Patent Publication Number: US-2021184645-A1

Title: Packaging module and packaging method of baw resonator

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation application of PCT Patent Application No. PCT/CN2019/109826, filed on Oct. 4, 2019, which claims priority to Chinese patent application No. 201910656689.6, filed on Jul. 19, 2019, the entirety of all of which is incorporated herein by reference. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure generally relates to the field of radio frequency product packaging technology and, more particularly, relates to a packaging module and packaging method of a bulk acoustic wave (BAW) resonator. 
     BACKGROUND 
     With continuous development of wireless communication technology, to meet multifunctional needs of various wireless communication terminals, terminal device needs to be able to use different carrier frequency spectrums to transmit data. Meanwhile, to support sufficient: data transmission rates within a limited bandwidth, strict performance requirements are raised for a radio frequency system. A radio frequency filter is an important part of the radio frequency system, which can filter out interference and noise outside a communication spectrum to meet requirements of the radio frequency system and the communication protocol for a signal-to-noise ratio. Take a mobile phone as an example, since each frequency band needs a corresponding filter, one mobile phone may need to include dozens of filters. 
     Surface acoustic wave (SAW) filters and bulk acoustic wave (BAW) filters are two most mainstream implementations of the radio frequency filter. An SAW filter has a simple structure. Compared with conventional cavity filter and ceramic filter, the SAW filter has a small size, and combines low insertion loss and good suppression performance. The SAW filter is widely used in 2G and 3G receiver front ends, duplexers and receivers, and can satisfy frequency bands including GSM, CDMA, 3G and a portion of 4G. But an operating frequency of the SAW filter is inversely proportional to an electrode linewidth. When the frequency is greater than 1 GHz, selectivity of this filter decreases, and when the operating frequency is above 2.4 GHz, the electrode line width is less than 0.5 μm, which leads to large high-frequency insertion loss, and degradation of quality factor and power tolerance. The SAW filter is also very sensitive to temperature (temperature rise will cause a drop of frequency response up to 4 MHz). Thus, application is extremely limited. In comparison, because sound wave in a BAW filter propagates vertically, its frequency is inversely proportional to a thickness of a film, that is, a. size can be reduced as the frequency rises, and communication requirements within 20 GHz can be met theoretically. Furthermore, the BAW filter has advantages of lower insertion loss, high quality factor (Q value above 1000), low sensitivity to temperature changes, and integration on IC chips. With popularization of 4G and gradual development of 5G, the BAW filter has become a preferred filter for solving many interference problems. 
     A BAW resonator is a basic component of the BAW filter. By cascading different BAW resonators, the BAW filter that meets different performance requirements can be made. The BAW resonator usually includes a substrate, a cavity formed on the substrate, and a resonant structure on the cavity. The resonant structure includes a lower electrode, a piezoelectric film, and an upper electrode. The resonant structure is used to convert electrical signals to elastic waves through physical vibration. 
     In a BAW resonator, a resonant structure including a piezoelectric layer is suspended on a cavity on a support substrate. During packaging, a circle of dry film is coated on a cap, the cap is aligned with a position of the cavity and bonded to the resonant structure, and then a through-silicon via (TSV) process is performed to electrically lead out upper and lower electrodes of the resonant structure to the cap. However, this method needs to etch through holes in the thick dry film, and process control difficulty is large. After the through holes are etched, conductive materials need to be deposited on the dry film. Due to poor stability of the dry film itself, the formed conductive plugs have poor quality and electrical contact is not stable. 
     BRIEF SUMMARY OF THE DISCLOSURE 
     One aspect of the present disclosure provides a packaging method of a bulk acoustic wave (BAW) resonator, including: providing a BAW resonant device, that the BAW resonant device includes a first substrate and a resonant structure disposed on the first substrate, and a first gap is formed between the resonant structure and the first substrate; bonding the BAW resonant device to a second substrate at a side of the resonant structure away from the first substrate through a bonding layer, that a second gap is provided between the resonant structure and the second substrate and is substantially surrounded by the bonding layer, and the second gap is at least partially aligned with the first gap; forming through holes that pass through the first substrate and expose corresponding electrical connection portions of the resonant structure around the first gap; and forming a conductive interconnection layer on inner surfaces of the through holes and on a portion of a surface of the first substrate at a periphery of the through holes. 
     Another aspect of the present disclosure provides a packaging module of a bulk acoustic wave (BAW) resonator, including: a second substrate and a bonding layer, that the bonding layer is formed on a portion of the second substrate; a BAW resonant device, that the BAW resonant device is bonded to the bonding layer, the BAW resonant device includes a first substrate and a resonant structure provided on a surface of the first substrate facing toward the second substrate, a first gap is formed between the resonant structure and the first substrate, the bonding layer is sandwiched between the second substrate and the resonant structure, a second gap is provided between the second substrate and the resonant structure and is substantially surrounded by the bonding layer, the second gap is at least partially aligned with the first gap, the BAW resonant device is also provided with through holes, and the through holes pass through the first substrate on a periphery of the first gap and expose corresponding electrical connection portions of the resonant structure; and a conductive interconnection layer, that the conductive interconnection layer is formed on inner surfaces of the through holes and on a portion of a surface of the first substrate at a periphery of the through holes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following drawings are merely examples for illustrative purposes according to various disclosed embodiments and are not intended to limit the scope of the present disclosure. 
         FIG. 1  illustrates a schematic flowchart of an exemplary method for fabricating an exemplary BAW resonator according to some embodiments of the present disclosure. 
         FIG. 2  illustrates a schematic cross-sectional view of an exemplary BAW resonant device according to some embodiments of the present disclosure. 
         FIG. 3  illustrates a schematic cross-sectional view of an exemplary BAW resonant device according to some other embodiments of the present disclosure. 
         FIG. 4  illustrates a schematic cross-sectional view of an exemplary second substrate according to some embodiments of the present disclosure. 
         FIG. 5  illustrates a schematic cross-sectional view after bonding an exemplary second substrate by using an exemplary packaging method of a BAW resonator according to some embodiments of the present disclosure. 
         FIG. 6  illustrates a schematic cross-sectional view of an exemplary conductive interconnection layer formed by using an exemplary packaging method of a BAW resonator according to some embodiments of the present disclosure. 
