Abstract:
A pressure sensing apparatus including a thin disc of a metal having a ceramic material layer and piezoresistive elements formed thereon. A surface of the disc is bonded to a diaphragm assembly on a pressure port base constructed of a low cost metal. The bonding process is performed at low temperatures, (&lt;700° C.), so that the diaphragm assembly and pressure port do not require high temperature corrosion resistance, and can thus be formed of less expensive materials. The inventive apparatus provides a lower cost alternative to conventional high pressure sensors since less material is used, less expensive materials are used, and fabrication is less complex. The inventive apparatus is also more reliable and exhibits greater thermal stability than conventional high pressure sensors.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates to pressure sensors. In particular, it relates to high pressure sensors having increased reliability, thermal stability and adhesion properties, while being inexpensive to fabricate.  
         [0003]     2. Description of the Related Art  
         [0004]     For years, those skilled in the art have made continuous efforts to develop pressure sensors that are low in cost and capable of being mass produced, while exhibiting high reliability, sensitivity and linearity. Certain-conventionally known pressure sensors have been known to include semiconductor materials with a micromachined sensing diaphragm. In the processing of such structures, a thin diaphragm is often formed in a silicon wafer through chemical etching. Ion implantation and diffusion techniques are then used to drive doping elements into the diaphragm, forming piezoresistive elements whose electrical conductivity changes with strain, such that deflection or deformation of the diaphragm causes a change in resistance value of the piezoresistive element. These changes correspond to the magnitude of pressure applied to the diaphragm. However, silicon is susceptible to chemical attack and erosion, particularly in environments such as where a high-pressure medium is to be sensed, such as automotive applications that involve sensing pressures of brake fluid, oil, coolant, transmission fluid, hydraulic fluid, fuel, or steering fluid, at pressures of two atmospheres or more. For such applications, a pressure sensor must be physically resilient, and resistant to the hostile environment of a sensed medium.  
         [0005]     Presently used methods for producing media-compatible sensors include enclosing a silicon sensing chip in an inert fluid, such as a silicone oil or gel, and then further separating the sensing chip from a medium to be sensed with a metal diaphragm, such that pressure must be transmitted through the metal diaphragm and the fluid to the sensing chip. Unfortunately, the manufacturing processes for these sensors are expensive and complex. As a result, these sensors are unsuitable for mass production.  
         [0006]     Another known process involves the formation of a capacitor plate on a ceramic diaphragm, which is then bonded to a base with a second capacitor plate. The use of a chemically-resistant and mechanically-tough ceramic materials, such as aluminum oxide or zirconium oxide, allow the diaphragm to directly contact the medium whose pressure is to be measured. As the ceramic diaphragm deforms or deflects under the influence of pressure, the space between the capacitor plates changes, thereby causing a change in capacitance that corresponds to the applied pressure. However, the circuits required to detect such capacitance changes are often complex and subject to noise corruption. Furthermore, attaining an adequate seal between the ceramic diaphragm and the base for high pressure applications is often difficult.  
         [0007]     Yet another known approach involves the use of a chemically resistant ceramic diaphragm having thick-film piezoresistors screen-printed thereon. As with ceramic capacitive pressure sensors, the ceramic material allows direct contact with the medium whose pressure is to be sensed, eliminating the need for protective packaging. However, while the signal detection circuitry may be less complicated, it is difficult to reliably seal the ceramic diaphragm with a base.  
