Composite Substrates for Acoustic Wave Devices

Abstract

[Problem] To improve the bonding strength between a piezoelectric material substrate and a support substrate, effectively reduce the reflection of bulk waves, and suppress spurious emissions. [Solution] This composite substrate 7A for an elastic wave device comprises a piezoelectric material layer PZ, a support substrate S, and x layers (x is an integer of 3 or more) of intermediate layers 1, 2, X between the piezoelectric material layer and the support substrate. The piezoelectric material layer, the support substrate, and the intermediate layers satisfy formula (1), formula (2) is satisfied when x is an even number, and formula (3) is satisfied when x is an odd number. Formula (1): Rn < Rn+1 (n represents all integers from 1 to x, Rn is the arithmetic mean roughness of the surface of the n-th intermediate layer on the piezoelectric material layer side when viewed from the piezoelectric material layer, and Rx+1 is the arithmetic mean roughness of the surface of the support substrate on the piezoelectric material layer side). Formula (2): Vn-1 < Vn (n is 2 or more and represents all even numbers equal to or less than x, and Vn is the acoustic velocity of the n-th intermediate layer when viewed from the piezoelectric material layer). Formula (3): Vn-1 > Vn (n is 1 or more and represents all odd numbers equal to or less than x, Vn is the acoustic velocity of the n-th intermediate layer when viewed from the piezoelectric material layer, and V0 represents the acoustic velocity of the piezoelectric material layer).

Description

[Technical field]

[0001] The present invention relates to a composite substrate for an acoustic wave device. [Background technology]

[0002] It is known that surface acoustic wave filters made by bonding lithium tantalate and sapphire with a silicon oxide layer in between generate bulk waves at the bonding interface, resulting in unwanted responses in the passband and high frequency range.To prevent this, a method has been proposed in which a rough surface is introduced at the bonding interface to scatter the bulk waves and suppress the unwanted responses (Patent Document 1, Patent Document 2).

[0003] In Patent Document 1, when the joining surface is roughened, the geometric specifications of the roughened surface are that the ratio of the average length RSm of the elements in the cross-sectional curve of the uneven structure constituting the roughened surface to the wavelength λ of the surface acoustic wave is 0.2 to 7.0, and the arithmetic mean roughness Ra of the cross-sectional curve of the uneven structure is 100 nm or more. On the other hand, Patent Document 2 specifies the height difference of the roughened surface. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 6250856 [Patent Document 2] US Publication No. 2017-063333 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in order to obtain a sufficiently high spurious suppression effect, it is necessary to process the back surface of the piezoelectric material substrate to make it significantly rough. However, if a bonded body is made using a piezoelectric material substrate whose back surface has been processed in this way, a process-induced layer appears on the surface when the piezoelectric material is thinned, and the characteristics are likely to deteriorate. In addition, when bonding the piezoelectric material substrate to the intermediate layer on the support substrate, it is difficult to increase the bonding strength if the back surface of the piezoelectric material substrate is highly rough.

[0006] The object of the present invention is to improve the bonding strength between a piezoelectric material substrate and a support substrate in a composite substrate for an elastic wave device, which is a bonded structure of a piezoelectric material substrate and a support substrate, and to efficiently reduce the reflection of bulk waves to suppress spurious waves. [Means for solving the problem]

[0007] The present invention relates to a composite substrate for an acoustic wave device, comprising a piezoelectric material layer, a support substrate, and an intermediate layer of x layers (x is an integer of 3 or more) between the piezoelectric material layer and the support substrate, wherein the piezoelectric material layer, the support substrate, and the intermediate layer satisfy the following formula (1), and also satisfy the following formula (2) when x is an even number, and satisfy the following formula (3) when x is an odd number. R n <R n+1 ... (1) (In formula (1), n represents all integers from 1 to x, R n is the arithmetic mean roughness of the surface of the n-th intermediate layer facing the piezoelectric material layer, as viewed from the piezoelectric material layer, R n and R n+1 The difference is 0.2 nm or more. R x+1 is the arithmetic mean roughness of the surface of the support substrate on the side of the piezoelectric material layer, That is, 1.5 to 5 nm. ( V n-1<V n (2) (In formula (2), n denotes every even number between 2 and x, V n is the sound velocity of the n-th intermediate layer as viewed from the piezoelectric material layer.) V n-1 >V n ... (3) (In formula (3), n denotes all odd numbers between 1 and x, V n is the sound velocity of the n-th intermediate layer as viewed from the piezoelectric material layer, V0 is the sound velocity in the piezoelectric material layer. Effect of the Invention

