Abstract:
The invention relates to a microphone membrane (M 1 ) comprising two piezoelectric layers (PS 1,  PS 2 ) with c-axes oriented in the same direction. A first electroconductive surface (E 11 ) is formed in the central metal layer and subjected to a first electrical potential. The piezoelectric layers (PS 1,  PS 2 ) are respectively arranged between the central metal layer (ML 2 ) and an outer metal layer (ML 1,  ML 3 ). In a preferred embodiment, the membrane (M 1 ) has a largely symmetrical structure in terms of the layer sequence and the layer thickness thereof.

Description:
TECHNICAL FIELD 
       [0001]    This patent application describes a microphone membrane that comprises at least one piezoelectric layer. 
       BACKGROUND 
       [0002]    U.S. Pat. No. 4,816,125 describes a microphone membrane with a piezoelectric layer comprising ZnO and several concentrically arranged electrodes. 
         [0003]    The following publication describes a piezoelectric microphone: Mang-Nian Niu and Eun Sok Kim in the Journal of Microelectromechanical Systems, Volume 12, 2003 IEEE, pages 892 through 898, entitled “Piezoelectric Bimorph Microphone Built on Micromachined Parylene Diaphragm.” 
       SUMMARY 
       [0004]    Described herein is a piezoelectric microphone membrane with a high signal/noise ratio. 
         [0005]    A microphone membrane is described that comprises two piezoelectric layers arranged one above the other with a central metal layer located in between them, wherein the c-axes of the two piezoelectric layers are oriented in the same direction. 
         [0006]    The membrane may have an essentially symmetrical structure in terms of layer sequence and layer thickness. Even with considerable and abrupt changes in temperature, compensation is thus provided, especially in regard to the bending moments that are produced as a result of the different expansion coefficients of layers that follow one another sequentially. In this way, warping of the membrane can be avoided over a wide temperature range. The central metal layer may be in the plane of symmetry. 
         [0007]    The microphone membrane may be used in a microphone. The microphone may be in the form of a microphone chip with a carrier substrate that has a recess, above which the membrane is mounted, and is thereby capable of vibrating. The microphone chip has external contacts on its surface, which are accessible from the outside. The microphone chip can be arranged in a housing with an acoustic back volume. 
         [0008]    Silicon, for example, is suitable as the material for the supporting substrate. ZnO, lead zirconate-titanate (PZT), and aluminum nitride are well-suited for the piezoelectric layer. 
         [0009]    The piezoelectric layers are each arranged between the central metal layer and a respective external metal layer. A first electrically conductive surface is constructed in the central metal layer. This electrically conductive surface is subjected to a first electrical potential and forms a first internal electrode of the microphone. 
         [0010]    In an embodiment, a second electrically conductive surface, which is subjected to a second electrical potential and which forms a second internal electrode of the microphone, can be arranged in the same metal layer as the first internal electrode. In this way, at least one floating structure may be constructed in each of the metal layers that faces outward. This floating structure is located opposite the first and second electrically conductive surfaces. However, the second internal electrode can also be formed by conductive surfaces that are arranged in the external metal layers. 
         [0011]    Metal structures that are subjected to an electrical potential are termed internal electrodes or electrodes. The internal electrodes are connected to the external electrodes of the microphone chip via strip conductors and, optionally, vertical electrical connections. For example, the external electrodes can be constructed in one of the externally located layers. The internal electrodes are connected to the external electrodes via electrical leads and vertical electrical connections (i.e., plated through-holes are arranged in the piezoelectric layer in question). 
         [0012]    In the case of a bimorphous membrane structure, two capacitors arranged one above the other, with a common electrode, are formed by three metal layers and the piezoelectric layers that are arranged between them. In the event of flexing, the first piezoelectric layer experiences extension and the second piezoelectric layer experiences contraction, or vice versa. In this way, oppositely directed piezo-potentials are produced in the two piezoelectric layers that have the same orientation of their c-axes. These piezo-potentials are, however, additive to one another when the capacitors, which are arranged one above the other, are connected in parallel. Their common electrode, in particular, is constructed in the plane that is arranged between the two piezoelectric layers. The common electrode, which corresponds to the first or second internal electrode, is thus subjected to an electrical potential, and may be connected to an external contact of the microphone chip. In one embodiment, the metal structures that are constructed in the external metal layers and are located opposite the common electrode, are conductively connected to one another and to an additional external contact of the microphone chip via electrical leads and interlayer contacts, for example. 
