Patent Application
20140055207
1. Technical Field
The present invention relates to a surface acoustic wave resonator, a surface acoustic wave oscillator in which the resonator is mounted, and an electronic apparatus, and more particularly to a type of surface acoustic wave resonator where grooves are provided in a substrate surface, and a surface acoustic wave oscillator in which the resonator is mounted.
2. Related Art
In a surface acoustic wave (SAW) device (for example, a SAW resonator), the effect of a SAW stop band, piezoelectric substrate (for example, quartz crystal substrate) cut angle, IDT (interdigital transducer) formation shape, and the like, on changes in frequency-temperature characteristics is considerable.
For example, a configuration exciting each of a SAW stop band upper end mode and lower end mode, the distribution of standing waves in each of the stop band upper end mode and lower end mode, and the like, are disclosed in JP-A-11-214958.
In addition, points for which the SAW stop band upper end mode has better frequency-temperature characteristics than the stop band lower end mode are described in JP-A-2006-148622, JP-A-2007-208871, JP-A-2007-267033 and JP-A-2002-100959. Then, it is described in JP-A-2006-148622 and JP-A-2007-208871 that, in order to obtain favorable frequency-temperature characteristics in a SAW device utilizing a Rayleigh wave, as well as adjusting the cut angle of the quartz crystal substrate, the electrode standardizing film thickness (H/λ) is increased to around 0.1.
Further, it is described in JP-A-2007-267033 that, as well as adjusting the cut angle of the quartz crystal substrate in a SAW device utilizing a Rayleigh wave, the electrode standardizing film thickness (H/λ) is increased by around 0.045 or more.
In addition, it is described in JP-A-2002-100959 that, by using a rotated Y-cut, X-propagating quartz crystal substrate, and utilizing the stop band upper end resonance, the frequency-temperature characteristics improve more than in the case of using the stop band lower end resonance.
It is described in JP-A-57-5418 and “Manufacturing Conditions and Characteristics of Grooved SAW Resonators” (Institute of Electronics and Communication Engineers of Japan Technical Research Report MW82-59 (1982)), that grooves are provided between the electrode fingers configuring the IDT, and between the conductor strips configuring the reflectors, in a SAW device using an ST cut quartz crystal substrate. Also, it is described in “Manufacturing Conditions and Characteristics of Grooved SAW Resonators” (Institute of Electronics and Communication Engineers of Japan Technical Research Report MW82-59 (1982)), that a peak temperature in the frequency-temperature characteristics having a quadratic curve shape changes depending on the depth of the grooves, and a second-order temperature coefficient is approximately −3.4×10−8/° C.2.
In Japanese Patent No. 3,851,336, as well as describing a configuration for making a curve indicating the frequency-temperature characteristics a cubic curve in a SAW device using an LST cut quartz crystal substrate, it is described that, in a SAW device using a Rayleigh wave, it has not been possible to find a cut angle substrate having the kind of temperature characteristics indicated by a cubic curve.
As described above, there is a wide range of elements for improving the frequency-temperature characteristics, and it is thought that, particularly with a SAW device using a Rayleigh wave, increasing the film thickness of the electrodes configuring the IDT is one factor contributing to the frequency-temperature characteristics. However, the applicant has found experimentally that on increasing the film thickness of the electrodes, environmental resistance characteristics, such as temporal change characteristics and temperature and shock resistance characteristics, are deteriorated. Further, when having the improvement of frequency-temperature characteristics as a principal object, it is necessary to increase the electrode film thickness, as described above, and an accompanying deterioration of temporal change characteristics, temperature and shock resistance characteristics, and the like, is unavoidable. Since this also applies to the Q value, it is difficult to realize a higher Q without increasing the electrode film thickness.
In order to solve the problem, Pamphlet of International Publication WO2010/098139 discloses a configuration in which grooves are formed on a quartz crystal substrate in a direction perpendicular to the propagation direction of the surface acoustic wave, and electrodes are formed on convex portions formed by the grooves. With this, environmental resistance characteristics such as temporal change characteristics and temperature and shock resistance characteristics are improved, thereby realizing a high Q value. In addition, JP-A-61-220513 or JP-A-61-220514 discloses a configuration in which, in order to realize a high Q value, grooves are formed between stripe-shaped metal films constituting reflectors which are disposed between IDT electrodes or at both sides of the IDT electrode.
Further, in Pamphlet of International Publication WO2010/098139, a groove depth, a film thickness of an electrode formed on the groove, and a line occupation rate of the groove are systematically investigated. In addition, in a case where the surface acoustic wave resonator is excited in a stop band upper end mode, the condition is ascertained that an absolute value of the second-order temperature coefficient of the surface acoustic wave is 0.01 ppm/° C.2 or less by adjusting the line occupation rate with respect to a given groove depth and electrode film thickness. Thereby, since the frequency-temperature characteristics of the surface acoustic wave show a cubic curve, it is expected that a frequency deviation can be suppressed in a temperature range around the inflection point.
In addition, JP-A-2009-225420 discloses a configuration in which a frequency deviation in an operating temperature range of the surface acoustic wave resonator is reduced when the line width of the electrode finger constituting the IDT electrode, that is, a line occupation rate fluctuates.
However, there is a strong demand for reduction in loss in the surface acoustic wave resonator in the surface acoustic wave resonator disclosed in Pamphlet of International Publication WO2010/098139, JP-A-61-220513, JP-A-61-220514, and JP-A-2009-225420 as well, but there is no detailed disclosure thereof at present.
An advantage of some aspects of the invention is to provide a surface acoustic wave resonator which reduces a frequency deviation of a surface acoustic wave and decreases loss in the surface acoustic wave resonator, and a surface acoustic wave oscillator and an electronic apparatus using the surface acoustic wave resonator.
This application example of the invention is directed to a surface acoustic wave resonator including a quartz crystal substrate with Euler angles (−1.5°≦φ≦1.5°, 117°≦θ≦142°, and 42.79°≦|Ψ|≦49.57°); and an IDT that is provided on the quartz crystal substrate, includes a plurality of electrode fingers, and excites a stop band upper end mode surface acoustic wave, wherein inter-electrode finger grooves which are depressions of the quartz crystal substrate are provided between the electrode fingers in a plan view, and wherein if a line occupation rate of convex portions of the quartz crystal substrate disposed between the inter-electrode finger grooves is ηg, and a line occupation rate of the electrode fingers disposed on the convex portions is ηe, the following relationships are satisfied in a case where an effective line occupation rate ηeff of the IDT is an arithmetic mean of the line occupation rate ηg and the line occupation rate ηe.
ηg>ηe,
0.59<ηeff<0.73
With the configuration, it is possible to reduce loss in a surface acoustic wave resonator by increasing excitation efficiency, and to suppress a fluctuation amount of the first-order temperature coefficient of the surface acoustic wave resonator so as to suppress variations in the resonance frequency.
