JP2020178109A - Piezoelectric element - Google Patents

Abstract

To provide a piezoelectric element having high sensitivity and an improved signal-to-noise ratio while suppressing an influence of residual stress of a piezoelectric film.SOLUTION: The piezoelectric element includes a diaphragm 2 made of piezoelectric films 5a, 5b supported by a support substrate 3 and a diaphragm 2, penetrating the piezoelectric films, partitioned by a slit 1 for partitioning the piezoelectric films as diaphragms. The slit is filled with an ionic liquid 11. The ionic liquid can keep the slit closed by following a vibration of the piezoelectric films and configure a piezoelectric element without deterioration of characteristics.SELECTED DRAWING: Figure 1

Description

The present invention relates to a piezoelectric element, and particularly to a piezoelectric element that can be used for a piezoelectric MEMS microphone or the like having high sensitivity and low noise.

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

Most of these types of MEMS elements are capacitive elements that capture the 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, attention has been paid to a piezoelectric element capable of extracting acoustic pressure or the like as a voltage change due to distortion of a single diaphragm made of a thin film (piezoelectric film) made of a piezoelectric material.

In the conventional piezoelectric element, a slit 1 having a desired shape as shown in FIG. 6 is formed in the piezoelectric film to form a diaphragm having a cantilever structure. In FIG. 6A, two quadrangular diaphragms 2 are formed, and in FIG. 6B, four triangular diaphragms 2 are formed. This type of piezoelectric element is disclosed in, for example, Patent Document 1.

FIG. 7 is a cross-sectional view of the piezoelectric element. As shown in FIG. 7, a multilayer structure piezoelectric film 5a and 5b is supported and fixed on a silicon substrate 3 serving as a support substrate via an insulating film 4, the piezoelectric film 5a is supported and fixed by electrodes 6a and 6b, and the piezoelectric film 5b is an electrode. The structure is sandwiched between the 6b and the electrode 6c, respectively. A pore 7 is formed in the silicon substrate 3, and the piezoelectric film and electrodes partitioned by the slit 1 form a diaphragm 2 in which one end is fixed to the silicon substrate 3 and the other end is an open end. ..

In such a piezoelectric element, when the vibrating plate 2 receives acoustic pressure or the like, the piezoelectric film 5a is distorted and polarization occurs inside the piezoelectric film 5a, and a voltage is generated from the wiring metal 8a connected to the electrode 6a and the wiring metal 8b connected to the electrode 6b. It becomes possible to take out a signal. Similarly, when the piezoelectric film 5b is distorted, polarization occurs inside the piezoelectric film 5b, and it becomes possible to extract a voltage signal from the wiring metal 8a connected to the electrode 6c and the wiring metal 8b connected to the electrode 6b.

By the way, in such a piezoelectric film having a cantilever structure, residual stress is released and warpage occurs, and the opening width of the slit 1 is widened. If a piezoelectric element in which the opening width of the slit is equal to or larger than the design value due to such warpage is used as a microphone, the acoustic resistance is lowered and the characteristics such as low frequency sensitivity are deteriorated.

Therefore, the applicant of the present application has proposed a piezoelectric element in which a thin film 9 is laminated and formed on a diaphragm as shown in FIG. 8 (Patent Document 2).

Japanese Patent No. 5936154 JP-A-2018-137297

Since the piezoelectric element previously proposed by the applicant of the present application has a structure including the thin film 9, it is possible to suppress the expansion of the opening width of the slit 1 due to the residual stress of the piezoelectric film and the accompanying deterioration of characteristics. However, in the manufacturing process, after the slit 1 is formed, the opening of the thin film 9 is formed so as to communicate with the slit 1, so that the opening of the thin film 9 is taken into consideration in consideration of the variation in the manufacturing process (misalignment). If the width is formed wider than the opening width of the slit 1, the thin film 9 has the effect of suppressing the deformation of the diaphragm by increasing the elastic modulus of the diaphragm, but even if the thin film 9 is formed, it vibrates. If the deformation of the plate cannot be completely suppressed, the opening width of the slit 1 will exceed the design value. When such a piezoelectric element is used as a microphone, it is not possible to reduce the acoustic resistance and completely suppress the deterioration of characteristics such as the decrease in low frequency sensitivity. An object of the present invention is to solve such a problem and to provide a piezoelectric element which suppresses the influence of residual stress of a piezoelectric film and has high sensitivity and an improved signal-to-noise ratio.

