JPH0927645A - Manufacture of composite substrate and piezoelectric element using that - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent substrates from being broken at the time of heating of the substrates due to the difference of the thermal expansion coefficients of the substrates by a method wherein a first heat treatment is conducted at a temperature lower than a temperature to cause fixing of the substrates and after a piezoelectric composite substrate consisting of the substrates is split into two or more small pieces in a state that the first substrate is superposed on the second substrate, a second heat treatment is conducted at a temperature to make the fixing generate. SOLUTION: The surfaces of a quartz 2 crystal substrate 1 and a silicon substrate 2 are mirror-polished and the surface layers of the substrates 1 and 2 are removed. Then, the surfaces of the substrates 1 and 2 are subjected to hydrophilic treatment and moreover are cleaned fully with pure water. Then, the two substrates are superposed and the substrate 1 is attracted to the substrate 2 with a van der Waals force. Then, a first heat treatment is conducted for removing excess water or the like which adheres to the substrates. After the first heat treatment, a piezoelectric composite substrate consisting of the substrates is split into small pieces and the junction area per one sheet of the substrate is made small, whereby small a mount of water constituent molecules and gas left on a junction interface 22 between the substrates 1 and 2 though the amount of the molecules and the gas are removed during a second heat treatment. Then, the second heat treatment is conducted so as to cause fixing of the substrates 1 and 2 using ionic bonds or covalent bonds.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a piezoelectric composite substrate constituted by directly bonding a piezoelectric body and another substrate, and a structure of the piezoelectric composite substrate. Piezoelectric materials are widely used as constituent materials for devices used in communication equipment, information equipment and the like.

[0002]

2. Description of the Related Art In recent years, various piezoelectric materials have been used as constituent materials for communication devices and the like. In particular, single crystal piezoelectrics such as quartz, lithium niobate, and lithium tantalate are widely used in bulk wave devices such as vibrators and surface acoustic wave devices such as filters.

Direct bonding technology and anodic bonding technology are available as technologies for manufacturing such a communication device in a small size and with good mass productivity. The direct bonding technology is a technology for fixing two substrates, regardless of the same type or different types, by covalent bond or ionic bond between atoms constituting the substrate surface, without interposing an intermediate adhesive layer such as an adhesive. is there. Direct bonding is performed by superposing two mirror-finished substrates and heat-treating them. Anodic bonding is performed by stacking two mirror-finished substrates and heating them while applying a voltage to the substrate interface. The bonding strength, which is an index of how firmly the two substrates are fixed to each other, depends on the heat treatment temperature at the time of bonding. Generally, the higher the heat treatment temperature, the higher the bonding strength.

However, if the heat treatment temperature is increased,
The following adverse effects occur. That is, when substrates having different coefficients of thermal expansion are directly bonded or anodically bonded, there arises a problem that the substrates are damaged during heating or the stacked substrates are separated due to the difference in the coefficient of thermal expansion of the substrates. The same applies to the manufactured piezoelectric composite substrate and the piezoelectric device.

Specifically, the average coefficient of thermal expansion at 25 to 300 ° C. is 3.4 × 10 ⁻⁶ / ° C. in the case of silicon, whereas it is 15.2 × 10 ^{− in} the case of quartz. ⁶ / ° C., 18.3 × 10 ⁻⁶ / ° C. for lithium niobate, and 19.9 × 10 ⁻⁶ / ° C. for lithium tantalate. Here, the piezoelectric material indicates the average coefficient of thermal expansion of the crystal in the X-axis direction. That is, for example, the coefficient of thermal expansion of the crystal in the X-axis direction is about 5 times that of silicon, and the difference in the coefficient of thermal expansion causes a problem such as damage to the substrate.

Japanese Unexamined Patent Publication (Kokai) No. 5-327383 describes the relationship between the thickness of the crystal substrate and the temperature at which the substrate is damaged when the crystal substrate and the semiconductor substrate are directly bonded. That is, it has been empirically reported that the thinner the quartz substrate is, the less the stress applied to the bonding portion is, and the higher the temperature at which the substrate is broken. For example, in a silicon substrate of a certain size, when the thickness of the quartz substrate is 80 μm, the temperature at which the substrate breaks is 350 ° C., and when the thickness of the quartz substrate is 40 μm, the temperature at which the substrate breaks is 450 ° C. Things have been reported. Note that these values are temperatures that change depending on the size and shape of the substrate.

Therefore, the heat treatment temperature for direct bonding is
It is necessary to treat at a temperature lower than the temperature at which this damage occurs, but with only a single heat treatment step,
The following adverse effects occur.

That is, in the case of direct bonding at the interface between the substrates via the water constituent molecules in the initial superposition, the water constituent molecules are present at the bonding interface in the initial bonding at low temperature. As the heat treatment temperature rises, most of the water-constituting molecules are removed, but there are some that are left behind at the joint interface because the periphery is blocked by the joint. This portion becomes a void that is not joined. The void portion is not joined, and the strongly bonded joint portion and void portion are present at the joint interface, resulting in non-uniform distribution of thermal stress. As a result, there is a problem that the substrates are damaged and the stacked substrates are separated. Even when other joining methods are used, voids are generated due to the gas existing at the superposition interface from the initial superposition and the gas generated during the heat treatment, and the above problems occur.

The above is the problem that occurs during the substrate bonding process, but the same problem may occur in the thermal process after the bonding process. For example, thermal stress due to heating during solder reflow causes voids, which may cause damage or peeling of the substrate.

In order to solve such a problem, in the prior art, a method of performing direct bonding by dividing the heat treatment temperature into two steps and thinning the substrate is used. That is, after the temporary bonding of the substrate at a relatively low first temperature, the substrate is thinned by a mechanical method or chemical etching, and then a strong direct bonding is realized at a relatively high second temperature. Kaihei 5-327383,
JP-A-4-286310 and JP-A-3-97215
Issue). That is, after the substrates are temporarily bonded by heat treatment at a first temperature that does not cause a problem such as breakage of the substrates, the substrates are thinned by polishing or the like, and the substrates are heated at a high temperature such that sufficient bonding strength is obtained. It is a method of heat treatment.

[0011]

However, even in the above method, it is difficult to completely prevent the problems such as the damage and the void, and there is a problem that the yield is lowered in the actual manufacturing process. there were. That is, it is difficult to completely remove problems such as damage due to stress generated in the central part of the substrate, because the removal of water and the like at the bonding interface in the central part of the substrate becomes insufficient as compared with the peripheral part of the substrate. . This problem becomes a big problem in expanding the substrate size in order to reduce the manufacturing cost.

Even when problems such as breakage and peeling of the substrate can be avoided, the stress applied to the substrate due to insufficient bonding may adversely affect the characteristics of the element formed on the composite substrate. Specifically, in a piezoelectric element such as a piezoelectric vibrator or a piezoelectric filter, the temperature characteristics of frequency may be affected. That is, the thermal stress applied to the substrate due to the temperature change is further exaggerated.

Further, the stress due to the difference in thermal expansion coefficient may cause a change in crystal structure. That is, when various crystal structures may exist even if they have the same molecular structure, the crystal structure may be changed by applying pressure or temperature. For example, 57
When heated above 3 ° C., the crystal undergoes a transition from the α phase to the β phase. Here, the phase transition may have various forms other than the α-β transition. An example of this is the Dauphine twin-type phase transition that occurs when stress is applied to the crystal. ), vol.3
1, p171 (1977)). When an element is formed on a substrate in which these phase transitions have once occurred, as a result, expected characteristics of the element cannot be obtained. Specifically, in the case of AT-cut quartz, phase transition causes a large dependence of frequency on temperature, making it difficult to obtain a piezoelectric element such as a quartz oscillator or a quartz filter having stable operation.

In order to solve the above-mentioned conventional problems, the present invention prevents the substrates from being damaged during heating due to the difference in the thermal expansion coefficient of the substrates when directly bonding or anodic bonding the substrates having different thermal expansion coefficients. And a method for manufacturing a composite substrate, which prevents the stacked substrates from peeling off, prevents the crystal structure of the substrates from changing due to thermal stress, and prevents the piezoelectric composite substrate from being distorted due to residual stress. It is an object to provide a piezoelectric element using the same.

Further, according to the present invention, a piezoelectric device using a piezoelectric composite substrate composed of substrates having different coefficients of thermal expansion has a substrate damage, peeling, a change in crystal structure, characteristic deterioration due to residual stress, and ambient temperature. An object of the present invention is to provide a method for manufacturing a composite substrate that does not cause characteristic deterioration due to stress generated by changes and a piezoelectric element using the same.

