JP2006295311A - Surface acoustic wave element chip and surface acoustic wave device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide technology capable of keeping the frequency of spurious radiation caused in a longitudinal mode at a distance from the principal resonant frequency by a prescribed frequency or over when exciting a crystal plate in an upper limit mode of the stop band. <P>SOLUTION: IDTs each comprising corded electrodes are provided to the front side of the crystal plate in the surface acoustic wave element chip. The cut angle of the crystal plate is expressed in the Euler's angle representation as (0°, θ, 0°≤¾ψ¾≤90°). The number of pairs of electrode digits in the surface acoustic wave element chip is made to be 140 or below in the case of H/λ=0.09, the number of pairs of the electrode digits is made to be 130 or below in the case of H/λ=0.10, and the number of pairs of the electrode digits is made to be 80 or below in the case of H/λ=0.11, wherein λ is the wavelength of a surface acoustic wave generated in the crystal plate by the IDTs, and H is the film thickness of the electrode digits of the IDTs. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a surface acoustic wave element piece using surface acoustic wave (SAW), and more particularly to a surface acoustic wave element piece and a surface acoustic wave device in which a piezoelectric substrate is made of quartz.

  Surface acoustic wave devices such as SAW resonators and SAW filters are used in various electronic devices because they are easy to cope with high frequencies, and are small and mass-productive. In particular, recently, it has been widely used in the communication field with an increase in communication speed, a reduction in the size and frequency of communication devices. Such a surface acoustic wave device includes a surface acoustic wave element that generates surface acoustic waves on a piezoelectric substrate by interdigital electrodes provided on the surface of the piezoelectric substrate.

  In particular, a surface acoustic wave element piece using a quartz substrate typified by ST cut can be a highly accurate surface acoustic wave element piece because quartz has high temperature stability. However, in recent years, portable communication devices that are remarkably popular have demanded surface acoustic wave devices with higher frequencies, smaller sizes, and higher accuracy that are stable with respect to temperature. Therefore, the present applicant has developed a surface acoustic wave element using an in-plane rotating ST-cut quartz plate (for example, Patent Document 1). Since the in-plane rotating ST-cut quartz plate has a temperature characteristic of a cubic function, it can be a highly accurate surface acoustic wave element piece as compared with a normal ST-cut quartz plate.

  By the way, when an IDT (Interdigital Transducer) composed of interdigital electrodes is provided on a piezoelectric substrate such as quartz and the Rayleigh wave, which is a surface acoustic wave, is generated by exciting the piezoelectric substrate, the lower limit of a region called a stop band is calculated. It is known that a frequency solution can be obtained at the upper limit and the upper limit. Comparing the lower limit frequency of the stopband (lower limit mode) and the upper limit frequency (upper limit mode), the upper limit mode has a smaller absolute value of the secondary temperature coefficient of the frequency temperature characteristic than the lower limit mode, and the electrode film thickness of the IDT It is known that the change in the absolute value of the secondary temperature coefficient is small when the value is increased. Therefore, if the piezoelectric substrate can be excited in the upper limit mode of the stop band, it is advantageous for increasing the frequency of the surface acoustic wave element, and high accuracy can be achieved.

  Therefore, the inventors of the present invention have made extensive studies and made various studies. As a result, when the Euler angle display is (φ, θ, ψ), the angle of the crystal plate is (0 °, θ, ψ). It was found that Rayleigh waves can be excited in the upper limit mode of the stop band by appropriately selecting ψ. FIG. 6 shows the frequency characteristics in the upper limit mode of the stop band of a surface acoustic wave element using a quartz plate with Euler angles (0 °, 123 °, 42 °) as a substrate. In FIG. 6, the horizontal axis represents frequency (MHz), and the vertical axis represents impedance (Ω).