In order to achieve the above object, the invention according to claim 1 penetrates the piezoelectric film, a vibrating plate made of a piezoelectric film having at least one end supported by a support substrate, a pair of electrodes arranged with the piezoelectric film interposed therebetween. A piezoelectric element provided with a slit for partitioning the piezoelectric film as the vibrating plate is characterized in that the slit is filled with an ionic liquid.

The invention according to claim 2 is the piezoelectric element according to claim 1, in which a dam portion is arranged along the slit, and the inside of the slit sandwiched between the dam portions or the inside of the slit surrounded by the dam portion. It is characterized in that it is filled with an ionic liquid.

In the invention according to claim 3, in the piezoelectric element according to any one of claims 1 or 2, a liquid reservoir communicating with the opposite side surface of the slit is arranged, and the slit including the liquid reservoir is filled with an ionic liquid. It is characterized by that.

By filling the slit with an ionic liquid, the piezoelectric element of the present invention can maintain a state in which the slit is closed by the ionic liquid that deforms following the vibration of the piezoelectric film, and provides a piezoelectric element having desired characteristics. It becomes possible to do.

By filling the area surrounded by the dam portion arranged along the slit with the ionic liquid, the slit can be reliably closed. Further, by arranging a liquid reservoir communicating with the slit and filling the slit containing the liquid reservoir with an ionic liquid, the slit can be reliably closed. Further, even if a part of the slit is a void that is not filled with the ionic liquid, the characteristics of the piezoelectric element can be improved.

It is explanatory drawing of the piezoelectric element of the 1st Example of this invention. It is explanatory drawing of the piezoelectric element of the 1st Example of this invention. It is explanatory drawing of the piezoelectric element of the 2nd Example of this invention. It is explanatory drawing of the piezoelectric element of the 3rd Example of this invention. It is explanatory drawing of the piezoelectric element of the 4th Example of this invention. It is explanatory drawing of the conventional piezoelectric element. It is explanatory drawing of the conventional piezoelectric element. It is explanatory drawing of another conventional piezoelectric element.

The piezoelectric element according to the present invention is provided with a diaphragm in which a piezoelectric film supported by a support substrate is partitioned by slits having a desired shape, and the slits are filled with an ionic liquid. The ionic liquid can keep the state of closing the slit by following the vibration of the piezoelectric film, and can form a piezoelectric element without deterioration of characteristics. The ionic liquid can be filled in the slit by dropping the ionic liquid onto the slit. Further, by variously changing the shape of the slit and its vicinity, it is possible to simplify the filling of the ionic liquid into the slit. Hereinafter, an embodiment in which the shape of the slit and its vicinity is changed will be described in detail.

A first embodiment of the present invention will be described. The piezoelectric element of the present invention forms a slit 1 having a desired shape as shown in FIG. 6 described above in a conventional example to form a piezoelectric element having a cantilever structure. Further, in the present invention, the slit 1 is filled with the ionic liquid by dropping the ionic liquid into the slit 1 from above the slit 1, so that the dam portion 10 is provided.

FIG. 1 is a cross-sectional view of the piezoelectric element of this embodiment. As shown in FIG. 1, a multilayer structure piezoelectric film 5a and 5b are supported and fixed on a silicon substrate 3 serving as a support substrate via an insulating film 4, the piezoelectric film 5a is supported and fixed by electrodes 6a and 6b, and the piezoelectric film 5b is formed by electrodes 6a and 6b. The structure is sandwiched between the electrodes 6b and 6c, respectively. Since the silicon substrate 3 is formed with holes 7, the piezoelectric film and electrodes partitioned by the slit 1 form a diaphragm 2 in which one end is fixed to the silicon substrate 3 and the other end is an open end. become.

