JPH09221392A - Composite piezoelectric substrate and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a substrate having excellent temp. characteristics without depending on the combination of materials or formation of films by directly joining a langasite substrate and a silicon substrate with hydrogen bonds of hydroxyl groups or covalent bonds of oxygen on the interface. SOLUTION: A langasite substrate 1 and a silicon substrate 2 shown in the figure (a) are directly joined by hydrogen bonds or covalent bonds. The single crystal langasite is expressed by La3 Ga5 SiO14 and it has the same crystal symmetry as quartz and a crystalline direction which gives almost zero dependence on temp. the same as quartz. The langasite has no crystal dislocation temp. to the melting point of 1470 deg.C, which is higher than quartz or gallium phosphate. The figure (b) shows an inorg. thin film layer 3 comprising silicon oxide, silicon nitride or silicon between the langasite substrate 1 and the silicon substrate 2. By forming the inorg. thin film layer 3 between substrates, the obtd. substrate can be used for various applications such as to embed electrodes in the layer 3. If the inorg. thin film layer 3 contains silicon, covalent bonds with contribution of oxygen can be easily formed for direct joining and the substrates can be directly joined at a lower heat treatment temp.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a composite piezoelectric substrate using Langasite.

[0002]

2. Description of the Related Art Piezoelectric devices such as crystal oscillators, lithium tantalate, lithium niobate, lithium borate, gallium phosphate, and piezoelectric devices such as surface acoustic wave devices are used as oscillators and filters for various oscillators in wireless communication equipment. Widely used in. Since these elements vibrate mechanically, how they are fixed is closely related to their performance. The method of mechanically fixing with a spring or a screw is simple, but it is difficult to obtain a stable one for a long time against thermal change or mechanical vibration. There are also known methods of fixing with various organic adhesives, but these adhesives still do not have sufficient heat resistance and the frequency of vibration may change when solder reflow is performed. Also, it is not preferable in terms of production.

An electronic circuit using an electroacoustic element, for example, an oscillating circuit such as an oscillator using a piezoelectric resonator, or an amplifying circuit using a piezoelectric filter is an electronic element for causing or amplifying oscillation. It is composed of a transistor, a resonator for oscillating at a desired frequency, a filter for extracting only a desired frequency, and some electric parts such as a capacitor and a resistor. The resonator or filter used here has a predetermined value as its vibration frequency or selected frequency, and is hermetically sealed in a container such as a metal tube so that its performance is stable for a long period of time.
Therefore, the size of the resonator and the filter becomes several times the size of the resonator and the filter itself, and miniaturization is an extremely important issue for devices such as mobile phones where small size is extremely important. Has become.

On the other hand, a piezoelectric thin film such as ZnO or AlN is formed on a semiconductor substrate such as Si by a thin film technique such as sputtering, and a resonator or the like is formed from this piezoelectric thin film to form a Si electronic element and an electroacoustic element. There is known an example of integrating the above.

As one method for solving these problems, Japanese Patent Laid-Open Nos. 4-283957 and 4-164 are known.
No. 452, JP-A No. 6-120416, JP-A No. 6-125036, etc., a method of directly bonding a piezoelectric device such as quartz, lithium borate, lithium niobate, or lithium tantalate to a silicon substrate is known. There is. These structures have excellent characteristics that they are extremely stable against thermal and mechanical changes and can be integrally formed on a semiconductor substrate.

[0006]

There is a method of fixing a piezoelectric substrate on another substrate by using various adhesives. In the pasting method using an adhesive, the thickness accuracy of the adhesive interface is improved. Is not enough, and there are problems such as insufficient heat resistance and reliability.

Further, in the method of forming on a substrate such as a semiconductor substrate by the thin film forming technique, the film that can be formed is limited to some materials such as ZnO and AlN, and its characteristics are inferior to those of the bulk. However, there is no crystal that exhibits excellent temperature dependence like quartz. Also, any crystal orientation cannot be obtained.

