JP6819002B2 - Piezoelectric element - Google Patents

Description

The present invention relates to a piezoelectric element, and more particularly to a piezoelectric element having high sensitivity and low noise.

In recent years, smartphones, whose demand is rapidly expanding, are often used for microphones that are small and thin and use MEMS (Micro Electro Mechanical System) technology that is resistant 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 thin film has been attracting attention.

By the way, it is known that in a piezoelectric element, when there is no acoustic pressure or the like, the residual stress and temperature fluctuation of the piezoelectric thin film are output as unnecessary signals to deteriorate the characteristics. Therefore, a technique for releasing residual stress by adopting a cantilever structure in which one end of a piezoelectric thin film is a free end is disclosed (for example, Patent Document 1).

FIG. 9 shows a cross-sectional view of the piezoelectric element having a cantilever structure. As shown in FIG. 9, the piezoelectric thin films 3a and 3b having a multilayer structure are fixed to the silicon substrate 1 serving as the support substrate via the insulating film 2, and the piezoelectric thin films 3a are formed from above and below by the electrodes 4a and 4b. Has a structure sandwiched between the electrodes 4b and 4c, respectively. The piezoelectric thin film and the electrode each have a rectangular planar shape, one end of which is fixed to the silicon substrate 1 and the other end of which is a free end. Further, the electrode 4a and the electrode 4c are connected to one wiring electrode 5a, and the electrode 4b is connected to another wiring metal 5b.

In such a piezoelectric element, when the piezoelectric thin film 3a is distorted by receiving acoustic pressure or the like, polarization occurs inside the piezoelectric thin film 3a, and a voltage signal is extracted from the wiring metal 5a connected to the electrode 4a and the wiring metal 5b connected to the electrode 4b. Is possible. Similarly, when the piezoelectric thin film 3b is distorted, polarization occurs inside the piezoelectric thin film 3b, and it becomes possible to extract a voltage signal from the wiring metal 5a connected to the electrode 4c and the wiring metal 5b connected to the electrode 4b.

By the way, the cantilever structure releases the residual stress of the piezoelectric thin film, but as a result, the piezoelectric thin film warps, and the gap between adjacent piezoelectric thin films (inter-beam gap G) and the side surface of the piezoelectric thin film (beam) and the support substrate The size of the substantial gap is widened. When the piezoelectric element is used as a microphone, the occurrence of a gap larger than the design value reduces the acoustic resistance and causes deterioration of characteristics such as a decrease in low frequency sensitivity.

Therefore, in order to solve this problem, the shape of the piezoelectric thin film is not a rectangle but a triangle, for example, by arranging the piezoelectric thin films so that the vertices of each of the four triangles are located in the center, even if the piezoelectric thin film is warped. (Patent Document 2) discloses a technique in which similar warpage occurs in adjacent piezoelectric thin films, and as a result, the dimensions of gaps between adjacent piezoelectric thin films are not significantly changed (Patent Document 2).

Japanese Patent No. 5707323 Japanese Patent Publication No. 2014-515214

In order to prevent the deterioration of characteristics due to the residual stress of the piezoelectric thin film, the shape of the piezoelectric thin film is triangular in the conventional piezoelectric element, and the dimensions of the gap between the piezoelectric thin films are arranged so that the vertices of the four triangles are centered. It was made possible not to change significantly. However, in order to match the resonance frequencies of the beams of each triangle, it is necessary to combine triangles having the same shape, and the outer shape of the piezoelectric MEMS element is limited to a square, which causes a problem that the degree of freedom in design is lost. An object of the present invention is to provide a piezoelectric element having high sensitivity and an improved signal-to-noise ratio by suppressing the influence of residual stress of a piezoelectric thin film and solving the problem of limiting the outer shape.

