Composite substrate, surface acoustic wave device, and method for manufacturing composite substrate

Abstract

There are provided a method for manufacturing a substrate excellent in heat dissipation with a small loss in radio frequencies with no need of a high temperature process in which a metal impurity is diffused, and a substrate of high thermal conductivity. A composite substrate according to the present invention is a composite substrate having a piezoelectric single crystal substrate, a support substrate, and an intermediate layer provided between the piezoelectric single crystal substrate and the support substrate. The intermediate layer is a film formed of an inorganic material, and at least a part of the film is thermally synthesized silica. The intermediate layer may be separated into at least two layers along the bonding surface of the composite substrate. The first intermediate layer in contact with the support substrate may be a layer including thermally synthesized silica.

Claims

1. A composite substrate comprising: a piezoelectric single crystal substrate; a support substrate; and an intermediate layer provided between the piezoelectric single crystal substrate and the support substrate, wherein the intermediate layer is a film formed of an inorganic material, and the intermediate layer is separated into at least three layers along a bonding surface of the composite substrate, and the intermediate layer includes: a first intermediate layer containing a thermally synthesized silica, the first intermediate layer being in contact with the support substrate; a second intermediate layer provided on the piezoelectric single crystal substrate side of the piezoelectric single crystal substrate; and a third intermediate layer formed of amorphous silicon, wherein the first intermediate layer is joined to the second intermediate layer as the third intermediate layer is sandwiched between the first intermediate layer and the second intermediate layer.

2. The composite substrate according to claim 1, wherein a material of the support substrate is a silicon substrate, and the thermally synthesized silica is synthesized by thermal oxidation of the silicon substrate.

3. The composite substrate according to claim 1, wherein the thermally synthesized silica is a sintered body of synthetic silica.

4. The composite substrate according to claim 1, wherein the intermediate layer is separated into at least two layers along a bonding surface of the composite substrate, and a material of a second intermediate layer provided on the piezoelectric single crystal substrate side of a first intermediate layer in contact with the support substrate includes at least one of SiOx, Al.sub.2O.sub.3, AlN, SiN, SiON, and Ta.sub.2O.sub.5.

5. The composite substrate according to claim 4, wherein the second intermediate layer has at least two layers having different materials.

6. The composite substrate according to claim 1, wherein a thickness of the first intermediate layer is 20 nm or more.

7. The composite substrate according to claim 1, wherein a thickness of the second intermediate layer is 25 μm or less.

8. The composite substrate according to claim 1, further comprising a thermally synthesized silica layer on a back surface of the support substrate.

9. The composite substrate according to claim 1, wherein the third intermediate layer is formed of amorphous silicon in a thickness of 50 nm or less.

10. The composite substrate according to claim 1, wherein a thickness of the intermediate layer is 8 μm or less.

11. The composite substrate according to claim 10, wherein a thickness of the intermediate layer is 7 μm or less.

12. The composite substrate according to claim 1, wherein a thickness of the piezoelectric single crystal is 20 μm or less.

13. The composite substrate according to claim 1, wherein an interface between the piezoelectric single crystal and the intermediate layer has an irregular structure.

14. A surface acoustic wave device comprising the composite substrate according to claim 1.

15. The surface acoustic wave device according to claim 14, wherein a thickness of the piezoelectric single crystal is 1.0 times or more and 3.5 times or less of a wavelength a surface acoustic wave.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a graph of thermal expansion coefficients of various materials in comparison.

(2) FIG. 2 shows a spectrum of the reflection coefficient of a resonator prepared from a conventional composite substrate.

(3) FIG. 3 shows the temperature dependence of amounts of out gas emission from intermediate layers.

(4) FIG. 4 shows a processing method of a support substrate before bonding.

(5) FIG. 5 shows a processing method of a piezoelectric single crystal substrate before bonding.

(6) FIG. 6 shows a bonding method and a processing method after bonding.

(7) FIG. 7 schematically shows the structure of a composite substrate provided with an amorphous silicon layer between a first intermediate layer and a second intermediate layer.

(8) FIG. 8 shows the cross section of a microphotograph of a composite substrate prepared in Example 1.

(9) FIG. 9 shows a spectrum of the reflection coefficient of a resonator prepared in Example 8.

DESCRIPTION OF EMBODIMENTS

(10) A composite substrate 1 according an embodiment is manufactured through processes in which a support substrate 100 and a piezoelectric single crystal substrate 200 are processed before bonding and then these two substrates are bonded to each other. In the following, manufacturing methods will be described with reference to FIGS. 4 to 6.

(11) [Process of the Support Substrate]

(12) First, the support substrate 100 is prepared (FIG. 4(a)). The support substrate 100 is selected from silicon, sapphire, silicon carbide, silicon nitride, aluminum nitride, and silica glass.

(13) Subsequently, a first intermediate layer 310 including thermally synthesized silica is formed on the surface of the support substrate 100 (FIG. 4(b)). At this time, a thermally synthesized silica layer having the same material as the first intermediate layer 310 may be formed on the back surface of the support substrate 100. Note that after the support substrate 100 is bonded to the piezoelectric single crystal substrate 200, the silica layer on the back surface may be appropriately removed using hydrofluoric acid, for example. In the case in which the material of the support substrate 100 is silicon, a thermally oxidized silica film to be the first intermediate layer 310 can be formed by thermal oxidation of a silicon substrate. Since thermally oxidized silica is grown at high temperature, the thermally oxidized silica has the properties of being fine with few impurities and can absorb gases to some extent.

(14) The first intermediate layer 310 can be formed by methods below regardless of whether the material of the support substrate 100 is silicon or not. That is, the first intermediate layer 310 may be formed by sintering a silica layer deposited by chemical vapor deposition (CVD) at a temperature of 800° C. or more. The first intermediate layer 310 may be formed by sintering a silica layer deposited by physical vapor deposition (PVD) at a temperature of 800° C. or more. Alternatively, the first intermediate layer 310 may be formed in which a solution of an organic silicon compound is applied and then sintered at a temperature of 800° C. or more. In the case in which the first intermediate layer 310 is formed in a silica sintered body, a heat resistant substrate is preferably used such that synthetic silica deposited on the support substrate 100 can be sintered together with the support substrate 100.

