High-frequency device and multiplexer

A high-frequency device includes: a circuit substrate including dielectric layers that are stacked, wiring patterns located on at least one of the dielectric layers, and a passive element formed of at least one of the wiring patterns, the circuit substrate having a first surface that is a surface of an outermost dielectric layer in a stacking direction of the dielectric layers; a terminal for connecting the high-frequency device to an external circuit, the terminal being located on the first surface and electrically connected to the passive element through a first path in the circuit substrate; and an acoustic wave element located on the first surface and electrically connected to the passive element through a second path in the circuit substrate.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2019-017367, filed on Feb. 1, 2019, the entire contents of which are incorporated herein by reference.

FIELD

A certain aspect of the present embodiments relates to a high-frequency device and a multiplexer.

BACKGROUND

There has been known a filter in which an acoustic wave resonator is provided to an LC circuit formed of a capacitor and an inductor as disclosed in, for example, Japanese Patent Application Publication Nos. 2018-129680 and 2018-129683 (hereinafter, referred to as Patent Documents 1 and 2, respectively). It has been known to mount an acoustic wave element on a circuit substrate as disclosed in, for example, Japanese Patent Application Publication No. 2014-14131 (hereinafter, referred to as a Patent Document 3).

SUMMARY OF THE INVENTION

According to a first aspect of the present embodiments, there is provided a high-frequency device including: a circuit substrate including dielectric layers that are stacked, wiring patterns located on at least one of the dielectric layers, and a passive element formed of at least one of the wiring patterns, the circuit substrate having a first surface that is a surface of an outermost dielectric layer in a stacking direction of the dielectric layers; a terminal for connecting the high-frequency device to an external circuit, the terminal being located on the first surface and electrically connected to the passive element through a first path in the circuit substrate; and an acoustic wave element located on the first surface and electrically connected to the passive element through a second path in the circuit substrate.

According to a second aspect of the present embodiments, there is provided a multiplexer including the above high-frequency device.

DETAILED DESCRIPTION

When an acoustic wave element is embedded in a circuit substrate, the acoustic wave element deteriorates due to the heat and pressure generated when the circuit substrate is formed. When an acoustic wave element is mounted on the upper surface of the circuit substrate, the acoustic wave element may be broken by impact.

Hereinafter, a description will be given of embodiments with reference to the accompanying drawings.

First Embodiment

A first embodiment is an exemplary filter as a high-frequency device.FIG. 1is a circuit diagram of a high-frequency device in accordance with the first embodiment. As illustrated inFIG. 1, a high-frequency device100in accordance with the first embodiment includes a resonant circuit60and an acoustic wave resonator R. The resonant circuit60includes capacitors C1and C2and an inductor L. The capacitors C1and C2are connected in series between terminals T1and T2. The inductor L is connected in parallel to the capacitors C1and C2between a node N1, which is located between the terminal T1and the capacitor C1, and a node N2, which is located between the terminal T2and the capacitor C2. A first end of the acoustic wave resonator R is connected to a node N3between the capacitors C1and C2, and a second end of the acoustic wave resonator R is connected to a ground terminal Tg.

The high-frequency device100functions as a low-pass filter or a high-pass filter. The high-frequency device100transmits signals in the passband to the terminal T2among high-frequency signals input to the terminal T1, and suppresses signals in other bands. For example, the capacitances of the capacitors C1and C2are configured to be 5.5 pF, the inductance of the inductor L is configured to be 1.5 nH, the resonant frequency of the acoustic wave resonator R is configured to be 2.26 GHz, and the antiresonant frequency of the acoustic wave resonator R is configured to be 2.33 GHz. This configuration causes the high-frequency device100to function as a low-pass filter having a passband that is a frequency band lower than the resonant frequency.

For example, the capacitances of the capacitors C1and C2are configured to be 7.1 pF, the inductance of the inductor L is configured to be 2 nH, the resonant frequency of the acoustic wave resonator R is configured to be 2.26 GHz, and the antiresonant frequency of the acoustic wave resonator R is configured to be 2.33 GHz. This configuration causes the high-frequency device100to function as a high-pass filter having a passband that is a frequency band higher than the resonant frequency. A filter having steep transition from the passband to the stopband is achieved by configuring the resonant frequency of the acoustic wave resonator to be located near the passband.

