Abstract:
Suspending a microelectromechanical system (MEMS) pressure sensing element inside a cavity using spring-like corrugations or serpentine crenellations, reduces thermally-mismatched mechanical stress on the sensing element. Overlaying the spring-like structures and the sensing element with a gel further reduces thermally-mismatched stress and vibrational dynamic stress.

Description:
BACKGROUND 
       [0001]    Microelectromechanical system (MEMS) pressure sensors are well known. Examples of such sensors are disclosed in various U.S. patents including but not limited to: U.S. Pat. No. 7,997,142 issued Aug. 16, 2011, entitled, “Low pressure sensor device with high accuracy and high sensitivity,” U.S. Pat. No. 8,215,176 issued Jul. 12, 2013, entitled “Pressure sensor for harsh media sensing and flexible packaging,” and U.S. Pat. No. 8,833,172 issued Sep. 16, 2014, entitled “Pressure sensing device with stepped cavity to minimize thermal noise,” the contents of each being incorporated by reference in their entireties. 
         [0002]    Those of ordinary skill in the MEMS pressure sensing art know that the thermally-mismatched mechanical stress or vibrational dynamic stress on a MEMS pressure sensing element adversely affects the device&#39;s accuracy. Reducing or eliminating thermally-mismatched stress and vibrational dynamic stress is therefore important to improving MEMS pressure sensing element accuracy. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0003]      FIG. 1A  is a perspective view of a pressure sensor employing a MEMS pressure sensing element in the prior art; 
           [0004]      FIG. 1B  shows a cross-sectional diagram of the MEMS pressure sensing element in  FIG. 1A ; 
           [0005]      FIG. 2  is a cross-sectional diagram of a stress-isolated MEMS pressure sensor; 
           [0006]      FIG. 3  is a top or plan view of a stress-isolated MEMS pressure sensor shown in  FIG. 2 ; 
           [0007]      FIG. 4A  is a cross-sectional diagram of an overmolded stress-isolated pressure sensor employing a MEMS pressure sensing element using bond wires as interconnections; 
           [0008]      FIG. 4B  is a cross-sectional diagram of an overmolded stress-isolated pressure sensor employing a MEMS pressure sensing element using conductive through vias as interconnections; 
           [0009]      FIG. 5  shows a perspective view and a cross-sectional view of corrugated suspenders that support the MEMS pressure sensing element in the stress-isolated pressure of  FIG. 2 ; 
           [0010]      FIG. 6A  is a perspective view of serpentine suspenders that support the MEMS pressure sensing element in  FIG. 2 ; 
           [0011]      FIG. 6B  is a top view of the serpentine suspender; 
           [0012]      FIG. 6C  is a side view of the serpentine suspender; 
           [0013]      FIG. 7  is a cross-sectional diagram of an embodiment of a stress-isolated MEMS pressure sensor; 
           [0014]      FIG. 8  is a cross-sectional diagram of another embodiment of a stress-isolated MEMS pressure sensor; 
           [0015]      FIG. 9  is a cross-sectional diagram of a stress-isolated MEMS pressure sensor mounted onto an application-specific integrated circuit (ASIC) and connected by conductive vias; and 
           [0016]      FIG. 10  is a cross-sectional diagram of a stress-isolated MEMS pressure sensing element mounted onto an ASIC using conductive vias and being overmolded 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Referring now to  FIG. 1  in an exploded view, a MEMS pressure sensor  100  comprises a main housing  102  having a port  104  that conducts pressurized fluid to a MEMS pressure sensing element  106  located inside a housing cavity  108 , which is inside the plastic housing  102 . A layer of viscous gel  110  is overlaid both the MEMS pressure sensing element  106  and an application specific-integrated circuit (ASIC)  112 , also mounted inside the housing cavity  108 . 
