Patent ID: 12218648

DESCRIPTION OF EMBODIMENTS

Below, an acoustic wave element, filter element, and communication apparatus according to embodiments of the present disclosure will be explained with reference to the drawings. Note that, the drawings used in the following explanation are schematic ones. Size ratios etc. in the drawings do not always coincide with the actual ones.

In the acoustic wave element, any direction may be defined as “above” or “below”. In the following explanation, however, for convenience, an orthogonal coordinate system xyz will be defined, and sometimes the “upper surface” or “lower surface”, and other terms will be used where the positive side of the z-direction is the upper part.

<Outline of Configuration of Acoustic Wave Element>

FIG.1is a plan view showing the configuration of an acoustic wave (SAW: surface acoustic wave) element1according to one embodiment of the present disclosure.FIG.2is an enlarged cross-sectional view of a principal part taken along the II-II line inFIG.1. The SAW element1, as shown inFIG.1, has a piezoelectric substrate2and an excitation (IDT: interdigital transducer) electrode3and reflectors4which are all provided on the upper surface2A of the piezoelectric substrate2.

The piezoelectric substrate2is configured by a substrate of a single crystal having a piezoelectric characteristic comprised of a lithium niobate crystal or lithium tantalate (LiTaO3, below, referred to as an “LT”) crystal. In the present example, the explanation will be given taking as an example a case where use is made of a Y-rotated and X-propagated LT substrate. Note that, the cut angle of the LT substrate will be explained later. The planar shape and various dimensions of the piezoelectric substrate2may be suitably set. As one example, the thickness (z-direction) of the piezoelectric substrate2is 0.2 mm to 0.5 mm.

The IDT electrode3, as shown inFIG.1, has two comb-shaped electrodes30. The comb-shaped electrodes30, as shown inFIG.1, have two bus bars31facing each other and pluralities of electrode fingers32extending from the bus bars31toward the other bus bar31sides. Further, the pair of comb-shaped electrodes30are arranged so that the electrode fingers32connected with one and the electrode fingers32connected with the other intermesh (intersect) with each other in a direction of propagation of the acoustic wave.

Further, the comb-shaped electrodes30have dummy electrode fingers33facing electrode fingers32. Note that, dummy electrode fingers33need not be arranged.

The bus bars31are for example substantially formed in long shapes so as to linearly extend with constant widths. Accordingly, the edge parts in the bus bars31on the sides where they face each other are linear-shaped. The pluralities of electrode fingers32are for example substantially formed in long shapes so as to linearly extend with constant widths and are arranged in the direction of propagation of the acoustic wave at substantially constant intervals.

The pluralities of electrode fingers32in the pair of comb-shaped electrodes30configuring the IDT electrode3are set so as to have a pitch Pt1. The pitch Pt1is for example set so as to be equal to the half wavelength of the wavelength λ of the acoustic wave at a frequency at which resonance is intended to be caused. The wavelength λ (that is 2×Pt1) is for example 1.5 um to 6 μm. The IDT electrode3is arranged so that the pluralities of electrode fingers32have a constant cycle by arranging almost all of the pluralities of electrode fingers32to have the pitch Pt1, so can generate an acoustic wave with a high efficiency.

Here, the “pitch Pt1” designates the interval from the center of one electrode finger32to the center of the other electrode finger32which is adjacent to the one electrode finger32in the direction of propagation. In each electrode finger32, the width w1in the direction of propagation of the acoustic wave is suitably set in accordance with the electrical characteristics demanded from the SAW element1and so on. The width w1of the electrode finger32is for example 0.3 time to 0.7 time the pitch Pt1.

By arranging the electrode fingers32in this way, an acoustic wave propagating in a direction perpendicular to the pluralities of electrode fingers32is generated. Accordingly, after considering the crystal orientation of the piezoelectric substrate2, the two bus bars31are arranged so as to face each other in a direction crossing the direction in which the acoustic wave is intended to be propagated. The pluralities of electrode fingers32are formed so as to extend in the direction perpendicular with respect to the direction in which the acoustic wave is intended to be propagated. Note that, the direction of propagation of the acoustic wave is defined according to the orientation of the pluralities of electrode fingers32and so on. In the present embodiment, however, for convenience, sometimes the orientation of the pluralities of electrode fingers32etc. will be explained using the direction of propagation of the acoustic wave as the standard.

The number of the electrode fingers32is 50 to 350 per one side. The lengths of the pluralities of electrode fingers32(length from the bus bar to the tip ends) are for example set substantially the same. The length of mutual intermeshing of the facing electrode fingers32(intersecting width) is 10 to 300 μm.

