JP2008182543A - Thin film piezoelectric resonator and thin film piezoelectric filter using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thin film piezoelectric resonator having a high Q value in which generation of a transverse acoustic mode is suppressed. <P>SOLUTION: The thin film piezoelectric resonator includes: a piezoelectric resonance stack 12 having a piezoelectric layer 2 and an upper electrode 10 and a lower electrode 8 formed so as to be opposite to each other on both sides of the piezoelectric layer 2; a gap 4 formed under the piezoelectric resonance stack 12; and a substrate 6 supporting the piezoelectric resonance stack 12. The piezoelectric resonance stack 12 is comprised of: an oscillation area 18 in which the upper electrode 10 and the lower electrode 8 are opposite to each other via a piezoelectric layer 2 and located in accordance with the gap 4; a supporting area 22 which contacts the substrate; and a buffer area 20 located between the oscillation area 18 and the support area 22. The buffer area 20 is constituted so that a resonance frequency of primary thickness longitudinal vibration becomes lower than in the oscillation area 18. The piezoelectric layer 2 is comprised of materials indicating a dispersion curve of a high-frequency cutoff type. The buffer area 20 includes the oscillation area 18. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention belongs to the technical field of communication equipment, and particularly relates to a thin film piezoelectric resonator and a thin film piezoelectric filter using the same.

  The RF circuit part of a cellular telephone is always required to be downsized. Recently, it has been demanded to give various functions to cellular telephones, and it is preferable to incorporate as many components as possible in order to realize such functions. On the other hand, since the size of cellular phones is limited, after all, there is a strict requirement to reduce the occupied area (mounting area) and height of each component in the equipment, and therefore the components that make up the RF circuit section are also dedicated. What has a small area and a low height is desired.

  Under such circumstances, a thin film piezoelectric filter using a thin film piezoelectric resonator that is small and can be reduced in weight is used as a band pass filter used in an RF circuit. Such a thin film piezoelectric filter forms a piezoelectric layer made of aluminum nitride (AlN), zinc oxide (ZnO) or the like so as to be sandwiched between upper and lower electrodes on a semiconductor substrate, and elastic wave energy leaks into the semiconductor substrate. Therefore, the RF filter uses a thin-film piezoelectric resonator in which a cavity or an acoustic reflection layer is provided immediately below.

  As described above, there are two types of thin film piezoelectric resonators. The first is a Film Bulk Acoustic Resonator (FBAR) in which a cavity is provided immediately below a piezoelectric resonance stack composed of an upper electrode, a lower electrode, and a piezoelectric layer, and the second one has an acoustic impedance on the substrate. This is a Surface Mounted Resonator (SMR) in which a piezoelectric resonance stack is formed on an acoustic reflection layer formed by alternately stacking two different types of layers.

  The FBAR and SMR are resonators using elastic waves (longitudinal acoustic mode) propagating in the thickness direction of the piezoelectric resonance stack. The elastic wave excited by the AC signal applied to the upper electrode and the lower electrode propagates in the thickness direction of the piezoelectric resonance stack, and is in contact with the air or the acoustic reflection layer on the upper surface of the upper electrode and the lower surface of the lower electrode. And reflected. For this reason, elastic resonance occurs when the distance between the upper surface of the upper electrode and the lower surface of the lower electrode is an integral multiple of half the wavelength of the elastic wave.

On the other hand, propagation of elastic waves in a direction parallel to the substrate surface also occurs inside the piezoelectric layer. Acoustic wave of the piezoelectric layer in this case becomes a cut-off mode at a frequency lower than a frequency omega _c, the propagation mode at a frequency higher than a certain frequency omega _c. Or becomes a propagating mode at a frequency lower than a frequency omega _c, the cutoff mode at a frequency higher than a certain frequency omega _c. This frequency ω _c is a cut-off frequency, and the cut-off frequency ω _c is the elastic wave when the thickness of the piezoelectric layer coincides with a half wavelength of the elastic wave propagating in the thickness direction of the piezoelectric layer. Corresponds to the frequency of. Whether a blocking mode becomes one propagation mode at a frequency higher than the omega _c is determined by the dispersion curve of the piezoelectric layer (Dispersion Curve). The dispersion curve shows the relationship between the wave number k of the elastic wave propagating through the piezoelectric layer and the frequency ω.

FIG. 6 schematically shows a dispersion curve. As shown in FIG. 6, ZnO thin films, since the cutoff mode than omega _c low frequency side, showing the dispersion curve of the low-cut type. On the other hand, AlN thin film, since the cut-off mode at a higher frequency side than omega _c, shows a dispersion curve of the high-cut type.

