Abstract:
An electrical module includes a base plate having an acoustic channel that opens into a first cavity at a first end and that is closed off by a microphone chip at a second end. The microphone chip borders a second cavity that opens to an exterior of the electrical module. The second cavity is separated from the acoustic channel by the microphone chip.

Description:
TECHNICAL FIELD 
     What is described is an electrical module with a microphone integrated therein. 
     BACKGROUND 
     A microphone module with an encapsulated MEMS microphone (MEMS=Micro Electromechanical System) is known from the publication J. J. Neumann, Jr., and K. J. Gabriel, “A fully-integrated CMOS-MEMS audio microphone,” 12th International Conference on Solid State Sensors, Actuators and Microsystems, 2003 IEEE, pp. 230-233. Described therein is a module having a volume for pressure equalization (back volume). 
     An electrical module with a built-in MEMS piezoresistive microphone is known from the publication D. P. Arnold et al., “A directional acoustic array using silicon micromachined piezoresisitive microphones,” J. Acoust. Soc. Am., vol. 113(1), 2003, pp. 289-298. 
     In the publication 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, a piezoelectric microphone having two piezoelectric layers of ZnO and a floating electrode arranged therebetween is described. 
     SUMMARY 
     Described herein is an electrical module with a built-in MEMS microphone that has a high signal-to-noise ratio. 
     An electrical module with a built-in microphone is described. The module comprises a base plate with an acoustic channel formed therein. One embodiment includes a first cavity connected via a sound inlet opening to the exterior, with a MEMS microphone chip arranged therein, and a second cavity, suitable as an acoustic back volume and connected to the acoustic channel. The microphone chip may be connected to the base plate, arranged above an opening formed in the base plate, and connected via this opening to the acoustic channel buried in the base plate. 
     The first cavity can coincide with the exterior. The diaphragm of the microphone chip separates the first cavity from the acoustic channel, which opens into a second cavity. The second cavity may be alongside the first cavity. The acoustic channel may run at least in part beneath the two cavities. 
     A pressure balance between the second cavity and the acoustic channel is possible by air interchange. A fast air interchange between the first and the second cavity—i.e., an air interchange in a period on the order of the oscillation period of the microphone diaphragm in the operating frequency range—may be prevented by the diaphragm of the microphone chip. A slow air interchange (in a period that is longer than the largest oscillation period of the microphone diaphragm in the operating frequency range) between the two cavities is nevertheless possible via a ventilation opening whose cross-sectional size is clearly smaller than the cross-sectional size of the diaphragm. 
     The acoustic channel may be at least sufficiently large in cross section that the pressure change in the proximity of the diaphragm of the microphone chip can be rapidly compensated. The cross-sectional size of the acoustic channel or the channel openings may be at least 5% of the diaphragm surface area. 
     Microphones that detect sound pressure via diaphragms are dependent on a large diaphragm excursion in reaction to the sound intensity in order to obtain the desired characteristics regarding sensitivity and noise behavior. For small components with built-in microphones, the attainable excursion is limited by the small diaphragm surface area. For this reason, only weak electrical signals can be obtained when the diaphragm excursion is transformed into an electrical variable. The compliance of a diaphragm manufactured in a separate procedure can be worsened by a high internal mechanical stress caused by a biasing of the diaphragm. 
     MEMS microphones described here have an air chamber (first cavity) connected to a sound input opening as well as a back volume formed by the acoustic channel and the second area. Back volume refers to enclosed air volumes, with which an acoustic short-circuit—an undesired pressure balance between front and back of the vibrating diaphragm—is prevented. For each diaphragm deflection, this air volume produces a restoring force in addition to the restoring force caused by the flexible diaphragm characteristics. 
     In the microphone specified here, the back volume may be formed by a horizontal acoustic channel formed in the carrier substrate below the two side-by-side cavities, and by the volume of the other cavity. With this particularly large back volume, the relative pressure changes in the back volume, caused by the diaphragm vibrations, and the associated restoring force affecting the diaphragm can be kept small. 
