JP4356675B2 - Surface acoustic wave device - Google Patents

Description

  The present invention relates to a surface acoustic wave device that uses a quartz substrate to obtain excellent frequency temperature characteristics in a wide band.

In recent years, surface acoustic wave (hereinafter, referred to as SAW) devices have been widely used as components for mobile communication terminals, in-vehicle devices, and the like, and have been strongly required to have a wide band and excellent frequency temperature characteristics.
There is a SAW device using an ST cut quartz substrate as a conventional SAW device. The ST cut quartz substrate is a cut name of a quartz crystal plate having a plane (XZ ′ plane) obtained by rotating the XZ plane by 42.75 ° with the crystal X axis as a rotation axis and a positive counterclockwise direction from the crystal Z axis. A SAW (hereinafter referred to as ST cut quartz SAW) which is a (P + SV) wave called a Rayleigh wave propagating in the direction is used. ST-cut quartz SAW devices have a wide range of applications, such as SAW resonators used as oscillation elements and IF filters arranged between the RF stage and the IC of mobile communication terminals. FIG. 7 shows the structure of the ST-cut quartz crystal SAW resonator 1. The comb-shaped electrodes 2 a and 2 b each having a plurality of electrode fingers that are interleaved with each other are arranged on the ST-cut quartz crystal substrate 4. Grating reflectors 3a and 3b for reflecting SAW are disposed on both sides of a comb-like electrode (IDT: Interdigital Transducer) formed by combining a pair of electrodes. The electrode material of the IDT and the grating reflectors 3a and 3b is made of Al or an alloy containing Al as a main component.

The ST-cut quartz SAW device has a first-order temperature coefficient of frequency temperature characteristics of zero, and the characteristics are represented by a quadratic curve. When the apex temperature is adjusted to be located at the center of the operating temperature range, the frequency fluctuation amount is It is generally known that the frequency stability is excellent because it is much smaller.
Japanese Patent No. 3353742 Michio Kadota, “Small-sized resonator IF filter using shear horizontal wave on heavy metal film / quartz substrate”, IEEE International Frequency Control Symp. Proc., Pp. 50-54 (2002)

However, the above-described ST-cut quartz SAW device has a primary temperature coefficient of zero, but a secondary temperature coefficient of -3.2 (10 ⁻⁸ / ° C. ² ) is relatively large. There has been a problem that the amount of frequency fluctuation becomes extremely large. Moreover, ST cut quartz SAW device small electromechanical coupling coefficient k ^2, when a filter has been difficult to realize a wide band characteristic.

In addition, if a piezoelectric substrate having a large electromechanical coupling coefficient k ² such as Li ₂ B ₄ O ₇ is used in order to increase the bandwidth, the frequency temperature characteristics deteriorate, and the insertion loss in the operating temperature range, the attenuation in the vicinity of the pass band, etc. There is a problem that a large margin of the guarantee standard must be secured and the manufacturing yield deteriorates.

  On the other hand, Patent Document 1 and Non-Patent Document 1 use SH waves propagating in a direction rotated in-plane by 90 ° with respect to the propagation direction of the ST-cut quartz SAW, and the electrode material is more specific than the piezoelectric substrate. By using a large metal such as W or Ta, an electromechanical coupling coefficient larger than that of the ST cut quartz SAW can be obtained, and a wide band characteristic can be realized when a filter is configured.

  However, although the SAW devices disclosed in Patent Document 1 and Non-Patent Document 1 can realize a wider band characteristic than the ST-cut quartz SAW device, the second-order temperature coefficient of the frequency temperature characteristic is the ST-cut quartz SAW device. The problem that the amount of frequency fluctuation increases when the operating temperature range is expanded remains unresolved.

The present invention has been made to solve the above-described problems, and realizes a wide band and excellent frequency-temperature characteristics in a SAW device using a quartz substrate as a piezoelectric substrate and utilizing SH (shear horizontal) waves. With the goal.
Another object of the present invention is to provide a composite device using the SAW device as described above.

