Abstract:
The voltages output from a low-pressure MEMS sensor are increased by increasing the sensitivity of the sensor. Sensitivity is increased by thinning the diaphragm of the low pressure sensor device with corner trench. Nonlinearity increased by thinning the diaphragm is reduced by simultaneously creating a cross stiffener to the bottom side of the diaphragm. A rim, anchors, and a stiffener pad can also be added to further stiffen the thinner diaphragm and further reduce pressure nonlinearity.

Description:
BACKGROUND 
     Solid state micro-electro-mechanical 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 to Johnson, et al. also discloses a solid state pressure transducer. U.S. Pat. No. 6,006,607 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. 
     A well-known problem with prior art MEMS pressure sensors, which use piezoresistive devices formed into a thin silicon diaphragm, is pressure non-linearity or PNL. The PNL is a function of the silicon diaphragm&#39;s deflection. The greater the diaphragm deflection, the greater degree of output non-linearity, whether the piezoresistance is detected and measured as a voltage or current. 
     Output non-linearity becomes more problematic in sensors that are intended to detect low pressures, e.g., pressures below 10 kPa. Since low pressure sensing devices require very thin silicon diaphragms, the diaphragm deflection in a thin diaphragm tends to aggravate the PNL in pressure sensors that are designed to measure low pressures. Another problem with thin silicon diaphragms is that they are fragile. A major challenge is to create a diaphragm to lower or reduce PNL while improving pressure sensitivity without increasing the die size for a low pressure sensor. A solid state piezoresistive pressure sensor that can be used at low pressures and which has an improved output linearity and which is more rugged and more sensitive than those in the prior art would be an improvement. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  and  FIG. 2  are perspective views of a pressure sensor; 
         FIG. 3  is a cross-sectional view of the pressure sensor shown in  FIG. 1  and  FIG. 2 ; 
         FIG. 4  depicts the topology of a Wheatstone bridge circuit; 
         FIG. 5  is a perspective view of a pressure sensing element with high sensitivity, high accuracy and having a thin diaphragm and a cross stiffener; 
         FIG. 6-1  is a perspective view of the bottom view of a pressure sensing element shown in  FIG. 5 ; 
         FIG. 6-2  is a plan view of the bottom of an alternate embodiment of a pressure sensing element; 
         FIG. 7  is a cross sectional view of the pressure sensing element shown in  FIG. 5  and  FIG. 6-1 ; 
         FIG. 8  is a plan view of the top side of a pressure sensing element showing a distributed Wheastone bridge formed from piezoresistors deposited into the top side of an epitaxial layer; 
         FIG. 9  is a perspective view of the bottom of a preferred embodiment of a pressure sensing element; 
         FIG. 10  is a plan view of the bottom of the pressure sensing element shown in  FIG. 9 ; 
         FIG. 11  is a cross sectional view of a differential pressure sensor; 
         FIG. 12  is a cross sectional view of a top side absolute pressure sensor; 
         FIG. 13  is a cross sectional view of a backside absolute pressure sensor; 
         FIGS. 14-1 through 14-4  are plan views of alternate embodiments of a sensing element; and 
         FIG. 15  is a flow chart showing steps of a method of forming a pressure sensor device having high sensitivity and high accuracy using a cross stiffener. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a perspective view of a pressure sensor  10  for use in automotive and industrial pressure sensing applications.  FIG. 2  is a side view of the pressure sensor  10 .  FIG. 3  is a cross-sectional diagram of the pressure sensor  10  shown in  FIG. 1  and  FIG. 2 . 
     In  FIGS. 1, 2 and 3 , the sensor  10  comprises an injection molded plastic housing  12  that comprises an elongated, hollow shroud portion  13 . The shroud  13  receives a male connector and protects one or more lead frame  24  that pass through the plastic material from which the housing  12  is made. The lead frame  24  provides electrical pathways into a pocket  16  inside the housing  12  where a pressure sensing element, identified by reference numeral  14 , is mounted with die-mount adhesive above a hole  18  formed through a substrate  26 . The hole  18  is aligned with an open passageway  30  of a port  32 . A liquid or gas, the pressure of which is to be measured by the pressure sensing element  14 , is able to pass through the passageway  30  and hole  18  in the substrate  26  and exert pressure on a diaphragm from which the pressure sensing element is made. 
     The pressure sensing element  14  is a diaphragm-type pressure sensing element  14  located inside a pocket  16  of the housing  12 . An application-specific integrated circuit (ASIC)  18 , also located inside the pocket  16  includes electronic devices to output a voltage that is proportional to changes in the resistance of one or more “distributed” piezoresistive devices formed in the sensing element  14  and which are electrically connected to each other to form a Wheatstone bridge circuit. 
