JPH08153915A - Composite piezoelectric substrate and its manufacture - Google Patents

Abstract

PURPOSE: To provide a structure of a composite piezoelectric substrate and its manufacturing method wherein a gallium phosphate substrate is used as a piezoelectric substrate, design freedom of piezoelectric characteristics is large due to the combination of various kinds of substrates, high precision fine work is enabled, thermal and mechanical stability is obtained, and a thicker substrate can be directly bonded at a higher temperature. CONSTITUTION: Parts to be bonded with a gallium phosphate substrate 1 and a retaining substrate 2 are extremly cleaned, subjected to the treatment for obtaining hydrophilic nature, stacked, and heated. Thereby the gallium phosphate substrate 1 is bonded to the retaining substrate 2 by hydrogen bond of hydroxyl group of an interface or covalent bond of oxygen.

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 gallium phosphate.

[0002]

2. Description of the Related Art Piezoelectric devices such as crystal oscillators, lithium tantalate, lithium niobate, and lithium borate, surface acoustic wave devices, and other piezoelectric devices are widely used in radio communication equipment as oscillators and filters for various oscillators. Has been done. 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.

Further, in order to use these piezoelectric devices in mobile communication equipment, it is necessary to use a small piezoelectric device having good characteristics. In the case of a filter, the insertion loss and its temperature dependence are important characteristics of the piezoelectric device. In the case of a resonator, the resonance Q (corresponding to the reciprocal of the loss) and the resonance / anti-resonance ratio (capacity ratio) and its It is temperature dependent. When using the capacitance ratio for a resonator type filter,
It is directly related to the pass band. The insertion loss, the Q of resonance, and the capacitance ratio depend on the electromechanical coupling coefficient of the piezoelectric body used, and the temperature dependence involves the temperature dependence of the sound velocity of the piezoelectric body used. Also, the speed of sound is related to the resonance frequency.

The electromechanical coupling coefficient and temperature dependence greatly vary depending on the material used. For lithium niobate,
Electromechanical coupling coefficient is large, but temperature dependence is also large.
It is about 0-70 ppm / ° C. In the case of lithium tantalate, the electromechanical coupling coefficient is smaller than that of lithium niobate, but the temperature dependence is about 30-50 ppm / ° C,
Better than lithium niobate. Quartz has a very small electromechanical coupling coefficient, but its temperature dependence has a cut angle close to zero. Lithium borate has a good cut angle with good temperature dependence, but also has a small electromechanical coupling coefficient.
In addition, since it is water-soluble, it is difficult to manufacture. The sound velocity is also peculiar to each material.

From the standpoint of electromechanical coupling coefficient, lithium niobate is generally desirable. However, quartz is most preferable in terms of temperature dependence. From the viewpoint of design flexibility, it is preferable that the electromechanical coupling coefficient is large and the temperature dependence is small. However, these conventional materials are not sufficient.

Further, an electronic circuit using an electroacoustic element, for example, an oscillation circuit such as an oscillator using a piezoelectric resonator, or an amplifier 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 by 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-6-120416, JP-A-6-125036, and the like disclose piezoelectric devices such as quartz, lithium borate, lithium niobate, and lithium tantalate as silicon or various holding substrates for holding. A method of directly joining is known. These structures are extremely stable against thermal and mechanical changes,
It has an excellent feature that it can be formed integrally with a semiconductor substrate or another substrate.

[0009]

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.

In the method of forming the composite piezoelectric substrate by various epitaxial growth methods, only a 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.

In the method of directly bonding quartz and silicon, or lithium niobate, lithium tantalate, lithium borate and various holding substrates, any piezoelectric substrate
There is a restriction on the temperature and substrate thickness at which direct bonding is possible. If a thick substrate is directly bonded, heat treatment cannot be performed at a high temperature, resulting in weak bonding strength or poor manufacturing yield. Quartz, which has particularly excellent temperature characteristics, undergoes a phase transition of the crystal phase at 573 ° C, so it cannot be heat treated at high temperatures, making it difficult to manufacture those with sufficiently strong bonding strength, and having a large shape and size for direct bonding. There were issues such as restrictions.

