JP4693407B2 - Piezoelectric thin film device and manufacturing method thereof - Google Patents

Description

  The present invention relates to a piezoelectric thin film device and a manufacturing method thereof, and more particularly to an acoustic multilayer film type piezoelectric thin film device using a multilayer acoustic reflection layer and a manufacturing method thereof.

  In electronic devices such as computers and communication devices, various processes are performed based on regular signals (high-frequency signals) obtained from vibrators that are one of electronic components. In these electronic devices, a frequency filter using a specific vibration mode of the vibrator is also used. As described above, a vibrator (used as a piezoelectric vibrator when a piezoelectric material is used as a vibration element) is obtained by processing a piezoelectric material such as quartz into a flat plate shape. A piezoelectric element plate is used. In a crystal resonator using crystal as a piezoelectric material, the thickness of the piezoelectric element plate made of crystal can be reduced in order to increase the fundamental resonance frequency. For example, in the case of an AT-cut quartz base plate that excites the thickness-shear vibration mode in quartz, a fundamental resonance frequency of about 67 MHz can be obtained by setting the thickness of the base plate to about 25 μm.

  However, in order to obtain a higher fundamental resonance frequency (for example, several hundred MHz or more), the thickness of the piezoelectric element plate is further reduced. However, the thinner the plate thickness is, the more difficult the machining is. Moreover, practical mechanical strength cannot be obtained. A SAW device (Surface Acoustic Wave Device) that uses surface acoustic waves is used as a device that supports such high frequencies. Compared with this, a piezoelectric that can handle higher frequencies (higher frequency). Thin film devices are attracting attention, and among them, an acoustic multilayer film (SMR (Solidly Mounted Resonator)) type piezoelectric thin film device in which a piezoelectric layer is provided on a substrate via a multilayer acoustic reflection layer has been developed ( Non-patent document 1).

  In recent years, with the shift of communication systems to the ultra-high frequency band, among piezoelectric thin film devices, for example, the piezoelectric thin film vibrator is an ultra-high frequency acoustic wave element that can be stably operated in the ultra-high frequency band. Attention has been paid. 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 multi-layered acoustic reflection layer, 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 reflection layer, stable operation is possible. Further, a technique for improving temperature characteristics by adding a thin film of SiO2 to an acoustic multilayer film 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.

  The structure of such an acoustic multilayer film type piezoelectric thin film device does not require three-dimensional processing in order to create a void structure for opening to the atmosphere, compared to the conventional diaphragm type. Further, it is not necessary to form a thin film such as a piezoelectric layer or a conductive layer, and it has a strong structure against external and internal forces that destroy the device, such as external stress and internal stress. Thus, the acoustic multilayer film type piezoelectric thin film device has excellent characteristics as a device for wireless communication.

  In this way, the acoustic multilayer film type piezoelectric thin film device has very good characteristics, but in order to prevent acoustic waves from leaking to the substrate, the substrate is in contact with the vibration part (piezoelectric layer and conductive layer). It is necessary to form a plurality of acoustic reflection layers. That is, a plurality of acoustic reflection layers are required in addition to the piezoelectric layer and the conductive layer.

  When a piezoelectric thin film device is formed by using a normal thin film forming apparatus, it is impossible to form different layer materials with the same apparatus because there is a problem in the stable production of a thin film layer. Therefore, forming thin film layers of different materials increases the necessary facilities for the different layer types. Further, since the film forming conditions for different thin film layers are managed, necessary management items are increased.

  Therefore, the inventor has developed an acoustic multilayer film type piezoelectric thin film device in which at least one of the piezoelectric layer and the acoustic reflection layer is made of the same material. However, simply using the same material for the piezoelectric layer and the acoustic reflection layer causes various problems, making it difficult to obtain excellent resonance characteristics. For example, when the same material as the piezoelectric layer, such as ZnO, is used for the high acoustic impedance layer constituting the acoustic reflection layer, the piezoelectric effect also occurs in the high acoustic impedance layer when acoustic vibration is applied, which generates charges and loss. It becomes.

  Therefore, by making the manufacturing conditions, the manufacturing means or the physical properties of the layers different between the piezoelectric layer and the acoustic reflection layer, the piezoelectric layer and the acoustic reflection layer can be made of the same material, and the acoustic multilayer film has high-performance resonance characteristics. It is an object of the present invention to produce a type of piezoelectric thin film device.

