JP5859355B2 - Elastic wave device and elastic wave device using the same - Google Patents

Description

  The present invention relates to an elastic wave device such as a surface acoustic wave (SAW) device and an elastic wave device using the same.

  An acoustic wave element having a piezoelectric substrate and an IDT (InterDigital Transducer) provided on the main surface of the piezoelectric substrate is known (for example, Patent Document 1). The IDT applies an electric field corresponding to an input electric signal to the main surface of the piezoelectric substrate to vibrate the main surface, and reflects a SAW generated by the vibration to form a standing wave having a predetermined period. . The standing wave generates an electric charge (electric signal) on the main surface of the piezoelectric substrate, and the electric signal is taken out by the IDT. As described above, the IDT has a function of applying a voltage to the piezoelectric substrate, extracting an electric signal from the piezoelectric substrate, and a function of reflecting the SAW.

JP 2004-112748 A

  The IDT is formed of a conductive material (generally metal) in order to apply an electric field to the piezoelectric substrate and to extract an electric signal from the piezoelectric substrate. However, metal may not necessarily be an optimal material for the function of reflecting SAW. That is, the IDT is required to be electrically conductive, so that the degree of freedom in selecting a material is lowered, and as a result, there is a possibility that the function of reflecting the SAW is not sufficiently ensured.

For example, the metal constituting the main component of IDT is preferably Al from the viewpoint of cost, etc. If the IDT made of Al is covered with a protective layer made of SiO ₂ in order to maintain the characteristics of the acoustic wave device, Al and SiO _{Since the} acoustic impedance is close to 2, the IDT is acoustically integrated with the protective layer, and the IDT may not sufficiently reflect the SAW.

  An object of the present invention is to provide an elastic wave element capable of suitably setting a SAW reflection coefficient and an elastic wave device having the elastic wave element.

  An acoustic wave device according to an aspect of the present invention includes an insulating material having a piezoelectric substrate and a plurality of elongated portions that are located on the upper surface of the piezoelectric substrate and arranged at intervals from one another. And a pair of comb-like electrodes each having an electrode finger made of a conductive material and laminated on the upper surface of each of the elongated portions.

  Preferably, the protective layer mainly composed of silicon oxide covers the pair of comb-like electrodes and has a thickness from the upper surface of the piezoelectric substrate larger than the total thickness of the insulating film and the comb-like electrodes. The pair of comb-like electrodes are made of a material mainly composed of Al, and the insulating film has an acoustic impedance larger than that of the electrode finger material and the protective layer material, and an elastic wave. It is made of a material with a slow propagation speed.

Preferably, the insulating film is made of a material mainly composed of _Ta 2 _{O 5.}

  Preferably, the pair of comb-like electrodes have a pair of bus bars arranged so as to face each other when seen in a plan view, and the electrode fingers extend from each of the pair of bus bars to the opposite bus bar. A gap is formed between the bus bar and the bus bar, and the elongated portion has a portion located in the gap.

  An elastic wave device according to one aspect of the present invention includes the above-described elastic wave element and a circuit board on which the elastic wave element is mounted.

  According to said structure, it has the insulating film which consists of an insulating material which has a some elongate part, and the electrode finger which consists of an electroconductive material laminated | stacked on each upper surface of the said elongate part. Therefore, the SAW reflection coefficient can be suitably set.

FIG. 1A is a plan view of a SAW element according to the first embodiment of the present invention, and FIG. 1B is a cross-sectional view taken along line Ib-Ib in FIG. FIG. 2A to FIG. 2E are cross-sectional views corresponding to FIG. 1B for explaining the method of manufacturing the SAW element according to the first embodiment. FIG. 3A to FIG. 3C are diagrams for explaining the operation of the first and second comparative examples and the SAW element of the first embodiment. FIG. 4 is a diagram corresponding to FIG. 1B showing the configuration of the third comparative example. FIG. 5A and FIG. 5B are graphs showing the electromechanical coupling coefficient K ² and the reflection coefficient Γ per electrode finger for the example and the third comparative example. FIG. 6A and FIG. 6B are diagrams showing a TCF calculation method and calculation results. It is a schematic diagram which shows the structure of a ladder type SAW filter. FIG. 8A and FIG. 8B are diagrams showing temperature characteristics in the ladder-type SAW filter with respect to the example and the comparative example. 1 is a cross-sectional view showing a SAW device according to a first embodiment of the present invention. FIGS. 10A and 10B are a plan view and a partially enlarged perspective view of a SAW element according to the second embodiment. FIG. 11A and FIG. 11B are a plan view and a partially enlarged perspective view of a SAW element according to the third embodiment. FIG. 12A and FIG. 12B are a plan view and a partially enlarged perspective view of a SAW element according to the fourth embodiment.

  Hereinafter, SAW elements and SAW devices according to embodiments of the present invention will be described with reference to the drawings. Note that the drawings used in the following description are schematic, and the dimensional ratios and the like on the drawings do not necessarily match the actual ones. In the second and subsequent embodiments, the same or similar configurations as those of the first embodiment may be denoted by the same reference numerals as those of the first embodiment, and description thereof may be omitted.