         [0018]    Electrical signals are provided to and received from the ASIC  112  through metal “lead frames”  114  that extend through the plastic housing  102 . The lead frames  114  are electrically coupled to the ASIC  112  through small bond wires  116 . On the other side of the ASIC  112 , bond wires  118  connect the ASIC  112  with the MEMS pressure sensing element  106 . The cross-sectional diagram of the MEMS pressure sensing element  106  in the prior art is depicted in  FIG. 1B . The MEMS pressure sensing element  106  comprises a silicon substrate  120  with a diaphragm  124  and a cavity  126  and anodically bonded to a glass substrate  122 . The cavity  126  is near vacuum for topside pressure sensing by a plurality of piezoresistors  128  formed near the edge of the diaphragm  124  on the top side. 
         [0019]    Inaccuracy or noise caused by thermally-induced and/or vibrational mechanical stress on the MEMS pressure sensing element  106  can be reduced by an improved MEMS pressure sensing element  201  with flexible “springs” or “suspenders”, which are formed from the same semiconductor material as shown in  FIG. 2 . In the preferred embodiment, the springs are made to be electrically conductive by doping and carry signals to and from a plurality of piezoresistors  304  as shown in  FIG. 3 . Additional mechanical stress isolation is provided to the stress-isolated MEMS pressure sensor  200  by overlaying the springs with the viscous gel  110  to damp out and reduce the vibrational dynamic stress. 
         [0020]      FIG. 2  is a cross-sectional diagram of a preferred embodiment of a stress-isolated pressure sensor  200  that comprises a MEMS pressure sensing element  201  with an internal vacuum cavity  204 . The cavity  204  has a bottom surface  205  and is defined by or bounded by sidewalls  207 , which are substantially orthogonal to the bottom surface  205 . 
         [0021]    A thin semiconductor diaphragm  206  having piezoresistors, as described in the aforementioned prior art patents, is formed and attached to the substrate  202  by a thin layer of silicon dioxide  209 . 
         [0022]    As used herein, the term “suspender” refers to a structure by which something is suspended or supported. As shown in  FIG. 2 , the MEMS pressure sensing element  201  having the diaphragm  206  is suspended in a cavity  210  by a plurality of suspenders  208  formed from the same semiconductor material from which the substrate  202  and the diaphragm  206  are made. The cavity  210  is formed by bonding a bottom surface  213  of a silicon rim  212  to a substrate  214 . 
         [0023]    If the substrate  214  is a glass substrate, the silicon rim  212  can be anodically bonded onto the substrate  214 . If the substrate  214  is a silicon substrate, the silicon rim  212  can be fusion-bonded onto the substrate  214  or glass-frit-bonded onto the substrate  214 . 
         [0024]      FIG. 3  is a top or plan view of the stress-isolated pressure sensor  200  shown in  FIG. 2 . The diaphragm  206  is essentially square and thus has four edges  305 . Piezoresistors  304  are formed to be located adjacent to each edge  305  of the diaphragm  206 . The piezoresistors  304  are electrically connected with each other by a plurality of P+interconnects  306  and bond pads  215  to form a Wheatstone bridge circuit  320 . As is well known, and explained in at least the issued patents noted above, deformation of the diaphragm  206  responsive to pressure applied to its top surfaces causes the piezoresistors  304  to deform and stressed. When they deform and are stressed, their resistance changes. When a constant-value input voltage is applied to input terminals of the Wheatstone bridge circuit, the voltage output from the Wheatstone bridge changes responsive to deformation and stresses, i.e., responsive to diaphragm deflection and stresses due to the applied pressure. 
         [0025]    Each piezoresistor  304  has of course two terminal ends. Each end of each piezoresistor  304  is connected to a suspender  208 , each of which is doped to make the suspenders  208  electrically conductive. 
         [0026]    Each suspender  208  is “connected” to a bond pad  215  on the top surface  211  of the diaphragm  206 . The suspenders  208  thus perform three functions: supporting the diaphragm  206 , providing stress isolation, and providing a conductive signal path to and from the piezoresistors  304 . 