         [0008]     Further, it is known in the art to employ a metal diaphragm as the sensing element. Because metal diaphragms generally deform more for a given thickness and pressure than ceramic diaphragms, which exhibit lower elongations before breaking and are therefore designed to deform less under pressure, sensing is performed by thin-film polysilicon resistors. The metal diaphragm is first coated with a thin-film layer of typically silicon dioxide or silicon nitride to electrically insulate the diaphragm from thin-film resistors and their conductors. A thin-film polysilicon layer is then deposited on the insulating layer by chemical vapor deposition (CVD), and then etched through a mask formed by spinning a liquid photoresistive material onto the polysilicon layer. The photoresistive material is exposed and developed to obtain the pattern required for the resistors, and thin-film conductors are then formed by evaporation. However, the equipment necessary to deposit the insulating and polysilicon layers is expensive, and deposition rates are extremely slow. Deposition of the thin-film layers requires multiple patterning, exposure, developing and stripping steps for the required thin-film spun-on photoresists and metallization, and must be carried out in a controlled environment to avoid surface defects. In addition, such processes typically deposit thin-films no thicker than about 10,000 angstroms, requiring the surface of the metal diaphragm to be extremely smooth to avoid defects.  
         [0009]     Metal diaphragm sensors have also been produced with thick-film metal oxide resistors. A thick-film oxide layer is formed by printing and firing a thick-film ink, after which a thick-film metal oxide layer is printed and fired on the insulating layer to form resistors. Thick-film conductors are typically employed with this type of sensor. However, since thick film materials fire at very high temperatures, for example in the range of 800° C. to 1000° C. and above, the diaphragm must then be formed of a metal having excellent high temperature corrosion resistance. While several high temperature resistant specialty metals are known in the art, they are very expensive and difficult to machine. Another option is to attach a small metal diaphragm “button” of specialty metal to a pressure port base made of a cheaper material. However, these structures are often unreliable at higher pressures.  
         [0010]     Clearly, a need for further improvement exists in the art of pressure sensors, particularly in the formation of pressure sensors which are lower in cost to produce, while still being reliable at high pressures. The present invention provides a solution to this problem. The invention provides a pressure sensing apparatus which includes a thin disc of a specialty metal or ceramic, having a ceramic material layer and piezoresistive elements formed thereon. This structure is bonded to a diaphragm assembly on a pressure port base constructed of a low cost metal. The bonding process is performed at low temperatures, (&lt;700° C.), so that the diaphragm assembly and pressure port do not require high temperature corrosion resistance, and can thus be formed of less expensive materials.  
         [0011]     The inventive pressure sensing apparatus provides a lower cost alternative to conventional high pressure sensors since less material is used, less expensive materials are used, and fabrication is less complex. The inventive apparatus is also more reliable and exhibits greater thermal stability than conventional high pressure sensors.  
       SUMMARY OF THE INVENTION  
       [0012]     The invention provides a pressure sensing apparatus which comprises:  
         [0000]     a) a pressure port having an upper surface and a base portion;  
         [0000]     b) an elastically deformable pressure-sensitive diaphragm assembly having upper and lower surfaces and a central pressure-sensitive diaphragm; the lower surface of the diaphragm assembly being attached onto the upper surface of the pressure port;  
         [0000]     c) a substantially planar disc having upper and lower surfaces; the lower surface of the disc being bonded to the upper surface of the diaphragm assembly via a layer of a bonding material;  
         [0000]     d) a ceramic material layer on the upper surface of the disc; and  
         [0000]     e) a plurality of piezoresistive elements on the ceramic material layer.  
         [0013]     The present invention further provides a method for forming a pressure sensing apparatus which comprises:  
         [0000]     a) providing a substantially planar disc having upper and lower surfaces;  
         [0000]     b) forming a ceramic material layer on the upper surface of the disc;  
         [0000]     c) forming a plurality of piezoresistive elements on the ceramic material layer;  
         [0000]     d) providing a pressure port having an upper surface and a base portion;  
         [0014]     e) attaching a deformable pressure-sensitive diaphragm assembly, having upper and lower surfaces and a central pressure-sensitive diaphragm, onto the pressure port such that the lower surface of the diaphragm assembly is attached onto the upper surface of the pressure port; and  
         [0000]     f) bonding the lower surface of the disc to the upper surface of the diaphragm assembly.  
