JP2002223143A - Surface acoustic wave device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface acoustic wave device that can be downsized and cope with low frequencies. SOLUTION: In the surface acoustic wave device provided with an interdigital electrode 13 and a reflector 14 provided by forming an interdigital electrode 13a to a piezoelectric substrate 12, a metallization ratio MR that is a ratio of a line width LW and a space width SW (LW/(LW+SW) of the interdigital electrode being a component of the reflector is 60% or more and an oxide film is coated to the interdigital electrode of the reflector.

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 having an interdigital transducer for converting between an electric signal and a surface acoustic wave, and more particularly to a small surface acoustic wave device compatible with low frequencies. Things.

[0002]

2. Description of the Related Art In recent years, in electronic parts and communication parts such as mobile phones and television receivers, surface acoustic wave devices (hereinafter referred to as "SAW (Surface)"
Acoustic Wave) devices are used.

FIG. 7 is a plan view showing an example of a conventional SAW device.

The SAW device 1 is, for example, a one-port SAW resonator, and includes a piezoelectric substrate 2 and interdigital electrodes (hereinafter referred to as “IDTs”).
3) and a reflector 4.

The piezoelectric substrate 2 is formed in a rectangular plate shape, for example, from quartz. The IDT 3 and the reflector 4 are formed in such a manner that a conductive metal is formed on the surface of the piezoelectric substrate 2 into a thin film by vapor deposition, sputtering, or the like, and then interdigitated by photolithography or the like.

Specifically, the IDT 3 includes a plurality of electrode fingers 3
a are arranged in parallel at a predetermined pitch so that the respective ends in the longitudinal direction are alternately conductive. That is, the respective comb teeth of the two comb-shaped electrodes are formed so as to alternately enter at a predetermined distance. The IDT 3 has a function of converting between an electric signal and a surface acoustic wave (SAW) via an external terminal 5 that is electrically connected. In this case, as shown in FIG.
The space width SW between the electrode fingers 3a opposing a is set equal to the line width LW of each electrode finger 3a.

The reflector 4 is formed such that a plurality of electrode fingers 4a are juxtaposed at a predetermined pitch and both ends in the longitudinal direction are electrically connected. Then, for example, two reflectors 4 having the same configuration are arranged so that the electrode finger 4a is parallel to the electrode finger 3a of the IDT 3, and the IDT 3 is set in the propagation direction of the surface acoustic wave.
That is, it is formed so as to be sandwiched at a predetermined distance in a direction perpendicular to the longitudinal direction of the electrode finger 3a of the IDT 3. The reflector 4 has a function of reflecting the surface acoustic wave propagating from the IDT 3 and confining the energy of the surface acoustic wave inside. In this case, as shown in FIG. 8, the space width SW between the electrode finger 4a of the reflector 4 and the adjacent electrode finger 4a is set equal to the line width LW of each electrode finger 4a. Then, each electrode finger 3 of the IDT 3 and the reflector 4
The widths LW of a and 4a are equal to each other.

In such a configuration, when an electric signal is input to the IDT 3 via the external terminal 5, it is converted into a surface acoustic wave by a piezoelectric effect. This surface acoustic wave has an ID
The light propagates in a direction perpendicular to the longitudinal direction of the electrode finger 3a of the T3, and is radiated to the reflector 4 from both sides of the IDT3. At this time, the propagation speed determined by the material of the piezoelectric substrate 2, the thickness of the electrode, the width of the electrode, and the like, and the electrode period LW of the electrode finger 3a of the IDT 3
A surface acoustic wave having a wavelength equal to twice the + SW is most strongly excited. This surface acoustic wave is reflected by the reflector 4 in multiple stages and returned to the IDT 3, where it is converted into an electric signal having only a frequency (operating frequency) f _v near the resonance frequency, and the IDT 3
3 via the external terminal 5.

Here, the SAW device 1 is, for example, 10
It is formed so as to correspond to a low frequency of about 6 MHz, and has a logarithm of the IDT 3 (the number of pairs of the electrode fingers 3a to be paired).
The number of pairs is 0. The number of electrode fingers of the reflector 4 is about 150.

[0010]

However, recently, SA
Various types of information devices on which W devices are mounted tend to be extremely miniaturized, and various SAW devices such as a high-frequency compatible SAW device and a low-frequency compatible SAW device are used depending on the purpose of the mounted device. Also need to be miniaturized.

