Abstract:
A packaged sensor assembly includes: a packaging structure, having at least one opening; a humidity sensor and a pressure sensor, which are housed inside the packaging structure and communicate fluidically with the outside through the opening, and a control circuit, operatively coupled to the humidity sensor and to the pressure sensor; wherein the humidity sensor and the control circuit are integrated in a first chip, and the pressure sensor is integrated in a second chip distinct from the first chip and bonded to the first chip.

Description:
BACKGROUND 
     Technical Field 
     The present disclosure relates to a packaged sensor assembly. 
     Description of the Related Art 
     As is known, attention to the development and integration of microelectromechanical environmental sensors is progressively increasing as the use of portable electronic devices, such as smartphones and tablets or other so-called “wearable” electronic devices, increases. In particular, there is noted a specific interest to enclose a microelectromechanical pressure sensor and a microelectromechanical humidity sensor within a single packaging structure for electronic devices, together with a control circuit such as an application-specific integrated circuit (ASIC). The control circuit may serve for controlling operation of the sensors and as an interface for converting the electrical signals supplied by the sensors into data that may be used by further processing stages for performing various functions. 
     A microelectromechanical pressure sensor generally comprises a flexible membrane, suspended over a cavity in a semiconductor substrate. The membrane is deformed by the pressure difference between the two sides. Sensitive elements, in general of a piezoelectric type, are coupled to the faces of the membrane and enable detection of the degree of deformation. 
     Microelectromechanical humidity sensors are, instead, usually of a capacitive type and comprise electrodes coupled together to form a capacitor and separated by a hygroscopic polymer, the dielectric constant of which varies as a function of the humidity absorbed. 
     A problem generally to be tackled in the integration process is determined by the contrasting application preferences of pressure sensors on the one hand and humidity sensors on the other. Both types of sensors, in fact, have to be exposed to external environmental conditions through openings in the packaging structure to be in condition of operating correctly. However, for humidity sensors it is important to maximize exposure to the external environment to favor absorption of humidity by the hygroscopic polymer, while pressure sensors prefer protection from electromagnetic radiation in the spectrum of the visible and of the infrared. In fact, incident radiation causes parasitic currents and consequent voltage drops that may alter the useful signals. Exposure to the external environment is therefore preferred to be sufficient to provide suitable fluidic connection therewith and, at the same time, should minimize the intensity of incident radiation on the membrane and, in particular, on the piezoresistive elements. 
     In addition to the desire to balance the contrasting preferences for pressure sensors and humidity sensors, it is also desirable to satisfy the general tendency to reduce the dimensions of devices and of the packaging structure as a whole, to render use of electronic devices more flexible and convenient. 
     BRIEF SUMMARY 
     One or more embodiments of the present disclosure is to provide a packaged sensor assembly. 
     One embodiment of the present disclosure is directed to a packaged sensor assembly including a packaging structure having an opening. The sensory assembly further includes a first semiconductor chip that integrates a humidity sensor and a control circuit. The control circuit is operatively coupled to the humidity sensor. The humidity sensor is housed inside the packaging structure and in fluid communication with an environment external to the sensor assembly through the opening. The sensor assembly further includes a second semiconductor chip that integrates a MEMS pressure sensor bonded to the first semiconductor chip. The pressure sensor is operatively coupled to control circuit. The pressure sensor is housed inside the packaging structure and in fluid communication with the environment external to the sensor assembly through the opening. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       For a better understanding of the disclosure, some embodiments thereof will now be described purely by way of non-limiting example and with reference to the attached drawings, wherein: 
         FIG. 1  is a cross-sectional view of a packaged sensor assembly according to one embodiment of the present disclosure; 
         FIG. 2  is a top plan view of an enlarged detail of the packaged sensor assembly of  FIG. 1 ; 
         FIG. 3  is a top plan view of a component of the packaged sensor assembly of  FIG. 1 ; 
         FIG. 4  is an enlarged cross-sectional view of the component of  FIG. 3 ; 
         FIG. 5  is a cross-sectional view of a packaged sensor assembly according to a different embodiment of the present disclosure; 
         FIG. 6  is a cross-sectional view of a packaged sensor assembly according to a further embodiment of the present disclosure; 
         FIG. 7  is a cross-sectional view of a packaged sensor assembly according to yet a further embodiment of the present disclosure; and 
         FIG. 8  is a simplified block diagram of an electronic system incorporating a packaged sensor assembly according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 , a packaged sensor assembly according to one embodiment of the present disclosure is designated as a whole by  1  and comprises a packaging structure  2 , a first chip  3 , in which a humidity sensor  5  and a control circuit or ASIC  7  are integrated, and a second chip  8 , in which an electromechanical transducer, such as a pressure sensor  10 , is integrated. 
