Patent Publication Number: US-7911316-B2

Title: Sensor array for a high temperature pressure transducer employing a metal diaphragm

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This Application is a divisional application claiming priority under 35 U.S.C. §121 to U.S. patent application Ser. No. 11/453,445, entitled “HIGH TEMPERATURE PRESSURE TRANSDUCER EMPLOYING A METAL DIAPHRAGM”, filed Jun. 15, 2006, the entirety of which is incorporated herein as if fully set forth in its entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to pressure transducers and more particularly to a high temperature pressure transducer. 
     BACKGROUND OF THE INVENTION 
     It has been a desire to provide pressure transducers which can operate at high temperatures and which basically are simple to construct. The prior art is replete with patents and devices which can operate at relatively high temperatures. Such devices include silicon sensors which operate for example at temperatures of 600° C. or higher. Other patents which are assigned to the assignee herein, namely Kulite Semiconductor Products, Inc., depict silicon carbide transducers which are capable of extremely high temperature operation. The prior art is replete with patents which utilize metal as deflecting diaphragms. Onto these metal diaphragms are affixed piezoresistive or other type of sensors. The diaphragms being made of metal are capable of operating at high temperatures. Sensors affixed to the diaphragm can include wire strain gauges or other types of semiconductor strain gauges. Such gauges have to be placed onto the diaphragm in specific positions and each individually affixed to the diaphragm at specific positions. Accordingly, the placement of the sensors on the diaphragm can be a time consuming task. 
     In the prior art, there were two principal methods of making miniature high temperature pressure transducers. In the first method, a thin metalized isolation diaphragm was mounted in front of the sensor and the pressure was transmitted to the sensor by a small volume of oil. As pressure was applied to the isolation diaphragm, pressure was transmitted to the sensor by the oil, which is a non-compressible fluid. As long as the oil retains its property as a non-compressible fluid, such a pressure transducer operates properly. However, as the temperature increases, the vapor pressure of the oil increases to a point where the oil no longer transmits the pressure to the sensor, setting an upper temperature limit on the operation of the transducer. In the second method, as individual gauges are affixed to the diaphragm, using either a high temperature cement or glass. In this method the only dielectric isolation between the gauge and the metal diaphragm is the cement itself. Any variations in the thickness of the cement can cause electrical breakdowns at relatively low temperatures, causing the transducer to fail. Furthermore, if the individual gauges are made from silicon, in order to obtain sufficient resistance, the gauge must be rather long and exhibit rather high resistivity. Thus it becomes difficult to place the individual gauge in a resistor in a region of high stress. As the temperature is increased, the relatively high resistivity material used for the sensor changes its value at a non-linear rate as the temperature is increased to a higher value, making thermal compensation very difficult. Moreover, each individual gauge must be separately applied to the diaphragm, or alternatively, if the various sensors are interconnected prior to application to the diaphragm, the structure is complex, fragile, and difficult to handle, so that the resulting transducers are of dubious quality. 
     SUMMARY OF THE INVENTION 
     A sensor array for a pressure transducer having a diaphragm with an active region, and capable of deflecting when a force is applied to the diaphragm. The sensor array disposed on a single substrate, the substrate secured to the diaphragm. The sensor array having a first outer sensor near an edge of the diaphragm at a first location and on the active region, a second outer sensor near an edge of the diaphragm at a second location and on the active region, and at least one center sensor substantially overlying a center of the diaphragm. The sensors connected in a bridge array to provide an output voltage proportional to the force applied to the diaphragm. The sensors dielectrically isolated from the substrate. 
     A sensor array in accordance with an embodiment of the invention includes an elongated substrate having a top and a bottom surface and longitudinally opposing ends. The array may further include a first semiconductor sensor on said top surface close to one of said opposing ends and a second semiconductor sensor on said top surface close to the other of said opposing ends. The array may also include third and fourth semiconductor sensors positioned substantially at a center of said substrate. Said first, second, third, and fourth sensors may be connected together by contact areas located on said substrate to form a bridge configuration. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a cross-sectional view of a pressure transducer taken through line  1 - 1  of  FIG. 2  and in accordance with an embodiment of this invention. 
