JPH11191720A - Surface acoustic wave device and surface accosting wave filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance the pass band characteristics and the attenuation characteristics of the SAW filer by making a Δf adjustable of each SAW device formed on a piezoelectric substrate. SOLUTION: The surface acoustic wave SAW device D1 is formed by forming at least a couple of inter-digital transducers IDTs on a principal surface of a piezoelectric substrate, and a relation of 0.04<=w2/w1<=0.20 is established, where w1 is a length of an electrode finger 30a of the IDT electrode 30 and w2 is an interval between the tip of the electrode finger 30a and a bus bar 30 of the opposed IDT electrode 30. Then a small electrode finger 30b is projected from the bus bar 30c toward to the interval.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave device and a surface acoustic wave filter used as a resonator or a frequency band filter incorporated in a communication device such as a cellular phone or a cellular phone.

[0002]

2. Description of the Related Art Conventional surface acoustic waves (Surface Acoustic W)
ave, hereinafter abbreviated as SAW) Apparatus D is shown in FIG. In the figure, 35 is an IDT of a comb-shaped electrode made of Al or the like.
(Inter Digital Transducer) electrode, 35a is an electrode finger of the IDT electrode 35, 35b is a bus bar which is a common electrode for providing a plurality of electrode fingers 35a, 36 is the IDT electrode 35
Reflectors disposed at both ends of the SAW propagation path for efficiently resonating the SAW, w1 is the length of the electrode finger 35a, and w2 is the distance between the electrode finger 35a and the bus bar 35b on the other side.
These parts are formed on a piezoelectric substrate (not shown) such as LiTaO _{3. The} number of electrode fingers 35 a is several tens to several hundreds, and the number of electrode fingers of the reflector 36 is several. Since the number ranges from ten to several hundreds, it is simplified in FIG.

FIG. 5 is a circuit diagram of a conventional three-stage ladder type (ladder type) SAW filter F1. This ladder-type SAW filter F1 has a series S
The AW resonators 2a to 2c and the parallel SAW resonators 3a to 3c are connected alternately. In the figure, reference numeral 4 denotes a series SAW resonator 2a to 2c and a parallel SAW resonator 3a to 3c.
Wiring patterns 5 for connecting c are wires for connecting to an external drive circuit or the like.

As shown in FIG. 4, the SAW device (SAW resonator) D has a resonance frequency (fr) 10 at which the impedance is minimized by frequency and an anti-resonance frequency (fa) 11 at which the impedance is maximized. And the double resonance characteristic having the following. In the figure, 9 is the impedance | Z | -frequency characteristic (hereinafter referred to as impedance characteristic) of the SAW resonator, and 12 is the equivalent capacitance. As for the ladder type SAW filter F1, as shown in FIG. 6, the resonance frequency 7a of the series SAW resonators 2a to 2c and the anti-resonance frequency 8b of the parallel SAW resonators 3a to 3c do not cross each other. It has been proposed that a band-pass filter having a pass band of 7 f near these frequencies can be configured by setting (refer to JP-A-6-23268).
No. 2).

In FIG. 6, 7 is the impedance characteristic of the series SAW resonators 2a to 2c, 7b is the anti-resonance frequency of the series SAW resonators 2a to 2c, and 7c is the steepness improved by the present invention as described later. Is the anti-resonance frequency of the series SAW resonators 2a to 2c. 8 is a parallel S
The impedance characteristics of the AW resonators 3a to 3c, 8a are the resonance frequencies of the parallel SAW resonators 3a to 3c, and 8c is the parallel S when the steepness is improved by the present invention as described later.
This is the resonance frequency of the AW resonators 3a to 3c.

A SAW for a communication device such as a mobile phone
In the filter, the impedance characteristics of the SAW resonator are optimally designed according to required specifications. Representative parameters for determining the impedance characteristics of the SAW resonator include a resonance frequency fr, an anti-resonance frequency fa, and Δf = fa−
fr and equivalent capacitance, these parameters are
It can be adjusted by designing the IDT electrode 35 of the resonator, fr can be controlled by the pitch of the electrode fingers 35a of the IDT electrode 35, and the equivalent capacitance can be controlled by the intersection width (w1 -w2) of the electrode fingers 35a and the logarithm of the electrode fingers 35a.

[0007]

However, since fa is mainly determined by the electromechanical coupling coefficient of the piezoelectric substrate 1, it is almost determined when the material and the film thickness of the IDT electrode 35 are determined. However, there is a problem that fa cannot be set freely for each SAW resonator.
Therefore, in order to change the pass bandwidth of the SAW filter, it is necessary to solve the problem by changing the material of the piezoelectric substrate 1 or the crystal orientation of the wafer. When the pass bandwidth of the SAW filter is determined, there is no method for improving the attenuation gradient (attenuation bandwidth / pass bandwidth).

