Abstract:
A method of detecting defects in a high impedance network of a MEMs microphone sensor interface circuit. The method includes adding a high-voltage reset switch to a high-voltage high impedance network, closing the high-voltage reset switch during a start-up phase of the MEMs microphone sensor interface circuit, simultaneously closing a low-voltage reset switch of a low-voltage high impedance network during the start-up phase, simultaneously opening the high-voltage reset switch and the low-voltage reset switch at the end of the start-up phase, and detecting a defect in the high-voltage high impedance network or the low-voltage high impedance network immediately after opening the high-voltage reset switch and the low-voltage reset switch.

Description:
RELATED APPLICATION 
       [0001]    This patent claims the benefit of prior filed co-pending U.S. Provisional Patent Application No. 62/048,571; filed on Sep. 10, 2014; and is a continuation of co-pending U.S. Pat. No. 9,743,203, filed Sep. 10, 2015, the entire contents of which are hereby incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    This patent relates to MEMS microphone incorporating a reset for a high-voltage high-impedance network for the microphone bias node allowing faster, more efficient testing of the MEMS microphone. 
         [0003]    MEMS capacitive microphones operate utilizing conservation of charge. A high impedance network, usually consisting of two anti-parallel diodes is used to apply a fixed charge across two plates of a capacitor. A high impedance network on a sense node is necessary to create the charge conservation node. A high impedance switch network on a bias node is followed by a capacitor to ground which is large in comparison to the capacitance from the sense node to ground. This capacitor serves two purposes. First the capacitor creates an AC ground on the bias side of the sensor so that in the presence of an acoustic signal, the voltage on the sense node changes. Second, the capacitor along with the high impedance network creates a low-pass filter for noise generated by the biasing circuits. 
         [0004]    Leakage current from the high-impedance bias node to ground can degrade the performance of the microphone. The leakage current from bias to ground lowers the impedance of the high impedance network and if large enough can compromise the noise filtering of the bias circuits, ultimately degrading the noise performance of the entire microphone. Similarly, leakage current from the bias node to the sense node flows into the sense node diodes giving rise to shot-noise which also degrades the noise performance of the entire microphone. Defects near these high impedance nodes which can be due to particles, surface contamination, or bulk material defects can give rise to these leakage currents which will affect the high impedance network. Furthermore these defects can be exacerbated through reliability and environmental stresses making early detection even more important to ensuring the quality of the microphone. 
         [0005]    U.S. patent application Ser. No. 13/040,466 describes one of many implementations for realizing high-voltage high impedance circuits. 
       SUMMARY 
       [0006]    This patent describes a microphone design and corresponding test for early detection of defects to the high impedance nodes. Design of a microphone in which both high-impedance networks attached to the sensor are switched during start-up allows for a test which can identify defects in the high impedance nodes leading to compromised microphone performance. 
         [0007]    In one embodiment, the patent provides a method of detecting defects in a high impedance network of a MEMs microphone sensor interface circuit. The method includes adding a high-voltage reset switch to a high-voltage high impedance network, closing the high-voltage reset switch during a start-up phase of the MEMs microphone sensor interface circuit, simultaneously closing a low-voltage reset switch of a low-voltage high impedance network during the start-up phase, simultaneously opening the high-voltage reset switch and the low-voltage reset switch at the end of the start-up phase, and detecting a defect in the high-voltage high impedance network or the low-voltage high impedance network immediately after opening the high-voltage reset switch and the low-voltage reset switch. 
         [0008]    In another embodiment, the patent provides a high-voltage reset MEMs microphone sensor interface circuit. The circuit includes a charge pump, a low-voltage high impedance network, a high-voltage high impedance network, a sense capacitor, a high impedance amplifier, and an output capacitor. The low-voltage high impedance network is coupled to a direct current potential and to a sense node. The low-voltage high impedance network includes a set of anti-parallel diodes and a low-voltage reset switch. The high-voltage high impedance network is coupled to the charge pump and to a bias node. The high-voltage high impedance network includes a set of anti-parallel diodes and a high-voltage reset switch. The sense capacitor is coupled between the sense node and the bias node. The high impedance amplifier is coupled to the sense node. The output capacitor coupled between the bias node and ground. The low-voltage reset switch and the high-voltage reset switch are closed during a start-up phase of the MEMs microphone sensor interface circuit and simultaneously opened at an end of the start-up phase. 
