Abstract:
A MEMS based pressure sensor for flow measurements includes a pressure sense die located between a media seal and a conductive seal. Such a system includes a pressure sense die located between a media seal and a conductive seal. A sensing diaphragm is generally associated with the pressure sense die, wherein the sensing diaphragm deflects when a pressure is applied thereto. An impedance circuit is generally embedded with one or more resistors on the sensing diaphragm to which the pressure to be detected is applied. An ASIC is generally associated with the impedance circuit and the sense die, wherein the ASIC is placed on a lead frame for signal conditioning in order to detect a change in the pressure.

Description:
TECHNICAL FIELD 
       [0001]    Embodiments are generally related to sensor methods and systems. Embodiments are also related to MEMS (Microelectromechanical System) based pressure sensors. Embodiments are additionally related to pressure sensors that incorporate ASIC (Application Specific Integrated Circuit) components for signal conditioning and/or amplification. 
       BACKGROUND OF THE INVENTION 
       [0002]    Many processes and devices have been used for measuring flow rate in different applications. A miniature MEMS based pressure sensor can be used to measure very low flow rates and with a reliable accuracy. Such MEMS based pressure sensors have been implemented, for example, in various flow sensing devices, such as medical applications, some of which utilize silicon piezoresistive sensing technology for measuring very low pressures. Other flow sensing implementations, for example, include environmental applications. 
         [0003]    MEMS involve the integration of micro-mechanical elements, sensor actuators, and electronic components on a common silicon substrate through the use of microfabrication technology. While the electronics can be fabricated using integrated circuit (IC) process sequences (e.g., CMOS, Bipolar, or BICMOS processes), the micromechanical components can be fabricated using compatible “micromachining” processes that selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical and electromechanical devices. MEMS promises to revolutionize nearly every product category by bringing together silicon based microelectronics with micromachining technology, making possible the realization of complete “system-on-a-chip”. MEMS is an enabling technology allowing the development of “smart” products, while augmenting the computational ability of microelectronics with the perception and control capabilities of microsensors and microactuators, while expanding the space of possible designs and applications. 
         [0004]    The majority of prior art MEMS pressure transducers produced for the automotive market, for example, typically include a four-resistor Wheatstone bridge fabricated on a single monolithic die using bulk-etch micromachining technology. The piezoresistive elements integrated into the sensor die can be located along the periphery of the pressure sensing diaphragm at points appropriate for strain measurement. Such sensors are inexpensive to produce and can be processed in association with integrated circuits on a wafer that may contain a few hundred to a few thousand sensing elements. 
         [0005]    In a bridge configuration, the resistance of diagonally opposed legs varies equally and in the same direction, as a function of the mechanical deformation caused by pressure. As the resistance of one set of diagonally opposed legs increases under pressure, the resistance of the other set decreases, and vice versa. Bridge excitation in the form of voltage or current is applied across two opposite corners of the bridge. Any change in voltage (e.g., pressure) can be detected as a voltage difference across the other two corners of the bridge, typically referred to as signal output. Unfortunately for silicon piezoresistive sensors, this voltage difference is quite small. Thus, the sensor must be compensated before it can be used. 
         [0006]    Bulk-micromachined silicon pressure sensors typically incorporate the use of a diaphragm that deflects when subjected to a pressure load, and also include a piezoresistive transducer that translates strain to a differential voltage. Metal pads can be used to interface with other system components. Signal-conditioning circuitry for calibration or amplification is optional, but is often included as well. The piezoresistive transducer is strategically placed near the edge of the diaphragm, since that is a high-strain location and each sensor is designed, such that its output voltage is linearly proportional to the applied pressure in its operating range. 
         [0007]    In some applications, it is preferred that a signal conditioning/signal amplification capability be incorporated into the sensors. It is believed that there is currently no amplified flow through sensors based on piezoresistive sensing technology in an integrated package. It is further believed that if such a sensor could be implemented, this would help in lowering installation and development costs, while eliminating secondary operations and shortening the design cycle time. 
