JP2001285020A - Surface acoustic wave device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface acoustic wave device which can be made monolithic with another element, using a semiconductor substrate and has superior characteristics. SOLUTION: The surface acoustic wave device has a PZT film, formed of PZT crystal on a semiconductor substrate and a interdigital electrode or it and a reflector on the PZT film or between the PZT film and semiconductor substrate, and the PZT crystal which has an axis (c) aligned almost perpendicular to the film surface is present in the PZT film.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave device using a semiconductor substrate used for an oscillator or a filter in a mobile communication device, a television, a VCR, or the like.

[0002]

2. Description of the Related Art SAW (hereinafter, referred to as SAW) resonators are small in size, can be used without adjustment, have high Q, and have small resonance loss. Therefore, SAW
If the resonator is used as an oscillator for a VCO such as an oscillator module, miniaturization and non-adjustment are possible.
Phase noise can be reduced. However, when a SAW resonator is used, the frequency variable width is limited by the electromechanical coupling coefficient k ² of the piezoelectric substrate used. The reason will be described below.
The SAW resonator has a resonance frequency between the resonance frequency and the anti-resonance frequency,
The impedance becomes inductive. The frequency range of this guidance region is the maximum value that the frequency variable width can take,
In practice, the control voltage that can be applied is limited, and S
Since the AW resonator needs to have an appropriate impedance, it is empirically known that the frequency variable width is 0.07 times the induction region. On the other hand, the electromechanical coupling coefficient k ² of the piezoelectric substrate is given by 2 times the frequency range of the induction area normalized. Accordingly, the electromechanical coupling coefficient k ² is required to piezoelectric substrate, it comes to about 30 times the normalized frequency variable width required by the VCO circuit.

Further, since the specific bandwidth of the SAW resonator filter is about 1/2 of the electromechanical coupling coefficient k ^2, a piezoelectric substrate having a large electromechanical coupling coefficient is required to obtain a wide band filter. You.

[0004] Recently, mobile communication systems such as mobile phones such as GSM have formed a large market. In pursuit of further convenience, GSM and IMT20
The size of mobile phones such as the 00 system has been reduced, and wristwatch-type phones are being studied in the future. In the case of miniaturization in this way, there is a limit only to miniaturizing semiconductor circuits such as ICs and LSIs, or passive components, etc., and monolithicization has been studied for a long time. For example, Proceedings 1987 IEEE published in 1988
On page 641 of Ultrasonics Symposium, Monolithic SAW d with c-axis oriented ZnO thin film formed on Si substrate
ual delay-line oscillator is described.

However, c-axis oriented ZnO has an electromechanical coupling coefficient k ² of about 1% and cannot be used for applications requiring a wide band such as mobile phones.

[0006] PZT (lead zirconate titanate), which is a piezoelectric ceramic, has a wide electromechanical coupling coefficient and is therefore widely used. For example, the Technical Report of IEICE published in July 1997
US Pat. No. 97-31, pages 39 to 44, describe a SAW filter having a PZT film. This SAW
In the filter, barium strontium titanate is formed as a buffer layer on a Si substrate, and a thickness of 0.64
A Pb (Zr _0.52 Ti _0.48 ) O ₃ film having a thickness of μm was formed by a sol-gel method, and a comb-shaped electrode having a period of 8 μm was formed thereon. In this document, the frequency response of the insertion loss is measured for this SAW, and as a result, a peak with an insertion loss of about 40 dB is observed around a center frequency of about 400 MHz. Note that PZT in this SAW filter
The normalized thickness 2πh / λ of the film is 0.5. here,
h is the actual thickness of the PZT film, and λ is the wavelength of the surface acoustic wave.

[0007]

Since piezoelectric ceramics are polycrystalline, loss occurs due to the presence of crystal grain boundaries. In particular, the propagation loss increases at high frequencies exceeding 100 MHz.

