Boundary acoustic wave device

A boundary acoustic wave device includes a first boundary acoustic wave element, and a second boundary acoustic wave element disposed on the first boundary acoustic wave element. Each boundary acoustic wave element includes a first solid layer made of a piezoelectric material, a second solid layer made of one of a piezoelectric material and an insulating material, and disposed on the first solid layer, and a boundary acoustic wave exciting interdigital transducer arranged in the boundary between the first and second solid layers.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a boundary acoustic wave device using a boundary acoustic wave traveling in a boundary between two solid layers and, in particular, to a boundary acoustic wave device having a laminate structure in which one boundary acoustic wave element or one surface acoustic wave element is laminated to at least another boundary acoustic wave element.

2. Description of the Related Art

A variety of surface acoustic wave devices have been conventionally used for an RF filter and an IF filter for cellular phones, a VCO resonator, and a VIF filter for televisions. The surface acoustic wave devices use a surface acoustic wave such as a Reyleigh wave or a first leakage wave traveling along the surface of a medium.

The surface acoustic wave is sensitive to variations in the state of the surface of the medium because the surface acoustic wave travels along the surface of the medium. For this reason, the surface acoustic element is housed in a cavity of a package to protect the wave propagating surface of the medium. The use of the package having the cavity increases the cost of the surface acoustic wave device. In addition, since the package is substantially larger than the surface acoustic wave element, the size of the surface acoustic wave device is increased.

One type of surface acoustic waves travels along a boundary between solid bodies.

Japanese Unexamined Patent Application Publication No. 2-47899 discloses a surface acoustic wave device that uses a Stonley wave, which is one type of surface acoustic wave.FIG. 23is a sectional view diagrammatically illustrating a surface acoustic wave device500. An interdigital transducer (IDT)502is disposed on a glass substrate501. A piezoelectric medium layer503made of zinc oxide (ZnO) covers the surface of the glass substrate501having the IDT502provided thereon. The thickness of each of the glass substrate501and the piezoelectric medium layer503is not less than five wavelengths of the surface acoustic wave, and the glass substrate501has a required density and modulus of rigidity. In this arrangement, a Stonley wave travels along the device.

The energy of the boundary acoustic wave concentrates in the boundary between the glass substrate501and the piezoelectric medium layer503in the surface acoustic wave device500. Little or no energy of the surface acoustic wave exists on the surface501aof the glass substrate501and the surface503aof the piezoelectric medium layer503. The surface501aand the surface503arespectively refer to the surfaces of the glass substrate501and the piezoelectric medium layer503that are opposite from the boundary surfaces thereof. Since there is little or no energy of the surface acoustic wave on the surface501aof the glass substrate501and the surface503aof the piezoelectric medium layer503, the characteristics of the device are not sensitive to variations in the state of the surface501aof the glass substrate501and the surface503aof the piezoelectric medium layer503. The surface acoustic wave device500disclosed in the above-described patent publication does not require a package having a cavity.

A leakage type boundary acoustic wave called a Maerfeld Tournois (MT) wave propagating in a [001]—Si(110)/SiO2/X—LiNbO3structure is disclosed in a non-patent document entitled “Highly Piezoelectric Boundary Method in Si/SiO3/LiNbO3Structural Propagation”, 26thEM Symposium, May 1997, pp. 53–58. The boundary acoustic wave of this type has an electromechanical coupling factor k2that is greater than the Stonley wave.FIG. 24is a sectional view illustrating a boundary acoustic wave device510disclosed in the above-mentioned non-patent document. An IDT512is disposed on a Y—X LiNbO3substrate511in the boundary acoustic wave device510. An SiO2layer513covers the IDT512. A [001]—Si(100) layer514is then disposed on the SiO2layer513.

Since the MT wave is a boundary acoustic wave, the boundary acoustic wave device510using the MT wave does not require a package having a cavity.

As cellular phones with multiple functions become smaller, flatter, and lighter, miniaturization of electronic components is accordingly required. For example, area requirements of flat RF interstage SAW filters were 3.8 by 3.8 mm2in 1998, 3.0 by 3.0 mm2in 2000, and 2.0 by 1.6 mm2or less in 2002. In the future, area requirements are expected to be less than 1 mm2.

Since the package has no cavity, the above-referenced boundary acoustic wave devices have dimensional advantages over the other devices. However, in the known boundary acoustic wave devices, the dimensions of the element remains unchanged from the known surface acoustic wave devices. Further miniaturization is thus impossible.FIG. 25is a plan sectional view illustrating electrodes of a 1 GHz band RF interstage filter that is designed in accordance with the known boundary acoustic wave device. As shown, a boundary surface bearing the electrodes is exposed in the sectional view.

As shown inFIG. 25, a number of IDTs523a–523e, reflectors524a–524j, electrode pads525a–525c, and wiring electrodes must be disposed on the top surface of a solid layer522made of an LiNbO3substrate in a boundary acoustic wave device521to manufacture the 1 GHz band RF interstage filter. These elements make it difficult to miniaturize the two dimensional area of the boundary acoustic wave device to less than 1 mm2.

SUMMARY OF THE INVENTION

To overcome the problems described above, preferred embodiments of the present invention provide a boundary acoustic wave device that satisfies miniaturization requirements of element size and has excellent characteristics and performance.

In a first preferred embodiment of the present invention, a boundary acoustic wave device includes a laminate of a plurality of boundary acoustic wave elements, wherein each boundary acoustic wave element includes a first solid layer made of a piezoelectric material, a second solid layer disposed on the first solid layer and made of one of a piezoelectric material and an insulating material, and a boundary acoustic wave exciting interdigital transducer disposed in a boundary between the first and second solid layers.

In a second preferred embodiment of the present invention, a boundary acoustic wave device includes a first solid layer made of a piezoelectric material, a second solid layer disposed on the first solid layer and made of one of a piezoelectric material and an insulating material, a boundary acoustic wave exciting interdigital transducer disposed in a boundary between the first and second solid layers, and a surface acoustic wave exciting interdigital transducer disposed on the surface of the first solid layer.

A surface acoustic wave element is preferably disposed on the external surface of the laminate, wherein the surface acoustic wave element includes a piezoelectric body and a surface acoustic wave exciting interdigital transducer disposed on one surface of the piezoelectric body. Since the surface acoustic wave element is arranged on the external surface, an even smaller device with multiple functions is provided.

The boundary acoustic wave device may further include a conductive layer disposed at at least one of an external surface of the laminate, a boundary between adjacent boundary acoustic wave elements, and a location between one of the boundary acoustic wave elements and the external surface of the laminate. This arrangement controls direct-path electromagnetic waves traveling between the boundary acoustic wave elements, thereby assuring a large attenuation in a filter.

