JPH11150438A - Band rejection type filter using IDT type surface acoustic wave resonator - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To realize a band rejection filter having a large attenuation in a stop band. SOLUTION: One or more IDT surface acoustic wave resonators using a θYX-LT piezoelectric substrate and an Al electrode are connected in series. The logarithm of the resonator, the electrode period, the film thickness, and θ are set to desired values. [Effect] The propagation loss of the surface acoustic wave near the anti-resonance frequency of the resonator can be reduced to a negligible level. As a result, the attenuation can be improved by 4 dB compared to a band rejection filter using a 36YX-LT piezoelectric substrate.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a band rejection filter using a steep impedance characteristic near an anti-resonance frequency of an IDT type surface acoustic wave resonator.

[0002]

2. Description of the Related Art Heretofore, IDT (Interdi) has been used as a band-rejection filter using a 36-degree rotated Y-cut X-propagating lithium tantalate single crystal (36YX-LT) substrate.
2. Description of the Related Art A filter using a digital transducer type surface acoustic wave resonator (FIG. 1) is known. 36YX
-An IDT-type resonator was formed from an aluminum (Al) film having a thickness of, for example, 100 nm in an 800 MHz band on an LT substrate, and was electrically connected in series. A filter using a reflector type resonator shown in FIG. 2 instead of the IDT type resonator is also known.

The reflector type resonator shown in FIG. 2 mainly confines a surface acoustic wave inside the IDT and the reflector by the reflectors 2 provided at both ends of the IDT 1, whereas the IDT type resonator shown in FIG. The device increases the number of electrode pairs of the IDT 1 and confines the surface acoustic wave inside the IDT by utilizing the reflection characteristics of the surface acoustic wave at the electrode finger. Since the potential of the metal part differs between the IDT and the reflector, the propagation characteristics of the surface acoustic wave also differ. For this reason, in the reflector type resonator, it is necessary to optimize the Al film thickness and the like in consideration of the propagation characteristics of the surface acoustic wave in both the IDT and the reflector.
In general, ripples due to the difference in the characteristics tend to occur near the anti-resonance frequency, and a band-rejection filter in which resonators are connected in series tends to have insufficient attenuation.

On the other hand, the IDT type surface acoustic wave resonator can confine most surface acoustic waves inside the IDT.
The Al film thickness and the like are optimized by considering only the propagation characteristics of the surface acoustic wave in the DT. Therefore, a Q value near the anti-resonance frequency is extremely excellent, and a filter with a large attenuation can be realized. In the 800MHz band, one IDT type resonator
As shown in FIGS. 6 to 10, when the electrode period (λ) is 5 μm, the number of electrode pairs is 200, and the aperture length is 20λ, a maximum attenuation of 12 dB can be realized. When a plurality of resonators having different electrode periods are connected in series, an attenuation of about 17 dB can be realized in a 25 MHz bandwidth.

[0005]

In a band rejection filter used for mobile communication, a larger amount of attenuation is required. For example, the GSM system, which is a typical mobile communication system, requires a 25 MHz bandwidth and an attenuation of 20 dB or more. It is necessary to improve the attenuation of the resonator by 3 dB or more.

[0006] The causes of the deterioration of the attenuation in the IDT type resonator are deterioration caused by the electrical sheet resistance of the Al film and deterioration caused by the propagation loss of the surface acoustic wave. Degradation due to sheet resistance can be reduced by increasing the thickness of the Al film, but degradation due to propagation loss increases rapidly. For this reason, even if the thickness of the Al film is simply increased, the attenuation cannot be improved.

An object of the present invention is to provide a band rejection type filter in which a plurality of IDT type surface acoustic wave resonators are connected in series and which have excellent attenuation.

[0008]

In order to achieve the above object, in a band rejection filter in which a plurality of IDT surface acoustic wave resonators are connected in series, the electrode period λ of the IDT, the film thickness h of the Al electrode, the substrate The relationship between the cut angle θ and the attenuation was examined in detail by simulation and experiment. As a result, they have found that the above object can be achieved by setting λ, h, and θ in a desired relationship, and have realized a band rejection filter that can achieve the above object by setting the IDT to a desired number of electrode pairs. That is, claim 1
Alternatively, as set forth in claim 2, an electrode cycle formed of a metal film having a thickness of h and containing Al as a main component is λ on the surface of the θ-degree rotated Y-cut X-propagating lithium tantalate (θYX-LT) piezoelectric substrate.
In a band-rejection filter in which at least one or more IDT surface acoustic wave resonators having IDTs are connected in series, the range of θ is 31+ (h / λ) × 80 + (h / λ) × (h / λ) × 790 <θ <38+ (h / λ)
× 100 + (h / λ) × (h / λ) × 500 More preferably, 33+ (h / λ) × 140 + (h / λ) × (h / λ) × 170 <θ <35+ (h /
λ) × 150 + (h / λ) × (h / λ) × 200. Further, the number of electrode pairs of the IDT is 90 or more, and 0.12> h / λ> 0.05.

