JP2005086233A - Surface acoustic wave device and method of adjusting frequency temperature characteristic thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of adjusting frequency temperature characteristic of a surface acoustic wave device in which a thin film is formed on a crystal substrate. <P>SOLUTION: A surface acoustic wave device 10 provided with a crystal substrate 12, a thin film 20 formed on the crystal substrate 12, and an interdigital electrode 14 formed between the crystal substrate 12 and the thin film 20. In the method of adjusting frequency temperature characteristic of the device, any one of ratios of an electrode pitch P to the cut angle of the crystal substrate 12, to the film thickness of the interdigital electrode 14, and to the electrode width L of the interdigital electrode 14 or the arbitrarily combined value of these is varied to adjust the frequency temperature characteristic. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method for adjusting a frequency temperature characteristic of a surface acoustic wave device in which a piezoelectric thin film is formed on a quartz substrate and a surface acoustic wave device.

  Examples of SAW devices that use surface acoustic waves (hereinafter referred to as SAWs) include SAW resonators, SAW oscillators, and SAW filters. These SAW devices are applied as reference frequency sources for electronic equipment, frequency selectors in transmission circuits, and the like. In the SAW device constituting the SAW device, interdigital electrodes (hereinafter referred to as IDT) are formed on the surface of a piezoelectric substrate made of a material that generates a piezoelectric effect, and reflectors are provided at both ends of the IDT. Things are the main structure. In some SAW devices, a protective film is formed on the IDT and the reflector, or a thin film is formed for the purpose of temperature compensation of the piezoelectric substrate.

Patent document 1 is mentioned as invention described about the said protective film. The invention of Patent Document 1 is a SAW device in which an IDT and a reflector are formed on a lithium tantalate (LiTaO ₃ ) substrate, and a silicon oxide (SiO ₂ ) film serving as a protective film is formed between the IDT and the reflector. The structure is formed on the LiTaO ₃ substrate on which the IDT and the reflector are not formed while being formed on the upper surface and the side surface. It is described that this configuration prevents the IDT from being short-circuited by the conductive material dropped on the IDT and protects the IDT and the reflector from contaminants.

The invention of Patent Document 2 is an invention in which a thin film is formed for the purpose of temperature compensation of a piezoelectric substrate. In the invention of Patent Document 2, an SiO ₂ film having a frequency temperature characteristic opposite to that of these piezoelectric substrates is formed on a lithium niobate (LiNbO ₃ ) substrate or LiTaO ₃ substrate, and the piezoelectric substrate and the SiO ₂ film are interposed between them. In this configuration, an IDT, a reflector, and the like are formed. This configuration describes that a SAW device having a wide band, low insertion loss, and excellent temperature stability can be obtained.

JP-A-9-186542 Japanese Patent Laid-Open No. 7-15274

By the way, LiNbO ₃ and LiTaO ₃ have a negative value in the primary temperature coefficient, and therefore the frequency temperature characteristics are poor. For this reason, a thin film having a positive primary temperature coefficient is formed on these piezoelectric substrates to compensate for the temperature of the piezoelectric substrate. On the other hand, since the original primary temperature coefficient value of the crystal is zero, the frequency temperature characteristic is good.

FIG. 7 shows the relationship between frequency deviation and temperature. (A) shows the frequency temperature characteristics of the SAW device in which the IDT and the reflector are made of aluminum (Al) on the quartz substrate, and (b) shows the frequency temperature characteristics of the SiO ₂ film that becomes the piezoelectric thin film. FIG. 6C shows frequency temperature characteristics when an SiO ₂ film is formed on a quartz substrate on which an IDT and a reflector are formed of Al. Here, the quartz substrate is expressed in Euler angles (0 °, 123 °, 43.1 °), the ratio of the Al film thickness to the wavelength is 0.0368, and the ratio of the SiO ₂ film thickness to the wavelength is 0.0016. The frequency temperature characteristic of the SiO ₂ film is expressed by a linear function, and the slope of the frequency temperature characteristic changes depending on the film thickness of SiO ₂ . 7A, the frequency deviation is in the range of 0 ppm to −55 ppm in the temperature range of −40 ° C. to + 85 ° C., whereas in FIG. 7C, the frequency deviation is in the range of 0 ppm to −80 ppm. It has become. As a result, when the SiO ₂ film, which is a piezoelectric thin film, is formed on the quartz substrate, the frequency temperature characteristic of the quartz substrate is shifted due to the primary temperature coefficient of the SiO ₂ . That is, if a SiO ₂ film is formed on a quartz substrate having good frequency temperature characteristics, the frequency temperature characteristics are deteriorated. There is no report on correction of temperature characteristics when a piezoelectric thin film (SiO ₂ film) is formed on a quartz substrate.

