JP2001111376A - Surface acoustic wave element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To constitute a transmission type surface acoustic wave filter with a low loss. SOLUTION: This surface acoustic wave element has a surface acoustic wave converter constituted of a positive electrode finger 102 and a negative electrode finger 204 and a floating electrode 300 arranged between them which are formed on the surface of langisite single crystal substrate whose substrate azimuth and surface acoustic wave propagating direction are selected so that natural uni-directivity can be obtained. When the wavelength of the surface acoustic wave is defined as λ, the width of the positive electrode finger and the negative electrode finger is set so as to be about λ/8, and the central interval of the both electrode fingers is set so as to be about 6/8λ, and a distance (g) between the center of the positive electrode finger and the center of the floating electrode 300 is set so as to be 13/40λ<=g<=14/40λ. In this case, each electrode is formed along the propagating direction of the surface acoustic wave so that width W of the floating electrode can be set so as to be 11/40λ<=W<=13/40λ.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a surface acoustic wave device used for mobile communication equipment and the like.

[0002]

2. Description of the Related Art In recent years, mobile communication devices such as mobile phones and mobile terminals have been widely used. Filters used in these terminals are required to have characteristics such as low loss, wide band, and small size. As a device satisfying the above characteristics, a transmission type surface acoustic wave (SAW) filter having a single-phase unidirectional converter has been put to practical use. In a single-phase unidirectional filter,
The phase difference between the excitation wave and the reflected wave is in phase in the forward direction (forward direction), and the two waves strengthen each other. In the opposite direction (reverse direction), the two waves cancel each other out, so the surface acoustic wave is strong only in the forward direction. Excited. This allows the transmission electrode and the reception electrode to face each other in one direction, theoretically 1 dB.
The following low-loss filter can be realized.

[0003] As a technique for realizing a unidirectional converter,
EWC-SPUDT and DART-SPUDT using an asymmetric electrode structure have been devised. In addition to these filters using the asymmetry of the electrode structure, a natural unidirectional filter (NSPUDT: N
atural Single Phase Uni d irecitonal Transduce
r). The natural one-way filter realizes one-way by utilizing the asymmetry of the substrate crystal. For this reason, a regular interdigital transducer (ID
T) The electrode width and the electrode interval are both λ /
Unidirectionality can be realized with a converter having a structure in which a plurality of positive and negative electrode fingers 4 are periodically and continuously arranged.

[0004] Even when a regular IDT is formed on an ST-X quartz substrate, surface acoustic waves generated by driving the regular IDT are excited by the regular IDT on the ST-X quartz substrate.
It propagates in both directions of T and cannot realize one-way. That is, the natural one-way characteristic indicates a characteristic of a substrate in which a surface acoustic wave is strongly excited in one direction when a regular type IDT is formed on the surface of a piezoelectric substrate. In the surface acoustic wave converter using the natural unidirectional substrate, since the anisotropy of the substrate itself is used, it is not possible to face the forward direction of the transmitting converter and the receiving converter. Unless the transmitting and receiving electrodes can face each other in one direction, it is impossible to produce a low-loss filter.

As a means for solving this problem, an electrode structure for reversing the natural unidirectional direction by Takeuchi et al. Is disclosed in Japanese Unexamined Patent Publication No. 8-125484. There has been proposed a surface acoustic wave converter composed of positive and negative electrode fingers and floating electrodes having an electrode width of 3 / 8λ arranged between the electrode fingers at an edge interval of approximately λ / 8.

[0006]

The characteristics of a surface acoustic wave device depend on the characteristics of a piezoelectric crystal used as a substrate. It is important for the piezoelectric crystal to have a large electromechanical coupling coefficient and good frequency temperature characteristics. At present, langasite is attracting attention as a crystal that satisfies these two characteristics simultaneously. −5 ° ≦ φ ≦ 5 °, 1 when Euler angle is expressed as (φ, θ, ψ)
Langasite within the range of 35 ° ≦ θ ≦ 145 ° and 20 ° ≦ ψ ≦ 30 ° has an electromechanical coupling coefficient of 0.3% to 0.1%.
4%, the frequency-temperature characteristic shows second-order dependence, and a peak temperature exists near room temperature. The electromechanical coupling coefficient is about three times that of ST quartz, and the second-order temperature coefficient in frequency temperature characteristics is about twice that of quartz, which is very good, and is expected to be applied to low-loss surface acoustic wave filters. Crystal.

