JP2010088132A - Surface acoustic wave device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve electrical characteristics for an SAW device using a piezo electric substrate containing an addition by showing a more proper angle for cutting a substrate. <P>SOLUTION: The surface acoustic wave device includes a monocrystal piezo-electric substrate and an intersected finger type electrode which is formed with aluminum located on the surface of the piezo-electric substrate as a main composition of material. The normal film thickness h/λ, which the thickness h of the intersected finger type electrode is normalized by the electrode interval λ of the intersected finger type electrode, is 7-11%, and the monocrystal piezo-electric substrate is a substrate of lithium tantalate, contains iron as an addition, and has a bearing which rotates an X axis as a center in a range 46°±0.3° in the direction from a Y axis to a Z axis. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a surface acoustic wave device, and more particularly to a substrate cut angle suitable when a surface acoustic wave device is configured by adding an additive to a single crystal piezoelectric substrate.

  Surface acoustic wave devices (hereinafter referred to as SAW devices) that use surface acoustic waves (SAW) generated by the piezoelectric effect are small, light, and have excellent reliability. Therefore, they are used as electronic devices such as filters and resonators. Widely used in mobile phones and other electronic devices.

Such a SAW device generally receives an electrode that excites a surface acoustic wave by applying a high-frequency signal, and a surface acoustic wave that is propagated by being excited by the electrode, and converts it into an electric signal again to generate a specific frequency. It is possible to form a signal processing device such as a filter, a resonator, a duplexer or the like that extracts a signal of a specific frequency by forming an electrode for extracting the above signal on a piezoelectric substrate.
Various proposals have been made on the cut angle of the piezoelectric substrate to be used in order to obtain optimum electrical characteristics as a filter or a resonator (see the following patent document).

JP-T-2004-507960 JP-A-9-167936

  By the way, a piezoelectric substrate generally has both pyroelectricity and pyroelectricity. For this reason, non-uniform charge distribution occurs on the substrate surface as the temperature changes, and the electrical characteristics of the device deteriorate, or discharge occurs between minute IDT electrodes due to charge accumulation, and the electrode fingers are damaged, Furthermore, electrostatic breakdown may occur.

  Therefore, as one method for solving the problems associated with pyroelectricity, a trace additive such as iron (Fe) may be added to the piezoelectric substrate. By adding an additive, the volume resistivity of the piezoelectric substrate is lowered, charge accumulation is prevented, and pyroelectric characteristics are improved.

  However, as a result of various examinations on the electrical characteristics of the SAW device using the piezoelectric substrate containing the trace additive as described above, the present inventor found that the optimum substrate cut angle is different between the substrate containing the additive and the substrate not containing the additive. I found it. That is, in the case of a substrate containing the additive as described above, it is not appropriate to use the conventional design method for the substrate cut angle (for the substrate not containing the additive) as it is, and it is specific to the substrate containing the additive. There are suitable cut angles. Therefore, the SAW device using the substrate containing the additive has room for further improving the electrical characteristics in this respect.

  Accordingly, an object of the present invention is to provide a more appropriate cut angle for a SAW device using a piezoelectric substrate containing an additive, and to further improve the electrical characteristics of the SAW device.

  In order to solve the above problems and achieve the object, the SAW device according to the present invention comprises a single crystal piezoelectric substrate and a material (Al or Al alloy) mainly composed of aluminum (Al) provided on the surface of the piezoelectric substrate. A SAW device including a formed IDT (intersecting finger electrode), wherein the single crystal piezoelectric substrate includes an additive, and is 42 ° to 48 in the Z-axis direction from the Y-axis centering on the X-axis. It has an orientation rotated at an angle in the range of ° (more preferably 46 ° ± 0.3 °).

  When a SAW device (for example, a SAW filter) is configured using a piezoelectric substrate containing an additive, if the cut angle of the piezoelectric substrate is set as described above, a high Q value and a low insertion loss in the passband can be obtained. A SAW device having good electrical characteristics can be realized. This point will be further described based on the measurement results (FIGS. 3 to 5) in the description of the later embodiments.

The piezoelectric substrate is typically a lithium tantalate (LiTaO ₃ / hereinafter referred to as LT) substrate or a lithium niobate (LiNbO ₃ / hereinafter referred to as LN) substrate. However, the present invention is not necessarily limited to these, and quartz or other piezoelectric substrates may be used.

