The present invention relates to an acoustic wave element such as a surface acoustic wave (SAW) element and an acoustic wave device using the same.
Known in the art is an acoustic wave element which has a piezoelectric substrate and an IDT (interdigital transducer) electrode (excitation electrode) which is provided on a major surface of the piezoelectric substrate (for example, Patent Literature 1 or 2). The IDT electrode has a plurality of electrode fingers which extend in a direction perpendicular to the direction of advance of the acoustic wave. Further, the acoustic wave element utilizes the piezoelectric effect to convert an electrical signal to an acoustic wave and convert an acoustic wave to an electrical signal.
Note that, in the arts of Patent Literature 1 and Patent Literature 2, a protective layer made of SiO2 (SiO2 film) is covered on the major surface of the piezoelectric substrate from the top of the IDT electrode. The protective layer contributes to suppression of corrosion of the IDT electrode, compensation for a change of characteristics of the IDT electrode according to a change of temperature, and so on. Further, in order to improve contact between the IDT electrode and the protective layer, Patent Literature 1 and Patent Literature 2 propose formation of a bonding layer between them (paragraph 0011 in Patent Literature 1 and paragraph 0107 in Patent Literature 2). In Patent Literature 1 and Patent Literature 2, the bonding layer is formed thin so as not to exert an influence upon the propagation of the SAW. Specifically, the bonding layer is controlled to 50 to 100 Å (paragraph 0009 in Patent Literature 1) or 1% or less based on the wavelength of the SAW (paragraph 0108 in Patent Literature 2).
In the acoustic wave element, improvement of the electromechanical coupling factor is sometimes desired. For example, by making the electromechanical coupling factor large, a high bandwidth filter can be realized.
Accordingly, desirably there are provided an acoustic wave element and acoustic wave device capable of raising the electromechanical coupling factor.
An acoustic wave element according to an aspect of the present invention has a piezoelectric substrate, electrode fingers arranged on an upper surface of the piezoelectric substrate, and mass-adding films arranged on the upper surfaces of the electrode fingers, wherein, when viewing cross-sections perpendicular to the extending directions of the electrode fingers, the mass-adding films have the narrowest widths at an upper sides in the cross-sections.
An acoustic wave device according to an aspect of the present invention has the above acoustic wave element and a circuit board to which the acoustic wave element is attached.
According to the above configuration, by arranging the mass-adding films on the upper surfaces of the electrode fingers and making the widths of the mass-adding films narrowest at the upper sides on their cross-sections when viewing cross-sections perpendicular to the extending directions of the electrode fingers, the electromechanical coupling factor can be made high.
Below, a SAW element and a SAW device according to an embodiment of the present invention is explained with reference to the drawings. Note that, the drawings used in the following explanation are diagrammatical ones. Dimensions, ratios, etc. on the drawings do not always match the actual ones.
(Configuration and Method of Production of SAW Element)
The SAW element 1 has a substrate 3, an IDT electrode 5, and reflectors 7 which are provided on an upper surface 3a of the substrate 3, mass-adding films 9 (
The substrate 3 is configured by a piezoelectric substrate. Specifically, for example, the substrate 3 is configured by a substrate of a single crystal having piezoelectricity such as a lithium tantalate (LiTaO3) single crystal or lithium niobate (LiNbO3) single crystal. More preferably, the substrate 3 is configured by a 128°±10° Y-X cut LiNbO3 substrate. The planar shape and various dimensions of the substrate 3 may be suitably set. As an example, the thickness of the substrate 3 (z-direction) is 0.2 mm to 0.5 mm.
The IDT electrode 5 has a pair of comb-shaped electrodes 13. Each comb-shaped electrode 13 has a bus bar 13a (
Note that,
The IDT electrode 5 is formed by for example a material containing Al as a major component (including an Al alloy). The Al alloy is for example an Al—Cu alloy. Note that, the term “containing Al as a major component” means that Al is basically used as the material, but a material mixed with impurities other than Al which may be naturally mixed in during for example the manufacturing process of the SAW device 1 is included as well. Below, a case using an expression such as “a major component” means the same as well. Further, the IDT electrode 5 may be configured by a plurality of metal layers as well. The various dimensions of the IDT electrode 5 are suitably set in accordance with the electrical characteristic etc. requested from the SAW element 1. As an example, the thickness “e” of the IDT electrode 5 (
Note that, the IDT electrode 5 may be directly arranged upon the upper surface 3a of the substrate 3 or may be arranged on the upper surface 3a of the substrate 3 through another member. The other member is for example Ti, Cr, or an alloy of the same. When the IDT electrode 5 is arranged on the upper surface 3a of the substrate 3 through another member in this way, the thickness of the other member is set to an extent such that almost no influence is exerted upon the electrical characteristics of the IDT electrode 5 (for example a thickness of 5% based on the thickness of the IDT electrode 5 in the case of Ti).
