JPH10209794A - Piezoelectric thin-film resonator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To facilitate the manufacture and to obtain a high Q-value by forming a lower electrode, a PZT thin film or PT thin film as a piezoelectric body thin film, and upper electrodes in order on a single-crystal or polycrystalline diamond thin film formed on an Si substrate, and making the interval between the two upper electrodes larger than the film thickness of the piezoelectric thin film. SOLUTION: On a Si substrate 2 which has a diamond thin film 1 on the top surface and an SiO2 film 2A formed on the reverse surface, a Ti layer with 50 to 500Å thickness is formed and on it, a conductive meal layer with 1000 to 2000Å in film thickness is formed of Pt, Ir, etc., as a lower electrode 3. On the lower electrode 3, the PZT thin film or PT thin film 4 with 0.1 to 10μm thickness as the piezoelectric thin film is formed by a sol-gel method and on it, the upper electrodes 5A and 5B are formed similarly to the lower electrode 3. The intervals between those upper electrodes is made 2 to 200 times as large as the film thickness of the PT thin film 4. Consequently the resonator is usable for a wide-band filter in a simple structure and a resonator with a wide range of oscillation frequency and can easily be manufactured.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a piezoelectric thin-film resonator, and more particularly to a piezoelectric thin-film resonator suitable for a resonator, a filter, and the like that operate in a high-frequency band and use a bulk wave in a thickness direction of the piezoelectric thin film.

[0002]

2. Description of the Related Art In general, a piezoelectric vibrator used in a high frequency band utilizes thickness vibration of a thin plate. Conventionally provided piezoelectric vibrators for high frequency include those having the following configurations (1) to (4).

[0003] A piezoelectric vibrator that uses a fundamental vibration of a piezoelectric plate made of quartz, piezoelectric ceramic, or the like, which is thinly polished. Higher-order mode vibrators utilizing higher-order vibrations such as quartz and piezoelectric ceramics. A composite vibrator in which a piezoelectric vapor-deposited film is formed on a substrate, and the piezoelectric vapor-deposited film is excited to vibrate the substrate to a higher order.

[0004] Of the above-mentioned conventional piezoelectric vibrators, the basic resonance frequency increases in inverse proportion to the plate thickness when the thickness of a piezoelectric plate such as quartz or piezoelectric ceramics is reduced.
The thinner the plate, the more difficult it is to machine. For this reason, at present, at a plate thickness of 30 to 40 μm and a resonance frequency of 5
The limit is about 0 MHz.

In the case of the structure described above, since the higher-order vibration is used, the electromechanical coupling coefficient is reduced, and the frequency bandwidth is too small to be practical. In addition, low-order vibration having a large electromechanical coupling coefficient causes spurious. There are drawbacks. Also,
The same configuration has the same disadvantage.

[0006] In the case of using such a higher-order vibration mode, the piezoelectric thin film becomes considerably thinner than the substrate, so that the resonance Q and temperature characteristics are mainly determined by the substrate material. Therefore, the constituent material of the substrate is extremely important.

The high frequency piezoelectric material for a piezoelectric element is, for example, a paraelectric AlN, CdS, ZnO
Etc. are used. Even if these materials are thinned by machining, the thickness is limited to about 40 μm, and the resonance frequency of the fundamental wave is limited to several tens of MHz for any material having such a thickness. In a high-frequency piezoelectric thin-film resonator using these materials, for example, in order to obtain a fundamental vibration having a high resonance frequency of 500 MHz or more, the plate thickness needs to be 10 μm or less.

On the other hand, in a high frequency band of several hundred MHz,
As a method for obtaining a piezoelectric vibrator having a large electromechanical coupling coefficient, there is a method using a thin film manufacturing technique such as a sputtering method and an etching technique. For example, JP-A-60-31305 discloses a method using zinc oxide and A piezoelectric element formed with a thin film of lead titanate is described. This Japanese Unexamined Patent Publication No.
In the piezoelectric element described in Japanese Patent No. 31305, a part of the substrate is removed by etching in order to eliminate the influence of the substrate and utilize the vibration characteristics of the piezoelectric thin film.

