Abstract:
A packaged MEMS device, wherein at least two support structures are stacked on each other and are formed both by a support layer and a wall layer coupled to each other and delimiting a respective chamber. The chamber of the first support structure is upwardly delimited by the support layer of the second support structure. A first and a second dice are accommodated in a respective chamber, carried by the respective support layer of the first support structure. The support layer of the second support structure has a through hole allowing wire connections to directly couple the first and the second dice. A lid substrate, coupled to the second support structure, closes the chamber of the second support structure.

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a package for a MEMS (Micro-Electro-Mechanical System) sensor, in particular a capacitive microphone, and to the manufacturing process thereof. 
     2. Description of the Related Art 
     As is known, a MEMS sensor, e.g., an acoustic transducer such as a capacitive microphone, generally comprises a micromechanical sensing structure, designed to convert a mechanical stress (e.g., acoustic pressure waves) into an electrical quantity (for example, by exploiting variations of the electrical quantity in a capacitive structure of the MEMS sensor due to the acoustic pressure waves), and reading electronics, designed to carry out appropriate processing operations (including amplification and filtering) of the electrical quantity for supplying an electrical output signal (for example, a voltage). 
     Generally, an MEMS acoustic transducer is formed in a die including a structural layer of semiconductor material, for example silicon, accommodating a cavity. A membrane, or diaphragm, extends on top of the cavity; the membrane is flexible and, in use, undergoes deformation as a function of the pressure of the incident sound waves. A rigid plate (generally referred to as “back-plate”) extends at a distance from the diaphragm. The back-plate and the diaphragm thus form a movable electrode and a fixed electrode of a variable capacitor. The die further comprises contacts, used for biasing the membrane and the back-plate and for receiving an electrical signal resulting from the deformation of the membrane caused by the incident acoustic pressure waves. 
     The die implementing the acoustic transducer is enclosed in a package, accommodating also reading electronics associated thereto, generally provided as an ASIC in a respective die of semiconductor material. 
     A known package as above described is shown for clarity in  FIG. 1 . 
       FIG. 1  shows a package  1  including a cap  2  and a substrate  3 . A first die  10 , forming the MEMS sensing structure, and a second die  11 , forming an ASIC integrating a reading electronics, are coupled side-by-side on the substrate  3 . The first die  10  has a cavity  14  delimiting a diaphragm  15  (for simplicity, no back plate has been shown herein). Electrical connections  12  between the first and second dice  10 ,  11  and between the second die  11  and the substrate  3 , are provided using a wire-bonding technique. Metallization layers and vias (not shown) are provided through the substrate  3  for routing the electrical signals towards the outside. Pads (in the case of an LGA—Land-Grid Array—package), or conductive spherical elements (in the case of a BGA—Ball-Grid Array—package), or similar connection elements, are moreover provided on the underside of the substrate  3  for soldering and electrical connection to an external printed circuit of a corresponding electronic device. 
     The cap  2  may be made of metal, or of a pre-molded plastic coated in the inside with a metallization layer, so as to prevent noise due to external electromagnetic signals (by providing a sort of Faraday cage). The cap  2  is generally attached to the substrate  12  by a conductive glue  17  so as to obtain also a ground connection towards the substrate  3 . The cap  2  further has an opening  18  allowing acoustic pressure waves from the external environment to enter the package  1 . 
     This known solution is susceptible of improvements. In particular, since the cap  2  is made by a molding technique, it requires specific and dedicated molding tools (comprising, for example, dies and punches), for each possible variation of dimensions and shapes, for example in case of variations of the silicon dimensions or in presence of different customer requirements. In addition, the pitch and layout of the molding and punching tools are not always compatible with the dimensions and configuration of the array of contacts. 
     Furthermore, this known solution has large dimensions for accommodating two dice side-by-side and arranging the cap, and in general it does not offer the designer a sufficient design freedom in sizing the front and back chambers of the acoustic transducer. 
     EP-1 755 360A discloses a package wherein the metal cap is secured to the substrate by welding rather than using a conductive epoxy. 
     US 2008/0063232 discloses a method of enclosing a silicon microphone in a plastic molded cap on which a metal layer has been deposited. 