The langasite single crystal within the above range in the Euler angle display has NSPUDT characteristics. To realize a low-loss filter using this substrate, it is necessary to use an electrode in which the transmitting and receiving electrodes are unidirectionally opposed. The structure must be constructed. Therefore, when a normal type IDT in which a plurality of positive and negative electrode fingers whose electrode width and electrode interval are both λ / 4 is periodically and continuously arranged is used for the transmitting electrode, the receiving electrode has a unidirectionality. Although an inverted structure must be used, the electrode structure proposed by Takeuchi et al. Cannot meet the demand for lower loss of the filter. The present invention has been made in view of such circumstances, and it is an object of the present invention to provide a surface acoustic wave device capable of forming a transmission-type surface acoustic wave (SAW) filter with lower loss. .

[0008]

In order to achieve the above object, the present invention is directed to a langasite single crystal having a substrate orientation and a surface acoustic wave propagation direction selected to have a natural unidirectionality. A surface acoustic wave device having a surface acoustic wave converter formed on a substrate surface and including a positive electrode finger, a negative electrode finger, and a floating electrode disposed therebetween, wherein the surface acoustic wave converter is naturally unidirectional. Each of the electrodes is formed along the propagation direction of the surface acoustic wave so that is inverted.

According to a second aspect of the present invention, in the surface acoustic wave device according to the first aspect, the langasite single crystal substrate has a substrate orientation, a substrate orientation, and a surface acoustic wave propagation direction expressed in Euler angles ( −5 when φ, θ, ψ)
° ≦ φ ≦ 5 °, 135 ° ≦ θ ≦ 145 °, 20 ° ≦ ψ ≦
It is characterized by being in the range of 30 ° or a crystallographically equivalent orientation.

According to a third aspect of the present invention, in the surface acoustic wave device according to the second aspect, the distance relationship between the positive electrode finger, the negative electrode finger, and the floating electrode in the surface acoustic wave converter is a surface elastic wave element. Assuming that the wavelength of the wave is λ, the width of the positive electrode finger and the negative electrode finger is approximately λ / 8, the center interval between the two electrode fingers is approximately 6 / 8λ, and the center of the positive electrode finger and the center of the floating electrode are set. The distance g is 13 / 40λ ≦ g ≦ 14/4
0λ, and the width W of the floating electrode is 11 / 40λ ≦ W ≦
13 / 40λ.

[0011]

Embodiments of the present invention will be described below in detail with reference to the drawings. First, on a langasite piezoelectric substrate, a so-called regular electrode (regular IDT) in which a plurality of positive and negative electrode fingers whose electrode width and electrode interval are both λ / 4 are periodically and continuously arranged, is formed. The principle of having a natural one-way property when the excitation drive is performed will be described with reference to FIG. FIG. 1 shows a schematic diagram of a regular electrode.
In FIG. 1, the normal electrode includes a positive electrode 1 and a negative electrode 2, and a positive electrode finger 1A constituting the positive electrode 1 and a negative electrode 2 constituting left and right sides of the positive electrode finger 1A. An electric field is generated between the electrode fingers 2A and 2B. At this time, the excitation center of the surface acoustic wave generated on the langasite piezoelectric substrate by being excited by this electric field becomes substantially the center A of the positive electrode finger 1A.