(15) Subsequently, the surface of the first intermediate layer 310 is flattened as necessary (FIG. 4(c)). This flattening may be performed by chemical-mechanical polishing. Note that in the case in which a silicon substrate having a mirror-finished surface is used as the support substrate 100 and the first intermediate layer 310 is formed by thermal oxidation, the surface of the first intermediate layer 310 has a mirror surface similar to the base, and hence this flattening process is unnecessary.

(16) Although not shown in the drawings, processes below may be performed as necessary in order to improve joining strength in bonding. For example, amorphous silicon may be deposited on the surface of the first intermediate layer 310. Stacking amorphous silicon makes the bonding interface that is Si/SiO.sub.2 or Si/Si, and this provides joining strength slightly higher than SiO.sub.2/SiO.sub.2 bonding (see detail, Tong Q. Y. and Gosele U., Semiconductor Wafer Bonding, Science and Technology, Chapter 4.7.1, 1999). The thickness of the amorphous silicon to be deposited is preferably 50 nm or less such that gas transmission is not inhibited. The surface of the first intermediate layer 310 may be subjected to activation. The surface activation process may be any one of ozone water treatment, UV ozone treatment, ion beam treatment, and plasma treatment, for example.

(17) With the processes above, the processes before bonding for the support substrate 100 is finished.

(18) [Processes of the Piezoelectric Single Crystal Substrate]

(19) First, the piezoelectric single crystal substrate 200 is prepared (FIG. 5 (a)). The piezoelectric single crystal substrate 200 is a piezoelectric single crystal, such as lithium tantalate (LT) and lithium niobate (LN). Subsequently, irregularities are formed on the surface of the piezoelectric single crystal substrate 200 as necessary (FIG. 5 (b)). The irregularities have the effect that suppresses the spurious phenomenon of a resonator prepared from the composite substrate 1. In the case in which the composite substrate 1 is used for the application that spuriousness causes no problem, this process may be omitted.

(20) Subsequently, a second intermediate layer 320 is formed by depositing the second intermediate layer 320 including an inorganic material on the surface of the piezoelectric single crystal substrate 200 (FIG. 5 (c)). The material of the second intermediate layer 320 includes any one of SiOx (e.g. SiO.sub.2), Al.sub.2O.sub.3, AlN, SiN, SiON, and Ta.sub.2O.sub.5. The second intermediate layer 320 can be formed by methods below. That is, the second intermediate layer 320 may be deposited by chemical vapor deposition (CVD). The second intermediate layer may be deposited by physical vapor deposition (PVD). Alternatively, the second intermediate layer 320 may be deposited in which a solution of an organic silicon compound is applied and then hardened. In order to reduce residual gases, the second intermediate layer 320 formed by any one of the methods may be heated at a temperature lower than the Curie temperature of the piezoelectric crystal. In order to prevent the second intermediate layer 320 from cracking due to the difference in the thermal expansion coefficient between the piezoelectric single crystal substrate 200 and the second intermediate layer 320, a heating temperature is preferably at a temperature of 600° C. or less. The second intermediate layer 320 may be configured in which the layer 320 has at least two layers having different materials.

(21) Subsequently, the surface of the formed second intermediate layer 320 is flattened (FIG. 5 (d)). This flattening may be performed by chemical-mechanical polishing.

(22) Although not shown in the drawings, processes below may be performed as necessary in order to improve joining strength in bonding. For example, amorphous silicon may be deposited on the surface of the second intermediate layer 320. The thickness of the amorphous silicon to be deposited is preferably 50 nm or less such that gas transmission is not inhibited. Note that in the case in which amorphous silicon is also deposited on the first intermediate layer 310, the total thickness may be 50 nm or less. The surface of the second intermediate layer 320 may be subjected to activation. The surface activation process may be any one of ozone water treatment, UV ozone treatment, ion beam treatment, and plasma treatment, for example.

(23) With the processes above, the processes for the piezoelectric single crystal substrate 200 before bonding are finished.

(24) [Bonding and Processes after Bonding]

(25) On the support substrate 100 and the piezoelectric single crystal substrate 200 processed as described above, the surface of the first intermediate layer 310 is bonded to the surface of the second intermediate layer 320 (FIG. 6 (a)). At this time, joining strength may be improved by heating the bonded substrates at a low temperature (e.g., 120° C.)

(26) Subsequently, the piezoelectric single crystal substrate 200 is ground and polished to reduce the thickness. For example, the thickness of the piezoelectric single crystal substrate 200 is reduced to a thickness of about 20 μm (FIG. 6 (b)). After that, additional heat treatment is performed further as necessary, and joining force may be reinforced. The first intermediate layer 310 and the second intermediate layer 320 thus bonded and joined configure an intermediate layer 300. As shown in FIG. 7, in the configuration of the composite substrate 1 provided with an amorphous silicon layer (a third intermediate layer) 330 between the first intermediate layer 310 and the second intermediate layer 320, the first intermediate layer 310, the second intermediate layer 320, and the third intermediate layer 330 configure the intermediate layer 300.

(27) By the manufacturing method described above, the composite substrate 1 having the piezoelectric single crystal substrate 200 joined to the support substrate 100 with the intermediate layer 300 sandwiched can be manufactured.

(28) [Evaluation of the Surface Acoustic Wave Properties]

(29) On the surface of the piezoelectric single crystal substrate 200 of the prepared composite substrate 1, an aluminum (Al) thin film in a thickness of 0.4 μm was sputtered, electrodes were formed by photolithography, and then a four-stage rudder filter formed of a two-stage parallel resonators and a four-stage series resonator at a wavelength of 5 μm and a resonator were prepared. For photolithographic exposure, a g-line stepper was used, and for Al etching, a mixed gas of Cl.sub.2, BCl.sub.3, N.sub.2, and CF.sub.4 was used. With the use of a network analyzer, the reflection coefficient (S11) of the prepared four-stage rudder filter was measured. The difference between the peak and bottom of observed spuriousness was evaluated as spurious intensity.

