Acoustic wave resonator, acoustic wave filter, multiplexer, communication apparatus, and method designing acoustic wave resonator

An acoustic wave resonator includes a piezoelectric substrate and an IDT electrode on the top surface of the piezoelectric substrate. Between a resonance frequency and anti-resonance frequency due to a surface acoustic wave, one to four of at least one of resonance frequencies or anti-resonance frequencies due to bulk waves are located.

TECHNICAL FIELD

The present disclosure relates to an acoustic wave resonator using an acoustic wave, an acoustic wave filter having the acoustic wave resonator, a multiplexer having the acoustic wave filter, a communication apparatus having the multiplexer, and a method for designing the acoustic wave resonator.

BACKGROUND ART

Known in the art is a surface acoustic wave resonator (SAW resonator) having a piezoelectric substrate and an IDT (InterDigital Transducer) electrode provided on the top surface of the piezoelectric substrate and exciting a surface acoustic wave (SAW) (for example Patent Literatures 1 and 2).

In Patent Literature 1, a capacity element is connected parallel to the IDT electrode. It is known that by providing such a capacity element, the anti-resonance frequency of a SAW can be moved to a low frequency side and a difference of frequencies from the resonance frequency to the anti-resonance frequency can be made narrower. Note that, in Patent Literature 1, a reflector is dually used as a capacity element so as to reduce the size of the SAW resonator.

In Patent Literature 2, the piezoelectric substrate is not used for the SAW resonator alone. A bonded substrate formed by bonding together a piezoelectric substrate and a support substrate having a smaller thermal expansion coefficient compared with the piezoelectric substrate is used for the SAW resonator. By utilizing such a bonded substrate, for example, a change of electrical characteristics of the SAW resonator due to temperature is compensated for. Patent Literature 2 discloses that spurious emission is generated if using a bonded substrate and that the factor behind that spurious emission is a bulk wave. Further, Patent Literature 2 proposes an electrode structure for cancelling out bulk waves by each other.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

An acoustic wave resonator according to one aspect of the present disclosure includes a piezoelectric substrate and an IDT electrode on a top surface of the piezoelectric substrate. Between a resonance frequency and anti-resonance frequency due to the surface acoustic wave, one to four of at least one of resonance frequencies or anti-resonance frequencies due to bulk waves are located.

An acoustic wave filter according to one aspect of the present disclosure includes a piezoelectric substrate, a support substrate bonded to a bottom surface of the piezoelectric substrate, and a plurality of IDT electrodes on a top surface of the piezoelectric substrate. The plurality of IDT electrodes include a first IDT electrode and a second IDT electrode which is different in thickness from the first IDT electrode.

A multiplexer according to one aspect of the present disclosure includes an antenna terminal, a transmission filter which filters a transmission signal and outputs the result to the antenna terminal, and a receiving filter which filters the reception signal from the antenna terminal. At least one of the transmission filter and the receiving filter includes the above acoustic wave filter.

A communication apparatus according to one aspect of the present disclosure includes an antenna, a multiplexer described above in which the antenna terminal is connected to the antenna, and an IC connected to the transmission filter and the receiving filter.

A method designing an acoustic wave resonator according to one aspect of the present disclosure, specifies a thickness of electrode fingers of an IDT electrode whereby in a case where a pitch of the electrode fingers is a predetermined initial value, a resonance frequency and an anti-resonance frequency due to a surface acoustic wave are located on the two sides of at least one of a resonance frequency and anti-resonance frequency due to a bulk wave; and specifies the pitch of the electrode fingers by which the above one frequency coincides with a predetermined target frequency by the thickness of the electrode fingers specified in the electrode thickness setting step.

DESCRIPTION OF EMBODIMENTS

Below, an acoustic wave resonator according to an embodiment 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 resonator, any direction may be defined as “above” or “below”. In the following description, however, for convenience, an orthogonal coordinate system comprised of a D1 axis, D2 axis, and D3 axis will be defined, and the “top surface”, “bottom surface”, and other terms will be sometimes used while using the positive side of the D3 axis as “above”.

(Outline of Configuration of Acoustic Wave Resonator)

FIG. 1is a plan view showing the configuration of an acoustic wave resonator1according to one embodiment of the present disclosure.FIG. 2is a cross-sectional view taken along the II-II line inFIG. 1. However, inFIG. 2, the number of the electrode fingers explained later is drawn smaller than that inFIG. 1.

The acoustic wave resonator1is a resonator based on a new principle of utilizing a SAW and bulk wave as acoustic waves. However, the configuration of the acoustic wave resonator1, except for various dimensions etc., may be basically made the same as the configuration of a SAW resonator. Specifically, this is as follows.

The acoustic wave resonator1for example has a bonded substrate3and an electrode portion5configured on the top surface of the bonded substrate3. Although not particularly shown, the acoustic wave resonator1, other than these, may have a protective layer configured by SiO2etc. covering the electrode portion5and so on.

The bonded substrate3for example has a piezoelectric substrate7and a support substrate9(FIG. 2) bonded to the bottom surface of the piezoelectric substrate7. Note that,FIG. 1shows an example of an X-axis, Y-axis, and Z-axis of the piezoelectric substrate7.

The piezoelectric substrate7is for example configured by a single crystal substrate having a piezoelectric characteristic. The single crystal substrate is for example comprised of lithium tantalate (LiTaO3), lithium niobate (LiNbO3), or quartz crystal (SiO2). The cut angle may be suitably set. For example, the lithium tantalate is a 42°±10° Y-plate or 0°±10° X-plate etc. The lithium niobate may be a 128°±10° Y-plate or 64°±10° Y-plate etc.

Note that, below, an aspect where the piezoelectric substrate7is configured by a 38° to 48° Y-plate made of lithium tantalate will be mainly explained as an example. Unless otherwise indicated, the results of simulation etc. explained later are for a 38° to 48° Y-plate made of lithium tantalite. Describing this for confirmation, in this Y-plate, the major surfaces (upper surface and lower surface) are perpendicular to a Y′-axis (not shown) obtained by rotation around the X-axis from the Y-axis to the Z-axis by an angle of 38° to 48°.

The thickness ts(FIG. 2) of the piezoelectric substrate7is for example constant over the entire of the piezoelectric substrate7in the surface direction. As will be explained later, in the acoustic wave resonator1in the present embodiment, unlike a SAW resonator, this thickness tsalso becomes a parameter defining the resonator characteristics.

The support substrate9is for example formed by a material having a smaller thermal expansion coefficient than that of the material of the piezoelectric substrate7. Due to this, a change due to temperature of electrical characteristics of the acoustic wave resonator1can be compensated for. As such a material, for example, silicon or other semiconductor, sapphire or other single crystal, and an aluminum oxide sintered body or other ceramic may be mentioned. Note that, the support substrate9may also be obtained by stacking a plurality of layers made of materials which are different from each other.

The thickness of the support substrate9is for example constant over the entire of the support substrate9in the surface direction. The size thereof may be suitably set in accordance with specifications etc. demanded from the acoustic wave resonator1. For example, the thickness of the support substrate9is made thicker than the thickness of the piezoelectric substrate7. In this case, for example, the action of temperature compensation becomes stronger, and the strength of the piezoelectric substrate7is reinforced. As an example, the thickness of the support substrate9is 100 μm to 300 μm. The planar shape and various dimensions of the support substrate9are for example equal to those of the piezoelectric substrate7.

The piezoelectric substrate7and the support substrate9are bonded to each other through for example a not shown bonding layer. The material of the bonding layer may be an organic material or may be inorganic material. As the organic material, for example, a thermosetting resin or other resin may be mentioned. As an inorganic material, for example, SiO2may be mentioned. Further, the piezoelectric substrate7and the support substrate9may be bonded to each other by so-called “direct bonding” activating the bonding surfaces by plasma or the like, then bonding them together without a bonding layer.

The configuration of the electrode portion5is for example made the same as the configuration of the electrode portion for a so-called 1-port SAW resonator. That is, the electrode portion5has an IDT electrode11and a pair of reflectors13positioned on the two sides of the IDT electrode11.

The IDT electrode11is configured by conductive patterns (conductive layer) formed on the top surface of the piezoelectric substrate7and has a pair of comb-shaped electrodes15as shown inFIG. 1.

The pair of comb-shaped electrodes15for example have bus bars17(FIG. 1) facing each other, pluralities of electrode fingers19extending from the bus bars17in the facing directions of the bus bars17, and dummy electrodes21projecting from the bus bars17between pluralities of electrode fingers19. Further, the pair of comb-shaped electrodes15are arranged so that the pluralities of electrode fingers19intermesh (cross) with each other.

The bus bars17are for example substantially formed in a long shapes so as to linearly extend in the direction of propagation of the SAW (D1-axis direction, X-axis direction) with constant widths. The bus bars17of the pair of comb-shaped electrodes15face each other in the direction (D2-axis direction) crossing the direction of propagation of the SAW.

The pluralities of electrode fingers19are for example substantially formed in long shapes so as to linearly extend in the direction (D2-axis direction) perpendicular to the direction of propagation of the SAW with constant widths and are arranged at substantially constant intervals in the direction of propagation of the SAW (D1-axis direction).

In general, in the SAW resonator, the pluralities of electrode fingers19in the pair of comb-shaped electrodes15are provided so that their pitch “p” (for example distance between the centers of the electrode fingers19) becomes equal to a half wavelength (λ/2) of the wavelength λ of the SAW at the frequency at which resonation is desired. On the other hand, as will be understood from the explanation given later, in the acoustic wave resonator in the present embodiment, the pitch “p” does not always become such a size. Note that, the wavelength λ of the SAW is for example 1.5 μm to 6 μm.

