JPH10261934A - Piezoelectric thin film resonator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a piezoelectric thin film resonator with a wide band width and a wide oscillating frequency range when it is applied to a resonator, a filter or the like and having a large electromechanical coupling coefficient. SOLUTION: This resonator is provided with a single crystal substrate 3, a piezoelectric thin film 2 formed on the single crystal substrate 3 and two electrodes 1 made of a conductive film formed on the piezoelectric thin film 2. In this case, the piezoelectric thin film 2 is made of e.g. lead zirconate titanate (PZT) or lead titanate (PT) thin film whose the thickness is 0.1-10 μm formed by a sol-gel method, an electric field is applied between the two electrodes 1 to apply polarization processing and to make the piezoelectric thin film. As required, an interval between the two electrodes 1 is made to 0.5-10 μm and a part of a rear side of the single crystal substrate is removed by etching.

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 suitable for a resonator, a filter, and the like utilizing a bulk wave of a piezoelectric thin film operating in a high-frequency range in a lateral direction.

[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.

Incidentally, as a high frequency piezoelectric material for a piezoelectric element, for example, 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, as represented by a piezoelectric thin-film resonator described in Japanese Patent Application Laid-Open No. 8-148968, the thickness of the piezoelectric film is controlled by the upper and lower electrodes of the piezoelectric film. Was excited. However, in the case of a piezoelectric thin-film vibrator utilizing a bulk wave in the thickness direction as in the past, since the lower electrode is an inactive region, the lower electrode is hindered and the Q factor, which is an important element of the resonator, is obstructed. Becomes smaller.

[0009]

As described above, all of the conventional piezoelectric materials have a small electromechanical coupling coefficient of about 20 to 30%.
The bandwidth and oscillation frequency range are limited. In Japanese Patent Application Laid-Open No. Sho 60-31305, a part of the substrate is removed in order to improve this point. However, when the substrate is removed in this way, there is a disadvantage that the element strength is reduced.

Hitherto, there has been a problem in forming a piezoelectric thin film material having a large electromechanical coupling coefficient, such as PZT, in the film forming process, and it has been difficult to obtain a piezoelectric thin film of good quality. . In addition, a piezoelectric thin film material having features such as PT having a small dielectric constant, a high Curie point of about 500 degrees, and a large difference between the coupling coefficient in the thickness direction and the coupling coefficient in the spreading direction is provided. Has a problem in the film forming process, and it is difficult to obtain a piezoelectric thin film having good film quality. The process for exposing the lower electrode is complicated, such as etching the piezoelectric film to expose the lower electrode as a terminal electrode, or forming the piezoelectric film on the upper surface while leaving a part of the lower electrode. It may be. In addition, since the acoustically inactive region hinders the inactive region, the Q value becomes small.

The present invention solves the above-mentioned conventional problems and provides a piezoelectric thin-film resonator having a large electromechanical coupling coefficient and a wide bandwidth and oscillation frequency range when applied to a resonator, a filter, and the like. It is an object of the present invention to provide a desired piezoelectric thin-film resonator which can obtain a high Q value and is therefore easy to manufacture.

[0012]

According to the present invention, there is provided a piezoelectric thin film resonator comprising a single crystal substrate, a piezoelectric film on the single crystal substrate, and a conductive film formed on the piezoelectric thin film. In the piezoelectric thin film resonator including the electrodes, the piezoelectric film is formed by a sol-gel method and has a thickness of 0.1 to 10 μm, which is lead zirconate titanate (PZT) or lead titanate (PT) (hereinafter simply referred to as PZT or PTZ). A thin film is formed, and two simple upper electrodes are provided on the thin film, and an electric field is applied to the two upper electrodes to perform a polarization process, thereby forming a piezoelectric thin film. Therefore, since the lower electrode does not exist, the polarization process is performed in the horizontal direction. In addition, a high Q value is obtained because there is no acoustically inactive region.

