JP2019140638A - Piezoelectric element - Google Patents

Abstract

To provide a piezoelectric element that suppresses influence of residual stress of a piezoelectric film and solves the problem of limiting an outer shape, and suppresses a decrease in low-frequency sensitivity.SOLUTION: The piezoelectric element includes: a piezoelectric film 8 having at least one end supported and fixed to a support substrate 1 and the other end being a free end by forming a slit 12; and a movable film 3 provided between the support substrate. The movable film is supported and fixed at a peripheral edge to the support substrate and vibrates in response to acoustic pressure or the like. A displacement of the movable film is transmitted to the piezoelectric film by a transmission unit 5 having one end bonded to the piezoelectric film and the other end bonded to the movable film, and an output signal is output.SELECTED DRAWING: Figure 6

Description

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

  2. Description of the Related Art In recent years, smartphones whose demand has been rapidly expanding are often small-sized, thin-type microphones using MEMS (Micro Electro Mechanical System) technology having high-temperature processing resistance in an assembly solder reflow process. In addition to MEMS microphones, other MEMS elements are rapidly spreading in various fields.

  Most of this type of MEMS element is a capacitive element that detects a vibration displacement of a diaphragm due to an acoustic pressure or the like as a change in capacitance with an opposing fixed plate, converts it into an electric signal, and outputs it. However, improvement in the signal-to-noise ratio of the capacitive element is becoming a limit due to acoustic resistance generated by the flow of air in the gap between the diaphragm and the fixed plate.

  Accordingly, attention has been paid to a piezoelectric element that can take out acoustic pressure or the like as a voltage change by distortion of a single diaphragm formed of a thin film (piezoelectric film) made of a piezoelectric material.

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

  FIG. 8 shows a cross-sectional view of a piezoelectric element having a cantilever structure. As shown in FIG. 8, a piezoelectric film 8 sandwiched between a lower electrode 7 and an upper electrode 9 is fixed via an insulating film 6 on a silicon substrate 1 serving as a support substrate, and a protective film 10 is formed on the uppermost surface. It has a structure. The diaphragm made of the insulating film 6, the lower electrode 7, the piezoelectric film 8, the upper electrode 9, and the protective film 10 has a rectangular planar shape by forming another slit (not shown), and one end of the diaphragm 1 The other end (the end portion constituting the slit 12 shown in the figure) is a free end. The lower electrode 7 is connected to the wiring metal 11a, and the upper electrode 9 is connected to another wiring metal 11b.

  In such a piezoelectric element, when the diaphragm vibrates due to an acoustic pressure or the like, the piezoelectric film 8 is distorted and polarization is generated therein, and the wiring metal 11 a connected to the lower electrode 7 and the wiring metal connected to the upper electrode 9. It becomes possible to take out a voltage signal from 11b.

  By the way, although the residual stress of the piezoelectric film 8 is released by adopting the cantilever structure as shown in FIG. 8, as a result, the piezoelectric film warps, and the gap (slit 12) between adjacent piezoelectric films or the piezoelectric film (beam) ) The size of the substantial gap between the side surface and the support substrate increases. The generation of such a gap exceeding the design value reduces acoustic resistance when the piezoelectric element is used as a microphone, leading to characteristic deterioration such as low frequency sensitivity reduction.

  In order to solve this problem, a piezoelectric element having a structure in which a square back chamber is formed on a support substrate and a cross-shaped slit 12 as shown in FIG. Yes. In the piezoelectric element having such a structure, by arranging each of the four triangles at the center of the piezoelectric film 8, even if the piezoelectric film warps due to residual stress, Has also been disclosed a technique in which the same warpage occurs, and as a result, the size of the gap (slit 12) between adjacent piezoelectric films is not greatly changed (Patent Document 2).

Japanese Patent No. 5707323 Special table 2014-515214 gazette

  In order to prevent characteristic deterioration due to the residual stress of the piezoelectric film, in the conventional piezoelectric element, the shape of the piezoelectric film is a triangle, and the gaps between adjacent piezoelectric films are arranged by collecting the four triangles at the center. It was made possible not to greatly change the width dimension. However, in order to match the resonance frequency of each triangular piezoelectric film, it is necessary to combine triangles having the same shape, and there is a problem that the outer shape of the piezoelectric element is limited to a square and there is no degree of freedom in design. In addition, the residual stress varies even within one piezoelectric element during the manufacturing process, the degree of warping of the four triangles is different, and the width of the gap between adjacent piezoelectric films expands beyond the design value, etc. Therefore, it is very difficult to control the width of the gap between adjacent piezoelectric films at the mass production level. Therefore, an object of the present invention is to provide a piezoelectric element that suppresses the influence of residual stress of a piezoelectric film and solves the problem of limiting the outer shape, and suppresses a decrease in low-frequency sensitivity.

