Abstract:
A microphone includes a first diaphragm and a second diaphragm coupled to the first diaphragm by a closed air volume. The first diaphragm and the second diaphragm each constitutes a piezoelectric diaphragm. The first diaphragm and the second diaphragm are electrically coupled so that movement of the first diaphragm causes movement of the second diaphragm.

Description:
TECHNICAL FIELD 
     This patent application describes a MEMS microphone (MEMS=Micro Electromechanical System). 
     BACKGROUND 
     U.S. Pat. No. 4,816,125 describes a MEMS microphone with a piezoelectric layer made from ZnO and several electrodes connected to this layer that are arranged concentrically. 
     The following publication describes a microphone module with an encapsulated MEMS microphone, in which an enclosed air volume (back volume) is in a housing underneath the microphone&#39;s diaphragm: J. J. Neumann, Jr. and K. J. Gabriel, “A fully integrated CMOS-MEMS audio microphone,” the 12th International Conference on Solid State Sensors, Actuators, and Microsystems, 2003 IEEE, pp. 230-233. 
     The following publication describes an electrical module with an installed MEMS piezoresistive microphone: D. P. Arnold, et al., “A directional acoustic array using silicon micromachined piezoresistive microphones,” J. Acoust. Soc. Am., Vol. 113(1), 2003, pp. 289-298. 
     The following publication describes a piezoelectric microphone, which has two piezoelectric layers made from ZnO and a floating electrode arranged in-between: Mang-Nian Niu and Eun Sok Kim, “Piezoelectric Bimorph Microphone Built on Micromachined Parylene Diaphragm,” Journal of Microelectromechanical Systems, Vol. 12, 2003 IEEE, pp. 892-898. 
     SUMMARY 
     Described herein is a sensitive microphone with a high signal-to-noise ratio. 
     It has been found that microphones that detect sound pressure using diaphragms are usually dependent on a large diaphragm displacement as a reaction to sound intensity in order to achieve desired characteristics in terms of sensitivity and noise behavior. For small components with built-in microphones, achievable displacement is limited by small diaphragm area. When diaphragm displacement is converted into an electrical quantity, only weak electrical signals can be obtained. The elasticity of a diaphragm produced in a deposition process can be negatively affected by a bias caused by a high internal mechanical stress. 
     MEMS microphones described here have an air chamber connected to a sound inlet opening and also a back volume. An enclosed air volume that prevents an acoustic short circuit—an undesired pressure balance between the front and back sides of the oscillating diaphragm—is referred to as a back volume. This air volume generates a restoring force for each diaphragm displacement in addition to the restoring force caused by the elastic diaphragm characteristics. For small components, the back volume is so small that even small diaphragm displacements lead to a considerable increase in pressure in the back volume, which can be on the order of magnitude of the sound level to be detected. The additional restoring force decreases the elasticity and the displacement of the diaphragm. 
     A microphone is described with a first and a second diaphragm, which are each connected to one and the same closed air volume and are thus coupled to each other so that, for a displacement of the first diaphragm, a simultaneous displacement of the second diaphragm is generated. 
     The first diaphragm is a microphone diaphragm, i.e., a “passive” diaphragm, which detects the sound pressure or converts an acoustic signal into an electrical signal. The second diaphragm is an auxiliary diaphragm or an “active” diaphragm, whose displacement generated by electrical driving interacts with the “passive” diaphragm via the closed air volume. 
     Two different strategies are described for the electrically driving the active diaphragm: 
     1) “Holding the enclosed air volume constant”: For this purpose, a signal derived from the passive diaphragm and amplified is fed to the active diaphragm such that the active diaphragm performs an opposite but equal-magnitude motion that is similar or identical to that of the passive diaphragm. For example, if the passive diaphragm is driven to a certain volume displacement towards the interior of the cavity by the external sound pressure, then an electrical driving of the active diaphragm by the approximately equivalent volume displacement away from the interior of the cavity is realized. As a result, the fluctuation of the chamber volume is reduced or eliminated. In this way, it is possible to reduce pressure fluctuations caused by the sound pressure in the closed air volume considerably, e.g., by at least a factor of two, in one embodiment by at least a factor of five. This reduction in internal pressure fluctuations, however, also means a corresponding reduction in the diaphragm restoring force. The effective back volume thus appears significantly enlarged, in the limiting case as infinite. 