         FIG. 2  is a top view of a pressure transducer in accordance with an embodiment of this invention. 
         FIG. 3  is a top plan view of a sensor array of the transducer of  FIG. 2 , taken along line  3 - 3  of  FIG. 2 . 
         FIG. 3A  is an enlarged view of the portion of  FIG. 3  as indicated in  FIG. 3 . 
         FIG. 3B  is a schematic showing a Wheatstone bridge array formed by the sensor array of  FIG. 3 . 
         FIG. 4  is a view of a typical masked assembly utilized to produce the array of  FIG. 3 . 
         FIG. 5  shows multiple arrays similar to those of  FIG. 3  formed on a single semiconductor wafer. 
         FIG. 6  shows the array of sensor arrays of  FIG. 5  separated into individual arrays for use on separate metal diaphragms. 
         FIG. 7  shows a cross-sectional view of the sensor array of  FIG. 3  secured to a metal diaphragm by a high temperature glass frit. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIGS. 1 and 2  there is shown a pressure transducer in accordance with an embodiment of the present invention.  FIG. 2  depicts a pressure transducer  10 , which has a housing  25  and which has a diaphragm  12 .  FIG. 1  depicts a cross-sectional view taken through line  1 - 1  of  FIG. 2 . Diaphragm  12  may be of metal and may be of uniform thickness. The particular thickness, radius and material may be selected depending on the particular application. Diaphragm  12  may be supported exclusively at a circumference thereof, and may be supported around the entire circumference thereof. In the absence of an applied force, diaphragm  12  is planar or substantially planar. In the presence of an applied force, diaphragm  12  is capable of deflection. An active area of diaphragm  12  deflects upon application of a force. Diaphragm  12  may be a continuous end wall of a closed cylindrical member  15  having an inner diameter approximately one-half that of its outer diameter. If closed cylindrical member  15  is circular, diaphragm  12  may be circular.  FIG. 2  also depicts in dashed lines sensor array  20 . Diaphragm  12  has a top surface external to transducer  10 , and a bottom surface interior to transducer  10 . Sensor array  20  is affixed to the bottom surface of the active area of diaphragm  12 . 
     As one can see from  FIG. 1 , sensor array  20  is completely isolated from the environment external to transducer  10  by closed cylindrical member  15 . Leads from sensor array  20  are directed to terminal pins,  17  and  18 , which pins connect with connecting terminal port  30 , which is in turn connected to output connector  14 . 
     Referring now to  FIG. 3 , sensor array  20  contains four sensors  25 ,  27 ,  41 ,  42 , which are interconnected to form a bridge circuit, such as a Wheatstone bridge. Sensors  25 ,  27 ,  41 ,  42  may be piezoresistive devices. As explained below in greater detail, sensor array  20  is so secured to the bottom surface of diaphragm  12  that deflection of diaphragm  12  causes sensors  25 ,  27 ,  41 ,  42  to change resistance. Thus, resistance changes upon application of an applied force F as shown in  FIG. 1  to the top surface of diaphragm  12 . Sensor array  20  is dielectrically isolated from diaphragm  12 . Sensor array  20  is defined on a substrate. The substrate may be a wafer, such as a single-crystal silicon wafer. 
     It is well known that when a circular deflecting diaphragm having a clamped edge is exposed to a pressure normal to its surface, the areas near the clamped edge experience compressive stresses while in the central region the area experiences tensile stresses. These stresses are given by the following formula. 
             σ   =     3   ⁢           ⁢   P   ⁢       [         a   2     ⁡     (     1   +   v     )       -       r   2     ⁡     (     3   +   v     )         ]       8   ⁢           ⁢     t   D   2                 
Where P is the applied pressure, a is the radius of the diaphragm, v is Poisson&#39;s ratio, t D  is the thickness of the diaphragm, and r is the radius at the location of interest.
