Surface Acoustic Wave Device

The present invention relates to a surface acoustic wave device which is suited, for example, to a transceiver or a transmitter and a receiver for mobile communications such as cellular radio.

A conventional surface acoustic wave filter comprises an input or output interdigital transducer mounted on a piezoelectric substrate for converting an electric signal to a surface acoustic wave or a surface acoustic wave to an electric signal and an intermediate interdigital transducer for connecting inputs when necessary. Such a surface acoustic wave filter is described, for example, in IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-33, No. 6, pp. 510-518, June 1985.

In the prior art, assuming that a wavelength of a surface acoustic wave propagating at a frequency equivalent to the center frequency (frequency at the center of the pass band) of the filter is .lambda.o, width of each of interdigital electrode fingers is about 1/4 of the wavelength or .lambda.o/4. The value of .lambda.o is proportional to the reciprocal of the center frequency of the surface acoustic wave filter; that is, the value of .lambda.o decreases as the frequency increases. When the frequency is 1 GHz, for example, the value of .lambda.o is about 5 .mu.m, and the electrode finger width becomes 1.0 to 1.25 .mu.m.

There has been a request for a high frequency surface acoustic wave filter of 1 GHz or higher in mobile and optical communications. However, the prior art does not consider productivity of such high frequency surface acoustic wave filters. Because the electrode finger width of .lambda.o/4 becomes submicrons for frequencies of 1 GHz or higher, a very fine process technic is required.

A transducer of the prior art, as shown in FIG. 3(a), excites or receives a surface acoustic wave by electrode fingers 14 and 15 which are alternately applied with a positive and a negative voltage. As shown in the sectional view in FIG. 3(b), electrode fingers 15 and 14 of the transducer exist at the peaks and bottoms of the oscillation generated by the surface acoustic wave, and the electrode fingers are applied with a positive or negative voltage or a negative or positive voltage in correspondence with each peak or bottom. As shown in FIG. 3(b), the electrode finger width of a transducer of the prior art is generally about a quarter of the wavelength.

The propagation speed of a surface acoustic wave is generally 4,000 to 5,000 m/s though it varies with a piezoelectric substrate. Therefore, the electrode finger width of a transducer is 1.0 to 1.25 .mu.m in a frequency band of about 1 GHz.

In recent mobile or optical communications, frequencies of about 1 GHz are often used. In the near future, higher frequencies will be required.

A surface acoustic wave device is possibly necessary for miniaturization of mobile communications and for performance of optical communications functions such as timing extraction. Use of high frequencies requires a very fine processing technic for forming minute surface acoustic wave devices. A surface acoustic wave device of a frequency band of 2 GHz, for example, requires an electrode finger width of 0.5 to 0.6 .mu.m. The production of such fine fingers is near the limits of the current processing. Although it is possible to form devices for higher frequencies, the yield rate is extremely low.

An object of the present invention is to provide a surface acoustic wave device for high frequencies which can be produced without a very fine processing technic.

The above object is accomplished by the present invention which provides a surface acoustic wave device comprising a piezoelectric substrate for propagating a surface acoustic wave and a plurality of electrode fingers mounted on the substrate along the wave propagation direction where the electrode fingers are electrically independent of the external circuit of every other finger.

In the accompanying drawings:

FIG. 1 is a perspective view of the first embodiment of the present invention;

FIG. 2 is a sectional view of electrode fingers of the embodiment shown in FIG. 1 for explaining the layout of the electrode fingers;

FIGS. 3(a) and 3(b) are a plan view and a sectional view for showing the layout of electrode fingers of a transducer of the prior art;

FIGS. 4(a) and 4(b) and 4(c) are plan views for showing the layout of electrode fingers of the second, third, and fourth embodiments of the present invention;

FIG. 5 is a plan view for showing the layout of electrode fingers of the fifth embodiment of the present invention;

FIG. 6 is a plan view for showing the layout of electrode fingers of a surface acoustic wave filter of the sixth embodiment of the present invention;

FIG. 7 is a graph for showing the frequency characteristics of the filter shown in FIG. 6; and

FIGS. 8(a), 8(b) and 8(c) are drawings for explaining the propagation status of a surface acoustic wave.

