Abstract:
A pressure sensor module is provided with an isolation platform which isolates stress. The pressure sensor module includes a base structure and a cantilever member formed in the base structure by an isolation gap. A pressure sensing element is located on the cantilever member such that the cantilever member provides stress isolation to the pressure sensing element.

Description:
TECHNICAL FIELD  
       [0001]     The present invention generally relates to pressure sensing and, more particularly, relates to a pressure sensor module that realizes minimal package stress.  
       BACKGROUND OF THE INVENTION  
       [0002]     Pressure sensors are commonly employed in automotive vehicle applications to control and monitor various aspects of vehicle operation. The pressure sensors are typically required to provide an accurate analog voltage output representative of the pressure applied to a sensing element. In automotive applications, the pressure sensor is generally required to be accurate over a large temperature range of approximately −40° to +125° C. throughout the life of the vehicle.  
         [0003]     A typical pressure sensor is shown in  FIGS. 1 and 2  including two components, namely a pressure sensing element shown in  FIG. 1  and a compensation circuit shown in  FIG. 2 . The pressure sensing element  10  shown and described herein is a piezo-resistance sensor having four resistors R 1 -R 4  configured in a Wheatestone Bridge. Input terminals  16  and  18  are coupled to voltage supply Vs and ground, respectively. Resistors R 1  and R 3  decrease in magnitude proportional to the applied pressure, and resistors R 2  and R 4  increase in magnitude proportional to the applied pressure. An increase in sensed pressure causes an increase in the voltage Vo+ on terminal  14 , and a decrease in the voltage Vo− on terminal  12 , thus producing a differential output voltage Vo+ minus Vo− that is proportional to the pressure applied to the sensing element  10 . Pressure sensing elements have alternately been configured to include a variable capacitance type element.  
         [0004]     The compensation circuit  20  shown in  FIG. 2  can be a separate integrated circuit (IC) or may be integrated with the pressure sensing element  10  or other circuitry. The compensation circuit  20  receives the differential voltage inputs Vo+ and Vo− at terminals  14  and  12 , respectively, and applies a differential voltage to a voltage-to-current converter and multiplier  22 . In addition to converting the differential voltage to a current signal, the multiplier compensates for gain at room temperature and temperature dependent gain. This is achieved by controlling current sources I A  and I B  via a programmed function, such as lookup table  24 . The temperature compensated current signal is then applied to a negative terminal of an amplifier  28 .  
         [0005]     The compensation circuit  20  also has a current source I S  applied to the negative terminal of the amplifier  28 . The current source I S  compensates for sensor offsets at room temperature and temperature dependent sensor offsets. This is achieved by controlling current source I S  via a programmed function, such as lookup table  26 .  
         [0006]     The resultant current is converted to an output voltage V OUT  across the amplifier  28  and feedback resistor R FB . The resistor R LD  applies current to place the output at a desired direct current (DC) offset. The resultant output voltage V OUT  at output terminal  30  is the desired compensated output signal ranging in value between ground and supply voltage V DD . The output voltage V OUT  is proportional to the pressure applied as an input to the sensing element  10 . Linear errors in gain, offset, and temperature dependency are thus compensated with the compensation circuit  20 .  
         [0007]     With many pressure sensors, the sensing element is packaged in a module that is easily susceptible to module package stress. Such module package stress generally causes a differential voltage from the pressure sensor to produce non-linear temperature effects. The resultant stresses on the module exhibited with conventional pressure sensing modules typically change over the life of the sensor package and cause the output voltage V OUT  signal to drift over time. The aforementioned non-linear temperature effects and the long-term drift generally cannot be easily calibrated out of the sensor arrangement, and thus will generally cause errors in the sensor output. These resultant errors limit the accuracy of the pressure sensor and complicate the sensor module design.  
         [0008]     It is therefore desirable to provide for a pressure sensor module that experiences reduced or minimal package stress. It is further desirable to provide for such a pressure sensor module that experiences reduced or minimal non-linear temperature effects and sensor signal drift.  
       SUMMARY OF THE INVENTION  
       [0009]     In accordance with the teachings of the present invention, a pressure sensor module is provided which offer isolation to reduce or minimize stress. The pressure sensor module includes a base structure and a cantilever member formed in the base structure. The pressure sensor module also includes a pressure sensing element located on the cantilever member. The cantilever member serves as a stress isolated platform.  
         [0010]     According to another aspect of the present invention, the base structure includes a first member connected to a second member. The cantilever member is formed by an isolation gap formed between the first and second members. According to a further aspect of the present invention, the cantilever member is formed by an isolation gap extending into an underlying housing. By arranging the pressure sensing element on the cantilever member, the sensor element is less susceptible to package stress.  
         [0011]     These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
         [0013]      FIG. 1  is a circuit diagram illustrating a piezo-resistive pressure sensing element configured as a Wheatestone Bridge;  
         [0014]      FIG. 2  is a compensation circuit for processing the sensed pressure signal;  
         [0015]      FIG. 3  is a top view of a pressure sensing module employing a stress isolated platform according to a first embodiment of the present invention;  
         [0016]      FIG. 4  is a cross-sectional view of the pressure sensing module taken through lines IV-IV of  FIG. 3 ;  
         [0017]      FIG. 5  is a top view of a pressure sensing module employing a stress isolated platform according to a second embodiment of the present invention; and  
         [0018]      FIG. 6  is a cross-sectional view of the pressure sensing module taken through lines VI-VI of  FIG. 5 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]     A pressure sensing module is shown and described herein having a pressure sensing element mounted on a cantilever member supported on a base structure according to the present invention. The arrangement of the pressure sensing element on the cantilever member reduces stress and improves the performance of the pressure sensor in a cost-effective manner. By reducing package stress, enhanced accuracy calibration of the pressure sensor may be achieved by eliminating non-linearities. The pressure sensor module is shown and described herein according to first and second embodiments, but is not intended to be limited to the specific embodiments shown.  