[0008] According to the present invention, in a composite substrate for an acoustic wave device consisting of a bonded body of a piezoelectric material substrate and a support substrate, the arithmetic mean roughness of the intermediate layer is gradually decreased from the support substrate to the piezoelectric material layer, thereby improving the bonding strength to the piezoelectric material substrate. In addition, it has been found that by sequentially providing an intermediate layer with a high sound velocity and an intermediate layer with a low sound velocity adjacent to each other from the piezoelectric material layer to the support substrate, the reflection of bulk waves is efficiently reduced and spurious waves are significantly suppressed, thereby completing the present invention. [Brief description of the drawings]

[0009] [Figure 1] 1A is a schematic cross-sectional view showing an intermediate layer X, 2 and a bonding layer M provided on a support substrate, FIG. 1B is a cross-sectional view showing a bonding layer Y provided on a piezoelectric material substrate, and FIG. 1C is a cross-sectional view showing a bonded body 7A of a support substrate and a piezoelectric material substrate. [Diagram 2]1A is a schematic cross-sectional view showing a state in which intermediate layers X, 3, 2 and a bonding layer M are provided on a support substrate, FIG. 1B is a cross-sectional view showing a state in which a bonding layer Y is provided on a piezoelectric material substrate, and FIG. 1C is a cross-sectional view showing a bonded body 7B of a support substrate and a piezoelectric material substrate. [Diagram 3] 1(a) shows a state in which the piezoelectric material substrate of the bonded body has been thinned by processing, and FIG. 1(b) shows an acoustic wave element 8. FIG. [Figure 4] 1 is a graph illustrating the sound speed of an intermediate layer on a supporting substrate. [Diagram 5] 1 is a graph illustrating the sound speed of an intermediate layer on a supporting substrate. [Figure 6] 13 is a graph showing frequency characteristics of S11 of the acoustic wave element of the example. [Figure 7] 13 is a graph showing frequency characteristics of S11 of an acoustic wave element of a comparative example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] The present invention will now be described in further detail with reference to the accompanying drawings. First, as shown in Fig. 1(a), an intermediate layer X, 2 and a bonding layer M are sequentially formed on a surface Sa of a support substrate S. Next, a surface Ma of the bonding layer M is precision polished, for example, chemical mechanical polished. Next, the surface Ma of the intermediate layer M is surface activated.

[0011] On the other hand, as shown in Fig. 1(b), a bonding layer Y is provided on the main surface PZa of the piezoelectric material layer PZ. The surface Ya of the bonding layer Y is surface activated. Next, the surface Ma of the bonding layer M and the surface Ya of the bonding layer Y are brought into contact with each other and directly bonded to each other, thereby obtaining the bonded body 7A shown in Fig. 1(c). When the bonding layer Y and the bonding layer M are made of the same material, the two are substantially integrated to form the intermediate layer 1.

[0012] FIG. 2 shows an embodiment in which four intermediate layers are provided between the piezoelectric material layer and the supporting substrate. First, as shown in Fig. 2(a), intermediate layers X, 3, 2 and a bonding layer M are sequentially formed on a surface Sa of a support substrate S. Next, a surface Ma of the uppermost bonding layer M is precision polished, for example, chemical mechanical polished. Next, the surface Ma of the bonding layer M is surface activated.

[0013] On the other hand, as shown in Fig. 2(b), the surface PZa of the piezoelectric material substrate PZ is roughened. A bonding layer Y is provided on the main surface PZa of the piezoelectric material substrate PZ. The surface Ya of the bonding layer Y is surface activated. Next, the surface Ma of the bonding layer M and the surface Ya of the bonding layer Y are brought into contact with each other and directly bonded to each other, thereby obtaining the bonded body 7B shown in Fig. 2(c).

[0014] Next, as shown in Fig. 3(a), the piezoelectric material substrate PZ is processed to make it thinner, and a piezoelectric material substrate PZC is formed to obtain a bonded body 7C. In this state, electrodes may be provided on the piezoelectric material substrate PZC. However, preferably, as shown in Fig. 3(b), a predetermined electrode 9 is formed on the processed surface of the piezoelectric material substrate PZC to obtain an acoustic wave element 8.

