Patent Application
20070194657
The present invention relates to a surface acoustic wave device using a quartz substrate in which a device size is reduced, a Q value is increased, and a frequency-temperature characteristic is improved.
In recent years, a surface acoustic wave (hereinafter, SAW) device is widely utilized as apart for a terminal for mobile communication, a vehicle-mounted equipment, or the like, and it is strongly demanded to be downsized, have a high Q value, and be excellent in frequency stability.
As a SAW device realizing these demands, there is a SAW device using an ST cut quartz substrate. The ST cut quartz substrate is a cut name of a quartz plate having a plane (XZ′ plane) obtained by rotating an XZ plane from a crystal Z-axis in a counterclockwise direction by an angle of 42.75° utilizing a crystal X-axis as a rotation axis, and it utilizes a SAW (hereinafter, called ST cut quartz SAW) that is a (P+SV) wave propagated in a crystal X direction, called “Rayleigh wave”. An application of the ST cut quartz SAW device widely lies in a SAW resonator used as an oscillator, a filter for IF disposed between an RF stage and an IC in a mobile communication terminal, or the like.
The reason why the ST cut quartz SAW device allows realization of a downsized device with a high Q value includes a point such that reflection of SAW can be utilized efficiently. An ST cut quartz SAW resonator shown in
As an important factor in use of the SAW device, there is a frequency-temperature characteristic. In the ST cut quartz SAW, it is generally known that the primary temperature coefficient of the frequency-temperature characteristic of the SAW is zero, a characteristic thereof is expressed by a quadratic curve, and the SAW is excellent in frequency stability because a frequency fluctuation amount is significantly reduced by making an adjustment so that a turnover temperature is positioned at the center in a usage temperature range.
However, in the ST cut quartz SAW device, while the primary temperature coefficient is zero, the secondary temperature coefficient is relatively large such as −0.034 (ppm/° C.2). Accordingly, when the usage temperature range is expanded, the frequency fluctuation amount becomes extremely large.
As an approach for solving the problem, there is a SAW device disclosed in Meirion Lewis, “Surface Skimming Bulk Wave, SSBW”, IEEE Ultrasonics Symp. Proc., pp. 744 to 752 (1977), and Japanese Examined Patent Publication No. 62-016050. As shown in
However, since the SH wave is a wave that fundamentally submerges in a substrate to advance, a reflecting efficiency of a SAW obtained by a grating reflector is poorer than that in the ST cut quartz SAW propagated along the piezoelectric substrate surface. Accordingly, there is a problem that it is difficult to realize a downsized SAW device with high Q. Since the prior Publication includes disclosure about application as a delay line which does not utilize reflection of a SAW but it does not propose any means for utilizing reflection of a SAW, it is said to be difficult to put the SAW device in a practical use.
In order to solve the problem, Japanese Examined Patent Publication No. 01-034411 discloses a so-called multi-pair IDT type SAW resonator in which, as shown in
However, the multi-pair IDT type SAW resonator cannot obtain efficient energy confining effect as compared with a SAW resonator including a grating reflector. Since the number of IDT pairs required to obtain a high Q value is increased considerably to 800±200 pairs, there is a problem that a device size becomes larger than an ST cut quartz SAW resonator, so that a recent demand for size reduction cannot be satisfied.
In the SAW resonator disclosed in Japanese Examined Patent Publication No. 01-034411, when a wavelength of a SAW excited by the IDT is represented as λ, a Q value can be increased by setting an electrode film thickness to 2%λ or more, preferably 4%λ or less. However, when a resonant frequency is 200 MHz, the Q value reaches saturation near 4%λ, but the Q value obtained at that time becomes only about 20000 and only the Q value approximately equal to that in the ST cut quartz SAW resonator can be obtained in a comparison with the ST cut quartz SAW resonator. As its reason, it is thought that since, when the film thickness is in a range of 2%λ or more to 4%λ or less, a SAW are not concentrated sufficiently on a piezoelectric substrate surface, reflection cannot be utilized efficiently.
