 μm or more and 25 μm or less. If the average thickness T1 of the piezoelectric substrate 5 is less than 0.1 μm, the loss of the surface acoustic wave may be large, or the processing of the piezoelectric substrate 5 may be difficult. If the average thickness T1 of the piezoelectric substrate 5 exceeds 25 μm, the expansion or contraction of the piezoelectric substrate 5 due to temperature change may be large, or the bonded body 2 may be unnecessarily thick. The lower limit of the average thickness T1 of the piezoelectric substrate 5 is preferably 0.1 μm, more preferably 1 μm, and even more preferably 3 μm. On the other hand, the upper limit of the average thickness T1 of the piezoelectric substrate 5 is preferably 25 μm, more preferably 15 μm, and even more preferably 10 μm. The average thickness of the piezoelectric substrate 5 is preferably 1 μm or more and 15 μm or less, and even more preferably 3 μm or more and 10 μm or less.

[0039] The thickness of the piezoelectric substrate can be confirmed by measuring the thickness of the bonded body and the thickness of the spinel polycrystalline substrate, and subtracting the thickness of the spinel polycrystalline substrate from the thickness of the bonded body. The thickness of the bonded body can be measured with a digital micrometer. The thickness of the spinel polycrystalline substrate can be measured with a micrometer.

[0040] As far as the applicant has measured, it has been confirmed that, as long as measurements are taken on the same sample, there is almost no variation in the calculation results even when the average thickness of the piezoelectric substrate is calculated multiple times by changing the selected location of the observation cross-section, and that setting the observation cross-section arbitrarily will not be arbitrary.

[0041] The upper limit of the arithmetic mean roughness (Ra) of the first main surface 5a of the piezoelectric substrate 5 is preferably 1 nm, more preferably 0.8 nm. If the arithmetic mean roughness (Ra) of the first main surface 5a of the piezoelectric substrate 5 exceeds 1 nm, the adhesive strength with the spinel polycrystalline substrate 1 due to van der Waals forces may decrease. On the other hand, the lower limit of the arithmetic mean roughness (Ra) of the first main surface 5a of the piezoelectric substrate 5 is preferably 0.05 nm. If the arithmetic mean roughness (Ra) of the first main surface 5a of the piezoelectric substrate 5 is less than 0.05 nm, the bulk wave may be easily reflected between the piezoelectric substrate 5 and the spinel polycrystalline substrate 1, which may increase the spurious response, or the adhesive area between the piezoelectric substrate 5 and the spinel polycrystalline substrate 1 may be reduced, which may decrease the adhesive strength between the piezoelectric substrate 5 and the spinel polycrystalline substrate 1 due to van der Waals forces.

[0042] In this specification, the arithmetic mean roughness (Ra) means the arithmetic mean roughness defined in JIS B 0601. The arithmetic mean roughness is a value measured by an atomic force microscope (AFM).

[0043] The upper limit of the linear expansion coefficient of the piezoelectric substrate 5 is 30×10 -6 / ℃ is preferred, 20×10 -6 / ° C. It is more preferable that the linear expansion coefficient of the piezoelectric substrate 5 is 30×10 -6If the linear expansion coefficient of the piezoelectric substrate 5 exceeds 8×10 / ° C., the piezoelectric substrate may expand or contract significantly due to temperature changes. -6 / ℃ is preferred, 10×10 -6 is more preferred.

[0044] The linear expansion coefficient is a value measured by a compression load method (thermomechanical analyzer TMA8310 manufactured by Rigaku Corporation).

[0045] The piezoelectric substrate 5 may contain components other than the above-mentioned main components, for example, metals may be added. By adding metal elements, the mechanical strength, heat resistance, and the like of the piezoelectric substrate 5 can be improved.

[0046] <Spinel polycrystalline substrate> The spinel polycrystalline substrate 1 is a substrate made of a spinel sintered body. The spinel polycrystalline substrate 1 is a support base material for increasing the strength of the bonded body 2 and suppressing the thermal expansion of the piezoelectric substrate. Examples of spinels constituting the spinel polycrystalline substrate 1 include MgO·nAl2O3 (1≦n≦3). The lower limit of the value of n is preferably 1, more preferably 1.03, and even more preferably 1.05. If the value of n is less than 1, MgO tends to increase locally and the porosity tends to increase. The upper limit of the value of n is preferably 3, more preferably 2, and even more preferably 1.5. If the value of n exceeds 3, Al2O3 tends to increase locally, the Al2O3 tends to be unevenly distributed together with the pores, and the opacity tends to increase visually.

[0047] The composition of the spinel polycrystalline substrate and the value of n in the composition formula MgO·nAl2O3 can be measured by the X-ray diffraction matrix flashing method.

[0048] The spinel polycrystalline substrate 1 may contain components other than spinel, for example, an alumina component. The upper limit of the content of components other than spinel in the spinel polycrystalline substrate is preferably 10 mass%, more preferably 5 mass%. The lower limit of the content of components other than spinel in the spinel polycrystalline substrate is preferably 3 mass%, more preferably 1 mass%, and even more preferably 0, i.e., no components other than spinel are contained.

