JPH03182111A - Surface acoustic wave device - Google Patents

Abstract

PURPOSE:To increase Q value of the surface acoustic wave device by specifying the dimension of a reflector and the electrode dimension of an interdigital electrode. CONSTITUTION:For a pair of reflectors 102, reflection structures 105-107 forming the periodical arrangement of a period length (PR) are equipped with 1/2 PR width. Further, in an interdigital electrode(IDT) 101, with an end part T on the IDT side of a pair of reflectors 102 as the origin, an area [width(s)], where an electrode conductor does not exist from the origin T to the center of the IDT 101, and an area, [electrode width (l)], where the electrode conductor exists, are alternatively arranged with a period length (PT) close to the reflection structures 105-107 PR of the reflector 102 and the ratio of l/PT is 0.5<l/PT<0.8. Thus, the Q value of the surface acoustic wave device can be increased.

Description

[Detailed description of the invention] [Industrial Application Field J The present invention relates to an electrode structure for a surface acoustic wave device.

[Prior Art] As an example of the electrode structure of a conventional surface acoustic wave device, an arrangement of metal conductors of an IDT formed on a piezoelectric flat plate and a pair of reflectors on both sides of the IDT is described as an example of a prior application. Taking the wavelength of the surface acoustic wave as an example, the electrode width of the metal conductor is 1/4, and the spacing between the electrode conductors is the same 1/4, making a uniform periodic arrangement, or the width of the metal conductor. β and the distance S between the electrode conductors
There was an example in which the ratio β/S of 1 or different from 1 was arranged uniformly and periodically. (See Patent Publication No. 1-33969-9.) [Problems to be Solved by the Invention] However, the above-mentioned conventional technology has problems in that the series resonance resistance of the surface acoustic wave device is high or the resonance Q value is low. has. Therefore, the present invention aims to solve these problems, and its purpose is to improve the elasticity of a SAW resonator that is small, has a high Q value, and has excellent stability, and a double mode filter made of a SAW resonator. The objective is to provide surface wave devices to the market.

[Means to solve the problem]

The surface acoustic wave device of the present invention includes (1) an interdigital electrode (IDT) that transmits and receives surface acoustic waves at the center of the surface of a piezoelectric flat plate, and electrodes that reflect the surface acoustic wave on both sides of the interdigital electrode (IDT). In a surface acoustic wave device in which a pair of reflectors is formed, the reflector has a periodic array of reflecting structures having a period length PR, and a width of approximately H of PR; With the end on the interdigital electrode (IDT) side as the origin, the electrode conductor exists from the origin toward the center of the interdigital electrode (IDT) with a period length PT close to the period PR of the reflective structure of the reflector as 6. regions (width s1. regions where electrode conductors exist (if pole width β) are arranged alternately, and
The ratio of 12/PT is 0.5 and J2. /Have interdigital electrodes (IDT) with PT<0,8.

(2) The interdigital electrodes according to claim (1) are formed at both ends of the IDT except for the central part.

[Function] According to the above configuration of the present invention, a reflector that can best reflect a surface acoustic wave and interdigital electrodes that are phase-matched to the phase of the surface acoustic wave from the reflector are configured, and the generated polarization is Since the width of the interdigital electrodes was selected to enable efficient charge detection, the Q value of the surface acoustic wave device is high and the series resonance resistance R of the resonator is low.
1 can be made smaller.