         FIG. 7  illustrates a schematic cross-sectional view of exemplary contact pads formed by using an exemplary packaging method of a BAW resonator according to some embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The packaging module and packaging method of a BAW resonator and a BAW filter of the present disclosure will be described in further detail below with reference to the accompanying drawings and exemplary embodiments. According to the following description, advantages and features of the present disclosure will be clearer. It should be noted that the drawings are in a very simplified form and all use imprecise proportions, which are only used to conveniently and clearly assist in explaining the embodiments of the present disclosure. The embodiments of the present disclosure should not be considered as limited to those alternative shapes of areas shown in the drawings. For the sake of clarity, in all the drawings used to assist in describing the embodiments of the present disclosure, same components are marked with same reference numerals in principle, and repeated descriptions thereof are omitted. 
     It should be noted that the terms “first”, “second”, etc., hereinafter are used to distinguish between similar elements, and are not necessarily used to describe a specific order or time sequence. It is to be understood that, under appropriate circumstances, these terms so used can be replaced, for example, to enable the embodiments of the present disclosure described herein to be operated in other sequences than described or shown herein. Similarly, if the method described herein includes a series of steps, and an order of these steps presented herein is not necessarily the only order in which these steps can be performed, and some of the described steps may be omitted and/or some other steps that are not described herein can be added to the method. 
       FIG. 1  illustrates a schematic flowchart of an exemplary method for fabricating an exemplary BAW resonator according to some embodiments of the present disclosure. Referring to  FIG. 1 , a packaging method of a BAW resonator according to some embodiments of the present disclosure includes the following steps: 
     S 1 : providing a BAW resonant device, that the BAW resonant device includes a first substrate and a resonant structure disposed on the first substrate, and a first gap is formed between the resonant structure and the first substrate; 
     S 2 : bonding the BAW resonant device with a second substrate at one side of the resonant structure through a bonding layer, that a second gap substantially surrounded by the bonding layer is provided between the resonant structure and the second substrate, and the second gap and the first gap are at least partially aligned with each other; 
     S 3 : forming through holes that pass through the first substrate and expose corresponding electrical connection portions of the resonant structure around the first gap; 
     S 4 : forming a conductive interconnection layer on inner surfaces of the through holes and on a portion of a surface of the first substrate at a periphery of the through holes; and 
     S 5 : forming a passivation layer, that the passivation layer fills the through holes and exposes a portion of the conductive interconnection layer on a surface of a resonator cover at the periphery of the through holes, and the exposed conductive interconnection layer forms contact pads. 
     The packaging method of the BAW resonator according to the embodiments of the present disclosure will be further described below in conjunction with schematic cross-sectional views during implementation processes. 
       FIG. 2  illustrates a schematic cross-sectional view of an exemplary BAW resonant device according to some embodiments of the present disclosure.  FIG. 3  illustrates a schematic cross-sectional view of an exemplary BAW resonant device according to some other embodiments of the present disclosure. Referring to  FIG. 2  or  FIG. 3 , in step S 1 , a BAW resonant device  100  is provided. The BAW resonant device  100  includes a first substrate  110  and a resonant structure  120  disposed on the first substrate  110 . A first gap  10  is formed between the resonant structure  120  and the first substrate  110 . 
     Alternatively, the resonant structure  120  includes a first electrode  121  close to the first substrate  110 , a piezoelectric layer  122  on the first electrode  121 , and a second electrode  123  on the piezoelectric layer  122 . The first electrode  121  may be used as an input (or output) electrode that receives (or provides) electrical signals such as radio frequency (RF) signals. For example, when the second electrode  123  is used as the input electrode, the first electrode  121  may be used as the output electrode, and when the second electrode  123  is used as the output electrode, the first electrode  121  may be used as the input electrode. The piezoelectric layer  122  converts the electrical signals input through the first electrode  121  or the second electrode  123  into bulk acoustic waves. For example, the piezoelectric layer  122  converts the electrical signals into the bulk acoustic waves through physical vibration. 
     Referring to  FIG. 2 , in some embodiments, the BAW resonant device  100  includes a support layer  130  disposed on the first substrate  110 , the first gap  10  is substantially defined or enclosed by the support layer  130 , and the resonant structure  120  (here, alternatively, is a layer where the first electrode  121  of the resonant structure  120  is located) is partially in contact with the support layer  130  around the first gap  10 . But not limited to this, referring to  FIG. 3 , in some other embodiments of the present disclosure, in the BAW resonant device, a groove is formed in the first substrate  110 , and the groove defines the first gap  10  between the resonant structure  120  and the first substrate  110 . The resonant structure  120  (here alternatively refers to the layer where the first electrode  121  of the resonant structure  120  is located) and the first substrate  110  around the groove overlap each other. 
     Alternatively, the first substrate  110  may be made of any suitable substrate known to those skilled in the art. For example, the first substrate  110  may be made of at least one of the following materials: silicon (Si), germanium (Ge), germanium silicon (SiGe), carbon silicon 
     (SiC), silicon germanium (SiGeC), indium arsenide (InAs), gallium arsenide (GaAs), indium phosphide (InP) or other III/V compound semiconductors; or multi-layer structures formed by these semiconductors, such as silicon-on-insulator (SOI), silicon-on-insulator (SSOI), silicon-germanium-on-insulator (S—SiGeOI), silicon germanium-on-insulator (SiGeOI), and germanium on insulator (GeOI)); or Double Side Polished Wafers (DSP); or ceramic substrate, quartz or glass substrate, etc., such as alumina, etc. In some embodiments, the first substrate  110  is made of a P-type high-resistance single crystal silicon wafer with a &lt;100&gt; crystal orientation. 
