Abstract:
A differential pressure transducer comprising: a housing having a first end containing a first recess, and an oppositely disposed second end containing a second recess; first and second isolation diaphragms respectively enclosing said first and second recesses and forming in conjunction therewith first and second cavities; first and second headers secured within said housing and being electronically coupled together; and, a pressure sensor being adapted to withstand an excess in either said first or second pressures and secured to said first header so as to isolate said first and second cavities from one another and being responsive to a first pressure applied to said first diaphragm and a second pressure applied to said second diaphragm to produce at least one signal indicative of a difference between said first and second pressures.

Description:
FIELD OF INVENTION 
     The present invention relates to pressure sensing devices, and more particularly to an oil-filled pressure transducer adapted for use in hostile environments which subject the transducer to harsh resonances. 
     BACKGROUND OF INVENTION 
     Oil-filled transducers are well known in the art. It is also well known that resonances can occur within a pressure cavity and be transferred through an isolation diaphragm, through the oil-filled cavity to a sensor itself. In extreme conditions these resonances can exert significant stresses on gold wires bonded to the sensor chip sufficient to cause either ball bonds to fracture or the gold wire itself to fail. In addition, when the transducer is used to measure a differential pressure across a pump, large pressure pulses result when cavitation occurs in the pump, which can also cause similar results. In any event, for a variety of reasons severe pressure spikes may be transmitted through the oil of an oil-filled pressure transducer putting excess stress on gold wires of the sensor as well as generating excessive stresses in the sensor itself, undesirably resulting in premature failure of the sensor. 
     It is therefore an object of the present invention to render such excess stresses harmless to the sensor to insure its survivability under these adverse conditions. 
     SUMMARY OF INVENTION 
     A differential pressure transducer including: a housing having a first end containing a first recess, an oppositely disposed second end containing a second recess; first and second isolation diaphragms respectively enclosing said first and second recesses and forming in conjunction therewith first and second cavities; first and second headers secured within said housing and being electronically coupled together; and, a silicon sensor secured to said first header so as to isolate said first and second cavities from one another and being responsive to a first pressure applied to said first diaphragm and a second pressure applied to said second diaphragm to produce at least one signal indicative of a difference between said first and second pressures and being adapted to withstand an excess in either said first or second pressures. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 illustrates a plan-view of a sensor structure according to a preferred form of the invention. 
     FIG. 2 illustrates a cross-section of the sensor of FIG.  1 . 
     FIG. 3 illustrates an end-view of a first header utilized according to a preferred form of the invention. 
     FIG. 4 illustrates a cross-section of the header of FIG.  3 . 
     FIG. 5 illustrates a cross-section of the sensor of FIG. 1 attached to the header of FIG.  3 . 
     FIG. 6 illustrates a cross-section of a pressure transducer according to a preferred form of the invention. 
     FIG. 7 illustrates an end-view of the device of FIG.  6 . 
     FIG. 8 illustrates an enlarged cross-section of the second header  80  of FIG.  6 . 
    
    
     DETAILED DESCRIPTION OF INVENTION 
     The basic approach according to the present invention includes using a specially designed “leadless” sensor chip having “through” apertures such that a separate pressure can be applied to each side of the chip, the chip itself being mounted on a specially designed header. By being secured to the specially designed header, the chip is more robust and less likely to be damaged by severe stresses caused by pump cavitation for example. The sensor chip can be either an open or closed bridge. The dieletrically isolated sensor chip can be manufactured in accordance with the teachings of commonly assigned U.S. Pat. No. 5,955,771 entitled “SENSORS FOR USE IN HIGH VIBRATIONAL APPLICATIONS AND METHODS FOR FABRICATING SAME” issued Sep. 21, 1999 and commonly assigned U.S. Pat. No. 5,973,590 entitled “ULTRA THIN SURFACE MOUNT WAFER SENSOR STRUCTURES AND METHODS FOR FABRICATING SAME” issued Oct. 26, 1999, the entire disclosures of which are hereby incorporated as if being set forth in their respective entireties. Therein, a P+ region is bonded to an oxide layer on top of a silicon carrier wafer in accordance with the “diffusion enhanced bonding” technique as taught in commonly assigned U.S. Pat. No. 5,286,671, entitled “FUSION BONDING TECHNIQUE FOR USE IN FABRICATING SEMICONDUCTOR DEVICES” issued Feb. 15, 1994, the entire disclosure of which is also incorporated by reference herein as if being set forth in its entirety. 