[0013]

In order to solve the above problems, the gallium phosphate substrate and the holding substrate are directly bonded by hydrogen bond of the hydroxyl group at the interface or covalent bond with oxygen.

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

Further, the thickness of the direct joint portion is set to 20 nm or less. A semiconductor can be used for the holding substrate, and silicon can be used as the semiconductor.

Further, glass or a piezoelectric material can be used for the holding substrate. Further, gallium phosphate can be used for the holding substrate.

Silicon or a silicon compound can be used for the inorganic thin film layer. Further, the gallium phosphate substrate may have at least a pair of comb-shaped electrodes so that the gallium phosphate substrate can excite surface acoustic waves.

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

Further, by providing at least a pair of counter electrodes on the surfaces of the gallium phosphate substrate and the holding substrate which are directly bonded to each other, a bulk wave is excited in the composite piezoelectric substrate composed of the gallium phosphate substrate and the holding substrate. May be.

A semiconductor may be used for the holding substrate, and the semiconductor substrate may have an electronic element or an electronic circuit.

In addition, the gallium phosphate substrate and the holding substrate,
The surfaces to be joined are made extremely clean, further subjected to a hydrophilic treatment, superposed and heated to directly join the gallium phosphate substrate to the holding substrate.

In addition, the gallium phosphate substrate and the holding substrate,
The inorganic gallium phosphate substrate is formed by forming an inorganic thin film layer on at least one surface to be bonded, and making the surface and the other substrate surface to be bonded extremely clean, further hydrophilizing, and superposing and heating. Is directly bonded to the holding substrate via the inorganic thin film layer.

[0023]

With the above-described manufacturing method and the structure obtained by the above method, it is possible to perform fine processing with high accuracy, which is thermally and mechanically stable, and the material combination and the piezoelectric material when applied as a piezoelectric device. The degree of freedom in selecting the characteristics is greatly increased, and it becomes possible to form a composite piezoelectric substrate by direct bonding using a higher temperature and thicker piezoelectric substrate, which was difficult with conventional piezoelectric materials. , Which is mechanically stable and has a good manufacturing yield.

[0024]

Embodiments of the composite piezoelectric substrate of the present invention will be described below with reference to the drawings.

(Embodiment 1) FIG. 1A shows a sectional structure of a first embodiment of the structure of the composite piezoelectric substrate of the present invention. FIG.
In (a), 1 is a gallium phosphate substrate which is a piezoelectric substrate, and 2 is a holding substrate, and these two substrates are directly bonded by hydrogen bond by a hydroxyl group at the interface or covalent bond by oxygen. Suitable holding substrates include Group 4 semiconductors such as Si, compound semiconductors such as GaAs and InP, various glass substrates containing silicic acid, single crystal piezoelectric substrates such as quartz, and single crystal insulating substrates such as sapphire. Alternatively, gallium phosphate can be used. Single crystal gallium phosphate has the same crystal symmetry as that of quartz, and has a crystal orientation in which the temperature dependence is almost zero as in quartz. In addition, the characteristics such as the electromechanical coupling coefficient are better than those of quartz. Its crystal phase transition temperature is 933 ° C.,
Much higher than the crystal's 573 ° C.

Another embodiment of such a configuration is shown in FIG.
It shows in (b). In FIG. 1B, reference numerals 1 and 2 are the same as those in FIG.
Same as (a). Reference numeral 3 is an inorganic thin film layer formed between the gallium phosphate substrate 1 and the holding 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 the gallium phosphate substrate 1 or the holding substrate 2.
Vacuum deposition, sputtering, chemical vapor deposition (CV
D) and the like. The gallium phosphate substrate 1 and the holding substrate 2 are directly bonded to each other at the interface between the inorganic thin film layer 3 and the holding substrate 2 or the interface with gallium phosphate as in the case of FIG. 1A.

