Crystal oscillators and methods for manufacturing the same

Abstract

A method for manufacturing a plurality of crystal oscillators includes providing a first wafer including a piezoelectric crystal, forming a first patterned metal layer on a first surface of the first wafer, wherein the first patterned metal layer includes a plurality of bottom electrodes, providing a second wafer, forming a second patterned metal layer on a first surface of the second wafer, wherein the second patterned metal layer includes a plurality of connecting pads, defining a plurality of first cavities in the second wafer below the first surface of the second wafer, bonding the first wafer and the second wafer to form a bonded structure by bonding the first patterned metal layer and the second patterned metal layer through direct metal bonding, thinning the first wafer from a second surface opposite the first surface of the first wafer, patterning the thinned first wafer to define a plurality of first suspended portions, and dicing the bonded structure into a plurality of crystal oscillators.

Description

[0001] CRYSTAL OSCILLATORS AND METHODS FOR MANUFACTURING THE SAME

[0002] BACKGROUND

[0003] Related Application

[0004] This application claims priority to U.S. Provisional Patent Application Ser. No. 63 / 454,962, filed on Mar. 28, 2023, entitled “Quartz Device and Manufacturing Method thereof,” and Ser. No. 63 / 470,177, filed on Jun. 01, 2023, entitled “Quartz Device and Manufacturing Method thereof,” which are incorporated herein by reference in their entirety.

[0005] Field of the Invention

[0006] The present disclosure relates to crystal oscillators and methods for manufacturing the same. In particular, some embodiments of the present disclosure relate to crystal oscillators and methods for manufacturing a plurality of crystal oscillators.

[0007] Description of Related Art

[0008] Piezoelectric oscillators are widely used as reference sources for frequency and time in compact portable electronic devices including, among others, portable telephones, because of their small size and light weight. Piezoelectric oscillators may serve to provide high-end reference clock signals and receive transmission signals, for example, for 5G technology.

[0009] In order to further increase the output frequency of piezoelectric oscillator, there is a need to further decrease the thickness of a vibrating element used in the piezoelectric oscillator. However, the rigidity of the vibrating portion decreases when the vibrating element is made thinner, and the manufacturing of the piezoelectric oscillator may suffer from difficulties in handling and mounting of the vibrating element.

[0010] SUMMARY

[0011] According to the present disclosure, a method for manufacturing a plurality of crystal oscillators is provided. The method comprises providing a first wafer comprising piezoelectric crystal (step (a)). The method comprises forming a first patterned metal layer on a first surface of the first wafer, wherein the first patterned metal layer comprises a plurality of bottom electrodes (step (b)). The method comprises providing a second wafer (step (c)). The method comprises forming a second patterned metal layer on a first surface of the second wafer, wherein the second patterned metal layer comprises a plurality of connecting pads (step (d)). The method comprises defining a plurality of first cavities in the second wafer below the first surface of the second wafer (step (e)). The method comprises bonding the first wafer and the second wafer to form a bonded structure by bonding the first patterned metal layer and the second patterned metal layer through direct metal bonding (step (f)). The method comprises thinning the first wafer from a second surface opposite the first surface of the first wafer (step (g)). The method comprises patterning the thinned first wafer to define a plurality of first suspended portions (step (h)). The method comprises dicing the bonded structure into a plurality of crystal oscillators (step (i)).

[0012] In one embodiment, the method further comprises forming a third patterned metal layer on an exposed surface of the thinned first wafer after the step (g) (step(j)) .

[0013] In one embodiment, the method further comprises forming first vias extending through the first wafer (step(kl)).

[0014] In one embodiment, the method further comprises forming second vias extending through the second wafer (step(k2)).

[0015] In one embodiment, the step (f) comprises bonding each of the bottom electrodes to a respective connecting pad of the connecting pads through direct metal bonding.

[0016] In one embodiment, the first patterned metal layer further comprises a plurality of first dummy pads, the second patterned metal layer further comprises a plurality of second dummy pads, and the step (f) comprises bonding each of the first dummy pads to a respective second dummy pads of the second dummy pads through direct metal bonding.

[0017] In one embodiment, the step (e) comprises defining the plurality of first cavities each having a depth in a range between 0.1 pm and 10 pm.

[0018] In one embodiment, the step (e) comprises etching the second wafer from the first surface to define the plurality of first cavities.

[0019] In one embodiment, the step (h) comprises defining a through hole extending through at least one of the first suspended portions.

[0020] In one embodiment, the step (h) comprises pattering the thinned first wafer to define a plurality of second suspended portions.

[0021] In one embodiment, the first wafer comprises quartz crystal.

[0022] According to the present disclosure, a crystal oscillator is provided. The crystal oscillator comprises a piezoelectric crystal plate, a first patterned metal layer, a substrate, a second patterned metal layer, a third patterned metal layer, and a direct metal bonding interface. The piezoelectric crystal plate comprises an anchoring portion and a first suspended portion connecting with the anchoring portion. The first patterned metal layer is disposed on a first surface of the crystal plate. The first patterned metal layer comprises a first bottom electrode overlapped with the first suspended portion. The substrate has a first cavity below a first surface of the substrate. The first suspended portion is overlapped with the first cavity. The second patterned metal layer is disposed on the first surface of the substrate. The second patterned metal layer comprises a connecting pad. The third patterned metal layer is disposed on a second surface opposite the first surface of the crystal plate. The third patterned metal layer comprises a first top electrode overlapped with the first suspended portion. The direct metal bonding interface extends between the first patterned metal layer and the second patterned metal layer.

[0023] In one embodiment, the first bottom electrode comprises an excitation portion overlapped with the first suspended portion, a pad portion overlapped with the anchoring portion, and a connecting portion continuously connected to both the excitation portion and the pad portion.

[0024] In one embodiment, the direct metal bonding interface extends between the connecting pad and the pad portion of the first bottom electrode.

[0025] In one embodiment, the crystal oscillator further comprises a first via extending through the crystal plate and electrically connected to the first top electrode.

[0026] In one embodiment, the crystal oscillator further comprises a second via extending through the substrate and electrically connected to the connecting pad.

[0027] In one embodiment, the first patterned metal layer further comprises at least one first dummy pad, the second patterned metal layer further comprises at least one second dummy pad, and the direct metal bonding interface extends between the first dummy pad and the second dummy pad.

[0028] In one embodiment, the first suspended portion has a substantially rectangular- shaped configuration.

