JP2012209841A - Acoustic wave element and acoustic wave device including the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an acoustic wave element high in reflection coefficient and low in temperature coefficient.SOLUTION: An SAW element 1 includes: a piezoelectric substrate 3; an IDT electrode 5 arranged on an upper face 3a of the piezoelectric substrate 3; a spacer 18 arranged on an upper face of the IDT electrode 5 and comprising an insulation material; an additional film 9 arranged on an upper face of the spacer 18; and an insulation layer 11 covering the IDT electrode 5, above which the additional film 9 is arranged, together with a part of the piezoelectric substrate 3, the part being exposed from the IDT electrode 5, having a thickness from the upper face 3a, thicker than the thickness between the upper face 3a of the piezoelectric substrate 3 and the upper face of the additional film 9, and comprising SiO. The additional film 9 comprises a material having an acoustic impedance greater than those of a material of the IDT electrode 5, a material of the spacer 18 and SiO, having a density higher than that of the spacer 18, and having a propagation velocity of elastic wave slower than those of the material of the IDT electrode 5 and SiO.

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 device having a piezoelectric substrate, an IDT (Inter Digital Transducer) electrode (excitation electrode) provided on the main surface of the piezoelectric substrate, and a protective layer covering the IDT electrode is known (for example, Patent Document 1 or 2). The protective layer is formed of, for example, SiO ₂ and contributes to the suppression of corrosion of the IDT electrode, the reduction of the temperature coefficient of the characteristics of the acoustic wave element, and the like.

In Patent Document 2, in the acoustic wave device having the protective layer (SiO ₂ film) as described above, when the IDT electrode is formed of Al or an alloy containing Al as a main component, a sufficient reflection coefficient cannot be obtained. (Paragraph 0009). Then, it has been proposed to obtain a relatively high reflection coefficient by forming the IDT electrode with a metal having a density higher than that of Al or an alloy containing the metal as a main component.

Although not related to the reflection coefficient, Patent Documents 1 and 2 propose forming an adhesion layer between the IDT electrode and the SiO ₂ film in order to improve adhesion ( Paragraph 0011 of Patent Document 1 and Paragraph 0107 of Patent Document 2). In Patent Documents 1 and 2, the adhesion layer is formed thin so as not to affect the propagation of SAW. Specifically, the adhesion layer is 50 to 100 mm (paragraph 0009 of Patent Document 1) or 1% or less of the wavelength of SAW (paragraph 0108 of Patent Document 2).

Japanese Patent Laid-Open No. 08-204493 JP 2004-112748 A

  As described above, Patent Document 2 proposes to form the IDT electrode with a material having a higher density than Al. However, it may be desired to use Al or an alloy containing Al as a main component as a material for the IDT electrode from the viewpoint of electrical characteristics, processability, material cost, and the like. For this reason, it is desired to provide a method for increasing the reflection coefficient by a method different from Patent Document 2. Furthermore, an elastic wave device having a smaller temperature coefficient is desired due to recent advances in wireless communication technology.

  An object of the present invention is to provide an elastic wave element and an elastic wave device that can further reduce the temperature coefficient of characteristics and increase the reflection coefficient.

An acoustic wave device according to an embodiment of the present invention includes a piezoelectric substrate, an IDT electrode disposed on the upper surface of the piezoelectric substrate, a spacer made of an insulating material disposed on the upper surface of the IDT electrode, and an upper surface of the spacer. And the IDT electrode on which the additional film is disposed together with a portion of the piezoelectric substrate exposed from the IDT electrode, and the thickness from the upper surface of the piezoelectric substrate is from the upper surface of the piezoelectric substrate. An insulating layer made of SiO ₂ having a thickness up to the upper surface of the additional film, and the additional film includes the IDT electrode material, the spacer material, and SiO 2
_It is made of a material having an acoustic impedance larger than _{2, a} density larger than that of the spacer, and a material having a slower propagation speed of elastic waves than the material of the IDT electrode and SiO ₂ .

  An elastic wave device according to an embodiment of the present invention includes the above elastic wave element and a circuit board on which the elastic wave element is mounted.

According to the arrangement, above the IDT electrode was placed an additional membrane material and SiO ₂ greater acoustic impedance than the IDT electrode, and the material and the propagation velocity of the acoustic wave from the SiO ₂ of the IDT electrodes made of slow material Thus, the reflection coefficient of the elastic wave element in which the IDT electrode is covered with SiO ₂ can be increased, and an elastic wave element having excellent temperature compensation characteristics and resonance characteristics can be obtained.

Further, a spacer is interposed between the IDT electrode and the additional film. That is, since the additional film is disposed apart from the IDT electrode, the additional film can be selected independently with respect to the material, thickness, and width of the IDT electrode, and the degree of freedom in design can be increased. Can be improved. In addition, since the energy of the elastic wave can be further distributed on the SiO ₂ side by the additional film and the spacer, the temperature coefficient of the characteristics of the elastic wave element can be further reduced.

FIG. 1A is a plan view of a SAW element according to an 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 a method of manufacturing a SAW element. FIG. 3A to FIG. 3C are diagrams for explaining the operation of the SAW element of the comparative example and the embodiment. It is a graph which shows the relationship between the thickness d of a spacer, and the frequency temperature characteristic of a SAW element. It is a graph which shows the reflection coefficient (GAMMA) ₁ per electrode finger. It is a graph showing the electromechanical coupling coefficient K ² per one electrode finger.

