JPH10209792A - SAW resonator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a stable and highly reliable SA without frequency jump by suppressing spurious due to a transverse mode.
A W resonator is provided. SOLUTION: In the interdigital transducers of the one-port and two-port SAW resonators, the mass difference between the left and right electrode fingers with respect to the center of the intersection width of the positive and negative electrode fingers is set to 5% or less, and the fundamental resonance as the main resonance is obtained. It is characterized in that occurrence of a fundamental wave obliquely symmetric transverse mode existing between the resonance frequency of the transverse mode and the anti-resonance frequency is suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a one-port and two-port SAW resonator constituted by utilizing surface acoustic waves, and to the dimensional accuracy of interdigital electrodes usually used for exciting surface acoustic waves. SAW that prevents unnecessary mode (fundamental transverse mode) that is unnecessary resonance (spurious) caused by
Related to a resonator.

[0002]

2. Description of the Related Art Among conventional SAW resonators, one-port type and two-port type SAW resonators are well known as those used for frequency stabilization and frequency control (US Pat. No. 4,144). , 507). Above all, those using ST cut, which is a rotation Y cut of about 30 to 45 degrees using quartz for the substrate, have a zero temperature coefficient at room temperature and have high accuracy.

[0003]

However, in the prior art of the SAW resonator described above, when the width of the interdigital electrode of the comb-shaped electrode, that is, the interdigital electrode is set to be about 30 wavelengths or more of the surface acoustic wave, The frequency of the fundamental oblique symmetric transverse mode is located between the antiresonant frequency and the resonant frequency of the SAW resonator, and depending on the manufacturing conditions, the fundamental oblique oblique transverse mode is excited to affect the oscillation frequency as spurious, The frequency jump has adversely affected the communication state.

Accordingly, the present invention is to solve such a problem, and an object of the present invention is to produce a spurious-free SAW resonator with a high yield and reduce the cost.

[0005]

[Means for Solving the Problems]

(1) The SAW resonator according to the present invention comprises at least one interdigital electrode on a piezoelectric flat plate and at least one IDT on both sides thereof.
In the SAW resonator composed of a pair of reflectors, the interdigital transducers are arranged so that the positive and negative electrode fingers are arranged in parallel with a specific intersection width substantially orthogonal to the propagation direction of the surface acoustic wave to be used, The average of the mass difference between the left and right electrode leaders with respect to the center of the intersection width is 5% or less.

(2) In the above (1), the difference between the average value of the width of the left and right electrode fingers with respect to the center of the intersection width is 5 in the positive and negative electrode fingers of the IDT.
% Or less.

(3) In the above (1), the difference between the average value of the film thickness of the left and right electrode fingers with respect to the center of the intersection width is 5 for the positive and negative electrode fingers of the interdigital electrode. % Or less.

[0008]

In order to solve the technical problems in the SAW resonator according to the present invention, the contents will be described in order because the following theory is the basis.

First, a description will be given of a fundamental wave obliquely symmetric transverse mode (hereinafter abbreviated as _A0 mode) which becomes spurious. Wherein A ₀ mode is a so-called one-port and two-port resonator, and more generally in the transversal filter, the width direction substantially perpendicular to the propagation direction of the surface acoustic wave used in these devices This is a vibration mode in which the amplitude changes, and has a node of vibration at the center of the intersection width of the electrode fingers of the interdigital transducer used in the element. On the other hand, the fundamental symmetric transverse mode S _0, called the primary vibration mode is used in the device has a maximum amplitude at the intersection width center of the electrode finger has a displacement of damping in the form of an exponential function on both sides. To supplement the description, the SAW resonator targeted by the present invention is a piezoelectric flat plate called a wafer cut in a specific direction from a piezoelectric material such as quartz or lithium tantalate, lithium niobate, lithium tetraborate, or the like. Is formed by forming a metal conductor thin film of aluminum or the like on the surface of the substrate, and then forming a fine pattern that performs a specific function by photolithography.
SAW resonators are roughly classified into a one-port type and a two-port type. The one-port type includes a single interdigital electrode and a pair of reflectors located on both sides thereof and having a surface acoustic wave reflecting function. The two-port type has a plurality of interdigital electrodes serving as input and output terminals, and a pair of reflectors located on both sides thereof. It should be added that these have already been described in a large number of well-known materials and will not be described in detail. In addition, the cross width of the electrode fingers of the interdigital transducer is a dimension in the width direction of the SAW resonator in which the positive and negative electrode fingers are arranged to overlap each other. FIG. 3 shows the modes of vibration displacement in the A ₀ mode and the S ₀ mode (calculation results). In the figure, 300 is the A ₀ mode, and 301 is the S ₀ mode. The piezoelectric plate in this case is a quartz ST-CUT,
The IDT arranged at the center is the intersection width of the electrode fingers of the IDT, and corresponds to 30 wavelengths of the surface acoustic wave. Said S
₀ mode it can be seen that even function, A ₀ mode is an odd function.