         [0015]     The present invention still further provides a method for sensing pressure, which comprises:  
         [0000]     I) providing a pressure sensing apparatus comprising:  
         [0000]    
       
         
           
              a) a pressure port having an upper surface and a base portion;  
              b) an elastically deformable pressure-sensitive diaphragm assembly having upper and lower surfaces and a central pressure-sensitive diaphragm; the lower surface of the diaphragm assembly being attached onto the upper surface of the pressure port;  
              c) a substantially planar disc having upper and lower surfaces; the lower surface of the disc being bonded to the upper surface of the diaphragm assembly via a layer of a bonding material;  
              d) a ceramic material layer on the upper surface of the disc; and  
              e) a plurality of piezoresistive elements on the ceramic material layer; 
 
 II) placing the pressure sensing apparatus into a pressurized environment, thereby causing the diaphragm to deform in response to changes in pressure; 
 
 III) detecting any changes in pressure of the pressurized environment via the piezoresistive elements; and 
 
 IV) analyzing the pressure of the pressurized environment by observing changes in the piezoresistive elements.
 
           
         
       
     
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]      FIG. 1  shows a schematic side-view of a sensing component and a base component, of which the inventive pressure sensing apparatus is formed.  
         [0022]      FIG. 2  shows a schematic side view of a pressure sensing apparatus of the invention.  
         [0023]      FIG. 3  shows a schematic side view of a pressure sensing apparatus of the invention having an additional ceramic material layer.  
         [0024]      FIGS. 4   a - 4   c  show top views of various disc configurations according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]     The invention provides a pressure sensing apparatus for use in various pressure environments and which performs particularly well in very high pressure environments.  
         [0026]     As the  FIGS. 1-3  show, the inventive pressure sensing apparatus  1 , includes two main components, namely a sensing component  5 , and a base component  3 . As shown in  FIG. 1 , the base component  3  comprises a pressure-sensitive diaphragm assembly  13  attached onto a pressure port  7 . The pressure port  7  includes an upper surface  9  and a base portion  11  surrounding a central inlet  10 . Such pressure ports are known in the art, and serve to connect the sensor to pressure media. Typical pressure ports are cylindrical in shape, and range from about ¼ inch (6.35 mm) to about 1 inch (25.4 mm) in diameter, and from about ½ inch (12.7 mm) to 1.5 inches (38.1 mm) in height. The pressure port&#39;s cylindrical base typically includes connection threads around an outer surface of its base portion, for threadably attaching the pressure port to other devices. Such threads typically range from about ¼ inch (6.35 mm) to about 9/16 inch (14.29 mm) in thickness. The pressure port preferably further includes a hexagonally threaded portion around the outer surface of its base portion, to allow for tightening the pressure port with a wrench. The pressure port may comprise a variety of materials known in the art of pressure port formation, such as metals including stainless steel, specialty stainless steels, superalloys and the like.  
         [0027]      FIGS. 1 and 2  further show a pressure-sensitive diaphragm assembly  13  attached onto the pressure port  7 . The diaphragm assembly  13  preferably comprises a substantially flat, substantially circular upper surface  15 , a pressure-sensitive diaphragm  14 , and a lower surface  17  having a central inlet  12 . This lower surface  17  of the diaphragm assembly  13  is to be attached onto the upper surface  9  of the pressure port  7 . The diaphragm assembly  13  and the pressure port  7  are preferably attached via welding or the like.  
         [0028]     Such diaphragm assemblies are typically formed by machining a flat metal disc. The resulting diaphragm assembly  13  includes a diaphragm  14 , preferably centrally located at an upper surface  15  of the diaphragm assembly  13 , and a protruding lower surface  17  at a bottom of the assembly  13 . The protruding lower surface  17  typically comprises an annular ring or the like having a central inlet  12 . When attached to a pressure port  7  having a central inlet  10 , it is preferred that the pressure port&#39;s central inlet  10  corresponds with the diaphragm assembly&#39;s central inlet  12  such that pressures exerted on the pressure port  7  and the diaphragm assembly  13  are indicated by deformations of the diaphragm  14 .  