     U.S. Pat. No. 7,166,910 in FIGS. 6-10 describes and shows a package layout with a MEMS transducer mounted on the top side of the package. 
     PCT/EP2010/070608, filed on 29 Dec. 2010 in the name of the same Applicant, discloses a package including a substrate carrying two dice including a MEMS chip and an ASIC; a wall structure, formed from a board and attached to the substrate to define a chamber accommodating the dice; and a cap layer upwardly closing the cavity. The dice are directly connected to connection elements formed on the face of the wall structure looking toward the cap layer. 
     All the above solutions may be improved in order to better exploit the available space and reduce the general bulk of the package for a given dimensions of the dice. In addition, a higher design freedom in sizing the back volume, on the back of the MEMS chip, is desirable. 
     BRIEF SUMMARY 
     One embodiment of the present disclosure is directed to a packaged MEMS device having a first support structure that includes a first chamber, a first support layer, and a first wall layer coupled to the first support layer, the first wall layer and the first support layer delimiting the first chamber. The MEMS device also includes a second support structure stacked on the first support structure, the second support structure including a second chamber, a second support layer, and a second wall layer coupled to the second support layer, the second wall layer and the second support layer delimiting the second chamber, the first chamber being delimited, at least in part, by the second support layer of the second support structure. A first die is on the first support layer in the first chamber and a second die is on the second support layer in the second chamber, and a lid is coupled to the second support structure, the lid configured to close the second chamber of the second support structure. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       For a better understanding of the present disclosure, preferred embodiments thereof are now described, purely by way of non-limiting example and with reference to the attached drawings, wherein: 
         FIG. 1  is a schematic cross-section of a known packaged MEMS acoustic transducer; 
         FIGS. 2-6 ,  8 ,  10 ,  12  and  14  show cross-sections of intermediate structures, in subsequent manufacturing steps, according to an embodiment; 
         FIGS. 7 ,  9  and  16  are a perspective schematic views of the intermediate structures of  FIGS. 6 ,  8  and  14 , respectively; 
         FIG. 11  is a perspective, cross-sectional view of the intermediate structures during a stacking step; 
         FIGS. 13 and 15  are perspective, cross-sectional views of the intermediate structures of  FIGS. 12 and 16 , respectively; 
         FIG. 17  is a cross-section of a packaged MEMS acoustic transducer obtained with the present process; 
         FIG. 18  is a cross-section of an intermediate structure, according to a different embodiment; and 
         FIG. 19  shows a general block diagram of an electronic device incorporating the present MEMS acoustic transducer. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 2-17  show an embodiment of a process for manufacturing a package for a MEMS device and is based on the use of composite substrates that are stacked and bonded to each other. In particular, the following description regards the manufacture of a packaged device through stacking of at least two composite structures, each supporting at least one die, wherein each composite structure may be manufactured basically using the technology described in PCT/EP2010/070608 cited above. 
       FIGS. 2-17  show only a portion of layers and structures intended to form a single packaged MEMS device, but the same features shown and described repeat on the sides of the illustrated portions, for simultaneous manufacturing the a plurality of packaged MEMS devices. 
       FIG. 2  shows the coupling of a first and a second package layers  20 ,  21  (also indicated as a support layer and a wall layer) of a same plastic material, in particular an epoxy resin, and specifically a laminated BT (bismaleimide triazine) board, to form a first package substrate, also called first support substrate  25  ( FIG. 3 ). Package layers  20 ,  21  may be made of the same plastic material. A first main face  20   a  of the first package layer  20  and first and second main faces  21   a ,  21   b  of the second package layer  21  are coated, using standard techniques, by thin, first metal layers  18 ,  19 ,  22 . In addition, an adhesion layer  23 , of non-conductive adhesive material, is formed e.g., on the first metal layer  19  covering the first main face  21   a  of the second package layer  21 , which is intended to be bonded to the first package layer  20 . 
     A chamber  24  has been formed (e.g., by a conventional routing process, laser drilling etc.) throughout the thickness of the second package layer  21 , so as to locally remove also the first metal layers  19 ,  22  and the adhesive layer  23 . Cavity  24  may have, for example, a rectangular or a circular shape and a bigger area than a MEMS device to be packaged. 
     Thereafter, as indicated by an arrow in  FIG. 2 , the first and second package layers  20 ,  21  are joined together, in a stacked way, to form a first composite substrate  25 , shown in  FIG. 3 . Thereby a chamber, still designated by number  24  for simplicity, is formed, having lateral walls and a bottom. A seed layer  26  is grown over the remaining portions of the first metal layer  22  on the second main face  21   b  of the second package layer  21 , the lateral walls and the bottom of chamber  24 . Then, for example using an electroplating technique or a sputtering technique, a second metal layer  27  is grown on the seed layer  26  and coats, in particular, the inside of the chamber  24 , forming, together with layers  18 ,  22  and  26 , a coating layer  28 , covering, i.e., the walls and the bottom of chamber  24 . 
     Then,  FIG. 4 , an access port  29 , forming an acoustic access port, is formed through the first package layer  20 , the first metal layer  18  and the coating layer  28 , using standard micromachining techniques. The access port  29  has, for example, circular cross-section and, as will be clarified hereinafter, is designed to enable entry of acoustic pressure waves into the package of the MEMS acoustic transducer. 
     In this step, in a not shown manner, marking of the package can advantageously be carried out. 
     Thereafter,  FIG. 5 , the first composite substrate  25  is turned upside down and a first die, indicated again by  10  and integrating a micromechanical sensing structure, is bonded to the first composite substrate  25  above the access port  29 . In particular, the first die  10  is arranged so that the cavity  14  defining the diaphragm  15  faces and is in fluid communication with the access port  29  to form a front chamber of a MEMS acoustic transducer. 
     Meanwhile,  FIGS. 6 and 7 , a second composite substrate, indicated at  25 ′, is worked as discussed above with reference to  FIGS. 2-4 . Thus, the second composite substrate  25 ′ has a first package layer  20 ′, a second package layer  21 ′, a chamber  24 ′ and a coating layer  28 ′. Here, a connection port  30  is formed instead of the access port  29 . Connection port  30  may be larger than access port  29  and differently positioned, as discussed hereinafter. In particular, connection port  30  is arranged in a position such as to extend approximately above the first die  10 , after coupling the composite substrates. Also chamber  24 ′ may have a different shape from chamber  24  of the first composite substrate  25  (see  FIG. 7 ). In addition, conductive vias  38  may be provided in the first package layer  20 ′, to connect the coating layer  28 ′ with the back of the second composite substrate  25 ′, as discussed below. 
     Then, in  FIGS. 8 and 9 , the second composite substrate  25 ′ undergoes a cutting process to form contact regions for connecting the device to the exterior. To this end, as disclosed in International Application PCT/EP2010/070608, cited above, standard cutting processes are used (so-called “sawing” operation), for example using a diamond-saw cutting tool. Thus, coating layer  28 ′ and a surface portion of second package layer  21 ′ are selectively removed for providing, at first surface  21   a ′ of the second package layer  21 ′, a plurality of contact regions  32 , electrically insulated from one another (see  FIG. 9 ). In particular, contact regions  32  may comprise a frame-shaped contact region  32   a , extending all around chamber  24 ′ and device contact regions  32   b , of smaller dimensions and e.g., square shape. In addition, frame-shaped contact region  32   a  may be connected to the coating layer  28 ′ on the bottom of the chamber  24 ′ and/or be formed by a plurality of electrically insulated regions, if so desired. 
     Then, in  FIG. 10 , a second die  11 , e.g., the ASIC integrating a reading electronics of  FIG. 1 , is bonded to the second composite substrate  25 ′, laterally with respect to the connection port  30 , for example using adhesive material (not shown), so that, after bonding the first and the second composite substrates  25 ,  25 ′, the second die  11  is laterally offset with respect to the first die  10 . 
