Patent Publication Number: US-8995690-B2

Title: Microphone and method for calibrating a microphone

Description:
TECHNICAL FIELD 
     The present invention relates generally to a microphone and a method for calibrating a microphone. 
     BACKGROUND 
     MEMS (microelectromechancial system) devices are generally manufactured in large numbers on semiconductor wafers. 
     A significant problem in the production of MEMS devices is the control of physical and mechanical parameters of these devices. For example, parameters such as mechanical stiffness, electrical resistance, diaphragm area, air gap height, etc. may vary by about +/−20% or more. 
     The variations of these parameters on the uniformity and performance of the MEMS devices may be significant. In particular, parameter variations are especially significant in a high volume and low-cost MEMS (microphones) manufacturing process where the complexity is low. Consequently, it would be advantageous to compensate for these parameter variations. 
     SUMMARY OF THE INVENTION 
     In accordance with an embodiment of the present invention a method for calibrating a MEMS comprises operating a MEMS based a first AC bias voltage, measuring a pull-in voltage of the MEMS, calculating a second AC bias voltage or DC bias voltage, and operating the MEMS based the second AC bias voltage. 
     In accordance with an embodiment of the present invention a method for calibrating a MEMS comprises increasing a first AC bias voltage at the membrane, detecting a first pull-in voltage and setting a second AC bias voltage or DC bias voltage for the membrane based on the first pull-in voltage. The method further comprises applying a first defined acoustic signal to the membrane and measuring a first sensitivity of the microphone. 
     In accordance with an embodiment of the invention a microphone comprises a MEMS device comprising membrane and a backplate, an AC bias voltage source connected to the membrane, and a DC bias voltage source connected to the backplate. 
     In accordance with an embodiment of the invention an apparatus comprises a MEMS device for sensing an acoustic signal, a bias voltage source for providing an AC bias voltage to the MEMS device, and a control unit for detecting a pull-in voltage and for setting the AC bias voltage or a DC bias voltage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
         FIG. 1  shows a block diagram of a microphone; 
         FIGS. 2   a - 2   c  show functional diagrams; and 
         FIG. 3  shows a flow chart of an embodiment of a calibration of a microphone. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention. 
     The present invention will be described with respect to embodiments in a specific context, namely a microphone. The invention may also be applied, however, to other types of systems such as audio systems, communication systems, or sensor systems. 
     In a condenser microphone or capacitor microphone, a diaphragm or membrane and a backplate form the electrodes of a capacitor. The diaphragm responds to sound pressure levels and produces electrical signals by changing the capacitance of the capacitor. 
     The capacitance of the microphone is a function of the applied bias voltage. At negative bias voltage the microphone exhibits a small capacitance and at positive bias voltages the microphone exhibits increased capacitances. The capacitance of the microphone as a function of the bias voltage is not linear. Especially at distances close to zero the capacity increases suddenly. 
     A sensitivity of a microphone is the electrical output for a certain sound pressure input (amplitude of acoustic signals). If two microphones are subject to the same sound pressure level and one has a higher output voltage (stronger signal amplitude) than the other, the microphone with the higher output voltage is considered having a higher sensitivity. 
     The sensitivity of the microphone may also be affected by other parameters such as size and strength of the diaphragm, the air gap distance, and other factors. 
     The condenser microphone may be connected to an integrated circuit such as an amplifier, a buffer or an analog-to-digital converter (ADC). The electrical signal may drive the integrated circuit and may produce an output signal. In one embodiment, the gain of a feedback amplifier may be adjusted or calibrated by varying the ratio of a set of resistors, a set of capacitors, or a set of resistors and capacitors that are coupled as a feedback network to the amplifier. The feedback amplifier can be either single ended or differential. 
     In a MEMS manufacturing process the pressure-sensitive diaphragm is etched directly into a silicon chip. The MEMS device is usually accompanied with integrated preamplifier. MEMS microphones may also have built in analog-to-digital converter (ADC) circuits on the same CMOS chip making the chip a digital microphone and so more readily integrated with modern digital products. 
     According to an embodiment of the invention, a combination of AC bias voltage adjustment and an amplifier gain adjustment allows the adjustment of the microphone. According to an embodiment of the invention, the microphone is calibrated during operation with an AC bias voltage. In one embodiment of the invention the operating AC bias voltage is set based on a pull-in voltage of the membrane. 