In order to achieve the above object, the invention according to claim 1 of the present application is a piezoelectric element in which a piezoelectric thin film having both ends fixed to a support substrate and a pair of electrodes arranged so as to sandwich the piezoelectric thin film is provided. Is composed of a laminated structure including at least a first piezoelectric thin film and a second piezoelectric thin film, and includes a plurality of sets of the pair of electrodes arranged so as to sandwich a part of the first piezoelectric thin film, and at least the first one. A piezoelectric element, a second piezoelectric element, and a third piezoelectric element are formed, and a plurality of sets of the pair of electrodes arranged so as to sandwich a part of the second piezoelectric thin film are provided, and at least a fourth piezoelectric element is provided. The element, the fifth piezoelectric element and the sixth piezoelectric element are formed, and the first piezoelectric element, the second piezoelectric element and the third piezoelectric element are from one end side to the other end of the both ends. The fourth piezoelectric element, the fifth piezoelectric element, and the sixth piezoelectric element are arranged in order from one end side to the other end side of both ends. , The first piezoelectric element and the fourth piezoelectric element, the second piezoelectric element and the fifth piezoelectric element, or the third piezoelectric element and the sixth piezoelectric element pair of piezoelectric elements are arranged in parallel. The first piezoelectric element, the second piezoelectric element, and the third piezoelectric element are connected in series, and the fourth piezoelectric element is connected in series. The piezoelectric element, the fifth piezoelectric element, and the sixth piezoelectric element are connected in series, and the two sets of piezoelectric elements connected in series are connected in parallel, and the first piezoelectric element The fourth piezoelectric element is vertically symmetrically laminated, the second piezoelectric element and the fifth piezoelectric element are vertically symmetrically laminated, and the third piezoelectric element and the sixth piezoelectric element are laminated vertically. It is characterized in that and are vertically symmetrically laminated.

The invention according to claim 2 of the present application is the piezoelectric element according to claim 1, wherein the set of piezoelectric elements connected in parallel is the front surface, the back surface, or between the films of the first piezoelectric thin film or the second piezoelectric thin film. It is characterized in that it is connected in series by an extension portion continuous from the electrode of the arranged piezoelectric element .

According to the third aspect of the present application, in the piezoelectric element according to any one of claims 1 or 2, when the piezoelectric thin film is curved and displaced due to vibration, at least for each region partitioned by the bending point of the displacement. Either the first piezoelectric element and the fourth piezoelectric element, the second piezoelectric element and the fifth piezoelectric element, or the third piezoelectric element and the sixth piezoelectric element, which are vertically symmetrically laminated. The feature is that the piezo is arranged .

The invention according to claim 4 of the present application is the piezoelectric element according to any one of claims 1 to 3.
The piezoelectric thin film is characterized in that it is a film that vibrates due to acoustic pressure .

In the piezoelectric element of the present invention, the piezoelectric element formed on the first piezoelectric thin film and the piezoelectric element formed on the second piezoelectric thin film are arranged so as to overlap vertically symmetrically, so that the residual stress and temperature fluctuation of the overlapping piezoelectric thin films After canceling each other out the piezoelectric voltage generated due to the above to reduce the influence of the residual stress of the piezoelectric thin film, the piezoelectric voltage generated by the first piezoelectric thin film and the piezoelectric voltage generated by the second piezoelectric thin film are combined. It is possible to raise the level of the output signal by superimposing it.

Further, according to the present invention, by adopting a double-sided beam structure in which both ends of the piezoelectric thin film are fixed to the support substrate, the piezoelectric thin film is suppressed from being significantly deformed, and the shape is not limited to a square shape, and the degree of freedom in design It is possible to secure.

Furthermore, according to the present invention, when the piezoelectric thin film is curved and deformed by vibration, the piezoelectric element is formed on the first piezoelectric thin film and the second piezoelectric thin film is formed on each region defined by the bending point of the displacement. By arranging the pair with the piezoelectric element, the piezoelectric element is separated in the tensile stress region and the compressive stress region generated in the stretching direction of the beam for each partitioned region, and the voltage signals generated in each region are superimposed. By connecting them in such a way, it is possible to efficiently convert them into electrical energy and take them out.

According to the present invention, the connection between the piezoelectric elements can be performed by extending the electrodes of the piezoelectric elements, and electricity is efficiently applied in that a connecting means such as a through hole that affects the displacement of the piezoelectric thin film is not required. It has the advantage of being able to be converted into energy.

In particular, when the piezoelectric thin 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 piezoelectric element of the 1st Example of this invention. It is sectional drawing of the piezoelectric element of 1st Example of this invention. It is explanatory drawing when an acoustic pressure signal is applied to a piezoelectric element and a piezoelectric thin film is displaced. It is a graph which shows the film thickness dependence of the piezoelectric thin film made of aluminum nitride of the signal-to-noise ratio. It is a graph which shows the relationship between the microphone characteristic and the slit width. It is a figure explaining the manufacturing method of the piezoelectric element of the 1st Example of this invention. It is a graph which shows the film thickness dependence of the piezoelectric thin film made of scandium aluminum nitride of the signal-to-noise ratio. It is sectional drawing of the piezoelectric element of the 2nd Example of this invention. It is explanatory drawing of the conventional piezoelectric type MEMS element.