In the same way as the SAW resonator, in portions of the pluralities of electrode fingers19, the pitch “p” thereof may be made relatively small. Conversely, it may be made relatively large as well. Further, so-called thinning may be carried out as well so that the pitch “p” becomes a whole multiple of the normal pitch “p”. Note that, in the present embodiment, when simply referring to the pitch “p”, unless otherwise indicated, the pitch “p” of a portion (major portion of the plurality of electrode fingers19) excluding special portions as described above (narrow pitch portion, wide pitch portion, or thinned out portion) or a mean value thereof is meant. Further, in the same way, when simply referring to electrode fingers19, unless otherwise indicated, this designates the electrode fingers19other than at the special portions.

The numbers, lengths (D2-axis direction), and widths (D1-axis direction) of the pluralities of electrode fingers19may be suitably set in accordance with the electrical characteristics etc. demanded from the acoustic wave resonator1. In setting these, as will be understood from the explanation which will be given later, basically the same thinking as in a SAW resonator can be utilized. As one example, the numbers of electrode fingers19are 100 to 400. The lengths and widths of the electrode fingers19are for example equal to each other among the plurality of electrode fingers19.

The dummy electrodes21for example project from the bus bar17at intermediate positions of the pluralities of electrode fingers19in one comb-shaped electrode15. The tip ends thereof face the tip ends of the electrode fingers19of the other comb-shaped electrode15over a gap. The lengths and widths of the dummy electrodes21are for example equal to each other among the plurality of dummy electrodes21.

The reflectors13are for example configured by conductive patterns (conductive layer) formed on the top surface of the piezoelectric substrate7and are formed in lattice shapes when viewed on a plane. That is, the reflectors13have pairs of bus bars (notation omitted) which face each other in the direction crossing the direction of propagation of the SAW and pluralities of strip electrodes (notation omitted) which extend in the direction (D2-axis direction) perpendicular to the direction of propagation of the acoustic wave (for example SAW) between these bus bars.

The pluralities of strip electrodes in the reflectors13are aligned in the D1-axis direction so as to continue from the array of the pluralities of electrode fingers19. The numbers and widths of the strip electrodes may be suitably set in accordance with the electrical characteristics etc. demanded from the acoustic wave resonator1. The pitch of the pluralities of strip electrodes is for example equal to the pitch of the pluralities of electrode fingers19. Further, the intervals between the strip electrodes at the end parts of the reflectors13and the electrode fingers19at the end parts of the IDT electrode11are for example equal to the pitch “p” of the pluralities of electrode fingers19(may be whole multiple of the pitch “p” as well).

The conductive layer configuring the IDT electrode11and reflectors13etc. is for example configured by a metal. As this metal, for example Al or an alloy containing Al as a main ingredient (Al alloy) may be mentioned. The Al alloy is for example an Al—Cu alloy. Note that, the conductive layer may be configured by a plurality of metal layers as well.

The thickness te(FIG. 2) of the IDT electrode11and the reflectors13is for example constant over their entire regions. As will be explained later, in the acoustic wave resonator1in the present embodiment, this thickness teis utilized as a parameter defining the resonator characteristics.

In the acoustic wave resonator1having the configuration as described above, first, the same action as that in a SAW resonator is caused. Specifically, when an electrical signal is input to one comb-shaped electrode15and voltage is applied to the piezoelectric substrate7by the pluralities of electrode fingers19, in the vicinity of the top surface of the piezoelectric substrate7, a SAW propagating along the top surface is induced. This SAW is reflected by the pluralities of electrode fingers19and pluralities of strip electrodes in the reflectors13. As a result, a standing wave of a SAW having a pitch “p” of the pluralities of electrode fingers19as substantially the half wavelength (λ/2) is formed. The standing wave generates an electrical charge (electrical signal having the same frequency as that of the standing wave) on the top surface of the piezoelectric substrate7. That electrical signal is extracted by the plurality of electrode fingers19in the other comb-shaped electrode15.

Further, in the acoustic wave resonator1, when voltage is applied to the piezoelectric substrate7by the pluralities of electrode fingers19as described above, not only a SAW, but also a bulk wave propagating inside the piezoelectric substrate7are excited. Patent Literature 2 discloses that the bulk wave becomes a factor of spurious emission if the piezoelectric substrate is thin like the piezoelectric substrate7in the bonded substrate3. In the present embodiment, this bulk wave spurious emission is utilized for making the difference Δf between the resonance frequency and the anti-resonance frequency narrower.

(Principle of New Acoustic Wave Resonator)

FIG. 3AandFIG. 3Bare diagrams for explaining the principle of the acoustic wave resonator1. InFIG. 3AandFIG. 3B, the abscissas indicate frequencies “f” (Hz), and the ordinates indicate the absolute values |Z| (Q) of impedance.

Note that, in the following explanation, for convenience, sometimes use will be made of the same notation for a resonance point and resonance frequency. In the same way, sometimes use will be made of the same notation for an anti-resonance point and anti-resonance frequency.

InFIG. 3A, a broken line L0 indicates the resonance characteristics in a usual SAW resonator different from the acoustic wave resonator1in the present embodiment. As is well known, in a SAW resonator, a SAW resonance point fsrhaving an impedance taking the minimum value and a SAW anti-resonance point fsahaving an impedance taking the maximum value appear. The SAW anti-resonance frequency fsais higher than the SAW resonance frequency fsr. Further, if the frequency difference Δfsbetween the two (=fsa−fsr) becomes narrow, for example, when configuring a filter by a SAW resonator, the rise or fall of an amount of attenuation with respect to a change of the frequency becomes sharp, so the filter characteristics are improved.

Note that, this SAW resonator is explained by taking as an example a case where there is no bulk wave spurious emission between the SAW resonance point fsrand the SAW anti-resonance point fsa.

InFIG. 3B, a solid line L1 indicates the resonance characteristic in the acoustic wave resonator1in the present embodiment. In the acoustic wave resonator1, due to the piezoelectric substrate7being thin, a bulk wave spurious emission SPO appears. In the bulk wave spurious emission SPO, for example, a bulk wave resonance point fbrhaving an impedance taking the minimum value and a bulk wave anti-resonance point fbahaving an impedance taking the maximum value appear. The relationship between the bulk wave resonance frequency fbrand the bulk wave anti-resonance frequency fbawith respect to their heights is for example inverse to the relationship between the SAW resonance frequency fsrand the SAW anti-resonance frequency fsawith respect to their heights. That is, the bulk wave resonance frequency fbris higher than the bulk wave anti-resonance frequency fba. Further, the frequency difference Δfbbetween the two (=fbr−fba) is for example narrower compared with the difference Δfsof frequencies in the SAW.

Here, when suitably setting the thickness tsof the piezoelectric substrate7and thickness teof the electrode portion5and also the interval between the electrode fingers and so on, the bulk wave spurious emission SPO (bulk wave resonance frequency fbrand bulk wave anti-resonance frequency fba) is positioned between the SAW resonance frequency fsrand the SAW anti-resonance frequency fsa. As a result, by the SAW resonance point fsrand bulk wave anti-resonance point fba, the resonance point and anti-resonance point of the frequency difference Δf1(=fba−fsr) is configured. In the same way, by the bulk wave resonance point fbrand SAW anti-resonance point fsa, the resonance point and anti-resonance point of the frequency difference Δf2(=fsa−fbr) are configured.

Therefore, in the acoustic wave resonator1in the present embodiment, the combination of the SAW resonance point fsrand bulk wave anti-resonance point fba(difference Δf1of frequencies) or the combination of the bulk wave resonance point fbrand SAW anti-resonance point fsa(difference Δf2of frequencies) described above is utilized as the combination of the normal resonance point and anti-resonance point. The differences Δf1and Δf2of frequencies are narrower than Δfs, therefore a resonance characteristic with a narrow difference Δf of frequencies is realized. This is the principle of the new acoustic wave resonator.

Note that, in this way, in the present embodiment, sometimes bulk wave spurious emission is not handled as a spurious emission. However, for convenience, a bulk wave in which the resonance point or anti-resonance point is utilized will be sometimes referred to as a “bulk wave spurious emission”.

(Setting of Various Dimensions)

Below, the influence of various dimensions of the acoustic wave resonator1upon the resonance characteristics by the SAW and bulk wave will be shown and the concrete method of setting various dimensions for utilizing the new principle explained above will be explained.

The resonance characteristics were found by simulation calculations assuming a plurality of acoustic wave resonators1with different thicknesses teof the electrode portions5(electrode fingers19).

Support substrate: Silicon

Note that, the duty ratio is the “electrode finger width/p”.

FIG. 4is a graph showing results of simulation calculations described above.

In this graph, the abscissa indicates the frequency “f” (MHz), and the ordinate indicates the absolute value |Z| (Ω) of impedance. The correspondence between the lines L51 to L57 and the thickness teof the electrode are as follows. Note that, numerals in parentheses indicate the values of normalized thickness te/2p obtained by normalization of the thicknesses teby the pitch “p” of the electrode fingers19. L51:121 nm (about 0.075), L52:131 nm (about 0.081), L53:141 nm (abut 0.087), L54:151 nm (about 0.093), L55:161 nm (about 0.099), L56:171 nm (about 0.105), and L57:181 nm (about 0.111).

A region Rr surrounded by a broken line indicates a region in which resonance points of the lines L51 to L57 due to the SAW appear. Further, a region Ra surrounded by a broken line indicates a region in which anti-resonance points of the lines L51 to L57 due to the SAW appear. Regions R1 to R4 indicated by arrows show the regions in which bulk wave spurious emissions appear.