The plurality of electrode fingers 13b are provided so that their pitch (repetition interval) “p” (
The reflectors 7 are formed in a lattice-shape having substantially an equal pitch to the pitch “p” of the electrode fingers 13b of the IDT electrode 5. The reflectors 7 are for example formed by the same material as that of the IDT electrode 5 and are formed to a thickness equivalent to that of the IDT electrode 5.
The mass-adding films 9 are for improving the electrical characteristics of the IDT electrode 5 and the reflectors 7. The mass-adding films 9 are for example provided over the entire surfaces of the upper surfaces of the IDT electrode 5 and reflectors 7. The material configuring the mass-adding films 9 is comprised of for example a material containing as a major component a material satisfying the conditions of at least one of a material by which the propagation velocity becomes slow compared with the material configuring the IDT electrode 5 and reflectors 7 (Al or Al alloy etc.) and a material having a different acoustic impedance compared with a material configuring the IDT electrode 5 and reflectors 7 (Al or Al alloy etc.) and the material configuring the protective layer 11 (which is explained later). The difference of the acoustic impedance is preferably a certain extent or more. For example, it is preferably 15 MRayl or more, more preferably 20 MRayl or more. The preferred material of the mass-adding films 9 and the preferred thickness “t” (
The mass-adding films 9 are formed so that the widths in the cross-sections become the narrowest at the upper sides when viewing the cross-sections in a direction perpendicular to the longitudinal directions (y-directions) of the electrode fingers 13b. Further, the widths in the cross-sections become larger at the lower sides than that at the upper sides. In other words, the mass-adding films 9 are formed narrower at the upper surface side portions than the lower surface side portions when viewed in the y-directions. In the SAW element 1, the cross-sectional shapes of the mass-adding films 9 become trapezoidal shapes. The lengths of the lower bases of the trapezoidal shapes of the mass-adding films 9 are for example equivalent to the widths w1 of the electrode fingers 13b. The preferred range of the lengths of the upper bases of the trapezoidal shapes (widths w2) is explained later.
The protective layer 11 is for example provided over substantially the entire surface of the upper surface 3a of the substrate 3, covers the IDT electrode 9 and reflectors 7 which are provided with the mass-adding films 9, and covers the portion of the upper surface 3a which is exposed from the IDT electrode 5 and the reflectors 7. The thickness T (
The protective layer 11 is made of a material containing as a major component a material having an insulation property. Preferably, the protective layer 11 is formed by a material containing as a major component a material by which the propagation velocity of the acoustic wave becomes fast when the temperature rises such as SiO2. The change of the characteristics according to the change of the temperature can be kept small by this. That is, an acoustic wave element excellent in temperature compensation can be obtained. Note that, in the material configuring the substrate 3 and other general material, the propagation velocity of the acoustic wave becomes slow when the temperature rises.
Further, the surface of the protective layer 11 is desirably made free from large concave-convex shapes. The propagation velocity of the acoustic wave propagating on the piezoelectric substrate changes when influenced by concave-convex shapes of the surface of the protective layer 11. Therefore, if large concave-convex shapes exist on the surface of the protective layer 11, there arises a large variation in the resonant frequencies of produced acoustic wave elements. Accordingly, when making the surface of the protective layer 11 flat, the resonant frequency of each acoustic wave element is stabilized. Specifically, desirably the flatness of the surface of the protective layer 11 is made 1% or less based on the wavelength of the acoustic wave propagating on the piezoelectric substrate.
As shown in
When the additional layer 17 is formed, as shown in
Next, as shown in
Further, as shown in
In etching of the additional layer 17 and conductive layer 15, the resist layer 19 which serves as the mask is etched as well though the extent is very small. Accordingly, as shown in
That is, in an initial stage of etching (resist layer 19 indicated by the solid line in
Therefore, if the etching conditions (for example ratio of composition of etching gas and applied voltage in the case of etching by RIE) are set so that the side surface of the resist layer 19 is etched much more, the side surface of the resist layer 19 exhibits a more inclined state. The side surfaces of the mass-adding film 9 are inclined more along with this. That is, by changing the conditions of etching, the shape of the mass-adding film 9 can be controlled.