         [0013]    For the same membrane deflection, a bimorphous membrane structure can successfully produce an electrical signal that is twice as large as that in the case of a membrane with only one piezoelectric layer, because the piezo-potentials of the two piezoelectric layers are additive to one another with appropriate circuitry. 
         [0014]    In the case of the deflection of a membrane that is firmly clamped at the edge, it is especially the edge region thereof, along with its central region, that is exposed to the greatest mechanical stresses. In this way, the edge region is extended in the event of contraction of the central region, and vice versa. Therefore high, opposing electrical potentials, which are essentially equal in terms of magnitude, are produced in the (ring-shaped) edge region and in the (circular) central region. A region of the piezoelectric layer that lies below the potential limit of 70% of the maximum potential is designated a region of high potential. Furthermore, the centrally arranged region of high potential is termed the first region of high potential, and the region of high potential which is concentric therewith and which is arranged in the edge region, is termed the second region of high potential. The electrodes, which are arranged in different regions of high potential in the same metal layer and which are connected to external electrodes of opposite polarity, may be insulated from one another since potential equalization would otherwise take place. 
         [0015]    It is possible to implement an internal electrode via conductive surfaces that are constructed in different metal layers and that are connected to one another electrically, e.g., by interlayer contacts. In one embodiment, a first conductive surface and a second conductive surface are arranged in the central metal layer, where the first conductive surface is located opposite third conductive surfaces arranged in external metal layers, and where the second conductive surface is located opposite fourth conductive surfaces located in external metal layers. The first conductive surface here is connected to the first external electrode, and the fourth conductive surfaces are connected to the second external electrode. The second conductive surface is connected, in an electrically conductive manner, to the third conductive surfaces by interlayer contacts that are arranged in the adjacent piezoelectric layer. 
         [0016]    The first conductive surface can be allotted to a first region of high potential, and the second conductive surface can be allotted to a second region of high potential, or vice versa. 
         [0017]    The electrodes of opposite polarity may be arranged in the same (central) metal layer. In the second metal layer, at least one floating conductive structure or surface that is capacitively coupled to the electrode in question via the piezoelectric layer located between them is then constructed. Two capacitors connected in series, the galvanic electrodes of which are formed by the floating conductive structure, are formed in this way. In order to reduce the stray capacitance, the floating conductive surface can be structured in such a way that it forms two comparatively broad regions, essentially repeating the shape of the opposite electrode of the capacitor in question, which are connected to one another by, e.g., a narrow strip conductor. 
         [0018]    In order to form electrodes, it is advantageous to structure the metal layer in such a way that the intermediate region—a region of low potential—arranged between the central region and the edge region, remains essentially free from metallization. 
         [0019]    A region of high potential (which is associated with the first metal layer) can be subdivided into at least two subregions. A first electrode is arranged in the first subregion, and this first electrode is electrically insulated from a second electrode that is associated with the second subregion. Both electrodes are located opposite a floating conductive surface, which is optionally subdivided into two portions connected galvanically to one another, and opposite the electrodes. The two electrodes may have the same surface area. Two capacitors are formed in this way that are connected in series via the floating conductive surface. It is possible to successfully increase the signal potential by a factor of two with such an electrode subdivision, relative to an implementation with non-subdivided electrodes of the same membrane dimensions. It is also possible to connect more than merely two capacitors, formed as above in series. These capacitors may be identical. 
         [0020]    In one embodiment, the galvanic connection of the serially connected capacitors takes place via a floating conductive surface. In the case of more than two capacitors that are connected one behind the other, these surfaces are arranged in the first and second metal layer. 
         [0021]    In another embodiment, the series connection of the capacitors is possible via vertical electrical connections, e.g., via interlayer contacts that are arranged in the piezoelectric layer. 
         [0022]    The two high-potential regions of opposite polarity can also be subdivided, as described above, into subregions with assigned electrodes in order to form several capacitors that are connected one behind the other. 