According to common sense considerations, the A ₀ mode just cancels out because the charges generated on the left and right sides with respect to the center of the intersection width are positive and negative, and the resonance current to the SAW resonator should not flow. It is. However, in the case of the SAW resonator which is an object of the present invention, the above-mentioned A ₀ mode is excited with considerable vibration and sub-intensity, and the A ₀ mode is the main resonance S ₀ mode resonance frequency fr and anti-resonance frequency fa. An intermediate frequency was observed. FIG. 4 shows the aforementioned SAW
It is a frequency characteristic of the impedance Z (ω) of the resonator. In the figure, 401 is the resonance frequency of the S ₀ mode, 40
Reference numeral 2 denotes the anti-resonance frequency of the S ₀ mode, and reference numeral 403 denotes the A ₀ mode which is a spurious problem. Therefore, the A ₀ mode is presumed to have been excited by not cause the conventional description. In this analysis, the basic formula is introduced as follows.

Regarding the S ₀ and A ₀ modes referred to as the transverse modes, the authors have already derived and published the differential equations governing them (Takagi, Momozaki, et al .: “Dynamic and static at room temperature”). K-cut quartz SAW resonator having a typical zero temperature coefficient, "The 25th EM Symposium, IEICE Electronics Engineering Committee, pp. 79-80, (1996)). If this equation is newly described, it becomes Equation (1).

[0012]

(Equation 1)

Here, ω is the angular frequency, ω ₀ (Y) is the element angular frequency of the corresponding area, a is the effective shear rigidity constant in the width direction, and V (Y) is the amplitude of the surface acoustic wave displacement in the width direction. , Y
Is the Y coordinate normalized by the wavelength of the surface acoustic wave. Also,
ω ₀ (Y) is an amount obtained by converting the velocity of the surface acoustic wave at the coordinate Y into an angular frequency, and is referred to as a frequency potential function. This frequency potential function changes near the operating point of the SAW resonator by a function of the thickness H (Y) of the aluminum metal conductor film existing on the surface acoustic wave propagation path. More generally, it has been found that it varies as a function of the mass m (Y) of the aluminum metal. Accordingly, in the interdigital transducer constituting the main part of the SAW resonator, ω is determined by the mass m (Y) of the interdigital transducer.
₀ (Y) is determined. That is, ω ₀ (m (Y)). In the case of the crystal ST-cut, the ω ₀ (Y) falls almost linearly with respect to m because the film thickness is small. Here, in order to simplify the calculation, in Expression (1), when dividing by the reference frequency ω ₀₀ ² ,

[0014]

(Equation 2)

Here, Ω = ω / ω ₀₀ is a normalized frequency, Q
(M (Y)) is a normalized potential function.

The method for obtaining the displacement amplitude V (Y) can be calculated by successive integration as follows, for example.

[0017]

(Equation 3)

Although V (Y, Ω) in equation (3) is a function of the normalized frequency, the displacement amplitude that actually occurs is obtained in Ω given by the following equation, which is the principle of minimum energy.

