JP2012227626A - Surface acoustic wave element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress deterioration of passing properties as a filter in a surface acoustic wave element coated with a dielectric film.SOLUTION: A surface acoustic wave element includes: a piezoelectric substrate; at least a pair of IDT electrodes formed on the piezoelectric substrate; and a dielectric film that coats at least a part of each of the piezoelectric substrate and the IDT electrodes. The piezoelectric substrate is a lithium tantalate substrate having a cut angle in a range of Euler angle (-10 degrees to 10 degrees, 125 degrees to 140 degrees, and -10 degrees to 10 degrees). The dielectric film includes: a first dielectric film which is composed mainly of silicon dioxide, and coats a region where a plurality of electrode fingers included in at least the IDT electrodes are arranged; and a second dielectric film which is composed mainly of silicon nitride, and coats the first dielectric film. An acoustic velocity of a surface acoustic wave propagated on the electrode fingers of the IDT electrodes is in a range of 4000 m/s to 4100 m/s.

Description

  The present invention relates to a surface acoustic wave device used for a band-pass filter or the like.

  In recent years, surface acoustic wave elements in which electrodes are formed on the surface of a piezoelectric substrate have been used as circuit elements such as resonators and filters in the field of information communication devices such as cellular phones. An example of such a surface acoustic wave element is shown in FIG. FIG. 12A is a top view of the surface acoustic wave element 900. The surface acoustic wave element 900 is formed by arranging a pair of comb-shaped IDT electrodes 902 and two reflectors 903 on a piezoelectric substrate 901. Each IDT electrode 902 has a bus bar 911 and a plurality of electrode fingers 912 extending from the bus bar 911. The electrode fingers 912 of the IDT electrode 902 are arranged so as to be alternately arranged with the electrode fingers 912 of the other IDT electrode 902. Further, the reflectors 903 are arranged one by one with the IDT electrodes 902 interposed therebetween. FIG. 12B is a top view of another surface acoustic wave element 920. The surface acoustic wave element 920 in the surface acoustic wave element 900 further includes dummy electrode fingers 913 in which the IDT electrodes 902 extend alternately with the electrode fingers 912 from the bus bar 911, and the electrode fingers 912 are dummy electrodes of the other IDT electrode 902. It is arranged to face the finger 913. Among the regions where the electrode fingers 912 are alternately arranged, an intersecting region which is a band-like region including a region excluding the tip portion of each electrode finger 912 is used as a main propagation path of elastic waves.

  A surface acoustic wave element in which an electrode is coated with a dielectric film has been proposed. For example, Patent Document 1 discloses a surface acoustic wave element 940 shown in FIG. FIG. 13 is a top view transparently depicting the surface acoustic wave element 940 and a sectional view taken along the line AA ′. In the surface acoustic wave element 940, the piezoelectric substrate 941 is covered with the first dielectric film 944, two pairs of IDT electrodes 942 are disposed on the dielectric film 941, and the first dielectric film 944 and the IDT are further disposed. The electrode 942 is formed by covering with a second dielectric film 945 having moisture resistance. This prevents corrosion of the IDT electrode 942 due to moisture.

  Patent Document 2 discloses a surface acoustic wave element 960 shown in FIG. FIG. 14 is a top view transparently depicting the surface acoustic wave element 960 and a cross-sectional view along the line AA ′. The surface acoustic wave element 960 includes a pair of IDT electrodes 962 having a predetermined film thickness on a quartz substrate 961 which is a piezoelectric substrate, so that the TCF (frequency temperature coefficient) value in the operating environment temperature range of the quartz substrate 961 is negative. The reflector 963 is disposed, and the IDT electrode 962 and the reflector 963 are formed by covering with a thin film such as a dielectric having a positive TCF value at the temperature. As a result, the TCF value is canceled out at the center of the use environment temperature range and becomes 0, and the frequency variation in the temperature range is reduced.

Japanese Patent Laid-Open No. 2002-151997 JP 2005-65160 A

  As described above, when a piezoelectric substrate or IDT electrode is covered with a dielectric film in order to protect the surface acoustic wave element or reduce the TCF value, the Q value at the resonance frequency of the IDT electrode is reduced, and the surface acoustic wave element There is a problem that the pass characteristic as a filter deteriorates.

  Therefore, an object of the present invention is to suppress the deterioration of the pass characteristics as a filter in a surface acoustic wave element coated with a dielectric film.

