Piezoelectric element

Abstract

A piezoelectric element that has the following characteristics: a piezoelectric layer comprising a first surface, a second surface positioned opposite the first surface, and a through-hole extending from the first surface to the second surface; a first electrode layer that is located adjacent to the first surface of the piezoelectric layer; a second electrode layer arranged adjacent to the second surface of the piezoelectric layer, wherein at least one section of the second electrode layer is positioned opposite the first electrode layer, the piezoelectric layer being arranged between them, the second electrode layer facing the through-hole; and a connecting electrode arranged on a connecting surface of the second electrode layer, wherein the connecting surface faces the through hole, wherein the second electrode layer contains silicon as its main component, the connecting electrode is made of a metal and a surface roughness Ra of the connection surface is greater than a surface roughness Ra of a main surface, wherein the main surface is a section other than the connection surface of a surface of the second electrode layer, the surface being adjacent to the piezoelectric layer.

Description

Technical area

[0001] The present invention relates to a piezoelectric element. Background technology

[0002] Formations of piezoelectric elements are disclosed in JP 2018-41788 A and JP 2009-302661 A. The piezoelectric element described in JP 2018-41788 A includes a piezoelectric thin film and a pair of electrodes. The piezoelectric thin film is disposed on a support substrate. The pair of electrodes is positioned so that the piezoelectric thin film is sandwiched between them. The electrode formed on the back of the piezoelectric thin film is connected to a wiring electrode. The electrodes and a wiring metal may be formed from a metallic thin film.

[0003] The piezoelectric element described in JP 2009-302661 A includes a silicon substrate, a piezoelectric film, and a conductor film. The piezoelectric film is made of a piezoelectric material, such as aluminum nitride (AlN), and is disposed on the silicon substrate. The conductor film is made of a conductive material and disposed on the piezoelectric film. The conductor film is made of, for example, a metal or an alloy. The conductor film is positioned on the piezoelectric film and between portions of the piezoelectric film, and is in contact with an n-type region of a silicon layer and the piezoelectric film. Summary of the inventionTechnical problem

[0004] In the piezoelectric element described in JP 2018-41788 A, the wiring electrode and the electrode formed on the back of the piezoelectric thin film are both made of a metal material, thus ensuring adhesion between the wiring electrode and the electrode. However, in cases where one of the electrode layers includes silicon as a main component, such as in the piezoelectric element described in JP 2009-302661 A, adhesion between the electrode layer and the metal electrode is insufficient, thus increasing electrical resistance at a bonding interface between the electrode layer and the metal electrode.

[0005] The present invention has been made in view of the above problem, and an object of the present invention is to provide a piezoelectric element in which an increase in electric resistance at a bonding portion where an electrode layer including silicon as a main component is bonded to a connecting electrode made of a metal can be inhibited. Solution to the problem

[0006] A piezoelectric element according to the present invention comprises a piezoelectric layer, a first electrode layer, a second electrode layer, and a connection electrode. The piezoelectric layer has a first surface, a second surface, and a through-hole. The second surface is positioned opposite the first surface. The through-hole extends from the first surface to the second surface. The first electrode layer is disposed adjacent to the first surface of the piezoelectric layer. The second electrode layer is disposed adjacent to the second surface of the piezoelectric layer. At least a portion of the second electrode layer is positioned opposite the first electrode layer such that the piezoelectric layer is sandwiched therebetween. The second electrode layer faces the through-hole.The second electrode layer includes silicon as its main component. The connecting electrode is disposed on a connecting surface of the second electrode layer, and the connecting surface faces the through-hole. The connecting electrode is made of a metal. A surface roughness Ra of the connecting surface is greater than a surface roughness Ra of a main surface. The main surface is a portion, other than the connecting surface, of a surface of the second electrode layer, and the surface is adjacent to the piezoelectric layer. Advantageous effects of the invention