In this electrode structure, periodically arranged electrode fingers having an electrode width of λ / 4 serve as a reflection source of surface acoustic waves. Since the reflection is caused by the discontinuity of the acoustic impedance, the surface acoustic wave is reflected at the end of each electrode finger. As described above, the surface acoustic waves are reflected at the two positions at both ends of the electrode finger, but there is no problem in that the surface acoustic wave is equivalently reflected at the center of the electrode finger. At this time, the phase of the reflected wave changes. The amount of change depends on the type of the piezoelectric substrate, the cut surface thereof, the propagation direction of the surface acoustic wave, and the electrode material and its thickness. For example, when ST-cut X-propagating quartz crystal is used for the piezoelectric substrate and Al is used as the metal material, the phase of the reflected wave is delayed by 90 °, that is, the amount of phase change is 90 °.

On the other hand, as a piezoelectric crystal, the substrate direction and the surface acoustic wave propagation direction are expressed in Euler angles (φ, θ, ψ).
-5 ° ≦ φ ≦ 5 °, 135 ° ≦ θ ≦ 145
°, 20 ° ≤ ψ ≤ 30 °, or a langasite single crystal having a crystallographically equivalent orientation as a substrate, and using Al as an electrode material to form a normal type IDT.
Is formed, the phase change amount of the surface acoustic wave reflected by the electrode finger is -90 + 2α. When this 2α is considered as a phase shift at the time of reflection, if the reflection center is defined as being shifted from the center of the electrode finger by an amount corresponding to 2α, the shift δ of the reflection center becomes

(Equation 1) Becomes When δ is positive, the reflection center is shifted to the right from the center of the electrode finger, and when δ is negative, the reflection center is shifted to the left.

When the magnitude of deviation between the reflection center and the center of the electrode finger is λ / 8, the wave excited by the positive electrode finger 1A and the reflection center B of the adjacent negative electrode finger 2A and the reflection center B of the positive electrode finger 1A Considering the phase at the point A of the wave reflected at the end C with reference to FIG.
The phase at point A of the wave reflected on the path A → B → A is

(Equation 2) And are in phase with the excitation wave. On the other hand, A → C
→ The phase at point A of the wave reflected on the path of A is

(Equation 3) Which is in antiphase with the excitation wave. For this reason, the surface acoustic wave is strongly excited in the right direction in FIG. 1, and the unidirectionality is realized.

From the above, as shown in FIG. 2, the distance between the excitation center and the reflection center is

(Equation 4) Then, it becomes possible to realize unidirectionality in the direction from the excitation center to the reflection center. That is, when a periodic electrode structure (IDT) capable of exciting surface acoustic waves is formed on an arbitrary crystal, whether the surface acoustic wave converter has one-sidedness depends on the positions of the excitation center and the reflection center. If it can be identified, it can be determined. The positions of the excitation center and the reflection center are described by mode coupling parameters when the mode coupling theory is used.

The mode coupling parameters include a self-coupling coefficient, an inter-mode coupling coefficient, an excitation coefficient, and a capacitance C. Here, the coupling coefficient between modes is

(Equation 5) Where the phase component corresponds to the shift of the reflection center from the reference plane, and the magnitude of the shift is expressed by equation (1). Also,
The excitation coefficient ζ

(Equation 6) And from the reference plane

(Equation 7) However, it can be considered that there is an excitation center at a distance. Therefore, in order for the difference between the reflection center and the excitation center to satisfy Equation (4), the phase between the inter-mode coupling coefficient and the excitation coefficient ζ

(Equation 8) It is only necessary that there be a relationship.

Here, Takeuchi discloses a technique disclosed in Japanese Unexamined Patent Publication No.
No. 4 publication discloses a unidirectional inversion electrode structure (TCS
-RDT: Translation Center Shift type Reversal of Di
A result obtained by analyzing the positions of the excitation center and the reflection center in the electrode structure of the surface acoustic wave device according to the embodiment of the present invention by the mode coupling theory is shown. The cut surface and propagation direction of the Langasite substrate shown here are represented by Euler angles (0 °, 140 °, 24 °)
It is. In addition, Al is used as an electrode material. Figure 3 shows T
FIG. 4 shows a CS-RDT structure and an electrode structure of a surface acoustic wave device according to an embodiment of the present invention.

