Patent Publication Number: US-11646712-B2

Title: Bulk acoustic wave structure and bulk acoustic wave device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional application of and claims the priority benefit of U.S. patent application Ser. No. 16/231,621, filed on Dec. 24, 2018, now allowed. The prior application Ser. No. 16/231,621 claims the priority benefit of Taiwan application serial no. 107131208, filed on Sep. 5, 2018. The entirety of each of the above-mentioned patent applications is hereby incorporated by reference herein and made a part of specification. 
    
    
     BACKGROUND 
     Technical Field 
     The disclosure relates to a bulk acoustic wave (BAW) structure, a BAW device and a manufacturing method thereof, particularly to a BAW structure having a single crystal piezoelectric material layer, a BAW device, and a manufacturing method thereof. 
     Description of Related Art 
     A BAW structure basically includes two metal electrodes sandwiching a piezoelectric material layer. When an electric field is applied to the metal electrodes, the piezoelectric material layer generates an acoustic wave due to vibration, and the acoustic wave oscillates in the piezoelectric material layer to form a standing wave, so as to reduce energy loss. 
     Since acoustic signals of the BAW structure are transmitted inside a medium, the BAW structure may be small in size and suitable for use in various portable electronic products, such as a bandpass filter for a mobile communication product. 
     However, when the BAW structure is applied in the bandpass filter, requirements for frequency specifications of the BAW structure are quite strict. For example, due to the manufacturing process, the piezoelectric material layer may be unlikely to grow into a single crystal material, which results in a reduction in a Q value (or quality factor) of the BAW filter, thereby affecting the reliability of the product. 
     SUMMARY 
     A BAW structure of the disclosure includes a single crystal piezoelectric material layer, a first electrode, a second electrode and an acoustic reflector. The first electrode and the second electrode are respectively located on a first surface and a second surface of the single crystal piezoelectric material layer, wherein an area of the second electrode is greater than or equal to an area of the second surface of the single crystal piezoelectric material layer, and a contact area of the single crystal piezoelectric material layer with the second electrode is equal to the area of the second surface of the single crystal piezoelectric material layer. The acoustic reflector is disposed on a surface of the first electrode. 
     A BAW device of the disclosure includes the aforementioned BAW structure, a plurality of bumps, a carrier and a package. The bumps are disposed below the BAW structure and electrically connected to the first electrode and the second electrode respectively. The carrier is electrically connected to the first electrode and the second electrode respectively via the bumps. The package is located on the carrier and encapsulates the BAW structure. 
     To make the aforementioned more comprehensible, several embodiments accompanied with drawings are described in detail as follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the disclosure, and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the disclosure and, together with the description, serve to explain the principles of the disclosure. 
         FIG.  1    is a schematic view of a BAW structure according to a first embodiment of the disclosure. 
         FIG.  2    is a schematic view of another BAW structure of the first embodiment. 
         FIG.  3    is a schematic view of a BAW device according to a second embodiment of the disclosure. 
         FIG.  4    is a schematic view of another BAW device of the second embodiment. 
         FIGS.  5 A to  5 F  are schematic cross-sectional views of a manufacturing process of a BAW structure according to a third embodiment of the disclosure. 
         FIGS.  6 A to  6 D  are schematic cross-sectional views of a manufacturing process of a BAW device according to a fourth embodiment of the disclosure. 
         FIGS.  7 A to  7 C  are schematic cross-sectional views of a manufacturing process of a BAW device according to a fifth embodiment of the disclosure. 
         FIG.  8    is a schematic view of a BAW device according to a sixth embodiment of the disclosure. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Several embodiments are described in detail below with reference to the accompanying drawings. However, the embodiments provided herein are not intended to limit the scope of the disclosure. In addition, the drawings are for illustrative purposes only and are not illustrated according to actual dimensions. To facilitate understanding, the same elements will hereinafter be denoted by the same reference numerals. In addition, terms such as “contain,” “include,” “have” and the like used herein are all open terms, which mean including but not limited to. Moreover, directional terms mentioned herein, such as “on” and “below,” are only directions relative to the drawings. Therefore, the directional terms are used to illustrate rather than limit the disclosure. 
