JP2005051354A - End face reflection type surface wave apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an end face relation type surface wave apparatus having a structure which is cheaply manufacturable, improves the productivity of the surface wave apparatus, and suppresses unwanted ripples appearing in the resonance characteristics. <P>SOLUTION: The apparatus comprises an interdigital transducer (IDT) having a plurality of electrode digits on a piezoelectric board. The board has two end faces opposite in the propagating direction of surface waves for reflecting SH type surface waves and outermost electrode digits provided at both ends of the IDT so as to extend in a direction parallel to the two end faces. If the wavelength of the SH type surface wave is λ, the widths W of the outermost electrode digits are within a range of λ/8<W≤(λ/8+7λ/64), and the piezoelectric board is of piezoelectric ceramics containing lead titanate or sodium niobate as a main component. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an end surface reflection type surface wave device including an interdigital transducer (IDT) on a piezoelectric substrate, and more particularly to an end surface reflection type surface wave device using SH type surface waves applied to a resonator, a filter, and the like.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, end surface reflection type surface wave devices using SH type surface waves are known. The SH type surface wave is a surface wave whose main component is a component whose displacement is perpendicular to the surface wave propagation direction and parallel to the substrate surface, such as a BGS wave or a Love wave.
The SH type surface wave has the property of being completely reflected by the end face existing in the propagation direction. The end-surface reflection type surface wave device utilizes this property, and the excited SH type surface wave is completely reflected at the two end faces facing the surface wave propagation direction, and the surface wave is confined between both end faces. The resonance phenomenon of generating a standing wave is caused, and the function as a surface wave device is expressed by utilizing the resonance characteristic.
[0003]
An example of the end surface reflection type surface acoustic wave device will be described with reference to FIG. FIG. 1 is a perspective view of an end surface reflection type surface acoustic wave device that uses a BGS wave as an SH type surface acoustic wave. In an end surface reflection type surface acoustic wave device using BGS waves, an interdigital transducer (IDT) 2 having a plurality of electrode fingers is formed on a piezoelectric substrate 1 having two opposite end surfaces 5a and 5b. Here, the piezoelectric substrate 1 is composed of piezoelectric ceramics or a piezoelectric single crystal. When the piezoelectric substrate 1 is ceramic, the direction of the arrow 4 indicates the polarization direction.
A plurality of electrode fingers of the interdigital transducer (IDT) extend in a direction parallel to the substrate end faces 5a and 5b. For the electrode fingers 3a and 3b on the outermost sides, the end faces 5a and 5b of the piezoelectric substrate are used. It is arranged along. Here, when the wavelength of the excited surface wave is λ, the width of the electrode finger is λ / 4 except for the outermost electrode fingers 3a and 3b, and the outermost electrode fingers 3a and 3b. Is a width of λ / 8. Further, the interval between adjacent electrode fingers is λ / 4.
[0004]
[Problems to be solved by the invention]
Conventionally, in order to obtain desired characteristics in the above-described end surface reflection type surface acoustic wave device, it has been considered that the width of the outermost electrode finger must be accurately set to λ / 8, which deviates from λ / 8. In this case, it was thought that unnecessary vibrations were excited, and ripples were generated between and in the vicinity of resonance and anti-resonance in the main vibration.
Therefore, in order to obtain the desired resonance characteristics without unnecessary ripples, the width of the outermost electrode finger needs to be exactly λ / 8, so that it is very accurate for porcelain processing to cause end face reflection. Was requested. As a result, there is a problem that productivity is lowered and cost is increased.
On the other hand, in order to effectively reduce unnecessary ripples on the lower frequency side than the resonance point, lead zirconate titanate ceramics or LiNbO ₃ piezoelectric single crystal is used as the piezoelectric substrate, and the width of the outermost electrode finger is λ / 64. An end surface reflection type surface acoustic wave device having a wavelength of ˜7λ / 64 is known (see Patent Document 1).
However, according to the present inventors, when the piezoelectric substrate is formed of piezoelectric ceramics mainly composed of lead titanate or sodium niobate, the width W of the outermost electrode finger is λ / 8 <W ≦ (λ / 8 + 7λ). / 64), it has been found that the generation of ripples between the resonance and anti-resonance of the main vibration and in the vicinity thereof can be effectively suppressed, and the present invention has been achieved.
