JPH01292908A - Surface acoustic wave filter - Google Patents

Abstract

PURPOSE:To prevent the preparation of a device from being difficult even when a frequency goes to be high while an unnecessary reflected wave is suppressed by selecting the material and film thickness of an electrode so that the reflection factor of a surface acoustic wave, which is caused by short-circuit due to the metallic electrode of a substrate electric field, can be zero. CONSTITUTION:A surface acoustic filter is composed of a substrate 1 and two interdigital solid type electrodes 2 and the material and film thickness of the electrode are selected so that the reflection factor of the surface acoustic wave, which is caused by the short-circuit due to the metallic electrode 2 in the electric field of the substrate 1, can be zero. Accordingly, in comparison with a conventional method, an electrode goes to be almost double and electrode operation goes to be easy. Thus, the necessary reflected wave is suppressed and an amplitude ripple and a group delay time ripple are decreased. Then, a problem that the width of the electrode goes to be narrow and the preparation of the device goes to be difficult with accompanying the high frequency is erased.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is directed to the treatment of unnecessary reflected waves (Maze Electrical Load
ing.

[Prior art] Regarding the suppression of unnecessary reflected waves caused by short circuits of the substrate electric field by metal electrodes, there is a conventional technique for suppressing unnecessary reflected waves caused by short circuits of the substrate electric field by metal electrodes.
1974, MTT 22, No. 960-964
Page (IEEE Trans, 1974. MTT
-22°9p, 960-964). This is a structure that will be deleted soon.

[Problem to be solved by the invention]

The conventional technology described above has a problem in that as the frequency of the surface acoustic wave device increases, the electrode width becomes narrower, making it difficult to fabricate the device.

SUMMARY OF THE INVENTION An object of the present invention is to provide a surface acoustic wave filter having a structure that does not make it difficult to manufacture a device even when the frequency becomes high.

[Means to solve the problem]

In order to solve the above problems, in the present invention, the electrode material and the electrode film thickness are selected so that the surface acoustic wave reflectance caused by the short circuit due to the metal electrode of the substrate electric field becomes zero. The reflectance ε of a surface acoustic wave caused by a short circuit caused by an electrode is generally expressed as a function of the normalized film thickness as follows.

However, λ: surface acoustic wave wavelength, n: electrode film thickness C1, C
8: Substrate constant The purpose can be achieved by selecting a normalized film thickness that makes e according to the above equation zero.

[Effect]

Unwanted reflected waves are suppressed by selecting a standardized film thickness such that the surface acoustic wave reflectance caused by short circuits caused by metal electrodes in the substrate electric field becomes zero, thereby eliminating amplitude ripples and clusters, which have been problems in the past. Delay time ripples and the like can be reduced. Furthermore, with the conventional method, it becomes difficult to create electrodes as the frequency increases, but according to the present invention, the width of the electrode finger is approximately twice that of the conventional method, so the electrode function becomes easier.

〔Example〕

The present invention will be explained in more detail using FIGS. 1 to 6. In a surface acoustic wave filter, the electrode width is λ/4 as shown in Figure 1.
When the solid type electrode 2 (1 is a surface acoustic wave substrate) is used as an input/output electrode, in the conventional structure, unnecessary reflected waves are generated between the input and output electrodes.

This is a reflected wave (MEL) generated because the solid type electrode 2 short-circuits the substrate electric field, and causes amplitude ripples and group delay ripples. In order to suppress this reflected wave, a split type electrode 3 shown in FIG. 2 has been used. In this case, the electrode width is set to λ/8 so that the reflected waves cancel each other out. Although this method is an effective means, the width of the electrode fingers of the split type electrode is 1/2 that of the solid type electrode, so it is difficult to manufacture at higher frequencies. but,
Using the relationship between normalized film thickness and reflectance ε shown in Figure 3,
If the reflected wave is suppressed by selecting a film thickness such that e becomes zero, the first
Reflected waves can be suppressed even with solid electrodes as shown in the figure (
Therefore, FIG. 1 is an example diagram). The characteristics shown in FIG. 3 are for the case where crystal is used as the substrate and aluminum is used as the electrode material.