FIG. 1 shows a surface acoustic wave device of the first embodiment of the present invention. The surface acoustic wave device of the first embodiment comprises an input transducer for converting an electric signal to a surface acoustic wave and an output transducer for converting a surface acoustic wave to an electric signal basically in the same way as a surface acoustic wave device of the prior art. The electrical characteristics of the surface acoustic wave device of the first embodiment are basically the same as those of a surface acoustic wave device of the prior art. There exists only one difference between the surface acoustic wave device of the first embodiment and a surface acoustic wave device of the prior art which is the electrode configuration of each transducer. In FIG. 1, a reference numeral 1 designates a piezoelectric substrate. Reference numerals 2, 3 and 4 designate electrode fingers mounted on the piezoelectric substrate 1 for forming an input transducer. Reference numerals 6, 7 and 8 designate electrode fingers mounted on the piezoelectric substrate 1 for forming an output transducer. Reference numerals 5 and 9 designate an input terminal and an output terminal of the surface acoustic wave device. A reference numeral 10 designates a surface acoustic wave which propagates from the input transducer to the output transducer. A positive or negative voltage is applied to the electrode fingers 2 and 4 of the input transducer as an input signal. The electrode fingers 3 is located between the electrode fingers 2 and 4. The electrode finger 3 is not connected to an external circuit and is electrically independent of the external circuit. A positive or negative voltage is delivered out of the electrode fingers 6 and 8 of the output transducer as an output signal. The electrode finger 7 is located between the electrode fingers 6 and 8. The electrode finger 7 is not connected to an external circuit and is electrically independent of the external circuit.

FIG. 2 shows a sectional view of the input transducer of the surface acoustic wave device shown in FIG. 1. The distance between the centers of the electrode fingers 2 and 4 which are applied with a positive or negative voltage is assumed as L. A frequency of a surface acoustic wave excited by the voltages applied to the electrode fingers 2 and 4 is obtained from the expression indicated below.

where symbol n indicates zero or a positive integer and symbol .lambda. indicates a wavelength of a surface acoustic wave which can propagate through the piezoelectric substrate.

In the case of a surface acoustic wave device, the frequency of a surface acoustic wave can be represented by the center frequency (for example, the frequency at the center of the pass band of the surface acoustic wave filter) of the surface acoustic wave device. Assuming, therefore, a wavelength of the surface acoustic wave propagating through the piezoelectric substrate 1 at a frequency equivalent to the center frequency of the surface acoustic wave device as .lambda.o, the above expression can be represented as follows:

According to the present invention, a symbol n can be set to 1, 2, or 3. The reason will be described later. Since the electrode finger 3 is located between the electrode fingers 2 and 4 in the present invention, the location pitch of the electrode fingers 2, 3 and 4 is about L/2=(n+0.5).lambda.o/2. The width of each of the electrode fingers 2, 3 and 4 in the present invention is about L/4=(n+0.5).lambda.o/4.

FIG. 2 shows an embodiment in which a symbol n is set to 1. When n=1, the location pitch of the electrode fingers 2, 3 and 4 is about 1.5.lambda.o/2, and the width of each of the electrode fingers 2, 3 and 4 is about 1.5.lambda.o/4. The width of each of the electrode fingers of a sutface acoustic wave device of the prior art is about .lambda.o/4. Since the width of each of the electrode fingers of the embodiment of the present invention shown in FIG. 2 is about 1.5.lambda.o/4 and the space between the neighboring electrode fingers is also about 1.5.lambda.o/4the fineness required of the processing technic for electrode fingers is eased by 50%.

When n=0, the width of each of the electrode fingers is about .lambda.o/8 and the space between the neighboring electrode fingers is also about .lambda.o/8. Therefore, a very fine processing technic is required. From a processing technic viewpoint, it is desirable that the value of n is large, n=3 is a practical upper limit from a viewpoint of the excitation efficiency of a surface acoustic wave. According to the present invention, therefore, a positive integer of 3 or smaller is selected as a value of n.

According to the present invention, both the width of each electrode finger and the space between the neighboring electrode fingers are equal to each other (i.e., (n+0.5).lambda.o/4). This means the fineness of processing of the width of electrode fingers and the space between neighboring electrode fingers are the same. This is extremely desirable from a viewpoint of the processing technic.

