Abstract:
Pressure sensor methods and systems are disclosed. In general, two micromachined die and a diaphragm for a pressure sensor can be provided. The two micromachined die can be embedded in a glass adhesive on a surface of the diaphragm, such that the glass adhesive possesses a large size relative to the two micromachined die. The large size of the glass adhesive creates a large planar target for placement of the two micromachined die upon the diaphragm, thereby providing a size difference between the glass adhesive and the two micromachined die thereby creates an optimum strain transfer, while maintaining stability for the pressure sensor.

Description:
[0001]     Embodiments are generally related to sensing methods and systems. Embodiments are also related to micromachined sensing devices. Embodiments are additionally related to hermetically sealed sensing devices. Embodiments are additionally related to pressure sensors.  
       BACKGROUND OF THE INVENTION  
       [0002]     High-pressure sensors, including hermetically sealed pressure sensors, have found utility in a number of different applications. For example, high-pressure sensors are often employed in the area of automotive controls to obtain a measurement of certain pressure parameters such as engine oil pressure, transmission fluid pressure or brake pressure. High-pressure applications generally utilize an integral stainless steel housing and pressure port, which attaches to the pressure vessel by a threaded fitting, for example.  
         [0003]     There currently exists a large demand for low cost hermetic pressure sensors for automotive, industrial, and other applications. One of the problems with conventional pressure sensors, particularly those involving the use of piezoelectric components, is that such devices are expensive to produce and subject to errors based on inaccurately combined sensor components.  
         [0004]     The pressure sensor systems and methods described herein therefore overcome the aforementioned problems by providing an efficient methodology and system for creating a low cost hermetic pressure sensor, which can be efficiently fabricated at a low cost for automotive, industrial, and other applications.  
       BRIEF SUMMARY  
       [0005]     The following summary is provided to facilitate an understanding of some of the innovative features unique to the present invention and is not intended to be a full description. A full appreciation of the various aspects of the invention can be gained by taking the entire specification, claims, drawings, and abstract as a whole.  
         [0006]     It is, therefore, one aspect of the present invention to provide for improved sensor-based systems and methods.  
         [0007]     It is another aspect of the present invention to provide for improved pressure systems and methods.  
         [0008]     It is yet a further aspect of the present invention to provide for a pressure sensor system formed utilizing a single deposit glass adhesive for piezoelectric die placement thereof.  
         [0009]     It is therefore another aspect of the present invention to provide for a hermetically sealed pressure sensor.  
         [0010]     The aforementioned aspects of the invention and other objectives and advantages can now be achieved as described herein. Pressure sensor methods and systems are disclosed. In general, two micro-machined die and a diaphragm for a pressure sensor can be provided. The two micro-machined die can be embedded in a glass adhesive on a surface of the diaphragm, such that the glass adhesive possesses a large size relative to the two micro-machined die. The large size of the glass adhesive creates a large planar target for placement of the two micromachined die upon the diaphragm, thereby providing a size difference between the glass adhesive and the two micromachined die thereby creates an optimum strain transfer, while maintaining stability for the pressure sensor.  
         [0011]     In general, the embodiments disclosed herein are directed toward low cost, high yield technique and system for creating a pressure sensor. The core of the pressure sensor can be, for example, a piezoresistive micromachined silicon die that is embedded in glass on a metal diaphragm. When the diaphragm is pressurized, it flexes. This flexing creates a strain field that is transferred through the glass and read by the two silicon sensing dice.  
         [0012]     Embodiments are described in which two small piezoresistive die can be embedded in a single deposit of glass paste. The large size of the single paste deposit relative to the small size of the die creates a large planar target in which to place the die. This size difference keeps the die planar to the surface of the diaphragm, which is critical for optimum strain transfer.  
         [0013]     The planar nature of the large single deposit of glass paste allows full embedding of all of the die edges, which aids in long-term stability. The large size of the single deposit also allows the die to be registered to the port instead of to two individual small glass deposits. The improved registration of the die to the port creates a more accurate sensor.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The accompanying figures, in which like reference numerals refer to identical or functionally-similar elements throughout the separate views and which are incorporated in and form a part of the specification, further illustrate the embodiments and, together with the detailed description of the invention, serve to explain the principles of the present invention.  
         [0015]      FIG. 1  illustrates a top view of a system that includes a port comprising a port surface, in accordance with a preferred embodiment.  