As described above, in a conventional ladder type SAW filter or lattice type SAW filter, Δf of the SAW resonator is determined by the electromechanical coupling coefficient of the piezoelectric substrate 1 used. For this reason, a plurality of S formed on the same piezoelectric substrate 1
The AW resonators have different impedances, but have the same impedance characteristics and Δf.

Therefore, in the SAW filter constituted by such a SAW resonator, the attenuation gradient is limited, and it is difficult to satisfy both the desired passband characteristics and the desired attenuation characteristics. In recent years, there has been a strong demand for performance improvement for electronic components for frequency control such as SAW filters.
For example, as shown by the dotted line in FIG. 6, there is a need to make the attenuation characteristic steep without changing the pass bandwidth of the SAW filter.

Therefore, the present invention has been completed in view of the above circumstances, and an object of the present invention is to reduce Δf of a SAW device (SAW resonator) formed on the same piezoelectric substrate to each SAW device.
An object of the present invention is to make adjustments possible for each device and to improve the pass band characteristics and attenuation characteristics of the SAW filter.

[0011]

A surface acoustic wave device according to the present invention is a surface acoustic wave device comprising at least one set of comb-like electrodes formed on a main surface of a piezoelectric substrate, wherein the comb-like electrode is provided. Where w1 is the length of the electrode finger and w2 is the distance between the tip of the electrode finger and the bus bar opposite thereto, 0.04≤w2 / w1≤
0.20, and a small electrode finger protrudes from the bus bar facing the tip of the electrode finger at the interval.

In the present invention, preferably, the small electrode finger is tapered from the bus bar toward the tip of the electrode finger.

The surface acoustic wave filter according to the present invention is a ladder type surface acoustic wave filter or a balanced surface acoustic wave filter formed by connecting a plurality of surface acoustic wave devices on the same piezoelectric substrate. At least one of the surface acoustic wave devices
This is characterized in that the individual is the surface acoustic wave device described above.

According to the present invention, the following effects can be obtained by the above configuration. Lithium tantalate (Li
TaO ₃ ) single crystal or lithium niobate (LiNb)
In the case of O ₃ ) single crystal, the electromechanical coupling coefficient k ² is clearly different between the intersecting portion and the non-intersecting portion (small electrode finger portion) of the electrode finger of the IDT electrode. Then, the aperture ratio (w2 / w1 × 100)
%) By changing the electromechanical coupling coefficient k ² of the electrode fingers decreases under the influence of small electrode fingers.

FIG. 8 shows the relationship between the aperture ratio and Δf. If the aperture ratio is less than 4%, the change rate of Δf is large and the degree of freedom in design is large, but the influence of the manufacturing deviation on the pass band is so large that it cannot be neglected and the yield of fine processing by photolithography or the like is low. Degrades rapidly. For example, when considering a 950 MHz lithium tantalate single crystal, it is often appropriate that the SAW wavelength λ is about 3.7 μm, the intersection width (w 1 −w 2) of the electrode fingers is about 10 λ, and the aperture ratio 4% is about 0 μm. If the precision is less than this, it is difficult to form accurately by photolithography. On the other hand, when the aperture ratio exceeds 20%, Δf
Changes saturate.

In the SAW device D shown in FIG. 3, when a high-frequency signal is input from the outside, a pseudo SAW is generated in the electrode finger 35a.
A wave is generated that spreads from the SAW propagation axis at the center a and propagates through the interval w2. The electrode finger 35a corresponding to the interval w2 is referred to as an electrode finger neck (non-intersecting portion). Since the electrode finger has a smaller electrode area than the electrode finger 35a, a speed difference occurs between the SAW propagating through the electrode finger neck and the SAW propagating through the center (intersection) of the electrode finger 35a.
This shows the same phenomenon as when the number k ^{2 of} electromechanical coupling grandchildren becomes small, and Δf becomes small.

The resonance characteristic of the SAW resonator is the same as that of the electrode finger 35a.
Since it has a combined characteristic of the SAW of the electrode finger and the SAW of the electrode finger, Δf can be controlled by adjusting the aperture ratio. In addition, when the aperture ratio is relatively large, the amount of exposure at the electrode finger in the photolithography process becomes large, and the line width of the electrode finger becomes narrower than a set value due to so-called halation. In this case, since the electrical resistance of the thinned portion increases, the insertion loss increases and the power durability (reliability) decreases.