A problem to be solved is that, when an ST cut quartz substrate is used as a piezoelectric substrate, the secondary temperature coefficient of a frequency-temperature characteristic is large as −0.034(ppm/° C.2) so that a frequency fluctuation amount becomes considerably large practically. The structure of the SAW device disclosed in Japanese Examined Patent Publication No. 01-034411 includes a point that, since the number of pairs of IDTs has to be made considerably large, a device is increased in size.
In order to solve the problem, the invention according to claim 1 of the present invention provides a SAW device comprising a piezoelectric substrate and an IDT that is formed on the piezoelectric substrate and is made from Al or alloy including Al as a main component, an excited wave being an SH wave, characterized in that the piezoelectric substrate is a quartz flat plate where a cut angle θ of a rotation Y cut quartz substrate is set in a range of −64.0°<θ<−49.3° in a counterclockwise direction from a crystal Z-axis, and a propagation direction of a SAW is set to 90°±5° to a crystal X-axis, and when a wavelength of the SAW excited is represented as λ, an electrode film thickness H/λ standardized by a wavelength of the IDT is set to be 0.04<H/λ<0.12.
The invention according to claim 2 provides the SAW device, characterized in that a relationship between the cut angle θ and the electrode film thickness H/λ satisfies
−1.34082×10−4×θ3−2.34969×10−2×θ2−1.37506×θ−26.7895<H/λ<−1.025 86×10−4×θ3−1.73238×10−2×θ2−0.977607×θ−18.3420.
The invention according to claim 3 provides the SAW device, characterized in that, when an electrode finger width of electrode fingers constituting the IDT/(electrode finger width+space between electrode fingers) is defined as a line metalization ratio mr, a relationship between the cut angle θ and a product H/λ×mr of the electrode film thickness and the line metalization ratio satisfies −8.04489×10−5×θ3−1.40981×10−2×θ2−0.825038×θ−16.0737<H/λ×mr<−6.15517×10−5×θ3−1.03943×10−2×θ2−0.586564×θ−11.0052.
The invention according to claim 4 provides a SAW device comprising a piezoelectric substrate and an IDT that is formed on the piezoelectric substrate and is made from Al or alloy including Al as a main component, an excited wave being an SH wave, characterized in that the piezoelectric substrate is a quartz flat plate where a cut angle θ of a rotation Y cut quartz substrate is set to satisfy a range of −61.4°<θ<−51.1° in a counterclockwise direction from a crystal Z-axis, and a propagation direction of a SAW is set to 90°±5° to a crystal X-axis, and when a wavelength of the SAW excited is represented as λ, an electrode film thickness H/λ standardized by a wavelength of the IDT is set to satisfy 0.05<H/λ<0.10.
The invention according to claim 5 provides the SAW device, characterized in that a relationship between the cut angle θ and the electrode film thickness H/λ satisfies −1.44605×10−4×θ3−2.50690×10−2×θ2−1.45086×θ−27.9464<H/λ<−9.875 91×10−5×θ3−1.70304×10−2×θ2−0.981173×θ−8.7946.
The invention according to claim 6 provides the SAW device, characterized in that, when an electrode finger width of electrode fingers constituting the IDT/(electrode finger width+space between electrode fingers) is defined as a line metalization ratio mr, a relationship between the cut angle θ and a product H/λ×mr of the electrode film thickness and the line metalization ratio satisfies −8.67632×10−5×θ3−1.50414×10−2×θ2−0.870514×θ−16.7678<H/λ×mr<−5.92554×10−5×θ3−1.02183×10−2×θ2−0.588704×θ−11.2768.
The inventions according to claims 7 to 14 provides the SAW device, characterized in that the SAW device is any one of a one-port SAW resonator, a two-port SAW resonator, a lateral coupling type multi-mode filter, a vertical coupling type multi-mode filter, a ladder type SAW filter, a transversal SAW filter where an IDT propagating a SAW in both directions or in one direction is disposed, or a SAW sensor.
The invention according to claim 15 provides the SAW device, characterized in that the SAW device has grating reflectors on both sides of the IDT.
The invention according to claim 16 provides a module device or an oscillation circuit using the SAW device according to any one of claims 1 to 15.