As will be understood from comparison of the lines L51 to L57, if the thickness teof the electrode portion5is made thick, the resonance frequency and anti-resonance frequency due to the SAW move to a low frequency side. On the other hand, even if the thickness teof the electrode portion5is made thick, the frequency of the bulk wave spurious emission does not change much at all compared with the resonance frequency and anti-resonance frequency due to the SAW.

Accordingly, by making the thickness teof the electrode portion5thick or thin, the resonance frequency and anti-resonance frequency due to the bulk wave (bulk wave spurious emission in the region R1 in the example inFIG. 4) can be positioned between the resonance frequency and the anti-resonance frequency due to the SAW. Further, the frequency difference Δf1(FIG. 3) between the resonance frequency of the SAW and the anti-resonance frequency of the bulk wave or the frequency difference Δf2(FIG. 3) between the resonance frequency of the bulk wave and the anti-resonance frequency of the SAW can be adjusted.

Note that, as will be explained later, if the thickness of the piezoelectric substrate7changes, the frequency of the bulk wave spurious emission changes. Accordingly, according to a certain thickness of the piezoelectric substrate7, even if the thickness teof the electrode portion5is not made thick or thin, the bulk wave spurious emission is positioned between the resonance frequency and the anti-resonance frequency due to the SAW. That is, in realization of the acoustic wave resonator1in the present embodiment, adjustment of the thickness teis not an essential factor.

Bulk wave spurious emission does not appear only in one frequency domain, but appears in a plurality of frequency domains R1 to R4. The bulk wave spurious emission which is positioned between the resonance frequency and the anti-resonance frequency due to the SAW may be the bulk wave spurious emission of any domain.

In the lines L51 and L52, at a glance, it looks as if no bulk wave spurious emission was generated in the region R1. Further, in the lines L53 to L57, the thicker the thickness teof the electrode portion5, the larger the deflection width of the bulk wave spurious emission in the region R1. This is because the frequency having a high excitation efficiency of bulk wave spurious emission moves to a lower frequency side as the thickness tebecomes thicker. That is, when the thickness teis made thick, not only can the relative relationships between the frequency of the bulk wave spurious emission and the resonance frequency and anti-resonance frequency due to the SAW be changed, but also the amount of deflection of the bulk wave spurious emission can be made larger.

FIG. 4indicates not only a qualitative influence of the thickness teof the electrode portion5exerted upon the resonance characteristics as described above, but also an example of the quantitative influence. In the following description, for the lines L51 to L57 inFIG. 4, a table of the anti-resonance frequency fsadue to the SAW (region Ra), anti-resonance frequency fb2adue to the bulk wave in the region R2, and frequency difference between them (fb2a−fsa) is shown. Note that, the reason why not the bulk wave spurious emission in the region R1, but the bulk wave spurious emission in the region R2 are used is that a bulk wave spurious emission was not generated in the region R1 for the lines L51 and L52 as explained above.

As will be understood from a comparison between the case of te=121 nm (te/2p≈0.075) and the case of te=181 nm (te/2p≈0.111), if the thickness teis changed by 60 nm (0.036 for te/2p) (if the thickness is increased by about 50% from te=121 nm), the resonance frequency and anti-resonance frequency due to the SAW can be changed by 90 MHz or more (when normalized by 2500 MHz, 90/2500×100=3.6% or more) with respect to the frequency of the bulk wave spurious emission. Accordingly, for example, it was confirmed that the adjustment of relative relationships of frequency between the bulk wave and the SAW for realizing the acoustic wave resonator1in the present embodiment could be sufficiently realized by adjusting the thickness tewithin a realistic range.

Further, inFIG. 4, the line L51 in which the bulk wave spurious emission R1 seemed to be not positioned between the resonance frequency and the anti-resonance frequency of the SAW since the bulk wave spurious emission R1 is small can be grasped as a resonance characteristic of a conventional SAW resonator. Further, in the line L51, the frequency difference Δfsbetween the SAW resonance frequency and the SAW anti-resonance frequency is about 100 MHz. On the other hand, for example, in the line L57, the frequency difference Δf1between the bulk wave resonance frequency and the SAW anti-resonance frequency is about 30 MHz. Accordingly, in the example inFIG. 4, by increasing the thickness teof the electrode portion5by just 50% relative to the conventional SAW resonator, the difference Δf of frequencies can be made narrower up to 30% compared with the conventional resonator, therefore a remarkable effect is exerted.

In order to realize the acoustic wave resonator1in the present embodiment, the thickness teof the electrode portion5may be made thicker or thinner compared with the thickness teof the electrode portion5in the usual SAW resonator. For example, the electrode portion5in the acoustic wave resonator1may be made thicker compared with the electrode portion5in the usual SAW resonator. In this case, for example, it is easy to utilize a bulk wave spurious emission having a relatively low frequency while employing the material and cut angle etc. of the piezoelectric substrate7which are being actually utilized or are easily utilized. The effect by utilizing the bulk wave spurious emission having a relatively low frequency will be explained later.

In a usual SAW resonator, the thickness teof the electrode portion5(electrode fingers19) is set so that the excitation efficiency of the SAW becomes the highest. In general, the normalized thickness te/2p obtained by normalizing the thickness teby the pitch “p” of the electrode fingers19is about 0.070. Accordingly, for example, if the normalized thickness te/2p is 0.075 or more, there is a possibility that a bulk wave is considered. Further, if the normalized thickness te/2p exceeds 0.080, this means that the thickness has become thicker by about 15% from the normalized thickness te/2p (0.07) of the usual SAW resonator and easily exceeds the error range, so it can be almost certainly said that a bulk wave is considered.

Further, if the thickness teof the electrode portion5becomes 0.06 or less or 0.09 or more in terms of the normalized thickness, the loss becomes large, so this thickness is not employed in a usual design. In this way, even in a case where the thickness is too thick or too thin, it can be said that a bulk wave is considered.

Note that, in a usual SAW resonator, the thickness of the electrode fingers19designates the thickness in the vicinity of the center of the intersection areas of the electrode fingers19.

Although particularly not shown, if the pitch “p” of the electrode fingers19is changed, the frequencies of both of the standing wave of the SAW and the standing wave of the bulk wave (bulk wave spurious emission) change. That is, if the pitch “p” is made small, the frequencies of the standing wave of the SAW and the standing wave of the bulk wave become higher and consequently the resonance frequencies and anti-resonance frequencies due to the SAW and bulk wave become higher. This is obvious from the principle of excitation of the standing wave by the IDT electrode11.

Accordingly, if the thickness teof the electrode fingers19(electrode portion5) is set so as to obtain the desired frequency difference Δf1or Δf2, and the pitch “p” is suitably set, the desired combination of the frequency difference Δf1and the SAW resonance frequency fsrand bulk wave anti-resonance frequency fbaor the desired combination of the frequency difference Δf2and the bulk wave resonance frequency fbrand SAW anti-resonance frequency fsais realized.

Specifically, for example, a suitable value is assumed first as the pitch “p”. For example, the pitch “p” is set in the same way as the case where the SAW resonance frequency fsror SAW anti-resonance frequency fsawhich is to be obtained in the acoustic wave resonator1is obtained in the usual SAW resonator. Next, under such an assumption, the thickness teof the electrode fingers19enabling the desired frequency difference Δf1or Δf2to be obtained is calculated. Next, at that calculated thickness te, the pitch “p” capable of obtaining the desired SAW resonance frequency fsrand bulk wave anti-resonance frequency fbaor the desired bulk wave resonance frequency fbrand SAW anti-resonance frequency fsais calculated. Even if the pitch “p” is changed from the value which was assumed first, the frequencies of both of the SAW and bulk wave change together, therefore the desired fsrand fbaor desired fbrand fsaare realized while maintaining the desired frequency difference Δf1or Δf2.

In the setting operation as described above, when the thickness teof the electrode fingers19is made thicker, the resonance frequency and anti-resonance frequency of the SAW move to a low frequency side, therefore the pitch “p” of the electrode fingers19is made narrower so that these frequencies become higher. Conversely, when the thickness teof the electrode fingers19is made thinner, the resonance frequency and anti-resonance frequency of the SAW move to a high frequency side, therefore the pitch “p” of the electrode fingers19is made wider so that these frequencies become lower.

Note, as already explained, in the case where the acoustic wave resonator1in the present embodiment is realized by the thickness teof the electrode fingers19being made thicker than the thickness tein the usual SAW resonator, it is easy to utilize a bulk wave spurious emission having a relatively low frequency while employing as the material and cut angle etc. of the piezoelectric substrate7ones which are being actually utilized or are easily utilized. Accordingly, it is considered that the pitch “p” of the electrode fingers19becomes narrower in the acoustic wave resonator1compared with the pitch “p” in the usual SAW resonator in many cases.

Here, the pitch “p” in the usual SAW resonator basically has become a half (λ0/2) of a wavelength λ0=V/fsrwhich is found from the propagation velocity V of the SAW and the resonance frequency fsr. Accordingly, in the acoustic wave resonator1(finished product) having a frequency adjusted by narrowing the pitch “p”, when the propagation velocity V is specified based on the material and cut angle of the piezoelectric substrate7(may be measured as well), an actual resonance frequency fr(fsror fbr(>fsr)) is measured, and λ0=V/fris calculated, “p” becomes smaller than λ0/2. Note that, when referring to “the pitch “p” is smaller than the half wavelength λ0/2”, a case where such a state occurs due to manufacturing error is excluded. The manufacturing error of the pitch “p” is for example 50 nm.

