JP2008278349A - Saw resonator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a small SAW resonator which has an excellent frequency temperature characteristic in a large temperature range. <P>SOLUTION: In an SAW resonator 10, an IDT 20 is formed on a crystal substrate 5, a resonance lies in the vicinity of a stop band upper side and under side of the IDT 20, and the crystal substrate 5 has an oblique symmetrical property. A high-order mode resonance frequency which the resonance in the vicinity of the stop band upper side has and a fundamental wave mode resonance frequency which the resonance in the vicinity of the stop band under side has are made coincident with each other to be formed in a coupling state. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a SAW resonator that excites a Rayleigh wave, and more particularly to a SAW resonator that improves frequency temperature characteristics.

In the SAW resonator of the prior art, an electrode pattern is formed on the surface of a quartz substrate using a metal such as aluminum, copper, titanium, tungsten, or the like as an electrode to constitute a SAW resonator. The orientation of the quartz substrate is Euler angle display (φ, θ, ψ) on the fundamental axis of the quartz crystal. First, φ is in the range of 0 ° ± 1 ° counterclockwise around the optical axis Z. Next, θ is in the range of 122 ° to 127 ° counterclockwise around the X axis, which is the electric axis, and the X axis is the starting point in the piezoelectric plate around the newly generated axis Z ′. There is an in-plane rotation ST cut that rotates in the counterclockwise direction and the direction in which ψ is in the range of 40 ° to 44 ° is the phase propagation azimuth of the surface acoustic wave.
In this in-plane rotating ST cut substrate, the surface acoustic wave used as the SAW resonator is a Rayleigh wave. Since this elastic wave is similar to the surface wave used for the SAW resonator using the well-known ST cut substrate and has similar attributes, it has a stable frequency, resistance to driving power, and residual stress of the electrode. Is low and can maintain excellent frequency stability (see Patent Document 1).

When configuring an oscillator using such a SAW resonator, it is required to improve the accuracy of the frequency temperature characteristics as well as the frequency accuracy. In particular, there is a strong demand for frequency stability in the high-speed network market.
Among frequency stability, several methods are known for improving frequency temperature characteristics. In Patent Document 2, a plurality of SAW resonators are arranged on a substrate so that the propagation directions of surface acoustic waves are different. The frequency temperature characteristics are improved by combining them and combining them.
Furthermore, in Patent Document 3, two SAW resonators having different frequency temperature characteristics are arranged in parallel, and these are excited so as to be in an obliquely symmetric mode and are coupled in a horizontal elastic manner to obtain a flat frequency temperature characteristic. I am trying.

Japanese Patent No. 3216137 U.S. Pat. No. 4,193,045 US Pat. No. 5,912,602

The SAW resonator using the in-plane rotating ST-cut quartz substrate as in Patent Document 1 has a frequency accuracy twice as high as the frequency temperature characteristic of the SAW resonator manufactured using the ST-cut substrate. However, it is inadequate for recent accuracy requirements.
Specifically, the second-order temperature coefficient of the frequency temperature characteristic of the SAW resonator is −1.6 × 10 ⁻⁸ / ° C. ² and its apex temperature is assumed to be within the operating temperature range (−35 ° C. to + 85 ° C. ), The frequency fluctuation amount due to temperature change is about 58 ppm, and it is difficult to further improve the frequency temperature characteristics.
In Patent Documents 2 and 3, frequency temperature characteristics are improved by using a plurality of SAW resonators, and it is difficult to reduce the size of the SAW resonators.
The present invention has been made to solve the above problems, and an object of the present invention is to provide a SAW resonator having a small size and excellent frequency temperature characteristics over a wide temperature range.

  In order to solve the above-described problems, the present invention is a SAW resonator in which an IDT is formed on a quartz substrate, and the quartz substrate has resonance in the vicinity of stopbands on the upper and lower sides of the IDT. The resonance state of the higher-order mode possessed by the resonance near the upper stop band and the resonance frequency of the fundamental mode possessed by the resonance near the lower stop band are matched to form a coupled state. And

According to this configuration, the higher-order mode of the resonance near the upper stop band and the fundamental mode in the resonance near the lower stop band simultaneously exist and are excited by the drive voltage. The resonance of each mode has slightly different frequency temperature characteristics, and specifically, the peak temperatures of the frequency temperature characteristics appear differently. In this state, the resonance frequencies of the two resonance modes are matched to generate a slight weak coupling between resonances, thereby synthesizing each frequency temperature characteristic and realizing a flat frequency temperature characteristic over a wide temperature range. it can.
Further, since the frequency temperature characteristic can be improved by combining two resonances existing in one SAW resonator, the SAW resonator having excellent frequency temperature characteristic can be downsized.