     The P+ fingers are sealed to a glass wafer containing apertures suitable for filling with a metal-glass frit, the purpose of which is to make electrical contact with to the contact fingers of the Wheatstone bridge. In addition, there is provided at least one additional aperture in the glass wafer which access the active portion of the sensor such that pressure applied through this central aperture will cause the sensor to deflect. Around the central aperture is a shallow depression substantially equal in size to the deflecting portion of the sensor, the depth of the depression being sufficient enough to permit deflection of the active portion of the sensor but shallow enough to act as an overpressure stop. 
     To the inactive side of the sensor diaphragm is sealed a second glass wafer which includes at least one aperture being aligned with the deflecting portion for the sensor. Around this aperture is also provided a small shallow depression approximately the size of the deflecting portion of the sensor and also deep enough to permit deflection of the active portion of the sensor yet shallow enough to act as an overpressure stop. 
     Referring now to the figures, like references identify like elements of the invention. A plan view of a circuit structure  160  of a sensor chip  20  preferably utilized according to the present invention is shown in FIG. 1 and a cross section is shown in FIG.  2 . Such a sensor structure  160  is described in commonly assigned United States Patent No. 5,891,751, entitled “HERMETICALLY SEALED TRANSDUCERS AND METHODS FOR PRODUCING THE SAME”, issued Apr. 6, 1999, the entire disclosure of which is also incorporated by reference as if being set forth in its entirety herein. While the particularly illustrated sensor structure  160  is well suited for the preferred embodiment of the present invention, other suitable circuit structures could of course be utilized. 
     In the preferred form, the circuit structure  160  takes the form of a piezoresistive bridge structure. This pressure sensitive structure  160  is of the type having serpentine or tortuous piezoresistors  170 ,  180 ,  190 ,  200  composed of highly doped P+ silicon. Each piezoresistor  170   180 ,  190 ,  200  is essentially a variable resistor in one of four legs of a Wheatstone bridge circuit with each of the respective resistances varying in response to an applied force or pressure to the sensor  20 . Referring also to FIG. 2, the portion  220  of the bridge structure  160  is generally referred to as the “active” area of the sensor  20  as it overlays a thinner region of the wafer  5 , e.g. a sensor diaphragm  8 , that deflects upon the application of a pressure to the sensor  20 . The areas of the sensor  20  that are external to the active area  220 , e.g. around the periphery of the wafer  5 , are termed the “non-active” areas. 
     Referring still to FIG. 1, for a closed bridge the four circuit nodes of the Be Wheatstone bridge consist of electrical contacts  230 ,  240 ,  250 ,  260 , and are located in the non-active areas of the transducer. For an open bridge separate contact areas can be provided for each arm of the Wheatstone bridge as will be understood by those possessing ordinary skill in the pertinent art. Interconnecting the contacts  230 ,  240 ,  250 ,  260  with the piezoresistors  170 ,  180 ,  190 ,  200  are electrical interconnections or “fingers”  270 ,  280 ,  290 ,  300  which are also formed of P+ silicon. It is noted that the contacts  230 ,  240 ,  250 ,  260  being doped P+ are conductive, as are the interconnections  270 ,  280 ,  290 ,  300  to allow ohmic contact between the piezoresistive Wheatstone bridge comprised of piezoresistors  170 ,  180 ,  190 ,  200  and the contacts  230 ,  240 ,  250 ,  260 . It should be understood, while the terms “electrical contacts” , “interconnections” and “fingers” are used for convenience, these terms can each be considered to essentially be the electrical paths that electrically couple the piezoresistor elements  170 ,  180 ,  190 ,  200  with the contact areas  230 ,  240 ,  250 ,  260 . The interconnections  270 ,  280 ,  290 ,  300  are wider than the piezoresistors  170 ,  180 ,  190 ,  200  to provide a low resistance path to the contacts  230 ,  240 ,  250 ,  260 , while the long, tortuous lengths and narrow widths of the piezoresistors  170 ,  180 ,  190 ,  200  are designed to provide the desired resistances. 