[0007] According to the present invention, an increase in electrical resistance at a bonding portion where a second electrode layer including silicon as a main component is bonded to a connecting electrode made of a metal can be inhibited. Brief description of the drawings Fig. 1 is a schematic plan view of a piezoelectric element according to a first embodiment of the present invention. Fig. 2 is a cross-sectional view of the piezoelectric element of Fig. 1 along a line II-II when viewed in the direction of the arrow. Fig. 3 is a cross-sectional view illustrating a configuration of a piezoelectric element according to a first modification of the first embodiment of the present invention. Fig. 4 is a cross-sectional view illustrating a configuration of a piezoelectric element according to a second modification of the first embodiment of the present invention. Fig. 5 is a cross-sectional view of a piezoelectric substrate provided in a method of manufacturing the piezoelectric element according to the first embodiment of the present invention. Fig. 6 is a cross-sectional view of a multilayer substrate having a second electrode layer provided in the method for manufacturing the piezoelectric element according to the first embodiment of the present invention. Fig. 7 is a cross-sectional view illustrating a state in which the piezoelectric substrate is bonded to the multilayer substrate having the second electrode layer in the method of manufacturing the piezoelectric element according to the first embodiment of the present invention. Fig. 8 is a cross-sectional view illustrating a state in which a piezoelectric layer is formed by grinding the piezoelectric substrate in the method of manufacturing the piezoelectric element according to the first embodiment of the present invention. Fig. 9 is a cross-sectional view illustrating a state in which a first electrode layer is provided in the method of manufacturing the piezoelectric element according to the first embodiment of the present invention. Fig. 10 is a cross-sectional view illustrating a state in which a through hole is provided in the method of manufacturing the piezoelectric element according to the first embodiment of the present invention. Fig. 11 is a cross-sectional view illustrating a state where a connection electrode is provided in the method of manufacturing the piezoelectric element according to the first embodiment of the present invention. Fig. 12 is a cross-sectional view illustrating a configuration of a piezoelectric element according to a second embodiment of the present invention. Description of the embodiments

[0008] Piezoelectric elements according to embodiments of the present invention will now be described with reference to the drawings. In the following description of the embodiments, like reference numerals are used to refer to the same or similar parts in the drawings, and descriptions thereof will not be repeated. (First embodiment)

[0009] Fig. 1 is a schematic plan view of a piezoelectric element according to a first embodiment of the present invention. Fig. 2 is a cross-sectional view of the piezoelectric element of Fig. 1 along a line II-II when viewed in the direction of the arrow.

[0010] As in the Fig. 1 and Fig. 2, a piezoelectric element 100 according to the first embodiment of the present invention includes a piezoelectric layer 110, a first electrode layer 120, a second electrode layer 130, a connection electrode 140, an external connection electrode 145, and a base portion 150.

[0011] As in Fig. As shown in Figure 2, the piezoelectric layer 110 has a first surface 111, a second surface 112, and a through-hole 113. The second surface 112 is positioned opposite the first surface 111. The through-hole 113 extends from the first surface 111 to the second surface 112.

[0012] In the present embodiment, the piezoelectric layer 110 is formed from an alkaline niobate-based compound or an alkaline tantalate-based compound. In the present embodiment, the alkali metal present in the alkaline niobate-based compound or the alkaline tantalate-based compound includes lithium, rubidium, and / or cesium. Specifically, in the present embodiment, the piezoelectric layer 110 is made of a single-crystal material including lithium niobate (LiNbO3) or lithium tantalate (LiTaO3); however, the piezoelectric layer 110 need not be formed from a single-crystal material.

[0013] The first electrode layer 120 is disposed adjacent to the first surface 111 of the piezoelectric layer 110. An adhesion layer may be disposed between the first electrode layer 120 and the piezoelectric layer 110. The first electrode layer 120 is formed from, for example, a metal such as Al or Pt. The adhesion layer is formed from, for example, Ti or NiCr.

[0014] The second electrode layer 130 is disposed adjacent to the second surface 112 of the piezoelectric layer 110. At least a portion of the second electrode layer 130 is positioned opposite the first electrode layer 120 such that the piezoelectric layer 110 is sandwiched therebetween. In the present embodiment, the second electrode layer 130 is positioned opposite the first electrode layer 120 such that only the piezoelectric layer 110 and a natural oxide film layer (described later) derived from the second electrode layer 130 are sandwiched therebetween.