  The quartz substrate is a flat plate obtained by rotating a plane perpendicular to the mechanical axis Y of a quartz single crystal in a counterclockwise direction around the electric axis X within a range of an angle θ = 32 ° to 37 °, and about the electric axis X The IDT is arranged along the newly defined X ′ axis by rotating the Y ′ axis newly defined by the rotation of the angle θ of the angle ++ or −ψ with the counterclockwise direction being positive. It is desirable that the angle be in the range of 40 ° to 44 °.

  According to this configuration, since the coupling resonator is configured using a quartz substrate having a small second-order temperature coefficient in the frequency temperature characteristics, a SAW resonator having further excellent frequency temperature characteristics over a wide temperature range can be obtained. Can be provided.

  The SAW resonator of the present invention is preferably a one-port SAW resonator in which one IDT and a pair of reflectors are arranged on both sides of the IDT.

  According to this configuration, it is possible to provide a one-port SAW resonator that is downsized and has good frequency temperature characteristics.

  The SAW resonator of the present invention is preferably a two-port SAW resonator in which two IDTs and a pair of reflectors are arranged on both sides of the two IDTs.

  According to this configuration, it is possible to provide a two-port SAW resonator that is downsized and has good frequency temperature characteristics.

DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, embodiments of the invention will be described with reference to the drawings.
(First embodiment)

1A and 1B are configuration diagrams showing the configuration of a 1-port SAW resonator according to the present embodiment. FIG. 1A is a schematic plan view, and FIG. 1B is along the A-A disconnection in FIG. It is a schematic cross section.
The SAW resonator 10 includes an IDT 20 and a reflector 30 formed on the main surface of the quartz substrate 5. The IDT 20 is made of a metal such as aluminum, copper, titanium, or tungsten, and is configured such that the interdigitated electrodes 21 and 22 having different polarities are alternately inserted. On both sides of the IDT 20, a grid-like reflector 30 made of a metal such as aluminum, copper, titanium, or tungsten is provided.

Next, the quartz substrate 5 will be described.
FIG. 2 is an explanatory view showing the azimuth angle of the quartz substrate. First, the quartz wafer 1 that is a base material of the quartz substrate 5 will be described. The crystal wafer 1 has a surface obtained by rotating a so-called Y plate 2 perpendicular to the mechanical axis Y (also referred to as Y axis) of the crystal, counterclockwise by an angle θ around the electrical axis X (also referred to as X axis) of the crystal. I have. The rotated surface defines new Y′-axis and Z′-axis which are rotated by θ of the mechanical axis Y and the optical axis Z (also referred to as Z-axis).
Further, the IDT 20 and the reflector 30 constituting the SAW resonator 10 are disposed along the X ′ axis obtained by rotating the electrical axis X of the crystal around the Y ′ axis by + ψ or −ψ with the counterclockwise direction being positive. The quartz substrate 5 constituting the SAW resonator 10 is cut from the quartz wafer 1 so as to have a main surface 6 defined by the Z ″ axis and the X ′ axis obtained by rotating the Z ′ axis by ± ψ.
Here, in the quartz substrate of the present embodiment, θ is set in the range of 32 ° to 37 ° and ψ = 40 ° to 44 °.
Note that the above azimuth is represented by Euler angle display (0 °, θ + 90 °, ± ψ). The sign of the angle is positive (+) in the counterclockwise direction. Regarding the angle ψ, the characteristics shown are the same when rotated in the + direction and when rotated in the − direction.
As described above, the crystal substrate having such an orientation is called an in-plane rotating ST cut substrate in a broad sense.

Here, in order to advance the understanding of the present invention, a SAW resonator having a constant electrode period length of an IDT using such an in-plane rotating ST cut substrate will be described.
FIG. 3 is a graph showing impedance characteristics of a SAW resonator using the in-plane rotating ST cut substrate.
In a SAW resonator having a constant electrode period length of an IDT using such an in-plane rotating ST cut substrate, a Rayleigh wave is excited and resonance E exists in the vicinity of the lower stop band. Usually, this resonance E is used to form a SAW resonator. In addition to the resonance E, a resonance H is confirmed near the upper stop band. This resonance H is considered to be caused by the oblique symmetry of the quartz substrate.
This oblique symmetry is interpreted to be caused by the fact that the reflected wave phase differences in the left and right traveling directions are not equal for each section of the IDT electrode conductors constituting the SAW resonator. In the case of quartz, since the piezoelectric constant is small, it is presumed that the attribute of the resonance characteristic is caused by the oblique symmetry of the mechanical structure in a state where the SAW resonator maintains the resonance state. For example, it is presumed that this hypothesis is correct considering that the vibration state can be maintained for a while even if the drive signal of the resonator is stopped.
Thus, due to the oblique symmetry of the quartz substrate, resonance near the upper stop band that does not exist in the ST cut substrate appears. In the present invention, a quartz substrate in which resonance appears in the vicinity of the upper stop band is defined as a quartz substrate having oblique symmetry.
The inventor conceived the present invention by paying attention to the resonance near the upper stop band. The details will be described below.