In the method of forming the composite piezoelectric substrate by various epitaxial growth methods, only the film suitable for the lattice constant of the substrate can be formed, so that the combination of materials is limited. Further, the crystallographic orientation of the film that can be formed depends on the crystallographic orientation of the substrate, and therefore, there is a limitation in obtaining a film having a preferable crystallographic orientation, so that the film cannot be formed freely. There are various epitaxial growth techniques such as liquid phase epitaxial growth, chemical vapor phase epitaxial growth, and molecular beam epitaxial growth, but it is difficult to obtain a superior single bulk crystal characteristic even if any of them is used.

Further, in the method of directly bonding quartz and silicon, or lithium niobate, lithium tantalate, lithium borate and various silicon substrates, there is a limitation in temperature and substrate thickness at which any piezoelectric substrate can be directly bonded.
If a substrate having a large thickness is to be directly joined, heat treatment cannot be performed at a high temperature, resulting in weak joining strength or poor manufacturing yield. Quartz, which is particularly excellent in temperature characteristics, cannot undergo a heat treatment at a high temperature because it causes a phase transition of a crystal phase at 573 ° C. Therefore, there are problems that it is difficult to manufacture a product having a sufficiently high bonding strength, and that there are large restrictions on the shape and size that can be directly bonded.

[0010]

In order to solve the above-mentioned problems, the Langasite substrate and the silicon substrate are directly bonded by hydrogen bond of the hydroxyl group at the interface or covalent bond with oxygen.

Further, it is composed of a Langasite substrate and a silicon substrate, and has an inorganic thin film layer on the surface of at least one of the substrates, and the substrates have hydrogen of a hydroxyl group at the interface between the inorganic thin film layer and the other substrate. A direct bond is formed by a bond or a covalent bond with oxygen.

The thickness of the direct joint portion is set to 20 nm or less. Silicon or a silicon compound can be used for the inorganic thin film layer.

The Langasite substrate may have at least a pair of comb-shaped electrodes so that the Langasite substrate can excite surface acoustic waves.

The Langasite substrate may be provided with at least a pair of opposing electrodes on both sides thereof so that the Langasite substrate is excited with a bulk wave.

Further, a silicon substrate may have an electronic element or an electronic circuit.

Further, the surfaces of the langasite substrate and the silicon substrate, which are to be joined, are extremely cleaned, hydroxyl groups are further attached, and the langasite substrate is directly joined to the silicon substrate by heating them in superposition. It is a thing.

Further, an inorganic thin film layer is formed on at least one surface of the Langasite substrate and the silicon substrate to be bonded, and the surface and the surface of the other substrate to be bonded are extremely cleaned, and hydroxyl groups are further attached. By overlapping and heating, the Langasite substrate is directly bonded to the silicon substrate via the inorganic thin film layer.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, composite piezoelectric substrates according to embodiments of the present invention will be described with reference to the drawings.

(Embodiment 1) FIG. 1A shows a sectional structure of Embodiment 1 of the structure of the composite piezoelectric substrate of the present invention. In FIG. 1A, 1 is a langasite substrate which is a piezoelectric substrate, 2 is a silicon substrate, and these two substrates are directly bonded by hydrogen bond by a hydroxyl group or covalent bond by oxygen at the interface. Single crystal langasite is represented by the chemical formula La3Ga5SiO14. There is a crystal orientation that has the same crystal symmetry as crystal and has a temperature dependence of almost 0 as in crystal. In addition, the characteristics such as the electromechanical coupling coefficient are better than those of quartz. Specifically, an electromechanical coupling coefficient of 2.2%, which is 3-5 times that of quartz, is obtained for bulk waves. Also, its crystal phase transition temperature is
It does not exist up to a melting point of 1470 ° C, much higher than 573 ° C of quartz and 933 ° C of gallium phosphate.

Another configuration of the first embodiment is shown in FIG. 1 (b). In FIG. 1B, 1 and 2 are the same as those in FIG. An inorganic thin film layer 3 is formed between the Langasite substrate 1 and the silicon substrate 2, and is, for example, a silicon compound such as silicon oxide or silicon nitride or a silicon layer. The inorganic thin film layer 3 is formed and attached to the langasite substrate 1 or the silicon substrate 2 by vacuum vapor deposition, sputtering, chemical vapor deposition (CVD) or the like. The langasite substrate 1 and the silicon substrate 2 are directly bonded to each other at the interface between the inorganic thin film layer 3 and the silicon substrate 2 or the interface with the langasite, as in the case of FIG.