The IDT electrode3is for example configured by a conductive layer15of metal. The material of this metal and the thickness S (z-direction) of this conductor layer15will be explained later.

The IDT electrode3may be directly arranged on the upper surface2A of the piezoelectric substrate2or may be arranged on the upper surface2A of the piezoelectric substrate2through an underlying layer configured by another member. The other member is for example comprised of Ti, Cr, or an alloy of the same or the like. When the IDT electrode3is arranged on the upper surface2A of the piezoelectric substrate2through an underlying layer, the thickness of the other member is set to a thickness to an extent where almost no influence is exerted upon the electrical characteristics of the IDT electrode3(for example within a thickness of 5% of the thickness of the IDT electrode3in the case of Ti).

This underlying layer, when viewed by a cross-section, may be made larger in a width of a part contacting the piezoelectric substrate2than a width of a part contacting the electrode fingers32in the IDT electrode3. In that case, the electric power resistance can also be raised by the underlying layer.

When a voltage is applied, the IDT electrode3excites an acoustic wave propagating in the x-direction near the upper surface2A of the piezoelectric substrate2. The excited acoustic wave is reflected at a boundary of the electrode fingers32with a region where the electrode fingers32are not arranged (long-shaped region between neighboring electrode fingers32). Further, a standing wave having the pitch Pt1of the electrode fingers32as a half wavelength is formed. The standing wave is converted to an electrical signal having the same frequency as that of this standing wave and is extracted by the electrode fingers32. In this way, the SAW element1functions as a 1 port resonator.

The reflectors4are arranged so as to sandwich the IDT electrode3in the direction of propagation of the acoustic wave. The reflector4is formed in substantially a slit shape.

The protective layer5, as shown inFIG.2, is provided on the piezoelectric substrate2so as to cover the tops of the IDT electrode3and reflectors4. Specifically, the protective layer5covers the surfaces of the IDT electrode3and reflectors4and covers the part of the upper surface2A of the piezoelectric substrate2which is exposed from the IDT electrode3and reflectors4. The thickness of the protective layer5is for example 1 nm or more, 20% or less of the thickness of the IDT electrode3.

The protective layer5is made of a material having an insulation characteristic and contributes to protection from corrosion and the like. Further, the protective layer5may be formed by SiO2or another material making the propagation velocity of the acoustic wave faster when the temperature rises as well. Due to this, it is also possible to suppress a change of the electrical characteristics due to a change of the temperature of the acoustic wave element1. Further, SiNx or another material may be used for improving a moisture resistance as well.

(Relationship between Piezoelectric Substrate2and IDT Electrode3)

Conventionally, in an acoustic wave element, considering the excitation efficiency of the acoustic wave, radiation loss, electric resistance etc., here a configuration in which an IDT electrode comprised of Al having a thickness of about 8% of the wavelength ratio of the acoustic wave is provided on an LT substrate having a cut angle of 42° has been generally used (below, an acoustic wave element having this configuration will be referred to as a “comparative model”).

As opposed to this, in recent years, the electric power of the high frequency signal input to an acoustic wave element has become larger, therefore an electrode which is more excellent in electric power resistance than an electrode comprised of Al has been demanded.

However, with just replacement by Mo or another electrode material which has a high strength, characteristics of the excitation efficiency of the acoustic wave, radiation loss, and electric resistance enough to enable replacement for an electrode made of Al could not be manifested since there is a tradeoff in these characteristics.

Therefore, in the SAW element1according to the present embodiment, first, Cu was selected as a material improving the electric power resistance and having the possibility of manifestation of characteristics substantially equal to those of current Al electrodes. Below, the specific configuration of the electrodes will be studied.

<Composition Ratio of Layer Containing Cu>

First, a composition ratio capable of raising the electric power resistance to the highest level as the layer containing Cu was studied. Specifically, an IDT electrode3formed by changing the composition ratio of Cu and Al was formed, and the electric power resistance thereof was measured.