In the piezoelectric thin film resonator of the present invention, the piezoelectric film is made of PZT or PZT which is a piezoelectric material having a large electromechanical coupling coefficient.
Since it is formed of T, a filter having a wide band in a high frequency region and a resonator having a wide oscillation frequency range can be realized.

Moreover, since two upper electrodes are formed at an interval, the piezoelectric film can be polarized in the lateral direction by applying an electric field between the upper electrodes and the piezoelectric film without the lower electrodes.

By the way, it is usually difficult to obtain a piezoelectric thin film of high quality, especially with PZT or PT. For example, in the sputtering method, a PZT or PT thin film which is sufficiently good to sufficiently confirm vibration resonance cannot be formed.

On the other hand, if a PZT or PT thin film is formed by the sol-gel method, a PZT or PT thin film having good film quality that exhibits high resonance with respect to thickness vibration and effectively functions as a piezoelectric thin film is formed. can do. The PZT or PT thin film preferably has a thickness of 0.1 to 10 μm.

Further, from the viewpoint of the polarization treatment effect, the interval between the upper electrodes (hereinafter, referred to as “upper electrode interval”) is
It is preferably from 0.1 to 10 μm, more preferably from 0.5 to 5 μm.

In the present invention, the single crystal substrate may be partially removed by etching to form a concave portion, whereby the characteristics of the transmission frequency and the insertion loss can be improved.

According to the present invention, the PZT or PT thin film has a thickness of 0.1 to 10 μm and an upper electrode interval of 0.5 to 10 μm.
10 μm, resonance frequency band 200 MHz to 10 GHz
, A high-performance piezoelectric thin-film resonator of the present invention is provided.

[0020]

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.

The substrate used in the piezoelectric thin-film resonator of the present invention comprises:
For example, a single crystal substrate of sapphire, MgO, SrTiO ₃ or the like can be used. The single crystal substrate 1 has a smooth surface and a sufficient mechanical strength, so that the P film having a good film quality can be relatively easily formed by the sol-gel method.
A ZT or PT thin film can be formed. Also, 2
Al, Pt, Au
Can be used.

The piezoelectric thin film resonator of the present invention is obtained by sequentially forming a PZT or PT thin film and an upper electrode layer on the above-mentioned single crystal substrate. The PZT or PT thin film as the piezoelectric thin film is, for example, a thin film having a thickness of 0.1 to 10 μm formed by a sol-gel method. As the conductive film forming the two electrodes, the above-mentioned metal or alloy film is used. It can be formed by a sputtering method or the like, and its thickness is usually about 1000 to 2000 °. Piezoelectricity is imparted by applying an electric field to these two electrodes and performing polarization processing.

In the present invention, PZ as a piezoelectric thin film is used.
The T or PT thin film 3 has a thickness of 1
0 μm or less, preferably 0.1 μm or less.
The range is from 10 to 10 μm, more preferably from 0.2 to 3 μm. In addition, PZT or P
If the thickness of the T thin film is too small, a sufficient piezoelectric effect cannot be obtained, while if it is too large, good film quality cannot be obtained.

Upper electrode 4 on PZT or PT thin film 3
As (4A, 4B), the above-described conductive metal layer can be patterned by a sputtering method or the like.

In the present invention, as shown in FIG. 2, it is better to form the concave portion 5 by etching the back surface of the single crystal substrate 1. By forming the concave portion 5 in this way, the piezoelectric thin film resonator Although the mechanical strength is slightly inferior, it is possible to excite the lower order mode more strongly, and it is possible to improve the characteristics of the transmission frequency and the insertion loss.

The recess 5 has a thickness of 50 to 100% of the thickness of the single crystal substrate 1 at a position facing the position where the upper electrodes 4A and 4B are formed (a position where the upper electrode is transparently projected on the substrate in the thickness direction). Preferably, it is formed at a depth. At this time, the substrate is made several times as large as the piezoelectric film.

