JP2020022049A - Composite substrate for surface acoustic wave device and manufacturing method thereof - Google Patents

Abstract

To provide a composite substrate for a surface acoustic wave device which can increase the frequency of the surface acoustic wave device and solve the problem of the fluctuation of frequency characteristics due to temperature changes, and a manufacturing method thereof.SOLUTION: A composite substrate for a surface acoustic wave device includes a piezoelectric substrate 1, a support substrate 2 having a smaller coefficient of thermal expansion than the piezoelectric substrate, and a polycrystalline diamond thin film layer 3 formed on one main surface of the support substrate, and in the piezoelectric substrate 1, the polycrystalline diamond thin film layer is directly bonded to a bonding piezoelectric substrate (bulk crystal), and the thickness of the bonding piezoelectric substrate is reduced to 5 μm or more and 30 μm or less by laser ablation, and the surface roughness Ra is 10 μm or less. In the surface acoustic wave device manufactured using the composite substrate, a high propagation speed is obtained because the surface acoustic wave propagates through the polycrystalline diamond thin film layer 3, and the thermal expansion can be suppressed by the action of the support substrate 2 having a small coefficient of thermal expansion, thereby realizing the good frequency temperature characteristics.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a surface acoustic wave device applied to a band-pass filter, a resonator, or the like. In particular, it is possible to increase the frequency of the surface acoustic wave device, and also to solve the problem that the frequency characteristics shift (change) due to a temperature change. The present invention relates to a composite substrate for a surface acoustic wave device and a method for manufacturing the same.

  2. Description of the Related Art In recent years, high frequency and miniaturization of communication devices such as mobile phones have been progressing, so that high performance and miniaturization of RF circuit units have been required. Among them, a surface acoustic wave device (Surface Acoustic Wave Device) (hereinafter sometimes abbreviated as SAW device) is used as an electronic element such as a high-frequency filter used in a transmission / reception unit of a communication device or a resonator used in an oscillator. is there. The SAW device is an element utilizing a phenomenon in which a specific frequency is selected in a process of converting a high-frequency signal into a surface acoustic wave using a piezoelectric material and converting the signal into a high-frequency signal again. Compared to dielectric filters and ceramic filters that have been used in the high frequency band, cell phones and smartphones can take advantage of the sharpness and frequency design of frequency characteristics, easy surface mounting, small size and light weight. , And various other communication devices such as sensors and touch panels. In particular, with the explosive development of miniaturized and high-frequency devices such as mobile phones in recent years, the demand has been greatly expanded.

  As the surface acoustic wave element, a piezoelectric layer as a surface acoustic wave propagation medium and a pair of comb-shaped electrodes [IDT: Interdigital Transducer] (hereinafter, referred to as IDT, IDT electrode, or electrode) are formed on a substrate. ) Are sequentially laminated. Usually, the IDT electrode is formed by forming a metal material layer on a piezoelectric layer and then etching the metal material layer.

  In this surface acoustic wave device, when an electric signal (AC power) is supplied to the input IDT, the piezoelectric layer is distorted by the electric field due to the electric signal. Since the electrodes have a comb-like shape, a difference in density occurs in the piezoelectric layer, and surface acoustic waves are generated. The surface acoustic wave is propagated to the output IDT, and the energy of the surface acoustic wave is converted into electrical energy by the output IDT and output.

The center frequency f ₀ of the transmission band of the surface acoustic wave element is obtained from the interval λ ₀ between the comb-shaped electrodes and the propagation velocity V of the elastic wave on the surface of the piezoelectric layer.
f ₀ = V / λ ₀
Given by

However, it is difficult to manufacture a surface acoustic wave device that operates well at 2.5 GHz or higher. In order to increase the center frequency f ₀ of the transmission band, either the interval λ ₀ between the comb-shaped electrodes is reduced or the propagation velocity V of the surface acoustic wave is increased, as is clear from the above relational expression. However, λ ₀ is significantly limited by processing techniques such as photolithography. At the present mass production level, the width of the comb-shaped electrodes is about 0.4 μm, and the interval λ ₀ between the comb-shaped electrodes is about 1.6 μm, which is a lithium tantalate substrate (LT) often used recently for SAW devices. 2400 MHz is the limit at a propagation speed of 3800 m / s. Therefore, in order to obtain a surface acoustic wave device operating in a high frequency band, it is necessary to increase the propagation speed V of the acoustic wave.

  As a high-frequency device, a film bulk acoustic resonator (FBAR) using, for example, AlN as a piezoelectric material has been studied. However, the manufacturing process of the piezoelectric thin film resonator FBAR is complicated and expensive, so that it is used only for some devices.

  Therefore, studies on a surface acoustic wave device operating in a high frequency band are being repeated. For example, since a diamond crystal has a very high sound velocity of 18000 m / s, research and development of a surface acoustic wave device using this high sound velocity characteristic (see Patent Documents 1 and 2) are proceeding. Non-Patent Document 1 introduces the production of a surface acoustic wave device in which a polycrystalline diamond crystal is formed on a silicon substrate and then a comb-shaped electrode and a zinc oxide layer are formed. Non-Patent Document 1 discloses that the manufactured surface acoustic wave device has a sound velocity of 10,000 m / s or more and a sufficiently high excitation efficiency.

  On the other hand, in a surface acoustic wave device required to operate in a high frequency band, there is another problem that a frequency characteristic shifts (fluctuates) because a piezoelectric substrate (piezoelectric layer) expands and contracts due to a temperature change. . In order to improve this temperature characteristic, a composite substrate has been proposed in which a piezoelectric substrate on which an IDT electrode is formed and an auxiliary substrate having a smaller thermal expansion coefficient than the piezoelectric substrate and a larger thickness than the piezoelectric substrate are directly bonded. (See, for example, Patent Document 3). Since the auxiliary substrate made of glass, silicon, or the like is directly bonded to the piezoelectric substrate, expansion and contraction of the piezoelectric substrate is suppressed by the auxiliary substrate, so that the temperature characteristics can be improved. The effect of improving the temperature characteristics increases as the difference between the thermal expansion coefficients of the piezoelectric substrate and the auxiliary substrate increases. However, a composite substrate (wafer) in which a piezoelectric substrate and an auxiliary substrate having different coefficients of thermal expansion are directly bonded to each other may warp due to a temperature change such as a heat treatment during the surface acoustic wave device manufacturing process, or the directly bonded substrates may be bonded to each other. Since the surface acoustic wave device may be peeled off, the precision of patterning is deteriorated in the manufacturing process of the surface acoustic wave device which has passed the process temperature, and there has been a problem that automatic handling becomes difficult. This situation is remarkable because the larger the difference between the coefficients of thermal expansion of the piezoelectric substrate and the auxiliary substrate, and the larger the size of the composite substrate (wafer), the greater the warpage of the wafer, and the more easily the directly bonded substrates are easily separated from each other. I know it will be. Then, when the warpage of the composite substrate (wafer) exceeds an allowable value or the directly bonded substrates are separated from each other, there is a problem that the composite substrate (wafer) cannot be flowed to the manufacturing process.

  Therefore, Patent Literature 4 discloses a composite substrate in which a piezoelectric substrate and a supporting substrate having a smaller coefficient of thermal expansion than the piezoelectric substrate (corresponding to the auxiliary substrate of Patent Literature 3) are bonded to each other. The first substrate and the second substrate are bonded by direct bonding with a strength that can be peeled off with a blade, and the first substrate is bonded to the piezoelectric substrate on a surface of the first substrate opposite to the bonding surface with the second substrate. 2 discloses a combined composite substrate. In this composite substrate, the warpage of the composite substrate generated in response to a change in process temperature is suppressed, and after the surface acoustic wave device is manufactured, the second substrate is peeled off from the first substrate with a blade and removed. This has the advantage that the thickness of the substrate can be easily reduced, and that it can meet the demand for thinner devices. However, while it is possible to cope with a demand for a thinner device, the temperature characteristics of the surface acoustic wave element deteriorate as the thickness of the supporting substrate becomes thinner.

  The possibility that the frequency characteristics shift (fluctuate) due to a change in temperature in this way can be solved by employing the methods of Patent Documents 3 and 4. However, in order to obtain a surface acoustic wave device that operates in a high frequency band, a surface acoustic wave device capable of improving a signal propagation speed and suppressing a temperature change in frequency characteristics has not been found yet.

JP-A-9-051248 (see paragraphs 0053-0055) JP-A-6-268463 (see paragraphs 0031-0034) JP-A-11-55070 WO2014 / 129432 JP-A-2002-94355 (see paragraphs 0017 and 0031) Japanese Patent Application No. 2018-052203 (refer to claim 6) JP-A-9-269430 (see paragraphs 0008-0012) JP 2001-33604 A (see paragraphs 0045-0046, FIG. 6)

Proceedings of the 6th Diamond Symposium (November 26-27, 1992), pp. 90-91: P21 "Surface elastic waves of ZnO / polycrystalline diamond structures and their application to high-frequency filters"

  In the field of communication equipment, the use of frequency resources has been depleted due to the exhaustion of available frequency band resources, and techniques for further increasing the frequency of surface acoustic wave devices have been required. To increase the frequency of the surface acoustic wave device, a method of miniaturizing the electrode size has been mainly used until now. However, the miniaturization of the electrode interval for determining the frequency has been described in the current lithography technology as described above. We are approaching the limit. Further, even if the frequency can be increased by miniaturization of the electrode dimensions, the thinning of the electrodes and the miniaturization of the electrode intervals cause a problem that the element structure itself is easily broken and power characteristics cannot be obtained.

  Therefore, as an element for transmitting a surface acoustic wave at high speed, in Patent Document 1, a diamond thin film is formed on a silicon substrate by a microwave plasma CVD method, and the diamond thin film surface is polished and flattened. A surface acoustic wave device in which a piezoelectric (ZnO) film is deposited on a surface of a diamond thin film by a sputtering method and then a comb-like electrode (metal aluminum) is formed on the piezoelectric film is disclosed. In Reference 2, a diamond film is formed on a silicon substrate by a combustion flame method using an oxygen-acetylene flame burner, a comb-like electrode (metal aluminum) is formed on the diamond film, and a piezoelectric material is formed by a high-frequency magnetron sputtering method. A surface acoustic wave device on which a (ZnO) layer is formed is disclosed.

  However, it has been pointed out that a piezoelectric (ZnO) layer formed on a diamond thin film by a film forming method such as a CVD method or a sputtering method has poor film quality, and a sufficient electromechanical coupling coefficient cannot be obtained. (See paragraph 0017 of Patent Document 5).

  In order to solve this problem, Patent Document 5 discloses an electromechanical conversion electrode formed on a first main surface of a piezoelectric layer (piezoelectric bulk single crystal) and a second main surface of the piezoelectric layer. Have proposed a surface acoustic wave device comprising a diamond layer formed on a substrate and a support substrate bonded to the diamond layer via an adhesive.

