JP2017098719A - Joined substrate, and elastic surface wave device using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a joined substrate superior in temperature characteristic, which is arranged so as to suppress an unwanted response owing to the reflection of an elastic wave at a junction interface.SOLUTION: A joined substrate is arranged by joining a LiTaOsubstrate with a base. The Li concentration in a joint face at which the LiTaOsubstrate is in contact with the base is larger than the Li concentration in a surface of the joined substrate on the side of the LiTaOsubstrate. The LiTaOsubstrate has a multi-domain structure in a range on the joint face side with a large Li concentration; its Li concentration in the joint face is given by Li/(Li+Ta)×100=(50+α) mol%, where α satisfies -1.0<α<0.5, and the range is formed so as to extend to 0.5-5 times of the wavelength of a surface acoustic wave or leak surface acoustic wave from the joint face of the LiTaOsubstrate toward the surface of the joined substrate on the LiTaOsubstrate side.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a bonded substrate obtained by bonding a lithium tantalate single crystal substrate to a base substrate, and a surface acoustic wave device using the same.

  Surface acoustic wave (SAW) devices in which interdigital transducers (IDT) for exciting surface acoustic waves are formed on piezoelectric substrates are used as components for frequency adjustment and selection such as cellular phones. It has been.

The surface acoustic wave device is required to have a small size, a low insertion loss, and a performance that does not pass unnecessary waves. As a material thereof, lithium tantalate (LiTaO ₃ ; LT) or lithium niobate (LiNbO ₃ ; LN) is used. A piezoelectric material such as is used.

  Communication bands used in mobile phones in recent years tend to have narrower inter-band spacings and wider individual bandwidths. Therefore, for surface acoustic wave devices whose characteristics change when the temperature changes, the temperature characteristics There is a need for something superior.

  As an example of a piezoelectric material that meets these requirements, for example, Non-Patent Document 1 reports that the use of a substrate bonded with lithium tantalate and sapphire can reduce fluctuations in the frequency of the surface acoustic wave device due to temperature. ing.

M. Miura, T. Matsuda, Y. Satoh, M. Ueda, O. Ikata, Y. Ebata, and H. Takagi, “Temperature Compensated LiTaO3 / SapphireBonded SAW Substrate with Low Loss and High Coupling Factor Suitable for US-PCS Application , ”Proc. IEEE. Ultrason. Symp., Pp. 1322-1325, 2004.

JP2013-46107A

  However, in the substrate reported in Non-Patent Document 1, since two types of members having different acoustic impedances are combined, the elastic wave excited on the lithium tantalate surface is reflected at the bonding interface and manifests as an unnecessary response. There is.

  According to Patent Document 1, it is reported that spurious response can be suppressed by bonding a lithium tantalate supporting substrate having a multi-domain to a lithium tantalate element substrate. However, although spurious response can be suppressed, since the same type of lithium tantalate substrate is used for the element substrate and the support substrate, sufficient temperature characteristics can be obtained even if the direction of the support substrate relative to the element substrate is a direction with a low coefficient of thermal expansion. There is a problem that cannot be obtained.

  Therefore, the present invention has been made in view of the above circumstances, and an object of the present invention is to provide a bonded substrate that has excellent temperature characteristics and suppresses unnecessary response due to reflection of elastic waves at the bonded interface, and a surface acoustic wave using the bonded substrate. Is to provide a device.

The present inventors have made intensive studies on LiTaO ₃ substrates constituting the bonded substrate, LiTaO ₃ Li concentration of the bonding surface side of the substrate is greater than the Li concentration of the surface side of the LiTaO ₃ substrate, the wave of the elastic wave Has the property of moving toward a region with low Li concentration (low velocity region), so energy is confined near the surface of the LiTaO ₃ substrate where the velocity of elastic waves is low (region with low Li concentration). It has been found that elastic wave energy is concentrated and bulk waves are not easily radiated into the substrate. Furthermore, further investigation has revealed that the spurious response due to the reflection of bulk waves can be further suppressed by making the range on the joint surface side where the Li concentration is larger into a multi-domain state, leading to the present invention. It is a thing.

That is, the present invention provides a bonded substrate formed by bonding a LiTaO ₃ substrate and the base substrate, Li concentration in the joint surfaces in contact with the base substrate LiTaO ₃ substrate, Li the LiTaO ₃ substrate side surface of the bonded substrate It is characterized by a multi-domain structure in the range of the bonding surface side where the Li concentration is higher than the concentration.