[0016]

In order to achieve the above object, the first method for producing a composite substrate of the present invention is
It is a manufacturing method of a composite substrate including steps F to F. A. B. mirror finishing at least one main surface of the first substrate; At least one main surface of the second substrate having a different coefficient of thermal expansion from the first substrate is mirror-finished; Superimposing the main surface of the first substrate and the main surface of the second substrate on each other, D. Then, a first heat treatment is performed at a temperature lower than a temperature at which the first substrate and the second substrate are fixed to each other, and E. After that, the first substrate and the second substrate are overlapped and divided into at least two pieces, and F. The small piece is subjected to a second heat treatment at a temperature at which the first substrate and the second substrate are fixed to each other.

The "one main surface" in the step A of the first method is a surface to be superposed on the second substrate in the step C. Further, "mirror finish" means that the center line average roughness Ra of the substrate is finished to 10 nm or less.

The "coefficient of thermal expansion" in the step B is
For example, silicon is 3.4 × 10^-6 / ° C, crystal is x-axis direction
At 15.2 × 10^-6/ ° C, lithium niobate in the x-axis direction
18.3 x 10^-6/ ° C, lithium tantalate in the x-axis direction
19.9 x 10^-6/ ° C, change the material composition of the glass substrate.
Although various thermal expansion coefficients can be achieved with and, the main ones are 3 x 10
^-6/ ° C ~ 15 x 10^-6It has a coefficient of thermal expansion of about / ° C. these
With the combination of materials, the maximum coefficient of thermal expansion is about 6 times.
There is a difference. In addition, the piezoelectric substrate is an anisotropic material and has a coefficient of thermal expansion.
Has anisotropy. For example, in AT-cut crystal, x-axis thermal expansion
The stretch rate is 15.2 × 10 ^-6/ ° C, but z'perpendicular to the x-axis
Coefficient of thermal expansion in the axial direction (axis tilted by 35 ° 22 'from the z-axis)
Is 11 × 10^-6/ ° C, isotropic for glass, semiconductors, etc.
Matching the coefficient of thermal expansion perfectly with the material with the coefficient of thermal expansion of
Can not. Therefore, in the present invention,
“Different” means that there is a difference in coefficient of thermal expansion of 10% or more.
Much more preferably, like the example of AT-cut quartz
And the coefficient of thermal expansion on the x-axis: 15.2 × 10^-6/ ° C and z'axis
Thermal expansion coefficient: 11 × 10^-6/ The difference in coefficient of thermal expansion is about ℃ or more
You.

The "first heat treatment temperature" in the step D is a temperature at which adsorption by Van der Waals force is maintained and ionic bonds or covalent bonds are not yet sufficiently generated, and the removal of water and the like. The temperature that can be achieved. Specifically, the bonded state of the substrates after the first heat treatment is a state in which the two substrates can still be separated by a mechanical method.

In the first method, it is preferable that the first substrate is made of a piezoelectric material. Piezoelectric bodies are widely used as constituent materials for elements such as filters and oscillators used in communication equipment. By using the piezoelectric substrate as the first substrate, the composite substrate used for manufacturing these elements can be used without impairing the original characteristics of the piezoelectric body.
In addition, it can be manufactured with high yield.

In the first method, it is preferable that the second substrate is made of at least one substance selected from semiconductor and glass. (Please explain why semiconductor or glass is preferable.
If a semiconductor is used as the substrate of (1), a composite substrate used for manufacturing an oscillator or the like in which a circuit and a piezoelectric element are integrated can be manufactured with high yield without impairing the original characteristics of the piezoelectric body.

In the first method, it is preferable that the second substrate is made of a piezoelectric material. A composite substrate used for manufacturing an oscillator or the like in which a plurality of piezoelectric elements are integrated can be manufactured with high yield without impairing the original characteristics of the piezoelectric body.

In the first method, it is preferable that the first substrate is a quartz substrate and the temperature of the second heat treatment is lower than the temperature at which the quartz substrate undergoes a phase transition. The “phase transition” here means that the crystal structure changes at high temperature and high pressure. When substrates with different thermal expansion coefficients are joined and heat treated, due to the difference in thermal expansion coefficient,
The substrate will be pressurized. For example, if a glass substrate of about 8 × 10 ^-6 / ° C and an AT-cut quartz crystal substrate are joined and heat-treated, the AT heat-treatment temperature is 300 ° C or less even at a low temperature.
The cut quartz substrate undergoes a phase transition, and the stability of frequency with respect to temperature decreases. Therefore, it is preferable to perform the second heat treatment at a temperature lower than the temperature at which the crystal substrate undergoes a phase transition.

Next, the second method for manufacturing a composite substrate of the present invention is a method for manufacturing a composite substrate including the following steps a to f. a. Mirror finishing at least one main surface of the first substrate; b. At least one main surface of the second substrate having a different coefficient of thermal expansion from the first substrate is mirror-finished; c. Superimposing the main surface of the first substrate and the main surface of the second substrate on each other, d. Then, a first heat treatment is performed at a temperature lower than a temperature at which the first substrate and the second substrate are fixed to each other, and e. Then, a portion of the second substrate is removed until it reaches the first substrate, f. The second heat treatment is performed at a temperature at which the first substrate and the second substrate are fixed to each other.

In the above description, "removing part of the substrate until reaching the first substrate" in the step e means removing part of the second substrate until reaching the first substrate. That is, removing the second substrate portion on which the piezoelectric element is formed or which is to be formed with the first substrate portion on which the piezoelectric element is to be formed. It is sufficient that the second substrate has at least a portion overlapping with the first substrate removed, and the second substrate may not be penetrated.

In the second method, the first method is used.
It is preferable that the substrate is made of a piezoelectric material. The second
In the second method, it is preferable that the second substrate is any material selected from semiconductors and glass.

In the second method, it is preferable that the second substrate is made of a piezoelectric material. In the second method, the first substrate is a substrate that undergoes a phase transition, and the temperature of the second heat treatment is a temperature other than a bonding portion between the first substrate and the second substrate. First
It is preferable that the temperature is lower than the temperature at which the substrate of (1) undergoes phase transition.

In the second method, it is preferable that the first substrate is made of quartz. Further, in the second method, it is preferable that the second substrate is a silicon substrate on which an integrated circuit is formed.

In the second method, the overlapped portion of the first substrate and the second substrate is a substantially rectangular shape, and the first substrate in the long side direction of the rectangle. The coefficient of thermal expansion differs from the coefficient of thermal expansion in the short side direction of the rectangle of the first substrate, and the difference between the coefficient of thermal expansion in the long side direction of the first substrate and the coefficient of thermal expansion of the second substrate. Is preferably smaller than the difference between the coefficient of thermal expansion of the first substrate in the short side direction and the coefficient of thermal expansion of the second substrate.

Next, the first piezoelectric element of the present invention is a piezoelectric substrate made of a piezoelectric material on which electrodes for excitation are formed, and a covalent bond or an ionic bond to a joint provided on a part of the piezoelectric substrate. In the piezoelectric element provided with the holding substrate bonded by, the crystal structure of the piezoelectric substrate at the portion where the excitation electrode is formed is different from the crystal structure of the piezoelectric substrate at the bonding portion. In the above,
"The crystal structure of the piezoelectric substrate is different from the crystal structure of the piezoelectric substrate at the joint portion" means that the joint portion with the holding substrate undergoes phase transition and the piezoelectric substrate at the portion where the excitation electrode is formed undergoes phase transition. It does not mean that it has the original characteristics of quartz. The joint has undergone a phase transition, is firmly held, and may have a function of relieving residual stress due to the joint.

In the piezoelectric element, the piezoelectric substrate is preferably a quartz substrate. Further, in the piezoelectric element, it is preferable that a width of the piezoelectric substrate in the long side direction of the rectangle around the bonding portion is smaller than a length of the long side of the rectangle. When the coefficient of thermal expansion differs by 2 times or more, the length ratio is preferably 2 times or more.

Further, in the piezoelectric element, it is preferable that the holding substrate is a silicon substrate on which an integrated circuit is formed. Further, in the piezoelectric element, it is preferable that the holding substrate is made of either semiconductor or glass.

Next, a second piezoelectric element of the present invention is bonded to a piezoelectric substrate made of a piezoelectric material on which an electrode is formed and a bonding portion provided on a part of the piezoelectric substrate by a covalent bond or an ionic bond. In the piezoelectric element having a holding substrate, the joint portion is substantially rectangular, and the coefficient of thermal expansion in the long side direction of the rectangle of the first substrate is the rectangle of the rectangle of the first substrate. Unlike the coefficient of thermal expansion in the direction of the short side, the difference between the coefficient of thermal expansion in the direction of the long side of the first substrate and the coefficient of thermal expansion of the second substrate is the heat in the direction of the short side of the first substrate. It is smaller than the difference between the coefficient of expansion and the coefficient of thermal expansion of the second substrate.