As explained above, depending on the thickness etc. of the piezoelectric substrate7explained later, even if the thickness teof the electrode fingers19is not adjusted, sometimes the desired frequency difference Δf1or Δf2is obtained. In this case, for example, adjustment of only the pitch “p” of the electrode fingers19is sufficient. Further, for example, as a result of suitably setting the initial value of the pitch “p”, it is conceivable that the anti-resonance frequency or resonance frequency due to the bulk wave will substantially match with the bulk wave anti-resonance frequency fbaor bulk wave resonance frequency fbrtrying to be obtained. In this case, adjustment of only the thickness teof the electrode fingers19is sufficient. Naturally, sometimes adjustment of neither the thickness tenor pitch “p” is necessary.

(Qualitative Influence of Thickness of Piezoelectric Substrate)

The present inventors engaged in repeated intense studies and consequently guessed that bulk wave spurious emissions having various frequencies are generated by the following mechanism:

When applying voltage to the piezoelectric substrate by the IDT electrode11, a plurality of types of bulk waves differing from each other in at least one of the mode of vibration direction and mode of order are generated. The modes of vibration direction are for example the mode of vibration in the D3 axis direction, the mode of vibration in the D2-axis direction, and the mode of vibration in the D1-axis direction. Each of the modes of vibration direction includes a plurality of modes of order. The modes of order are defined according to for example numbers of nodes and antinodes in the depth direction (D3 axis direction).

Therefore, assuming a plurality of SAW resonators1given thicknesses tsof piezoelectric substrate7made different from each other (unlike the acoustic wave resonator1in the present embodiment, ones without adjustment of the thickness teand pitch “p” of the electrode fingers19), the influences of thicknesses of the piezoelectric substrates7exerted upon the frequency of the bulk wave of each mode were checked. Specifically, by simulation calculations, the frequencies of bulk waves of the different modes generated on the piezoelectric substrates7having various thicknesses were calculated.

FIG. 5Ais a graph showing the results of simulation calculations as described above.

In this graph, the abscissa (ts) shows the thicknesses of the piezoelectric substrates7. The ordinate (f) shows the frequencies of the bulk waves (appearing as the bulk wave resonance frequencies fbrin the acoustic wave resonators1). The plurality of lines L11 to L17 show the frequencies of a plurality of types of bulk waves between which at least one of the modes of vibration direction and modes of order differ from each other.

Note that, in this graph, the plots of the lines L15, L16, and L17 were shown up to the middle. In actuality, however, the lines continue to drop in frequencies along with an increase of thicknesses in the same way as the lines L11 to L14. Further, although not shown, even after the line L17 (line L18, line L19, . . . ), there are numerous lines exhibiting the same trends as those of L11 to L17. Further, in a usual bonded substrate, 20 μm is recommended in many cases as the thickness of the piezoelectric substrate7. That is, the thickness of the usual bonded substrate is further greater than the thicknesses in the thickness range shown inFIG. 5A.

As shown in this graph, in any mode of bulk wave, the thinner the thickness of the piezoelectric substrate7, the higher the frequency.

The line L11 and the line L12 indicate the frequencies of the bulk waves between which the modes of vibration direction are the same as each other and the modes of order are different from each other. As indicated by the arrows, the thinner the thickness of piezoelectric substrate7, the larger the frequency interval of these two bulk waves. Note that, this is true also for the other bulk waves between which the modes of vibration direction are the same as each other and the modes of order are different from each other (for example lines L13 and L14).

FIG. 5Bis a graph showing the relationships between the thickness of the piezoelectric substrate7and the frequency interval of the bulk waves of the same mode of vibration direction, but which are different in the mode of order as described above. This graph is obtained from the results of simulation calculations.

The abscissa Df indicates the frequency interval. The ordinate ts/2p indicates the normalized thickness of the piezoelectric substrate7. The normalized thickness ts/2p is obtained by dividing the thickness tsof the piezoelectric substrate7by two times the pitch “p” of the electrode fingers19(here, basically the same as the wavelength λ of the SAW) and is a dimensionless quantity (there is no unit). In this graph, each plot indicates the frequency interval of the bulk waves obtained by the simulation calculation, and the line indicates an approximation curve.

As shown in this graph, the frequency interval of the bulk waves where the normalized thickness of the piezoelectric substrate7is made thin increases abruptly more as the normalized thickness of the piezoelectric substrate7is thinner. For example, when the normalized thickness ts/2p is 5 or more, the frequency interval does not change so much. On the other hand, when the normalized thickness ts/2p becomes 3 or less, the frequency interval suddenly increases. Note that, the inclination of the curve approaches a constant level if the normalized thickness ts/2p becomes 3 or less.

Accordingly, for example, if the thickness tsof the piezoelectric substrate (normalized thickness ts/2p) is made relatively thin, the frequency interval between the bulk wave spurious emissions becomes wide. Therefore, in the frequency domain between the SAW resonance frequency fsrand the SAW anti-resonance frequency fsaand on the periphery, only the frequency of the bulk wave spurious emission which is to be utilized for making Δf narrow is positioned, and the other bulk wave spurious emissions which truly become spurious emissions can be kept away from the frequency domain described before.

Further, for example, if the thickness ts(normalized thickness ts/2p) is made relatively thin, the frequency between the bulk wave spurious emissions becomes high. As a result, for example, among numerous bulk wave spurious emissions, the bulk wave spurious emission having the lowest frequency (line L11) becomes easy to approach the resonance frequency and anti-resonance frequency which are to be realized in the acoustic wave resonator1. Due to this, as the bulk wave spurious emission which is utilized for making Δf narrow, it becomes easy to select the bulk wave spurious emission having the lowest frequency. The effect by this will be explained later.

(Quantitative Influence of Thickness of Piezoelectric Substrate)

Referring toFIG. 6, the influence of the thickness of the piezoelectric substrate7will be quantitatively evaluated and an example of the range of the thickness of the piezoelectric substrate7will be explained.

FIG. 6is a graph showing the relationships between the thickness of the piezoelectric substrate7and the frequency of the bulk wave as inFIG. 5Aand shows the frequencies of three bulk waves on a side where the frequency is low within a range where the thickness of the piezoelectric substrate7is relatively thin.

FIG. 6is obtained based on the simulation calculations. The conditions of simulation will be shown below:

Support substrate: Silicon

Note that, the duty ratio is “electrode finger width/p”.

InFIG. 6, the abscissa indicates the normalized thickness ts/2p, while the ordinate indicates the normalized frequency f×2p. The normalized frequency f×2p is the product of the frequency “f” and two times the pitch “p” of the electrode fingers19(here, basically the same as the wavelength λ of the SAW).

The line L21 indicates a bulk wave having the lowest frequency in a shown range (the range where ts/2p is 1 to 3 and the periphery of the same). This bulk wave will be called “the bulk wave of the first type in the order mode of the first vibration direction mode”. Note that, the vibration direction of the first vibration direction mode is a bulk wave which vibrates substantially in the D3-axis direction in the lithium tantalate. Note that, this line L21 is generated on the lowest frequency side among the bulk waves which may be generated.

The line L22 indicates a bulk wave having the next lowest order (frequency from another viewpoint) relative to the bulk wave of the line L21 among the bulk waves having the same vibration direction mode as that of the bulk wave of the line L21. This bulk wave will be called “the bulk wave of the second type in the order mode of the first vibration direction mode”.

The line L23 is a bulk wave having the lowest frequency in the shown range among the bulk waves which are different in the vibration direction mode from that of the bulk waves of lines L21 and L22. This will be called “the bulk wave of the first type in the order mode of the second vibration direction mode”. The line L23 is higher in frequency than the line L21, but crosses the line L22 and is lower in frequency than the line L22 in a range where the normalized thickness ts/2p is thinner than the intersection. Note that, the vibration direction of the second vibration direction mode is the bulk wave which vibrates substantially in the D2-axis direction in the lithium tantalate.

The lines L21 to L23 correspond to the lines L11 to L13 inFIG. 5A. As understood from the explanation for the lines L21 to L23 explained above and comparison betweenFIG. 6andFIG. 5A, in the shown range, there is no bulk wave drawing a line positioned under the line L21 (the frequency is lower). Further, in the shown range, there is no bulk wave drawing a line positioned between the line L21 and the line L22 or L23 either. In other words, the other bulk waves, in the shown range, are positioned above the lines L22 and L23 (frequencies are higher).

Accordingly, so far as the SAW resonance frequency fsris positioned on a lower frequency side than the line L21 and the SAW anti-resonance frequency fsais kept in a region surrounded by the lines L21 to L23, the bulk wave spurious emission of the line L21 can be utilized for making the difference Δf of frequencies narrow. In the acoustic wave resonator1, the thickness ts(normalized thickness ts/2p) of the piezoelectric substrate7may be set so that such relationships of frequencies are obtained.

When looking at an acoustic wave resonator as one prepared product, this product has only one value as the normalized thickness ts/2p, therefore the SAW anti-resonance frequency fsais kept between the frequency of the bulk wave spurious emission having the lowest frequency and the bulk wave frequency having the next lowest frequency. Further, the bulk wave frequency having the next lowest frequency described above is one of the line L22 or one of the line L23 (one by the two at the intersection point).

The region on a lower frequency side than the line L21 or the region surrounded by the lines L21 to L23 is a region in which another bulk wave is not generated as explained above. Such a region is a unique region which becomes extremely broad even compared with the other regions which are surrounded by a variety of lines in any combination. This can realize the advantage in the ordinate direction in the graph that no bulk wave spurious emission at all is generated in a certain frequency range (for example the range on the periphery of the SAW resonance frequency fsror SAW anti-resonance frequency fsa) and also the advantage in the abscissa direction in the graph that no bulk wave spurious emission is generated even if the thickness of the piezoelectric substrate7varies a little.