The Li concentration at the joint surface in contact with the base substrate of the LiTaO ₃ substrate of the present invention is Li / (Li + Ta) × 100 = (50 + α) mol%, and α is in the range of −0.5 <α <0.5. it is preferable, the range toward the LiTaO ₃ substrate side surface of the bonded substrate from the bonding surface of the LiTaO ₃ substrate, it is preferably formed over the 0.5 to 5 times the wavelength of the surface acoustic wave or leaky surface acoustic wave .

The bonding substrate of the present invention, towards the LiTaO ₃ substrate side surface of the bonded substrate from the bonding surface of the LiTaO ₃ substrate has a range of Li concentration decreases, a range in which the Li concentration is decreased, the surface acoustic wave Alternatively, it is preferably formed over 1 to 5 times the wavelength of the leaky surface acoustic wave. In addition, in the range where the Li concentration decreases, the range from the joining surface side to 0.5 times is preferably a multi-domain structure, and further the range from the joining surface side to 0.8 times is more preferably a multi-domain structure. .

Further, the Li concentration of the LiTaO ₃ substrate side surface of the bonding substrate of the present invention is Li / (Li + Ta) × 100 = (48.5 + β) mol%, and β is in the range of −0.25 <β <0.25. Preferably, the range is formed from 1 to 10 times the wavelength of the surface acoustic wave or leaky surface acoustic wave from the LiTaO ₃ substrate side surface of the bonded substrate toward the bonded surface of the LiTaO ₃ substrate. .

The crystal orientation of the LiTaO ₃ substrate of the present invention is preferably a rotational 36 ° Y to 49 ° Y cut, and the base substrate used in the present invention is preferably selected from Si, SiC, and spinel.

   The bonded substrates of the present invention are preferably used for surface acoustic wave devices.

  According to the present invention, it is possible to eliminate a response due to an unnecessary reflected wave that causes a problem in a surface acoustic wave bonded substrate, and therefore, an elastic surface that has a small loss and can improve the out-of-band suppression degree when the filter is used. It is possible to provide a wave bonding substrate.

It is a diagram illustrating a LiTaO ₃ model of Li content profiles of substrates constituting the bonded substrate of Example 1. It is the result of having calculated the transverse wave amplitude in the depth direction from the substrate surface of the bonded substrate model of Example 1 and the bonded substrate model of Comparative Example 1. FIG. It is the result of having calculated the frequency response of the SAW resonator using the joint substrate model of Example 1 and Comparative Example 1. FIG. It is a diagram illustrating a LiTaO ₃ Li content profiles of substrates constituting the bonded substrate of Example 2. It is the result of having measured the frequency response of the SAW resonator using the joining board | substrate of Example 2 and Comparative Example 2. FIG.

  Hereinafter, although an embodiment of the present invention is described in detail, the present invention is not limited to this.

The bonded substrate of the present invention is configured by bonding a LiTaO ₃ substrate and a base substrate. As a method for bonding the LiTaO ₃ substrate and the base substrate, a known method may be used, but considering the influence on the warpage of the substrate, it is preferable to use a room temperature bonding method.

Bonded substrate of the present invention is characterized in that Li concentration at the joint surface in contact with the base substrate of LiTaO ₃ substrate is greater than the Li concentration of the LiTaO ₃ substrate side surface of the bonded substrate. Therefore, the LiTaO ₃ substrate bonded to the base substrate has a difference in Li concentration in the thickness direction.

Such a LiTaO ₃ substrate can be obtained, for example, by diffusing Li from the surface of the LiTaO ₃ substrate by a known method such as a vapor phase diffusion method. Since the substrate thus obtained has a large Li concentration on the substrate surface and a small Li concentration inside the substrate, the interior of the LiTaO ₃ substrate is formed so that the substrate surface becomes a bonding surface with the base substrate. If processing such as grinding and polishing is performed, the bonded substrate of the present invention can be obtained.