FIG. 7A and FIG. 7B are graphs showing the reflection coefficient Γ ₁ and the electromechanical coupling coefficient K ² per electrode finger. FIGS. 8A and 8B are other graphs showing the reflection coefficient Γ ₁ and the electromechanical coupling coefficient K ² per electrode finger. It is another graph which shows the reflection coefficient (GAMMA) ₁ per electrode finger. FIG. 10A and FIG. 10B are diagrams for explaining how to obtain the lower limit of the preferred range of the thickness of the additional film. The preferred range is graph showing the lower limit of the thickness of the additional film made of Ta ₂ O _5. Is a graph showing the lower limit of the preferred range of additional film thickness made of TaSi _2. W is a graph showing the lower limit of the preferred range of the thickness of the additional film made of ₅ Si _2. It is a graph showing a preferable range of the thickness of the additional film made of Ta ₂ O _5. It is sectional drawing which shows the SAW apparatus which concerns on embodiment of this invention.

  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.

(Configuration and manufacturing method of SAW element)
1A is a plan view of an acoustic wave element (hereinafter referred to as a SAW element) 1 according to an embodiment of the present invention, and FIG. 1B is a cross-sectional view taken along 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 substrate 3, an IDT electrode 5 and a reflector 7 provided on the upper surface 3a of the substrate 3, a spacer 18 (FIG. 1B) provided on the IDT electrode 5 and the reflector 7, An additional film 9 (FIG. 1B) provided on the spacer and a protective layer 11 (FIG. 1B) covering the upper surface 3a from above the additional film 9 are provided. The additional film 9 is spaced from the IDT electrode 5 and the reflector 7 by a spacer 18. In addition to this, the SAW element 1 may have wiring for inputting / outputting signals to / from the IDT electrode 5.

The substrate 3 is constituted by a piezoelectric substrate. Specifically, for example, the 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 substrate 3 is configured by a 128 ° ± 10 ° YX cut LiNbO ₃ substrate. The planar shape and various dimensions of the substrate 3 may be set as appropriate. As an example, the thickness (z direction) of the substrate 3 is 0.2 mm to 0.5 mm.

  The IDT electrode 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. 3 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 IDT electrodes 5 are connected by a system such as a series connection or a parallel connection may be configured, or a dual mode SAW resonance in which a plurality of IDT electrodes 5 are arranged along the X direction. A filter or the like may be configured. Further, weighting by apodization may be performed by making the lengths of the plurality of electrode fingers different.

  The IDT electrode 5 is made of, for example, metal. More preferably, it is made of a conductive material such as Al or an alloy containing Al as a main component (Al alloy), Au, or Cu. The Al alloy is, for example, an Al—Cu alloy. Various dimensions of the IDT electrode 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 electrode 5 is 100 nm to 300 nm.

The IDT electrode 5 may be disposed directly on the upper surface 3a of the substrate 3 or may be disposed on the upper surface 3a of the substrate 3 via another member. Another member is, for example, Ti, Cr, or an alloy thereof. Thus, when the IDT electrode 5 is disposed on the upper surface 3a of the piezoelectric substrate 3 via another member, the thickness of the other member has a thickness that does not substantially affect the electrical characteristics of the IDT electrode 5 (for example, Ti In this case, the thickness is set to 5% of the thickness of the IDT electrode 5). Such another member is used, for example, to increase the adhesion of the IDT electrode to the substrate or to improve the crystallinity of the IDT electrode to lower the conductivity.

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

  The spacer 18 is provided on the upper surface of the IDT electrode 5 in order to separate the IDT electrode 5 and the additional film 9 described later, and is formed of an insulating material having a density and an acoustic impedance smaller than that of the additional film 9. More preferably, the same material as the protective layer 11 described later is used. Further, it may be formed integrally with the protective layer 11. A suitable thickness d of the spacer 18 (FIG. 1B) will be described later.

  The additional film 9 is for increasing the reflection coefficient of the IDT electrode 5 and the reflector 7. The additional film 9 is disposed on the upper surface of the spacer 18. For example, the additional film 9 is disposed so as to be opposed to the entire upper surfaces of the IDT electrode 5 and the reflector 7 via the spacer 18. The additional film 9 is made of a material (for example, Al or Al alloy) constituting the IDT electrode 5 and the reflector 7 and a material having an acoustic impedance different from that of the material constituting the protective layer 11 (described later). The difference in the acoustic impedance is preferably a certain amount or more, for example, 15 MRayl or more, more preferably 20 MRayl or more.

  Here, the additional film 9 is separated from the IDT electrode 5 by the spacer 18. For this reason, for example, when Al is used as the IDT electrode 5, a metal or semiconductor with a low ionization tendency that causes electrochemical corrosion in Al by being laminated with Al may be adopted as the additional film 9. it can. A suitable material for the additional film 9 and a suitable thickness t of the additional film 9 (FIG. 1B) will be described later.

  The protective layer 11 is provided, for example, over substantially the entire upper surface 3a of the substrate 3, covers the IDT electrode 5 and the reflector 7 on which the additional film 9 is disposed above, and the IDT electrode of the upper surface 3a. 5 and the part exposed from the reflector 7 are covered. A thickness T (FIG. 1B) from the upper surface 3a of the protective layer 11 is set to be larger than the thickness e of the IDT electrode 5 and the reflector 7. For example, the thickness T is 200 nm to 700 nm, which is 100 nm or more thicker than the thickness e.

The protective layer 11 is made of an insulating material. Further, the protective layer 11 is formed of a material such as SiO ₂ that increases the propagation speed of the elastic wave when the temperature rises, and this makes it possible to suppress a change in characteristics due to a change in temperature. That is, an acoustic wave device excellent in temperature compensation can be obtained. Note that the propagation speed of an elastic wave of a general material such as a material constituting the substrate 3 becomes slow as the temperature rises.

  Further, it is desirable that the surface of the protective layer 11 be free from large irregularities. 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.

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). The various layers change in shape and the like with the progress of the process, but common symbols may be used before and after the change.