Therefore, in order to prevent a decrease in the line width of the electrode finger neck portion when the aperture ratio is changed, a small electrode finger projecting from the bus bar and not in contact with the electrode finger 35a is provided at the interval w2. At this time, if the shape of the small electrode finger is the same as that of the electrode finger 35a, the speed difference becomes small, and it becomes difficult to control Δf. Therefore, by forming the small electrode finger so as to be tapered from the bus bar toward the tip of the electrode finger 35a, in other words, to reduce the line width, the line width of the electrode finger neck can be reduced without losing the speed difference. Can be prevented.

In the SAW filter of the present invention, as shown in FIG. 6, by reducing Δf of one SAW resonator, for example, the impedance characteristic from the resonance frequency fr to the anti-resonance frequency fa of this SAW resonator is reduced. , Will change more rapidly than other SAW resonators. As a result, S
The characteristics of the AW filter are such that the attenuation gradient becomes steep while maintaining the pass band 7f width. More specifically, as shown in FIG. 7, since the attenuation pole of the SAW filter moves from 24a to 24b, the attenuation can be changed from 20a to 20b. still,
In the figure, 19 is a pass band, and 20 is an attenuation band.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The SAW device according to the present invention will be described below. 1 and 2 show a SAW device D according to the present invention.
1 is a plan view of a basic configuration of D2. In FIG. 1, 3
0 is an IDT electrode, 30a is an electrode finger, 30b is a small electrode finger,
30c is a bus bar, and 31 is a reflector. Electrode finger 30a
Where w1 is the length of the electrode finger 30a and w2 is the distance between the end of the electrode finger 30a and the bus bar 30c of the IDT electrode 30.
04 ≦ w2 / w1 ≦ 0.20, and a small electrode finger 30b protruding from the bus bar 30c is provided at the interval.

In the example shown in FIG. 1, the small electrode finger 30b has a triangular shape, and the bottom of the small electrode finger 30b is widened as shown by a dotted line, or the small electrode finger 30b is formed in a curved line. Is also good. In the example of FIG. 2, the small electrode finger 30bb may be formed of two triangular portions and divided into three or more.

In the present invention, the tip of the small electrode finger and the electrode finger 3
The distance from the tip of 5a is preferably from 0.15λ to 0.35λ, and if it is less than 0.15λ, the yield of fine processing by photolithography or the like is rapidly deteriorated. Also, 0.35
When the distance exceeds λ, the distance between the electrode finger 35a and the small electrode finger is increased, so that thinning due to halation becomes remarkable.

Also, the area of the small electrode finger is 50% of that of the case where the small electrode finger is square (the dashed line in FIG. 1).
1 to 150% (dotted line in FIG. 1). If it is less than 50%, the effect of applying a small electrode finger, that is, the effect of suppressing thinning due to halation decreases, and if it exceeds 150%, Δ
It becomes difficult to control f.

The case where the present invention is applied to the ladder type SAW filter F1 shown in FIG. 5 will be described below. Series SA
Two W resonators 2b and 2c are configured as shown in FIG. FIG. 6 shows the frequency characteristics of the pass band of the ladder type SAW filter F1 (upper graph) and the impedance characteristics of the SAW resonator (upper graph). The impedance characteristic 7 of the series SAW resonator 2a is as follows.
The anti-resonance frequency 7b moves to the lower frequency side while maintaining a, and becomes 7c. In response to this change in the impedance characteristic, the frequency characteristic of the pass band of the SAW filter F1 has a steep attenuation characteristic on the high frequency side.

In the impedance characteristic 8 of the parallel SAW resonator 3b, Δf is reduced by increasing w 2 / w 1 as compared with the prior art, the anti-resonance frequency 8b is maintained, and the resonance frequency 8a shifts to the high frequency side and 8c Becomes In response to the change in the impedance characteristic, the frequency characteristic of the pass band of the SAW filter F1 has a sharp change in the attenuation characteristic on the low frequency side.

The variation width of Δf obtained by such adjustment of w 2 / w 1 is the LiT of 42 ° Y cut-X propagation.
It is about 10% of Δf determined by the piezoelectric substrate 1 made of the aO ₃ single crystal, and the steepness of the attenuation characteristic of the SAW filter can be sufficiently improved.

