FIELD OF THE INVENTION 
     The present invention relates to a micromechanical component and a manufacturing method for producing it. 
     BACKGROUND INFORMATION 
     Membranes are usually manufactured by bulk or surface micromechanics. Bulk micromechanical designs have the disadvantage that they are relatively complex to manufacture and are therefore expensive. Surface micromechanical variants have the disadvantage that it is generally not possible to manufacture monocrystalline membranes. 
     Monocrystalline membranes have the advantage that the mechanical properties are more defined than in polycrystalline membranes. Moreover, it is possible to manufacture piezoresistive resistors having a significantly better long-term stability and higher piezoelectric coefficients using monocrystalline membranes than piezoresistive resistors in polycrystalline membranes. 
     SUMMARY OF THE INVENTION 
     The micromechanical component according to the present invention and the corresponding manufacturing method for producing the micromechanical component provide the advantage that a cavern having a superimposed monocrystalline membrane may be manufactured simply and cost-effectively using surface micromechanics. The monocrystalline membrane may be used, for example, for pressure sensors. 
     In accordance with the present invention, to manufacture the membrane, n + - or p + -doped areas are first selectively anodized (porous etching), which is carried out locally by means of a monocrystalline cover layer, e.g., an epitaxial layer. This is followed by a time-controlled switch to selective electropolishing of an n + - or p + -doped layer buried under the membrane. In this way, a cavity or a cavern is produced under the cover layer. Optionally, the porous n + - or p + -doped areas in the cover layer are finally sealed to enclose a defined gas pressure in the cavity produced. 
     Advantages of the present invention include simple integration into a semiconductor circuit process, consequently making it possible, for example, to integrate a membrane having an evaluation circuit on a chip (e.g., as a pressure sensor). In addition, little fluctuation due to underetchings occurs, i.e., it is possible to implement exactly specifiable dimensions. Moreover, simple sealing of the access openings is possible, if desired. 
     According to an exemplary embodiment, one or more sealing layers are provided above the monocrystalline layer to seal the porous areas. 
     According to another exemplary embodiment, the porous areas are sealed by oxidation. This is a particularly effective sealing method. 
     According to another exemplary embodiment, the monocrystalline layer and the porous areas are of the same doping type. 
     According to another exemplary embodiment, the monocrystalline layer and the porous areas are of different doping types. 
     According to another exemplary embodiment, the monocrystalline layer is provided by epitaxy. 
     According to another exemplary embodiment, the substrate is of a first conduction type and the buried layer is of a second conduction type. In the buried layer, one or more areas of the first conduction type are provided, which have higher doping than the substrate. This makes it possible to concentrate the lines of force during electropolishing and to avoid undesirable residues in the cavity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1   a - 1   c  show cross-sectional views illustrating different stages of the manufacturing process for a micromechanical component according to a first embodiment of the present invention. 
         FIGS. 2   a  and  2   b  show cross-sectional views illustrating different stages of the manufacturing process for a micromechanical component according to a second embodiment of the present invention. 
         FIG. 3  shows a possible complication in manufacturing the micromechanical component according to the present invention. 
         FIGS. 4   a  and  4   b  show cross-sectional views illustrating different stages of the manufacturing process for a micromechanical component according to a third embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Identical reference numerals in the figures denote identical components or components having an identical function. 
       FIGS. 1   a - 1   c  show cross-sectional views illustrating different stages of the manufacturing process for a micromechanical component according to a first embodiment of the present invention. 
     In  FIGS. 1   a - 1   c , reference numeral  1  denotes a p-doped silicon substrate;  5  denotes a buried n + -doped layer;  10  denotes an n-epitaxial layer;  15  denotes n + -doped areas in n-epitaxial layer  10 ;  10   a  denotes a later membrane area;  20  denotes a mask; and  30  denotes a sealing layer of, e.g., metal, oxide, nitride, BPSG, etc. 
     According to  FIG. 1   a , buried n + -doped layer  5  in p-silicon substrate  1  is produced under n-doped epitaxial layer  10  by standard process steps, e.g., by implantation. In addition, n + -doping areas  15  are incorporated in epitaxial layer  10  at selected sites, e.g., at points or in the form of strips or rings, to produce n + -doped connections from the surface to buried n + -layer  5 . Optionally, it is possible to deposit and structure a masking layer  20  or a plurality of such masking layers (e.g., of nitride) on epitaxial layer  1 . 
