JP4339604B2 - Piezoelectric thin film element - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a piezoelectric thin film element including a piezoelectric thin film provided on a substrate via an acoustic multilayer film.
[0002]
[Prior art]
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. To increase the fundamental resonance frequency, a piezoelectric plate is used. You can make it thinner. For example, the resonant frequency of about 50 MHz is obtained by setting the plate thickness to about 30 to 40 μm.
[0003]
However, in order to obtain a higher fundamental resonance frequency than this, the plate thickness is made thinner. However, as the plate thickness is made thinner, machining becomes more difficult and practical mechanical strength is reduced. I can't get it.
On the other hand, an 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). ).
[0004]
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.
[0005]
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).
[0006]
[Non-Patent Document 1]
KMLakin, et al. "Development of Miniature Filters for Wireless Applications", IEEE Transactions on Microwave Theory and Techniques, Vol. 43, No. 12, p2933, December 1995.
[Non-Patent Document 2]
KMLakin, et al. "Temperature Compensated Bulk Acoustic Thin Film Resonator", IEEE Ultrasonics Symposium paper 3H-2, October 24, 2000.
[0007]
[Problems to be solved by the invention]
However, the above-described conventional technique has a problem that a process for manufacturing an element becomes long because a new film needs to be added in order to improve temperature characteristics.
The present invention has been made to solve the above-described problems, and in a state in which an increase in new manufacturing processes is suppressed, a piezoelectric thin film element such as a SMR (Solidly Mounted Resonator) type piezoelectric thin film vibrator. The purpose is to improve the temperature characteristics of.
[0008]
[Means for Solving the Problems]
The piezoelectric thin film element according to the present invention is
An acoustic multilayer film that is placed on a substrate and laminated in the order of a high acoustic impedance layer and a low acoustic impedance layer having a lower acoustic impedance, and the uppermost layer is a low acoustic impedance layer, and is formed on this acoustic multilayer film A delay time temperature coefficient of the low acoustic impedance layer, comprising: a first electrode film formed; a piezoelectric thin film formed on the first electrode film; and a second electrode film formed on the piezoelectric thin film. Is a sign opposite to the delay time temperature coefficient of the piezoelectric thin film, and the uppermost low acoustic impedance layer is 1 / of the wavelength when the sound wave having the resonance frequency of the piezoelectric thin film propagates through the uppermost low acoustic impedance layer. It is formed to be thicker or thinner than 4.
In this piezoelectric thin film element, the frequency acoustic 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 uppermost low acoustic impedance layer is changed to the resonance frequency of the piezoelectric thin film. The resonance frequency temperature coefficient of the piezoelectric thin film element is brought close to 0 by making the thickness of the acoustic wave thicker or thinner than ¼ of the wavelength when the sound wave propagates through the uppermost low acoustic impedance layer.
[0009]
In the piezoelectric thin film element, the resonance frequency temperature coefficient of the piezoelectric thin film element is ¼ of the wavelength when the sound wave having the resonance frequency of the piezoelectric thin film propagates through the uppermost low acoustic impedance layer through the uppermost low acoustic impedance layer. The resonance frequency temperature coefficient in the case of the thickness is closer to zero.
In the piezoelectric thin film element, the low acoustic impedance layer excluding the uppermost low acoustic impedance layer is formed to have a thickness of ¼ of the wavelength when the sound wave having the resonance frequency of the piezoelectric thin film propagates through the low acoustic impedance layer. and, the high acoustic impedance layer is All Ku formed 1/4 the thickness of the wavelength when the sound waves of the resonant frequency of the piezoelectric thin film propagates the high acoustic impedance layer. In this state, if the uppermost low acoustic impedance layer is formed to a thickness substantially 0.47 times the wavelength when the sound wave having the resonance frequency of the piezoelectric thin film propagates through the uppermost low acoustic impedance layer, The resonance frequency temperature coefficient of the piezoelectric thin film element can be made substantially “0”.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
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, ZnO layers (high acoustic impedance layers) 102 and SiO ₂ layers (low acoustic impedance layers) 103 are alternately laminated on a substrate 101 made of single crystal silicon or the like. An acoustic multilayer film 105 having an SiO ₂ layer 104 as an upper layer is provided. The acoustic multilayer film 105 is laminated, for example, by 8 layers or more.