FIG. 4 is an explanatory diagram showing the frequency characteristics of each part constituting the SAW resonator of this embodiment.
FIG. 4A shows the reflection characteristic of the IDT, which has a lower stop band 41 and an upper stop band 42.
4B and 4C show the admittance characteristics of the IDT resonator. In the quartz substrate used in this embodiment, the so-called IDT resonator in which the IDT is formed on the substrate has two admittance characteristics. It has characteristics. This resonance corresponds to Yup0 (fundamental wave mode) and Ylow0 (fundamental wave mode) shown in FIGS. 4B and 4C, and exists near the upper and lower stopbands, respectively.
Further, both resonances are accompanied by higher-order longitudinal modes, and in this embodiment, by appropriately selecting the value of the logarithm M of the IDT constituting the SAW resonator, the fundamental mode of the lower resonance is selected. The resonance frequency of Ylow0 and the longitudinal higher-order mode Yup1 of the upper resonance are matched. Here, the coincidence of resonance frequencies means a range of about 0 to ± 20 ppm with respect to the target frequency.
In this state, as shown in the admittance characteristics of the SAW resonator in FIG. 4E, the modes of both resonances are excited by the drive voltage, and two resonances exist simultaneously to form a coupled state. The cause of coupling is due to the oblique symmetry of the substrate.
Further, as shown in the reflection characteristic of the reflector in FIG. 4D, the reflector is set so that the resonance frequency of the IDT to be selected is arranged at the center position.
4A and 4D, the vertical axis represents the reflection coefficient Γ, and in FIGS. 4B, 4C, and 4E, the vertical axis represents the admittance Y, and the horizontal axis represents the frequency f in common. ing.

In such a resonance state, two resonances exist simultaneously and are excited. Each resonance has a different frequency temperature characteristic, and specifically appears at a different apex temperature.
FIG. 5 is an explanatory diagram for explaining the coupling of the respective frequency temperature characteristics. As for the vertex temperature, the longitudinal higher-order mode Yup1 of the upper resonance appears at a higher temperature than the fundamental wave mode Ylow0 of the lower resonance, and there is a difference in the vertex temperature between the two. In this state, there is a coupling between the resonances of the two, and the synthesized frequency temperature characteristic is nearly flat, and the temperature characteristic is remarkably improved.

The coupling between the fundamental mode Ylow0 of the lower resonance and the longitudinal higher-order mode Yup1 of the upper resonance can be considered as follows.
When operating in the fundamental wave mode Ylow0, a diagonally symmetric component is generated by the fundamental wave mode. This obliquely symmetric component induces a higher-order component of the longitudinal higher-order mode Yup1, so that Ylow0 and Yup1 are in a coupled state. FIG. 6 is an electrical equivalent circuit diagram showing the coupling state of each mode.
This equivalent circuit is configured by connecting a fundamental wave mode Ylow0, an upper resonance longitudinal higher-order mode Yup1 and a capacitor 150 in parallel, and the capacitor 150 represents a coupling portion. The capacitor 150 is generated by a vibration phenomenon including a mechanical oblique component.

Next, the vibration mode used in the present invention will be described with reference to the drawings.
FIG. 7 is an explanatory diagram illustrating an example of a vibration mode used in the present invention. The lower graph display shows a state viewed from the direction of the arrow U, the horizontal axis X indicates the vibration region of the SAW resonator, and the vertical axis indicates the displacement amplitude.
LS0 in the figure indicates the envelope displacement state of the amplitude in the fundamental wave mode Ylow0 in the vicinity of the lower stop band. On the other hand, LS1 illustrates the envelope displacement state of the first-order mode amplitude, which is one of the longitudinal higher-order modes belonging to the resonance system near the upper stop band. It can also be seen from the state of the displacement amplitude that excitation can be performed with an electric drive voltage. Accordingly, the impedance of the entire SAW resonator can be realized even when both modes are coupled. The ratio between the pitch PT of the IDT 20 and the pitch PR of the reflector 30 was set to PT / PR = 0.993.