[0005]
An object of the present invention is to provide an end surface reflection type surface acoustic wave device having a structure that improves productivity of the end surface reflection type surface acoustic wave device, is easy to manufacture at low cost, and can suppress unnecessary ripples appearing in resonance characteristics. There is to do.
[0006]
[Patent Document 1]
Japanese Patent Laid-Open No. 7-131283
[Means for Solving the Problems]
The present inventor has conducted intensive research in order to improve the productivity of an end surface reflection type surface acoustic wave device that utilizes the reflection of SH type surface waves at the two opposing end surfaces.
As a result, as described above, the width W of the outermost electrode finger of the interdigital transducer (IDT) formed on the piezoelectric substrate using a specific piezoelectric ceramic is wider by a predetermined amount than λ / 8. In this case, the present inventors have found that the desired ripple is suppressed and the desired resonance characteristics can be obtained.
[0008]
That is, the present invention includes an interdigital transducer (IDT) having a plurality of electrode fingers on a piezoelectric substrate, and the piezoelectric substrate faces the propagation direction of the surface wave in order to reflect the SH type surface wave. An end surface reflection type surface acoustic wave device having an end surface and having outermost electrode fingers provided on both sides of the interdigital transducer (IDT) so as to extend in a direction parallel to the two end surfaces, the SH type The width W of the outermost electrode finger is in the range of λ / 8 <W ≦ (λ / 8 + 7λ / 64) when the wavelength of the surface wave is λ, and the piezoelectric substrate is lead titanate or The present invention relates to an end surface reflection type surface acoustic wave device characterized by being a piezoelectric ceramic mainly composed of sodium niobate.
Furthermore, in the present invention, it is desirable that the load of the electrode part in the interdigital transducer (IDT) is in the range of 1 × 10 ^{−5 to} 3 × 10 ⁻³ g / cm ² .
[0009]
In the present invention, a piezoelectric substrate whose main component is a lead titanate (PbTiO ₃ ) system or a sodium niobate (NaNbO ₃ ) system is used. This will be described.
When a piezoelectric substrate is used as a resonator of an oscillation circuit, a DC voltage drive can be applied. Therefore, it is important to design and divide the voltage so that the impedance is relatively high. For that purpose, it is important to apply a material having a small relative dielectric constant to reduce the capacitance of the electrode portion formed on the piezoelectric substrate.
Therefore, a piezoelectric ceramic mainly composed of PbTiO ₃ or NaNbO ₃ having a relative dielectric constant of 360 or less and a large electromechanical coupling coefficient is desirable.
Lead titanate has a smaller dielectric constant than lead zirconate titanate and can provide a larger P / V, and is therefore suitable for use as an oscillator.
In addition, the reason for using sodium niobate is that lead-containing compounds have been pointed to the risk of causing environmental pollution, and it is desirable to use piezoelectric materials that do not contain lead, and the same perovskite structure as lead titanate This is because a large P / V can be obtained. Accordingly, it is desirable to use sodium niobate as a main component. Specifically, for example, Na _0.98 K _0.02 NbO ₃ is used as a main component, which corresponds to 0.5% by weight with respect to 100 parts by mass of the main component. composition, etc. plus MnO ₂ which are mentioned.
[0010]
[Action]
According to the end surface reflection type surface acoustic wave device of the present invention, an interdigital transducer (IDT) having a plurality of electrode fingers is provided on a piezoelectric substrate, and the piezoelectric substrate reflects the surface wave of the SH type in order to reflect the SH type surface wave. In an end surface reflection type surface acoustic wave device having two end faces opposed to a propagation direction and having outermost electrode fingers provided on both sides of the interdigital transducer (IDT) so as to extend in a direction parallel to the two end faces When the wavelength of the SH type surface wave is λ, the width W of the outermost electrode finger is set in the range of λ / 8 <W ≦ (λ / 8 + 7λ / 64), respectively, thereby resonating the main vibration. In addition, the generation of ripples between and near the anti-resonance can be remarkably suppressed.
When the width W of the outermost electrode finger exceeds (λ / 8 + 7λ / 64), the phase of the traveling surface wave and the phase of the reflected wave at the end face cannot be matched, and an unnecessary ripple is generated in the resonance characteristics. appear.
[0011]
The piezoelectric substrate used in the present invention is made of piezoelectric ceramics mainly composed of lead titanate (PbTiO ₃ ) and sodium niobate (NaNbO ₃ ), which have a large P / V value and a small relative dielectric constant. is important.
