JP2002232255A - Surface acoustic wave device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce loss in a pass band frequency and also to obtain a desired width for a pass band frequency. SOLUTION: The average electrode line width (L2) of a grating reflector has L2<L1 relation with respect to the average electrode line width (L1) of the electrodes of an interdigital transducer part, and the radio (L2/P2) of the average electrode line width (L2) of the electrodes of the grating reflector to the average electrode period (P2) of the electrodes of the grating reflector is defined as about 0.5 value.

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 used as a component such as a resonator, a resonator type filter and an oscillator.

[0002]

2. Description of the Related Art Surface acoustic wave devices
An e-device (also called a SAW filter) has comb-like electrodes formed on a piezoelectric substrate. The surface acoustic wave device is suitable for use in a mobile communication device such as a mobile phone because it is small and lightweight.

FIG. 5 schematically shows an electrode structure of a conventional surface acoustic wave device. The surface acoustic wave device includes a piezoelectric substrate 1
0 on the surface of the interdigital transducer (ID
T) as a basic configuration. In the surface acoustic wave device, in order to meet the requirement of low-loss frequency characteristics over a wide band, materials such as lithium tantalate (lithium niobate) that have small fluctuations in vibration characteristics due to temperature changes and have a large electromechanical coupling constant are used. A piezoelectric substrate is used.

The IDT section 11 includes a comb-shaped input electrode 12 for generating a surface acoustic wave on the surface of the piezoelectric substrate 10,
And a comb-shaped output electrode 13 for receiving a surface acoustic wave to derive an output signal. Further, the IDT unit 11
Grating reflectors 14 and 15 having grid-like electrodes on the piezoelectric substrate 10 are provided on both sides. Each of the grating reflectors 14 and 15 has a plurality of reflective electrodes, each of which has a similar pattern, and a plurality of strip electrodes parallel to the comb-tooth electrodes (perpendicular to the propagation direction of the surface acoustic wave). Having.

[0005] By providing the grating reflectors 14 and 15, the surface acoustic waves propagated from the IDT section 11 are reflected by the grating reflectors 14 and 15, thereby reducing the loss of surface acoustic wave energy of the IDT section 11. be able to.

The characteristics of the above-described surface acoustic wave device are affected by the propagation characteristics of the piezoelectric substrate 11 used.
The center frequency can be adjusted by the average electrode period of the IDT unit 11, and the pass bandwidth can be controlled by the number of electrodes of the IDT unit. Further, in order to adjust the impedance of the surface acoustic wave device and the external impedance, it is designed by adjusting the number of electrodes or the opening length (length of the electrodes) of the electrodes of the IDT unit 11. Here, the electrodes of the IDT section 11 and the electrodes of the grating reflectors 14 and 15 are made of a metal film formed by a sputtering method, and a desired frequency characteristic is obtained based on the above-described electrode design. Here, the electrodes of the IDT unit 11, the grating reflectors 14, 15
Of the electrode line width / electrode period (L3 / P3) and (L
4 / P4) are designed to be almost the same.

[0007]

However, despite the selection of the number of electrode pairs and the aperture length of the IDT portion and the grating reflector so as to obtain good characteristics in design, desired characteristics are obtained. There is a problem that it is not possible to obtain suppression of the loss in the frequency pass band and the frequency pass band. This does not satisfy the requirement of further reducing the loss of the frequency pass band.

SUMMARY OF THE INVENTION An object of the present invention is to provide a surface acoustic wave device capable of reducing a loss in a frequency pass band and obtaining a desired width as a frequency pass bandwidth.

[0009]

In order to achieve the above object, the present invention is directed to a surface acoustic wave device having a grating reflector, wherein the average electrode line width (L2) of the grating reflector is equal to the interdigital transducer. L2 <L with respect to the average electrode line width (L1)
1, the line width of the electrode of the grating reflector is formed to be narrower than the line width of the electrode of the interdigital transducer section, and the ratio of the line width of the electrode of the grating reflector to its electrode period ( Duty ratio) of approximately 0.5 (± 0.). According to this configuration, the efficiency of sealing the surface acoustic wave energy into the electrodes of the interdigital transducer section is improved,
The performance as a device can be improved. That is,
The frequency pass bandwidth can be made wider than before,
Further, loss in a desired frequency pass band can be suppressed.

[0010]

Embodiments of the present invention will be described below with reference to the drawings.