         [0016]      FIG. 2  illustrates a cross sectional view of system illustrated in  FIG. 1 , in accordance with a preferred embodiment;  
         [0017]      FIG. 3  illustrates a top view of a silicon die, which can be placed upon a diaphragm associated with the port surface indicated in  FIGS. 1-2 , in accordance with a preferred embodiment;  
         [0018]      FIG. 4  illustrates an electrical implementation for a pressure sensor system in association with the components depicted in  FIG. 1-3  in accordance with a preferred embodiment;  
         [0019]      FIG. 5  illustrates a pressure sensor, which can be implemented in accordance with a preferred embodiment;  
         [0020]      FIG. 6  illustrates a Wheatstone die schematic diagram in accordance with a preferred embodiment;  
         [0021]      FIG. 7  illustrates a diagram illustrating die layout on the diaphragm in accordance with one embodiment; and  
         [0022]      FIG. 8  illustrates a diagram depicting the reaction to full-scale pressurization in accordance with one embodiment.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]     The particular values and configurations discussed in these non-limiting examples can be varied and are cited merely to illustrate embodiments of the present invention and are not intended to limit the scope of the invention.  
         [0024]      FIG. 1  illustrates a top view of a system  100  that includes a port  206  comprising a port surface  100 , in accordance with a preferred embodiment. In general, port surface  100  includes a diaphragm  102  upon which is located a single deposit glass adhesive  106  and two piezoresistive die  104 ,  105 .  FIG. 2  illustrates a cross-sectional view of system  100  illustrated in  FIG. 1 , in accordance with a preferred embodiment. Note that in  FIGS. 1-3 , identical or similar parts are generally indicated by identical reference numerals. As indicated in  FIG. 2 a  pressure port  206  is located below diaphragm  102 . A pressure inlet  204  is formed from pressure port  206 .  
         [0025]     In general, piezoresistive die  104 ,  105  are embedded in a single deposit of glass adhesive  106 , which may be, for example, in the form of a glass paste. The large size of the glass paste deposit  106  relative to the small size of the die  104 ,  105  creates a large planar target in which to place the die  104 ,  105 . This size difference maintains the die  104 ,  105  planar to the surface of diaphragm  102 , which is critical for optimum strain transfer. The planar nature of the large single deposit of glass paste  106  allows for the full embedment of all of the edges of die  104 ,  105 , which aides in long term stability. The large size of the single deposit glass adhesive  106  also allows the die  104 ,  105  to be registered to the pressure port  206  instead of as two individual small glass deposits. The improved registration of the die  104 ,  105  to the port  206  creates a more accurate pressure sensor thereof.  
         [0026]      FIG. 3  illustrates a top view of a silicon die  104 , which can be placed upon the diaphragm  102  associated with the surface of port  206  indicated in  FIGS. 1-2 , in accordance with a preferred embodiment. Note that silicon die  104  is depicted in  FIGS. 1-2  and is generally identical in structure and shape to die  105 . Die  104 ,  105  can be formed, for example, as 0.5×0.5×0.05 mm thick micromachined silicon blocks, depending upon design considerations.  
         [0027]     Each die  104 ,  105  can be configured to contain a full Wheatstone bridge, including a plurality of piezoresistors  306 ,  312 ,  318 ,  324 . The four piezoresistors  306 ,  312 ,  318 ,  324  depicted in  FIG. 3 , which are arranged in a Wheatstone bridge arrangement, can be respectively electrically attached via electrical connections  302 ,  308 ,  314 ,  320  at four respective corner intersections to four respective wirebond pads  304 ,  310 ,  316 ,  322 . The piezoresistors  306 ,  312 ,  318 ,  324  change resistance when placed under strain. Such a resistance change can be either positive when the piezoresistors  306 ,  312 ,  318 ,  324  are placed in tension or negative when the piezoresistors  306 ,  312 ,  318 ,  324  are placed under compression.  
         [0028]     Each die  104 ,  105  can be placed at 0 and 90 degrees on the metal diaphragm. The radial placement locations of the two die  104 ,  105  maximize the difference between circumferential strain minus radial strain at each die  104 ,  105  location during port pressurization of pressure port  206 . This maximization of circumferential minus radial strain creates the maximum potential output from the die&#39;s Wheatstone bridge for a given diaphragm strain field. The die placement locations, from the center of the diaphragm to the center of the silicon die, can be, for example, 1.5 mm for a 5000-psi full-scale pressure sensor.  
