JP2006109128A - Piezoelectric thin film element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce a loss in a high acoustic impedance layer of an SMR type piezoelectric thin film element. <P>SOLUTION: The piezoelectric thin film element is provided with an acoustic multilayer film 105 formed by alternately laminating an Al<SB>2</SB>O<SB>3</SB>layer (high acoustic impedance layer) 102 and a SiO<SB>2</SB>layer (low acoustic impedance layer) 103 on a substrate 101 comprising single crystal silicon or the like and the uppermost layer of which is a SiO<SB>2</SB>layer 104. The acoustic multilayer film 105 may be laminated by, e.g. 8 layers or more. A piezoelectric thin film 107 comprising, e.g. ZnO clamped between electrode films 106, and 108 is formed on the acoustic multilayer film 105. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a piezoelectric thin film element including a piezoelectric thin film provided on a substrate via an acoustic multilayer film.

  For example, electronic devices such as computers and communication devices are processed based on regular signals (high-frequency signals) obtained from vibrators. Moreover, in these electronic devices, it is used also for the frequency filter using the thickness vibration of the vibrator. As described above, a piezoelectric plate made of a piezoelectric material such as quartz has been used for a vibrator (piezoelectric vibrator) used in an electronic device. In order to increase the fundamental resonance frequency, a piezoelectric plate is used. You can make it thinner. For example, in a quartz AT plate, a resonance frequency of about 67 MHz is obtained by setting the plate thickness to about 25 μm.

However, in order to obtain a higher fundamental resonance frequency than this, the plate thickness is made thinner. However, the thinner the plate thickness, the more difficult the machining is and the practical mechanical strength cannot be obtained.
On the other hand, a SMR (Solidly Mounted Resonator) type piezoelectric thin film vibrator in which a piezoelectric thin film having a thickness of λ / 2 is provided on a substrate via an acoustic multilayer film has been developed (Non-patent Document 1). ).

In recent years, the transition of the communication system to the super-high frequency band has been promoted, and the piezoelectric thin film vibrator has attracted attention as an ultra-high frequency acoustic wave element that can be stably operated in the super-high frequency band.
In the SMR type piezoelectric thin film vibrator, a piezoelectric thin film having a thickness of λ / 2 is formed on an acoustic multilayer film in which two kinds of thin films (λ / 4) having different acoustic impedances are alternately laminated on a substrate. Is.

According to this configuration, the piezoelectric thin film is acoustically separated from the substrate by the acoustic multilayer film, and resonance with a high Q value can be obtained. In addition, since the entire lower surface of the piezoelectric thin film is fixed by the acoustic multilayer film, stable operation is possible.
A technique for improving temperature characteristics by adding a SiO ₂ thin film to an SMR type piezoelectric thin film vibrator has been proposed (Non-patent Document 2).
A technique for improving temperature characteristics without adding a new film has also been proposed.

The applicant has not yet found prior art documents related to the present invention by the time of filing other than the prior art documents specified by the prior art document information described in this specification.
KMLakin, et al. "Development of Miniature Filters for Wireless Applications", IEEE Transactions on Microwave Theory and Techniques, Vol. 43, No. 12, p2933, December 1995. KMLakin, et al. "Temperature Compensated Bulk Acoustic Thin Film Resonator", IEEE Ultrasonics Symposium paper 3H-2, October 24, 2000.

By the way, as an SMR type piezoelectric thin film vibrator, a low acoustic impedance layer made of SiO ₂ and a high acoustic impedance layer made of ZnO are laminated to form an acoustic multilayer film, and a piezoelectric thin film made of ZnO is used thereon. Has become commonplace. These are formed using a technique of crystal growth in a gas phase, but ZnO has good characteristics as a piezoelectric material and is easy to use as a high acoustic impedance layer. For this reason, the SMR-type piezoelectric thin film vibrator having the above-described material structure has many advantages such that it is not necessary to use various materials and the manufacture is easy.