[0029] In one embodiment, a length of the first suspended portion is in a range between 100 pm and 1000 pm.

[0030] In another embodiment, a length of the first suspended portion is in a range between 10 pm and 500 pm.

[0031] In one embodiment, a depth of the first cavity is in a range between 0.1 pm and 10 pm.

[0032] In one embodiment, the crystal plate has a through hole extending through the first suspended portion.

[0033] In one embodiment, the crystal plate further comprises a second suspended portion extending from the anchoring portion. In one embodiment, the substrate has a second cavity below the first surface of the substrate, and the second suspended portion is overlapped with the second cavity.

[0034] In one embodiment, the crystal oscillator further comprises a cap over and overlapped with both the first suspended portion and the second suspended portion.

[0035] In one embodiment, the second suspended portion is overlapped with the first cavity.

[0036] In one embodiment, the first patterned metal layer further comprises a second bottom electrode overlapped with the second suspended portion, and the third patterned metal layer further comprises a second top electrode overlapped with the second suspended portion.

[0037] In one embodiment, the first top electrode is electrically coupled to the second bottom electrode, and the second top electrode is electrically coupled to the first bottom electrode.

[0038] In one embodiment, the first top electrode is electrically coupled to the second top electrode, and the first bottom electrode is electrically coupled to the second bottom electrode.

[0039] In one embodiment, the first top electrode and the first bottom electrode are electrically coupled to a first functional circuit, and the second top electrode and the second bottom electrode are electrically coupled to a second functional circuit.

[0040] In one embodiment, the crystal oscillator is a strip resonator.

[0041] BRIEF DESCRIPTION OF THE DRAWINGS

[0042] FIGS. 1 to 9, FIGS. 10A to 10B, FIGS. 11A to 11B, FIGS. 12A to 12B, FIG. 13, and FIGS. 14A to 14C are schematic diagrams illustrating intermediate stages in the manufacture of a plurality of crystal oscillators according to one embodiment of the present disclosure.

[0043] FIGS. 15 to 20 are schematic diagrams illustrating intermediate stages in the manufacture of a plurality of crystal oscillators according to one embodiment of the present disclosure.

[0044] FIGS. 21A to 21C are schematic views to illustrate an embodiment of a crystal oscillator according to the present disclosure.

[0045] FIG. 22 is a schematic view to illustrate an embodiment of a crystal oscillator according to the present disclosure.

[0046] FIG. 23 is a schematic view to illustrate an embodiment of a crystal oscillator according to the present disclosure.

[0047] FIGS. 24A to 24C are schematic views to illustrate an embodiment of a crystal oscillator according to the present disclosure.

[0048] FIGS. 25 A to 25C are schematic views to illustrate embodiments of crystal oscillators according to the present disclosure. DETAILED DESCRIPTION OF EMBODIMENTS

[0049] The terminology used in the description presented below is intended to be interpreted in its broadest reasonable manner, even though it is used in conjunction with a detailed description of certain specific embodiments of the technology. Certain terms may even be emphasized below; however, any terminology intended to be interpreted in any restricted manner will be specifically defined as such in this Detailed Description section. Components and achievement of a structure or a device, according to the present disclosure may be illustrated in the following drawings and embodiments. However, the size and shape shown on drawings for the structure(s) or the device(s) do not limit the features of the present disclosure.

[0050] The phrase “on” used in this application can mean directly on or indirectly on with intervening elements or layers. The spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature’s relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as being “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the exemplary term “below” can encompass both an orientation of above and below. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

[0051] FIGS. 1 to 9, FIGS. 10A to 10B, FIGS. 11A to 11B, FIGS. 12A to 12B, FIG. 13, and FIGS. 14A to 14C are schematic diagrams illustrating intermediate stages in the manufacture of a plurality of crystal oscillators according to one embodiment of the present disclosure.

[0052] As shown in FIG. 1, a first wafer 10 is provided (step (a)). The first wafer 10 comprises piezoelectric crystal. In some embodiments, the first wafer 10 comprises quartz crystal. For example, an AT-cut quartz crystal wafer may be provided. However, the present disclosure may be applied to various piezoelectric crystal wafers, such as BT-cut or other type of crystal wafer. In some embodiments, the first wafer 10 may be a wafer with a diameter of 2 inches or larger. In the present embodiment, the thickness of the first wafer 10 may be at about 200 pm. However, the present disclosure is not limited thereto. A first surface 10S1 of the first wafer 10 may be polished, and the first wafer 10 may be sufficiently washed and dried.

[0053] FIGS. 1 to 4 show an enlarged view of a region A of the first wafer 10. Despite that only the region A is shown in the figures, the manufacturing process discussed herein may be wafer level process(es) and may simultaneously apply to other regions of first wafer 10, such that a plurality of crystal oscillators may be manufactured simultaneously by the process discussed herein.

[0054] As shown in FIG. 2, through holes Hl and H3 are defined in the first wafer 10. The through holes Hl and H3 penetrate through the first wafer 10 and extend from the first surface

[0055] 1051 of the first wafer 10 to a second surface 10S2 of the first wafer 10 opposite the first surface 10S1 of the first wafer 10. In other words, the through holes Hl and H3 are defined by inner sidewalls of the first wafer 10. In some embodiments, the through holes Hl and H3 may be cylindrical holes, or the like. The cross-sectional shape of the through holes Hl and H3 may be rectangle, square, or the like, and the top view of the through holes Hl and H3 may be circular, oval, or the like. However, the disclosure is not limited thereto. The through holes Hl and H3 may be formed in any suitable shapes. In one embodiment, the through holes Hl and H3 may each have a radius of about, but not limited to, 0.03 mm. In some embodiments, the through holes Hl and H3 may be formed using laser. In some embodiments, the through holes Hl and H3 may be formed using photolithography and etching processes.

[0056] As shown in FIG. 3, first vias VI and third vias V3 are formed (step (kl)). The first vias VI and the third vias V3 may extend through the first wafer 10. A conductive material fills the through holes Hl and H3 to form the first vias VI and the third vias V3, respectively. In some embodiments, copper is used to fill the through holes Hl and H3 by electrochemical plating (ECP). However, embodiments of the present disclosure are not so limited. The first vias VI and the third vias V3 can be formed using any suitable conductive material and processes. For example, a conductive adhesive may be applied into the through holes Hl and H3. In some embodiments, the first surface 10S1 of the first wafer 10 and / or the second surface

[0057] 1052 of the first wafer 10 may be polished after the formation of the first vias V 1 and the third vias V3. In the embodiment shown in FIGS. 1 to 14C, the first vias VI and the third vias V3 may be formed before bonding the first wafer 10 and the second wafer 20.

