Abstract:
An electronic pressure-sensing device is isolated from corrosive, conductive gasses and fluids by a corrosion resistant metal diaphragm welded to a pressure port. The pressure-sensing device is attached to a support structure with a hole that provides a path from the diaphragm area to the pressure-sensing device. A fill fluid is sealed behind the diaphragm and fills the hole through the support structure to the electronic pressure-sensing device. In this design, any hostile chemical applied is completely isolated from the electronic sensor and associated adhesive seals by the metal diaphragm.

Description:
BACKGROUND  
         [0001]    The present invention relates generally to pressure sensing transducers and pertains particularly to a package for transducers that is resistant to corrosive or conductive gasses and liquids.  
           [0002]    Due to the hostile environment from highly corrosive fluids and the like, packages for electronic sensors measuring pressures in such environments are typically highly specialized, difficult to calibrate and expensive.  
           [0003]    A pressure sensor (or pressure transducer) converts pressure to an electrical signal that can be easily measured. Sensors that incorporate micro-machining or MEMS (Micro-Electro-Mechanical System) technology are small and very accurate. Because they are fabricated similarly to the fabrication of commercial semiconductors they are also inexpensive to produce. FIG. 1 illustrates a MEMS pressure sensor  2  manufactured in accordance with the prior art. The topside  4  of the sensing element  6  (typically a silicon die) has defined resistors exhibiting a resistance that charges in magnitude in proportion to mechanical strain applied to die  6 . Such resistors are called piezoresistive. The backside  8  of die  6  has a cavity  10  such that a thin diaphragm  12  of die material is formed. The alignment of the topside resistors and backside cavity  10  is such that the resistors are strategically placed in strain fields. When pressure is applied across diaphragm  12 , diaphragm  12  flexes. The strain sensitive resistors and an associated circuit coupled thereto (not shown in FIG. 1) provide an electrical signal constituting a measure of this pressure.  
           [0004]    Often, silicon die  6  is bonded to a support structure  14  with a bonding adhesive  15  or other method such as anodic bonding. Support structure  14 , is bonded to a stainless steel plate  16  with a bonding adhesive  17 . (Plate  16  is sometimes referred to as a header). Support structure  14  is made from a material such as glass or silicon, and helps isolate diaphragm  12  from sources of strain that are unrelated to pressure, e.g. thermal expansion or contraction of header  16 . Support structure  14  includes a centrally defined opening  18  directly adjacent to and in fluid communication with cavity  10 . Header  16  comprises a pressure port  19  in fluid communication with opening  18 . This port  19  can be used to seal a vacuum in cavity  10 . Alternatively, port  19  can be used to permit cavity  10  to be maintained at ambient pressure.  
           [0005]    Header  16  is welded to a second port  20 . Port  20  is connected to a body (e.g. a pipe, container or other chamber, not shown) containing fluid (e.g. a gas or a liquid) whose pressure is to be measured by sensor  2 . Port  20  serves as a conduit for applying this fluid to sensor  2 .  
           [0006]    A drawback to MEMS sensors is that conductive and corrosive fluids (gases and liquids) can damage the sensor and the electronic structures (e.g. resistors) that are used to measure the pressure. Backside  8  of die  6  and adhesive bonds  15  and  17  are also susceptible to corrosion. To be used with corrosive or conductive fluids these sensors require some kind of isolation technique.  
           [0007]    A popular isolation technique is to interpose a stainless steel diaphragm  22  between die  6  and port  20 . Diaphragm  22  is welded to port  20  and header  16 . A cavity  23  is thus formed between diaphragm  22  and header  16  and this cavity  23  is filled with a non-corrosive, non-conductive liquid such as silicone oil  24 . Thus, diaphragm  22  and oil  24  isolate die  6  from any corrosive material in port  20 .  
           [0008]    When pressure is applied by the fluid in port  20  to diaphragm  22 , diaphragm  22  deflects slightly, pressing on oil  24 , which in turn presses on die  6 . The pressure on die  6  is then detected by measuring the resistance of the piezoresistive resistors formed in diaphragm  12  of die  6 . Corrosive media, the pressure of which is being measured, is kept away from the electronics by stainless steel diaphragm  22  and oil  24 .  