Here, the frequency of the SAW device is f = v
Since / λ (v = the sound velocity of the piezoelectric substrate, λ = wavelength of the surface wave), if the frequency f is low, the wavelength becomes long and the interval between the IDT and the reflector becomes large, so that the SAW device becomes large. For this reason, it is effective to reduce the logarithm of the IDT and the number of electrodes of the reflector in reducing the size of the SAW device corresponding to a low frequency.

However, especially when the number of electrodes of the reflector is reduced, the reflection efficiency of the surface wave is reduced, and the surface wave reaching the chip end face is converted into a bulk wave at the end face and becomes spurious. In addition, there is a problem that the surface waves reflected by the chip end face return in an opposite phase and hinder the propagating surface waves.

Therefore, as a method of increasing the reflectance of the reflector 4 without increasing the number of electrodes of the reflector 4, there is a method of increasing the thickness of the electrode. That is, when the thickness of the electrode finger 4a of the reflector 4 is increased, the weight increases, the reflectance of the surface acoustic wave increases, and the effect of confining the energy is improved, so that the number of electrodes can be reduced accordingly. Becomes

FIG. 9 shows two types of SAs having different electrode thicknesses.
5 is a graph comparing the relationship between the temperature and the frequency change of a W device. In the figure, A is the electrode thickness H /
B shows how a frequency change occurs due to a temperature change for a SAW device with λ = 2.1% and B for an electrode thickness H / λ = 2.7%. Here, H is the height of the electrode film (film thickness of the electrode), and λ is the wavelength of the surface wave.

As shown in the figure, in the SAW device, as shown from A to B, when the thickness of the electrode is increased to improve the reflectance of the reflector 4 as described above,
There is a disadvantage that the peak temperature of the frequency change curve changes from P1 to P2, and the frequency change becomes large at the use environment temperature when the SAW device 1 is mounted on various devices and used.

An object of the present invention is to solve the above problems, improve the reflection efficiency of surface acoustic waves without changing the frequency-temperature characteristics, and reduce the number of electrode fingers for forming a reflector. It is an object of the present invention to provide a surface acoustic wave device that can be formed in a small size and is compatible with low frequencies.

[0017]

According to a first aspect of the present invention, there is provided a surface acoustic wave device comprising a comb-shaped electrode formed by forming an interdigital electrode on a piezoelectric substrate and a reflector. The interdigitated electrodes constituting the container have a metallization ratio MR, which is a ratio (LW / (LW + SW)) of the line width LW and the space width SW with respect to the line width LW and the space width SW, is 60% or more; Further, the present invention is achieved by a surface acoustic wave device in which the interdigital electrodes of the reflector are coated with an oxide film.

According to the first aspect of the present invention, the metallization ratio MR of the interdigital transducer constituting the reflector is 6
By setting it to 0% or more, the reflectance of the surface acoustic wave is improved, and the number of IDTs constituting the reflector can be reduced without lowering the performance. further,
By coating the interdigital electrode of the reflector with an oxide film, the reflectance of the surface acoustic wave is further improved.

Here, as the metallization ratio MR is increased, the reflectivity of the surface acoustic wave is correspondingly increased. However, as the size of the device is reduced, it is necessary to increase the metallization ratio MR between the electrodes. Is minimized, making it difficult to manufacture, and lowering the yield. For this reason, as a result of various attempts by the present inventor, it has been found that the same effect can be obtained by coating the interdigital electrode of the reflector with an oxide film. In addition to not only increasing the metallization ratio MR, but also covering the interdigital electrode of the reflector with an oxide film, it is possible to obtain a desired reflectivity without causing difficulty in manufacturing.

According to a second aspect of the present invention, in the configuration of the first aspect, the interdigital transducers constituting the reflector and the comb-shaped electrode have a line width LW and a space width SW as a ratio of the line width to the space width. LW / (LW + S
W), wherein the metallization ratio MR is 60% or more.

According to the second aspect of the present invention, not only the reflector but also the interdigital electrodes constituting the comb-shaped electrode are provided with a metallization ratio MR of 60% or more, so that the reflectance of the surface acoustic wave is increased. It is possible to reduce the number of IDTs constituting the reflector without increasing the performance and reducing the performance.