[0015] Here, by adjusting the arithmetic mean roughness Ra and the sound velocity of each of the multiple intermediate layers between the piezoelectric material layer and the support substrate, it is possible to increase the bonding strength between the support substrate and the piezoelectric material substrate and to suppress spurious waves. This configuration will be further described.

[0016] First, x layers (x is 3 or more) of intermediate layers are provided between the piezoelectric material layer PZ (PZC) and the surface Sa of the support substrate S. Three or more intermediate layers are required in order to gradually lower the arithmetic mean roughness of the intermediate layers and efficiently reduce the reflection of bulk waves to suppress spurious signals. From this perspective, it is more preferable that the number of intermediate layers is 4 or more. There is no particular upper limit to the number of intermediate layers, but since an increase in the number of intermediate layers increases the manufacturing cost, from this perspective, it is preferable that the number of intermediate layers is 10 or less.

[0017] The bonded body satisfies formula (1), and also satisfies the following formula (2) when x is an even number, and satisfies the following formula (3) when x is an odd number. First, formula (1) defines the arithmetic mean roughness of each surface of the supporting substrate and each intermediate layer.

[0018] R n <R n+1 ... (1) (In formula (1), n represents all integers from 1 to x, R n is the arithmetic mean roughness of the surface of the n-th intermediate layer facing the piezoelectric material layer, as viewed from the piezoelectric material layer, R n and R n+1 The difference is 0.2 nm or more. R x+1 is the arithmetic mean roughness of the surface of the support substrate on the side of the piezoelectric material layer, That is, 1.5 to 5 nm. (

[0019] That is, the arithmetic mean roughness R1 of the piezoelectric material layer side surface 1a of the first intermediate layer 1 as viewed from the piezoelectric material layer PZ is lower than the arithmetic mean roughness R2 of the piezoelectric material layer side surface 2a of the second intermediate layer 2, which in turn is lower than the arithmetic mean roughness R3 of the piezoelectric material layer side surface 3a of the third intermediate layer 3, and so on, with the arithmetic mean roughness of the piezoelectric material layer side surfaces of the intermediate layers successively increasing toward the support substrate.The arithmetic mean roughness Rx of the piezoelectric material layer side surface Xa of the intermediate layer X closest to the support substrate S is smaller than the arithmetic mean roughness Rx of the piezoelectric material layer side surface Xa of the intermediate layer X. x+1 is higher. In other words, the arithmetic mean roughness of each surface of each intermediate layer on the piezoelectric material layer side is successively decreased from the support substrate to the piezoelectric material layer.

[0020] 1(c), there are three intermediate layers 1, 2, and X (x=3) between the piezoelectric material layer PZ and the support substrate S. In this case, the arithmetic mean roughness R1 of the piezoelectric material layer side surface 1a of the first intermediate layer 1 as viewed from the piezoelectric material layer PZ is lower than the arithmetic mean roughness R2 of the piezoelectric material layer side surface 2a of the second intermediate layer 2, and the arithmetic mean roughness R2 of the surface 2a of the second intermediate layer 2 is lower than the arithmetic mean roughness R of the surface Xa of the third (xth) intermediate layer X. x The arithmetic mean roughness R of the surface Xa of the third intermediate layer X is lower than x is the arithmetic mean roughness R of the surface Sa of the support substrate S. x+1 That is, the arithmetic mean roughness of the surface of the intermediate layer increases stepwise with increasing distance from the piezoelectric material layer.

[0021] 2(c), there are four intermediate layers 1, 2, 3, and X (x=4) between the piezoelectric material layer PZ and the support substrate S. In this case, the arithmetic mean roughness R1 of the piezoelectric material layer side surface 1a of the first intermediate layer 1 as viewed from the piezoelectric material layer PZ is lower than the arithmetic mean roughness R2 of the piezoelectric material layer side surface 2a of the second intermediate layer 2, the arithmetic mean roughness R2 of the surface 2a of the second intermediate layer 2 is lower than the arithmetic mean roughness R3 of the surface 3a of the third intermediate layer 3 (xth layer), and the arithmetic mean roughness R3 of the surface 3a of the third intermediate layer 3 is lower than the arithmetic mean roughness R of the surface Xa of the fourth intermediate layer X (xth layer). x The arithmetic mean roughness R of the surface Xa of the fourth intermediate layer X is lower than x is the arithmetic mean roughness R of the surface Sa of the support substrate S. x+1 That is, the arithmetic mean roughness of the surface of the intermediate layer increases stepwise with increasing distance from the piezoelectric material layer.