Further, the LiTaO ₃ substrate constituting the bonded substrate of the present invention may be a bonded substrate composed of a plurality of LiTaO ₃ substrates. The bonded substrate of the present invention can also be manufactured by preparing a LiTaO ₃ substrate having a low Li concentration and a LiTaO ₃ substrate having a high Li concentration and bonding them to a base substrate. For example, a small LiTaO ₃ substrate of Li concentration, consistent prepared by conventional pulling method melted using LiTaO ₃ substrate (congruent) composition, whereas, as a large LiTaO ₃ substrate of Li concentration, double crucible method It is possible to use a LiTaO ₃ substrate with a stoichiometric composition manufactured by

Further, the LiTaO ₃ substrate constituting the bonded substrate of the present invention may be doped with a metal element such as Fe as necessary, or may be subjected to a reduction treatment for suppressing pyroelectricity. .

Key features of the present invention is greater than the Li concentration of the LiTaO ₃ substrate-side surface of the Li concentration bonding substrate in the bonding surface in contact with the base substrate LiTaO ₃ substrate, and perhaps pass structure, the bonding surface of the LiTaO ₃ substrate The Li concentration in is preferably Li / (Li + Ta) × 100 = (50 + α) mol%, and α is preferably in the range of −1.0 <α <0.5.
Here, the “multidomain structure” in the present invention represents a state disordered from the structure of a single domain state. This multi-domain structure is not necessarily a state in which the piezoelectric characteristics are completely lost, and may exhibit a response of the piezoelectric characteristics, but is not in a uniform single-domain state.

The LiTaO ₃ substrate that constitutes the bonded substrate of the present invention is produced by a vapor phase method on a LiTaO ₃ substrate having a congruent composition produced by a normal pulling method from the viewpoint of ease of production and manufacturing cost. It is preferable to manufacture by performing Li diffusion treatment. When the LiTaO ₃ substrate was fabricated in this way, the Li concentration on the substrate surface was Li / (Li + Ta) × 100 = (50 + α) mol% (stoichiometric composition). -1.0 <α <0.5).

Also, when the Li diffusion treatment is performed, the longer the time, the more likely the substrate is warped or cracked, so the Li concentration is Li / (Li + Ta) × 100 = (50 + α) mol% and α is -1.0 <alpha <range is 0.5, towards the LiTaO ₃ substrate side surface of the bonded substrate from the bonding surface of the LiTaO ₃ substrate, is formed over the 0.5 to 5 times the wavelength of the surface acoustic wave or leaky surface acoustic wave Preferably it is. In this way, it is possible to suppress the occurrence of warping and cracking of the substrate.

In the present invention, the LiTaO ₃ substrate subjected to the Li diffusion treatment is processed at a temperature higher than the Curie temperature, whereby the entire substrate can have a multi-domain structure. Then, by performing a poling process using a difference in Curie temperature caused by a difference in Li concentration, only the surface side of the LiTaO ₃ substrate can be in a single domain state, and the base substrate side can have a multi-domain structure. .

By performing such a polling process, it is possible to suppress reflection of unnecessary waves at the bonding interface between the LiTaO ₃ substrate and the base substrate. Incidentally, since the Curie temperature of the coincidence melting (congruent) composition is about 600 ° C. and the Curie temperature of the stoichiometric composition is about 700 ° C., it is preferable to perform the treatment at a temperature in between. In addition, in the case of performing Li diffusion treatment, it is possible to make the entire substrate have a multi-domain structure by raising the LiTaO ₃ substrate above the Curie temperature.

Further, in the present invention, from the joint surface of the LiTaO ₃ substrate LiTaO ₃ substrate side surface of the bonded substrate, it is preferable to have a range of Li concentration decreases. By using a LiTaO ₃ substrate that has been subjected to Li diffusion treatment by a vapor phase method as a LiTaO ₃ substrate as a bonded substrate, such a bonded substrate can be manufactured. In this case, the junction substrate with a sharp change in Li concentration can suppress the reflection of unwanted waves at the junction interface between the LiTaO ₃ substrate and the base substrate, but response noise may occur due to a sharp change in Li concentration. There is.

Therefore, towards the LiTaO ₃ substrate side surface of the bonded substrate from the bonding surface of the LiTaO ₃ substrate, the range Li concentration is decreased, that is formed over 1-5 times the wavelength of the surface acoustic wave or leaky surface acoustic wave Is preferred. In this way, not only manufacturing is easy, but also response noise due to a sharp change in Li concentration can be suppressed. In the range where the Li concentration decreases, the range from the junction surface side to 0.5 times is preferably a multi-domain state, and the range from the joint surface side to 0.8 times is more preferably a multi-domain structure.

