JPH11136086A - Electrode structure of serial arm resonator of acoustic wave filter for very high frequency - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve a frequency characteristic and also to prevent electrostatic destruction by adjusting an interval in a boundary part of a grating reflector (ground) and a comb-shaped electrode. SOLUTION: An electrode structure of a serial arm resonator of an acoustic wave filter of a ladder circuit configuration which has a dual mode resonance characteristic is provided with a comb-shaped electrode 7 which is formed on a piezo-electric substrate 1 and also is formed by interlocking an interdigital electrode 8 that is connected to an input terminal with an interdigital electrode 9 that is connected to an output terminal and grating reflectors 10 which are arranged at both sides of the electrode 7. The intervals of the outermost electrode fingers of the electrode 7 and the electrode fingers of the reflectors 10 which face the electrode fingers are adjusted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrode structure of a series arm resonator of a surface acoustic wave filter for interstages used in mobile communication equipment, particularly, a super-high frequency portable telephone.

[0002]

2. Description of the Related Art Conventionally, techniques in such a field include:
For example, (1) “Resonator type SAW filter” Oki R & D 173 Vol. 64, no. 1 pp. 79-84,
(2) There is one disclosed in JP-A-9-167937. For example, the transmission frequency bandwidth of a mobile communication terminal of a Japanese NTT car and a mobile phone is ± 7.5 MHz centered on 932.5 MHz,
The bandwidth of the reception frequency is centered around 877.5 MHz ±
7.5 MHz. The surface acoustic wave filter needs to satisfy such specifications.

[0003] Such a surface acoustic wave filter is configured as a resonator type filter in which a surface acoustic wave element is used as a resonator and connected to a ladder type. FIG. 4 is a circuit diagram of such a conventional surface acoustic wave filter for ultrahigh frequency, and FIG. 5 is a diagram showing an electrode structure of a conventional surface acoustic wave resonator. First,
A circuit diagram of the surface acoustic wave filter for ultrahigh frequency will be described.

[0004] As shown in FIG.
00 is, for example, the first surface acoustic wave resonator 110 and the second surface acoustic wave resonator 110.
And a surface acoustic wave resonator 120 of the present embodiment. The first surface acoustic wave resonator 110 has a parallel arm 11
Second, the second surface acoustic wave resonator 120 is connected to the series arm 122. These surface acoustic wave resonators 110 and 120 are connected to the input IDT 13 as shown in FIG.
1 and a comb-shaped electrode 130 composed of an output interdigital electrode 132. In this figure, 133 grating reflector is grounded, x ₁ is the interval between the grating reflector electrode fingers facing the electrode finger and the outermost electrode fingers of the comb-shaped electrodes formed on a regular, lambda is comb This is the electrode cycle.

The comb electrode 130 is represented by an equivalent circuit 140 as shown in FIG. 6 ignoring its resistance. Here, Cd is the capacitance of the comb-shaped electrode, C ₁ and L ₁ are equivalent constants, and this equivalent circuit 140 is shown in FIG. FIGS. 8A and 8B show the comb-shaped electrode 1.
The frequency characteristics of impedance (x) and admittance (B) when 30 is represented by an equivalent circuit 140 are shown.

As can be seen from these figures, a double resonance characteristic having two resonance frequencies fg and fh is obtained. here,
fg is called a resonance frequency, and fh is called an anti-resonance frequency.
Resonators having such double resonance characteristics are arranged in the series arm and the parallel arm, respectively, and the anti-resonance frequency f
When hp is substantially equal to fgs of the serial arm, a circuit showing the characteristics of the band-pass filter having these frequencies as center frequencies is obtained.

The reason will be described below.
As shown in the frequency characteristic of the immittance of fhp ≒ fg
This is because a pass band is formed near the center frequency s, and an attenuation band is formed in a region far from the center frequency. Therefore, the resonator type filter 100 having the configuration shown in FIG.
The frequency characteristic is as shown in FIG.