     2) “Compensation of the passive diaphragm displacement”: Here, the electrical driving of the active diaphragm is part of a control circuit that reduces or even eliminates the displacement of the passive diaphragm, despite the effect of the external acoustic field on the passive diaphragm. A measure for this displacement is the electrical output signal of the passive diaphragm, which is held close to zero by the control circuit. At each moment, the active diaphragm establishes, for this purpose, an internal pressure in the chamber, which is close or equal to the external pressure (sound pressure). The resulting differential pressure for the passive diaphragm is reduced or disappears completely, which also applies to its displacement. Without significant diaphragm displacement of the passive diaphragm, however, the back volume causes, in turn, no relevant restoring forces on this diaphragm. The output signal of the arrangement in this case is not that of the passive diaphragm (which is definitely driven to zero in the described way), but instead the drive signal of the active diaphragm formed in the control circuit. 
     In both cases, a virtual back volume is achieved that is greater than the real back volume by a multiple (in one construction by at least two times, in one embodiment construction by at least five times). 
     The two circuit-related strategies for reducing the effective restoring force run the risk of building up feedback oscillations in the entire system. In one embodiment, therefore, circuit-related measures are provided for recognizing and preventing such conditions. 
     In a first construction, a microphone is specified with a body in which two openings are provided, which open into a cavity formed in the body. A first diaphragm is arranged over a first opening and a second diaphragm (auxiliary diaphragm) is arranged over a second opening, so that an air volume is enclosed in the cavity. The second diaphragm may be decoupled acoustically from the exterior by another cavity. A space in which the source of an acoustic input signal is located is referred to as the exterior. 
     A chamber that is connected to the exterior and isolated from the cavity is arranged over the first diaphragm. The cavity is designated below as the back volume. 
     The first diaphragm is arranged in a first cavity wall over an opening formed in this wall. In one embodiment, the second diaphragm is arranged in a second cavity wall. The diaphragms may be arranged in opposite cavity walls. Because the acoustic pressure change is transmitted equally in all directions when the diaphragm is dispersed, it is also possible to arrange the two diaphragms in walls standing at right angles to each other. The two diaphragms can be arranged in the same cavity wall. 
     The two diaphragms may have essentially the same mass and can be formed identically. The (passive) first diaphragm acts as a microphone diaphragm, while the (driven) second diaphragm functions as a loudspeaker diaphragm. In the case of a piezoelectric MEMS microphone based on the direct piezoelectric effect, for example, the displacement of the first diaphragm is converted into an electrical signal. In a capacitive MEMS microphone, the relative position of the electrodes of the microphone changes. The associated change in capacitance is converted into an electrical signal. The respective diaphragm can be basically an electromechanical converter operating with an electric field or magnetic field. 
     The displacement of the second diaphragm can be generated like in a loudspeaker, e.g., by a changing electric or magnetic field. The displacement of the second diaphragm with piezoelectric properties can be generated on the basis of the inverse piezoelectric effect. 
     In an embodiment, both diaphragms each have at least one piezoelectric layer. Both diaphragms may be constructed identically. Alternatively, it is possible for the electromechanical conversion in the diaphragms to be based on different electromechanical effects. For example, the first diaphragm can function as a capacitive MEMS microphone and the second diaphragm can function as a piezoelectric converter. 
     In one embodiment, a vent opening can be provided, which connects the enclosed air volumes (back volume of the microphone) and the exterior and which is small relative to the cross-sectional size of the diaphragm and which is used for slow pressure balancing, e.g., in the range of ≧100 ms. The pressure balancing is performed slowly relative to the period of an acoustic signal with the largest wavelength in the operating range of the microphone. This opening can be arranged in the diaphragm or in a wall of the container that encloses the acoustic back volume. 