Further, Pb (Zr _0.52 Ti
_0.48 ) The SAW filter having an O ₃ film may have a large electromechanical coupling coefficient by analogy with piezoelectric ceramics. However, this one has an insertion loss of 40
Since it is so large as dB, it cannot be used practically. The cause is described in the literature as deterioration of the film quality. The inventors of the present invention considered that the PZT film was formed by the sol-gel method, so that the PZT film became a polycrystalline film. As a result, at a high frequency of 400 MHz, the propagation loss due to the crystal grain boundaries was significantly increased. Can be For practical use, the propagation direction of surface acoustic waves and PZT
It is necessary to optimize the film thickness, but these have not been studied in the above-mentioned literature.

An object of the present invention is to provide a surface acoustic wave device which can be monolithically integrated with other elements using a semiconductor substrate and has excellent characteristics.

[0010]

The above object is achieved by the following (1).
This is achieved by the present invention according to (5). (1) A PZT film made of a PZT crystal is formed on a semiconductor substrate, and a comb-shaped electrode or a reflector is provided on the PZT film or between the PZT film and the semiconductor substrate. 3. The surface acoustic wave device according to claim 1, wherein a PZT crystal whose c-axis is oriented substantially perpendicular to the film surface is present. (2) The surface acoustic wave device according to the above (1), further comprising a short-circuit electrode in contact with a surface of the PZT film where no comb-shaped electrode is present. (3) When the thickness of the PZT film is h and the wavelength of the surface acoustic wave is λ, the normalized thickness of the PZT film is 2πh /
The surface acoustic wave device according to the above (1) or (2), wherein λ is 0.6 ≦ 2πh / λ ≦ 1.4. (4) The PZT film is a single crystal film in which the directions of the a-axis of the PZT crystal are substantially aligned, or a c-domain in which the c-axis is oriented substantially perpendicular to the film surface, The surface acoustic wave device according to any one of (1) to (3) above, wherein the surface acoustic wave device is a multi-domain structure film having an a domain oriented substantially parallel to the film surface. (5) The surface acoustic wave device according to any one of (1) to (4), wherein the semiconductor substrate is made of Si.

[0011]

FIG. 1 is a plan view showing an example of the configuration of a SAW device according to the present invention. Also, FIG.
2 shows a sectional view. In this specification, this structure is called a first structure. This SAW device comprises a zirconium oxide film 21, a strontium titanate film 22, a PZT film 4, and a pair of comb-shaped electrodes 51, 52 for exciting or receiving SAW on a Si substrate 1 made of Si single crystal. Having. Sound absorbing materials 91 and 92 are provided on the PZT film 4 with the interdigital electrodes 51 and 52 interposed therebetween.

FIG. 3 shows a configuration example of a SAW device having the second structure. This SAW device has a short-circuit electrode 8 provided between the strontium titanate film 22 and the PZT film 4 in the device shown in FIG.

FIG. 4 shows a configuration example of a SAW device having a third structure. This SAW device has a zirconium oxide film 21, a strontium titanate film 22, comb-shaped electrodes 51 and 52, and a PZT film 4 in this order on a Si substrate 1. On the PZT film 4, sound absorbing materials 91 and 92 are provided at positions sandwiching the comb-shaped electrodes 51 and 52.

FIG. 5 shows a configuration example of a SAW device having a fourth structure. This SAW device has a short-circuit electrode 8 provided on the PZT film 4 of the device shown in FIG.

FIG. 6 shows a configuration example of a SAW device having a fifth structure. This SAW device has a comb-shaped electrode 5 on a PZT film 4.
Is provided, and a pair of reflectors 71 and 72 are provided on both sides thereof.

The PZT film 4 in each of the above devices is made of PZT
(Lead zirconate titanate) crystal, a zirconium oxide film 21 and a strontium titanate film 22
Functions as a buffer layer when the PZT film 4 is grown on the Si substrate 1. However, the buffer layer is not limited to these, and may be appropriately selected so that the crystal orientation of the PZT film can be made desired. If a desired orientation can be obtained, a PZT film may be formed directly on the semiconductor substrate.

The PZT crystal contained in the PZT film has a tetragonal system. In the present invention, the PZT film is c-crystal of the PZT crystal.
Any film may be used as long as its axis is oriented substantially perpendicular to the film surface. However, it is most preferable that each crystal be a single crystal film in which the a-axis directions are almost aligned, that is, a (001) oriented film. . However, the PZT film is a multi-domain structure film having a c domain whose c axis is oriented substantially perpendicular to the film surface and an a domain whose c axis is oriented substantially parallel to the film surface, that is, ( A 90 ° domain structure film in which (100) orientation and (001) orientation are mixed may be used. Even with such a domain structure film, sufficiently good impedance characteristics and filter characteristics can be obtained by using the c domain. The spontaneous polarization is oriented in the c-axis direction.