[0019]

(Equation 4)

The above equations (2) to (4) are basic equations.

Next, the generation principle of the anti-symmetric transverse modes A ₀ will be described with reference to these basic equations. First, a theoretical explanation will be given using equation (2).

Now, it is assumed that there is a mass difference between the center of the interdigital transducer of the SAW resonator in the width direction Y (Y = 0). At this time, Q ² which is the square of the potential function has an odd function component with respect to the origin of Y = 0. That is, when e is an even function and o is an odd function, Q ² = e
(Y) + o (Y). Therefore, in order to obtain the displacement amplitude function V (Y) in this state, V (Y) is also considered to be the sum of the even function Ψs and the odd function Ψa. Ψs can correspond to the fundamental symmetric transverse mode S ₀ , and Ψa can correspond to the oblique symmetric transverse mode A ₀ . That is, V (Y) =
Ψs + Ψa. By substituting these Q ² and V (Y) into equation (2) and integrating and organizing both sides with respect to Y, the product of the even function and the odd function becomes zero, so the following equation (5) is obtained.

[0023]

(Equation 5)

From equation (5), Δs satisfying the following equation (6)
And Ψa are solutions, and the A ₀ mode of Ψa (Y) exists.

[0025]

(Equation 6)

FIG. 5 shows the relationship between the displacement amplitude V (Y) calculated by the equations (3) and (4) and the normalized potential Q (Y). The straight lines 500 and 501 in the figure indicate the shape of Q (Y), and the lines 502 and 503 indicate the shape of V (Y). 50
A Q (Y) of 0 is a case where there is a frequency drop of 7300 ppm with respect to the free surface and a case where a frequency drop of 10% with respect to the center position is at the end (15 wavelengths), and conversely, 501 is 10% This is the case where the frequency rise of the edge occurs at the end. 502 corresponds to the straight line 500, and 503 corresponds to 501. Normalized potential function 501
And their displacement amplitudes were folded back to the positive region of the coordinate Y and compared for easy understanding. As is clear from FIG. 5, a difference occurs between the amplitude displacements 502 and 503, and half of the difference between the displacement amplitudes 502 and 503 is the fundamental oblique symmetric transverse mode displacement, whereas the difference is 50%.
Half of the sum of 2 and 503 is the fundamental symmetric transverse mode displacement.

As described above, in the interdigital transducer of the SAW resonator, if there is a mass difference in the width direction,
As a result of generating an odd function component in the frequency potential function,
It will excite the fundamental anti-symmetric transverse modes A _0. It is added that the difference in mass between the electrodes may be a difference in film thickness or a difference in electrode width. Therefore, it is essential to the suppression of A ₀ mode to be the spurious prepared as such differences do not exist.

A specific example of this phenomenon will be described in the following embodiment, including numerical values.

[0029]

(Embodiment 1) Embodiments of the present invention will be described below in order from FIG. FIG. 1 is an enlarged view of an embodiment of an IDT used in a SAW resonator according to the present invention. In FIG. 1, 100 is a piezoelectric flat plate, 101 is an interdigital electrode, 102 is a positive electrode finger, and 103 is a negative electrode finger. The piezoelectric flat plate 100 is composed of a substrate on which a piezoelectric single crystal such as quartz or lithium tantalate and a piezoelectric thin film such as ZnO are formed. The interdigital electrode consisting of 101 formed on the above-mentioned 100 is formed by forming a thin film of a conductive metal film such as aluminum and gold by means of vapor deposition, sputtering or the like, and then forming a pattern by photolithography. . A large number of the interdigital transducers are arranged at right angles to the phase traveling direction (longitudinal direction X) of the surface acoustic waves (Rayleigh wave, Leaky wave, etc.) to be used, parallel and periodically. A point to be noted in the present invention is that the intersection width Wc (104) is a width dimension in which the positive and negative electrode fingers of the interdigital electrode engage in the width direction Y.
(Y = 0) is processed with a total difference of 5% or less on the left and right. In order to maintain the processing accuracy of this mass, since the mass is the product of the width B, the length Wc, and the thickness H of the electrode finger, at least each of B and H is equally processed with a difference of 5% or less. Must be done.

