Joint body and elastic wave element

Abstract

To provide a joint body which can increase the Q-value of an elastic wave element.SOLUTION: A joint body 7 includes a supporting substrate 1 and a piezoelectric material layer 2C joined to the supporting substrate 1. The piezoelectric material layer 2C has a first main surface 9 joined to the supporting substrate 1 and a second main surface 3a opposed to the first main surface 9. The piezoelectric material layer 2C has an argon atom-containing layer 3 exposed to the second main surface 3a.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a bonded body of a piezoelectric material layer and a support substrate, and an elastic wave device.

Background Art

[0002] Elastic wave devices such as surface acoustic wave devices that can function as filter elements or oscillators used in mobile phones and the like, and Lamb wave devices and thin film resonators (FBAR: Film Bulk Acoustic Resonator) using piezoelectric thin films are known. As such an elastic wave device, one in which a support substrate and a piezoelectric material substrate for propagating a surface acoustic wave are bonded together, and a comb-shaped electrode capable of exciting a surface acoustic wave is provided on the surface of the piezoelectric material substrate is known. By attaching a support substrate having a thermal expansion coefficient smaller than that of the piezoelectric material substrate to the piezoelectric material substrate in this way, changes in the size of the piezoelectric material substrate when the temperature changes are suppressed, and changes in the frequency characteristics as a surface acoustic wave device are suppressed.

[0003] When manufacturing such a surface acoustic wave device, after bonding the piezoelectric material substrate on the support substrate, the exposed surface of the piezoelectric material substrate is ground and polished to reduce the thickness of the piezoelectric material substrate to, for example, 20 μm or less. Thereby, the characteristics of the surface acoustic wave can be improved.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, it has been found that there is still room for further improvement in the Q value of the surface acoustic wave device thus obtained. In particular, there is room for improvement in the range of 0.3 to 6.0 GHz.

[0006] An object of the present invention is to provide a bonded body capable of improving the Q value of an elastic wave element.

Means for Solving the Problems

[0007] The present invention relates to a bonded body including a support substrate and a piezoelectric material layer bonded to the support substrate, wherein the piezoelectric material layer has a first main surface bonded to the support substrate and a second main surface opposite to the first main surface, and has an argon atom-containing layer exposed on the second main surface.

[0008] Further, the present invention relates to an elastic wave element including the bonded body and an electrode provided on the second main surface of the piezoelectric material layer.

Effects of the Invention

[0009] The inventor bonded a piezoelectric material substrate to a support substrate, then thinned the surface (exposed surface) of the piezoelectric material substrate by polishing to form a piezoelectric material layer, and then variously examined the surface state of the piezoelectric material layer. However, significant improvement in the Q value of elastic waves could not be achieved by changing the degree of polishing, the polishing method, the grinding stone, etc.

[0010] Therefore, various methods for processing the surface of the piezoelectric material substrate were examined, and attempts were made to perform ion trimming with argon ions. As a result, a thin processed and modified layer was formed on the surface of the piezoelectric material layer. An electrode was formed thereon and an elastic wave element was fabricated, but there was still a limit to the improvement of the Q value.

[0011] When the ratio of each atom in the surface region of such a piezoelectric material layer was measured by EDX, it was found that in the processed and modified layer on the surface, the niobium atoms and tantalum atoms were few, and gradually increased in the depth direction from the surface. And when reaching a thickness of several nanometers from the surface of the piezoelectric material layer, it was found that the ratio of niobium atoms and tantalum atoms became 30 - 40 atom% and became almost stable. This is considered that the crystal structures of lithium niobate and lithium tantalum are considerably destroyed in the vicinity of the surface of the piezoelectric material layer. On the other hand, although there are no argon atoms in this processed and modified layer, it was found that there is an argon atom-containing layer containing a relatively large amount of argon atoms under it. Therefore, the inventor of the present invention removed the processed and modified layer, exposed the argon atom-containing layer, formed an electrode thereon, and tried to fabricate an elastic wave element. As a result, it was found that the Q value was remarkably improved, and the present invention was achieved.

[0012] The reason for obtaining such an effect is not clear, but it is considered that the propagation loss in the surface region of the piezoelectric material layer is reduced by the argon atom-containing layer, and the Q value is remarkably improved.