According to a third aspect of the present invention, in the configuration of the first or second aspect, the metallization ratio MR is 6
It is not less than 0% and not more than 80%.

According to the third aspect of the present invention, the metallization ratio MR of the interdigital transducer constituting the reflector and / or the comb-shaped electrode is set to 60% or more.
The reflectivity of the surface acoustic wave is improved, and the number of IDTs constituting the reflector can be reduced without deteriorating the performance. Here, if the metallization ratio MR is higher than 80%, the interval between the electrodes becomes too small when the electrodes are formed by photolithography, and the production becomes difficult.

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a plan view showing a schematic configuration of a first embodiment of the SAW device of the present invention.

The SAW device 10 includes a piezoelectric substrate 12
And an IDT 13 which is an interdigital transducer and reflectors 14 arranged on both sides of the IDT 13, respectively.
It is a W resonator.

The piezoelectric substrate 12 is made of, for example, a single crystal substrate such as quartz, lithium tantalate, LBO, or a multilayer film substrate such as ZnO / Si as a piezoelectric material. In the present embodiment, for example, a Rayleigh wave is propagated. As a piezoelectric material
The cut angle orientation (φ, θ, ψ) of the crystal material is (0, 12
3,0) quartz substrate is used. The shape of the piezoelectric substrate 12 is, for example, a rectangular plate as illustrated.

The IDT 13 and the reflector 14 correspond to the piezoelectric substrate 1
A conductive metal such as aluminum or titanium is formed into a thin film on the surface of the substrate 2 by vapor deposition or sputtering, and is then formed into an interdigital shape by photolithography or the like.

More specifically, the IDT 13 is formed such that a plurality of electrode fingers 13a are juxtaposed at a predetermined pitch, which is a space to be described later, so that the ends in the longitudinal direction are alternately conductive. That is, each comb tooth portion of the two comb-shaped electrodes is
It is formed so as to alternately enter at a predetermined distance. The IDT 13 has a function of converting between an electric signal and a surface acoustic wave (SAW) via an external terminal 15 that is electrically connected.

Reflectors 14 are provided on both sides of the IDT 13 with a gap G therebetween. The reflector 14 is formed such that a plurality of electrode fingers 14a are arranged side by side at a predetermined pitch with a space in the same manner as the IDT 13, so that both ends in the longitudinal direction are conductive.

Then, for example, the two reflectors 14 having the same configuration are connected to the electrode finger 13a of the IDT 13 by the electrode finger 14a.
The IDT 13 is formed so as to be parallel to a and a predetermined distance in the propagation direction of the surface acoustic wave, that is, a direction orthogonal to the longitudinal direction of the electrode finger 13a of the IDT 13. The reflectors 14 have the function of reflecting the surface acoustic waves propagating from the IDT 13 and confining the energy of the surface acoustic waves inside.

FIG. 2 is an enlarged sectional view of a predetermined portion of FIG. 1. FIG. 2A is a partially enlarged sectional view of the IDT 13 cut in a direction perpendicular to the direction in which the electrode fingers 13a extend.
(B) is a partial enlarged sectional view cut in a direction orthogonal to the direction in which the electrode fingers 14a of the reflectors 14 and 14 extend.

As shown in FIG. 2A, the space width SW2 between each electrode finger 13a of the IDT 13 and the adjacent electrode finger 13a is set equal to the line width LW2 of each electrode finger 13a. LW2 + SW2, which is the sum of the line width LW2 and the space width SW2, is set to λ / 2, where λ is the wavelength of the surface acoustic wave of the SAW device 10, which is a SAW resonator.

Incidentally, an oxide film 13b may be provided on each electrode finger 13a of the IDT 13 (see FIG. 2A). When the oxide film 13b is provided on each electrode finger 13a of the IDT 13, a short circuit of each electrode finger 13a can be effectively prevented.

On the other hand, as shown in FIG.
Line width LW1 of each electrode finger 14a of the reflectors 14, 14
Is formed to be larger than the space width SW1 between the electrode fingers 14a of the reflectors 14, 14. Specifically, in this embodiment, regarding the line width LW1 and the space width SW1 of each electrode finger 14a of the reflectors 14, 14,
The ratio of the line width to the space width, that is, LW1 /
The metallization ratio MR represented by (LW1 + SW1) is set to 60% or more. The metallization ratio MR is preferably 80% or less. Note that LW1 + SW1, which is the sum of the line width LW1 and the space width SW1, is SAW
Assuming that the wavelength of the surface acoustic wave of the SAW device 10 as a resonator is λ, it is set to λ / 2.