Specifically, for a 900 MHz band SAW filter F1, two SAW resonators 2b,
The electrode finger 30a of 2c has 86 pairs and 94 pairs, and the electrode finger 30a
The length w1 of the electrode fingers 30a was 10λ, the material of the electrode fingers 30a was Al formed by vapor deposition, the thickness of the electrode fingers 30a was 410 nm, and the interval w2 was 12.5% of w1. The electrode finger 30a width and the electrode finger 30a interval are each about 1 μm. FIG. 11 shows the frequency characteristics of the signal level before the improvement, and FIG. 12 shows the frequency durability obtained by the present invention. The present invention has improved damping characteristics.

FIG. 9 is a circuit diagram of a balanced (bridge-type) SAW filter F2 having a symmetrical lattice shape according to the present invention. The present invention is applied to two SAW resonators 40 and 41 connected to a serial arm. Incidentally, in FIG.
43 is two SAW resonators connected to the parallel arm;
a and 45b are input terminals, and 46a and 46b are output terminals.

FIG. 10 shows the frequency characteristics of the pass band of the balanced SAW filter F2 and the impedance characteristics of the SAW resonators 40 to 43. The impedance characteristic 17 of the SAW resonator 40 is reduced by Δf due to the increase of w2 / w1 as compared with the prior art, and the anti-resonance frequency 17b moves to the lower frequency side while maintaining the resonance frequency 17a to become 17c.
In response to this change in the impedance characteristic, the frequency characteristic of the pass band 17f of the SAW filter F2 has a steep attenuation characteristic on the high frequency side.

Further, the impedance characteristic 18 of the SAW arm resonators 42 and 43 is reduced in Δf by increasing w 2 / w 1 as compared with the prior art, the anti-resonance frequency 18b is maintained, and the resonance frequency 18a is shifted to the high frequency side. 18c. In response to this change in impedance characteristics, SAW
In the frequency characteristic of the pass band 17f of the filter F2, the attenuation characteristic on the low frequency side becomes steep.

The width of change of Δf obtained by such adjustment of w 2 / w 1 is the same as that of ladder type SAW filter F 1, with respect to Δf determined by piezoelectric substrate 1 made of LiTaO ₃ single crystal of 42 ° Y cut-X propagation. About 10%,
The steepness of the attenuation characteristic of the SAW filter can be sufficiently improved.

Specifically, for a 900 MHz band SAW filter F2, two SAW resonators 40,
41 electrode fingers 30a are 60 pairs and 60 pairs, electrode fingers 30a
Are 15λ, the electrode material is Al formed by vapor deposition, and the thickness of the electrode finger 30a is 410n.
m and the interval w2 were both set to 12.0% of w1. The electrode finger 30a width and the electrode finger 30a interval are each about 1 μm.
m. FIG. 13 shows the frequency characteristics of the signal level before the improvement, and FIG. 14 shows the frequency durability obtained by the present invention. The present invention has improved damping characteristics.

In the present invention, the IDT electrode 30 is made of Al or an Al alloy (Al-Cu system, Al-Ti system, etc.), and Al is particularly preferable because of its high excitation efficiency and low material cost. The IDT electrode 30 is formed by a thin film forming method such as an evaporation method, a sputtering method, or a CVD method.

The logarithm of the electrode finger 30a of the IDT electrode 30 is about 50 to 200, and the line width of the electrode finger 30a is 0.1.
About 10.0 μm, and the interval between the electrode fingers 30 a is 0.1 to 1 μm.
The opening width (intersection width) of the electrode finger 30a is about 0.0 μm,
The thickness of the IDT electrode 30a is about 0.2 to 100 μm.
It is preferable that the thickness be about 0.4 μm in order to obtain desired characteristics as a SAW resonator or a SAW filter. Also, zinc oxide, between electrode fingers of the IDT electrode 30a,
It is preferable to form a piezoelectric material such as aluminum oxide because the SAW resonance efficiency is improved.

As the piezoelectric substrate, 36 ° ± 10 ° Y-cut-X propagating LiTaO ₃ single crystal, 64 ° Y-cut-X
Propagating LiNbO ₃ single crystal, 45 ° X cut-Z propagating LiB ₄ O ₇ single crystal, and the like are preferable because the electromechanical coupling coefficient is large and the group delay time temperature coefficient is small. LiTaO ₃ single crystal of ± 10 ° Y cut-X propagation is preferred. Further, the cut angle in the crystal Y-axis direction may be within a range of 36 ° ± 10 °,
In that case, sufficient piezoelectric characteristics can be obtained. The thickness of the piezoelectric substrate is preferably about 0.1 to 0.5 mm. If the thickness is less than 0.1 mm, the piezoelectric substrate becomes brittle, and if it exceeds 0.5 mm, the material cost increases.