FIG. 7 schematically shows the relationship of dispersion curves between the electrode forming portion (vibration region) and the electrode non-forming portion (buffer region) of the piezoelectric resonance stack. The cut-off frequency ω _vc of the vibration region where the upper electrode is formed is lower than the cut-off frequency ω _{bc of the} buffer region where the upper electrode does not exist, due to the increase in thickness and mass addition due to the electrode. When an AlN thin film is used as the piezoelectric layer, the dispersion curve shows a high-frequency cutoff type, so that the elastic wave excited in the vibration region becomes a propagation mode in the buffer region. On the other hand, the cut-off frequency ω _sc of the support region supported by the substrate is lower than the cut-off frequency ω _vc of the vibration region due to the increase in thickness and mass addition by the support substrate. For this reason, it becomes a cutoff mode in a support area, and an elastic wave does not propagate. Therefore, the elastic wave propagated through the buffer region is reflected at the boundary between the support region and the buffer region.

  The elastic wave excited in the vibration region does not propagate at the resonance frequency of the thickness longitudinal vibration because it is in the cutoff mode in the support region, and is reflected at the boundary between the buffer region and the support region. In addition, the elastic wave becomes an evanescent wave and attenuates on the support region side from the boundary between the buffer region and the support region. Thus, even when an elastic wave is reflected at the boundary between the buffer region and the support region, the dissipation of elastic energy to the support substrate is not completely absent. Even a slight loss of elastic energy due to the dissipation of elastic energy to the support substrate has caused a decrease in the quality factor Q value of the thin film piezoelectric resonator.

  In Patent Document 1, by providing an annular band on the outer peripheral portion of the upper electrode, the acoustic impedance between the inner part surrounded by the annular band and the annular band is made different to suppress the dissipation of elastic waves outside the vibration region. It has been shown to obtain a thin film piezoelectric resonator having a high Q value.

Non-Patent Document 1 discloses that the dispersion curve of an AlN thin film is changed from a high-frequency cutoff type to a low-frequency cutoff type by optimizing the layer structure of the acoustic reflection layer for SMR using an AlN thin film as a piezoelectric layer. Furthermore, it has been shown that the spurious mode can be suppressed and the acoustic wave energy can be confined by providing an annular band on the outer periphery of the upper electrode.
JP 2006-109472 A “Optimization of Acoustic Dispersion for High Performance Thin Film BAW Resonators”, Processeds of IEEE Ultrasonics Symposium 2005, pp. 5J-1

  Thin film piezoelectric filters are required to suppress spurious characteristics appearing in the passband and realize low insertion loss. For this reason, the thin film piezoelectric resonator used in the thin film piezoelectric resonator is required to suppress the transverse acoustic mode, which is an unnecessary vibration, and to realize a high quality factor Q value.

  As the material of the piezoelectric layer of the thin film piezoelectric resonator, AlN or ZnO is used. Among these, in a conventional thin film piezoelectric resonator having a piezoelectric layer made of ZnO, it is difficult to suppress the elastic loss converted into thermal energy by the piezoelectric layer and realize a high Q value. On the other hand, since AlN exhibits a high-frequency cutoff type dispersion curve as described above, the conventional thin-film piezoelectric resonator having a piezoelectric layer made of AlN does not have sufficient elastic energy confinement inside the vibration region and is high. It was difficult to realize the Q value.

  The technique described in Patent Document 1 can increase the Q value of the thin film piezoelectric resonator, but has a drawback of increasing spurious near the resonance frequency. Further, the method described in Non-Patent Document 1 is limited to application to an SMR type thin film piezoelectric resonator and is difficult to apply to FBAR.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a thin film piezoelectric resonator having a high Q value in which generation of a transverse acoustic mode is suppressed. Another object of the present invention is to provide a thin film piezoelectric filter capable of realizing a low insertion loss.

According to the present invention, the object as described above is achieved.
A piezoelectric resonance stack having a piezoelectric layer and an upper electrode and a lower electrode formed to face each other with the piezoelectric layer interposed therebetween, a gap or an acoustic reflection layer formed under the piezoelectric resonance stack, and the piezoelectric resonance A thin film piezoelectric resonator comprising a substrate supporting the stack,
The piezoelectric resonance stack includes a vibration region in which the upper electrode and the lower electrode face each other through the piezoelectric layer and are positioned corresponding to the gap or the acoustic reflection layer, a support region in contact with the substrate, and the vibration A buffer area located between the area and the support area,
The buffer region is configured such that the resonance frequency of the primary thickness longitudinal vibration is lower than the vibration region, the thin film piezoelectric resonator,
Is provided.