[0058] As shown in FIG. 4, a first patterned metal layer 120L is formed on a first surface 10S1 of the first wafer 10 (step (b)). The first patterned metal layer 120L may comprise a plurality of bottom electrodes 122, wherein each of the bottom electrodes 122 may further comprise an excitation portion 122a, a pad portion 122c, and a connecting portion 122b continuously connected to both the excitation portion 122a and the pad portion 122c. In some embodiments, the first patterned metal layer 120L may further comprise a plurality of first dummy pads 124, such as first dummy pads 124a, 124b, 124c, 124d shown in FIG. 4. In some embodiments, the first patterned metal layer 120L may further comprise a plurality of connecting pads 125. In one embodiment, a metal film in which gold (Au) is stacked on a base of chromium (Cr) is formed on the first surface 10S1 of the first wafer 10 by sputtering, evaporation, or the like. In one embodiment, a thickness of the gold layer may be about 100 nm, and a thickness of the chromium layer may be about 5 nm. Then, a first mask defining the pattern of the first patterned metal layer 120L may be formed on the metal film, and the metal film may be patterned through, for example, removing the portion of metal film exposed by the first mask using suitable solution such as aqua regia or the like. The first dummy pads 124a, 124b, 124c, 124d as well as the connecting pads 125 may be patterned by the same mask as that of the bottom electrodes 122. In some embodiments, the first patterned metal layer 120L may be formed using the first vias VI and / or the third vias V3 as alignment mark.

[0059] As shown in FIG. 5, a second wafer 20 is provided (step (c)). The second wafer 20 may comprise any suitable material such as silicon, silicon oxide, quartz, or ceramics, e.g., low- temperature cofired ceramics (LTCC); or, the second wafer 20 may be a printed circuit board. In the present embodiment, the second wafer 20 may be a quartz crystal wafer. In some embodiments, the second wafer 20 may be a wafer with a diameter of 2 inches or larger. In the present embodiment, the thickness of the second wafer 20 may be at about 400 pm. However, the present disclosure is not limited thereto. A first surface 20S1 of the second wafer 20 may be polished, and the second wafer 20 may be sufficiently washed and dried.

[0060] FIGS. 5 to 9 show an enlarged view of a region B of the second wafer 20. Despite that only the region B is shown in the figures, the manufacturing process discussed herein may be wafer level process(es) and may simultaneously apply to other regions of second wafer 20, such that a plurality of crystal oscillators may be manufactured simultaneously by the process discussed herein.

[0061] As shown in FIG. 6, through holes H2 and H4 are defined in the second wafer 20. The through holes H2 and H4 penetrate through the second wafer 20 and extend from the first surface 20S1 of the second wafer 20 to a second surface 20S2 of the second wafer 20 opposite the first surface 20S1 of the second wafer 20. In other words, the through holes H2 and H4 are defined by inner sidewalls of the second wafer 20. In some embodiments, the through holes H2 and H4 may be cylindrical holes, or the like, as that discussed above with respect to the through holes Hl and H3. However, the disclosure is not limited thereto. The through holes H2 and H4 may be formed in any suitable shapes. In one embodiment, the through holes H2 and H4 may each have a radius of about, but not limited to, 0.03 mm. The through holes H2 and H4 may be formed using laser. In some embodiments, the through holes H2 and H4 may be formed using photolithography and etching processes. As shown in FIG. 7, second vias V2 and fourth vias V4 are formed (step (k2)). The second vias V2 and the fourth vias V4 may extend through the second wafer 20. A conductive material fills the through holes H2 and H4 to form the second vias V2 and the fourth vias V4. In some embodiments, copper is used to fill the through holes H2 and H4 by electrochemical plating (ECP). However, embodiments of the present disclosure are not so limited. The second vias V2 and the fourth vias V4 can be formed using any suitable conductive material and processes. For example, a conductive adhesive may be applied into the through holes H2 and H4. In some embodiments, the first surface 20S1 of the second wafer 20 and / or the second surface 20S2 of the second wafer 20 may be polished after the formation of the second vias V2 and the fourth vias V4. In the embodiment shown in FIGS. 1 to 14C, the second vias V2 and the fourth vias V4 may be formed before bonding the first wafer 10 and the second wafer 20.

[0062] As shown in FIG. 8, a second patterned metal layer 140L is formed on a first surface 20S1 of the second wafer 20 (step (d)). The second patterned metal layer 140L may comprise a plurality of connecting pads 142. In some embodiments, the second patterned metal layer 140L may further comprise a plurality of second dummy pads 144, such as second dummy pads 144a, 144b, 144c, 144d shown in FIG. 8. In some embodiments, the second patterned metal layer 140L may further comprise a plurality of connecting pads 145. The second patterned metal layer 140L may have similar composition as that of the first patterned metal layer 120L and may be formed using a method similar to that described above with reference to FIG. 4. Specifically, a metal film is formed on the first surface 20S1 of the second wafer 20 by sputtering, evaporation, or the like. Then, a second mask defining the pattern of the second patterned metal layer 140L may be formed on the metal film, and the metal film may be patterned through, for example, removing the portion of metal film exposed by the second mask using suitable solution such as aqua regia or the like. The second dummy pads 144a, 144b, 144c, 144d as well as the connecting pads 145 may be patterned by the same mask as that of the connecting pads 142. In some embodiments, the second patterned metal layer 140L may be formed using the second vias V2 and / or the fourth vias V4 as alignment mark.

[0063] As shown in FIG. 9, a plurality of first cavities Cl in the second wafer 20 are defined (step (e)), wherein each of the first cavities Cl is a cavity below (recessed from) the first surface 20S1 of the second wafer 20. In some embodiments, the plurality of first cavities Cl are defined such that each of the first cavities Cl has a depth in a range between 0.1 pm and 10 pm. In some embodiments, the plurality of first cavities Cl may be defined by etching the second wafer 20 from the first surface 20S 1. For example, the first surface 20S 1 of the second wafer 20 exposed by a photoresist are etched to a desired depth using suitable solution such as a mixed solution of hydrofluoric acid and ammonium fluoride.

