Abstract:
A pressure transducer including: a silicon substrate including: a first surface adapted for receiving a pressure applied thereto, an oppositely disposed second surface, and a flexing portion adapted to deflect when pressure is applied to the first surface; at least a first sensor formed on the second surface and adjacent to a center of the flexing portion, and adapted to measure the pressure applied to the first surface; at least a second gauge sensor formed on the second surface and adjacent to a periphery of the flexing portion, and adapted to measure the pressure applied to the first surface; a glass substrate secured to the second surface of the silicon wafer.

Description:
This application is a divisional of U.S. patent application Ser. No. 09/245,158, filed Feb. 4, 1999, now U.S. Pat. No. 6,272,929. 
    
    
     FIELD OF INVENTION 
     The present invention relates to pressure transducers and more particularly to an improved high pressure piezoresistive transducer which is suitable for use in hostile environments and a novel, advantageous method for making the same. 
     BACKGROUND OF THE INVENTION 
     Kulite Semiconductor Products, Inc., the assignee herein, has previously made and patented a method for fabricating high pressure piezoresistive transducers using both longitudinal and transverse piezoresistive coefficients U.S. Pat. No. 5,702,619, entitled “Method of Fabricating a High-Pressure Piezoresistive Transducer”, filed Sep. 30, 1996, and assigned to the assignee herein, the entire disclosure of which is hereby incorporated by reference. Therein, a basic sensor is formed from a piece of single crystal silicon to which sensors are dielectrically bonded on one surface and the other surface of the silicon is bonded to a glass support member. In those structures the piezoresistive elements were formed on the surface of the transducer that is directly exposed to the pressure media. Additionally, electrical contacts and lead wires are also exposed to the media. 
     This structure is undesirable in some situations, where exposure of the piezoresistive elements, electrical contacts and lead wires to the media shortens the life expectancy of the pressure transducer. Accordingly, it is an object of the present invention to provide a high pressure transducer less sensitive to the media. 
     SUMMARY OF INVENTION 
     A pressure transducer including: a silicon substrate including: a first surface adapted for receiving a pressure applied thereto, an oppositely disposed second surface, and a flexing portion adapted to deflect when pressure is applied to the first surface; at least a first sensor formed on the second surface and adjacent to a center of the flexing portion, and adapted to measure the pressure applied to the first surface; at least a second gauge sensor formed on the second surface and adjacent to a periphery of the flexing portion, and adapted to measure the pressure applied to the first surface; a glass substrate secured to the second surface of the silicon wafer. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 illustrates a diagram of the approximate shape of a diaphragm with the resistor placement according to the present invention. 
     FIG. 2 illustrates a top view of a glass support to which the diaphragm of FIG. 1 is mounted according to the present invention. 
     FIG. 3 illustrates a cross-section A—A of FIG.  2 . 
     FIG. 4 illustrates the mounting of the diaphragm/support assembly to a header according to the present invention. 
     FIG. 5 illustrates a cross-section of a diaphragm/support and header according to the present invention. 
     FIG. 6 illustrates a perspective view of gauge placement according to the present invention. 
     FIG. 7 illustrates a side view of a sensor assembly as described in U.S. Pat. No. 5,614,678. 
     FIG. 8 illustrates a stress diagram for the sensor of FIG.  7 . 
     FIG. 9 illustrates a sensor assembly according to the present invention. 
     FIG. 10 illustrates a stress diagram for the sensor of FIG.  9 . 
    