In one aspect of the present invention, the piezoelectric layer is made of a material exhibiting a high-frequency cutoff type dispersion curve. In one aspect of the present invention, the buffer region includes the vibration region. In one aspect of the present invention, the lower electrode is present in a portion of the support region adjacent to the buffer region. In one aspect of the present invention, a mass addition layer is formed on the piezoelectric layer in the buffer region. In one aspect of the present invention, the sound velocity V _{l of the} material constituting the mass addition layer and the sound velocity V _{u of the} material constituting the upper electrode satisfy a relationship of V _l <1.5 V _u . Fulfill. In one aspect of the present invention, the width w of the buffer region and the thickness t of the vibration region satisfy a relationship of w ≧ t.

  Further, according to the present invention, a thin film piezoelectric filter formed by connecting the plurality of thin film piezoelectric resonators so as to form a filter circuit is provided to achieve the above object.

  According to the thin film piezoelectric resonator of the present invention, since the resonance frequency of the primary thickness longitudinal vibration in the buffer region is set lower than the resonance frequency of the primary thickness longitudinal vibration in the vibration region, the occurrence of the transverse acoustic mode is suppressed. Thus, a thin film piezoelectric resonator having a high Q value can be realized.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 shows one embodiment of the thin film piezoelectric resonator of the present invention, FIG. 1 (a) is a schematic plan view thereof, and FIG. 1 (b) and FIG. 1 (c) are respectively shown in FIG. These are XX typical sectional drawing and YY typical sectional drawing.

  The thin film piezoelectric resonator of this embodiment includes a piezoelectric resonance stack 12, a gap 4 formed under the piezoelectric resonance stack, and a substrate 6 that supports the piezoelectric resonance stack so as to form the gap. .

  The piezoelectric resonance stack 12 is a laminate including the piezoelectric layer 2 and a lower electrode 8 and an upper electrode 10 formed so as to face each other with the piezoelectric layer interposed therebetween. The piezoelectric resonance stack 12 is not limited to the region where the laminated structure of the piezoelectric layer, the upper electrode, and the lower electrode is formed, but includes the region where the upper electrode or the lower electrode is not formed. The piezoelectric resonance stack 12 is a vibration region 18 which is a region where the lower electrode 8 and the upper electrode 10 overlap when viewed in a plane parallel to a portion in contact with the piezoelectric resonance stack 12 on the upper surface of the substrate 6, that is, when viewed in the vertical direction. And a support region 22 in contact with the substrate 6 and a buffer region 20 located between the vibration region 18 and the support region 22. The support region 22 is shown as a ring-shaped part adjacently disposed outside the buffer region 20 in the figure. The lower electrode 8 is present in the ring-shaped portion of the support region 22 adjacent to the buffer region 20. Thereby, durability at the time of vibration of the piezoelectric resonance stack 12 is improved. In addition, the part located on the board | substrate 6 around the said ring-shaped part shall also belong to a support area | region. The buffer region 20 surrounds or encloses the vibration region 18 when viewed in the vertical direction. In this way, by forming the buffer region 20 so as to include the vibration region 18, it is possible to sufficiently suppress the dissipation of elastic energy in the horizontal direction, and a thin film piezoelectric resonator having a high Q value is obtained. be able to.

  A gap 4 is formed under the vibration region 18 and the buffer region 20. That is, the vibration region 18 and the buffer region 20 are located corresponding to the gap 4.

  In the present embodiment, the shape of the vibration region 18, which is a region in which the piezoelectric layer 2 is sandwiched between the lower electrode 8 and the upper electrode 10, is a circle (a = b) or an ellipse (a ≠ b). Here, conductive thin films (referred to as connection conductors) 14A and 14B formed to connect the lower electrode 8 and the upper electrode 10 to an external circuit are not included in the lower electrode and the upper electrode, respectively. That is, the region where the connection conductor is formed is not regarded as the vibration region 18, and each boundary between the connection conductors 14 A and 14 B and the lower electrode 8 and the upper electrode 10 is one of the boundaries between the vibration region 18 and the buffer region 20. Parts. Thus, the boundary between the vibration region 18 and the buffer region 20 is obtained by extending the outline of the portion of the upper electrode 10 or the lower electrode 8 that is not in contact with the connection conductor. The buffer region 20 includes a portion where the connection conductor 14B in contact with the upper electrode 10 and the lower electrode 8 overlap when viewed in the vertical direction.

