JPH1188100A - Surface acoustic wave device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a SAW device of a small capacity rate by making the cut angle of a rotary Y cut lithium niobate substrate smaller than a specified value, weighting the facing electrode fingers of IDT by crossing width and selecting the reference film thickness of IDT to be a specified range. SOLUTION: At least one comb-like electrode (IDT) is arranged on the rotated Y cut lithium niobate substrate. The cut angle θ of the piezoelectric substrate is θ<=40 degrees, the facing electrode fingers of IDT are weighted by crossing width and the reference film thickness H/λ of IDT is set to be within the range of 0.053<H/λ<0.102. The cut angle θ of the piezoelectric substrate is θ<=40 degrees, the crossing widths of the facing electrode fingers of IDT are equal and the reference film thickness H/λ of IDT is set to be within the range of 0.053<H/λ<0.075. When the line width of the facing electrode fingers of IDT is set to be Lm and space width to be Lg, the relation of Lm/(Lm-+Lg)<0.62 is satisfied.

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 using lithium niobate for a piezoelectric substrate, and more particularly to a surface acoustic wave device having a leaky SAW resonator with a capacitance ratio improved from that of the prior art.

[0002]

2. Description of the Related Art In recent years, there have been remarkable reductions in size, loss, and cost of surface acoustic wave devices, which have greatly contributed to the spread of various communication devices. In particular, the spread of mobile communication devices has been rapid, and in response to the demand, there has been a shift from an analog system to an increase in communication channels and a digital system suitable for various services. As described above, with the digitization of the communication system, the communication band is widened, and a filter used in the RF stage and the IF stage of the communication device is required to have a broader pass band. In addition, a VCO (voltage controlled oscillator) used for the mixer stage is required to have a variable oscillation frequency range wider than before.

[0003] Among communication devices, a ladder type surface acoustic wave filter (hereinafter, referred to as an RF stage) is most commonly used for an RF stage of a mobile phone or the like.
A ladder-type SAW filter is often used. As is well known, a ladder type SAW filter is a resonator type SAW filter configured by arranging a plurality of SAW resonators in a ladder type as shown in FIG. FIG. 6 shows a configuration in which a parallel resonator (SAW resonator in a parallel arm) Sp is arranged at the input and output. There is also a configuration in which a series resonator (SAW resonator in a series arm) Ss is provided at the input and the output. Both types have the same filter characteristics but different input / output impedance characteristics. Each of the series SAW resonators Ss and the parallel SAW resonators Sp of the ladder-type SAW filter is constituted by a pair of comb-shaped electrodes (IDT) having a plurality of electrode fingers interposed on one piezoelectric substrate. A ladder-type SAW filter is configured by disposing lead electrodes between SAW resonators. Further, each SAW resonator has an ID as shown in a plan view in FIG.
There are cases where reflectors are arranged on both sides of the T electrode and cases where no reflectors are arranged as shown in FIG.

FIG. 8A shows a frequency-impedance characteristic of one section of a ladder-type SAW filter composed of one parallel SAW resonator Sp and one series SAW resonator Ss. By making the anti-resonance frequency fap of the parallel SAW resonator Sp substantially equal to the resonance frequency frs of the series SAW resonator Ss, as shown in FIG. 8B, the resonance frequency frp of the parallel SAW resonator Sp on both sides of the pass band. And series SAW
A filter characteristic having an attenuation pole composed of the anti-resonance frequency fas of the resonator Ss is obtained.

FIG. 9A shows an electrical equivalent circuit of a single SAW resonator, and FIG. 9B shows a frequency-impedance characteristic thereof. In order to widen the pass band in the ladder type SAW filter, a SAW having a large difference frequency between the resonance frequency fr and the anti-resonance frequency fa shown in FIG.
A resonator is required. Generally, there is a relationship of fa ² = fr ² (1 + 1 / γ) (1) between the resonance frequency fr and the anti-resonance frequency fa. Here, γ is the capacitance ratio of the SAW resonator, and is represented by the ratio (C ₀ / C ₁ ) between the capacitance C ₀ and the motional capacitance C ₁ of the electric equivalent circuit shown in FIG. As is well known, the capacitance ratio γ of the SAW resonator is substantially determined by the type of piezoelectric material and the cutting direction. From equation (1), as the capacitance ratio γ decreases, the resonance frequency fr and the anti-resonance frequency fa decrease.
Thus, a SAW resonator having a large difference frequency from the above can be obtained. Therefore, by using a SAW resonator having a small capacitance ratio γ, the pass band width of the ladder-type SAW filter can be widened.

