Abstract:
A pressure sensor assembly includes a first die assembly, a second die assembly, and a conducting member. The first die assembly includes a MEMS pressure sensor. The second die assembly includes an ASIC configured to generate an electrical output corresponding to a pressure sensed by the MEMS pressure sensor. The conducting member is positioned between the first die assembly and the second die assembly and is configured and to electrically connect the MEMS pressure sensor to the ASIC.

Description:
FIELD 
       [0001]    This disclosure relates generally to semiconductor devices and particularly to a microelectromechanical system (MEMS) pressure sensor. 
       BACKGROUND 
       [0002]    Microelectromechanical systems (MEMS) have proven to be effective solutions in various applications due to the sensitivity, spatial and temporal resolutions, and lower power requirements exhibited by MEMS devices. Consequently, MEMS-based sensors, such as accelerometers, gyroscopes, acoustic sensors, optical sensors, and pressure sensors, have been developed for use in a wide variety of applications. 
         [0003]    MEMS pressure sensors are often packaged in either a ceramic or a pre-mold package. Ceramic and pre-mold packages function well to contain MEMS pressure sensors. For some sensor applications, however, these types of packages are simply too large. For example, the package may define a substrate contact area that exceeds the area available for mounting the pressure sensor. Also, the package may exceed a height limitation of the sensor application, especially when wire bonds are used to electrically connect the package to the circuit/sensor. Additionally, ceramic and pre-mold packages are typically expensive to manufacture compared to some other packaging approaches. 
         [0004]    Therefore, in an effort to make MEMS pressure sensors usable in even more sensor applications, it is desirable to reduce the size of the package and also the cost to package MEMS pressure sensors. 
       SUMMARY 
       [0005]    According to one embodiment of the present disclosure, a sensor assembly includes a first die assembly, a second die assembly, and a conducting member. The first die assembly includes a MEMS sensor. The second die assembly includes an ASIC configured to generate an electrical output corresponding to a pressure sensed by the MEMS sensor. The conducting member is positioned between the first die assembly and the second die assembly and is configured and to electrically connect the MEMS sensor to the ASIC. 
         [0006]    According to another embodiment of the present disclosure, a sensor assembly includes a first die assembly and a second die assembly. The first die assembly includes a MEMS sensor. The second die assembly includes an ASIC configured to generate an electrical output corresponding to a pressure sensed by the MEMS sensor. The ASIC is electrically connected to the MEMS sensor. The first die assembly is attached to the second die assembly in a stacked configuration. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0007]    The above-described features and advantages, as well as others, should become more readily apparent to those of ordinary skill in the art by reference to the following detailed description and the accompanying figures in which: 
           [0008]      FIG. 1  is a perspective view of a MEMS sensor assembly, as described herein; and 
           [0009]      FIG. 2  is a cross-sectional view taken along line II-II of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    For the purpose of promoting an understanding of the principles of the disclosure, reference will now be made to the embodiments illustrated in the drawings and described in the following written specification. It is understood that no limitation to the scope of the disclosure is thereby intended. It is further understood that this disclosure includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the disclosure as would normally occur to one skilled in the art to which this disclosure pertains. 
         [0011]    As shown in  FIG. 1 , a pressure sensor assembly  100  includes an upper die assembly  108 , a conducting member  116 , a conducting member  120 , a bonding member  122 , and a lower die assembly  124 . The pressure sensor assembly  100  is shown positioned on a substrate  132 , such as a printed circuit board or any other substrate that is suitable for mounting electrical components. 
         [0012]    With reference to  FIG. 2 , the upper die assembly  108  is formed from silicon and includes a MEMS pressure sensor  140 . The pressure sensor  140  is a capacitive pressure sensor that defines a cavity  172  and includes an upper electrode  180  and a membrane  188  that is movable with respect to the upper electrode. The membrane  188  is preferably made of epitaxial silicon. 
         [0013]    The upper electrode  180  is defined in the upper die assembly  108  and is formed by doping a portion of the upper die assembly. Alternatively, the upper electrode  180  is formed by using a doped silicon layer on an insulating film above the substrate of the upper die assembly  108 . The area of the upper electrode  180  is approximately 0.01 to 1 square millimeter (0.01-1 mm 2 ). An electrical lead  156  connects the upper electrode  180  to the conducting member  116 . 