         [0009]    Other aspects of the patent will become apparent by consideration of the detailed description and accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic diagram of a prior art MEMs microphone sensor interface circuit. 
           [0011]      FIG. 2  is a graph of node voltages versus time for the sensor interface circuit of  FIG. 1 . 
           [0012]      FIG. 3  is a schematic diagram of a MEMs microphone high-voltage reset sensor interface circuit in accordance to an embodiment of the disclosure. 
           [0013]      FIG. 4  is a graph of node voltages versus time for the high-voltage reset sensor interface circuit of  FIG. 3 . 
           [0014]      FIG. 5  is a graph comparing sense node voltage for the sensor interface circuit of  FIG. 1  and the high-voltage reset sensor interface circuit of  FIG. 3  versus diode characteristics. 
           [0015]      FIG. 6  is a graph of sense node voltage for the sensor interface circuit of  FIG. 1  with 100 fA leakage from bias to ground and 10 fA from bias to sense. 
           [0016]      FIG. 7  is a graph of sense node voltage for the high-voltage reset sensor interface circuit of  FIG. 3  with 100 fA leakage from bias to ground and 10 fA from bias to sense. 
           [0017]      FIG. 8  is a graph of leakage current versus noise for the high-voltage reset sensor interface circuit of  FIG. 3 . 
           [0018]      FIG. 9  is a graph of leakage current versus sense node dV/dT for the high-voltage reset sensor interface circuit of  FIG. 3 . 
           [0019]      FIG. 10  is a graph of leakage current versus sense node maximum voltage change time for the high-voltage reset sensor interface circuit of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    Before any embodiments of the patent are explained in detail, it is to be understood that the patent is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The patent is capable of other embodiments and of being practiced or of being carried out in various ways. 
         [0021]    A MOS switch is typically connected in parallel with a high-impedance switch on the sense node, allowing the sense node to be held at a fixed potential while the other side of a capacitor is charged. Due to complexity, the high-impedance node on the high-voltage side of the capacitor is typically not switched. Inclusion of a switch on the high-voltage high-impedance network allows for microphone tests which indicate the presence of small leakage currents associated with high impedance defects. Additionally the level of leakage current which can be detected in these tests is much smaller than the leakage levels which will increase the noise of the microphone. 
         [0022]      FIG. 1  shows a schematic representation of a prior art sensor interface circuit  100 . The sensor interface circuit  100  includes a low-voltage high impedance network  105 , a high-voltage high impedance network  110 , a charge pump  115 , an output capacitor  120 , a sense capacitor  125 , and a high input impedance amplifier  130 . The low-voltage high impedance network  105  includes a first diode  135 , a second diode  140 , a third diode  145 , and a low-voltage reset switch  150 . The first and second diodes  135  and  140  are configured as anti-parallel diodes. The high-voltage high impedance network  110  includes a fourth diode  155 , and a fifth diode  160 . The fourth and fifth diodes  155  and  160  are configured as anti-parallel diodes. A first connection of the low-voltage high impedance network  105  is coupled to a direct current (DC) potential, which can be ground, and a second connection is coupled to the sense capacitor  125  and an input of the high impedance amplifier  130 . A sense node  165  is formed by the connection of the second connection, the sense capacitor  125 , and the input of the high impedance amplifier  130 . 
         [0023]    An output of the charge pump  115  is connected to a first connection of the high-voltage high impedance network  110 , and a second connection of the high-voltage high impedance network  110  is connected to the output capacitor  120  and the sense capacitor  125  forming a bias node  170 . 
         [0024]    The charge pump  115  creates a high-voltage for biasing the sense capacitor  125  and the high input impedance amplifier  130  buffers the signal produced at the sense node  165 . The low- voltage high impedance network  105  at the sense node  125  creates a conservation of charge node. During start-up the low-voltage reset switch  150  is turned on (typically less than 100 ms) while the bias node  170  is charged through the charge pump  115  and the high-voltage high impedance network  110 . This phase is used to put a fixed charge on the sense capacitor  125 . After the start-up, the low-voltage reset switch  150  is opened the microphone becomes operational and changes in the value of the capacitance of the sense capacitor  125  produce a proportional voltage change on the sense node  165  which is buffered by the high impedance amplifier  130 . 
