Patent ID: 12199590

DETAILED DESCRIPTION

Embodiments will be described with reference to the accompanying drawings.

First Embodiment

FIG.1Ais a plan view of an acoustic wave device in accordance with a first embodiment, andFIG.1Bis a cross-sectional view taken along line A-A inFIG.1A. InFIG.1A, resonance regions50aand50band an insulating film18are indicated by hatching.

As illustrated inFIG.1AandFIG.1B, a recessed portion is formed on the upper surface of a substrate10, and an air gap30(an air layer) is provided in the recessed portion. Lower electrodes12aand12bare provided on the substrate10and the air gap30. The lower electrodes12aand12bare separated from each other on the air gap30, and a groove55is formed between the lower electrodes12aand12b. The insulating film18is provided in the groove55. The insulating film18extends inside the groove55and reaches the top of the substrate10outside the air gap30(seeFIG.1A). A piezoelectric film14is continuously provided on the lower electrodes12aand12band the insulating film18. An upper electrode16is continuously provided on the piezoelectric film14. A frequency adjusting film or a protective film may be provided on the upper electrode16. In the parallel resonator of the ladder-type filter, a mass load film may be interposed between the upper electrode16and the frequency adjusting film or the protective film, or the upper electrode16may be formed of two layers, and a mass load film may be interposed between the two layers of the upper electrode16.

The region where the lower electrode12aand the upper electrode16overlap with each other across at least a part of the piezoelectric film14is a resonance region50a, and the region where the lower electrode12band the upper electrode16overlap with each other across at least a part of the piezoelectric film14is a resonance region50b. In a plan view, the resonance regions50aand50bare provided inside a single air gap30. Each of the resonance regions50aand50bhas a semi-elliptical shape, and is a region where the acoustic wave in the thickness extension mode or the thickness-shear mode resonates. The lower electrode12a, the piezoelectric film14, and the upper electrode16form a piezoelectric thin film resonator11a, while the lower electrode12b, the piezoelectric film14, and the upper electrode16form a piezoelectric thin film resonator11b. The resonators11aand11bare connected in series between the lower electrodes12aand12b.

The planar shape of the air gap30is an elliptical shape, and the center of the elliptical shape is a center58. The region that overlaps with the upper electrode16and is located between the resonance regions50aand50bin a plan view is a dividing region56. The dividing region56includes the center58. The resonance regions50aand50bare located at both sides of the dividing region56, the planar shape of the combined region of the resonance regions50aand50band the dividing region56is similar to the shape of the air gap30(i.e., an elliptical shape), and is included in the air gap30. In a plan view, the planar shape of the combined region of the resonance regions50aand50band the dividing region56may be congruent to the shape of the air gap30and be in agreement with the air gap30. The areas of the resonance regions50aand50bare, for example, substantially equal. The planar shape of the air gap30and the planar shape of the combined region of the resonance regions50aand50band the dividing region56may be polygonal shapes such as, but not limited to, quadrangle shapes or pentagonal shapes instead of elliptical shapes. The upper electrode16is not extracted to the outside of the air gap30, and is not coupled to any resonator other than the resonators11aand11b.

The substrate10is an insulating substrate such as, but not limited to, a silicon substrate, a sapphire substrate, a spinel substrate, an alumina substrate, a quartz substrate, a glass substrate, a crystal substrate, a ceramic substrate, or a GaAs substrate, or a semiconductor substrate. The lower electrodes12aand12band the upper electrode16are formed of a single-layer film made of a metal such as, but not limited to, ruthenium (Ru), chrome (Cr), aluminum (Al), titanium (Ti), copper (Cu), molybdenum (Mo), tungsten (W), tantalum (Ta), platinum (Pt), rhodium (Rh) or iridium (Ir), or a multilayered film of any combination of them.

The piezoelectric film14is a film mainly composed of aluminum nitride (AlN), zinc oxide (ZnO), lead zirconate titanate (PZT), lead titanate (PbTiO3), mono-crystalline lithium tantalate (LiTaO3), or mono-crystalline lithium niobate (LiNbO3). For example, the piezoelectric film14may be mainly composed of aluminum nitride and contain other elements for improving the resonance characteristics or the piezoelectricity. For example, use of scandium (Sc), use of a Group II element and a Group IV element, or use of a Group II element and a Group V element as additive elements improves the piezoelectricity of the piezoelectric film14. Therefore, the effective electromechanical coupling coefficient of the piezoelectric thin film resonator can be improved. Examples of the Group II element include, but are not limited to, calcium (Ca), magnesium (Mg), strontium (Sr), and zinc (Zn). Examples of the Group IV element include, but are not limited to, titanium, zirconium (Zr), and hafnium (Hf). Examples of the Group V element include, but are not limited to, tantalum, niobium (Nb), and vanadium (V). The piezoelectric film14may be mainly composed of aluminum nitride and contain boron (B).