In the piezoelectric element, the piezoelectric substrate is preferably a quartz substrate. Further, in the piezoelectric element, it is preferable that a width of the piezoelectric substrate in the long side direction of the rectangle around the bonding portion is smaller than a length of the long side of the rectangle.

Further, in the piezoelectric element, it is preferable that the substrate is a silicon substrate on which an integrated circuit is formed. Further, in the piezoelectric element, the holding substrate is preferably made of at least one substance selected from semiconductor and glass.

With the above-described structure, the present invention can significantly reduce the stress in the joint portion even when the substrates having different thermal expansion coefficients are directly joined, solve the problems such as the damage of the substrate, and mass-produce the composite substrate. Can be increased. Further, since the stress on the substrate is reduced, problems such as deterioration of characteristics due to stress of elements formed on the substrate can be solved. further,
In the quartz substrate, the problem of stress-induced phase transition can be solved.

[0037]

The present invention will be described more specifically with reference to the following examples. (Embodiment 1) A first embodiment of the manufacturing method of the present invention will be described with reference to FIG. FIG. 1A is a sectional view of a substrate to be used, where 1 is a quartz substrate and 2 is a silicon substrate. The size of the silicon substrate 2 is, for example, length: 12 m × width: 12 mm.
The thickness was 450 μm, and the size of the crystal substrate 1 was, for example, vertical: 12 mm × horizontal: 12 mm, and the thickness was 80 μm.

First, the surfaces of the quartz substrate 1 and the silicon substrate 2 were mirror-polished, and the surface layer was removed with a hydrofluoric acid-based etching solution. Next, the quartz substrate 1 and the silicon substrate 2 were immersed in a mixed solution of ammonia, hydrogen peroxide and pure water to make the surface hydrophilic, and then sufficiently washed with pure water. The surface of each substrate was terminated with hydroxyl groups by the hydrophilic treatment. Next, 2
The mirror-polished surfaces of the substrates were superposed. The two substrates were adsorbed by Van der Waals force.

Next, the first heat treatment was performed at 150 ° C. for 5 hours. This heat treatment was performed to remove excess water and the like attached to the substrate. The first heat treatment is usually 100-
It may be carried out at a temperature of 300 ° C. for several minutes to several tens of hours, preferably at a temperature range of 100 to 200 ° C. at which the substrate does not stick, for 1 hour to several tens of hours. That is, although some heating is required to remove excess water, if ionic bonding or covalent bonding is caused on the entire substrate, stress is generated at the bonding interface, which may cause damage to the substrate. This is because it may cause a problem. That is, the temperature of the first heat treatment is a temperature at which adsorption by Van der Waals forces is maintained and ionic bonds or covalent bonds are not sufficiently generated, and at which water and the like can be removed. Specifically, the bonding state of the substrates after the first heat treatment is a state in which the two substrates can still be separated from the bonding interface by a mechanical method.

Next, the second heat treatment was performed at 300 ° C. for 3 hours. Such heat treatment is performed to remove moisture and the like in the bonded portion and cause fixation by ionic bond or covalent bond. In the case of a quartz substrate and a silicon substrate, the second heat treatment is performed at a temperature higher than the first temperature and at a temperature lower than the crystal phase transition temperature of 573 ° C. for several minutes to several tens hours, preferably 200 to 500. A temperature of ℃ is good. FIG. 1B shows the piezoelectric composite substrate after the first heat treatment.
As described above, the two substrates are adsorbed at the bonding interface 21 mainly by the bond of the hydroxyl group or the water constituent molecule. Here, the water constituent molecule is a molecule based on water molecules, and is a molecule obtained by adding atoms or molecules existing at various interfaces to water molecules.

These bonds were relatively weak, and removal of water and other gases from the bonding interface between the two substrates was performed smoothly. Therefore, almost no void was formed at the bonding interface of the substrate. FIG. 1C shows the piezoelectric composite substrate after the second heat treatment. At the bonding interface 22, the two substrates were bonded very strongly at the atomic level by a bond mainly consisting of a covalent bond (siloxane bond).

According to the manufacturing method described above, it is possible to obtain a direct bond in which stress is reduced by suppressing voids generated during the heat treatment only by the heat treatment step without requiring the step of thinning the substrate. As a result, it has become possible to improve the manufacturing yield of the piezoelectric composite substrate.

If the first heat treatment is divided into a low temperature side and a high temperature side, the effect of removing water and gas at the joint interface is further increased. That is, even if the heating is performed rapidly, the water or the like can be smoothly removed by heating stepwise or gradually. For example, the low temperature side of the first heat treatment is usually performed at 100 to 200 ° C. for several minutes to several tens of hours, preferably at 100 to 180 ° C. for one hour to several tens of hours. The high temperature side is usually higher than the heat treatment temperature on the low temperature side,
The heat treatment is performed at 300 ° C. or lower for several minutes to several tens of hours, preferably at a temperature of the low temperature side heat treatment temperature of 200 ° C. or lower for 1 hour to several tens hours.

Further, in the above embodiments, the piezoelectric substrate may be a single crystal piezoelectric material such as lithium tantalate or lithium niobate, or other piezoelectric material capable of mirror polishing. Further, the present invention can be applied to a semiconductor substrate such as gallium arsenide or indium phosphide, a glass substrate, or another piezoelectric substrate having a thermal expansion coefficient different from that of the piezoelectric substrate as a substrate material bonded to the piezoelectric substrate.

Also in the direct bonding between piezoelectric substrates of the same type, when the substrates having the anisotropy of thermal expansion are bonded by shifting the crystal orientation, the same stress problem as in the case of bonding different substrates is encountered. Occurs. Even in this case, the problem of stress can be solved by implementing the present invention.

(Embodiment 2) A second embodiment of the manufacturing method of the present invention will be described with reference to FIG. As shown in FIG. 2A, the substrate material and shape are the same as those in the first embodiment. Similar to Example 1, steps up to superposition were performed. Next, the first heat treatment was performed at 250 ° C. for 5 hours. In FIG. 2B, as compared with Example 1, the heat treatment temperature was slightly increased to prevent peeling due to dicing in the next step, and the bonding strength was increased.

Next, the piezoelectric composite substrate is vertically 3 mm and horizontally 3
It was cut with a dicing saw into a size of mm. FIG.
C is a piezoelectric composite substrate cut after the first heat treatment. The second heat treatment was performed at 350 ° C. for 3 hours. Figure 2D is second
It is the piezoelectric composite substrate after the heat treatment of.

In the present embodiment, after the first heat treatment, the piezoelectric composite substrate is divided into small pieces to reduce the bonding area per substrate, so that a small amount remains at the bonding interface 22 during the second heat treatment. Removal of water constituent molecules and gas was progressed smoothly. As a result, the thermal stress applied non-uniformly during the second heat treatment was further reduced as compared with the manufacturing method of Example 1.

This method is very effective when the size of the substrate is large. That is, it is possible to completely remove water and the like remaining in the central portion of the substrate.

According to the manufacturing method described above, it is possible to obtain a direct bond with reduced stress by suppressing voids generated during the heat treatment only by the heat treatment step without requiring the step of thinning the substrate. As a result, it has become possible to improve the manufacturing yield of the piezoelectric composite substrate. Especially, when the size of the substrate is large, the manufacturing yield of the piezoelectric composite substrate is significantly improved.

If the first heat treatment is divided into a low temperature side and a high temperature side, the effect of removing water and gas at the joint interface is further increased. That is, even if the heating is performed rapidly, the water or the like can be smoothly removed by heating stepwise or gradually. For example, the low temperature side of the first heat treatment is usually performed at 100 to 200 ° C. for several minutes to several tens of hours, preferably at 100 to 180 ° C. for one hour to several tens of hours. The high temperature side is usually higher than the heat treatment temperature on the low temperature side,
The heat treatment is performed at 300 ° C. or lower for several minutes to several tens of hours, preferably at a temperature of the low temperature side heat treatment temperature of 200 ° C. or lower for 1 hour to several tens hours.

In the above embodiments, the piezoelectric substrate may be a single crystal piezoelectric material such as lithium tantalate or lithium niobate, or other piezoelectric material capable of mirror polishing. Further, it can be applied to a semiconductor substrate such as gallium arsenide or indium phosphide, a glass substrate, or another piezoelectric substrate having a thermal expansion coefficient different from that of the piezoelectric substrate as a substrate material to be bonded to the piezoelectric substrate.

