Abstract:
Measures for dynamically regulating the microphone sensitivity of a MEMS microphone component at low frequencies by way of variable roll-off behavior are proposed. The micromechanical microphone structure of the component, which is implemented in a layer structure on a semiconductor substrate, encompasses an acoustically active diaphragm having leakage openings which spans a sound opening in the substrate back side, and a stationary acoustically permeable counterelement having through openings which is disposed in the layer structure above/below the diaphragm. The component furthermore encompasses a capacitor assemblage for signal sensing, having at least one deflectable electrode on the diaphragm and at least one stationary electrode on the counterelement, and an arrangement for implementing a relative motion between the diaphragm and counterelement parallel to the layer planes.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a component having a micromechanical microphone structure that is implemented in a layer structure on a semiconductor substrate. The microphone structure encompasses an acoustically active diaphragm having leakage openings which spans a sound opening in the substrate back side, and a stationary acoustically permeable counterelement having through openings which is disposed in the layer structure above or below the diaphragm. The component is equipped with a capacitor assemblage for signal sensing, which encompasses at least one deflectable electrode on the diaphragm and at least one stationary electrode on the counterelement. In addition, an arrangement is provided for implementing a relative motion between the diaphragm and counterelement parallel to the layer planes. 
       BACKGROUND INFORMATION 
       [0002]    Sound impingement onto the diaphragm occurs via the sound opening in the substrate and/or via the through openings in the counterelement. The diaphragm deflections resulting therefrom perpendicular to the layer planes are sensed, as changes in capacitances, with the aid of the capacitor assemblage. 
         [0003]    The diaphragm structure reacts, however, not only to acoustic pressure but also to fluctuations in ambient pressure and to air-flow-related low-frequency pressure fluctuations such as those caused, for example, by wind. Spurious influences of this kind on the microphone signal can be reduced by a slow pressure equalization between the two sides of the diaphragm. The speed at which such a pressure equalization occurs depends substantially on the flow resistance of the corresponding flow paths. The lower the flow resistance, the more quickly a pressure equalization between the diaphragm front side and diaphragm back side takes place, and the less influence atmospheric pressure fluctuations and air flows have on the microphone signal. As the flow resistance decreases, however, so too does the microphone&#39;s sensitivity to low-frequency acoustic signals, referred to as the “roll-off” at low frequencies. 
         [0004]    U.S. Published Patent Appln. No. 2012/0033831 describes a microphone component having variable roll-off behavior. The known microphone component encompasses an acoustically active diaphragm that functions as a movable electrode of a capacitor assemblage for signal sensing. Leakage openings for pressure equalization between the diaphragm front side and diaphragm back side are embodied in the diaphragm. The known microphone component furthermore encompasses a counterelement constituting a carrier of a stationary electrode off the capacitor assemblage. The counterelement is disposed at a distance from the diaphragm and has through openings, so that it is acoustically permeable. Because of the very short distance between the diaphragm and counterelement, in the case of the known microphone component the flow resistance between the front and back sides of the diaphragm depends substantially on the offset between the through openings of the counterelement and the leakage openings of the diaphragm, and can accordingly be varied in controlled fashion by a parallel displacement between the diaphragm and counterelement. This relative motion is produced with the aid of a drivable actuator arrangement, for example capacitively or piezoelectrically. 
         [0005]    Although the roll-off behavior of the known microphone component can in this manner be dynamically adapted to the ambient situation, the overall sensitivity of the known microphone component is nevertheless very limited. This is attributable to the very short distance d between the diaphragm and counterelement. 
         [0006]    The mechanical sensitivity of the diaphragm of a microphone component can be appreciably increased by application of a bias voltage. The closer this bias voltage U bias  is to the so-called “pull-in” voltage U pull-in  (i.e. the voltage at which the return force of the diaphragm is overcome and the diaphragm is pulled against the counterelement), the greater the acoustic sensitivity of the diaphragm. The pull-in voltage U pull-in  rises with the distance between the diaphragm and counterelement, specifically as a power of 3/2. The sensitivity of a microphone component correspondingly also rises with the distance d, specifically as a power of 1/2, when the diaphragm is acted upon by a bias voltage U bias  close to the pull-in voltage U pull-in . 