However, when the same material as the piezoelectric thin film such as ZnO is used for the high acoustic impedance layer, when acoustic vibration is applied, electric charges are generated due to the piezoelectric effect, resulting in a loss.
The present invention has been made to solve the above-described problems, and an object thereof is to reduce the loss in the high acoustic impedance layer of the SMR type piezoelectric thin film vibrator.

The piezoelectric thin film element according to the present invention is disposed on a substrate, laminated in the order of a high acoustic impedance layer and a low acoustic impedance layer having a lower acoustic impedance, and an acoustic multilayer film in which the uppermost layer is a low acoustic impedance layer. A first electrode film formed on the acoustic multilayer film, a piezoelectric thin film formed on the first electrode film, and a second electrode film formed on the piezoelectric thin film, The high acoustic impedance layer is made of a material having no piezoelectricity.
In this piezoelectric thin film element, in the high acoustic impedance layer constituting the acoustic multilayer film, electric charges are not generated by the acoustic vibration that propagates and is applied.

In the piezoelectric thin film element, the high acoustic impedance layer may be made of a dielectric crystal such as Al ₂ O ₃ , for example. The high acoustic impedance layer may be made of an amorphous piezoelectric material such as an amorphous ZnO.
Further, in the piezoelectric thin film element, the delay time temperature coefficient of the low acoustic impedance layer is opposite in sign to the delay time temperature coefficient of the piezoelectric thin film, and the uppermost low acoustic impedance layer is an acoustic wave having a resonance frequency of the piezoelectric thin film. May be thicker than ¼ of the wavelength when propagating through the uppermost low acoustic impedance layer.

  As described above, according to the present invention, since the high acoustic impedance layer constituting the acoustic multilayer film is made of a material having no piezoelectricity, the loss in the high acoustic impedance layer of the SMR type piezoelectric thin film vibrator is determined. As a result, an excellent effect can be obtained.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a schematic cross-sectional view schematically showing a configuration example of a piezoelectric thin film element (SMR type piezoelectric thin film vibrator) according to an embodiment of the present invention. In this piezoelectric thin film element, first, Al ₂ O ₃ layers (high acoustic impedance layers) 102 and SiO ₂ layers (low acoustic impedance layers) 103 are alternately stacked on a substrate 101 made of single crystal silicon or the like. Thus, an acoustic multilayer film 105 having the uppermost layer as the SiO ₂ layer 104 is provided. The acoustic multilayer film 105 is laminated, for example, by 8 layers or more. These can be formed by, for example, a vacuum deposition method, a sputtering method, a CVD method, or the like. Note that the lowermost layer of the acoustic multilayer film 105 on the substrate 101 may be the SiO ₂ layer 103. The acoustic multilayer film 105 may be formed by alternately laminating a plurality of layers having different acoustic impedances, and the uppermost layer may be a layer having a lower acoustic impedance.

On the acoustic multilayer film 105, an electrode film 106 and a piezoelectric thin film 107 made of ZnO, for example, are formed. The electrode film 106 and the electrode film 108 are composed of, for example, a Cr film having a thickness of about 5 nm and an Au film having a thickness of about 30 nm. Here, assuming that the resonance frequency of the piezoelectric thin film 107 in a single state is F ₀ , the film thickness of the piezoelectric thin film 107 is half of the wavelength λ when the sound wave having the frequency F ₀ propagates through the single piezoelectric thin film 107. Is.

By the way, the film thickness is generally expressed as “the film thickness of the piezoelectric thin film 107 has a thickness of ½λ”. Hereinafter, “λ” is used for the film thickness of each layer. For example, "film thickness of the SiO ₂ layer 103, a is 1 / 4.lamda" refers to the thickness of the SiO ₂ layer 103, the wavelength λ when a sound wave of frequency F ₀ is propagated SiO ₂ layer 103 alone It shows that it is 1/4. Therefore, the value of “λ” is different in each layer.