         [0029]      FIG. 4  illustrates an electrical implementation for a pressure sensor system  400  in association with the components depicted in  FIG. 1-3  in accordance with a preferred embodiment. Note that in  FIGS. 1-4 , identical or similar parts can be indicated by identical reference numerals. Thus, the configuration depicted in  FIGS. 1-3  can be adapted for use with system  400  depicted in  FIG. 4 . System  400  generally includes a circuit board portion  410  and  412  to which a pressure port  206  can be attached. Pressure inlet  204  is surrounded by pressure port  206  and located below diaphragm  102 . The single deposit glass adhesive  106  is formed above diaphragm  102 . Die  104  is formed in glass adhesive  106  as indicated earlier. Wirebonds  408  can be connected to the wirebond pads  304 ,  310 ,  316 ,  322  depicted in  FIG. 3 . Wirebonds  408  are electrically connected to circuit board portion  410 . Note that a single circuit board can be formed from circuit board portions  410 ,  412 .  
         [0030]      FIG. 5  illustrates a pressure sensor system  500 , which can be implemented in accordance with a preferred embodiment. Note that in  FIGS. 1-5 , identical or similar parts or components can be indicated by identical reference numerals. System  400  of  FIG. 4  and the components depicted in  FIGS. 1-3  can therefore be adapted for utilization with system  500  illustrated in  FIG. 5 . System  500  can be implemented as a hermetically sealed device. Note that a solder joint  510  can be located within a connector shell  502  and can further connect to a terminal  504 .  
         [0031]     In general, system  500  can be configured to function as a hermetically sealed pressure sensor that includes pressure port  206 , which is surrounded by an epoxy  522  and a retaining ring  520 . A retaining ring protector  518  is also provided to protect retaining ring  520 . A hex installation housing  516  can also be provided through which pressure port  206  is maintained. An O-ring  514  can also surround pressure port  206 . A solder joint  510  is located within a connector shell  502  and connects to a terminal  504 . A terminal holding plastic portion  506  is also located within connector shell  502 . Additionally, an EMC cover  508  is located above diaphragm  102 . Note that the glass and/or die are not depicted in  FIG. 5 .  
         [0032]     In a typical 5000 psi full-scale pressure sensor, the metal pressure port is screw machined from 17-4 H1150 stainless steel. It has a 3 mm inner diameter that allows pressure to reach the diaphragm. The diaphragm is 0.5 mm thick and the radius between the diaphragm and inner diameter is 1 mm. The diaphragm surface that the glass and die are placed onto is 5.5 mm in diameter.  
         [0033]     At the die  104 ,  105  placement locations, the die  104 ,  105  are embedded in a single large glass deposit  106 . This glass deposit  106  can be formed in the shape of two connected circles that may be, for example, approximately 2 to 2.5 mm in diameter. The single deposit  106  has a resulting oblong shape that goes well beyond the placement of both die  104 ,  105 . The glass deposit  106  can be, for example, 0.25 mm in thickness. The large size of the glass relative to the die  104 ,  105  creates a large planar target in which to embed the die  104 ,  105 . The large size of the glass also aids in die  104 ,  105  placements, because less exacting placement parameters for the glass  106  and die  104 ,  105  are necessary.  
         [0034]     The glass  106  can be, for example, vitreous glass with solvents and binders. The vitreous glass can be provided as a lead borate glass. The solvents and binders keep the glass particles together during the stencil application of the glass paste onto the metal diaphragm  102 . The binders also keep the glass particles together during transporting the unfired populated port  206 . The glass properties are chosen such that the firing temperature is less than the degradation temperature of either the port  206  or the piezoresistive die  104 ,  105 . The glass coefficient of thermal expansion is also chosen to be in a range that buffers the difference in coefficient of thermal expansions between the metal diaphragm and the die  104 ,  105 .  
         [0035]     The silicon die  104 ,  105  can be embedded in the unfired glass paste until their wirebond plane surfaces are flush with the surface of the unfired single deposit of glass paste  106 . After the die  104 ,  105  are placed, the metal pressure port with integral diaphragm  102 , the glass  106 , and the die  104 ,  105  can be are taken to the temperature at which the glass melts. This process is often referred to as firing. The unit is then cooled to room temperature. At this point, the die  104 ,  105  can be permanently adhered to the diaphragm in the single deposit  106  of glass.  