     The packaging structure  2  is of a molded type, for example of the land grid array (LGA) type, and comprises a substrate  11 , a lateral structure  12 , and a first cap  13 . The substrate  11  is made, for example, of FR-4 and is provided with metal contact pads or lands  15  on an outer face. The lateral structure  12  may be made of resin and surrounds the first chip  3  and the second chip  8 . In one embodiment, the lateral structure  12  is obtained by molding, for example using a technique of transfer molding, in particular film-assisted molding. 
     The first cap  13  defines a wall of the packaging structure  2  opposite to the substrate  11  and forms a die with the first chip  3 , the second chip  8 , and a second cap  17 . 
     An opening  18  in the first cap  13  sets the inside of the packaging structure  2  in fluidic communication with an environment outside the package. The opening  18  is configured to enable air external to the package to exchange within the packaging structure  2  so that the humidity sensor  5  is reached in a short time by a gaseous mixture in which the packaged sensor assembly  1  is immersed. In one embodiment, the dimensions of the opening  18  are equal to or greater than the dimensions of the humidity sensor. 
     The humidity sensor  5  is based upon MEMS (microelectromechanical systems) technology and, in one embodiment, is of a capacitive type. For instance ( FIG. 2 ), the humidity sensor  5  comprises first electrodes  5   a  and second electrodes  5   b  comb-fingered and coupled together to form a capacitor  5   c . The space between the first electrodes  5   a  and the second electrodes  5   b  is at least in part occupied by a dielectric region  5   d  of hygroscopic material, the electrical permittivity of which is a function of the humidity absorbed. In one embodiment, the dielectric region  5   d  is made of polyimide. With reference once again to  FIG. 1 , the humidity sensor  5  is integrated in the first chip  3 , together with the control circuit  7 , and is aligned to the opening  18  in the first cap  13 . 
     In one embodiment, the pressure sensor  10  is a microelectromechanical membrane sensor, with piezoelectric type or capacitive type detection, for example (see also  FIGS. 3 and 4 ). As already mentioned, the pressure sensor  10  is integrated in the second chip  8 . The second chip  8  comprises a frame-like supporting portion  8   a , a sensor portion  8   b , and elastic connection elements  8   c  that connect the sensor portion  8   b  elastically to the supporting portion  8   a . The supporting portion  8   a  and the sensor portion  8   b  are separated by a through trench  20 , extending across which are the elastic connection elements  8   c . The sensor portion  8   b , which is, for example, quadrangular in shape, has a cavity  21  sealed on one side by a membrane  22  that defines a portion of a face  8   d  of the second chip  8 . The elastic connection elements  8   c  are configured to enable relative movements of the sensor portion  8   b  with respect to the supporting portion  8   a , in particular translations in two independent directions parallel to the face  8   d  and rotations about axes perpendicular to the face  8   d . The elastic connection elements  8   c  enable accommodation of the effects of thermal expansion, of deformations, and of mechanical stresses that may arise during the packaging steps and during the useful service life of the sensor assembly  1 . 
     With reference once again to  FIG. 1 , the second chip  8  is sandwiched between the first cap  13  and the second cap  17 . More precisely, the face  8   d  of the second chip  8  is bonded to the first cap  13  by bonding regions  24 , whereas a face  8   e , opposite to the face  8   d , is bonded to the second cap  17  by bonding regions  25 . The bonding regions  24 ,  25  are, for example, of a type used in wafer-to-wafer bonding techniques and, according to the technique specifically used, may have a thickness, for example, between 3 μm and 50 μm. The membrane  22  of the pressure sensor  10  thus faces the first cap  13 , from which it is separated by a distance substantially equal to the thickness of the bonding regions  24 . In addition, the membrane  22  is arranged facing the opposite side with respect to the first chip  3 , in which the humidity sensor  5  and the control circuit  7  are integrated. 
     The second chip  8  projects laterally with respect to the first cap  13  and houses pads  27  for electrical connection to the control circuit  7  by wire bonding with respective pads  28  on the first chip  3 . 