  The substrate 6 is, for example, a silicon substrate, a gallium arsenide substrate, a glass substrate, or the like. The air gap 4 can be formed by anisotropic wet etching, RIE (Reactive Ion Etching), or the like. The piezoelectric layer 2 is made of, for example, a piezoelectric material that can be manufactured as a thin film such as zinc oxide (ZnO) or aluminum nitride (AlN). As the material of the piezoelectric layer 2, a material exhibiting a high-frequency cutoff type dispersion curve such as AlN is particularly preferable. By using such a piezoelectric layer 2, a thin film piezoelectric resonator having a higher Q value can be realized. The materials of the upper electrode 10 and the lower electrode 8 are aluminum (Al), tungsten (W), molybdenum (Mo), platinum (Pt), ruthenium (Ru), iridium (Ir), and gold (Au). In addition, a metal material that can be manufactured as a thin film and that can be patterned can be used. The upper electrode 10 and the lower electrode 8 may be made of a laminated body in addition to the single layer film made of the metal material.

The buffer region 20 is configured so that the resonance frequency of the primary thickness longitudinal vibration is lower than that of the vibration region 18. That is, when the resonance frequency of the primary thickness longitudinal vibration of the vibration region 18 is ω _vc and the resonance frequency of the primary thickness longitudinal vibration of the buffer region 20 is ω _bc , ω _vc > ω _bc is satisfied. . FIG. 8 schematically shows a dispersion curve of the piezoelectric resonance stack 12 of the thin film piezoelectric resonator when such a buffer region 20 is provided. Since the buffer region 20 is in the cutoff mode at the cutoff frequency (resonance frequency of the primary thickness longitudinal vibration) ω _vc of the vibration region 18, no elastic wave propagates. Thus, the dissipation of elastic energy to the support substrate 6 is suppressed, and the elastic wave energy is confined in the vibration region 18, and a thin film piezoelectric resonator having a high Q value can be obtained.

  Such a buffer region 20 is preferably realized by forming the mass addition layer 16 on the piezoelectric layer 2. The material and thickness of the mass addition layer 16 are appropriately set so that the buffer region 20 has a lower resonance frequency of the primary thickness longitudinal vibration than the vibration region 18.

The mass addition layer 16 is preferably composed of a dielectric layer, and more preferably composed of either silicon oxide (SiO ₂ ) or silicon nitride (Si ₃ N ₄ ). When the mass addition layer 16 is formed of a conductor layer, an unintended capacitance may be formed between the upper electrode and the lower electrode and the mass addition layer, which may deteriorate the resonator characteristics. If it is composed of body layers, there is no such worry. Further, by using SiO ₂ and Si ₃ N ₄ as the mass addition layer, there is an advantage that a thin film manufacturing apparatus used in a general semiconductor process can be used in manufacturing a thin film piezoelectric resonator.

Furthermore, when the sound velocity of the material constituting the mass addition layer 16 is V _l and the sound velocity of the material constituting the upper electrode 10 is V _u , V _l <1.5 V _u is satisfied. Is preferred. Thereby, the resonance frequency of the buffer region 20 is made lower than the resonance frequency of the vibration region 18 without making the thickness of the mass addition layer 16 too large, and a thin film piezoelectric resonator having a high Q value is easily manufactured. can do.

  Further, the width of the buffer region 20 (that is, the distance between the boundary between the buffer region 20 and the vibration region 18 and the boundary between the buffer region 20 and the support region 16) w and the thickness t of the vibration region 18 are: , W ≧ t is preferably satisfied. Thereby, the attenuation of the elastic energy in the buffer region 20 is sufficient, and a thin film piezoelectric resonator having a higher Q value can be realized.