The SAW device according to claim 1 or 4 of the present invention has a configuration that a rotation Y cut quartz substrate whose cut angle θ is in a range of −64.0°<θ<−49.3°, preferably −61.4°<θ<−51.1° is used, an SH wave excited such that a propagation direction of a SAW is (90°±5°) to a crystal X-axis is used, an electrode material for the IDT and for the grating reflector is Al or alloy mainly containing Al, and the electrode film thickness H/λ standardized by a wavelength is set to satisfy 0.04<H/λ<0.12, preferably 0.05<H/λ<0.10. Since the present invention is configured to concentrate waves which originally submerge in a substrate to advance on a substrate surface to allow the grating reflector or the like to utilize reflection of a SAW efficiently, a SAW device, which is reduced in size, has a high Q value, and is excellent in frequency stability as compared with an ST cut quartz SAW device, can be provided.
By satisfying the condition of the electrode film thickness H/λ and the cut angle θ described in claim 2 or 5, a turnover temperature Tp (° C.) can be set in a practical temperature range.
By satisfying the condition of the product H/λ×mr of the electrode film thickness and the line metalization ratio and the cut angle θ described in claim 3 or 6, a turnover temperature Tp (° C.) can be set in a practical temperature range.
By using the SAW device of the various systems described in any one of claims 7 to 14, a SAW device, which is reduced in size, has a high Q value, and is excellent in frequency stability can be provided.
Since the SAW device described in claim 15 can confine energy of a SAW in an IDT sufficiently by disposing grating reflectors on both sides of the IDT, a downsized SAW device having a high Q value can be provided.
Since the module device or the oscillation circuit described in claim 16 uses the SAW device according to the present invention, a downsized and high performance module device or an oscillation circuit can be provided.
The present invention will be explained below in detail based on embodiments shown in the drawings.
In the present invention, by setting the electrode film thickness H/λ to be larger than a conventional one in view of the defect in the conventional technique, a SAW are concentrated on a piezoelectric substrate surface, efficient utilization of reflection of a SAW by the grating reflectors is allowed, and SAW energy is confined in the IDT even in a reduced number of IDT pairs or grating reflectors so that a device size is reduced.
Generally, in an optimal design for a SAW resonator, it is important that a frequency-temperature characteristic is excellent, Q is high, and a capacitance ratio γ is small, namely, figure of merit (Q/γ) is large. Various characteristics of the SAW resonator according to the present invention are examined here.
With reference to
The SAW resonator according to the present invention can reduce the size of the piezoelectric substrate while maintaining a Q value better than that in the ST cut quartz SAW resonator. This is because an amount of increase of a reflection amount of SAW at an IDT or a grating reflector to increase of the electrode film thickness H/λ of the SAW resonator according to the present invention is considerably larger than that in the ST cut quartz SAW resonator. That is, the SAW resonator according to the present invention can realize a high Q value in the number of pairs of IDTs or grating reflectors less than that in the ST cut quartz SAW resonator by making the electrode film thickness H/λ large.
In a comparison of the Q values of the multi-pair IDT type SAW resonator shown in Japanese Examined Patent Publication No. 01-034411 and the SAW resonator according to the present invention, the Q value obtained in the patent application is a value when the resonant frequency is 207.561 (MHz), and the Q value is about 15000 when the resonant frequency is converted to 315 (MHz) that is the resonant frequency applied in the embodiment, where the Q value is approximately equivalent to that in the ST cut quartz SAW resonator. In a comparison of sizes of the resonators, the multi-pair IDT type SAW resonator described in the patent application requires 800±200 IDT pairs, but the present invention only requires a space accommodating 200 IDT pairs for both the IDT and the grating reflectors so that the present invention can be significantly reduced in size. Accordingly, by setting the electrode film thickness in a range of 0.04<H/λ<0.12 and providing grating reflectors to reflect a SAW efficiently, a SAW device, which is more downsized and higher in a Q value than the multi-pair IDT type SAW resonator disclosed in Japanese Examined Patent Publication No. 01-034411, can be realized.