The insulating film is an inorganic insulating film, and is, for example, a metal oxide film such as a silicon oxide film or an aluminum oxide film, or a metal nitride film such as a silicon nitride film or an oxynitride silicon film. The frequency adjusting film and the protective film are insulating films such as, but not limited to, silicon oxide films, silicon nitride films, or aluminum oxide films. The mass load film is a metal film made of one of the materials exemplified as the material of the lower electrodes12aand12band the upper electrode16, or an insulating film such as, but not limited to, a silicon oxide film, a silicon nitride film, or an aluminum oxide film.

In the case that the piezoelectric thin film resonator has a resonant frequency of 2 GHz, the lower electrodes12aand12bare formed of a chrome film with a film thickness of 100 nm and a ruthenium film with a film thickness of 250 nm that are stacked in this order from the substrate10side. The piezoelectric film14is an aluminum nitride film with a film thickness of 1100 nm. The upper electrode16is formed of a ruthenium film with a film thickness of 250 nm and a chrome film with a film thickness of 50 nm that are stacked in this order from the piezoelectric film14side. The film thickness of each layer can be freely selected to obtain the desired resonance characteristics.

Manufacturing Method of the First Embodiment

FIG.2AtoFIG.3Care cross-sectional views illustrating a method of manufacturing the acoustic wave device in accordance with the first embodiment. As illustrated inFIG.2A, a recessed portion is formed on the upper surface of the substrate10. A sacrifice layer38is formed in the recessed portion by, for example, sputtering, vacuum evaporation, or chemical vapor deposition (CVD). The sacrifice layer38is made of, for example, magnesium oxide (MgO), zinc oxide (ZnO), germanium (Ge), or silicon oxide (SiO2). The upper surfaces of the substrate10and the sacrifice layer38are planarized by chemical mechanical polishing (CMP).

As illustrated inFIG.2B, a lower electrode12is formed on the sacrifice layer38and the substrate10by, for example, sputtering, vacuum evaporation, or CVD. As illustrated inFIG.2C, a mask layer60having an aperture61is formed on the lower electrode12. The mask layer60is made of, for example, a photoresist, and is formed using photolithography. The lower electrode12is removed using the mask layer60as a mask to form the lower electrodes12aand12band the groove55. The lower electrode12is removed by, for example, dry etching.

As illustrated inFIG.3A, insulating films18and19are formed in the groove55and on the mask layer60. The insulating films18and19are formed by, for example, CVD, sputtering, or vacuum evaporation. By removing the mask layer60as illustrated inFIG.3B, the insulating film18is left in the groove55.

As illustrated inFIG.3C, the piezoelectric film14is formed on the lower electrodes12aand12band the insulating film18by, for example, sputtering or vacuum evaporation. The upper electrode16is formed on the piezoelectric film14by, for example, sputtering, vacuum evaporation, or CVD. The upper electrode16and the piezoelectric film14are patterned by, for example, photolithography and etching. The above processes form the resonance regions50aand50band the dividing region56. Thereafter, the sacrifice layer38is removed using an etching medium (an etching liquid). The etching medium is preferably a medium that does not etch the substrate10, the lower electrodes12a,12b, the piezoelectric film14, the upper electrode16, or the insulating film18other than the sacrifice layer38. Examples of the etching medium include, but are not limited to, a hydrofluoric acid or a nitric acid. The removal of the sacrifice layer38forms the air gap30between the lower electrode12and the substrate10. Through the above steps, the acoustic wave device illustrated inFIG.1AandFIG.1Bis manufactured.

FIG.4AandFIG.4Bare enlarged cross-sectional views of acoustic wave devices in accordance with a first comparative example and the first embodiment, respectively. As illustrated inFIG.4A, in the first comparative example, no insulating film18is provided. The lower surface of the piezoelectric film14between the lower electrodes12aand12band the lower surfaces of the lower electrodes12aand12bform a substantially flat surface. The piezoelectric film14is formed so that the lower surface of the piezoelectric film14has a level difference having a height equal to the thicknesses T1aand T1bof the lower electrodes12aand12a. Therefore, cracks62may be formed in the edges of the level difference. In addition, a region63where the piezoelectric film14have reduced crystallinity may be formed near the ends of the lower electrodes12aand12b. InFIG.3C, when the sacrifice layer38is etched, the etching liquid may erode the piezoelectric film14as indicated by arrows64. In particular, in the case that the crack62or the region63where the crystallinity is reduced is formed in the piezoelectric film14, the piezoelectric film14is more likely to be eroded. Therefore, in the first comparative example, the piezoelectric film14may deteriorate.

As illustrated inFIG.4B, in the first embodiment, the insulating film18is formed in the groove55. The thickness T2of the insulating film18is adjusted to be substantially equal to the thicknesses T1aand T1bof the lower electrodes12aand12b. The thicknesses T1aand T1bare substantially equal, and the upper surface of the insulating film18and the upper surfaces of the lower electrodes12aand12bare substantially in the same plane. This structure inhibits cracks and the region63where the crystallinity is reduced from being formed near the ends of the lower electrodes12aand12b. In addition, the insulating film18protects the lower surface of the piezoelectric film14in the dividing region56. Therefore, the piezoelectric film14is inhibited from being eroded when the sacrifice layer38is etched. As described above, in the first embodiment, the deterioration of the piezoelectric film14is reduced.

