Piezoelectric device

Abstract

To provide a piezoelectric device having a structure in which a base plate, a diaphragm plate, and a lid plate are joined by metal-to-metal bonding, which can improve the reliability of wiring.SOLUTION: A diaphragm 13 includes a vibrating portion 13a and a frame portion 13d, and a base plate 11 includes a terminal electrode 17 provided on the surface opposite the vibrating plate, a through-hole 11a which serves as a path for wiring 19 from the vibrating portion to the terminal electrode, and the wiring itself. The through-hole includes a step 11aa of a predetermined depth around the opening on the vibrating plate side, and includes a first wiring portion 19a, which is a part of the wiring, in a first portion of the side wall of the through-hole that faces the terminal electrode, and a second wiring portion 19b, which is a part of the wiring, extending from above the first wiring portion on the side wall to the step. The terminal electrode and first wiring portion are each made of a laminated film of titanium, nickel, and gold, and the second wiring portion is made of titanium and nickel films.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a piezoelectric device having a structure in which a base plate, a vibration plate, and a lid plate are joined by metal-to-metal bonding. [Background technology]

[0002] Piezoelectric devices require a structure that can hermetically seal the piezoelectric element. Currently, the mainstream structure uses a ceramic base. However, as piezoelectric devices become smaller and thinner, there are concerns that the ceramic base will reach its limits in terms of precision and cost. Therefore, alternative structures are being considered.

[0003] For example, Patent Document 1 discloses a piezoelectric device having a structure in which a quartz base plate, a diaphragm, and a lid plate are laminated in this order and joined together by metal-to-metal bonding (see, for example, Figure 1 of Patent Document 1). Specifically, the diaphragm has a vibrating portion and a frame portion provided around the vibrating portion. A laminated film of titanium and gold (Au) films is provided around the front and back of the frame portion as a sealing film (ibid., paragraph 39). A similar sealing film is also provided on the regions of the base plate and the lid plate corresponding to the sealing film (ibid., paragraphs 40 and 41). The base plate, diaphragm, and lid plate are then joined together by a diffusion bonding method, known as metal-to-metal bonding, using these sealing films to achieve an airtight sealing structure (ibid., paragraph 44).

[0004] Furthermore, a terminal electrode is provided on the surface of the base plate opposite the diaphragm (ibid., paragraph 41). Wiring is provided from the excitation electrode of the vibrating part to the terminal electrode. Specifically, a through hole is provided in the base plate, and the excitation electrode and the terminal electrode are electrically connected by wiring that uses this through hole as part of the wiring path (ibid., paragraph 42). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2023-42421 Summary of the Invention [Problem to be solved by the invention]

[0006] The piezoelectric device described in Patent Document 1 has great potential for miniaturization and thinning. However, the structure of Patent Document 1 still needs to be improved in terms of increasing the reliability of the wiring. Specifically, during the soldering process to mount a piezoelectric device on an electronic device, solder flows through the wiring inside the through-hole in the base plate, eroding the gold film on the surface of the wiring and spreading to the diaphragm (so-called solder erosion), which may result in an increase in wiring resistance or lead to breakage. This application has been made in consideration of the above points, and therefore, the object of this application is to provide a piezoelectric device having a structure in which a base plate, a vibration plate, and a lid plate are joined by metal-to-metal bonding, and having a novel structure that can improve the reliability of the wiring. [Means for solving the problem]

[0007] In order to achieve this object, according to the present invention, there is provided a piezoelectric device in which a base plate, a vibration plate, and a lid plate are laminated in this order and joined together by metal-to-metal bonding, wherein the vibration plate has a vibration portion and a frame portion provided around the vibration portion, and the base plate has terminal electrodes provided on the surface opposite to the vibration plate side, through holes that serve as paths for wiring from the vibration portion to the terminal electrodes, and the wiring, the through-hole has a step of a predetermined depth around the opening on the diaphragm side, a first wiring portion that is a part of the wiring, provided at a first portion of a side wall of the through hole that is on the terminal electrode side; a second wiring portion that is a part of the wiring and extends from above the first wiring portion on the side wall of the through hole to the step; the terminal electrode and the first wiring portion are each made of a first laminated film having an adhesive film and a top film made of a gold film; The second wiring portion is characterized in that it is made up of a second laminated film in which an adhesive film and a top film are solder erosion prevention films that are non-gold (non-Au) films.