Hereinafter, the effects of the present invention will be described with reference to Table 1.

[0010]

[Table 1]

Table 1 shows the propagation loss at the antiresonance frequency of the surface acoustic wave when the Al IDT electrode is formed on the θYX-LT piezoelectric substrate, and the Al film thickness h converted by the rotation angle θ and the wavelength of the surface acoustic wave. 6 is a table showing a relationship of / λ. These are calculated from an analysis simulator for a surface acoustic wave resonator (1994 Ultrasonic Symposium, pages 275 to 276, 15th ultrasonic symposium). In this simulator, the number of electrode pairs of the IDT is approximated by infinite pairs. It has been found that in an actual resonator, when the number of electrode pairs of the IDT is 90 or more, the surface acoustic wave leaking from the end of the resonator is sufficiently small, and the influence on the Q value can be neglected.

Conventionally, a 36YX-LT piezoelectric substrate used for a surface acoustic wave filter does not satisfy the condition (<0.003 dB / λ) where propagation loss is almost negligible when h / λ> 0.05. On the other hand, even when h / λ> 0.05, 31+ (h / λ) × 80 + (h / λ) × (h / λ) × 790 <θ <38+ (h / λ)
× 100 + (h / λ) × (h / λ) × 500 More preferably, 33+ (h / λ) × 140 + (h / λ) × (h / λ) × 170 <θ <35+ (h /
When λ) × 150 + (h / λ) × (h / λ) × 200, it was found that the propagation loss can be reduced to a negligible level. From this, it has been clarified that a band rejection filter having a large attenuation can be realized by using the vicinity of the anti-resonance frequency of the resonator as the stop band.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments according to the present invention will be described in detail. FIG. 3 is a plan view showing one embodiment of the surface acoustic wave resonator according to the present invention, and FIG. 4 is a partial cross-sectional view of the AA section of FIG. The present embodiment is a single-aperture resonator, and has the same configuration as the conventional configuration except for the electrode thickness and θ.
That is, on the substrate surface of the θYX-LT single crystal 5, the IDT electrode 1 is patterned with a metal film containing Al as a main component, and a high-frequency signal is applied between two electrodes 1A and 1B in which comb-shaped electrode fingers are interposed. Have been added. In each electrode finger, the thickness of the comb-shaped electrode is h, the width is L, and the electrode period is λ (substantially the same as the propagation wavelength of the surface acoustic wave). A gap having a width S between adjacent electrode fingers is provided. The electrode structure is h = 0.308 μm,
L / S = 1.18, the number of electrode finger pairs is 200, and the aperture length is 20λ.

With the connection shown in FIG. 5, the resonator 6 shown in FIG.
6 to 10 show the maximum attenuation at the anti-resonance frequency when the pass characteristics of the above are measured. θ = 36, 38, 40, 42, 44, 4
Using the θYX-LT piezoelectric substrate that is 6, 50, 52, λ = 8.33μ
m (h / λ = 0.037), 6.25 μm (h / λ = 0.049), 5 μm (h /
λ = 0.062), 4.12 μm (h / λ = 0.075), 3.57 μm (h / λ =
0.086).

When h / λ ≦ 0.049, the attenuation is not improved for any θ as compared with the conventional 36YX-LT piezoelectric substrate (θ = 36). On the other hand, when h / λ ≧ 0.062, the attenuation was improved. Especially at h / λ = 0.062, 44YX-LT, h / λ = 0.
At 075, the attenuation was most improved at 44-46YX-LT, and at h / λ = 0.086, at 46YX-LT.

FIG. 11 is a graph showing the pass characteristics of a band rejection filter manufactured using the above resonator. 46YX
-An IDT surface acoustic wave resonator patterned with an Al metal film is formed on an LT substrate. The thickness h of the comb-shaped electrode 1 is 308
The electrode structure is such that the electrode finger width L = l / 4 (l = 3.57 mm), the number of electrode finger pairs is 200, and the opening length is 20 l. According to FIG. 11, a band rejection filter having a maximum attenuation of 15.8 dB, which is about 4 dB larger than that of the conventional filter, can be realized.