       FIG.  1    is a schematic view of a BAW structure according to a first embodiment of the disclosure. 
     Referring to  FIG.  1   , a BAW structure  100  of the first embodiment includes a single crystal piezoelectric material layer  102 , a first electrode  104 , a second electrode  106 , and a first acoustic reflector  108 , wherein a material of the single crystal piezoelectric material layer  102  includes, for example, aluminum nitride, LiTaO 3 , LiNbO 3 , quartz or diamond. In an embodiment, a thickness of the single crystal piezoelectric material layer  102  is, for example, between 100 nm and 200 nm. The first electrode  104  and the second electrode  106  are respectively located on a first surface  102   a  and a second surface  102   b  of the single crystal piezoelectric material layer  102 , wherein materials of the first electrode  104  and the second electrode  106  each independently include molybdenum (Mo), tungsten (W), ruthenium (Ru), tantalum (Ta), platinum (Pt), titanium (Ti), gold (Au) or aluminum (Al). The acoustic reflector  108  is disposed on a surface  104   a  of the first electrode  104 . The first acoustic reflector  108  includes a first sublayer  110   a  and a second sublayer  110   b  that are alternately stacked, and is, for example, a Bragg reflector, wherein the first sublayer  110   a  and the second sublayer  110   b  are respectively materials having different impedances (Z). In addition, metal materials, dielectric materials or semiconductor materials having different impedances may be applied to the first sublayer  110   a  and the second sublayer  110   b  of the first acoustic reflector  108 . In another embodiment, different types of materials may also be used in combination as the first sublayer  110   a  and the second sublayer  110   b  of the first acoustic reflector  108 . They include, for example, alternately stacked metal/dielectric materials, alternately stacked metal/semiconductor materials, alternately stacked dielectric/semiconductor materials, and so on. In the case of alternately stacked metal/dielectric materials, the material of the first sublayer  110   a  includes, for example, silicon oxide (SiO 2 ), and the material of the second sublayer  110   b  includes, for example, molybdenum (Mo). 
     Referring still to  FIG.  1   , the BAW structure  100  of the present embodiment has no substrate. Thus, a contact area of the single crystal piezoelectric material layer  102  with the second electrode  106  is equal to an area of the second surface  102   b  of the single crystal piezoelectric material layer  102 , and an area of the second electrode  106  should be greater than or equal to the area of the second surface  102   b  of the single crystal piezoelectric material layer  102 , so as to facilitate circuit design. In  FIG.  1    as an example, the area of the second electrode  106  is equal to the area of the second surface  102   b  of the single crystal piezoelectric material layer  102 . However, in practice, the second electrode  106  may extend beyond an edge of the single crystal piezoelectric material layer  102 . 
       FIG.  2    is a schematic view of another BAW structure of the first embodiment, wherein the reference numerals in  FIG.  1    are used to denote the same components. 
     In  FIG.  2   , in addition to the single crystal piezoelectric material layer  102 , the first electrode  104 , the second electrode  106  and the first acoustic reflector  108 , a second acoustic reflector  200  is further disposed on a surface  106   a  of the second electrode  106 . The second acoustic reflector  200  includes a first sublayer  202   a  and a second sublayer  202   b  that are alternately stacked, wherein the choice of materials of the first sublayer  202   a  and the second sublayer  202   b  may refer to the first sublayer  110   a  and the second sublayer  110   b  of the first acoustic reflector  108 , and description thereof is thus omitted. In addition, the first sublayers  110   a  and  202   a  in the second acoustic reflector  200  and the first acoustic reflector  108  may be the same or different materials, and the second sublayers  110   b  and  202   b  in the second acoustic reflector  200  and the first acoustic reflector  108  may be the same or different materials. In view of simplifying the manufacturing process, the first sublayers  110   a  and  202   a  are preferably the same material, and the second sublayers  110   b  and  202   b  are preferably the same material. 
       FIG.  3    is a schematic view of a BAW device according to a second embodiment of the disclosure. 