The boundary acoustic wave device preferably includes an acoustic wave absorbing layer disposed at at least one of an external surface of the laminate, a boundary between adjacent boundary acoustic wave elements, and a location between one of the boundary acoustic wave elements and the external surface of the laminate. This arrangement enables direct-path acoustic waves to be absorbed, thereby controlling crosstalk between the boundary acoustic wave elements. If a filter is constructed in this arrangement, aging of the filter characteristics is controlled.

The boundary acoustic wave device preferably includes a third solid layer having an acoustic impedance that is different from the first and second solid layers, disposed at at least one of an external surface of the laminate, a boundary between adjacent boundary acoustic wave elements, and a location between one of the boundary acoustic wave elements and the external surface of the laminate. The third solid layer having a different acoustic impedance reflects direct-path acoustic waves, thereby controlling crosstalk between the boundary acoustic wave elements. A filter having this arrangement is less susceptible to aging in the filter characteristics.

Preferably, a boundary between the solid layers, other than the boundary having the boundary acoustic wave exciting interdigital transducer, is not as flat as the surface having the surface acoustic wave exciting interdigital transducer. Direct bulk waves and unwanted bulk waves reflected from the boundary are controlled because the boundary between the solid layers other than the boundary having the boundary acoustic wave exciting interdigital transducer is not flat.

The boundary acoustic wave exciting interdigital transducer disposed on one boundary acoustic wave element is preferably shifted in position in a laminating direction from the boundary acoustic wave exciting interdigital transducer disposed on another boundary acoustic wave element. Direct electromagnetic waves are controlled, thereby increasing an attenuation in a filter having that arrangement. Direct-path capacitance is thus greatly reduced.

Preferably, the boundary acoustic wave device further includes an inductance element connected between the boundary acoustic wave exciting interdigital transducers of different boundary acoustic wave elements. Parallel resonance reduces direct-path capacitance in the vicinity of a passband of the filter. Outstanding filter characteristics are thus achieved.

Preferably, the boundary acoustic wave device further includes at least two boundary acoustic wave elements electrically connected to each other through a through-hole electrode provided in the laminate, wherein an elastic material fills the inside of the through-hole electrode. The use of at least two boundary acoustic wave elements electrically connected to each other through the through-hole electrode enables the surface of the boundary acoustic wave element to be planarized. The processing of the element is greatly simplified. Unwanted reflection of the acoustic waves is greatly reduced and minimized. The elastic material filling the inside of the through-hole electrode controls gas explosions within the through-hole during heating.

The boundary acoustic wave device preferably includes at least two boundary acoustic wave elements electrically connected to each other through a through-hole electrode arranged in the laminate, the through-hole electrode includes a plurality of through-hole electrode sections, upper and lower through-hole electrode sections are arranged at different positions in a plane at a height within the laminate, and the two through-hole electrode sections are electrically connected to each other through an electrical connection junction extending in the plane. If the upper and lower through-hole electrode sections are arranged at different positions in a plane at the height within the laminate, corrosion that begins in the vicinity of the through-hole is controlled. Thus, the device is corrosion resistant and moisture resistant.

At least two boundary acoustic wave elements are preferably electrically connected to each other via a wiring electrode arranged on the external surface of the laminate. In this case, the wiring electrode is easily formed. The device prevents unwanted reflection of the acoustic waves, which waves are generated if a through-hole electrode is provided.

The laminate preferably includes a step on the external surface thereof, wherein the wiring electrode is disposed on the step. The formation of the step increases the reliability of the electrical connection of the IDT arranged on the wiring electrode and the boundary surface. A collective wiring method is performed in a wafer process.

Preferably, the boundary acoustic wave device further includes a temperature characteristic improving layer disposed at at least one of an external surface of the laminate, a boundary between adjacent boundary acoustic wave elements, and a location between one of the boundary acoustic wave elements and the external surface of the laminate, wherein the temperature characteristic improving layer has a linear expansion coefficient in a direction that is substantially parallel to the boundary plane of the boundary acoustic wave element having the boundary acoustic wave exciting interdigital transducer therein that is less than a linear expansion coefficient of the first and second solid layers of the boundary acoustic wave element, or has a linear expansion coefficient that is different in polarity from the linear expansion coefficient of the first and second solid layers. The use of the temperature characteristic improving layer greatly improves the temperature characteristics of the boundary acoustic wave device.

Preferably, the boundary acoustic wave device includes a high thermal conductivity layer, disposed at at least one of an external surface of the laminate, a boundary between adjacent boundary acoustic wave elements, and a location between one of the boundary acoustic wave elements and the external surface of the laminate, wherein the high thermal conductivity layer has a greater thermal conductivity than an adjacent boundary acoustic wave element. In this arrangement, heat dissipation and power withstand ability of the boundary acoustic wave device are greatly improved.

The boundary acoustic wave elements may be bonded together using a cavity forming member such that a cavity is provided in at least one of the boundaries of the boundary acoustic wave elements. The boundary acoustic wave elements may be bonded together directly or with the cavity forming member interposed therebetween.

The boundary acoustic wave device preferably includes a circuit element disposed at at least one of an external surface of the laminate, a boundary between adjacent boundary acoustic wave elements, and a location between one of the boundary acoustic wave elements and the external surface of the laminate. The formation of the circuit element between the boundary acoustic wave elements or on the external surface of the laminate permits an impedance matching circuit to be incorporated in the boundary acoustic wave device. The boundary acoustic wave device having multiple functions is thus provided.

The boundary acoustic wave device may further include a base substrate which defines a stress relaxation body and on which the laminate is mounted. Stress due to the bending of a printed circuit board is greatly reduced by the base substrate. Variations in the characteristics of the boundary acoustic wave device are effectively controlled, and the reliability of the boundary acoustic wave device is greatly improved.

The base substrate may be made of a material that is harder than the first and second solid layers which define the boundary acoustic wave element.

The thickness of each of the first and second solid layers is preferably equal to or greater than the wavelength of a boundary acoustic wave. In the arrangement, little or no boundary acoustic wave energy exists on the surfaces of the first and second solid layers.

In another preferred embodiment of the present invention, a method for manufacturing a boundary acoustic wave device includes a step of manufacturing the boundary acoustic wave element by laminating the first solid layer made of the piezoelectric material and the second solid layer made of the one of the piezoelectric material and the insulating material, with a boundary acoustic wave exciting interdigital transducer being interposed therebetween, repeating the step of manufacturing the boundary acoustic wave element to produce a plurality of boundary acoustic wave substrates, and producing a laminate by laminating the plurality of boundary acoustic wave elements.

The method preferably includes a step for providing an acoustic wave absorbing layer at at least one of an external surface of the laminate, a boundary between adjacent boundary acoustic wave elements, and a location between one of the boundary acoustic wave elements and the external surface of the laminate, wherein the acoustic wave absorbing layer is degassed when the first and second solid layers are bonded together with the acoustic wave absorbing layer and the boundary acoustic wave exciting interdigital transducer interposed therebetween. Alternatively, the acoustic wave absorbing layer is degassed when the boundary acoustic wave elements are bonded together via the acoustic wave absorbing layer interposed therebetween.