[0008] In carrying out this invention, it is preferable that the adhesion film is a titanium film or a chromium film, and the solder leach prevention film is a nickel film. When the adhesion film is a titanium film or a chromium film, it is easier to ensure adhesion of the second laminate film to the base plate compared to when it is not a titanium film or a chromium film. However, considering the drawbacks of a chromium film, such as its relatively large stress and its tendency to diffuse into upper films, it is more preferable that the adhesion film be a titanium film. Furthermore, when the solder erosion prevention film is a nickel film, it is easier to form the film and also easier to prevent solder erosion than when it is not made of nickel.

[0009] In carrying out this invention, it is preferable that the diaphragm is made of a quartz plate, and that the base plate and the lid plate are each made of a glass plate or a quartz plate, and more preferably made of a quartz plate. When the base plate, diaphragm, and lid plate are each made of a quartz plate, they can be manufactured with high precision using photolithography and wet etching techniques, and since the base plate, diaphragm, and lid plate are made of the same material, it is possible to provide a piezoelectric device that is less affected by factors such as the coefficient of thermal expansion.

[0010] In carrying out this invention, the depth of the step is preferably deeper than the thickness of the metal film including the second stack formed in the step and shallower than 10% of the thickness of the base plate, preferably shallower than 5%. If the depth of the step is deeper than the above-mentioned predetermined depth, the upper surface of the metal film including the second stack formed in the step can be prevented from protruding from the surface of the base plate, thereby preventing the metal film including the second stack from adversely affecting the metal-to-metal bond. Furthermore, if the step is too deep, there is a risk of reducing the strength of the base plate, but if the depth of the step is shallower than 10% of the thickness of the base plate, preferably shallower than 5%. [Effects of the Invention]

[0011] According to this invention, the terminal electrode and the first wiring portion in the through hole are each made of a first laminated film having an adhesive film and a gold film as the top film, which makes it easy to ensure solderability, and the piezoelectric device can be easily soldered to any electronic device. Furthermore, the second wiring section, which extends from the first wiring section of the through-hole to the stepped section, is composed of an adhesive film and a second laminated film whose top film is a non-gold (non-Au) solder erosion prevention film, thereby preventing solder erosion in the wiring and improving the reliability of the wiring. Furthermore, because a step is provided on the diaphragm side of the through-hole, even if the thickness of the solder erosion prevention film is increased, the surface of the metal film including the second laminated film does not protrude above the surface of the base plate, preventing the metal film including the second laminated film formed within the step from adversely affecting the intermetallic bond between the base plate and the diaphragm. These features provide a piezoelectric device having a structure in which the base plate, diaphragm, and lid plate are bonded together by intermetallic bonding, which improves the reliability of the wiring and the intermetallic bond. [Brief explanation of the drawings]

[0012] [Figure 1] 1A and 1B are a perspective view and a cross-sectional view of a main part of a piezoelectric device according to an embodiment of the present invention; [Figure 2] FIG. 2 is an exploded perspective view showing the internal structure of the piezoelectric device according to the embodiment. [Figure 3] 3A and 3B are a perspective view and a cross-sectional view of a main part for explaining a through hole and a solder erosion structure provided in a base plate of a piezoelectric device according to an embodiment of the present invention; [Figure 4] 3A to 3C are diagrams illustrating an example of a method for manufacturing the piezoelectric device according to the embodiment and an example of a joining structure in the vicinity of the through-hole 11a of the base plate 11 and the diaphragm 13. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, embodiments of the piezoelectric device of the present invention will be described with reference to the drawings. Note that each drawing used for the description is merely a schematic illustration to enable understanding of the present invention. Furthermore, in each drawing used for the description, similar components are designated by the same numbers, and their description may be omitted. Furthermore, the shapes, dimensions, materials, etc. described in the following embodiments are merely preferred examples within the scope of the present invention. Therefore, the present invention is not limited to the following embodiments.

[0014] 1, 2, and 3 are diagrams illustrating a piezoelectric device 10 according to an embodiment. In particular, FIG. 1A is a perspective view of the piezoelectric device 10, and FIG. 1B is a cross-sectional view of the piezoelectric device 10 taken along line II in FIG. 1A. FIG. 2 is an exploded perspective view of the piezoelectric device 10 according to an embodiment. FIG. 3A is an enlarged perspective view of a portion (portion M in FIG. 2) near a through-hole 11a, which is a characteristic feature of the present invention, and FIG. 3B is a cross-sectional view of the portion shown in FIG. 3A taken along line II-II in FIG. 3A.

