Patent Publication Number: US-2023138460-A1

Title: Method and system for manufacturing a semiconductor package structure

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 17/086,179 filed Oct. 30, 2020, now issued as U.S. Pat. No. 11,538,787, the contents of which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     1. Field of the Disclosure 
     The present disclosure relates to a method and system for manufacturing a semiconductor package structure, and to a method including a thinning step and a system for accomplishing the same. 
     2. Description of the Related Art 
     In order to reduce a thickness of a semiconductor package structure, a molded wafer that is in an intermediate state of fabrication is thinned. However, the molded wafer may include a semiconductor die and a molding compound covering the semiconductor die. The thinned molded wafer may have a severe warpage due to the small thickness of the thinned molded wafer, and the CTE mismatch between the semiconductor die and the molding compound. Thus, the chuck may not handle the thinned molded wafer successfully. Thus, the following step may be difficult to be conducted to the warped and thinned molded wafer. 
     SUMMARY 
     In some embodiments, a method for manufacturing a semiconductor package structure includes: (a) providing a package body including at least one semiconductor device encapsulated in an encapsulant; (b) providing a flattening force to the package body; (c) thinning the package body after (b); (d) attaching a film to the package body; and (e) releasing the flattening force after (d). 
     In some embodiments, a system for manufacturing a semiconductor package structure includes a grinding unit, a film attaching unit and a tape removing unit. The grinding unit is used for grinding a package body with a tape. The film attaching unit is used for attaching a film to the package body. The tape removing unit is used for removing the tape from the package body. The film attaching unit is disposed between the grinding unit and the tape removing unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of some embodiments of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that various structures may not be drawn to scale, and dimensions of the various structures may be arbitrarily increased or reduced for clarity of discussion. 
         FIG.  1    illustrates a schematic arrangement of a system for manufacturing a semiconductor package structure according to some embodiments of the present disclosure. 
         FIG.  2    illustrates one or more stages of an example of a method for manufacturing a semiconductor package structure according to some embodiments of the present disclosure. 
         FIG.  3    illustrates one or more stages of an example of a method for manufacturing a semiconductor package structure according to some embodiments of the present disclosure. 
         FIG.  4    illustrates one or more stages of an example of a method for manufacturing a semiconductor package structure according to some embodiments of the present disclosure. 
         FIG.  4 A  illustrates one or more stages of an example of a method for manufacturing a semiconductor package structure according to some embodiments of the present disclosure. 
         FIG.  5    illustrates one or more stages of an example of a method for manufacturing a semiconductor package structure according to some embodiments of the present disclosure. 
         FIG.  6    illustrates one or more stages of an example of a method for manufacturing a semiconductor package structure according to some embodiments of the present disclosure. 
         FIG.  7    illustrates one or more stages of an example of a method for manufacturing a semiconductor package structure according to some embodiments of the present disclosure. 
         FIG.  8    illustrates one or more stages of an example of a method for manufacturing a semiconductor package structure according to some embodiments of the present disclosure. 
         FIG.  9    illustrates one or more stages of an example of a method for manufacturing a semiconductor package structure according to some embodiments of the present disclosure. 
         FIG.  10    illustrates one or more stages of an example of a method for manufacturing a semiconductor package structure according to some embodiments of the present disclosure. 
         FIG.  11    illustrates one or more stages of an example of a method for manufacturing a semiconductor package structure according to some embodiments of the present disclosure. 
         FIG.  12    illustrates one or more stages of an example of a method for manufacturing a semiconductor package structure according to some embodiments of the present disclosure. 
         FIG.  13    illustrates a schematic arrangement of a system for manufacturing a semiconductor package structure according to some embodiments of the present disclosure. 
         FIG.  14    illustrates one or more stages of an example of a method for manufacturing a semiconductor package structure according to some embodiments of the present disclosure. 
         FIG.  15    illustrates one or more stages of an example of a method for manufacturing a semiconductor package structure according to some embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Common reference numerals are used throughout the drawings and the detailed description to indicate the same or similar components. Embodiments of the present disclosure will be readily understood from the following detailed description taken in conjunction with the accompanying drawings. 