[0022] In this way, by making the surface of the intermediate layer rougher with increasing distance from the piezoelectric material layer, the bonding strength with the piezoelectric material substrate can be increased.

[0023] From this perspective, R x+1 and R x The difference between R and R is preferably 0.2 nm or more, and more preferably 0.5 nm or more.x+1 and R x The difference between them is often less than 1 nm. In addition, from the viewpoint of the present invention, R n-1 and R n The difference between R and R is preferably 0.2 nm or more, and more preferably 0.5 nm or more. n-1 and R n The difference between them is often less than 1 nm.

[0024] In addition, the arithmetic mean roughness R of the surface Sa of the support substrate S x+1 teeth, 1.5 From the viewpoint of bonding strength, the arithmetic mean roughness R1 of the surface 1a of the intermediate layer 1 (the first layer) closest to the piezoelectric material layer is preferably 1 nm or less, and more preferably 0.3 nm or less.

[0025] Furthermore, the bonded structure of the present invention must satisfy a specific relationship between the sound velocities of the layers, that is, when x is an even number, the following formula (2) must be satisfied, and when x is an odd number, the following formula (3) must be satisfied. V n-1 <V n (2) (In formula (2), n denotes every even number between 2 and x, V n is the sound velocity of the n-th intermediate layer as viewed from the piezoelectric material layer.) V n-1 >V n ... (3) (In formula (3), n denotes all odd numbers between 1 and x, V n is the sound velocity of the n-th intermediate layer as viewed from the piezoelectric material layer, V0 indicates the sound velocity of the piezoelectric material layer.

[0026] For example, in the example of Fig. 1(c), three intermediate layers 1, 2, and X are formed between the piezoelectric material layer and the supporting substrate. Since x = 3, formula (3) is satisfied. Therefore, equation (3) can be expressed as shown in FIG. x+1 is the sound velocity in the supporting substrate S.

[0027] In general, when x is an odd number, n is an odd number that is 1 or more and is 0 or less. Therefore, from the support substrate to the piezoelectric material layer, the arrangement is as follows. V x-1 >V x :V x-3 >V x-2 ...V2>V3:V0>V1 In this case, V n-1 and V n-2 There is no restriction on the size relationship between V n-2 <V n-1 It is particularly preferable that the following relationship is satisfied.

[0028] If x is even, V n-1 <V n (2) Since the above equation is satisfied, the sound velocity of each intermediate layer from the piezoelectric material layer to the supporting substrate is as follows: V x-1 <V x :V x-3 <V x-2 ...V3 <V4:V1<V2

[0029] For example, in the example of Fig. 2(c), four intermediate layers 1, 2, 3, and X are formed between the piezoelectric material layer and the support substrate. Since x = 4, formula (2) becomes as follows. V3 <V x :V1 <V2 Therefore, for example, it will be as shown in Figure 5. Note that V x+1 is the sound velocity in the supporting substrate S. In this case, V n-1 and V n-2 There is no restriction on the size relationship between V n-2 >V n-1 It is preferable that the following relationship is satisfied.

[0030] From the viewpoint of the present invention, in formula (2) and formula (3), V n-1 and V n The difference between V and V is preferably 200 m / sec or more, and more preferably 500 m / sec or more. n-1 and V n The difference between these is often less than 3000 m / sec. From the viewpoint of the present invention, the difference between V0 and V1 is preferably 200 m / sec or more, more preferably 500 m / sec or more. There is no particular upper limit to the difference between V0 and V1, but from a practical viewpoint, it may be 3000 m / sec or less. Also, V n-2 and V n-1 If different, V n-2 and V n-1 The difference between V and V is preferably 1000 m / sec or more, and more preferably 3000 m / sec or more. n-2 and V n-1 The difference between these is often less than 10,000 m / sec.

[0031] The material of the support substrate is not particularly limited, but is preferably a material selected from the group consisting of silicon, quartz crystal, and sapphire, which can further improve the temperature characteristic of the frequency of the acoustic wave element. The surface of the support substrate facing the piezoelectric material layer may be roughened by processing with a grinding wheel or blasting. Blasting is a process in which an abrasive material is blown onto a surface using compressed air.

[0032] The method for forming each intermediate layer and each intermediate layer on the piezoelectric material substrate is not limited, but examples thereof include sputtering, chemical vapor deposition (CVD), and deposition.

