JP2019097145A - Composite substrate for surface acoustic wave element, and manufacturing method thereof - Google Patents

Abstract

To provide a composite substrate for surface acoustic wave element which is capable of increasing a frequency of a surface acoustic wave element, solves the problem that a frequency characteristic is fluctuated by a temperature change, suppresses power resistance deterioration or signal loss caused by mismatching of acoustic impedance between a piezoelectric layer and a diamond crystal and further comprises the piezoelectric layer of high film quality, and a manufacturing method thereof.SOLUTION: A composite substrate for surface acoustic wave element comprises: a piezoelectric substrate 1; a support substrate 2 having a smaller thermal expansion coefficient than that of the piezoelectric substrate; and a polycrystal diamond thin film layer 3 formed on one principal surface of the support substrate. An acoustic impedance layer 4 of which the acoustic impedance is smaller than the polycrystal diamond thin film layer and the piezoelectric substrate is formed on the polycrystal diamond thin film layer, and the acoustic impedance layer and the piezoelectric substrate are directly bonded.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, and more particularly to a composite substrate for a surface acoustic wave device suitably used for a surface acoustic wave device and a method of manufacturing the same.

  In recent years, as communication devices such as mobile phones are advancing in frequency and size, the RF circuit unit is required to have higher performance and size reduction. Among these, as a high frequency filter used for a transmitting and receiving unit of a communication device, and an electronic element such as a resonator used for an oscillator, a surface acoustic wave device (Surface Acoustic Wave Device) (hereinafter, may be abbreviated as a SAW device) Is used. The SAW device is an element utilizing a phenomenon in which a specific frequency is selected in the process of converting a high frequency signal into a surface acoustic wave and converting it again into a high frequency signal by using a piezoelectric material. And, taking advantage of the steepness of the frequency characteristics and the ability to design waveforms compared to dielectric filters and ceramic filters that have been used in the high frequency band conventionally, the ease of surface mounting, and the small size and light weight, a mobile phone They are rapidly being adopted in mobile communication devices represented by smartphones and other communication devices such as various sensors and touch panels. In particular, with the explosive development of small-sized and high-frequency devices such as mobile phones in recent years, the demand is being greatly expanded.

  As this surface acoustic wave element, a piezoelectric layer as a propagation medium of surface acoustic waves on a substrate, and a pair of interdigital electrodes [IDT: Interdigital Transducer] (hereinafter referred to as an IDT, an IDT electrode, or an electrode) There is known one configured by sequentially stacking layers. Usually, the IDT electrode is formed by forming a metal material layer on a piezoelectric layer and then etching the metal material layer.

  In the surface acoustic wave device, when an electric signal (AC power) is supplied to the input IDT, distortion occurs in the piezoelectric layer due to the electric field. And since the said electrode is a comb-tooth shape shape, the difference in density arises in a piezoelectric material layer, and a surface acoustic wave generate | occur | produces. 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 device is determined by the distance λ ₀ of the comb-like electrodes and the propagation velocity V of the elastic wave on the surface of the piezoelectric layer,
f ₀ = V / λ ₀ (Expression 1)
Given by

However, it is difficult to fabricate a surface acoustic wave device that operates well at 2.5 GHz or higher. In order to increase the central frequency f ₀ of the transmission band, either the interval λ ₀ of the interdigital electrodes is decreased or the propagation velocity V of the surface acoustic wave is increased as is apparent from the above (formula 1) However, λ ₀ is extremely limited by processing techniques such as photolithography. At the current mass production level, the width of the comb-like electrodes is about 0.4 μm, the spacing λ ₀ of the comb-like electrodes is about 1.6 μm, and a lithium tantalate substrate often used recently for surface acoustic wave devices Hereinafter, at a propagation speed of 3800 m / s (which may be abbreviated as LT substrate), the center frequency f ₀ of the transmission band is limited to 2400 MHz. For this reason, in order to obtain a surface acoustic wave device operating in a high frequency band, it is necessary to increase the propagation velocity V.

On the other hand, when a surface acoustic wave device is used as the above-mentioned resonator, as shown in FIG. 3, the reflection comprising the IDT electrode 21 on the piezoelectric substrate 20 and the pair of strip electrodes 22 on both sides of the IDT electrode 21 The configuration to place the container is taken. The reflector is realized by periodically changing the acoustic impedance of the surface of the piezoelectric substrate 20 by periodically forming a lattice-like thin film made of metal or dielectric. Since the value of acoustic impedance differs between the portion where the lattice thin film exists and the portion where the lattice thin film does not exist, the surface acoustic wave propagating on the surface of the piezoelectric substrate 20 has different acoustic impedance as described above. It will be reflected at the portion where the thin film is present to produce a reflected wave. If you leave arranging the strip-like electrode 22 in a cycle of lambda _0/2, a surface acoustic wave which is reflected constructive in-phase, most SAW energy is returned to the IDT electrode 21. As a result, the surface acoustic wave traveling in the left and right at the frequency f ₀ is synchronized becomes a standing wave produces a high resonant phenomena Q. This frequency is called a resonant frequency, and satisfies the same relationship as the above-mentioned (Equation 1). As the operation of the SAW resonator, it is known that there exist two resonance frequencies: a resonance frequency fr at which the impedance becomes a minimum value, and an antiresonance frequency fa at which the impedance becomes a maximum value. As the characteristics of the SAW resonator, the larger the difference between the minimum value and the maximum value of the impedance, the better. Also in this resonator, a surface acoustic wave device that operates in a high frequency band is desired.

  In addition, as a device for high frequency, a piezoelectric thin film resonator FBAR (Film Bulk Acoustic Resonator) using, for example, AlN as a piezoelectric material is considered. However, since the piezoelectric thin film resonator FBAR is complicated and expensive to manufacture, it is used only for some devices.

  Then, examination of the surface acoustic wave element which operates in a high frequency band is repeated. For example, since a diamond crystal has a very high sound velocity of 18000 m / s, research and development (see Patent Documents 1 and 2) of a surface acoustic wave device utilizing this high sound velocity characteristic has been advanced. Then, Non-Patent Document 1 discloses the production of a surface acoustic wave device in which a comb-like electrode and a zinc oxide layer are formed after polycrystalline diamond crystals are formed on a silicon substrate. Non-Patent Document 1 discloses that the manufactured surface acoustic wave device has an acoustic velocity of 10000 m / s or more and sufficiently high excitation efficiency.

  And, in the surface acoustic wave device in which the piezoelectric layer (zinc oxide layer) is formed on the diamond crystal layer, there is an advantage that the frequency can be easily increased as described in Non-Patent Document 1. However, while the acoustic impedance of the piezoelectric layer is generally low, the acoustic impedance of the diamond crystal layer is high, so that the acoustic impedance mismatch occurs between the piezoelectric layer and the diamond crystal layer, and the interface between the piezoelectric layer and the diamond crystal layer The reflection of the acoustic wave at this time may confine the elastic wave to the inside of the piezoelectric layer, whereby the vibration displacement may be extremely concentrated on the surface of the piezoelectric layer. The acoustic impedance is related to the velocity of sound and substrate density at each substrate. Then, if displacement of the surface wave is concentrated on the surface layer, there is a possibility that breakage of the surface layer of the piezoelectric body (rupture due to stress migration due to increase in displacement) may occur, so the power durability of the surface acoustic wave device (device) Problem of getting worse. Also, there is a problem that bulk wave conversion loss is increased due to partial conversion to a bulk wave at the electrode end when a surface wave is incident on the electrode, and the performance of the surface acoustic wave device (device) is deteriorated. . Furthermore, since the surface acoustic wave is concentrated on the surface layer of the piezoelectric body, the reflection coefficient of the electrode is increased, so that the design freedom of the surface acoustic wave device (device) is reduced.

Therefore, a base layer (a base layer made of diamond or diamond-like carbon), a piezoelectric layer disposed on the base layer, and an electrode formed on either the front or back surface of the piezoelectric layer are used. Patent Document 3 describes an acoustic relaxation layer (a base layer and a base layer) between a base layer and a piezoelectric layer, for the problem of the surface acoustic wave device (such as deterioration of power resistance due to mismatch of acoustic impedance). Propose a device in which a material with small acoustic impedance and larger acoustic impedance than the piezoelectric layer, such as an acoustic relaxation layer composed of AlN, Si ₃ N ₄ , Al ₂ O ₃ , SrTiO ₃ etc., is interposed There is. And, by interposing the acoustic relaxation layer between the base layer and the piezoelectric layer, even in the case of achieving high frequency using a base layer (such as diamond) having a high phase velocity of the elastic wave, propagation in the inside of the piezoelectric layer Of the surface acoustic wave is reflected at the boundary between the base layer and the piezoelectric layer and confined in the piezoelectric layer, so that concentration of displacement on either the front or back surface of the piezoelectric layer is suppressed. As a result, the power resistance of the device can be improved, and the bulk wave conversion loss in the electrode structure formed on either the front or back surface of the piezoelectric layer is reduced, and the reflection coefficient of the electrode structure is It is said that the degree of freedom in device design is improved due to the reduction. Furthermore, when the acoustic impedance of each layer present from the piezoelectric layer to the base layer is configured to increase sequentially, an acoustic relaxation layer and other layers having an acoustic impedance intermediate between the base layer and the piezoelectric layer become In any case, since the penetration of the displacement component of the surface acoustic wave into the base layer is not inhibited, the improvement effect of the phase velocity of the surface acoustic wave by the base layer can be secured, and it is not disturbed to increase the frequency of the device.

  On the other hand, in surface acoustic wave devices which are required to operate in a high frequency band, another problem exists that the frequency characteristics shift (fluctuate) because the piezoelectric substrate (piezoelectric layer) expands and contracts due to temperature change. . Therefore, in order to improve this temperature characteristic, a composite substrate is proposed in which a piezoelectric substrate on which an IDT electrode is formed and an auxiliary substrate having a thermal expansion coefficient smaller than that of the piezoelectric substrate and larger than that of the piezoelectric substrate are directly bonded. (See, for example, Patent Document 4). Since the auxiliary substrate is directly bonded to the piezoelectric substrate, expansion and contraction of the piezoelectric substrate is suppressed by the auxiliary substrate having a smaller thermal expansion coefficient and a larger thickness than the piezoelectric substrate, so that the temperature characteristics can be improved. As the difference between the thermal expansion coefficients of the piezoelectric substrate and the auxiliary substrate is increased, the effect of improving the temperature characteristics is greater. However, a composite substrate (wafer) in which a piezoelectric substrate and an auxiliary substrate having different thermal expansion coefficients are directly bonded is warped due to temperature change such as heat treatment in the surface acoustic wave device manufacturing process, or directly bonded substrates are peeled off. As the surface acoustic wave device passes through the process temperature, the patterning accuracy is deteriorated in the process of manufacturing the surface acoustic wave device, which makes automatic handling difficult. In this situation, the larger the difference between the thermal expansion coefficients of the piezoelectric substrate and the auxiliary substrate, and the larger the size of the composite substrate (wafer), the larger the warpage of the wafer, and the directly bonded substrates are easily peeled off. When the warpage of the composite substrate (wafer) exceeds the allowable value or the substrates directly bonded to each other are peeled off, the composite substrate (wafer) can not flow in the manufacturing process.

  Therefore, Patent Document 5 is a composite substrate in which a piezoelectric substrate and a supporting substrate having a thermal expansion coefficient smaller than that of the piezoelectric substrate (corresponding to the auxiliary substrate in Patent Document 4) are bonded, and the supporting substrate is the same material Of the first substrate and the second substrate are bonded by direct bonding with a peelable strength with a blade, and the first substrate is bonded to the piezoelectric substrate on the side opposite to the bonding surface with the two substrates. A combined composite substrate is disclosed. In this composite substrate, warpage of the composite substrate generated according to the process temperature change is suppressed to a small level, and after the surface acoustic wave device is manufactured, the second substrate is peeled off from the first substrate by a blade and removed. The thickness of the substrate can be easily reduced, which has the advantage of being able to meet the demand for thinner devices.

  However, although the methods of Patent Documents 4 to 5 can cope with the problem that the frequency characteristic is shifted (varied) due to temperature change, the signal propagation speed for obtaining the surface acoustic wave device operating in the high frequency band is The functions to be improved are not considered at all in Patent Documents 4 to 5.

Unexamined-Japanese-Patent No. 9-051248 (refer to paragraph 0053-0055) Unexamined-Japanese-Patent No. 6-268463 (refer to paragraph 0031-0034) JP, 2007-228225, A (refer to paragraph 0011, paragraph 0018-0025) Japanese Patent Application Laid-Open No. 11-55070 WO 2014/129432 JP 2002-94355 A (see paragraphs 0017 and 0031) JP, 2015-73331, A (refer to paragraph 0138-0142)

Proceedings of the Sixth Diamond Symposium (November 26-27, 1992) pp. 90-91: P21 "Application of ZnO / Polycrystalline Diamond Structure to Surface Acoustic Wave and High Frequency Filter"

  In the field of communication equipment, the use of higher frequency bands has been directed by the exhaustion of available frequency band resources, and even in surface acoustic wave devices, a technology for further high frequency bands is required. In order to increase the frequency of the surface acoustic wave device, the method of mainly reducing the electrode size has been performed until now, but the reduction of the electrode spacing for determining the frequency is as described above in the current lithography technology. It is approaching its limit. Further, even if the frequency can be increased by the miniaturization of the electrode dimensions, the thinning of the electrodes and the miniaturization of the electrode spacing cause the problem that the element structure itself is easily broken and power characteristics can not be obtained.