This application example of the invention is directed to the surface acoustic wave resonator according to the above application example of the invention, wherein, if a wavelength of the surface acoustic wave is λ, a depth of the inter-electrode finger groove is G, an electrode film thickness of the IDT is H, and plane coordinates (G/λ, ηeff) are indicated using the value G/λ obtained by dividing a depth G of the inter-electrode finger groove by the wavelength λ of the surface acoustic wave, and the effective line occupation rate ηeff, the plane coordinates (G/λ, ηeff) are included in one of
a range surrounded by a line connecting (0.010, 0.710), (0.020, 0.710), (0.030, 0.710), (0.040, 0.710), (0.050, 0.710), (0.060, 0.710), (0.070, 0.710), (0.080, 0.710), (0.090, 0.710), (0.090, 0.420), (0.080, 0.570), (0.070, 0.590), (0.060, 0.615), (0.050, 0.630), (0.040, 0.635), (0.030, 0.650), (0.020, 0.670), and (0.010, 0.710) in this order, and
a range surrounded by a line connecting (0.030, 0.590), (0.040, 0.580), (0.050, 0.550), (0.060, 0.520), (0.070, 0.480), (0.080, 0.450), (0.090, 0.400), (0.090, 0.180), (0.080, 0.340), (0.070, 0.410), (0.060, 0.460), (0.050, 0.490), (0.040, 0.520), (0.030, 0.550), and (0.030, 0.590) in this order,
a range surrounded by a line connecting (0.010, 0.770), (0.020, 0.740), (0.030, 0.715), (0.040, 0.730), (0.050, 0.740), (0.060, 0.730), (0.070, 0.730), (0.080, 0.730), (0.080, 0.500), (0.070, 0.570), (0.060, 0.610), (0.050, 0.630), (0.040, 0.635), (0.030, 0.655), (0.020, 0.680), (0.010, 0.760), and (0.010, 0.770) in this order, and
a range surrounded by a line connecting (0.020, 0.650), (0.030, 0.610), (0.040, 0.570), (0.050, 0.550), (0.060, 0.520), (0.070, 0.470), (0.070, 0.370), (0.060, 0.440), (0.050, 0.480), (0.040, 0.520), (0.030, 0.550), (0.020, 0.590), and (0.020, 0.650) in this order,
a range surrounded by a line connecting (0.010, 0.770), (0.020, 0.760), (0.030, 0.760), (0.040, 0.750), (0.050, 0.750), (0.060, 0.750), (0.070, 0.740), (0.080, 0.740), (0.080, 0.340), (0.070, 0.545), (0.060, 0.590), (0.050, 0.620), (0.040, 0.645), (0.030, 0.670), (0.020, 0.705), (0.010, 0.760), and (0.010, 0.770) in this order, and
a range surrounded by a line connecting (0.010, 0.740), (0.020, 0.650), (0.030, 0.610), (0.040, 0.570), (0.050, 0.540), (0.060, 0.480), (0.070, 0.430), (0.070, 0.350), (0.060, 0.420), (0.050, 0.470), (0.040, 0.510), (0.030, 0.550), (0.020, 0.610), (0.010, 0.700), and (0.010, 0.740) in this order,
a range surrounded by a line connecting (0.010, 0.770), (0.020, 0.770), (0.030, 0.760), (0.040, 0.760), (0.050, 0.760), (0.060, 0.750), (0.070, 0.750), (0.070, 0.510), (0.060, 0.570), (0.050, 0.620), (0.040, 0.640), (0.030, 0.660), (0.020, 0.675), (0.010, 0.700), and (0.010, 0.770) in this order, and
a range surrounded by a line connecting (0.010, 0.690), (0.020, 0.640), (0.030, 0.590), (0.040, 0.550), (0.050, 0.510), (0.060, 0.470), (0.070, 0.415), (0.070, 0.280), (0.060, 0.380), (0.050, 0.470), (0.040, 0.510), (0.030, 0.550), (0.020, 0.610), (0.010, 0.680), and (0.010, 0.690) in this order,
a range surrounded by a line connecting (0.010, 0.770), (0.020, 0.770), (0.030, 0.760), (0.040, 0.760), (0.050, 0.760), (0.060, 0.760), (0.070, 0.760), (0.070, 0.550), (0.060, 0.545), (0.050, 0.590), (0.040, 0.620), (0.030, 0.645), (0.020, 0.680), (0.010, 0.700), and (0.010, 0.770) in this order, and
a range surrounded by a line connecting (0.010, 0.690), (0.020, 0.640), (0.030, 0.590), (0.040, 0.550), (0.050, 0.510), (0.060, 0.420), (0.070, 0.415), (0.070, 0.340), (0.060, 0.340), (0.050, 0.420), (0.040, 0.470), (0.030, 0.520), (0.020, 0.580), (0.010, 0.650), and (0.010, 0.690) in this order,
a range surrounded by a line connecting (0.010, 0.770), (0.020, 0.770), (0.030, 0.770), (0.040, 0.760), (0.050, 0.760), (0.060, 0.760), (0.070, 0.760), (0.070, 0.550), (0.060, 0.505), (0.050, 0.590), (0.040, 0.620), (0.030, 0.645), (0.020, 0.680), (0.010, 0.700), and (0.010, 0.770) in this order, and
a range surrounded by a line connecting (0.010, 0.670), (0.020, 0.605), (0.030, 0.560), (0.040, 0.520), (0.050, 0.470), (0.060, 0.395), (0.070, 0.500), (0.070, 0.490), (0.060, 0.270), (0.050, 0.410), (0.040, 0.470), (0.030, 0.520), (0.020, 0.580), (0.010, 0.620), and (0.010, 0.670) in this order
a range surrounded by a line connecting (0.010, 0.770), (0.020, 0.770), (0.030, 0.770), (0.040, 0.760), (0.050, 0.760), (0.060, 0.760), (0.070, 0.760), (0.070, 0.550), (0.060, 0.500), (0.050, 0.545), (0.040, 0.590), (0.030, 0.625), (0.020, 0.650), (0.010, 0.680), and (0.010, 0.770) in this order
a range surrounded by a line connecting (0.010, 0.655), (0.020, 0.590), (0.030, 0.540), (0.040, 0.495), (0.050, 0.435), (0.060, 0.395), (0.070, 0.500), (0.070, 0.550), (0.060, 0.380), (0.050, 0.330), (0.040, 0.410), (0.030, 0.470), (0.020, 0.520), (0.010, 0.590), and (0.010, 0.655) in this order.
With the configuration, it is possible to suppress an absolute value of the second-order temperature coefficient of the surface acoustic wave resonator to 0.01 ppm/° C.2 or less so as to correspond to the thickness of H.
This application example of the invention is directed to the surface acoustic wave resonator according to the above application example of the invention, wherein the depth G of the inter-electrode finger groove and the effective line occupation rate ηeff satisfy the following relationships.
−2.0000×G/λ+0.7200≦ηeff−2.5000×G/λ+0.7775
provided that 0.0100λ≦G≦0.0500%
−3.5898×G/λ+0.7995≦ηeff≦−2.5000×G/λ+0.7775
provided that 0.0500λ<G≦0.0695λ
With the configuration, it is possible to suppress an absolute value of the second-order temperature coefficient of the surface acoustic wave resonator to 0.01 ppm/° C.2 or less.
This application example of the invention is directed to the surface acoustic wave resonator according to Application Example 3, wherein the electrode film thickness H of the IDT satisfies the following relationship.
0<H≦0.035λ
With the surface acoustic wave resonator having such a characteristic, it is possible to realize favorable frequency-temperature characteristics in the operating temperature range. In addition, with the characteristics, it is possible to suppress deterioration in environmental resistance characteristics due to the increase in the electrode film thickness.
This application example of the invention is directed to the surface acoustic wave resonator according to Application Example 4, wherein the effective line occupation rate ηeff satisfies the following relationship.
By fixing ηeff to satisfy the above Equation in the electrode film thickness range in Application Example 4, it is possible to suppress an absolute value of the second-order temperature coefficient to 0.01 ppm/° C.2 or less.
This application example of the invention is directed to the surface acoustic wave resonator according to any one of Application Examples 2, 4 and 5, wherein the sum of the depth G of the inter-electrode finger groove and the electrode film thickness H satisfies the following relationship.
0.0407λ≦G+H
By setting the sum of the depth G of the inter-electrode finger groove and the electrode film thickness H as in the above Expression, it is possible to obtain a higher Q value than that of the surface acoustic wave resonator in the related art.
