JPH0614608B2 - Elastic wave device - Google Patents

Description

Description: TECHNICAL FIELD OF THE INVENTION The present invention relates to an acoustic wave device, particularly lithium tantalate (LiTaO ₃ ).
The present invention relates to improvement of an acoustic wave device that uses a piezoelectric substrate as a base material and has a large coupling coefficient and a small temperature coefficient.

(2) Background of the technology The acoustic wave device can easily realize a VHF band or UHF band filter and a resonator by forming a comb-shaped electrode body on the surface of a piezoelectric substrate. Although it is realized as a PIF filter for some TV receivers and an oscillating element of an RF converter, new functions and performance improvements are required for acoustic wave elements with the aim of expanding their applications. The performance of these acoustic wave devices depends largely on the piezoelectric substrate used as well as the design of the interdigital electrode body. Lithium niobate (LiNbO ₃ ) and lithium tantalate (LiTa
O ₃ ), quartz, and various performances depending on the cutting plane direction and the propagation direction.

[Conventional technology and problems]

There are a coupling coefficient and a temperature coefficient as indexes for displaying the characteristics of the acoustic wave device substrate. The coupling coefficient is an index indicating the efficiency with which electric energy is converted into vibration energy, and is the propagation velocity of surface waves when a metal layer or the like is not attached to the surface of the voltage substrate that constitutes the acoustic wave device and is free. Vf, on the other hand, when the propagation velocity of the surface wave when a metal layer or the like is attached to the surface and is short-circuited is Vs, the coupling coefficient K ² is Is defined as On the other hand, the temperature coefficient Kt is the rate of change of the speed at which the surface wave propagates in the piezoelectric medium substrate with respect to temperature, and when the phase change that occurs when the temperature is changed by ΔT at a certain phase (H) is ΔH. , Kt = ΔH / H / ΔT.

The above-mentioned coupling coefficient and temperature characteristic of the substrate have a close relationship with the characteristic of the acoustic wave element. The larger the coupling coefficient is, the wider band filter or VCO element can be obtained. This is advantageous in realizing a highly stable oscillator. Table 1 shows the coupling coefficient K ² of the most typical substrate used for the acoustic wave device and the frequency temperature coefficient TCF of the acoustic wave device.
Shown in.

That is, when lithium niobate is used, the temperature coefficient is poor, and when lithium tantalate or quartz is used, the coupling coefficient is poor.

(4) Object of the invention The object of the present invention is to eliminate this drawback, and to provide a surface acoustic wave element having a large coupling coefficient using lithium tantalate, which originally has a good temperature coefficient, as the piezoelectric medium substrate. Especially.

[Structure of Invention]

According to the present invention, the object is to cut at the first rotation angle φ rotated about the Z axis with the positive counterclockwise direction and the second rotation angle θ rotated about the X axis with the positive counterclockwise direction. In the Euler angle display (φ, θ, ψ), where the surface is displayed and the third rotation angle ψ rotated about the Z axis after rotation with the counterclockwise direction as positive is the surface wave propagation direction in the substrate surface, φ = 0 ± 5 °, θ = 1
Lithium tantalate (LiTaO ₃ ) having a cutting direction and a propagation direction represented by 30 ° ± 5 ° and ψ = 1 ° to 3 °
A piezoelectric substrate, and at least one comb-shaped electrode body in which at least two comb-shaped electrodes are meshed with each other so that each finger is positioned on the surface of the substrate substantially at right angles to the surface wave propagation direction, The elastic wave device is characterized in that the surface wave is a surface slip wave, and the slip wave is an end face reflection type piezoelectric slip wave resonator that reflects both end faces of the substrate to give a desired resonance frequency.

Example of Invention

 Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a definition of an Euler angle for Euler display of a cut surface of a piezoelectric substrate made of lithium tantalate (LiTaO ₃ ) used for an end face reflection type piezoelectric shear wave resonator according to the present invention and a propagation direction of an elastic wave. FIG.

The first rotation angle φ rotates about the Z axis, the second rotation angle θ rotates about the X axis with the counterclockwise direction being positive, and the cutting plane orientation is displayed, and the third rotation angle ψ is the inside of the substrate after rotation. The rotation angle from the X-axis represents the propagation direction, and is displayed as (φ, θ, ψ).

The Euler angle indications φ and θ of the LiTaO ₃ substrate used are 0 ° and 130 °, respectively, and are generally 40 ° rot (rotation) YLi.
Also expressed as TaO ₃ .