By interposing the inorganic thin film layer at the interface, it is possible to embed an electrode inside the interface, so that the range of application is expanded. 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, covalent bonds involving oxygen are easily formed, the bonding strength is increased, and the direct bonding can be performed 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 state 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 the bond is mainly mediated by oxygen. 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 affected 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 in 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 of using an organic material or another adhesive 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
It becomes 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 making a surface acoustic wave device, it is necessary to form fine comb-shaped electrodes on the surface of the gallium phosphate 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 microfabrication such as wet etching or dry etching, there is a case where it is exposed to acid as an etching agent or various gases, or exposed to 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, since the combination of the holding substrate and the gallium phosphate substrate is not limited like the epitaxial growth, different elements, crystal structures, crystal orientations,
Since it is possible to combine materials having lattice constants, 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 the constituent elements of the substrate is allowed to intervene, 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 and the like to a substrate such as a semiconductor substrate or a glass substrate. However, by using the gallium phosphate substrate, it becomes possible to perform the direct bonding under the condition that the conventional direct bonding is difficult for these substrates.

3 (a) and 3 (b) show the crystal and the Si substrate, and the coefficient of thermal expansion (10X) which is relatively close to that of the crystal.
The conditions under which direct bonding with a high thermal expansion coefficient glass having a temperature of 10 ⁻⁶ / ° C.) is possible are compared with those in the case of a gallium phosphate substrate.

In the direct bonding, when the thickness of the holding 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. 3A, 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 gallium phosphate, when the thickness is 50 μm, heat treatment can be performed up to about 550 ° 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 production, the substrate thickness that can be easily handled in production 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, gallium phosphate is preferable.

In the case of the bulk wave oscillator, the thickness of the piezoelectric substrate is inversely proportional to the resonance frequency. Therefore, the wider the range of substrate thickness that can be handled, the wider the manufacturable frequency range.
From this viewpoint as well, gallium phosphate is preferable.

From these viewpoints, gallium phosphate is clearly preferable because it allows direct bonding at a higher temperature and a thicker substrate than that of quartz.

FIG. 3B shows the case where the holding substrate is a glass substrate. In this case as well, the heat treatment temperature is obviously higher in the gallium phosphate substrate than in the case of quartz, and the substrate thickness is also higher. May be thick. In this case, since the characteristics of the glass substrate change at 600 ° C. or higher, the measurement is not performed, but if a glass substrate having a high softening temperature is used, heat treatment at a higher temperature is possible.

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.
It shows in (b). FIG. 4A shows the case where the holding substrate is a Si substrate, and FIG. 4B shows the case where the holding substrate is a glass having a high coefficient of thermal expansion. In this case, too, in the case of gallium phosphate, direct bonding can be performed at a higher temperature than that of lithium niobate, lithium tantalate, and lithium borate, or even a thicker substrate can be directly bonded at the same temperature. , Again, favorable results have been obtained.

These results indicate that the coefficient of thermal expansion of quartz, lithium niobate, lithium tantalate, etc. all has a large value and is 10 × 10 ⁻⁶ / ° C. or more, whereas the coefficient of thermal expansion of gallium phosphate is higher. Has an azimuth of 10 × 10 ⁻⁶ / ° C. or less, and particularly in the minimum azimuth (Z-axis direction), 3
x10 ^-6 / ° C, which is almost the same value as Si, and the phase change temperature of the crystal phase is 933 ° C, which is higher than the crystal phase change temperature of 573 ° C and the Curie temperature of lithium tantalate 607 ° C. It is considered that the effect is due to the fact that it is much higher and that it is not water-soluble like lithium borate.

Also in the structure 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.

(Embodiment 2) The composite piezoelectric substrate having the structure described in Embodiment 1 is used for various purposes.

A first example of an applied device using the structure described in the first embodiment is shown in FIGS. 5 (a) and 5 (b). FIG. 5 shows a surface acoustic wave device formed on a gallium phosphate substrate. In FIG. 5, 1 and 2 are the gallium phosphate substrate and the holding substrate, and 4 and 4 ′ are the comb-shaped electrodes formed on the surface of the gallium phosphate substrate, as in the first embodiment. Here, two sets of comb-shaped electrode configurations are shown, but this is a representative configuration, and the electrode configuration normally used in a device using surface acoustic waves can be similarly configured.