         [0007]    With the known microphone component, however, the distance d between the diaphragm and counterelement cannot be increased arbitrarily if the roll-off behavior is to be varied by a parallel displacement between the diaphragm and counterelement. The greater the distance d, or gap, between the diaphragm and counterelement, the lower the flow resistance in the gap becomes. The influence exerted on the roll-off behavior of the known microphone component by the offset between the through openings in the counterelement and the leakage openings in the diaphragm thus also becomes less as the distance d increases. Especially when the distance d between the diaphragm and counterelement is on the order of the diameter of the through openings in the counterelement, the flow resistance in the gap becomes so low that the alignment of the through openings in the counterelement and the leakage openings in the diaphragm has practically no further influence on the roll-off behavior of the known microphone component. 
       SUMMARY 
       [0008]    The present invention further develops the microphone element with variable roll-off behavior known from U.S. Published Patent Appln. No. 2012/0033831 in order to improve the overall microphone sensitivity. 
         [0009]    According to the present invention, the counterelement of such a microphone component is equipped for that purpose with a stop structure which is designed so that the number of leakage openings in the diaphragm that are overlapped by the stop structure depends on the relative position between the diaphragm and counterelement. According to the present invention, the degree of overlap between the stop structure and the assemblage of leakage openings can thus be modified in controlled fashion by a parallel displacement between the diaphragm and counterelement. 
         [0010]    With the microphone component according to the present invention, the flow resistance upon pressure equalization between the diaphragm front side and diaphragm back side is determined substantially by the degree of overlap between the stop structure and the leakage openings. The magnitude of the distance d between the diaphragm and counterelement plays only a subordinate role in this context, so that, in particular, greater distances d can also be implemented in order to increase the microphone sensitivity. The stop structure can moreover be used as a support for the biased diaphragm, and as an overload protector in the operating mode. 
         [0011]    There are in principle many possibilities for implementing a microphone component according to the present invention, in particular with regard to the layout of the diaphragm having the leakage openings and the layout of the counterelement. 
         [0012]    In a preferred embodiment of the invention, the disposition of the leakage openings and the layout of the stop structure are coordinated with one another in such a way that in an initial position of the diaphragm and counterelement only a few leakage openings (if any) are overlapped by the stop structure of the counterelement, and the number of overlapped leakage openings rises as the parallel displacement between the diaphragm and counterelement proceeds, at least up to a predefined offset between the diaphragm and counterelement. In this case the flow resistance between the two sides of the diaphragm can easily be regulated by way of the offset between the diaphragm and counterelement. 
         [0013]    The leakage openings and the corresponding stop structures are preferably disposed in the edge region of the diaphragm in order to make available the largest possible continuous, highly movable, diaphragm area for sound reception. 
         [0014]    As already mentioned previously, the sensitivity of a microphone component can increased by mechanically biasing the microphone diaphragm. In a preferred embodiment of the invention, the capacitor assemblage is used for this purpose for signal sensing, by the fact that an electrical bias voltage U bias  is applied between the diaphragm and counterelement, the result of which is that the diaphragm is electrostatically deflected. This electrostatic deflection of the diaphragm can of course also be brought about using a capacitor assemblage that is provided for the purpose and is not used for signal sensing, or also with arrangements of other kinds, for example with the aid of piezo actuators. In any case, the diaphragm can in this fashion be acted upon in controlled fashion with a mechanical bias, and pulled against the stop structure of the counterelement. 
         [0015]    A particularly high flow resistance can be achieved with a stop structure that encompasses a continuous sealing ring. The center region of the diaphragm can thereby be peripherally sealed acoustically. Alternatively or also in supplementary fashion thereto, it is often advantageous if the stop structure encompasses peg-like and/or finger-like structural elements whose disposition and geometry are adapted to the disposition and shape of the corresponding leakage openings in the diaphragm. 
         [0016]    The parallel displacement between the diaphragm and the counterelement of the microphone component according to the present invention can also be implemented in various ways. 
