Abstract:
An oil-filled pressure transducer having reduced back pressure, comprising an alignment plate having a sensor accommodating aperture, a sensor module inserted into the sensor accommodating aperture, a header surrounding the alignment plate, the header having a protruding top surface, and a diaphragm disposed on the protruding top surface to create a relatively small oil accommodating region between the diaphragm and the sensor. This configuration reduces the oil volume required for operation, which ultimately reduces the back pressure applied against the diaphragm.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application claiming priority under 35 U.S.C. §120 to U.S. patent application Ser. No. 13/467,207, filed May 9, 2012, which is a continuation application claiming priority under 35 U.S.C. §120 to U.S. patent application Ser. No. 13/025,257, filed Feb. 11, 2011, now U.S. Pat. No. 8,181,530, which is a continuation application claiming priority under 35 U.S.C. §120 to U.S. patent application Ser. No. 12/384,828, now U.S. Pat. No. 7,921,721, entitled “LEADLESS OIL FILLED PRESSURE TRANSDUCER,” filed Apr. 9, 2009 and issued Apr. 12, 2011, all of which are hereby incorporated by reference in their entirety as if fully set forth below. 
    
    
     FIELD OF INVENTION 
     The present invention relates generally to an oil filled pressure transducer and a method for making such a transducer and more particularly, to an oil filled pressure transducer having a lower backpressure when temperature is increased. 
     BACKGROUND OF THE INVENTION 
     Oil filled pressure transducers are well known. The prior art shows such devices that have been used with oil, where oil or fluid is employed as a force-transmitting medium. In such devices, the pressure is applied to a transducer diaphragm, and typically these devices employ a metal diaphragm as a force collector. Such oil filled transducers are known in the art. For example, see U.S. Pat. No. 4,406,993 entitled, “Oil Filled Pressure Transducers” issued on Sep. 27, 1983 to Anthony D. Kurtz, the inventor herein, and assigned to Kulite Semiconductor Products, Inc., the assignee herein. See also U.S. Pat. No. 6,330,829 entitled, “Oil Filled Pressure Transducer” issued Dec. 18, 2001 to A. D. Kurtz et al., the inventor herein, and assigned to Kulite Semiconductor Products, Inc., the assignee herein. 
     It is well known that the pressure caused by a deflection (δ) of a clamped edge diaphragm of thickness (t) and radius (a) is given by: 
             P   =         16   ⁢           ⁢   δ   ⁢           ⁢     Em   2       +   3       3   ⁢           ⁢       a   4     ⁡     (       m   2     -   1     )                 
Where m=reciprocal of Poisson&#39;s Ration, P=pressure, and E=Young Modulus
 