In order to achieve the above object, a surface acoustic wave device according to the present invention is a surface acoustic wave device including a piezoelectric substrate and an IDT formed on the piezoelectric substrate, and using an excitation wave as an SH wave. In the substrate, the cut angle θ is the rotation angle of the crystal Z axis with the crystal X axis as the rotation axis, and the direction of rotation from the crystal + Z axis to the crystal + Y axis side is the rotation direction in which the cut angle θ is negative. A rotating Y-cut quartz substrate made of a quartz plate set in a range of 62 ° ≦ θ ≦ −51 ° and having a surface acoustic wave propagation direction of 90 ° ± 5 ° with respect to the crystal X axis. It is characterized by being formed of W or an alloy mainly composed of W. According to the surface acoustic wave device having such a configuration, the vertex temperature can be set within a practical range. Therefore, a surface acoustic wave device having a wide band and excellent frequency temperature characteristics can be realized in a practical range of the ambient temperature.

In the surface acoustic wave device configured as described above, when the wavelength of the surface acoustic wave to be excited is λ, the electrode film thickness is H, and the line occupancy of the electrode fingers constituting the IDT is mr, the surface acoustic wave It is desirable to determine the product of the normalized film thickness H / λ and the line occupancy ratio mr of the electrode fingers so as to be in the range of 0.001 ≦ H / λ · mr <0.027. . According to the surface acoustic wave device having such a configuration, the secondary temperature coefficient can be set to a favorable value as compared with the conventional surface acoustic wave device. For this reason, the cut angle θ is the rotation angle of the crystal Z axis with the crystal X axis as the rotation axis, and the direction of rotation from the crystal + Z axis to the crystal + Y axis side is the rotation direction in which the cut angle θ is negative. In a rotated Y-cut quartz substrate made of a quartz plate set in a range of ° ≦ θ ≦ −51 ° and having a surface acoustic wave propagation direction of 90 ° ± 5 ° with respect to the crystal X axis , H / λ · mr Is set to 0.001 ≦ H / λ · mr <0.027, a surface acoustic wave device having a wide band and excellent frequency temperature characteristics can be obtained in a practical range of the ambient temperature.

  The composite device for achieving the above object may be an oscillation circuit, a module, or the like using the surface acoustic wave device having the above configuration. If it is a composite device of such a structure, the effect which acts on the surface acoustic wave device of the said structure can be acquired.

Hereinafter, embodiments of the SAW device of the present invention will be described in detail with reference to the drawings. The following embodiments are some embodiments according to the present invention, and the present invention assumes various forms as long as the main part is not changed.
First, a SAW resonator 100 according to the present invention will be described with reference to FIG. 1A is a diagram illustrating a planar configuration of the SAW resonator, and FIG. 1B is a diagram illustrating a cross section taken along the line AA in FIG. The SAW resonator 100 of this embodiment has a basic configuration of a SAW element piece 10 for exciting a surface acoustic wave and a package 110 on which the SAW element piece 10 is mounted.

The SAW element piece 10 includes an interdigital electrode (IDT) 14 on one main surface of a piezoelectric substrate 12 and a grating reflector (hereinafter simply referred to as a reflector) 22 (22a) sandwiching the IDT 14 in the propagation direction of the surface acoustic wave. , 22b).
The IDT 14 is formed by a bus bar 16 (16a, 16b) arranged along the propagation direction of the surface acoustic wave and a plurality of electrode fingers 20 (20a, 20b) formed perpendicular to the bus bar 16. A pair of comb-shaped electrodes are arranged so that the electrode fingers 20 intersect each other. The reflector 22 has a configuration in which a plurality of conductor strips formed in parallel to the electrode fingers 20 are connected to each other. The comb electrodes are provided with input / output pads 18 (18a, 18b) for supplying or outputting input / output power to / from the IDT 14 to the bus bar 16, respectively. As shown in FIG. 2, the piezoelectric substrate 12 on which the electrode pattern having such a configuration is formed on one main surface is rotated by −52 ° with the cut angle θ of the rotating Y-cut quartz plate being positive in the counterclockwise direction from the Z axis. The crystal plate is set in the vicinity, and the SAW propagation direction is set to the Z′-axis direction substantially perpendicular to the crystal X-axis (90 ° ± 5 °). Therefore, the surface acoustic wave (SAW) excited by the SAW element piece 10 having such a configuration is an SH wave. When the cut angle is expressed as (φ, θ, ψ) in Euler angle display, it is (0 ° ± 5 °, 38 °, 90 ° ± 5 °). Here, φ is expressed as 0 ° ± 5 °, but this range is an error range in the manufacturing stage, and does not affect the characteristics of the SAW element piece or the SAW resonator.