Since the acoustic wave absorbing layer is degassed, the device does not experience problems arising from residual gases. If any gas remains in a resin of the acoustic wave absorbing layer, cracks may occur in the boundary acoustic wave device during a reflow operation. Degassing over time changes stress in the boundary acoustic wave element, causing the frequency characteristics of the device to age. If the acoustic wave absorbing layer is degassed beforehand, the device does not experience these problems.

The above and other elements, characteristics, features, steps and advantages of the present invention will become clear from the following description of preferred embodiments taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will now be described.

FIG. 1is a front sectional view illustrating a boundary acoustic wave device1in accordance with a first preferred embodiment of the present invention. The boundary acoustic wave device1includes a laminate structure including a plurality of boundary acoustic wave elements2–4. The boundary acoustic wave elements2–4are laminated such that boundary acoustic wave propagating surfaces thereof are substantially parallel with each other. If the elements2–4are bonded using a bonding agent, the boundary acoustic wave propagating surfaces of upper and lower boundary acoustic wave elements will not be parallel to each other due to variations in thickness of the bonding agent. The boundary acoustic wave propagating surfaces may have an angle of ±30 degrees therebetween. In the first preferred embodiment, the boundary acoustic wave elements2–4are preferably identically manufactured. In the boundary acoustic wave element2, an IDT2bis disposed on a first solid layer2a. A second solid layer2cis disposed on the first solid layer2ahaving the IDT2bthereon. In other words, the IDT2bis provided in the boundary between the first and second solid layers2aand2c.

The first solid layer2ais preferably made of a lithium niobate (LiNbO3) monocrystal, in the first preferred embodiment. Besides the lithium niobate monocrystal, the first solid layer2amay be made of a piezoelectric monocrystal, or a piezoelectric ceramic, or other suitable material. The first solid layer2ais made of any suitable piezoelectric material.

The material of the IDT2bis not limited to any particular material. For example, the IDT2bis made of any suitable metal, such as aluminum (Al) or copper (Cu), or any alloy. The IDT2bis preferably made of aluminum in the first preferred embodiment.

The second solid layer2cis preferably made of one of a piezoelectric material and an insulating material. The second solid layer2cis preferably made of silicon dioxide (SiO2) in the first preferred embodiment. A piezoelectric monocrystal such as lithium niobate or a piezoelectric ceramic may be used as the piezoelectric material. Glass or any other appropriate insulating material other than silicon dioxide may be used as the insulating material.

The boundary acoustic wave elements3and4, which are preferably substantially identical in structure to the boundary acoustic wave element2, respectively include a first solid layer3a, an IDT3b, and a second solid layer3c, and a first solid layer4a, an IDT4b, and a second solid layer4c.

A boundary acoustic wave is excited in the boundary between the first solid layers2aand2cwhen an alternating current is applied to the IDT2b. Most of the energy of the boundary acoustic wave concentrates in a region of a thickness that is substantially equal to a wavelength λ from the boundary between the first and second solid layers2aand2c. The wavelength λ is that of the excited boundary acoustic wave.

Almost all energy of the boundary acoustic wave device is confined to a region within a thickness range of about λ to about 3λ from the boundary. The thickness of the solid layers2aand2cis preferably at least about λ, and more preferably at least about 3λ.

The boundary acoustic wave is not adversely affected by the state of the surfaces2a1and2c1of the first and second solid layers2aand2cif the thickness of the first and second solid layers2aand2cis greater than about λ or greater than about 3λ.

The surfaces2a1and2c1of the first and second solid layers2aand2care those of the first and second solid layers2aand2copposite to the boundary of the first and second solid layers2aand2c. In the discussion that follows, the surface of the solid layer refers to the surface opposite to the acoustic wave propagating boundary.

The boundary acoustic wave used in preferred embodiments of the present invention includes but is not limited to the above-mentioned Stonley wave and the MT wave.

The boundary acoustic wave device1of the first preferred embodiment, including the laminate structure of the plurality of boundary acoustic wave elements2–4, achieves an area dimension that is less than the boundary acoustic wave device521illustrated inFIG. 25.

The acoustic speed of the boundary acoustic wave is about 2,000 m/s to about 10,000 m/s. If the boundary acoustic wave device1is operated at a frequency of about 1 GHz, the wavelength of the wave is within a range of about 2 μm to about 10 μm. If the boundary acoustic wave elements2–4are laminated, an increase in overall thickness of the device is not a problem. Without a significant thickness increase, a boundary acoustic wave device having a very small area is achieved.

A manufacturing method of the boundary acoustic wave device1will now be described. The IDT2bpreferably made of aluminum is formed on the first solid layer2amade of the LiNbO3substrate using a photolithographic technique. The silicon dioxide layer is deposited preferably using the RF magnetron sputter deposition. The silicon dioxide layer is planarized using an etch-back technique. The second solid layer2c, made of silicon dioxide, is preferably formed in this manner.

The surface2c1of the second solid layer2cis preferably planarized for the following reason. If the silicon dioxide layer is deposited after the formation of the IDT2b, the surface of the silicon dioxide layer is not flat because of the height of the IDT2b. If the silicon dioxide layer is not flat, the IDT bearing surface of the first solid layer3aof the boundary acoustic wave element3to be produced next is also not flat. Thus, the boundary of the second boundary acoustic wave element3is not flat. The surface2c1of the second solid layer2cis thus planarized.

The first solid layer3ais made of lithium niobate using the laser ablation. The IDT3bis preferably formed by depositing aluminum, and the silicon dioxide layer is deposited using the RF magnetron sputter deposition, and then planarized using the etch-back technique. In this way, the boundary acoustic wave element3is laminated on the boundary acoustic wave element2. Similarly, the boundary acoustic wave element4is laminated on the boundary acoustic wave element3, thereby completing the boundary acoustic wave device1.

A thickness of at least about 100λ of the first solid layer2aof the boundary acoustic wave element2preferably made of the lithium niobate monocrystal substrate is sufficient. The thickness of the first solid layer3aand the first solid layer4a, preferably made of lithium niobate, is about 10λ, the thickness of the second solid layers2c,3c, and4c, made of silicon dioxide, is about 10λ, and the thickness of the IDTs is about 0.02λ. The overall thickness of the boundary acoustic wave device1is about 150λ.

FIG. 2is a front sectional view illustrating a boundary acoustic wave device11in accordance with a second preferred embodiment of the present invention. The boundary acoustic wave device11includes boundary acoustic wave elements12–14. In the second preferred embodiment, first solid layers12a,13a, and14aof the boundary acoustic wave elements12–14are preferably made of zinc oxide, and second solid layers12c,13c, and14care preferably made of borosilic acid glass. IDT12b, IDT13b, and IDT14bare preferably made of aluminum.

