JP2006074136A - Surface acoustic wave element and surface acoustic wave device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To decrease a constant k in a current square characteristic of a surface acoustic wave element chip. <P>SOLUTION: The surface acoustic wave element chip is provided with an IDT comprising interdigital electrodes. Electrode fingers of each interdigital electrode are alternately arranged at an equal interval in parallel in the IDT. A projected area S of the electrode fingers included in a region within a cross width b of the electrode fingers onto a crystal substrate in the surface acoustic wave element chip is selected to be 0.014 mm<SP>2</SP>≤S≤0.019 mm<SP>2</SP>. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a surface acoustic wave element piece, and more particularly to a surface acoustic wave element piece and a surface acoustic wave device using a quartz crystal.

として表され、電流二乗特性と呼ばれている。数式１に示された定数κは、水晶のカット角、振動モード、水晶および電極の寸法によって決まる固有定数であるとされている（特許文献１）。そして、ＡＴカット水晶の振動片の場合、κ＝０．２×１０^−６［Ａ^−２］程度であるとされている。
特開平１０−１４５１４０号公報 段落番号０００６ It is known that the oscillation frequency (resonance frequency) of the crystal resonator element constituting the crystal resonator varies depending on the drive level, that is, the current level flowing through the crystal resonator element. The drive level characteristics of the quartz crystal resonator element are as follows: the resonance frequency is f, the frequency variation is df, and the current flowing through the crystal oscillator is I.

And is called the current square characteristic. The constant κ shown in Expression 1 is an intrinsic constant determined by the cut angle of the crystal, the vibration mode, and the dimensions of the crystal and the electrode (Patent Document 1). In the case of an AT-cut quartz crystal resonator element, it is assumed that κ = 0.2 × 10 ⁻⁶ [A ⁻² ].
Japanese Patent Laid-Open No. 10-145140, paragraph number 0006

  As shown in Equation 1, the current square characteristic has a frequency variation amount that varies depending on the value of the constant κ even when the amount of current flowing through the quartz crystal resonator element is the same. When the amount of frequency fluctuation increases, the phase noise is affected and the phase noise increases. For this reason, when an oscillator using a crystal vibrating piece having a large constant κ is used, problems such as inability to synchronize between electronic devices and inability to communicate. Therefore, it is required to make the current square characteristic constant κ as small as possible.

  By the way, in recent years, high-speed communication using a frequency in the GHz band is performed with the progress of electronic technology. In such a high frequency band, surface acoustic wave devices such as SAW filters and SAW resonators using surface acoustic wave element pieces made of quartz using surface acoustic waves (SAW) are often used. However, the surface acoustic wave element piece has a frequency characteristic worse than that of the AT-cut quartz crystal piece, and how to reduce the frequency fluctuation is one of the major problems. However, research on the current square characteristic of the surface acoustic wave element piece has not sufficiently progressed, and the conditions for reducing κ in Formula 1 are not clear.

  An object of the present invention is to reduce the constant κ in the current square characteristic of a surface acoustic wave element.

  The inventors of the present invention conducted various experiments by examining the current square characteristics of the surface acoustic wave element piece in detail, and found that the constant κ of the current square characteristic is an interdigital transducer (IDT) of the surface acoustic wave element piece. It was found that it depends on the area of the electrode. The present invention has been made based on such knowledge.

That is, in the surface acoustic wave element according to the present invention, the projected area S on the quartz substrate within the intersecting width of the electrode fingers constituting the interdigital electrode formed on the quartz substrate is 0.014 mm ² ≦ S ≦ 0.019 mm. It is characterized by ² . In the present invention configured as described above, the constant κ of the current square characteristic is set to ± 0.02 × 10 ⁻⁶ [A ⁻² ] or less and 1/10 or less of the value of κ for the AT-cut quartz crystal resonator element. be able to. Therefore, the surface acoustic wave element piece according to the present invention can greatly reduce the amount of frequency fluctuation due to the drive level.

  As the quartz substrate on which the surface acoustic wave element piece is formed, an ST-cut quartz plate with a cut angle represented by Euler angles (0 °, 113 ° to 135 °, 0 °) can be used. The quartz substrate having such a cut angle has excellent frequency-temperature characteristics, and can be a surface acoustic wave element having good frequency stability against temperature changes in the usage environment. Further, as the quartz substrate, an in-plane rotated ST-cut quartz plate with cut angles represented by Euler angles (0 °, 113 ° to 135 °, ± (40 ° to 49 °)) can be used. The quartz substrate having such a cut angle can further improve the frequency temperature characteristics. In addition, the longitudinal mode spurious of the surface acoustic wave element can be made very small.