  As for the frequency adjustment in both modes, it is possible to perform a parallel shift of both frequencies in the region B and to perform a frequency difference between the two in the region A. For example, by etching the quartz substrate in the region B, both frequencies are shifted in parallel in the lower direction, and by etching the quartz substrate in the region A, the frequency of the longitudinal higher order mode belonging to the resonance system near the upper stop band is obtained. It is possible to make the frequency difference between the two close by adjusting it low. Since the resonance spectrum of each mode is intense for both LS0 and LS1, it can be observed with a network analyzer.

Next, a method for matching the resonance frequencies of the upper resonance series primary mode LS1 and the lower resonance series fundamental wave mode LS0 will be described.
FIG. 8 is a graph showing the relation between the number of crossed finger electrodes and the frequency in IDT. FIG. 8 shows that the resonance frequencies of the upper resonance series primary mode LS1 and the lower resonance series fundamental wave mode LS0 of FIG. 7 can be matched.
The upper resonance series fundamental wave mode LS0 and the lower resonance series fundamental wave mode LS0 change with substantially the same slope curve depending on the logarithm M of the IDT of the SAW resonator. On the other hand, the upper resonance series primary mode LS has a steep characteristic that is more dependent on M than the above-described LS0, and therefore, both characteristic curves can cross at a logarithm M0 of a specific IDT, and at an operating frequency F0. The upper resonance series primary mode LS1 and the lower resonance series fundamental mode LS0 can be operated. In the figure, LA0 indicates an obliquely symmetric fundamental wave mode.

Next, an example of the characteristics of a SAW resonator designed in consideration of the above will be described.
FIG. 9 is an explanatory diagram for explaining an example of the characteristics of a SAW resonator using the present invention. FIG. 9A is a graph showing admittance characteristics, and FIG. 9B is a SAW resonator without coupling. FIG. 9C is a graph showing the phase characteristics, and FIG. 9C is a graph showing the SAW resonator phase characteristics when coupling occurs.
In the manufacturing conditions where the ratio of the film thickness H of the IDT cross-finger electrode to the surface wave wavelength λ is H / λ = 2.5% and the corresponding reflection coefficient is about 1%, the upper resonance series first order The number of electrode pairs for matching the resonance frequencies of the mode LS1 and the lower resonance series fundamental wave mode LS0 was M0 = 150 pairs. The crossing width was 35 wavelengths and the number of reflectors N was 150.

In such a SAW resonator, an admittance characteristic as shown in FIG. 9A is shown, and when there is no coupling between the upper resonance series primary mode LS1 and the lower resonance series fundamental wave mode LS0, FIG. Such SAW resonator phase characteristics are shown. In this case, the frequency difference between the upper resonance series primary mode LS1 and the lower resonance series fundamental wave mode LS0 is about 5000 ppm in terms of frequency change rate.
When the coupling between the upper resonance series primary mode LS1 and the lower resonance series fundamental wave mode LS0 occurs, as shown in the phase characteristics of FIG. 9C, the upper resonance series primary mode LS1 and the lower resonance series It can be seen that the fundamental wave mode LS0 matches.

FIG. 10 is a graph showing an example of frequency temperature characteristics of the SAW resonator.
As shown in this graph, the upper resonance series primary mode LS1 and the lower resonance series fundamental wave mode LS0 are combined, and a flat temperature characteristic curve appears.

As described above, the SAW resonator according to the present embodiment is excited by the drive voltage in which the higher-order mode of the resonance near the upper stop band and the fundamental wave mode in the resonance near the lower stop band simultaneously exist. The resonance of each mode has slightly different frequency temperature characteristics, and specifically, the peak temperatures of the frequency temperature characteristics appear differently. In this state, the resonance frequencies of the two resonance modes are matched to generate a slight weak coupling between resonances, thereby synthesizing each frequency temperature characteristic and realizing a flat frequency temperature characteristic over a wide temperature range. it can.
In addition, since the frequency temperature characteristic is improved by combining two resonances existing in one SAW resonator, it is possible to reduce the size of the SAW resonator excellent in the frequency temperature characteristic.
Furthermore, since the coupled oscillator is configured using a quartz substrate having a small secondary temperature coefficient in frequency temperature characteristics, a SAW resonator excellent in frequency temperature characteristics over a wide temperature range can be provided. .
Further, it is possible to provide a one-port SAW resonator that is downsized and has good frequency temperature characteristics.
(Second Embodiment)

Next, a second embodiment will be described. Although the 1-port SAW resonator has been described in the first embodiment, the invention can also be implemented in a 2-port SAW resonator.
11A and 11B are configuration diagrams showing the configuration of the two-port SAW resonator according to the present embodiment. FIG. 11A is a schematic plan view, and FIG. 11B is along the BB disconnection in FIG. It is a schematic cross section.
The SAW resonator 50 includes IDTs 60 and 70 and a reflector 80 formed on the main surface of the quartz substrate 5. The IDTs 60 and 70 are made of a metal such as aluminum, copper, titanium, or tungsten, and are configured such that interdigitated electrodes 61, 62, 71, and 72 having different polarities are alternately inserted. A grid-like reflector 80 formed of a metal such as aluminum, copper, titanium, or tungsten is provided so as to sandwich the IDTs 60 and 70.
The quartz substrate 5 is a substrate similar to the substrate described in FIG. 2 of the above embodiment.