The piezoelectric element of the present invention has a double-sided beam structure in which both ends of the piezoelectric thin film are fixed to a support substrate. The piezoelectric thin film has a laminated structure including at least two layers of piezoelectric thin films. Each piezoelectric thin film is formed with a plurality of piezoelectric elements in which electrodes are arranged so as to sandwich a part thereof, and each piezoelectric element is connected in parallel or in series. In particular, in the present invention, the piezoelectric elements are arranged so as to be vertically symmetrically overlapped with each other. By adopting the above configuration, the present invention aims to improve the signal-to-noise ratio by mutually canceling the piezoelectric voltage generated by the residual stress and the temperature fluctuation due to the output of the piezoelectric elements overlapping vertically. Furthermore, by arranging a set of piezoelectric elements that are vertically symmetrically overlapped at a predetermined position, the signal can be efficiently extracted. 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.

FIG. 1 shows a plan view of the piezoelectric element according to the first embodiment of the invention, and FIG. 2 shows a cross-sectional view of the piezoelectric element shown in FIG. 1 on the AA plane. As shown in FIG. 2, piezoelectric thin films 3a and 3b are laminated and formed on a silicon substrate 1 serving as a support substrate via an insulating film 2 made of a silicon oxide film (SiO ₂ ). In this embodiment, in order to have a double-sided beam structure, a slit 6 extending in the lateral direction of the drawing is formed as shown in FIG. For example, aluminum nitride (AlN) can be used as the piezoelectric thin film, and the crystal orientation (piezoelectric orientation) thereof is formed so as to be the same direction in each of the laminated piezoelectric thin films.

Explaining the structure of the piezoelectric element of this embodiment in detail, the electrodes 4a1 and 4a2 are formed on the back surface side of the piezoelectric thin film 3a, and the electrodes 4a1 are connected to the wiring electrodes 5a. The electrode 4a2 is not connected to the wiring electrode 5a or other electrodes, and is in a floating state. Further, electrodes 4b1 and 4b2 are formed on the upper surface side of the piezoelectric thin film 3a and on the lower surface side (corresponding to the space between the films) of the piezoelectric element 3b, and the electrodes 4b2 are connected to the wiring electrode 5b. The electrode 4b1 is not connected to the wiring electrode 5b or other electrodes, and is in a floating state. Further, an electrode 4c1 and an electrode 4c2 are formed on the upper surface side of the piezoelectric thin film 3b, the electrode 4c1 is connected to the wiring electrode 5a, and the electrode 4c2 is not connected to the wiring electrode 5a or other electrodes. , It is in a floating state. 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, the piezoelectric element C1 (corresponding to the first piezoelectric element) is formed in the region where the electrode 4a1, the piezoelectric thin film 3a (corresponding to the first piezoelectric thin film), and the electrode 4b1 overlap. Similarly, the piezoelectric element C2 (corresponding to the second piezoelectric element) in the region where the electrode 4a2, the piezoelectric thin film 3a and the electrode 4b1 overlap, and the piezoelectric element C3 (third piezoelectric element) in the region where the electrode 4a2, the piezoelectric thin film 3a and the electrode 4b2 overlap. The element (corresponding to the element) is the piezoelectric element C4 (corresponding to the fourth piezoelectric element), the electrode 4c2, the piezoelectric thin film 3b and the electrode 4b1 in the region where the electrode 4c1, the piezoelectric thin film 3b (corresponding to the second piezoelectric thin film) and the electrode 4b1 overlap. The piezoelectric element C5 (corresponding to the fifth piezoelectric element), the electrode 4c2, the piezoelectric thin film 3b, and the piezoelectric element C6 (corresponding to the sixth piezoelectric element) are formed in the region where the electrodes 4b2 overlap.

As a result, the first piezoelectric element C1 and the fourth piezoelectric element C4 are connected in parallel, the second piezoelectric element C2 and the third piezoelectric element C3 are connected in series, and the fifth piezoelectric element C5 and the sixth piezoelectric element are connected in series. The series connection of the elements C6 is connected in parallel, and these parallel-connected piezoelectric elements are connected in series between the wiring electrode 5a and the wiring metal 5b.