(30) The relationship between the Q-value and the resonance frequency of the prepared resonator was found by Equation (1) below.

(31) $\begin{array}{cc}\left[\mathrm{Equation}1\right]& \\ Q=\frac{\omega .Math.\tau .Math..Math.S_{11}.Math.}{1-{.Math.S_{11}.Math.}^2}& \left(1\right)\end{array}$

(32) Here, ω is the angular frequency, and τ is the group delay time. The Q-value is the value indicating the sharpness of the waveform of resonance, and a filter having a higher Q value can be of excellent properties with a small loss. Since the Q-value has frequency dependency, the maximum value (Q.sub.max) of the Q-value was evaluated.

EXAMPLES

Example 1

(33) An LT substrate in a diameter of six inches having a roughness of about 230 nm (in P-V, about 1.7 μm) at Ra (arithmetic average roughness) was prepared as a piezoelectric single crystal substrate. On the LT substrate, an SiO.sub.2 film was formed by deposition in a thickness of about 10 μm by plasma CVD using silane and oxygen gas as raw material gases. This SiO.sub.2 film was subjected to heat treatment at a temperature of about 400° C., the surface was polished and mirror-finished, and the SiO.sub.2 film was finished in a film thickness of about 2 μm.

(34) An Si substrate in a diameter of six inches having a thermal oxidation film grown in a thickness of 500 nm was prepared as a support substrate. After that, plasma surface activation was applied to both of the LT substrate deposited with the SiO.sub.2 film and the Si substrate having the thermal oxidation film grown. The two substrates were bonded to each other and subjected to heat treatment at a temperature of 120° C., and then the LT was ground and polished to reduce the thickness to about 20 The cross section of a microphotograph of the composite substrate thus finished is shown in FIG. 8.

(35) A heat resistance test was examined in which the wafer of the composite substrate was diced in two-millimeter squares and reciprocated between a hot plate at a temperature of 200° C. and a metal cooling stage (the substrate was held for 30 seconds each on the hot plate and on the cooling stage). The wafer was reciprocated for 100 times. However, peeling, for example, was not observed.

(36) Note that the similar experiment was performed in which an oxide film was formed on the surface of a mirror-finished LT substrate under the same conditions. However, the entirely similar result was obtained regarding heat resistance. In the case of applications in which spuriousness causes no problem, an oxide film was formed on the mirror-finished LT substrate for performing the similar methods, and hence this shown that the present invention is applicable as it is.

Comparative Example 1

(37) An experiment similar to Example 1 was performed using an LT substrate provided with an SiO.sub.2 film similarly to Example 1 and an Si substrate without thermal oxidation. As a result of a heat resistance test similar to Example 1, peeling was observed at edges in the fifth reciprocation. With the comparison between Example 1 and Comparative Example 1, the LT substrate provided with the SiO.sub.2 film by plasma CVD was bonded to the Si substrate having the thermal oxidation film grown, and hence it was shown that peeling can be suppressed compared with the case in which the Si substrate with no thermal oxidation film grown is bonded.

Example 2

(38) An LT substrate in a diameter of six inches having a roughness of about 230 nm (in P-V, about 1.7 μm) in Ra was prepared. On the LT substrate, a process was repeated for a plurality of times in which a solution of an organic silicon compound was spin-coated and heated at a temperature of 350° C., and hence an SiO.sub.2 layer in a thickness of about 5 μm was obtained. The solution of the organic silicon compound used here is two types of perhydropolysilazane (a solvent was dibutylether) and methyltrimethoxysilane (a solvent was propylene glycol monoethyl ether).

(39) After heat treatment at a temperature of about 400° C., the surface of this SiO.sub.2 film was polished and mirror-finished. An Si substrate in a diameter of six inches having a thermal oxidation film grown in a thickness of 500 nm was prepared. Plasma surface activation was applied to the two substrates. The two substrates were bonded to each other and subjected to heat treatment at a temperature of 120° C., the LT was ground and polished to reduce the thickness to 20 μm, and then a composite substrate was obtained.

(40) A heat resistance test was examined in which the wafer of the composite substrate was diced in two-millimeter squares and reciprocated between a hot plate at a temperature of 200° C. and a metal cooling stage (the substrate was held for 30 seconds each on the hot plate and on the cooling stage). The wafer was reciprocated for 100 times. However, peeling, for example, was not observed.

Comparative Example 2

(41) An experiment similar to Example 2 was performed using an LT substrate provided with an SiO.sub.2 film similarly to Example 2 and an Si substrate without thermal oxidation. As a result of a heat resistance test similar to Example 2, peeling was observed at edges in the seventh reciprocation. With the comparison between Example 2 and Comparative Example 2, the LT substrate provided with the SiO.sub.2 film by spin-coating and heating the solution of the organic silicon compound was bonded to the Si substrate having the thermal oxidation film grown, and hence it was shown that peeling can be suppressed compared with the case in which the Si substrate with no thermal oxidation film grown is bonded.

Example 3

(42) An LT substrate in a diameter of six inches having a roughness of about 230 nm (in P-V, about 1.7 μm) in Ra was prepared. On the LT substrate, an SiO.sub.2 film was deposited in a thickness of about 10 μm by PVD (here, magnetron sputtering). After heat treatment at a temperature of about 400° C., the surface of this SiO.sub.2 film was polished and mirror-finished. An Si substrate in a diameter of six inches having a thermal oxidation film grown in a thickness of 500 nm was prepared. Plasma surface activation was applied to the two substrates. The two substrates were bonded to each other and subjected to heat treatment at a temperature of 120° C., the LT was ground and polished to reduce the thickness to 20 μm, and then a composite substrate was obtained.

(43) A heat resistance test was examined in which the wafer of the composite substrate was diced in two-millimeter squares and reciprocated between a hot plate at a temperature of 200° C. and a metal cooling stage (the substrate was held for 30 seconds each on the hot plate and on the cooling stage). The wafer was reciprocated for 100 times. However, peeling, for example, was not observed.

