JP6908324B2 - Piezoelectric element - Google Patents

Description

The present invention relates to a piezoelectric element, and more particularly to a piezoelectric element utilizing a high-sensitivity, low-noise transverse piezoelectric effect.

In recent years, smartphones, whose demand is rapidly expanding, are often used for microphones using MEMS (Micro Electro Mechanical System) technology, which is small and thin and has resistance to high temperature processing in the solder reflow process of assembly. Further, not only MEMS microphones but also other MEMS elements are rapidly becoming widespread in various fields.

Most of these types of MEMS elements are capacitive elements that capture the vibration displacement of the diaphragm due to acoustic pressure or the like as a capacitance change with the opposing fixed plate, convert it into an electric signal, and output it. However, the improvement of the signal-to-noise ratio of the capacitive element is becoming a limit due to the acoustic resistance caused by the flow of air in the gap between the diaphragm and the fixing plate.

Therefore, a piezoelectric element capable of extracting acoustic pressure or the like as a voltage change due to distortion of a single diaphragm composed of a piezoelectric film has been attracting attention.

By the way, in this type of piezoelectric element, it is known that when the piezoelectric film constituting the diaphragm is vibrated and displaced due to acoustic pressure or the like, the strain or stress applied to the piezoelectric film in the thickness direction of the piezoelectric film is in the opposite direction. .. FIG. 7 is an explanatory diagram schematically showing strain or stress applied to the piezoelectric film in a piezoelectric element having a general structure. In the piezoelectric element shown in FIG. 7, a piezoelectric film 3 is laminated on a silicon substrate 1 serving as a support substrate via an insulating film 2 made _{of a silicon oxide film (SiO 2).} Further, a slit (not shown) is formed to form a double-sided beam structure. A pair of electrodes 4 are formed on the front surface and the back surface of the piezoelectric film 3 so as to sandwich the piezoelectric film 3, and each of the electrodes 4 is connected to a wiring electrode 5.

In a piezoelectric element having such a structure, for example, when an acoustic pressure signal is applied from the silicon substrate 1 side as shown in FIG. 7, tensile stress is generated on the silicon substrate side of the piezoelectric film 3 in regions A and C. Tensile stress is generated on the surface side. On the other hand, in region B, compressive stress is generated on the silicon substrate side of the piezoelectric film, and tensile stress is generated on the surface side.

In the piezoelectric film sandwiched between the pair of electrodes in this way, the polarities of the generated voltages are opposite in the region fixed to the support substrate (silicon substrate 1) (fixed portion side) and the region distant (center side). Further, the polarities of the voltages generated on the surface side of the piezoelectric film and the silicon substrate side are reversed, and an output signal cannot be obtained.

Therefore, in order to effectively utilize the energy generated in the piezoelectric film, a piezoelectric element as shown in FIG. 8 has been proposed (Patent Document 1). In the piezoelectric element shown in FIG. 8, a multilayer structure piezoelectric film 3a and 3b is fixed on a silicon substrate 1 serving as a support substrate via an insulating film 2, and the piezoelectric film 3a is piezoelectricized by electrodes 4a and 4b from above and below. The film 3b has a structure in which the electrodes 4b and 4c are sandwiched from above and below, respectively. Each of the piezoelectric film and the electrode has a rectangular planar structure, and has a cantilever structure in which one end is fixed to the silicon substrate 1 and the other end is a free end.

In such a piezoelectric element, when the piezoelectric film 3a is distorted by receiving acoustic pressure or the like, polarization occurs inside the piezoelectric film 3a, and a voltage signal is taken out from the wiring electrode 5a connected to the electrode 4a and the wiring electrode 5b connected to the electrode 4b. Is possible. Similarly, when the piezoelectric film 3b is distorted, polarization occurs inside the piezoelectric film 3b, and it becomes possible to extract a piezoelectric signal from the wiring electrode 5a connected to the electrode 4c and the wiring electrode 5b connected to the electrode 4b. However, when forming a piezoelectric element having such a structure, it is necessary to repeatedly form an electrode and a piezoelectric film, which makes the manufacturing process long and complicated.

