JP3378182B2 - Electrode structure of series arm resonator of surface acoustic wave filter for ultra-high frequency - Google Patents

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 an inter-stage surface acoustic wave filter used in mobile communication equipment, especially in ultra high frequency portable telephones.

[0002]

2. Description of the Related Art Conventionally, as a technique in such a field,
For example, (1) "Resonator type SAW filter" Oki R & D 173 Vol. 64, No. 1 pages 79-84,
(2) Some are disclosed in Japanese Patent Laid-Open No. 9-167937.

For example, the bandwidth of the transmission frequency of mobile communication terminals of NTT car phones and mobile phones in Japan is ± 7.5 MHz centered on 932.5 MHz,
The bandwidth of the reception frequency is centered around 877.5 MHz ±
It is 7.5 MHz. The surface acoustic wave filter needs to meet such specifications.

Such a surface acoustic wave filter is constructed as a resonator type filter in which the surface acoustic wave element is a resonator and is connected in a ladder type.

FIG. 4 is a circuit diagram of such a conventional surface acoustic wave filter for super high frequencies, and FIG. 5 is a view showing an electrode structure of a conventional surface acoustic wave resonator.

First, a circuit diagram of a surface acoustic wave filter for ultrahigh frequencies will be described.

As shown in FIG. 4, a resonator type filter 1
00 is, for example, the first surface acoustic wave resonator 110 and the second surface acoustic wave resonator 110.
The surface acoustic wave resonator 120 is a multi-stage filter. The first surface acoustic wave resonator 110 includes the 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 1
20 is composed of a comb-shaped electrode 130 composed of an input interdigital electrode 131 and an output interdigital electrode 132, as shown in FIG. In this figure, 133 is a grounded grating reflector, x ₁ is the distance between the outermost electrode finger of a generally formed comb-shaped electrode and the electrode finger of the grating reflector facing this electrode finger, and λ is Comb electrode period.

Further, the comb-shaped electrode 130 is shown by an equivalent circuit 140 as shown in FIG. 6 when its resistance is neglected. 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.

Further, FIGS. 8A and 8B show frequency characteristics of impedance (X) and admittance (B) when the comb-shaped electrode 130 is shown by an equivalent circuit 140.

As can be seen from these figures, the double resonance characteristic has two resonance frequencies fg and fh. 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 of the parallel arm is further increased.
When hp is made to substantially match the fgs of the series arm, a circuit showing the characteristics of the bandpass filter having these frequencies as the center frequencies can be formed.

The reason for this will be described with reference to FIG.
As shown in the immittance frequency characteristics of, fhp≈fg
This is because the pass band is in the vicinity of the center frequency of s and the attenuation band is in the area away from the center frequency. Therefore, the resonator type filter 100 having the configuration shown in FIG.
The frequency characteristic shown in FIG. 9B is obtained.

[0013]

However, in particular,
When it comes to filters for ultra high frequencies (1.9 GHz),
Measures against harmonics are needed.

Further, in the surface acoustic wave resonator having the ladder type circuit structure having the conventional double mode resonance characteristic described above, the input / output terminals of the circuit and the pattern for wire bonding for grounding are super high frequency (1 Since it is a filter for .9 GHz), it is sufficiently larger than an extremely thin electrode finger,
Due to the curing after the die bonding in the assembly process and the heat history applied to the chip during the wire bonding, the static electricity charged in the pattern for wire bonding is applied to the boundary between the grounded grating reflector 133 and the comb-shaped electrode 130 shown in FIG. There is a problem in that electrostatic breakdown is caused in the portion (x ₁ portion).

The present invention eliminates the above-mentioned problems and widens the gap at the boundary between the grating reflector (ground) and the comb-shaped electrode, thereby improving the frequency characteristics of the surface acoustic wave filter for ultrahigh frequencies. An object is to provide an electrode structure for a series arm resonator.

[0016]

In order to achieve the above-mentioned object, the present invention provides [1] an electrode structure of a series arm resonator of a surface acoustic wave filter having a ladder type circuit configuration having double mode resonance characteristics,
A comb-shaped electrode formed on a piezoelectric substrate and meshing with a comb-shaped electrode connected to the input terminal and a comb-shaped electrode connected to the output terminal, and grating reflections arranged on both sides of the comb-shaped electrode. And a comb shape
The grates closest to the outermost electrode fingers of the electrode
One electrode finger of the ring reflector is removed, and the outermost electrode finger of the comb-shaped electrode and the electrode finger of the grating reflector facing the electrode finger are widened .