FIG. 2is a cross-sectional view of the filter in accordance with the first embodiment. As illustrated inFIG. 2, the direction in which dielectric layers20ato20fare stacked (the stacking direction of the dielectric layers20ato20f) is defined as a Z direction, and the planar directions of the dielectric layers20ato20fare defined as an X direction and a Y direction. In a circuit substrate20, the dielectric layers20ato20fare stacked in the Z direction. The lower surface of the dielectric layer20ais a lower surface21a(i.e., a first surface that is a surface of the outermost dielectric layer of the dielectric layers20ato20fin the Z direction) of the circuit substrate20, and the upper surface of the dielectric layer20fis an upper surface21b(i.e., a second surface that is an opposite surface of the circuit substrate20from the lower surface21a) of the circuit substrate20. A region66ais a first region located closer to the lower surface21athan a center64in the Z direction in the circuit substrate20, and a region66bis a second region located closer to the upper surface21bthan the center64.

Wiring patterns22ato22eare respectively located on the dielectric layers20ato20e. Via wirings24ato24erespectively penetrating through the dielectric layers20ato20eare provided. A direction identification mark21is provided on the upper surface21bof the circuit substrate20. Terminals23,25a, and25bare located on the lower surface21aof the circuit substrate20. The terminals23include the terminals T1and T2and the ground terminal Tg. Bumps26are located on the terminals23. Terminals33of an acoustic wave element10are bonded to the terminals25aand25bthrough bumps28. This structure mounts the acoustic wave element10on the terminals25aand25b.

The wiring patterns22band22cface each other across the dielectric layer20c. This structure allows the wiring patterns22band22cand the dielectric layer20cto form the capacitors C1and C2. The wiring patterns22dand22eform the inductor L. The capacitors C1and C2are located in the region66ain the circuit substrate20, and the inductor L is located in the region66bof the circuit substrate20.

The capacitors C1and C2are electrically connected to the terminals T1and T2through the wiring patterns22cand the via wirings24ato24c. The inductor L is electrically connected to the terminals T1and T2through the via wirings24ato24e. A node to which the wiring patter22c, and the via wirings24ato24cconnected to the terminal T1connect is the node N1, and a node to which the wiring patter22c, and the via wirings24ato24cconnected to the terminal T2connect is the node N2. A path L1is a path connecting the node N1and the terminal T1.

The capacitors C1and C2are electrically connected to the terminal25bthrough the via wirings24aand24b. The terminal25ais electrically connected to the ground terminal Tg through the via wiring24aand the wiring patter22a. A node to which the wiring pattern22b, and the via wirings24aand24bconnected to the terminal25bconnect is the node N3. A path L2is a path connecting the node N3and the terminal25b. A path L3is a path connecting the terminal25aand the ground terminal Tg.

The dielectric layers20ato20fare made of an inorganic insulating material such as, but not limited to, a ceramic material or an organic insulating material such as, but not limited to, resin. The dielectric layers20ato20fcontain an oxide of silicon (Si), calcium (Ca), and magnesium (Mg) (for example, diopside (CaMgSi2O6)) as a main component. The wiring patterns22ato22e, the terminals23,25a, and25b, and the direction identification mark21are formed of metal layers containing, for example, silver (Ag), palladium (Pd), platinum (Pt), copper (Cu), nickel (Ni), gold (Au), a gold-palladium alloy, or a silver-platinum alloy. The terminals23,25a, and25band the direction identification mark21may include a plated layer made of nickel or the like on the aforementioned metal layer. The bumps26and28are metal bumps such as, but not limited to, solder bumps, gold bumps, or copper bumps. The terminals23and the bumps26function as terminals for connecting the circuit substrate20to an external element.

