Lithium tantalate single-crystal substrate, its use for a bonded substrate and a surface acoustic wave device, and manufacturing method of the bonded substrate

Abstract

A rotated Y-cut lithium tantalate (LiTaO3) single crystal substrate having a crystal orientation of 36°Y - 49°Y-cut, characterized in that: the substrate is diffused with Li from its surface to its depth so as to have a Li concentration profile showing a difference in Li concentration between the substrate surface and the depth of the substrate; and the substrate is treated with a singular polarization treatment such that the Li concentration is substantially uniform from the substrate surface to a depth corresponding to 5 to 15 times the wavelength of either a surface acoustic wave or a volume fraction surface acoustic wave (LSAW;leaky surface acoustic wave) propagating in the LiTaO3 substrate surface, wherein the substrate is doped with Fe in an amount of 25 ppm - 150 ppm and the Li concentration profile is one in which the Li concentration is higher in a region closer to the substrate surface of the rotated Y-cut LiTaO3 substrate and lower in a region closer to the center of the substrate, and wherein the region extending from the substrate surface in which the Li concentration is substantially uniform is a region in which the half-width of the Raman shift peak is around 600 cm; -1 in the range of ±0.2cm -1 which is at the surface of the substrate, or a region in which the value Li / (Li + Ta) is in the range of ±0.001 of that at the surface of the substrate.

Description

Technical area

[0001] The present invention relates to a lithium tantalate single crystal substrate, its use in a bonded substrate, a manufacturing method of the bonded substrate, and use in a surface acoustic wave device using such a bonded substrate. Technical background

[0002] A surface acoustic wave (SAW) device formed with a comb-like electrode (IDT: interdigital transducer) for exciting a surface acoustic wave on a piezoelectric substrate is used as a component for frequency adjustment and selection of a mobile phone, etc.

[0003] For this surface acoustic wave device, a piezoelectric material such as lithium tantalate (LiTaO3 or LT) and lithium niobate (LiNbO3 or LN) is used to fabricate the base substrate because piezoelectric materials meet the requirements of small size, low insertion loss, and the ability to stop the passage of unnecessary waves.

[0004] On the one hand, communication standards for fourth-generation mobile phones require a narrow frequency band difference between transmission and reception, as well as a wide bandwidth. However, under such communication standards, unless the property changes induced by temperature changes in the material of the acoustic harmonic wave device are sufficiently small, a shift in the frequency selection range occurs, resulting in problematic interference with the filtering and duplexing functions of the devices. Therefore, a material for a surface acoustic wave device with a low tendency to undergo property fluctuation with temperature change and with a wide bandwidth is desired.

[0005] Regarding such a material for a harmonic acoustic device, for example, IP Document 1 teaches that a stoichiometric composition LT composed of copper used as an electrode material, which is generally obtained by a gas-phase method, is preferable because the breakdown mode of sudden breakage is difficult to occur at a time when high power is applied to the IDT electrode. Likewise, IP Document 2 has a detailed description of the stoichiometric composition LT obtained by the gas-phase method; and similarly, IP Document 3 describes a detailed method of performing annealing on a waveguide formed of a ferroelectric crystal of lithium tantalate or lithium niobate.

[0006] Furthermore, IP Document 4 describes a piezoelectric substrate for a surface acoustic wave device obtained by subjecting a single crystal substrate of lithium tantalate or lithium niobate to Li diffusion treatment, and IP Document 5 and Non-IP Document 1 also report that when LT in which the LT composition is uniformly transformed to Li-rich from the surface to a depth by a gas-phase equilibrium method was used to fabricate the surface acoustic wave element, its frequency stability against the temperature change was improved, which was advantageous. State-of-the-art documentsIP publications IP Publication 1: Japanese Patent Application, Publication No. JP 2011-135245 A IP Publication 2: US Pat. No. 6,652,644 B1 IP Publication 3: Japanese Patent Application, Publication No. JP 2003-207671 A IP Publication 4: Japanese Patent Application, Publication No. JP 2013-66032 A IP Publication 5: WO 2013 / 135886 A1

[0007] Further prior art can be found in DE 11 2015 002 181 T5, JP 2014-157911A, JP 2013-81099 A and DE 698 36 719 T2. Non-IP publications

[0008] Bartasyte, A. et al., “Reduction of temperature coefficient of frequency in LiTaO3 single crystals for surface acoustic wave applications” Applications of Ferroelectrics, held jointly with the 2012 European Conference on the Applications of Polar Dielectrics and the 2012 International Symp Piezoresponse Force Microscopy and Nanoscale Phenomena in Polar Materials (ISAF / ECAPD / PFM), 2012 Intl Symp, 2012, page(s): 1-3 Summary of the inventionProblems to be solved by the invention

[0009] However, when the present inventors investigated the methods described in the publications, they found that these methods do not necessarily provide favorable results. Specifically, according to the method described in IP Document 5, the wafer is manufactured at a high temperature of about 1300°C in the gas phase, and the manufacturing temperature must also be as high as about 1300°C. Therefore, the resulting wafer warpage would be large and cracks (or fractures) might occur with a high frequency, resulting in poor productivity. There is also a problem that the product becomes excessively expensive as a material for a surface acoustic wave device.Moreover, in this process, the vapor pressure of Li2O is so low that the modification degree of the sample to be modified varies significantly depending on the distance from the Li source, and the resulting problem of fluctuation in the quality of the product hinders its industrialization.

[0010] Moreover, in the manufacturing method described in IP Document 5, no singular polarization treatment is performed on the lithium-rich LT after modification by the gas phase equilibrium method, and as a result of the present inventors' investigation on this point, it has recently been found that with the LT modified with Li-rich but not subjected to singular polarization treatment, there is a problem that the Q value of the SAW device is low.