Here, in the case of piezoelectric ceramics mainly composed of PbTiO ₃ or NaNbO ₃ , the Poisson's ratio of material properties is less than 0.3, and a material of 0.3 or more typified by lead zirconate titanate (PZT) Since the vibration form is different, the frequency and level of the ripple are also different. Therefore, it is important to set the width W of the outermost electrode finger to λ / 8 <W ≦ (λ / 8 + 7λ / 64).
[0012]
Moreover, in the electrode part of the interdigital transducer (IDT) comprising a plurality of electrode fingers on the piezoelectric substrate, the load per unit area of the electrode part is in the range of 1 × 10 ^{−5 to} 3 × 10 ⁻³ g / cm ² . By making it inside, the mass load effect is increased by the electrode load applied to the piezoelectric substrate, and with the increase in the energy confinement effect, the generation of ripples between and near the main vibration resonance and anti-resonance can be further suppressed. .
[0013]
When the load of the electrode part of the interdigital transducer (IDT) is 1 × 10 ⁻⁵ g / cm ² or more, an effect of suppressing the generation of ripples due to the small mass load effect is obtained. On the other hand, in the case of 3 × 10 ⁻³ g / cm ² or less, it is possible to prevent the P / V from being reduced by suppressing the excitation in the interdigital transducer due to the large mass load effect. is there. Here, P / V is represented by the ratio of the anti-resonance resistance value to the resonance resistance value, and serves as an index representing the strength of the excited piezoelectric vibration.
[0014]
That is, in the past, it was considered that the outermost electrode finger width W had to be exactly λ / 8. However, in the present invention, when a specific piezoelectric ceramic is used for the piezoelectric substrate, It has been found that if R is within a range of λ / 8 <W ≦ (λ / 8 + 7λ / 64), unnecessary ripples are suppressed and desired resonance characteristics can be obtained. As a result, it is possible to give a margin to the processing accuracy of the reflection end surface, suppress unnecessary ripples, improve the processing yield of the end surface reflection type surface acoustic wave device, and greatly improve productivity.
Furthermore, in the electrode part of the interdigital transducer (IDT) composed of a plurality of electrode fingers on the piezoelectric substrate, the load of the electrode part is set within the range of 1 × 10 ^{−5 to} 3 × 10 ⁻³ g / cm ^2. Thus, a desired resonance characteristic can be obtained. In order to generate a stable standing wave in the surface wave device, it is necessary to confine the energy of the surface wave by the mass load effect of the electrode. Therefore, in order to obtain a desired resonance characteristic, This is because an electrode finger having a mass of 1 is required.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is an external perspective view of an end surface reflection type surface acoustic wave device of the present invention, and FIG. 2 is a front view on which an IDT is formed.
1 and 2, reference numeral 1 is a piezoelectric substrate, 2 is an electrode portion of an interdigital transducer (IDT), 3a and 3b are outermost electrode fingers, 4 is a polarization axis direction of the piezoelectric ceramic, and 5a and 5b are SH types. It is an end face that reflects surface waves.
The piezoelectric substrate 1 is mainly composed of lead titanate (PT), specifically, Pb partially substituted with Sr, Ba, Ca and rare earth La, Pr, Nb, etc., and Mn is added. It is the composition contained. Further, it is a composition containing NaNbO ₃ as a main component, specifically, a component in which Na is partially substituted with K, Ca, Bi or the like as a main component, and adding Mn. In this case, ₂ parts by mass or less of MnO ₂ can be added to 100 parts by mass of these main components to form the piezoelectric substrate 1.
Thereby, the electromechanical coupling coefficient can be increased and a large P / V value can be obtained.
[0016]
The electrode width of the interdigital transducer (IDT) is λ / 4 except for the outermost electrode fingers, and the interval between adjacent electrode fingers is λ / 4, and the outermost electrode fingers 3a and 3b The electrode width W is set in a range of λ / 8 <W ≦ (λ / 8 + 7λ / 64). These outermost electrode fingers 3a, 3b are arranged so as to be in contact with the reflection end faces 5a, 5b.
In particular, W is preferably (λ / 8 + λ / 64) to (λ / 8 + 4λ / 64) because the ripple level can be reduced while obtaining a large characteristic with a main vibration P / V value of 55 dB or more.