     As an example, fabrication of the BAW resonant device may include the following processes. First, a piezoelectric stack film layer is fabricated on a preparation substrate, that the piezoelectric stack film layer includes a first electrode layer, a piezoelectric layer, and a second electrode layer, stacked in sequence. Before the piezoelectric stack film layer is fabricated, an isolation layer with a thickness of about  1  um can be formed on the preparation substrate, and the isolation layer can be used as a barrier material for removing the preparation substrate. Then, the support layer  130  is formed on the piezoelectric stack film layer, and the first gap is formed in the support layer  130  through a photomask and etching process. Next, the first substrate  110  is bonded on the support layer  130  to transfer the piezoelectric stack film layer to the first substrate  110 , and a backside etching process is used to remove the preparation substrate. Then, the photomask and etching process is used to process the piezoelectric stack film to form the resonant structure  120 , and the support layer  130  encloses the first gap  10  between the resonant structure  120  and the first substrate  110 , as shown in  FIG. 2 . 
     The fabrication of the BAW resonant device  100  of the present disclosure is not limited to the above method. For example, in some other embodiments, the first gap  10  may not be formed by the support layer, but a sacrificial layer (not shown) may be used to directly form the first gap  10  and the resonant structure  120  on the first substrate  110 , to obtain the BAW resonant device  100 . Referring to  FIG. 3 , alternative processes are included below. 
     First, a portion of a thickness in a partial area of the first substrate  110  is removed by etching to form a groove (not shown) for forming the first gap. Here, the first substrate  110  may include a substrate base (not shown), and at least one thin film (not shown) covering the substrate base (not shown), or a bare chip made of a semiconductor material. 
     Then, a sacrificial layer (not shown) is filled in the groove, and a top surface of the sacrificial layer is flush with a top surface of the first substrate  110 , or may be higher than the top surface of the first substrate  110 , or slightly lower than the top surface of the first substrate  110 . The sacrificial layer may have a single-layer structure or a stacked-layer structure. 
     Afterwards, the top surfaces of the first substrate  110  and the sacrificial layer are covered with the piezoelectric stack film layer, that the piezoelectric stack film layer includes the first electrode layer, the piezoelectric layer, and the second electrode layer, stacked in sequence. Through exposure, development, and etching processes, the first electrode layer, the piezoelectric layer, and the second electrode layer or the second electrode layer, the piezoelectric layer, and the first electrode layer, are sequentially patterned, to define the first electrode  121 , the piezoelectric layer  122  on the first electrode  121 , and the second electrode  123  on the piezoelectric layer  122 , thereby forming the resonant structure  120 . 
     Then, a release hole (not shown) is opened in the resonant structure  120  near an edge in a region of the first gap  10 , and the sacrificial layer is removed by introducing an etchant into the release hole to re-empty the groove, thereby obtaining the first gap  10  between the resonant structure  120  and the first substrate  110 . The first gap  10  is a groove structure with an entire bottom recessed in the first substrate  110 . So far, the processes of providing the BAW resonant device  100  in step S 1  are completed. 
     In some other embodiments of the present disclosure, another method of using a sacrificial layer to directly form the first gap  10  and the resonant structure  120  on the first substrate  110  to obtain the BAW resonant device  100  may be provided. Alternative processes are included below. 
     First, a sacrificial layer (not shown) is fully covered on the first substrate  110 . The sacrificial layer may be a single-layer structure or a stacked-layer structure. 
     Then, exposure, development, and etching processes are performed to etch the sacrificial layer to pattern it to form a patterned sacrificial layer for forming the first gap  10 . 
     Next, top surfaces of the first substrate  110  and the sacrificial layer are sequentially covered with the piezoelectric stack film layer, that the piezoelectric stack film layer includes the first electrode layer, the piezoelectric layer, and the second electrode layer, stacked in sequence. Through exposure, development, and etching processes, the first electrode layer, the piezoelectric layer, and the second electrode layer are sequentially patterned or the second electrode layer, the piezoelectric layer, and the first electrode layer are sequentially patterned, to define the first electrode  121 , the piezoelectric layer  122  on the first electrode  121 , and the second electrode  123  on the piezoelectric layer  122 , to form the resonant structure  120 . 
     Then, a release hole (not shown) may be opened on an edge area of the resonant structure  120 , and the sacrificial layer may be removed by introducing an etchant into the release hole, thereby obtaining the first gap  10  between the resonant structure  120  and the first substrate  110 . The first gap  10  is projected on the first substrate  110 . So far, the processes of providing the BAW resonant device  100  in step S 1  are completed. 
     In addition, it should be noted that when the first substrate  110  is a wafer, providing the BAW resonant device  100  may not be an independent device, that is, a number of the resonant structure  120  on the first substrate  110  is not limited to one. When multiple resonant structures  120  are formed on the first substrate  110  at a same time, there is the first gap  10  between each resonant structure  120  and the first substrate  110 . Adjacent first gaps can be separated through corresponding support layers or a first substrate material, and adjacent resonant structures  120  may be disconnected from each other, or some of film layers may be connected to each other. 
     In addition, cross-sections of the resonant structure  120  are not limited to the structures shown in  FIG. 2  and  FIG. 3 , and can be alternatively designed according to alternative needs. A planar shape of a resonant part that performs resonant operation in the resonant structure  120  may be a rectangle, or may be a circle, an ellipse, or a polygon other than a rectangle, such as a pentagon, a hexagon, and the like. Furthermore, a planar shape of the first gap  10  may be a rectangle, or a circle, an ellipse, or a polygon other than a rectangle, such as a pentagon, a hexagon, and the like. 
     In some embodiments, taking the BAW resonant device  100  shown in  FIG. 2  as an example, the resonant structure  120  includes the first electrode  121  facing towards the first substrate  100 , the piezoelectric layer  122  on the first electrode  121 , and the second electrode  123  on the piezoelectric layer  122 . The first electrode  121 , the piezoelectric layer  122 , and the second electrode  123  are formed by a patterning process. The first electrode  121  and the second electrode  123  are exposed in a direction away from the first substrate  110 , and have a step difference. 
     Alternatively, the resonant structure  120  may be formed in a double trench (Air Trench) structure, such as a first trench  120   a  and a second trench  120   b  in  FIG. 2  and  FIG. 3 , that the first trench  120   a  penetrates the second electrode  123  and the piezoelectric layer  122 , and exposes the first electrode  121 ; the second trench  120   b  penetrates the first electrode  121  and the piezoelectric layer  122 , and exposes the second electrode  123 ; and the second trench  120   b  is connected to the first gap  10 . 