     Referring to  FIG.  3   , a BAW device  300  of the second embodiment includes a BAW structure  302 , a plurality of bumps  304 , a carrier  306 , and a package  308 . The BAW structure  302  may be the BAW structure of the first embodiment, the bumps  304  are disposed below the BAW structure  302 , and the bumps  304  are electrically connected to a first electrode (not illustrated) and a second electrode (not illustrated) respectively within the BAW structure  302 . Although the first and second electrodes are not shown in  FIG.  3   , it should be understood that the first and second electrodes can be connected to the same surface of the BAW structure  302  by current technology, and then packaged by flip chip. For example, a contact pad  310  may be formed first, the bumps  304  are formed on the contact pad  310 , then the bumps  304  face a contact pad  312  of the carrier  306 , and the bumps  304  are then remelted (e.g., by hot air reflow soldering), such that the contact pad  312  of the carrier  306  is electrically connected to the first and second electrodes respectively via the bumps  304 . The carrier  306  is, for example, a lead frame, a substrate or a printed circuit board. The package  308  is located on the carrier  306  and encapsulates the BAW structure  302 . In general, a gap  314  is formed between the carrier  306  and the BAW structure  302 , and thus the gap  314  is unfilled by the package  308 . Alternatively, as shown in  FIG.  4   , the gap  314  is filled with the package  308  in the BAW device  400 . If the package  308  does not completely fill the gap  314 , the BAW structure of  FIG.  1    or  FIG.  2    may be used, and the BAW structure of  FIG.  1    which has an acoustic reflector on a single surface is preferred. If the package  308  fills up the gap  314 , it is more suitable to use the BAW structure of  FIG.  2    which has an acoustic reflector on both surfaces. In addition, a passivation layer  316  may be provided as a protective layer on the BAW structure  302 , and a material thereof includes, for example, silicon oxide (SiO 2 ) or a corrosion-resistant polymer. 
       FIGS.  5 A to  5 F  are schematic cross-sectional views of a manufacturing process of a BAW structure according to a third embodiment of the disclosure. 
     Referring first to  FIG.  5 A , in the manufacturing method of the third embodiment, a single crystal substrate  500  is provided first, and a material thereof includes, for example, zinc oxide, silicon carbide or silicon. A single crystal piezoelectric material layer  502  is formed on the single crystal substrate  500  by, for example, single crystal epitaxy. Due to the structure property of the single crystal substrate  500  itself, it is ensured that a piezoelectric material grown therefrom is also single crystal, and thereby a Q value of the subsequently formed BAW structure is improved. The single crystal piezoelectric material layer  502  has a first surface  502   a  and a second surface  502   b  opposing each other, and the second surface  502   b  is in direct contact with the single crystal substrate  500 . 
     Referring still to  FIG.  5 A , a material of the single crystal piezoelectric material layer  502  includes, for example, aluminum nitride, LiTaO 3 , LiNbO 3 , quartz or diamond. From the viewpoint of manufacturing process control, There should be a high etching selectivity ratio between the single crystal substrate  500  and the single crystal piezoelectric material layer  502 , for example, greater than 10:1, preferably greater than 20:1. For example, if the material of the single crystal substrate  500  is zinc oxide, the material of the single crystal piezoelectric material layer  502  may be aluminum nitride, but the disclosure is not limited thereto. 
     Then, referring to  FIG.  5 B , a first electrode  504  is formed on the first surface  502   a  of the single crystal piezoelectric material layer  502 . A method of forming the first electrode  504  is, for example, sputtering or other suitable method. The choice of the material of the first electrode  504  may refer to the first electrode  104  in the first embodiment, and description thereof is thus omitted. 
     Thereafter, referring to  FIG.  5 C , a first acoustic reflector  506  is formed on a surface  504   a  of the first electrode  504 . The first acoustic reflector  506  is formed by, for example, alternately depositing a first sublayer  508   a  and a second sublayer  508   b , wherein the choice of materials of the first sublayer  508   a  and the second sublayer  508   b  may refer to the first sublayer  110   a  and the second sublayer  110   b  of the first acoustic reflector  108  in the first embodiment, and description thereof is thus omitted. 
     Next, the single crystal substrate  500  is removed to obtain a structure as shown in  FIG.  5 D . A method of removing the single crystal substrate  500  is, for example, wet etching. If zinc oxide is used as the material of the single crystal substrate  500 , sulfur hexafluoride (SF 6 ) or citric acid may be used as an etchant. Moreover, to protect the first acoustic reflector  506 , a passivation layer  510  may be formed on the first acoustic reflector  506  before the single crystal substrate  500  is removed. The choice of material of the passivation layer  510  may refer to the passivation layer  316  in the second embodiment, and description thereof is thus omitted. 