The results thereof will be shown inFIG.3. InFIG.3, an abscissa shows the ratio of Cu and Al, and an ordinate shows the electric power resistance (dBm). Note that, as the composition ratio of Cu and Al, a value which existed as a Cu—Al alloy was set. Note that, in the case of 100% of Cu, the Cu was oxidized during an electric power resistance test and a low electric power resistance was exhibited. Further, in a case of 100% of Al, the electric power resistance was that of the comparative model. As a result of this, it was confirmed that the electric power resistance was improved compared with the comparative model in any case other than a composition ratio of Cu and Al of 3:2. In particular, it was confirmed that the electric power resistance could be raised the most when using an alloy of a composition ratio of Cu and Al of 1:2 (that is, CuAl2alloy). Due to this, the layer containing Cu was configured by a CuAl2alloy. By configuring the layer containing Cu by an alloy, oxidation reduction become harder compared with the layer configured by Cu alone, therefore the reliability as an electrode can be raised.

<Electrode Configuration of IDT Electrode3>

From the viewpoint of electric power resistance, preferably use is made of a layer configured by CuAl2as the IDT electrode3. However, the inventors studied the configuration of the IDT electrode3in order to simultaneously improve the excitation efficiency of the acoustic wave, radiation loss, and other characteristics. Specifically, as shown inFIG.4, a model configured by a multilayer structure of a first layer35comprised of Al and a second layer36containing Cu (comprised of CuAl2) was prepared as the IDT electrode3, and the excitation efficiency of the acoustic wave, radiation loss and other characteristics at the time when the relationships between the cut angle of the piezoelectric substrate2and the thicknesses of the layers in the IDT electrode3were made different were simulated. Specifically, a combination of thicknesses giving the smallest loss in the resonance frequency in each model was found. The optimum ratios of the thickness of the first layer35and the second layer37at the different cut angles found from the results of simulation are shown inFIG.5.

InFIG.5, the abscissa shows the total of the thicknesses (below, also referred to as the “total thickness”) of the IDT electrode3by the wavelength ratio, while the ordinate shows the ratio of the second layer37relative to the total thickness of the IDT electrode3. Note that, basically, the multilayer structure of the first layer35and the second layer37such as shown inFIG.4is employed. However, simulation is also carried out for both of a case where the second layer37is 0% and a case where it is 100%. That is, the simulation is also carried out for the case where the IDT electrode3is configured by only the first layer35and for the case where it is configured by only the second layer37.

As clear also fromFIG.5, it could be confirmed that the total thickness of the IDT electrode3became thicker as the cut angle of the piezoelectric substrate2was increased from 42° to 48°. The state of change of the optimum thickness ratio at the different cut angles trended the same without regard to the cut angle. That is, it could be confirmed that the trend of the ratio of the second layer37relative to the total thickness of the IDT electrode3was the same also in the case where the cut angle was made different. Further, as confirmed, even when the ratio of the second layer37was changed within a range from 15% to 50% (range of 0.15 to 0.5), the total thickness of the electrode for minimizing the loss became a constant value.

When the total thickness, the thickness of the first layer35, and the thickness of the second layer37of the IDT electrode3satisfy the relationships shown inFIG.5, due to the presence of the second layer37configured by a material having a larger tensile strength than the first layer35, in addition to the electric power resistance of the IDT electrode3being able to be raised, the radiation loss of the acoustic wave is suppressed and the excitation efficiency can be raised. From the above, it becomes possible to provide a SAW element1which is excellent in electric power resistance and has a small loss.

Note that, the relationships explained above were obtained by finding by simulation the values giving the smallest radiation loss of the acoustic wave at the resonance frequency when changing the cut angle of the piezoelectric substrate2and the material of the IDT electrode3(multilayer structure) and showing the correlation among them. Further, the validity was verified by measured values.

As such a first layer35, use can be made of Al or an Al alloy containing Al as the principal ingredient. That is, use can be made of an Al alloy containing a sub-component in an amount less than 10%. For example, use can be made of an Al—Cu alloy formed by adding about 1% to 3% of Cu to Al, an Al—Cu—Mg alloy formed by further adding 0.5% to 3% of Mg, or the like. In this case, Cu and Mg mainly become positioned at a grain boundary of the crystal grains of Al. Note that, the first layer35may be made of Al containing a sub-component in a concentration of less than 5% as well. In that case, the crystallinity of Al can be raised, therefore the reliability and electrical characteristics can be improved.

Further, as the second layer37, use can be made of a CuAl2alloy. The second layer37may contain impurities to an extent where segregation at the grain boundary or formation of a solid solution in the CuAl2crystal occurs. Such impurities may be for example less than 5% as well. The concentration of impurities may be made less than 2% as well.

Further,FIG.4showed an example of arranging the second layer37on the side closer to the piezoelectric substrate2. However, the arrangement is not limited to this example. For example, the first layer35may be arranged on the piezoelectric substrate2side as well.