  The thin film piezoelectric resonator of this embodiment can be manufactured as follows. After a pit portion is formed on a substrate 6 such as a silicon wafer by a technique such as wet etching, a sacrificial layer is formed by a film forming technique such as a CVD method. Thereafter, the surface of the substrate and the surface of the sacrificial layer in the pit part are planarized by a planarization technique such as a CMP method to obtain a substrate on which the sacrificial layer is deposited only inside the pits. As the sacrificial layer, a material that is easily etched, such as PSG (phospho-silicate glass), is suitable. On the substrate obtained as described above, the lower electrode 8, the piezoelectric layer 2, the upper electrode 10, and the mass addition layer 16 are formed by a film forming method such as sputtering or vapor deposition, and wet etching, RIE, Each layer is patterned using a patterning technique such as a lift-off method. At this time, the material, thickness, and shape of the mass addition layer 16 are determined so that the resonance frequency of the primary thickness longitudinal vibration of the buffer region 20 is lower than the resonance frequency of the primary thickness longitudinal vibration of the vibration region 18. . Furthermore, after forming the through-hole 28 which reaches from the upper electrode 10 to the sacrificial layer using the patterning technique, an etching solution is supplied through the through-hole to perform an etching process, thereby removing the sacrificial layer. Thereby, the space | gap 4 is formed in a pit part.

Moreover, as an embodiment of the present invention, there is an embodiment as shown in FIGS. 2 and 3 in addition to the embodiment shown in FIG. FIG. 2 is a schematic cross-sectional view showing another embodiment of the thin film piezoelectric resonator of the present invention. In the embodiment of FIG. 1, the pit portion is formed on the substrate 6, and the gap 4 is formed here. In the embodiment of FIG. 2, a silicon oxide (SiO ₂ ) layer 30 is formed on the flat upper surface of the substrate 6, and the through opening formed here is used as the gap 4. In this embodiment, the whole of the substrate 6 and the SiO ₂ layer 30 corresponds to the substrate referred to in the present invention. The embodiment of FIG. 3 is the same as the embodiment of FIG. 1 in that the gap 4 is formed in the substrate 6, but is different in that a through hole formed in the substrate 6 is used as the gap 4.

The thin film piezoelectric resonator shown in FIG. 2 can be manufactured, for example, as follows. A silicon oxide (SiO ₂ ) layer 30 is formed on a substrate 6 such as a silicon wafer by a film forming technique such as sputtering or CVD, or thermal oxidation. Thereafter, a sacrificial layer that is easily dissolved by an etching solution is formed by a film formation method such as sputtering or vapor deposition, and patterning is performed using a patterning technique such as wet etching, RIE, or lift-off. As the sacrificial layer, a metal such as germanium (Ge), aluminum (Al), titanium (Ti), magnesium (Mg), or a metal oxide thereof is suitable. Thereafter, the lower electrode 8, the piezoelectric layer 2, the upper electrode 10, and the mass addition layer 16 are formed by a film forming method such as sputtering or vapor deposition, and each layer is formed using a patterning technique such as wet etching, RIE, or lift-off method. Is patterned. At this time, the material, thickness, and shape of the mass addition layer 16 are determined so that the resonance frequency of the primary thickness longitudinal vibration of the buffer region 20 is lower than the resonance frequency of the primary thickness longitudinal vibration of the vibration region 18. Furthermore, after forming the through-hole 28 which reaches from the upper electrode 10 to the sacrificial layer using the patterning technique, an etching solution is supplied through the through-hole to perform an etching process, thereby removing the sacrificial layer. Furthermore, selecting the etching capable etchant SiO ₂ layer, by etching the SiO ₂ layer can be etched SiO ₂ layer with a sacrificial layer of the same pattern. Thus, the gap 4 is formed in a portion was removed and the sacrificial layer and the SiO ₂ layer.

  The thin film piezoelectric resonator shown in FIG. 3 can be manufactured, for example, as follows. The lower electrode 8, the piezoelectric layer 2, the upper electrode 10, and the mass addition layer 16 are formed on the substrate 6 by a film forming method such as sputtering or vapor deposition, and patterning techniques such as wet etching, RIE, and lift-off method are used. Then, each layer is patterned. Thereafter, the void 4 can be formed by etching from the back surface (lower surface) of the substrate 6 using a deep etching technique such as anisotropic wet etching or Deep-RIE until the lower electrode 8 is exposed.

  As still another embodiment of the present invention, there is a thin film piezoelectric resonator shown in FIG. 4A shows a schematic plan view thereof, and FIG. 4B and FIG. 4C show its XX sectional view and YY sectional view, respectively. As shown in FIG. 4, this embodiment is different from the embodiment of FIGS. 1 to 3 in that an acoustic reflection layer 32 is provided below the piezoelectric resonance stack 12 instead of the gap.

