JPH0337765B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a surface acoustic wave device designed to reduce the influence of surface acoustic waves due to mechanical reflection.

A surface acoustic wave device uses a piezoelectric single crystal material such as quartz, LiNbO ₃ (lithium niobate), a piezoelectric ceramic material, or a piezoelectric thin film material provided on a non-piezoelectric substrate, and is formed on the piezoelectric substrate. It is configured to use a transducer to convert an electric signal into a surface acoustic wave and propagate it on the surface of a substrate, and is being applied to various electronic components such as filters.

Figure 1 shows a filter as an example, in which 1 is a piezoelectric substrate, 2 is a pair of comb-shaped electrodes 2
An input transducer is formed by crossing A and 2B, and 3 is an output transducer by crossing a pair of comb-shaped electrodes 3A and 3B, and the electrical signal applied to the input terminal IN is transferred to the input transformer. It is converted into a surface acoustic wave by the transducer 2, propagates through the surface of the piezoelectric substrate 1 as shown by the arrow, and reaches the output transducer 3, whereupon it is converted back into an electrical signal and sent to the output terminal OUT. configured to be retrieved from. Here, the input transducer 2 and the output transducer 3
Each electrode width W and each electrode crossing interval L of each pair of comb-shaped electrodes 2A, 2B and 3A, 3B constituting the comb-shaped electrodes 2A, 2B and 3A, 3B are respectively λ, where λ ₀ is the wavelength of the center frequency ₀ of the surface acoustic wave used. It is designed to have a constant value of _0/4 and is formed as a so-called regular type electrode.

By the way, a transducer consisting of interdigitated electrodes designed with such dimensions causes mechanical reflections such as multiple reflections of surface acoustic waves, so-called TTE (triple transit echo), which deteriorates the transmission characteristics of filters, etc. was.

To prevent such mechanical reflections, the second
As shown in the figure, the intersecting comb portions (finger portions) of each pair of comb-shaped electrodes 2A, 2B and 3A, 3B are divided into two, and each electrode width W and interval L are designed to be λ ₀ /8. Electrode type transducers are known. According to this structure, the phases of the reflected waves at the ends of each electrode finger differ by 180 degrees and are in opposite phases, so that they are canceled out.

However, on the other hand, the higher the frequency used, the smaller the wavelength λ ₀ becomes, so high precision is required in the processing technology of the comb-shaped electrode. Since short circuits and electrode breaks between opposing electrodes are likely to occur, a decrease in manufacturing yield is inevitable.

In order to overcome these drawbacks, a single-phase transducer having a structure as shown in FIGS. 3a and 3b is known. This structure uses a piezoelectric substrate 1 consisting of an elastic substrate 4 and a piezoelectric thin film 5 provided thereon, and an electrode corresponding to one of the interdigitated electrodes is provided on the elastic substrate 1 as a lower electrode 6. , the lower electrode 6 is disposed on the piezoelectric thin film 5.
The width W and spacing L of each electrode at a position opposite to λ/
Upper electrodes 7A and 7B corresponding to the other comb-shaped electrodes designed in 2 are provided.

According to this structure, a signal source 8 is connected between the upper electrode 7A and the lower electrode 6 to propagate surface acoustic waves, and an electrode signal is extracted from the end of the load 9 between the upper electrode 7B and the lower electrode 6, so that the counter electrode It is possible to prevent short circuits between the two and to reduce the accuracy of processing techniques, thereby improving manufacturing yield.

However, since this structure cannot be driven by balanced power feeding by the signal source 8, the influence of electrical signals that propagate as direct waves without passing through surface acoustic waves (so-called feedthrough) is not canceled out, and the passing characteristics deteriorate. .

In order to further eliminate this drawback, a structure has been proposed in which two single-phase transducers are arranged facing each other in parallel, as shown in FIG. This structure includes the upper electrode 7A,
The other upper electrodes, comb-shaped electrodes 8A and 8B, are shifted by λ ₀ /2 and provided with a space a interposed therebetween, so as to face the upper electrodes 7A and 8A.
By adding electrical signals that are 180° out of phase with each other between the two, it is possible to drive with balanced power supply, thereby reducing the effects of feedthrough.

However, even with this structure, problems cannot be avoided, and due to the existence of the space a provided to prevent short circuit between the opposing electrodes, each electrode 7A, 8
There is a drawback that disturbance occurs in the wavefront of the combined surface acoustic waves S ₁ and S ₂ excited from A.

The present invention has been made in response to the above problems, and includes a piezoelectric substrate having a piezoelectric thin film provided on an elastic substrate, and a first electrode formed on a single phase transducer on the piezoelectric thin film. , a second electrode formed on a single phase transducer is provided on the elastic substrate, and a third electrode is provided in a planar shape within the piezoelectric thin film,
It is an object of the present invention to provide a surface acoustic wave device in which the conventional drawbacks are eliminated by arranging the electrode fingers of the first electrode and the electrode fingers of the second electrode in a vertically overlapping position. Embodiments of the present invention will be described below with reference to the drawings.

5a and 5b are a top view and a sectional view showing a surface acoustic wave device according to an embodiment of the present invention, in which an elastic substrate 4
A piezoelectric substrate 1 having a piezoelectric thin film 5 provided thereon has comb-shaped electrodes 10A and 10B on the piezoelectric thin film 5.
A first electrode consisting of a
The electrode width W and spacing L of 1B are designed to be approximately λ ₀ /2.