  As shown in FIG. 2A, first, on the upper surface 3a of the substrate 3, the conductive layer 15 that becomes the IDT electrode 5 and the reflector 7, the spacer layer 18A that becomes the spacer 18, and the addition film 9 that becomes the additional film 9 are provided. Layer 17 is formed. Specifically, first, the conductive layer 15 is formed on the upper surface 3a by a thin film forming method such as a sputtering method, a vapor deposition method, or a CVD (Chemical Vapor Deposition) method. Next, the spacer layer 18A and the additional layer 17 are formed by the same thin film forming method.

  When the additional layer 17 is formed, as shown in FIG. 2B, a resist layer 19 is formed as a mask for etching the additional layer 17, the spacer layer 18A, and the conductive layer 15. Specifically, a negative or positive photosensitive resin thin film 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 electrode 5 and the reflector 7 by a photolithography method or the like. Is done.

  Next, as shown in FIG. 2C, the spacer layer 18A, the additional layer 17 and the conductive layer 15 are etched by an appropriate etching method such as RIE (Reactive Ion Etching). As a result, the IDT electrode 5, the reflector 7 and the spacer 18 in which the additional film 9 and the spacer layer 18A are provided apart from each other 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 time, unevenness is formed on the surface of the thin film to be the protective layer 11 due to the thickness of the IDT electrode 5 and 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. Note that a part of the protective layer 11 may be removed by a photolithography method or the like in order to expose a pad 39 (FIG. 15) described later or the like before or after planarization.

  With reference to FIG. 3A to FIG. 3C, the operation of the comparative example and the operation of the SAW element 1 of the 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 additional film 9, the spacer layer 18, and the protective layer 11 are not provided in the SAW element 1 of the embodiment.

  When a voltage is applied to the substrate 3 by the IDT electrode 5, SAW propagating along the upper surface 3a is induced in the vicinity of the upper surface 3a of the substrate 3 as indicated by an arrow y1. Further, the SAW is reflected at the boundary between the electrode finger 13b and the gap portion (non-arrangement region of the electrode finger 13b) as indicated by the arrow y2. 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 is converted into an electric signal having the same frequency as that of the standing wave, and is taken out by the electrode finger 13b. 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 substrate 3 becomes slow, and the gap portion becomes large due to thermal expansion. 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 is in a state where the additional film 9 and the spacer layer 18 are not present in the SAW element 1 of the 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 substrate 3 but also in the protective layer 11, as indicated by the arrow y3. Here, as described above, the protective layer 11 is formed of a material (SiO ₂ ) that increases the propagation speed of elastic waves when the temperature rises. Therefore, as a whole SAW propagating through the substrate 3 and the protective layer 11, the speed change due to the temperature rise is suppressed. That is, the protective layer 11 compensates for a change in the characteristics of the substrate 3 due to a temperature rise.

However, when the IDT electrode 5 is made of Al or an Al alloy and the protective layer 11 is made of SiO ₂ , the acoustic properties of the IDT electrode 5 and the protective layer 11 are approximate, and the electrode finger 13b and the gap The boundary with the part becomes acoustically ambiguous. In other words, the reflection coefficient at the boundary between the electrode finger 13b and the gap portion 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.3 (c) is sectional drawing explaining the effect | action of the SAW element 1 of 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 additional film 9, and the additional film 9 has a higher acoustic impedance than the material of the IDT electrode 5, the material of the spacer 18, and the protective layer 11, and the material of the IDT electrode 5 and the protective layer 11. It is made of a material with a slow propagation speed of elastic waves. Since the acoustic wave distribution is attracted to a material having a large acoustic impedance and a low propagation speed, the acoustic wave propagates further on the protective layer 11 side than in the case of FIG. 3B, as indicated by an arrow y5. To come. For this reason, the temperature coefficient of the characteristic of the SAW element 1 can be reduced as compared with the case of FIG. And since the ratio of the acoustic wave distributed on the protective layer 11 side can be adjusted by the thickness d of the spacer 18, the temperature coefficient can be adjusted.

  Further, the additional film 9 is made of a material whose acoustic impedance is somewhat different from that of the IDT electrode 5 and the protective layer 11. Accordingly, the reflection coefficient at the boundary position between the electrode finger 13b and the gap portion is increased. As a result, even when the IDT electrode 5 is formed of Al or an Al alloy, it is possible to obtain a sufficient reflected wave of SAW as indicated by the arrow y2.

  In the configuration of the present invention, when the IDT electrode 5 is made of a material having a high density such as Cu, Au, or Pt, a film similar to the additional film 9 is provided between the IDT electrode 5 and the spacer 18. It is also possible to use it together. Even in this case, since the elastic wave further propagates on the protective layer 11 side, the temperature coefficient of the characteristics of the SAW element 1 can be reduced.

  Thus, by arranging the additional film 9 on the IDT electrode 5 via the spacer 18, the temperature coefficient of the SAW element 1 of the second comparative example shown in FIG. This can be further reduced as compared with the surface acoustic wave element 201.

In particular, when the spacer 18 is made of SiO ₂ , the propagation speed of the elastic wave increases as the temperature of the spacer 18 increases. As a result, a member having an effect of reducing the temperature coefficient by increasing the speed of the elastic wave is disposed in the region where the elastic wave is distributed in the IDT electrode 5 forming portion, and the temperature coefficient can be further reduced. Further, as shown in FIGS. 1B and 3C, when the protective layer 11 is on the upper surface of the additional film 9, the upper and lower sides of the protective layer 11 above the additional film 9 and the spacer 18 below A member having an effect of reducing the temperature coefficient by increasing the velocity of the elastic wave in the direction is arranged, and the temperature coefficient can be further reduced.