Japanese Patent No. 5707323

The piezoelectric element having the structure shown in FIG. 7 has a problem that the energy generated in the piezoelectric film due to the displacement of the piezoelectric thin film cannot be effectively utilized. Further, the piezoelectric element having the structure shown in FIG. 8 needs to repeatedly form an electrode and a piezoelectric film, which causes a problem that the manufacturing process becomes long and complicated. An object of the present invention is to solve such a problem and to provide a piezoelectric element which can effectively utilize the energy generated in the piezoelectric film and can be easily formed.

In order to achieve the above object, the invention according to claim 1 of the present application utilizes a transverse piezoelectric effect including a piezoelectric film whose circumference is fixed to a support substrate and a pair of electrodes arranged so as to sandwich the piezoelectric film. In the piezoelectric element, the piezoelectric film has a laminated structure in which at least the first piezoelectric film and the second piezoelectric film are directly overlapped with each other, and is a crystal exhibiting the piezoelectricity of each of the first piezoelectric film and the second piezoelectric film. When one of the orientation directions is upward and the other is downward, a plurality of sets of the pair of electrodes arranged so as to sandwich the piezoelectric film of the laminated structure are provided, and at least the first piezoelectric element and the second piezoelectric element are provided. The first piezoelectric element is arranged on the fixed portion side of the piezoelectric film whose circumference is fixed to the support substrate, and the second piezoelectric element is arranged on the center side of the piezoelectric film. The first piezoelectric element and the second piezoelectric element are connected in series by an extension portion continuous from the electrode of the piezoelectric element.

The invention according to claim 2 of the present application is characterized in that, in the piezoelectric element according to claim 1, the piezoelectric film is a film that vibrates due to acoustic pressure .

The piezoelectric element of the present invention has a structure in which piezoelectric films having different crystal orientation directions indicating piezoelectricity (one is upward and the other is downward) are stacked, so that the voltage has opposite polarity in the thickness direction of the piezoelectric film. Even when the above occurs, the voltage generated in the piezoelectric film whose crystal orientation direction is upward and the voltage generated in the piezoelectric film whose crystal orientation direction is downward can be superimposed and extracted, and the crystal orientation direction having piezoelectricity can be determined. There is an advantage that a large output signal can be obtained as compared with the case of using a single-layer film structure in one direction.

In particular, in the present invention, the piezoelectric voltage is superimposed and the level of the output signal can be raised by connecting a plurality of sets of piezoelectric elements in which a plurality of piezoelectric elements are connected.

Further, according to the present invention, when the piezoelectric film is curved and deformed by vibration, a piezoelectric element is formed for each region defined by the bending point of the displacement, and a voltage signal output from each piezoelectric element is superimposed. By connecting so as to output, it is possible to superimpose and take out the voltage generated in each region, and it is possible to raise the level of the output signal.

When the piezoelectric film of the present invention is formed by sandwiching a dielectric film having no piezoelectric characteristics, the piezoelectric film has a large stress relative to the central surface in the thickness direction of the piezoelectric film from the upper and lower surface sides of the piezoelectric film. An output signal can be obtained, and improvement in characteristics can be expected. Further, for example, _{a structure in which the layers are laminated via a silicon oxide film (SiO 2} ) is preferable because the dielectric loss is smaller than that of the piezoelectric thin film.

Note that the connection between the piezoelectric elements can be performed by an extension portion formed by extending the electrodes of the piezoelectric elements, and it is also efficient in that a connecting means such as a through hole that affects the displacement of the piezoelectric film is not required. It has the advantage of being able to be converted into electrical energy.

When the piezoelectric film of the piezoelectric element of the present invention is set to a thickness that vibrates due to acoustic pressure and is used as an acoustic transducer, high sensitivity and improvement in the signal-to-noise ratio are expected.