Further, since SAW resonators and SAW oscillators are required to have accuracy of ± several tens of ppm or less by combining various factors, SAW devices used in these SAW devices are required to resonate at a very constant frequency. . For this reason, when the SiO ₂ film is formed on the piezoelectric substrate, a desired resonance frequency cannot be obtained due to the frequency-temperature characteristics of the SiO ₂ film, resulting in a problem that the accuracy cannot be achieved.

  The present invention has been made to solve the above-described problems. Frequency temperature characteristics of a surface acoustic wave device that can obtain desired frequency temperature characteristics even when a piezoelectric thin film is formed on a quartz substrate provided with interdigital electrodes. An object is to provide an adjustment method and a surface acoustic wave device.

  In order to achieve the above object, a method for adjusting frequency temperature characteristics of a surface acoustic wave device according to the present invention includes a quartz substrate, a piezoelectric thin film formed on the quartz substrate, and the quartz substrate and the piezoelectric thin film. A method of adjusting temperature characteristics of a surface acoustic wave device including a formed interdigital electrode, wherein the ratio of the cut angle of the quartz substrate, the film thickness of the interdigital electrode, the electrode width and the electrode pitch of the interdigital electrode It is characterized in that the temperature characteristic is adjusted by changing the value of any one or any combination. In this case, the crystal substrate can have an Euler angle of (0 °, 113 ° to 135 °, ± (40 ° to 49 °)) or (0 °, 113 ° to 135 °, 0 °). The piezoelectric thin film can be a silicon oxide film.

  The frequency temperature characteristic of the surface acoustic wave device is a combination of the frequency temperature characteristic of the piezoelectric thin film itself and the frequency temperature characteristic of the quartz crystal substrate provided with the interdigital electrode. That is, when a piezoelectric thin film is formed on a quartz substrate provided with interdigital electrodes, the frequency temperature characteristics change. For this reason, if the interdigital electrode and the quartz substrate are formed by adjusting in advance the amount of change in the frequency temperature characteristic that changes due to the formation of the piezoelectric thin film, the frequency temperature characteristic after the piezoelectric thin film is formed can be obtained as a desired frequency. Temperature characteristics can be obtained. In order to adjust the frequency temperature characteristics by forming the interdigital electrode and the quartz substrate, the cut angle of the quartz substrate, the thickness of the interdigital electrode, that is, the ratio of the interdigital electrode thickness H to the wavelength λ (H / Λ), the ratio (L / P) of the electrode width L to the electrode pitch P (L / P) of the interdigital electrodes, or any value in any combination may be changed. An in-plane rotated ST-cut quartz substrate whose Euler angles are represented by (0 °, 113 ° to 135 °, ± (40 ° to 49 °)), (0 °, 113 ° to 135 °, 0 °) The above-described adjustment can be performed in the surface acoustic wave device using the ST cut quartz crystal substrate. The above-described adjustment can also be performed when a silicon oxide film is used as the piezoelectric thin film.

  Further, in order to adjust the frequency temperature characteristic by changing the cut angle of the quartz substrate, the frequency temperature characteristic is adjusted by changing ψ among Euler angles (φ, θ, ψ). The frequency temperature characteristic can be easily adjusted by changing the in-plane rotation angle ψ. This adjustment is particularly effective when adjusting a quartz substrate having Euler angles (0 °, 113 ° to 135 °, ± (40 ° to 49 °)).