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

  However, it has been pointed out that the film quality of the piezoelectric (ZnO) layer formed on the diamond thin film by the film forming method such as the CVD method or the sputtering method is not good and a sufficient electromechanical coupling coefficient can not be obtained. (See paragraph 0017 of Patent Document 6). Further, also in Patent Document 3 for improving the problem of the surface acoustic wave device caused by the mismatch of the acoustic impedance, the acoustic relaxation layer (the acoustic impedance is smaller than that of the base layer by the film forming method such as CVD or sputtering) The piezoelectric material layer is formed on the acoustic relaxation layer having an acoustic impedance larger than that of the body layer (see paragraph 0025 of Patent Document 3), and the film quality of the piezoelectric layer is not good as in Patent Documents 1 and 2. Have.

  In order to solve these problems, in Patent Document 6, an electromechanical conversion electrode formed on a first main surface of a piezoelectric layer (piezoelectric bulk single crystal) and a second main of the piezoelectric layer are disclosed. A surface acoustic wave device is proposed which comprises a diamond layer formed on a surface and a supporting substrate bonded to the diamond layer via an adhesive.

  However, as described in paragraph 0031 of Patent Document 6, a substrate (piezoelectric material) when a polycrystalline diamond layer is formed on a piezoelectric bulk single crystal substrate (piezoelectric material layer) using microwave plasma CVD method Bulk single crystal substrate) is deposited at 850 ° C., and if the substrate temperature rises or drops rapidly, the piezoelectric bulk single crystal substrate is formed by the pyroelectricity of the piezoelectric bulk single crystal substrate (piezoelectric layer). Another problem is that the piezoelectric bulk single crystal substrate is degraded due to destruction or heating of the substrate due to microwave power or substrate heating, resulting in deterioration of the piezoelectric characteristics. For example, when a lithium tantalate substrate is used, since the Curie temperature is 650 ° C., after forming a polycrystalline diamond layer on the lithium tantalate substrate, the piezoelectricity is lost and the function as a SAW device can not be obtained. There was a problem that Furthermore, in Patent Document 6, the diamond layer and a support substrate (silicon substrate, glass substrate, ceramic substrate, etc.) are bonded (adhered) using an adhesive (epoxy resin adhesive, solder alloy, etc.) Therefore, the adhesive softens due to heat and moves due to stress, and there is also a problem that in the thermal cycle test, there is a problem such as lack of reliability, such as peeling in part.

In Patent Document 7, made of a high acoustic velocity film between the (aluminum nitride, aluminum oxide, made of diamond or the like) and a piezoelectric film (made of LiTaO _3), a low speed of sound film (silicon oxide, glass, tantalum oxide etc. Although the sound velocity of the elastic wave is lowered by arranging the acoustic wave device, an elastic wave device is proposed in which elastic wave energy is concentrated on a low sound velocity medium. Since the confinement effect of the elastic wave energy in the piezoelectric film and in the IDT electrode in which the elastic wave is excited can be enhanced, the loss is reduced compared to the case where the low sound velocity film is not provided, and the Q value is improved. It can be raised. Further, the high sound velocity film is described as confining the elastic wave in a portion where the piezoelectric film and the low sound velocity film are stacked and functioning so as not to leak to a structure below the high sound velocity film. However, Patent Document 7, sequentially forming a high acoustic velocity film made of low sound speed film, an aluminum nitride or the like made of silicon oxide on a piezoelectric substrate such as made of LiTaO _3, mirror polishing an exposed surface of the high acoustic velocity film Only the manufacturing procedure for bonding to a supporting substrate (made of sapphire, silicon, glass, etc.) is described (see paragraphs 0138-0142), for example, using a film forming method such as CVD or sputtering. When the crystalline aluminum nitride film is formed on the piezoelectric substrate, the film is formed with the temperature of the piezoelectric substrate set to a high temperature as in Patent Document 6, so the piezoelectric substrate is broken due to the pyroelectricity of the piezoelectric substrate. Alternatively, there is a problem that the piezoelectric substrate is degraded due to microwave power or substrate heating to deteriorate the piezoelectric characteristics. Further, when the low sound velocity film or the high sound velocity film is formed using the transfer method regardless of the film forming method, as in Patent Document 6, there are concerns about various problems of lack of reliability due to heat softening of the adhesive. Ru.

  The present invention has been made focusing on such problems, and the problem to be solved is to increase the propagation velocity of the surface acoustic wave to realize higher frequency of the surface acoustic wave element, and to obtain frequency characteristics. While improving the problem of shifting (fluctuation) due to temperature change, deterioration in power resistance and increase in signal loss due to mismatch in acoustic impedance between the piezoelectric layer and the diamond crystal are suppressed and good. It is an object of the present invention to provide a composite substrate for a surface acoustic wave device having a film quality piezoelectric layer and a method of manufacturing the same.

  Therefore, the inventor has found that the frequency characteristics are changed due to the increase in frequency and the temperature change required for the surface acoustic wave device, and the acoustic impedance mismatch between the piezoelectric layer and the diamond crystal. In order to solve the problem of deterioration in power resistance and increase in signal loss, a method of increasing surface acoustic wave propagation speed, improving frequency temperature characteristics and suppressing deterioration in power resistance and increase in signal loss investigated.

  As a result of examination, in order to solve the above-mentioned subject, a piezoelectric substrate composed of bulk crystal is applied as a piezoelectric layer, and a polycrystalline diamond thin film having a thermal expansion coefficient smaller than that of the piezoelectric substrate and on one main surface A support substrate on which a layer and an acoustic impedance layer (an acoustic impedance layer composed of a material having an acoustic impedance smaller than that of the piezoelectric layer and the polycrystalline diamond thin film layer) is sequentially applied, and the piezoelectric substrate and the acoustic impedance layer It has been found that this can be achieved by directly bonding and further polishing and thinning the non-bonded surface of the directly bonded piezoelectric substrate. The present invention is completed by such technical discovery.

That is, the first invention according to the present invention is
A piezoelectric substrate,
A support substrate having a thermal expansion coefficient smaller than that of the piezoelectric substrate;
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 an acoustic impedance layer having an acoustic impedance smaller than that of the piezoelectric substrate are formed on the polycrystalline diamond thin film layer, and the acoustic impedance layer and the piezoelectric substrate are directly bonded. age,
The second invention is
A piezoelectric substrate,
A support substrate having a thermal expansion coefficient smaller than that of the piezoelectric substrate;
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 an acoustic impedance layer having an acoustic impedance smaller than that of the piezoelectric substrate are formed on the polycrystalline diamond thin film layer, and the acoustic impedance layer and the piezoelectric substrate are directly bonded via the metal thin film. It is characterized by

In addition, the third invention according to the present invention is
In the composite substrate for a surface acoustic wave device according to the second invention,
Characterized in that the metal thin film is a titanium film or a chromium film;
The fourth invention is
In the composite substrate for a surface acoustic wave device according to any one of the first to third inventions,
The acoustic impedance layer is composed of a SiO ₂ film,
The fifth invention is
In the composite substrate for a surface acoustic wave device according to any one of the first to fourth inventions,
The supporting substrate is made of one selected from silicon, sapphire, aluminum nitride, aluminum oxide, borosilicate glass and quartz glass;
The sixth invention is
In the composite substrate for a surface acoustic wave device according to any one of the first to fifth inventions,
The piezoelectric substrate is one or more bulk crystals selected from lithium tantalate, lithium niobate, lithium niobate-lithium tantalate solid solution single crystal, quartz, lithium borate, zinc oxide, aluminum nitride, langasite, langaite It is characterized by being composed of

Next, a seventh invention according to the present invention is
A piezoelectric substrate,
A support substrate having a thermal expansion coefficient smaller than that of the piezoelectric substrate;
A polycrystalline diamond thin film layer formed on one main surface of the support substrate;
A method of manufacturing a composite substrate for a surface acoustic wave device, comprising the polycrystalline diamond thin film layer and the acoustic impedance layer having an acoustic impedance smaller than that of the piezoelectric substrate, which is formed on the polycrystalline diamond thin film layer.
Forming a polycrystalline diamond thin film layer on one main surface of the support substrate;
Forming a polycrystalline diamond thin film layer and an acoustic impedance layer having an acoustic impedance smaller than that of the piezoelectric layer on the polycrystalline diamond thin film layer;
Directly bonding the acoustic impedance layer and the piezoelectric substrate by surface activation room temperature bonding;
Polishing the non-bonding surface of the piezoelectric substrate directly bonded to the acoustic impedance layer;
Characterized in that
The eighth invention is
A method of manufacturing a composite substrate for a surface acoustic wave device according to a seventh invention,
In the step of directly bonding the acoustic impedance layer and the piezoelectric substrate by surface activation room temperature bonding method,
The acoustic impedance layer is directly bonded to the piezoelectric substrate through a metal thin film.

The ninth invention is
A method of manufacturing a composite substrate for a surface acoustic wave device according to a seventh invention,
In the step of forming a polycrystalline diamond thin film layer on one main surface of the support substrate,
Forming the polycrystalline diamond thin film layer by microwave plasma CVD method;
The tenth invention is
In the method of manufacturing the composite substrate for a surface acoustic wave device according to the seventh invention,
Polishing the surface of the polycrystalline diamond thin film layer formed on one main surface of the supporting substrate;
The eleventh invention is
A method of manufacturing a composite substrate for a surface acoustic wave device according to a seventh invention,
Polishing the non-bonding surface of the piezoelectric substrate directly bonded to the acoustic impedance layer,
Polishing is performed until the thickness of the piezoelectric substrate becomes 0.3 to 25 μm.

The twelfth invention is
A method of manufacturing a composite substrate for a surface acoustic wave device according to a seventh invention,
In the step of directly bonding the acoustic impedance layer and the piezoelectric substrate by surface activation room temperature bonding method,
The bonding surfaces of the acoustic impedance layer and the piezoelectric substrate before bonding are cleaned, and the bonding surfaces are irradiated with an ion beam to remove residual impurities, and then bonding is performed directly at room temperature in vacuum.
The thirteenth invention is
It is a manufacturing method of the compound substrate for surface acoustic wave elements given in the 8th invention,
In the step of directly bonding the acoustic impedance layer and the piezoelectric substrate via a metal thin film,
The bonding surfaces of the acoustic impedance layer and the piezoelectric substrate before bonding are cleaned, and ion beams are irradiated to the bonding surfaces to remove residual impurities, and the bonding surface of at least one of the acoustic impedance layer and the piezoelectric substrate After forming a metal thin film on the upper surface, it is characterized by direct bonding at normal temperature in vacuum,
The fourteenth invention is
In the method of manufacturing a composite substrate for a surface acoustic wave device according to the thirteenth invention,
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 comprises a piezoelectric substrate, a support substrate having a thermal expansion coefficient smaller than that of the piezoelectric substrate, and a polycrystalline diamond thin film layer formed on one main surface of the support substrate. An acoustic impedance layer having an acoustic impedance smaller than that of the polycrystalline diamond thin film layer and the piezoelectric substrate is formed on the polycrystalline diamond thin film layer, and the acoustic impedance layer and the piezoelectric substrate are directly bonded to each other. It is characterized by

  And, in a 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 supporting substrate having a thermal expansion coefficient smaller than that of a piezoelectric substrate. Since the surface acoustic wave propagates, it is possible to realize an extremely high propagation speed.

  Further, in the surface acoustic wave device using the composite substrate for surface acoustic wave devices according to the present invention, since the supporting substrate having a thermal expansion coefficient smaller than that of the piezoelectric substrate is applied, the frequency characteristic is shifted due to the temperature change ( It is possible to provide good frequency temperature characteristics without fluctuation.

  Furthermore, the composite substrate for a surface acoustic wave device according to the present invention comprises a piezoelectric substrate made of a bulk crystal, and an acoustic wave on a polycrystalline diamond thin film layer provided on a supporting substrate having a smaller thermal expansion coefficient than the piezoelectric substrate. It is obtained by directly bonding to the impedance layer and thinning the non-bonding surface of the bonded piezoelectric substrate by polishing.

  The piezoelectric substrate (piezoelectric layer) made of bulk crystal is deposited on the diamond thin film or on the acoustic relaxation layer (the acoustic relaxation layer having a smaller acoustic impedance than the base layer and a larger acoustic impedance than the piezoelectric layer) The film quality is good compared to the piezoelectric layers of Patent Documents 1 to 3 formed by sputtering or the like, and the acoustic impedance layer on the polycrystalline diamond thin film layer provided on the supporting substrate is directly bonded to the piezoelectric substrate. As a result, there is no problem of destruction of the piezoelectric substrate and deterioration of the piezoelectric characteristics described in Patent Documents 6 to 7 and various problems caused by heat softening of the adhesive because the bonding method by the adhesive is not adopted. Is also resolved.