[0015] The second electrode layer 130 faces the through-hole 113. A connection surface 131 of the second electrode layer 130 faces the through-hole 113. Furthermore, the second electrode layer 130 faces the second surface 112 of the piezoelectric layer 110. A main surface 132 of the second electrode layer 130 faces the piezoelectric layer 110. The connection surface 131 is located further inside the second electrode layer 130 relative to the main surface 132 in a direction perpendicular to the second surface 112. A minimum distance between the connection surface 131 and the main surface 132 in the direction perpendicular to the second surface 112 is greater than or equal to 5 nm. The minimum distance between the connection surface 131 and the main surface 132 in the direction perpendicular to the second surface 112 could, for example, be less than or equal to 100 nm.It should be noted that the minimum distance between the connecting surface 131 and the main surface 132 is a distance in the direction perpendicular to the second surface 112 between a point closest to the main surface 132 in the direction perpendicular to the second surface 112, among all points on the connecting surface 131, and a point closest to the connecting surface 131 in the direction perpendicular to the second surface 112, among all points on the main surface 132. The second electrode layer 130 further has inner surfaces 133 that connect the connecting surface 131 and the main surface 132 to each other in the direction perpendicular to the second surface 112.

[0016] In the present embodiment, the second electrode layer 130 includes silicon as a main component. Specifically, the second electrode layer 130 is a doped single-crystal silicon layer. In the second electrode layer 130, the main surface 132 is covered with a silicon oxide film 135. The silicon oxide film 135 has a film thickness of approximately 2 nm or less. Furthermore, in the present embodiment, the bonding surface 131 is not covered with the silicon oxide film 135. Note that in the present specification, the term "includes silicon as a main component" means "includes silicon in an amount of 50 atm% or more."

[0017] It is noted that the connection surface 131 may be covered with a silicon oxide film, which may be formed when silicon present in the second electrode layer 130 is naturally oxidized on a surface of the second electrode layer 130. Fig. 3 is a cross-sectional view illustrating a configuration of a piezoelectric element according to a first modification of the first embodiment of the present invention. As shown in Fig. As shown in Figure 3, in a piezoelectric element 100a according to the first modification of the first embodiment of the present invention, the bonding surface 131 is covered with a silicon oxide film 135a. Note that in this modification, the silicon oxide film 135a covering the bonding surface 131 is thinner than the silicon oxide film 135 covering the main surface 132. In this modification, the minimum distance between the bonding surface 131 and the main surface 132 in the direction perpendicular to the second surface 112 is less than 5 nm.

[0018] As in Fig. As shown in FIG. 2, in the piezoelectric element 100 according to the first embodiment of the present invention, a surface roughness Ra of the bonding surface 131 is greater than a surface roughness Ra of the main surface 132. The main surface 132 is a portion, other than the bonding surface 131, of a surface of the second electrode layer 130, and the surface is adjacent to the piezoelectric layer 110. Specifically, the surface roughness Ra of the bonding surface 131 is 30 nm or more and less than 1 μm. The surface roughness Ra of the main surface 132 is greater than 0.05 nm and less than 1 nm.

[0019] In the present embodiment, the surface roughness Ra of the joint surface 131 and the surface roughness Ra of the main surface 132 are calculated by directly examining a cross-section of the piezoelectric element 100 taken perpendicular to the main surface 132 with a transmission electron microscope (TEM). In the present description, the surface roughness Ra is a value expressed in micrometers and is determined as follows. From a roughness curve, only a reference length 1 is extracted in the center line direction, and the roughness curve is expressed as y = f(x), where the X-axis is taken in the center line direction of the extracted portion and the Y-axis is taken in the longitudinal magnification direction of the extracted portion. The surface roughness Ra is then calculated by the following equation (1).This means that in this specification, the surface roughness Ra is defined according to JIS B 0601:2013. In this embodiment, the reference length 1 for calculating the surface roughness Ra is 1 µm. Ra=1l∫0l|f(x)|dx

[0020] As in Fig. 2, the connection electrode 140 is disposed on the connection surface 131 of the second electrode layer 130, which is a surface facing the through-hole 113. The connection electrode 140 is also disposed on the inner surfaces 133. The connection electrode 140 is continuously disposed along the connection surface 131, inner surfaces 114 of the through-hole 113, and the first surface 111. Accordingly, an outer edge of the connection electrode 140 is located outside the through-hole 113 when viewed in a direction perpendicular to the first surface 111. Further, a width of a portion of the connection electrode 140 in a direction parallel to the second surface 112, the portion being a portion on the connection surface 131, is greater than the minimum distance between the connection surface 131 and the main surface 132 in the direction perpendicular to the second surface 112.It is noted that the width of the portion of the connection electrode 140 on the connection surface 131 in the direction parallel to the second surface 112 is determined as follows: a cross section of the piezoelectric element 100 is taken perpendicular to the second surface 112 such that a spacing between the inner surfaces 114 positioned opposite to each other in the piezoelectric layer 110 is minimal, and the cross section is directly examined with a transmission electron microscope (TEM) to measure a length of a portion of the connection electrode 140 in contact with the connection surface 131.