Here, for example, the first piezoelectric element C1 and the second piezoelectric element C2 commonly use the electrodes 4b1 constituting the piezoelectric element, so that the electrodes 4b1 do not overlap with the opposing electrodes (electrodes 4a1 and 4a2, respectively). It is connected by the area (corresponding to the extension). Similarly, the fourth piezoelectric element C4 and the fifth piezoelectric element C5 commonly use the electrodes 4b1 constituting the piezoelectric element, so that the regions of the electrodes 4b1 that do not overlap with the opposing electrodes (electrodes 4c1 and 4c2, respectively) It is connected by (corresponding to the extension). Further, the second piezoelectric element C2 and the third piezoelectric element C3 are provided by an electrode 4a2, and the fifth piezoelectric element C5 and the sixth piezoelectric element C6 are provided by an electrode 4c2, respectively, so that the electrodes 4a2 do not overlap with the opposing electrodes. They are connected by an extension (corresponding to an extension) or by an electrode 4c2 region (corresponding to an extension). With such a connection, it is not necessary to form a connecting means that affects the displacement of the piezoelectric thin film such as a through hole in the piezoelectric thin film.

Further, as is clear from FIG. 2, the first piezoelectric element C1 and the fourth piezoelectric element C4, the second piezoelectric element C2 and the fifth piezoelectric element C5, the third piezoelectric element C3 and the sixth piezoelectric element C6 Is vertically symmetrical with respect to the plane passing through the center in the thickness direction of the electrodes 4b1 and 4b2, respectively, at least in the region forming each piezoelectric element.

A part of the back surface side of the silicon substrate 1 is removed to form a hole 7, and the electrodes 4a1, 4a2 and the piezoelectric thin film 3a are exposed in the hole 7. The pores 7 communicate with the surface side of the silicon substrate 1 through the slit 6 shown in FIG.

With this configuration, the piezoelectric element of this embodiment has a double-sided beam structure in which a plurality of electrode pairs are formed on a piezoelectric thin film in which both ends are supported on a silicon substrate 1 (support substrate).

When the piezoelectric element of the present invention is configured as an acoustic transducer, acoustic pressure is applied from the pores 7 formed in the silicon substrate 1. The beam structure containing the piezoelectric thin film subjected to acoustic pressure is curved and displaced upward. As a result, tensile stress and compressive stress are generated in the aluminum nitride constituting the piezoelectric thin film.

FIG. 3 shows an example when an acoustic pressure signal is applied and the piezoelectric thin film is displaced. In this case, two inflection points are generated and divided into three regions according to the direction of stress on the piezoelectric thin film. For example, in the regions 1 and 3, the region 1 and the region 3 are curved and displaced downward in a convex shape, and a tensile stress is generated in the first piezoelectric thin film 3a and a tensile stress is generated in the second piezoelectric thin film 3b. On the other hand, in the region 2, the displacement is curved upward in a convex shape, and compressive stress is generated in the first piezoelectric thin film 3a and tensile stress is generated in the second piezoelectric thin film 3b.

By the way, in the piezoelectric element of this embodiment, as shown in FIG. 2, the piezoelectric element C1 and the piezoelectric element C4 have a series connection of the piezoelectric element C2 and the piezoelectric element C3 and a series connection of the piezoelectric element C5 and the piezoelectric element C6. , Each is connected in parallel, and has a vertically symmetrical structure. Therefore, the voltages generated in each of the regions 1 to 3 have opposite polarities and have the same value, so that the in-phase voltage caused by the residual stress and the temperature fluctuation is canceled out.

As a result, the output signal of each region based on the acoustic pressure signal is applied (voltage) is superimposed added without the signal due to residual stresses and temperature change, the output voltage for the acoustic pressure (P _a) ( It is possible to increase the sensitivity as an acoustic transducer defined by the ratio of V _out ) (V _out / P _a ).

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.

Specifically, a design example of the dimensions in the case of a rectangular double-sided beam, the film thickness of each piezoelectric thin film, and the size of the electrodes is shown. For example, the input signal is human voice, and the resonance frequency of the double-sided beam is 20 kHz. In addition, the plane dimensions are set so that they can be mounted on electronic devices such as smartphones. The length of the double-sided beam (corresponding to the length of the slit in FIG. 1) is 0.7 mm, and the width (upper and lower in FIG. 1) is 1.4 mm. The thickness of the piezoelectric thin films 3a and 3b made of aluminum nitride is 0.5 μm, and the thickness of the electrodes 4a1 to 4c1 and 4a2-4c2 made of molybdenum is 0.1 μm. The extension length of the electrodes 4a1, 4c1 and 4b2 from the support end (the end of the hole 7) is 90 μm, and the length from the support end of the electrodes 4b1, 4a2 and 4c2 to the electrode end is 500 μm. The width of the slit 6 is 1 μm.