[0049] The average thickness T2 of the spinel polycrystalline substrate 1 is preferably 100 μm or more and 500 μm or less. If the average thickness T2 of the spinel polycrystalline substrate 1 is less than 100 μm, the thermal expansion of the piezoelectric substrate may not be sufficiently suppressed. If the average thickness T2 of the spinel polycrystalline substrate 1 exceeds 500 μm, the bulk wave may be easily reflected at the boundary between the piezoelectric substrate 5 and the spinel polycrystalline substrate 1, or the bonded body 2 may become unnecessarily thick. The lower limit of the average thickness T2 of the spinel polycrystalline substrate 1 is preferably 100 μm, more preferably 150 μm, and even more preferably 200 μm. On the other hand, the upper limit of the average thickness T2 of the spinel polycrystalline substrate 1 is preferably 500 μm, more preferably 400 μm, and even more preferably 300 μm. The average thickness T2 of the spinel polycrystalline substrate 1 is more preferably 150 μm or more and 400 μm or less, and even more preferably 200 μm or more and 300 μm or less.

[0050] The thickness of the spinel polycrystalline substrate is a value measured with a micrometer on a cross section parallel to the normal direction of the back surface 1a of the spinel polycrystalline substrate. Measurements are taken at three points on each cross section, and the average value of the three points is taken as the average thickness of the piezoelectric substrate.

[0051] As far as the applicant has measured, it has been confirmed that, as long as measurements are performed on the same sample, there is almost no variation in the calculation results even when the average thickness of the spinel polycrystalline substrate is calculated multiple times by changing the selected location of the observation cross section, and therefore, setting an arbitrary observation cross section will not be arbitrary.

[0052] The ratio T1 / T2 of the average thickness T1 of the piezoelectric substrate 5 to the average thickness T2 of the spinel polycrystalline substrate 1 is 0.1 or less, and the average thickness T2 of the spinel polycrystalline substrate is sufficiently larger than the average thickness T1 of the piezoelectric substrate. Therefore, the spinel polycrystalline substrate can sufficiently suppress the thermal expansion of the piezoelectric substrate, and the bonded body can have excellent frequency-temperature characteristics with little variation.

[0053] The upper limit of the ratio T1 / T2 is 0.1, preferably 0.04, and more preferably 0.02. If the ratio T1 / T2 exceeds 0.1, the temperature coefficient of frequency (TCF) tends to deteriorate.

[0054] On the other hand, the lower limit of the ratio T1 / T2 is preferably 0.0002, more preferably 0.002, and even more preferably 0.006. If the ratio T1 / T2 is less than 0.0002, the processing accuracy of the piezoelectric substrate 5 may decrease. Furthermore, the deformation of the bonded body 4 due to temperature change may increase, and the mechanical strength may decrease.

[0055] The ratio T1 / T2 is preferably 0.0002 or more and 0.1 or less, more preferably 0.002 or more and 0.04 or less, and even more preferably 0.006 or more and 0.02 or less.

[0056] The spinel polycrystalline substrate 1 has a TTV of 1.5 μm or less on the main surface (corresponding to the opposing surface 1b) that contacts the piezoelectric substrate 5. This allows the spinel polycrystalline substrate to maintain a uniform film thickness, and the thermal expansion of the piezoelectric substrate can be sufficiently suppressed over the entire surface of the spinel polycrystalline substrate. This allows the bonded body to have excellent frequency-temperature characteristics with little variation.

[0057] The upper limit of the TTV is 1.5 μm, preferably 1.2 μm, more preferably 0.9 μm, and even more preferably 0.7 μm. If the TTV exceeds 1.5 μm, the thickness of the spinel polycrystalline substrate becomes non-uniform, and the frequency-temperature characteristics of the bonded body tend to vary.

[0058] On the other hand, the lower limit of the TTV is not particularly limited, but from the viewpoint of production, it is preferably 0.1 μm, and more preferably 0.5 μm.

[0059] The TTV is preferably from 0.1 μm to 1.5 μm, more preferably from 0.1 μm to 1.2 μm, and further preferably from 0.5 μm to 0.7 μm.

[0060] The TTV is measured by a flatness measuring device (Tropell's "FlatMaster 200XRA-Wafer") The measurement area is the entire main surface (corresponding to the opposing surface 1b) of the spinel polycrystalline substrate in the bonded body that is in contact with the piezoelectric substrate.

[0061] When the total thickness of T1 and T2 is 100.1 μm or more and 525 μm or less, the main surface of the spinel polycrystalline substrate is a circle with a diameter of 100 mm or more and 200 mm or less, or the circle has an orientation flat formed thereon, and the TTV of the main surface of the spinel polycrystalline substrate that is in contact with the piezoelectric substrate is 1.5 μm or less, the TTV of the outer surface of the bonded body can be 10 μm or less.

[0062] The spinel polycrystalline substrate preferably contains a plurality of crystal grains, the average grain size of which is preferably 1 μm or more and 60 μm or less, which makes it possible to more effectively suppress loss of elastic waves.