  The surface acoustic wave device according to the present invention is characterized by being manufactured using the surface acoustic wave temperature adjusting method described above. Even when a piezoelectric thin film is formed on a quartz substrate provided with interdigital electrodes, a surface acoustic wave device having desired frequency-temperature characteristics can be obtained.

  Hereinafter, a method for adjusting a frequency temperature characteristic of a surface acoustic wave device and a surface acoustic wave device according to the present invention will be described. In addition, what is described below is only one embodiment of the present invention, and the present invention is not limited to this. FIG. 1 is an explanatory diagram of the cut angle of the quartz substrate. The crystal axis of quartz is defined by the X axis (electric axis), the Y axis (mechanical axis), and the Z axis (optical axis). The surface acoustic wave (SAW) device 10 includes an ST-cut quartz substrate whose Euler angles (φ, θ, ψ) are represented by (0 °, 113 ° to 135 °, 0 °), and (0 °, 113). An in-plane rotated ST-cut quartz substrate represented by ° to 135 ° and ± (40 to 49 °) is mainly used. The ST-cut quartz substrate is obtained by rotating a quartz crystal Z plate 2 whose Euler angles (φ, θ, ψ) are (0 °, 0 °, 0 °) around the X axis by θ = 113 ° to 135 °. A quartz substrate 3 cut out along the surface to be cut. The axes obtained by rotating the Y axis and the Z axis by θ around the X axis are defined as the Y ′ axis and the Z ′ axis, respectively. The in-plane rotated ST-cut substrate is a quartz substrate obtained by rotating the ST-cut quartz substrate in the XY ′ plane, that is, rotating ψ = ± (40 ° to 49 °) around the Z ′ axis.

  The frequency-temperature characteristics of these quartz substrates are expressed by a quadratic function for the ST-cut quartz substrate and a cubic function for the in-plane rotated ST-cut quartz substrate. By the way, the inflection point temperature of the in-plane rotation ST-cut quartz substrate is located around 110 ° C. which is higher than the normal use temperature range (for example, −40 ° C. to + 85 ° C.). For this reason, the frequency temperature characteristic curve has a maximum value within the operating temperature range, and this maximum value becomes the apex temperature.

FIG. 2 is an explanatory diagram of the SAW device. FIG. 2A is a plan view of the SAW device, and FIG. 2B is an enlarged view of the cross section of the SAW device. The SAW device 10 has a configuration in which a pair of interdigital electrodes 14 (IDT 14) is formed on the above-described quartz substrate 12, and reflectors 16 are formed at both ends of the IDT 14 along the propagation direction of the surface acoustic wave. In addition, a bonding pad 19 is formed on the quartz substrate 12, and the bonding pad 19 and the IDT 14 are electrically connected via the extraction electrode 18. Further, a silicon oxide (SiO ₂ ) film 20 to be the piezoelectric thin film 20 is formed on the quartz substrate 12, the IDT 14 and the reflector 16. The piezoelectric thin film 20 is not limited to the SiO ₂ film 20.

Next, an adjustment method for the SAW device 10 to obtain a desired frequency temperature characteristic will be described. In this adjustment method, the thickness of the SiO ₂ film 20 is first determined. The SiO ₂ film 20 is formed by a chemical vapor deposition method, a sputtering method, a vapor deposition method, or the like after the thickness necessary for protecting the IDT 14 or the like or the thickness necessary for correcting the frequency temperature characteristic is appropriately designed. . The thickness of the SiO ₂ film 20, the frequency temperature characteristic represented by a linear function of the SiO ₂ film 20 is determined. In order to obtain a desired frequency temperature characteristic of the SAW device 10, the frequency temperature characteristic of the quartz substrate 12 provided with the IDT 14 and the like is uniquely determined by the frequency temperature characteristic of the SiO ₂ film 20. That is, since the desired frequency temperature characteristic of the SAW device 10 is a combination of the frequency temperature characteristic of the quartz substrate 12 provided with the IDT 14 and the like and the frequency temperature characteristic of the SiO ₂ film 20, the quartz substrate 12 provided with the IDT 14 and the like. The frequency temperature characteristic may be a value obtained by adjusting the amount of change in the frequency temperature characteristic due to the formation of the SiO ₂ film 20.