         [0027]      FIG. 4A  is a cross-sectional diagram of an overmolded stress-isolated pressure sensor  400 . The stress-isolated pressure sensor  200  is attached to a printed circuit board (PCB)  412  or the lead frames  114 , with adhesive  411 . An overmold  402  using materials such as thermoplastic materials to overmold the stress-isolated pressure sensor  200  and bond wires  406 , which connects the stress-isolated pressure sensor  200  to the ASIC  112  or the lead frame  114 . Gel  404  is dispensed to cover the top surface  211  of the stress-isolated pressure sensor  200  to protect the Wheatstone bridge circuit  320 . A through hole  418  is formed by the overmold  402  to allow the pressure media to pass through the hole  418  to reach the top surface of the gel  404 . The viscous gel  404  is soft enough to transmit pressure applied thereon to the diaphragm  206  of the MEMS pressure sensing element  201 . The gel  404  is also filled into the cavity  212 . The gel  404  can damp down the dynamic stress and reduce the noise sensed by the piezoresistors  304  on the diaphragm  206  during vibration or impact. 
         [0028]      FIG. 4B  is a cross-sectional diagram of an overmoled stress-isolated pressure sensor  401 , which comprises a stress-isolated pressure sensor  200  using different electrical interconnections with through-vias  408  and solder bumps  410  to connect to a PCB  412  or the lead frames  114  for voltage signal input and output. The gel  404  fills in the cavity completely over the MEMS pressure sensing element  201  and is overmolded by the overmold  402  without an open space. A membrane  420  is formed by the overmold  402  on the top of the gel  404 . The membrane  420  is flexible to transmit pressure applied thereon to the gel and the diaphragm  206  of the MEMS pressure sensing element  201 . 
         [0029]    The stress-isolated pressure sensor can be also contained in the injection-molded plastic housing  102  in the prior art as shown in  FIG. 1A  by replacing the MEMS pressure sensing element  106  by the stress-isolated pressure sensor  200  in  FIG. 2 . 
         [0030]    The suspenders  208  that support and isolate the MEMS pressure sensing element  201  from stress are formed by either etching the top surface  211  or the bottom surface  213  or etching both surfaces  211  and  213  of the stress-isolated pressure sensor  200 . The etching required to form a preferred-embodiment suspender is a multi-step process that forms corrugated suspenders or serpentine suspenders in the material from which the stress-isolated pressure sensor  200  is made. The corrugated suspenders or serpentine suspenders expand and contract responsive to acceleration or movement of the MEMS pressure sensing element  201  and thus act as springs, absorbing mechanical forces that would otherwise be applied to the diaphragm and distort its output signals. Stated another way, the suspenders  208  isolate or alleviate the diaphragm  206  from mechanical stress. 
         [0031]      FIG. 5  shows a corrugated suspender  500 , which can be one kind of the suspenders  208  used in the stress-isolated pressure sensor  201  as shown in  FIG. 2  and  FIG. 3  for stress isolation. The corrugated suspender  500  comprises a wavy top surface  501  and a wavy bottom surface  502 . The top surface  501  is boron-doped with P+ conductive material as an interconnect connecting the P+ interconnect  306  as shown in  FIG. 3 . The corrugated suspenders  500  have several substantially planar and horizontal merlons  504 , which are “attached” to inclined sections referred to herein as crenels  506 . The substantially planar merlons  504  are substantially parallel to the substantially planar top and bottom surfaces of the diaphragm  206  of the stress-isolated MEMS pressure sensing element  201 . 
         [0032]      FIG. 6A  is a perspective view of a serpentine suspender  600  that is an alternate for the corrugated suspender  500 .  FIG. 6B  is a top view of the serpentine suspender  600  and  FIG. 6C  is a side view of the serpentine suspender  600 . The material from which the suspender  600  is formed can be doped to be conductive and carry electrical current. It can also support a pressure sensing element in a cavity. Unlike the corrugated suspenders shown in  FIG. 5 , which have vertically-oriented crenellations, the serpentine suspender  600  has crenellations considered herein to be horizontally-oriented. The serpentine suspender  600  shown in  FIG. 6  can be more easily formed by fewer etching steps than the corrugated suspenders shown in  FIG. 5  and can thus be considered “preferred.” 
         [0033]    Each crenellated section  610  has a merlon  612  and a crenel  614 . In a preferred embodiment the suspenders  600  are doped to be P+using conventional processes to make them electrically conductive while at the same time being mechanically flexible. 
