JPS6145891B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to selection of a cutting direction with a large phase velocity and good frequency-temperature characteristics in a surface acoustic wave device using quartz crystal. Conventional surface acoustic wave devices using crystals mainly use ST-cut and X-axis Rayleigh waveform surface waves with good temperature characteristics.

Filters, oscillators, resonators, etc. that use this surface wave can be realized relatively easily up to several hundred MHZ band, but as the frequency approaches the GHZ band, the wavelength becomes shorter, so interdigital transducer electrodes are used. Since the electrode finger width is less than 1μ, which is close to the wavelength of light,
It has been difficult to realize patterns using conventional optical exposure methods.

In order to deal with this problem, it has been proposed to use the Overton mode or to use an electron beam exposure method. However, in the former case, deterioration of characteristics such as increased insertion loss becomes a problem.
Moreover, the equipment of the latter is very expensive, and there remains a problem in terms of economic efficiency.

On the other hand, recently a device using bulk waves propagating near the surface of a piezoelectric plate called surface-skimming bulkwave has been reported. According to this report, the phase velocity of this bulk wave is
It has a speed approximately 1.6 times that of an axially propagating surface wave, and has relatively good temperature characteristics, so it is considered suitable as a high-frequency device.

However, this wave relates to a bulk wave propagating on a propagation path with no mass load, and as the wave propagates, the displacement on the surface is attenuated and losses increase. Furthermore, there is no description whatsoever regarding the influence of mass load due to the electrode film thickness of the interdigitated finger electrode portions.

The present invention was made in view of the above-mentioned drawbacks, and its purpose is to provide a surface acoustic wave device with a large phase velocity, good frequency-temperature characteristics, sufficient concentration of displacement on the surface by mass loading, and low loss. It is about providing.

To achieve the above object, the present invention is based on the IRE standard, in which an interdigital transducer made of aluminum electrodes is attached to the surface of the crystal substrate in the direction of the Z' axis after rotation in a rotating crystal Y plate represented by (YXI)Θ. The electrode film thickness of the transducer is ho, the electrode finger width is a, the gap between the electrode fingers is b, and the wavelength of the surface acoustic wave excited by the transducer and having a principal displacement in the X-axis direction. is characterized by the relationship between X expressed by x=a/a+b・ho/λ and the cutting direction Θ as follows: Θ=(28333X ² −11.667X+36.083) (degrees), Furthermore, an aluminum electrode is attached on the propagation path between adjacent interdigital transducers, and the displacement is concentrated on the surface by the mass load of the aluminum electrode.

The present invention will be explained in detail below.

First, the outline of the present invention will be explained using FIG. 1.

In the present invention, a rotary Y-cut plate is used, which is obtained by cutting the Y-cut plate shown by the dotted line in the figure at an angle of θ degrees with respect to the X-axis. This is shown as a solid line.

A cross-finger type transducer T is formed in the direction of the Z' axis of the rotary Y-cut plate cut in this way.

Transducer T is made of aluminum. The reason for using a surface wave propagating in the Z-axis direction will be explained with reference to FIG.

As shown in Figure 2, the coordinate system xj (j=1, 2,
3), and the displacement uj (j=1, 2,
3) Take. Also, considering the uniformity of the potential 4 in the x1 (X-axis) direction, we solved the equation of motion and the law of conservation of charge for waves propagating in the ^x3 (Z'-axis) direction, and found that in the case of a rotating crystal Y plate, the displacement u1 is not combined with displacements u2 and u3,
And the piezoelectrically excited displacement is only u1, u2, u3
was found to be not piezoelectrically excited. As a result, it was found that there was no spurious as seen in the conventional ST cut and X-axis propagation surface waves, and that it had single mode characteristics. In the figure, 1 is an electrode film, and 2 is a crystal substrate. Also, as a boundary condition,
In the interdigitated electrode part, it can be approximated that an electrode with a film thickness of h'=a/a+b・ho is attached to the entire surface, and when x2=0, the displacement of the crystal substrate 2 and the electrode film 1,
The stress is continuous and the potential becomes zero. or
It may be considered that the stress in the electrode film 1 is zero at x2=+h'.

Based on the above, the configuration of the present invention will be explained below. In a Y-plate rotated by 36 degrees and 7 minutes, which is an example of the present invention, the value The distribution in the depth direction (x2 direction) is x2 = 0
Fig. 3 shows this as a reference. In the figure, the depth from the surface is normalized by λ. In the figure, curve X1
If X=0.5X10 ^-2 , the curve x2 is x=1.010 ^-2 ,
Curve x3 shows the case where X=2.010 ^-2 .