The normalized thickness ts/2p may be for example 1 to 3. In this case, for example, the bulk wave spurious emission having the lowest frequency as described above (line L21) can be utilized.

If ts/2p is less than 1, for example, the loss of the SAW becomes large. Further, for example, the frequency of the SAW becomes susceptible to the influence of the state of the bottom surface of the piezoelectric substrate7, and the variation of frequency characteristics becomes large among a plurality of acoustic wave resonators1. Further, for example, it becomes difficult to secure the strength of the piezoelectric substrate7. Conversely speaking, if ts/2p is 1 or more, such an inconvenience is solved or reduced.

Further, if ts/2p is 3 or less, for example, as already alluded to, the frequency interval between the bulk waves which are different in mode from each other is relatively wide. Further, for example, when considering an actual propagation speed of the SAW and so on, the frequency of the bulk wave spurious emission having the lowest frequency is easily positioned between the SAW resonance frequency fsrand the SAW anti-resonance frequency fsa.

Note that, the normalized thickness ts/2p being 1 to 3 is just one example of the range. Within a range where the normalized thickness ts/2p is less than 1 or exceeding 3, the frequency of the bulk wave spurious emission having the lowest frequency may be positioned between the SAW resonance frequency fsrand the SAW anti-resonance frequency fsaas well.

Two times the pitch “p” (2p) in the acoustic wave resonator1is for example 1.5 μm to 6 μm. Accordingly, tsis for example 1.5 μm to 18 μm. Targeting other effects accompanied with reducing the thickness of the piezoelectric substrate7(for example increase of temperature compensation effect of the support substrate9) and so on, tsmay be made further thinner than that in the range described above and set to 1.5 μm to less than m as well.

In the example explained above, the case of using the Si substrate as the support substrate9was explained as an example. However, it is confirmed that the same applies to the case of using a sapphire substrate. Specifically, when expressing the lines L21 to L23 shown inFIG. 6by equations, although the coefficients determining inclination etc. are different, the same trend is shown. Specifically, where the normalized thickness is “x” and the normalized frequency is “y”, the approximate equations of the lines L21 to L23 become as follows in the case where use is made of an Si substrate as the support substrate:
L21:y=71.865x4−706.82x3+2641.5x2−4567.1x+6518.1
L22:y=466.89x4−2884x3+6768x2−7310.5x+7544.4
L23:y=−66.245x3+689.86x2−2546x+6941.6

In the same way, when use is made of a sapphire substrate, the approximate equations of the lines L21 to L23 become as follows:
L21:y=33.795x4−419.77x3+1966.9x2−4212.8x+6990.5
L22:y=−54.624x3+625.48x2−2533.6x+7334.6
L23:y=−258.23x3+1477.7x2−2912.2x+6418.1

(Combination of Electrode Thickness and Electrode Finger Pitch)

As already explained, for example, the thicker the thickness teof the electrode portion5, the lower the SAW resonance frequency fsrand SAW anti-resonance frequency fsa. Further, this drop in the frequency can be compensated for by narrowing the pitch “p” of the electrode fingers19. At this time, the higher the order mode of bulk wave, the higher frequency. As a result, for example, utilization of the bulk wave spurious emission is facilitated more. This will be shown below.

FIG. 7is a graph corresponding toFIG. 6in a case where the thickness teof the IDT electrode11is made thicker than that inFIG. 6.

FIG. 7is obtained based on the simulation calculations in the same way asFIG. 6. The simulation conditions different from those inFIG. 6will be shown below:

The lines L31 to L33 correspond to the lines L21 to L23. That is, the lines L31 to L33 correspond to the first type in the order mode of the first vibration direction mode, the second type in the order mode of the first vibration direction mode, and the first type in the order mode of the second vibration direction mode. Note that, the abscissa in the graph is made the same as that inFIG. 6. That is, this indicates values before adjustment of the thickness and pitch of the IDT electrode11. Further, the line L31 indicates the simulation results in the case of the same thickness as that inFIG. 6, while the line L32 and line L33 indicate the simulation results in the case where the thicknesses are determined as explained above.

InFIG. 7, compared withFIG. 6, the frequencies of the lines L32 and L33 (particularly the line L32) become high, and consequently the width of the frequency in the region surrounded by the lines L31 to L33 becomes broad. Due to this, when it is desired to position the SAW resonance frequency fsrand the SAW anti-resonance frequency fsaon a higher frequency side than that inFIG. 6and/or when the frequency of the bulk wave is to be positioned on a higher frequency side than that inFIG. 6, there is reduced apprehension that the bulk wave spurious emissions by the lines L32 and L33 which truly become spurious emissions appear in the frequency domain on the periphery of the same.

In this way, the results inFIG. 7show that the specific range surrounded by the lines L21 to L23 inFIG. 6can be offset to the desired position by adjusting the thickness and pitch of the IDT electrode11. That is, the specific region can be shifted to a high frequency side or can be offset to a low frequency side. Further, it is possible to adjust the thickness range of the piezoelectric substrate7capable of utilizing the bulk wave spurious emission having the lowest frequency to a practical region or possible to widen the thickness range.

FIG. 8is a flow chart showing an example of the procedure of design of the thickness teof the electrode portion5and pitch “p” of the electrode fingers19etc.

This procedure is shown further detailing the procedure of the method of design explained above. As explained with reference toFIG. 3B, the acoustic wave resonator1includes the aspect of utilizing the SAW resonance frequency fsrand the bulk wave anti-resonance frequency fba(frequency difference Δf1) and the aspect of utilizing the bulk wave resonance frequency fbrand the SAW anti-resonance frequency fsa(frequency difference Δf2). An explanation will be given taking the former as an example.

At step S1, the various design conditions or design values of the acoustic wave resonator1are initially set. For example, the material, cut angle, and thickness tsof the piezoelectric substrate7and the material, thickness te, crossing width, pitch “p”, duty ratio, and number of the electrode fingers19and so on are suitably selected. At this time, provisional values are set also for the thickness teand the pitch “p” which are changed after step ST1. Further, for example, as already explained, the values may be initially set in the same way as the case where the SAW resonance frequency fsror SAW anti-resonance frequency fsa(SAW resonance frequency fsrin the procedure inFIG. 8) which is to be obtained in the acoustic wave resonator1is obtained in a usual SAW resonator.

At step S2, the resonance characteristics are calculated based on the designing conditions or design values set at step ST1. Specifically, for example, simulation calculation is carried out, and the SAW resonance frequency fsrand bulk wave anti-resonance frequency fba, and the difference Δf1of frequency between them are calculated.

At step S3, it is judged whether the frequency difference Δf1calculated at step ST2coincides with the targeted frequency difference Δft. Note that, the judgment of whether they coincide referred to here includes judgment of whether the difference of the two is kept in a predetermined permissible range. This same is also true for steps ST6and ST9which will be explained later. Further, when judging that they do not coincide, the routine proceeds to step ST4. Otherwise, when judging that they coincide, the routine skips steps ST4and ST5and proceeds to step ST6.

At step S4, the design value of the thickness teof the electrode portion5is changed so that the calculated frequency difference Δf1approaches the targeted frequency difference Δft. That is, if Δf1(=fba−fsr)<Δf (including also Δf1≤0), the design value of the thickness teis made thick so that the SAW resonance frequency fsrmoves to a low frequency side. Conversely, if Δf1>Δft, the design value of the thickness teis made thin so that the SAW resonance frequency fsrmoves to a high frequency side. The amount of change at this time may be suitably set. Further, it may be a constant amount or may be adjusted in accordance with the magnitude of the difference between Δf1and Δft.

At step S5, the same calculation as that at step ST2is carried out. Further, the routine returns to step ST3. Due to this, the design value of the thickness teis changed until the positive judgment is carried out at step ST3.

At step S6, it is judged whether the SAW resonance frequency fsrcoincides with the target resonance frequency ftr. Further, when judging it does not coincide, the routine proceeds to step ST7. When judging it coincides, the routine skips steps ST7and ST8and proceeds to step ST9.

At step S7, the design value of the pitch “p” of the electrode fingers19is changed so that the SAW resonance frequency fsrapproaches the target resonance frequency ftr. That is, if fsr<ftr, the design value of the pitch “p” is made narrow so that the SAW resonance frequency fsrmoves to the high frequency side. Conversely, if fsr>ftr, the design value of the pitch “p” is made wide so that the SAW resonance frequency fsrmoves to the low frequency side. The amount of change at this time may be suitably set. Further, it may be a constant amount or may be adjusted in accordance with the magnitude of difference between fsrand ftr.

At step S8, the same calculation as that at step ST2is carried out. Further, the routine returns to step ST6. Due to this, the design value of the pitch “p” is changed until the judgment at step ST6is yes.

At step S9, it is judged whether the bulk wave anti-resonance frequency fbacoincides with the target anti-resonance frequency fta. Basically, when the SAW resonance frequency fsrcoincides with the target value at step ST6after the frequency difference Δf1coincides with the target value at step ST3, the bulk wave anti-resonance frequency fba(=fsr+Δf1) also coincides with the target anti-resonance frequency fta. However, the pitch “p” affects the difference Δf1somewhat, therefore such a judgment is carried out for confirmation.

Further, when judging noncoincidence at step ST9, the routine returns to step ST3. Due to this, step ST3and the following steps are repeated until both of the frequency difference Δf1and the SAW resonance frequency fsr(consequently the bulk wave anti-resonance frequency fba) coincide with the target values. Further, when judging coincidence, the design procedure is completed.

