JPH03165116A - Surface acoustic wave device - Google Patents

Abstract

PURPOSE:To suppress bulk acoustic wave resonance by providing strip shape slots to the rear side opposite to the output electrode of a piezoelectric substrate and specifying the depth, width and pitch of the slots to distribute the in-phase component and the reverse phase component of a reflected wave on the rear side nearly equal. CONSTITUTION:A couple of comb-shaped electrodes 2, 2' converting an electric signal into a surface acoustic wave are formed to one major side of a piezoelectric substrate 1, and an output electrode 3 is formed to a position clipped by the couple of comb-shaped electrodes. Then strip shape slots 6 whose depth is (d), whose width p1 and whose pitch is p2 are provided onto other side of the piezoelectric substrate 1 at least opposite to the output electrode 3 in a way of nearly satisfying the relations of d=vb/4.f0 and p1=p2. Thus, the in- phase component and the reverse phase component of a reflected wave on the rear side are almost equally distributed, then bulk acoustic wave resonance caused between the output gate electrode and the rear side of the substrate is suppressed and the frequency characteristic and the spurious characteristic are improved.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a surface acoustic wave device, and in particular to an improvement of a surface acoustic wave convolver used in a spread spectrum communication system.

[Summary of the Invention] The present invention is a surface acoustic wave convolver in which bulk wave resonance occurring between an output gate electrode and the back surface of a substrate is suppressed by means that are easy to design, and frequency characteristics and spurious characteristics are improved. It is something.

[Prior Art] Surface acoustic wave (SAW) devices are currently undergoing a wide range of applications and research and development, and in particular, SAW convolvers with real-time convolution and integration functions are expected to be used as next-generation communication systems for consumer use, for example. spread spectrum (SS)
It is attracting attention as a major device in demodulators for communication systems.

In this SS communication system, a carrier wave carrying information is further spread over a wide frequency band by a pseudo-noise code (PN code), so the SAW convolver is required to have wide-band frequency characteristics.

-[- SAW convolvers are roughly divided into three types based on their structure: (1) elastic convolvers that utilize elastic nonlinearity; It is known as an air-gap type convolver, and a monolithic type convolver made by laminating a piezoelectric thin film on a semiconductor substrate1.

Basically, as shown in FIG. 5, two input comb-shaped electrodes (ID"f") 2°2'
and an output gate electrode 3 formed therebetween.
In the convolution integral operation, two signals are converted into SAW by [DT2.2'', propagate in the interaction region under the output gate electrode in opposite directions, and are multiplied using elastic nonlinearity or semiconductor nonlinearity. Gate '11!
This is done by integrating by the poles.

[Problem to be solved by the invention] By the way, among the three types of SAW convolvers mentioned above,
The air gap type allows for easy control of the minute gap between the piezoelectric body and the semiconductor, and other elastic types and monolithic types are advantageous in terms of practical application, but in both of these types, problems occur at the output gate fli pole. The integrated output depends uniformly on the electrode surface, and therefore, as shown in FIG. 6, a bulk wave is generated in the thickness direction of the substrate 1, just as if an alternating current signal was applied to the piezoelectric resonator. It resonates at a frequency that depends on the sound speed and thickness of the substrate. As a result,
As shown in the graph of FIG. 7, dips occur at equal intervals on the frequency axis in the frequency characteristic 4 of the output signal of the SAW convolver. This is a cause of narrowing the bandwidth of the SAW convolver or deteriorating the ratio of the desired output to unnecessary output (spurious ratio) when correlation operation is performed.

Deterioration of characteristics due to the generation of bulk waves causes problems such as a decrease in resolution in VIFSAW filters used in television receivers, so as shown in Fig. As a countermeasure, we roughen the surface by applying

However, because the depth and pitch of the grooves are random, there are problems with the reproducibility of the effect, or even when the depth and pitch of the grooves are specified, the purpose is to diffusely reflect the bulk wave 5 and suppress the response. Therefore, the necessary depth and pitch values for the frequency band used must be optimized through repeated experiments.

In addition, in the first place, these conventional measures require two human output IDs.
This is for the bulk wave propagation response between T, and resonance in the thickness direction has not been considered.

[Object of the Invention] The present invention provides an easy-to-design means for suppressing thickness resonance of a bulk wave whose source is an integral output generated at an output gate and improving frequency characteristics and spurious ratio in a SAW convolver. It is intended to provide.

[Means for Solving the Problems] In order to achieve the above-mentioned [1], the surface acoustic wave device according to the present invention has a pair of comb shapes on one main surface of a piezoelectric substrate for converting electrical signals into bony surface waves. While forming the electrode, the front side d
An output electrode is formed at a position sandwiched between a pair of comb-shaped electrodes,
On the other side of the piezoelectric substrate, at least at a position facing the output electrode, a depth d, a width P1, a distance P2, an output, a center frequency of No. 1 is f0, a thickness of the piezoelectric substrate t, and a bulk in the substrate. When the sound speed of the wave is ■b, d=vb /4 ・f
The gist is that a strip-shaped groove that substantially satisfies o *P 1 = P 2 is provided.