[0021] It should be noted that the connecting electrode may be spaced from the inner surfaces 114 of the through-hole 113. Fig. Fig. 4 is a cross-sectional view illustrating a configuration of a piezoelectric element according to a second modification of the first embodiment of the present invention. In this modification, a connection electrode 140b is spaced apart from the inner surfaces 114 of the through-hole 113, as shown in Fig. 4. A width of a portion of the connection electrode 140b in the direction parallel to the second surface 112, the portion being a portion on the connection surface 131, is greater than a maximum distance between the connection electrode 140b and the inner surfaces 114 of the through-hole 113.

[0022] As in Fig. As shown in Figure 2, the connecting electrode 140 is made of a metal. The connecting electrode 140 is formed, for example, from Au. An adhesion layer may be formed between the connecting electrode 140 and the second electrode layer 130. The adhesion layer is formed, for example, from Ti or NiCr. Furthermore, the connecting electrode 140 is in ohmic contact with the connection surface 131 of the second electrode layer 130.

[0023] In the present embodiment, a multi-layer body 101 includes at least the first electrode layer 120, the piezoelectric layer 110 and the second electrode layer 130, as shown in Fig. 2. The multilayer body 101 further includes the connecting electrode 140 and the external connecting electrode 145. The base portion 150 supports the multilayer body 101.

[0024] As in Fig. 2, the base portion 150 is arranged adjacent to the second electrode layer 130 of the multilayer body 101. As shown in Fig. 1, when viewed in a stacking direction of the multi-layer body 101, the base portion 150 has a ring shape along a periphery of a surface of the multi-layer body 101, which is a surface adjacent to the base portion 150.

[0025] In the present embodiment, the base portion 150 comprises a silicon oxide layer 151 and a base portion body 152, as shown in Fig. 2. The silicon oxide layer 151 is in contact with the second electrode layer 130. The base portion body 152 is in contact with the silicon oxide layer 151 on an opposite side of the silicon oxide layer 151 from the second electrode layer 130. In the present embodiment, the base portion body 152 is formed of single-crystal silicon, although the material constituting the base portion body 152 is not particularly limited.

[0026] As in Fig. 2, an opening 103 is located within the base portion 150 when viewed in the stacking direction of the multilayer body 101. As shown in Fig. As shown in Figure 1, the opening 103 has an edge with a rectangular outer shape when viewed in the stacking direction and extends in the stacking direction. Furthermore, the outer shape of the opening 103 is not particularly limited.

[0027] In the present embodiment, the multi-layer body 101 comprises a membrane section 104, as shown in the Fig. 1 and Fig. 2. The membrane portion 104 overlaps the opening 103 and does not overlap the base portion 150 when viewed in the stacking direction. The multilayer body 101 has a through-slot 105 communicating with the opening 103, which is located within the base portion 150 when viewed in the stacking direction. In particular, the through-slot 105 is arranged in the membrane portion 104.

[0028] In the piezoelectric element 100 according to the present embodiment, applying a voltage between the external connection electrode 145 and the connection electrode 140 as shown in Fig. 2 that a voltage is applied between the first electrode layer 120 and the second electrode layer 130, as in Fig. 2. Accordingly, the piezoelectric layer 110, located between the first electrode layer 120 and the second electrode layer 130, is actuated to deform. Accordingly, the membrane portion 104 can flexurally vibrate significantly in the stacking direction of the multilayer body 101.

[0029] A method for manufacturing the piezoelectric element according to the first embodiment of the present invention will now be described. It should be noted that the conditions shown in the Fig. 5 to 11, which are described below, are in cross-sectional views, as in Fig. 2.

[0030] Fig. 5 is a cross-sectional view of a piezoelectric substrate provided in the method for manufacturing the piezoelectric element according to the first embodiment of the present invention. As shown in Fig. 5, a piezoelectric substrate 110S has a first main surface 111S and a second main surface 112S opposite to the first main surface 111S.

