Composite substrates for surface acoustic wave devices

Abstract

To provide a composite substrate for a surface acoustic wave device having less loss in a filter passband, fewer spurious components, and excellent performance and reliability.SOLUTION: A composite substrate for a surface acoustic wave device includes a piezoelectric single crystal thin film, a support substrate, and at least one type of intervening layer disposed between the piezoelectric single crystal thin film and the support substrate, and the intervening layer is in contact with the piezoelectric single crystal thin film, the total thickness of the intervening layer is equal to or less than twice the wavelength of the surface acoustic wave, and the attenuation of longitudinal waves of the intervening layer calculated at frequencies between 40 GHz and 60 GHz using the Brillouin oscillation method is equal to or less than 8×10-2 (nm-1 THz-2).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a composite substrate for a surface acoustic wave device, which comprises a piezoelectric single crystal thin film and a support substrate. [Background technology]

[0002] Surface acoustic wave (SAW) devices, which have interdigital transducers (IDTs) formed on a piezoelectric substrate to excite surface acoustic waves, are used in mobile phones and other devices as frequency adjustment and selection components. Surface acoustic wave devices are required to be small, have low insertion loss, and block unwanted waves, and are therefore made from piezoelectric materials such as lithium tantalate (LiTaO3; LT) and lithium niobate (LiNbO3; LN).

[0003] On the other hand, under the communication standards for fourth-generation and later mobile phones, the piezoelectric materials used in surface acoustic wave devices must have sufficiently small temperature-related fluctuations in their characteristics. Furthermore, to prevent unwanted noise from spilling over between bands, filters, duplexers, and multiplexers must have extremely small insertion loss and extremely steep shoulder characteristics, requiring the resonators that make up the filters to have a high Q (Quality Factor). Furthermore, filters must be able to handle a wide bandwidth depending on the band they are used in.

[0004] Regarding materials used in such surface acoustic wave devices, composite substrates made of piezoelectric materials and other materials have been considered. For example, Patent Document 1 discloses an acoustic wave device having a piezoelectric film, the acoustic wave device including a support substrate, a high acoustic velocity film formed on the support substrate and having a bulk wave acoustic velocity faster than the acoustic wave acoustic velocity propagating through the piezoelectric film, a low acoustic velocity film laminated on the high acoustic velocity film and having a bulk wave acoustic velocity slower than the bulk wave acoustic velocity propagating through the piezoelectric film, the piezoelectric film laminated on the low acoustic velocity film, and an IDT electrode formed on one surface of the piezoelectric film.

[0005] Furthermore, Patent Document 2 includes a support substrate, a medium layer laminated on the support substrate, a piezoelectric body laminated on the medium layer through which a bulk wave propagates, and an IDT electrode formed on one surface of the piezoelectric body. The medium layer includes a low-speed medium in which the propagation speed of the bulk wave, which is the main component of the elastic wave, is lower than the sound speed of the elastic wave propagating through the piezoelectric body, and a high-speed medium in which the propagation speed of the bulk wave, which is the main component of the elastic wave, is higher than the sound speed of the elastic wave propagating through the piezoelectric body. When the sound speed of the main vibration mode when the medium layer is formed of the high-speed medium is VH and the sound speed of the main vibration mode when the medium layer is formed of the low-speed medium is VL, the medium layer is formed such that VL < sound speed of the main vibration mode < VH in the elastic wave device in which the medium layer is formed, and the thickness of the medium layer is 1λ or more when the period of the IDT is λ. An elastic wave device is disclosed, which is characterized by the above.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] However, when a surface acoustic wave filter is fabricated using the composite substrate of Patent Document 1 or Patent Document 2, the energy of the acoustic waves leaks from the piezoelectric body into the low-velocity medium and remains there, resulting in the generation of noise called spurious or ripple within the passband of the surface acoustic wave filter or at higher frequencies. This noise is generated by wave reflection at the bonding interface between the piezoelectric crystal film and the support substrate, or by acoustic waves being trapped in an intervening layer between the piezoelectric crystal film and the support substrate, and is undesirable because it deteriorates the frequency characteristics of the surface acoustic wave filter and causes an increase in loss in the passband of the filter.

[0008] The present invention has been made in view of the above circumstances, and has an object to provide a composite substrate for a surface acoustic wave device that has small loss in the filter passband, few spurious components, excellent performance, and high reliability. [Means for solving the problem]

[0009] As a result of extensive research, the inventors discovered that by providing a piezoelectric single crystal thin film, a support substrate, and at least one intervening layer between the piezoelectric single crystal thin film and the support substrate, and by specifying the total thickness of the intervening layer and the attenuation of longitudinal waves, it is possible to reduce loss in the passband of the filter and obtain high reliability, and thus completed the present invention. [1] A composite substrate comprising a piezoelectric single crystal thin film, a support substrate, and at least one intervening layer provided between the piezoelectric single crystal thin film and the support substrate, wherein the intervening layer is in contact with the piezoelectric single crystal thin film, the total thickness of the intervening layer is equal to or less than twice the wavelength of the surface acoustic wave, and the attenuation of the longitudinal wave of the intervening layer calculated at a frequency of 40 GHz to 60 GHz by the Brillouin oscillation method is 8×10 -2 (nm -1 THz -2 ) or less. [2] The composite substrate for a surface acoustic wave device according to [1], wherein the sound velocity of the shear wave in the intermediate layer is faster than the sound velocity of the slow shear wave in the piezoelectric single crystal thin film. [3] The piezoelectric single crystal thin film contains iron at 120 ppm or less, and the volume resistivity of the piezoelectric single crystal thin film is 2×10 11 Ω·cm or less, which is the composite substrate for surface acoustic wave devices described in [1] or [2]. [4] The intermediate layer is either SiOx (1.8 < x < 2.05) or SiOyNz (0.02 < z / (y + z) < 0.1), which is the composite substrate for surface acoustic wave devices described in any one of [1] to [3]. [5] The main component of the piezoelectric single crystal thin film is lithium tantalate or lithium niobate, which is the composite substrate for surface acoustic wave devices described in any one of [1] to [4]. [6] The support substrate is any one of a silicon substrate, a sapphire substrate, an alumina substrate, a silicon carbide substrate, an aluminum nitride substrate, a silicon nitride substrate, and a quartz substrate, which is the composite substrate for surface acoustic wave devices described in any one of [1] to [5]. [7] The support substrate consists of a Si single crystal and a polysilicon layer formed on the Si single crystal, which is the composite substrate for surface acoustic wave devices described in any one of [1] to [5]. [8] The content of Li ions in the intermediate layer is 1×10 17 atom / cm 3 or less, which is the composite substrate for surface acoustic wave devices described in any one of [1] to [7]. [Effect of the Invention]