     The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below to explain certain aspects of the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed or disposed in direct contact, and may also include embodiments in which additional features may be formed or disposed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
       FIG.  1    illustrates a schematic arrangement of a system  5  for manufacturing a semiconductor package structure according to some embodiments of the present disclosure. The system  5  may include a tape attaching unit  50 , a grinding unit  51 , an adhesive releasing unit  52 , a film attaching unit  53 , an overturning unit  54 , a tape removing unit  55 , a curing unit  56 , a marking unit  57  and a sawing unit  58 . 
     As shown in  FIG.  1   , the tape attaching unit  50 , the grinding unit  51 , the adhesive releasing unit  52 , the film attaching unit  53 , the overturning unit  54 , the tape removing unit  55 , the curing unit  56 , the marking unit  57  and the sawing unit  58  may be disposed or arranged in sequence. That is, a workpiece may be sequentially processed in the tape attaching unit  50 , the grinding unit  51 , the adhesive releasing unit  52 , the film attaching unit  53 , the overturning unit  54 , the tape removing unit  55 , the curing unit  56 , the marking unit  57  and the sawing unit  58 . That is, the workpiece may be delivered or transferred in such order. For example, according to such arrangement, the film attaching unit  53  is disposed between the grinding unit  51  and the tape removing unit  55 , and the overturning unit  54  is disposed between the film attaching unit  53  and the tape removing unit  55 . 
     The tape attaching unit  50  and the film attaching unit  53  are used for attaching a tape of a film onto the workpiece. The grinding unit  51  is used for grinding a surface of the workpiece so as to thin the workpiece. The adhesive releasing unit  52  is used for releasing the adhesive of a tape on the workpiece. The overturning unit  54  is used for turning the workpiece over. The tape removing unit  55  is used for removing a tape from the workpiece. The curing unit  56  is used for curing or solidifying at least a portion of the workpiece. The marking unit  57  is used for forming a mark on or in the workpiece. The sawing unit  58  is used for sawing or cutting the workpiece into a plurality of singulated devices. 
       FIG.  2    through  FIG.  11    illustrate a method for manufacturing a semiconductor package structure according to some embodiments of the present disclosure. In some embodiments, the method is for manufacturing the semiconductor package structure  6  shown in  FIG.  11   . 
     Referring to  FIG.  2    and  FIG.  3   , wherein  FIG.  3    illustrates a partially enlarged view of a region of the package body  1  in  FIG.  2   , a package body  1  may be formed on a carrier  9 . The carrier  9  may be a glass carrier, and may be in a wafer type, a panel type or a strip type. The package body  1  may be in a molded wafer type or in a molded panel type, and may have a first surface  11  (e.g., a top surface) and a second surface  12  (e.g., a bottom surface) opposite to the first surface  11 . The package body  1  may include at least one semiconductor device  13 , an encapsulant  14 , a redistribution structure  15 , a plurality of under bump metallurgies (UBMs)  154  and a plurality of external connectors  16 . The at least one semiconductor device  13  may include a plurality of semiconductor dice  13  disposed side by side. The semiconductor device  13  has a first surface  131  (e.g., an active surface), a second surface  132  (e.g., a backside surface) and a lateral side surface  133 . The first surface  131  (e.g., an active surface) of the semiconductor device  13  is adjacent to the first surface  11  of the package body  1 . That is, the first surface  11  of the package body  1  is nearer the first surface  131  (e.g., an active surface) of the semiconductor device  13  than the second surface  12  of the package body  1  is. A distance between the first surface  11  of the package body  1  and the first surface  131  (e.g., an active surface) is less than a distance between the second surface  12  of the package body  1  and the first surface  131  (e.g., an active surface) of the semiconductor device  13 . The second surface  132  is opposite to the first surface  131 , and the lateral side surface  133  extends between the first surface  131  and the second surface  132 . The second surface  132  of the semiconductor device  13  may be disposed on or attached to the carrier  9 . The semiconductor device  13  may include a plurality of conductive pads  133  and a plurality of stud bumps  134 . The conductive pads  133  may include copper, aluminum or gold, and may be disposed adjacent to or exposed from the first surface  131  of the semiconductor device  13 . The stud bumps  134  may be disposed on the conductive pads  133  and may protrude from the first surface  131  of the semiconductor device  13 . In some embodiments, the stud bump  134  may include copper (Cu), and may be in a pillar form. 