[0008]

However, in particular,
When it comes to a filter for ultra-high frequency (1.9 GHz),
It is necessary to take measures against harmonics. Further, in the above-described conventional surface acoustic wave resonator having a ladder-type circuit configuration having a dual mode resonance characteristic, the input / output terminals of the circuit and the pattern for performing wire bonding for grounding have an extremely high frequency (1.
9 GHz), it is sufficiently large compared to extremely thin electrode fingers, and is charged in the wire bonding pattern by curing after die bonding in the assembly process and heat history applied to the chip during wire bonding. but there is a problem that at the boundary of the grating reflector 133 and comb electrodes 130 which is grounded is shown in FIG. 5 (x ₁ portion) and cause electrostatic breakdown.

The present invention eliminates the above problems and adjusts the spacing at the boundary between the grating reflector (ground) and the comb-shaped electrode, thereby improving the frequency characteristics. It is an object of the present invention to provide an electrode structure of the series arm resonator.

[0010]

According to the present invention, there is provided an electrode structure of a series arm resonator of a surface acoustic wave filter having a ladder type circuit configuration having dual mode resonance characteristics. Comb-shaped electrodes formed on the piezoelectric substrate and interdigitated with interdigital electrodes connected to the input terminals and output terminals, and grating reflections arranged on both sides of the interdigital electrodes Vessels,
The apparatus includes a means for adjusting a distance between an outermost electrode finger of the comb-shaped electrode and an electrode finger of the grating reflector facing the electrode finger.

[2] In the electrode structure of the series arm resonator of the surface acoustic wave filter for ultra-high frequency waves described in [1], the outermost electrode finger of the comb-shaped electrode of the grating reflector to be properly arranged. The electrode fingers facing away are eliminated, and the distance between the outermost electrode finger of the comb-shaped electrode and the electrode finger of the grating reflector facing this electrode finger is increased.

[3] In the electrode structure of the series arm resonator of the surface acoustic wave filter for ultrahigh frequency waves described in [2], the widened interval is 3 × s / 4 μm (where s is a reduction ratio). It is like that.

[0013]

Embodiments of the present invention will be described below in detail. FIG. 1 is a circuit diagram of an ultra-high frequency surface acoustic wave filter according to the present invention, and FIG. 2 is a circuit diagram of a series arm resonator of an ultra-high frequency surface acoustic wave filter according to a first embodiment of the present invention, which has taken measures against electrostatic breakdown. It is a figure showing an electrode structure.

In FIG. 1, a resonator type filter 2 comprises:
For example, the filter is a multi-stage filter including the first surface acoustic wave resonator 3 and the second surface acoustic wave resonator 4. The first surface acoustic wave resonator 3 is connected to the parallel arm 5 and the second surface acoustic wave resonator 4 is connected to the series arm 6. As shown in FIG. 2, a second surface acoustic wave resonator 4 connected to the series arm 6 is a comb-shaped electrode composed of an input interdigital electrode 8 and an output interdigital electrode 9 formed on the piezoelectric substrate 1. 7. Grating reflectors 10 that are grounded are arranged on both sides of the comb-shaped electrode 7. In this figure, x
Is the distance between the outermost electrode finger of the comb-shaped electrode 7 and the electrode finger of the grating reflector 10 facing this electrode finger;
Is the comb electrode period.

FIG. 1 shows that a surface acoustic wave resonator is composed of serial arms IDTS1... Sn and parallel arms IDTP1.
FIG. 2 shows a surface acoustic wave filter having a ladder-type circuit configuration having a dual mode resonance characteristic, and the electrode structure of the surface acoustic wave filter for ultra-high frequency shown in FIG.
Used for S1... Sn. In this embodiment, the distance between the boundary between the grating reflector (ground) 10 and the comb-shaped electrode 7, that is, the outermost electrode finger of the comb-shaped electrode 7, and the electrode finger of the grating reflector 10 facing this electrode finger the distance x, by adjusting the distance x ₁ regular shown in the conventional example, it is possible to improve the frequency characteristics.