In addition, each electrode finger 14a of the reflector 14 is covered with an oxide film 14b. This oxide film 14b
For example, when the electrode finger 14a is formed of aluminum, after forming the electrode finger 14a,
_An anodic oxide film made of ₂ O ₃ can be provided.

The present embodiment is configured as described above. By setting the metallization ratio MR of the interdigital transducer 14a constituting the reflector 14 to 60% or more, the reflectance of the surface acoustic wave is improved. The number of interdigital transducers constituting the reflector can be reduced without lowering the performance.

Here, as the metallization ratio MR is increased, the reflectivity of the surface acoustic wave is correspondingly increased. However, as the size of the device is reduced, it is necessary to increase the metallization ratio MR between the electrodes. Is minimized, and in the process of forming electrodes by photolithography, manufacturing becomes difficult, and the yield decreases. In this regard, the metallization ratio MR is 8
Preferably it is less than 0 percent.

Further, the interdigital transducer 14a of the reflector 14
By coating the oxide film 14b as described above, the reflectance of the surface acoustic wave is further improved.

That is, the metallization ratio MR
Since there is a manufacturing limit as described above in increasing the value of, the metallization ratio MR is set to less than 80%. Then, in order to further increase the reflectance of the surface acoustic wave, the interdigital electrode of the reflector 14 is coated with the oxide film 14b, so that a high reflectance can be easily obtained. In this case, the oxide film 14b
Since it is formed by the anodic oxidation method, there is no difficulty in the manufacturing process as in the case of forming the electrode. Therefore, the metallization ratio MR is increased, and the interdigital electrode of the reflector 14 is coated with the oxide film 14b. Thereby, the SA having the reflector 14 which can be easily manufactured and has a desired reflectance is provided.
The SAW device 10 that is a W resonator can be obtained.

FIG. 3 is a graph showing the relationship between the metallization ratio MR of the SAW 10 and the CI (crystal impedance) value. In this case, the SAW device 10 which is a SAW resonator is formed so as to correspond to a low frequency of about 106 MHz, for example, and the IDT 13
(The number of pairs of electrode fingers 13a to be paired) is 60, the number of electrode fingers of the reflector 14 is about 100, and the thickness H / λ of the electrode is about 2.1%. I have.

In a SAW device, if the crystal impedance (hereinafter, referred to as “CI value”) is higher than 40Ω, the device may not oscillate when combined with an oscillating circuit, or may operate normally at normal temperature, but may not operate properly due to a change in temperature environment. Oscillation may stop. However, in this embodiment, S
In the SAW device 10, which is an AW resonator, as shown in FIG.
%, The CI value is 40Ω or less. Therefore, even if the number of electrodes of the reflector 14 is reduced,
It can exhibit sufficient performance.

FIG. 4 shows the frequency distribution corresponding to the CI value when the metallization ratio MR is changed for the SAW device 10 which is a SAW resonator. FIG. 4A shows the metallization ratio MR. Is 53.3
Shows a frequency distribution corresponding to the CI value in the case of percent,
FIG. 4 (b) shows that the metallization ratio MR is 59.
FIG. 4C shows a frequency distribution corresponding to the CI value in the case of 9%, and FIG. 4C shows that the metallization ratio MR is 6%.
The frequency distribution corresponding to the CI value of 4.7% is shown.

As shown in these figures, when the metallization ratio MR is 53.3%, there are very few cases where the CI value satisfies the condition of 40Ω or less, and the metallization ratio MR is 59. In the case of 9%, the number satisfying the condition that the CI value is 40Ω or less increases considerably, and exceeds 70%. When the metallization ratio MR is 64.7%, most of those satisfying the condition that the CI value is 40Ω or less.

FIG. 5 is a block diagram of a second embodiment of the present invention.
The AW device 30 is shown. In the figure, the same reference numerals are given to configurations common to the first embodiment,
A duplicate description will be omitted, and the description will focus on the differences.

FIG. 6 is an enlarged sectional view of a predetermined portion of FIG. 2, and shows the electrode fingers 13 constituting the IDT 33 and the reflector 14.
FIG. 2 is a partially enlarged cross-sectional view cut in a direction orthogonal to a direction in which a and 14a extend.