  However, as described in paragraph 0031 of Patent Document 5, when a polycrystalline diamond layer is formed on a piezoelectric bulk single crystal substrate (piezoelectric layer) by using a microwave plasma CVD method, the substrate (piezoelectric layer) is formed. The film is formed at a temperature of 850 ° C., and if the temperature of the substrate rapidly rises or falls, the pyroelectricity of the piezoelectric bulk single crystal substrate (piezoelectric layer) causes the piezoelectric bulk single crystal substrate to move. There has been another problem that the piezoelectric bulk single crystal substrate is degraded by microwave power or substrate heating, and the characteristics of the piezoelectric material are degraded. For example, when a lithium tantalate substrate is used, since the Curie temperature is 650 ° C., the piezoelectricity is lost after forming a polycrystalline diamond layer on the lithium tantalate substrate, and the function as a SAW device cannot be obtained. There was a problem. Further, in Patent Document 5, the diamond layer and a supporting substrate (silicon substrate, glass substrate, ceramic substrate, etc.) are joined (adhered) using an adhesive (epoxy resin adhesive, solder alloy, etc.). For this reason, the adhesive material is softened by heat and moved by stress, and there is also a problem that the peeling is partially observed in the thermal cycle test and the reliability is lacking.

  Under such technical background, the present inventor has realized an increase in the propagation speed of the surface acoustic wave to realize a higher frequency of the surface acoustic wave element, and has improved the problem that the frequency characteristics are shifted (changed) due to a temperature change. In addition, a composite substrate for a surface acoustic wave device having a piezoelectric substrate made of a bulk crystal and a method for manufacturing the same have already been proposed (see Patent Document 6).

  That is, the composite substrate for a surface acoustic wave device is composed of a piezoelectric substrate made of a bulk crystal and a polycrystalline diamond thin film layer formed on one main surface of a supporting substrate having a smaller thermal expansion coefficient than the piezoelectric substrate. Are directly bonded by a room temperature bonding method, and the non-bonded surface of the piezoelectric substrate directly bonded to the polycrystalline diamond thin film layer is polished to a thin film.

  In the proposed surface acoustic wave device using the composite substrate for a surface acoustic wave device, a polycrystalline diamond thin film layer formed on one main surface of a supporting substrate having a smaller thermal expansion coefficient than that of a piezoelectric substrate is used. Since elastic waves propagate, it is possible to achieve extremely high propagation speeds, and frequency characteristics shift due to temperature changes because a support substrate with a smaller thermal expansion coefficient than that of a piezoelectric substrate is applied. The problem of (variation) is also improved, and the piezoelectric substrate made of a bulk crystal has better film quality than the piezoelectric layers of Patent Documents 1 and 2 formed by CVD or sputtering on a diamond thin film, and In addition, since the piezoelectric substrate and the polycrystalline diamond thin film layer are bonded at room temperature, a remarkable effect of preventing the piezoelectric substrate from being destroyed and the characteristics of the piezoelectric body from deteriorating described in Patent Document 5 It was having.

  However, in the composite substrate for a surface acoustic wave device proposed by the present inventor, the non-bonding surface of the piezoelectric substrate made of a bulk crystal is thinned by a polishing method. In consideration of the cost, the thickness of the piezoelectric substrate to be thinned by the polishing method is about 12 μm to 25 μm (see Examples 1 and 2 of Patent Document 6). For this reason, there is a production cost and a technical limit in thinning the piezoelectric substrate by the polishing method, and there is room for further improvement in improving the propagation speed and frequency temperature characteristics of the surface acoustic wave device. Was.

  SUMMARY OF THE INVENTION The present invention has been made in view of such a problem, and an object of the present invention is to improve a propagation speed and a frequency temperature characteristic of a surface acoustic wave element and to shorten a processing time for a composite for a surface acoustic wave element. It is to provide a substrate and a manufacturing method thereof.

  In order to solve the above-mentioned problems, the present inventor tried to employ a “laser ablation processing method” described in Patent Documents 7 and 8 instead of thinning the piezoelectric substrate using the conventional polishing method.

  Patent Literature 7 discloses a method of manufacturing an optical waveguide device including a substrate made of an oxide single crystal such as lithium niobate and lithium tantalate, and a ridge-type optical waveguide protruding on a main surface of the substrate. The ridge-type optical waveguide is formed by a “laser ablation method” using an excimer laser in an ultraviolet region having a wavelength of 150 to 300 nm.

  The “laser ablation processing method” is a method in which high-energy light such as excimer laser is applied to the material to be processed, thereby instantaneously decomposing and evaporating the irradiated area to form a desired shape. It is a processing method to obtain.

  Patent Document 8 discloses manufacturing of an optical component having a silicon substrate and an optical element (for example, an optical path conversion mirror, a polarization splitting prism, or the like) formed to include a plurality of surfaces (that is, inclined surfaces) that are not parallel to the substrate surface. A method is disclosed, in which a polymer material applied on a silicon substrate is processed by a "laser ablation etching method" to form a 45-degree slope forming the optical element.

  Therefore, in the method of manufacturing a composite substrate for a surface acoustic wave device proposed by the inventor of the present invention, a method for thinning a piezoelectric substrate directly bonded to a polycrystalline diamond thin film layer is described in Japanese Patent Application Laid-Open No. H11-157300, instead of the conventional polishing method. And the like, the thickness of the piezoelectric substrate directly bonded to the crystal diamond thin film layer can be set to 30 μm or less without greatly increasing the production cost, and the “laser ablation processing method” can be set. It has been confirmed that the surface roughness Ra of the thinned piezoelectric substrate can be set to 10 nm or less with the adoption of the “working method”, and the propagation velocity of the surface acoustic wave element is further reduced with the thinning of the piezoelectric substrate (30 μm or less). And it came to find that frequency temperature characteristic was improved. The present invention has been completed based on such technical findings.

That is, the first invention according to the present invention is:
A piezoelectric substrate,
A supporting substrate having a smaller coefficient of thermal expansion than the piezoelectric substrate,
In a composite substrate for a surface acoustic wave device comprising a polycrystalline diamond thin film layer formed on one main surface of the support substrate,
The polycrystalline diamond thin film layer and the piezoelectric substrate are directly bonded, and the thickness of the piezoelectric substrate is 5 μm or more and 30 μm or less, and the surface roughness Ra of the piezoelectric substrate is 10 nm or less,
The second invention is
A piezoelectric substrate,
A supporting substrate having a smaller coefficient of thermal expansion than the piezoelectric substrate,
In a composite substrate for a surface acoustic wave device comprising a polycrystalline diamond thin film layer formed on one main surface of the support substrate,
The polycrystalline diamond thin film layer and the piezoelectric substrate are directly bonded via a metal thin film, the thickness of the piezoelectric substrate is 5 μm or more and 30 μm or less, and the surface roughness Ra of the piezoelectric substrate is 10 nm or less. It is assumed that.

Further, a third invention according to the present invention provides:
In the composite substrate for a surface acoustic wave device according to the second invention,
The metal thin film is a titanium film or a chromium film,
The fourth invention is
The composite substrate for a surface acoustic wave device according to any one of the first invention to the third invention,
The support substrate is characterized by being constituted by one selected from silicon, sapphire, aluminum nitride, aluminum oxide, borosilicate glass, and quartz glass,
The fifth invention is
The composite substrate for a surface acoustic wave device according to any one of the first invention to the third invention,
The piezoelectric substrate is made of at least one kind of bulk crystal selected from lithium tantalate, lithium niobate, lithium niobate-lithium tantalate solid solution single crystal, crystal, lithium borate, zinc oxide, aluminum nitride, langasite, and langatate. It is characterized by comprising.

Next, the sixth invention is
A piezoelectric substrate,
A supporting substrate having a smaller coefficient of thermal expansion than the piezoelectric substrate,
In a method for manufacturing a composite substrate for a surface acoustic wave device comprising a polycrystalline diamond thin film layer formed on one main surface of the support substrate,
Forming a polycrystalline diamond thin film layer on one main surface of the support substrate;
A step of forming a bonded body by directly bonding the polycrystalline diamond thin film layer formed on the support substrate and one main surface of the bonding piezoelectric substrate by a surface activation room temperature bonding method,
The non-bonded surface of the bonding piezoelectric substrate in the bonded body is irradiated with a laser having a wavelength of 150 to 300 nm to ablate to decompose and volatilize the non-bonded surface of the bonding piezoelectric substrate, and has a thickness of 5 μm or more and 30 μm or less. And forming a piezoelectric substrate having a surface roughness Ra of 10 nm or less,
Characterized by having
The seventh invention is
A method for manufacturing a composite substrate for a surface acoustic wave device according to a sixth aspect,
In the step of forming a bonded body by directly bonding the polycrystalline diamond thin film layer formed on the support substrate and one main surface of the bonding piezoelectric substrate by a surface activation room temperature bonding method,
Characterized in that the polycrystalline diamond thin film layer and one main surface of the bonding piezoelectric substrate are directly bonded by a surface activated room temperature bonding method via a metal thin film,
The eighth invention is
A method for manufacturing a composite substrate for a surface acoustic wave device according to a sixth aspect,
In the step of forming a polycrystalline diamond thin film layer on one main surface of the support substrate,
The polycrystalline diamond thin film layer is formed by a microwave plasma CVD method,
The ninth invention is
In the method for manufacturing a composite substrate for a surface acoustic wave device according to the sixth or eighth invention,
Polishing the surface of the polycrystalline diamond thin film layer formed on one main surface of the support substrate,
The tenth invention is
A method for manufacturing a composite substrate for a surface acoustic wave device according to a sixth aspect,
The non-bonded surface of the bonding piezoelectric substrate in the bonded body is irradiated with a laser having a wavelength of 150 to 300 nm to ablate to decompose and volatilize the non-bonded surface of the bonding piezoelectric substrate, and has a thickness of 5 μm or more and 30 μm or less. And forming a piezoelectric substrate having a surface roughness Ra of 10 nm or less,
The laser having a wavelength of 150 to 300 nm is an excimer laser.

Also, the eleventh invention is
A method for manufacturing a composite substrate for a surface acoustic wave device according to a sixth aspect,
In the step of forming a bonded body by directly bonding the polycrystalline diamond thin film layer formed on the support substrate and one main surface of the bonding piezoelectric substrate by a surface activation room temperature bonding method,
After cleaning each bonding surface of the polycrystalline diamond thin film layer and the bonding piezoelectric substrate before bonding, irradiating each bonding surface with an ion beam to remove and activate residual impurities, and then directly bonding in vacuum at room temperature Characterized in that
The twelfth invention is
A method of manufacturing a composite substrate for a surface acoustic wave device according to a seventh invention,
In a step of directly bonding the polycrystalline diamond thin film layer and one main surface of the bonding piezoelectric substrate by a surface activated room temperature bonding method via a metal thin film,
The bonding surfaces of the polycrystalline diamond thin film layer and the bonding piezoelectric substrate before bonding are cleaned, and each bonding surface is irradiated with an ion beam to remove residual impurities, and the polycrystalline diamond thin film layer and the bonding piezoelectric substrate are bonded. After forming a metal thin film on at least one of the joining surfaces, in vacuum, characterized by direct joining at room temperature,
The thirteenth invention is
In the method for manufacturing a composite substrate for a surface acoustic wave device according to a twelfth aspect,
The metal thin film is a titanium film or a chromium film having a thickness of 5 to 10 nm.

