Bonded body and acoustic wave element

Abstract

[Problem] To provide a joint body that enables improvement of a Q value of an elastic wave element. [Solution] A joint body 7 is provided with a support substrate 1 and a piezoelectric material layer 2C joined to the support substrate 1. The piezoelectric material layer 2C has a first main surface 9 joined to the support substrate 1 and a second main surface 3a opposite to the first main surface 9. The piezoelectric material layer 2C has an argon atom-containing layer 3 exposed at the second main surface 3a.

Description

[Technical field]

[0001] The present invention relates to a bond between a piezoelectric material layer and a supporting substrate, and to an acoustic wave element. [Background technology]

[0002] There are known acoustic wave devices such as 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 (FBAR: 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 a comb-shaped electrode capable of exciting surface acoustic waves is 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 in this way, the change in size of the piezoelectric material substrate when the temperature changes is suppressed, and the change in frequency characteristics as a surface acoustic wave element is suppressed.

[0003] When manufacturing such a surface acoustic wave element, the piezoelectric material substrate is bonded onto the 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 body 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, 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 to the second main surface, Manufacturing method This concerns.

[0008] That is, a piezoelectric material substrate is bonded to the support substrate, the surface of the piezoelectric material substrate is ion-trimmed with argon ions to generate a processing-affected layer and the argon atom-containing layer, the processing-affected layer is removed to form the piezoelectric material layer, and the argon atom-containing layer is exposed on the surface of the piezoelectric material layer. Effect of the Invention

[0009] The inventors of the present invention bonded a piezoelectric material substrate to a support substrate, then polished the surface (exposed surface) of the piezoelectric material substrate to thin it to form a piezoelectric material layer, and then investigated the surface condition of the piezoelectric material layer in various ways. However, no significant improvement in the Q value of the elastic wave could be achieved by changing the degree of polishing, the polishing method, the grindstone, etc.

[0010] We therefore 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-induced deterioration layer was formed on the surface of the piezoelectric material layer. We formed electrodes on this and fabricated an acoustic wave element, but there was still a limit to how much we could improve 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 there were few niobium and tantalum atoms in the process-affected layer on the surface, and that they gradually increased in the depth direction from the surface. It was also found that when the thickness reached a few nm from the surface of the piezoelectric material layer, the ratio of niobium and tantalum atoms reached 30-40 atom%, and became almost stable. This is because lithium niobate and tantalum atoms are present near the surface of the piezoelectric material layer. acid It is believed that the crystal structure of lithium is considerably destroyed. On the other hand, it was found that there are no argon atoms in this process-affected layer, but an argon-atom-containing layer containing a relatively large amount of argon atoms exists below it. Therefore, the inventor removed the process-affected layer to expose the argon-atom-containing layer, and then formed an electrode on it to fabricate an acoustic wave element. As a result, it was found that the Q value was significantly improved, and the present invention was achieved.

[0012] The reason why such an effect was obtained is not clear, but it is believed 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 description of the drawings]

[0013] [Figure 1] 1A is a schematic diagram showing a bonded body of a support substrate 1 and a piezoelectric material substrate 2, FIG. 1B is a schematic diagram showing the state in which the piezoelectric material substrate is thinned to form a piezoelectric material layer 2A, and FIG. 1C is a schematic diagram showing the state in which argon ion trimming is being performed on the piezoelectric material layer 2A. [Diagram 2] 1A is a schematic diagram showing the piezoelectric material layer 2B after argon ion trimming, FIG. 1B is a schematic diagram showing the piezoelectric material layer 2C with the argon atom-containing layer 3 exposed, FIG. 1C is a diagram showing a bonded body 7 of the piezoelectric material layer 2C and a supporting substrate 1, and FIG. 1D is a diagram showing an acoustic wave element 8 having an electrode 6 provided on the piezoelectric material layer 2C of the bonded body. [Diagram 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. [Diagram 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] 4 is a chart showing S11 characteristics in Example 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] The present invention will now be described in detail with reference to the accompanying drawings. As shown in Fig. 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 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. This produces a piezoelectric material layer 2B as shown in the enlarged view of Fig. 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 a non-altered portion that has not been altered by processing.