<First Embodiment>
(Configuration and manufacturing method of SAW element)
FIG. 1A is a plan view of the SAW element 1 according to the first embodiment of the present invention, and FIG. 1B is a cross-sectional view taken along the line Ib-Ib in FIG. Note that the SAW element 1 may be either upward or downward, but hereinafter, for convenience, the orthogonal coordinate system xyz is defined and the positive side in the z direction (FIG. 1 (a ) On the front side of the paper surface and the upper side of the paper surface in FIG.

  The SAW element 1 includes a piezoelectric substrate 3, an insulating film 9 (FIG. 1B) provided on the upper surface 3a of the piezoelectric substrate 3, an IDT 5 and a reflector 7 provided on the insulating film 9, and an upper surface 3a. The protective layer 11 (FIG. 1B) covering the IDT 5 and the reflector 7 is provided. In addition, the SAW element 1 may have wiring or the like for inputting / outputting signals to / from the IDT 5.

The piezoelectric substrate 3 is composed of a single crystal substrate having piezoelectricity such as a lithium tantalate (LiTaO ₃ ) single crystal, a lithium niobate (LiNbO ₃ ) single crystal, or the like. More preferably, the piezoelectric substrate 3 is composed of a 128 ° ± 10 ° YX cut LiNbO ₃ substrate. The planar shape and various dimensions of the piezoelectric substrate 3 may be set as appropriate. As an example, the thickness (z direction) of the piezoelectric substrate 3 is 0.2 mm to 0.5 mm.

  The IDT 5 has a pair of comb-like electrodes 13. Each comb-like electrode 13 includes a bus bar 13a (FIG. 1A) extending in the SAW propagation direction (x direction) and a plurality of electrode fingers 13b extending from the bus bar 13a in a direction orthogonal to the propagation direction (y direction). And have. The two comb-like electrodes 13 are provided so as to mesh with each other (the electrode fingers 13b cross each other). The plurality of electrode fingers 13b are provided so that the pitch thereof is equivalent to, for example, a half wavelength of the SAW wavelength λ at a frequency to be resonated.

  Note that FIG. 1 and the like are schematic diagrams, and actually, a plurality of pairs of comb-like electrodes having a larger number of electrode fingers may be provided. Further, a ladder-type SAW filter in which a plurality of IDTs 5 are connected in a series connection or a parallel connection may be configured, or a dual mode SAW resonator filter in which a plurality of IDTs 5 are arranged along the x direction. May be configured.

  The IDT 5 is made of, for example, a material containing Al as a main component (including an Al alloy). The Al alloy is, for example, an Al—Cu alloy. “Al as a main component” basically means that Al is used as a material. However, a material mixed with impurities other than Al that can be naturally mixed in the manufacturing process of the SAW element 1 is also possible. Including the case where the mass percent concentration of Al is 95 wt% or more. Hereinafter, the same meaning is used when the expression “principal component” is used. Various dimensions of the IDT 5 are appropriately set according to electrical characteristics required for the SAW element 1. As an example, the thickness e (FIG. 1B) of the IDT 5 is 100 nm to 300 nm.

  In addition, IDT5 may be arrange | positioned directly on the insulating film 9, and may be arrange | positioned on the insulating film 9 through another member. Another member is, for example, Ti, Cr, or an alloy thereof. When the IDT 5 is thus disposed on the insulating film 9 via another member, the thickness of the other member has a thickness that hardly affects the electrical characteristics of the IDT 5 (for example, the thickness of the IDT 5 in the case of Ti). 5% of the thickness).

  The reflector 7 is formed in a lattice shape having a pitch substantially equal to the pitch of the electrode fingers 13b of the IDT 5. For example, the reflector 7 is formed of the same material as the IDT 5 and has a thickness equivalent to that of the IDT 5.

  For example, the protective layer 11 is provided over substantially the entire upper surface 3 a of the piezoelectric substrate 3, covers the IDT 5 and the reflector 7, and covers a portion of the upper surface 3 a exposed from the IDT 5 and the reflector 7. . The thickness T (FIG. 1B) from the upper surface 3a of the protective layer 11 is at least larger than the thickness t of the insulating film 9, and preferably the thickness t of the insulating film 9, and the thickness e of the IDT 5 and the reflector 7 And greater than the total thickness. For example, the thickness T is 200 nm to 700 nm.

  It is desirable that the surface of the protective layer 11 has no large unevenness. Since the propagation speed of the elastic wave propagating on the piezoelectric substrate changes under the influence of the irregularities on the surface of the protective layer 11, if there are large irregularities on the surface of the protective layer 11, the resonance frequency of each manufactured acoustic wave element. A large variation will occur in this case. Therefore, if the surface of the protective layer 11 is made flat, the resonance frequency of each acoustic wave element is stabilized. Specifically, it is desirable that the flatness of the surface of the protective layer 11 is 1% or less of the wavelength of the elastic wave propagating on the piezoelectric substrate.

The protective layer 11 is made of an insulating material. The protective layer 11 is formed of a material mainly composed of a material such as silicon oxide (for example, SiO ₂ ) that increases the propagation speed of elastic waves when the temperature rises. As a result, the SAW element 1 is formed as described later. The change in characteristics due to the change in temperature can be kept small. Note that in a general material such as a material constituting the piezoelectric substrate 3, the propagation speed of the elastic wave becomes slow as the temperature rises.

  The insulating film 9 is for increasing the SAW reflection coefficient at the positions of the IDT 5 and the reflector 7. For example, the insulating film 9 is formed in the same shape as the planar shape of the IDT 5 and the reflector 7 in plan view, and overlaps the IDT 5 and the reflector 7 without excess or deficiency.