(Spacer thickness: Additional film height)
Consider the characteristics of the SAW element when the thickness d of the spacer 18 is changed.

  4 to 6 show changes in the frequency temperature coefficient, the reflection coefficient per electrode finger, and the electromechanical coupling coefficient when the spacer thickness d is changed.

4 to 6 are obtained by simulation calculation. The calculation conditions are as follows. Various dimensions are indicated by the ratio of the SAW to the wavelength λ.
Substrate 3 Material: 127 ° YX Cut LiNbO ₃ Substrate IDT Electrode 5 Material: Al
Spacer 18 material: SiO ₂
Material of the protective layer 11: SiO ₂
Normalized thickness e of the IDT electrode 5: 0.08λ
Normalized thickness T of the protective layer 11: 0.25λ
Two calculation results of normalized thickness t of additional film 9: 0.03 and 0.05λ were shown.
Material of additional film 9: Ta ₂ O ₅
In FIG. 4, the vertical axis represents the frequency temperature coefficient (unit: ppm / ° C.), and the horizontal axis represents the thickness d (unit: λm) of the spacer 18. FIG. 4 shows that the frequency temperature coefficient is reduced when the thickness d of the spacer is increased from 0 (in contact with the IDT electrode 5). Thus, it has been confirmed that the temperature coefficient can be reduced by separating the IDT electrode 5 and the additional film 9 by the spacer 18 and that the temperature coefficient can be adjusted by the separation distance.

Next, in FIG. 5, the vertical axis represents the reflection coefficient Γ ₁ , and the horizontal axis represents the thickness d (unit: λ) of the spacer 18. From this figure, it was confirmed that the reflected wave can be sufficiently obtained even when the additional film 9 contributing to the reflection characteristic of the elastic wave is arranged away from the IDT electrode 5.

Furthermore, it was confirmed from FIG. 5 that the reflection coefficient slightly increased as the thickness d of the spacer 18 increased. This is because SAW vibrations tend to collect near the surface of the IDT electrode 5, and the effect of the additional film 9 is enhanced by arranging the additional film 9 in this portion. For this reason, it was confirmed that the reflective characteristic can be improved while lowering the temperature coefficient of the SAW element 1 by arranging the additional film 9 apart from the IDT electrode 5.

Next, in FIG. 6, the ordinate indicates the electro-mechanical coupling coefficient K ^2, the horizontal axis is the thickness of the spacer 18 d (unit: lambda) shows. From FIG. 6, it was confirmed that the electromechanical coupling coefficient slightly decreased as the spacer thickness d increased. Such a slight decrease in the electromechanical coupling coefficient does not significantly affect the performance of the SAW element in terms of conversion efficiency from elastic waves to electric signals. On the other hand, the electromechanical coupling coefficient is also related to the band of the filter. The larger the value, the wider the band. Here, the substrate 3 has a different electromechanical coupling coefficient depending on the cut angle, and may have a wider value than a desired band. In such a case, the bandwidth can be adjusted by the thickness d of the spacer 18. By adjusting the band according to the thickness d of the spacer 18, the bandwidth can be adjusted without requiring a complicated design change of the IDT electrode 5 and the like, and maintaining a low temperature coefficient and good reflection characteristics. . Further, it is possible to use a substrate having a cut angle, which has conventionally been unusable due to an excessively large electromechanical coupling coefficient.

  The thickness d of the spacer 18 can be appropriately set by a desired frequency temperature coefficient, reflectance, and electromechanical coupling coefficient. However, if the total thickness of the thickness e of the IDT electrode 5, the thickness d of the spacer 18, and the thickness t of the additional film 9 becomes too large, the aspect ratio becomes too large, and the IDT electrode 5, the spacer 19 and the additional film 9. There is a risk that the processing accuracy will be reduced. Therefore, the total thickness of the thickness e of the DT electrode 5, the thickness d of the spacer 18, and the thickness t of the additional film 9 is preferably equal to or less than the gap between the IDT electrodes 5 (generally 0.25λ). Since the thickness e of the IDT electrode 5 is 0.08λ and the thickness t of the additional film 9 is about 0.05λ, the thickness d of the spacer 18 is preferably 0.12λ or less.

  The SAW element 1 used in the above description can be variously modified without departing from the gist thereof. For example, in the SAW element 1, the protective layer 11 is disposed above the additional film 9, but the upper surface of the additional film 9 and the upper surface of the protective layer 11 coincide with each other, and the upper surface of the additional film 9 is exposed. It may be a thing. In other words, the thickness T of the protective layer 11 may be equal to the sum of the thickness e of the IDT electrode 5, the thickness d of the spacer 18, and the thickness t of the additional film 9. In this case, after the protective layer 11 covering the IDT electrode 5 is formed, a concave portion can be formed in the protective layer 11 above the IDT electrode 5, and the additional film 9 can be formed in the concave portion, which facilitates manufacture. .

  In the SAW element 1, the cross-sectional shape in the thickness direction of the additional film 9 is a rectangular shape having a width equal to the width of the IDT electrode 5, but may be different from the width of the IDT electrode 5. The shape may be trapezoidal or the like. In particular, a trapezoidal cross-sectional shape is preferable because the electromechanical coupling coefficient can be increased.

(Suitable material and thickness of additional film)
Hereinafter, a suitable material and thickness t of the additional film 9 will be examined. In the following examination, unless otherwise specified, the substrate 3 is a 128 ° YX cut LiNbO ₃ substrate, the IDT electrode 5 is made of Al, and the protective layer 11 is made of SiO ₂ . For simplicity, the calculation was performed for the case where the additional film 9 is in contact with the IDT electrode 5. In the present embodiment, the additional film 9 is provided above the IDT electrode 5 and separated from the IDT electrode 5. However, the additional film 9 is in contact with the IDT electrode 5 for a suitable material and thickness of the additional film. It is almost the same as the case where it exists (refer FIG. 5).