     The encapsulant  14  may be a cured molding compound with or without fillers. The encapsulant  14  may cover the carrier  9 , the second surface  132  of the semiconductor device  13 , the lateral side surface  133  of the semiconductor device  13  and the stud bumps  134  of the semiconductor device  13 . Thus, the encapsulant  14  encapsulates the stud bumps  134  and the semiconductor device  13 . The semiconductor device  13  is encapsulated in the encapsulant  14 . The encapsulant  14  has a first surface  141  and a second surface  142  opposite to the first surface  141 . In some embodiments, the first surface  141  of the encapsulant  14  may be ground, and the top surfaces of the stud bumps  134  may be substantially coplanar with the first surface  141  of the encapsulant  14 . Thus, the top surfaces of the stud bumps  134  may be exposed from the first surface  141  of the encapsulant  14 . The second surface  132  of the semiconductor device  13  may be substantially coplanar with the second surface  142  of the encapsulant  14 , and they may be the second surface  12  of the package body  1 . 
     The redistribution structure  15  may be disposed on the first surface  141  of the encapsulant  14 , and may include a first dielectric layer  151 , an interconnection structure  152  and a second dielectric layer  153 . The first dielectric layer  151  may cover the encapsulant  14 , and may define a plurality of openings  1511  to expose the top surfaces of the stud bumps  134 . In some embodiments, the first dielectric layer  151  may include, or be formed from, a photoresist layer, a cured photosensitive material, a cured photoimageable dielectric (PID) material such as a polyamide (PA), an Ajinomoto build-up film (ABF), a bismaleimide-triazine (BT), a polyimide (PI), epoxy or polybenzoxazole (PBO), or a combination of two or more thereof. 
     The interconnection structure  152  may be formed on a top surface of the first dielectric layer  151  and in the openings  1511  of the first dielectric layer  151 . The interconnection structure  152  may be a fan-out redistribution layer or a fan-in redistribution layer. For example, the interconnection structure  152  may include a plurality of conductive traces  1521  and a plurality of conductive pads  1522 ,  1523 . The conductive pads  1522  may be disposed in the openings  1511  of the first dielectric layer  151  and contact the top surfaces of the stud bumps  134 . The conductive pads  1523  may be disposed right under the UBMs  154  and may be also referred to as “capture lands”. The conductive traces  1521  extend between the conductive pads  1522 ,  1523 . In some embodiments, the conductive traces  1521  and the conductive pads  1522 ,  1523  may be formed integrally and concurrently. As shown in  FIG.  2    and  FIG.  3   , the interconnection structure  152  may include one circuit layer; however, in other embodiments, the interconnection structure  152  may include a plurality of circuit layers electrically connected to one another. 
     The second dielectric layer  153  may cover the first dielectric layer  151  and the interconnection structure  152 , and may define a plurality of openings  1531  to expose the conductive pads  1523  (i.e., the capture lands) of the interconnection structure  152 . In some embodiments, the second dielectric layer  153  may include, or be formed from, a photoresist layer, a cured photosensitive material, a cured photoimageable dielectric (PID) material such as a polyamide (PA), an Ajinomoto build-up film (ABF), a bismaleimide-triazine (BT), a polyimide (PI), epoxy or polybenzoxazole (PBO), or a combination of two or more thereof. 
     The UBMs  154  may be formed on a top surface of the second dielectric layer  153  and in the openings  1531  of the second dielectric layer  153  so as to contact the conductive pads  1523  (i.e., the capture lands) of the interconnection structure  152 . The external connectors  16  (e.g., solder balls) may be formed or disposed on the UBMs  154 . Thus, the external connectors  16  are disposed on the redistribution structure  15 . 