It is a top view of the electrode of the piezoelectric element of the 1st Example of this invention. It is a partial cross-sectional view of the piezoelectric element of the 1st Example of this invention. It is explanatory drawing of 1st Example of this invention. It is explanatory drawing of 1st Example of this invention. It is a partial cross-sectional view of the piezoelectric element of the 2nd Example of this invention. It is a partial cross-sectional view of the piezoelectric element of the 3rd Example of this invention. It is explanatory drawing of the conventional piezoelectric element. It is explanatory drawing of another conventional piezoelectric element.

The piezoelectric element of the present invention has a piezoelectric film having a periphery fixed to a support substrate having a laminated structure including at least two layers of piezoelectric films, one of which has a piezoelectricity and a crystal orientation direction facing upward, and the other. The piezoelectric thin film has a piezoelectricity, and the crystal orientation direction is a downward film. A plurality of piezoelectric elements in which electrodes are arranged so as to sandwich a part of the piezoelectric film having a laminated structure are formed, and each piezoelectric element is connected in series. In particular, in the present invention, the signal can be efficiently extracted by arranging the piezoelectric element at a predetermined position. Hereinafter, a case where the piezoelectric element of the present invention is configured as an acoustic transducer will be described in detail as an example.

A first embodiment of the present invention will be described. FIG. 1 schematically shows a plan view of the electrodes of the piezoelectric element of the first embodiment, and FIG. 2 schematically shows cross-sectional views of the piezoelectric elements shown in FIG. 1 on the A and B sides of which the electrodes of the piezoelectric element are arranged. ing. As shown in FIG. 1A, the lower layer electrode is composed of a plurality of electrodes 4a1 to 4a4, and has a structure in which two adjacent electrodes are connected to each other. For example, the electrode 4a1 has a structure in which two electrode portions constituting the two piezoelectric elements and an extension portion for connecting the electrode portions are integrated. Similarly, as shown in FIG. 1B, the upper layer electrode is also composed of a plurality of electrodes 4b1 to 4b5, and the electrodes 4b1 to 4b4 have a structure in which two adjacent electrodes are connected via an extension portion, and the electrode 4b5 has a structure. It has an independent structure.

FIG. 2 shows a cross-sectional view of a plane A and a plane B in which the lower layer electrode and the upper layer electrode of the piezoelectric element shown in FIG. As shown in FIG. 2, piezoelectric films 3a and 3b are laminated on a silicon substrate 1 serving as a support substrate _{via an insulating film 2 made of a silicon oxide film (SiO 2).} The piezoelectric films 3a and 3b are fixed around the silicon substrate 1 via the insulating film 2 to form a circular diaphragm. A vent hole is formed in the central portion.

Explaining the structure of the piezoelectric element of this embodiment in detail, electrodes 4a1, electrodes 4a2, electrodes 4a3, and electrodes 4a4, which are lower layer electrodes, are formed on the back surface side of the piezoelectric film 3a. Further, on the upper surface side of the piezoelectric film 3b, electrodes 4b1, electrodes 4b2, electrodes 4b3, electrodes 4b4, and electrodes 4b5, which are upper layer electrodes, are formed. You are connected. These electrodes are arranged in a circle as shown in FIGS. 1 (a) and 1 (b). The electrode can be formed of a metal thin film such as molybdenum (Mo), platinum (Pt), titanium (Ti), iridium (Ir), and ruthenium (Ru).

With this configuration, as shown in the cross-sectional view in FIG. 2, the region where the electrode 4a1, the piezoelectric film 3a (corresponding to the first piezoelectric film), the piezoelectric film 3b (corresponding to the second piezoelectric film), and the electrode 4b1 overlap each other. The piezoelectric element C1 (corresponding to the first piezoelectric element) is formed. Similarly, the piezoelectric element C2 (corresponding to the second piezoelectric element) is formed in the region where the electrodes 4a1, the piezoelectric film 3a, the piezoelectric film 3b, and the electrodes 4b2 overlap.