The thickness of the piezoelectric thin films 3a and 3b and the length of the double-sided beam (slit) can be determined as follows. FIG. 4 is a graph showing the film thickness dependence of the piezoelectric thin film made of aluminum nitride with a signal-to-noise ratio. The resonance frequency was constant (20 kHz), the beam width was 1.4 mm, and the slit width was 1 μm. In addition, the volume of the holes was set to a relatively small value of 3 mm ^{2 in} consideration of being limited by the size of the mounting housing when mounting on an electronic device such as a smartphone. Further, in addition to the piezoelectric element (acoustic transducer) of the present invention, it is necessary to implement an amplifier circuit for processing the output signal of the piezoelectric element in a small housing such as a smartphone. Therefore, in consideration of the allowable chip size of the piezoelectric element, the following studies were conducted when the beam lengths were 0.6 mm, 0.7 mm, and 0.8 mm.

As shown in FIG. 4, when the film thickness of each piezoelectric thin film made of aluminum nitride is made thinner than 0.4 μm, the signal-to-noise ratio is slightly improved. However, since the substantial acoustic-electric conversion coefficient is limited due to the acoustic compliance of the holes, the improvement effect by the thinning layer is not remarkable, and it can be seen that the dependence on the beam length is not large. On the contrary, when the thickness of the piezoelectric thin film is increased, the signal-to-noise ratio drops sharply. In addition, the dependence on the length of the beam becomes remarkable. That is, in order to keep the resonance frequency constant, it is necessary to increase the length of the beam when increasing the thickness of the piezoelectric thin film. In the example shown in FIG. 4, when the thickness of the piezoelectric thin film is 0.5 μm, the length of the beam is 0.7 mm, and when the thickness of the piezoelectric thin film is 0.6 μm, the length of the beam is set. It can be seen that 0.8 mm is preferable.

Next, the slit width will be described. FIG. 5 is a graph showing the relationship between the rate of decrease in sensitivity of 100 Hz and the slit width with reference to the sensitivity of 1 kHz, which is one of the main characteristics of the microphone. Similarly to the above, the resonance frequency of the double-sided beam is 20 kHz, the length of the beam is 0.7 mm, the width is 1.4 mm, and the volume of the holes is 3 mm ² . When the thickness of the piezoelectric thin film made of aluminum nitride was 0.4 μm, 0.5 μm, and 0.6 μm, the relationship with the slit width was examined.

As shown in FIG. 5, almost no dependence on the thickness of the piezoelectric thin film is observed, and when the slit width exceeds 1 μm, the sensitivity decreases significantly at 100 Hz. When the frequency is lowered below 100 Hz, this decrease in sensitivity becomes even more severe. That is, it can be seen that the slit width needs to be 1 μm or less in order to suppress the decrease in sensitivity at low frequencies.

As described above, in order to increase the sensitivity, the length of the double-sided beam, the thickness of the piezoelectric thin film, the slit width, and the like may be appropriately adjusted and set.

The piezoelectric element of the present invention can be formed by using a conventional method for manufacturing a semiconductor device. FIG. 6 is an explanatory diagram of a method for manufacturing the piezoelectric element of this embodiment. First, an insulating film 2 made of a silicon oxide film (SiO ₂ ) is formed on the silicon substrate 1 by a thermal oxidation method. A molybdenum (Mo) film having a thickness of 0.1 μm is laminated on the insulating film 2 by a sputtering method, and patterning is performed by a normal lithographic method to form electrodes 4a1 and 4a2 (FIG. 6a).

After that, an aluminum nitride film having a thickness of 0.5 μm is laminated on the entire surface by a sputtering method to form a piezoelectric thin film 3a corresponding to the first piezoelectric thin film. Then, a molybdenum (Mo) film having a thickness of 0.1 μm is laminated on the piezoelectric thin film 3a by a sputtering method, and patterning is performed by a normal lithograph method to form electrodes 4b1 and 4b2. Further, an aluminum nitride film having a thickness of 0.5 μm is laminated on the entire surface by a sputtering method to form a piezoelectric thin film 3b corresponding to the second piezoelectric thin film. A molybdenum (Mo) film having a thickness of 0.1 μm is laminated by a sputtering method and patterned by a normal lithographic method to form electrodes 4c1 and 4c2 (FIG. 6b).