Next,
By setting the electrode film thickness H/λ in a range of 0.04<H/λ<0.12 based on the above, a SAW device, which is more downsized and higher in a Q value than the ST cut quartz SAW device and the SAW device disclosed in Japanese Examined Patent Publication No. 01-034411 and is excellent in frequency stability, can be provided.
While only the case that the cut angle θ is set to −51° has been described in the above, even if the cut angle θ is changed in the SAW resonator according to the present invention, a film thickness dependency is not changed so much, so that an excellent Q value and a excellent secondary temperature coefficient can be obtained even at a cut angle shifted from −51° by several degrees by setting the electrode film thickness in a range of 0.04<H/λ<0.12.
The SAW resonator according to the present invention takes a cubic temperature characteristic in a very wide temperature range, but the temperature characteristic can be regarded as a quadratic characteristic in a specific narrow temperature range and a turnover temperature Tp in the temperature range varies depending on the electrode film thickness and the cut angle. Therefore, no matter how excellent the frequency-temperature characteristic is, the frequency stability remarkably degrades when the turnover temperature Tp fluctuates to outside of the usage temperature range. In order to realize excellent frequency stability in a practical usage temperature range (−50° C. to +125° C.), it is necessary to examine not only the secondary temperature coefficient but also the turnover temperature Tp in detail.
Tp(H/λ)=−41825×(H/λ)2+2855.4×(H/λ)−26.42 (1)
The approximate expression (1) can be applied in the cut angle near −50°, except for its intercept.
Tp(θ)=−43.5372×θ−2197.14 (2)
Next, it is understood from the expression (1) and the expression (2) that, when the electrode film thickness H/λ is set to satisfy 0.04<H/λ<0.12, it is sufficient to set the cut angle θ in a range of −59.9°≦θ≦−48.9° for setting the turnover temperature Tp in the practical usage temperature range (−50° C. to +125° C.)
When both the electrode film thickness H/λ and the cut angle θ are taken in consideration, the turnover temperature Tp is represented by the following approximate expression utilizing the expression (1) and the expression (2).
Tp(H/λ,θ)=Tp(H/λ)+Tp(θ)=−41825×(H/λ)2+2855.4×(H/λ)−43.5372×θ−2223.56 (3)
In order to set the turnover temperature Tp in the usage temperature range (−50° C. to +125° C.), the electrode film thickness H/λ and the cut angle θ can be set in a range represented by the following expression from the approximate expression (3).
0.9613≦−18.498×(H/λ)2+1.2629×(H/λ)−0.019255×θ≦1.0387 (4)
Thus, in the present invention, the rotation Y cut quartz substrate where the cut angle θ is in a range of −59.9°≦θ≦−48.9° is used, the SH wave excited such that a propagation direction of a SAW is a direction approximately perpendicular to the X-axis is used, the electrode material for the IDT and the grating reflector is Al or alloy including Al as a main component, and the electrode film thickness H/λ is set to satisfy 0.04<H/λ<0.12. With this configuration, a SAW device, which is more downsized and higher in a Q value than the ST cut quartz SAW device and is excellent in frequency stability, can be realized.
An optimal condition will be examined here. It is preferable that the electrode film thickness H/λ is set in a range of 0.05<H/λ<0.10 where the Q value of 20000 or more can be obtained as shown in
0.9845≦−18.518×(H/λ)2+1.2643×(H/λ)−0.019277×θ≦1.0155 (5)
In the above explanation, a relational expression between the electrode film thickness H/λ and the cut angle θ where the turnover temperature Tp falls in the practical usage temperature range has been derived from the relationship between the electrode film thickness H/λ and the turnover temperature Tp obtained when the cut angle θ shown in
Tp=−50(° C.):H/λ≈1.02586×10−4×θ3−1.73238×10−2×θ2−0.977607×θ−18.3420
Tp=0(° C.):H/λ≈−9.87591×10−5×θ3−1.70304×10−2×θ2−0.981173×θ−18.7946
Tp=+70(° C.):H/λ≈1.44605×10−4×θ3−2.50690×10−2×θ2−1.45086×θ−27.9464
Tp=+125(° C.):H/λ≈−1.34082×10−4×θ3−2.34969×10−2×θ2−1.37506×θ−26.7895
It is understood from
Further, in an examination about the more optimal condition, it is preferable that the turnover temperature Tp (° C.) is set to satisfy 0≦Tp≦+70 that corresponds to a further practical usage temperature range. The cut angle θ and the electrode film thickness H/λ can be set to satisfy a region surrounded by curves of Tp=0° C. and Tp=+70° C. shown in
As the result of the detailed examination explained above, it has been found that an excellent temperature characteristic where the Q value is larger than that in the ST cut quartz SAW device can be obtained, and the turnover temperature Tp can be set in a practical usage temperature range by using a rotation Y cut quartz substrate where the cut angle θ satisfies −64.0°<θ<−49.3°, preferably −61.4°<θ<−51.1°, using an SH wave excited such that a propagation direction of a SAW is a direction approximately perpendicular to the X-axis, forming an electrode material for the IDT or the grating reflector from Al or alloy mainly including Al, and setting the electrode film thickness H/λ to satisfy 0.04<H/λ<0.12, preferably 0.05<H/λ<0.10.