  The present invention is a surface acoustic wave device including a piezoelectric substrate, at least one set of IDT electrodes formed on the piezoelectric substrate, and a dielectric film covering at least a part of the piezoelectric substrate and the IDT electrode. The piezoelectric substrate is a lithium tantalate substrate having a cut angle in the range of Euler angles (-10 ° to 10 °, 125 ° to 140 °, -10 ° to 10 °), and the dielectric film is made of silicon dioxide. A first dielectric film that covers at least a region in which a plurality of electrode fingers of the IDT electrode are arranged, and a second dielectric that has silicon nitride as a main component and covers the first dielectric film The speed of sound of the surface acoustic wave including the film and propagating on the electrode finger of the IDT electrode is in the range of 4000 m / s to 4100 m / s.

  Further, it is preferable that the film thicknesses H1 and H2 of the first and second dielectric films satisfy the relational expression 0.3 ≦ H2 / H1 ≦ 1.2.

Alternatively, normalized film thicknesses Hλ1 and Hλ2 obtained by dividing the film thicknesses of the first and second dielectric films by the wavelength λ of the surface acoustic wave excited by the IDT electrode are expressed by the relational expression 0.9 × (−0. 5 × Hλ1 ² + 0.295 × Hλ1 + 0.0362) ≦ Hλ2 ≦ 1.1 × (−0.5 × Hλ1 ² + 0.295 × Hλ1 + 0.0362) is preferably satisfied.

  Alternatively, the thickness of the first dielectric film is preferably equal to or less than the thickness of the IDT electrode.

  ADVANTAGE OF THE INVENTION According to this invention, the deterioration of the passage characteristic as a filter can be suppressed in the surface acoustic wave element which coat | covered the dielectric film.

The top view and expanded sectional view of the surface acoustic wave element concerning the embodiment of the present invention The figure which shows the frequency characteristic of the surface acoustic wave element which concerns on embodiment of this invention The figure which shows the frequency characteristic which concerns on the examination example of the surface acoustic wave element which concerns on embodiment of this invention The figure which shows the frequency characteristic which concerns on the examination example of the surface acoustic wave element which concerns on embodiment of this invention The figure which shows the frequency characteristic which concerns on the examination example of the surface acoustic wave element which concerns on embodiment of this invention The figure which shows Q value which concerns on the examination example of the surface acoustic wave element which concerns on embodiment of this invention The figure which shows Q value which concerns on the examination example of the surface acoustic wave element which concerns on embodiment of this invention The figure which shows the film thickness which concerns on the examination example of the surface acoustic wave element concerning embodiment of this invention Enlarged sectional view of a surface acoustic wave device according to an embodiment of the present invention The figure which shows the TCF value which concerns on the examination example of the surface acoustic wave element which concerns on embodiment of this invention Enlarged sectional view showing a modification of the surface acoustic wave element according to the embodiment of the present invention Top view of a conventional surface acoustic wave device Top view and sectional view of a conventional surface acoustic wave device Top view and sectional view of a conventional surface acoustic wave device

(First embodiment)
A first embodiment of the present invention will be described below. FIG. 1 is a top view transparently depicting the surface acoustic wave device 100 according to the present embodiment and an enlarged cross-sectional view along the line AA ′. The surface acoustic wave element 100 is a resonator configured by arranging two IDT electrodes 102 and two reflectors 103 on a piezoelectric substrate 101. Each IDT electrode 102 has a bus bar 111 and a plurality of electrode fingers 112 and dummy electrode fingers 113 extending alternately from the bus bar 111. The electrode fingers 112 of the IDT electrode 102 are alternately arranged with the electrode fingers 112 of the other IDT electrode 102, and are arranged so that the tip faces the tip of the dummy electrode finger 113 of the other IDT electrode 102. Further, the reflectors 103 are arranged one by one with these IDT electrodes sandwiched therebetween. In addition, the piezoelectric substrate 101, the IDT electrode 102, and the reflector 103 are covered with a first dielectric film 104 and a second dielectric film 105 whose sound speed is faster than that of the first dielectric film. An intersecting region, which is a region where the electrode fingers 112 on the IDT electrode 102 are alternately arranged, is used as a main propagation path of an elastic wave and constitutes a resonator.

