Abstract:
A micromechanical structure and a method for producing a micromechanical structure are provided, the micromechanical structure being configured for receiving and/or generating acoustic signals in a medium at least partially surrounding the structure. The structure includes a first counterelement that has first openings and essentially forms a first side of the structure, a second counterelement that has second openings and essentially forms a second side of the structure, and an essentially closed diaphragm disposed between the first counterelement and the second counterelement.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a micromechanical structure for receiving and/or generating acoustic signals in a medium at least partially surrounding the structure. 
     2. Description of Related Art 
     U.S. Patent Application 2002/0151100 A1 discloses a monolithically integrated pressure sensor having a microphone cavity, a backplate being disposed above an acoustic diaphragm located in a middle plane, the diaphragm being disposed above a cavity, the cavity being closed off toward the bottom by a substrate. A disadvantage here is that because of the substrate being closed off toward the bottom, no top- or bottom-side incoupling or outcoupling of acoustic signals is possible. It is additionally disadvantageous that the sensitivity of the assemblage is thereby reduced. 
     BRIEF SUMMARY OF THE INVENTION 
     The micromechanical structure according to the present invention for receiving and/or generating acoustic signals in a medium at least partially surrounding the structure, and the method for producing a micromechanical structure and the use of a micromechanical structure according to the present invention have the advantage that with simple means, an improvement in the acoustic properties of the micromechanical structure is possible, and the micromechanical structure is nevertheless producible by way of a comparatively simple and robust production method. The micromechanical structure according to the present invention exhibits great mechanical stability because of the embedding of the diaphragm (buried diaphragm) between the first and the second counterelement. 
     It is particularly preferred that a first cavity be configured between the first counterelement and the diaphragm and that a second cavity be configured between the diaphragm and the second counterelement, and that the first counterelement have a mass several times greater as compared with the diaphragm and/or that the second counterelement have a mass several times greater as compared with the diaphragm. This makes possible, with simple means, a further improvement in the acoustic properties of the micromechanical structure. 
     It is also possible for the micromechanical structure to be provided in monolithically integrated fashion together with an electronic circuit. This makes it possible, using a so-called one-chip solution, to group together the entire unit made up of a micromechanical structure for converting between an acoustic signal and an electrical signal, and an electronic circuit for evaluating and preparing the electronic signals. 
     It is further preferred that the first and/or second counterelement be provided in a manner produced essentially from semiconductor material, and that the diaphragm encompass semiconductor material, and that the first counterelement have a first electrode, the second counterelement have a second electrode, and the diaphragm have a third electrode. It is thereby advantageously possible for the electrical properties of the micromechanical structure to be optimized to the extent that differential evaluation of the change in capacitance between the electrodes is enabled. 
     A further subject of the present invention is a method for producing a micromechanical structure according to the present invention, such that for production of the second cavity, a first sacrificial layer either is applied in patterned fashion onto a raw substrate or is introduced in patterned fashion into the raw substrate, and a first precursor structure is obtained; that then, for production of the diaphragm, at least one first diaphragm layer is applied onto the first precursor structure; that then, for production of the first cavity, a second sacrificial layer is applied; and that then, for production of the first counterelement, an epitaxic layer is applied, the first and second openings then being introduced into the counterelements and the first and the second sacrificial layer being removed in order to constitute the first and the second cavity. This makes it possible, in particularly advantageous fashion, to produce the micromechanical structure according to the present invention by way of a relatively robust and comparatively inexpensive production process. 
     It is also possible for an electronic circuit to be produced, after production of the micromechanical structure, in monolithically integrated fashion with the micromechanical structure, the electronic circuit being disposed either on the first side or on the second side. Monolithic integration of the electronic circuit enables a complete sensor unit or a complete microphone unit to be implemented integrally. 
    
    
     
       BRIEF DESCRIPTION OF THE VARIOUS VIEWS OF THE DRAWING 
         FIGS. 1 and 2  schematically depict micromechanical structures known from the existing art. 
         FIG. 3  schematically depicts a micromechanical structure according to the present invention. 
