Elastic wave device

An elastic wave device includes an elastic wave resonator which includes a comb-shaped electrode pair including a pair of com-shaped electrodes interdigitating with each other and provided on a piezoelectric substrate and which is configured to trap energy of the elastic wave therein. Each of the pair of comb-shaped electrodes includes interdigital electrode fingers connected to a common. A pitch of the interdigital electrode fingers changes along a direction perpendicular to a propagation direction of elastic wave. The elastic wave device has a small insertion loss and operates efficiently.

This application is a U.S. National phase application of PCT International application No. PCT/JP2011/001048.

TECHNICAL FIELD

The present invention relates to an elastic wave device to be used chiefly in mobile communication devices.

BACKGROUND ART

In recent years, a ladder filter formed by combining elastic wave resonators, each of which has a terminal pair, has been widely used at an RF stage of portable phones. A longitudinally coupled resonator type elastic filter including plural electrode pairs has been also widely used in the RF stage.

FIG. 11shows an electrode pattern of conventional elastic wave device501. Elastic eave device501includes piezoelectric substrate1, a pair of reflecting electrodes2and a pair of comb-shaped electrodes5. Both of electrodes2and5are formed on substrate1. Comb-shaped electrode pair5is disposed between the pair of reflecting electrodes2, and electrode pair5interdigitates. Comb-shaped electrode5includes common electrode3and plural electrode fingers4connected to common electrode3. Elastic wave device501constitutes an acoustic surface wave resonator on piezoelectric substrate1for trapping energy of the elastic wave therein.

In recent years communication devices have been downsized and the frequency band to be used has been densified due to a large number of users, so that a highly efficient elastic wave device having less insertion loss is required for those communication devices to operate reliably.

A conventional elastic wave device similar to elastic wave device501is disclosed in Patent Literature 1.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

An elastic wave device includes an elastic wave resonator which includes a comb-shaped electrode pair including a pair of com-shaped electrodes interdigitating with each other and provided on a piezoelectric substrate and which is configured to trap energy of the elastic wave therein. Each of the pair of comb-shaped electrodes includes interdigital electrode fingers connected to a common. A pitch of the interdigital electrode fingers changes along a direction perpendicular to a propagation direction of elastic wave.

The elastic wave device has a small insertion loss and operates efficiently.

DETAIL DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1Ais an electrode pattern of elastic wave device1001in accordance with Exemplary Embodiment 1 of the present invention.FIG. 1Bis an enlarged view of the device. Elastic wave device1001includes piezoelectric substrate11made of rotated Y-cut and X-propagating lithium tantalate monocrystal, and elastic wave resonator12provided on surface111of piezoelectric substrate11. Elastic wave resonator12constitutes a one-terminal-pair resonator which includes a pair of reflecting electrodes13and comb-shaped electrode pair14disposed between reflecting electrodes13. Comb-shaped electrode pair14excites an elastic wave. The pair of reflecting electrodes13and comb-shaped electrode pair14are arranged along propagating direction D1of the elastic wave, so that they can trap energy of the elastic wave on piezoelectric substrate11. Comb-shaped electrode pair14includes comb-shaped electrodes51A and51B interdigitating with each other. Comb-shaped electrode51A includes common electrode (busbar)15A, plural interdigital electrode fingers16A connected to common electrode15A, and plural dummy electrode fingers17A connected to common electrode15A. Comb-shaped electrode51B includes common electrode (busbar)15B extending in parallel with common electrode15A, plural interdigital electrode fingers16B connected to common electrode15B, and plural dummy electrode fingers17B connected to common electrode15B. Plural interdigital electrode fingers16A and16B interdigitate with each other in interdigital region19. As shown inFIG. 1B, each of interdigital electrode fingers16A has end216A connected to common electrode15A and tip116A opposite to end216A while each of interdigital electrode fingers16B has end216B connected to common electrode15B and tip116B opposite to end216B. Each one of dummy electrode fingers17A has end217A connected to common electrode15A and tip117A opposite to end217A. Each one of dummy electrode fingers17B has end217B connected to common electrode15B and tip117B opposite to end217B. Tip116A of interdigital electrode finger16A faces tip117B of dummy electrode finger17B while tip116B of interdigital electrode finger16B faces tip117A of dummy electrode finger17A.

As shown inFIG. 1A, in comb-shaped electrode pair14, interdigital region19in which interdigital electrode fingers16A and16B interdigitate with each other, has width WA along direction D2perpendicular to direction D1. No electrode16A interdigitates with electrode16B but dummy electrode fingers17A are disposed in dummy region22A. Dummy region22A has width WDA along direction D2. No electrode16A interdigitates with electrodes16B but dummy electrode fingers17B are disposed in dummy region22B. Dummy region22B has width WDB along direction D2. According to Embodiment 1, width WDA is equal to width WDB. Both ports of comb-shaped electrode pair14, namely, common electrodes15A and15B are connected to input-output terminals18A and18B, respectively.