[0011] The present invention has been completed in view of the above circumstances, and an object of the present invention is to provide a manufacturing method of a lithium tantalate single crystal substrate that suffers only little warpage (or bulging), hardly has cracks or scratches, and undergoes less property change with temperature than a conventional rotated Y-cut LiTaO3 substrate, and provides a high electromechanical coupling coefficient and high device Q values; the invention also aims to provide a bonded substrate obtained by bonding the above-mentioned lithium tantalate single crystal substrate, a method for manufacturing the above bonded substrate, and ultimately a surface acoustic wave device using such a substrate.

[0012] As a result of extensive studies to achieve the above object, the inventors of the present invention found that it is possible to obtain a piezoelectric oxide single-crystal substrate which, when used as a surface acoustic wave element or the like, suffers only little warpage, hardly has cracks or scratches, and undergoes reduced property changes with temperature without having to go so far as to modify the substrate to have a crystalline structure with a uniform Li concentration in a region close to the core of the substrate in the thickness direction, if the following procedure is carried out, namely, applying a gas-phase Li diffusion treatment to a substrate having a substantially congruent composition to thereby create therein such a modified region in which a Li concentration profile taken in a thickness direction,a higher Li concentration at a measurement point closer to the substrate surface and a lower Li concentration at a measurement point closer to the substrate core. Additionally, the inventors found that the area of modification by Li diffusion, as well as whether or not the singular polarization treatment is performed, tend to affect the Q value of the device, and thus, they acquired the present invention. Means to solve the problem

[0013] Therefore, the lithium tantalate single crystal substrate of the present invention is a rotated Y-cut LiTaO3 single crystal substrate according to claim 1 having a crystal orientation of 36°Y - 49°Y -cut, characterized in that it has undergone Li diffusion from its surface to its depth, with the result that the Li concentration profile shows a difference in Li concentration between the surface of the substrate and an inner part of the substrate; and it has undergone a singular polarization treatment, with the result that the Li concentration is roughly uniform from the surface of the substrate to a depth which is about 5 to 15 times the wavelength of the surface acoustic wave or a leaky surface acoustic wave (LSAW) propagating in the LiTaO3 substrate surface.

[0014] In the present invention, the Li concentration profile shows that the Li concentration is higher at a point closer to the surface of the rotated Y-cut LiTaO3 substrate, and the Li concentration is lower at a point closer to the core of the substrate. In the present invention, it is preferable that the ratio of Li to Ta at the surface of the substrate is such that: Li:Ta = 50 - α:50 + α, where α is in the range of -0.5 < α < 0.5. In the present invention, Fe is doped in the substrate at a concentration of 25 ppm to 150 ppm.

[0015] Additionally, the lithium tantalate single-crystal substrate of the present invention may be bonded to a base substrate to form a bonded substrate. In this case, it is preferable to remove the LiTaO3 surface layer from the surface opposite to the bonding surface in such a manner that at least a portion where the Li concentration is substantially uniform is removed to form a bonded substrate. Also, the base substrate is preferably made of Si, SiC, or spinel. Moreover, the manufacturing method of a bonded substrate according to the present invention is characterized in that a LiTaO3 single crystal substrate having a substantially uniform Li concentration is bonded to a base substrate to thereby leave at least a part of the portion in which the Li concentration is substantially uniform, or that the LiTaO3 surface layer is removed from the surface facing away from the bonding surface to leave only the portion in which the Li concentration is substantially uniform, and the method is also characterized in that the portion in which the Li concentration is substantially uniform has a pseudo-stoichiometric composition.

[0016] The lithium tantalate single crystal substrate and the bonded substrate of the present invention are suitable as a material for the surface acoustic wave device. Effects of the invention

[0017] According to the present invention, it is possible to provide a lithium tantalate single-crystal substrate with better temperature independence characteristics than conventional rotated Y-cut LiTaO3 substrates, with a large electromechanical coupling coefficient, and with a high device Q. In addition, the surface acoustic wave device using this single-crystal substrate can be provided at a low cost and is suitable for the broadband band required for smartphones. BRIEF DESCRIPTION OF THE DRAWINGS [ Fig. 1] A diagram showing a Raman profile of Example 1. [ Fig.2] A diagram showing an insertion loss waveform of the SAW filter of Example 1. [ Fig. 3] A diagram showing SAW resonator waveforms of Example 1. [ Fig. 4] A graph showing values calculated using the SAW resonator waveform, the input impedance (Zin) real part / imaginary part display waveform, and the BVD model of Example 1. [ Fig. 5] A graph showing values calculated using measured values of the SAW resonator input impedance (Zin) in the cases of Example 1 and Comparative Examples 2 and 4 and the calculated value in the case of the BVD model, where the real part is taken on the horizontal axis, and the imaginary part is taken on the vertical axis. [ Fig. 6] A transmission electron micrograph of a bonded substrate of Example 5 taken over a region of the interface between LiTaO3 and Si. Embodiments for carrying out the invention

[0018] Hereinafter, embodiments of the present invention will be described in detail, but the present invention is not limited to this embodiment.

[0019] The lithium tantalate single crystal substrate of the present invention has a concentration profile in which the Li concentration differs between the substrate surface and an inner part of the substrate. From the viewpoint of ease of manufacture, it is preferable that the substrate have a region in which the concentration profile is such that the Li concentration is higher in a region closer to the substrate surface in the thickness direction and the substrate, and the Li concentration is lower in a region closer to the substrate core. Such a substrate having a region exhibiting the above-described Li concentration profile can be easily manufactured by diffusing Li from the substrate surface by any known method. Here, the "concentration profile" refers to a continuous (not stepwise) change in the concentration.

[0020] The lithium tantalate single-crystal substrate of the present invention is characterized by having a substantially uniform Li concentration in a region between its surface and its depth, which is 5 to 15 times the wavelength of a surface acoustic wave or a bulk acoustic wave propagating in the surface of the LiTaO3 substrate. This is because a LiTaO3 substrate having a region where the Li concentration is substantially uniform from the substrate surface to a depth equivalent to at least 5 times the wavelength of a surface acoustic wave or a bulk acoustic wave propagating in the surface of the LiTaO3 substrate has approximately the same or larger Q value compared to a LiTaO3 substrate not subjected to Li diffusion treatment.When the region with the substantially uniform Li concentration is set to a depth exceeding 15 times the wavelength, it requires a ridiculously long time to diffuse Li, resulting in poor productivity, and in addition, the longer the time of Li diffusion, the greater the probability that the substrate suffers warpage or cracking.