Exemplary dimensions are as follows. The thickness D1of the circuit substrate20is 0.4 mm, the distance D2between the lower surface of the circuit substrate20and the lower surface of the bump26is 0.3 mm, the distance D3between the lower surface of the circuit substrate20and the upper surface of the acoustic wave element10is 0.02 mm, the height D4of the acoustic wave element10is 0.1 mm, and the width W1of the circuit substrate20is 2.5 mm. Since D2is greater than D3+D4, the acoustic wave element10is protected by the bumps26. The dielectric layers20ato20fare, for example, 6 layers to 12 layers.

FIG. 3Ais a plan view of an acoustic wave resonator in the first embodiment, andFIG. 3Bis a cross-sectional view of another acoustic wave resonator of the first embodiment. In the example ofFIG. 3A, the acoustic wave resonator12is a surface acoustic wave resonator. An interdigital transducer (IDT)50and reflectors52are located on the upper surface of the substrate11. The IDT50includes a pair of comb-shaped electrodes50afacing each other. The comb-shaped electrode50aIncludes electrode fingers50band a bus bar50cconnecting the electrode fingers50b. The reflectors52are located at both sides of the IDT50. The IDT50excites the surface acoustic wave in the substrate11. The substrate11is a piezoelectric substrate such as, but not limited to, a lithium tantalate substrate, a lithium niobate substrate, or a crystal substrate. The substrate11may be a composite substrate in which a piezoelectric substrate is bonded on a support substrate such as, but not limited to, a sapphire substrate, a spinel substrate, an alumina substrate, a crystal substrate, or a silicon substrate. An insulating film such as a silicon oxide film or an aluminum nitride film may be located between the support substrate and the piezoelectric substrate. The IDT50and the reflectors52are formed of, for example, an aluminum film or a copper film. A protective film or a temperature compensation film may be located on the substrate11so as to cover the IDT50and the reflectors52.

In the example ofFIG. 3B, the acoustic wave resonator12is a piezoelectric thin film resonator. A piezoelectric film56is located on the substrate11. A lower electrode54and an upper electrode58are provided so as to sandwich the piezoelectric film56between them. An air gap55is formed between the lower electrode54and the substrate11. The region where the lower electrode54and the upper electrode58face each other across at least a part of the piezoelectric film56is a resonance region57. The lower electrode54and the upper electrode58in the resonance region57excite the acoustic wave in the thickness extension mode in the piezoelectric film56. The substrate11is, for example, a sapphire substrate, a spinel substrate, an alumina substrate, a glass substrate, a crystal substrate, or a silicon substrate. The lower electrode54and the upper electrode58are formed of a metal film such as, but not limited to, a ruthenium film. The piezoelectric film56is, for example, an aluminum nitride film.

FIG. 4is a cross-sectional view of an acoustic wave element in the first embodiment. As illustrated inFIG. 4, in the acoustic wave element10, a substrate11is mounted on a wiring substrate30. The wiring substrate30includes insulating layers30aand30bthat are stacked. The insulating layers30aand30bare, for example, resin layers or ceramic layers. Wiring layers32band a ring-shaped metal layer36are located on the upper surface of the wiring substrate30. Wiring layers32aare located on the insulating layer30a. Via wirings34aand34brespectively penetrating through insulating layers30aand30bare provided. The terminals33are located on the lower surface of the wiring substrate30. The wiring layers32aand32b, the via wirings34aand34b, and the terminals33are formed of metal layers such as, but not limited to, copper layers, gold layers, aluminum layers, or nickel layers.

An acoustic wave resonator12and wiring lines14are located on the lower surface of the substrate11. The acoustic wave resonator12is the acoustic wave resonator illustrated inFIG. 3AorFIG. 3B. The wiring lines14are formed of a metal layer such as, but not limited to, a copper layer, a gold layer, and an aluminum layer. The wiring lines14and the wiring layers32bare bonded by bumps16. The bumps16are metal bumps such as, but not limited to, gold bumps, copper bumps, or solder bumps. The substrate11is flip-chip mounted on the wiring substrate30with use of the bumps16such that the acoustic wave resonator12faces the wiring substrate30across an air gap18. The terminals33are electrically connected to the acoustic wave resonator12through the via wirings34a, the wiring layers32a, the via wirings34b, the wiring layers32b, the bumps16, and the wiring lines14.