Variation 1 of the First Embodiment

FIG.5AtoFIG.9Care cross-sectional views illustrating acoustic wave devices in accordance with variations 1 to 18 of the first embodiment, respectively. As illustrated inFIG.5A, in the variation 1 of the first embodiment, the thickness T2of the insulating film18is less than the thicknesses T1aand T1bof the lower electrodes12aand12b. Even in the variation 1 of the first embodiment, the level difference D1between the upper surfaces of the lower electrodes12aand12band the upper surface of the insulating film18is smaller than the level difference of the lower surface of the piezoelectric film14corresponding to the heights T1aand T1bof the lower electrodes12aand12bin the first comparative example. Therefore, the formation of the crack62and the region63is inhibited. Because the insulating film18protects the lower surface of the piezoelectric film14in the dividing region56, the erosion of the piezoelectric film14is inhibited. The level difference D1is preferably equal to or less than ¾ of the thicknesses T1aand T1b, more preferably equal to or less than ½ of the thicknesses T1aand T1b. The insulating film18may be thicker than the lower electrodes12aand12b. Even in this case, the level difference D1is preferably equal to or less than ¾ of the thicknesses T1aand T1b, more preferably equal to or less than ½ of the thicknesses T1aand T1b. Other structures are the same as those of the first embodiment, and the description thereof is thus omitted.

Variation 2 of the First Embodiment

As illustrated inFIG.5B, in the variation 2 of the first embodiment, the insulating film18includes a part18alocated in the groove55and a part18blocated under the lower electrodes12aand12band the part18a. The thickness T2aof the part18ais substantially equal to the thicknesses T1aand T2bof the lower electrodes12aand12b. This structure inhibits the formation of the crack62and the region63as in the first embodiment. The part18bis provided under the lower electrodes12aand12bover the entire air gap30, thus reinforcing the resonance regions50aand50b. In the case that the part18bis thick, the resonance characteristics of the resonators11aand11bdecrease. Therefore, the thickness T2bof the part18bis preferably less than the thicknesses T1aand T1bof the lower electrodes12aand12b, more preferably equal to or less than ½ of T1aand T1b, further preferably equal to or less than ¼ of T1aand T1b. Too thin T2bmakes the reinforcement impossible. Therefore, the thickness T2bis preferably equal to or greater than 1/10 of T1aand T1b. Other structures are the same as those of the first embodiment, and the description thereof is thus omitted.

Variation 3 of the First Embodiment

As illustrated inFIG.5C, in the variation 3 of the first embodiment, the thickness T2aof the part18ais thinner than the thicknesses T1aand T1bof the lower electrodes12aand12b. Even when the part18ais thin as described above, the level difference D1between the upper surface of the part18aand the upper surfaces of the lower electrodes12aand12bis smaller than the thicknesses T1aand T1b. Therefore, as in the variation 2 of the first embodiment, the formation of the crack62and the region63is inhibited. The preferable range of the level difference D1is the same as the preferable range of the level difference D1of the variation 1 of the first embodiment. The part18bis located under the lower electrodes12aand12bover the entire air gap30. This structure reinforces the resonance regions50aand50b. Other structures are the same as those of the variation 2 of the first embodiment, and the description thereof is thus omitted.

Variation 4 of the First Embodiment

As illustrated inFIG.6A, in the variation 4 of the first embodiment, the part18bextends to a part outside the dividing region56. The width of the part18bin each of the resonance regions50aand50bis D2. The width D2is preferably equal to or greater than the alignment margin between the part18band the lower electrodes12aand12b. Therefore, the width D2is preferably equal to or greater than 1/10 of the thicknesses T1aand T1b, more preferably equal to or greater than ½ of the thicknesses T1aand T1b. The large width D2deteriorates the resonance characteristics of the resonators11aand11b. Therefore, the width D2is preferably equal to or less than two times the thicknesses T1aand T1b. Other structures are the same as those of the variation 2 of the first embodiment, and the description thereof is thus omitted.

Variation 5 of the First Embodiment

As illustrated inFIG.6B, in the variation 5 of the first embodiment, the thickness T2aof the part18ais less than the thicknesses T1aand T1bof the lower electrodes12aand12b. Other structures are the same as those of the variation 4 of the first embodiment, and the description thereof is thus omitted.