Note that, in place of step ST9, the same judgment as that at step ST3may be carried out as well. Further, the judgment at step ST6and the judgment at step ST9can be reversed. That is, the step of making the SAW resonance frequency fsrcoincide with the target value and the step of making the bulk wave anti-resonance frequency fbacoincide with the target value may be grasped as the same step as well. In the same way, when the difference Δf2of frequencies is utilized, the step of making the SAW anti-resonance frequency fsacoincide with the target value and the step of making the bulk wave resonance frequency fbrcoincide with the target value may be grasped as the same step.

Example of Utilization of Acoustic Wave Resonator

Below, as examples of utilization of the acoustic wave resonator1, an acoustic wave filter, multiplexer, and communication apparatus will be explained.

FIG. 9Aschematically shows an acoustic wave filter51including the acoustic wave resonator1. The acoustic wave filter51is a so-called ladder type resonator filter and has one or more (two inFIG. 9A) serial resonators53A and53B and one or more (three inFIG. 9A) parallel resonators55A to55C which are connected in a ladder configuration. Note that, in the following description, “A”, “B”, or “C” of these notations will be sometimes omitted.

Each of the serial resonators53and parallel resonators55is for example a 1-port resonator including an IDT electrode11and reflectors13on the two sides thereof. The IDT electrodes11and pairs of reflectors13(electrode portions5) in these plurality of resonators are for example provided on a common piezoelectric substrate7.

One or more serial resonators53are for example connected in series between a pair of terminals57(may be lines in place of the terminals as well). That is, one of the pair of comb-shaped electrodes15is directly or indirectly connected to one of the pair of terminals57, and the other of the pair of comb-shaped electrodes15is directly or indirectly connected to the other of the pair of terminals57.

One or more parallel resonators55are for example connected between a part between a pair of terminals57(from another viewpoint, before or after any one serial resonator53) and the reference potential portion. That is, one of the pair of comb-shaped electrodes15is connected to the part between the pair of terminals57, and the other of the pair of comb-shaped electrodes15is connected to the reference potential portion.

The serial resonators53and parallel resonators55are configured so that the anti-resonance frequency of the parallel resonator55and the resonance frequency of the serial resonator53coincide. Due to this, between the pair of terminals57, a filter having the anti-resonance frequency of the parallel resonator55and the resonance frequency of the serial resonator53as the center of the passing band is configured.

Further, at least one among the one or more serial resonators53and one or more parallel resonators55is configured by the acoustic wave resonator1in the present embodiment.

For example, as shown inFIG. 9Bwhich further schematically showsFIG. 9A, one of the parallel resonators55may be configured by the acoustic wave resonator1in the present embodiment while the serial resonators53and the other parallel resonator55may be configured by conventional SAW resonators59. If the acoustic wave resonator1in the present embodiment is used for the parallel resonator55in this way, the difference Δf of frequencies can be made narrow, therefore the rise of the amount of attenuation on the low frequency side in the passing band can be made steep, so the filter characteristics of the acoustic wave filter51are improved. In particular, it is preferable to use the acoustic wave resonator1in the present embodiment for the parallel resonator55having the highest resonance frequency among the plurality of parallel resonators55.

Further, for example, as shown inFIG. 9C, which is a schematic view of an example different from that inFIG. 9B, one of the serial resonators53may be configured by the acoustic wave resonator1in the present embodiment, while the other serial resonators53and the parallel resonators55may be configured by conventional SAW resonators59. If the acoustic wave resonator1in the present embodiment is used for a serial resonator53in this way, the difference Δf of frequencies can be made narrow, therefore the fall of the amount of attenuation on the high frequency side in the passing band can be made steep, so the filter characteristics of the acoustic wave filter51are improved. In particular, it is preferable to use the acoustic wave resonator1in the present embodiment for the serial resonator53having the lowest resonance frequency among a plurality of serial resonators53.

Here, the conventional SAW resonator59is a resonator provided with an IDT electrode exciting the surface acoustic wave. In the resonator, unlike the acoustic wave resonator1, no bulk wave spurious emission is positioned or three or more bulk wave spurious emissions are positioned between the resonance frequency and the anti-resonance frequency of the surface acoustic wave. That is, between the resonance frequency and the anti-resonance frequency of the surface acoustic wave, zero or five or more resonance frequencies and anti-resonance frequencies of bulk waves are included.

Note that, although not particularly shown, the acoustic wave resonator1in the present embodiment may be applied to both of the serial resonators53and the parallel resonators55as well. In this case, the steepness of change of the amount of attenuation can be improved at the both of the low frequency side and the high frequency side in the passing band. Further, inFIG. 9BandFIG. 9C, only one of the plurality of parallel resonators55or only one of the plurality of serial resonators53is configured as the acoustic wave resonator1in the present embodiment. However, two or more or all of them may be configured as the acoustic wave resonators1in the present embodiment as well.

In the case where the acoustic wave resonator1in the present embodiment is applied only for the resonators in a portion among one or more serial resonators53or one or more parallel resonators55as inFIG. 9BandFIG. 9C, for example, frequencies of various bulk wave spurious emissions are kept away from the passing band and the spurious emissions are reduced at the periphery of the passing band in the SAW resonator59while obtaining the effect of making the change of amount of attenuation steep at the end of the passing band as described above. Note that, in this case, for example, the frequency difference Δfsof the SAW resonator59is positioned in the range surrounded by the lines L21 to L23 inFIG. 6due to the piezoelectric substrate7being relatively thin (for example the thickness is 1λ to 3λ). Further, conversely, in a case where the acoustic wave resonator1in the present embodiment is applied for all or relatively many of the resonators, for example, the effect of making the amount of attenuation steep as described above can be increased.

When the acoustic wave resonator1in the present embodiment is applied for only a portion among the plurality of parallel resonators55as inFIG. 9B, the acoustic wave resonator1is different in the thickness teof the electrode portion5and in the pitch “p” of the electrode fingers19from the other parallel resonators55(SAW resonators59). In the same way, when the acoustic wave resonator1in the present embodiment is applied for only a portion among the plurality of serial resonators53as inFIG. 9C, the acoustic wave resonator1is different in the thickness teof the electrode portion5and in the pitch “p” of the electrode fingers19from the other serial resonators53(SAW resonators59). In general, a SAW resonator59of a parallel resonator55and a SAW resonator59of a serial resonator53are the same in the thicknesses teof the electrode portions5.

Accordingly, it is possible to judge whether an acoustic wave resonator1in the present embodiment is provided according to whether the plurality of IDT electrodes11configuring one or more serial resonators53and one or more parallel resonators55include a first IDT electrode11and a second IDT electrode11having a different thickness from that of the first IDT electrode11. Note that, as already explained, when utilizing a bulk wave spurious emission having a relatively low frequency while employing the material and cut angle etc. of the piezoelectric substrate7which are being actually utilized or are easily utilized, a probability that the thickness teof the electrode portion5becomes thicker and the pitch “p” of the electrode fingers19becomes narrower in the acoustic wave resonator1compared with a SAW resonator59is high.

IDT electrodes11having different thicknesses may be suitably formed. For example, after forming and etching a conductive layer for a thick (or thin) IDT electrode11, a conductive layer for a thin (or thick) IDT electrode11may be formed and etched. Further, for example, after forming and etching a conductive layer for forming a portion of the thickness of a thick IDT electrode11, a conductive layer for forming the remaining thickness of the thick IDT electrode11and forming the entire thin IDT electrode may be formed and etched. In a case of formation of the conductive layer through a mask as well, in the same way, the two may be formed in separate steps or the step for a portion for formation of the thick IDT electrode11may be made common with the step for formation of the thin IDT electrode11.

(Examples of Acoustic Wave Filter)

The specific conditions of the filter51were assumed and its filter characteristics were checked. The filter51was configured as one having three resonators of a serial resonator53A, parallel resonator55A, and parallel resonator55B. In the examples, the acoustic wave resonator1in the present embodiment was applied to the parallel resonator55A. In the comparative examples, all resonators were configured as usual SAW resonators59. Further, for two types of cases (Case 1 and Case 2) in which use was made of Δf2shown inFIG. 3as the difference Δf of frequencies and the magnitudes of them were made different, models of the examples and comparative examples were fabricated, simulation calculations were carried out, and the results thereof were compared.

The conditions (mainly design values) of the Case 1 (Comparative Example 1 and Example 1) will be shown below.

Support substrate: Silicon

Duty ratio of electrode fingers: 0.5

In Case 2 (Example 2), the pitch “p”, thickness te, etc. were adjusted so that Δf2became further smaller than that in Case 1.

FIG. 10AandFIG. 10Bshow simulation results of filter characteristics in Case 1.

In these graphs, the abscissas indicate the frequencies F (MHz), and the ordinates indicate the amounts of attenuation A (dB).FIG. 10Bis an enlarged graph of the low frequency side inFIG. 10A. InFIG. 10AandFIG. 10B, the line L51 indicates Comparative Example 1, and the line L52 indicates Example 1.

As shown in these graphs, it was confirmed that, even if using an acoustic wave resonator1which does not grasp a bulk wave spurious emission as a spurious emission, but utilizes a bulk wave spurious emission as a resonance point or anti-resonance point for the filter51, the filter51functioned as a filter in the same way as a filter51configured by only the usual SAW resonator59. Further, it was confirmed that, by making the frequency difference Δf narrow, the effect of making the change of amount of attenuation (rise on the low frequency side in the present embodiment) steep on the end of the passing band was obtained.