  That is, the insulating film 9 includes an IDT corresponding part 9v where the IDT 5 overlaps and a reflector corresponding part (illustration and reference numerals omitted) overlapping the reflector 7. The IDT corresponding portion 9v is formed in a pair of comb teeth (having a pair of comb electrode corresponding portions 9w), and each comb electrode corresponding portion 9w extends in the SAW propagation direction, A bus bar corresponding portion (not shown and reference numerals omitted) on which the bus bar 13a overlaps, and a plurality of electrode finger corresponding portions 9b extending from the bus bar corresponding portion in a direction orthogonal to the SAW propagation direction and on which the plurality of electrode fingers 13b overlap. . The reflector corresponding part is formed in a lattice shape.

  The thickness t (FIG. 1B) of the insulating film 9 is appropriate as long as an electric field is applied to the upper surface 3a of the piezoelectric substrate 3 by the IDT 5 and a change in charge on the upper surface 3a of the piezoelectric substrate 3 can be extracted as a signal. It may be a size. An example of the influence of the thickness t on the characteristics of the SAW element 1 and an example of a preferable range of the thickness t in consideration of the influence will be described later.

The difference in acoustic impedance between the material constituting the insulating film 9 and the material constituting the protective layer 11 is larger than the difference in acoustic impedance between the material constituting the IDT 5 and the reflector 7 and the material constituting the protective layer 11. The difference in acoustic impedance between the material composing the insulating film 9 and the material composing the protective layer 11 is preferably a certain level or more, for example, 15 MRayl or more, more preferably 20 MRayl or more. For example, when the material of the IDT 5 and the reflector 7 is mainly composed of Al (13.5 MRayl) and the material of the protective layer 11 is mainly composed of SiO ₂ (12.2 MRayl), the material is mainly composed of the insulating film 9. Can include WC (102.5 MRayl), TiN (56.0 MRayl), TaSi ₂ (40.6 MRayl), Ta ₂ O ₅ (33.8 MRayl), and W ₅ Si ₂ (67.4 MRayl).

Further, in addition to the above, the material of the insulating film 9 is preferably a material having a slower SAW propagation speed than the material constituting the IDT 5 and the reflector 7 and the material constituting the protective layer 11. For example, when the material of the IDT 5 and the reflector 7 is mainly composed of Al (5020 m / s) and the protective layer 11 is composed mainly of SiO ₂ (5560 m / s), the main component of the material of the insulating film 9 is Examples include TaSi ₂ (4438 m / s), Ta ₂ O ₅ (4352 m / s), and W ₅ Si ₂ (4465 m / s).

  FIG. 2A to FIG. 2E are cross-sectional views corresponding to FIG. 1B for explaining a method of manufacturing the SAW element 1. The manufacturing process proceeds in order from FIG. 2 (a) to FIG. 2 (e). Note that the various layers change in shape and the like as the process progresses, but common symbols may be used before and after the change.

  As shown in FIG. 2A, first, on the upper surface 3 a of the piezoelectric substrate 3, the additional layer 17 that becomes the insulating film 9 and the conductive layer 15 that becomes the IDT 5 and the reflector 7 are formed. Specifically, first, the additional layer 17 is formed on the upper surface 3a by a thin film forming method such as a sputtering method or a CVD (Chemical Vapor Deposition) method, and then the same appropriate thin film forming method. Thus, the conductive layer 15 is formed.

  When the conductive layer 15 is formed, a resist layer 19 as a mask for etching the additional layer 17 and the conductive layer 15 is formed as shown in FIG. Specifically, a thin film of a negative or positive photosensitive resin is formed by an appropriate thin film forming method, and a part of the thin film is removed at a non-arranged position of the IDT 5 and the reflector 7 by a photolithography method or the like. The

  Next, as shown in FIG. 2C, the additional layer 17 and the conductive layer 15 are etched by an appropriate etching method such as RIE (Reactive Ion Etching). Thereby, the insulating film 9, the IDT 5, and the reflector 7 are formed. Thereafter, as shown in FIG. 2D, the resist layer 19 is removed by using an appropriate chemical solution.

  Then, as shown in FIG. 2E, a thin film to be the protective layer 11 is formed by an appropriate thin film forming method such as a sputtering method or a CVD method. At this point, unevenness is formed on the surface of the thin film to be the protective layer 11 due to the thickness of the IDT 5 or the like. Then, if necessary, the surface is flattened by chemical mechanical polishing or the like, and a protective layer 11 is formed as shown in FIG.

  With reference to FIG. 3A to FIG. 3C, the operation of the comparative example will be described, and the operation of the SAW element 1 of the first embodiment will be described.

  FIG. 3A is a cross-sectional view illustrating the operation of the SAW element 101 of the first comparative example. The SAW element 101 is in a state where the insulating film 9 and the protective layer 11 are not provided in the SAW element 1 of the first embodiment.

