Abstract:
A MEMS sensor packaged with an integrated circuit includes switches and control circuitry. In a test mode, the control circuitry causes the switches to turn off and on such that the first and second capacitance of the MEMS sensor can be monitored individually. During a normal mode of operation, the switches are maintained such that the MEMS sensor packaged with the integrated circuit operates to produce a filtered and trimmed output reflecting the sensed phenomena.

Description:
FIELD OF THE INVENTION 
     This invention relates to MEMS circuits and, more specifically, to a method and circuit for testing the characteristics of the individual capacitances of a MEMS sensor that has been sealed or packaged that may be integrated with another circuit or packaged with another circuit. 
     BACKGROUND OF THE INVENTION 
     One prior art model of a MEMS sensor integrated with a circuit is shown in FIG.  1 . The MEMS sensor  100  is represented by a first capacitor  110  and a second capacitor  120  sharing a common node  130   a  that represents a moving element. When an excitation voltage is applied to the plates  110   a  and  120   a  of a MEMS sensor and fluctuations occur on the moving element  130   a  due to an input stimulus like acceleration or pressure, the moving element changes its position according to the input stimulus. When this occurs, capacitances between  110  and  120  change. The output  130  of the moving element of the MEMS sensor  100  is fed into a first amplifier input  141  of a capacitor-voltage (C-V) converter  143 . The other input to the amplifier is connected to a reference voltage  142 . During a reset, the reference voltage is also applied to the two plates  181  and  182  of the MEMS sensor  100 . The reference voltage  142  can be hardwired to the sensor and amplifier directly through the pins of the package or can be controlled by an on-chip control  150 , such as an ASIC or other control logic. In either case, after the packaged MEMS sensor and IC have been reset by switch  192 , the voltage applied to the plates of the sensor is excited by changing the voltage directly applied to the pins of the package or by programming the control logic to switch between various voltages supplied to the package. The excitation voltage applied to plates  181  and  182  start at the voltage reference after reset and then are excited to an excitation voltage that is equal in magnitude and opposite in polarity. For instance, the voltage applied to the first plate  181  would step from the voltage reference to an excitation voltage (Vexcite) at the same time that the voltage applied to the second plate  182  steps from the reference voltage Vref to a negative excitation voltage (−Vexcite). Each step function would then alternate to its original reference voltage state and back again so that any fluctuations on the moving element  130   a  would cause corresponding fluctuations on the capacitors  110  and  120 . 
     The amplifier  140  produces a C-V output voltage  155  reflecting the difference between the first and second capacitances  110  and  120  experienced by fluctuations in the moving member caused by the input stimulus. The C-V output voltage  155  is typically modified by a feedback capacitance Cref represented by feedback path  145  (and reset by switch  192 ) such as to produce an output voltage Vout=−[(C 1 −C 2 )/Cref]*(Vexcite−Vref). The C-V output voltage  155  is then signal conditioned as needed by other integrated circuitry  160 , such as filters, gain and offset trim and the like. The final output voltage  170  of the integrated device represents the physical activity of the MEMS sensor and is used in various applications such as accelerometers, pressure sensors, gyroscopes. 
     To reduce failure rates, the MEMS sensors are tested before being packaged. However, before and during packaging with the circuit, additional processing problems cause some MEMS sensors to malfunction or become damaged. Some problems may occur due to moisture ingress into the capacitor, for example. Common MEMS problems involve stiction where the moving element  130   a  or proof mass comes into contact with the fixed plates  181 ,  182 . Additionally, breakages or holes may occur in the moving element. Because the MEMS sensor&#39;s moving element is extremely delicate, often resulting in capacitance changes in the few femto-farad range, direct connections to sensor elements such as the moving element are problematic as any probing would make the measurements inaccurate. Additionally, once the circuit and sensor are packaged, the only measurement available always reflects the difference between both sensor capacitances and does not assist in identifying problems with the individual capacitances. 