Here, the first piezoelectric element C1 and the second piezoelectric element C2 use the electrodes 4a1 constituting the piezoelectric element in common, so that the electrodes 4a1 do not overlap with the opposing electrodes (electrodes 4b1 and electrodes 4b2, respectively). It is connected in series by an area (corresponding to an extension). With such a connection, it is not necessary to form a connecting means that affects the displacement of the piezoelectric element such as a through hole in the piezoelectric film.

In this embodiment, four sets of piezoelectric elements corresponding to the first and second piezoelectric elements are connected in series, and these piezoelectric elements C1 to C8 are provided between the wiring electrodes 5a and the wiring electrodes 5b. It becomes a connected configuration. Specifically, it has the configuration shown in FIG. In FIG. 3, the first piezoelectric element C1 is arranged on the outer peripheral side of the piezoelectric film, that is, the piezoelectric film on the fixed portion side fixed to the support substrate, and the second piezoelectric element C2 is on the central side of the piezoelectric film, that is, It is arranged on the piezoelectric film between the region where the first piezoelectric element is formed and the center of the piezoelectric film. Similarly, the piezoelectric elements C3, C5 and C7 are arranged on the fixed portion side of the piezoelectric film, and the piezoelectric elements C4, C6 and C8 are arranged on the central side of the piezoelectric film. That is, the piezoelectric elements C2, C4, C6 and C8 are arranged so as to surround the midpoint of the piezoelectric film serving as the vibrating portion, and the piezoelectric elements C1, C3, C5 and C7 are arranged outside the piezoelectric elements C2, C4, C6 and C8.

Next, the crystal orientation direction of the piezoelectric film of the present invention having piezoelectricity will be described. The piezoelectric film of this embodiment has a structure in which a film having a piezoelectricity and a crystal orientation direction facing upward and a film having a piezoelectricity facing downward are stacked, as shown by an arrow indicating the crystal orientation direction (piezoelectric polarity) having piezoelectricity in FIG. .. Specifically, when the c-axis orientation, which is the crystal orientation showing the piezoelectricity of the piezoelectric thin film 3a made of aluminum nitride (AlN), is downward, the c-axis orientation of the piezoelectric thin film 3b made of aluminum nitride is upward. Or vice versa.

The crystal orientation is controlled by a well-known method. Specifically, when forming a thin film of aluminum nitride having a wurtzite structure by a reactive sputtering method using nitrogen or oxygen gas as a reactive gas, the substrate temperature, sputtering pressure, nitrogen or oxygen concentration, power density, and film thickness are determined. By setting appropriately, it is possible to form a film having good crystal orientation and uniform c-axis orientation.

Further, by changing the sputtering conditions, it is possible to laminate and generate an aluminum nitride thin film whose c-axis direction is changed by 180 degrees.

The c-axis orientation is not limited to the case where the orientation is aligned perpendicular to the surface of the piezoelectric film as shown in FIG. 2, and may deviate from the vertical direction. Further, the upward c-axis direction and the downward c-axis direction may be directions opposite to each other, and may not be 180 degrees different as shown in FIG. Needless to say, the sensitivity is highest when the difference is 180 degrees, which is preferable.

When the piezoelectric element of the present invention configured in this way is configured as an acoustic transducer, acoustic pressure is applied from the pores 6 formed in the silicon substrate 1. The piezoelectric film that receives the acoustic pressure is curved and displaced upward. As a result, tensile stress and compressive stress are generated in the aluminum nitride constituting the piezoelectric film.

FIG. 4 shows an example in which an acoustic pressure is applied to the piezoelectric element in the region described in FIG. 2 and the piezoelectric thin film is displaced upward. As shown in FIG. 4, tensile stress and compressive stress are generated in the piezoelectric film, and the direction of stress in the piezoelectric film is divided into two regions depending on the inflection point of displacement. Specifically, at the outer peripheral portion near the fixed portion where the circular piezoelectric film is fixed to the silicon substrate 1 via the insulating film 2, tensile stress is generated in the piezoelectric film 3a, and compressive stress is applied to the piezoelectric film 3b. appear. On the other hand, in the central portion inside the piezoelectric film 3a, a compressive stress is generated, and a tensile stress is generated in the piezoelectric film 3b. In this way, it is divided into two regions where the stress direction differs depending on the inflection point of displacement.