[0010] According to the present invention, it is possible to provide a composite substrate for surface acoustic wave devices with a small loss in the filter passband and high reliability. [Brief Description of the Drawings]

[0011] [Figure 1] FIG. 1 is a diagram showing an example of the spectrum of Brillouin vibration of a SiO1.7 thin film. [Figure 2] FIG. 2 is a diagram showing the waveform obtained by Fourier-transforming only the reflected light by ultrasonic waves (longitudinal waves) in the thin film. [Figure 3]FIG. 3 shows the dependency relationship between the LSAW sound velocity of a thin film and the value obtained by normalizing the thin film thickness to the ultrasonic wavelength (thin film thickness / wavelength), as well as the calculation results of the sound velocity of elastic waves obtained by the finite element method. [Figure 4] FIG. 4 shows an example of the calculation result of the displacement distribution of elastic waves in which the energy is concentrated in the surface layer of a thin film having a SiO1.7 thin film / (111)Si substrate structure. [Figure 5] FIG. 5 is a diagram showing an example of a cross-sectional TEM photograph of the composite substrate produced in Example 1. [Figure 6] FIG. 6 is a diagram showing a composite substrate having Al fine electrodes formed on the surface. [Figure 7] FIG. 7 is a diagram showing the resonant waveform (input impedance (Zin) and Q value) of the SAW obtained in Example 1. [Figure 8] FIG. 8 is a diagram showing the resonant waveform (input impedance (Zin) and Q value) of the SAW obtained in Example 2. [Figure 9] FIG. 9 is a diagram showing the resonant waveform (input impedance (Zin) and Q value) of the SAW obtained in Comparative Example 1. [Figure 10] FIG. 10 is a diagram showing the resonant waveform (input impedance (Zin) and Q value) of the SAW obtained in Comparative Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0012] The composite substrate for a surface acoustic wave device of the present invention will be described in detail below, but the present invention is not limited to the embodiments. Furthermore, a numerical range defined using the symbol "to" is intended to include both the upper and lower limits of the range.

[0013] [Composite substrates for surface acoustic wave devices] The composite substrate for a surface acoustic wave device of the present invention is a composite substrate comprising a piezoelectric single crystal thin film, a support substrate, and at least one intervening layer provided between the piezoelectric single crystal thin film and the support substrate, wherein the intervening layer is in contact with the piezoelectric single crystal thin film, the total thickness of the intervening layer is equal to or less than twice the thickness of the wavelength of the surface acoustic wave, and the attenuation of the longitudinal wave of the intervening layer calculated at frequencies of 40 GHz to 60 GHz by the Brillouin oscillation method is 8×10 -2 (nm -1 THz -2 ) is as follows.

[0014] (Piezoelectric single crystal thin film) The piezoelectric single crystal thin film in the surface acoustic wave device composite substrate according to one embodiment of the present invention is not particularly limited as long as it can be used as a piezoelectric film, but it is preferable that the main component is lithium tantalate (LiTaO3:LT) or lithium niobate (LiNbO3:LN). By using these components as the main component of the piezoelectric single crystal thin film, an acoustic wave device with high electromechanical coupling can be obtained. In the present invention, the phrase "the main component of the piezoelectric single crystal thin film is lithium tantalate or lithium niobate" means that 50 mass % or more of the components constituting the piezoelectric single crystal thin film is lithium tantalate or lithium niobate. Furthermore, the piezoelectric single crystal thin film may contain components other than lithium tantalate and lithium niobate, such as iron and magnesium.

[0015] Furthermore, it is preferable that the piezoelectric single crystal thin film contains 120 ppm or less of iron. By containing a small amount of iron in the piezoelectric single crystal thin film, the electric field at which the polarization of the piezoelectric single crystal thin film is destroyed, i.e., the coercive electric field, increases, making it difficult for the polarization to be destroyed. From the above points of view, the iron content is more preferably 10 ppm or more and 120 ppm or less, and even more preferably 50 ppm or more and 100 ppm or less.

[0016] In addition, the initial volume resistivity of the piezoelectric single crystal thin film is 2×10 11 It is preferable that the initial volume resistivity of the piezoelectric single crystal thin film is 2×10 11By being below Ω·cm, the constant-temperature resistance of the composite substrate for the surface acoustic wave device of the present invention can be improved. The initial volume resistivity of the piezoelectric single-crystal thin film is 2×10 10 Ω·cm or more and 2×10 11 Ω·cm or less is more preferable, and 2×10 10 Ω·cm or more and 1×10 11 Ω·cm or less is even more preferable.