[Function] In the surface acoustic wave device having the above configuration, a strip-shaped groove is provided on the back surface of the piezoelectric substrate facing the output electrode, and by defining the depth, width, and interval of the groove, the reflected waves on the back surface are in phase. The component and the anti-phase component are distributed almost equally, and the bulk wave resonance is suppressed.

[Embodiment] FIG. 1 shows the main parts of a surface acoustic wave convolver according to an embodiment of the present invention.

In the figure, 1 is a piezoelectric substrate, 3 is an output gate electrode, and a strip-shaped groove 6 is provided on the back surface of the substrate facing the output gate electrode 3. t is the thickness of the piezoelectric substrate, d is the groove depth - Px is the groove width, and p2 is the groove interval.

Bulk resonance in the thickness direction at the output gate electrode portion of the SAW convolver occurs at a frequency fbm that satisfies equation (1), where t is the thickness of the substrate and vb is the sound speed of the bulk wave.

2.m=1.2,3. ...This is because the potential is uniform within the plane of the output gate electrode, the back surface is almost flat, and there is a -V line to the output gate electrode, so below the gate electrode it can be regarded as almost a plane wave. This is because bulk waves are generated in the same phase within the plane, or are repeatedly reflected in the same phase on the front and back surfaces. Therefore, in order to suppress resonance, it is necessary to set the phase of the bulk wave at the time of generation or reflection to rung 11 in the plane direction of the interaction region, or to distribute the in-phase component and the anti-phase component almost equally. SAW
In a convolver, it is essential that the potential on the gate electrode be uniform, and the phase at the time of generation cannot be distributed. Therefore, the phase distribution must be given under the reflection conditions of the back surface, but a random distribution poses a problem in terms of the reliability of the suppression effect.

Therefore, in the present invention, specifically, as in the embodiment shown in FIG. This is achieved by defining the depth d.

The depth d of the groove 6 is defined by equation (2), where fo is the center frequency of the output frequency band of the SAW convolver.

As a result, the bulk wave reflected from the groove and the bulk wave reflected from the back surface of the original have a phase difference of approximately 180'' due to the path difference 2d. In order to cancel the waves, the area of the groove and the remaining part must be approximately equal, so (3)
A condition based on an expression is required.

P1=Pz...(3)
The graph in Fig. 2 shows an example of the frequency characteristics when the present invention is applied to a ZnO/Si monolithic SAW convolver.0 The characteristics when the means of the present invention are not applied are shown in Fig. 7. However, according to the embodiment of the present invention, as shown in 1-frequency characteristic 7, it is clearly recognized that the response due to bulk resonance within the band has been eliminated.

Thus, with the above conditions, the shape of the groove can be easily designed based on the thickness of the substrate, the sound velocity of the bulk wave, and the output signal band, and the effect is remarkable.

The piezoelectric substrate constituting the surface acoustic wave device Y♂ includes:
As shown in Figure 3, lithium silicon niobate (Si
) laminated structure etc. are used.

[Effects of the Invention As described above, according to the present invention, bulk wave resonance occurring between the output gate electrode and the back surface of the substrate in a surface acoustic wave convolver can be suppressed by an easy-to-design method. It is possible to suppress frequency characteristics and spurious characteristics.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a main part of a surface acoustic wave device showing an embodiment of the present invention, and FIG. 2 is a graph showing frequency characteristics of an output signal.
3 and 4 are side views of a surface acoustic wave device showing an embodiment of the present invention, FIG. 5 is a perspective view of a SAW convolver with a conventional structure, and FIG. 6 is a schematic diagram illustrating bulk wave resonance. Figure 7 is a graph showing the frequency characteristics of the output signal, Figure 8 is a graph showing the frequency characteristics of the output signal.
The figure is an explanatory diagram showing an example of conventional bulk wave response suppression. 1...Piezoelectric substrate, 2, 2'...
...Comb-shaped electrode, 3... Output gate electrode, 6... Groove.

Claims (1)

[Claims] A pair of comb-shaped electrodes for converting electrical signals into surface acoustic waves are formed on one main surface of the piezoelectric substrate, and an output electrode is formed at a position sandwiched between the pair of comb-shaped electrodes. On the other side, at least at a position facing the output electrode, a depth d, a width p_1, a distance p_2, a center frequency of the output signal as f_0, a thickness of the piezoelectric substrate as t, and a sound speed of the bulk wave in the substrate as vb. When, d=vb/4・f_0, p_
A surface acoustic wave device characterized by being provided with a strip-shaped groove that substantially satisfies 1=p_2.