  When an electric signal is input to one comb-shaped electrode 13 and a voltage is applied to the piezoelectric substrate 3 by the electrode finger 13b of the comb-shaped electrode 13, the vicinity of the upper surface 3a of the piezoelectric substrate 3 is indicated by an arrow y1. , SAW propagating along the upper surface 3a is induced. Further, as indicated by the arrow y2, the SAW is reflected at the boundary between the electrode finger 13b of the IDT 5 and the gap between the electrode fingers (a region between the electrode fingers 13b intersecting each other, a region where the electrode fingers 13b are not disposed). And the standing wave which makes the pitch of the electrode finger 13b a half wavelength is formed by SAW shown by arrows y1 and y2. The standing wave generates an electric charge (an electric signal having the same frequency as that of the standing wave) on the upper surface 3 a, and the electric signal is taken out by the electrode finger 13 b of the other comb-like electrode 13. In this way, the SAW element 1 functions as a resonator or a filter.

  However, in the SAW element 101, when the temperature rises, the propagation speed of the elastic wave in the piezoelectric substrate 3 is reduced, and the gap between the electrode fingers is increased. As a result, the resonance frequency is lowered and the desired characteristics may not be obtained.

  FIG. 3B is a cross-sectional view illustrating the operation of the SAW element 201 of the second comparative example. The SAW element 201 has no insulating film 9 in the SAW element 1 of the first embodiment. In other words, the protective layer 11 is added to the SAW element 101 of the first comparative example.

Since the protective layer 11 is provided in the SAW element 201, the induced SAW propagates not only in the piezoelectric substrate 3 but also in the protective layer 11, as indicated by the arrow y3. Here, as described above, the protective layer 11 is made of a material whose propagation speed of elastic waves is increased as the temperature rises, for example, silicon oxide such as SiO ₂ . Therefore, as a whole SAW propagating through the piezoelectric substrate 3 and the protective layer 11, a change in speed due to a temperature rise is suppressed. That is, the protective layer 11 compensates for a change in characteristics of the piezoelectric substrate 3 due to a temperature rise.

However, when the IDT 5 is formed of Al or an Al alloy and the protective layer 11 is formed of SiO ₂ , the acoustic properties of the IDT 5 and the protective layer 11 are approximate, and the gap between the electrode fingers 13b and the electrode fingers The boundary of is obscured acoustically. In other words, the reflection coefficient at the boundary between the electrode finger 13b and the gap between the electrode fingers decreases. As a result, in FIG. 3B, as indicated by an arrow y4 smaller than the arrow y2 in FIG. 3A, the reflected wave of SAW cannot be obtained sufficiently, and desired characteristics may not be obtained.

  FIG. 3C is a cross-sectional view illustrating the operation of the SAW element 1 according to the first embodiment.

  Since the SAW element 1 has the protective layer 11, a temperature characteristic compensation effect can be obtained in the same manner as the SAW element 201 of the second comparative example. Further, the SAW element 1 has an insulating film 9, and the insulating film 9 is formed of a material having a larger acoustic impedance difference from the material of the protective layer 11 than the material of the IDT 5. Therefore, the reflection coefficient at the boundary position between the electrode finger 13b and the gap between the electrode fingers is increased. As a result, it is possible to obtain a sufficient reflected wave of SAW as indicated by the arrow y2.

  Further, if the material of the insulating film 9 is a material having a slow propagation speed of elastic waves, the vibration distribution is likely to concentrate on the insulating film 9, and the reflection coefficient is effectively increased. It should be noted that a material having a larger acoustic impedance than a material forming the protective layer 11 and the IDT 5 tends to satisfy the condition that the elastic wave propagation speed is slower than the material forming the protective layer 11 and the IDT 5. The material can be easily selected.

(Comparison with the case where the insulating film is provided on the IDT)
FIG. 4 is a cross-sectional view corresponding to FIG. 1B showing the configuration of the third comparative example.

  In the third comparative example, the vertical relationship between the insulating film 9 and the IDT 5 (and the reflector 7) is reversed. Other than that, the third comparative example and the first embodiment are substantially the same. In the third comparative example, the same effect as that of the first embodiment described with reference to FIG. That is, the effect of temperature compensation can be obtained by the protective layer 11 and the reflection coefficient can be increased by the insulating film 9.

  Therefore, specific materials and dimensions were set for the SAW elements of the first embodiment and the third comparative example, and simulation calculations were performed on the behavior thereof to examine differences in characteristics between the two. Specifically, it is as follows.

In the simulation calculation, the following conditions were assumed in common with the first embodiment (example) and the third comparative example. In the following, λ is the wavelength of the SAW that is desired to resonate, and is approximately twice the pitch of the electrode fingers 13b.
Piezoelectric substrate 3: 126 ° YX cut LiNbO ₃ substrate IDT5:
Material: Al
Thickness e: 0.08λ
Insulating film 9:
Material: Ta ₂ O ₅
Thickness t: variously changed.
Protective layer 11:
Material: SiO ₂
Thickness T: 0.33λ

  FIG. 5A and FIG. 5B are graphs showing simulation results.

5A and 5B, the horizontal axis indicates the normalized thickness t / λ of the insulating film 9. The vertical axis in FIG. 5 (a) shows the electromechanical coupling coefficient K ^2. In FIG. 5B, the vertical axis represents the reflection coefficient Γ per electrode finger 13b. 5A and 5B, a line Lc indicates the characteristic of the third comparative example, and a line Lp indicates the characteristic of the example.

As shown in FIG. 5 (a), examples are compared with the third comparative example, it was small electromechanical coupling coefficient K ^2. Therefore, the embodiment is more suitable for the narrow band filter than the third comparative example. In addition, about reflection coefficient (GAMMA), the Example became smaller than the 3rd comparative example.

