JPS61208306A - Surface acoustic wave resonator - Google Patents

Abstract

PURPOSE:To attain ease of design and manufacture of a high Q SAW resonator by forming most of reflecting electrodes arranged in a reed screen form of grating electrodes of a reflector type SAW resonator with the same width and pitch and increasing the remaining prescribed reflection electrode width or pitch than the others. CONSTITUTION:The width of two among lots of reflector electrodes 9, 9 of the reed screen shape in a couple of reflectors 7 is selected to d2 and the width of the other most electrodes is selected to d1 which is smaller than the d2 to form the reflector electrodes 9, 9 at a prescribed pitch P3. Thus, denoting the electrodes from the inside of the electrode 9 to the electrode 9' in a pitch d2 as the 1st electrode group M1, the electrodes from the electrode 9' to the electrode 9'' by a pitch d2 as the 2nd electrode group M2, and the remaining electrodes as the 3rd electrode group M3, then the wave propagating speed in the 1st-3rd electrode groups M1-M3 is the fastest at the 1st electrode group M1 and the slowest at the 3rd electrode group M3 because of the speed reduction at the reflecting electrodes 9', 9'' by the width d2.

Description

[Detailed description of the invention] One joza and one bean! This invention is a surface acoustic wave resonator used as an oscillator in various communication devices.
This invention relates to a reflector-type surface acoustic wave resonator with grating reflectors arranged on both sides.

A reflector-type surface acoustic wave resonator generally has a structure □ in which several ten pairs of IDTs are placed in the center of a crystal piezoelectric substrate, and several hundred pairs of grating reflectors are placed on both sides of the IDT. An example of the conventional basic structure will be explained below with reference to FIGS. 5 and 6.

In the figure, (1) is a crystal substrate which is a piezoelectric substrate, (2)
is the IDT formed in the center of the crystal substrate (1); (
3), (3) is a reflector for surface acoustic waves (SAW) excited by the IDT (2), which includes reflective electrodes (4) formed on the crystal substrate (1) on both sides of the IDT (2), (4).

When a pulse voltage is applied to the IDT (2) in this SAW resonator, the IDT is activated due to the piezoelectric effect.

Distortion occurs on the substrate surface between the pair of electrodes in (2), and a surface acoustic wave is excited, and this surface wave is transmitted to the reflectors (3) and (3).
) side and is reflected by the reflective electrodes (4), (4). If the wavelength of this surface wave is matched with the electrode pitch P1 of the reflective electrodes (4), (4), the surface wave will be reflected efficiently and a standing wave will be generated, with a frequency that matches the electrode pitch P2 of the IDT (2). Only the standing wave is received by the IDT (2) and extracted as a resonant frequency signal specific to the SAW resonator.

Here, the frequency-radiation conductance characteristic of the surface wave excited by the IDT (2) of the SAW resonator rises sharply at the center frequency f1, as shown by curve a in Figure 7, and the reflector (3), (3) The frequency-reflectance characteristic of the reflected wave from the laser beam has a predetermined bandwidth as shown by the curve in FIG. This bandwidth is called a stop band, and it is necessary for the center frequency f1 of the IDT (2) to fall within this stop band D to increase the Q of the SAW resonator. By matching f2 with the center frequency f1 of the IDT (2), the waves are completely reflected by the two-wavelengths (3) and (3), and the SAW vibration energy is confined on the surface of the crystal substrate (1) to minimize vibration loss. It is necessary to reduce the amount.

By the way, the resonant frequency f of the surface waves and reflected waves in the IDT (2) and the reflectors (3) and (3) is f='A (however, ■
is given by the propagation speed of the wave, λ is the wavelength), the wavelength λ is determined by the pinch between the electrodes P, , P2, and the propagation speed
It is largely determined by two factors: the periodic perturbation effect, in which the wave propagates faster in certain parts (the electrode) and not slower in some parts, and the reflection effect, in which the speed decreases when the wave is reflected by the metal (electrode). Here IDT(2
) is determined by the periodic perturbation effect and the reflection effect, and the wave propagation speed in the reflectors (3) and (3) is almost determined only by the reflection effect, so the IDT (2) and the reflector (
3) Thickness, material, pitch p of both electrodes in (3),
, p2 are made the same, a difference naturally occurs in the wave propagation speed between the two, and the center frequency f! , f2 cannot be matched, and the Q of the SAW resonator decreases.

Therefore, conventionally, for example, IDT (2) and reflectors (3), (3)
) between each electrode pitch P violet, P2 is their center frequency f!
, [2 are calculated and made different so that they match.

溌" (°゛- Aluminum is optimal for the electrode metal of the 3AW resonator because it does not disturb the resonance characteristics and can obtain a high Q, and the above ID
T(2) and reflectors (3), (3) are usually formed of a vapor-deposited aluminum film. However, due to the limits of processing accuracy of the mask used in aluminum vapor deposition, it is difficult to match the width and spacing of the vapor deposited film to the calculated values. In particular, when the resonant frequency exceeds several hundred MHz, the magnitude of the difference between the calculated value and the actual design value of the repetition pitch of the IDT and grating reflector becomes a problem. In other words, the center frequency of the reflector and the resonant frequency of the IDT no longer match. For this reason, the resonant frequency of the IDT may shift to a frequency portion with a relatively low reflectance within the stop band of the reflector, or may even deviate from the stop band.