     In an oil-filled pressure transducer, the sensor element is isolated from the pressure media by means of a thin metal diaphragm and the pressure is transmitted to the sensor by means of a volume of oil mainly in the form of a film between the metal diaphragm and the sensor. This is, of course, indicated in the above-noted patents. In any event, when the temperature increases, the oil expands and pushes against the metal diaphragm thus exerting a backpres sure against the sensor leading to an error signal. This problem is known and is addressed and explained in U.S. Pat. No. 5,999,082 entitled, “Compensated Oil Filled Pressure Transducer” issued on Dec. 7, 1999 to A. D. Kurtz et al., the inventor herein and assigned to the assignee herein. In that patent, the object was to reduce errors at very low pressure caused by the oil exerting a tension on the deflecting portion of the diaphragm. A first attempt to make a minimum oil volume transducer using a leadless sensor structure was shown in U.S. Pat. No. 6,591,686 entitled, “Oil Filled Pressure Transducer”. 
     The prior art describes an oil filled pressure transducer which utilizes a leadless sensor which is secured to a header comprising a glass pre-form and a header shell. The glass pre-form contains holes which accept header pins and another aperture or hole which accepts the oil fill tube. The diameter of the sensor was chosen to be almost as large as the inner diameter of the shell. In this manner, there was a small cut out over the portion of the sensor that would otherwise cover the oil fill tube. The sensor was mounted to the header using glass bonds. There was a very small space between the outer diameter of the sensor and the inner diameter header housing, which was filled with glass used to mount the sensor. Since there were no ball bonds or gold wires in the area between the surface of the sensor and the diaphragm the distance between the sensor and metal diaphragm was drastically reduced, thereby substantially reducing the backpres sure problem. However, there are still a number of remaining problems with this construction. Making the sensor diaphragm chip the same size as the inner hole is not appropriate. It is difficult to line up the location of the header pins with respect to the apertures in the sensing diaphragm. Using this approach, it is very difficult to maintain a close separation between the metal diaphragm and the underlying sensor over a large diameter. It is very difficult to orient the sensing diaphragm with respect to the pins, thus making the mounting exceedingly difficult. It is an object of this invention to reduce the oil volume while enabling easier sensor assembly. It is another object of the present invention to make the effective diameter of the isolation diaphragm much larger to reduce the back pressure. 
     It is an object of the present invention to reduce the oil volume to a minimum volume expansion and thus reduce the error causing backpressure. 
     SUMMARY OF INVENTION 
     An oil filled pressure transducer utilizes a leadless sensor which is secured to a header comprising a glass pre-form and a header shell. The glass pre-form contains holes which accept header pins and another aperture or hole which accepts the oil fill tube. There is now employed an alignment glass which becomes part of the transducer structure. The alignment glass is self locating due to the fact that the oil filled tube is raised above the surface of the header glass. The alignment glass has a diameter essentially equal to the inner diameter of the header shell. It also contains a rectangular cut out which conforms to the size of the sensor die and external from this cut out is a hole through which the oil filled tube may pass. The sensing diaphragm is mounted to the header in the following way: The alignment glass is mounted to the header using a glass bond; the sensor is mounted to the header using glass bonds during the same operation. In this way the rectangular hole in the alignment glass defines where the header pins are relative to the sensing element. In this way it becomes comparatively easy to mount the sensor on top of the header pins to insure a good contact between the header pins and the sensing network on the diaphragm. 
     Moreover, the diaphragm and the alignment glass can be made significantly larger than the dimensions of the sensor and since the alignment glass is of known uniform thickness it is easier to maintain a very narrow gap between the sensor and the metal diaphragm. This method gives rise to a means of obtaining relatively much smaller oil volumes than in the prior art. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a perspective view of a prior art leadless oil filled transducer without a metal diaphragm cover. 
         FIG. 2  consists of  FIG. 2A  and  FIG. 2B  which show a top plan view and a cross-sectioned view of the prior art transducer  08   FIG. 1 . 
         FIG. 3  is a perspective view of an improved leadless oil filled transducer according to this invention. 
         FIG. 4  consists of a top view  FIG. 4A  of the prior art transducer and a cross sectional view  FIG. 4B  of the prior art transducer as shown in  FIG. 1 . 
         FIG. 5  is a perspective view of a glass alignment structure according to this invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1  there is shown a prior art transducer useful in explaining the present invention. The figure shown in  FIG. 1  is FIG. 2 of U.S. Pat. No. 6,591,686 entitled “Oil Filled Pressure Transducer” issued on Jul. 15, 2003 to A. D. Kurtz and assigned to Kulite Semiconductor Products, Inc., the assignee herein. That patent, as indicated above, describes an oil filled pressure transducer using a leadless sensor which is secured to a header. In any event, referring to  FIG. 1 , there is shown the prior art device. As seen in  FIG. 1 , the transducer includes a header shell  30  which includes pins  31  and  32  and an oil fill tube  33 . As will be explained the unit includes a leadless sensor device  41  which is associated with a small oil cavity  36  as compared to multiple oil cavities of prior art devices. It is noted that FIG. 1 of U.S. Pat. No. 6,591,686 as indicated above, shows a prior art device which was prior to the device depicted in the above-noted patent. It is also indicated that the above-noted U.S. Pat. No. 6,691,686 is incorporated herein in its entirety. 