           [0009]    Header  16  often has at least one small hole  25  used to fill cavity  23  with oil  24 . After cavity  23  is filled with oil  24 , hole  25  is welded shut, e.g. with a welded ball  29 . The design of FIG. 1 also includes metal pins  26  that are hermetically sealed to, but pass through, header  16 . (Pins  26  are typically gold plated.) Gold or aluminum wires  28  are bonded to and electrically connect die  6  to metal pins  26 . Pins  26  and wires  28  are used to connect die  6  to electronic circuitry (not shown in FIG. 1, but located below header  16 ) so that the resistance of resistors within die  6  can be measured.  
           [0010]    A significant drawback the design of FIG. 1 is that when the temperature is increased, oil  24  expands and exerts pressure on stainless steel diaphragm  22  and sensor die  6 . The resulting pressure change due to temperature causes the calibration of the sensor to change with temperature. The resulting errors introduced into the sensor measurements may contain linear and nonlinear components, and are hard to correct. The extent of this error is proportional to the amount of oil  24  contained in cavity  23 . The more oil contained in cavity  23 , the more oil there is to expand and thus more error over temperature. Currently existing designs require a substantial amount of oil for at least the following reasons: a) pressure sensing die  6  is enclosed inside oil filled cavity  23 , and thus cavity  23  must be large enough to accommodate die  6 ; b) there are four hermetic pins  26  that must be wire bonded to die  6  (only two of which are shown in FIG. 1) so cavity  23  must also accommodate pins  26  and bonding wires  28 ; and c) cavity  23  must also accommodate manufacturing tolerances that are large enough to permit assembly of die  6 , wiring  28  and the associated housing.  
           [0011]    Another drawback to this design arises out of the fact that die  6  is made of silicon, which has a low coefficient of thermal expansion. Because die  6  must be mounted to stainless steel, and stainless steel has a relatively high coefficient of thermal expansion, a compliant die attach structure must be used. Typically this compliant die attach structure is a silicone elastomer. Because the silicone elastomers are not hermetic, when high vacuums are present, gas is in drawn through the silicone and into the oil. This causes large shifts in the offset calibration of the sensor due to the pressure of the gas drawn into cavity  23 .  
           [0012]    A third drawback to this design is the fact that hermetic feedthrough pins  26  are costly and problematic. In particular, this design requires metal pins  26  extending through glass regions  30  that serve as the hermetic seals. Glass  30  can crack. Also, pins  26  must be gold plated and flat on top to permit wire bonding. These designs are difficult to customize and the hermetic seals can be a leak point that must be checked before the sensor is assembled.  
           [0013]    Attempts have been made to provide a corrosion resistant package using a non-fluid filled housing and polymeric or hermetic seals to seal the housing directly to the die. These methods allow corrosive material to travel inside and contact the die and sealing surfaces. Here, the amount of corrosion protection is limited because the sensor and associated seals are subject to damage by corrosive and possibly conductive materials. There have been some attempts to provide a polymeric barrier on the inside of the die and seal area. Conformal coatings such as Parylene or silicone materials only provide minimal corrosion improvement.  
           [0014]    To maintain high quality and low cost it is desirable to construct an isolation technique that holds as little oil as possible, is readily assembled by automated processes, is easily modified for custom applications, and avoids unnecessary machining and assembly costs for hermetic feed through pins.  
         SUMMARY  
         [0015]    A pressure sensor in accordance with the invention comprises a die having pressure-sensing electrical components formed in a first side of the die. The pressure-sensing electrical components are typically resistors whose resistance changes as a function of pressure. Alternatively, the pressure-sensing electrical components can be capacitors whose capacitance changes as a function of pressure. The electrical components within the die are coupled to bonding structures such as bonding wires.  
           [0016]    In one embodiment, instead of placing the die inside an oil filled cavity with the pressure-sensing electrical components and electrical bonding structures on the side of the die facing oil, the side of the die containing the electrical components and the bonding structures coupled thereto do not face an oil-filled cavity.  
           [0017]    In one embodiment, a second side of the die contacts oil in an oil-filled cavity, The die is bonded and sealed to a plate (i.e. a leader) such that the oil is kept away from the first side of the die. Because of this, the volume of oil in the oil-filled cavity can be greatly reduced compared to the sensor of FIG. 1. This is because the oil-filled cavity does not have to be large enough to surround the die and house bonding wires and pins coupled thereto. In particular, the cavity does not have to be large enough to accommodate pins that are hermetically sealed to the header. Further, the oil-filled cavity does not have to be large enough to accommodate electrical assembly tolerances.  