     Thereafter, in  FIGS. 11-13 , the first and the second composite substrates  25 ,  25 ′ are bonded together, for example by interposing a thin attach film  35  of attach material (not shown). The attach material may be a conductive adhesive medium if a full shielding of both chambers  24  and  24 ′ is desired (with coating layers  28  and  28 ′ electrically coupled through the conductive vias  38  in the first package layer  21 ′, or not conductive, if a full shielding is not a requirement. 
     The thin attach film  35  may be applied to either the back surface of the second composite substrate  25 ′ or to the front surface of the first composite substrate  25  and may be applied by any technique, such as screen printing or standard pump dispensing. After application of the thin attach film  35 , the first and second composite substrates  25 ,  25 ′ are stacked on each other and then bonded, using a standard curing process. Thereby, a multi-level stack substrate  36  is obtained, wherein the thin attach film  35  also acts as a sealing structure for the final packaged device. 
     Then,  FIGS. 14-16 , electrical connections between the first and second dice  10 ,  11  the second die  11  and the device contact regions  32   b  are formed using a wire-bonding technique, forming wires  37 . 
     Thereafter, a sealing material  44  (for example, a conductive resin) is applied to the second composite substrate  25 ′, to seal each final device, after singulation. According to  FIG. 17 , a printed-circuit substrate  45  is attached to the multi-level stack substrate  36 , using a flip-chip technique, after applying conductive bonding regions  46 , e.g., of a solder-paste, to the contact regions  32 . The printed-circuit substrate  45  has contact regions  47  on both its faces, to allow electrical connection with the multi-level stack substrate  36  and with the exterior. To this end, conductive, built-in through vias may be provided in the printed-circuit substrate  45  to electrically connect bottom contact regions  47  to upper contact regions  47 . In some embodiments, the face of the printed-circuit substrate  45  looking toward the multi-level stack substrate  36  may carry additional components (not shown), electrically connected by other contact regions, similar to contact regions  47 . 
     Subsequently, the stacked assembly of the printed-circuit substrate  45  and the multi-level stack substrate  36  is subjected to brazing (so-called “reflow”), so as to obtain their mechanical and electrical bonding, by soldering. Thereby, a package structure  48  is formed. 
     Finally, using traditional cutting techniques, the package structure  48  is cut to obtain a plurality of packaged devices  49  ( FIG. 19 ), e.g., packaged MEMS acoustic transducers. The finished devices are then subjected to the usual testing procedures (so-called sorting operation). 
     The final packaged device  49  thus obtained includes a back chamber of the first die  10  (formed by the chamber  24  internal to the first composite substrate  25 ) that is generally distinct from the chamber  24 ′ defined by the second composite substrate  25 ′ by being upwardly closed by the first layer  20 ′ of the second composite structure  25 ′. 
     This structure allows the first die  10  to be separated from other components (such as the second die  11  or any further components bonded to the printed-circuit substrate  45 ), and the optimization of the dimensions of the back chamber  24  to the specific requirements. In fact, the thickness of the back chamber  24  may be modified by appropriately dimensioning the second package layer  21  and its area may also be optimized during the machining of the chamber  24 . Also the dimensions of the connection port  30  may be selected in order to optimize the separation of the back chamber  24  from the second chamber  24 ′. In fact, in acoustic transducers, the volume of the front chamber  14  (i.e., the space traversed in use by acoustic pressure waves coming from the outside through access port  29 ), and the volume of the back-chamber  24 , set in use at a reference pressure, directly affect the acoustic performance of the transducer. 
     By virtue of the stacking, the bulk of the packaged device  48  is greatly optimized, since it is possible to arrange the dice in a stacked way. On the other hand, since each die is supported by a separate bearing layer (second package layer  21 ), the overall packaged device  48  is very robust, thus allowing also multiple stacking. In practice, the overall package is of a modular structure, including a plurality of stacked, distinct chambers that may be generally separated from each other, wherein the level of separation depends on the dimensions of the through holes in the first package layers (support layers) of the overlying composite structures  25 ,  25 ′. 
     In addition, since each level includes a support layer, a high level of overlapping of the dice may be obtained, if desired, thereby increasing the reduction in the overall bulk. 