In the SAW device 10 in which the SiO ₂ film 20 is not formed on the quartz substrate 12 provided with the IDT 14 or the like, the apex temperature of the frequency temperature characteristic is approximately the middle temperature (25 ° C. to −40 ° C. to + 85 ° C.). ). However, in this embodiment, since the SiO ₂ film 20 is formed on the quartz substrate 12 provided with the IDT 14 and the like, the vertex temperature is moved to the low temperature side or the high temperature side according to the frequency temperature characteristics of the SiO ₂ film 20. There is a need.

Since the in-plane rotation ST-cut quartz substrate has a frequency temperature characteristic of a cubic function, the same result as when the vertex temperature is moved can be obtained by rotating the frequency temperature characteristic curve around the inflection point. For this reason, the vertex temperature is moved by first adjusting the amount of in-plane rotation around the Z ′ axis. That is, it is performed by adjusting the in-plane rotation angle ψ. FIG. 3 shows the relationship between the frequency deviation and the temperature when the in-plane rotation angle ψ is changed. In FIG. 3, the ratio (H / λ) between the film thickness H of the IDT 14 and the wavelength λ (H / λ) is 0.0368, the ratio (η) between the electrode width L and the electrode pitch P of the IDT 14 is 0.5, and the crystal substrate 12 is cut. The angles (φ, θ, ψ) are (0 °, 123 °, ψ). From FIG. 3, when the in-plane rotation angle ψ is changed in the range from 42.4 ° to 43.2 °, the vertex temperature moves to the low temperature side when the in-plane rotation angle ψ is large, and the in-plane rotation angle When ψ is small, it can be seen that the vertex temperature moves to the high temperature side. Thereby, if the in-plane rotation angle ψ is changed, the vertex temperature moves. Therefore, if the in-plane rotation angle ψ of the quartz substrate 12 is appropriately adjusted according to the frequency temperature characteristic of the SiO ₂ film 20, a desired frequency temperature can be obtained. The SAW device 10 having the characteristics can be manufactured.

  The adjustment of the cut angle will be described next. First, an ST-cut quartz wafer is prepared, and an in-plane rotation angle ψ is given using an orientation flat provided on the quartz wafer. Then, the IDT 14, the reflector 16, and the like are formed in each SAW device 10 region so that the surface acoustic wave propagates in a direction along the in-plane rotation angle ψ (direction along the X ′ axis in FIG. 1). Thereby, the quartz substrate 12 on which the IDT 14 and the like having the in-plane rotation angle ψ are formed, that is, the in-plane rotation ST-cut quartz substrate can be formed.

Second, the vertex temperature is moved by adjusting the value of H / λ. That is, the vertex temperature is moved by changing the film thickness of the IDT 14. FIG. 4 shows the relationship between the frequency deviation and temperature when the value of H / λ is changed. In FIG. 4, η is 0.5, and the cut angles (φ, θ, ψ) of the quartz substrate 12 are (0 °, 123 °, 43 °). As shown in FIG. 4, when the value of H / λ is changed in the range from 0.25 to 0.45, the vertex temperature moves to the low temperature side when the value of H / λ is large, and the value of H / λ is small. It can be seen that the apex temperature moves to the high temperature side. Accordingly, if the value of H / λ is changed, the vertex temperature moves. Therefore, if the value of H / λ, that is, the film thickness of the IDT 14 is appropriately adjusted according to the frequency temperature characteristic of the SiO ₂ film 20, a desired value can be obtained. The SAW device 10 having frequency temperature characteristics can be manufactured.