Since the electrode fingers 3 of the embodiment of the present invention shown in FIG. 2 are not connected to the external circuit, the electrode fingers 3 are electrically floating. The potential of this floating electrode finger 3 is almost an intermediate of the voltages applied to the electrode fingers 2 and 4 on both sides. The installation of the floating electrode finger 3 between the changed electrode fingers 2 and 4 represents a novel feature of the present invention. As described later in detail, it is desirable that each of the floating electrode fingers are electrically connected to each other so as to provide the same potential.

In the embodiment shown in FIG. 1, the electrode fingers 6, 7 and 8 form the output transducer and are located on the piezoelectric substrate 1 at the same pitch as the electrode fingers 2, 3 and 4 of the input transducer. The width the electrode fingers 6, 7 and 8 is also the same as that of the electrode fingers 2, 3 and 4. The electrode finger 7 between the electrode fingers 6 and 8, which supply output voltages, is also a floating electrode finger like the electrode finger 3 of the input transducer. Similarly to the floating electrode fingers of the input transducer, it is desirable that each of the floating electrode fingers of the output transducer are electrically connected to each other so as to provide the same potential.

The transducer of the configuration shown in FIG. 2 can excite a surface acoustic wave in similar way as with the configuration shown in FIGS. 3(a) and 3(b) and will be described hereunder. It is necessary to consider an interval of L first. Although lines of electric force run from an electrode finger which is applied with a positive voltage to an electrode finger which is applied with a negative voltage, very few lines of electric force run directly from a positive electrode finger to a negative electrode finger. The oscillation field distribution of the surface acoustic wave shown in FIG. 2 oscillates by the distance equivalent to about 1.5 times of the wavelength within an interval of L. The above lines of electric force in this section run in a fixed direction predetermined by the relative position of the positive and negative electrode fingers. Very few directly running electric force lines are coupled to the oscillation field distribution of the surface acoustic wave via the piezoelectric effect off the substrate 1. Since few lines of electric force run directly between the positive and negative electrode fingers and few lines of electric force directly running are coupled to the surface acoustic wave, excitation of the surface acoustic wave by the lines of electric force directly running between the positive and negative electrode fingers can be ignored.

Lines of electric force, which run from the positive electrode finger to the intermediate floating electrode finger and then from the floating electrode finger, to the negative electrode finger, exist. The above lines of electric force between the positive and floating electrode fingers are, as shown in FIG. 2, synchronized with the period of the oscillation field distribution of the surface acoustic wave in similar way as the lines of electric force running from the positive electrode finger to the negative electrode finger shown in FIG. 3(b). The lines of electric force running from the floating electrode finger to the negative electrode finger are also synchronized with the period of the oscillation field distribution of the surface acoustic wave. Therefore, the lines of electric force, which run from the positive electrode finger to the intermediate floating electrode finger and then from the floating electrode finger to the negative electrode finger, are intensely coupled to the surface acoustic wave via the piezoelectric effect of the substrate. It has been recorded that the coupling intensity is almost equal to that when electrode fingers with the width of about a quarter of the wavelength shown in FIGS. 3(a) and 3(b) are used. The foregoing is the configuration of the first embodiment of the present invention. Assuming that a wavelength of a surface acoustic wave, which can propagate at a frequency equivalent to the center frequency of a surface acoustic wave device, is .lambda.o, by setting the width of electrode fingers, which are applied with positive and negative voltages, to about 1.5.lambda.o/4 and installing electrically floating electrode fingers with a width of about 1.5.lambda.o/4 between the electrode fingers, a surface acoustic wave device, which provides a function almost equal to that of a surface acoustic wave device of the prior art with a configuration of electrode fingers with a width of about .lambda.o/4 and which eases the processing load required for forming electrode fingers to two thirds of the load required for forming the conventional electrode fingers, can be provided.

Next, the second, third, and fourth embodiments of the present invention will be described according to the present invention. FIGS. 4(a), 4(b) and 4(c) show various configuration examples of transducers according to the present invention. FIG. 4(a) shows a transducer with a configuration which is the same as that of the transducer shown in FIG. 1 or 2.