  By the way, regarding the wave passing through the domain of the multi-domain structure, the longitudinal domain of the multi-domain structure has no piezoelectricity because it does not have piezoelectricity, Attenuation occurs because it is a region. On the other hand, the transverse wave that passes through the region of the multi-domain structure usually has an electrical mismatch with the LT composition part due to incomplete piezoelectricity. In addition, since the region has a multi-domain structure, attenuation occurs.

This region of the multi-domain structure is elastically faster in sound speed than that of the LT composition part. Therefore, especially for a wave that penetrates into the region of the multi-domain structure at a shallow angle, the wave part normally enters the LT composition part. It will work to rebound. Therefore, in general, in the composition LT, particularly in the rotation Y-cut, the main wave shear horizontal (wave near the transverse wave having a displacement component that travels in the X direction and is horizontal on the substrate surface) is a rough congruent of the bonded substrate of the present invention. Energy is concentrated on the surface layer of LiTaO _{3 having} the ent composition.

In the bonding substrate of the present invention, the Li concentration at the bonding surface of the LiTaO ₃ substrate is Li / (Li + Ta) × 100 = (50 + α) mol%, and α is in the range of −1.0 <α <0.5. On the other hand, the Li concentration on the LiTaO ₃ substrate side surface is Li / (Li + Ta) × 100 = (48.5 + β) mol%, and β is in the range of −0.25 <β <0.25. Is preferred.

As described above, the bonded substrate of the present invention is Li diffusion by vapor phase method to LiTaO ₃ substrate of congruent melting (congruent) composition manufactured by a normal pulling method from the viewpoint of ease of manufacture and manufacturing cost. It is preferable to produce by processing. Therefore, the Li concentration inside the LiTaO ₃ substrate is Li / (Li + Ta) × 100 = (48.5 + β) mol% (-0.25 <β <0.25), which is close to the congruent composition. ₃ substrate after bonding the base substrate, grinding to expose the interior of the LiTaO ₃ substrate, if Hodokose machining such as polishing, Li concentration of LiTaO ₃ substrate side surface of the bonded substrate is, Li / (Li + Ta) X100 = (48.5 + β) mol%, and a bonded substrate in which β is in the range of −0.25 <β <0.25 can be obtained.

Further, the range in which the Li concentration is Li / (Li + Ta) × 100 = (48.5 + β) mol% and β is −0.25 <β <0.25 can be arbitrarily determined. However, if this range is narrowed relative to the thickness of the LiTaO ₃ substrate that constitutes the bonded substrate, it is necessary to lengthen the Li diffusion treatment time, and the substrate tends to warp and crack, so this Li concentration range Is preferably somewhat wide. It is preferable that the bonding substrate is formed over the surface of the LiTaO ₃ substrate side from the surface of the LiTaO ₃ substrate to the bonding surface of the LiTaO ₃ substrate over about 1 to 10 times the wavelength of the surface acoustic wave or leakage surface acoustic wave.

The crystal orientation of the LiTaO ₃ substrate constituting the present invention can be arbitrarily selected, but is preferably a rotation of 36 ° Y to 49 ° Y from the viewpoint of characteristics. The substrate used as the base substrate is not particularly limited, but is preferably selected from Si, SiC, and spinel.

  If a surface acoustic wave device is manufactured using such a bonded substrate of the present invention, the response due to unnecessary reflected waves at the bonded interface can be removed, so that loss is small and out-of-band suppression when filtering is performed. The degree can be improved.

The Li concentration of the LiTaO ₃ substrate constituting the bonded substrate of the present invention may be measured by a known method, but can be evaluated by, for example, Raman spectroscopy. For LiTaO ₃ single crystal, it is known that there is a roughly linear relationship between the half-width of the Raman shift peak and the Li concentration (Li / (Li + Ta) value) (2012 IEEE International Ultrasonics Symposium Proceedings, Page (s): 1252-1255, Applied Physics A 56, 311-315 (1993)).
Therefore, by using an expression representing such a relationship, the composition at an arbitrary position of the oxide single crystal substrate can be evaluated.

  The relationship between the half-width of the Raman shift peak and the Li concentration is obtained by measuring the Raman half-width of several samples with known compositions and different Li concentrations, but the Raman measurement conditions are the same. If so, a relational expression that has already been clarified in literature may be used.