This application example of the invention is directed to the surface acoustic wave resonator according to any one of Application Examples 1 to 6, wherein Ψ and θ satisfy the following relationship.
ψ=1.191×10−3×−4.490×10−1×θ2+5.646×101×θ−2.324×103±1.0
By manufacturing a surface acoustic wave resonator using a quartz crystal substrate which is cut with the cut angle having the characteristics, it is possible to realize a surface acoustic wave resonator showing favorable frequency-temperature characteristics in a wide range.
This application example of the invention is directed to the surface acoustic wave resonator according to any one of Application Example 1 to 7, wherein, if a stop band upper end frequency of the IDT is ft2, a stop band lower end mode frequency of reflectors provided so as to sandwich the IDT in the propagation direction of the surface acoustic wave is fr1, and a stop band upper end mode frequency of the reflectors is fr2, the following relationship is satisfied.
fr1<ft2<fr2
With the characteristics, the reflection coefficient |Γ| of the reflectors increases in the stop band upper end mode frequency ft2 of the IDT, and thus the stop band upper end mode surface acoustic wave excited from the IDT is reflected to the IDT side by the reflectors at a high reflection coefficient. Then, energy confinement of the stop band upper end mode surface acoustic wave becomes strong, and thereby it is possible to realize a low-loss surface acoustic wave resonator.
This application example of the invention is directed to the surface acoustic wave resonator according to any one of Application Examples 1 to 8, an inter-conductor strip groove is provided between conductor strips forming the reflectors, wherein a depth of the inter-conductor strip groove is smaller than that of the inter-electrode finger groove.
With the characteristics, it is possible to shift the reflector stop band to a higher frequency than the IDT stop band. Therefore, it is possible to realize the relationship of the equation described in Application Example 8.
This application example of the invention is directed to a surface acoustic wave oscillator including the surface acoustic wave resonator according to any one of Application Examples 1 to 9 and a circuit that drives the IDT.
This application example of the invention is directed to an electronic apparatus including the surface acoustic wave resonator according to any one of Application Examples 1 to 9.
The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements.
Hereinafter, a surface acoustic wave resonator and surface acoustic wave oscillator according to embodiments of the invention will be described in detail with reference to the drawings. However, constituent elements, and kinds, combinations, shapes, relative dispositions thereof, and the like are not intended to limit the scope of the invention unless particularly described but are only description examples.
First, a surface acoustic wave (SAW) resonator according to a first embodiment of the invention will be described referring to
The SAW resonator 10 according to the embodiment is basically constituted by a quartz crystal substrate 30, an IDT 12, and reflectors 20.
In the embodiment, an in-plane rotation ST cut quartz crystal substrate expressed by Euler angles (−1.5°≦φ≦1.5°, 117°≦θ≦142°, and 42.79≦|Ψ|≦149.57°) is employed as the quartz crystal substrate 30. Here, a description will be made of the Euler angles with reference to
Here, φ of Euler angles (φ, θ, and Ψ), relating to a first rotation of the Z cut substrate 3, is a first rotation angle, with the Z axis as a rotation axis, and with a direction rotating from the +X axis to a +Y axis side as a positive rotation angle. The X axis and Y axis after a first rotation are respectively an X′ axis and a Y′ axis. In addition, in
The Euler angle θ, relating to a second rotation carried out after the first rotation of the Z cut substrate 3, is a second rotation angle, with the X axis (that is, the X′ axis) after the first rotation as a rotation axis, and with a direction rotating from the +Y axis (that is, +Y′ axis) after the first rotation to the +Z axis as a positive rotation angle. The cut surface of a piezoelectric substrate, that is, the cut surface of the above-described wafer 1 is determined by the first rotation angle φ and the second rotation angle θ. That is to say, when the Y axis after the second rotation is a Y″ axis and the Z axis after the second rotation is a Z′ axis, a surface parallel to both the X′ axis and the Y″ axis is the cut surface of the piezoelectric substrate, and the Z′ axis is a normal of the cut surface.
The Euler angle Ψ, relating to a third rotation carried out after the second rotation of the Z cut substrate 3, is a third rotation angle, with the Z′ axis which is the Z axis after the second rotation as a rotation axis, and with a direction rotating, from the +X axis (that is, +X′ axis) after the second rotation to the +Y axis (that is, +Y″ axis) side after the second rotation as a positive rotation angle. A propagation direction of the SAW is expressed by the third rotation angle Ψ with respect to the X axis (that is, the X′ axis) after the second rotation. That is to say, when the X axis after the third rotation is an X″ axis and the Y axis after the third rotation is a Y′″ axis, a surface parallel to both the X″ axis and the Y′″ axis is the cut surface of the piezoelectric substrate, and the Z′ axis is a normal of the cut surface. As such, since the normal does not vary even if the third rotation is performed, the cut surface of the piezoelectric substrate is also the cut surface of the wafer 1. In addition, a direction parallel to the X″ axis is a propagation direction of the SAW.
Further, a SAW phase velocity direction is a direction parallel to the X″ axis direction. A phenomenon called a SAW power flow is a phenomenon in which a direction where a phase of the SAW progresses (phase velocity direction) and a direction where energy of the SAW progresses (group velocity direction) are deviated from each other. An angle formed by the phase velocity direction and the group velocity direction is called a power flow angle (refer to
The IDT 12 including a pair of pectinate electrodes 14a and 14b where the base end portions of a plurality of electrode fingers 18a and 18b are connected by bus bars 16a and 16b respectively, and the electrode fingers 18a configuring one of the pectinate electrodes 14a, and the electrode fingers 18b configuring the other pectinate electrode 14b, are alternately disposed with a predetermined space between them. Furthermore, the electrode fingers 18a and 18b are disposed in such a way that the extension direction of the electrode fingers 18a and 18b is perpendicular to the X″ axis, which is the propagation direction of the surface acoustic wave, as shown in
In addition, furthermore, it is possible to give the SAW resonator 10 according to a modified example of the first embodiment the kinds of shape shown in
Whichever kind of tilted type IDT is used, by disposing the electrode fingers in such a way that a direction perpendicular to the X″ axis is the extension direction of the electrode fingers, as in the embodiments, it is possible to realize a low-loss SAW resonator, while maintaining good temperature characteristics in the invention.
Here, a description will be made of the relationship between a stop band upper end mode SAW and a lower end mode SAW.
According to
In addition, one pair of the reflectors 20 is provided so as to sandwich the IDT 12 in the SAW propagation direction. As a specific configuration example, both ends of each of a plurality of conductor strips 22, provided parallel to the electrode fingers 18 configuring the IDT 12, are connected.
Here, in another embodiment, only one set of ends of a plurality of conductor strips 22 may be connected to each other. Further, in still another embodiment, parts (for example, centers in the extension direction of a plurality of conductor strips 22) other than both ends of a plurality of conductor strips 22 may be connected to each other.
With an edge reflection type SAW resonator which actively utilizes a reflected wave from the SAW propagation direction end face of the quartz crystal substrate, or a multi-pair IDT type SAW resonator which excites the SAW standing wave with the IDT itself by increasing the number of pairs of IDT electrode fingers, the reflectors are not absolutely necessary.
As the material of the electrode film configuring the IDT 12 and reflectors 20 configured in this way, it is possible to use aluminum (Al), or a metal alloy with Al as its base.
By making the electrode thickness of the electrode film configuring the IDT 12 and reflectors 20 extremely small, the effect of the temperature characteristics possessed by the electrodes is kept to a minimum. Furthermore, by making the depth of the quartz crystal substrate portion grooves large, good frequency-temperature characteristics are derived from the performance of the quartz crystal substrate portion grooves, that is, by utilizing the good temperature characteristics of the quartz crystal. Because of this, it is possible to reduce the effect of the electrode temperature characteristics on the temperature characteristics of the SAW resonator and, provided that the fluctuation of the electrode mass is within 10%, it is possible to maintain good temperature characteristics.