[0017] (Support substrate) In the surface acoustic wave device composite substrate in one embodiment of the present invention, the support substrate can be any of a silicon substrate, a sapphire substrate, an alumina substrate, a silicon carbide substrate, an aluminum nitride substrate, a silicon nitride substrate, and a quartz substrate. The surface acoustic wave resonator manufactured using the composite substrate for the surface acoustic wave device using the above substrate as the support substrate has a high Q value, further suppresses spurious signals outside the band, and further, a surface acoustic wave device with excellent temperature characteristics can be obtained.

[0018] Also, as the support substrate, a support substrate composed of a Si single crystal and a polysilicon layer formed on the Si single crystal can be used. By appropriately adjusting the thickness of the polysilicon layer as the support substrate, spurious signals outside the band can be further suppressed. The thickness of the polysilicon layer is preferably 0.2 μm or more and 2 μm or less, more preferably 0.5 μm or more and 1.9 μm or less, and even more preferably 0.5 μm or more and 1.2 μm or less.

[0019] (Intervening layer) The composite substrate for the surface acoustic wave device of the present invention includes an intervening layer between the above piezoelectric single-crystal thin film and the support substrate, and the intervening layer is provided in contact with the piezoelectric single-crystal thin film. In the surface acoustic wave device composite substrate in one embodiment of the present invention, the intervening layer is preferably either SiOx (1.8 < x < 2.05) or SiOyNz (0.02 < z / (y + z) < 0.1). The composite substrate for a surface acoustic wave device of the present invention also includes at least one intermediate layer, which may be one layer or two or more layers, and in the case of two or more layers, each layer may be made of the same material or different materials.

[0020] The content of Li ions in the intervening layer is 1×10 17 atom / cm 3 Li ions mainly diffuse from the piezoelectric single crystal thin film to the intermediate layer, and the content of Li ions in the intermediate layer is preferably 1×10 or less. 17 atom / cm 3 By setting the thickness to the value below, it is possible to prevent the intermediate layer from softening and to suppress in-band and out-of-band spurious emissions.

[0021] Furthermore, the thickness of the intermediate layer is, if there is one intermediate layer, the thickness of the intermediate layer itself, and if there are two or more intermediate layers, the total thickness is less than or equal to twice the wavelength of the surface acoustic wave of the intermediate layer. If the thickness of the intermediate layer exceeds twice the wavelength of the surface acoustic wave of the intermediate layer, the acoustic wave is more likely to be trapped within the intermediate layer, resulting in increased out-of-band spurious. The total thickness of the intermediate layer is preferably less than or equal to twice the wavelength of the surface acoustic wave, and more preferably less than or equal to one time.

[0022] In addition, the attenuation of longitudinal waves in the intervening layer calculated at frequencies between 40 GHz and 60 GHz using the Brillouin oscillation method was 8 × 10 -2 (nm -1 THz -2 ) is as follows. If the acoustic attenuation of longitudinal waves in the intermediate layer is large, the acoustic waves are easily trapped in the intermediate layer. In this case, the surface acoustic wave resonator made of the composite substrate for a surface acoustic wave device of the present invention will have the problem of increased spurious out-of-band. -2 (nm -1 THz -2 ) or less, the Q value of the surface acoustic wave resonator is high, and out-of-band spurious can be further suppressed. The attenuation of the longitudinal wave in the intermediate layer is 2×10 -2 (nm-1 THz -2 ) or less, and is preferably 0.5 × 10 -2 (nm -1 THz -2 ) or less is more preferable.

[0023] Furthermore, it is preferable that the acoustic velocity of the shear waves in the intervening layer is faster than the acoustic velocity of the slow shear waves in the piezoelectric single crystal thin film. By making the shear wave (bulk wave) velocity in the intervening layer faster than the slow shear wave (bulk wave) in the piezoelectric single crystal thin film, it is possible to improve the passband loss of the surface acoustic wave filter obtained using the composite substrate for surface acoustic wave devices. If the shear wave velocity in the intervening layer is slower than the slow shear wave in the piezoelectric single crystal thin film, there is a concern that the acoustic wave may be more easily trapped in the intervening layer. Note that, since the piezoelectric single crystal thin film is an anisotropic material, there are two types of shear waves in the piezoelectric single crystal thin film: fast shear waves and slow shear waves.

[0024] For example, in LiTaO3, which is an example of a piezoelectric single crystal thin film, the slow shear wave of 46° rotated Y-cut LiTaO3 is 3330 m / s. 1.85 , or SiO 1.94 N 0.06 In SiO 1.85 , or SiO 1.94 N 0.06 The sound velocities of the shear waves in SiO are high, at 3850 m / s and 3750 m / s, respectively. 1.85 , or SiO 1.94 N 0.06 The sound velocity of the shear wave of LSAW in a piezoelectric single crystal thin film will be described in detail later, but it can be determined from the measured sound velocity of the shear wave of LSAW in the piezoelectric single crystal thin film using a linear convergent beam acoustic microscope and analysis by the finite element method, as described in, for example, Tatsuya Omori1, Kensuke Sakamoto, Satoshi Suzuki, Jun-ichi Kushibiki, Satoru Matsuda, and Ken-ya Hashimoto, “Characterization of Elastic Properties of SiO2 Thin Films by Ultrasonic Microscopy.” The intervening layer is SiO 1.85 , or SiO 1.94 N 0.06 By using LiTaO3 for the piezoelectric single crystal thin film, the sound velocity of the transverse waves in the intervening layer can be made faster than the sound velocity of the slow transverse waves in the piezoelectric single crystal thin film.

