JP3255502B2 - Highly stable surface acoustic wave device - Google Patents

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 resonator or a filter, and more particularly to a highly stable surface acoustic wave device having a small frequency variation with respect to a temperature change.

[0002]

2. Description of the Related Art In recent years, a surface acoustic wave (SAW: Surface Acoustic) propagating near the surface of an elastic body has been developed.
Electromechanical functional elements using Wave) are used as resonators or filters mainly in the field of electronic and communication devices. For example, recently, applications as filters for mobile communication such as pagers and mobile phones have been advanced. ing. In the field of such communication, there is a strong demand for higher frequency devices and higher frequency stability with respect to temperature due to the demand for effective use of frequency.

As a resonator having a stable frequency-temperature characteristic, an AT cut using a bulk wave (Euler angle θ = 125)
{; 35 ° rotation Y-cut) Quartz resonator is common,
As shown in FIG. 1, it exhibits a frequency temperature characteristic of a cubic curve having an inflection point near normal temperature, and a frequency fluctuation rate (frequency temperature change rate) with respect to a temperature change over a relatively wide temperature range around the inflection point. Can be reduced, and -4
The frequency temperature change rate is about 2 for a temperature change of 0 to 80 ° C.
It is known to be 0 ppm. However, although resonators using bulk waves of quartz have excellent frequency stability against temperature changes, the resonance frequency is inversely proportional to the thickness of the substrate. And as a result, the mechanical strength is reduced, making the production difficult.
Hz is the limit of high frequency. On the other hand, since the surface acoustic wave element has its resonance frequency determined by the electrode period, it is easy to raise the frequency of the fundamental wave vibration to about 1 GHz, and further higher frequency is expected.

However, a piezoelectric substrate for a surface acoustic wave device generally has a drawback that the frequency temperature characteristic is significantly inferior to that of an AT-cut quartz crystal, and as shown in FIG. Or SA
ST cut (Eulerian angle θ = 132.75 °; 42.75 ° rotation Y cut X widely used in W filters and the like)
Propagation) Even in the case of a surface acoustic wave device using a quartz substrate, the frequency temperature change rate with respect to a temperature change of -40 to 80 ° C. is about 120 ppm, which is 6 times as large as the AT cut. There were defects.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in order to eliminate the above-mentioned drawbacks of the conventional surface acoustic wave device, and has an object to improve the frequency-temperature characteristics over a relatively wide temperature range near room temperature, and to improve the ST characteristics. It is an object of the present invention to provide a surface acoustic wave device which is far superior to a surface acoustic wave device using cut quartz, and preferably exhibits a frequency temperature characteristic equal to or higher than that of an AT cut using a bulk wave.

[0006]

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention provides a piezoelectric substrate having at least one relatively heavy metal material on its surface to utilize SH-type surface acoustic waves propagating near the surface of the substrate. In a surface acoustic wave device provided with a digital transducer (IDT) electrode,
A quartz substrate cut out so that a cut angle θ with respect to an XY plane is in a range of 27 ° to 37 ° with respect to a crystal X axis as a center of rotation, is used as the piezoelectric substrate, and a phase of the SH type surface acoustic wave is used. The in-plane rotation angle 成 between the velocity propagation direction and the crystal X axis is 75 ゜ ≦ | ゜ | <90
ＩＤ, and the IDT electrode is configured such that the in-plane rotation angle ψ and the cut angle θ are | ψ | = (1.1 θ + 48) ± 5 (deg.), Where | ψ | <90 °, or when the wavelength of the SH type surface acoustic wave is λ and the thickness of the IDT electrode is h 2, h / λ is 0. 01 through 0
. 018 or the IDT
At least one interdigital transducer (IDT) electrode made of gold on the surface of the piezoelectric substrate in order to utilize SH-type surface acoustic waves that propagate near the surface of the piezoelectric substrate, ideally using gold as the material of the electrode Wherein the quartz substrate cut out so that the cut angle θ with respect to the XY plane is 30 ° with respect to the XY plane with the crystal X axis as the center of rotation is used as the piezoelectric substrate. The in-plane rotation angle す between the phase velocity propagation direction of the SH type surface acoustic wave and the crystal X axis is 81. 6
The IDT electrode is configured to be ゜,
The IDT electrodes are arranged along the group velocity propagation direction of the SH type surface acoustic wave.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to the drawings showing embodiments. According to the present invention, as shown in FIG. 3, when one of two rotating Y-cut quartz substrates orthogonal to each other is assumed to be an AT-cut substrate 1, an IDT electrode is provided on the surface of the other substrate 2 to obtain a frequency-temperature characteristic. SH type elastic surface having the same propagation direction as the bulk wave of the AT-cut substrate 1 shown by the black arrow in the figure and having particle displacement in the direction shown by the white arrow in the figure perpendicular to this propagation direction. This is based on the knowledge that a wave (for example, a surface acoustic wave such as a so-called Love wave) exists.