  A composite substrate for a surface acoustic wave device according to the present invention includes a piezoelectric substrate, a support substrate having a smaller coefficient of thermal expansion than the piezoelectric substrate, and a polycrystalline diamond thin film layer formed on one main surface of the support substrate. Wherein the polycrystalline diamond thin film layer and the piezoelectric substrate are directly joined, the thickness of the piezoelectric substrate is 5 μm or more and 30 μm or less, and the surface roughness Ra of the piezoelectric substrate is 10 nm or less. And

  In the surface acoustic wave device using the composite substrate for a surface acoustic wave device according to the present invention, a polycrystalline diamond thin film layer formed on one main surface of a support substrate having a smaller thermal expansion coefficient than a piezoelectric substrate Since the thickness of the piezoelectric substrate directly bonded to the substrate is extremely thin, 5 μm or more and 30 μm or less, the piezoelectric substrate is significantly influenced by the polycrystalline diamond thin film layer, and approaches the hardness of the polycrystalline diamond thin film layer. Since the surface acoustic wave propagates through the diamond thin film layer, it is possible to realize a higher propagation speed than before.

  Further, in the surface acoustic wave device using the composite substrate for a surface acoustic wave device according to the present invention, a support substrate having a smaller coefficient of thermal expansion than the piezoelectric substrate is applied, and is formed on one main surface of the support substrate. The frequency characteristic shifts (fluctuates) due to a temperature change because the piezoelectric substrate directly bonded to the formed polycrystalline diamond thin film layer is remarkably influenced by the polycrystalline diamond thin film layer and approaches the hardness of the polycrystalline diamond thin film layer. Therefore, it is possible to provide better frequency temperature characteristics than before.

  Furthermore, the composite substrate for a surface acoustic wave device according to the present invention is formed on one main surface of a bonding piezoelectric substrate made of a bulk crystal and a supporting substrate having a smaller thermal expansion coefficient than the bonding piezoelectric substrate. Since the bonded piezoelectric substrate is thinned by the “laser ablation method” at room temperature, the thickness is 5 μm or more and 30 μm or less, and the surface roughness Ra is 10 nm or less. Obtained as a piezoelectric substrate.

  The bonding piezoelectric substrate made of a bulk crystal has a better film quality than the piezoelectric layers of Patent Documents 1 and 2 formed on a diamond thin film by a CVD method or a sputtering method. Since the polycrystalline diamond thin film layer and the polycrystalline diamond thin film layer are bonded at room temperature, the breakage of the bonding piezoelectric substrate and the deterioration of the piezoelectric characteristics described in Patent Document 5 do not occur.

  Further, since the bonding piezoelectric substrate is thinned by the “laser ablation processing method”, the thinned piezoelectric substrate has a surface roughness Ra at a level that can be directly applied to the manufacture of a surface acoustic wave device (SAW filter). Can be set to a surface state of 10 nm or less.

FIG. 1 is a configuration explanatory view of a surface acoustic wave device using a composite substrate for a surface acoustic wave device according to a first embodiment of the present invention. FIG. 4 is a configuration explanatory view of a surface acoustic wave device using a composite substrate for a surface acoustic wave device according to a second embodiment of the present invention. FIG. 1 is an explanatory diagram illustrating a schematic configuration of a microwave plasma CVD apparatus. FIG. 1 is an explanatory diagram showing a schematic configuration of a laser processing device for ablation processing.

  Hereinafter, a composite substrate for a surface acoustic wave device according to the first and second embodiments of the present invention and a method for manufacturing the same will be described in detail.

1. 1. Composite substrate for surface acoustic wave device (A) Composite substrate for surface acoustic wave device according to first embodiment of the present invention As shown in FIG. A piezoelectric substrate, a support substrate 2 having a smaller thermal expansion coefficient than the piezoelectric substrate 1, and a polycrystalline diamond thin film layer 3 formed on one main surface of the support substrate 2. The diamond thin film layer 3 and the piezoelectric substrate 1 are directly bonded, and the thickness of the piezoelectric substrate 1 is 5 μm or more and 30 μm or less, and the surface roughness Ra of the piezoelectric substrate is 10 nm or less, The surface acoustic wave device configured by using the composite substrate for a surface acoustic wave device according to the first embodiment has a configuration in which a comb-shaped electrode 4 is formed on a non-bonding surface of the piezoelectric substrate 1.

(B) Composite substrate for surface acoustic wave device according to second embodiment of the present invention A composite substrate for surface acoustic wave device according to the second embodiment of the present invention, as shown in FIG. A supporting substrate having a smaller coefficient of thermal expansion than the piezoelectric substrate; and a polycrystalline diamond thin film layer formed on one main surface of the supporting substrate. 1 is directly bonded via the metal thin film 5, the thickness of the piezoelectric substrate 1 is 5 μm or more and 30 μm or less, and the surface roughness Ra of the piezoelectric substrate is 10 nm or less. The surface acoustic wave device configured by using the composite substrate for a surface acoustic wave device according to the embodiment has a configuration in which a comb-shaped electrode 4 is formed on a non-bonding surface of the piezoelectric substrate 1.

  Hereinafter, (1) a piezoelectric substrate, (2) a support substrate, (3) a polycrystalline diamond thin film layer, (4) a metal thin film, (5) a composite substrate for a surface acoustic wave device, and (6) a surface acoustic wave device It will be described in order.

(1) Piezoelectric substrate The piezoelectric substrate according to the present invention is a substrate through which elastic waves can propagate, and the piezoelectric substrate used in the composite substrate for a surface acoustic wave device according to the present invention includes lithium tantalate, lithium niobate, and niobium. It is preferably one or more bulk crystals selected from lithium oxide-lithium tantalate solid solution single crystal, quartz, lithium borate, zinc oxide, aluminum nitride, langasite, and langate, and lithium tantalate or lithium niobate is preferably used. More preferred. This is because lithium tantalate and lithium niobate have a high surface acoustic wave propagation speed and a large electromechanical coupling coefficient, and thus are suitable for high-frequency and wide-band frequency surface acoustic wave devices. The piezoelectric substrate 1 is directly joined to the polycrystalline diamond thin film layer 3 formed on one main surface of the support substrate 2 to constitute the composite substrate for a surface acoustic wave device according to the first embodiment.

  By the way, the piezoelectric substrate 1 is formed by directly bonding a polycrystalline diamond thin film layer formed on a supporting substrate and one main surface of a bonding piezoelectric substrate by a surface activation room temperature bonding method to form a bonded body. The non-bonding surface of the bonding piezoelectric substrate is thinned by a “laser ablation method” to form a thin film having a thickness of 5 μm or more and 30 μm or less.

  That is, the polycrystalline diamond thin film layer formed on the support substrate and one main surface of the bonding piezoelectric substrate are directly bonded by a surface activation room temperature bonding method to form a bonded body. A non-bonded surface is irradiated with a laser having a wavelength of 150 to 300 nm, and ablation processing for decomposing and volatilizing the non-bonded surface of the bonding piezoelectric substrate forms a piezoelectric substrate having a thickness of 5 μm or more and 30 μm or less. Further, in the ablation process, a laser beam having a shorter wavelength than the absorption edge of the material constituting the bonding piezoelectric substrate (that is, a laser having a wavelength of 150 to 300 nm) is used. Since the laser light irradiated to the surface is absorbed in the very surface layer, only the surface layer of the non-bonding surface is decomposed and instantaneously decomposed and vaporized, so that no processing damage is given to the inside of the bonding piezoelectric substrate. In addition, no process-affected layer is generated, and the surface roughness Ra of the formed piezoelectric substrate is 10 nm or less.

  The size of the bonding piezoelectric substrate is not particularly limited, but a bonding piezoelectric substrate having a diameter of about 50 to 200 mm is exemplified. With respect to the thickness of the bonding piezoelectric substrate before being directly bonded to the polycrystalline diamond thin film layer, the thickness of the polycrystalline diamond thin film layer formed on the supporting substrate and one of the principal surfaces of the bonding piezoelectric substrate are set to the surface. It is appropriately set in consideration of the workability and the like at the time of the above-mentioned “laser ablation processing” with respect to the bonding piezoelectric substrate of the bonded body formed by direct bonding by the activated room temperature bonding method.

(2) Support Substrate The support substrate 2 used in the composite substrate for a surface acoustic wave device according to the present invention needs to be made of a material having a smaller coefficient of thermal expansion than the piezoelectric substrate 1. The support substrate 2 is made of a material having a smaller coefficient of thermal expansion than the piezoelectric substrate 1. The support substrate 2, the polycrystalline diamond thin film layer 3 formed on one main surface of the support substrate 2, and the piezoelectric substrate 1 Since the expansion and contraction of the piezoelectric substrate 1 when the temperature changes is suppressed by using the composite substrate provided, when the composite substrate is used as a SAW device, the problem that the frequency characteristic shifts (changes) due to the temperature change is solved. It becomes possible.

  The material of the support substrate 2 is preferably one selected from silicon, sapphire, aluminum nitride, aluminum oxide, borosilicate glass, and quartz glass. In terms of hardness, an inexpensive general-purpose soda glass substrate has a Vickers hardness of 500 to 600, a silicon substrate is about 1040, and a sapphire substrate is 2300, and a sapphire substrate is more preferable. However, a glass substrate or a silicon substrate is inexpensive, mass-produced, and inexpensive. Therefore, a silicon substrate is preferable as the support substrate 2 when viewed comprehensively.

Therefore, by using an inexpensive silicon substrate and forming a polycrystalline diamond thin film layer having high hardness on the support substrate, the hardness of the support substrate is increased, and the polycrystalline diamond thin film layer 3 and the piezoelectric substrate 1 are separated. By direct bonding, the resulting composite substrate has a higher propagation speed than the piezoelectric substrate alone. Further, the silicon substrate has a thermal expansion coefficient of 3.9 × 10 ⁻⁶ / K, which is much smaller than that of a piezoelectric substrate made of lithium tantalate or the like, and can suppress the temperature change of the frequency characteristics of the SAW device.

  As the size of the support substrate 2, for example, one having a diameter of 50 to 200 mm and a thickness of 200 to 1200 μm is suitably used.

(3) Polycrystalline Diamond Thin Film Layer In the composite substrate for a surface acoustic wave device according to the present invention, a polycrystalline diamond thin film layer 3 is formed on one main surface of the support substrate 2. Further, a bonding body is formed by directly bonding the bonding piezoelectric substrate and the polycrystalline diamond thin film layer 3 by a surface activation room temperature bonding method, and a laser having a wavelength of 150 to 300 nm is applied to the non-bonding surface of the bonding piezoelectric substrate. The piezoelectric substrate for bonding is thinned to a thickness of 5 μm or more and 30 μm or less, and the surface roughness Ra is reduced to 10 nm or less by ablation processing for irradiating the non-bonded surface of the bonding piezoelectric substrate to decompose and volatilize the non-bonded surface. A substrate 1 is formed. Thereafter, by applying a voltage using a pair of comb-shaped electrodes 4 formed on the piezoelectric substrate 1 of the obtained composite substrate for a surface acoustic wave device, a surface acoustic wave is excited, and the surface acoustic wave is The light propagates through the polycrystalline diamond thin film layer 3 and is converted again into an electric signal by the piezoelectric substrate 1 by another pair of comb-like electrodes. The diamond layer is a material having the highest sound propagation speed among materials, and can achieve a propagation speed of 10,000 m / s or more even when integrated with a piezoelectric substrate. The composite substrate according to the present invention has a high-frequency surface. It can be used for an elastic wave device.