[0011]
On the acoustic multilayer film 105, a piezoelectric thin film 107 made of, for example, ZnO is formed between the electrode films 106 and 108. The electrode films 106 and 108 are made 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.
[0012]
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.
[0013]
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 affected not only by the thickness of the piezoelectric thin film 107 but also by the electrode films 106 and 108, and deviates from the theoretical value calculated from the thickness of the piezoelectric thin film 107. It is. If the influence of the electrode films 106 and 108 is large, for example, the thickness of the piezoelectric thin film 107 may be changed as appropriate.
[0014]
In the acoustic multilayer film 105, the ZnO 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 thicknesses of the ZnO layer 102 and the SiO ₂ layer 103 constituting the acoustic multilayer film 105 may be λ / 4, respectively. For example, the ZnO layer 102 is about 0.41 μm and the SiO ₂ layer 103 is about 0.46 μm. And it is sufficient.
The piezoelectric multilayer film 105 acoustically isolates the piezoelectric thin film 107 and the substrate 101, 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.
[0015]
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 addition, in the present embodiment, the film thickness d ₁ of the SiO ₂ layer 104, which is one of the low acoustic impedance layers, is formed to be thicker than the film thickness d ₂ of the other SiO ₂ layer 103, so that FIG. The resonance frequency temperature coefficient (TCF) of the piezoelectric thin film element shown was made closer to “0”. The fact that TCF is close to “0” means that the frequency-temperature characteristic is good.
[0016]
According to the investigation by the inventors, the TCF of the piezoelectric thin film element shown in FIG. 1 is set to “0” by making the thickness of the SiO ₂ layer 104, which is the uppermost SiO ₂ layer closest to the piezoelectric thin film 107, larger than λ / 4. It became clear that it became possible to approach (FIG. 2). As is apparent from FIG. 2, the TCF can be made substantially zero by setting the film thickness d ₁ of the SiO ₂ layer 104 to about 0.47λ. FIG. 2 shows the results of analysis with the electrode films 106 and 108 having a thickness of zero.
[0017]
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 described above, the TCF of the piezoelectric thin film element shown in FIG. 1 can be brought close to “0”. Further, according to the present embodiment, the TCF of the piezoelectric thin film element shown in FIG. 1 can be brought close to “0” without adding a new layer.
[0018]
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”.
[0019]
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.
[0020]
Here, as a result of investigations by the inventors, if the thickness of the SiO ₂ layer 104, which is the uppermost layer of the acoustic multilayer film 105, is thicker than λ / 4, other characteristics of the piezoelectric thin film element are affected as shown below. Turned out to give. FIG. 3 shows that the effective electromechanical coupling coefficient (K ² ) of the piezoelectric thin film element is lowered by making the thickness of the SiO ₂ layer 104, which is the uppermost layer of the acoustic multilayer film 105, larger than λ / 4. Show. This is considered because the vibration energy contained in the SiO ₂ layer 104 is increased. FIG. 3 shows the results of analysis with the electrode films 106 and 108 having a thickness of zero.
[0021]
Next, the vibration energy at each part of the piezoelectric thin film element is shown as the vibration displacement | u | in FIG. 4 and FIG. 5 with the film thickness of the SiO ₂ layer 104 as a parameter. 4 and 5, a comparison of peak change, when the SiO ₂ layer 104 and the film thickness lambda / 4 than the vibration energy contained in the SiO ₂ layer 104 (FIG. 4), the film of the SiO ₂ layer 104 When the thickness (d ₁ ) is 0.47λ (FIG. 5), it is larger. 4 and 5 show an example of the acoustic multilayer film 105 in which the number of stacked layers of the ZnO layer 102 and the SiO ₂ layer 103 is eight.
[0022]
As is clear from the comparison between FIG. 4 and FIG. 5, the energy leaking to the substrate 101 is larger when d ₁ is set to 0.47λ, which is a factor for lowering the Q value. . Note that the Q value represents the degree of sharpness of resonance characteristics when natural vibration occurs.