     In addition, both the first and the second dice  11 ,  12  are sealed from the external environment and also shielded from electromagnetic disturbance by virtue of the coating layers  28 ,  28 ′ surrounding chambers  24 ,  24 ′. However, the shielding may be applied to only one of the dice  11 ,  12 , if so desired, in which case no through vias  38  need to be formed. 
     The use of a multi-level cavity structure allows the first die  10  to be assembled directly on the package access port  29 , for optimal frequency response, and allows stacking of other devices while keeping the first die  10  substantially separated therefrom, as above discussed. 
     The connection between the first die  10  and the second die  11  through wires allows any parasitic capacitance to be kept to a minimum. 
     In practice, the present package and method improves upon the technique taught in PCT/EP2010/070608 for manufacturing a multiple-chamber package that allows an optimal fine tuning of both the front and the back chambers for high performance, minimizing the floor space by stacking two or more cavities on top of each other and attaining direct connection of a MEMS to an ASIC. This is particularly advantageous when the MEMS device is a capacitive sensor, for example an acoustic transducer, since in this case parasitic capacitances are crucial for the capacitive sensor performance and thus a very good connection between the sensor and the control dice is very critical. 
     According to a different embodiment, the bottom of chamber  24  (formed by the first package layer  20 ) is connected to the top of the first composite substrate  25  by through vias. In this case, of  FIG. 18 , before bonding the first and second package layers  20 ,  21  and before forming the first metal layers  19 ,  22  on the first and second main faces  21   a ,  21   b  of the second package layer  21 , holes  50  are formed throughout the thickness of layer  21 , the walls of the holes  50  are covered by a metal layer  51  and, possibly, the holes are filled with conductive or insulating material  52 , thereby forming vias  53 . Then, the first metal layers  19 ,  22  on the first and second main faces  21   a ,  21   b  of the second package layer  21  are deposited, as well as an adhesion layer  23 . Thereafter, the same step above discussed with reference to  FIGS. 3-17  are performed, including forming the first chamber  24  and bonding the first and second layers  20 ,  21  to obtain the first composite layer  25 , forming the second composite layer  25 ′, bonding the dice  10 ,  11 ; bonding the first and second composite layers  25 ,  25 ; wire bonding the dice  10 ,  11  to each other and to the contact regions  32 ; bonding the printed-circuit substrate  45  and singulating the packaged devices  49 . 
     The packaged device  49  may be used in an electronic device  60 , as shown in  FIG. 19 . The electronic device  60  is preferably a mobile communications device, such as, for example, a cell phone, a PDA, a notebook, but also a voice recorder, an audio-file reader with voice-recording capacity, etc. Alternatively, the electronic device  60  may be a hydrophone, capable of working under water, or else a hearing-aid device. 
     The electronic device  60  may comprise a microprocessor (CPU—central processing unit)  61 , a memory block  62 , connected to the microprocessor  61 , and an input/output interface  63 , for example provided with a keyboard and a display, which is also connected to the microprocessor  61 . The MEMS packaged device  49  communicates with the microprocessor  61 . In particular, the ASIC in the second die  11  sends electrical output signals to the microprocessor  61  (a further electronic circuit for processing these electrical output signals  65  may possibly be present). A loudspeaker  66  is also provided for generation of sounds on an audio output (not shown) of the electronic device  60 . As shown schematically, the MEMS packaged device  49 , the microprocessor  61 , the memory block  62 , the input/output interface  63 , and the possible further electronic components are coupled to a single printed circuit board  67 , for example using the SMD technique. 
     Finally, it is clear that modifications and variations may be made to what has been described and illustrated herein, without thereby departing from the scope of the present disclosure. 
     In particular, the connections between the coating layers  28 ,  28 ′ in the first and second composite layers  25 ,  25 ′ may differ with respect to the described solution and may be obtained using any known technique and means; coating layers  28  and/or  28 ′ may be missing, if not needed; the connections to the outside may be obtained in any known way, and the stacked solution may be applied to any dice wherein distinct back chambers are desired. 
     The stacking procedure may be repeated so as to obtain multiple-level composite structures. 
     The various embodiments described above can be combined to provide further embodiments. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, application and publications to provide yet further embodiments. 
     These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.