In this example, the resist layer 19 is formed by positive type photolithography. Accordingly, as shown in
At this time, the resist layer 19 located under the light-shielding part of the mask 21 is basically not removed since it is not irradiated by light, but the portions located under the periphery of the light-shielding part of the mask 21 are removed at their upper surface sides since they are irradiated by the light diffracted at the edges of the light-shielding part. As a result, the resist layer 19 has a trapezoidal shape in which the upper surface side portion is smaller than the lower surface side portion. Further, as explained in
Referring to
When voltage is applied to the substrate 3 by the IDT electrode 5, as indicated by an arrow y1, near the upper surface 3a of the substrate 3, a SAW propagating along the upper surface 3a is induced. Further, as indicated by the arrows y2, the SAW is reflected at a boundary between an electrode finger 13b and a gap portion (a region in which no electrode finger 13b is arranged). Further, a standing wave which has the pitch of the electrode fingers 13b as the half wavelength is formed by the SAW indicated by the arrows y1 and y2. The standing wave is converted to an electrical signal having the same frequency as that of the standing wave and is extracted by the electrode fingers 13b. In this way, the SAW element 1 functions as a resonator or filter.
In the SAW element 101, however, when the temperature rises, the propagation velocity of the acoustic wave on the substrate 3 becomes slow, and the gap portion becomes large. As a result, the resonant frequency becomes low, so the desired characteristics are liable to not be obtained. Further, the IDT electrode 5 is exposed upward, therefore it easily contacts moisture, so it is liable to corrode.
In the SAW element 201, since the protective layer 11 is provided, as indicated by the arrow y3, the induced SAW is propagated not only on the substrate 3, but also on the protective layer 11. Here, for example, the protective layer is formed by the material by which the propagation velocity of the acoustic wave becomes faster when the temperature rises such as SiO2. Accordingly, in the SAW as a whole which propagates on the substrate 3 and the protective layer 11, the change of the velocity due to the temperature rise is suppressed. That is, by the protective layer 11, the change of characteristics of the substrate 3 due to a temperature rise is compensated for. Further, by the protective layer 11, the probability of contact of the IDT electrode 5 with moisture is reduced, and consequently the liability of corrosion is reduced.
However, if the vibration of the SAW is transferred from the substrate 3 to the protective layer 11 too much, the conversion from the SAW to an electrical signal or the like is no longer carried out sufficiently. That is, the electromechanical coupling factor falls. Further, in a case where the IDT electrode 5 is formed by Al or an Al alloy and the protective layer 11 is formed by SiO2, the acoustical properties of the IDT electrode 5 and the protective layer 11 become similar, so the boundary between an electrode finger 13b and a gap portion acoustically becomes vague. In other words, the reflection coefficient at the boundary between an electrode finger 13b and a gap portion falls. As a result, as indicated by the arrows y4 in
Since the SAW element 1 has the protective layer 11, in the same way as the SAW element 201 of the second comparative example, the effect of compensation for the temperature characteristics and so on are obtained. Further, in a case where the mass-adding films 9 are formed by a material whereby the propagation velocity of the acoustic wave becomes slower than that on the IDT electrode 5, as indicated by an arrow y5 having a position made lower than the position of the arrow y3, excessive transfer of the SAW to the protective layer 11 near the electrode finger 13b is suppressed. As a result, the electromechanical coupling factor becomes high. Further, in a case where the mass-adding films 9 are formed by a material having an acoustic impedance which is different from the acoustic impedances of the IDT electrode 5 and protective layer 11 to a certain extent, the reflection coefficient at the boundary position between an electrode finger 13b and a gap portion becomes high. As a result, as indicated by the arrows y2, it becomes possible to obtain a sufficient reflection wave of the SAW.
In
In
The computation conditions were as follows.
Material of substrate 3: 128° Y-X cut LiNbO3 substrate
Material of IDT electrode 5: Al
Material of protective layer 11: SiO2
Material of mass-adding film 9: Ta2O5
Normalized thickness e/λ of IDT electrode 5: 0.08
Normalized thickness T/λ of protective layer 11: 0.33
Normalized thickness t/λ of mass-adding films 9: 0.05.