[0021] The Li concentration of a lithium tantalate single crystal can be evaluated by measuring the Raman shift peak. Regarding a lithium tantalate single crystal, it is known that a substantially linear relationship can be obtained between the half-width of the Raman shift peak and the Li concentration, i.e., Li / (Li + Ta). [See non-IP publication: 2012 IEEE International Ultrasonics Symposium Proceedings, pages 1252-1255, Applied Physics A 56, 311-315 (1993)]. Therefore, by using a formula representing such a relationship, it is possible to evaluate the composition at any position on the oxide single crystal substrate.

[0022] A formula representing a relationship between the half-width of the Raman shift peak and the Li concentration is obtained by measuring the Raman half-width for some samples with a known composition and different Li concentrations; as long as the Raman measurement conditions are the same, it will be sufficient to use a formula already described in the literature, etc. For example, for a lithium tantalate single crystal, the following formula (1) can be used. Li / (Li+Ta)=(53.15−0.5 FWHM1) / 100 where “FWHM1” is the half-width of the Raman shift peak around 600 cm -1 For details of the measurement conditions, reference is made to any relevant publications.

[0023] For the purpose of the present invention, “the region in which the Li concentration is substantially uniform, ranging from the substrate surface” means a region in which the half-width of the Raman shift peak is around 600 cm -1 in the range of ±0.2cm -1 or so of that at the surface of the substrate, or the range in which the value Li / (Li + Ta) is in the range of ±0.001 or so of that at the surface of the substrate.

[0024] The lithium tantalate single crystal substrate of the present invention is characterized by having undergone a singular polarization treatment, since this treatment makes the Q value of the substrate larger than in the case of a substrate without polarization treatment; it is preferable that this polarization treatment be performed after the Li diffusion treatment.

[0025] Furthermore, in the lithium tantalate single crystal substrate of the present invention, the ratio of Li to Ta at the substrate surface is Li: Ta = 50 - α: 50 + α, where α is in the range of -0.5 < α < 0.5. This is because when the ratio of Li to Ta at the substrate surface is within the above range, the surface can be considered to have a pseudo-stoichiometric composition and exhibits particularly excellent temperature independence characteristics.

[0026] The lithium tantalate single crystal substrate of the present invention can be manufactured, for example, by subjecting an oxide single crystal substrate having a substantially congruent composition to a vapor phase treatment for diffusing Li from the surface of the substrate into the interior thereof. Oxide single crystal substrates having a substantially congruent composition can be obtained by producing a single crystal ingot by a known method such as the Czochralski method, cutting the ingot into wafers, and lapping or polishing the wafer as needed.

[0027] Furthermore, the lithium tantalate single crystal substrate of the present invention is doped with Fe at a concentration of 25 ppm to 150 ppm. This is because the lithium tantalate single crystal substrate doped with Fe at a concentration of 25 ppm to 150 ppm allows Li to diffuse at a rate approximately 20% faster than in the case without Fe doping, thus significantly improving the productivity of the Li-diffused lithium tantalate wafer—hence its preference. As a procedure for doping Fe into a lithium tantalate single crystal substrate, it is possible to add an appropriate amount of Fe2O3 to the raw material when pulling a single crystal ingot by the Czochralski method.

[0028] Furthermore, the polarization treatment to be performed in the present invention can be performed by any known method. Regarding the vapor phase treatment, although it is performed in the following examples with the substrate embedded in a powder consisting mainly of Li3TaO4, it should be noted that the invention is not limited to the type or form of the materials used in the vapor phase treatment performed in the examples. Furthermore, with respect to the substrate subjected to the vapor phase treatment, additional processing or treatment may be performed as needed.

[0029] The lithium tantalate single-crystal substrate of the present invention can be bonded to various base substrates to form a bonded substrate. The base substrate to which the inventive substrate is bonded is not particularly limited and can be selected according to the purpose; however, it is preferably made of Si, SiC, or spinel.

[0030] In the case of manufacturing a bonded substrate of the present invention, it is also possible to remove the LiTaO3 surface layer from the surface facing away from the bonding surface in such a manner that at least a part of the region in which the Li concentration is substantially uniform remains, so as to obtain a bonded substrate having excellent characteristics for a surface acoustic wave device.

[0031] The surface acoustic wave device obtained using the lithium tantalate single crystal substrate or the bonded substrate of the present invention will have excellent temperature independence characteristics and is particularly suitable for a component of a fourth-generation mobile phone or the like. Examples

[0032] Hereinafter, examples of the present invention and comparative examples will be described more specifically. (Example 1)

[0033] In Example 1, a 10.16 cm (4 inch) diameter singly polarized lithium tantalate single-crystal ingot with a substantially congruent composition and a Li:Ta ratio of 48.5:51.5 was first sliced to obtain several 370-µm-thick, 42° rotated Y-cut lithium tantalate substrates. Then, for protocol purposes, the surface roughness of each sliced wafer was adjusted to 0.15 µm in terms of arithmetic average roughness Ra with one lapping step, and the machined thickness was adjusted to 350 µm (micrometers).

[0034] Subsequently, both side surfaces of the substrates (wafers) were finished to a quasi-mirror finish with an Ra value of 0.01 µm by planar polishing, and the substrates were embedded in a powder composed of Li, Ta, and O, mainly in the form of Li3TaO4. The powder, mainly in the form of Li3TaO4, used in this example was prepared by mixing Li2CaO3 and Ta2O5 powders in a molar ratio of 7:3 in this order and subjecting the resulting mixture to calcination at 1300°C for 12 hours. The powder, mainly in the form of Li3TaO4, was spread in a small container, and a plurality of slice wafers were embedded in the Li3TaO4 powder.