FIG. 7A and FIG. 7B are graphs showing the reflection coefficient Γ ₁ and the electromechanical coupling coefficient K ² per electrode finger 13b.

FIG. 7A and FIG. 7B are obtained by simulation calculation. The calculation conditions are as follows. Various dimensions are indicated by the ratio of the SAW to the wavelength λ.
Normalized thickness e of the IDT electrode 5: 0.08λ
Normalized thickness T of the protective layer 11: 0.25λ
The normalized thickness t of the additional film 9 was changed in the range of 0.01λ to 0.05λ.
Material of additional film 9: WC, TiN, TaSi ₂
Acoustic impedance of each material (unit: MRayl):
SiO ₂ : 12.2 Al: 13.5
_{WC: 102.5 TiN: 56.0 TaSi 2} : 40.6
In FIG. 7A and FIG. 7B, the horizontal axis indicates the normalized thickness t of the additional film 9. In FIG. 7A, the vertical axis indicates the reflection coefficient Γ ₁ per electrode finger 13b. The vertical axis in FIG. 7 (b) shows the electromechanical coupling coefficient K ^2.

In FIGS. 7 (a) and 7 (b), the line L1, L2 and L3, respectively, the additional film 9 corresponds to the case comprising WC, a TiN and TaSi _2. In FIG. 7 (a), line LS1 indicates the lower limit of the range are generally preferred reflection coefficient gamma _1. In FIG. 7 (b), line LS2 indicates the lower limit of the range are generally the electromechanical coupling coefficient K ² preferred.

From these figures, by providing the additional film 9, the reflection coefficient Γ _{1 is set} to a generally preferred range while the electromechanical coupling coefficient K ² is kept within a generally preferred range. It was confirmed that it was possible.

Moreover, it can be seen from FIG. 7A that the reflection coefficient Γ ₁ increases as the normalized thickness t of the additional film 9 increases. From FIG. 7 (b), the normalized thickness t of the additional film 9, suggests that have had little impact on the electro-mechanical coupling coefficient K ^2. However, when the normalized thickness t of the additional film 9 is increased slightly, the electromechanical coefficient K ² is increased. Such a tendency occurs even when the additional film 9 is formed of any material.

In general, the larger the difference in acoustic impedance between media through which sound waves propagate, the greater the reflected wave. However, TaSi ₂ (line L3) has a smaller acoustic impedance than TiN (line L2), and thus the reflection coefficient Γ ₁ is increased despite the small difference in acoustic impedance with SiO _2. Yes. In the following, this reason will be examined.

When the additional film 9 is formed of various virtual materials having the same acoustic impedance Z _S and different Young's modulus E and density ρ (cases No. 1 to No. 7), the reflection coefficient Γ ₁ and the electromechanical coupling the coefficient ^{K 2} was calculated.

The calculation conditions are as follows.
Normalized thickness e of the IDT electrode 5: 0.08λ
Normalized thickness T of the protective layer 11: 0.30λ
Normalized thickness t of additional film 9: 0.03λ
Physical properties of additional film 9:
Z _S E ρ
(MRayl) (GPa) (10 ³ kg / m ³ )
No. 1: 50 100 25.0
No. 2: 50 200 12.5
No. 3: 50 300 8.33
No. 4: 50 400 6.25
No. 5: 50 500 5.00
No. 6: 50 600 4.17
No. 7: 50 700 3.57
Note that Z _S = √ (ρE).

FIG. 8A and FIG. 8B are graphs showing the results calculated based on the above conditions. The horizontal axis is No. The vertical axis indicates the reflection coefficient Γ ₁ or the electromechanical coupling coefficient K ² per electrode finger 13b. A line L5 indicates the calculation result.

In FIG. 8A, the reflection coefficient Γ ₁ increases as the Young's modulus E decreases and the density ρ increases even if the acoustic impedance Z _S is the same. No. 1-No. The rate of change of the reflection coefficient Γ ₁ in FIG. 3-No. 7 is larger than the rate of change of the reflection coefficient Γ ₁ in FIG. In other words, No. In the vicinity of 3, there is room to find critical significance.

Such a change in the reflection coefficient Γ ₁ is considered to be caused by the difference in the propagation speed of the elastic wave of the material constituting the additional film 9 as follows. First, from the waveguide theory, the vibration distribution becomes larger in the medium region where the propagation speed of the elastic wave is slower. On the other hand, no. 1-No. The elastic wave propagation velocity V of the virtual material 7 is as follows (unit: m / s). Note that V = √ (E / ρ).
No. 1: 2000 No. 2: 4000 No. 3: 6000
No. 3: 8000 No. 4: 10000 No. 6: 12000
No. 7: 14000
Therefore, even if the additional film 9 has the same acoustic impedance, the additional film 9 in which the vibration distribution is concentrated on the additional film 9 and the propagation speed of the elastic wave is slower is higher than that of the elastic wave in which the vibration distribution is dispersed. It is considered that the reflection coefficient is effectively higher than that of the additional film 9 having a high propagation speed.

The propagation speed of the elastic wave of SiO ₂ is 5560 m / s, and the propagation speed of the elastic wave of Al is 5020 m / s. Therefore, no. The elastic wave propagation speed of the additional film 9 of 1 and 2 is slower than the propagation speed of the elastic wave of the protective layer 11 and the IDT electrode 5. The propagation speed of elastic waves of the additional films 9 of 3 to 7 is faster than the propagation speed of elastic waves of the protective layer 11 and the IDT electrode 5. Therefore, the above-mentioned No. The change in the change rate of the reflection coefficient in the vicinity of 3 can also be explained by the propagation speed of the elastic wave.