     Referring then to  FIG.  5 E , the structure in  FIG.  5 D  is inverted and a second electrode  512  is formed on the second surface  502   b  of the single crystal piezoelectric material layer  502 , wherein an area of the second electrode  512  is greater than or equal to an area of the second surface  502   b  of the single crystal piezoelectric material layer  502 , and a contact area of the single crystal piezoelectric material layer  502  with the second electrode  512  is equal to the area of the second surface  502   b  of the single crystal piezoelectric material layer  502 . A method of forming the second electrode  512  may refer to the method of forming the first electrode  504 ; the choice of material of the second electrode  512  may refer to the second electrode  106  in the first embodiment, and description thereof is thus omitted. 
     Basically, by performing the steps of  FIGS.  5 A to  5 E , a BAW structure having an acoustic reflector on a single surface can be completed. If a BAW structure having an acoustic reflector on both surfaces needs to be manufactured, the step of  FIG.  5 F  may be performed subsequently. A second acoustic reflector  514  is formed on a surface  512   a  of the second electrode  512 , wherein the second acoustic reflector  514  is formed in the same manner as the first acoustic reflector  506 , and the choice of materials of a first sublayer  516   a  and a second sublayer  516   b  in the second acoustic reflector  514  may refer to the first sublayer  110   a  and the second sublayer  110   b  of the first acoustic reflector  108  in the first embodiment, and description thereof is thus omitted. Finally, another passivation layer  518  may be formed on the second acoustic reflector  514 . The choice of material of the passivation layer  518  may refer to the passivation layer  510 , and description thereof is thus omitted. 
       FIGS.  6 A to  6 D  are schematic cross-sectional views of a manufacturing process of a BAW device according to a fourth embodiment of the disclosure. 
     Referring first to  FIG.  6 A , at least one BAW structure  600  is formed, and a manufacturing method of the BAW structure  600  may refer to the third embodiment, and description thereof is thus omitted. A contact pad  602  may be formed below the BAW structure  600 , and then a plurality of bumps  604  are formed on the contact pad  602 . The bumps  604  are electrically connected to first and second electrodes (not illustrated) respectively of the BAW structure  600  via the contact pad  602 . Although the first and second electrodes are not shown in  FIG.  6 A , it should be understood that the first and second electrodes can be pulled to the same surface of the BAW structure  600  by the related art, and the contact pad  602  and the bumps  604  are then formed on the above surface. In addition, before the contact pad  602  is formed, a passivation layer  606  may be formed as a protective layer on a surface of the BAW structure  600 . The choice of material of the passivation layer  606  may refer to the passivation layer  316  in the second embodiment, and description thereof is thus omitted. 
     Then, referring to  FIG.  6 B , the BAW structure  600  is flip-chip bonded to a carrier  608  via the bumps  604 . A surface of the carrier  608  with respect to the bumps  604  has a contact pad  610  thereon. A gap  612  is formed between the carrier  608  and the BAW structure  600  after bonding. The flip-chip bonding is performed by, for example, aligning the bumps  604  with the contact pad  610  of the carrier  608 , and then remelting the bumps  604  (e.g., by hot air reflow soldering). Thus, the carrier  608  is bonded to the bumps  604  and electrically connected to the first and second electrodes respectively via the bumps  604 . The carrier  608  may refer to the carrier  306  of the second embodiment, and description thereof is thus omitted. 
     Next, referring to  FIG.  6 C , a solid-state package  614  may be pressed against the BAW structure  600  by heating and pressurization in order to perform packaging. 
     Thereafter, referring to  FIG.  6 D , in a packaged BAW device, since the package  614  is solid-state, the gap  612  is not filled and a cavity is formed for sound wave reflection. Therefore, the manufacturing method of the BAW device of the present embodiment preferably uses a BAW structure having an acoustic reflector on a single surface (as shown in  FIG.  5 E ). 
       FIGS.  7 A to  7 C  are schematic cross-sectional views of a manufacturing process of a BAW device according to a fifth embodiment of the disclosure. 