[0035] This small container was then placed in an electric furnace, and the interior of the furnace was filled with an N2 atmosphere. The furnace was then electrically heated to a temperature of 975°C for 100 hours, causing Li to diffuse from the surface of the sliced wafer toward its center. Afterward, while the wafer temperature decreased, the wafer was subjected to a 12-hour annealing treatment at 800°C. Then, as the temperature decreased from 770°C to 500°C, an electric field of approximately 4000 V / m was applied substantially in the +Z direction; and then the temperature was allowed to drop to room temperature.After this treatment, one side of the wafer was subjected to a final processing consisting of sandblasting, which gave this side an Ra value of approximately 0.15 µm; in contrast, the other, quasi-mirror-machined surface was subjected to 3 µm polishing, and in this way, a plurality of lithium tantalate single-crystal substrates were prepared.

[0036] Regarding one of these lithium tantalate single crystal substrates, a laser Raman spectrometer (LabRam HR series, manufactured by HORIBA Scientific Inc., Ar ion laser, spot size 1 µm, room temperature) was used to measure the half-width of the Raman shift peak by 600 cm -1 which is an indicator of the amount of Li diffusion with respect to a depth-related distance from the surface at any position which was 1 cm or more away from the outer periphery of the disc-shaped substrate; and as the result, a value as shown in Fig.The Raman profile shown in Figure 1 was obtained.

[0037] According to the result of the Fig. 1, while the value of the Raman half-width at the surface of this lithium tantalate single crystal substrate was different from that in a deep part of the substrate, the value of the Raman half-width was more or less constant, namely between 5.9 and 6.0 cm -1 in the range from a depth of 0 µm to approximately 18 µm in the thickness direction. In the deeper range, it was confirmed that the Raman half-width value tends to increase as the measurement point approaches the center of the substrate.

[0038] The Raman half-width at a depth of 80 µm in the thickness direction of the lithium tantalate single crystal substrate was 9.3 cm -1 and, although not shown in the figure, the Raman half-width at the thickness-center position of the substrate was also 9.3 cm -1 .

[0039] From the above results of Fig. 1, it was confirmed that in Example 1, the Li concentration in the vicinity of the substrate surface and that inside the substrate differ from each other, and that the substrate has a region exhibiting a concentration profile such that the Li concentration is higher in regions closer to the substrate surface, and the Li concentration decreases with the depth of the substrate in the thickness direction. It was also confirmed that the Li concentration was generally uniform up to a depth of 18 μm from the LiTaO3 substrate surface.

[0040] Furthermore, the results of Fig. 1 the Raman half-width is about 5.9 - 6.0 cm -1from the surface of the lithium tantalate single crystal substrate to the depth of 18 µm in the thickness direction, therefore, using the equation (1), the composition in this region is approximately Li / (Li + Ta) = 0.515 to 0.52 inclusive, so that it was confirmed that the composition was pseudo-stoichiometric.

[0041] Furthermore, since the Raman half-width at the central part in the thickness direction of the substrate of the lithium tantalate single crystal is about 9.3 cm -1 , if formula (1) is assumed similarly to the above, the value of Li / (Li + Ta) becomes 0.485, therefore it was confirmed that the central part of the substrate had a substantially congruent composition.

[0042] In the case of the rotated Y-cut LiTaO3 substrate of Example 1, the region between the substrate surface and the position where the Li concentration begins to decrease, as well as the region between the position where the Li concentration stops increasing and the other side surface of the substrate, have a pseudo-stoichiometric composition, and the central part in the thickness direction has a substantially congruent composition. The position where the Li concentration begins to decrease or the position where the Li concentration stops increasing were each 20 μm from the substrate surface in the thickness direction.

[0043] Next, the warpage of this 10.16 cm (4 in) lithium tantalate single crystal substrate subjected to Li diffusion was measured by an interference measurement method using laser light, and the value was as small as 60 µm, and no chipping or cracking was observed.

[0044] Next, a small piece was cut from the 10.16 cm (4 inch) Li-diffused 42° rotated Y-cut lithium tantalate single-crystal substrate. In a d33 / d15 piezometer (model ZJ-3BN) manufactured by the Institute of Acoustics of the Chinese Academy of Sciences, the small piece was subjected to vertical vibration in the thickness direction toward the main surface and also toward the back surface to observe the voltage waveform induced thereby. A waveform was observed at every position over the entire wafer, indicating the presence of a piezoelectric response. Thus, it was confirmed that the lithium tantalate single-crystal substrate of Example 1 exhibits piezoelectricity at every location on the substrate surface and can thus be utilized as a singularly polarized surface acoustic wave device.

[0045] Next, a 42° Y-cut lithium tantalate single crystal substrate of Example 1, which had been subjected to Li diffusion treatment, was subjected to sputtering treatment to form an Al film with a thickness of 0.2 µm on its surface, and a resist material was applied to the thus-treated substrate; then, a single-stage ladder-type filter and an electrode pattern for a resonator were exposed and developed in a stepper, and an electrode for a SAW device was fabricated by RIE (Reactive Ion Etching).

[0046] The wavelength of this patterned single-stage ladder-type filter electrode was set to 2.33 µm for the series resonator, and the wavelength of the parallel resonator was set to 2.47 µm. Furthermore, a single resonator was configured to have a wavelength of 2.50 µm for evaluation purposes.

[0047] Regarding this single-stage ladder-type filter, the SAW waveform characteristics were investigated using an RF probe, and the Fig. 2 results were obtained. In Fig. 2 are the results of the measurement of the SAW waveform in the case of a 42° Y-cut lithium tantalate single crystal substrate which was not subjected to Li diffusion treatment and was formed with a similar electrode as described above, as well as in Fig. 2 shown.