[0063] If the average grain size of the crystal grains is less than 1 μm, the porosity of the spinel polycrystalline substrate becomes too large, and the elastic modulus decreases, so that it tends to be difficult to obtain the effect of the support substrate of suppressing the thermal expansion of the piezoelectric substrate. Furthermore, the contact area at the bonding interface between the spinel polycrystalline substrate and the piezoelectric substrate decreases, so that the bonding strength decreases. On the other hand, if the average grain size of the crystal grains exceeds 60 μm, the pores present at the grain boundaries between the spinel particles tend to be unevenly distributed. The average grain size of the spinel polycrystalline substrate is more preferably 5 μm to 30 μm, and even more preferably 8 μm to 30 μm.

[0064] In this specification, the term "average particle size" refers to the median diameter (d50) in a volume-based particle size distribution (volume distribution), and refers to the average particle size of all spinel particles contained in a spinel polycrystalline substrate.

[0065] The particle size of each particle for calculating the average particle size of spinel particles can be measured by the following method. First, the surface of the spinel polycrystalline substrate is mirror-polished, and a measurement field of 10 mm x 10 mm is determined on the polished surface. A backscattered electron image of the spinel polycrystalline substrate in the measurement field is observed at a magnification of 5000 times using an electron microscope. Next, in this backscattered electron image, the diameter of a circle circumscribing the particle constituting the spinel polycrystalline substrate (i.e., the diameter equivalent to the circumscribing circle) is measured, and this diameter is taken as the particle size of the spinel particle.

[0066] As far as the applicant has measured, it has been confirmed that, as long as measurements are performed on the same sample, there is almost no variation in the calculation results even when the average grain size of the crystal grains contained in the spinel polycrystalline substrate is calculated multiple times by changing the selected location of the measurement field of view, and that setting the measurement field of view arbitrarily will not be arbitrary.

[0067] The lower limit of the arithmetic mean roughness (Ra) of the surface of the spinel polycrystalline substrate 1 is preferably 0.01 nm, more preferably 0.1 nm. If the arithmetic mean roughness (Ra) of the surface of the spinel polycrystalline substrate 1 is less than 0.01 nm, the surface of the spinel polycrystalline substrate 1 needs to be processed to be very flat, which tends to increase the processing cost. On the other hand, the upper limit of the arithmetic mean roughness (Ra) of the surface of the spinel polycrystalline substrate 1 is preferably 3.0 nm, more preferably 2.0 nm. If the arithmetic mean roughness (Ra) of the surface of the spinel polycrystalline substrate 1 exceeds 3.0 nm, there is a risk of a decrease in adhesive strength when the piezoelectric substrate 5 and the spinel polycrystalline substrate 1 are bonded together by van der Waals forces.

[0068] The upper limit of the linear expansion coefficient of the spinel polycrystalline substrate 1 is 16×10 -6 / ℃ is preferred, 8×10 -6 / °C. The linear expansion coefficient of the spinel polycrystalline substrate 1 is preferably 16×10 -6If the linear expansion coefficient exceeds 1×10 / ° C., the thermal expansion of the piezoelectric substrate may not be sufficiently suppressed. -6 / ℃ is preferred, 3×10 -6 / °C. The linear expansion coefficient of the spinel polycrystalline substrate 1 is preferably 1×10 -6 If it is less than 1 / ° C., the difference in linear expansion coefficient between the piezoelectric substrate 5 and the spinel polycrystalline substrate 1 may become large, resulting in large distortion of the spinel polycrystalline substrate 1 when the temperature changes.

[0069] The linear expansion coefficient is a value obtained by measuring a material molded into a rod shape using a linear thermal expansion measuring device.

[0070] The lower limit of the difference in linear expansion coefficient between the spinel polycrystalline substrate 1 and the piezoelectric substrate 5 is 5×10 -6 / ℃ is preferred, 7×10 -6 / °C. The difference in linear expansion coefficient between the spinel polycrystalline substrate 1 and the piezoelectric substrate 5 is preferably 5×10 -6 If the difference in linear expansion coefficient between the spinel polycrystalline substrate 1 and the piezoelectric substrate 5 is less than 20×10 / ° C., the thermal expansion of the piezoelectric substrate may not be sufficiently suppressed. -6 / ° C. The difference in linear expansion coefficient between the spinel polycrystalline substrate 1 and the piezoelectric substrate 5 is preferably 20×10 -6 If it exceeds 100 / ° C., there is a risk that distortion of the spinel polycrystalline substrate 1 will become significant when the temperature changes.

[0071] Spinel polycrystalline substrate 1 supports piezoelectric substrate 5, which vibrates in response to an electric signal. As a result, a considerable stress is applied to spinel polycrystalline substrate 1. Furthermore, when piezoelectric substrate 5 operates, it generates heat, and this heat is also transmitted to spinel polycrystalline substrate 1. At this time, thermal stress is generated in spinel polycrystalline substrate 1. For this reason, it is preferable that spinel polycrystalline substrate 1 have an appropriate strength.

[0072] The lower limit of the Young's modulus of the spinel polycrystalline substrate 1 is preferably 100 GPa, more preferably 150 GPa, and even more preferably 180 GPa. If the Young's modulus of the spinel polycrystalline substrate 1 is less than 100 GPa, the spinel polycrystalline substrate 1 may be easily cracked. On the other hand, the upper limit of the Young's modulus of the spinel polycrystalline substrate 1 is preferably 400 GPa, more preferably 350 GPa, and even more preferably 300 GPa. If the Young's modulus of the spinel polycrystalline substrate 1 exceeds 400 GPa, the hardness of the spinel polycrystalline substrate 1 becomes excessively high, increasing the possibility of chipping. Furthermore, the hardness of the spinel polycrystalline substrate 1 becomes excessively high, and processing may become difficult.

