Abstract:
Described herein is the sense element assembly for a capacitive pressure sensor and method for creating same that has increased sensitivity without additional size. The sense element assembly and method includes fabricating an off-centered elliptically shaped center electrode, at least one elliptical annular-like electrode around the center electrode, a ground electrode and a method for fusing the layers together to optimize sensitivity.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to capacitive pressure sensors and more specifically to sense element assemblies and methods of creating same for capacitive pressure sensors. 
       BACKGROUND OF THE INVENTION 
       [0002]    Ceramic capacitive pressure sensors are well known in the art. In order to maintain maximum deflection with a minimal amount of bending stress, capacitive sensors generally use circular diaphragms centered in circular housings and two electrodes. The first electrode, which responds to pressure changes, will be referred to throughout as the “CP” electrode. The second electrode, which is the reference electrode, will be referred to as the “CR” electrode. The housings usually also contain the electrical connections necessary to activate the diaphragm electrodes. 
         [0003]    Thus, it is desirable to create a pressure sensor containing a sensor assembly that has maximized deflection with minimal bending stress, detects large gradients of pressure, and is easy to fabricate. 
       SUMMARY OF THE PREFERRED EMBODIMENTS 
       [0004]    In the present invention, a capacitive pressure sensor element assembly is provided having a substrate assembly and a diaphragm assembly. The substrate assembly comprises a substrate onto which a first electrode and second electrode is fabricated. The first electrode is elliptically shaped and has a first conductive terminal. The second electrode is substantially annular and elliptically shaped and surrounds the first electrode. The second electrode has a second conductive terminal. The diaphragm assembly comprises a diaphragm onto which a ground electrode is fabricated, the ground electrode having a ground conductive terminal. The diaphragm assembly and the substrate assembly are sealed together to form the sensor element assembly. 
         [0005]    In the preferred embodiment, the first and second electrodes are fabricated so that they are located off-center on the substrate. 
         [0006]    According to another aspect of the present invention, there is provided a method for fabricating the pressure sense element assembly. The method comprises assembling the substrate by printing CP and CR electrodes onto a substrate, drying and firing the electrodes, printing a frit on top of the substrate, drying the frit; assembling the diaphragm by printing the common/ground electrode, drying and firing the electrode, printing a frit onto the diaphragm, drying the frit. In the method of the present invention, it does not matter which of the assemblies—either the substrate assembly or the diaphragm assembly is manufactured first. However, the method of the present invention calls for firing the frit of one of the assemblies prior to aligning the substrate assembly with the diaphragm assembly and fusing them together to create the sensor assembly. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The invention may be more readily understood by referring to the accompanying drawings in which: 
           [0008]      FIG. 1  is an exploded perspective view of the integrated pressure sensor element assembly in accordance with an embodiment of the present invention; 
           [0009]      FIG. 2  is a partial cutaway side perspective view of the integrated pressure sensor element assembly of  FIG. 1 ; 
           [0010]      FIG. 3  is a cross-sectional plan view of the substrate assembly of pressure sensor element assembly of  FIG. 1 ; and 
           [0011]      FIG. 4  is a perspective view of the frit layer of the diaphragm assembly portion of the pressure sensor element assembly of  FIG. 1 . 
       
    
    
       [0012]    Like numerals refer to like parts throughout the several views of the drawings. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0013]    In the following descriptions of the invention, terms such as “front,” “back,” “top,” “bottom,” “side,” and the like are used herein merely for ease of description and refer to the orientation of the components as shown in the Figures. 
         [0014]    Generally, the present invention may be briefly described as follows. Referring first to  FIGS. 1 and 2 , a preferred embodiment of an integrated pressure sense element assembly  100  of the present invention is shown. 
         [0015]    The sense element assembly  100  comprises two assemblies—a substrate assembly  102  and a diaphragm assembly  104 . The substrate assembly comprises a substrate  106 , a CP electrode  108 , a CR electrode  110  and a sealing layer  112 . In a preferred embodiment, the sealing layer  112  is a frit. However, any other material that provides a hermetic seal and provides for thermal expansion can be used. 
         [0016]    The CP electrode  108  is preferably elliptical in shape as shown more fully in  FIG. 3  and is connected to CP terminal  114 . The CR electrode  110  is also preferably elliptical in shape and is substantially an annular ring surrounding the CP electrode  108  as shown in  FIGS. 1 and 3 . In a preferred embodiment, the CR electrode  110  is C-shaped. However, the CR electrode  110  may be a complete annulus surrounding the CP electrode  108  providing that there is an electrical connection possible directly with the CP electrode  108  without touching the CR electrode  110 . 
         [0017]    The CR electrode  110  is connected on one end  116  to a CR terminal  118 . The CP electrode  108  is connected to the CP terminal  114  by a conductor which extends from the CP electrode  108  and through the gap between the two ends  116  and  122  of the CR electrode  110 . In a preferred embodiment, a guard electrode  120  is provided which is connected from CR terminal  118  around CP terminal  114  to end  122  of the CR electrode  108 . The reason for this guard electrode  120  is to protect, or isolate the CP electrode that when the sensor is operational, there should be no temperature effect that affects the relative capacitance of the CP  108  and CR  110  electrodes. 
         [0018]    In an preferred embodiment in which a frit is used, the frit layer  112  will become di-electric after it is fired. Thus, it will respond to outside temperature changes by a change in the dielectric constant. Therefore, if the areas covered by the frit are different, as the temperature changes, the capacitances as measured at CP  108  and CR  110  electrodes will deviate from one another making the sensor less accurate. If the CR  110  and CP  108  electrodes have substantially the same area covered by the frit layer  112  then there will be very little capacitance deviation between the two electrodes. Thus, in a preferred embodiment a guard electrode  120  is placed around the terminal  114  of the CP electrode  108  to act as a shield so that the only capacitance that the CP electrode  108  will be exposed to will be the capacitance created by the CR electrode  110 . 