[0048] From the Fig.From the results shown in Figure 2, it was confirmed that in the SAW filter made from a 42° Y-cut lithium tantalate single crystal substrate subjected to Li diffusion treatment, the frequency range at which the insertion loss was 3 dB or less was 93 MHz and the center frequency was 1745 MHz. In contrast, in the SAW filter made from a 42° Y-cut lithium tantalate single crystal substrate not subjected to Li diffusion treatment, the frequency range at which the insertion loss was 3 dB or less was 80 MHz and the center frequency was 1710 MHz.

[0049] Likewise, as the temperature of the stage was raised from about 16°C to 70°C, the antiresonance frequency, which corresponds to the frequency of the dip on the right in Fig.2, and the temperature coefficient of the resonant frequency corresponding to the frequency of the dip on the left were examined. As a result, it was confirmed that since the temperature coefficient of the resonant frequency was -21 ppm / °C and the temperature coefficient of the anti-resonant frequency was -42 ppm / °C, the average frequency temperature coefficient was -31.5 ppm / °C. For comparison, the temperature coefficient of the 42° Y-cut lithium tantalate single crystal substrate that was not subjected to Li diffusion treatment was similarly examined. As a result, it was confirmed that since the temperature coefficient of the resonant frequency was -33 ppm / °C and the temperature coefficient of the anti-resonant frequency was -43 ppm / °C, the average frequency temperature coefficient was -38 ppm / °C.

[0050] Therefore, from the above results, it was confirmed that in the lithium tantalate single-crystal substrate of Example 1, the band in which the filter insertion loss was 3 dB or less was 1.2 times wider than that of the substrate that had not undergone Li diffusion treatment. Regarding the temperature independence characteristics, the average frequency temperature coefficient was approximately 6.5 ppm / °C lower than that of the substrate that had not undergone Li diffusion treatment, so the property fluctuation with temperature was small, and thus the stability against temperature change was confirmed to be good.

[0051] Next, a 1-port SAW resonator with a wavelength of 2.5 µm was fabricated from a 42° Y-cut lithium tantalate single crystal substrate subjected to the Li diffusion treatment of Example 1, and the Fig. 3 was obtained. In Fig.For comparison, in Figure 3, a similar 1-port SAW resonator is also fabricated from a 42° Y-cut lithium tantalate single crystal substrate which was not subjected to Li diffusion treatment, and the results in the case of the thus obtained SAW waveforms are also shown in the figure.

[0052] From the results of the SAW waveforms of Fig. 3, the values of the antiresonance frequency and the resonance frequency were obtained, and the electromechanical coupling coefficient k2 was calculated based on the following equation 2; as shown in Table 1, in the case of the 42° Y-cut lithium tantalate single crystal substrate subjected to the Li diffusion treatment of Example 1, the electromechanical coupling coefficient k2 was 7.7%, which was about 1.2 times larger than that in the case of the 42° Y-cut lithium tantalate single crystal substrate not subjected to Li diffusion treatment.

[0053] Equation to obtain K2: K2=(πfr / 2fa) / tan(πfr / 2fa) Where fr is the resonance frequency and fa is the antiresonance frequency.

[0054] Fig. 4 shows the relationship between the real / imaginary parts of the input impedance (Zin) and the frequency for the SAW resonator of Example 1, and also shows Fig. 4 the calculated value of the input impedance obtained using the following equation (3) according to the BVD model (see John D. et al., “Modified Butterworth-Van Dyke Circuit for FBAR Resonators and Automated Measurement System”, IEEE ULTRASONICS SYMPOSIUM, 2000, pp. 863-868). From the results of graphs A and B in Fig. 4, it was confirmed that the input impedance value measured in Example 1 is in very good agreement with the calculated value according to the BVD model.

[0055] Furthermore, Table 1 shows the results of the Q value as calculated using the following formula (3), and Fig. Figure 5 shows the measured values of the Q-circle of the SAW resonator together with the calculated values according to the BVD model. Now in the Q-circle the real part of the input impedance (Zin) is plotted against the horizontal axis and the imaginary part of the input impedance (Zin) is plotted against the vertical axis.

[0056] From the result of the Q-circle curve C in Fig. 5, it was confirmed that the input impedance value measured in Example 1 and the calculated value according to the BVD model were in good agreement, so the Q values obtained by Equation (3) shown below according to the BVD model can be considered reasonable values. Furthermore, with the Q compass, it can be judged that when the radius is approximately large, the Q value is also large.

[0057] Additionally, Table 1 and Fig. 5 For comparison purposes, the results in the case of a 42° Y-cut lithium tantalate single crystal substrate that has not been subjected to Li diffusion treatment (see the Q-circle of curve D in Fig. 5), and it was confirmed that the Q of Example 1 shows a value equal to or even higher than the Q of the 42° Y-cut lithium tantalate single crystal substrate which has not been subjected to Li diffusion treatment. z(ω)=Xpj⋅(ωωp)⋅[1−(ωωs)2+j⋅(ωωs)⋅1Qso][1−(ωωp)2+j⋅(ωωp)1Qpo] where: r=c0c1 ωs=1L1⋅C1 (ωpωs)2=1+1r Xp=1ωp⋅C0 1Qs=ωs⋅R1⋅C1 1Qe=ωs⋅R0⋅C0r 1Qso=1Qs⋅(1+RsR1) 1Qpo=(ωpωs)⋅(1Qs+1Qc) <Beispiel 2>

[0058] In Example 2, a lithium tantalate single crystal substrate with a roughly uniform Li concentration in a range from the surface of the substrate to a depth of 18 µm was first prepared using the same method as in Example 1. Next, the surface of the substrate was lapped to a depth of 2 µm, thereby obtaining a lithium tantalate single crystal substrate with a roughly uniform Li concentration in a range from the surface of the substrate to a depth of 16 µm.

[0059] Then, the thus obtained lithium tantalate single crystal substrate was evaluated in the same manner as in Example 1, and the results are shown in Table 1. When normalized with the wavelength of the leaky surface acoustic wave (LSAW) propagating in the X direction of the wafer, the region in which the Li concentration is uniform ranged from the substrate surface to a depth equivalent to 6.4 times the wavelength.