A piezoelectric thin film device according to the present invention includes a substrate, at least one piezoelectric layer made of zinc oxide or a material mainly composed of zinc oxide formed on the substrate, and a plurality of conductive layers above and below the piezoelectric layer. A piezoelectric thin film device having an acoustic reflection layer group formed by a plurality of acoustic reflection layers between the substrate and the piezoelectric layer, wherein the acoustic reflection layer group includes the first acoustic reflection layer and the second acoustic reflection layer. The first acoustic reflection layer is a high acoustic impedance layer formed of zinc oxide or a material mainly composed of zinc oxide, and the second acoustic reflection layer. Is a low acoustic impedance layer formed of silicon dioxide or a material mainly composed of silicon dioxide, the surface form of the first acoustic reflection layer is an arbitrary arithmetic surface roughness (Ra1), and the second The surface form of the acoustic reflection film is Characterized in that is formed by the arithmetic surface roughness of one acoustic reflection layer (Ra1) smaller arithmetic surface roughness (Ra2).

Further, the second acoustic reflection layer is formed by using an RF sputtering method under a vacuum degree of 0.9 Pa or less and / or a temperature environment of 300 ° C. or more and a power density of 10.0 W / cm 2 or more. The piezoelectric thin film device according to paragraph 0012 , wherein the piezoelectric thin film device is manufactured.

A method of manufacturing a piezoelectric thin film device according to the present invention includes a substrate, at least one piezoelectric layer made of zinc oxide or a material mainly composed of zinc oxide formed on the substrate, and applying a voltage to the piezoelectric layer. a plurality of conductive layers for, between the substrate and the piezoelectric layer, in the manufacturing method of the piezoelectric thin film device having an acoustic reflective layer group including a plurality of acoustic reflection layer for preventing the acoustic wave leakage, oxidation A first acoustic reflection layer, which is a high acoustic impedance layer made of zinc or zinc oxide as a main component, is formed so that its surface form has an arbitrary arithmetic surface roughness (Ra1), and silicon dioxide Alternatively, the second acoustic reflection layer, which is a low acoustic impedance layer formed of a material mainly composed of silicon dioxide, has an arithmetic surface whose surface morphology is smaller than the arithmetic surface roughness (Ra1) of the first acoustic reflection layer. Coarse Formed to have a (Ra2), by stacking a plurality of the said first acoustic reflection layer and said second acoustic reflection layer are alternately and forming the acoustic reflective layer group.

The second acoustic reflection layer, by an RF sputtering method, that under the following vacuum degree and / or 300 ° C. or higher temperature environment 0.9 Pa, where and power density to form at 10.0 W / cm @ 2 or more It is also a method for manufacturing a piezoelectric thin film device according to paragraph 0018 .

As described above, according to the piezoelectric thin film device and a manufacturing method thereof according to the present invention, as a main component and at least one piezoelectric layer is made of a material whose main component is zinc oxide or zinc oxide, zinc oxide or zinc oxide material The electrical properties of the piezoelectric thin film device are remarkably improved by making the physical properties or manufacturing means of the piezoelectric layer and the acoustic reflection layer different from each other in at least one acoustic reflection layer prepared in (1). Furthermore, since the piezoelectric layer and the acoustic reflection layer can be made of the same material, it is possible to reduce manufacturing equipment and manufacturing cost, and to provide an acoustic multilayer type piezoelectric thin film device having high-performance resonance characteristics. There is an effect that makes possible.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In addition, in order to clarify the description in each drawing, a part of the drawing is not shown. In addition, each dimension in the drawings is partially exaggerated, and the same reference numerals denote the same objects in each figure.

  FIG. 1 shows a schematic diagram of a piezoelectric thin film vibrator 1 which is one of the piezoelectric thin film devices of the present invention. The embodiment of the present invention will be described in order based on the example with reference to FIG.

  First, a substrate 2 for forming the piezoelectric thin film vibrator 1 is prepared. In this embodiment, a Si (111) substrate is used. The Si substrate 2 is a substrate having a high resistance and a specific resistance of 1000 Ω / cm or more. The surface of the Si substrate 2 has a thickness corresponding to the target resonance frequency of the Si thermal oxide film. It is formed with. The other substrate conditions are the same as those of a normal semiconductor Si substrate. However, the material of the substrate is not limited to Si, and other materials such as a quartz substrate and a glass substrate can be used. Preferably, an insulating material is desired, but a conductive substrate such as low-resistance Si can be used by performing an appropriate insulating treatment.

