JP2007267033A - Surface acoustic wave element and surface acoustic wave device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress variations in temperature characteristics to be suppressed in a SAW device, and to enable miniaturization to be suited for mass production. <P>SOLUTION: A single type IDT 13 composed of crossed-finger electrodes 12a, 12b each having thickness H to be H/λ≤0.085 relating to wave length λ of the SAW is to be formed on a main surface of a crystal substrate 11 composed of an in-plane rotation ST cut crystal having an Eiler angle to be indicated by (0°,θ,0°<¾Ψ¾<90°), preferably by (0°,θ,9°<¾Ψ¾<46°), more preferably by (0°,95°<θ<155°,33°<¾Ψ¾<46°). Thereby good temperature characteristics for making frequency variation width Δfab suppressed to be equal to or lower than 25 ppm within a usage temperature range (0 to 70°C) including variations in manufacturing can be acquired by exciting in an upper bound mode of a stop band. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a SAW element using a surface acoustic wave (SAW) and a SAW device having such a SAW element.

  Conventionally, SAW devices such as resonators, filters, oscillators, and the like having SAW elements using surface acoustic waves excited by IDTs (interdigital transducers) formed of interdigitated electrodes formed on the surface of a piezoelectric substrate are various electronic devices. Widely used. Recently, due to the development of mobile communication devices and the like, miniaturization is required along with higher frequency and higher accuracy of SAW devices that support higher communication speeds.

  A crystal having high temperature stability is generally used for the piezoelectric substrate of the SAW element, and among them, an ST-cut quartz plate having a good frequency temperature characteristic is often used. As illustrated in FIG. 3, three orthogonal crystal axes of crystal are an electric axis (X axis), a mechanical axis (Y axis), and an optical axis (Z axis), and Euler angles (φ, θ, ψ) are ( ST cut out a new crystal axis (X, Y ′, Z ′) obtained by rotating the crystal Z plate 1 of 0 °, 0 °, 0 °) around the X axis by an angle θ = 113 to 135 °. This is a cut crystal plate 2.

  In order to improve the frequency temperature characteristics of the SAW element, an in-plane rotated ST-cut quartz plate 3 obtained by rotating the ST-cut quartz plate 2 by an angle ψ about the Z ′ axis is known. Since the temperature characteristics of the in-plane rotating ST-cut quartz plate 3 are expressed by a cubic function, the Euler angles are set to (0 °, θ = 113 to 135 °, ψ = ± (40 ° to 49 °)). In addition, a SAW device has been proposed in which the temperature characteristic is rotated around an inflection point to adjust the frequency fluctuation range in the operating temperature range to a minimum (see, for example, Patent Document 1).

  Furthermore, in the SAW device using the in-plane rotating ST-cut quartz plate described in Patent Document 1, the ratio of the electrode width to the electrode pitch of the IDT is set to 0 in order to reduce the variation of the resonance frequency with respect to the variation in the electrode width of the IDT. It is known that it is set to .5 or more and 0.65 or less (for example, see Patent Document 2).

  On the other hand, a typical SAW excited by the IDT is a Rayleigh wave, but it is known that two frequency solutions called stop bands can be obtained by calculation. The SAW element in which the Rayleigh wave is excited uses either the lower frequency (lower limit mode) or the higher frequency (upper limit mode). Comparing these frequencies, the absolute value of the secondary temperature coefficient of the frequency temperature characteristic is smaller in the upper limit mode than in the lower limit mode, and the absolute value of the secondary temperature coefficient is increased when the IDT electrode film thickness is increased. It is known that the change is small (see, for example, Non-Patent Document 1). Therefore, it has been found that the upper limit mode has better frequency temperature characteristics and is suitable for higher frequencies.

  However, the SAW element using the ST cut quartz substrate is excited only in the lower limit mode, not the upper limit mode of the stop band, when the IDT is a single type in which two electrode fingers are provided in one wavelength of SAW. The Therefore, a SAW device having a reflection inversion type IDT electrode in which three or more electrode fingers are provided in one SAW wavelength and enabling excitation in an upper limit mode of a stop band is known (for example, , See Patent Document 3).