Brief Description of the Drawings

[0013]

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Mode for Carrying Out the Invention

[0014] Hereinafter, the present invention will be described in detail with reference to the drawings as appropriate. As shown in FIG. 1(a), the support substrate 1 and the piezoelectric material substrate 2 are joined to obtain a joined body. The piezoelectric material substrate 2 has a first main surface 9 and a second main surface 2a. Next, the second main surface 2a of the piezoelectric material substrate 2 is thinned by polishing to form a thin piezoelectric material layer base 2A as shown in FIG. 1(b). 2b is the polished surface.

[0015] Next, as shown in FIG. 1(c), argon ion trimming is performed on the polished surface 2b of the piezoelectric material layer 2A as indicated by the arrow A. As a result, as shown in the enlarged view in FIG. 2(a), a piezoelectric material layer 2B is generated. On the surface of the piezoelectric material layer 2B, a processed altered layer 4 is exposed, and an argon atom-containing layer 3 is generated directly below the processed altered layer 4. Reference numeral 5 is an unaltered portion that has not been altered by the processing.

[0016] Next, by removing the processed altered layer 4 by processing, a piezoelectric material layer 2C as shown in FIG. 2(b) is obtained. An argon atom-containing layer 3 is generated and exposed on the second main surface 3a side of the argon atom-containing layer 2C. As a result, a joined body 7 as shown in FIG. 2(c) is obtained. The joined body 7 is composed of the support substrate 1 and the piezoelectric material layer 2C joined to the support substrate 1. Next, as shown in FIG. 2(d), the elastic wave element 8 is fabricated by forming a predetermined electrode 6 on the second main surface 3a of the piezoelectric material layer 2C.

[0017] In the present invention, the support substrate may be made of a single crystal or a polycrystal. The material of the support substrate is preferably selected from the group consisting of silicon, sialon, sapphire, cordierite, mullite, and alumina. Alumina is preferably translucent alumina.

[0018] Silicon may be single-crystal silicon or polycrystalline silicon, and may also be high-resistance silicon. Sialon is a ceramic obtained by sintering a mixture of silicon nitride and alumina, and has the following composition. Si 6-w Al w O w N 8-w It is more preferable that w is 0.5 or more. Further, it is more preferable that w is 4.0 or less. Sapphire is a single crystal having the composition of Al2O3, and alumina is a polycrystal having the composition of Al2O3. Cordierite is a ceramic having the composition of 2MgO·2Al2O3·5SiO2. Mullite is a ceramic having a composition in the range of 3Al2O3·2SiO2 to 2Al2O3·SiO2.

[0019] The material of the piezoelectric material substrate is not limited as long as it has the necessary piezoelectricity, but a single crystal having the composition of LiAO3 is preferable. Here, A is one or more elements selected from the group consisting of niobium and tantalum. Therefore, LiAO3 may be lithium niobate, lithium tantalate, or a lithium niobate-lithium tantalate solid solution.

[0020] The support substrate and the piezoelectric material substrate may be directly bonded. The direct bonding method may be surface activation by plasma or a surface activation method by a neutralized atomic beam.

[0021] In a preferred embodiment, one or more bonding layers can be provided between the piezoelectric material substrate and the support substrate. Examples of the material of such a bonding layer include the following. SiO2, Si (1-v) O v (0.008 ≦ v ≦ 0.408) 、 Ta2O5, Al2O 3、 Nb2O5, TiO2

[0022] In the present invention, the piezoelectric material layer has an argon atom-containing layer exposed on the second main surface. Here, the argon atom-containing layer is a layer in which argon atoms are contained in the piezoelectric material. Specifically, when measured by EDX, a portion where the atomic ratio of argon atoms is 1 atom% or more is defined as the argon atom-containing layer. However, the atomic ratio of argon atoms in the argon atom-containing layer is usually 5 atom% or less.

[0023] In a preferred embodiment, the thickness of the argon atom-containing layer is 1 to 10 nm, more preferably 3 to 8 nm.

[0024] Also, in a preferred embodiment, the argon atom content in the argon atom-containing layer is on average 5 to 7 atomic%, more preferably 7 to 10 atomic%.