     Referring to  FIG.  4   , a flattening force may be provided or applied to the package body  1 . In some embodiments, the package body  1  and the carrier  9  may be provided or moved to the tape attaching unit  50  ( FIG.  1   ). In the tape attaching unit  50 , a tape  2  may be attached to or disposed on the first surface  11  of the package body  1  to cover the redistribution structure  15  of the package body  1  and the external connectors  16 . That is, the tape attaching unit  50  may be used for attaching the tape  2  to the first surface  11  of the package body  1 . In some embodiments, the tape  2  may be a back grinding (BG) tape, and a thickness of the tape  2  may be greater than a thickness of the package body  1 . For example, the thickness of the tape  2  may be about 500 μm, and the thickness of the package body  1  may be about 300 μm. Meanwhile, an assembly  4  including the tape  2 , the package body  1  and the carrier  9  is formed. In addition, the thick tape  2  (e.g., the BG tape) may provide stiffness and rigidity, which may provide flattening force to the package body  1  and reduce the warpage of the package body  1 . 
     Referring to  FIG.  4 A , the flattening force may be further provided or applied to the package body  1  by holding the package body  1  on a flatterning chuck  9 ′. As shown in  FIG.  4 A , the assembly  4  of the tape  2 , the package body  1  and the carrier  9  is attached to or fixed on a flatterning chuck  9 ′ by suction, for example, vacuum suction. That is, the flatterning chuck  9 ′ is utilized to suck the assembly  4  of the tape  2 , the package body  1  and the carrier  9 . The flatterning chuck  9 ′ has a receiving surface  91 ′, and includes a plurality of suction holes  93 ′. Each of the suction holes  93 ′ is communicated with a vacuum source, and has an opening at the receiving surface  91 ′. In some embodiments, the assembly  4  is sucked on the receiving surface  91 ′ of the flatterning chuck  9 ′ through the suction holes  93 ′. The carrier  9  of the assembly  4  may contact the receiving surface  91 ′ of the flatterning chuck  9 ′. The flatterning chuck  9 ′ may provide suction force to the entire package body  1 , which may provide flattening force to the package body  1  and reduce the warpage of the package body  1 . 
     Referring to  FIG.  5   , the assembly  4  of the tape  2 , the package body  1  and the carrier  9  is attached to or fixed on a first chuck  8  by suction, for example, vacuum suction. That is, the first chuck  8  is utilized to suck the assembly  4  of the tape  2 , the package body  1  and the carrier  9 . The first chuck  8  has a receiving surface  81 , and includes a plurality of suction holes  83 . Each of the suction holes  83  is communicated with a vacuum source, and has an opening at the receiving surface  81 . In some embodiments, the assembly  4  is sucked on the receiving surface  81  of the first chuck  8  through the suction holes  83 . The tape  2  of the assembly  4  may contact the receiving surface  81  of the first chuck  8 . 
     Referring to  FIG.  6   , the assembly  4  and the first chuck  8  may be provided or moved to the grinding unit  51  ( FIG.  1   ). In the grinding unit  51 , the carrier  9  is removed, and the second surface  12  of the package body  1  is ground by the grinding head  7 . Thus, the package body  1  is thinned from the second surface  12  of the package body  1 . During the grinding process, the tape  2  (e.g., a back grinding (BG) tape) can provide buffer, so as to protect the external connectors  16  from damage caused by the press of the grinding head  7 . 
     Referring to  FIG.  7   , after the grinding head  7  is removed from the thinned package body  1 , the thinned package body  1  may have a large warpage due to the thinness of the thinned package body  1 , and the CTE (coefficient of thermal expansion) mismatch between the encapsulant  14 , the semiconductor devices  13  and the redistribution structure  15 . In some embodiments, the large warpage may be greater than 7 μm. 
     Then, the thinned package body  1  and the tape  2  that are sucked on the first chuck  8  may be provided or moved to the adhesive releasing unit  52  ( FIG.  1   ). The adhesive releasing unit  52  may include a light irradiation source such as a UV (Ultra Violet) light source. In the adhesive releasing unit  52 , the tape  2  is irradiated by the UV light so that the adhesive of the tape  2  is reduced or released. That is, the adhesive releasing unit  52  may be used for releasing the adhesive of the tape  2 . 
