JP2018170697A - Piezoelectric element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a piezoelectric element which suppresses influence of residual stress of a piezoelectric film, solves the problem of restricting an outer shape, and has high sensitivity and improved signal to noise ratio.SOLUTION: Slits 7 are formed in a diaphragm whose peripheral edge portion is supported and fixed to a supporting substrate. The slits 7 extend from the peripheral portion toward a center side of the diaphragm, do not cross each other, and extension lines do not converge to one point. Further, a slit extending along the peripheral edge portion may be added to the slits 7. Furthermore, a slit extending along the peripheral edge direction may be added to a slit extending along the peripheral edge portion.SELECTED DRAWING: Figure 1

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 is rapidly expanding are often used with microphones using a micro electro mechanical system (MEMS) technology that is small and thin and has 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 piezoelectric material.

  FIG. 7 shows a cross-sectional view of a conventional piezoelectric element. As shown in FIG. 7, piezoelectric films 3a and 3b having a multilayer structure are supported and fixed on a silicon substrate 1 serving as a support substrate via an insulating film 2. The piezoelectric film 3a is formed by an electrode 4a and an electrode 4b from above and below. 3b has a structure sandwiched between the electrode 4b and the electrode 4c. A part of the silicon substrate is formed with, for example, a circular hole 6 that is removed in a circular shape, and the peripheral portions of the piezoelectric films 3 a and 3 b are supported and fixed to the remaining silicon substrate 1. The piezoelectric films 3a and 3b on the holes 6 serve as diaphragms. The electrodes 4a to 4c arranged with the piezoelectric films 3a and 3b constituting the diaphragm sandwiched therebetween are arranged in a donut shape.

  In such a piezoelectric element, when the piezoelectric film 3a is distorted by receiving acoustic pressure or the like, polarization occurs inside due to the transverse piezoelectric effect, and the wiring electrode 5a connected to the electrode 4a and the wiring electrode 5b connected to the electrode 4b The voltage signal can be extracted. Similarly, when the piezoelectric film 3b is distorted, polarization occurs in the piezoelectric film 3b, and a voltage signal can be extracted from the wiring electrode 5a connected to the electrode 4c and the wiring electrode 5b connected to the electrode 4b.

  However, in the structure in which the peripheral portion is supported and fixed to the silicon substrate 1, the piezoelectric film constituting the diaphragm is effectively hardened due to the effect of the residual effect of the piezoelectric films 3a and 3b constituting the diaphragm, and the resonance frequency is increased. Therefore, there has been a problem that the conversion rate (sensitivity) for converting an acoustic pressure signal or the like into an electric signal has not reached a satisfactory level. Further, it is known that the residual stress and temperature fluctuation of the piezoelectric film are output as unnecessary signals and the characteristics are deteriorated.

  Therefore, in order to effectively utilize the energy generated in the piezoelectric film and release the residual stress, a piezoelectric element as shown in FIG. 8 has been proposed (Patent Document 1). In the piezoelectric element shown in FIG. 8, a piezoelectric film 3a, 3b having a multilayer structure is supported and fixed on a silicon substrate 1 having a square hole 6 via an insulating film 2, and the piezoelectric film 3a is connected to an electrode 4a and an electrode from above and below. Due to 4b, the piezoelectric film 3b is sandwiched between the electrodes 4b and 4c from above and below. The piezoelectric film has a rectangular planar structure by forming slits, 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 an acoustic pressure or the like, polarization occurs in 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 in the piezoelectric film 3b, and a piezoelectric signal can be extracted from the wiring electrode 5a connected to the electrode 4c and the wiring electrode 5b connected to the electrode 4b.

