Patent Publication Number: US-2022231651-A1

Title: Method for forming film bulk acoustic resonator

Description:
TECHNICAL FIELD 
     The present invention relates to the technical field of filters and, more specifically, to a method for forming a film bulk acoustic resonator (FBAR). 
     BACKGROUND 
     The ever-developing wireless communication technology requires various wireless communication terminals to be versatile and able to transfer data over different frequency spectrums. In addition, in order to support a sufficient data rate with a limited bandwidth, it also places demanding requirements on the performance of radio-frequency (RF) systems. As a key component of RF systems, RF filters are used to filter out interference and noise that are not in their communication spectra and thus provide for a signal-to-noise ratio as required by the RF systems and communication protocols. For example, it may be necessary for a mobile phone to incorporate tens of filters, each for a frequency band in which the mobile phone may operate. 
     Filters are typically constructed from inductors, capacitors and resonators. In resonators operating based on piezoelectricity, an acoustic wave resonates in a piezoelectric material and is then converted into an electric wave for use. Among piezoelectric resonators, different numbers of bulk acoustic wave (BAW) resonators can be cascaded to form BAW filters that satisfy various performance requirements. As a subcategory of BAW resonators, film bulk acoustic resonators (FBAR) include a BAW film stack seated over a cavity that is formed in a substrate to serve as a reflector. The BAW film stack generally includes a piezoelectric film sandwiched between two electrodes, and an acoustic wave resonates in the piezoelectric film at a frequency depending on the material thereof. Thanks to a wide range of advantages including a high quality factor (Q-factor), ability to integrate to an IC chip and compatibility with CMOS fabrication processes, FBARs have undergone rapid development in recent years. 
     An existing FBAR fabrication method involves etching a substrate to form therein a recess and filling a sacrificial material therein. A BAW film stack is then formed over the sacrificial material, and a window is opened in the stack using an etching process, followed by removal of the sacrificial material via the window. In this method, since the BAW film stack is formed over the sacrificial material, surface roughness of the underlying material is critical to the performance of the stack. Therefore, particular care must be taken in roughness control, making the overall process complicated. In addition, it is difficult for this method to result in a high-quality monocrystalline piezoelectric film that is crucial to the performance of the resulting FBAR. 
     Instead of using a sacrificial material, another existing FBAR fabrication method involves fabricating a BAW film stack directly on a substrate and a support structure on the BAW film stack, bonding the whole to another substrate via the support structure, and removing the first substrate so that electrodes are separated and remain over a cavity. In order to provide sufficient support to the BAW film stack, in addition to support walls that delimit the cavity, the support structure also includes secondary support pillars that are formed inside the cavity and removed after the completion of the resonator. However, a process for removing the support pillars tends to etch away part of the support walls and thus impair the resonator&#39;s performance. 
     SUMMARY OF THE INVENTION 
     In view of the problems with the conventional methods, the present invention proposes a method for forming a film bulk acoustic resonator (FBAR) with improved reliability using a simpler process. 
     The proposed method comprises the steps of: providing a first substrate; forming an isolation layer on the first substrate and a bulk acoustic wave (BAW) film stack on the isolation layer; forming a support structure comprising an primary support wall, an isolation wall internal to the primary support wall, and a secondary support pillar internal to the isolation wall, which are disposed across a top surface of the BAW film stack, both the primary support wall and the isolation wall being annular, the isolation wall surrounded by the primary support wall, the secondary support pillar in turn surrounded by the isolation wall; bonding a side of the first substrate with the support structure formed thereon to a second substrate and removing the first substrate; forming a release window in the BAW film stack, which brings a space delimited by the isolation wall into communication with the outside; and removing both the secondary support pillar and the isolation wall via the release window. 
     In the proposed method, the secondary support pillar contributes to effective support provided during transfer of the films and any other process carried out above the support structure, and the isolation wall between the primary support wall and the secondary support pillar can minimize or eliminate the possibility of the primary support wall being eroded during a process for removing the secondary support pillar, providing for high reliability of a cavity subsequently formed within an area demarcated by the primary support wall and improved resonant performance of the fabricated FBAR. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flowchart schematically illustrating a method for forming a film bulk acoustic resonator (FBAR) according to embodiments of the present invention. 
         FIGS. 2 to 8  are schematic cross-sectional views of structures resulting from steps in a method for forming an FBAR according to embodiments of the present invention. 
     
    
    
     In these figures, 
       100  denotes a first substrate;  200 , a second substrate;  110 , an isolation layer;  120 , a BAW film stack;  121 , a first electrode layer;  122 , a piezoelectric layer;  123 , a second electrode layer;  130 , a support structure;  131 , a primary support wall;  132 , an isolation wall;  133 , a secondary support pillar;  123   a,  a peripheral trimmed region;  120   a,  a release window; and  140 , a cavity. 
     DETAILED DESCRIPTION 
     BAW resonator fabrication methods provided in the present invention will be described below in greater detail with reference to particular embodiments and to the accompanying drawings. Features and advantages of the invention will be more apparent from the following description. Note that the figures are provided in a very simplified form not necessarily drawn to exact scale for the only purpose of helping to explain the disclosed exemplary embodiments in a more convenient and clearer way. Embodiments of the invention should not be construed as limited to the particular shapes of the regions illustrated in the figures. For the sake of clarity, throughout the figures that help illustrate the embodiments disclosed herein, like elements are in principle labeled with like reference numbers, and repeated descriptions thereof are omitted. 
     It is to be understood that the terms “first”, “second” and so on, as used hereinafter, may be used to distinguish between similar elements without necessarily implying any particular ordinal or chronological sequence. It is to be understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in sequences other than those illustrated or otherwise described herein. Similarly, if a method is described herein as comprising a series of steps, the order of such steps as presented herein is not necessarily the only order in which such steps may be performed, and certain of the stated steps may possibly be omitted and/or certain other steps not described herein may possibly be added to the method. 