     Using for example an adhesive layer  30 , the second cap  17  is bonded to the first chip  3 , which projects laterally for housing the pads  28  and further pads  31  for connection with respective pads or paths  32  on the substrate  11  by wire bonding. The wire bondings between the pads  27  and the pads  28  and the wire bondings between the pads  31  and the paths  32  are incorporated in the lateral structure  12 . The paths  32  are located on the face of the substrate  11  bonded to the first chip  3  and are in turn coupled, by through connections (not shown), to respective contact pads  15  on the outer face of the substrate  11 . 
     The first chip  3  is finally bonded to the substrate  11  through an adhesive layer  33 . 
     The supporting portion  8   a  of the second chip  8  and the second cap  17  have respective through openings aligned to the opening  18  in the first cap  13  (the opening in the supporting portion  8   a  is designated by  8   f  in  FIGS. 3 and 4 ). In one embodiment, the openings of the supporting portion  8   a  of the second chip  8  and of the second cap  17  have the same dimensions as the opening  18  in the first cap  18  and as the humidity sensor  5 . Also the adhesive layer  30  is interrupted in a region corresponding to the humidity sensor  5 , which is thus exposed to the external atmosphere through a direct path. Advantageously, instead, the openings are provided at a distance from the pressure sensor  10 , in particular, as regards the second chip  8 , in the supporting portion  8   a . The pressure sensor  10  thus communicates with the outside through a gap between the second chip  8  and the first cap  13 , which has the thickness of the bonding regions  24 . The gap enables the membrane  22  to be kept at the external atmospheric pressure, without there being a significant exposure to light radiation. In particular, direct exposure is prevented or minimized such that only a small fraction of the radiation that enters from the opening  18  may reach the membrane through multiple reflections along the gap between the second chip  8  and the first cap  13 . 
     Another advantage of the sensor assembly  1  is represented by the connection of the sensor portion  8   b  of the second chip  8  to the supporting portion  8   a  by the elastic connection elements  8   c . This type of connection, in fact, affords a good level of mechanical decoupling between the supporting portion  8   a  and the sensor portion  8   b . The stresses that are frequently set up during the packaging steps or as a result of thermal and mechanical stresses are absorbed by the elastic connection elements  8   c  and are not transmitted to the membrane  22 . The membrane  22  is thus free from stresses that might affect its natural deformations and, consequently, the process of electromechanical transduction of the pressure. The use of the elastic elements  8   c  enables in particular incorporation of the second chip  8  with the pressure sensor  10  in a molded packaging structure, instead of in a structure of the cavity type. The benefit reflects especially on the dimensions of the sensor assembly as a whole and on abatement of light radiation incident on the pressure sensor. 
       FIG. 5  illustrates a different embodiment of the present disclosure. In this case, a packaged sensor assembly  100  comprises a packaging structure  102 , a first chip  103 , in which a humidity sensor  105  and a control circuit  107  are integrated, and a second chip  108 , in which a pressure sensor  110  is integrated. 
     The humidity sensor  105  and the pressure sensor  110  are of the type already described with reference to  FIGS. 1-4 . In particular, the pressure sensor  110  is integrated in the second chip  108 , which comprises a supporting portion  108   a , a sensor portion  108   b  connected to the supporting portion  108   a  by elastic connection elements  108   c , and a membrane  122  that extends so to close a cavity  121  and defines a portion of a face  108   d  of the second chip  108 . 
     The packaging structure  102  is of a molded type, for example of the LGA type, and comprises a substrate  111 , a lateral structure  112 , and a cap  113 . The substrate  111  is made, for example, of FR-4 and is provided with metal contact pads or lands  115  on an outer face. The lateral structure  112  may be obtained by molding resin and surrounds the first chip  103  and the second chip  108 . 
     A cap  113  defines a wall of the packaging structure  102  opposite to the substrate  111  and forms a die with the first chip  103  and the second chip  108 . 
     The inside of the packaging structure  102  is in fluidic communication with the environment outside of the package through an opening  118  in the cap  113 . The opening  118 , of dimensions substantially equal to those of the humidity sensor  105 , enables air external to the package to exchange within the packaging structure  102  so that the humidity sensor  105  is reached in a short time by the gaseous mixture in which the packaged sensor assembly  100  is immersed. 