  By the way, although the residual stress of the piezoelectric film is released by adopting the cantilever structure, as a result, the piezoelectric thin film warps, and the gap between the adjacent piezoelectric films (gap G) and the substantial gap between the side surfaces of the piezoelectric film and the support substrate are reduced. The dimensions will spread. 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 hole is formed in a support substrate and a gap G as shown in FIG. In the piezoelectric element having such a structure, the adjacent piezoelectric films 3 are arranged even when the piezoelectric films 3 are warped by residual stress by arranging each of the four triangles at the center of the diaphragm. Has also been disclosed in which a similar warp occurs, and as a result, the size of the gap between adjacent piezoelectric films 3 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 diaphragm is a triangle, and the dimensions of the gap of the piezoelectric film are arranged by collecting the apexes of four triangles at the center. It was made possible not to change greatly. However, in order to match the resonance frequencies of the triangular diaphragms, 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, there is a variation in residual stress even within one piezoelectric element in the manufacturing process, the degree of warping of the four triangles is different, and the width of the gap G is wider than the design value. It was very difficult to control the width of the gap G. Accordingly, an object of the present invention is to provide a piezoelectric element that suppresses the influence of the residual stress of the piezoelectric film and solves the problem that the outer shape is limited, and has a high sensitivity and an improved signal-to-noise ratio.

  In order to achieve the above object, an invention according to claim 1 of the present application provides a piezoelectric film comprising a piezoelectric film having a peripheral portion supported and fixed on a support substrate and constituting a vibration plate, and a pair of electrodes arranged with the piezoelectric film interposed therebetween. The element includes a slit that penetrates the piezoelectric film or the piezoelectric film and the electrode, and the slit includes a first slit group including a plurality of slits, and the slit of the first slit group includes the peripheral portion. And extending toward the center of the diaphragm and intersecting each other, the extension lines do not converge to one point, and a part of the piezoelectric element constitutes a connecting portion.

  The invention according to claim 2 of the present application is the piezoelectric element according to claim 1, wherein the slit includes the first slit group and a second slit group including a plurality of slits. The slit extends in a direction along the peripheral edge portion, and one of the slits of the second slit group is connected to one end of the slit of the first slit group.

  The invention according to claim 3 of the present application is the piezoelectric element according to claim 2, wherein the slit includes the first slit group, the second slit group, and a third slit group including a plurality of slits. The slits of the third slit group extend in the peripheral direction, and one of the slits of the third slit group is connected to one of the slits of the first slit group at one end. The other end and one end of the slit of the slit group are connected, and the other end extends in the peripheral portion direction.

  The invention according to claim 4 of the present application is the piezoelectric element according to any one of claims 1 to 3, wherein the slits of the first slit group are directed from the edge of the slits toward the center of the diaphragm. It arrange | positions so that it may become the extended line inclined in the same direction from the line.

  The invention according to claim 5 of the present application is characterized in that in the piezoelectric element according to any one of claims 1 to 4, the diaphragm including the piezoelectric film is a film that vibrates due to an acoustic pressure.

  In the piezoelectric element of the present invention, the piezoelectric film constituting the diaphragm is supported and fixed to the support substrate at the peripheral portion, and the residual stress can be released by providing a slit having a predetermined shape. Furthermore, by providing a connecting portion in which a part of diaphragms having a conventional cantilever structure are connected to each other, warping due to residual stress can be effectively suppressed. As a result, the interval between the slits formed in the piezoelectric film (the width of the gap G) can be controlled within a predetermined range.

  In particular, the slits of the piezoelectric element of the present invention extend from the peripheral portion toward the center of the diaphragm, and are arranged so that the extension lines do not converge at one point without crossing each other. By arranging it to be an inclined extension line, when compressive stress or tensile stress is applied to the diaphragm, the residual stress is made into a rotational moment that rotates the central part of the diaphragm, so that the width of the gap G can be reduced. It becomes possible to greatly suppress deformation such as spreading.

  Therefore, when the piezoelectric element of the present invention is used as an acoustic transducer, the adverse effect due to residual stress can be alleviated, a high signal-to-noise ratio can be realized with high sensitivity, and the gap G becomes as designed, and the decrease in acoustic resistance is suppressed and the low frequency roll. The off frequency becomes high, and it is possible to effectively suppress the sensitivity decrease in the low frequency region.