     Referring to  FIG.  8   , the thinned package body  1  and the tape  2  that are sucked on the first chuck  8  may be provided or moved to the film attaching unit  53  ( FIG.  1   ). In the film attaching unit  53 , a film  3  may be attached to or disposed on the second surface  12  of the package body  1 . That is, the film attaching unit  53  may be used for attaching the film  3  to the second surface  12  of the package body  1 . In some embodiments, the film  3  may be a pre-cut lamination film. For example, the pre-cut lamination film may be a double-layered structure that includes a back side coating film (or back side film)  31  and a dicing tape  32 . That is, the pre-cut lamination film may be a combination of the dicing tape  32  and the back side coating film  31 . The back side coating film  31  may be used for forming a mark thereon, and a thickness of the back side coating film  31  may be about 25 μm to about 40 μm. The dicing tape  32  may be used in a dicing step or a sawing step, and a thickness of the dicing tape  32  may be about 80 μm to about 100 μm. Meanwhile, an assembly  4 ′ including the tape  2 , the package body  1  and the film  3  is formed. 
     In the illustrated embodiment, the film  3  is attached to the package body  1  before the tape  2  (e.g., the BG tape) is removed. That is, the package body  1  may be interposed between the tape  2  (e.g., the BG tape) and the film  3 . Thus, the film  3  can be attached to the package body  1  easily since the package body  1  has a small warpage such as a warpage of less than 7 μm. In a comparative embodiment, the film  3  is attached to the package body  1  after the tape  2  (e.g., the BG tape) is removed. When the tape  2  (e.g., the BG tape) is removed from the thinned package body  1 , the thinned package body  1  may have a severe warpage (e.g., a warpage of greater than 7 μm) and may be difficult to be sucked by a chuck. As a result, the film  3  is difficult to be attached to the package body  1  in the subsequent stage. Further, in the illustrated embodiment, the thick tape  2  (e.g., the BG tape) may provide stiffness and rigidity, which may reduce the warpage of the package body  1 . In addition, in the illustrated embodiment, the film  3  may be attached to the second surface  12  of the package body  1  by a roller. Thus, the warpage of the package body  1  may be reduced due to the press force of the roller. 
     Referring to  FIG.  9   , the assembly  4 ′ of the tape  2 , the package body  1  and the film  3  sucked by the first chuck  8  is turned over in the overturning unit  54  ( FIG.  1   ) so that the assembly  4 ′ of the tape  2 , the package body  1  and the film  3  is attached to or fixed on a second chuck  8 ′ by suction, for example, vacuum suction. That is, the overturning unit  54  is used for turning over the assembly  4 ′ of the package body  1 , the tape  2  and the film  3 . In addition, the second chuck  8 ′ is utilized to suck the assembly  4 ′ of the tape  2 , the package body  1  and the film  3 . The second chuck  8 ′ has a receiving surface  81 ′, and includes a plurality of suction holes  83 ′. Each of the suction holes  83 ′ is communicated with a vacuum source, and has an opening at the receiving surface  81 ′. In some embodiments, the assembly  4 ′ is sucked on the receiving surface  81 ′ of the second chuck  8 ′ through the suction holes  83 ′. The film  3  of the assembly  4 ′ may contact the receiving surface  81 ′ of the second chuck  8 ′. Then, the first chuck  8  is removed. 
     Referring to  FIG.  10   , the assembly  4 ′ and the second chuck  8 ′ may be provided or moved to the tape removing unit  55  ( FIG.  1   ). In the tape removing unit  55 , the tape  2  is removed from the package body land the external connectors  16  are exposed. That is, the film-removing unit  53  may be used for removing the tape  2 . Since the adhesive of the tape  2  is released or reduced, the tape  2  may be removed easily and no residue will be left on the package body  1  after removal. 
     Then, the package body  1  and the film  3  that are sucked on the second chuck  8 ′ may be provided or moved to the curing unit  56  ( FIG.  1   ). In the curing unit  56 , the back side coating film  31  of the film  3  may be cured or solidified. That is, the curing unit  56  may be used for curing or solidifying the back side coating film  31  of the film  3 . 
     Then, the package body  1  and the film  3  are removed from the second chuck  8 ′, and then may be provided or moved to the marking unit  57  ( FIG.  1   ). In the marking unit  57 , a mark may be formed on or in the back side coating film  31  of the film  3 . That is, the marking unit  57  may be used for forming a mark on or in the back side coating film  31  of the film  3 . It is noted that during the formation of the mark, the dicing tape  32  is still on the back side coating film  31 . Thus, the light irradiation source (e.g., laser light) that is used for forming the mark may pass through the dicing tape  32  of the film  3 . 
