JPH0583076A - Surface acoustic wave device - Google Patents

Abstract

PURPOSE:To reduce the propagation loss of the surface acoustic wave used for signal processing of a quasi-microwave band being 1GHz or over. CONSTITUTION:A distance between input and output electrodes 3, 4 is selected to be a distance being a length of a multiple of 25 of a wavelength of a surface acoustic wave or below with respect to a center frequency of a signal replied by the said input and output electrodes in a multi-electrode type surface acoustic wave device employing a 36 deg. Y-X LiTaO3 substrate and whose electrode and propagation path are made of an aluminum thin film whose thickness is 100nm. Thus, the loss of the surface acoustic wave device for a quasi-microwave band is selected to be nearly equal to that of a surface acoustic wave device for a low frequency band having been conventionally used.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to 36 ° Y-X LiT.
The present invention relates to a high frequency surface acoustic wave device configured by using an aO ₃ substrate.

[0002]

2. Description of the Related Art A surface acoustic wave device is excellent in mass productivity because it can be manufactured by a photolithography technique like a semiconductor device such as an LSI, and is a signal processing device suitable for the VHF band or the UHF band as an intermediate frequency of a television receiver. Widely used as a resonator for filters and VTRs.

Further, recently, it has been applied as a filter device for a high frequency part of a mobile phone or a mobile phone by taking advantage of its small size and light weight. In the field of mobile communication, the 800 MHz band or 900 MHz band is mainly used as a signal frequency band at present, but in the future, the use of the quasi-microwave band of 1 GHz or more, for example, the 1.5 GHz band is planned. ing.

The surface acoustic wave device for mobile band communication is required to have low loss characteristic and small frequency fluctuation. Therefore, among the practical substrate materials for the piezoelectric substrate, the electromechanical coupling coefficient is relatively large and the temperature characteristic is 36 °.
Y-X LiTaO ₃ single crystal substrate is mainly used.

Further, this 36 ° Y-X LiTaO ₃ single crystal substrate has a large attenuation when the propagation path is a free surface.
Wave propagates, so the surface of the substrate is covered with a thin metal film,
A mode wave called a pseudo surface wave propagating by this is used.

Further, in order to reduce the loss as much as possible,
The interdigital transducer is of a multi-electrode type. The multi-electrode type interdigital transducer is configured by alternately arranging a plurality of input / output electrodes,
The surface acoustic wave excited by the interdigital transducer improves the loss caused by propagating to the left and right of the electrode. In the case of a normal pair of input and output electrodes, a bidirectional loss of 6 dB will occur.

Techniques relating to such a multi-electrode type surface acoustic wave device include Japanese Patent Publication No. 54-34518, Japanese Patent Publication No. 54-35478, Japanese Patent Publication No. 55-34607 and Japanese Patent Publication No. 58-26688. JP-A-57-1
The technique described in Japanese Patent Publication No. 29512 is known.

[0008]

By the way, the loss of the surface acoustic wave device includes mismatch loss, resistance loss, propagation loss and diffraction loss in addition to the bidirectional loss described above.

Further, the conventional surface acoustic wave device has been used in a frequency band not so high, and 128 ° LiNbO ₃ is used as a piezoelectric positive substrate to utilize a Rayleigh wave having a not so large propagation loss. Therefore, no special consideration was given to the propagation loss.

However, the inventors have found that, as will be described later, in the quasi-microwave band, the propagation loss of the pseudo surface wave becomes larger than that conventionally known as compared with the Rayleigh wave.

Therefore, according to the present invention, the propagation loss can be suppressed to an appropriate value even at a high frequency, 36 ° Y-X L.
It is an object of the present invention to provide a surface acoustic wave device using an iTaO ₃ substrate.