A sealing portion35is provided so as to surround the substrate11. The lower surface of the sealing portion35is bonded to the ring-shaped metal layer36. The sealing portion35is made of a metal such as, but not limited to, solder or an insulating material such as, but not limited to, resin. A lid38is located on the upper surfaces of the substrate11and the sealing portion35. The acoustic wave element10may be a bare chip in which the acoustic wave resonator12is not sealed. The lid38is a metal plate or an insulator plate. InFIG. 2, the terminals33are connected to the terminals25aand25bof the circuit substrate20through the bumps28.

FIG. 5AthroughFIG. 6Bare cross-sectional views illustrating a method of manufacturing the filter in accordance with the first embodiment.FIG. 5AthroughFIG. 6Aare upside down views ofFIG. 2. That is, the lower surface21aof the circuit substrate20is illustrated at the upper side, and the upper surface21bis illustrated at the lower side. As illustrated inFIG. 5A, the circuit substrate20is formed. As illustrated inFIG. 5B, the acoustic wave element10is mounted on the circuit substrate20by using the bumps28. The bumps28are, for example, solder bumps.

As illustrated inFIG. 6A, the bumps26are formed on the terminals23. The bump26includes a core26aand an outer layer26bcovering the core26a. The outer layer26bis, for example, a solder layer, and is, for example, a tin silver solder layer. The core26ais made of a metal or an insulating material having a melting point greater than the melting point of the outer layer26b, and is made of, for example, copper. For example, when the outer layer26bis made of tin silver, the melting point of the core26ais 220° C. or greater. The height D5of the core26ais greater than D3+D4. The step ofFIG. 5Band the step ofFIG. 6Amay be carried out in reverse order, or the step ofFIG. 5Band the step ofFIG. 6Amay be simultaneously carried out. Through the above process, the filter of the first embodiment is completed.

As illustrated inFIG. 6B, terminals42are located on the upper surface of a mounting board40. The bumps26are bonded to the terminals42. This process mounts the high-frequency device100on the mounting board40. The cores26ainhibit the lower surface of the acoustic wave element10from coming in contact with the mounting board40.

First Comparative Example

FIG. 7is a cross-sectional view of a high-frequency device in accordance with a first comparative example. As illustrated inFIG. 7, in a high-frequency device110in accordance with the first comparative example, the terminals25aand25bare located on the upper surface21bof the circuit substrate20. The acoustic wave element10is mounted on the upper surface21bof the circuit substrate20. The terminal25band the node N3are electrically connected through via wirings24cto24f. The terminal25aand the ground terminal Tg are electrically connected through the via wirings24ato24f. Other structures are the same as those of the first embodiment, and the description thereof is thus omitted.

In the first comparative example, the acoustic wave element10is located on the upper surface21bopposite from the lower surface21aon which the terminals23and the bumps26are located. Thus, when the circuit substrate20is mounted on the mounting board40, a collet holding the circuit substrate20comes in contact with the acoustic wave element10. Therefore, an object may come in contact with the acoustic wave element10from the above. This may break the acoustic wave element10.

In the first embodiment, as illustrated inFIG. 2, the circuit substrate20includes the dielectric layers20ato20fand the wiring patterns22ato22e. The wiring patterns22ato22eare located on at least one of the dielectric layers20ato20f. One or more capacitors C1and C2and the inductor L (a passive element) are formed of at least one of the wiring patterns22ato22e. The terminals23and the bumps26(a terminal) are located on the lower surface21a(a first surface) of the circuit substrate20, and the terminals23are electrically connected to the capacitors C1and C2and the inductor L through the path L1(a first path) in the circuit substrate20. The acoustic wave element10is located on the lower surface21aof the circuit substrate20, and is electrically connected to the capacitors C1and C2and the inductor L through the path L2(a second path) in the circuit substrate20. Since the acoustic wave element10is located on the lower surface21a, on which the terminals23are also located, of the circuit substrate20, an object is inhibited from coming in contact with the acoustic wave element10from above.

The height D2from the lower surface21aof the circuit substrate20to the lower surface (the surface furthest away from the first surface) of the bump26in the Z direction is greater than the height D3+D4from the lower surface21ato the lower surface (the surface furthest away from the first surface) of the acoustic wave element10. This structure inhibits the acoustic wave element10from coming in contact with the mounting board40and breaking when the high-frequency device100is mounted on the mounting board40as illustrated inFIG. 6B.

