JP4134627B2 - High frequency acoustic wave device using aluminum nitride piezoelectric thin film - Google Patents

Description

[0001]
[Technical field to which the invention belongs]
The present invention relates to an elastic wave element such as a lamb wave type elastic wave resonator used in communication equipment or the like.
[0002]
[Prior art]
Conventionally, a surface acoustic wave element using diamond is known as an acoustic wave resonator for high frequency applications (for example, Japanese Patent No. 02885349). Since the sound velocity is as high as 8000 m / s, it is used as an elastic wave device in the 2.5 GHz band.
[0003]
[Problems to be solved by the invention]
The acoustic wave device for communication equipment is desired to have a higher sound speed in consideration of reduction of process load. Diamond, which is the highest sound speed material, has a very high sound speed of 10000 m / s or more, but since it does not have piezoelectricity, it is necessary to add a piezoelectric material, so that the available sound speed is 8000 m / s There is a problem that it will be lowered.
[0004]
In a SAW element for communication equipment, it is possible to obtain electrical characteristics with low loss by using a resonator having a larger electromechanical coupling coefficient (k ² ). When producing a high-performance SAW element, k ^{2 of a} surface acoustic wave element using diamond is 2% or less, which is a problem.
An object of the present invention is to solve the above-described problems, that is, to provide an acoustic wave device having a very high sound velocity (> 9000 m / s). The second object is to provide an elastic wave device having a large k ² (> 2%).
[0005]
[Means for Solving the Problems]
In the Lamb wave type acoustic wave device comprising a piezoelectric film mainly composed of aluminum nitride (AlN) and an electrode pattern formed on the surface of the piezoelectric film and having at least two electrode fingers, The polarization orientation (c-axis) of the AlN piezoelectric film is substantially perpendicular to the normal line of the piezoelectric film and substantially perpendicular to the electrode fingers, and the thickness of the piezoelectric film is smaller than the electrode pitch of the electrode pattern. This is solved by the Lamb wave type elastic wave element. Here, the Lamb wave type elastic wave is an elastic wave propagating through a flat plate and shows a substantially uniform energy distribution in the thickness direction of the flat plate (for example, Japanese Patent No. 2936210).
[0006]
The operation of the present invention will be described below with reference to the drawings. FIG. 1 is a cross-sectional view illustrating the structure of a Lamb wave type elastic wave device. FIG. 2 shows the dependence of the sound velocity of Lamb wave type elastic wave on the plate thickness of k2. On the surface of the AlN piezoelectric film 1 having a thickness h, electrode fingers (3-1, 3-2) of two kinds of potentials are alternately formed with a period P. The ratio of the electrode finger width L to the interelectrode gap S is 2 to 3. The AlN piezoelectric film 1 is made of a material whose main component is an AlN piezoelectric body polarized in the direction indicated by the arrow 9. The calculation is based on a finite element-analytic solution coupling method (for example, Satoshi Isobe, Mitsutaka Hamada, Kengo Asai, Junji Sumioka, "Search for a new mode-type SAW by the finite element-analytic solution coupling method and its application to mobile communication devices" Electronic Information The communication journal CI Vol. J 82-CI No. 12 pp. 697-705 (December 1999) was calculated by adding the electrical opening and virtual opening conditions at the virtual boundary. As shown in FIG. 2, it was found that a high acoustic velocity (> 9000 m / s) and a high k ² (> 2%) can be obtained with a Lamb wave type elastic wave of h <P. Figure 3 shows the electrode finger direction dependence of the sound velocity and k ² of the Lamb wave type elastic wave. The horizontal axis represents the angle formed between the polarization direction of the AlN piezoelectric film and the direction of the electrode fingers. When the polarization direction and the direction of the electrode finger are orthogonal (90 degrees), the sound velocity and k2 are the largest. When the angle is between 55 ° and 125 °, a high k ² (> 2%) is obtained. That is, when the AlN piezoelectric single crystal substrate is made thinner than the pitch of the electrode fingers, and the polarization direction and the direction of the electrode fingers are at an angle between 55 ° and 125 °, the Lamb wave type elastic wave device having a high sound speed and a high k ² Can be formed. From the above consideration, as shown in FIG. 1 as an example, a comb-shaped electrode pattern 3 formed on the surface of the piezoelectric film 1 sandwiched between the gaps and having at least two electrode fingers is formed. In the acoustic wave element in which the thickness h of the piezoelectric film 1 is smaller than the pitch P of the electrode fingers 3, the main component of the piezoelectric film 1 is aluminum nitride, and the polarization direction 9 of the piezoelectric film 1 is the normal line of the piezoelectric film A high-frequency acoustic wave device using an aluminum nitride piezoelectric thin film characterized in that the electrode finger 3 is set at an angle in the range of 55 to 125 ° with respect to the polarization direction of the piezoelectric film 1 To do.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, preferred embodiments of the present invention will be described in detail.