For example, for lithium tantalate single crystal, the following formula (1) may be used (see 2012 IEEE International Ultrasonics Symposium Proceedings, Page (s): 1252-1255).
Li / (Li + Ta) = (53.15-0.5FWHM1) / 100 (1)
Here, “FWHM1” is the full width at half maximum of the Raman shift peak near 600 cm ⁻¹ . Refer to the literature for details of the measurement conditions.

  Hereinafter, examples and comparative examples of the present invention will be described more specifically.

<Example 1>
Example 1 relates to a bonded substrate formed by bonding a 42 ° rotated Y-cut LiTaO ₃ substrate and a base substrate. In Example 1, a bonded substrate was prepared in which the Li concentration of the bonding surface contacting the base substrate of the LiTaO ₃ substrate was first higher than the Li concentration of the LiTaO ₃ substrate side surface of the bonded substrate, and the bonding surface side had a multi-domain structure. . From the model in which LiTaO ₃ with a large Li concentration has a multidomain structure, the horizontal displacement intensity of shear horizontal waves (SH waves) propagating in the X-axis direction of the LiTaO ₃ crystal, which is the main wave mode, The change with thickness direction was calculated by computer analysis. In this calculation, the surface layer of LiTaO ₃ is assumed to be electrically open.

Specifically, with reference to the wavelength of the SH wave, a rough congruent with a Li concentration of 48.5 mol% in the surface layer range of 3 wavelengths out of the 20 wavelength thickness range from the surface to the bonding surface position shown in FIG. The range of the ent composition LT (CLT) up to 17 wavelengths deeper than that is a junction substrate having a Li concentration change of SLT in which the Li concentration is 50 mol%. Then, the horizontal displacement of the shear horizontal wave (SH wave) propagating in the X-axis direction of the LiTaO ₃ crystal was calculated for the multi-domain structure of LiTaO ₃ having a large Li concentration.

  FIG. 2 shows the calculation result. According to the result of FIG. 2, the amplitude of the SH wave is a wave in which energy is concentrated on the surface layer that becomes almost zero in the range of about 4 wavelengths from the surface.

Next, for the same bonded substrate model, the input impedance when the gap between the Al electrode line of the Inter digital Transducer (IDT) electrode made of Al with a thickness of 0.08 wavelength and the gap is 1: 1 with respect to the wavelength of the SH wave. As a result of calculation, the result of the broken line shown in FIG. 3 was obtained.
Here, the wavelength of the SH wave was 0.47 μm.

According to the result of FIG. 3, in Example 1, spurious modes other than the main resonance / antiresonance peak were not observed. Therefore, in Example 1, since there was no reflection leaking from the surface layer of LiTaO ₃ , it was confirmed that the response due to unnecessary reflected waves was eliminated by suppressing the wave component reflected at the bonding interface.

<Comparative Example 1>
In Comparative Example 1, a bonded substrate model was assumed in which a LiTaO ₃ substrate (SLT) having a 42 ° rotated Y-cut Li concentration of 50 mol% and a base substrate were bonded. Incidentally, the bonding substrate model, the Li concentration of the Li concentration and LiTaO ₃ substrate-side surface of the joint surface in contact with the base substrate of LiTaO ₃ substrate is the same 50 mol%.

Next, for a bonded substrate model in which a LiTaO ₃ substrate with a Li concentration of 50 mol% is bonded, a shear wave that propagates in the X-axis direction of a LiTaO ₃ crystal with a Li concentration of 50 mol% as a main wave mode ( Based on the wavelength of the SH wave, the change in the horizontal displacement intensity of the shear horizontal wave (SH wave) propagating in the X-axis direction in the thickness range of 20 wavelengths was calculated by computer analysis.
In this calculation, the surface layer of LiTaO ₃ is assumed to be electrically open.

  The calculation results are shown in FIG. According to the result of Comparative Example 1 in FIG. 2, it was found that the amplitude component of the SH wave still has an amplitude component even in the region of 7 wavelengths, and the wave leaks also in a region deeper than 7 wavelengths.

Next, for the same bonded substrate model, the input impedance when the gap between the Al electrode line of the Inter digital Transducer (IDT) electrode made of Al with a thickness of 0.08 wavelength and the gap is 1: 1 with respect to the wavelength of the SH wave. As a result, a result as shown by a solid line in FIG. 3 was obtained.
Here, the wavelength of the SH wave was 0.47 μm.