         [0017]    In a particularly advantageous embodiment of the invention, the counterelement is incorporated into the layer structure via a resilient mount that enables a rotational or translational motion of the counterelement parallel to the layer planes, but does not permit any “out-of-plane” motion of the counterelement. In this case the counterelement is displaced within the layer plane, i.e. parallel to the diaphragm, by controlled driving of the resilient mount. The resilient mount is advantageously driven capacitively. This variant is appropriate in particular when the counterelement with its resilient elements has been patterned out of a thick epi-polysilicon layer of the layer structure. In this case even relatively large-area electrodes of a corresponding capacitor assemblage can be implemented in the epi-polysilicon layer, for example in the form of mutually interengaging comb electrodes. The resilient mount of the counterelement can, however, be driven in a different manner, for example using a piezo actuator. 
         [0018]    Alternatively or also as a supplement thereto, the diaphragm can also be displaced in the layer plane, i.e. parallel to the counterelement. In this case the resilient mount of the diaphragm is designed so that it allows not only out-of-plane motion resulting from acoustic pressure, but also an “in-plane” motion. In addition, as in the case of the drivable resilient mount of the counterelement, an arrangement for a controlled production of such a lateral motion is provided. 
         [0019]    Particularly high microphone sensitivity can also be achieved, in the context of the microphone component according to the present invention, with a so-called flexural beam diaphragm, i.e. a diaphragm that is incorporated into the layer structure of the component only via a resilient element similar to a strut or flexural beam, and that consequently, in response to sound, deflects chiefly in plane-parallel fashion and is in practice does not become warped. Lateral movability of the diaphragm can furthermore be implemented by way of constrictions in the flexural beam so that it can be utilized for the parallel displacement between the diaphragm and counterelement. 
         [0020]    Ideally, the flow resistance between the two sides of the diaphragm, or the air leakage rate, can be regulated as a function of the ambient conditions of the microphone component, specifically during operation, in order to achieve consistently good microphone performance even in varying ambient conditions. A preferred embodiment of the microphone component according to the present invention is therefore equipped with an arrangement for regulating the relative position between the diaphragm and counterelement as a function of the occurrence and intensity of low-frequency pressure fluctuations. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1   a  is a schematic sectioned view through a first microphone component  10  according to the present invention with the counterelement in an idle position, so that the stop structure of the counterelement overlaps the leakage openings in the diaphragm. 
           [0022]      FIG. 1   b  is a schematic sectioned view through said microphone component  10  with the counterelement displaced in parallel fashion, so that the leakage openings in the diaphragm are open. 
           [0023]      FIG. 2   a  is a plan view of a second microphone component  20  according to the present invention with the counterelement displaced in parallel fashion. 
           [0024]      FIG. 2   b  is a schematic sectioned view through said microphone component  20 . 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    The microphone structure of the MEMS microphone component  10  depicted in  FIGS. 1   a  and  1   b  is implemented in a layer structure on a semiconductor substrate  1 , for example on a silicon substrate. It encompasses an acoustically active diaphragm  11  having leakage openings  12  which spans a sound opening  14  in the substrate back side, and a stationary acoustically permeable counterelement  15  having through openings  16  which is disposed in the layer structure above diaphragm  11 . Diaphragm deflections resulting from acoustic pressure are sensed capacitively, in which context diaphragm  11  functions as a movable electrode and the stationary counterelement  15  is equipped with an immovable electrode of a microphone capacitor assemblage. 
         [0026]    In the present exemplifying embodiment, the diameter of diaphragm  11  is greater than the cross-sectional area of sound opening  14 . Diaphragm  11  is incorporated peripherally into the layer structure of component  10 , so that diaphragm  11  is continuous over sound opening  14  except for leakage openings  12 . Diaphragm  11  is electrically insulated by corresponding intermediate layers  2  and  3  of the layer structure with respect to substrate  1  on the one hand, and with respect to a thick epi-polysilicon layer  4  on the other hand. 