As shown in FIGS. 5 and 6, in this embodiment, the metallization ratio MR of the IDT 33 and the reflector 14 is the same. That is, IDT
33 and the electrode fingers 13a, 14a of the reflectors 14, 14 have the same line width and are commonly LW1.
The line width LW1 is equal to the IDT 33 and the reflector 1
Between the four electrode fingers 13a, 13a,
It is formed to be larger than the space width SW1 between a and 14a. Specifically, in this embodiment, with respect to the line width LW1 and the space width SW1, the ratio of the line width to the space width, that is, LW1 / (LW1 + SW)
The metallization ratio MR represented by 1) is set to 60% or more. The metallization ratio MR is preferably 80% or less.

The electrode fingers 13a and 14a are covered with oxide films 13b and 14b.

The present embodiment is configured as described above, and since the metallization ratio MR of the IDT 33 and the reflector 14 is set to 60% or more, the same operation and effect as those of the first embodiment are exhibited. can do.

The present invention is not limited to the above-described embodiment, but may be applied to any form of SAW device such as a SAW resonator, a SAW filter, a SAW oscillator and the like without departing from the spirit of the invention described in the claims. Applied.

In particular, the individual configuration of each of the above embodiments is
If necessary, the configuration may be omitted, or another configuration different from these, or individual configurations may be arbitrarily combined.

[0052]

As described above, according to the present invention, it is possible to improve the reflection efficiency of surface acoustic waves and reduce the number of electrode fingers for forming a reflector without changing the frequency-temperature characteristics. It is possible to provide a surface acoustic wave device that can be formed in a small size and that can handle low frequencies.

[Brief description of the drawings]

FIG. 1 is a schematic plan view showing a first embodiment of a SAW device of the present invention.

FIG. 2 is a partially enlarged cross-sectional view of the SAW device of FIG. 1; FIG. 2A is a partially enlarged cross-sectional view cut in a direction orthogonal to a direction in which each electrode finger 13a of the IDT 13 extends;
(B) is a partial enlarged sectional view cut in a direction perpendicular to the direction in which the electrode fingers 14a of the reflectors 14 and 14 extend.

FIG. 3 is a graph showing the relationship between the metallization ratio MR of FIG. 1 and the average of the crystal impedance values.

FIG. 4 is a graph showing a frequency distribution corresponding to a CI value when the metallization ratio MR of the SAW device in FIG. 1 is changed.

FIG. 5 is a schematic plan view showing a second embodiment of the SAW device of the present invention.

FIG. 6 is a partially enlarged cross-sectional view cut in a direction orthogonal to a direction in which electrode fingers 13a and 14a of the IDT 13 and the reflector 14 of the SAW device of FIG. 5 extend.

FIG. 7 is a schematic plan view of a conventional SAW device.

8 is an IDT 3 and a reflector 4 of the SAW device of FIG.
FIG. 3 is a partially enlarged cross-sectional view cut in a direction orthogonal to a direction in which each of the electrode fingers 3a and 4a of the first embodiment extends.

FIG. 9 is a graph comparing the relationship between temperature and frequency change for two types of SAW devices having different electrode thicknesses.

[Explanation of symbols]

 10, 30 SAW device (surface acoustic wave device) 13 IDT (comb-shaped electrode) 13a electrode finger 14 reflector 14a, 14b, 14c electrode finger 15 reflection end face 16 reflection end face

Claims (3)

[Claims] 1. A surface acoustic wave device comprising a comb-shaped electrode formed on a piezoelectric substrate and provided with interdigital electrodes and a reflector, wherein the interdigital transducer constituting the reflector has a line width L.
Regarding W and the space width SW, the metallization ratio MR which is the ratio (LW / (LW + SW)) of the line width LW to the space width SW is 60% or more, and the interdigital transducer of the reflector has: A surface acoustic wave device comprising an oxide film. 2. The interdigital transducer constituting the reflector and the comb-shaped electrode has a line width LW and a space width SW, and a ratio of the line width to the space width, LW /
(LW + SW) metallization ratio MR
Is not less than 60%, the surface acoustic wave device according to claim 1, wherein 3. The metallization ratio MR,
The surface acoustic wave device according to claim 1, wherein the surface acoustic wave device has a range of 60% or more and 80% or less.