By the way, as shown in FIG. 2, the piezoelectric element of this embodiment is connected in series with the piezoelectric element C1 and the piezoelectric element C2. Here, the voltage generated in the outer peripheral portion and the voltage generated in the central portion have opposite polarities and can be the same value, and it is possible to cancel out the in-phase voltage caused by the residual stress and the temperature fluctuation. It becomes.

Since the piezoelectric thin films 3a and 3b have a structure in which a film having piezoelectricity in the crystal orientation direction upward and a film having a downward crystal orientation are stacked, the direction of the electric field generated by the transverse piezoelectric effect due to the tensile stress of the piezoelectric thin film 3a and the piezoelectric thin film 3b. The direction of the electric field due to the compressive stress of is the same.

On the contrary, when the piezoelectric film is displaced downward, a compressive stress is generated in the piezoelectric film 3a and a tensile stress is generated in the piezoelectric film 3b between the two electrodes 4a1 and 4b1. Also in this case, the direction of the electric field generated by the piezoelectric film 3a and the direction of the electric field generated by the piezoelectric film 3b are the same. As a result, in any displacement, the voltage generated by each piezoelectric film is superimposed and output between the electrodes 4a1 and 4b1.

Similarly, even between the electrodes 4a2 and 4b2, when the piezoelectric film is displaced upward, compressive stress is generated in the piezoelectric film 3a, tensile stress is generated in the piezoelectric thin film 3b, and the voltage generated in each piezoelectric film is generated. Is weighted and output. When the piezoelectric film is displaced downward, tensile stress is generated in the piezoelectric film 3a, compressive stress is generated in the piezoelectric film 3b, and the voltages generated in the respective piezoelectric films are superimposed and output.

Further, since the piezoelectric element of the present invention has a structure in which four sets of piezoelectric elements composed of the piezoelectric element C1 and the piezoelectric element C2 are connected in series, each of the four sets of piezoelectric elements is based on the application of an acoustic pressure signal. The output signal (voltage) of the set of elements is superimposed and added without including the signal due to residual stress or temperature fluctuation, and the ratio of the output voltage (V _{out ) to the acoustic pressure (P a ) (V out} / P _a ). It is possible to increase the sensitivity as an acoustic transducer defined in.

It is desirable that the size of each electrode is optimized from the viewpoint of maximizing the signal-to-noise ratio. This wiring electrodes 5a, the capacity of the equivalent capacitor as seen from 5b when the C _out, of the electrodes so as to maximize the energy stored in the equivalent capacitor ^{_{(C out · V out 2/}} 2) You just have to decide the size.

Next, a second embodiment of the present invention will be described. FIG. 5 is a cross-sectional view of the piezoelectric element of the second embodiment of the present invention. As shown in FIG. 5, the piezoelectric element of this embodiment is piezoelectric, as in the first embodiment, via an insulating film 2 made _{of a silicon oxide film (SiO 2) on a silicon substrate 1 serving as a support substrate.} A piezoelectric film 3a and a piezoelectric film 3b having different crystal orientation directions are formed. In this embodiment, instead of the structure in which the piezoelectric film 3a and the piezoelectric film 3b are directly overlapped, the structure is such that the dielectric film 7 having no piezoelectric property is interposed.

By selecting the dielectric film 7 from dielectrics that do not have piezoelectric characteristics, the magnitude of compressive stress or tensile stress becomes a thin film when an acoustic pressure signal or the like is applied to the piezoelectric film having a laminated structure to cause displacement. The structure may be such that the piezoelectric film is arranged only on the front surface portion or the back surface portion of the thin film which is relatively large with respect to the central surface in the thickness direction of the thin film. As a result, it is possible to form a piezoelectric element having a high ratio (sensitivity) of the output signal voltage to the applied acoustic signal pressure.