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

[0017] In the present invention, the support substrate may be made of a single crystal or may be made of 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 either single crystal silicon or polycrystalline silicon, or may be high resistance silicon. Sialon is a ceramic obtained by sintering a mixture of silicon nitride and alumina, and has the following composition. S 6-w Al w O w N 8-w More preferably, w is 0.5 or more, and even more preferably, w is 4.0 or less. Sapphire is Al 2 O 3 Alumina is a single crystal with the composition of Al 2 O 3 Cordierite is a polycrystalline material with the composition 2MgO·2Al 2 O 3 5SiO 2 Mullite is a ceramic with the composition: 3Al 2 O 3 2SiO 2 ~2Al 2 O 3 SiO 2 The ceramics have a composition in the range of

[0019] The material of the piezoelectric substrate is not limited as long as it has the necessary piezoelectricity. 3 A single crystal having the composition LiAO is preferred, where A is one or more elements selected from the group consisting of niobium and tantalum. 3may be lithium niobate, may be lithium tantalate, or may be 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. SiO 2 , Si (1-v) O v (0.008≦v≦0.408) 、 Ta 2 O 5 , Al 2 O 3、 Nb 2 O 5 ,TiO 2

[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 a piezoelectric material. Specifically, a portion in which the atomic ratio of argon atoms is 1 atom % or more when measured by EDX 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 originating from the material constituting the non-altered 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 non-altered portion of the piezoelectric material layer refers to the piezoelectric material. When the piezoelectric material is LiAO 3 In this case, the atomic ratio is the sum of the atomic ratio of element A and the atomic ratio of O (the atomic ratio of lithium is immeasurable).

[0026] Here, the measurement of each atomic ratio by TEM-EDX is carried out as follows. First, the subject of analysis is irradiated with an electron beam. This causes the subject of analysis to emit characteristic X-rays. 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] In order to obtain the conjugate of the present invention, the following method is preferred. 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 the flattening method includes lap polishing, chemical mechanical polishing (CMP), etc. In addition, the flatness of the main surface is preferably Ra≦1 nm, and more preferably 0.3 nm or less.

[0028] Next, it is preferable to clean the main surface of the piezoelectric material layer in order to remove the abrasive residue and the process-degraded layer. Methods for cleaning the main surface include wet cleaning, dry cleaning, scrub cleaning, etc., but scrub cleaning is preferable in order to obtain a clean surface simply and efficiently.

[0029] Next, the main surface of the piezoelectric material layer is subjected to argon ion trimming, thereby forming a process-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, in which Ar atoms accelerated by an electric field are bombarded against the workpiece to sputter off atoms on the surface of the workpiece. In this case, a focused argon ion beam is bombarded against 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 is configured such that an input-side IDT (Interdigital Transducer) electrode (also called a comb-shaped electrode or interdigital electrode) that excites a surface acoustic wave and an output-side IDT electrode that receives the surface acoustic wave 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, and the surface acoustic wave is excited and propagates on the piezoelectric material substrate. The propagated surface acoustic wave 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 in the vicinity of 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 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 metal films include aluminum, aluminum alloy, copper, and gold. When manufacturing a Lamb wave element, a composite substrate including a piezoelectric material layer that does not have a metal film on its bottom surface may be used.

[0033] The bottom surface of the piezoelectric material substrate may have a metal film and an insulating film. The metal film plays the role of 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 is made into a cavity so that 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° (e.g., 128°) 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 reduce 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, because this reduces 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. EXAMPLES

[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 a rotational Y-cut plate cut out at an angle of 42°, with the propagation direction of a surface acoustic wave (SAW) being X. The first main surface 9 of the piezoelectric material substrate 2 was mirror-polished to have an arithmetic mean roughness Ra of 0.3 nm. Here, Ra was measured in a field of view of 10 μm×10 μm using an atomic force microscope (AFM).