With such a structure, the characteristics of the surface acoustic wave can be controlled in various ways. For example, when a piezoelectric substrate is also used as the holding substrate, if the thickness of the gallium phosphate substrate or the piezoelectric substrate on the holding substrate side is set to about 1 wavelength or less of the surface acoustic wave to be operated, the surface elasticity propagating in this layered structure will be reduced. The wave property is a composite property of gallium phosphate and a piezoelectric substrate. For example, when quartz is used for the holding substrate, the temperature dependence is very small, and the piezoelectric constant is obtained by combining the characteristics of gallium phosphate and quartz. When lithium niobate or lithium tantalate is used for the holding substrate, a composite piezoelectric bulk wave device having excellent temperature dependence of gallium phosphate and a large electromechanical coupling coefficient of lithium niobate or lithium tantalate can be obtained. Moreover, even if it is a non-piezoelectric holding substrate,
By directly bonding to a single crystal substrate such as sapphire or Si, it becomes possible to excite surface acoustic waves having different sound velocities. When the glass substrate is used as the holding substrate, it is particularly advantageous to obtain a substrate having a slow sound velocity (1500-2500 m / sec).

In this case, the thickness of the direct joint is 2 in each case.
Since it can be realized with a thickness of 0 nm or less, propagation of surface acoustic waves between substrates having a layered structure is performed with almost no loss, and a characteristic of a composite surface acoustic wave in a layered structure with a low loss that cannot be realized by ordinary adhesive bonding is obtained. .

FIG. 5B shows a structure in which the inorganic thin film layer 3 is interposed between the gallium phosphate substrate and the holding 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.

(Embodiment 3) A second example of an applied device using the structure of this embodiment is shown in FIGS. 6 (a) and 6 (b). Figure 6
Is a bulk wave device formed on a gallium phosphate substrate. In FIG. 6, 1 and 2 are the gallium phosphate substrate and the holding substrate as in the first embodiment, 5 is a cavity formed by removing a part of the holding substrate by etching or the like, and 6 and 6 ′ are formed facing the gallium phosphate substrate. And a pair of electrodes for exciting a bulk wave. Although one pair of counter electrode configurations is shown here,
This shows a typical configuration, and the electrode configuration normally used in a device using bulk waves can be similarly configured.

By adopting such a configuration, it becomes possible to maintain the characteristics of the bulk wave device while thermally and mechanically maintaining it stable. For example, when a single crystal semiconductor such as Si is used for the holding substrate, the fine processing of the cavity can be processed with high accuracy by utilizing anisotropic etching of the single crystal. Further, when glass is used for the holding substrate, workability is also easy, which is preferable for forming the cavity. Further, when the same material, that is, a gallium phosphate substrate is used for the holding substrate, no thermal stress is applied to the bonding interface, so that the temperature stability of the bulk wave characteristic is improved.

Further, in this embodiment, since the thickness of the direct bonding portion is 20 nm or less, the gallium phosphate substrate after the direct bonding can be thinned with high accuracy by mechanical processing such as polishing. Become. 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. 500
When operating above MHz, it is necessary to reduce the thickness to 3-5 μm or less. When bonded to a holding substrate using a normal adhesive, the thickness of the adhesive is on the order of μm, so using a normal adhesive, a thin plate of 3-5 μm or less can be highly accurately (in-plane uniform). It cannot be processed. The accuracy in this case is about 3-5 μm + −1 μm.
However, when the direct bonding of this embodiment is used, it is 3-5 μm +
It is possible to make a thin plate with an accuracy of about -20 nm.

FIG. 6B shows a structure in which the inorganic thin film layer 3 is interposed between the gallium phosphate substrate and the holding 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.

(Embodiment 4) A third example of an applied device using the structure of this embodiment is shown in FIGS. 7 (a) and 7 (b). Figure 7
Is a bulk wave device formed on a holding substrate directly bonded to a gallium phosphate substrate. In FIG. 7, 1,
Similar to the first embodiment, 2 is a gallium phosphate substrate and a holding substrate, and 6 and 6 ′ are a pair of bulk wave excitation electrodes formed so as to face the surface of the gallium phosphate substrate and the holding substrate facing the direct bonding interface. . 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.

