JP2007300174A - Frequency temperature characteristic adjustment method of surface acoustic wave element chip, surface acoustic wave element chip, and surface acoustic wave device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a temperature characteristic adjustment method of a surface acoustic wave device for improving the temperature characteristic by utilizing the effect caused by provision of a protection film. <P>SOLUTION: In the adjustment method of the frequency temperature characteristic of the surface acoustic wave element chip using an inplane rotation ST-cut crystal substrate on the condition that the cut angle is within (0°, 95°≤θ≤155°, 33°≤¾ψ¾≤46°) in the Euler's representation, a relationship between the temperature and the frequency in the surface acoustic wave element chip is obtained, a deviation between the obtained frequency and a reference frequency at a reference temperature is obtained, the obtained relationship between the deviation and the temperature is approximated by a linear expression, and the gradient of the linear expression for the approximation is made closer to zero by forming the protection film at an exciting electrode on the basis of the relationship obtained in advance between the thickness of the protection film formed on the exciting electrode and the gradient of the linear expression. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for adjusting the frequency temperature characteristic of a surface acoustic wave element piece using an in-plane rotating ST-cut quartz substrate, a surface acoustic wave element piece whose frequency temperature characteristic is adjusted using this method, and the surface acoustic wave. The present invention relates to a surface acoustic wave device mounted with an element piece.

  Conventionally, it has been known that among piezoelectric devices, those using an element piece constituted by a quartz substrate have good frequency temperature characteristics (frequency fluctuation characteristics with respect to temperature changes). It is also known that among piezoelectric devices using a crystal resonator element, an AT-cut crystal resonator using an AT-cut crystal substrate as the resonator element has particularly good frequency temperature characteristics.

  Further, in the field of piezoelectric devices, it is desired to increase the frequency of vibration to be excited. Correspondingly, as a resonator element having a configuration capable of exciting a high frequency, a surface acoustic wave using an ST cut quartz substrate is adopted. A surface acoustic wave device (SAW device) using a (SAW: surface acoustic wave) element piece is known. However, SAW devices have poor frequency temperature characteristics compared to AT-cut quartz resonators. For this reason, various techniques for improving the frequency temperature characteristics of the SAW device have been proposed.

For example, Patent Document 1 proposes improving the frequency temperature characteristics of a SAW element piece (SAW device) by giving a specific in-plane rotation angle ψ to an ST-cut quartz crystal substrate.
Japanese Patent No. 3622202 JP 2005-57666 A

  As shown in Patent Document 1, by appropriately changing the in-plane rotation angle ψ of the SAW element piece constituted by the ST cut quartz substrate, the frequency temperature characteristic of the SAW element piece can surely be improved. It is considered possible.

  However, while the applicant of the present application intensively researches, even with a SAW element piece in which the in-plane rotation angle of the quartz crystal substrate is adjusted and the frequency temperature characteristics are improved as described above, the resonance frequency is adjusted and It has been found that the frequency temperature characteristic is changed by applying a protective film to an IDT (interdigital transducer) constituting the excitation electrode for the purpose of preventing a short circuit or the like.

  As described above, the provision of a protective film to the excitation electrode is also disclosed in Patent Document 2, but the provision of the protective film in the prior art including Patent Document 2 only gives the resonance frequency of the SAW element piece. The purpose is to adjust, and the influence on the temperature characteristics was not considered. In general, when a protective film is applied to the excitation electrode of the SAW element piece employing the ST cut quartz substrate, there is almost no influence on the frequency temperature characteristics. For this reason, conventionally, it has been considered that the SAW element piece using the in-plane rotation ST-cut quartz substrate is not affected by the provision of the protective film.

  For this reason, in the case where the design by the applicant of the present application is such that the frequency temperature characteristic becomes an optimum value within the operating temperature range under the resonance frequency at which the in-plane rotation angle ψ of the ST cut quartz substrate is set. However, it became clear for the first time that the above-described phenomenon that the frequency temperature characteristics deteriorate due to the application of the protective film to the excitation electrode as the final stage of production.

  Therefore, in the present invention, the frequency temperature characteristic adjusting method for a surface acoustic wave element piece that improves the temperature characteristic by using the effect of applying a protective film to the surface acoustic wave element piece using the in-plane rotating ST-cut quartz substrate, and the elastic surface An object is to provide a wave element piece and a surface acoustic wave device.