     In an embodiment, the module comprises a cover that has a separation ridge connecting two opposite side faces of the cover and sealing to the base plate. Between the base plate and the cover, for instance, the first cavity connected via a sound inlet opening to the exterior and the second cavity isolated from it by the separation ridge of the cover are formed. The sound inlet opening may be arranged in the cover. 
     The acoustic channel and the second cavity together form a back volume. An advantage of this embodiment is that the back volume is arranged partly in the base plate and partly thereabove. Thus a large portion of the module volume is used as the acoustic back volume. 
     A microphone chip comprises a carrier substrate, with a diaphragm capable of vibrating. The microphone chip may have a piezoelectric layer as well metal layers and/or electrode structures connected thereto, clamped therein above a recess or an opening. 
     There may be an adhesion-promoting and/or sealing mechanism, e.g., a glue layer or an epoxy resin layer, between the cover and the plate. 
     The microphone chip is arranged above a first opening provided in the base plate, which opens into the acoustic channel and the first cavity. The base plate has a second opening, through which the acoustic channel is connected to the second cavity. The cross-sectional size of the first opening may correspond to roughly the cross-sectional size of the microphone chip&#39;s diaphragm. The cross-sectional size of the second opening may be selected in such a way that a fast air interchange between the acoustic channel and the second cavity is possible. 
     With the microphone, the restoring force acting on the diaphragm can be reduced and the diaphragm excursion increased. 
     In an embodiment, the base plate comprises a first layer with a recess associated with the acoustic channel formed in it, and a second layer arranged above the first layer, which partly covers the recess for forming an acoustic channel up to the above-mentioned openings. The separation ridge of the cover may seal to the second layer. 
     The first layer of the base plate can optionally be a glass-fiber reinforced organic laminate, or contain ceramics. The first layer of the base plate can comprise several homogeneous or different dielectric sublayers arranged one above another, between which structured metal layers are arranged. The sublayers can be formed from a glass-fiber reinforced organic laminate or ceramics. The second layer of the base plate is formed with a first layer, which may include another material such as ceramic, formed like a solder stop mask (of acrylate or epoxy resin in one embodiment). 
     The second cavity can house at least one chip element, e.g., a resistor, a capacitor, an inductor, a filter, an impedance transformer and an amplifier. The chip element may be suitable for surface mounting. 
     The microphone chip can also be mounted with a flip-chip construction method and electrically connected by bumps to electrical contacts arranged on the upper side of the base plate. In another embodiment, the microphone chip can be electrically connected by bonding wires to electrical contacts arranged on the upper side of the base plate. The interfaces formed by the opposing mounting surfaces of the chip and the base plate may be sealed by adhesion, underfilling or soldering, for example. A solder frame or a frame made of potting compound may be arranged between the chip and base plate. In the case of a solder frame, it is expedient to form a solderable metallization, whose outline corresponds to the form of the frame in the lateral plane, on the upper side of the base plate and the lower surface of the microphone chip. 
     In one embodiment, the cover comprises a cap of plastic or ceramic coated with a conductive layer. The cover can also be formed from metal. 
     In case of a large acoustic pressure, such large peak-to-peak oscillation amplitudes of the diaphragm can occur that a nonlinearity in the electroacoustic transformation of the diaphragm oscillation into an electrical signal, and therefore signal distortion, results. This problem is remedied in one embodiment, in which a negative-feedback compensation circuit is connected to a diaphragm. Deflection of the diaphragm provoked by the compensation circuit opposes the deflection of the diaphragm caused by the acoustic pressure, and compensates it to a large extent, so that the diaphragm oscillates with a reduced amplitude or does not oscillate at all. The electrical parameter produced by the compensation circuit, for example, a potential, is proportional to the acoustic pressure or the signal to be detected. Any desired electrical circuit suitable for negative feedback is applicable as the compensation circuit. 
     According to another embodiment of the microphone, the diaphragm is clamped to the carrier substrate only at one side, while its end opposite the clamped end can oscillate freely above an opening formed in the carrier substrate when an acoustic signal is applied. Clamping on both sides is also possible, in which case only two opposite diaphragm ends are arranged above the carrier substrate. A diaphragm carrier capable of oscillation that is sealed to the carrier substrate on all sides, e.g., a flexible film, may be stretched over the opening in the carrier substrate. The diaphragm is located on the diaphragm carrier. 