Reflecting electrode13includes common electrodes52A and52B extending in parallel with common electrodes15A and15B, plural reflecting electrode fingers53disposed between common electrodes52A and52B. Plural reflecting electrode fingers53are connected to common electrodes52A and52B, and arranged in direction D1.

Center region20and side regions21A,21B are provided in interdigital region19of comb-shaped electrode pair14and reflecting electrodes13. Center region20extends along direction D1at the center between common electrodes15A and15B and at the center between common electrodes52A and52B. Side region21A is adjacent to center region20in direction D2and faces common electrodes15A and52A. Side region21B is adjacent to center region20in direction D2and faces common electrodes15B and52B. Center region20has width WB in direction D2, and side regions21A and21B have widths WCA and WCB in direction D2, respectively. According to Embodiment 1, width WCA is equal to width WCB.

A distance in direction D1between respective centers of two adjacent electrode fingers out of interdigital electrode fingers16A and16B of comb-shaped electrode pair14, reflecting electrodes13, dummy electrode fingers17A and17B, and reflecting electrode fingers53is defined as a pitch of the electrode fingers. In center region20, a pitch of electrode fingers16A,16B,17A,17B, and53is constant along direction D2; however, the pitch in center region20may change gradually along in direction D1in which the elastic wave propagates. This structure efficiently reduces loss of elastic wave energy, thus improving electrical characteristics of elastic wave device1001.

Side regions21A,21B are adjacent to center region20in direction D2, and located at positions opposite to each other. In side regions21A and21B, each pitch of the electrode fingers becomes wider gradually as located away from center region20.

Dummy region22A is located between side region21A and common electrode15A, and has dummy electrode fingers17A disposed therein. Dummy region22B is located between side region21B and common electrode15B, and has dummy electrode fingers17B disposed therein. In dummy region22A, the pitch which is the distance between the center of interdigital electrode finger16A and the center of dummy electrode finger17A adjacent to each other becomes wider gradually as located away from center region20. Similarly, a pitch which is the distance between the center of interdigital electrode finger16B and the center of dummy electrode finger17B adjacent to each other becomes wider gradually as away from center region20.

As shown inFIG. 1B, pitches P1and P2are the distances in direction D1between respective centers of interdigital electrode fingers16A and16B adjacent to each other in side regions21A and21B. The position of pitch P2is farther from center region20than the position of pitch P1is. Pitches P3and P4are the distances in direction D1between respective centers of interdigital electrode finger16A and dummy electrode finger17A adjacent to each other in dummy region22A, and they are also the distances in direction D1between respective centers of interdigital electrode finger16B and dummy electrode finger17B adjacent to each other in dummy region22B. The position of pitch P3is farther from center region20than the position of pitch P2is. The position of pitch P4is farther from center region20than the position of pitch P3is. Pitches P1, P2, P3, and P4become wider in this order. Namely, pitch P2is wider than pitch P1, and pitch P3is wider than pitch P2, and pitch P4is wider than pitch P3.

As shown inFIG. 1B, pitches P5to P8are the distances in direction D1between the respective centers of each one of plural reflecting electrode fingers53adjacent to each other. Pitches P5to P8are farther from center region20in this order. Namely, the position of pitch P6is farther from center region20than the position of pitch P5is. The position of pitch P7is farther from the center region20than the position of pitch P6is. The position of pitch P8is farther from the center region20than the position of pitch P7is. Pitches P5, P6, P7and P8become wider in this order. Namely, pitch P7is wider than pitch P6, and pitch P8is wider than pitch P7. Center region20, side regions21A and21B extend in parallel to common electrodes51A,51B,52A, and52B across comb-shaped electrode pair14and two reflecting electrodes13.

In a gap between tip116A of electrode finger16A and tip117B of dummy electrode finger17B, and in a gap between tip116B of electrode finger16B and tip117A of dummy electrode finger17A, the pitch in direction D1between electrode fingers16A and16B is measured as the distance between lines extending into the gaps along respective centers of electrode fingers16A and16B.

In elastic wave device1001in accordance with Embodiment 1, the width (WA+WDA+WDB) in direction D2between common electrodes15A and15B is 45 μm. Each of widths WDA and WDB of dummy regions22A and22B is 2.5 μm. The gap between tip116A of electrode finger16A and tip117B of dummy electrode finger17B is 0.5 μm in direction D2. The pitch of interdigital electrode fingers16A and16B in center region20is 1 μm. Width WA of interdigital region19in which interdigital electrode fingers16A and16B interdigitate with each other is 40 μm.

The electrode fingers extend along a continuous and smooth curved line from boundary61A (61B) between center region20and side region21A (21B) to common electrodes15A and52A (15B,52B). According Embodiment 1, the pitch of between the electrode fingers of comb-shaped electrode pair14and reflecting electrodes13changes according to a quadratic function of a distance from boundary61A (61B) in direction D2by a changing amount increasing as approaching common electrodes15A,15B,52A and52B. The ratio of the width of the electrode fingers to the pitch of the electrode fingers is ½ in any of center region20, side regions21A and21B, and dummy regions22A and22B.

