JPS63184411A - Surface acoustic wave device - Google Patents

Abstract

PURPOSE:To prevent the titled device from reduction in Q in a no load state with small size by providing an electrode digit comprising the reflector of the titled device or the segment part tying the center of a groove end part with the prescribed angle to the perpendicular line in the digit or the groove lengthwise direction so as to prevent the energy of the surface wave from being leaked to the outside of a reflector. CONSTITUTION:An LiTaO3 single crystal substrate of Y propagation with power flow angle of Xcut-112 deg. is used for the substrate 1 formed with an electrode pattern. An exciting electrode 2 made of aluminum and a reflecting electrode 3 to clip it are formed on the substrate 1 as a pattern. Plural electrode digits 201 are short-circuited by a bus bar 202 to the electrode 2 to constitute a comb- tooth electrode 203 in crossing and plural electrode digits 301 are short-circuited to the reflecting electrode 3 by using a short-circuit bus bar 302. A part of the line segment tying electrode digits 201, 301 constituting the reflector of the surface acoustic wave device or mid points 204, 303 of the slot end has a prescribed angle with respect to the vertical line in the length direction of the slot to prevent the reduction of Q in a no load state.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a surface acoustic wave device particularly suitable for application to a surface acoustic wave resonator.

(Prior Art) Generally, in a surface acoustic wave device as a surface acoustic wave resonator, an excitation electrode formed by crossing a pair of Ia tooth electrodes is formed approximately at the center of a piezoelectric substrate. A pair of reflective electrodes are formed.

The comb-teeth electrode and reflective electrode are composed of a plurality of electrode fingers. Furthermore, since these electrode patterns are formed by photo-etching, which is a well-known technique, if there is a part surrounded by the electrode pattern on the piezoelectric substrate, the etching solution will accumulate during the photo-etching described above, which will have an adverse effect on the electrode pattern. It may be harmful. For this reason, each of the above-mentioned reflective electrodes has conventionally had a comb-teeth structure in which electrode fingers protrude from the short bus bar toward one side, thereby ensuring a flow path for the etching solution. That is, it has a structure in which only one side of a plurality of electrode fingers is short-circuited with a short bus bar.

By the way, in the surface acoustic wave resonator configured in this way, for example, one with a resonant frequency of 60 MHz or less is
In order to obtain the same level of no-load Q as that of 00 MHz or higher, conceivable methods include increasing the thickness of the electrode pattern and increasing the number of electrode fingers of the reflective electrode.

However, in the method of increasing the thickness of the electrode pattern, it is extremely difficult to increase the thickness extremely using the well-known photoetching technique described above. Therefore, in order to obtain a sufficient unloaded Q at a conventionally low resonant frequency, a method is used in which the thickness of the electrode pattern is made as thick as possible by photoetching and the number of electrode fingers of the reflective electrode is increased.

(Problems to be Solved by the Invention) By the way, the piezoelectric substrate power flow angle (Powe
For example, as a substrate where the r Flow Angle) is not 06 (zero degree), Li with Xcut-112°Y propagation
When a Ta0° single crystal substrate is used, as shown in Figure 7, the direction of the group velocity of the surface acoustic wave excited from the excitation electrode (
In other words, the energy flow (direction of energy flow) propagates on the piezoelectric substrate with a predetermined angle θ difference from the direction of phase velocity, which is perpendicular to the excitation electrode and the reflection electrode. Propagation on the piezoelectric substrate with a difference of a predetermined angle θ with respect to the X axis in this manner is called beam steering, and this angle θ is called a power flow angle.

By the way, an example of research on a substrate having this power flow angle is shown in Japanese Patent Laid-Open No. 51-89365. The technology described in this publication is a transversal type filter, and as shown in Figure 8 (numeral 11 indicates an excitation electrode and numeral 12 indicates a receiving electrode), beam steering is applied to a substrate with a power flow angle. This is a technique of configuring interdigital electrodes (excitation electrodes and reception electrodes) along the direction. That is, this technique has a structure in which the peak of the surface acoustic wave excited by the excitation electrode is always located at the center in the length direction of the interdigital electrode. Thereby, the surface acoustic wave excited by the excitation electrode is efficiently converted into an electric signal by the reception electrode.