     The following materials are suitable as a piezoelectric layer in all embodiments: ZnO, lead zirconate titanate (PZT) and aluminum nitride. 
     It is proposed that a diaphragm with at least one piezoelectric layer be to a large extent symmetrical regarding its layer succession and layer thickness. In particular, bending moments that develop due to different coefficients of expansion of sequential layers are compensated even for substantial temperature discontinuities. Warping of the diaphragm can therefore be avoided over a broad temperature range. This measure is particularly applicable to a bimorph diaphragm structure. 
     A microphone will be described in detail below on the basis of embodiments and the related figures. The figures show embodiments of different versions of the microphone on the basis of schematic representations, not drawn to scale. Identical or identically functioning parts are labeled with the same reference symbols. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically shows an example of an electrical module with a built-in microphone; 
         FIG. 2A , an electrical module with a microphone chip, an acoustic channel and two cavities in cross section; 
         FIG. 2B , the view of the module of  FIG. 2A  from above; 
         FIGS. 3A ,  3 B, another electrical module; 
         FIG. 4 , a microphone with a diaphragm comprising a piezoelectric layer; 
         FIG. 5 , a microphone with a diaphragm having a bimorph structure. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 ,  2 A,  2 B,  3 A,  3 B each show an electrical module with a built-in microphone chip MCH. The microphone chip can be formed, for example, in accordance with one of the configurations presented in  FIGS. 4 and 5 . 
     Microphone chip MCH is arranged on a base plate BP above an opening formed therein—sound opening IN in  FIG. 1  and/or opening W 1  in  FIG. 2A . Microphone chip MCH may be tightly sealed on all sides to the upper side of base plate BP, on which a cover CAP is arranged. 
     A closed cavity, which is used as an acoustic back volume, is formed between microphone chip MCH, the upper side of the base plate and cover CAP. In addition, a chip component BE 1  electrically connected to microphone chip MCH is arranged in this cavity. Additional chip components BE 2  are located on the base plate BP outside of the closed cavity. The electrical connections between the module components just mentioned are buried in part in multilayered base plate BP. 
       FIGS. 2A ,  2 B show another embodiment of an electrical module, in which the sound inlet opening IN is formed in the cover CAP.  FIG. 2A  shows the electrical module in a schematic cross section, and  FIG. 2B  shows a schematic view of this module from above through its cover. 
     Base plate BP comprises a lower layer S 2  and an upper layer S 1  arranged thereon. In layer S 2 , an acoustic channel AK is provided in the form of a blind hole or a trench extending in a longitudinal direction. As viewed from above, layer S 1  may cover this blind hole completely, up to a first opening W 1  and a second opening W 2 . Layer S 1  can be formed, for instance, as a solder stop mask. 
     A cover CAP, which has a separation ridge TS that interconnects two opposing sides of the cover, is arranged on layer S 1 . Cover CAP may be sealed tightly on all sides to the upper side of base plate BP or to its upper layer S 1 . A glue layer KS can be arranged between them for adhesion promotion or sealing. 
     A microphone chip MCH, which is sealed tightly on all sides to layer S 1 , is arranged above first opening W 1  of layer S 1 . A sealing frame KS 1  is arranged between microphone chip MCH and layer S 1 . Sealing frame KS 1  can be formed from potting compound in one embodiment. In another embodiment, sealing frame KS 1  can be formed as a solder frame. 
     Two cavities HR 1 , HR 2 , which are connected by acoustic channel AK and isolated from one another by microphone chip MCH arranged in first cavity HR 1 , are formed between the S 1  and cover CAP by separation ridge TS of the cover. First cavity HR 1  is connected via sound inlet opening IN to the outside. 
     Chip components BE 1 , BE 2 , which are electrically connected via contacts K 1 -K 3  arranged on the base plate to one another and to microphone chip MCH, are arranged in second cavity HR 2 . 
     On the upper side of microphone chip MCH, a contact surface AF is arranged, which is electrically connected, e.g., to the first electrode of the microphone, and to an electrical contact K 1  located on layer S 1  via a bonding wire. The contact K 2  shown in  FIG. 2B  may be electrically connected to the second electrode of the microphone. 
