Abstract:
The voltage output span and sensitivity from a MEMS pressure sensor are increased and pressure nonlinearity is reduced by thinning a diaphragm and forming the diaphragm to include anchors that are not connected to or joined to diaphragm-stiffening beams or thickened regions of the diaphragm.

Description:
BACKGROUND 
       [0001]    Micro-electromechanical system (MEMS) pressure sensors are well known. For example, U.S. Pat. No. 4,236,137 to Kurtz, et al. discloses a semiconductor pressure transducer. U.S. Pat. No. 5,156,052 issued to Johnson, et al. also discloses a solid state pressure transducer. U.S. Pat. No. 6,006,607 issued to Bryzek, et al. discloses a pressure sensor that uses a piezoresistive device. U.S. Pat. Nos. 5,178,016 and 6,093,579 also discloses solid state pressure sensors. See also U.S. Pat. No. 8,881,596 entitled, “Semiconductor sensing device to minimize thermal noise,” which is owned by the Applicant of this application and which is also incorporated by reference in its entirety. 
     
    
       [0002]    A well-known problem with prior art MEMS pressure sensors is pressure nonlinearity or “PNL.” PNL is a function of the silicon diaphragm&#39;s deflection. Diaphragm deflection, however, determines a MEMS pressure sensor&#39;s ability to detect small pressure changes. Unfortunately, as diaphragm deflection increases, so does output nonlinearity. See for example, U.S. pre-grant publication 20150330856, entitled, “PRESSURE SENSOR DEVICE WITH HIGH SENSITIVITY AND HIGH ACCURACY,” published Nov. 19, 2015, assigned to the same applicant and incorporated herein by reference in its entirety. 
         [0003]    Sensitivity becomes more problematic due to a smaller diaphragm in a shrunken MEMS pressure sensor that is required to sense low pressures, i.e., pressures below about 100 kPa. A solid state piezoresistive pressure sensor that can be used at low pressures and which has a smaller diaphragm in a smaller die with an improved output linearity and which is more sensitive than those in the prior art would be an improvement. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0004]      FIG. 1  is a graph showing pressure nonlinearity or PNL which is a difference in the real output voltage of the MEMS pressure sensor across a linear voltage output range divided by the full scale span output voltage; 
           [0005]      FIG. 2  is a perspective view of a first embodiment of a pressure sensing element having a cavity formed into the bottom of a substrate which confines a diaphragm in the substrate, the top of which is formed to have a recess with anchors; 
           [0006]      FIG. 3  is a cutaway view of the pressure sensing element of  FIG. 2  showing one-quarter thereof; 
           [0007]      FIG. 4  is a bottom view of the pressure sensing element depicted in  FIG. 2 ; 
           [0008]      FIG. 5A  is a bottom view of the pressure sensing element shown in  FIG. 2  but showing the location of the recess formed into the top surface of the diaphragm which is formed into the bottom surface of the substrate; 
           [0009]      FIG. 5B  is a bottom view of an alternate embodiment of the pressure sensing element shown in  FIG. 2  depicting a recess formed into the top surface of the diaphragm, the shape of which is different from that shown in  FIG. 5A ; 
           [0010]      FIG. 6A  shows a cross-section view of the pressure sensing element with topside anchors bonded with a substrate at the bottom for topside absolute pressure sensing; 
           [0011]      FIG. 6B  shows a cross-section view of the pressure sensing element with topside anchors bonded with a substrate at the bottom with a through-hole and another substrate as a cap at the top for backside absolute pressure sensing; 
           [0012]      FIG. 7  is a bottom view of a pressure sensing element comprising a recess formed into the bottom of a substrate to define a rim and anchors after the cavity is etched; 
           [0013]      FIG. 8  is a cross-sectional view of the pressure sensing element with backside anchors as shown in  FIG. 7  plus a glass pedestal with a through-hole at the bottom and a cap on the top; 
           [0014]      FIG. 9  is a chart of output voltage spans, span ratios, pressure and nonlinearity percentages for prior art MEMS pressure sensing elements, the pressure sensing element described herein and using both 7 micrometer and 10 micrometer thicknesses; 
           [0015]      FIG. 10  is a top view of a pressure sensing element with symmetric topside anchors; and 
           [0016]      FIG. 11  is a top view of a pressure sensing element with asymmetric topside anchors. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    For clarity purposes, pressure nonlinearity or “PNL” is considered herein to be the maximum voltage difference between an idealized linear voltage output and an actual voltage output from a MEMS pressure sensor embodied as a Wheatstone bridge circuit formed from piezoresistors. The maximum difference in the real voltage output from an idealized linear voltage output divided by the full scale or simply “span” defines a PNL. 