[0025] In addition, the attenuation of longitudinal waves in the intervening layer calculated at frequencies of 40 GHz to 60 GHz by the Brillouin oscillation method was found to be 1.85 is the attenuation of 1.2 x 10 -2 (nm -1 THz -2 ), SiO 1.94 N 0.06 The attenuation is 0.1×10 -2 (nm -1 THz -2 ) and the attenuation of the longitudinal wave is 8×10 -2 (nm -1 THz -2 ) can be as follows:

[0026] Here, the Brillouin oscillation method can be performed using the method described in H. Ogi et al., "Elastic constant and Brillouin oscillations in sputtered vitreous SiO2 thin films", PHISICAL REVIEW B 78, 13204 (2008). 1.7 In this case, the SiO 1.7 An example of the spectrum of Brillouin oscillations of a thin film is shown.

[0027] The SiO2 used in the measurement of Figure 1 1.7 The thin film (hereinafter simply referred to as "thin film") is made of SiO 2 having the same composition and by the same manufacturing method as the intermediate layer. 1.7 A thin film was formed on a Si substrate to a thickness of 1055 nm (the thickness is defined as d), and then SiO 1.7 An Al film of 10 nm was formed on the thin film. The method for producing the thin film is not particularly limited, but may be, for example, a CVD method. 1.7Ultra-short pulsed light is irradiated onto a structure consisting of a thin film and a Si substrate to generate ultra-high frequency ultrasonic waves. Furthermore, delayed pulsed light with a reference wavelength (λ) of 400 nm is irradiated. The longitudinal wave velocity of the thin film can be determined by detecting the light diffracted by the ultrasonic waves within the thin film.

[0028] At this time, the density of the thin film and the refractive index at 400 nm were determined in advance. The refractive index (n) of the thin film at 400 nm was determined to be 1.728 by the prism coupler method. The density (ρ) of the thin film was determined by XPS (X-ray photoelectron spectroscopy) and found to be 2300 kg / m 3 The first half of the Brillouin spectrum shown in Figure 1 shows the intensity of reflected light due to diffraction by ultrasonic waves (longitudinal waves) within the thin film. By performing a Fourier transform on only the light reflected by ultrasonic waves (longitudinal waves) within the thin film, the frequency of the ultrasonic waves (longitudinal waves) within the thin film shown in Figure 2 can be obtained. From Figure 2, the frequency (f) of the ultrasonic waves (longitudinal waves) within the thin film can be calculated to be 54.5 GHz.

[0029] At this time, the sound velocity vl of the ultrasonic waves (longitudinal waves) in the thin film can be calculated by the following formula (1). vl=f×λ / (2×n)...Equation (1) The sound velocity of the ultrasonic waves (longitudinal waves) in the thin film shown in Figure 2, calculated using equation (1), was 6325 m / s. The elastic constant C11 of the thin film was calculated using vl = √(C11 / ρ), and was found to be 92 GPa.

[0030] Next, SiO 1.7 A method for determining the sound velocity of shear waves in a film using a linear focused beam acoustic microscope and the finite element method is described. First, using a linear convergent beam acoustic microscope, as described in the aforementioned Tatsuya Omori1, Kensuke Sakamoto, Satoshi Suzuki, Jun-ichi Kushibiki, Satoru Matsuda, and Ken-ya Hashimoto, "Characterization of Elastic Properties of SiO2 Thin Films by Ultrasonic Microscopy," we determined the dependency of the LSAW sound velocity of the thin film on the ultrasonic wavelength (thin film thickness / wavelength) by varying the frequency from 160 MHz to 275 MHz. The relationship is shown in Figure 3.

[0031] The LSAW sound velocity (vertical axis) measured by a linear focused beam acoustic microscope in Figure 3 is the SiO 1.7 A thin film was formed on a Si(111)-oriented substrate with a thickness of 2830 nm. The SiO was then applied to the substrate via pure water as a couplant so that the linear convergent beam of the linear convergent beam acoustic microscope was shifted 45 degrees clockwise from the (110) direction in the Si substrate plane as viewed from the thin film side. 1.7 This was achieved by propagating LSAW on the surface of the thin film / Si substrate structure.

[0032] On the other hand, a two-dimensional finite element method was used to analyze SiO 1.7 On the surface of the thin film / (111)Si substrate, a ZnO film and an Al electrode, which are sufficiently thin compared to the electrode period, are placed. <110> The elastic wave was excited in a direction shifted 45 degrees clockwise from the direction of the electrode, and the wavelength determined by the electrode period was changed in the finite element method calculation model to calculate the SiO 1.7 The relationship between the acoustic wave velocity of the thin film / (111)Si substrate and the intervening layer normalized by wavelength was calculated. 1.7 Because the elastic constant C12 of the thin film is an unknown quantity, a tentative value was input for C12, and C12 was determined so as to match the measurement results of the LSAW sound velocity using a linear convergent beam acoustic microscope in Figure 3. The other constants (C11, ρ) were calculated using the values ​​mentioned above.

[0033] The C12 calculated above was 20 GPa. 1.7 Figure 3 shows the results of calculations of the acoustic velocity of elastic waves using the finite element method when the elastic constant C12 of the thin film is 20 GPa. As an example, in a calculation model using the finite element method when the elastic wave wavelength is 18 μm, the elastic wave is 1.7 Figure 4 shows an example of the calculated displacement distribution of waves whose energy is concentrated on the surface layer of a thin film with a thin film / (111)Si substrate structure.

[0034] From the above considerations, SiO 1.7 The shear wave acoustic velocity Vs (=√((C11-C12) / ρ)) of the thin film alone was calculated, and Vs = 3960 m / s was obtained. In the calculation model using the finite element method of the present application, the elastic constants of the Si substrate, ZnO film, and Al film were taken from the values ​​given in the Surface Acoustic Wave Data Book (edited by the Japan Electronics Industry Association), (p. 66 (ZnO), p. 165 (Si), p. 172 (Al)).