     The support layer  130  can be made of silicon oxide, silicon nitride, silicon oxynitride, aluminum oxide, aluminum nitride, titanium oxide, titanium nitride, amorphous carbon, etc. The support layer  130  can also be a stacked layer of two or more materials. For example, the support layer  130  may have a stacked structure of silicon oxide and silicon nitride, and the silicon nitride layer is in contact with the resonant structure  120 . The first electrode  121  and the second electrode  123  can be made of one or more of conductive materials such as molybdenum (Mo), tungsten (W), aluminum, copper, iridium (Ir), rubidium (Ru), and doped polysilicon. The piezoelectric layer  122  can be made of one or more of piezoelectric materials such as quartz, aluminum nitride (AlN), zinc oxide (ZnO), lead zirconate titanate (PZT), lithium niobium oxide (LiNbO 3 ), lithium tantalum oxide (LiTaO 3 ), etc. The piezoelectric layer  122  may also be doped with rare earth elements. In some embodiments, the first electrode  121  and the second electrode  123  are made of, for example, molybdenum, and the piezoelectric layer  122  is made of, for example, aluminum nitride. A thickness of the first electrode  121  and the second electrode  123  is approximately in a range of about 100 nm to about 200 nm. A thickness of the piezoelectric layer  122  is in a range of about 1 μm to about 3 Molybdenum can be deposited by a PVD (physical vapor deposition) process or a magnetron sputtering process, and aluminum nitride can be deposited by the PVD (physical vapor deposition) process or an MOCVD (metal organic chemical vapor deposition) process. 
     After the BAW resonant device  100  is formed, the resonant structure  120  needs to be covered at a side of the resonant structure  120  away from the first substrate  110 , so that both sides of the resonant structure  120  have vibration spaces. In some embodiments, in step S 2 , the second substrate is provided as a cap wafer, and the first substrate  110  is used as a carrier wafer.  FIG. 4  illustrates a schematic cross-sectional view of an exemplary second substrate according to some embodiments of the present disclosure. Referring to  FIG. 4 , the second substrate  200  may use a substrate material similar to that of the first substrate  110 . Moreover, to better cover the BAW resonant device  100  and provide sufficient vibration space for the resonant structure  120 , a bonding layer material with a certain thickness is formed on the second substrate  200 , and the bonding layer material is patterned, to form a bonding layer  210  having a second gap  20 . 
     The second substrate  200  may be bonded to the first substrate  110  by vacuum bonding or bonding in a vacuum environment, so that the bonding layer  210  may be a conventional bonding material, such as silicon oxide, silicon nitride, silicon oxynitride, ethyl silicate, etc., and may also be an adhesive such as a light-curing material or a heat-curing material, such as an adhesive film (Die Attach Film, DAF) or a dry film (Dry Film). When the first substrate  110  and the second substrate  200  are bonded, the resonant structure  120  is formed on the first substrate  110 . A contact area between the second substrate  200  and the first substrate  110  includes both a surface of the support layer  130  and a surface of the resonant structure  120 , and there is a certain step difference between the two surfaces. For the resonant structure  120  shown in  FIG. 2  or  FIG. 3 , a side of the resonant structure  120  facing towards the second substrate has an opening, that the opening is located at a periphery of the first gap  10  and the second gap  20  and exposes at least a portion of a surface of corresponding electrical connection portions of the resonant structure  120  facing towards the second substrate. In other words, the first electrode  121  and the second electrode  123  have a step difference at the opening that exposes the electrical connection portions of the resonant structure  120 . Therefore, to form a better seal between the first substrate  110  and the second substrate  200  and tolerate the step difference of the resonant structure  120  on the periphery of the first gap  10 , a material selected for the bonding layer  210  needs to be able to: be patterned, be cured under certain conditions, stably adhere to upper and lower materials, and be elastic (be tolerant of a material with certain deformation or a material with less hardness), so as to tolerate a certain step height difference when the second substrate  200  and the resonant structure  120  are subsequently bonded. The material of the bonding layer  210  is, for example, a light-curing material, a heat-curing material, or a combination of the light-curing material and the heat-curing material, which can lose its elasticity through light, and cooling after heating. In some embodiments, the bonding layer  210  is, for example, a dry film. A thickness of the dry film is about 10 μm to about 20 μm, and its formation method can adopt a process of “sticking dry film-exposure developing-etching-demolding” or a process of “substrate cleaning treatment-sticking dry film-exposing developing-etching-demolding” for formation. Process conditions of “sticking dry film” include: a process temperature of about 80° C. to about 120° C. (for example, about 110° C.), and a process environment of being vacuum. Process conditions of “exposure and development” include: a UV exposure under vacuum conditions, and standing for a while after exposure, that a radiation dose of the UV exposure is, for example, about 200 mJ/cm 2 to about 300 mJ/cm 2 . The exposed bonding layer  210  is pre-baked at a temperature of about 100° C. to about 150° C. (for example, about 130° C.) for about 100 seconds to about 300 seconds (for example, about 200 seconds). At a room temperature, a developer is spin-sprayed on the pre-baked bonding layer  210  many times (for example, 3 times) to develop the pre-baked bonding layer  210 , that the developer is PGMEA, and its components include propylene-glycol-methyl-ether-acetate, and a molecular formula of propylene-glycol-methyl-ether-acetate is C 6 H 12 O 3 . The dry film formed on the second substrate  200  is, for example, a ring structure, and a range defined by the dry film may be at least partially aligned with a defined range of the support layer in the BAW resonant device  100  (that is, the second gap formed subsequently and the first gap are at least partially aligned with each other), to form a cavity defining the resonant structure  120  between the first substrate  110  and the second substrate  200  after bonding. 