When the second layer37is arranged on the piezoelectric substrate2side, a second layer37having a stronger strength can be provided on the side closer to the piezoelectric substrate2having a stronger vibration, therefore it becomes possible to provide a SAW element1excellent in electric power resistance. Further, the center of gravity of the IDT electrode3can be moved downward, therefore the electromechanical coupling coefficient can be made smaller, and the propagation loss can be made smaller.

Note that, the radiation loss of the acoustic wave is also related to the density of the material configuring the IDT electrode3. Therefore, in a case where a so-called mass-addition film is provided on the IDT electrode3or a case where the IDT electrode3is buried in the insulation material, the relationships explained above do not stand. The term “buried” means for example a case where the thickness of the insulation material is half or more of the thickness of the IDT electrode3or the like.

Further, the crystallinity of the second layer37is not particularly limited. The layer may be amorphous in state or may be polycrystalline.

Modification 1

A modification of the SAW element1will be explained.

The larger the ratio of the second layer37, the higher the electric power resistance. However, the larger the ratio of the second layer37, the higher the electric resistance of the electrode. As a result, an electrical loss becomes larger.

Therefore, the inventors studied combinations of film thicknesses not degrading the electric resistance while raising the electric power resistance by the second layer37. Specifically, with respect to a SAW element1having an IDT electrode3satisfying the optimum film thickness ratio of the first layer35and the second layer37which was clarified inFIG.5, the inventors ran simulations on the maximum stress applied to each of the first layer35and the second layer37and the relative electrical resistivity of the IDT electrode3to the comparative model. The results thereof will be shown inFIG.6.

InFIG.6, the abscissa shows the ratio of the thickness of the second layer37relative to the total thickness of the IDT electrode3, while the ordinate shows the maximum stress (unit: MPa) on the left side and the electrical resistivity when compared with the electric resistance in a case where the IDT electrode3is configured by only the first layer35on the right side. Note that, inFIG.6, the case where the cut angle of the piezoelectric substrate2is 46° is exemplified.

As clear also fromFIG.6, the state of reduction of the stress applied to the first layer35as the thickness of the second layer37became thicker could be confirmed. On the other hand, the state of the electrical resistivity rapidly becoming higher when the ratio of the second layer37exceeded 56% could be confirmed. The same trend of degradation of the electrical characteristics described above occurred at other cut angles as well. For this reason, the ratio of film thickness of the second layer37relative to the total thickness of the IDT electrode3may be controlled to 56% or less as well. By employing such a configuration, a SAW element1excellent in electric power resistance and reduced in occurrence of loss due to the electric resistance can be provided.

Further, when the cut angle of the piezoelectric substrate2is 46°, if the ratio of the second layer37is controlled to 27% or less, the electric resistance can be made substantially equal to that at the time when the IDT electrode3is configured only by the first layer35.

Next, a SAW element1having an IDT electrode3satisfying the optimum film thickness ratio of the first layer35and second layer37was prepared and was measured for breakdown power. The results thereof are shown inFIG.7. InFIG.7, an abscissa shows the ratio of the thickness of the second layer37relative to the total thickness of the IDT electrode3, while an ordinate shows the breakdown power (unit: dBm). Note that, the breakdown of the IDT electrode3includes two modes of migration and spark. The electric power where each occurs is plotted.

As clear also from this graph, it was seen that the breakdown power became sharply lower if the thickness of the second layer37relative to the total thickness of the IDT electrode3became less than 20%. It was confirmed from the above that the electric power resistance could be raised by making the ratio of the thickness of the second layer37relative to the total thickness of the IDT electrode320% or more. Note that, from the fact that in a case where the ratio of the thickness of the second layer37relative to the total thickness of the IDT electrode3exceeds 20%, the maximum stress applied to the first layer35becomes 50% or less compared with the case where the IDT is configured by only the first layer35as shown inFIG.6, it can be confirmed that the electric power resistance of the entire IDT electrode3can be raised.

Modification 2

In the modification 1, the configuration optimizing the ratio of the first layer35and second layer37to thereby reduce an increase of the electric resistance was explained. However, it is also possible to adjust the cut angle of the piezoelectric substrate2to lower the electric resistance.

If the ratio of the second layer37becomes larger, the electric resistance of the electrode rises. As a result, the electrical loss becomes larger. On the other hand, as shown inFIG.5, it is seen that the total thickness can be made thicker as the cut angle of the piezoelectric substrate2is made larger.