[0033] The material of each intermediate layer is not particularly limited as long as it can be surface-activated, but a metal oxide film is preferable, and a material selected from the group consisting of silicon, silicon oxide, alumina, tantalum pentoxide, niobium pentoxide, and titanium oxide is particularly preferable. In addition, a surface activation method can be appropriately selected depending on the material of the bonding layer used. Examples of such surface activation methods include plasma activation and FAB (Ar atomic beam).

[0034] From the viewpoint of the present invention, the thickness of each intermediate layer is preferably 0.02 μm or more, more preferably 0.05 μm or more, and particularly preferably 0.1 μm or more. Also, the thickness of each intermediate layer is preferably 3 μm or less, more preferably 2 μm or less, and more preferably 1 μm or less.

[0035] From the viewpoint of the present invention, the total thickness of the multiple intermediate layers is preferably from 0.1 to 5 μm, and more preferably from 0.2 to 2 μm.

[0036] The piezoelectric material substrate used in the present invention is preferably lithium tantalate (LT) single crystal, lithium niobate (LN) single crystal, or lithium niobate-lithium tantalate solid solution, which have a fast acoustic wave propagation speed and a large electromechanical coupling coefficient and are therefore suitable for use as a surface acoustic wave device for high and wide frequency bands.

[0037] In addition, the normal direction of the main surface of the piezoelectric material substrate is not particularly limited, but for example, when the piezoelectric material substrate is made of LT, it is preferable to use a direction rotated 32 to 55 degrees from the Y axis to the Z axis around the X axis, which is the propagation direction of the surface acoustic wave, and (180°, 58 to 35°, 180°) in Euler angle representation, because the propagation loss is small. When the piezoelectric material substrate is made of LN, it is preferable to use (a) a direction rotated 37.8° from the Z axis to the -Y axis around the X axis, which is the propagation direction of the surface acoustic wave, and (0°, 37.8°, 0°) in Euler angle representation, because the electromechanical coupling coefficient is large, or (b) a direction rotated 40 to 65° from the Y axis to the Z axis around the X axis, which is the propagation direction of the surface acoustic wave, and (180°, 50 to 25°, 180°) in Euler angle representation, because a high sound velocity can be obtained. Furthermore, the size of the piezoelectric material substrate is not particularly limited, but may be, for example, 100 to 200 mm in diameter and 0.15 to 1 μm in thickness.

[0038] For example, the surface of the outermost bonding layer M on the support substrate and the support substrate side surface PZa of the piezoelectric material substrate PZ or the surface Ya of the bonding layer Y on the piezoelectric material substrate are surface-activated and directly bonded. For example, each surface is irradiated with plasma at 150° C. or less to activate the bonding surfaces. From the viewpoint of the present invention, it is preferable to irradiate with nitrogen plasma, but it is also possible to obtain the bonded body of the present invention when irradiating with oxygen plasma.

[0039] The pressure during surface activation is preferably 100 Pa or less, more preferably 80 Pa or less. The atmosphere may be only nitrogen or only oxygen, or may be a mixture of nitrogen and oxygen.

[0040] The temperature during plasma irradiation is preferably 150° C. or less. This makes it possible to obtain a bonded body with high bonding strength and no deterioration in crystallinity. From this viewpoint, the temperature during plasma irradiation is set to 150° C. or less, and more preferably 100° C. or less. The energy during plasma irradiation is preferably 30 to 150 W. The product of the energy and irradiation time during plasma irradiation is preferably 0.12 to 1.0 Wh.

[0041] The bonding surface of the plasma-treated piezoelectric material substrate and the bonding surface of the bonding layer are brought into contact with each other at room temperature, which may be in a vacuum, but is more preferably in the atmosphere.

[0042] When performing surface activation using an argon atomic beam, it is preferable to generate and irradiate an argon atomic beam using an apparatus such as that described in JP 2014-086400 A. That is, a saddle field type fast atom beam source is used as a beam source. Then, an inert gas is introduced into a chamber, and a high voltage is applied to the electrode from a DC power source. As a result, a saddle field type electric field generated between the electrode (positive electrode) and the housing (negative electrode) moves electrons e, generating a beam of argon atoms and ions. Of the beams that reach the grid, the ion beam is neutralized by the grid, and therefore a beam of argon atoms is emitted from the fast atom beam source. The voltage during activation by beam irradiation is preferably 0.5 to 2.0 kV, and the current is preferably 50 to 200 mA.