       FIG. 4  illustrates the topology of a Wheatstone bridge circuit  400 , and which is used in the pressure sensing element  14 . The circuit  400  comprises four separate piezoresistors  402 ,  404 ,  406  and  408  connected to each other, “end-to-end” forming a loop having four nodes where the four piezoresistors are connected together. The piezoresistors  402 ,  404 ,  406  and  408  are formed by depositing a P− semiconductor material into the top surface of a thin diaphragm made of an N epitaxial layer. The piezoresistors  402 ,  404 ,  406  and  408  are interconnected with conductors. The conductors are formed by depositing a P+ semiconductor material into the top surface of the epitaxial layer. The P− semiconductor material comprising a resistor is limited to a small area and is thus localized. The resistors are connected to each other by conductors and metal runners deposited onto the top surface of the diaphragm. Two diagonally-opposite nodes  410 ,  412  are considered to be input terminals; the other two diagonally-opposite nodes  414 ,  416  are considered to be output terminals. 
     Those of ordinary skill in the art will recognize that the magnitude of a voltage applied to the input terminals  410 ,  412  will be divided by the ratio of the piezoresistors&#39; resistance values and output across the output terminals  414 ,  416 . Since the piezoresistors  402 ,  404 ,  406  and  408  are formed into a thin silicon diaphragm that deflects when a pressure is applied to the diaphragm, the physical size, shape and electrical resistance of the piezoresistors  402 ,  404 ,  406  and  408  will change responsive to diaphragm deflection caused by pressure applied to the diaphragm. 
     Referring again to  FIG. 3 , the ASIC  18  includes circuitry that applies a voltage to the input terminals  410 ,  412  and measures the output voltage at the output terminals  414 ,  416 . The IC  18  thus generates an electrically measurable output signal, which changes responsive to changes in the resistance of one or more resistors formed into the thin diaphragm comprising the sensing element  14 . As best seen in  FIG. 3 , electrical signals from the electronic devices inside the ASIC  18  are routed through the housing  12  through the lead frame  24  that extends into the shroud  13  that surrounds the lead frame  24 . 
     As used herein, a rectangle is a parallelogram, adjacent sides of which form right angles. A square is a rectangle, the sides of which have equal lengths. As described below, the pressure sensing element  14  is formed of a thin and substantially square-shaped silicon diaphragm having a cross stiffener, which can be sized, shaped and arranged to reduce non-linearity by controlling deflection of the thin diaphragm. The diaphragm and cross stiffener are formed together, i.e., at the same time, by etching material from one side of a relatively thick silicon substrate until an N epitaxial layer is reached, then etching the epitaxial layer to form the diaphragm and cross stiffener from the epitaxial layer material. 
       FIG. 5  is a perspective view of the silicon substrate  500 , viewed from above.  FIG. 6-1  is a perspective view of the bottom of the substrate  500  shown in  FIG. 5 .  FIG. 7  is a cross-sectional view of the substrate  500  taken through section lines  7 - 7 . 
     The thickness of the diaphragm  606  is preferably between about 2.5 microns and about 5.0 microns. The diaphragm  606  having a thickness within that range easily deflects when a pressure is applied to it. As described below and as can be seen in the other figures, a cross stiffener formed under the diaphragm limits diaphragm deflection and reduces non-linearity of signals output from a pressure sensor constructed using such a pressure sensing element. 
     Referring now to  FIG. 5 , the pressure sensing element  14  comprises an essentially a rectangular parallelepiped-shaped substrate  502  made of P-type single crystalline silicon. The substrate  502  has a substantially planar top side  504  and an opposing substantially planar bottom side  506 . The substrate  502  also has four substantially vertical side faces or edges  508 ,  510 ,  512  and  514 . An epitaxial layer  516 , formed using conventional processes, is grown or deposited on the top side  504  of the substrate  502 . One or more passivation layers  517 , typically formed from silicon nitride, can be deposited over the epitaxial layer  516 . 
       FIG. 6-1  is a perspective view of the bottom  506  of the substrate  502  shown in  FIG. 5 , i.e., the substrate  502  when it is viewed from below. The substrate  502  has a cavity  602  formed into the bottom side  506  by a first etch, which is preferably deep reactive ion etching (DRIE) or a “wet” etch performed using potassium hydroxide (KOH). The first etch removes substrate material all the way “down” to the epitaxial layer  516  that is formed on the top side  504  of the substrate  502 . Stated another way, the first etch forms a cavity  602  into the bottom side  506  of the substrate  502  that extends upwardly from the bottom  506  of the substrate  502  to the epitaxial layer  516 . 
