JPH10261933A - 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 filter or the like and in which no lower electrode is required to be exposed. SOLUTION: This resonator is provided with a piezoelectric thin film 2 consisting of lead zirconate titanate (PZT) or lead titanate (PT) thin film whose thickness is 0.1-10 μm formed by a sol-gel method and two electrodes 1 made of a conductive film formed on the piezoelectric thin film 2. In this case, the resonator is formed on a buffer layer 3 of a dielectric thin film 2 formed on a semiconductor substrate 5, an electric field is applied between the two electrodes 1 to apply polarization processing and to make the piezoelectric thin film 2. As required, the buffer layer 3 is made as a thin film made of barium strontium titanate(BST), strontium titanate(STO) or barium titanate(BTO) whose the thickness is 0.01-1.0 μm formed by the sol-gel method and an interval between the two electrodes 1 is made to 0.5-10 μm and a part of a rear side of the semiconductor substrate 5 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.

Conventionally, 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 film 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 lower electrode is an acoustically inactive region, it is hindered, and the Q value is reduced.

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]

A piezoelectric thin film resonator according to the present invention comprises, for example, a semiconductor substrate having an insulating film, a piezoelectric film on a buffer layer formed on the semiconductor substrate, and a piezoelectric thin film. In a piezoelectric thin-film resonator including two electrodes formed of a conductive film formed thereon, the piezoelectric film is formed of a zirconate titanate (PZT) having a thickness of 0.1 to 10 μm and formed by a sol-gel method. Lead titanate (PT) (hereinafter, referred to as
It has a thin film, has two simple upper electrodes on the thin film, and applies an electric field to the two upper electrodes to perform a polarization process to form 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, it is possible to realize a filter having a wide band and a resonator having a wide oscillation frequency range in a high frequency region.

Moreover, since two upper electrodes are formed at an interval, the piezoelectric film can be horizontally polarized by applying an electric field between the upper electrodes.

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

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 surface of the semiconductor substrate may be partly removed by etching to form a concave portion, whereby the characteristics of transmission frequency and 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 semiconductor substrate such as Si or GaAs having an insulating film such as SiO ₂ or SiN formed on the surface can be used. With the semiconductor substrate 1 having an insulating film, the surface is smooth, the diffusion of elements during heat treatment can be prevented, and the mechanical strength is sufficient. PZT or PT thin film can be formed.

The insulating film 1A of the semiconductor substrate 1 with the insulating film
If the thickness is too thin, there is no diffusion prevention effect, and if it is too thick, there is a problem of crack generation. Therefore, the thickness is preferably about 0.5 to 2 μm.

Further, as the semiconductor substrate 1 with an insulating film,
It is desirable for the use as a thin film piezoelectric element to be as thin as possible. However, if the thickness is too small, the mechanical strength is reduced. Therefore, the thickness is preferably about 100 to 600 μm.

In the piezoelectric thin-film resonator of the present invention, barium strontium titanate (BST), strontium titanate (STO) or barium titanate (BTO) is provided on such a semiconductor substrate 1 with an insulating film as a buffer layer.
(Hereinafter, simply referred to as BST, STO, or BTO, respectively) and a PZT or PT thin film and an upper electrode layer are sequentially formed by using a dielectric thin film. The BST, STO, or BTO thin film as a buffer layer on an insulating film is a thin film formed by a sol-gel method with a thickness of 0.01 to 1.0 μm, and a PZT or PT thin film as a piezoelectric thin film thereon is A thin film formed by a sol-gel method having a thickness of 0.1 to 10 μm is formed by applying an electric field to two electrodes and performing a polarization treatment. As the conductive film forming the two electrodes, a film containing Al, Pt, Ir, Au, or the like as a main component can be formed by a sputtering method or the like, and the thickness is usually about 1000 to 2000 °. .

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 6 by etching one surface of the semiconductor substrate 1. By forming the concave portion 6 in this manner,
Although the mechanical strength of the piezoelectric thin film resonator is slightly inferior, it is possible to excite the lower order mode more strongly,
The characteristics of insertion loss can be improved.