[0064] FIG. 10A is a perspective view of a bonded structure 30, and FIG. 10B is a cross- sectional view of the bonded structure 30 along line A-A’ in FIG. 10A. Specifically, FIGS. 10A, 11A, 12A, and 13 show an enlarged view of a region of the bonded structure 30 corresponding to the region A of the first wafer 10 and the region B of the second wafer 20. The manufacturing process discussed herein may be wafer level process(es) and may simultaneously apply to other regions of the bonded structure 30, such that a plurality of crystal oscillators may be manufactured simultaneously.

[0065] As shown in FIGS. 10A and 10B, the first wafer 10 and the second wafer 20 are bonded to form a bonded structure 30 by bonding the first patterned metal layer 120L and the second patterned metal layer 140L through direct metal bonding (step (f)). A wafer level metal to metal direct bonding process may be performed. The first patterned metal layer 120L and the second patterned metal layer 140L may be bonded by thermal compression bonding, eutectic bonding, or reactive bonding. For example, the first patterned metal layer 120L and the second patterned metal layer 140L may be bonded by thermal compression bonding process, such as diffusion bonding. In one embodiment, both the first patterned metal layer 120L and the second patterned metal layer 140L are cleaned by conventional cleaning techniques such as dry etching and plasma surface treatment. In one embodiment, external pressure is applied on the stacked first wafer 10, the first patterned metal layer 120L, the second patterned metal layer 140L, and the second wafer 20 during a thermal process. In some embodiments, the first patterned metal layer 120L contacts the second patterned metal layer 140L, and the atoms on the surfaces of the first patterned metal layer 120L and the second patterned metal layer 140L may interdiffuse, so that metal bonds may be formed at a metal bonding interface MI extending between the first patterned metal layer 120L and the second patterned metal layer 140L.

[0066] Referring to FIGS. 10A and 14A, in some embodiments, bonding the first patterned metal layer 120L and the second patterned metal layer 140L comprises bonding the bottom electrode 122 (e.g., the pad portion 122c of the bottom electrode 122) of the first patterned metal layer 120L to a respective connecting pad 142 of the second patterned metal layer 140L through direct metal bonding. In some embodiments, bonding the first patterned metal layer 120L and the second patterned metal layer 140L comprises bonding each of the first dummy pads 124 of the first patterned metal layer 120 to respective second dummy pads 144 of the second patterned metal layer 140L through direct metal bonding. For example, bonding the first dummy pad 124a to the second dummy pad 144a, the first dummy pad 124b to the second dummy pad 144b, the first dummy pad 124c to the second dummy pad 144c, the first dummy pad 124d to the second dummy pad 144d, through direct metal bonding.

[0067] FIG. 11A is a perspective view of the bonded structure 30, and FIG. 1 IB is a cross- sectional view of the bonded structure 30 along line A-A’ in FIG. 11A. As shown in FIGS. 11A and 11B, the first wafer 10 is thinned from the second surface 10S2 of the first wafer 10 (step (g)). The first wafer 10 may be thinned by any suitable method, such as grinding, wet etching, plasma etching and / or polishing. In one embodiment, the bonded structure 30 is soaked in an etchant for a predetermined time period. In some embodiments, both the first wafer 10 and the second wafer 20 may be thinned. In some embodiments, the thinned first wafer 10’ may have a thickness T1 of about 10 pm to 100 pm, for example, of about 35 pm. However, the present disclosure is not limited thereto, the thinned first wafer may be thinner or thicker according to, for example, the desired characteristics of the oscillators.

[0068] FIG. 12A is a perspective view of the bonded structure 30, and FIG. 12B is a cross- sectional view of the bonded structure 30 along line A-A’ in FIG. 12A. As shown in FIGS. 12A and 12B, a third patterned metal layer 150L is formed on an exposed surface 10S2’ of the thinned first wafer 10’ (step (j)) after thinning the first wafer 10. The third patterned metal layer 150L may comprise a plurality of top electrodes 152, wherein each of the top electrodes 152 may further comprise an excitation portion 152a, a pad portion 152c, and a connecting portion 152b continuously connected to both the excitation portion 152a and the pad portion 152c. In some embodiments, the third patterned metal layer 150L may further comprise a plurality of connecting pads 155. The third patterned metal layer 150L may have similar composition as that of the first patterned metal layer 120L and may be formed using a method similar to that described above with reference to FIG. 4. In some embodiments, the thicknesses of the third patterned metal layer 150L may be measured and finely adjusted in order to achieve a predetermined frequency. In one embodiment, the third patterned metal layer 150L may have a thickness of about 0.3 pm. However, the present disclosure is not limited thereto.

[0069] As shown in FIG. 13, the thinned first wafer 10’ may be patterned to define a plurality of first suspended portions 114 (step (h)). In the present embodiment, first trenches TRI extending through the thinned first wafer 10’ may be formed, for example, using photolithography and etching processes. In the embodiment shown in FIG. 13, first trenches TRI in a configuration of right angled “U” shaped in top view are formed, and each of the first trenches TRI defines a first suspended portion 114 having a substantially rectangular- shaped configuration. In some embodiments, the etchant applied to the thinned first wafer 10’ during the etching process(es) may flow through the first trenches TRI into the first cavities Cl. The “stagnant” etchant in the first cavities Cl may be drained through gaps between adjacent dummy pads.

[0070] As shown in FIG. 14A, the bonded structure 30 is diced into a plurality of crystal oscillators including a crystal oscillator 100A (step (i)). Laser cutting process(es) or other suitable process(es) may be performed to the bonded structure 30. In some embodiments, the bonded structure 30 may be cut with a cutting width of about 16 pm to 20 pm.

[0071] FIG. 14A is a perspective view of the crystal oscillator 100A, FIG. 14B is a cross- sectional view of the crystal oscillator 100A along line B-B’ in FIG. 14A, and FIG. 14C is a cross-sectional view of the crystal oscillator 100A along line C-C’ in FIG. 14A. Referring to FIGS. 14A to 14C, the crystal oscillator 100A is provided. The crystal oscillator 100A comprises a piezoelectric crystal plate 110, a first patterned metal layer 120, a substrate 130, a second patterned metal layer 140, and a third patterned metal layer 150.

[0072] In some embodiments, the crystal plate 110 comprises quartz crystal. For example, the crystal plate 110 may be an AT-cut quartz crystal plate, and thickness-shear vibration may be excited as main vibration. However, the present disclosure may be applied to various piezoelectric crystal plates, such as BT-cut or other type of crystal plate. In the present embodiment, the thickness of the crystal plate 110 may be from about 10 pm to about 100 pm.