  The package 110 includes a base 112 on which the SAW element piece 10 is mounted, and a lid body (lid) 120 for sealing an opening of the base 112. The base 112 has a cavity formed in the shape of a staircase inside, and mounting terminals 114 (114a, 114b) for mounting the SAW element piece 10 are formed in the cavity. The lid 120 is a flat plate for hermetically sealing the opening of the base 112, and is preferably formed of a member having a thermal expansion coefficient close to that of the base 112 as a constituent member.

  In the SAW resonator 100 shown in the present embodiment, the SAW element piece 10 having the above-described configuration is mounted on the base 112 having the above-described configuration using an adhesive or the like with the active surface thereof facing upward. Mounting is performed by connecting the input / output pad 18 formed on the substrate and the mounting terminal 114 formed on the base 112 with metal wires 116 (116a, 116b). After mounting the SAW element piece 10 on the base 112, the body of the SAW resonator 100 is formed by sealing the opening of the base 112 with a lid 120 (not shown).

  In the SAW resonator 100 having the above-described configuration, in the present invention, W (tungsten) is used as a constituent material for forming the excitation electrodes formed on the piezoelectric substrate 12, that is, the IDT 14, the reflector 22, and the input / output pad 18. ) Or an alloy containing W as a main component. FIG. 3 shows a partially enlarged cross-sectional view of the IDT 14. In the embodiment described below, the wavelength of the SAW excited on the IDT 14 is λ. Further, the film thickness of the electrode such as IDT 14 and the value obtained by standardizing the electrode film thickness H by the wavelength λ, that is, the normalized film thickness is represented by H / λ. Also, a value obtained by dividing the electrode finger width L by the sum of the electrode finger width L and the inter-electrode finger space S (L / (L + S)), that is, the line occupancy is represented by mr.

  In the SAW resonator 100 having the above-described configuration, the apex temperature is kept good by adjusting the cut angle of the piezoelectric substrate 12 and the product (H / λ · mr) of the normalized film thickness and the line occupation ratio. It becomes possible to keep the secondary temperature characteristics better than a SAW resonator in which a SAW element piece in which an Al film is formed on a conventional ST-cut quartz substrate is mounted.

  First, with reference to FIG. 4, an investigation result of the relationship between the normalized film thickness, the line occupation ratio (H / λ · mr), and the vertex temperature Tp will be described. FIG. 4 shows the relationship between the normalized film thickness, the line occupancy (H / λ · mr), and the apex temperature Tp, with the cut angle θ of the rotated Y-cut quartz plate set to −50 counterclockwise from the Z axis. The calculation result when calculating from 0 ° to −63 ° at intervals of 1 ° is shown. As for the Z ′ axis, the SAW propagation direction is substantially perpendicular (90 ° ± 5 °) to the crystal X axis in the same manner as described above. When represented as (φ, θ, ψ) in the Euler angle display, (0 ° ± 5 °, 27 ° ≦ θ ≦ 40 °, 90 ° ± 5 °).