Generally, in the case of a transversal type filter, it is desirable that all beams (propagation direction of the surface acoustic wave) of the surface acoustic waves excited by the excitation electrode reach the excitation electrode. However, if there is a shift in the positional relationship of interdigital electrodes formed on a substrate with a power flow angle, the receiving electrode will not be able to receive all the surface acoustic waves, and the insertion loss will increase. There was a problem that deterioration of characteristics such as distortion occurred, and desired characteristics could not be obtained.

By the way, the power flow angle of the surface acoustic wave resonator is 0''
There has been almost no research on substrates that do not have the same structure, and the present inventors have conducted extensive research in order to bring an invention to this field.

In other words, for surface acoustic waves where the power flow angle is not 0°, a surface acoustic wave resonator with a conventional structure leaks a portion of the surface acoustic wave (SAW) energy from a reflector, such as a reflective electrode, so Energy is not confined, leading to a decrease in the no-load Q of the resonator. Moreover, the surface acoustic waves leaking from the reflective electrode are reflected at the ends of the piezoelectric substrate and return into the reflective electrode, causing interference in the reflected waves and causing fluctuations in the no-load Q. To prevent this, it is possible to widen the electrode crossing width.

For example, LiTa0. of Xcut-1126Y propagation. The electrode crossing width of a surface acoustic wave resonator is constructed by forming electrodes on the substrate using an AQ film with a thickness of 2.3 tnn, the resonance frequency is 100 MHz, the number of electrode fingers of the reflective electrode is 250, and the number of pairs of excitation electrodes is 19. 0.103mn (3 wavelengths) and 0.2 respectively
40mn (7 wavelengths).

0.500nm (15 wavelengths), 1.030mn (3
0 wavelength), the unloaded Q is 7500.
It is improved to 14000.17500°20000. However, the above method results in a decrease in impedance and an increase in the size of the surface acoustic wave device, which is not a realistic method.

The present invention has been made in response to the above-mentioned circumstances, and an object of the present invention is to provide a surface acoustic wave device that is small and has a high no-load Q.

[Structure of the invention]

(Means for Solving the Problems) That is, in the surface acoustic wave device of the present invention, a line segment connecting the midpoints of the ends of the electrode fingers or grooves constituting the reflector extends in the longitudinal direction of the electrode fingers or grooves of the reflector. It is characterized by being formed at a predetermined angle with respect to the perpendicular line.

(Function) In the surface acoustic wave device of the present invention, the line segment connecting the midpoints of the electrode fingers or groove ends constituting the reflector is at a predetermined level with respect to the perpendicular line in the length direction of the electrode fingers or grooves of the reflector. Since the reflector has an angle, the phase front of the surface acoustic wave remains aligned with the electrode finger, and the energy of the surface acoustic wave does not leak to the outside of the reflector, thereby preventing a decrease in the no-load Q.

(Example) An embodiment of the present invention will be described below with reference to FIG.

In FIG. 1, the substrate (2) on which the electrode pattern is formed is a LiTaO3 single crystal substrate with Xcut-112°Y propagation and the power flow angle is not Oo. On the substrate, a pattern is formed of an excitation electrode (2) made of aluminum and a reflector, such as a grating reflection electrode (2), placed on both sides of the excitation electrode (2). This excitation electrode ■ has multiple electrode fingers (2
The comb-teeth electrodes (203) are constructed by short-circuiting the electrodes (01) with a bus bar (202) and intersect with each other. On the other hand, the grating reflective electrode (2) is constructed by short-circuiting a plurality of electrode fingers with a bus bar (302).

Now, in order to maintain a high no-load Q of a surface acoustic wave resonator, it is necessary for the surface wave guided inside the resonator to be in a complete waveguide mode, so that it is not excited by the excitation electrode ■. The surface waves continue to propagate without leaking out of the waveguide, making it possible to realize a resonator with a higher no-load Q. However, this is because some of the surface waves propagating in the conventional waveguide reach the bus A- (302) through the critical angle and leak out of the waveguide.

