Abstract:
A micro-electromechanical pressure transducer formed from a silicon die centers itself on a pedestal, formed from either a metal or a dielectric, by applying a predetermined amount of liquid epoxy adhesive to the square, top surface of the pedestal and allowing the liquid adhesive to distribute itself over the top surface. A MEMS die placed atop the liquid adhesive is centered on the top surface by surface tension between sides of the die and the top surface.

Description:
BACKGROUND 
       [0001]    Silicon-based micro-sensors use so-called MEMS (micro-electromechanical systems) technology to achieve low cost and high performance. One such a device is a MEMS pressure sensor, which is comprises a small, thin silicon diaphragm onto which a piezoresistive circuit, normally a Wheatstone bridge, is formed. Diaphragm stresses caused by pressure applied to the diaphragm change the resistance values of the piezoresistors in the bridge circuit. An electronic circuit detects the resistance changes of the piezoresistive bridge circuit and outputs an electrical signal representative of the applied pressure. One such device is the “Differential Pressure Sensor Device” disclosed in U.S. Pat. No. 8,466,523, the content of which is incorporated herein by reference. 
         [0002]    In order to sense the pressure of a liquid or gas, both of which are fluids, the fluid&#39;s pressure needs to be applied to the silicon diaphragm. Applying a fluid&#39;s pressure to a silicon diaphragm is usually accomplished using a port or hole formed into a spacer to which the silicon die having the piezoresistors is attached. Precise alignment of the port or hole through the spacer is thus important. As micro-sensors get smaller, however, assembling them so that their structures are properly aligned with each other keeps getting more challenging. A bond or connection, formed between a MEMS silicon die and a spacer or pedestal, which will automatically align the silicon die onto the spacer or pedestal would be an improvement over the prior art. 
       BRIEF SUMMARY 
       [0003]    Embodiments of the invention are directed to a micro-electromechanical pressure transducer formed from a silicon die centers itself on a pedestal, formed from either a metal or a dielectric, by applying a predetermined amount of liquid epoxy adhesive to the square, top surface of the pedestal and allowing the liquid adhesive to distribute itself over the top surface. A MEMS die placed atop the liquid adhesive is centered on the top surface by surface tension between sides of the die and the top surface. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]      FIGS. 1A-1C  are perspective views of examples of right-rectangular prisms; 
           [0005]      FIG. 1D  is a top view of a right-rectangular prism, vertical corners of which are provided with a radius; 
           [0006]      FIG. 2  is a cross section of the curved upper surface of a column of liquid, also known as a meniscus and which is also referred to herein as a fillet; 
           [0007]      FIG. 3  is a perspective view of a MEMS pressure sensor; 
           [0008]      FIG. 4  is a top view of the sensor shown in  FIG. 3 ; 
           [0009]      FIG. 5  is a cross sectional view of the sensor shown in  FIG. 3 ; and 
           [0010]      FIG. 6  is a flow chart, showing steps of a method of attaching a MEMS die to a pedestal. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    As used herein, a right-rectangular prism is a convex polyhedron bounded by six quadrilateral faces. The quadrilateral faces can be either rectangles or squares. 
         [0012]      FIG. 1A  is a perspective view of one embodiment of a right-rectangular prism  100 - 1 , the six faces of which are squares. The prism  100 - 1  has a top surface  102 - 1 , an opposing bottom surface  104 - 1  and four side surfaces  106 - 1 ,  108 - 1 ,  110 - 1 , and  112 - 1 . Since the exterior surfaces are squares, the right-rectangular prism  100 - 1  shown in  FIG. 1A  is a cube. 
         [0013]      FIG. 1B  depicts another embodiment of a right-rectangular prism  100 - 2 . It&#39;s top and bottom surfaces  102 - 2  and  104 - 2  are squares, but the side surfaces  106 - 2 ,  108 - 2 ,  110 - 2 , and  112 - 2  are rectangles. The side surface height dimension  114  is less than the side surface width dimension  116 . The height dimension  114  is also less than the depth dimension  118  of the prism  100 - 2 . 