       FIG. 1  is a flowchart schematically illustrating a method of forming a film bulk acoustic resonator (FBAR) according to embodiments of the present invention. Referring to  FIG. 1 , the method includes the steps of: 
     (S 1 ) providing a first substrate; 
     (S 2 ) forming an isolation layer on the first substrate and a BAW film stack on the isolation layer; 
     (S 3 ) forming a support structure comprising an primary support wall, an isolation wall internal to the primary support wall, and a secondary support pillar internal to the isolation wall, which are disposed across a top surface of the BAW film stack, both the primary support wall and the isolation wall being annular, the isolation wall surrounded by the primary support wall, the secondary support pillar in turn surrounded by the isolation wall; 
     (S 4 ) bonding the side of the first substrate where the support structure is formed to a second substrate and removing the first substrate; 
     (S 5 ) forming a release window in the BAW film stack, which brings a space delimited by the isolation wall into communication with the outside; and 
     (S 6 ) removing the secondary support pillar and the isolation wall via the release window. 
       FIGS. 2 to 8  are schematic cross-sectional views of structures resulting from steps in a method of forming an FBAR according to embodiments of the present invention. This method will be described in greater detail below with reference to  FIGS. 2 to 8 . 
     First of all, in step S 1 , a first substrate  100  is provided. In the illustrated embodiment, the first substrate  100  serves to bear both the subsequent-fabricated BAW film stack and support structure. 
     The first substrate  100  may be selected from device substrates and carrier substrates commonly used in the art. Specifically, the first substrate  100  may be fabricated from any suitable substrate material well known to those skilled in the art. Examples of such materials may include at least one of silicon (Si), germanium (Ge), silicon germanium (SiGe), silicon carbide (SiC), silicon germanium carbide (SiGeC), indium arsenide (InAs), gallium arsenide (GaAs), indium phosphide (InP) or other III-V compound semiconductors. Alternatively, the substrate may be a multilayer structure or the like of one or more of those materials. Still alternatively, it may be a silicon on insulator (SOI), strained silicon on insulator (SSOI), strained silicon germanium on insulator (S—SiGeOI), silicon germanium on insulator (SiGeOI), germanium on insulator (GeOI), double side polished (DSP), alumina or like ceramic, quartz, glass or like substrate. In the illustrated embodiment, the first substrate  100  may be, for example, a P-type high-resistance monocrystalline silicon wafer with a (100) crystal plane on the top side. Of course, the first substrate  100  may include any other suitable material known in the art. 
       FIG. 2  shows a schematic cross-sectional view of a structure resulting from the formation of a BAW film stack in the method according to an embodiment of the present invention. Referring to  FIGS. 1 and 2 , in step S 2 , an isolation layer  110  is formed on the first substrate  100  and a BAW film stack  120  on the isolation layer  110 . 
     The isolation layer  110  on the first substrate  100  may serve as a buffer material for the BAW film stack  120  and may be formed on the first substrate  100  using a suitable method (e.g., chemical vapor deposition, physical vapor deposition, atomic layer deposition, coating, thermal oxidation method, etc.). The isolation layer  110  may be made of any suitable material that can be easily coated on the first substrate  100  but does not easily react with the subsequent-formed BAW film stack  120 , such as a dielectric material including, but not limited to, at least one of silicon oxide, silicon nitride, silicon oxynitride, aluminum oxide, aluminum nitride, titanium oxide, titanium nitride, fluorocarbon, carbon-doped silicon oxide, silicon carbide nitride and the like. In other embodiments, the isolation layer  110  may be made of any suitable material that can be easily coated on the first substrate  100  but does not easily react with the subsequent-formed BAW film stack  120 , such as amorphous carbon, a light-curing adhesive, a hot melt adhesive, a laserable bonding layer (e.g., a polymer), etc. Advantageously, the isolation layer  110  can mitigate any adverse impact of possible defects present on the surface of the first substrate  100  on the BAW film stack  120 , thus helping improve the performance and reliability of the device being fabricated. Moreover, during the subsequent removal of the first substrate  100  using a backside thinning process (e.g., chemical mechanical planarization), the isolation layer  110  can facilitate controlling stopping point for the etching process for removing the first substrate  100 , avoiding possible damage to the subsequently-formed BAW film stack  120 . The isolation layer  110  may have a thickness in the range of from 0.1 μm to 2 μm, optionally below 1 μm. 
     The isolation layer  110  may include an upper etch stop layer (not shown) and a sacrificial layer (not shown) between the etch stop layer and the first substrate  100 . The etch stop layer in the isolation layer  110  may have a relatively small thickness (e.g., 1000 Å), and each of the sacrificial layer and the subsequently-formed BAW film stack  120  (more exactly, the electrode layer closer to the first substrate  100 ) exhibit a high etch rate ratio to the etch stop layer. This provides for a stop point for the subsequent process for separating the BAW film stack  120  from the first substrate  100 , which can avoid unwanted damage to the BAW film stack  120  during the removal of the first substrate  100 . For example, the etch stop layer may be made of silicon oxide, silicon nitride or silicon oxynitride. The sacrificial layer in the isolation layer  110  may be any suitable material that allows easy separation of the BAW film stack  120  from the first substrate  100  using a simpler process. 
     Referring to  FIG. 2 , in the illustrated embodiment, the BAW film stack  120  may include a first electrode layer  121 , a piezoelectric layer  122  and a second electrode layer  123 , which are sequentially stacked one above another away from the isolation layer  10 . The first electrode  121 , piezoelectric  122  and second electrode  123  layers may have either the same or different shapes or areas. When the resonator is subsequently completed by patterning the first and second substrates  100 ,  200  that are bonded together, the remainders of the first and second electrode layers  121 ,  123  remaining from the patterning process may provide top and bottom electrodes of the resonator, respectively. In other non-limiting embodiments of the present invention, the BAW film stack  120  may also include other film(s), in addition to the first electrode layer  121 , the piezoelectric layer  122  and the second electrode layer  123 , as appropriately required by the device being fabricated. 