FIG. 2 is a perspective view showing an end face reflection type piezoelectric shear wave resonator according to the present invention using this 40 ° rotYLiTaO ₃ substrate 1, in which a surface slip wave is generated by the interdigital transducer body 2 and the free end face 3, It is reflected at 4 and a resonance characteristic is obtained. This surface slip wave changes in a direction parallel to the fingers 2c, 2d of the comb-teeth electrodes 2a, 2b of the interdigital electrode body 2 as shown by an arrow 5 and propagates in a direction substantially perpendicular thereto, so that the free end faces 3, 4
The surface slip wave is reflected as it is without conversion to other modes. Further, when the wavelength of the surface slip wave is λ, its energy is concentrated 90% or more within a depth of 10λ from the surface.

The admittance of such a resonator is generally Is represented.

Here, W is the angular frequency, W _R is the resonance angular frequency, Cd is the parallel capacitance between the electrodes of the interdigital transducer, R is the resonance resistance that represents loss, and K ² is the electromechanical coupling coefficient.

Further, the expression (2) is simply expressed as Y = jwCd + 1 / (R + jwL + 1 / jwC) (3), and its equivalent circuit expression is expressed as a two-terminal resonance circuit as shown in FIG.

In FIG. 3, Cd is the parallel capacitance, C is the series capacitance of the interdigital transducer, R is the resonance resistance, and L is the series inductance.

On the other hand, the resonance angular frequency W _R and the resonance sharpness Q of the resonator are W _R = 1 / LC (4) and Q = WL / R (5). The capacitance ratio γ, which is a measure of the width of the inductive region of the oscillator, is expressed by γ = Cd / C ……………… (6).

As shown in FIG. 4, the method of manufacturing a resonator according to the present invention uses a finger so that the propagation direction (arrow SAW) φ of the surface slip wave is 1 to 5 °, preferably 1 ° on a wafer-shaped 40rotYLiTaO ₃ substrate 1a. After forming a plurality of interdigital electrode strips 2e having equal pitches in parallel, the wafer is cut with a dicing saw or the like. This cleavage divided into division (f ...... f ¹⁾ and, piece by a predetermined number of fingers as further present resonator the divided IDT band 2e is obtained in each interdigital electrode strip 2e (g ...... ) There is a process to do.

Further, the interdigital electrode strip 2e is formed by nichrome (NiCr) 2f for gold adhesion on the surface of the substrate 1a of about 500 A, and gold (A
u) 2 g is formed by vapor deposition to a film thickness of about 2500 A, and then patterned by etching by photolithography technology.

Further, in order to adjust the resonance frequency change by chipping of the end faces 3 and 4 of the substrate 1 at the time of division, the gold 2g on the upper layer of the interdigital electrode body 2 is plated with gold or the gold 2g is dissolved by etching. It

In this way, the end face reflection type piezoelectric shear wave resonator of FIG. 2 according to the present invention is formed.

FIG. 6 is a diagram showing the temperature characteristic TCF of the resonance frequency with respect to the change of Q when the surface shear wave propagation direction 4 of the substrate 1 according to the present invention is changed.

The resonator used here has a substrate 1 of 40 ° rotYLiTaO.
₃ , the interdigital electrode body 2 on the surface of the substrate 1 is formed,
Further, the interdigital electrode 2 has a finger length W (finger overlapping length) W, a finger pitch P, the number of fingers N (the number of fingers of the comb-teeth electrodes on both sides) is 410 μm, 25.9 μm and 90 (as shown in FIG. 2), respectively. Finger logarithm is 45
Pair).

As can be seen from FIG. 6, the propagation direction ψ of 1 ° ± 5 ° can be used, 1 ° to 3 ° is favorable, and the highest Q value 600 was obtained especially at 1 °. The substrate characteristics at this time are shown in Table 2. Compared with the conventional piezoelectric substrate shown in Table 1, the coupling coefficient is large and the temperature coefficient is good.

The inventors have also investigated φ and θ by variously cutting the LiTaO ₃ piezoelectric crystal, and as a result, φ = 0 ± 5 ° and θ = 130 °
± 5 ° was good, the other cut angles had a small coupling coefficient, and the temperature coefficient was not good either.

FIG. 7 is a diagram showing the relationship between the finger lengths W and Q of the resonator according to the present invention described in FIG.

This is data as a result of fixing the finger logarithm to 60 pairs (the number of fingers is 120) and changing the finger length W. From the figure, the finger length W is preferably 5λ to 15λ, and particularly 10λ is optimal.

In addition, λ is the wavelength of the surface wave slip wave.