         [0014]      FIG. 1C  is another embodiment of a right-rectangular prism  100 - 3 . It too has a top surface  102 - 3 , an opposing bottom surface  104 - 3 , but the top and bottom surfaces are rectangles. Unlike the prisms  100 - 1  and  100 - 2  shown in  FIG. 1A and 1  B, however, the right-rectangular prism  100 - 3  shown in  FIG. 1C  is taller than its length or width. Its side surface height dimension  120  is greater than its side surface width  124  and its depth  122 . 
         [0015]      FIG. 1D  shows the top surface  102 - 4  of another embodiment of a right-rectangular prism  100 - 4 . In  FIG. 1D , the vertically-oriented corners  150  are rounded over and have a radius of curvature R. 
         [0016]      FIG. 2  is a cross-sectional view of a typical fillet  200 , which is a curved upper surface of a volume of liquid  201  formed when the volume or liquid  201  contacts two surfaces  204  and  206 . The fillet  200  has a vertical height  208  and a horizontal width  210 . The height  208  and the width  210  depend on the viscosity of the liquid  201 , its surface tension, as well as the surface roughness and wetability of the two surfaces  204  and  206 . 
         [0017]      FIG. 3  is a perspective view of a pressure sensor  300 .  FIG. 4  is a top view of the pressure sensor  300 .  FIG. 5  is a cross-sectional view of the sensor  300 . 
         [0018]    The sensor  300  comprises a micro-electro-mechanical system (MEMS) pressure sensor semiconductor die  301 , referred to hereafter as a MEMS sensor die  301 , the shape of which is essentially a right-rectangular prism. The MEMS sensor die  301  has a top surface  302 , an opposing bottom surface  304 , and four vertical side surfaces  306 ,  308 ,  310 , and  312 . 
         [0019]    The MEMS sensor die  301  is attached to a comparatively thin pedestal  314  by way of a die bond  330 , which will automatically center the MEMS sensor die  301  on the pedestal  314 . The shape of the pedestal  314  is also a right-rectangular prism, however, the corners  316  on its substantially planar top surface  318  are preferably rounded as explained below. The pedestal  314  has four, substantially vertical side surfaces  320 ,  322 ,  324 , and  326 . 
         [0020]    The pedestal  314  is preferably a dielectric but it can also be formed of a metal. In the preferred embodiment, however, the pedestal is a plastic and is formed by molding a trench around and into the material that forms a substrate for the pressure sensor. In a preferred embodiment, the pedestal  314  has a vertical height of about 0.1 millimeters. In alternate embodiments, however, the height of the pedestal  314  is increased to provide additional stress isolation of the MEMS die from a substrate on which the pedestal is attached or from which the pedestal is formed. The pedestal is thus considered to be a stress isolator for the MEMS die. 
         [0021]    The pedestal  314  is formed with a through hole  328 , which is centered in the top surface  318  and centered in the bottom surface  319  of the pedestal. The hole  328  extends completely through the top surface  318  and the bottom surface  319  and thus provides a passage way through the pedestal, through which fluids (liquids and gases) can readily pass and exert force on a diaphragm formed in the top surface  302  of the MEMS sensor die  301 . 
         [0022]    The top surface  318  of the pedestal  314  “faces” the bottom surface  304  of the MEMS sensor die  301  but the two surfaces  318  and  304  do not engage or contact each other. The MEMS sensor die  301  is embedded in an epoxy adhesive and its bottom surface  304  is separated from the top surface  318  of the pedestal  314  by a short distance that is equal to the thickness of the die bonding adhesive layer, also known as a die bond layer  330 , which is located between them. 