Further, as is well known, since the capacitance ratio γ is inversely proportional to the square k ² of the electromechanical coupling coefficient k, to realize a SAW resonator having a small capacitance ratio γ, the electromechanical coupling coefficient k
² large piezoelectric substrates are required. `` J. Appl. Phys., Vol.
43, No. 3, pp. 856-862 (March1972) ”(referred to as Reference 1), a SAW device using a 41-degree rotated Y-cut X-propagating lithium niobate for a piezoelectric substrate has a large electric power. It is known that a mechanical coupling coefficient k ² can be obtained.
Here, the θ-degree rotated Y-cut substrate is obtained by rotating the Y-cut plate around the X-axis by θ degrees as is well known. Japanese Patent Application Laid-Open No. Hei 9-121136 discloses that a 41-degree rotated Y-cut X-propagating lithium niobate is used as a piezoelectric substrate, and the IDT period λ
And the thickness of the electrode H, that is, the normalized thickness H / λ is 0.0
It is stated that by selecting the range of 25 ≦ H / λ ≦ 0.075, a wide band ladder type SAW filter having no spurious can be realized.

FIGS. 10A to 10C show frequency-impedance characteristics of a SAW resonator using a 41-degree rotated Y-cut X-propagating lithium niobate as a piezoelectric substrate, obtained by an experiment conducted by the present inventors. is there. The horizontal axis represents the frequency, and the vertical axis represents the absolute value (Ω) of the impedance Z. FIG.
(A) shows the case where the normalized thickness H / λ of the IDT disposed on lithium niobate is set to 0.06, and (b) shows the case where the normalized thickness H / λ is set to 0.08 and the same. (C) is the normalized film thickness H
It is a frequency-impedance characteristic when / λ is set to 0.10. In this prototype, all parameters other than the standardized film thickness H / λ are the same. JP-A-9-12113
6 As described, the normalized film thickness H / λ is 0.07
In the frequency-impedance characteristics shown in FIGS. 10B and 10C, which are larger than 5, small periodic spurious components are superimposed on the resonance characteristics between the resonance frequency and the antiresonance frequency. On the other hand, when the normalized film thickness is 0.025 ≦ H / λ ≦ 0.
It can be seen that the above-mentioned spurious is not superimposed on the frequency-impedance characteristic of FIG. 10A in the range of 075.

Japanese Patent Application Laid-Open No. Hei 9-121136 discloses a flat broadband ladder that does not generate spurious components in a pass band by weighting an intersection of electrodes facing an IDT disposed on a lithium niobate substrate. It is described that a type SAW filter is obtained. When the line width of the IDT electrode finger is Lm and the space width is Lg, the relationship between the line width and the space width is 0.2 ≦ Lm / (Lm + Lg).
It is also described that a ladder type SAW filter having a flat passband and a wide band can be obtained by configuring so as to satisfy ≦ 0.5.

In this specification, Lm / (Lm + Lg) is referred to as a line occupancy t. Unless otherwise specified, the line occupancy of the reflector and the line occupancy t of the IDT are substantially the same.
To confirm the contents regarding the line occupancy t, the IDT
SAW in which the electrode fingers of FIG.
A resonator was prototyped. The line occupancy t is 0.3, 0.4,
Five types of 0.5, 0.6 and 0.7, and a standardized film thickness H / λ
Are three types of 0.06, 0.08, and 0.10.
Kind. FIG. 12 shows the result of the detailed measurement of the capacitance ratio γ of each SAW resonator. As is clear from FIG. 12, the minimum value of the capacitance ratio γ of the SAW resonator is approximately equal in each case. It turns out that it is 4.64.

[0010]

However, as is apparent from the above experimental results, the capacitance ratio of a SAW resonator using a 41-degree rotated Y-cut X-propagating lithium niobate as a piezoelectric substrate varies with various parameters. However, the minimum value is about 4.64, and there is a problem that a smaller capacity ratio cannot be obtained. The present invention has been made in order to solve the above-mentioned problems, and has a small capacity ratio SAW.
The purpose is to provide a device.