[0023]
Based on the above, the inventors changed the film thickness (d ₂ ) of the SiO ₂ layer 103 and the film thickness of the ZnO layer 102 in addition to the film thickness (d ₁ ) of the SiO ₂ layer 104 so that the TCF is The combinations that become “0” were investigated. The results of this investigation are shown in FIG. As shown in FIG. 6, K ² and Q value also change as d ₂ changes. It can also be seen that there are two regions for d ₁ that satisfy TCF = 0.
[0024]
Among these, in the vicinity of d ₁ = 0.47λ, the Q value tends to decrease as K ² increases, and it is understood that it is better to set d ₁ according to the characteristics required for the piezoelectric thin film element. .
In the vicinity of d ₁ = 0.75λ, both K ² and the Q value peak at the same position. This is considered to state that the film thickness of the SiO ₂ layer 104 (d ₁₎ is 3 / corresponds to 4λ become place, SiO ₂ layer having a thickness of lambda / 2 the SiO ₂ layer 104 is added . As a result, it is considered that the peak of K ² and the Q value coincide in the vicinity of d ₁ = 0.75λ. However, as shown in FIG. 7, since the film thickness of the SiO ₂ layer 104 is increased lambda / 2, the vibration energy contained in the SiO ₂ layer 104 is increased, K ² is, d ₁ = 0.46λ~0.48λ Lower than nearby.
[0025]
As described above, according to this embodiment, first, the low acoustic impedance layer of the acoustic multilayer film 105 is made of SiO ₂ having a frequency temperature characteristic with a sign different from that of the piezoelectric thin film 107. In addition, the frequency temperature characteristic of the piezoelectric thin film element is improved by making the thickness of the SiO ₂ layer 104 larger than λ / 4 while the thickness of the ZnO layer 102 and the SiO ₂ layer 103 is λ / 4. I did it. Among these, by setting the film thickness of the SiO ₂ layer 104 to 0.47λ, the TCF of the piezoelectric thin film element of FIG. 1 can be made almost “0”.
[0026]
Further, as shown in FIG. 6, the SiO ₂ layer 103 is formed thicker than λ / 4, and the SiO ₂ layer 104 is formed thicker than the SiO ₂ layer 103, thereby satisfying TCF = 0 and Q Values and K ² can be required characteristics.
In recent years, with the increase in communication volume, there has been a demand for higher frequency of communication systems. According to the above-described piezoelectric thin film element in the present embodiment, the temperature characteristics that can be used at ultra-high frequencies. A good acoustic wave device can be provided.
[0027]
In the above description, zinc oxide is used as the piezoelectric thin film, a zinc oxide layer is used as the high acoustic impedance layer, and a SiO ₂ layer is used as the low acoustic impedance layer. However, the present invention is not limited to this.
The same applies to the case where another piezoelectric material is used as the piezoelectric thin film, a material having a TCD having a sign different from that of the piezoelectric material is used for the low acoustic impedance layer, and a material having a higher acoustic impedance is used for the high acoustic impedance layer.
[0028]
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 AlN may be used for the high acoustic impedance layer.
[0029]
【The invention's effect】
As described above, in the present invention, a material having a delay time temperature coefficient having a sign opposite to the delay time temperature coefficient of the piezoelectric thin film is used for the low acoustic impedance layer having a low acoustic impedance constituting the acoustic multilayer film. And, the uppermost low acoustic impedance layer has a resonance frequency of the piezoelectric thin film such that the sound wave having the resonance frequency of the piezoelectric thin film is thicker or thinner than ¼ of the wavelength when propagating through the uppermost low acoustic impedance layer. The thickness was different from ¼ of the wavelength when the sound wave propagated through the uppermost low acoustic impedance layer.
As a result, according to the present invention, the frequency temperature characteristic of the piezoelectric thin film is changed in the opposite direction without increasing a new manufacturing process, and the frequency temperature characteristic of the piezoelectric thin film element is improved. An excellent effect of being able to approach 0 is obtained.
[0030]
Further, for example, the low acoustic impedance layer other than the uppermost low acoustic impedance layer is formed to be thicker than 1 / 4λ of the wavelength when the sound wave having the resonance frequency of the piezoelectric thin film propagates through the low acoustic impedance layer. The low acoustic impedance layer is formed thicker than the other low acoustic impedance layers, and by setting each film thickness appropriately, the frequency temperature characteristic of the piezoelectric thin film element is substantially “0”. It is possible to change K ² to a desired value.