  In addition, a SAW element in which an IDT and reflectors on both sides thereof are provided on an ST-cut quartz substrate has a reflection coefficient per electrode of a reflector formed of aluminum of about 0.02, and in order to obtain high reflection efficiency. There is a problem that the number of electrodes increases and hinders downsizing. Therefore, an ST-cut quartz substrate with Euler angles (0 °, 113 ° to 135 °, 0 °) or in-plane rotation with (0 °, 113 ° to 135 °, ± (40 ° to 49 °)) A SAW chip that uses a ST-cut quartz substrate to excite Rayleigh waves and that is made of a metal material heavier than the metal material that forms the IDT, thereby reducing the chip size and improving the frequency temperature characteristics. It is known (see, for example, Patent Document 4).

IEICE Technical Report, The Institute of Electronics, Information and Communication Engineers, US99-20 (1999-06), p. 37-42 JP 2003-152487 A JP 2003-258601 A JP 2002-100959 A JP 2005-184340 A

  In general, in a SAW element using an ST cut quartz substrate, the oscillation frequency is determined by the electrode pitch of the IDT, and it is necessary to reduce the electrode pitch of the IDT in order to increase the frequency. In the IDT design, when the electrode pitch is made smaller, the electrode width and the electrode film thickness are usually made smaller at the same time. However, this is not preferable because the resistance value of the IDT also increases and the impedance of the SAW element increases.

  On the other hand, if the electrode film thickness of the IDT is increased, a problem that is well known to those skilled in the art that the oscillation frequency generally decreases greatly occurs. Therefore, if the electrode width of the IDT is further reduced to increase the frequency, the manufacturing process using wet etching generally employed may cause a problem of processing accuracy, which may reduce the manufacturing yield and increase the cost. is there. In particular, in the case of the SAW element having the reflection inversion type IDT described above, it is necessary to make the electrode width finer than that of the single type IDT in order to increase the frequency. In addition, by increasing the electrode film thickness of the IDT, the absolute value of the second-order temperature coefficient in the frequency temperature characteristic increases, and as a result, the temperature stability inherent to the quartz substrate may be impaired (for example, non-patent) Reference 1).

  Further, the SAW element using the in-plane rotating ST-cut quartz plate described in Patent Document 1 has a reflection coefficient of about 0.015 per electrode and is smaller than that of the ST-cut quartz plate. Therefore, the number of electrode fingers increases, and there is a problem that it is difficult to reduce the chip size of the SAW element.

  As a result of further various studies on the in-plane rotated ST-cut quartz plate with Euler angles (0 °, θ, ψ), the applicant of the present application appropriately selects the angle ψ, and more preferably appropriately selects the angle θ. Thus, it has been found that a Rayleigh wave can be excited in the upper limit mode of the stop band with a single type IDT. Further, in that case, it was found that the reflection coefficient of the electrode finger of the IDT is comparable to that of the ST cut quartz plate.

  Further, by utilizing the analysis method described in Non-Patent Document 1, in the in-plane rotation ST-cut quartz plate, the frequency fluctuation range Δf with respect to the temperature change in the operating temperature range (0 to 70 °), that is, the temperature characteristic is It has been found that variation occurs depending on the angle ψ. That is, it was found that when the change in the temperature characteristic was observed while changing the angle ψ in various ways, the change in the frequency fluctuation range Δf with respect to the change in the angle ψ could be minimized. It was also found that the change in the frequency fluctuation width Δf due to the change in the angle ψ is affected by the change in the electrode film thickness of the IDT. In this case, with respect to the change in the angle ψ, the variation in Δf increases as the electrode film thickness of IDT increases, and conversely, the variation in Δf decreases as the electrode film thickness of IDT decreases.

  On the other hand, in the mass production of industrial SAW elements, it is a matter of course that an allowable error range, that is, tolerance, must be taken into consideration. As described above, the frequency fluctuation width Δf due to the change in the in-plane rotation angle ψ is greatly affected by the electrode film thickness of the IDT. It is difficult to realize a SAW element that suppresses variations in temperature characteristics.

  Accordingly, an object of the present invention is to eliminate the problems of the prior art. The inventors of the present invention have made various studies on the SAW element using the in-plane rotated ST-cut quartz plate as described above, and related to the frequency temperature characteristics, the in-plane rotation angle ψ, the IDT electrode pitch, and the film thickness. As a result of sufficiently examining the mutual relationship between the line width and the line width, the present invention has been devised based on the knowledge. Accordingly, an object of the present invention is to suppress variations in temperature characteristics due to fluctuations in the angle ψ, while taking into account manufacturing errors and variations, especially in a SAW device using an in-plane rotated ST-cut quartz plate, and in manufacturing. An object of the present invention is to provide a SAW element and a SAW device that can reduce the burden of the device and can be downsized suitable for mass production.