Also in the direct bonding of piezoelectric substrates of the same kind, when the substrates having the anisotropy of the thermal expansion coefficient are bonded by shifting the crystal orientation, the same problem of stress as in the bonding of different substrates is caused. appear. Even in this case, the problem of stress can be solved by implementing the present invention.

(Embodiment 3) A third embodiment of the manufacturing method of the present invention will be described with reference to FIG. As shown in FIG. 3A, the substrate material and shape are the same as those in the first embodiment. Similar to Example 2, steps up to superposition were performed (FIG. 3B). Reference numeral 21 is a bonding interface. Next, the first heat treatment was performed at 250 ° C. for 5 hours.

Next, as shown in FIG. 3C, a part of the silicon substrate 2 was removed from the surface facing the bonding surface 21 with a hydrofluoric acid-based etching solution. Next, a second heat treatment is performed 35
It was carried out at 0 ° C. for 3 hours.

In the first heat treatment of this embodiment, water and other gases from the bonding interface between the two substrates are smoothly removed,
The piezoelectric composite substrate for the etching process of the silicon substrate 2 was temporarily joined. In the second heat treatment, since the bonding area of the piezoelectric composite substrate was reduced, water constituent molecules at the bonding interface,
Gas etc. were removed smoothly.

By the above-mentioned manufacturing method, it is possible to obtain the direct bonding in which the voids generated during the heat treatment are suppressed and the stress is reduced. As a result, it has become possible to improve the manufacturing yield of the piezoelectric composite substrate. Especially, when the size of the substrate is large, the manufacturing yield of the piezoelectric composite substrate is significantly improved.

If the first heat treatment is divided into a low temperature side and a high temperature side, the effect of removing water and gas at the joint interface is further increased. That is, even if the heating is performed rapidly, the water or the like can be smoothly removed by heating stepwise or gradually. For example, the low temperature side of the first heat treatment is usually performed at 100 to 200 ° C. for several minutes to several tens of hours, preferably at 100 to 180 ° C. for one hour to several tens of hours. The high temperature side is usually higher than the heat treatment temperature on the low temperature side,
The heat treatment is performed at 300 ° C. or lower for several minutes to several tens of hours, preferably at a temperature of the low temperature side heat treatment temperature of 200 ° C. or lower for 1 hour to several tens hours.

In the above embodiments, the piezoelectric substrate may be a single crystal piezoelectric material such as lithium tantalate or lithium niobate, or other piezoelectric material capable of mirror polishing. Further, it can be applied to a semiconductor substrate such as gallium arsenide or indium phosphide, a glass substrate, or another piezoelectric substrate having a thermal expansion coefficient different from that of the piezoelectric substrate as a substrate material to be bonded to the piezoelectric substrate.

Also in the direct bonding of piezoelectric substrates of the same kind, when the substrates having the anisotropy of thermal expansion are bonded by shifting the crystal orientation, the same problem of stress as in the bonding of different substrates is caused. appear. Even in this case, the problem of stress can be solved by implementing the present invention.

(Embodiment 4) A fourth embodiment of the manufacturing method of the present invention will be described. In this embodiment, a glass substrate is used instead of the silicon substrate. It should be noted that, in order to simplify the description, only the case of using a glass substrate in the case of Example 1 will be described, but Example 2 or Example 3
If it is carried out in this mode, the effect of reducing the stress in the central portion of the substrate shown in each of the embodiments can be similarly obtained.

In the case of this embodiment, the use of the glass substrate has the following advantages. The coefficient of thermal expansion of the glass substrate could be changed by mixing an alkaline component such as sodium hydroxide. By bringing the coefficient of thermal expansion of the glass substrate close to that of the quartz substrate, the temperature of the second heat treatment could be increased, and the firmly fixed piezoelectric composite substrate could be manufactured with high yield. Further, glass is inexpensive as a substrate material, and the cost can be reduced.

The size of the crystal substrate 1 is vertical: 12 mm ×
Width: 12 mm, thickness 80 μm, glass substrate length: 1
2 mm × width: 12 mm, and the thickness was 400 μm. The coefficient of thermal expansion of the glass substrate was 14 × 10 ^-6 / ° C.

The manufacturing method is as follows. First, the surfaces of the glass substrate and the quartz substrate are mirror-polished, and a hydrofluoric acid-based etching solution is used.
The surface layer was removed. Next, the glass substrate and the quartz substrate were made into a mixed solution of ammonia, hydrogen peroxide, and pure water, the surface was hydrophilized, and further thoroughly washed with pure water. The surface of each substrate was terminated with hydroxyl groups by the hydrophilic treatment. Next, 2
The mirror-polished surfaces of the substrates were superposed. The two substrates were adsorbed by Van der Waals force.

Next, the first heat treatment was performed at 150 ° C. for 5 hours. This heat treatment was performed to remove excess water and the like attached to the substrate. The first heat treatment is usually 100-
The heating is carried out at a temperature of 300 ° C. for several minutes to several tens of hours, preferably at a temperature of 100 to 200 ° C. at which the substrate is not fixed, and for one hour to several tens of hours. That is, although some heating is required to remove excess water, if ionic bonding or covalent bonding is caused on the entire substrate, stress is generated at the bonding interface, which may cause damage to the substrate. This is because it may cause a problem. That is, the temperature of the first heat treatment is a temperature at which adsorption by Van der Waals forces is maintained and ionic bonds or covalent bonds are not sufficiently generated, and at which water and the like can be removed. Specifically, the bonded state of the substrates after the first heat treatment is a state in which the two substrates can still be separated by a mechanical method.

Next, the second heat treatment was performed at 300 ° C. for 3 hours. Such heat treatment is performed to remove moisture and the like in the bonded portion and cause fixation by ionic bond or covalent bond. The second heat treatment was performed at a temperature higher than the first temperature.

As described above, the piezoelectric composite substrate after the first heat treatment is adsorbed at the bonding interface mainly by the bond of the hydroxyl group or the water constituent molecule. These bonds are relatively weak bonds, and removal of water and other gas from the bonding interface between the two substrates is facilitated. Therefore,
Almost no void is formed at the bonding interface of the substrate. As a result, after the second heat treatment, the two substrates are very firmly fixed at the atomic level at the bonding interface by a bond mainly including a covalent bond (siloxane bond).

If the first heat treatment is divided into a low temperature side and a high temperature side, the effect of removing water and gas at the joint interface is further increased. That is, even if the heating is performed rapidly, the water or the like can be smoothly removed by heating stepwise or gradually. For example, the low temperature side of the first heat treatment is usually performed at 100 to 200 ° C. for several minutes to several tens of hours, preferably at 100 to 180 ° C. for one hour to several tens of hours. The high temperature side is usually higher than the heat treatment temperature on the low temperature side,
The heat treatment is performed at 300 ° C. or lower for several minutes to several tens of hours, preferably at a temperature of the low temperature side heat treatment temperature of 200 ° C. or lower for 1 hour to several tens hours.

According to the manufacturing method described above, even in the direct bonding with the glass substrate, which has the advantages that the coefficient of thermal expansion can be adjusted and the cost can be reduced, the step of heat treatment does not require the step of thinning the substrate. Only by doing so, it is possible to suppress the voids generated during the heat treatment and obtain a direct bond with reduced stress. That is, since the coefficient of thermal expansion can be adjusted according to the substrates to be bonded, the stress can be further reduced. As a result, the manufacturing yield of the piezoelectric composite substrate can be improved.

In the above embodiments, the piezoelectric substrate may be a single crystal piezoelectric material such as lithium tantalate or lithium niobate, or another piezoelectric material capable of mirror polishing.

(Embodiment 5) A fifth embodiment of the manufacturing method of the present invention will be described. The substrates used are a lithium niobate substrate and a silicon substrate. It should be noted that, in order to simplify the description, only the case of using the lithium niobate substrate in the case of Example 1 will be described, but if it is carried out in the mode of Example 2 or Example 3, the substrate center shown in each Example will be described. The effect of reducing the stress in the portion is similarly obtained.

In this embodiment, the size of the silicon substrate is, for example, length: 12 mm × width: 12 mm, and the thickness is 450 μm.
m, the size of the lithium niobate substrate 4 is, for example, vertical:
12 mm × width: 12 mm, and the thickness was 80 μm.