FIG. 3 is a diagram showing the adjustment of the distance x between the grating reflector (ground) and the comb-shaped electrode of the present invention and the frequency characteristics. In this figure, the vertical axis shows the attenuation, and the horizontal axis shows the frequency (f). As is clear from this figure, when the distance x between the boundary between the grating reflector (ground) and the comb-shaped electrode becomes larger than the reference curve (solid line), the resonance point of the frequency decreases, and as shown by the curve a (dotted line). When the distance x becomes smaller than the reference curve (solid line), the resonance point of the frequency increases, and the curve b (dotted line)
It can be adjusted as follows.

Further, by changing both or one of the intervals between the comb-shaped electrode and the reflector, the phase of the energy reflected by the reflector becomes different from the phase of the energy excited by the comb-shaped electrode. By combining the phases, the frequency characteristics of the electrodes can be varied. As described above, according to the first embodiment, the frequency characteristic can be changed by arbitrarily changing the interval between the comb-shaped electrode and the reflector.

Next, a second embodiment of the present invention will be described. Returning to FIG. 2, in this embodiment, the distance x between the grounded grating reflector 10 and the boundary between the comb-shaped electrode 7 is adjusted. In this case, the comb-shaped electrode 130 shown in the conventional example is adjusted.
The outermost electrode fingers, but is intended to widen the distance x ₁ between the electrode fingers of the grating reflector 133 facing to the electrode fingers, in which case, as shown in the conventional example, the comb-shaped electrodes of the grating reflector 133 One electrode finger facing 130 is eliminated to increase the interval.

In the conventional comb-shaped electrode shown in FIG. 5, if the period of the comb-shaped electrode is λ, the center of the band frequency is f, and the sound speed is v, v = fλ, λ = v / f, and λ is about 3 μm
It is. Further, since x ₁ = λs / 4 [where s is the reduction ratio (about 0.65)], x ₁ is about 0.5 μm. That is, in the case of series arm resonators of the super high frequency surface acoustic wave filter of 1.9 GHz, the distance (interval) x ₁ 0.5
μm.

In the second embodiment of the present invention, as shown in FIG. 2, the distance (interval) x ₁ is 0.5 μm, but one electrode finger facing the comb-shaped electrode 7 of the grating reflector 10 is used. , Ie, the distance (interval) x is 3 × x ₁ =
Spread to 1.5 μm. Therefore, in FIG. 3, the resonance point can be adjusted so as to be lowered, and the electrostatic breakdown at the boundary x between the grounded grating reflector and the comb-shaped electrode can be prevented.

In the manufacturing process of this product, a wafer process by photolithography is performed several times, and the diced chips are die-bonded in a package using a paste. To fix the chip, apply some heat to harden the paste,
To perform wire bonding, some heat is applied again.

Normally, a load due to heat is applied to the chip twice in the manufacturing process as described above. However, the product to which the present invention is applied is an ultra-high frequency surface acoustic wave receiving filter. The electrode intersection length is very short, and
Since it is for ultrahigh frequency (1.9 GHz), the pattern width is very narrow (about 0.5 μm), so that the electrode structure is very easily destroyed even by some static electricity. Incidentally, for example, the number of electrode fingers of the input interdigital transducer is 90, and the number of electrode fingers of the output interdigital transducer is 91.
The number of electrode fingers of this and the grating reflector is 100.

Therefore, for an electrode having a short intersection length, FIG.
As shown in FIG. 5, the boundary between the grounded grating reflector and the comb-shaped electrode is reduced by one electrode finger of the conventional grating reflector shown in FIG. 5 so that it can withstand the potential difference of the discharge due to the charged electric charge. The electrode structure is such that the space between the ground portion and the electrode fingers of the comb-shaped electrode is sufficiently spaced. Of course, a similar configuration is applied to the grating reflector facing the opposite side of the comb-shaped electrode.

Here, by reducing the number of electrode fingers of the grating reflector on both sides of the comb-shaped electrode one by one, regularity arranged at a period of 1/2 wavelength is lost, and the frequency of the attenuation pole is reduced by the original pattern. Although it fluctuates slightly, correction is performed by changing the pattern width (wavelength) of only the electrode. As described above, according to the second embodiment, the pattern width is 0.1 mm.
The electrode finger of about 5 μm can withstand the potential difference of the discharge due to the electric charge charged by the heating during the process and the assembling, and the electrostatic breakdown can be avoided.

