Abstract:
A media isolated pressure sensor is disclosed that helps improve performance and reduce cost. In one illustrative embodiment, a pressure sensor may include a carrier having one or more holes defined therein. The carrier may be coupled to a diaphragm on one side, and a pressure sensing die on the other, with the pressure sensing die in fluid communication with the hole in the carrier. The carrier, diaphragm and pressure sense die may form a transfer fluid cavity, which is filled with a pressure transfer fluid. An input pressure from a media to be sensed may be provided to the diaphragm, which transmits the pressure to the pressure sensing die via the pressure transfer fluid. The pressure sensing die remains isolated from the media to be sensed.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates generally to pressure sensors, and more particularly, to media isolated pressure sensors that utilize a pressure transfer fluid between a pressure sense element and a media to be sensed. 
       BACKGROUND 
       [0002]    Pressure sensors are used in a wide variety of applications including, for example, commercial, automotive, aerospace, industrial, and medical applications. Pressure sensors often use a pressure sense element that is configured to detect a pressure of a media to be sensed by converting mechanical stress caused by the incoming pressure of the media into an electrical output signal. In some applications, it may be desirable to isolate the pressure sense element from the media. This is sometimes accomplished by providing a pressure transfer fluid (such as oil) between the pressure sense element and a diaphragm. Typically, pressure from the media is applied to the diaphragm, which is then transferred through the pressure transfer fluid and ultimately to the pressure sense element. While such configurations may help isolate and protect the pressure sense element from the media, they can be expensive to build, and they can have relatively high offset variation with temperature, often caused by relatively high aspect ratios (height to width, traditionally ranging from 1:1 to 1:10) of the cavity holding the pressure transfer fluid, which must be calibrated out to produce accurate results. 
       SUMMARY 
       [0003]    The present disclosure relates generally to pressure sensors, and more particularly, to media isolated pressure sensors that utilize a pressure transfer fluid between a pressure sense element and a media to be sensed. The media to be sensed may be a liquid or gas, depending on the application. 
         [0004]    In one illustrative embodiment, a pressure sensor may include a housing that defines a fluid passageway. A diaphragm may be positioned to seal one end of the fluid passageway, such that a media entering the other end of the fluid passageway may engage and apply pressure to the diaphragm. The diaphragm may include metal, plastic, ceramic, or any other suitable material. A carrier may be secured to the diaphragm opposite the housing. In some instances, the carrier may have a side wall that extends out from the carrier to help form a transfer fluid cavity between the diaphragm and the carrier. 
         [0005]    The carrier may include an opening extending through the carrier. A pressure sensing die may be mounted to the carrier. A pressure sensing diaphragm of the pressure sensing die may be in fluid communication with the opening in the carrier, and thus in fluid communication with the transfer fluid cavity. When so provided, the transfer fluid cavity may be formed, at least in part, by the diaphragm, the carrier and the pressure sensing diaphragm of the pressure sensing die. A pressure transfer fluid may be disposed within the transfer fluid cavity. 
         [0006]    In some instances, a substrate such as a printed circuit board may be secured to the carrier, adjacent to the pressure sensing die. Wire bonds may be used to electrically connect one or more bond pads of the pressure sensing die to one or more bond pads of the printed circuit board. Signal conditioning circuitry may be provided on the printed circuit board to receive a signal from the pressure sensing die and condition the signal to provide a conditioned output signal from the pressure sensor. 
         [0007]    The preceding summary is provided to facilitate a general understanding of some of the innovative features of the present disclosure, and is not intended to be a full description. A full appreciation of the disclosure can be gained by taking the entire specification, claims, drawings, and abstract as a whole. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0008]    The disclosure may be more completely understood in consideration of the following description of various illustrative embodiments in connection with the accompanying drawing, in which: 
           [0009]      FIG. 1  is a schematic cross-sectional view of an illustrative pressure sensor. 
       
    
    
       [0010]    While the disclosure is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawing and will be described in detail. It should be understood, however, that the intention is not to limit the disclosure to the particular illustrative embodiments described herein. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the disclosure. 
       DESCRIPTION 
       [0011]    The following description should be read with reference to the drawing. The description and the drawing, which is not necessarily to scale, depicts an illustrative embodiment and is not intended to limit the scope of the disclosure. The illustrative embodiment depicted is intended only as exemplary. 