       BRIEF SUMMARY 
       [0008]    The following summary is provided to facilitate an understanding of some of the innovative features unique to the embodiments disclosed and is not intended to be a full description. A full appreciation of the various aspects of the embodiments can be gained by taking the entire specification, claims, drawings, and abstract as a whole. 
         [0009]    It is, therefore, one aspect of the present invention to provide for an improved pressure sensor for measuring very low flow rates with a reliable accuracy. 
         [0010]    It is another aspect of the present invention to provide for a pressure sensor that integrates an ASIC for signal conditioning and isolates the ASIC from a pressure sensing die cavity. 
         [0011]    It is a further aspect of the present invention to provide for a miniature amplified and temperature compensated flow through sensor in a single package. 
         [0012]    The aforementioned aspects and other objectives and advantages can now be achieved as described herein. A pressure sensor is system and/or pressure sensing device are disclosed. Such a device includes a housing partitioned into two halves. In one half, a pressure sensing die with a conductive seal and a media seal can be packaged and in the other half, an ASIC can be provided. The ASIC is preferably placed on the lead frame so that the temperature sensor in the ASIC is used for temperature compensation. The whole housing can then be packaged with a top case, which can be heat-sealed ultrasonic welded or any other plastic joining process. 
         [0013]    The pressure sensor can be implemented in the context of a MEMS based pressure sensing system for flow rate measurements. Such a system includes a pressure sense die located between a media seal and a conductive seal. A sensing diaphragm is generally associated with the pressure sense die, wherein the sensing diaphragm deflects when a pressure is applied thereto. An impedance circuit is generally embedded with one or more resistors on the sensing diaphragm to which the pressure to be detected is applied. An ASIC is generally associated with the impedance circuit and the sense die, wherein the ASIC is placed on a lead frame for signal conditioning in order to detect a change in the pressure. 
     
    
     
       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, serve to explain the embodiments disclosed herein. 
           [0015]      FIG. 1  illustrates exploded-perspective view of an amplified flow through pressure sensor incorporating an ASIC, which can be implemented in accordance with a preferred embodiment; 
           [0016]      FIG. 2  illustrates a plan view of a flow through pressure sensor package, which can be implemented in accordance with a preferred embodiment; 
           [0017]      FIG. 3-5  illustrates cross-sectional views taken along lines A-A, B-B and C-C respectively, with respect to the illustration depicted in  FIG. 2 , in accordance with a preferred embodiment; 
           [0018]      FIG. 6  illustrates a detailed functional block diagram of a pressure sensor incorporating an ASIC as depicted in  FIG. 1 , where pressure is applied to the bottom side, in accordance with a preferred embodiment; and 
           [0019]      FIG. 7  illustrates a detailed functional block diagram of a pressure sensor incorporating an ASIC as depicted in  FIG. 1 , where pressure is applied to the top side, in accordance with an alternative embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    The particular values and configurations discussed in these non-limiting examples can be varied and are cited merely to illustrate at least one embodiment and are not intended to limit the scope thereof. 
         [0021]      FIG. 1  illustrates an exploded-perspective view of an amplified flow through pressure sensor  100 , which can be implemented in accordance with a preferred embodiment. The amplified flow through sensor  100  can be partitioned into two halves, first half  101  and second half  102 . A tube  103  is generally located between and proximate to the first half  101  and second half  102 . In first half  101 , a pressure sense die  105  can be packaged with a media seal  106  and a conductive seal  107 . A pressure inducing substance can flow through tube  103 . In the second half  102 , an ASIC  108  can be located and packaged. The sensor  100  can be placed in a desired base with the assistance of a connector  104 . 
         [0022]      FIG. 1  further illustrates a second half  102  of the pressure sensor  100  wherein the ASIC  108  is placed. The ASIC  108  is generally placed on a lead frame  109  so that a temperature sensor in the ASIC  108  is used for temperature compensation. The ASIC  108  placed on the lead frame  109  can be located on a plastic housing  110  on the second half  102  of the sensor  100 . The entire sensor  100  can be heat-sealed, ultrasonic welded or joined by any other plastic joining process. The ASIC  108  incorporated in the second half  102  of the sensor brings about a signal amplification. 