As for the piezoelectric substrate 101, when the cut angle and propagation angle are represented by Euler angles (φ, θ, ψ) of right-handed orthogonal coordinates, φ is −10 ° to 10 °, and θ is 125 ° to 140 °. , Ψ is preferably lithium tantalate (LiTaO ₃ ) in the range of −10 ° to 10 °. When lithium tantalate having this Euler angle is used as the piezoelectric substrate 101, the main elastic wave excited by the IDT electrode 102 can be a SH (Shear Horizontal) wave. Further, as the IDT electrode 102 and the reflector 103, for example, a single metal made of aluminum, copper, silver, gold, titanium, tungsten, molybdenum, platinum, or chromium, an alloy containing these as a main component, or these metals An electrode made of a laminate obtained by laminating is used. The first dielectric film 104 is preferably mainly composed of silicon dioxide. The second dielectric film 105 is preferably composed mainly of silicon nitride.

  A study example of the conditions of the thicknesses of the first dielectric film 104 and the second dielectric film 105 for improving the Q value of the surface acoustic wave element 100 will be described below.

2 shows that when the thickness of the second dielectric film 105 is 0.01λ, the thickness of the first dielectric film 104 is 0λ (none), 0.05λ, 0.10λ, and 0. when changing the 15Ramuda, a diagram showing the S ₂₁ value of the surface acoustic wave element 100 (transmission coefficient) at each frequency. Here, λ is the wavelength of the excitation wave determined by twice the interval between the electrode fingers 112 of the IDT electrode 102. 3, 4, and 5 are views similar to FIG. 1 when the thickness of the second dielectric film 105 is set to 0.03λ, 0.05λ, and 0.07λ. Here, the film thickness of the IDT electrode 102 is 0.08λ. When the film thickness of the first dielectric film 104 is the same, the S ₂₁ value at the resonance frequency decreases as the film thickness of the second dielectric film 105 increases within the range shown in FIGS. It can be seen that the Q value at the antiresonance frequency of the surface acoustic wave device 100 tends to be improved.

  FIG. 6 is a diagram showing the relationship between the film thickness of the second dielectric film 105 and the Q value at the antiresonance frequency of the surface acoustic wave element 100 for each film thickness of the first dielectric film 104. . When each film thickness is changed, the sound velocity of the surface acoustic wave at the resonance frequency also changes. FIG. 7 is a diagram in which the horizontal axis represents the speed of sound of the surface acoustic wave at the resonance frequency instead of the thickness of the second dielectric film 105 in FIG. According to FIG. 7, regardless of the thickness of the first dielectric film 104, the film thickness of the second dielectric film 105 is set so that the sound velocity is in the range of 4000 m / s or more and 4100 m / s. By selecting, a large Q value can be obtained. In this range, if the thickness of the first dielectric film 104 is H1 and the thickness of the second dielectric film 105 is H2, the value of H2 / H1 is approximately 0.3 or more and 1.2 or less. It becomes. Therefore, a large Q value can be obtained by setting H1 and H2 within this range.

FIG. 8 is a graph obtained by plotting the thickness of the second dielectric film 105 having the maximum Q value at each film thickness of the first dielectric film 104 and interpolating with a quadratic function based on FIG. . This quadratic function interpolation formula uses Hλ2 = −0 using normalized film thicknesses Hλ1 and Hλ2 obtained by dividing the film thicknesses of the first dielectric film 104 and the second dielectric film 105 by the excitation wavelength λ. .5 × Hλ1 ² + 0.295 × Hλ1 + 0.0362. Here, let the right side be f (Hλ1). The determination coefficient R ² is 0.9966, and it can be said that Hλ2 = f (Hλ1) is a good interpolation formula. A large Q value is obtained when Hλ2 is set to 0.9 × f (Hλ1) or more and 1.1 × f (Hλ1) with respect to Hλ1.

  The film thickness H1 of the first dielectric film 104 may be equal to or less than the film thickness H3 of the IDT electrode 102. FIG. 9 shows an enlarged cross-sectional view of the surface acoustic wave element 100 in this case. By setting the film thickness H1 of the first dielectric film 104 to be equal to or less than the film thickness H3 of the IDT electrode 102, the reflectance of the elastic wave propagating through the IDT electrode 102 is sufficiently secured, and the surface acoustic wave element 100 has An electromechanical coupling coefficient can be ensured.