     The first and second electrode layers  121  and  123  may be each made of any suitable conductive or semiconductor material known to those skilled in the art. The conductive material may be a metal material with conductive properties. For example, each of the layers may consist of a single layer or a stack of layers of one or more of molybdenum (Mo), aluminum (Al), copper (Cu), tungsten (W), tantalum (Ta), platinum (Pt), ruthenium (Ru), rhodium (Rh), iridium (Ir), chromium (Cr), titanium (Ti), silver (Ag), gold (Au), osmium (Os), rhenium (Re), palladium (Pd), tin (Sn) and other metals. The semiconductor material may be, for example, Si, Ge, SiGe, SiC, SiGeC or the like. The first and second electrode layers  121  and  123  may be formed using a physical vapor deposition technique, such as magnetron sputtering or evaporation, or using a chemical vapor deposition technique. Preferably, the first and second electrode layers  121  and  123  may be formed of the same material. However, in practice, they may also be constructed from different conductive materials, as actually needed. The piezoelectric layer  122 , that can be also referred to as a piezoelectric resonator layer or a piezoelectric resonator structure, may be fabricated from one or more piezoelectric materials such as quartz, aluminum nitride (AlN), zinc oxide (ZnO), lead zirconate titanate (PZT), lithium niobate (LiNbO3), lithium tantalate (LiTaO3), etc. The piezoelectric layer  122  may be further doped with one or more rare-earth elements. In the illustrated embodiment, the first and second electrode layers  121  and  123  are for example molybdenum layers, and the piezoelectric layer  122  is for example an aluminum nitride layer. The first and second electrode layers  121  and  123  may each have a thickness in the range of approximately 100-200 nm, and the piezoelectric layer  122  may have a thickness ranging from 1 μm to 3 μm, depending on the desired resonant frequency. For example, the thickness of the piezoelectric layer  122  may be configured to be equal to ½ of the resonant wavelength. Each molybdenum layer may be deposited using physical vapor deposition (PVD) or magnetron sputtering, and the aluminum nitride layer may be deposited using PVD or metal organic chemical vapor deposition (MOCVD). 
       FIG. 3  shows a schematic cross-sectional view of a structure resulting from the formation of a support structure in the method according to an embodiment of the present invention.  FIG. 4  is a schematic top view of the support structure of  FIG. 3 . Referring to  FIGS. 3 and 4 , in step S 3 , a support structure  130  including a primary support wall  131 , an isolation wall  132  internal to the primary support wall  131 , and a secondary support pillar  133  internal to the isolation wall  132  is formed, which are spaced from one another across a top surface of the BAW film stack  120 . The primary support wall  131  and the isolation wall  132  are both annular walls, and the isolation wall  132  is surrounded by the primary support wall  131 , with the secondary support pillar  133  being in turn surrounded by the isolation wall  132 . The support structure  130  may be formed by depositing and patterning a support material over the second electrode layer  123 . The support material may be any suitable material that does not easily react with the BAW film stack. Non-limiting examples of the support material may include, but are not limited to, silicon nitride, silicon oxynitride, aluminum oxide, aluminum nitride, titanium oxide, titanium nitride, amorphous carbon, tetraethyl orthosilicate, etc. or a combination thereof. The support material may also be, for example, a dry film or any other suitable material well-known in the art. The support material may include a laminate of two or more materials. The support material may also be selected as at least one material with high mechanical strength such as silicon oxide, silicon nitride, silicon oxynitride, etc. In this case, the pillar in the support structure  130  can provide sufficient support to enhance structural integrity of the device and avoid undesirable deformation or breakage of the BAW film stack in a subsequent process due to a pressure difference between inside and outside of the cavity. In addition, risk of current leakage can be reduced, and adhesion can be enhanced, between the BAW film stack and the subsequently-bonded second substrate, resulting in a further increase in the performance and reliability of the device being fabricated. 
     The support structure  130  demarcates an area of the subsequently-formed air cavity (referred to as the “cavity” hereinafter for short) of the resonator on the first substrate  100 . This is made possible using a simple and easily controllable process without involving the formation and removal of a sacrificial layer. In the illustrated embodiment, the outermost primary support wall  131  in the support structure  130  is formed on the BAW film stack  120  to demarcate the location and area of the cavity of the BAW resonator. The primary support wall  131  may have a cross section parallel to the surface of the first substrate  100  in the shape of a rectangle, a circle, a pentagon, a hexagon or the like. The isolation wall  132  and the secondary support pillar  133  located within the area demarcated by the primary support wall  131  are formed to enhance the support to the BAW film stack during the bonding of the first substrate  100  to the second substrate  200 , transfer of the stack and the formation of the resonator until the resonator is packaged. This helps ensure sufficient reliability of the film stack to avoid undesirable collapse and allow for reduced process control complexity. Located within an area demarcated by the isolation wall  132 , at least one secondary support pillar  133  may be each formed as a solid pillar. For example, two or more secondary support pillars  133  each in the form as a column may be distributed equidistantly within the area demarcated by the isolation wall  132  to provide additional support, or at least one secondary support pillar  133  in the form of a continuous or non-continuous annulus spaced apart from the isolation wall  132  is disposed within the area demarcated by the isolation wall  132  to provide additional support. The following description will be made in the context of an example in which secondary support pillars  133  each in the form of a column are provided in such a manner that each of them is spaced apart from any adjacent secondary support pillar  133  and from the isolation wall  132  that is in turn spaced apart from the primary support wall  131 . With support from both the primary support wall  131  and the isolation wall  132 , the overlying films can be retained stably. Each secondary support pillar  133  may have a cross section parallel to the first substrate  100  in the shape of one or combinations of a circle, an ellipse, a quadrilateral, a pentagon and a hexagon. In the case of at least two secondary support pillars  133  being provided, they may assume either the same or different shapes. In addition, along a certain direction, these secondary support pillars  133  may be sized either equally or not. In the illustrated embodiment, each secondary support pillar  133  has a rectangular longitudinal cross section (see  FIG. 3 ), i.e., maintains a constant width from bottom to top. However, in particular embodiments, each secondary support pillar  133  may have an otherwise-shaped, e.g., regular or inverted trapezoidal, longitudinal cross section than a rectangular one, with the objects of the present invention being still achieved. Optionally, two or more identically shaped secondary support pillars  133  may be uniformly distributed within the area demarcated by the isolation wall  132  in order to provide more uniform support in the subsequent course of the completion of the resonator following the removal of the first substrate  100 . 