Third, the vertex temperature is moved by adjusting the value of η. FIG. 5 shows the relationship between the frequency deviation and temperature when the value of η is changed. In FIG. 5, H / λ is 0.0368, and the cut angles (φ, θ, ψ) of the quartz substrate 12 are (0 °, 123 °, 43 °). From FIG. 5, when the value of η is changed in the range from 0.3 to 0.7, the vertex temperature moves to the low temperature side when the value of η is large, and the vertex temperature is changed when the value of η is small. It can be seen that it has moved to the high temperature side. Accordingly, if the value of η is changed, the vertex temperature moves. Therefore, if the value of η, that is, the ratio between the electrode width L and the electrode pitch P of the IDT 14 is appropriately adjusted according to the frequency temperature characteristics of the SiO ₂ film 20. The SAW device 10 having a desired frequency temperature characteristic can be manufactured.

Further, the first to third adjustment methods described above can be arbitrarily combined, and the vertex temperature can be moved by changing the respective values for adjusting the frequency temperature characteristics.
Next, when the ST cut quartz substrate is used, the frequency temperature characteristics can be adjusted by adjusting any one of the Euler angles (φ, θ, ψ) to move the vertex temperature. However, since the ST-cut quartz substrate has Euler angles (0 °, 113 ° to 135 °, 0 °) and is easy to manufacture, any one of Euler angles (φ, θ, ψ) is adjusted. The method is not preferred. For this reason, when using an ST cut quartz substrate, it is preferable to adjust the frequency temperature characteristic by adjusting the value of H / λ to move the apex temperature. In this case, when the value of H / λ is large, the vertex temperature moves to the low temperature side, and when the value of H / λ is small, the vertex temperature moves to the high temperature side. This shows the same tendency as in the case of the in-plane rotation ST cut. Accordingly, if the value of H / λ is changed, the vertex temperature moves. Therefore, if the value of H / λ, that is, the thickness of the IDT is appropriately adjusted according to the frequency temperature characteristics of the SiO ₂ film 20, a desired value can be obtained. The SAW device 10 having frequency temperature characteristics can be manufactured. It is also possible to adjust the frequency temperature characteristic by adjusting the ratio of the electrode width L and the electrode pitch P of the IDT 14 formed on the ST cut quartz substrate to move the vertex temperature.

As described above, since the IDT 14 and the like are formed on the in-plane rotated ST-cut quartz substrate and the ST-cut quartz substrate and the SiO ₂ film 20 is formed thereon, the conductive material falls on the IDT 14 and the reflector 16. Can also prevent short circuit. Even if contaminants fall on the IDT 14 or the reflector 16, the IDT 14 or the reflector 16 can be protected by the SiO ₂ film 20.

Further, since the SiO ₂ film 20 is formed on the quartz substrate 12 provided with the IDT 14 and the like, the frequency temperature characteristics of the SAW device 10 are deteriorated. However, since the frequency temperature characteristic of the quartz crystal substrate 12 can be adjusted according to the frequency temperature characteristic of the SiO ₂ film 20, the desired frequency temperature characteristic of the SAW device 10 can be obtained. The frequency temperature characteristics of the quartz substrate 12 can be adjusted by changing the in-plane rotation angle ψ among the Euler angles (φ, θ, ψ) of the quartz substrate 12. It can also be performed by changing the ratio between the film thickness and the wavelength of the IDT 14, and can also be performed by changing the ratio between the electrode width L and the electrode pitch P of the IDT 14.

Further, even if the SiO ₂ film 20 is formed on the in-plane rotated ST-cut quartz substrate or the ST-cut quartz substrate for the purpose of correcting the frequency temperature characteristic, if the quartz substrate 12 is adjusted as described above, the desired frequency-temperature characteristic can be obtained. The SAW device 10 can be obtained.

