JPH0993072A - Surface acoustic wave filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface acoustic wave (SAW) filter with which frequency characteristics are improved and the rate of frequency shift change caused by dispersion in electrode finger width is reduced by forming the ratio of elec trode finger width at a specified level corresponding to the electrode finger pitch of comb-line electrode at a SAW resonator. SOLUTION: This SAW filter is formed by stepwisely connecting plural SAW resonators to serial SF1 and SF2 and parallel PF1-PF3. The respective SAW resonators are formed on a piezoelectric substrate as electrode patterns so that the respective electrode fingers of a pair of comb-line electrodes can be alternately inserted. Then, the electrode fingers of comb-line electrodes are formed in a certain pattern so that width W corresponding to its pitch P can be the ratio of 60% at least. Thus, the difference between a resonance frequency and an anti-resonance frequency can be relatively reduced.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a surface acoustic wave (SA).
The present invention relates to a resonator (SAW resonator) using an element utilizing W: Surface Acoustic Wave, and in particular, a filter (ladder type SAW) formed by combining a plurality of such SAW resonators in series and parallel and connecting them in a ladder shape. The present invention relates to a technique for improving frequency characteristics of a filter.

[0002]

2. Description of the Related Art FIG. 9 shows a structural example of a typical SAW resonator. In the figure, (a) schematically shows the structure of the SAW resonator, and (b) shows it in symbol notation. In FIG. 9A, 10 is a piezoelectric substrate, and 20 is a SAW resonator formed on the piezoelectric substrate 10. The piezoelectric substrate 10 is formed of a single crystal such as lithium niobate (LiNbO ₃ ) or lithium tantalate (LiTaO ₃ ), or a piezoelectric ceramic such as lead zirconate titanate (PZT). Further, the SAW resonator 20 is composed of a pair of excitation comb electrodes (IDT: Inter-Digi).
tal transducers) 21 and 22, and reflectors 23 and 24 arranged on both sides of the IDT, respectively.
The T21 and 22 and the reflectors 23 and 24 are formed by, for example, growing aluminum (Al) by sputtering and patterning it. At this time, input (I
IDT 21 on the N) side and IDT 22 on the output (OUT) side
Are patterned such that the respective electrode fingers F ₁ and F ₂ are alternately sandwiched. In addition, P _IDT is each _{IDT 2}
The distance (pitch) between adjacent electrode fingers of 1 and 22 is expressed as P
_REF represents an interval (pitch) between adjacent electrodes of the reflectors 23 and 24.

In a SAW resonator having such a structure,
The surface acoustic waves (SAW) generated by the excitation electrodes (IDTs 21, 22) are reflected by the reflectors 23, 24 arranged on both sides of the electrodes to generate a standing wave, thereby generating a vibration having a high Q. Functions to excite. In this case, the excited frequency is determined depending on the magnitudes of the pitches P _IDT and P _REF .

By appropriately combining a plurality of such SAW resonators in series and parallel and connecting them in a ladder shape, for example, a ladder type SAW filter as shown in FIG. 1A is constructed. In such a ladder type SAW filter, its filter characteristic is a combination of the frequency characteristic due to the resonance point of the SAW resonators connected in series and the frequency characteristic due to the resonance point and the antiresonance point of the SAW resonators connected in parallel. It is determined.

In the conventionally known technique, as shown in FIG. 10, the width (W ₀ ) of the electrode fingers of the comb-shaped electrodes forming the SAW resonator used in the ladder type SAW filter is
The pattern was formed at a ratio of 50% (= W ₀ / P) with respect to the pitch (P) of the electrode fingers, and was fixed at λ / 4. It should be noted that λ represents the wavelength of the surface acoustic wave at the resonance (or anti-resonance) frequency f ₀ and is represented by λ = V / f ₀ (where V is the propagation velocity of the surface acoustic wave).

[0006]

Generally, in order to improve the frequency pass characteristics of the SAW filter, it is necessary to increase the amount of signal attenuation outside the band. In other words, SA
The frequency pass characteristic of the W filter has a sharp rise in the characteristic curve at the boundary between the pass band and the out-of-band from the purpose of use (that is, use as a band pass filter).
Or, it is required that you are standing down.