The concave portion 6 has a depth of 50 to 100% of the thickness of the semiconductor 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). It is preferable to form it. 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 insulating film 4 can be formed of SiO ₂ , SiN or the like, and its thickness is preferably about 0.05 to 1 μm.

In the present invention, BS as a buffer layer
PZT of T, STO or BTO thin film and piezoelectric thin film
Alternatively, the PT thin film is preferably formed as follows.

First, barium compounds such as barium carboxylate such as barium 2-ethylhexanoate; strontium compounds such as strontium carboxylate such as strontium 2-ethylhexanoate; tetraethoxytitanium, tetraisopropoxytitanium, tetrabutoxytitanium; A titanium compound such as titanium alkoxide such as dimethoxydiisopropoxytitanium; and a carboxylic acid such as 2-ethylhexanoic acid and 2-ethylbutyric acid in a solvent such as isoamyl acetate at a predetermined molar ratio and a metal oxide. A composition for forming a BST, STO or BTO thin film obtained by dissolving so as to have a converted total concentration of about 0.5 to 8% by weight, for example, is applied to a single crystal Si substrate with an oxide film by a spin coater or the like. And dried at 400-600 ° C. This coating and drying are repeated until a thin film having a desired thickness is obtained.
BST, STO by firing at 00-700 ° C for 1 minute to 1 hour
Alternatively, a BTO thin film is obtained.

The BST and STO thus formed are
Or, if the thickness of the BTO thin film is too large, it affects the piezoelectric characteristics, and if the thickness is too thin, the effect of preventing lead diffusion is lost.
Usually, 0.01 to 1.0 μm, especially 0.01 to 1.0 μm
It is preferably 0.3 μm.

Next, on the buffer layer thus formed, a lead compound such as a lead carboxylate such as lead acetate or a lead alkoxide such as diisopropoxy lead; tetraethoxy zirconium, tetraisopropoxy zirconium, or tetrabutoxy. Zirconium, zirconium titanium such as zirconium alkoxide such as dimethoxydiisopropoxy zirconium, and titanium compound such as titanium alkoxide such as dimethoxydiisopropoxytitanium in a solvent such as 2-methoxyethanol at a predetermined molar ratio, and a metal oxide A PZT or PT thin film-forming composition obtained by dissolving so as to have a total converted concentration of about 10 to 20% by weight is applied to a single crystal substrate by a spin coater or the like, and then applied to a substrate.
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 PZT or PT thin film is polarized by applying a voltage of 20 to 50 V to the electrode formed on the PZT or PT thin film.
By applying a DC voltage of 1 to 60 minutes. Here, by performing sufficient polarization processing, the film functions as a piezoelectric thin film. However, if the film quality of the PZT or PT thin film is insufficient, the electric field of the polarization processing cannot be sufficiently applied, and the film functions as a piezoelectric thin film. Will not do.

[0037]

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

Example 1 A barium strontium titanate (BST) thin film having a thickness of 0.15 μm was formed by a sol-gel method as a buffer layer using a Si (100) substrate having 1 μm of SiO ₂ formed on its surface. Barium strontium titanate (BST) used for the sol-gel method is 7
% Solution is used. 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.
Then, 0.8 μm of P was formed on the buffer layer by a sol-gel method.
A ZT thin film was formed. PZT used for the sol-gel method is 18
% Solution is used. 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, two patterns of Al having a size of 70 × 70 μm were patterned on the PZT thin film at intervals of 3 μm. Then, as shown in FIG. 2, the rear surface of the substrate is set to a size of 70 × 10 μm square TM
Etched by AH. In the polarization treatment, a DC electric field of 300 kV / cm was applied between the upper electrodes at 150 ° C. for 10 minutes. By doing so, it was 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) was obtained.