Below, results of comparison of the numerical values between Comparative Example 1 and Example 1 will be shown.

Here, fLis the frequency at the time when the amount of attenuation is 0.6 dB, fAis the frequency when the amount of attenuation is 10 dB, and fDis fL−fA. Accordingly, the smaller the fD, the higher the steepness. Further, dB/fDis the ratio obtained by dividing 9.4 (=10−0.6) dB by fD(MHz).

It was confirmed from the above numerical values that the steepness became higher in Example 1 relative to Comparative Example 1. Specifically, for dB/fDof the two, (Example 1)/(Comparative Example 1)×100≈2.29/1.16× 100≈198%, therefore the steepness becomes about two times greater.

In Example 2, compared with Example 1, the magnitude of Δf2was adjusted considering the attenuation characteristic on the lower frequency side. In the case of Example 2, it was confirmed that the attenuation characteristic on the low frequency side outside of the passing band of the filter could be improved.

Note that, when paying attention to a shoulder characteristic on the low frequency side in the passing band, it was confirmed that the steepness was lowered more than that in Example 1, but the steepness was improved compared with Comparative Example 1. In this way, by adjusting the magnitude of Δf2, not only the shoulder characteristic, but also the attenuation characteristic outside of the passing band can be improved.

FIG. 11is a schematic view showing a multiplexer101as an example of utilization of the acoustic wave resonator1.

The multiplexer101for example has a transmission filter109which filters a transmission signal from a transmission terminal105and outputs the result to an antenna terminal103and has a receiving filter111which filters a reception signal from the antenna terminal103and outputs the result to a pair of reception terminals107.

The transmission filter109is for example given the same configuration as that of the acoustic wave filter51explained with reference toFIG. 9A. That is, the transmission filter109has one or more serial resonators and one or more parallel resonators which are connected in a ladder configuration. Further, at least one of these resonators is configured by the acoustic wave resonator1. In the example inFIG. 11, a case where one serial resonator and one parallel resonator are configured by acoustic wave resonators1, and the other serial resonators and other parallel resonator are configured by conventional SAW resonators59is exemplified. The IDT electrodes11and pairs of reflectors13(electrode portions5) configuring these plurality of resonators are for example provided on the same piezoelectric substrate7.

The receiving filter111is for example configured by a SAW resonator59and SAW filter61connected in series to each other. The IDT electrodes11and pairs of reflectors13configuring them are for example provided on the same piezoelectric substrate7. The piezoelectric substrate7on which the receiving filter111is configured may be the same as, or may be different from, the piezoelectric substrate7on which the transmission filter109is configured.

The SAW filter61is for example a longitudinal coupled multiplex mode (including double mode) type resonator filter and has a plurality of IDT electrodes11aligned in the direction of propagation of the SAW and a pair of reflectors13which are arranged on the two sides thereof.

FIG. 12is a block diagram showing a principal part of a communication apparatus151as an example of utilization of the acoustic wave resonator1.

The communication apparatus151performs wireless communications utilizing radio waves. The communication apparatus151utilizes the acoustic wave resonator1by having the multiplexer101explained above. Specifically, this is as follows.

In the communication apparatus151, a transmitting information signal TIS including information to be transmitted is modulated and boosted in frequency (converted to high frequency signal having a carrier wave frequency) by an RF-IC (radio frequency integrated circuit)153to be made the transmission signal TS. The transmission signal TS is stripped of unwanted components other than the transmission-use passband by a band pass filter155, is amplified by an amplifier157, and is input to the multiplexer101(transmission terminal105). Further, the multiplexer101strips unwanted components other than the transmission-use passband from the input transmission signal TS and outputs the stripped down transmission signal TS from the antenna terminal103to the antenna159. The antenna159converts the input electrical signal (transmission signal TS) to a wireless signal (radio waves) and transmits the result.

Further, in the communication apparatus151, the wireless signal (radio waves) received by the antenna159is converted to an electrical signal (reception signal RS) by the antenna159and is input to the multiplexer101. The multiplexer101strips unwanted components other than the reception-use passband from the input reception signal RS and outputs the result to the amplifier161. The output reception signal RS is amplified by the amplifier161and is stripped of the unwanted components other than the reception-use passband by the band pass filter163. Further, the reception signal RS is lowered in frequency and demodulated by the RF-IC153to become the reception information signal RIS.

Note that, the transmitting information signal TIS and the reception information signal RIS may be low frequency signals (baseband signals) containing suitable information and are for example analog audio signals or digital audio signals. The passband of the wireless signal may be one according to various standards such as the UMTS (Universal Mobile Telecommunications System). Usually, the passband for transmission and the passband for reception do not overlap each other. The modulation method may be either of phase modulation, amplitude modulation, frequency modulation, or a combination of two or more selected from among them. As the circuit system,FIG. 12exemplified a direct conversion method. However, a suitable one other than that may be employed as well. For example, it may be a double super-heterodyne method as well. Further,FIG. 12schematically shows only the principal part. A low pass filter or isolator etc. may be added at suitable positions. Further, positions of the amplifier etc. may be changed as well.

As described above, in the present embodiment, the acoustic wave resonator1has the piezoelectric substrate7and the IDT electrode11positioned on the top surface of the piezoelectric substrate7. Further, between the resonance frequency fsrand the anti-resonance frequency fsadue to the SAW, at least one of the resonance frequency fbrand anti-resonance frequency fbadue to the bulk wave is positioned.

Accordingly, as explained with reference toFIG. 3B, the resonance characteristic of the frequency difference Δf1can be realized by the SAW resonance frequency fsrand the bulk wave anti-resonance frequency fbaor the resonance characteristic of the frequency difference Δf2can be realized by the bulk wave resonance frequency fbrand the SAW anti-resonance frequency fsa. Further, Δf1or Δf2is narrower than the difference Δfsof frequency between the SAW resonance frequency fsrand the SAW anti-resonance frequency fsa. Therefore, compared with a SAW resonator, a resonance characteristic having a narrow frequency difference Δf is realized. The acoustic wave resonator1in the present embodiment utilizes a bulk wave which is handled as a spurious emission in a SAW resonator for realization of the resonance characteristic according to a reverse idea. This is epoch-making. Further, the acoustic wave resonator1, unlike a conventional SAW resonator or bulk wave resonator, is not predicated on one type of acoustic wave, but utilizes two types of acoustic waves (SAW and bulk wave). This is epoch-making also on this point.

Further, in the present embodiment, the acoustic wave filter51has one or more serial resonators53and one or more parallel resonators55which are connected in a ladder configuration. At least one of these resonators is configured by the acoustic wave resonator1of the present embodiment.

Accordingly, the acoustic wave resonator1having a narrow frequency difference Δf is included. Therefore, as explained with reference toFIG. 10AandFIG. 10B, the rise or fall of the amount of attenuation at the end of the passing band can be made steep, therefore the filter characteristics are improved. Specifically, so long as at least one of the one or more parallel resonators55is configured by the acoustic wave resonator1in the present embodiment, the rise of the amount of attenuation on the low frequency side in the passing band can be made steep. Further, if at least one of the one or more serial resonators53is configured by the acoustic wave resonator1in the present embodiment, the fall of the amount of attenuation on the high frequency side in the passing band can be made steep.

Further, from another viewpoint, in the present embodiment, the acoustic wave filter51has the piezoelectric substrate7, the support substrate9bonded to the bottom surface of the piezoelectric substrate7, and the plurality of IDT electrodes11which are positioned on the top surface of the piezoelectric substrate7and configure one or more serial resonators53and one or more parallel resonators55which are connected in a ladder configuration. The plurality of IDT electrodes11include the first IDT electrode11(IDT electrode11configuring the SAW resonator59) and the second IDT electrode11(IDT electrode11configuring the acoustic wave resonator1) which is different in thickness from the first IDT electrode11(for example thicker than the first IDT electrode11).

Accordingly, it is possible to use the first IDT electrode11to configure a usual SAW resonator59in which the bulk wave spurious emission is not positioned between the SAW resonance frequency and the SAW anti-resonance frequency while use the second IDT electrode11to configure an acoustic wave resonator1in the present embodiment in which the bulk wave spurious emission is positioned between the SAW resonance frequency and the SAW anti-resonance frequency. By provision of the acoustic wave resonator1, various effects explained above are obtained. Further, by mixing a SAW resonator59and an acoustic wave resonator1, the combination of merits of the two becomes possible.

Further, in the present embodiment, the method of design of the acoustic wave resonator1has an electrode thickness setting step (steps ST3to ST5) of specifying the thickness teof the electrode fingers19where the resonance frequency fsrand anti-resonance frequency fsadue to the SAW are positioned on the two sides of at least one of the resonance frequency fbrand the anti-resonance frequency fbadue to the bulk wave in the case where the pitch “p” of the electrode fingers19in the IDT electrode11is a predetermined initial value, and a step (steps ST6to ST8. Here, as already explained, steps ST6and ST9may be viewed as the same.) specifying the pitch “p” of the electrode fingers19with which either frequency described above (fbror fba) coincides with the predetermined target frequency by the thickness teof the electrode finger19specified in the electrode film thickness setting step.

Accordingly, first, the acoustic wave resonator1in the present embodiment in which at least one of the bulk wave resonance frequency fbrand the bulk wave anti-resonance frequency fbais positioned between the SAW resonance frequency fsrand the SAW anti-resonance frequency fsais realized. Further, if the thickness teof the electrode finger19is changed, the SAW resonance frequency fsrand the SAW anti-resonance frequency fsachange, but almost no change occurs in the frequency of the bulk wave spurious emission. Therefore, it is easy to realize the desired frequency difference Δf. On the other hand, if the pitch “p” is changed, the frequencies of the standing waves of the SAW and bulk wave can be changed by substantially equal amounts. Therefore, by setting the pitch “p” after setting the thickness te, a desired combination of the frequency difference Δf and the frequencies on the two ends thereof can be realized easily and conveniently as a whole.