In the second preferred embodiment, the boundary acoustic wave elements12–14are bonded together using epoxy based bonding agents15and16including a silver filler after each of the boundary acoustic wave elements12–14is produced. The laminated boundary acoustic wave device11is thus provided. In the first preferred embodiment, the laminate is produced by successively forming the boundary acoustic wave elements. In the second preferred embodiment, the boundary acoustic wave elements12–14are bonded together for lamination after each of the elements12–14is produced.

The thickness of the first solid layers12a,13a, and14ais not limited to a particular dimension, but is about 30 λ, for example. The thickness of the first solid layers12a,13a, and14a, made of zinc oxide, is about 10 λ, and the thickness of the IDTs12b,13b, and14bis about 0.02 λ.

To bond the boundary acoustic wave devices, the bonding agent is applied on one of the boundary acoustic wave elements using spin coating under room temperature. In a vacuum, the bonding agent is degassed, and another boundary acoustic wave element is attached to the first boundary acoustic wave element. The device with the bonding agent is then heated and cured at a temperature of about 150° C.

In the second preferred embodiment, the epoxy based bonding agents15and16are preferably used. Another bonding agent may be used to bond the plurality of boundary acoustic wave elements. The bonding agent not only bonds the boundary acoustic wave elements, but also absorbs bulk waves leaked from the boundary acoustic wave element. In other words, the bonding agent functions as and defines an acoustic wave absorbing layer. The bonding agent defining an acoustic wave absorbing layer is not limited to the epoxy based bonding agent. The bonding agent may be an agent including an appropriate material having a large propagation loss to acoustic waves as a major component, for example, a silicone based bonding agent, a urethane based bonding agent, and a polyimide based bonding agent, or other suitable bonding agent.

In the second preferred embodiment, the epoxy based bonding agents15and16preferably include a silver filler. The bonding agent including an electrically conductive filler such as silver controls direct-path electromagnetic waves between adjacent boundary acoustic wave elements.

The particle diameter of the filler in the bonding agent defining an acoustic wave absorbing layer is important. When the particle diameters become large, the acoustic wave absorbing layer becomes thick. If the amount of leakage of bulk waves is small, the acoustic wave absorbing layer may be thin, with the particle diameter being in a range of about 1 to about 20 μm. Conversely, if the amount of leakage of bulk waves is large, the acoustic wave absorbing layer may be thick, with the filler particle diameter being in the range of about 10 μm to about 100 μm.

If the filler is a metal, such as silver, having a thermal conductivity higher than the solid layer of the boundary acoustic wave element, thermal conductivity between adjacent boundary acoustic wave elements is increased. Heat dissipation of the entire boundary acoustic wave device11increases, a temperature increase is reduced at the input of high power, and power withstand ability is greatly improved.

Generally, the boundary acoustic wave is reflected from the boundary if the acoustic wave propagates across media having different acoustic impedances. In the boundary acoustic wave device11, the epoxy based bonding agents15and16have different acoustic impedances from adjacent boundary acoustic wave elements. The epoxy based bonding agents15and16reduce the amount of leakage of acoustic waves between the adjacent boundary acoustic wave elements.

More specifically, the bonding agent15, disposed between the second solid layer12cof the boundary acoustic wave element12and the first solid layer13aof the boundary acoustic wave element13, controls the leakage of the acoustic waves between the second solid layer12cand the first solid layer13a.

The acoustic impedance per unit volume depends on the density and Young's modulus of the material. To make the acoustic impedance different, the density and/or the Young's modulus of the epoxy based bonding agents15and16are different from those of the solid layers in contact with the epoxy based bonding agents15and16.

As already discussed, the epoxy based bonding agents15and16not only bond the boundary acoustic wave elements12–14, but also function as an acoustic wave absorbing layer including a conductive filler, as a high thermal conductivity layer including a metal filler, and as an acoustic wave reflecting layer having a different acoustic impedance. It is not a requirement that the epoxy based bonding agents15and16have all of the functions described above. The epoxy based bonding agents15and16may have one or more of the three functions.

A layer functioning as at least one of the acoustic wave absorbing layer, the high thermal conductivity layer, and the acoustic wave reflecting layer may be interposed instead of the bonding agent. In such a case, a bonding agent may also be used for the bonding purpose only.

FIG. 3is a front sectional view illustrating a boundary acoustic wave device21in accordance with a third preferred embodiment of the present invention.

The boundary acoustic wave device21includes a first solid layer21amade of a piezoelectric material, an IDT21b, and a second solid layer21cmade of an insulating material. The first solid layer21a, the IDT21b, and the second solid layer21cmay be respectively identical in structure to the first solid layer2a, the IDT2b, and the second solid layer2cin the boundary acoustic wave element1of the first preferred embodiment. The upper and lower positional relationship between the first and second solid layers21aand21cshown inFIG. 3is reversed from that of the first and second solid layers2aand2cshown inFIG. 1.

The third preferred embodiment includes a surface acoustic wave exciting IDT22that is disposed on a top surface21a1of the first solid layer21a. In other words, in the boundary acoustic wave device21, the surface acoustic wave exciting IDT22is combined with the boundary acoustic wave element including the first and second solid layers21aand21c, and the boundary acoustic wave exciting IDT21b. Since the boundary acoustic wave element and the surface acoustic wave element are combined, the boundary acoustic wave device21which performs multiple functions is miniaturized.

In this way, a surface acoustic wave device may be integrated by providing a surface acoustic wave exciting interdigital transducer on a first solid layer made of a piezoelectric material in preferred embodiments of the present invention.

The surface acoustic wave exciting IDT22is disposed on the surface21a1of the first solid layer21ain the boundary acoustic wave device21. If the second solid layer21cis made of a piezoelectric material, a surface acoustic wave exciting IDT may also be disposed on a surface21c1of the second solid layer21c.

FIG. 4is a front sectional view illustrating a boundary acoustic wave device31in accordance with a fourth preferred embodiment of the present invention.

The boundary acoustic wave device31includes a surface acoustic wave element32laminated on a boundary acoustic wave element including a first solid layer31a, an IDT31b, and a second solid layer31c. The surface acoustic wave element32includes a surface acoustic wave substrate32aand an IDT32bprovided on the surface acoustic wave substrate32a. The surface acoustic wave substrate32amay be configured to include a piezoelectric substrate and a piezoelectric thin film disposed on the piezoelectric substrate or may be configured to include an insulating substrate and a piezoelectric thin film disposed on the insulating substrate.

As shown inFIG. 4, a surface acoustic wave element may be disposed on a boundary acoustic wave element in preferred embodiments of the present invention. The boundary acoustic wave device31, including the boundary acoustic wave element and the surface acoustic wave element, produces a boundary acoustic wave device having very small dimensions and multiple functions.