From the above, it is possible to raise the ratio of the second layer37to raise the electric power resistance and make the cut angle of the piezoelectric substrate2larger to make the electric resistance smaller. Specifically, the electric resistance of the electrode may be made the same degree or less compared with the comparative model.

In order to compare the electric resistance characteristic of the electrodes, a FOM (figure of merit) is introduced. In this example, as the FOM, use is made of a value obtained by multiplying the conductivity and the electrode thickness. Note that, in the case of a multilayer structure, the conductivities and the electrode thicknesses are multiplied for the different electrode materials and the products added to calculate the FOM of the electrode as a whole. When defining the FOM in this way, the larger the FOM, the smaller the electric resistance per unit thickness of the electrode.

Note that, in the calculation, the conductivity is expressed in units of MS/cm (conductivity of Cu: 0.588 MS/cm, conductivity of Al: 0.370 MS/cm, and conductivity of CuAl2: 0.136 MS/cm) and the thickness of the electrode is represented by the ratio relative to the wavelength λ (=2×Pt). For example, in a case where the film thickness is 8% in terms of the wavelength ratio in an electrode of pure Al, FOM=0.370×0.08=0.0296 stands. Further, in a case where the film thickness of Cu is 3% and the film thickness of Al is 4%, FOM=0.588×0.03+0.370×0.04=0.0325 stands.

Therefore, for an IDT electrode3satisfying each optimum film thickness ratio shown inFIG.5, FOM values when making the cut angle of the piezoelectric substrate2different were calculated. Note that, the FOM in the comparative model is 0.0296.

As a result, it was confirmed that, when the cut angle was made larger, the FOM value became larger and the ratio of the second layer37capable of manifesting electrical characteristics at least equal to those in the comparative model could be made larger. Specifically, in terms of (cut angle, ratio of second layer37, total thickness), the FOM at the time of (42°, 0.3, 0.08λ) is 0.0330, and the FOM at the time of (46°, 0.3, 0.12λ) is 0.0360. In this way, it was seen that electrical characteristics at least equal to those in the comparative model were realized in any case. Further, the FOM becomes larger as the cut angle is made larger. The FOM is greatly improved compared with comparative model. Therefore, it was confirmed that electrical characteristics at least equal to those in the comparative model could be obtained even if the ratio of the second layer having a low conductivity was increased (in this case, even if the ratio of the second layer was made 0.5).

Modification 3

In the example explained above, the case where the first layer35and second layer37were stacked in order on the piezoelectric substrate2was explained. However, the configuration is not limited to this. For example, as shown inFIG.8, an intermediate layer38may be provided between the first layer35and the second layer37as well.

The intermediate layer38is configured by a material which is low in reactivity with Cu and Al and is higher in the strength than Al. For example, it may be configured by a layer made of a material which is chemically stable and has conductivity such as Ti or Cr or a laminate thereof. By providing the intermediate layer38, diffusion between the first layer35and the second layer37can be reduced. As a result, a SAW element1having stable characteristics can be provided.

Further, when Ti is used as the intermediate layer38and the first layer35is formed on that, the crystal of Al can be made grow in the <111> direction, therefore a first layer35excellent in crystallinity can be obtained For this reason, occurrence of a defect causing breakage can be reduced, so the electric power resistance can be raised.

The thickness of such an intermediate layer38is controlled to a thickness within a range not influencing the electrical characteristics etc. of the IDT electrode3. For example, it may be made a value of 3 nm or more and 5% or less of the total thickness of the IDT electrode3. By setting such a thickness, it is possible to realize a diffusion prevention function of the element between the layers which are positioned above and below each other and there is no adverse influence upon the excitation of the acoustic wave and the electric resistance of the electrode.

Note that, in the example shown inFIG.8, the underlying layer34is positioned on the lower side of the second layer37. As such an underlying layer34, for example use can be made of Ti etc.

Note that the thicknesses of the intermediate layer38and underlying layer34are excluded from the total thickness of the IDT electrode3and the relative film thickness of the second layer37etc. found inFIGS.5,7,8and the like.

Further, in the example shown inFIG.8, the explanation was given by taking as an example the case where one first layer35and one second layer37were provided. However, there may be pluralities of the first layer35and second layer37as well. In that case, each first layer35and each second layer37may be alternately stacked and the intermediate layer38may be provided between each two or the intermediate layer38may be provided between the two in at least part.