     The isolation wall  132  is situated between the primary support wall  131  and the secondary support pillars  133 , and has an annular shape. In a subsequent process for removing the secondary support pillars  133 , a gaseous or liquid etchant may be introduced via a release window formed within the area demarcated by the isolation wall  132  so that the etchant acts primarily on the secondary support pillars  133  and isolation wall  132  located within the area. Additionally, diffusion of the gaseous or liquid etchant toward the primary support wall  131  is blocked by the isolation wall  132 , significantly reducing the possibility of the primary support wall  131  being (e.g., laterally) etched. Since the primary support wall  131  is substantially prevented from erosion or damage, an improved integrity of the cavity (serving as a resonant cavity of the BAW resonator) that is delimited by the primary support wall  131  is achieved, which is favorable to the performance of the BAW resonator. 
     The primary support wall  131 , isolation wall  132  and secondary support pillars  133  in the support structure  130  may be configured to have equal heights in order to act simultaneously to provide support jointly. In the illustrated embodiment, the primary support wall  131 , isolation wall  132  and secondary support pillars  133  in the support structure  130  may be all about 3 μm high. However, the present invention is not so limited, because depending on the material used and on the process tolerance as well as a difference in distance between the top contact interface and the bottom contact interface, the support structure  130  may be somewhat elastic and allow height differences between top surfaces of the primary support wall  131 , isolation wall  132  and secondary support pillars  133 . Additionally, since the isolation wall  132  is intended mainly to protect the primary support wall  131  during the removal of the secondary support pillars, it is reasonable to allow it to be shorter than the primary support wall  131  and the secondary support pillars  133 . The spacings between, and dimensions of, the primary support wall  131 , isolation wall  132  and secondary support pillars  133  may also be designed according to practical process and structure constraints. For example, the isolation wall  132  may have a thickness designed according to both the number of the secondary support pillars and how difficult the secondary support pillars are to be etched away. Optionally, in a thickness-wise direction of the primary support wall  131 , dimensions of the secondary support pillars  133  and isolation wall  132  may be smaller than, e.g., equal to ⅓ or less of, the thickness of the primary support wall  131 . This is favorable to quick removal of both the secondary support pillars  133  and the isolation wall  132  in a subsequent process, with reduced or eliminated adverse impact on the primary support wall  131 . If necessary, more than one isolation wall  132  may be provided. For example, in another embodiment, in the support structure on the BAW film stack  120 , two or three isolation walls  132  may be arranged one within another between the primary support wall  131  and the secondary support pillars  133 . In the illustrated embodiment, each of the primary support wall  131  and isolation wall  132  has a rectangular longitudinal cross section in a thickness-wise direction (see  FIG. 3 ), i.e., maintains a constant width from bottom to top. However, in particular embodiments, each of the primary support wall  131  and isolation wall  132  may have an otherwise-shaped, e.g., regular or inverted trapezoidal, longitudinal cross section in the thickness-wise direction than a rectangular one, with the objects of the present invention being still achieved. 
     During a subsequent etching process for removing the secondary support pillars  133  from the support structure  130 , an inner surface of the isolation wall  132  facing the secondary support pillars  133  will also be etched. In order to ensure complete removal of the secondary support pillars  133  with minimized impact on the primary support wall  131 , the isolation wall  132  may have a width that is slightly greater than or equal to a dimension of each secondary support pillar  133  in a width-wise direction of the isolation wall  132 . Here, the width-wise direction of the isolation wall  132  is defined as a direction in a plane parallel to the surface of the first substrate  100  from the outside of the isolation wall  132  toward a center of the region demarcated thereby. Since the isolation wall  132  is etched on one side thereof while the secondary support pillars  133  are etched isotropically, the above width relationship can ensure that the secondary support pillars  133  are substantially etched away with the primary support wall  131  being unaffected. Moreover, the duration of the etching process may be optionally so controlled that both the isolation wall  132  and the secondary support pillars  133  are etched away, with the primary support wall  131  and the area demarcated thereby being not affected at all. This is helpful in improving the integrity of the resonant cavity and thus the BAW resonator&#39;s performance. Depending on the design requirements of the BAW resonator, the BAW film stack  120  may have a circular, elliptical, polygonal or otherwise-shaped resonant region (in the cross section in parallelism with the surface of the first substrate  100 ), and the support structure  130  may be appropriately shaped to save space.  FIG. 4  is a schematic plan view of the support structure formed in the method. Referring to  FIG. 4 , for example, the primary support wall  131  may have a similar shape (e.g., pentagonal, hexagonal, heptagonal, etc.) as the subsequently-formed resonator. Here, by “similar”, it is meant that, when viewed in the plan view, the primary support wall  131  and the resonator are proportionally sized polygons, circles or the like. The shape of the isolation wall  132  may be designed in accordance with that of the primary support wall  131  so that the isolation wall  132  is concentric with, and scaled down relative to, the primary support wall  131 . In this way, there is a gap with a consistent width between the primary support wall  131  and the isolation wall  132 , leading to space savings. The secondary support pillars  133  are arranged internal to the isolation wall  132 . As shown in  FIG. 4 , in the illustrated embodiment, both the primary support wall  131  and the isolation wall  132  have pentagonal cross-sections in a plane parallel to the surface of the first substrate  100 . 
     In order to reduce or eliminate any adverse impact of the subsequent etching process for removing the isolation wall  132  and the secondary support pillars  133  on the primary support wall  131 , according to another embodiment of the present invention, the primary support wall  131 , the isolation wall  132  and the secondary support pillars  133  in the support structure  130  are made of different materials. For example, if the subsequent etching process for the secondary support pillars  133  is a wet etching process, it is preferred that the etching rates for the secondary support pillars  133  and the isolation wall  132  are greater than that for the primary support wall  131 . In addition, depending on the design requirements, it is also possible that the isolation wall  132  and the secondary support pillars  133  are also made of different materials. In this case, in the wet etching process for the secondary support pillars  133 , the etching rates for the secondary support pillars  133  is greater than that for the isolation wall  132 , to avoid the liquid etchant from etching through the isolation wall  132  and reaching the primary support wall  131 . 
     By way of examples, several optional embodiments of forming the support structure  130  will be explained below. 
     In a first optional embodiment, forming the support structure  130  on the BAW film stack  120  involves forming a support layer with a predetermined thickness on the BAW film stack  120 . In particular, a chemical vapor deposition process may be carried out on the second electrode layer  123  of the BAW film stack  120  to form thereon an approximately 2 μm to 5 μm thick silicon dioxide film as the support layer, and the surface of the support layer may be then planarized using a CMP process. The support layer is then patterned to form the support structure  130 . The patterning process may include exposure, development, etching, demolding and other processes. 