In the present invention, as shown in FIG. 3, an insulating film 4 is partially formed on the PZT or PT thin film 3, and the upper electrodes 4A and 4B are connected to the insulating film 4 and the PZT or PT film 3. , The upper electrodes 4A and 4B as terminal electrodes can be formed at different positions to reinforce the structure. In this case, the thickness of the insulating film 4 is preferably about 0.05 to 1 μm.

The piezoelectric thin film PZT or P
The T thin film is formed, for example, as follows. Lead compounds such as lead carboxylate such as lead acetate and lead alkoxide such as diisopropoxy lead; zirconium alkoxide such as tetraethoxy zirconium, tetraisopropoxy zirconium, tetrabutoxy zirconium and dimethoxydiisopropoxy zirconium on a single crystal substrate. A titanium compound such as titanium alkoxide such as zirconium titanium or dimethoxydiisopropoxytitanium in a solvent such as 2-methoxyethanol at a predetermined molar ratio, and a total concentration in terms of metal oxide of 10 to 20% by weight. % Of the composition for forming a PZT or PT thin film obtained by dissolving the solution to about 400% on a single crystal substrate by a spin coater or the like.
Dry at 00 ° C. This coating and drying are performed using a PZ having a desired film thickness.
Repeat until a T or PT thin film is obtained and finally 60
PZT or PT fired at 0-700 ° C for 1 minute to 1 hour
Obtain a thin film.

The polarization treatment of the PZT or PT thin film is performed by applying 20 to 20 electrodes to the two electrodes formed on the PZT or PT thin film.
It can be performed by applying a DC voltage of 50 V for 1 to 60 minutes. Here, by performing a sufficient polarization treatment, the film functions as a piezoelectric thin film.
Alternatively, if the film quality of the PT thin film is insufficient, an electric field for polarization treatment cannot be sufficiently applied, and the PT thin film will not function as a piezoelectric thin film.

[0032]

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

Example 1 A 0.8 μm PZT thin film was formed on a sapphire substrate as a single crystal substrate by a sol-gel method. PZT used in the sol-gel method uses an 18% concentration solution. The coating is repeated by heat treatment at 400 ° C. to obtain a desired film thickness, and finally baked at a temperature of 650 ° C. Further, Al: 1500 ° is formed on the PZT thin film, and as shown in FIG. 1, two patterns of 70 × 70 μm square Al are formed at 3 μm intervals.
I did it. Then, as shown in FIG.
The opening of μm square was dry-etched by a reverse sputtering method. Then, the polarization process is performed at 150 ° C. between the upper electrodes.
A DC electric field of 300 kV / cm was applied for 10 minutes.
By doing so, it is polarized in the lateral direction, and the piezoelectric thin film resonator 1 of the present invention (hereinafter, referred to as the resonator 1 of the present invention)
I got

For forming a PZT thin film, a PZT having the following composition is used.
The composition for forming a thin film was applied by a spin coater. PZT thin film forming composition (total concentration in terms of metal oxide:
20% by weight) Lead acetate: 23.985% by weight Tetrabutoxyzirconium: 11.455% by weight Titanium isopropoxy titanium: 7.842% by weight 2-methoxyethanol: balance

Example 2 In Example 1, MgO was used for the single crystal substrate, and the opening on the back surface was formed by chemical etching, and the piezoelectric thin film resonator 2 of the present invention (hereinafter referred to as the resonator 2 of the present invention) was used. ).

Example 3 In Example 1, SrTiO ₃ was used for the single crystal substrate, and the opening on the back surface was formed by reverse sputtering, and the piezoelectric thin film resonator 3 of the present invention (hereinafter referred to as resonator 3 of the present invention) was used. ).

Example 4 In Example 1, the piezoelectric thin film resonator 4 of the present invention (hereinafter referred to as the piezoelectric thin film resonator) comprising a 0.64 μm PT thin film formed by a sol-gel method using a 10% concentration solution. Invented resonator 4) was manufactured.