[Brief description of 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) in an embodiment of the present invention.
FIG. 2 is a characteristic diagram showing a change in TCF depending on the thickness of a SiO ₂ layer 104 which is the uppermost layer of an acoustic multilayer film.
FIG. 3 is a characteristic diagram showing a change in K ² depending on the thickness of the SiO ₂ layer 104 which is the uppermost layer of the acoustic multilayer film.
FIG. 4 is a distribution diagram showing a vibration displacement distribution when the acoustic multilayer film 105 has eight layers.
FIG. 5 is a distribution diagram showing a vibration displacement distribution when the acoustic multilayer film 105 has eight layers and the thickness d ₁ of the SiO ₂ layer 104 is 0.47λ.
FIG. 6 is a correlation diagram showing the relationship between the thickness d ₁ of the SiO ₂ layer 104 and the K ² and Q values when TCF = 0 is satisfied.
FIG. 7 is a distribution diagram showing a vibration displacement distribution when the thickness d ₁ of the SiO ₂ layer 104 is 0.75λ and the thickness d ₂ of the SiO ₂ layer 103 is 0.25λ.
[Explanation of symbols]
101 ... substrate, 102 ... ZnO layer, 103 ... SiO ₂ layer, 104 ... SiO ₂ layer, 105 ... acoustic multilayer, 106 ... electrode film 107 ... piezoelectric thin film, 108 ... electrode film.

Claims (2)

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 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 low acoustic impedance layer of the uppermost layer is formed to be thicker than ¼ of the wavelength when the sound wave of the resonance frequency of the piezoelectric thin film propagates through the lower acoustic impedance layer of the uppermost layer ,
The resonance frequency temperature coefficient of the piezoelectric thin film element has a thickness that is ¼ of the wavelength when the sound wave having the resonance frequency of the piezoelectric thin film propagates through the low acoustic impedance layer of the uppermost layer through the low acoustic impedance layer of the uppermost layer. Near the resonance frequency temperature coefficient in the case of
The low acoustic impedance layer excluding the uppermost low acoustic impedance layer is formed to have a thickness of ¼ of the wavelength when the acoustic wave of the resonance frequency of the piezoelectric thin film propagates through the low acoustic impedance layer,
The high acoustic impedance layer is formed to a thickness of ¼ of a wavelength when a sound wave having a resonance frequency of the piezoelectric thin film propagates through the high acoustic impedance layer.
The top acoustic acoustic impedance layer is
A piezoelectric thin film element characterized in that the piezoelectric thin film element is formed to have a thickness substantially 0.47 times the wavelength when a sound wave having a resonance frequency of the piezoelectric thin film propagates through the uppermost low acoustic impedance layer . 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 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 lower acoustic impedance layer of the uppermost layer is formed to be thinner than ¼ of the wavelength when the sound wave of the resonance frequency of the piezoelectric thin film propagates through the lower acoustic impedance layer of the uppermost layer ,
The resonance frequency temperature coefficient of the piezoelectric thin film element has a thickness that is ¼ of the wavelength when the sound wave having the resonance frequency of the piezoelectric thin film propagates through the low acoustic impedance layer of the uppermost layer through the low acoustic impedance layer of the uppermost layer. Near the resonance frequency temperature coefficient in the case of
The low acoustic impedance layer excluding the uppermost low acoustic impedance layer is formed to have a thickness of ¼ of the wavelength when the acoustic wave of the resonance frequency of the piezoelectric thin film propagates through the low acoustic impedance layer,
The high acoustic impedance layer is formed to a thickness of ¼ of a wavelength when a sound wave having a resonance frequency of the piezoelectric thin film propagates through the high acoustic impedance layer.
The top acoustic acoustic impedance layer is
A piezoelectric thin film element characterized in that the piezoelectric thin film element is formed to have a thickness substantially 0.47 times the wavelength when a sound wave having a resonance frequency of the piezoelectric thin film propagates through the uppermost low acoustic impedance layer .

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