In FIG. 3, the electrodes of the TCS-RDT structure include a positive electrode 10 and a negative electrode 20, and the positive electrode fingers 12 and 14 constituting the positive electrode 10 and the negative electrode fingers 22 constituting the negative electrode 20. , 24 have an electrode width of λ / 8, and the center distance between the positive electrode finger 12 and the negative electrode finger 24 is 6λ / 8. The floating electrode 30 provided between the positive electrode finger 12 and the negative electrode finger 24 has an electrode width of 3λ / 8, and the center distance g between the positive electrode finger 12 and the floating electrode 30 is 3λ / 8. .

On the other hand, the electrodes of the surface acoustic wave converter used in the surface acoustic wave device according to the embodiment of the present invention are:
As shown in FIG. 4, the positive electrode fingers 102 and 104, which include the positive electrode 100 and the negative electrode 200 and constitute the positive electrode 100.
And negative electrode fingers 202 and 204 constituting the negative electrode 200
Have an electrode width of λ / 8, and the center distance between the positive electrode finger 102 and the negative electrode finger 204 is 6λ / 8. The floating electrode 300 provided between the positive electrode finger 102 and the negative electrode finger 204 has an electrode width of 11λ / 40,
The center distance g between the floating electrode 2 and the floating electrode 300 is 13λ / 40. 3 and 4, both the excitation coefficient ζ and the reference plane of the phase of the inter-mode coupling coefficient are negative electrode fingers 2 having a λ / 8 width.
4, 204, the center.

FIG. 5 shows the dependence of the phase term 2α on the phase term 2α of the inter-mode coupling coefficient of the TCS-RDT structure and the electrode structure of the surface acoustic wave converter used in the surface acoustic wave device according to the embodiment of the present invention. FIG. 6 shows the electrode thickness dependence of the phase term β of the excitation coefficient の of the TCS-RDT structure and the electrode structure of the present invention, respectively. FIG. 7 shows the electrode thickness dependence of the phase difference (α-β) between the excitation coefficient and the inter-mode coupling coefficient. Phase difference (α-β)
A negative sign means that the unidirectional direction of the TCS-RDT structure is opposite to the natural unidirectional direction. From this result, in the TCS-RDT structure, the normalized electrode thickness H / λ (H
Is the electrode thickness) between 0 and 0.05, and the size of the
° remained between -30 ° from the vicinity, it is at an angle to optimize the unidirectional As is apparent from equation (8) - does not reach 45 °. In contrast, by using the electrode structure according to the embodiment of the present invention, when the normalized film thickness is about 0.013, the value of the phase difference (α-β) optimizes the unidirectionality.
- It can be seen that the 45 °.

FIG. 8 shows the dependence of the positions of the excitation center and the reflection center on the electrode film thickness for the TCS-RDT structure (FIG. 3) based on the results of FIGS. 3 and 4, and FIG. FIG. 9 shows the electrode thickness dependence of the positions of the excitation center and the reflection center with respect to the electrode structure (FIG. 4) of the surface acoustic wave converter used in the surface acoustic wave device according to the first embodiment. 8 and 9, a plan view of the electrode structure is shown in the upper part, and a cross-sectional view of the electrode structure is shown in the lower graph in order to clarify the positional relationship in the propagation direction of the surface acoustic wave. Also in these figures,
The center of reflection is indicated by ○, and the center of excitation is indicated by ×.

As shown in FIG. 9, in the electrode structure of the surface acoustic wave transducer used in the surface acoustic wave device according to the embodiment of the present invention, the reflection center exists on the left side with respect to the excitation center, Since the difference between the distances is approximately λ / 8, the one-way direction is on the left side of the paper, and it can be seen that the one-way direction is reversed with respect to the natural one-way direction. Note that the distance relationship between the positive electrode finger, the negative electrode finger, and the floating electrode in the surface acoustic wave converter is such that when the wavelength of the surface acoustic wave is λ, the width of the positive electrode finger and the negative electrode finger is approximately λ / 8
Thus, the center distance between the two electrode fingers is approximately 6 / 8λ, and the distance g between the center of the positive electrode finger and the center of the floating electrode is 13 / 40λ.
≦ g ≦ 14 / 40λ, and the width W of the floating electrode is 11
If / 40λ ≦ W ≦ 13 / 40λ, the unidirectionality can be reversed with respect to the natural unidirectional direction.