[0031] Fig. 6 is a cross-sectional view of a multilayer substrate having a second electrode layer provided in the method for manufacturing the piezoelectric element according to the first embodiment of the present invention. As shown in Fig. As shown in Figure 6, a multilayer substrate 102S is provided, which includes a second electrode layer 130 and a base portion 150. In the present embodiment, the multilayer substrate 102S is a silicon-on-insulator (SOI) substrate.

[0032] Fig. Fig. 7 is a cross-sectional view illustrating a state in which the piezoelectric substrate is bonded to the multilayer substrate having the second electrode layer in the method for manufacturing the piezoelectric element according to the first embodiment of the present invention. As shown in Fig. As shown in Figure 7, the piezoelectric substrate 110S is bonded to the multilayer substrate 102S using surface-activated bonding or atomic diffusion bonding. Specifically, the piezoelectric substrate 110S is bonded to the second electrode layer 130. Specifically, the piezoelectric substrate 110S is bonded to a silicon oxide film 135 on the second electrode layer 130.

[0033] Fig. Fig. 8 is a cross-sectional view illustrating a state in which a piezoelectric layer is formed by grinding the piezoelectric substrate in the method for manufacturing the piezoelectric element according to the first embodiment of the present invention. As shown in Fig. As shown in Fig. 8, a piezoelectric layer 110 is formed by performing grinding and polishing using chemical mechanical polishing (CMP) on the piezoelectric substrate 110S.

[0034] Note that when the piezoelectric substrate 110S is to be ground, a release layer may be formed on the first main surface 111S side of the piezoelectric substrate 110S by previously implanting ions therein. In the case where the release layer is formed before the piezoelectric substrate 110S is bonded to the second electrode layer 130, the piezoelectric layer 110 may be formed by removing the release layer after bonding. The piezoelectric layer 111 may be formed by grinding the release layer, for example, by performing grinding and CMP.

[0035] Fig. 9 is a cross-sectional view illustrating a state in which a first electrode layer is provided in the method for manufacturing the piezoelectric element according to the first embodiment of the present invention. As shown in Fig. 9, a first electrode layer 120 is formed on a first surface 111 of the piezoelectric layer 111 by using a photolithography method, an application and lift-off method, or the like, so that a desired pattern is obtained.

[0036] Fig. 10 is a cross-sectional view illustrating a state in which a through-hole is provided in the method for manufacturing the piezoelectric element according to the first embodiment of the present invention. As shown in Fig. As shown in Figure 10, a through-hole 113 is provided in the piezoelectric layer 110, for example, by using etching such as reactive ion etching (RIE). In this case, a portion of a region of the second electrode layer 130 facing the through-hole 113 is etched away while forming the through-hole 113. Accordingly, a bonding surface 131 is formed in an exposed manner so that the entire silicon oxide film 135 of the second electrode layer 130 has been removed therefrom. The etching should be performed in such a manner that the bonding surface 131 has a desired surface roughness Ra. Note that in the first modification of the first embodiment of the present invention, it is sufficient that a portion of the silicon oxide film 135 of the second electrode layer 130 is removed.

[0037] Fig. 11 is a cross-sectional view illustrating a state in which a connection electrode is provided in the method for manufacturing the piezoelectric element according to the first embodiment of the present invention. As shown in Fig. 11, a connection electrode 140 and an external connection electrode 145 are formed by using a photolithography method, an application and lift-off method, or the like.

[0038] Next, a through-slot 105 is formed in a multilayer body 101 using RIE. Furthermore, a base portion body 152 is etched from an opposite side of the base portion 150 from the second electrode layer 130 using deep reactive ion etching (deep RIE). Finally, a silicon oxide layer 151 is etched using RIE to form an opening 103.

[0039] With the steps described above, the piezoelectric element 100 according to the first embodiment of the present invention is manufactured as shown in the Fig. 1 and Fig. 2 shown, manufactured.

[0040] As described above, in the piezoelectric element 100 according to the first embodiment of the present invention, the piezoelectric layer 110 has the first surface 111, the second surface 112, and the through-hole 113. The through-hole 113 extends from the first surface 111 to the second surface 112. The second electrode layer 130 is located adjacent to the second surface 112 of the piezoelectric layer 110. The second electrode layer 130 faces the through-hole 113. The second electrode layer 130 contains silicon as a main component. The connection electrode 140 is disposed on the connection surface 131 of the second electrode layer 130, which is a surface facing the through-hole 113. The connection electrode 140 is made of a metal. The surface roughness Ra of the connection surface 131 is greater than the surface roughness Ra of the main surface 132.The main surface 132 is a portion, other than the connection surface 131, of a surface of the second electrode layer 130, and the surface is adjacent to the piezoelectric layer 110.