     Referring to  FIG.  11   , the package body  1  and the film  3  may be provided or moved to the sawing unit  58  ( FIG.  1   ). In the sawing unit  58 , the package body  1  and the film  3  may be attached to or mounted on a cutting frame, and then they are sawed or singulated to form a plurality of semiconductor package structures  6  of  FIG.  11   . 
     As shown in  FIG.  11   , the semiconductor package structure  6  may include the singulated package body  1  and the singulated back side coating film  31 . The singulated package body  1  may include the semiconductor device  13 , the encapsulant  14 , the redistribution structure  15 , the UBMs  154  and the external connectors  16 . 
       FIG.  12    illustrates a method for manufacturing a semiconductor package structure according to some embodiments of the present disclosure. The initial stages of the illustrated process are the same as, or similar to, the stage illustrated in  FIG.  2    to  FIG.  7   .  FIG.  12    depicts a stage subsequent to that depicted in  FIG.  7   . 
     Referring to  FIG.  12   , the thinned package body  1  and the tape  2  that are sucked on the first chuck  8  may be provided or moved to the film attaching unit  53  ( FIG.  1   ). In the film attaching unit  53 , a film  3 ′ may be formed or disposed on the second surface  12  of the package body  1 . In some embodiments, the film  3 ′ may be a single layer structure that is a dicing tape  32 ′. The dicing tape  32 ′ may be used in a dicing step or a sawing step, and a thickness of the dicing tape  32 ′ may be about 80 μm to about 100 μm. 
     Then, the following stages of the method may be similar to the stages illustrated in  FIG.  9    to  FIG.  11    so as to obtain a plurality of semiconductor package structures. The semiconductor package structure formed from the illustrated process may be similar to the semiconductor package structure  6  of  FIG.  11   , except that the semiconductor package structure of the illustrated embodiment may not include the back side coating film  31 . 
       FIG.  13    illustrates a schematic arrangement of a system  5   a  for manufacturing a semiconductor package structure according to some embodiments of the present disclosure. The system  5   a  of  FIG.  13    is similar to the system  5  of  FIG.  1   , except that the system  5   a  of  FIG.  13    further include an additional film attaching unit  53   a  disposed between the film attaching unit  53  and the tape removing unit  55 . 
       FIG.  14  through  15    illustrates a method for manufacturing a semiconductor package structure according to some embodiments of the present disclosure. In some embodiments, the method is for manufacturing the semiconductor package structure  6  shown in  FIG.  11   . The initial stages of the illustrated process are the same as, or similar to, the stage illustrated in  FIG.  2    to  FIG.  7   .  FIG.  14    depicts a stage subsequent to that depicted in  FIG.  7   . 
     Referring to  FIG.  14   , the thinned package body  1  and the tape  2  that are sucked on the first chuck  8  may be provided or moved to the film attaching unit  53  ( FIG.  13   ). In the film attaching unit  53 , a film  3   a  may be formed or disposed on the second surface  12  of the package body  1 . In some embodiments, the film  3   a  may be a single layer structure that is a back side coating film  31 . 
     Referring to  FIG.  15   , the film  3   a , the thinned package body  1  and the tape  2  that are sucked on the first chuck  8  may be provided or moved to the additional film attaching unit  53   a  ( FIG.  13   ). In the additional film attaching unit  53   a , a film  3   b  may be attached or disposed on the film  3   a  (e.g., the back side coating film  31 ). In some embodiments, the film  3   b  may be a single layer structure that is a dicing tape. 
     Then, the following stages of the method may be similar to the stages illustrated in  FIG.  9    to  FIG.  11    so as to obtain a plurality of semiconductor package structures  6  of  FIG.  11   . 
     Spatial descriptions, such as “above,” “below,” “up,” “left,” “right,” “down,” “top,” “bottom,” “vertical,” “horizontal,” “side,” “higher,” “lower,” “upper,” “over,” “under,” and so forth, are indicated with respect to the orientation shown in the figures unless otherwise specified. It should be understood that the spatial descriptions used herein are for purposes of illustration only, and that practical implementations of the structures described herein can be spatially arranged in any orientation or manner, provided that the merits of embodiments of this disclosure are not deviated from by such an arrangement. 