  The substrate 2 is subjected to a cleaning process as used in a normal semiconductor process. Then, each acoustic reflection layer 3a and 3b which comprises the acoustic reflection layer group 3 is formed. The acoustic reflection layers 3a and 3b are made of materials having different acoustic impedances determined by the density and hardness of the material. FIG. 2 shows a schematic diagram disclosing the acoustic reflection layer group 3. As shown in FIG. 2, the acoustic energy generated in the piezoelectric layer 4 to be described later is satisfied by satisfying the acoustic wave reflection and transmission conditions by alternately and regularly forming the acoustic reflection layers 3 a and 3 b having different acoustic impedances. Without leaking to the substrate 2, energy is confined between the acoustic reflection layer group 3 and the uppermost surface of the piezoelectric layer 4, and a high-performance piezoelectric thin film device can be manufactured.

  In the present invention, the structure of the acoustic reflection layer group 3 is, as shown in FIG. 2, two layers of a high acoustic impedance layer 3b and a low acoustic impedance layer 3a are used as the acoustic reflection layer. In the configuration, the low acoustic impedance layer 3a is formed from the substrate 2 side, and the high acoustic impedance layer 3b is formed thereon. Three to four sets of this combination were formed, and finally the low acoustic impedance layer 3a was formed to complete the acoustic reflection layer group 3.

  In this example, zinc oxide was used for at least one layer of the high acoustic impedance layer 3b, and silicon dioxide was used as the low acoustic impedance layer 3a. Since zinc oxide has an acoustic impedance approximately 2.5 times that of silicon dioxide, these two types of materials can be used as substances for forming an acoustic reflection layer. Zinc oxide is an excellent piezoelectric material having a large electromechanical coupling coefficient compared to a piezoelectric material such as quartz. If this zinc oxide can be used for the piezoelectric layer and the high acoustic impedance layer 3b, it is possible to reduce manufacturing equipment and management costs.

  A lower conductive layer 5 for applying a voltage to the piezoelectric layer 4 is formed on the acoustic reflection layer group 3. In the present invention, the lower conductive layer 5 formed by laminating the Au layer 5b and the Ti layer 5a is used. Ti was used to improve the adhesion between silicon dioxide and Au layer 5b. The lower conductive layer 5 was formed by a lift-off method, and the conductive layer metal was formed by an evaporation method using an electron beam. In this embodiment, the lower conductive layer 5 is formed by the above method, but a known method such as an etching method or sputtering may be used.

  The piezoelectric layer 4 is formed so as to cover the lower conductive layer 5. In this embodiment, zinc oxide, which is the same material as at least one of the high acoustic impedance layers 3b constituting the acoustic reflection layer, is used as the piezoelectric layer 4. The manufacturing method used to create the piezoelectric layer 4 was the same as the method of creating the acoustic reflection layer 3a or 3b.

  On the piezoelectric layer 4, an upper conductive layer 6 that is paired with the previously formed lower conductive layer 5 is formed. In this embodiment, the manufacturing method is the same as that for the lower conductive layer 5. Also, the method for forming the upper conductive layer 6 is not limited to the use of a known method for forming, similarly to the lower conductive layer 5. If necessary, if the lower conductive layer 5 needs to be connected to a terminal outside the device, or if the lower conductive layer 5 and the upper conductive layer 6 need to be electrically connected, the upper conductive layer 6 Before forming, a part of the piezoelectric layer 4 may be removed.

  In addition, since zinc oxide has a negative frequency temperature coefficient, the temperature characteristic of the frequency is not zero. On the other hand, silicon dioxide has a positive frequency temperature coefficient contrary to zinc oxide. Therefore, it is possible to create a piezoelectric thin film device having a desired frequency temperature coefficient by combining zinc oxide and silicon dioxide. In this embodiment, the frequency temperature coefficient is adjusted by forming an additional silicon dioxide layer on the upper conductive layer 6 or between the upper conductive layer 6 and the piezoelectric layer 4.

  FIG. 3 shows the electrical characteristics of an acoustic multilayer film type piezoelectric thin film vibrator produced by a conventional method. As shown in FIG. 3, although a slight resonance characteristic is obtained at a desired frequency, its performance is not sufficient and it cannot withstand practical use as a vibrator.