While the example where the Line metalization ratio mr of the IDT is fixed at 0.60 has been explained above, an example where the Tp characteristic is obtained when the line metalization ratio is included in a variable is examined below.
Next,
Tp=−50(° C.):H/λ×mr≈−6.15517×10−5×θ3−1.03943×10−2×θ2−0.586564×θ−11.0052
Tp=0(° C.):H/λ×mr≈−5.92554×10−5×θ3−1.02183×10−2×θ2−0.588704×θ−11.2768
Tp=+70(° C.):H/λ×mr≈−8.67632×10−5×θ3−1.50414×10−2×θ2−0.870514 ×θ−16.7678
Tp=+125 (° C.):H/λ×mr≈−8.04489×10−5×θ3−1.40981×10−2×θ2−0.825038 ×θ−16.0737
It is understood from
The cut angle θ and the product H/λ×mr of the electrode film thickness and the line metalization ratio can be set to satisfy a region surrounded by curves of Tp=0° C. and Tp=+70° C. shown in
While only a one-port SAW resonator such as the one shown in
The transversal SAW filter has a problem that insertion loss of a filter becomes large because a SAW is propagated equally to right and left along a propagation direction. As a method for solving the problem, there is a transversal SAW filter where so-called single phase uni-directional transducers (SPUDT) 82 and 83 where excitation of a SAW is made uni-directional by changing an electrode finger arrangement or an electrode finger width to weight excitation and reflection of a SAW are disposed, as shown in
In the various SAW devices described above, it is apparent that an advantage similar to that obtained in the present invention can be obtained by using a quartz flat plate as a piezoelectric substrate where the cut angle θ of the rotation Y cut quartz substrate is set to satisfy a range of −64.0°<θ<−49.3°, preferably −61.4°<θ<−51.1°, in a counterclockwise direction from the crystal Z-axis, and a propagation direction of an surface acoustic wave is set to 90±5° to the crystal X-axis and setting the electrode film thickness H/λ to satisfy a range of 0.04<H/λ<0.12, preferably 0.05<H/λ<0.10.
In the above described SAW device, it is apparent that an advantage similar to that obtained in the present invention can be obtained even when such a protective film as a protective film made from SiO2 or the like, a protective film obtained by anode-oxidizing Al, or the like is formed on an IDT or a grating reflector, or a closely adhesive layer or another metal thin film for improvement in power withstanding is formed on an upper or lower portion of an Al electrode. It goes without saying that the SAW device according to the present invention is applicable to a sensor device or a module device, an oscillation circuit, or the like. Since the capacity ratio γ can be reduced by using the SAW device according to the present invention in a voltage control SAW oscillator (VCSO) or the like, a variable width of a frequency can be taken larger.
The SAW device according to the present invention can have a structure other than the structure where the SAW chip and the package are wire-bonded to each other, such as the one shown in
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| Number | Date | Country | Kind |
|---|---|---|---|
| 2004-108608 | Apr 2004 | JP | national |
| 2004-310452 | Oct 2004 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP05/03998 | 3/8/2005 | WO | 9/29/2006 |