    
     DETAILED DESCRIPTION OF INVENTION 
     Referring now to the numerous figures, wherein like references refer to like elements of the invention, FIG. 1 illustrates a diagram of the approximate shape of a diaphragm with the resistor placement according to the present invention. 
     According to the present invention, piezoresistive elements are placed on a side of the silicon structure  10  isolated, or away, or opposite from a media. Preferably, two elements or gauges,  15 ,  20  are located near a center of flexing portion  25  of the silicon member  10 , while two additional members or gauges  30 ,  35  are located just inside the flexing portion  25  area. 
     Contact areas  40  on the silicon structure  10  are sealed to a glass support structure  45  in a non-flexing area (complement of flexing area  25 ). Referring now also to FIGS. 2 and 3, holes  50  are provided in the glass support structure  45  to access the various contact areas  40  of the silicon structure  10  associated with sensors  15 ,  20 ,  30  and  35 . Additionally, a small depression  55  to allow the flexing area portion  25  of the silicon structure  10  to deflect is provided. The sensor network (sensors  15 ,  20 ,  30  and  35 ) and contact areas  40  are preferably dielectrically isolated from the silicon structure  10  in the same manner as U.S. patent application Ser. No. 09/047,548, entitled “Compensated Oil-Filled Pressure Transducers” filed Mar. 25, 1998, the entire disclosure of which is also incorporated by reference hereinto, including the seal of the glass to a rim structure and to the contact areas  40 . 
     Referring now also to FIGS. 4 and 5, the apertures, or holes,  50  in the glass structure  45  are preferably partially filled with a metallic frit  60  (and or an epoxy metal frit for example) and small copper balls  65  are inserted in the back areas of the apertures  50  (also see FIG.  9 ). The sensor-glass support structure (collectively  10  and  45 ) is then mounted to either a polymide structure  70  or ceramic structure with plated through holes  75  into which the exposed portions of the copper balls  65  will seat. 
     Contact can be made between the balls  65  and plated through holes  75  with a solder or braze. If a polymide structure  70  is used, the sensor structure can be secured with an epoxy or like material, while if a ceramic structure  70  is used the mounting may be made using a glass type frit. However, both mounting surfaces contain lead outs, or metallizations  90  to a series of holes  80 , sized in such a way to conform to the position of pins  95  on a header  85  preferably secured utilizing tapered glass  100 . 
     The composite structure ( 10 ,  45  and  70 ) is then mounted on the header  85  allowing the interconnects and the composite structure ( 10 ,  45  and  70 ) to be electrically connected to the pins of the header  85 . When pressure is applied from the side of the silicon not containing the sensor network, i.e. opposite thereof, the central portion of the silicon structure  10  deflects giving rise to a tensile surface strain in the center of the flexing member  25 , while the exterior portions of the flexing member  25  will be put in compression. 
     Referring now also to FIG. 6, methods of finite analysis were used to elucidate the various stresses within the plane of the silicon structure  10  (directions  105  and  110 ) and normal to it (direction  115 ). This analysis shows that the region of compressive surface stress in the flexing portion  25  where the sensor may be placed is very narrow. This is because the compressive normal stress in the center of the flexing region  25  is zero, but rises to its largest value at the outer edge of the flexing member  25 , and because of the negative sign of the transverse gage factor in the &lt; 110 &gt; direction is negative. If the outer gauge is in this region, the change in resistance will be positive [(−1)×(−1)] and there will be no output. 
     In general, each gauge sees three different stresses: a longitudinal stress in the plane of the diaphragm (direction  110 ), a transverse stress in the plane of the diaphragm (direction  105 ), and a transverse stress perpendicular to the diaphragm (direction  115 ). These stresses serve to change the resistivity of the gauge through piezoresistive effects. In general this change in resistivity can be broken down into a change for each separate stress, namely:                  Δ                 R     R     =         σ   x          π   x       +       σ   y          π   y       +       σ   z          π   z                 (   1   )                                
     where σ is the stress is one of the three directions and Π is the piezoresistive coefficient in that same direction. 
     By appropriate choice of crystallographic orientation, one skilled in the art can ensure the coefficient in the longitudinal in plane ( 110 ) and transverse out of plane ( 115 ) are equal in magnitude and opposite in sign, while the coefficient for the transverse in plane ( 105 ) is very close to 0. This leads for a final result for the change in resistance to be:                  Δ                 R     R     =         σ   long          (       π   44     2     )       -       σ   tran          (       π   44     2     )                 (   2   )                                
     By finite element analysis one can compute the transverse and longitudinal stresses that the gauges see and therefore choose the locations which yield the maximum change is resistance for a given load condition. 
     Referring now also to FIGS. 7-10, therein is illustrated a not to scale drawing and a graph of the relevant stresses for both a conventional high pressure sensor (FIGS. 7-8) and the new leadless one (FIGS.  9 - 10 ). FIGS. 7 and 9 are for reference only and should be used to clarify FIGS. 8 and 10. FIGS. 8 and 10 illustrate the transverse and longitudinal stresses in the appropriate part of the diaphragm. FIGS. 8 and 10 also have marked the approximate locations for the placement of the gauges. 
     Referring first to FIG. 7, therein is illustrated a conventional pressure transducer including supports  120 , diaphragm  125  and gauges  130 . 
     It can be seen from FIGS. 8 and 10 that for each sensor there are two gauges which will see a negative change in resistance and two which will receive a positive change in resistance. By combining these four gauges in a wheatstone bridge as set forth in U.S. Pat. No. 3,654,579, entitled “Electromechanical Transducers and Housings” filed May 11, 1970, is assigned to the assignee hereof, also herein incorporated by reference, one can to achieve the desired change in voltage. 
     This new structure has a number of unanticipated advantages. The position of both the inner and outer gages was only learned by computation using finite element analysis and would be different for each geometry of the sensor but the large difference in surface stress distributed from the top to the bottom surface of the silicon was not anticipated. However, the use of the finite analysis still makes possible the fabrication of a miniature sensor. 
     By putting the sensing network on the side of the silicon away from the media and using glass support structures with access holes to reach the contacts, it makes possible the construction of a “leadless” structure without fine gold wires and ball bonds as is illustrated in pending U.S. patent application Ser. No. 09/160,976 entitled “Hermetically Sealed Ultra High Temperature Silicon Carbide Pressure Transducers and Method for Fabricating Same” filed Sep. 25, 1998. It also makes possible higher temperature application of the device since the contact material in the apertures is sealed from any high temperatures, hostile environment while still retaining all of the advantages of the structure disclosed in pending U.S. patent application Ser. No. 09/160,976 entitled “Hermetically Sealed Ultra High Temperature Silicon Carbide Pressure Transducers and Method for Fabricating Same” filed Sep. 25, 1998. Additionally, the use of a separate mounting surface for the sensor structure makes possible the use of a header specifically designed for high pressure while still employing a miniature sensor. 
     Having described the preferred embodiment of this invention, it is evident that other embodiments incorporating these concepts may be used. Accordingly, although the invention has been described and pictured in a preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the detail of construction in combination and arrangement of parts may be made without departing from the spirit and scope of the invention as here and after claimed. It is intended that the patent shall cover by suitable expression in the appended claims, the whatever features of patentable novelty exist in the invention disclosed.