The electrode 2 of the interdigital transducer (IDT) is made of a metal such as Al, Cr, Ag, Au, Pt, or Cu, and the load on the electrode portion is 1 × 10 ^{−5 to} 3 × 10 ⁻³ g / cm ² . The film is formed by sputtering or vapor deposition so as to be within the range.
[0017]
【Example】
Hereinafter, the present invention will be further clarified by describing examples.
The present embodiment relates to an end surface reflection type surface wave resonator using BGS waves as an end surface reflection type surface acoustic wave device.
First, a piezoelectric substrate made of piezoelectric ceramic containing lead titanate Pb _0.9 La _0.1 TiO ₃ and 0.5 parts by mass of MnO ₂ with respect to 100 parts by mass of the main component is prepared, and an arrow 4 in FIG. As shown, the polarization treatment was performed in the thickness-sliding direction of the substrate. Next, Cr as a base and Ag as an electrode were sequentially deposited on the piezoelectric substrate, and an interdigital transducer (IDT) was formed so as to be parallel to the direction of polarization and the direction in which the electrode fingers extended. Here, the widths of the electrode fingers of the interdigital transducer (IDT) were all λ / 4 = 15 μm, and the interval between adjacent electrode fingers was also λ / 4 = 15 μm. The thickness of the electrode was approximately 1 μm. Thereafter, the piezoelectric substrate is cut in the thickness direction so that the width W of the outermost electrode finger becomes 8λ / 64 thicker than λ / 8 = 7.5 μm, and the end face reflection type surface wave resonance having the shape shown in FIG. A child was made. FIG. 2 is an electrode pattern diagram of an interdigital transducer (IDT).
When the outermost width W of the electrode finger of the produced end surface reflection type surface wave resonator is from (λ / 8 + λ / 64) to (λ / 8 + 7λ / 64), the P / V value of the main vibration is 50 dB or more, and for the resonator As a result, sufficiently large characteristics were obtained.
[0018]
FIG. 3 shows an example of the impedance waveform of the end surface reflection type surface wave resonator thus obtained. As can be seen from FIG. 3, when the width W of the outermost electrode finger is λ / 8 + 8λ / 64, a large ripple is generated between the resonance frequency 49.2 MHz and the anti-resonance frequency 50.5 MHz.
For each sample of the surface wave resonator obtained as described above, the magnitude of the ripple is measured, and plotted against the outermost electrode dimensions is shown in FIG. In FIG. 4, the horizontal axis represents the deviation of the dimension of the outermost electrode finger from λ / 8. That is, for example, a sample having a value on the horizontal axis of 5λ / 64 means that the dimension of the outermost electrode finger is (λ / 8 + 5λ / 64) = 13λ / 64. Also, the vertical axis of FIG. 4 shows the magnitude of the ripple P / V value.
[0019]
FIG. 4 shows that no ripple exists when the width of the outermost electrode finger is exactly λ / 8. That is, as previously considered, if processing is performed so that the wavelength is accurately λ / 8, desired resonance characteristics can be obtained in which no ripple occurs between the resonance frequency and the anti-resonance frequency.
Next, when the width of the outermost electrode finger is gradually increased, as is apparent from FIG. 4, no ripple occurs up to 4λ / 64, and a desired resonance characteristic can be obtained. When it becomes larger than 5λ / 64, ripples are generated, and the magnitude of the ripple increases as the width of the outermost electrode finger increases. However, in the case of 7λ / 64 or less, although the ripple is generated, the magnitude thereof is small and there is no practical problem.
It can be seen from FIG. 4 that the ripple increases rapidly when the width of the outermost electrode finger is 8λ / 64. In this case, desired characteristics as a resonator can no longer be obtained.
[0020]
Therefore, by making the width W of the outermost electrode finger within the range of λ / 8 <W ≦ (λ / 8 + 7λ / 64), the end face reflection that suppresses the generation of ripples between the resonance frequency and the anti-resonance frequency. A type surface acoustic wave resonator can be obtained.
Further, in the electrode part of an interdigital transducer (IDT) composed of a plurality of electrode fingers on the piezoelectric substrate, the load of the electrode part is set within a range of 1 × 10 ^{−5 to} 3 × 10 ⁻³ g / cm ^2. An electrode was prepared by sputtering. The load was adjusted by the thickness. At this time, the width of the outermost electrode finger was set to λ / 8 + 2λ / 64. The impedance waveform of the sample thus prepared was measured, and the P / V value of the main vibration was evaluated.