[0073] In this specification, Young's modulus is a value measured by a three-point bending test performed in accordance with JIS R 1602. For the measurement, a "Materials Tester AL-50NB" manufactured by Minebea Co., Ltd. is used.

[0074] The lower limit of the Knoop hardness of the spinel polycrystalline substrate 1 is preferably 1000, and more preferably 1200. If the Knoop hardness of the spinel polycrystalline substrate 1 is less than 1000, the spinel polycrystalline substrate 1 may be prone to cracking. On the other hand, the upper limit of the Knoop hardness of the spinel polycrystalline substrate 1 is preferably 2500, and more preferably 1800. If the Knoop hardness of the spinel polycrystalline substrate 1 exceeds 2500, processing of the spinel polycrystalline substrate 1 may become difficult.

[0075] In this specification, the Knoop hardness is a value measured by a "Hardness Testing Machine HM" manufactured by Mitutoyo Corporation.

[0076] <Method of manufacturing the bonded body> The method for manufacturing a bonded body according to this embodiment can include a step of preparing a piezoelectric substrate, a step of preparing a spinel polycrystalline substrate, a step of bonding the piezoelectric substrate and the spinel polycrystalline substrate to obtain a bonded body (hereinafter also referred to as a "bonding step"), and a step of adjusting the thickness of the piezoelectric substrate.

[0077] (Step of Preparing Piezoelectric Substrate) First, a piezoelectric substrate is prepared. A conventionally known piezoelectric substrate can be used as the piezoelectric substrate. Next, the main surface of the piezoelectric substrate is polished. Specifically, the main surface of the piezoelectric substrate is roughly polished by grinding, and then the surface roughness of the surface to be bonded to the spinel polycrystalline substrate is reduced by CMP (Chemical Mechanical Polishing) until the arithmetic mean roughness (Ra) is about 0.05 μm or more and 1 μm or less.

[0078] (Step of preparing a spinel polycrystalline substrate) The step of preparing the spinel polycrystalline substrate can include the steps of preparing a spinel powder, compacting, sintering, and processing.

[0079] In the spinel powder preparation step, powder made of spinel and having a composition formula of MgO·nAl2O3 (1≦n≦3) is prepared. The spinel powder preferably has an average particle size of 0.1 μm or more and 0.3 μm or less, and a purity of 99.5% or more.

[0080] In order to prepare a spinel powder having the above-mentioned composition, it is preferable to mix MgO (magnesium oxide) powder and Al2O3 (alumina) powder so that the mixing ratio (mass ratio) satisfies 1≦Al2O3 / MgO≦3.

[0081] The particle size of each particle used to calculate the average particle size of the spinel powder is measured using a particle size distribution measurement method based on the laser diffraction / scattering method. Specifically, this method measures the diameter of powder particles by analyzing the scattering intensity distribution of the light scattered by a laser beam irradiated onto the powder particles.

[0082] Next, the molding step is carried out. Specifically, a green body is obtained by pressing or CIP (Cold Isostatic Pressing). More specifically, it is preferable to first preform the MgO·nAl2O3 powder prepared in the spinel powder preparation step by pressing, and then perform CIP to obtain a green body. Note that only either press molding or CIP may be performed, or both may be performed, for example, by performing CIP after press molding.

[0083] In press molding, it is preferable to use a pressure of, for example, 1 MPa to 300 MPa, particularly 10 MPa to 100 MPa, whereas in CIP, it is preferable to use a pressure of, for example, 160 MPa to 250 MPa, particularly 180 MPa to 230 MPa.

[0084] Next, a sintering step is carried out. Specifically, the compact is sintered in a vacuum at a temperature condition of 1500°C to 1700°C for 60 to 600 minutes (first sintering step), and then sintered by HIP (Hot Isostatic Pressing) at a temperature condition of 1600°C to 1800°C for 60 to 240 minutes (second sintering step). This makes it possible to obtain a spinel ingot made of a spinel sintered body.

[0085] The second sintering step is preferably carried out while changing the pressure in multiple stages. More specifically, it preferably includes a 2a sintering step in which sintering is performed at a temperature of 1600°C to 1800°C, a pressure of 100 MPa to 200 MPa, and a time of 1 minute to 60 minutes, and a 2b sintering step in which sintering is performed at a temperature of 1600°C to 1800°C, a pressure of 150 MPa to 300 MPa, and a time of 10 minutes to 300 minutes.

[0086] Next, a processing step is performed. Specifically, the obtained spinel ingot is sliced ​​with a diamond wire saw to a desired thickness. This completes the base of the spinel polycrystalline substrate having the desired thickness. The desired thickness is preferably determined in consideration of the thickness of the spinel polycrystalline substrate to be finally formed and the polishing allowance of the main surface of the spinel polycrystalline substrate in the post-process. The thickness T2 of the spinel polycrystalline substrate to be finally formed can be, for example, 100 μm or more and 500 μm or less.