In FIG. 8A, the propagation speeds of the elastic waves of SiO ₂ and Al when the horizontal axis is regarded as the propagation speed of the elastic waves are indicated by lines LV1 and LV2. Also, the electromechanical coupling coefficient K ² shown in FIG. 8 (b), even if the Young's modulus E and density ρ is changed, a large change does not occur, is within the preferred range.

  As described above, the additional film 9 is made of a material whose acoustic impedance is different from that of the material forming the protective layer 11 and the IDT electrode 5 and whose acoustic wave propagation speed is slower than that of the material forming the protective layer 11 and the IDT electrode 5. It is preferable to become. Note that the material having a larger acoustic impedance than the material forming the protective layer 11 and the IDT electrode 5 has a slower propagation speed of the elastic wave than the material forming the protective layer 11 and the IDT electrode 5. It is easy to satisfy the conditions and the selection of materials is easy.

Examples of such a material include Ta ₂ O ₅ , TaSi ₂ , and W ₅ Si ₂ . These physical properties (acoustic impedance Z _S , elastic wave propagation velocity V, Young's modulus E, density ρ) are as follows.

Z _S V E ρ
(MRayl) (m / s) (GPa) (10 ³ kg / m ³ )
_{Ta 2 O 5: 33.8 4352 147} 7.76
TaSi _2: 40.6 4438 180 9.14
_{W 5 Si 2: 67.4 4465 301} 15.1
Note that WC and TiN exemplified in FIG. 7A do not satisfy the condition that the elastic wave propagation speed is slower than the material forming the protective layer 11 and the IDT electrode 5 (WC V: 6504 m / s, TiN V: 10721 m / s).

About Ta ₂ O ₅ (the difference in acoustic impedance between Al and SiO ₂ is about 20 MRayl) whose acoustic impedance is closer to that of the protective layer 11 and the IDT electrode 5 than TaSi ₂ (line L3 in FIG. 7A). The reflection coefficient was calculated and the knowledge about the above materials was confirmed.

  The calculation conditions are as follows.

Normalized thickness e of the IDT electrode 5: 0.08λ
Normalized thickness T of protective layer 11: 0.27λ, 0.30λ or 0.33λ
The normalized thickness t of the additional film 9 was changed in the range of 0.01λ to 0.09λ.

  FIG. 9 is a graph showing the results calculated based on the above conditions. The horizontal and vertical axes are the same as the vertical and horizontal axes in FIG. Lines L7, L8, and L9 correspond to cases where the normalized thickness T of the protective layer 11 is 0.27λ, 0.30λ, and 0.33λ, respectively (the lines L7, L8, and L9 are substantially overlapped). )

In FIG. 9, Ta ₂ O ₅ has a lower propagation speed of the elastic wave, although the acoustic impedance is close to the acoustic impedance of the protective layer 11 compared to TiN (line L2 in FIG. 7A). Therefore, the reflection coefficient is high.

  In FIG. 9, the normalized thickness T of the protective layer 11 generally has no effect on the reflection coefficient.

  Next, a preferable range of the normalized thickness t of the additional film 9 will be examined. First, a lower limit value of a preferable range of the thickness t of the additional film 9 (hereinafter, “preferable range” may be omitted and simply referred to as a “lower limit value”) is examined.

FIG. 10A is a graph schematically showing the reflection coefficient Γ _all of the IDT electrode 5 (all electrode fingers 13b). In FIG. 10A, the horizontal axis indicates the frequency f, and the vertical axis indicates the reflection coefficient Γ _all .

The frequency band (f _{1 to} f ₂ ) where the reflection coefficient Γ _all is approximately 1 (100%) is called a stop band. In practice, the reflection coefficient Γ _all in the stop band need not be completely 1. For example, a frequency band having a reflection coefficient Γ _all of 0.99 or more may be specified as the stop band. In general, since the reflection coefficient Γ _all changes abruptly at the lower end f1 or the upper end f2 of the stop band, the interval between the changes may be specified as the stop band.

The reflection coefficient Γ _all of the IDT electrode 5 is determined by the reflection coefficient Γ ₁ per electrode finger 13b, the number of electrode fingers 13b, and the like. It is generally known that when the reflection coefficient Γ ₁ becomes small, the width SB of the stop band becomes small.

  FIG. 10B is a graph schematically showing the electrical impedance Ze of the IDT electrode 5.

In FIG. 10B, the horizontal axis indicates the frequency f, and the vertical axis indicates the absolute value | Ze | of the impedance. As is generally known, | Ze | takes a minimum value at the resonance frequency f ₃ and takes a maximum value at the anti-resonance frequency f ₄ . Further, when the normalized thickness t of the additional film 9 is changed, the upper end f ₄ of the stop band and the anti-resonance frequency f ₄ change in a state where the lower end f _{1 of the} stop band and the resonance frequency f ₃ coincide with each other. Rate of change of this time, towards the stop band of the upper end f ₂ is greater than the anti-resonance frequency f _4.

Here, if the upper end f2 of the stop band, indicated by the line L11, assuming that the frequency is lower than the anti-resonance frequency f _4, as shown in phantom (two-dot chain line) in the region Sp1, the resonance frequency f ₃ spurious in the frequency band (width Delta] f) between the anti-resonance frequency f ₄ is generated. As a result, a desired filter characteristic or the like may not be obtained.

On the other hand, the upper end f2 of the stop band indicated by a line L12, assuming that the frequency higher than the antiresonance frequency f _4, as shown in phantom in the region Sp2 (two-dot chain line), spurious, antiresonance frequency It occurs at a higher frequency than f _4. In this case, the influence of spurious on the filter characteristics and the like is suppressed.