Conventionally, in a piezoelectric thin film resonator using thickness vibration, an electric field is applied to upper and lower electrodes of a piezoelectric film, as typified by a piezoelectric thin film resonator described in JP-A-8-148968. By applying the voltage, the bulk wave is excited.

[0010]

The zinc oxide formed by the sputtering method and other conventional piezoelectric materials all have a small electromechanical coupling coefficient of about 20 to 30%. The width and oscillation frequency range are limited. In Japanese Patent Application Laid-Open No. Sho 60-31305,
To improve this point, a part of the substrate is removed. However, when the substrate is removed in this way, there is a disadvantage that the element strength is reduced.

Further, in a conventionally provided piezoelectric thin film resonator using thickness vibration, a bulk wave is excited by applying an electric field to upper and lower electrodes of a piezoelectric film and performing polarization processing in a thickness direction. Therefore, it is necessary to expose the lower electrode in order to use the lower electrode as a terminal electrode in this polarization processing. Therefore, after the piezoelectric film is uniformly formed on the lower electrode, a part of the piezoelectric film is removed by etching to expose the lower electrode, or the lower electrode is partially formed in forming the piezoelectric film. It is necessary to form a piezoelectric film while leaving a part of the lower electrode by masking or the like, and there is a disadvantage that a complicated process for exposing the lower electrode is required.

As described above, in the case of using the higher-order vibration mode, the selection of the substrate material is extremely important because the characteristics of the piezoelectric thin film resonator are greatly affected by the substrate material. Investigation of the optimal substrate material for the resonator has not been sufficiently performed.

The present invention solves the above-mentioned conventional problems, and provides a piezoelectric thin film having a large electromechanical coupling coefficient, a wide bandwidth and oscillation frequency range when applied to a resonator, a filter, etc., and a high Q value. It is an object of the present invention to provide a piezoelectric thin film resonator which does not need to expose a lower electrode and is therefore easy to manufacture.

[0014]

According to the present invention, there is provided a piezoelectric thin film resonator comprising: a single crystal or polycrystalline substrate; a lower electrode formed on the substrate; a piezoelectric thin film formed on the lower electrode; A piezoelectric thin-film resonator comprising two upper electrodes formed on the piezoelectric thin film, wherein the piezoelectric thin film is a PZT thin film or a PT thin film, and a gap between the two upper electrodes is provided. The interval is larger than the thickness of the piezoelectric thin film.

In the piezoelectric thin film resonator of the present invention, the piezoelectric film is made of PZT (lead zirconate titanate), which is a piezoelectric material having a large electromechanical coupling coefficient, or the Curie point is as high as about 500 degrees, and the coupling in the thickness direction is high. (Lead titanate) which has the characteristic that the coefficient and the value of the coupling coefficient in the spreading direction are greatly different.
Therefore, a wide-band filter and a resonator having a wide oscillation frequency range can be realized.

Further, since a single crystal or polycrystal substrate such as a single crystal or polycrystal diamond substrate is used, a high Q value can be obtained, and therefore, insertion loss can be reduced. This single-crystal or polycrystalline substrate is particularly preferably formed by forming a diamond thin film having a thickness of 1 to 30 μm on a Si substrate.

Moreover, since two upper electrodes are formed at an interval, the piezoelectric film can be polarized in the thickness direction by applying an electric field between the upper electrodes. Therefore, it is not necessary to use the lower electrode as a terminal electrode, so that a complicated process for exposing the lower electrode is not required.

It is difficult to obtain a high quality piezoelectric thin film from PZT or PT. For example, the sputtering method cannot form a PZT thin film or a PT thin film that is good enough to sufficiently confirm thickness vibration resonance.