Further, in addition to the part between the first layer35and the second layer37, one or more intermediate layers38may be provided in the middle of the thickness of the first layer35. In this case, migration in the first layer35can be reduced, therefore the electric power resistance can be improved. Further, compared with the case where the intermediate layer38is not provided in the middle of the thickness of the first layer35, by providing the intermediate layer38in the middle of the thickness of the first layer35, the size of the Al crystal grain configuring the first layer35can be made smaller. From this fact as well, the electric power resistance of the IDT electrode3can be raised.

Modification 4

In the example explained above, the cross-sectional shape of the first layer35and the second layer37in the thickness direction was rectangular. However, the shape is not limited to this. For example, it may be a trapezoidal shape. Otherwise, as shown inFIG.9, a part having a thin thickness may be present at a position separated from the outer edge in the width direction of the electrode fingers32as well. By employing such a configuration, a SAW element capable of reducing the loss due to the electric resistance while raising the electric power resistance can be provided. Below, the mechanism thereof will be explained.

The stress distribution in the plane of the electrode fingers32in the IDT electrode3at the time when a high frequency signal was input to the IDT electrode3was simulated. The results thereof will be shown inFIG.10. InFIG.10, the abscissa shows the position in the width direction of the electrode fingers32, and the ordinate shows the maximum stress (dBm). As clear also fromFIG.10, a state where that the stress applied to the electrode fingers32was the largest at the outer edge and became smaller toward the vicinity of the center of the width was confirmed.

It is estimated from this that the electric power resistance can be maintained even if the second layer37excellent in electric power resistance is made larger in thickness at the peripheral part of the outer edge where the stress is larger and is made smaller in thickness at the inner side from the peripheral part of the outer edge. In particular, it is estimated that the electric power resistance is maintained even if the thickness is made smaller in the vicinity of the center.

The example shown inFIG.9shows a shape where the thickness of an outer peripheral part continuing from the outer edge is large and the thickness of the center part37bsurrounded by the outer peripheral part37ais small. Further, the first layer35enters into the region recessed by the center part37b. That is, the thickness of the center part37bbecomes smaller compared with the periphery. By making the outer peripheral part37ahaving a larger thickness positioned in a place where the stress is large and making the center part37bhaving a small thickness positioned in a place where the stress is small in this way, the electric power resistance can be improved.

Note that, in such a case, the ratios of the thicknesses of the first layer35and the second layer37are corrected in accordance with the volume.

Further, the configuration where a part having a small thickness is present at a position separated from the outer edge in the width direction of the electrode fingers32is not limited toFIG.9. For example, when viewed by a cross-section in the thickness direction, a region which continuously becomes smaller in thickness linearly or in a curve state the more separated from the outer edge may be provided, a region which becomes smaller in thickness stepwise may be provided, or the shape may be a V-shape or mortar shape. Further, as shown inFIG.11, two or more center parts37bmay be provided as well.

In these cases, the intermediate layer38may be provided as well. The intermediate layer38may be provided along the shape of the upper surface of the second layer37or may be provided at the height position matching with the upper surface of the outer peripheral part37aas shown inFIG.11.

Modification 5

In the example explained above, the explanation was given by taking as an example a case where the first layer35was a single layer of Al and there was no gradient in concentration in the thickness direction, but the first layer is not limited to this. That is, the first layer35may be provided with a gradient of concentration in the thickness direction as well. Specifically, the sub-component of the first layer35may be provided with a gradient of concentration in the thickness direction as well.

For example, as shown inFIG.15A, the first layer35may be provided with a layered region39containing a lot of Cu at the intermediate layer38side.

In the case of the configuration shown inFIG.15A, by providing the region39containing a lot of Cu having a higher electric power resistance compared with Al at a position near the piezoelectric substrate2having a large vibration, the electric power resistance of the SAW element can be raised.

Further, as shown inFIG.15B, two or more regions39may be provided in the middle of the thickness of the first layer35as well. Even in this case, the electric power resistance can be raised. This is because, in addition to the electric power resistance being raised by making the grain size of Al smaller, the electric power resistance can be raised by forming a pseudo Al—Cu added alloy by making Cu diffuse from the region39to the grain boundary of the crystal of Al.

Note that, in both ofFIG.15AandFIG.15B, the thickness and composition of the region39are adjusted so that the Cu concentration in the entire first layer35including the region39becomes the sub-component not more than 10%. The region39may contain Cu as the principal ingredient or may be an alloy such as CuAl2. Further, a gradient of concentration where the concentration gradually becomes smaller toward the two sides or one side of the thickness direction from the region39may be provided.