[0011]

According to a first aspect of the present invention, there is provided a surface acoustic wave device according to the present invention, wherein at least one IDT is disposed on a rotating Y-cut lithium niobate substrate. In the surface acoustic wave device, the cutting angle θ of the piezoelectric substrate is θ ≦ 40 degrees, and the electrode width of the IDT is set to 0.053 <H. /
A surface acoustic wave device characterized in that λ is set within a range of 0.102. According to the second aspect of the present invention, at least one I
In the surface acoustic wave device in which the DT is disposed, the cutting angle θ of the piezoelectric substrate is θ ≦ 40 degrees and the IDT
Wherein the intersection width of the opposing electrode fingers is the same, and the standardized film thickness H / λ of the IDT is set in the range of 0.053 <H / λ ≦ 0.075. It is. The invention according to claim 3 is characterized in that, when the line width of the electrode finger facing the IDT is Lm and the space width is Lg, the relationship of Lm / (Lm + Lg) <0.62 is satisfied. And 2 are surface acoustic wave devices. According to a fourth aspect of the present invention, there is provided the surface acoustic wave device according to any one of the first to third aspects, wherein reflectors are arranged on both sides of the IDT. According to a fifth aspect of the present invention, there is provided a surface acoustic wave filter in which a plurality of one-port pair surface acoustic wave resonators are formed on a rotating Y-cut lithium niobate substrate, and the one-port pair surface acoustic wave resonators are connected in a ladder form. 5. A surface acoustic wave filter according to claim 1, wherein at least one of the plurality of one-port surface acoustic wave resonators is the surface acoustic wave device according to any one of claims 1 to 4.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail based on an embodiment shown in the drawings. As described in the above-mentioned Document 1, the inventors of the present invention have developed a leaky surface acoustic wave (hereinafter, referred to as a “propagation”) propagating on a θ-degree rotated Y-cut lithium niobate substrate.
It is noted that the propagation speed (phase speed) of the leaky SAW greatly changes as the cutting angle of the piezoelectric substrate increases. In addition, the propagation speed of the leaky SAW when the substrate surface is metallized and the propagation speed of the leaky SAW when the substrate surface is electrically open differ depending on the cutting angle of the substrate. The propagation speed of the leaky SAW when the substrate surface is metallized is Vm, and the propagation speed of the leaky SAW when the substrate surface is electrically open is Vf.
Then, the electromechanical coupling coefficient k ² is k ² = 2 (Vf−V
m) / Vf. Therefore, by increasing the difference between the propagation speed Vf and Vm, it becomes possible to increase the electromechanical coupling coefficient k ^2.

As described in Document 1, the difference between the propagation velocities Vf and Vm becomes larger as the cutting angle of the lithium niobate substrate is made smaller than 41 degrees which has been said to be the best value. I understand. That is, it is expected that the electromechanical coupling coefficient k ² will increase as the cutting angle θ of the substrate becomes smaller than 41 degrees in the θ-degree rotated Y-cut lithium niobate substrate. As described above, since the capacitance ratio γ is inversely proportional to the square k ² of the electromechanical coupling coefficient k, the capacitance ratio γ should be able to be reduced by increasing the electromechanical coupling coefficient k ² . That is, if the cutting angle θ of the lithium niobate substrate is smaller than 41 degrees, the capacity ratio γ is expected to be small. However, according to Document 1,
If the surface of the lithium niobate substrate is metallized,
It is shown that when the cutting angle is smaller than about 60 degrees, the propagation loss of leaky SAW increases. Further, it is shown that when the surface of the lithium niobate substrate is electrically opened, the propagation loss of leaky SAW increases in a region where the angle θ is smaller than around 36 degrees.

Therefore, SA having a cutting angle smaller than 41 degrees
When a W resonator is manufactured, it is expected that the resonance characteristics deteriorate, and in particular, the resonance resistance R1 increases. Therefore, the inventors of the present application used a θ-degree rotated Y-cut lithium niobate substrate having a rotation angle smaller than 41 degrees, and set various SAW resonators using the normalized film thickness H / λ and the line occupancy t as parameters. Was prototyped, and the normalized thickness H / λ of the IDT arranged on the piezoelectric substrate was
And the influence of the line occupancy t on the electrical constant and resonance characteristics. Here, the material of the IDT electrode was an aluminum alloy.