[0025] Further, in the argon atom-containing layer, the atomic ratio (total value) of atoms derived from the material constituting the non-transformed portion of the piezoelectric material layer is on average 99.0 to 99.9 atomic%, more preferably 99.5 to 99.9 atomic%. Here, the material constituting the non-transformed portion of the piezoelectric material layer is the piezoelectric material. When the piezoelectric material is LiAO3, it is the total value of the atomic ratio of element A and the atomic ratio of O (the atomic ratio of lithium is unmeasurable).

[0026] Here, the measurement of each atomic ratio by TEM-EDX is performed as follows. First, irradiate the object to be analyzed with an electron beam. By irradiating with the electron beam, characteristic X-rays are generated from the object to be analyzed. Since the energy of the characteristic X-rays is specific to each element, by measuring the type and number of occurrences of this energy, the ratio of each element is measured.

[0027] In order to obtain the bonded body of the present invention, the following method is preferable. First, polish the second main surface of the piezoelectric material substrate to thin the piezoelectric material substrate and form a piezoelectric material layer. At this time, it is preferable to planarize the main surface by precision polishing. Examples of the planarization method include lap polishing and chemical mechanical polishing (CMP). Further, the flatness of the main surface is preferably Ra ≦ 1 nm, and more preferably 0.3 nm or less.

[0028] Next, in order to remove residues of the abrasive and the processed and modified layer, it is preferable to clean the main surface of the piezoelectric material layer. Examples of the method for cleaning the main surface include wet cleaning, dry cleaning, and scrub cleaning. However, scrub cleaning is preferable in order to obtain a clean surface simply and efficiently.

[0029] Next, by performing argon ion trimming on the main surface of the piezoelectric material layer, a processed and modified layer and an argon atom-containing layer can be formed on the main surface side of the piezoelectric material layer. Argon ion trimming is a processing method that uses the sputtering phenomenon of knocking off the atoms on the surface of the workpiece by hitting the workpiece with Ar atoms accelerated by an electric field. In this case, the argon ion beam is converged and hit on the workpiece. The preferred conditions for argon ion trimming are as follows. Diameter of the converged ion beam: 10 mm or less Acceleration output: 120 W

[0030] From the viewpoint of device characteristics, the thickness of the piezoelectric material layer is preferably 1 μm or less, and more preferably 0.5 μm or less. Further, from the viewpoint of processability, the thickness of the piezoelectric material layer is preferably 0.1 μm or more.

[0031] The use of the bonded body of the present invention is not particularly limited, and for example, it can be suitably applied to elastic wave elements and optical elements. As elastic wave elements, surface acoustic wave devices, Lamb wave elements, thin film bulk acoustic resonators (FBAR), etc. are known. For example, a surface acoustic wave device is provided with an input-side IDT (Interdigital Transducer) electrode (also called a comb-shaped electrode or a grating electrode) for exciting a surface acoustic wave and an output-side IDT electrode for receiving the surface acoustic wave on the surface of a piezoelectric material substrate. When a high-frequency signal is applied to the input-side IDT electrode, an electric field is generated between the electrodes, and a surface acoustic wave is excited and propagates on the piezoelectric material substrate. Then, the propagated surface acoustic wave can be taken out as an electric signal from the output-side IDT electrode provided in the propagation direction.

[0032] The bottom surface of the piezoelectric material substrate may have a metal film. The metal film plays a role of increasing the electromechanical coupling coefficient in the vicinity of the back surface of the piezoelectric material substrate when manufacturing a Lamb wave element as an elastic wave device. In this case, the Lamb wave element has a structure in which comb teeth electrodes are formed on the surface of the piezoelectric material substrate, and the metal film of the piezoelectric material substrate is exposed by a cavity provided in the support substrate. Examples of the material of such a metal film include aluminum, aluminum alloy, copper, gold, etc. When manufacturing a Lamb wave element, a composite substrate provided with a piezoelectric material layer having no metal film on the bottom surface may also be used.

[0033] Further, the bottom surface of the piezoelectric material substrate may have a metal film and an insulating film. The metal film serves as an electrode when manufacturing a thin film bulk acoustic resonator as an elastic wave device. In this case, the thin film bulk acoustic resonator has a structure in which electrodes are formed on the front and back surfaces of the piezoelectric material substrate, and the metal film of the piezoelectric material substrate is exposed by making the insulating film into a cavity. Examples of the material of such a metal film include molybdenum, ruthenium, tungsten, chromium, aluminum, etc. Examples of the material of the insulating film include silicon dioxide, phosphorus silicate glass, boron phosphorus silicate glass, etc.