         [0014]    The membrane  188  is positioned beneath the cavity  172  defined by the upper die assembly  108 . The membrane  188  includes an electrode defined therein. The area of the membrane  188  is approximately 0.01-1 square millimeter (0.01-1 mm 2 ). The membrane  188  is spaced apart from the upper electrode  180  by approximately 1 micrometer (1 μm). An electrical lead  164  connects the membrane  188  to the conducting member  120 . The epitaxial silicon membrane  188  in combination with the capacitive transduction principle makes the pressure sensor  140  mechanically robust, as compared to other types of pressure sensors. The thickness of  188  is about 1-20 um. 
         [0015]    The cavity  172  of the pressure sensor  140  is typically at or near vacuum; accordingly, the pressure sensor is an absolute pressure sensor. In other embodiments, the cavity  172  is at a pressure level other than at or near vacuum, depending on the operating environment of the pressure sensor assembly  100 , among other factors. 
         [0016]    The conducting members  116 ,  120  are positioned between the upper die assembly  108  and the lower die assembly  124 . The conducting member  116  is electrically isolated from the conducting member  120 . The conducting members  116 ,  120  electrically connect the upper die assembly  108  to the lower die assembly  124 . To this end, the conducting member  116  is positioned to make electrical contact with the electrical lead  156 , and the conducting member  120  is positioned to make electrical contact with the electrical lead  164 . Additionally, the conducting members  116 ,  120  make electrical contact with the lower die assembly  124 . The conducting members  116 ,  120  are formed from solder or any metal or conductive material. 
         [0017]    The bonding member  122  structurally connects the upper die assembly  108  to the lower die assembly  124  in a stacked configuration using a eutectic bonding procedure. The bonding member  122  spaces the upper die assembly  108  apart from the lower die assembly  124 , such that a cavity  196  is defined between the upper die assembly and the lower die assembly. A gap  204  ( FIG. 1 ) between the conducting members  116 ,  120  and the bonding member  122  exposes the cavity  196  to atmosphere (or to the fluid surrounding the pressure assembly  100 ). It is noted that in another embodiment, the structural connection of the upper die assembly  108  to the lower die assembly  124  is accomplished through a thermo-compression bonding procedure. In yet another embodiment, the structural connection of the upper die assembly  108  to the lower die assembly  124  is accomplished through solid-liquid-interdiffusion bonding or through metallic soldering, gluing, and/or using solder balls. In a further embodiment, the bonding member  122  and the conducting members  116 ,  120  are applied to the lower die assembly  124  (or the upper die assembly  108 ) during the same fabrication step when forming the pressure sensor assembly  100 . 
         [0018]    The lower die assembly  124  is formed from silicon. The lower die assembly  124  includes an ASIC  212  and defines a plurality of through silicon vias  220 . The ASIC  212  is electrically connected to the pressure sensor  140  through the conducting members  116 ,  120 . The ASIC  212  generates an electrical output that corresponds to a pressure sensed by the pressure sensor  140 . As shown in  FIGS. 1 and 2 , the “footprint” of upper die assembly  108  is approximately equal to the footprint of the lower die assembly  124 . In another embodiment, the footprint of the upper die assembly  108  is sized differently (either smaller or larger) than the footprint of the lower die assembly  124 . 
         [0019]    The through silicon vias  220  convey the electrical output of the pressure sensor assembly  100 . Additionally, the through silicon vias  220  may receive electrical signals from an external circuit (not shown), such as signals for configuring the ASIC  212 . The pressure sensor assembly  100  is shown as including three of the through silicon vias  220 , it should be understood, however, that the lower die assembly  124  includes as many of the through silicon vias as is used by the ASIC  212 . 
         [0020]    The pressure sensor assembly  100  is connectable directly to the substrate  132  without being mounted in a separate package. This mounting scheme is often referred to as a bare-die mounting/connection scheme. Since the pressure sensor assembly  100  is not mounted in a ceramic or pre-mold package, the manufacturing costs of the pressure sensor assembly are typically less than the manufacturing costs associated with conventional packaged pressure sensor assemblies. 
         [0021]    As shown in  FIG. 2 , solder balls  228  are used to structurally and electrically connect the pressure sensor assembly  100  to the substrate  132 . The solder balls  228  are positioned to make electrical contact with the through silicon vias  220 , in a process known to those of ordinary skill in the art. 