     Accordingly, what is needed is a packaged sensor device that allows for more accurate testing of the MEMS sensor after it has been packaged with an IC. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified electrical schematic of a prior art MEMS sensor integrated with an IC circuit; 
     FIG. 2 is a simplified electrical schematic of one embodiment of a MEMS sensor integrated with an IC circuit that includes testing circuitry; 
     FIGS. 3-5 are a series of simplified schematics during various timing cycles of testing the Sensor&#39;s first capacitance; 
     FIGS. 6-8 are a series of simplified schematics during various timing cycles of testing the Sensor&#39;s second capacitance; and 
     FIGS. 9-10 are flow charts demonstrating the steps required for testing the various capacitances of a MEMS sensor packaged with an IC. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 2, one model of an embodiment of a MEMS sensor  100  packaged with a circuit that includes testing circuitry is shown. For simplicity, similar reference numerals are used throughout the figures to represent similar features when possible. 
     The MEMS sensor  100  is represented by a first capacitor  110  and a second capacitor  120  such that when an excitation voltage is applied to the plates  181 ,  182  of a MEMS sensor and fluctuations occur on the moving element  130   a , a difference in the capacitances C 1   110  and C 2   120  may be observed. The output  130  of the moving element  130   a  of the MEMS sensor  100  is fed into a first input  141  of an amplifier  140  of C-V converter  143 . The other input  142  to the amplifier  140  is connected to a reference voltage Vref that is also typically applied to the first and second plates  181  and  182  of the MEMS sensor  100  during a reset stage. Switches  190  and  191  allow for blocking a voltage being applied to any combination of the plates of the sensor  181  and  182 . Switches  190  and  191  may be any electrical device that can operate in two states, one allowing current to flow and the other state preventing current from flowing across its terminals  190   a  to  190   b  and  191   a  to  191   b . For instance, the switch could be a CMOS transistor with gates  190   c / 191   c , sources  190   a / 191   a  and drains  190   b / 191   b . When the voltage applied to the gate reaches a point where the Voltage exceeds a known threshold the switch turns “ON” and allows the current to flow from the source to the drain. The nomenclature allows the switch to be “ON” in the sense that the transistor has been activated, but from the model of a switch  191  and  190 , it is also acceptable to consider the switch “CLOSED” when the transistor is “ON.” Accordingly, this nomenclature will be used throughout the description. It should be made clear that other nomenclatures could be adopted and are within the scope of the invention. In addition, other types of transistors or switches may be used that accomplish a similar electrical phenomena and any reference to a switch made in this description is likewise defined. 
     The voltage applied to input  142  of the amplifier  140  and to the source sides of the switch  190   a  and  191   a  may be hardwired to a pin on the package that may be manipulated on a testing bench or the package can have pins for multiple voltages as shown in FIG.  2 . In this case, the reference voltage  142  and another excitation voltage is hardwired to the circuit and is manipulated by a control logic, such as an ASIC for applying the various voltages to the switches  190  and  191  as well as the input of the amplifier  142 . The design may have more than two voltages coming into the control logic if needed and may apply varying voltages to the sources of the switches  190  and  191  such that the voltage appearing on source  191   a  could be different that that appearing on  190   a  and  142  for instance. Any combination of applied voltages is within the scope of the invention. 
     Therefore, the reference voltage Vref and the excitation voltage Vexcite can be hardwired to the sources of the switches and/or amplifier directly through the pins of the package or can be controlled by an on-chip control  150 , such as an ASIC or other control logic. ASIC designs and other control logic are well known in the electrical arts for controlling when and what duration to turn on switches and apply various voltages and accordingly is not discussed in detail. 
     The voltage applied to the amplifier and the plates of the sources of the switches can be altered by changing the voltage directly applied to the pins of the package or by programming the control logic to switch between various voltages supplied to the package. During normal operation, the amplifier  140  produces an C-V output voltage  155  reflecting the difference between the first capacitor C 1  and second capacitor C 2  experienced by fluctuations in the sensor&#39;s moving element. This output  155  is typically modified by a feedback capacitance Cref represented by feedback path  145  such as to produce an output voltage C-Vout=−[(C 1 −C 2 )/Cref]*(Vexcite−Vref). Another switch  192  provides for discharging the reference capacitance  145  when necessary for testing or resetting by closing the switch  192 . 
     The C-V output voltage  155  is then adjusted as needed by other integrated circuitry  160 , such as filters and trimmers. Another switch  193  in combination with switch  194  allows the additional integrated circuitry  160  to be bypassed when the switch  193  is closed and the switch  194  is opened allowing the final output voltage  170  to be the C-V output voltage of the amplifier appearing at  155 . During normal operation, the output voltage  170  of the integrated device represents the physical activity of the MEMS sensor and is used in various applications such as accelerometers, pressure sensors and gyroscopes. 
