JP2020080519A - Surface acoustic wave element - Google Patents

Abstract

To provide a small surface acoustic wave element capable of exciting a piston mode.SOLUTION: The surface acoustic wave element includes: a piezoelectric substrate 11; a pair of bus bars 2a, 2b formed on an upper surface of the piezoelectric substrate; and a pair of IDT electrodes provided with a plurality of electrode fingers 3a, 3b extending from the bus bars 2a, 2b in a comb shape. Gap regions RB are formed on both sides of an intersection region ZAB provided so that the electrode fingers 3a, 3b intersect. In the gap region RB, propagation velocity control films 4a, 4b for reducing a propagation velocity of a surface acoustic wave by an effect of mass loading to excite a surface acoustic wave in piston mode are provided.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a surface acoustic wave element that converts a frequency signal into a surface acoustic wave.

  As a surface acoustic wave element having a high Q value and low spurious, one operating in a piston mode has been developed (for example, Patent Documents 1 and 2). The piston mode is one of the vibration modes of a surface acoustic wave (SAW), and as shown in FIG. 2A described later, the SAW amplitude is substantially constant in the excitation region, An amplitude distribution in which the amplitude sharply decreases in the region on the side is shown.

  When exciting the SAW, comb-teeth electrodes in which a large number of electrode fingers are connected to a bus bar are used. The surface acoustic wave element (SAW element) is provided with an IDT (Interdigital Transducer) electrode in which two comb-teeth electrodes are arranged so as to face each other so that the electrode fingers cross each other when viewed in the arrangement direction of the electrode fingers. The SAW is excited by inputting a frequency signal having a predetermined frequency to one of the comb-teeth electrodes and propagates to the other comb-teeth electrode.

  The techniques described in Patent Documents 1 and 2 are, for a region between two bus bars arranged facing each other, a central region along the direction in which the bus bar extends, two regions on both sides thereof, and further two regions on both sides. It is divided into five (Patent Document 1: central region, edge region, gap region, and Patent Document 2: central excitation region, inner edge region, outer edge region). Then, the piston mode is obtained by increasing or decreasing the width of the electrode fingers located in the regions on both sides of the center (described in Patent Document 1 as "electrode part"). Further, Patent Document 1 describes that it is necessary to secure a gap length dimension of the gap region of about 1 to 3 acoustic wavelengths.

However, in these methods, the size of the gap length (the size of the gap region of Patent Document 1 or the outer edge region of Patent Document 2) becomes longer than that of a normal IDT electrode, so that the element (Patent Document 1: Acoustic Wave Device, Patent Document) 2: There is a problem that the electroacoustic transducer) becomes large.
Note that Patent Document 3 describes a technique of providing a dielectric film in a gap region (described as an intermediate region in Patent Document 3) where electrode fingers of an IDT electrode do not intersect for the purpose of confining a main elastic wave. However, it is not intended to excite the piston mode.

Japanese Patent No. 5221616 Japanese Patent No. 5503020 Japanese Patent No. 6181719

  The present invention has been made under such circumstances, and provides a small surface acoustic wave element capable of exciting a piston mode.

The surface acoustic wave device includes a piezoelectric substrate and
A pair of bus bars formed on the piezoelectric substrate, and a plurality of electrode fingers extending in a comb-like shape from each of the bus bars toward the opposing bus bars are provided. As seen from the above, an electrode finger connected to one of the busbars and an intersecting area that is an area where the electrode fingers connected to the other busbar intersect, and each of the busbars is positioned closer to the busbar than the intersecting area. A pair of IDT electrodes formed with a gap region which is a region where the electrode fingers connected to the bus bar of No. 1 and the electrode fingers connected to the other bus bar do not intersect,
The propagation velocity of the surface acoustic wave in the gap region is reduced by the mass loading effect, and the velocity difference from the propagation velocity of the surface acoustic wave in the intersecting region causes a velocity difference that excites the piston mode surface acoustic wave in the intersecting region. And a propagation velocity adjusting film formed on the upper surface side of the gap region.