First, the processes up to the superposition of two substrates were performed in the same process as in the first embodiment. Next, the first heat treatment was performed at 250 ° C. for 5 hours. The first heat treatment is usually 10
It is carried out at a temperature of 0 to 300 ° C. for several minutes to several tens of hours, but it is preferably carried out at a temperature range of 100 to 240 ° C. at which the adhesion of the substrate does not occur, for one hour to several tens of hours.

The second heat treatment was performed at 350 ° C. for 3 hours.
In the case of a lithium niobate substrate and a silicon substrate, the second heat treatment is usually performed at a temperature of 200 to 1000 ° C. for several minutes to several tens of hours, preferably 250 to 400 ° C. This is because 250 ° C. is a temperature at which silicon and lithium niobate start to adhere to each other, and the substrate may be damaged at 400 ° C. or higher.

The piezoelectric substrate is very fragile depending on the constituent material and the cut angle, and even if it is in a weakly coupled state, it is damaged due to uneven thermal stress. However, since the temperature of the crystal phase transition is much higher than the failure temperature as in the quartz substrate, it is not necessary to determine the upper limit of the second heat treatment due to the problem of the phase transition. In the present embodiment, the first heat treatment temperature is low, and thus the two substrates are joined by a relatively weak bond. Removal of water and other gases from the joint interface is done smoothly. Therefore, voids are hardly formed at the bonding interface of the substrate, and there is almost no in-plane variation in the bonding speed. As a result, the substrate is not damaged due to stress concentration. The piezoelectric composite substrate after the second heat treatment is very firmly fixed at the bonding interface 22 by bonding at the atomic level.

If the first heat treatment is divided into a low temperature side and a high temperature side, the effect of removing water and gas at the joint interface is further increased. That is, even if the heating is performed rapidly, the water or the like can be smoothly removed by heating stepwise or gradually. For example, the low temperature side of the first heat treatment is usually performed at 100 to 200 ° C. for several minutes to several tens of hours, preferably at 100 to 180 ° C. for one hour to several tens of hours. The high temperature side is usually higher than the heat treatment temperature on the low temperature side,
The heat treatment is performed at 300 ° C. or lower for several minutes to several tens of hours, preferably at a temperature of the low temperature side heat treatment temperature of 200 ° C. or lower for 1 hour to several tens hours.

According to the manufacturing method described above, even in the direct bonding between the lithium niobate and the silicon substrate, the step of thinning the substrate is not required, and the voids generated during the heat treatment are suppressed only by the heat treatment step. A direct bond with reduced stress can be obtained. As a result, the manufacturing yield of the piezoelectric composite substrate can be improved.

(Embodiment 6) A manufacturing method in a sixth embodiment of the present invention will be described. The substrates used are a quartz substrate and a silicon substrate. It should be noted that, for simplification of explanation, only the same case as the case of the first embodiment will be described. However, if it is carried out in the mode of the second embodiment or the third embodiment, the stress in the central portion of the substrate shown in each embodiment is reduced. The same effect can be obtained.

In this example, the size of the quartz substrate was, for example, 10 mm in length × 10 mm in width, and the thickness was 56 μm. The size of the silicon substrate is, for example, vertical: 10 mm
× width: 10 mm, and the thickness was 450 μm.

First, the surfaces of the quartz substrate and the silicon substrate were mirror-polished, and the surface layer was removed with a hydrofluoric acid-based etching solution. Next, the quartz substrate and the silicon substrate are ammonia,
It was immersed in a mixed solution of hydrogen peroxide and pure water to make the surface hydrophilic, and then thoroughly washed with pure water. The surface of each substrate was terminated with hydroxyl groups by the hydrophilic treatment. Next, the mirror-polished surfaces of the two substrates were overlapped. The two substrates were adsorbed by Van der Waals force.

Next, the first heat treatment was performed at 150 ° C. for 5 hours. This heat treatment was performed to remove excess water and the like attached to the substrate. The first heat treatment is usually 100-
It is carried out at a temperature of 300 ° C. for several minutes to several tens of hours, preferably at a temperature of 100 to 200 ° C. for one hour to several tens of hours. That is, although some heating is required to remove excess water, if ionic bonding or covalent bonding is caused on the entire substrate, stress is generated at the bonding interface, which may cause damage to the substrate. This is because it may cause a problem. That is, the temperature of the first heat treatment is a temperature at which adsorption by Van der Waals forces is maintained and ionic bonds or covalent bonds are not sufficiently generated, and at which water and the like can be removed. Specifically, the bonded state of the substrates after the first heat treatment is a state in which the two substrates can still be separated by a mechanical method.

Next, the second heat treatment was performed at 250 ° C. for 3 hours. Here, as described in Example 1, in the case of a quartz substrate, the temperature of the second heat treatment is α-β of quartz without pressure.
It may be 573 ° C. which is the phase transition temperature, but when stress is actually applied to the substrate due to bonding, the phase transition is 573 ° C.
It can occur at temperatures below ° C. Therefore, in this embodiment,
In order to completely prevent the problem of phase transition, the temperature was lower than that in Example 1 by 50 ° C.

In this example, the two substrates were adsorbed mainly by hydrogen bonds after the first heat treatment. The crystal substrate 1 had no change in crystal structure due to thermal stress, that is, no phase transition occurred.

In the present embodiment, during the first heat treatment at a low temperature, water molecules and gas are smoothly removed from the superposition interface, and the water constituent molecules and gas are uniformly spread at the superposition interface. The non-uniform thermal stress applied to the quartz substrate was greatly reduced. Therefore, there was no partial stress concentration in the plane of the crystal substrate, and the crystal substrate did not undergo phase transition. In addition, since the first heat treatment was performed during the second heat treatment as well, the thermal stress in the bonding surface was applied almost uniformly, and the crystal substrate 1 did not undergo phase transition.

According to the manufacturing method described above, it is possible to obtain a direct bond with reduced stress by suppressing voids generated during the heat treatment only by the heat treatment step without requiring the step of thinning the substrate. As a result, it has become possible to improve the manufacturing yield of the piezoelectric composite substrate. Moreover, the phase transition of the piezoelectric substrate can be completely prevented, and the piezoelectric composite substrate having desired characteristics can be manufactured.

If the first heat treatment is divided into a low temperature side and a high temperature side, the effect of removing water and gas at the joint interface is further increased. That is, it is possible to smoothly remove moisture and the like by heating stepwise or gradually even by rapidly heating. For example, the low temperature side of the first heat treatment is usually performed at 100 to 200 ° C. for several minutes to several tens hours, preferably at 100 to 180 ° C. for one hour to several tens hours. On the high temperature side, it is usually higher than the heat treatment temperature on the low temperature side, and it is better to carry out at 300 ° C. or lower for a few minutes to tens of hours, preferably at a temperature higher than the low temperature side heat treatment temperature and lower than 200 ° C. for 1 to tens of hours. .

In the above embodiments, the piezoelectric substrate may be a single crystal piezoelectric material such as lithium tantalate or lithium niobate, or another piezoelectric material capable of mirror polishing. Further, it can be applied to a semiconductor substrate such as gallium arsenide or indium phosphide, a glass substrate, or another piezoelectric substrate having a thermal expansion coefficient different from that of the piezoelectric substrate as a substrate material to be bonded to the piezoelectric substrate.

Also in the direct bonding of piezoelectric substrates of the same kind, when the substrates having the anisotropy of thermal expansion are bonded by shifting the crystal orientation, the same problem of stress as in the bonding of different substrates is caused. appear. Even in this case, the problem of stress can be solved by implementing the present invention.

(Embodiment 7) A seventh embodiment of the manufacturing method of the present invention will be described. The substrates used are a quartz substrate and a glass substrate. It should be noted that, for simplification of explanation, only the same case as the case of the first embodiment will be described. However, if it is carried out in the mode of the second embodiment or the third embodiment, the stress in the central portion of the substrate shown in each embodiment is reduced. The same effect can be obtained.

In this example, a glass substrate was used instead of the silicon substrate. The coefficient of thermal expansion of the glass substrate could be changed by changing the material forming it. By selecting a coefficient of thermal expansion close to that of the piezoelectric body to be bonded, it was possible to manufacture a firmly fixed piezoelectric composite substrate with high yield. Further, glass is an inexpensive substrate material, and when used as a holding substrate, the cost could be suppressed.

The size of the quartz substrate is vertical: 12 mm × horizontal:
12 mm, thickness 80 μm, glass substrate vertical: 12 m
m × width: 12 mm, and the thickness was 400 μm. The coefficient of thermal expansion of the glass substrate was 10 × 10 ⁻⁶ / ° C. The substrates were mirror-polished, washed, hydrophilized, and superposed. The first heat treatment was performed at 150 ° C. for 5 hours.