  Part of the cause of this characteristic deterioration is the formation conditions of silicon dioxide used as the low acoustic impedance layer 3a of the acoustic reflection layer group 3. That is, the Au / Ti lower conductive layer 5 manufactured under the same conditions and the same conditions on the two types of acoustic reflection layers with different formation conditions of silicon dioxide used as the low acoustic impedance layer 3a of the acoustic reflection layer group 3 and under the same conditions. Next, zinc oxide as the piezoelectric layer 4 was formed on the lower conductive layer 5 under the same conditions. Further, the upper conductive layer 6 was produced on the piezoelectric layer 4 under the same conditions, and the characteristics were compared. FIG. 4 shows the results of measuring the electrical characteristics of these two types of piezoelectric thin film vibrators. FIG. 4 shows the resonance characteristics of a piezoelectric thin film vibrator using an acoustic reflection layer formed of silicon dioxide. FIG. 4A shows the case where the temperature of the substrate 2 is 300 ° C. during the formation of the silicon dioxide layer. FIG. 4B shows the resonance characteristics of the piezoelectric thin film vibrator when the temperature of the substrate 2 is 350 ° C. during the formation of the silicon dioxide layer. As is clear from this measured value, FIG. 4B shows an excellent resonance characteristic.

  Conventionally, there are many reports on zinc oxide for piezoelectric layers, and there are also many reports on changes in crystallinity and piezoelectricity depending on the formation conditions of the piezoelectric layer. There is also a literature on silicon dioxide as a holding layer in a diaphragm type piezoelectric thin film device. However, there is no document disclosing the interaction of two types of acoustic reflection layers, the high acoustic impedance layer 3b and the low acoustic impedance layer 3a, in the acoustic multilayer film type piezoelectric thin film device.

  Therefore, in this embodiment, zinc oxide is used as at least one layer (first acoustic reflection layer) of the high acoustic impedance layer 3b, and silicon dioxide is used as the low acoustic impedance layer 3a (second acoustic reflection layer) thereon. used. And various examinations were repeated about the formation conditions of these two acoustic reflection layers. As a result, by producing silicon dioxide, which is the low acoustic impedance layer 3a, on at least one layer of the high acoustic impedance layer 3b having the arithmetic surface roughness (Ra1) = 1.913 nm, a low acoustic impedance is achieved. The arithmetic surface roughness (Ra2) of the impedance layer 3a is 1.020 nm, which can be made smaller than the arithmetic surface roughness (Ra1) of the high acoustic impedance layer 3b. The effect of the arithmetic surface roughness (Ra2) of the low acoustic impedance layer 3a and the arithmetic surface roughness (Ra1) of the high acoustic impedance layer 3b has a great influence on the electrical characteristics of the piezoelectric thin film device. The greater the difference, the better the electrical characteristics of the piezoelectric thin film device.

  Moreover, it was confirmed that the effect | action by this difference is greatly influenced by the formation conditions of the low acoustic impedance layer 3a. That is, when zinc oxide is used for at least one layer of the high acoustic impedance layer 3b and silicon dioxide is used for the low acoustic impedance layer 3b, for example, when this silicon oxide is formed by an RF magnetron sputtering apparatus, the vacuum during the formation is formed. The physical properties of the layer and the characteristics of the piezoelectric thin film device are greatly affected each time. When the degree of vacuum was 1 Pa or less, good arithmetic surface roughness difference action was obtained, and when 0.9 Pa or less, even better difference action was obtained. At the same time, it was found that the temperature at the time of formation has a great influence on this different action. According to this, the difference in arithmetic surface roughness was better as the temperature during formation was higher. In order to obtain a practical different action, it is necessary to form at a temperature of 250 ° C. or higher, and it is desirable to form it in a temperature environment of 300 ° C. or higher.

Furthermore, when zinc oxide is used for at least one layer of the high acoustic impedance layer 3b, silicon dioxide is used for the low acoustic impedance layer 3a, and silicon dioxide is formed by the RF magnetron sputtering apparatus, It has been found that the difference in arithmetic surface roughness is greatly influenced by the RF output. A good different action was obtained at 9.0 W / cm ² or more, and a better different action was obtained at 10.0 W / cm ² or more.