Center region20functions as a main exciting region of elastic wave resonator12. In center region20, the distance in direction D1between respective centers of two adjacent interdigital electrode fingers16A of comb-shaped electrode51A is defined as cycle λ. Cycle λ is a wavelength of the elastic wave in propagating direction D1excited by comb-shaped electrode pair14. In center region20, the distance in direction D1between respective centers of two adjacent interdigital electrode fingers16B of other comb-shaped electrode51B is also cycle λ. Pitch P0in direction D1between respective centers of two adjacent interdigital electrode fingers16A and16B in center region20is λ/2. The ratio of a pitch at ends216A and216B connected respectively with common electrodes15A and15B to pitch P0in center region20is defined as expansion ratio α. While elastic resonator12in center region20has width WE in propagation direction D1, elastic resonator12at ends216A and216B of interdigital electrode fingers16A and16B has width (α×WE) in propagation direction D1. Interdigital electrode finger16A is not connected to common electrode15B, thus being located away from common electrode15B. Interdigital electrode finger16B is not connected to common electrode15A, thus being located away from common electrode15A. The pitch of interdigital electrode fingers16A and16B at ends216A and216B is actually a half of the distance in direction D1between respective centers of ends216A of two adjacent interdigital electrode fingers16A of comb-shaped electrode51A. The pitch of interdigital electrode fingers16A and16B at ends216A and216B is actually a half of the distance in direction D1between respective centers of ends216B of two adjacent interdigital electrode fingers16B of the other comb-shaped electrode51B.

Appropriate ranges of expansion ratio α of the electrode fingers and widths WCA and WCB will be discussed below.FIG. 2Ais a circuit diagram of an evaluation circuit for evaluating elastic wave device1001as a series resonator which is connected in series to signal path1001A.

FIG. 2Bshows propagation characteristics of elastic wave device1001in accordance with Embodiment 1 in which elastic wave resonator12is connected in series to signal path1001A as a series resonator. For the evaluation purpose, both ports of resonator12are connected to capacitive elements which are grounded. InFIG. 2B, the horizontal axis represents a frequency of a signal, and the vertical axis represents attenuation of the signal. Propagation profile S501shows the characteristics of a comparative example having expansion ratio α of 1. Propagation profile S1shows the characteristics of elastic wave device1001having expansion ratio α of 1.01 while widths WCA and WCB of side regions21A and21B are 3λ. Elastic wave device1001as a series resonator having expansion ratio α of 1.01 and widths WCA and WCB of 3λ provides an insertion loss smaller, by an amount slightly more than 0.2 dB, than the comparative example having expansion ratio α of 1.

Next, expansion ratio α of the pitch of the electrode fingers is evaluated within a range from 0.995 to 1.020 and the widths WCA, and WCB of side regions21A,21B are evaluated within a range from 0 to 10λ to obtain the propagation characteristics of the series resonator for evaluating the insertion loss of elastic wave device1001. To be more specific, the insertion loss of elastic wave device1001is measured by changing the frequency of the signal to find a minimum loss from the measured insertion loss.

FIGS. 3A to 3Eshow the characteristics of minimum insertion loss of elastic wave resonator12of elastic wave device1001used as a series resonator. The minimum insertion loss is measured for various values of widths WCA, WCB of side regions21A,21B. InFIGS. 3A to 3E, the horizontal axis represents expansion ratio α of the pitch, and the vertical axis represents the minimum insertion loss. Width WCA (WCB) of side region21A (21B) takes values of 0, λ, 3λ, 6λ, and 10λ inFIGS. 3A,3B,3C,3D, and3E, respectively. The comparative example has expansion ratio α of 1 and widths WCA and WCB of side regions21A,21B of 0.

As shown inFIG. 3A to 3E, when elastic wave resonator12is used as a series resonator, the insertion loss can be reduced when expansion ratio α is not smaller than 1.005, and each of widths WCA and WCB of side regions21A and21B is not smaller than λ. Total evaluation of the propagation characteristics finds that the insertion loss can be reduced when expansion ratio α ranges from 1.005 to 1.015, and each of width WCA and WCB of side regions21A and21B ranges from λ to 6π. These ranges provide excellent propagation characteristics.

As discussed above, elastic wave resonator12connected in series to signal path1001A as a series resonator having expansion ratio α and widths WCA and WCB within the above ranges decreases the insertion loss.

FIG. 4Ais a circuit diagram of an evaluation circuit of elastic wave device1001used as a parallel resonator connected between signal path1001A and ground1001B. The evaluation result of the characteristics of device1001will be described below:

FIG. 4Bshows the propagation characteristics of elastic wave device1001in accordance with Embodiment 1 in which elastic wave resonator12is connected between signal path1001A and ground1001B as a parallel resonator. For the evaluation purpose, each of both ports of resonator12is connected to a capacitive element which is grounded. InFIG. 4B, the horizontal axis represents a frequency of a signal, and the vertical axis represents attenuation of the signal. Propagation profile S502shows the characteristics of a comparative example having expansion ratio α of 1. Propagation profile S2shows the characteristics of elastic wave device1001having expansion ratio α of 1.01 and widths WCA and WCB of side regions21A and21B of 3λ. Elastic wave device1001as a parallel resonator having expansion ratio α of 1.01 and widths WCA and WCB of 3λ has a smaller insertion loss reduced by 0.1 dB than the comparative example having expansion ratio α of 1.