  Next, the difference in frequency temperature coefficient (TCF: Temperature Coefficient of Frequency) was investigated based on the simulation results.

  FIG. 6A is a diagram for explaining a TCF calculation method. In FIG. 6A, the horizontal axis indicates the frequency f, and the vertical axis indicates the absolute value | Ze | of the electrical impedance. A line L25 indicates the characteristics of the SAW element (Example or Comparative Example) when the temperature is 25 ° C., and a line L85 indicates the characteristics of the SAW element when the temperature is 85 ° C.

As is generally known, | Ze | takes a minimum value at the resonance frequency fr (fr _{25 at} 25 ° C., fr _{85 at} 85 ° C.), and anti-resonance frequency fa (fa _{25 at} 25 ° C., fa at 85 ° C. ₈₅ ). Further, the resonance frequency fr and the antiresonance frequency fa change according to the temperature change. Therefore, the TCF of the resonance frequency fr and the antiresonance frequency fa was calculated by the following formula.
TCF = (fr ₈₅ −fr ₂₅ ) / (fr ₂₅ × (85−25)) × 10 ⁶
TCF = (fa ₈₅ −fa ₂₅ ) / (fa ₂₅ × (85−25)) × 10 ⁶

  FIG. 6B is a diagram illustrating a calculation result of TCF.

  In FIG. 6B, the horizontal axis represents the normalized thickness t / λ of the insulating film 9, and the vertical axis represents TCF. A line Lpr and a line Lpa indicate the TCF of the resonance frequency fr and the antiresonance frequency fa of the embodiment, respectively, and a line Lcr and a line Lca indicate the TCF of the resonance frequency fr and the antiresonance frequency fa of the third comparative example, respectively. ing.

  As shown in this figure, the TCF having the resonance frequency fr is different from the TCF having the anti-resonance frequency fa. And about the difference, in the range shown in FIG.6 (b), the difference Dp in an Example is smaller than the difference Dc in a 3rd comparative example.

  Therefore, in the example, it is expected that the change in the filter characteristics due to the temperature change is suppressed as compared with the comparative example. For example, taking a ladder-type SAW filter as an example, it is as follows.

  FIG. 7 is a schematic diagram showing the ladder-type SAW filter 27.

  The ladder-type SAW filter 27 includes a plurality of series resonators 1S connected in series and a plurality of parallel resonators 1P connected in parallel to the plurality of series resonators 1S. The series resonator 1S and the parallel resonator 1P are one-port SAW resonators, similarly to the SAW element 1 illustrated in FIG.

  8A and 8B are diagrams showing the characteristics of the ladder-type SAW filter 27. FIG. 8A corresponds to the case where the SAW element of the embodiment is used, and FIG. Corresponds to the case where the SAW element of the third comparative example is used.

  8A and 8B, the horizontal axis indicates the frequency f, and the vertical axis indicates the absolute value of electrical impedance | Ze | (lower part of the figure) and the pass characteristic S (upper part of the figure). Is shown. In FIGS. 8A and 8B, lines L25-S and L85-S indicate | Ze | at 25 ° C. and 85 ° C. of the series resonator 1S, respectively, and lines L25-P and L85- P indicates | Ze | at 25 ° C. and 85 ° C. of the parallel resonator 1P, respectively, and lines Ln25 and Ln85 indicate the pass characteristics S of the ladder-type SAW filter 27 at 25 ° C. and 85 ° C., respectively.

For example, the series resonator 1S and the parallel resonator 1P have a resonance frequency of the series resonator 1S at 25 ° C. as shown by lines L25-S and L25-P in FIG. 8A or 8B. The anti-resonance frequency fa ₂₅ -P of the parallel resonator 1P is made to coincide with fr ₂₅ -S (see line Li). Therefore, the component having a frequency equivalent to the resonance frequency fr ₂₅ -S among the signals input to the input terminal 29I (FIG. 7) is connected to the series resonator 1S without being short-circuited to the ground by the parallel resonator 1P. Pass through and reach the output terminal 29O (FIG. 7).

When the temperature reaches 85 ° C., as indicated by lines L85-S and L85-P, the values of the resonance frequency and antiresonance frequency of the series resonator 1S and the parallel resonator 1P deviate from the values at 25 ° C. At this time, as described with reference to FIGS. 6A and 6B, the resonance frequency and the anti-resonance frequency have different amounts of change (TCF) with respect to temperature. The resonance frequency fr ₈₅ -S of the parallel resonator 1P and the anti-resonance frequency fa ₈₅ -P of the parallel resonator 1P do not match (see line Lj).

Here, as described above, since the TCF is smaller than that of the third comparative example, the deviation amount between the resonance frequency fr ₈₅ -S and the anti-resonance frequency fa ₈₅ -P is relatively small. As shown by lines Ln25 and Ln85 in 8 (a), the pass characteristic S does not change much between 25 ° C and 85 ° C.

On the other hand, in the third comparative example, the amount of deviation between the resonance frequency fr ₈₅ -S and the anti-resonance frequency fa ₈₅ -P is relatively large, and as shown by lines Ln25 and Ln85 in FIG. The characteristic S is different between 25 ° C. and 85 ° C. Specifically, as shown in the region Ar, at 85 ° C., a part of the signal to be passed flows to the ground through the parallel resonator 1P, resulting in a passage loss.