         [0012]    As used herein, the term “fluid” is not intended to be limited to a liquid. Rather, the term “fluid” is intended to include any material subject to flow, such as, but not limited to, liquids and/or gases. 
         [0013]      FIG. 1  is a schematic cross-sectional view of an illustrative media isolated pressure sensor  10 . In the illustrated embodiment, the pressure sensor  10  may include a housing  12 , which is configured to provide a fluid passageway  14  that transmits a media (fluid or gas) from a customer application into the pressure sensor  10 . The housing  12  may be formed from plastic, polyamide, ceramic, metal, or any other suitable material. As shown in  FIG. 1 , the housing  12  may define a fluid passageway  14  that extends from a first end  16  to a second end  19 . The first end  16  may receive a media to be sensed, and may deliver the media to the second end  19 . 
         [0014]    In some embodiments, the second end  19  of the fluid passageway  14  is wider than the first end  16 , although this is not required. In some cases, a transition  18  or base may be provided, as shown in  FIG. 1 . In some instances, the housing  12  may include an elongated structure  20  extending up from the transition  18  or base, which may define at least part of the elongated fluid passageway  14 . In some cases, the elongated structure  20  may include a generally tubular member extending away from the transition  18  or base, but this is just one example. It is contemplated that the opening at the first end  16  of the fluid passageway may include one of a variety of interface options and/or connections, depending on the application. It is also contemplated that the elongated structure  20  may include an elbow, or may take on any other desired configuration or shape, depending on the application. In other embodiments, the elongated structure  20  may include a threaded region for threadably engaging another threaded component. In some embodiments, the elongated structure  20  may not be present at all, leaving only the transition  18  or base. It is contemplated that the fluid passageway  14  may be of any shape or size desired according to the user&#39;s needs. 
         [0015]    In some instances, the second end  19  of the fluid passageway of the housing may include one or more sidewalls extending downward from the transition  18  or base, as shown in  FIG. 1 . When so provided, this may help provide a space between the transition  18  or base and a diaphragm  22 . 
         [0016]    The diaphragm  22  may have a first side  21  and a second side  23 . In some instances, the first side  21  of the diaphragm  22  may be attached to the housing  12 , and may seal the second end  19  of the fluid passageway  14 . In some instances, the diaphragm  22  may be a thin, solid material configured to provide media isolation between the media to be sensed and a pressure sensing die  44 . In some instances, the diaphragm  22  may be attached such that the first side  21  of the diaphragm is bonded to the housing  12 , and the second side  23  of the diaphragm faces away from the housing  12 . It is contemplated that the diaphragm  22  may be bonded to the housing  12  using welding, an adhesive, a seal, a gasket, or any other suitable bonding or sealing mechanism (e.g. solder, eutectic, etc.). 
         [0017]    The illustrated pressure sensor  10  may further include a carrier  26 , which in some cases, may be attached to the second side  23  of the diaphragm  22 . It is contemplated that the carrier  26  may be formed from any suitable material, such as, but not limited to, ceramic, metal, plastic, etc. The carrier  26  may include a first side surface  32  and a second side surface  34 . In some embodiments, the carrier  26  may include one or more side walls  36  that extend upward from the first side surface  32 , as shown in  FIG. 1 . The side walls  36 , in combination with the first side surface  32  of the carrier  26  and the second side surface  23  of the diaphragm  22 , may form at least part of a fluid transfer cavity  38 , which as further described below, can receive a pressure transfer fluid  2 . In some embodiments, the pressure transfer fluid  24  may be an oil, a gel, or any other suitable pressure transfer fluid, as desired. In some embodiments, the oil may be a silicone based oil. In some instances, the pressure transfer fluid  24  may be an incompressible fluid, sometime with a relatively low temperature expansion coefficient. 