[0087] Next, the main surface of the spinel polycrystalline substrate is polished. Specifically, the main surface of the spinel polycrystalline substrate is roughly polished by grinding, and then the surface (opposing surface 1b) to which the piezoelectric substrate is bonded is subjected to lapping and rough polishing while maintaining a balance so that the load is evenly applied within the substrate surface, thereby reducing the variation in thickness from the back surface 1a so that the TTV is 1.5 μm or less.

[0088] Specifically, lapping is performed using a single-sided or double-sided lapping machine as the lapping plate and diamond abrasives as the lapping agent. Specifically, rough polishing is performed using a single-sided or double-sided polishing machine. In either case, the work thickness, jig dimensions, etc. are taken into consideration, and processing is performed while maintaining a balance so that the load is uniform.

[0089] Furthermore, it is preferable to reduce the surface roughness of the surface (opposing surface 1b) to which the piezoelectric substrate is bonded by CMP until the arithmetic mean roughness (Ra) is about 0.01 nm or more and 3.0 nm or less, which enables the spinel polycrystalline substrate to be bonded to the main surface of the piezoelectric substrate by van der Waals forces.

[0090] (Joining process) Next, the piezoelectric substrate and spinel polycrystalline substrate prepared above are bonded to obtain a bonded body. Specifically, the spinel polycrystalline substrate and the piezoelectric substrate are placed in a vacuum chamber with their polished surfaces facing each other. While maintaining this state, the internal gas in the chamber is evacuated to create a high vacuum. After that, the polished surfaces of both substrates are irradiated with a high-speed atomic beam of neutralized argon, and the two substrates are brought close to each other and bonded to obtain a bonded body.

[0091] (Process for adjusting the thickness of the piezoelectric substrate) In the bonded body obtained above, the second main surface 5b of the piezoelectric substrate is ground, lapped, and polished so that the thickness T1 of the piezoelectric substrate becomes a desired thickness. The desired thickness is a thickness where the value of T1 / T2 is 0.1 or less, and can be, for example, 0.1 μm to 25 μm. This makes it possible to obtain a bonded body where the value of T1 / T2 is 0.1 or less.

[0092] When the thickness of the spinel polycrystalline substrate in the bonded body was measured, it was confirmed that the thickness of the spinel polycrystalline substrate before bonding was maintained.

[0093] Furthermore, when the TTV of the main surface (opposing surface 1b) of the spinel polycrystalline substrate in contact with the piezoelectric substrate in the bonded body was measured, it was confirmed that the TTV of the spinel polycrystalline substrate before bonding was maintained.

[0094] [Embodiment 2: Surface Acoustic Wave Device] <Configuration of surface acoustic wave device> The surface acoustic wave device according to this embodiment will be described with reference to FIGS.

[0095] 2 and 3, a surface acoustic wave device 10 according to this embodiment includes the bonded body 2 described in embodiment 1, and an electrode 3 provided on a main surface (second main surface 5b) of a piezoelectric substrate 5 opposite to a surface (first main surface 5a) on which the spinel polycrystalline substrate 1 is provided. The bonded body 2 includes a piezoelectric substrate 5 and a spinel polycrystalline substrate provided on one main surface (first main surface 5a) of the piezoelectric substrate 5.

[0096] The structures of the bonded body, the piezoelectric substrate, and the spinel polycrystalline substrate used in this embodiment are similar to those described in the first embodiment, and therefore the description thereof will not be repeated.

[0097] The electrode 3 may include a first pole 3a and a second pole 3b. For example, an AC voltage is applied between the first pole 3a and the second pole 3b. Then, an electric signal is input to the current caused by the AC voltage applied between the first pole 3a and the second pole 3b. Then, the crystal particles (atoms) constituting the piezoelectric substrate 5 on which the electrode 3 is provided are subjected to stress, and move closer to or farther away from each other due to the piezoelectric effect, causing the main surface of the piezoelectric substrate 5 to vibrate in a wavy manner.

[0098] The first pole 3a and the second pole 3b may each have a comb-shaped shape. With this, for example, among the electric signals input to the electrode 3, only the electric signal having a wavelength corresponding to the distance between the comb-shaped component 3c and the comb-shaped component 3d of the first pole 3a is propagated to the outside. In other words, the electric signals having wavelengths other than the above-mentioned wavelengths are not propagated to the outside and are blocked inside the joint body 2. Based on this principle, the joint body 2 can block electric signals other than the desired wavelength (i.e., noise) by outputting only the electric signal having the desired wavelength to the outside, and eliminate noise in the output signal.

[0099] As shown in FIGS. 2 and 3, an electrode member 6 may be further provided on the second main surface 5b of the piezoelectric substrate.

[0100] 2 and 3, the piezoelectric substrate 5, the electrode 3 provided on the second main surface 5b of the piezoelectric substrate, and the spinel polycrystalline substrate 1 bonded to the first main surface 5a of the piezoelectric substrate form a bonded substrate 4. The bonded substrate 4 may further include a first electrode member 6.

[0101] In this specification, the surface acoustic wave device may be formed only from the bonding substrate 4, or may include other components in addition to the bonding substrate 4, as described below. An example of a surface acoustic wave device including other components in addition to the bonding substrate 4 will be described with reference to FIG.