  The surface acoustic wave device according to the present invention includes any one of the surface acoustic wave element pieces described above. Such a surface acoustic wave device can achieve the effects described above.

Preferred embodiments of a surface acoustic wave element and a surface acoustic wave device according to the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 is a plan view for explaining a main part of a surface acoustic wave element according to an embodiment of the present invention. In FIG. 1, a surface acoustic wave element piece 10 is formed from a quartz crystal substrate 12 which is a piezoelectric body. In the case of the embodiment, the quartz substrate 12 is an ST-cut quartz plate whose cut angle is Euler angle display (0 °, 113 ° to 135 °, 0 °), or whose cut angle is Euler angle display (0 °, 113 °). It consists of a so-called in-plane rotating ST-cut quartz plate that is ˜135 °, ± (40 ° -49 °). The quartz substrate 12 is formed in a rectangular shape, and an IDT 14 is provided at the center of the main surface.

  The IDT 14 is composed of a pair of comb-shaped electrodes 16 (16a, 16b) and has an interdigital shape. The electrode fingers 18 (18a, 18b) corresponding to the comb teeth of each comb-shaped electrode 16 are alternately and in parallel at equal intervals. It is arranged with. The IDT 14 generates a surface acoustic wave having a predetermined frequency on the surface layer portion of the quartz substrate 12 by applying a signal voltage between the comb electrode 16a and the comb electrode 16b. The surface acoustic wave element piece 10 is provided with reflectors 20 on both sides of the IDT 14. Each reflector 20 is formed in the same manner and has a plurality of conductor strips 22. Each reflector 20 has a lattice shape in which both ends of a plurality of conductor strips 22 are connected to each other. In the embodiment, the IDT 14 and the pair of reflectors 20 are formed from a thin film of aluminum or an aluminum alloy (hereinafter simply referred to as aluminum). That is, the IDT 14 and the reflector 20 are formed by etching an aluminum thin film formed on the surface of a quartz wafer by vapor deposition or sputtering into a predetermined shape.

  According to the research by the inventors, it has been clarified that the constant κ of the current square characteristic of the surface acoustic wave element piece 10 depends on the effective area of the electrode of the IDT 14. That is, it has been found that the constant κ is inversely proportional to the projected area of the electrode fingers 18a and 18b in the region 24 indicated by the broken line 24 in FIG. In addition, the code | symbol a shown in FIG. 1 has shown the width | variety of the electrode finger 18, and the code | symbol b has shown the crossing width of the electrode finger 18. As shown in FIG.

  The inventors produced surface acoustic wave element pieces of samples A to C shown in FIG. 2, conducted detailed experiments on these samples A to C, and obtained the relationship between the drive level and the resonance frequency. Sample A is a surface acoustic wave element piece made of an ST-cut quartz plate having a cut angle of Euler display (0 °, 113 ° to 135 °, 0 °) and having a resonance frequency of 620 MHz band. Sample A has a pair of electrode fingers n = 125, an electrode finger width a = 1.24 μm, and an electrode finger crossing width b = 49.6 μm. Sample B is a surface acoustic wave element having a resonance frequency of 1 GHz made of an ST-cut quartz plate similar to the above, and n = 180, a = 0.76 μm, and B = 30.4 μm. Sample C is an elastic surface having a resonance frequency of a 640 MHz band made of an in-plane rotating ST-cut quartz plate with a cut angle represented by Euler angles (0 °, 113 ° to 135 °, ± (40 ° to 49 °)). It is a wave element piece. Sample C has n = 250, a = 0.92 μm, and b = 36.8 μm.