         [0022]    With reference again to  FIG. 1 , the pressure sensor assembly  100  defines a length L, a width W, and a height H. Since the pressure sensor assembly  100  is not mounted in a package it exhibits a comparatively small size as compared to other package-mounted pressure sensor assemblies. In particular, the contact area of the pressure sensor assembly  100  that is positioned against the substrate  132  is less than approximately two square millimeters (2 mm 2 ). The contact area (also referred to as a “footprint”) is equal to the length L times the width W of the pressure sensor assembly  100 . Additionally, the height H of the pressure sensor assembly is less than approximately one millimeter (1 mm). It is noted that the height H is less than 1 mm even when the pressure sensor assembly  100  is electrically connected to the substrate  132 , since wire bonds are not used to electrically connect the pressure sensor assembly. As the sensitive membrane  188  is facing the ASIC  212 , there is also no protective housing needed (package is protection itself). 
         [0023]    In operation, the pressure sensor assembly  100  senses the pressure of the fluid (not shown) surrounding the pressure sensor assembly. In particular, the pressure sensor assembly  100  exhibits an electric output that corresponds to the pressure imparted on the membrane  188  by the fluid in the cavity  196 , as described below. 
         [0024]    The pressure of the fluid in the cavity  196  causes the membrane  188  to move relative to the electrode  180 . This is because the cavity  196  is fluidly connected to the environment/atmosphere, since the connecting members  116 ,  120  and the bonding member  122  do not form a closed perimeter. Typically, an increase in pressure causes the membrane  188  to move closer to the electrode  180 . This movement results in a change in capacitance between the electrode  180  and the membrane  188 . 
         [0025]    The ASIC  212  exhibits an electrical output signal that is dependent on the capacitance sensed between the electrode  180  and the membrane  188 . The electrical output signal of the ASIC  212  changes in a known way in response to the change in capacitance between the electrode  180  and the membrane  188 . Accordingly, the electrical output signal of the ASIC  212  corresponds to the pressure exerted on the membrane  188  by the fluid in the cavity  196 . 
         [0026]    The comparatively small size of the pressure sensor assembly  100  makes it particularly suited for consumer electronics, such as mobile telephones and smart phones. Additionally, the robust composition of the pressure sensor assembly  100  makes it useful in automotive applications, such as tire pressure monitoring systems, as well as any application in which a very small, robust, and low cost pressure sensor is desirable. Furthermore, the pressure sensor assembly  100  may be implemented in or associated with a variety of applications such as home appliances, laptops, handheld or portable computers, wireless devices, tablets, personal data assistants (PDAs), MP3 players, camera, GPS receivers or navigation systems, electronic reading displays, projectors, cockpit controls, game consoles, earpieces, headsets, hearing aids, wearable display devices, security systems, and etc. 
         [0027]    In an alternative embodiment of the pressure sensor assembly  100 , the pressure sensor assembly is mounted to the substrate  132  in an inverted orientation with the upper die assembly  108  positioned against the solder balls  228  and the substrate. In this embodiment, the through silicon vias  220  are formed in the upper die assembly  108  and are electrically connected to the ASIC  212  through at least the conducting members  116 ,  120 . 
         [0028]    Also in another embodiment of the pressure sensor assembly  100 , the upper die assembly  108  includes a gel or a polymer coating (not shown). The gel or the polymer coating protects the epitaxial silicon membrane  188 . 
         [0029]    Furthermore, in some embodiments, the pressure sensor assembly  100  is coated by a conformal coating process. The coating (not shown) protects the pressure sensor assembly  100  against harsh environments. The coating is applied to the pressure sensor assembly  100 , in some of the embodiments, by atomic layer deposition. The coating applied to the pressure sensor assembly  100  is formed from materials including, but not limited to, Al203, HfO2, ZrO2, SiC, parylene, and combinations thereof. 
         [0030]    In another embodiment of the pressure sensor assembly  100 , the connecting members  116 ,  120  electrically connect the upper die assembly  108  to the lower die assembly  124  and also structurally connect the upper die assembly to the lower die assembly in the stacked configuration. Accordingly, in this embodiment, a separate bonding member  122  is not included since the connecting member  116  and the connecting member  120  perform both the electrical and structural connection. 
         [0031]    While the disclosure has been illustrated and described in detail in the drawings and foregoing description, the same should be considered as illustrative and not restrictive in character. It is understood that only the preferred embodiments have been presented and that all changes, modifications and further applications that come within the spirit of the disclosure are desired to be protected.