Other Modifications

Further, in the example shown inFIG.2, the explanation was given by taking as an example a case where use was made of a sufficiently thick piezoelectric substrate2. However, a so-called bonded substrate formed by using a thin piezoelectric substrate2and bonding a support substrate to the lower surface of the piezoelectric substrate may be used as well. In this case, the thickness of the piezoelectric substrate2may be made for example about 0.5 μm to 20 μm, and the support substrate may be given a thickness larger enough to support the piezoelectric substrate2. In particular, where a sapphire substrate or Si substrate etc. is used, deformation of the piezoelectric substrate2due to a temperature change can be reduced, therefore a SAW element excellent in temperature characteristic can be provided. Note that, the thickness of the piezoelectric substrate2may be made 0.2λ to 10λ in terms of wavelength ratio. Further, between the piezoelectric substrate2and the support substrate, a junction layer, an acoustic intermediate layer, or the like may be positioned as well.

Further, in the example explained above, the explanation was given by taking as an example a case where the IDT electrode3had a uniform layer configuration and thickness. However, it is sufficient that the electrode fingers32satisfy the relationships explained above in the region where the electrode fingers32intersect each other. The configuration is not limited to this. For example, the bus bars31may have a larger thickness compared with the electrode fingers32or may be provided with a different layer configuration.

Note that, the characteristics of the formulas derived in the examples explained above were confirmed to be similar in the trend of increase of the total thickness of the IDT electrode3even if the cut angle of the piezoelectric substrate2is 50° or more. Note that, the match between the measured values and the simulation values is confirmed up to when the cut angle is 50°.

Example

The SAW element1shown inFIG.8and the SAW element in the comparative model were prepared, and the breakdown powers were measured. The specific conditions were as follows:Piezoelectric substrate2: 46°-rotated, Y-cut, X-propagated lithium tantalate substrateIDT electrode3: 2.7 μm of pitch, 0.5 of duty, and 100 electrode fingers32First layer35: Material . . . Al, Thickness . . . 541 nmSecond layer37: Material . . . CuAl2, Thickness . . . 54 nmUnderlying layer34: Material . . . Ti, Thickness . . . 6 nmIntermediate layer38: Material . . . Ti, Thickness . . . 6 nm

The breakdown power of the SAW element according to the example was improved by 3 dB compared with the breakdown power of the SAW element in the comparative model. This is synonymous with improvement about 1000 times when converted into time. As described above, it was confirmed that the SAW element in the present disclosure was provided with a high electric power resistance. Further, the results of measurement of the frequency characteristics of the SAW element according to the example will be shown inFIG.14. As clear also fromFIG.14, it was confirmed that the both of the SAW elements in the comparative model and the example realized good resonance characteristics.

<Filter Element and Communication Apparatus>

FIG.12is a block diagram showing the principal part in a communication apparatus101according to an embodiment of the present disclosure. The communication apparatus101performs wireless communication utilizing radio waves. The multiplexer7has the function of branching a signal having the transmission frequency and a signal having the reception frequency in the communication apparatus101.

In the communication apparatus101, a transmission information signal TIS including information to be transmitted is modulated and raised in frequency (conversion to a high frequency signal having a carrier frequency) by an RF-IC103to become a transmission signal TS. The transmission signal TS is stripped of unwanted components other than the transmission-use passing band by a band pass filter105, is amplified by an amplifier107, and is input to the multiplexer7. The multiplexer7strips the unwanted components other than the transmission-use passing band from the input transmission signal TS and outputs the result to an antenna109. The antenna109converts the input electrical signal (transmission signal TS) to a wireless signal and transmits the result.

In the communication apparatus101, the wireless signal received by the antenna109is converted to an electrical signal (reception signal RS) by the antenna109and is input to the multiplexer7. The multiplexer7strips unwanted components other than the reception-use passing band from the input reception signal RS and outputs the result to an amplifier111. The output reception signal RS is amplified by the amplifier111and is stripped of unwanted components other than the reception-use passing band by a band pass filter113. Further, the reception signal RS is boosted down in frequency and demodulated by the RF-IC103to become the reception information signal RIS.

The transmission information signal TIS and reception information signal RIS may be low frequency signals (baseband signals) containing suitable information. For example, they are analog audio signals or digital audio signals. The passing band of the radio signal may be one according to the UMTS (Universal Mobile Telecommunications System) or other various standards. The modulation scheme may be a phase modulation, amplitude modulation, frequency modulation, or a combination of any two or more among them.