  For forming the polycrystalline diamond thin film layer 3, it is preferable to form a film on the support substrate 2 by using a microwave plasma CVD method. The diamond layer is formed by a gas phase synthesis method using a hydrocarbon or the like as a raw material gas, for example, a method of activating a raw material gas by heating an electron emitting material, a method of exciting a raw material gas by plasma, and a method of decomposing and exciting a gas by light. It can be formed by a method, a method of growing polycrystalline diamond from a source gas by ion bombardment, or the like. The polycrystalline diamond thin film layer 3 according to the present invention is formed by using a microwave plasma CVD method among the above-described film forming methods. Is preferred. The microwave plasma CVD method is a synthesis method using electrodeless discharge using microwaves (usually a frequency of 2.45 GHz is used). The microwave plasma CVD method is a film forming method using plasma only by microwaves in a pressure range of about 1.3 to 8.0 kPa. The microwave plasma CVD method is a method in which a raw material gas is excited by plasma, and a denser thin film can be formed at a lower temperature than a thermal CVD method or the like. It has the advantages that the speed is high, damage due to heat can be suppressed, and interdiffusion between stacked films can be suppressed.

  The magnetic field microwave CVD method, which is a kind of the microwave plasma CVD method, makes it possible to produce a stable high-density plasma even at a low pressure by utilizing the interaction between the plasma and the magnetic field. In this case, a stronger plasma state by electron cyclotron resonance (ECR) is used in a lower pressure (1.3 to 133 Pa) region than the CVD method. Since the plasma is low-pressure plasma, a uniform polycrystalline diamond thin film can be formed over a large area, which is suitable for forming the polycrystalline diamond thin film layer according to the present invention.

  The thickness of the polycrystalline diamond thin film layer 3 is preferably set to about 5 μm on the support substrate 2. The reason is that the surface of the formed polycrystalline diamond thin film layer 3 has irregularities, and it is necessary to grind and smooth the bonding surface before directly bonding to the bonding piezoelectric substrate. . In addition, if there is unevenness on the bonding surface, there is a possibility that floating may occur without being completely bonded at the atomic level. For example, in the case of the above film thickness (about 5 μm), it is preferable that the surface is polished to about 3 μm to make the surface roughness Ra 0.5 nm or less, preferably 0.3 nm or less.

(4) Metal Thin Film In the composite substrate for a surface acoustic wave device according to the first embodiment of the present invention, the polycrystalline diamond thin film layer 3 formed on one main surface of the support substrate 2 and the piezoelectric substrate 1 are directly bonded. Have been. In order to directly join the piezoelectric substrate 1 and the polycrystalline diamond thin film layer 3, the bonding surfaces of the polycrystalline diamond thin film layer 3 and the bonding piezoelectric substrate before joining are cleaned, and the cleaned polycrystalline diamond thin film layer 3 is joined. Place the piezoelectric substrate for use in a vacuum vessel, irradiate each joint surface with an ion beam in ultra-high vacuum to remove residual impurities, activate each joint surface, and then apply an appropriate load to join. Thereby, the diffusion of the atoms at the bonding interface proceeds, the bonding interface becomes amorphous, and the bonding interface can be directly bonded at the atomic level. It is preferable that the above-mentioned joining be performed at normal temperature and no voltage.

  In the composite substrate for a surface acoustic wave device according to the second embodiment of the present invention, the polycrystalline diamond thin film layer 3 formed on one main surface of the support substrate 2 and the piezoelectric substrate 1 are directly joined via the metal thin film 5. Have been. In order for the piezoelectric substrate 1 and the polycrystalline diamond thin film layer 3 to be directly bonded via the metal thin film 5, the bonding surfaces of the polycrystalline diamond thin film layer 3 and the bonding piezoelectric substrate before bonding are cleaned, and the cleaned polycrystalline diamond film 3 is cleaned. The diamond thin film layer 3 and the bonding piezoelectric substrate are placed in a vacuum vessel, and each bonding surface is irradiated with an ion beam in an ultra-high vacuum to remove residual impurities and activate each bonding surface. A metal thin film 5 is formed on the bonding surface of at least one of the polycrystalline diamond thin film layer 3 and the bonding piezoelectric substrate by a film forming method such as that described above. -Direct bonding can be performed without voltage. The metal thin film 5 exists at the interface between the polycrystalline diamond thin film layer 3 and the bonding piezoelectric substrate, and the metal thin film 5 can be bonded by atomic diffusion.

  As the metal thin film 5, a thin film such as chromium or titanium which has a strong bonding force with oxygen and a high diffusion coefficient is preferable. The thickness of the metal thin film 5 is preferably 5 to 10 nm. If the film thickness is too thin, less than 5 nm, the film becomes discontinuous and diffusion becomes discontinuous. On the other hand, if the film thickness exceeds 10 nm and is too thick, a continuous film may be formed before diffusion, and the film may be interposed between the polycrystalline diamond thin film layer 3 and the bonding piezoelectric substrate and may not function as a diffusion layer. There is. Due to the presence of the metal thin film 5, the surface roughness at both bonding surfaces may be coarser than when the metal thin film 5 is not interposed, and there is a merit of reducing the polishing cost.

(5) Composite substrate for surface acoustic wave device The composite for a surface acoustic wave device according to the first embodiment in which the polycrystalline diamond thin film layer 3 formed on one main surface of the support substrate 2 and the piezoelectric substrate 1 are directly joined. A composite substrate for a surface acoustic wave device according to a second embodiment in which a substrate and a polycrystalline diamond thin film layer 3 formed on one main surface of a support substrate 2 and a piezoelectric substrate 1 are directly bonded via a metal thin film 5. In the above, the piezoelectric substrate 1 of each composite substrate is formed as a thin film having a thickness of 5 μm or more and 30 μm or less by thinning the non-joining surface of the joining piezoelectric substrate by “laser ablation processing”. That is, after directly bonding one main surface of the bonding piezoelectric substrate and the polycrystalline diamond thin film layer formed on the support substrate to form a bonded body, the non-bonded surface of the bonded piezoelectric substrate in the bonded body is formed. Is irradiated with a laser having a wavelength of 150 to 300 nm to decompose and volatilize the non-bonding surface of the bonding piezoelectric substrate, thereby forming a piezoelectric substrate 1 having a thickness of 5 μm or more and 30 μm or less. .

  The ablation process uses a laser beam (for example, an excimer laser having a wavelength of 150 to 300 nm) shorter than the absorption end of the material constituting the piezoelectric substrate for bonding as described above. Since the laser light applied to the non-bonded surface of the substrate is absorbed in the very surface layer, only the surface layer of the non-bonded surface is decomposed and instantly decomposed and vaporized. No damage is caused, no deteriorated layer is formed at all, the flatness is high, and the surface roughness Ra of the formed piezoelectric substrate is 10 nm or less. Therefore, the non-bonded surface of the piezoelectric substrate 1 thinned by the “laser ablation processing method” can be applied to the manufacture of a SAW filter without further polishing unless there is a special use. Becomes

  Further, by making the thickness of the piezoelectric substrate 1 sufficiently smaller than the total thickness of the support substrate 2 on which the polycrystalline diamond thin film layer 3 is formed (the thickness of the polycrystalline diamond thin film layer + the thickness of the support substrate), The warping force of the composite substrate caused by the difference in the thermal expansion coefficient between the support substrate 2 on which the diamond thin film layer 3 is formed and the piezoelectric substrate 1 is reduced so that the composite substrate can be kept parallel and the polycrystalline diamond thin film layer can be used as the composite substrate. A state approaching a hardness of 3 is obtained.

  The ratio of the total thickness of the support substrate 2 on which the polycrystalline diamond thin film layer 3 is formed (the thickness of the polycrystalline diamond thin film layer + the thickness of the support substrate) to the thickness of the piezoelectric substrate 1 is expressed as (polycrystalline diamond thin film layer). The thickness of the piezoelectric substrate 1 is preferably 1/10 or less, and more preferably 1/20 or less of (thickness + supporting substrate thickness). If there is the difference in the film thickness, the warpage of the composite substrate due to the difference in the thermal expansion is suppressed even when the ambient temperature is about 120 ° C.

  In the composite substrate for a surface acoustic wave device, the thickness of the piezoelectric substrate 1 thinned by the “laser ablation processing method” needs to be 5 μm or more and 30 μm or less. When the thickness of the piezoelectric substrate 1 is increased, the characteristics of the piezoelectric substrate (for example, lithium tantalate) appear, so when a surface acoustic wave device is configured, the thermal expansion of the piezoelectric substrate becomes dominant and the surface acoustic wave device electrode is formed. This is because expansion and contraction of the surface acoustic wave element increase, the frequency temperature characteristic of the surface acoustic wave element decreases, and the hardness as the composite substrate decreases, and the propagation speed also decreases.

(6) Surface acoustic wave device The surface acoustic wave device using the composite substrate for a surface acoustic wave device according to the present invention is, as shown in FIGS. An electrode (comb-shaped electrode) 4 is formed. The surface of the piezoelectric substrate 1 is partitioned so that a large number of surface acoustic wave devices are formed, and a pair of comb-like electrodes (IDT electrodes) for the acoustic wave device are provided at positions corresponding to the respective surface acoustic wave devices. Are formed using a photolithography technique.

  Finally, a large number of SAW devices can be obtained by dicing along the sections. In the obtained SAW device, when a high-frequency signal is applied to the input-side IDT electrode, an electric field is generated between the electrodes, and a surface acoustic wave is excited and propagates on the piezoelectric substrate. Then, the propagated surface acoustic wave can be extracted as an electric signal from the output-side IDT electrode provided in the propagation direction.

2. Method for manufacturing composite substrate for surface acoustic wave device (1) Method for manufacturing composite substrate for surface acoustic wave device according to first embodiment of the present invention Piezoelectric substrate 1 and support having a smaller coefficient of thermal expansion than piezoelectric substrate 1 A first embodiment in which a substrate 2 and a polycrystalline diamond thin film layer 3 formed on one main surface of the support substrate 2 are provided, and the polycrystalline diamond thin film layer 3 and the piezoelectric substrate 1 are directly bonded. The method for manufacturing a composite substrate for a surface acoustic wave device is as follows.
Forming a polycrystalline diamond thin film layer 3 on one main surface of the support substrate 2;
Forming a bonded body by directly bonding the polycrystalline diamond thin film layer 3 formed on the support substrate 2 and one main surface of the bonding piezoelectric substrate by a surface activated room temperature bonding method;
The non-bonded surface of the bonding piezoelectric substrate in the bonded body is irradiated with a laser having a wavelength of 150 to 300 nm to ablate to decompose and volatilize the non-bonded surface of the bonding piezoelectric substrate, and has a thickness of 5 μm or more and 30 μm or less. And forming a piezoelectric substrate having a surface roughness Ra of 10 nm or less,
It is characterized by having.

Hereinafter, each step will be described.
<a> Step of Forming Polycrystalline Diamond Thin-Film Layer 3 on One Main Surface of Supporting Substrate 2 The method for manufacturing a composite substrate for a surface acoustic wave device according to the first embodiment is described in 1. 1. On one main surface of the support substrate 2 having a smaller coefficient of thermal expansion than the piezoelectric substrate described in (2). The polycrystalline diamond thin film layer 3 described in (3) is formed.