Variation 6 of the First Embodiment

In the first embodiment, the side surfaces of the insulating film18are inclined with respect to the lower surface of the insulating film18. Therefore, in the lower ends of the side surfaces of the insulating film18, the insulating film18is in contact with the lower electrodes12aand12b, while near the upper surface of the insulating film18, the insulating film18is separated from the lower electrodes12aand12b. Therefore, a recessed portion is formed between each of the upper surfaces of the lower electrodes12aand12band the upper surface of the insulating film18. As illustrated inFIG.6C, in the variation 6 of the first embodiment, the side surfaces of the insulating film18are in contact with the side surfaces of the lower electrodes12aand12b. Therefore, unlike the first embodiment, the upper surfaces of the lower electrodes12aand12band the insulating film18form a substantially flat surface. Thus, the formation of the crack62and the region63where the crystallinity is reduced in the piezoelectric film14is further inhibited. Other structures are the same as those of the first embodiment, and the description thereof is thus omitted.

Variation 7 of the First Embodiment

As illustrated inFIG.6D, in the variation 7 of the first embodiment, the thickness T2of the insulating film18is less than the thicknesses T1aand T1bof the lower electrodes12aand12b. The side surfaces of the insulating film18are in contact with the side surfaces of the lower electrodes12aand12b. This structure inhibits the formation of a recessed portion between each of the upper surfaces of the lower electrodes12aand12band the upper surface of the insulating film18compared with the structure of the variation 1 of the first embodiment illustrated inFIG.5A. Thus, the formation of the crack62and the region63where the crystallinity is reduced in the piezoelectric film14is further inhibited. Other structures are the same as those of the variation 1 of the first embodiment, and the description thereof is thus omitted.

Variation 8 of the First Embodiment

As illustrated inFIG.7A, in the variation 8 of the first embodiment, the side surfaces of the lower electrodes12aand12bare inclined with respect to the lower surfaces of the lower electrodes12aand12b. The angle (the interior angle) between the lower surface and the side surface of each of the lower electrodes12aand12bis an acute angle, and is, for example, 20° to 60°. Other structures are the same as those of the variation 1 of the first embodiment, and the description thereof is thus omitted.

Variation 9 of the First Embodiment

As illustrated inFIG.7B, in the variation 9 of the first embodiment, the side surfaces of the lower electrodes12aand12bare inclined with respect to the lower surfaces of the lower electrodes12aand12b. Other structures are the same as those of the variation 1 of the first embodiment, and the description thereof is thus omitted.

Variation 10 of the First Embodiment

As illustrated inFIG.7C, in the variation 10 of the first embodiment, the side surfaces of the lower electrodes12aand12bare inclined with respect to the lower surfaces of the lower electrodes12aand12b. Other structures are the same as those of the variation 2 of the first embodiment, and the description thereof is thus omitted.

Variation 11 of the First Embodiment

As illustrated inFIG.7D, in the variation 11 of the first embodiment, the side surfaces of the lower electrodes12aand12bare inclined with respect to the lower surfaces of the lower electrodes12aand12b. Other structures are the same as those of the variation 3 of the first embodiment, and the description thereof is thus omitted.

Variation 12 of the First Embodiment

As illustrated inFIG.8A, in the variation 12 of the first embodiment, the side surfaces of the lower electrodes12aand12bare inclined with respect to the lower surfaces of the lower electrodes12aand12b. Other structures are the same as those of the variation 4 of the first embodiment, and the description thereof is thus omitted.

Variation 13 of the First Embodiment

As illustrated inFIG.8B, in the variation 13 of the first embodiment, the side surfaces of the lower electrodes12aand12bare inclined with respect to the lower surfaces of the lower electrodes12aand12b. Other structures are the same as those of the variation 5 of the first embodiment 5, and the description thereof is thus omitted. As in the variations 8 to 13 of the first embodiment, the side surfaces of the lower electrodes12aand12bmay be inclined with respect to the lower surfaces. The angle between the lower surface and the side surface of each of the lower electrodes12aand12bis an acute angle. In the case that the angle between the lower surface and the side surface of the insulating film18is an acute angle, the angle between the side surface of each of the lower electrodes12aand12band the side surface of the insulating film18is greater than those in the variation 1 to 5 of the first embodiment. Therefore, the formation of the crack62and the region63where the crystallinity is reduced in the piezoelectric film14is further inhibited.

Variation 14 of the First Embodiment

As illustrated inFIG.8C, in the variation 14 of the first embodiment, the side surfaces of the insulating film18are in contact with the side surfaces of the lower electrodes12aand12b. Therefore, the upper surfaces of the lower electrodes12aand12band the upper surface of the insulating film18form a substantially flat surface. Other structures are the same as those of the variation 8 of the first embodiment, and the description thereof is thus omitted.

Variation 15 of the First Embodiment

As illustrated inFIG.8D, in the variation 15 of the first embodiment, the side surfaces of the insulating film18are in contact with the side surfaces of the lower electrodes12aand12b. Other structures are the same as those of the variation 9 of the first embodiment, and the description thereof is thus omitted.

Variation 16 of the First Embodiment

As illustrated inFIG.9A, in the variation 16 of the first embodiment, the side surfaces of the insulating film18are in contact with the side surfaces of the lower electrodes12aand12b. Other structures are the same as those of the variation 10 of the first embodiment, and the description thereof is thus omitted.