[0036] On the other hand, 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 as the support substrate 1. The surface of the support substrate 1 was finished by chemical mechanical polishing (CMP) to have an arithmetic mean roughness Ra of 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.

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

[0038] A photograph of the surface vicinity of the obtained piezoelectric material layer 2B is shown in Fig. 3, and its explanatory diagram is shown in Fig. 4. In Fig. 3, the bright area on the upper side is the protective film 10, and at the bottom is the unaltered part 5 of the piezoelectric material layer. On the unaltered part 5 are the argon atom-containing layer 3 and the process-altered layer 4. FIG. 5 shows the results of EDX measurement of the surface region of the piezoelectric material layer in FIG. 3 and FIG. 4. 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 a depth of about 5 nm, the oxygen atomic ratio decreases from 100 atomic % to about 60 atomic %, while at the same time the niobium atomic ratio increases from 0 atomic % to about 30 atomic %. This corresponds to the process-affected layer. Since the process-affected layer is generated by the destruction of the crystal structure of lithium niobate, 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 about 5 nm from the main surface of the piezoelectric material layer.

[0039] On the other hand, in a region about 5 nm to 10 nm from the main surface of the piezoelectric material layer, an argon atom-containing layer having a thickness of 5 nm is formed. The argon atom content in the argon atom-containing layer is 2 to 6 atomic %, and contains 4 atomic % of argon atoms on average. And, both the oxygen atomic ratio and the niobium atomic ratio are stable under the argon atom-containing layer, forming a non-altered 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 Fig. 6, and an explanatory diagram of Fig. 6 is shown in Fig. 7. In Fig. 7, the bright area on the upper side is the protective film 10, and underneath is the unaltered portion 5 of the piezoelectric material layer. The argon atom-containing layer 3 is present 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 is removed by polishing to a depth of about 5 nm, exposing the argon atom-containing layer on the main surface of the piezoelectric material layer. Therefore, as mentioned above, argon atoms are contained in the range of about 5 nm from the main surface of the piezoelectric material layer, and an unaffected area exists 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-tooth electrode consisting of 100 sets of electrode fingers. The electrode period was 5.66 μm for both the comb-tooth electrode and the reflector. The frequency characteristic S 11 The measurement results are shown in Figure 9. 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 When the Q value (Bode-Q) was calculated from the parameters, a maximum value of 2800 was obtained.

[0043] Example 2 A bonded body was obtained between a piezoelectric material layer having an argon atom-containing layer exposed on the second main surface side and a supporting substrate 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-atom-containing layer was formed within a range of 4 nm from the main surface, with the maximum argon atomic ratio 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 2400.

[0044] Comparative Example 1 A bonded body of a piezoelectric material layer and a supporting 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 neither a process-affected layer nor an argon atom-containing layer was 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 1,800.

[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, after the ion trimming, polishing was not performed. Therefore, a process-affected layer not containing argon atoms was generated 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 1,150.

Claims

1. A supporting substrate, and A bonded structure comprising a piezoelectric material layer bonded to the support substrate, A method for producing a bonded body, the 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 to the second main surface, comprising: A method for manufacturing a bonded body, comprising the steps of: bonding a piezoelectric material substrate to a supporting substrate; ion-trimming a surface of the piezoelectric material substrate with argon ions to generate a process-affected layer and the argon atom-containing layer; removing the process-affected layer to form the piezoelectric material layer; and exposing the argon atom-containing layer on a surface of the piezoelectric material layer.

2. 2. The method for producing a bonded body according to claim 1, wherein the argon atom-containing layer has a thickness of 1 to 10 nm.

3. 3. The method for producing a joint body according to claim 1, wherein the piezoelectric material layer is made of lithium niobate, lithium tantalate, or lithium niobate-lithium tantalate.

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