JPH0224048B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a surface acoustic wave device (hereinafter, surface acoustic wave is abbreviated as SAW) using split interdigital electrodes.

Generally, the interdigital electrode used in the transducer of a SAW device has a configuration in which a set of interdigitated electrodes consisting of a large number of electrode fingers are inserted into each other, and the capacitance of the transducer is set between each opposing electrode finger. If the dielectric constant of the piezoelectric substrate is large, it becomes relatively large. Therefore, when incorporating a SAW device into a corridor, it may be difficult to achieve impedance matching between the front and rear circuits. A method has been proposed to reduce the capacity by connecting. According to this method, the electrode pattern is configured symmetrically (including point symmetry) in order to prevent deterioration of frequency characteristics due to division. Such symmetrically arranged interdigital electrodes can provide sufficiently good frequency characteristics when focusing only on the electrodes.

However, such interdigital electrode 2
is constructed on a piezoelectric substrate 1 as shown in FIG. SAW
When the device is incorporated into a circuit, observing the frequency characteristics shows that the passband characteristics are good, as shown by the thin line in Figure 6, but the surface wave diffraction loss or
Due to the following effects, etc., a phenomenon occurs in which the insertion loss at the attenuation pole, that is, the amount of attenuation decreases. Therefore, the inventor conducted various experiments to investigate the cause of the deterioration of the attenuation peak, and in one of the experiments, the frequency characteristics were calculated by intentionally changing the capacitance of the divided partial electrodes. However, the calculation results shown in FIG. 5 were obtained. The above calculation was performed based on a weighting method using Fourier transform, which is commonly used in the analysis of surface acoustic wave devices. That is, the seventh
As schematically shown in the figure, assuming an epicenter in which comb-shaped electrodes Q ₁ to _{Q 3} are configured on a piezoelectric substrate P, the upward arrow in FIG. 1'' and calculated based on the following formula (a). That is, the frequency characteristics of the surface acoustic wave device can be considered as follows: H(f)= _N 〓 ⁿ⁼¹ _ao expj (2π+T) = _N 〓 ⁿ⁼¹ _ao expj (k _R x _o )...(a).

However, a _o is the intersection width, x _o is the position of the vibration source, and n
In this case, 1, -1, and 0 may be given.

The above formula (a) is described as the formula on page 66 (4, 23) of "Surface Acoustic Wave Engineering" published by the Institute of Electronics and Communication Engineers.

If the interdigital electrode is a regular regular electrode that is not divided, "1", "-1",
"1", "-1", "1", "-1", etc. On the other hand, in the case of the split-type interdigital electrode shown in FIG.
1”, “1”, “-1”, “1”, “-1”, “1”,
“-
1”, “1”, “0”, “1”, “-1”, “1”, “
-1'',
"1", "-1", "1", "-1", "1".

However, in the split electrode shown in Figure 1, the capacitance of the left and right split electrodes is not the same during mounting, so the capacitance of the left and right split electrodes is 10% and
We performed calculations assuming a 20% deviation.

When the capacitance of the left and right divided electrodes is 10% larger than that of the left divided electrode with respect to the common electrode, the impedance and excited voltage will be higher than that of the left divided electrode.
10% smaller, and the vibration sources are (1/
1.1) The ratio is 1 to 1. In other words, the calculation is "0.9",
"-0.9", "0.9", "-0.9", "0.9", "-0.9"
,
"0.9", "-0.9", "0.9", "0", "1", "-
1”,
"1", "-1", "1", "-1", "1", "-1"
”,
It can be calculated as "1". In Figure 5,
All data are based on an example in which the capacitance was divided into two. Curve A shows the capacitance of the two partial electrodes shifted by 20% from each other, curve B shows the capacitance shifted by 10%, and curve C shows the capacitance of the two partial electrodes made equal. Inferring from this result, the above-mentioned deterioration of the attenuation pole is due to the capacitance due to the effects of the back electrode, conductive adhesive, shield electrode, etc., due to mounting, even though the electrode patterns are arranged symmetrically in advance. This is thought to be due to the non-uniformity of the

The present invention has been made in view of the conventional technical situation described above, and it integrates the capacitance between opposing electrode fingers of each partial electrode so that the capacitance when each partial electrode is mounted is substantially equal. The purpose is to provide SAW devices with different capacities.