  The supporting substrate 2 needs to have a smaller thermal expansion coefficient than the selected piezoelectric substrate 1. Specifically, the material of the supporting substrate 2 is preferably one selected from silicon, sapphire, aluminum nitride, aluminum oxide, borosilicate glass, and quartz glass. Further, in consideration of hardness and material cost, a silicon substrate is more preferable. By using a silicon substrate and forming a polycrystalline diamond thin film layer having high hardness on the silicon substrate, the hardness as a support substrate is increased, and by directly bonding the polycrystalline diamond thin film layer and the piezoelectric substrate. The resulting composite substrate for a surface acoustic wave device has a higher propagation speed than the piezoelectric substrate alone.

  For forming the polycrystalline diamond thin film layer, it is preferable to use a microwave plasma CVD method. The microwave plasma CVD method is a synthesis method using electrodeless discharge using microwaves (usually a frequency of 2.45 GHz is used). The microwave plasma CVD method is a film forming method using plasma only by microwaves in a pressure range of about 1.3 to 8.0 kPa. The microwave plasma CVD method is a method of exciting a source gas by plasma, and can form a denser thin film at a lower temperature than a thermal CVD method or the like, and has a higher film forming rate than other CVD methods. It has the advantages that damage due to heat can be suppressed and interdiffusion between the stacked films can be suppressed.

  The magnetic field microwave CVD method, which is a kind of the microwave plasma CVD method, makes it possible to produce a stable high-density plasma even at a low pressure by utilizing the interaction between the plasma and the magnetic field. This uses a strong plasma state by electron cyclotron resonance (ECR) in a lower pressure (1.3 to 133 Pa) region than the CVD method. Since the plasma is low-pressure plasma, a uniform polycrystalline diamond thin film can be formed over a large area, which is suitable for forming the polycrystalline diamond thin film layer according to the present invention.

  FIG. 3 shows a schematic configuration of the microwave plasma CVD apparatus.

  The microwave plasma CVD apparatus includes a magnetron 6, a rectangular waveguide 7, a matching device 8, a plunger 9, a reaction tube 10, a susceptor (substrate holder) 11, an exhaust pump 12, a power monitor 13, and the like. In FIG. 3, reference numeral 14 denotes a reaction gas.

  The support substrate 2 used in the present invention is placed on a susceptor (substrate holder) 11 in a reaction tube 10 of a microwave plasma CVD apparatus, and a predetermined reaction gas is introduced to adjust the pressure. The microwave generated by the magnetron 6 travels through the rectangular waveguide 7 and reaches the plunger 9. The plunger 9 functions to change the length of the rectangular waveguide 7. By this adjustment, a predetermined wave is generated so that a strong electric field is set in the reaction tube 10, and a plasma P is generated in the reaction tube 10. . The substrate heating generally utilizes dielectric heating of the substrate holder 11 by microwaves. For the reaction tube 10, transparent quartz having low microwave absorption and high heat resistance is used. The matching unit 8 is for reducing the reflected wave to zero and efficiently transmitting the microwave to the reaction tube.

As the film forming conditions, a reaction pressure of 1.3 to 8.0 kPa, a methane gas (CH ₄ ) as a reaction gas, and a hydrogen gas (H ₂ ) as a carrier gas are generally used. / H _2, etc. other types of gas can also be used. The ratio of the H ₂ gas is preferably 80% or more. If the ratio of the H ₂ gas is low, the non-diamond component of the obtained polycrystalline diamond thin film layer is increased, and the film quality is deteriorated.

Since microwave plasma has high energy, the film formation rate is higher than other CVD methods. Further, since the microwave plasma P exists near the supporting substrate 2 and becomes high temperature, direct heating of the substrate holder 11 is not required. The substrate temperature during film formation is preferably adjusted to 700 to 800 ° C. If the substrate temperature is low, the ratio of sp ³ C—C bonds in the thin film to be formed decreases, and the diamond structure decreases and the hardness decreases. If the substrate temperature becomes too high, the influence on the substrate is increased, and the substrate may be deformed.

  The polycrystalline diamond thin film layer 3 is formed on the support substrate 2 by adjusting the above film forming conditions. The thickness of the polycrystalline diamond thin film layer 3 is preferably set to about 5 μm on the support substrate 2. The reason for this is that irregularities are present on the surface of the formed polycrystalline diamond thin film layer 3, and it is preferable to polish the bonding surface before directly bonding to the bonding piezoelectric substrate for forming the piezoelectric substrate 1. It is. If there are irregularities, there is a possibility that floating will not occur due to complete bonding at the atomic level. For example, if the thickness of the polycrystalline diamond thin film layer 3 is about 5 μm, it is desirable that the surface roughness Ra is about 0.5 μm or less, preferably 0.3 nm or less by polishing to about 3 μm.

  As a method for polishing the surface of the polycrystalline diamond thin film layer 3, for example, direct polishing using a diamond electrodeposition wheel or diamond abrasive grains, or polishing using a metal such as iron or nickel heated at a high temperature (polishing using a thermochemical reaction) ) Etc. are available.

  In the next step, since the polycrystalline diamond thin film layer 3 formed on the support substrate 2 and the bonding piezoelectric substrate are directly bonded by the surface activation room temperature bonding method, the bonding surface of the bonding piezoelectric substrate has no irregularities. Preferably, the surface roughness is Ra 0.5 nm or less, preferably 0.3 nm or less. The reason for reducing the surface roughness is that smoothness is important for directly bonding the polycrystalline diamond thin film layer 3 and the bonding piezoelectric substrate in the next step. If there are irregularities, there is a possibility that floating will occur without complete bonding at the atomic level.

<B> Step of directly bonding the polycrystalline diamond thin film layer 3 formed on the support substrate 2 and one main surface of the bonding piezoelectric substrate by a surface activation room temperature bonding method to form a bonded body. The polycrystalline diamond thin film layer 3 formed on the support substrate 2 and one main surface of the bonding piezoelectric substrate are directly bonded by a surface activated room temperature bonding method to form a bonded body.

  Usually, in order to join the polycrystalline diamond thin film layer 3 and the joining piezoelectric substrate, an organic adhesive, an inorganic adhesive, a UV adhesive, heat diffusion bonding, or the like is used. However, since the various adhesives soften as the temperature rises, when the obtained composite substrate is used for a SAW device, the comb-shaped electrodes 4 formed on the piezoelectric substrate 1 also move at the same time as the piezoelectric substrate 1, causing resonance. The frequency is likely to change. Further, in order to join the polycrystalline diamond thin film layer 3 and the piezoelectric substrate for bonding by thermal diffusion, heating at 1000 ° C. or more is required, and the temperature exceeds the Curie temperature of the piezoelectric substrate for bonding.

  In order to avoid this problem, it is desirable to use room temperature bonding that can be bonded at room temperature and is bonded at the atomic level. In order to bond at room temperature, the bonding surface of the polycrystalline diamond thin film layer 3 formed on the support substrate 2 and one main surface which is the bonding surface of the bonding piezoelectric substrate are sufficiently washed, and the washed polycrystalline diamond thin film is washed. The layer 3 and the bonding piezoelectric substrate are placed in a vacuum vessel, and each bonding surface is irradiated with an ion (argon) beam in an ultra-high vacuum to remove residual impurities and activate each bonding surface. Join with pressure and no voltage. After cleaning the bonding surface of the polycrystalline diamond thin film layer 3 and the bonding surface of the bonding piezoelectric substrate, it is also preferable to further perform UV irradiation on each of the bonding surfaces.

<C> A non-bonding surface of the bonding piezoelectric substrate in the bonded body is irradiated with a laser having a wavelength of 150 to 300 nm to form a piezoelectric substrate thinned by ablation processing for decomposing and volatilizing the non-bonding surface of the bonding piezoelectric substrate. Step Next, a laser (for example, an excimer laser) having a wavelength of 150 to 300 nm is applied to the non-bonding surface of the bonding piezoelectric substrate in the bonded body of the polycrystalline diamond thin film layer 3 formed on the support substrate 2 and the bonding piezoelectric substrate. To form a piezoelectric substrate having a thickness of 5 μm or more and 30 μm or less and a surface roughness Ra of 10 nm or less by ablation processing for decomposing and volatilizing the non-bonding surface of the bonding piezoelectric substrate. By the ablation process, the bonding piezoelectric substrate directly bonded to the polycrystalline diamond thin film layer 3 is thinned, and the piezoelectric substrate 1 having a thickness of 5 μm or more and 30 μm or less is formed.

  As a light source for ablation processing, it is preferable to use light having a wavelength shorter than the absorption end of the material constituting the bonding piezoelectric substrate, and usually, light having a wavelength of 350 nm or less is preferable. In particular, when processing a substrate made of an oxide single crystal, by using light having a wavelength of 300 nm or less, light irradiated to the substrate is absorbed in the extremely surface layer. Decomposes only the layer and does not damage the inside of the substrate.

  The wavelength of the light for ablation processing for thinning the non-bonding surface of the bonding piezoelectric substrate is more preferably 300 nm or less as described above. However, from a practical viewpoint, the thickness is preferably 150 nm or more. As specific light sources, in addition to excimer laser light sources, fourth-order harmonics of YAG (laser light of 266 nm), excimer lamps, and the like are currently practical.

  FIG. 4 is an explanatory diagram showing a schematic configuration of a laser processing apparatus for ablation processing.

  The laser processing apparatus 30 is a non-bonded surface of the bonded piezoelectric substrate 21 of a bonded body composed of the bonded piezoelectric substrate 21 / (direct bonding) / polycrystalline diamond thin film layer / support substrate (silicon substrate) 22 shown in FIG. Is a device for making a thin film by ablation processing, and is configured as follows. That is, in the laser processing apparatus 30, a bonded body composed of the bonding piezoelectric substrate 21 / (direct bonding) / polycrystalline diamond thin film layer / support substrate (silicon substrate) 22 is formed on the support base 37 by, for example, a vacuum suction method. Fixedly held on top. The joined body fixed and held by the support table 37 is irradiated with the laser beam LB by controlling the X-axis direction moving means and the Y-axis direction moving means of the stage 41 provided below the support table 37. Is moved directly below the irradiation unit 36 to be irradiated.

  In the laser processing apparatus 30, a laser light source 31 that outputs a laser beam LB is used. The laser beam LB output from the laser light source 31 in the horizontal direction is shaped into a beam shape by an irradiation unit 36, and The non-bonded surface of the bonding piezoelectric substrate 21 of the bonded body fixed to the support table 37 is irradiated.

The irradiating unit 36 includes a mirror 61 that reflects the laser beam LB and guides the laser beam LB downward, two cylindrical lenses 62 and 63 that shape the laser beam LB reflected by the mirror 61, and these cylindrical lenses. A condensing lens 64 for condensing the laser beam LB shaped by the lenses 62 and 63 inside the bonding piezoelectric substrate 21. Then, in the laser processing apparatus 30, the irradiation section 36 shapes the laser beam LB into a desired beam shape, and adjusts the irradiation beam size, that is, the irradiation area to be focused inside the bonding piezoelectric substrate 21. be able to. By using a short wavelength excimer laser or the like and condensing the laser light with the lens of the irradiation unit 36, the shorter the laser wavelength, the smaller the focused spot diameter can be reduced to several μm. By raising the peak value of the energy, the power density per unit area can be set to 10 ⁷ to 10 ⁹ W / cm ^2, and the desired thickness of the bonding piezoelectric substrate 21 to be irradiated with the laser beam can be obtained. It can be instantaneously vaporized just before dissolution. Further, not only the portion having the spot diameter but also the portion exceeding the evaporation temperature of the piezoelectric substrate material is vaporized. Since the piezoelectric substrate material is vaporized without passing through a molten state, the surface state of the piezoelectric substrate can be smoothed at the nano level. Depending on the energy density of the irradiated laser beam, it can be vaporized to a thickness of about 1 to 2 mm in the thickness of the piezoelectric substrate. Therefore, the thickness of the piezoelectric substrate can be easily set to 5 μm or more and 30 μm or less. By scanning the non-bonding surface of the bonding piezoelectric substrate 21 with the laser light, a large area can be decomposed and volatilized to be thin. In addition, the surface of the thinned piezoelectric substrate has a surface state with a surface roughness Ra of 10 nm or less, which is a level that can be directly applied to the manufacture of a SAW filter.