[0043] Next, the surface Ma of the bonding layer M on the outermost surface of the support substrate is brought into contact with the surface PZa of the piezoelectric material substrate PZ or the surface Ya of the bonding layer Y on the piezoelectric material substrate, and bonded. After this, it is preferable to improve the bonding strength by performing an annealing treatment. The temperature during the annealing treatment is preferably 100°C or higher and 300°C or lower.

[0044] The bonded structure of the present invention can be suitably used for an acoustic wave element, that is, an acoustic wave element including the bonded structure of the present invention and an electrode provided on a piezoelectric material substrate. Specifically, surface acoustic wave devices, Lamb wave elements, and thin-film resonators (FBARs) are known as acoustic wave elements. For example, a surface acoustic wave device is provided on the surface of a piezoelectric material substrate with 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. 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 electric signal from the output-side IDT electrode provided in the propagation direction.

[0045] The material constituting the electrodes on the piezoelectric substrate is preferably aluminum, an aluminum alloy, copper, or gold, and more preferably aluminum or an aluminum alloy. The aluminum alloy is preferably an aluminum alloy mixed with 0.3 to 5% by weight of Cu. In this case, Ti, Mg, Ni, Mo, or Ta may be used instead of Cu. EXAMPLES

[0046] Example 1 Acoustic wave element 8 shown in FIG. 3(b) was fabricated according to the method described with reference to FIGS. Specifically, we prepared a 42Y-cut X-propagation LiTaO3 substrate (piezoelectric material substrate) PZ with a thickness of 350 μm and mirror-finished on both sides. We also prepared a high-resistance (>2 kΩ·cm) Si(100) substrate (support substrate) S with a thickness of 675 μm. The size of each substrate is 150 mm.

[0047] Next, the surface Sa of the support substrate S was ground with a grinding wheel equivalent to GC#6000. The grinding amount was set to 5 μm so that the entire surface would be ground uniformly. When the processed support substrate surface Sa was measured with an optical interference type roughness measuring device, the arithmetic average roughness R x+1 was 3.2 nm.

[0048] After cleaning the surface Sa of the support substrate S, an intermediate layer X made of Ta2O5 was formed by a sputtering device. The thickness of the intermediate layer X at this time was 1200 nm. The wafer after the film formation was temporarily removed, and the arithmetic mean roughness R of the surface Xa was measured. x was measured and found to have been greatly reduced to 1.9 nm. An intermediate layer 2 made of silicon was formed continuously on the intermediate layer X to a thickness of 800 nm. The arithmetic mean roughness R2 of the surface 2a of the intermediate layer 2 was 1.3 nm. Furthermore, a bonding layer M made of silicon oxide was formed to a thickness of 400 nm, finally forming a three-layer laminate structure. The arithmetic mean roughness of the surface Ma of the bonding layer M was 1.0 nm, which was a much smoother surface than the initial 3.2 nm. The surface of the outermost bonding layer was subjected to CMP processing to remove approximately 30 nm. As a result, the arithmetic mean roughness of the surface was reduced to 0.6 nm.

[0049] In addition, a bonding layer Y made of silicon oxide was formed to a thickness of 100 nm on the surface PZa of the piezoelectric material substrate PZ. The arithmetic mean roughness of the surface Ya of the bonding layer Y at this time was 1.2 nm. When this surface was processed by CMP to a depth of about 50 nm, the arithmetic mean roughness was 0.3 nm.

[0050] The bonding surface of the bonding layer on the piezoelectric material substrate thus obtained and the surface of the outermost bonding layer on the support substrate were cleaned and surface-activated, respectively. Specifically, ultrasonic cleaning was performed using pure water, and the substrate surface was dried by spin drying. Next, the cleaned support substrate was introduced into a plasma activation chamber, and the bonding surface was activated at 30°C with nitrogen gas plasma. Similarly, the piezoelectric material substrate was introduced into a plasma activation chamber, and the bonding surface was surface-activated at 30°C with nitrogen gas plasma. The surface activation time was 40 seconds, and the energy was 100W. In order to remove particles that had adhered during surface activation, the same ultrasonic cleaning and spin drying as described above were performed again.