As an example, the frequency used is 800MHZ (λ = 6.4
μ), the film thickness of the interdigitated electrodes ho=1400Å
(b/ ^a =1), the value of It can be seen that it becomes a normal surface wave. Next, FIG. 4 shows the influence of the mass load on the frequency-temperature characteristics of the electrode for concentrating displacement on the surface in the cutting direction shown in FIG. 3. In the figure, the horizontal axis shows temperature (° C.) and the vertical axis shows frequency change rate Δf/fo. fo is the center frequency. Also, the curve x11 is X=0.5×10 ^-2 ,
Curve x22 is X=1.010 ^-2 , curve X33 is X=1.5×10 ^-2
Curve X44 is for X=2.0.

As is clear from the figure, the influence of the mass load on the temperature characteristics is large, and especially when used in communication equipment, etc., it is necessary to select the optimal cutting direction in consideration of the influence of the mass load. This relationship was determined in FIG. 5, which shows the relationship between the cutting direction Θ, which has a zero temperature coefficient at a room temperature of 25° C., and the mass load X caused by the aluminum electrode. The relationship between X and Θ is approximately Θ=
It is shown as 2833X ² −11667+36.083…(1). By selecting Θ and X according to FIG. 5 (Equation I), a surface acoustic wave device having good frequency-temperature characteristics can be obtained.

Also, in this cutting direction, the phase velocity is 1.6 times higher than that of the conventional ST cut X-axis propagation surface wave, so it is possible to generate a frequency up to 1.6 times higher.

Furthermore, another feature of the present invention is that the aluminum electrode 3 is attached on the propagation path as in the first case, and the displacement is sufficiently concentrated on the surface due to the influence of the mass load.
A surface acoustic wave device with low loss, good temperature characteristics, and long delay time characteristics can be provided.

To summarize the advantages of the present invention as described above, it is possible to achieve high frequencies up to approximately 1.6 times the frequency band that can be achieved with conventional ST cut and X-axis propagation surface waves.

It is possible to obtain frequency-temperature characteristics as good as those of conventional ST cut X-axis propagation surface waves, and to select the optimum cutting direction while taking into account the influence of the mass load on the electrode.

Since displacements u2 and u3 are not piezoelectrically excited,
A surface acoustic wave device without spurious components can be obtained.

By attaching aluminum electrodes to the propagation path, the displacement on the propagation path is concentrated on the surface to form a surface wave, and a surface acoustic wave device with low loss, good temperature characteristics, and long delay characteristics can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a diagram for explaining the outline of the present invention, and FIG. 2 is a diagram showing a coordinate system of a rotating Y plate of crystal and Z-axis propagation, where 1 is an electrode film and 2 is a crystal plate. Figure 3 is a diagram showing changes in the displacement distribution in the depth direction of the mass load due to the aluminum electrode on a Y plate rotated by 39 degrees and 7 minutes, which is an example of the present invention, and Figure 4 shows the mass in the cutting direction in Figure 3. FIG. 5 is a diagram showing the influence of load on frequency-temperature characteristics, and is a diagram showing the optimum cutting direction in consideration of the mass load of the aluminum electrode according to the present invention.

Claims (1)

[Claims] 1. In a crystal rotating Y plate represented by (YXI)Θ according to the IRE standard, interdigital transducers for transmitting and receiving made of aluminum electrodes are arranged in the direction of the Z' axis after rotation. The interdigital transducer is configured on the surface of a crystal substrate, has an electrode film thickness ho, an electrode finger width a, a gap between the electrode fingers b, and is mainly excited in the X-axis direction by the transducer. When the wavelength of a ^surface acoustic wave with displacement is λ, it is understood that X expressed as Featured surface acoustic wave device. 2 In a crystal rotating Y plate represented by (YXI)Θ according to the IRE standard, an interdigitated transducer for transmitting and receiving made of aluminum electrodes is configured on the surface of the crystal substrate in the direction of the Z' axis after rotation. , an aluminum electrode is attached on the propagation between the interdigital transducers, the displacement is concentrated on the surface by the mass load of the aluminum electrode, the electrode film thickness ho of the interdigital transducer, the electrode When the finger width is a, the gap between the electrode fingers is b, and the wavelength of the surface acoustic wave with the principal displacement in the X-axis direction excited by the transducer is λ, then X=a/a+b・ho/λ A surface acoustic wave device characterized in that X and the cutting direction Θ are related by Θ = (28333X ² −11.667X + 36.083) (degrees).