         [0025]      FIG. 2  shows the transient waveform of the charge pump  115  output voltage (rawhv), bias node  170  voltage (mic_bias) and sense node  165  voltage (ampin) for sensor interface circuit  100 . During the start-up/charging phase (10 ms in  FIG. 2 ) the sense node  165  is held at ground while the charge pump  115  and bias node  170  charge. As the charge pump  115  voltage rises an output capacitor  120  and sense capacitor  125  are charged through the high-voltage high impedance network  110 . During this phase, there is a large enough voltage difference between the charge pump  115  output and the bias node  170  that one of the high-voltage high impedance diodes  155  or  160  forward conducts and charges the output and sense capacitors  120  and  125  to within a diode drop of the charge pump  115  voltage. This charging occurs quickly as the small signal impedance of the diode  155  or  160  is small when in the forward biased region. After the start-up/charging phase there exists a voltage drop from the charge pump  115  to the bias node  170 . With no static current flowing off the bias node  170 , the bias node  170  will continue to charge. However, the charging will become slower as the voltage drop decreases and the small signal impedance of the diode  155  or  160  increases. In  FIG. 2  it can be seen that the bias node  170  has not fully charged by the time the start-up phase has ended and continues to charge through 50 seconds. The rising voltage on the bias node  170  creates a transient current through the sense capacitor  125  and, since the low-voltage high impedance network  105  is in its high impedance state, a voltage develops from the sense node  165  to ground in response to the transient current. Once the bias node  170  is fully charged to a charge pump  115  voltage the transient current does not exist and the sense node  165  will leak back to ground. In  FIG. 2  this occurs from 50 to 80 seconds. After all of the currents have settled then the sense node  165  voltage returns to the voltage that it was reset to during the start-up phase; 0V in  FIG. 2 . 
         [0026]      FIG. 3  shows a schematic representation of a high-voltage reset sensor interface circuit  300 . The high-voltage reset sensor interface circuit  300  is similar to the sensor interface circuit  100  ( FIG. 1 ) except that the high-voltage high impedance network  302  includes a high-voltage reset switch  305  and a sixth diode  310 . 
         [0027]    This high-voltage reset sensor interface circuit  300  operates in the same manner as the sensor interface circuit  100  described in  FIG. 1 ; however, during the start-up phase both the low and high voltage reset switches  150  and  305  are closed at the start of the start-up phase and simultaneously opened at the end of the start-up phase. 
         [0028]      FIG. 4  shows the transient waveform of the charge pump  115  output voltage (rawhv), bias node  170  voltage (mic_bias) and sense node  165  voltage (ampin) for the high-voltage reset sensor interface  300 . During the start-up/charging phase (10 ms) the sense node  165  is held at ground through the switch  150  across the low-voltage high impedance network  105  while the bias node  170  is connected to the charge pump  115  through the high-voltage reset switch  305  across the high-voltage high impedance network  302 . As the charge pump  115  voltage rises, the bias node  170  is held to the charge pump  115  voltage. At the end of the start-up phase the charge pump  115  and the bias node  170  are at the same potential. When the switches  150  and  305  across both the high-voltage and low-voltage high impedance networks  105  and  302  open there are no transient currents that flow due to charging and in the absence of any leakage currents from the bias node  170  the sense node  165  will remain at the voltage that it was reset to; 0V in  FIG. 4 . 
         [0029]    In the sensor interface  100 , the transient sense voltage heavily depends on the I-V characteristics of the diodes which are used in both high-impedance networks. Variations of these diodes  135 ,  140 ,  155 , and  160  from chip-to-chip, wafer-to-wafer and lot-to-lot will affect the charging of the bias node  170  and subsequently the settling of the sense node  165 . In contrast, with high-voltage reset sensor interface  300 , the diode characteristics are negligible due to the high-voltage reset switch  305  pre-setting both the sense node  165  and bias node  170 .  FIG. 5  shows the sense node  165  voltage over time with three different sets of diodes  135 ,  140 ,  155 , and  160  for both the sensor interface  100  and the high-voltage reset sensor interface  300 . It should be observed that with the sensor interface  100 , three distinct sense  125  voltage curves are produced. Conversely, only one sense  125  voltage curve is produced with the high-voltage reset sensor interface  300 . This shows that the sense node  165  voltage is not impacted by the diode characteristics with the high-voltage reset sensor interface  300 . 