Variation 17 of the First Embodiment

As illustrated inFIG.9B, in the variation 17 of the first embodiment, the side surfaces of the insulating film18are in contact with the side surfaces of the lower electrodes12aand12b. Other structures are the same as those of the variation 10 of the first embodiment, and the description thereof is thus omitted. As in the variations 14 to 17 of the first embodiment, in the case that the angle between the lower surface and the side surface of each of the lower electrodes12aand12bis an acute angle, the side surface of the insulating film18may be in contact with the side surfaces of the lower electrodes12aand12b.

Variation 18 of the First Embodiment

As illustrated inFIG.9C, in the variation 18 of the first embodiment, the angle between the lower surface and the side surface of each of the lower electrodes12aand12bis an obtuse angle. The side surfaces of the insulating film18are in contact with the side surfaces of the lower electrodes12aand12b. Other structures are the same as those of the variation 16 of the first embodiment, and the description thereof is thus omitted. As in the variation 18 of the first embodiment, in the case that the angle between the lower surface and the side surface of each of the lower electrodes12aand12bis an obtuse angle, the side surfaces of the insulating film18may be in contact with the side surfaces of the lower electrodes12aand12b.

Variation 19 of the First Embodiment

FIG.10Ais a plan view of an acoustic wave device in accordance with a variation 19 of the first embodiment, andFIG.10Bis a cross-sectional view taken along line A-A inFIG.10A. InFIG.10A, an insertion film28is indicated by hatching. As illustrated inFIG.10AandFIG.10B, the piezoelectric film14includes a lower piezoelectric film14aand an upper piezoelectric film14b. The insertion film28is interposed between the lower piezoelectric film14aand the upper piezoelectric film14b. The insertion film28is provided in outer peripheral regions52of the resonance regions50aand50band is not provided in center regions54of the resonance regions50aand50b. The outer peripheral regions52are the regions within the resonance regions50aand50b, and are the regions that include respective outer peripheries of the resonance regions50aand50band are located along the respective outer peripheries. The outer peripheral region52has, for example, a strip shape. The center regions54are regions that are within the resonance regions50aand50band include the respective centers of the resonance regions50aand50b. The center does not have to be the geometric center. The insertion film28is continuously provided from the outer peripheral regions52to the outsides of the resonance regions50aand50b, and is provided in the dividing region56. The side surface of the lower piezoelectric film14ais located further out than the side surface of the upper piezoelectric film14b, and the insertion film28is provided on the lower piezoelectric film14alocated outside of the upper piezoelectric film14b.

The acoustic impedance of the insertion film28is preferably less than the acoustic impedance of the piezoelectric film14. In the case that the piezoelectric film14is mainly composed of aluminum nitride, the insertion film28is, for example, a silicon oxide film, an aluminum film, a gold film, a copper film, a titanium film, a platinum film, or a tantalum film. By providing the insertion film28in the region where the resonance regions50aand50bare adjacent to each other, the interference between the acoustic waves of the resonators11aand11bis reduced. In addition, the acoustic waves are inhibited from leaking from the resonators11aand11bto the substrate10, and therefore, the Q factor can be improved. Other structures are the same as those of the first embodiment, and the description thereof is thus omitted.

Variation 20 of the First Embodiment

FIG.11Ais a cross-sectional view of an acoustic wave device in accordance with a variation 20 of the first embodiment. As illustrated inFIG.11A, a temperature compensation film26is provided between the lower piezoelectric film14aand the upper piezoelectric film14b. The temperature compensation film26is provided across the entire resonance regions50aand50band the entire dividing region56. The temperature coefficient of the elastic constant of the temperature compensation film26is opposite in sign to the temperature coefficient of the elastic constant of the piezoelectric film14. This makes the temperature coefficient of frequency of each of the resonators11aand11bsmall. The temperature compensation film26is, for example, a silicon oxide film to which no impurities are intentionally added, or a silicon oxide film to which impurities such as, but not limited to, fluorine are added. Other structures are the same as those of the first embodiment, and the description thereof is thus omitted.

Variation 21 of the First Embodiment

FIG.11Bis a cross-sectional view of an acoustic wave device in accordance with a variation 21 of the first embodiment. As illustrated inFIG.11B, the temperature compensation film26and the insertion film28are provided between the lower piezoelectric film14aand the upper piezoelectric film14b. Other structures are the same as those of the variations 19 and 20 of the first embodiment, and the description thereof is thus omitted. As in the variations 19 to 21 of the first embodiment, the insertion film28and/or the temperature compensation film26may be provided in the first embodiment and the variations 1 to 18 of the first embodiment.

