Abstract:
A displacement transducer includes a load cell structure having a thick outer peripheral area, a thick inner central area and two symmetrical thin beams. The two beams are disposed along a common diameter of the structure and joins the outer peripheral area and the inner central area. At least one strain gauge is placed on a surface of one beam and at least one strain gauge is placed on a surface of the second beam. A top diaphragm cover member is secured to a top surface of the outer peripheral area and covers the two beams.

Description:
CLAIM FOR PRIORITY 
   This application is a continuation of application Ser. No. 11/322,721 filed Dec. 30, 2005, now U.S. Pat. No. 7,284,444 the entire disclosure of which is hereby incorporated by reference as if being set forth in its entirety herein 

   FIELD OF THE INVENTION 
   The present invention relates generally to displacement sensors and more particularly to a hermetically sealed displacement sensor using piezoresistors. 
   BACKGROUND OF THE INVENTION 
   Semiconductor piezoresistive transducers and displacement sensors have been widely known and are used in a great variety of applications including those applications having extremely harsh environments. Such devices may employ semiconductor or metal strain gages depending on the environment and application. For such applications, sensors have to be protected from the environment. For many applications the sensors must be contained within a cavity (usually a vacuum) to protect the strain gages. This vacuum cavity is hermetically sealed to maintain the vacuum and protect the sensing elements. Such protected sensors may be employed in displacement pressure sensors and used in many applications including, for example, medical, automotive, and aerospace applications. 
   Techniques for hermetically sealing semiconductor piezoresistors from hostile environments have generally limited the size of such devices. The reason for this is that additional lateral space is required to accommodate the hermetically sealing cover member. The piezoresistive transducer employs silicon resistive elements whose resistance varies according to intensity or magnitude of applied displacement upon an associated diaphragm. These resistors must be hermetically isolated from the external environment to ensure proper sensing performance and avoid destruction in harsh conditions. 
   For an example of hermetically sealed environmentally protected transducers, reference is made to U.S. Pat. No. 5,002,901 entitled “Method of Making Integral Transducer Structures Employing High Conductivity Surface Features”, issued on Mar. 26, 1991 to A. D. Kurtz, et al and assigned to the assignee herein. In this patent the piezoresistive elements are formed over the central region of a dielectric layer which overlays a silicon diaphragm. The elements are arranged to form a Wheatstone bridge where four circuit nodes of the bridge are configured as four p+ silicon electrical contact posts disposed on the peripheral corners of the device. Electrical interconnections also comprised of p+ silicon interconnect the contact posts with the piezoresistive transducer elements. A bias voltage is brought to the two contacts where the voltage is measured between the other two contacts. In this manner the hermetic seal for this device is provided by fabricating the peripheral flange on the device&#39;s outer periphery beyond the contact posts, and an absolute cavity can be made which provides a vacuum reference. A glass sheet cover is then bonded to the top of the flange to create the hermetic seal. U.S. Pat. No. 5,891,751 entitled “Hermetically Sealed Transducers and Methods for Producing the Same”, issued on Apr. 6, 1999 to A. D. Kurtz, et al and assigned to Kulite Semiconductor Products, Inc., the assignee herein, teaches a hermetically sealed semiconductor transducer and methods for fabricating the same. In this patent a sealing member hermetically seals an aperture whereby a vacuum is maintained between the transducer element and the cover member. The transducer element is hermetically sealed from the external environment while at least a portion of the electrical contact remains exposed to enable subsequent wire bonding thereto. 
   Reference is also made to U.S. Pat. No. 5,461,001 entitled “Methods for Making Semiconductor Structures Having Environmentally Isolated Elements”, issued on Oct. 24, 1995 to A. D. Kurtz, et al and assigned to the assignee herein. This patent shows a method of fabricating semiconductor structures where one can provide a great number of hermetically sealed individual circuit devices using the methods described in the above noted patent. Reference is also made to U.S. Pat. No. 6,229,427 entitled “Covered Sealed Pressure Transducers and Methods for Making the Same”, issued on May 8, 2001 to A. D. Kurtz, et al and assigned to the assignee herein. That patent shows a method which can be utilized to hermetically seal a raised feature of a sensing network of a silicon on oxide pressure transducer. The invention described in the &#39;427 patent can also be utilized to hermetically seal the depressed feature sensing network of a diffused pressure transducer. 
   U.S. Pat. No. 5,286,671 entitled “Fusion Bonding Technique for Use in Fabricating Semi Conductor Devices”, issued to A. D. Kurtz, et al on February 1994, relates to silicon oxide pressure transducers and methods for fabricating and bonding to such devices. 
   In view of the above it is extremely desirable to produce a hermetically sealed displacement sensor, which is easy to fabricate and which provides improved operation over prior art devices. 