A boundary acoustic wave device51of a fifth preferred embodiment of the present invention will now be discussed with reference toFIGS. 5A and 5B, andFIGS. 6–9.

FIGS. 5A and 5Bare front sectional views of the boundary acoustic wave device51of the fifth preferred embodiment of the present invention. The boundary acoustic wave device51includes boundary acoustic wave elements52–56. The boundary acoustic wave element56, shown in enlargement inFIG. 5B, includes a laminate structure including a first solid layer56a, an IDT56b, a second solid layer56c, and an electrically conductive layer56d. The electrically conductive layer56dcovers a surface56c, of the second solid layer56c. The other boundary acoustic wave elements52–55preferably have substantially identical structure to the boundary acoustic wave element56. As shown inFIG. 5A, an electrically conductive film57is disposed on the boundary acoustic wave element56to provide electromagnetic shielding.

As shown inFIG. 6, the second solid layer52cis smaller in area than the first solid layer52ain a lower portion of the boundary acoustic wave element52. A portion of the boundary surface of the first solid layer52ais thus exposed. Electrode pads52dand52eare arranged on the exposed portions. The electrode pads52dand52eare arranged to provide electrical connection to the outside.

FIGS. 7A–7Care plan views diagrammatically illustrating the configuration of electrodes and electrical connections provided in each of the boundaries of the boundary acoustic wave elements52–56of the boundary acoustic wave device51.FIGS. 7B and 7Care plan views illustrating the shapes of conductive layers56dand57.

More specifically, each of the boundary acoustic wave elements52–56includes an IDT and reflectors arranged on both sides of the IDT along a direction of propagation of surface acoustic wave as shown inFIG. 7A. As shown, the letter A represents the IDT, and the letter B represents the reflectors. In other words, each of the boundary acoustic wave elements52–56is a one-port type boundary acoustic wave resonator. As shown, the boundary acoustic wave elements52–56are electrically connected to provide a ladder type filter.FIG. 8is a circuit diagram of the ladder type filter.

In accordance with preferred embodiments of the present invention, a plurality of boundary acoustic wave elements are laminated to define a complex circuit such as a ladder type filter without increasing a chip area dimension.

The circuit arrangement shown inFIGS. 7A–7CandFIG. 8is identical to the circuit arrangement of the known boundary acoustic wave device shown inFIG. 25. The fifth preferred embodiment provides a chip size of about 0.6×0.4 mm2, which is one-eighth the area of the known device shown inFIG. 25. Substantial miniaturization is thus achieved.

The first and second solid layers and the IDTs of the boundary acoustic wave elements52–56preferably have substantially identical structure to that of the boundary acoustic wave element2in the first preferred embodiment.

If an input IDT and an output IDT overlap in the laminate of a plurality of boundary acoustic wave elements as in the fifth preferred embodiment, a strong electromagnetic wave signal may be transmitted from the input IDT to the output IDT in a direct-path propagation. If the electromagnetic wave signal propagating in direct path is strong, out-of-band attenuation is reduced in the filter.

FIG. 10illustrates attenuation versus frequency characteristics of a band pass filter defined by the boundary acoustic wave device. When the direct-path electromagnetic wave signal is strong, the attenuation versus frequency characteristics are plotted as represented by the broken line. In the fifth preferred embodiment, the conductive layer56d, interposed between the boundary acoustic wave elements, controls the direct-path propagating electromagnetic wave. As represented by the solid line inFIG. 10, the out-of-band attenuation is increased. The conductive layers56dand57are preferably made of an appropriate metal such as aluminum.

The electrical interconnection of the boundary acoustic wave elements52–57, schematically shown inFIG. 7A, is achieved via through-hole electrodes. If the boundary acoustic wave travels along the boundary between the first and second solid layers of the boundary acoustic wave element, the boundary acoustic wave travels not only in the region of the IDT, but also travels and leaks outwardly from the IDT in a direction that is substantially perpendicular to the surface acoustic wave propagation direction as shown inFIG. 9.

A through-hole electrode is an electrode that is disposed in a through hole provided in a solid layer, in this case, the through-hole electrode may include at least three configurations. First, the electrode may be disposed on the surface of the through hole and the hole may have a cavity. Second, the electrode may be filled in the through hole and there is no cavity, and third the electrode may be disposed on the surface of the through hole and an elastic material may be filled in the cavity of the through hole with the electrode provided on the surface thereof. The elastic material may be made of a conductive material, a resin material, or a glass material. Further, the electrode and the elastic material may be made of same conductive material. If a through-hole electrode has a cavity, a difference between the acoustic impedance of the through-hole electrode and the acoustic impedance of the first and second solid layers increases, and the boundary acoustic wave may be reflected, scattered, or resonated. As a result, unwanted spurious waves and ripples may be included in the characteristics, and the attenuation may be reduced.

Preferably, the through-hole is filled with an elastic material so as not to cause a void after the through-hole electrode is produced by disposing a conductive film in a through hole. With the through-hole electrode filled with the elastic material, the acoustic impedance difference between the through-hole electrode and the first and second solid layers is greatly reduced.

The IDTs are preferably formed by using a photolithographic technique. If the through-hole electrode remains hollow, a problem may arise in the application of a resist and the vacuum chucking process of a wafer. By filling the through-hole electrode with the elastic material, such problems are prevented.

If a conductor, such as copper, is used as an elastic material, wiring resistance of the through-hole electrode is decreased.

If the through-hole portion of the through-hole electrode is not filled with the elastic material, gas tends to enter the central portion of the boundary acoustic wave element from the outside. If a corrosive gas ingresses, performance may be degraded.

Even if the cavity inside the through-hole electrode is filled with the elastic material, the solid layers of the boundary acoustic wave element have a different thermal expansion coefficient from the elastic material. Stress occurs in the device, thereby leading to a crack. There is a possibility that a corrosive gas enters from the outside. If some of the solid layers of the boundary acoustic wave element are made of a polycrystal, such as silicon dioxide or zinc oxide, a corrosive gas may enter through the crack, thereby corroding the electrodes.

Through-hole electrodes61and65shown inFIGS. 11 and 12, respectively, are preferred, if any through-hole is provided. As shown inFIG. 11, the through-hole electrode61in a boundary acoustic wave device including boundary acoustic wave elements62–64includes a through-hole section61athat extends from the boundary along which the boundary acoustic wave of the boundary acoustic wave element64travels to the boundary along which the boundary acoustic wave element63travels, a through-hole section61bthat extends from the boundary along which the boundary acoustic wave of the boundary acoustic wave element63travels to the boundary along which the boundary acoustic wave of the boundary acoustic wave element62travels, and a through-hole section61cthat extends from the boundary along which the boundary acoustic wave of the boundary acoustic wave element62travels to a surface62a1of the solid layer62aof the boundary acoustic wave element62. The through-hole sections61a–61care arranged so that they do not overlap in the laminate structure in the direction of thickness thereof. The through-hole section61aand through-hole section61bare electrically connected to each other by an interconnect conductor61d, and the through-hole section61band the through-hole section61care electrically connected to each other by an interconnect conductor61e. The interconnect conductors61dand61eare provided in the boundary surfaces of the boundary acoustic wave elements63and62, respectively.