By interposing the inorganic thin film layer at the interface, it is possible to embed an electrode in the inside of the interface, thereby expanding the range of application. Specifically, electrode wiring is formed on one surface of the substrate, and a high resistance inorganic thin film layer such as silicon oxide or silicon nitride is deposited on the surface of the substrate sufficiently thicker than the electrode wiring thickness by a thin film forming technique such as sputtering. Then, the steps of the electrode wiring portion can be flattened by polishing or the like, and then directly joined.

In addition, when the inorganic thin film layer contains silicon, in the direct bonding, oxygen-related covalent bonds are easily formed, the bonding strength is increased, and the direct bonding is possible at a lower heat treatment temperature. .

Here, the direct joining described in the present invention will be described. When the surface of the substrate is made extremely clean, and the surface is made hydrophilic and immersed in pure water, a large number of hydroxyl groups are adsorbed on the surface of the substrate. This situation is shown in FIG. The figure schematically shows a very typical case. The hydroxyl group consists of oxygen and hydrogen. When the substrates are overlapped with each other in this state, the initial bonding between the substrates is performed mainly by hydrogen bonding via the hydroxyl group. This state is shown in FIG. This also represents a typical example.

When heating is performed in this state, water molecules or hydrogen are gradually released from the joint interface, and the joint is strengthened. This strengthening of bonding is performed by increasing the ratio of covalent bonds centering on oxygen from hydrogen bonds by the hydroxyl groups. In this state, the heat treatment temperature is 200-500 ° C.
Are often found in.

When the temperature is further raised, hydrogen is further released, and bonds mainly via oxygen become main. This state is shown in FIG. This bond becomes covalent and the bond strength is further enhanced. If there is silicon at the interface,
Silicon also promotes covalent bond strengthening.

FIG. 2 schematically shows a typical case in any of the drawings, and the details are influenced by the constituent elements of the substrate and the surface condition.

With respect to the bonding strength, a value of several tens of kg / cm ² is easily obtained even by heat treatment at 200 ° C. for about 1 hour, which is sufficiently practical.

According to TEM (transmission electron microscope) observation, the bonding interface is bonded on the atomic order, and the thickness of the direct bonding portion is usually 10 nm or less, and it is easy to suppress it to 20 nm or less.

Differences and effects between the case where the organic material or other adhesive is used and the direct bonding described in this embodiment will be described. First, when bonding is performed using an adhesive, an adhesive layer always remains at the bonding interface. This is usually several μm to several tens
μm. In this embodiment, the bonding can be performed with the thickness of several molecular layers of hydroxyl groups. Therefore, the parallelism of the upper and lower surfaces of the substrates after bonding becomes extremely good. When an adhesive is used, it is virtually impossible to control the thickness of the adhesive on the atomic order.

From the above, in the case of the composite piezoelectric substrate described in this embodiment, fine processing of the surface of the composite piezoelectric substrate becomes possible.
For example, when manufacturing a surface acoustic wave device, it is necessary to form fine comb-shaped electrodes on the surface of a Langasite substrate. Sub-micron precision is required for the width of the comb electrodes. A common method for forming the comb-shaped electrodes is to form the electrodes by vacuum vapor deposition or the like, form a mask using photolithography, and form the mask by etching. When sub-micron photolithography is performed, unless the parallelism of the upper and lower surfaces of the substrate is sufficient, the exposure accuracy cannot be sufficient and good processing cannot be obtained.

Further, in fine processing such as wet etching or dry etching, there is a case where it is exposed to an acid as an etching agent or various gases, or exposed to a high temperature.
When various adhesives are used, their chemical and thermal stability are important problems, but in the case of this embodiment, such problems do not occur and high-precision fine processing is possible.