  According to the present invention, in order to achieve the above object, a quartz substrate comprising an in-plane rotating ST-cut quartz plate and an IDT comprising at least one pair of interdigitated electrodes formed on the main surface of the quartz substrate are provided. A SAW element is provided in which the thickness H of the interdigitated electrode is set to H / λ ≦ 0.085 with respect to the wavelength λ of the SAW excited by the IDT.

  As described later, the inventors of the present application use the in-plane rotating ST-cut quartz plate and set the electrode film thickness ratio H / λ of the IDT with respect to the wavelength of the SAW. The frequency temperature characteristics of the SAW element, that is, the frequency fluctuation range can be suppressed to a small level. Thereby, the dispersion | variation in a temperature characteristic can be suppressed with respect to the fluctuation | variation of the electrode film thickness of IDT in a use temperature range.

  In one embodiment, by using a quartz substrate with Euler angles (0 °, θ, 0 ° <| ψ | <90 °), it is possible to excite the SAW element in the stopband upper limit mode, Thus, the frequency drop can be suppressed even if the electrode film thickness is increased. Therefore, it is possible to realize a SAW element that has excellent frequency temperature characteristics, can suppress variations thereof, and can achieve high frequency and high accuracy.

  In another embodiment, by setting the Euler angles of the quartz substrate to (0 °, θ, 9 ° <| ψ | <46 °), further excellent frequency temperature characteristics can be exhibited, and the amount of frequency fluctuation with respect to temperature change Can be made smaller. In still another embodiment, even better frequency temperature characteristics can be obtained by setting the Euler angles of the quartz substrate to (0 °, 95 ° <θ <155 °, 33 ° <| ψ | <46 °). Can do.

  According to another aspect of the present invention, by providing the above-described SAW element of the present invention, a SAW device having excellent frequency temperature characteristics, capable of high frequency and high accuracy, and suitable for miniaturization is provided. The

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1A shows a SAW element 10 for a one-port resonator to which the present invention is applied. The SAW element 10 has a rectangular quartz substrate 11 made of the in-plane rotating ST-cut quartz plate described above with reference to FIG. On the main surface of the quartz substrate 11, an IDT 13 for SAW excitation comprising a pair of crossed finger electrodes 12 a and 12 b is formed at substantially the center, and lattice-like reflectors 14 and 14 are formed on both sides in the longitudinal direction. Has been. The cross finger electrode and the reflector are formed of an electrode film made of Al from the viewpoint of workability and cost. In another embodiment, an Al / Cu film can be used as the electrode film.

  FIG. 1B shows a cross section of the crossed finger electrodes 12a and 12b along the SAW propagation direction. In the present embodiment, when the thickness of the cross finger electrode is H and the wavelength of the SAW excited by the IDT 13 is λ, the ratio thereof, that is, the thickness ratio H / λ is preferably 0.085. The IDT 13 is formed so as to be 0.06 to 0.07. By setting the film thickness ratio in this way, the change in the frequency fluctuation range with respect to the fluctuation of the film thickness H is reduced to about 25 ppm in the predetermined operating temperature range (0 to 70 ° C.), thereby suppressing the variation in temperature characteristics. can do. Furthermore, unlike the conventional SAW device using the in-plane rotating ST-cut quartz plate of Patent Document 1 described above, the reflection coefficient of the interdigital electrodes 12a and 12b can be made as large as that of the ST-cut quartz plate.

  In this embodiment, if the electrode pitch of the IDT 13, that is, the center distance between adjacent crossed finger electrodes is p and the line width in the SAW propagation direction of the electrodes 12 a and 12 b is d, the ratio η = d / p is 0 It is preferable to set to about .8. As a result, even when the SAW element 10 itself is downsized or the IDT 13 is narrowed, when the IDT is patterned on the quartz substrate 11, the line width of the crossed finger electrode is excessively narrowed, resulting in reduced processing accuracy. The possibility of disconnection can be eliminated or reduced. Therefore, the miniaturization of the quartz substrate 11, that is, the SAW element 10, does not impose an excessive burden on the formation of the IDT in manufacturing.