With such a structure, the characteristics of the bulk wave can be controlled in various ways. For example, when a piezoelectric substrate is also used as the holding substrate, the properties of the entire bulk wave can be obtained by combining gallium phosphate and the piezoelectric substrate. For example, when quartz is used for the holding substrate, the temperature dependence is very small, and the piezoelectric constant is obtained by combining the characteristics of gallium phosphate and quartz. When lithium niobate or lithium tantalate is used for the holding substrate, a composite piezoelectric bulk wave device having excellent temperature dependence of gallium phosphate and a large electromechanical coupling coefficient of lithium niobate or lithium tantalate can be obtained. Even if it is a non-piezoelectric holding substrate, by directly joining it with a single crystal having good crystallinity such as sapphire or Si,
It is possible to increase the Q of resonance.

In this case, since the thickness of the direct bonding portion is 20 nm or less, the propagation of the bulk wave between the substrates having the laminated structure is carried out with almost no loss, and the low-loss laminated structure which cannot be realized by the usual bonding with an adhesive. The characteristics of the composite bulk wave in the structure are obtained.

FIG. 7B shows a structure in which the inorganic thin film layer 3 is interposed between the gallium phosphate substrate and the holding 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.

(Embodiment 5) A fourth example of an applied device using the structure of this embodiment is shown in FIGS. 8 (a) and 8 (b), and a fifth example is shown in FIGS. 9 (a) and 9 (b). Show. 8 (a) and 8 (b),
A gallium phosphate substrate is directly bonded to an electronic element such as a transistor or a semiconductor substrate having an electronic circuit including them, a piezoelectric device is formed on the gallium phosphate substrate, and the element is connected to the electronic element or the electronic circuit. In FIG. 8, 1 and 2 are gallium phosphate substrates and a holding substrate as in the first embodiment. In this case, the holding substrate is a semiconductor substrate such as Si, and 4 is a comb-shaped electrode for exciting surface acoustic waves (in the figure, The comb-shaped electrode is omitted in the drawing. 6'is a counter electrode for exciting a bulk wave (in this case, it is provided so as to face both upper and lower surfaces, but only the upper electrode is shown in the figure). 5 is a cavity portion for exciting a bulk wave, 7 is an electronic element such as a transistor formed on the semiconductor holding substrate 2, or an electronic circuit portion including them.
Reference numerals 8 and 8 ′ are wiring portions that connect the electrodes of the piezoelectric device to the electronic elements or electronic circuits. 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 gallium phosphate substrate and the semiconductor holding substrate are directly joined, thermal,
It is mechanically extremely reliable. 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 gallium phosphate substrate and the holding 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.

Example 6 An example of a method for manufacturing a composite piezoelectric substrate having the structure shown in Example 1 and FIG. 1A will be described.

The surfaces of the holding substrate and the gallium phosphate 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 subjected to a hydrophilic treatment by irradiation with ultraviolet rays. After that, the surface is thoroughly washed with pure water, and the surfaces are evenly overlaid, so that bonding can be easily obtained. By performing heat treatment in this state, the bonding strength is enhanced. When the heat treatment is performed at a temperature of 100 to 900 ° C., the bond is further strengthened.

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 holding substrate and the gallium phosphate substrate are denatured.

Those which can be directly bonded to gallium phosphate by such a method include Si, GaAs, and I as semiconductors.
nP, the piezoelectric body is a piezoelectric body containing gallium phosphate, quartz, lithium niobate, lithium tantalate, lithium borate, lead zirconate titanate as a main component, and the insulator is a glass such as borosilicate glass or quartz glass, For example, sapphire.

When the gallium phosphate substrate and the holding substrate are made of the same material or substrates having substantially the same coefficient of thermal expansion, heat treatment can be performed at a higher temperature, and a stronger bonding strength can be obtained. When glass is used as the holding substrate, the coefficient of thermal expansion can be varied over a wide range, and therefore glass having a coefficient of thermal expansion that matches the gallium phosphate substrate used can be used as the holding substrate.

The substrate used for 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 direct bonding.

