JPH07226638A - Composite piezoelectric substrate and its production - Google Patents

Abstract

PURPOSE:To provide a composite piezoeiectric substrate stable against thermal fluctuation and mechanical vibration and suitable for miniaturization, weight reduction and performance improvement by directly joining a piezoelectric substrate and a metallic holding base by the hydrogen bonding or covalent bonding of the hydrogen, oxygen or hydroxyl group of a boundary. CONSTITUTION:The surfaces of substrates 1 and 2 are hydrophilicity impartation processed and immersed in demineralized water, the substrates are joined together and heated in a state where moisture elements and the hydroxyl group are adsorbed, dehydration from a joint boundary is performed and joining is reinforced by the polymerization of the hydroxyl group. Further, a heating temperature is raised, joining strength is reinforced in therms of the covalent bonding and direct joining is performed. In such a manner, since an adhesive material is not used, stabilization against the thermal fluctuation and the mechanical vibration is performed and highly accurate microfabrication such as etching or the like is made possible. For instance, in a piezoelectric device 3, since a metallic holding plate 2 holds the piezoelectric substrate 2 operated as a piezoelectric vibrator, also acts as a wiring terminal and is on the same plane as an electrode 4, the degree of freedom of the wiring of an electric wire 6 is increased and the miniaturization is performed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention provides an electrode directly bonded to a piezoelectric substrate, which is suitable for small size, light weight, high performance of piezoelectric filters, piezoelectric vibrators, etc. and has excellent stability against heat and mechanical vibration. And a composite piezoelectric substrate having the same.

[0002]

2. Description of the Related Art Piezoelectric devices utilizing the piezoelectric effect, represented by crystal oscillators, are widely used in wireless communication devices as resonators of various oscillators and as resonance type filters. 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 changes and mechanical vibrations. 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 for producing the product.

As one method for solving these problems, a method of directly bonding a crystal to silicon is known from Japanese Patent Laid-Open No. 4-283957. These structures have the excellent feature that they are extremely stable against thermal and mechanical changes. The manufacturing method described here is a method in which the surfaces of the quartz and silicon to be joined are cleaned, and then the surfaces to be joined are subjected to a hydrophilic treatment and an overlay heat treatment.

[0004]

In the method of fixing the piezoelectric device using the adhesive, the characteristics are not stable against thermal fluctuation and mechanical vibration. Further, when the holding substrate and the piezoelectric device are directly bonded, it is difficult to directly bond the surface by hydrophilizing the surface of the electrode layer and heat treatment for superposition. Therefore, various restrictions have been added, such as the need for via holes on the wiring when connecting an oscillation circuit or the like to a crystal unit.

[0005]

In order to solve the above-mentioned problems, the piezoelectric substrate and the metal holding substrate are bonded by hydrogen bond or covalent bond of hydrogen or oxygen or hydroxyl group at the interface.

[0006]

With the above structure and manufacturing method, it is stable against thermal fluctuations and mechanical vibrations, has a high degree of freedom in the electric field applying electrode structure and the electrode drawing structure, and is easy to hermetically seal. Is obtained.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The structure of a composite piezoelectric substrate according to an embodiment of the present invention and its manufacturing method will be described below with reference to the drawings.

(Embodiment 1) FIG. 1 shows a sectional structure of a first embodiment of the structure of the composite piezoelectric substrate of the present invention.

In FIG. 1, 1 is a metal holding substrate, 2 is a piezoelectric substrate, which is directly bonded to the metal holding substrate 1 by hydrogen bond or covalent bond by hydrogen, oxygen or hydroxyl group.

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, many water molecules and hydroxyl groups are adsorbed on the surface of the substrate. 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 by hydrogen bonding or covalent bonding via water molecules, hydroxyl groups and the like. This state is shown in FIG. The figure schematically shows a very typical case.