In the through-hole electrode65shown inFIG. 12, through-hole sections65a,65b,65c, and65dare connected by interconnect conductors65e,65f, and65g. The through-hole sections65a,65b,65c, and65drespectively penetrate a first solid layer66aof a boundary acoustic wave element66, a second solid layer67cof a second boundary acoustic wave element67, a first solid layer67aof the second boundary acoustic wave element67, and a second solid layer68cof a first boundary acoustic wave element68. In the through-hole electrode65shown inFIG. 12as well, adjacent through-hole sections do not overlap in the direction of lamination.

As shown inFIGS. 11 and 12, the location of upper and lower through-hole sections are shifted and an interconnect conductor provided in a plane at a height substantially in parallel with the boundary acoustic wave propagating surface of the laminate structure connects the upper and the lower through-hole sections. This arrangement controls the ingress of corrosive gas, thereby greatly improving the reliability of the device. In this case, the plane at the height substantially in parallel with the boundary acoustic wave propagating surface may be the boundary acoustic wave propagating surface, a boundary between adjacent boundary acoustic wave elements, or a plane extending at a middle height of one of the first and second solid layers.

In other words, a plurality of through hole electrodes are provided in a plurality of layers of the laminate. Among the layers, the location of the through hole at the upper layer is different from that of the through hole at the lower layer. The upper through hole is connected via a connection electrode to the lower through hole. The connection electrode is arranged on a layer between the upper and lower through holes. The connection electrode is arranged along a surface of the layer.

FIGS. 13is an exploded perspective view illustrating a boundary acoustic wave device71in accordance with a sixth preferred embodiment of the present invention, andFIG. 14is a plan view diagrammatically illustrating the layout of electrodes in the boundary acoustic wave device71.

As shown inFIG. 13, the boundary acoustic wave device71includes first and second boundary acoustic wave elements72and73. The first boundary acoustic wave element72includes a first solid layer72a. Disposed on the first solid layer72aare IDTs72band72c, reflectors72dand72earranged on both sides of the IDTs72band72c, and input electrode pads72fand72g. The boundary acoustic wave element72is preferably formed by laminating a second solid layer72hon the first solid layer72a.

Similarly, IDTs73band73cand reflectors73dand73eare disposed on a first solid layer73ain the second boundary acoustic wave element73. Input electrode pads73fand73gare further disposed on the first solid layer73a. A second solid layer73his laminated on the first solid layer73a.

The first and second boundary acoustic wave elements72and73respectively form longitudinally coupled boundary acoustic wave filters.

Through-hole electrodes74and75are connected to the IDT72c. The through-hole electrodes74and75penetrate the first solid layer72aand the second solid layer73hand electrically connect the IDT72cand the IDT73b. In the boundary acoustic wave device71, two stages of the longitudinally coupled boundary acoustic wave filters are thus cascaded. By cascading the two stages of the longitudinally coupled boundary acoustic filters, the two-stage cascaded boundary acoustic wave device has a greatly reduced area size.

As clearly seen inFIGS. 13 and 14, the size of the first solid layer73a, the second solid layer73h, and the second solid layer72hdecreases from bottom to top in a direction that is substantially perpendicular to the boundary acoustic wave propagating direction. More specifically, c represents a width dimension of the first solid layer73a, b represents a width dimension of the second solid layer73h, b represents a width dimension of the first solid layer72a, and a represents a width dimension of the second solid layer72h, and a relationship of c>b>a is maintained as shown inFIG. 14.

In the boundary acoustic wave device71, the input electrode pads72fand72g, and output electrode pads73fand73g, which are outwardly exposed, provide electrical connection to the outside.

The input IDT72barranged in the first boundary acoustic wave element and the output IDT73carranged in the second boundary acoustic wave element are shifted in location from each other in the direction of lamination in the boundary acoustic wave device71. This arrangement controls direct-path electromagnetic waves and the leakage of bulk waves.

Electromagnetic waves directly traveling across two IDTs provided in different layers depends on a (direct-path) capacitance caused between the two IDTs. The direct-path capacitance is proportional to an area of overlap of the two IDTs. The direct-path capacitance is controlled in proportion to the area of overlap by reducing the area of overlap.

As shown inFIGS. 13 and 14, any limitation to the area dimensions of the electrodes and the elements eliminates the need to shift the input IDT72band the output IDT73cto a nonoverlapping state in a plan view. For example, if the overlapping area is reduced by 10%, the direct-path capacitance is also reduced by 10%. If the location of the input IDT72band the output IDT73care shifted with respect to each other, out-of-band attenuation in the filter is improved accordingly, and a 100% nonoverlapping arrangement is not a requirement.

In a structure where at least three boundary acoustic wave elements are laminated, a direct-path capacitance at an intermediate stage is reduced not only by shifting an IDT connected to an input terminal from an IDT connected to an output terminal in position, but also by shifting an IDT arranged at the intermediate stage. In this case, the out-of-band attenuation of the filter is also increased.

If a plurality of IDTs are longitudinally coupled to produce a longitudinally coupled filter or a transversal type filter, IDTs, other than IDTs which are electrically connected through conductive wiring for longitudinal connection, are preferably shifted from each other in the direction of lamination.

Not only the overlapping area of the IDTs, but also the overlapping areas of bus bars and wiring electrodes connected to the IDTs are preferably reduced. The direct-path capacitance is further reduced.

If a reflector, which is electrically unconnected to the IDT, is placed close to an IDT within a range of about 3λ, a direct-path capacitance is created from a path of the IDT provided on one boundary acoustic wave element—the reflector provided on the boundary acoustic wave element—a reflector provided on another boundary acoustic wave element—and an IDT provided on the other boundary acoustic wave element. Thus, the location of the reflector is also preferably shifted. The direct-path capacitance is further reduced, and the out-of-band attenuation is further increased.

If a plurality of boundary acoustic wave element are electrically connected via the though-hole electrodes, the location of the through-hole electrode is flexibly set. The area dimensions of the boundary acoustic wave device are further reduced.

When a monocrystal material is used for the solid layer of the boundary acoustic wave element, formation of the through-hole electrode is difficult. For example, a through-hole may be formed via reactive etching using a gas mixture of Ar and CF4. If the boundary acoustic wave element is thick, assuring vertical stability and integrity of the through-hole becomes difficult, and process time increases. The solid layer with a through-hole drilled therein has a lower mechanical strength. Stress due to a mounting operation of a circuit board and ambient temperature change may lead to cracks.

