JPH0228921B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a surface acoustic wave device without velocity dispersion that is used, for example, in high frequency oscillators, filters, etc.

The conventional intersecting finger electrodes of this type of device have a structure in which intersecting finger electrodes e ₁ and e ₂ are opposed to each other on a piezoelectric substrate 1 as shown in Fig. 1, and the area other than the intersecting finger electrodes is an elastic surface. The piezoelectric substrate 1 for wave propagation was exposed.

At this time, if the surface acoustic wave propagation speed of only the piezoelectric substrate is V ₀ and the wavelength determined by the interdigital electrodes is λ, the operating frequency of the surface acoustic wave excited by the interdigital electrodes is approximately = V ₀ /λ given.

However, in reality, the surface acoustic wave propagation velocity V ₀ in the piezoelectric substrate alone is different from the surface acoustic wave propagation velocity V ₁ in the place where the interdigitated finger electrodes are placed, and the difference is different when the electrodes are placed. Because it varies depending on the thickness of the metal layer, it exhibits velocity dispersion characteristics, and the linearity of phase delay and group delay deteriorates, making it unsuitable as a retardation medium. That is, when the desired operating frequency is given, the electrode pitch T (corresponding to the wavelength λ) to be formed is T=V ₀ /
However, since there is a region in the propagation path with a propagation velocity V ₁ different from V ₀ , strictly speaking, T = (V ₀ +
V ₁ )/2. In recent years, there have been proposals to apply surface acoustic waves to narrowband devices, but this has been hindered by the difficulty of setting the frequency.

In order to eliminate these drawbacks, the present invention provides a surface acoustic wave device that excites surface acoustic waves using crossed finger electrodes. The purpose is to provide a non-dispersive surface acoustic wave device with respect to velocity.

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 2 shows an embodiment of the present invention, in which 1 is a piezoelectric substrate, e ₁ and e ₂ are interdigitated finger electrodes made of metal, and C is an insulating film between the interdigitated finger electrodes e ₁ and e ₂ . It is formed. The operating frequency x of the circuit formed by the intersecting finger electrodes e ₁ and e ₂ of this structure is the speed V ₁ of the intersecting finger electrode part made of metal.
From the average velocity V ₃ of the velocity V ₂ of the insulating film portion and the wavelength λ, it can be expressed as x=Vs/λ.

Figure 3 shows a conventional structure of Al formed on the piezoelectric substrate 1 using ST cut
The velocity dispersion characteristics (changes in operating frequency) due to the electrodes are shown by the solid line α. The 〇 marks in the figure are measured values. In this characteristic, the mass of the electrode metal and the anti-piezoelectric effect contribute to lowering the operating frequency from the frequency determined by the speed of the substrate itself and the pitch of the electrodes.

The two-dot chain line in Fig. 3 shows the velocity determined from the velocity of the aluminum oxide film (Al ₂ O ₃ ) produced by anodization as the insulating film C on the ST cut X propagation crystal, which is the piezoelectric substrate 1. Dispersion characteristics (change in operating frequency)
It is β. From this speed dispersion characteristic β, it can be seen that the aluminum oxide film (Al ₂ O ₃ ) as the insulating film C has the effect of increasing the speed of the substrate.

Therefore, the velocity dispersion characteristic α due to the Al electrode and
By combining the velocity dispersion characteristic β of the Al ₂ O ₃ insulating film (Fig. 2), it is possible to construct a circuit without velocity dispersion like γ.

In Fig. 3, the x mark is the actual measured value when kH = 0.067 with the configuration shown in Fig. 2, but the characteristic γ without velocity dispersion is
It can be seen that the value is very close to .

From the above, it can be seen that even when many surface acoustic wave circuits are formed on one substrate, by using the configuration shown in FIG. 2, it is possible to save frequency fluctuations due to fluctuations in electrode film thickness.

Al− on the ST Cut X Denpan crystal used in the experiment
In the combination of Al ₂ O ₃ , the velocity dispersion characteristics α and β were almost symmetrical with respect to γ, but when the slope of β is small, the slope of β can be increased by increasing the thickness of the insulating film. If it is larger than , it can be handled by making the insulating film thinner than the electrode metal film. In this case, the kH of the metal electrode and the kH of the insulator are not made equal, but are made equal to the kH of the insulating film, which has a magnitude of frequency change that matches the dispersion characteristics of the metal electrode.

Note that d in Fig. 3 is the shoot effect of the electrode metal;
In other words, the anti-piezoelectric effect (the amount is 1/4 of the coupling coefficient k ² )
This amount is not caused by the insulator inserted between the crossed finger electrodes.

Note that the formation of the insulating film is not limited to the above embodiment, and an insulating material may be added to the insulating portion between the interdigital electrodes by a method such as vapor deposition or sputtering.

As explained above, the present invention makes the velocity dispersion characteristic of the interdigitated finger electrodes of the conventional configuration give the insulator inserted between the intersected finger electrodes a dispersion characteristic opposite to the velocity dispersion characteristic of the intersected finger electrodes. As a result, a surface acoustic wave device with no velocity dispersion can be constructed for the electrode as a whole. Therefore, it is possible to eliminate the difference between phase delay and group delay, and it is possible to provide a high-quality delay medium. Furthermore, if the surface acoustic wave velocity and electromechanical coupling coefficient of a single substrate are given, it becomes easy to design an electrode that obtains a desired operating frequency, and it can be realized with high precision. Again, there are many advantages such as being able to match the operating frequency even for samples with different film thicknesses.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view of an interdigitated finger electrode portion of a conventional surface acoustic wave device, FIG. 2 is a cross-sectional view showing an example of an interdigitated finger electrode portion of a non-dispersive surface acoustic wave device according to the present invention, and FIG.
The figure is a curve diagram showing an example of the velocity dispersion characteristics of a surface acoustic wave device using Al and Al ₂ O ₃ for interdigital electrodes according to the present invention. e ₁ , e ₂ ... interdigitated finger electrode, C ... insulating film, 1 ... piezoelectric base, λ ... wavelength, T ... pitch, V ₀ ... speed of piezoelectric base itself, V ₁ ... speed when electrode metal falls, _V2
...Velocity of the substrate when the insulator is stretched, d...Amount of frequency reduction due to anti-piezoelectric effect, α...Dispersion characteristics due to Al electrode, β...Dispersion characteristics due to Al ₂ O ₃ electrode, 〇...Actual measurement value using conventional electrode, ×...actual value measured by Al−Al ₂ O ₃ electrode, γ...dispersion property to be sought.

Claims (1)

[Claims] 1. A surface acoustic wave device in which intersecting finger electrodes are provided on a piezoelectric substrate made of ST cut An insulator film made of Al ₂ O ₃ having a velocity dispersion characteristic opposite to that of the interdigitated finger electrodes is provided in the region between the electrode fingers, and the velocity dispersion characteristic of the insulator film is equal to the velocity dispersion characteristic of the interdigitated finger electrodes. A surface acoustic wave device characterized in that the thickness of the insulating film is adjusted so as to be symmetrical with the velocity dispersion characteristic of the index finger electrode, and the device as a whole is configured so that there is no velocity dispersion.