When heating is performed in this state, dehydration gradually occurs from the bonding interface and hydrogen is released, so that
Polymerization of the hydroxyl groups proceeds and the bond is strengthened. The strengthening is performed by increasing the ratio of covalent bonds centered on oxygen from hydrogen bonds due to water molecules and hydroxyl groups. Figure 2
It shows in (b). This also represents a typical example. In this state, the heat treatment temperature is 300-500 ° C.
Are often found in.

[0012] When the temperature is further raised, hydrogen is further released, and the bond through oxygen becomes the 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 can be easily obtained even by heat treatment at 300 ° C. for about 1 hour, which is sufficiently practical.

According to TEM (transmission electron microscope) observation, the bonding interface is bonded in atomic order.

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, in the case where the adhesive is used for adhesion, an adhesive layer always remains at the adhesion interface. This is normally several μm to several 100 μm. In this embodiment, bonding can be performed with a thickness of several molecular layers to several atomic layers. This is confirmed by TEM observation. 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 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 piezoelectric 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 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. Piezoelectric devices that use the characteristics of the substrate surface, such as surface acoustic wave devices, whose characteristics change in the heating process such as soldering or solder reflow (about 230 ° C) that occurs during manufacturing, or gas is generated. Adversely affect. 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 metal holding substrate and the piezoelectric substrate is not limited like the epitaxial growth, it is possible to combine the materials having different elements, crystal structures, crystal orientations and lattice constants. Therefore, 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 interposed, the bonding performed by such a method is hereinafter referred to as direct bonding. I will decide.

As the piezoelectric substrate, it is possible to use a piezoelectric substrate such as quartz, lithium niobate, lithium tantalate, lithium borate, lead zirconate titanate (PZT) or PLZT containing PZT as a main component and lanthanum added thereto.

As the metal holding substrate, a metal substrate made of iron, copper, tantalum, tungsten or the like, or an alloy substrate made of stainless steel, aluminum bronze, brass or the like can be used.

The composite piezoelectric substrate having such a structure is used for various purposes. Specifically, for example, there is a piezoelectric device as shown in FIG.

In FIG. 3, reference numerals 1 and 2 are a metal holding substrate and a piezoelectric substrate, and 3 is a metal holding substrate 1 as in FIG.
4 is an electrode formed on the lower surface of the piezoelectric substrate 2 exposed in the through hole 3, and 5 is provided on the piezoelectric substrate 2 facing the electrode 4. It is an electrode. Reference numeral 6 is a wiring for electrically connecting the electrode 4 and the metal holding substrate 1.

In this structure, the piezoelectric substrate 2 operates as a piezoelectric vibrator, and the metal holding substrate 1 serves as one electric wiring portion or an electric terminal for holding and electrically connecting to the excitation electrode 4. Play a role.

In such a structure, since the electrode 4 which is the lower electrode for excitation of the piezoelectric device and the holding substrate 1 which functions as wiring and electric terminals are present on the same plane, a via hole or the like is formed in the holding substrate. This is advantageous because the wiring is not required to be routed below the holding substrate, and the degree of freedom in wiring is increased.

FIG. 4 shows another configuration example.
In FIG. 4, 1 and 2 are a metal holding substrate and a piezoelectric substrate as in FIG. Reference numeral 5 denotes one electrode provided on the piezoelectric substrate 2 for applying an electric field to the piezoelectric substrate 2, and reference numeral 7 denotes a thin portion obtained by thinning a part of the metal holding substrate 1 by etching. In this case, the thin portion 7 serves as the other electrode for applying an electric field to the piezoelectric substrate 2.

In this structure, the piezoelectric substrate 2 operates as a piezoelectric vibrator, and the metal holding substrate 1 plays a role as one electrode for holding and exciting the metal holding substrate 1.

In such a structure, the thin portion 7 of the metal holding substrate 1 which plays the role of the lower excitation electrode of the piezoelectric device.
Since the holding substrate 1 that plays the role of wiring and electric terminals exists on the same plane, it is not necessary to provide a via hole or the like in the holding base and route the wiring from below the holding base. It is more advantageous.