         [0023]    In order for the MEMS die  301  to automatically align itself with the center of the top surface  318  of the pedestal  314 , the surface area “A” of the top surface  318  of the pedestal  314  should be greater than the area B of the bottom surface  304  of the MEMS sensor die  301 , as can be seen in  FIG. 4 . In a preferred embodiment, as can be seen in  FIG. 4 , each side of the MEMS die  301  is about two millimeters in length. Each side surface of the pedestal is about  0 . 65  millimeters away from the side surfaces of the MEMS sensor die  301 . The liquid epoxy having a viscosity of about 6500 centipoise was able to form uniform-size fillets on all four sides of the MEMS die  301 . It was also determined that the corners  316  of the top surface  318  of the pedestal should be rounded in order to facilitate distribution of liquid adhesive around the MEMS sensor die  301  and thereby help the formation of fillets of uniform size. 
         [0024]    Between the MEMS sensor die  301  and the pedestal  314  is the previously mentioned die bond layer  330 , best seen in  FIG. 5 , which is also referred to herein as a bonding layer  330  and die bond  330 . The die bond  330  is a layer of a hardened epoxy, also referred to herein as being a cured epoxy. In its pre-cured liquid state, the epoxy has a viscosity, which is preferably about 6500 centipoise. The pre-cured, liquid epoxy is applied to the top surface  318  of the pedestal  314 . 
         [0025]    The volume of liquid epoxy applied to the top surface  318  should be predetermined, i.e., determined before it is applied to the top surface  318  of the pedestal  314 . The volume of liquid adhesive should be just enough to be able to flow over the entire area A, of the top surface  318  but not spill into the through hole  328 . The amount of liquid epoxy that is applied will thus depend on the area A, of the top surface  318 , the roughness of the top surface  318 , and the viscosity of the particular liquid adhesive being used. A through hole  328  in the pedestal, which becomes plugged or even partially blocked with epoxy during assembly because too much epoxy was applied to the top surface  318  of the pedestal  314 , will render the pressure sensor  300  inoperative. 
         [0026]    Almost immediately after the liquid epoxy is applied to the top surface  318  of the pedestal  314 , the MEMS sensor die  301  is placed onto the liquid. When the MEMS sensor die  301  is placed on top of the liquid epoxy, it will start to sink into the liquid adhesive. As the MEMS sensor die  301  sinks, surface tension of the liquid epoxy causes the liquid to adhere to, i.e., wet, the vertical side surfaces  306 ,  308 ,  310 ,  312  of the MEMS sensor die  301  as well as the horizontal top surface  318  of the dielectric pedestal  314 . 
         [0027]    As the MEMS sensor sinks into the liquid adhesive, surface tension causes fillets  331  to form on each of the side surfaces  306 ,  308 ,  310 ,  312  of the MEMS sensor die  301  and horizontally across the top surface  318  of the pedestal  314 . If the fillets  331  formed on opposite sides of the MEMS sensor die  301  are not the same size, a larger fillet  331  on one side will exert a tensile force on its side of the die that is greater than the tensile force exerted on the opposite side of the MEMS sensor die  301 . A greater tensile force exerted by a larger fillet  331  on one side of the die  301  tends to pull the MEMS sensor die  301  in the direction of the larger fillet  331 , opening up area on the top surface  318  of the pedestal  314 , which allows more liquid adhesive to flow over the opened area, which also provides more liquid adhesive to equalize the size of the fillets  331  until the fillets equalize due to the force exerted by the fillets on the opposite sides becoming equal in magnitude but in opposite directions. Since the top surface  318  of the pedestal  314  is square and since the bottom surface  304  of the MEMS sensor die  301  is also square, liquid adhesive will eventually distribute itself evenly across the top surface  318  of the pedestal  314  to form equal-size fillets  331  against each side of the MEMS sensor die  301 . After the liquid adhesive has distributed itself evenly across the top surface  318  of the pedestal  314  and around the vertical side surfaces  306 ,  308 ,  310 , and  312  of the MEMS sensor die  301 , the equal-sized fillets  331  will exert equal magnitude but opposite-direction forces on opposing surfaces of the pedestal. Such forces will urge the MEMS sensor die  301  to the exact center of the pedestal. 