     According to  FIG. 1   b , n + -doping areas  15  may be converted into porous n + -areas or entirely dissolved by electrochemical etching in solutions containing hydrofluoric acid (“anodization”), depending on the anodization conditions (hydrofluoric acid concentration, current density, etc.). The anodization rate is strongly dependent on the doping of the silicon. Low-doped n-silicon (n-epitaxial layer) is barely attacked while n + -doped silicon is readily attacked. This selectivity is used to advantage in this embodiment. 
     In a first, time-controlled anodization step, n+-doped areas  15  in epitaxial layer  1  are etched to more or less complete porosity. The porosity is preferably greater than 50%. A change of the anodization conditions causes buried n+-doped layer  5  to be dissolved away when an etchant penetrates through the now porous areas  150  to the buried n+-doped layer  5 . In the transitional area from n+-area  5  to p-doped substrate  1 , there is a weakly n-doped area which acts as an anodization limit. The form of buried n+-doping  5  defines the area dissolved out. 
     According to  FIG. 1   c , porous areas  150  may—if desired—be closed very simply in a subsequent process step after removal of mask  20  because they have a nearly flat surface having very small holes. This is a significant advantage compared to standard micromechanical surface manufacturing methods in which holes usually having diameters greater than one μm must be sealed. The sealing may be performed, for example, by deposition of metal layer  30  or several layers (oxide, nitride, metal BPSG, . . . ) or by oxidation. The process pressure during deposition defines the internal gas pressure in cavity  50 . 
       FIGS. 2   a  and  2   b  show cross-sectional views illustrating different stages of the manufacturing process for a micromechanical component according to a second embodiment of the present invention. The dopings used for the etched areas have been varied in this case. 
     In addition to the reference symbols already introduced in  FIGS. 2   a  and  2   b ,  5 ′ denotes a buried p + -doped layer and  15 ′ denotes p + -doped feed-through areas. 
     In this example, a p + -doping is incorporated in p-substrate  1  for buried layer  5 ′. In addition, n-epitaxial layer  10  is grown epitaxially over it and provided with p + -feedthroughs  15 ′. 
     According to  FIG. 2   b , p + -doped area  15 ′ is selectively anodized to form porous p + -doped area  150 ′. n-epitaxial layer  10  is not attacked in this case, and p-substrate  1  is attacked only slightly since the anodization rate of p +  is significantly higher than that of p and n. 
       FIG. 3  shows a possible complication in manufacturing the micromechanical component according to the present invention. 
     When the buried doping layer  5 ′ is dissolved out via an etchant penetrating through the porous areas  150 ′, there is the danger that a silicon web  151  will remain at the point at which two etch fronts meet. This web  151  could cause membrane  10   a  not to be completely freed, thus adversely affecting its function. 
       FIGS. 4   a  and  4   b  show cross-sectional views illustrating different stages of the manufacturing process for a micromechanical component according to a third embodiment of the present invention. 
     In the third embodiment, the danger described in connection with  FIG. 3  may be confronted by providing a buried p+-doping area  5 ″ in buried n+-layer  5  where the etch fronts meet during the subsequent anodization. This p+-doping area  5 ″ causes lines of force S to be guided selectively during the subsequent anodization so that no web remains after the silicon is dissolved out via an etchant penetrating through the porous areas  150  to the buried n+-layer  5 . 
     Although the present invention was described on the basis of exemplary embodiments, it is not limited to them but instead may be modified in various ways. 
     The described and illustrated embodiments are only exemplary of the manufacturing sequence. Optionally, additional dopings may be implemented next to the membrane or in the membrane, for example, to manufacture piezoresistors in the membrane and an evaluation circuit next to the membrane for an integrated pressure sensor. The buried n + -doped layer and the n + -doped feeds through the epitaxial layer may be designed in such a way that the buried layer is dissolved out through lateral n + -etch channels, which are connected with the surface of the epitaxial layer at the channel end via the n + -feeds.
A micromechanical component is described which includes a substrate; a monocrystalline layer, which is provided above the substrate and which has a membrane area; a cavity that is provided underneath the membrane area; and one or more porous areas, which are provided inside the monocrystalline layer and which have a doping that is higher than that of the surrounding layer.