By forming such a PZT film on a semiconductor substrate, an electromechanical coupling coefficient of 10% or more can be obtained. Since a semiconductor substrate such as a Si substrate is used, the semiconductor device can be monolithically integrated with an active device such as an IC or an LSI using the semiconductor substrate, which is extremely useful in industry. In addition, if the PZT film is a single crystal film, unlike a substrate made of polycrystalline ceramics or a substrate having a PZT film formed by a sol-gel method, 100 MHz
Even at the high frequencies described above, the propagation loss can be kept small.

In the present invention, when the PZT film is a single crystal film, the direction of the a-axis existing in the plane of the PZT film is not limited. That is, the direction of the opening length of the comb electrode and the PZT
The angle between the film 4 and the a-axis (hereinafter, referred to as angle q) is not particularly limited. For example, as is apparent from FIG. 12, the effective electromechanical coupling coefficient is substantially constant regardless of the angle q.
That is, the propagation angle dependence of the effective electromechanical coupling coefficient is hardly recognized.

The displacement direction of the propagation mode is, for example, q
In the case of = 0 ° and q = 45 °, there is a feature that the component is composed of a component in the propagation direction and a component in the thickness direction of the substrate and does not include an SH (shear horizontal) component.

Here, the propagation direction of the propagation mode is defined as a direction perpendicular to the equal phase plane of the propagation mode. That is,
Even when the power flow angle is not zero, the propagation direction is a direction perpendicular to the equal phase plane of the propagation mode. In this case, the opening length direction and the equal phase plane are parallel.

In the SAW device of the present invention, a normalized thickness 2πh / λ obtained by standardizing the actual thickness h of the PZT film using the wavelength λ of the SAW greatly affects the electromechanical coupling coefficient.
The preferred range of the normalized film thickness 2πh / λ differs depending on the semiconductor substrate used, but when a Si substrate or a semiconductor substrate having a material constant close to this is used, 0.6 ≦ 2πh / λ ≦ 1.4. . By setting the normalized film thickness in this range,
A SAW device having a large electromechanical coupling is realized.

PZT means Pb (Zr, Ti) O ₃ , that is, PbZrO ₃ -PbTiO ₃ solid solution. In the present invention, the molar ratio Ti / (Ti + Z) in the PZT film is used.
r) is not particularly limited and may be appropriately determined so as to obtain a desired orientation, and is preferably set to 0.6 to 0.9. If the ratio of Ti is too low, it is difficult to obtain good ferroelectric characteristics and resonance characteristics. On the other hand, if the ratio of Ti is too high,
It becomes difficult to obtain good electrical insulation.

The molar ratio Pb / (Ti +) in the PZT film
Zr) need not be 1, but is preferably 0.8 to
1.3, and more preferably 0.9 to 1.2.
By setting Pb / (Ti + Zr) within such a range, good crystallinity can be obtained. The ratio of O to Ti + Zr is not limited to three. In some cases, a stable perovskite structure is formed due to oxygen deficiency or excess oxygen. The molar ratio O / (Ti + Zr) is usually 2.
It is about 7 to 3.3. The composition of the PZT film can be measured by X-ray fluorescence analysis.

In the present invention, the PZT film is preferably made of Pb, Zr and Ti, but may contain an additional element or an impurity element in addition to these. For example, it is difficult to separate ZrO ₂ and HfO ₂ with the current high-purity technology, so that HfO ₂ is contained as an impurity in the PZT film.
May be mixed. However, HfO ₂ is mixed in PZ
There is no particular problem since the characteristics of the T film are not significantly affected. Examples of the impurity element and the additional element present in the PZT film include rare earth elements (including Sc and Y), B
i, Ba, Sr, Ca, Cd, K, Na, Mg, Nb,
Ta, Hf, Fe, Sn, Al, Mn, Cr, W, Ru
Is mentioned. When these substitution elements or impurity elements are contained, rare earth elements, Bi, Ba, Sr, Ca, C
d, K, Na, and Mg substitute for Zr, and Nb,
Ta, Hf, Fe, Sn, Al, Mn, Cr, W, Ru
Calculates Ti / (Ti + Zr) as a substitute for Ti, and the result is preferably within the above range. P
The substitution rate of the substitution element or impurity element in b, Zr and Ti is preferably 10% or less, more preferably 5% or less. The PZT film contains other elements such as Ar, N, H, Cl, C, Cu, and N.
i, Pt, etc. may be contained as a trace additive or an unavoidable impurity.