         [0029]     The diaphragm assembly  13 , including the diaphragm  14  is preferably elastically deformable, and is capable of deforming in response to, and correspondingly with, changes in pressure of the medium or environment to be sensed. The diaphragm assembly  13  may comprise a variety of materials such as metals, ceramics, and the like. Factors such as performance, cost, operating environment, and specific customer needs will dictate material selection. In one preferred embodiment, the diaphragm assembly  13  comprises a low cost metal or metal alloy. Suitable materials for the diaphragm assembly nonexclusively include stainless steel, nickel, or combinations and/or alloys thereof. Preferred diaphragm assembly materials nonexclusively include common stainless steels, such as 300- and 400-series stainless steels and the like; precipitation-hardened stainless steels such as 17-4PH, 15-5PH, 13-8PH and the like; and superalloys. Superalloys are specialty alloy materials known in the art which include a high nickel content and exhibit desirable properties such as high strength, high temperature resistance, oxidation resistance, and the like. Examples of superalloys include Inconel™ materials, Hastelloy™ materials and Haynes™ materials, which are commercially available.  
         [0030]     The diaphragm  14  preferably has a substantially consistent thickness throughout, such that its thickness is substantially the same at its center and its edges. Size and shape characteristics of the diaphragm  14  will depend on environmental factors and be determined by those skilled in the art, since the diaphragm&#39;s geometry, thickness, and diameter will determine the usable pressure range of the overall pressure sensor. For example, for use in a high pressure environment, the thickness of the diaphragm is increased and its diameter is decreased. For lower pressure environments, the thickness of the diaphragm is decreased and its diameter is increased. Preferably the diaphragm  14  has a thickness ranging from about 0.025 mm to about 4.5 mm, more preferably from about 0.051 mm to about 1.83 mm, and most preferably from about 0.076 mm to about 0.91 mm. Preferably the diaphragm  14  has a diameter ranging from about 0.3 mm to about 30 mm or more, more preferably from about 0.51 mm to about 1.83 mm, and most preferably about 0.76 mm to about 0.91 mm.  
         [0031]     The diameter of the diaphragm assembly  13  will vary, depending on the diameter of the diaphragm  14 . That is, the diameter of the diaphragm assembly  13  must be at least equal to the diameter of the diaphragm  14 . Preferably the ratio of the entire diaphragm assembly&#39;s diameter to diaphragm&#39;s diameter ranges from about 1.1:1 to about 5:1.  
         [0032]     Furthermore, it is preferred that, in those embodiments wherein the lower surface  15  of the diaphragm assembly  13  comprises an annular ring or the like, the wall thickness of the annular ring or the like is greater than or equal to the thickness of the diaphragm  14 .  
         [0033]     The invention further comprises a sensing component  5 , which is to be attached to the base component  3  via the upper surface  15  of the diaphragm assembly  13 . It is preferred that at least a portion of the sensing component  5  is attached onto a surface of the diaphragm  14 . The sensing component  5  first comprises a substantially planar disc  19  having upper and lower surfaces,  21  and  23  respectively. The substantially planar disc  19 , which preferably comprises a metal or ceramic material, should have a substantially similar coefficient of thermal expansion as the material of the diaphragm assembly  13  as described above, to thereby minimize stress caused by differential expansion of the diaphragm  14  and the disc  19 . Again, factors such as performance, cost, operating environment, and specific customer needs will dictate material selection. Some suitable materials for the substantially planar disc  19  nonexclusively include metals such as stainless steel, nickel, aluminum, or combinations and/or alloys thereof; ceramics such as alumina, zirconia, silica, and combinations thereof. Preferred disc materials nonexclusively include common stainless steels such as 300- and 400-series stainless steels and the like; precipitation-hardened stainless steels such as 17-4PH, 15-5PH, 13-8PH and the like; superalloys such as Inconel™ materials, Hastelloy™ materials and Haynes™ materials, which are commercially available; and oxidation-resistant aluminum-containing alloys such as Haynes 214™, FeCr alloys, 18SR stainless and the like; and combinations and/or alloys thereof.  