Next, two types of transmission type surface acoustic wave filters constructed using the electrode structure of the surface acoustic wave converter used in the surface acoustic wave device according to the embodiment of the present invention were prototyped,
The results of evaluating the characteristics are shown. The cut surface and propagation direction of the used Langasite substrate are represented by Euler angles (0 °, 14 °).
0 °, 24 °). In addition, Al was used as an electrode material. FIG. 10 shows the configuration of a first transmission type surface acoustic wave filter (referred to as filter # 1) as a sample under test. In the figure, a normal type IDT 310 as a transmission electrode and an IDT 320 as a reception electrode are provided on the langasite substrate 300 along the propagation direction (+ X direction) of the surface acoustic wave. The regular type IDT 310 has a positive electrode 312.
And the negative electrode 314 are formed so that a plurality of positive electrode fingers 313 and a plurality of negative electrode fingers 315 whose electrode width and electrode interval are both λ / 4 are periodically and continuously arranged, and utilizing the NPUDT characteristic. It is one-way.

The IDT 320 as a receiving electrode uses the electrode structure of the present invention, and includes a positive electrode 322 and a negative electrode 32.
4 and the floating electrode 330. Here, the positive electrode finger 323
And the electrode width of the negative electrode finger 325 is λ / 8 and both electrode fingers 3
23, 325 becomes 6λ / 8, and the positive electrode finger 3
23 and the distance g between the centers of the floating electrodes 330 is 13λ /
40, and the width W of the floating electrode 330 is 11λ / 40.
It is. The structure of this receiving electrode is the same as the structure shown in FIG.

The second transmission type surface acoustic wave filter (hereinafter referred to as filter # 2) as a test sample uses the same normal type IDT as the first transmission type surface acoustic wave filter for the transmission electrode and receives the same. The electrode has an IDT of the TCS-RDT structure shown in FIG.
Was used. As shown in FIG. 10, both filters are arranged so that the one-way characteristics of the transmitting and receiving electrodes are opposed to each other. A damper agent 340 for absorbing the reflection of the surface acoustic wave at the end is applied to both ends of the langasite substrate 300. The cycle length λ of the electrode fingers of the filters # 1 and 2 is 3
The electrode Al film thickness is 5000 ° at 2.15 μm. The transmitting and receiving electrodes are weighted by thinning.

FIGS. 11 and 12 show the measurement results of the frequency characteristics of the filters # 1 and # 2. FIG. 12 shows the frequency characteristic shown in FIG.
It is the figure which expanded. It can be seen from FIGS. 11 and 12 that the pass band insertion loss, in-band ripple, and in-band delay ripple of the filter of the present invention are improved. Specifically, as shown in Table 1, the passband insertion loss of the filter # 1 is-
Whereas the filter # 2 is 8.0 dB, the filter # 2 is -9.0.
and the in-band ripple was 0.24 for filter # 1.
In contrast to dB, it is 0.58 in the filter # 2. The filter # 1 has 69.5 in-band delay ripples.
80.0 nsec for filter # 2
It is.

[0027]

As described above, according to the present invention, according to the present invention, it is possible to form a positive electrode on a langasite single crystal substrate whose substrate orientation and surface acoustic wave propagation direction are selected so as to have a natural unidirectionality. A surface acoustic wave device having a surface acoustic wave converter comprising an electrode finger, a negative electrode finger, and a floating electrode disposed therebetween, wherein the surface acoustic wave converter has a surface acoustic wave such that a natural unidirectionality is reversed. The respective electrodes are formed along the propagation direction, so parameters for specifying the electrode structure, that is, the width of the positive electrode finger and the negative electrode finger, the center distance between the positive electrode finger and the negative electrode finger, By appropriately selecting the center distance between the positive electrode finger and the floating electrode and the width of the floating electrode, a low-loss transmission type surface acoustic wave filter can be configured.