  In order to achieve the above object, the method for adjusting the frequency-temperature characteristics of a surface acoustic wave element according to the present invention has a cut angle represented by Euler angles (0 °, 95 ° ≦ θ ≦ 155 °, 33 ° ≦ | ψ | ≦ 46 °) is a method for adjusting the frequency-temperature characteristics of a surface acoustic wave element piece using an in-plane rotated ST-cut quartz crystal substrate, and the temperature and frequency in the surface acoustic wave element piece. The relationship between the obtained frequency and the reference frequency at the reference temperature is obtained, the relationship between the obtained deviation and the temperature is approximated by a linear expression, and the thickness of the protective film formed on the excitation electrode obtained in advance is A protective film is formed on the excitation electrode on the basis of the relationship with the slope of the linear expression so that the slope of the approximated primary expression approaches 0.

  As described above, by adjusting the film thickness of the protective film, it is possible to change the variation amount of the temperature characteristic in the surface acoustic wave element piece. Further, when this fluctuation amount (slope) is approximated by a linear expression, the slope becomes 0, so that the temperature characteristics are optimal. For this reason, the frequency temperature characteristic of the surface acoustic wave element can be improved by adjusting the slope of the linear function to 0 by adjusting the thickness of the protective film.

  In order to achieve the above object, the method for adjusting the frequency temperature characteristics of the surface acoustic wave element according to the present invention has a cut angle represented by Euler angles (0 °, 113 ° ≦ θ ≦ 135 °, 40 ° ≦ | ψ | ≦ 49 °) is a method for adjusting the frequency-temperature characteristics of a surface acoustic wave element piece using an in-plane rotated ST-cut quartz crystal substrate, which is basically within the range of | The relationship between the obtained frequency and the reference frequency at the reference temperature is obtained, the relationship between the obtained deviation and the temperature is approximated by a linear expression, and the protective film formed on the excitation electrode obtained in advance is obtained. Based on the relationship between the thickness and the slope of the linear expression, a protective film may be formed on the excitation electrode so that the approximate slope of the primary expression approaches 0.

  Even in the case of a surface acoustic wave element piece using an in-plane rotating ST cut quartz substrate having such cuts, the frequency temperature characteristics of the surface acoustic wave element piece can be improved by the characteristic method described above. it can.

  In the method for adjusting the frequency temperature characteristics of the surface acoustic wave element as described above, the excitation electrode is preferably made of aluminum or an alloy mainly composed of aluminum, and the protective film is preferably an anodic oxide film.

The anodic oxide film is formed by obtaining a step of applying a voltage by immersing the excitation electrode in an electrolytic solution, and can be easily obtained. Further, since the film thickness can be adjusted by the applied voltage, it is advantageous when adjusting the temperature characteristics depending on the thickness of the film. Furthermore, since the anodic oxide film can be easily made thicker than the SiO ₂ film or the like, the adjustment range of the temperature characteristic can be widened.

Furthermore, when the inclination in the linear expression is α and the resonance frequency of the manufactured surface acoustic wave device is f, an anodic oxidation voltage V applied to the excitation electrode in the anodizing step for forming the anodized film is
And an anodic oxide film having a film thickness obtained by the anodic oxidation voltage thus determined may be applied to the surface of the excitation electrode.

  By determining the film thickness of the anodic oxide film formed on the surface of the excitation electrode in this way, the film thickness of the anodic oxide film to be formed can be set to an optimum value for bringing the inclination close to zero.

  The approximated primary expression may be based on a measurement frequency in a temperature range of −10 ° C. to 70 ° C. This is because the temperature characteristic can be approximated to a linear expression within such a temperature range.

  The surface acoustic wave element according to the present invention for achieving the above object is characterized in that the frequency temperature characteristic is adjusted by any one of the above-described frequency temperature characteristic adjusting methods.

  Furthermore, the surface acoustic wave device according to the present invention is characterized in that the surface acoustic wave element piece is mounted.

  Hereinafter, embodiments of the method for adjusting the frequency temperature characteristics of a surface acoustic wave element piece, the surface acoustic wave element piece, and the surface acoustic wave device according to the present invention will be described in detail with reference to the drawings. In addition, this invention shall have a various form in the limit which does not change the principal part.

  In the present embodiment, the cut angle of the crystal substrate will be described with reference to the Euler angle display (φ, θ, ψ), which is well known in describing the cut angle of the crystal substrate. In addition, the SAW element piece and the SAW device in the present embodiment use the upper limit mode of the stop band. The upper limit mode in the stop band of the surface acoustic wave has a smaller absolute value of the secondary temperature coefficient in the frequency temperature characteristics than the lower limit mode, and the change in the absolute value of the secondary temperature coefficient when the thickness of the IDT electrode is increased. Is also small. Further, the upper limit mode in the stop band of the surface acoustic wave also has a feature that the change amount of the oscillation frequency when the thickness of the IDT constituting the excitation electrode is increased is smaller than that in the lower limit mode.

  FIG. 1 shows the configuration of a SAW element piece according to the present invention. 1A is a plan view showing the configuration of the SAW element piece, and FIG. 1B is a cross-sectional view taken along the line AA in FIG. 1A.

  The SAW element piece 10 of this embodiment is basically composed of a piezoelectric substrate 12 and an excitation electrode formed on one main surface of the piezoelectric substrate 12. The piezoelectric substrate 12 is made of quartz, and its cut angle and surface acoustic wave propagation direction are expressed in Euler angles (0 °, 95 ° ≦ θ ≦ 155 °, 33 ° ≦ | ψ | ≦ 46 °). The in-plane rotation angle ψ is determined in detail by the resonance frequency f determined by the configuration of the excitation electrode described later, the setting of the protective film 20 applied to the excitation electrode body 18, and the like. And

  The excitation electrode body 18 can be composed of various conductive materials. In the present embodiment, a case where aluminum (Al) or an alloy mainly composed of Al is used as a constituent material will be described. And

  The excitation electrode is basically composed of an IDT 14 composed of a pair of comb electrodes and a pair of reflectors 16 arranged so as to sandwich the IDT 14. The IDT 14 is configured by connecting a plurality of electrode fingers 14a provided so as to be orthogonal to the propagation direction of the surface acoustic wave by bus bars 14b arranged along the propagation direction of the surface acoustic wave. Composed of two comb electrodes. The arrangement relationship between the two comb-shaped electrodes is that the electrode fingers 14a extending from the two bus bars 14b are arranged so as to alternately engage with each other. The reflector 16 is a grid-like electrode configured by connecting both ends of a plurality of conductor strips 16a arranged in parallel with the electrode fingers 14a of the comb-shaped electrode to a bus bar 16b.

The protective film 20 applied to the excitation electrode body 18 can be variously selected such as an anodic oxide film or SiO ₂ film. In this embodiment, the protective film 20 is obtained by anodizing Al as the protective film. A description will be given by taking an anodic oxide film as an example. Anodization is configured by applying a voltage by immersing the piezoelectric substrate 12 on which the excitation electrode body 18 is formed in an electric field solution (for example, ammonium phosphate). Here, the film thickness of the anodic oxide film coated (applied) to the excitation electrode body 18 is the voltage applied to the excitation electrode body 18 during an immersion (anodization voltage), the time during which the voltage is applied (anodization time), etc. Can be determined by. When the voltage is constant, the thickness of the anodic oxide film after a certain time is constant. In addition, the maximum value of the thickness of the anodized film (protective film 20) coated on the excitation electrode body increases in proportion to the anodizing voltage if the anodizing time is sufficient.

  Here, when the applicant of the present invention applies an anodic oxide film (protective film) to the excitation electrode of the SAW element piece using the in-plane rotating ST-cut quartz substrate, a change occurs in the frequency temperature characteristics of the manufactured SAW device. I found. As described above, the phenomenon of the change in the frequency temperature characteristic due to the application of such a protective film is caused by a normal ST-cut quartz substrate (for example, the cut angle is displayed in Euler angle (0 °, 113 ° ≦ θ ≦ 135 °, 0 It was almost impossible to confirm with a SAW device using a quartz substrate), and it was thought that this could not occur in the past.

  The characteristic of the change in frequency temperature characteristics by applying a protective film is that the line (temperature characteristic curve) showing the temperature characteristics by applying a protective film rotates counterclockwise around the reference point of the measured temperature. It is to show. This tends to be obtained by sequentially measuring the frequency temperature characteristics of the SAW element piece without the protective film and the SAW element piece after the protective film is provided. Therefore, the SAW element piece 10 in the present embodiment is preferably designed so that the temperature characteristic curve shows a state of lowering right in a state where the protective film 20 is not provided. Specifically, the in-plane rotation angle ψ may be set to be slightly increased with respect to the design value of the piezoelectric substrate in which the temperature characteristics are optimal when the protective film 20 is not provided. For reference, the increase range of the in-plane rotation angle ψ may be less than 1 °, and within this range, the temperature characteristics can be improved to the optimum value by applying the protective film 20.

  The present applicant has also found that the amount of change in the temperature characteristic curve due to the provision of the protective film is determined by the relative relationship between the thickness of the excitation electrode and the protective film. When an anodic oxide film is employed as the protective film 20, the thickness of the anodic oxide film depends on the voltage applied during anodization and the resonance frequency determined by the film thickness and pitch of the IDT 14. The relationship between the voltage and the film thickness is proportional, and the relationship between the film thickness and the resonance frequency is also proportional. Further, since the anodic oxide film tends to increase the thickness while oxidizing the excitation electrode body 18, the ratio of the thickness of the anodic oxide film to the thickness of the excitation electrode body 18 changes relatively. .

FIG. 2 shows the temperature in the operating temperature range (-10 ° C. to 70 ° C. in this embodiment) of the SAW device before applying the protective film to the excitation electrode and the SAW element piece after applying the protective curtain to the excitation electrode. And frequency, that is, frequency temperature characteristics are approximated by a linear function. Specifically, the linear function indicating the temperature characteristics of the SAW element piece before the protective film is applied is
And the linear function indicating the temperature characteristics of the SAW element piece after applying the protective film is
It is. Here, the resonance frequency (reference frequency) of the SAW element piece shown in FIG. 2 is 314.85 MHz. Further, when the reference temperature (reference point) is around 35 ° C., it is necessary to approximate the linear coefficient (gradient) in the linear function shown in Equation 1 to 0 (approximate to the state shown in Equation 2). The anodic oxidation film thickness can be obtained by applying an anodic oxidation voltage of 60 V to the excitation electrode.

Such a relationship can be expressed by the following mathematical formula. That is, when the slope (primary coefficient) of the linear expression approximated to the variation amount of the temperature characteristic of the SAW element piece before the protective film is applied is expressed by α (ppm / ° C.) and the resonance frequency is expressed by f (MHz), The anodic oxidation voltage V (V) required to obtain the anodic oxide film thickness for approximating α to 0 is
Can be shown.

  As described above, the anodic oxidation voltage and the anodic oxidation film thickness are in a proportional relationship, and the maximum value of the obtained anodic oxide film thickness is determined by determining the anodic oxidation voltage. For this reason, by applying an anodic oxide film having a film thickness obtained by the anodic oxidation voltage V derived from Equation 3 to the excitation electrode of the SAW element piece that requires adjustment of the temperature characteristics, the variation amount of the temperature characteristics is reduced to 0. Can be approximated. That is, the temperature characteristics of the SAW element piece can be improved.

  Note that the anodic oxide film thickness determined by the specific anodic oxidation voltage is the maximum film thickness of the anodic oxide film obtained during anodic oxidation. If the anodic oxidation time required to obtain the film thickness is obtained in advance, the anodic oxidation may be performed at a voltage equal to or higher than the anodic oxidation voltage derived from Equation 3. In this case, the thickness of the anodic oxide film may be controlled by the anodic oxidation time to obtain the anodic oxide film having the specific thickness.

  The description of the above embodiment is based on an in-plane rotated ST-cut quartz substrate having a cut angle obtained in the range of Euler angle display (0 °, 95 ° ≦ θ ≦ 155 °, 33 ° ≦ | ψ | ≦ 46 °). It was. However, the SAW element piece to which the present invention can be applied is not limited to this configuration, and is expressed in Euler angles (0 °, 113 ° ≦ θ ≦ 135 °, 40 ° ≦ | ψ | ≦ 49 °). An in-plane rotated ST-cut quartz substrate having a cut angle obtained in a range may be used. This is because even in such a case, the applicant of the present application has found that the frequency temperature characteristic changes due to the application of the protective film, and the change in the frequency temperature characteristic shows the same tendency.

In the above embodiment, as the protective film, but mentions only the anodic oxide film, SiO ₂ film or the like, be other insulating protective film, the change in frequency temperature characteristics. Therefore, even when another protective film such as the SiO ₂ film is applied, the temperature characteristics of the SAW device can be improved by adjusting the film thickness.

  Next, an embodiment of the SAW device of the present invention will be described with reference to FIG. The SAW device shown in FIG. 3 is an example of a SAW oscillator, but the SAW device according to the present invention may be a SAW resonator or a SAW filter.

  The SAW device 30 shown in FIG. 3 is based on a package 36, a SAW element piece 10 mounted on the package, and an IC 40. The package 36 according to this embodiment includes a package base 32 and the lid 34 for housing the SAW element piece 10 and the IC, respectively. The package base 32 includes a concave portion for accommodating the SAW element piece and a concave portion for accommodating the IC, and the two concave portions share the respective bottom plates. It can have a mold structure. The package base is provided with a mounting terminal 38 for mounting the SAW device 30 on a substrate such as an electronic device. The mounting terminal is electrically connected to an internal terminal (not shown) provided on the package base 32, and supplies power to the SAW element piece 10 and the IC 40 accommodated in the recessed portion to take out an electrical signal. Making it possible.

  The lid 34 is a lid for sealing the recessed portion that accommodates the SAW element piece 10. The concave portion for accommodating the SAW element piece 10 may be evacuated, and the concave portion for accommodating the IC may be filled with resin and sealed with this.

  Note that, as described above, the SAW device 30 having the above-described configuration is a part of the SAW device according to the present invention, and the configuration other than the SAW element piece 10 that is a characteristic portion is appropriately changed. However, it goes without saying that it can be regarded as a part of the present invention.

It is a figure which shows the structure of a SAW element piece. It is the figure which showed by the linear function which approximates the temperature characteristic of the surface acoustic wave apparatus before provision of a protective film, and the temperature characteristic of the surface acoustic wave apparatus after provision of a protective film, respectively. It is a figure which shows an example of the SAW device which concerns on this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... SAW element piece (surface acoustic wave element piece), 12 ......... Piezoelectric substrate, 14 ... IDT, 16 ... Reflector, 18 ... Excitation electrode body, 20 ... Protection film.

Claims (7)

Elasticity using an in-plane rotated ST-cut quartz substrate whose cut angle is in the range of Euler angle display (0 °, 95 ° ≦ θ ≦ 155 °, 33 ° ≦ | ψ | ≦ 46 °) A method for adjusting the frequency temperature characteristics of a surface acoustic wave element piece,
Finding the relationship between temperature and frequency in the surface acoustic wave element piece,
Find the deviation between the calculated frequency and the reference frequency at the reference temperature,
Approximate the relationship between the calculated deviation and temperature with a linear equation,
Based on the relationship between the thickness of the protective film formed on the excitation electrode obtained in advance and the slope of the linear expression, a protective film is formed on the excitation electrode so that the slope of the approximated primary expression approaches 0. A method for adjusting frequency-temperature characteristics of a surface acoustic wave element. An in-plane rotation ST-cut quartz substrate based on Euler angle display (0 °, 113 ° ≦ θ ≦ 135 °, 40 ° ≦ | ψ | ≦ 49 °) was used. A method for adjusting the frequency-temperature characteristics of a surface acoustic wave element,
Finding the relationship between temperature and frequency in the surface acoustic wave element piece,
Find the deviation between the calculated frequency and the reference frequency at the reference temperature,
Approximate the relationship between the calculated deviation and temperature with a linear equation,
Based on the relationship between the thickness of the protective film formed on the excitation electrode obtained in advance and the slope of the linear expression, a protective film is formed on the excitation electrode so that the slope of the approximated primary expression approaches 0. A method for adjusting frequency-temperature characteristics of a surface acoustic wave element.   3. The frequency temperature characteristic adjustment of a surface acoustic wave element according to claim 1, wherein the excitation electrode is made of aluminum or an alloy mainly composed of aluminum, and the protective film is an anodic oxide film. Method. The slope in the linear expression is α,
When the resonance frequency of the manufactured surface acoustic wave device is f,
An anodic oxidation voltage V applied to the excitation electrode in the anodic oxidation process for forming the anodic oxide film is
Determined by
4. The method for adjusting the frequency temperature characteristics of a surface acoustic wave element according to claim 3, wherein an anodic oxide film having a film thickness obtained by the predetermined anodic oxidation voltage is applied to the surface of the excitation electrode.   5. The method of adjusting the frequency-temperature characteristics of a surface acoustic wave element according to claim 1, wherein the approximated primary expression is based on a measurement frequency in a temperature range of −10 ° C. to 70 ° C. 6.   A surface acoustic wave element piece, wherein the frequency temperature characteristic is adjusted by the frequency temperature characteristic adjusting method according to any one of claims 1 to 5.   A surface acoustic wave device comprising the surface acoustic wave element according to claim 6.