FIG. 13AtoFIG. 13Cshow various modifications of the acoustic wave resonator1in the present embodiment in which at least one of the bulk wave resonance frequency fbrand bulk wave anti-resonance frequency fbais positioned between the SAW resonance frequency fsrand the SAW anti-resonance frequency fsa.

As shown inFIG. 13A, an additional film201given a substantially equal shape to the shape of the electrode fingers19(electrode portion5) in a plane view may be provided on the electrode fingers19(electrode portion5) as well. The additional film201may be made of a conductor or may be made of an insulator. Note that, it is also possible to provide the additional film201under the electrode fingers19.

Such an additional film201contributes to an increase of the reflection coefficient of the acoustic wave in the electrode fingers19. In particular, it is effective at the time when a not shown protective layer is formed thicker than the electrode finger19and the material of the protective layer (for example SiO2) and the material of the electrode fingers19(for example Al or Al alloy) are acoustically close. If the additional film201is made of an insulator, the additional film201does not always have to have the completely the same shape as the shape of the electrode portion5when viewed on a plane. For example, it may have portions positioned between the electrode fingers19and the dummy electrodes21(FIG. 1) as well.

In a configuration having the additional film201, the same effect as that by making the thickness teof the electrode finger19thick can be obtained even by formation of the additional film201given a thickness tmmade thick. That is, the resonance frequency and anti-resonance frequency due to the SAW can be changed without changing the frequency of the bulk wave spurious emission so much. Note that, it may also be grasped that the electrode fingers203are configured by the electrode fingers19(metal layer) and an additional film201(may be conductor or insulator).

In the embodiments, by changing the thickness teof the electrode finger19(or201), the resonance frequency and anti-resonance frequency due to the SAW were changed and the frequency having a high excitation efficiency of bulk wave spurious emission was shifted without changing the frequency of the bulk wave spurious emission so much. On the other hand, there are various parameters exerting the same effect other than these. Accordingly, in addition to or in place of the change of the thickness te, the other parameters may be suitably set and the acoustic wave resonator1in the present embodiment realized by this.

For example, even if the width “w” (duty ratio w/p) of the electrode finger19is changed, the same effect as that in the case where the thickness teof the electrode finger19is changed can be obtained. Specifically, when making the duty ratio w/p large, in the same way as the case where the thickness teof the electrode finger19is made thick, without changing the frequency of bulk wave spurious emission so much, the resonance frequency and anti-resonance frequency due to the SAW can be made low and the frequency at which the excitation efficiency of bulk wave spurious emission is high can be shifted to the low frequency side.

However, the effect of lowering the resonance frequency and anti-resonance frequency due to the SAW and so on is more remarkable in the case of change of the thickness teof the electrode fingers19than the case of change of the duty ratio of the electrode fingers19. Further, if the duty ratio is made too large, short-circuiting is liable to occur, therefore there is a limit to the amount of adjustment.

Further, for example, as shown inFIG. 13B, in the case where the top surface of the piezoelectric substrate7is covered by the protective layer205from the top of the electrode portion5, the same effect as that in the case of change of the thickness teof the electrode fingers19can be obtained even if changing the thickness tpof the protective layer205. Specifically, if the thickness tpof the protective layer205is made thick, in the same way as the case where the thickness teof the electrode fingers19is made thick, the resonance frequency and anti-resonance frequency due to the SAW can be made low without changing the frequency of the bulk wave spurious emission so much, and the frequency at which the excitation efficiency of the bulk wave spurious emission is high can be shifted to the low frequency side.

Further, although not particularly shown, by changing the cut angle of the piezoelectric substrate7, the frequency at which the excitation efficiency of bulk wave is high can be shifted. The magnitude of the bulk wave can be adjusted by this as well. For example, in a Y-plate of a lithium tantalate single crystal, the frequency at which the excitation efficiency of the bulk wave is high shifts to a higher frequency side as the cut angle is made larger.

In the embodiments, only one bulk wave spurious emission was positioned between the SAW resonance frequency fsrand the SAW anti-resonance frequency fsa. However, as shown inFIG. 13C, two bulk wave spurious emissions (two bulk wave spurious emissions SP1and SP2in the shown example) may be positioned between the SAW resonance frequency fsrand the SAW anti-resonance frequency fsaas well. In this case as well, the resonance characteristic of the frequency difference Δf1can be realized due to the SAW resonance frequency fsrand the anti-resonance frequency fbaof the bulk wave spurious emission SP1, and the resonance characteristic of the frequency difference Δf2can be realized by the resonance frequency fbrof the bulk wave spurious emission SP2and the SAW anti-resonance frequency fsa.

If the number of the bulk wave spurious emissions becomes three or more (if the number of the resonance frequencies and anti-resonance frequencies of the bulk wave becomes five or more), adjustment by the film thickness and pitch of the IDT electrodes substantially becomes difficult. Note that, when there are two bulk wave spurious emissions, the two of them may be utilized as shown inFIG. 13Cor either may be positioned in the vicinity of the resonance frequency or anti-resonance frequency of the surface acoustic wave to reduce its influence or may be positioned at a frequency at which the excitation efficiency of the bulk wave is extremely low to reduce its influence.

Further, although not particularly shown, in the bulk wave spurious emission, both of the resonance frequency fbrand the anti-resonance frequency fbado not have to be positioned between the SAW resonance frequency fsrand the SAW anti-resonance frequency fsa. For example, if the resonance characteristic of the difference Δf1of the frequencies is to be realized, the bulk wave anti-resonance frequency fbaonly has to be positioned between the SAW resonance frequency fsrand the SAW anti-resonance frequency fsa. If the resonance characteristic of the difference Δf2of the frequencies is to be realized, the bulk wave resonance frequency fbronly has to be positioned between the SAW resonance frequency fsrand the SAW anti-resonance frequency fsa. Note, in general, the difference of frequency between the bulk wave resonance frequency fbrand the bulk wave anti-resonance frequency fbais narrower than the difference Δfsof frequency between the SAW resonance frequency fsrand the SAW anti-resonance frequency fsa. Therefore, realistically, there may be many cases where both of the bulk wave resonance frequency fbrand the bulk wave anti-resonance frequency fbaare positioned between the SAW resonance frequency fsrand the SAW anti-resonance frequency fsa.

Further, in the embodiments, as the acoustic wave filter5, the explanation was given by taking as an example a case where the acoustic wave resonator1was used in a ladder type filter, but the present invention is not limited to this. For the acoustic wave filter51, use can be made of the acoustic wave resonator1even in a filter provided with the longitudinal coupled type resonators13as in the receiving filter111shown inFIG. 11.

Specifically, as shown inFIG. 14, provision may be made of a filter51A having a longitudinal coupled type (serial connection type) resonator61between the terminals103and107, and a parallel resonators58arranged between it and the reference potential are provided, and the acoustic wave resonator1may be used as this parallel resonator58.

Such a parallel resonator58may be arranged on the side nearer the antenna terminal103than the resonator61as shown inFIG. 14or may be arranged on the reception terminal107side.

The art according to the present disclosure is not limited to the above embodiments or modifications and may be executed in various ways.

The shape of the IDT electrode is not limited to the one shown. For example, the IDT electrode may be one without dummy electrode fingers as well. Further, for example, the IDT electrode may be a so-called apodized one in which the lengths etc. of the electrode fingers change in the direction of propagation of the SAW. The bus bars may be inclined relative to the direction of propagation of the SAW as well.

In the acoustic wave resonator in the present embodiment, even if not providing a capacity element connected in parallel to the IDT electrode, the difference Δf of frequency between the resonance frequency and the anti-resonance frequency can be made narrow. However, a capacity element which is connected in parallel to the IDT electrode may be provided as well.

The bulk wave spurious emission can be generated if the piezoelectric substrate is relatively thin (for example the thickness tsis 30 μm or less or the normalized thickness ts/2p is 60 or less). Accordingly, the support substrate is not an essential factor. However, when the support substrate is bonded to the bottom surface of the piezoelectric substrate, for example, in the manufacturing process, the strength of a wafer for production of multiple acoustic wave resonators (thin piezoelectric substrates) can be improved. Further, the support substrate need not have a temperature compensation function either.

The bulk wave spurious emission utilized for the resonance point or anti-resonance point is not limited to the bulk wave spurious emission having the lowest frequency (for example line L21 inFIG. 6). For example, the bulk wave spurious emission having the second lowest frequency (for example line L22 or line L23 inFIG. 6) may be utilized as well.

In the method of design explained with reference toFIG. 8, the resonance characteristics were estimated by simulation calculations and various dimensions (thickness teand pitch “p” in the embodiment) satisfying various conditions (steps ST3, ST6, and ST9) were specified. Note, in addition to or place of the simulation calculations, a prototype may be prepared and its resonance characteristics may be measured, and various dimensions satisfying the conditions may be specified. That is, the method of design in the present embodiment is not limited to one realized by software.

In the method of design in the embodiment, various dimensions satisfying the conditions were specified by the idea of assuming various dimensions of the usual SAW resonator and changing these dimensions. However, considering the influence of various dimensions exerted upon the SAW and bulk wave, various dimensions of the acoustic wave resonator may also be calculated from the beginning or adjusted based on the results of calculation.

REFERENCE SIGNS LIST