           [0018]    The passages and cavities are very small and thus the oil fill fluid volume is small. Finally, because there is no need for hermetic feed through pins, the reliability and cost of the sensor package is greatly improved.  
           [0019]    In one embodiment, the die is bonded to the header using a hermetic die attach material. By using a hermetic die attach material (e.g. glass, solder or braze), gas cannot be pulled through the adhesive. Because of the use of hermetic die attach material, the sensor package can withstands high vacuum for extended periods of time without suffering damage. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1 illustrates in cross section a pressure sensor constructed in accordance with the prior art.  
         [0021]    [0021]FIG. 2 illustrates in cross section a pressure sensor in accordance with the present invention comprising a flat header and an oil-bearing cavity in which oil is not exposed to the sensor resistors.  
         [0022]    [0022]FIG. 2A illustrates in cross section a modified version of the pressure sensor of FIG. 2 in which a raised area is provided in a header. This raised area is bonded to a support structure which, in turns is bonded to the sensor die.  
         [0023]    [0023]FIG. 2B illustrates in cross section a portion of a pressure sensor in accordance with the invention where a header, stainless steel diaphragm, housing and port are welded together.  
         [0024]    [0024]FIG. 3 illustrates in cross section an embodiment of the invention in which the header comprises a set of annular grooves for isolating a sensor die from externally applied mechanical stresses. The FIG. 3 embodiment also includes a glass feedthrough for facilitating the attachment of the sensor die to the header.  
         [0025]    [0025]FIG. 4 illustrates in cross section an embodiment similar to FIG. 3, except that the top surface of the feedthrough extends above the top surface of the header, and a tube extends through the header so that oil can be provided in the oil-filed cavity.  
         [0026]    [0026]FIG. 5 illustrates in cross section an embodiment similar to FIG. 4, except that the oil input tube extends through the glass feedthrough. Also, another metal tube extends through the glass feedthrough to facilitate fluid communication to the pressure sensor.  
         [0027]    [0027]FIG. 5A illustrates in cross section an embodiment similar to FIG. 5, except in FIG. 5A a fill tube extends above the top surface of a glass feed through.  
         [0028]    [0028]FIG. 5B illustrates in cross section an embodiment similar to FIG. 5A, except the fill tube extends slightly further above the top surface of a glass feed through, and a support structure is bonded to the fill tube.  
         [0029]    [0029]FIG. 6 illustrates in cross section an embodiment in which a cap is placed over the pressure sensor die.  
         [0030]    [0030]FIG. 6A illustrates a modified version of the embodiment of FIG. 6 using a capacitive sensing mechanism to sense pressure. 
     
    
     DETAILED DESCRIPTION  
       [0031]    While the invention is described below with reference to certain illustrated embodiments, it is understood that these embodiments are presented by way of example and not by way of limitation.  
         [0032]    [0032]FIG. 2 illustrates in cross section a pressure sensor assembly  100  comprising a micro-machined silicon pressure sensor die  101  comprising a frame portion  101   a  surrounding a thinned diaphragm portion  101   b.  (Diaphragm portion  101   b  is typically formed by thinning a portion of a silicon wafer using either a liquid or dry etching process.) Piezoresistive resistors are formed in the top surface of die  101  in diaphragm portion  101   b,  e.g. by ion implantation or diffusion. These resistors are formed in locations on diaphragm  101   b  where the strain is greatest when diaphragm  101   b  is exposed to fluid under pressure.  
         [0033]    Die  101  is anodically bonded to a support structure  102 . Support structure  102  is sometimes referred to as a “constraint,” and is typically silicon or glass. In one embodiment, diaphragm portion  101   b  of pressure sensor die  101  is between 15 and 100 microns thick. (The exact thickness depends upon the pressure range that the sensor is to measure.) Frame portion  101   a  of die  101  is typically between 300 and 650 microns thick (e.g. 375 microns). Die  101  is typically square or rectangular, and is between 40 and 200 mils on a side. Support structure  102  is typically between 15 and 70 mils thick (usually but not necessarily thicker than die  101 ), is square or rectangular, and is between 40 and 200 mils on a side. Die  101  and support structure  102  can be bonded together in wafer form using an anodic bonding process, e.g. as described U.S. Pat. No. 3,397,278, issued to Pomerantz, and U.S. Pat. No. 3,697,917, issued to Orth et al. The &#39;278 and &#39;917 patents are incorporated herein by reference. Die  101  and support structure  102  are then sawed into the assembly shown. Other methods can be used to bond support structure  102  to die  101  such as silicon fusion bonding, glass frit bonding, or other commonly known techniques.  