In both the embodiments shown in FIGS. 3 and 4, when the vibrating portion is hermetically sealed, only the upper portion of the piezoelectric substrate 2 needs to be hermetically sealed because the wiring portions are all on the same plane. Therefore, it is advantageous in terms of mounting.

The piezoelectric substrate 2 and the metal holding substrate 1 are directly bonded by a hydrogen bond or a covalent bond, are extremely stable against thermal changes and mechanical vibrations, and have excellent long-term reliability. Is obtained.

Further, since no adhesive in the ordinary sense is used for the bonding interface, the parallelism between the upper and lower surfaces of the substrate does not deteriorate due to the adhesive at the interface, so that the surface acoustic wave ( When forming a (SAW) filter, high precision fine processing such as photolithography is possible.

(Embodiment 2) FIG. 5 shows a first embodiment of a preferred structure of the composite piezoelectric substrate of the present invention. In FIG.
Reference numeral 11 is a metal holding substrate, specifically, stainless steel or aluminum bronze (for example, 90% Cu, 5% Al, 5%
% Alloy) or copper or brass, 12 is lithium tantalate or lithium niobate, and the metal holding substrate 11 and the lithium tantalate or lithium niobate substrate 12 are bonded by the direct bonding described in Example 1. ing.

In the case of direct bonding, if the thermal expansion coefficient of the substrates to be bonded is matched, the heat treatment temperature can be increased, so that the bonding strength is improved and a highly reliable product can be obtained.
Further, the yield of direct bonding is improved, which is advantageous in production.

The coefficient of thermal expansion of lithium tantalate or lithium niobate differs depending on the crystal orientation, and in the direction with a large coefficient of thermal expansion, the a-axis direction, lithium tantalate is 16 × 10.
^-6 , and lithium niobate is about 15 x 10 ^-6 .

The coefficient of thermal expansion of stainless steel is 15 × 10 ⁻⁶ ,
Aluminum bronze has a coefficient of thermal expansion of 16x10 ^-6 , copper has a coefficient of thermal expansion of 17x10 ^-6 brass has a coefficient of thermal expansion of 18x10.
^-6 .

The coefficient of thermal expansion of lithium tantalate or lithium niobate varies depending on the crystal orientation, but by matching the value of the larger coefficient of thermal expansion with the coefficient of thermal expansion of the holding substrate,
With these combinations, a good direct bond is obtained,
In either case, good direct bonding can be obtained.

(Embodiment 3) A second embodiment of a preferred structure of the composite piezoelectric substrate of the present invention is shown in FIG. In FIG.
Reference numeral 21 is a metal holding substrate, specifically iron or stainless steel or copper, 22 is quartz or lithium borate, and the metal holding substrate 21 and quartz or lithium borate 22 are bonded by the direct bonding described in the first embodiment. There is.

The coefficient of thermal expansion of quartz and lithium borate is
The crystal size is 12-
14 × 10 ⁻⁶ , lithium borate is about 11 × 10 ⁻⁶ , iron has a thermal expansion coefficient of 12 × 10 ⁻⁶ , and stainless steel has a thermal expansion coefficient of 15 × 10 ⁻⁶ , so that a good direct bonding can be obtained.

As a result, the thermal expansion coefficients of the substrates are substantially the same, so that the bonding strength is improved and a highly reliable product is obtained. Further, the yield of direct bonding is improved, which is advantageous in production.

(Embodiment 4) A third embodiment of a preferred structure of the composite piezoelectric substrate of the present invention is shown in FIG. In FIG.
Reference numeral 31 is a metal holding substrate, specifically tungsten or tantalum, 32 is PZT or PLZT, and the metal holding substrate 31 and PZT or PLZT 32 are joined by the direct joining described in the first embodiment.

The thermal expansion coefficient of PZT or PLZT is 3-
It is about 6x10 ^-6, the coefficient of thermal expansion of tungsten 4.5 × 10 ^-6, the coefficient of thermal expansion of tantalum 6.3X10 ^-6
Therefore, good direct bonding can be obtained.

As a result, the thermal expansion coefficients of the substrates are substantially the same, so that the bonding strength is improved and a highly reliable product is obtained. Further, the yield of direct bonding is improved, which is advantageous in production.