     In this first embodiment, the primary support wall  131 , isolation wall  132  and secondary support pillars  133  are formed by etching the same support layer. Therefore, they are of the same material and etched at the same rate. 
     In a second optional embodiment, forming the support structure  130  on the BAW film stack  120  includes the steps of: first, forming a first support layer with a predetermined thickness on the BAW film stack  120 ; then etching the first support layer to form the primary support wall  131 ; subsequently, depositing a second support layer within the area demarcated by the primary support wall  131  and making a top surface of the second support layer flush with that of the primary support wall  131 ; and afterward, etching the second support layer to form both the isolation wall  132  and the secondary support pillars  133 . 
     In this second embodiment, the primary support wall  131 , the isolation wall  132  and secondary support pillars  133  are formed in different etching processes, and the first support layer from which the primary support wall  131  is fabricated and the second support layer from which both the isolation wall  132  and secondary support pillars  133  are fabricated may be of different materials and thus etched at different rates when the same etching technique is used. The materials of the first and second support layers may be so selected that the secondary support pillars  133  and the isolation wall  132  are etched faster than the primary support wall  131 . In this way, the isolation wall  132  may be removed in the same etching process for removing the secondary support pillars  133 , and due to the slower etching rate for the primary support wall  131 , it will be effectively protected against the etching process proceeding in the isolation wall  132 . 
     In a third optional embodiment, forming the support structure  130  on the BAW film stack  120  includes the steps of: first, forming a first support layer with a predetermined thickness on the BAW film stack  120 ; then etching the first support layer to form both the primary support wall  131  and the isolation wall  132 ; subsequently, depositing a second support layer within the area demarcated by the isolation wall  132  and making a top surface of the second support layer flush with that of the primary support wall  131 ; and afterward, etching the second support layer to form the secondary support pillars  133 . 
     In this third embodiment, the primary support wall  131 , the isolation wall  132  and secondary support pillars  133  are formed in different etching processes, and the first support layer from which both the primary support wall  131  and the isolation wall  132  are fabricated and the second support layer from which secondary support pillars  133  are fabricated may be of different materials and thus etched at different rates when the same etching technique is used. The materials of the first and second support layers may be so selected that the secondary support pillars  133  are etched faster than the isolation wall  132  (or the primary support wall  131 ). In this way, during the etching of the secondary support pillars  133 , although the used etchant, e.g., a liquid etchant, may come into contact with the isolation wall  132 , since the isolation wall  132  is etched slower, i.e., more resistant to the etching process, than the secondary support pillars  133 , it is less likely for the etchant used in the etching process for removing the secondary support pillars  133  to etch through the isolation wall  132  and cause any damage to the primary support wall  131 . As a result, the primary support wall  131  is well isolated. Further, compared with the case in which the isolation wall  132  and the secondary support pillars  133  are of the same material (i.e., the first embodiment), the width of the isolation wall  132  may be reduced while providing the same isolation effect. 
     In a fourth optional embodiment, forming the support structure  130  on the BAW film stack  120  includes the steps of: first, forming a first support layer with a predetermined thickness on the BAW film stack  120 ; then etching the first support layer to form the primary support wall  131 ; subsequently, depositing a second support layer within the area demarcated by the primary support wall  131  and making a top surface of the second support layer flush with that of the primary support wall  131 ; afterward, etching the second support layer to form the isolation wall  132 ; then depositing a third support layer within the area demarcated by the isolation wall  132  and making a top surface of the third support layer flush with that of the isolation wall  132 ; and finally, etching the third support layer to form the secondary support pillars  133 . 
     In this fourth embodiment, each of the primary support wall  131 , isolation wall  132  and secondary support pillars  133  is formed in a separate etching process, and the first support layer from which the primary support wall  131  is fabricated, the second support layer from which the isolation wall  132  is fabricated and the third support layer from which the secondary support pillars  133  are fabricated may be of distinct materials and thus etched at different rates when the same etching technique is used. For example, the first, second and third support layers may be so selected that, for the etching process for removing the secondary support pillars  133 , an etching rate for the secondary support pillars  133  is faster than an etching rate for the isolation wall  132 , which is in turn faster than an etching rate for the primary support wall  131 . In this way, during the etching of the secondary support pillars  133 , although the used etchant, e.g., a liquid etchant, may come into contact with the isolation wall  132 , since the isolation wall  132  is etched slower, i.e., more resistant to the etching process, than the secondary support pillars  133 , it is less likely for the etchant used in the etching process for removing the secondary support pillars  133  to etch through the isolation wall  132  and cause any damage to the primary support wall  131 . As a result, the primary support wall  131  is well isolated. Further, in the subsequent etching process for removing the isolation wall  132  performed after the secondary support pillars  133  have been completely removed, since the primary support wall  131  is even more etch-resistant, the possibility of the used etchant to partially etch away and cause damage to the primary support wall  131  is minimized. 
       FIG. 5  is a schematic cross-sectional view of a structure resulting from bonding the first substrate to a second substrate in the method according to an embodiment of the present invention. Referring to  FIGS. 1 and 5 , in step S 4 , the side of the first substrate  100  with the support structure  130  formed thereon is bonded to the second substrate  200 , and the first substrate  100  is then removed. 
     In the illustrated embodiment, the second substrate  200  is provided as a carrier substrate, and the first substrate  100  is so bonded to the second substrate  200  that the BAW film stack  120  is sandwiched between the two substrates. A backside etching and thinning process may be then performed to substantially remove the first substrate  100 , followed by removal of the secondary support pillars  133  and isolation wall  132  from the support structure  130 , resulting in the formation of air interfaces on opposing sides of the BAW film stack and the formation of a main body of the FBAR. 