It is desirable that the electrically floating electrode fingers 3 are all electrically connected to each other as shown in FIG. 4(a). By doing this, the excitation of the surface acoustic wave described in FIG. 2 will not be affected greatly. The excited surface acoustic wave propagates horizontally, though the surface acoustic wave is converted to a bulk wave or scattered by an effect of the electrically floating finger during propagation, increasing losses. Various experiments show that the losses can be greatly decreased by electrically connecting the electrically floating electrode fingers 3 to each other as shown in FIG. 4(a). Possible causes are as follows.

When a surface acoustic wave propagates through a piezoelectric substrate, the concentration of the surface acoustic wave energy on the substrate varies with the status of the propagation path surface whether or not the surface is deposited with a thin metallic film. The energy concentration on the substrate surface coated with a thin metallic film is high. This means that the substrate surface coated with a thin metallic film prevents the substrate surface from disturbance and minimizes conversion of the surface acoustic wave to a bulk wave. The same may be said with FIG. 4(a). When an electrically floating electrode finger is fully independent of other floating electrode fingers, the floating electrode finger produces little effect on the energy concentration of a surface acoustic wave during propagation on the substrate surface. When an electrode finger is on the substrate surface, therefore, the surface is greatly disturbed and the surface acoustic wave is converted to a bulk wave during propagation, resulting in an increase in losses.

When the electrically floating electrode fingers are electrically connected to each other as shown in FIG. 4(a), the excitation of the surface acoustic wave, as described above, is not different from that when the floating electrode fingers are not electrically connected to each other. The propagation of the excited surface acoustic wave takes place in a state very close to the state when the propagation path of the surface acoustic wave, i.e., the substrate surface, is coated with a thin metallic film. Therefore, the conversion of the surface acoustic wave to a bulk wave, which is caused by the disturbance of the substrate surface where electrode fingers exist, is extremely minimized, resulting in a great decrease in the loss by conversion.

This is the reason why the electrically floating electrode fingers are electrically connected to each other as shown in FIG. 4(a). However, it is not always advisable, in consideration of the yield rate of forming of devices, to electrically connect all floating electrode fingers. Various experiments show that partial connections between several floating electrode fingers as shown in FIGS. 4(b) and 4(c) produce the same effect. The experiments show that connections of at least two floating electrode fingers to each other produce almost the same loss characteristics as those in FIG. 4(a).

When creating frequency characteristics of a surface acoustic wave device or filter, it is necessary to weight the transducers. A weighting example of a transducer comprising electrode fingers with the configuration shown in FIG. 4(a) (see introduction of "New Phase Weighting" indicated in IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-33, No. 6, pp. 510-518, June 1985) will be described hereunder.

FIG. 5 represents the fifth embodiment of the present invention which is a transducer comprising electrode fingers with the configuration similar to that shown in FIG. 4 (a), which is subject to "New Phase Weighting". It is important that when a transducer is weighted, broad electrodes 2b, which are not directly related to the excitation of a surface acoustic wave or similar portions, are formed as shown in FIG. 5. The transducer, the back of the piezoelectric substrate and the substrate act as a capacitor. This capacitor adversely affects high frequency characteristics of the surface acoustic wave device. To minimize this effect of the earth capacitor, it is desirable to form broad electrodes 2b with a large area on the ground side of the transducer as shown in FIG. 5. By doing this, a device which is not affected by the earth capacitor of the broad electrodes 2b can be produced.

A transducer with the electrode finger configuration like that shown in FIG. 4(a) is applied, as shown in FIG. 6, to image impedance connection transducers 20 and 21 of a low-loss surface acoustic wave filter using image impedance connection transducers described in the previous literature, and also to input and output transducers 19 and 22. FIG. 6 shows a filter of the sixth embodiment of the present invention.

FIG. 7 shows the response of the filter configuration shown in FIG. 6 when the filter is simulated in the 1.5 GHz frequency band by the simulation method indicated in the literature. The simulation result shows that the obtained filter frequency characteristics are almost the same as those obtained when the transducers comprise electrode fingers with a width of about a quarter of the wavelength, which are indicated in the literature, are used and that the electrode finger configuration of the present invention is extremely effective. Each arrow shown in FIG. 6 indicates the propagation direction of the surface acoustic wave or the flow direction of the electrical signal.