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

Example 7 An example of a method of manufacturing the composite piezoelectric substrate having the structure shown in Example 1 and FIG. 1B will be described.

The surfaces of the holding substrate and the gallium phosphate 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. An inorganic thin film layer is formed on at least one surface of the substrate to be bonded next by sputtering, vacuum deposition, chemical vapor deposition, 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, each of the surfaces to be joined is irradiated with ultraviolet rays to be hydrophilized. After that, the surface is sufficiently washed with pure water, and the surfaces are evenly overlaid, so that bonding can be easily obtained as in the sixth embodiment. By performing heat treatment in this state, the bonding strength is enhanced. 100 ° C to 900 ° C
When the heat treatment is performed at the temperature of, the bond is further strengthened.
The effect of the heat treatment is almost the same as in Example 6, and the temperature is 200 ° C.
Then, even if it is held for about 1 hour, the bonding strength is increased several times, and a strength of several tens kg / square cm can be obtained.

The heat treatment temperature must be lower than the temperature at which the holding substrate, the gallium phosphate substrate and the inorganic thin film layer are denatured.

Regarding the inorganic thin film layer, the example in which it is attached to the gallium phosphate substrate side has been described, but it can be directly joined in the same manner even if it is attached to the holding 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.

In the direct bonding, covalent bonds are predominant in the high temperature heat treatment, so that the inorganic insulating layer containing silicon is likely to form a covalent bond and easily has high bonding strength.

By such a method, through the inorganic thin film layer,
Similar to the sixth embodiment, those which can be directly bonded to gallium phosphate are Si, GaAs, InP as the semiconductor, and gallium phosphate, quartz, lithium niobate, lithium tantalate, lithium borate, zircon titanate as the piezoelectric body. Piezoelectric materials containing lead as a main component and insulators include glass such as borosilicate glass and quartz glass, and sapphire.

When the gallium phosphate substrate and the holding substrate are substrates made of the same material or having substantially the same coefficient of thermal expansion, heat treatment can be performed at a higher temperature, and a stronger bonding strength can be obtained. When glass is used as the holding substrate, the coefficient of thermal expansion can be varied over a wide range, and therefore glass having a coefficient of thermal expansion that matches the gallium phosphate substrate used can be used as the holding substrate.

Further, as compared with the manufacturing method of Example 6, since the inorganic thin film layer serves as a buffer at the time of direct bonding, there is an effect that the difference in the coefficient of thermal expansion between the gallium phosphate substrate and the holding 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 Example 6, the composite piezoelectric substrate stable against thermal changes and mechanical vibrations. Is obtained.

In this embodiment, an example of hydrophilization is shown.
The method is not limited to this.

[0085]

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 300 ° C. or higher is easy, a piezoelectric device extremely stable at a temperature of 300 ° C. or lower can be easily obtained.

Since the holding substrate and the gallium phosphate substrate have a large degree of freedom in combination, the degree of freedom in selecting the piezoelectric characteristics of the obtained composite piezoelectric substrate is greatly increased. For example, in the case of compounding a quartz crystal, piezoelectric characteristics with a temperature coefficient of 0 can be obtained. When combined with lithium niobate and lithium tantalate, an electromechanical coupling coefficient larger than that of gallium phosphate itself is obtained. Further, when a glass substrate is used, the degree of freedom in selecting the sound velocity is increased and the shape of the holding portion is easily processed. When combined with a semiconductor substrate, a device in which a piezoelectric device and a semiconductor device are integrated can be obtained by forming various electronic elements and electronic circuits on the semiconductor substrate.

Further, compared with direct bonding when using a piezoelectric substrate such as quartz, lithium niobate, lithium tantalate, or lithium borate, heat treatment can be performed at a higher temperature, and direct bonding on a thicker substrate is possible. is there. More specifically, with a substrate thickness of 50 μm or more, a heat treatment temperature at the time of direct bonding at 300 ° C. or more becomes possible. As a result, the joint strength of the direct joint portion is enhanced, and a more thermally and mechanically stable joint is obtained. In addition, since the substrates can be directly bonded to each other with a thickness of 50 μm or more, the wafer can be processed in 2-3 inches, which is suitable for mass production. In addition, the direct bonding yield is significantly improved. Specifically, when the thickness of the piezoelectric substrate is 50 μm and the heat treatment temperature is 280 ° C., the yield is about 10% in the quartz-Si direct bonding, whereas it is 90% or more in the case of gallium phosphate-Si.