  Further, the piezoelectric element of the present invention has an advantage that the shape of the diaphragm can be freely designed because the diaphragm is not separated by the slit.

It is explanatory drawing of the 1st Example of this invention. It is explanatory drawing of the 1st Example of this invention. It is the figure which demonstrated notionally the deformation | transformation of a diaphragm. It is explanatory drawing of the 2nd Example of this invention. It is explanatory drawing of the 3rd Example of this invention. It is explanatory drawing of the 4th Example of this invention. It is explanatory drawing of the conventional piezoelectric element. It is explanatory drawing of another conventional piezoelectric element. It is explanatory drawing of another conventional piezoelectric element.

  In the piezoelectric element of the present invention, a plurality of slits having a desired shape are formed in a piezoelectric film constituting a diaphragm whose peripheral portion is supported and fixed to a support substrate so as to partially divide the piezoelectric film. Hereinafter, the piezoelectric element of the present invention will be described in detail.

  1 and 2 are explanatory views of a first embodiment of the present invention. FIG. 1 is a plan view of the piezoelectric element, and FIG. 2 is a sectional view of a central portion of the piezoelectric element of FIG. As shown in FIG. 2, in the piezoelectric element of this embodiment, as in the conventional piezoelectric element, piezoelectric films 3a and 3b having a multilayer structure are supported and fixed on a silicon substrate 1 serving as a supporting substrate via an insulating film 2. Is sandwiched between the electrodes 4a, 4b, 4c from above and below. The piezoelectric element shown in FIG. 1 has a hole 6 removed in a square shape in a part of a silicon substrate 1, and the peripheral portions of the piezoelectric films 3 a and 3 b are supported and fixed to the remaining silicon substrate 1. . In FIG. 1 and FIG. 2, description of the wiring electrodes 5a and 5b is omitted.

  Here, the present invention has a great feature in the shape of the slit 7. That is, the slit 7 of the present invention extends from the peripheral portion toward the center of the diaphragm, and does not cross each other, and the extension line does not converge to one point. In the example shown in FIG. 1, four slits 7 extend from the apexes of the square diaphragm toward the center of the diaphragm. Further, the slits 7 do not cross each other. Further, each extension line (indicated by a broken line in FIG. 1) has a shape that does not converge to one point. Specifically, each slit 7 is disposed at a position where the slit 7 extending toward the center of the diaphragm is inclined leftward, and has a length that does not cross each other. The slit shown in FIG. 1 corresponds to a first slit group.

  In this way, the slits 7 do not cross each other, and the extension line does not converge to one point, so that the diaphragm is not separated and independent, and a part of the diaphragm of the conventional cantilever structure is mutually connected. It becomes a monolithic structure that is continuous. A region surrounded by a circle in FIG. 1 is a connecting portion 8, and the diaphragm is integrated.

  In the piezoelectric element having such a structure, the residual stress of the piezoelectric films 3 a and 3 b is released by forming the slit 7. As a result, the piezoelectric films 3 a and 3 b are deformed, but the deformation is suppressed by the connecting portion 8. Further, by extending the shape of the slit 7 in a direction inclined from the center of the piezoelectric film, the stress to be deformed is a rotational moment that rotates the central portion of the diaphragm (the portion that becomes the connecting portion 8 in FIG. 1). Therefore, deformation that widens the slit gap is suppressed. As a result, it is possible to form a piezoelectric element that maintains high signal-noise ratio with high sensitivity and suppresses characteristic deterioration due to the formation of the slit 7.

  FIG. 3 is a diagram conceptually illustrating the deformation of the diaphragm. In FIG. 3A, reference numerals 10a to 10d denote fixing portions which are supported and fixed to a silicon substrate which is a supporting substrate, and show a region of the piezoelectric film between two slits 7 of the piezoelectric element having the structure shown in FIG. Yes. Reference numeral 11 corresponds to the connecting portion 8. The extending direction of the slit 7 is inclined without facing the center of the piezoelectric film as shown in FIG.