  Here, the excimer laser will be described. An excimer laser is a pulse repetition oscillation laser of ultraviolet light. The excimer laser emits ultraviolet light oscillated by a gaseous compound such as ArF (wavelength 193 nm), KrF (wavelength 248 nm), XeCl (wavelength 308 nm), and directs the light by an optical resonator. It was taken out together. Since the excimer laser is a short-wavelength laser of ultraviolet light, it is possible to decompose the bonds of atoms and molecules constituting a substance with the energy of photons, and applications based on this chemical action have been developed.

  By the way, the following three modes can be mentioned as a method of forming a piezoelectric substrate by thinning the non-bonding surface of the bonding piezoelectric substrate directly bonded to the polycrystalline diamond thin film layer 3 by the excimer laser.

(1) Spot scan processing;
A non-bonding surface of the bonding piezoelectric substrate is irradiated with a spot-like light beam so that the optical axis of the laser is perpendicular, and the spot-like light beam is scanned to decompose and volatilize a portion through which the light beam has passed, thereby forming a thin film. A piezoelectric substrate is formed.

(2) Batch transfer processing;
A non-bonding surface of the bonding piezoelectric substrate is directly irradiated with a laser beam which has been passed through a mask of a predetermined pattern in advance, and the irradiated portion is decomposed and volatilized in a plane without moving the laser beam to form a thin film to form a piezoelectric substrate. I do. Since the bonding piezoelectric substrate can be thinned at one time, the processing efficiency is high and the flatness is extremely good. However, it is necessary to oscillate a large area laser beam, and it is necessary to increase the precision of the optical system.

(3) Slit scan processing;
The laser beam is transmitted through a mask having slits of an elongated pattern to obtain a laser beam having an elongated rectangular planar shape. Then, the laser beam is irradiated and moved on the non-bonding surface of the bonding piezoelectric substrate, thereby being decomposed and volatilized into a thin film to form a piezoelectric substrate. According to the slit scan processing method, the irradiation site can be decomposed and volatilized in a planar manner, so that the surface smoothness is ensured.

  As described above, the non-bonding surface of the bonding piezoelectric substrate is irradiated with a laser having a wavelength of 150 to 300 nm, and the thickness of the non-bonding surface of the bonding piezoelectric substrate is 5 μm or more and 30 μm or less by ablation processing for decomposing and volatilizing the non-bonding surface. A piezoelectric substrate can be formed. Since the thickness of the piezoelectric substrate is extremely small, the piezoelectric substrate is significantly affected by the polycrystalline diamond thin film layer and approaches the hardness of the polycrystalline diamond thin film layer. Since the signal is propagated, it is possible to realize a higher propagation speed than before.

  Further, the piezoelectric substrate directly bonded to a polycrystalline diamond thin film layer formed on one main surface of a support substrate (for example, a silicon substrate) having a smaller coefficient of thermal expansion than the piezoelectric substrate, Since the hardness is remarkably influenced and approaches the hardness of the polycrystalline diamond thin film layer, the frequency characteristics do not shift (fluctuate) due to a temperature change, and it is possible to provide better frequency-temperature characteristics than before.

  According to the steps <a> to <c>, the composite substrate for a surface acoustic wave device according to the first embodiment in which the frequency is increased and the frequency temperature characteristic is improved can be obtained.

(2) Method of manufacturing composite substrate for surface acoustic wave device according to second embodiment of the present invention Piezoelectric substrate 1, support substrate 2 having a smaller coefficient of thermal expansion than piezoelectric substrate 1, and one of support substrates 2 For the surface acoustic wave device according to the second embodiment in which the polycrystalline diamond thin film layer 3 is formed on the main surface of the substrate and the piezoelectric thin film layer 3 and the piezoelectric substrate 1 are directly bonded via the metal thin film 5. The manufacturing method of the composite substrate
Forming a polycrystalline diamond thin film layer 3 on one main surface of the support substrate 2;
The above-mentioned polycrystalline diamond thin film layer 3 formed on the support substrate 2 and one main surface of the bonding piezoelectric substrate are directly bonded via the metal thin film 5 by a surface activated room temperature bonding method to form a bonded body. Process and
The non-bonded surface of the bonding piezoelectric substrate in the bonded body is irradiated with a laser having a wavelength of 150 to 300 nm to ablate to decompose and volatilize the non-bonded surface of the bonding piezoelectric substrate, and has a thickness of 5 μm or more and 30 μm or less. And forming a piezoelectric substrate having a surface roughness Ra of 10 nm or less,
Having, and
In the step of directly bonding the polycrystalline diamond thin film layer 3 and one main surface of the bonding piezoelectric substrate by the surface activation room temperature bonding method, the bonding surfaces of the polycrystalline diamond thin film layer 3 and the bonding piezoelectric substrate before bonding are formed. Is cleaned, and each bonding surface is irradiated with an ion beam to remove residual impurities, and after the metal thin film 3 is formed on at least one of the bonding surfaces of the polycrystalline diamond thin film layer 3 and the bonding piezoelectric substrate. It is characterized by direct bonding at room temperature in vacuum.

Of the above manufacturing process,
<a> Step of forming polycrystalline diamond thin film layer 3 on one main surface of support substrate 2 <c> Irradiating a laser having a wavelength of 150 to 300 nm to a non-bonding surface of the bonding piezoelectric substrate in the bonded body, Regarding each step of the step of forming the piezoelectric substrate thinned by ablation processing for decomposing and volatilizing the non-bonding surface of the bonding piezoelectric substrate, <a> in the manufacturing method of the composite substrate for a surface acoustic wave device according to the first embodiment Step and <c> step,
Step <b> in the method of manufacturing the composite substrate for a surface acoustic wave device according to the second embodiment, that is, the polycrystalline diamond thin film layer 3 formed on the support substrate 2 via the metal thin film 5 and the bonding piezoelectric The process is different from the first embodiment in that a main body is directly bonded to one main surface of the substrate by a surface activated room temperature bonding method to form a bonded body.

  Therefore, the step <b> in the method for manufacturing a composite substrate for a surface acoustic wave device according to the second embodiment will be described below.

<B> A bonded body obtained by directly bonding the polycrystalline diamond thin film layer 3 formed on the support substrate 2 and one main surface of the bonding piezoelectric substrate via the metal thin film 5 by a surface activated room temperature bonding method. Step of Forming Polycrystalline diamond thin film layer 3 is formed on one main surface of support substrate 2 by the step <a> in the method of manufacturing a composite substrate for a surface acoustic wave device according to the first embodiment.

  Next, the above-mentioned polycrystalline diamond thin film layer 3 formed on the support substrate 2 and one main surface of the bonding piezoelectric substrate are directly bonded via the metal thin film 5 by a surface activated room temperature bonding method to form a joined body. To form

  In order to directly join the polycrystalline diamond thin film layer 3 and the bonding piezoelectric substrate via the metal thin film 5, the bonding surface of the polycrystalline diamond thin film layer 3 before bonding and one main surface of the bonding piezoelectric substrate are washed. The bonding surface between the cleaned polycrystalline diamond thin film layer 3 and the bonding piezoelectric substrate is placed in a vacuum vessel so that the ion beam can be irradiated, and each bonding surface is irradiated with an ion beam in an ultra-high vacuum to remove residual impurities. And activate each joint surface.

  Next, a metal thin film 5 is formed on at least one of the bonding surfaces of the polycrystalline diamond thin film layer 3 and the bonding piezoelectric substrate by a sputtering method. The metal thin film 5 is preferably a chromium film, a titanium film, or the like having a strong bonding force with oxygen and a high diffusion coefficient, and particularly preferably a titanium film. The thickness of the metal thin film 5 formed on at least one of the bonding surfaces of the polycrystalline diamond thin film layer 3 and the bonding piezoelectric substrate is preferably 5 to 10 nm. If the film thickness is too thin, less than 5 nm, the film becomes discontinuous, and diffusion to the formed bonding surface becomes discontinuous. On the other hand, if the film thickness exceeds 10 nm and is too thick, a continuous film is formed before diffusion, and the film is interposed between the polycrystalline diamond thin film layer 3 and the bonding piezoelectric substrate, and may not function as a diffusion layer. There is.

  After the metal thin film 5 is formed, the polycrystalline diamond thin film layer 3 and the main surface of the bonding piezoelectric substrate are bonded at room temperature, without pressure, and without voltage by utilizing the large atomic diffusion of the metal thin film 5. The metal thin film 5 exists on these bonding surfaces, and the metal thin film 5 can be bonded by atomic diffusion. Thereby, the polycrystalline diamond thin film layer 3 and the bonding piezoelectric substrate are directly bonded via the metal thin film 5, and the polycrystalline diamond thin film layer 3 formed on one main surface of the support substrate 2 and the bonding piezoelectric substrate are connected to each other. Are obtained. With the metal thin film 5 interposed, the surface roughness at both bonding surfaces of the polycrystalline diamond thin film layer 3 and the bonding piezoelectric substrate may be higher than when the metal thin film 5 is not provided, and the polishing cost before bonding is reduced. There is a merit to lower.

  Next, in the <c> step in the method for manufacturing a composite substrate for a surface acoustic wave device according to the first embodiment, a non-bonded surface of the bonded piezoelectric substrate in the bonded body is irradiated with a laser having a wavelength of 150 to 300 nm. Then, a piezoelectric substrate thinned by ablation processing for decomposing and volatilizing the non-bonding surface of the bonding piezoelectric substrate is formed.

  As a result, a composite substrate for a surface acoustic wave device according to the second embodiment in which the polycrystalline diamond thin film layer 3 formed on the support substrate 2 and the thinned piezoelectric substrate 1 are directly bonded via the metal thin film 5 is obtained. Can be That is, it is possible to obtain the composite substrate for a surface acoustic wave device according to the second embodiment in which the frequency is increased and the frequency temperature characteristic is improved.

3. Method for Manufacturing Surface Acoustic Wave Device The composite substrate for a surface acoustic wave device according to the first embodiment manufactured by the method described above, and the composite substrate for a surface acoustic wave device according to the second embodiment are thinned by laser ablation processing. The surface acoustic wave device electrode (IDT electrode) 4 having the above-described function is formed on the non-bonded surface of the piezoelectric substrate 1 thus manufactured, and the surface acoustic wave device is manufactured. When the surface acoustic wave device is used as a resonator, an IDT electrode and a pair of reflectors are arranged on the piezoelectric substrate on both sides of the IDT electrode.