FIG. 8 is a diagram according to the present invention showing the relationship between the finger logarithm and Q at each finger length in FIG.

As can be understood from this figure, the logarithm of fingers is 40 to 80.
The number of pairs is good, and especially 60 pairs are optimum.

FIG. 9 is a diagram according to the present invention showing changes in the cutting position error of the substrate 1 and the resonance frequency.

In the resonator according to the present invention, when cutting the substrate 1 as shown in FIG. 2, there is almost no frequency change when cutting at the center position of the finger.

However, such cutting causes some variation, but it appears remarkably when the number of finger pairs is small (for example, 20 pairs in FIG. 9).

On the other hand, when the number of finger pairs is 40 to 80, the change is small, and the desired resonance frequency can be obtained by eliminating the variation by the above-mentioned frequency adjusting method by plating or etching.

Next, (φ, θ, ψ) is (0, 130 °, 1 °) LiTaO ₃
Table 3 shows an equivalent circuit constant string of the end face reflection type piezoelectric shear wave resonator according to the present invention using a substrate.

The interdigital transducer electrode body 2 of this resonator is as described in FIG.

Fig. 10 shows the temperature characteristics of the resonance frequency and anti-resonance frequency of the resonator in Table 3, which are -28ppm respectively.
/ ° C and -39ppm / ° C were obtained.

Further, FIG. 11 is a diagram according to the present invention showing the amplitude and phase characteristics in Table 3.

〔The invention's effect〕

As described above, according to the present invention, it is possible to provide an elastic wave element having a large coupling coefficient, using lithium tantalate, which originally has an excellent temperature coefficient, as a piezoelectric medium base material.

[Brief description of drawings]

FIG. 1 is a diagram that defines a cutting plane direction of a piezoelectric substrate used for a resonator according to the present invention and a propagation direction of elastic waves, and FIG. 2 is a diagram showing a structure of the resonator according to the present invention, FIG. 2 is an equivalent circuit of the resonator, FIGS. 4 and 5 are drawings for explaining the method of manufacturing the resonator according to the present invention, and FIG. 6 is a graph showing Q and Q when the propagation direction 4 of the surface wave is changed. Figures showing the relationship of the resonance frequency according to the present invention, FIGS. 7 and 8 and 9 show the characteristics of the interdigital transducer electrode of the resonator according to the present invention, and FIG. 10 shows the resonance according to the present invention. FIG. 11 is a diagram showing temperature characteristics of the resonance frequency and anti-resonance frequency of the child, and FIG. 11 is a view showing amplitude and phase characteristics of the resonator according to the present invention. [Description of symbols] 1 ... Piezoelectric substrate, 2 ... Interdigital electrode body, 3, 4 ... Free end face, 5 ... Displacement direction of surface slip wave

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Noboru Wakatsuki 1015 Kamiodanaka, Nakahara-ku, Kawasaki-shi, Kanagawa Fujitsu Limited (56) Reference JP 58-33310 (JP, A) JP 52- 130586 (JP, A)

Claims (5)

[Claims] 1. A cutting plane is displayed with a first rotation angle φ rotated about the Z axis with a positive counterclockwise direction and a second rotation angle θ rotated about the X axis with a positive counterclockwise direction, Euler angle display (φ, 3rd rotation angle ψ rotated about Z axis after rotation with positive counterclockwise direction as surface wave propagation direction in substrate surface)
θ, ψ), φ = 0 ± 5 °, θ = 130 ° ± 5 °, ψ = 1
A lithium tantalate (LiTaO ₃ ) piezoelectric substrate having a cutting direction and a propagation direction represented by ° to 3 ° is provided, and at least two fingers are placed on the surface of the substrate so that each finger is positioned substantially at right angles to the surface wave propagation direction. One interdigital electrode body in which a number of comb-teeth electrodes are intermeshed with each other, and the surface wave is a surface slip wave, and the slip wave reflects both end surfaces of the substrate to give a desired resonance frequency. An acoustic wave device characterized by being a piezoelectric piezoelectric shear wave resonator. 2. The finger length, which is the overlapping length of the fingers of the interdigital electrode body, is 5λ to 15λ when the wavelength of the surface slip wave is λ. Elastic wave device. 3. The acoustic wave device according to claim 2, wherein the finger length is about 10λ. 4. The number of finger pairs of the interdigital transducer is 40.
The elastic wave device according to claim 1, wherein the acoustic wave device is composed of ~ 80 pairs. 5. The acoustic wave device according to claim 4, wherein the number of finger pairs is about 60.