Normalized length w1/p of lower base of mass-adding film 9: 0.50
Normalized length w2/p of upper base of mass-adding film 9: Changed within range of 0.35 to 0.50
In
It was confirmed from this computation result that the electromechanical coupling factor K2 became high by forming a mass-adding film 9 in a trapezoidal shape (by making w2/p less than 0.5). More specifically, it was confirmed that the electromechanical coupling factor K2 became high when w2/w1 was 0.7 or more, but was less than 1.0. Note that, it is considered that the effect of raising the electromechanical coupling factor K2 is obtained if the mass-adding film 9 is changed from a rectangular shape to a trapezoidal shape even a little. However, in simulation, it has been confirmed that the effect is manifested when w2/w1 is 0.98 (when w2/p is 0.49).
Here, when a mass-adding film 9 is formed to a trapezoidal shape, the volume of the mass-adding film 9 is reduced as well as a whole. There is a possibility that the electromechanical coupling factor K2 has become high due to simple reduction of the volume of the mass-adding film irrespective of the shape of the mass-adding film. Therefore, the influence of reduction of the volume of the mass-adding film when reducing the volume of the mass-adding film 9 by changing the thickness “t” of the mass-adding film was checked.
It is seen from
Further, in
In the SAW element in
The SAW elements in
In all of
Note that, in the same way as the mass-adding films 9, the electrode fingers 25 in
The SAW elements in
Specifically, the mass-adding film 26 in
The mass-adding film 27 in
The mass-adding film 28 in
Note that, in the SAW elements in
In all of the mass-adding films in
(Preferred Material and Thickness of Mass-Adding Films)
Below, the preferred material and thickness “t” of the mass-adding films 9 are studied. Note, in the simulation in the following study, the mass-adding films are formed in rectangles (the mass-adding films 309 in the third comparative example). However, the mass-adding films 9 are obtained by improving the mass-adding films 309, therefore the preferred material and thickness in the mass-adding films 309 are the preferred material and thickness also in the mass-adding films 9.
Further, in the following study, among the actions exhibited by the mass-adding films, the action of increase of the reflection coefficient is focused on. Note, it is confirmed everywhere that the preferred material and thickness set by focusing on the action of increase of the reflection coefficient are preferred ones concerning the electromechanical coupling factor K2 as well.
In the following study, so long not otherwise indicated, the substrate 3 is the 128° Y-X cut LiNbO3 substrate, the IDT electrode 5 is made of Al, and the protective layer 11 is made of SiO2.
Normalized thickness e/λ of IDT electrode 5: 0.08
Normalized thickness T/λ of protective layer 11: 0.25
Normalized thickness t/λ of mass-adding films 309: Changed within a range of 0.01 to 0.05.
Material of mass-adding films 309: WC, TiN, TaSi2
Acoustic impedances of materials (unit is MRayl):
In
In
It was confirmed from these diagrams that, by provision of the mass-adding films 309, it was possible to keep the reflection coefficient Γ1 in the generally preferred range while keeping the electromechanical coupling factor K2 in the generally preferred range.
Further, it is suggested from these diagrams that, the larger the normalized thickness t/λ of the mass-adding films 309, the higher the reflection coefficient Γ1 and electromechanical coupling factor K2. Such a tendency occurs no matter what the material is used to form the mass-adding films 309.
In general, the larger the difference of acoustic impedance among the media through which sound wave is propagated, the larger the reflection wave. However, with TaSi2 (line L3), compared with TiN (line L2), irrespective of the fact that the acoustic impedance is small and the difference of the acoustic impedance from SiO2 is small, the reflection coefficient Γ1 becomes large. Below, the reason for this is studied.
The reflection coefficients Γ1 and the electromechanical coupling factors K2 were computed for cases (Case No. 1 to No. 7) where the mass-adding films 309 were formed by various hypothetical materials having acoustic impedances Zs which were the same as each other, but having Young's moduli E and densities p which were different from each other.
The computation conditions were as follows.
Normalized thickness e/λ of IDT electrode 5: 0.08
Normalized thickness T/λ of protective layer 11: 0.30
Normalized thickness t/λ of mass-adding films 309: 0.03
Physical property values of mass-adding films 309:
In
It is considered that such a change of reflection coefficient Γ1 is caused due to a difference of the propagation velocity of the acoustic wave among the materials configuring the mass-adding films 309. First, by waveguide theory, the vibration distribution becomes larger in a region of a medium having a slower propagation velocity of the acoustic wave. On the other hand, the propagation velocities V of acoustic waves in the hypothetical materials No. 1 to No. 7 become as follows (unit: m/s). Note that, V=√(E/ρ).