       FIG. 5  illustrates a schematic cross-sectional view after bonding an exemplary second substrate by using an exemplary packaging method of a BAW resonator according to some embodiments of the present disclosure. Referring to  FIG. 5 , bonding operations in step S 2  of the packaging method of the BAW resonator of some embodiments can be performed. Alternatively, the BAW resonant device  100  is bonded to the second substrate  200  at the side of the resonant structure  120  through the bonding layer  210 . The second gap  20  substantially surrounded by the bonding layer  210  is provided between the resonant structure  120  and the second substrate  200 , and the second gap  20  surrounded by the bonding layer  210  and the first gap  10  are at least partially aligned with each other. The first gap  10  and the second gap  20  may not be connected, or may be connected. When the first gap  10  and the second gap  20  are connected, they form a cavity  30  between the first substrate  110  and the second substrate  200 , and a portion of the resonant structure  120  used for vibration is confined in the cavity  30 . Process conditions of bonding the second substrate  200  and the BAW resonant device  100  include: a process pressure of about 1 Pa to about 10 5  Pa, bonding in a vacuum environment, a temperature of about 150° C. to about 200° C. (for example, about 150° C.), and a pressure time of about 20 min to about 30 min. Therefore, under a premise of ensuring bonding performance, resonance performance of the product is prevented from being affected. After the bonding is completed, the bonding layer  210  is solidified by light, and cooling after heating, that is, the bonding layer  210  loses its elasticity, so that the second substrate  200  and the BAW resonant device  100  are reliably bonded together. A process of curing the bonding layer  210  can be a high-temperature curing process, where a curing temperature is about 180° C. to about 220° C. (for example, about 190° C.), and a curing time is about 1.5 hours to about 2 hours (for example, about 2 hours). In some other embodiments of the present disclosure, the process of curing the bonding layer  210  can also be a UV curing process, where a radiation dose of the UV curing can be selected from about 200 mJ/cm 2  to about 300 mJ/cm 2 , which is the same as the light used by the UV exposure process to expose the bonding layer  210 , to simplify the process and reduce costs. 
     Alternatively, after the first substrate  110  and the second substrate  200  are bonded, the bonding layer  210  has a certain thickness and is a hollow structure, thereby defining the second gap  20  between the resonant structure  120  and the second substrate  200 . The second gap  20  and the above-mentioned first gap  10  are respectively arranged on upper and lower sides of the resonant structure  120  and connected to each other, thereby forming the cavity  30  between the first substrate  110  and the second substrate  200 , and confining the resonant structure  120  in the cavity  30 . In other words, a shape and size of the second gap may be the same as the first gap  10 , or not completely the same, as long as after bonding, the second gap  20  can make the first electrode, the piezoelectric layer, and the second electrode of the resonant structure  120  to have a sufficient portion overlapping the first gap  10  and the second gap  20  at the same time, thereby forming an effective resonance region of the resonator. 
     In some embodiments, a shape of a bottom surface of the cavity  30  is a rectangle, but in some other embodiments, according to designs of the resonant operation of the resonant device, the shape of the bottom surface of the cavity  30  may also be a circle, an ellipse or a polygon other than a rectangle, such as a pentagon, a hexagon, etc. 
     In some embodiments, the dry film can be used to bond the BAW resonant device  100  and the second substrate  200 . The dry film has a low hardness and can tolerate a certain step height difference, so that it can form good contact with the support layer  130 , and the electrode  121  and the second electrode  123  of the resonant structure  120 , and the bonding effect is good. 
     It should be noted that, in some other embodiments of the present disclosure, when the bonding layer  210  does not have elasticity, it can be chosen to first cover the surface of the resonant structure  120  facing away from the first substrate  110  with a sufficiently thick bonding material and a surface of the bonding material is planarized. At this time, when the resonant structure  120  has the opening at the periphery of the first gap exposing the corresponding electrical connection portions, the bonding layer  210  can also fill the opening. Next, the planarized bonding material is etched, until the surface of the resonant structure  120  facing away from the first substrate  110  is exposed to form the bonding layer  210  with the second gap  20 . After that, through direct molecular adsorption force (or molecular bonding force) formed between the bonding layer  210  and the second substrate  200 , the second substrate  200  is bonded to the bonding layer  210 , to close the second gap  20 . Since in this method of forming the bonding layer  210  with the second gap  20 , the bonding layer  210  can fill up the opening (not shown) of the resonant structure  120  in the periphery of the first gap  10  and has a flattened surface, it can tolerate a certain step height difference, and can form good contact with the support layer  130  and the first electrode  121  and the second electrode  123  of the resonant structure  120 , so that the second substrate  200  can have good bonding effect. 
     Next, the first electrode  121  and the second electrode  123  of the resonant structure  120  need to be led out, to control from outside of the package structure and obtain relevant electrical parameters in the package structure from the outside. In some embodiments, due to a low hardness of the bonding layer  210 , if a through hole etching is performed and a conductive material is deposited therein, it is difficult to control the process on one hand, and on the other hand the conductive material is not easy to form a continuous film, and conductive performance and stability are poor. Therefore, the following processes are avoided to be performed at a side of the second substrate  200 , but are performed at a side of the first substrate  110 . 
       FIG. 6  illustrates a schematic cross-sectional view of an exemplary conductive interconnection layer formed by using an exemplary packaging method of a BAW resonator according to some embodiments of the present disclosure. Referring to  FIG. 6 , after performing step S 3  in some embodiments, a first through hole  140  and a second through hole  150  can be formed in the BAW resonant device  100 . Alternatively, the following processes may be included. 
     First, the first substrate  110  is thinned at a side away from the second substrate  200 . Thinning the first substrate  110  facilitates an etching process corresponding to the through holes. Generally, a thickness of the first substrate  110  needs to be reduced to be less than about and the thickness of the thinned first substrate  110  in some embodiments is about 60 μm. 
     Then, for example, using TSV technology, the etching process is performed on the side of the first substrate  110  to form the first through hole  140  and the second through hole  150  respectively. The first through hole  140  exposes a portion of the first electrode  121  (i.e., a first electrical connection portion) of the resonant structure  120 , and the second through hole  150  exposes a portion of the second electrode  123  (i.e., a second electrical connection portion) of the resonant structure  120 . Alternatively, the first through hole  140  may be formed through a first photomask process and etching process, and the etching process for forming the first through hole  140  stops when the first electrode  121  is exposed, and then through a second photomask process to cover other areas, the second through hole  150  is formed by an etching process, and the etching process of the second through hole  150  stops when the second electrode  123  is exposed. Sizes of the first through hole  140  and the second through hole  150  may be determined according to ranges of the electrodes to be exposed and etching conditions. In some embodiments, an upper opening diameter of the first through hole  140  and the second through hole  150  is about 20 μm to about 70 μm, and a depth of the first through hole  140  and the second through hole  150  is about 60 μm to about 100 μm, alternatively about 70 μm to about 80 μm. 