  Furthermore, in the composite substrate for a surface acoustic wave device according to the present invention, the acoustic impedance layer (the piezoelectric substrate and the polycrystalline diamond thin film layer) has an acoustic impedance smaller than that of the piezoelectric substrate (piezoelectric layer) and the polycrystalline diamond thin film. Layer), the surface acoustic wave device using the composite substrate for a surface acoustic wave device according to the present invention includes a piezoelectric layer associated with a mismatch in acoustic impedance between the piezoelectric substrate (piezoelectric layer) and the polycrystalline diamond thin film layer. The reflection of the elastic wave at the interface is reduced, thereby suppressing the phenomenon in which the vibration displacement of the surface elastic wave is concentrated in the vicinity of the surface of the piezoelectric layer, thereby preventing the breakage on the surface of the piezoelectric layer and preventing the surface acoustic wave element Power property can be improved, and a signal propagating on the surface of the piezoelectric layer is spread inside the acoustic impedance layer. When, since the signal at the interface between the acoustic impedance layer and the polycrystalline diamond thin film layer is returned to the piezoelectric layer is reflected, it is possible to reduce the loss of propagation signal in the surface acoustic wave element.

FIG. 1 is a cross-sectional 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. 7 is a cross-sectional view of a surface acoustic wave device using the composite substrate for a surface acoustic wave device according to a second embodiment of the present invention. A configuration example in which a surface acoustic wave device is used as a resonator is shown, and a reflector comprising a piezoelectric substrate, an IDT electrode provided on the piezoelectric substrate, and strip electrodes disposed on both sides of the IDT electrode And FIG. 7 is a schematic plan view of a resonator configured by Explanatory drawing which shows schematic structure of a microwave plasma CVD apparatus.

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

1. Composite substrate for surface acoustic wave device (A) Composite substrate for surface acoustic wave device according to the first embodiment of the present invention The composite substrate for surface acoustic wave device according to the first embodiment of the present invention is as shown in FIG. A piezoelectric substrate 1, a supporting substrate 2 having a thermal expansion coefficient smaller than that of the piezoelectric substrate 1, and a polycrystalline diamond thin film layer 3 formed on one main surface of the supporting substrate 2; The polycrystalline diamond thin film layer 3 and an acoustic impedance layer 4 having an acoustic impedance smaller than that of the piezoelectric substrate 1 are formed on the diamond thin film layer 3, and the acoustic impedance layer 4 and the piezoelectric substrate 1 are directly bonded. Also, in the surface acoustic wave device configured using the composite substrate for a surface acoustic wave device according to the first embodiment, the comb-like electrode 6 is formed on the non-bonding surface of the piezoelectric substrate 1. What It is.

(B) Composite Substrate for Surface Acoustic Wave Element According to Second Embodiment of the Present Invention The composite substrate for a surface acoustic wave element according to the second embodiment of the present invention is, as shown in FIG. A support substrate 2 having a thermal expansion coefficient smaller than that of the piezoelectric substrate 1 and a polycrystalline diamond thin film layer 3 formed on one main surface of the support substrate 2 are provided. A polycrystalline diamond thin film layer 3 and an acoustic impedance layer 4 having an acoustic impedance smaller than that of the piezoelectric substrate 1 are formed, and the acoustic impedance layer 4 and the piezoelectric substrate 1 are directly bonded via the metal thin film 5. The surface acoustic wave device is characterized by using the composite substrate for a surface acoustic wave device according to the second embodiment, wherein the comb-like electrode 6 is formed on the non-bonding surface of the piezoelectric substrate 1 described above. It is.

  (1) piezoelectric substrate, (2) support substrate, (3) polycrystalline diamond thin film layer, (4) acoustic impedance layer, (5) metal thin film, (6) composite substrate for surface acoustic wave device, 7) The surface acoustic wave elements will be described in order.

(1) Piezoelectric Substrate The piezoelectric substrate 1 is a substrate having piezoelectricity capable of propagating elastic waves, and lithium tantalate, lithium niobate, niobium as a piezoelectric substrate used for the composite substrate for a surface acoustic wave device according to the present invention. Lithium tantalate solid solution single crystal, quartz crystal, lithium borate, zinc oxide, aluminum oxide, langasite, langasite, preferably bulk crystal selected from lithium tantalate or lithium niobate More preferable. It is because lithium tantalate and lithium niobate are suitable for surface acoustic wave devices of high frequency and wide band frequency because the propagation speed of surface acoustic wave is high and the electromechanical coupling coefficient is large.

  The piezoelectric substrate 1 is directly bonded to the acoustic impedance layer 4 provided on the polycrystalline diamond thin film layer 3 formed on one main surface of the support substrate 2 and the composite substrate for the surface acoustic wave device according to the present invention Configure It should be noted that if the surface of the piezoelectric substrate 1 is uneven, it can not be completely bonded to the acoustic impedance layer 4 at the atomic level, which may cause floating, so the surface of the piezoelectric substrate 1 has a surface roughness Ra 0 It is preferable to make the surface smooth to 0.5 nm or less, preferably 0.3 nm or less.

  Although the size of the piezoelectric substrate 1 is not particularly limited, for example, one having a diameter of 50 to 200 mm and a thickness of 0.3 to 25 μm is suitably used.

(2) Support Substrate The support substrate 2 used for the composite substrate for surface acoustic wave devices according to the present invention needs to be made of a material having a thermal expansion coefficient smaller than that of the piezoelectric substrate 1. Using a material having a thermal expansion coefficient smaller than that of the piezoelectric substrate 1 as the supporting substrate 2, the supporting substrate 2, a polycrystalline diamond thin film layer 3 formed on one main surface of the supporting substrate 2, and the polycrystalline diamond thin film By setting it as the composite substrate provided with the acoustic impedance layer 4 provided on the layer 3 and the piezoelectric substrate 1, the polycrystalline diamond thin film layer 3 and the acoustic impedance layer 4 are interposed between the support substrate 2 and the piezoelectric substrate 1. Even if the composite substrate is used as a SAW device, the frequency characteristics shift (fluctuate) due to the temperature change because expansion and contraction of the piezoelectric substrate 1 when the temperature changes are suppressed by the action of the support substrate 2. It becomes possible to solve the problem.

  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 and general-purpose soda glass substrate has a Vickers hardness of 500 to 600, a silicon substrate of about 1040, and a sapphire substrate of 2300, and a sapphire substrate is preferable. However, since a glass substrate and a silicon substrate are inexpensive and mass-produced and inexpensive, the silicon substrate is preferable as the support substrate 2 from a comprehensive viewpoint.

Therefore, the hardness of the support substrate 2 is increased by forming the polycrystalline diamond thin film layer 3 having high hardness on the support substrate using an inexpensive silicon substrate, and the above-mentioned polycrystalline diamond thin film layer 3 is formed. By directly bonding the provided acoustic impedance layer 4 and the piezoelectric substrate 1, the composite substrate obtained can obtain a propagation velocity faster than that of the piezoelectric substrate alone. Furthermore, the silicon substrate has a thermal expansion coefficient of 3.9 × 10 ^-6 / K, which is much smaller than that of the piezoelectric substrate 1 such as lithium tantalate, and can suppress the temperature change of the frequency characteristic 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, polycrystalline diamond thin film layer 3 is formed on one main surface of support substrate 2. Furthermore, by forming the acoustic impedance layer 4 provided on the polycrystalline diamond thin film layer 3 and the piezoelectric substrate 1 directly bonded to each other, for example, a pair of comb-like electrodes 6 is formed on the piezoelectric substrate 1 The surface acoustic wave is excited by applying a voltage. Since the acoustic impedance layer 4 having a smaller acoustic impedance than the piezoelectric substrate 1 and the polycrystalline diamond thin film layer 3 is interposed between the piezoelectric substrate 1 and the polycrystalline diamond thin film layer 3, the excited surface acoustic wave is The surface acoustic wave is propagated through the polycrystalline diamond thin film layer 3 and converted into an electrical signal by the piezoelectric substrate 1 again by another pair of comb-like electrodes. As a result, when the piezoelectric substrate 1 and the polycrystalline diamond thin film layer 3 are directly bonded, various problems associated with the mismatch of the acoustic impedance of the two layers (such as the deterioration of the power resistance of the SAW device and the increase of the signal loss) It is possible to improve the

  The diamond layer is a material having the highest sound propagation speed in the substance, and it is possible to realize a propagation speed of 10000 m / s or more even by laminating a piezoelectric thin film, so the composite substrate according to the present invention has high frequency surface elasticity. It can be used for wave elements.

  In order to form the polycrystalline diamond thin film layer 3, it is preferable to form a film on the supporting substrate 2 using a microwave plasma CVD method. The polycrystalline diamond thin film layer 3 is a vapor phase synthesis method using hydrocarbon etc. as a source gas, for example, a method of heating the electron emitting material to activate the source gas, a method of exciting the source gas by plasma, a gas by light The polycrystalline diamond thin film layer 3 according to the present invention can be formed by the microwave plasma CVD method among the film forming methods described above. It is preferable to form a film. Microwave plasma CVD is a synthesis method by electrodeless discharge using microwaves (a frequency of 2.45 GHz is usually used). The microwave plasma CVD method is a film forming method using plasma with only 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 the film forming speed is faster than other CVD methods, It is advantageous in that damage due to heat can be suppressed, and mutual diffusion between laminated films can be suppressed.

  Magnetic field microwave CVD, which is a type of microwave plasma CVD, makes it possible to create a stable high density plasma even at low pressure by utilizing the interaction between plasma and magnetic field, and microwave plasma It uses a strong plasma state by electron cyclotron resonance (ECR) in a lower pressure (1.3 to 133 Pa) region further than the CVD method. And since it is low pressure plasma, a uniform polycrystalline diamond thin film can be formed into a large area, and it is suitable for forming a polycrystalline diamond thin film layer concerning the present invention.

  The film thickness of the polycrystalline diamond thin film layer 3 is preferably about 5 μm on the support substrate 2. The reason is that since the unevenness is present on the surface of the polycrystalline diamond thin film layer 3 formed, the unevenness is also transferred to the surface of the acoustic impedance layer 4 formed on the surface of the polycrystalline diamond thin film layer 3 I will. And, if unevenness is present on the surface of the acoustic impedance layer 4 to be directly bonded to the piezoelectric substrate 1, as described in the above (1) Piezoelectric substrate column, floatation can be generated without being completely bonded at the atomic level To avoid this. That is, it is necessary to polish and smooth the uneven surface of the surface of the polycrystalline diamond thin film layer 3 before the acoustic impedance layer 4 is formed. For example, in the case of the above film thickness (about 5 μm), it is preferable to polish to about 3 μm and to make the surface roughness Ra 0.5 nm or less, preferably 0.3 nm or less. However, in consideration of the polishing cost, the polishing is not performed until the surface roughness Ra is reached, and the acoustic impedance layer 4 is equal to or half the polycrystalline diamond thin film layer 3 on the polycrystalline diamond thin film layer 3. The film is formed to have a film thickness within a certain range, and the uneven surface of the surface of the acoustic impedance layer 4 having the surface roughness Ra to which the surface roughness Ra of the polycrystalline diamond thin film layer 3 is transferred is polished and smoothed. The surface roughness Ra is preferably 0.5 nm or less, more preferably 0.3 nm or less.

(4) Acoustic Impedance Layer In the composite substrate for surface acoustic wave devices according to the present invention, the acoustic impedance layer 4 is formed on the polycrystalline diamond thin film layer 3 provided on one main surface of the support substrate 2, The acoustic impedance layer 4 and the piezoelectric substrate 1 are directly bonded.

  The acoustic impedance layer 4 is made of a material having an acoustic impedance smaller than that of the polycrystalline diamond thin film layer 3 and the piezoelectric layer 1 as described above.

  Since the acoustic impedance is represented by the product of "sound velocity in the medium" and "density of the medium", the acoustic impedance Z of the polycrystalline diamond thin film layer 3 is Z = density ρ · sound velocity VL = 61.6 It is [Pa · s / m]. For this reason, it is necessary to use an acoustic impedance layer 4 having an acoustic impedance lower than the acoustic impedance Z = 61.6 [Pa · s / m] of the polycrystalline diamond thin film layer 3.

On the other hand, when the piezoelectric substrate is made of zinc oxide, the acoustic impedance Z of the piezoelectric substrate 1 is made of ZnO (Z = 34.6 [Pa.s / m]) and lithium tantalate, for example. In this case, LiTaO ₃ (as an example, Z = 31.2 [Pa · s / m]), and when the piezoelectric substrate is made of aluminum nitride, it is AlN (Z = 38.4 [Pa · s / m]). is there. Therefore, it is necessary to use an acoustic impedance layer 4 having an acoustic impedance lower than the acoustic impedance of these piezoelectric substrates.