  Here, if the film thickness of the piezoelectric thin film 107 is formed to about 700 nm, for example, the resonance frequency of the piezoelectric thin film element of FIG. 1 is about 3 GHz. However, in the case of the piezoelectric thin film element shown in FIG. 1, the resonance frequency is influenced not only by the thickness of the piezoelectric thin film 107 but also by the electrode film 106 and the electrode film 108, and from the theoretical value calculated from the thickness of the piezoelectric thin film 107. It is a deviation. When the influence of the electrode film 106 and the electrode film 108 is large, for example, the thickness of the piezoelectric thin film 107 may be changed as appropriate.

In the acoustic multilayer film 105, the Al ₂ O ₃ layer 102 functions as a high acoustic impedance layer, and the SiO ₂ layer 103 functions as a low acoustic impedance layer. These are relative. The film thicknesses of the Al ₂ O ₃ layer 102 and the SiO ₂ layer 103 constituting the acoustic multilayer film 105 may be λ / 4, respectively. For example, the Al ₂ O ₃ layer 102 has a thickness of 0.41 μm and a SiO ₂ layer. 103 may be about 0.46 μm.

Further, according to the piezoelectric thin film element shown in FIG. 1, since the high acoustic impedance layer is made of Al ₂ O ₃ which is a dielectric material having no piezoelectricity, energy loss in the acoustic multilayer film 105 can be suppressed. . An acoustic vibration is applied to the acoustic multilayer film 105. If a material having piezoelectricity is present here, an electric charge is generated by the applied acoustic vibration, and this is moved and lost as thermal energy. . On the other hand, in the piezoelectric thin film element shown in FIG. 1, since the acoustic multilayer film 105 is made of a material having no piezoelectricity, energy loss can be prevented even when acoustic vibration is applied. A high state is obtained.

  In the piezoelectric thin film element shown in FIG. 1, the piezoelectric thin film 107 and the substrate 101 are acoustically isolated by the acoustic multilayer film 105, and resonance with a high Q value can be obtained. Further, since the piezoelectric thin film 107 is held on the entire surface by the acoustic multilayer film 105, stable operation is possible.

  As the number of layers of the acoustic multilayer film 105 increases, the load impedance as viewed from the piezoelectric thin film 107 toward the substrate 101 decreases. Therefore, by making the acoustic multilayer film 105 a multilayer of, for example, eight layers or more, the piezoelectric thin film 107 can be brought into a free state on both sides, and resonance with a high Q value can be realized.

In the above description, the Al ₂ O ₃ layer is used as the high acoustic impedance layer, but the present invention is not limited to this. The high acoustic impedance layer may be made of a material having no piezoelectricity that has an impedance difference of 3 or more with respect to the low acoustic impedance layer. For example, the high acoustic impedance layer may be made of amorphous ZnO. Even if the crystal is a material exhibiting piezoelectricity, the loss of energy of the acoustic multilayer film can be suppressed by using it for the high acoustic impedance layer as an amorphous material having no crystal orientation.

  It should be noted that the frequency temperature characteristic of the piezoelectric thin film is changed in the opposite direction by the uppermost low acoustic impedance layer constituting the acoustic multilayer film, and the acoustic wave of the resonance frequency of the piezoelectric thin film is the highest in the uppermost low acoustic impedance layer. The resonant frequency temperature coefficient of the piezoelectric thin film element can be made close to 0 by making it thicker or thinner than ¼ of the wavelength when propagating through the upper low acoustic impedance layer.

For example, in the configuration shown in FIG. 1, the resonance frequency temperature coefficient (TCF) of the piezoelectric thin film element can be increased by forming the film thickness d ₁ of the SiO ₂ layer 104 larger than the film thickness d ₂ of the other SiO ₂ layer 103. , It can be closer to “0”. The fact that TCF is close to “0” means that the frequency-temperature characteristic is good.

The SiO ₂ thin film (amorphous) has a delay time temperature coefficient (TCD) opposite to that of the piezoelectric thin film 107 made of zinc oxide, and such a material is used as the low acoustic impedance layer of the acoustic multilayer film 105. As a result, the TCF of the piezoelectric thin film element shown in FIG. 1 can be made close to “0”. With this configuration, the TCF of the piezoelectric thin film element shown in FIG. 1 can be brought close to “0” without adding a new layer.