         [0030]    In both systems the sense node  165  voltage is altered in the presence of leakage currents on the bias node  170 , either from the bias node  170  to ground or from the bias node  170  to the sense node  165 . These leakages can be due to particles, surface contamination or bulk material defects. In the sensor interface  100 , current flow from the bias node  170  to ground will be in the opposite direction as the transient charging current and therefore will only have a small impact on the sense node  165  voltage. When a leakage current exists between the bias node  170  and sense node  165  it will sum with the transient current. As the transient current is largest immediately after the start-up phase it will be difficult to observe the change in the sense node  165  voltage due to this leakage until the transient current has completely settled out.  FIG. 6  shows the sense node  165  voltage with a 100 fA leakage current from bias node  170  to ground and with a 10 fA leakage current from bias node  170  to sense node  165 . Given that the diode characteristics on any given device are not known and can have a significant impact on the sense node  165  voltage it would be difficult to identify leakages from bias node  170  to ground due to defects on the high impedance networks  105  and  110 . Furthermore, it would only be possible to identify leakages from bias node  170  to sense node  165  after the transient currents have fully settled which would require prohibitive test times for volume production. 
         [0031]    In the high-voltage reset sensor interface  300 , since there are no inherent transient currents due to charging and the start-up is not impacted by the diode characteristics, any leakage currents due to defects are easily detectable immediately after start-up (i.e., within milliseconds). While the high-voltage reset switch  305  is closed, the bias node  170  is held to the charge pump  115  voltage. If a leakage current is present from the bias node  170  to ground then this current will flow through the diodes  155  and  160  on the high voltage impedance network  302  when the switch  305  is opened. The bias node  170  will then fall by a voltage determined by the amount of leakage current and the I-V curve of the diode  310 . As the voltage on the bias node  170  falls, a transient current through the sense capacitor  125  will pull the sense node  165  below its reset value causing the sense node  165  to decrease in voltage. Once the voltage on the bias node  170  has settled, the transient current will be gone and the sense node  165  will leak back to ground through the low-voltage high impedance diodes  105 . If a leakage current from bias node  170  to sense node  165  is present, this current will have to flow through the low voltage impedance network  105  diodes  135  and  140  and the sense node  165  will rise by a voltage determined by the amount of leakage current and the I-V curve of the low voltage impedance network  105  diodes  135  and  140 .  FIG. 7  shows the sense voltage with a 100 fA leakage current from bias node  170  to ground and with a 10 fA leakage current from bias  140  to sense node  165 . 
         [0032]      FIG. 8  shows an example which illustrates the impact of various leakage currents on the noise performance of the microphone with the high-voltage reset sensor interface  300 . In  FIG. 8  leakages above 10 fA from bias node  170  to sense node  165  create shot noise through the low-voltage high impedance network  105  which leads to degraded noise performance of the entire microphone. Similarly, leakage current from bias node  170  to ground lowers the impedance of the high voltage-impedance network  302  and subsequently reduces the noise filtering of the biasing circuits. Above 10 pA the bias node  170  to ground leakage leads to degraded noise performance of the entire microphone. 
         [0033]      FIG. 9  illustrates the impact of various leakage currents on the slope of the of the sense node  165  voltage after the start-up phase for the high-voltage reset sensor interface  300 .  FIG. 9  shows that, like the noise, the rate of change in voltage at the sense node  165  can be correlated to the leakage current. 
         [0034]      FIG. 10  illustrates the impact of various leakage currents on the maximum voltage change on the sense node  165  over 2 minutes for the high-voltage reset sensor interface  300 .  FIG. 10  shows that, like the noise, the absolute change in voltage on the sense node  165  can be correlated to the leakage current. 
         [0035]    By including a high-voltage reset switch  305 , the I-V characteristics of the diodes  155  and  160  do not affect the transient voltage on the sense node  165  after the start-up phase. Deviations in the sense node  165  voltage after start-up can be correlated to leakage level currents, with the magnitude and rate of the sense node  165  voltage change corresponding to the amount of leakage. With the high-voltage reset sensor interface  300 , various tests can be implemented in order to identify defects to the high impedance nodes  105  and  302  which can lead to increased noise and ultimately degrade microphone performance.