         [0023]    In accordance with another embodiment, a piezoelectric microphone is described with a supporting substrate and a membrane that is mounted above a recess constructed therein. The membrane is clamped only on one side to the supporting substrate, and its end opposite the clamped end can vibrate freely upon the application of an acoustic signal. The membrane may have a bimorphous structure. 
         [0024]    In one embodiment, the membrane can be clamped to the supporting substrate in a bridge-like manner. The two opposite ends of the membrane are fastened to the supporting substrate, and the two additional ends of it are not fastened. 
         [0025]    The microphone can comprise a vibratable support, e.g., an elastic film (e.g., one comprising a metal or a polymer) or a thin SiO 2  layer on which the membrane is arranged. The vibratable support extends beyond the free end of the membrane and thereby connects the opposite walls of the recess to one another. 
         [0026]    Microphone membranes will be explained in detail below by examples and the drawings associated therewith. The drawings show various examples through schematic illustrations that are not true to scale. Identical components, or identically operating components, are labeled with identical reference symbols. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0027]      FIG. 1A , a microphone with a membrane that has a bimorphous structure; 
           [0028]      FIG. 1B , an equivalent circuit diagram of the microphone in accordance with  FIG. 1A ; 
           [0029]      FIG. 2A , an embodiment of the microphone shown in  FIG. 1A , with a structured central metal layer; 
           [0030]      FIG. 2B , an equivalent circuit diagram of the microphone in accordance with  FIG. 2A ; 
           [0031]      FIG. 3 , an embodiment of the microphone shown in  FIG. 1A , with metal layers structured into electrodes; 
           [0032]      FIG. 4A , in a cutout, the interconnection of the electrodes in a microphone in accordance with  FIG. 2 ; 
           [0033]      FIG. 4B , an equivalent circuit diagram of the microphone in accordance with  FIG. 4A ; 
           [0034]      FIGS. 5 ,  6 A,  7 A and  7 B, a first metal layer (on the left), a second metal layer (in the center), and a third metal layer (on the right) of a microphone with a bimorphous membrane; 
           [0035]      FIG. 6B , in a schematic cross section, a membrane with metal layers that have been structured in accordance with  FIG. 6A ; 
           [0036]      FIGS. 8A ,  8 B and  8 C, a microphone with a unilaterally clamped membrane that comprises a piezoelectric layer; and 
           [0037]      FIGS. 9 through 14 , a microphone with a unilaterally clamped membrane comprising two piezoelectric layers. 
       
    
    
     DETAILED DESCRIPTION  
       [0038]      FIG. 1A  shows, in a schematic cross section, a microphone chip with a supporting substrate SU and a membrane M 1  with a bimorphous structure that is mounted thereon. The membrane M 1  can vibrate above a recess AU that is constructed in the supporting substrate. 
         [0039]    The membrane M 1  has a first piezoelectric layer PS 1 , which is arranged between an external metal layer ML 3  and a central metal layer ML 2 , as well as a second piezoelectric layer PS 2  that is arranged between an external metal layer ML 1  and the central metal layer ML 2 . The direction of the c-axis in the two piezoelectric layers PSI and PS 2  is marked by the arrows. 
         [0040]      FIG. 1B  shows that a first capacitor C 1  is formed between the conductive surfaces E 11  and E 31  that are located opposite one another and that are constructed in the metal layers ML 2  and ML 3 . A second capacitor C 2  is formed between the conductive surfaces E 11  and E 21  constructed in the metal layers ML 1  and ML 2 . These capacitors have a common first electrode that is connected to a first external contact AE 1 . The second electrodes of these capacitors are connected to a second external contact AE 2 . The capacitors C 1  and C 2  are connected in parallel between the external contacts AE 1  and AE 2 . 
         [0041]    The thicknesses of the layers that form the membrane M 1  are related to a plane of symmetry that corresponds to the metal layer ML 2 , and may be symmetric. In this way, the piezoelectric layers have the same thickness and a unidirectional orientation of the c-axes. The two external metal layers ML 1  and ML 3  are constructed equally thickly as well. 
         [0042]    In  FIG. 1A , the electrodes, which have opposite polarity and are connected to different external contacts of the microphone, are arranged one above the other. The arrangement of the two electrodes in a plane is shown in  FIG. 2A . 