  In order to achieve the above object, an invention according to claim 1 of the present application is a piezoelectric element comprising a piezoelectric film having at least one end supported and the other end being a free end, and a pair of electrodes disposed with the piezoelectric film interposed therebetween. A support substrate having a back chamber, a peripheral portion fixed on the support substrate, a movable film disposed opposite to the piezoelectric film, one end bonded to the piezoelectric film, and the other end bonded to the movable film, And a transmission unit that transmits the displacement of the movable film to the piezoelectric film.

  The invention according to claim 2 of the present application is the piezoelectric element according to claim 1, wherein the transmission portion is joined to one end on the center side of the movable film and the other end on the free end side of the piezoelectric film. It is characterized by being.

  In the piezoelectric element of the present invention, since a low path such as acoustic resistance is not formed by forming a movable film on the back chamber, it is possible to suppress a decrease in low frequency sensitivity. Therefore, the number of slits formed in the piezoelectric film can be increased and the slit width can be increased, and there is an advantage that the degree of freedom in designing the piezoelectric element is increased.

  Further, by forming the transmission part of the present invention so as to be joined to the center side of the movable film and the free end side of the piezoelectric film, a large displacement of the movable film is transmitted to the piezoelectric film via the transmission part, and a large output A signal can be obtained.

  Furthermore, in the case where there is no acoustic pressure or the like, which is a problem with conventional piezoelectric elements, the output of unnecessary signals due to the residual stress or temperature fluctuation of the piezoelectric film is limited to a certain extent by the transmission part, but the transmission part is limited by the transmission part. Is smaller than the area of the piezoelectric film, so that the influence of the residual stress of the piezoelectric film can be suppressed. Furthermore, since the piezoelectric film is fixed by the transmission unit, it is possible to suppress warping of the plurality of piezoelectric films, and to align the characteristics of the respective piezoelectric films.

  In addition, the piezoelectric element of the present invention generates noise in the air gap due to the flow of air and the S / N value decreases, but the dimension between the movable film and the piezoelectric film (interval of the air gap) is increased. By doing so, the influence of noise can be reduced. In addition, due to the structure in which the transmission part is joined to the movable film and the piezoelectric film, even if the dimension between the movable film and the piezoelectric film is increased, the displacement of the movable film is transmitted to the piezoelectric film without being reduced. Therefore, a decrease in S / N value can be suppressed.

It is explanatory drawing of the manufacturing process of the piezoelectric element of 1st Example of this invention. It is explanatory drawing of the manufacturing process of the piezoelectric element of 1st Example of this invention. It is explanatory drawing of the manufacturing process of the piezoelectric element of 1st Example of this invention. It is explanatory drawing of the manufacturing process of the piezoelectric element of 1st Example of this invention. It is explanatory drawing of the manufacturing process of the piezoelectric element of 1st Example of this invention. It is explanatory drawing of the piezoelectric element of 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 conventional piezoelectric element. It is explanatory drawing of another conventional piezoelectric element.

  The piezoelectric element of the present invention includes a movable film constituting a vibration film between a support film and a piezoelectric film in which at least one end is supported and fixed to a support substrate and a slit is formed so that the other end is a free end. The configuration. The movable film is supported and fixed at the peripheral edge to the support substrate, and vibrates in response to acoustic pressure or the like. The displacement of the movable film is transmitted to the piezoelectric film by a transmission unit in which one end is joined to the movable film and the other end is joined to the piezoelectric film, and the piezoelectric film vibrates and outputs a signal.

  If comprised in this way, since the low path | routes, such as acoustic resistance through which an acoustic pressure etc. pass into a back chamber, are not formed, it will become possible to suppress the fall of a low frequency sensitivity, etc. Hereinafter, the piezoelectric element of the present invention will be described in detail.