Variation 22 of the First Embodiment

In variations 22 and 23 of the first embodiment, the structure of the air gap is changed.FIG.12AandFIG.12Bare cross-sectional views of the vicinity of the resonance region in the variations 22 and 23 of the first embodiment, respectively. As illustrated in FIG.12A, the upper surface of the substrate10is flat. A dome-shaped air gap30is formed between the upper surface of the substrate10and the lower surfaces of the lower electrodes12aand12band the insulating film18. The dome shape is a shape in which the height of the air gap30is small in the periphery of the air gap30, and increases at closer distances to the center of the air gap30. To form the dome-shaped air gap30, the total internal stress of the lower electrodes12aand12b, the piezoelectric film14, and the upper electrode16is preferably adjusted to be a compression stress. Other structures are the same as those of the first embodiment, and the description thereof is thus omitted.

Variation 23 of the First Embodiment

As illustrated inFIG.12B, instead of the air gap30of the first embodiment, an acoustic mirror31is formed. The acoustic mirror31has a structure in which a film31ahaving a low acoustic impedance and a film31bhaving a high acoustic impedance are alternately provided. The film thickness of each of the films31aand31bis, for example, substantially λ/4 (λ is the wavelength of the acoustic wave). The number of the films31aand31bthat are stacked can be freely selected. For example, the acoustic mirror31may have a structure in which a film having an acoustic impedance different from that of the substrate10is provided in the substrate10. Other structures are the same as those of the first embodiment, and the description thereof is thus omitted.

In the first embodiment and the variations 1 to 21 of the first embodiment, the dome-shaped air gap30may be formed as in the variation 22 of the first embodiment, or the acoustic mirror31may be formed instead of the air gap30as in the variation 23 of the first embodiment.

As in the first embodiment and the variations 1 to 22 of the first embodiment, the resonators11aand11bmay be a film bulk acoustic resonator (FBAR). As in the variation 23 of the first embodiment, the resonators11aand11bmay be a solidly mounted resonator (SMR). As described above, the acoustic layer provided in or on the substrate10is the air gap30or the acoustic mirror31in which at least two layers with different acoustic characteristics are stacked.

In the first embodiment and the variations thereof, the acoustic layer is the air gap30or the acoustic mirror31in which at least two layers with different acoustic characteristics are stacked is provided on the substrate10. The lower electrodes12aand12bare arranged so that the lower electrodes12aand12bshare a single acoustic layer between the lower electrodes12aand12band the substrate10and are separated from each other across the groove55. The insulating film18is provided in the groove55on the acoustic layer. The piezoelectric film14is continuously provided on the lower electrodes12aand12band the groove55. The upper electrode16is continuously provided on the piezoelectric film14, and forms the resonators11aand11bby sandwiching the piezoelectric film14between the upper electrode16and the lower electrodes12aand12b.

Accordingly, as described with use ofFIG.4B, the crack62and the region63where the crystallinity is reduced, which are formed in the first comparative example illustrated inFIG.4A, are inhibited from being formed in the piezoelectric film14. When the sacrifice layer38is removed inFIG.3C, since the insulating film18protects the piezoelectric film14, the erosion of the piezoelectric film14is inhibited. Thus, the deterioration of the piezoelectric film14can be inhibited.

The upper electrode16is not extracted to the outside of the acoustic layer in a plan view. That is, no resonator is connected to the upper electrode16. In this structure, the resonators11aand11bare connected in series, and the polarization direction of the piezoelectric film14of the resonator11ais opposite to that of the piezoelectric film14of the resonator11b. Therefore, as described in Patent Document 1, the secondary distortion is reduced. In addition, since there is air above the upper electrode16, the parasitic capacitance added to the upper electrode16in the dividing region56can be reduced. Therefore, the secondary distortion can be further reduced as described in FIG. 4 of Patent Document 1. To reduce the secondary distortion, the areas of the resonance regions50aand50bare preferably substantially equal to each other within a range of about the manufacturing error. In addition, the planar shapes of the resonance regions50aand50bare preferably symmetric (for example, point-symmetric, line-symmetric, or mirror-symmetric). The planar shapes of the resonance regions50aand50bmay be asymmetric. This can inhibit the unnecessary waves of the resonators11aand11bfrom interfering with each other.

In addition, in the case that the upper electrode16is not extracted to the outside of the acoustic layer, the upper electrode16and the piezoelectric film14are included in the air gap30in a plan view. Therefore, the resonators11aand11bare supported by the substrate10through mainly the lower electrodes12aand12b, and the support strength is weak. By providing the insulating film18, the resonators11aand11bcan be reinforced.

The insulating film18is in contact with each of the lower electrodes12aand12b. For example, the insulating film18is in contact with at least the lower ends of the lower electrodes12aand12b. The insulating film18can reinforce the lower electrodes12aand12b. Additionally, the piezoelectric film14is inhibited from being exposed to the etching liquid for the sacrifice layer38as indicated by the arrows64inFIG.4Aof the first comparative example. Even when the insulating film18is in no contact with each of the lower electrodes12aand12b, the effect to inhibit the formation of the crack62and the region63can be obtained.