According to the result of Comparative Example 1 in FIG. 3, many spurious modes were observed in addition to the main resonance / antiresonance peak. These spurious modes are responses caused by reflected waves that are reflected by the base substrate and returned by the waves excited in the LiTaO ₃ surface layer of the bonded substrate leaking in the depth direction. The response by these reflected waves is an unnecessary mode when configuring a filter or the like, and it is not preferable for the filter or the like that there are many unnecessary modes.

<Example 2>
In Example 2, first, a 4-inch diameter LiTaO ₃ single crystal ingot having a substantially congruent composition with Li: Ta = 48.5: 51.5 subjected to single domain treatment was sliced, and a 42 ° rotation Y cut was performed. A LiTaO ₃ substrate was cut out to a thickness of 370 μm. Thereafter, if necessary, the surface roughness of each slice wafer was adjusted to 0.15 μm in arithmetic mean roughness Ra value by a lapping process, and the finished thickness was set to 350 μm.

Next, a substrate whose front and back surfaces were finished to a quasi-mirror surface with an Ra value of 0.01 μm by planar polishing was embedded in a powder composed of Li, Ta, and O containing Li ₃ TaO ₄ as main components. At this time, as a powder mainly composed of Li ₃ TaO ₄ , Li ₂ CO ₃ : Ta ₂ O ₅ powder was mixed at a molar ratio of 7: 3 and baked at 1300 ° C. for 12 hours. . Then, such a powder mainly composed of Li ₃ TaO ₄ was spread in a small container, and a plurality of sliced wafers were embedded in the Li ₃ TaO ₄ powder. Then, this small container was set in an electric furnace, and the inside of the furnace was made an N ₂ atmosphere and heated at 900 ° C. for 20 hours to diffuse Li from the surface of the slice wafer to the center. After that, in the temperature lowering process of this process, an annealing process is performed at 800 ° C. for 12 hours, and an electric field of about 4000 V / m is applied in the + Z-axis direction between 630 ° C. and 500 ° C. in the process of further cooling the wafer. The temperature was lowered to room temperature.

In addition, after this treatment, the rough surface side is finished by sandblasting to an Ra value of about 0.15 μm, and the rough mirror side is polished by 3 μm to produce a plurality of LiTaO ₃ single crystal substrates. did. When this substrate was observed with a piezo d33 / d15 meter (model ZJ-3BN) manufactured by the Institute of Chinese Vocal Music, the vertical and vertical vibrations in the thickness direction were applied to the main and back surfaces, respectively. The voltage was 0V and the piezoelectric constant d33 was also zero.

Further, a LiTaO ₃ single crystal substrate having a surface layer / back surface polished by 10 μm was prepared for another wafer prepared in the same manner as described above. When this was observed with a piezo d33 / d15 meter (model ZJ-3BN), the voltage waveform induced by applying vertical vibration in the thickness direction to the main and back surfaces of each was observed, and voltage response was observed. d33 is 13.5 pc / N, which is the value indicated by the normal congruent composition LiTaO ₃ . From this result, it was confirmed that the surface layer of 10 μm has a multidomain structure.

Next, a LiTaO ₃ substrate and a 500 μm thick Si substrate that are not subjected to 10 μm polishing as described above on the surface layer and the back surface are referred to as “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.

Specifically, a cleaned substrate is set in a high vacuum chamber, and a high-speed atomic beam of argon with neutralized ion beam is irradiated on the substrate surface for activation treatment, and then a LiTaO ₃ single crystal substrate And Si substrate were joined.

In addition, grinding and polishing were performed to leave a range of 30 μm from the bonding interface of this bonded substrate to the LiTaO ₃ side, and a bonded substrate of a rotating Y-cut LiTaO ₃ substrate and a Si substrate in which Li was diffused was produced. .

Then, for one of the bonded substrates thus produced, using a laser Raman spectrometer (LabRam HR series manufactured by HORIBA Scientific, Ar ion laser, spot size 1 μm, room temperature) From the surface of the LiTaO ₃ side, the half-value width “FWHM1” of the Raman shift peak near 600 cm ^−1, which is an index of the amount of Li diffusion, is measured from the LiTaO ₃ side surface, and the Li concentration is obtained from the above equation (1). The profile of Li concentration shown was obtained.