Thus, the SAW device of the present invention has the IDT
By adjusting the aperture ratio (w2 / w1 × 100%) of the electrode fingers of the electrode, Δf can be controlled, and at the time of forming the IDT electrode in the photolithography process, the electrode finger neck can be prevented from being thinned. When a SAW filter is formed by the SAW device according to the present invention, there is an operational effect that the attenuation characteristic at the end of the pass band is improved.

Further, in the SAW filter of the present invention, at least one SAW device provided with small electrode fingers with 0.04 ≦ w 2 / w 1 ≦ 0.20 only needs to be provided. The above configuration may be adopted. Further, a SAW device or a SAW filter may be provided on both main surfaces (front and back surfaces) of the piezoelectric substrate.

Note that the present invention is not limited to the above-described embodiment, and various changes may be made without departing from the scope of the present invention.

[0039]

According to the present invention, when the length of the electrode finger of the IDT electrode is w1 and the distance between the tip of the electrode finger and the bus bar of the IDT electrode is w2, 0.04≤w2 / w1≤0.2.
0, the small electrode finger protrudes from the bus bar at the interval, so that Δf of the SAW device can be controlled, and the electrode finger neck portion becomes thin when forming the IDT electrode in the photolithography process. Can be prevented,
Further, when the SAW filter is configured, the attenuation characteristics at the end of the pass band are improved.

[Brief description of the drawings]

FIG. 1 is a plan view of a basic configuration of a SAW device D1 of the present invention.

FIG. 2 is a plan view of a basic configuration of a SAW device D2 of the present invention.

FIG. 3 is a plan view of a basic configuration of a conventional SAW device D.

FIG. 4 is a graph showing impedance characteristics of a conventional SAW device D;

FIG. 5 shows a ladder type SAW filter F to which the present invention can be applied.
1 is a circuit diagram of FIG.

FIG. 6 is a graph of a frequency characteristic of a pass band of the ladder-type SAW filter F1, and a graph of impedance characteristics of a series SAW resonator and a parallel SAW resonator.

FIG. 7 is a graph for describing an improvement in attenuation characteristics at a passband end of a ladder-type SAW filter F1.

FIG. 8 is a graph showing the relationship between the aperture ratio and Δf of the SAW device of the present invention.

FIG. 9 is a circuit diagram of a balanced SAW filter F2 according to the present invention.

FIG. 10 is a graph of a frequency characteristic of a pass band of the balanced SAW filter F2, and a graph of impedance characteristics of a SAW resonator connected to a series arm and a SAW resonator connected to a parallel arm.

FIG. 11 is a graph showing frequency characteristics of a pass band of a conventional ladder-type SAW filter F1.

FIG. 12 is a graph of a passband frequency characteristic of the ladder-type SAW filter F1 according to the present invention.

FIG. 13 is a graph showing frequency characteristics of a pass band of the conventional balanced SAW filter F2.

FIG. 14 is a graph of a passband frequency characteristic of the balanced SAW filter F2 according to the present invention.

[Explanation of symbols]

 1: piezoelectric substrate 2a: series SAW resonator 2b: series SAW resonator 2c: series SAW resonator 3a: parallel SAW resonator 3b: parallel SAW resonator 3c: parallel SAW resonator 4: wiring pattern 5: wire 7: series Impedance characteristics of SAW resonator 7a: Resonance frequency 7b: Anti-resonance frequency 7c: Anti-resonance frequency 8: Impedance characteristics of parallel SAW resonator 8a: Resonance frequency 8b: Anti-resonance frequency 8c: Resonance frequency 9: Impedance characteristics of SAW resonator 30: IDT electrode 30a: Electrode finger 30b: Small electrode finger 30c: Bus bar 31: Reflector

Claims (3)

[Claims] 1. A surface acoustic wave device comprising at least one pair of comb-shaped electrodes formed on a main surface of a piezoelectric substrate, wherein a length of an electrode finger of the comb-shaped electrode is w1, and a tip of the electrode finger is 0.04 ≦ w2, where w2 is the distance between the busbar and
/W1≦0.20, and a small electrode finger protrudes from the bus bar facing the tip of the electrode finger at the interval. 2. The surface acoustic wave device according to claim 1, wherein the small electrode finger is tapered from the bus bar toward a tip end of the electrode finger. 3. A ladder-type surface acoustic wave filter or a balanced surface acoustic wave filter formed by connecting a plurality of surface acoustic wave devices on the same piezoelectric substrate, wherein at least one of the surface acoustic wave devices is connected. Item 3. A surface acoustic wave filter according to item 1 or 2.