     The face  108   d  of the second chip  108  is bonded to the cap  113  by bonding regions  124 , whereas a face  108   e , opposite to the face  108   d , is bonded to the first chip  103  by conductive bonding regions  125 . In this case, the first chip  103  functions as protective cap for the pressure sensor  110 . The bonding regions  124  are, for example, of a type used in wafer-to-wafer bonding techniques. The bonding regions  125 , instead, may be of a type used in ball bonding techniques and enable electrical coupling between the pressure sensor  110  and the control circuit  107 . The electrical connection between the structures on the face  108   d  of the second chip  108  and the bonding regions  125  is obtained with conductive through silicon vias (TSVs)  126 . A layer of filler material  123  surrounds and seals the second chip  108  to prevent the liquid resin from penetrating between the first chip  103  and the second chip  108  during fabrication by molding of the lateral structure  112 . 
     The supporting portion  108   a  of the second chip  108  has a through opening aligned with the opening  118  in the cap  113 . Also in this embodiment, the pressure sensor  110  is located at a distance from the opening  118  and is accessible through a gap between the second chip  108  and the cap  113 , of a thickness equal to the thickness of the bonding regions  124 . The gap is sufficient for setting the pressure sensor  110  in fluidic communication with the environment outside of the package and, at the same time, eliminates or at least reduces substantially the incident light radiation. 
     The first chip  103  is bonded to the substrate  111  by an adhesive layer  133 . The humidity sensor  105  is aligned to the opening  118  and to the opening in the supporting portion  108   a  of the second chip  108  and is thus exposed to the external atmosphere. 
     The first chip  103  projects laterally with respect to the second chip  108  for housing pads  131  for connection with respective pads or paths  132  on the substrate  111  by wire bonding, which are incorporated in the lateral structure  112 . The paths  132  are located on the face of the substrate  111  bonded to the first chip  103  and are in turn coupled, by through connections (not shown), to respective contact pads  115  on the outer face of the substrate  111 . 
     The sensor assembly  100  utilizes one protective cap for the pressure sensor  110 , since on one side, the first chip  103  has the purpose of providing protection. The overall thickness of the sensor assembly  100  is thus advantageously reduced. 
     With reference to  FIG. 6 , a packaged sensor assembly  200  according to one embodiment of the present disclosure comprises a packaging structure  202 , a first chip  203 , in which a humidity sensor  205  and a control circuit  207  are integrated, and a second chip  208 , in which a pressure sensor  210  is integrated. 
     The humidity sensor  205  and the pressure sensor  210  are of the type already described with reference to  FIGS. 1-4 . In particular, the pressure sensor  210  is integrated in the second chip  208 , which comprises a supporting portion  208   a , a sensor portion  208   b  connected to the supporting portion  208   a  by elastic connection elements  208   c , and a membrane  222 , which closes a cavity  221  and defines a portion of a face  208   d  of the second chip  208 . 
     The packaging structure  202  is of a molded type, for example of the LGA type, and comprises a substrate  211  and a lateral structure  212 . The first chip  203  forms an integral part of the packaging structure  202 , of which it defines a protective cap. The substrate  211  is made, for example, of FR-4 and is provided with metal contact pads or lands  215  on an outer face. The lateral structure  212  may be obtained by molding resin. The first chip  203  is arranged for closing the lateral structure  212  and has an opening  218  that sets the inside of the packaging structure  202  in communication with the environment outside of the package. In the embodiment of  FIG. 6 , the opening  218  is arranged at the center with respect to the first chip  203 . However, in other embodiments (not illustrated) the opening may be off-center. The opening  218  may be obtained with a step of dry chemical etching carried out following upon steps of production of electronic components, for example with a CMOS process, to obtain the control circuit  207 . 
     The humidity sensor  205  is arranged on the face of the first chip  203  facing the inside of the packaging structure  202 , in the immediate vicinity of the opening  218 . 
     The first chip  203  is bonded to a face  208   e  of the second chip  208  by conductive bonding regions  224 , which enable electrical connection. The face  208   e  is opposite to the face  208   d  on which the membrane  222  is located. In one embodiment, further, the second chip  208  is provided with conductive through vias  226 , which enable electrical connection of the bonding regions  224  to pads  227  on an inner face of the substrate  211 . 