         [0034]    Support structure  102  provides mechanical isolation between sensor die  101  and a plate or header  103 . For example, the coefficient of thermal expansion of die  101  is typically less than that of header  103 . Support structure  102  serves as a mechanical buffer to limit or reduce the amount of stress applied to die  101  caused by the thermal expansion or contraction of header  103 . Also, if some external force is applied to header  103 , causing it to bend or flex, support structure  102  tends to reduce the amount of stress applied to die  101  as a result of that bending or flexing. If support structure  102  is formed from an electrically insulating material, it will electrically insulate die  101  from header  103 . (The body of die  101  is typically positively biased. Accordingly, it is advantageous to insulate die  101  from electrically conductive portions of the sensor package.) Lastly, if die  101  were attached directly to header  103 , the die attach area would be equal to the area of the bottom surface  101   c  of frame region  101   a  of die  101 . In contrast, the bonding area  102   a  between support structure  102  and header  103  is typically larger than bottom surface  101   c  of frame region  101   a.  Thus, one can form a stronger bond between support structure  102  and header  103  than one could form between die  101  and header  103  if die  101  were bonded directly to header  103 .  
         [0035]    Support structure  102  is attached to a header  103  with a low temperature glass or solder  105 . (By low temperature glass we mean a glass having a relatively low melting temperature, e.g. below about 750° C.)  
         [0036]    Header  103  is typically an alloy in which iron is not the major component. In one embodiment, the alloy from which header  103  is fabricated is substantially free of iron. For example, in one embodiment, header  103  comprises Hastalloy. (Hastalloy is a nickel alloy.) Hastalloy has the following advantages:  
         [0037]    1. Hastalloy resists corrosion.  
         [0038]    2. As explained below, header  103  is welded to one or more structures comprising stainless steel. One can weld Hastalloy to stainless steel using a weld that does not tend to corrode.  
         [0039]    3. Hastalloy has a relatively low coefficient of thermal expansion. Thus, the thermal expansion of Hastalloy is closer to that of silicon than other commonly used materials, e.g. stainless steel.  
         [0040]    While Hastalloy is advantageous, in other embodiments, other materials are used for header  103 , e.g. 400 series stainless steel, cold roll steel (i.e. typical carbon steel), kovar, alloy 42, or other controlled expansion metals. In one embodiment, header  103  is a controlled expansion metal, e.g. having a coefficient of thermal expansion less than 13×10 −6 /° C.  
         [0041]    A diaphragm  108  is attached, e.g. by welding, soldering or brazing to header  103 . Diaphragm  108  is typically stainless steel, and can have convolutions as schematically shown in FIG. 2. Diaphragm  108  can also be made of Hastalloy, Inconnel, brass, or other corrosion resistant material. In one embodiment, welding is accomplished using TIG (tungsten inert gas). In another embodiment, welding is accomplished using an e-beam or a laser. A port  104  (typically a stainless steel alloy such as 316 stainless steel, and typically structurally rigid) is affixed, e.g. by welding or brazing to header  103  at the same time as diaphragm  108  so that only one joint is needed. Port  104  is typically connected to a cavity or conduit containing a medium the pressure of which is to be measured using pressure-sensing die  101 .  
         [0042]    A housing  107  may also be attached to header  103  at this time so that a single weld joins housing  107 , header  103 , diaphragm  108  and port  104 . Housing  107  surrounds and protects die  101 . A fill fluid such as silicone oil  109  is degassed and sealed inside a space comprising a) a conduit  110  and b) the volume  111  between diaphragm  108  and header  103 . The fill fluid is introduced inside this space via a conduit  112  that is then sealed by a welded ball  113 . Other methods may be used to seal oil  109  inside this space such as crimping a tube, re-flowing solder or other methods known to the art. All structure materials and seal materials to which oil  109  is exposed are selected such that no gas may pass therethrough into oil  109 , even with a high differential pressure or vacuum applied to the pressure sensor.  
         [0043]    [0043]FIG. 21B illustrates in cross section a portion of the pressure sensor where header  103 , port  104 , housing  107  and diaphragm  110  are welded together at a weld point WA. As can be seen, an outer portion  103   b  of header  103  is narrowed to facilitate such a weld point. Also shown is an indentation  107   a  in housing  107  and an indentation  104   a  in port  104  where housing  107  meets header  103 . These indentations facilitate welding by reducing thermal conduction away from the weld point. Also, they are particularly useful for arc welding, since the arc tends to jump to the highest point.  