Then, as the condition is satisfied acoustic impedance layer 4 such as _{this, SiO 2 (Z = 15.6 [} Pa · s / m]), Al (Z = 17.0 [Pa · s / m]), and, Si (Z = 19.7 [Pa · s / m]) and the like can be mentioned, and among them, a SiO ₂ film is preferable.

  An acoustic impedance layer 4 having an acoustic impedance smaller than that of the polycrystalline diamond thin film layer 3 and the piezoelectric layer 1 is formed on the polycrystalline diamond thin film layer 3 and the acoustic impedance layer 4 and the piezoelectric substrate 1 are directly bonded. With the construction of the surface acoustic wave device, reflection of elastic waves at the interface of the piezoelectric layer 1 is reduced, as compared to the structure in which the polycrystalline diamond thin film layer 3 and the piezoelectric layer are directly bonded. Since the phenomenon in which the vibrational displacement of the elastic wave is concentrated in the vicinity of the surface of the piezoelectric layer can be suppressed, the breakdown on the surface of the piezoelectric layer can be prevented and the power resistance of the surface acoustic wave device can be improved. When the signal propagating on the surface of layer 1 is diffused into acoustic impedance layer 4, the signal is reflected at the interface between acoustic impedance layer 4 and polycrystalline diamond thin film layer 3. Re order to be returned to the piezoelectric layer 1, it is possible to reduce the loss of propagation signal in the surface acoustic wave element.

When an SiO ₂ film is used as the acoustic impedance layer 4, the SiO ₂ film can be obtained by a film forming method such as a sputtering method or a CVD method. When forming into a film by sputtering method, it can obtain by carrying out sputtering film-forming, introduce | transducing oxygen by using Si or SiO as a target. Although it is possible to use SiO ₂ as a target, it is preferable to use Si or SiO as a target because the film forming rate is slow.

  Further, as described above, the film thickness of the acoustic impedance layer 4 is desirably in the range equivalent to or about half that of the polycrystalline diamond thin film layer 3. When the film thickness of the acoustic impedance layer 4 is increased, stress is generated to cause warpage in the composite substrate, which causes problems such as difficulty in handling.

  The polycrystalline diamond thin film layer 3 is polished to about 3 μm if the film thickness at the time of film formation is about 5 μm, and the surface roughness Ra is preferably 0.5 nm or less, preferably 0.3 nm or less . The acoustic impedance layer 4 is formed on the polycrystalline diamond thin film layer 3 having the surface roughness Ra so as to have a film thickness equal to or approximately half that of the polycrystalline diamond thin film layer 3. The surface roughness of the polycrystalline diamond thin film layer 3 is transferred to the surface of the layer 4, and the surface roughness of the acoustic impedance layer 4 may be reduced to a surface roughness Ra of 0.5 nm or less, preferably 0.3 nm or less. it can. When the surface roughness of the acoustic impedance layer 4 is larger than the above range, the surface roughness Ra is preferably 0.5 nm or less, preferably 0.3 nm or less, by polishing. However, when the polishing cost of the polycrystalline diamond thin film layer 3 is also taken into consideration, the surface roughness Ra of the polycrystalline diamond thin film layer 3 is not polished to the above range, and on the polycrystalline diamond thin film layer 3 as described above. The acoustic impedance layer 4 is formed to have a film thickness equal to or half of that of the polycrystalline diamond thin film layer 3, and the surface roughness to which the surface roughness Ra of the polycrystalline diamond thin film layer 3 has been transferred It is also preferable to polish and smooth the uneven surface of the said acoustic impedance layer 4 surface which has Ra, and to make surface roughness Ra 0.5 nm or less, preferably 0.3 nm or less.

  In the composite substrate for a surface acoustic wave device according to the first embodiment of the present invention, polycrystalline diamond thin film layer 3 is formed on one main surface of support substrate 2, and on polycrystalline diamond thin film layer 3. The polycrystalline diamond thin film layer 3 and the acoustic impedance layer 4 having an acoustic impedance smaller than that of the piezoelectric substrate 1 are formed, and the acoustic impedance layer 4 and the piezoelectric substrate 1 are directly bonded.

  In order to directly bond the acoustic impedance layer 4 and the piezoelectric substrate 1, each bonding surface of the acoustic impedance layer 4 before bonding and the piezoelectric substrate 1 is cleaned, and the cleaned acoustic impedance layer 4 and the piezoelectric substrate 1 are placed in a vacuum vessel. Arrange and irradiate the ion beam to each bonding surface in ultra high vacuum to remove residual impurities and activate each bonding surface, and then apply appropriate load to bond and the bonding interface promotes atomic diffusion , And can be directly bonded at the atomic level. The direct bonding is preferably performed at normal temperature and without voltage.

(5) Metal Thin Film 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 is formed on one main surface of the support substrate 2 and the polycrystalline diamond A polycrystalline diamond thin film layer 3 and an acoustic impedance layer 4 having an acoustic impedance smaller than that of the piezoelectric substrate 1 are formed on the thin film layer 3, and the acoustic impedance layer 4 and the piezoelectric substrate 1 are directly bonded via the metal thin film 5. It is done.

  In order to directly bond the acoustic impedance layer 4 and the piezoelectric substrate 1 via the metal thin film 5, the bonding surface of the acoustic impedance layer 4 before bonding and the piezoelectric substrate 1 is cleaned and cleaned. 1 is placed in a vacuum vessel, and each bonding surface is irradiated with an ion beam in ultrahigh vacuum to remove residual impurities and activate each bonding surface, and then an acoustic impedance layer 4 is formed by a film forming method such as sputtering. It is possible to form a metal thin film 5 on at least one bonding surface of the piezoelectric substrate 1 and directly bond at normal temperature, no pressure and no voltage by utilizing the large atomic diffusion of the metal thin film 5. The metal thin film 5 is present at the interface between the acoustic impedance layer 4 and the piezoelectric substrate 1 and can be bonded by atomic diffusion of the metal thin film 5.

  The metal thin film 5 is preferably a thin film having a strong bonding force with oxygen such as chromium and titanium and a high diffusion coefficient. Moreover, as for the film thickness of the metal thin film 5, 5-10 nm is preferable. When the film thickness is too thin, less than 5 nm, the film becomes discontinuous and the diffusion 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 it may intervene as a film between the polycrystalline diamond thin film layer 3 and the piezoelectric substrate 1 and may not function as a diffusion layer is there. Due to the presence of the metal thin film 5, the surface roughness at both bonding surfaces may be rougher than when the metal thin film 5 is not present, and there is an advantage of reducing the polishing cost.

(6) Composite substrate for surface acoustic wave device The polycrystalline diamond thin film layer 3 is formed on one main surface of the supporting substrate 2, and the polycrystalline diamond thin film layer 3 and the above-mentioned piezoelectric material are formed on the polycrystalline diamond thin film layer 3. The composite substrate for a surface acoustic wave device according to the first embodiment, in which the acoustic impedance layer 4 having an acoustic impedance smaller than that of the substrate 1 is formed, and the acoustic impedance layer 4 and the piezoelectric substrate 1 are directly bonded A polycrystalline diamond thin film layer 3 is formed on one of the main surfaces of 2 and the polycrystalline diamond thin film layer 3 on the polycrystalline diamond thin film layer 3 and an acoustic impedance layer 4 having an acoustic impedance smaller than that of the piezoelectric substrate 1 In the second embodiment, the acoustic impedance layer 4 and the piezoelectric substrate 1 are directly bonded to each other through the metal thin film 5. With respect to the composite substrate for a surface acoustic wave device, the non-bonding surface of the piezoelectric substrate 1 in the composite substrate is polished to adjust the thickness of the piezoelectric substrate 1 to be thin.

  The difference in the thermal expansion coefficients of the support substrate 2 on which the polycrystalline diamond thin film layer 3 and the acoustic impedance layer 4 are formed and the piezoelectric substrate 1 makes it possible to prevent the composite substrate from warping due to temperature changes. The total thickness of the support substrate 2 on which the polycrystalline diamond thin film layer 3 and the acoustic impedance layer 4 are formed (acoustic impedance layer 4 thickness + polycrystalline diamond thin film layer 3 thickness + support substrate 2 thickness) You need to make it thinner. By reducing the thickness of the piezoelectric substrate 1, the warping force of the composite substrate is reduced and the composite substrate can be kept parallel, and a polycrystal bonded to the piezoelectric substrate 1 via the acoustic impedance layer 4 as a composite substrate A state in which the hardness of the diamond thin film layer 3 is as close as possible is obtained.

  Total thickness of support substrate 2 on which polycrystalline diamond thin film layer 3 and acoustic impedance layer 4 are formed (acoustic impedance layer 4 thickness + polycrystalline diamond thin film layer 3 thickness + support substrate 2 thickness) and thickness of piezoelectric substrate 1 As for the thickness, the thickness of the piezoelectric substrate 1 is reduced, and the ratio is 1/10 or less of (Acoustic impedance layer 4 thickness + polycrystalline diamond thin film layer 3 thickness + supporting substrate 2 thickness) Is preferably, more preferably 1/20. If there is a difference in the film thickness, the warpage of the composite substrate due to the difference in thermal expansion is suppressed even when the ambient temperature reaches about 120 ° C.

  The thickness of the piezoelectric substrate after polishing the non-bonding surface of the piezoelectric substrate 1 in the composite substrate is preferably 0.3 to 25 μm. When the polishing cost is also considered, it is desirable to set it as 1-25 micrometers. In addition, in consideration of performance such as suppression of warpage of the composite substrate, it is preferable to set the thickness to 0.3 to 5 μm. If the thickness of the piezoelectric substrate after polishing is less than 0.3 μm, the polishing cost may increase, but the influence of the bonding surface smoothness on the acoustic impedance layer 4 / polycrystalline diamond thin film layer 3 / supporting substrate 2 Thus, the thickness of the piezoelectric substrate 1 can not be maintained, and the thickness of the piezoelectric substrate 1 may be discontinuous. On the other hand, when the thickness of the piezoelectric substrate after polishing exceeds 25 μm, the warpage of the composite substrate increases, and the frequency temperature characteristic and the propagation speed decrease. That is, when the thickness of the piezoelectric substrate 1 increases, the characteristics of the piezoelectric substrate (for example, lithium tantalate) appear, the thermal expansion of the piezoelectric substrate becomes dominant, and the expansion and contraction of the surface acoustic wave device electrode becomes large. This is because the characteristics are lowered and the hardness as the composite substrate is lowered and the propagation speed is also lowered.

(7) Surface Acoustic Wave Element The surface acoustic wave element using the composite substrate for surface acoustic wave elements according to the present invention is for surface acoustic wave elements on the surface of the composite substrate on the side of the piezoelectric substrate 1 as shown in FIGS. An electrode (comb-like electrode) 6 is formed. The surface of the piezoelectric substrate 1 is partitioned such that a large number of surface acoustic wave devices are formed, and a pair of comb-like electrodes (IDT electrodes) for the elastic wave devices are provided at positions corresponding to the respective surface acoustic wave devices. And reflectors (for SAW resonators) are formed using photolithography technology.

  Finally, by dicing along the sections, a large number of SAW devices can be obtained. In the obtained SAW device, when a high frequency signal is applied to the IDT electrode on the input side, an electric field is generated between the electrodes, and a surface acoustic wave is excited to propagate on the piezoelectric substrate.

  In the case of a resonator, a reflector consisting of strip-like electrodes formed of the same electrode film is disposed on both sides of the IDT electrode, and the surface acoustic wave is slightly reflected at the end of the strip-like electrodes, so that the reflected wave is Return to IDT electrode. When the strip electrodes are arranged at a period of λ / 2, the reflected surface acoustic waves are in-phase and constructive, and most of the surface acoustic wave energy is returned to the IDT electrode. Then, the propagated surface acoustic wave can be extracted as an electrical signal from the other output IDT electrode.

2. Method of manufacturing composite substrate for surface acoustic wave device (1) Method of manufacturing composite substrate for surface acoustic wave device according to the first embodiment of the present invention Piezoelectric substrate 1 and support having a smaller thermal expansion coefficient than that of piezoelectric substrate 1 A substrate 2, a polycrystalline diamond thin film layer 3 formed on one main surface of the support substrate 2, an acoustic wave formed on the polycrystalline diamond thin film layer 3 and more than the crystalline diamond thin film layer 3 and the piezoelectric substrate 1 A method of manufacturing a composite substrate for a surface acoustic wave device according to the first embodiment, comprising the acoustic impedance layer 4 having a small impedance, wherein the acoustic impedance layer 4 and the piezoelectric substrate 1 are directly bonded to each other,
Forming a polycrystalline diamond thin film layer 3 on one main surface of the support substrate 2;
Forming a polycrystal diamond thin film layer 3 and an acoustic impedance layer 4 having a smaller acoustic impedance than the piezoelectric substrate 1 on the polycrystal diamond thin film layer 3;
Directly bonding the acoustic impedance layer 4 and the piezoelectric substrate 1 by surface activation room temperature bonding;
And polishing the non-bonding surface of the piezoelectric substrate 1 directly bonded to the acoustic impedance layer 4.