It should be noted that the present invention is not limited to the above-described embodiment, and various modifications are possible based on the spirit of the present invention, and these are not excluded from the scope of the present invention.

[0026]

As described above, according to the present invention, the following effects can be obtained. (1) According to the first aspect of the invention, the distance between the grounded grating reflector and the boundary between the comb-shaped electrode, that is,
The most and outer electrode fingers of the interdigital electrodes, the distance x between the electrode fingers of the grating reflector facing the electrode fingers, by adjusting the distance x ₁ regular shown in the conventional example, to improve the frequency characteristic be able to.

(2) According to the second aspect of the invention, the distance x between the outermost electrode finger of the comb-shaped electrode and the electrode finger of the grating reflector facing this electrode finger is increased. The frequency resonance point can be lowered, and electrostatic breakdown can be prevented. (3) According to the third aspect of the invention, by removing a part of the electrode of the series arm resonator of the conventional surface acoustic wave filter for ultrahigh frequency, the resonance point of the frequency can be easily lowered and the electrostatic force can be reduced. Destruction can be prevented.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of an ultra-high frequency surface acoustic wave filter according to the present invention.

FIG. 2 is a view showing an electrode structure of a series arm resonator of a surface acoustic wave filter for ultra-high frequency in which a countermeasure against electrostatic destruction according to an embodiment of the present invention is taken.

FIG. 3 is a diagram illustrating adjustment of a distance x between a grating reflector (ground) and a comb-shaped electrode and frequency characteristics according to the present invention.

FIG. 4 is a circuit diagram of a conventional ultra-high frequency surface acoustic wave filter.

FIG. 5 is a diagram showing an electrode structure of a conventional surface acoustic wave resonator.

FIG. 6 is an equivalent circuit diagram of a conventional surface acoustic wave resonator.

FIG. 7 is a diagram showing symbols of a conventional surface acoustic wave resonator.

FIG. 8 is a diagram showing impedance and admittance of a conventional resonator.

FIG. 9 is a diagram showing immittance and pass characteristics of a conventional filter.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Piezoelectric substrate 2 Resonator type filter 3 First surface acoustic wave resonator 4 Second surface acoustic wave resonator 5 Parallel arm 6 Series arm 7 Comb electrode 8 Input interdigital electrode 9 Output interdigital electrode 10 Grating reflection vessel

Claims (3)

[Claims] 1. An electrode structure of a series arm resonator of a surface acoustic wave filter having a ladder type circuit configuration having a dual mode resonance characteristic, wherein (a) the electrode structure is formed on a piezoelectric substrate and connected to an input terminal. A comb-shaped electrode formed by meshing the IDT and the IDT connected to the output terminal;
(B) adjusting the distance between the grating reflectors arranged on both sides of the comb-shaped electrode, and (c) adjusting the distance between the outermost electrode finger of the comb-shaped electrode and the electrode finger of the grating reflector facing the electrode finger. And an electrode structure of a series arm resonator of the surface acoustic wave filter for ultrahigh frequency. 2. The electrode structure of a series arm resonator of a surface acoustic wave filter for ultra-high frequency waves according to claim 1, wherein said grating reflector faces the outermost electrode finger of said comb-shaped electrode to be properly arranged. A series of super-high-frequency surface acoustic wave filters, characterized in that the electrode fingers are eliminated and the outermost electrode fingers of the comb-shaped electrode and the electrode fingers of the grating reflector facing the electrode fingers are widened. Electrode structure of arm resonator. 3. The electrode structure of a series arm resonator of a surface acoustic wave filter for ultra-high frequency waves according to claim 2, wherein the widened interval is 3 × s / 4 μm (where s is a reduction ratio). An electrode structure of a series arm resonator of a surface acoustic wave filter for ultra-high frequency, characterized in that: