JP2006186623A - Surface acoustic wave element, manufacturing method thereof, and surface acoustic wave device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an SAW element along with its manufacturing method capable of keeping desired frequency characteristics and performance even if film thickness of an electrode of IDT is increased. <P>SOLUTION: Relating to the surface acoustic wave element, an Al electrode film 15 of thickness H, where H/λ=0.05 to 0.15 for wavelength λ of SAW, is formed on the main surface of a quartz substrate 11 consisting of an in-plane rotation ST cut quartz plate. A resist pattern 16 is formed on the electrode film. The exposed portion of the electrode film is dry-etched to remove a part of it while a remaining part 15a is totally removed by wet-etching, thus forming interdigital electrodes 12a and 12b of IDT13. Here, line widths d1 and d2 at a lower and upper sides of the interdigital electrode are so set that (d1-d2)/electrode pitch p=0 to 0.16 in the propagation direction of SAW. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a SAW element using a surface acoustic wave (SAW), a manufacturing method thereof, 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. In particular, in recent years, in the field of communication equipment and the like, there has been a demand for higher frequency and higher accuracy of SAW devices corresponding to higher communication speeds.

  The IDT of the SAW element is formed by patterning an electrode film such as Al formed on the surface of the piezoelectric substrate using a photolithography technique. When the electrode film is patterned by wet etching, there is a problem that undercuts occur due to a large amount of side etching, the side surfaces of the electrode are tapered, and the line width is difficult to control and tends to vary. On the other hand, when dry etching such as reactive ion etching is performed, the surface of the piezoelectric substrate may be damaged and vibration characteristics may be adversely affected. In order to solve this problem and pattern the electrode with high accuracy, a method has been proposed in which the electrode film is first partially removed by dry etching and the remaining portion is completely removed by wet etching (for example, Patent Documents 1 and 2).

  As a piezoelectric substrate of a SAW element, 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 ( A crystal Z plate 1 of 0 °, 0 °, 0 °) cut out along a new coordinate axis (X, Y ′, Z ′) obtained by rotating an angle θ = 113 to 135 ° around the X axis, This is an ST cut quartz plate 2. Since the ST cut quartz plate has a temperature characteristic of a cubic function, the applicant of the present application further rotates the ST cut quartz plate 2 around the Z ′ axis by an angle ψ = ± (40 to 49) ° to propagate the SAW. In-plane rotation ST-cut quartz plate 3 that exhibits better frequency temperature characteristics is proposed (see Patent Document 3).

JP-A 63-285011 JP-A-10-135759 JP 2003-152487 A

  Further, the applicant of the present invention appropriately stops the Rayleigh wave by the single IDT by appropriately selecting the angle ψ, more preferably appropriately selecting the angle θ in the Euler angle (0 °, θ, ψ) quartz substrate. We found that it can be excited in the upper band mode. Thereby, in addition to further improving the frequency temperature characteristics, the frequency drop can be suppressed even if the film thickness of the electrode is increased, which is suitable for miniaturization and higher frequency of the electrode. However, in the electrode patterning according to the conventional method described above, as the electrode film thickness increases, the side surface of the electrode is easily tapered, and it becomes more difficult to control the electrode line width with high accuracy. In particular, when the line width causes a large difference between the lower side and the upper side due to the taper of the electrode side surface, the reflection efficiency is lowered, and the CI value and Q value of the SAW element are lowered.

  Therefore, the present invention has been made in view of the problems of the prior art, and the object thereof is to substantially reduce the desired frequency characteristics and performance even if the film thickness of the IDT electrode is increased. An object of the present invention is to provide a SAW element that can be maintained and secured and a method for manufacturing the SAW element.

  A further object of the present invention is to provide a SAW element having excellent frequency temperature characteristics, high stability, high frequency and high accuracy, and a method for manufacturing the SAW element.

  According to the present invention, in order to achieve the above object, a quartz substrate made of an in-plane rotated ST-cut quartz plate and at least one set of interdigital electrodes formed on the principal surface of the quartz substrate with a predetermined pitch p. And the thickness H of the interdigitated electrode is set to be H / λ = 0.05 to 0.15 with respect to the wavelength λ of the SAW excited by the IDT, in the SAW propagation direction. A SAW element in which the line widths d1 and d2 on the lower side and the upper side of the cross finger electrode are set to be (d1−d2) / p = 0 to 0.16 is provided.

  Ideally, it is desirable that the line widths of the cross finger electrodes coincide with each other along the SAW propagation direction. The inventor of the present application sets the line widths d1 and d2 on the lower side and the upper side of the cross finger electrode within the above range in the SAW element using the quartz substrate made of the in-plane rotated ST-cut quartz plate. It has been found that even if the film thickness of the electrode is increased, the desired frequency characteristics and performance can be maintained and secured without substantially degrading, and the present invention has been created. As a result, the electrode pitch can be further reduced to achieve higher frequency and higher accuracy.

  Conventionally, in patterning of the crossed finger electrodes, the electrode pitch is very stably and accurately controlled. By using this as a reference, the line widths at the lower and upper sides of the crossed finger electrode can be set more accurately. And can be formed.

  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 having excellent frequency temperature characteristics, high stability, high frequency and high accuracy.

  In another embodiment, by setting the Euler angles of the quartz substrate to (0 °, θ, 9 ° <| ψ | <46 °), a frequency temperature characteristic superior to that of a conventional ST-cut quartz plate is exhibited, The frequency fluctuation amount with respect to the temperature change can be reduced, and in another embodiment, the Euler angles of the crystal substrate are set to (0 °, 95 ° <θ <155 °, 33 ° <| ψ | <46 °). As a result, even better frequency temperature characteristics can be obtained.

  According to another aspect of the present invention, a method of manufacturing a SAW element suitable for manufacturing the SAW element of the present invention is provided. In this manufacturing method, an electrode film having a thickness H is formed on the main surface of the quartz substrate so that H / λ = 0.05 to 0.15 with respect to the wavelength λ of SAW excited by the IDT. A step of patterning a resist material applied on the electrode film to form a resist pattern of a crossed finger electrode; a portion of the electrode film exposed from the resist pattern is dry-etched to remove a part thereof; and an electrode film Are completely removed by wet etching, so that the line widths d1 and d2 on the lower side and the upper side along the SAW propagation direction are (d1−d2) / p = 0 to 0.16. A step of forming such that

  The electrode film can be formed by controlling the film thickness H relatively accurately, for example, by vapor deposition or sputtering. In the dry etching of the electrode film, the etching amount can be controlled relatively accurately, and the etching rate of the wet etching of the remaining part of the electrode film is also known in advance by the etching solution used. Therefore, according to the present invention, the line widths d1 and d2 of the upper and lower sides of each cross-finger electrode are calculated in advance by calculating the etching amounts of dry etching and wet etching with respect to the thickness of the deposited electrode film. Can be formed in a desired range relatively accurately and easily.

  Furthermore, according to another aspect of the present invention, there is provided a SAW device that has the above-described surface acoustic wave device of the present invention and has excellent frequency temperature characteristics, high stability, high frequency, and high accuracy. Provided.

  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. As shown in the figure, the side surfaces of the electrodes 12a and 12b are tapered by the patterning process. In this embodiment, when the electrode pitch of the IDT 13, that is, the center distance between the adjacent interdigitated electrodes is p, and the line widths of the lower and upper sides of the electrodes 12 a and 12 b in the SAW propagation direction are d1 and d2, respectively (lower side d1− The IDT 13 is formed so that the upper side d2) / electrode pitch p = R preferably falls within the range of 0 to 0.16, preferably about 0.1. The case of R = 0 is an ideal case where the lower side d1 = the upper side d2.

  Conventionally, in the patterning of the interdigitated electrodes in the manufacture of SAW elements, the electrode pitch can be controlled very stably and accurately. Therefore, by using the electrode pitch as a reference, the line widths d1 and d2 on the lower side and the upper side of the cross finger electrode can be set and formed more accurately.

  Similarly, the reflectors 14 and 14 are also formed so as to have the same taper angle as the crossed finger electrodes 12a and 12b of the IDT 13 in the cross section along the SAW propagation direction.

  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. Thereby, 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 preferably has an Euler angle (0 °, θ, 9 ° <| ψ | <46 °), thereby exhibiting a frequency temperature characteristic superior to that of a conventional ST-cut quartz plate, The amount of frequency fluctuation with respect to the change can be reduced. 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.