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 2a is a schematic diagram showing the capacitance of each part when the SAW device is mounted, for explaining the present invention. Corresponding to the SAW device shown in FIG. 1, 1 is a piezoelectric substrate, 2 is a divided It is a type interdigital electrode. 5 indicates a state in which one comb-shaped electrode of one partial electrode of the electrode 2 is grouped together, 6 indicates a state in which one comb-shaped electrode of the other partial electrode is grouped together, and 7 indicates a state in which the comb-shaped electrode 5 is grouped together. , 6, and are electrically connected in series. 4 is a shield electrode provided between the other interdigital electrode, and 8 is a ground electrode provided on the back surface of the piezoelectric substrate 1. The comb-shaped electrodes 5, 6, and 7 constitute a segmented interdigital electrode 2. Then, a voltage is applied between electrodes 5 and 6, and the voltage is applied between electrodes 5 and 7 and between electrodes 7 and 6.
divided between. Referring to the figure, looking at the capacitance between electrodes 5 and 7 and between electrodes 7 and 6, we find that electrode 5
The capacitance C5 to C7 between C5 and C7 is as shown in b in the same figure,
Further, the capacitance C7-6 between the electrodes 7 to 6 is as shown in c in the figure. Comparing C5-7 and C7-6, the capacitance of the partial electrode on the earthed side C5-7
becomes larger. Therefore, C5-7 and C7
-6 to be substantially equal, the electrode pattern of the partial electrodes is set asymmetrically, as shown in FIG. Figure a shows an example in which a new electrode finger 10 is added to the partial electrode 2b (corresponding to electrodes 7 and 6 in Figure 2a) on the side that is not grounded.
It is set to have the same potential as that so that no new surface wave excitation source is generated. Similarly, in FIG. 2B, the partial electrode 2b on the side that is not earthed is the same as the partial electrode 2a on the side that is earthed (electrode 5 in FIG. 2A,
This is an example in which the number of surface wave excitation sources is larger than that in the case (corresponding to No. 7), and the number of vibration sources in the left and right partial electrodes is changed to create an asymmetric structure. Figure c shows a diagram in which the electrode finger on the side that is not grounded has a wider electrode width. Figures d and e both show examples in which unnecessary electrode portions 15 and 16 are similarly added to the partial electrode 2b. Figure f shows an example in which a winding electrode 17 is similarly provided on the partial electrode 2b, and a terminal portion 18 is provided at the tip thereof. In each of the above examples, the capacitance due to the partial electrode 2b is higher than the capacitance due to the partial electrode 2a. This will be explained with reference to FIG. Now, as shown in FIG. 8a, the partial electrode 2
Assume that a shield electrode V connected to a ground potential is configured on the side of an interdigital electrode in which electrodes a and 2b are connected in series. In this case, the equivalent circuit between the terminal electrodes S, U, the connection point T, and the shield electrode V of the surface acoustic wave filter is shown in Figure 8b.
As shown below.

That is, the capacitance C _ST between ST is the shield electrode V
(earth potential), C _ST = C ₁ /C ₀ ),
C _ST > C _TU . On the other hand, since the capacitance of the interdigital electrode increases in proportion to the number of electrode fingers, C _ST =C _TU can be achieved by increasing the number of electrode fingers of the partial electrode 2b.

In the structure shown in FIG. 4a, the capacitance of the partial electrode 2b is increased by providing an additional electrode finger 10 on the partial electrode 2b on the side that is not grounded, and the capacitance of the partial electrode 2b is increased. In the structural example, by making the number of oscillation sources (logarithms) different for the left and right electrodes, by making the electrode finger of one partial electrode wider, and by providing unnecessary electrode parts 15 and 16 or a rotating electrode 17, The capacitance due to the partial electrode 2b increases, and the partial electrode 2b increases in the mounted state.
The capacitance due to a and the capacitance due to the partial electrode 2b can be made equal. When a SAW device as shown in FIG. 1 is constructed using the electrode pattern shown in FIG. 4b, as is clear from the calculation method based on the weighting method explained referring to FIG. The capacitances due to the partial electrodes are made equal in the mounted state, so that the amount of attenuation of the attenuation pole can be made sufficiently large, as shown by the bold line in FIG. The thin lines in the figure are from the conventional example in which the electrode pattern is symmetrically divided, and it can be seen that the attenuation amount of the attenuation pole cannot be sufficiently obtained. Further, the broken line in the figure shows, for reference, the result when an additional electrode finger is added to the earthed partial electrode 2a side. Each of the above examples shows an example in which the partial electrode 2b on the side that is not earthed is modified, but according to the present invention, the partial electrode 2a on the side that is earthed is focused on, and the most The outer electrode finger may be deleted or the outermost finger may be configured to have a narrow width.
The capacitance of the partial electrode on the earthed side may be set to be smaller than the capacitance of the partial electrode on the side not earthed.