     After the P-type single crystalline silicon forming the substrate  502  is removed to form the cavity  602  and expose the epitaxial layer  516 , a second, “dry” etch is performed inside the cavity  602  and against the bottom surface  604  of the epitaxial layer  516 . The dry etch is preferably performed using “SF6,” well known to those of ordinary skill in the semiconductor processing art. The second etch removes material from the epitaxial layer to thin the epitaxial layer and thus form a very thin diaphragm  606 , which also has a cross stiffener  608 , best seen in  FIG. 6-1  and shown in cross section in  FIG. 7 . 
     In a preferred embodiment, a corner-rounding etch step is performed after the wet etch and prior to the dry etch. The corner-rounding etch eliminates or at least reduces sharp corners between intersecting surfaces formed by the wet etch and thus reduces or eliminates stress concentrations at intersecting surfaces. 
     Still referring to  FIGS. 5, 6-1 and 7 , the cross stiffer  608  essentially comprises two fixed beams  608 A and  608 B, i.e., both ends of each beam  608 A,  608 B are fixed. The beams  608 A,  608 B support and stiffen the diaphragm  606  and against which a pressurized fluid, i.e., a liquid or gas, applies pressure. The cross stiffener  608  thus supports a load on the diaphragm  606  that is distributed across the diaphragm  606 . 
     The ends  609  of the beams  608 A,  608 B are considered to be “fixed” because the beams  608 A,  608 B and their ends  609  are formed as part of a rim  618 , also part of the epitaxial layer  516 , which extends around the perimeter of the diaphragm  606 . 
     Although the beams  608 A,  608 B are formed by etching away the epitaxial layer  516 , the beams are nevertheless considered herein to be joined to each other at their respective midpoints  610  and are at right angles to each other. The cross stiffener  608  is thus considered herein to be a complex fixed beam. It stiffens the diaphragm  606  thus reduces deflection of the diaphragm  606  when a pressure is applied to the diaphragm. 
       FIG. 6-2  is a plan view of the bottom of an alternate embodiment  600  of a pressure sensing element  500 . Unlike the epitaxial layer  516  shown in  FIG. 6-1 , which has a rim  618  that extends around and supports the periphery of the diaphragm, in  FIG. 6-2  the cross stiffener  629  formed from the epitaxial layer  516  does not have a rim. The cross stiffener  629  instead extends completely across the diaphragm  607  to where the arms of the cross stiffener are joined to, or extend from, the sloped side walls  620 ,  622 ,  624  and  626  surrounding the cavity  603 , and which are formed during the first etch. Stated another way, the pressure sensing element  600  shown in  FIG. 6-2  comprises a silicon substrate  503 , the bottom side  507  of which is etched away to form a cavity  603  and exposes an epitaxial layer  516 . The epitaxial layer  516  is then etched by SF6 to a required thickness level of a cross stiffener  629 . With a photoresist covering the backside of the etched silicon substrate and patterned with the cross stiffener  629  feature, further etching with SF6 provides a corner trench to form the cross stiffener  629  without a rim. As with the embodiment shown in  FIG. 6-1 , the cross stiffener  629  shown in  FIG. 6-2  stiffens and thus reduces deflection of the diaphragm  607  when a pressure is applied to the diaphragm  607 . 
     In one embodiment, the sensing element is able to measure pressures between about 1.0 kilopascal and about 10.0 kilopascals. In each embodiment, the rigidity or stiffness of the cross stiffener is determined by its dimensions as well as the characteristics of the material from which it the cross stiffener is formed. 
     The dimensions of the cross stiffener were determined analytically and experimentally via design of experiments (DOE). For a diaphragm having a thickness between about 2.5 micrometers and about 5.0 micrometers, the height of the cross stiffener  608  should be between about 7.0 micrometers and about 10.0 micrometers and preferably about 8.5 micrometers. 
       FIG. 8  is a plan view of the top surface  802  of the epitaxial layer  516 . The epitaxial layer is essentially square having four sides. The diaphragm region or portion  606  of the epitaxial layer  516  is also square. It too has four sides  804 ,  806 ,  808  and  810 . 
     Four piezoresistors  812 ,  814 ,  816  and  818  are formed in small, localized regions in the top surface  802  and within the diaphragm region  606  by depositing P− type semiconductor material into the N epitaxial layer  516 . As can be seen in the figure, the piezoresistors  812 ,  814 ,  816  and  818  are located at the midpoints  820  of each side  804 ,  806 ,  808  and  810  of the square-shaped diaphragm region  606 . 