[0060] Compared with the 42° Y-cut lithium tantalate single crystal substrate which was not subjected to the Li diffusion treatment, the lithium tantalate single crystal substrate of Example 2 had a larger electromechanical coupling coefficient k2, a better temperature independence characteristic, and the Q values were similar to or larger on average than those of the previous one. <Beispiel 3>

[0061] Likewise, in Example 3, a lithium tantalate single crystal substrate having a region in which the Li concentration was substantially uniform from the substrate surface to a depth of 18 µm was first prepared in the same manner as in Example 1. Next, the surface of the substrate was lapped to a depth of 4 µm, thereby obtaining a lithium tantalate single crystal substrate having a substantially uniform Li concentration in a region from the surface of the substrate to a depth of 14 µm.

[0062] Then, the thus obtained lithium tantalate single crystal substrate was evaluated in the same manner as in Example 1, and the results are shown in Table 1. When normalized with the wavelength of the LSAW propagating in the X direction of the wafer, the range in which the Li concentration is uniform also ranged from the substrate surface to a depth equivalent to 5.6 times the wavelength.

[0063] Compared with the 42° Y-cut lithium tantalate single crystal substrate which was not subjected to Li diffusion treatment, the lithium tantalate single crystal substrate of Example 3 exhibited a larger electromechanical coupling coefficient k 2 showed a better temperature independence characteristic and the Q values were similar or on average larger than those of the previous one. <Beispiel 4>

[0064] Likewise, in Example 4, a lithium tantalate single crystal substrate having a region in which the Li concentration was substantially uniform from the substrate surface to a depth of 18 µm was first prepared in the same manner as in Example 1. Next, the surface of the substrate was lapped to a depth of 5.5 µm, thereby obtaining a lithium tantalate single crystal substrate having a substantially uniform Li concentration in a region from the surface of the substrate to a depth of 12.5 µm.

[0065] Then, the thus obtained lithium tantalate single crystal substrate was evaluated in the same manner as in Example 1, and the results are shown in Table 1. When normalized with the wavelength of the LSAW propagating in the X direction of the wafer, the range in which the Li concentration is uniform also ranged from the substrate surface to a depth equivalent to 5.0 times the wavelength.

[0066] Compared with the 42° Y-cut lithium tantalate single crystal substrate which was not subjected to the Li diffusion treatment, the lithium tantalate single crystal substrate of Example 4 had a larger electromechanical coupling coefficient k2, a better temperature independence characteristic, and the Q values were similar to or larger on average than those of the previous one. <Beispiel 5>

[0067] In Example 5, a lithium tantalate single-crystal substrate having a region in which the Li concentration is substantially uniform, ranging from the substrate surface to a depth of 18 µm, was first prepared in the same manner as in Example 1. Next, this substrate and a 200 µm-thick Si substrate were bonded to each other using a room-temperature bonding method described in the non-IP publication [Takagi H. et al., “Room-temperature wafer bonding using argon beam activation” from Proceedings-Electrochemical Society (2001), 99-35 (Semiconductor Wafer Bonding: Science, Technology, and Applications V), 265-274.], and a bonded substrate was fabricated.Specifically, a cleaned substrate was placed in a high vacuum chamber, and an activation treatment was performed on the substrate by irradiating it with a high-speed atom beam of argon in which the ion beam was neutralized at the substrate surface; thereafter, the lithium tantalate single crystal substrate and the Si substrate were bonded together.

[0068] The bonding interface of the bonded substrate was examined with a scanning electron microscope and it was observed that Fig. 6 showed that the pseudo-stoichiometric composition LiTaO3 and the atoms of Si were mixed together at the bonding interface to form a tight bond.

[0069] In addition, this bonded substrate consisting of the rotated Y-cut LiTaO3 substrate diffused with Li and the silicon substrate was lapped and polished on the LiTaO3 side in such a manner that a LiTaO3 layer with a thickness of 18 µm, measured from the bonding interface, remained, whereupon the bonded substrate of the present invention was completed.

[0070] Next, the bonded substrate thus obtained was evaluated in the same manner as in Example 1, and the results are as shown in Table 2. From these results, it was also confirmed that the bonded substrate of Example 5 also had a large electromechanical coupling coefficient value and a large Q value and had excellent temperature independence characteristics. <Beispiel 6>

[0071] In Example 6, first, a lithium tantalate single crystal substrate having a region in which the Li concentration was substantially uniform, ranging from the substrate surface to a depth of 18 µm, was prepared in the same manner as in Example 1. Next, this substrate and a Si substrate having a thickness of 200 µm were bonded by a normal temperature bonding method described in the above-mentioned non-IP publication, and thus a bonded substrate was obtained.

[0072] The bonding interface of this bonded substrate was observed with a scanning electron microscope, and it was observed, as in the case of Example 5, that the pseudo-stoichiometric composition LiTaO3 and the atoms of Si were mixed together at the bonding interface to form a strong bond.

[0073] In addition, this bonded substrate consisting of the rotated Y-cut LiTaO3 substrate diffused with Li and the silicon substrate was lapped and polished on the LiTaO3 side in such a manner that a LiTaO3 layer with a thickness of 1.2 µm, measured from the bonding interface, remained, whereupon the bonded substrate of the present invention was completed.

[0074] Next, the bonded substrate thus obtained was evaluated in the same manner as in Example 1, and the results are as shown in Table 2. From these results, it was also confirmed that the bonded substrate of Example 5 also had a large electromechanical coupling coefficient value and a large Q value and had excellent temperature independence characteristics. Comparison examples

[0075] In the comparative examples shown below, lithium tantalate single crystal substrates were prepared by the same method as in Example 1, except that no singular polarization treatment was performed thereon. <Vergleichsbeispiel 1 >

[0076] In Comparative Example 1, no electric field was applied in approximately the +Z direction during the temperature reduction period from 770°C to 500°C after the Li diffusion treatment (thus, the singular polarization treatment was not performed), but with respect to other points, the lithium tantalate single crystal substrate was prepared in the same manner as in Example 1.