  With this configuration, the four fixing portions 10a to 10d function as springs, transmit residual stress to the central connecting portion 11, and release or relieve the residual stress by rotationally displacing the connecting portion 11. For example, when compressive stress remains in the piezoelectric film, the force applied from the fixing portions 10a to 10d in the direction indicated by the arrow in FIG. 3B rotates and displaces the connecting portion 11 counterclockwise. At this time, when the force applied to each slit 7 is viewed, the forces applied to the opposite sides of the connecting portion 11 are opposite to each other. Therefore, even if one force acts in the direction of expanding the slit, the other force is This acts in the direction of narrowing the slit, and as a result, the slit width is not significantly changed.

  Similarly, when the tensile effect remains in the piezoelectric film, the force applied from the fixing portions 10a to 10d in the direction indicated by the arrow in FIG. 3C causes the connecting portion 11 to rotate and displace clockwise. At this time, when the force applied to each slit 7 is viewed, the forces applied to the opposing sides of the connecting portion 11 are opposite to each other. Therefore, even if one force acts in the direction of narrowing the slit, the other force is applied. Will act in the direction of expanding the slit, and as a result, will not change the slit width significantly.

  Further, since the diaphragm has an integral structure unlike the conventional cantilever structure, the slit width is not significantly changed because the displacement in the direction perpendicular to the drawing is also suppressed.

  In addition, the strength of the springs of the fixing portions 10a to 10d can be set as appropriate depending on the length and width thereof. The assumed residual stress, the planar dimensions of the diaphragm, the thickness of the film constituting the diaphragm, and the elasticity It may be set appropriately in consideration of the rate and the like. Therefore, changing the slit shape as described below is also an effective method. Hereinafter, examples in which the shape and the like of the slit are changed will be described in detail.

  FIG. 4 is an explanatory diagram of the second embodiment of the present invention. The slit of this embodiment has a shape in which the slit 7b is connected to the tip of the slit 7a corresponding to the slit 7 described in the first embodiment. As shown in FIG. 4, the slit 7b extends in a direction along the peripheral edge. The slit 7a shown in FIG. 4 corresponds to the first slit group, and the slit 7b corresponds to the second slit group.

  With such a shape, the slit length becomes longer than that of the slit structure described in the first embodiment shown in FIG. 1, and the effect of releasing the residual stress of the piezoelectric films 3a and 3b is increased. Further, the piezoelectric film remaining between the slit 7b and the adjacent slit 7b corresponds to the fixed portion described in FIG. 3, and compressive stress or tensile stress is concentrated on the fixed portion, and the center of the piezoelectric film corresponding to the connecting portion is formed. A large rotational moment is applied to the part.

  FIG. 5 is an explanatory diagram of the third embodiment of the present invention. The slit of this embodiment has a shape in which the slit 7c is connected to the tip of the slit 7b described in the second embodiment. As shown in FIG. 5, the slit 7c is parallel to the slit 7b and extends in the peripheral direction. The slit 7a shown in FIG. 5 corresponds to the first slit group, the slit 7b corresponds to the second slit group, and the slit 7c corresponds to the third slit group.

  With such a shape, the slit length is further increased and the effect of releasing the residual stress of the piezoelectric films 3a and 3b is further increased as compared with the slit structure described in the second embodiment shown in FIG. . Further, the piezoelectric film remaining between the slit 7c and the adjacent slit 7b corresponds to the fixed portion described in FIG. 3, and compressive stress or tensile stress is further concentrated on the fixed portion, and the piezoelectric film corresponding to the connecting portion. A large rotational moment is applied to the central portion of the film.

  FIG. 6 is an explanatory diagram of the fourth embodiment of the present invention. This embodiment shown in FIG. 6 shows an example in which the shape of the holes formed in the silicon substrate serving as the support substrate is circular and the number of slits is increased to six as compared with the third embodiment. Yes. Further, the slit 7c is parallel to the slit 7a and extends in the peripheral portion direction. The slit 7a shown in FIG. 6 corresponds to the first slit group, the slit 7b corresponds to the second slit group, and the slit 7c corresponds to the third slit group.

