JP2011035872A - Surface acoustic wave device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a surface acoustic wave (SAW) device which has infinitely reducible temperature characteristics while preventing deliquescence of an LBO substrate with respect to water and has superior characteristics in frequency bands of 2 to 3 GHz. <P>SOLUTION: The SAW device 1 includes: the LBO substrate 10 with Euler angles (0°, 35°-50°, 90°); a protecting film 11 which is provided on a surface of the LBO substrate 10 and includes an oxide film or nitride film having a KH of 0.05 to 0.10; and an electrode 20 having a KH of 0.04 to 0.15 provided on the protecting film 11. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a surface acoustic wave device.

In recent years, in a radio system using a frequency of 2 GHz or higher such as LTE (Long Term Evolution), a high-precision RF filter is required in a transmission or reception module.
A conventional surface acoustic wave device (SAW filter) has a sound velocity V of only about 4000 m / s, and it is difficult to form an electrode with an i-line stepper (a stepper that uses the i-line of a mercury lamp as a light source). Further, even if it can be formed, there is a problem that the reliability is poor.
Therefore, it has been proposed to use an LBO substrate having good temperature characteristics, a sound velocity V of 6000 m / s or more, and an electromechanical coupling coefficient K ² of 1% or more.

However, since the LBO substrate has a deliquescent property with respect to water, a protective film such as SiO ₂ or TaO ₃ is required in order to obtain a stable element using the LBO substrate. Thereby, a board | substrate can be protected from the etching liquid, the washing | cleaning liquid, etc. which are used when forming a comb-tooth electrode.

In Patent Document 1, by using an LBO substrate with a cut angle of Euler angles (45 °, 90 °, 90 °), sound velocity V: 3000 m / s, electromechanical coupling coefficient K ² : 1%, frequency temperature coefficient TCF : A SAW filter of 0 ppm / ° C. is realized. According to this configuration, the TCF is as low as 0 ppm / ° C. and good temperature characteristics can be obtained, but the sound velocity V can be obtained only about 3000 m / s, which is insufficient as a high-frequency SAW in the GHz band.

JP H8-268759 A

“Propagation Properties of Longitudinal Leaky Surface Waves on Lithium Tetraborate”, ieee transactions on ultrasonics, ferroelectrics, and frequency control, vol. 45, no. 1, january 1998, T. Sato and H. Abe, pp. 36

In Non-Patent Document 1, the sound velocity V6500 m / s, K is obtained by a structure having a cut angle substrate with Euler angles (0 °, 47.3 °, 90 °) and an electrode film thickness (h / λ = 2%). ² : 1% and TCF: 10 ppm are achieved. However, in this structure, although the sound speed is fast, there remain problems of temperature characteristics due to the deliquescent nature due to water and the large TCF of 10 ppm.

  The present invention has been made in view of the above-mentioned problems of the prior art, and can reduce the temperature characteristics as much as possible while preventing deliquescence of the LBO substrate with respect to water, and has good characteristics in the frequency band of 2 to 3 GHz. One of the objects is to provide a surface acoustic wave device.

In the surface acoustic wave device of the present invention, in order to solve the above-described problem, the surface orientation is in the range of (0 °, 35 ° to 50 °, 90 °) in the Euler display (φ, θ, ψ). An LBO substrate made of lithium borate single crystal (Li ₂ B ₄ O ₇ ), a protective film made of an oxide or nitride having a KH of 0.05 or more and 0.10 or less provided on the surface of the LBO substrate; And a comb-teeth electrode provided on the protective film and having a KH of 0.04 or more and 0.15 or less. However, KH is a value represented by the formula [(2π / λ) × film thickness], and λ is the wavelength of the surface acoustic wave.

  According to the present invention, it is possible to reduce the temperature characteristics as much as possible while preventing the deliquescence of the LBO substrate with respect to water. Further, by setting the thickness of the protective film or the comb electrode within the above range, the quality factor (Q value) can be maintained without increasing the propagation loss. As a result, a surface acoustic wave device having good frequency characteristics can be obtained.

The protective film is preferably a silicon oxide film or a silicon nitride film.
According to the present invention, it can be formed on a substrate by sputtering or the like.

The protective film is preferably a piezoelectric body.
According to the present invention, the phase velocity is remarkably improved, and the operating frequency can be remarkably increased.

The protective film is preferably made of AlN or ZnO.
In the present invention, when the protective film is made of ZnO, there is an advantage that the electromechanical conversion efficiency is higher than that of other piezoelectric bodies. On the other hand, when it is made of AlN, there is an advantage that the sound velocity of the ultrasonic wave propagating inside is faster than other piezoelectric bodies, and the resonance frequency is easily increased.