[0073] As shown in FIGS. 14A to 14C, the piezoelectric crystal plate 110 may comprise an anchoring portion 112 and a first suspended portion 114. The first suspended portion 114 is a “tongue portion” of the piezoelectric crystal plate 110 extending from and connecting with the rest of the piezoelectric crystal plate 110 (the anchoring portion 112). As discussed above, the first suspended portion 114 and the anchoring portion 112 are integrally formed such that the first suspended portion 114 is seamlessly connecting with the anchoring portion 112. The first suspended portion 114 substantially defines a vibration region of the crystal oscillator 100A in which vibration (e.g., thickness- shear vibration) energy is confined. In the present embodiment, the first suspended portion 114 has a substantially rectangular- shaped configuration. The first suspended portion 114 may substantially constitute a crystal strip for the crystal oscillator 100A, and the crystal oscillator 100A may be a strip resonator. In some embodiments, the first suspended portion 114 may have a length L in a range between 100 pm and 1000 pm. In some embodiments, the first suspended portion 114 may have a length L in a range between 10 pm and 500 pm. For example, a first suspended portion 114 having a length L of about 300 pm may be provided. A first suspended portion 114 having a length L less than 300 pm may also be achieved. These values are merely examples and are not intended to be limiting. In the embodiment shown in FIGS. 14A to 14C, the first suspended portion 114 is surrounded by the anchoring portion 112.

[0074] The first patterned metal layer 120 is disposed on a first surface 110S 1 of the crystal plate 110. The first patterned metal layer 120 may comprise any suitable metal material and may have a multilayer structure. The first patterned metal layer 120 may have similar composition and thickness as that of the first patterned metal layer 120L. The first patterned metal layer 120 comprises a first bottom electrode 122. The first bottom electrode 122 is overlapped with the first suspended portion 114 and may drive the first suspended portion 114. Specifically, the first bottom electrode 122 may comprise an excitation portion 122a, a pad portion 122c, and a connecting portion 122b. The excitation portion 122a is disposed overlapped with the first suspended portion 114 of the crystal plate 110. The pad portion 122c is disposed overlapped with the anchoring portion 112 of the crystal plate 110. The connecting portion 122b is continuously connected to both the excitation portion 122a and the pad portion 122c, such that the excitation portion 122a on the first suspended portion 114 is electrically connected to the pad portion 122c on the anchoring portion 112 by the connecting portion 122b. In the embodiment shown in FIGS. 14A to 14C, the excitation portion 122a of the first bottom electrode 122 may have a rectangular shape. However, the present disclosure is not limited thereto.

[0075] The third patterned metal layer 150 is disposed on a second surface 110S2 of the crystal plate 110 opposite the first surface 110S 1 of the crystal plate 110. The third patterned metal layer 150 may comprise any suitable metal material and may have a multilayer structure. The third patterned metal layer 150 may have similar composition and thickness as that of the third patterned metal layer 150L. The third patterned metal layer 150 comprises a first top electrode 152. The first top electrode 152 is overlapped with the first suspended portion 114 and may drive the first suspended portion 114. Specifically, the first top electrode 152 may comprise an excitation portion 152a, a pad portion 152c, and a connecting portion 152b. The excitation portion 152a is disposed overlapped with the first suspended portion 114 of the crystal plate 110. The pad portion 152c is disposed overlapped with the anchoring portion 112 of the crystal plate 110. The connecting portion 152b is continuously connected to both the excitation portion 152a and the pad portion 152c, such the excitation portion 152a on the first suspended portion 114 is electrically connected to the pad portion 152c on the anchoring portion 112 by the connecting portion 152b. In the embodiment shown in FIGS. 14A to 14C, the excitation portion 152a of the first top electrode 152 may have a rectangular shape. However, the present disclosure is not limited thereto. The substrate 130 may comprise any suitable material such as silicon, silicon oxide, quartz, or ceramics, e.g., low-temperature cofired ceramics (LTCC). In the present embodiment, the substrate 130 may comprise quartz crystal. As shown in FIGS. 14A to 14C, the substrate 130 has a first cavity Cl below a first surface 130S1 of the substrate 130. The first suspended portion 114 is overlapped with the first cavity Cl. In some embodiments, a depth D of the first cavity Cl, which is calculated by the depth that the first cavity Cl being recessed from the first surface 130S1 of the substrate 130, is in a range between 0.1 pm and 10 pm. The first cavity Cl may be formed of a sufficient depth to decrease the probability that the first suspended portion 114 being stuck to substrate 130 during manufacturing process, e.g., etching process(es) for defining the first suspended portion 114.

[0076] The second patterned metal layer 140 is disposed on the first surface 130S1 of the substrate 130. The second patterned metal layer 140 may comprise any suitable metal material and may have a multilayer structure. The second patterned metal layer 140 may have similar composition and thickness as that of the second patterned metal layer 140L. The second patterned metal layer 140 may comprise a connecting pad 142. In the present embodiment, the connecting pad 142 is disposed in a position corresponding to that of the pad portion 122c of the first bottom electrode 122.

[0077] The crystal oscillator 100A comprises a direct metal bonding interface MI extending between the first patterned metal layer 120 and the second patterned metal layer 140. Specifically, the first patterned metal layer 120 contacts the second patterned metal layer 140 to form a direct metal bonding interface MI. The direct metal bonding interface MI may be formed by metal to metal direct bonding process(es) during manufacturing of the crystal oscillator 100A, as discussed above. For example, wafer level metal to metal direct bonding process(es) may be performed to the first patterned metal layer and the second patterned metal layer to form the direct metal bonding interface MI. In some embodiments, the atoms on the surfaces of the first patterned metal layer 120 and the second patterned metal layer 140 may inter-diffuse, so that metal bonds may be formed at the direct metal bonding interface MI. The direct metal bonding interface MI may extend between the pad portion 122c of the first bottom electrode 122 and the connecting pad 142. As such, the pad portion 122c of the first bottom electrode 122 is in direct contact with and secured to the connecting pad 142, and the first bottom electrode 122 is electrically connected to the connecting pad 142.