Comparative Example 3

(44) An experiment similar to Example 3 was performed using an LT substrate provided with an SiO.sub.2 film similarly to Example 3 and an Si substrate without thermal oxidation. As a result of a heat resistance test similar to Example 3, peeling was observed at edges in the second reciprocation. With the comparison between Example 3 and Comparative Example 3, the LT substrate provided with the SiO.sub.2 film by PVD was bonded to the Si substrate having the thermal oxidation film grown, and hence it was shown that peeling can be suppressed compared with the case in which the Si substrate with no thermal oxidation film grown is bonded.

Example 4

(45) A plurality of LT substrates in a diameter of six inches having a roughness of about 230 nm (in P-V, about 1.7 μm) in Ra was prepared. On the prepared LT substrates, an SiO.sub.2 film was deposited in a thickness of about 10 μm by plasma CVD. Heat treatment at a temperature of about 400° C. was applied to the SiO.sub.2 film, the surface was polished and mirror-finished, and then amorphous silicon (a-Si) was deposited in various thicknesses as shown in Table 1. The amorphous silicon was deposited by varying thicknesses by PVD (magnetron sputtering) and by CVD. Plasma surface activation was applied to both of the LT substrate and the Si substrate in a diameter of six inches having the thermal oxidation film grown in a thickness of 500 nm. The two substrates were bonded to each other and subjected to heat treatment at a temperature of 120° C., the LT was ground and polished to reduce the thickness to 20 μm, and then a composite substrate was obtained.

(46) A heat resistance test was examined in which the wafer of the obtained composite substrate was diced in two-millimeter squares and reciprocated between a hot plate at a temperature of 200° C. and a metal cooling stage (the substrate was held for 30 seconds each on the hot plate and on the cooling stage). The result is shown in Table 1. It is shown that after the thickness of the amorphous silicon exceeds 50 nm both by PVD and by CVD, the resistance is rapidly degraded. It is considered that gases generated from SiO.sub.2 on the LT side fail to transmit the amorphous silicon film due to an increase in the thickness of the amorphous silicon and the absorption of the gasses to the Si thermal oxidation film is inhibited.

(47) TABLE-US-00001 TABLE 1 a-Si Thickness (nm) 3 5 10 20 35 50 65 80 80 100 PVD >100Times >100Times >100Times >100Times >100Times >100Times 75Times 52Times 12Times 3Times CVD >100Times >100Times >100Times >100Times >100Times >100Times 19Times 11Times  5Times 2Times

Example 5

(48) Amorphous silicon was deposited on the side of an Si substrate having a thermal oxidation film grown, not on the LT substrate side, and an experiment similar to Example 4 was performed. The result was almost the same as the result in Example 4. From the result, it was shown that amorphous silicon can be provided on any of the LT substrate side and the Si substrate side.

Example 6

(49) Amorphous silicon was deposited on both of an LT substrate and an oxidation Si substrate side, and an experiment similar to Example 4 was performed. The thickness of the amorphous silicon was the total thickness of the amorphous silicon films deposited on both substrates. The result was almost the same as the result in Example 4. From the result, it was shown that amorphous silicon can be provided on both of the LT substrate side and the Si substrate side.

Example 7

(50) The experiment was performed as the surface activation method was changed to ozone water treatment, UV ozone treatment, and ion beam treatment. However, no difference was found in the results of bonding. It is thought that the present invention does not strongly depend on activation methods. No great difference was observed when surface activation was applied to only one substrate.

Example 8

(51) A four-stage rudder filter was formed on a composite substrate prepared by the method in Example 1, the spectrum of the reflection coefficient (S11) was measured, and the spurious properties were evaluated. As a result, as shown in FIG. 9, it was confirmed that spurious strength is 1 dB or less. It was revealed that the resonator prepared by this method can effectively reduce spuriousness.

Example 9

(52) An experiment similar to Example 1 was performed by changing the thickness of a thermal oxidation film grown on an Si substrate. As a result, an effect was confirmed in the thermal oxidation films in a thickness of 20 nm or more. However, in thermal oxidation films in a thickness of less than 20 nm, out gas absorption capability was insufficient, and peeling was sometimes observed in heating and cooling cycle tests.

Example 10

(53) An experiment similar to Example 1 was performed by changing the thickness of an SiO.sub.2 film deposited on an LT substrate. The result is shown in Table 2. The thickness of the deposited SiO.sub.2 shown in Table 2 is the thickness after the surface was flattened. This result revealed that when the thickness of the SiO.sub.2 film (only the deposited film, the thermal oxidation film is not included) exceeds 25 μm, cracks were generated in the LT layer. It is thought that the cracks were generated by stress due to the difference in the expansion coefficient between LT and SiO.sub.2. In the case in which the thickness of SiO.sub.2 is 25 μm or less, SiO.sub.2 can be deformed following the difference in the expansion coefficient to some extent. However, it is thought that when the thickness is 25 μm or more, cracks are generated due to stress relaxation.

(54) TABLE-US-00002 TABLE 2 Thickness of deposited SiO.sub.2 (μm) 1 2 5 10 20 25 30 Crack No crack No crack No crack No crack No crack Some cracks Cracks observed generation state on edges on whole wafer

Example 11

(55) An LT substrate in a diameter of six inches having a mirror-finished surface on one side was prepared. On the mirror surface side of the LT substrate, an Al.sub.2O.sub.3 film was deposited in a thickness of 1 μm by sputtering. On the Al.sub.2O.sub.3 film of the Al.sub.2O.sub.3 film attached LT substrate, an SiO.sub.2 film was deposited in a thickness of about 10 μm by plasma CVD using silane and oxygen gas as raw material gases. The film stack of Al.sub.2O.sub.3 and SiO.sub.2 was subjected to heat treatment at a temperature of about 400° C., the surface was polished and mirror-finished, and the SiO.sub.2 film was finished in a film thickness of about 2 μm.