The critical angle mentioned here will be explained using FIG. 2. In the figure, velocity V is the propagation velocity of a surface wave when the substrate surface is a free plane (a surface on which no electrode film is deposited), and velocity V' is the velocity of the surface wave propagating within the waveguide. The condition for the surface wave of this velocity V' to satisfy the waveguide mode is that the surface wave s
This is when the surface wave is incident at an angle greater than or equal to the angle ψ that satisfies inψ = V'/V, and at this time, the surface wave is totally reflected at the bus bar and the waveguide mode is maintained. Further, when a surface wave is incident at an angle of less than ψ, the surface wave passes through the bus bar, is refracted, and leaks out of the waveguide, resulting in a low unloaded Q. The angle ψ at this time is called the critical angle, and in order to reduce it, the width of the electrode intersection is widened. Methods such as increasing the electrode film thickness (reducing V') are taken.

By the way, the propagation velocity V' of the surface wave propagating in the waveguide is
In general, it differs depending on the angle with respect to the phase propagation direction. That is, in FIG. 9, the propagation velocity V 11 of the wave propagating in the direction of φ1 and the propagation velocity V 1 of the wave propagating in the direction φ2.
Different from 2. However, if the substrate has a symmetrical crystal structure with respect to the propagation direction, the sound speed of the wave propagating with the angle φ□ and the sound velocity of the wave propagating with the angle −φ1 are both v'1.

However, in the case of an anisotropic substrate as shown in Figure 1 (a substrate where the power flow angle is not 00), a wave propagating at an angle φ1 with respect to the phase propagation direction and a wave propagating at an angle −φ1 The speed of the wave that propagates with the wave is different. In other words, Xcut
In a substrate having a power flow angle of approximately +1.5° with respect to the phase propagation direction, such as a LiTaO3 substrate with −112′ Y propagation, a wave propagating at an angle of φ is faster than a wave propagating at an angle of −φ1. In other words, as shown in Figure 10,
The wave with velocity v1 that is incident on the busbar at angle φ□ has angle φ2
It will be reflected by ■2. In other words, even if the incident wave at angle φ1 satisfies the critical angle, the incident angle φ2 of the reflected wave does not necessarily satisfy the critical angle, and this may distort the waveguide mode and reduce the no-load Q. be. In addition, since the state of reflection at the bus bar (boundary of the waveguide) is different on the left and right sides when viewed from the phase propagation direction, 1゛- widening the electrode crossing width and increasing the electrode winding thickness are necessary to improve the no-load Q. There is a drawback that incorporating design methods such as these does not work as expected.

In order to make the angles of the incident wave and the reflected wave equal, it is best to tilt the boundary of the waveguide with respect to the phase propagation direction as shown in FIG. To explain this inclination, for example, in Fig. 1, the midpoint (
303) is the electrode finger (303) of this reflective electrode (3).
301) has a predetermined angle O with respect to the vertical line (A) in the length direction. In addition, the electrode fingers (20
The line segment connecting the midpoint (204) of the end of 1) is at a predetermined angle θ with respect to the perpendicular (to) in the length direction of this electrode finger (201). is equal to the angle θ). As a result, the apparent critical angle at the boundary between the left and right waveguides as seen from the phase propagation direction becomes equal despite the difference in wave propagation speed, the waveguide mode is maintained, and a resonator with a higher unloaded Q is created. Not only is this possible, but design factors such as electrode crossing width and electrode film thickness can be easily manipulated.

Next, other embodiments of the present invention will be described with reference to FIGS. 3 to 6. FIG. 3 shows an embodiment in which only the reflective electrode (2) has a predetermined angle with respect to the perpendicular line. The device shown in FIG. 3 also has the same effect as the device shown in FIG.

FIG. 4 shows an embodiment in which the excitation electrode (2) and the reflective electrode (3) have a predetermined angle with respect to the perpendicular line, and the electrode fingers of both reflective electrodes (3) protrude in the same direction. In this way, the absence of a short busbar has the effect of eliminating etching deposits when etching electrodes.

FIG. 5 shows an embodiment in which the excitation electrode (2) and the reflective electrode (3) 1 have a predetermined angle with respect to the perpendicular line, and the electrode fingers of both reflective electrodes (3) protrude in opposite directions. be.