Current product (200M quartz ST-X CUT)
Looking at the current state of processing accuracy of a 1 Hz SAW resonator, a thickness H of about 5000 ° is 0.1 to 0.1 mm.
For B of about 3.9 μm, a difference of up to 20% was found within one SAW resonator, while it was within 2% (difference between the left and right ends of the electrode fingers). , B
1, B2). Therefore, the relationships of the difference between the resonance amplitudes of the A ₀ mode B tried to measure is a diagram 2. Since the width of the electrode finger changes continuously with one electrode finger, the average width can be regarded as half the sum of the minimum value and the maximum value.
The average of the widths was taken as B on the horizontal axis. The normalized amplitude ratio obtained by dividing the amplitude of the A ₀ mode by the amplitude of the S ₀ mode is shown on the vertical axis (D ₁ / D _{0 in} FIG. 2). The amplitude referred to here is 20xLOG ₁₀ Z, which is the logarithmic value of the impedance characteristic Z (ω) of the SAW resonator, and represents the difference between the resonance and the anti-resonance amplitude (D
₁ and D ₂ ). As shown in the figure, when B is 5% or less, the _A0 mode does not remarkably occur, and the normalized amplitude ratio is several percent or less, which is a degree that does not hinder normal operation. Above that, the A ₀ mode was remarkably generated, and the normalized amplitude ratio was about 30% when the difference of B reached 10% (see FIG. 4). This phenomenon is observed not only in the case of the one-port SAW resonator shown in FIG. 2 but also in the case of the two-port SAW resonator. As a method of reducing the mass difference, uniform application of a resist or the like can be cited as one measure.

[0031]

As described above, according to the present invention, the difference between the left and right electrode finger masses with respect to the center in the width direction of the positive and negative electrode fingers of the interdigital transducer of the one-port SAW resonator is 5% or less. By doing so, it is possible to suppress the fundamental wave obliquely symmetric transverse mode A ₀ and prevent the occurrence thereof, thereby providing a SAW resonator having excellent spurious quality to the market. Also, 1
In the port-type and 2-port-type SAW resonators, the cost can be reduced by remarkably improving the yield.

[Brief description of the drawings]

FIG. 1 is a plan view showing one embodiment of an interdigital electrode of a SAW resonator according to the present invention.

FIG. 2 is a characteristic diagram according to the embodiment of the present invention.

FIG. 3 is a lateral mode displacement diagram.

FIG. 4 is a characteristic diagram of a conventional SAW resonator.

FIG. 5 is a diagram illustrating the principle of the present invention.

[Explanation of symbols]

 REFERENCE SIGNS LIST 100 piezoelectric flat plate 101 interdigital electrode 102 positive electrode finger 103 negative electrode finger 104 intersection width

Claims (3)

[Claims] 1. A SAW resonator comprising at least one interdigital electrode on a piezoelectric plate and at least one pair of reflectors on both sides thereof, wherein the interdigital electrode transmits a surface acoustic wave to be used. The electrode fingers of the positive electrode and the negative electrode are arranged substantially perpendicular to the direction and in parallel with a specific intersection width, and the average of the mass difference between the left and right electrode leaders with respect to the center of the intersection width is 5% or less. A SAW resonator. 2. The method according to claim 1, wherein the difference between the average value of the width of the left and right electrode fingers with respect to the center of the intersection width is 5% or less in the positive and negative electrode fingers of the interdigital electrode. The SAW resonator according to claim 1, wherein 3. The method according to claim 1, wherein the difference between the average value of the thickness of the left and right electrode fingers with respect to the center of the intersection width is 5% or less in the positive and negative electrode fingers of the interdigital electrode. The SAW resonator according to claim 1, wherein