Next, examples will be described. In this embodiment, an example is shown in which the in-plane rotation angle ψ is changed to adjust the frequency temperature characteristics of a quartz substrate provided with an IDT or the like. First, H / λ of the SiO ₂ film formed on the quartz substrate is 0.0016, and the frequency-temperature characteristics of this SiO ₂ film are shown in FIG. The frequency temperature characteristic of the quartz substrate provided with IDT or the like is uniquely determined by the frequency temperature characteristic of the SiO ₂ film. That is, the frequency temperature characteristics of the SAW device are adjusted so that the frequency fluctuation amount is minimized in the operating temperature range (−40 ° C. to + 85 ° C.) centering on the normal operating temperature of 25 ° C. For this reason, the frequency temperature characteristic of the quartz substrate provided with IDT or the like is a value obtained by adjusting the amount of change in the frequency temperature characteristic that changes after the SiO ₂ film is formed. This value is calculated, and an IDT or the like is formed on the quartz substrate so as to satisfy this value.

FIG. 6 shows the relationship between the frequency deviation and the temperature according to this example. FIG. 6A shows frequency temperature characteristics obtained by adjusting the amount of change in frequency temperature characteristics of the quartz substrate provided with IDT and the like, and FIG. 6B shows the SiO ₂ film on the quartz substrate provided with IDT and the like. It is the frequency temperature characteristic of the SAW device which formed. The value obtained by calculating the amount of change in the frequency temperature characteristic is that the Euler angles of the quartz substrate are (0 °, 123 °, 43.27 °), and an IDT or the like is formed on the quartz substrate so as to satisfy this value. The film thickness of IDT is H / λ = 0.0368. The frequency temperature characteristic of the quartz substrate provided with the IDT or the like is in a state where the vertex temperature has moved to the low temperature side (see FIG. 6A). Then, when the above-described SiO ₂ film is formed on this quartz substrate, a SAW device in which the amount of frequency fluctuation is minimized within the operating temperature range is formed (see FIG. 6B). Thus, it can be seen that the frequency temperature characteristic of the quartz substrate can be adjusted according to the frequency temperature characteristic of the SiO ₂ film to obtain the desired frequency temperature characteristic of the SAW device.

It is explanatory drawing of the cut angle of a quartz substrate. It is explanatory drawing of the surface acoustic wave apparatus which concerns on this Embodiment. It is a figure which shows the relationship between a frequency deviation and temperature when changing in-plane rotation angle (psi). It is a figure which shows the relationship between a frequency deviation when changing the film thickness of a comb-shaped electrode, and temperature. It is a figure which shows the relationship between a frequency deviation and temperature when changing the ratio of the electrode width of an interdigital electrode, and an electrode pitch. It is a frequency temperature characteristic which shows the relationship between the frequency deviation and temperature which concern on a present Example. It is the frequency temperature characteristic which shows the relationship between a frequency deviation and temperature.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ......... Surface acoustic wave (SAW) apparatus, 12 ......... Quartz substrate, 14 ......... IDT (interdigital electrode), 16 ......... Reflector, 18 ...... Extraction electrode, 20 ...... Silicon oxide Film (piezoelectric thin film).

Claims (5)

  A method for adjusting a frequency temperature characteristic of a surface acoustic wave device comprising a quartz substrate, a piezoelectric thin film formed on the quartz substrate, and an interdigital electrode formed between the quartz substrate and the piezoelectric thin film, Elasticity characterized by adjusting the temperature characteristics by changing the value of any one or any combination of the cut angle of the quartz substrate, the film thickness of the interdigital electrode, the ratio of the electrode width and electrode pitch of the interdigital electrode Method for adjusting frequency temperature characteristics of surface wave device.   The Euler angle of the quartz substrate is (0 °, 113 ° to 135 °, ± (40 ° to 49 °)) or (0 °, 113 ° to 135 °, 0 °). 2. A frequency temperature adjusting method for a surface acoustic wave device according to 1.   The temperature characteristic is adjusted by changing ψ among Euler angles (φ, θ, ψ) in order to adjust the temperature characteristic by changing the cut angle of the quartz crystal substrate. The frequency temperature adjustment method of the surface acoustic wave apparatus of description.   4. A method of adjusting a temperature of a frequency of a surface acoustic wave device according to claim 1, wherein the piezoelectric thin film is a silicon oxide film. A surface acoustic wave device manufactured using the surface acoustic wave frequency temperature adjusting method according to claim 1.

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