According to the result of FIG. 4, in this bonded substrate, the Li concentration in the bonded surface contacting the base substrate of the LiTaO ₃ substrate is 49.6 mol%, and the Li concentration on the LiTaO ₃ substrate side surface of the bonded substrate is 48.5 mol%. some because, Li concentration in the joint surfaces in contact with the base substrate LiTaO ₃ substrate, it was confirmed that is larger than the Li concentration of the LiTaO ₃ substrate side surface of the bonded substrate.

Further, in the range from 0 μm to about 3 μm from the bonding interface toward the LiTaO ₃ substrate side surface of the bonding substrate, Li / (Li + Ta) = 49.6 mol%, which is a value indicating a pseudo stoichiometric composition. The range close to the surface layer of the bonded substrate, a transition layer as Li concentration close to LiTaO ₃ substrate side surface of the bonding substrate is reduced has over a range of about 12 [mu] m, from LiTaO ₃ substrate side surface of the bonded substrate of 13μm Over the depth, it was confirmed that Li / (Li + Ta) = 48.5 mol% and a Li concentration exhibiting a rough congruent composition.

  Further, when the warpage of the bonded substrate was measured by an interference method using a laser beam, the value was as small as 60 μm, and cracks and cracks were not observed.

Next, an Al film having a thickness of 0.2 μm is formed by sputtering on the LiTaO ₃ substrate side surface of the bonding substrate thus obtained, and one wavelength is about 4 μm by photolithography using a g-line as a light source. A ladder filter was formed by connecting one resonator in series and one in parallel.
The Al electrode was formed by RIE (reactive ion etching), and a gas mixture of BCl ₃ , Cl ₂ , CF ₄ , and N ₂ was used as the RIE gas.

  And when the characteristic of this produced ladder filter for evaluation was measured with AR prober, the measurement result is as shown in FIG.

According to the result of FIG. 5, in Example 2, spurious modes other than the main resonance / antiresonance peak were not observed. Therefore, even in Example 2, since there is no reflection leaking from the surface layer of LiTaO ₃ , it was confirmed that the response due to unnecessary reflected waves was eliminated by suppressing the wave component reflected at the bonding interface.

  Next, the ladder filter chip is cut out by dicing, fixed to the evaluation package, wired by wire bonding, and the package is hermetically sealed with AuSn with LID, and the temperature coefficient of the hermetically sealed ladder filter is calculated. It was measured.

  When the frequency of the insertion loss of the left shoulder of the filter was −20 dB and the temperature was changed in the range of −30 ° C. to 85 ° C., the temperature coefficient was −13 ppm / ° C. Further, the frequency at which the insertion loss of the right shoulder of the filter was −30 dB was measured by changing the temperature in the range of −30 ° C. to 85 ° C., and the temperature coefficient was −27 ppm / ° C.

<Comparative Example 2>
In Comparative Example 2, a single-domain treatment Li: Ta = 48.5: 51.5-sliced 4 inch diameter LiTaO ₃ single crystal ingot with a roughly congruent composition and a 42 ° rotated Y-cut LiTaO ₃ substrate Was cut into a thickness of 370 μm. Thereafter, if necessary, the surface roughness of each slice wafer was adjusted to 0.15 μm in arithmetic mean roughness Ra value by a lapping process, and the finished thickness was set to 350 μm.

Next, the surface roughness of both surfaces of the wafer after lapping is finished to a quasi-mirror surface with a Ra value of 0.01 μm by surface polishing, and one side is finished to about 0.15 μm with a Ra value by sandblasting, and the other side The rough mirror surface side of this was polished to 3 μm to prepare a plurality of LiTaO ₃ single crystal substrates having a substantially congruent composition. After that, as in Example 2, this LiTaO ₃ substrate and a 230 μm thick Si substrate were bonded, and further, grinding and polishing were performed so as to leave a range of 30 μm on the LiTaO ₃ side from the bonding interface of this bonded substrate. Thus, a bonded substrate of a rotating Y-cut LiTaO ₃ substrate and a Si substrate was produced.

Then, with respect to one of the bonded substrates thus manufactured, in the same manner as in Example 2, a laser Raman spectrometer was used to diffuse Li from the LiTaO ₃ side surface to the depth direction at the center of the bonded substrate. The half-value width “FWHM1” of the Raman shift peak near 600 cm ^−1, which is an index of the amount, was measured and the Li concentration was determined from the above equation (1). The LiTaO ₃ substrate constituting the bonded substrate was It was confirmed that a substantially congruent composition of 48.5 mol% was exhibited in the almost constant direction.