     The piezoresistors  812 ,  814 ,  816  and  818  are considered to be “distributed” elements because they are not confined to one side or edge of the diaphragm  606  but are instead separated from each other and located along the sides  804 ,  806 ,  808  and  810  of the square-shaped diaphragm  606 . 
     The piezoresistors are connected to each other by conductors  824 , which are formed by P+ conductive material deposited into the N epitaxial layer. The P+ conductors  824  extends from each end of a piezoresistor outwardly to metal runners  826 , which connect to metal bond pads  830 ,  832 ,  834 ,  836  for the input and output voltages to form a Wheatstone bridge circuit. 
     Two of the P+ connectors are connected to metal runners that extend from them to metal bond pads  830  and  832  to which an input signal can be applied. Two other P+ conductors are connected to other metal runners  834 ,  836  which extend to a second pair of metal pads from which an output signal can be taken from the Wheatstone bridge. 
       FIG. 9  is a perspective view of the bottom a preferred embodiment of a sensing element  900 .  FIG. 10  is a plan view of the same preferred embodiment of a sensing element  900 . 
     As best seen in  FIG. 9 , the opposing ends  902  of the two substantially orthogonal arms  904 ,  906  forming a cross stiffener  908  are formed, and extend away from, four cross stiffener anchors  910 . Stated another way, the anchors  910 , cross stiffener  908 , and rim  920  are formed together by etching an epitaxial layer  912  deposited onto the top surface  914  of a single crystalline silicon substrate  916  into which a cavity  918  is etched as described above. 
     The anchors  910  are substantially parallelepiped-shaped blocks and are located against and extend from a rim  920 , which is also formed by etching the epitaxial layer  912 . The anchors  910  are formed to be located directly beneath piezoresistors (See  FIG. 8 .) which are formed into the top surface of the epitaxial layer  912 , which is not visible in  FIG. 9  or  FIG. 10 . The anchors  910  have a thickness selected to prevent leakage current from passing from P+ and P− into the N epitaxial layer  912  through P-N junctions. 
     As best seen in  FIG. 10 , the cross stiffener  908  is attached to the rim  920  through the anchors  910 . As stated above, the cross stiffener  908  is a beam that supports a diaphragm  924  formed by thinning the epitaxial layer  1012 . The cross stiffener  1008  defines four corner trenches  926 . 
     The rim  920  supports and stiffens the perimeter of the diaphragm  924 . The thinned epitaxial layer that forms the “bottom” of the trenches  926  will deflect responsive to a pressure applied to them and thereby change the shape or dimensions of the piezoresistors formed in the top surface of the epitaxial layer. The cross stiffener  908 , which is a beam supporting or stiffening the diaphragm, reduces the deflection of the diaphragm  924 . 
       FIG. 11  is a cross sectional diagram of a differential pressure sensor  1100 . The pressure sensor  1100  can be formed from a sensing element  500  shown in  FIG. 5 , which has a rim  618  or formed from a sensing element  600  shown in  FIG. 6-2  without a rim, or formed from a sensing element  900  shown in  FIG. 9  with a rim and four anchors. In  FIG. 11 , a first substrate  1102  comprising a sensing element as depicted in  FIG. 6-2  is formed in the first substrate  1102 , which is then attached to and supported by a second substrate  1108 . The second substrate  1108  is typically glass but can also be formed of silicon. The second substrate  1108  is attached to the bottom side  1110  of the first substrate  1102 . 
     A hole  1112  is formed through the second substrate. The hole  1112  is aligned with the diaphragm  1114  formed in the first substrate  1102 . The hole  1112  is sized, shaped and arranged to conduct a fluid, i.e. a liquid or a gas, into the cavity  1116  formed into the bottom side  1110  of the first substrate  1102 . The fluid will thus exert a pressure on the bottom side of the diaphragm  1114 . The deflection of the diaphragm  1114  is thus dependent on the difference between the pressure applied to the top side of the diaphragm  1114 , i.e., the pressure inside the top cavity  1006 , and the pressure applied to the bottom side of the diaphragm, i.e., the pressure inside the bottom or lower cavity  1116 . 
     Changing the height  1120  and/or width  1122  of a cross stiffener  1124  supporting the diaphragm  1114  determines the deflection of the diaphragm and cross stiffener  1124  responsive to a pressure applied to them and hence the sensitivity of the sensor. Changing the dimensions of the cross stiffener  1124  thus allows the pressure sensitivity and nonlinearity to be precisely controlled. The sizes of the rim  618  in the sensing element  500  or the rim  920  and the anchors  910  in sensing element  900  also influence the pressure sensitivity and nonlinearity. 