[Brief description of the drawings]

FIG. 1 is a plan view showing an electrode structure of a regular type IDT.

FIG. 2 is an explanatory diagram showing a positional relationship between an excitation center and a reflection center for realizing unidirectionality by the normal type IDT shown in FIG.

FIG. 3 is a plan view showing an IDT having a conventional TCS-RDT structure.

FIG. 4 is a plan view showing an electrode structure of the IDT used in the surface acoustic wave device according to the embodiment of the present invention.

[5] intermodal coupling coefficient kappa ₁₂ characteristic diagram showing the electrode film thickness dependency of the phase term.

FIG. 6 is a characteristic diagram showing the dependence of the phase term of the excitation coefficient 電極 on the electrode film thickness.

FIG. 7 shows the phase difference (α−
FIG. 6 is a characteristic diagram showing the dependence of β) on the electrode film thickness.

FIG. 8 is a characteristic diagram showing the electrode thickness dependence of the positions of the excitation center and the reflection center in the IDT of the TCS-RDT structure.

FIG. 9 is a characteristic diagram showing the electrode thickness dependence of the positions of the excitation center and the reflection center in the IDT used in the surface acoustic wave device according to the embodiment of the present invention.

FIG. 10 is a plan view showing a configuration of a transmission type surface acoustic wave filter to which the present invention is applied.

FIG. 11 is a characteristic diagram showing frequency characteristics of a transmission type surface acoustic wave filter to which the present invention is applied and a transmission type surface acoustic wave filter using an IDT having a TCS-RDT structure as a reception electrode.

FIG. 12 is an enlarged characteristic diagram of the vicinity of the pass band of the filter in the frequency characteristics shown in FIG. 11;

[Explanation of symbols]

 1, 10, 100 Positive electrode 1A, 12, 14, 102, 104 Positive electrode finger 2, 20, 200 Negative electrode 2A, 2B, 22, 24, 202, 204 Negative electrode finger

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5J097 AA01 AA15 BB11 CC15 DD04 DD05 DD10 DD28 FF01 GG01 GG05 KK04 KK05

Claims (3)

[Claims] 1. A positive electrode finger, a negative electrode finger, and a float disposed between the positive electrode finger and the negative electrode finger formed on the surface of a langasite single crystal substrate having a substrate orientation and a surface acoustic wave propagation direction selected to have a natural unidirectionality. A surface acoustic wave device having a surface acoustic wave converter composed of electrodes, wherein each of the electrodes is formed along the propagation direction of the surface acoustic wave such that the natural surface acoustic wave converter reverses its natural unidirectionality. A surface acoustic wave device comprising: 2. The langasite single-crystal substrate has a substrate orientation, a substrate orientation, and a surface acoustic wave propagation direction expressed by Euler angles (φ, θ, ψ), −5 ° ≦ φ ≦ 5 °, 1
2. The surface acoustic wave device according to claim 1, wherein the angle is in a range of 35 ° ≦ θ ≦ 145 ° and 20 ° ≦ ψ ≦ 30 °, or an equivalent direction. 3. The distance relationship between the positive electrode finger, the negative electrode finger, and the floating electrode in the surface acoustic wave converter is represented by the width of the positive electrode finger and the negative electrode finger when the wavelength of the surface acoustic wave is λ. Is approximately λ / 8, the center distance between both electrode fingers is approximately 6 / 8λ, the distance g between the center of the positive electrode finger and the center of the floating electrode is 13 / 40λ ≦ g ≦ 14 / 40λ, and the width of the floating electrode 3. The surface acoustic wave device according to claim 2, wherein W is 11 / 40λ ≦ W ≦ 13 / 40λ. 4.