(Embodiment 5) A fourth embodiment of the preferred structure of the composite piezoelectric substrate of the present invention is shown in FIG. In FIG.
Reference numeral 11 is a metal holding substrate, specifically, stainless steel or aluminum bronze (for example, 90% Cu, 5% Al, 5%
% Alloy) or copper or brass, 12 is lithium tantalate or lithium niobate. An inorganic thin film layer 8 made of silicon or a silicon compound is formed on the surface of the metal holding substrate 11 by vacuum vapor deposition, sputtering, chemical vapor deposition, or the like. The thickness of the inorganic thin film layer 8 is about 0.1-10 μm.

The metal holding substrate 11 and the lithium tantalate or lithium niobate substrate 12 are bonded at the interface between the inorganic thin film layer 8 and the piezoelectric substrate 12 by the direct bonding described in the first embodiment.

Even in such a structure, good direct bonding can be obtained as in the second embodiment. By providing the inorganic thin film layer at the bonding interface, even if some dust is present at the interface during direct bonding, it is taken into the inorganic thin film layer, so that the bonding strength is improved and the manufacturing yield is improved. Also, by using silicon or a silicon compound for the inorganic thin film layer, good direct bonding can be obtained because silicon easily forms a covalent bond in the process of strengthening the bonding strength during direct bonding. Silicon may be polycrystalline or amorphous, and as the silicon compound,
Examples thereof include silicon oxide and silicon nitride.

The effect of the coefficient of thermal expansion on the direct bonding is the same as in Example 2. (Embodiment 6) A fifth embodiment of the preferred structure of the composite piezoelectric substrate of the present invention is shown in FIG. In FIG. 9, 21 is a metal holding substrate, specifically iron or stainless steel. An inorganic thin film layer 8 made of silicon or a silicon compound is formed on the surface of the metal holding substrate 21 by vacuum vapor deposition, sputtering, chemical vapor deposition, or the like, as in the fifth embodiment.
The thickness of the inorganic thin film layer 8 is about 0.1-10 μm. Two
2 is quartz or lithium borate. The metal holding substrate 21 and the crystal or lithium borate substrate 22 are bonded at the interface between the inorganic thin film layer 8 and the piezoelectric substrate 22 by the direct bonding described in the first embodiment.

Even in such a structure, good direct bonding can be obtained as in the third embodiment. The effect of providing the inorganic thin film layer at the bonding interface is the same as in Example 5. Examples of the silicon compound include silicon oxide and silicon nitride. The effect of the coefficient of thermal expansion on the direct bonding is the same as in Example 3.

(Embodiment 7) A sixth embodiment of the preferred structure of the composite piezoelectric substrate of the present invention is shown in FIG. In FIG. 10, reference numeral 31 is a metal holding substrate, specifically, tungsten or tantalum. The inorganic thin film layer 8 made of silicon or a silicon compound is formed on the surface of the metal holding substrate 31 by vacuum vapor deposition, sputtering, chemical vapor deposition, or the like, as in the fifth embodiment. The thickness of the inorganic thin film layer 8 is 0.1
It is about −10 μm. 32 is PZT or PLZT
Is. The metal holding substrate 31 and the PZT or PLZT substrate 32 are bonded at the interface between the inorganic thin film layer 8 and the piezoelectric substrate 32 by the direct bonding described in the first embodiment.

Even in such a structure, good direct bonding can be obtained as in the fourth embodiment. The effect of providing the inorganic thin film layer at the bonding interface is the same as in Example 5. Examples of the silicon compound include silicon oxide and silicon nitride. The effect of the coefficient of thermal expansion on the direct bonding is the same as in Example 4.