         [0020]     Referring to  FIGS. 3 and 4 , a pressure sensor module  40  is illustrated according to a first embodiment of the present invention. The pressure sensor module  40  includes a pressure sensing element  10  mounted on a sensor cell  42 . The sensor cell  42  is a supporting base member that may be made of silicon and supports the sensing element  10  and electrical circuitry including electrical contact pads  48 . The sensor cell  42 , in turn, is adhered onto the upper surface of an underlying substrate  46  via an adhesive  44 .  
         [0021]     Adhesive  44  is applied to fill in only a portion of the region between the bottom surface of sensor cell  42  and upper surface of substrate  46 . In particular, the application of adhesive  44  is limited to an inactive sensing region of sensor cell  42  so as to create an isolation gap  50  between a portion of sensor cell  42  and substrate  46 . The isolation gap  50  results in the formation of a cantilever member formed by the portion of sensor cell  42  including the sensing element  10  extending over isolation gap  50 .  
         [0022]     The pressure sensing element  10  is arranged on the sensor cell  42  in a region on or over the isolation gap  50  and, thus, is arranged on the cantilever member. By arranging the sensing element  10  on the cantilever member, the amount of stress realized by the pressure sensor module  40  is advantageously minimized with the present invention. By minimizing the resultant stress that is experienced, the pressure sensor module  40  achieves enhanced pressure sensing accuracy.  
         [0023]     The pressure sensing element  10  may include any of a number of pressure sensing elements, such as piezo-resistive elements and variable capacitance type sensors. The pressure sensing element  10  has contact terminals which, in turn, are electrically coupled to contact pads  48  formed on the upper surface of the sensor cell  42 . The contact terminals  48  may in turn be electrically coupled to a compensation circuit which may be integrated with or separate from an electronic control module for further processing the output signal generated with the pressure sensor.  
         [0024]     Referring to  FIGS. 5 and 6 , a pressure sensing module  40  is illustrated according to a second embodiment of the present invention. The pressure sensing element  40 ′ includes a pressure sensing element  10  mounted on a sensor cell  42 . The sensor cell  42 , in turn, is adhered on its lower surface to an upper surface of an underlying substrate  46 . In contrast to the first embodiment, sensor cell  42  and substrate  46  are adhered together via a substantially continuous layer of adhesive  44  so as not to form a gap therebetween. The substrate  46 , in turn, is adhered on its lower surface to an upper surface of a housing  54  via adhesive layer  52 . Housing  54  may include a ceramic material, such as low temperature co-fired ceramic (LTCC) made from multiple thin layers of ceramic. Together, the sensor cell  42 , substrate  46 , and housing  54  form a base structure.  
         [0025]     The pressure sensing module  40 ′ includes the presence of a cantilever member formed in the housing  54  which, in turn, supports the substrate  46  and sensor cell  42  containing the pressure sensing element  10 . The cantilever member is labeled as member  56  of housing  54  formed by an isolation gap  60  extending into housing  54 . The isolation gap  60  may be formed by cutting, etching, or otherwise forming a slot, channel, or other opening into housing  54  from the upper surface vertically downward and then orthogonal thereto, as shown. Alternately, the isolation gap  60  may be formed extending from one side edge extending into but not completely through to the opposite side edge. The isolation gap  60  may be formed in housing  54  during formation of housing  54  by patterning layers to add channels and cavities in the final substrate material of housing  54 . The ceramic layers may then be sandwiched together and then co-fired to create the final substrate including the isolation gap  60 . Alternately, it should be appreciated that the isolation gap  60  may be formed by etching, cutting, or other known removal techniques for forming a slot, channel, or other opening to provide the isolation gap and cantilever member.  
         [0026]     According to this arrangement of pressure sensor module  40 ′, the pressure sensing element  10  is supported on a base structure made up of sensor cell  42 , substrate  46 , and cantilever member  56  of housing  54  so as to realize reduced package stress. The pressure sensing element  10  includes electrical terminals electrically coupled to contact pads  48 . Contact pads  48  in turn may be electrically coupled to compensation circuitry which may be integrated with or separate from an electronic control module which further processes the output signal generated with the pressure sensor.  
         [0027]     The pressure sensing modules  40  and  40 ′ employing the cantilever arrangement of the present invention achieve significantly lower stress levels in the active region of the pressure sensor element  10 . It is further possible to control the amount of cantilever stress reduction in the pressure sensor and substrate to achieve certain resulting characteristics. It should be appreciated that the sensing element  10  may be formed in various types of sensor cells and the cantilever member may be formed in any of a number of base structure members including, but not limited, the sensor cell  42  itself, the substrate  46 , the housing  54 , or any intermediary layers.  
         [0028]     Accordingly, the pressure sensing modules  40  and  40 ′ of the present invention advantageously reduce the amount of stress that is experienced and improve the performance of the pressure sensor in a cost-effective manner. The reduction of the stress achieved with the present invention allows for enhanced accuracy calibration of the pressure sensor by eliminating or reducing non-linearities. The elimination or reduction of non-linearities produces a more accurate pressure sensor that can be reliably manufactured. Additionally, by sufficiently isolating the stress, it is possible to calibrate sensing circuits in bulk, and singulate the sensor module packages. Testing in bulk is highly desirable, because the manufacturing costs of the pressure sensor can be significantly reduced.  
         [0029]     It will be understood by those who practice the invention and those skilled in the art, that various modifications and improvements may be made to the invention without departing from the spirit of the disclosed concept. The scope of protection afforded is to be determined by the claims and by the breadth of interpretation allowed by law.