 
     As can be seen from the dashed line depiction in  FIG. 2 , sensor array  20  has an elongated form, in which sensor array  20  extends across diaphragm  12  substantially from one edge to an opposing edge, while sensor array  20  is relatively narrow. Sensor array  20  may be in the form generally of a narrow rectangle. The length of sensor array  20  is less than the diameter of an active region of diaphragm  12 . Sensor array  20  may be a silicon-on-insulator (SOI) structure. Each sensor may be a P+ piezoresistor element, each separated by an oxide layer from the underlying silicon substrate. The substrate may be affixed to diaphragm  12  by a high temperature glass frit. High temperature glass frits are well known in the art and many patents assigned to the assignee of the present application, Kulite Semiconductor Products, Inc., disclose such frits as well as the composition of the same. 
     Referring again to  FIG. 3 , first outer sensor  25  is at, or close to, a first end of sensor array  20 . Second outer sensor  27  is at, or close to, a second end of sensor array  20 . First and second ends are opposing ends. First and second outer sensors  25 ,  27 , are each positioned near an edge of diaphragm  12 , and are on the active region of diaphragm  12 . First outer sensor  25  is near an edge of diaphragm  12 , at a first location, and second outer sensor  27  is also near an edge of diaphragm  12 , at a second location. First outer sensor  25  and second outer sensor  27  are thus both near an edge of diaphragm  12 , but at different locations, and may be positioned at locations on opposite edges of diaphragm  12 . First center sensor  41  and second center sensor  42  are located at the center of sensor array  20 , and overlying or near a center of diaphragm  12 . Center sensors  41  and  42  are interconnected on the array by, for example, suitable lead areas which are deposited on the array. 
     Continuing to refer to  FIG. 3 , first outer sensor  25  is associated with contact  22  which connects one terminal of first outer sensor  25  to one terminal of first center sensor  41 . The other terminal of first outer sensor  25  is connected by a contact  21  to one terminal of second center sensor  42 . In a similar manner, one terminal of first center sensor  41  is connected to contact  28 . The other terminal of second center sensor  42  is connected to contact  23  which connects to one terminal of second outer sensor  27 . The other terminal of second outer sensor  27  is connected to contact  24 . The schematic or equivalent diagram of sensor array  20  depicted in  FIG. 3  is shown in  FIG. 3B . As is evident from  FIG. 3B , sensors  25 ,  27 ,  41 , and  42 , which, as noted, are piezoresistors, define a bridge circuit, and more particularly a Wheatstone bridge. Contacts  24 ,  28  are the output leads of the circuit defined by sensor array  20 . Contacts  24 ,  28 , are connected to cable  14  of  FIG. 1 . The outputs from contacts  24  and  28  may be connected to various devices and circuits for compensating for such factors as variation in temperature. Such devices may include, by way of example, compensating resistors. As one can also see from  FIG. 3B , a biasing potential may be applied to contact  22  indicated as+IN; a reference potential may be applied to contact  23  which is indicated as−IN. One output of sensor array  20  is taken from terminal  21 , indicated as−OUT, while the other output is taken between terminals  28  and  24 , indicated as+OUT. 
     Shown in  FIG. 3A , is an enlarged view of an exemplary embodiment of second end sensor  27 . Second end sensor  27  includes a series of interfolded or interconnected lines in a serpentine pattern. Such a series of interfolded or interconnected lines may be advantageous in providing a resistor of relatively high value in a relatively small area.  FIG. 3  includes exemplary dimensions of such sensors and of a sensor array  20 . By way of example, a piezoresistor employed as a sensor may have dimensions of about 7.6 mils by 2.76 mils. As one can see from  FIG. 3  the length of the array from the front of piezoresistor  25  to the back of piezoresistor  27  is typically 91 mils. 
     The sensor array may be fabricated on a wafer of silicon. The wafer of silicon has deposited thereon a layer of silicon dioxide, and sensors  25 ,  27 ,  41  and  42  are deposited on the layer of silicon dioxide by suitable techniques. The sensor array itself may be termed a SOI structure with four P+ resistor elements separated by an oxide layer from the underneath silicon material. Thus, each sensor is dielectrically isolated from the silicon substrate. 