  In the case of the in-plane rotating ST-cut quartz plate as in the present invention, if the ratio η between the electrode line width and the electrode pitch of the IDT is increased as in the present embodiment, the frequency fluctuation width with respect to the temperature change is increased. (For example, refer to Patent Document 2). However, according to the present invention, by setting the film thickness ratio of the IDT as described above, it is possible to suppress the variation in the frequency fluctuation range even when the manufacturing error and variation are taken into consideration, and to obtain a favorable temperature characteristic. Can be maintained.

  The quartz substrate 11 of this embodiment is an in-plane rotated ST-cut quartz plate with Euler angles (0 °, θ, 0 ° <| ψ | <90 °), and the X′-axis direction coincides with the SAW propagation direction. To be formed. As a result, in addition to suppressing variation in temperature characteristics, it is possible to excite in the upper limit mode of the stop band in the single type IDT as in this embodiment, and even if the electrode film thickness is increased, the frequency drop Can be suppressed. Therefore, the electrode pitch can be further reduced to achieve higher frequency and higher accuracy.

  The quartz substrate 11 more preferably has an Euler angle (0 °, θ, 9 ° <| ψ | <46 °), and further exhibits a frequency temperature characteristic superior to that of a conventional ST-cut quartz plate. An effect of reducing the amount of frequency fluctuation with respect to the change can be obtained. More preferably, the Euler angles of the quartz substrate 11 are (0 °, 95 ° <θ <155 °, 33 ° <| ψ | <46 °), and even better frequency temperature characteristics can be obtained.

  The SAW device of this embodiment shown in FIG. 1 in which an IDT is formed on an in-plane rotating ST-cut quartz substrate with Euler angles (0 °, 95 ° <θ <155 °, 33 ° <| ψ | <46 °). The frequency-temperature characteristic was simulated using the analysis method described in Non-Patent Document 1 above. The results are shown in FIGS.

  FIG. 2 shows changes in the frequency fluctuation width Δf with respect to changes in the film thickness ratio H / λ in the operating temperature range (0 to 70 ° C.) when the electrode line width to electrode pitch ratio η = 0.8 of the IDT is set. Show. In the figure, Δf indicated by a solid line is a case where the film thickness ratio H / λ is manufactured at an ideal optimum value that does not include manufacturing errors and variations such as the electrode film thickness H of the IDT. On the other hand, Δf indicated by a broken line is a case where the film thickness ratio H / λ is assumed to have a variation width of 2% due to manufacturing errors and variations in IDT. Therefore, the difference in the numerical value between the solid line and the broken line indicates the influence of the fluctuation width of the film thickness ratio H / λ on the frequency fluctuation width Δf due to manufacturing errors and variations. Moreover, p0 has shown the case where the conventional ST cut quartz plate is used.

  From the same figure, when η = 0.8, in the ideal case where there is no fluctuation width of the film thickness ratio H / λ, the frequency fluctuation width Δf is over the entire range of H / λ = 0.045 to 0.105. It can be seen that the amount is stably suppressed to 10 ppm or less. On the other hand, when the fluctuation range of the film thickness ratio H / λ is 2%, the frequency fluctuation range Δf is stably reduced to 20 ppm or less when H / λ = 0.06 to 0.07. However, when H / λ> 0.085, there is a tendency to begin to increase rapidly. Regardless of the fluctuation range of the film thickness ratio H / λ, it is considered that the influence of this on the change of the frequency fluctuation range Δf shows the same tendency as the broken line in FIG. Therefore, the frequency fluctuation width Δf is more stable than the case of using the conventional ST-cut quartz plate in the film thickness ratio H / λ ≦ 0.085, even if the fluctuation width due to IDT manufacturing error and variation is taken into consideration. And it turns out that it suppresses favorably.

  3 and 4 show changes in the frequency fluctuation width Δf with respect to changes in the film thickness ratio H / λ in the operating temperature range when the electrode line width to pitch ratio η = 0.7 and 0.6, respectively. Show. 3 and 4, the solid line is Δf when the film thickness ratio H / λ is manufactured with an ideal optimum value that does not include manufacturing errors and variations of IDT, whereas the broken line indicates the film thickness. The ratio H / λ is Δf when it is assumed that the fluctuation range due to IDT manufacturing error and variation is 2%. In these cases, the same results as in FIG. 2 were obtained. That is, when the fluctuation range of the film thickness ratio H / λ is 0, the frequency fluctuation range Δf as a whole is stably suppressed to about 15 ppm or less. On the other hand, when the variation width of the film thickness ratio H / λ is 2%, the frequency variation width Δf tends to increase rapidly when H / λ> 0.085. From these results, according to the present invention, even if the electrode line width to pitch ratio η is relatively large, variation in temperature characteristics can be satisfactorily suppressed with respect to changes in the electrode film thickness ratio H / λ. I understand.