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

The supporting substrate 2 needs to have a thermal expansion coefficient smaller than that of the selected piezoelectric substrate 1 as compared with the selected piezoelectric substrate 1. Specifically, the material of the support substrate 2 is preferably one selected from silicon, sapphire, aluminum nitride, aluminum oxide, borosilicate glass, and quartz glass. Furthermore, a silicon substrate is more preferable in terms of hardness and material cost. The silicon substrate has a thermal expansion coefficient of 3.9 × 10 ⁻⁶ / K, which is much smaller than that of the piezoelectric substrate 1 such as lithium tantalate, and can suppress the temperature change of the frequency characteristic of the SAW device. By using a silicon substrate and depositing a polycrystalline diamond thin film layer having high hardness on the silicon substrate, the hardness as a support substrate is increased, and polycrystal is formed via an acoustic impedance layer directly bonded to the piezoelectric substrate. By laminating the diamond thin film layer and the piezoelectric substrate, the composite substrate for a surface acoustic wave device obtained can obtain a propagation velocity faster than that of the piezoelectric substrate alone.

  It is preferable to use the microwave plasma CVD method for film-forming of the said polycrystalline diamond thin film layer. Microwave plasma CVD is a synthesis method by electrodeless discharge using microwaves (a frequency of 2.45 GHz is usually used). The microwave plasma CVD method is a film forming method using plasma with only microwaves in a pressure range of about 1.3 to 8.0 kPa. The microwave plasma CVD method is a method of exciting the source gas by plasma, and can form a denser thin film at a lower temperature than the thermal CVD method etc., and the film forming speed is faster than other CVD methods, It is advantageous in that damage due to heat can be suppressed, and mutual diffusion between laminated films can be suppressed.

  Magnetic field microwave CVD, which is a type of microwave plasma CVD, makes it possible to create a stable high density plasma even at low pressure by utilizing the interaction between plasma and magnetic field, and microwave plasma It uses a strong plasma state by cyclotron resonance (ECR) of electrons at a lower pressure (1.3 to 133 Pa) region than the CVD method. And since it is low pressure plasma, a uniform polycrystalline diamond thin film can be formed into a large area, and it is suitable for forming a polycrystalline diamond thin film layer concerning the present invention.

  FIG. 4 shows a schematic configuration of the microwave plasma CVD apparatus.

  The microwave plasma CVD apparatus comprises a magnetron 10, a rectangular waveguide 7, a matching device 8, a plunger 9, a reaction tube 14, a susceptor (substrate holder) 11, an exhaust pump 12, a power monitor 13 and the like. In addition, the code | symbol 15 shows reaction gas in FIG.

  The supporting substrate 2 used in the present invention is disposed on the susceptor (substrate holder) 11 in the reaction tube 14 of the microwave plasma CVD apparatus, and a predetermined reaction gas is introduced to adjust the pressure. The microwave generated by the magnetron 10 travels in 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 14, and a plasma P is generated in the reaction tube 14. . Substrate heating generally utilizes dielectric heating of the substrate holder 11 by microwaves. The reaction tube 14 is made of transparent quartz that has low absorption of microwaves and high heat resistance. The matching unit 8 is for efficiently transmitting microwaves to the reaction tube with the reflected wave being zero.

As film forming conditions, it is general to use a reaction pressure of 1.3 to 8.0 kPa, a reaction gas using methane gas (CH ₄ ), and a carrier gas using hydrogen gas (H ₂ ), but CO / H _2, etc. other types of gas can also be used. The proportion of H ₂ gas is preferably 80% or more. If the proportion of H ₂ gas is low, the non-diamond component of the obtained polycrystalline diamond thin film layer will increase and the film quality will deteriorate.

Since the energy of microwave plasma is high, the deposition rate is faster than other CVD methods. Further, since the microwave plasma P is present in the vicinity of the support substrate 2 and becomes high temperature, direct heating or the like of the substrate holder 11 is not necessary. It is preferable to adjust the substrate temperature at the time of film formation 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 is reduced, the diamond structure is reduced, and the hardness is reduced. When the substrate temperature becomes too high, the influence on the substrate becomes large, and the substrate may be deformed.

  The polycrystalline diamond thin film layer 3 is formed on the supporting substrate 2 by adjusting the above-mentioned film forming conditions. The film thickness of the polycrystalline diamond thin film layer 3 is preferably about 5 μm on the support substrate 2. The reason for this is that unevenness is present on the surface of the deposited polycrystalline diamond thin film layer 3, and it is preferable to polish the uneven surface of the polycrystalline diamond thin film layer 3 before depositing the acoustic impedance layer 4. It is from. When the unevenness is present, the unevenness of the surface of the polycrystalline diamond thin film layer 3 is transferred to the acoustic impedance layer 4 formed on the polycrystalline diamond thin film layer 3, and the piezoelectric substrate 1 and the acoustic impedance layer 4 are formed. When direct bonding is performed, there is a possibility that floating may not occur due to complete bonding at the atomic level.

  For example, if the film thickness of the polycrystalline diamond thin film layer 3 is about 5 μm, the surface roughness Ra is preferably 0.5 nm or less, preferably 0.3 nm or less by polishing to about 3 μm. However, in consideration of the polishing cost, the polishing is not performed until the surface roughness Ra is reached, and the acoustic impedance layer 4 is formed on the polycrystalline diamond thin film layer 3 to be equivalent to or half the polycrystalline diamond thin film layer 3. Film is formed to a film thickness within the range, and the uneven surface of the surface of the acoustic impedance layer 4 having the surface roughness Ra to which the surface roughness Ra of the polycrystalline diamond thin film layer 3 is transferred is polished and smoothed, The surface roughness Ra is preferably 0.5 nm or less, more preferably 0.3 nm or less.

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

  In the next step, the acoustic impedance layer 4 is formed on the polycrystalline diamond thin film layer 3, and then the acoustic impedance layer 4 and the piezoelectric substrate 1 are directly bonded by the surface activation room temperature bonding method. It is preferable that no unevenness be present, and the surface roughness should be Ra 0.5 nm or less, preferably 0.3 nm or less.

<B> Step of Forming Acoustic Impedance Layer 4 on Polycrystalline Diamond Thin Film Layer 3 Next, on polycrystalline diamond thin film layer 3 formed on supporting substrate 2, the polycrystalline diamond thin film layer 3 and the piezoelectric material are formed. An acoustic impedance layer 4 having an acoustic impedance smaller than that of the substrate 1 is formed.

The acoustic impedance layer 4 is made of a material having a smaller acoustic impedance than the polycrystalline diamond thin film layer 3 and the piezoelectric layer 1 as described above. As the acoustic impedance layer 4 satisfying the above conditions, SiO ₂ (Z = 15.6 [Pa · s / m]), Al (Z = 17.0 [Pa · s / m]), and Si (Z =) 19.7 [Pa · s / m]) and the like, and among them, a SiO ₂ film is preferable.

When the SiO ₂ film is formed as the acoustic impedance layer 4, the SiO ₂ film can be obtained by a film forming method such as a sputtering method or a CVD method. When forming into a film by sputtering method, it can obtain by carrying out sputtering film-forming, making Si or SiO into a target and introduce | transducing oxygen.

By laminating a SiO ₂ film as the acoustic impedance layer 4 on the polycrystalline diamond thin film layer, the surface acoustic wave propagating from the piezoelectric substrate 1 is reflected to the piezoelectric substrate 1 side by the interface between the polycrystalline diamond thin film layer and the SiO ₂ film. It has the effect of returning.

  The film thickness of the acoustic impedance layer 4 is preferably in the range equivalent to or half of that of the polycrystalline diamond thin film layer 3 as described above. Specifically, the film thickness is preferably 50 nm or more and 2 μm or less. If the thickness is less than 50 nm, the effect as an acoustic impedance layer is less likely to be exhibited, and the characteristics targeted by the present invention may not be obtained. If the film thickness is more than 2 μm, the effect of improving the propagation velocity of the surface acoustic wave is suppressed, and there is a possibility that the stable crystallinity can not be obtained. Contribute to

  The polycrystalline diamond thin film layer 3 is polished to about 3 μm if the film thickness at the time of film formation is about 5 μm, and the surface roughness Ra is preferably 0.5 nm or less, preferably 0.3 nm or less . The acoustic impedance layer 4 is formed on the polycrystalline diamond thin film layer 3 having the surface roughness Ra so as to have a film thickness equal to or approximately half that of the polycrystalline diamond thin film layer 3. The surface roughness of the polycrystalline diamond thin film layer 3 is transferred to the surface of the layer 4, and the surface roughness of the acoustic impedance layer 4 may be reduced to a surface roughness Ra of 0.5 nm or less, preferably 0.3 nm or less. it can. When the surface roughness of the acoustic impedance layer 4 is larger than the above range, the surface roughness Ra is preferably 0.5 nm or less, preferably 0.3 nm or less, by polishing. However, when the polishing cost of the polycrystalline diamond thin film layer 3 is also taken into consideration, the surface roughness Ra of the polycrystalline diamond thin film layer 3 is not polished to the above range, and on the polycrystalline diamond thin film layer 3 as described above. The acoustic impedance layer 4 is formed to have a film thickness equal to or half of that of the polycrystalline diamond thin film layer 3, and the surface roughness to which the surface roughness Ra of the polycrystalline diamond thin film layer 3 has been transferred It is also preferable to polish and smooth the uneven surface of the said acoustic impedance layer 4 surface which has Ra, and to make surface roughness Ra 0.5 nm or less, preferably 0.3 nm or less.

<C> Step of Directly Bonding the Acoustic Impedance Layer 4 Formed on the Polycrystalline Diamond Thin Film Layer 3 with the Piezoelectric Substrate 1 by Surface Activated Room Temperature Bonding Method Next, the acoustic form formed on the polycrystalline diamond thin film layer 3 The impedance layer 4 and the piezoelectric substrate 1 are directly bonded by surface activation room temperature bonding.

  In general, an organic adhesive, an inorganic adhesive, a UV adhesive, a thermal diffusion bonding, or the like is used to bond the acoustic impedance layer 4 and the piezoelectric substrate 1. However, since various adhesives soften as the temperature rises, when the composite substrate is used for a SAW device, the comb-like electrodes 4 formed on the piezoelectric substrate 1 also move simultaneously with the piezoelectric substrate 1, and the resonance frequency changes There is a high possibility of doing. Further, in order to bond the acoustic impedance layer 4 and the piezoelectric substrate 1 by heat diffusion, heating at 1000 ° C. or more is required, and the Curie temperature of the piezoelectric substrate is exceeded, which causes a problem of reduction in piezoelectricity.

  In order to avoid this problem, it is desirable that direct normal temperature bonding (hereinafter sometimes referred to as normal temperature bonding), which can be bonded at normal temperature and bonded at the atomic level, is desirable. For normal temperature bonding, the bonding surface of the acoustic impedance layer 4 formed on the polycrystalline diamond thin film layer 3 and the bonding surface of the piezoelectric substrate 1 are sufficiently cleaned, and the cleaned acoustic impedance layer 4 and the piezoelectric substrate 1 are vacuum vessels. It is disposed inside, and each bonding surface is irradiated with an ion (argon) beam in an ultrahigh vacuum to remove residual impurities and activate each bonding surface, and bonding is performed at normal temperature, no pressure and no voltage. After cleaning the bonding surface of the acoustic impedance layer 4 and the bonding surface of the piezoelectric substrate 1, it is also preferable to further perform UV irradiation on the bonding surfaces.

<D> Step of polishing the non-bonding surface of the piezoelectric substrate 1 bonded to the acoustic impedance layer 4 Next, the obtained composite substrate is mounted on a polishing machine, and the non-bonding surface of the piezoelectric substrate 1 in the composite substrate is polished The thickness of the piezoelectric substrate 1 is adjusted to be thin. The difference between the thermal expansion coefficients of the support substrate 2 on which the polycrystalline diamond thin film layer 3 and the acoustic impedance layer 4 are formed and the piezoelectric substrate 1 makes it possible to prevent the composite substrate from warping due to temperature change. The thickness is greater than the total thickness of the support substrate 2 on which the polycrystalline diamond thin film layer 3 and the acoustic impedance layer 4 are formed (acoustic impedance layer 4 thickness + polycrystalline diamond thin film layer 3 thickness + support substrate 2 thickness) You need to make it thin enough. By reducing the thickness of the piezoelectric substrate 1, the warping force of the composite substrate is reduced, and the warping of the composite substrate is suppressed. In addition, under the influence of the polycrystalline diamond thin film layer 3 through the acoustic impedance layer 4 directly bonded by thinning the piezoelectric substrate 1, the composite substrate is as close as possible to the hardness of the polycrystalline diamond thin film layer 3. can get.