First, the results of a theoretical analysis of the frequency-temperature characteristics will be described, and then the results of experiments performed based on the results will be described. In discussing the cut surface of the quartz substrate and the propagation characteristics of surface waves, Euler angles are generally used. Here, FIG.
The Euler angles (φ, θ, ψ) of the right-handed system shown in FIG. In this figure, when X, Y and Z are crystal axes of quartz, respectively, the first rotation angle φ is centered on the Z axis, and the second rotation angle θ is centered on the X ′ axis which is the X axis converted by φ. The third rotation angle ψ is the rotation angle from the X ′ axis in the plane determined by the two rotations, and is the propagation direction of the surface wave. Using this Euler angle, a new coordinate system (X
₁ , X ₂ , X ₃ ). For example, in the case of a −60 ° rotated Y-cut Z ′ propagation quartz substrate, it is described as (0 °, 30 °, 90 °), and θ is hereinafter referred to as a cut angle, and ψ as an in-plane rotation angle.

[0009] surface of the rotating Y-cut quartz substrate provided with a metal film having a thickness h, and analyzes the SH type surface acoustic wave propagating in the X ₁ direction. In this case the SH type surface acoustic wave is a transverse wave having a particle displacement in the X ₂ direction, and utilizes the so-called Love waves. FIG. 5 is a diagram showing an analysis model,
Since an IDT electrode having a relatively large mass is required to excite an ove wave, an analysis was performed using gold (Au) as a material of a metal film having a film thickness h.

In the analysis, independent displacements and potentials are assumed in the quartz substrate and the metal film, respectively, and a constitutive equation T _ij = C ^E _ijkl S _kl -e _klj E _k (1) D _i = ε in each region. ^S _ij E _j + e _ijk S _jk (2) where C ^E : elastic stiffness constant at constant electric field T: stress ε ^S : dielectric constant at constant strain e: piezoelectric constant E: electric field S: strain D: electric Bundle degree and Newton's equation of motion

Equation 1 was used. However, the dot on the character indicates the time integration, and the comma indicates the spatial integration in the direction of the subsequent numeral. Furthermore, Gauss's law D _{i, j} = 0 (i = 1, 2, 3) (4) is simultaneously established with these, and the boundary conditions at each boundary are (1) particle displacement is continuous (2) stress is continuous (3) The normal component of the electric flux density is continuous. (4) The analysis was performed assuming that the potential was continuous.

[0011] of the wave propagating in the X ₁ direction of the phase velocity V,
Assuming that one cycle length of the IDT is L, the center frequency f is f = V / L (5). Although the phase velocity V is determined by the material constant of the substrate and the metal film thickness, the material constant fluctuates depending on the temperature. As a result, V changes depending on the temperature. Here, as a material constant that changes with temperature and affects V, 1) an elastic constant of the substrate material 2) a density of the substrate material 3) an elastic constant of the electrode metal film 4) a film thickness of the electrode metal film 5) an electrode metal film Further, the following factors were considered as affecting the period L of the IDT: 6) thermal expansion of the substrate material; and 7) thermal expansion of the electrode metal film.