[0035] Furthermore, in the study of this application, as a unique attempt, attention was focused on the fact that the reflected light intensity due to diffraction by ultrasonic waves (longitudinal waves) within the thin film layer, which is the first half of the Brillouin spectrum shown in Figure 1, exponentially decays over time. That is, in the envelope of the reflected light intensity shown by the dotted line in Figure 1, if the amplitude intensity at time 0 psec is A0 and the amplitude intensity at the time when the ultrasonic waves reach Si is A, the attenuation constant (β) of the ultrasonic waves (longitudinal waves) within the thin film can be described by equation (2). A=A0·exp(-β·f 2 ·d) ··········Formula (2) In the case of Figure 2, A0 was 0.03 and A was 0.004. From the above, the damping constant β is calculated using equation (3). β=-1 / (f 2 d) In(A / A0) Equation (3) In the case of Figure 2, β is 10.8×10 -2 (nm -1 THz -2 ) was.

[0036] For the various intervening layers described above, the attenuation of longitudinal waves was calculated at frequencies of 40 GHz to 60 GHz by the Brillouin oscillation method using the same method as above. When the acoustic attenuation of longitudinal waves in the intervening layer is large, the acoustic waves tend to be trapped within the intervening layer. In this case, we confirmed that the surface acoustic wave resonator made of the composite substrate for surface acoustic wave devices has the problem of large out-of-band spurious emissions. On the other hand, if the intermediate layer of the present invention has a high acoustic velocity and little attenuation of longitudinal waves, the acoustic waves cannot be concentrated excessively on the intermediate layer, and wave energy can be concentrated on the piezoelectric single crystal thin film. Therefore, the Q value of the surface acoustic wave resonator using the composite substrate for surface acoustic wave devices of the present invention is high, and out-of-band spurious emissions are further suppressed, which is preferable. [Example]

[0037] The present invention will be explained in more detail below by showing examples, but the present invention is not limited to these examples.

[0038] Example 1 A polysilicon layer of 1.7 μm was formed on the surface of a 150 mm diameter (111) oriented high resistance silicon substrate.

[0039] Next, a melt having a composition in which the amount of lithium was reduced based on the amount of lithium contained in the lithium tantalate crystal of the congruent composition was prepared, and an iron-substituted lithium tantalate crystal was grown from the melt having the prepared composition. The amount of iron added was adjusted so that the iron content in the iron-substituted lithium tantalate crystal was 95 mass ppm. A 6-inch iron-containing 42° Y-cut lithium tantalate (LT) substrate was prepared. The iron-containing 42° Y-cut lithium tantalate (LT) substrate was subjected to a reduction treatment to reduce the volume resistivity to 2.2 × 10 10 It was adjusted to Ω·cm.

[0040] Next, hydrogen molecular ions were implanted into the 6-inch iron-containing lithium tantalate (LT) substrate from the side to be bonded. The dose was 9×10 16 atm / cm2 The acceleration voltage was 160 KeV. Next, a 0.4 μm thick layer of SiO 2 was applied to the 6-inch iron-containing lithium tantalate (LT) substrate ion-implanted surface. 1.85 The intermediate layer was formed by CVD. Next, the silicon substrate on which the polysilicon layer was formed and a 0.4 μm thick SiO 1.85 The iron-containing LT substrate with the interposed layer was subjected to surface activation treatment by plasma treatment. Furthermore, the silicon substrate with the polysilicon layer and the 0.4 μm thick SiO 1.85 The iron-containing LT substrate was bonded to the substrate with an intervening layer made of .

[0041] Next, the bonded body was heat-treated at 350°C in a nitrogen atmosphere to prevent the introduction of crystal defects due to misalignment at the bonding interface. The heat-treated bonded body was then heated to 110°C, and a wedge was driven into one end of the ion-implanted portion of the iron-containing lithium tantalate (LT) substrate to separate it into the iron-containing lithium tantalate (LT) piezoelectric single crystal thin film layer bonded to the silicon substrate and the remaining iron-containing lithium tantalate (LT) substrate. The thickness of the iron-containing lithium tantalate (LT) piezoelectric single crystal thin film layer bonded to the silicon substrate after separation was measured using a spectrophotometer and found to be 0.52 μm. Next, the composite substrate consisting of the iron-containing lithium tantalate (LT) piezoelectric single crystal thin film layer bonded to the silicon substrate was heated at 500°C for 6 hours.

[0042] Furthermore, the surface layer of the iron-containing lithium tantalate (LT) piezoelectric single crystal thin film of the composite substrate was polished by 20 nm, and the thickness of the iron-containing lithium tantalate (LT) piezoelectric single crystal thin film layer bonded to the silicon substrate was measured using a spectrophotometer, and was found to be 0.5 μm. FIG. 5 shows an example of a cross-sectional TEM observation photograph of the composite substrate of Example 1 produced by the above method, and FIG. 5(b) is a further enlarged photograph of a part of FIG. 5(a).

[0043] The intermediate layer (SiO 1.85The amount of Li in the SiO 1.85 The amount of Li in the layer is up to 2×10 16 atom / cm 3 In addition, the intermediate layer (SiO 1.85 The composition ratio of each layer was determined by X-ray photoelectron spectroscopy (XPS).

[0044] Furthermore, an intermediate layer (SiO 1.85 The density of the layer was determined by XPS (X-ray photoelectron spectroscopy) and found to be 2240 g / cm 3 In addition, the intervening layer (SiO 1.85 The longitudinal wave velocity and attenuation of the intermediate layer (SiO 1.85 The longitudinal wave velocity in the layer is 6200 m / s, and the acoustic attenuation rate is 1.2 × 10 at 55 GHz. -3 (nm -1 THz -2 ) was. In addition, the intermediate layer (SiO 1.85 The shear wave velocity of the intervening layer (SiO 1.85 The shear wave velocity of the single layer was 3850 m / s.