  Further, when the warpage of the bonded substrate was measured by an interference method using a laser beam, the value was as small as 60 μm, and cracks and cracks were not observed.

Next, an Al film having a thickness of 0.4 μm is formed on the LiTaO ₃ substrate side surface of the bonding substrate thus obtained by sputtering, and one wavelength is about 5 μm by photolithography using g-line as a light source. A ladder filter having four resonators connected in series and two in parallel was formed.
The Al electrode was formed by RIE (reactive ion etching), and a gas mixture of BCl ₃ , Cl ₂ , CF ₄ , and N ₂ was used as the RIE gas.

  And when the characteristic of this produced ladder filter for evaluation was measured with AR prober, the result as shown in FIG. 5 was obtained.

  According to the result of Comparative Example 2 in FIG. 5, since many spurious modes were observed in addition to the main resonance / anti-resonance peak, it was confirmed that undesirable reflected waves were generated in the filter and the like. .

  In addition, the ladder filter chip was cut out by dicing, fixed to the evaluation package, wired by wire bonding, and the package was hermetically sealed with AuSn with LID, and the temperature coefficient of the hermetically sealed ladder filter was measured. .

When the frequency of the insertion loss of the left shoulder of the filter was −20 dB and the temperature was changed in the range of −30 ° C. to 85 ° C., the temperature coefficient was −13 ppm / ° C. Further, the frequency coefficient of the insertion loss of the right shoulder of the filter was -30 dB, and the temperature coefficient was -27 ppm / ° C when the temperature was measured in the range of -30 ° C to 85 ° C.
Therefore, it was confirmed that the temperature coefficient of Comparative Example 2 was the same as that of Example 2 and there was no change.

Claims (10)

LiTaO ₃ a bonded substrate formed by bonding a substrate and a base substrate, the Li concentration in the joint surfaces in contact with the base substrate of the LiTaO ₃ substrate is greater than the Li concentration of the LiTaO ₃ substrate-side surface of the bonded substrate And a multi-domain structure in the range of the bonding surface side where the Li concentration is high. The Li concentration at the joint surface in contact with the base substrate of the LiTaO ₃ substrate is Li / (Li + Ta) × 100 = (50 + α) mol%, and α is in the range of −1.0 <α <0.5. The bonding substrate according to claim 1, characterized in that: The Li / (Li + Ta) × 100 = (50 + α) is mol%, alpha is -0.5 <alpha <0.5 range are the LiTaO ₃ substrate side surface of the bonded substrate from the bonding surface of the LiTaO ₃ substrate The bonded substrate according to claim 1, wherein the bonded substrate is formed over a range of 0.5 to 5 times the wavelength of the surface acoustic wave or the leaky surface acoustic wave. Towards the LiTaO ₃ substrate side surface of the bonded substrate from the bonding surface of the LiTaO ₃ substrate, the bonded substrate according to any one of claims 1 to 3 having a range of Li concentration decreases.   The bonding substrate according to claim 4, wherein the range in which the Li concentration decreases is formed over 1 to 5 times the wavelength of the surface acoustic wave or the leaky surface acoustic wave. The Li concentration of the LiTaO ₃ substrate side surface of the bonding substrate is Li / (Li + Ta) × 100 = (48.5 + β) mol%, and β is in a range of −0.25 <β <0.25. The bonded substrate according to any one of claims 1 to 5. Li / (Li + Ta) × 100 = (48.5 + β) mol%, and β is in the range of −0.25 <β <0.25 from the LiTaO ₃ substrate side surface of the bonding substrate to the bonding surface of the LiTaO ₃ substrate. 7. The bonding substrate according to claim 1, wherein the bonding substrate is formed over a wavelength of 1 to 10 times the wavelength of the surface acoustic wave or the leaky surface acoustic wave. The bonded substrate according to claim 1, wherein the crystal orientation of the LiTaO ₃ substrate is a rotation of 36 ° Y to 49 ° Y cut.    The bonded substrate according to claim 1, wherein the base substrate is selected from Si, SiC, and spinel. A surface acoustic wave device comprising the bonded substrate according to claim 1.

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