[0041] As a result, adhesion between the second electrode layer 130, which contains silicon as a main component, and the connecting electrode 140 made of a metal is improved at the connecting surface 131. Thus, an increase in electrical resistance at the bonding portion where the second electrode layer 130 is bonded to the connecting electrode 140 can be inhibited.

[0042] In the present embodiment, the connecting surface 131 is arranged further inside the second electrode layer 130 relative to the main surface 132 in the direction perpendicular to the second surface 112.

[0043] Since the surface of the second electrode layer 130 adjacent to the piezoelectric layer 110 is processed to form the bonding surface 131 as described above, a natural oxide film layer originating from the second electrode layer 130 is removed at the bonding surface 131. Consequently, contact resistance between the second electrode layer 130 and the bonding electrode 140 at the bonding surface 131 can be reduced.

[0044] In the piezoelectric element 100 according to the first embodiment of the present invention and the piezoelectric element 100a according to the first modification of the first embodiment of the present invention, the main surface 132 is covered with the silicon oxide film 135. The bonding surface 131 is not covered with a silicon oxide film or is covered with a silicon oxide film 135a. The silicon oxide film 135a covering the bonding surface 131 is thinner than the silicon oxide film 135 covering the main surface 132.

[0045] Consequently, the contact resistance between the second electrode layer 130 and the connection electrode 140 can be reduced.

[0046] In the piezoelectric element 100 according to the first embodiment of the present invention, the minimum distance between the connecting surface 131 and the main surface 132 in the direction perpendicular to the second surface 112 is greater than or equal to 5 nm.

[0047] Since the main surface 132 of the second electrode layer 130 is processed such that the connection surface 131 and the main surface 132 are sufficiently spaced apart from each other, as described above, the natural oxide film layer resulting from the second electrode layer 130 is sufficiently removed at the connection surface 131. Consequently, the contact resistance between the second electrode layer 130 and the connection electrode 140 at the connection surface 131 can be sufficiently reduced.

[0048] In the present embodiment, the surface roughness Ra of the bonding surface 131 is 30 nm or more and less than 1 µm. The surface roughness Ra of the main surface 132 is less than 1 nm.

[0049] When the surface roughness Ra of the bonding surface 131 is 20 nm or more and less than 1 μm, adhesion between the second electrode layer 130 and the bonding electrode 140 is further improved, which reduces the contact resistance between the second electrode layer 130 and the bonding electrode 140. Furthermore, when the surface roughness Ra of the main surface 132 is less than 1 nm, bonding strength between the piezoelectric layer 110 and the second electrode layer 130 is improved.

[0050] In the present embodiment, the surface roughness Ra of the main surface 132 is greater than 0.05 nm. Thus, it is easy to adjust the surface roughness Ra of the main surface so that the bonding strength between the piezoelectric layer 110 and the second electrode layer 130 can be improved.

[0051] In the present embodiment, the width of a portion of the connection electrode 140 in the direction parallel to the second surface 112, which portion is a portion on the connection surface 131, is greater than the minimum distance between the connection surface 131 and the main surface 132 in the direction perpendicular to the second surface 112.

[0052] Accordingly, on the connection surface 131 having a surface roughness Ra as described above, a contact area between the connection electrode 140 and the second electrode layer 130 can be increased, which in turn reduces the contact resistance between the connection electrode 140 and the second electrode layer 130.

[0053] In the present embodiment, the second electrode layer 130 further includes the inner surfaces 133 that connect the connecting surface 131 and the main surface 132 to each other in the direction perpendicular to the second surface 112. The connecting electrode 140 is also arranged on the inner surfaces 133.

[0054] Accordingly, the connecting electrode 140 and the second electrode layer 130 are bonded to each other at the inner surfaces 133 and thus adhesion between the second electrode layer 130 and the connecting electrode 140 is further improved.

[0055] In the present embodiment, the connecting electrode 140 is in ohmic contact with the connecting surface 131 of the second electrode layer 130.

[0056] Accordingly, there is no Schottky barrier between the connecting electrode 140 and the second electrode layer 130, and therefore the contact resistance between the connecting electrode 140 and the second electrode layer 130 is reduced, which improves the electromechanical conversion efficiency of the piezoelectric element 100.