The PZT film is preferably formed by a reactive multi-source evaporation method using an oxidizing gas such as oxygen as a reaction gas, but may be formed by, for example, an MBE method or an RF magnetron sputtering method. In the case of using these forming methods, in particular, the multi-source evaporation method, P
Since the surface of the ZT film can be made smooth at the molecular level, the comb-shaped electrode formed thereon is less likely to be broken, and a comb-shaped electrode having electrode fingers with good linearity can be easily formed.

The buffer layer may be selected so that the PZT film grows so that the c-axis is parallel to the film surface. For the formation of the buffer layer, it is preferable to use a reactive evaporation method, an MBE method, an RF magnetron sputtering method, or the like.
It is preferable to use the method described in JP-A-10-17394.

The comb-shaped electrode is preferably formed by patterning a conductive film formed by vapor deposition, MBE, RF magnetron sputtering or the like by photolithography. The thickness of the comb electrode is usually 50 to 5
It is preferably set to 00 nm.

Although the above description has been given of the case where a Si substrate is used, the present invention is also applicable to a SAW device using a semiconductor substrate other than Si, for example, GaAs, InP, silicon germanium, or the like. However, in order to obtain a sufficient mechanical strength, it is preferable to use a substrate made of Si or silicon / germanium, and it is preferable to use a Si substrate in order to reduce costs.

Next, examples supporting the limitations in the present invention will be described.

Example 1 (First Structure) A SAW device having the structure shown in FIG. 2 was manufactured by the following procedure.

The Si substrate 1 has a thickness of 250 μm (10
0) A Si substrate was used. The zirconium oxide film 21 has a thickness of 10 nm, and the strontium titanate film 22 has a thickness of 50 nm, all of which are epitaxially grown by a reactive evaporation method. The PZT film was 0.77 μm thick and was epitaxially grown by a reactive multi-source evaporation method. The molar ratio Ti / (Ti + Zr) in the PZT film was set to 0.75.
When analyzed using X-ray diffraction, the PZT film 4 was a tetragonal system, and its c-axis was almost perpendicular to the film, and the direction of the a-axis was almost aligned, ie, a single crystal film. Was confirmed.

Next, a pair of comb electrodes 51 and 52 were formed by forming an Al film on the PZT film by an evaporation method and patterning the Al film by a photolithography technique. The comb-shaped electrodes 51 and 52 are formed by
The ZT film 4 was provided such that an angle q between the ZT film 4 and the a-axis was 45 °. The comb electrode has a thickness of 200 nm and a line width of the electrode finger of 1
μm, metallization ratio 0.5, aperture length 100 μm,
The number of electrode fingers was 7.5. In addition, the comb-shaped electrodes 51, 5
The distance between the two (the distance between the centers of the electrode fingers closest to each other) is 5
It was 0 μm. Therefore, the wavelength λ of the fundamental wave is 4 μm, and P
The normalized thickness 2πh / λ of the ZT film 4 is 1.2.

Next, the Si substrate was diced to have an element size to obtain a chip. At this time, sound absorbing materials 91 and 92 are applied between the outside of the comb-shaped electrodes and the end face of the substrate so that the SAW propagated outside the pair of comb-shaped electrodes is reflected on the end face of the substrate and does not affect the filter characteristics. The substrate was cut obliquely while roughening the end face of the substrate.

Next, the chip was mounted on a package using a die bonding agent and sealed with a lid. At this time,
The input / output terminals of the package and the comb-shaped electrodes 51 and 52 were connected with Al wires, respectively.