         [0018]      FIG. 1  is a graph depicting how PNL is determined. Paraphrased, PNL is determined as the maximum output voltage deviation of a MEMS pressure sensor from an ideal linear output voltage over a range of input pressures. As shown in  FIG. 1 , PNL is expressed as: 
         [0019]    Those of ordinary skill in the MEMS pressure sensor art know that PNL should be reduced as low as possible, usually to less than about  1 . 5 % for automotive applications. 
         [0020]      FIG. 2  depicts a first embodiment of a pressure sensing element  200 , which comprises a cube-shaped substrate  202  having a top surface  204  and a bottom surface  206 . The substrate  202  also has a recess  208  etched into the top surface  204 , the recess  208  being sized and shaped to define four, “anchors,” which are substantially rectangular-shaped blocks or cuboids identified by reference numeral  210 . Unlike the pressure sensing elements disclosed in the Applicant&#39;s co-pending applications noted above, the anchors disclosed herein are not connected or coupled to each other with or by, a diaphragm stiffener that extends between opposing anchors. 
         [0021]    In  FIG. 2 , the anchors  210  are spaced apart from each other “in” the recess  208  such that each of the anchors  210  is located near each diaphragm edge center. Stated another way, each two anchors  210  opposite from each other are evenly spaced on the diaphragm  208 . 
         [0022]      FIG. 3  is a perspective view of a quarter-section of the pressure sensing element  200  shown in  FIG. 2 . The bottom side  206  of the substrate  202  can be seen in  FIG. 3  as having a substantially pyramid-shaped cavity  300  formed into the bottom side  206  of the substrate  202 . The sides of the cavity  300  are slightly inclined as a consequence of the etching process by which the cavity is formed. 
         [0023]      FIG. 4  is a perspective view of the bottom side  206  of the pressure sensing element  200  shown in  FIG. 2 . The cavity  300  extends from the bottom surface  206  upwardly through most of the material from which the substrate  202  is made. The depth of the cavity  300  is selected such that material not etched away and left at the top  304  of the cavity  300  defines a substantially square-shaped diaphragm  306 , the thickness of which is between about 7.0 micrometers up to about 15.0 micrometers. 
         [0024]      FIG. 5A  is a bottom view of the pressure sensing element  200  depicted in  FIG. 2 .  FIG. 5A  also shows the shape of the recess  208  formed into the top side  204  and anchors  210  formed by etching the substrate material.  FIG. 5B  is a bottom view of an alternate embodiment of the pressure sensing element  200  shown in  FIG. 2  depicting an asymmetric recess  258  formed into the top surface of the diaphragm  306 , the shape of which is different from that shown in  FIG. 5A . Two pairs of anchors  210  and  260  are formed from the asymmetric recess  258 . 
         [0025]    It is well known that a polygon is “regular” when all of its angles are equal and the lengths of all of its sides are equal, otherwise a polygon is “irregular.” The shape of the recess  208  is considered herein to be a closed irregularly-shaped polygon due mainly to the size of the anchors relative to the perimeter of the recess. 
         [0026]    As best seen in  FIG. 3 , the thickness, t, of the diaphragm  306  is quite thin, preferably between about 2.0 and about 5.0 microns. The anchors  210 , however, have a greater thickness, about 7.0 up to about 15.0 microns. 
         [0027]    As best seen in  FIG. 6A , the pressure sensing element with topside anchors  210  shown in  FIGS. 2-5  is preferably supported by a second substrate or pedestal  602 , preferably made of glass, for topside pressure sensing. The second substrate  602  is preferably made of either silicon or glass and is attached to the bottom surface  206  of the first substrate  202 . In one embodiment, the second substrate  602  is provided with a through-hole  604  as shown in  FIG. 6B . The through-hole  604  allows fluid in the hole  604  to exert pressure against the backside of the diaphragm  306  formed into the top of the first substrate  202  for differential pressure sensing. An optional cap  606  can be placed over the top side of the diaphragm  306 , i.e. over the top surface  204  of the substrate  202  to define an evacuated cavity  608  above the top side of the diaphragm  306 . The cap  606  protects the top side of the diaphragm  306  and prevents pressure from being applied to the top surface of the diaphragm  306  for backside pressure sensing. 
         [0028]      FIG. 7  depicts a bottom view of an alternate embodiment of a pressure sensing element  700 . The pressure sensing element  700  comprises a substrate  702  having a top side  704 , not visible in  FIG. 7 , and a bottom side  706 . The bottom side  706  has a cavity  708  having a pyramid-like shape, similar to the shape of the cavity  300  formed into the substrate shown in  FIGS. 2-6 . 
