JP2008005186A - Thin film piezoelectric resonator and thin film piezoelectric filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thin film piezoelectric resonator which has a high Q value, and also to provide a thin film piezoelectric resonator filter which has superior filter characteristics. <P>SOLUTION: The thin film piezoelectric resonator comprises: a piezoelectric resonator stack having a piezoelectric layer, and an upper electrode and a lower electrode formed to be opposed across the piezoelectric layer; a gap part formed below the piezoelectric resonator stack; and a substrate which supports the piezoelectric resonator stack so that the gap part is formed. and the resonator satisfies t<SB>2</SB>>5t<SB>1</SB>/E<SB>1</SB>, where t<SB>1</SB>is the thickness of the piezoelectric layer of a vibration part where the upper electrode and lower electrode overlap each other along the thickness, E<SB>1</SB>is the specific dielectric constant of the piezoelectric layer, and t<SB>2</SB>is the depth of the gap part. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention belongs to the technical field of communication equipment, and relates to a thin film piezoelectric resonator and a thin film piezoelectric filter using the thin film piezoelectric resonator, and particularly to a resonator structure in which loss is suppressed.

  The RF circuit part of a cellular telephone is always required to be downsized. Recently, it has been demanded to add various functions to cellular phones, and it is preferable to incorporate as many components as possible in order to realize them, while the size of cellular phones is limited. Therefore, there is a strict demand for reduction of the exclusive area (mounting area) and height in the equipment, and therefore, components constituting the RF circuit portion are required to have a small exclusive area and a low height.

  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. The thin film piezoelectric filter as described above forms a piezoelectric thin film such as aluminum nitride (AlN) or zinc oxide (ZnO) so as to be sandwiched between upper and lower electrodes on a semiconductor substrate as shown in the literature, and is elastic. This is an RF filter composed of a thin film bulk acoustic resonator (FBAR) in which a cavity is provided immediately below so that wave energy does not leak into the semiconductor substrate (see, for example, Patent Document 1 and Patent Document 2). .

  FIG. 7 shows one mode of a conventional thin film piezoelectric resonator, FIG. 7 (a) is a schematic plan view thereof, and FIG. 7 (b) is a sectional view taken along line XX of FIG. 7 (a). . Since FIG. 7 is a schematic drawing, it does not accurately reflect the relative sizes of the piezoelectric layer, the cavity, and the like. The thin film piezoelectric resonator of FIG. 1 includes a piezoelectric thin film, and a lower electrode 8 and an upper electrode 10 formed so as to sandwich the piezoelectric thin film. The thin film piezoelectric resonator includes a substrate 6 having a cavity 4 and a piezoelectric resonator stack 12 having a peripheral portion supported on an edge on the upper surface of the substrate 6 and suspended. The piezoelectric resonator stack 12 includes a piezoelectric thin film (piezoelectric layer) 2, a lower electrode (lower electrode layer) 8 and an upper electrode (upper electrode layer) 10 formed so as to sandwich the piezoelectric thin film.

A piezoelectric resonator stack 12 composed of a laminate of the piezoelectric layer 2 and the electrode layers 8 and 10 is suspended at the peripheral edge thereof, and both main surfaces thereof are in contact with air or other surrounding gas or vacuum. Yes. In this case, the piezoelectric resonator stack 12 forms an acoustic resonator having a high Q. The AC signal applied to the electrode layers 8 and 10 has a frequency equal to a value obtained by dividing the speed of sound in the piezoelectric resonator stack 12 by twice the weighted thickness of the stack 12. That is, in the case of fr = v / 2t ₀ (where fr is the resonance frequency, v is the speed of sound in the stack 12, and t ₀ is the weighted thickness of the stack 12), The piezoelectric resonator stack 12 resonates. Since the sound velocity in the layers constituting the piezoelectric resonator stack 12 differs depending on the material constituting each layer, the resonance frequency of the piezoelectric resonator stack 12 is not the physical thickness but the piezoelectric layer 2 or the electrode layers 8 and 10. It is determined by the weighted thickness considering the speed of sound and their physical thickness. In the form of FIG. 7, the outer shape of the portion where the upper electrode 10 and the lower electrode 8 overlap each other in the thickness direction is a quadrangle.