Adhesives, especially organic ones, are difficult to keep stable at high temperatures. The characteristics may change in the heating process such as soldering and solder reflow (about 230 ° C) that occurs during the manufacturing process, or gas may be generated, which may adversely affect surface acoustic wave devices that utilize the characteristics of the substrate surface. give. In this embodiment, it is extremely strong and stable below the heat treatment temperature for joining. Further, since only oxygen and hydrogen are present at the interface, there is no generation of gas that would adversely affect device fabrication.

Further, in the direct junction of this embodiment, it is not necessary to select an angle having a lattice constant between the silicon substrate and the Langasite substrate as in the case of epitaxial growth, so that the degree of freedom in material combination becomes extremely large.

Since the bonding interface is bonded at the atomic level and no special element other than hydrogen, oxygen and substrate constituent elements is interposed, the bonding performed by such a method is hereinafter referred to as direct bonding. I will decide.

As an example of such direct bonding, there is known a method of directly bonding quartz, lithium niobate, lithium tantalate, lithium borate, etc. to a substrate such as a semiconductor substrate or a glass substrate. However, by using the Langasite substrate, it becomes possible to perform the direct bonding under the condition that the conventional direct bonding is difficult for these substrates.

FIG. 3 compares the conditions under which direct bonding of quartz or gallium phosphate and the Si substrate is possible with the case of the Langasite substrate.

In the direct bonding, when the thickness of the silicon substrate is constant, the thicker the piezoelectric substrate is and the higher the heat treatment temperature is, the thermal stress is generated at the interface, and the direct bonding becomes difficult. As can be seen from FIG. 3, in the case of direct bonding of quartz and Si, for example, when the thickness of quartz is 50 μm, about 30
Only heat treatment at 0 ° C or lower is possible. On the other hand, in the case of langasite, when the thickness is 50 μm, the melting point of silicon is 1410.
The heat treatment can be performed up to a temperature of about 1400 ° C in the vicinity of ° C. The higher the temperature that can be treated, the stronger and stable the bonding strength, which is preferable in terms of reliability.

In terms of manufacturing, the substrate thickness that can be easily handled in manufacturing is 50 μm or more when considering a wafer having a diameter of 2 inches. Therefore, the bondable substrate thickness is preferably at least 50 μm or more. From this point of view, langasite is preferable.

Further, in the case of a bulk wave oscillator, the thickness of the piezoelectric substrate is inversely proportional to the resonance frequency, so the wider the range of substrate thickness that can be handled, the wider the manufacturable frequency range.
Langasite is preferable also from this viewpoint.

This situation applies to other piezoelectric substrates, such as
The same applies when compared with lithium niobate, lithium tantalate, and lithium borate. This situation is shown in FIG. FIG. 4 shows the bondable range in the case of bonding lithium niobate, lithium tantalate, and lithium borate to a silicon substrate in comparison with the case of langasite.

In this case as well, in the case of langasite, it is possible to directly bond at a higher temperature than lithium niobate, lithium tantalate, or lithium borate, or even a thicker substrate at the same temperature. And
Again, favorable results have been obtained.

These are the fact that the phase change temperature of the crystalline phase of the Langasite substrate is 1470 ° C., the phase change temperature of quartz is 573 ° C. and the Curie temperature of lithium tantalate is 60.
This is an effect obtained when the temperature is 7 ° C., which is far higher than the phase change temperature of gallium phosphate of 933 ° C.

Further, in the method of direct bonding, the effect obtained by experimentally investigating that the Langasite substrate can be bonded to a silicon substrate up to an extremely high temperature or a large thickness, in particular, is obtained. Is.

Also in the configuration of FIG. 1B, vacuum evaporation,
By forming an inorganic thin film layer by sputtering, chemical vapor deposition, etc., it is possible to control the film thickness with high accuracy. Therefore, in photolithography, even for surface acoustic wave devices where parallelism between the upper and lower surfaces of the substrate is important. , Can be applied.

(Second Embodiment) The composite piezoelectric substrate having the structure described in the first embodiment is used for various purposes.

A first example of an applied device using the structure described in the first embodiment is shown in FIG.
(B).