The surface acoustic wave device described above may have the following configuration.
(A) The propagation velocity adjusting film is formed in a grating shape between the tip of each electrode finger in the intersecting region and the bus bar.
(B) The width of the gap region as viewed along the extending direction of the electrode fingers is within the range of 0.1d to 2d with respect to the distance d between the center lines of the electrode fingers arranged adjacent to each other.
(C) An adjustment film is formed on the upper surface side of the bus bar so that the attenuation of elastic waves in each bus bar is larger than that in the intersection region and the gap region.
(D) A dielectric film having a temperature-frequency characteristic in which the frequency changes in a direction opposite to the temperature-frequency characteristic of the piezoelectric substrate is formed on the upper surface side of the surface acoustic wave element on which the electrode fingers are formed. The propagation velocity adjusting film is formed on the upper surface side of the dielectric film. Here, for example, the piezoelectric substrate is LiNbO ₃ , and the dielectric film is silicon oxide, silicon oxynitride, or fluorine-doped silicon oxide.
(E) The propagation velocity adjusting film is a dielectric film formed in contact with the upper surface of the piezoelectric substrate in the gap region.

  According to the present invention, a propagation velocity adjusting film for reducing the propagation velocity of a surface acoustic wave is provided in the gap regions formed on both sides of the intersection region where the electrode fingers of the pair of IDT electrodes intersect, and the elastic surface in the intersection region is provided. Since the velocity difference from the wave propagation velocity is adjusted, the piston mode surface acoustic wave can be excited without providing the conventional IDT electrode with a long gap length dimension.

3A and 3B are a plan view and a vertical sectional side view of a SAW element according to an embodiment. It is an explanatory view of an operation of the SAW element. It is the top view and vertical side view of the SAW element concerning other embodiments. It is a frequency-admittance characteristic figure concerning the SAW element of an example. It is a frequency-admittance characteristic figure concerning the SAW element of a comparative example.

Hereinafter, a configuration example of the surface acoustic wave element (SAW element) according to the embodiment will be described with reference to FIG. FIG. 1A is an enlarged plan view schematically showing the SAW element of this example, and FIGS. 1B and 1C schematically show positions AA′ and BB′, respectively. FIG.
The SAW element shown in FIG. 1 includes an IDT electrode formed on a rectangular piezoelectric substrate 11 that excites SAW. In the following description, the direction along the long side of the rectangular piezoelectric substrate 11 is the vertical direction (X direction in FIG. 1A), and the direction along the short side is the horizontal direction (Y direction in FIG. 1). Also called.

The IDT electrode is provided, for example, so as to extend in the vertical direction along each long side of the piezoelectric substrate 11, and has two bus bars 2a and 2b respectively connected to the signal ports 12a and 12b and laterally from each bus bar 2a and 2b. It is provided with a large number of electrode fingers 3a, 3b formed so as to extend in the direction.
Examples of the piezoelectric material forming the piezoelectric substrate 11 include the case where lithium niobate (LiNbO ₃ ), aluminum nitride (AlN), scandium aluminum nitride (ScAlN), lithium tantalate (LiTaO ₃ ) or the like is used. .

As shown in FIG. 1A, the electrode fingers 3a connected to one of the bus bars 2a are provided so as to extend toward the side of the bus bar 2b arranged at the opposite position. The electrode fingers 3b connected to the other bus bar 2b are provided so as to extend toward the one bus bar 2a. When viewed along the arrangement direction of the electrode fingers 3a and 3b, the electrode fingers 3a connected to the one bus bar 2a and the electrode fingers 3b connected to the other bus bar 2b are arranged so as to alternate with each other. ing.
The bus bars 2a and 2b and the electrode fingers 3a and 3b are made of, for example, copper (Cu).