Accordingly, it is considered that, even in the mass-adding films 309 which have equivalent acoustic impedances, it is believed that the reflection coefficient becomes effectively higher in the mass-adding film 309 having a slow propagation velocity of the acoustic wave in which the vibration distribution is concentrated to the mass-adding film 309 than a mass-adding film 309 having a fast propagation velocity of the acoustic wave in which the vibration distribution is dispersed to the periphery.
Further, the propagation velocity of the acoustic wave of SiO2 is 5560 m/s, and the propagation velocity of the acoustic wave of Al is 5020 m/s. Accordingly, the propagation velocities of the acoustic wave of the mass-adding films 309 in No. 1 and No. 2 are slower than the propagation velocities of the acoustic wave through the protective layer 11 and IDT electrode 5, and the propagation velocities of the acoustic wave of the mass-adding films 309 in No. 3 to No. 7 are faster than the propagation velocities of the acoustic wave through the protective layer 11 and IDT electrode 5. Accordingly, the change of the ratio of change of the reflection coefficient near No. 3 explained above can also be explained by the propagation velocities of the acoustic wave.
Note that, in
As described above, the mass-adding films 309 is preferably made of a material which has a different acoustic impedance from the materials forming the protective layer 11 and the IDT electrode 5 and which has a slower propagation velocity of the acoustic wave than the materials forming the protective layer 11 and the IDT electrode 5. Note that, materials having acoustic impedances larger than the materials forming the protective layer 11 and the IDT electrode 5, compared with materials having smaller acoustic impedances, tend to satisfy the condition that the propagation velocity of the acoustic wave is slower than the materials forming the protective layer 11 and the IDT electrode 5, and are easily selected.
As such a material, for example, there can be mentioned Ta2O5, TaSi2, and W5Si2. Their physical property values (acoustic impedance Zs, propagation velocity V of acoustic wave, Young's moduli E, and density ρ) are as follows.
Note that, WC and TiN exemplified in
The reflection coefficient was calculated for Ta2O5 (difference of acoustic impedance between it and Al or SiO2 is about 20 MRayl) which has an acoustic impedance further closer to the acoustic impedances of the protective layer 11 and the IDT electrode 5 than even TaSi2 (
The calculation conditions were as follows.
Normalized thickness e/λ of IDT electrode 5: 0.08
Normalized thickness T/λ of protective layer 11: 0.27, 0.30, or 0.33
Normalized thickness t/λ of mass-adding film 309: Changed within a range of 0.01 to 0.09.
In
Next, the preferred range of the normalized thickness t/λ of the mass-adding film 309 is studied. First, the lower limit value of the preferred range of the normalized thickness t/λ of the mass-adding film 309 (hereinafter sometimes “of the preferred range” is omitted and the “lower limit value” is simply referred to) is studied.
The frequency band (f1 to f2) in which the reflection coefficient Γall substantially becomes 1 (100%) is called the “stop band”. Note that, in practical use, the reflection coefficient Γall in the stop band does not have to be exactly 1. For example, a frequency band in which the reflection coefficient Γall is 0.99 or more may be specified as the stop band. Further, in general, at the lower end f1 and upper end f2 of the stop band, the reflection coefficient Γall rapidly changes, therefore the interval between these changes may be specified as the stop band as well.
The reflection coefficient Γall of the IDT electrode is determined by the reflection coefficient Γ1 per electrode finger 13b and the number of electrode fingers 13b and so on. Further, as generally known, the smaller the reflection coefficient Γ1, the smaller the width SB of the stop band.
In
Here, when assuming that the upper end f2 of the stop band is a frequency indicated by the line L11 which is lower than the antiresonant frequency f4, as indicated by an imaginary line (two dotted chain line) in a region Sp1, a spurious wave is generated in a frequency band (width Δf) between the resonant frequency f3 and the antiresonant frequency f4. As a result, the desired filter characteristics etc. are liable to not be obtained.