FIG. 5 shows a surface acoustic wave device formed on a Langasite substrate. In FIG. 5, reference numerals 1 and 2 are the Langasite substrate and the silicon substrate, and 4 and 4 ′ are comb-shaped electrodes formed on the surface of the Langasite substrate, as in the first embodiment. Although two sets of comb-shaped electrodes are shown here,
This shows a typical configuration, and the electrode configuration normally used in a device using surface acoustic waves can be similarly configured.

FIG. 5B shows a structure in which the inorganic thin film layer 3 is interposed between the Langasite substrate and the silicon substrate. Also in this case, various comb-shaped electrodes can be formed.
Further, in this case, the electrode can be embedded in the inorganic thin film layer, and the degree of freedom of the electrode configuration is further increased.

(Third Embodiment) A second example of an applied device using the structure of the first embodiment is shown in FIGS. 6 (a) and 6 (b) as a third embodiment. FIG. 6 shows a bulk wave device formed on a Langasite substrate. In FIG. 6, 1 and 2 are the same as in the first embodiment, a langasite substrate and a silicon substrate, 5 is a cavity formed by removing a part of the silicon substrate by etching, and 6 and 6 ′ are opposed to the langasite substrate. It is a pair of formed bulk wave excitation electrodes. Here, a pair of counter electrode configurations is shown, but this is a representative configuration, and the electrode configuration normally used for a device using a bulk wave can be similarly configured.

By adopting such a configuration, it becomes possible to maintain the characteristics of the bulk wave device while keeping them thermally and mechanically stable. For example, in the case of a silicon substrate, the fine processing of the cavity can be processed with high accuracy by utilizing anisotropic etching of a single crystal.

Further, in the third embodiment, since the thickness of the direct bonding portion is 20 nm or less, the Langasite substrate after the direct bonding can be thinned with high precision by mechanical processing such as polishing. It will be possible. Since the resonance frequency of the bulk wave device is generally inversely proportional to the thickness of the piezoelectric substrate, thinning is required to increase the frequency.
When operating at 500 MHz or higher, it is necessary to reduce the thickness to 3-5 μm or less. When a normal adhesive is used to bond to a silicon substrate, the thickness of the adhesive is on the order of μm. Therefore, using a normal adhesive, a thin plate of 3-5 μm or less can be obtained with high accuracy (in-plane uniformity). It cannot be processed. The accuracy in this case is about 3-5 μm + −1 μm. However, when the direct joining of this embodiment is used, 3
It is possible to make a thin plate with an accuracy of about −5 μm + −20 nm.

FIG. 6B shows a structure in which the inorganic thin film layer 3 is interposed between the Langasite substrate and the silicon substrate. Also in this case, various electrode configurations are possible. Further, in this case, the electrode can be embedded in the inorganic thin film layer, and the degree of freedom of the electrode configuration is further increased.

(Fourth Embodiment) A third example of an applied device using the structure of the first embodiment will be described as a fourth embodiment.
This is shown in FIGS. 7 (a) and 7 (b). FIG. 7 shows a bulk wave device formed on a composite substrate in which a Langasite substrate and a silicon substrate are directly bonded. In FIG. 7, reference numerals 1 and 2 are the same as in the first embodiment, and 6 and 6 ′ are a pair of facing surfaces that are opposed to the direct bonding interface between the Langasite substrate and the silicon substrate. It is an electrode for bulk wave excitation. Here, a pair of counter electrode configurations is shown, but this is a typical configuration, and the electrode configuration normally used for a device using bulk waves is
It is similarly configurable.

FIG. 7B shows a structure in which the inorganic thin film layer 3 is interposed between the Langasite substrate and the silicon substrate. Also in this case, various electrode configurations are possible. Further, in this case, the electrode can be embedded in the inorganic thin film layer, and the degree of freedom of the electrode configuration is further increased.

(Fifth Embodiment) A fifth example of an applied device using the structure of the first embodiment will be described as a fifth embodiment.
8A and 8B, another example of the fifth embodiment is shown in FIG.
(A) and (b) show.