     In one embodiment it is advantageous to operate the microphone with the highest possible bias voltage. The higher the bias voltage the more sensible is the microphone. The higher the sensibility of the microphone the better is the signal to noise ratio (SNR) the microphone system. 
       FIG. 1  shows a block diagram of a microphone  100 . The microphone  100  comprises a MEMS device  110 , an amplifier unit  120 , an AC bias voltage source  130  and a digital control unit  140 . 
     The AC bias voltage source  130  is electrically connected to the MEMS device  110  via resistor R charge pump    150 . In particular, the AC bias voltage source  130  is connected to the membrane or diaphragm  112  of the MEMS device  110 . The backplate  114  of the MEMS device  110  is connected to the DC bias voltage source  160  via the resistor R inbias    170 . The MEMS device  110  is electrically connected to an input of an amplifier unit  120 . An output of the amplifier unit  120  is electrically connected to an output terminal  180  of the microphone  100  or to an analog-digital converter ADC (not shown). 
     Digital control lines connect the digital control unit  140  to the amplifier unit  120  and the AC bias voltage source  130 . The digital control unit  140  may comprise a glitch detection circuit. An embodiment of the glitch detection circuit is disclosed in co-pending application application Ser. No. 13/299,098 which is incorporated herein by reference in its entirety. The digital control unit  140  or the glitch detection circuit detects a pull-in or collapse voltage (V pull-in ) at an input of the amplifier unit  120 . The digital control unit  140  also measures the sensitivity of the output signal of the amplifier unit  120  and controls the AC bias voltage source  130 . Memory elements such as volatile or non-volatile may be embedded in the digital control unit  140  or may be a separate element in the microphone  100 . 
     During calibration operation of the microphone  100  a first AC bias voltage (comprising of an AC component provided by the AC bias voltage source  130  and a DC component provided by the bias voltage source  160 ) is applied to the MEMS device  110 . The first AC bias voltage is increased until the backplate  114  and the membrane  112  collapse or until the distance between the backplate  114  and the membrane  112  is minimized, e.g., zero. The pull in voltage (V pull-in ) is measured or detected by the digital control unit  140 . The pull in voltage (V pull-in ) may be detected by a voltage jump at the input of the amplifier unit  120 . A second AC bias voltage is derived from the pull in voltage (V pull-in ). The second AC bias voltage may be stored in the memory elements. 
     The first AC bias voltage may comprise a maximal amplitude of an AC component of about 1% to about 20% of a value of the DC component. Alternatively, the AC component may be about 10% to about 20% of the value of the DC component. For example, the DC voltage V DC  may be about 5 V and the AC voltage V AC  may be about 0.5 V to about 1 V. Alternatively, the AC component may comprise other values of the DC component, e.g., higher values or lower values. The second AC bias voltage may comprise a maximal amplitude of an AC component comprises about 0% to about 20% of a value of the DC component because the microphone can also be operated with a DC bias voltage. 
     According to an embodiment of the invention, a DC voltage is superimposed with an AC voltage. The first AC bias voltage may comprise a low frequency such as a frequency of up to 500 Hz or up to 200 Hz. Alternatively, the first AC bias voltage may comprise a frequency from about 1 Hz to about 50 Hz. The second AC bias voltage may comprise a low frequency such as a frequency of up to 500 Hz or up to 200 Hz. Alternatively, the second AC bias voltage may comprise a frequency from about 0 Hz to about 50 Hz. 
     After setting the second AC bias voltage a defined acoustic signal is applied to the microphone  100 . The sensitivity of the microphone  100  is measured at the output terminal  180  and compared to a target sensitivity of the microphone  100 . The control unit  140  calculates a gain setting so that the microphone meets its target sensitivity. The gain setting is also stored in the memory elements. 
       FIGS. 2   a - 2   c  show different functional diagrams.  FIG. 2   a  shows a diagram wherein the vertical axis corresponds to the AC bias voltage V bias  and the horizontal axis represents the time t. The AC bias voltage V bias  comprises a DC component and an AC component.  FIG. 2   a  shows the AC bias voltage V bias  as DC voltage superimposed with an AC voltage. In one embodiment the AC bias voltage V bias  can be increased/decreased by increasing the DC component and by keeping the AC component constant. Alternatively, the AC bias voltage V bias  can be increased by increasing/decreasing the DC component and increasing/decreasing the AC component. The AC bias voltage may be a periodic sine voltage or a periodic square wave voltage. The AC component may be set for the possible tolerance of the pull in voltage. 