On the other hand, when assuming that the upper end f2 of the stop band is a frequency indicated by the line L12 which is higher than the antiresonant frequency f4, as indicated by the imaginary line (two dotted chain line) in a region Sp2, a spurious wave is generated at a frequency higher than the antiresonant frequency f4. In this case, the influence of the spurious wave exerted upon the filter characteristic etc. is suppressed.
Accordingly, the upper end f2 of the stop band is preferably a higher frequency than the antiresonant frequency f4. Here, the upper end f2 of the stop band depends upon the reflection coefficient, therefore the reflection coefficient of the IDT electrode 5 may be adjusted so that the upper end f2 of the stop band becomes a frequency higher than the antiresonant frequency f4. Further, the reflection coefficient of the IDT electrode 5 linearly increases as the normalized thickness t/λ of the mass-adding film 309 becomes larger as shown in
Here, as shown in
Therefore, the normalized thickness t/λ by which the upper end f2 of the stop band becomes equivalent to the antiresonant frequency f4 was calculated by changing the normalized thickness T/λ of the protective layer 11. Based on the calculated result, the lower limit value of the normalized thickness t/λ was defined by the normalized thickness T/λ.
In
As indicated by the solid line LN1, for the normalized thickness t/λ with which the upper end f2 of the stop band becomes equivalent to the antiresonant frequency f4, approximation curve could be suitably derived by second order curve.
Specifically, this is as follows.
t/λ=0.5706(T/λ)2−0.3867T/λ+0.0913
t/λ=0.3995(T/λ)2−0.2675T/λ+0.0657
t/λ=0.2978(T/λ)2−0.1966T/λ+0.0433
Note that, in all equations of the lower limit value, the minimum value of the normalized thickness t/λ is larger than the largest value (0.01) of the normalized thickness of the bonding layer shown in Patent Literature 2. In Patent Literature 1, the thickness of the bonding layer is not normalized by wavelength, therefore comparison is difficult. However, even when the frequency is made high (for example the largest frequency of UMTS is 2690 MHz) and the propagation velocity of acoustic wave is made slow (for example 3000m/s) so that the normalized thickness becomes large, λ=1.1 μm, and the largest value (100 Å) of the thickness of the bonding layer in Patent Literature 1 is less than 0.01 when normalized.
Next, the upper limit value of the preferred range (hereinafter, sometimes “of the preferred range” is omitted and the “upper limit value” is simply referred to) of the normalized thickness t/λ of the mass-adding films 309 is studied.
As shown in
In the same way as the lower limit value of the normalized thickness t/λ, when the upper limit value of the normalized thickness t/λ is defined according to an equation, for example, this can be defined as in the following equation by estimating the normalized thickness e/λ of the IDT electrode 5 as less than 0.1 in comparison with the normalized thickness e/λ in the general SAW element.
Upper limit value: t/λ=T/λ−0.1
The preferred range of the normalized thickness t/λ derived from the above study is shown in
(Configuration of SAW Device)
The SAW device 51 configures for example a filter or duplexer. The SAW device 51 has a SAW element 31 and a circuit board 53 on which the SAW element 31 is mounted.
The SAW element 31 is for example configured as a SAW element of a so-called wafer level package. The SAW element 31 has the SAW element 1 explained above, a cover 33 which covers the SAW element 1 side of the substrate 3, terminals 35 which pass through the cover 33, and a back surface portion 37 which covers the opposite side to the SAW element 1 of the substrate 3.
The cover 33 is configured by a resin or the like and forms a vibration space 33a for facilitating the propagation of the SAW above the IDT electrode 5 and reflectors 7 (positive side in the z-direction). On the upper surface 3a of the substrate 3, lines 38 which are connected to the IDT electrode 5 and pads 39 which are connected to the lines 38 are formed. The terminals 35 are formed on the pads and are electrically connected to the IDT electrode 5. Though particularly not shown, the back surface portion 37 for example has a back surface electrode for discharging electrical charges charged in the surface of the substrate 3 due to temperature variation etc. and an insulation layer covering the back surface electrode.
The circuit board 53 is configured by a for example so-called rigid type printed circuit board. On a mount surface 53a of the circuit board 53, mount-use pads 55 are formed.
The SAW element 31 is arranged so that the cover 33 side faces the mount surface 53a. Further, the terminals 35 and the mount-use pads 55 are bonded by solder 57. After that, the SAW element 31 is sealed by a seal resin 59.
Note that, in the above embodiments, the substrate 3 is one example of the piezoelectric substrate, and the protective layer 11 is an example of the insulation layer.