Next, expansion ratio α of the pitch of the electrode fingers is evaluated within a range from 0.995 to 1.020, and the widths WCA and WCB of side regions21A,21B are evaluated within a range from 0 to 10λ to obtain the propagation characteristics of the parallel resonator for evaluating the insertion loss of elastic wave device1001. To be more specific, the insertion loss of elastic wave device1001is measured while changing the frequency of the signal to measure a minimum loss from the measured insertion losses.

FIGS. 5A to 5Eshow the characteristics of the minimum insertion loss of elastic wave resonator12of elastic wave device1001used as a series resonator. The minimum insertion loss is measured for various values of widths WCA and WCB of side regions21A and21B. InFIGS. 5A to 5E, the horizontal axis represents expansion ratio α of the pitch, and the vertical axis represents the minimum insertion loss. Width WCA (WCB) of side region21A (21B) takes values of 0, λ, 3λ, 6λ, and 10λ inFIGS. 5A,5B,5C,5D, and5E, respectively. The comparative example takes a value of expansion ratio α of 1 and each of widths WCA and WCB of side regions21A and21B of 0.

As shown inFIG. 5A to 5E, when elastic wave resonator12is used as a parallel resonator, the insertion loss can be reduced effectively with expansion ratio α not smaller than 1.005, and the insertion loss can be reduced with each of widths WCA and WCB of side regions21A,21B not smaller than λ. For total evaluation of the propagation characteristics, the insertion loss can be reduced when expansion ratio α ranges from 1.01 to 1.015. The insertion loss becomes particularly small when each of widths WCA and WCB of side regions21A and21B ranges from λ to 6λ. These ranges provide excellent propagation characteristics.

As discussed above, elastic wave resonator12connected between signal path1001A and ground1001B as a parallel resonator having expansion ratio α widths WCA and WCB in the above ranges decreases the insertion loss.

As discussed above, side regions21A and21B in which the pitch of the electrode fingers becomes wider gradually as located away from center region20, thereby decreasing the insertion loss of elastic wave resonator12.

Piezoelectric substrate11is made of piezoelectric mono-crystal, such as rotated Y-cut and X-propagating lithium tantalate mono-crystal, having a concave reciprocal velocity plane of elastic wave. In elastic wave resonator12employing piezoelectric substrate11, the pitch of the electrode fingers within side regions21A and21B becomes wider gradually as located away from center region20functioning as a main exciting region. This structure allows the velocity of the elastic wave in side regions21A and21B to be slower than that in center region20functioning as the main exciting region. This mechanism allows the energy of resonating elastic wave to be trapped within a guided-wave path of the elastic wave, thereby reducing energy loss and the insertion loss.

In order to trap the energy within a guided-wave path by making a velocity of the elastic wave in both sides of the main exciting region than a velocity in the main exciting region, a ratio of a width of each of electrode fingers16A,16B,17A, and17B to the pitch of electrode fingers16A,16B,17A, and17B in dummy regions22A and22B can be increased, or the pitch of the electrode fingers in dummy regions22A and22B can be increased. However, in the case that the ratio of the width to the pitch is increased, if the electrode fingers are thin and arranged densely, the electrode fingers may touch each other even after the dingers are etched, thus being prevented from being formed. This method is thus limited to an elastic wave device that includes electrode fingers arranged at a low density. In the case that the pitch of the electrode fingers are increased in dummy regions22A and22B, a large number of electrode fingers accumulate differences between the pitch in dummy regions22A and22B, and cause a large deviation. This may provide discontinuity between interdigital region19and dummy regions22A and22B, and disperse the elastic wave, thus causing the energy of the elastic wave to be lost. In the case that the electrode fingers are thin, elastic wave device1001in accordance with Embodiment 1 can reduce the insertion loss without lowering the yield rate of electrode pattern.

In elastic wave device1001in accordance with Embodiment 1, electrode fingers16A,16B,17A, and17B in side regions21A,21B and dummy regions22A,22B extend along continuous and smooth curved lines. Interdigital electrode fingers16A and16B in side regions21A,21B and center region20extend along continuous and smooth curved lines. This structure eliminates discontinuous steps between electrode fingers16A and16B at positions where center region20is connected to side regions21A and21B and within side regions21A and21B, thereby reducing dispersion loss of the elastic wave at these discontinuous positions, and reducing the insertion loss.

FIG. 5Fis an enlarged view of another example of electrode fingers16A,16B,17A, and17B of elastic wave device1001in accordance with Embodiment 1. InFIGS. 1A and 2B, electrode fingers16A and16B extend along continuous and smooth curved lines in side regions21A,21B and dummy regions22A,22B. However, as shown inFIG. 5F, electrode fingers16A,16B,17A, and17B can extend along approximate curved line L2including plural straight lines L1connected to each other, providing effects similar to the structure discussed above.