  As described above, the region where the electrode fingers 3a and 3b are arranged so as to intersect each other corresponds to the intersection region ZAB of the IDT electrodes. Further, since the tip ends of the electrode fingers 3a and 3b connected to one of the bus bars 2a and 2b do not reach the other bus bar 2b and 2a on the bus bar 2a and 2b side as viewed from the intersection area ZAB, , Two regions where the electrode fingers 3a and 3b do not intersect are formed. These regions correspond to the gap region RB. The two regions in which the bus bars 2a and 2b are formed are also called bus bars 2a and 2b.

Further, the SAW element of this example is configured as a TC (Temperature Compensate)-SAW element having a temperature compensation function for compensating the influence of the temperature-frequency characteristics of the piezoelectric material forming the piezoelectric substrate 11.
As shown in the vertical side views of FIGS. 1B and 1C, in the TC-SAW element, on the upper surface of the piezoelectric substrate 11 on which the IDT electrodes (bus bars 2a, 2b, electrode fingers 3a, 3b) are formed, The dielectric film 5 is formed (hereinafter, also referred to as “loading”). In FIG. 1A, in order to show that the dielectric film 5 is loaded on the upper surface side of the electrode fingers 3a and 3b, the formation positions (contour lines) of the electrode fingers 3a and 3b are indicated by broken lines.

  As the dielectric film 5 loaded on the TC-SAW element, one having a frequency temperature characteristic opposite to that of the piezoelectric material of the piezoelectric substrate 11 is used. For example, when the piezoelectric material of the piezoelectric substrate 11 has a negative frequency-temperature characteristic in which the excited frequency decreases with increasing temperature, the positive frequency temperature increases with increasing temperature. A dielectric film 5 having characteristics is loaded. On the contrary, the piezoelectric substrate 11 made of a piezoelectric material having a positive frequency temperature characteristic is loaded with the dielectric film 5 having a negative frequency temperature characteristic. As described above, by loading the dielectric film 5 having the frequency-temperature characteristic opposite to that of the piezoelectric substrate 11, the influence of the temperature change around the SAW element can be reduced.

For example, the above-mentioned LiNbO ₃ has a negative frequency temperature characteristic. Therefore, for the piezoelectric substrate 11 made of LiNbO ₃ , silicon dioxide or silicon oxynitride having a positive frequency-temperature characteristic (the content ratio of elements is not particularly limited, SiNO may be used, or SiO 2 may be used). ₂ may be doped with nitrogen) or fluorine-doped silicon dioxide dielectric film 5 may be loaded. The dielectric film 5 made of these materials can be loaded by a technique such as CVD (Chemical Vapor Deposition) or sputtering.

The SAW element of this example has a configuration for exciting piston mode SAW in addition to the configuration described above.
When viewed in the lateral direction, the inventor of the present application makes the SAW propagation speed on the gap region RB side sufficiently higher than the SAW propagation speed on the intersection region ZAB, as shown by a thin solid line in FIG. It is understood that it is possible to excite the piston mode SAW having the amplitude distribution indicated by the thick solid line in the figure by lowering it.

Therefore, in the SAW element of this example, as shown in FIGS. 1A to 1C, the SAW of each gap region RB is provided on the upper surface side of the dielectric film 5 loaded on the SAW element due to the mass load effect. Propagation velocity adjusting films 4a and 4b for reducing the propagation velocity are formed.
There is no particular limitation on the constituent material of the propagation velocity adjusting films 4a and 4b formed for the purpose of obtaining the mass loading effect, but in consideration of the ease of film formation and the size of the specific gravity, a metal or a dielectric is used. Is selected. In the SAW element of this example, titanium (Ti), which is relatively easy to form and has a large specific gravity, is used as the propagation velocity adjusting films 4a and 4b.

  In order to excite the piston mode SAW, as the speed difference between the SAW propagation speed on the crossing area ZAB side and the SAW propagation speed on the gap area RB side increases, the lateral width dimension of the gap area RB increases. It is necessary to design the SAW element so that it becomes smaller. Conversely, when designing the SAW element having the gap region RB having a predetermined width dimension, it can be said that the piston mode SAW can be excited if the speed difference is sufficiently large.