  With such a shape, the slit length is further increased as compared with the slit structure described in the third embodiment shown in FIG. 5, and the effect of releasing the residual stress of the piezoelectric films 3a and 3b is increased. Further, the piezoelectric film remaining between the slit 7c and the adjacent slit 7a corresponds to the fixed portion described in FIG. 3, and compressive stress or tensile stress is concentrated on the fixed portion. A large rotational moment is applied to the central portion. In particular, in this embodiment, a rotational moment along the rotational direction is added to the connecting portion, and the effect is great.

  When the piezoelectric element of the present invention is configured as an acoustic transducer, acoustic pressure is applied from the cavity 6 formed in the silicon substrate 1. The piezoelectric films 3a and 3b constituting the diaphragm that has received the acoustic pressure are curvedly displaced upward. As a result, the piezoelectric film 3a is polarized inside due to the transverse piezoelectric effect, and voltage signals can be taken out from the electrodes 4a and 4b connected to wiring electrodes (not shown). Similarly, in the piezoelectric film 3b, it is possible to take out a voltage signal from the electrodes 4c and 4b connected to wiring electrodes (not shown).

  In addition, the residual stress is eased by providing the slit 7, the slit width becomes almost the design value, the signal-to-noise ratio is maintained with high sensitivity, and the slit width becomes as designed, and the acoustic resistance is lowered and the low frequency sensitivity is lowered. Thus, it is possible to provide an acoustic transducer that does not cause deterioration of characteristics such as the above.

  The embodiments of the present invention have been described above. Needless to say, the present invention is not limited to the above embodiments. Specifically, the shape of the cavity, the number of slits, the extending direction, and the like can be changed as appropriate. 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 portion corresponding to the fixed portion, various changes can be made such as reducing the thickness of the piezoelectric film or forming a through hole.

1: silicon substrate, 2: insulating film, 3, 3a, 3b: piezoelectric film, 4, 4a, 4b: electrode, 5a, 5b: wiring electrode, 6: hole, 7: slit, 8: connecting portion,
10a to 10d: fixed part, 11: connecting part

Claims (5)

In a piezoelectric element comprising a piezoelectric film having a peripheral portion supported and fixed on a support substrate and constituting a diaphragm, and a pair of electrodes arranged with the piezoelectric film interposed therebetween,
Comprising a slit penetrating the piezoelectric film or the piezoelectric film and the electrode;
The slit comprises a first slit group comprising a plurality of slits,
The slits of the first slit group extend from the peripheral edge toward the center of the diaphragm, do not cross each other, and the extension line does not converge to one point;
A part of said piezoelectric element comprises a connection part, The piezoelectric element characterized by the above-mentioned. The piezoelectric element according to claim 1, wherein
The slit comprises the first slit group and a second slit group comprising a plurality of slits,
The slits of the second slit group extend in the direction along the peripheral edge, and one of the slits of the second slit group is connected to one end of the slit of the first slit group. A piezoelectric element characterized by that. The piezoelectric element according to claim 2, wherein
The slit comprises the first slit group, the second slit group, and a third slit group comprising a plurality of slits,
The slits of the third slit group extend in the peripheral direction, and one of the slits of the third slit group is connected to one of the slits of the first slit group at one end. One end of the slit of the slit group is connected to one end, and the other end extends in the peripheral portion direction. The piezoelectric element according to any one of claims 1 to 3,
The slits of the first slit group are arranged so as to be extended lines inclined in the same direction from a line extending from an end on the peripheral edge side of the slit toward the center of the diaphragm. Piezoelectric element. The piezoelectric element according to any one of claims 1 to 4,
The piezoelectric element including the piezoelectric film is a film that vibrates by an acoustic pressure.

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