(56) An Si substrate in a diameter of six inches having a thermal oxidation film grown in a thickness of 500 nm was prepared. Plasma surface activation was applied to the two substrates. The two substrates were bonded to each other and subjected to heat treatment at a temperature of 120° C., the LT was ground and polished to reduce the thickness to about 6 μm, and then a composite substrate was obtained.

(57) A heat resistance test was examined in which the wafer of the composite substrate was diced in two-millimeter squares and reciprocated between a hot plate at a temperature of 200° C. and a metal cooling stage (the substrate was held for 30 seconds each on the hot plate and on the cooling stage). The wafer was reciprocated up to 200 reciprocations. However, peeling, for example, was not observed. From the example, it was shown that a multi-layered intermediate layer (i.e., the second intermediate layer) may be provided on the LT substrate side before bonding.

Example 12

(58) An LT substrate in a diameter of six inches having a roughness of about 230 nm (in P-V, about 1.7 μm) at Ra (arithmetic average roughness) was prepared. On the LT substrate, an SiO.sub.2 film was formed by deposition in a thickness of about 10 μm by plasma CVD using silane and oxygen gas as raw material gases. This SiO.sub.2 film was subjected to heat treatment at a temperature of about 400° C., the surface was polished and mirror-finished, and the SiO.sub.2 film was finished in a film thickness of about 2 μm.

(59) A sapphire substrate in a diameter of six inches was prepared, and on the substrate, an SiO.sub.2 film was deposited in a thickness of about 5 μm by plasma CVD. The SiO.sub.2 film on the sapphire substrate was sintered by heat treatment at a temperature of 800° C., the surface was polished and mirror-finished, and the SiO.sub.2 film was finished in a film thickness of 1 μm.

(60) A plasma activation process was applied to both of the SiO.sub.2 film deposited on the LT substrate and the sintered SiO.sub.2 film deposited on the sapphire substrate, the two substrates were bonded to each other and subjected to heat treatment at a temperature of 120° C., the LT was ground and polished to reduce the thickness to 20 μm, and then a composite substrate was obtained.

(61) A heat resistance test was examined in which the wafer of the composite substrate was diced in two-millimeter squares and reciprocated between a hot plate at a temperature of 200° C. and a metal cooling stage (the substrate was held for 30 seconds each on the hot plate and on the cooling stage). The wafer was reciprocated for 100 times. However, peeling, for example, was not observed. From the example, it was shown that a sapphire substrate can be used as a support substrate. It was shown that thermally synthesized silica obtained by heating and sintering SiO.sub.2 deposited by CVD can be used for an intermediate layer (i.e., the first intermediate layer) provided on the support substrate side before bonding.

Example 13

(62) An LT substrate in a diameter of six inches having a roughness of about 230 nm (in P-V, about 1.7 μm) at Ra (arithmetic average roughness) was prepared. On the LT substrate, an SiO.sub.2 film was formed by deposition in a thickness of about 10 μm by plasma CVD using silane and oxygen gas as raw material gases. This SiO.sub.2 film was subjected to heat treatment at a temperature of about 400° C., the surface was polished and mirror-finished, and the SiO.sub.2 film was finished in a film thickness of about 2 μm.

(63) A sapphire substrate in a diameter of six inches was prepared, and on the substrate, an SiO.sub.2 film was deposited in a thickness of about 10 μm by PVD (magnetron sputtering). The SiO.sub.2 film on the sapphire substrate was sintered by heat treatment at a temperature of 900° C., the surface was polished and mirror-finished, and SiO.sub.2 film was finished in a film thickness of 1 μm.

(64) A plasma activation process was applied to both of the SiO.sub.2 film deposited on the LT substrate and the sintered SiO.sub.2 film deposited on the sapphire substrate, the two substrates were bonded to each other and subjected to heat treatment at a temperature of 120° C., the LT was ground and polished to reduce the thickness to 20 μm, and then a composite substrate was obtained.

(65) A heat resistance test was examined in which the wafer of the composite substrate was diced in two-millimeter squares and reciprocated between a hot plate at a temperature of 200° C. and a metal cooling stage (the substrate was held for 30 seconds each on the hot plate and on the cooling stage). The wafer was reciprocated for 100 times. However, peeling, for example, was not observed. From the example, it was shown that a sapphire substrate can be used as a support substrate. It was shown that thermally synthesized silica obtained by heating and sintering SiO.sub.2 deposited by PVD can be used for an intermediate layer (i.e., the first intermediate layer) provided on the support substrate side before bonding.

Example 14

(66) An LT substrate in a diameter of six inches having a roughness of about 230 nm (in P-V, about 1.7 μm) at Ra (arithmetic average roughness) was prepared. On the LT substrate, an SiO.sub.2 film was formed by deposition in a thickness of about 10 μm by plasma CVD using silane and oxygen gas as raw material gases. This SiO.sub.2 film was subjected to heat treatment at a temperature of about 400° C., the surface was polished and mirror-finished, and the SiO.sub.2 film was finished in a film thickness of about 2 μm.

(67) A sapphire substrate in a diameter of six inches was prepared, and on the substrate, an SiO.sub.2 film was deposited in a thickness of about 3 μm by repeating a process of spin coating a solution of the organic silicon compound (a dibutylether solution of perhydropolysilazane) and a process of thermosetting at a temperature of 350° C. for a few times. The SiO.sub.2 film on the sapphire substrate was sintered by heat treatment at a temperature of 900° C., the surface was polished and mirror-finished, and the thickness of the SiO.sub.2 film was finished to a film thickness of 0.5 μm.

(68) A plasma activation process was applied to both of the SiO.sub.2 film deposited on the LT substrate and the sintered SiO.sub.2 film deposited on the sapphire substrate, the two substrates were bonded to each other and subjected to heat treatment at a temperature of 120° C., the LT was ground and polished to reduce the thickness to 20 μm, and then a composite substrate was obtained.

(69) A heat resistance test was examined in which the wafer of the composite substrate was diced in two-millimeter squares and reciprocated between a hot plate at a temperature of 200° C. and a metal cooling stage (the substrate was held for 30 seconds each on the hot plate and on the cooling stage). The wafer was reciprocated for 100 times. However, peeling, for example, was not observed. From the example, it was shown that a sapphire substrate can be used as a support substrate. It was shown that thermally synthesized silica obtained by applying, heating, and sintering organic silicon can be used for an intermediate layer (i.e., the first intermediate layer) provided on the support substrate side before bonding.