  From this point of view, the general design variables of the SAW element of this example are shown below. For the wavelength λ corresponding to the design frequency of the SAW element, the distance d between the center lines of the electrode fingers 3a and 3b is λ/2. Is set. From the viewpoint of configuring a small SAW element, the width dimension of the gap region RB is 0.1d to 2d (0.05λ to 1λ), preferably 0.8d to 1.0d (0.4λ to 0. 5λ) is set.

Whether or not the piston mode is actually excited (whether or not the speed difference at which the piston mode is excited is obtained) can be confirmed in advance by simulation.
In the SAW element of the present example utilizing the mass loading effect, the width of decrease in the SAW propagation velocity in the gap region RB depends on the selection of the constituent material (specific gravity) of the propagation velocity adjusting films 4a and 4b and the propagation velocity adjusting films 4a and 4b. The thickness can be adjusted by changing the thickness.

  Here, as shown in FIG. 1(a) with sand-like hatching, the SAW element of this example has a tip of each electrode finger 3a, 3b arranged in the intersection area ZAB adjacent to the gap area RB. It has a grating structure in which the propagation velocity adjusting films 4a and 4b are formed like electrode fingers at positions adjacent to the portions. The inventor has found that the propagation velocity adjusting films 4a and 4b having such a grating structure have an effect of reducing the propagation velocity of the SAW as compared with the case where the propagation velocity adjusting film 42 is formed on the entire surface of the gap region RB. I know it is big.

In the SAW element of this example, the same material as the propagation speed adjusting films 4a and 4b is formed on the upper surface side of the dielectric film 5 and on the upper surface side of each bus bar region SB in which the bus bars 2a and 2b are formed. The bus bar region film 41 is formed. The above-described propagation velocity adjusting films 4a and 4b are formed integrally with the bus bar region film 41.
Thus, by forming the propagation velocity adjusting films 4a and 4b together with the bus bar region film 41 having a relatively large area, the arrangement positions and shapes of the propagation velocity adjusting films 4a and 4b each having a very small grating shape, The dimensions and the like can be formed more accurately.

  By providing the propagation velocity adjusting films 4a and 4b described above, the SAW element of this example has a configuration capable of exciting the piston mode SAW. On the other hand, when the attenuation of the elastic wave in the busbar region SB arranged adjacent to the gap region RB is small, the SAW leaks to the busbars 2a, 2b side, and the piston mode having a sufficiently large amplitude cannot be excited. There are cases.

From the viewpoint of suppressing the leakage of SAW, it is preferable that the attenuation of elastic waves in the busbar region SB is significantly larger than that in the intersection region ZAB and the gap region RB, as indicated by the thin solid line in FIG. 2B. .
Therefore, in the SAW element of this example, the attenuation of the elastic wave in the region is increased by forming the adjustment film 6 in the busbar region SB on the upper side of each busbar 2a, 2b.

As a constituent material of the adjustment film 6, aluminum (Al), which is common to constituent materials such as wirings connected to the signal ports 12a and 12b, can be exemplified. As a result, the adjustment film 6 can be formed integrally with the wiring, and an increase in the number of steps for manufacturing the SAW element can be suppressed.
In order to obtain a sufficient amount of elastic wave attenuation, the copper film forming the bus bars 2a, 2b, the electrode fingers 3a, 3b, the dielectric film 5, the propagation speed adjusting film 42, and the metal film forming the propagation speed adjusting film 42. The adjustment film 6 thicker than the dielectric film is formed. If the SAW element can excite the piston mode SAW, the thickness of the adjustment film 6 is not particularly limited, but the thickness of about 40% or more of the wavelength λ described above can be exemplified.