         [0043]    A variant of a bimorphous membrane is presented in  FIG. 2A . Floating conductive surfaces FE 1  and FE 2  have been constructed in the two external metal layers ML 1  and ML 3 . These floating conductive surfaces are located opposite the conductive surfaces E 11  and E 12  that are connected to the external contacts. The first conductive surface E 11 , which is arranged in the central region of high potential and may be round or square, is connected to the external contact AE 1 . The ring-shaped second conductive surface E 12 , which is arranged in the second region of high potential, is connected to the external contact AE 2 . 
         [0044]    The replacement circuit diagram is shown in  FIG. 2B . A first capacitor C 1  is formed between the conductive surface E 11  and the floating surface E 12 . A second capacitor C 2  is formed between the conductive surface E 11  and the floating surface FE 1 . In a similar way, the third or fourth capacitor C 3  or C 4  is formed between the conductive surface E 12  and the floating surfaces FE 1  and FE 2 , respectively. The series connection of the capacitors C 1  and C 3  is connected in parallel to the series connection of the capacitors C 2 and C 4 . 
         [0045]      FIG. 5  shows a plan view of the metal layers of the membrane in accordance with  FIG. 2A . 
         [0046]    It is specified in  FIG. 3  that all three metal layers ML 1  through ML 3  can be structured to form the conductive surfaces E 11 , E 12 , E 21 , E 22 , E 31  and E 32 . In an embodiment, the centrally arranged conductive surfaces E 11 , E 21  and E 31 , which may be round or square, and/or the conductive surfaces E 12 , E 22  and E 32 , which are arranged in the edge region and may be ring-shaped, can be structured into subsurfaces; see  FIG. 7B , for example. 
         [0047]      FIGS. 4A and 4B , in the form of a cross section, show an embodiment with an advantageous connection of conductive surfaces that are constructed in three different metal layers in order to form several capacitors, which are connected to one another in series and in parallel, along with the corresponding replacement circuit diagram.  FIG. 4A  shows the microphone chip only, in the form of a cutout. The conductive surfaces may be constructed in cross-section as in  FIG. 3 , i.e., essentially concentrically. 
         [0048]    A first conductive surface E 11  and a second conductive surface E 12  are constructed in the central metal layer. A third conductive surface E 21  and E 31  and a fourth conductive surface E 22  and E 32  are respectively constructed in the two external metal layers. 
         [0049]    The first conductive surface E 11  is connected to an external contact AE 1  and is arranged between the third conductive surfaces E 21  and E 31 . Two capacitors that are connected one behind the another are formed as a result of this. The first conductive surface E 11  here forms a common electrode of these capacitors. 
         [0050]    The second conductive surface E 12  is arranged between the fourth conductive surfaces E 22  and E 32 . Two capacitors C 3  and C 4  that are connected one behind another are formed as a result of this. The second conductive surface E 12  here forms a common electrode of these capacitors. The second conductive surface E 12  is electrically connected to the two third conductive surfaces E 21  and E 31  by interlayer contacts DK. The second conductive surface forms a floating conductive structure with these two third conductive surfaces. The fourth conductive surfaces E 22  and E 32  are connected to a second external contact AE 2 . 
         [0051]    For example, the first conductive surface E 11  is arranged in the centrally located first region of high potential, and the second conductive surface E 12  is arranged in the edge region of the membrane, i.e., in the second region of high potential. 
         [0052]    The connection of the conductive surfaces is presented in  FIGS. 4A and 4B , wherein the parallel connection of the capacitors C 1  and C 2  is connected in series with the parallel connection of additional capacitors C 3  and C 4 . It is also possible to arrange more than merely two parallel connections of capacitors one behind the other and to connect them between the external contacts AE 1  and AE 2 . In this way, for example, the fourth conductive surfaces E 22  and E 32  can be connected, via vertical electrical connections, to an additional conductive surface, arranged in the central metal layer, and forming floating structure, instead of to the external contact AE 2 . The arrangement of the additional conductive surface between two conductive surfaces, not illustrated here, or their coupling, may correspond to the arrangement of the second conductive surface E 12 . 