[0045] Next, an Al film was sputtered to a thickness of 0.14 μm on the surface of the composite substrate obtained above, and after applying a resist, a resist pattern with a line width of approximately 0.5 μm was formed by i-line exposure. Then, the Al was etched by dry etching to form the first layer of a one-port SAW resonator. At this time, the wavelength of the surface acoustic wave was 2 μm, and the intermediate layer (SiO 1.85 The thickness of the layer is 0.2 wavelengths. Furthermore, a second layer of pads, an Al film 0.6 μm thick, was formed on the composite substrate by the lift-off method. Figure 6 shows the composite substrate with Al microelectrodes formed on its surface.

[0046] Next, the electrical characteristics of the one-port SAW resonator on the iron-containing lithium tantalate (LT) piezoelectric single crystal thin film substrate bonded to the silicon substrate prepared above were measured using a network analyzer. The obtained SAW resonance waveform (input impedance (Zin) and Q value) is shown in Figure 7. In addition, the resonant frequency (fr), anti-resonant frequency (fa), and electromechanical coupling coefficient (k 2 ), maximum Q value (Qmax), amplitude of input impedance (ΔZ), fractional bandwidth value (fractional bandwidth = (anti-resonance frequency - resonance frequency) / resonance frequency), and spurious intensity between 2400 and 2800 MHz are shown in Table 1.

[0047] [Table 1]

[0048] Next, the iron-containing lithium tantalate (LT) piezoelectric single crystal thin film substrate bonded to the silicon substrate with the one-port SAW resonator pattern created above was placed on a hot plate heated to 350°C for 10 minutes, then placed on a cooling plate at room temperature to cool, and the electrical characteristics of the one-port SAW resonator were measured again with a network analyzer. The resulting characteristics were the same as those in Table 1. Furthermore, this process of hot plate heating at 350°C, cooling, and measurement was repeated for a total cumulative time of 4 hours, and the electrical characteristics of the one-port SAW resonator after each cumulative heating time were no different from those in Table 1.

[0049] Example 2 In Example 1, the thickness is 0.4 μm and the composition is SiO 1.85 Instead of an intermediate layer of SiO 1.94 N 0.06 A composite substrate was prepared in the same manner as in Example 1, except that an intervening layer of iron-containing lithium tantalate (LT) was used.

[0050] The interlayer (SiO ) of the iron-containing lithium tantalate (LT) piezoelectric single crystal thin film substrate bonded to the silicon substrate 1.94 N0.06 The amount of Li in the SiO layer was measured by secondary ion mass spectrometry (SIMS). 1.94 N 0.06 The amount of Li in the layer is 1×10 14 atom / cm 3 In addition, the intermediate layer (SiO 1.94 N 0.06 The composition ratio of each layer was determined by X-ray photoelectron spectroscopy (XPS).

[0051] In addition, the LT layer of a composite substrate consisting of an iron-containing lithium tantalate (LT) piezoelectric single crystal thin film bonded to a silicon substrate was removed, and the intervening layer (SiO 1.94 N 0.06 The density of the intervening layer (SiO 1.94 N 0.06 The density of the layer is 2210 g / cm 3 In addition, the LT layer of the iron-containing lithium tantalate (LT) piezoelectric single crystal thin film substrate bonded to the silicon substrate was removed, and the intermediate layer (SiO 1.94 N 0.06 The longitudinal wave velocity and attenuation of the intermediate layer (SiO 1.94 N 0.06 The longitudinal wave velocity in the layer is 6190 m / s, and the acoustic attenuation rate is 1.2 × 10 at 55 GHz. -3 (nm -1 THz -2 ) was. In addition, the intervening layer SiO 1.94 N 0.06 The shear wave velocity of the layer itself was calculated by combining the linear convergent beam acoustic microscope and finite element analysis. 1.94 N 0.06 The shear wave velocity of the layer alone was 3750 m / s.

[0052] Next, the electrical characteristics of the one-port SAW resonator on the iron-containing lithium tantalate (LT) piezoelectric single crystal thin film substrate bonded to the silicon substrate prepared above were measured using a network analyzer. The obtained SAW resonance waveform (input impedance (Zin) and Q value) is shown in Figure 8. At this time, the wavelength of the surface acoustic wave was 2 μm, and the intermediate layer (SiO 1.94 N 0.06 The thickness of the layer is 0.15 wavelengths. In addition, the resonant frequency (fr), anti-resonant frequency (fa), and electromechanical coupling coefficient (k 2 ), the maximum value of Q (Qmax), the amplitude of the input impedance (ΔZ), the fractional bandwidth, and the spurious intensity between 2400 and 2800 MHz are shown in Table 1.

[0053] [Table 2]

[0054] Next, an iron-containing lithium tantalate (LT) piezoelectric single crystal thin film substrate bonded to a silicon substrate with a one-port SAW resonator pattern, fabricated in the same manner as in Example 1, was placed on a hot plate heated to 350°C for 10 minutes, then placed on a cooling plate at room temperature to cool, and the electrical characteristics of the one-port SAW resonator were measured again using a network analyzer. The resulting characteristics were the same as those in Table 2. Furthermore, this process of hot plate heating at 350°C, cooling, and measurement was repeated for a total cumulative time of 4 hours, and the electrical characteristics of the one-port SAW resonator after each cumulative heating time were no different from those in Table 2.

[0055] (Comparative Example 1) A polysilicon layer of 1.7 μm was formed on the surface of a 150 mm diameter (111) oriented high resistance silicon substrate.

[0056] Next, the amount of lithium contained in the lithium tantalate crystal of the congruent composition was used as a reference to adjust the melt to a composition in which the amount of lithium was reduced, and an iron-substituted lithium tantalate crystal was grown from the melt of the adjusted composition. A 6-inch iron-containing 42° Y-cut lithium tantalate (LT) substrate was prepared by adding iron so that the iron content in the iron-substituted lithium tantalate crystal was 95 mass ppm. The iron-containing 42° Y-cut lithium tantalate (LT) substrate was subjected to a reduction treatment to reduce the volume resistivity to 2.2 × 10 10 It was adjusted to Ω·cm.