The first heat treatment is usually carried out in the temperature range of 90 to 350 ° C., preferably 150 to 300 ° C. Next, the second heat treatment was performed at 250 ° C. for 3 hours. Here, as described in Example 1, in the case of the quartz substrate, the temperature of the second heat treatment may theoretically be 573 ° C. which is the phase transition temperature of the quartz, but it is actually bonded to the substrate. When stressed, the phase transition can partially occur at temperatures below 573 ° C. Therefore, in this example, in order to completely prevent the problem of phase transition, the temperature was lower by 50 ° C. than in the case of Example 1.

The second heat treatment is usually 100 to 573 ° C.
The temperature range of 250 to 573 ° C. is preferable. By the manufacturing method described above, in the case of using a glass substrate having an advantage that the coefficient of thermal expansion can be adjusted, the step of thinning the substrate is not required,
Only by the heat treatment step, it was possible to obtain voids generated during the heat treatment and obtain a direct bond with reduced stress. As a result, it has become possible to improve the manufacturing yield of the piezoelectric composite substrate. Moreover, the phase transition of the piezoelectric substrate can be completely prevented, and the piezoelectric composite substrate having desired characteristics can be manufactured.

The piezoelectric substrate may be a single crystal piezoelectric material such as lithium tantalate or lithium niobate, or another piezoelectric material capable of mirror polishing. (Embodiment 8) An eighth embodiment of the manufacturing method of the present invention is shown in FIG. In FIG. 4A, 1 is a quartz substrate and 2 is a silicon substrate. Here, an integrated circuit for signal processing may be formed on the silicon substrate. That is, since direct bonding is sufficient with a heat treatment at a low temperature, there is no problem that the characteristics of the integrated circuit are adversely affected by heating.

First, in the same steps as in Example 1, the substrates were mirror-polished, washed, hydrophilized, and superposed. Next, the first
Was heat-treated at 180 ° C. for 5 hours (FIG. 5B). The first heat treatment is usually carried out in the range of 100 to 300 ° C., but preferably a temperature of 100 to 200 ° C. at which substrate adhesion does not occur.

Next, a part of the silicon substrate 2 was removed from the surface facing the bonding surface with a hydrofluoric acid-based etching solution (FIG. 5C). In an actual piezoelectric element, it is necessary to remove the substrate in the portion where the vibrator of the piezoelectric element is formed in this way.

Next, the second heat treatment was performed at 380 ° C. for 3 hours (FIG. 5D). Second for crystal and silicon substrates
The heat treatment (1) is usually higher than the first heat treatment temperature and is performed at a temperature of 573 ° C. or lower for several minutes to several hours, preferably 200 to 500 ° C.

In the first heat treatment in this embodiment, the piezoelectric composite substrate is temporarily joined to perform the etching process of the silicon substrate 2. In addition, the second heat treatment was performed to firmly bond the silicon substrate 2 and the crystal substrate 1.
The portion 5 of the quartz substrate 1 in FIG. 5D was firmly fixed at the atomic level.

Here, the crystal substrate 5 at the junction with silicon and the periphery of the junction undergoes phase transition, but the crystal substrate 6 at the portion where the silicon substrate is removed does not undergo phase transition. That is, according to the manufacturing method of the present example, in the second heat treatment, even if the bonding portion undergoes a phase transition, a substrate in which the portion of the piezoelectric element forming the vibrator is not affected by the phase transition is produced. can do. In other words, the second heat treatment can be performed without being limited by the upper limit of the phase transition temperature.

As a result, a device having desired characteristics can be manufactured by using the part of the crystal that does not undergo phase transition.
The piezoelectric device could be manufactured with high yield. By the manufacturing method described above, it is possible to obtain voids generated during heat treatment and to obtain direct bonding with reduced stress. As a result, it has become possible to improve the manufacturing yield of the piezoelectric composite substrate. Further, the phase transition of the piezoelectric substrate in the portion that affects the characteristics of the piezoelectric element can be completely prevented, and the piezoelectric composite substrate having desired characteristics can be manufactured.

If the first heat treatment is divided into a low temperature side and a high temperature side, the effect of removing water and gas at the joint interface is further increased. That is, even if the heating is performed rapidly, the water or the like can be smoothly removed by heating stepwise or gradually. For example, the low temperature side of the first heat treatment is usually performed at 100 to 200 ° C. for several minutes to several tens of hours, preferably at 100 to 180 ° C. for one hour to several tens of hours. The high temperature side is usually higher than the heat treatment temperature on the low temperature side,
The heat treatment is performed at 300 ° C. or lower for several minutes to several tens of hours, preferably at a temperature of the low temperature side heat treatment temperature of 200 ° C. or lower for 1 hour to several tens hours.

In the above embodiments, the piezoelectric substrate may be a single crystal piezoelectric material such as lithium tantalate or lithium niobate, or another piezoelectric material capable of mirror polishing. Further, it can be applied to a semiconductor substrate such as gallium arsenide or indium phosphide, a glass substrate, or another piezoelectric substrate having a thermal expansion coefficient different from that of the piezoelectric substrate as a substrate material to be bonded to the piezoelectric substrate.

(Embodiment 9) FIG. 5 shows a perspective view and a top structure of a piezoelectric element when the piezoelectric composite substrate of the present invention is used.
In the examples subsequent to this example, the case where a composite substrate is used for an actual piezoelectric element will be described.

In FIG. 5, 11 is a piezoelectric substrate, for example, AT
It is a cut crystal substrate, and the size is, for example, vertical: 4 mm ×
The width: 8 mm, and the thickness was 50 μm. 12 is a semiconductor substrate, for example, a silicon substrate is used. The size is vertical: 8
mm × width: 12 mm, and the thickness was 450 μm. Thirteen
Is the joint between the piezoelectric substrate and the semiconductor substrate, and the size is vertical: 4 m
m × width: 1 mm. Here, an integrated circuit for signal processing may be formed on the silicon substrate. That is,
This is because heat treatment at a low temperature is sufficient for direct bonding, and there is no problem that the characteristics of the integrated circuit are adversely affected by heating.

In this embodiment, the shape of the joint portion 13 is a rectangle, the short side direction of the rectangle is the x-axis direction, and the long side direction is z '.
The axial direction, that is, the direction tilted by 35 ° 22 'from the z-axis direction. In the present embodiment, excitation electrodes 14a and 14b were further provided on the upper and lower surfaces of the piezoelectric substrate 2 to form a piezoelectric vibrator (FIG. 6).

Next, a method of manufacturing a crystal resonator, which is an application of the piezoelectric composite substrate having the structure of this embodiment, will be described. (A) The surfaces of the AT-cut quartz crystal substrate 11 and the silicon substrate 12 were mirror-polished and washed. AT cut crystal substrate 11
Was thinned to a predetermined thickness, for example 50 μm, at this stage. (B) The silicon substrate 12 was subjected to hollow etching with a hydrofluoric acid-based etching solution to form a shape corresponding to the semiconductor substrate 12 shown in FIG. (C) On the upper and lower surfaces of the AT-cut crystal substrate 11, 14 shown in FIG. 6 is formed by using a technique such as photolithography and vacuum deposition.
Excitation electrodes corresponding to a and 14b were formed. The back surface electrode 14b is an end surface of the AT-cut quartz crystal substrate 11 and the electrode is drawn out to the upper surface of the substrate. (D) The surface was immersed in a mixed solution of ammonia, hydrogen peroxide and pure water to make the surface hydrophilic, and then thoroughly washed with pure water.
Through the above treatment, water constituent molecules were attached to the surface of each substrate. (E) The mirror-polished surfaces of the two substrates were superposed. The two substrates were adsorbed by the Van der Waals force due to the water constituent molecules. (F) After temporarily adhering the substrates by the first heat treatment at 150 ° C. for 5 hours, heat treatment was performed at 350 ° C. for 2 hours. In the case of the AT-cut quartz crystal substrate and the silicon substrate, the second heat treatment is usually performed at a temperature of about 100 to 573 ° C. for several minutes to several tens of hours, but a temperature of about 250 to 500 ° C. is preferable.

Here, one of the characteristics of the piezoelectric vibrator of the present embodiment is that the holding portion of the piezoelectric substrate by direct bonding has a rectangular shape, and the long side direction of the rectangle is the direction in which the coefficient of thermal expansion is close to that of the semiconductor substrate. That is what I did.

The thermal expansion coefficient of AT-cut quartz is 1 in the X-axis direction.
Since it is 5.2 × 10 ⁻⁶ and 11 × 10 ⁻⁶ in the Z ′ direction, in this example, the Z ′ axis direction closer to silicon was set to the long side direction.