The results of analysis based on the above theoretical formula are shown below. First, FIG. 6 shows the result of analysis when the in-plane rotation angle ψ is 90 ° (Z propagation). In the figure, the horizontal axis is the cut angle θ, and the vertical axis is the film thickness h / λ normalized by the wavelength λ of the propagating SH type surface acoustic wave, and the numbers in the contour lines are −40 to 80 °. It is the amount of change of the frequency temperature change rate in C, and the unit is ppm. The hatched portion is a region where the frequency temperature change rate is 100 ppm or less. When the in-plane rotation angle ψ = 90 °, the frequency temperature change amount becomes ST by selecting the optimal cut although the frequency temperature characteristic of the secondary curvature is exhibited as in the case of the ST cut.
It is 以下 or less of the cut. However, this is about three times the frequency temperature change rate of the AT cut, and the analysis was performed by changing the condition of the in-plane rotation angle ψ stepwise in order to further improve the frequency temperature characteristics.

FIG. 7 summarizes the results.
The horizontal axis represents the cut angle θ, and the vertical axis represents the in-plane rotation angle ψ. In each combination, the normalized film thickness h / λ at which the variation of the frequency temperature change rate at −40 to 80 ° C. is minimum is defined as It is a plot. According to the figure, when the crystal substrate cut out so that the cut angle θ is in the range of 27 ° to 37 ° satisfies the following expression: {1.1θ + 48 (deg.) (6) Optimum frequency temperature characteristics can be obtained by appropriately selecting. FIG. 8 shows the normalized film thickness h / λ.
= 0.015 ° C. The amount of change of the frequency temperature change rate at −40 ° C. to 80 ° C. is shown as an isometric diagram, and the number on the isoline is the amount of change of the frequency temperature change rate. Is pp
m. The shaded portion is a region where the variation of the frequency temperature change rate is 100 ppm or less which is smaller than that of the surface acoustic wave device using the ST cut quartz substrate, and the cut angle θ is in the range of 27 ° to 42 °. , The in-plane rotation angle 存在 is generally in a range larger than 70 ° and smaller than 90 °. 9 and 10 show the normalized film thickness h / λ =
Cut angle θ and in-plane rotation angle ψ at 0.015
FIG. 11 shows the difference between the frequency temperature change rate curves at −40 ° C. to 80 ° C., and FIG. 11 shows the Euler angles (0 °, 29.
9 ゜, 81.55 ゜), the normalized film thickness h /
7 is a frequency temperature change rate curve when λ is changed.
Under the above-mentioned minimum condition, 1/1 / AT cut
This indicates the realization of a SAW device having an extremely high stability of about 6 ppm or less of 3 or less.

The cut angle θ with respect to the above optimum conditions
When only the value is changed, about ± 0.85 ゜, the in-plane rotation angleψ
When only the thickness is changed, about ± 1.4 °, and when only the film thickness is changed, the normalized film thickness h / λ is about ± 5% in a relatively wide range of −40 to 80 °. The amount of change in the frequency temperature change rate at C is 30 ppm or less. If such good frequency temperature characteristics exist over a wide range,
This is advantageous in realizing a device having a sufficient frequency-temperature characteristic by permitting even a misalignment in mask alignment when actually cutting the substrate or forming an electrode on the substrate. Therefore, in order to obtain a characteristic superior to the frequency temperature characteristic of the conventional surface acoustic wave element using the ST cut quartz substrate, は = (1.1θ + 50) ± 5 (deg.) Where ψ <90 ゜(7) What is necessary is just to comprise so that the following formula may be satisfied.