Example 15

(70) An LT substrate in a diameter of six inches having a roughness of about 230 nm (in P-V, about 1.7 μm) at Ra (arithmetic average roughness) was prepared. On the LT substrate, an SiO.sub.2 film was formed by deposition in a thickness of about 10 μm by plasma CVD using silane and oxygen gas as raw material gases. This SiO.sub.2 film was subjected to heat treatment at a temperature of about 400° C., the surface was polished and mirror-finished, and the SiO.sub.2 film was finished in a film thickness of about 2 μm.

(71) A silica glass substrate in a diameter of six inches was prepared, and on the substrate, an SiO.sub.2 film was deposited in a thickness of about 3 μm by repeating a process of spin coating a solution of the organic silicon compound (a propylene glycol monoethyl ether solution of methyltrimethoxysilane) and a process of thermosetting at a temperature of 350° C. for a few times. The SiO.sub.2 film on the silica glass substrate was sintered by heat treatment at a temperature of 1,000° C., the surface was polished and mirror-finished, and the thickness of the SiO.sub.2 film was finished to a film thickness of 0.5 μm.

(72) A plasma activation process was applied to both of the SiO.sub.2 film deposited on the LT substrate and the sintered SiO.sub.2 film deposited on the silica glass substrate, the two substrates were bonded to each other and subjected to heat treatment at a temperature of 120° C., the LT was ground and polished to reduce the thickness to 20 μm, and then a composite substrate was obtained.

(73) A heat resistance test was examined in which the wafer of the composite substrate was diced in two-millimeter squares and reciprocated between a hot plate at a temperature of 200° C. and a metal cooling stage (the substrate was held for 30 seconds each on the hot plate and on the cooling stage). The wafer was reciprocated for 100 times. However, peeling, for example, was not observed. From the example, it was shown that a silica glass substrate can be used as a support substrate. It was shown that thermally synthesized silica obtained by applying, heating, and sintering organic silicon can be used for an intermediate layer (i.e., the first intermediate layer) provided on the support substrate side before bonding.

Example 16

(74) A plurality of LT substrates in irregular structures having similar Ra (arithmetic average roughness) and RSm (The Mean width of profile elements (roughness) was prepared (Ra=300 nm±10%, RSm=3 μm±10%, and Rz=2.0±10%). Here, the irregular structure of the LT substrate was formed by polishing using free abrasive grains. Here, the definitions of Ra and RSm were in compliance with JIS B 0601:2001 and ISO 4287:1997, and calculated from the profiles measured using an atomic force microscope (AFM).

(75) Subsequently, on the surface having the irregular structure of the LT substrate, SiO.sub.2 was deposited in a thickness of about 10 μm at a temperature of 35° C. using plasma CVD, the surface having SiO.sub.2 deposited was polished to form a mirror surface. At this time, the polishing amount was changed depending on the LT substrates, and the thickness of SiO.sub.2 was from 1.5 μm to 9.5 μm.

(76) An Si substrate to be a support substrate was subjected to heat treatment at a temperature of 850° C. in an oxygen atmosphere, and hence thermally oxidized silica in a thickness of 0.5 μm was formed on the surface of the Si substrate.

(77) Thermally oxidized silica formed on both of the SiO.sub.2 mirror surface and the surface of the Si substrate was subjected to plasma surface activation, the LT substrate was bonded to the support substrate, the LT substrate was polished, and then a composite substrate was prepared. At this time, the polishing amount was changed depending on the substrates, and the thickness of the LT substrate was 5 μm to 25 μm.

(78) The composite substrate wafers were loaded into an oven and gradually heated, and a temperature at which the LT substrate starts peeling was measured. Electrodes were formed on the surfaces of the composite substrates, and the evaluation of the surface acoustic wave properties was performed. The evaluation results are shown in Tables 3 and 4.