FIGS. 1A and 1B are diagrams schematically showing a surface acoustic wave device according to an embodiment of the present invention.
FIG. 1A is a plan view, and FIG. 1B is a partially sectional view. An interdigital transducer (IDT) is basically provided on the surface of the piezoelectric substrate 100. In the surface acoustic wave device, in order to satisfy the requirement of low-loss frequency characteristics over a wide band, materials such as lithium tantalate (lithium niobate), which have small fluctuations in vibration characteristics due to temperature changes and have a large electromechanical coupling constant, are used. A piezoelectric substrate is used.

The IDT section 101 has a comb-shaped input electrode 10 for generating a surface acoustic wave on the surface of the piezoelectric substrate 100.
2 and a comb-shaped output-side electrode 103 for receiving a surface acoustic wave to derive an output signal. Furthermore, this ID
On both sides of the T portion 101, reflection grating electrode portions 104 and 105 are provided on the piezoelectric substrate 100.
Each of the grating reflectors 104 and 105 is a plurality of reflection electrodes, and each of them has the same pattern, and a plurality of strip electrodes parallel to the comb-tooth electrode (perpendicular to the propagation direction of the surface acoustic wave). Have.

The above electrode pattern is made of aluminum (A
l) Alternatively, after an aluminum alloy film is deposited by a sputtering method, it is formed by a photography technique. Here, the piezoelectric substrate 100 is made of 36 ° Y-cut lithium tantalate.

FIG. 2 shows the result of measuring the reflection efficiency of an Al (aluminum) electrode using lithium tantalate for the piezoelectric substrate. The horizontal axis represents the normalized film thickness, and the vertical axis represents the ratio (%) between the electrode line width and the electrode period. The normalized thick film is the ratio (%) of the electrode thickness to twice the electrode period. That is, P1 (λ wavelength) is twice as thick as h, and h / λ (%) is a normalized thick film. According to the measurement results, the larger the normalized film thickness and the closer the ratio between the electrode line width and the electrode period is to 50%, the better the reflection efficiency. That is, even if the film thickness is the same, the ratio (L2 / P2) of the electrode line width (L2) to the electrode period (P2) is 5
At 0%, the reflection efficiency is good.

Here, in the frequency pass band characteristics of the surface acoustic wave device, the ratio of the frequency pass band width at the level lowered by 3.5 dB from the upper limit of the level to the line width of the electrode of the grating reflector and the electrode period ( L2 / P2) could be measured as shown in FIG.

According to the measurement results, the ratio (L2 / P
It can be seen that the maximum pass bandwidth is obtained when 2) is 0.5.

These are grating reflectors 103, 10
5, the efficiency of trapping the elastic wave energy in the IDT unit 101 is increased, and the loss in the frequency pass band is suppressed.

On the other hand, in the conventional surface acoustic wave device, in order to increase the energy confinement efficiency (that is, to reduce the bulk wave), the electrode period (P3) of the IDT section and the electrode period (P4) of the grating reflector are changed. There is a proposal to change the ratio and set (P3 / P4) to approximately 0.98 (Japanese Patent Laid-Open No. Hei 10-276606).

In such an apparatus, P3 is set simply by focusing only on the grating reflectors 104 and 105, and then the electrode line width is set so that the duty ratio on the reflector side becomes approximately 0.5. When L2) is simply changed (decreased), the propagation speed in the grating reflectors 104 and 105 increases, and the frequency of the stop band shifts to a higher direction. Such a phenomenon affects the frequency pass band of the surface acoustic wave device.

Therefore, in order to suppress such an effect,
In the present invention, as a correction for the frequency shift,
In the electrode period P2 of the grating reflectors 104 and 105 and the electrode period P1 of the IDT section, (P1 / P2) is equal to 0.1.
It is set to be between 97 and 0.98, but adjustment is also made on the IDT side.

Here, the ratio (L1 / P1) between the electrode line width and the electrode period in the IDT section 101 is constant and is selected to be 0.7, and the impedance of the device is adjusted along with the aperture length and the number of IDTs. , To obtain good frequency characteristics. Thus, the duty ratio (L1 / P
The reason why 1) is set to 0.7 is that if the line width of the electrode is adjusted to be smaller than necessary, the electrode resistance will increase, or the capacity adjustment will not work in the limited number of IDT electrodes with a 50Ω impedance. This is because loss occurs in the frequency pass band. However, in the IDT, by adjusting Haikou town and the number of IDTs as design parameters, (L1 / P1) becomes
If it is about 0.5 or more, characteristics satisfying practical use can be obtained in the used frequency band.

As a result of the above, the average electrode line width (L1) of the IDT portion and the average electrode line width (L
The ratio (L1 / L2) of 2) is 1.01 to 1.6.