  From the calculation result, the cut angle of θ that can make the vertex temperature within a practical range, that is, a range of about 0 ° C. to 70 ° C. is extracted. As can be seen from FIG. 4, the range where the vertex temperature is 0 ° C. to 70 ° C. is a range surrounded by a broken line. Therefore, the θ cut angle at which the vertex temperature can be set within this range is −62 ° ≦ θ ≦ −51 in the range of 0.001 ≦ H / λ · mr ≦ 0.030 in the rotated Y-cut quartz plate. °. When this is displayed in Euler angles, it is (0 ° ± 5 °, 28 ° ≦ θ ≦ 39 °, 90 ° ± 5 °). Here, if the cut angle θ in the rotated Y-cut quartz plate is larger than −51 °, the vertex temperature Tp cannot be set near room temperature (in the range of 0 ° C. to 70 ° C.), and if the cut angle θ is smaller than −62 °, the reference is obtained. The sensitivity of the apex temperature Tp with respect to the chemical film thickness H / λ is remarkably increased, and it becomes very difficult to control the apex temperature. Therefore, it is not practical to set the cut angle θ to θ> −51 ° and θ <−62 °.

Next, the relationship between the normalized film thickness, the line occupation rate (H / λ · mr), and the secondary temperature characteristic b will be described. FIG. 5 shows the calculation of the relationship between H / λ · mr and b when each cut θ is changed in the range of −50.0 ° to −63.0 ° in a rotated Y-cut quartz plate. The results are shown. A broken line parallel to the axis of H / λ · mr shown in FIG. 5 is a line indicating −3.2 (10 ⁻⁸ / ° C. ² ). This value is the secondary temperature coefficient b in the SAW device shown in the prior art, and the range above this line in the figure, that is, the secondary temperature coefficient b is −3.2 (10 ⁻⁸ / ° C. ² ). It can be said that the SAW resonator 100 of the present embodiment can obtain a temperature coefficient better than that of the conventional SAW device in a range lower than the absolute value of. In order for the secondary temperature coefficient b to be lower than the absolute value of −3.2 (10 ⁻⁸ / ° C. ² ) in the range where the cut angle θ is −62 ° ≦ θ ≦ −51 °, it can be read from FIG. Thus, H / λ and mr may be adjusted so that the value of H / λ · mr is lower than 0.027. That is, adjustment is performed so that H / λ · mr <0.027.

  From the above, the SAW in which the cut angle θ is set to −62 ° ≦ θ ≦ −51 ° and the SAW propagation direction is set to about 90 ° with respect to the crystal X axis, the electrode is formed with the W film on the rotated Y-cut quartz substrate. In the resonator 100, by setting the product H / λ · mr of the standardized film thickness and the line occupancy ratio to 0.001 ≦ H / λ · mr ≦ 0.030, the vertex temperature is around normal temperature (0 ° C. to 70 ° C. The secondary temperature coefficient can be made smaller than that of the conventional product by setting H / λ · mr <0.027. Therefore, if H / λ · mr is set in the range of 0.001 ≦ H / λ · mr <0.027, the apex temperature can be easily set to around room temperature, and the secondary temperature coefficient is higher than that of the conventional product. Can be small. As a specific example, for example, when H / λ is 0.02, mr may be set to 0.06. In this case, H / λ · mr is 0.012, which can be within the above range. Note that the normalized film thickness H / λ is preferably 0.01 or more. This is because when the electrode film thickness is extremely thin, the resistance value of the electrode itself increases and the loss increases. Naturally, for the metallization ratio mr, a numerical value of 0 or 1 is outside the adjustment range. This is because when mr is 0, there are no electrodes, and when mr is 1, the positive and negative electrodes are short-circuited, and the IDT is not formed.

  Up to now, only a 1-port SAW resonator as shown in FIG. 1 has been mentioned, but a 2-port SAW resonator, a dual-mode SAW (DMS) filter utilizing the acoustic coupling of the SAW resonator, and a SAW resonator are used. The present invention is also applicable to various types of SAW devices such as a ladder-type SAW filter arranged in a ladder form on a series arm and a parallel arm, and a transversal SAW filter in which an input IDT and an output IDT are arranged with a predetermined gap. It goes without saying that the same effect can be obtained by applying.