         [0018]    In some embodiments, the carrier  26  may include a first opening  28  extending from the first side surface  32  to the second side surface  34 . A pressure sensing die  44  may be mounted to the carrier  26  adjacent to the first opening  28 . In some cases, the pressure sensing die  44  may be mounted to the carrier  26  using an adhesive such as a silicone, RTV (room temperature vulcanizing), a silicone-epoxy, a soft epoxy, or a regular or hard epoxy. In some cases, the adhesive may have a thickness suitable for providing mechanical stress isolation between the pressure sensing die  44  and the carrier  26 , such that the pressure sensing die  44  is effectively unconstrained relative to the carrier  26 . In some cases, the thickness of adhesive may be thick enough for adequate adherence of pressure sense die  44  to carrier  26 , but not so thick so as to interfere with the bonding or diaphragm of pressure sense die  44 . In other cases, the pressure sensing die  44  may be mounted to the carrier  26  using other suitable bonding mechanisms, such as solder, eutectic, etc. 
         [0019]    In some cases, the pressure sensing die  44  may be mounted over the first opening  28  in carrier  26  as shown, and may be sized to expose the back side of the pressure sensing diaphragm  46  to the second side  34  of the carrier  26  and/or the second side  23  of the diaphragm  22 . In some instances, the pressure sensing die  44  may include a front side  51  and a back side  53 . The back side  53  may include an etched cavity that defines at least part of the pressure sensing diaphragm  46 , which extends along the front side  51  of the pressure sensing die  44 . The pressure sensing diaphragm  46  may include one or more piezoresistive elements (not explicitly shown). The front side  51  of the pressure sensing die  44  may also include one or more bond pads, and the back side  53  of the pressure sensing die  44  may face the second side  34  of the carrier  26 . In this instance, a pressure applied to the fluid passageway  14  of the housing via a media to be sensed may be transmitted to the back side  53  of pressure sensing diaphragm  46  via the diaphragm  22 , the fluid transfer fluid  24 , and the first opening  28  in the carrier  26 . However, the pressure sensing diaphragm  46  may remain isolated from the media to be sensed. 
         [0020]    In some embodiments, the pressure sensing die  44  may be a micromechanical sensor element fabricated using a silicon wafer and suitable fabrication techniques. The pressure sensing die  44  may have one or more pressure sensing elements and/or other circuitry (e.g. trim circuitry, signal conditioning circuitry, etc.) formed using any suitable fabrication or printing techniques. In some cases, the pressure sensing die  44  may include a pressure sensing diaphragm  46  including one or more sensing elements, such as piezoresistive sensing components, formed thereon for sensing a deflection and thus a pressure differential between a top  51  and bottom  53  side of the pressure sensing diaphragm  46 . As indicated above, in some cases, the pressure sensing diaphragm  46  may be fabricated by back-side etching a silicon die, however, it is contemplated that any other suitable process may be used, as desired. 
         [0021]    When provided, the piezoresistive components may be configured to have an electrical resistance that varies according to an applied mechanical stress (e.g. pressure sensing diaphragm  46  deflection). In some cases, the piezoresistive components may include a silicon piezoresistive material, however, other non-silicon materials may be used. In some cases, the piezoresistive components may be connected in a Wheatstone bridge configuration (full or half bridge). It is to be understood that the piezoresistive components are only one example of a pressure sensing element that can be used, and it is contemplated that any other suitable sensing elements may be used, as desired. 
         [0022]    In some embodiments, the carrier  26  may include a second, or auxiliary, opening  30  extending from the first side surface  32  to the second side surface  34 . The second opening  30  may be configured such that the transfer fluid cavity  38  may be filled with a pressure transfer fluid  24  in a vacuum once the pressure sensing die  44  has been mounted to seal the first opening  28 . Prior to filling the transfer fluid cavity  38 , the transfer fluid cavity formed by the diaphragm  22 , carrier  26  and pressure sensing die  44  may be cleaned using any suitable method to remove any residual debris or other unwanted material or residue. Once the transfer fluid cavity  38  has been filled, and in some embodiments, the second opening may be sealed with a swedged (and possibly welded) metal ball  40 . It is contemplated that in some embodiments, the sealing mechanism may be a ceramic ball. However, it is contemplated that other suitable technique of sealing the second opening  30  may be used as desired, such as, but not limited to, adhesives, solder, eutectic, etc. 