[0034] When the object of the present invention is an elastic wave element and the material of the piezoelectric material substrate is lithium tantalate, it is preferable to use a direction rotated 123 to 133° (for example, 128°) from the Y-axis to the Z-axis around the X-axis, which is the propagation direction of the elastic surface wave, because the propagation loss is small. When the piezoelectric material substrate is made of lithium niobate, it is preferable to use a direction rotated 86 to 94° (for example, 90°) from the Y-axis to the Z-axis around the X-axis, which is the propagation direction of the elastic surface wave, because the propagation loss is small. Further, the size of the piezoelectric material substrate is not particularly limited, but for example, it has a diameter of 50 to 150 mm and a thickness of 0.2 to 60 μm.

Example

[0035] (Example 1) An elastic surface wave element was prototyped by the method described with reference to FIGS. 1 and 2. Specifically, a lithium niobate substrate (LN substrate) having an OF portion, a diameter of 4 inches, and a thickness of 250 μm was used as the piezoelectric material substrate 2. The LN substrate had the propagation direction of the elastic surface wave (SAW) as X, and a 42° Y-cut X-propagation LN substrate with a cut-out angle being a rotated Y-cut plate was used. The first main surface 9 of the piezoelectric material substrate 2 was mirror-polished so that the arithmetic mean roughness Ra was 0.3 nm. However, Ra is measured in a 10 μm × 10 μm field of view by an atomic force microscope (AFM).

[0036] On the other hand, as the support substrate 1, a support substrate 1 made of silicon (Si(111)) having an orientation flat (OF) portion, a diameter of 4 inches, and a thickness of 500 μm was prepared. The surface of the support substrate 1 was finished by chemical mechanical polishing (CMP), and the arithmetic mean roughness Ra was 0.2 nm. Next, the main surface 9 of the piezoelectric material substrate 2 and the surface of the support substrate 1 were irradiated with plasma to activate the surface and directly bonded.

[0037] Next, the main surface 2a of the piezoelectric material substrate 2 was ground and polished so that the thickness became from the initial 250 μm to 20 μm, and the piezoelectric material layer 2A was formed. Argon ion trimming was performed on the main surface 2b of the piezoelectric material layer 2A under the following conditions. Gas flow rate: 6 sccm Output: 120 W

[0038] A photograph of the vicinity of the surface of the obtained piezoelectric material layer 2B is shown in FIG. 3, and an explanatory diagram thereof is shown in FIG. 4. In FIG. 3, the upper bright region is the protective film 10, and the unaltered portion 5 of the piezoelectric material layer exists at the bottom most. An argon atom-containing layer 3 and a processed altered layer 4 exist on the unaltered portion 5. The measurement results by EDX of the surface region of the piezoelectric material layer in FIGS. 3 and 4 are shown in FIG. 5. The horizontal axis is the distance from the surface (main surface) of the piezoelectric material layer, and the vertical axis is the ratio of oxygen atoms, argon atoms, and niobium atoms. From the surface of the piezoelectric material substrate to about 5 nm, the oxygen atom ratio decreases from 100 atomic % to about 60 atomic %, and at the same time the niobium atom ratio increases from 0 atomic % to about 30 atomic %. This corresponds to the processed altered layer. Since the processed altered layer is generated by the destruction of the crystal structure of lithium niobate, the niobium ratio becomes lower as it approaches the surface, and the oxygen ratio becomes higher. Note that the lithium atom ratio has not been measured. On the other hand, almost no argon atoms were detected in the range of about 5 nm from the main surface of the piezoelectric material layer.

[0039] On the other hand, in the region from about 5 nm to 10 nm from the main surface of the piezoelectric material layer, an argon atom-containing layer with a thickness of 5 nm is formed. The argon atom content in the argon atom-containing layer is 2 to 6 atomic %, and on average it contains 4 atomic % of argon atoms. And below the argon atom-containing layer, both the oxygen atom ratio and the niobium atom ratio are stable, forming an unaltered portion.