  A first embodiment of the present invention will be described in accordance with its manufacturing process. First, an insulating film 2 made of a silicon oxide film having a thickness of about 0.3 to 1.0 μm is formed by thermal oxidation on a silicon substrate 1 having a crystal orientation (100) plane of 420 μm to be a support substrate, Further, a polysilicon film having a thickness of about 0.2 to 1.0 μm is laminated on the insulating film 2 by a CVD (Chemical Vapor Deposition) method, and patterning is performed by a normal photolithography method to form the movable film 3. Thereafter, an insulating film 4 made of a silicon oxide film having a thickness of about 0.5 to 1.0 μm is formed on the movable film 3 by CVD (FIG. 1).

  Patterning is performed by a normal photolithography method, and the insulating film 4 in the region where the transmission part is to be formed is removed by etching to expose the movable film 3. Here, the transmission unit has a shape that can transmit the vibration of the movable film 3 to the piezoelectric film and can receive the acoustic pressure and the like on the entire movable film 3 so that the vibration of the movable film 3 is greatly vibrated. Is preferred. For example, since it is assumed to be a columnar shape as an example, the transmission portion formation scheduled region can be a cylindrical concave portion. Here, there are four transmission part formation scheduled regions as described below.

  Filling the etched region where the transmission part is to be formed, forming polysilicon with a planarized surface on the insulating film 4, and then etching back to expose the surface of the insulating film 4, thereby forming the transmission part The planned region can be selectively filled with polysilicon. This polysilicon becomes the transmission portion 5. Next, an insulating film 6 made of a silicon nitride film having a thickness of about 0.1 to 1.0 μm is formed on the transmission portion 5 and the insulating film 4 by CVD (FIG. 2).

  Thereafter, molybdenum or the like having a thickness of about 0.1 to 1.0 μm is stacked on the insulating film 6 by patterning, and patterning is performed by a normal photolithography method to form the lower electrode 7. Next, a piezoelectric film 8 made of aluminum nitride or the like having a thickness of about 0.1 to 1.0 μm is formed on the entire surface by sputtering. Further, molybdenum or the like having a thickness of about 0.1 to 1.0 μm is laminated on the piezoelectric film 8 and patterned by a normal photolithography method to form the upper electrode 9 (FIG. 3).

  After a protective film 10 made of a silicon nitride film having a thickness of about 1.0 μm is deposited on the entire surface by CVD, a part of the protective film 10 and the piezoelectric film 8 is removed by etching, and the wiring electrode 11a connected to the lower electrode 7 and the upper part A wiring electrode 11b connected to the electrode 9 is formed. The wiring electrodes 11a and 11b are made of aluminum and can be formed by a normal photolithography method (FIG. 4).

  A part of the protective film 10, the piezoelectric film 8 and the insulating film 6 in the slit formation region is removed by a dry etching method by a normal photolithography method to form a slit 12, and the insulating film 4 is exposed. Thereafter, a part of the insulating film 4 made of a silicon oxide film is removed through the slit 12 to form an air gap 13 (FIG. 5). The planar shape of the slit 12 is, for example, a structure that opens in a cross shape as shown in FIG. 9, and is separated into four piezoelectric films.

  As shown in FIG. 5, it is understood that the transmission unit 5 has a shape in which one end is bonded to the movable film 3 and the other end is bonded to the piezoelectric film 8 via the insulating film 6 and the lower electrode 7. Furthermore, it turns out that it has joined to the free end side of the piezoelectric film. In addition, the four transmission parts 5 previously formed on the four piezoelectric films 8 separated by the slits 12 are joined one by one.

  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 a back chamber 14 (FIG. 6).

  As shown in FIG. 6, when the back chamber 14 is formed, the movable film 3 is fixed to the silicon substrate (supporting substrate) at the periphery, and can be vibrated. Further, the joint between the transmission unit 5 and the movable film 3 is near the center (center side) of the back chamber 14 and is at a position where the displacement of the movable film 3 is large.

  When the piezoelectric element of the present invention formed as described above receives acoustic pressure or the like from the surface side where the slit 12 is formed, the piezoelectric film 8 directly receives and displaces the acoustic pressure or the like, and is movable via the transmission unit 5. The membrane 3 is displaced. At the same time, acoustic pressure or the like is transmitted from the slit 12 into the air gap 13, and the movable film 3 is displaced. The displacement of the movable film 3 is transmitted to the piezoelectric film 8 through the transmission unit 5.