In the crystal resonator having the above structure, the change in resonance frequency in the temperature range of 0 to 60 ° C. was 10 ppm or less. Further, the substrate was not damaged or peeled off even when heat shock was applied from room temperature to 260 ° C.

Further, although phase transition was observed at the joint between the quartz substrate and the silicon substrate, no phase transition was observed at the portion where the piezoelectric element was actually produced. Here, the presence or absence of the phase transition could be confirmed by observing the surface state of the surface after etching using a hydrofluoric acid-based etchant. Moreover, it could be confirmed by measuring the plane spacing of the crystal lattice using X-ray diffraction.

With the above structure, the quartz substrate 11 is less susceptible to the thermal stress caused by the difference in the thermal expansion coefficient from the silicon substrate 12, and the piezoelectric vibration may be hindered by the thermal stress. Absent. In addition, since the phase transition does not occur in the crystal substrate where the piezoelectric vibrator is formed, the stability of the resonance frequency with respect to temperature can be the same as that of the original AT-cut crystal. Further, since the substrates are securely joined by the direct joining, the substrates are not damaged even if a heat shock such as solder reflow is applied.

The same effect can be expected if the corners of the rectangle are curved. Even if the shape of the holding portion is other than that, the same effect can be expected by shortening the direction in which the coefficient of thermal expansion of the piezoelectric substrate is closer to the semiconductor substrate than the shape of the holding portion.

By forming the joints at both ends of the piezoelectric substrate, it is possible to increase the strength against a shock such as a drop. If the first heat treatment is divided into a low temperature side and a high temperature side, the effect of removing water and gas at the joint interface is further increased. That is, even if the heating is performed rapidly, the water or the like can be smoothly removed by heating stepwise or gradually. For example, the low temperature side of the first heat treatment is usually performed at 100 to 200 ° C. for several minutes to several tens of hours, preferably at 100 to 180 ° C. for one hour to several tens of hours. The high temperature side is usually higher than the heat treatment temperature on the low temperature side,
The heat treatment is performed at 300 ° C. or lower for several minutes to several tens of hours, preferably at a temperature of the low temperature side heat treatment temperature of 200 ° C. or lower for 1 hour to several tens hours.

In the above embodiments, the piezoelectric substrate may be a single crystal piezoelectric material such as lithium tantalate or lithium niobate, or another piezoelectric material capable of mirror polishing. Further, the present invention can be applied to a semiconductor substrate such as gallium arsenide or indium phosphide, a glass substrate, or another piezoelectric substrate having a thermal expansion coefficient different from that of the piezoelectric substrate as a substrate material bonded to the piezoelectric substrate.

(Embodiment 10) FIG. 7 shows a top structure and a sectional structure of a tenth embodiment of the present invention. 11 in FIG.
Is a piezoelectric substrate, for example, an AT-cut quartz substrate, and has a size of, for example, length: 4 mm × width: 8 mm, and a thickness of 50 μm.
m. Reference numeral 12 is a semiconductor substrate, for example, a silicon substrate, and the size is vertical: 8 mm × horizontal: 12 mm, and the thickness is 4
It was 50 μm. Here, an integrated circuit for signal processing may be formed on the silicon substrate. That is, since direct bonding is sufficient with a heat treatment at a low temperature, there is no problem that the characteristics of the integrated circuit are adversely affected by heating.

Reference numeral 13 denotes a joint portion between the piezoelectric substrate and the semiconductor substrate,
The size is length: 4 mm x width: 1 mm. The shape of the silicon joint 13 is a substantially rectangular shape with curved corners, and the short side direction is the x-axis direction and the long side direction is the z'axis direction, that is,
The direction was inclined by 35 ° 22 'from the z-axis direction. Also,
A constricted structure for reducing stress is provided near the boundary between the bonded portion and the non-bonded portion on the crystal substrate 11. In this example, as shown in FIG. 8, excitation electrodes 14a and 14b were further provided on the upper and lower surfaces of the quartz substrate 11 to form a quartz oscillator.

A feature of the crystal resonator to which the piezoelectric composite substrate of this embodiment is applied is that a constricted structure is provided near the boundary between the bonded portion and the non-bonded portion on the quartz substrate 11. With such a constricted structure, the influence of thermal stress on the vibrating part due to damage to the crystal, thermal stress, and residual stress was reduced.

With the above-described structure, the quartz substrate 11 becomes even less susceptible to the thermal stress due to the difference in thermal expansion coefficient from the silicon substrate 12, and the piezoelectric vibration is hindered by the thermal stress. There was no In addition, since the phase transition does not occur in the crystal substrate where the piezoelectric vibrator is formed, the stability of the resonance frequency with respect to the temperature is the same as that of the original AT-cut crystal. Furthermore, since the substrates are securely joined by the direct joining, the substrates were not damaged even when a heat shock such as solder reflow was applied.

In this embodiment, an AT is used as the piezoelectric substrate.
Although the cut quartz is used, a quartz substrate having another cut angle may be used depending on the application. Further, although the frequency stability with respect to temperature change is inferior to that of quartz, the same effect can be obtained by using lithium niobate or lithium tantalate having a high electromechanical coupling coefficient as the piezoelectric substrate.

(Embodiment 11) An eleventh embodiment of a piezoelectric vibrator, which is an application of the piezoelectric composite substrate of the present invention, will be described. The structure is a structure using X-cut lithium tantalate 15 instead of the AT-cut crystal substrate 1 of Example 9 (FIGS. 5 and 6). The size of the X-cut lithium tantalate substrate 15 is, for example, length: 4 mm × width: 8 mm, and thickness is 50.
μm. At the joint, the short side direction of the rectangle is the x-axis direction,
The long-side direction was the z-axis direction.

This example is different from Example 9 in that lithium tantalate is used as the piezoelectric substrate. Lithium tantalate is inferior to quartz in frequency stability against temperature changes, but has a high electromechanical coupling coefficient and a low Q value, so it is widely used in voltage controlled oscillators that require high efficiency and a relatively wide band. To be For the same reason, lithium niobate may be used as the piezoelectric substrate.

With the above structure, the lithium tantalate substrate 15 is less susceptible to the thermal stress due to the difference in the thermal expansion coefficient from the silicon substrate 12, and the piezoelectric vibration is hindered by the thermal stress. Never. In addition, since the phase transition does not occur in the lithium tantalate substrate in the portion forming the piezoelectric vibrator, the stability of the resonance frequency with respect to temperature can be obtained which is the same as the original AT-cut quartz crystal. Further, since the substrates are securely joined by the direct joining, the substrates are not damaged even if a heat shock such as solder reflow is applied.

The same effect can be expected when the corners of the rectangle are curved. Even if the shape of the holding portion is other than that, the same effect can be expected by shortening the direction in which the coefficient of thermal expansion of the piezoelectric substrate is closer to the semiconductor substrate than the shape of the holding portion.

By forming the joints at both ends of the piezoelectric substrate, it is possible to increase the strength against a shock such as a drop. (Embodiment 12) A twelfth embodiment of a piezoelectric vibrator, which is an application of the piezoelectric composite substrate of the present invention, will be described. The structure is shown in Example 9.
In this structure, the X-cut lithium tantalate 15 is used instead of the AT-cut quartz crystal substrate 1 of FIGS. 5 and 6 and the glass substrate 16 is used instead of the silicon substrate 12. The size of the x-cut lithium tantalate substrate 15 is, for example, vertical:
4 mm × width: 8 mm, and the thickness was 50 μm. The size of the glass substrate 16 is vertical: 8 mm x horizontal: 12 mm,
The thickness was 400 μm, and the coefficient of thermal expansion was 10 × 10 ⁻⁶ / ° C. In the joint portion, the short side direction of the rectangle is the x-axis direction and the long side direction is the z-axis direction.

The present embodiment differs from the ninth embodiment in that a glass substrate is used instead of the silicon substrate. The coefficient of thermal expansion of the glass substrate can be changed, and the coefficient of thermal expansion can be selected according to the piezoelectric body to be bonded. Further, the glass substrate is inexpensive, and the cost can be reduced when used as the holding substrate.

With the above structure, the lithium tantalate substrate 15 is less susceptible to the thermal stress due to the difference in the thermal expansion coefficient from the glass substrate 16, and the piezoelectric vibration is hindered by the thermal stress. Never happened. Also,
Since the phase transition does not occur in the lithium tantalate substrate in the portion where the piezoelectric vibrator is formed, the stability of the resonance frequency with respect to the temperature is the same as the original AT cut quartz crystal. Furthermore, since the substrates are securely joined by the direct joining, the substrates were not damaged even when a heat shock such as solder reflow was applied.