     The acoustic back volume is formed by an air volume enclosed in acoustic channel AK and second cavity HR 2 . The essential point is that acoustic channel AK connects the remote cavity HR 2  to the rear side of microphone chip MCH and thus makes available an expanded back volume. 
       FIGS. 3A and 3B  represent another electrical module with a built-in MEMS microphone in cross section and in plan view through cover CAP, respectively. Layer S 1  here covers only one part of the recess provided in layer S 2  for the formation of acoustic channel AK. Opening W 1 , which issues into acoustic channel AK and is isolated by the lower surface of microphone chip MCH and a sealing frame KS 1  from first cavity HR 1 , is provided in layer S 1 . 
     Opening W 2  connecting acoustic channel AK to second cavity HR 2  is formed in that a part of layer S 2 , in the area of the recess formed therein, is not covered by layer S 1 . 
     In an embodiment, layer S 1  is completely covered by the cover CAP, wherein separation ridge TS rests upon on this layer and is fixedly connected thereto by glue layer KS. In this example, the height of separation ridge TS is less than the height of the external walls of the cover. 
     Microphone chip MCH is fixedly connected to layer S 1  by a frame-like glue layer KS 1  (or solder layer) arranged in the peripheral area of microphone chip MCH. Thus, opening W 1  of the acoustic channel is isolated from first cavity HR 1 . Layer KS 1  serves to seal off the interface between microphone chip MCH and layer S 1 . 
       FIG. 4  shows an example of a MEMS microphone chip with a piezoelectric microphone. The microphone chip comprises a carrier substrate SU in which an opening is formed, above which a diaphragm M 1  is arranged on a carrier TD capable of oscillating. The diaphragm has a piezoelectric layer PS 1  arranged between two metal layers ML 1 , ML 2 . Contact surfaces AF, which are electrically connected to the electrodes formed in metal layers ML 1  and/or ML 2 , are arranged on the upper side of carrier substrate SU. 
       FIG. 5  shows in schematic cross section a microphone chip with a carrier substrate SU and a diaphragm M 1  with a bimorph structure stretched out thereabove. Diaphragm M 1  has a first piezoelectric layer PS 1  arranged between an outer metal layer ML 3  and a central metal layer ML 2 , as well as a second piezoelectric layer PS 2  arranged between an outer metal layer ML 1  and a central metal layer ML 2 . The piezoelectric axes in the two layers PS 1 , PS 2  can be arranged in the same direction or in opposite directions. 
     A bimorph diaphragm structure has the advantage over a diaphragm with only one piezoelectric layer in that it is possible to obtain twice as large an electrical signal for the same diaphragm curvature, since the potentials of the two piezoelectric layers are additive. 
     The layer thicknesses of the layers forming diaphragm M 1  may be chosen to be symmetrical relative to metal layer ML 2 . The piezoelectric layers have the same thickness and the same orientation of their piezoelectric axes. The two outside metal layers ML 1 , ML 3  may be formed with equal thickness. 
     On the upper side of carrier substrate SU, electrical contacts AE 1 , AE 2  are arranged, which are electrically connected on the one hand via electrical leads to electrodes formed in metal layers ML 1  and ML 2 , and on the other hand, via plated-through holes DK to contact surfaces AF arranged on the lower surface of the carrier substrate SU. 
     In an embodiment, a ventilation opening, which is small in relation to the cross-sectional size of the diaphragm and serves for a slow pressure equalization in the range of ≧100 ms, can be provided to connect the enclosed air volume (back volume of the microphone) to the outside. The pressure is equalized slowly in relation 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 enclosing the acoustic back volume. 
     The module is not limited to the number or the special form of the elements, microphones and/or microphone chips shown in the figures, or to the audible acoustic range from 20 Hz to 20 kHz. Additional piezoelectric acoustic sensors, such as distance sensors operating with ultrasound, are also possible. A microphone chip can be used in any desired signal processing module. Different embodiments can be combined. 
     It is possible to form the carrier substrate as a multilayer structure with structured printed conductors integrated therein to realize, for instance, electrical leads, inductors, capacitors and resistors.