         [0053]    Instead of connecting the first conductive surface E 11  to the contact AE 1 , it is also possible to assign this conductive surface to an additional floating structure. The arrangement of the first conductive surface E 11  between two conductive surfaces, not illustrated here, or their coupling, may correspond to the arrangement of the second conductive surface E 12 . 
         [0054]    Thus it is possible, with good success, to increase the number of capacitors per membrane via vertical electrical connections, and hence to increase the signal potential as well. 
         [0055]      FIGS. 5 ,  6 A,  6 B,  7 A and  7 B show different embodiments for the construction of electrode structures in the three metal layers ML 1 , ML 2  and ML 3  in a membrane with a bimorphous structure.  FIGS. 5 ,  6 A,  7 A and  7 B show, in the center, the central metal layer ML 2  of the membrane with metal structures constructed therein. 
         [0056]    In  FIG. 5 , a round first conductive surface E 11  is arranged in the first region of high potential, and a ring-shaped second conductive surface E 12  is arranged in the second region of high potential. The conductive surfaces E 11  and E 12  form an internal electrode and are respectively connected, via horizontally running strip conductors and vertical electrical connections—interlayer contacts DK 1  and DK 2 —to an external contact AE 1  or AE 2  that is arranged in the external metal layer ML 3 , which is the upper one here. In an embodiment, the external contacts AE 1  and AE 2  of the microphone can be arranged in the same metal layer as the conductive surfaces E 11  and E 12 , and they can be connected to the conductive surfaces E 11  and E 12  via horizontal electrical connections (electrical leads). 
         [0057]    In the two external metal layers ML 1  and ML 3 , respectively, a continuous floating conductive surface FE 1  and FE 2  is constructed. On the one hand, a continuous floating conductive surface is located opposite the first conductive surface E 11  and, on the other hand, a continuous floating conductive surface is located opposite the second conductive surface E 12 . 
         [0058]    In order to give slow pressure equalization, a ventilation opening VE, where the cross-sectional opening size is significantly smaller than the cross-sectional size of the membrane, is provided that passes through the membrane. 
         [0059]    A modification of the membrane in accordance with  FIG. 5  is presented in  FIGS. 6A and 6B . Here, structured floating surfaces are provided instead of continuous floating conductive surfaces FE 1  and FE 2 . The circular first conductive surface E 11  is arranged between two surfaces FE 11  and FE 21  that have essentially the same shape. The ring-shaped second conductive surface E 12  is arranged between two surfaces FE 12  and FE 22  that have essentially the same shape. The surfaces FE 11  and FE 12 , which are arranged in the central region and in the edge region, respectively, are connected to one another by narrow strip conductors. The surfaces FE 21  and FE 22 , which are arranged in the central region and in the edge region, respectively, are also connected to one another by narrow strip conductors. This embodiment is characterized by low parasitic capacitors. 
         [0060]    The membrane with metal layers ML 1 , ML 2  and ML 3 , which are constructed in accordance with  FIG. 6A , is shown in the form of a schematic cross section in  FIG. 6B . 
         [0061]    An additional embodiment of the construction of metal layers of a bimorphous membrane is shown in  FIG. 7A . 
         [0062]    A first floating structure, having a first subsurface E 12   b  and a second subsurface E 11   a  connected thereto by a narrow strip conductor, is constructed in the central metal layer ML 2 . 
         [0063]    A second floating structure FE 1   a  and a third floating structure FE 1   b , which is electrically insulated therefrom, are arranged in the first external metal layer ML 1 . A second floating structure FE 2   a  and a third floating structure FE 2   b,  which is electrically insulated therefrom, and external contacts AE 1  and AE 2  are arranged in the second external metal layer ML 3 . 
         [0064]    The second floating structures FE 1   b  and FE 2   b  are located opposite the first conductive surface E 11   b  and a first subsurface E 12   b  of the first floating structure. The third floating structures FE 1   a  and FE 2   a  are located opposite the second conductive surface E 12   a  and a second subsurface Eli a of the first floating structure. In this example, a total of eight capacitors, which are connected to one another, are implemented because the metal structures located opposite one another are coupled capacitively. The equivalent circuit diagram corresponds to the connection one behind the other of the two capacitor circuits in accordance with  FIG. 2B . 