         [0034]    Referring now to  FIG. 7  there is depicted a cross section and method of forming a stress-isolated pressure sensor. A silicon-on-insulator (SOI) substrate  701  with an internal cavity  704  can be formed, which consists of a silicon support substrate  702 , a silicon dioxide layer  706 , and a silicon device layer  708 . The SOI silicon support substrate  702  is etched by deep reactive ion etching (DRIE) at the center of the bottom surface  703  to effectively form a shallow cavity  712 . 
         [0035]    The corrugated or serpentine suspenders  208 , as described above, are formed after a further DRIE etching process framing a deeper cavity  714  surrounding the MEMS pressure sensing element  201 . The shallow cavity  712  and the deeper cavity  714  surrounding the MEMS pressure sensing element  201  constitute the cavity  210 . The suspenders  208  support and mechanically isolate the pressure sensing element  201  from the remaining SOI substrate  701 . 
         [0036]    The etched SOI substrate consisting of the MEMS pressure sensing element  201  is attached to a substrate  720 . The entire structure is supported on the substrate  720 , which can be glass or silicon. 
         [0037]      FIG. 8  depicts an alternate method of forming a stress-isolated pressure sensor. A first silicon substrate  802  is etched to form a shallow cavity  804  and attached to a SOI substrate  810  to form a bonded substrate. The SOI substrate  810  comprises a silicon dioxide layer  812 , a silicon device layer  814 , a silicon dioxide layer  816 , and a silicon support substrate  818 . The rim or edge portion of the bonded substrate is etched from the topside to expose the silicon device layer  814 . A silicon or glass cap  820  with a deep cavity  822  formed is then attached to the top surface of the exposed silicon device layer  814  of the bonded substrate  810 . If the cap  820  is glass, the cap  820  can be anodically bonded onto the bonded substrate  810 . If the cap  820  is silicon, the cap  820  can be fusion-bonded onto the bonded substrate  810  or glass-frit-bonded onto the bonded substrate  810 . 
         [0038]    The bottom surface of the SOI substrate  810  is etched to define a substantially square-shaped MEMS pressure sensing element  201  having a diaphragm  206  and a plurality of supporting suspenders  208  to form a stress-isolated pressure sensor. 
         [0039]      FIG. 9  depicts a stress-isolated pressure sensor  900  comprising a MEMS pressure sensing element  901  attached on top of a silicon or glass substrate  904 . A cavity  906  in which the MEMS pressure sensing element  901  is suspended by suspenders  908  is filled with a viscous gel  910  in which the MEMS pressure sensing element  901  is suspended. 
         [0040]    Conductive vias  914  extend between the top surface  916  of the substrate  902  and the bottom surface  918  of the substrate  904  and provide conductive pathways to an ASIC  920  having its own conductive through-silicon vias  922 . Signals of the ASIC  920  can thus be conducted to and from the MEMS pressure sensing element  901  suspended in a viscous gel by corrugated or serpentine suspenders, which are doped to be electrically conductive. 
         [0041]    Referring now to  FIG. 10 , the structure shown in  FIG. 9  is shown as being overmolded in an overmold  1002 . The stress-isolated pressure sensor  1000  and its overmold  1002  include PCB with conductive traces or conductive lead frames  1004  which extend from exterior surfaces of the overmold  1002  through the overmolding material to the bond pads  1006  on the ASIC  920 . 
         [0042]    Those of ordinary skill in the art should know that a catenary is the curve assumed by a cord of uniform density and cross section that is perfectly flexible but not capable of being stretched to be horizontal and which hangs freely from two fixed points. Examples of catenaries are power lines and telephone lines suspended from towers or posts. 
         [0043]    Those of ordinary skill in the art should recognize that the support or suspension of a MEMS pressure sensing element in a cavity by springs, regardless of the springs&#39; shape will have a shape that is inherently catenary. The spring-like suspenders disclosed herein are thus considered herein to have a shape that is at least partly catenary. The shape assumed by the opposing spring and the MEMS pressure sensing element is also assumed to be at least partially catenary. 
         [0044]    The foregoing description is for purposes of illustration only. The true scope of the invention is set forth in the following claims.