[Embodiment] FIG. 1 is a plan view of an embodiment of the surface acoustic wave device of the present invention, and the names of the various parts in the figure are as follows: 100 is a piezoelectric flat plate, 101 is an interdigital electrode (IDT). ), 102 is a reflector,
103 and 104 are the positive and negative polarity electrodes of the interdigitated electrode (IDT) 101, respectively, and 105, 106, and 107 are reflective conductors that are the reflective structure of the reflector 102.6 In the surface acoustic wave device of the present invention, The surface acoustic waves used are crystal, LiT
aos, L, 1NbOx, Lf* B40? This is an elastic wave that locally propagates on the surface of the piezoelectric flat plate 100 such as the above. The interdigitated fitly (IDT) 101 and the metal conductor of the reflector 102 are formed by mirror-finishing the surface of the piezoelectric flat plate and then depositing a thin film of a conductive metal such as AE, Ag, or Cn by means of vapor deposition. After formation, a pattern is formed by photolithography. The interdigital electrode (IDT) can excite surface acoustic waves by applying an alternating voltage to the positive and negative electrodes 103 and 104. Further, the function of the reflector 102 is based on the ID
It reflects the surface acoustic waves radiated from T in both directions and returns them to the original state, and is usually caused by the acoustic mismatch of the surface acoustic wave propagation paths created by the respective conductors of 105.106.107. The device is constructed on both sides of the IDT. The detailed dimensional relationship of FIG. 1 of the present invention is shown in FIG. 2 as an A-A cross-sectional view. The names of each part in FIG. 201, 202, 203 are interdigital electrodes (IDT)
, 204, 205, etc. are reflective conductors of the reflector. In the same figure, the period length of the reflector 204 and 6 reflecting conductors 204 and 205, which cover the area on the right side from the left end T of the reflecting conductor, is PR, and PR is approximately the equivalent of the wavelength likelihood of the surface acoustic wave to be used. 206 and 207 in the figure represent the amplitude change of the stress wave of the surface acoustic wave that is excited and stands on the surface.
and 207 indicate a state with a 180° phase difference, and the area from the left end T of the reflector to the right end of another reflector placed opposite to each other is an interdigital electrode (IDT) region. The interdigitated electrode (IDT) has a period length PT that is close to and appropriately set to the period length PR of the reflective conductor that is the reflective structure of the reflector, and extends I from the origin T toward the center of the IDT.
Regions of the DT where no electrode conductor exists (width S) and regions where the electrode conductor exists (electrode width I2) are arranged alternately, and the ratio of the electrode width 4 to PT is 0.5<β/PT<0. The length T from the origin T to the left end of the second electrode is approximately the same as the wavelength of the surface acoustic wave. According to the results of various experiments and calculations, the node of the stress wave coincides with the lower left end of the reflector, and also forms a node at the left end of the interdigital electrode near the reflector. There is. Of course, in the case where the reflector is placed on the left side of the IDT, the above description applies to the case where the lower left end of the reflector becomes the lower right end, and the left end of the electrode becomes the right end.

Next, Fig. 3 shows the same A-A cross-sectional structure as Fig. 2 for the case where the reflection structure of the reflector is a long and narrow groove, that is, a group, dug over the propagation width (aperture length) of one surface acoustic wave. The names of the parts in Figure 0 are: 300 is a piezoelectric flat plate, 301, 302, and 303 are interdigital electrodes (IDT).
The electrode conductors 304 and 305 are the reflection structure of the reflector, and 306 and 307 are the stress waves of the surface acoustic waves.The meanings of the dimension symbols in Fig. 3 are the same as in Fig. 2. be.

Next, the validity of the dimensional relationship adopted by the surface acoustic wave device of the present invention will be explained using FIGS. 4 and 5. First, the validity of the reflector adopted by the present invention will be explained with reference to FIG.
In the figure, the horizontal axis is the ratio of the radiation WR of a reflecting structure to the periodic length PR of one reflecting structure, and the vertical axis shows the reflection coefficient γ of the reflector.The larger γ is, the more the radiation enters the reflector. Surface acoustic waves are largely reflected.From Figure 6, at WR given 34PH, γ
shows the maximum value of However, piezoelectric materials with such characteristics include crystal, Li*BnO? , L i T
ags etc. are known. When γ takes the maximum value, the Q value of the surface acoustic wave device has the maximum value because the energy of the wave lost after passing through one reflector is minimized.