[0102] 4, the surface acoustic wave device 410 may further include a sealing substrate 7 for sealing the electrode 3, in addition to the bonding substrate 4. A second electrode member 9 made of a metal thin film is formed on one of the main surfaces of the sealing substrate 7 that faces the bonding substrate 4, and an external terminal 11 is formed on the other main surface. The second electrode member 9 and the external terminal 11 are electrically connected to each other through via wiring 8 that penetrates the sealing substrate 7.

[0103] In the surface acoustic wave device 410, the first electrode member 6 and the second electrode member 9 are arranged so as to be in contact with each other, and therefore the first electrode member 6 and the external terminal 11 are also electrically connected via the second electrode member 9 and the via wiring 8.

[0104] The bonding substrate 4 and the sealing substrate 7 are bonded via an adhesive member 13. The adhesive member 13 is provided so as to surround the electrode 3, the first electrode member 6, and the second electrode member 9. Therefore, the electrode 3, the first electrode member, and the second electrode member are isolated from the outside and hermetically sealed. The bonding substrate 4 and the sealing substrate 7 may be bonded directly by pressure bonding without using an adhesive member.

[0105] Metal or resin can be used as the adhesive member 13. Metals that can be used include gold, platinum, silver, copper, aluminum, nickel, titanium, gold alloys, and metals whose surfaces are coated with gold. Resins that can be used include epoxy resin, acrylic resin, silicone resin, polyimide resin, polyamide resin, and polyamideimide resin.

[0106] It is preferable to use spinel as the sealing substrate 7. The average thickness of the sealing substrate 7 is preferably 10 μm or more and 1000 μm or less.

[0107] The thickness of the surface acoustic wave device 410 is preferably 110.1 μm or more and 1550 μm or less. The main surface of the surface acoustic wave device 10 is preferably a rectangle with each side having a length of 0.1 mm or more and 10 mm or less.

[0108] <Method of manufacturing a surface acoustic wave device> The method for manufacturing a surface acoustic wave device according to this embodiment can include the steps of preparing a bonding substrate, preparing a sealing substrate, and bonding the bonding substrate and the sealing substrate to obtain a surface acoustic wave device.

[0109] (Step of preparing a bonded substrate) First, a bonded substrate is prepared. The bonded substrate can be obtained by forming electrodes on the main surface of the bonded body described in the first embodiment. Specifically, first, the surface of the piezoelectric substrate 5 in the bonded body 2 is cleaned by RCA cleaning. Next, an aluminum-based material that will become an electrode is deposited on the polished surface of the piezoelectric substrate 5 by electron beam deposition to a thickness of 100 to several hundreds of angstroms.

[0110] Next, a resist is applied to the polished surface of the piezoelectric substrate using a coater, and after hardening by baking, a pattern of a surface acoustic wave device is exposed using a stepper exposure machine, and developed using a developer.

[0111] Next, the aluminum-based material in the portion not masked by the resist is removed using an RIE (Reactive Ion Etching) device, leaving the aluminum-based material only in the electrode portion. The resist is removed to complete the electrode. This results in a bonded substrate 4 including the piezoelectric substrate 5, the electrode 3 provided on the second main surface 5b of the piezoelectric substrate 5, and the spinel polycrystalline substrate 1 bonded to the first main surface 5a of the piezoelectric substrate 5. The first electrode member 6 can also be formed on the second main surface 5b of the piezoelectric substrate 5 by a method similar to that for producing the electrode 3 described above.

[0112] 2 and 3, when the surface acoustic wave device is formed only from the bonded substrate, the bonded substrate obtained above corresponds to the surface acoustic wave device 10. When the surface acoustic wave device further includes other components in addition to the bonded substrate as shown in FIG. 4, the following steps are further performed.

[0113] (Process of preparing sealing substrate) As the sealing substrate 7, for example, a substrate made of spinel is prepared. Via wiring 8 penetrating the substrate is formed. Next, a second electrode member 9 is formed on one main surface of the substrate so as to cover the via wiring 8. In addition, an external terminal 11 is formed on the other main surface of the substrate so as to cover the via wiring 8. In this manner, the sealing substrate 7 can be obtained.

[0114] (Process for obtaining a surface acoustic wave device) Next, the resulting bonded substrate 4 and sealing substrate 7 are bonded. First, an adhesive member 13 made of metal or resin is placed on the second main surface 5b of the piezoelectric substrate 5. Next, the sealing substrate 7 is placed so as to face the second main surface 5b of the piezoelectric substrate 5. At this time, the first electrode member 6 and the second electrode member 9 are placed so as to be in contact with each other. Next, the sealing substrate 7 is pressed against the piezoelectric substrate 5 at a constant heating temperature, and the sealing substrate 7 and the piezoelectric substrate 5 are bonded with the adhesive member 13, and the electrode 3 is hermetically sealed.

[0115] Next, the bonding substrate 4 and the sealing substrate 7 are cut to a desired size while keeping the electrodes 3 hermetically sealed, thereby obtaining the surface acoustic wave device 410. Alternatively, the bonding substrate 4 may be cut to a desired size and then bonded to the sealing substrate 7 to obtain the surface acoustic wave device 410.