(Preferred thickness of insulating film)
In FIG. 6B, regarding the example, the line Lpa indicating the TCF of the anti-resonance frequency fa and the line Lpr indicating the TCF of the resonance frequency fr both extend linearly, and the thickness t / As λ increases, they approach each other. That is, the difference in TCF between the antiresonance frequency fa and the resonance frequency fr becomes small. The line Lpa and the line Lpr intersect each other when the thickness t / λ becomes 0.0945, and the difference in TCF becomes zero. Further, when the thickness t / λ of the insulating film is increased, the lines Lpa and Lpr are separated from each other. That is, the TCF difference is increased.

  When the thickness t / λ is 0.01, the difference in TCF is less than 10 ppm / ° C. Although not particularly shown, when the thickness t / λ is larger than 0.0945, the TCF difference is 10 ppm / ° C. when the thickness t / λ is 0.17. On the other hand, if the difference in TCF is about 10 ppm / ° C., the SAW element is considered to be suitably used. In the third comparative example, the difference in TCF is about 10 ppm / ° C. in the range of FIG.

  From the above, from the viewpoint of TCF, the thickness t / λ of the insulating film 9 is preferably 0.01 or more and 0.17 or less, more preferably around 0.0945 (for example, 0.0945 ± 0.01). ).

(Configuration of SAW device)
FIG. 9 is a cross-sectional view showing the SAW device 51 according to the present embodiment.

  The SAW device 51 configures, for example, a filter or a duplexer. The SAW device 51 includes a SAW element 31 and a circuit board 53 on which the SAW element 31 is mounted.

  The SAW element 31 is configured as, for example, a so-called wafer level package SAW element. The SAW element 31 includes the above-described SAW element 1, a cover 33 that covers the upper surface 3 a side of the SAW element 1, and a back surface portion 37 that covers the opposite side of the piezoelectric substrate 3 from the SAW element 1.

  The cover 33 is made of resin or the like, and forms a vibration space 33 a on the upper surface 3 a side of the SAW element 1. Although not shown in FIG. 9, the IDT 5 and the reflector 7 are provided on the upper surface 3a in the vibration space 33a. SAW propagation (vibration of the upper surface 3a) is facilitated by the vibration space 33a. For example, although not particularly illustrated, the back surface portion 37 includes a back surface electrode for discharging electric charges charged on the surface of the piezoelectric substrate 3 due to a temperature change or the like, and an insulating layer covering the back surface electrode.

  The SAW element 1 has a pad 35 connected to the IDT 5 through a wiring (not shown) on the upper surface 3a. The pad 35 is exposed above the SAW element 1 (on the positive side in the z direction) by forming an opening in the cover 33.

  The circuit board 53 is constituted by, for example, a so-called rigid printed wiring board. A mounting pad 55 is formed on the mounting surface 53 a of the circuit board 53.

  The SAW element 31 is arranged with the cover 33 side facing the mounting surface 53a. Then, the pad 35 and the mounting pad 55 are bonded by solder 57. Thereafter, the SAW element 31 is sealed with a sealing resin 59.

  For example, the circuit board 53 may constitute a mother board, or may be a package for mounting the SAW element 1 on another circuit board. Further, electronic elements other than the SAW element 31 may be mounted on the circuit board 53 together with the SAW element 31.

<Second Embodiment>
FIG. 10A is a plan view showing a SAW element 301 according to the second embodiment.

  Similar to the IDT 5 of the first embodiment, the IDT 305 of the SAW element 301 is made of Al or an alloy containing Al as a main component, and has a pair of comb-like electrodes 313. Each comb-like electrode 313 includes a bus bar 313a and a plurality of electrode fingers 313b, like the comb-like electrode 13 of the first embodiment.

  Further, each comb-like electrode 313 has a plurality of dummy electrodes 313c extending from the bus bar 313a toward the bus bar 313a side of the other comb-like electrode 313 between the plurality of electrode fingers 313b. The tips of the plurality of dummy electrodes 313c of each comb-like electrode 313 are opposed to the tips of the plurality of electrode fingers 313b of the other comb-like electrode 313 via the tip gap G1. The width of the dummy electrode 313c (size in the x direction) is, for example, equal to the width of the electrode finger 313b. The length of the tip gap G1 (size in the y direction) is, for example, about a half wavelength (λ / 2).

  The IDT 305 is so-called apodized. That is, the length in the y direction of the range (intersection range R1) where the plurality of electrode fingers 313b of the pair of comb-like electrodes 313 intersect by changing the length of the plurality of electrode fingers 313b according to the position in the x direction. W changes in accordance with the position in the x direction. FIG. 10A illustrates a case where the width W increases at the center in the x direction of the IDT 5 and decreases at both sides in the x direction. The plurality of dummy electrodes 313c have different lengths corresponding to the change in the width W so that the lengths of the plurality of tip gaps G1 are substantially the same. The IDT 305 may not be apodized.

  FIG. 10B is a perspective view in the region Xb of FIG. In the figure, the protective layer 11 is seen through.

  As in the first embodiment, an insulating film 9 is provided between the upper surface 3a and the IDT 305 and between the upper surface 3a and the reflector 7. The insulating film 9 is provided not only in the arrangement region of the bus bar 313a (not shown in FIG. 10B) and the electrode finger 313b but also in the arrangement region of the dummy electrode 313c. In the insulating film 9, a portion extending from the bus bar corresponding portion in a direction orthogonal to the SAW propagation direction and overlapping with the dummy electrode 313c is the dummy electrode corresponding portion 9c. Further, in the SAW element 301, the insulating film 9 is also provided in the tip gap G1. A portion of the insulating film 9 that is positioned at the tip gap G1 is a gap corresponding portion 9g.