Japanese Patent Laid-Open No. 06-204776 JP 2000-69594 A

  In the piezoelectric resonator, as described above, it is known to form a cavity in order to ensure longitudinal vibration in the thickness direction. However, even in this type of piezoelectric resonator, an excellent filter characteristic having a higher Q value is desired.

  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 and to easily provide a thin film piezoelectric resonator filter having excellent filter characteristics. Is.

  The inventors of the present invention have found that when Si, which is a semiconductor, is used for the substrate, if the depth of the cavity to be formed is insufficient, there is a capacitance in the cavity between the lower electrode and the Si substrate, causing deterioration of characteristics. The present invention has been reached.

That is, the present invention relates to a piezoelectric resonator stack having a piezoelectric layer and an upper electrode and a lower electrode formed so as to face each other with the piezoelectric layer interposed therebetween, and a gap portion formed under the piezoelectric resonator stack. A thin film piezoelectric resonator comprising a substrate that supports the piezoelectric resonator stack so as to form the gap portion, and the thickness of the piezoelectric layer of the vibrating portion in which the upper electrode and the lower electrode overlap in the thickness direction is t _{1. The} thin film piezoelectric resonator according to claim ₁ , wherein t ₂ > 5t ₁ / E ₁ where E ₁ is a dielectric constant of the piezoelectric layer and t ₂ is a depth of the gap.

Further, preferably, _{t 2} the depth of the gap _{portion, 5t 1 / E 1 <t} 2 <25t 1 / E 1
It is.

  The present invention also relates to a thin film piezoelectric filter using the thin film piezoelectric resonator.

According to the thin film piezoelectric resonator of the present invention, the thickness of the piezoelectric layer of the vibrating portion where the upper electrode and the lower electrode overlap in the thickness direction is t ₁ , the relative dielectric constant of the piezoelectric layer is E ₁ , and the depth of the gap is set. When t ₂ is t ₂ > 5t ₁ / E ₁ , a thin film piezoelectric resonator having a high Q value can be realized without causing deterioration of characteristics. Further, it is possible to provide a thin film piezoelectric filter using these thin film piezoelectric resonators and having excellent filter characteristics.

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

FIG. 1 shows an embodiment of the thin film piezoelectric resonator of the present invention, FIG. 1 (a) is a schematic plan view thereof, and FIG. 1 (b) is a sectional view taken along line XX of FIG. 1 (a). . The thin film piezoelectric resonator of the present invention includes a piezoelectric resonator stack 12 including a piezoelectric thin film (piezoelectric layer) 2, a lower electrode 8 and an upper electrode 10 formed so as to sandwich the piezoelectric thin film, and the piezoelectric resonator stack. And a substrate 6 that supports the piezoelectric resonator stack so as to form the gap 4. When the thickness of the piezoelectric layer 2 is t ₁ , the relative dielectric constant of the piezoelectric layer 2 is E ₁ , and the depth of the gap 4 is t ₂ in the vibrating portion where the upper electrode 10 and the lower electrode 8 overlap in the thickness direction. , T ₂ > 5t ₁ / E ₁ . The depth of the gap 4 refers to the distance between the lower electrode and the bottom of the gap.