[0077] It was confirmed that the lithium tantalate single crystal substrate of Comparative Example 1 shows a similar Raman profile to that in Example 1, and that the lithium tantalate single crystal substrate has a substantially uniform Li concentration up to a depth of 18 µm from the substrate surface.

[0078] Next, a small piece was cut out of the 10.16 cm (4 inch) Li-diffused 42°Y-cut lithium tantalate single-crystal substrate obtained in Comparative Example 1. In a piezometer d33 / d15 (model ZJ-3BN) manufactured by the Institute of Acoustics of the Chinese Academy of Sciences, the small piece was subjected to vertical vibration in the thickness direction toward the main surface and the back surface, respectively, to observe the voltage waveform induced thereby. The observation indicated the absence of piezoelectric response from any part of the wafer. Thus, it was confirmed that the lithium tantalate single-crystal substrate of Example 1 did not exhibit thickness-wise piezoelectricity at any part of the substrate surface and was not singularly polarized.

[0079] On the other hand, when this small piece was placed in the d15 unit and a vibration in the horizontal direction parallel to the substrate was applied, a piezoelectric response in the thickness direction could be recorded, so that the lithium tantalate single crystal substrate of Comparative Example 1 was found to be converted into a conventional piezoelectric body which exhibits piezoelectricity when subjected to vibration in the horizontal direction parallel to the substrate surface, although it does not exhibit a piezoelectric response in the thickness direction in response to a vibration received in the thickness direction.

[0080] The same evaluation as in Example 1 was performed on the lithium tantalate single crystal substrate of Comparative Example 1, and the results are as shown in Table 1. From these results, it was confirmed that, compared with the 42° Y-cut lithium tantalate single crystal substrate that had not undergone Li diffusion treatment, the lithium tantalate single crystal substrate of Comparative Example 1 had a larger electromechanical coupling coefficient k2 and superior temperature independence characteristics, while its Q values were lower. <Vergleichsbeispiel 2>

[0081] In Comparative Example 2, first, a lithium tantalate single crystal substrate having a substantially uniform Li concentration in a region ranging from the substrate surface to a depth of 18 µm was prepared by the same method as in Example 1. Next, the surface of this substrate was polished by 8 µm to prepare a lithium tantalate single crystal substrate having a substantially uniform Li concentration to a depth of 10 µm from the substrate surface.

[0082] The lithium tantalate single crystal substrate of Comparative Example 2 was evaluated in the same manner as in Example 1, and the results are shown in Table 1. Moreover, when normalized with the wavelength of the LSAW propagating in the X direction of the wafer, the region in which the Li concentration was uniform existed from the substrate surface to a depth of 4 times the wavelength.

[0083] From these results, it was confirmed that, compared with the 42° Y-cut lithium tantalate single crystal substrate which had not undergone Li diffusion treatment, the lithium tantalate single crystal substrate of Comparative Example 2 had a larger electromechanical coupling coefficient k2 and exhibited superior temperature independence characteristics, while its Q values were lower, as shown by the Q-circle curve in Fig. 5 shown. <Vergleichsbeispiel 3>

[0084] In Comparative Example 3, first, a lithium tantalate single crystal substrate having a substantially uniform Li concentration in a region ranging from the substrate surface to a depth of 18 µm was prepared by the same method as in Example 1. Next, the surface of this substrate was polished by 12 µm to prepare a lithium tantalate single crystal substrate having a substantially uniform Li concentration to a depth of 8 µm from the substrate surface.

[0085] The lithium tantalate single crystal substrate of Comparative Example 3 was evaluated in the same manner as in Example 1, and the results are shown in Table 1. Moreover, when normalized with the wavelength of the LSAW propagating in the X direction of the wafer, the region in which the Li concentration was uniform existed from the substrate surface to a depth of 3.2 times the wavelength.

[0086] From these results, it was confirmed that, compared with the 42° Y-cut lithium tantalate single crystal substrate that had not undergone Li diffusion treatment, the lithium tantalate single crystal substrate of Comparative Example 3 had a larger electromechanical coupling coefficient k2 and exhibited superior temperature independence characteristics, while its Q values were lower. <Vergleichsbeispiel 4>

[0087] In Comparative Example 4, first, a lithium tantalate single crystal substrate having a substantially uniform Li concentration in a region ranging from the substrate surface to a depth of 18 µm was prepared by the same method as in Example 1. Next, the surface of this substrate was polished by 14 µm to prepare a lithium tantalate single crystal substrate having a substantially uniform Li concentration to a depth of 6 µm from the substrate surface.

[0088] The lithium tantalate single crystal substrate of Comparative Example 4 was evaluated in the same manner as in Example 1, and the results are shown in Table 1. Moreover, when normalized with the wavelength of the LSAW propagating in the X direction of the wafer, the region in which the Li concentration was uniform existed from the substrate surface to a depth of 2.4 times the wavelength.