In elastic wave device1001in accordance with Embodiment 1, interdigital electrode fingers16A and16B in center region20have pitch P0between electrode fingers16A and16B while electrode fingers16A and16B have the maximum pitch not smaller than 1.005×P0between electrode fingers16A and16B in side regions21A and21B and dummy regions22A and22B. This structure efficiently suppresses the energy loss of the elastic wave, thus improving the electrical characteristics of elastic wave device1001.

Interdigital electrode fingers16A and16B have pitch P0between electrode fingers16A and16B in center region20while electrode fingers16A and16B have the maximum pitch not larger than 1.020×P0in side regions21A and21B and dummy regions22A and22B. This structure efficiently suppresses the energy loss of the elastic wave, thus improving the electrical characteristics of elastic wave device1001.

In elastic wave device1001in accordance with Embodiment 1, the pitch of the electrode fingers in side regions21A and21B and dummy regions22A and22B can be not greater than 1.015×P0. This structure efficiently suppresses the energy loss of the elastic wave, thus improving the electrical characteristics of elastic wave device1001.

In elastic wave device1001in accordance with Embodiment 1, widths WCA and WCB of side regions21A and21B in direction D2perpendicular to the propagating direction D1may be not smaller than λ. This structure efficiently suppresses the energy loss of the elastic wave, thus improving the electrical characteristics of elastic wave device1001.

When the elastic wave device constitutes a ladder-type filter, at least one of series-arm resonators and parallel arm resonators may be elastic wave resonator12according to Embodiment 1, thereby reducing the insertion loss.

Since elastic wave device1001in accordance with Embodiment 1 includes piezoelectric substrate11made of rotated Y-cut and X-propagating lithium tantalate monocrystal, no lateral mode spurious occurs. The electrode fingers may not necessarily be designed for canceling the lateral mode spurious, such as weighting (apodizing) the interdigital length of electrode fingers or weighting (apodizing) the length of dummy electrode fingers. The apodization is usually employed when quartz crystal or lithium niobate is used.

Material of piezoelectric substrate11is not limited to the rotated Y-cut and X-propagating lithium tantalate monocrystal, but any piezoelectric monocrystal having a reciprocal velocity plane which is concave in a direction in which a surface acoustic wave propagates can provide an effect similar to that discussed above.

As discussed above, the reciprocal velocity plane of piezoelectric substrate11is concave in propagating direction D1in which the elastic wave propagates. Elastic wave resonator12includes comb-shaped electrode pair14including comb-shaped electrodes51A and51B which are formed on substrate11and which interdigitate with each other. Elastic wave resonator12has center region20and side regions21A and21B. In center region20, plural interdigital electrode fingers16A and16B interdigitate with each other. Pitch of interdigital electrode fingers16A and16B are constant along direction D2perpendicular to propagating direction D1. In side regions21A and21B, pitch of interdigital electrode fingers16A and16B are is than the pitch in center region20. This structure allows elastic wave device1001to trap the elastic wave in elastic wave resonator12efficiently, thus allowing elastic wave device1001to work efficiently with a small insertion loss.

Each one of plural interdigital electrode fingers16A and respective one of plural dummy electrode fingers17B extend along a line including plural straight lines connected together or a smooth curved line. Each one of plural interdigital electrode fingers16B and respective one of plural dummy electrode fingers17A extend along a line including plural straight lines connected together or a smooth curved line.

While interdigital electrode fingers16A and16B are arranged at pitch P0between interdigital electrode fingers16A and16B in center region20, the maximum pitch of plural interdigital electrode fingers16A and plural dummy electrode fingers17A is not smaller than 1.005×P0. The maximum pitch of plural interdigital electrode fingers16B and plural dummy electrode fingers17B is not smaller than 1.005×P0.

While interdigital electrode fingers16A and16B are arranged at pitch P0between interdigital electrode fingers16A and16B in center region20, the maximum pitch between plural interdigital electrode fingers16A and plural dummy electrode fingers17A is not greater than 1.020×P0, and the maximum pitch between plural interdigital electrode fingers16B and plural dummy electrode fingers17B is not greater than 1.020×P0.

In side regions21A and21B, each of interdigital electrode fingers16A extends along a continuous curved line or a line including plural straight lines connected together. In side regions21A and21B, each of interdigital electrode fingers16B extends along a continuous curved line or a line including plural straight lines connected together.

Plural interdigital electrode fingers16A extend along smooth curved lines from side region21A to center region20, and plural interdigital electrode fingers16B extend along smooth curved lines from side region21B to center region20.

While interdigital electrode fingers16A and16B are arranged at pitch P0between interdigital electrode fingers16A and16B in center region20, the maximum pitch in side regions21A and21B is not smaller than 1.005×P0and not greater than 1.020×P0.

FIG. 6is an electrode pattern diagram of elastic wave device1002in accordance with Exemplary Embodiment 2 of the present invention. InFIG. 6, components identical to those of elastic wave device1001in accordance with Embodiment 1 shown inFIG. 1are denoted by the same reference numerals.