     That is to say, in some embodiments, the corresponding electrical connection portions of the resonant structure  120  include: the first electrical connection portion, including a portion of the first electrode  121  extending out of the first gap  10 , and the second electrical connection portion, including a portion of the second electrode  123  extending out of the first gap  10 . 
     Continuing to refer to  FIG. 6 , after performing step S 4  in some embodiments, a conductive interconnection layer can be formed on inner surfaces of the first through hole  140  and the second through hole  150  and on a portion of the surface of the first substrate  110  around the two through holes. Alternatively, the following processes can be included. First, a seed layer is formed on surfaces of the first through hole  140 , the second through hole  150 , and the first substrate  110 . The seed layer may be formed by PVD or sputtering. In some embodiments, when a main material of the conductive interconnection layer is copper, TiCu can be used as a seed layer material. Then, the BAW resonant device  100  including the seed layer is placed in an electrolytic tank of an electrolytic apparatus or an electroless plating solution of an electroless plating apparatus, and is taken out after a set time, and a copper conductive layer is formed on the surfaces of the first through hole  140 , the second through hole  150 , and the first substrate  110 . Then, a photolithography combined with etching process can be used to remove unnecessary copper conductive layer and seed layer on the surface of the first substrate  110  to form the conductive interconnection layer. The conductive interconnection layer covering the first through hole  140  is defined as a first conductive interconnection layer  141 , and the conductive interconnection layer covering the second through hole  150  is defined as a second conductive interconnection layer  151 . In some embodiments, since the support layer  130  is provided between the first substrate  110  and the first electrode  121  of the resonant structure  120 , the first through hole  140  and the second through hole  150  both pass through the first substrate  110  and the support layer  130 . Therefore, the first conductive interconnection layer  141  is in electrical contact with the first electrode  121  of the resonant structure  120 , and the second conductive interconnection layer  151  is in electrical contact with the second electrode  123  of the resonant structure  120 . In some other embodiments (as shown in  FIG. 3 ), there is no need to provide the support layer about a height of the first gap  10  between the resonant structure  120  and the first substrate  110 , so that the through holes can be reduced in depth compared to the through holes in  FIG. 6 . In addition, in some other embodiments of the present disclosure, the deposited conductive material may include one or more of metal materials such as copper, nickel, zinc, tin, silver, gold, tungsten, and magnesium, or include alloys including elements such as copper, nickel, zinc, tin, silver, gold, tungsten, and magnesium. The conductive material can be deposited using a process such as physical vapor deposition. Since electroplating and electroless plating processes have good hole filling effects, electroplating or electroless plating processes are preferably used to deposit the conductive material. In some other embodiments of the present disclosure, the material of the conductive interconnection layer may be the same as the material of the second electrode and the first electrode, and the deposition process conditions and the process conditions of etching the second electrode and the first electrode are also the same, to maximize compatibility with the processes of step S 1  and simplify the processes. 
     In addition, in some embodiments shown in  FIG. 6 , since the second through hole  150  needs to pass through a material layer where the first electrode  121  is located, and a material layer where the piezoelectric layer  122  is located to expose the second electrode  123 , when the electroplating process is performed in the first through hole  140  and the second through hole  150 , to prevent the conductive material from covering sidewalls of the first electrode  121  material layer and the piezoelectric layer  122  material layer in the first through hole  140  and the second through hole  150  to cause adverse effect to conductive performance of the first conductive interconnection layer  141  and the second conductive interconnection layer  151 , in some embodiments of the present disclosure, before the conductive material is deposited, a sidewall protection layer  111  is formed on inner sidewalls of the first through hole  140  and the second through hole 150  respectively. A method for forming the sidewall protection layer  111  may include the following processes: forming a dielectric layer that fills the first through hole  140  and the second through hole  150 ; and etching the dielectric layer vertically to make a remaining dielectric layer as the sidewall protection layer  111  that only covers the sidewalls of the first through hole  140  and the second through hole  150 . After the sidewall protection layer  111  is formed, bottom surfaces of the first through hole  140  and the second through hole  150  still expose the corresponding electrical connection portions (i.e., electrodes) of the resonant structure  120 , that the conductive interconnection layer capable of making electrical contact with the resonant structure  120  can be formed by the above-mentioned electroplating or electroless plating process. Since the sidewalls of the first through hole  140  and the second through hole  150  are different, that the sidewall of the first through hole  140  does not expose sidewalls of the first electrode  121  material layer and the piezoelectric layer  122  material layer, the sidewall protection layer  111  may also cover only the sidewall of the second through hole  150 . 
     In addition, the method of forming the conductive interconnection layer on the surface of the first substrate  110  is not limited to this. In some other embodiments, after a portion of the conductive interconnection layer covering the inner surfaces of the first through hole  140  and the second through hole  150  may be formed first, a conductive material is deposited to form a portion of the conductive interconnection layer covering the surface of the first substrate  110  away from the second substrate  200 , so that the portion of the conductive interconnection layer on the first substrate  110  is electrically connected to the portion of the conductive interconnection layer in the through hole  140  and the second through hole  150  respectively. 
     After the above processes, the formed first conductive interconnection layer  141  and the second conductive interconnection layer  151  electrically connect the first electrode  121  and the second electrode  123  respectively to the surface of the first substrate  110  away from the second substrate  200 . 
       FIG. 7  illustrates a schematic cross-sectional view of an exemplary passivation layer and exemplary contact pads formed by using an exemplary packaging method of a BAW resonator according to some embodiments of the present disclosure. Referring to  FIG. 7 , further, after step S 5  is performed in some embodiments, the first through hole  140  and the second through hole  150  can be filled with a passivation layer, and contact pads can be formed on the first substrate  110 . Alternatively, forming the contact pads may include the following processes. 