[0078] In some embodiments, the first patterned metal layer 120 may further comprise at least one first dummy pad 124, such as first dummy pads 124a, 124b, 124c, 124d shown in FIGS. 14A to 14C. The top surfaces of the first dummy pads 124a, 124b, 124c, 124d may be substantially level with the top surface of the first bottom electrode 122. In some embodiments, the second patterned metal layer 140 may further comprise at least one second dummy pad 144, such as second dummy pads 144a, 144b, 144c, 144d shown in FIGS. 14A to 14C. The top surfaces of the second dummy pads 144a, 144b, 144c, 144d may be substantially level with the top surface of the connecting pad 142. As shown in FIGS. 14A to 14C, the direct metal bonding interface MI may also extend between the first dummy pad 124a and the respective second dummy pad 144a, between the first dummy pad 124b and the respective second dummy pad 144b, between the first dummy pad 124c and the respective second dummy pad 144c, and between the first dummy pad 124d and the respective second dummy pad 144d.

[0079] In the embodiment shown in FIGS. 14A to 14C, the first dummy pads 124a, 124b, 124c, 124d collectively surround the bottom electrodes 122 with at least a gap between adjacent first dummy pads (for example, between the first dummy pad 124c and the first dummy pad 124d or between the first dummy pad 124b and the first dummy pad 124c), and the second dummy pads 144a, 144b, 144c, 144d collectively surround the first cavity Cl with at least a gap between adjacent second dummy pads (for example, between the second dummy pad 144c and the second dummy pad 144d or between the second dummy pad 144b and the second dummy pad 144c). A width of one of the gaps (for example, a width W of the gap between the second dummy pad 144b and the second dummy pad 144c) may be larger than 60 pm, such that the “stagnant” etchant in the first cavities Cl during the manufacturing of the crystal oscillator 100A, as discussed above, may be drained through the gaps between adjacent dummy pads.

[0080] In the embodiment shown in FIGS. 14A to 14C, the crystal oscillator 100A further comprises a first via VI extending through the crystal plate 110 and electrically connected to the first top electrode 152. The first patterned metal layer 120 may further comprise a connecting pad 125 electrically connected to the first via VI. As shown in FIGS. 14A to 14C, the second patterned metal layer 140 may further comprise a connecting pad 145 disposed in a position corresponding to that of the connecting pad 125, and the direct metal bonding interface MI may also extend between the connecting pad 125 and the connecting pad 145, such that the connecting pad 125 is secured to and electrically connected with the connecting pad 145. As shown in FIGS. 14A to 14C, the crystal oscillator 100A may further comprises a fourth via V4 extending through the substrate 130 and electrically connected to the connecting pad 145. As such, the first top electrode 152 can be electrically connected to the fourth via V4 through the first via VI and the connecting pads 125 and 145.

[0081] In the embodiment shown in FIGS. 14A to 14C, the crystal oscillator 100A further comprises a second via V2 extending through the substrate 130 and electrically connected to the connecting pad 142. As such, the first bottom electrode 122 can be electrically connected to the second via V2 through the connecting pad 142. With this arrangement, the crystal oscillator 100A can be mounted on a circuit substrate and can and be electrically connected to a circuit, for example, through the second via V2 and the fourth via V4.

[0082] By methods disclosed herein, the crystal oscillator 100A may be fabricated with a thinner and / or shorter strip, such that higher frequencies and / or higher vibration resistance may be achieved. In some embodiments, the crystal oscillator 100A may have a fundamental frequency larger than 50 MHz. Generally, a crystal oscillator is manufactured by mounting an individual vibrating element onto a respective substrate, for example, through conductive adhesive. However, the manufacturing process may suffer from handling difficulties, especially when the thickness of a vibrating element decreases. By methods disclosed herein, manufacturing costs of the crystal oscillators can be reduced and / or manufacturing yield can be improved.FIGS. 15 to 20 are schematic diagrams illustrating intermediate stages in the manufacture of a plurality of crystal oscillators according to one embodiment of the present disclosure. The manufacturing process shown in FIGS. 15 to 20 may be substantially similar to the process shown in FIGS. 1 to 14C, where like reference numerals indicate like elements.

[0083] As shown in FIG. 15, a first wafer 10 is provided (step (a)), and a first patterned metal layer 120L is formed on a first surface 10S1 of the first wafer 10 (step (b)).

[0084] As shown in FIG. 16, a second wafer 20 is provided (step (c)), and a second patterned metal layer 140L is formed on a first surface 20S1 of the second wafer 20 (step (d)). Then, a plurality of first cavities Cl in the second wafer 20 are defined (step (e)).

[0085] As shown in FIG. 17, the first wafer 10 and the second wafer 20 are bonded to form a bonded structure 30 by bonding the first patterned metal layer 120L and the second patterned metal layer 140L through direct metal bonding (step (f)). Then, the first wafer 10 is thinned from the second surface 10S2 of the first wafer 10 (step (g)).

[0086] As shown in FIG. 18, a third patterned metal layer 150L is formed on an exposed surface 10S2’ of the thinned first wafer 10’ (step (j)) after thinning the first wafer 10.

[0087] As shown in FIG. 19, the thinned first wafer 10’ is patterned to define a plurality of first suspended portions 114 (step (h)). First trenches TRI similar to that described above with respect to FIG. 13 may be formed. In the present embodiment, second trenches TR2a and TR2b extending through the thinned first wafer 10’ may also be formed by the same or separate photolithography and etching processes. The connecting pads 142 of the second patterned metal layer 140L may be exposed by the second trenches TR2a, and the connecting pads 145 of the second patterned metal layer 140L may be exposed by the second trenches TR2b. As shown in FIG. 20, a conductive adhesive 157 is applied to and filled at least a portion of the second trenches TR2a, and the bonded structure is diced into a plurality of crystal oscillators including a crystal oscillator 100B (step (i)).

[0088] Referring to FIGS. 15 to 20, the crystal oscillator 100B may be substantially similar to the crystal oscillator 100A described above with respect to FIGS. 14A to 14C, where like reference numerals indicate like elements, except that in the present embodiment, the crystal oscillator 100B may not include through vias (e.g., the vias VI, V2, V3, and V4 shown in FIG. 14A to 14C), and the bottom electrode 122 may be electrically connected to a connecting pad 155 of the third patterned metal layer 150L through conductive adhesive 157 and probably through the connecting pad 142.

[0089] FIGS. 21A to 21C are schematic views to illustrate an embodiment of a crystal oscillator according to the present disclosure. Specifically, FIG. 21A is a perspective view of a crystal oscillator 100C, FIG. 21B is a cross-sectional view of the crystal oscillator 100C along line B-B’ in FIG. 21A, and FIG. 21C is a cross-sectional view of the crystal oscillator 100C along line C-C’ in FIG. 21A.