     Referring first to  FIG.  7 A , at least one BAW structure  700  is formed, and a manufacturing method of the BAW structure  700  may refer to the third embodiment, and description thereof is thus omitted. A plurality of contact pads  702  are formed below the BAW structure  700 , and then a plurality of bumps  704  is formed on the contact pads  702 . The bumps  704  are electrically connected to first and second electrodes (not illustrated) respectively of the BAW structure  700 . In addition, before the contact pads  702  are formed, a passivation layer  706  may be formed as a protective layer on a surface of the BAW structure  700 . The choice of material of the passivation layer  706  may refer to the passivation layer  316  in the second embodiment, and description thereof is thus omitted. 
     Then, referring to  FIG.  7 B , the BAW structure  700  is flip-chip bonded to a contact pad  710  of a carrier  608  via the bumps  704 . A gap  712  is formed between the carrier  708  and the BAW structure  700 . The flip-chip bonding may refer to the fourth embodiment, and description thereof is thus omitted. The carrier  708  may refer to the carrier  306  of the second embodiment, and description thereof is thus omitted. 
     Next, referring to  FIG.  7 C , packaging may be performed by glue injection using a colloidal state packaging material, thus forming a package  714  on the carrier  708 , and the package  714  encapsulates the BAW structure  700 . Compared with  FIG.  6 D , since the gap  712  is filled, it cannot be used as a cavity for sound wave reflection. Due to the use of the colloidal state packaging material, the package  714  fills up the gap  712 . Therefore, the manufacturing method of the BAW device of the present embodiment preferably uses a BAW structure having an acoustic reflector on both surfaces (as shown in  FIG.  5 F ). 
       FIG.  8    is a schematic view of a BAW device according to a sixth embodiment of the disclosure. 
     Referring to  FIG.  8   , a BAW device of the sixth embodiment at least includes a plurality of BAW structures  800   a  to  800   c  connected to each other in series, a plurality of bumps  802 , a carrier  804  and a package  808 . Basically, the bumps  802 , the carrier  804  and the package  808  may all be deduced with reference to the second embodiment, and description thereof is thus omitted. In addition, the BAW structures  800   a  to  800   c  may further have a contact pad  810  and a passivation layer  812 , and the carrier  804  may further have a contact pad  814 . The BAW structures  800   a  to  800   c  may have different filtering effects for different bands (frequencies), and therefore, the BAW device of the present embodiment may be applied for different frequency ranges. Although  FIG.  8    shows three BAW structures  800   a  to  800   c , the disclosure is not limited thereto, and the number of BAW structures may be increased or decreased as needed. 
     Moreover, the BAW device as shown in  FIG.  8    can also be manufactured by the method of the fourth embodiment or the fifth embodiment. For example, in the flip-chip bonding step of  FIG.  6 B  or  FIG.  7 B , the BAW structures  800   a  to  800   c  may all be bonded to the carrier  804 , and the carrier  804  is electrically connected to first and second electrodes (not illustrated) respectively of each of the BAW structures  800   a ,  800   b  and  800   c  via the bumps  802 . The second electrodes of the BAW structures  800   a ,  800   b  and  800   c  may be connected to each other through leads in the carrier  804  to form a common electrode (not illustrated), and different voltages are applied to the first electrodes of the BAW structures  800   a ,  800   b  and  800   c , so as to achieve the application in different frequency ranges. In addition, although  FIG.  8    shows that the package  808  fills up the gap  806 , the package  808  may not completely fill the gap  806 . 
     In summary, in the disclosure, a single crystal piezoelectric material layer is formed on a single crystal substrate. Therefore, the Q value of the BAW structure is effectively improved, and the thus obtained BAW structure has no substrate, and the need for miniaturized products can thus be met in terms of structural size (thickness). Also, with a specific package structure, the disclosure can be used as a BAW device (such as a filter). In addition, if BAW structures with different filtering frequencies are disposed in the same BAW device, the application for different frequency ranges can be performed. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed embodiments without departing from the scope or spirit of the disclosure. In view of the foregoing, it is intended that the disclosure covers modifications and variations provided that they fall within the scope of the following claims and their equivalents.