         FIGS. 4 and 5  show precursor structures of the micromechanical structure according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1 and 2  depict two micromechanical structures  100  known according to the existing art, which each have a diaphragm  120  and a grid-shaped counterelectrode  130 . In one case, diaphragm  120  constitutes the surface of the micromechanical structure on a first side  111  ( FIG. 1 ), and in the other case diaphragm  120  is provided in buried fashion, i.e. counterelectrode  130  of micromechanical structure  100  constitutes the surface of micromechanical structure  100  on first side  111  ( FIG. 2 ). 
       FIG. 3  depicts a micromechanical structure  10  according to the present invention.  FIG. 4  depicts a first precursor structure  50 , and  FIG. 5  a second precursor structure  60 .  FIGS. 3 to 5  are hereinafter described together. Micromechanical structure  10  according to the present invention has a first counterelement  20 , a diaphragm  30 , and a second counterelement  40 . First counterelement  20  has first openings  21 , and second counterelement  40  has second openings  41 . According to the present invention, first and second openings  21 ,  41  are implemented in particular by the fact that first and second counterelement  20 ,  40  have a grid-like structure. First counterelement  20  constitutes, according to the present invention, a first side  11  of micromechanical structure  10 , and second counterelement  40  constitutes, according to the present invention, a second side  12  of micromechanical structure  10 . 
     Micromechanical structure  10  according to the present invention is particularly suitable for being used as a microphone or loudspeaker, and for this application in particular combines high sensitivity to material vibrations of the medium surrounding micromechanical structure  10  with great robustness especially with respect to mechanical influences, since the (comparatively sensitive) diaphragm  30  is disposed in buried and generally protected fashion in the interior of micromechanical structure  10  between the two counterelements  20 ,  40 . Provision is thus made according to the present invention, in particular, that diaphragm  30 , which is comparatively thin compared with the thickness of both the first and the second counterelement  20 ,  40 , is also protected from the back side (second side)  12 , so that it is not exposed to direct mechanical contact during wafer handling in the semiconductor production process, the testing process, and the packaging process. In the installed state, the comparatively stiff structures of counterelements  20 ,  40  enhance the robustness of the micromechanical structure. The construction according to the present invention of micromechanical structure  10  is flip-chip-capable for both a microphone application and a loudspeaker application, since there is comparatively little topography on the surface and the topography thus also combinable with modern low-voltage CMOS methods. The flip-chip connections can be made via metal connector points (not depicted) via first side  11  of structure  10 . The first and the second counterelement  20 ,  40  are hereinafter also respectively referred to as the first and second counterelectrode  20 ,  40 . First and second openings  21 ,  41  in first and second counterelectrodes  20 ,  40 , respectively, are introduced in order to achieve pressure equalization respectively between the first and the second cavity and the exterior of micromechanical structure  10  according to the present invention. According to the present invention it is also possible for diaphragm  30  to be provided in partly open fashion, or for diaphragm  30  to have an opening (not depicted) for static pressure equalization. As an alternative to an opening in diaphragm  30 , it is also possible for an opening for pressure equalization to be present in other regions of the micromechanical structure. 
     Diaphragm  30  is provided in freely movable fashion, and upon excitation by acoustic signals (waves) of a medium (in particular a gas, and in particular air) surrounding micromechanical structure  10 , is caused to move so that diaphragm  30  vibrates. The motion of diaphragm  30  causes the spacing from first counterelement  20 , located above diaphragm  30  (i.e. on a first side  11  of micromechanical structure  10 ) to decrease and increase. This change in spacing can, according to the present invention, be evaluated capacitatively. For this, provision is advantageously made according to the present invention for first counterelement  20  to have a first electrode, diaphragm  30  to have a second electrode  32 , and second counterelement  40  to have a third electrode.  FIG. 3  also schematically depicts the corresponding capacitor assemblages C 1  and C 2 , which are constituted by the shape of counterelements  20 ,  40  and of diaphragm  30 . A first capacitor C 1  is implemented between first counterelement  20  and diaphragm  30 , and a second capacitor C 2  between diaphragm  30  and second counterelement  40 . A small spacing between diaphragm  30  and first counterelement  20  advantageously allows a high electrical sensitivity to be achieved. This makes it possible for diaphragm  30  to be embodied under a controlled tensile stress, and nevertheless permits high sensitivity. 