         [0034]     The disc  19  preferably has a thickness ranging from about 0.025 mm to about 1.5 mm, more preferably from about 0.025 mm to about 0.75 mm, and most preferably from about 0.040 mm to about 0.75 mm. The disc  19  preferably has a diameter ranging from about 0.3 mm to about 30 mm or more, more preferably from about 1.6 mm to about 25 mm, and most preferably from about 6.4 mm to about 12.7 mm.  
         [0035]     The disc  19  may have a variety of shapes. In one embodiment, shown in  FIG. 4   a , the disc is present in the form of a substantially flat circular disc. In another embodiment, shown in  FIG. 4   b , the disc  19  is in the form of a substantially flat annular ring interconnected by a single cross bridge. In yet another embodiment, the shown in  FIG. 4   c  the disc  19  is present in the shape of a substantially flat annular ring interconnected by multiple cross bridges.  
         [0036]     The disc  19  serves as a substrate on which a ceramic material layer and piezoelectric elements are to be formed. Additionally, prior to the formation or application of such materials onto the disc, the disc surface may optionally but preferably be washed, roughened, and/or oxidized using conventional methods. It is an important feature of this invention that the ceramic material layer and/or the piezoelectric elements are formed onto the upper surface  21  of the disc  19  prior to bonding the disc to the diaphragm assembly  13 , as described below.  
         [0037]     A ceramic material layer  25  is formed on the upper surface of the disc  19  as shown in  FIGS. 1 and 2 . The ceramic material layer  25  serves as an insulator. Any suitable known ceramic material may be used for the ceramic material layer  25 . Examples of suitable ceramic materials nonexclusively include silica, alumina, zirconia, glass, and combinations thereof. Additional materials include crystallizable glass, filled glass, and the like. The thickness of ceramic material layer  25  on the disc  19  may be determined by those skilled in the art. Preferably, the ceramic material layer  25  has a thickness ranging from about 0.025 mm to about 0.1 mm, preferably from about 0.025 mm to about 0.075 mm, and most preferably from about 0.040 mm to about 0.060 mm.  
         [0038]     The ceramic material layer  25  may be formed on the disc  19  using a variety of conventional methods. In certain embodiments of the invention, the ceramic material layer  25  is present in the form of a ceramic coating which has been coated onto the upper surface  21  of the disc  19  using conventional coating methods. In forming such ceramic coatings, a commercial ceramic dielectric paste may be screen printed on the disc  19 , dried in an oven at a temperature of about 150° C., and fired at temperatures of about 850° C. to 950° C. to form such a ceramic coating. Several layers of ceramic paste can be dried independently and fired (co-fired) together in forming the ceramic coating. As few as one, or up to 8 or more ceramic paste layers may be applied in this way. Preferably the thickness of the ceramic coating after firing ranges from about 0.01 mm to about 0.2 mm microns in thickness, preferably from about 0.03 mm to about 0.09 mm in thickness, and most preferably from about 0.04 mm to about 0.06 mm in thickness.  
         [0039]     As shown in  FIG. 3 , an additional ceramic material layer  25   b  may be present on the lower surface  23  of the disc  19 . In an alternate embodiment, the ceramic material layer  25  is present in the form of a ceramic substrate which has been attached onto the upper surface  21  of the disc  19  using conventional bonding methods. Such methods nonexclusively include glass bonding, gluing, soldering, brazing and the like.  