In the first embodiment and the variations 2, 4, 6, 8, 10, 12, 14, 16, and 18 thereof, a first surface, which is located closer to the piezoelectric film14, of the insulating film18and first surfaces, which are located closer to the piezoelectric film14, of the lower electrodes12aand12bare substantially in the same plane. This structure inhibits the formation of the crack62and the region63in the piezoelectric film14. When two surfaces are substantially in the same plane, this means that the levels of the two surfaces may differ within a range of about the manufacturing error. In addition, the substantially flat surface means that there is substantially no difference on the surface, and the surface may curve or slope within a range of about the manufacturing error. For example, in the case that the air gap30is dome-shaped as illustrated inFIG.12Aof the variation 22 of the first embodiment, the height of the air gap30is approximately 1/100 of the width of the air gap30, and the upper surface of the air gap30can be considered to be the substantially flat surface. A recessed portion may be provided between the first surface, which is located closer to the piezoelectric film14, of the insulating film18and the first surfaces, which are located closer to the piezoelectric film14, of the lower electrodes12aand12b.

In the variations 1, 3, 5, 7, 9, 11, 13, 15, and 17 of the first embodiment, the first surface, which is located closer to the piezoelectric film14, of the insulating film18is located closer to the substrate10than the first surfaces, which are located closer to the piezoelectric film14, of the lower electrodes12aand12b, and is located closer to the piezoelectric film14than second surfaces, which are located closer to the substrate10, of the lower electrodes12aand12b. In this structure, the level difference D1is less than the thicknesses T1aand T1b. Thus, the formation of the crack62and the region63in the piezoelectric film14is inhibited.

As in the variations 6, 7, and 14 to 18 of the first embodiment, the side surfaces of the insulating film18are in contact with the side surfaces of the lower electrodes12aand12b. In this structure, no recessed portion is formed between the upper surface of the insulating film18and the upper surfaces of the lower electrodes12aand12b. Therefore, the formation of the crack62and the region63in the piezoelectric film14is further inhibited.

In the variations 6, 7, 14, 16, and 18 of the first embodiment, the first surface, which is located closer to the piezoelectric film14, of the insulating film18and the first surfaces, which are located closer to the piezoelectric film14, of the lower electrodes12aand12bform a substantially flat surface within a range of about the manufacturing error. That is, no recessed portion is provided between the first surface, which is located closer to the piezoelectric film14, of the insulating film18and the first surfaces, which are located closer to the piezoelectric film14, of the lower electrodes12aand12b. This structure further inhibits the formation of the crack62and the region63in the piezoelectric film14.

In the first embodiment and the variations 1, 6 to 9, 14, and 15 of the first embodiment, a second surface, which is located closer to the substrate10, of the insulating film18and the second surfaces, which are located closer to the substrate10, of the lower electrodes12aand12bform a substantially flat surface within a range of about the manufacturing error. That is, there is substantially no level difference between the second surface of the insulating film18and each of the second surfaces of the lower electrodes12aand12b. In this structure, the insulating film18can be easily formed because it is sufficient if the lower electrodes12aand12band the insulating film18are formed on the upper surface of the sacrifice layer38as illustrated inFIG.3B.

In the variations 2 to 5, 10 to 13, and 16 to 18 of the first embodiment, a part of the insulating film18is provided on the second surfaces, which are located closer to the substrate10, of the lower electrodes12aand12b. This structure allows the insulating film18to reinforce the lower electrodes12aand12b.

In the variations 2, 3, 10, 11, and 16 to 18 of the first embodiment, the insulating film18overlaps the entire acoustic layer in a plan view. This structure allows the insulating film18to reinforce the lower electrodes12aand12b.

In the case that the acoustic layer is the air gap30as in the variations 1 to 22 of the first embodiment, the strength of the dividing region56is low without the insulating film18. Therefore, by providing the insulating film18in the dividing region56, the lower electrodes12aand12bcan be reinforced.

In the case that the surface located closer to the air gap30of the substrate10is a substantially flat surface within a range of about the manufacturing error as in the variation 22 of the first embodiment, the lower electrodes12aand12bcurve upward. The strength of the dividing region56is low without the insulating film18. Therefore, by providing the insulating film18in the dividing region56, the lower electrodes12aand12bcan be reinforced.

As in the variations 19 to 21 of the first embodiment, the insertion film28having a material different from the material of the piezoelectric film14and/or the temperature compensation film26having a material different from the material of the piezoelectric film14may be interposed between the upper electrode16and at least one of the lower electrodes12aand12b.