In the SAW element having the above-described configuration, when a frequency signal of the design frequency is applied, the piston mode SAW is excited by the action of the propagation velocity adjusting films 4a and 4b provided in the gap region RB.
Moreover, since the attenuation of the elastic wave in the busbar area SB is increased by the adjustment film 6, it is possible to suppress the leakage of the SAW and excite the SAW having a large amplitude in the intersection area ZAB.

  Here, by increasing the attenuation of the elastic wave in the busbar area SB and suppressing the amplitude of the higher-order transverse mode in the busbar area SB, the positive and negative amplitudes of the higher-order transverse mode are sufficient in the entire SAW element. It can be configured to cancel each other. The higher-order transverse mode is observed as a spurious response, which causes an increase in insertion loss of the SAW element.

  For example, in FIG. 2B, the amplitude distribution of the second-order higher-order transverse mode is indicated by a thick line. By increasing the attenuation of the elastic wave in the busbar region SB, the amplitude of the higher-order transverse mode is suppressed as compared with the amplitude distribution (indicated by the broken line in the figure) when the adjustment film 6 is not provided. Be done. As a result, even if higher-order transverse modes occur, they can cancel each other out in the SAW element and suppress the generation of spurious responses.

  The SAW element of this embodiment has the following effects. Propagation velocity adjusting films 4a and 4b for reducing the propagation velocity of SAW are provided in the gap region RB formed on both sides of the intersection region ZAB where the electrode fingers 3a and 3b of the pair of IDT electrodes intersect, and the SAW in the intersection region ZAB is provided. Since the velocity difference with the propagation velocity of the is adjusted, the piston mode SAW can be excited without adding a new region to the conventional IDT electrode.

Here, it is not an essential requirement to provide the propagation velocity adjusting films 4a and 4b having the grating structure as in the example described with reference to FIG. When the effect of reducing the propagation velocity of the SAW on the gap region RB side can be sufficiently obtained by selecting the constituent material and adjusting the film thickness, the propagation velocity adjusting film covering the entire surface of the gap region RB as shown in FIG. 42 may be formed.
Further, it is not essential to integrally form the propagation velocity adjusting films 4a, 4b, 42 with the busbar region film 41 on the busbar region SB side, and when sufficient processing accuracy can be obtained, the busbar region film 41 may be omitted. Thus, only the propagation velocity adjusting films 4a, 4b, 42 may be provided.

  Furthermore, the SAW element that excites the piston mode may not be the TC-SAW element loaded with the dielectric film 5. In this case, the propagation velocity adjusting films 4a, 4b and 42 are directly formed on the piezoelectric substrate 11 on which the electrode fingers 3a and 3b are formed. Therefore, from the viewpoint of preventing short-circuiting of the electrode fingers 3a, 3b and the bus bars 2a, 2b, it is preferable to form the propagation velocity adjusting films 4a, 4b, 42 with a dielectric material, for example. At this time, the propagation speed adjusting films 4a, 4b, 42 may be made of a dielectric material having a low SAW propagation speed, such as tellurium oxide. However, the formation of the propagation velocity adjusting films 4a and 4b is not prohibited by the metal, and the metal propagation velocity adjusting film is formed in the gap region RB while avoiding contact with the electrode fingers 3a and 3b and the bus bars 2a and 2b. 4a and 4b may be formed in a grating shape.

The SAW element described above can be used as an electronic component in the single SAW element shown in FIGS. 1 and 3. Further, the piezoelectric substrate 11 may be expanded in the vertical direction, and grating reflectors may be provided before and after the IDT electrode.
Further, the SAW element can also be applied to a case where a plurality of IDT electrodes are provided on the common piezoelectric substrate 11 like a ladder type filter in which a plurality of SAW elements are connected in a ladder shape.

  The SAW element of this example can be used for a filter, a duplexer, a quadplexer, and other devices having a filter function. In addition, the device can be incorporated into a power amplifier duplexer module (Power Amp Integrated Duplexer) called PAMiD, PAiD, PAD, etc., and a diversity reception module (Diversity Front End Module) called DiFEM.