The thin film piezoelectric resonator as shown in FIG. 4 can be manufactured as follows, for example. After a pit portion is formed on a substrate 6 such as a silicon wafer by a technique such as wet etching, an acoustic reflection layer 32 is formed by a film forming technique such as sputtering or CVD. Thereafter, the substrate surface is flattened by a flattening technique such as a CMP method to obtain a substrate on which an acoustic reflection layer is deposited only inside the pits. As the acoustic reflection layer, it is preferable that the low impedance layer is made of a material having a small acoustic impedance such as SiO ₂ or AlN, and the high impedance layer is made of a material having a large acoustic impedance such as Mo or W. By setting the thicknesses of the low-impedance layer and the high-impedance layer so as to correspond to a quarter wavelength of the elastic wave, an action as an acoustic reflection layer can be obtained. On the substrate obtained as described above, the lower electrode 8, the piezoelectric layer 2, the upper electrode 10, and the mass addition layer 16 are formed by a film forming method such as sputtering or vapor deposition, and wet etching, RIE, Each layer is patterned using a patterning technique such as a lift-off method. At this time, the material, thickness, and shape of the mass addition layer 16 are determined so that the resonance frequency of the primary thickness longitudinal vibration of the buffer region is lower than the resonance frequency of the primary thickness longitudinal vibration of the vibration region.

  In the thin film piezoelectric resonator according to the present invention, as shown in FIG. 5, there is an embodiment in which the mass addition layer 16 is formed to extend to the outer peripheral edge portion 18 ′ of the vibration region 18. FIG. 5A shows a schematic plan view, and FIGS. 5B and 5C show an XX schematic cross-sectional view and a YY schematic cross-sectional view, respectively.

  In the present embodiment, the mass addition layer is formed such that the resonance frequency of the primary thickness longitudinal vibration of the buffer region 20 is lower than the frequency of the primary thickness longitudinal vibration of the portion of the vibration region 18 where the mass addition layer 16 is not formed. A thickness of 16 is set. Further, the width w2 of the portion of the mass addition layer 16 formed on the outer peripheral edge portion 18 'of the vibration region is set so as to satisfy w2 ≦ w. Thereby, good resonance characteristics similar to those in the case where the mass addition layer 16 is not formed on the vibration region 18 can be obtained. On the other hand, when w2> w, a resonance phenomenon is likely to occur even at a frequency different from the desired frequency, and problems such as occurrence of spurious out of band are likely to occur in applications to thin film piezoelectric filters and the like. .

  The thin film piezoelectric resonator shown in FIG. 5 can be manufactured using the same material and manufacturing method as the thin film piezoelectric resonator shown in FIG.

In the thin film piezoelectric resonator according to the present invention, as shown in FIG. 9, there is an embodiment in which a dielectric layer 24 is formed below the lower electrode 8 and a dielectric layer 26 is formed above the upper electrode 10. As a material of the dielectric layers 24 and 26, a material having a relatively large elastic modulus such as AlN, AlON, Si ₃ N ₄ , and SiAlON is preferable. The configuration other than the presence of the dielectric layers 24 and 26 is the same as that of the embodiment described with reference to FIG. Even in the thin film piezoelectric resonator having the lower dielectric layer 24 and / or the upper dielectric layer 26 as shown in FIG. 9, the transverse acoustic mode is used as in the thin film piezoelectric resonator shown in FIGS. A thin film piezoelectric resonator having a high Q value can be obtained without deteriorating characteristics. By providing the lower dielectric layer 24 and / or the upper dielectric layer 26, it is possible to prevent oxidative degradation of the lower electrode 8 and / or the upper electrode 10.

  FIG. 10 shows an example of a ladder type filter circuit which is an embodiment of the thin film piezoelectric filter of the present invention. In this thin film piezoelectric filter 100, the thin film piezoelectric resonators 111, 113, 115 according to the present invention as in the above embodiment are used as series elements, and the thin film piezoelectric resonators 112, 114, according to the present invention as in the above embodiment are further used. 116 is used as a shunt element (parallel element). 101 and 102 are input / output ports. The thin film piezoelectric filter of the present invention is not limited to the one having the circuit configuration shown in FIG. 10, but a lower loss thin film piezoelectric filter can be configured by using a ladder circuit.

(Example 1)
A thin film piezoelectric resonator having the form shown in FIG. 1 was manufactured with the diameter d (= a = b) of the upper electrode being 120 μm. The material and thickness of each constituent layer in this example were set as follows. The lower electrode was 300 nm thick with Mo, the piezoelectric layer was 1700 nm thick with AlN, the upper electrode was 200 nm thick with Mo, and the mass addition layer was 400 nm with Si ₃ N ₄ . The thickness t of the vibration region was 2.2 μm, and the width w of the buffer region was 4.0 μm.