When the load of the electrode part in the interdigital transducer (IDT) is 1 × 10 ⁻⁵ g / cm ² , 1 × 10 ⁻³ g / cm ² , 3 × 10 ⁻³ g / cm ² , the P / V value is respectively It was sufficiently large as 50 dB, 58 dB, and 67 dB, and no ripple was observed.
[0021]
On the other hand, when the load on the electrode portion was 4 × 10 ⁻³ g / cm ² , no ripple was observed in the impedance waveform, but the P / V value was as small as 45 dB. Also, when the load on the electrode portion was 0.8 × 10 ⁻⁵ g / cm ² , the ripple was not recognized, but the P / V value was as small as 42 dB.
From this embodiment, the weight of the electrode part per unit area in the electrode part of the interdigital transducer (IDT) is set in the range of 1 × 10 ^{−5 to} 3 × 10 ⁻³ g / cm ² , which is unnecessary. It is possible to obtain a surface wave resonator that does not generate ripples and has a large P / V value.
Also, a piezoelectric substrate containing sodium niobate, specifically, Na _0.98 K _0.02 NbO ₃ as a main component and containing 0.5 part by mass of MnO ₂ with respect to 100 parts by mass of the main component is used. When a surface wave resonator was fabricated and evaluated in the same manner as in the above example, the same effect as in the above example was obtained.
In the above embodiments, the end surface reflection type surface wave resonator has been described. However, the present invention can be applied to any device that uses complete reflection at the end surface of an SH type surface wave, such as an end surface reflection type filter. Is possible.
[0022]
【The invention's effect】
According to the end surface reflection type surface acoustic wave device of the present invention, in the end surface reflection type surface acoustic wave device using the reflection of the SH type surface wave at the two end surfaces facing the direction of propagation of the surface wave of the piezoelectric substrate, When the width W of the electrode finger is in the range of λ / 8 <W ≦ (λ / 8 + 7λ / 64) when the wavelength of the surface wave is λ, unnecessary ripples appearing in the resonance characteristics are suppressed, In addition, it is possible to provide a margin for the processing accuracy of the end surface, the processing yield of the end surface reflection type surface acoustic wave device is improved, and the productivity is greatly improved.
Furthermore, in the electrode part of the interdigital transducer (IDT) composed of a plurality of electrode fingers on the piezoelectric substrate, the load of the electrode part is set within the range of 1 × 10 ^{−5 to} 3 × 10 ⁻³ g / cm ^2. As a result, the mass load effect due to the electrode load applied to the piezoelectric substrate is increased, and the energy confinement effect is increased, and the generation of ripples between and near the main vibration resonance and anti-resonance can be suppressed.
[Brief description of the drawings]
FIG. 1 is an external perspective view showing an end surface reflection type surface acoustic wave device of the present invention.
2 is a front view showing an IDT formed on the piezoelectric substrate of FIG. 1. FIG.
FIG. 3 is a diagram showing electrical characteristics of the end surface reflection type surface acoustic wave device of the present invention.
FIG. 4 is a diagram showing the relationship between the width of the outermost electrode finger and the magnitude of ripple in the end face reflection type surface acoustic wave resonator according to the embodiment of the present invention.
[Explanation of symbols]
1: Piezoelectric substrate 2: Interdigital transducer (IDT) electrode parts 3a, 3b: IDT outermost electrode finger 4: Polarization axis direction of piezoelectric ceramic.
5a, 5b: End faces for reflecting SH type surface waves

Claims (2)

An interdigital transducer (IDT) having a plurality of electrode fingers on a piezoelectric substrate, the piezoelectric substrate having two end faces opposed to the propagation direction of the surface wave in order to reflect the SH type surface wave; and An end surface reflection type surface acoustic wave device in which outermost electrode fingers are provided on both sides of the interdigital transducer (IDT) so as to extend in a direction parallel to the two end surfaces, wherein the wavelength of the SH type surface wave is determined. When λ, the width W of the outermost electrode finger is in the range of λ / 8 <W ≦ (λ / 8 + 7λ / 64), respectively, and the piezoelectric substrate is mainly composed of lead titanate or sodium niobate An end face reflection type surface acoustic wave device characterized by comprising: The end reflection type surface acoustic wave according to claim 1, wherein a load of an electrode portion in the interdigital transducer (IDT) is in a range of 1 × 10 ^{−5 to} 3 × 10 ⁻³ g / cm ^2. apparatus.

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