     First, on a surface of the first through hole  140  where the first conductive interconnection layer  141  is formed, a surface of the second through hole  150  where the second conductive interconnection layer  151  is formed, and a surface of the first substrate  110  where the conductive interconnection layer is formed, a passivation layer material is deposited, so that the passivation layer material fills the first through hole  140  and the second through hole  150  and covers the first substrate  110  with a certain thickness. Then a planarization process, such as a CMP process, is performed to remove a portion of a thickness of the passivation layer material, and a remaining passivation layer material is used as a passivation layer  160  to fill the first through hole  140  and the second through hole  150  and be a flat surface on the first substrate  110 . Then the passivation layer  160  is etched to form contact openings that expose at least a portion of the first conductive interconnection layer  141  above the first substrate  110  and at least a portion of the second conductive interconnection layer  151  above the first substrate  110 , respectively. The exposed first conductive interconnection layer  141  serves as a first contact pad  142 , and the exposed second conductive interconnection layer  151  serves as a second contact pad  152 . 
     The passivation layer  160  is used to define positions of the contact pads and protect the formed packaging structure. A material of the passivation layer  160  may include any one or more of dielectric materials such as magnesium oxide (MgO), zirconium oxide (ZrO 2 ), aluminum nitride (AlN), lead zirconate titanate (PZT), gallium arsenide (GaAs), hafnium oxide (HfO 2 ), aluminum oxide (Al 2 O 3 ), titanium oxide (TiO 2 ), and zinc oxide (ZnO). The material of the passivation layer  160  can be the same as that of the piezoelectric layer  122 , and a same deposition process as that of the piezoelectric layer  122  can be used, so as to be compatible with the fabrication process of the BAW resonant device  100  to a maximum extent, while avoiding problems of temperature drift and introduction of unnecessary stress when other material is used to make the passivation layer, thereby improving the resonance performance of the resonator. In addition, the passivation layer  160  fills the first through hole  140  and the second through hole  150 , which can also enhance mechanical support performance of the BAW resonant device  100 . 
     In some embodiments, to control the first electrode  121  and the second electrode  123  of the resonant structure  120  respectively, the first contact pad  142  and the second contact pad  152  are formed respectively. The first contact pad  142  is electrically connected to the first electrode  121  through the first conductive interconnection layer  141 , and the second contact pad  152  is electrically connected to the second electrode  123  through the second conductive interconnection layer  151 , so that the resonant structure can be electrically controlled from outside of the packaging module. 
     In the above-mentioned packaging method of the BAW resonator, the BAW resonant device  100  is bonded to the second substrate  200  through the bonding layer  210 , and then the through holes are formed at the side of the first substrate  110  to expose the electrical connection portions of the resonant structure  120  in the BAW resonant device  100 , and the conductive interconnection layer is formed on the inner surfaces of the through holes and on a portion of the surface of the first substrate, thereby avoiding the steps of etching the through holes and depositing conductive materials from the bonding layer  210 . The material of the bonding layer  210  can be selected to provide good bonding effect, which helps to reduce the process difficulty, and improves stability of the conductive interconnection layer and the formed packaging module, thereby helping to improve the performance of the BAW resonator. 
     The embodiments also include a packaging module of a BAW resonator, which can be fabricated by using the packaging method of the BAW resonator according to the embodiments. Referring to  FIG. 7 , the packaging module includes the BAW resonant device  100 , the second substrate  200  bonded to the BAW resonant device  100 , the first through hole  140  and the second through hole  150  formed on the BAW resonant device  100 , and, the conductive interconnection layer formed on the BAW resonant device  100 . 
     Alternatively, the BAW resonant device  100  includes the first substrate  110  and the resonant structure  120  disposed at the side of the first substrate  110  facing towards the second substrate  200 . The first gap  10  is formed between the resonant structure  120  and the first substrate  110 . The BAW resonant device  100  is bonded to the second substrate  200  through the bonding layer  210 , and the second gap  20  substantially surrounded by the bonding layer  210  is provided between the resonant structure  120  and the second substrate  200 . The second gap  20  is at least partially aligned with the first gap  10  to confine a portion of the resonant structure  120  between the second gap  20  and the first gap  10 . The first gap  10  and the second gap  20  may be connected to form the cavity  30 , or may not be connected. A portion of the resonant structure  120  overlapping the first gap  10  and the second gap  20  is used as an effective operating area. 
     In some embodiments, the resonant structure  120  includes the first electrode  121  close to the first substrate  100 , the piezoelectric layer  122  on the first electrode  121 , and the second electrode  123  on the piezoelectric layer  122 . The first through hole  140  and the second through hole  150  are both located at the periphery of the first gap  10  and respectively pass through the first substrate  110  to expose the corresponding electrical connection portions of the resonant structure  120 . The electrical connection portions include: the first electrical connection portion including a portion of the first electrode  121  extending out of the first gap  10 ; and the second electrical connection portion including a portion of the second electrode  123  extending out of the first gap  10 . The conductive interconnection layer includes: the first conductive interconnection layer  141  formed on the inner surface of the first through hole  140  and a portion of the first substrate  110  around the first through hole  140 , that the first conductive interconnection layer  141  is electrically connected to the first electrode  121  by electrically contacting the first electrical connection portion; and, the second conductive interconnection layer  151  formed on the inner surface of the second through hole  150  and a portion of the first substrate  110  around the second through hole  150 , that the second conductive interconnection layer  151  is electrically connected to the second electrode  123  by electrically contacting the second electrical connection portion. 
     In addition, the packaging module of the above-mentioned BAW resonator may further include the passivation layer  160 , which fills the first through hole  140  and the second through hole  150  and exposes at least a portion of the first conductive interconnection layer  141  and the second conductive interconnection layer  151  on the surface of the first substrate  110  at the periphery of the first through hole  140  and the second through hole  150 , respectively. The exposed first conductive interconnection layer  141  serves as the first contact pad  142 , and the exposed second conductive interconnection layer  151  serves as the second contact pad  152 . The first contact pad  142  and the second contact pad  152  may be connected to an external controller to control operations of the resonant structure  120 . 