Furthermore, as illustrated inFIG. 6B, the bump26includes the outer layer26bmade of solder and the core26ahaving a melting point greater than the melting point of solder and covered with the outer layer26b. The height D5of the core26ain the Z direction is greater than the height D3+D4from the lower surface21aof the circuit substrate20to the lower surface of the acoustic wave element10. This structure inhibits the acoustic wave element10from coming in contact with the mounting board40even when solder of the outer layer26bis melted at the time of mounting the circuit substrate20on the mounting board40.

In the first comparative example, since the acoustic wave element10is located on the upper surface21bof the circuit substrate20as illustrated inFIG. 7, the direction identification mark21is poorly-visible. In the first embodiment, the direction identification mark21is located on the upper surface21bof the circuit substrate20as illustrated inFIG. 2. Since the acoustic wave element10is not mounted on the upper surface21bof the circuit substrate20, the visibility of the direction identification mark21is improved. The direction identification mark21is a mark for identifying the orientation of the circuit substrate20when the circuit substrate20is viewed from above. For example, the terminal23closest to the direction identification mark21can be identified as a specific terminal.

As illustrated inFIG. 7, in the first comparative example, each of the length of the path L2connecting the node N3and the terminal25band the length of the path L3connecting the terminal25aand the ground terminal Tg is approximately the thickness D1of the circuit substrate20.

FIG. 8illustrates an equivalent circuit of the high-frequency device in accordance with the first comparative example. As illustrated inFIG. 8, in the first comparative example, the path L2is connected between the node N3and the acoustic wave resonator R, and the path L3is connected between the acoustic wave resonator R and the ground terminal Tg.

To examine the influence of the paths L2and L3, the capacitor C1and the acoustic wave resonator R are considered.FIG. 9Ais a cross-sectional view of the high-frequency device in accordance with the first embodiment, andFIG. 9Billustrates an equivalent circuit of the high-frequency device in accordance with the first embodiment. As illustrated inFIG. 9AandFIG. 9B, the capacitor C1and the acoustic wave resonator R are connected in series between the terminal T1and the ground terminal Tg. The capacitor C1is located in the region66athat is located lower than the center64in the Z direction of the circuit substrate20. This is because, when the inductor L is located in the region66a, the distance between the inductor L and the metal layer of the mounting board40decreases, and the Q-value of the inductor L decreases due to eddy-current loss. Provision of the capacitor C2in the region66areduces the lengths of the path L2between the node N3and the terminal25band the path L3between the terminal25aand the ground terminal Tg.

FIG. 10Ais a cross-sectional view of the high-frequency device in accordance with the first comparative example, andFIG. 10Billustrates an equivalent circuit of the high-frequency device in accordance with the first comparative example. As illustrated inFIG. 10AandFIG. 10B, the length of the path L2is equal to or greater than one-half of the thickness D1of the circuit substrate20, and the length of the path L3is equal to or greater than the thickness D1.

As illustrated inFIG. 9BandFIG. 10B, the acoustic wave resonator R and the paths L2and L3form a series resonant circuit62. The resonant frequency f0of the series resonant circuit62is f0=1/(2π√{square root over (L0×C0)}). L0represents the inductance of the paths L2and L3, and C0represents the capacitance of the acoustic wave resonator R. The capacitance C0is, for example, 1 pF to 3 pF. As the inductance of the paths L2and L3increases, the resonant frequency f0decreases.

FIG. 11is a schematic view of transmission characteristics of the first embodiment and the first comparative example. As illustrated inFIG. 11, in the first embodiment, attenuation poles A1to A3are formed at frequencies higher than the passband Pass. The attenuation pole A1is formed mainly by the resonant frequency of the acoustic wave resonator R. The attenuation pole A2is formed mainly by the resonant circuit60. The attenuation pole A3is formed mainly by the resonant frequency f0of the series resonant circuit62. The stopband higher than the passband Pass in frequency is widened by forming three attenuation poles A1to A3.