FIG. 11 shows the frequency-impedance (Z) and phase characteristics of the thin film piezoelectric resonator fabricated in this manner. Table 1 shows the electromechanical coupling coefficient kt ² and the Q value of the thin film piezoelectric resonator. Compared to the results of Comparative Example 1 described later shown in FIG. 12 and Table 1, it can be seen that the resonance characteristics are excellent with a large impedance at the antiresonance frequency and a large Q value.

(Examples 2 to 4)
A thin film piezoelectric resonator having the configuration shown in FIG. 1 was fabricated in the same manner as in Example 1 except that the width w of the buffer region was 3.5 μm, 3.0 μm, and 1.0 μm, respectively. The results are shown in Table 1. In Examples 2 and 3 in which the width w of the buffer region is in the range of w ≧ t with respect to the thickness t of the vibration region, a thin film piezoelectric resonator having a higher Q value than that in Example 4 was obtained. It can be seen that more excellent resonator characteristics can be obtained by setting the width w of the buffer region to be w ≧ t.

(Comparative Example 1)
A piezoelectric thin film resonator having the configuration shown in FIG. 1 was fabricated in the same manner as in Example 1 except that the mass addition layer was not formed. The results are shown in FIG. As can be seen from FIG. 12 and Table 1, when the mass-added layer is not formed, the impedance at the antiresonance frequency becomes small and the Q value becomes small, which is not preferable.

(Example 5)
A thin film piezoelectric resonator having the configuration shown in FIG. 1 was fabricated in the same manner as in Example 1 except that the mass addition layer was made of SiO ₂ and had a thickness of 350 nm. The results are shown in Table 1. As can be seen from Table 1, the Q value is also large and shows excellent resonance characteristics, and it can be seen that the SiO ₂ layer is suitable as the mass addition layer.

(Example 6)
A thin-film piezoelectric resonator having the configuration shown in FIG. 1 was prepared in the same manner as in Example 1 except that the shape of the upper electrode, that is, the shape of the vibration region was changed to a major axis (a) of 144 μm and a minor axis (b) of 102 μm. The results are shown in Table 1. As can be seen from Table 1, a thin film piezoelectric resonator having a high Q value can be obtained even when the vibration region has an elliptical shape.

(Comparative Example 2)
A piezoelectric thin film resonator having the configuration shown in FIG. 1 was fabricated in the same manner as in Example 6 except that the mass addition layer was not formed. The results are shown in Table 1. As can be seen from Table 1, when the mass addition layer is not formed, the Q value decreases, which is not preferable.

(Example 7)
Example 1 except that the shape of the upper electrode, that is, the shape of the vibration region is a polygon having no parallel sides as shown in FIG. 13 and the area is the same as that of Example 1. A thin film piezoelectric resonator was fabricated in the same manner as described above. The results are shown in Table 1. Although the noise due to the transverse acoustic mode of the obtained thin film piezoelectric resonator was suppressed, the Q value at the antiresonance frequency was 960, which was lower than that of the vibration region having a circular or elliptical shape. It can be seen that the electrode shape is preferably circular or elliptical rather than polygonal.

（実施例８）
実施例１と同様にして６個の薄膜圧電共振器を作製し、これらを用いて図１０に示す形態の薄膜圧電フィルタを作製した。図１４に作製した薄膜圧電フィルタの通過特性を示す。図１４には、比較例１と同様にして作製された６個の薄膜圧電共振器を用いた以外は同様にして作製された比較例の薄膜圧電フィルタの通過特性も示されている。図１４より明らかなように、質量付加層を形成した緩衝領域がある薄膜圧電共振器を用いた本発明実施例の薄膜圧電フィルタは比較例に比べて、挿入損失が小さくなっており、優れた特性を示すことがわかる。

(Example 8)
Six thin film piezoelectric resonators were produced in the same manner as in Example 1, and a thin film piezoelectric filter having the form shown in FIG. 10 was produced using them. FIG. 14 shows the pass characteristics of the thin film piezoelectric filter fabricated. FIG. 14 also shows the pass characteristics of a thin film piezoelectric filter of a comparative example manufactured in the same manner except that six thin film piezoelectric resonators manufactured in the same manner as in Comparative Example 1 were used. As is clear from FIG. 14, the thin film piezoelectric filter of the embodiment of the present invention using the thin film piezoelectric resonator having the buffer region in which the mass addition layer is formed has an insertion loss smaller than that of the comparative example and is excellent. It can be seen that the characteristics are shown.