       FIG. 2A  and  FIG. 2B  depict the prior art sensor as shown in U.S. Pat. No. 6,691,686 indicated above. Essentially  FIGS. 2A and 2B  are  FIGS. 4A and 4B  of U.S. Pat. No. 6,691,681. As seen from  FIG. 2A  there is shown a top view of the sensor of  FIG. 1  and a cross-sectional view  FIG. 2B  of the sensor. The sensor of the prior art as indicated is a leadless sensor device as can be seen from  FIGS. 2A and 2B , the header includes a header shell  30  and a glass pre-form  32 . The oil fill tube  44  is directed to an aperture  64  in the glass  42  and communicates with an aperture  63  in the transducer. The glass pre-form  46  has a plurality of holes which accept the header pins as hole  64  for pin  44  and hole  65  for the oil fill tube  45 . As seen in  FIG. 2B  the metal diaphragm  60  is secured to the header and basically covers the top portion of the sensor  41 . The region requiring oil is shown by reference numeral  61  and is the region between the metal diaphragm  60  and the top portion of the sensor  41 . As indicated in U.S. Pat. No. 6,591,686, which is incorporated herein in its entirety, it is shown that the distance between the sensor  41  and the metal diaphragm is reduced thus exhibiting a marked improvement in performance and assembly over prior art devices as those prior to that device depicted in U.S. Pat. No. 6,591,686. In any event, as indicated above, it is an object of the present invention to still improve the design. 
     Referring to  FIG. 3  there is shown on oil filled pressure transducer according to this invention. It is noted that in  FIG. 3  the same reference numerals have been utilized to depict the same parts as shown in the prior art  FIGS. 1 and 2 . Essentially, the transducer of  FIG. 3  includes a header shell  30  which includes pins  31  and an oil fill tube  33 . As will be explained, this unit includes a leadless sensor device  41 , an alignment glass  47  which is associated with a small oil cavity  36 , and to align the sensor  41  for ease of mounting, as compared to the prior art device. As can be seen, there is a similarity between the general appearance of the prior art device of  FIGS. 1 and 2  and that device of  FIG. 3 . The header  30  also has a peripheral flange  50  about the top opening  51 . The opening is covered by a metal diaphragm  60  ( FIG. 4B ). The peripheral flange is of a given height which height partly determines the oil volume. 
     Referring to  FIG. 4A , there is shown a top view of the sensor of  FIG. 3  and a cross sectional view ( FIG. 4B ) of the sensor of  FIG. 3 . The sensor of this invention is a leadless sensor device  41 . Leadless sensors have been developed by Kulite Semiconductor Products, Inc., the assignee herein. For examples of such sensor devices, reference is made to U.S. Pat. No. 6,272,929 entitled, “High Pressure Piezoresistor Transducer Suitable for Use in Hostile Environments” issued on Aug. 14, 2001 to Anthony D. Kurtz et al. See also U.S. Pat. No. 6,210,989 entitled, “Ultra Thin Surface Mount Wafer Sensor Structure and Methods for Fabricating the Same” issued on Apr. 3, 2001 to A. D. Kurtz et al. and assigned to Kulite Semiconductor Products, Inc., the assignee herein. See also U.S. Pat. No. 5,973,590 entitled, “Ultra Thin Surface Mount Wafer Sensor Structure and Methods of Fabricating the Same” issued on Oct. 26, 1999 to A. D. Kurtz et al. 
     Basically, these patents show leadless semiconductor sensor devices which include a semiconductor diaphragm having a top surface coating of an oxide layer. There are P+ sensor elements fusion bonded to the oxide layer at a relatively central area of the diaphragm. The P+ finger elements are fusion bonded to the oxide layer and extend from the sensors to an outer contact location of the diaphragm for each of the fingers. There is an external layer with P+ material fusion bonded to the oxide layer and surrounding the sensors and fingers. A glass wafer is electrostatically bonded at the bottom surface to the fingers and rim to hermetically seal the sensors and fingers of the diaphragm member. The glass wafer includes a depression above the sensors and has a plurality of apertures where each aperture is associated a separate finger at the contact location, and each aperture is smaller than the associated finger lining up with the contact location. Each contact location can be accessed via the associated aperture. The sensors contain piezoresistive P and elements normally arranged in a Wheatstone bridge configuration. 
     The sensor basically is a leadless device in the sense that the header pins of the diaphragm are coplanar with the top surface of the glass wafer and hence these transducers are extremely thin and do not have the conventional wire leads. The sensors described in the above-noted patents do not have wires bonded to pins, but utilize the P+ finger elements which are fusion bonded to outer contact locations as shown in the above patents, all of which are incorporated herein by reference. 
     The header includes a header shell  30  and a glass pre-form  42 . The oil fill tube  33  is directed through an aperture  64  in the glass  42 . The glass pre-form  42  contains five holes. Four of the holes accept the header pins as hole  65  for pin  45 , while the other hole  64  accepts the oil fill tube  33 . The arrangement is like the prior art U.S. Pat. No. 6,591,686 device. The pins  45  are about 0.01 inches in diameter and protrude about 0.005 inches from the top surface of the glass pre-form  42 . The pin  45  is shown extending into the contact terminal present in the leadless transducer  41 . In addition, the oil filled tube  44  is located near the outside diameter of the glass and is about 0.03 inches in diameter. The pins  45  and the oil fill tube  44  are hermetically sealed to the glass pre-form during firing, as well as the header shell. The entire structure as shown in  FIG. 4  is subjected to heat which causes the glass associated with the sensor  41  to be secured to the glass pre-form  42  by means of heat and using a frit, which essentially causes a hermetic seal to form. This seal secures the glass pre-form  42  to the glass layer  46  of the leadless sensor  41 . The firing also secures the header glass  42  as well as the glass section  46  of the sensor to the header shell  30 . The alignment glass  47  is selected to be almost as large as the inner diameter of the metal shell  30 . The firing also secures the alignment glass  47  to the glass pre-form  42 . There is a small cut out  36  which is shown in the top view and also indicated in  FIG. 2 . The small cut out  36  is positioned over a portion of the alignment glass that would otherwise cover the oil fill tube  44 . 