A part of the piezoelectric thin films 3a and 3b is removed by etching to form a wiring electrode 5a connected to the electrode 4a1 and the electrode 4c1 and a wiring electrode 5b connected to the electrode 4b2. The wiring electrodes 5a and 5b are made of aluminum and can be formed by a conventional lithographic method (FIG. 6c).
When a part of the piezoelectric thin films 3a and 3b is removed by etching, a part of the piezoelectric thin films 3a and 3b corresponding to the slit 6 shown in FIG. 1 is also removed by etching to form a recess, and an insulating film 2 is formed on the bottom thereof. Leave it exposed.

Finally, a part of the back surface of the silicon substrate 1 is removed by a dry etching method, and a part of the exposed insulating film 2 is also removed to form the pores 7. A part of the electrode 4a1, the electrode 4b1 and the piezoelectric thin film 3a formed earlier is exposed in the pore 7. By forming the pores 7, the insulating film 2 exposed at the bottom of the recess formed to form the slit 6 is also removed, and the slit 6 through which the front surface side and the back surface side of the piezoelectric thin film penetrate is formed. Due to the slit 6, the piezoelectric thin film has a double-sided beam structure (FIG. 6d).

Although the piezoelectric element of the present embodiment and the manufacturing method thereof have been described above, it goes without saying that the present invention is not limited to aluminum nitride as the piezoelectric thin film. Table 1 affects the characteristics of piezoelectric microphones for typical piezoelectric materials such as aluminum nitride, scandium aluminum nitride (Al _1-x Sc _x N), zinc oxide (ZnO), and lead zirconate titanate (PZT). It is a table comparing material constants such as Young's modulus and transverse piezoelectric strain coefficient.

Performance index corresponding to the signal-to-noise ratio shown in Table 1 (FOM) is represented by the ratio of the coupling coefficient (k ₃₁ ²⁾ and the loss angle (tan [delta), as the value is larger, at approximately proportional to the form to the value Improvement of signal-to-noise ratio can be expected. As shown in Table 1, aluminum nitride has a figure of merit 6 to 40 times larger than that of zinc oxide and lead zirconate titanate, indicating that it is a suitable material for piezoelectric transducers. Further, it is known that scandium aluminum nitride (Al _1-x Sc _x N) obtained by adding scandium to aluminum nitride has a higher transverse piezoelectric strain coefficient than aluminum nitride. For example, the ratio of scandium is set to 35%. In this case, the performance index can be expected to be improved about 7 times as much as that of aluminum nitride.

FIG. 7 is a graph showing the film thickness dependence of the piezoelectric thin film made of scandium aluminum nitride (Al _1-x Sc _x N: x = 0.35) having a signal-to-noise ratio. Comparing with the graph showing the film thickness dependence of the piezoelectric thin film made of aluminum nitride shown in FIG. 4, it can be seen that an improvement in the signal-to-noise ratio of about 8 dB, which corresponds to 7 times the figure of merit, can be expected. It was also confirmed that the film thickness dependence shows the same tendency regardless of the type of piezoelectric thin film. Specifically, when the thickness of the piezoelectric thin film is 0.4 μm, the length of the beam is 0.6 mm, and when the thickness of the piezoelectric thin film is 0.6 μm, the length of the beam is 0. It can be seen that when the thickness of the piezoelectric thin film is 7 mm and the thickness of the piezoelectric thin film is 0.7 μm, the length of the beam is preferably 0.8 mm.

Next, a second embodiment of the present invention will be described. FIG. 8 is a cross-sectional view of the piezoelectric element of the second embodiment of the present invention. Compared with the piezoelectric element shown in FIG. 2 described above, it is different in that it includes a dielectric film 8, an electrode 4d1, an electrode 4d2, and the like.

That is, as shown in FIG. 8, the piezoelectric thin film 3a, the dielectric film 8 and the piezoelectric thin film 3b are laminated and formed on the silicon substrate 1 serving as the support substrate via the insulating film 2 made of the silicon oxide film (SiO ₂ ). Has been done. In this embodiment, in order to have a double-sided beam structure, a slit 6 extending in the lateral direction of the drawing is formed as described with reference to FIG. For the piezoelectric thin film, for example, aluminum nitride (AlN) can be used, and the crystal orientation (piezoelectric orientation) thereof is formed to be the same direction.

In the piezoelectric element of this embodiment, electrodes 4a1 and 4a2 are formed on the back surface side of the piezoelectric thin film 3a, and the electrodes 4a1 are connected to the wiring electrodes 5a. The electrode 4a2 is not connected to the wiring electrode 5a or other electrodes, and is in a floating state. Further, an electrode 4b1 and an electrode 4b2 are formed on the upper surface side of the piezoelectric thin film 3a, and the electrode 4b2 is connected to the wiring electrode 5b. The electrode 4b1 is not connected to the wiring electrode 5b or other electrodes, and is in a floating state.