FIGS. 12 and 13 show resonators 6 having different values of l.
Are a circuit diagram and a plan view of a band-rejection type filter manufactured by electrically connecting a plurality of filters in series.
According to FIG. 11, it is possible to realize a band rejection filter having an attenuation amount improved by 4 dB as compared with the conventional one without affecting the loss characteristics of the pass band.

[0018]

As described above, according to the present invention,
By forming a desired IDT surface acoustic wave resonator using a θ-degree rotated Y-cut X-propagating lithium tantalate piezoelectric substrate, a band-rejection filter with a large attenuation can be manufactured. For example, in the 800 MHz band, a band rejection filter having an attenuation of 21 dB in a 25 MHz bandwidth can be provided.

[Brief description of the drawings]

FIG. 1 is a diagram showing an electrode structure of an IDT type surface acoustic wave resonator.

FIG. 2 is a diagram showing an electrode structure of a reflector type surface acoustic wave resonator.

FIG. 3 is a plan view showing a band-rejection type filter using an IDT type surface acoustic wave resonator to which the present invention is applied.

FIG. 4 is a sectional view taken along the line AA in FIG. 3;

FIG. 5 is a circuit diagram of a band rejection type filter using an IDT type surface acoustic wave resonator.

FIG. 6 is a diagram showing the cut angle θ dependence of the attenuation in the pass characteristic of a band rejection filter having an electrode period converted Al film thickness of 0.037.

FIG. 7 is a diagram showing the cut angle θ dependence of the attenuation in the pass characteristic of a band rejection filter having an electrode period converted Al film thickness of 0.049.

FIG. 8 is a graph showing an electrode period converted Al film thickness of 0.062 according to the present invention.
FIG. 7 is a diagram showing the cut angle θ dependence of the attenuation in the pass characteristic of the band rejection filter.

FIG. 9 shows that the electrode period converted Al film thickness to which the present invention is applied is 0.075.
FIG. 7 is a diagram showing the cut angle θ dependence of the attenuation in the pass characteristic of the band rejection filter.

FIG. 10 is a graph showing an electrode period converted Al film thickness of 0.08 according to the present invention.
FIG. 6 is a diagram illustrating the cut angle θ dependence of the attenuation in the pass characteristic of the band rejection filter of No. 6;

FIG. 11 is a graph showing an electrode period converted Al film thickness of 0.08 according to the present invention.
FIG. 6 is a bandpass characteristic diagram of a band rejection filter having a cut angle θ of 46 degrees in FIG.

FIG. 12 is a circuit diagram of a band rejection filter in which a plurality of IDT surface acoustic wave resonators according to the present invention are electrically connected in series.

FIG. 13 is a plan view of FIG. 12;

FIG. 14 is a diagram showing ranges of θ and h / λ described in claims 1 and 2 of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... IDT electrode, 2 ... Reflector, 3 ... Input terminal, 4 ... Output terminal, 5 ... ThetaYX-LT piezoelectric substrate cut out by a desired cut angle, 6 ... IDT surface acoustic wave resonator, 7 ... Claim. 3. The range of θ and h / λ described in 1, 8... The range of θ and h / λ described in claim 2.

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Kengo Asai 1-280 Higashi Koigakubo, Kokubunji-shi, Tokyo Inside Central Research Laboratory, Hitachi, Ltd.

Claims (3)

[Claims] 1. A film having aluminum as a main component is formed on a surface of a piezoelectric substrate rotated by θ degrees rotated Y-cut X-propagation lithium tantalate.
In a band rejection type surface acoustic wave filter having an attenuation band in a frequency band higher than a pass frequency band in which at least one or more IDT surface acoustic wave resonators made of a metal film having an electrode period of λ are connected in series, + (h / λ) × 80 + (h / λ) × (h / λ) × 790 <θ <38+ (h / λ)
A surface acoustic wave filter characterized by × 100 + (h / λ) × (h / λ) × 500, and 0.12> h / λ> 0.05. 2. The surface acoustic wave filter according to claim 1, wherein 33+ (h / λ) × 140 + (h / λ) × (h / λ) × 170 <θ <35+ (h /
λ) × 150 + (h / λ) × (h / λ) × 200. A surface acoustic wave filter. 3. A surface acoustic wave filter according to claim 2, wherein the number of IDT electrode pairs is 90 or more.