Moreover, it is preferable that an insulating film is provided on the comb electrode.
According to the present invention, it is possible to prevent an electrode short-circuit when a conductive foreign matter adheres to the substrate.

1 is a perspective view showing a surface acoustic wave device according to the present invention. 1 is a side sectional view showing a surface acoustic wave device according to the present invention. Propagation loss (dB / λ) and surface Euler angle (°) of surface acoustic wave The graph which shows a frequency impedance characteristic. The graph which shows a frequency temperature characteristic. (A) characteristic diagram showing the relationship between the Al electrode film thickness and the Q value, characteristic diagram showing the relationship between the Q value and (b) the thickness of the SiO ₂ protective film (KH). The characteristic view which shows the relationship between the film thickness (KH) of an AlN protective film, and Q value. The characteristic view which shows the relationship.

  Embodiments of the present invention will be described below with reference to the drawings. In each drawing used for the following description, the scale of each member is appropriately changed to make each member a recognizable size.

A surface acoustic wave device according to an embodiment of the present invention will be described. FIG. 1 is a sectional view showing a surface acoustic wave device according to the present invention, and FIG. 2 is a perspective view of the surface acoustic wave device.
As shown in FIGS. 1 and 2, the surface acoustic wave device 1 according to the present embodiment is a one-port type resonance device, and has an LBO substrate 10 made of a lithium tetraborate single crystal. A protective film 11 made of oxide or nitride is laminated on the LBO substrate 10, and an electrode 20 and reflective electrodes 31 and 32 are formed on the protective film 11.

The LBO substrate 10 is a substrate in which the cut-out angle from the lithium tetraborate single crystal base crystal and the Euler angle indicating the surface acoustic wave propagation direction are represented by (0 °, 35 ° to 50 °, 90 °). That is, it is a substrate whose surface orientation is expressed in the range of (0 °, 35 ° to 50 °, 90 °) in Euler display (φ, θ, ψ).
The frequency temperature coefficient (TCF) and the electromechanical coupling coefficient (K ² ) are important for the substrate used in the surface acoustic wave device, the frequency temperature coefficient is close to 0, and the electromechanical coupling coefficient K ² is 0.1%. The above is preferable.

The protective film 11 is a dielectric film made of oxide or nitride, and here is made of a silicon oxide film or a silicon nitride film. The protective film 11 can be formed on the LBO substrate 10 by sputtering or the like, and plays a role of protecting the LBO substrate from an etching solution or a cleaning solution used when forming the IDT electrode on the substrate.
The KH of the protective film 11 is 0.04 to 0.15. Here, KH is a value represented by the product of K and H, where λ is the wavelength of the surface acoustic wave, K (= 2π / λ) is the wave number, and H is the thickness of the protective film.

The electrode 20 is an interdigital type electrode (Inter-Digital Transducer: hereinafter referred to as “IDT electrode”), and has a pair of comb electrodes 21 and 22 as shown in FIG. The comb electrodes 21 and 22 each have a trunk portion 20a and a plurality of branch portions 20b connected by the trunk portion 20a. 1 and 2 show three branch portions 20b, but a large number are actually provided.
A high-frequency signal source 50 is connected to one comb electrode 21 constituting the IDT electrode 20, and a signal line is connected to the other comb electrode 22.

  The IDT electrode 20 corresponds to an electric signal application electrode, and the reflection electrodes 31 and 32 correspond to resonance electrodes that resonate a specific frequency component. The reflective electrodes 31 and 32 are arranged on the SAW propagation path in order to confine the SAW excited by the IDT electrode 20 in the IDT electrode 20.

The IDT electrode 20 and the reflective electrodes 31 and 32 are made of Al or an Al alloy, and KH is in the range of 0.04 to 0.15. Here, KH is a value represented by the product of K and H, where λ is the surface acoustic wave wavelength, K (= 2π / λ) is the wave number, and H is the electrode thickness. If KH is 0.1 or less, a small effect on the reduction in propagation speed and the electromechanical coupling coefficient K ² of the surface acoustic wave.

An insulating film 23 is formed on the IDT electrode 20 and the reflective electrodes 31 and 32. The insulating film 23 is made of Al ₂ O ₃ and is formed by anodic oxidation so that KH is in the range of 0.01 to 0.03. As described above, since the LBO substrate 10 is used in the present embodiment, conductive foreign matter is adhered to the insulating film 23 between the comb electrodes 21 and 22 or between the IDT electrode 20 and the reflective electrodes 31 and 32. In this case, it functions as a protective film for preventing an electrode short circuit.

  In the above configuration, when a high-frequency signal is output from the high-frequency signal source 50, this high-frequency signal is applied to one comb-tooth electrode 21 of the IDT electrode 20, thereby propagating to the reflective electrode 31 side on the upper surface of the LBO substrate 10. The surface acoustic wave that propagates and the surface acoustic wave that propagates toward the reflective electrode 32 are generated.