         [0065]    The first conductive surface El lb and the second subsurface E 11   a  of the first floating structure are arranged in the first region of high potential. The second conductive surface E 12   a  and the first subsurface E 12   b  of the first floating structure are arranged in a second region of high potential. 
         [0066]      FIG. 7B  shows a modification of the embodiment in accordance with  FIG. 7A . The floating structures FE 1   a , FE 1   b , FE 2   a  and FE 2   b,  which are constructed in the external metal layers ML 1  and ML 3 , are, in each case, structured in such a way that they have subsurfaces conductively connected to one another by narrow strip conductors. The shape of the subsurfaces corresponds essentially to the shape of the structures E 11   a , E 11   b,  E 12   a  and E 12   b  that are located opposite them. 
         [0067]    The structures, which are arranged in the same metal layers and which are conductively connected to one another, can basically be replaced by a continuous conductive surface (without cutouts). A continuous conductive surface can be replaced by subsurfaces that are conductively connected to one another and the shape of which has been adapted to that of the opposite metal structures. 
         [0068]      FIGS. 8A-8C  show the construction of a microphone chip with a unilaterally clamped membrane M 1 , whose free end is quasi-elastically connected to the supporting substrate TS. The membrane M 1  has a piezoelectric layer PS that is arranged between the structured metal layers ML 1  and ML 2 . First conductive surfaces E 11  and E 12  are constructed in the metal layer ML 1 , and second conductive surfaces E 21  and E 22  are constructed in the metal layer ML 2 . The membrane M 1  is arranged above a recess AU, which is formed in the substrate TS, and it is arranged above the supporting substrate SU on one side only, so that one end of the membrane can vibrate freely. The recess AU may be a continuous opening in the supporting substrate. 
         [0069]    In the embodiment shown in  FIG. 8A , the free end of the membrane is connected quasi-elastically to the supporting substrate SU via a conductive surface E 11  constructed in the lower metal layer ML 1 . 
         [0070]    In  FIG. 8B , a support TD, which can vibrate, and the membrane M 1  arranged thereon and firmly connected thereto, is mounted above the recess AU. The support TD, which can vibrate, may be highly elastic and allows a large deflection amplitude for the free end of the membrane, and hence a large degree of membrane travel. 
         [0071]    In  FIG. 8C , the membrane M 1  additionally comprises a layer S 11 , e.g., one comprising silicon dioxide. A support TD, which can vibrate, e.g., an elastic film such as a plastic film, which connects the free end of the membrane to the supporting substrate, is coated on, or laminated on to, the upper side of the membrane. The film here runs down as far as the lowermost membrane layer. 
         [0072]    Different embodiments of a unilaterally clamped membrane with a bimorphous structure are shown in  FIGS. 9 through 14 . 
         [0073]    The quasi-elastic coupling of the free end of the membrane can take place, as in  FIG. 3 , via a metal structure E that is constructed in the lowermost metal layer ( FIG. 9 ). The metal structure E can also be constructed in the upper or central metal layer and it can run down as far as the plane that corresponds to the lowermost membrane layer ( FIGS. 10 and 11 ). 
         [0074]    A unilaterally clamped bimorphous membrane, the free end of which is connected to the supporting substrate SU by a vibratable support TD, is shown (on the left) in the embodiment of  FIG. 12 . Here, the support TD, which can vibrate, covers only a portion of the upper side of the membrane, but it can completely cover the upper side of the membrane as in  FIG. 4 . 
         [0075]      FIG. 13  shows an embodiment of the coupling of the free end of the membrane arranged on a vibratable support TD by the vibratable support TD, and an additional metal structure E, missing in  FIG. 14 , that is arranged above it. 
         [0076]    An additional metal structure which connects the upper side of the membrane, at its clamped end, to the upper side of the supporting substrate, is arranged in  FIGS. 9 through 13 . 
         [0077]    The microphone membranes can also be used in additional piezoelectric acoustic sensors, e.g., distance sensors that operate via ultrasound. A microphone chip with a microphone membrane can be inserted into any desired signal processing module. 
         [0078]    Different embodiments can be combined with one another.