         [0023]      FIG. 2  illustrates a plan view  200  of the flow through pressure sensor  100  according to a preferred embodiment. Note that in  FIGS. 1-2 , identical or similar parts or elements are generally indicated by identical reference numerals. For example, the first half  101  and second half  102  of the sensor and the flow through tube  103  depicted in  FIG. 1  also appears in  FIG. 2 . The plane A-A illustrates the horizontal cross-section of flow through pressure sensor and the lines B-B and C-C illustrates the vertical cross-section of the flow through pressure sensor  100 . The first half of the amplified flow through sensor  100  is illustrated by  101  and second half is illustrated by  102 . The sensor  100  can be placed in a desired base with the assistance of a connector  104 , depending upon design considerations. 
         [0024]      FIG. 3  illustrates a cross-sectional view  300  along the plane A-A that includes the first half  101  and second half  102  of the amplified flow through pressure sensor  100 . Note that in  FIGS. 1-3 , identical or similar parts or elements are generally indicated by identical reference numerals. Thus the reference numerals  101 ,  102 ,  103 ,  104 ,  105 ,  106 ,  107 ,  110  as depicted in  FIG. 1  refer to the same components in  FIG. 3 . A pressure sense die  105  can be packaged with a media seal  106  and a conductive seal  107 . A pressure inducing substance can flow through tube  103 . The second half  102  has a plastic housing  110  where the ASIC  108  is placed. The sensor  100  can be placed in a desired base with the assistance of a connector  104 . 
         [0025]      FIG. 4  illustrates a cross sectional view  400  of the amplified flow through pressure sensor along the plane B-B. Note that in  FIGS. 1-3 , identical or similar parts or elements are generally indicated by identical reference numerals. Thus, the reference numerals  101 ,  104 ,  105 ,  106 , and  107  as depicted in  FIG. 1  refer to the same components in  FIG. 4 . The figure illustrates the first half  101  of the pressure sensor. A pressure sense die  105  can be packaged with a media seal  106  and a conductive seal  107  as illustrated in the figure. The sensor  100  can be placed in a desired base with the assistance of a connector  104 . 
         [0026]      FIG. 5  illustrates a cross-sectional view  500  of the flow through sensor along the section C-C that includes the first half  101  and second half  102  of the amplified flow through pressure sensor  100 . Note that in  FIGS. 1-5  identical or similar parts or elements are generally indicated by identical reference numerals. Thus, the reference numerals  101  and  102  as depicted in  FIG. 1  refer to the same components in  FIG. 4 . 
         [0027]      FIG. 6  illustrates a functional block diagram  600  of a pressure sensor  100  with an ASIC  108  as depicted in  FIG. 1  incorporated for signal conditioning and amplification when pressure is applied on the pressure die  105 , in accordance with a preferred embodiment. Note that in  FIGS. 1 and 6 , identical or similar parts and/or elements are generally indicated by identical reference numerals. Thus reference numerals  100  and  108  depicted in  FIG. 1  refers to the same components in  FIG. 6 . Pressure sensor  100  generally includes a pressure block  202  in association with a first signal conditioning unit  206 , which is connected to a second signal conditioning unit  208 . The first signal conditioning unit  206  generates one or more signals that can be transmitted to the second signal conditioning unit  208 . An external supply voltage  205  provides a voltage to the first signal conditioning unit  206  and the second signal conditioning unit  208 . The pressure sensor  100  additionally includes an ASIC  108  in the signal conditioning unit  208 . 