The dielectric film 8 is laminated on the piezoelectric thin film 3a and the electrodes 4b1 and 4b2, and the electrodes 4d1 and 4d2 are formed on the dielectric film 8. The electrodes 4d1 and 4d2 have the same shape as the previously formed electrodes 4b1 and 4b2. Further, the electrode 4d2 is connected to the wiring electrode 5b, and the electrode 4d1 is not connected to the wiring electrode 5b or other electrodes, and is in a floating state.

Specifically, the electrode 4d1 and the electrode 4d2 are formed on the upper surface side of the dielectric film 8 and the lower surface side (corresponding to the space between the films) of the piezoelectric element 3b, and the electrode 4d2 is connected to the wiring electrode 5b. ing. The electrode 4d1 is not connected to the wiring electrode 5b or other electrodes, and is in a floating state. Further, an electrode 4c1 and an electrode 4c2 are formed on the upper surface side of the piezoelectric thin film 3b, the electrode 4c1 is connected to the wiring electrode 5a, and the electrode 4c2 is not connected to the wiring electrode 5a or other electrodes. , It is in a floating state. 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, the piezoelectric element C1 (corresponding to the first piezoelectric element) is formed in the region where the electrode 4a1, the piezoelectric thin film 3a (corresponding to the first piezoelectric thin film), and the electrode 4b1 overlap. Similarly, the piezoelectric element C2 (corresponding to the second piezoelectric element) is in the region where the electrode 4a2, the piezoelectric thin film 3a and the electrode 4b2 overlap, and the piezoelectric element C3 (third) is in the region where the electrode 4a2, the piezoelectric thin film 3a and the electrode 4b2 overlap. Piezoelectric element C4 (corresponding to the fourth piezoelectric element) is the region where the electrode 4c1 and the piezoelectric thin film 3b (corresponding to the second piezoelectric thin film) and the electrode 4d1 overlap each other. And the piezoelectric element C5 (corresponding to the fifth piezoelectric element) is formed in the region where the electrodes 4d1 overlap, and the piezoelectric element C6 (corresponding to the sixth piezoelectric element) is formed in the region where the electrodes 4c2, the piezoelectric thin film 3b and the electrodes 4d2 overlap. Will be done.

As a result, the first piezoelectric element C1, the second piezoelectric element C2, and the third piezoelectric element C3 are connected in series between the wiring electrode 5a and the wiring metal 5b. Similarly, the fourth piezoelectric element C4, the fifth piezoelectric element C5, and the sixth piezoelectric element C6 are connected in series between the wiring electrode 5a and the wiring metal 5b. Further, a set of these piezoelectric elements connected in series is also configured to be connected in parallel.

Even if it is formed in this way, as is clear from FIG. 8, the first piezoelectric element C1 and the fourth piezoelectric element C4, the second piezoelectric element C2 and the fifth piezoelectric element C5, and the third piezoelectric element C3 The sixth piezoelectric element C6 has a structure that is vertically symmetrical with respect to a surface passing through the center of the dielectric film 8 in the thickness direction, at least in a region forming each piezoelectric element.

On the back surface side of the silicon substrate 1, a hole 7 from which a part thereof has been removed is formed, and the electrodes 4a1, 4a2 and the piezoelectric thin film 3a are exposed in the hole 7. The pores 7 communicate with the surface side of the silicon substrate 1 through the slit 6 shown in FIG.

With this configuration, the piezoelectric element of this embodiment can also have a double-sided beam structure in which a plurality of electrode pairs are formed on a piezoelectric thin film in which both ends are supported by a silicon substrate 1 (support substrate).

When the piezoelectric element of this embodiment is configured as an acoustic transducer, acoustic pressure is applied from the pores 7 formed in the silicon substrate 1. The beam structure containing the piezoelectric thin film subjected to acoustic pressure is curved and displaced upward. As a result, tensile stress and compressive stress are generated in the aluminum nitride constituting the piezoelectric thin film.

However, as in the first embodiment, the piezoelectric element C1 and the piezoelectric element C4, the piezoelectric element C2 and the piezoelectric element C5, and the piezoelectric element C3 and the piezoelectric element C6 have vertically symmetrical structures, respectively. As in the case described above, the voltages generated in each region have opposite polarities and have the same value, so that in-phase voltages caused by residual stress and temperature fluctuations can be canceled out.