Further, a third electrode 1 is provided in the piezoelectric thin film 5 in a planar manner.
2, and the third electrode 12 is formed between the first electrodes 10A, 10B and the second electrodes 11A, 11B at a position where it is approximately equally spaced from both electrodes.

In the above configuration, when the signal source 8 is connected between the first electrode 10A and the second electrode 11A and the third electrode 12 is connected to the ground potential, the first electrode 10A and the second electrode 1
Since signal voltages with approximately equal amplitude and a phase difference of 180° are applied between the 1A and 1A, electric fields with the same direction and magnitude are generated between the first electrode 10A and the second electrode 11A and the third electrode 12, respectively. As a result, the first electrode 10A and the second electrode 11A
In-phase surface acoustic waves S ₁ and S ₂ are excited from each of the regions including the two regions. Therefore, these excited surface acoustic waves S ₁ and S ₂ propagate so as to reinforce each other.

In this structure, the first and second electrodes 1
Since the electrode width W and interval L of each comb-shaped electrode constituting 0A and 11A are approximately λ ₀ /2, W and L are different from the conventional values of λ ₀ /4 or λ ₀ /8. Since the accuracy of the processing technology is less than that in the case of 2, it becomes easier to manufacture high-frequency transducers.

Therefore, it is possible to reduce short circuits and electrode breakages between the electrodes, thereby improving manufacturing yield, especially for the first and second electrodes 10 to which electrical signals are applied.
Since A and 11A are provided through the piezoelectric thin film 5, short circuits hardly occur.

Furthermore, since the electrode width W is set to λ ₀ /2, mechanical reflection of surface acoustic waves does not occur, which is effective in suppressing TTE. Furthermore, since the electrode width is twice as wide as that of the conventional structure and the effective separation between the electrodes is 1/2, the capacitance between the electrodes can be increased.

As a result, the input impedance of the transducer can be reduced, providing the advantage that matching with external circuits can be easily achieved.

In the structure of the embodiment shown in FIG. 5, in order to cancel the feedthrough by balanced power supply,
Two sets of comb-shaped electrodes 10A, 1 via piezoelectric thin film 5
Although 1A, 10B, and 11B are configured to overlap each other when viewed from above the substrate, their positions can also be shifted relative to each other as follows.

In other words, without considering field through,
When the phase difference of the signal input between the first and second electrodes is θ (0θ2π), the relative position between both electrodes is approximately (θ) in the propagation direction of the surface acoustic wave.
−π) λ ₀ /2π shift, the same operation as in the embodiment can be performed.

FIG. 6 shows another shape of the first and second electrodes, and shows an example in which the first and second electrodes have a slit shape.

Further, by using a semiconductor material as the elastic substrate 4, it is possible to integrate electronic devices and surface wave devices, so that it can be applied to a wide range of uses. Furthermore, it is advantageous in terms of characteristics to use zinc oxide (ZnO) or aluminum nitride (AlN) as the piezoelectric thin film 5.

As is clear from the above explanation, according to the present invention,
The manufacturing process and characteristic defects of the conventional surface acoustic wave device described above can be eliminated.

[Brief explanation of the drawing]

Figures 1, 2, 3a, 4 and 3
FIG. 5B is a top view and a sectional view showing a conventional example, FIGS. 5A and 5B are a top view and a sectional view showing an embodiment of the present invention, and FIG. 6 is a schematic diagram showing an embodiment of the present invention. 2... Input transducer, 3... Output transducer, 4... Elastic substrate, 5... Piezoelectric thin film, 10A, 10B... First electrode, 11A,
11B...second electrode, 12...third electrode.

Claims (1)

[Claims] 1. A piezoelectric substrate comprising a piezoelectric thin film provided on an elastic substrate, a first electrode formed on the single phase transducer on the piezoelectric thin film, and a first electrode formed on the elastic substrate on the piezoelectric thin film. second electrodes formed on the single phase transducer, and a third electrode arranged in a planar manner within the piezoelectric thin film;
A surface acoustic wave device characterized in that the electrode fingers of the first electrode and the electrode fingers of the second electrode are arranged in a vertically overlapping position. 2. The surface acoustic wave device according to claim 1, wherein the electrical signals applied between the first electrode and the second electrode have a phase difference of approximately 180°. 3. The surface acoustic wave device according to claim 1 or 2, wherein the third electrode is connected to a ground potential. 4. The surface acoustic wave device according to claim 1, wherein the first electrode and the second electrode are formed in the same shape. 5. Claim 1, wherein the first electrode and the second electrode have a plurality of electrode fingers whose width and spacing are set to approximately λ/2, where λ is the wavelength of the surface acoustic wave. 5. The surface acoustic wave device according to any one of items 4 to 4. 6. Claim 1, wherein the third electrode is located between the first electrode and the second electrode, and is formed at a substantially equal distance from both electrodes.
6. The surface acoustic wave device according to any one of items 5 to 6. 7. The surface acoustic wave device according to claim 1, wherein the first electrode and the second electrode have a comb-like or slit-like shape. 8. The surface acoustic wave device according to claim 1, wherein the elastic substrate is made of a semiconductor material.