However, none of the conventionally known SAW filters can satisfy such demands satisfactorily. Also, when manufacturing a SAW filter,
Due to the variation in the process, the width of the electrode fingers of the comb-shaped electrodes forming the SAW resonator also varies (that is, becomes nonuniform). As a result, the propagation velocity (V) of the surface acoustic wave in macroscopic view changes, and the resonance frequency (f ₀ ) shifts. This is an obstacle to stable production of products and is a factor of reducing the yield.

The present invention has been made in view of the above-mentioned problems in the prior art. It improves frequency characteristics, reduces the frequency shift change rate due to variations in electrode finger width caused by the manufacturing process, and thus improves the yield. An object is to provide a surface acoustic wave (SAW) filter that can contribute to improvement.

[0009]

In order to solve the above-mentioned problems of the prior art, according to the present invention, a SAW filter comprising a plurality of SAW resonators connected in series and in a ladder shape is used. Each of the resonators has a pair of excitation comb-shaped electrodes formed by forming an electrode pattern on a piezoelectric substrate such that the electrode fingers are alternately sandwiched between the comb-shaped electrodes.
Reflectors each having an electrode pattern formed on both sides of a pair of comb-shaped electrodes, and the electrode fingers are patterned at a ratio of at least 60% to the pitch of the electrode fingers of the comb-shaped electrodes. A SAW filter is provided.

When the ratio of the electrode finger width to the electrode finger pitch of the comb-shaped electrode is increased from the conventional 50% (to at least 60% in the preferred embodiment of the present invention), the electromechanical coupling coefficient of the comb-shaped electrode becomes higher. It gets smaller. Therefore,
The difference between the resonance frequency and the anti-resonance frequency of the SAW filter (Δf
And) becomes smaller. This makes it possible to make the rising and falling of the characteristic curve steep at the boundary between the pass band and the outside of the band. That is, the frequency pass characteristic of the SAW filter can be improved.

Further, when the ratio of the electrode finger width is changed, the propagation velocity of the surface acoustic wave in macro view changes. This acts in the direction of lowering the resonance frequency and the anti-resonance frequency (frequency shift). On the other hand, the electromechanical coupling coefficient also changes, shifting the resonance frequency and the anti-resonance frequency as described above. When both of these frequency shift amounts are combined, the frequency shift change rate with respect to the electrode finger width change amount becomes smaller as the ratio of the electrode finger width increases.

Therefore, by increasing the ratio of the electrode finger width, it is possible to suppress the frequency fluctuation due to the fluctuation factor of the electrode finger width, and it is possible to stably manufacture the product. This contributes to the improvement of yield. Furthermore, when the ratio of the electrode finger width is changed, the propagation velocity of the surface acoustic wave and the electrostatic capacitance of the comb-shaped electrode are changed, and a difference appears between the resonance frequency and the anti-resonance frequency. Therefore, by making good use of these changes, it becomes possible to finely adjust factors such as the pass band width, impedance matching, and center frequency that influence the characteristics of the SAW filter.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows the structure of a SAW filter according to an embodiment of the present invention. In the figure, (a) is S
The circuit structure of an AW filter is shown, and (b) shows a part of the pattern of the comb-shaped electrode which comprises the SAW resonator used in the circuit of (a). As shown in FIG. 1A, the SAW filter according to this embodiment has, as a basic configuration, two Ss connected in series with respect to input / output (IN / OUT).
AW resonators SF1 and SF2 and input / output (IN / OU
T) three SAW resonators PF1 connected in parallel
To PF3 are combined in a ladder shape to form a ladder SAW filter having a four-stage structure. Furthermore, these SAW resonators SF1, SF2, PF1, PF2 and P
Inductors L1, L2, L3, in series with F3, respectively.
L4 and L5 are connected.