As shown in FIG. 6, the spurious S ₁ due to the longitudinal mode appears near the high frequency side of the main resonance (main vibration) S ₀ , and the spurious S ₂ appears further on the high frequency side of the spurious S ₁ . Since the spurious S ₁ is close to the main resonance S ₀ , the characteristics are degraded by the spurious S ₁ when used in a surface acoustic wave device such as a resonator or a filter. For this reason, it is necessary to keep the spurious S ₁ away from the main resonance S ₀ . In Patent Document 2, 36 ° Y-cut X-propagation LiTaO ₃ is used as the piezoelectric substrate, and the number of IDTs of the high-frequency surface acoustic wave resonator filter and the low-frequency surface acoustic wave resonator filter (of the electrode finger) The logarithm) is made different, and the position of the spurious is controlled to obtain the optimum filter characteristic.
JP 2003-152487 A JP 2000-59176 A

By the way, the relationship between the number of IDT electrode fingers and the position of the spurious relative to the target frequency (main resonance frequency) varies depending on the type of piezoelectric substrate, the cut angle of the piezoelectric substrate, and the like. For this reason, even if the logarithm of the electrode finger of IDT provided in 36 ° Y-cut X-propagating LiTaO ₃ as in Patent Document 2 is applied as it is to a crystal plate with cut angles (0 °, 123 °, 41 °). A surface acoustic wave element having desired characteristics is not necessarily obtained.
An object of the present invention is to enable the spurious due to the longitudinal mode to be separated from the main resonance by a predetermined frequency or more when the quartz plate is excited in the upper limit mode of the stop band.

  In order to achieve the above object, a surface acoustic wave element piece according to the present invention is a surface acoustic wave element piece in which interdigital electrodes are provided on the surface of a piezoelectric substrate, and the piezoelectric substrate displays an Euler angle display. When (φ, θ, ψ), the cut angle is (0 °, θ, 0 ° ≦ | ψ | ≦ 90 °), and the wavelength of the surface acoustic wave generated on the crystal plate is λ, where H is the film thickness of the electrode fingers constituting the interdigital electrode, H / λ = 0.09, the number of pairs of electrode fingers is 140 or less, and H / λ = 0.10 The number of pairs of electrode fingers is 130 or less, and when H / λ = 0.11, the number of pairs of electrode fingers is 80 or less. θ may be 0 ° or more and 180 ° or less, but is more preferably 95 ° or more and 155 ° or less.

The present invention thus configured can generate an upper limit mode in the stopband of the Rayleigh wave on the quartz plate, and can keep the spurious frequency due to the longitudinal mode away from the main resonance frequency by 500 ppm or more. Note that ppm is (spurious frequency−main resonance frequency) ÷ main resonance frequency, and represents a deviation amount of the spurious frequency with respect to the main resonance frequency.
A surface acoustic wave device according to the present invention includes the above-described surface acoustic wave element. The surface acoustic wave device according to the present invention thus configured can reduce the influence of spurious due to the longitudinal mode, and can avoid deterioration of characteristics.

Preferred embodiments of a surface acoustic wave element piece and a surface acoustic wave device according to embodiments of the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 is a plan view schematically showing a surface acoustic wave element according to an embodiment of the present invention, and FIG. 2 is a partial sectional view taken along the line AA of FIG. In these figures, the surface acoustic wave element piece 10 is composed of a rectangular crystal plate 12 which is a piezoelectric substrate, and an IDT 14 is formed at the center of the surface of the crystal plate 12. The surface acoustic wave element piece 10 is provided with a pair of reflectors 16. The pair of reflectors 16 are provided along the propagation direction of the surface acoustic wave excited by the IDT 14 and are disposed with the IDT 14 interposed therebetween.