   SUMMARY OF THE INVENTION 
   A hermetically sealed displacement transducer, comprising a load cell structure having a top surface and a bottom surface and having a thick outer peripheral area and a thick inner concentric central area with said outer peripheral area and said inner central area joined by two symmetrical thin beams directed along a common diameter with a first beam directed from an inner edge of said peripheral area to an outer edge of said central area, with a second beam directed along said diameter from an oppositely opposed inner edge of said peripheral area to an oppositely opposed outer edge of said inner area, said beams below the top surfaces of said peripheral and central areas and above the bottom surfaces thereof, at least a first strain gauge positioned on a surface of one beam, and at least a second strain gauge positioned on a corresponding surface of said second beam, a bottom cover member secured to said bottom surface of said peripheral area to cover and enclose said beams and said strain gauges, a top diaphragm cover member secured to said top surface of said peripheral area to cover and enclose said beams, with said cover member and said diaphragm member forming a hermetic cavity for said beams and strain gauges. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
     Understanding of the present invention will be facilitated by consideration of the following detailed description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which like numerals refer to like parts and in which: 
       FIG. 1  is a top view of a displacement sensor beam according to this invention; 
       FIG. 2  is a cross sectional view taken through line  2 - 2  of the load cell shown in  FIG. 1 .  FIG. 2  includes an isolation diaphragm and a bottom cover member; 
       FIG. 3  is a top plan view of a convoluted isolation diaphragm shown in this invention; 
       FIG. 4  is a sectional view showing the shape of a particular convolution; 
       FIG. 5  is a detailed view depicting the area shown in  FIG. 2  within a dashed circle; and, 
       FIG. 6  is an enlarged sectional view depicting the area shown within the dashed circle of  FIG. 2  designated as  FIG. 6 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   It is to be understood that the figures and descriptions of the present invention have been simplified to illustrate elements that are relevant for a clear understanding of the present invention, while eliminating, for purposes of clarity, many other elements found in typical displacement sensor systems and methods. However, because such elements are well known in the art, and because they do not facilitate a better understanding of the present invention, a discussion of such elements is not provided herein. The disclosure herein is directed to all such variations and modifications known to those skilled in the art. 
   Referring to  FIG. 1  there is shown a top view of a load cell beam arrangement  10 . According to an embodiment of the present invention, the load cell and beams are integrally formed from a suitable metal.  FIG. 2  depicts a cross sectional view of the load beam cell configuration of  FIG. 1  taken through line  2 - 2  of  FIG. 1 . The load beam cell of  FIG. 1  is shown in a top view and essentially is circular in configuration, but other geometrical configurations can be employed as well, including but not limited to rectangular, oval, square, and/or other geometries. As one can ascertain from  FIG. 1 , the load cell  10  has a thickened outer peripheral area  35  which is depicted in  FIG. 2  as well. The outer area  35  is integral with two beams  11  and  12 . While two beams  11  and  12  are shown along the common diameter  26 , it is understood that additional beam pairs may be disposed along common diameters of the load cell. For example, two beams may be placed along a diameter which is transverse to diameter  26 . These beams do not necessarily require strain gages, but are formed with the load cell to prevent twisting and/or sideloading. The beams  11  and  12  as shown in  FIG. 2  are thinned areas capable of deflecting. Both beams  11  and  12  are generally triangular in shape and extend from a central hub  19 , which central hub  19  is common to both beams  11  and  12  and each beam  11 ,  12  is directed along a common diameter  26 . The beams  11  and  12  as seen extend from the central hub  19  along a common diameter  26  and gradually increase in size as they meet with the circular edge  40  of the load beam cell  10 . The beams  11  and  12  are generally triangular having the base of the triangle at the edge  40  of the inner surface of area  35 , and with the truncated apex integral with the outer periphery of hub  19 . Thus, the corresponding beams  11  and  12  with the common central hub  19  are of a bow-tie configuration. However, other configurations may be used as well. 
   Each beam  11 ,  12  has located thereon piezoresistive sensors as  14  and  15  associated with beam  11  and  16  and  17  associated with beam  12 . Each of the piezoresistive sensors  14 - 17  is arranged so that one is positioned in a longitudinal direction and the other is in a transverse direction with respect to diameter  26 . Piezoresistive sensors  14 - 17  are silicon devices and are well known as being fabricated by many different techniques. In any event, each of the piezoresistive silicon devices  14 - 17  has leads such as lead  23  associated with device  16 . The leads  23  are directed through a common channel  20  which is directed from the periphery of the inner circle  40  to the outside of the load cell  10 . This channel accommodates wires from each of the piezoresistive sensors. Typically the piezoresistive sensors  14 - 17  are connected to form a full bridge such as a Wheatstone bridge whereby the output of the bridge would be proportional to a stress supplied to the beams  11  and  12 . As seen in  FIG. 2 , each beam such as  11  and  12  is thin compared to the thicker outer peripheral member  35 . The beams  11  and  12  are integrally formed therewith and are much thinner than the outer peripheral area  35 . In a typical example, the outer peripheral area  35  may be about 0.15 inches thick with the beams being about 0.04 inches thick. The diameter of the cell  10  shown in  FIG. 1 , which is the outer diameter, is typically about three (3) inches. The beams  11  and  12  as provided are two constant moment beams joined at the center via this central hub  19 . As seen the central hub  19  has an outer area  18  which is depressed or located below the inner area  19 . This depression is approximately 0.002 inches in height and is a recess to accommodate a top convoluted diaphragm cover. Peripheral region  35  also has apertures such as  25  located about the periphery, which apertures serve as mounting apertures to enable the entire load cell  10  to be mounted on a surface. Therefore, a pressure can be applied to the cell  10  and which pressure would be directed to the thin beam sections  11  and  12  of the load cell  10 ( FIG. 1 ). 