     Referring again to FIG. 2, the wafer  5  includes a thinned portion which defines the active area  220  and deflectable sensor diaphragm  8  as has been described. The diaphragm  8  defines a recessed area  7  within the wafer  5  which is aligned with the active area  220  of the sensor  20 . The recessed area  7  is preferably deep enough to permit adequate deflection of the diaphragm  8  for sensing purposes, yet shallow enough to act as an over-pressure stop. Likewise a small cavity or recess is preferably formed in the member  10  around the aperture  14  to enable the diaphragm  8  to deflect, yet shallow enough to serve as an over-pressure stop. 
     Still referring to FIG. 2, the side of the wafer  5  having the structure  160  formed thereon, opposite to the recess  7 , is referred to herein as the “active” side, while the oppositely disposed side is referred to as the “inactive” side for purposes of explanation. To the inactive side of the wafer  5  is sealed a glass wafer or covering member  15  which includes an aperture  17  with is preferably central to the deflecting portion of the diaphragm  8 , e.g. active area  220 , and hence accesses the recess  7 . When coupled to the wafer  5 , the aperture  17  is surrounded by the small, shallow depression or recess  7  preferably equal in size to deflecting diaphragm  8  which forms the active area  220 , the depth of the depression  7  being great enough to allow the sensor diaphragm  8 , e.g. active area  220 , to deflect but shallow enough to allow the member  15  to act as an over-pressure stop. 
     Still referring to FIG. 2, a second covering or glass member  10  is preferably sealed to the P+ fingers  260 ,  270 ,  280 ,  290  formed on the active side of the wafer  5 . The glass member  10  includes apertures  12  suitable for filling with a metal-glass frit, the purpose of which is to respectively make electrical contact with the contact areas  230 ,  240 ,  250 ,  260  of the structure  160 . In addition, there is provided at least one additional aperture  14  in the glass wafer  10  which accesses the diaphragm  8  of the sensor  20 . A pressure applied through this aperture  14  will cause the sensor diaphragm  8  of the sensor  20 , e.g. the active area  220 , to deflect. The wafer  5  is preferably composed of silicon while the covering members  10  and  15  are preferably composed of a suitable glass. 
     In FIG. 3 is illustrated an end view of a first header  30  preferably used in connection with the sensor  20 . For a closed bridge, the header has four pins  40  (two input and two output); while for an open bridge it has 5 to 6 pins  40  (to provide individual connectivity for each of the Wheatstone bridge arms). In addition, the header  30  includes an aperture  37 . To mount the sensor  20  to the  20  header  30 , apertures  12  in the glass member  10 , which is bonded to the active side of the sensor  20 , are filled with a conductive metal-glass frit and subsequently affixed to the pins  10  of the header  30  such that the aperture  14  of the glass member  10  is aligned with the central aperture  37  of the header  30 . 
     Referring now also to FIG. 4, there is illustrated a cross-section of the header  30  of FIG.  3 . The header  30  preferably includes a cylindrical main body  50  having a glass portion  70  affixed therein so as to leave recessed portions  55  and  57  in each longitudinal end of the body  50 . The glass portion  70  serves to electrically insulate each of the pins  40  from one another as well as from the body  50 . Preferably a Kovar tube  60  is affixed within the aperture  37  which also passes through the glass portion  70 . The pins  40  each preferably extend into the recessed portions  55 ,  57  to enable electrical connectivity between the oppositely disposed longitudinal ends of cylindrical body  50 . However, each of the pins  40  further preferably protrudes out of and hence completely through the recessed portion  57  and one end of the body  50 . The pins  40  of the header  30  which extend into the recessed portion  57  are preferably very short, e.g. on the order of 0.005 to 0.015 inches. Further, the recessed portion  55  is preferably formed so as to have suitable dimensions to at least partially accommodate the sensor  20  therein. 