         [0040]     Next, the invention includes a plurality of piezoresistive elements  27  on the ceramic material layer  25  on the upper surface  21  of the disc  19 . In this invention, the piezoresistive elements  27  serve to translate pressure into electrical signals. It is preferred that at least a portion of the piezoresistive elements are present at a location above the diaphragm  14 .  
         [0041]     Such piezoresistive elements  27  are well known in the art, and are commercially available. They often comprise a composite of ruthenium oxide in a glass matrix. Such piezoresistive elements  27  are preferably formed on the ceramic material layer  25  to form a Wheatstone bridge. The individual piezoresistive elements  27  can vary in size as determined by those skilled in the art. Generally, these elements range in size from about 0.5 mm×0.5 mm to about 2.5 mm×2.5 mm.  
         [0042]     The piezoresistive elements  27  may further be electrically connected to a signal detection circuitry, such as an Application Specific Integrated Circuit (ASIC).  
         [0043]     In attaching the sensing component  5  of the present invention with the base component  3 , the lower surface  23  of the disc  19  is bonded onto the upper surface  15  of the diaphragm assembly  13  via a bonding material layer  29 . It is preferred that at least a portion of the disc  19  is attached onto a surface of the diaphragm  14 . While it is known in the art to bond individual components onto a diaphragm assembly via a bead of bonding material, it is preferred according to this invention that the bonding material be present as a substantially uniform thickness layer between the surfaces of the diaphragm assembly  13  and the disc  19 . That is, the upper surface  15  of the diaphragm assembly  13  and the lower surface  23  of the disc  19  are directly bonded via such a bonding material layer  29  having substantially uniform thickness.  
         [0044]     Bonding may be conducted using any conventionally known method such as by melt bonding, gluing, soldering, brazing, and the like. Suitable bonding materials nonexclusively include organic adhesives such as structural epoxies, perform epoxy films, epoxy paste; glass materials such as glass performs, glass paste; ceramics; metal materials such as metal solder or braze, or combinations thereof. In one preferred embodiment the bonding material comprises an epoxy adhesive. In another preferred embodiment the bonding material comprises a metal solder or braze.  
         [0045]     Bonding temperature and processes depend on the bonding materials used. Epoxies normally cure at room temperature to about 200° C., solders melt at about 180° C. and higher, preferred glass pastes or performs melt at temperatures ranging from about 500° C. to about 700° C.  
         [0046]     It is most preferred that the diaphragm assembly  13  and pressure port  7  are not exposed to high temperature processing. That is, it is preferred that the diaphragm assembly  13  and pressure port  7  are not subjected to temperatures above about 700° C. during the fabrication of the inventive pressure sensing apparatus  1 . Eliminating the need for high cost heat-sensitive materials for the diaphragm assembly and/or pressure port results in a great cost savings. Accordingly, it is preferred that any high-heat steps (above about 700° C.) such as steps required to form the ceramic material layer  25  on the disc  19 , are conducted prior to attaching the disc  19  to the diaphragm assembly  13 . In one preferred embodiment, the pressure sensing apparatus is assembled as two separate units, wherein a base component  3  and a sensing component  5  are separately formed and thereafter attached by bonding the disc  19  of the sensing component  5  to the diaphragm assembly  13  of the base component  3 , via a layer of bonding material  29 .  
         [0047]     The pressure sensing apparatus  1  of the present invention may be used in a variety of applications, and are particularly well suited for very high pressure environments. The apparatus  1  is also useful in medium or low pressure environments as well. For example, the inventive pressure sensing apparatus  1  may be useful at pressures ranging from as low as 50 psi to about 20,000 psi or above, depending on diaphragm design and diaphragm assembly design. The inventive apparatus may also be used at a variety of temperatures, such as within the range of from about −40° C. to about 150° C.  