On the other hand, examples of the additive include iron (Fe), manganese (Mn), copper (Cu), and titanium (Ti). Two or more of these may be included. Of these, Fe is particularly preferable in that it can be easily mixed uniformly with the piezoelectric substrate (LT, LN) and a good crystal quality can be obtained. In addition, when using Fe as said additive, it is desirable that the addition rate of the additive shall be 1.24 wt% or less. In this case, a piezoelectric substrate having a volume resistivity of 3.6 × 10 ¹⁰ Ω · cm or more is obtained. When the Fe addition rate exceeds 1.24 wt%, the volume resistivity of the piezoelectric substrate is excessively reduced, so that electricity easily flows to the substrate, the electrode fingers of the IDT 12 are short-circuited, and the electrical characteristics deteriorate.

  Piezoelectric substrates generally have pyroelectricity as well as piezoelectricity, so nonuniform charge distribution occurs on the surface of the substrate with changes in temperature, and when this charge is accumulated, it is used after the SAW device is manufactured or mounted on various electronic devices. In actual use, there is a risk of causing characteristic deterioration and damage to the electrode due to discharge of the IDT portion. On the other hand, when an additive such as Fe is added to the piezoelectric substrate as described above, the volume resistance of the substrate can be reduced, and this problem can be avoided to manufacture a highly reliable SAW device. In addition, by setting the cut angle of the substrate within the above range according to the present invention, a SAW device having excellent electrical characteristics can be realized.

  Although the pyroelectric characteristics can be improved by the addition of the additive as described above, the present invention does not limit the purpose of the additive to the improvement of the pyroelectric characteristics. That is, even if it is added for other purposes than improvement of the pyroelectric characteristics, if the additive is included as a result, it corresponds to the above-described piezoelectric substrate in the present invention. Even if it is such a board | substrate, it is because there is no change in containing an additive, and since the favorable electrical property which this invention intends similarly is provided by providing the said cut angle which this invention shows.

  As for the external dimensions of the electrode (IDT) formed on the surface of the piezoelectric substrate, for example, the normalized film thickness h / λ, which is a value obtained by normalizing the IDT thickness h by the electrode interval λ of the IDT, is approximately 7 to 11. % (H / λ≈0.07 to 0.11) is preferable. However, the external dimension of the IDT in the present invention is not necessarily limited to the range, and other values (for example, h / λ = 1% to 15%) may be adopted.

  The SAW device of the present invention includes various devices using surface acoustic waves generated on a piezoelectric substrate such as an IDT or a SAW resonator including a reflector and a SAW filter, a SAW filter, and a SAW duplexer.

  According to the present invention, when a SAW device is configured using a piezoelectric substrate to which an additive has been added, a more appropriate substrate cut angle can be presented, and good electrical characteristics (high Q, low insertion loss, etc.) It is possible to obtain a SAW device provided with

  Other objects, features and advantages of the present invention will become apparent from the following description of embodiments of the present invention.

It is a mimetic diagram showing a SAW device concerning one embodiment of the present invention. It is a conceptual diagram explaining the cutting angle of a single crystal piezoelectric substrate. And LiTaO ₃ single-crystal substrate cut angle with the addition of Fe as an additive, is a graph showing the relationship between the resonant resistance. And the cut angle of the LiTaO ₃ single crystal substrate added with Fe as an additive, is a graph showing the relationship between the anti-resonance resistance. And LiTaO ₃ single-crystal substrate cut angle with the addition of Fe as an additive, resonant resistance - is a graph showing the relationship between the anti-resonance resistance ratio.

FIG. 1 is a conceptual diagram showing a SAW device according to an embodiment of the present invention. As shown in the figure, this SAW apparatus is provided with an IDT (interstitial finger electrode) 12 on the surface of an LT (LiTaO ₃ ) single crystal substrate 11, and Fe is added to the LT substrate 11 as a trace additive. It is. As a method for adding Fe, for example, when a single crystal constituting a substrate (wafer) is generated in the Czochralski method, a method such as adding Fe to a molten LT material may be used.