[0089] From these results, it was confirmed that, compared with the 42° Y-cut lithium tantalate single crystal substrate which had not undergone Li diffusion treatment, the lithium tantalate single crystal substrate of Comparative Example 3 had a larger electromechanical coupling coefficient k2 and exhibited superior temperature independence characteristics, while its Q values were lower, as shown by the Q-circle curve in Fig. 5 shown. [Table 1] Depth from the substrate surface over which the Li concentration is uniform (µm) Depth from the substrate surface over which the Li concentration is uniform, normalized with respect to the multiple of the wavelength of the LSAW propagating in the LiTaO3 substrate surface (x wavelength) Qs Qe Q sO Q pO Q mittel Resonance frequency (MHz) Antiresonance frequency (MHz) k2 (%) Temperature coefficient (ppm / °C) Example 1 18 7,2 957 1118 900 500 869 1658,0 1712,0 7,7 -21 Example 2 16 6,4 1070 1204 600 550 856 1659,0 1712,0 7,5 -22 Example 3 14 5,6 957 1118 900 500 869 1659,0 1712,5 7,6 -21 Example 4 12,5 5,0 1020 1100 750 550 855 1658,0 1712,0 7,7 -23 Comparison example 1 18 7,2 700 370 274 455 450 1653,0 1709,0 7,9 -23 Comparison example 2 10 4,0 801 682 750 360 648 1659,0 1712,8 7,6 -22 Comparison example 3 8 3,2 773 603 600 330 577 1658,0 1713,0 7,8 -23 Comparison example 4 6 2,4 804 241 150 180 344 1653,5 1708,0 7,7 -23 Without eyelid diffusion treatment - - 1106 1202 500 560 842 1628,0 1672,0 6,4 -33 [Table 2] Thickness of LiTaO3 in the bonded substrate (µm) Qs Qe Q sO Q pO Q mittel Resonance frequency (MHz) Antiresonance frequency (MHz) k2 (%) Temperature coefficient of resonance frequency (ppm / °C) Temperature coefficient of the antiresonance frequency (ppm / °C) Example 5 18 1535 1453 1500 1150 1410 1685,0 1743,0 8,1 -10 -20 Example 6 12 1950 1847 1700 1837 1834 1724,0 1803,0 10,5 10 -10 Description of reference symbols A: Curves (solid line and dotted line) showing the Im (Zin) measured values and values calculated according to the BVD model in Fig. 4 represent. B: Curves (solid line and dotted line) showing the Re (Zin) measured values and values calculated according to the BVD model in Fig. 4 represent. C: Q-circle curves in Fig.5, showing the measured values of the input impedance (Zin) of Example 1 (solid line) and the values calculated according to the BVD model (dotted line). D: Q-circle curves in Fig. 5, which show the measured values of the input impedance (Zin) in the case of no Li diffusion treatment (solid line) and calculated values according to the BVD model (dotted line). E: Q-circle curves in Fig. 5, which represent measured values of the input impedance (Zin) of Comparative Example 2 (in the case where the depth of the uniform Li concentration region from the surface is 10 µm) (solid line) and calculated values according to the BVD model (dotted line). F: Q-circle curves in Fig.5, which represent measured values of the input impedance (Zin) of Comparative Example 4 (in the case where the depth of the uniform Li concentration region from the surface is 6 µm) (solid line) and calculated values according to the BVD model (dotted line).

Claims

[1] A rotated Y-cut lithium tantalate (LiTaO3) single crystal substrate with a crystal orientation of 36°Y - 49°Y-cut, characterized bythat: the substrate is diffused with Li from its surface to its depth such that it has a Li concentration profile that shows a difference in Li concentration between the substrate surface and the depth of the substrate; and the substrate is treated with a singular polarization treatment such that the Li concentration is substantially uniform from the substrate surface to a depth corresponding to 5 to 15 times the wavelength of either a surface acoustic wave or a volume fraction surface acoustic wave (LSAW;leaky surface acoustic wave) propagating in the LiTaO3 substrate surface, wherein the substrate is doped with Fe in an amount of 25 ppm - 150 ppm and the Li concentration profile is one in which the Li concentration is higher in a region closer to the substrate surface of the rotated Y-cut LiTaO3 substrate and lower in a region closer to the center of the substrate, and wherein the region extending from the substrate surface in which the Li concentration is substantially uniform is a region in which the half-width of the Raman shift peak is around 600 cm; -1 in the range of ±0.2cm -1 which is at the surface of the substrate, or a region in which the value Li / (Li + Ta) is in the range of ±0.001 of that at the surface of the substrate. [2] Lithium tantalate single crystal substrate according to claim 1, characterized bythat a ratio of Li to Ta at the substrate surface is such that Li: Ta = 50 - α : 50 + α, where α is in the range of -0.5 < α < 0.

5. [3] Use of a lithium tantalate single crystal substrate according to claim 1 or 2 in a bonded substrate comprising a base substrate and the lithium tantalate single crystal substrate, wherein the lithium tantalate single crystal substrate is bonded to the base substrate. [4] Use according to claim 3, characterized by that a LiTaO3 surface layer facing away from the bonding surface of the lithium tantalate single crystal substrate is removed in such a way that at least a part of the area where the Li concentration is substantially uniform remains. [5] Use according to claim 3 or 4, characterized by that the base substrate is made of Si, SiC or spinel. [6] Use of a lithium tantalate single crystal substrate according to claim 1 or 2 in a surface acoustic wave device, characterized by that it contains either the lithium tantalate single crystal substrate itself or as the bonded substrate according to any one of claims 3 to 5. [7] Method for producing a bonded substrate, characterized bythe steps of bonding a base substrate to a LiTaO3 single-crystal substrate according to claim 1 or 2, which has a concentration profile in which the Li concentration is different between the substrate surface and an inner part of the substrate, and wherein the Li concentration is substantially uniform in a region extending from at least one of the surfaces of the substrate to a depth, and removing a LiTaO3 surface layer facing away from the bonding surface in such a manner that at least a part of the region where the Li concentration is substantially uniform remains, wherein the substrate is doped with Fe in an amount of 25 ppm - 150 ppm and the Li concentration profile is one in which the Li concentration is higher in a region closer to the substrate surface of the rotated Y-cut LiTaO3 substrate and lower in a region closer to the center of the substrate, and wherein the region,which extends from the substrate surface in which the Li concentration is substantially uniform, is a region in which the half-width of the Raman shift peak is around 600 cm, -1 in the range of ±0.2cm -1 which is at the surface of the substrate, or a region in which the value Li / (Li + Ta) is in the range of ±0.001 of that at the surface of the substrate. [8] A manufacturing method according to claim 7, wherein the removal of a LiTaO3 surface layer is carried out in such a manner that only the region where the Li concentration is substantially uniform remains. [9] A method of manufacturing a bonded substrate according to any one of claims 7 or 8, characterized by that the region in which the Li concentration is essentially uniform has a pseudo-stoichiometric composition.

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