[0012]

[Means for Solving the Problems] To achieve the above object,
The present invention is a surface acoustic wave device having a center frequency of 1 GHz or more, including a 36 ° Y—X LiTaO ₃ piezoelectric single crystal substrate and an input interdigital transducer electrode formed by a metal thin film on the piezoelectric single crystal substrate. An output interdigital electrode, a pseudo surface wave propagation path between the input interdigital electrode and the output interdigital electrode, and in a propagation direction of a surface acoustic wave between the centers of the input interdigital electrode and the output interdigital electrode. There is provided a surface acoustic wave device characterized in that the measured distance is a distance of about 25 times or less the wavelength of the surface acoustic wave at the center frequency.

[0013]

According to the surface acoustic wave device of the present invention, the distance measured between the centers of the input interdigital electrode and the output interdigital electrode in the propagation direction of the surface acoustic wave is the wavelength of the surface acoustic wave at the center frequency. Since the distance is limited to about 25 times or less, the loss of the surface acoustic wave device in the quasi-microwave band can be made comparable to that of the surface acoustic wave device in the lower frequency band used conventionally. it can.

[0014]

EXAMPLE An example of the surface acoustic wave device according to the present invention will be described below.

First, in the above-mentioned quasi-microwave band,
The propagation loss of the pseudo surface wave will be described.

The three-electrode method was used to measure the propagation loss.

The film thickness of the aluminum thin film of the interdigital transducer of the surface acoustic wave device used for the measurement was 100 nm. This is because when the thickness is 100 nm or less, the resistance loss rapidly increases due to the effect of the thin film, and conversely, when the film thickness is too thick, the reflection of the surface wave inside the electrode cannot be ignored.

As mentioned above, 36 ° Y-X L
Since the iTaO ₃ substrate uses pseudo surface waves, an aluminum thin film having the same film thickness as the electrodes was formed in the surface wave propagation path. The opening length of the electrode was set to 150λ, and the loss due to the diffraction effect was set to be negligible.

FIG. 4 shows the structure of the electrodes used for measuring the propagation loss.

In the figure, the central interdigital electrode is an input electrode, and the interdigital electrodes on both sides are output electrodes. The difference between the propagation distances to the two output electrodes is λ, where λ is the wavelength of the surface acoustic wave at the center frequency of the signal responding at the input and output electrodes.
It was set to 300λ. Then, the loss of each output signal was compared, and the relationship between the propagation distance and the propagation loss was obtained.

Further, the frequency is 425 MHz, 850 MH
In the case of z and 1.7 GHz, the electrodes shown in FIG. 4 were manufactured and the respective measurements were performed.

FIG. 5 shows the measurement result of the propagation loss. As a result, the propagation loss of the conventionally used Rayleigh wave increases with the power of the frequency and becomes 1 × 10 to ⁴ d at 1 GHz.
Compared with the propagation loss of about B / λ, the propagation loss of the pseudo surface wave shows an exponential increase that is steeper than the power of the frequency and is 5 × 10 to ³ dB / λ at 1 GHz, which is more than double at 1 GHz. The result showing the large value of was obtained.

The distance between the input and output electrodes of the conventional surface acoustic wave device is about 50 to 60λ, and in this case, 850 MH.
The propagation loss of the surface acoustic wave device in the z band is 0.2 to 0.3 dB.
However, in the surface acoustic wave device in the 1.5 GHz band, a propagation loss of about 0.5 dB occurs. In the conventional low-frequency band multi-electrode surface acoustic wave device, the total loss is about 3 dB, so the propagation loss in this 1.5 GHz band is equivalent to 17% of the conventional surface acoustic wave device and cannot be ignored. It is a value.

Next, the first surface acoustic wave device according to the present invention will be described.
An example will be described.

FIG. 1 shows the structure of a surface acoustic wave device according to the first embodiment.

The surface acoustic wave device shown in FIG.
This is a one-stage multi-electrode surface acoustic wave device with a center frequency of 1.495 GHz used in the z band, and is expected to be applied to a transmission filter having a bandwidth of 36 MHz.

The piezoelectric substrate 1 in the figure has 36 ° Y-X Li.
A TaO ₃ substrate was used, and the input electrode 2, the output electrode 3 and the ground electrode 4 were formed of an aluminum thin film having a film thickness of 100 nm.

