Bonded body and acoustic 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 structure of a piezoelectric material layer and a supporting substrate, and an acoustic wave element. [Background technology]

[0002] Known acoustic wave devices include surface acoustic wave devices that can function as filter elements or oscillators used in mobile phones, Lamb wave elements using piezoelectric thin films, and thin film resonators (FBARs: Film Bulk Acoustic Resonators). One such acoustic wave device is one in which a support substrate and a piezoelectric material substrate that propagates surface acoustic waves are bonded together, and comb electrodes capable of exciting surface acoustic waves are provided on the surface of the piezoelectric material substrate. By bonding a support substrate with a smaller thermal expansion coefficient than the piezoelectric material substrate to the piezoelectric material substrate, changes in the size of the piezoelectric material substrate due to temperature changes are suppressed, thereby suppressing changes in the frequency characteristics of the surface acoustic wave device.

[0003] When fabricating such a surface acoustic wave element, a piezoelectric material substrate is bonded onto a support substrate, and then 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 improving the characteristics of the surface acoustic wave. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] WO 2020-250490A1 Summary of the Invention [Problem to be solved by the invention]

[0005] However, it was found that there was room for further improvement in the Q value of the surface acoustic wave element thus obtained, especially in the range of 0.3 to 6.0 GHz.

[0006] An object of the present invention is to provide a bonded structure that enables an improvement in the Q value of an acoustic wave element. [Means for solving the problem]

[0007] The present invention relates to a support substrate, and A bonded structure comprising a piezoelectric material layer bonded to the support substrate, 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 to the second main surface. The thickness of the argon atom-containing layer is 1 to 10 nm. The present invention relates to a bonded body characterized by the above.

[0008] The present invention also relates to the above-mentioned bonded body, and an electrode provided on the second main surface of the piezoelectric material layer; The present invention relates to an acoustic wave device comprising: [Effects of the Invention]

[0009] The inventors bonded a piezoelectric material substrate to a support substrate, then polished the surface (exposed surface) of the piezoelectric material substrate to thin it, forming a piezoelectric material layer. After that, they investigated the surface condition of the piezoelectric material layer in various ways. However, they were unable to achieve a significant improvement in the Q value of the elastic wave by changing the degree of polishing, the polishing method, the grindstone, etc.

[0010] Therefore, we investigated various methods for processing the surface of the piezoelectric material substrate, and tried ion trimming with argon ions. As a result, a thin process-affected layer was formed on the surface of the piezoelectric material layer. We formed electrodes on this and tried to fabricate an acoustic wave element, but there was still a limit to the improvement in the Q value.

[0011] When the atomic ratios in the surface region of such a piezoelectric material layer were measured using EDX, it was found that niobium and tantalum atoms were scarce in the surface process-affected layer and gradually increased from the surface toward the depth direction. Furthermore, it was found that the ratios of niobium and tantalum atoms reached 30-40 atom% and stabilized at a depth of a few nanometers from the surface of the piezoelectric material layer. This suggests that the crystal structure of lithium niobate or lithium tantalum is significantly destroyed near the surface of the piezoelectric material layer. Meanwhile, it was found that there were no argon atoms in the process-affected layer, but that an argon-containing layer containing a relatively large amount of argon atoms existed underneath. Therefore, the inventors removed the process-affected layer to expose the argon-containing layer, and then formed electrodes on it to fabricate prototype acoustic wave devices. As a result, they found that the Q value was significantly improved, leading to the present invention.

[0012] The reason why such an effect was obtained is not clear, but it is thought that the propagation loss in the surface region of the piezoelectric material layer was reduced by the argon atom-containing layer, resulting in a significant improvement in the Q value. [Brief explanation of the drawings]

[0013] [Figure 1] (a) is a schematic diagram showing a bonded body of a support substrate 1 and a piezoelectric material substrate 2, (b) is a schematic diagram showing the state in which the piezoelectric material substrate is thinned to form a piezoelectric material layer 2A, and (c) is a schematic diagram showing the state in which argon ion trimming is being performed on the piezoelectric material layer 2A. [Figure 2] (a) is a schematic diagram showing the piezoelectric material layer 2B after argon ion trimming, (b) is a schematic diagram showing the piezoelectric material layer 2C with the argon atom-containing layer 3 exposed, (c) shows a bonded body 7 of the piezoelectric material layer 2C and a support substrate 1, and (d) shows an acoustic wave element 8 in which an electrode 6 is provided on the piezoelectric material layer 2C of the bonded body. [Figure 3] 1 is a transmission electron microscope photograph showing the surface state of a piezoelectric material substrate after argon ion trimming. [Figure 4] FIG. 4 is a schematic diagram corresponding to FIG. 3. [Figure 5] 4 is a graph showing EDX data of the surface region of the piezoelectric material substrate of FIG. 3. [Figure 6] 1 is a transmission electron microscope photograph showing the vicinity of an argon atom-containing layer of a piezoelectric material substrate. [Figure 7] FIG. 7 is an explanatory diagram of the photograph in FIG. 6. [Figure 8] 7 is a graph showing EDX data of the surface region of the piezoelectric material substrate of FIG. 6. [Figure 9] 10 is a chart showing the S11 characteristic in Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0014] The present invention will be described in detail below with reference to the drawings as appropriate. As shown in Figure 1(a), a support substrate 1 and a piezoelectric material substrate 2 are bonded to obtain a bonded 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 polished to thin it, forming a thin piezoelectric material layer 2A as shown in Figure 1(b). 2b is the polished surface.