In the first comparative example, since the paths L2and L3are long, the inductance of the paths L2and L3Increases, and the resonant frequency f0of the series resonant circuit62thereby lowers. Thus, the attenuation poles A2and A3are combined and form an attenuation pole A4. This narrows the stopband. In the first embodiment, the paths L2and L3are shortened by mounting the acoustic wave element10on the lower surface21a. Thus, the stopband is widened.

In the first embodiment, a first end of the capacitor C1(a first capacitor) is electrically connected to the terminal T1(a first signal terminal) through the first path L1. A second end of the capacitor C1is electrically connected to a first end of the acoustic wave resonator R through the path L2. A second end of the acoustic wave resonator R is electrically connected to the ground terminal Tg through the path L3(a third path) in the circuit substrate20. This structure shortens the paths L2and L3as illustrated inFIG. 9AandFIG. 9B.

In the first embodiment, the total length of the paths L2and L3is less than the sum of the thickness D1of the circuit substrate20and the distance between the second end of the capacitor C1and the upper surface21b(a second surface) of the circuit substrate20. This configuration allows the total length of the paths L2and L3to be less than that of the first comparative example.

A first end of the capacitor C2(a second capacitor) is electrically connected to the second end of the capacitor C1and the first end (i.e., the node N3) of the acoustic wave resonator R. A second end of the capacitor C2is electrically connected to the terminal T2(a second signal terminal). The inductor L is connected in parallel to the capacitor C1and the capacitor C2in the path between the terminals T1and T2. This structure achieves a low-pass filter and a high-pass filter. The stopband of the filter is widened.

The acoustic wave resonator R, the capacitors C1and C2, and the inductor L form a low-pass filter. As illustrated inFIG. 11, the resonant frequency of the acoustic wave resonator is higher than the passband Pass of the low-pass filter. This configuration makes the transition region between the passband and the stopband steep.

As illustrated inFIG. 11, the resonant frequency f0of the series resonant circuit62formed by the paths L1and L2of the acoustic wave resonator R is higher than the resonant frequency of the acoustic wave resonator R. This configuration widens the stopband.

The attenuation pole A2due mainly to the resonant circuit60is preferably located between the attenuation pole A1due mainly to the resonant frequency of the acoustic wave resonator R and the attenuation pole A3due mainly to the resonant frequency f0of the series resonant circuit62. This configuration widens the stopband of the low-pass filter. Therefore, the resonant frequency f0is inhibited from shifting to a lower frequency to combine the attenuation poles A2and A3as the lengths of the paths L2and L3increase as in the first comparative example.

The capacitor may be located in the region66b, and the inductor may be located in the region66a. However, to improve the Q-value of the inductor L, one or more inductors L are preferably located in the region66b(a second region), and one or more capacitors C1and C2are preferably located in the region66a(a first region). In the first embodiment and the first comparative example, the capacitors C1and C2are electrically connected to the acoustic wave resonator R through the path L2, and the inductor L is electrically connected to the acoustic wave resonator R only through the capacitors C1and C2. In this case, in the first comparative example, the paths L2and L3pass through the dielectric layers20dto20fprovided with the inductor L as illustrated inFIG. 7. The diameters of the via wirings24ato24fare, for example, 10 μm to 120 μm. It is impossible to provide the wiring patterns22ato22ein the region around the via wirings24ato24f. The diameter of the region is 110 μm to 200 μm. Thus, in the first comparative example, the planar area in which the wiring patterns22dand22ecan be provided decreases, and the high-frequency device110thereby increases in size. In the first embodiment, since the paths L2and L3do not pass through the dielectric layers20dto20f, the planar area in which the wiring patterns22dand22ecan be provided increases, and the high-frequency device110is reduced in size.