         [0027]    Counterelement  15  was patterned out of said epi-polysilicon layer  4  together with a resilient mount made up of two resilient elements  171  and  172  and the associated actuator components  181  and  182 . Resilient mount  171 ,  172  and actuator components  181 ,  182  extend, just like counterelement  15 , over the entire thickness of epi-polysilicon layer  4 . This resilient mount  171 ,  172  also correspondingly allows only an in-plane deflection of counterelement  15 , i.e. in the layer plane. Resilient mount  171 ,  172  is flexurally stable perpendicularly to the layer planes, so that counterelement  15  is not deflected by acoustic pressure. Also contributing to this are through openings  16  in counterelement  15 . Counterelement  15  is furthermore equipped with a stop structure  19  for diaphragm  11 , which in the exemplifying embodiment depicted here is implemented in the form of a peripheral sealing ring  19  at the edge of counterelement  15 . 
         [0028]    Stop structure  19  (i.e. in this case sealing ring  19 ) is designed according to the present invention so that the degree of overlap of stop structure  19  and leakage openings  12  depends on the relative position between diaphragm  11  and counterelement  15 . In the present exemplifying embodiment this relative position can be varied by an in-plane deflection of counterelement  15 . For this, resilient elements  171  and  172  are driven in controlled fashion with the aid of the corresponding actuator components  181  and  182 . Actuator components  181 ,  182  can be, for example, capacitor comb structures having an asymmetrical electrode position in the idle state, so that a directed motion in only one preferred direction is brought about by application of a voltage. 
         [0029]    The microphone structure of component  10  depicted in  FIGS. 1   a  and  1   b  is designed so that the degree of overlap between stop structure  19  of counterelement  15  and leakage openings  12  in diaphragm  11  is greatest when counterelement  15  is in its idle position, i.e. when the actuator mechanism on resilient mount  171 ,  172  is not being actuated. This situation is depicted in  FIG. 1   a.  Here both resilient elements  171 ,  172  are in the same stress state. The flow resistance between the two sides of the diaphragm is maximal. This operating mode is adapted to a normal ambient situation with no low-frequency interference noise, and provides good microphone performance with a high signal-to-noise ratio. 
         [0030]    Upon occurrence of large low-frequency pressure fluctuations, leakage openings  12  are exposed by way of a parallel displacement of counterelement  15 , in order to decrease the flow resistance between the two sides of diaphragm  11  and thus also to decrease the microphone&#39;s sensitivity to low-frequency interference noise. This situation is depicted in  FIG. 1   b,  where resilient element  171  on the left side of counterelement  15  is compressed, while resilient element  172  on the right side of counterelement  15  is elongated. 
         [0031]    This adaptation or modulation of the acoustic leakage flow resistance usefully occurs automatically whenever a previously defined threshold value for the occurrence of large low-frequency pressure fluctuations is exceeded. This can be done, for example, by monitoring the total sound level summed over all frequencies below 200 Hz, and defining for it a threshold value of &gt;50 dB. The acoustic leakage flow resistance can then be regulated as a function of this total sound level by way of a parallel displacement of counterelement  15 .  FIGS. 1   a  and  1   b  show microphone component  10  during a regulation operation of this kind, since diaphragm  11  is not respectively biased in this case. 
         [0032]    This is because for signal sensing, diaphragm  11  is biased, and pulled against stop structure  19 , by application of a voltage U bias  between diaphragm  11  and counterelement  15 . The result thereof is to increase the mechanical sensitivity of diaphragm  11  and the acoustic sealing effect of stop structure  19 , which has as a whole a positive effect on microphone performance. In order to modulate the leakage flow resistance, however, this voltage U bias  applied between diaphragm  11  and counterelement  15  is switched off in order to release diaphragm  11  from stop structure  19  and thus enable a parallel displacement of counterelement  15 . Only thereafter is diaphragm  11  biased again by re-applying voltage U bias . 