[ 2 ] In the electrode structure of the series arm resonator of the surface acoustic wave filter for super high frequencies described in [ 1 ] above, the widened spacing is 3 × s / 4 μm (where s is a reduction ratio). It is characterized by doing so.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below.

FIG. 1 is a circuit diagram of a surface acoustic wave filter for ultra high frequencies according to the present invention, and FIG. 2 is a series arm of a surface acoustic wave filter for ultra high frequencies in which electrostatic discharge countermeasures have been taken to show a first embodiment of the present invention. It is a figure which shows the electrode structure of a resonator.

In FIG. 1, the resonator type filter 2 is
For example, it is a filter having a multi-stage structure including a first surface acoustic wave resonator 3 and a 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.

The second surface acoustic wave resonator 4 connected to the series arm 6 includes an input interdigital electrode 8 and an output interdigital electrode 9 formed on the piezoelectric substrate 1, as shown in FIG. It is composed of a comb-shaped electrode 7. Grounded grating reflectors 10 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 the electrode finger, and λ is the comb-shaped electrode period.

Further, in FIG. 1, the surface acoustic wave resonators are shown as serial arms IDTS1 ... Sn and parallel arms IDTP1 ... IDTPn.
2 shows a surface acoustic wave filter having a ladder-type circuit configuration having dual-mode resonance characteristics arranged in the same manner. The electrode structure of the surface acoustic wave filter for superhigh frequencies 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 frequency characteristic is adjusted by adjusting the interval x ( the general interval x ₁ shown in the conventional example ) between the outermost electrode finger of the comb-shaped electrode 7 and the electrode finger of the grating reflector 10 facing the electrode finger. Can be improved.

FIG. 3 is a diagram showing the adjustment of the distance x and the frequency characteristic at the boundary portion between the grating reflector (ground) and the comb-shaped electrode of the present invention. In this figure, the vertical axis represents the amount of attenuation and the horizontal axis represents the frequency (f).

As is clear from this figure, when the distance x between the boundary of 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 the a curve (dotted line). ) And its distance x
When becomes smaller than the reference curve (solid line), the resonance point of the frequency rises and can be adjusted as shown by the b curve (dotted line).

Further, by changing both or one of the distance <br/> comb-shaped electrode and the grating reflectors, energy of the phase that is reflected by the reflector, different from the excited energy of the phase by the comb-shaped electrode phase Therefore, the frequency characteristics of the electrodes can be changed by combining the different phases.

As described above, according to the first embodiment, the frequency characteristic can be changed by arbitrarily changing the distance between the comb-shaped electrode and the reflector.

Next, a second embodiment of the present invention will be described.

Returning to FIG. 2, this embodiment adjusts the distance x at the boundary between the grounded grating reflector 10 and the comb-shaped electrode 7. In that case, the most of the comb-shaped electrode 130 shown in the conventional example is adjusted. The distance x ₁ between the outer electrode finger and the electrode finger of the grating reflector 133 facing the electrode finger is widened. In that case, the comb-shaped electrode 130 of the grating reflector 133 shown in the conventional example is used. The one electrode finger facing is eliminated to widen the gap.

In the conventional comb-shaped electrode shown in FIG. 5, when the comb-shaped electrode period is λ, the band frequency center is f, and the sound velocity is v, v = fλ, λ = v / f, and λ is about 3 μm.
Is. Since x ₁ = λs / 4 [where s is a reduction ratio (about 0.65)], x ₁ is about 0.5 μm. That is, in the case of a series arm resonator of a surface acoustic wave filter for ultrahigh frequencies of 1.9 GHz, the distance (spacing) x ₁ is 0.5.
μm.

In the second embodiment of the present invention, as shown in FIG. 2, one electrode finger facing the comb-shaped electrode 7 of the grating reflector 10 having a distance (spacing) x ₁ of 0.5 μm. , That is, the distance (interval) x is 3 × x ₁ =
Spread so that it becomes 1.5 μm.

Therefore, in FIG. 3, the resonance point can be adjusted so as to be lowered, and electrostatic breakdown at the boundary portion 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 dicing-processed chips are die-bonded in the package by using a paste. To fix this tip, apply some heat to cure the paste, then
To perform wire bonding, some heat is applied again.