As a result, the output signal of each region based on the acoustic pressure signal is applied (voltage) is superimposed added without the signal due to residual stresses and temperature change, the output voltage for the acoustic pressure (P _a) ( It is possible to increase the sensitivity as an acoustic transducer defined by the ratio of V _out ) (V _out / P _a ). In particular, the piezoelectric signal is extracted from the top layer piezoelectric thin film 3b and the bottom layer piezoelectric thin film 3a of the laminated structure, and the piezoelectric signal is extracted from the portion where the stress is relatively large, so that the signal noise ratio is expected to be further improved. Will be done.

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) The same applies to the fact that the size may be determined. Further, the thickness and material of the dielectric film may be appropriately selected in order to obtain desired characteristics. The dielectric film may be aluminum nitride. When three layers of aluminum nitride are laminated, the thickness of each layer may be 0.33 μm.

When the bending displacement is caused by the vibration of the piezoelectric thin film, when the inflection point of the displacement is 2 or more, the number of piezoelectric elements is increased for each region partitioned by the inflection point, not limited to the above embodiment. Alternatively, a plurality of elements may be arranged in each region.

By the way, when a piezoelectric element as in the present invention is used as an acoustic transducer, an amplifier circuit for processing a signal output from the piezoelectric element is required. Generally, integrated circuits are formed on different silicon substrates and mounted on the mounting substrate as separate elements. In the piezoelectric element of the present invention, there is no problem even if an element for signal processing is formed on a support substrate, for example. In that case, the piezoelectric thin film may be used as an interlayer insulating film, and the electrodes may be used as wiring metal.

1: Silicon substrate 2: Insulating film, 3a, 3b: Piezoelectric thin film, 4a, 4b, 4c, 4d: Electrode, 5a, 5b: Wiring electrode, 6: Slit, 7: Pore, 8: Dielectric film

Claims (4)

In a piezoelectric element provided with a piezoelectric thin film having both ends fixed to a support substrate and a pair of electrodes arranged so as to sandwich the piezoelectric thin film.
The piezoelectric thin film has a laminated structure including at least a first piezoelectric thin film and a second piezoelectric thin film.
A plurality of sets of the pair of electrodes arranged so as to sandwich a part of the first piezoelectric thin film are provided, and at least the first piezoelectric element, the second piezoelectric element, and the third piezoelectric element are formed.
A plurality of sets of the pair of electrodes arranged so as to sandwich a part of the second piezoelectric thin film are provided, and at least a fourth piezoelectric element, a fifth piezoelectric element, and a sixth piezoelectric element are formed.
The first piezoelectric element, the second piezoelectric element, and the third piezoelectric element are arranged side by side in order from one end side to the other end side of both ends.
The fourth piezoelectric element, the fifth piezoelectric element, and the sixth piezoelectric element are arranged side by side in order from one end side to the other end side of both ends.
The first piezoelectric element and the fourth piezoelectric element, the second piezoelectric element and the fifth piezoelectric element, or the third piezoelectric element and the sixth piezoelectric element pair of piezoelectric elements are arranged in parallel. That the piezoelectric elements that are connected and connected in parallel are connected in series,
The first piezoelectric element, the second piezoelectric element, and the third piezoelectric element are connected in series, and the fourth piezoelectric element, the fifth piezoelectric element, and the sixth piezoelectric element are connected in series. However, the two sets of piezoelectric elements connected in series are connected in parallel, and
The first piezoelectric element and the fourth piezoelectric element are vertically symmetrically laminated, and the second piezoelectric element and the fifth piezoelectric element are vertically symmetrically laminated to form the third piezoelectric element. A piezoelectric element characterized in that the sixth piezoelectric element and the sixth piezoelectric element are vertically symmetrically laminated. In the piezoelectric element according to claim 1,
The set of piezoelectric elements connected in parallel is connected in series by an extension portion continuous from the electrodes of the piezoelectric elements arranged on the front surface, the back surface, or between the films of the first piezoelectric thin film or the second piezoelectric thin film. the piezoelectric element characterized by there. In the piezoelectric element according to any one of claims 1 or 2.
When the piezoelectric thin film is curved and displaced due to vibration, the first piezoelectric element and the fourth piezoelectric element, which are formed by stacking at least vertically symmetrically for each region defined by the bending point of the displacement, the said. A piezoelectric element characterized in that any one of the second piezoelectric element and the fifth piezoelectric element or the third piezoelectric element and the sixth piezoelectric element is arranged . In the piezoelectric element according to any one of claims 1 to 3,
The piezoelectric thin film is a piezoelectric element that vibrates due to acoustic pressure .

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