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

[Brief description of drawings]

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

FIG. 2 is an explanatory diagram of the principle of the present invention.

FIG. 3 shows a piezoelectric substrate (a) Si according to an embodiment of the present invention.
The figure which shows the relationship between the heat treatment temperature and the thickness of the piezoelectric substrate that can be directly bonded to the glass substrate (b).

FIG. 4 (a) is a piezoelectric substrate according to another embodiment of the present invention.
The figure which shows the relationship between the heat treatment temperature and the thickness of the piezoelectric substrate which can be directly bonded to the Si substrate and (b) the glass substrate.

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

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

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

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

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

[Explanation of symbols]

 1 gallium phosphate substrate 2 holding substrate 3 inorganic thin film layer 4 comb-shaped electrode 5 cavity part 6 electrode 7 electronic element or electronic circuit part 8 wiring

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Reference number within the agency FI Technical indication location H03H 9/25 C 7259-5J (72) Inventor Yutaka Taguchi 1006 Kadoma, Kadoma-shi, Osaka Matsushita Electric Sangyo Co., Ltd.

Claims (16)

[Claims] 1. A composite piezoelectric substrate, wherein the gallium phosphate substrate and the holding substrate are directly bonded to each other by hydrogen bonds of hydroxyl groups at the interface or covalent bonds with oxygen. 2. A gallium phosphate substrate and a holding substrate, wherein an inorganic thin film layer is provided on the surface of at least one of the substrates, 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 1, wherein the holding substrate is a semiconductor. 5. The composite piezoelectric substrate according to claim 4, wherein the semiconductor is silicon. 6. The composite piezoelectric substrate according to claim 1, wherein the holding substrate is glass. 7. The composite piezoelectric substrate according to claim 1, wherein the holding substrate is a piezoelectric body. 8. The composite piezoelectric substrate according to claim 7, wherein the holding substrate is gallium phosphate. 9. The composite piezoelectric substrate according to claim 2, wherein the inorganic thin film layer is silicon or a silicon compound. 10. The composite piezoelectric substrate according to claim 1, wherein the gallium phosphate substrate has at least a pair of comb-shaped electrodes so that a surface acoustic wave is excited in the gallium phosphate substrate. 11. The composite piezoelectric substrate according to claim 1, wherein the gallium phosphate substrate is provided with at least a pair of opposing electrodes on both sides to excite a bulk wave in the gallium phosphate substrate. 12. A bulk wave is excited on a composite piezoelectric substrate composed of the gallium phosphate substrate and the holding substrate by providing at least a pair of counter electrodes on the surfaces of the gallium phosphate substrate and the holding substrate which are directly bonded to each other. The composite piezoelectric substrate according to claim 1 or 2, which is characterized in that. 13. The composite piezoelectric substrate according to claim 1, wherein a semiconductor is used for the holding substrate, and the semiconductor substrate has an electronic element or an electronic circuit. 14. A gallium phosphate substrate and a holding substrate are made to have extremely clean surfaces to be joined, and further subjected to hydrophilic treatment,
A method for manufacturing a composite piezoelectric substrate, wherein the gallium phosphate substrate is directly bonded to the holding substrate by superposing and heating. 15. An inorganic thin film layer is formed on at least one surface of a gallium phosphate substrate and a holding substrate where bonding is to be performed, and the surface and the other surface of the other substrate to be bonded are extremely cleaned, and further subjected to a hydrophilic treatment. A method for manufacturing a composite piezoelectric substrate, characterized in that the gallium phosphate substrate is directly bonded to the holding substrate via the inorganic thin film layer by superposing and heating. 16. The method for manufacturing a composite piezoelectric substrate according to claim 15, wherein the inorganic thin film layer is silicon or a silicon compound.