Each of the interdigital transducers is composed of a solid electrode, and the number of pairs of input interdigital transducers is 31 pairs.
The number of output electrode pairs is 15 and the center frequency is 1.495.
Since the frequency is GHz, the electrode width is about 0.7 μm. In an actual device, a substrate on which electrodes are formed is placed in a package, terminals and wiring are provided, and then sealed, but the package and the like are omitted in the figure for clarity of explanation.

Although FIG. 1 shows the case where the total number of interdigital electrodes is 5, the basic configuration is the same even when the number of electrodes is different from this.

Next, FIG. 2 shows a cross section between the input and output electrodes of the surface acoustic wave device according to the first embodiment.

As shown in the figure, an aluminum thin film which also serves as the ground electrode 4 is formed in the propagation path. Where
The distance between the output electrodes is defined by the distance d between the centers of the electrodes as shown in FIG.

Usually, the distances d ₁ , d ₂ , d ₃ and d ₄ between the input and output electrodes shown in FIG. 1 are not necessarily the same depending on the design conditions, but they are symmetrical with respect to the center for the sake of simplicity of design. ,
For example, in FIG. 1, it is often configured as d ₁ = d ₂ and d ₃ = d ₄ .

In the first embodiment, the distance d between the output electrodes is
The longest one among ₁ , d ₂ , d ₃ and d ₄ was configured to be 25λ or less. That is, in FIG. 1, d
_{Since 1} (= d ₄ )> d ₂ (= d ₃ ), d ₁ and d ₄ were set to 25λ (121 μm).

The loss of the surface acoustic wave device according to the first embodiment thus constructed and the distance between the input and output electrodes are 50λ.
FIG. 3 shows the loss of the surface acoustic wave device.

In FIG. 3, I shows the loss of 10 surface acoustic wave devices with the distance between the input and output electrodes being 50λ, and II shows the loss of 10 surface acoustic wave devices according to the first embodiment. White triangles indicate the average value of loss.

As shown in the figure, the loss of the conventional surface acoustic wave device is 2.85 dB, whereas the loss of the surface acoustic wave device according to the first embodiment is 2.66 dB, which is about 0.2 dB. Improvements are being made. That is, the propagation loss of the surface acoustic wave device in the 1.5 GHz band could be made comparable to the propagation loss of the conventional surface acoustic wave device in the 850 MHz band.

Hereinafter, the second surface acoustic wave device according to the present invention will be described.
An example will be described.

FIG. 6 shows the structure of the surface acoustic wave device according to the second embodiment.

As shown in the figure, the second embodiment is a two-stage multi-electrode type surface acoustic wave device, except that a common electrode 5 is provided between the first and second stage interdigital transducers. The surface acoustic wave device according to the first embodiment has the same structure as that of the surface acoustic wave device, and has the longest distance between the output electrodes of 25λ or less.

That is, the second embodiment is intended to improve the out-of-band suppression degree by forming the electrodes in two stages.

In the surface acoustic wave device according to the second embodiment, the surface acoustic wave device according to the first embodiment is the same as the device, and the circle indicates a receiving side filter having a center frequency of 1.447 GHz and a bandwidth of 36 MHz. Can be applied.

As a third embodiment of the present invention, an antenna duplexer using the surface acoustic wave device according to each of the above embodiments as a filter will be described below.

FIG. 7 shows an embodiment of an antenna demultiplexer for a mobile phone or a mobile phone, which is constructed by using the surface acoustic wave device of the present invention.

The surface acoustic wave device 6 operates as a filter on the receiving side, and the surface acoustic wave device 7 operates as a filter on the transmitting side. Each device is connected to an antenna 9 via a microstrip line 8. The surface acoustic wave device that constitutes the antenna demultiplexer is configured to have sufficiently low loss in each pass band and conversely to have sufficient suppression degree in the stop band.