     During normal operation, switches  190 ,  191  and  194  are closed or “ON”, switch  193  is open or “OFF”, and switch  192  is alternately “on” and “off” during operation as needed. This allows the circuit to perform like traditional MEMS sensors packaged with an integrated circuit where the voltage appearing at  155  reflects the total action of the sensor&#39;s moving element by measuring the difference of the representative capacitances, such that C-Vout=−[(C 1 −C 2 )/Cref]*(Vexcite−Vref) and the voltage appearing at  170  is the C-V output voltage modified by any circuitry in  160  such as filters or trimmers. 
     FIGS. 3-5 are embodiments of the same circuit during various timing sequences of a test operation where the first capacitance C 1  of the sensor is being tested. In FIG. 3, the control circuitry in a first instance opens switch  190  and  194  while closing switches  191 ,  192  and  193 . The reference voltage is applied to both switches  190  and  191  as well as to the amplifier  140  through input  142 . However, as the switch is opened leading to the plate of second capacitor C 2 , no voltage actually gets applied to C 2 . During the first instance shown in FIG. 3, the reference capacitance Cref  145  is discharged when switch  192  is closed. 
     In a second instance, switch  192  is opened resulting in the circuit shown in Figure. And, in a final instance, the reference voltage applied to switch  191  is stepped to a known excitation voltage Vexcite as shown in FIG.  5 . The switching of voltages allows the first capacitance C 1  of the sensor to be evaluated as the output voltage appearing at  155  and  170  (because switch  193  is closed and switch  194  is opened)=−[C 1 /Cref]*(Vexcite−Vref). The circuit can then be switched between the circuits shown in FIGS. 3-5 as needed. 
     Similarly, a cycle of switching can be programmed for testing the second capacitance C 2  of the sensor as shown in FIGS. 6-8. FIGS. 6-8 are embodiments of the same circuit during various timing sequences of a test operation where the second capacitance C 2  of the sensor is being tested. In FIG. 6, the control circuitry in a first instance opens switch  191  and  194  while closing switches  190 ,  192  and  193 . The reference voltage is still supplied to the switch  190  and to the amplifier  140 . During the first instance shown in FIG. 6, the capacitance Cref is discharged by closing switch  192 . 
     In a second instance, switch  192  is opened resulting in the circuit shown in FIG.  7 . And, in a final instance, the reference voltage applied to switch  190  is switched to a known excitation voltage −Vexcite as shown in FIG.  8 . The application of the excitation voltage allows the second capacitance of the sensor to be evaluated as the output voltage appearing at  155  and  170  (because switch  193  is closed)=[C 2 /Cref]*(Vexcite−Vref). The circuit can then be switched between the circuits shown in FIGS. 6-8 as needed. 
     FIG. 9 is a flow chart showing how the various capacitances are monitored on the sealed MEMS sensor integrated with a circuit. In step  910 , the first capacitance C 1  is enabled while the second capacitance C 2  is disabled by opening switch  190  for disabling power to the second capacitance. In step  920 , the MEMS sensor packaged with the IC is reset by discharging the reference capacitance Cref of the amplifier. In step  930 , the voltage applied to the first capacitor is excited at the plate  181  of the sensor. This allows the individual capacitance C 1  to be observed in step  940  as C-Vout  155  equals the ratio—C 1 /Cref(Vexcite−Vref). 
     Similarly, FIG. 10 is a flow chart showing how the second capacitance is monitored. In step  1010 , the second capacitance is enabled while the first capacitance is disabled for testing by disabling the power to the first capacitance (turning switch  191  off). In step  1020 , the MEMS sensor packaged with an IC is reset by discharging the amplifier&#39;s feedback capacitance Cref. In step  1030 , the voltage applied to the second capacitor is excited at the plate  182  of the sensor. This allows the individual capacitance C 2  to be observed in step  1040  as C-Vout  155  equals the ratio [C 2 /Cref](Vexcite−Vref). 
     Once the testing cycle has been initiated by discharging the reference capacitance Cref  145 , the excitation voltage applied may be a step function with the reference capacitance Cref  145  intermittently reset so the individual capacitances can be monitored while other environmental conditions are adjusted, such as temperature. By isolating the various capacitances of the sensor a more accurate understanding of a defect can take place such that processes can be improved and failing devices better identified reducing defective parts shipped to customers. 
     While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.