FIG. 4 is a plan view showing the configuration of the one-opening resonator according to the first embodiment of the present invention. The one-opening resonator is formed by an AlN piezoelectric thin film 1, a comb electrode formed on the surface, and a reflector. The comb-shaped electrode is composed of two bus bars 2 parallel to the polarization direction 9 of the AlN piezoelectric thin film 1, eleven pairs of electrode fingers 3 perpendicular to the polarization direction 9, and input / output terminals 4 and 5. Has been. Input terminal 4 is connected to bus bar 2-1 connected to eleven electrode fingers 3-1, and output terminal is connected to bus bar 2-2 connected to twelve electrode fingers 3-2. The electrode fingers 3-1 and the output-side electrode fingers 3-2 are alternately arranged. Reflectors are installed on both sides of the comb electrode. Each reflector is composed of two bus bars 6 parallel to the polarization direction of the AlN piezoelectric thin film and 18 reflective electrode fingers 10 perpendicular to the polarization direction, and both ends of all the reflective electrode fingers 10 are connected to the bus bar 6. ing. The comb electrode, the input / output terminal, and the reflector are made of an Al-0.7% Cu alloy.
[0008]
FIG. 5 is a cross-sectional view of the portion shown in FIG. 4A-A ′. The thickness h of the AlN piezoelectric thin film 1 is set to 0.03P (P = 20 μm). The electrode finger line width L is 0.4P. The AlN piezoelectric thin film 1 is supported by a silicon single crystal substrate 7.
In the one-opening resonator configured as described above, since the thickness of the AlN piezoelectric thin film 1 in the comb-shaped electrode portion is extremely smaller than the electrode finger pitch P, the elastic vibration excited by the comb-shaped electrode instantaneously causes the AlN piezoelectric thin film 1 Therefore, it propagates as a Lamb wave instead of a surface acoustic wave. In addition, the surface acoustic waves are distributed in the range of about 2P in the depth direction from the electrode, whereas this one-opening resonator is concentrated in the region very close to the electrode (<0.03P). Greater piezoelectric effect. In addition, since the polarization direction of the AlN piezoelectric thin film 1 matches the propagation direction of the Lamb wave, the main component of the elastic vibration becomes a longitudinal wave, resulting in a higher sound speed and a strong piezoelectric effect. As a result, it is possible to realize a Lamb wave type elastic wave resonator having a very large sound velocity (> 9000 m / s) and a specific bandwidth (> 0.02).
FIG. 6 is a plan view showing a configuration of a one-opening resonator according to another embodiment.
FIG. 7 is a cross-sectional view of a portion indicated by BB ′ in FIG. An AlN piezoelectric thin film 1 having a thickness h = 0.1 P, a comb-shaped electrode formed on the surface thereof, a reflector, an input / output terminal, and a silicon single crystal substrate 7 having a recess are formed. Although the shape of the silicon single crystal substrate 7 is different from that of the first embodiment, a lamb wave type elastic wave resonator having exactly the same function is realized.
[0009]
【The invention's effect】
As described above, according to the present invention, it is possible to realize a Lamb wave type acoustic wave device having an extremely high sound velocity (> 9000 m / s) and k ² (> 0.02).
Since the i-line exposure machine widely used in the surface acoustic wave device process has a resolution of 0.30 μm or more, the applicable frequency of ordinary surface acoustic wave devices (sonic velocity 4000 m / s) is 3.3 GHz or less. The surface acoustic wave device using diamond (sonic speed of 8000 m / s) is 6.7 GHz or lower, but by using the present invention (sound speed of 9000 m / s), it can be expanded to 7.5 GHz.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view illustrating the structure of an acoustic wave device of the present invention.
FIG. 2 is a graph showing the dependence of sound velocity and k ^{2 on} the AlN piezoelectric thin film thickness of the acoustic wave device of the present invention.
FIG. 3 is a graph showing the dependence of acoustic velocity and k ^{2 on} the polarization direction of an AlN piezoelectric thin film of the acoustic wave device of the present invention.
FIG. 4 is a perspective view showing an acoustic wave resonator according to an embodiment of the present invention.
FIG. 5 is a cross-sectional view taken along line AA ′ of FIG.
FIG. 6 is a perspective view showing an acoustic wave resonator according to an embodiment of the present invention.
7 is a cross-sectional view taken along the line BB ′ in FIG.
[Explanation of symbols]
1 ... AlN piezoelectric thin film.
2 ... Bus bar.
3 ... Electrode fingers.
4, 5 ... Input / output terminals.
6 ... Reflector busbar.
7: A silicon single crystal substrate.
8: Void.
9: Polarization direction of AlN.