[0051] Next, the substrates were aligned, and the activated bonding surfaces of both substrates were brought into contact with each other at room temperature. The substrates were brought into contact with each other with the piezoelectric material substrate side facing up. As a result, the adhesion between the substrates was observed to spread (a so-called bonding wave), and it was confirmed that preliminary bonding had been performed satisfactorily. Next, in order to increase the bonding strength, the bonded body was placed in an oven with a nitrogen atmosphere and kept at 150°C for 10 hours. The bonding strength of the bonded body taken out of the oven was measured by the crack opening method, and was found to be 2.3 J / m 2 It was found that sufficient bonding strength was obtained.

[0052] After heating, the surface of the piezoelectric material substrate of the bonded body was subjected to grinding, lapping and CMP processing so that the thickness of the piezoelectric material substrate became 20 μm. Next, in order to confirm the effect of the present invention, a comb electrode made of metal aluminum was formed on the piezoelectric material substrate of the bonded body to fabricate a resonator of a surface acoustic wave element. The specifications are shown below. IDT period 6μm IDT opening length 300um Number of IDTs: 80 Number of reflectors: 40

[0053] When the reflection characteristics were measured using a network analyzer, the maximum spurious level in the region above the anti-resonance frequency was 2.2 dB, as shown in FIG.

[0054] The physical properties of the support substrate, intermediate layer, and piezoelectric material substrate in this example are summarized below. The sound velocity of each part is also shown in FIG. Arithmetic mean surface roughness (nm) Sound speed (m / sec) Piezoelectric material layer PZC - 5,574 First layer 1 1.0 4,173 Second tier middle tier 2 1.3 7,458 Third layer middle layer X 1.9 5,235 Support board S 3.2 ━

[0055] However, the sound speed in each part is defined as follows. That is, if the elastic modulus of a material is E and its density is ρ, the sound speed V is calculated using the following formula:

number

[0056] Example 2 A bonded body 7B as shown in FIG. 2(c) was produced in the same manner as in Example 1, and was then subjected to the treatment as shown in FIG. 3 to obtain a SAW element. However, intermediate layers X, 3, 2, and a bonding layer M were provided on a support substrate. Specifically, after cleaning the surface Sa of a support substrate S made of silicon with an arithmetic mean roughness Ra of 3.2 nm, an intermediate layer X made of aluminum oxide was formed using a sputtering device. The thickness of the intermediate layer X at this time was 600 nm. The wafer after film formation was temporarily removed, and the arithmetic mean roughness R xThe roughness was measured and found to have decreased to 2.7 nm. An intermediate layer 3 made of silicon oxide was formed again on the same wafer to a thickness of 1000 nm. The arithmetic mean roughness at this time was 1.6 nm. An intermediate layer 2 made of silicon was further formed to a thickness of 300 nm, and then a bonding layer M made of silicon oxide was continuously formed to a thickness of 350 nm, finally obtaining a support substrate with a four-layer film structure. The arithmetic mean roughnesses of the intermediate layer 2 and bonding layer M at this time were 1.3 nm and 1.2 nm, respectively, and the surfaces were significantly smoother than the initial 3.2 nm. The surface Ma of the outermost bonding layer M was processed by CMP to remove approximately 30 nm. As a result, the arithmetic mean roughness of the surface Ma of the bonding layer M was reduced to 0.55 nm.

[0057] A SAW element as shown in FIG. 3(c) was fabricated in the same manner as in Example 1, and similar measurements were carried out, revealing that the maximum spurious level was 1.3 dB.

[0058] The surface roughness and sound velocity of each layer are as follows. The sound velocity of each part is shown in Figure 5. Arithmetic mean surface roughness (nm) Sound speed (m / sec) Piezoelectric material layer PZC ━ 5,574 First layer 1 1.2 5,235 Second tier middle tier 2 1.3 6,872 Third tier middle tier 3 1.6 5,235 4th layer middle layer X 2.7 7,458 Support board S 3.2 ━

[0059] Comparative Example 1 In the same manner as in Example 1, a bonded body as shown in FIG. 1 was produced, and further subjected to the treatment as shown in FIG. 3 to obtain a SAW element. However, in this example, the material of each intermediate layer was changed from that of Example 1. Specifically, an intermediate layer X (600 nm) made of aluminum oxide, an intermediate layer 2 (1200 nm) made of silicon oxide, and an outermost bonding layer M (400 nm) made of silicon were successively formed on the surface Sa of a support substrate S made of silicon with an arithmetic mean roughness Ra=2.8 nm, to obtain a three-layer structure. The arithmetic mean roughnesses of the intermediate layers 2 and bonding layers M were 1.2 nm, 1.0 nm, and 0.9 nm, respectively. The outermost bonding layer was polished by CMP to a thickness of about 20 nm to obtain a mirror surface. Next, the surface of the bonding layer on the piezoelectric material substrate and the surface of the outermost intermediate layer on the support substrate were irradiated with neutral atoms of Ar, and then directly bonded.