         [0028]    As depicted by arrow  201  in  FIG. 6 , input pressure (P) can be applied to the pressure block  202 . This pressure change causes a deflection of a membrane as indicated by arrow  203 , and as a result deflection to resistance change of the resistive network  204  occurs in the signal conditioning unit  206 . This process can be referred to as “signal conditioning”. The external supply voltage (Vs)  205  can be provided to the signal conditioning units  206  and  208 , which in turn produces the output signal (V o )  207 , which occurs via the first signal conditioning unit  206 . The output signal  207  is then passed to the second signal conditioning unit  208 . ASIC  108  generally performs signal conditioning/amplification at the second signal conditioning unit  208  and generates an amplified output signal  209  (i.e., V o  amplified). The sensor signal conditioning units  206  and  208  work together to perform all necessary functions for calibration, temperature compensation influence and linearizing the amplified output signal  209 , and thus improvement is obtained by integrating the ASIC  108  for signal conditioning and its isolation from the pressure block  202 . 
         [0029]      FIG. 7  illustrates a high-level block diagram  400  of a pressure sensor  100  incorporating an ASIC  108  depicted in  FIG. 1 , in accordance with a preferred embodiment. Note that in  FIGS. 1-7 , identical or similar parts and/or elements are generally indicated by identical reference numerals. Thus reference numerals  105  and  108  as depicted in  FIG. 1  and reference numerals  202 ,  203 ,  204 ,  205 ,  206 ,  207 ,  208  and  209  depicted in  FIG. 6  refers to the same components in  FIG. 7 . As indicated by arrow  401  in  FIG. 7 , input pressure can be applied to the pressure block  202 . Pressure sense die  105  senses the pressure change and causes a deflection of membrane as indicated by the arrow  203  and as a result change in resistance of the resistive network  204  occur in the first signal conditioning unit  206 . The signal conditioning unit  206  generates an output signal (Vo)  207 . The ASIC  108  performs signal conditioning and amplification at the second signal conditioning unit  208  and generates the amplified output signal (V o  amplified)  209  and corrects the output digitally. In some implementations, the ASIC  108  can be powered by a 5 Volt DC source (V s )  205 . 
         [0030]    Positioned within the second cavity is a signal amplifier  108  which is in electrical contact with the pressure sensor and amplifies a signal from the pressure sensor representative of the pressure of a fluid such as air or air and water vapor within the first cavity. The signal amplifier is preferably an application specific integrated circuit (ASIC), which is well known in the art. ASIC&#39;s can also be used for signal-conditioning a MEMS silicon piezoresistive sensor. The ASIC may be employed to calibrate and compensate the pressure sensor with a total error of less than ±1% full scale output (FSO) over its operating pressure range. The total error includes effects due to offset and sensitivity, as well as the offset and sensitivity temperature coefficients. 
         [0031]    ASIC&#39;s are useful because a typical output signal for a piezoresistive pressure sensor depends on temperature. Useful ASIC&#39;s include, for example, ASIC devices such as the DSP-based circuit from Fujikura Ltd. in Tohoku, Japan that may correct for the sensor&#39;s offset and sensitivity. Such an example Fujikura&#39;s circuit can operate in a temperature range from −30° C. to 80° C. The example Fujikura&#39;s ASIC can be configured on a 0.7-pm double-polysilicon, double-metal, n-well CMOS process. It has a sigma-delta 16-bit analog-to-digital converter, a reference voltage with a built-in temperature sensor, the 16-bit DSP core, 101 polysilicon fuses, a step-up voltage regulator, a 10-bit digital-to-analog converter (DAC), and a 4-MHz oscillator. Corrected coefficients are stored using the polysilicon fuses. The output code is accessible with a serial interface or an analog signal provided by a 10-bit DAC. This circuit also may compensate for secondary temperature characteristics and can utilize, for example, a 120 serial interface. A built-in charge pump permits the device to function within circuits rated under 3 V. A “sleep” mode reduces power consumption. 