If a silicon oxide film (SiO ₂ ) is selected as the dielectric film 7, the dielectric loss (loss angle tan δ) of the dielectric film 7 can be made smaller than that of the piezoelectric film made of aluminum nitride, and the sensitivity and signal can be increased. It is expected that the noise ratio will be improved, which is preferable.

Next, a third embodiment will be described. Generally, when a piezoelectric film such as aluminum nitride is deposited by a sputtering method, it is known that it is affected by the deposited substrate. In the case of the piezoelectric element described in the first embodiment, when the piezoelectric film 3b is deposited, the portion where the electrode 4b1 or the like made of molybdenum is formed and the electrode are not formed on the base surface, and the piezoelectric film 3a is not formed. There is a part where is greatly exposed. It is very difficult to form a piezoelectric film having uniform characteristics on such a base surface.

Therefore, the portion where the electrode is not formed may be coated with the same material as the electrode. Specifically, at the same time as the electrodes 4a1 to 4a4 to be the lower layer electrodes, a dummy electrode (an electrode including the electrode 4c1 shown in FIG. 6) that is not connected to these electrodes is formed in a region where these electrodes are not formed. Similarly, at the same time as the electrodes 4b1 to 4b5 to be the lower layer electrodes, a dummy electrode (an electrode including the electrode 4c2 shown in FIG. 6) that is not connected to these electrodes is formed in a region where these electrodes are not formed. The dummy electrode formed in this way makes it possible to form a piezoelectric film having uniform characteristics. This dummy electrode can have a structure that is vertically symmetrical with respect to the surface passing through the center of the piezoelectric films 3a and 3b in the thickness direction, and the central axial surface in which the stress of tension and compression is balanced and the stress becomes zero. Is used as the interface between the piezoelectric films 3a and 3b, and the acoustic pressure can be efficiently taken out as the output voltage.

The example shown in FIG. 6 is an example in which a dummy electrode is added to the piezoelectric element described in the first embodiment, but a dummy electrode is added to the piezoelectric element described in the second embodiment provided with a dielectric film. It is also possible to add.

Although the piezoelectric element of the present embodiment has been described above, it goes without saying that the present invention is not limited to the above embodiment. Specifically, the piezoelectric thin film is not limited to aluminum nitride, and scandium aluminum nitride (Al _1-x Sc _x N), zinc oxide (ZnO), and lead zirconate titanate (PZT) can also be used. It is possible. Further, the size of the pores 6, the shape of each electrode, the number of piezoelectric elements to be connected, and the connection can be appropriately changed.

1: Silicon substrate 2: Insulating film, 3a, 3b: Piezoelectric thin film, 4a, 4b, 4c: Electrodes, 5a, 5b: Wiring electrodes, 6: Pore, 7: Dielectric film

Claims (2)

In a piezoelectric element utilizing a transverse piezoelectric effect, which comprises a piezoelectric film whose circumference is fixed to a support substrate and a pair of electrodes arranged so as to sandwich the piezoelectric film.
The piezoelectric film has a laminated structure in which at least the first piezoelectric film and the second piezoelectric film are directly overlapped with each other, and the crystal orientation direction indicating the piezoelectricity of each of the first piezoelectric film and the second piezoelectric film is determined. When one is facing up, the other is facing down,
A plurality of sets of the pair of electrodes arranged so as to sandwich the piezoelectric film having the laminated structure are provided, and at least the first piezoelectric element and the second piezoelectric element are formed.
The first piezoelectric element is arranged on the fixed portion side of the piezoelectric film whose circumference is fixed to the support substrate, and the second piezoelectric element is arranged on the center side of the piezoelectric film.
A piezoelectric element characterized in that the first piezoelectric element and the second piezoelectric element are connected in series by an extension portion continuous from the electrode of the piezoelectric element. The piezoelectric element according to claim 1, wherein the piezoelectric film is a film that vibrates due to acoustic pressure .

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