On the other hand, if a PZT thin film or PT thin film is formed by the sol-gel method, a PZT thin film or PT thin film of good film quality which exhibits high resonance with respect to thickness vibration and effectively functions as a piezoelectric thin film. Can be formed.
The thickness of this PZT thin film or PT thin film is 0.1 to 10 μm
It is preferred that

From the viewpoint of the polarization effect, the distance between the upper electrodes (hereinafter, referred to as the “upper electrode distance”) is determined.
2 to 20 times the thickness of the PZT thin film or PT thin film of the piezoelectric thin film
It is preferably 0 times.

In the present invention, a concave portion may be formed by partially removing the surface of the substrate opposite to the surface on which the lower electrode is formed by etching, thereby improving the characteristics of transmission frequency and insertion loss. be able to. In this case, since the substrate has the diamond thin film, the strength can be sufficiently maintained even when the concave portion is formed.

According to the present invention, the thickness of the PZT thin film or PT thin film is 0.1 to 10 μm, and the upper electrode interval is 0.2 μm.
m to 2 mm, resonance frequency band 50 MHz to 10 GHz
, A high-performance piezoelectric thin-film resonator of the present invention is provided.

[0023]

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

FIG. 1 is a perspective view showing an embodiment of the present invention.
FIG. 2 is a front view showing another embodiment of the present invention, FIG. 3 is a perspective view showing another embodiment of the present invention, and FIG. 4 is a view showing another embodiment of the present invention. 4 (a) is a front view, and FIG. 4 (b) is a side view.

In FIGS. 1 to 4, members having the same functions are denoted by the same reference numerals.

In this embodiment, an Si substrate 2 having a diamond thin film 1 formed on the surface is used as the substrate. Since the mechanical strength of the Si substrate 2 with the diamond thin film 1 is sufficient, a PZT thin film having good film quality can be formed by the sol-gel method. 2A is an oxide film (SiO ₂ film) as an insulating film.

If the thickness of the diamond thin film 1 of the Si substrate 2 with the diamond thin film is too thin, the orientation of the diamond is not uniform and the diamond is difficult to be oriented. Further, smoothing is difficult and the effect of improving strength is low. Conversely, if the thickness is too large, it takes a long time to form a film, which is disadvantageous in terms of cost, and may cause acoustic loss. Therefore, the thickness is preferably about 1 to 30 μm.

The SiO ₂ film 2 on the back side of the Si substrate 2
The thickness of A is preferably 0.5 to 1.5 μm.

The Si substrate 2 with a diamond thin film is desirably as thin as possible for use as a thin film piezoelectric element. However, if the thickness is excessively small, the mechanical strength is reduced. Is preferred.

Such a Si substrate 2 with a diamond thin film can be manufactured by forming a diamond thin film having a desired thickness on a Si substrate by a vapor phase synthesis method.

The piezoelectric thin-film resonator of the present invention comprises a lower electrode 3, a PZ
It is obtained by sequentially forming a T thin film or PT thin film 4 and an upper electrode 5 (5A, 5B).
The Ti layer (not shown) is formed by a PZT thin film or a PZT thin film.
This is advantageous in forming the T thin film 4.

That is, the PZT thin film or P
In the formation of the T thin film, shrinkage occurs during drying and firing, so that
Although it is difficult to form a thin film having a thickness exceeding μm, a PZT thin film or a PT thin film having a thickness of 0.1 μm or less is not preferable because it is too thin to function as a piezoelectric thin film. On the other hand, by forming the Ti layer on the substrate, the P layer acts as an adhesion layer between the substrate and the underlying electrode.
The ZT thin film or the PT thin film can be formed as a relatively thick thin film.

This Ti layer can be formed by a sputtering method or the like, and its thickness is preferably about 50 to 500 °. If the thickness of the Ti layer is less than 50 °, the effect of forming the Ti layer cannot be sufficiently obtained.

As the lower electrode 3, a conductive metal layer such as Pt, Ir, Al or the like can be formed by a sputtering method or the like.
Its thickness is usually about 1000 to 2000 °.