  Total thickness of support substrate 2 on which polycrystalline diamond thin film layer 3 and acoustic impedance layer 4 are formed (acoustic impedance layer 4 thickness + polycrystalline diamond thin film layer 3 thickness + support substrate 2 thickness) and thickness of piezoelectric substrate 1 As to the thickness, as described above, the thickness of the piezoelectric substrate 1 is reduced, and the ratio thereof is 1 for (acoustic impedance layer 4 thickness + polycrystalline diamond thin film layer 3 thickness + support substrate 2 thickness). It is preferably 10 or less, more preferably 1/20. If there is a difference in the film thickness, the warpage of the composite substrate due to the difference in thermal expansion is suppressed even when the ambient temperature reaches about 120 ° C.

  The thickness of the composite substrate after polishing the non-bonding surface of the piezoelectric substrate 1 is preferably 0.3 to 25 μm. When the polishing cost is also considered, it is desirable to set it as 1-25 micrometers. In addition, in consideration of performance such as suppression of warpage of the composite substrate, it is preferable to set the thickness to 0.3 to 5 μm. If the thickness after polishing is less than 0.3 μm, the polishing cost may increase, but the surface smoothness of the polycrystalline diamond thin film layer 3 affects the acoustic impedance layer 4 and the acoustic impedance layer The piezoelectric substrate 1 directly bonded to 4 is not preferable because the thickness can not be maintained during surface polishing and the thickness of the piezoelectric substrate 1 may be discontinuous. On the other hand, when the thickness after polishing exceeds 25 μm, the warp of the composite substrate increases, and the frequency temperature characteristic and the propagation speed decrease. That is, when the thickness of the piezoelectric substrate 1 is large, the characteristics of the piezoelectric substrate 1 (for example, lithium tantalate) are obtained, the thermal expansion of the piezoelectric substrate becomes dominant, and the expansion and contraction of the surface acoustic wave device electrode becomes large. This is because the temperature characteristic is lowered and the hardness as the composite substrate is lowered and the propagation speed is also lowered.

  By the above steps <a> to <d>, high frequency and frequency temperature characteristics are improved, and the deterioration of power resistance and signal loss due to the mismatch of the acoustic impedance between the piezoelectric layer and the diamond crystal are suppressed. The composite substrate for a surface acoustic wave device according to the first embodiment 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 thermal expansion coefficient smaller than that of piezoelectric substrate 1, and one of support substrate 2 And an acoustic impedance layer 4 formed on the polycrystalline diamond thin film layer 3 and having an acoustic impedance smaller than that of the crystalline diamond thin film layer 3 and the piezoelectric substrate 1. A method of manufacturing a composite substrate for a surface acoustic wave device according to a second embodiment, wherein the acoustic impedance layer 4 and the piezoelectric substrate 1 are directly bonded via the metal thin film 5,
Forming a polycrystalline diamond thin film layer 3 on one main surface of the support substrate 2;
Forming a polycrystal diamond thin film layer 3 and an acoustic impedance layer 4 having a smaller acoustic impedance than the piezoelectric substrate 1 on the polycrystal diamond thin film layer 3;
Directly bonding the acoustic impedance layer 4 and the piezoelectric substrate 1 through the metal thin film 5 by surface activation room temperature bonding;
Polishing the non-bonding surface of the piezoelectric substrate 1 directly bonded to the acoustic impedance layer 4;
In the step of directly bonding the acoustic impedance layer 4 and the piezoelectric substrate 1 by the surface activation room temperature bonding method through the metal thin film 5, the bonding surfaces of the acoustic impedance layer 4 and the piezoelectric substrate 1 before bonding are cleaned and bonded The surface is irradiated with an ion (argon) beam to remove residual impurities, and the metal thin film is formed on the bonding surface of at least one of the acoustic impedance layer 4 and the piezoelectric substrate 1, and then directly in vacuum at room temperature. It is characterized by joining.

Of the above manufacturing processes,
<a> Step of forming polycrystalline diamond thin film layer 3 on one main surface of support substrate 2 (including polishing) <b> Step of forming acoustic impedance layer 4 on polycrystalline diamond thin film layer 3 <d> About each process of the process of grind | polishing the non-joining surface of the piezoelectric substrate in a composite substrate, the <a> process in the manufacturing method of the composite substrate for surface acoustic wave elements which concerns on said 1st embodiment, <b> process, <d> Similar to the process,
The step <c> in the method of manufacturing the composite substrate for a surface acoustic wave device according to the second embodiment, that is, the step of directly bonding the acoustic impedance layer 4 and the piezoelectric substrate 1 through the metal thin film 5 by surface activation room temperature bonding method It is different.

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

<C> Step of Directly Bonding the Acoustic Impedance Layer 4 with the Piezoelectric Substrate 1 Through the Metal Thin Film 5 by Surface Activated Room Temperature Bonding Method <a> Step in the Method of Manufacturing the Composite Substrate for Surface Acoustic Wave Element According to the Second Embodiment By the <b> step, the support substrate 2 having the polycrystalline diamond thin film layer 3 and the acoustic impedance layer 4 on one main surface on the support substrate 2 is obtained.

  Next, the acoustic impedance layer 4 formed on the polycrystalline diamond thin film layer 3 and the piezoelectric substrate 1 are directly bonded via the metal thin film 5 by surface activation room temperature bonding method.

  In order to directly bond the acoustic impedance layer 4 and the piezoelectric substrate 1 through the metal thin film 5, the bonding surface of the acoustic impedance layer 4 before bonding and the piezoelectric substrate 1 is cleaned and cleaned. Is placed in a vacuum vessel, and each bonding surface is irradiated with an ion beam in an ultrahigh vacuum to remove residual impurities and to activate each bonding surface.

  Next, a metal thin film 5 is formed on the bonding surface of at least one of the acoustic impedance layer 4 and the piezoelectric substrate 1 by sputtering. The metal thin film 5 is preferably a film such as a chromium film or a titanium film having a strong bonding force with oxygen and a high diffusion coefficient, and particularly preferably a titanium film. The film thickness of the metal thin film 5 formed on the bonding surface of at least one of the acoustic impedance layer 4 and the piezoelectric substrate 1 is preferably 5 to 10 nm. When the film thickness is too thin at less than 5 nm, the film becomes discontinuous and the diffusion to the formed junction surface becomes discontinuous. On the other hand, if the film thickness exceeds 10 nm and is too thick, it may intervene as a film between the acoustic impedance layer 4 on which the continuous film is formed before diffusion and the piezoelectric substrate 1 and may not function as a diffusion layer .

  After the metal thin film 5 is formed, the acoustic impedance layer 4 and the piezoelectric substrate 1 are bonded at normal temperature, no pressure, and no voltage by utilizing the large atomic diffusion of the metal thin film 5. The metal thin film 5 exists on these bonding surfaces, and bonding can be performed by atomic diffusion of the metal thin film 5. Thereby, the acoustic impedance layer 4 and the piezoelectric substrate 1 are directly bonded to each other through the metal thin film 5, and the composite substrate for a surface acoustic wave device according to the second embodiment can be obtained. By interposing the metal thin film 5, the surface roughness of both the bonding surfaces of the acoustic impedance layer 4 and the piezoelectric substrate 1 may be rougher than when the metal thin film 5 is not present, and the polishing cost before bonding is reduced. There is a merit.

  Thereafter, it is obtained according to the step <d> in the method of manufacturing the composite substrate for a surface acoustic wave device according to the first embodiment, that is, the step of polishing the non-bonding surface of the piezoelectric substrate 1 bonded to the acoustic impedance layer 4 The composite substrate for a surface acoustic wave device is mounted on a polishing machine, and the surface of the piezoelectric substrate 1 of the composite substrate is polished, thereby directly bonding the acoustic impedance layer 4 and the piezoelectric substrate 1 via the metal thin film 5. The composite substrate for a surface acoustic wave device according to the embodiment is obtained. That is, the surface elasticity according to the second embodiment is improved in frequency resistance and frequency temperature characteristics, and in which deterioration of the power resistance and signal loss due to the mismatch of the acoustic impedance between the piezoelectric layer and the diamond crystal are suppressed. A composite substrate for wave elements can be obtained.

3. Method of 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 non-bonding of the piezoelectric substrate 1 in the composite substrate for a surface acoustic wave device according to the second embodiment A surface acoustic wave element is produced by forming the surface acoustic wave element electrode (IDT electrode) 6 having the above-described function on the surface. When the surface acoustic wave device is used as a resonator, an IDT electrode and a reflector formed of a pair of strip electrodes on both sides of the IDT electrode are disposed on a piezoelectric substrate.

  Hereinafter, the method of 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 surface acoustic wave devices, on this conductive material layer, it corresponds to an IDT electrode and a reflector by photolithography. Form a shaped resist layer.

  Then, using the resist layer as a mask, the conductive material layer in a portion where the resist layer is not formed is removed by dry etching such as reactive ion etching (RIE), thereby reflecting the IDT electrode of a predetermined pattern and reflection. The vessel is formed.

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

The conductive material for the electrode has a small mass, a low electric resistance value, and a demand for electric power resistance. Therefore, aluminum or aluminum with a small amount of dissimilar metals (eg Cu, Si, Ti, HfB ₂ etc.) An aluminum-based alloy (which may not necessarily be a solid solution) to which is mentioned is preferred. For example, an aluminum-based alloy to which a small amount of copper is added by sputtering deposition, which has a reputation for being resistant to migration in the field of semiconductor devices, from the viewpoint of power resistance of IDT electrodes affecting the life of surface acoustic wave devices It is preferred to use. However, the present invention is not limited to the above-mentioned 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 surface acoustic wave devices according to the first embodiment and the second embodiment has one of the main surfaces of the support substrate having a thermal expansion coefficient smaller than that of the piezoelectric substrate. Since the surface acoustic wave propagates through the polycrystalline diamond thin film layer formed on the substrate, it is possible to realize an extremely high propagation velocity, and a support substrate having a thermal expansion coefficient smaller than that of the piezoelectric substrate is applied. Therefore, the frequency characteristics do not shift (fluctuate) due to the temperature change, and it is possible to provide good frequency temperature characteristics.

  Moreover, the composite substrate for surface acoustic wave devices according to the first embodiment and the second embodiment includes multiple piezoelectric substrates formed of bulk crystals and a supporting substrate having a thermal expansion coefficient smaller than that of the piezoelectric substrates. It is obtained by directly bonding the acoustic impedance layer on the crystalline diamond thin film layer, and thinning the non-bonding surface of the bonded piezoelectric substrate by polishing. The piezoelectric substrate (piezoelectric layer) made of bulk crystal has a better film quality than a piezoelectric layer formed by CVD or sputtering on a diamond thin film or the acoustic relaxation layer of Patent Document 3, and Since the acoustic impedance layer on the polycrystalline diamond thin film layer provided on the supporting substrate is directly bonded to the piezoelectric substrate, there is no problem of destruction of the piezoelectric substrate or deterioration of the piezoelectric characteristics described in Patent Document 6; In addition, since no bonding method using an adhesive is adopted, various problems caused by heat softening of the adhesive are solved.

  Furthermore, in the composite substrate for surface acoustic wave devices according to the first embodiment and the second embodiment, an acoustic impedance layer (piezoelectric substrate and polycrystalline diamond thin film layer) is provided between the piezoelectric substrate (piezoelectric layer) and polycrystalline diamond thin film layer. In the surface acoustic wave device using the above composite substrate for surface acoustic wave devices, the acoustic impedance layer between the piezoelectric substrate (piezoelectric layer) and the polycrystalline diamond thin film layer is used. The reflection of the elastic wave at the interface of the piezoelectric layer due to the misalignment is reduced, which can suppress the phenomenon that the vibration displacement of the surface elastic wave concentrates in the vicinity of the surface of the piezoelectric layer, thereby preventing the breakage on the surface of the piezoelectric layer Power resistance of the surface acoustic wave device can be improved, and the signal propagating on the surface of When it diffuses into the inside of the impedance layer, the signal is reflected at the interface between the acoustic impedance layer and the polycrystalline diamond thin film layer and returned to the piezoelectric layer, so that it is possible to reduce the loss of the propagation signal in the surface acoustic wave device. It becomes.

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

Example 1
(1) Formation of polycrystalline diamond thin film layer on supporting substrate (silicon substrate) A 2 inch diameter single crystal silicon substrate (diameter 2 inches × thickness 200 μm) is prepared as a supporting substrate, and the silicon substrate is made of acetone. And ultrasonic cleaning.

  Next, a polycrystalline diamond thin film layer was formed 5.0 [mu] m on the cleaned silicon substrate under the following film forming conditions using a microwave plasma CVD apparatus.

<Deposition conditions of polycrystalline diamond thin film layer>
Pretreatment: Ar bombardment (10 minutes)
・ Used reaction gas / carrier gas: methane (CH ₄ ) / hydrogen, (methane concentration 3%)
Gas flow rate: 100 sccm
・ Power source microwave frequency; 2.45 GHz
・ Microwave output: 600W
-Pressure during film formation: 1.3 kPa
・ Substrate temperature during film formation: 700 ° C
Film formation time: 180 minutes

As a result of measuring the Raman spectrum of the obtained thin film using a Raman spectrophotometer, a high peak was confirmed at 1333 cm ⁻¹ . It was confirmed from this measurement result that it is a diamond film.