As already discussed, the boundary acoustic wave may be reflected or scattered by the through-hole electrode, and corrosive gas may ingress into the device.

The plurality of boundary acoustic wave elements that are laminated together may be electrically connected through a wiring electrode that is routed to the external surface of the boundary acoustic wave device rather than by using the through-hole electrode.

FIG. 15is a perspective view illustrating wiring electrodes84and85mounted on the external surface of such a boundary acoustic wave device81. The boundary acoustic wave device81includes a plurality of steps83a–82cformed on the external surface thereof. The steps82a–82care easily provided by differentiating the two dimensional shape. More specifically, the steps are easily provided by differentiating the width dimension of the solid layer to be laminated or a depth dimension perpendicular to the width of the solid layer as shown inFIG. 13.

The steps83a–83care formed in a wafer resulting in several tens to several tens of thousands of boundary acoustic wave elements, and wiring electrodes are then collectively disposed using a photolithographic technique, screen printing, and a plating method. After that, the wafer is diced into individual boundary acoustic wave devices. The wiring electrodes are also diced. In this way, the electrical connection between the boundary acoustic wave elements is achieved in a cost effective manner.

FIG. 16is a front sectional view illustrating a boundary acoustic wave device91in accordance with a seventh preferred embodiment of the present invention.

The boundary acoustic wave device91includes boundary acoustic wave elements92–94. The boundary acoustic wave element92includes a first solid layer92apreferably made of lithium tantalate, an IDT92b, and a second solid layer92cpreferably made of silicon dioxide. Each of the boundary acoustic wave element93and the boundary acoustic wave element94preferably has substantially identical structure to the boundary acoustic wave element92.

The seventh preferred embodiment of the present invention includes a high thermal-conductivity layer95, a circuit element layer99, and a high thermal-conductivity layer96interposed between the boundary acoustic wave element92and the boundary acoustic wave element93, and a high thermal-conductivity layer97, a circuit element layer100, and a high thermal-conductivity layer98interposed between the boundary acoustic wave element93and the boundary acoustic wave element94. More specifically, the circuit element layer99is interposed between the boundary acoustic wave element92and the boundary acoustic wave element93. The circuit element layer99includes electrodes99band99cdisposed on an epoxy substrate99athat is softer than and has a larger dielectric constant than lithium tantalate. The electrodes99band99cdefines an impedance matching circuit, for example. The shape and number of the electrodes99band99care determined depending on functions of the device.

Epoxy bonding agent layers99dand99e, disposed above and below, cover the epoxy substrate99aand the electrodes99band99cin the circuit element layer99. The circuit element layer99is bonded to the high thermal-conductivity layers95and96using the epoxy bonding agent layers99dand99e. The high thermal-conductivity layers95and96respectively cover a surface92c1of the second solid layer92cof the first boundary acoustic wave element92and a surface93a1of the first solid layer93aof the second boundary acoustic wave element93.

The circuit element layer100and the high thermal-conductivity layers97and98are also provided between the boundary acoustic wave element93and the boundary acoustic wave element94.

The high thermal-conductivity layers95and96are preferably made of carbon film. The material of the high thermal-conductivity layers95and96is not limited to any particular material. Any material is acceptable as the material for the high thermal-conductivity layers95and96as long as the material has a thermal conductivity greater than that of the material of the first and second solid layers of the boundary acoustic wave element in contact therewith. With the high thermal-conductivity layers95and96being provided, heat dissipation performance of the boundary acoustic wave device91is greatly improved. This arrangement controls degradation due to direct-path waves when high power is applied.

Variations in frequency and temperature characteristics of a solid layer through which the boundary acoustic wave propagates are determined by a variation in acoustic wave speed responsive to unit temperature change, and a variation in the length of the substrate along the wave propagation direction responsive to unit temperature change. Variations in frequency and temperature characteristics are reduced if the contraction and expansion of the substrate with temperature are controlled. The linear expansion coefficient of the carbon film defining the high thermal-conductivity layers95–98is less than the linear expansion coefficient of the solid layers of the boundary acoustic wave elements92–94. The carbon film restricts the thermal expansion of the boundary acoustic wave elements92–94in the horizontal direction. This arrangement controls breakdown of the boundary acoustic wave device91due to stress arising from a thermal expansion coefficient difference if the boundary acoustic wave device91is mounted on a hard ceramic substrate having a small thermal expansion coefficient. In preferred embodiments of the present invention, a low linear thermal expansion coefficient layer that is less than the solid layer of the boundary acoustic wave element in linear thermal expansion coefficient, such as the carbon thin film, may be provided. In the seventh preferred embodiment, the carbon thin film functions as both the high thermal conductivity layer and the low linear thermal expansion coefficient layer. Alternatively, a material layer providing only one of the two functions may be provided.

A linear thermal expansion coefficient layer having a polarity that is opposite to the linear thermal expansion coefficient of the solid layer of the boundary acoustic wave element may be provided instead of the low linear thermal expansion coefficient layer.

With the circuit element layers99and100interposed, the boundary acoustic wave device91includes an appropriate circuit such as an impedance matching circuit.

Any appropriate electrode may be provided as the electrodes99band99cdepending on a desired function. For example, a circuit element to be incorporated may be a capacitance element, a strip line, a micro strip filter including a stub and a strip line, or a mixer. A resistor element may be included using a resistance material instead of the electrode. The circuit element provided on the circuit element layers99and100is not limited to any particular element.

If an impedance matching circuit or a modulator circuit of the boundary acoustic wave element is provided using the circuit element layers99and100, a corresponding external circuit may be omitted.

To perform wire bonding or bump bonding on the external surface of the boundary acoustic wave device91, an electrical interconnect line that has a length as long as several tens to several thousands μm is provided. The characteristic impedance of such a line is typically different from the input and output impedance of a boundary acoustic wave filter. This can increase reflection loss due to impedance mismatching of the line. If the circuit element layers99and100are incorporated, no long line is required, and the reflection loss is greatly reduced. In a frequency range above 1 GHz, in particular, shortening of the line substantially improves the characteristics.

In the seventh preferred embodiment, the epoxy substrate99ais covered with the epoxy bonding agent layers99dand99e, which are soft dielectric layers. Thus, direct-path bulk waves leaked from between boundary acoustic wave elements are absorbed.