The same effect can be expected when the corners of the rectangle are curved. Even if the shape of the holding portion is other than that, the same effect can be expected by shortening the direction in which the coefficient of thermal expansion of the piezoelectric substrate is closer to the semiconductor substrate than the shape of the holding portion.

By forming the joints at both ends of the piezoelectric substrate, it is possible to increase the strength against a shock such as a drop.

[0129]

As described above, the first and second aspects of the present invention
According to the second manufacturing method and the first to second piezoelectric elements,
Even when directly joining substrates with different thermal expansion coefficients,
It is possible to significantly reduce the stress at the joint portion, solve problems such as breakage of the substrate, and improve mass productivity of the composite substrate. Further, since the stress on the substrate is reduced, problems such as deterioration of characteristics due to stress of elements formed on the substrate can be solved. Further, in the quartz substrate, the problem of phase transition promoted by stress can be solved.

[Brief description of drawings]

1A to 1D are process explanatory views for manufacturing a piezoelectric composite substrate according to a first embodiment of the present invention.

2A to 2D are process explanatory views for manufacturing a piezoelectric composite substrate according to a second embodiment of the present invention.

3A to 3D are process explanatory views for manufacturing a piezoelectric composite substrate of Example 3 of the present invention.

4A to 4D are process explanatory views for manufacturing the piezoelectric composite substrate of Example 8 of the present invention.

5A and 5B are configuration diagrams of a piezoelectric element configured using the piezoelectric composite substrate of Example 9 of the present invention, where A is a perspective view and B.
Is a plan view

6A and 6B are configuration diagrams of a piezoelectric vibrator configured by using a piezoelectric composite substrate of Example 9 of the present invention, where A is a perspective view,
B is a plan view

FIG. 7 is a configuration diagram of a piezoelectric vibrator configured by using a piezoelectric composite substrate of Example 10 of the present invention, in which A is a plan view and B is I.
-I line cross section

FIG. 8 is a configuration diagram of a piezoelectric vibrator configured using a piezoelectric composite substrate of Example 10 of the present invention, in which A is a plan view and B is II.
-II cross section

[Explanation of symbols]

1: Crystal Substrate (Piezoelectric Substrate) 2: Silicon Substrate (Semiconductor Substrate) 3: Phase Transition Substrate 4: Non-Phase Transition Substrate 11: AT-Cut Quartz Substrate 12: Single Crystal Silicon Substrate 13: Holding Part by Direct Bonding 14a : Excitation electrode 14b: Excitation electrode 15: X-cut lithium tantalate substrate 16: Glass substrate 21: Bonding interface adsorbed by Van der Waals force 22: Bonding interface fixed by ionic bond or covalent bond

Continuation of the front page (51) Int.Cl. ⁶ Identification number Reference number within the agency FI Technical indication C30B 33/02 7202-4G C30B 33/02 33/06 7202-4G 33/06 H01L 21/02 H01L 21/02 C 25/16 25/16 41/09 H03H 3/02 B H03H 3/02 9/02 K 9/02 9/05 9/05 H01L 41/08 C (72) Inventor Kazuo Eda Kadoma City, Osaka 1006 Kadoma Matsushita Electric Industrial Co., Ltd.

Claims (23)

[Claims] 1. A method for manufacturing a composite substrate, which includes the following steps A to F: A. B. mirror finishing at least one main surface of the first substrate; At least one main surface of the second substrate having a different coefficient of thermal expansion from the first substrate is mirror-finished; Superimposing the main surface of the first substrate and the main surface of the second substrate on each other, D. Then, a first heat treatment is performed at a temperature lower than a temperature at which the first substrate and the second substrate are fixed to each other, and E. After that, the first substrate and the second substrate are overlapped and divided into at least two pieces, and F. The small piece is subjected to a second heat treatment at a temperature at which the first substrate and the second substrate are fixed to each other. 2. The method for manufacturing a composite substrate according to claim 1, wherein the first substrate is made of a piezoelectric material. 3. The method for manufacturing a composite substrate according to claim 1, wherein the second substrate is made of at least one substance selected from a semiconductor and glass. 4. The method for manufacturing a composite substrate according to claim 1, wherein the second substrate is made of a piezoelectric material. 5. The method of manufacturing a composite substrate according to claim 1, wherein the first substrate is a quartz substrate, and the temperature of the second heat treatment is lower than the temperature at which the quartz substrate undergoes a phase transition. 6. A method of manufacturing a composite substrate, which includes the following steps a to f. a. Mirror finishing at least one main surface of the first substrate; b. At least one main surface of the second substrate having a different coefficient of thermal expansion from the first substrate is mirror-finished; c. Superimposing the main surface of the first substrate and the main surface of the second substrate on each other, d. Then, a first heat treatment is performed at a temperature lower than a temperature at which the first substrate and the second substrate are fixed to each other, and e. Then, a portion of the second substrate is removed until it reaches the first substrate, f. The second heat treatment is performed at a temperature at which the first substrate and the second substrate are fixed to each other. 7. The method for manufacturing a composite substrate according to claim 6, wherein the first substrate is made of a piezoelectric material. 8. The method for manufacturing a composite substrate according to claim 6, wherein the second substrate is any substance selected from a semiconductor and glass. 9. The method for manufacturing a composite substrate according to claim 6, wherein the second substrate is made of a piezoelectric material. 10. The first substrate is a substrate that undergoes a phase transition, and the temperature of the second heat treatment is different from that of the first substrate except for a bonding portion between the first substrate and the second substrate. The method for producing a composite substrate according to claim 6, wherein the temperature is lower than the temperature at which the phase transition occurs. 11. The method of manufacturing a composite substrate according to claim 10, wherein the first substrate is made of quartz. 12. The method of manufacturing a composite substrate according to claim 6, wherein the second substrate is a silicon substrate on which an integrated circuit is formed. 13. The overlapped portion of the first substrate and the second substrate is a substantially rectangular shape, and the coefficient of thermal expansion in the long side direction of the rectangle of the first substrate is the first rectangular shape. Different from the coefficient of thermal expansion of the substrate in the short side direction of the rectangle, the difference between the coefficient of thermal expansion of the first substrate in the direction of the long side and the coefficient of thermal expansion of the second substrate is equal to that of the first substrate. The method for manufacturing a composite substrate according to claim 6, wherein the difference between the coefficient of thermal expansion in the short side direction and the coefficient of thermal expansion of the second substrate is smaller than that of the second substrate. 14. A piezoelectric substrate made of a piezoelectric material on which an excitation electrode is formed, and a holding substrate bonded to a bonding portion provided on a part of the piezoelectric substrate by covalent bonding or ionic bonding. In the piezoelectric element, the crystal structure of the piezoelectric substrate in a portion where the excitation electrode is formed is different from the crystal structure of the piezoelectric substrate in the bonding portion. 15. The piezoelectric element according to claim 14, wherein the piezoelectric substrate is a quartz substrate. 16. The piezoelectric element according to claim 14, wherein a width in the long side direction of the rectangle of the piezoelectric substrate around the joint is smaller than a length of the long side of the rectangle. 17. The piezoelectric element according to claim 14, wherein the holding substrate is made of a silicon substrate on which an integrated circuit is formed. 18. The piezoelectric element according to claim 14, wherein the holding substrate is made of either semiconductor or glass. 19. A piezoelectric element provided with a piezoelectric substrate made of a piezoelectric body on which electrodes are formed, and a holding substrate bonded to a bonding portion provided on a part of the piezoelectric substrate by covalent bond or ionic bond. In, the joint portion is substantially rectangular, the coefficient of thermal expansion in the long side direction of the rectangle of the first substrate is different from the coefficient of thermal expansion in the short side direction of the rectangle of the first substrate, The difference between the coefficient of thermal expansion in the long side direction of the first substrate and the coefficient of thermal expansion of the second substrate is the coefficient of thermal expansion in the direction of the short side of the first substrate and the coefficient of thermal expansion of the second substrate. A piezoelectric element characterized by being smaller than the difference in the ratio. 20. The piezoelectric element according to claim 19, wherein the piezoelectric substrate is a quartz substrate. 21. The piezoelectric element according to claim 19, wherein a width in a long side direction of the rectangle of the piezoelectric substrate around the joint is smaller than a length of a long side of the rectangle. 22. The piezoelectric element according to claim 19, wherein the substrate is a silicon substrate on which an integrated circuit is formed. 23. The holding substrate is made of at least one substance selected from semiconductor and glass.
The piezoelectric element according to 1.