In the present invention, PZ as a piezoelectric thin film
The T thin film or the PT thin film 4 needs to have a film thickness of 10 μm or less in order to be compatible with a high frequency.
It is appropriately determined according to the purpose of use within a range of 1 to 10 μm, more preferably 0.2 to 3 μm. If the PZT thin film or the PT thin film is too thin, a sufficient piezoelectric effect cannot be obtained, and if it is too thick, good film quality cannot be obtained.

Upper electrode 5 on PZT thin film or PT thin film 4
As (5A, 5B), a conductive metal layer similar to that of the lower electrode 3 described above can be formed by patterning by a sputtering method or the like.
It is about 00 °.

In the present invention, two upper electrodes 5A and 5B are formed as upper electrodes, and these upper electrodes 5A and 5B are formed.
The interval between them is made larger than the thickness of the PZT thin film or PT thin film 4.

The interval between the upper electrodes is a PZT thin film or PT thin film 4
If the film thickness is less than the thickness of the upper electrodes 5A, 5
When an electric field is applied between B and B, a short circuit occurs and the PZT thin film or PT thin film 4 cannot be polarized in the thickness direction.
However, if the interval between the upper electrodes is excessively large, the element dimensions unnecessarily increase, which is not preferable.

The distance between the upper electrodes is 2 to 200 times the thickness of the PZT thin film or PT thin film, ie, 0.1 to 10 μm.
Of the upper electrode for PZT thin film or PT thin film of 0.2
Preferably it is ~ 2000 μm. The interval between the upper electrodes is particularly preferably 10 to 100 times the thickness of the PZT thin film or PT thin film.

In the present invention, as shown in FIG.
The recess 6 may be formed by etching the surface of the substrate 2 on which the lower electrode 3 is formed and the surface on the reaction side. By forming the recess 6 in this manner, the mechanical strength of the piezoelectric thin film resonator is slightly inferior. The lower-order mode can be more strongly excited, and the characteristics of the transmission frequency and the insertion loss can be improved. In this case, since the diamond thin film 1 is formed on the Si substrate 2, a decrease in strength due to etching is suppressed, and the strength of the substrate can be maintained.

The recess 6 has a depth of 50 to 100% of the thickness of the Si substrate 2 at a position facing the position where the upper electrodes 5A and 5B are formed (a position where the upper electrode is transparently projected on the substrate in the thickness direction). It is preferable to form it.

In the present invention, as shown in FIG. 3, an insulating film 7 such as a SiO ₂ film is partially formed on the PZT thin film or the PT thin film 4, and the upper electrodes 5A and 5B are And the PZT film or the exposed surface of the PT thin film 4 so as to extend the formation positions of the upper electrodes 5A and 5B as terminal electrodes, so that structural reinforcement can be achieved. In this case, the insulating film 7 is made of SiO ₂ , Si
It can be formed of N, AlN, TiO ₂ , Al ₂ O ₃ or the like, and its thickness is preferably about 0.05 to 1 μm.

Also, when the insulating film 7 is formed as described above, the concave portion 6 may be formed in the Si substrate 2 as shown in FIG.

Next, a preferred example of the method for manufacturing a piezoelectric thin film resonator of the present invention will be described.

First, a Ti layer and a lower electrode are sequentially formed on the surface of a Si substrate with a diamond thin film by sputtering.

Next, a lead compound such as lead acetate and a titanium compound such as titanium isopropoxide and titanium butoxide (in the case of a PT thin film) or a zirconium compound such as zirconium butoxide and zirconium propoxide (PZT thin film) are formed on the lower electrode. Is coated with a composition for forming a PZT thin film or PT thin film dissolved in a solvent such as methoxyethanol or acetate so that the total concentration becomes about 10 to 20% by weight at a predetermined molar ratio.
The coating is dried and dried at 50 to 400 ° C. so as to have a predetermined film thickness, and finally baked at 500 to 800 ° C. for 0.1 to 2 hours.