Normally, as described above, a load is applied to the chip by heat twice in the manufacturing process. However, since the product in which the present invention is used is a super high frequency surface acoustic wave reception filter, the design The crossing length of the electrodes is very short, and
Since it is for ultra-high frequency (1.9 GHz), the pattern width is also very narrow (about 0.5 μm), so that the electrode structure is very easily destroyed 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.
And the number of electrode fingers of the grating reflector is 100.

Therefore, as shown in FIG.
As shown in FIG. 5, the boundary portion 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 as to withstand the potential difference of discharge due to the charged electric charge. The electrode structure is such that a sufficient distance is provided between the grounded portion and the electrode fingers of the comb-shaped electrode. Of course, the same configuration is applied to the grating reflector facing the opposite side of the comb electrode.

Here, by reducing the number of electrode fingers of the grating reflector on both sides of the comb-shaped electrode one by one, the regularity arranged in the cycle of 1/2 wavelength is lost, and the frequency of the attenuation pole is changed by the original pattern. Although it fluctuates to some extent, this is corrected by changing the pattern width (wavelength) of only that electrode.

As described above, according to the second embodiment, the electrode fingers having the pattern width of about 0.5 μm can withstand the electric potential difference of the discharge due to the electric charges charged by the heating during the process and the assembly. Electrostatic damage can be avoided.

The present invention is not limited to the above embodiments, and various modifications can be made based on the spirit of the present invention, and these modifications are not excluded from the scope of the present invention.

[0039]

As described in detail above, according to the present invention, the following effects can be obtained.

(A) The distance between the boundary between the grounded grating reflector and the comb-shaped electrode, that is, 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. and by increasing the general <br/> intervals x ₁ shown in the conventional example, it is possible to improve the frequency characteristics.

(B) Since 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 widened, the resonance point of the frequency is lowered and the electrostatic capacitance is reduced. It is possible to prevent destruction.

(C) By removing a part of the electrodes of the series arm resonator of the conventional surface acoustic wave filter for super high frequencies,
It is possible to easily lower the frequency resonance point and prevent electrostatic breakdown.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a surface acoustic wave filter for ultrahigh frequencies according to the present invention.

FIG. 2 is a diagram showing an electrode structure of a series arm resonator of a surface acoustic wave filter for ultrahigh frequencies on which electrostatic discharge countermeasures according to an embodiment of the present invention are taken.

FIG. 3 is a diagram showing the adjustment of the distance x and the frequency characteristic of the boundary portion between the grating reflector (ground) and the comb-shaped electrode of the present invention.

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

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 an equivalent circuit of a conventional surface acoustic wave resonator.

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

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

[Explanation of symbols]

1 Piezoelectric substrate 2 Resonator type filter 3 First surface acoustic wave resonator 4 Second surface acoustic wave resonator 5 parallel arms 6 series arms 7 Comb-shaped electrode 8-input interdigital transducer 9 Output interdigital transducer 10 Grating reflector

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-7-46077 (JP, A) JP-A-10-200374 (JP, A) JP-A-9-232905 (JP, A) Actual development Sho-55- 133617 (JP, U) Actual development Sho 62-73622 (JP, U) (58) Fields investigated (Int.Cl. ⁷ , DB name) H03H 9/25 H03H 3/10 H03H 9/145 H03H 9/64

Claims (2)

(57) [Claims] 1. In an electrode structure of a series arm resonator of a surface acoustic wave filter having a ladder type circuit configuration having double mode resonance characteristics, (a) is formed on a piezoelectric substrate and is connected to an input terminal. A comb-shaped electrode formed by interlocking a comb-shaped electrode with a comb-shaped electrode connected to the output terminal; and (b) a grating reflector arranged on both sides of the comb-shaped electrode , and (c) most of the comb-shaped electrodes. Closest to outer electrode fingers
One electrode finger of the grating reflector is deleted, and an interval between the outermost electrode finger of the comb-shaped electrode and the electrode finger of the grating reflector facing the electrode finger is widened. Electrode structure of series arm resonator of surface acoustic wave filter for ultra high frequency. 2. The electrode structure of a series arm resonator of a surface acoustic wave filter for ultrahigh frequencies according to claim 1 , wherein the widened spacing is 3 × s / 4 μm (where s is a reduction ratio). The electrode structure of the series arm resonator of the surface acoustic wave filter for ultra high frequencies characterized by the above.