  FIGS. 5 to 7 show changes in the frequency fluctuation width Δf with respect to the electrode line width to electrode pitch ratio η when the film thickness ratios H / λ = 0.08, 0.07, and 0.06, respectively. Yes. Also in each of these figures, the solid line is Δf when the film thickness ratio H / λ is manufactured at an ideal optimum value that does not include manufacturing errors and variations of IDT, and the broken line is the film thickness ratio H / λ. Is Δf when it is assumed that the fluctuation range due to manufacturing errors and variations of IDT is 2%. Further, the frequency fluctuation width Δf at the left end of each graph shown in these drawings, that is, η = 0.45, is almost the same as that when the conventional ST-cut quartz plate is used.

  5 to 7, when the fluctuation width of the film thickness ratio H / λ is 0, the frequency fluctuation width Δf is stably suppressed to 20 ppm or less as a whole. When the fluctuation width of the film thickness ratio H / λ is 2%, the frequency fluctuation width Δf fluctuates relatively large at the electrode line width to electrode pitch ratio η <0.65, whereas η = 0.65. It is stable at about 60 ppm or less at ˜0.85. As can be seen from these results, even when the electrode line width to electrode pitch ratio η is set large in the in-plane rotating ST cut quartz plate of the present invention, the temperature ratio of the IDT is set as described above. It can be seen that variation in characteristics can be satisfactorily suppressed.

  Although the preferred embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and it will be apparent to those skilled in the art that various modifications and changes can be made thereto. For example, in addition to the 1-port type and single-type IDT of the above-described embodiment, the present invention is similarly applied to SAW elements having various configurations such as those having no reflector, 2-port type, and transversal type. be able to. Needless to say, the present invention can be applied to various SAW devices such as an oscillator in addition to the resonator.

(A) The figure is a top view which shows the SAW element to which this invention is applied, (B) The figure is the elements on larger scale in the II line. FIG. 2 is a diagram showing the temperature characteristics when the electrode line width to pitch ratio is η = 0.8 in the SAW element of FIG. 1 as a frequency variation width Δfab with respect to the IDT film thickness ratio H / λ. FIG. 3 is a similar diagram of FIG. 2 showing temperature characteristics when the electrode line width to pitch ratio η = 0.7. FIG. 3 is a similar diagram of FIG. 2 showing the temperature characteristics when the electrode line width to pitch ratio η = 0.6. FIG. 2 is a diagram showing the temperature characteristics when the film thickness ratio of IDT is H / λ = 8% in the SAW element of FIG. 1 as a frequency variation width Δfab with respect to the electrode line width to pitch ratio η. FIG. 6 is a similar diagram of FIG. 5 showing temperature characteristics when the IDT film thickness ratio H / λ = 7%. FIG. 6 is a similar diagram of FIG. 5 showing temperature characteristics when the IDT film thickness ratio H / λ = 6%. Explanatory drawing which shows the in-plane rotation ST cut quartz plate used for this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Crystal Z plate, 2 ... ST cut crystal plate, 3 ... In-plane rotation ST cut crystal plate, 10 ... SAW element, 11 ... Crystal substrate, 12a, 12b ... Interstitial electrode, 13 ... IDT, 14 ... Reflector.

Claims (5)

A quartz substrate made of an in-plane rotating ST-cut quartz plate, and an IDT made of at least one pair of crossed finger electrodes formed on the principal surface of the quartz substrate,
The surface acoustic wave device according to claim 1, wherein the thickness H of the interdigitated electrode is set to H / λ ≦ 0.085 with respect to the wavelength λ of the surface acoustic wave (SAW) excited by the IDT.   2. The surface acoustic wave device according to claim 1, wherein an Euler angle of the quartz crystal substrate is (0 °, θ, 0 ° <| ψ | <90 °).   3. The surface acoustic wave device according to claim 2, wherein the Euler angles of the quartz substrate are (0 °, θ, 9 ° <| ψ | <46 °).   4. The surface acoustic wave device according to claim 3, wherein the Euler angles of the quartz substrate are (0 °, 95 ° <θ <155 °, 33 ° <| ψ | <46 °).   A surface acoustic wave device comprising the surface acoustic wave element according to claim 1.

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