         [0033]    The microphone structure of the MEMS microphone component  20  depicted in  FIGS. 2   a  and  2   b  is also implemented in a layer structure on a semiconductor substrate  1 . It encompasses an acoustically active diaphragm  21  having leakage openings  22  which spans a sound opening  24  in the substrate back side, and a stationary acoustically permeable counterelement  25  having passthrough openings  26  which is disposed in the layer structure above diaphragm  21 . Diaphragm  21  serves as a deflectable electrode of a microphone capacitor assemblage for signal sensing, and is electrically insulated by corresponding intermediate layers  2  and  3  of the layer structure with respect to substrate  1  on the one hand and with respect to a thick epi-polysilicon layer  4  on the other hand. Counterelement  25  having the stationary electrode of the microphone capacitor assemblage is embodied in this epi-polysilicon layer  4 . 
         [0034]    In the present exemplifying embodiment, diaphragm  21  is a circular flexural-beam diaphragm that is incorporated on only one side, via a flexural beam  23 , into the layer structure of component  20 . 
         [0035]    The plan view of  FIG. 2   a  illustrates the layout of counterelement  25 , which has been patterned out of epi-polysilicon layer  4  together with its resilient mount, resilient elements  271 ,  272 , and associated actuator components  281 ,  282 . As in the case of microphone component  10 , all these components  25 ,  271 ,  272 ,  281 , and  282  extend over the entire thickness of epi-polysilicon layer  4 . The circular counterelement  25  is attached to epi-polysilicon layer  4 , and thus incorporated into the layer structure of component  20 , only at two oppositely located edge segments, in each case via a respective actuator component  281  and  282  and a respective resilient element  271  and  272 . This layout makes possible a translational motion of counterelement  25  along the axis of resilient mount  271  and  272 , i.e. within the layer plane. Resilient mount  271 ,  272  is flexurally stable perpendicular to the layer planes, so that counterelement  25  having through openings  26  in the center region is acoustically permeable. 
         [0036]    Counterelement  25  is equipped with a stop structure for diaphragm  21 , which can be made of an insulating material. The sectioned view of  FIG. 2   b  illustrates the fact that this stop structure encompasses a continuous sealing ring  291  embodied at the edge of counterelement  25 , as well as peg-like structural elements  292 , as is evident from  FIG. 2   a . These structural elements  292  are disposed on the inner edge of sealing ring  291  in correspondence with the positions of individual leakage openings  22  in diaphragm  21 . The stop structure (i.e. in this case sealing ring  291  and structural elements  292 ) are designed according to the present invention in such a way that the degree of overlap between stop structure  291 ,  292  and leakage openings  22  depends on the relative position between diaphragm  21  and counterelement  25 . This can be modified in controlled fashion by way of a translational motion of counterelement  25 . This purpose is served by the drivable actuator components  281  and  282 , which interact with resilient mount  271 ,  272  of counterelement  25 . 
         [0037]    In the case of microphone component  20  depicted here, the microphone structure is again designed so that the degree of overlap between stop structure  291 ,  292  of counterelement  25  and leakage openings  22  in diaphragm  21  is greatest when counterelement  25  is in its idle position, i.e. the two resilient elements  271  and  272  are in the same stress state. This operating mode with maximum leakage flow resistance is adapted to a normal ambient situation with no low-frequency interference noise, and provides good microphone performance with a high signal-to-noise ratio. 
         [0038]      FIGS. 2   a  and  2   b  show the operating mode of microphone component  20  upon occurrence of large low-frequency pressure fluctuations, when leakage openings  22  in diaphragm  21  have been exposed by a translational motion of counterelement  25  in order to decrease the flow resistance between the two sides of diaphragm  21 , and thus also to decrease the microphone&#39;s sensitivity to low-frequency noise. Resilient element  271  on the left side of counterelement  25  is in this case compressed, while resilient element  272  on the right side of counterelement  25  is elongated. 
         [0039]    In order to decrease the leakage flow via the diaphragm edge and resilient elements  271 ,  272 , microphone component  20  is equipped with a further sealing structure  41  that is implemented here in the form of two sealing rings disposed concentrically with respect to counterelement  25  and to sealing ring  281 . 
         [0040]    In conclusion, be it noted once again that the present invention not only can be implemented in the context of microphone components having a front-plate counterelectrode, as described above with reference to the exemplifying embodiments, but can just as easily be realized with microphone components having a back-plate counterelectrode.