Second Embodiment

A second embodiment is an exemplary duplexer.FIG.13is a circuit diagram of a duplexer in accordance with the second embodiment. As illustrated inFIG.13, a common terminal Ant is coupled to an antenna44. A transmit filter40is connected between the common terminal Ant and a transmit terminal Tx. A receive filter42is connected between the common terminal Ant and a receive terminal Rx. An inductor L1, as a matching circuit, is connected between the common terminal Ant and a ground. The transmit filter40transmits signals in the transmit band to the common terminal Ant as transmission signals among signals input from the transmit terminal Tx, and suppresses signals with other frequencies. The receive filter42transmits signals in the receive band to the receive terminal Rx as reception signals among signals input from the common terminal Ant, and suppresses signals with other frequencies. The inductor L1matches the impedance so that the transmission signal transmitted through the transmit filter40is output from the common terminal Ant without leaking to the receive filter42.

The transmit filter40is a ladder-type filter. Series resonators S1to S4are connected in series between the common terminal Ant and the transmit terminal Tx. Parallel resonators P1to P3are connected in parallel between the common terminal Ant and the transmit terminal Tx. First ends of the parallel resonators P1to P3are jointly grounded through an inductor L2. The parallel resonator P1is divided in series into resonators P1aand P1b.

FIG.14Ais a plan view of the transmit filter in the second embodiment, andFIG.14Bis a plan view of the air gap. As illustrated inFIG.14A, the series resonators S1to S4, the parallel resonators P1to P3, wiring lines45, and terminals46are formed on the substrate10. Each of the series resonators S1to S4and the parallel resonators P1to P3has the resonance region50of its own. The wiring lines45connect the resonance regions50to each other, and connect the resonance regions50to the terminals46. The terminals46include an input terminal Tin, an output terminal Tout and ground terminals Tg. The wiring lines45and the terminals46are formed of the lower electrodes12or the upper electrodes16.

As illustrated inFIG.14AandFIG.14B, in the resonators other than the parallel resonator P1, a single resonance region50is provided for the single air gap30. The resonators P1aand P1bcorrespond to the resonators11aand11bin accordance with the first embodiment and the variations of the first embodiment, and two resonance regions50aand50band the dividing region56are provided for a single air gap30a. The air gap30afor the resonators P1aand P1band other air gaps30have elliptical shapes. Since the air gaps30aand30have the same shape, the stress condition for forming the piezoelectric film14and the like on the air gaps30aand30can be adjusted to be substantially the same. Therefore, the air gaps30aand30, the piezoelectric film14, and the like can be easily formed.

In the second embodiment, the transmit filter40includes piezoelectric thin film resonators corresponding to respective resonance regions50provided for the single air gap30in accordance with the first embodiment and the variations of the first embodiment. Therefore, the parasitic capacitance of the wiring line between the resonators P1aand P1bcan be reduced. Thus, the secondary distortion can be reduced as in Patent Document 1. In addition, since the insulating film18is provided in the groove55between the lower electrodes12aand12b, the deterioration of the piezoelectric film or the like can be inhibited.

The second embodiment has described an example in which the first embodiment and the variations of the first embodiment are applied to the transmit filter40, but the first embodiment and the variations of the first embodiment may be applied to the receive filter42. An example in which the first embodiment and the variations of the first embodiment are applied to the ladder-type filter has been described, but the first embodiment and the variations of the first embodiment may be applied to a lattice-type filter or a multimode filter. The duplexer has been described as an example of the multiplexer, but the multiplexer may be a triplexer or a quadplexer.

The parallel resonator P1is divided into the resonators P1aand P1b(divided resonators), and the respective resonance regions50corresponding to the resonators P1aand P1bshare the air gap30a. This structure reduces the secondary distortion. The resonator to be divided is at least one of the one or more series resonators and the one or more parallel resonators. To reduce the secondary distortion, the series resonator S1electrically closest to the output terminal and or the parallel resonator P1electrically closest to the output terminal is preferably divided. The number of series resonators and the number of parallel resonators can be freely selected.

Third Embodiment

A third embodiment is an example in which the first embodiment and the variations of the first embodiment are applied to a ladder-type filter.FIG.15is a plan view of a filter in accordance with the third embodiment. The resonance regions50ato50cand the insulating film18are illustrated by hatching. As illustrated inFIG.15, three resonance regions50ato50ccorresponding to the series resonators S1and S2and the parallel resonator P1, respectively share the air gap30. The regions between each two of the resonance regions50aand50care the dividing regions56. The grooves55are located between each two of the lower electrodes12ato12c. The insulating film18is provided in the grooves55. The upper electrode16is shared by the resonance regions50ato50c. The three resonators are electrically connected by the upper electrode16. The lower electrodes12a,12b, and12care coupled to the input terminal Tin, the output terminal Tout, and the ground terminal Tg, respectively. Therefore, the series resonators S1and S2are connected in series between the input terminal Tin and the output terminal Tout, and the parallel resonator P1is connected in parallel between the input terminal Tin and the output terminal Tout.

In the third embodiment, the resonance regions50corresponding to at least one of one or more series resonators and at least one of one or more parallel resonators share the air gap30. This structure reduces the parasitic capacitance of the wiring line between the series resonator and the parallel resonator.

Although the embodiments of the present invention have been described in detail, it is to be understood that the various change, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.