The PZT thin film or P
On the T thin film, if necessary, heat treatment, CVD, etc.
After forming an insulating film such as O ₂ , two upper electrodes are formed in a predetermined pattern by a sputtering method. Thereafter, 200-500 kV at 120-200 ° C. between the two upper electrodes
/ Cm is applied for about 10 to 60 minutes.
The T thin film or the PT thin film is polarized. By performing this polarization treatment, the film functions as a PZT thin film or a PT thin film vibrating piezoelectric thin film.

In the piezoelectric thin-film resonator, P of good film quality is used.
It is extremely important to form a ZT thin film or a PT thin film. That is, the PZT thin film or PT thin film functions as a piezoelectric thin film by performing sufficient polarization processing, but does not function sufficiently as a piezoelectric thin film because a polarization processing electric field having poor film quality cannot be sufficiently applied. . For this reason, in the present invention, it is important to form a PZT thin film or PT thin film having good film quality by optimizing the conditions for forming the substrate, the lower electrode, the PZT thin film or the PT thin film, and the upper electrode.

When a concave portion is formed by etching a Si substrate, TMAH (tetramethyl ammonium) is used.
The substrate may be partially removed using an etching solution such as onium hydroxide.

[0050]

The present invention will be described more specifically with reference to the following examples.

Examples 1 and 2 As a substrate, a 250 μm-thick Si substrate having a 20 μm-thick diamond thin film formed on its surface was used.
A 0 ° Ti layer and a 2000 ° thick Pt lower electrode layer were sequentially formed.

On this Pt lower electrode layer, a PZT thin film or a PT thin film having a thickness of 0.8 μm was formed by a sol-gel method.

For forming the PZT thin film, a PZT thin film forming solution in which lead acetate, zirconium butoxide, and titanium isopropoxide are dissolved at a predetermined molar ratio so as to have a total concentration of 18% by weight is used. To form a PT thin film, lead acetate and titanium isopropoxide were dissolved at a predetermined molar ratio to give a total concentration of 10% by weight.
Using a solution for forming a T thin film, each was applied by spin coating, dried at 400 ° C., and this application and drying were repeated until a predetermined film thickness was reached, and finally baked at 650 ° C. for 1 hour.

Further, as shown in FIG. 1, a PZT thin film or a PT thin film having a thickness of 1500.degree.
Two rectangular Al upper electrodes of μm × 75 μm were formed by patterning at an upper electrode interval of 75 μm.

Then, at 150 ° C. between the upper electrodes,
When a direct current electric field of 00 kV / cm was applied for 10 minutes, the PZT thin film or PT thin film was polarized in the thickness direction, and a piezoelectric thin film resonator was obtained.

Table 1 shows the basic resonance frequency and the insertion loss of the thickness vibration of the obtained piezoelectric thin film resonator.

Comparative Examples 1 and 2 In Examples 1 and 2, the substrate was 1 μm thick on the surface.
A piezoelectric thin-film resonator was manufactured in the same manner except that a 250 μm-thick Si substrate on which an oxide film was formed was used, and the basic resonance frequency and the insertion loss of the thickness vibration of the obtained piezoelectric thin-film resonator were shown in Table 1. It was shown to.

[0058]

[Table 1]

Comparative Examples 3 and 4 In Examples 1 and 2, except that the interval between the upper electrodes was set to 0.2 μm which was smaller than the thickness of the PZT thin film or the PT thin film, the same procedure was performed. Was applied, a short circuit occurred and the polarization treatment could not be performed.

Example 3 In Example 1, the PZT film thickness was set to 0.8 μm,
A piezoelectric thin-film resonator was fabricated in the same manner except that two rectangular Al upper electrodes of μm × 75 μm were formed at an upper electrode interval of 75 μm.
It was shown to.

Example 4 In Example 3, as shown in FIG. 2, a 235 μm × 75 μm
A piezoelectric thin-film resonator was manufactured in the same manner except that a concave portion having a depth of × 235 μm was formed, and its resonance frequency and insertion loss are shown in Table 2. In the present embodiment, the concave portion was formed in the substrate, but there was no problem of strength reduction due to the concave portion formation because the diamond thin film was formed on the substrate surface.

