JPS62254509A - Surface acoustic wave element - Google Patents

Abstract

PURPOSE:To improve the selectivity of a single frequency by setting a maximum cross width of an interdigital electrode on a specific piezoelectric substrate so as to give a specific value with respect to a wavelength thereby improving the no load Q. CONSTITUTION:At least a couple of interdigital electrodes are provided on a piezoelectric substrate of shear horizontal type made of a 30-50 deg. rotation Y axis-cut LiNbO3 single crystal on which a surface acoustic wave is propagated and the maximum cross width WMAX of the interdigital electrode is selected to be 2<=WMAX/lambdaIDT<=10 with respect to the wavelength lambdaIDT. In setting the maximum cross width WMAX of the interdigital electrode to the said range, the no load Q is increased and the selectivity of a single frequency is improved. Thus, in applying the method to, e.g., a surface acoustic wave resonator for VCO, the C/N ratio as the VCO is improved. In order to the higher no load Q, it is preferred that the maximum cross width WMAX of the interdigital electrode is set to be 2<=WMAX/lambdaIDT<=8.

Description

Detailed Description of the Invention [Technical Field] The present invention relates to a resonator, a filter, a delay line, etc. having a reflector made of a metal strip, an interdigital electrode, etc. on a piezoelectric substrate on which a shear horizontal type surface acoustic wave propagates. between surface acoustic wave elements.

[Prior art and its problems] Surface acoustic wave elements have traditionally been used for special purposes in military applications, but in recent years they have begun to be used in civilian equipment such as FM tuners and TVs, and have suddenly come into the spotlight. Specifically, surface acoustic wave elements that have become popular include delay elements, oscillators,
It has been commercialized as a filter, etc. The characteristics of these various surface acoustic wave devices are that they are small, lightweight, and highly reliable, and that their manufacturing process is similar to that of integrated circuits, making them suitable for mass production. Nowadays, it is mass-produced as an indispensable electronic component.

There are various types of surface acoustic waves that propagate on the surface of piezoelectric media, but
Generally used is Raylei (Raylei9
h) It is called a wave. Incidentally, there are a coupling coefficient and a temperature coefficient as indicators for evaluating the performance of a piezoelectric substrate. The coupling coefficient is an index representing the efficiency with which electrical energy is converted into vibration energy, and the temperature coefficient is an index representing the temperature coefficient of propagation delay time of a surface acoustic wave propagating through a piezoelectric medium. Furthermore, surface acoustic waves include a shear horizontal type surface acoustic wave in which particles are displaced in a direction perpendicular to the direction in which the surface acoustic waves propagate within the surface layer of the piezoelectric substrate through which the surface acoustic waves propagate, and the coupling coefficient is It is starting to attract attention due to its large size.

As an example of a conventional surface acoustic wave element having a reflector using a metal strip, an example of a surface acoustic wave resonator is shown in FIGS. 2 and M3.

In other words, this surface acoustic wave resonator has a piezoelectric substrate 1 on which surface acoustic waves propagate, an interdigital electrode 2 for excitation of surface acoustic waves, and a large number of metal strips arranged periodically at right angles to the propagation direction of the surface acoustic waves. It is constructed by forming reflectors arranged at 3.3 degrees. When a voltage of a specific frequency is applied to the interdigital electrodes 2, the piezoelectric substrate 1 in the gap between the interdigital electrodes 2
When an electric field is applied to the surface, a strain proportional to the voltage is generated due to the piezoelectricity of the piezoelectric substrate 1, and the strain propagates to both sides as a surface wave at a sound speed determined by the material of the piezoelectric substrate 1.

This surface wave is reflected by the grid-like reflectors 3.3 degrees on both sides, returns to the interdigital electrode 2 again, and resonates.

In such a surface acoustic wave element, the higher the unloaded Q (quality factor) expressed by a single element, the better the single frequency selectivity.

Conventionally, in piezoelectric substrates such as crystal, interdigital electrodes (I
Although the relationship between the intersection width of DT) and the no-load Q of the resonant element has been investigated experimentally and theoretically, there has been no report on a piezoelectric substrate made of LiNbO3 rotated by 30 to 50 degrees and cut on the Y axis.

“Object of the Invention” The object of the present invention is to provide a Y-axis cut
In a surface acoustic wave device using a piezoelectric substrate on which a shear horizontal type surface acoustic wave made of an O3 single crystal propagates,
The purpose is to increase the unloaded Q and increase the selectivity of a single frequency.

"Structure of the Invention" The surface acoustic wave element of the present invention includes at least one set of interdigital electrodes on a piezoelectric substrate on which a shear horizontal surface acoustic wave propagates, which is made of a LiNbOi single crystal rotated by 30 to 50 degrees and cut on the Y axis. and a maximum crossing width W of the interdigital electrodes.
MAX is 2≦Lmyo/λIII for wavelength λIOT
It is characterized in that it is set so that T≦10.