Thus, the upper end f ₂ of the stop band is preferably higher frequency than the anti-resonance frequency f _4. Here, since the upper end f ₂ of the stop band depends on the reflection coefficient, the reflection coefficient of the IDT electrode 5 may be adjusted so that the upper end f _{2 of the} stop band is higher than the antiresonance frequency f ₄ . Since the reflection coefficient of the IDT electrode 5 increases linearly as the normalized thickness t of the additional film 9 increases as shown in FIGS. 7 and 9, the normalized thickness t of the additional film 9 is adjusted. the upper end f ₂ of the stop band may be a frequency higher than the antiresonance frequency f _4. That is, the frequency band between the normalized thickness t of the additional film 9 the upper end f ₂ of the stop band by a higher becomes thicker than the anti-resonance frequency f _4, the resonance frequency f ₃ and the anti-resonance frequency f ₄ ( Spurious generation is suppressed in the width Δf).

Here, as shown in FIG. 9, the reflection coefficient Γ ₁ is affected by the normalized thickness T of the protective layer 11. Further, the width Δf is affected by the normalized thickness T of the protective layer 11. Therefore, the normalized thickness t of the additional film 9 is preferably determined according to the normalized thickness T of the protective layer 11.

Therefore, the normalized thickness t at which the upper end f _{2 of the} stop band is equivalent to the antiresonance frequency f ₄ is calculated by changing the normalized thickness T of the protective layer 11, and the normalized thickness t is calculated based on the calculation result. The lower limit was defined by the normalized thickness T.

11 to 13 are graphs for explaining the normalized thickness t at which the upper end f _{2 of the} stop band is higher than the antiresonance frequency f _4. The materials of the additional film 9 are Ta ₂ O ₅ , TaSi ₂ , This corresponds to the case of W ₅ Si ₂ .

11 to 13, the horizontal axis represents the normalized thickness T of the protective layer 11, and the vertical axis represents the normalized thickness t of the additional film 9. Solid LN1~LN3 shown in each figure is the upper end f ₂ of the stop band showed the calculation result of the normalization thickness t to be equal to the antiresonant frequency f _4. In the calculation, the normalized thickness e of the IDT electrode 5 was set to 0.08λ.

  As indicated by the solid lines LN1 to LN3 in each figure, the normalized thickness t of the additional film 9 was able to suitably derive an approximate curve from a quadratic curve.

  Specifically, it is as follows. In the following equation, it is assumed that t and T are normalized by being divided by the SAW wavelength λ.

Ta ₂ O ₅ (FIG. 11):
Lower limit (solid line LN1): t = 0.5706T ² −0.3867T + 0.0913
TaSi ₂ (FIG. 12):
Lower limit (solid line LN2): t = 0.3995T ² −0.2675T + 0.0657
W ₅ Si ₂ (FIG. 13):
Lower limit (solid line LN3): t = 0.2978T ² −0.1966T + 0.0433
In any of the lower limit expressions, the minimum value of the normalized thickness t is larger than the maximum value (0.01λ) of the thickness of the adhesion layer shown in Patent Document 2. Patent Document 1 is difficult to compare because the thickness of the adhesion layer is not normalized by the wavelength. However, even if the frequency is increased (for example, the maximum frequency of 2690 MHz of UMTS) and the propagation speed of the elastic wave is decreased (for example, 3000 m / s) so that the normalized thickness is increased, λ = 1.1 μm, The maximum value (100 mm) of the thickness of the adhesion layer of Patent Document 1 is less than 0.01λ.

  Next, an upper limit value of a preferable range of the normalized thickness t of the additional film 9 (hereinafter, “preferable range” may be omitted and simply referred to as “upper limit value”) is examined.

  As shown in FIGS. 7A and 9, the reflection coefficient increases as the normalized thickness t of the additional film 9 increases. Therefore, the upper limit value of the normalized thickness t is a range in which the additional film 9 is not exposed from the protective layer 11.

Similarly to the lower limit value of the normalized thickness t, if the upper limit value of the normalized thickness t is defined by an equation, for example, the thickness e of the IDT electrode 5 is 0 in light of the thickness e of a general SAW element. Estimated to be less than 1λ and can be defined as:
Upper limit value: t = T−0.1
Note that t and T are normalized by being divided by λ.

A preferable range of the normalized thickness t derived from the above study is shown in FIG. 14 taking Ta ₂ O ₅ as an example.

  In FIG. 14, the horizontal axis and the vertical axis indicate the normalized thickness T of the protective layer 11 and the normalized thickness t of the additional film 9, as in FIG. 11. A line LL1 indicates a lower limit value, and a line LH1 indicates an upper limit value. A hatched area between these lines is a preferable range of the normalized thickness t of the additional film 9. The line LH5 indicates the upper limit value (0.01λ) of the adhesion layer shown in Patent Document 2.

As described above, when the additional film 9 is made of Ta ₂ O ₅ , it is desirable that the normalized thickness t of the additional film 9 be in the range of the following formula (1).

0.5706T ² −0.3867T + 0.0913 ≦ t ≦ T−0.1 Formula (1)
However, T is the normalized thickness of the insulating layer 11, and T and t are normalized by dividing by the wavelength of the elastic wave.

Similarly, when the additional film 9 is made of TaSi ₂ , it is desirable that the normalized thickness t of the additional film 9 be in the range of the following formula (2).

0.3995T ² −0.2675T + 0.0657 ≦ t ≦ T−0.1 Formula (2)
However, T is the normalized thickness of the insulating layer 11, and T and t are normalized by dividing by the wavelength of the elastic wave.

When the additional film 9 is made of W ₅ Si ₂ , it is desirable that the normalized thickness t of the additional film 9 be in the range of the following formula (3).