  Among these surface acoustic waves, a surface acoustic wave having a specific frequency component is reflected by the reflective electrode 31 and the reflective electrode 32, and a standing wave is generated between the reflective electrode 31 and the reflective electrode 32. The surface acoustic wave having the specific frequency component is repeatedly reflected by the reflection electrodes 31 and 32, whereby the specific frequency component or the frequency component in the specific band resonates and the amplitude increases.

  A part of the surface acoustic wave of the specific frequency component or the frequency component of the specific band is extracted from the other comb-teeth electrode 22 of the IDT electrode 20 and has a frequency corresponding to the resonance frequency of the reflective electrode 31 and the reflective electrode 32. An electrical signal (or a frequency having a certain band) is taken out to the terminals 55a and 55b.

The surface acoustic wave device 1 of the present embodiment includes an LBO substrate with Euler angles (0 °, 35 ° to 50 °, 90 °), and a protective film made of oxide or nitride with KH = 0.05 to 0.10. 11 and KH = 0.04 to 0.15 comb-teeth electrodes 21 and 22 are mainly configured. By adopting such a configuration, the temperature characteristics of the LBO substrate 10 can be prevented while preventing deliquescent properties against water. It can be made as small as possible. Moreover, since the quality factor (Q value) can be maintained without increasing the propagation loss by setting the film thicknesses of the protective film 11 and the comb-tooth electrodes 21 and 22 within the above ranges, the frequency characteristics are good. The surface acoustic wave device 1 is obtained.
In addition, since the insulating film 23 provided on the comb-tooth electrodes 21 and 22 can prevent a short-circuit between the comb-tooth electrodes 21 and 22 when conductive foreign matter adheres to the LBO substrate 10, Increases nature.

  In the present embodiment, a dielectric film made of oxide or nitride is used as the protective film 11, but a piezoelectric film such as AlN or ZnO may be used. However, when KH is 0.04 or less, it does not function as a piezoelectric body. Therefore, in order to secure the function as a piezoelectric body, the lower limit of KH is set to 0.04.

  Thus, by employing a piezoelectric film as the protective film 11, the phase velocity can be remarkably improved and the operating frequency can be remarkably increased. For example, when the protective film 11 is made of ZnO, there is an advantage that the electromechanical conversion efficiency is higher than that of other piezoelectric bodies. On the other hand, when the protective film 11 is made of AlN, it has an advantage that the ultrasonic wave propagating through the inside is faster than other piezoelectric bodies, and the resonance frequency is easily increased.

  With the configuration as described above, in the formation of elements in the frequency 3 GHz band, the electrode line width can be achieved with a line width of 0.4 to 0.5 μm, and it is formed using conventional photolithography technology. Can do. Therefore, it can be used as a filter for next-generation wireless systems such as LTE (Long Term Evolution).

Next, the configuration and characteristics of Example 1 will be described.
In the surface acoustic wave device according to the first embodiment, an IDT electrode made of an SiO ₂ protective film having a thickness of 300 mm and an aluminum film having a thickness of 600 mm is formed on an LBO substrate having an Euler angle (0 °, 46 °, 90 °). It is configured. The IDT electrode has 300 electrode pairs and an intersection width of 20λ. With this configuration, the wavelength λ of the generated SAW zero-order vibration mode is 2.8 μm.

FIG. 3 is a graph showing the relationship between the Euler angles (45 °, θ °, 90 °) of the LBO substrate and the propagation loss (dB / λ).
As shown in FIG. 3, regarding the relationship between the Euler angle of the LBO substrate and the propagation loss, the propagation loss rapidly increases when θ is less than 35 ° or θ exceeds 50 °. If the propagation loss exceeds 0.05 dB / λ, sufficient characteristics as a resonator cannot be obtained. Therefore, it is preferable that θ is in the range of 35 ° to 50 °. In order to further suppress the propagation loss, it is more preferable that θ is in the range of 35 ° to 43.5 °, 47.5 ° to 50 °.