  In the first embodiment, the insulating film 9 includes a pair of comb-like portions (a pair of comb-like electrode corresponding portions 9w) corresponding to the IDT 5, whereas the second embodiment In the form, the insulating film 9 includes a lattice portion like the shape of the reflector 7 corresponding to the IDT 5. However, any insulating film 9 still includes a plurality of long portions (electrode finger corresponding portions 9b and the like) arranged in the short direction and overlapping the plurality of electrode fingers. The width (x direction) of the gap corresponding part 9g is, for example, equal to the widths of the electrode finger 313b and the dummy electrode 313c.

  In such a SAW element 301, for example, the additional layer 17 and the conductive layer 15 (FIG. 2A) are not etched together using one mask, but the conductive layer 15 is formed after the additional layer 17 is etched. It can be produced by filming and etching.

  According to the second embodiment, similar to the first embodiment, the insulating film 9 can improve the reflection coefficient at the arrangement position of the IDT 5 and the reflector 7. Further, since the insulating film 9 is also located in the tip gap G1, SAW scattering in the tip gap G1 is suppressed, and consequently, SAW propagation loss can be suppressed. This effect is particularly effective in an IDT in which the position of the tip gap G1 changes in the y direction, for example, an IDT in which the width of the crossing range changes and an IDT in which the crossing range is inclined with respect to the SAW propagation direction.

  In the third comparative example shown in FIG. 4 as well, it is conceivable that the insulating film 9 is formed so that the insulating film 9 is positioned in the tip gap G1. However, in this case, since the portion of the insulating film 9 located in the tip gap G1 is located on the upper surface 3a of the piezoelectric substrate 3, a step is generated between the portion and the portion of the insulating film 9 located on the IDT 5. There may be a propagation loss due to the step. On the other hand, in the second embodiment, such a step does not occur.

<Third Embodiment>
FIG. 11A is a plan view showing a SAW element 401 according to the third embodiment.

  The IDT 5 of the SAW element 401 is the same as the IDT 5 of the first embodiment. That is, the dummy electrode 313c shown in the second embodiment is not provided. And the front-end | tip of the some electrode finger 13b of each comb-tooth shaped electrode 13 has opposed the bus-bar 13a of the other comb-tooth-shaped electrode 13 via the some front-end | tip gap G2. The tip gap G2 is the same as the tip gap G1 of the second embodiment in that it is a gap between a pair of comb-like electrodes in front of the tip of the electrode finger. The IDT 5 may be apodized as in the second embodiment.

  FIG. 16B is a perspective view of the region XIb in FIG. In the figure, the protective layer 11 is seen through.

  An insulating film 9 is provided between the upper surface 3a and the IDT 5 and between the upper surface 3a and the reflector 7, as in the first embodiment. Furthermore, the insulating film 9 is provided not only in the region overlapping the IDT 5 but also in the tip gap G2 (having a gap corresponding portion 9g). In other words, the insulating film 9 has a lattice-like portion like the shape of the reflector 7 corresponding to the IDT 5 as in the second embodiment.

  According to the third embodiment described above, the provision of the insulating film 9 provides an effect that the reflection coefficient can be increased as in the first embodiment. Further, since the insulating film 9 has the gap corresponding portion 9g, the SAW scattering can be suppressed in the tip gap G2 as in the second embodiment.

  As described above, the SAW element 401 can be regarded as having the shape of the insulating film 9 changed in the first embodiment, but is regarded as having the dummy electrode 313c eliminated in the second embodiment. You can also. In the second embodiment, the SAW speed differs in the arrangement region of the intersection range R1, the tip gap G1, and the dummy electrode 313c, and a speed discontinuity occurs. On the other hand, in the third embodiment, since the dummy electrode 313c is eliminated, the number of speed discontinuities is reduced. As a result, it is expected that the scattering of SAW in the discontinuous portion of the velocity is suppressed.

<Fourth Embodiment>
FIG. 12A is a plan view showing a SAW element 501 according to the fourth embodiment.

  The IDT 505 of the SAW element 501 has a dummy electrode 513c as in the second embodiment, and is not apodized as in the first embodiment. However, the IDT 505 may be apodized as in the second embodiment.

  FIG. 17B is a perspective view of the region XIIb in FIG. In the figure, the protective layer 11 is seen through.

  As in the other embodiments, an insulating film 9 is provided between the upper surface 3a and the IDT 505 (and the reflector 7). Furthermore, the insulating film 9 is also provided in the side gap G3 between the dummy electrodes 513c and the electrode fingers 513b belonging to the same comb-like electrode 513 and adjacent to each other. For example, the insulating film 9 completely fills the side gap G3 in plan view.

  According to the fourth embodiment described above, the provision of the insulating film 9 provides an effect that the reflection coefficient can be increased as in the first embodiment. Further, in the fourth embodiment, the speed difference between the SAW propagating in the intersecting region of the plurality of electrode fingers 513b and the SAW propagating in the region of the dummy electrode 513b and the side gap G3 is increased, and the SAW in the intersecting region is increased. The confinement effect is expected to increase. As a result, loss is expected to be suppressed.