         [0032]    Another example of an ASIC is a device produced by The Institute of Microelectronics in Singapore, which is a fully customized analog ASIC with a fusible-link array that achieves the aforementioned performance from −40° C. to 125° C. Such an ASIC can be configured via a 0.8-pm double-polysilicon, double-metal CMOS process. This type of an ASIC includes a core analog signal processor, a 64-bit fusible link array, and a serial fusible-link interface. The ASIC&#39;s digital portion provides the interface between the analog signal processor and controller. This controller writes data to an interface and reads data back from it by a serial-in and serial-out communications protocol. Data in the serial interface can be loaded into the fusible-link array to control various resistor networks in the analog signal processor. These resistor networks are used for various programmable functions. 
         [0033]    All of these programmable elements make it possible to compensate for the calibration, sensitivity, and temperature effects to the first order. The ASIC features an output of 0.5 to 4.5 V using a 5-V power supply. Other suitable signal amplifiers, which may be adapted in accordance with alternative embodiments, include for example, a ZMD31050 RB IC  series advanced differential sensor signal conditioner, commercially available from ZMD America Inc, of Melville, N.Y. 
         [0034]    The signal path of the ASIC  108  is partly analog and partly digital. The analog part is realized differentially. Consequently, it is possible to amplify positive and negative input signals, which are located in the common mode range of the signal input. The electrical output signal (V o )  207  from the signal conditioning unit  206  can be pre-amplified by a programmable gain amplifier (PGA)  402 . A multiplexer (MUX)  403  can be utilized to transmit signals generated by signal conditioning unit  206  or a separate temperature sensor (TS)  404  to an analog-to-digital converter (ADC)  405  in a certain sequence. Thereafter, the ADC  405  converts these signals to digital values. A digital signal correction takes place in a calibration microcontroller (CMC)  406  and is based on a special correction formula located in an ROM  407  and on sensor-specific coefficients stored into an EEPROM  408  during calibration. 
         [0035]    The output signal (V o  Amplified)  209  can be provided at a serial interface (SIF)  409 . The final value can be converted to an analog voltage via an 11-bit Digital-to-Analog converter (DAC)  410 . The analog output  411  possesses registers which can store the actual pressure and the results of temperature measurement. According to the programmed output configuration, the corrected sensor amplified signal output (V o  amplified)  209  is produced as the analog value. The configuration data and the correction parameters can be programmed into the EEPROM  408  via the serial digital interfaces  409 . The signal paths from pressure sense die  105  to the amplified output signal  209  (V o  amplified) is generally analog-digital-analog for isolation and compensation flexibility. 
         [0036]    The correction values at each temperature of calibration can be recorded utilizing a computer. A calculation can then be utilized to fabricate a multi-order equation that corrects the sensor&#39;s output over temperature. The coefficients for that equation can be loaded into the unit&#39;s EEPROM  408  after the final calibration temperature data is taken. After the correction equation coefficients are loaded into the unit&#39;s EEPROM  408 , the device is considered fully calibrated. The calibration procedure should preferably include the set of coefficients of calibration calculation and depending on the configuration, the adjustment of the extended offset compensation, the zero compensation of temperature measurement, and the adjustment of the bridge current. 
         [0037]    The pressure sensor described herein can be inexpensively manufactured and marketed and can include temperature compensation and calibration capabilities, along with media flow-through ports and true “wet” differential sensing and is also operable after exposure to frozen conditions with a choice of termination for gage sensors. Such a sensor can also provide interchangeability, proven elastomeric construction, ASIC based signal conditioning and digital output and can be used to measure vacuum or positive pressure. 
         [0038]    The disclosed pressure sensor device and system can find usefulness in a wide range of application, such as, for example, medical applications, including but not limited to dental chairs, nebulizers, kidney dialysis machines, blood cell separators and so forth. Such products not only simplify the testing, monitoring, and treatment process, but will also assist in improving the quality of life for patients by minimizing time spent in hospitals and providing automatic and continuous treatment of chronic conditions. Such a device and/or system can also be employed in environmental applications, such as water control valves, instrumentation, irrigation equipment, and so forth, whenever flow monitoring and control are important. 
         [0039]    It will be appreciated that variations of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.