FIG.13is a circuit diagram showing the configuration of a multiplexer7according to one embodiment of the present disclosure. The multiplexer7is a multiplexer used in the communication apparatus101inFIG.12. The multiplexer7has a filter element configuring a transmission filter11and/or receiving filter12. The filter element configuring the transmission filter11and/or receiving filter12is configured by the SAW element1and a resonator arranged on the piezoelectric substrate2.

The SAW element1is for example a SAW element configuring part of a ladder type filter circuit in the transmission filter11in the multiplexer7shown inFIG.12. The transmission filter11, as shown inFIG.13, has the piezoelectric substrate2and serial resonators S1to S3and parallel resonators P1to P3which are all formed on the piezoelectric substrate2.

The multiplexer7is mainly configured by an antenna terminal8, transmission terminal9, reception terminals10, transmission filter11arranged between the antenna terminal8and the transmission terminal9, and receiving filter12arranged between the antenna terminal8and the reception terminals10.

The transmission terminal9receives as input the transmission signal TS from the amplifier107. The transmission signal TS input to the transmission terminal9is stripped of unwanted components other than the transmission-use passing band in the transmission filter11and the result is output to the antenna terminal8. Further, the antenna terminal8receives as input the reception signal RS from the antenna109. The signal is stripped of unwanted components other than the reception-use passing band in the reception filter12and the result is output to the reception terminals10.

The transmission filter11is for example configured by a ladder type SAW filter. Specifically, the transmission filter11has three serial resonators S1, S2, and S3which are connected in series between its input side and the output side and three parallel resonators P1, P2, and P3which are provided between a serial arm forming a wiring for connecting the serial resonators to each other and a reference potential part Gnd. That is, the transmission filter11is a ladder type filter having a 3-stage configuration. However, in the transmission filter11, there may be any number of stages of the ladder type filter.

Between the parallel resonators P1, P2, and P3and the reference potential part Gnd, an inductor L is provided. By setting an inductance of this inductor L to a predetermined magnitude, an attenuation pole is formed out of the band of the passing frequency of the transmission signal and thereby make the out-of-band attenuation larger. Each of the plurality of serial resonators S1, S2, and S3and plurality of parallel resonators P1, P2, and P3is configured by a SAW resonator like the SAW element1.

The receiving filter12for example has a multimode type SAW filter17and an auxiliary resonator18connected in series to the input side of the multimode type SAW filter17. Note that, in the present embodiment, the multimode includes a double mode. The multimode type SAW filter17has a balance-unbalance conversion function, and the receiving filter12is connected to the two reception terminals10from which the balanced signals are output. The receiving filter12is not limited to one configured by the multimode type SAW filter17and may be configured by a ladder type filter or may be a filter without having a balance-unbalance conversion function.

Between the connection point of the transmission filter11, receiving filter12, and antenna terminal8and the ground potential part an impedance matching-use circuit configured by an inductor or the like may be inserted as well.

The SAW element in the present embodiment may be used in any of the serial resonators S1to S3or any of the parallel resonators P1to P3. By using the SAW element1in at least one of the parallel resonators P1to P3, the electric power resistance of the filter can be raised.

Further, the electrode design of the SAW element1may be made different for each frequency as well. Specifically, for example, the electrode configuration and the cut angle of the piezoelectric substrate which configure the SAW element1may be different between the transmission filter11and the receiving filter12as well. Further, irrespective of transmission/reception, the electrode configuration and the cut angle of the piezoelectric substrate which configure the SAW element1may be different between a filter having a high passing band and a filter having a low passing band as well. For example, in the filter on a low frequency side, the cut angle of the piezoelectric substrate may be made larger than that in the filter on a high frequency side to design the IDT electrodes3in the present application as well.

Further, on the side of the filter having a multimode type SAW filter having a lower electric power resistance compared with the ladder type filter, IDT electrodes3having a higher electric power resistance (having a ratio of second layer made larger) by making the cut angle larger may be employed as well.

REFERENCE SIGNS LIST

1acoustic wave element (SAW element)2piezoelectric substrate2A upper surface3excitation (IDT) electrode35first layer37second layer38intermediate layer4reflector5protective layer7multiplexer8antenna terminal9transmission terminal10reception terminal11transmission filter12receiving filter15conductive layer17multimode type SAW filter18auxiliary resonator101communication apparatus103RF-IC105bandpass filter107amplifier109antenna111amplifier113bandpass filterS1to S3serial resonatorsP1to P3parallel resonators