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

(3) Formation of SiO ₂ Film (Acoustic Impedance Layer) on Polycrystalline Diamond Thin Film Layer Next, a DC magnetron sputtering apparatus (Shibaura) is used for the silicon substrate having the polycrystalline diamond thin film layer subjected to the polishing treatment of (2). It was set to Mfg. Co., Ltd., model CFS-4ES, and a SiO target with a diameter of 3 inches was attached to the apparatus as a sputtering target. In addition, the ultimate vacuum before the start of sputtering was 6.5 × 10 ⁻³ Pa.

  Then, while heating the substrate temperature to 120 ° C., while supplying a mixed gas of argon and oxygen (argon gas flow rate: 15 sccm, oxygen flow rate: 10 sccm) as a reactive gas, sputtering output: 200 W, film forming time 35 minutes Sputtering was performed to form a silicon oxide film having a thickness of 0.5 μm as an acoustic impedance layer.

When the crystallinity of the obtained silicon oxide thin film is examined by thin film X-ray diffraction (XRD), a (002) plane peak (half value width is 12.4 °) is confirmed, and a (002) oriented SiO ₂ crystal film It turned out that it was. Subsequently, the surface roughness of the obtained silicon oxide thin film was measured by a three-dimensional optical profiler Nexview apparatus (manufactured by Canon), and polishing was performed until the surface roughness Ra was 0.3 nm.

(4) Polishing of piezoelectric substrate (lithium tantalate substrate) Next, the surface of a lithium tantalate substrate [Sumitomo Metal Mining Co., Ltd. product] having a diameter of 2 inches and a thickness of 350 μm is the same as in the case of the above diamond thin film layer. The surface roughness Ra was 0.3 nm.

(5) Room temperature bonding of SiO ₂ film (acoustic impedance layer) and piezoelectric substrate Next, the SiO ₂ film / surface-polished polycrystalline diamond thin film layer / silicon substrate obtained in the above (3), and the above (4 After ultrasonically cleaning the surface-polished lithium tantalate substrate obtained in 2.) in acetone solution, UV irradiation is further performed for 60 seconds on the SiO ₂ film surface on the silicon substrate and the polished lithium tantalate substrate surface The

Next, place both substrates after cleaning and UV irradiation in a surface activation bonding type room temperature bonding apparatus [made by Musashino Engineering Co., Ltd.], and evacuate to an ultra-high vacuum of 2 × 10 ^-6 Pa, and silicon substrate Ar beam irradiation is performed on the SiO ₂ film surface and the polished lithium tantalate substrate surface (irradiation conditions: acceleration voltage 50 kV, beam diameter 1.2 mm, irradiation amount 2 × 10 ¹⁴ ions / cm ² ), and these surfaces are After activation, the activated surfaces of both substrates are made to face each other, and both surfaces are joined at room temperature without applying heat, pressure or the like, and the composite substrate for a surface acoustic wave device according to Example 1 (composite substrate) Made.

(6) Polishing of non-bonding surface of lithium tantalate substrate in composite substrate Lithium tantalate substrate in the above composite substrate having the configuration of lithium tantalate substrate / (direct bonding) / SiO ₂ film / polycrystalline diamond thin film layer / silicon substrate The non-bonding surface of the above was polished to a thickness of 22 μm using a surface polisher [DGP 8761 manufactured by Disco Corporation]. Furthermore, the non-bonded surface of the lithium tantalate substrate was mirror-polished by mechanochemical polishing using colloidal silica to obtain a surface roughness Ra of 4 nm of the non-bonded surface.

  The above production conditions are shown in Table 1.

(7) Production of Surface Acoustic Wave Element (SAW Device) The thickness of the non-bonded surface of the lithium tantalate substrate of the composite substrate for a surface acoustic wave element according to Example 1 subjected to polishing treatment is first deposited by vacuum evaporation. A 5 nm thick Cr film was formed, and then a 0.15 μm thick Cu film was formed.

  Next, a resist layer having a shape corresponding to the IDT electrode is formed by photolithography on the Cu film, and the resist layer is used as a mask to form a resist layer by dry etching using reactive ion etching (RIE). The Cu film and the Cr film in the unexposed portions were removed. Thus, an IDT electrode having a predetermined pattern was formed, and a SAW device according to Example 1 was manufactured.

  The obtained SAW device had a propagation velocity of 7420 m / s, a frequency temperature characteristic of −12.2 ppm / ° C., and an electromechanical coupling coefficient of 9.5%. Also, the Q value indicating the loss of signal was 1800.

  From these evaluation results, in the SAW device according to Example 1, the propagation velocity, frequency temperature characteristics, and Q value exceed those of the conventional SAW device (see Comparative Example 1 and Comparative Example 2) to which a lithium tantalate substrate is applied. It was confirmed that it was obtained.

  Further, by setting the width of the IDT electrode to 0.4 μm (the wavelength λ of the surface acoustic wave is 0.4 × 4 = 1.6 μm), it is possible to obtain a SAW device having a resonance frequency of 4637 MHz.

  The characteristics of the obtained SAW device are shown in Table 2.

Example 2
The steps (1) to (5) of Example 1 are carried out in the same manner, and in the step (6), it has a structure of lithium tantalate substrate / (direct bonding) / SiO ₂ film / polycrystalline diamond thin film layer / silicon substrate The non-bonded surface of the lithium tantalate substrate in the composite substrate was polished to a thickness of 40 μm using a surface polisher [DGP 8761 manufactured by Disco Corporation]. Furthermore, the non-bonded surface of the lithium tantalate substrate was mirror-polished by mechanochemical polishing using colloidal silica to obtain a surface roughness Ra of 4 nm of the non-bonded surface.

  Then, the SAW device according to Example 2 was manufactured according to the process of (7) "Preparation of surface acoustic wave device (SAW device)" of Example 1.

  The obtained SAW device had a propagation velocity of 6080 m / s, a frequency temperature characteristic of −13.6 ppm / ° C., an electromechanical coupling coefficient of 7.2%, and a Q value of 1750.

  From these evaluation results, the SAW device according to Example 2 has a propagation speed, frequency temperature characteristics and Q value that exceed those of the conventional SAW device (see Comparative Example 1 and Comparative Example 2) to which a lithium tantalate substrate is applied. It was confirmed that it was obtained.

  Further, by setting the width of the IDT electrode to 0.4 μm (the wavelength λ of the surface acoustic wave is 0.4 × 4 = 1.6 μm), a SAW device with a resonance frequency of 3800 MHz can be obtained.

  The production conditions of the composite substrate are shown in Table 1, and the characteristics of the SAW device are shown in Table 2.

Comparative Example 1
Conducted using a lithium tantalate substrate [Sumitomo Metal Mining Co., Ltd.] alone with a diameter of 2 inches, a thickness of 350 μm, and a surface roughness of Ra 3 nm without bonding SiO ₂ film / polycrystalline diamond thin film layer / silicon substrate A SAW device according to Comparative Example 1 was manufactured according to the process (7) "Production of surface acoustic wave device (SAW device)" in Example 1.

  The obtained SAW device had a propagation velocity of 3850 m / s, a frequency temperature characteristic of -38.0 ppm / ° C, an electromechanical coupling coefficient of 7.0%, and a Q value of 1520 (see Table 2). ).

  Further, by setting the width of the IDT electrode to 0.4 μm (the wavelength λ of the surface acoustic wave is 0.4 × 4 = 1.6 μm), a SAW device with a resonance frequency of 2406 MHz was obtained.

In Comparative Example 1, the structure of the composite substrate in which the SiO ₂ film / polycrystalline diamond thin film layer / silicon substrate are bonded is not employed, and the thermal expansion of the lithium tantalate substrate is 15.7 × 10 ⁻⁶ / K. Since the silicon substrate is 3 × 10 ^-6 / K, the lithium tantalate substrate expands with the temperature rise and the electrode interval also spreads, and the resonance frequency is lowered, and there is a danger of cross-wires of other bands.

Comparative Example 2
A 2 inch diameter single crystal silicon substrate (diameter 2 inches × thickness 350 μm, surface roughness Ra 0.3 nm) was prepared as a supporting substrate.

  In addition, as a piezoelectric substrate, a lithium tantalate substrate [Sumitomo Metal Mining Co., Ltd. product] having a diameter of 2 inches, a thickness of 350 μm, and a surface roughness Ra of 0.3 nm was prepared.

  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 after cleaning and UV irradiation are placed in a surface activation bonding type room temperature bonding apparatus [made by Musashino Engineering Co., Ltd.], and vacuum drawn up to 2 × 10 ^-6 Pa of ultra-high vacuum. Ar beam is irradiated on the surface of the substrate (irradiation condition: acceleration voltage 50kV, beam diameter 1.2mm, irradiation dose 2 × 10 ¹⁴ ions / cm ² ), and after activating the surfaces of both substrates, the surfaces of both substrates are made to face each other Comparative example having a structure of lithium tantalate substrate / (direct bonding) / silicon substrate, bonding both surfaces at room temperature without applying heat, pressure, etc. and not having a SiO ₂ film / polycrystalline diamond thin film layer The composite substrate (composite substrate) for surface acoustic wave elements which concerns on 2 was produced.

(6) Polishing the non-bonding surface of lithium tantalate substrate in the composite substrate The surface polishing machine of the non-bonding surface of the lithium tantalate substrate in the above composite substrate having the configuration of lithium tantalate substrate / (direct bonding) / silicon substrate It was polished to a thickness of 40 μm using DGP 8761 manufactured by Disco Corporation. Furthermore, the non-bonded surface of the lithium tantalate substrate was mirror-polished by mechanochemical polishing using colloidal silica to obtain a surface roughness Ra of 4 nm of the non-bonded surface.

  The above production conditions are shown in Table 1.

  Using the composite substrate thus obtained, a SAW device according to Comparative Example 2 was produced according to the process of (7) "Production of surface acoustic wave device (SAW device)" in Example 1.

  The obtained SAW device had a propagation velocity of 4180 m / s, a frequency temperature characteristic of -23.0 ppm / ° C, an electromechanical coupling coefficient of 7.2, and a Q value of 1680 (see Table 2). Further, by setting the width of the IDT electrode to 0.4 μm (the wavelength λ of the surface acoustic wave is 0.4 × 4 = 1.6 μm), a SAW device with a resonance frequency of 2612 MHz can be obtained.

  In Comparative Example 2, a structure of a composite substrate in which a silicon substrate and a lithium tantalate substrate are directly bonded is taken, and since the silicon substrate has a smaller thermal expansion than the lithium tantalate substrate, the lithium tantalate substrate is used. Elongation was suppressed, electrode spacing did not increase, and the change in resonant frequency was negligible. However, no result was obtained that greatly exceeded the propagation velocity of the SAW device (conventional SAW device) according to Comparative Example 1 using a lithium tantalate substrate.

Comparative Example 3
The surface roughness Ra of the polycrystalline diamond thin film layer is the same as the step (1) (2) of Example 1 without performing the step (3) of Example 1 for forming a SiO ₂ film (acoustic impedance layer). Prepare a laminated substrate of polycrystalline diamond thin film layer / silicon substrate whose surface was polished to 0.3 nm, and polished the surface to a surface roughness Ra of 0.3 nm in the same manner as the step (4) of Example 1 The prepared lithium tantalate substrate was prepared.

  Next, each polished surface of the surface-polished polycrystalline diamond thin film layer / silicon substrate laminated substrate and the surface-polished lithium tantalate substrate is ultrasonically cleaned in an acetone solution, and further, each cleaned surface is cleaned. UV irradiation was performed for 60 seconds.

Next, both substrates after cleaning and UV irradiation are placed on a surface activation bonding type room temperature bonding apparatus [made by Musashino Engineering Co., Ltd.], and vacuumed up to 2 × 10 ^-6 Pa of ultra-high vacuum, both substrates Ar beam irradiation is performed on the polished surface (irradiation conditions: acceleration voltage 50 kV, beam diameter 1.2 mm, irradiation dose 2 × 10 ¹⁴ ions / cm ² ), and after these surfaces are activated, both substrates are polished The produced surfaces were made to face each other, and both surfaces were joined at room temperature without applying heat, pressure or the like, to prepare a composite substrate (composite substrate) for a surface acoustic wave element according to Comparative Example 3.

  Next, in the same manner as in step (6) of Example 1, the non-bonding surface of the lithium tantalate substrate in the composite substrate having the structure of lithium tantalate substrate / (direct bonding) / polycrystalline diamond thin film layer / silicon substrate is obtained. Polished to a thickness of 20 μm using a surface polisher [DGP 8761 manufactured by Disco Corporation], and further mirror-polish the non-bonded surface of the lithium tantalate substrate by mechanochemical polishing using colloidal silica, The surface roughness Ra of the non-bonding surface was 4 nm.

  Furthermore, a SAW device according to Comparative Example 3 was manufactured according to the process of (7) "Preparation of surface acoustic wave device (SAW device)" in Example 1.

  These manufacturing conditions are shown in Table 1.