         [0029]    The cavity  708  shown in  FIG. 7  is formed into the bottom surface  706  of the substrate  702 . The cavity  708  is formed to provide a substantially planar surface  711 . The planar surface  711  is further etched to form a recess  712  with a rim  710  and four anchors  714  surrounding the recess  712 . Each of the four anchors  714  is located near the center of each diaphragm edge at the bottom surface which is at the same elevation as the planar surface  711 . The four anchors  714  are extended inwardly from each side of the rim  710 . A thinner corrugated diaphragm  719  as shown in  FIG. 8  is thus formed by the rim  710 , the recess  712 , and the four anchors  714 . The thinner portion of the diaphragm  719  is used to increase pressure sensitivity and the rim and anchors are used to reduce pressure nonlinearity. 
         [0030]      FIG. 8  is a cross-sectional view of the pressure sensing element depicted in  FIG. 7 , showing the cavity  708  in the bottom side and a recess  712  further formed into the cavity  708  of the substrate  702 . 
         [0031]    As with the pressure sensing element depicted in  FIGS. 2-6 , the recess  712  formed in the bottom surface of the substrate  702  is a closed polygon shape. The diaphragm  719  thickness away from the anchors  714  is between about 2.0 and about 5.0 microns. The anchors  714  and the rim  710 , however, are thicker, having thicknesses between about 7.0 and 15.0 microns. 
         [0032]    Similar to the pressure sensing element  200  shown in  FIG. 2 , the pressure sensing element  700  shown in  FIG. 7  can also be provided with a second substrate  602  as shown in  FIG. 8  with a through-hole  604  for differential pressure sensing. The second substrate  602 , also known as a pedestal, is preferably made of glass. A cap  824  with a vacuum cavity  808  can also be placed over the top surface  704  with an evacuated cavity  808  for backside absolute pressure sensing. 
         [0033]      FIG. 9  is a table depicting performances of prior art pressure sensing elements and the pressure sensing elements depicted herein for 1 Bar topside absolute pressure sensing using a 300 um diaphragm. For a diaphragm thickness of about 7 micrometers, a conventional prior art flat diaphragm has a span voltage of about 22.36 millivolts. The top cross, described in claim in Applicant&#39;s co-pending application number 2014P05613US has an improved span voltage at 32.99 millivolts, improved by about  48 %; however, the span voltage of a diaphragm supported by anchors only, and as described herein has yet an even greater span voltage of about 43.14 millivolts. The span ratios also improve over the prior art structures although the pressure nonlinearity is slightly higher but still within the pressure nonlinearity requirement, +/−1.5%. 
         [0034]    Similar performance improvements have been realized with diaphragms of about 10 micrometers in thickness. Span voltages between the top cross braced diaphragm versus the anchor-only diaphragm are nearly 50% better. The pressure nonlinearity is pretty low within +/−1.5%. 
         [0035]    Finally,  FIGS. 10 and 11  depict symmetric and asymmetric topside anchors, respectively. In  FIG. 10 , the anchors  1002  are substantially the same size and have substantially the same surface area. In  FIG. 11 , however, two longer anchors  1102  which are on opposite sides  1106  and  1108  of a diaphragm  306  are larger or longer than shorter anchors  1112  on opposite sides  1114  and  1116  of the diaphragm  306 . 
         [0036]      FIG. 10  depicts a plan view of the top surface  204  of the pressure sensing element  200  shown in  FIG. 6A  with symmetric topside anchors. Four piezoresistors  1012 ,  1014 ,  1016  and  1018  are formed by depositing P- type semiconductor material into the area of the anchors  1002  on the top surface  204  of the pressure sensing element  200 . The piezoresistors  1012 ,  1014 ,  1016  and  1018  are considered to be “distributed” elements because they are not confined to one side or edge of the diaphragm  306  but are instead separated from each other and located along the sides  306 A,  306 B,  306 C and  306 D of the square-shaped diaphragm  306 . 
         [0037]    The piezoresistors are connected to each other by conductors  1020 , which are formed by P+ conductive material deposited into the top surface  204  of the pressure sensing element  200 . The P+ conductors  1020  extend from each end of a piezoresistor outwardly and connect to metal bond pads  1032 ,  1034 ,  1036 , and  1038  for the input and output voltages to form a Wheatstone bridge circuit. 
         [0038]    Two loops of circuits are used to connect the metal bond pad  1032  to the metal bond pad  1034  for an input signal and each loop comprises two piezoresistors and two pairs of the P+ connectors. Another two loops of circuits are used to connect the metal bond pad  1036  to the metal bond pad  1038  for an output signal and similarly each loop consists of two piezoresistors and two pairs of the P+ connectors. 
         [0039]    As shown in  FIG. 11 , the asymmetric anchors have been experimentally determined to increase the span or sensitivity of the diaphragm but may introduce some higher pressure nonlinearity or electrical noise. 
         [0040]    The foregoing description is for purposes of illustration only. The true scope of the invention is set forth in the following claims.