The received signal is transmitted from the antenna through the microstrip line, the surface acoustic wave device 6 and the terminal 10 to the receiving system. Further, the transmission signal is guided from the terminal 11 of the transmission system to the antenna through the surface acoustic wave device 7 and the microstrip line.

As described above, by using the surface acoustic wave device according to each of the above-mentioned embodiments as a filter, it is possible to obtain a small-sized and lightweight duplexer with less loss.

Although the high frequency filter is shown in the present embodiment, the surface acoustic wave device of the present invention is not limited to this, and the surface acoustic wave device according to the present embodiment includes an oscillator, a correlator, and It can also be applied to RF filters, etc.

[0048]

As described above, according to the present invention, there is provided a surface acoustic wave device using a 36 ° Y-X LiTaO ₃ substrate capable of suppressing the propagation loss to an appropriate value even at high frequencies. be able to.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a configuration of a surface acoustic wave device according to a first embodiment of the present invention.

FIG. 2 is a sectional view showing a partial section of a surface acoustic wave device according to a first embodiment of the present invention.

FIG. 3 is a characteristic diagram showing a loss of the surface acoustic wave device according to the first embodiment of the present invention.

FIG. 4 is an explanatory diagram showing a configuration of a comb-like electrode used for measuring a propagation loss.

FIG. 5 is a characteristic diagram showing the frequency dependence of the propagation loss of a surface acoustic wave device in which an aluminum thin film is formed to a thickness of 100 nm on a 36 ° Y-X LiTaO ₃ substrate.

FIG. 6 is an explanatory diagram showing a configuration of a surface acoustic wave device according to a second embodiment of the present invention.

FIG. 7 is a block diagram showing a configuration of an antenna duplexer according to a third embodiment of the present invention.

[Explanation of symbols]

 1 Piezoelectric Substrate 2 Input Electrode 3 Output Electrode 4 Grounding Electrode 5 Common Electrode, 6, 7 Surface Acoustic Wave Device 8 Microstrip Line 9 Antenna 10 Reception Side Terminal 11 Transmission Side Terminal

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kazushi Watanabe 292 Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa Prefecture Hitachi Ltd. Video Media Research Laboratory

Claims (4)

[Claims] 1. A surface acoustic wave device having a center frequency of 1 GHz or higher, comprising a 36 ° Y—X LiTaO ₃ piezoelectric single crystal substrate and an input interdigital electrode formed by a metal thin film on the piezoelectric single crystal substrate. And an output interdigital electrode, a pseudo surface wave propagation path between the input interdigital electrode and the output interdigital electrode, and a propagation direction of a surface acoustic wave between the centers of the input interdigital electrode and the output interdigital electrode. The surface acoustic wave device is characterized in that the measured distance is about 25 times or less the wavelength of the surface acoustic wave at the center frequency. 2. A surface acoustic wave device using a 36 ° Y-XLiTaO ₃ piezoelectric single crystal substrate having a multi-electrode interdigital transducer having a center frequency of 1 GHz or higher, wherein the multi-electrode interdigital transducer is configured. Each input interdigital transducer and each output interdigital transducer are connected to the 36 ° Y-X LiT
The propagation direction of the surface acoustic wave between the center of the input interdigital transducer and the output interdigital transducer receiving the surface acoustic wave transmitted by the input interdigital transducer was measured by a metal thin film on the aO ₃ piezoelectric single crystal substrate. Each distance is formed to be a distance of about 25 times or less the wavelength of the surface acoustic wave at the center frequency, and the input interdigital transducer and the transmitted interdigital elasticity of the input interdigital transducer. A surface acoustic wave device having a pseudo surface wave propagation path formed of a metal thin film, between an output interdigital transducer for receiving surface waves. 3. The surface acoustic wave device according to claim 1, wherein the metal thin film has a thickness of 100 nm or more. 4. An antenna duplexer comprising the surface acoustic wave device according to claim 1, 2 or 3 as a reception filter or a transmission filter.