     The disposition of counterelements  20 ,  40  on both sides relative to diaphragm  30  allows micromechanical structure  10  according to the present invention to be used for differential evaluation of the change in capacitance, which enables higher sensitivity. Associated with this is the possibility for coupling in the acoustic oscillation or acoustic signal of the medium surrounding the micromechanical structure both from first side  11  of structure  10  and from second side  12  of structure  10 . If diaphragm  30  is contacted as a measurement electrode, it is additionally possible for first counterelement  20  and second counterelement  40  to be connected to ground potential, thereby reducing the electrical sensitivity to contaminants and charges from the environment. In addition to its function as first electrode, first counterelement  20  can also be used in the microphone design for other mechanical or electrical functions (configuring springs and movable diaphragm clamping systems, electrical contacting of individual components, e.g. for electrical adjustment of sensitivity). 
     In order to illustrate the method according to the present invention for producing micromechanical structure  10 ,  FIG. 4  depicts first precursor structure  50  of micromechanical structure  10 . First precursor structure  50  encompasses a raw substrate  15  of micromechanical structure  10 , into which substrate a first sacrificial layer  49  is introduced. Raw substrate  15  is, in particular, a doped silicon substrate. First sacrificial layer  49  is, for example, an oxidized region of raw substrate  15 , i.e., first sacrificial layer  49  is provided in a manner introduced into raw substrate  15 . Alternatively thereto, provision can also be made that first sacrificial layer  49  is applied in patterned fashion onto the raw substrate  15 , for example has been deposited. 
       FIG. 5  depicts a second precursor structure  60 , at least one first diaphragm layer  31  being provided, in the diaphragm region above first sacrificial layer  49  and outside the diaphragm above raw substrate  15 , in a manner applied onto first precursor structure  50 . According to the present invention, provision is made in particular for a plurality of, for example, three (or even a number greater or less than three) diaphragm layers to be applied.  FIG. 5  depicts, in addition to first diaphragm layer  31 , a second diaphragm layer  32  and a third diaphragm layer  33 . Diaphragm layers  31 ,  32 ,  33  together constitute diaphragm  30 . According to the present invention, a second sacrificial layer  29  is applied above diaphragm  30 . An epitaxic layer  16  is then applied in order to constitute the second precursor structure  60 . 
     In order to constitute micromechanical structure  10  according to the present invention, first openings  21  are then introduced from first side  11  into epitaxic layer  16 , in particular using an anisotropic trench etching process. Second sacrificial layer  29  is then etched, likewise from first side  11 , thereby creating first cavity  25 . Subsequent thereto, second openings  41  are introduced from second side  12  into raw substrate  15 , in particular using an anisotropic trench etching process. First sacrificial layer  49  is then etched, likewise from second side  12 , thereby creating second cavity  35 . One skilled in the art will recognize that the treatment of second side  12  can also be performed before the treatment of first side  11 . 
     In order to constitute the first electrode, either epitaxic layer  16  is provided in in-situ-doped fashion, or else a doping region is introduced into epitaxic layer  16 . In order to constitute the third electrode, second counterelement  40  or raw substrate  15  is provided in doped fashion, or else a doping region is introduced into second counterelement  40 . In the example depicted, second diaphragm layer  32  is provided inside diaphragm  30  as a correspondingly conductive layer, in particular of polycrystalline silicon, having a corresponding doping. 
     The layer stack of diaphragm  30 , made up of first, second, and third diaphragm layers  31 ,  32 ,  33 , can be made up, for example, of a sequence of silicon nitride, polysilicon, silicon nitride. A diaphragm construction of five diaphragm layers can be made up, for example, of nitride, oxide, polysilicon, oxide, nitride. A diaphragm construction of four diaphragm layers can be made up, for example, of oxide, polysilicon, nitride, and reoxidized nitride. In constructing the diaphragm, care should preferably be taken that the diaphragm as a whole is placed under tensile stress; this can be achieved, for example, by introducing a tensile-stressed layer into the layer sequence of diaphragm  30 , for example by way of a low-pressure chemical vapor deposition (LPCVD) silicon nitride layer. It is preferred to use, in order to bring about the tensile stress in the diaphragm, materials whose mechanical properties are readily adjustable (such as thermal oxide, LPCVD nitride). The polysilicon layer is in all cases doped, and serves as an electrically conductive capacitor plate of second electrode  32 . The layer thickness of the polysilicon layer is selected in such a way that the layer stress of the polysilicon has only a small effect on the overall stress.