JPH10247835A - Love wave-type surface acoustic wave device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce cost by providing a Love wave-type surface acoustic wave by using a comparatively inexpensive base metal with large specific gravity, such as Ta(tantalum), W(tungsten) and Pd(palladium), instead of noble metal such as gold. SOLUTION: An interdigital electrode(IDT electrode) 21 of a first layer made of Al(aluminum) is formed on a rotary Y cut-X transmission LiNb O3 piezoelectric substrate 1. Then, an IDT electrode 22 of a second layer formed of Ta, W or Pd is formed on the electrode with a prescribed film thickness. The IDT electrode 21 of the first layer of Al is formed on -10 deg. to +50 deg. rotary Y cut LiTaO3 piezoelectric substrate 1, and the IDT electrode 22 of the second layer of Ta, W or PD is formed on the electrode with a prescribed thickness. The both IDT electrodes are made into two layers. The layer below aluminum is a layer, where electric resistance is lowered and the upper layer formed of Ta, W or Pd is the layer formed making the Love-wave and it can control the film thickness more easily as compared to gold.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to the, LiNbO ₃ (lithium niobate) or LiTaO ₃ using a piezoelectric single crystal substrate (lithium tantalate), surface acoustic wave using a Love wave type surface acoustic wave (Surface Acoustic Wave: (Hereinafter abbreviated as SAW).

[0002]

2. Description of the Related Art First, SAW using a LiNbO ₃ substrate
The device will be described. Y cut-X propagation LiNbO
A love wave type SAW using _three substrates has an electromechanical coupling coefficient k
² is much larger than a Rayleigh-wave type SAW, and is therefore applied to resonators and the like that require broadband characteristics.

[0003] By depositing a thin film of gold (Au) having a large mass and a surface wave velocity lower than that of a substrate on a LiNbO ₃ substrate, a pseudo-acoustic surface wave having a large propagation attenuation is converted into a love-wave type SAW without attenuation. be able to. That is,
The pseudo acoustic surface wave existing on the piezoelectric substrate is deposited on a thin film of gold (Au) having a large mass and a low acoustic velocity to reduce the acoustic velocity of the pseudo acoustic surface wave.
079 m / s), a love wave type SAW without attenuation can be obtained. The gold thin film to be deposited does not need to be provided on the entire surface of the piezoelectric substrate, and only the interdigital electrode (IDT electrode) for exciting the SAW is formed of a love wave type S-shaped electrode.
An AW device is formed.

As a typical example of a Love wave type SAW device,
The SAW resonator will be described below. FIG. 2A is a plan view showing an example of the simplest conventional Love-wave type SAW resonator, in which only an IDT electrode 2 is provided on a piezoelectric substrate 1. FIG. 2B is a cross-sectional end view taken along the line AA ′ of FIG. 2A and shows the configuration of the IDT electrode 2. In FIG. 2, the number of pairs of IDT electrodes is three.
Needless to say, it is not necessary to limit to pairs. FIG.
Has described an example of a SAW device in which only the IDT electrode 2 is provided on the substrate for the sake of simplicity, but a configuration in which reflectors are provided on both sides of the IDT electrode 2 may be used.

The material of the IDT electrode 2 is gold (Au).
Although gold is generally used, gold has poor adhesion to the piezoelectric substrate 1, and therefore, as shown in FIG. 2B, an adhesive layer 23 such as Cr is usually provided as a base, and Is provided with gold (Au) 24. However, since gold is expensive, there is a problem that the cost of the SAW device is increased in terms of cost.

Therefore, instead of a noble metal such as gold, T
There is a configuration in which a relatively inexpensive base metal such as a (tantalum), W (tungsten), and Pd (palladium) is used to obtain a Love-wave type surface wave by using a base metal having a large specific gravity.

Next, a SAW device using a LiTaO ₃ substrate will be described. Heretofore, there has been a Love wave type SAW device using a LiTaO ₃ substrate which has been previously proposed by the present inventors (see Japanese Patent Application No. 4-57231).

FIG. 3A is a characteristic diagram of the surface wave velocity (phase velocity) with respect to the rotation angle θ of the LiTaO ₃ substrate. As shown in FIG. 3B, the horizontal axis is in the YZ plane. Indicates the cutting rotation angle θ from the Y axis, and the surface wave propagates in the X axis direction.

As shown in FIG. 3, a rotating Y-cut LiTa
It is known that a Rayleigh wave indicated by a broken line and a pseudo elastic surface wave (leaky wave) indicated by a solid line exist on the O ₃ piezoelectric substrate.

[0010] Also, the slow transverse wave (3380 m / s) is shown in FIG.
However, when the surface wave velocity is higher than this slow transverse wave, such as a pseudo-surface acoustic wave, it is a so-called leaky wave that propagates while converting energy into a bulk wave while propagating. It is not practical except for the Y plate. If the surface wave velocity is lower than the slow transverse wave, such as a Rayleigh wave, the surface wave has no propagation attenuation.