Although individual data are omitted because the specification becomes complicated, the analysis results at each plot point shown in FIG. 7 are substantially the same as those when the normalized film thickness h / λ = 0.015. The optimum condition of the film thickness decreases as the in-plane rotation angle ψ approaches 90 ° or the cut angle θ increases as shown in FIG. Further, since the amount of change in the frequency temperature change rate at -40 to 80 ° C. is 30 ppm or less, the region where the cut angle θ, the in-plane rotation angle ψ, and the normalized film thickness h / λ can take is thick. It becomes wider,
There was a tendency to become narrower as the film thickness became thinner.

Although the analysis results have been described focusing only on the frequency temperature characteristics, other characteristics such as the electromechanical coupling coefficient (K ² ) and the power flow angle of the SAW device also need to be considered. Here, as is well known, the electromechanical coupling coefficient is an amount indicating the magnitude of the piezoelectric effect, and is preferably large when used as a substrate of a surface acoustic wave element. FIG. 12 shows the normalized film thickness h / λ = 0.015.
7 is an isometric diagram showing an analysis result of the electromechanical coupling coefficient at the time of (1), wherein a solid line indicates an electromechanical coupling coefficient of an SH-type surface acoustic wave and a broken line indicates an electromechanical coupling coefficient of a general Rayleigh wave. The electromechanical coupling coefficient of SH-type surface acoustic waves is about twice as large as the general value of ST-cut quartz substrates, and although the Rayleigh wave has an unnecessary spurious response for the SH-type surface acoustic waves used in this study, The electromechanical coupling coefficient of the surface wave is sufficiently large as compared with that of the Rayleigh wave, and it is considered that the influence of the Rayleigh wave is small. However, it is desirable that the cut angle θ is small and the in-plane rotation angle 近 is close to 90 °.
If the angle is 5 ° or less, it is considered that it is difficult to obtain an electromechanical coupling coefficient sufficient to constitute a surface acoustic wave device. Although not shown, it has been found that as the film thickness increases, the electromechanical coupling coefficient of the SH-type surface acoustic wave increases and the response of the Rayleigh wave decreases.

On the other hand, the power flow angle is the angle between the propagation direction of the phase velocity and the propagation velocity of the group velocity of the surface acoustic wave excited by the IDT electrode disposed on the substrate as shown in FIG. It is considered that it is desirable to set the value to zero because a larger flow angle causes an increase in propagation loss. FIG. 14 is an isometric diagram showing the analysis result of the power flow angle. In the case of the SH type surface acoustic wave, the power flow angle becomes zero when the in-plane rotation angle ψ is 90 °, and as the deviation from 90 ° occurs. The power flow angle tends to increase. That is, taking into account the effects of the electromechanical coupling coefficient and the power flow angle, the cut angle θ is 27 ° to 37 °.
It can be said that it is practical to select the range of ゜ and the in-plane rotation angle で in the range of approximately greater than 75 ° and smaller than 90 ° as the surface acoustic wave element.

On the basis of the above analysis results, a sample was made as a prototype, and the rate of frequency temperature change in the temperature range of -40 to 80 ° C. was measured. Hereinafter, in order to avoid complication, a quartz substrate (−60) having an Euler angle (0 °, 30 °, ψ) is used.
A case where electrodes are formed on a (rotated Y-cut quartz substrate) will be described. Since the adhesion between gold and the quartz substrate was weak, thin (several hundred angstroms) titanium was deposited on the substrate, and then gold was deposited, followed by photoetching to form electrodes. The IDT electrodes were 60 pairs and 40 pairs for transmission and reception, respectively, and the intersection width was 20 wavelengths.
A grating for 20 wavelengths is provided between the transmitting and receiving IDTs, and a uniform film for 50 wavelengths is provided outside the IDTs. FIG. 15 is a schematic layout diagram of the electrode patterns used in the experiment, and was arranged along the group velocity propagation direction in consideration of the power flow angle.