     In a MEMS calibration process, the AC bias voltage V bias  may be increased up to the pull-in voltage event and then decreased until at least the release voltage event.  FIG. 2   b  shows a diagram wherein the vertical axis corresponds to the capacity of the MEMS C 0  and the horizontal axis corresponds to the time t. The graph in  FIG. 2   b  shows the capacitance changes of the MEMS C 0  over time with increasing/decreasing AC bias voltage V bias . The graph shows two significant steps. The capacitance of the MEMS C 0  changes slightly in a first region up to the pull in voltage event. Around and at the pull-in voltage event the capacitance C 0  increases substantially. Thereafter, the AC bias voltage V bias  is decreased and the capacitance C 0  does not change or barely changes the capacitance C 0  until the pull out voltage event (or release voltage event). Around and at the pull-out voltage event the capacitance decreases substantially. 
       FIG. 2   c  shows a diagram wherein the y-axis corresponds to the input voltage V in  at the input of the amplifier unit and the horizontal axis represents the time t. The input voltage V in  shows small positive and negative amplitudes or voltage impulses. In the event that the membrane and the backplate touch each other, the amplitude is substantially larger than the regular voltage impulses. Similar, in the event that the membrane and the backplate are released from each other, the amplitude is substantially larger than the regular voltages impulses. 
     When the AC bias voltage V bias  is increased until the membrane and the backplate touch each other and the pull in voltage is reached, the MEMS capacitance changes substantially. A glitch occurs at the input of the amplifier unit  120  and the information is processed in the control logic unit  140 . After this event, the AC bias voltage V bias  can be reduced in one embodiment, until the membrane and the backplate separate. At this event, the MEMS capacitance C 0  is reduced to its original value and a voltage glitch at the input of the amplifier unit  120  is visible again. This indicates the pull out voltage or release voltage. 
       FIG. 3  shows a flow chart of a calibration process for a microphone. The flow chart includes two global steps and eight detailed steps. In a first global step a second AC bias voltage is set and in a second global step the amplifier gain is calculated based on the measured sensitivity of the microphone. To measure the sensitivity of the microphone a first AC bias voltage is applied to the membrane wherein the first AC bias voltage comprises an AC component from the AC bias voltage source and a DC component from the DC bias voltage source applied to the backplate. 
     In a first detail step  302  the digital control unit starts the calibration process by increasing a first AC bias voltage biasing the MEMS device. The AC bias voltage may be increased as shown in  FIG. 2   a . Increasing the first AC bias voltage leads eventually to a collapse of the membrane and the backplate. In step  304  the collapse or pull-in voltage is detected by a significant positive jump of the input voltage V in  as soon as the membrane and the backplate touch each other. An example can be seen in  FIG. 2   c . The pull-in voltage (V pull-in ) may be defined as the pull-in voltage with the lowest voltage necessary to collapse the two plates. This event may be detected by the digital control unit at the input of the amplifier unit. After detecting the pull-in voltage, the digital control unit may stop increasing the AC bias voltage. 
     In optional step  306  the digital control unit may decrease the AC bias voltage (through the AC bias voltage source). The AC bias voltage may be decreased as shown in  FIG. 2   a . The release voltage or pull-out voltage is detected by a significant negative jump of the input voltage V in  as soon as the membrane and the backplate release or separate from each other. An example can be seen in  FIG. 2   c . This event may be detected by the digital control unit at the input of the amplifier unit. After detecting the release voltage, the digital control unit may stop decreasing the AC bias voltage. 
     In step  308 , the digital control unit sets a second AC bias voltage or a DC bias voltage based on the detected pull-in voltage (V pull-in ) and, optionally, based on the release voltage V release . For example, the second AC bias voltage or DC bias voltage (V FAC ) can be set as V FAC =V release −V margin , wherein V margin  depends on the expected sound levels. The value of V FAC  may be stored in the memory elements. 
     In step  310 , a defined acoustic signal is applied to the MEMS device. The MEMS device is biased with the second AC bias voltage V FAC  or the DC bias voltage. The digital control unit may measure an output sensitivity of the amplifier unit at the output terminal (step  312 ). Then, in step  314 , the digital control unit may calculate a difference between the target sensitivity and the measured output sensitivity. Finally, in step  316 , the digital control unit calculates a gain setting for the amplifier unit in order to match the measured output sensitivity with the target output sensitivity. The digital control unit may store the gain setting parameters in or outside of the digital control unit. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. 
     Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.