     The second chip  208  is bonded to the substrate  211  by conductive bonding regions  225 . The pads  227  and the bonding regions  225  are coupled to respective contact pads  215  on the outer face of the substrate  211 . Further, the second chip  208  is oriented so that the membrane  222  faces the substrate  211 , on the side opposite to the opening  218 . 
     A layer of filler material  223  surrounds the first chip  203  at the interface with the second chip  208  and seals the gaps between the first chip  203  and the second chip  208 . Likewise, a layer of filler material  230  surrounds the second chip  208  at the interface with the substrate  211  and seals the gaps between the second chip  208  and the substrate  211 . 
     According to a further embodiment of the present disclosure, illustrated in  FIG. 7 , a packaged sensor assembly  300  comprises a packaging structure  302 , a first chip  303 , in which a humidity sensor  305  and a control circuit  307  are integrated, and a second chip  308 , in which a pressure sensor  310  is integrated. 
     The humidity sensor  305  and the pressure sensor  310  are of the type already described with reference to  FIGS. 1-4 . In particular, the pressure sensor  310  is integrated in the second chip  308 , which comprises a supporting portion  308   a , a sensor portion  308   b  connected to the supporting portion  308   a  by elastic connection elements  308   c , and a membrane  322 , which closes a cavity  321  and defines a portion of a face  308   d  of the second chip  308 . 
     The packaging structure  302  is of a molded type, for example of the LGA type, and comprises a substrate  311  and a cap  312 . The substrate  311  is made, for example, of FR-4 and is provided with metal contact pads or lands  315  on an outer face. The cap  312  may be obtained by molding resin and englobes the first chip  303  and the second chip  308 . 
     The first chip  303  is bonded to the substrate  311  with an adhesive layer  333  and is electrically connected to pads  327  on the substrate  311  by wire bonding. 
     An opening  318  through the substrate  311  and the first chip  303  sets the inside of the packaging structure  302  in fluidic communication with the environment outside of the package. In the embodiment of  FIG. 7 , the opening  318  is arranged at the center with respect to the substrate  311  and to the first chip  303 . In other embodiments (not illustrated), however, the opening may be off-center. 
     The second chip  308  is sandwiched between a cap  313  and the first chip  303  and is bonded thereto by bonding regions  324  and bonding regions  325 . Further, the second chip  308  is electrically coupled to the first chip  302  by wire bondings. 
     The second chip  308  is oriented so that the membrane  322  faces the cap  311 , thus on a side opposite to the opening  318 . 
       FIG. 8  illustrates a portion of an electronic system  400  according to one embodiment of the present disclosure. The system  400  incorporates the sensor assembly or electromechanical transducer  1  and may be used in devices such as, for example, a laptop computer or tablet, possibly with wireless-connection capacity, a cellphone, a smartphone, a messaging device, a digital music player, a digital camera, or other devices designed to process, store, transmit, or receive information. In particular, the electroacoustic transducer  1  may be used for providing voice-control functions, for example, in a motion-activated user interface for computers or consoles for videogames or in a satellite-navigation device. 
     The electronic system  400  may comprise a control unit  410 , an input/output (I/O) device  420  (for example, a keyboard or a display), the electroacoustic transducer  1 , a wireless interface  440 , and a memory  460 , of a volatile or non-volatile type, which are coupled together through a bus  450 . In one embodiment, a battery  480  may be used for supplying electrically energy to the system  400 . It should be noted that the scope of the present disclosure is not limited to embodiments that present necessarily one or all of the devices listed. 
     The control unit  410  may comprise, for example, one or more microprocessors, microcontrollers, and the like. 
     The I/O device  420  may be used for generating a message. The system  400  may use the wireless interface  440  for transmitting and receiving messages to and from a wireless communication network with a radio-frequency (RF) signal. Examples of wireless interface may comprise an antenna, a wireless transceiver, such as a dipole antenna, even though the scope of the present disclosure is not limited from this standpoint. In addition, the I/O device  420  may supply a voltage representing what is stored either in the form of digital output (if digital information has been stored) or in the form of analog output (if analog information has been stored). 
     The electronic system  400  may be any electronic system, such as portable electronic devices, such as smartphones, cameras, video recording devices, tablets, wearable devices, such as smart watches, or any other electronic device. 
     Finally, it is evident that modifications and variations may be made to the device and to the method described, without thereby departing from the scope of the present disclosure. 
     The various embodiments described above can be combined to provide 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.