Elastic wave device1002in accordance with Embodiment 2 includes dual-terminal-pair resonator23of two electrodes type as an elastic wave resonator. Dual-terminal-pair resonator23constitutes a longitudinally-coupled resonator type elastic wave filter, and includes two comb-shaped electrode pairs14disposed adjacently to each other along propagating direction D1of the elastic wave.

As shown inFIG. 6, elastic wave device1002includes piezoelectric substrate11made of rotated Y-cut and X-propagating lithium tantalate monocrystal, and two comb-shaped electrode pairs14disposed between a pair of reflecting electrodes13, thus providing dual-terminal-pair resonator23of two electrodes type. Input-output terminal18A is connected to common electrode15A of one comb-shaped electrode pair14of two comb-shaped electrode pairs14while grounding terminal24B is connected to common electrode15B of the one comb-shaped electrode pair. Grounding terminal24A is connected to common electrode15A of the other comb-shaped electrode pair14of the two comb-shaped electrode pairs14while input-output terminal18B is connected to common electrode15B of the other comb-shaped electrode pair14.

Center region20and side regions21A and21B are provided in interdigital region19of comb-shaped electrode pairs14and reflecting electrodes13. Center region20has width WB in direction D2perpendicular to propagating direction D1of the elastic wave, and side regions21A and21B have width WCA and WCB in direction D2, respectively.

A pitch which is the distance in direction D1between a center of electrode finger16A and a center of electrode finger16B adjacent to each other is constant along direction D2. Side regions21A and21B are provided on both sides of center region20in direction D2. The pitch of electrode fingers16A and16B in side regions21A and21B is wider than the pitch in center region20, and become wider gradually as located away from center region20.

In dummy regions22A and22B, a pitch which is a distance in direction D1between respective centers of any two adjacent electrode fingers of interdigital electrode fingers16A and16B and dummy electrode fingers17A and17B is wider than the pitch in side regions21A and21B, and becomes wider gradually as located away from side regions21A and21B.

In dual-terminal-pair resonator23in accordance with Embodiment 2, width WA in direction D2of interdigital region19in which interdigital electrode fingers16A and16B interdigitate with each other is 40 μm, and the distance (WA+WDA+WDB) between common electrodes15A and15B is 45 μm. Pitch P0of interdigital electrode fingers16A and16B in center region20is 1 μm. An elastic wave device includes dummy regions22A and22B, namely, dummy electrode fingers17A and17B, is compared with another elastic wave device that has no dummy electrode finger17A or17B. The elastic wave device has dummy regions22A and22B having widths WDA and WDB in direction D2of 2.5 μm. The gap between the tip of electrode finger16A and the tip of dummy electrode finger17B is 0.5 μm, and the gap between the tip of electrode finger16B and the tip of dummy electrode finger17A is 0.5 μm. On the other hand, in the elastic wave device without dummy regions or dummy electrode finger17A or17B, a gap between interdigital electrode finger16A and common electrode15B, and a gap between interdigital electrode finger16B and common electrode15A are 0.5 μm.

The fingers of comb-shaped electrode pairs14and reflecting electrodes13extend along continuous curved lines from center region20to common electrodes15A and15B. According to Embodiment 2, the pitch of the electrode fingers changes according to a quadratic function of a distance from center region20by a changing amount increasing as approaching common electrodes15A and15B. The ratio of a width of the electrode fingers to the pitch of the electrode fingers is ½ in any of center region20, side regions21A and21B, and dummy regions22A and22B.

Center region20functions as a main exciting region for dual-terminal-pair resonator23. In center region20, the distance in direction D1between respective centers of two adjacent interdigital electrode fingers16A of comb-shaped electrode51A is defined as cycle λ. In center region20, the distance in direction D1between respective centers of two adjacent interdigital electrode fingers16B of the other comb-shaped electrode51B is defined also as cycle λ. Pitch P0in direction D1between respective centers of two adjacent interdigital electrode fingers16A and16B in center region20is λ/2. The ratio of the pitch at end216A and end216B (shown inFIG. 1B) connected respectively to common electrodes15A and15B to the pitch in center region20is defined as expansion ratio α. While dual-terminal-pair resonator23has width WE in propagation direction D1in center region20, the width of dual-terminal-pair resonator23in propagating direction D1at positions where ends216A and216B of electrode fingers16A and16B and ends217A and217B of dummy electrode fingers17A and17B are connected to common electrodes15A and15B is expressed as α×WE. Interdigital electrode finger16A is not connected to common electrode15B, thus being located away from common electrode15B. Interdigital electrode finger16B is not connected to common electrode15A, thus being located away from common electrode15A. The pitch of interdigital electrode fingers16A and16B at respective ends216A and216B is actually a half of the distance in direction D1between respective centers of ends216A of two adjacent interdigital electrode fingers16A of comb-shaped electrode51A, and actually a half of the distance in direction D1between respective centers of ends216B of two adjacent interdigital electrode fingers16B of the other comb-shaped electrode51B.