The present invention is not limited to the above embodiments and may be worked in various ways.
The acoustic wave element is not limited to a SAW element (in a narrow sense). For example, it may also be a so-called elastic boundary wave element (note, included in a SAW element in a broad sense) in which the thickness of the insulation layer (11) is relatively large (for example 0.5λ to 2λ). Note that, in an elastic boundary wave element, the formation of the vibration space (33a) is unnecessary, and accordingly the cover 33 etc. are unnecessary too.
In the acoustic wave element, the insulation layer (11) is not an essential factor. The insulation layer may be provided for only the purpose of preventing corrosion and may be made thinner than the thickness of the electrode fingers. In these cases as well, for example, by the formation of the mass-adding films by a material having a slower propagation velocity of the acoustic wave than that of the material for the electrode fingers, the reflection coefficient can be made large, and the reflection efficiency of the SAW becomes good, therefore the effect of sealing the SAW in the resonator is improved. Due to this, for example, such effect that a loss can be reduced is exhibited. Further, in these cases as well, in the same way as the embodiment, by the formation of the additional layer so that the upper surface side portion becomes narrower than the lower surface side portion, rapid transition of the vibration center of SAW to the surfaces of the electrode fingers is suppressed, so the effect of improvement of the electromechanical coupling factor is exhibited.
The acoustic wave element is not limited to the wafer level packaged one. For example, in the SAW element, the cover 33 and terminal 35 etc. need not be provided, and the pad 39 on the upper surface 3a of the substrate 3 and the mount-use pad 55 of the circuit board 53 may be directly bonded by solder 57 as well. Further, the vibration space may be formed by a clearance between the SAW element 1 (protective layer 11) and the mount surface 53a of the circuit board 53.
The mass-adding films are preferably provided over the entire surface of the electrode. Note, the mass-adding films may be provided only at a portion of the electrode, for example, may be provided only on the electrode fingers. Further, the mass-adding films may be provided only at portions on the center sides when viewed in the longitudinal directions of the electrode fingers. Furthermore, the mass-adding films may be provided not only on the upper surface of the electrode, but also over the side surfaces. The material of the mass-adding films may be a conductive material or insulation material. Specifically, tungsten, iridium, tantalum, copper, or another conductive material, BaxSr1-xO3, PbxZn1-xO3, ZnO3, or another insulation material can be mentioned as the materials of the mass-adding films. Further, WC etc. which were not considered to be preferred materials in
By forming the mass-adding films by an insulation material, compared with forming the mass-adding films by a metal material, corrosion of the electrode is suppressed, and the electrical characteristics of the acoustic wave element can be stabilized. The reason for this is as follows: Pinholes are sometimes formed in an insulation layer made of SiO2. When such pinholes are formed, moisture intrudes up to the electrode portion through them. However, if a metal film made of a material different from the electrode material is arranged on the electrode, corrosion is liable to occur due to a battery effect between dissimilar metals caused by the intruded moisture. Accordingly, if the mass-adding films are formed by insulation material such as Ta2O5, almost no battery effect occurs between the electrode and the mass-adding films, therefore an acoustic wave element suppressed in corrosion of electrode, so having a high reliability can be obtained.
The upper surface of the insulation layer (11) may have concave-convex shapes so as to form projecting shapes at the positions of the electrode fingers. In this case, the reflection coefficient can be made further higher. The concave-convex shapes may be formed due to the thickness of the electrode fingers at the time of formation of the protective layer as explained with reference to
For the substrate, other than the 128°±10° Y-X cut LiNbO3 substrate, for example, use can be made of 38.7°±Y-X cut LiTaO3 etc. The material of the electrode (electrode fingers) is not limited to Al and an alloy containing Al as the major component and may be for example Cu, Ag, Au, Pt, W, Ta, Mo, Ni, Co, Cr, Fe, Mn, Zn, or Ti. The material of the insulation layer is not limited to SiO2, but may be for example a silicon oxide other than SiO2.
1 . . . SAW element (acoustic wave element), 3 . . . substrate (piezoelectric substrate), 3a . . . upper surface, 5 . . . IDT electrode (electrode), 9 . . . first film, and 11 . . . protective layer (insulation layer).
Number | Date | Country | Kind |
---|---|---|---|
2011-015649 | Jan 2011 | JP | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/JP2012/050821 | 1/17/2012 | WO | 00 | 7/26/2013 |