     By virtue of the described compensation measures according to the first and the second embodiment, it is possible to reduce the real acoustic back volume (i.e., the closed air volume) relative to known microphones without an auxiliary diaphragm, so that space savings can be achieved. Nevertheless, because the virtual back volume can be kept sufficiently large, no disadvantageous consequences (loss of sensitivity) occur due to the smaller construction. 
     To prevent an acoustic short circuit of a driven auxiliary diaphragm to the exterior or to the sound inlet opening, an additional cavity isolated from the exterior is provided in an advantageous variant as an acoustic back volume for the auxiliary diaphragm. The additional cavity is separated by the auxiliary diaphragm from the closed air volume. The additional cavity can be smaller than the closed air volume, because the auxiliary diaphragm is driven actively and thus its displacement is set. The space requirements of the microphone arrangement can accordingly be kept small overall. 
     A microphone will be explained in detail below on the basis of embodiments and the associated figures. The figures show different embodiments of the microphone on the basis of schematic representations that are not to scale. Parts that are identical or that have identical functions are labeled with the same reference symbols. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1A , a part of a microphone according to a first embodiment, comprising two electrically coupled diaphragms in a schematic cross section; 
         FIG. 1B , equivalent circuit diagram of the microphone according to  FIG. 1A ; 
         FIGS. 2 ,  3 , each a variant of the embodiment shown in  FIG. 1 ; 
         FIG. 4A , a part of a microphone according to the second variant; 
         FIG. 4B , equivalent circuit diagram of the microphone according to  FIG. 4A ; 
         FIG. 5 , an example microphone diaphragm in a schematic cross section; 
         FIG. 6 , a metal layer, in which two electrodes connected electrically to external contacts are formed. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1A  shows a microphone with a body GH, which has openings AU 1 , AU 2  opening into a cavity HR 2  on its opposing walls HW 1 , HW 2 . A first diaphragm M 1  (microphone diaphragm, passive diaphragm) is arranged over the first opening AU 1  and a second diaphragm M 2  (auxiliary diaphragm, driven diaphragm) is arranged over the second opening AU 2 . 
     The diaphragm M 1 , M 2  can be mounted on the walls of the body GH. Alternatively, the diaphragm M 1 , M 2  can be replaced by a microphone chip with a carrier substrate and a diaphragm mounted thereon. The microphone chip can be connected fixedly to the body GH, e.g., by an adhesive layer. 
     The first diaphragm M 1  separates the cavity HR 2  from a chamber HR 1 , which is connected to the exterior via a sound inlet opening IN. The first diaphragm M 1  begins to vibrate as soon as an acoustic pressure p is exerted on it. The change in pressure in the chamber HR 1  and the vibration of the diaphragm M 1  would lead to a change in volume or pressure in the cavity HR 2  (without the auxiliary diaphragm M 2 ) and an associated restoring force, which acts on the first diaphragm M 1  and reduces the vibration amplitude. Due to an electrical coupling of the two diaphragms M 1 , M 2 , they vibrate in such a manner that the displacement of the first diaphragm M 1  is towards the interior of the cavity HR 2  and the displacement of the second diaphragm M 2  is realized with the same amplitude towards the outside. The active diaphragm M 2  is driven in a push-pull way with respect to the passive first diaphragm M 1 . Here, a reduced change or no change at all in the volume of the cavity HR 2  occurs. 
     The second diaphragm M 2  separates the cavity HR 2  from an additional closed cavity HR 3 , which is isolated from a space connected to a sound source, i.e., the exterior and the chamber HR 1 . The additional cavity HR 3  prevents feedback of the active diaphragm onto the passive diaphragm on the outer path. 
     The additional cavity HR 3  and/or the chamber HR 1  can be created, e.g., by a cap-shaped, dimensionally stable cover. 
     In  FIG. 1B , a simplified equivalent circuit diagram of diaphragms M 1 , M 2  coupled by a control circuit V 1  is shown. For a displacement of the passive diaphragm M 1  caused by the sound pressure, an electrical signal is generated that can be tapped at the output OUT as a usable signal for further processing. A part of the electrical signal is used for generating a control signal at the output of the control circuit V 1 , with which the auxiliary diaphragm M 2  is driven in a push-pull way (relative to the internal pressure established in the cavity HR 2 ) with respect to the passive diaphragm. 