  The obtained SAW device has a propagation velocity of 7410 m / s, a frequency temperature characteristic of −14.6 ppm / ° C., an electromechanical coupling coefficient of 6.8%, and a Q value of 1600 (see Table 2). 4 is lower than the Q value of the SAW device according to Examples 1 and 2 in which the polycrystalline diamond thin film layer is incorporated. Further, by setting the width of the IDT electrode to 0.4 μm (the wavelength λ of the surface acoustic wave is 0.4 × 4 = 1.6 μm), a SAW device with a resonance frequency of 4631 MHz can be obtained.

In Comparative Example 3, a composite substrate structure in which the polycrystalline diamond thin film layer / silicon substrate and the lithium tantalate substrate are directly bonded and the thickness of the lithium tantalate substrate is polished to 20 μm is taken. Unlike Examples 1 and ₂ , the SiO ₂ film (acoustic impedance layer) is not formed on the polycrystalline diamond thin film layer. Then, in the SAW device according to Comparative Example 3, the signal propagating in the lithium tantalate substrate is diffused to the inside of the lithium tantalate substrate and the polycrystalline diamond thin film layer directly bonded to the lithium tantalate substrate. It is considered that a loss occurred and the Q value decreased as compared with the SAW devices according to Examples 1 and 2.

[Example 3]
Example 1 (1) A polycrystalline diamond thin film layer was formed on a silicon substrate according to the process of "formation of polycrystalline diamond thin film layer on supporting substrate (silicon substrate)".

(2) Polishing of Polycrystalline Diamond Thin Film Layer Next, the polycrystalline diamond thin film layer formed on the silicon substrate was polished using a diamond nano-polisher [manufactured by Abiko Giken Lab.]. The surface roughness of the polycrystalline diamond thin film layer after polishing was measured by a three-dimensional optical profiler Nexview apparatus (manufactured by Canon), and polishing was performed until the surface roughness Ra became 0.4 nm.

(3) Formation of SiO ₂ Film (Acoustic Impedance Layer) on Polycrystalline Diamond Thin Film Layer Next, a DC magnetron sputtering apparatus (Shibaura) is used for the silicon substrate having the polycrystalline diamond thin film layer subjected to the polishing treatment of (2). It was set to Mfg. Co., Ltd., model CFS-4ES, and a SiO target with a diameter of 3 inches was attached to the apparatus as a sputtering target. In addition, the ultimate vacuum before the start of sputtering was 6.5 × 10 ⁻³ Pa.

  Then, while heating the substrate temperature to 120 ° C., while supplying a mixed gas of argon and oxygen (argon gas flow rate: 15 sccm, oxygen flow rate: 10 sccm) as a reactive gas, sputtering output: 200 W, film forming time 35 minutes Sputtering was performed to form a silicon oxide film having a thickness of 0.5 μm as an acoustic impedance layer.

When the crystallinity of the obtained silicon oxide thin film is examined by thin film X-ray diffraction (XRD), a (002) plane peak (half value width is 12.4 °) is confirmed, and a (002) oriented SiO ₂ crystal film It turned out that it was. Subsequently, the surface roughness of the obtained silicon oxide thin film was measured by a three-dimensional optical profiler Nexview apparatus (manufactured by Canon), and polishing was performed until the surface roughness Ra was 0.3 nm.

(4) Polishing of piezoelectric substrate (lithium tantalate substrate) Next, the surface of a lithium tantalate substrate [Sumitomo Metal Mining Co., Ltd. product] having a diameter of 2 inches and a thickness of 350 μm is the same as in the case of the above diamond thin film layer. To a surface roughness Ra of 0.4 nm.

(5) Room temperature bonding of SiO ₂ film (acoustic impedance layer) and piezoelectric substrate through metal thin film Next, the SiO ₂ film obtained in the above (3) / surface-polished polycrystalline diamond thin film layer / silicon substrate After ultrasonically cleaning the surface-polished lithium tantalate substrate obtained in (4) above in an acetone solution, UV is further applied to the surface of the SiO ₂ film on the silicon substrate and the surface of the lithium tantalate substrate polished. Irradiation was performed for 60 seconds.

Next, place both substrates after cleaning and UV irradiation in a surface activation bonding type room temperature bonding apparatus [made by Musashino Engineering Co., Ltd.], and evacuate to an ultra-high vacuum of 2 × 10 ^-6 Pa, and silicon substrate Ar beam irradiation is performed on the SiO ₂ film surface and the polished lithium tantalate substrate surface (irradiation conditions: acceleration voltage 50 kV, beam diameter 1.2 mm, irradiation amount 2 × 10 ¹⁴ ions / cm ² ), and these surfaces are After activation, a titanium film was formed to a thickness of 7 nm on the surface of the SiO ₂ film on the silicon substrate using the sputtering method in the same chamber.

Next, the lithium tantalate substrate and the surface of the SiO ₂ film (acoustic impedance layer) on which a titanium film is formed are opposed to each other, and both surfaces are joined at room temperature without applying heat, pressure or the like. A composite substrate for a surface acoustic wave device (composite substrate) was produced.

(6) Polishing of non-bonded surface of lithium tantalate substrate in composite substrate Tantalum in the above composite substrate having the configuration of lithium tantalate substrate / (direct bonding: Ti film) / SiO ₂ film / polycrystalline diamond thin film layer / silicon substrate The non-bonding surface of the lithium oxide substrate is polished to a thickness of 20 μm using a surface polisher [DGP 8761 manufactured by Disco Corporation], and the non-bonding surface of the lithium tantalate substrate is further treated with colloidal silica. It mirror-polished by chemical polish and was set as surface roughness Ra4nm of the said non-joining surface.

  The above production conditions are shown in Table 1.

(7) Production of Surface Acoustic Wave Device (SAW Device) The thickness of the non-bonding surface of the lithium tantalate substrate of the composite substrate for a surface acoustic wave device according to Example 3 which has been subjected to polishing treatment is first formed by vacuum evaporation. A 5 nm thick Cr film was formed, and then a 0.15 μm thick Cu film was formed.

  Next, a resist layer having a shape corresponding to the IDT electrode is formed by photolithography on the Cu film, and the resist layer is used as a mask to form a resist layer by dry etching using reactive ion etching (RIE). The Cu film and the Cr film in the unexposed portions were removed. Thus, an IDT electrode having a predetermined pattern was formed, and a SAW device according to Example 3 was produced.

  The obtained SAW device had a propagation velocity of 7440 m / s, a frequency temperature characteristic of −11.6 ppm / ° C., an electromechanical coupling coefficient of 9.1%, and a Q value of 1780.

  The following is confirmed from these evaluation results.

  First, an acoustic impedance layer having a smaller acoustic impedance than the polycrystalline diamond thin film layer and the piezoelectric layer (lithium tantalate) is formed on the polycrystalline diamond thin film layer, and the acoustic impedance layer and the piezoelectric layer (tantalum layer) In the structure in which lithium oxide is directly bonded through the Ti film, the elastic wave at the interface of the piezoelectric layer is compared with the structure in which the polycrystalline diamond thin film layer and the piezoelectric layer (lithium tantalate) are directly bonded. The reflection is reduced, thereby suppressing the phenomenon in which the vibration displacement of the surface acoustic wave is concentrated in the vicinity of the surface of the piezoelectric layer, so that the breakage on the surface of the piezoelectric layer is prevented and the power resistance of the SAW device can be improved. Acoustic impedance when the signal propagating on the surface of the piezoelectric layer is diffused into the acoustic impedance layer. Since the signal is reflected at the interface between the thin film and the polycrystalline diamond thin film and returned to the piezoelectric layer, it is also possible to reduce the loss of the propagation signal in the SAW device, and as a result, the Q value of the SAW device according to the third embodiment. It is also confirmed that propagation speed and frequency temperature characteristics can be obtained over conventional SAW devices to which a lithium tantalate substrate is applied.

  Further, by setting the width of the IDT electrode to 0.4 μm (the wavelength λ of the surface acoustic wave is 0.4 × 4 = 1.6 μm), it is possible to obtain a 4650 MHz SAW device.

  The characteristics of the obtained SAW device are shown in Table 2.

  Furthermore, the acoustic impedance layer and the piezoelectric layer (lithium tantalate) are directly bonded via a Ti film, and by diffusion bonding of Ti atoms, larger bonding adhesion can be obtained as compared with bonding of only Ar beam irradiation. Also confirmed.

  A surface acoustic wave device using the composite substrate for a surface acoustic wave device according to the present invention can increase the frequency and can solve the problem that the frequency characteristics shift (fluctuate) due to the temperature change. Has industrial applicability to be used.

P plasma 1 piezoelectric substrate 2 support substrate 3 polycrystalline diamond thin film layer 4 acoustic impedance layer 5 metal thin film 6 comb-like electrode (IDT electrode)
7 rectangular waveguide 8 alignment device 9 plunger 10 magnetron 11 susceptor (substrate holder)
12 exhaust pump 13 power monitor 14 reaction tube 15 reaction gas 20 piezoelectric substrate 21 IDT electrode 22 strip electrode

Claims (14)

A piezoelectric substrate,
A support substrate having a thermal expansion coefficient smaller than that of the piezoelectric substrate;
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 an acoustic impedance layer having an acoustic impedance smaller than that of the piezoelectric substrate are formed on the polycrystalline diamond thin film layer, and the acoustic impedance layer and the piezoelectric substrate are directly bonded. Composite substrate for surface acoustic wave devices. A piezoelectric substrate,
A support substrate having a thermal expansion coefficient smaller than that of the piezoelectric substrate;
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 an acoustic impedance layer having an acoustic impedance smaller than that of the piezoelectric substrate are formed on the polycrystalline diamond thin film layer, and the acoustic impedance layer and the piezoelectric substrate are directly bonded via the metal thin film. A composite substrate for a surface acoustic wave device, characterized in that   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 composite substrate for a surface acoustic wave device according to any one of claims 1 to 3, wherein the acoustic impedance layer is formed of a SiO ₂ film.   The surface acoustic wave according to any one of claims 1 to 4, wherein the supporting 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 one or more bulk crystals selected from lithium tantalate, lithium niobate, lithium niobate-lithium tantalate solid solution single crystal, quartz, lithium borate, zinc oxide, aluminum nitride, langasite, langaite The composite substrate for a surface acoustic wave device according to any one of claims 1 to 5, characterized in that A piezoelectric substrate,
A support substrate having a thermal expansion coefficient smaller than that of the piezoelectric substrate;
A polycrystalline diamond thin film layer formed on one main surface of the support substrate;
A method of manufacturing a composite substrate for a surface acoustic wave device, comprising the polycrystalline diamond thin film layer and the acoustic impedance layer having an acoustic impedance smaller than that of the piezoelectric substrate, which is formed on the polycrystalline diamond thin film layer.
Forming a polycrystalline diamond thin film layer on one main surface of the support substrate;
Forming a polycrystalline diamond thin film layer and an acoustic impedance layer having an acoustic impedance smaller than that of the piezoelectric layer on the polycrystalline diamond thin film layer;
Directly bonding the acoustic impedance layer and the piezoelectric substrate by surface activation room temperature bonding;
Polishing the non-bonding surface of the piezoelectric substrate directly bonded to the acoustic impedance layer;
A method of manufacturing a composite substrate for a surface acoustic wave device, comprising: In the step of directly bonding the acoustic impedance layer and the piezoelectric substrate by surface activation room temperature bonding method,
8. The method of manufacturing a composite substrate for a surface acoustic wave device according to claim 7, wherein the acoustic impedance layer and the piezoelectric substrate are directly bonded via a metal thin film. In the step of forming a polycrystalline diamond thin film layer on one main surface of the support substrate,
The method for producing a composite substrate for a surface acoustic wave device according to claim 7, wherein the polycrystalline diamond thin film layer is formed by a microwave plasma CVD method.   8. A method of manufacturing a composite substrate for a surface acoustic wave device according to claim 7, wherein the surface of the polycrystalline diamond thin film layer formed on one main surface of the support substrate is polished. Polishing the non-bonding surface of the piezoelectric substrate directly bonded to the acoustic impedance layer,
8. A method of manufacturing a composite substrate for a surface acoustic wave device according to claim 7, wherein the piezoelectric substrate is polished until its thickness becomes 0.3 to 25 [mu] m. In the step of directly bonding the acoustic impedance layer and the piezoelectric substrate by surface activation room temperature bonding method,
Each bonding surface of the acoustic impedance layer and the piezoelectric substrate before bonding is cleaned, and ion beams are irradiated to each bonding surface to remove residual impurities, and then direct bonding is performed at room temperature in vacuum. The manufacturing method of the composite substrate for surface acoustic wave elements as described in 7. In the step of directly bonding the acoustic impedance layer and the piezoelectric substrate via a metal thin film,
The bonding surfaces of the acoustic impedance layer and the piezoelectric substrate before bonding are cleaned, and ion beams are irradiated to the bonding surfaces to remove residual impurities, and the bonding surface of at least one of the acoustic impedance layer and the piezoelectric substrate 9. A method of manufacturing a composite substrate for a surface acoustic wave device according to claim 8, wherein a metal thin film is formed thereon and then directly bonded at room temperature in vacuum.   The method according to claim 13, 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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