(Embodiment 8) A first example of a method of manufacturing a composite piezoelectric substrate having the structure of this embodiment will be described. The piezoelectric substrate to be bonded is flattened by polishing and further polished until the surface becomes a mirror surface. Next, the surfaces to be joined are made extremely clean with detergent and various solvents. The metal holding substrate side surface is similarly treated. Next, the surfaces of the planned joining portions are made hydrophilic.
Specifically, for example, an ammonia-hydrogen peroxide aqueous solution is used as the hydrophilic treatment liquid, and after dipping in this solution, pure water cleaning may be performed.

After that, the substrates are superposed on each other and heated in a clean atmosphere, so that the van der Waals forces of hydrogen, oxygen, and hydroxyl groups, which are water constituents adsorbed on the surface, are applied to the interface. Substrates are joined together and strengthened by heating. At the beginning of joining, hydrogen adsorbed on the surface,
It is carried out by hydrogen bond or covalent bond due to a hydroxyl group.

When the heat treatment is carried out at a temperature of about 100 ° C. to 1000 ° C. for several minutes to several hours, hydrogen is released from the interface to form a covalent bond in which oxygen participates and strengthen the bond.

As the piezoelectric substrate, quartz, lithium niobate, lithium tantalate, lithium borate and PZ are used.
T, PLZT, etc., and stainless steel, aluminum bronze, copper, brass, tungsten, tantalum substrate, etc. as the metal holding substrate, the above-mentioned manufacturing method allows the bonding to be thermally or mechanically performed by hydrogen bonding or covalent bonding. A stable and airtight joint is obtained. Since no adhesive is used at the interface, it is directly bonded.

Since the composite piezoelectric substrate manufactured by such a method is bonded at the atomic level, extremely stable characteristics with respect to thermal changes and mechanical fluctuations were obtained. Specifically, when heat-treated at 400 ° C. or higher, 32
With respect to the soldering treatment at 0 ° C., there is almost no change in the initial characteristics.

(Embodiment 9) A second example of a method of manufacturing the composite piezoelectric substrate having the structure of this embodiment will be described.

As in Example 8, the metal holding substrates to be joined are flattened by polishing and further polished until the surface becomes a mirror surface. Then, a silicon or silicon compound film is vacuum-deposited, sputtered, on the surface of the bonding side,
Formed by chemical vapor deposition or the like. What is the film thickness? 0.1
It is about −10 μm.

The surface on the piezoelectric substrate side is polished in the same manner as in Example 8. Thereafter, in the same manner as in Example 8, the surfaces to be joined of both substrates are made extremely clean with a detergent and various solvents to make them hydrophilic. Specifically, as a hydrophilic treatment liquid,
For example, an ammonia-hydrogen peroxide aqueous solution may be used, and after soaking in this solution, pure water may be washed.

After that, the substrates are superposed on each other and heated in a clean atmosphere, so that the van der Waals forces of hydrogen, oxygen, and hydroxyl groups, which are water constituents adsorbed on the surface, are applied at the interface. Substrates are joined together and strengthened by heating. At the beginning of joining, hydrogen adsorbed on the surface,
It is carried out by hydrogen bond or covalent bond due to a hydroxyl group.

When the heat treatment is carried out at a temperature of about 100 ° C. to 1000 ° C. for several minutes to several hours, hydrogen is released from the interface to form a covalent bond in which oxygen participates and strengthen the bond.

By using a silicon or silicon compound layer as the inorganic thin film layer, silicon in the film promotes covalent bonding, and bonding is easier than in the case of Example 8.

As the piezoelectric substrate, quartz, lithium niobate, lithium tantalate, lithium borate and PZ are used.
T, PLZT, etc., and stainless steel, aluminum bronze, copper, brass, tungsten, tantalum substrate, etc. as the metal holding substrate, the above-mentioned manufacturing method allows the bonding to be thermally or mechanically performed by hydrogen bonding or covalent bonding. A stable and airtight joint is obtained. Since no adhesive is used at the interface, it is directly bonded.

Since the composite piezoelectric substrate manufactured by such a method is bonded at the atomic level, extremely stable characteristics were obtained against thermal changes and mechanical fluctuations. Specifically, when heat-treated at 400 ° C. or higher, 32
With respect to the soldering treatment at 0 ° C., there is almost no change in the initial characteristics.