FIG. 16 shows that the in-plane rotation angle ８ is 81.53 °, the electrode film thickness is 8720 Å, and the wavelength λ is 52 μm.
m, the ratio of the width of the electrode finger to the pitch of the IDT electrode w /
It is the result of having measured the frequency transmission characteristic by IDT with the 50 ohm network analyzer about the sample which p is 47%. From the figure, the response of the Rayleigh wave can be confirmed on the frequency side about 4.5% lower than the center frequency of the SH type surface acoustic wave, but the response of the SH type surface acoustic wave is smaller than the frequency temperature change rate of the Rayleigh wave. It turns out that it is very small.

FIG. 17 is a graph showing the change in the rate of temperature change of the frequency of the sample in which the in-plane rotation angle ８１ is 81.6 °. It shows that the frequency temperature change characteristic shows a linear change with a large slope and a downward slope, and the slope decreases as the film thickness gradually increases and exhibits a cubic curve having an inflection point near 20 ° C. A trend was seen. Although the graph is complicated, the number of samples has been thinned out.
-40 to 8 in the range of 00 to 8600 angstroms
The amount of change in the frequency temperature change rate at 0 ° C. is 70 ppm or less.
0 ppm, almost the same characteristics as in the case of a bulk wave in a target AT-cut quartz substrate were obtained. When the film thickness is further increased, the line again shows a downward-sloping straight line similarly to the theoretical value.

Variations in the rate of temperature change of each sample due to variations in manufacturing such as the in-plane rotation angle ψ, w / p, the film thickness of titanium used as the base of the gold thin film, and mask accuracy were also observed. However, when the correction was performed in consideration of these variations, a result almost equivalent to the theoretical value could be obtained. 19 and 20 show the experimental results of the samples in which the in-plane rotation angles ８３ were 83.0 ° and 85.0 °, respectively.
Shown in Also in this case, a result almost equal to the theoretical value could be obtained. Although the experimental results were omitted, it was confirmed that substantially the same results were obtained under conditions other than the cut angle θ of 30 °.

Although the present invention has been described with reference to the case where the in-plane rotation angle ψ is positive, the present invention is not limited to this, and ψ is shifted in the negative direction due to crystal symmetry. It goes without saying that the same characteristics can be obtained even when rotated. That is, the frequency temperature change rate change amount is 100 ppm or less, and a practical region as a surface acoustic wave element in consideration of the influence of the electromechanical coupling coefficient and the power flow angle is a cut angle θ of 27 ° to 42 °.
In the range of ゜, the in-plane rotation angle ψ is approximately 75 ° ≦ | ψ | <90.
Equation (7) can be rewritten as | | = (1.1θ + 48) ± 5 (deg.) Where | ψ | <90 ゜. Although the present invention has been described by way of an example of a surface acoustic wave resonator in which two IDT electrodes are formed on a quartz substrate, the present invention is not limited to this. Needless to say, the present invention can be applied to any surface acoustic wave element such as a type in which one IDT electrode and a reflector are arranged on both sides thereof, or a type in which a number of IDT electrodes are arranged and cascade-connected.

[0023]

Since the present invention is constructed as described above, a SAW device using an SH type surface acoustic wave is cut out so that the cut angle θ is in the range of 27 ° to 37 °. The substrate is used as the piezoelectric substrate, and the in-plane rotation angle ψ is appropriately selected so as to be approximately 75 ° ≦ | ψ | <90 °, and the electrode is formed only by forming an electrode. This has a remarkable effect in realizing frequency temperature characteristics equivalent to those of bulk waves.

[0024]

[Brief description of the drawings]

FIG. 1 is a diagram showing frequency temperature characteristics of an AT-cut quartz resonator.

FIG. 2 is a diagram showing frequency temperature characteristics of a surface acoustic wave resonator using an ST cut quartz substrate.

FIG. 3 is a diagram illustrating an SH-type surface acoustic wave.

FIG. 4 is a diagram showing a definition of Euler angles.