Claims (6)

A piezoelectric film sandwiched between gaps;
A comb-shaped electrode pattern formed on the piezoelectric film and having at least two electrode fingers;
The thickness of the piezoelectric film is smaller than the pitch of the electrode fingers, and a lamb wave type elastic wave propagates in the piezoelectric film,
The direction of the electrode finger is set to an angle in a range of 55 ° to 125 ° with respect to the polarization direction of the piezoelectric film, and the polarization direction of the piezoelectric film is substantially perpendicular to the normal line of the piezoelectric film. And a direction of the electrode finger is substantially perpendicular to a normal line of the piezoelectric film . In claim 1,
The high frequency acoustic wave device, wherein the comb electrode pattern is formed on a surface of the piezoelectric film. In claim 1,
The high-frequency acoustic wave device according to claim 1, wherein the piezoelectric film comprises aluminum nitride. In any one of Claims 1 thru | or 3,
The high-frequency acoustic wave device according to claim 1, wherein a ratio of a width of the electrode finger to a gap between the electrode fingers is 2 to 3. In any of claims 1 to 4,
The high-frequency acoustic wave device according to claim 1, wherein the pitch is half the wavelength of the acoustic wave. A substrate having a first surface and a hollow portion having an opening opening in the first surface;
A piezoelectric film formed on the first surface having a portion overlapping the first opening;
A first electrode pattern formed on the piezoelectric film and having a first plurality of electrode fingers;
A second electrode pattern formed on the piezoelectric film and having a second plurality of electrode fingers;
The first plurality of electrode fingers and the second plurality of electrode fingers are alternately arranged with a predetermined period,
The piezoelectric film has a thickness smaller than the predetermined period, and is configured such that a Lamb wave type elastic wave propagates in the piezoelectric film,
The direction of the first and second electrode fingers is an angle in the range of 55 ° to 125 ° with respect to the polarization direction of the piezoelectric film, and the polarization direction of the piezoelectric film is that of the piezoelectric film. An acoustic wave resonator characterized by being substantially perpendicular to a normal line and a direction of the electrode finger being substantially perpendicular to a normal line of the piezoelectric film .

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