[0066]

The present invention is stable against thermal fluctuations and mechanical vibrations due to the above-described structure and manufacturing method.

Further, the present invention provides a piezoelectric device which has a high degree of freedom in the electrode structure for applying a voltage to the piezoelectric device and the electrode lead-out structure, and which is easily hermetically sealed.

In addition, the degree of freedom in material combination is large, and the degree of freedom in selecting crystal orientation is large.

[Brief description of drawings]

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

FIG. 2 is an explanatory principle diagram of direct joining according to the present invention.

FIG. 3 is a configuration diagram of application parts according to the first embodiment of the present invention.

FIG. 4 is a configuration diagram of application parts according to the first embodiment of the present invention.

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

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

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

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

FIG. 9 is a structural diagram of a fifth preferred embodiment of the present invention.

FIG. 10 is a structural diagram of a sixth preferred embodiment of the present invention.

[Explanation of symbols]

1 Metal Holding Substrate 2 Piezoelectric Substrate 3 Through Hole of Metal Holding Substrate 4, 5 Electrode 6 Wiring 7 Metal Holding Substrate Thin Section 8 Inorganic Thin Film Layer 11 Stainless Steel or Aluminum Bronze or Copper or Brass 12 Lithium Tantalate or Lithium Niobate 21 Iron Or stainless steel or copper 22 quartz or lithium borate 31 tungsten or tantalum 32 PZT or PLZT

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication location H03H 9/17 F 7719-5J G 7719-5J (72) Inventor Masato Sugimoto Daimon Kadoma, Kadoma City, Osaka Prefecture 1006 Matsushita Electric Industrial Co., Ltd.

Claims (10)

[Claims] 1. A composite piezoelectric substrate, wherein the piezoelectric substrate and the metal holding substrate are directly bonded to each other by hydrogen or oxygen at the interface or hydrogen bond or covalent bond of a hydroxyl group. 2. A piezoelectric substrate and a metal holding substrate, wherein at least one of the substrates has an inorganic thin film layer on the surface thereof.
A composite piezoelectric substrate, wherein the substrates are directly bonded to each other by hydrogen bonding or covalent bonding of hydrogen or oxygen or a hydroxyl group at the interface between the inorganic thin film layer and the other substrate. 3. The piezoelectric substrate is quartz, lithium niobate, lithium tantalate, or lithium borate,
3. The composite piezoelectric substrate according to claim 1, wherein the metal holding substrate is stainless steel, aluminum bronze, copper, brass, or iron. 4. The composite piezoelectric substrate according to claim 1, wherein the piezoelectric substrate contains lead zircon titanate as a main component, and the metal holding substrate is tungsten or tantalum. 5. The composite piezoelectric substrate according to claim 2, wherein the inorganic thin film layer is silicon or a silicon compound. 6. The composite piezoelectric substrate according to claim 5, wherein the silicon compound is silicon oxide or silicon nitride. 7. The composite piezoelectric substrate according to claim 1, wherein the piezoelectric substrate and the metal holding substrate have substantially the same coefficient of thermal expansion. 8. The metal holding substrate serves as at least one electrode for applying an electric field to the piezoelectric substrate, or is electrically connected to an electrode provided on the piezoelectric substrate. Item 3. The composite piezoelectric substrate according to Item 1 or 2. 9. The piezoelectric substrate and the metal holding substrate are directly joined to the metal holding substrate by extremely cleaning the surfaces to be joined of the piezoelectric substrate and the metal holding substrate, further hydrophilizing them, and superposing them on each other and then heat treating them. And a method for manufacturing a composite piezoelectric substrate. 10. An inorganic thin film layer is formed on at least one surface of a piezoelectric substrate and a metal holding substrate where bonding is to be performed, and the surface and the surface of the other substrate to be bonded are made extremely clean and further subjected to a hydrophilic treatment. Then, the piezoelectric substrate is directly bonded to the metal holding substrate through the inorganic thin film layer by superposing and heat-treating the composite piezoelectric substrate.