8 (a) and 8 (b), a Langasite substrate is directly bonded to an electronic element such as a transistor or a semiconductor substrate having an electronic circuit including them, and a piezoelectric device is formed on the Langasite substrate. It is connected to an electronic element or electronic circuit. In FIG. 8, 1 and 2 are the same as in the first embodiment, and the Langasite substrate and the silicon substrate are the comb-shaped electrodes when the surface acoustic wave is excited (the comb-shaped electrode shape is omitted in the figure). , 6'are counter electrodes when exciting a bulk wave (in this case, they are provided so as to face both upper and lower surfaces, but only the electrodes on the upper surface are shown in the figure), and 5 excite a bulk wave In the case, a hollow portion 7 is an electronic element such as a transistor formed on the semiconductor silicon substrate 2 or an electronic circuit portion composed of them, and 8 and 8'connect the electrodes of the piezoelectric device to the electronic element or the electronic circuit. It is a wiring part. The electronic element is a transistor, a resistor, a capacitor, an inductor, or the like, and in the case of an electronic circuit, an oscillation circuit using a surface acoustic wave or a bulk wave, an amplifier circuit having a filter, various sensor circuits, and the like can be configured.

Here, a typical configuration is shown,
Various structures can be configured as long as they include a piezoelectric device and an electronic circuit.

With this structure, the piezoelectric device and the semiconductor device can be integrated together, and the size can be greatly reduced. In addition, since the Langasite substrate and the semiconductor silicon substrate are directly bonded to each other, a thermally and mechanically highly reliable product can be obtained. If the package is hermetically sealed as it is, it is stable because there is no generation of gas from the joint, and the direct joint heat treatment temperature can be set to the solder reflow temperature even for primary heating during manufacturing such as solder reflow. If it is performed at a temperature higher than the temperature, extremely stable characteristics can be obtained.

FIGS. 9A and 9B show a structure in which the inorganic thin film layer 3 is interposed between the Langasite substrate and the silicon substrate. Also in this case, various electrode configurations are possible. Further, in this case, the electrode can be embedded in the inorganic thin film layer, and the degree of freedom of the electrode configuration is further increased.

(Sixth Embodiment) FIG. 1 of the first embodiment.
An example of a method of manufacturing the composite piezoelectric substrate having the structure shown in (a) will be described.

The surfaces of the silicon substrate and the langasite substrate are flattened by polishing, and further polished until the surfaces are in a mirror state. Next, the surfaces to be joined are made extremely clean with detergent and various solvents. Then, each surface is hydrophilized. For example, ultraviolet irradiation may be used. Then, the surface is thoroughly washed with pure water, and hydroxyl groups are attached to the surface. After that, the two substrates are uniformly overlapped with each other, so that the bonding can be easily obtained. By performing the heat treatment in this state, the bonding strength is enhanced. 100 ° C to 1400
The heat treatment at a temperature of ° C further strengthens the bond.

The heat treatment effect for strengthening the bond is, for example, 200
The bonding strength is increased several times and the strength of several tens Kg / square cm can be obtained even if it is held at ℃ for about 1 hour.

The heat treatment temperature needs to be lower than the temperature at which the silicon substrate and the Langasite substrate are altered.

The substrate used for the direct bonding is preferably a single crystal substrate which is easy to obtain a flat and clean surface because the unevenness of the substrate surface affects the yield of the direct bonding.

Since the bonding interface obtained by the present manufacturing method can be bonded on an atomic order without using an adhesive, a composite piezoelectric substrate that is stable against thermal changes and mechanical vibrations can be obtained.

(Embodiment 7) FIG. 1 of Embodiment 1
An example of a method of manufacturing the composite piezoelectric substrate having the structure shown in (b) will be described.