   Referring to the cross sectional view of  FIG. 2 , it is seen that the piezoresistor sensors  14 ,  15 ,  16  and  17  are enclosed within a hermetically sealed cavity  38 . The cavity  38  is formed by placing a cover member  30  over the bottom surface of the cell  10  shown in  FIG. 1 . The cover member  30  is attached (e.g. welded) to the outer concentric peripheral area  35  of the cell. As seen in  FIG. 2 , there is a step depression  37  whereby the outer periphery of the cover member  30  is thinner and fits within the peripheral depression  37  formed in the outer thick concentric portion  35 . A top cover  31  contains a series of convolutions ( 42   a ,  42   b , . . .  42   k ) and essentially acts as an isolation diaphragm for the sensing device. The convoluted isolation diaphragm deflects for forces applied thereto are transmitted to the triangular beams  11  and  19  and are responded to by the hermetically sealed sensors. The cover member  30  is spaced apart from the central hub  19  by a space  36  to enable the diaphragm to deflect upon application of a force thereto. The force is applied essentially at the center whereby the triangular shaped thin beams act as two constant moment beams which are joined at the center portion. 
     FIG. 3  depicts the top isolation diaphragm  31  showing the particular shape of the convolutions in  FIG. 4 . As seen in  FIG. 4 , the convoluted isolation diaphragm  41  has a central aperture  40  which central aperture surrounds the raised impression  19  (shown in  FIG. 1 ). The rim or concentric area  43  sits on top of the depressed area  18  allowing the isolation diaphragm  31  to be positioned as shown with a predetermined space  36  from the thin triangular beam sections  11  and  12 . The convolution  42  is shown in detail in  FIG. 4  and is round with the top facing the beams  11  and  12 . 
     FIGS. 5 and 6  show the encircled sections of  FIG. 2  in greater detail. From  FIG. 5  and  FIG. 6 , one can visualize the relationship of the convoluted isolation diaphragm  31  and the bottom sealing cover  30  including the spacing  36  between the bottom cover member  30  and the central hub portion  19  of the cell.  FIG. 6  again depicts the encircled area of  FIG. 2  designated by the dashed circle labeled  FIG. 6  and also shows the beam  12  which is integrally formed with the thick outer concentric peripheral area  35  showing the cover member  30  which is welded to the area  35  and showing the convoluted isolation diaphragm  31  in greater detail. The dimensions of course can vary for various structures but consistent with the above noted dimensions and referring to  FIG. 5  it is indicated that the radius  43  depicted is typically 0.01 inches with the thickness of the convoluted cover member  31  being 0.002 inches and with the spacing  36  from the bottom of the member  19  to the outside of the cover  30  being 0.04 inches. In  FIG. 6 , the thickness of the bottom cover  30 , which hermetically seals the sensors, is 0.015 inches. As previously indicated the thickness of the outer concentric area  35  is 0.14 inches while the thickness of the beams as  11  and  12  are about 0.02 inches. Thus, the device  10  has strain gauges or piezoresistors  14  to  17  placed on the flexible thin beams  11  and  12 . The gauges are in a hermetically sealed cavity  38  which is formed by a top cover diaphragm member  31  and a bottom cover  30 . The sensors  14 - 17  are placed on the beams  11  and  12  by use of an apoxy or bonding agent. The beams  11  and  12  are two constant moment beams joined at the center. The thin beams  11  and  12  provide a compliant member to allow displacement of the sensor. The displacement produces a strain in the thin beams  11  and  12  and by measuring the strain, one obtains an electrical output proportional to the displacements. The electrical output from the device is proportional to the deflection of the center of the sensor. While typical dimensions were given above for a typical sensor it is understood that dimensions of the beams as well as thickness and diameters are selected to give the required displacement with minimal force imparted on the measured device. 
   One skilled in the art will understand how to formulate such dimensions depending upon the application. It is therefore apparent that there are many modifications which can be imparted by one skilled in the art. All such modifications are deemed to be encompassed in the spirit and scope of the enclosed claims. 
   It will be further apparent to those skilled in the art that modifications and variations may be made in the apparatus and process of the present invention without departing from the spirit or scope of the invention. It is intended that the present invention cover the modification and variations of this invention provided they come within the scope of the appended claims and their equivalents.