     The second substrate  200  may be selected from carrier substrates commonly used in the art. Specifically, the second substrate  200  may be fabricated from any suitable substrate material well known to those skilled in the art. Examples of such materials may include at least one of silicon (Si), germanium (Ge), silicon germanium (SiGe), silicon carbide (SiC), silicon germanium carbide (SiGeC), indium arsenide (InAs), gallium arsenide (GaAs), indium phosphide (InP) or other III-V compound semiconductors. Alternatively, the substrate may be a multilayer structure or the like of one or more of those materials. Still alternatively, it may be a silicon on insulator (SOI), strained silicon on insulator (SSOI), strained silicon germanium on insulator (S—SiGeOI), silicon germanium on insulator (SiGeOI), germanium on insulator (GeOI), double side polished (DSP), alumina or like ceramic, quartz, glass or like substrate. In the illustrated embodiment, the second substrate  200  may be, for example, a P-type high-resistance monocrystalline silicon wafer with a ( 100 ) crystal plane on the top side. Of course, the second substrate  200  may include any other suitable material known in the art. 
     The bonding of the first substrate  100  to the second substrate  200  may accomplished with a fusion bonding process or a vacuum bonding process capable of forming strong covalent bonds between the surfaces of the second substrate  200  and the support structure  130  on the first substrate  100 . In another embodiment of the present invention, the bonding of the first substrate  100  to the second substrate  200  may also be accomplished with an adhesive such as a hot melt adhesive applied to the second substrate  200 . A vacuum bonding process may be then performed to bond a top surface of the support structure  130  (i.e., the surface thereof away from the BAW film stack  120 ) to the surface of the second substrate  200  in a vacuum environment optionally at a pressure of 1 Pa to 105 Pa and a temperature of 150° C. to 200° C. Such a vacuum bonding process can avoid the formation of bubbles and ensure a good bonding result. 
     After the first substrate  100  is bonded to the second substrate  200 , the whole may be flipped over so that the second substrate  200  serves as a carrier substrate. With the support structure  130  and the BAW film stack  120  having been transferred onto the second substrate  200 , the first substrate  100  may be stripped away. 
     The first substrate  100  may be removed using a backside etching and thinning process with the isolation layer  110  serving as an etch stop layer. This can avoid any adverse impact on the BAW film stack  120 , and at the point of removal of the first substrate  100 , the thickness of the isolation layer  110  may be significantly reduced, or the whole isolation layer  110  may be removed. For this reason, it is no longer shown in  FIG. 5 . In another embodiment, the first substrate  100  may be removed using a chemical mechanical polishing process, along with the isolation layer  110 . Optionally, when it is taken into consideration to protect the BAW film stack  120  with the isolation layer  110 , the isolation layer  110  may be alternatively partially retained. In this case, the remaining thickness of the isolation layer  110  may be determined by a processing limit of the employed chemical mechanical polishing equipment, e.g., 1000 Å. In yet another embodiment, the removal of the first substrate may be accomplished with a suitable process selected according to the material properties of the isolation layer  110  and the first substrate  100 . For example, in case of the isolation layer  110  being made of a light-curing adhesive, a chemical reagent may be used to dissolve the light-curing adhesive, separating the first substrate  100  from the BAW film stack  120  and thus allowing removal of the first substrate  100 . When the isolation layer  110  is made of a hot melt adhesive, a heat release process such as a thermal treatment may be employed to make the hot melt adhesive lose its adhesion, thus separating the first substrate  100  from the BAW film stack  120  and allowing removal of the first substrate  100 . Furthermore, when the isolation layer  110  is a stack of an etch stop layer and a sacrificial layer made of a laser release material, the sacrificial layer may be removed by a laser ablation process, allowing the first substrate  100  to be stripped away. In this case, the etch stop layer in the isolation layer  110  can function as a protective layer for the BAW film stack  120  in the laser ablation process. 
     As shown in  FIG. 5 , as a result of step S 4 , there are air interfaces on both sides of the BAW film stack  120  on the second substrate  200  (i.e., the opposing sides in the thickness-wise direction). Supported by the multiple features of the support structure  130 , the BAW film stack  120  is stable and does not tend to collapse when undergoing various processes performed thereon (e.g., patterning). This allows for reduced process control complexity. After the completion of each process step that requires the BAW film stack  120  to be highly stable, the secondary support pillars  133  and the isolation wall  132  may be removed. 
     Referring to  FIGS. 6 and 7 , subsequent to the removal of the first substrate  100 , step S 5  is carried out to form a release window  120   a  in the BAW film stack  120 , which brings a space delimited by the isolation wall  132  into communication with the outside. 
     Optionally, after the first substrate  100  has been removed in step S 4 , the first electrode layer  121  and piezoelectric layer  122  in the BAW film stack  120  may be partially removed using a cutting process or a photomask-based process, resulting in the formation of a peripheral trimmed region  123   a , in which part of the second electrode layer  123  is exposed. The peripheral trimmed region  123   a  may have a side wall that is perpendicular to a top surface of the second electrode layer  123  or slanted at the top towards the center of the area demarcated by the isolation wall  132 . The peripheral trimmed region  123   a  may be partially overlapped with the area demarcated by the isolation wall  132  along the thickness-wise direction thereof. In the overlap between the peripheral trimmed region  123   a  and the area demarcated by the isolation wall  132 , there may be a relatively small film thickness, which enables easy formation of the release window in the BAW film stack  120  and allows a relative large size of the release window. This allows the secondary support pillars  133  and isolation wall  132  to be subsequently removed easily and efficiently. 
     Additionally, after the first substrate  100  has been removed in step S 4 , the BAW film stack  120  may be optionally patterned (e.g., by a series of photolithography and etching processes) to form top and bottom electrodes, as well as both a resonant region and a non-resonant region above the area demarcated by the primary support wall  131 . The BAW film stack  120  in the resonant region may serve as a resonant structure of the FBAR device being fabricated. In addition, after the formation of the top and bottom electrodes, for example, a metal lift-off technique may be employed to form a metal bonding layer outside the resonant region, which is configured to allow the subsequent bonding of a third substrate serving as a cap substrate on the side of the BAW film stack  120  away from the second substrate  200 . With the support from the support structure  130 , the films within the boundary of the support structure  130  will not be pressured to experience an excessive degree of downward deformation or be broken in the processes involved in the formation of the top and bottom electrodes and the metal bonding layer. In alternative embodiments of the present invention, the formation of the BAW film stack  120  on the first substrate  100  in step S 2  may involve: patterning (e.g., by photolithography and etching processes) the first electrode layer  121  to form the top electrode of the BAW resonator before the piezoelectric layer  122  is formed thereon; patterning (e.g., by photolithography and etching processes) the piezoelectric layer  122  to form a piezoelectric layer in the resonant region of the BAW resonator before the second electrode layer  123  is formed thereon; and patterning (e.g., by photolithography and etching processes) the second electrode layer  123  to form the bottom electrode of the BAW resonator before the support material is formed thereon. It will be appreciated that, in particular embodiments, it is also possible to pattern any one, any two or all of the first electrode  121 , piezoelectric  122  and second electrode  123  layers in the BAW film stack  120  in step S 2  and pattern all the other layer(s) after the first substrate is removed in step S 4 . 