  In the embodiment, the crystal plate 12 has a cut angle of (0 °, θ, 0 ° ≦ | ψ | ≦ 90 °) when the Euler angle display is (φ, θ, ψ). That is, the quartz plate 12 has a cut angle as shown in FIG. 3 in the quartz crystal. This cut angle is obtained as follows. The quartz crystal Z plate 18 is perpendicular to the Z axis with Euler angles (0 °, 0 °, 0 °), where the electrical axis of the crystal is the X axis, the mechanical axis is the Y axis, and the optical axis is the Z axis. It becomes a crystal plate. When this quartz crystal Z plate 18 is rotated counterclockwise by an angle θ about the X axis as a rotation axis, newly obtained coordinate axes are taken as an X axis, a Y ′ axis, and a Z ′ axis. In this XY′Z ′ coordinate system, a new coordinate axis obtained by rotating the quartz crystal plate 20 parallel to the plane formed by the X axis and the Y ′ axis by an angle ψ about the Z ′ axis is defined as the X ′ axis, The Y ″ axis and the Z ′ axis. In this X′Y ″ Z ′ coordinate, the crystal plate parallel to the plane formed by the X ′ axis and the Y ″ axis is the Euler angle (0 °, θ, ψ) crystal plate. It becomes.

  The quartz crystal plate 12 according to the embodiment is a quartz crystal plate having a rotation angle ψ with the Z ′ axis as the rotation center of 0 ° to 90 °. Note that the rotation angle ψ about the Z ′ axis can be ± (0 ° to 90 °) because the quartz crystal is symmetric regardless of which direction the crystal is rotated about the Z ′ axis. That is, 0 ° ≦ | ψ | ≦ 90 °. Further, the rotation angle θ about the X-axis is preferably 0 ° ≦ θ ≦ 180 ° according to experiments by the inventors. More desirably, the crystal is 95 ° ≦ θ ≦ 155 °, 33 ° ≦ | ψ | ≦ 46 °, that is, a crystal having Euler angles (0 °, 95 ° ≦ θ ≦ 155 °, 33 ° ≦ | ψ | ≦ 46 °). It is more desirable to use a plate.

  The IDT 14 of the surface acoustic wave element piece 10 includes a pair of comb-shaped electrodes 22 (22a, 22b). Each comb-shaped electrode 22 includes a plurality of electrode fingers 26a and 26b each having one end connected to a bus bar 24 (24a and 24b). Each electrode finger 26 (26a, 26b) is formed along the Y ″ axis of the quartz plate 12. And, the IDT 14 has an interdigital shape. That is, the IDT 14 is a comb of each comb-shaped electrode 22. The electrode fingers 26 corresponding to the teeth are alternately arranged in parallel and at equal intervals so as to engage with each other, and the IDT 14 is applied with a signal voltage between the comb electrode 22a and the comb electrode 22b. Thus, a surface acoustic wave having a predetermined frequency is generated on the surface layer portion of the quartz plate 12. The surface acoustic wave propagates along the X ′ axis of the quartz plate 12 orthogonal to the electrode finger 26.

  Each reflector 16 includes a plurality of conductor strips 28 whose ends are connected to each other, and has a lattice shape. In the embodiment, the pair of reflectors 16 and the IDT 14 are formed from a thin film of aluminum or an aluminum alloy. That is, the IDT 14 and the reflector 16 are formed by photo-etching an aluminum or aluminum alloy thin film formed on the surface of a quartz wafer by vapor deposition or sputtering into a predetermined shape. The IDT 14 is electrically connected to a connection pad (not shown in FIG. 1).

The electrode fingers 26a and 26b of the comb-shaped electrodes 22a and 22b have a formation interval (pitch) of p as shown in a partial sectional view in FIG. The wavelength λ of the surface acoustic wave generated on the quartz plate 12 by the IDT 14 depends on the formation pitch p of the electrode fingers 26 as is well known. Further, in the case of the embodiment, the IDT 14 has a different number of pairs of electrode fingers 26a and 26b depending on the thickness of each electrode finger 26 (electrode film thickness). This is because the frequency deviation between the main resonance S ₀ and the spurious S ₁ depends on the electrode film thickness H and the number of pairs of electrode fingers.