(79) TABLE-US-00003 TABLE 3 Intermediate layer thickness [μm] Peeling Intermediate S11 Si LT LT starting layer LT spurious substrate substrate thickness temperature thickness/ thickness/ intensity side side Total [μm] [° C.] wavelength wavelength [dB] Qmax 0.5 1.5 2.0 5.0 350 0.4 1.0 0.20 4000 0.5 1.5 2.0 7.5 340 0.4 1.5 0.20 4000 0.5 1.5 2.0 10.0 320 0.4 2.0 0.40 4000 0.5 1.5 2.0 12.5 310 0.4 2.5 0.40 4000 0.5 1.5 2.0 15.0 300 0.4 3.0 0.40 4000 0.5 1.5 2.0 17.5 290 0.4 3.5 0.55 3250 0.5 1.5 2.0 20.0 280 0.4 4.0 0.70 2500 0.5 1.5 2.0 22.5 250 0.4 4.5 0.65 2150 0.5 1.5 2.0 25.0 220 0.4 5.0 0.60 1800 0.5 2.5 3.0 5.0 330 0.6 1.0 0.30 3500 0.5 2.5 3.0 7.5 335 0.6 1.5 0.35 4000 0.5 2.5 3.0 10.0 310 0.6 2.0 0.30 4000 0.5 2.5 3.0 12.5 303 0.6 2.5 0.28 4000 0.5 2.5 3.0 15.0 295 0.6 3.0 0.25 4000 0.5 2.5 3.0 17.5 283 0.6 3.5 0.35 3250 0.5 2.5 3.0 20.0 270 0.6 4.0 0.45 2500 0.5 2.5 3.0 22.5 235 0.6 4.5 0.63 2150 0.5 2.5 3.0 25.0 200 0.6 5.0 0.80 1800 0.5 3.5 4.0 5.0 320 0.8 1.0 0.40 3200 0.5 3.5 4.0 7.5 330 0.8 1.5 0.30 3800 0.5 3.5 4.0 10.0 300 0.8 2.0 0.20 4000 0.5 3.5 4.0 12.5 295 0.8 2.5 0.20 3900 0.5 3.5 4.0 15.0 290 0.8 3.0 0.20 3800 0.5 3.5 4.0 17.5 275 0.8 3.5 0.38 3150 0.5 3.5 4.0 20.0 260 0.8 4.0 0.55 2500 0.5 3.5 4.0 22.5 220 0.8 4.5 0.43 2150 0.5 3.5 4.0 25.0 180 0.8 5.0 0.30 1800 0.5 4.5 5.0 5.0 300 1.0 1.0 0.20 3000 0.5 4.5 5.0 7.5 320 1.0 1.5 0.20 3800 0.5 4.5 5.0 10.0 290 1.0 2.0 0.50 4000 0.5 4.5 5.0 12.5 288 1.0 2.5 0.48 3900 0.5 4.5 5.0 15.0 285 1.0 3.0 0.45 3800 0.5 4.5 5.0 17.5 270 1.0 3.5 0.38 3150 0.5 4.5 5.0 20.0 255 1.0 4.0 0.30 2500 0.5 4.5 5.0 22.5 213 1.0 4.5 0.50 2150 0.5 4.5 5.0 25.0 170 1.0 5.0 0.70 1800 0.5 5.5 6.0 5.0 280 1.2 1.0 0.30 2800 0.5 5.5 6.0 7.5 300 1.2 1.5 0.40 3700 0.5 5.5 6.0 10.0 280 1.2 2.0 0.20 3800 0.5 5.5 6.0 12.5 278 1.2 2.5 0.28 3750 0.5 5.5 6.0 15.0 275 1.2 3.0 0.35 3700 0.5 5.5 6.0 17.5 253 1.2 3.5 0.43 3100 0.5 5.5 6.0 20.0 230 1.2 4.0 0.50 2500 0.5 5.5 6.0 22.5 195 1.2 4.5 0.50 2150 0.5 5.5 6.0 25.0 160 1.2 5.0 0.50 1800

(80) TABLE-US-00004 TABLE 4 Intermediate layer thickness [μm] Peeling Intermediate S11 Si LT LT starting layer LT spurious substrate substrate thickness temperature thickness/ thickness/ intensity side side Total [μm] [° C.] wavelength wavelength [dB] Qmax 0.5 6.5 7.0 5.0 260 1.4 1.0 0.20 2500 0.5 6.5 7.0 7.5 275 1.4 1.5 0.30 3700 0.5 6.5 7.0 10.0 260 1.4 2.0 0.30 3600 0.5 6.5 7.0 12.5 263 1.4 2.5 0.30 3650 0.5 6.5 7.0 15.0 265 1.4 3.0 0.30 3700 0.5 6.5 7.0 17.5 243 1.4 3.5 0.50 3100 0.5 6.5 7.0 20.0 220 1.4 4.0 0.70 2500 0.5 6.5 7.0 22.5 185 1.4 4.5 0.63 2150 0.5 6.5 7.0 25.0 150 1.4 5.0 0.55 1800 0.5 7.5 8.0 5.0 240 1.6 1.0 0.30 2000 0.5 7.5 8.0 7.5 255 1.6 1.5 0.15 3600 0.5 7.5 8.0 10.0 240 1.6 2.0 0.40 3400 0.5 7.5 8.0 12.5 235 1.6 2.5 0.30 3500 0.5 7.5 8.0 15.0 230 1.6 3.0 0.20 3600 0.5 7.5 8.0 17.5 215 1.6 3.5 0.40 3050 0.5 7.5 8.0 20.0 200 1.6 4.0 0.60 2500 0.5 7.5 8.0 22.5 170 1.6 4.5 0.75 2150 0.5 7.5 8.0 25.0 140 1.6 5.0 0.90 1800 0.5 8.5 9.0 5.0 230 1.8 1.0 0.10 1800 0.5 8.5 9.0 7.5 228 1.8 1.5 0.20 3500 0.5 8.5 9.0 10.0 220 1.8 2.0 0.50 3200 0.5 8.5 9.0 12.5 215 1.8 2.5 0.45 3400 0.5 8.5 9.0 15.0 210 1.8 3.0 0.40 3600 0.5 8.5 9.0 17.5 200 1.8 3.5 0.38 3000 0.5 8.5 9.0 20.0 190 1.8 4.0 0.35 2400 0.5 8.5 9.0 22.5 160 1.8 4.5 0.58 2100 0.5 8.5 9.0 25.0 130 1.8 5.0 0.80 1800 0.5 9.5 10.0 5.0 220 2.0 1.0 0.20 1600 0.5 9.5 10.0 7.5 215 2.0 1.5 0.30 3400 0.5 9.5 10.0 10.0 210 2.0 2.0 0.40 3000 0.5 9.5 10.0 12.5 205 2.0 2.5 0.50 3300 0.5 9.5 10.0 15.0 200 2.0 3.0 0.60 3600 0.5 9.5 10.0 17.5 185 2.0 3.5 0.60 3000 0.5 9.5 10.0 20.0 170 2.0 4.0 0.60 2400 0.5 9.5 10.0 22.5 145 2.0 4.5 0.65 2100 0.5 9.5 10.0 25.0 120 2.0 5.0 0.70 1800

(81) These results show that wafers having an LT substrate in a smaller thickness and having an intermediate layer in a smaller thickness has higher peeling starting temperature and the wafers are of excellent heat resistance.

(82) In the case in which heating at a temperature of 200° C. is taken into account in the post-processes, an LT substrate in a thickness of 15 μm or less is usable regardless of an intermediate layer in a thickness of 2 to 10 μm, which is preferable. In this case, when the thickness of the LT substrate is 17 μm, the total thickness of the intermediate layer is preferably 9 μm or less. When the thickness of the LT substrate is increased to 20 μm, the total thickness of the intermediate layer is preferably 8 μm or less.