The normalized thickness of the PZT film 4 is set to 0.3 to
A SAW device was manufactured in the same manner as described above, except that the thickness was set to 5.0 (the maximum film thickness was 2 μm), and the line width of the comb-shaped electrode was changed within a predetermined range (the minimum line width was 0.5 μm). In addition,
The metallization ratio of the comb electrode was fixed at 0.5.

With respect to the SAW device manufactured as described above, the S parameter S21 was measured with a 50Ω network analyzer. Then, the center frequency fc was obtained from the frequency response of the insertion loss obtained from S21, and the effective propagation speed V _eff was estimated by V _eff = fc · λ.

The package was mounted and sealed in the same manner as described above, except that the substrate was cut so that only the comb electrodes 51 were on the chip. Then, the electrical terminal pair of the comb-shaped electrode 51 was connected to the input terminal of the network analyzer, and the S parameter S11 was measured and converted to admittance. Then, the obtained admittance frequency response was compared with a value calculated using the equivalent circuit model of the improved Smith, and the electromechanical coupling coefficient k ² was estimated.

As a result, the maximum frequency of the first mode is 12
00 MHz, and the insertion loss at that time was 20 dB. From this result, it can be seen that the insertion loss does not greatly deteriorate even at a high frequency of 1200 MHz and the propagation loss is small. FIG. 7 shows the normalized film thickness dependence of the effective propagation speed. Further, in FIG. 8 shows the normalized film thickness dependence of the electromechanical coefficient k ^2.

FIG. 8 shows that if the normalized film thickness 2πh / λ is in the range of 0.6 ≦ 2πh / λ ≦ 1.4, an electromechanical coupling coefficient of 10% or more can be obtained. For example, in these figures, when the normalized film thickness is 0.8, the effective electromechanical coupling coefficient k ² and the effective propagation velocity are 13.5% and 4235 m / s, respectively. Therefore, although the effective electromechanical coupling coefficient k ² is about 13%, a large effective propagation speed can be obtained.

Example 2 (Second Structure) An S-type semiconductor device having the structure shown in FIG. 3 was formed in the same manner as in Example 1 except that a short-circuit electrode 8 was provided between the Si substrate 1 and the PZT film 4.
An AW device was manufactured. That is, by changing the thickness of the PZT film 4 and changing the line width of the comb-shaped electrode with the metallization ratio fixed at 0.5, the normalized electromechanical coupling coefficient k ² and the effective propagation speed are normalized. Dependencies were investigated. As the short-circuit electrode 8, a 100 nm thick Pt film formed by a vapor deposition method was used.

For these devices, the effective electromechanical coupling coefficient k ² and the effective propagation speed were estimated in the same manner as in Example 1. Figure 9 shows the relationship between the normalized thickness and the effective electromechanical coupling coefficient k ² of the PZT film. The effective propagation speed is
In the case of the standardized film thickness range in which the electromechanical coupling coefficient is large, it is about 4000 to 50 in the first mode.
It was about 00 m / s. From FIG. 9, for example, when the normalized film thickness is 0.8, the effective electromechanical coupling coefficient k ² is 14.3%.
It can be seen that it is. The effective propagation speed at this time was 4204 m / s. The normalized film thickness range giving 70% of the peak value of the electromechanical coupling coefficient is the same as in the case of the first structure. If it this structure, although the effective electromechanical coupling coefficient k ² is about 14%, is relatively high effective propagation velocity.

Example 3 (Third Structure) As shown in FIG. 4, a SAW device having a structure in which comb electrodes 51 and 52 are arranged between a Si substrate 1 and a PZT film 4 was manufactured by the following procedure. .

First, as in the first embodiment, the Si substrate 1
A zirconium oxide film 21 and a strontium titanate film 22 were epitaxially grown thereon. Next, a Pt film having a thickness of 100 nm was epitaxially grown thereon by vapor deposition and patterned by photolithography to form comb-shaped electrodes 51 and 52. Then
A PZT film 4 was formed in the same manner as in Example 1, and the subsequent steps were the same as in Example 1 to obtain a SAW device.