[0057] In the present embodiment, the connection electrode 140 is arranged continuously along the connection surface 131, the inner surfaces 114 of the through hole 113 and the first surface 111.

[0058] Accordingly, an electrode extending from the second electrode layer 130 may be provided together with the first electrode layer 120 on the first surface 111. Thus, an external electrode (not shown) to be connected to the connecting electrode 140 can be efficiently routed.

[0059] In the present embodiment, the second electrode layer 130 is a doped single-crystal silicon layer. When the second electrode layer 130 is formed from a single crystal, the bonding strength to the piezoelectric layer 110 is improved, which improves the electromechanical conversion efficiency of the piezoelectric element 100. Furthermore, the electrical resistance of the second electrode layer 130 can be reduced compared to cases where the second electrode layer 130 is formed from an undoped single-crystal silicon layer.

[0060] In the present embodiment, the piezoelectric layer 110 is made of a single-crystal material including lithium niobate (LiNbO3) or lithium tantalate (LiTaO3). Consequently, the bonding strength between the piezoelectric layer 110 and the second electrode layer 130 is improved, which improves the electromechanical conversion efficiency of the piezoelectric element 100.

[0061] In the present embodiment, the piezoelectric element 100 further includes the base portion 150 supporting the multilayer body 101, which includes at least the first electrode layer 120, the piezoelectric layer 110, and the second electrode layer 130. The base portion 150 is adjacent to the second electrode layer 130 of the multilayer body 101, and when viewed in a stacking direction of the multilayer body 101, the base portion 150 has a ring shape along a periphery of a surface of the multilayer body 101, which is a surface adjacent to the base portion 150.

[0062] Accordingly, the actuation of the piezoelectric layer 110 can be converted into a bending vibration of the diaphragm portion 104, and consequently, the device characteristics of the piezoelectric element 100 can be improved.

[0063] In the present embodiment, the multi-layer body 101 has the through-slot 105 communicating with the opening 103 disposed within the base portion 150 when viewed in the stacking direction.

[0064] Consequently, the bending vibration of the diaphragm portion 104 is further increased and hence the device characteristics of the piezoelectric element 100 can be improved.

[0065] Furthermore, in the piezoelectric element 100b according to the second modification of the first embodiment of the present invention, the width of a portion of the connection electrode 140b in the direction parallel to the second surface 112, which portion is a portion on the connection surface 131, is larger than the maximum distance between the connection electrode 140b and the inner surfaces 140 of the through-hole 113.

[0066] Accordingly, on the connection surface 131, the contact area between the connection electrode 140 and the second electrode layer 130 is increased, which reduces the contact resistance between the connection electrode 140 and the second electrode layer 130. (Second embodiment)

[0067] A piezoelectric element according to a second embodiment of the present invention will now be described. The piezoelectric element according to the second embodiment of the present invention differs from the piezoelectric element according to the first embodiment of the present invention mainly in that the piezoelectric element according to the second embodiment includes an intermediate layer. Accordingly, descriptions of features similar to those of the piezoelectric element 100 according to the first embodiment of the present invention will not be repeated.

[0068] Fig. 12 is a cross-sectional view illustrating a configuration of the piezoelectric element according to the second embodiment of the present invention. Fig. 12 the representation is shown in a cross-sectional view as in Fig. 2.

[0069] In a piezoelectric element 200 according to the second embodiment of the present invention, an intermediate layer 260 is disposed between a second electrode layer 130 and a piezoelectric layer 110. Consequently, the bonding strength between the second electrode layer 130 and the piezoelectric layer 110 is improved. The intermediate layer 260 may be a metal layer or a dielectric layer including, for example, SiO2. In the present embodiment, a through-hole is disposed in the intermediate layer 260, and the through-hole is continuous with the through-hole 113 disposed in the piezoelectric layer 110.

[0070] In each of the above-described embodiments, a state of contact between the connecting electrode and the second electrode layer can be evaluated according to a behavior that the membrane portion exhibits when a voltage is applied between the upper electrode layer and the lower electrode layer.