For the above-mentioned reasons, in a case of using an alloy as the electrode film material, the proportion by weight of metals other than the main element Al should be 10% or less, and preferably 3% or less. With this, a case of using pure Al and a case of using an Al alloy can obtain good temperature characteristics or other electrical characteristics equivalent to each other.
In a case of using electrodes with a metal other than Al as a base, the electrode film thickness may be adjusted so that the mass of the electrodes is within ±10% of that when using Al. In this way, good temperature characteristics equivalent to those when using Al can be obtained.
The quartz crystal substrate 30 in the SAW resonator 10 with the above-described basic configuration is provided with the grooves (inter-electrode finger grooves) 32 between the electrode fingers of the IDT 12 and between the conductor strips of the reflectors 20.
When the SAW wavelength in the stop band upper end mode is λ, and the groove depth is G, the groves 32 provided in the quartz crystal substrate 30 may be such that
0.01λ≦G (1).
When fixing an upper limit for the groove depth G, as can be seen from
0.01λ≦G≦0.094λ (2).
This is because, by fixing the groove depth G within this range, it is possible to keep the frequency fluctuation amount within the operating temperature range (−40° C. to +85° C.) at or below the target value of 25 ppm, to be described in detail later. In addition, it is preferable that the groove depth G be within the range of
0.01λ≦G≦0.0695λ (3).
By fixing the groove depth G within this kind of range, even in a case where manufacturing variation occurs in the groove depth G, it is possible to keep the shift amount of resonance frequency between individual SAW resonators 10 within a correctable range.
In addition, the line occupation rate η is a value where the line width L of the electrode finger 18 (in a case of the quartz crystal protrusion only, the width of the protrusion) is divided by the pitch λ/2 (=L+S) between the electrode fingers 18, as shown in
η=L/(L+S) (4)
Here, in relation to the SAW resonator 10 according to the embodiment, the line occupation rate η may be fixed within a range which satisfies Expressions (5) and (6). As can also be seen from Expressions (5) and (6), η can be derived by fixing the depth G of the grooves 32.
−2.0000×G/λ+0.7200≦η≦−2.5000×G/λ+0.7775
provided that 0.0100λ≦G≦0.0500λ (5)
−3.5898×G/λ+0.7995≦η≦−2.5000×G/λ+0.7775
provided that 0.0500λ<G≦0.0695λ (6)
In addition, it is preferable that the film thickness of the electrode film material (IDT 12, reflectors 20, and the like) in the SAW resonator 10 according to the embodiment be within the range of
0<H≦0.035λ (7).
Furthermore, when taking into consideration the thickness of the electrode film shown in Equation (7) with regard to the line occupation rate η, η can be obtained from Equation (8)
With the manufacturing variation of the electrical characteristics (particularly the resonance frequency) being greater the greater the electrode film thickness, there is a high probability that the manufacturing variation of the line occupation rate η may be within ±0.04 or less when the electrode film thickness H is within the range of Expressions (5) and (6), and that a manufacturing variation greater than ±0.04 will occur when H>0.035λ. However, provided that the electrode film thickness H is within the range of Equations (5) and (6), and the variation of the line occupation rate η is within ±0.04 or less, it is possible to realize a SAW device with a low absolute value of second-order temperature coefficient β. That is, a line occupation rate η up to the range of Equation (9), where a tolerance of ±0.04 is added to Equation (8), is allowable.
In the SAW resonator 10 according to the embodiment with the above-described kind of configuration, in a case where the second-order temperature coefficient β is within ±0.01 (ppm/° C.2), and the SAW operating temperature range is preferably −40° C. to +85° C., the object is to improve the frequency-temperature characteristics to a degree whereby it is possible to keep a frequency fluctuation amount ΔF in the operating temperature range at or under 25 ppm.
Generally, however, the temperature characteristics of a surface acoustic wave resonator are expressed by the following equation.
Δf=α×(T−T0)+β×(T−T0)2
Here, Δf represents the frequency change amount (ppm) between the temperature T and the peak temperature T0, α represents the first-order temperature coefficient (ppm/° C.), β represents the second-order temperature coefficient (ppm/° C.2), T represents the temperature, and T0 represents the temperature at which the frequency is highest (the peak temperature).
For example, a case where the piezoelectric substrate is formed of a so-called ST cut (Euler angles (φ, θ, and Ψ)=(0°, 120° to 130°, and 0°) quartz crystal substrate, the first-order temperature coefficient α=0.0 (ppm/° C.), and the second-order temperature coefficient β=−0.034 (ppm/° C.2), is expressed in a graph is as in
In the SAW resonator shown in
Consequently, an advantage of some aspects of the invention is to solve the above-described kind of problem, making the frequency-temperature characteristics of the surface acoustic wave device extremely good ones, and realizing a surface acoustic wave device which operates with a stable frequency, even though the temperature changes.
How a solution to the above-described kind of problem may be realized with a SAW device to be configured including the above-described kind of technical sprit (technical components), that is, how the inventor arrived at the findings according to the invention by repeatedly carrying out simulations and experiments, will be described in detail and proved hereinafter.
In a SAW resonator using the above-described quartz crystal substrate called an ST cut, with the propagation direction as the crystal X axis direction, in a case where the operating temperature range is the same, the frequency fluctuation amount ΔF in the operating temperature range is approximately 133 (ppm), and the second-order temperature coefficient β is about −0.034 (ppm/° C.2). In addition, in a case of utilizing a stop band lower end mode excitation in a SAW resonator using an in-plane rotation ST cut quartz crystal substrate with the same operating temperature range, with the quartz crystal substrate cut angles and SAW propagation direction as (0, 123°, and 45°) in Euler angle representation, the frequency fluctuation amount ΔF is approximately 63 ppm, and the second-order temperature coefficient β is about −0.016 (ppm/° C.2).
The SAW resonators using the ST cut quartz crystal substrate and in-plane rotation ST cut quartz crystal substrate both utilizing surface acoustic waves called Rayleigh waves, have extremely small variation of frequency and frequency-temperature characteristics with respect to the machining accuracy of the quartz crystal substrate and electrodes in comparison with the surface acoustic wave, called a leaky wave, of an LST cut quartz crystal substrate, meaning that they are most suitable for mass production, and are used in various kinds of SAW device. However, with the SAW resonators using the ST cut quartz crystal substrate, in-plane rotation ST cut quartz crystal substrate, or the like utilized to date, as described above, the second-order temperature characteristics being such that the curve indicating the frequency-temperature characteristics is a quadratic curve, and furthermore, the absolute value of the second-order temperature coefficient of the second-order temperature characteristics being large, the frequency fluctuation amount in the operating temperature range is large, and they may not be utilized in a SAW device such as a resonator or oscillator used in a wired communication device or wireless communication device which requires a stability of frequency. For example, in a case where it is possible to obtain frequency-temperature characteristics which have second-order temperature characteristics where the second-order temperature coefficient β is ±0.01 (ppm/° C.2) or less, corresponding to an improvement in the ST cut quartz crystal substrate second-order temperature coefficient β of ⅓ or less, and in the in-plane rotation ST cut quartz crystal substrate second-order temperature coefficient β of 37% or more, it is possible to realize a device which requires that kind of frequency stability. Furthermore, in a case where it is possible to obtain third-order temperature characteristics, where the second-order temperature coefficient β is substantially zero, and the curve indicating the frequency-temperature characteristics is a cubic curve, it is more preferable, as the frequency stability in the operating temperature range further increases. In third-order temperature characteristics such as these, it is possible to obtain an extremely high frequency stability of ±25 ppm or less, which has not been realizable with the SAW device in the related art, even in the wide operating temperature range of −40° C. to +85° C.