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

[0016] Next, the damaged layer 4 is removed by processing, thereby obtaining a piezoelectric material layer 2C as shown in FIG. 2(b). An argon atom-containing layer 3 is formed and exposed on the second main surface 3a side of the argon atom-containing layer 2C. This results in a bonded structure 7 as shown in FIG. 2(c). The bonded structure 7 comprises a support substrate 1 and a piezoelectric material layer 2C bonded to the support substrate 1. Next, as shown in FIG. 2(d), predetermined electrodes 6 are formed on second main surface 3a of piezoelectric material layer 2C, thereby fabricating acoustic wave element 8.

[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. The alumina is preferably translucent alumina.

[0018] The silicon may be single crystal silicon, polycrystalline silicon, or high-resistivity 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 More preferably, w is 0.5 or more, and even more preferably 4.0 or less. Sapphire is a single crystal with the composition Al2O3, alumina is a polycrystal with the composition Al2O3, cordierite is a ceramic with the composition 2MgO·2Al2O3·5SiO2, and mullite is a ceramic with a composition ranging from 3Al2O3·2SiO2 to 2Al2O3·SiO2.

[0019] The material of the piezoelectric substrate is not limited as long as it has the required piezoelectricity, but a single crystal having the composition LiAO3 is preferred. 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 to each other by a surface activation method using plasma or a surface activation method using a neutral 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 materials for such bonding layers 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 an 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, and more preferably 3 to 8 nm.

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

[0025] In the argon atom-containing layer, the atomic ratio (total value) of atoms derived from the material constituting the unaltered portion of the piezoelectric material layer is 99.0 to 99.9 atomic % on average, and more preferably 99.5 to 99.9 atomic %. Here, the material constituting the unaltered portion of the piezoelectric material layer refers to the piezoelectric material. When the piezoelectric material is LiAO3, the atomic ratio is the sum of the atomic ratio of element A and the atomic ratio of O (the atomic ratio of lithium cannot be measured).

[0026] Here, the measurement of each atomic ratio by TEM-EDX is carried out as follows. First, the object to be analyzed is irradiated with an electron beam. This causes characteristic X-rays to be emitted from the object to be analyzed. The energy of the characteristic X-rays is specific to each element, so the ratio of each element can be determined by measuring the type of energy and the number of times it is emitted.

[0027] The following method is preferred for obtaining the conjugate of the present invention. First, the second main surface of the piezoelectric material substrate is polished to thin the piezoelectric material substrate and form a piezoelectric material layer. At this time, it is preferable to flatten the main surface by precision polishing, and flattening methods include lap polishing and chemical mechanical polishing (CMP). The flatness of the main surface is preferably Ra≦1 nm, and more preferably 0.3 nm or less.

[0028] Next, the main surface of the piezoelectric material layer is preferably cleaned to remove abrasive residues and layers affected by processing. Methods for cleaning the main surface include wet cleaning, dry cleaning, and scrub cleaning, but scrub cleaning is preferred to obtain a clean surface simply and efficiently.

[0029] Next, argon ion trimming is performed on the main surface of the piezoelectric material layer, forming a work-affected layer and an argon atom-containing layer on the main surface side of the piezoelectric material layer. Argon ion trimming is a processing technique that uses the sputtering phenomenon to strike the workpiece with argon atoms accelerated by an electric field, thereby scattering atoms from the workpiece surface. In this case, a focused argon ion beam is struck on the workpiece. The preferred conditions for argon ion trimming are as follows: Diameter of focused ion beam: 10mm or less Acceleration output: 120W

[0030] From the viewpoint of device characteristics, the thickness of the piezoelectric material layer is preferably 1 μm or less, more preferably 0.5 μm or less, and 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 it can be suitably applied to, for example, an acoustic wave element or an optical element. Known acoustic wave elements include surface acoustic wave devices, Lamb wave elements, and thin-film resonators (FBARs). For example, a surface acoustic wave device has an input-side interdigital transducer (IDT) electrode (also called an interdigital transducer) that excites surface acoustic waves and an output-side IDT electrode that receives surface acoustic waves, both of which are provided 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, exciting surface acoustic waves that propagate across the piezoelectric material substrate. The propagated surface acoustic waves can then be extracted as an electrical signal from the output-side IDT electrode provided in the propagation direction.