  Hereinafter, a method for manufacturing the surface acoustic wave device will be specifically described.

  First, after forming a conductive material layer for an electrode on the non-bonding surface of the piezoelectric substrate 1 in the composite substrate for a surface acoustic wave device, an IDT electrode and a reflector were formed on the conductive material layer by photolithography. A resist layer having a shape is formed.

  Then, by using the resist layer as a mask and removing the conductive material layer in a portion where the resist layer is not formed by a dry etching method such as reactive ion etching (RIE), the IDT electrode having a predetermined pattern and the reflective layer are removed. A vessel is formed.

  When forming the IDT electrode, patterning may be performed by a lift-off method instead of the etching method. The number of the reflectors is appropriately adjusted in consideration of necessary insertion loss, chip size, and the like.

As the conductive material for the electrode, aluminum or a trace amount of a different metal (for example, Cu, Si, Ti, HfB _2, etc.) is used for aluminum or aluminum because of its low mass, low electric resistance, and high power durability. (Preferably, not necessarily a solid solution). For example, from the viewpoint of the power durability of the IDT electrode that affects the life of the surface acoustic wave element, an aluminum alloy to which a trace amount of copper is added by sputtering film deposition, which has a reputation for being resistant to migration in the field of semiconductor devices, is used. Preferably, it is used. However, the present invention is not limited to the above-described aluminum-based alloy, and one selected from Cu, Au, Pt, Ag, and an alloy containing one of these metals as a main component can also be used.

  The surface acoustic wave device manufactured using the composite substrate for the surface acoustic wave device according to the first embodiment and the second embodiment, the propagation speed of the surface acoustic wave propagating on the surface is increased, the resonance frequency is increased, In addition, the problem that the frequency characteristics shift (fluctuate) due to a change in temperature is improved, and the piezoelectric layer has a good film quality.

  Hereinafter, examples of the present invention will be specifically described with reference to comparative examples.

[Example 1]
(1) Polishing of piezoelectric substrate for bonding One main surface of a lithium tantalate substrate (manufactured by Sumitomo Metal Mining Co., Ltd.) having a diameter of 2 inches and a thickness of 50 μm was coated with a diamond nano polishing machine [Abiko Giken Research Inc.] [In-house]. The surface roughness of the polished lithium tantalate substrate surface was measured with a three-dimensional optical profiler Nexview device (manufactured by Canon Inc.), and the substrate was polished until the surface roughness Ra became 0.3 nm.

(2) Formation of Polycrystalline Diamond Thin Film Layer on Support Substrate (Silicon Substrate) A 2-inch diameter single-crystal silicon substrate (2 inches in diameter × 200 μm in thickness) is prepared as a support substrate, and the silicon substrate is formed using acetone. Ultrasonic cleaning.

Next, a 5.0 μm-thick polycrystalline diamond thin film layer was formed on the cleaned silicon substrate using a microwave plasma CVD apparatus under the following film forming conditions.
<Deposition conditions of polycrystalline diamond thin film layer>
・ Pretreatment: Ar bombard (10 minutes)
・ Reaction gas / carrier gas used: methane (CH ₄ ) / hydrogen, (methane concentration 3%)
・ Gas flow rate: 100sccm
・ Power supply microwave frequency; 2.45 GHz
・ Microwave output: 600W
-Film forming pressure: 1.3 kPa
・ Substrate temperature during film formation: 700 ° C
・ Deposition time: 180 minutes

The Raman spectrum of the obtained thin film was measured using a Raman spectrophotometer. As a result, a high peak was confirmed at 1333 cm ^-1 . From this measurement result, it was confirmed that the film was a diamond film.

(3) Polishing of Polycrystalline Diamond Thin Film Layer Next, the polycrystalline diamond thin film layer formed on the supporting substrate (silicon substrate) was polished using a diamond nano-polisher [Abiko Giken Co., Ltd.]. The surface roughness of the polished polycrystalline diamond thin film layer was measured with a three-dimensional optical profiler Nexview apparatus (manufactured by Canon Inc.), and polished until the surface roughness Ra became 0.3 nm.

(4) Room Temperature Bonding of Polycrystalline Diamond Thin Film Layer and Piezoelectric Bonding Substrate Surface-polished polycrystalline diamond thin film layer / support substrate (silicon substrate) and lithium tantalate substrate whose main surface is polished (piezoelectric bonding substrate) ) Was subjected to ultrasonic cleaning in an acetone solution, and the polished surfaces of both substrates were subjected to UV irradiation for 60 seconds.

Next, the two substrates, which have been washed and irradiated with UV, are placed in a surface activated bonding type room temperature bonding apparatus [Musashino Engineering Co., Ltd.], and evacuated to an ultra-high vacuum of 2 × 10 ⁻⁶ Pa. The surface of the polished substrate is irradiated with an Ar beam (irradiation conditions: acceleration voltage 50 kV, beam diameter 1.2 mm, irradiation amount 2 × 10 ¹⁴ ions / cm ² ), and after activating the surface, polishing of both substrates is performed. The two surfaces were bonded at room temperature without applying heat, pressure, etc. to obtain a bonded body of a polycrystalline diamond thin film layer and a lithium tantalate substrate (piezoelectric substrate for bonding).

(5) Thinning of the piezoelectric substrate by ablation processing to decompose and volatilize the non-bonding surface of the bonding piezoelectric substrate in the bonded body Lithium tantalate substrate (piezoelectric bonding substrate) / (direct bonding) / polycrystalline diamond thin film layer / support substrate A laser ablation process was performed on the non-bonded surface of the lithium tantalate substrate in the bonded body having the configuration of (silicon substrate).
<Laser ablation processing conditions>
・ Laser device for laser ablation processing: Excimer laser LEAP 150K by Coherent
・ Laser wavelength: wavelength 248 nm
Laser mode: single mode Focused spot diameter 10 μm, pulse width 1 msec, output 0.8 J (peak value 80 J)
・ Power density: 8 × 10 ⁷ J / m ²
・ Laser irradiation conditions: scanning speed 5 mm / s, scanning width 2 mm

  When laser ablation processing was performed under the above conditions, the thickness of the lithium tantalate substrate (bonding piezoelectric substrate) was evaporated by about 5 μm by one scanning. This scanning was performed six times, and the thickness of the lithium tantalate substrate (piezoelectric substrate for bonding) was reduced by about 30 μm.

  By the above treatment, the thickness of the lithium tantalate substrate was 20 μm, the surface roughness Ra of the lithium tantalate substrate surface after laser ablation processing was 8 nm, and the smoothness was maintained. Therefore, it was possible to shift to the manufacturing process of the surface acoustic wave device in this state.

(6) Production of Surface Acoustic Wave Device A 5 nm-thick Cr was first deposited on the non-bonded surface of the laser ablation-processed lithium tantalate substrate of the composite substrate for a surface acoustic wave device according to Example 1 by a vacuum evaporation method. Was formed, and then a Cu film having a thickness of 0.15 μm was formed.

  Next, a resist layer having a shape corresponding to the IDT electrode is formed on the Cu film by a photolithography method, and the resist layer is formed by a dry etching method of reactive ion etching (RIE) using the resist layer as a mask. The portions of the Cu film and the Cr film not removed were removed. Thus, an IDT electrode having a predetermined pattern was formed, and the surface acoustic wave device (SAW filter) according to Example 1 was manufactured.

  Regarding the characteristics of the obtained SAW device, the propagation speed was 11,000 m / s, the frequency temperature characteristic was −12.8 ppm / ° C., and the electromechanical coupling coefficient was 7.2%.

  From these evaluation results, it was confirmed that the propagation speed and the frequency temperature characteristics were higher than those of the conventional SAW device using the lithium tantalate substrate.

  Further, by setting the width of the IDT electrode to 0.4 μm (the wavelength λ of the surface acoustic wave becomes 0.4 × 4 = 1.6 μm), a SAW device having a resonance frequency of 6875 MHz could be obtained. Table 2 shows the characteristics of the obtained SAW device.

[Example 2]
In the step (4) of the embodiment 1, except that the room temperature bonding between the polycrystalline diamond thin film layer and the bonding piezoelectric substrate was performed via the metal thin film, the same as the steps (1), (2) and (3) of the embodiment 1 Under the conditions described above, a joined body of the polycrystalline diamond thin film layer according to Example 2 and a lithium tantalate substrate (piezoelectric substrate for joining) was obtained.

  Step (4) in Example 2, that is, the room-temperature bonding between the polycrystalline diamond thin film layer and the bonding piezoelectric substrate via the metal thin film was performed as follows.

  The surface-polished polycrystalline diamond thin film layer / support substrate (silicon substrate) and the lithium tantalate substrate (bonding piezoelectric substrate) whose main surface has been polished are ultrasonically cleaned in an acetone solution. UV irradiation was performed on the polished surface for 60 seconds.

Next, both substrates that have been washed and irradiated with UV light are placed in a surface activated bonding type room-temperature bonding device [Musashino Engineering Co., Ltd.], and evacuated to an ultra-high vacuum of 2 × 10 ⁻⁶ Pa. Is irradiated with an Ar beam (irradiation conditions: acceleration voltage 50 kV, beam diameter 1.2 mm, irradiation amount 2 × 10 ¹⁴ ions / cm ² ), and after activating the surface, a support substrate (silicon) is placed in the same chamber. On the surface of the polycrystalline diamond thin film layer formed on the (substrate), a titanium film was formed to a thickness of 7 nm by a sputtering method.

  Then, the lithium tantalate substrate (piezoelectric substrate for bonding) and the surface of the polycrystalline diamond thin film layer on which the titanium film is formed are opposed to each other, and both surfaces are bonded at room temperature without applying heat, pressure and the like. A joined body of the thin film layer and the lithium tantalate substrate was obtained.

(5) Thinning the piezoelectric substrate by ablation processing to decompose and volatilize the non-bonding surface of the bonding piezoelectric substrate in the bonded body Lithium tantalate substrate (bonding piezoelectric substrate) / (direct bonding: Ti film) / polycrystalline diamond thin film layer Laser ablation processing was performed on the non-bonded surface of the lithium tantalate substrate in the bonded body having the structure of / supporting substrate (silicon substrate) under the same conditions as in Example 1.

  When laser ablation was performed under the same conditions as in Example 1, the thickness of the lithium tantalate substrate (bonding piezoelectric substrate) was evaporated by about 5 μm by one scanning. This scanning was performed six times, and the thickness of the lithium tantalate substrate (piezoelectric substrate for bonding) was reduced by about 30 μm.

  By the above treatment, the thickness of the lithium tantalate substrate was 20 μm, the surface roughness Ra of the surface of the lithium tantalate substrate after laser ablation processing was 8 nm, and the smoothness was maintained. Therefore, it was possible to shift to the manufacturing process of the surface acoustic wave device in this state.

  Table 1 shows these manufacturing conditions together with Example 1.

(6) Production of Surface Acoustic Wave Device A 5 nm-thick Cr layer was first formed on the non-bonding surface of the laser ablation-processed lithium tantalate substrate of the composite substrate for a surface acoustic wave device according to Example 2 by a vacuum evaporation method. Was formed, and then a Cu film having a thickness of 0.15 μm was formed.