       FIG. 12  is a cross sectional view of an absolute pressure sensor  1200 . The absolute pressure sensor  1200  has a top or first substrate  1102  that is the same as the first substrate  1102  shown in  FIG. 11  and described above. A second substrate  1202  is attached to the bottom side  1110  of the first substrate  1102 . The second substrate  1202  does not have a hole; it closes the cavity  1116  formed into the bottom side  1110  of the first substrate  1120 . 
     If the cavity  1116  is evacuated when the second substrate  1202  is attached, a pressure or vacuum applied to the top side  1103  of the first substrate  1102  will cause the diaphragm  1114  to deflect, changing the resistance of piezoresistors formed into the diaphragm. 
       FIG. 13  is a cross sectional view of a backside absolute pressure sensing element  1300 . A cap  1302  applied to the top side  1103  of the first substrate  1102  shown in  FIG. 11  provides an evacuated cavity  1304  above the diaphragm  1114 . A second substrate  1306  applied to the bottom side  1110  of the first substrate  1102  has a hole  1308  aligned with the diaphragm  1114  through which fluid can pass into the cavity  1116  formed in the first substrate  1102 . The hole  1308  enables pressurized fluids to exert pressure on the bottom or backside  1115  of the diaphragm  1114 . The net deflection of the diaphragm  1114  is determined by the pressure in the top cavity  1304  and the bottom or backside cavity  1116 . 
     Experimentation revealed that the cross stiffeners described above and shown in  FIGS. 3-13  can also be formed with a centrally-located diaphragm-stiffening pad.  FIGS. 14-1-14-4  are bottom vies of alternate embodiments of cross stiffeners. In  FIG. 14-1 , a cross stiffener  1402  is formed without anchors but extends from a centrally-located square pad  1404  outwardly to a rim  1406  that extends around the perimeter of a diaphragm  1408 . 
     In  FIG. 14-2 , the cross stiffener  1402  extends from a substantially round or circular pad  1410  to the rim  1406 . 
     In  FIG. 14-3 , the cross stiffener  1402  extend from anchors  1412  located at the midpoint of each side of the rim  1406  inwardly to a substantially square shaped pad  1404 . 
     Finally, in  FIG. 14-4 , the centrally-located pad  1410  is also circular or round. The cross stiffener  1402  extends outwardly from it to anchors  1412 . 
     In a preferred embodiment, the diaphragm thickness is between about 2.5 microns and about 5 microns. The cross stiffener, however, had a thickness between about 5.0 microns and about 10.0 microns but can be up to 15.0 microns in thickness. 
     As noted above, the stiffener and the diaphragm are formed from the same epitaxial layer. In an alternate embodiment, however, the stiffener can be deposited on to the bottom side of the epitaxial layer using a different material from which the diaphragm is formed. 
       FIG. 15  is a flow chart showing steps of the method of forming the pressure sensing element described above. 
     At a first step  1502 , an epitaxial layer is formed on a top side of a single crystal silicon substrate. The thickness of the epitaxial layer is thin, preferably less than about 20 microns. 
     At a second step  1504 , the bottom side of the single crystal silicon layer is etched to define or form a cavity. The first etch removes all of the single crystalline layer material “down” to the epitaxial layer exposing the bottom surface of the epitaxial layer for a subsequent etching step. 
     In a third step  1506 , a second etch is performed on the epitaxial layer to remove material from the epitaxial layer to define a cross stiffener, a rim, or anchors and if desired, a stiffener pad described above. 
     In a preferred embodiment, a corner rounding etching step  1505  is optionally performed after the cavity is etched into the bottom side of the first substrate and prior to etching the epitaxial layer. The cornering rounding etch step essentially rounds interior corners and reduces stress concentrations that would otherwise develop without the corner rounding etching. 
     At a fifth step  1508 , the substrate having the epitaxial layer is attached to a second substrate, which is considered to be a support for the first substrate. The second substrate can have a hole formed through it depending upon whether the resultant pressure sensing element is to be a differential pressure sensor or topside absolute pressure sensor. 
     Finally, at step  1510 , a cap is applied to the top side of the first substrate to define an evacuated cavity above the diaphragm. 
     Those of ordinary skill in the art will appreciate that a cross stiffener as described above will support and thus control deflection of a thin diaphragm. The cross stiffener dimensions, (height, width and length) can be determined experimentally or through computer modeling to stiffen a diaphragm as desired. A diaphragm can thus be made very thin, in combination with a cross stiffener, rim, and/or anchors, to increase its sensitivity and accuracy, as described in more detail above. 
     The foregoing description is for purposes of illustration only. The true scope of the invention is set forth in the following claims.