[0032] The piezoelectric material substrate may have a metal film on its bottom surface. The metal film serves to increase the electromechanical coupling coefficient near the back surface of the piezoelectric material substrate when a Lamb wave element is manufactured as an acoustic wave device. In this case, the Lamb wave element has a structure in which a comb-shaped electrode is 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 materials for such a metal film include aluminum, aluminum alloy, copper, and gold. When manufacturing a Lamb wave element, a composite substrate having a piezoelectric material layer without a metal film on its bottom surface may also be used.

[0033] The bottom surface of the piezoelectric material substrate may also have a metal film and an insulating film. The metal film serves as an electrode when a thin-film resonator is manufactured as an acoustic wave device. In this case, the thin-film resonator has electrodes formed on the front and back surfaces of the piezoelectric material substrate, and the insulating film has a cavity in which the metal film of the piezoelectric material substrate is exposed. Examples of materials for such metal films include molybdenum, ruthenium, tungsten, chromium, and aluminum. Examples of materials for the insulating film include silicon dioxide, phosphorus silica glass, and boron phosphorus silica glass.

[0034] When the subject of the present invention is an acoustic wave element and the material of the piezoelectric material substrate is lithium tantalate, it is preferable to use one 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 surface acoustic wave, as this reduces propagation loss. Furthermore, when the piezoelectric substrate is made of lithium niobate, it is preferable to use one 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 surface acoustic wave, in order to minimize propagation loss. Furthermore, the size of the piezoelectric substrate is not particularly limited, but is, for example, 50 to 150 mm in diameter and 0.2 to 60 μm in thickness. [Example]

[0035] Example 1 A surface acoustic wave element was fabricated by the method described with reference to FIGS. 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 used was a 42° Y-cut X-propagation LN substrate, with the propagation direction of the surface acoustic wave (SAW) set to X, and cut out at a rotated Y-cut angle. The first main surface 9 of the piezoelectric material substrate 2 was mirror-polished to an arithmetic mean roughness Ra of 0.3 nm. Ra was measured using an atomic force microscope (AFM) in a field of view of 10 μm × 10 μm.

[0036] On the other hand, a support substrate 1 made of silicon (Si(111)) with an orientation flat (OF) portion, a diameter of 4 inches, and a thickness of 500 μm was prepared as the support substrate 1. 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 surfaces, thereby directly bonding them together.

[0037] Next, the main surface 2a of the piezoelectric material substrate 2 was ground and polished so that the thickness was reduced from the original 250 μm to 20 μm, to form a piezoelectric material layer 2 A. The main surface 2b of the piezoelectric material layer 2 A was subjected to argon ion trimming under the following conditions. Gas flow rate: 6sccm Output: 120W

[0038] A photograph of the surface and vicinity of the resulting piezoelectric material layer 2B is shown in Figure 3, and an explanatory diagram thereof is shown in Figure 4. In Figure 3, the bright area on the top is the protective film 10, and underneath is the unaltered portion 5 of the piezoelectric material layer. Above the unaltered portion 5 are the argon atom-containing layer 3 and the process-altered layer 4. Figure 5 shows the results of EDX measurements of the surface regions of the piezoelectric material layers in Figures 3 and 4. The horizontal axis represents the distance from the surface (main surface) of the piezoelectric material layer, and the vertical axis represents the ratios of oxygen, argon, and niobium atoms. From the surface of the piezoelectric material substrate to a depth of approximately 5 nm, the oxygen atomic ratio decreases from 100 atomic % to approximately 60 atomic %, while the niobium atomic ratio increases from 0 atomic % to approximately 30 atomic %. This corresponds to the process-affected layer. The process-affected layer is caused by destruction of the crystalline structure of lithium niobate, so the niobium ratio decreases and the oxygen ratio increases toward the surface. Note that the lithium atomic ratio was not measured. On the other hand, almost no argon atoms were detected within a range of approximately 5 nm from the main surface of the piezoelectric material layer.

[0039] Meanwhile, a 5 nm thick argon atom-containing layer is formed in a region approximately 5 nm to 10 nm from the main surface of the piezoelectric material layer. The argon atom content in the argon atom-containing layer is 2 to 6 atomic %, with an average argon atom content of 4 atomic %. Beneath the argon atom-containing layer, both the oxygen atomic ratio and the niobium atomic ratio are stable, forming an unaltered portion.