According to the research by the inventors, when the electrode film thickness of the electrode finger 26 is H and the wavelength of the surface acoustic wave is λ, the logarithm of the electrode finger 26 and the spurious S ₁ are obtained when the film thickness ratio is H / λ. frequency deviation from the main resonance S ₀ is as shown in FIG. 4, the horizontal axis is the logarithm of the electrode finger 26, the vertical axis is the frequency deviation from the main resonance S ₀ spurious S ₁ shown in ppm. Here, f ₀ is the frequency of the main resonance S ₀ , and Δf is Δf = f _S −f ₀ , where f _S is the frequency of the spurious S ₁ . Also, the solid curve A in FIG. 4 is H / λ = 0.09 (= 9%), and the one-dot chain line curve B is H / λ = 0.10 (= 10%). The curve C of H / λ = 0.11 (= 11%). The quartz plate 12 used is a Euler angle (0 °, 123 °, 42 °) quartz plate, the width of the electrode finger 26 of the IDT 14 is B (see FIG. 2), and the pitch between the electrodes is p /. In the case of 2, η = B / (p / 2) = 0.7. The inter-electrode pitch p / 2 is 4.89 μm. The IDT 14 is made of aluminum.

As shown in FIG. 4, in order to reduce the influence of the spurious S ₁ , the frequency deviation amount Δf / f _{0 is set} to 500 ppm or more. When H / λ = 0.09, the electrode finger 26 needs to be 140 pairs or less. When H / λ = 0.10, it is necessary to have 130 pairs or less, and when H / λ = 0.11 it is necessary to have 80 pairs or less. As a result, the frequency of the spurious S ₁ can be separated from the frequency of the main resonance S ₀ by 500 ppm or more, and the surface acoustic wave element piece 10 having a high accuracy with little influence of the spurious S ₁ can be obtained.

  FIG. 5 is a cross-sectional view showing an example of a surface acoustic wave device according to an embodiment of the present invention, and is a cross-sectional view of a SAW resonator. In FIG. 5, the SAW resonator 30 has a package body 32 made of ceramic or the like. The package body 32 is formed in a box shape having an open upper end, and the surface acoustic wave element piece 10 is fixed to the bottom surface of the package body 32 with an adhesive. The surface acoustic wave element piece 10 is provided with connection pads 34 electrically connected to the IDT 14 on both sides of the IDT 14. These connection pads 34 are connected to electrodes 38 provided on the package body 32 via bonding wires 36. In the SAW resonator 30, a lid body 40 made of ceramic, glass, metal, or the like is disposed on the upper end of the package body 32 that houses the surface acoustic wave element piece 10, and the package body 32 is hermetically sealed by a sealing material (not shown). Sealed.

It is the top view which showed typically the surface acoustic wave element piece which concerns on embodiment. FIG. 2 is a partial cross-sectional view taken along line AA in FIG. 1. It is a figure explaining the cut angle of the crystal plate which concerns on embodiment. It is a figure which shows the relationship between the logarithm of the electrode finger | toe of the surface acoustic wave element piece which concerns on embodiment, and the deviation amount from the main resonance frequency of a spurious frequency. It is sectional drawing which shows an example of the surface acoustic wave apparatus which concerns on embodiment. It is a figure which shows the frequency characteristic of a surface acoustic wave element piece.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ......... Surface acoustic wave element piece, 12 ...... Quartz plate, 14 ...... IDT, 16 ...... Reflector, 22a, 22b ...... Comb-shaped electrode, 26a, 26b ...... Electrode finger, 30 .... Surface acoustic wave device (SAW resonator).

Claims (2)

A surface acoustic wave element piece in which interdigital electrodes are provided on the surface of a piezoelectric substrate,
The piezoelectric substrate is made of a quartz plate having a cut angle of (0 °, θ, 0 ° ≦ | ψ | ≦ 90 °) when Euler angle display is (φ, θ, ψ),
When the wavelength of the surface acoustic wave generated on the quartz plate is λ, and the film thickness of the electrode fingers constituting the interdigital electrode is H,
When H / λ = 0.09, the number of pairs of electrode fingers is 140 or less,
When H / λ = 0.10, the number of pairs of electrode fingers is 130 or less,
When H / λ = 0.11, the number of pairs of electrode fingers is 80 or less,
Is,
A surface acoustic wave element having the above structure.   A surface acoustic wave device comprising the surface acoustic wave element according to claim 1.

Images