It is a figure which shows one Embodiment of the thin film piezoelectric resonator of this invention, (a) is a typical top view, (b) is XX typical sectional drawing of (a), (c) is (a). It is YY typical sectional drawing. It is a typical sectional view showing one embodiment of a thin film piezoelectric resonator of the present invention. It is a typical sectional view showing one embodiment of a thin film piezoelectric resonator of the present invention. It is a figure which shows one Embodiment of the thin film piezoelectric resonator of this invention, (a) is a typical top view, (b) is XX typical sectional drawing of (a), (c) is (a). It is YY typical sectional drawing. It is a figure which shows one Embodiment of the thin film piezoelectric resonator of this invention, (a) is a typical top view, (b) is XX typical sectional drawing of (a), (c) is (a). It is YY typical sectional drawing. It is a schematic diagram of the dispersion curve of a piezoelectric thin film. It is a schematic diagram of a dispersion curve of a conventional thin film piezoelectric resonator using an AlN thin film. It is a schematic diagram of the dispersion curve of the thin film piezoelectric resonator of the present invention using an AlN thin film. It is a figure which shows one Embodiment of the thin film piezoelectric resonator of this invention, (a) is a typical top view, (b) is XX typical sectional drawing of (a), (c) is (a). It is YY typical sectional drawing. It is a figure which shows the ladder type filter circuit which is one Embodiment of the thin film piezoelectric filter using the thin film piezoelectric resonator of this invention. It is a figure which shows the characteristic of the impedance of the thin film piezoelectric resonator obtained in Example 1, and a phase. It is a figure which shows the characteristic of the impedance and phase of the thin film piezoelectric resonator obtained by the comparative example 1. It is a schematic diagram which shows the shape of the upper electrode of Example 7, ie, the shape of a vibration area | region. It is a figure which shows the passage amplitude characteristic of the thin film piezoelectric filter obtained in Example 7.

Explanation of symbols

2 Piezoelectric layer 4 Gap 6 Substrate 8 Lower electrode 10 Upper electrode 12 Piezoelectric resonance stacks 14A and 14B Connection conductor 16 Mass addition layer 18 Vibration region 18 ′ Vibration region outer peripheral edge 20 Buffer region 22 Support region 24 Lower dielectric layer 26 Upper dielectric layer Body layer 28 Sacrificial layer etching through hole 30 SiO ₂ layer 32 Acoustic reflection layer 100 Thin film piezoelectric filter 101, 102 Input / output ports 111, 113, 115 Serial thin film piezoelectric resonator 112, 114, 116 Parallel thin film piezoelectric resonator

Claims (8)

A piezoelectric resonance stack having a piezoelectric layer and an upper electrode and a lower electrode formed to face each other with the piezoelectric layer interposed therebetween, a gap or an acoustic reflection layer formed under the piezoelectric resonance stack, and the piezoelectric resonance A thin film piezoelectric resonator comprising a substrate supporting the stack,
The piezoelectric resonance stack includes a vibration region in which the upper electrode and the lower electrode face each other through the piezoelectric layer and are positioned corresponding to the gap or the acoustic reflection layer, a support region in contact with the substrate, and the vibration A buffer area located between the area and the support area,
The buffer region is configured such that a resonance frequency of primary thickness longitudinal vibration is lower than that of the vibration region. The thin film piezoelectric resonator according to claim 1, wherein the piezoelectric layer is made of a material exhibiting a high-frequency cutoff type dispersion curve. The thin-film piezoelectric resonator according to claim 1, wherein the buffer region includes the vibration region. The thin film piezoelectric resonator according to claim 1, wherein the lower electrode exists in a portion of the support region adjacent to the buffer region. The thin film piezoelectric resonator according to claim 1, wherein a mass addition layer is formed on the piezoelectric layer in the buffer region. The sound velocity V _{l of the} material constituting the mass addition layer and the sound velocity V _{u of the} material constituting the upper electrode satisfy a relationship of V _l <1.5V _u. Item 6. The thin film piezoelectric resonator as described in Item 5. The thin film piezoelectric resonator according to claim 1, wherein a width w of the buffer region and a thickness t of the vibration region satisfy a relationship of w ≧ t. A thin film piezoelectric filter formed by connecting a plurality of thin film piezoelectric resonators according to claim 1 so as to form a filter circuit.

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