[0015] The piezoelectric device 10 of this embodiment is a piezoelectric device in which a base plate 11, a vibration plate 13, and a lid plate 15 are stacked in this order and bonded together by metal-to-metal bonding. However, the metal film used for the metal-to-metal bonding is not shown in Fig. 1. The metal film used for the metal-to-metal bonding is shown in Figs. 2 and 3.

[0016] The base plate 11 and the lid plate 15 are preferably made of glass or quartz crystal plates. In this embodiment, the base plate 11 and the lid plate 15 are made of quartz crystal plates. An AT-cut quartz crystal plate or a Z-cut quartz crystal plate is preferable. This is because AT-cut quartz crystal plates or Z-cut quartz crystal plates are often used in quartz crystal units manufactured using photolithography technology, and these quartz crystal plates are inexpensive and easily available as the base plate and lid plate. When an AT-cut quartz crystal plate is used as the diaphragm 13, it is preferable that the base plate 11 and the lid plate 15 are also made of AT-cut quartz crystal plates in order to match the linear expansion coefficient to that of the diaphragm 13. When a diaphragm for a tuning fork quartz crystal unit is to be obtained as the diaphragm 13, it is preferable that the base plate 11 and the lid plate 15 are also made of Z-cut quartz crystal plates, since Z-cut quartz crystal plates are used in the manufacture of tuning fork quartz crystal units.

[0017] The planar shape of the base plate 11, diaphragm 13, and lid plate 15 can be any shape, but is typically rectangular. The sizes of the base plate 11, diaphragm 13, and lid plate 15 can be the same or different, but are typically the same. The thickness of the base plate 11, diaphragm 13, and lid plate 15 can be determined according to the design of the piezoelectric device 10, but if the diaphragm 13 is a thickness-shear vibration type, the thickness of the vibrating portion 13a is determined according to the required frequency. The base plate 11 also has a terminal electrode 17 on the surface opposite to the diaphragm 13 side, and further has a through hole 11a that serves as a path for wiring 19 from the excitation electrode 13e of the vibrating portion 13a of the diaphragm 13 to the terminal electrode 17, and a part of the wiring 19. The terminal electrode 17 is a terminal that connects the piezoelectric device 10 to an arbitrary electronic device. The through hole 11a, the terminal electrode 17, and the wiring 19 will be described in detail later.

[0018] As shown in FIG. 2 , a metal film 21 having a predetermined width W is provided around the edge of each of the base plate 11, diaphragm 13, and lid plate 15, running along the outer periphery of each plate. The width W of the metal film 21 may be the same or different for the base plate 11, diaphragm 13, and lid plate 15, but is typically the same. The metal film 21 can be any metal film that allows intermetallic bonding. Specific examples of the metal film 21 include a laminated film of a chromium film and a gold film disposed thereon, and a laminated film of a titanium film and a gold film disposed thereon. The metal film 21 may also be a laminated film that includes a barrier film as an intermediate film between the titanium film or chromium film and the gold film.

[0019] The diaphragm 13 has a vibrating portion 13a and a frame portion 13b surrounding the vibrating portion 13a. The diaphragm 13 can be made of a piezoelectric plate according to the design of the piezoelectric device 10, but is typically made of a quartz crystal plate. Specifically, to obtain a vibrating portion that vibrates in thickness shear mode, an AT-cut quartz crystal plate or a twice-rotated quartz crystal plate, such as an SC-cut, can be used. Furthermore, to obtain a diaphragm for a tuning fork-type quartz crystal unit, a Z-cut quartz crystal plate with a predetermined cutting angle can be used. Below, a specific structural example of the diaphragm 13 will be described with reference to FIG. 2. Diaphragm 13 of the embodiment is composed of vibrating portion 13a provided near the center of diaphragm 13 in a plan view, frame portion 13b provided around vibrating portion 13a, separation portion 13c separating vibrating portion 13a from frame portion 13b, and connecting portion 13d connecting a portion of vibrating portion 13a to frame portion 13b. Separation portion 13c is composed of a through region formed by removing a predetermined portion of diaphragm 13.