[0040] Next, the main surface of the piezoelectric material layer was treated by CMP (chemical mechanical polishing) to remove the process-affected layer. A transmission electron microscope photograph of the surface region of the piezoelectric material substrate is shown in Figure 6, and an explanatory diagram of Figure 6 is shown in Figure 7. In Figure 7, the bright area on the top is the protective film 10, and underneath is the unaltered portion 5 of the piezoelectric material layer. An argon atom-containing layer 3 exists on the unaltered portion 5. The process-altered layer has been removed.

[0041] The EDS results for this surface region are roughly as shown in Figure 8. That is, the process-affected layer of the piezoelectric material substrate was removed by polishing to a depth of about 5 nm, exposing an argon atom-containing layer on the main surface of the piezoelectric material layer. Therefore, as mentioned above, argon atoms are contained within a range of about 5 nm from the main surface of the piezoelectric material layer, and an unaffected area lies below that.

[0042] A measurement electrode pattern was formed on the surface of the argon atom-containing layer of the piezoelectric material layer to obtain a surface acoustic wave element. Specifically, a SAW (surface acoustic wave) resonator was formed on the surface of the wafer by photolithography. That is, 50 reflectors were provided on each side of a comb-shaped electrode consisting of 100 pairs of electrode fingers. The electrode period was 5.66 μm for both the comb-shaped electrode and the reflectors. The frequency characteristic S 11 The measurement results are shown in Figure 9. From the frequency characteristics obtained, the resonant frequency f r and its half-width Δf r Calculate f r / Δf r The Q value is obtained by solving The S thus obtained 11 The Q value (Bode-Q) was calculated from the parameters, and a maximum value of 2800 was obtained.

[0043] Example 2 A bonded body of a piezoelectric material layer having an argon atom-containing layer exposed on the second principal surface side and a supporting substrate was obtained in the same manner as in Example 1. However, in Example 2, unlike Example 1, the acceleration condition of argon ions during argon ion trimming was set to an output of 60 W. EDX analysis of the surface region of the piezoelectric material layer revealed that an argon-containing layer was formed within a range of 4 nm from the main surface, with the maximum argon atomic percentage being 3 atomic % and the average being 2 atomic %. Using this bonded body, a surface acoustic wave element was fabricated in the same manner as in Example 1, and the Q value was measured, revealing a maximum value of 2,400.

[0044] (Comparative Example 1) A bonded body of a piezoelectric material layer and a support substrate was obtained in the same manner as in Example 1. However, in Comparative Example 1, unlike Example 1, argon ion trimming was not performed on the main surface of the piezoelectric material layer. Therefore, lithium niobate was exposed on the main surface of the piezoelectric material layer, and a work-affected layer or an argon atom-containing layer was not formed. Using this bonded body, a surface acoustic wave element was fabricated in the same manner as in Example 1, and the Q value was measured, revealing a maximum value of 1800.

[0045] (Comparative Example 2) A bonded body of a piezoelectric material layer and a support substrate was obtained in the same manner as in Example 1. Here, in Comparative Example 2, argon ion trimming was performed on the main surface of the piezoelectric material layer under the same conditions as in Example 1. However, polishing was not performed after ion trimming. Therefore, a process-affected layer not containing argon atoms was formed on the main surface of the piezoelectric material layer. Using this bonded body, a surface acoustic wave element was fabricated in the same manner as in Example 1, and the Q value was measured, revealing a maximum value of 1150.

Claims

1. a supporting substrate, and A bonded structure comprising a piezoelectric material layer bonded to the support substrate, a piezoelectric material layer having a first main surface bonded to the support substrate and a second main surface opposite to the first main surface, and an argon atom-containing layer exposed on the second main surface, wherein the argon atom-containing layer has a thickness of 1 to 10 nm.

2. Between the piezoelectric material substrate and the support substrate, SiO ２ , Si （１－ｖ） O ｖ (0.008≦v≦0.408), Ta ２ O ５ , Al ２ O ３、 Nb ２ O ５ or TiO ２ 2. The bonded body according to claim 1, further comprising a bonding layer comprising:

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

4. 3. The bonded body according to claim 1, wherein the atomic ratio of argon atoms in said argon atom-containing layer is 1 atom % or more.

5. 5. The bonded body according to claim 4, wherein the atomic ratio of argon atoms in said argon atom-containing layer is 10 atom % or less.

6. 6. The bonded body according to claim 5, wherein the atomic ratio of argon atoms in said argon atom-containing layer is 2 atom % or more and 4 atom % or less.

7. The conjugate according to claim 1 or 2, and an electrode provided on the second main surface of the piezoelectric material layer; An acoustic wave element comprising:

Images