[0040] Next, the main surface of the piezoelectric material layer was processed by CMP (chemical mechanical polishing) to remove the processed altered layer. A transmission electron micrograph of the surface region of the piezoelectric material substrate is shown in Fig. 6, and an explanatory diagram of Fig. 6 is shown in Fig. 7. In Fig. 7, the upper bright region is the protective film 10, and the unaltered portion 5 of the piezoelectric material layer exists at the bottommost. An argon atom-containing layer 3 exists on the unaltered portion 5. The processed altered layer has been removed.

[0041] The EDS results of this surface region are approximately as shown in Fig. 8. That is, since the processed altered layer of the piezoelectric material substrate has been removed to about 5 nm by polishing, the argon atom-containing layer is exposed on the main surface of the piezoelectric material layer. Therefore, in the range of about 5 nm from the main surface of the piezoelectric material layer, argon atoms are contained as described above, and there is an unaltered portion below it.

[0042] A measurement electrode pattern was formed on the surface of the argon atom-containing layer of the piezoelectric material layer to obtain an elastic surface wave device. Specifically, a SAW (surface acoustic wave) resonator was formed on the surface of the wafer by photolithography. That is, 50 reflectors were provided on both sides of the comb-shaped electrode composed of 100 sets of electrode fingers. The period of the electrodes was 5.66 μm for both the comb-shaped electrode and the reflectors. The frequency characteristic S 11 was measured using a network analyzer "E5072A" manufactured by Keysight Technologies. The measurement results are shown in Fig. 9. From the frequency characteristics thus obtained, the resonance frequency f r , and its half-value width Δf r were calculated, and the Q value was obtained by obtaining f r / Δf r . When the Q value (Bode-Q) was calculated from the S 11 parameters thus obtained, a maximum value of 2800 was obtained.

[0043] (Example 2) In the same manner as in Example 1, a bonded body of a piezoelectric material layer with an argon atom-containing layer exposed on the second main surface side and a support substrate was obtained. However, in Example 2, different from Example 1, the acceleration condition of argon ions during argon ion trimming was set to an output of 60 W. When the surface region of the piezoelectric material layer was analyzed by EDX, an argon atom-containing layer was formed in the range of 4 nm from the main surface. Also, the maximum value of the argon atom ratio was 3 atomic %, and the average was 2 atomic %. An elastic surface wave device was fabricated in the same manner as in Example 1 using this bonded body, and when the Q value was measured, the maximum value was 2400.

[0044] (Comparative Example 1) In the same manner as in Example 1, a bonded body of a piezoelectric material layer and a support substrate was obtained. However, in Comparative Example 1, unlike Example 1, argon ion trimming of the main surface of the piezoelectric material layer was not performed. Therefore, lithium niobate was exposed on the main surface of the piezoelectric material layer, and no processed altered layer or argon atom-containing layer was formed. An elastic surface wave device was fabricated in the same manner as in Example 1 using this bonded body, and when the Q value was measured, the maximum value was 1800.

[0045] (Comparative Example 2) In the same manner as in Example 1, a bonded body of a piezoelectric material layer and a support substrate was obtained. Here, in Comparative Example 2, argon ion trimming of the main surface of the piezoelectric material layer was performed under the same conditions as in Example 1. However, after the ion trimming, polishing was not performed. Therefore, a processed altered layer containing no argon atoms was formed on the main surface of the piezoelectric material layer. An elastic surface wave device was fabricated in the same manner as in Example 1 using this bonded body, and when the Q value was measured, the maximum value was 1150.

Claims

1. A bonded body comprising a support substrate and a piezoelectric material layer bonded to the support substrate, wherein the piezoelectric material layer has a first main surface bonded to the support substrate and a second main surface opposite to the first main surface, and has an argon atom-containing layer exposed on the second main surface.

2. The bonded body according to claim 1, wherein the thickness of the argon atom-containing layer is 1 to 10 nm.

3. The bonded body according to claim 1 or 2, wherein the argon atom content in the argon atom-containing layer is on average 1 to 10 atomic %.

4. The bonded body according to any one of claims 1 to 3, wherein the piezoelectric material layer is made of lithium niobate, lithium tantalate or lithium niobate-lithium tantalate.

5. A surface acoustic wave device comprising the bonded body according to any one of claims 1 to 4 and an electrode provided on the second main surface of the piezoelectric material layer. ​

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