For the formation of a BST thin film, a BST having the following composition
The composition for forming a thin film was applied by a spin coater. Composition for forming BST thin film (total concentration in terms of metal oxide:
7% by weight) Barium 2-ethylhexanoate: 9.514% by weight Strontium 2-ethylhexanoate: 3.598% by weight Tetraisopropoxytitanium: 9.205% by weight 2-ethylhexanoic acid: 18.50% by weight acetic acid Isoamyl: balance

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

[Embodiment 2] A piezoelectric thin film resonator 2 of the present invention in which the buffer layer is strontium titanate (STO) in Embodiment 1 (hereinafter referred to as the present resonator 2).
For the formation of the STO thin film, a composition for forming an STO thin film having the following composition was used and applied by a spin coater. Composition for forming STO thin film (total concentration in terms of metal oxide:
7 wt%) strontium 2-ethylhexanoate: 14.269 wt% tetraisopropoxytitanium: 10.950 wt% 2-ethylhexanoic acid: 22.015 wt% isoamyl acetate: balance

[Embodiment 3] A piezoelectric thin-film resonator 3 of the present invention in which the buffer layer is barium titanate (BTO) in Embodiment 1 (hereinafter referred to as the resonator 3 of the present invention). BTO
For the formation of the thin film, a BTO thin film forming composition having the following composition was used and applied by a spin coater. Composition for forming BTO thin film (total concentration in terms of metal oxide:
7% by weight) Barium 2-ethylhexanoate: 12.722% by weight Titanium isopropoxytitanium: 8.616% by weight 2-ethylhexanoic acid: 17.31% by weight Isoamyl acetate: balance

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).

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

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

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) prepared similarly 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.

[0048]

[Table 1]

[0049]

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.

According to the present invention, barium strontium titanate (BST), strontium titanate (STO) or barium titanate (BTO) formed by a sol-gel method as a buffer layer on a single crystal semiconductor Si substrate is used. A dielectric thin film is formed, and a PZT or PT thin film, which is a piezoelectric thin film, is formed on the dielectric thin film. In particular, in the case of a PZT thin film, the lead deficiency of the buffer layer prevents the lead deficiency of the PZT film. This is effective in forming a good PZT thin film. If this buffer layer is not formed, PZT lead diffuses into Si, so that a PZT thin film cannot be formed on a single crystal semiconductor Si substrate. However, the buffer
Due to the presence of the layer, an inexpensive single crystal semiconductor Si substrate can be used, which is advantageous in terms of cost, and in particular, a piezoelectric thin film resonator on which a high quality PZT thin film is formed can be obtained.
It will greatly contribute in related fields.

[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 buffer layer 4 insulating layer 5 Si substrate 6 concave portion

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

Claims (6)

[Claims] A piezoelectric thin film resonator comprising a semiconductor substrate, a piezoelectric thin film formed on the semiconductor substrate, and two electrodes formed of a conductive film formed on the piezoelectric thin film; The body thin film is a piezoelectric thin film resonator formed on a buffer layer of a dielectric thin film formed on the semiconductor substrate. The piezoelectric thin film is formed of lead zirconate titanate (PZT) formed by a sol-gel method. ) Or a lead titanate (PT) thin film, and an electric field is applied between the two electrodes to polarize the lead zirconate titanate (PZT) or lead titanate (PT) thin film to form a piezoelectric thin film. A piezoelectric thin-film resonator characterized in that: 2. The method according to claim 1, wherein 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 Rugel method. Piezoelectric thin film resonator. 3. The buffer layer is a thin film of barium strontium titanate (BST), strontium titanate (STO) or barium titanate (BTO) having a thickness of 0.01 to 1.0 μm formed by a sol-gel method. 3. The piezoelectric thin-film resonator according to claim 1, wherein: 4. The distance between the two electrodes is 0.5 to 10 μm.
The piezoelectric thin-film resonator according to any one of claims 1 to 3, wherein: 5. The piezoelectric thin-film resonator according to claim 1, wherein a part of the back surface of said semiconductor substrate is removed by etching. 6. The semiconductor substrate according to claim 1, wherein said semiconductor substrate is a semiconductor substrate provided with an insulating film having an insulating film formed on at least a surface on which a piezoelectric thin film is formed. Piezoelectric thin film resonator.