  The above embodiments may be implemented in combination as appropriate. For example, in the fourth embodiment, the IDT may be an IDT 5 having no dummy electrode as in the first embodiment. In the fourth embodiment, the insulating film may have a portion located in the tip gap G1 as in the second embodiment.

  In addition, the electrode finger corresponding part 9b in the first and fourth embodiments, the electrode finger corresponding part 9b, the dummy electrode corresponding part 9c and the gap corresponding part 9g in the second embodiment, and the electrode finger in the third embodiment The corresponding portion 9b and the gap corresponding portion 9g are an example of a long portion of the insulating film.

  The present invention is not limited to the above embodiment, and may be implemented in various aspects.

  The acoustic wave element is not limited to a SAW element (in the narrow sense). For example, the insulating layer (11) may be a so-called boundary acoustic wave element (however, included in a broad sense SAW element) in which the thickness of the insulating layer (11) is relatively large (for example, 0.5λ to 2λ). In the boundary acoustic wave element, it is not necessary to form the vibration space (33a).

  Further, the acoustic wave element is not limited to a wafer level package. For example, the SAW element may have a vibration space formed by a gap between the SAW element 1 (protective layer 11) and the mounting surface 53a of the circuit board 53. In the case where the acoustic wave element is of a wafer level package, the acoustic wave element may have a terminal penetrating the cover.

  The acoustic wave device may not have a protective layer. Even in this case, the material that defines the reflection coefficient can be selected not only from the conductive material but also from the insulating material, and the effect of improving the degree of freedom in design is achieved.

  The insulating film is preferably provided over the entire surface of the IDT and the reflector. However, the insulating film may be provided only on a part of the IDT, such as provided only on the electrode fingers. Moreover, the edge part of the insulating film may be located outside the edge part of IDT and a reflector in planar view, and may be located inside. In addition, the insulating film may have a shape in which the upper surface is narrower than the lower surface, in other words, the cross-sectional shape may be tapered (trapezoid), or the upper surface may be wider than the lower surface. The insulating film is not limited to a single layer, and may have a multilayer structure of two or more layers.

The piezoelectric substrate 3 may be, for example, 38.7 ° ± YX cut LiTaO ₃ in addition to the 128 ° ± 10 ° YX cut LiNbO ₃ substrate.

  Since the reflector does not need to have conductivity, it may be formed of only an insulating film.

  DESCRIPTION OF SYMBOLS 1 ... SAW element (elastic wave element), 3 ... Substrate (piezoelectric substrate), 3a ... Upper surface, 5 ... IDT, 9 ... Insulating film, 11 ... Protective layer.

Claims (7)

A piezoelectric substrate;
An insulating film made of an insulating material having a plurality of elongated portions located on the upper surface of the piezoelectric substrate and arranged at intervals from each other in one direction;
A pair of bus bars arranged opposite to each other when viewed in plan, and electrode fingers extending from each of the pair of bus bars toward the other bus bar and stacked on the upper surface of each of the elongated portions A pair of comb-like electrodes made of a conductive material,
The insulating layer is located on the tip gap interposed between the tip of the electrode fingers of each of said pair of comb-shaped electrodes, and the other interdigital electrode of the pair of comb-shaped electrodes An elastic wave element having a portion. A protective layer covering silicon oxide as a main component, covering the pair of comb-like electrodes, and having a thickness from the upper surface of the piezoelectric substrate larger than the total thickness of the insulating film and the comb-like electrodes;
The pair of comb electrodes are made of a material mainly composed of Al,
The acoustic wave device according to claim 1, wherein the insulating film is made of a material having an acoustic impedance larger than that of the electrode finger material and the protective layer material and having a slow propagation speed of elastic waves. The pair of comb-like electrodes protrude between the plurality of electrode fingers from each of the pair of bus bars, and tips of the plurality of electrode fingers and a plurality of the tips that extend from the other bus bar. The acoustic wave device according to claim 1, further comprising a plurality of dummy electrodes facing each other through a gap. The first electrode fingers extending from each of the pair of bus bars, the elastic wave element according to claim 1 or 2 the tip is opposed through the tip formic cap with the edge of the other bus bar. The insulating film has a portion located in a side gap between the plurality of electrode fingers and the plurality of dummy electrodes adjacent to each other of each of the pair of comb- like electrodes. Item 4. The acoustic wave device according to Item 3. A piezoelectric substrate;
An insulating film made of an insulating material having a plurality of elongated portions located on the upper surface of the piezoelectric substrate and arranged at intervals from each other in one direction;
A pair of bus bars arranged opposite to each other when viewed in plan, and electrode fingers extending from each of the pair of bus bars toward the other bus bar and stacked on the upper surface of each of the elongated portions A pair of comb-like electrodes made of a conductive material,
The pair of comb-like electrodes protrude between the plurality of electrode fingers from each of the pair of bus bars, and tips of the plurality of electrode fingers and a plurality of the tips that extend from the other bus bar. It has a plurality of dummy electrodes facing each other through a gap,
The insulating film has a portion located in a lateral gap between the plurality of electrode fingers and the plurality of dummy electrodes adjacent to each other of each of the pair of comb- like electrodes. Wave element. The elastic wave device according to any one of claims 1 to 6,
An acoustic wave device comprising: a circuit board on which the acoustic wave element is mounted.

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