[0060] As in Example 1, the thickness of the piezoelectric material substrate was processed to 20 μm, and then the frequency characteristics were measured. As shown in FIG. 11 A chart of the frequency change was obtained. The maximum spurious level was 14.4 dB.

[0061] The surface roughness and sound velocity of each layer are as follows: Arithmetic mean surface roughness (nm) Sound speed (m / sec) Piezoelectric material layer PZC ━ 5,574 First layer 1 0.9 6,872 Second tier middle tier 2 1.0 5,235 Third layer middle layer X 1.2 7,458 Support board S 2.8 ━

[0062] Comparative Example 2 In the same manner as in Example 1, a bonded body as shown in FIG. 1 was produced, and further subjected to the treatment as shown in FIG. 3 to obtain a SAW element. However, in this example, the material of each intermediate layer was changed from that of Example 1. Specifically, an intermediate layer X (600 nm) made of silicon, an intermediate layer 2 (1200 nm) made of aluminum oxide, and an outermost bonding layer M (400 nm) made of silicon were successively formed on the surface Sa of a support substrate S made of silicon with an arithmetic mean roughness Ra=2.9 nm. The arithmetic mean roughnesses of the intermediate layer 2 and the bonding layer M at the time of formation were 2.2 nm, 1.7 nm, and 1.6 nm, respectively. The outermost bonding layer was polished by CMP to a thickness of about 80 nm to obtain a mirror surface. Next, the surface of the piezoelectric material substrate and the surface of the outermost bonding layer on the support substrate were irradiated with neutral atoms of Ar, and then directly bonded.

[0063] As in Example 1, the piezoelectric material substrate was processed to a thickness of 20 μm, and then the frequency characteristics were measured, and the maximum spurious amplitude was 17.8 dB.

[0064] The surface roughness and sound velocity of each layer are as follows: Arithmetic mean surface roughness (nm) Sound speed (m / sec) Piezoelectric material layer PZC ━ 5,574 First layer 1 1.6 6,872 Second tier middle tier 2 1.7 7,458 Third layer middle layer X 2.2 6,872 Support board S 2.9 ━

Claims

1. A composite substrate for an acoustic wave device comprising: a piezoelectric material layer, a support substrate, and an intermediate layer of x layers (x is an integer of 3 or more) between the piezoelectric material layer and the support substrate, wherein the piezoelectric material layer, the support substrate, and the intermediate layer satisfy the following formula (1), and also satisfy the following formula (2) when x is an even number, and satisfy the following formula (3) when x is an odd number. R ｎ <R ｎ＋1 ・・・ (1) (In formula (1), n represents every integer from 1 to x; R ｎ is the arithmetic mean roughness of the surface of the n-th intermediate layer on the side of the piezoelectric material layer, as viewed from the piezoelectric material layer, The difference between R n and R n+1 is 0.2 nm or more; R ｘ＋1 is the arithmetic mean roughness of the surface of the support substrate on the side of the piezoelectric material layer, and is 1.5 to 5 nm. V ｎ－１ <V ｎ ・・・ (2) (In formula (2), n denotes every even number between 2 and x, V ｎ is the sound velocity of the n-th intermediate layer as viewed from the piezoelectric material layer.) V ｎ－１ >V ｎ ・・・ (3) (In formula (3), n is any odd number between 1 and x, V ｎ is the sound velocity of the n-th intermediate layer as viewed from the piezoelectric material layer, V ０ is the sound velocity in the piezoelectric layer.

2. 2. The composite substrate for an acoustic wave device according to claim 1, wherein the intermediate layer is made of a material selected from the group consisting of silicon, silicon oxide, alumina, tantalum pentoxide, niobium pentoxide, hafnium oxide and titanium oxide.

3. 3. The composite substrate for an acoustic wave device according to claim 1, wherein the surface of the support substrate facing the piezoelectric material layer is roughened by processing with a grinding wheel or by blasting.

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