When the depth t ₂ of the gap portion 4 is t ₂ > 5t ₁ / E ₁ , the capacitance C ₁ formed between the upper electrode and the lower electrode of the vibrating portion is formed in the hollow portion below the vibrating portion. Since it is much larger than the capacitance C ₂ to be formed, the characteristics of the vibration part are not affected by the capacitance of the cavity part, and this makes it possible to provide a resonator having a high Q with little deterioration of the resonance characteristics. . Since t2 is preferably as large as possible in terms of filter characteristics, there is no upper limit. However, in manufacturing, if the depth of the gap portion 4 is increased, if the depth of the cavity portion is increased, it takes time for manufacturing and increases costs, which is not practical. . Further, since the resonator becomes large, t ₂ <25t ₁ / E ₁ is practical. The form in which t ₂ is infinitely large is a thin film piezoelectric resonator having a structure in which the lower part of the substrate is opened. However, since the opened hole becomes larger in the lower part, a plurality of resonators are used when a filter is formed. It cannot be arranged precisely and the filter becomes large. The thin film piezoelectric resonator of the present invention is excellent in filter characteristics and easy to arrange.

  In the embodiment of FIG. 1, the outer shape of the portion of the piezoelectric thin film 2 sandwiched between the lower electrode 8 and the upper electrode 10 is circular, but is not limited thereto. A conductive thin film (referred to as a connection conductor) is connected around the upper electrode and a part of the lower electrode to connect to an external circuit, but these are not included in the upper electrode and the lower electrode. To do. Further, the boundary between the connection conductor and the upper electrode or the lower electrode is obtained by extending a line of the outer shape of another part of the upper electrode or the lower electrode. In the embodiment of FIG. 1, the portion where the upper electrode and the lower electrode overlap with each other in the thickness direction is circular, and further, the region includes a straight line having a length of 1/50 of the diameter passing through the center of the circle. Does not form the upper electrode. Thereby, characteristic deterioration by transverse acoustic mode can be prevented.

  As the configuration and material of the thin film piezoelectric resonator of the present invention, the same configuration and material as those of a conventional thin film piezoelectric resonator can be applied. For example, the substrate 6 may be made of a silicon substrate, a gallium arsenide substrate, a glass substrate, or the like, and the piezoelectric thin film (piezoelectric layer) 2 can be manufactured as a thin film such as zinc oxide (ZnO) or aluminum nitride (AlN). It may be made of a piezoelectric material.

  The thin film piezoelectric resonator as described above 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 is planarized by a planarization technique such as a CMP method to obtain a substrate on which a sacrificial layer is deposited only inside the pits. The lower electrode 8, the piezoelectric layer 2, and the upper electrode 10 are formed by a film forming method such as sputtering or vapor deposition, and each layer is patterned using a patterning technique such as wet etching, RIE, or lift-off. At this time, a region 40 where the upper electrode is not formed is formed at the center of the upper electrode. Further, after forming the through hole 30 reaching from the upper surface of the substrate to the sacrificial layer using the patterning technique, the sacrificial layer is removed with an etching solution. As a result, the pit portion becomes the hollow portion 4.

In the embodiment of FIG. 1, the outer shape of the portion of the piezoelectric thin film 2 sandwiched between the lower electrode 8 and the upper electrode 10 is circular, but as another embodiment, the thin film piezoelectric resonator shown in FIG. Is mentioned. Also in the embodiment of FIG. 2, in the vibrating portion where the upper electrode 10 and the lower electrode 8 overlap in the thickness direction, the thickness of the piezoelectric layer 2 is t ₁ , the relative dielectric constant of the piezoelectric layer 2 is E ₁ , and the gap portion 4 when the _{t 2} depth _{is t 2> 5t 1 / E 1} . Accordingly, it is possible to provide a resonator having a high Q with little deterioration in resonance characteristics. Further, in the embodiment of FIG. 2, the outer shape of the portion where the upper electrode and the lower electrode overlap each other in the thickness direction is a quadrangle, includes the intersection of two diagonal lines of the quadrangle, and is 1 / of one of the diagonal lines. In the region including 10, the upper electrode is not formed. Thereby, characteristic deterioration by transverse acoustic mode can be prevented.