The fact that the electrode finger 18 line occupation rate η in the IDT 12, electrode film thickness H, groove depth G, and the like, are related to the change in the frequency-temperature characteristics of the SAW resonator 10, as described above, has been made clear by the findings based on the simulations and experiments carried out by the inventor. Then, the SAW resonator 10 according to the embodiment utilizes the excitation of the stop band upper end mode.
From
From this, it is clear that in order to obtain good frequency-temperature characteristics in a SAW device, it is preferable to use the oscillation of the stop band upper end mode.
Next, the inventor investigated the relationship between the line occupation rate η and second-order temperature coefficient β when propagating a stop band upper end mode SAW in quartz crystal substrates with variously changed groove depths G.
Regarding this, it is possible to increase an understanding thereof by referring to
According to the above-described tendency, it can be supposed that it is preferable to employ the β=0 point at which the frequency fluctuation amount with respect to the fluctuation in the groove depth G is smaller, that is, η1, for a mass production article in which discrepancies are liable to occur when manufacturing.
In
The graph shown in
0.01λ≦G≦0.094λ (2).
The groove depth G has a maximum variation of around ±0.001λ in the mass production process. Therefore, the individual frequency fluctuation amounts Δf of the SAW resonator 10 in a case in which the groove depth G deviates by ±0.001λ, when the line occupation rate η is taken to be a constant, are shown in
Here, in a case where the frequency fluctuation amount ΔF is less than ±1000 ppm, frequency adjustment is possible using various frequency fine adjustment unit. However, in a case where the frequency fluctuation amount Δf is ±1000 ppm or more, adjusting the frequency has an effect on static characteristics such as the Q value and CI (crystal impedance) value, and on long-term reliability, leading to a reduction in the yield rate as the SAW resonator 10.
By deriving an approximate equation indicating the relationship between the frequency fluctuation amount Δf (ppm) and groove depth G for the straight line linking the plots shown in
Δf=16334(G/λ)−137 (10)
Here, on calculating the values of G at which Δf<1000 ppm, it is found that G≦0.0695%. Consequently, it can be said that it is preferable that the range of the groove depth G according to the embodiment be optimally
0.01λ≦G≦0.0695λ (3).
Next,
From the evaluation results shown in
The coordinates of points a to h in
η≦−2.5000×G/λ+0.7775
provided that 0.0100λ≦G≦0.0695λ (11)
η≦−2.0000×G/λ+0.7200
provided that 0.0100λ≦G≦0.0500λ (12)
η3.5898×G/λ+0.7995
provided that 0.0500λ<G≦0.0695λ (13)
From Equations (11), (12), and (13), it can be said that it is possible to specify the line occupation rates η in the range surrounded by the solid line in
−2.0000×G/λ+0.7200≦η≦−−2.5000×G/λ+0.7775
provided that 0.0100λ≦G≦0.0500λ (5)
−3.5898×G/λ+0.7995≦η≦−−2.5000×G/λ+0.7775
provided that 0.0500λ<G≦0.0695λ (6)
Here, in a case of allowing the second-order temperature coefficient β to be within ±0.01 (ppm/° C.2), it is confirmed that by configuring so as to satisfy both Equation (3) and Equation (5) when 0.0100λ≦G≦0.0500λ, and satisfy both Equation (3) and Equation (6) when 0.0500λ≦G≦0.0695λ, the second-order temperature coefficient β comes within ±0.01 (ppm/° C.2).
The values of the second-order temperature coefficient β (ppm/° C.2) for each electrode film thickness H at the points a to h are shown in Table 2 below. From Table 2, it can be confirmed that |β|≦0.01 (ppm/° C.2) at all of the points.
In addition, when the relationship between the groove depth G and line occupation rate η at each point at which β=0 for SAW resonators 10 in which the electrode film thickness H≅0, 0.01λ, 0.02λ, 0.03λ, based on Equations (11) to (13) and Equations (5) and (6) derived therefrom, is indicated by an approximate line, the result is as in
When changing the electrode film thickness H at 3.0% λ(0.030λ) or less, the frequency-temperature characteristics of β=0, that is, the cubic curve, can be obtained. At this time, a relational equation for G and η when the frequency-temperature characteristics are good can be expressed by Equation (8).
Here, the units of G and H are λ. It is noted that Equation (8) is established when the electrode film thickness H is in the range of 0<H≦0.030λ.
With the manufacturing variation of the electrical characteristics (particularly the resonance frequency) being greater the greater the electrode film thickness, there is a high probability that the manufacturing variation of the line occupation rate η may be ±0.04 or less when the electrode film thickness H is within the range of Equations (5) and (6), and that a manufacturing variation greater than ±0.04 will occur when H>0.035×. However, provided that the electrode film thickness H is within the range of Equations (5) and (6), and the variation of the line occupation rate η is ±0.04 or less, it is possible to realize a SAW device with a low second-order temperature coefficient β. That is, when taking into consideration the manufacturing variation of the line occupation rate, and keeping the second-order temperature coefficient β within ±0.01 ppm/° C.2, a line occupation rate η up to the range of Equation (9), where a tolerance of ±0.04 is added to Equation (8), is allowable.
In addition,
Here,
In addition,
In addition,
In addition,
In addition,
In addition,
Although there are slight differences in all of the graphs in these diagrams (
That is, it can be said that an advantage according to the embodiment of the invention is that it can be achieved even when propagating a surface acoustic wave on an individual quartz crystal substrate 30 from which the electrode film is omitted.
For each of the two points η1 and η2 at which the second-order temperature coefficient β becomes zero, a simulation is performed for each of the range of η1 and η2 when the range of β is expanded to |β|≦0.01 (ppm/° C.2), and the case in which the range of the electrode film thickness H is fixed, and the groove depth G is changed. Of η1 and η2, the larger η at which |β|≦0.01 (ppm/° C.2) is taken to be η1, and the smaller at which |β|≦0.01 (ppm/° C.2) is η2. The quartz crystal substrates used are all ones with Euler angles (0°, 123°, and Ψ), and with regard to Ψ, an angle at which ΔF is smallest is appropriately selected.