Although not shown, each SAW resonator is formed with an electrode pattern such that the electrode fingers are alternately sandwiched on the piezoelectric substrate, as shown in FIG. 9A. A pair of comb-shaped electrodes for excitation and a reflector formed by forming electrode patterns on both sides of the pair of comb-shaped electrodes. In FIG. 1B, P represents the pitch of the electrode fingers of the comb-shaped electrodes (the interval between the adjacent electrode fingers of the pair of comb-shaped electrodes), and W represents the width of the electrode fingers.

In the present embodiment, the width (W) of the electrode fingers of the comb-shaped electrode of the SAW resonator is larger than 50% (conventional type) with respect to the pitch (P) of the electrode fingers (W / P> 50).
%). The selection of the ratio (W / P) of the electrode finger width can be easily dealt with by appropriately changing the exposure time when forming the electrode pattern.

As will be described later, in the preferred embodiment of the present invention, the electrode finger width ratio (W / P) is preferably selected to be 60% to 80%.

[0017]

EXAMPLE The present inventor changed the electrode finger width of a comb-shaped electrode with respect to the SAW resonator used in the ladder SAW filter having a four-stage structure according to the embodiment shown in FIGS. 1 (a) and 1 (b). The changes in various characteristics were investigated. A SAW filter having a center frequency of about 950 MHz was used as the sample filter. In addition, in the configuration of FIG. 1A, the inductance of each of the inductors L1 to L5 was set to 1.5 mH.

FIG. 2 shows the frequency pass characteristics of the SAW filter with respect to changes in the electrode finger width, and FIG. 3 shows the reflection characteristics of the SAW filter with respect to changes in the electrode finger width. The following data was obtained from these characteristic graphs. FIG. 4 shows changes in the difference (Δf) between the resonance frequency and the anti-resonance frequency with respect to changes in the electrode finger width ratio (W / P).

As can be seen from FIG. 4, the value of Δf becomes smaller as the ratio of the electrode finger width increases. Where Δf
2 represents the degree of steep change in the rising and falling edges of the curve at the boundary between the pass band and the out-of-band in the frequency pass characteristic shown in FIG. 2, and the smaller the value of Δf, the smaller the value of Δf. The rising and falling changes are sharp.

FIG. 5 shows changes in the frequency shifts at the resonance point and the anti-resonance point with respect to changes in the electrode finger width ratio (W / P). Further, FIG. 6 shows changes in the pass band width with respect to changes in the electrode finger width ratio (W / P). As can be seen from FIG. 5, the rate of change in frequency shift decreases as the electrode finger width ratio (W / P) increases. Further, it can be seen that the rate of change at the resonance point is smaller than the rate of change at the anti-resonance point. Therefore, the pass band width is also reduced (see FIG. 6).

As can be seen from FIG. 6, when the electrode finger width ratio (W / P) is 60% or less, the pass band width becomes too large, which is not preferable.
P) is preferably at least 60%. However, if the electrode finger width (W) is excessively increased, SAW
Since the power withstand of the filter is deteriorated and the life of the SAW filter is shortened, it is desirable to keep the ratio to an appropriate level.

FIG. 7 shows S with respect to changes in the electrode finger width (W).
The change in the life of the AW filter is shown. In the example shown,
The change in the life (T) with respect to the electrode finger width (W) is shown in logarithmic display. In the current test environment (1 W input, chip temperature 85 ° C), there is no problem in using the product if it has a life of 8 hours or more, so electrode finger width ratio (W / P)
Is 80% or less, there is no problem.

Further, in FIG. 8, the ratio of electrode finger width (W / P)
The change in yield is shown with respect to the change. However, the illustrated relationship is that the data of the variation (standard deviation) of the electrode finger width and the frequency shift amount with respect to the change rate of the electrode finger width (see FIG. 5).
It was calculated based on Since the frequency shift change rate becomes small up to the electrode finger width ratio (W / P) of 75% (see FIG. 5), the yield is improved (see FIG. 8), but above that, the variation in electrode finger width is large. Therefore, the yield tends to decrease.

The pitch P (in this embodiment, P = 2.
If the ratio (W / P) of the electrode finger width W to 5 μm) is set to 80% or more, the width (P−W) of the insulating portion is 0.5 μm.
Since it is less than m, it is difficult to form the electrode pattern of the SAW resonator on the piezoelectric substrate under the current technology. That is, it becomes difficult to manufacture the SAW filter. Therefore, the electrode finger width ratio (W / P) is preferably 80% or less.