For these devices, the same measurements as in Example 1 were performed. Figure 10 shows the relationship between the normalized thickness and the effective electromechanical coupling coefficient k ² of the PZT film. Note that the effective propagation speed is about the same as that of the first structure, and is in the range of 4000 to 5500 in the normalized film thickness range where the electromechanical coupling coefficient is large.
m / s. From FIG. 10, for example, the normalized film thickness 1.
When 6, the effective electromechanical coupling coefficient k ² is found to be 11%. The effective propagation speed at this time is 5154
m / s. If it this structure, although the effective electromechanical coupling coefficient k ² is about 10%, is relatively large effective propagation velocity.

Example 4 (Fourth Structure) A SAW device having the structure shown in FIG. 5 was manufactured in the same manner as in Example 3 except that the short-circuit electrode 8 was formed on the PZT film 4.
The short-circuit electrode 8 was formed in the same manner as in Example 3.

For these devices, the insertion loss and the frequency response of the admittance were measured in the same manner as in Example 1, and from the results, the effective electromechanical coupling coefficient k ² and the effective propagation speed were estimated. Figure 11 shows the relationship between the normalized thickness and the effective electromechanical coupling coefficient k ² of the PZT film. In addition,
The effective propagation speed is about the same as that of the third structure, and is 4,000 to 4,000 in the normalized film thickness range where the electromechanical coupling coefficient is large.
It was about 5500 m / s. From FIG. 11, for example, when the normalized film thickness is 1.4, the effective electromechanical coupling coefficient k ² is 7.1.
%It can be seen that it is. The effective propagation speed at this time was 5269 m / s. If it this structure, although the effective electromechanical coupling coefficient k ² is about 7%, is relatively large effective propagation velocity.

Fifth Embodiment (First to Fourth Structures) The angle q between the opening length direction and the a-axis of the PZT film is 0 °, 1 °.
5 °, 30 ° or 45 ° so that the comb-shaped electrode 51
SAW devices were fabricated in the same manner as in Examples 1, 2, 3, and 4 except that 52 was formed.

The same measurement as in Example 1 was performed for these devices, and the normalized film thickness dependence was determined for each of the propagation speed and the electromechanical coupling coefficient. First, second, third,
FIG. 12 shows the relationship between the angle q and the maximum value of the electromechanical coupling coefficient at that angle for each of the fourth structures.
In the figure, curves corresponding to the first, second, third and fourth structures are shown as A1, A2, A3 and A4, respectively. From this figure, it can be seen that in the first, second and third structures, an electromechanical coupling coefficient of 10% or more can be obtained regardless of the angle q. Further, it can be seen that the fourth structure can provide an electromechanical coupling coefficient of 7% or more.

Example 6 (Fifth Structure) A SAW device having the structure shown in FIG. 6 was manufactured by the following procedure.

First, as in the first embodiment, the Si substrate 1
A zirconium oxide film 21, a strontium titanate film 22, and a PZT film 4 were epitaxially grown thereon.
However, the thickness of the PZT film 4 was 0.6 μm.

Next, the PZT film having a thickness of 20
By forming an Al film of 0 nm and patterning it by photolithography, the comb-shaped electrode 5 is formed.
And reflectors 71 and 72 on both sides thereof. The comb-shaped electrode 5 was provided such that the angle q between the opening length direction and the a-axis of the PZT film 4 was 45 °. For the comb-shaped electrode, the line width of the electrode finger was 1.2 μm, the metallization ratio was 0.5, the opening length was 80 μm, and the logarithm of the electrode finger was 30.5. The line width of the reflectors 71 and 72 was slightly larger than that of the comb-shaped electrode 5. The number of electrode fingers of each reflector was 300. Comb electrode 5
The distance between the reflector and the reflector (the distance between the centers of the electrode fingers closest to each other) was 2.4 μm. The wavelength λ of the fundamental wave is 4.8μ
m, the normalized thickness 2πh / λ of the PZT film 4 is 0.8.

Next, the Si substrate was diced to have a device size to obtain a chip. The chip was mounted on a package using a die bonding agent, and then sealed with a lid. At this time, the electric terminals of the package and the comb-shaped electrode 5 were connected with an Al wire.