         [0028]    In a preferred embodiment, the fillets  331  are preferably symmetrical in order to reduce bonding stress exerted on the MEMS sensor die  301  by the cured adhesive, which is quite rigid. To make the fillets  331  symmetrical, the area of the top surface  318  of the pedestal  314 , the volume of the liquid adhesive applied to the top surface  318  of the pedestal  314 , its viscosity and wetability of the MEMS sensor die and pedestal surfaces are preferably selected such that, when the MEMS sensor die  301  is placed into the liquid adhesive, the fillets  331  will extend upwardly on the side surfaces  306 ,  308 ,  310 , and  312  of the MEMS sensor die  301 , by a vertical distance that is equal to the distance that the fillets extend horizontally across the top surface  318  of the pedestal  314  from the side surfaces of the MEMS sensor die  301  to the side surfaces  320 ,  322 ,  324 ,  326  of the pedestal  314 . 
         [0029]    Those of ordinary skill in the art will recognize that the pressure sensor  300  comprising the MEMS sensor die  301 , the die bond layer  330  and the pedestal  314  will usually be mounted on some other surface made of a material that differs from the pedestal material. When so mounted, the pedestal height can be increased to reduce thermally-induced stress on the MEMS sensor die. 
         [0030]    As shown in  FIG. 5 , the pedestal height  500  is relatively short, i.e., about 0.1 millimeter, as compared to the height  502  of the MEMS sensor die  301 . When the pressure sensor  300  is mounted on a plastic substrate  504 , differences in thermal expansion coefficients can cause thermally-induced stress on the pedestal  314  by the substrate  504  to be transmitted to the MEMS sensor die  301 . In alternate embodiments, the pedestal height  500  is increased as needed in order to provide more isolation of the MEMS sensor die  301  from thermally-induced stresses on the pedestal and thereafter on the MEMS sensor die  301 . 
         [0031]    In a preferred embodiment, the vertically-oriented corners formed by adjacent vertical faces of the pedestal should be rounded to insure that the radii of the fillets  331  are uniform.  FIG. 1D  is a view of the top surface  102 - 4  of a right-rectangular prism  100 - 4  having vertical corners  150  that are formed to have a radius, R.  FIG. 1D  thus depicts the top surface of a pedestal the vertically-oriented corners of which are rounded to have a radius. In a preferred embodiment, the radius of the rounded corners  316  of a pedestal  314  should be substantially equal to the horizontal width  210  of a fillet  200  of liquid adhesive. 
         [0032]      FIG. 6  is a flowchart of a method  600  of assembling a pressure sensor. In a first step  602 , a predetermined amount of a predetermined adhesive is applied to the top surface of a plastic pedestal, the amount being determined to just cover the top surface of a pedestal. In steps  604  and  606 , the method waits for a predetermined amount of time until the liquid adhesive has settled itself across the top surface of the pedestal. A MEMS sensor die is applied at step  608 , which is followed by a “wait” time in step  610 , during which the liquid adhesive cures to hardness. 
         [0033]    By properly sizing the right-rectangular prisms so that the lower prism is just slightly larger than the upper prism and by properly sizing the amount of adhesive and selecting an adhesive with the proper viscosity, a MEMS sensor applied to liquid adhesive on the top of a pedestal will cause the MEMS sensor to center itself on the pedestal&#39;s top surface. After the epoxy cures, the resultant structure can be attached to a circuit board or onto a housing after the epoxy cures. The through hole through the pedestal will be automatically centered with the through hole in the MEMS sensor by the surface tension of the liquid adhesive as exerted on the sides of the MEMS sensor. 
         [0034]    The foregoing description is for purposes of illustration only. The scope of the invention is defined by the following claims.