  Next, a resist layer having a shape corresponding to the IDT electrode is formed on the Cu film by a photolithography method, and the resist layer is formed by a dry etching method of reactive ion etching (RIE) using the resist layer as a mask. The portions of the Cu film and the Cr film not removed were removed. Thus, an IDT electrode having a predetermined pattern was formed, and a surface acoustic wave device (SAW filter) according to Example 2 was manufactured.

  Regarding the characteristics of the obtained SAW device, the propagation speed was 8,900 m / s, the frequency temperature characteristic was -22.2 ppm / ° C., and the electromechanical coupling coefficient was 7.4%.

  From these evaluation results, it was confirmed that the propagation speed and the frequency temperature characteristics were higher than those of the conventional SAW device using the lithium tantalate substrate.

  Further, by setting the width of the IDT electrode to 0.4 μm (the wavelength λ of the surface acoustic wave becomes 0.4 × 4 = 1.6 μm), a SAW device having a resonance frequency of 5563 MHz was able to be obtained. Table 2 shows the characteristics of the obtained SAW device.

[Comparative Example 1]
The steps (1) to (5) of Example 1 were not performed, and the surface of the lithium tantalate substrate on which the IDT electrode was formed was polished using a lithium tantalate substrate having a diameter of 2 inches and a thickness of 50 μm. A surface acoustic wave device (SAW filter) according to Comparative Example 1 was manufactured in accordance with the step (6) of “producing a surface acoustic wave device” of Example 1.

  Regarding the characteristics of the obtained SAW filter, the propagation speed was 3900 m / s, the frequency temperature characteristic was -38.2 ppm / ° C., and the electromechanical coupling coefficient was 8.0%. These evaluation results were the same propagation speed and frequency temperature characteristics as those of the conventional SAW filter.

  Further, by setting the width of the IDT electrode to 0.4 μm (the wavelength λ of the surface acoustic wave becomes 0.4 × 4 = 1.6 μm), a SAW device having a resonance frequency of 2438 MHz was obtained.

  With the above propagation speed (3900 m / s), a higher frequency of the SAW device could not be realized. Table 2 shows the characteristics of the obtained SAW device.

[Comparative Example 2]
A 2-inch diameter single crystal silicon substrate (2 inches in diameter × 200 μm in thickness, surface roughness Ra 0.3 nm) was prepared as a supporting substrate.

  Further, as a piezoelectric substrate, a lithium tantalate substrate [manufactured by Sumitomo Metal Mining Co., Ltd.] having a diameter of 2 inches, a thickness of 50 μm, and a surface roughness Ra of 0.3 nm was prepared.

  Then, after ultrasonic cleaning of both substrates in an acetone solution, UV irradiation was further performed on the surfaces of both substrates for 60 seconds.

Next, both substrates that have been washed and irradiated with UV light are placed in a surface activated bonding type room-temperature bonding device [Musashino Engineering Co., Ltd.], and evacuated to an ultra-high vacuum of 2 × 10 ⁻⁶ Pa. Is irradiated with an Ar beam (irradiation conditions: acceleration voltage 50 kV, beam diameter 1.2 mm, irradiation amount 2 × 10 ¹⁴ ions / cm ² ), and after activating the surfaces of both substrates, the surfaces of both substrates are opposed to each other. By bonding both surfaces at room temperature without applying heat, pressure, etc., a composite substrate (composite substrate) for a surface acoustic wave device according to Comparative Example 2 in which a support substrate (silicon substrate) and a lithium tantalate substrate are bonded is produced. did.

  The non-bonding surface of the lithium tantalate substrate in the composite substrate was polished without performing the step (5) of Example 1, "thinning by ablation processing to decompose and volatilize the non-bonding surface of the piezoelectric substrate in the bonded body". .

  That is, the non-bonding surface of the lithium tantalate substrate in the composite substrate having the structure of lithium tantalate substrate / (direct bonding) / silicon substrate is reduced to a thickness of 31 μm using a surface polisher [DGP8761 manufactured by Disco Corporation]. Polished. Further, the non-bonded surface of the lithium tantalate substrate was mirror-polished by mechanochemical polishing using colloidal silica to have a surface roughness Ra of 4 nm for the non-bonded surface.

  Table 1 shows the above manufacturing conditions.

  Using the obtained composite substrate, a SAW device according to Comparative Example 2 was manufactured in accordance with the step (6) “Production of surface acoustic wave device” of Example 1.

  As for the characteristics of the obtained SAW device, the propagation speed was 4100 m / s, the frequency temperature characteristic was −14.8 ppm / ° C., and the electromechanical coupling coefficient was 7.3 (see Table 2).

  By setting the width of the IDT electrode to 0.4 μm (the wavelength λ of the surface acoustic wave becomes 0.4 × 4 = 1.6 μm), a SAW device having a resonance frequency of 2563 MHz was obtained.

  Comparative Example 2 employs a composite substrate structure in which a silicon substrate and a lithium tantalate substrate are directly bonded to each other. The silicon substrate has a smaller thermal expansion than the lithium tantalate substrate, and thus the elongation of the lithium tantalate substrate is reduced. It was suppressed, the electrode spacing was not widened, and the change in resonance frequency was very small. However, a propagation speed higher than that of a conventional SAW device using a lithium tantalate substrate was not obtained.

  The surface acoustic wave device using the composite substrate for a surface acoustic wave device according to the present invention can be used as a substrate for a surface acoustic wave device because the frequency can be increased and the frequency characteristics can be shifted (fluctuated) due to a temperature change. Has industrial applicability used.

P plasma 1 Piezoelectric substrate 2 Support substrate 3 Polycrystalline diamond thin film layer 4 Comb-shaped electrode (IDT electrode)
Reference Signs List 5 metal thin film 6 magnetron 7 rectangular waveguide 8 matching device 9 plunger 10 reaction tube 11 susceptor (substrate holder)
12 Exhaust pump 13 Power monitor 14 Reaction gas 21 Piezoelectric substrate for bonding 22 Polycrystalline diamond thin film layer / support substrate (silicon substrate)
Reference Signs List 30 laser processing device 31 laser light source 34 control unit 36 irradiation unit 37 support base 41 stage 61 mirror 62, 63 cylindrical lens 64 condensing lens LB laser beam

Claims (13)

A piezoelectric substrate,
A supporting substrate having a smaller coefficient of thermal expansion than the piezoelectric substrate,
In a composite substrate for a surface acoustic wave device comprising a polycrystalline diamond thin film layer formed on one main surface of the support substrate,
The surface elasticity, wherein the polycrystalline diamond thin film layer and the piezoelectric substrate are directly joined, the thickness of the piezoelectric substrate is 5 μm or more and 30 μm or less, and the surface roughness Ra of the piezoelectric substrate is 10 nm or less. Composite substrate for wave elements. A piezoelectric substrate,
A supporting substrate having a smaller coefficient of thermal expansion than the piezoelectric substrate,
In a composite substrate for a surface acoustic wave device comprising a polycrystalline diamond thin film layer formed on one main surface of the support substrate,
The polycrystalline diamond thin film layer and the piezoelectric substrate are directly bonded via a metal thin film, the thickness of the piezoelectric substrate is 5 μm or more and 30 μm or less, and the surface roughness Ra of the piezoelectric substrate is 10 nm or less. Composite substrate for a surface acoustic wave device.   The composite substrate for a surface acoustic wave device according to claim 2, wherein the metal thin film is a titanium film or a chromium film.   The surface acoustic wave according to any one of claims 1 to 3, wherein the support substrate is made of one selected from silicon, sapphire, aluminum nitride, aluminum oxide, borosilicate glass, and quartz glass. Composite substrate for devices.   The piezoelectric substrate is made of at least one kind of bulk crystal selected from lithium tantalate, lithium niobate, lithium niobate-lithium tantalate solid solution single crystal, crystal, lithium borate, zinc oxide, aluminum nitride, langasite, and langatate. The composite substrate for a surface acoustic wave device according to claim 1, wherein the composite substrate is configured. A piezoelectric substrate,
A supporting substrate having a smaller coefficient of thermal expansion than the piezoelectric substrate,
In a method for manufacturing a composite substrate for a surface acoustic wave device comprising a polycrystalline diamond thin film layer formed on one main surface of the support substrate,
Forming a polycrystalline diamond thin film layer on one main surface of the support substrate;
A step of forming a bonded body by directly bonding the polycrystalline diamond thin film layer formed on the support substrate and one main surface of the bonding piezoelectric substrate by a surface activation room temperature bonding method,
The non-bonded surface of the bonding piezoelectric substrate in the bonded body is irradiated with a laser having a wavelength of 150 to 300 nm to ablate to decompose and volatilize the non-bonded surface of the bonding piezoelectric substrate, and has a thickness of 5 μm or more and 30 μm or less. And forming a piezoelectric substrate having a surface roughness Ra of 10 nm or less,
A method for manufacturing a composite substrate for a surface acoustic wave device, comprising: In the step of forming a bonded body by directly bonding the polycrystalline diamond thin film layer formed on the support substrate and one main surface of the bonding piezoelectric substrate by a surface activation room temperature bonding method,
7. The surface acoustic wave device according to claim 6, wherein the polycrystalline diamond thin film layer and one main surface of the bonding piezoelectric substrate are directly bonded via a metal thin film by a surface activation room temperature bonding method. A method for manufacturing a composite substrate. In the step of forming a polycrystalline diamond thin film layer on one main surface of the support substrate,
7. The method according to claim 6, wherein the polycrystalline diamond thin film layer is formed by a microwave plasma CVD method.   9. The method of manufacturing a composite substrate for a surface acoustic wave device according to claim 6, wherein a surface of the polycrystalline diamond thin film layer formed on one main surface of the support substrate is polished. The non-bonded surface of the bonding piezoelectric substrate in the bonded body is irradiated with a laser having a wavelength of 150 to 300 nm to ablate to decompose and volatilize the non-bonded surface of the bonding piezoelectric substrate, and has a thickness of 5 μm or more and 30 μm or less. And forming a piezoelectric substrate having a surface roughness Ra of 10 nm or less,
The method according to claim 6, wherein the laser having a wavelength of 150 to 300 nm is an excimer laser. In the step of forming a bonded body by directly bonding the polycrystalline diamond thin film layer formed on the support substrate and one main surface of the bonding piezoelectric substrate by a surface activation room temperature bonding method,
After cleaning each bonding surface of the polycrystalline diamond thin film layer and the bonding piezoelectric substrate before bonding, irradiating each bonding surface with an ion beam to remove and activate residual impurities, and then directly bonding in vacuum at room temperature The method for manufacturing a composite substrate for a surface acoustic wave device according to claim 6, wherein: In a step of directly bonding the polycrystalline diamond thin film layer and one main surface of the bonding piezoelectric substrate by a surface activated room temperature bonding method via a metal thin film,
The bonding surfaces of the polycrystalline diamond thin film layer and the bonding piezoelectric substrate before bonding are cleaned, and each bonding surface is irradiated with an ion beam to remove residual impurities, and the polycrystalline diamond thin film layer and the bonding piezoelectric substrate are bonded. 8. The method for manufacturing a composite substrate for a surface acoustic wave device according to claim 7, wherein a metal thin film is formed on at least one of the bonding surfaces, and then directly bonded at room temperature in a vacuum.   The method for manufacturing a composite substrate for a surface acoustic wave device according to claim 12, wherein the metal thin film is a titanium film or a chromium film having a thickness of 5 to 10 nm.

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