[0062]

[Table 2]

Example 5 In Example 1, after forming the PZT thin film, as shown in FIG. 3, an SiO ₂ film having a thickness of 1 μm was formed by sputtering except in the vibration region, and thereafter, this SiO ₂ film and the PZT thin film were formed. A piezoelectric thin-film resonator was manufactured in the same manner as above except that an Al upper electrode having a thickness of 1500 ° was formed at an upper electrode interval of 180 μm. The upper electrode of this piezoelectric thin-film resonator has an area of 70 μm formed on the PZT thin film.
m × 70 μm, the area of the portion formed on the SiO ₂ thin film is 100 μm × 100 μm, and the width of the connecting portion is 20 μm.

For this piezoelectric thin-film resonator, the impedance due to the difference in the area of the upper electrode was examined.
When compared with the piezoelectric thin film resonator of Example 1, the piezoelectric thin film resonator of Example 1 is around 50Ω, while the piezoelectric thin film resonator of this embodiment is also around 50Ω. There was no change, and it was found that the reinforcing structure could be formed by shifting the upper electrode forming position.

[0065]

As described in detail above, according to the piezoelectric thin film resonator of the present invention, a PZT thin film having a large electromechanical coupling coefficient as a piezoelectric film or a coupling coefficient in the thickness direction having a high Curie point of about 500 degrees. Since a PT thin film having characteristics such as a large difference in the coupling coefficient in the direction of spread and the like is used, it is possible to obtain a piezoelectric thin film resonator that realizes a wide band filter and a resonator having a wide oscillation frequency even with a simple structure.
Also, since two upper electrodes are provided at a predetermined interval,
There is no need to use the lower electrode as a terminal electrode. As a result, a complicated process for exposing the lower electrode is not required, and the piezoelectric thin film resonator can be easily manufactured. Moreover, since a single-crystal or polycrystalline substrate is used as the substrate, a high Q value can be obtained.

[Brief description of the drawings]

FIG. 1 is a perspective view showing an embodiment of the present invention.

FIG. 2 is a front view showing another embodiment of the present invention.

FIG. 3 is a perspective view showing another embodiment of the present invention.

FIG. 4 shows a different embodiment of the present invention,
FIG. 4A is a front view, and FIG. 4B is a side view.

[Explanation of symbols]

 Reference Signs List 1 diamond thin film 2 Si substrate 3 lower electrode 4 PZT thin film or PT thin film 5A, 5B upper electrode 6 concave portion 7 insulating film

Claims (7)

[Claims] 1. A single-crystal or polycrystalline substrate, a lower electrode formed on the substrate, a piezoelectric thin film formed on the lower electrode, and two upper portions formed on the piezoelectric thin film A piezoelectric thin-film resonator comprising: an electrode; wherein the piezoelectric thin-film is a PZT thin-film or a PT thin-film; and a distance between the two upper electrodes is larger than a thickness of the piezoelectric thin-film. A piezoelectric thin-film resonator characterized in that: 2. The piezoelectric thin film resonator according to claim 1, wherein said single crystal or polycrystalline substrate is a single crystal or polycrystalline diamond substrate. 3. The single crystal or polycrystalline substrate according to claim 2, wherein the single crystal or polycrystalline diamond thin film is formed on a Si substrate, and the lower electrode is formed on the diamond thin film of the substrate. A piezoelectric thin-film resonator characterized in that: 4. The piezoelectric thin film resonator according to claim 3, wherein said diamond thin film has a thickness of 1 to 20 μm. 5. The PZT thin film or PT thin film according to claim 1, wherein the PZT thin film or the PT thin film is formed by a sol-gel method, and has a thickness of 0.1 to 10 μm. Piezoelectric thin film resonator. 6. The piezoelectric thin film according to claim 1, wherein an interval between the two upper electrodes is 2 to 200 times a thickness of the piezoelectric thin film. Resonator. 7. The piezoelectric thin-film resonator according to claim 1, wherein a concave portion is formed by etching on a surface of the substrate opposite to a surface on which the lower electrode is formed.