Further, in the above-described SAW device, a protective film such as an anodic oxide film or SiO ₂ is applied on the IDT or the grating reflector, or another metal thin film is formed on the upper or lower portion of the electrode for the purpose of improving the adhesion layer or power durability. Even when it is formed, it is obvious that the same effect as the present invention can be obtained. Needless to say, the SAW device of the present invention can be applied to a sensor device, a module device, an oscillation circuit, and the like. For example, as a SAW oscillator using the SAW device of the above embodiment, the one shown in FIG. 6 can be exemplified. A SAW oscillator 200 shown in FIG. 6 includes a SAW element piece 10 and an IC 230 for controlling excitation of the SAW element piece 10 in a package 210 including a base 212 and a lid 220. .
Further, if the SAW device of the present invention is used for a voltage controlled SAW oscillator (VCSO) or the like, the capacitance ratio γ can be reduced, so that the frequency variable width can be increased.

  Further, the SAW device of the present invention may have a structure other than the structure in which the SAW element piece and the package are wire-bonded as shown in FIG. 1, for example, the input / output pads of the SAW element piece and the package mounting terminals are connected by metal bumps. A flip chip bonding (FCB) structure, a CSP (Chip Size Package) structure in which a SAW element piece is flip-chip bonded on a wiring board and the periphery of the SAW element piece is resin-sealed, or a metal film or resin layer on the SAW element piece It is also possible to form a WLCSP (Wafer Level Chip Size Package) structure that eliminates the need for a package or a wiring board by forming the. Furthermore, an AQP (All Quartz Package) structure in which a quartz crystal device is sandwiched between quartz or glass substrates and sealed. Since the AQP structure is simply sandwiched between crystal or glass substrates, a package is not required and the thickness can be reduced. If it is sealed with a low melting point glass or directly joined, outgas due to the adhesive is reduced and aging characteristics are achieved. Excellent effect.

It is a figure explaining the SAW resonator of this invention, (A) is a top view, (B) is a figure which shows the AA cross section of the figure (A). It is a figure for demonstrating the cut angle of the piezoelectric substrate which comprises the SAW element piece used for the SAW resonator of this invention. It is a partial expanded sectional view which shows the cross section of IDT. The relationship between the normalized film thickness and the line occupation ratio H / λ · mr and the apex temperature Tp when the cut angle θ of the SAW resonator of the present invention is −50 ° to −63 ° is shown. The relationship between the normalized film thickness and the product of line occupancy H / λ · mr and the secondary temperature coefficient b when the cut angle θ of the SAW resonator of the present invention is −50 ° to −63 ° is shown. It is a figure explaining the SAW oscillator using the SAW resonator of this invention. It is a figure explaining the SAW device using the conventional ST cut quartz substrate.

Explanation of symbols

10 ......... SAW element piece, 12 ...... Piezoelectric substrate, 14 ... Interdigital electrode (IDT), 16 (16a, 16b) ......... Bus bar, 18 (18a, 18b) ... I / O pad, 20 (20a, 20b) ......... Electrode finger, 22 (22a, 22b) ......... Grating reflector, 100 ... SAW resonator, 110 ......... Package, 112 ......... Base, 114 (114a, 114b) ) ... Mounting terminals, 116 (116a, 116b) ... Metal wires, 120 ... Lids.

Claims (2)

A surface acoustic wave device comprising a piezoelectric substrate and an IDT formed on the piezoelectric substrate, wherein the excitation wave is an SH wave,
In the piezoelectric substrate, the cut angle θ is the rotation angle of the crystal Z axis with the crystal X axis as the rotation axis, and the direction of rotation from the crystal + Z axis to the crystal + Y axis side is the rotation direction in which the cut angle θ is negative. , A rotated Y-cut quartz substrate made of a quartz plate that is set in a range of −62 ° ≦ θ ≦ −51 ° and the propagation direction of the surface acoustic wave is 90 ° ± 5 ° with respect to the crystal X axis,
The IDT is formed of W or an alloy containing W as a main component,
When the wavelength of the surface acoustic wave to be excited is λ, the electrode film thickness is H, and the line occupancy of the electrode fingers constituting the IDT is mr, the standardized film thickness H / λ based on the wavelength of the surface acoustic wave The product of the electrode finger line occupation ratio mr is
0.001 ≦ H / λ · mr <0.027
A surface acoustic wave device characterized by being determined to be within the range of A composite device of an oscillation circuit and a module using the surface acoustic wave device according to claim 1 .

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