In this way, in the present invention, the maximum crossing width W of the interdigital electrodes is
Since MAX is set within the above range, the unloaded Q can be increased to increase single frequency selectivity. Therefore, when applied to a surface acoustic wave resonator for vCO, for example, the C/N ratio as vCO can be improved.

In the above, if W□8/λ1° is less than 2 or more than 10, a sufficiently high unnegative mQ cannot be obtained. In general, in the present invention, a higher
In order to obtain the load Q, 2≦WM□/in. It is preferable to set the maximum crossing width VIMAX of the interdigital electrodes so that , ≦8.

Note that the present invention can be applied to a surface acoustic wave device having reflectors on both sides of the interdigital electrode, and can also be applied to a surface acoustic wave device having only the interdigital electrode without a reflector.

Embodiments of the Invention FIG. 1 shows an interdigital electrode portion of a surface acoustic wave device according to the present invention. That is, in the present invention, the maximum crossing width WMAX of the interdigital electrode 2 is set so that 2≦L, /λ1°a≦10 with respect to the wavelength λ, 0□.

Example 1 Using a 41 degree rotated Y-axis cut LiNbO3 single crystal as a substrate, an AI film with a thickness of +oooA, interdigital electrodes 2 with a logarithm of 10 pairs, and 200 reflectors 3.3゛ on each side. A 450 MHz band one-boat type surface acoustic wave resonator was fabricated. For the above-mentioned surface acoustic wave resonator, the maximum crossing width 'LAx! The impedance characteristics of each surface acoustic wave resonator were measured with various changes and various acids, and the no-load Q was calculated. The relationship between W□X/λIOf and no-load Q obtained in this way is shown in FIG.

From Figure 4, 1 of W□XZλ1゜T is less than 10 and Q
The improvement in W, , /λ, is remarkable. It can be seen that when the value of □ is less than 2, Q deteriorates. It can be seen that in order to obtain a sufficiently high value of Q, it is preferable to set 2≦WMAX/λIOf≦8.

Example 2 A 41 degree rotated Y-axis cut L+NbO3 single crystal was used as a substrate, and a 100 OA thick ^l film was used to form interdigital electrodes 2 with 50 pairs of logarithms to generate a 450 MHz band 1-boat type surface acoustic wave. Created a resonator. Then, with respect to the above-mentioned surface acoustic wave resonator 1, the maximum crossing width WMA of the interdigital electrode 2 is
Various surface acoustic wave resonators were created with various X values, and the impedance characteristics of each surface acoustic wave resonator were measured to calculate the no-load Q.

Thus obtained W, , /λ,. When the relationship between A and no-load Q was determined, almost the same results as in Example 1 were obtained.

"Effects of the Invention" As explained above, according to the present invention, the maximum crossing width WMAX of the interdigital electrodes is 2≦W□X for the wavelength λ1°7.
Since it is set so that /λ1°7≦10, the no-load Q can be increased and the selectivity of a single frequency can be improved. Therefore, for example, when applied to a surface acoustic wave resonator for vCO, the C7N ratio as vCO can be improved.

[Brief explanation of drawings]

FIG. 1 is a partially enlarged plan view showing a sagging electrode portion of a surface acoustic wave device according to the present invention, FIG. 2 is a plan view showing an example of a conventional surface acoustic wave device, and FIG. 3 is a plan view of the same surface acoustic wave device. The cross-sectional view and FIG. 4 show W□8/λ when the present invention is applied to a surface acoustic wave resonator. It is a chart showing the relationship between □ and no-load Q. In the figure, 1 is the piezoelectric substrate, 2.2″ is the interdigital electrode, 3.3
゛ is a reflector. Individual IDT-1 Figure 1 Figure 2 WMAX/Input IDT Figure 4

Claims (2)

[Claims] (1) In a surface acoustic wave element comprising at least one set of interdigital electrodes on a piezoelectric substrate on which shear-horizontal surface acoustic waves made of LiNbO_3 single crystal rotated by 30 to 50 degrees and cut on the Y axis, the interdigital The maximum crossing width W_M_A_X of the shape electrode is 2≦ with respect to the wavelength λ_I_D_T.
A surface acoustic wave element characterized in that W_m_a_x/λ_I_D_T≦10. (2) The surface acoustic wave element according to claim 1, wherein the maximum crossing width W_M_A_X of the interdigital electrodes is set such that 2≦W_m_a_x/λ_I_D_T≦8 with respect to the wavelength λ_I_D_T.