  Next, a process of forming the IDT 13 of FIG. 1 will be described with reference to FIG. First, as shown in FIG. 2A, an Al electrode film 15 is formed on the surface of the quartz substrate 11 to have a predetermined film thickness H by vapor deposition or sputtering. A resist material is applied on the Al electrode film 15 and patterned by a photolithography technique to form a resist pattern 16. A part of the Al electrode film 15 exposed from the resist pattern 16 is removed by dry etching such as reactive ion etching (FIG. 2B). Next, the remaining portion 15a of the Al electrode film 15 is removed by wet etching using an appropriate etching solution (FIG. 2C). Finally, when the resist pattern 16 is removed, an IDT 13 including the interdigital electrodes 12a and 12b having a desired structure shown in FIG. 1B is formed (FIG. 2D).

  The film thickness H of the Al electrode film 15 is controlled relatively accurately by vapor deposition or sputtering, the etching amount of the Al electrode film 15 by dry etching can be controlled relatively accurately, and wet etching of the Al electrode film remaining portion 15a is also used. The etching rate is known in advance by the etching solution. In the present embodiment, as shown in the drawing, the electrode film 15 is removed to the vicinity of the surface of the quartz substrate 11 by the first dry etching. However, by calculating each etching amount of dry etching and wet etching in advance, The line widths d1 and d2 of the upper and lower sides can be formed in the above-described desired range relatively accurately.

  Actually, a quartz substrate having Euler angles (0 °, 95 ° <θ <155 °, 33 ° <| ψ | <46 °) is used, and the surface is thickened according to the method of the present invention shown in FIG. An Al electrode film having a thickness of 1 μm was formed and patterned to form an IDT composed of cross-finger electrodes having an electrode pitch of 5 μm and a line width (= d1) of 3 μm. The line widths of the lower side and the upper side in the SAW propagation direction of each electrode were set as d1 and d2, and the value R of (lower side d1−upper side d2) / electrode pitch p was set within the range of 0 to 0.16. As a result, it was confirmed that the obtained SAW element had substantially the same reflection efficiency as that of an ideal R = 0, and therefore exhibited substantially the same frequency characteristics.

  Further, the SAW element made of this quartz substrate is advantageous because the frequency drop is small even if the electrode film thickness is increased as described above. When the conventional ST cut quartz plate is used, the relationship between the electrode film thickness (H) and the SAW wavelength (λ) is H / λ = about 0.03. On the other hand, the SAW element of the present embodiment can be set to H / λ = 0.05 to 0.15. Therefore, according to the present invention, even if the electrode film thickness is increased in this way, the reflection efficiency and the frequency characteristics can be maintained well.

  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. And can be used in various SAW devices other than resonators.

(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. FIGS. 4A to 4D are partial enlarged cross-sectional views similar to FIG. 1B showing steps for manufacturing the SAW element of FIG. 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, 15 ... Al electrode film, 15a ... remaining portion, 16 ... resist pattern.

Claims (6)

A quartz substrate made of an in-plane rotating ST-cut quartz plate, and an IDT made of at least one set of interdigitated electrodes formed at a predetermined pitch p on the principal surface of the quartz substrate,
The thickness H of the interdigitated electrode is set to be H / λ = 0.05 to 0.15 with respect to the wavelength λ of the surface acoustic wave (SAW) excited by the IDT,
Line widths d1 and d2 on the lower side and the upper side of the cross-finger electrode along the SAW propagation direction are set to be (d1−d2) / p = 0 to 0.16. Surface acoustic wave device.   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 °).   5. In order to manufacture the surface acoustic wave device according to claim 1, H / λ = 0.05 to the wavelength λ of the SAW excited by the IDT on the main surface of the quartz substrate. A step of forming an electrode film having a thickness H so as to be 0.15, a step of patterning a resist material applied on the electrode film to form a resist pattern of the crossed finger electrode, The exposed electrode film is dry-etched to remove a part thereof, and the remaining part of the electrode film is completely removed by wet etching, so that the cross finger electrode is moved along the propagation direction of the SAW. A method of manufacturing a surface acoustic wave device, comprising a step of forming line widths d1 and d2 on a lower side and an upper side to be (d1-d2) / p = 0 to 0.16.   A surface acoustic wave device comprising the surface acoustic wave element according to claim 1.

Images