[0090] Referring to FIGS. 21A to 21C, the crystal oscillator 100C may be substantially similar to the crystal oscillator 100A described above with respect to FIGS. 14A to 14C, where like reference numerals indicate like elements, except that the crystal plate 110’ has a through hole TH1 extending through the first suspended portion 114’. In some embodiments, the top view of the through hole TH1 may be rectangular shaped. However, the present disclosure is not limited thereto. With such configuration, Q factor of the crystal oscillator 100C may be further increased, for example, by about 2 to 3 times.

[0091] The crystal oscillator 100C may be formed by processes similar to that described above with respect to FIGS. 1 to 14C and / or FIGS. 15 to 20, wherein patterning the thinned first wafer 10’ (the step (h)) may further comprise defining a through hole TH1 extending through at least one of the first suspended portions 114’.

[0092] FIG. 22 is a schematic view to illustrate an embodiment of a crystal oscillator according to the present disclosure.

[0093] Referring to FIG. 22, a crystal oscillator 100D is provided. The crystal oscillator 100D may be substantially similar to the crystal oscillator 100A described above with respect to FIGS. 14A to 14C, where like reference numerals indicate like elements, except that the crystal plate 110 further comprises a second suspended portion 116 extending from the anchoring portion 112. In the present embodiment, the second suspended portion 116 is overlapped with the first cavity Cl. As shown in FIG. 22, the crystal oscillator 100D may further comprise a cap 160 over and overlapped with both the first suspended portion 114 and the second suspended portion 116.

[0094] The first patterned metal layer 120 may further comprise a second bottom electrode 126 overlapped with the second suspended portion 116, and the third patterned metal layer 150 may further comprise a second top electrode 156 overlapped with the second suspended portion 116. In the present embodiment, the first top electrode 152 is electrically coupled to the second bottom electrode 126, and the second top electrode 156 is electrically coupled to the first bottom electrode 122. With such arrangement, the crystal oscillator 100D may achieve improved vibration resistance and / or lower temperature drift.

[0095] The crystal oscillator 100D may be formed by processes similar to that described above with respect to FIGS. 1 to 14C and / or FIGS. 15 to 20, wherein patterning the thinned first wafer 10’ (the step (h)) may further comprise pattering the thinned first wafer 10’ to define a plurality of second suspended portions 116.

[0096] FIG. 23 is a schematic view to illustrate an embodiment of a crystal oscillator according to the present disclosure.

[0097] Referring to FIG. 23, a crystal oscillator 100E is provided. The crystal oscillator 100E may be substantially similar to the crystal oscillator 100D described above with respect to FIG. 22, where like reference numerals indicate like elements, except that the first top electrode 152 and the first bottom electrode 122 are electrically coupled to a first functional circuit CR1, and the second top electrode 156 and the second bottom electrode 126 are electrically coupled to a second functional circuit CR2. In one embodiment, the second functional circuit CR2 may be a sensor circuit configured to derive a data, such as temperature data, gas data, inertial data, etc., from the vibration of the second suspended portion 116, and the first functional circuit CR1 is configured to be fed with and function with reference to the data. However, the present disclosure is not limited thereto.

[0098] FIGS. 24A to 24C are schematic views to illustrate an embodiment of a crystal oscillator according to the present disclosure. FIG. 24A is a perspective view of the crystal oscillator 100F, FIG. 24B is a transparent view of the crystal oscillator 100F, and FIG. 24C is a cross-sectional view of the crystal oscillator 100F along line D-D’ in FIG. 24B.

[0099] Referring to FIGS. 24A to 24C, a crystal oscillator 100F is provided. The crystal oscillator 100F may be substantially similar to the crystal oscillator 100A described above with respect to FIGS. 14A to 14C, where like reference numerals indicate like elements. In the present embodiment, the crystal plate 110 further comprises a second suspended portion 116 extending from the anchoring portion 112. In the present embodiment, the anchoring portion 112 is a portion of the crystal plate 110 secured to the substratel30 through metal bonding between the connecting pads 125a and 125b and respective connecting pads 145a and 145b and between the bottom electrodes 122 and 126 and respective connecting pads 142a and 142b, and each of the first suspended portion 114 and the second suspended portion 116 is a crystal strip extending from the anchoring portion 112. The crystal oscillator further comprises a cap 160 over and overlapped with both the first suspended portion 114 and the second suspended portion 116.

[0100] As shown in FIGS. 24A to 24C, the substrate 130 has a second cavity C2 below the first surface 130S1 of the substrate 130. The second suspended portion 116 is overlapped with the second cavity C2, while the first suspended portion 114 is overlapped with the first cavity Cl.

[0101] The first patterned metal layer 120 may further comprise a second bottom electrode 126 overlapped with the second suspended portion 116. The third patterned metal layer 150 may further comprise a second top electrode 156 overlapped with the second suspended portion 116.

[0102] FIGS. 25 A to 25C are schematic views to illustrate embodiments of crystal oscillators according to the present disclosure.

[0103] Referring to FIG. 25 A, a crystal oscillator 100G is provided. The crystal oscillator 100G may be substantially similar to the crystal oscillator 100F described above with respect to FIGS. 24A to 24C, where like reference numerals indicate like elements. In the present embodiment, the first top electrode 152 is electrically coupled to the second top electrode 156, and the first bottom electrode 122 is electrically coupled to the second bottom electrode 126. With such arrangement, the crystal oscillator 100G may achieve improved vibration resistance, for example, due to a shorter the first suspended portion 114 and / or the second suspended portion 116.

[0104] Referring to FIG. 25B, a crystal oscillator 100H is provided. The crystal oscillator 100H may be substantially similar to the crystal oscillator 100F described above with respect to FIGS. 24A to 24C, where like reference numerals indicate like elements. In the present embodiment, the first top electrode 152 is electrically coupled to the second bottom electrode 126, and the second top electrode 156 is electrically coupled to the first bottom electrode 122. With such arrangement, the crystal oscillator 100F may achieve improved vibration resistance and / or lower temperature drift.