Here, the reflectance ε is expressed as follows.

ε −c, + c, · (le/λ) 10, · (A
/λ)2・・・・・・・・・・・・ (1) However, C
I, ('2, C3 is the substrate constant and experimentally determined that cl
−0,005* CI depth −1,2+ 's −''. From this result, the reflectance 6 is zero and h/λ≒ 4
．． 5 x 10-' l 5.5 x 10-'' If you create a surface acoustic wave filter with this standardized film thickness, unnecessary reflected waves can be suppressed and amplitude ripples and group delay ripples can be reduced. In the example, the center frequency is 280
3fHz, normalized film thickness at which ε becomes zero: 5.3x11)-z
Therefore, the structure was made such that the electrode film thickness was 5BOF&s+substrate 42.75°y-x crystal, the number of pairs of solid type input electrodes was 30, and the number of pairs of solid type output electrodes was 48 pairs. The frequency characteristics at this time are
As shown in the figure. Further, for comparison, the frequency characteristics when the electrode film thickness is 500 am are shown in FIG. From these two frequency characteristics, it can be seen that in the embodiment of the present invention, hardly any amplitude ripple or group delay ripple is observed, and unnecessary reflected waves are suppressed. In addition, in this example, a value of 5.3 x 10-" was selected as the normalized film thickness at which the reflectance ε becomes zero, but the same effect can be obtained even if a value of 4.5 x 10-" is selected. is obtained. Another embodiment is shown in FIG. This is because the reflectance ε has a positive or negative value depending on the value of the normalized film thickness, so by selecting normalized film thicknesses with equal absolute values and opposite signs and arranging them alternately, each reflected wave can be struck. The above conditions are met. Further, in this embodiment, a solid type electrode is used, but when a split Wt electrode is used, there is an advantage that unnecessary reflected waves can be suppressed even if the film thickness varies.

In addition, the literature [Positive and negative reflection surface acoustic wave reflector and resonator]
, Propensity, Yamanouchi Institute of Electronics and Communication Engineers Technical Research Report, USB5-
11, from the characteristics shown in FIG. 7, when lithium tantalate is used for the substrate 1, gold is used for the electrode material, the standardized film thickness is T #5.6 x 10
-", the reflectance e can be made zero. From the characteristics shown in Fig. 8, when lithium niobate is used for the substrate 1, the normalized film thickness h/λ is By setting h/λ≈2.5×10−”, the reflectance e can be made zero.

Although this embodiment has been described using pure aluminum and gold, it goes without saying that the same effect can be obtained even when the above materials are used as main materials and impurities are included therein. In addition, experiments have shown that when an electrode material with a mixing ratio of aluminum and gold of approximately 1:1 is used on a quartz substrate, the reflectance is almost zero regardless of the film thickness. Reflected waves can be suppressed. Other differences 2
In the structure shown in Figure 10, in which different types of electrode materials are arranged alternately, if two types of electrode materials, aluminum and gold, are used and quartz or lithium tantalate is used for the substrate, the sign of the reflection coefficient will change depending on the electrode material. Because of the difference, the phases of the reflected waves at the aluminum electrode portion and the gold electrode portion are opposite to each other.

Therefore, reflected waves can be suppressed. In addition, as shown in FIG. 9, each half of the electrode ring has a different film thickness,
By arranging half of the adjacent electrode fingers facing each other so that they have the same film thickness, the reflected waves can be similarly suppressed because they are applied in opposite phases. In addition, the structure shown in FIG. 11 can be used as a structure for suppressing reflected waves. In this method, a part of the interdigital electrode is buried in the surface acoustic wave substrate to reduce the reflectance to zero and suppress reflected waves.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to obtain the effect of reducing amplitude ripples and group delay ripples while suppressing unnecessary reflected waves using solid electrodes. In the example,
Amplitude ripple is 3dB, group delay ripple is 100n
zImproved.