By adding Fe, the volume resistivity of the LT substrate 11 is lowered. In this case, it is desirable that the volume resistivity of the LT substrate 11 be in the range of 3.6 × 10 ^{10 to} 1.5 × 10 ¹⁴ Ω · cm by adjusting the addition amount of Fe. By setting the volume resistivity to 1.5 × 10 ¹⁴ Ω · cm or less, accumulation of electric charges on the LT substrate 11 can be prevented, and the electrode 12 can be prevented from being damaged or electrostatically damaged by electric discharge. This is to prevent the electrode fingers of the IDT 12 from being short-circuited by setting the resistivity to 3.6 × 10 ¹⁰ Ω · cm or more.

  FIG. 2 is a conceptual diagram showing the cutting angle (cut angle) of the LT substrate. An LT single crystal having crystal axes X, Y, and Z is rotated around the crystal axis X by a rotation angle θ from the Y axis to the Z axis. The state cut out by the rotated angle is shown. If such a single crystal piezoelectric substrate is referred to as a θ rotation YX substrate, in this embodiment, a substrate having θ of 46 °, that is, a 46 ° rotation YX substrate is used as the LT substrate 11. Since actual manufacturing includes manufacturing errors (tolerances), θ in the present embodiment is 46 °, which is 46 ° ± 0.3 ° (45.7 ° ≦ θ ≦ 46.3 °). Includes angles within range. In the present embodiment, θ is set to 46 ° based on the following reason.

  FIG. 3 shows the relationship between the cutting angle (cut angle) θ of the substrate and the resonance resistance value of the resonator when a resonator is formed on a θ-rotation YX substrate of an LT single crystal containing Fe as an additive. FIG. 4 shows the relationship between the substrate cut angle θ and the antiresonance resistance value, and FIG. 5 shows the relationship between the substrate cut angle θ and the resonance resistance-antiresonance resistance ratio. The electrodes of the resonator (IDT) used for the measurement are made of Al, and the normalized film thickness h / λ (h is the electrode thickness and λ is the electrode interval) are 7%, 9% and 11%. It was. In each of the diagrams of FIGS. 3 to 5, the case where h / λ = 7% is set as “●”, and the case where h / λ = 9% is set as “Δ” where h / λ = 11%. Each case is indicated by an “x” point.

  As is apparent from these diagrams, the LT single crystal substrate containing Fe has the lowest resonance resistance and the highest when the substrate cut angle θ is 46 ° compared to 42 °, 44 ° and 48 °. It is found that anti-resonance resistance as well as resonance resistance-anti-resonance resistance ratio can be obtained. Therefore, in the SAW device of this embodiment, the substrate cut angle θ is set to 46 ° as a particularly preferable configuration example. As a result, a SAW resonator excellent in electrical characteristics (high Q and low insertion loss) can be formed on the surface of the LT single crystal substrate containing Fe.

  The IDT 12 is made of Al, but may be made of an Al alloy. As for the IDT electrode dimensions, the normalized film thickness h / λ is set to 7 to 11%, but other values (for example, 1% to 15%) may be adopted. Although not shown in FIG. 1, the surface of the substrate 11 is provided with a reflector, a conductor line, an input / output electrode pad, etc. in addition to the IDT 12. Furthermore, a plurality of resonators can be formed on the LT substrate 11 to constitute a SAW filter, a SAW duplexer, etc., but various connection structures, IDT electrode arrangement structures, etc. are employed. Is possible and is not particularly limited.

  As mentioned above, although embodiment of this invention was described, this invention is not limited to these, It is clear to those skilled in the art that a various change can be made within the range as described in a claim. .

11 Fe-added LT single crystal substrate 12 IDT electrode

Claims (1)

A surface acoustic wave device comprising a single crystal piezoelectric substrate and an interdigitated electrode formed of a material mainly composed of aluminum provided on the surface of the piezoelectric substrate,
The normalized film thickness h / λ obtained by normalizing the thickness h of the interdigital electrode with the electrode interval λ of the interdigital electrode is 7 to 11%,
The single crystal piezoelectric substrate is a lithium tantalate substrate, contains iron as an additive, and is rotated at an angle in the range of 46 ° ± 0.3 ° from the Y axis to the Z axis around the X axis. A surface acoustic wave device characterized by having an orientation.

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