     The drive circuit V 1  may contain an amplifier for amplifying the signal tapped at the diaphragm M 1 . 
       FIG. 2  shows an embodiment of the microphone presented in  FIG. 1 , in which both diaphragms M 1 , M 2  are arranged in the same cavity wall HW 1 . In a cavity wall of the cavity HR 2 , a small ventilation opening VE connecting this cavity and the exterior is provided, whose cross-sectional size is clearly smaller (e.g., by at least a factor of 100) than the cross-sectional size of the diaphragm or the openings AU 1  or AU 2  and which is used for slow pressure balancing, e.g., in the range of ≧100 ms. In a cavity wall of the cavity HR 3 , a small ventilation opening VE′ connecting this cavity and the exterior is also provided. 
     In  FIG. 3 , the openings AU 1 , AU 2  are provided in mutually perpendicular walls. The ventilation opening VE is formed here in the diaphragm M 1 . 
     The direction of the diaphragm displacement is indicated with arrows in  FIGS. 1 to 4A , B. 
     In a variant of the embodiment presented in  FIG. 4A , the active second diaphragm M 2  is driven in a push-pull way (relative to the internal pressure) with the passive first diaphragm M 1  in contrast to  FIG. 1A . Here, the displacements of the two diaphragms are directed towards the interior of the air volume enclosed in the cavity HR 2 . In  FIG. 4A , a dashed line shows how the passive diaphragm M 1  would deform due to external sound pressure. A solid line shows the actual position of the diaphragm M 1  achieved due to the compensating effect of the active diaphragm M 2 , wherein the diaphragm M 1  remains practically in its rest position or oscillates with a very small amplitude relative to the displacement of the active diaphragm M 2 . 
       FIG. 4B  shows an equivalent circuit diagram to the embodiment according to  FIG. 4A . The electrical signal tapped at the diaphragm M 1  is processed by the control circuit RK. On one hand, a control signal for driving the diaphragm M 2  is output and, on the other, another control signal, which is superimposed on the signal tapped at the diaphragm M 1  and damps the oscillation amplitude of the diaphragm M 1 . An output signal at the output OUT can be evaluated. The output OUT is connected here to the diaphragm M 2 . 
     In the variants presented in  FIGS. 2 and 3 , it is also possible to drive the active diaphragm M 2  in common mode relative to the passive diaphragm M 1 , in order to damp the displacement amplitude of the passive diaphragm M 1  in addition to the restoring force acting on this diaphragm. 
       FIG. 4B  shows the equivalent circuit diagram of a microphone, which comprises a control circuit RK for compensating the displacement of the diaphragm M 1 . The output signal OUT 2  is obtained here from the control circuit, while the signal of the converter M 1  is held close to zero by the effect of the control. An example of a diaphragm with a piezoelectric layer PS arranged between two metal layers ML 1 , ML 2  is shown in  FIGS. 5 and 6 . Electrodes E 11  and E 12  connected to the external contacts AE 1 , AE 2  are arranged in the first metal layer ML 1 . A floating conductive area, which lies opposite the two electrodes E 11 , E 12 , is formed in the second metal layer ML 2 . Here, two capacitors connected to each other in series are formed. 
     In  FIG. 6 , a first metal layer ML 1  of the diaphragm presented in  FIG. 5  is shown. The round electrode E 11  is arranged in a first high-potential region and the annular electrode E 12  is arranged in a second high-potential region. The two high-potential regions have opposite polarity. The electrodes E 11 , E 12  are each connected to external contacts AE 1  and AE 2 , respectively. In a metal layer ML 2  arranged underneath or above and shown in  FIG. 5 , a continuous, floating, conductive surface may be arranged, which is opposite the two electrodes E 11 , E 12 . 
     The microphone is not limited to the number of elements shown in the figures or to the acoustically audible range from 20 Hz to 20 kHz. The microphone can also be used in other piezoelectric acoustic sensors, e.g., distance sensors operating with ultrasound. A microphone chip with a described microphone can be used in any signal-processing module. Different embodiments can also be combined with each other.