Therefore, the reflection of the surface acoustic wave propagating toward one reflective electrode (3) is symmetrical with the reflection of the surface acoustic wave propagating toward the other reflective electrode (2).

Therefore, there is no difference in the phase of the received surface acoustic waves, and there is an effect that the no-load Q becomes higher than that of FIG. 1.

What is shown in FIG. 6 is an embodiment of the structure shown in FIG. 5, in which the electrode fingers of the reflective electrode (2) protrude in the opposite direction. This provides the same effect as shown in FIG. 5.

In addition, the angle at which the reflective electrode, etc. has a predetermined angle with respect to the perpendicular line is the power flow angle (the power flow angle of the wave guided in the electrode pattern after forming the aluminum electrode).
If the surface acoustic wave device is equal to the flow angle (referred to as the flow angle), the reflection electrode is present in the excitation direction of the surface acoustic wave, so the no-load Q becomes higher than that of the surface acoustic wave device shown in FIG.

Note that when the surface acoustic wave propagates from the excitation electrode to the reflection electrode, it is desirable that these electrodes improve the power flow. However, even if there is a leakage of surface acoustic wave energy at the end of the reflective electrode (boundary between the reflective electrode and the substrate) due to a slight deviation in this angle, the no-load Q will only decrease slightly, and the waveform will be distorted. It is different from a transversal type filter in that it does not have any adverse effects such as the occurrence of. In other words, in the surface acoustic wave resonator, even if there is a leakage of surface acoustic wave energy, the no-load Q is only slightly lowered, and the effect of the present invention is hardly affected. The range of angles where this little effect occurs is LiTa0. For the substrate, it is sufficient that it is within the range of 1.5±1''.

Further, in the above embodiments, the reflective electrode made up of electrode fingers has been described, but it goes without saying that a surface acoustic wave resonator made of a reflector made up of grooves formed on the substrate surface can also be used. Furthermore, in the surface acoustic wave device of the present invention, at least a part of the line segment connecting the midpoints of the electrode fingers or groove ends constituting the reflector is in the longitudinal direction of the electrode fingers of the reflector, etc. It is clear that a certain angle with respect to the normal will achieve the desired effect of the invention.

〔Effect of the invention〕

By adopting the above configuration, the surface acoustic wave device of the present invention efficiently confines the energy of the surface acoustic wave excited from the excitation electrode within the reflector, so that the no-load Q becomes higher.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing one embodiment of the surface acoustic wave device of the present invention, FIG. 2 is a schematic diagram showing the critical angle, and FIGS. 3 to 6 are other embodiments of the surface acoustic wave device of the present invention. A schematic diagram showing an example, FIGS. 7 and 8 are schematic diagrams for explaining a conventional surface acoustic wave device, and FIG. 9 is a diagram showing the relationship between sound speed and propagation angle when the substrate is an isotropic body. , FIG. 10 is a schematic diagram showing the opposite state of waves propagating on the surface of an anisotropic body at the waveguide boundary. ■... Piezoelectric substrate ■... Excitation electrode ■... Reflective electrode (201), (301)... Electrode finger (204), (303)... Midpoint Figure 3 Figure 4 Figure 5 Figure 6 Figure 7

Claims (3)

[Claims] (1) A substrate that propagates surface acoustic waves, an excitation electrode formed by intersecting comb-shaped electrodes consisting of multiple electrode fingers formed on one main surface of this substrate, and an excitation electrode formed on one main surface of this substrate. and a reflector configured of a plurality of electrode fingers or grooves that reflect surface acoustic waves excited from the excitation electrode, at least the ends of the electrode fingers or grooves constituting the reflector. A surface acoustic wave device, wherein at least a portion of a line segment connecting midpoints of the reflector has a predetermined angle with respect to a perpendicular to a longitudinal direction of the electrode fingers or grooves of the reflector. (2) At least a portion of the line segment connecting the midpoints of the electrode finger ends constituting the excitation electrode has a predetermined angle with respect to the perpendicular to the longitudinal direction of the electrode fingers or grooves constituting the reflector. A surface acoustic wave device according to claim 1, characterized in that: (3) The surface acoustic wave device according to any one of claims 1 to 2, wherein the predetermined angle is in the same direction as a power flow angle of the substrate.