  As a result, the piezoelectric film 8 is distorted and a partial pressure is generated therein, and a voltage signal can be taken out as an output signal from the wiring metal 11 a in contact with the lower electrode 7 and the wiring metal 11 b in contact with the upper electrode 9.

  In particular, in this embodiment, all of the acoustic pressure transmitted from the slit 12 into the air gap 13 becomes energy to vibrate the movable film 3, and the movable film 3 can be vibrated greatly, and a large output signal can be obtained. .

  Further, the area of the transmission unit 5 is smaller than the area of the piezoelectric film 8, and the piezoelectric element characteristics can be improved by suppressing the variation in the warp of the piezoelectric film while releasing the residual stress of the piezoelectric film. .

  By the way, in the air gap 13, noise is generated due to the flow of air, and the S / N value is lowered. However, by forming a large dimension (interval of the air gap 13) between the movable film 3 and the piezoelectric film 8. The effect of noise can be reduced. Further, due to the structure in which the transmission unit 5 is joined to the movable film 3 and the piezoelectric film 8, even if the dimension between the movable film 3 and the piezoelectric film 8 is formed large, the displacement of the movable film 3 is not reduced. Since it is transmitted to the piezoelectric film 8, it is possible to suppress a decrease in the S / N value.

  Next, a second embodiment will be described. As described in the first embodiment, the piezoelectric element of the present invention can obtain a large output signal even when the width and number of the slits 12 are increased, in other words, the opening area of the slits 12 is increased. . Therefore, there is no problem even if the shape of the slit 12 is as shown in FIG. In this case, it is necessary to dispose the transmission unit 5 so that the displacement of the movable film 3 is transmitted to each piezoelectric film 8. Even when a plurality of transmission portions 5 are formed as shown in FIG. 7, a gap remains between adjacent transmission portions 5, and the entire movable film can receive acoustic pressure and the like, and a large output signal can be obtained. Can do.

  Next, a third embodiment will be described. In the first and second embodiments, the movable film 3 has been described as a structure that covers the entire upper surface of the back chamber 14. However, when the back chamber 14 is configured to be a sealed space, when the piezoelectric element is mounted on the mounting substrate, it is heated and then cooled in order to fix the silicon substrate as the supporting substrate to the mounting substrate. It is not preferable because it is necessary to go through a manufacturing process. Therefore, it is possible to form a through hole in a part of the movable film 3. In this case, it is not preferable that the acoustic pressure transmitted from the slit 12 into the air gap 13 passes through the through hole and does not contribute to the displacement of the movable film 3. Therefore, it is preferable to set the opening area of the through hole to be sufficiently smaller than the opening area of the slit. The formation of the through hole is preferable from the viewpoint of preventing damage to the piezoelectric element even when the atmospheric pressure in the surrounding environment of the piezoelectric element changes.

  Although the embodiments of the present invention have been described above, it is needless to say that the present invention is not limited to the above-described embodiments. Specifically, the shape of the cavity, the number of slits, the extending direction, and the like can be appropriately changed. . The slit is not limited to a straight line, but can be formed of a curved line, so that unnecessary stress concentration can be reduced. Furthermore, in order to change the spring property of the movable film, its material, manufacturing method, and the like can be appropriately changed. Therefore, the degree of freedom in designing the piezoelectric element is increased, and for example, the resonance frequency can be set freely.

1: silicon substrate, 2: insulating film, 3: movable film, 4: insulating film, 5: transmission part, 6: insulating film, 7: lower electrode, 8: piezoelectric film, 9: upper electrode, 10: protective film, 11a, 11b: wiring electrode, 12: slit, 13: air gap, 14: back chamber

Claims (2)

In a piezoelectric element comprising a piezoelectric film having at least one end supported and the other end being a free end, and a pair of electrodes arranged with the piezoelectric film interposed therebetween,
A support substrate with a back chamber;
A movable film having a peripheral portion fixed on the support substrate and disposed opposite to the piezoelectric film;
A piezoelectric element, comprising: a transmitting portion that joins one end to the piezoelectric film and joins the other end to the movable film, and transmits a displacement of the movable film to the piezoelectric film. The piezoelectric element according to claim 1, wherein
One end of the transmission section is joined to the center side of the movable film, and the other end is joined to the free end side of the piezoelectric film.

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