       FIG. 7  is a sectional view through diaphragm  12  and sensor array  20 . Secured to metal diaphragm  12  is sensor array  20 . Sensor array  20  is provided on substrate  101 , which may be a wafer of single-crystal silicon, for example. Substrate  101  is secured to diaphragm  12  by a high temperature glass frit  102 . Silicon substrate  101  has a layer  100  of silicon dioxide formed thereon. Upon layer  100  of silicon dioxide are first and second outer sensors  25  and  27  and center sensors generally designated  26 , and contacts  21 ,  23 . 
     Methods for fabrication of the devices described herein are well known to those of skill in the art. By way of example, the assignee of the present application, Kulite Semiconductor Corp., has many patents which teach techniques for fabrication of SOI devices which techniques are among those which may be employed to fabricate sensor array  20 . See, for example, U.S. Pat. No. 5,973,590 entitled “Ultra Thin Surface Mount Sensor Structures and Methods of Fabrication” issued on Apr. 3, 2001 and assigned to Kulite Semiconductor Products, Inc. the assignee herein. 
     Sensor array  20  is defined on substrate  101 . The shape and dimensions of substrate  101  are adapted to permit outer sensors  25 ,  27  to be formed close within the active region of diaphragm  12  and close to the circumference of diaphragm  12 , while permitting center sensors  41 ,  42 , to be formed close to the center of diaphragm  12 . In the illustrated embodiment, this is accomplished by providing substrate  101  to have a generally rectangular form having a first width, and having an end width, narrower than the first width, at each end to permit outer sensors  25 ,  27  to be closer to the curved circumference of diaphragm  12 . This also enhances the amount of stress induced in outer sensors  25  and  27 . In a central region, the width of substrate  101  may be a center width less than the first width, to enhance the amount of stress induced in center sensors  41 ,  42 . 
     The dimensions described below and illustrated in  FIG. 3  provide a non-limiting example of dimensions for a sensor array for use with a circular diaphragm  12  having a diameter of 100 mils. Substrate  101  may be approximately 95 mils long, by approximately 20 mils wide. The sensor array  20  may be 91 mils long by 13.5 mils wide. The areas of reduced width containing outer sensors  25  and  27  may be 12-13 mils wide. The area of reduced width at the center containing center sensors  41 ,  42  may be about 10 mils wide for a length of about 10 mils. 
     According to the above-noted stress equation, stress at any given point on diaphragm  12  is dependent on the distance of the point from the center of the diaphragm, and decreases significantly as the distance from the center of the diaphragm decreases. In fact the stress actually changes in sign at about ⅔ of the radius. Thus, as the size of the sensors, particularly along the axis of the diaphragm, is kept relatively small, averaging of the detected stress at any given sensor is reduced. By way of example, in a 100 mil diameter diaphragm, the resistor length may be no more than about 10 mils. 
       FIG. 3A  shows in detail an exemplary structure of outer sensor  27 , and terminals coupled to contacts  23  and  24 . The other sensors may have the same structure. The line widths of sensor  27  may be extremely small, such as about 0.17 mils. The spacing between each line may be about 0.2 mils. In the illustrated example, since the separation of the lines is on the order of 1 mil, with 5 back and forth lines, the resistance may be about 2000 ohms or in excess of about 2000 ohms, in a width of less than about 5 mils. The width of the resistor as depicted in  FIG. 3A  is about 3 mils. 
     One obtains a high resistance in a small area, such as a resistance in excess of 2000 ohms, in an area of about 5 mils by about 7.6 ohms. Such a high resistance is advantageous in that there is relatively little self heating, which tends to provide a more stable transducer. 
     A variety of methods for fabricating sensor array  20 , which may be a dielectrically isolated SOI sensor bridge array as depicted in  FIG. 3 , are known in the art. One exemplary technique commences with two substrates. The two substrates may be wafers, such as wafers of single crystal silicon. A first wafer may be designated as a pattern wafer and may be selected to optimize the piezoresistive performance characteristics of each of the sensors as  25 ,  27 ,  41  and  42 . The second wafer also fabricated from silicon is designated as a substrate wafer and acts as a base. The thickness of the substrate wafer is reduced during the process, as described in greater detail below. 