From the above, the ratio (W / P) of the width of the comb electrodes to the pitch of the electrode fingers is preferably in the range of 60% to 80%. As described above, according to the configuration of the SAW filter according to the present embodiment, the electrode finger width ratio (W / P) is larger than the conventional 50% (60% to 80%).
%), The difference (Δf) between the resonance frequency and the anti-resonance frequency can be made relatively small, and as a result, the rise of the curve at the boundary between the pass band and the out-of-band can be achieved. And, the change of the falling edge can be made steep. This contributes to the improvement of frequency characteristics.

Further, as can be seen from FIG. 5, the rate of change in frequency shift can be suppressed by increasing the ratio (W / P) of the electrode finger widths, so that stable production of products becomes possible. . This contributes to the improvement of yield. Furthermore, by appropriately changing the ratio (W / P) of the electrode finger width within a preferable range (60% to 80%),
It is possible to finely adjust the factors such as the pass band width and the center frequency that determine the characteristics of the SAW filter.

[0027]

As described above, according to the present invention, S
In the AW filter, the frequency characteristic can be improved by setting the ratio of the electrode finger width to the pitch of the electrode fingers of the comb-shaped electrodes forming the SAW resonator to a specific ratio larger than the conventional 50%. The rate of change in frequency shift with respect to the variation in the electrode finger width caused by the manufacturing process can be reduced, and the yield can be improved.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of a SAW filter according to an embodiment of the present invention, in which (a) is a circuit configuration diagram of the SAW filter;
(B) is a pattern diagram of a part of the comb-shaped electrode that constitutes the SAW resonator.

FIG. 2 is a diagram showing a frequency pass characteristic of a SAW filter with respect to a change in a ratio of electrode finger widths.

FIG. 3 is a diagram showing a reflection characteristic of a SAW filter with respect to a change in a ratio of electrode finger widths.

FIG. 4 is a diagram showing changes in the difference between the resonance frequency and the anti-resonance frequency with respect to changes in the ratio of the electrode finger width.

FIG. 5 is a diagram showing changes in frequency shift at a resonance point and an anti-resonance point with respect to a change in electrode finger width ratio.

FIG. 6 is a diagram showing a change in pass band width with respect to a change in electrode finger width ratio.

FIG. 7 is a diagram showing changes in the life of the SAW filter with respect to changes in the electrode finger width.

FIG. 8 is a diagram showing a change in yield with respect to a change in a ratio of electrode finger widths.

FIG. 9 is a diagram showing a configuration example of a typical SAW resonator,
(A) is a perspective view schematically showing the structure of a SAW resonator,
(B) is the figure which showed it in the symbol notation.

FIG. 10 is a pattern diagram of a part of a comb-shaped electrode forming a conventional SAW resonator.

[Explanation of symbols]

 PF1 to PF3 ... SAW resonators connected in parallel SF1, SF2 ... SAW resonators connected in series L1 to L5 ... Inductor P ... Pitch of electrode fingers of comb-shaped electrodes W ... Width of electrode fingers

Claims (2)

[Claims] 1. A surface acoustic wave filter comprising a plurality of surface acoustic wave resonators connected in series in parallel in a ladder shape, wherein each of the plurality of surface acoustic wave resonators has a respective electrode on a piezoelectric substrate. A pair of excitation comb-shaped electrodes formed by forming electrode patterns so that fingers are alternately sandwiched;
And a reflector formed by forming electrode patterns on both sides of a pair of comb-shaped electrodes, and patterning the electrode finger width (W) with respect to the electrode finger pitch (P) of the comb-shaped electrodes at a ratio of at least 60%. A surface acoustic wave filter characterized in that 2. The surface acoustic wave according to claim 1, wherein the width (W) of the electrode fingers with respect to the pitch (P) of the electrode fingers of the comb-shaped electrode is patterned at a ratio of 60% to 80%. filter.