Example 5 In Example 2, the piezoelectric thin film resonator 5 of the present invention (hereinafter referred to as the piezoelectric thin film resonator) was formed of a 0.64 μm PT thin film formed by a sol-gel method using a 10% concentration solution. Invented resonator 5) was manufactured.

Example 6 In Example 3, the piezoelectric thin film resonator 6 of the present invention (hereinafter referred to as the piezoelectric thin film resonator) comprising a 0.64 μm PT thin film formed by a sol-gel method using a 10% concentration solution. Invented resonator 6 was manufactured).

Example 7 In Example 1, after the PZT thin film was formed, as shown in FIG.
A SiO ₂ film having a thickness of 1500 μm is formed by sputtering, and then a thickness of 1500 is formed so as to extend over the SiO ₂ film and the PZT thin film.
The piezoelectric thin-film resonator 7 of the present invention (hereinafter referred to as the resonator 7 of the present invention) produced in the same manner except that two Al upper electrodes were formed. With respect to the resonator 7 of the present invention, the impedance due to the difference in the area of the upper electrode was examined and compared with the resonator 1 of the present invention. There was no change in the characteristics, and it was confirmed that the terminal position of the upper electrode could be secured.

As described above, the obtained resonators 1 to 7 of the present invention are obtained.
The basic resonance frequency of the thickness vibration of the piezoelectric thin-film resonator was measured, and the results are shown in Table 1.

[0042]

[Table 1]

[0043]

As described above, by the sol-gel method, PZT having a large electromechanical coupling coefficient can be used, the dielectric constant is small, the Curie point is as high as about 500 degrees, the coupling coefficient in the thickness direction and the expansion coefficient in the spreading direction are small. Since a PT having characteristics such as a large difference in the value of the coupling coefficient is used, a piezoelectric thin-film resonator that realizes a broadband filter and a resonator having a wide oscillation frequency can be obtained even with a simple structure as shown in FIG. I can do it. In addition, since two upper electrodes are provided at a predetermined interval, a large Q value can be obtained by utilizing a bulk acoustic wave in the lateral direction, since there is no lower electrode in an inactive region acoustically. Further, by forming the terminal electrode on the insulating layer, the terminal electrode can be secured, the resonance under the terminal electrode can be suppressed, and the piezoelectric thin film resonator can be easily manufactured.

[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.

4A and 4B are diagrams showing different embodiments of the present invention, wherein FIG. 4A is a front view and FIG. 4B is a side view.

[Explanation of symbols]

 Reference Signs List 1 upper electrode 2 piezoelectric thin film 3 single crystal substrate 4 insulating film 5 concave portion

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Kenya Hashimoto 1-33 Yayoi-cho, Inage-ku, Chiba-shi, Chiba Chiba Univ.

Claims (4)

[Claims] 1. A piezoelectric thin film resonator comprising a single crystal substrate, a piezoelectric thin film formed on the single crystal substrate, and two electrodes formed of a conductive film formed on the piezoelectric thin film. The piezoelectric thin film is made of a lead zirconate titanate (PZT) or lead titanate (PT) thin film formed by a sol-gel method, and an electric field is applied between the two electrodes to lead zirconate titanate (PZT) or titanium. A piezoelectric thin film resonator characterized in that a lead oxide (PT) thin film is subjected to a polarization treatment to form a piezoelectric thin film. 2. The lead zirconate titanate (PZT) or lead titanate (PT) thin film is a thin film having a thickness of 0.1 to 10 μm formed by a sol-gel method. Piezoelectric thin film resonator. 3. An interval between the two electrodes is 0.5 to 10 μm.
3. The piezoelectric thin-film resonator according to claim 1, wherein the piezoelectric thin-film resonator has a structure of m. 4. The piezoelectric thin film resonator according to claim 1, wherein a part of the back surface of the single crystal substrate is removed by etching.