FIG. 1 shows that the cutting angle of the lithium niobate substrate is θ = 35 degrees, the line occupancy of the IDT is t = 0.4,
SA in which the cross width is weighted to the opposing electrode fingers of the IDT
When the normalized film thickness H / λ is changed in the W resonator,
FIG. 9 is a diagram in which a relationship between a normalized film thickness H / λ and a capacitance ratio γ is plotted. As is clear from this figure, the normalized film thickness H / λ
Is selected in the range of 0.053 to 0.102, the SAW using the 41-degree rotated Y-cut X-propagating lithium niobate substrate
It can be seen that the capacitance ratio γ can be made smaller than the capacitance ratio γ of the resonator. FIG. 2 shows that the cutting angle of the lithium niobate substrate is θ = 35 degrees, the standardized film thickness H / λ is 0.08,
FIG. 13 is a diagram plotting the relationship between the line occupancy t and the capacitance ratio γ when the line occupancy t of the IDT is changed in the SAW resonator in which the electrode fingers opposed to T are weighted with the cross width. From this figure, it is understood that when the line occupancy t is set to be smaller than 0.62, the capacitance ratio γ can be made smaller than the capacitance ratio γ of the SAW resonator using the 41-degree rotated Y-cut X-propagation lithium niobate substrate.

FIG. 3 shows the normalized film thickness H / λ of 0.08, ID
In a SAW resonator in which the line occupation ratio t of T is 0.4 and the electrode fingers facing each other of the IDT are weighted by the cross width,
FIG. 6 is a diagram plotting the relationship between the cutting angle θ and the capacity ratio γ when the cutting angle θ of the lithium niobate substrate is changed.
If the cutting angle θ is selected to be smaller than 41 degrees as shown in FIG.
Degree rotation Y-cut X-propagation lithium niobate substrate
It can be seen that the capacitance ratio γ can be made smaller than the capacitance ratio γ of the AW resonator. FIG. 4 shows the normalized film thickness H / λ of 0.08 and ID.
In a SAW resonator in which the line occupation ratio t of T is 0.4 and the electrode fingers facing each other of the IDT are weighted by the cross width,
FIG. 9 is a diagram plotting the relationship between the cutting angle θ and the resonance resistance R1 when the cutting angle θ of the lithium niobate substrate is changed. When the cutting angle was reduced, the resonance resistance R1, which was a concern before the start of the experiment, did not increase, but rather, as the cutting angle θ was smaller than 41 degrees, the resonance resistance was reduced. Although the experimental results did not agree with the relationship between the cutting angle of the lithium niobate substrate and the propagation loss of leaky SAW shown in Document 1, the IDT
It is assumed that the propagation loss of leaky SAW is reduced due to the effect of the mass load effect due to the electrode film thickness.

FIG. 5 shows a conventional SAW resonator (dashed line) using a 41-degree rotated Y-cut X-propagating lithium niobate substrate,
4 is a comparison of impedance characteristics with a SAW resonator (solid line) using a 35-degree rotated Y-cut X-propagating lithium niobate substrate according to one embodiment of the present invention. In each resonator, the IDT electrode shape, the number of electrode pairs, the intersection width weighting, the standardized film thickness, and the line occupancy are the same. As is clear from this figure, the capacitance ratio could be increased without deteriorating the resonance characteristics. That is, it is possible to widen the difference between the resonance frequency and the anti-resonance frequency.

In the above description, an example of the SAW resonator in which the electrode fingers facing each other of the IDT are weighted by the intersection width has been described.
If the intersection width of each electrode finger facing the IDT is the same,
When the standardized film thickness H / λ is set to be larger than 0.075, a periodic small spurious is generated between the resonance frequency and the antiresonance frequency. Therefore, the standardized film thickness is 0.053 <H / λ ≦
It has been confirmed by experiments that it is preferable to select a value within the range of 0.075. To summarize the above results, in the SAW resonator in which the cutting angle θ of the θ-degree rotated Y-cut lithium niobate substrate is smaller than 41 degrees and the width of the electrode finger facing the IDT is weighted by the cross width, the standardized film thickness H / Λ is 0.053
<H / λ <0.102, and when the intersection width of each electrode finger facing the IDT is the same, the standardized film thickness H /
If λ is selected in the range of 0.053 <H / λ ≦ 0.075,
The capacitance ratio γ can be reduced without deteriorating the resonance characteristics. By setting the line occupancy t of the IDT to t <0.62, the capacitance ratio γ can be further reduced without deteriorating the resonance characteristics. It is possible.