         [0019]    In addition, if the sensor is exposed to high frequency signals, without the guard electrode  120 , terminal  118  would act like an antenna causing the sensitivity of the sensor to decrease. However, it is to be understood that the guard electrode  120  can be omitted. 
         [0020]    As shown in  FIG. 3 , in a preferred embodiment of the present invention, the CP  108  and CR  110  electrodes are fabricated by printing the CP and CR electrodes onto the substrate  106  so that they are off center on the substrate  106 . Because they are elliptical in shape, by placing them off center onto the substrate  106 , the electrodes  108  and  110  can be fabricated on a larger area while still retaining sufficient space between the perimeter of the outer CR electrode  110  and the circumference of the substrate to be fused together. Thus, when the diaphragm  104  and the substrate  102  assemblies layers are fused together, there is still enough space for the sealing layer  112  so that an effective seal is still possible. 
         [0021]    In a preferred embodiment, the elliptical electrodes are shaped such that the outer electrode  110  will provide essentially the same outside exposed area as the inner electrode  108 . Further, in a preferred embodiment, the major diameter of the outer electrode  110  is no more than approximately twenty percent (20%) larger than the minor diameter so that the increase in stress caused by the non-circular shape will remain negligible but the sensitivity to pressure will be nearly increased proportionally to the increased size of the major diameter of the outer electrode. Accordingly, this configuration allows a more sensitive sensor assembly to be packaged in a smaller diameter housing. 
         [0022]    The substrate assembly preferably is fabricated by printing the CP  108 , CR  110  and guard  120  electrodes and the CR  118  and CP  114  terminals onto the substrate  106 , drying and firing them, printing the sealing layer  112 , drying the seal to drive off any volatile organic binders to leave a material that is approximately the consistency and hardness of chalk. 
         [0023]    Referring again to  FIGS. 1-3 , the diaphragm assembly  104  is shown comprising a diaphragm  120 , a ground electrode  122  having a terminal  126  connected thereto and a sealing layer  124 . A shield electrode which is on the bottom of the diaphragm assembly is not shown. 
         [0024]    In a preferred embodiment, the ground  122  and shield (not shown) electrodes and their terminals are fabricated by printing them onto the diaphragm  120  so that the electrodes are centered on the diaphragm  120  and correspond to the outer perimeter of the CR electrode  110 . Thereafter, the electrodes are dried then fired. Once the electrodes have cooled, the sealing layer  124  is printed and dried to drive off any volatile organic binders to leave a material that is approximately the consistency and hardness of chalk. 
         [0025]    The sense element  100  is completed when the substrate  102  and diaphragm  104  assemblies are fused together by aligning them along alignment notches  130  and  136  and then firing them so that the sealing layers  112  and  124  melt and fuse the assemblies  102  and  104  together. In a preferred method of the present invention, either the sealing layer  112  on the substrate assembly  102  or the sealing layer  124  on the diaphragm assembly  104  is fired prior to joining the assemblies together. The firing is done to partial completion so that the sealing layer will still have a coarse texture. After the sealing layer has cooled, the two assemblies  102  and  104  are then joined together by firing. This reduces the amount of outgassing during the joining process and it produces a stronger and less porous bond, reducing the chance of leakage in the final product. 
         [0026]    In a preferred embodiment, the sealing layer  112  is non-symmetrical due to the elliptical shapes of the electrodes  108  and  110 . Since the sealing layers  112  and  124  provide support for the both the diaphragm  120  and the substrate  106  during the firing process and control the final gap between the electrodes, the sealing layers  112  and  124  must be held as constant and uniform as possible. To overcome the lack of symmetry, the sealing layers  112  and  124  are fabricated so that they have gaps  140  and  142  to balance out each sealing area so as to improve slumping when the layers of the sensor assembly  100  are fused together. This way, the centroid of each of the sealing layers  112  and  124  are kept as close as possible to the geometric center of the diaphragm  120 . 
         [0027]    Another feature that is offered is the placement of an additional terminal  144  on the ground electrode. This way, it does not matter on which side of the diaphragm  120  that the ground electrode  122  is placed during fabrication so long as the shield electrode (not shown) is placed on the opposite side and they are connected through via holes  146   a  or  146   b.    
         [0028]    The substrate  106  is manufactured so that it has  3  holes  150   a, b  and  c  corresponding to terminals  114 ,  118 , and  126  of the CP, CR and ground electrodes  108 ,  110  and  122 , respectively. In addition, each of the sealing layers  112  and  124  also have holes therein which correspond to terminals  114 ,  118 , and  124  of the CP, CR and ground electrodes. Once the assemblies are aligned to be fused together, the holes in the substrate  106  and in the sealing layers  112  and  124  are also aligned and filled with conductive epoxy to provide the necessary conduit for the electrical connections. 
         [0029]    IT will be understood that in the present invention, the sensitivity of the sense element  100  will increase without having to increase the overall size of the housing. 
         [0030]    Those skilled in the art will understand that this type of sensor can be used in the automotive, airplane, heating, ventilating, and air conditioning systems (HVAC) industries, among other applications. 
         [0031]    The embodiments described above are exemplary embodiments of the present invention. Those skilled in the art may now make numerous uses of, and departures from, the above-described embodiments without departing from the inventive concepts disclosed herein. Thus, the construction of the embodiments disclosed herein is not a limitation of the invention. Accordingly, the present invention is to be defined solely by the scope of the following claims.