(83) In the case in which heating at a temperature of 250° C. is further taken into account in the post-processes, when the thickness of the LT substrate is 15 μm or less, the thickness of the intermediate layer is 7 μm or less, which is more preferable. When the thickness of the LT substrate is increased to 20 μm, the thickness of the intermediate layer is 6 μm or less, which is more preferable.

(84) On the other hand, it is shown that as the thickness of the intermediate layer is smaller, the Q-value is more increased. When the thickness of the LT substrate is less than 1.5 wavelength or exceeds 3.0 wavelength, the Q-value tend to decrease.

(85) Regardless of the thickness of the LT substrate or the thickness of the intermediate layer, the spurious intensity is suppressed as low as 1.0 dB or less.

Example 17

(86) An LT substrate in a diameter of 100 mm and a thickness of 0.35 mm having a roughness of 20 nm in Ra (arithmetic average roughness) was prepared. On the LT substrate, a 200 nm SiO.sub.2 film was deposited in a thickness of about 10 μm film by PVD, the film was polished to a thickness of 50 nm, the surface was mirror-finished. It was confirmed that the surface roughness was 1.0 nm or less in RMS. Subsequently, to the LT substrate formed with the SiO.sub.2 film, hydrogen ions (H.sup.+) were implanted under in which the dose amount was 7.0×10.sup.16 atoms/cm.sup.2 and the accelerating voltage was 100 KeV. An Si substrate is prepared as a support substrate, and the thermal oxidation film in a thickness of 500 nm was grown. A plasma activation process was applied to the LT substrate and the Si substrate, and surface activation was performed. The two substrates were bonded to each other, and subjected to heat treatment at a temperature of 100° C. for 24 hours. Subsequently, mechanical peeling was performed in which a blade in a wedge shaped was placed near the ion implantation interface on the side surface of the substrates thus bonded to each other. Thus, a composite substrate having a stack of the LT thin film in a thickness of about 600 nm on the Si substrate through the SiO.sub.2 layer can be obtained. After polishing to provide a mirror surface, evaluation was performed. However, in a thermal endurance test, peeling was not observed.

(87) Note that in Example 17, the implantation of hydrogen ions to the LT substrate can also be performed before the SiO.sub.2 film is deposited. Also with this configuration, the effect similar to the case can be obtained in which the SiO.sub.2 film was deposited and then hydrogen ions were implanted. When the dose amount of hydrogen ions ranges from 6.0×10.sup.16 atoms/cm.sup.2 to 2.75×10.sup.17 atoms/cm.sup.2, the similar effect can be obtained. Instead of hydrogen ions, the similar effect can be obtained by implanting hydrogen molecule ions (H.sub.2.sup.+) in the range of 3.0×10.sup.16 atoms/cm.sup.2 to 1.37×10.sup.17 atoms/cm.sup.2.

Example 18

(88) An LT substrate in a diameter of 100 mm and a thickness of 0.35 mm having a roughness of 20 nm in Ra (arithmetic average roughness) was prepared. On the LT substrate, a 200 nm SiO.sub.2 film was deposited in a thickness of about 10 μm film by PVD, the film was polished to a thickness of 50 nm, the surface was mirror-finished. It was confirmed that the surface roughness was 1.0 nm or less in RMS. Subsequently, to the LT substrate formed with the SiO.sub.2 film, hydrogen ions were implanted under in which the dose amount was 7.0×10.sup.16 atoms/cm.sup.2 and the accelerating voltage was 100 KeV. An Si substrate is prepared as a support substrate, and the thermal oxidation film in a thickness of 500 nm was grown. A plasma activation process was applied to the LT substrate and the Si substrate, and surface activation was performed. The two substrates were bonded to each other, and subjected to heat treatment at a temperature of 100° C. for 24 hours. Subsequently, peeling was performed at the ion implantation interface in which flash light was irradiated to the bonded substrates from the LT side using a flash lamp annealing (FLA) device. Thus, a composite substrate having a stack of the LT thin film in a thickness of about 600 nm on the Si substrate through the SiO.sub.2 layer can be obtained. After polishing to provide a mirror surface, evaluation was performed. However, in a thermal endurance test, peeling was not observed.

(89) Note that in Example 18, the implantation of hydrogen ions to the LT substrate can also be performed before the SiO.sub.2 film is deposited. Also with this configuration, the effect similar to the case can be obtained in which the SiO.sub.2 film was deposited and then hydrogen ions were implanted. When the dose amount of hydrogen ions ranges from 6.0×10.sup.16 atoms/cm.sup.2 to 2.75×10.sup.17 atoms/cm.sup.2, the similar effect can be obtained. Instead of hydrogen ions, the similar effect can be obtained by implanting hydrogen molecule ions (H.sub.2.sup.+) in the range of 3.0×10.sup.16 atoms/cm.sup.2 to 1.37×10.sup.17 atoms/cm.sup.2.

Modifications of Examples

(90) In all the examples and comparative examples, the LT substrate is used as a piezoelectric single crystal substrate. However, instead of the LT substrate, results in exactly the same tendency were obtained even using an LN substrate. Even though the material of the second intermediate layer was changed from SiO.sub.2 to SiOx, Al.sub.2O.sub.3, AlN, SiN, SiON, and Ta.sub.2O.sub.5, for example, other than SiO.sub.2, the similar results were obtained. For the film material of the intermediate layer, SiO.sub.2 was used for all the investigation. However, the effect was exactly the same even on materials that are not strictly stoichiometric like SiO.sub.2±0.5. It is considered that the effect is exerted by the intermediate layer mainly burying the irregularities. Even in the case in which the material of the support substrate was changed to silicon, silicon carbide, silicon nitride, or aluminum nitride, a synthetic silica film was formed on the surface by CVD, PVD, or by applying organic silicon, the substrate was heated and sintered at a temperature of 800° C. or more, and then thermally synthesized silica was formed, the substrates were able to be excellently joined to each other. The substrates were diced in two-millimeter squares for a temperature cycle test. However, no peeling was observed.

REFERENCE SIGNS LIST

(91) 100 support substrate 200 piezoelectric single crystal substrate 300 intermediate layer 310 first intermediate layer 320 second intermediate layer 330 third intermediate layer