         [0023]    The illustrative design of the pressure sensor  10  may allow for the height of the pressure transfer fluid  24  to be relatively small. As a result, the aspect ratio (height to width in  FIG. 1 ) of the pressure transfer fluid may help to reduce the effect of temperature expansion of the pressure transfer fluid  24  on the output response of the pressure sensor  10 . As used herein, the aspect ratio is generally defined as the distance (height) of the pressure fluid  24  between the second side  23  of the diaphragm  22  and the first side surface  32  of the carrier  26  to the width of the pressure fluid  24  between each of the side walls  36  of the carrier  26 . It is contemplated that the aspect ratio of the pressure sensor  10  may be less than a 1:10. For example, the aspect ratio of the pressure sensor  10  may be 1:20 or lower without negatively affecting the overall size of the pressure sensor  10 . 
         [0024]    With the transfer fluid cavity  38  has been filled with the pressure transfer fluid  24  and sealed, a printed circuit board (PCB) or other substrate  42  may be mounted on the second side surface  34  of the carrier  26 , if desired. As shown in  FIG. 1 , the placement of substrate  42  may allow the substrate  42  to be electrically connected to the pressure sensing die  44  via, for example, direct wire bonds  50 , but this is not required. In some embodiments, the substrate  42  may include one or more bond pads, and the pressure sense die  44  may include one or more bond pads. Wire bonds may be used to electrically connect one or more of the bond pads of the pressure sense die  44  and one or more bond pads of the substrate  42 . It is further contemplated that in some embodiments, the substrate  42  may include circuitry for compensation of bridge signals from the pressure sense die  44 . However, in other embodiments, the substrate may not include circuitry for compentation. Also, in some instances, the pressure sensor  10  may include one or more terminals  54  for connecting the substrate  42  to additional boards, interconnect, and/or other components as desired. In some instances, the terminals  54  may be surface mount terminals. However, it is contemplated the terminals  54  may be any type of terminal desired for a particular applications. 
         [0025]    In operation, a media to be sensed can be provided to the first end  16  of the fluid passageway  14 . The media to be sensed may have a pressure, which is transmitted through the media to the diaphragm  22 . The applied pressure deforms the diaphragm  22 , and transmits the pressure to the back side  51  of the pressure sensing die  44  via the pressure transfer fluid  24 . A pressure difference between the pressure of the media to be sensed and atmosphere causes a deflection of the pressure sensing diaphragm  46  of the pressure sense die  44 , which then stresses one or more piezoresistive elements on the pressure sensing diaphragm  46 . Applying a current through the piezoresistive elements may provide a signal that corresponds to the applied pressure in the media to be sensed. In some cases, the resulting signal may be conditioned by conditioning circuitry  48  and output via electrical leads (not explicitly shown). 
         [0026]    As discussed above, in some instances, the pressure sensor  10  may include optional signal conditioning circuitry  48  mounted on the substrate  42 . In some cases, the signal conditioning circuitry may include an ASIC (Application Specific Integrated Circuit) or other electronics. In some cases, the optional signal conditioning circuitry  48  may include amplification, analog-to-digital conversion, offset compensation circuitry, and/or other suitable signal conditioning electronics. When so provided, the signal conditioning circuitry  48  may receive a signal from the pressure sensing die  44 , and condition and/or process the signal for transmission from the pressure sensor  10 . While an ASIC die is shown, it is contemplated that signal conditioning circuitry  48  may include any suitable signal conditioning circuitry including any suitable microprocessor or microcontroller, discrete components, or any other suitable circuitry, as desired. 
         [0027]    In some cases, signal conditioning circuitry  48  may be mounted to the substrate  42  using an adhesive or any other suitable bonding mechanism (e.g. solder, eutectic, etc.). In some embodiments, the signal conditioning circuitry  48  may be fabricated directly on the substrate  42 . As shown, signal conditioning circuitry  48  may be secured to the substrate  42  adjacent to the pressure sense die  44 , and may be electrically connected to the substrate  42  via direct wire bonds  52 , but this is not required. In some embodiments, the signal conditioning circuit  48  may be electrically connected to pressure sensing die  44  via direct die-to-die wire bonds. Trace conductors may be connected to connectors, leads or terminals of the pressure sensor  10 , when desired. 
         [0028]    Those skilled in the art will recognize that the present disclosure may be manifested in a variety of forms other than the specific embodiments described and contemplated herein. Accordingly, departure in form and detail may be made without departing from the scope and spirit of the present disclosure.