     The packaging module of the BAW resonator of the embodiments can be packaged by using the above-mentioned packaging method of the BAW resonator. The bonding layer  210  can be made of a material with a lower hardness, such as a light-curing material (including a dry film) and/or a heat-cured material, so as to use its better step tolerance to be implemented between materials with poor flatness to achieve good bonding, so that the second substrate  200  (as the cap wafer) has a good packaging quality for the BAW resonant device  100 . The bonding layer  210  can also be made of a material with a higher hardness, such as at least one of silicon dioxide, nitride, ethyl orthosilicate, and a high-K medium with a dielectric constant K greater than  4 . In addition, structures such as the through holes and the conductive interconnection layer for electrically leading out the electrical connection portions of the BAW resonant device  100  are formed at the side of the first substrate  110  away from the second substrate  200 , which can avoid undesirable effect on a cap wafer structure where the second gap  20  is located, and at the same time helps to improve the quality of the through holes and the conductive interconnect layer. 
     In the packaging module of some embodiments, the BAW resonant device may have the structure as shown in  FIG. 2  or  FIG. 3 . Taking the BAW resonant device shown in  FIG. 2  as an example, the BAW resonant device  100  includes the support layer  130  disposed on the first substrate  110 . A hardness of the support layer  130  may be greater than that of the bonding layer  210 . The first gap  10  is substantially defined by the support layer  130 , and the resonant structure  120  overlaps the support layer  130 . In addition, as shown in  FIG. 7 , in the packaging module, the first through hole  140  and the second through hole  150  are both provided through the first substrate  110  and the support layer  130 . In some other embodiments (referring to FIG. 
       3 ), the groove is formed in the first substrate  110  of the BAW resonant device  100 , and the resonant structure  120  is located above the groove and overlaps the first substrate  110  surrounding the groove. In those embodiments, the first through hole and the second through hole provided in the BAW resonant device  100  can only pass through the first substrate  110 . 
     Taking the BAW resonant device shown in  FIG. 2  as an example, the support layer  130  can be made of materials with a higher hardness than the dry film, such as at least one of silicon dioxide, silicon nitride, silicon oxynitride, aluminum nitride, titanium oxide, and titanium nitride. On one hand, higher support strength can be provided, and on another hand, process control of a through hole etching process is less difficult. When a conductive material is deposited in the holes to form a conductive interconnection layer, a film layer of the conductive interconnection layer is continuous, the quality is higher, and the stability of the conductive interconnection layer is better, which is beneficial to improve the performance of the BAW resonator packaging structure. 
     The embodiments also include a filter which includes the packaging module of the above-mentioned BAW resonator. The filter may be a radio frequency filter. By improving the packaging method of the BAW resonant device, the manufacturing difficulty is reduced, the performance and reliability of the resonant device are improved, and the performance and reliability of the filter are also improved. 
     The method and structure in the embodiments are described in a progressive manner. Following method and structure mainly describe differences from previous method and structure, and relevant points can be understood by reference. 
     As disclosed, the technical solutions of the present disclosure have the following advantages. 
     The technical solutions of the present disclosure are to bond the BAW resonant device with the second substrate through the bonding layer, form the through holes at the side of the first substrate to expose the electrical connection portions of the resonant structure in the BAW resonant device, and form the conductive interconnection layer on the inner surfaces of the through holes and on a portion of the surface of the first substrate, thereby avoiding steps of through hole etching and depositing conductive material from the bonding layer, so that the material of the bonding layer can be selected with materials providing good bonding effects, which helps reduce process difficulty, and improves the stability of the through holes and the formed packaging module, thereby helping to improve the performance of the BAW resonator packaging structure. 
     In the technical solutions of the present disclosure, further, the BAW resonant device includes the support layer disposed on the first substrate. The resonant structure is overlapped on the support layer, and the first gap between the resonant structure and the first substrates is substantially surrounded by the support layer. The through holes pass through the first substrate and the support layer, and the support layer can be made of materials with a higher hardness, thereby making the process difficulty of forming through-holes in the support layer lower than that of forming through-holes in the bonding layer, and improving the quality and stability of the through-holes. 
     In the technical solutions of the present disclosure, further, the material of the bonding layer is a non-metallic material, for example, including at least one of a light-curing material, a heat-curing material, silicon dioxide, nitride, tetraethyl orthosilicate, and a high-K dielectric with a dielectric constant K greater than  4 , which can reduce the process difficulty and cost of combining the second substrate and the BAW resonant device, can be highly compatible with the process of the BAW resonant device, and will not cause pollution issues caused by metal bonding process such as an Au—Au bonding, etc. 
     In the technical solutions of the present disclosure, the same material as the piezoelectric layer in the BAW resonant structure can be used to make the passivation layer, which can be compatible with the first gap process to a greatest extent, while avoiding the problems of temperature drift and introduction of unnecessary stress caused by using other materials to make the passivation layer, thereby improving the resonance performance of the resonator. In addition, the passivation layer fills the through holes, which can enhance the mechanical strength of the BAW resonant device, thereby increasing supporting force of the sidewalls of the first gap of the resonator, and preventing the deformation of the first gap from affecting the resonance performance and reliability of the BAW resonator. 
     In the technical solutions of the present disclosure, it is possible to choose to form the sidewall protection layer on the sidewalls of the formed through holes before forming the conductive interconnection layer to avoid unnecessary electrical connection problems between the conductive interconnection layer and other structures, thereby ensuring the reliability of the electrical connection between the formed conductive interconnection layer and the resonant structure. 
     In the technical solutions of the present disclosure, further, the bonding layer can fill the opening on the resonant structure at the periphery of the second gap, thereby tolerating a certain step height difference of the resonant structure in the area outside the second gap. Furthermore, after the second substrate is bonded to the bonding layer, not only the surfaces of the second substrate and the first substrate facing away from each other do not tilt, but also the step height difference of the resonant structure located at the periphery of the second gap facing towards the side of the second substrate can be compensated, to ensure the reliability and stability of the bonding. Moreover, because the side of the first substrate facing away from the second substrate is horizontal, a flat process window can be provided for the manufacturing process of the through holes and the conductive interconnection layer, thereby ensuring the performance of the formed through holes and the conductive interconnection layer. 
     The foregoing description is only a description of the embodiments of the present disclosure, and does not limit the scope of the present disclosure in any way. Any person skilled in the art can use the methods and technical content disclosed above to make possible changes and modifications of the technical solutions of the present disclosure, without departing from the spirit and scope of the present disclosure. Therefore, all simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present disclosure without departing from the technical solutions of the present disclosure, belong to the protected range of the technical solutions of the present disclosure.