The frequency characteristics of the SAW element depending on the presence or absence of the propagation velocity adjusting films 4a and 4b were confirmed by simulation.
A. Simulation conditions (Example)
A simulation model of a SAW element having the propagation velocity adjusting films 4a and 4b having the grating structure in the gap region RB described with reference to FIG. 1 was prepared, and the frequency-admittance characteristic was investigated. The result is shown in FIG.
(Comparative example)
A simulation model of a SAW device having the same configuration as that of the example was prepared except that the propagation velocity adjusting films 4a and 4b and the bus bar region film 41 were not provided, and the frequency-admittance characteristic was investigated. The result is shown in FIG.

B. Simulation Results According to the simulation results of the example shown in FIG. 4, the resonance point and anti-resonance point of the SAW were confirmed, and it was confirmed that the SAW element has the characteristics that can be used. Further, spurious caused by higher-order transverse modes was not observed, and a SAW element with small insertion loss could be constructed.

On the other hand, as shown in FIG. 5, in the SAW element according to the comparative example, many spurious components due to the higher-order transverse modes were confirmed, and it can be seen that the insertion loss becomes larger than that of the SAW element of the example.
As described above, also from the comparison between the example and the comparative example, it was confirmed that the SAW element according to the example including the propagation velocity adjusting films 4a and 4b had good characteristics with less spurious.

ZAB Crossing area RB Gap area 11 Piezoelectric substrates 2a, 2b Busbars 3a, 3b Electrode fingers 4a, 4b Propagation speed adjusting film 5 Dielectric film 6 Adjusting film

Claims (7)

A piezoelectric substrate,
A pair of bus bars formed on the piezoelectric substrate and a plurality of electrode fingers extending in a comb-like shape from each of the bus bars toward the opposing bus bars are provided. As seen from the above, an electrode finger connected to one of the busbars and an intersecting area that is an area where the electrode fingers connected to the other busbar intersect, and each of the busbars is positioned closer to the busbar than the intersecting area. A pair of IDT electrodes in which a gap region which is a region where the electrode finger connected to the bus bar of No. 1 and the electrode finger connected to the other bus bar do not intersect,
The propagation velocity of the surface acoustic wave in the gap region is reduced by the mass loading effect, and the velocity difference from the propagation velocity of the surface acoustic wave in the intersecting region causes a velocity difference that excites the piston mode surface acoustic wave in the intersecting region. And a propagation velocity adjusting film formed on the upper surface side of the gap region.   The surface acoustic wave device according to claim 1, wherein the propagation velocity adjusting film is formed in a grating shape between the tip end portion of each electrode finger in the intersecting region and the bus bar.   The width of the gap region as viewed along the extending direction of the electrode fingers is within the range of 0.1d to 2d with respect to the distance d between the center lines of the electrode fingers arranged adjacent to each other. The surface acoustic wave device according to claim 1.   The adjustment film for increasing the attenuation of elastic waves in each bus bar is formed on the upper surface side of the bus bar more than in the intersection region and the gap region. The surface acoustic wave device according to any one of claims.   A dielectric film having a temperature-frequency characteristic in which a frequency changes in a direction opposite to a temperature-frequency characteristic of the piezoelectric substrate is formed on an upper surface side of the surface acoustic wave element on which the electrode fingers are formed, and the propagation is performed. 5. The surface acoustic wave device according to claim 1, wherein the speed adjustment film is formed on the upper surface side of the dielectric film. The surface acoustic wave device according to claim 5, wherein the piezoelectric substrate is LiNbO ₃ , and the dielectric film is silicon oxide, silicon oxynitride, or fluorine-doped silicon oxide.   The surface acoustic wave device according to claim 1, wherein the propagation velocity adjusting film is a dielectric film formed so as to be in contact with an upper surface of the piezoelectric substrate in the gap region.

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