In such a piezoelectric element, when the vibrating plate 2 is displaced by receiving acoustic pressure or the like, the piezoelectric film 5a is distorted and polarization occurs inside the piezoelectric film 5a, so that the wiring metal 8a connected to the electrode 6a and the wiring metal 8b connected to the electrode 6b are connected. It is possible to take out the voltage signal from. Similarly, when the piezoelectric film 5b is distorted, polarization occurs inside the piezoelectric film 5b, and it becomes possible to extract a voltage signal from the wiring metal 8a connected to the electrode 6c and the wiring metal 8b connected to the electrode 6b.

Here, in this embodiment, the dam portion 10 is arranged on the piezoelectric film along the slit 1. As shown in FIG. 1, the dam portion 10 is set to a height sufficient to fill the inside with the ionic liquid 11.

The dam portion 10 can be arranged as shown in FIG. The piezoelectric element shown in FIG. 2A has a structure in which a dam portion 10 is arranged around a slit 1 forming two square diaphragms 2 and an ionic liquid 11 is filled inside the dam portion 10. Further, the piezoelectric element shown in FIG. 2B has a structure in which a dam portion 10 is arranged around a slit 1 forming four triangular diaphragms 2 and an ionic liquid 11 is filled inside the dam portion 10.

The ionic liquid filled in the slit 1 has non-volatility, flame retardancy, and high heat resistance. The ionic liquid can have a desired viscosity depending on the constituent cations and anions. Further, as a mixed liquid containing an ionic liquid, it is possible to adjust the characteristics to be suitable for filling the slit. The ionic liquid filled in the slit 1 is deformed following the movement of the diaphragm 2, and the slit 1 can be kept in a closed state.

If the entire slit 1 is closed with the ionic liquid 11 as shown in FIG. 2, characteristic fluctuations and the like caused by fluctuations in the width of the slits 1 are eliminated. When such a piezoelectric element is used as a microphone, it is preferable to configure a microphone that does not deteriorate in characteristics such as a decrease in acoustic resistance and a decrease in low frequency sensitivity.

Next, a second embodiment will be described. In the first embodiment described above, an example in which the dam portion 10 is formed so as to surround the entire periphery of the slit 1 has been described. However, even if the structure does not completely block the slit 1, the characteristics of the piezoelectric element can be improved. For example, as shown in FIG. 3, the dam portion 10 can be arranged leaving a part of the slit 1. In the piezoelectric element shown in FIG. 3A, dam portions 10 are arranged at both ends of the slits 1 forming the two quadrangular diaphragms 2 with the slits 1 extending in the vertical direction of the drawing interposed therebetween. It has a structure in which only the ionic liquid 11 is filled. In this case, the slit 1 extending in the left-right direction of the drawing is not filled with the ionic liquid, and the void 12 remains. Further, the piezoelectric element shown in FIG. 3B has a structure in which dam portions 10 are arranged at both ends of the slit 1 forming the four triangular diaphragms 2 and the ionic liquid 11 is filled only inside the dam portions 10. ing. In this case, the end portion of the slit 1 is not filled with the ionic liquid, and the void 12 remains.

Since the slit 1 is filled so as to leave a void 12, the ionic liquid or the mixed liquid containing the ionic liquid may be appropriately selected or adjusted.

When the slit 1 is closed with the ionic liquid 11 as shown in FIG. 3, characteristic fluctuations and the like caused by fluctuations in the width of the slits 1 are suppressed. When such a piezoelectric element is used as a microphone, it is preferable to configure a microphone that suppresses characteristic deterioration such as a decrease in acoustic resistance and a decrease in low frequency sensitivity. In particular, in this embodiment, the structure may include a gap 12, and the gap 12 can function as a vent hole.