The direct-path capacitance is reduced by reducing the overlapping area between the input IDT and the output IDT in the sixth preferred embodiment. If a plurality of boundary acoustic wave elements are laminated at intervals of about 10λ, such an arrangement may not be sufficient. If a piezoelectric substrate having a large electromagnetic factor is used for the first solid layer, a direct-path capacitance is produced between the boundary acoustic wave elements because of the large dielectric constant thereof. Sometimes, a direct-path capacitance greater than about 10 pF may occur. Thus, configurations other than restricting the direct-path capacitance by reducing the overlapping area of the IDTs are preferred.FIG. 17diagrammatically illustrates a direct-path capacitance C that occurs between an input terminal and an output terminal with longitudinally coupled boundary acoustic wave filters111and112cascaded. Let f0represent a center frequency of the boundary acoustic wave filter, and a parallel circuit of the direct-patch capacitance C and an inductor L is set in an anti-resonance state at the center frequency f0if the direct-path capacitance C and the inductor L having an inductance of {(2×π×f0)2×C}−1are connected as shown inFIG. 18. Thus, no degradation in performance is caused by the direct-path capacitance.

If the parallel inductor L is merely connected as discussed above, improvements in the characteristics are limited to a region at and around the center frequency f0. Preferably, a micro strip line filter or an LC filter, having a wide bandpass characteristic, is disposed in each of the circuit element layers99and100shown inFIG. 16. This arrangement provides wide band and increased attenuation filter characteristics.

FIG. 19is a front sectional view illustrating a boundary acoustic wave device121in accordance with an eighth preferred embodiment of the present invention. The boundary acoustic wave device121includes a laminate of boundary acoustic wave elements122,123,124, and125. The eighth preferred embodiment is different from the first preferred embodiment in that a cavity D is provided between the boundary acoustic wave elements123and124. The boundary acoustic wave element123is electrically connected to the boundary acoustic wave element124via metal bumps126and127. The height of the cavity D is defined by a height dimension of the metal bumps126and127. Element129is an epoxy resin.

It is not a requirement that the boundary acoustic wave elements be closely in contact with each other. With the cavity D, the boundary acoustic wave elements may be laminated. In this arrangement, thermal source is distributed in the boundary acoustic wave device121, and a temperature increase in the entire boundary acoustic wave device121is controlled.

The boundary acoustic wave device of preferred embodiments of the present invention has a laminate structure in which a plurality of boundary acoustic wave elements are laminated. The boundary acoustic wave does not propagate along the surface of each boundary acoustic wave element. Thus, a package structure having a cavity is not required. In addition, it is not necessary to encapsulate the boundary acoustic wave device in a package.

If the boundary acoustic wave device is used only for a short period of time, the addition of a housing or a package to the device is not required.

If the boundary acoustic wave device is intended for use for a long period of time, the boundary acoustic wave device is subject to a breakdown due to a corrosion of the solid layer defining the boundary acoustic wave element, corrosion of the electrode caused by a corrosive gas ingressing into the solid layer, and stress generated due to a difference in thermal expansion coefficient between the circuit board and the boundary acoustic wave device in the course of repeated ambient temperature cycles. The use of the housing is preferred to enhance the reliability of the device for use for a long period of time.

The formation of a cavity as in the boundary acoustic wave device or a monolithic filter is not required. This is because the energy of boundary acoustic waves reaching the chip surface is extremely small.

FIGS. 20 through 22are front sectional views illustrating modifications of the boundary acoustic wave device of preferred embodiments of the present invention.

A boundary acoustic wave device131shown inFIG. 20includes an electrically conductive layer133and an insulating layer134disposed on the surface of a boundary acoustic wave device chip132. The conductive layer133may be a laminated metal layer made of a gold film and a nickel film laminated in that order, or a single metal film. The use of the conductive layer133electromagnetically shields the boundary acoustic wave device chip132. The insulating layer134made of an insulating material, such as aluminum oxide (Al2O3) or aluminum nitride (AlN), protects the boundary acoustic wave device chip132. The boundary acoustic wave device chip132is entirely covered with an encapsulation resin layer137except for the connection electrode pads135and136. The use of the encapsulation resin layer137reliably keeps out corrosive gases.

The boundary acoustic wave device chip132is shown as having a single layer. As already discussed in connection with preferred embodiments described above, in practice, the boundary acoustic wave device chip includes a plurality of boundary acoustic wave elements.

A boundary acoustic wave device141shown inFIG. 21includes an electrically conductive layer133, an insulating layer134, and an encapsulation resin layer137in the boundary acoustic wave device chip thereof as in the boundary acoustic wave device131. The boundary acoustic wave device141is different from the boundary acoustic wave device131in that the boundary acoustic wave device chip132is bonded onto electrode pads143and144mounted on a hard ceramic substrate142via ultrasonic welding. After being bonded, the boundary acoustic wave device chip132is covered with an encapsulation resin layer137. The boundary acoustic wave device chip132is rigidly secured to the substrate142. If the boundary acoustic wave device141is mounted on a printed circuit board, stress acting on the boundary acoustic wave device chip132from the printed circuit board is reduced by the ceramic substrate142. In other words, the ceramic substrate142functions as a stress relaxation layer.

A boundary acoustic wave device151shown inFIG. 22includes a laminate of a plurality of boundary acoustic wave elements152–156. The two dimensional shapes of the boundary acoustic wave elements152–156are different from each other. Steps are produced on the external surface as shown, and a conductive layer133is arranged on the steps. The conductive layer133on the steps electrically connects the boundary acoustic wave elements152–156. The conductive layer133is provided using a conductive paste. The flexibility of the conductive paste alleviates stress acting on the boundary acoustic wave device151when being mounted on the printed circuit board. The external conductive layer, made of the flexible conductive paste, functions as a stress relaxation layer.

Since the boundary acoustic wave device of the preferred embodiments of the present invention includes the laminate of the plurality of boundary acoustic wave elements, the two dimensional area size of the device is less than the known boundary acoustic wave device. Thus, the boundary acoustic wave device for use in a band pass filter or a resonator is further miniaturized.

Since the boundary acoustic wave device uses the boundary acoustic wave, a package having a cavity, which is expensive to produce, in not required. Thus, the cost of the band pass filter or the resonator is greatly reduced, and the manufacturing process of the band pass filter or the resonator is greatly simplified.

The boundary acoustic wave device of the preferred embodiments of the present invention includes the first solid layer made of the piezoelectric material, the second solid layer laminated on the first solid layer and made of one of the piezoelectric material and the insulating material, the boundary acoustic wave exciting IDT arranged in the boundary between the first and second solid layers, and the surface acoustic wave exciting IDT arranged on the surface of the first solid layer. The boundary acoustic wave element and the surface acoustic wave element are combined into a unitary body. A miniaturized boundary acoustic wave device including the boundary acoustic wave element and the surface acoustic wave element combined in the unitary body is provided.

A miniaturized band pass filter and resonator for use in a high frequency region above 1 GHz are thus provided.

The present invention is not limited to each of the above-described preferred embodiments, and various modifications are possible within the range described in the claims. An embodiment obtained by appropriately combining technical features disclosed in each of the different preferred embodiments is included in the technical scope of the present invention.