As a solution to this problem, for example, the pitch of several hundred reflective electrodes arranged in a comb-like pattern on the reflector can be gradually increased in μm units to increase the wave propagation speed of the reflector, and the center frequency of the IDT and the reflector can be increased. It is known to bring the However, in this case as well, it was difficult to manufacture the electrodes according to the calculated values due to manufacturing limitations of the electrodes.

The present invention has been made in view of the above-mentioned problems and has solved the problem by forming most of the reflecting electrodes arranged in the interdigital shape of the grating electrodes of the reflector-type SAW co-layer with the same width and pitch. , is characterized in that it is formed with a predetermined remaining reflective electrode width or pitch between reflective electrodes larger than others.

Tatsuri When the electrode pattern of the reflector is set as described above in the present invention, the size, number, and position of the electrodes with a larger width than other electrodes or pitches at the selected positions of the interdigital reflective electrodes in this electrode pattern can be set. The center frequency is determined and the ID
It becomes easy to match the center frequency of T in terms of manufacturing.

An embodiment of the present invention will be described based on FIGS. 1 to 3. In the figures, (5) is a piezoelectric substrate, (6) is an IDT formed on the piezoelectric substrate (5), and (7) is an IDT formed on the piezoelectric substrate (5). ), (7) are IDT
Grating reflector formed on both sides of (6), consisting of reflective electrodes (9), (9).

The feature of this embodiment is that, for example, the width of two of the reflective electrodes (9), (9), etc. of the pair of reflectors (7) is set to d2, and most of the other widths are set to d2. width d smaller than d2
1, and the reflective electrodes (9), (9) - are formed at a constant pitch P3. From the inside of one reflective electrode (9) to the first @d2 reflective electrode (9") is the first electrode group M, and from this reflective electrode (9°) to the next reflective electrode (9") with width d2. is the second electrode group M2 and the rest is the third electrode group M3, the wave propagation speed in each of the first to third electrode groups M1 to M3 is the reflective electrode (9゛) and (9'') with width d2. Due to the relationship that causes a decrease in speed when propagating, the speed is the highest at the first electrode group M1 (and the slowest at the third electrode group M3.From this relationship of propagation speed, the first to third electrode groups M1 to
The frequency-reflectance characteristics of the reflector using M3 alone are shown, for example, by curves c1, c2, and C3 in FIG.

In other words, the first electrode group M! The center frequency f3 when alone is the highest, the second electrode group Mz INN standby center frequency f4 is intermediate, and the center frequency f5 of the third electrode group M3 is the smallest, and the first to third electrode groups M1 to M3. reflector (
The frequency-reflectance characteristic of 7) is curve C4 in FIG.

This corresponds to the combination of the above three curves CI, c2, and c.

Therefore, the stop band D' of the reflector (7) can be set large, and if f4 is designed to be close to to, f3
From the relationship >r4>f5, it can be seen that the center frequency fo of the IDT (6) always falls within the stop band D' of the reflector (7), and furthermore, the center frequency of the stop band D' of the reflector (7) f6 is 'II d z reflective electrode (9°),
By calculating the width, number, and position of (9°°), it can be matched with fo. In addition, a reflective electrode (9゛) with a width d2, (9°
) can be approximately several times the width of the pond, so the electrode can be easily formed according to the above calculation.

Fig. 4 shows an embodiment of the present invention.The feature of this is the reflectors (10), (10) on both sides of the IDT (6). (12) ---
The method is to arrange a large number of holes at the same pitch P4 in the form of a blind, and to form only a part of the pitch P5 several times larger than the pond. In this way, the wave propagation speed does not decrease at the pitch P5, and the wave propagation speed in one reflector (10) differs depending on the position as in the above embodiment, producing the same effect.

As described above, according to the present invention, it is easy to match the center frequencies of the IDT and the reflector both in terms of calculation and manufacturing.
Furthermore, it becomes easier to widen the stop band of the reflector, and therefore it becomes easier to design and manufacture a high-Q SAW resonator.

[Brief explanation of the drawing]

1 and 2 are a plan view and a sectional view taken along the line A-A showing one embodiment of the present invention, FIG. 3 is a frequency characteristic diagram of the SAW resonator shown in FIG. 1, and FIG. 4 is a diagram showing an embodiment of the present invention. It is a top view showing an example of. 5 and 6 are a plan view and a sectional view taken along line B--B of a conventional SAW resonator, and FIG. 7 is a frequency characteristic diagram of the SAW resonator shown in FIG. 5. (5)---Piezoelectric substrate, (6)---IDT, (7)
...Reflector, (9) (9°) (9”°) ---
Reflecting electrode, (10) --Reflector, (12) -Reflecting electrode. "Figure 8, Figure 5, Figure 6, Figure 7, Enkosan--

Claims (1)

[Claims] (1) In a resonator in which an IDT and a grating reflector are arranged on both sides of the IDT on a piezoelectric substrate, a large number of reflective electrodes arranged in a comb shape in the grating reflector are formed with most of them having the same width and pitch, 1. A surface acoustic wave resonator characterized in that a predetermined remaining reflective electrode width or pitch between electrodes is formed larger than others.