FIG. 3 shows another embodiment of the thin film piezoelectric resonator of the present invention. Also in the embodiment of FIG. 3, in the vibrating portion where the upper electrode 10 and the lower electrode 8 overlap in the thickness direction, the thickness of the piezoelectric layer 2 is t ₁ , the relative dielectric constant of the piezoelectric layer 2 is E ₁ , and the gap portion 4 when the _{t 2} depth _{is t 2> 5t 1 / E 1} . Accordingly, it is possible to provide a resonator having a high Q with little deterioration in resonance characteristics. Further, in the embodiment of FIG. 3, the portion of the piezoelectric thin film 2 sandwiched between the lower electrode 8 and the upper electrode 10 is a parallelogram. Thus, even when the portion where the lower electrode and the upper electrode overlap with each other in the thickness direction is a parallelogram, it includes the intersection of two diagonal lines of the parallelogram and is at least 1 / on one of the diagonal lines. An excellent thin film piezoelectric resonator that suppresses the generation of noise due to the transverse acoustic mode without impairing the high Q value can be obtained by providing the region including 10 and not forming the upper electrode.

FIG. 4 shows another embodiment of the thin film piezoelectric resonator of the present invention. Also in the embodiment of FIG. 4, the vibration part where the upper electrode 10 and the lower electrode 8 overlap in the thickness direction has a thickness t _{1 of} the piezoelectric layer 2, a dielectric constant E _{1 of} the piezoelectric layer 2, and when the _{t 2} depth _{is t 2> 5t 1 / E 1} . Accordingly, it is possible to provide a resonator having a high Q with little deterioration in resonance characteristics. Furthermore, in the embodiment of FIG. 4, the portion of the piezoelectric thin film 2 sandwiched between the lower electrode 8 and the upper electrode 10 is elliptical. Thus, even when the portion where the lower electrode and the upper electrode overlap with each other in the thickness direction is elliptical, it passes through the center of the circle inscribed in the elliptical shape and the length is 1/50 of the diameter of the circle. By not forming the upper electrode in the region including the straight line, an excellent thin film piezoelectric resonator that suppresses the generation of noise due to the transverse acoustic mode without impairing the high Q value can be obtained.

As described above, the depth t ₂ portion of the cavity from which the sacrificial layer has been removed forms a capacitance between the lower electrode and the Si of the substrate. Therefore, if t ₂ is too small, the additional capacitance near the center of the vibration portion is increased. It becomes a large value and causes the characteristics of the resonator to deteriorate. In addition, it is desirable that the sufficient amount is a factor of high cost in terms of manufacturing, and it is desirable to define the necessary minimum. T ₂ when the thickness of the piezoelectric vibrating portion is t _1, the
_{5t 1 / E 1 <t 2} <25t 1 / E 1
It is preferable to set in the range represented by.

  Furthermore, the thin film piezoelectric filter of the present invention is a filter using the thin film piezoelectric resonator of the present invention, and is configured by arranging the thin film piezoelectric resonator of the present invention in a plurality of ladder types as shown in FIG. However, the present invention is not limited to this. The thin film piezoelectric filter of the present invention has a high Q and is excellent in filter characteristics.

(Example 1)
A thin film piezoelectric resonator having a resonance frequency in the 2 GHz band shown in FIG. 1 in which the upper electrode has a circular shape with a radius of 100 μm and a square portion in which the upper electrode with a side of 10 μm is removed exists in the center. The thickness of each constituent layer in this example was set as follows. The lower electrode was made of Mo with a thickness of 300 nm, the piezoelectric layer was made of AlN with a thickness t ₁ of 1500 nm, and the upper electrode was made of Mo with a thickness of 200 nm. The depth t ₂ of the cavity below the vibrating part was set to 1.5 μm, which is the same as the film thickness of the piezoelectric layer. The relative dielectric constant of AlN used is 10. That is, in the thin film piezoelectric resonator of this example, t ₂ is in a relationship greater than 5t ₁ / E ₁ . A solid line in FIG. 6 shows the pass characteristics of the filter formed by the ladder circuit shown in FIG. 5 using the resonator formed as described above. It can be seen that the filter formed with the fabricated thin film piezoelectric resonator has low loss and shows good filter characteristics.