[0020] In this example, the vibrating portion 13a has a rectangular shape in a plan view. Excitation electrodes 13e of a predetermined shape and size are provided at predetermined portions on the front and back sides of the vibrating portion 13a. A connecting portion 13d is provided from a portion of one side of the vibrating portion 13a toward the frame portion 13b. Therefore, this connecting portion 13d provides a limited connection of a portion of the vibrating portion 13a to the frame portion 13b. Furthermore, a contact hole 13f is provided in a portion of the frame portion 13b, penetrating the diaphragm 13. This contact hole 13f connects the excitation electrode 13e on the lid plate 15 side of the vibrating portion 13a to the base plate 11 side, and is used as part of the path of the wiring 19. Furthermore, this contact hole 13f, in cooperation with the through hole 11a provided in the base plate 11, connects the wiring 19 to the terminal electrode 17. The excitation electrode 13e and the wiring 19 can be made of any suitable metal film. In the case of this diaphragm 13, the vibrating portion 13a and the frame portion 13b are separated by the separating portion 13c, which reduces mutual interference between the vibrating portion 13a and the frame portion 13b, i.e., vibration leakage from the vibrating portion to the frame portion and the influence of external forces from the frame portion to the vibrating portion. In addition, the excitation electrode 13e of the vibrating portion 13a can be connected to the terminal electrode 17 by the wiring 19.

[0021] Next, with reference mainly to FIGS. 2 and 3, the details of the structures of the through-hole 11a, the terminal electrode 17, and the wiring 19, which are features of the present invention, will be described. The through holes 11a are provided in predetermined portions of the base plate 11. Specifically, in this example, the through holes 11a are provided near two diagonally opposite corners of the four corners of the base plate 11, at positions that do not interfere with the metal film 21 for intermetallic bonding. Of course, the positions of the through holes 11a are not limited to the above example. Furthermore, a step 11aa of a predetermined depth d (see FIG. 3(B)) is provided around the opening of each of these through holes 11a on the diaphragm 13 side. Furthermore, a first wiring portion 19a (see FIG. 3(B)), which is a part of the wiring 19, is provided in a first portion A (see FIG. 3(B)) that is on the terminal electrode 17 side of the side wall of the through hole 11a, and a second wiring portion 19b (see FIG. 3(B)), which is a part of the wiring 19, is provided in an area extending from above the first wiring portion 19a on the side wall of the through hole 11c to above the step 11aa. The terminal electrode 17 and the first wiring portion 19a are each made of a first laminated film including an adhesive film 19c, an intermediate film 19d, and a gold film 19e as the top film, while the second wiring portion 19b is made of a second laminated film including an adhesive film 19f and a non-gold (non-Au) solder erosion prevention film 19g as the top film.

[0022] In the illustrated example, the cross-sectional shape of through hole 11a cut in the thickness direction of base plate 11 is such that the diameter of the through hole becomes smaller halfway through the hole compared to the opening side. This cross-sectional shape results from the fact that the quartz crystal plate, which is base plate 11, is formed by wet etching, and the etchant progresses less near the center of the hole. It is also preferable to separate first wiring portion 19a and second wiring portion 19b near the point where the diameter of through hole 11a becomes smaller, in order to prevent solder erosion. In this embodiment, the planar shape of step 11aa is substantially the same as the planar shape of through hole 11a, i.e., circular. However, the planar shapes of through hole 111a and step 11aa may be any shape, such as elliptical, rectangular, or polygonal.

[0023] The adhesive film 19c of the first wiring portion 19a can be made of, for example, a chromium film or a titanium film. In this example, the first wiring portion 19a also includes an intermediate film 19d, which is continuous with the solder erosion prevention film 19f of the second wiring layer 19b. In this example, the intermediate film 19d is made of a nickel film. The top film of the first wiring portion 19a is made of a gold (Au) film 19e. The adhesive film 19f of the second wiring layer 19b can be made of, for example, a chromium film or a titanium film. The solder erosion prevention film 19f of the second wiring portion 19b can be any film that can prevent or reduce solder flow compared to a gold film, but in this example it is made of a nickel film. Therefore, the first wiring portion 19a has a gold film surface, and the second wiring portion 19b has a nickel film surface.

[0024] The thickness of each film constituting the wiring 19 can be any thickness depending on the design of the piezoelectric device. While not limited to this, the adhesion films 19c and 19f are each made of a titanium film with a thickness of approximately 5 nm, the intermediate film 19d and the solder erosion prevention film 19g are each made of a nickel film with a thickness of approximately 300 nm, and the metal film 19e, which is the top film of the first wiring portion, is made of a gold film with a thickness of approximately 50 nm. The adhesion film 21a of the metal film 21 for intermetallic bonding is made of a titanium film with a thickness of approximately 5 nm, and the upper layer 21b of the metal film 21 for intermetallic bonding is made of a gold film with a thickness of approximately 200 nm. The metal film 19, which will be described later, is made of a laminated film of a titanium film with a thickness of approximately 5 nm and a gold film with a thickness of approximately 200 nm, similar to the metal film 21. Therefore, the total thickness of the metal film formed in the step 11aa in the case of these exemplary film thicknesses is approximately 550 nm, which is the thickness resulting from the metal films 19f, 19g, and 19h. Therefore, the depth of the step 11aa is set to an appropriate value that is deeper than this total thickness.

