Patent Publication Number: US-2019194014-A1

Title: Pressure sensor structure configured for wafer-level calibration

Description:
BACKGROUND 
     Pressure sensors, including monolithic pressure sensors, are individually calibrated, or calibrated in small groups, for example 8 pressure sensors at a time, as fully assembled modules. A monolithic pressure sensor has both MEMS (Microelectromechanical systems) and ASIC (application-specific integrated circuit) co-processed (i.e., both created) on the same wafer. 
     Calibration requires application of precise pressure and exposure to well controlled temperature of each pressure sensor which, in turn, requires bulky, complicated, and expensive test equipment, including a connector and a communication board for each pressure sensor being simultaneously calibrated. 
     Improved techniques for calibrating pressure sensors more efficiently would be an improvement. 
     BRIEF SUMMARY 
     Embodiments of the invention are directed to a wafer structure configured for wafer-level calibration of a plurality of pressure sensors, the wafer structure includes: a microelectromechanical systems (MEMS) wafer that includes a plurality of MEMS dice that are separated by a plurality of MEMS-wafer dicing areas; an application-specific integrated circuit (ASIC) wafer that includes a plurality of ASIC-wafer dice that are separated by a plurality of ASIC-wafer dicing areas; a Film on Wafer (FOW) that bonds the MEMS wafer to the ASIC wafer; a plurality of thru silicon vias (TSVs) extending through the ASIC wafer; and a plurality of metallizations extending through the FOW thereby creating an electrical connection between the ASIC wafer and the MEMS wafer thereby enabling wafer-level calibration of the plurality of pressure sensors. The MEMS wafer and the ASIC wafer may each include alignment features for aligning the MEMS wafer with the ASIC wafer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a wafer structure for a differential sensor that can be used for wafer-level calibration of pressure sensors. 
         FIG. 2  shows a wafer structure for an absolute sensor that can be used for wafer-level calibration of pressure sensors. 
         FIG. 3  shows wafer-level calibration of a wafer structure for an absolute sensor that can be used for wafer-level calibration of pressure sensors. 
         FIG. 4  shows wafer-level calibration of a wafer structure for a differential sensor that can be used for wafer-level calibration of pressure sensors. 
         FIG. 5  shows an after-sawing individual differential pressure sensor in accordance with embodiments of the invention. 
         FIG. 6  shows an after-sawing individual absolute pressure sensor in accordance with embodiments of the invention. 
         FIG. 7  shows an application of after-sawing individual differential pressure sensor in accordance with embodiments of the invention. 
         FIG. 8  shows an application of after-sawing individual absolute pressure sensor in accordance with embodiments of the invention. 
         FIG. 9  shows a wafer structure for a differential sensor that can be used for wafer-level calibration of pressure sensors and that is similar to the wafer structure of  FIG. 1 . 
         FIG. 10  shows a wafer structure for an absolute sensor that can be used for wafer-level calibration of pressure sensors and that is similar to the wafer structure of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     Wafer-level calibration offers calibration of many devices “at the same time”. For example, pressure and temperature are applied to many sensors (e.g., 4,000 sensors on a single wafer) at the same time, and multiple (e.g.,  16 ) sensors may be probed (i.e., an electrical connection may be established) simultaneously. This offers speedy, cost-reduced calibration relative to the individual calibration procedure described above. 
     Wafer-level calibration would require a wafer structure suitable for this kind of operation. Such a wafer structure may be created at a packaging foundry. 
     As such, embodiments of the invention result in cost-effective structure for calibration of pressure sensors at the wafer level by leveraging advantages, including cost efficiencies, of processing MEMS wafers and ASIC wafers at their own separate dedicated foundries. 
     To create such a wafer structure, post-processing at a packaging foundry, may comprise the following steps:
         Differential or Absolute MEMS wafers come in as finished, not yet sawn, products from a MEMS foundry;   ASIC wafer comes in as a finished, not sawn, product from ASIC foundry;       

     Both wafers (i.e., MEMS and ASIC) are of the same size and have appropriate alignment features so that the MEMS and ASIC wafers can be efficiently aligned with each other; Wafer-alignment features may be some unique marks on both wafers. For example, 3 crosses, in three different places on MEMS wafer and ASIC wafer. They may be placed precisely in the same location (from the same reference point) on the MEMS wafer and the ASIC wafer. 
     TSVs (Thru Silicon Vias) are created in the ASIC wafer along with a “vent hole” using DRIE (Deep Reactive Ion Etching);
         External metallization is deposited on MEMS and ASIC wafers; and   Wafers are aligned and joined together using FOW (Film On Wafer) with via interconnects in-between.       