[0116] [Example] The present embodiment will be described more specifically with reference to examples, although the present embodiment is not limited to these examples.

[0117] <Preparation of the junction> (Step of Preparing Piezoelectric Substrate) A piezoelectric substrate made of lithium tantalate and having an average thickness of 250 μm (shown as "average thickness P-T1" in Table 1) was prepared as the piezoelectric substrate. The main surface of the piezoelectric substrate was circular, and its diameter was Φ98 mm.

[0118] The main surfaces of the above-mentioned piezoelectric substrate were roughly polished by grinding, and then the surface roughness of the surface to be bonded to the spinel polycrystalline substrate was reduced by CMP (Chemical Mechanical Polishing) until the arithmetic mean roughness (Ra) was approximately 0.5 nm.

[0119] (Step of preparing a spinel polycrystalline substrate) Powder consisting of spinel with the formula MgO·nAl2O3 (n = 1.03) was prepared. The spinel powder had an average particle size of 17 μm and a purity of 99.9%.

[0120] Next, the powder made of the above-mentioned spinel was press-molded at a pressure of 10 MPa to obtain a green body.

[0121] Next, the above-mentioned compact was sintered in a vacuum at 1675° C. for 4 hours, and then sintered for 120 minutes under conditions of a pressure of 200 MPa and a temperature of 1780° C. As a result, a spinel ingot consisting of a spinel sintered body was obtained.

[0122] Next, the obtained spinel ingot was sliced ​​with a diamond wire saw to a desired average thickness to obtain a spinel polycrystalline substrate. The desired average thickness was determined by taking into consideration the thickness of the spinel polycrystalline substrate shown in the "Average thickness P-T2" column of the spinel polycrystalline substrate before bonding in Table 1, and the polishing allowance of the main surface of the spinel polycrystalline substrate in the post-processing. The shape of the main surface of the spinel polycrystalline substrate was circular, and its diameter was Φ100 mm.

[0123] Next, the main surface of the spinel polycrystalline substrate was roughly polished by grinding, and then the surface to which the piezoelectric substrate was bonded was subjected to lapping and rough polishing while maintaining a balance so that the load was evenly applied to the substrate surface. This reduced the variation in thickness from the back surface so that the TTV was the value shown in the "P-TTV" column of the spinel polycrystalline substrate before bonding in Table 1. Specifically, the lapping was performed using a single-sided or double-sided lapping machine as the lapping plate and diamond abrasive grains as the lapping agent. Specifically, the rough polishing was performed using a single-sided or double-sided polishing machine. In either case, the work thickness, the jig dimensions, etc. were taken into consideration, and the processing was performed while maintaining a balance so that the load was evenly applied. Furthermore, the surface roughness was reduced by CMP until the arithmetic mean roughness (Ra) was about 0.5 nm.

[0124] (Joining process) Next, the spinel polycrystalline substrate and the piezoelectric substrate were placed in a vacuum chamber so that their polished surfaces faced each other. While maintaining this state, the gas inside the chamber was evacuated to create a high vacuum. After that, the polished surfaces of both substrates were irradiated with a high-speed atomic beam of neutral argon, and the two substrates were brought close to each other and bonded to obtain a bonded body.

[0125] (Process for adjusting the thickness of the piezoelectric substrate) In the bonded body obtained above, the second main surface of the piezoelectric substrate was subjected to grinding, lapping and polishing so that the thickness T1 of the piezoelectric substrate became the thickness shown in the "Average thickness T1" column of the piezoelectric substrate in the bonded substrate in Table 1.

[0126] In the obtained bonded body, the average thickness T2 of the spinel polycrystalline substrate was measured. The results are shown in the "Average thickness T2" column of the spinel crystal substrate in the bonded body in Table 1. From the results in Table 1, it was confirmed that the average thickness P-T2 of the spinel polycrystalline substrate before bonding was the same as the average thickness T2 of the spinel polycrystalline substrate in the bonded body. The method of calculating the average thickness T1 and the average thickness T2 is described in the first embodiment, and therefore the description will not be repeated.

[0127] The T1 / T2 value was calculated from the obtained T1 and T2 values. The results are shown in the "T1 / T2" column in Table 1.

[0128] In the obtained bonded body, the TTV of the interface between the spinel polycrystalline substrate and the piezoelectric substrate (corresponding to the opposing surface 1b of the spinel polycrystalline substrate) was measured. The results are shown in the "TTV" column for the spinel polycrystalline substrate in the bonded body in Table 1. It was confirmed from the results in Table 1 that the TTV of the spinel polycrystalline substrate before bonding ("P-TTV" in Table 1) was the same as the TTV of the spinel polycrystalline substrate in the bonded body. Note that the method for measuring the TTV is described in the first embodiment, and therefore the description will not be repeated.

[0129] [Table 1]

[0130] <Fabrication of surface acoustic wave devices> (Step of preparing a bonded substrate) The surfaces of the piezoelectric substrates in the above bonded bodies were cleaned by RCA cleaning. Next, an aluminum-based material that would become an electrode was deposited on the cleaned surfaces of the piezoelectric substrates to a thickness of several hundred Å by electron beam deposition.

[0131] Next, a resist was applied to the polished surface of the piezoelectric substrate using a coater, and after hardening by baking, a pattern of a surface acoustic wave device was exposed using a stepper exposure machine and developed using a developer.