From
From
From
From
From
From
From
From
From
From
From
From
From
From
The relationship between Ψ and the groove depth G obtained from η1 in the graphs shown in
In the same way as described above, the relationships between the groove depth G and Ψ when the second-order temperature coefficient β=−0.01 (ppm/° C.2), and Ψ when β=+0.01 (ppm/° C.2), are obtained, and summarized in
A simulation is carried out for the range of Ψ which satisfies the requirement |β|≦0.01 (ppm/° C.2) when changing the groove depth G, in the case of changing the electrode film thickness H. The results of the simulation are shown in
In the groove depth G in the range of 0.01λ≦G≦0.0695λ, when approximating the range of the solid line and broken line shown in
Ψ≦3.0×G/λ+43.92
provided that 0.0100λ≦G≦0.0695λ (14)
Ψ≧−48.0×G/λ+44.35
provided that 0.0100λ≦G≦0.0695λ (15)
In the groove depth G in the range of 0.01λ≦G≦0.0695λ, when approximating the range of the solid line and broken line shown in
Ψ≦8.0×G/λ+43.60
provided that 0.0100λ≦G≦0.0695λ (16)
Ψ≧−48.0×G/λ+44.00
provided that 0.0100λ≦G≦0.0695λ (17)
In the groove depth G in the range of 0.01λ≦G≦0.0695λ, when approximating the range of the solid line and broken line shown in
Ψ≦10.0×G/λ+43.40
provided that 0.0100λ≦G≦0.0695λ (18)
Ψ≧−44.0×G/λ+43.80
provided that 0.0100λ≦G≦0.0695λ (19)
In the groove depth G in the range of 0.01λ≦G≦0.0695λ, when approximating the range of the solid line and broken line shown in
Ψ≦12.0×G/λ+43.31
provided that 0.0100λ≦G≦0.0695λ (20)
Ψ≧−30.0×G/λ+43.40
provided that 0.0100λ≦G≦0.0695λ (21)
In the groove depth G in the range of 0.01λ≦G≦0.0695λ, when approximating the range of the solid line and broken line shown in
Ψ≦14.0×G/λ+43.16
provided that 0.0100λ≦G≦0.0695λ (22)
Ψ≧−45.0×G/λ+43.45
provided that 0.0100λ≦G≦0.0600λ (23)
Ψ≧367.368×G/λ+18.608
provided that 0.0600λ≦G≦0.0695λ (24)
In the groove depth G in the range of 0.01λ≦G≦0.0695λ, when approximating the range of the solid line and broken line shown in
Ψ≦12.0×G/λ+43.25
provided that 0.0100λ≦G≦0.0695λ (25)
Ψ≧−50.0×G/λ+43.32
provided that 0.0100λ≦G≦0.0500λ (26)
Ψ≦167.692×G/λ+32.435
provided that 0.0500λ≦G≦0.0695λ (27)
In the groove depth G in the range of 0.01λ≦G≦0.0695λ, when approximating the range of the solid line and broken line shown in
Ψ≦12.0×G/λ+43.35
provided that 0.0100λ≦G≦0.0695λ (28)
Ψ≧−45.0×G/λ+42.80
provided that 0.0100λ≦G≦0.0500λ (29)
Ψ≧186.667×G/λ+31.217
provided that 0.0500λ≦G≦0.0695λ (30)
Next, the change in the second-order temperature coefficient β when the angle θ is altered, that is, the relationship between θ and the second-order temperature coefficient β, is shown in
Under these conditions, from
Tables 17 to 19 are shown as simulation data proving the relationship between θ and the second-order temperature coefficient β.
Table 17 being a table illustrating the relationship between θ and the second-order temperature coefficient β when the electrode film thickness H is changed, it shows the values of the second-order temperature coefficient β at the critical values (117° and 142°) of θ when the electrode film thickness H is 0.01% λ, and when the electrode film thickness H is 3.50% λ. The groove depths G in the simulation are all 4% λ. From Table 17, it can be seen that, in the range of 117°≦θ≦142°, even though the thickness of the electrode film thickness H is changed (0≅0.01% λ and 3.5% λstipulated as critical values of the electrode film thickness), |β|≦0.01 (ppm/° C.2) is satisfied regardless of the thickness.
Table 18 being a table illustrating the relationship between θ and the second-order temperature coefficient β when the groove depth G is changed, it shows the values of the second-order temperature coefficient β at the critical values (117° and 142°) of θ when the groove depth G is 1.00% λ and 6.95% λ. The electrode film thicknesses H in the simulation are all 2.00% λ. From Table 18, it can be seen that, in the range of 117°≦θ≦142°, even though the groove depth G is changed (1.00% λ, and 6.95% λstipulated as critical values of the groove depth G), |β|≦0.01 (ppm/° C.2) is satisfied regardless of the depth.
Table 19 being a table illustrating the relationship between θ and the second-order temperature coefficient β when the line occupation rate η is changed, it shows the values of the second-order temperature coefficient β at the critical values (117° and 142°) of θ when the line occupation rate η is 0.62 and 0.76. The electrode film thicknesses H in the simulation are all 2.00% λ, and the groove depths G are all 4.00% λ. From Table 19, it can be seen that, in the range of 117°≦θ≦142°, even though the line occupation rate η is changed (η=0.62 and 0.76 are the minimum value and maximum value of 1 when the groove depth is 4% λ in
From
In the above description, the range of optimum values of each of φ, θ, and Ψ is derived for the relationship with the groove depth G under specific conditions. In contrast to this, an extremely preferable relationship between θ and Ψ, where the frequency fluctuation amount at −40° C. to +85° C. is smallest, is shown in
Ψ=1.19024×10−3×θ3−4.48775×10−1×θ2+5.64362×101×θ−2.32327×103±1.0 (31)
Because of this, it is possible to fix Ψ by θ being fixed, and it is possible to make the range of Ψ 42.79°≦Ψ≦49.57° when the range of θ is 117°≦θ≦142°. The groove depth G and electrode film thickness H in the simulation are G=0.04λ, and H=0.02λ.
Due to the above-described kinds of reason, by configuring the SAW resonator 10 in accordance with the various conditions fixed in the embodiment, it is possible to obtain a SAW resonator which can realize good frequency-temperature characteristics fulfilling the target values.
In addition, with the SAW resonator 10 according to the embodiment, the improvement in the frequency-temperature characteristics is sought by having the film thickness H of the electrode film in the range of 0<H≦0.035λ, as shown in Equation (7) and
In addition, on carrying out a high temperature storage test on a SAW resonator manufactured under the same conditions as those in
In the SAW resonator 10 manufactured under the above-described conditions, conditions where H+G=0.067λ (aluminum film thickness 2,000 angstrom, groove depth 4,700 ANGSTROM), the IDT line occupation rate ηi=0.6, the reflector line occupation rate ηr=0.8, the Euler angles are (0°, 123°, and 43.5°), the IDT pair number is 120, the intersection width is 40λ (λ=10 μm), the number of reflectors (one side) is 72 (36 pairs), and the electrode fingers have no angle of tilt (the electrode finger array direction and SAW phase velocity direction correspond with each other), frequency-temperature characteristics like those shown in
In
In the embodiment, a description has been made of the effect on the frequency-temperature characteristics of the groove depth G, electrode film thickness H, and the like. However, the combined depth of the groove depth G and electrode film thickness H (the level difference) also has an effect on static characteristics such as the equivalent circuit constant and CI value, and on the Q value. For example,
The frequency, equivalent circuit constant, and static characteristics of the SAW resonator 10 exhibiting frequency-temperature characteristics like those shown in
In addition,
The basic data of the SAW resonators according to the simulation is as follows.
Basic data of SAW resonator 10 according to the embodiment
G: changes
IDT line occupation rate ηi: 0.6
Reflector line occupation rate ηr: 0.8
Euler angles (0°, 123°, and 43.5°)
Pair number: 120
Intersection width: 40λ (λ=10 μm)
Reflector number (one side): 60
No electrode finger tilt angle
Basic data of SAW resonator in the related art
H: changes
G: zero
IDT line occupation rate ηi: 0.4
Reflector line occupation rate ηr: 0.3
Euler angles (0°, 123°, and 43.5°)
Pair number: 120
Intersection width: 40λ (λ=10 μm)
Reflector number (one side): 60
No electrode finger tilt angle
When referring to
In order to efficiently confine the energy of a surface acoustic wave excited in the stop band upper end mode, a stop band upper end frequency ft2 of the IDT 12 may be set between a stop band lower end frequency fr1 of the reflectors 20 and a stop band upper end frequency fr2 of the reflectors 20, as in
fr1<ft2<fr2 (32)
With this, a reflection coefficient Γ of the reflectors 20 at the stop band upper end frequency ft2 of the IDT 12 increases, and the stop band upper end mode SAW excited with the IDT 12 is reflected by the reflectors 20 to the IDT 12 side with a high reflection coefficient. Then, the stop band upper end mode SAW energy confinement becomes stronger, and it is possible to realize a low-loss resonator.
As opposed to this, in a case where the relationship between the stop band upper end frequency ft2 of the IDT 12 and the stop band lower end frequency fr1 of the reflectors 20 and stop band upper end frequency fr2 of the reflectors 20 is set to the condition of ft2<fr1, or the condition of fr2<ft2, the reflection coefficient Γ of the reflectors 20 at the stop band upper end frequency ft2 of the IDT 12 decreases, and it becomes difficult to realize a strong energy confinement condition.