An appropriate range for expansion ratio α of the pitch is studied in the range from 0.995 to 1.020, and appropriate ranges of widths WCA and WCB of side regions21A and21B are studied in the range from 0 to 10λ.

Next, the characteristics of elastic wave device1002in accordance with Embodiment 2 will be described below. A bandwidth at which an insertion loss becomes 1.5 dB is found based on the waveform of propagation characteristics of dual-terminal-pair resonator23. The wider bandwidth produces the smaller insertion loss of elastic wave device1002.

FIGS. 7A to 7Eshow bandwidth characteristics of elastic wave device1002including dummy regions22A and22B for various values of widths WCA and WCB of side regions21A and21B of device1002. Widths WDA and WDB of dummy regions22A and22B are 2.5 μm (=1.25λ). InFIGS. 7A to 7E, the horizontal axis represents expansion ratio α, and the vertical axis represents the bandwidth. Width WCA (WCB) of side region21A (21B) takes values of 0, λ, 3λ, 6λ, and 10λ inFIGS. 7A,7B,7C,7D, and7E, respectively. The comparative example has expansion ratio α of 1 and widths WCA and WCB of side regions21A and21B of 0.

As shown inFIG. 7A, in dual-terminal-pair resonator23having dummy regions22A and22B, when widths WCA and WCB of side regions21A and21B is 0 and the pitch of the electrode fingers is widened only in dummy regions22A and22B, there is little difference from the comparative example having expansion ratio α of 1. The structure shown inFIG. 7Athus does not produce any advantage.

As shown inFIGS. 7B and 7C, in dual-terminal-pair resonator23having dummy regions22A and22B, when each of widths WCA and WCB of side regions21A and21B ranges from λ to 3λ, the bandwidth at which insertion loss is 1.5 dB is wider than that of the comparative example. In the case that widths WCA and WCB are λ, the bandwidth becomes wider by about 13% while expansion ratio α ranges from 1.01 to 1.020. In the case that widths WCA and WCB are 3λ, the bandwidth becomes wider by about 25% while expansion ratio α ranges from 1.01 to 1.020. The structures shown inFIGS. 7B and 7Cthus can advantageously reduce the insertion loss.

As shown inFIGS. 7D and 7E, in dual-terminal-pair resonator23having dummy regions22A and22B, when each of widths WCA and WCB of side regions21A and21B ranges from 6λ to 10λ, the bandwidth at which insertion loss is 1.5 dB becomes narrower than that of the comparative example, so that those structures produce adversely a larger insertion loss.

As described above, dual-terminal-pair resonator23having dummy regions22A and22B, i.e., including dummy electrode fingers17A and17B, can produce a smaller insertion loss than the comparative example under the condition that each of widths WCA and WCB of side regions21A and21B ranges from λ to 3λ and expansion ratio α ranges from 1.01 to 1.020. The electrical characteristics of elastic wave device1002can be thus improved.

FIGS. 8A to 8Dshow the bandwidth characteristics of elastic wave device1002having none of dummy regions22A and22B under the condition of various values of widths WCA and WCB of side regions21A and21B. Since widths WDA and WDB of dummy regions described inFIGS. 8A to 8Dare 0, elastic wave devices1002include none of dummy electrode fingers17A and17B. InFIGS. 8A to 8g, the horizontal axis represents expansion ratio α, and the vertical axis represents the bandwidth. Width WCA (WCB) of side region21A (21B) takes values of λ, 3λ, 6λ, and 10λ inFIGS. 8A,8B,8C, and8D, respectively. The comparative example has expansion ratio α of 1 and each of widths WCA and WCB of side regions21A and21B of 0.

As shown inFIGS. 8A to 8D, in dual-terminal-pair resonator23having none of dummy regions22A and22B, i.e., having none of dummy electrode fingers17A and17B, when each of widths WCA and WCB of side regions21A and21B ranges from λ to 3λ and expansion ratio α ranges from 1.005 to 1.01, the bandwidth is wider by about 13 to 25% than that of the comparative example. The insertion loss can be thus slightly lowered. When each of widths WCA and WCB of side regions21A and21B ranges from 6λ to 10λ, the bandwidth becomes narrower than that of the comparative example, so that the insertion loss increases.

As discussed above, dual-terminal-pair resonator23having none of dummy regions22A and22B, i.e., having none of dummy electrode fingers17A and17B, can produce a smaller insertion loss than the comparative example under the condition that each of widths WCA and WCB of side regions21A and21B ranges from λ to 3λ and expansion ratio α ranges from 1.005 to 1.01, thus improving the electrical characteristics of elastic wave device1002.

Side regions21A and21B in which the pitch of interdigital electrode fingers16A and16B becomes wider gradually as located away from center region20provides elastic wave device1002in accordance with Embodiment 2 with a small insertion loss of dual-terminal-pair resonator23.

The elastic wave device having none of dummy regions22A and22B, i.e., having none of dummy electrode fingers17A and17B, can include side regions21A and21B to reduce the insertion loss.