Figure 3 shows the case where the split interdigital electrodes are not grounded, and the capacitance between electrodes 5 to 7 in figure a is shown in figure b, and the capacitance between electrodes 7 to 6 is shown in figure c. become. Comparing b and c in the same figure, since C4>C2>C3, the partial electrode 2b adjacent to the shield electrode 4 (electrode 7 in Fig. 3a,
6) is larger. Therefore, the means shown in FIG. 4 may be appropriately selected and applied to the partial electrode 2a.

In addition to those described in Figures 2 and 3, there are cases in which the partial electrode to be grounded is changed, cases in which one of the partial electrodes is grounded without a back electrode, and cases in which a back electrode is provided but not grounded. Although various embodiments can be realized, all of them can be considered similar to those shown in FIG. 2 or 3, so
Detailed explanation will be omitted. Although each of the above embodiments shows a two-part interdigital electrode, the present invention can also be applied to three or more parts. In this case, all the divided electrodes may be configured to be asymmetrical, or only the grounded partial electrodes or the partial electrodes that are close to other conductive members and the adjacent partial electrodes may be configured to be asymmetrical. It may be configured as follows. In short, it is sufficient to set the capacitance of each partial electrode to be substantially equal in the mounted state. Furthermore, although the above embodiments have exemplified regular electrodes, the present invention can also be applied to weighted electrodes, and may also be applied to the other interdigital electrode.

As explained above, the present invention integrates the capacitance between opposing electrode fingers of each partial electrode so that the capacitance of at least two partial electrodes of a split type interdigital electrode when mounted is substantially equal. Since the capacitances are different, it is possible to reduce the capacitance by division, and it has the effect that the attenuation amount of the attenuation pole in the frequency characteristic can be made sufficiently large.

[Brief explanation of drawings]

Fig. 1 is a plan view showing a general SAW device, Fig. 2 a is a schematic diagram for explaining the present invention, and Fig. 2 b is a schematic diagram for explaining the present invention.
and c are equivalent circuit diagrams, FIG. 3 a is a schematic diagram for explaining the present invention, FIG. 3 b and c are equivalent circuit diagrams, and FIGS.
A pattern diagram of a split type interdigital electrode by a SAW device, FIG. 5 is a frequency characteristic diagram for explaining the present invention, and FIG. 6 is a diagram of the conventional and present invention.
A frequency characteristic diagram obtained by the SAW device, Fig. 7 is a schematic cross-sectional view showing an example of the vibration source of the interdigital electrode used to calculate the frequency characteristic, and Fig. 8 a and b respectively explain the difference in capacitance due to partial electrodes. FIG. 8a is a plan view of the surface acoustic wave filter, and FIG. 8b is a diagram showing an equivalent circuit of the divided interdigital electrode portion.

Claims (1)

[Scope of Claims] 1. A surface acoustic wave device comprising interdigital electrodes that are divided into at least two parts in the surface acoustic wave propagation direction and electrically connected to each other in series, comprising: at least two of the divided partial electrodes; is the capacitance of the at least two partial electrodes, which is the total capacitance between each opposing electrode finger in the at least two partial electrodes, such that each capacitance during mounting is substantially equal to the geometry of the electrode fingers. A surface acoustic wave device characterized in that the surface acoustic wave devices are different from each other depending on their chemical arrangement. 2. One of the comb-shaped electrodes of any one of the divided partial electrodes is grounded, and at least the grounded partial electrode and the adjacent partial electrode have substantially the same capacitance when mounted. The capacitance of each partial electrode, which is the total capacitance between each opposing electrode finger, is made to be equal to
A surface acoustic wave device according to claim 1. 3. A shield electrode adjacent to the interdigital electrode is provided, none of the divided partial electrodes is grounded, and at least the partial electrode adjacent to the shield electrode and the partial electrode adjacent thereto are connected to each other during mounting. Claim 1: The capacitance of each partial electrode, which is the sum of the capacitances between opposing electrode fingers as seen from the electrode pattern, is made different from each other by the geometrical arrangement of the electrode fingers so that the capacitances are substantially equal. The surface acoustic wave device described in .