         [0036]     The fired single deposit paste now transfers the strain field developed on the diaphragm  102  of the pressure port  206  during pressurization to the silicon die  104 ,  105 . The die  104 ,  105  can be wirebonded with four wirebonds  408  each to the circuit board formed by circuit portions  410 ,  412 . The wirebonds  408 , currently gold, transfer electrical signals from the wirebond pads  304 ,  310 ,  316 ,  322  on the silicon die to wirebond pads on the circuit board. Both die  104 ,  105  can be connected through routing on the circuit board formed by circuit portions  410 ,  412  so that their Wheatstone components are in parallel. Both die  104 ,  105  react to the strain field with a similar Wheatstone output upon pressurization of diaphragm  102 .  
         [0037]     The placement of the die  104 ,  105  is such that the circumferential piezoresistors go into tension and the radial piezoresistors go into compression during pressurization. Both of these strain inputs to the die  104 ,  105  unbalance the Wheatstone bridge. This unbalancing is repeatable with respect to pressure. This unbalancing of the Wheatstone bridge is the basis for creating the electrical signal for pressure sensor system  500 . The level of die Wheatstone output for a 5000-psi sensor, for example, can be approximately 50 mV/V at a full-scale pressure input.  
         [0038]     In a typical 5000-psi full-scale pressure sensor, for example, the metal pressure port  206  can be screw machined from stainless steel. Such a device may possess a 3 mm inner diameter that allows pressure to reach the diaphragm  102 . The diaphragm  102  can be, for example, 0.5 mm thick and the radius between the diaphragm  102  and inner diameter can be approximately 1 mm. The diaphragm  102  surface that the glass  106  and die  104 ,  105  can be placed onto can be, for example, approximately 5.5 mm in diameter.  
         [0039]      FIG. 6  illustrates a Wheatstone die schematic diagram in accordance with a preferred embodiment  600 . Note that in  FIGS. 1-6  identical or similar parts are generally indicated by identical reference numerals. Note that the level of imbalance is turned into a useful output for the end user through the use of an application specific integrated circuit (ASIC). The ASIC can provided with the Wheatstone components of the two silicon die through routing on the circuit board. The ASIC turns the Wheatstone imbalance into a repeatable, useful pressure related quantity for the end user.  
         [0040]     The ASIC powers the die with roughly 1 Volt across the Wheatstone bridge of both die  104 ,  105  in parallel. Gain can be applied in the ASIC to the dies&#39; Wheatstone output to get the output into a useful range for the end user. The ASIC typically can be programmed so that the pressure sensor output is 0.5 V relative to a 5 V input Voltage at zero pressure. The ASIC typically is programmed so that the pressure sensor output is 4.5 V relative to a 5V input Voltage at full scale pressure.  
         [0041]     The ASIC can be utilized to maintain the output at these zero and full scale pressure targets within a tolerance band regardless of temperature over the sensor&#39;s full operating temperature. This 0.5V to 4.5 V correlation to pressure of the sensor output is often referred to as an analog output. The analog output allows the end user to correlate a sensor output Voltage to the pressure present in the port of the pressure sensors.  
         [0042]      FIG. 7  illustrates a diagram illustrating die layout  700  on the diaphragm  102  in accordance with one embodiment.  FIG. 8  illustrates a diagram depicting the reaction to full scale pressurization in accordance with one embodiment. Note that in  FIGS. 7-8  identical parts or components are generally indicated by identical reference numerals.  
         [0043]     Note that the analog output of the ASIC can be presented to the end user through a three pin connector comprised of three terminals. One terminal provides a regulated 5 Volt power supply to the ASIC. A second terminal provides a ground to the ASIC. The third terminal provides the end user with the analog output of the ASIC. This analog output can be correlated to the input pressure that the sensor&#39;s diaphragm is presented with through the pressure port.  
         [0044]     It can be appreciated that various other alternatives, modifications, variations, improvements, equivalents, or substantial equivalents of the teachings herein that, for example, are or may be presently unforeseen, unappreciated, or subsequently arrived at by applicants or others are also intended to be encompassed by the claims and amendments thereto.