As described above, in the present invention, the reflection efficiency of the grating reflector is selected to be the best. Next, an electrode cycle (IDT section side) / electrode cycle (grating reflector side) with high energy confinement efficiency is obtained in relation to the grating reflector and the IDT section. Here, the average electrode line width on the IDT portion side is also adjusted to prevent the frequency characteristics from deteriorating.

As a result, in the surface acoustic wave device according to the present invention, the average electrode line width (L2) of the grating reflector is related to the average electrode line width (L1) of the electrodes of the IDT section 101 by L2 <L1. Becomes Also, the ratio of the average electrode line width of the grating reflectors 104 and 105 to the electrode period (L
2 / P2) (see FIG. 3) is approximately 0.5. At this time, it is possible to obtain a good filter characteristic with a wide pass bandwidth and a small loss.

In the surface acoustic wave device of the present invention, in the above example, the electrode width of the IDT section 101 is 1.67 μm,
The electrode length is 250 μm, the thickness is 0.35 μm, and the electrode width in the grating reflectors 104 and 105 is:
The thickness was set to 1.22 μm and the thickness to 0.35 μm.

In the above embodiment, lithium niobate is used as the piezoelectric substrate. However, lithium tetraborate or the like may be used, and the material of the electrode and the forming method are limited to the above examples. Not something.

FIG. 4 is a diagram showing the frequency characteristics (solid line) of the surface acoustic wave device of the present invention measured and compared with the characteristics (dotted line) of the conventional device. As can be seen from this comparison, it is understood that the surface acoustic wave device of the present invention has a wider bandwidth and a reduced loss.

[0028]

As described above, according to the present invention,
The loss of the frequency pass band can be reduced, and a desired width can be obtained as the frequency pass band width.

[Brief description of the drawings]

FIG. 1 is a diagram showing an embodiment of the present invention.

FIG. 2 is a graph showing measurement results of the reflection efficiency of an electrode by lithium tantalate.

FIG. 3 shows a ratio (L2 / P2) between the frequency pass bandwidth at a level lowered by 3.5 dB from the upper limit of the frequency pass band characteristic of the surface acoustic wave device and the line width of the electrode of the grating reflector and the electrode period. FIG. 7 is a diagram showing the results of measuring the relationship with ()).

FIG. 4 is a frequency characteristic of the surface acoustic wave device of the present invention (solid line).
FIG. 7 is a graph showing measured values and comparing them with characteristics (dotted lines) of a conventional device.

FIG. 5 is a diagram showing a configuration of a conventional surface acoustic wave device.

[Explanation of symbols]

100: piezoelectric substrate, 101: interdigital transducer section, 102: input side comb-shaped electrode, 103: output side comb-shaped electrode,

Claims (4)

[Claims] 1. A surface acoustic wave device comprising: an interdigital transducer section on a piezoelectric substrate; and a grating reflector disposed on the piezoelectric substrate so as to sandwich the interdigital transducer section. Average electrode line width (L2) is
L2 <L1 with respect to the average electrode line width (L1) of the electrodes of the interdigital transducer section; and the average electrode line width (L) of the electrodes of the grating reflector.
2) and the ratio (L2 / P2) of the average electrode period (P2) of the electrodes of the grating reflector is approximately 0.5. 2. A surface acoustic wave device comprising: an interdigital transducer section on a piezoelectric substrate; and a grating reflector disposed on the piezoelectric substrate so as to sandwich the interdigital transducer section. Ratio (L1 / L1) of the average electrode line width (L1) of the electrodes of the interdigital transducer unit to the average electrode period (P1) of the electrodes of the interdigital transducer unit.
P1) is a value larger than 0.5, and the average electrode line width (L
2) and the ratio (L2 / P2) of the average electrode period (P2) of the electrodes of the grating reflector is approximately 0.5. 3. A surface acoustic wave device having an interdigital transducer section on a piezoelectric substrate, and a grating reflector disposed on the piezoelectric substrate so as to sandwich the interdigital transducer section. Average electrode line width (L2) is
L2 <L1 with respect to the average electrode line width (L1) of the electrodes of the interdigital transducer section, and the average electrode line width (L1) of the electrodes of the interdigital transducer section and the electrodes of the interdigital transducer section. Of the average electrode period (P1) and the ratio (L1 / P
1) is a value greater than 0.5, and the average electrode line width (L
2) and the ratio (L2 / P2) of the average electrode period (P2) of the electrodes of the grating reflector is approximately 0.5. 4. The ratio (L1 / L) of the average electrode line width (L1) of the electrodes of the interdigital transducer to the average electrode line width (L2) of the electrodes of the grating reflector.
4. The surface acoustic wave device according to claim 1, wherein 2) is 1.01 to 1.6.