[0025] Next, to deepen understanding of the present invention, an example of a method for manufacturing the piezoelectric device 10 and an example of a connection structure near the through-hole 11a will be briefly described with reference to FIG. The piezoelectric device 10 of the present invention is preferably manufactured in wafer form and then singulated. The base plate wafer (base plate wafer) 11w, the diaphragm wafer (diaphragm wafer) 13w, and the lid plate wafer (lid plate wafer) 15w are each manufactured using photolithography and wet etching techniques. Specifically, wafers 11W, 13W, and 15W are prepared, each equipped with the metal film for intermetallic bonding, the vibrating portion, the frame portion, wiring, contact holes, through-holes, and the like, as described above. These are then bonded together using a predetermined bond. The bonded wafer is then divided using a cutting tool such as a dicing saw to obtain a large number of piezoelectric devices 10.

[0026] The metal-to-metal bonding between the base plate 11 and the diaphragm 13, and the connection of the wiring 19 near the through-hole 11a of the base plate 11 and the diaphragm 13 can be performed, for example, as follows. That is, as shown in the latter part of FIG. 4, the base plate 11 and the diaphragm 13 are bonded by a metal film 21 to ensure an airtight structure. Although not shown, the base plate and the lid plate are also bonded by a metal film 21 to ensure an airtight structure. Furthermore, the wiring 19 of the diaphragm 13 and the metal film 19h provided in the through-hole 11a of the base plate are electrically connected by a metal film 19h. However, the metal film 19 of the diaphragm 13 and the wiring 19h provided in the through-hole 11a of the base plate may be connected by a method other than metal film bonding. [Explanation of symbols]

[0027] 10: Piezoelectric device of an embodiment 11: Base plate 11a: Through hole 11aa: Step 13: Vibration plate 13a: Vibration part 13b: Holding frame 13c: Separation part 13d: Excitation electrode 13e: Extension part 13f: Contact hole 15: Lid plate 17: Terminal electrode 19: Wiring 19a: First wiring portion (first laminated film) 19b: Second wiring portion (second laminated film) 19c, 19f: Adhesive film 19d: Interlayer 19e: Gold (Au) 19g: Solder erosion prevention film 19h: Part of the wiring 21: Metallic films for metal-to-metal bonding

Claims

1. A piezoelectric device in which a base plate, a vibration plate, and a lid plate are laminated in this order and joined together by metal-to-metal bonding, wherein the vibration plate has a vibration part and a frame part provided around the vibration part, and the base plate has terminal electrodes provided on the surface opposite to the vibration plate side, through holes that serve as paths for wiring from the vibration part to the terminal electrodes, and the wiring, the through-hole has a step of a predetermined depth around the opening on the diaphragm side, a first wiring portion that is a part of the wiring, provided at a first portion of a side wall of the through hole that is on the terminal electrode side; a second wiring portion that is a part of the wiring and extends from above the first wiring portion on the side wall of the through hole to the step; the terminal electrode and the first wiring portion are each made of a first laminated film having an adhesive film and a top film made of a gold film, The piezoelectric device is characterized in that the second wiring portion is composed of a second laminated film having an adhesive film and a top film that is a solder erosion prevention film that is a non-gold film.

2. 2. The piezoelectric device according to claim 1, wherein the depth of the step is greater than the thickness of the metal film including the second stacked layer formed within the step.

3. 2. The piezoelectric device according to claim 1, wherein the depth of the step is less than 10% of the thickness of the base plate.

4. 2. The piezoelectric device according to claim 1, wherein the first laminated film comprises a nickel film as an intermediate film between an adhesive film and a gold layer as an uppermost film.

5. 2. The piezoelectric device according to claim 1, wherein the second laminated film has a solder erosion prevention film made of nickel.

6. the first laminated film includes a nickel film as an intermediate film between an adhesive film and a gold layer as a top film, The second laminated film has a solder erosion prevention film made of nickel, 2. The piezoelectric device according to claim 1, wherein the nickel film of the intermediate film and the nickel film of the solder erosion prevention film are continuous nickel films.

7. 7. The piezoelectric device according to claim 1, wherein the base plate, the diaphragm, and the lid plate are each made of a quartz crystal plate.

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