Here, in order to realize the condition of Equation (32), it is necessary to make a frequency shift of the stop band of the reflectors 20 to an area higher than the stop band of the IDT 12. Specifically, this can be realized by making the array cycle of the conductor strips 22 of the reflectors 20 shorter than that of the array cycle of the electrode fingers 18 of the IDT 12. In addition, as in other methods, it can be realized by making the thickness of the electrode film formed as the conductor strips 22 of the reflectors 20 less than the thickness of the electrode film formed as the electrode fingers 18 of the IDT 12, or by making the depth of the inter-conductor strip grooves of the reflectors 20 less than the depth of the inter-electrode finger grooves of the IDT 12. In addition, a plurality of these methods may be employed in combination.
According to
As described above, as the SAW resonator according to the embodiment of the invention has inflection points within the operating temperature range (temperature range to be used: −40° C. to +85° C.), as shown in
As opposed to this, the SAW resonator according to the embodiment of the invention, with the frequency fluctuation amount describing a cubic curve, or an approximate cubic curve, within the operating temperature range, realizes a dramatic reduction of the frequency fluctuation amount. Changes in the frequency fluctuation amount within the operating range for a SAW resonator whose IDT and reflectors are covered with a protective film are shown in
The example shown in
Basic data of SAW resonator according to example shown in
H: (material: aluminum): 2,000 (ANGSTROM)
(H+G=0.067λ)
IDT line occupation rate ηi: 0.6
Reflector line occupation rate ηr: 0.8
In-plane rotation ST cut substrate with Euler angles (0°, 123°, and 43.5°)
Pair number: 120
Intersection width: 40λ (λ=10 μm)
Reflector number (one side): 36
No electrode finger tilt angle
Protective film (alumina) thickness 400 (ANGSTROM)
Second-order temperature coefficient β=+0.0007 (ppm/° C.2)
The example shown in
Basic data of SAW resonator according to example shown in
H: (material: aluminum): 2,000 (ANGSTROM)
(H+G=0.067λ)
IDT line occupation rate ηi: 0.6
Reflector line occupation rate ηr: 0.8
In-plane rotation ST cut substrate with Euler angles (0°, 123°, and 43.5°)
Pair number: 120
Intersection width: 40λ (λ=10 μm)
Reflector number (one side): 36
No electrode finger tilt angle
Protective film (SiO2) thickness 400 (ANGSTROM)
Second-order temperature coefficient β=+0.0039 (ppm/° C.2)
The inventor has described that the absolute value of the second-order temperature coefficient β can be made 0.01 ppm/° C.2 or less by adjusting the line occupation rate η for the above-described designed G and H. On the other hand, the inventor has found that, in a case where the line occupation rate η fluctuates when excitation is performed in the stop band upper end mode, the frequency-temperature characteristics of the surface acoustic wave fluctuate. When a plurality of surface acoustic wave resonators are manufactured, there are cases in which it is difficult to match all the surface acoustic wave resonators with a desired design line occupation rate and thus surface acoustic wave resonators with a line occupation rate different from the design line occupation rate may be manufactured. At this time, fluctuation amounts of the line occupation rates with the design line occupation rate are not constant, and variations in the line occupation rates occur. Therefore, when a plurality of surface acoustic wave resonators are manufactured, variations occur in the frequency-temperature characteristics of the surface acoustic wave. The inventor has found that this causes variations in frequency deviations in an operating temperature range of the surface acoustic wave element, which is a cause of reducing a yield of surface acoustic wave resonators.
In addition, as described above, the inventor has found that a value of the line occupation rate η when a fluctuation amount of the first-order temperature coefficient is the minimum in a case where variations occur in the line occupation rate η and a value of the line occupation rate η where the second-order temperature coefficient β is the minimum (−0.01≦β≦0.01) do not correspond with each other. Therefore, in the second embodiment, a description will be made of a SAW device which can suppress a value of the second-order temperature coefficient β to the above-described range, and can minimize a fluctuation amount of the first-order temperature coefficient which dominantly functions in the frequency-temperature characteristics in the operating temperature range.
A basic configuration of the SAW resonator according to the second embodiment is similar to that of the surface acoustic wave resonator 10 according to the first embodiment. That is to say, the SAW resonator 110 includes a quartz crystal substrate 30, an IDT 112 formed on the quartz crystal substrate, and reflectors 120 formed at both sides of the IDT 112. The IDT 112 extends in a direction perpendicular to the direction where surface acoustic wave (wavelength λ) propagates, and is formed by pectinate electrodes 114a and 114b which intersect each other. The pectinate electrode 114a is constituted by a plurality of electrode fingers 118a which are arranged in the propagation direction of the surface acoustic wave at the same interval (λ) and a bus bar 116a which connects a plurality of electrode fingers 118a in parallel to each other. In a similar way, the pectinate electrode 114b is constituted by a plurality of electrode fingers 118b which are arranged in the propagation direction of the surface acoustic wave at the same interval (λ) and a bus bar 116b which connects a plurality of electrode fingers 118b in parallel to each other.
Therefore, the electrode fingers 118a and the electrode fingers 118b are alternately disposed in the propagation direction of the surface acoustic wave at the same interval (λ/2). The reflectors 120 have conductor strips 122 which are disposed in the propagation direction of the surface acoustic wave at the same interval (λ/2). In addition, the SAW resonators 110 excite the surface acoustic wave in the stop band upper end mode.
In the first embodiment, the widths of the convex portions 34 formed by the inter-electrode finger grooves 32 correspond with those of the electrode fingers 18a and 18b in the propagation direction of the surface acoustic wave. However, the second embodiment is different from the first embodiment in that the widths of the electrode fingers 118a and 118b in the propagation direction of the surface acoustic wave on the convex portions 134 formed by the inter-electrode finger grooves 132 are smaller than the widths of the convex portions in the propagation direction of the surface acoustic wave. In addition, both ends of the electrode fingers 118a and 118b in the propagation direction of the surface acoustic wave are disposed at inner sides than both ends of the convex portions 134 in the propagation direction of the surface acoustic wave in a plan view. Therefore, both the ends of the convex portions 134 in the propagation direction of the surface acoustic wave are not covered with the electrode fingers 118a and 118b and thus are exposed.
In addition, the same configuration is formed in the reflectors 120 which are disposed at both sides of the IDT 112.
In
L
g
>L
e (33).
With this, since an amount (solid angle) where lines of electric force emitted from the electrode fingers 118a and 118b are absorbed by the convex portions 134 increases, an excitation efficiency of the surface acoustic wave increases, thereby reducing loss in the SAW resonator 110 in comparison with the first embodiment.
However, in the SAW resonator 10 according to the first embodiment, the surface acoustic wave is reflected at the parts where step differences of both ends of the convex portion 34 (refer to
However, since the widths of the convex portions 134 corresponding to the line occupation rate η are also L, in the configuration according to the first embodiment, it is difficult to simultaneously specify a line occupation rate η which minimizes the second-order temperature coefficient β and a line occupation rate η which minimizes a fluctuation amount of the first-order temperature coefficient.
On the other hand, in the SAW resonator 110 according to the second embodiment, the surface acoustic wave is reflected at the parts where step differences of both ends of the convex portion 134 with the width Lg in the thickness (G) direction rise and at the parts where step differences of both ends of each of the electrode fingers 118a and 118b with the width Le in the thickness (H) direction rise. Therefore, in the second embodiment, it is supposed that the electrode fingers 118a and 118b have
L
eff=(Lg+Le)/2 (34)
as an effective width and reflect the surface acoustic wave at the both end positions.