0.2978T ² −0.1966T + 0.0433 ≦ t ≦ T−0.1 Formula (3)
However, T is the normalized thickness of the insulating layer 11, and T and t are normalized by dividing by the wavelength of the elastic wave.

(Configuration of SAW device)
FIG. 15 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 SAW element 1 side of the substrate 3, a terminal 35 that penetrates the cover 33, and a back surface portion 37 that covers the opposite side of the substrate 3 from the SAW element 1. have.

  The cover 33 is made of resin or the like, and a vibration space 33a for facilitating SAW propagation is formed above the IDT electrode 5 and the reflector 7 (on the positive side in the z direction). A wiring 38 connected to the IDT electrode 5 and a pad 39 connected to the wiring 38 are formed on the upper surface 3 a of the substrate 3. The terminal 35 is formed on the pad 39 and is electrically connected to the IDT electrode 5. For example, although not particularly illustrated, the back surface portion 37 includes a back electrode for discharging a charge charged on the surface of the substrate 3 due to a temperature change or the like, and an insulating layer covering the back electrode.

  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. The terminals 35 and the mounting pads 55 are bonded by solder 57. Thereafter, the SAW element 31 is sealed with a sealing resin 59.

  In the above embodiment, the substrate 3 is an example of the piezoelectric substrate of the present invention, the IDT electrode 5 is an example of the electrode of the present invention, and the protective layer 11 is an example of the insulating layer of the present invention.

  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 protective layer (11) may be a relatively large thickness (for example, 0.5λ to 2λ), a so-called boundary acoustic wave device (however, included in a broad sense SAW device). In the boundary acoustic wave element, it is not necessary to form the vibration space (33a), and thus the cover 33 and the like are not necessary.

  Further, the acoustic wave element is not limited to a wafer level package. For example, the SAW element does not have the cover 33 and the terminal 35, and the pad 39 on the upper surface 3 a of the substrate 3 and the mounting pad 55 of the circuit substrate 53 may be directly bonded by the solder 57. A vibration space may be formed by a gap between the SAW element 1 (protective layer 11) and the mounting surface 53a of the circuit board 53.

The additional film is preferably provided over the entire surface of the electrode. However, the additional film may be provided only on a part of the electrode, such as provided only on the electrode finger. The additional film may be the same width as the electrode, or may be narrower or wider than the electrode. The material of the additional film may be a conductive material or an insulating material. Specifically, conductive materials such as tungsten, iridium, tantalum, and copper, and insulating materials such as Ba _x Sr _1-x O ₃ , Pb _x Zn _1-x O ₃ , and ZnO ₃ may be cited as the material for the additional film. it can.

By forming the additional film from an insulating material, it is possible to suppress the corrosion of the electrode and stabilize the electrical characteristics of the acoustic wave device, compared to the case where the additional film is formed from a metal material. This is because a pinhole may be formed in the insulating layer made of SiO ₂ , and when this pinhole is formed, moisture penetrates into the electrode portion, but the electrode is formed on the electrode. This is because if a metal film made of a material different from the material is disposed, corrosion due to the battery effect between different metals occurs due to the infiltrated moisture. Therefore, if the additional film is formed of an insulating material such as Ta ₂ O ₅ , the battery effect hardly occurs between the electrode and the additional film, so that a highly reliable acoustic wave element in which corrosion of the electrode is suppressed is obtained. be able to.

  The upper surface of the protective layer may have irregularities so as to be convex at the position of the electrode finger. In this case, the reflection coefficient can be further increased. As described with reference to FIG. 2 (e), the unevenness may be formed due to the thickness of the electrode finger when the protective layer is formed, or the surface of the protective layer may be formed on the electrode finger. It may be formed by etching in the region between.

In addition to the LiNbO ₃ substrate of 128 ° ± 10 ° YX cut, for example, 38.7 ° ± YX cut LiTaO ₃ can be used as the substrate ₃ .

  DESCRIPTION OF SYMBOLS 1 ... SAW element (elastic wave element), 3 ... Substrate (piezoelectric substrate), 3a ... Upper surface, 5 ... IDT electrode (electrode), 9 ... Additional film, 11 ... Protective layer (insulating layer), 18 ... Spacer.

Claims (7)

A piezoelectric substrate;
An IDT electrode disposed on the upper surface of the piezoelectric substrate;
A spacer made of an insulating material disposed on the upper surface of the IDT electrode;
An additional film disposed on the upper surface of the spacer;
The IDT electrode on which the additional film is disposed is covered together with a portion of the piezoelectric substrate exposed from the IDT electrode, and the thickness from the upper surface of the piezoelectric substrate is greater than the thickness from the upper surface of the piezoelectric substrate to the upper surface of the additional film. An insulating layer made of SiO ₂ ,
The additional film has an acoustic impedance larger than that of the IDT electrode material, the spacer material, and SiO ₂ , a density higher than that of the spacer, and an elastic wave propagation speed higher than that of the IDT electrode material and SiO _2. An acoustic wave device made of a slow material. The acoustic wave device according to claim 1, wherein the IDT electrode is made of Al or an alloy containing Al as a main component. The acoustic wave device according to claim 1, wherein the spacer is made of SiO ₂ . 4. The acoustic wave device according to claim 1, wherein the piezoelectric substrate is a LiNbO ₃ substrate of 128 ° ± 10 ° Y—X cut. 5. The acoustic wave device according to claim 1, wherein the additional film is made of an insulating material. The acoustic wave device according to claim 1, wherein the additional film is made of Ta ₂ O ₅ , TaSi _2, or W ₅ Si ₂ . The acoustic wave device according to any one of claims 1 to 6,
A circuit board on which the acoustic wave element is mounted;
An elastic wave device comprising:

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