In the case of the piezoelectric thin film element of FIG. 1 using the piezoelectric thin film 107 made of ZnO, assuming that the thickness of the uppermost SiO ₂ layer 104 is λ / 4, the absolute value of the TCF of the piezoelectric thin film element shown in FIG. Is greater than “0”. Since the sign of TCF of the piezoelectric thin film element in this case is the same as that of the TCD of the piezoelectric thin film 107, in this embodiment, by making the film thickness of the SiO ₂ layer 104 larger than λ / 4, FIG. The TCF of the piezoelectric thin film element shown was made closer to “0”.

On the other hand, when the thickness of the low acoustic impedance layer having the reverse sign TCD in the uppermost layer is made larger than λ / 4, the TCF of the piezoelectric thin film element may be separated from “0”. In such a case, the TCF of the piezoelectric thin film element may be made closer to “0” by making the film thickness of the low acoustic impedance layer having the TCD of the opposite sign in the uppermost layer thinner than λ / 4.
For example, by setting the thickness of the SiO ₂ layer 104 to 0.47λ, the TCF of the piezoelectric thin film element shown in FIG. 1 can be made almost “0”.

Further, the SiO ₂ layer 103 is formed to be thicker than λ / 4, and the SiO ₂ layer 104 is formed to be thicker than the SiO ₂ layer 103, thereby satisfying TCF = 0 and requiring Q value and K ^2. It becomes possible.

In the above description, zinc oxide is used as the piezoelectric thin film, but the present invention is not limited to this. For example, aluminum nitride (AlN), cadmium sulfide (CdS), PZT (PbZrO ₃ —PbTiO ₃ solid solution), crystal, potassium niobate (KNbO ₃ ), or the like can be used as the material for the piezoelectric thin film. When AlN is used for the piezoelectric material, SiO ₂ may be used for the low acoustic impedance layer, and amorphous AlN may be used for the high acoustic impedance layer.

1 is a schematic cross-sectional view schematically showing a configuration example of a piezoelectric thin film element (SMR type piezoelectric thin film vibrator) in an embodiment of the present invention.

Explanation of symbols

101 ... substrate, 102 ... Al ₂ O ₃ layer (the high acoustic impedance layer), 103 ... SiO ₂ layer, 104 ... SiO ₂ layer, 105 ... acoustic multilayer, 106 ... electrode film 107 ... piezoelectric thin film, 108 ... electrode film .

Claims (6)

An acoustic multilayer film disposed on a substrate, laminated in the order of a high acoustic impedance layer, a low acoustic impedance layer having a lower acoustic impedance, and the uppermost layer being the low acoustic impedance layer;
A first electrode film formed on the acoustic multilayer film;
A piezoelectric thin film formed on the first electrode film;
And at least a second electrode film formed on the piezoelectric thin film,
The high acoustic impedance layer is made of a material having no piezoelectricity. A piezoelectric thin film element. The piezoelectric thin film element according to claim 1, wherein
The piezoelectric thin film element, wherein the high acoustic impedance layer is made of a dielectric crystal. The piezoelectric thin film element according to claim 2,
The high acoustic impedance layer is made of Al ₂ O ₃ . The piezoelectric thin film element according to claim 1, wherein
The high acoustic impedance layer is made of an amorphous piezoelectric material. A piezoelectric thin film element. The piezoelectric thin film element according to claim 4,
The piezoelectric thin film element, wherein the high acoustic impedance layer is made of an amorphous ZnO. In the piezoelectric thin film element according to any one of claims 1 to 5,
The delay time temperature coefficient of the low acoustic impedance layer is opposite in sign to the delay time temperature coefficient of the piezoelectric thin film,
The uppermost low acoustic impedance layer is formed to be thicker than 1/4 of a wavelength when a sound wave having a resonance frequency of the piezoelectric thin film propagates through the uppermost low acoustic impedance layer. element.

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