Therefore, if a line occupation rate of the convex portions 134 is ηg (=Lg/P), and a line occupation rate of the electrode fingers 118a and 118b is ηe (=Le/P), the effective line occupation rate ηeff at this time becomes
ηeff=(ηg+ηe)/2 (35).
At this time,
ηg>ηe (36).
Therefore, the second-order temperature coefficient can be adjusted by adjusting ηeff. On the other hand, a fluctuation amount of the first-order temperature coefficient can be adjusted using ηg. Thus, the second-order temperature coefficient can be adjusted by adjusting ηeff, and a fluctuation amount of the first-order temperature coefficient can be adjusted by adjusting ηg.
Further, in a case where, as in
Here, Le and Lg are defined as
The inventor investigated a frequency deviation, a CI value, and a fluctuation amount of the first-order temperature coefficient in a case (type 1) where the width Le of the electrode finger corresponds with the width Lg of the convex portion as in the first embodiment and in a case (type 2) of Lg>Le as in the second embodiment.
In the SAW device used in the investigation for both of the types 1 and 2, the Euler angles were (φ=0°, θ=123°, and) Ψ=44°), G=0.046λ, H=0.021λ, the IDT electrode pair number was 210, and the number of the reflectors disposed at both ends of the IDT electrodes was 94 at one sides.
First, as an example 1, in the type 1, the line occupation rate η of the electrode fingers was 0.64, and the line occupation rate η 1g of the convex portions 34 (refer to
Next, as an example 2, in the type 1, the line occupation rate η of the electrode fingers 118 and 118b was 0.66, and the line occupation rate ηg of the convex portions 134 was 0.66 such that the line occupation rate η was 0.66. In the type 2, the ηe was 0.59 and ηg was 0.73 such that the effective line occupation rate ηeff was 0.66. That is to say, in both the examples, ηeff in the type 2 maintains the value of η in the type 1, and is adjusted such that ηg>ηe. In addition, with regard to a CI value, a plurality of samples (1784) were prepared for each type, and an average value thereof was calculated.
As shown in
Further, in the example 1, the CI value of the type 1 was 23.8Ω, but that of the type 2 was improved to 20.1Ω. In addition, in the example 2, the CI value of the type 1 was 22.4Ω, but that of the type 2 was improved to 19.2Ω, and thus the type 2 realizes a low-loss SAW resonator in the examples 1 and 2. It is thought that this is because the solid angle at which the convex portions anticipate the lines of electric force generated from the electrode fingers is greater in the type 2 than in the type 1, and thus the excitation efficiency increases, as described above.
Next, changes in the fluctuation amounts of the first-order temperature coefficient of the type 1 and the type 2 were investigated.
As shown in
On the other hand, as shown in
As described above, the type 1 and the type 2 in the example 1 have a favorable frequency deviation of 12 ppm as shown in
0.58<ηeff<0.73 (39)
In addition, a plurality of samples (1784) for the type 1 and the type 2 in the example 1 were prepared, and variations in the first-order temperature coefficient and variations in the resonance frequency at 25° C. were investigated. If the magnitude of the variation in the first-order temperature coefficient of the type 1 is 1, the magnitude of the variation in the first-order temperature coefficient of the type 2 is improved up to 0.2. As described above, in the type 1, the line occupation rate ηg of the convex portions 34 (refer to
In addition, from the above description, the type 2 maintains the frequency-temperature characteristics of the type 1 which has η equal to a value of ηeff. Therefore, it may be considered that η is replaced with ηeff in all the drawings where η is expressed by the above-described transverse axis. Therefore, ηeff is designed so as to be included in the range of Equation (39) and the range which is surrounded by the line connected so as to make a round in an alphabetical order, that is, the range satisfying the above Equations (3), (5) and (6) in
Further, ηeff is set so as to be included in the range of Equation (39), and a range included in the range surrounded by the line connecting the respective points shown with the plane coordinates (G/λ, ηeff) in the figures in an alphabetical order so as to make a round, in
In the embodiments, the IDT 12 forming the SAW resonator 10 and the IDT 112 forming the SAW resonator 110 are shown such that all the electrode fingers alternately intersect each other. However, the SAW resonator 10 and the SAW resonator 110 according to the embodiments of the invention can achieve considerable effects only with the quartz crystal substrate. For this reason, even in a case where the electrode fingers 18 in the IDT 12 and the electrode fingers 118a and 118b in the IDT 112 are thinned out, it is possible to achieve the same effect.
In addition, in the first embodiment, the grooves 32 may be partially provided between the electrode fingers 18 and between the conductor strips 22 of the reflectors 20. In particular, as the central portion of the IDT 12, which has a high oscillatory displacement, has a dominant effect on the frequency-temperature characteristics, a structure may be adopted where the grooves 32 are provided only in that portion. In this kind of structure as well, it is possible to achieve a SAW resonator 10 with good frequency-temperature characteristics.
In addition, in the above-described embodiment, it is noted that Al or an Al-based alloy is used as the electrode film. However, the electrode film may be configured using another metal material, provided that it is a metal which can achieve the same effect as the embodiment. Further, a protective film such as SiO2 or alumina may be provided on the electrode film.
In addition, although the above-described embodiment is a one-terminal pair SAW resonator in which only one IDT is provided, the invention is also applicable to a two-terminal pair SAW resonator in which a plurality of IDTs are provided, and is also applicable to a longitudinally coupled or transversally coupled double mode SAW filter or multiple mode SAW filter.
Next, a description will be made, referring to
In the SAW oscillator 100 according to the embodiment, the SAW resonator 10 and IC 50 are housed in the same package 56, and electrode patterns 54a to 54g formed on a bottom plate 56a of the package 56, and the pectinate electrodes 14a and 14b of the SAW resonator 10 and pads 52a to 52f of the IC 50, are connected by metal wires 60. Then, a cavity of the package 56 housing the SAW resonator 10 (the SAW resonator 110) and IC 50 is hermetically sealed with a lid 58. By adopting this configuration, it is possible to electrically connect the IDT 12 (refer to
Therefore, in response to a demand for an expansion of operating temperature range and higher accuracy of internally mounted electronic devices, with the effect of internal heat generation increasing along with the miniaturization of blade servers and other packages, in addition to a higher reference clock frequency due to the speeding-up of information communication in recent years, and furthermore, in response to a market which needs long-term, stable operating in environments from low temperature to high temperature, such as wireless base stations installed outdoors, the SAW oscillator according to the embodiments of the invention is preferred, as it has extremely good frequency-temperature characteristics of a frequency fluctuation amount of approximately 20 (ppm) or less in its operating temperature range (temperature range to be used: −40° C. to +85° C.)
Furthermore, as the SAW resonator according to the invention, or SAW oscillator including the SAW resonator, realizes a significant improvement in frequency-temperature characteristics, it contributes largely to realizing a product with, as well as extremely good frequency-temperature characteristics, excellent jitter characteristics and phase noise characteristics, for example, a mobile telephone, a hard disk, a personal computer, a tuner receiving a BS and CS broadcast, an instrument processing a high frequency signal transmitted through a coaxial cable or an optical signal transmitted through an optical cable, or an electronic instrument such as a server network instrument or wireless communication instrument which needs a high frequency, high accuracy clock (low jitter, low phase noise) in a wide temperature range, and it goes without saying that it contributes greatly to further system reliability and quality improvement.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2010-270953 | Dec 2010 | JP | national |
This is a continuation application of U.S. application Ser. No. 13/310,123 filed Dec. 2, 2011 which claims priority to Japanese Patent Application No. 2010-270953 filed Dec. 3, 2010 all of which are incorporated by reference herein in their entireties.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 13310123 | Dec 2011 | US |
| Child | 14066959 | US |