     The sensor  41  is mounted to the header  30  using a metal glass frit (conductive) to bond the pins to the sensor contacts and a ceramic glass frit to join the rest of the sensor surface to the glass header. This enables the unit to then be fired using the glass frit and the ceramic glass frit to secure the parts together. Any space between the outer diameter of the alignment glass  47  and the inner diameter of the header housing  30  is filled by the glass frit, which is used to mount the sensor and the alignment glass. This occurs when the unit is fired and the glass flows and fills the voids. It can easily be seen that the only region requiring oil is the region between the non-contact surface of the sensor, alignment glass and the metal diaphragm  60 . 
     As seen in  FIG. 4 , the metal diaphragm  60  is secured to the header and basically covers the top of the sensor  41 . The region requiring oil is shown by reference numeral  61  and is the region between the metal diaphragm  60  and the top portion of the sensor  41  and the alignment glass  47 . Since there are no gold bonds or gold wires in the area between the surface and the sensor of the metal diaphragm, this region  61  is the only region requiring oil. Thus, the distance between the sensor  41  and the alignment glass  47  and the metal diaphragm is drastically reduced, leaving a gap, for example, of between 0.001 inch to 0.002 inch. This small gap and the ability to reduce almost to zero, the space between the outer diameter of the sensor  41  and the alignment glass  47  ( FIG. 5 ) and the inner diameter of the header housing serves to reduce oil volume by at least an order of magnitude. As seen in  FIG. 4A  there is an alignment glass  47 . The alignment glass  47 , as seen in  FIG. 5 , basically is a glass member which has an alignment aperture  48  and a chip accommodating aperture  49 . When the unit is fabricated the oil fill tube  33  expends above the top surface of the glass member  42 . In this manner, the alignment glass is placed so that the aperture  48  encircles the oil fill tube  33 , as shown in  FIG. 4B . In this manner, the sensor accommodating aperture  49  is precisely positioned with respect to the pins as  45 . Thus, one now places the sensor chip  41  in the pin area surrounded by the walls of aperture  49 . This assures that the sensor chip  41  is properly aligned. It further assures that the spacing between the diaphragm and the top portion of the sensor is minimized because of the precise alignment. This reduces the gap and can reduce it to almost zero, because of the method of aligning. As one can see, the top surface of the alignment glass member  47  protrudes into the oil fill area thus further reducing the amount of oil, while the aperture  49  allows oil to be directly coupled to the deflecting diaphragm associated with the sensor  41 . Thus the use of the alignment plate substantially reduces the amount of oil required to fill the region  61 . Essentially, one can readily compute the expected deflection of the metal diaphragm for the two different structures and thus, the backpressure, (new  FIGS. 3 and 4 ) and prior art ( FIGS. 1 and 2 ). 
     The deflection (δ) for an increase ΔP is given by the equation δ=h×(dB/dT)×ΔT. Where (h) is the height of the oil gap, ΔT is the temperature increase, (B) is the bulk compressibility of the oil and dB/dT is the coefficient of thermal expansion of the oil. For silicon oil dB/dT=100×10 −5 /° K. 
     Thus, for a difference of temperature of 100° K, δ=0.1 h, when h=0.001 inch, δ=0.0001 inch. However, when h=0.015 inches, δ=0.0015 inches, which is 15 times larger. Since the backpres sure is directly proportional to δ, the wider gap oil filled unit exhibits a backpres sure 15 times larger, thus leading to a substantial error signal. It is therefore apparent that the apparatus shown, for example in  FIGS. 2 and 4  are extremely desirable and exhibits a marked improvement in performance over prior art devices. 
     From the two oil heights assuming the following: 
     t=0.001 inches 
     E=30×10 6  lb/in t    
     M=3.3 
     The actual backpressures with given radius are: 
     
       
         
               
               
               
             
           
               
                   
               
               
                   
                 Wide Gap 
                 Narrow Gap 
               
               
                   
                 .015 
                 0.001 
               
               
                 Radius 
                 Oil Height 
                 Oil Height 
               
               
                   
               
             
             
               
                 .10” 
                 2.64 PSI 
                  .18 PSI 
               
               
                 .20” 
                 .165 PSI 
                 .011 PSI 
               
               
                   
               
             
          
         
       
     
     Where P 1  is the backpressure for the wide gap case, which is the prior art device shown, for example, in  FIG. 1  and in  FIG. 2  and P 2  is the backpressure for the narrow gap case, which is the improved oil filled transducer shown in  FIG. 3  and  FIG. 4 .