     As used herein, the terms “approximately,” “substantially,” “substantial” and “about” are used to describe and account for small variations. When used in conjunction with an event or circumstance, the terms can refer to instances in which the event or circumstance occurs precisely as well as instances in which the event or circumstance occurs to a close approximation. For example, when used in conjunction with a numerical value, the terms can refer to a range of variation of less than or equal to ±10% of that numerical value, such as less than or equal to ±5%, less than or equal to ±4%, less than or equal to ±3%, less than or equal to ±2%, less than or equal to ±1%, less than or equal to ±0.5%, less than or equal to ±0.1%, or less than or equal to ±0.05%. For example, a first numerical value can be deemed to be “substantially” the same or equal to a second numerical value if the first numerical value is within a range of variation of less than or equal to ±10% of the second numerical value, such as less than or equal to ±5%, less than or equal to ±4%, less than or equal to ±3%, less than or equal to ±2%, less than or equal to ±1%, less than or equal to ±0.5%, less than or equal to ±0.1%, or less than or equal to ±0.05%. For example, “substantially” perpendicular can refer to a range of angular variation relative to 90° that is less than or equal to ±10°, such as less than or equal to ±5°, less than or equal to ±4°, less than or equal to ±3°, less than or equal to ±2°, less than or equal to ±1°, less than or equal to ±0.5°, less than or equal to ±0.1°, or less than or equal to ±0.05°. For example, a characteristic or quantity can be deemed to be “substantially” consistent if a maximum numerical value of the characteristic or quantity is within a range of variation of less than or equal to +10% of a minimum numerical value of the characteristic or quantity, such as less than or equal to +5%, less than or equal to +4%, less than or equal to +3%, less than or equal to +2%, less than or equal to +1%, less than or equal to +0.5%, less than or equal to +0.1%, or less than or equal to +0.05%. 
     Two surfaces can be deemed to be coplanar or substantially coplanar if a displacement between the two surfaces is no greater than 5 μm, no greater than 2 μm, no greater than 1 μm, or no greater than 0.5 μm. A surface can be deemed to be substantially flat if a displacement between a highest point and a lowest point of the surface is no greater than 5 μm, no greater than 2 μm, no greater than 1 μm, or no greater than 0.5 μm. 
     As used herein, the singular terms “a,” “an,” and “the” may include plural referents unless the context clearly dictates otherwise. 
     As used herein, the terms “conductive,” “electrically conductive” and “electrical conductivity” refer to an ability to transport an electric current. Electrically conductive materials typically indicate those materials that exhibit little or no opposition to the flow of an electric current. One measure of electrical conductivity is Siemens per meter (S/m). Typically, an electrically conductive material is one having a conductivity greater than approximately 10 4  S/m, such as at least 10 5  S/m or at least 10 6  S/m. The electrical conductivity of a material can sometimes vary with temperature. Unless otherwise specified, the electrical conductivity of a material is measured at room temperature. 
     Additionally, amounts, ratios, and other numerical values are sometimes presented herein in a range format. It is to be understood that such range format is used for convenience and brevity and should be understood flexibly to include numerical values explicitly specified as limits of a range, but also to include all individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly specified. 
     While the present disclosure has been described and illustrated with reference to specific embodiments thereof, these descriptions and illustrations are not limiting. It should be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the true spirit and scope of the present disclosure as defined by the appended claims. The illustrations may not be necessarily drawn to scale. There may be distinctions between the artistic renditions in the present disclosure and the actual apparatus due to manufacturing processes and tolerances. There may be other embodiments of the present disclosure which are not specifically illustrated. The specification and drawings are to be regarded as illustrative rather than restrictive. Modifications may be made to adapt a particular situation, material, composition of matter, method, or process to the objective, spirit and scope of the present disclosure. All such modifications are intended to be within the scope of the claims appended hereto. While the methods disclosed herein have been described with reference to particular operations performed in a particular order, it will be understood that these operations may be combined, sub-divided, or re-ordered to form an equivalent method without departing from the teachings of the present disclosure. Accordingly, unless specifically indicated herein, the order and grouping of the operations are not limitations of the present disclosure.