Finally, Figure 4 shows the characteristics of the series resonant resistance R1 and resonance dynamic capacitance CI of the surface acoustic wave device when the electrode width β of the interdigital electrode (IDT) is changed. IDT
The ratio of the IDT electrode width 2 to the period length PT! 2/PT
, and the vertical axis is the relative value of the series resonant resistance R1 (501) and the resonant active capacitance C1 (502). First, the reason why C1 of song 11502 has the characteristics as shown in the figure is as follows. It has already been explained that stress waves of surface acoustic waves are present as shown in FIGS. 2 and 3. This stress wave F (
Considering the polarization distribution on the piezoelectric pressure surface that occurs in response to x), the strain wave S (x) accompanying one surface acoustic wave is S
(x)=eF (x) (1) However, in equation 1), X is the position coordinate in the propagation direction of the surface acoustic wave. Corresponding to the above S (X), the polarization wave P (x) generated on the surface of the piezoelectric body is P (X)=ds (x) (2)=
It is given by edF (x). Here, e is a piezoelectric constant, and d is a piezoelectric coefficient, which is a constant given by the material and orientation. (
2) In the formula, F (x) is expressed as follows, where k is the wave number and B is the amplitude.
Since F (x) = Bs i n (kx) is approximately given, the total charge M collected per interdigital electrode is given by the following integral.

Considering that C1 of the resonator is proportional to M, 502 in Figure 5
The characteristics of can be explained. Furthermore, the series resonant resistance R1 is calculated from the above C, using the Q value of the resonator of the surface acoustic wave device, so that R2=
Since it is given by 1/(QwC+), the characteristic of R1 at 501 in the figure can be explained.

However, W is the operating frequency. Therefore, β/Pt is 0.
5 or more, R3 decreases. However (2/P
If T is 0.8 or more, it will be extremely difficult to process the electrodes in the IDT portion, and the practical effect will be diminished. In fact, in a surface acoustic wave device that operates using the crystal ST cut s*0.8 μm, making processing by photoetching extremely difficult.

Finally, regarding the region of the IDT where the interdigital electrodes take 0.5 (J2/PT < 0°8), the stress waves of at least 6 surface acoustic waves are generated at the ends of the electrodes at the center of the IDT. Since there is no knot in the IDT, a further effect can be recognized by approving the electrode width of the above-mentioned value of I2/PT on both sides of the IDT except for the central part.

[Effect of the invention] As described above, according to the present invention, the dimensions and I
By appropriately setting the electrode dimensions of the DT, the Q value of the surface acoustic wave device is high, and as a result, the electromechanical coupling with the surface acoustic wave is large, the dynamic capacitance C1 of the resonator is large, and the series resonance resistance R is also large. Since a small power can be realized, when used as a resonator, stable oscillation can be maintained due to excellent Q and R1, and the device can also be made smaller. Furthermore, when the present invention is applied to a dual mode filter, a good filter with low intrusion loss can be realized because R+ is small.

[Brief explanation of drawings]

FIG. 1 is a plan view showing an embodiment of the surface acoustic wave device of the present invention, FIG. 2 is a sectional view taken along the line AA in FIG. 1, and FIG.
The figure is a sectional view taken along the line A-A of another embodiment of the present invention, and FIG. 4 is a diagram showing the reflection characteristics of a reflector. Figure 5 shows the series resonance resistance R shown by the surface acoustic wave device of the present invention.
1 and dynamic capacitance C1. 00 ・Piezoelectric flat plate 0 ・Interdigital electrode (IDT) 02 ・Reflector or more

Claims (2)

[Claims] (1) An elastic surface formed with an interdigital electrode (IDT) that transmits and receives surface acoustic waves at the center of the surface of a piezoelectric flat plate, and a pair of reflectors that reflect the surface acoustic wave on both sides of the interdigital electrode (IDT). In the wave device, the reflector has reflective structures arranged periodically with a period length PR, and the width thereof is approximately 1/2 of PR;
Furthermore, with the end of the pair of reflectors on the interdigital electrode (IDT) side as the origin, the interdigital electrode (IDT) is
a region (width s) where no electrode conductor exists toward the center of
It is characterized by having interdigital electrodes (IDT) in which regions where electrode conductors exist (electrode width l) are arranged alternately and the ratio of E/PT is 0.5<l/PT<0.8. surface acoustic wave device. (2) A surface acoustic wave device characterized in that the interdigital electrodes according to claim 1 are formed at both ends of the IDT except for the central part.