JPH0119291B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a surface acoustic wave filter element.

Generally, a surface acoustic wave filter element has an interdigital type for converting an electric signal into a surface acoustic wave on a piezoelectric substrate 1 made of LiTaO ₃ , LiNbO ₃ , crystal, ceramics, etc., as shown in FIGS. 1 and 2. input transducer (hereinafter referred to as input IDT)
)2, and a shield electrode 3 is connected to this input IDT2.
An output transducer (hereinafter referred to as output IDT) having an interdigital type that converts surface acoustic waves into electrical signals, which are arranged at a predetermined distance via
4 and a sound absorbing layer 5 provided near the end of the piezoelectric substrate 1 in the surface acoustic wave propagation direction, and the characteristics of this surface acoustic wave filter element are mainly determined by the input.
It is determined by the intersection width and pitch of IDT2 and output IDT4.

This kind of surface acoustic wave filter element is used as input IDT.
2. Either one of the output IDT4 (in this example, the output
IDT) has a constant crossover width, while the other (in this example, the input
IDT) is formed with a so-called apodized IDT that changes the crossover width and pitch. In this configuration, the frequency characteristics of the surface acoustic wave filter element are determined by the convolution of the frequency characteristics of the input IDT 2 and the output IDT 4, so that the design becomes easy. In addition, a dummy electrode 6 is provided in the non-intersecting portion of the apodized IDT to compensate for the phase shift of the surface acoustic waves, but the surface acoustic waves traveling in the opposite direction to the output IDT 4 do not contribute to signal propagation. Therefore, there is no need for phase compensation, and the area near the maximum weighting width in the direction opposite to the output IDT 4 is filled with a conductive member except for the crossing portion, thereby forming a so-called filled electrode 7. Also, among the surface acoustic waves excited from the input IDT 2, those directed in the opposite direction to the output IDT 4 are absorbed by the sound absorbing material layer 5 provided on the piezoelectric substrate, and are prevented from being received by the output IDT 4 as unnecessary reflected waves. There is.

However, in such a surface acoustic wave filter element, the side edge ₇₁ of the filled electrode 7 is almost perpendicular to the surface acoustic wave propagation direction, and the surface acoustic wave is reflected.
It is formed to have a side edge 5 ₁ that covers the side edge 7 ₁ of and obliquely intersects with the surface acoustic wave propagation direction. In this case, due to variations in the coating process of the sound absorbing material layer 5, as shown in an enlarged view in _FIG . become,
When the amount of the uncoated portion 7a is large, the incident surface acoustic wave is reflected by this side edge 7a and reaches the peak of the filter characteristic curve 8 of the surface acoustic wave filter element shown in terms of frequency and relative level as shown in FIG. Ripples ₈₁ were generated, which caused deterioration of characteristics.

The present invention has been made in view of the above-mentioned problems, and it is an object of the present invention to provide a surface acoustic wave filter element that does not generate reflected waves from the side edges of filled electrodes, even if the positional accuracy of the sound absorbing material layer is poor. The purpose is

Next, one embodiment of the surface acoustic wave filter element of the present invention will be described with reference to FIGS. 4 and 5.

That is, on a piezoelectric substrate 11 made of LiTaO ₃ , LiNbO ₃ , crystal, ceramics, etc., there is an input IDT 12 for converting an electric signal into a surface acoustic wave, and an input IDT 12 is arranged at a predetermined distance from this input IDT 12 via a shield electrode 13 . It is equipped with an output IDT 14 that converts the provided surface acoustic waves into an electric signal, and a sound absorbing material layer 15 provided near the end of the piezoelectric substrate 11 in the surface acoustic wave propagation direction, so that the intersection width of the output IDT 14 is kept constant. , input IDT12 changes the crossover width and pitch,
Formed with so-called apodized IDT, this input
A dummy electrode 16 is provided in the non-intersecting part of the IDT 12 to compensate for the phase shift of the surface acoustic waves, but since the surface acoustic waves traveling in the opposite direction to the output IDT 14 do not contribute to signal propagation, the phase is compensated. It is almost the same as a conventional surface acoustic wave filter element, but the crossing part is filled with a conductive material in the opposite direction to the output IDT 14 from the vicinity of the maximum width of normal weighting, and is filled with a conductive material to form a so-called fill electrode 17. In the example, the fill electrode 17 is extended to the vicinity of the end of the piezoelectric substrate 11, and the sound absorbing material layer 15 is provided so as to cover a part of the side edge 17 ₁ and opposing portion 17 ₂ of the fill electrode 17. It is characterized by

With this structure, the side edge ₁₇₁ of the fill-in electrode 17 always completely enters the sound-absorbing material layer 15 even if there are variations in the coating process of the sound-absorbing material layer 15, unlike the conventional method. The exposed portion 7a of the side edge ₇₁ of the fill-in electrode 7 is completely eliminated, and reflected waves from the side edge ₁₇₁ can be prevented, so that the filter of the surface acoustic wave filter element shown in frequency and relative level in FIG. The top of the characteristic curve 18 became flat, and the filter characteristics became extremely good.

Next, another embodiment of the present invention is shown in FIG. In the figures, the same reference numerals as in the previous embodiment indicate the same parts and will not be particularly described.

That is, in this embodiment, the filling electrode extends to the vicinity of the end of the piezoelectric substrate 11, but the side edge 17 ₁ and opposing portion 17 ₂ of the filling electrode 17 are both elastic. It is provided obliquely to the surface wave propagation direction, making it possible to more completely prevent reflected waves than in the previous embodiment. This may be done so that only the opposing portions 17 ₂ intersect obliquely.

As described above, the surface acoustic wave filter of the present invention makes it possible to always cover the side edges of the filled electrode with a sound absorbing layer, and has the effect of preventing characteristic deterioration due to waves reflected from the side edges of the filled electrode. It is expected that the characteristics will improve. The effect is particularly remarkable when a material with a large electromechanical coupling coefficient, such as LiNbO ₃ , is used as the piezoelectric substrate.

[Brief explanation of drawings]

Figures 1 to 3 are diagrams showing an example of a conventional surface acoustic wave filter element, in which Figure 1 is a plan view, Figure 2 is an enlarged view of section A in Figure 1, and Figure 3 is a frequency characteristic diagram. , FIG. 4 and FIG. 5 are diagrams showing one embodiment of the surface acoustic wave filter element of the present invention, in which FIG. 4 is a plan view, FIG. 5 is a frequency characteristic diagram, and FIG. 6 is a diagram showing the elasticity of the present invention. FIG. 7 is a plan view showing another example of the surface wave filter element. DESCRIPTION OF SYMBOLS 1, 11... Piezoelectric substrate, 2, 12... Interdigital type input transducer, 3, 13... Shield electrode, 4, 14... Interdigital type output transducer, 5, 15... Sound absorbing material layer, 6, 16...
Dummy electrode, 7, 17... Filled electrode.

Claims (1)

[Claims] 1. A piezoelectric substrate, an input transducer having an interdigital type provided on the piezoelectric substrate, and an interdigital type input transducer provided on the piezoelectric substrate and facing the input transducer. a surface acoustic wave filter element comprising: an output transducer having a shape, wherein at least one of the input transducer and the output transducer is weighted by changing the width of an intersection part; The filled electrode, which is formed by filling the cross section of the transducer with a conductive material except for the cross section, extends from the end of the cross section to near the end of the piezoelectric substrate, and extends at least substantially perpendicular to the surface acoustic wave propagation direction. A surface acoustic wave filter element characterized in that a side edge of the filled electrode is coated with a sound absorbing layer.