     An oxide layer, such as oxide layer  100  of  FIG. 7 , may be thermally grown on the surface of the substrate wafer. Separately, the pattern wafer may be patterned with the piezoresistive pattern of the sensors. The piezoresistive patterns may be diffused to the highest concentration level (solid solubility) in order to achieve the most stable, long term electrical performance characteristics of the sensing network. 
     Once the pattern and the substrate wafers are appropriately processed, the two wafers may be fusion bonded together using a diffusion enhanced bonding technique. Such a technique is shown in U.S. Pat. No. 5,286,671 designated as “Fusion Bonding Technique for Use in Fabricating Semiconductor Devices” issued on Feb. 15, 1994, to A. D. Kurtz et al and assigned to the assignee herein. 
     The resulting molecular bond between the two wafers is as strong as the silicon itself. Since both the lines of the sensors and the base are of the same silicon material, there is no thermal mismatch between the two, thus resulting in a very stable and accurate performance characteristic with temperature. The presence of dielectric isolation enables the sensor bridge array to function at very high temperatures without any current leakage effects which are typically associated with ordinary pn junction type devices. 
     Referring to  FIG. 4  there is shown an exemplary masked wafer for forming a sensor array such as array depicted in  FIG. 3 . As seen in  FIG. 4 , there is a surrounding layer  100  which may be a P+ material; layer  100  may be used to separate various arrays which are formed on a common wafer as will be explained.  FIG. 4  utilizes the same reference numerals as depicted in  FIG. 3  to designate the same parts. It is understood that multiple masks may be employed in order to fabricate the entire structure but such techniques are well known to those skilled in the art. 
       FIG. 5  shows a pattern  500  for multiple sensor arrays all fabricated on a single wafer. The wafer can be sliced to separate the sensor arrays, after measurement of their electrical characteristics, for fabrication of multiple transducers. and which sensor arrays can be separated after their electrical characteristics are measured. In this manner, one can produce numerous sensor arrays as the array of  FIG. 3  on one common wafer and select all arrays which are within desired specifications. The fabrication of multiple semiconductor devices or multiple devices on a single wafer is well known. Thus  FIG. 5  shows for example a portion of the semiconductor wafer which contains 12 sensor arrays as the array of  FIG. 5 . 
       FIG. 6  shows such sensor arrays  505  after a step of separation. 
     As depicted in  FIG. 3 , an entire Wheatstone bridge array is provided, on a substrate of silicon having an oxide layer, with the piezoresistors of the Wheatstone bridge array deposited thereon. The oxide layer provides isolation between the silicon and the sensing devices. The use of dielectric isolation enables the sensor bridge to function at very high temperatures without any current leakage effects, in contrast to pn junction type devices. 
     A sensor array in accordance with the invention has many advantages over prior art devices, including the exemplary advantages described herein. In a sensor array in accordance with an embodiment of the invention, the geometry of the individual sensor elements may be defined by photolithographic techniques, so that the widths and thicknesses of the sensors can be on the order of fractions of a mil; as a result, very compact sensors can be formed. As each piezoresistive sensor may be made of a series of P+ regions of width of about 0.1 mils, a thickness of about 0.05 mils and lengths of about 7.5 mils, each sensor may have a relatively high resistivity. For example, each back and forth leg of a sensor, with P+ doping densities of above 10.sup.20 boron atoms/cm.sup.3, may have a resistance in excess of 350 ohms. Since the separation of the legs may be on the order of 1 mil, and there may be, for example, 5 back and forth legs in a single sensor, such a sensor may provide a resistance in excess 2000 ohms in a width of less than 5 mils. Such a high resistance reduces self heating and thus provides a more stable transducer. 
     While the foregoing invention has been described with reference to the above embodiments, various modifications and changes can be made without departing from the spirit of the invention. Accordingly, all such modifications and changes are considered to be within the scope of the appended claims.