         [0044]    A plurality of wires connects die  101  to a compensation circuit  114 . In one embodiment, die  101  is coupled to a board  115  by a set of wires, one of which is shown as wire  116 . (Bonding pads are typically formed on die  101  and board  115  to facilitate bonding wire  116  thereto.) A conductive trace on board  115  (not shown) electrically couples wire  116  to wire  117 . Wire  117  extends upward to and electrically contacts a conductive trace (not shown) on a PC board  118 , which in turn electrically couples wire  117  to a leg or pin  104   a  of compensation circuit  114 . (There are other wires and traces, not shown in FIG. 2, that couple other bonding pads on die  101  to the other legs or pills of circuit  114  in a manner similar to wires  116  and  117  and the above-described traces on boards  115  and  118 .) Compensation circuit  114  is mounted on PC board  118 , which in turn is affixed to housing  107 . Connections to compensation circuit  114  through housing  107  can be made through a connector or a plurality of wires extending through housing  107  (not shown). Compensation circuit  114  can be a device similar to the circuit described in “Solid-State Pressure Sensors Handbook”, Vol. 16, published by Sensym, Inc. of Milpitas Calif. in 1998, incorporated herein by reference. See, for example, pages 8-70 to 8-73 and 8-92 to 8-93.  
         [0045]    Although board  115  is illustrated as being on one side oft die  101  (the left side), board  115  typically extends in front of and in back of die  101 , and thus typically surrounds die  101  on three sides.  
         [0046]    As mentioned header  103  is typically made from an alloy such as Hastalloy. Hastalloy has several characteristics that make it desirable for manufacturing header  103 . First, Hastalloy resists corrosion. Second, as mentioned above, header  103  is typically welded to one or more structures made of stainless steel. When welding Hastalloy to stainless steel, one can form welds that resist corrosion.  
         [0047]    Hastalloy also enjoys the advantage of a relatively low coefficient of thermal expansion. This is important because silicon has a relatively low coefficient of thermal expansion, e.g. between 2×10 −6  and 2.3×10 −6 /° C. 316 stainless steel has a coefficient of thermal expansion of about 18×10 −6 /° C. Because of this mismatch in thermal expansion between silicon and stainless steel, if one made header  103  out of stainless steel, temperature changes would result in stress applied to silicon sensor die  101 . Such a stress would introduce inaccuracies into the pressure measurements provided using die  101 . By using a material like Hastalloy (which has a coefficient of thermal expansion of only 12×10 −6 /° C.) the mismatch in thermal expansion between the silicon and header  103  is minimized.  
         [0048]    The embodiment of FIG. 2 has the following additional features:  
         [0049]    First, only one diaphragm  101   b  is included in sensor  101 , and pressure is only measured from a side  101   d  of sensor  101  that is not exposed to oil. In other words, piezoresistive resistors are formed in silicon on side  101   d  of sensor  101  facing away from oil  109 . In addition, wires  116 , bonded to these resistors, are not exposed to oil  109 . This is advantageous because it avoids having to extend pins through a hermetic seal, e.g. as in the design of FIG. 1. It is also advantageous because a smaller volume of oil can be used when the oil is not exposed to side  101   d  of die  101 . The reason is that the cavity  107   a  on side  101   d  of die  101  must be sufficiently large to accommodate bonding wires, and structures that the bonding wires connect to. It requires more oil to fill this volume than the volume of oil required to fill cavity  111  and conduit  110 . Because less oil is required to fill cavity  111  and conduit  110 , sensor  101  encounters less thermal expansion of oil if the temperature increases. This smaller amount of thermal expansion of oil results in application of less pressure to die  101 , thereby reducing distortion of the pressure measurements provided by die  101 .  
         [0050]    Second, header  103  is relatively flat. Thus, it is easy to fabricate a header  103  in accordance with the invention. For example, header  103  can be formed by stamping. Alternatively, header  103  can be formed by machining, etching or sintering.  