FIG. 5 is a view showing an analysis model used for analysis.

FIG. 6 is a diagram showing analysis results at Euler angles (0, θ, 90 °).

FIG. 7 is a diagram showing an analysis result when the in-plane rotation angle ψ is changed stepwise.

FIG. 8 is a diagram showing an amount of change of a frequency temperature change rate when a normalized film thickness h / λ = 0.015 as an iso-diagram.

FIG. 9 is a diagram showing a frequency temperature change rate curve when the cut angle θ is changed stepwise.

FIG. 10 is a diagram showing a frequency temperature change rate curve when the in-plane rotation angle ψ is changed stepwise.

FIG. 11 is a diagram showing a frequency temperature change rate curve when the normalized film thickness h / λ is changed stepwise.

FIG. 12 is a diagram showing, as a contour map, an analysis result of an electromechanical coupling coefficient when a normalized film thickness h / λ = 0.015.

FIG. 13 is a diagram illustrating a power flow angle.

FIG. 14 is a diagram showing an analysis result of a power flow angle as a contour map.

FIG. 15 is a schematic layout diagram of an electrode pattern.

FIG. 16 is a diagram showing a result of measuring frequency transmission characteristics.

FIG. 17 is a diagram showing a frequency temperature change rate curve of a sample whose in-plane rotation angle ψ is 81.6 °.

FIG. 18 is a diagram showing a frequency temperature change rate curve when the in-plane rotation angle に よ り obtained by analysis is 81.6 °.

FIG. 19 is a diagram illustrating a frequency temperature change rate curve of a sample having an in-plane rotation angle ψ of 83.0 °.

FIG. 20 is a diagram showing a frequency temperature change rate curve of a sample whose in-plane rotation angle ψ is 85.0 °.

Claims (6)

(57) [Claims] 1. An elastic substrate having an interdigital transducer (IDT) electrode made of at least one relatively heavy metal material on the surface of a piezoelectric substrate in order to utilize an SH-type surface acoustic wave propagating near the surface of the piezoelectric substrate. In the surface acoustic wave device, a quartz substrate cut out so that a cut angle θ with respect to an XY plane is in a range of 27 ° to 37 ° with respect to a crystal X axis as a rotation center is used as the piezoelectric substrate, The in-plane rotation angle の between the phase velocity propagation direction of the SH-type surface acoustic wave and the crystal X-axis is 75 ゜ ≦ | ψ | <9
A highly stable surface acoustic wave device, wherein the IDT electrode is configured to be at 0 ° . 2. The in-plane rotation angle ψ and the cut angle θ.
Is | ψ | = (1.1θ + 48) ± 5 (deg.) Where
2. The highly stable surface acoustic wave device according to claim 1, wherein | ψ | <90 ゜ is satisfied. 3. The wavelength of the SH surface acoustic wave is λ,
When the thickness of the DT electrode is h, h / λ is 0.01 to
The highly stable surface acoustic wave device according to claim 1 or 2, wherein the surface acoustic wave device is configured to be 0.018 . 4. The highly stable surface acoustic wave device according to claim 1, wherein gold is used as a material of said IDT electrode. 5. An at least one interdigital transducer (ID) made of gold on the surface of the piezoelectric substrate so as to utilize an SH-type surface acoustic wave propagating near the surface of the piezoelectric substrate.
T) In a surface acoustic wave device provided with electrodes, the cut angle θ with respect to the XY plane with the crystal X axis as the center of rotation.
Is used as the piezoelectric substrate, and the in- plane rotation angle ψ between the phase velocity propagation direction of the SH surface acoustic wave and the crystal X axis is 8 °.
A highly stable surface acoustic wave device, wherein the IDT electrode is configured to be 1.6 ° . 6. The highly stable surface acoustic wave device according to claim 1, wherein said IDT electrode is arranged along a group velocity propagation direction of said SH type surface acoustic wave.