         [0048]     The pressure sensing apparatus  1  of the present invention provides several advantages over the prior art. The inventive apparatus can be produced at a lower cost due to the requiring of less material, the use of less expensive materials, less reject cost, and less expensive fabrication cost due to a simpler fabrication process. Furthermore, multiple sensors can be fabricated on a single substrate, significantly reducing labor costs and processing time. The inventive pressure sensing apparatus further exhibits greater adhesion, greater thermal stability, and higher reliability due to a match of thermal expansion between the diaphragm assembly  13  and the disc  19 .  
         [0049]     The following non-limiting examples serve to illustrate the invention. It will be appreciated that variations in proportions and alternatives in elements of the components of the invention will be apparent to those skilled in the art and are within the scope of the present invention.  
       EXAMPLE 1  
       [0050]     A circular, flat disc of 430 stainless steel is machined to have a diameter of 0.5 mm and a thickness of 0.1 mm, and is washed in detergent. A ceramic dielectric paste comprising alumina is screen printed onto an upper surface of the disc. The ceramic paste is dried on the disc in an oven for 10-15 minutes at  150 C. The dried paste is then fired on the disc for 60-90 minutes at 850-950° C. to form a ceramic coating having a thickness of about 0.05 mm. Piezoresistive elements are then formed on the ceramic coating by first screen printing a conductive gold paste onto the ceramic coating. The conductive paste is then dried on the ceramic coating in an oven for 10-15 minutes at 150° C. The conductive paste is then fired on the ceramic coating for 60-90 minutes at 850° C. to form a conductive layer having a thickness of about 0.01 mm. A resistor paste is then screen printed onto the conductive layer. The resistor paste is then dried on the conductive layer in an oven for 10-15 minutes at 150° C. The resistor paste is then fired on the conductive layer for 60-90 minutes at 850° C. to form a resistor layer having at thickness of about 0.02 mm.  
         [0051]     Separately, a 17-4PH pressure port is welded to a 17-4PH stainless steel diaphragm assembly. The diaphragm assembly includes a central diaphragm having a diameter of 0.6 mm and a thickness of 0.1 mm. A lower surface of the metal disc above is bonded to an upper surface of the diaphragm assembly using an epoxy paste bonding material.  
       EXAMPLE 2  
       [0052]     A circular, flat ceramic disc comprising alumina is formed, having a diameter of 0.5 mm and a thickness of 0.1 mm, and is washed in detergent. Piezoresistive elements are then formed on the ceramic coating by first screen printing a conductive gold paste onto the ceramic coating. The conductive paste is then dried on the ceramic coating in an oven for 10-15 minutes at 150° C. The conductive paste is then fired on the ceramic coating for 60-90 minutes at 850° C. to form a conductive layer having a thickness of about 10 microns. A resistor paste is then screen printed onto the conductive layer. The resistor paste is then dried on the conductive layer in an oven for 10-15 minutes at 150° C. The resistor paste is then fired on the conductive layer for 60-90 minutes at 850° C. to form a resistor layer having at thickness of about 0.02 mm.  
         [0053]     Separately, a 316 stainless steel pressure port is welded to a 316 stainless steel diaphragm assembly. The diaphragm assembly includes a central diaphragm having a diameter of 0.6 mm and a thickness of 0.1 mm. A lower surface of the ceramic disc above is bonded to an upper surface of the diaphragm assembly using an epoxy paste bonding material.  
       EXAMPLE 3  
       [0054]     Example 1 is repeated except that after firing the resistor paste, an overglaze glass sealant paste is screen printed onto the resistor layer and dried in an oven for 10-15 minutes at 150° C. The overglaze glass sealant paste is then fired for 30-60 minutes at 500-700° C. to form an overglaze glass sealant layer having a thickness of 0.01-0.02 mm.  
         [0055]     While the present invention has been particularly shown and described with reference to preferred embodiments, it will be readily appreciated by those of ordinary skill in the art that various changes and modifications may be made without departing from the spirit and scope of the invention. It is intended that the claims be interpreted to cover the disclosed embodiment, those alternatives which have been discussed above, and all equivalents thereto.