Next, a third embodiment will be described. FIG. 4 is a cross-sectional view of the piezoelectric element of this embodiment. As shown in FIG. 4, in order to increase the amount of the ionic liquid 11 filled in the slit 1, it is possible to arrange the liquid reservoir 13 in the slit 1. The liquid reservoir 13 can be formed between the diaphragms 2. For example, as an example, after forming the piezoelectric film 5a, a part of the piezoelectric film 5a is removed to form a sacrificial film in the region where the liquid pool is to be formed, and the piezoelectric film 5b is formed on the sacrificial film. If the sacrificial film is selectively removed after the slit 1 is formed, the liquid reservoir 13 can be formed. Alternatively, the sacrificial layer may be formed without removing a part of the piezoelectric film 5a, and then removed.

The liquid reservoir 13 of this embodiment can be formed inside the diaphragm 2 so as to communicate with the slit 1, and can be formed in the entire slit 1 as in the arrangement of the dam portion of the first embodiment. It can be formed in a part of the slit 1 as in the arrangement of the dam portion in the second embodiment. Alternatively, a plurality of independent liquid reservoirs 13 may be formed.

By arranging such a liquid reservoir 13, the amount of the ionic liquid 11 staying in the slit 1 can be increased, and the diaphragm 1 is deformed following a large movement of the diaphragm 2 to close the slit 1. It becomes possible to keep the state.

The next fourth embodiment will be described. FIG. 5 is a cross-sectional view of the piezoelectric element of this embodiment. As shown in FIG. 5, in order to simplify the filling of the ionic liquid 11 into the slit 1, the liquid reservoir 13 formed in the slit 1 can be arranged on the surface of the diaphragm 2. The liquid reservoir 13 can be formed by removing a part of the surface of the diaphragm 2. For example, after forming the piezoelectric film 5b, a part of the surface may be removed to form a liquid reservoir 13, and then the slit 1 may be formed. Alternatively, after forming the slit 1, the liquid reservoir 13 may be formed.

The liquid reservoir 13 of this embodiment can also be formed so as to communicate with the slit 1, and can be formed in the entire slit 1 as in the arrangement of the dam portion of the first embodiment, or the dam of the second embodiment. It can be formed in a part of the slit 1 like the arrangement of the portions. Alternatively, a plurality of independent liquid reservoirs 13 may be formed.

By arranging such a liquid reservoir 13, the amount of the ionic liquid 11 that stays in the slit 1 can be increased, and the diaphragm 2 is deformed following a large movement of the diaphragm 2 to close the slit 1. It becomes possible to keep the state.

Although the examples of the present invention have been described above, the present invention is not limited to the above examples. For example, it may be configured to include both a dam portion and a liquid reservoir.

1: Slit, 2: Diaphragm, 3: Silicon substrate, 4: Insulating film, 5a, 5b: Piezoelectric film, 6a, 6b, 6c: Electrode, 7: Pore, 8a, 8b: Wiring metal, 9: Thin film, 10: Dam part, 11: Ionic liquid, 12: Void, 13: Liquid reservoir

Claims (3)

A vibrating plate made of a piezoelectric film having at least one end supported by a support substrate, a pair of electrodes arranged with the piezoelectric film interposed therebetween, and a slit penetrating the piezoelectric film to partition the piezoelectric film as the vibrating plate. In the piezoelectric element
A piezoelectric element characterized in that the slit is filled with an ionic liquid. In the piezoelectric element according to claim 1,
A piezoelectric element in which a dam portion is arranged along the slit, and an ionic liquid is filled in the slit sandwiched between the dam portions or in the slit surrounded by the dam portion. In the piezoelectric element according to any one of claims 1 or 2.
A piezoelectric element characterized in that a liquid reservoir communicating with the opposite side surface of the slit is arranged, and the slit containing the liquid reservoir is filled with an ionic liquid.

Images