The S-parameter S11 of the device manufactured as described above was measured using a network analyzer and converted to impedance. FIG. 13 shows the frequency response of the amplitude of the impedance. From this figure, it can be seen that the resonance frequency is 882.3 MHz. When the effective electromechanical coupling coefficient k ² and the effective propagation velocity were estimated, they were 13.5% and 4235 m / s, respectively.

[0055]

According to the SAW device of the present invention, at least a PZT film and a comb electrode are formed on a semiconductor substrate. In the present invention, since a semiconductor substrate is used, it is easy to make it monolithic with another element that also uses a semiconductor substrate.

In the PZT film of the present invention, the c-axis of the PZT crystal is oriented perpendicular to the film surface. Therefore, it is possible to increase the electromechanical coupling coefficient. Therefore, a filter having a wide bandwidth and a VCO oscillator having a wide frequency variable width can be realized and can be used at a relatively high operating frequency.

In the present invention, if the PZT film is formed as a single crystal film, a multi-domain structure film or a film having good crystal orientation, the loss can be reduced even at a high frequency, for example, a frequency of 100 MHz or more.

[0058]

[Brief description of the drawings]

FIG. 1 is a plan view showing a configuration example of a SAW device of the present invention.

FIG. 2 is a cross-sectional view illustrating a configuration example of a SAW device according to the present invention.

FIG. 3 is a cross-sectional view illustrating a configuration example of a SAW device according to the present invention.

FIG. 4 is a cross-sectional view illustrating a configuration example of a SAW device according to the present invention.

FIG. 5 is a cross-sectional view illustrating a configuration example of a SAW device according to the present invention.

FIG. 6 is a cross-sectional view illustrating a configuration example of a SAW device according to the present invention.

FIG. 7 is a graph showing the normalized film thickness dependence of the effective propagation speed.

FIG. 8 is a graph showing the dependence of the effective electromechanical coupling coefficient on the normalized film thickness.

FIG. 9 is a graph showing the dependence of the effective electromechanical coupling coefficient on the normalized film thickness.

FIG. 10 is a graph showing normalized film thickness dependence of an effective electromechanical coupling coefficient.

FIG. 11 is a graph showing the dependence of the effective electromechanical coupling coefficient on the normalized film thickness.

FIG. 12 is a graph showing a relationship between an angle q formed by a direction of an a-axis of a PZT film and a direction of an opening length of a comb-shaped electrode, and an effective electromechanical coupling coefficient.

FIG. 13 is a graph showing the frequency response of impedance.

[Explanation of symbols]

 Reference Signs List 1 Si substrate 21 zirconium oxide film 22 strontium titanate film 4 PZT film 5, 51, 52 comb-shaped electrode 71, 72 reflector 8 short-circuit electrode 91, 92 sound absorbing material

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Takao Noguchi 1-13-1 Nihonbashi, Chuo-ku, Tokyo TDC Corporation F-term (reference) 5J097 AA06 AA19 BB01 BB11 DD28 EE07 EE08 EE10 FF02 FF04 HA02 HA03 HA08 HA09 HB08 JJ01 KK05 KK09

Claims (5)

[Claims] 1. A PZT crystal comprising a PZT crystal on a semiconductor substrate.
A ZT film, and a comb-shaped electrode or a reflector on the PZT film or between the PZT film and the semiconductor substrate; and in the PZT film, the c-axis is substantially perpendicular to the film surface. A surface acoustic wave device in which oriented PZT crystals exist. 2. The surface acoustic wave device according to claim 1, further comprising a short-circuit electrode in contact with a surface of the PZT film where no comb-shaped electrode is present. 3. When the thickness of the PZT film is h and the wavelength of the surface acoustic wave is λ, the normalized thickness of the PZT film is 2.
3. The surface acoustic wave device according to claim 1, wherein πh / λ is 0.6 ≦ 2πh / λ ≦ 1.4. 4. The PZT film is a single crystal film in which the directions of the a-axis of the PZT crystal are substantially aligned, or a c-domain in which the c-axis is substantially perpendicular to the film surface; 4. The surface acoustic wave device according to claim 1, wherein the surface acoustic wave device is a multi-domain structure film having an a domain whose axis is oriented substantially parallel to the film surface. 5. The surface acoustic wave device according to claim 1, wherein said semiconductor substrate is made of Si.