(Comparative Example 1)
FBAR, except that the depth t ₂ of the cavity of the vibrating subordinate unit was set to 0.4μm which is about 1/4 of the thickness of the piezoelectric layer has the same configuration as in Example 1. A pass characteristic of a filter formed by the ladder circuit shown in FIG. 5 using this resonator is shown by a dotted line in FIG. t ₂ is smaller than half of the thickness t ₁ of the piezoelectric layer, and t ₂ is smaller than 5t ₁ / E ₁ . Therefore, this comparative example was out of the scope of the present invention, and the pass characteristic of the filter showed a filter characteristic with a large ripple and a large loss.

(Example 2)
FBAR, except that the depth t ₂ of the cavity of the vibrating subordinate unit was set to 3μm is twice the thickness t ₁ of the piezoelectric layer have the same structure as in Example 1. In this case, the relative permittivity of the piezoelectric layer is 10, _{t 2} is the _5t 1 / _{E 1} greater than relationship. The pass characteristic of a filter formed by the ladder circuit shown in FIG. 5 using this resonator is shown by a broken line in FIG. It can be seen that the pass characteristics of the filter show good filter characteristics with no ripple and small loss.

It is (a) typical top view and (b) sectional view showing one embodiment of a thin film piezoelectric resonator of the present invention. It is (a) typical top view and (b) sectional view showing other embodiments of the thin film piezoelectric resonator of the present invention. It is (a) typical top view and (b) sectional view showing other embodiments of the thin film piezoelectric resonator of the present invention. It is (a) typical top view and (b) sectional view showing other embodiments of the thin film piezoelectric resonator of the present invention. It is a figure which shows the ladder type circuit which is one Embodiment of the filter using the thin film piezoelectric resonator of this invention. It is a figure which shows the filter characteristic of the filter using (a) thin film piezoelectric resonator obtained in Example 1, 2, and the comparative example 1, and (b) the filter using a thin film piezoelectric resonator. It is (a) typical top view of the conventional thin film piezoelectric resonator, and (b) sectional drawing.

Explanation of symbols

2 Piezoelectric Layer 4 Cavity 6 Substrate 8 Lower Electrode 10 Upper Electrode 12 Piezoelectric Resonator Stack 14 Connection Conductor 30 Sacrificial Layer Etching Through Hole 40 Region Where Upper Electrode is Not Formed (Upper Electrode Layer Removal Portion)
44 Region where the upper electrode is not formed (upper electrode, lower electrode and piezoelectric layer removing portion)
DESCRIPTION OF SYMBOLS 100 Thin film piezoelectric filter 101,102 Input / output port 111,113,115 Series resonance element of thin film piezoelectric filter 112,114,116 Shunt resonance element of thin film piezoelectric filter

Claims (3)

Piezoelectric resonator stack having a piezoelectric layer and an upper electrode and a lower electrode formed so as to face each other with the piezoelectric layer interposed therebetween, a void portion formed under the piezoelectric resonator stack, and forming the void portion A thin film piezoelectric resonator comprising a substrate supporting a piezoelectric resonator stack, wherein the piezoelectric layer has a thickness t ₁ , wherein the upper electrode and the lower electrode overlap each other in the thickness direction, and the piezoelectric layer has a thickness t ₁ . dielectric constant _{E 1} a, when the depth of the gap portion was _{t 2,} the FBAR, which is a t _2> 5t ₁ / _E 1 of the. The cavity depth t ₂ is:
t ₂ <25t ₁ / E ₁
The thin film piezoelectric resonator according to claim 1, wherein:   A thin film piezoelectric filter using the thin film piezoelectric resonator according to claim 1.

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