     The header  30  further preferably includes an external flange  55  adjacent to the recessed portion  57  and of sufficient shape and thickness to make it suitable to be discharge welded to the body  90  of a transducer  100 . 
     The header  30  with the sensor chip  20  mounted thereon is shown in FIG.  5 . Basically, the sensor chip  20  is at least partially accommodated in the recess  55  of the body  50  of the header  30  such that pins  40  are in electrical contact with the metal-glass frits in the apertures  12  which are in electrical contact with the contact areas  230 ,  240 ,  250 ,  260 . Accordingly, a pressure can be communicated through the apertures  37  and  14  to the active area  220  of the sensor  20  and electrical signals indicative of an amount of deflection of the diaphragm  8  can be sensed using the pins  40 . 
     As is shown in FIG. 6, the assembly of FIG. 5 including the sensor chip  20  and header  30  is welded or otherwise suitably affixed within a recess  95  of a body  90  of a transducer  100 . The flange  55  is preferably used to suitably seat the header  30  within a recessed area  95  of the body  90  of the transducer  100 . A second header  80  is also welded within a recess  97  of the body  90  of the transducer  100 . The second header  80  preferably includes external leads  110  which access the exterior of the transducer body  90 . A flexible circuit  120  is preferably used to electrically interconnect the two headers  30 ,  80 . The flexible circuit  120  can readily be made to electrically contact the portions of the pins  40  which extend through the recess  57  of the body  50  of the header  30  using conventional methodology. 
     The transducer body  90  preferably takes the form of a cylindrical body having recesses  92 ,  94  in each end. Isolation diaphragms  140 ,  150  are preferably affixed over each recess  92 ,  94  thereby forming cavities which are preferably oil-filled. There is further included channel  160  through the transducer body  90  which opens to both recess  94  and recess  95  which in turn opens to recess  92 . Oil-fill tubes  130  access the cavities formed by the diaphragms  140 ,  150  and recesses  94 ,  92  and are positioned so as to extend out a side-wall of the body  90  of the transducer  100 . The tubes  130  can be used to fill the recesses  94 ,  92  with a suitably non-compressible oil and hence the channel  160 , recess  95 , and apertures  37 ,  14  and  17  as well. The lead wires  110  also preferably extend out the same sidewall substantially adjacently to the oil-fill tubes  130  according to the preferred form of the present invention. 
     Accordingly, a positive pressure applied to the isolation diaphragm  140  will be communicated through the oil-filled recess  94  and channel  160  through the aperture  37  and aperture  14  and will cause the diaphragm  8 , and hence active area  220  of the sensor  20  to deflect. Further, a positive pressure applied to the isolation diaphragm  150  will be communicated through the oil-filled recess  92  aperture  17  and will cause the diaphragm  8  and hence active area  220  of the sensor  20  to deflect in an opposite direction. 
     When the two headers  30 ,  80  are welded to the transducer body  90 , the cavities formed by the diaphragms  140 ,  150  are isolated from one-another by the sensor chip  20 . Thus, when a positive pressure is exerted on the first isolation diaphragm  140 , it will cause a deflection of the sensor active area  220  because of the pressure difference between the two oil cavities. Similarly, when a positive pressure is exerted on the second isolation diaphragm  150 , it will cause a deflection of the same area  220  but in the opposite direction, and when equal pressures are exerted on both isolation diaphragms  140 ,  150  no deflection will result. Thus, the deflection of the active area  220  is caused by the difference in pressure applied to the two isolation diaphragms  140 ,  150 . 
     It should also be noted that the short pins  40  on both headers  30 ,  80  and the flex circuit  120  insures that the oil-filled cavity formed by the diaphragm  150  has a minimal oil volume, resulting in better thermal performance. 
     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 details of construction and combination and arrangement of parts may be made without departing from the spirit and scope of the invention as hereinafter claimed. It is intended that the patent shall cover by suitable expression in the appended claim, whatever features of patentable novelty exist in the invention disclosed.