  The film thickness H3 of the IDT electrode 102 is smaller than the sum H1 + H2 of the film thickness H1 of the first dielectric film 104 and the film thickness H2 of the second dielectric film 105. It is desirable to improve the moisture resistance by covering with the second dielectric film 105. However, the film thickness H3 of the IDT electrode 102 is not less than the sum H1 + H2 of the film thickness H1 of the first dielectric film 104 and the film thickness H2 of the second dielectric film 105, and the upper surface of the IDT electrode 102 is The dielectric film 105 may be exposed.

  FIG. 10 is a diagram showing the relationship between the film thickness of the first dielectric film 104 and the TCF value at the resonance frequency and antiresonance frequency of the surface acoustic wave element 100. In a generally used acoustic wave device using lithium tantalate as a piezoelectric substrate, the TCF value is about −50 ppm / K. However, in the surface acoustic wave device 100, the TCF value is further improved. It can be confirmed that the effect of improving the temperature characteristics by providing the first dielectric film 104 is not impaired.

  In the present embodiment, the first dielectric film 104 and the second dielectric film 105 are formed so that the upper surfaces thereof are flat, but the protrusions may be formed. Good. FIGS. 11A and 11B are enlarged cross-sectional views of the surface acoustic wave element 100 in the case where such convex portions are formed. In the first dielectric film 104 and the second dielectric film 105, a portion covering the IDT electrode 102 is raised and a convex portion is formed. As shown in FIG. 11B, if the film thickness H1 of the first dielectric film 104 is set to be equal to or less than the film thickness H3 of the IDT electrode 102, the elastic wave propagating through the IDT electrode 102 as described above. Of the surface acoustic wave element 100 can be secured.

  Further, in this embodiment, the IDT electrode 102 includes the dummy electrode finger 113, but the dummy electrode finger 113 may not be provided as illustrated in FIG.

  The present invention is useful in a surface acoustic wave device used for information communication equipment and the like.

100, 900, 920, 940, 960 Surface acoustic wave element 101, 901, 941, 961 Piezoelectric substrate 102, 902, 942, 962 IDT electrode 103, 903, 963 Reflector 104, 944 First dielectric film 105, 945 Second dielectric film 111, 911 Bus bar 112, 912 Electrode finger 113, 913 Dummy electrode finger

Claims (4)

A surface acoustic wave device comprising a piezoelectric substrate, at least one set of IDT electrodes formed on the piezoelectric substrate, and a dielectric film covering at least a part of the piezoelectric substrate and the IDT electrode,
The piezoelectric substrate is a lithium tantalate substrate having a cut angle in the range of Euler angles (-10 ° to 10 °, 125 ° to 140 °, -10 ° to 10 °),
The dielectric film includes silicon dioxide as a main component, and includes at least a first dielectric film covering a region where a plurality of electrode fingers of the IDT electrode are arranged, and silicon nitride as a main component, and the first dielectric A second dielectric film covering the body film;
A surface acoustic wave device in which the speed of sound of the surface acoustic wave propagating on the electrode finger of the IDT electrode is within a range of 4000 m / s to 4100 m / s. A surface acoustic wave device comprising a piezoelectric substrate, at least one set of IDT electrodes formed on the piezoelectric substrate, and a dielectric film covering at least a part of the piezoelectric substrate and the IDT electrode,
The piezoelectric substrate is a lithium tantalate substrate having a cut angle in the range of Euler angles (-10 ° to 10 °, 125 ° to 140 °, -10 ° to 10 °),
The dielectric film includes silicon dioxide as a main component, and includes at least a first dielectric film covering a region where a plurality of electrode fingers of the IDT electrode are arranged, and silicon nitride as a main component, and the first dielectric A second dielectric film covering the body film;
A surface acoustic wave device in which the film thicknesses H1 and H2 of the first and second dielectric films satisfy the relational expression 0.3 ≦ H2 / H1 ≦ 1.2. Normalized film thicknesses Hλ1 and Hλ2 obtained by dividing the film thicknesses of the first and second dielectric films by the wavelength λ of the surface acoustic wave excited by the IDT electrode are expressed by a relational expression 0.9 × (−0. 5. The surface acoustic wave device according to claim 1, wherein 5 × Hλ1 ² + 0.295 × Hλ1 + 0.0362) ≦ Hλ2 ≦ 1.1 × (−0.5 × Hλ1 ² + 0.295 × Hλ1 + 0.0362) is satisfied. .   3. The surface acoustic wave device according to claim 1, wherein a thickness of the first dielectric film is equal to or less than a thickness of the IDT electrode.

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