         [0051]    As mentioned above, the above-described embodiment uses a low temperature glass to bond support, structure  102  to header  103 . However, in another embodiment, support structure  102  is bonded to header  103  by soldering or brazing. For the case of a Hastalloy header, this can be done by a) plating nickel on the bonding area of header  103 ; and b) using a solder or brazing material to attach suppose structure  102  to the bonding area. The solder or brazing material can be a eutectic material such as AuSi, AuSn or SuPb.  
         [0052]    In an alternative embodiment using a Hastalloy header, gold is plated onto the nickel prior to the above-mentioned brazing or soldering. For an embodiment in which header  103  is ceramic, it is preferable to use low temperature glass to bond support structure  102  to header  103 .  
         [0053]    [0053]FIG. 2A shows a modified embodiment of the invention in which header  103  comprises a raised section  103   a  in the bonding area so as to a) define the sealing area (where support structure  102  is to be sealed to header  103 ) and b) to be used as a guide during assembly. In this embodiment, width W of raised section  103   a  is greater than or equal to the width of support structure  102  and die  101 .  
         [0054]    [0054]FIG. 3 illustrates in cross section a sensor assembly similar to that of FIG. 2. However, in FIG. 3, support structure  102  is attached to a glass feedthrough  120  that is hermetically sealed to header  103  through a glass seal. (The manner in which glass feedthrough  120  is hermetically sealed to header  103  is similar to seals in the hermetic connector industry.) Glass feedthrough  120  provides improved electrical insulation between die  101  and header  103  compared to that of the header design in FIG. 2. FIG. 3 also shows a low thermal expansion bonding area  121  where support  102  is bonded to feedthrough  120 . This is especially advantageous if a low temperature glass is used for bonding support structure  102  to feedthrough  120 . As mentioned above, silicon  101  has a thermal expansion coefficient between 2×10 −6  and 2.3×10 −6 /° C. Hastalloy has a thermal expansion coefficient of about 12×10 −6 /° C., and sealing glass has a thermal expansion coefficient of about 9×10 −6 /° C. By bonding support structure  102  to glass feedthrough  120 , less thermal stress is applied to bonding area  121  than if support structure  102  were bonded directly to header  103 .  
         [0055]    If support structure  102  is a material such as silicon, typically a metallic material is applied to the top surface of glass feedthrough  120  to facilitate bonding of support structure  102  to feedthrough  120 . On one embodiment, a material such as nickel or chromium is deposited on feedthrough  120  (e.g. by sputtering, or sputtering followed by plating), and then support structure  102  is soldered or brazed to the nickel or chromium.  
         [0056]    Glass feedthrough  120  can be provided in header  103  with a compression seal. In other words, glass feedthrough  120  is provided in header  103  when both the glass and the header are hot. As the temperature drops because header  103  has a higher coefficient of thermal expansion, it will contract around feedthrough  120  and apply a compressive mechanical force on feedthrough  120 , thus adding to the forces that tend to hold feedthrough in place.  
         [0057]    Also shown in FIG. 3 are annular grooves  122 , which are provided in header  103  to help isolate outside strain due to welding or installation from the inside assembly. In particular, header  103  will bend at annular grooves  122 , thereby mitigating the amount of stress applied to sensor  101 .  
         [0058]    [0058]FIG. 4 shows another embodiment where glass feedthrough  120  extends above the header top surface  103   c  to provide additional electrical isolation and package strain isolation between header  103  and die  101 . In one embodiment, feedthrough  120  extends above surface  103   c  by a distance D less than 20 mils, e.g. between 5 and 20 mils, and typically about 10 mils. Also, in one embodiment feedthrough  120  has a width W less than about 200 mils, and typically about 160 mils. The aspect ratio of the portion  120   a  of feedthrough  120  extending above header top surface  103   c  is typically 8 to 1 (width to height) or greater.  
         [0059]    Also shown in FIG. 4 is a crimped tube type fill fluid seal  126  for introducing silicone oil into the sensor. Here a tube  126   a  is sealed to header  103  by a braze or glass seal. Thereafter, an end  126   b  of tube  116   a  is hermetically sealed by crimping or soldering after filling the inner cavity with fill fluid  109  (again, typically a liquid such as oil).  
         [0060]    It is noted that prior art U.S. Pat. No. 5,635,649 discusses an embodiment of a sensor mechanism comprising a stationary base  2  extending above a housing  4  for supporting a die  1  (see &#39;649 FIG. 1). Feedthrough  120  is different from &#39;649 stationary base  2  in several regards. For example, the &#39;649 patent requires a thin walled region  22  for absorbing thermal strains from &#39;649 housing  4  and pressure strains due to application of a static pressure. In order to perform this function, thin wall region  22  has a width that is less than the width of &#39;649 pressure sensing chip  1 . In stark contrast, feedthrough  120  has a width W′ that is substantially equal to or greater than the width of die  101 .  