     In the illustrated embodiment, the method may further include, subsequent to the removal of the first substrate  100  and prior to the formation of the release window  120   a , a first sub-step in which the first electrode layer  121  and the piezoelectric layer  122  are etched using a photomask with a first pattern so that the side of the second electrode layer  123  away from the second substrate is exposed (as shown in  FIG. 6 ) and the exposed portion of the second electrode layer encompasses part of the area demarcated by the isolation wall  132 ; and a subsequent second sub-step in which the exposed portion of the second electrode layer  123  is etched using a photomask with a second pattern to form the release window  120   a  in the area demarcated by the isolation wall  132  (as shown in  FIG. 7 ). 
     In other embodiments of the present invention, the side of the second electrode layer  123  away from the second substrate may be exposed during the formation of the peripheral trimmed region  123   a  or of the top and bottom electrodes. Forming the release window  120   a  in the exposed portion of the second electrode layer  123  can reduce process complexity while avoiding imposing any adverse impact on the resonant region. 
     In particular, the release window  120   a  may extend through the BAW film stack  120  into a gap between the isolation wall  132  and one secondary support pillar. Alternatively, the release window  120   a  may extend through the BAW film stack  120  so that the top of the secondary support pillars  133  is partially exposed. The formation of the release window  120   a  may be accomplished with a dry etching or a wet etching. Examples of the dry etching process may include, but are not limited to, reactive-ion etching (RIE), ion beam etching, plasma etching and like processes. For example, the release window  120   a  may be formed by performing a reactive-ion etching process using a fluorine-based gaseous etchant on the exposed portion of the second electrode layer  123 . The fluorine-based gaseous etchant may include at least one of CF 4 , CHF 3 , C 2 F 6 , CH 2 F 2 , C 4 F 8 , NF 3  and SF 4 , and the process may be carried out at a power level of 0-500 W in order to ensure a good yield. Alternatively, the release window  120   a  may be formed in the area demarcated by the isolation wall  132  by performing a laser drilling process on the second electrode layer  123  exposed in the peripheral trimmed region  123   a.    
     In other embodiments of the present invention, instead of forming the peripheral trimmed region  123   a , the release window  120   a  may be directly formed in the BAW film stack  120  so as to extend through the first electrode  121 , piezoelectric  122  and second electrode  123  layers. In this case, the release window  120   a  may be formed using an etching process including multiple steps for individually etching through the first electrode  121 , piezoelectric  122  and second electrode  123  layers. The size of the resulting release window  120   a  may be large enough to allow easy evacuation of unwanted reaction by-products from the etching process from the inside of the support structure via the window. For example, the window may be a circular aperture with a diameter ranging from 10 μm to 30 μm, or a square aperture with a side length of about 10-30 μm. 
     In order to minimize any adverse impact of the process for removing the secondary support pillars  133  on the primary support wall  131 , an orthographic projection of the release window  120   a  on the surface of the second substrate  200  is desirably within the area demarcated by the isolation wall  132 . In other words, a gaseous or liquid etchant may be introduced through the release window  120   a  and react with the isolation wall  132  and secondary support pillars  133  to remove the secondary support pillars  133  from the space delimited by the isolation wall  132 . 
     The release window  120   a  may have an opening size depending on the area where the release window is allowed to be formed. Moreover, more than one release window  120   a  may be formed. Optionally, in order to expedite the removal of the secondary support pillars  133  and isolation wall  132 , two or more release windows  120   a  may be formed in the BAW film stack  120 . The multiple release windows  120   a  may be scattered across the second electrode layer  304  in the area demarcated by the isolation wall  132 . Optionally, each release window  120   a  may be formed at a marginal location of the area demarcated by the isolation wall  132 . This avoids imposing any adverse impact on the resonant region, allows a high Q value of the resulting BAW resonator, and facilitates easy evacuation of undesirable substances resulting from subsequent etching and cleaning processes from the cavity and drying of the cavity. Further, this can minimize the areas of possible parasitic devices. 
     Referring to  FIG. 8 , in step S 6 , the secondary support pillars  133  and the isolation wall  132  are removed via the release window  120   a.    
     Depending on the material(s) of the secondary support pillars  133  and the isolation wall  132 , they can be removed either by wet or dry etching. In the former case, for example, if the secondary support pillars and isolation wall  132  are made of silicon oxide, an etching solution include a liquid etchant that etches silicon oxide, such as dilute hydrochloric acid, buffered oxide etchant (BOE) or dilute hydrofluoric acid (DHF), may be introduced through the release window  120   a  into the space delimited by the isolation wall together with the second substrate  200  and the second electrode layer  123  to remove the secondary support pillars  133  and the isolation wall  132 . The BOE solution may be a mixture of hydrofluoric acid (HF), ammonium fluoride (NH 4 F) and wafer. For example, it may be obtained by mixing 40% NH4F, 49% HF and H 2 O at a ratio in the range from 10:1:0 to 200:1:10. The DHF solution may be prepared by mixing 49% HF and H 2 O at a ratio of, for example, from 30:1 to 500:1. When introduced into the space delimited by the isolation wall  132  and the BAW film stack  120  via the release window  120   a,  the liquid etchant may come into contact with side walls of the isolation wall  132  and the secondary support pillars  133 . Alternatively, it may first come into contact with the top of the secondary support pillar  133  exposed in the release window  120   a  and then flow through gaps surrounding the secondary support pillar  133  and reach the side walls of any other secondary support pillar  133 , if present, and of the isolation wall  132 . Further, a process for removing the secondary support pillars  133  and the isolation wall  132  using a BOE or DHF solution may include a short over-etching period (i.e., a cleaning period) for initial cleaning of the cavity delimited by the primary support wall  131  with the BOE or DHF solution through evacuating by-product particles, metal ions and other contaminants resulting from the etching process. This allows a good cleaning effect of the cavity within a short period of time and results in an addition increase in the performance of the resulting device. 