The surfaces of the silicon substrate and the langasite substrate are flattened by polishing and further polished until the surfaces are in a mirror state. Next, the surfaces to be joined are made extremely clean with detergent and various solvents. Next, on at least one surface of the substrate to be bonded, an inorganic thin film layer, sputtering, vacuum deposition,
It is formed by a chemical vapor deposition method or the like. The inorganic thin film layer is preferably silicon or a silicon compound, and silicon may be polycrystalline or amorphous. The silicon compound is preferably silicon oxide or silicon nitride. After the surface of the formed inorganic thin film layer is extremely cleaned, the surfaces of the respective portions to be joined are subjected to hydrophilic treatment by irradiating with ultraviolet rays. Then, the surface is thoroughly washed with pure water to attach hydroxyl groups to the surface. After that, the two substrates are uniformly overlapped with each other, so that the bonding can be easily obtained as in the sixth embodiment. By performing the heat treatment in this state, the bonding strength is enhanced. When the heat treatment is performed at a temperature of 100 ° C to 1400 ° C, the bond is further strengthened. The effect of the heat treatment is almost the same as that of the sixth embodiment, and the bonding strength is increased several times and the strength of several tens Kg / square cm can be obtained only by holding at 200 ° C. for about 1 hour.

The heat treatment temperature needs to be lower than the temperature at which the silicon substrate, the Langasite substrate and the inorganic thin film layer are altered.

Regarding the inorganic thin film layer, the example in which it is attached to the langasite substrate side has been described, but it can be directly joined in the same manner even if it is attached to the silicon substrate side. Further, even if the inorganic insulating layers are formed on the surfaces of both substrates, the direct bonding can be performed in the same manner. The functions and performances of the obtained composite piezoelectric substrate are almost the same in all cases.

Further, in the direct bonding, the covalent bond is predominant in the high temperature heat treatment, and therefore, the inorganic insulating layer containing silicon is likely to form the covalent bond and easily obtain the one having high bonding strength.

Further, as compared with the manufacturing method of Example 6, since the inorganic thin film layer plays a role of a buffer at the time of direct bonding, there is an effect that the difference in the coefficient of thermal expansion between the langasite substrate and the silicon substrate is slightly alleviated. Therefore, the joining heat treatment can be performed at a higher temperature, and the one having high joining strength can be obtained. Further, even if there is some dust on the bonding interface, it is taken into the inorganic thin film layer in the process of strengthening the bonding directly by the heat treatment, which has the effect of increasing the manufacturing yield.

Since the bonding interface obtained by this manufacturing method can be bonded in atomic order without using an adhesive as in the sixth embodiment, the composite piezoelectric element is stable against thermal changes and mechanical vibrations. A substrate is obtained.

In this embodiment, an example of hydrophilization is shown, but the method is not limited to this. Other hydrophilic treatments or methods of attaching hydroxyl groups to the surface may be used.

[0074]

According to the present invention, since a composite piezoelectric substrate can be formed with a bonding portion having a thickness of 20 nm or less without using an adhesive such as an organic substance, the parallelism between the upper and lower sides of the substrate is good and the substrate surface High-precision microfabrication is possible, and specifically, the film thickness can be controlled at 3 μm + −20 nm.

Further, since it is thermally and mechanically stable and specifically, heat treatment at 573 ° C. or higher is easy, an extremely stable piezoelectric device can be easily obtained at a temperature of 573 ° C. or lower.

Since heat treatment at 933 ° C. or higher is easy,
An extremely stable piezoelectric device can be easily obtained at a temperature of 933 ° C. or lower.

Further, between the silicon substrate and the langasite substrate,
Since the degree of freedom in combining crystal orientations is large, the degree of freedom in selecting materials is greatly increased. Further, since it is combined with the semiconductor substrate, various electronic elements and electronic circuits are formed on the semiconductor substrate to obtain a device in which the piezoelectric device and the semiconductor device are integrated.

Further, compared with direct bonding using a piezoelectric substrate of quartz, lithium niobate, lithium tantalate, lithium borate, gallium phosphate, etc., heat treatment can be performed at a higher temperature, and direct bonding on a thicker substrate is possible. Is possible. More specifically, with a substrate thickness of 50 μm or more, 3
Even if the temperature is 00 ° C. or higher, 573 ° C. or higher, or 933 ° C. or higher, the heat treatment temperature at the time of direct bonding is possible up to 1400 ° C. As a result, the joint strength of the direct joint portion is enhanced, and a more thermally and mechanically stable joint is obtained.