[0132] Next, the aluminum-based material in the areas not masked by the resist was removed using a RIE (Reactive Ion Etching) device, leaving the aluminum-based material only in the electrode area. The electrode was completed by removing the resist. The first electrode member was also formed on the second main surface of the piezoelectric substrate using the same method as the above-mentioned electrode fabrication method.

[0133] (Process of preparing sealing substrate) A substrate made of spinel was prepared as the sealing substrate. Via wiring penetrating the substrate was formed. Next, a second electrode member was formed on one main surface of the substrate so as to cover the via wiring. In addition, an external terminal was formed on the other main surface of the substrate so as to cover the via wiring. In this way, a sealing substrate was obtained.

[0134] (Process for obtaining a surface acoustic wave device) Next, the resulting bonded substrate and sealing substrate were bonded. First, an adhesive member made of resin was placed on the second main surface of the piezoelectric substrate. Next, the sealing substrate was placed so as to face the second main surface of the piezoelectric substrate. At this time, the first electrode member and the second electrode member were placed so as to be in contact with each other. Next, the sealing substrate was pressed against the piezoelectric substrate at a constant heating temperature, and the sealing substrate and the piezoelectric substrate were bonded with the adhesive member, and the electrodes were hermetically sealed.

[0135] Next, the bonding substrate and the sealing substrate were cut into pieces measuring 1 mm×1 mm while keeping the electrodes hermetically sealed, to obtain surface acoustic wave devices of Samples 1 to 19.

[0136] <Measurement of Temperature Coefficient of Frequency (TCF)> The frequency temperature coefficient of the obtained surface acoustic wave device was measured by the following specific method.

[0137] A network analyzer was connected to the SAW resonators of five test surface acoustic wave elements from one board, and the ambient temperature was changed from -40°C to 85°C to measure the temperature coefficient of frequency (TCF) of each sample surface acoustic wave device, and the average value and variation (standard deviation) were calculated. The results are shown in the "Average value" and "Variation" columns in Table 1.

[0138] <Evaluation> Samples 1 to 3, 5 to 14, and 16 to 19 have a ratio T1 / T2 of 0.1 or less between the average thickness T1 of the piezoelectric substrate and the average thickness T2 of the spinel polycrystalline substrate, and the spinel polycrystalline substrate has a TTV of 1.5 μm or less on the main surface in contact with the piezoelectric substrate, and thus correspond to the examples. Samples 1 to 3, 5 to 14, and 16 to 19 have good frequency-temperature characteristics with small variations.

[0139] Sample 4 has a T1 / T2 ratio exceeding 0.1 and corresponds to a comparative example. Compared to the above examples, Sample 4 had a deteriorated frequency-temperature characteristic.

[0140] Sample 15 has a TTV exceeding 1.5 μm and corresponds to a comparative example. Sample 15 had a large variation in frequency temperature characteristics compared to the above examples.

[0141] Although the embodiments and examples of the present disclosure have been described above, it is intended from the outset that the configurations of the above-described embodiments and examples may be appropriately combined or modified in various ways.

[0142] The embodiments and examples disclosed herein are illustrative in all respects and should not be considered as limiting. The scope of the present invention is indicated by the claims, not by the embodiments and examples described above, and is intended to include the meaning equivalent to the claims and all modifications within the scope. [Explanation of symbols]

[0143] 1,51 spinel polycrystalline substrate, 1a back surface, 1b opposing surface, 2 bonded body, 3 electrode, 3a first pole, 3b second pole, 4 bonded substrate, 5 piezoelectric substrate, 5a first main surface, 5b second main surface, 6 first electrode member, 7 sealing substrate, 8 via wiring, 9 second electrode member, 10,410 surface acoustic wave device, 11 external terminal, 13 adhesive member.

Claims

1. A piezoelectric substrate and a spinel polycrystalline substrate provided on one main surface of the piezoelectric substrate, a ratio T1 / T2 of an average thickness T1 of the piezoelectric substrate to an average thickness T2 of the spinel polycrystalline substrate is 0.1 or less; The T2 is 150 μm or more, the spinel polycrystalline substrate has a total thickness variation (TTV) of 1.5 μm or less on a main surface in contact with the piezoelectric substrate; The piezoelectric substrate is a bonded body made of lithium tantalate, lithium niobate, or lithium borate.

2. 2. The joint body according to claim 1, wherein the T1 / T2 is 0.0002 or more and 0.1 or less.

3. The T1 is 0.1 μm or more and 25 μm or less, 3. The bonded body according to claim 1, wherein T2 is 150 μm or more and 500 μm or less.

4. The total thickness of the T1 and T2 is 150.1 μm or more and 525 μm or less, the spinel polycrystalline substrate has a main surface that is circular and has a diameter of 100 mm or more and 200 mm or less, or has a circular shape with an orientation flat formed thereon; The bonded body according to claim 1 , wherein the total thickness variation (TTV) of the outer surface of the bonded body is 10 μm or less.

5. A joint body according to any one of claims 1 to 4, and an electrode provided on a main surface of the piezoelectric substrate opposite to the surface on which the spinel polycrystalline substrate is provided.

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