A quasi-surface acoustic wave (leaky wave) is propagated by depositing a heavy substance having a slow sound velocity on a LiTaO ₃ piezoelectric substrate with a predetermined film thickness to lower the surface wave velocity and make it slower than the slow transverse wave. Love wave type surface waves without attenuation can be obtained.

FIG. 4 shows the relationship between the film thickness and the surface wave velocity when gold (Au) as a heavy substance having a low sound velocity is uniformly deposited on a piezoelectric substrate at a cutting angle θ = 0 °. This is the result. As can be seen from FIG. 4, the surface wave velocity becomes 3380 m / s or less when the Au film thickness (Ha / λ) is 0.04 or more (λ: wavelength of the surface wave), and a Love wave type surface wave can be obtained. It is understood that it is possible.

Also, instead of attaching a heavy substance having a slow sound velocity uniformly on a piezoelectric substrate, gold (A) is applied to a IDT (Interdigital Transducer) for exciting a surface wave.
It is known that similar effects can be obtained by using a noble metal having a high specific gravity, such as u), silver (Ag), platinum (Pt), and the like and setting the thickness to a predetermined film thickness or more.

Further, as can be seen from FIG. 3, if the range of the cutting angle of the rotation Y cut is in the range of -10 ° to + 50 °, the rotation Y cut is equal to or larger than the rotation Y cut-X propagation LiTaO ₃ . electromechanical coupling coefficient k ² is obtained. The electromechanical coupling coefficient k ² is shown in FIG. 3 as open (the substrate surface is electrically open) and short (the substrate surface is electrically shorted).
Is proportional to the difference in sound speed.

As the electrode, gold (Au) is generally used, but since gold has poor adhesion to the piezoelectric substrate, as shown in FIG. A layer 23 is applied as a base, on which gold (Au) 24 is applied. However, since gold is expensive, there is a problem that the cost of the SAW resonator is increased in terms of cost.

Therefore, instead of a noble metal such as gold, T
There is a configuration in which a relatively inexpensive base metal such as a (tantalum), W (tungsten), and Pd (palladium) is used to obtain a Love-wave type surface wave by using a base metal having a large specific gravity.

[0017]

However, both in the case of LiNbO ₃ described above and in the case of LiTaO ₃ described above,
When the ID electrode is formed using only the base metal, the electric resistance (volume resistivity: Ωm) of the ID electrode is 2.4 × 10 ^{−8 of} gold.
In contrast, Ta: 15 × 10 ⁻⁸ , W: 5.5 × 10 ⁻⁸ ,
Since Pd is 10.8 × 10 ⁻⁸ , which is about 2 to 6 times that of gold, there are drawbacks such as a decrease in Q in a resonator and an increase in insertion loss in a filter. Also,
Since the density of Au is as large as 19.3 g / cm ³ and the characteristics of the device largely depend on the film thickness, control of the line width and film thickness is strictly required at the time of forming the electrode pattern, which makes the manufacturing process difficult. there were.

It is an object of the present invention to reduce costs due to gold, which is a problem of the prior art, and to reduce characteristic deterioration due to an increase in electrical resistance when the electrodes are only Ta, W or Pd, and to facilitate the manufacturing process. It is another object of the present invention to provide a Love wave type surface acoustic wave device using a LiNbO ₃ substrate or a LiTaO ₃ substrate.

[0019]

According to a first aspect of the present invention, there is provided a Love type surface acoustic wave device which excites a Love type surface acoustic wave on the surface of a rotating Y-cut and X-propagating LiNbO ₃ piezoelectric substrate. In an interdigital electrode, or in a Love-wave type surface acoustic wave device in which an interdigital electrode and a reflector electrode are disposed, the electrode has a two-layer structure that is vertically stacked,
As a lower layer of the two-layer structure, a first layer formed in contact with the substrate is formed of aluminum having a predetermined thickness, and a second layer laminated thereon is formed of tantalum, tungsten or palladium having a predetermined thickness. Characterized by being formed by any of the above.

Further, in the Love type surface acoustic wave device according to the second aspect of the present invention, the Y-axis is a normal line, and the Y-axis
IDTs or IDTs for exciting Love-type surface acoustic waves on the surface of a rotating Y-cut LiTaO ₃ piezoelectric substrate cut at a predetermined angle in the range of −10 ° to + 50 ° from the axis. In a Love-wave type surface acoustic wave device in which an electrode and a reflector electrode are provided, the electrode has a two-layer structure stacked vertically, and is formed as a lower layer of the two-layer structure in contact with the substrate. The first layer is formed of aluminum having a predetermined thickness, and the second layer laminated thereon is formed of tantalum, tungsten, or palladium having a predetermined thickness.

[0021]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. FIG. 1 is a plan view (a) showing an embodiment of the present invention, and FIG.
It is an end view (b) of -A 'cut part. Although the logarithm of the IDT in the figure is three, it goes without saying that regardless of the logarithm of the IDT, even if a reflector is provided, it is common to the electrodes.