[0057] Next, hydrogen molecular ions were implanted into the 6-inch iron-containing lithium tantalate (LT) substrate from the side to be bonded. The dose was 9×10 16 atm / cm 2 The acceleration voltage was 160 KeV. Next, a 0.4 μm thick layer of SiO 2 was applied to the 6-inch iron-containing lithium tantalate (LT) substrate ion-implanted surface. 1.7 The intermediate layer was formed by CVD. Next, the silicon substrate on which the polysilicon layer was formed and a 0.4 μm thick SiO 1.7 The iron-containing LT substrate with the interposed layer was subjected to surface activation treatment by plasma treatment. Furthermore, the silicon substrate with the polysilicon layer formed thereon and the SiO 1.7 The iron-containing LT substrate was bonded to the substrate with an intervening layer made of .

[0058] Next, the bonded body was heat-treated at 350°C in a nitrogen atmosphere to prevent the introduction of crystal defects due to misalignment at the bonding interface. The heat-treated bonded body was then heated to 110°C, and a wedge was driven into one end of the ion-implanted portion of the iron-containing lithium tantalate (LT) substrate to separate it into the iron-containing lithium tantalate (LT) piezoelectric single crystal thin film layer bonded to the support substrate and the remaining iron-containing lithium tantalate (LT) substrate. The thickness of the iron-containing lithium tantalate (LT) piezoelectric single crystal thin film layer bonded to the silicon substrate after separation was measured using a spectrophotometer and found to be 0.52 μm. Next, the composite substrate consisting of the iron-containing lithium tantalate (LT) piezoelectric single crystal thin film bonded to the silicon substrate was heated at 500°C for 6 hours.

[0059] Furthermore, the surface layer of the iron-containing lithium tantalate (LT) piezoelectric single crystal thin film of the composite substrate was polished by 20 nm, and the thickness of the iron-containing lithium tantalate (LT) piezoelectric single crystal thin film layer bonded to the silicon substrate was measured using a spectrophotometer, and was found to be 0.5 μm.

[0060] The intermediate layer (SiO 1.7 The amount of Li in the SiO 1.7 The amount of Li in the layer is up to 2×10 17 atom / cm 3 The intermediate layer (SiO 1.7 The composition ratio of each layer was determined by X-ray photoelectron spectroscopy (XPS).

[0061] In addition, the intermediate layer (SiO 1.7 The density of the layer was determined by XPS (X-ray photoelectron spectroscopy) and found to be 2240 g / cm 3 In addition, the intervening layer (SiO 1.7 The longitudinal wave velocity and attenuation of the intermediate layer (SiO 1.7 The longitudinal wave velocity in the layer is 6325 m / s, and the acoustic attenuation rate is 10.8 × 10 at 55 GHz. -2 (nm -1 THz -2 ) was. In addition, the intermediate layer (SiO 1.7 The shear wave velocity of the intervening layer (SiO 1.7 The shear wave velocity of the single layer was 3960 m / s.

[0062] Next, an Al film was sputtered onto the surface of the composite substrate to a thickness of 0.14 μm, and a resist was applied. After that, a resist pattern with a line width of approximately 0.5 μm was formed by i-line exposure. The Al was then etched by dry etching to form the first layer of a one-port SAW resonator. Furthermore, a second layer pad of 0.6 μm thick Al film was formed on the composite substrate by the lift-off method. At this time, the wavelength of the surface acoustic wave was 2 μm. 1.7 The thickness of the layer is 0.2 wavelengths.

[0063] Next, the electrical characteristics of the one-port SAW resonator on the iron-containing lithium tantalate (LT) piezoelectric single crystal thin film substrate bonded to the silicon substrate prepared above were measured using a network analyzer. The obtained SAW resonance waveform (input impedance (Zin) and Q value) is shown in Figure 9. In addition, the resonant frequency (fr), anti-resonant frequency (fa), and electromechanical coupling coefficient (k 2 ), the maximum value of Q (Qmax), the amplitude of the input impedance (ΔZ), the fractional bandwidth, and the spurious intensity between 2400 and 2800 MHz are shown in Table 3.

[0064] [Table 3]

[0065] Next, the iron-containing lithium tantalate (LT) piezoelectric single crystal thin film substrate bonded to the silicon substrate with the one-port SAW resonator pattern created above was placed on a hot plate heated to 350°C for 10 minutes, then placed on a cooling plate at room temperature to cool, and the electrical characteristics of the one-port SAW resonator were measured again using a network analyzer. The resulting characteristics were the same as those in Table 3. Furthermore, this process of hot plate heating at 350°C, cooling, and measurement was repeated for a total cumulative time of 4 hours, and the electrical characteristics of the one-port SAW resonator after each cumulative heating time were no different from those in Table 3.

[0066] (Comparative Example 2) A polysilicon layer of 1.7 μm was formed on the surface of a high-resistivity silicon substrate with a diameter of 150 mm.

[0067] Next, an iron-free lithium tantalate crystal was grown on the lithium tantalate crystal of the congruent composition. Iron-free lithium tantalate crystal was processed to prepare 6-inch iron-free 42° Y-cut lithium tantalate (LT) substrates. The iron-free 42° Y-cut lithium tantalate (LT) substrates were subjected to reduction treatment to reduce the volume resistivity to 2.2× 10 It was adjusted to Ω·cm.

[0068] Next, hydrogen molecular ions were implanted into the 6-inch iron-free lithium tantalate (LT) substrate from the side to be bonded. The dose was 9×10 16 atm / cm 2 The acceleration voltage was 160 KeV.