     Desirably, it shall be taken into consideration that the piezoelectric layer  122 , second electrode layer  123  or first electrode layer  121  shall be prevented from any damage during the removal of the secondary support pillars  133  and the isolation wall  132 . To this end, it may be appropriate to choose a liquid etchant with a relatively high etch rate ratio of the secondary support pillars  133  and isolation wall  132  to the BAW film stack. Such a liquid etchant can remove the secondary support pillars  133  and the isolation wall  132  while causing minimal or no damage to the BAW film stack. 
     In another embodiment of the present invention, the secondary support pillars  133  are made of a material easy to be ashed away, such as photoresist, dry film or amorphous carbon. In this case, after the other area than that of the release window  120   a  is covered by a protective layer, a plasma processing gas may be introduced through the release window  120   a  into the space delimited by the second substrate  200 , the second electrode layer  123  and the isolation wall  132  to remove the secondary support pillars  133 . Specific parameters of this process may be determined according to the etching technique used and the requirements of the actual application. 
     In the illustrated embodiment, during the etching process for removing the secondary support pillars  133 , although the isolation wall  132  that is exposed to the liquid etchant or gas etchant may be etched thereby, the possibility of the primary support wall  131  being eroded is minimized due to the blockage provided by the isolation wall  132 . Through controlling the number, width of such isolation walls as well as the duration of the etching process for the isolation walls, the isolation wall  132  may be removed simultaneously with the secondary support pillars  133  in step S 6 . Alternatively, after the secondary support pillars  133  have been completely removed, the etching process may be continued to further remove the isolation wall  132 . In other words, the secondary support pillars  133  and the isolation wall  132  may be removed successively. Desirably, the etching reaction is stopped as soon as possible after the isolation wall  132  has been removed, in order to avoid the primary support wall  131  from being affected due to an excessively long duration of the etching process. 
     After the secondary support pillars  133  and the isolation wall  132  are removed, the second substrate  200 , primary support wall  131  and BAW film stack  120  together delimit a cavity  140  acting as a resonant cavity of the BAW resonator. With the above method, no significant variations in the boundary or shape of the cavity  140  will be caused by the above-discussed etching process. That is, the cavity  140  can be fabricated with high consistency, which is favorable to the performance of the BAW resonator. 
     Optionally, following the removal of the secondary support pillars  133  and isolation wall  132 , the cavity  140  may be cleaned (i.e., rinsed) with deionized water injected therein via the release window  120   a  and dried by further injecting therein gaseous isopropyl alcohol (IPA) through the release window  120   a , which enables complete removal of any residual liquid from the cavity  140 , ensuring good performance of the resulting resonator. In addition, thanks to the presence of the release window  120   a , the inside and outside of the cavity  140  are kept in communication with each other during the cleaning and drying of the cavity  140 , resulting in a balance between internal and external pressures of the cavity  140  and avoid cracking of the cavity  140  under an excessively large difference between the pressures. 
     The cavity  140 , together with the BAW film stack and the second substrate  200 , constitutes a main body of the resonator. Subsequently, a third substrate may be bonded (as a cap substrate) above the primary support wall  131 , with a clearance left between the BAW film stack  120  and the third substrate. The clearance serves as another cavity of the BAW resonator, and is in communication with the cavity  140 . The third substrate is provided to encapsulate and protect the main body. Therefore, the secondary support pillars  133  and isolation wall  132  of the support structure can always provide support throughout the various processes involved in the fabrication of the resonator until they are removed prior to the bonding of the third substrate. In the illustrated embodiment, the release window may be located within the area encompassed by the third substrate. 
     Further, solder pads electrically connected to the first and second electrode layers  121  and  123  may be subsequently formed on the third substrate on both sides of the resonant region, thus completing the FBAR. The first electrode layer  121  may serve as an input or output electrode for receiving or providing electrical signals such as radio-frequency signals. For example, if the patterned second electrode layer  123  is used as an input electrode, the patterned first electrode layer  121  may act as an output electrode. If the patterned second electrode layer  123  is used as an output electrode, the patterned first electrode layer  121  may act as an input electrode. The piezoelectric layer  104  is adapted to convert electrical signals incoming from the patterned first electrode layer  121  or second electrode layer  123  into bulk acoustic waves, for example, by virtue of physical vibration. The support structure and cavity fabricated using the above-described method both have enhanced reliability, which provides for improved performance of the resulting BAW resonator. 
     In embodiments of the present invention, there is also provided a FBAR fabricated using the above method. The FBAR includes a second substrate  200 , a BAW film stack above the second substrate  200  and a support structure between the BAW film stack and the second substrate  200 . The support structure includes a primary support wall  131 , and the second substrate  200 , the primary support wall  131  and the BAW film stack together delimit a cavity  140 . The BAW film stack is situated in contact with the support structure above the cavity  140 . The above method provides for higher reliability of the cavity  140  and hence of the resulting FBAR, which helps in obtaining improved resonant performance. 
     In embodiments of the present invention, there is also provided a filter including at least one FBAR, which is fabricated using a method including the above method. The filter may be an RF filter. The above improved method allows for enhanced resonator performance and reliability, helping in improving the filter&#39;s performance and yield. 
     The method and device embodiments disclosed herein are described in a progressive manner, with the description of each succeeding embodiment focusing on its differences from one or more preceding embodiments, and reference may be made therebetween whenever appropriate. 
     The foregoing description merely explains and illustrates a few preferred embodiments of the present invention and is not intended to limit its scope in any sense. In light of the teachings disclosed above, any person of skill in the art may make various changes and modifications to the disclosed embodiments without departing from the scope of the present invention. Accordingly, any and all such simple changes, equivalent variations and modifications made to the above embodiments in light of the foregoing teachings without departing from the scope of the present invention are intended to fall within the scope.