  Even when zinc oxide is used for the piezoelectric layer 4, there is no limitation on the combination as long as the low acoustic impedance layer 3 a has a function and effect of the present disclosure, such as selecting a material such as aluminum oxide. In addition, in the present embodiment, a conductive layer formed of at least one layer of Au and Ti was used for the metal constituting the upper and lower conductive layers, but Al, Mo, W, Pt, or the like was used for the other conductive layers. However, the material is not limited. Similarly, in this example, both zinc oxide and silicon dioxide were produced using generally known film forming conditions using an RF magnetron sputtering apparatus. In addition to this RF magnetron sputtering method, an ECR sputtering method, a CVD method, or the like can be used, and the manufacturing method, film forming conditions, and the like are not limited.

FIG. 1 is a schematic perspective view schematically showing a configuration example of an embodiment of a piezoelectric thin film device (SMR type piezoelectric thin film vibrator) according to the present invention. FIG. 2 is a schematic partial cross-sectional view schematically showing a configuration example of an embodiment of a piezoelectric thin film device (SMR type piezoelectric thin film vibrator) according to the present invention. FIG. 3 shows the electrical characteristics of an acoustic multilayer film type piezoelectric thin film vibrator produced by a conventional method. FIG. 4 illustrates the resonance characteristics of a piezoelectric thin film vibrator using an acoustic reflection layer formed of silicon dioxide. FIG. 4A illustrates the case where the temperature during film formation of the silicon dioxide layer is 300 ° C. (B) shows the resonance characteristics of the piezoelectric thin film vibrator when the temperature during the formation of the silicon dioxide layer is 350.degree.

Explanation of symbols

1. Piezoelectric thin film device (piezoelectric thin film vibrator)
2 ... Substrate 3 ... Acoustic reflective film group 3a ... Acoustic reflective layer (low acoustic impedance layer)
3b ... Acoustic reflection layer (high acoustic impedance layer)
4 ... Piezoelectric layer 5 ... Lower conductive layer 5a ... Ti layer 5b ... Au layer

Claims (4)

A substrate, at least one piezoelectric layer made of zinc oxide or a material mainly composed of zinc oxide formed on the substrate, and a plurality of conductive layers above and below the piezoelectric layer, the substrate and the piezoelectric In a piezoelectric thin film device having an acoustic reflection layer group formed by a plurality of acoustic reflection layers between layers,
The acoustic reflection layer group is configured by alternately laminating a plurality of first acoustic reflection layers and second acoustic reflection layers,
The first acoustic reflection layer is a high acoustic impedance layer formed of zinc oxide or a material mainly composed of zinc oxide,
The second acoustic reflection layer is a low acoustic impedance layer formed of silicon dioxide or a material mainly composed of silicon dioxide,
The surface configuration of the first acoustic reflection layer is an arbitrary arithmetic surface roughness (Ra1), and the surface configuration of the second acoustic reflection layer is based on the arithmetic surface roughness (Ra1) of the first acoustic reflection layer. A piezoelectric thin film device having a small arithmetic surface roughness (Ra2). The second acoustic reflection layer is formed using an RF sputtering method under a vacuum degree of 0.9 Pa or less and / or a temperature environment of 300 ° C. or more and a power density of 10.0 W / cm 2 or more. the piezoelectric thin film device according to claim 1, characterized in that the. A substrate, at least one piezoelectric layer made of zinc oxide or a material mainly composed of zinc oxide formed on the substrate, and a plurality of conductive layers for applying a voltage to the piezoelectric layer, In the method of manufacturing a piezoelectric thin film device having an acoustic reflection layer group composed of a plurality of acoustic reflection layers between a substrate and the piezoelectric layer,
Forming a first acoustic reflection layer, which is a high acoustic impedance layer formed of zinc oxide or a material containing zinc oxide as a main component, so that its surface form has an arbitrary arithmetic surface roughness (Ra1);
The second acoustic reflection layer, which is a low acoustic impedance layer formed of silicon dioxide or a material mainly composed of silicon dioxide, has a surface form smaller than the arithmetic surface roughness (Ra1) of the first acoustic reflection layer. Formed to have an arithmetic surface roughness (Ra2),
A plurality of the first acoustic reflection layers and the second acoustic reflection layers are alternately stacked to form the acoustic reflection layer group.
A method for manufacturing a piezoelectric thin film device. The second acoustic reflection layer is formed using an RF sputtering method in a vacuum degree of 0.9 Pa or less and / or a temperature environment of 300 ° C. or more and a power density of 10.0 W / cm 2 or more. The method of manufacturing a piezoelectric thin film device according to claim 3.

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