Connections between electrically floating electrode fingers as shown in FIG. 4(a) may greatly affect not only the loss characteristics but also the excitation efficiency of the surface acoustic wave since the propagation mode of a surface acoustic wave depends on the cut angle of the piezoelectric substrate. This will be described hereunder.

In the case of a lithium niobate (LiNbO.sub.3) piezoelectric substrate, for example, the cut angle of LiNbO.sub.3, which is generally used as a TV intermediate frequency surface acoustic wave filter, is 128.degree. rotated Y-cut X-propagation. This surface acoustic wave is called a Rayleigh surface wave and is lower in propagation speed than bulk waves propagating in LiNbO.sub.3, i.e., a longitudinal wave and two transverse waves. When the crystal cut angle of LiNbO.sub.3 is changed, surface acoustic waves in various modes depending on the rotational angle as indicated in Journal of Applied Physics, Vol. 43, No. 3, pp. 856-862, March 1972. The literature shows that a leaky surface wave can propagate along the X axis of a 41.degree. or 64.degree. rotated Y-cut plane in LiNbO.sub.3. This leaky surface wave's propagation speed is, unlike a Rayleigh surface wave, higher than that of the slowest bulk wave, i.e., the slow transverse wave in the bulk waves which propagate through LiNbO.sub.3. Such a surface acoustic wave can be efficiently converted from an electrical signal to an elastic oscillation because the electromechanical coupling factor of the wave is extremely high. The temperature characteristics of the leaky surface wave are better than those of the Rayleigh surface wave. Therefore, the lithium niobate (LiNbO.sub.3) piezoelectric substrate is well suited as a piezoelectric substrate for current communication surface acoustic wave devices.

As described in the above literature, this leaky surface wave in the oscillation field distribution differs between the piezoelectric substrate surface with a thin metallic film coated and that without a thin metallic film. The leaky surface wave is compared in FIG. 8 with the Rayleigh surface wave.

FIG. 8(a) shows the propagation status of a Rayleigh surface wave; that is, a surface acoustic wave 27, which is excited by an input transducer 25, propagates on the surface of the substrate, and is received by an output transducer 26. There is no loss caused by propagation in principle on the propagation path from the input transducer to the output transducer. FIGS. 8(b) and 8(c) show the propagation statuses of the leaky surface wave. The leaky surface wave, which is excited by the input transducer 25, is propagated to the output transducer 26. When the propagation path between the transducers 25 and 26 is not coated with a thin metallic film, the propagation form of the leaky surface wave is close to that of a bulk wave, causing most of the energy to be discharged into the substrate, as shown in FIG. 8(b). When the substrate surface is coated with a thin metallic film 29, the energy discharge into the substrate is minimized and the loss caused by propagation is decreased to that of the Rayleigh surface wave, as shown in FIG. 8(a). Therefore, when forming devices using a leaky surface wave, it is necessary to use substrates with the surface coated with the thin metallic film 29 or substrates in a state similar to the coated state.

The above description shows that when a transducer with the electrode finger configuration of the present invention is formed with a substrate through which a leaky surface wave can propagate, electrical connections of the electrically floating electrode fingers shown in FIGS. 4(a), 4(b) and 4(c) provide a state equivalent to that when the substrate surface is coated with a thin metallic film.

As described above, a surface acoustic wave device of the prior art uses electrode fingers with a width of a quarter of the wavelength, and the processing difficulties are increased as the frequency increases. In comparison while the electrode finger width according to the present invention is at least 1.5/4 times the wavelength and provides the same function while its processing difficulties can be greatly eased.

A transducer introducing the present invention can be weighted in the same way as a transducer of the prior art. For example, the filter structure of the prior art can be used unchanged.

The description of the embodiments of the present invention uses examples of surface acoustic wave filters. However, there are various surface acoustic wave devices available besides filters, such as a signal processing device and a device using an interaction of light and a surface acoustic wave. The present invention can obviously be applied to those devices.