[0071] In the descriptions of the above embodiments, features that can be combined could be combined with each other. List of reference symbols

[0072] 100, 100a, 100b, 200 piezoelectric element, 101 multilayer body, 102S multilayer substrate, 103 opening, 104 diaphragm portion, 105 through-slot, 110 piezoelectric layer, 110S piezoelectric substrate, 111 first surface, 111S first main surface, 112 second surface, 112S second main surface, 113 through-hole, 114 inner surface, 120 first electrode layer, 130 second electrode layer, 131 connecting surface, 132 main surface, 133 inner surface, 135, 135a silicon oxide film, 140, 140b connecting electrode, 145 external connecting electrode, 150 base portion, 151 silicon oxide layer, 152 base portion body, 260 intermediate layer

Claims

[1] A piezoelectric element having the following characteristics: a piezoelectric layer having a first surface, a second surface positioned opposite the first surface, and a through-hole extending from the first surface to the second surface; a first electrode layer disposed adjacent to the first surface of the piezoelectric layer; a second electrode layer disposed adjacent to the second surface of the piezoelectric layer, wherein at least a portion of the second electrode layer is positioned opposite the first electrode layer, the piezoelectric layer being disposed therebetween, the second electrode layer facing the through-hole; and a connection electrode arranged on a connection surface of the second electrode layer, the connection surface facing the through-hole, wherein the second electrode layer contains silicon as its main component, the connecting electrode is made of a metal and a surface roughness Ra of the bonding surface is greater than a surface roughness Ra of a main surface, the main surface being a portion, other than the bonding surface, of a surface of the second electrode layer, the surface being adjacent to the piezoelectric layer. [2] The piezoelectric element according to claim 1, wherein the connection surface is located relative to the main surface in the second electrode layer in a direction perpendicular to the second surface inside. [3] The piezoelectric element according to claim 2, wherein the main surface is covered with a silicon oxide film, the bonding surface is not covered with a silicon oxide film or is covered with a silicon oxide film and the silicon oxide film covering the bonding surface is thinner than the silicon oxide film covering the main surface. [4] The piezoelectric element according to claim 2 or 3, wherein a minimum distance between the bonding surface and the main surface in the direction perpendicular to the second surface is greater than or equal to 5 nm. [5] The piezoelectric element according to any one of claims 1 to 4, wherein the surface roughness Ra of the joint surface is 30 nm or more and less than 1 µm and the surface roughness Ra of the main surface is less than 1 nm. [6] The piezoelectric element according to claim 5, wherein the surface roughness Ra of the main surface is greater than 0.05 nm. [7] The piezoelectric element according to claim 4, wherein a width of a portion of the connection electrode in a direction parallel to the second surface, the portion being a portion on the connection surface, is larger than the minimum distance between the connection surface and the main surface in the direction perpendicular to the second surface. [8] The piezoelectric element according to any one of claims 2 to 7, wherein a width of a portion of the connection electrode in a direction parallel to the second surface, the portion being a portion on the connection surface, is larger than a maximum distance between the connection electrode and an inner surface of the through-hole. [9] The piezoelectric element according to any one of claims 2 to 4, wherein the second electrode layer further has an inner surface connecting the connecting surface and the main surface to each other in the direction perpendicular to the second surface, and the connecting electrode is also arranged on the inner surface. [10] The piezoelectric element according to any one of claims 1 to 9, wherein the connecting electrode is in ohmic contact with the connecting surface of the second electrode layer. [11] The piezoelectric element according to any one of claims 1 to 8, wherein the connection electrode is continuously arranged along the connection surface, the inner surface of the through-hole and the first surface. [12] The piezoelectric element according to any one of claims 1 to 11, wherein the second electrode layer is a doped single crystal silicon layer. [13] The piezoelectric element according to any one of claims 1 to 12, wherein the piezoelectric layer is made of a single crystal material comprising lithium niobate (LiNbO3) or lithium tantalate (LiTaO3). [14] The piezoelectric element according to any one of claims 1 to 13, wherein an intermediate layer is disposed between the second electrode layer and the piezoelectric layer. [15] The piezoelectric element according to any one of claims 1 to 14, further comprising a base portion supporting a multi-layer body including at least the first electrode layer, the piezoelectric layer, and the second electrode layer, wherein the base portion is disposed adjacent to the second electrode layer of the multi-layer body, and when viewed in a stacking direction of the multi-layer body, the base portion has a ring shape along a periphery of a surface of the multi-layer body, the surface being a surface adjacent to the base portion. [16] The piezoelectric element according to claim 15, wherein the multi-layer body has a through-slit communicating with an opening located inside the base portion when viewed in the stacking direction.

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