Center region20and side regions21A and21B are provided in each of plural comb-shaped electrode pairs14reduces the insertion loss.

In elastic wave device1002shown inFIG. 6including dual-terminal-pair resonator23that has plural comb-shaped electrode pairs14, electrode fingers16A,16B,17A, and17B in side regions21A and21B and dummy regions22A and22B flare along direction D1oppositely to each other from one point, and the electrode fingers of plural comb-shaped electrode pairs14and reflecting electrodes13form the same curvature for effectively reducing the insertion loss.

In center region20, a pitch of electrode fingers16A and16B of dual-terminal-pair resonator23is constant along propagating direction D1of the elastic wave; however, the pitch in center region20may change along propagating direction D1. For instance, the pitch of interdigital electrode fingers16A and16B in center region20may change gradually like gradation between two comb-shaped electrode pairs14or at the vicinity between comb-shaped electrode pair14and reflecting electrode13along propagating direction D1. The structure discussed above can reduce the insertion loss.

Elastic wave device1002in accordance with Embodiment 2 includes dual-terminal-pair resonator23that includes two comb-shaped electrode pairs14; however, the present invention is applicable to a dual-terminal-pair resonator that includes three or more comb-shaped electrode pairs14, providing the same effect.

FIG. 9Ais a circuit diagram of elastic wave device1003in accordance with Exemplary Embodiment 3 of the present invention. InFIG. 9A, components identical to those of elastic wave device1002shown inFIG. 6according to Embodiment 2 are denoted by the same reference numerals. Elastic wave device1003includes two elastic wave devices1002each including dual-terminal-pair resonator23of two-electrode type. Each of two resonators23constitutes a longitudinally-coupled resonator type elastic wave filter.

FIG. 9Bshows propagation characteristics of elastic wave device1003. InFIG. 9B, the horizontal represents a frequency, and the vertical axis represents attenuation. Two dual terminal pair resonators23of two electrode-type shown inFIG. 9Ainclude dummy regions22A and22B (shown inFIG. 6), and are longitudinally coupled to each other to form a band-pass filter.FIG. 9Bshows propagation profile S3of elastic wave device1003and propagation profile S503of a comparative example having none of side regions21A and21B and having expansion ratio α of 1 of the pitch of the electrode fingers of each of the dual-terminal-pair resonators. Each of two dual-terminal-pair resonators23has expansion ratio α of 1.01 and widths WCA and WCB of 3λ of side regions21A,21B. As shown inFIG. 9B, elastic wave device1003having expansion ratio α of 1.01 and widths WCA and WCB of 3λ can reduce the insertion loss by 0.3 to 0.4 dB from that of the comparative example, thus improving the characteristics of device1003.

As discussed above, elastic wave device1003in accordance with Embodiment 3 is a band-pass filter including two dual-terminal-pair resonators23longitudinally coupled to each other. Side regions21A and21B in which the pitch of the electrode fingers becomes wider gradually as located away from center region20drastically reduce the insertion loss.

FIG. 10Ais a circuit diagram of elastic wave device1004in accordance with Exemplary Embodiment 4 of the present invention. InFIG. 10A, components identical to those of elastic wave device1002shown inFIG. 6according to Embodiment 2 are denoted by the same reference numerals. Elastic wave device1004includes four elastic wave devices1002each including dual-terminal-pair resonator23of two-electrode type. Each of four resonators23constitutes a longitudinally coupled resonator type elastic wave filter.FIG. 10Bshows propagation characteristics of elastic wave device1004. InFIG. 10B, the horizontal axis represents a frequency, and the vertical axis represents attenuation. Four dual terminal pair resonators23of two electrode-type shown inFIG. 10Ainclude dummy regions22A and22B (shown inFIG. 6). Two of the four resonators23are longitudinally coupled to each other to form longitudinally coupled circuit123, namely, two circuits123are connected together in parallel to each other to form a band-pass filter.FIG. 10Bshows propagation profile S4of elastic wave device1004and propagation profile S504of a comparative example including none of side regions21A and21B and having expansion ratio α of 1 of the pitch of the electrode fingers of each of the dual-terminal-pair resonators. Each of four dual terminal pair resonators23has expansion ratio α of 1.01 and widths WCA and WCB of 3λ of side regions21A and21B. As shown inFIG. 10B, elastic wave device1004having expansion ratio α of 1.01 and widths WCA and WCB of 3λ can reduce the insertion loss by about 4 dB from that of the comparative example, thus improving the characteristics of device1004.

As discussed above, elastic wave device1004in accordance with Embodiment 7 is a band-pass filter including four dual-terminal-pair resonators23. Side regions21A and21B in which the pitch of the electrode fingers becomes wider gradually as located away from center region20reduce the insertion loss.

INDUSTRIAL APPLICABILITY

An elastic wave device according to the present invention can reduce a loss of resonating energy, thereby reducing an insertion loss. The elastic device is useful for an elastic wave filter to be used mainly in mobile communication devices.

DESCRIPTION OF REFERENCE MARKS