[0069] Next, a 0.4 μm thick intervening layer having a composition of SiO 2 was formed by CVD on the ion-implanted surface of a 6-inch iron-free lithium tantalate (LT) substrate. Next, the silicon substrate with the polysilicon layer and the iron-free LT substrate with a 0.4 μm thick SiO2 interlayer were subjected to surface activation treatment using plasma.Furthermore, the silicon substrate with the polysilicon layer and the iron-free LT substrate were bonded together with the 0.4 μm thick SiO2 interlayer interposed therebetween to form a bonded assembly.

[0070] Next, the surface of the bonded body (bonded substrate) was removed by grinding and polishing. The thickness of the iron-free lithium tantalate (LT) piezoelectric single crystal thin film layer bonded to the silicon substrate after thinning was measured by a spectrophotometer and found to be 0.5 μm.

[0071] The amount of Li in the intermediate layer (SiO2 layer) of the composite substrate manufactured by the above method was measured by secondary ion mass spectrometry (SIMS). The amount of Li in the SiO2 layer was found to be up to 5 × 10 19 atom / cm 3 The composition ratio of the intervening layer (SiO2 layer) was determined by X-ray photoelectron spectroscopy (XPS).

[0072] In addition, the LT layer of the composite substrate manufactured by the above method was removed, and the density of the intermediate layer (SiO2 layer) was measured by X-ray reflectivity measurement (XRR). As a result, the density of the intermediate layer (SiO2 layer) was 2100 g / cm 3 In addition, the LT layer of the composite substrate was removed, and the longitudinal wave sound velocity and attenuation of the intervening layer (SiO2 layer) were determined using the Brillouin oscillation method. As a result, the longitudinal wave sound velocity of the intervening layer (SiO2 layer) was 5400 m / s, and the acoustic attenuation rate was 5 × 10 at 55 GHz. -1 (nm -1 THz -2 ) was. Furthermore, the shear wave acoustic velocity of the SiO2 layer alone, which is the intervening layer, was determined by combining the linear convergent beam acoustic microscope and finite element method analysis mentioned above, and was found to be 3150 m / s.

[0073] Next, the electrical characteristics of the one-port SAW resonator on the iron-free lithium tantalate (LT) piezoelectric single crystal thin film substrate bonded to the silicon substrate prepared above were measured using a network analyzer. The obtained SAW resonance waveform (input impedance (Zin) and Q value) is shown in Figure 10. In addition, the resonant frequency (fr), anti-resonant frequency (fa), and electromechanical coupling coefficient (k 2 ), maximum Q value (Qmax), amplitude of input impedance (ΔZ), relative bandwidth, and spurious intensity between 2400 and 2800 MHz are shown in Table 4. At this time, the wavelength of the surface acoustic wave is 2 μm, and the thickness of the intermediate layer (SiO2 layer) is 0.2 wavelengths.

[0074] [Table 4]

[0075] Next, the iron-free lithium tantalate (LT) piezoelectric single crystal thin film substrate bonded to the silicon substrate with the one-port SAW resonator pattern created above was placed on a hot plate heated to 350°C for 10 minutes, then placed on a cooling plate at room temperature to cool, and the electrical characteristics of the one-port SAW resonator were measured again using a network analyzer. The resulting characteristics were the same as those in Table 4. Furthermore, this process of hot plate heating at 350°C, cooling, and measurement was repeated for a total of four hours, with the cumulative heating time at 350°C, and the electromechanical coupling coefficient (k 2 ) begins to decrease from the value in Table 3, and after a cumulative heating time of 4 hours, the electromechanical coupling coefficient (k 2 ) had decreased to 5%.

Claims

1. A composite substrate comprising a piezoelectric single crystal thin film, a support substrate, and at least one intervening layer provided between the piezoelectric single crystal thin film and the support substrate, The intermediate layer is in contact with the piezoelectric single crystal thin film, the total thickness of the intermediate layer is equal to or less than twice the wavelength of the surface acoustic wave, and the attenuation of the longitudinal wave of the intermediate layer calculated at a frequency of 40 GHz to 60 GHz by the Brillouin oscillation method is 8×10 －２ (nm －１ ・THz －２ ) A composite substrate for a surface acoustic wave device, which is:

2. 2. The composite substrate for a surface acoustic wave device according to claim 1, wherein the sound velocity of the shear wave in the intermediate layer is faster than the sound velocity of the slow shear wave in the piezoelectric single crystal thin film.

3. the piezoelectric single crystal thin film contains 120 ppm or less of iron, The volume resistivity of the piezoelectric single crystal thin film is 2×10 １１ 3. The composite substrate for a surface acoustic wave device according to claim 1, wherein the resistivity is Ω·cm or less.

4. 3. The composite substrate for a surface acoustic wave device according to claim 1, wherein the intermediate layer is one of SiOx (1.8<x<2.05) and SiOyNz (0.02<z / (y+z)<0.1).

5. 3. The composite substrate for a surface acoustic wave device according to claim 1, wherein the main component of the piezoelectric single crystal thin film is lithium tantalate or lithium niobate.

6. 3. The composite substrate for a surface acoustic wave device according to claim 1, wherein the support substrate is any one of a silicon substrate, a sapphire substrate, an alumina substrate, a silicon carbide substrate, an aluminum nitride substrate, a silicon nitride substrate, and a quartz substrate.

7. 3. The composite substrate for a surface acoustic wave device according to claim 1, wherein the support substrate comprises a silicon single crystal and a polysilicon layer formed on the silicon single crystal.

8. The content of Li ions in the intervening layer is 1×10 １７ atom / cm ３ 3. The composite substrate for a surface acoustic wave device according to claim 1, wherein:

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