         [0061]    Also, the ratio of the height to width of the raised portion feedthrough  120  is much smaller than the ratio of the height of structure  2  to the width of structure  2  in the &#39;649 patent.  
         [0062]    [0062]FIG. 5 shows another embodiment where a single glass seal  120 ′ provides the seal for fill tube  126   a  and the bonding area for support structure  102 . In addition, FIG. 5 shows a tube  127  inserted in glass seal  120 ′ to provide a cost effective way of making a hole through glass seal  120 ′ to permit fluid communication of oil  109  die  101 . Tube  127 , if smaller in diameter than hole  102   b  in support structure  102 , can also be raised above the top surface of glass seal  120 ′ slightly so as to be used as an alignment fixture during assembly (see FIG. 5A). This configuration has the advantage of reducing cost compared with the embodiment of FIG. 4, as only one hole needs to be drilled in header  103  when manufacturing the embodiment of FIGS. 5 and 5A. Tube  127  is also advantageous, in that it is difficult to bore a small diameter fill hole directly through glass  120 ′. It is much easier and less expensive to insert metal fill tube  127  through glass seal  120 ′.  
         [0063]    The mechanical isolation between the header and the die may be further improved using an embodiment in accordance with FIG. 5B, in which a tube  127  includes a portion  127   a  extending above header  103  and into a region between header  103  and support structure  102 . In this embodiment, tube  127  is sealed to header  103  by a hermetic feed through  120 . Tube  127  is typically made of a controlled expansion material such as Kovar or Alloy 42. Support structure  102  and die  101  are joined together as in the above-described embodiments. Tube  127  is inserted inside support structure  102  providing a joined surface that has a large seal area  105   a  but small in diameter. Support structure  102  is then adhered to tube  127  with an adhesive or a hermetic material such as low temperature glass or solder. The oil fill fluid has a path  109  from header  103  to die  101  and tube  127  provides mechanical isolation.  
         [0064]    A bulge or shelf  127   a  is formed in tube  127  so that during assembly, support  102  does not fall past bulge or shelf  127   a.    
         [0065]    In lieu of glass feed through  120 , tube  127  can be sealed to header  103  by brazing, soldering or welding. This alternative embodiment has a cost advantage, but does not provide electrical isolation between header  103  and die support structure  102 .  
         [0066]    [0066]FIG. 6 shows a cap  119  attached to die  101  to provide a sealed absolute vacuum reference cavity  130 . Cap  119  is typically silicon or glass. Alternatively, cap  119  can be metal. Cap  119  can be positioned such that the clearance between diaphragm and cap is very small, thus limiting the diaphragm travel and effectively increasing the burst pressure of the diaphragm. Cap  119  can be used as a surface an electrode  119   a  if instead of using a piezoresistive die  101 , a capacitive die  101 ′ is used (FIG. 6A). (The other electrode  119   b  of the capacitive sensor is formed on die  101 ′, e.g. by sputtering or vacuum deposition.) Cap  119  can be between 300 and 650 microns thick, and can be bonded to die  101  by anodic bonding, silicon fusion, a glass frit or soldering.  
         [0067]    Thus specific embodiments of the invention have been described above, it is to be understood that numerous changes and modifications may be made therein without departing from the spirit and scope of the invention. For example, a pressure sensor in accordance with our invention can be used without oil isolation. Such an embodiment lacks a ball seal or a crimped tube as discussed above.  
         [0068]    In another embodiment, fluids (e.g. liquids) other than oil can be used to isolate a die from a medium whose pressure is to be measured.  
         [0069]    As mentioned above, support structure  102  can be silicon or glass. If support structure  102  is silicon, it can be bonded to die  101  using anodic bonding, silicon fusion bonding, or other silicon-to-silicon or silicon-oxide-silicon bonding methods.  
         [0070]    As mentioned above, header  103  is a low coefficient of thermal expansion material, preferably containing low or very little iron. Header  103  can be Hastalloy, or other alloys such as Inconnel. Header  103  can also be ceramic. Die  101  can be a material other than silicon. Also, die  101  can comprise more than one diaphragm. Accordingly, all such changes come within the invention.