[0105] Referring to FIG. 25C, a crystal oscillator 1001 is provided. The crystal oscillator 1001 may be substantially similar to the crystal oscillator 100F described above with respect to FIGS. 24A to 24C, where like reference numerals indicate like elements. In the present embodiment, the first top electrode 152 and the first bottom electrode 122 are electrically coupled to a first functional circuit CR1, and the second top electrode 156 and the second bottom electrode 126 are electrically coupled to a second functional circuit CR2. In one embodiment, the second functional circuit CR2 may be a sensor circuit configured to derive a data, such as temperature data, gas data, inertial data, etc., from the vibration of the second suspended portion 116, and the first functional circuit CR1 is configured to be fed with and function with reference to the data. However, the present disclosure is not limited thereto.

[0106] The foregoing description of embodiments is provided to enable any person skilled in the art to make and use the subject matter. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the novel principles and subject matter disclosed herein may be applied to other embodiments without the use of the innovative faculty. The claimed subject matter set forth in the claims is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein. It is contemplated that additional embodiments are within the spirit and true scope of the disclosed subject matter. Thus, it is intended that the present invention covers modifications and variations that come within the scope of the appended claims and their equivalents.

Claims

WHAT IS CLAIMED IS:

1. A method for manufacturing a plurality of crystal oscillators, comprising:(a) providing a first wafer comprising piezoelectric crystal;(b) forming a first patterned metal layer on a first surface of the first wafer, wherein the first patterned metal layer comprises a plurality of bottom electrodes;(c) providing a second wafer;(d) forming a second patterned metal layer on a first surface of the second wafer, wherein the second patterned metal layer comprises a plurality of connecting pads;(e) defining a plurality of first cavities in the second wafer below the first surface of the second wafer;(f) bonding the first wafer and the second wafer to form a bonded structure by bonding the first patterned metal layer and the second patterned metal layer through direct metal bonding;(g) thinning the first wafer from a second surface opposite the first surface of the first wafer;(h) patterning the thinned first wafer to define a plurality of first suspended portions; and(i) dicing the bonded structure into a plurality of crystal oscillators.

2. The method of claim 1 further comprising (j) forming a third patterned metal layer on an exposed surface of the thinned first wafer after the step (g).

3. The method of claim 1 further comprising (kl) forming first vias extending through the first wafer.

4. The method of claim 1 further comprising (k2) forming second vias extending through the second wafer.

5. The method of claim 1, wherein the step (f) comprises bonding each of the bottom electrodes to a respective connecting pad of the connecting pads through direct metal bonding.

6. The method of claim 1 , wherein the first patterned metal layer further comprises a plurality of first dummy pads, the second patterned metal layer further comprises a plurality of second dummy pads, and the step (f) comprises bonding each of the first dummy pads to a respective second dummy pads of the second dummy pads through direct metal bonding.

7. The method of claim 1, wherein the step (e) comprises defining the plurality of first cavities each having a depth in a range between 0.1 pm and 10 pm.

8. The method of claim 1, wherein the step (e) comprises etching the second wafer from the first surface to define the plurality of first cavities.

9. The method of claim 1, wherein the step (h) comprises defining a through hole extending through at least one of the first suspended portions.

10. The method of claim 1, wherein the step (h) comprises pattering the thinned first wafer to define a plurality of second suspended portions.

11. The method of claim 1, wherein the first wafer comprises quartz crystal.

12. A crystal oscillator, comprising: a piezoelectric crystal plate comprising an anchoring portion and a first suspended portion connecting with the anchoring portion; a first patterned metal layer disposed on a first surface of the crystal plate, the first patterned metal layer comprising a first bottom electrode overlapped with the first suspended portion; a substrate having a first cavity below a first surface of the substrate, wherein the first suspended portion is overlapped with the first cavity; a second patterned metal layer disposed on the first surface of the substrate, the second patterned metal layer comprising a connecting pad; a third patterned metal layer disposed on a second surface opposite the first surface of the crystal plate, the third patterned metal layer comprising a first top electrode overlapped with the first suspended portion; and a direct metal bonding interface extending between the first patterned metal layer and the second patterned metal layer.

13. The crystal oscillator of claim 12, wherein the first bottom electrode comprises an excitation portion overlapped with the first suspended portion, a pad portion overlapped with the anchoring portion, and a connecting portion continuously connected to both the excitation portion and the pad portion.

14. The crystal oscillator of claim 13, wherein the direct metal bonding interface extends between the connecting pad and the pad portion of the first bottom electrode.

15. The crystal oscillator of claim 12 further comprising a first via extending through the crystal plate and electrically connected to the first top electrode.

16. The crystal oscillator of claim 12 further comprising a second via extending through the substrate and electrically connected to the connecting pad.

17. The crystal oscillator of claim 12, wherein the first patterned metal layer further comprises at least one first dummy pad, the second patterned metal layer further comprises at least one second dummy pad, and the direct metal bonding interface extends between the first dummy pad and the second dummy pad.

18. The crystal oscillator of claim 12, wherein the first suspended portion has a substantially rectangular- shaped configuration.

19. The crystal oscillator of claim 12, wherein a length of the first suspended portion is in a range between 10 pm and 500 pm.

20. The crystal oscillator of claim 12, wherein a depth of the first cavity is in a range between 0.1 pm and 10 pm.

21. The crystal oscillator of claim 12, wherein the crystal plate has a through hole extending through the first suspended portion.

22. The crystal oscillator of claim 12, wherein the crystal plate further comprises a second suspended portion extending from the anchoring portion.

23. The crystal oscillator of claim 22, wherein the substrate has a second cavity below the first surface of the substrate, and the second suspended portion is overlapped with the second cavity.

24. The crystal oscillator of claim 22 further comprising a cap over and overlapped with both the first suspended portion and the second suspended portion.

25. The crystal oscillator of claim 22, wherein the second suspended portion is overlapped with the first cavity.

26. The crystal oscillator of claim 22, wherein the first patterned metal layer further comprises a second bottom electrode overlapped with the second suspended portion, and the third patterned metal layer further comprises a second top electrode overlapped with the second suspended portion.

27. The crystal oscillator of claim 26, wherein the first top electrode is electrically coupled to the second bottom electrode, and the second top electrode is electrically coupled to the first bottom electrode.

28. The crystal oscillator of claim 26, wherein the first top electrode is electrically coupled to the second top electrode, and the first bottom electrode is electrically coupled to the second bottom electrode.

29. The crystal oscillator of claim 26, wherein the first top electrode and the first bottom electrode are electrically coupled to a first functional circuit, and the second top electrode and the second bottom electrode are electrically coupled to a second functional circuit.

30. The crystal oscillator of claim 12, wherein the crystal oscillator is a strip resonator.

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