In the above description, a leaky SAW resonator is taken as an example, and if the resonator is applied to a ladder type SAW filter, the filter characteristics in which the pass band width is expanded without increasing the insertion loss in the pass band can be obtained. Said that can be realized. The present invention is not limited to this.
It is needless to say that the present invention can be applied to a lattice type SAW filter in which W resonators are connected in a lattice type, and a so-called multi-mode SAW filter in which a plurality of surface wave vibrations are confined between reflectors. Further, when the SAW resonator according to the present invention is applied to a VCO (voltage controlled oscillator), a VCO having a wide frequency variable width can be obtained.
Can be realized.

[0020]

According to the present invention, as described above, the capacitance ratio of a leaky SAW resonator can be made smaller than that of a conventional SAW resonator, and a wideband ladder-type SAW filter is realized. In doing so, it shows an excellent effect. On the other hand, if the SAW resonator according to the present invention is applied to a VCO, it is possible to realize an oscillator having a wide frequency variable width, and this has a remarkable effect on speeding up digital communication.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between a normalized film thickness H / λ and a capacitance ratio γ according to the present invention.

FIG. 2 is a diagram showing a relationship between a line occupation ratio t and a capacitance ratio γ according to the present invention.

FIG. 3 is a diagram showing a relationship between a cutting angle θ and a capacity ratio γ of the lithium niobate substrate according to the present invention.

FIG. 4 is a diagram showing a relationship between a cutting angle θ of a lithium niobate substrate according to the present invention and an equivalent resistance R1.

FIG. 5 is a diagram showing a comparison between the impedance characteristic of a leaky SAW resonator according to the present invention (solid line) and the impedance characteristic of a conventional leaky SAW resonator (dashed line).

FIG. 6 is a diagram illustrating a circuit configuration of a conventional ladder-type SAW filter.

7A is a plan view of a one-port SAW resonator with a reflector, and FIG. 7B is a plan view of a one-port SAW resonator without a reflector.

8A is a diagram showing a frequency-impedance characteristic of one section of a conventional ladder filter, and FIG. 8B is a diagram showing the filter characteristic.

9A is an electrical equivalent circuit diagram of a one-port SAW resonator, and FIG. 9B is a diagram illustrating impedance characteristics thereof.

FIGS. 10A to 10C show a case where the normalized film thickness H / λ is set to 0.0.
It is an impedance characteristic when changing from 6 to 0.10.

FIG. 11 is a plan view of a one-port SAW resonator in which an intersection width of an IDT is weighted.

FIG. 12 is a diagram showing a relationship between a standardized film thickness H / λ and a capacitance ratio γ when a line occupancy t of a conventional leaky SAW resonator is used as a parameter.

[Explanation of symbols]

 γ ·· Resonator capacitance ratio λ ·· IDT period (wavelength) H ··· IDT electrode film thickness H / λ ··· IDT standardized film thickness θ ·· Rotation of lithium niobate Y plate around X axis Angle R1 ··· Equivalent electrical resistance of SAW resonator Z ··· Impedance of SAW resonator

Claims (5)

[Claims] 1. A surface acoustic wave device in which at least one IDT is disposed on a rotating Y-cut lithium niobate substrate, wherein a cutting angle θ of the piezoelectric substrate is θ ≦ 40 degrees, and an electrode finger facing the IDT is provided. The intersection width is weighted, and the normalized thickness H / λ of the IDT is set to 0.053 <
A surface acoustic wave device wherein H / λ is set within the range of 0.102. (However, λ is the period of IDT) 2. A surface acoustic wave device in which at least one IDT is arranged on a rotating Y-cut lithium niobate substrate, wherein the cutting angle θ of the piezoelectric substrate is θ ≦ 40 degrees and the IDT of the opposing electrode fingers of the IDT is The intersection width is the same, and the standardized film thickness H / λ of the IDT is 0.053 <H
A surface acoustic wave device characterized by being set within the range of /λ≦0.075. (However, λ is the period of IDT) 3. When the line width of the electrode finger facing the IDT is Lm and the space width is Lg, Lm / (Lm +
3. The surface acoustic wave device according to claim 1, wherein a relationship of Lg) <0.62 is satisfied. 4. The surface acoustic wave device according to claim 1, wherein reflectors are arranged on both sides of the IDT. 5. A surface acoustic wave filter in which a plurality of one-port pair surface acoustic wave resonators are formed on a rotating Y-cut lithium niobate substrate, and the one-port pair surface acoustic wave resonators are connected in a ladder form. A surface acoustic wave filter characterized in that at least one of the plurality of one-port surface acoustic wave resonators is the surface acoustic wave device according to any one of claims 1 to 4.