The frequency characteristic of the said structure is shown.
FIG. 4 is a graph showing frequency impedance characteristics, and FIG. 5 is a graph showing frequency temperature characteristics. Here, the frequency at 25 ° C. was used as a reference.
In order to investigate the characteristics of the surface acoustic wave device, first, a high frequency signal was applied to the input terminal by a network analyzer, and the high frequency signal taken out from the output terminal was measured. As a result, a pass characteristic from the input terminal to the output terminal was obtained, and as shown in FIG. 4, it was possible to observe a resonance waveform with little influence of spurious. Further, as shown in FIG. 5, a parabola (second order coefficient) was obtained as a frequency temperature coefficient in the vicinity of 25 ° C., and the linear approximation (first order coefficient) could be set to 0 [ppm / ° C.].
[Characteristic]
Resonance frequency: 2.30 GHz
Q value: 200
Inductance L: 80 nH
Parallel capacitance C: 0.06pF
Inductor series resistance R: 5.4Ω
Electromechanical coupling coefficient K ² : 1.10%
Temperature characteristics (maximum frequency fluctuation range): 500 ppm (-20 ° C to 80 ° C)
According to the configuration of this example, excellent characteristics were shown in the 2 GHz band.

[Performance evaluation of frequency characteristics]
Next, in order to evaluate the propagation loss of the surface acoustic wave device 1, the Q value was evaluated.
Here, the characteristic evaluation was performed while changing the thickness of the Al electrode film of the surface acoustic wave device 1 and the thickness of the SiO ₂ protective film. Since it indicates that a steeper frequency characteristic is obtained as the Q value is larger, a resonator having a higher Q value is particularly desired.

FIG. 6A is a graph showing the relationship between the KH of the Al electrode and the Q value when the SiO ₂ protective film is KH0.045.
As shown in FIG. 6A, when the KH of the Al electrode is 0.15 or less, a Q value of 150 or more is obtained, and when the KH of the Al electrode is 0.10 or less, a Q value of 250 or more is obtained. The lower limit of KH is determined by an increase in the resistance value of Al, and about 0.025 seems to be the limit. Therefore, the Al electrode is preferably in the range of KH 0.025 to 0.15, and particularly preferably in the range of KH 0.025 to 0.1.

FIG. 6B is a graph showing the relationship between the KH and Q value of the SiO ₂ protective film when the Al electrode film is KH 0.045. As shown in FIG. 6B, a Q value of 150 or more is obtained when KH of the SiO ₂ protective film is 0.3 or less. For this reason, the Al electrode film is preferably formed with a KH of 0.3 or less, and the lower limit value is determined by whether or not the film is formed. If it is too thin, the number of pinholes increases, so KH of 0.010 or more is preferable.
Therefore, it is preferable that the SiO ₂ protective film is formed within a range of KH0.010 to 0.30.

FIG. 7 is a graph showing the relationship between the KH and Q value of the AlN film when the Al electrode is KH0.045.
As shown in FIG. 7, the thickness of the AlN protective film is preferably KH 0.3 or less, more preferably a Q value of 150 or more, and the lower limit value of KH is determined by whether or not the film is formed. As with the above-described SiO ₂ protective film, the AlN protective film is preferably KH 0.010 or more because if it is too thin, pinholes increase.
Therefore, the AlN protective film is preferably in the range of KH0.010 to 0.30.

  As described above, the LBO substrate 10 whose surface orientation range is represented by (φ, θ, ψ) = (45 °, 35 ° to 50 °, 90 °) is used, and the thicknesses of the protective film 11 and the electrode 20 are set. By setting within the above conditions, it is possible to set the SAW sound velocity V to 6500 m / s or more, the electromechanical coupling coefficient K2 to 1%, and the frequency temperature coefficient TCF to 0 [ppm / ° C.].

  As described above, the preferred embodiments according to the present invention have been described with reference to the accompanying drawings, but the present invention is not limited to the examples. It is obvious for those skilled in the art that various changes or modifications can be conceived within the scope of the technical idea described in the claims. It is understood that it belongs to.

DESCRIPTION OF SYMBOLS 1 ... Surface acoustic wave apparatus, 10 ... LBO board | substrate, 11 ... Protective film, 20 ... Electrode, 21, 22 ... Comb electrode, 23 ... Insulating film

Claims (5)

LBO substrate made of lithium tetraborate single crystal (Li ₂ B ₄ O ₇ ) whose surface orientation is in the range of (0 °, 35 ° to 50 °, 90 °) in Euler display (φ, θ, ψ) When,
A protective film provided on the surface of the LBO substrate and made of an oxide or nitride having a KH of 0.05 to 0.10;
A surface acoustic wave device comprising: a comb-like electrode provided on the protective film and having a KH of 0.04 to 0.15.
However, KH is a value represented by the formula [(2π / λ) × film thickness], and λ is the wavelength of the surface acoustic wave.   2. The surface acoustic wave device according to claim 1, wherein the protective film is a silicon oxide film or a silicon nitride film.   The surface acoustic wave device according to claim 1, wherein the protective film is a piezoelectric body.   4. The surface acoustic wave device according to claim 3, wherein the protective film is made of AlN or ZnO.   The surface acoustic wave device according to claim 1, wherein an insulating film is provided on the comb electrode.

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