The SAW resonator 50 also has resonance near the upper and lower stop bands. The resonance frequency of the fundamental mode in the resonance near the upper stop band and the resonance of the fundamental wave mode in the resonance near the lower stop band can be matched by selecting the logarithm of the IDTs 60 and 70.
The resonance of each mode has slightly different frequency temperature characteristics, and specifically, the peak temperatures of the frequency temperature characteristics appear differently. In this state, the resonance frequencies of the two resonance modes are matched to generate a slight weak coupling between resonances, thereby synthesizing each frequency temperature characteristic and realizing a flat frequency temperature characteristic over a wide temperature range. it can.
Thus, even if it is a 2-port SAW resonator, the same effect as the SAW resonator of the first embodiment can be obtained.

  Since the SAW resonator of the present invention has excellent frequency temperature characteristics over a wide temperature range, it has excellent frequency stability in the optical communication network market where it is necessary to transmit and receive a huge amount of data in the future. It can be used as an oscillator application.

It is a block diagram which shows the structure of the SAW resonator in 1st this embodiment, (a) is a schematic plan view, (b) is a schematic cross section along the AA disconnection of the same figure (a). Explanatory drawing which shows the azimuth which the crystal substrate of 1st Embodiment has. The graph which shows the impedance characteristic of the SAW resonator using the in-plane rotation ST cut board | substrate in 1st Embodiment. Explanatory drawing which shows the frequency characteristic which each site | part which comprises the SAW resonator of 1st Embodiment has. Explanatory drawing explaining the coupling | bonding of the frequency temperature characteristic in 1st Embodiment. The equivalent circuit diagram explaining the coupling | bonding of each mode in 1st Embodiment. Explanatory drawing which illustrated typically about one example of the vibration mode utilized for this invention. The graph which shows the relationship between the logarithm of the cross finger electrode in 1st Embodiment, and a frequency. It is explanatory drawing explaining one example of the characteristic of the SAW resonator in 1st Embodiment, (a) is a graph which shows an admittance characteristic, (b) is a graph which shows the SAW resonator phase characteristic when there is no coupling, (C) is a graph showing SAW resonator phase characteristics when coupling occurs. The graph which shows one example of the frequency temperature characteristic of the SAW resonator in 1st Embodiment. It is a block diagram which shows the structure of the 2 port type | mold SAW resonator in 2nd Embodiment, (a) is a schematic plan view, (b) is a schematic cross section along the BB broken line of the same figure (a).

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Quartz wafer, 2 ... Y plate, 5 ... Quartz substrate, 10 ... SAW resonator, 20 ... IDT, 21, 22 ... Interstitial electrode, 30 ... Reflector, 50 ... SAW resonator, 60, 70 ... IDT, 80: Reflector.

Claims (4)

IDT is formed on the quartz substrate,
A SAW resonator having resonance in the vicinity of the upper and lower stop bands of the IDT and the quartz crystal substrate having oblique symmetry;
A coupled state is formed by matching a resonance frequency of a higher-order mode possessed by resonance near the upper stop band with a resonance frequency of a fundamental wave mode possessed by resonance near the lower stop band. SAW resonator. The SAW resonator according to claim 1,
The quartz substrate is a flat plate obtained by rotating a plane perpendicular to the mechanical axis Y of the quartz single crystal in a counterclockwise direction around the electric axis X within a range of an angle θ = 32 ° to 37 °;
The IDT is arranged along the newly defined X ′ axis by rotating the Y ′ axis newly defined by rotation of the angle θ around the electrical axis X by the angle + ψ or −ψ with the counterclockwise direction being positive. A SAW resonator having a range of ψ = 40 ° to 44 °. The SAW resonator according to claim 1 or 2,
A SAW resonator, which is a one-port SAW resonator in which one IDT and a pair of reflectors are arranged on both sides of the IDT. The SAW resonator according to claim 1 or 2,
The SAW resonator is a two-port SAW resonator in which two IDTs and a pair of reflectors are arranged on both sides of the two IDTs.

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