である。したがって、試料Ａの投影面積Ｓは、約０．０１５ｍｍ^２、試料Ｂの投影面積Ｓは約０．００８ｍ^２、試料Ｃの投影面積Ｓは約０．０１７ｍ^２である。そして、これらの試料Ａ〜Ｃについて、ドライブレベルである弾性表面波素子片に流す電流量Ｉを０．１ｍＡ〜１．５ｍＡまで０．１ｍＡずつ変えて共振周波数ｆを繰り返し測定した。さらに、共振周波数ｆの測定結果から、Ｉ^２と（ｄｆ／ｆ）との関係を求めた。ただしｄｆは周波数変動量である。 In these surface acoustic wave elements, the projected area S of the electrode fingers 18 in the region 24 on the quartz substrate 12 is:

It is. Accordingly, the projected area S of the sample A is about 0.015 mm ² , the projected area S of the sample B is about 0.008 m ² , and the projected area S of the sample C is about 0.017 m ² . Then, for these samples A to C, the resonance frequency f was repeatedly measured by changing the current amount I flowing through the surface acoustic wave element at the drive level from 0.1 mA to 1.5 mA by 0.1 mA. Further, the relationship between I ² and (df / f) was obtained from the measurement result of the resonance frequency f. However, df is a frequency fluctuation amount.

Then, κ of the current square characteristic was calculated from the relationship between the current I ² and (df / f). As shown in FIG. 2, the obtained κ is κ = 0.018 × 10 ⁻⁶ [A ⁻² ] and κ = 0.014 × 10 ⁻⁶ [A ⁻² ] for the sample A, the sample For B, κ = 0.088 × 10 ⁻⁶ [A ⁻² ] and κ = 0.043 × 10 ⁻⁶ [A ⁻² ], and for Sample C, κ = −0.008 × 10 ⁻⁶ [ A ⁻² ] and κ = −0.003 × 10 ⁻⁶ [A ⁻² ] were obtained. Therefore, when the relationship between the projected area S and the constant κ is obtained, the relationship as shown in FIG. 3 is obtained.

As apparent from FIG. 3, in the case of the surface acoustic wave element piece 10, the projected area S of the electrode finger 18 on the crystal substrate 12 in the region 24 shown in FIG. 1 is set to 0.014 to 0.019 mm ² . Thus, κ = ± 0.02 × 10 ⁻⁶ [A ⁻² ] and 1/10 or less of the AT-cut quartz crystal vibrating piece can be achieved. Therefore, the surface acoustic wave element piece 10 in which the projected area S of the electrode finger 18 in the region 24 on the quartz substrate 12 is 0.014 to 0.019 mm ² can greatly reduce the frequency change due to the drive level. A surface acoustic wave device such as a highly accurate SAW resonator can be obtained.

  In addition, if the ST cut quartz plate whose cut angle is Euler angle display (0 °, 113 ° to 135 °, 0 °) is used for the quartz substrate 12, a surface acoustic wave element having excellent frequency temperature characteristics can be obtained. It is possible to obtain a surface acoustic wave element with good frequency stability against temperature changes in the usage environment. Furthermore, by using an in-plane rotated ST-cut quartz plate with a cut angle (0 °, 113 ° to 135 °, ± (40 ° to 49 °)) as a crystal substrate 12 as a Euler angle display, the frequency temperature is further increased. A surface acoustic wave element having excellent characteristics can be obtained. Further, the longitudinal mode spurious of the surface acoustic wave element can be made very small.

It is a top view which shows the principal part of the surface acoustic wave element piece which concerns on embodiment. It is explanatory drawing of the sample which concerns on embodiment of this invention. It is a figure which shows the relationship between the projection area of the electrode finger of the sample which concerns on embodiment, and the constant (kappa).

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ......... Surface acoustic wave element piece, 12 ...... Quartz substrate, 14 ...... IDT, 16a, 16b ...... Comb-shaped electrode, 18a, 18b ...... Electrode finger, a ...... Width of electrode finger , B ... Crossing width.

Claims (4)

A surface acoustic wave element having a projected area S on the quartz substrate within the intersection width of the electrode fingers constituting the interdigital electrodes formed on the quartz substrate is 0.014 mm ² ≦ S ≦ 0.019 mm ^2. . The surface acoustic wave element piece according to claim 1,
2. The surface acoustic wave element according to claim 1, wherein the quartz substrate is an ST cut quartz plate having a cut angle represented by Euler angles (0 °, 113 ° to 135 °, 0 °). The surface acoustic wave element piece according to claim 1,
The surface acoustic wave device is characterized in that the quartz substrate is an in-plane rotating ST-cut quartz plate whose cut angle is expressed by Euler angle (0 °, 113 ° to 135 °, ± (40 ° to 49 °)). Piece.   A surface acoustic wave device comprising the surface acoustic wave element according to claim 1.

Images