     A die in the context of integrated circuits is a small block of semiconducting material, on which a given functional circuit is fabricated. Typically, integrated circuits are produced in large batches on a single wafer of electronic-grade silicon (EGS) or other semiconductor (such as GaAs) through processes such as photolithography. The wafer is cut (“diced”) into many pieces, each containing one copy of the circuit. Each of these pieces is called a die. 
     There are three commonly used plural forms: dice, dies, and die. 
       FIG. 1  shows a wafer structure  100  for a differential sensor that can be used for wafer-level calibration of pressure sensors. The wafer structure  100  includes a MEMS wafer  106 , which includes MEMS die  102 - 1 , MEMS die  102 - 2 , and MEMS die  102 - 3 , which are separated by MEMS-wafer dicing areas  104 - 1  and  104 - 2 , which will later (i.e., after wafer-level calibration has been performed) be removed by sawing through the dicing areas  104 - 1  and  104 - 2  to separate the MEMS dice  102 - 1  through  102 - 3  from one another. 
     The wafer structure  100  also includes ASIC wafer  108 , which, similar to MEMS wafer  106 , includes ASIC-wafer dice  112 - 1  through  112 - 5 . ASIC-wafer dice  112 - 2  and  112 - 3  are separated by ASIC-wafer dicing area  114 - 1 , and ASIC-wafer dice  112 - 4  and  112 - 5  are separated by ASIC-wafer dicing area  114 - 2 . ASIC-wafer dicing areas  114 - 1  and  114 - 2  will later (i.e., after wafer-level calibration has been performed) be removed by sawing through the dicing areas  114 - 1  and  114 - 2  to separate ASIC die  112 - 2  from ASIC die  112 - 3  and to separate ASIC die  112 - 4  from ASIC die  112 - 5 , respectively. 
     The wafer structure  100  also includes Film on Wafer  110 , which bonds the MEMS wafer  106  to the ASIC wafer  108 . 
       FIG. 2  shows a wafer structure  200  for an absolute sensor that can be used for wafer-level calibration of pressure sensors. The wafer structure  200  of  FIG. 2  is the same as the wafer structure  100  of  FIG. 1  except that glass  202  is included above the MEMS wafer. 
       FIG. 3  shows wafer-level calibration  300  of wafer structure  200 . As shown in  FIG. 3 , wafer probing needles  302 - 1  and  302 - 2 ,  302 - 3  and  302 - 4 , and  302 - 5  and  302 - 6 , make necessary electrical connections to the ASIC wafer dies respectively, through metallizations  304 - 1  and  304 - 2 ,  304 - 3  and  304 - 4 , and  304 - 5  and  304 - 6  and thru-silicon vias (TSVs)  306 - 1  and  306 - 2 ,  306 - 3  and  306 - 4 , and  306 - 5  and  306 - 6 , respectively. Metallizations  308 - 1  and  308 - 2 ,  308 - 3  and  308 - 4 , and  308 - 5  and  308 - 6  represent electrical connections between ASIC and MEMS. 
       FIG. 4  shows wafer-level calibration  400  of wafer structure  100 .  FIG. 4  is the same as  FIG. 3 , except that glass  202  of  FIG. 3  is omitted from  FIG. 4 . 
       FIG. 5  shows an after-sawing individual differential pressure sensor  500  in accordance with embodiments of the invention. Active area  502  is where actual circuits are made in silicon. In case of MEMS, it will be diffusion area for piezo-resistors and interconnects, which ultimately ends up connected to MEMS metallized pads. In case of ASIC, active areas  504 - 1 ,  504 - 2 ,  506 - 1 , and  506 - 2  comprise diffusions that create transistors, resistors, or capacitors, and the like, in other words, ASIC circuitry. ASIC will have metallized pads for connections to MEMS and to the outside world. 
       FIG. 6  shows an after-sawing individual absolute pressure sensor  600  in accordance with embodiments of the invention.  FIG. 6  is the same as  FIG. 5  except that glass  202  appears in  FIG. 6 . 
       FIG. 7  shows an application  700  of after-sawing individual differential pressure sensor  500  in accordance with embodiments of the invention.  FIG. 7  is the same as  FIG. 5 , except that after-sawing individual differential pressure sensor  500  is bonded and electrically connected to leadframe  704  by electrically conductive adhesive (ECA)  702 . 
       FIG. 8  shows an application  800  of after-sawing individual absolute pressure sensor  600  in accordance with embodiments of the invention.  FIG. 8  is the same as  FIG. 7  except that glass  202  appears in  FIG. 8 . 
       FIG. 9  shows a wafer structure  900  for a differential sensor that can be used for wafer-level calibration of pressure sensors and that is similar to the wafer structure  100  of  FIG. 1  except that the MEMS wafer  906  is oriented “up-side down” relative to the MEMS wafer  106  of  FIG. 1 . 
       FIG. 10  shows a wafer structure  1000  for an absolute sensor that can be used for wafer-level calibration of pressure sensors and that is similar to the wafer structure  200  of  FIG. 2  except that the MEMS wafer  906  is oriented “up-side down” relative to the MEMS wafer of  FIG. 2  and that the through-FOW TSVs extend through glass  202  and through the MEMS  906 . 
     Because the MEMS wafers and ASIC wafers are processed (i.e., created) separately from one another, pressure sensors that are configured for wafer-level calibration can be packaged at a significantly reduced cost relative to pressure sensors for which both the MEMS and the ASIC are created on a single die. 
     While the present invention has been illustrated by a description of various embodiments and while these embodiments have been described in considerable detail, it is not the intention of the applicants to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and method, and illustrative example shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicant&#39;s general inventive concept.