First Variation of the First Embodiment

FIG. 12is a circuit diagram of a high-frequency device in accordance with a first variation of the first embodiment. As illustrated inFIG. 12, in a high-frequency device102, the acoustic wave resonator R is connected between the terminals T1and T2. The capacitors C1and C2are connected in parallel to the acoustic wave resonator R between the node N1, which is located between the acoustic wave resonator R and the terminal T1, and the node N2, which is located between the acoustic wave resonator R and the terminal T2. The capacitors C1and C2are connected in series with each other between the nodes N1and N2. The Inductor L is connected between the node N3, which is located between the capacitors C1and C2, and the ground terminal Tg. A path L5is formed between the acoustic wave resonator R and the node N1, and a path L6is formed between the acoustic wave resonator R and the node N2. Other structures are the same as those of the first embodiment, and the description thereof is thus omitted.

For example, the capacitances of the capacitors C1and C2are configured to be 0.2 pF, the inductance of the inductor L is configured to be 9.1 nH, the resonant frequency of the acoustic wave resonator R is configured to be 2.26 GHz, and the antiresonant frequency of the acoustic wave resonator R is configured to be 2.33 GHz. This configuration causes the high-frequency device102to function as a low-pass filter having a passband that is a frequency band lower than the resonant frequency.

For example, the capacitances of the capacitors C1and C2are configured to be 0.4 pF, the inductance of the inductor L is configured to be 8.2 nH, the resonant frequency of the acoustic wave resonator R is configured to be 2.26 GHz, and the antiresonant frequency of the acoustic wave resonator R is configured to be 2.33 GHz. This configuration causes the high-frequency device100to function as a high-pass filter having a passband that is a frequency band higher than the resonant frequency.

In the first variation of the first embodiment, the lengths of the paths L5and L6are shortened. As in the first variation of the first embodiment, when the acoustic wave resonator R is connected between the terminals T1and T2, the capacitance of the acoustic wave resonator R may be small according to the requirements on the circuit characteristics. Thus, even when the paths L5and L6are long, this does not affect the characteristics as much as in the first embodiment.

In the first embodiment and the first variation thereof, it is sufficient if the passive element is at least one of the capacitor and the inductor. The resonant frequency of the acoustic wave resonator R is located outside the passband of the filter. This configuration increases the steepness between the passband and the stopband. The resonant frequency of the acoustic wave resonator R may be located in the passband. The resonant frequency of the acoustic wave resonator R is, for example, 0.7 GHz to 100 GHz, preferably 0.7 GHz to 10 GHz.

Second Variation of the First Embodiment

FIG. 13AandFIG. 13Bare a cross-sectional view and a plan view in which a high-frequency device in accordance with a second variation of the first embodiment is mounted on a mounting board. The dimensions and arrangements do not match between the cross-sectional view and the plan view. As illustrated inFIG. 13AandFIG. 13B, no bump is provided to the terminals23,25a, and25b. A recessed portion44is located on the upper surface of the mounting board40. The acoustic wave element10fits inside the recessed portion44. Other structures are the same as those of the first embodiment, and the description thereof is thus omitted.

Third Variation of the First Embodiment

FIG. 14is a plan view in which a high-frequency device in accordance with a third variation of the first embodiment is mounted on a mounting board. As illustrated inFIG. 14, the mounting board40has the same size as the circuit substrate20or smaller than the circuit substrate20. Other structures are the same as those of the second variation of the first embodiment, and the description thereof is thus omitted.

As in the second and third variations of the first embodiment, the height of the terminal23may be lower than the acoustic wave element10.

Second Embodiment

FIG. 15is a circuit diagram of a diplexer in accordance with a second embodiment. As illustrated inFIG. 15, a high-pass filter46is connected between a common terminal TA and a terminal TH. A low-pass filter48is connected between the common terminal TA and a terminal TL. The high-pass filter transmits signals in the passband to the terminal TH or the common terminal TA among high-frequency signals input from the common terminal TA or the terminal TH, and suppresses signals with other frequencies. The low-pass filter48transmits signals in the passband to the terminal TL or the common terminal TA among high-frequency signals input from the common terminal TA or the terminal TL, and suppresses signals with other frequencies. A bandpass filter may be used instead of at least one of the high-pass filter46and the low-pass filter48. At least one of the high-pass filter46and the low-pass filter48can be the high-frequency device according to any one of the first embodiment and the variations thereof.

A diplexer has been described as an example of the multiplexer, but the multiplexer may be a triplexer or a quadplexer.