[Brief explanation of the drawing]

Figure 1 is a diagram of an embodiment of the present invention, Figure 2 is a diagram of a conventional example, and Figure 3 is a diagram of a conventional example.
The figure is a characteristic diagram showing the relationship between normalized film thickness and reflectance when using a crystal substrate and aluminum electrode, Figure 4 is a diagram of other examples, and Figure 5 is the frequency when the reflectance is not zero. Characteristic diagram, Figure 6 is the frequency characteristic diagram when the reflectance is zero, Figure 7 is the frequency characteristic diagram when the reflectance is zero.
The figure shows the reflectance characteristics versus normalized film thickness when using a lithium niobate substrate and an aluminum electrode, and Figure 8 shows the reflectance versus normalized film thickness when using a lithium tantalate substrate and gold electrode. Figures 9.10.11 are diagrams showing still another embodiment. 1......Substrate 2...Solid type electrode 3...
・・・・・・・・・Split-type electrode representative Patent attorney Katsutoshi Ogawa 41 Figure 2 Figure 5 Yuzuki L Ne 4 A, Membrane sword 1 Mukunu, (2 N A-A' cross-sectional view 〒5 figure circumference 51LfLmHzfudogatsu,ne7toy1:,Onotohe/>. Cross-sectional view A-A'm view

Claims (9)

[Claims] 1. In a surface acoustic wave filter in which at least two interdigital electrodes are arranged on a surface acoustic wave substrate, the electrode material and electrode are such that the reflectance of surface acoustic wave reflected waves caused by short circuit of the substrate electric field by the metal electrode is zero. A surface acoustic wave filter characterized by a thick film. 2. Using crystal as the surface acoustic wave substrate and a metal material mainly made of aluminum as the electrode material, when the electrode film thickness is expressed by h and the surface acoustic wave wavelength is expressed by λ, h/λ≒4.5×10^- ^3 or h/λ≒5.3×1
Claim 1 characterized in that 0^-^2.
The surface acoustic wave filter described. 3. Using lithium tantalate as the surface acoustic wave substrate and a metal material mainly composed of gold as the electrode material, when the electrode film thickness is expressed by h and the surface acoustic wave wavelength is expressed by λ, A/λ≒3.6×10
2. The surface acoustic wave filter according to claim 1, wherein the surface acoustic wave filter has an angle of . 4. Using lithium niopate as the surface acoustic wave substrate and a metal material mainly made of aluminum as the electrode material, when the electrode film thickness is expressed by h and the surface acoustic wave wavelength is expressed by λ, h/λ≒2.
．． 2. The surface acoustic wave filter according to claim 1, wherein the surface acoustic wave filter is 5×10^-^2. 5. In a surface acoustic wave filter in which at least two interdigital electrodes are arranged on a surface acoustic wave substrate, the electrode film thickness of one interdigital electrode is h_A, and the film thickness of the other electrode is h_
The surface acoustic wave reflectance caused by the short circuit of the substrate electric field due to the metal electrode at the film thickness h_A is ε_A, and the film thickness h_B is
When the reflectance at ε_B is ε_B, the above film thickness is ε_A=
2. The surface acoustic wave filter according to claim 1, wherein the surface acoustic wave filter satisfies the following relationship: -ε_B. 6. In a surface acoustic wave filter in which a surface acoustic wave substrate is made of quartz or lithium tantalate and at least two interdigital electrodes are arranged on the substrate, electrode fingers made of aluminum and electrode fingers made of gold are arranged alternately. 2. The surface acoustic wave filter according to claim 1, wherein the surface acoustic wave filter is comprised of interdigital electrodes disposed in the interdigitated electrodes. 7. The mixing ratio of aluminum and gold in the electrode material is approximately 1:1.
The elastic surface filter according to claim 1, characterized in that an alloy is used. 8. Each electrode finger of the interdigital electrode is configured such that 1/2 of the width of the electrode finger is different from the thickness of the remaining 1/2 width, and the thickness of the adjacent electrode finger end portions facing each other in the width direction is the same. 2. The surface acoustic wave filter according to claim 1, wherein the surface acoustic wave filter is arranged so that 9. 2. The surface acoustic wave filter according to claim 1, wherein the interdigital interdigital electrode is partially or entirely embedded within the surface acoustic wave substrate.