Further, since it is possible to directly bond the substrate with a thickness of 50 μm or more, it becomes possible to perform wafer processing in 2-3 inches, which is suitable for mass production. In addition, the direct bonding yield is significantly improved. Specifically, the thickness of the piezoelectric substrate is 50μ.
When the heat treatment temperature is 350 ° C., the yield is 5% or less in the case of quartz-Si direct bonding, whereas it is 95% or more in the case of Langasite-Si.

As a result, it is possible to obtain an effect that the degree of freedom in designing various applied devices such as surface acoustic wave devices, bulk wave devices, and piezoelectric semiconductor integrated devices is significantly increased.

[Brief description of drawings]

FIG. 1 is a structural diagram according to a first embodiment of the present invention.

FIG. 2 is an explanatory diagram of the principle according to the first embodiment of the present invention.

FIG. 3 is a diagram showing the relationship between the thickness of a substrate capable of directly bonding a Langasite substrate to a silicon substrate and the heat treatment temperature in comparison with the case of quartz and gallium phosphate in the embodiment of the present invention.

FIG. 4 is a diagram showing a relationship between a substrate thickness capable of directly joining a Langasite substrate to a silicon substrate and a heat treatment temperature in comparison with other piezoelectric substrates in an embodiment of the present invention.

FIG. 5 is a structural diagram according to a second embodiment of the present invention.

FIG. 6 is a structural diagram according to a third embodiment of the present invention.

FIG. 7 is a structural diagram according to a fourth embodiment of the present invention.

FIG. 8 is a structural diagram according to a fifth embodiment of the present invention.

FIG. 9 is a structural diagram when an inorganic thin film layer is interposed in the fifth embodiment.

[Explanation of symbols]

 1 Langasite Substrate 2 Silicon Substrate 3 Inorganic Thin Film Layer 4 Comb Electrode 5 Cavity 6 Electrode 7 Electronic Element or Electronic Circuit 8 Wiring

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location H03H 3/08 7259-5J H03H 9/25 C 9/17 H01L 41/08 C 9/25 41 / 22 Z (72) Inventor Osamu Kawasaki 1006 Kadoma, Kadoma-shi, Osaka Matsushita Electric Industrial Co., Ltd.

Claims (10)

[Claims] 1. A composite piezoelectric substrate, wherein a Langasite substrate and a silicon substrate are directly bonded by hydrogen bond of a hydroxyl group at the interface or covalent bond with oxygen. 2. A Langasite substrate and a silicon substrate, wherein at least one of the substrates has an inorganic thin film layer on the surface thereof, and the substrates have a hydroxyl group at the interface between the inorganic thin film layer and the other substrate. A composite piezoelectric substrate, which is directly bonded by a hydrogen bond or a covalent bond with oxygen. 3. The composite piezoelectric substrate according to claim 1, wherein the thickness of the direct bonding portion is 20 nm or less. 4. The composite piezoelectric substrate according to claim 2, wherein the inorganic thin film layer is silicon or a silicon compound. 5. The composite piezoelectric substrate according to claim 1, wherein the Langasite substrate is provided with at least a pair of comb-shaped electrodes so that surface acoustic waves are excited in the Langasite substrate. 6. The composite piezoelectric substrate according to claim 1, wherein at least one pair of counter electrodes are provided on both surfaces of the Langasite substrate to excite a bulk wave in the Langasite substrate. 7. The composite piezoelectric substrate according to claim 1, wherein the silicon substrate has an electronic element or an electronic circuit. 8. A Langasite substrate and a silicon substrate,
A method for manufacturing a composite piezoelectric substrate, which comprises directly bonding the Langasite substrate to the silicon substrate by extremely cleaning the surfaces of the intended bonding portions, further attaching a hydroxyl group, and superposing them on each other and heating them. 9. A Langasite substrate and a silicon substrate,
An inorganic thin film layer is formed on the surface of at least one part to be bonded, the surface and the part to be bonded on the surface of the other substrate are extremely cleaned, further hydroxyl groups are attached, and the langasite substrate is heated by stacking and heating. Is directly bonded to the silicon substrate via the inorganic thin film layer. 10. The method for producing a composite piezoelectric substrate according to claim 9, wherein the inorganic thin film layer is silicon or a silicon compound.