In the embodiment of the present invention, a first layer IDT electrode 21 of Al is formed on a rotating Y-cut and X-propagating LiNbO ₃ piezoelectric substrate 1, and Ta, W
Alternatively, the IDT electrode 22 of the second layer of Pd is formed with a predetermined thickness.

In the embodiment of the present invention, an Al first IDT electrode 21 is formed on a Y-cut LiTaO ₃ piezoelectric substrate 1 rotated from −10 ° to + 50 °, and an Al first IDT electrode 21 is formed thereon. An IDT electrode 22 of a second layer of Ta, W or Pd is formed with a predetermined thickness.

In the first and second aspects of the present invention,
The DT electrode 2 has two layers, the lower layer of aluminum performs the function of lowering the electric resistance, and the upper layer made of Ta, W or Pd is a layer for making a love wave. It has the feature that film thickness control is easier than before.

The density of Al in the first layer 21 is 2.96 g / c.
m ³ , and the densities of Au, Ta, W, and Pd are 19.3, 16.6, 19.1, and 12.16, respectively. In order to convert a quasi-surface acoustic wave with propagation attenuation existing on the piezoelectric substrate 1 into a Love-wave type surface wave without propagation attenuation,
A certain mass is required. Since the density of Al is about 1/6 of that of Au, if the Al layer 21 alone is used to make a love wave, a film thickness six times that of Au is required. The second layer 22 of Ta, W or Pd contributes to the generation of the love wave. Therefore, Al
The layer 21 does not contribute much to Love wave formation and can be deposited relatively thick.

The electrical resistance of Al (volume resistivity Ω ·
m) is 2.75 × 10 ^-8, not much different from 2.4 × 10 ^-8 gold, Ta, W, the volume resistivity of each 15 × 10 ^-8 of Pd, 5.5 × 10 ^{- 8} and 10.8 × 10 ⁻⁸ , the electric resistance of the IDT electrode 2 can be reduced as compared with the case where the IDT electrode is composed of only Ta, W and Pd.

Further, as described above, since the density of Ta, W, and Pd is smaller than the density of Au, the line width is the same in order to obtain the same effect as Au in the Love wave conversion. In this case, the film thickness must be set inversely proportional to the density, but from the viewpoint of the film thickness dependence of device characteristics,
Since the dependence is smaller than that of Au, strict film thickness control unlike the conventional Au is not required. In addition, since the total mass of the electrodes is involved in Love wave conversion,
From the viewpoint of the line width, Au is reduced by the amount corresponding to the decrease in the density.
Control accuracy is lower than that of. Also, the price of Ta, W, Pd is much lower than Au, so SAW
The cost as a resonator can be reduced.

[0028]

As described in detail above, by implementing the present invention, the material cost is significantly reduced as compared with the case where a conventional Au electrode is used as an IDT electrode, and Ta, W
Alternatively, deterioration of device characteristics due to an increase in electric resistance when only Pd is used is suppressed, and the manufacturing process is facilitated, so that the practical effect is extremely large.

[Brief description of the drawings]

FIG. 1 is a plan view showing an embodiment of the present invention and an end surface of an AA ′ cut portion thereof.

FIG. 2 is a plan view of a conventional SAW device and an end view of an AA ′ cut section thereof.

FIG. 3 is a diagram illustrating a relationship between a rotation angle and a surface wave velocity in a rotating Y-cut LiTaO ₃ substrate.

FIG. 4 is an explanatory diagram showing the film thickness dependence of the surface wave velocity in a rotating Y-plate LiTaO ₃ substrate.

[Explanation of symbols]

 Reference Signs List 1 piezoelectric substrate 2 IDT electrode 21 first layer of IDT electrode 22 second layer of IDT electrode 23 adhesive layer (Cr) 24 IDT electrode (Au)

Claims (2)

[Claims] 1. A rotation Y-cut -X propagation LiNbO ₃ interdigital electrode to excite the Love wave type surface acoustic wave on the surface of the piezoelectric substrate or the Love wave with interdigital electrodes and reflector electrodes are disposed, In the surface acoustic wave device, the electrode has a two-layer structure stacked vertically, and a first layer formed in contact with the substrate as a lower layer of the two-layer structure is formed of aluminum having a predetermined thickness. And a second layer laminated thereon is formed of one of tantalum, tungsten and palladium having a predetermined thickness. 2. On the surface of a rotating Y-cut LiTaO ₃ piezoelectric substrate cut at a predetermined angle in the range of −10 ° to + 50 ° from the Y axis on the YZ plane with the Y axis as a normal line. In a Love wave type surface acoustic wave device in which an interdigital transducer or an interdigital transducer and a reflector electrode are arranged so as to excite a Love acoustic wave, the electrode has a two-layer structure in which the electrodes are vertically stacked. A first layer formed on the substrate as a lower layer of the two-layer structure is formed of aluminum having a predetermined thickness, and a second layer laminated thereon is formed of tantalum having a predetermined thickness. A Love wave type surface acoustic wave device formed of either tungsten or palladium.