Abstract:
A micromechanical diaphragm is described which has a partially n-doped p-substrate on its surface and a top n-epitaxial layer, one or more n-epitaxial layers which are p-doped in the diaphragm area being arranged on the p-substrate.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims the benefit under 35 U.S.C. §120 of U.S. application Ser. No. ______, filed Sep. 26, 2001, which is incorporated herein by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to a micromechanical diaphragm having a partially n-doped p-substrate on its surface, a topmost layer being an n-epitaxial layer.  
         BACKGROUND INFORMATION  
         [0003]    In micromechanical pressure sensors, a diaphragm is defined by anisotropic etching with a pn-etch stop. The etch front stops at the boundary surface between the p-doped substrate and an n-doped layer. The clamping of the diaphragm is defined by the edge of the etched pit. The pressure-dependent deflection of the diaphragm is measured via the change in resistance of piezoresistive resistors on the surface of the diaphragm. Of critical importance for the sensitivity of the resistors is the position of the resistors relative to the clamping of the diaphragm.  
           [0004]    A micromechanical diaphragm is already known from German Published Patent Application No. 43 09 207. This publication describes a semiconductor device with a piezoresistive pressure sensor having a diaphragm formed by a conductive epitaxial layer (epitaxy layer) and applied to a semiconductor substrate of opposite conductivity. At least one piezoresistor is provided on the diaphragm surface facing away from the semiconductor and an opening penetrating the semiconductor substrate has been etched into the inner surface of the diaphragm. A conductive intermediate layer having an annular structure has been inserted between the semiconductor substrate and the epitaxial layer, the intermediate layer defining the area of the opening adjacent to the inner surface of the diaphragm. The intermediate layer possesses a conductivity which is opposite to that of the semiconductor substrate. This diaphragm with varying dopings ensures that there are only limited differences in thickness in the diaphragm which are determined by the penetration depth of the doping. In addition, the diaphragm described here has only one epitaxial layer.  
         SUMMARY OF THE INVENTION  
         [0005]    An object of the present invention is to design and arrange a micromechanical diaphragm in such a way that precisely formed clampings or diaphragm areas with strongly varying stiffniess are ensured.  
           [0006]    This object is achieved according to the present invention in that one or more n-epitaxial layers which are p-doped in the diaphragm area are arranged on the p-substrate. This ensures that the clamping or support points of the diaphragm are defined during the subsequent etching of the diaphragm, i.e. the p-doped area, and not by the underetching of the lateral surfaces. The edge area of the etching within the p-doped substrate is located below the additionally arranged n-epitaxial layers and does not influence the behavior or the clamping of the diaphragm. A precise definition of the clamping of the diaphragm is, for example, very important for pressure sensors since the position of the piezoresistive resistors relative to the clamping (diaphragm edge) influences the sensitivity with respect to pressure.  
           [0007]    It is advantageous in this regard that the substrate is locally n-doped before the n-epitaxial layer is deposited and that the n-epitaxial layer is n-doped during the deposition and is then locally p-doped or p-doped during the deposition and then locally n-doped in the edge area or the epitaxial layer is deposited undoped and subsequently the edge area is provided locally with an n-doping and the center area of the epitaxial layer is provided with a p-doping. This type of doping makes continuous doping possible so that the individual n-epitaxial layers are thicker. The variable number of n-epitaxial layers presented here also permits clearly greater thicknesses of the clamping of the diaphragm.  
           [0008]    According to a further development, an additional possibility is that an n-doped diaphragm layer or the diaphragm is in contact with the locally p-doped n-epitaxial layer or the locally n-doped p-substrate. The diaphragm thus formed or stiffening of the diaphragm is substantially thicker in the clamping area and accordingly significantly more stabile.  
           [0009]    In addition, it is advantageous that the p-doped area of the various n-epitaxial layers or of the locally n-doped p-substrate is formed with a varying size in the individual n-epitaxial layers or in the doping layer or extending from the diaphragm layer or a diaphragm, it has a larger or smaller surface area than in the preceding layer. It is thus possible to design any desired stiffening form below the diaphragm and the diaphragm can be adapted corresponding to its field of application.  
           [0010]    It is also advantageous in this regard that the p-doped area of the various n-epitaxial layers or of the locally n-doped p-substrate is arranged in various subareas of the respective layer, such as the center of the micromechanical diaphragm or below the diaphragm and the n-doped areas and the p-doped areas, always in alternation with an n-epitaxial layer, are arranged side by side and/or symmetric to the center of the micromechanical diaphragm.  
           [0011]    According to a preferred embodiment of the device according to the present invention, it is also provided that the various n-epitaxial layers are formed with varying thicknesses.  
           [0012]    It is of particular significance for the present invention that the areas previously identified as n-doped may also be p-doped and the areas previously identified as p-doped may also be n-doped. Accordingly, the base doping of the various layers may be selected freely and the corresponding locally provided doping may be adapted.  
           [0013]    Finally, it is of advantage that the diaphragm layer is formed as an n-epitaxial layer and the side of the p-substrate directed to the outside has an etch mask which opens up the area to be exposed below the diaphragm to the etching agent.  
           [0014]    Lastly, it is of advantage that one side of a p-substrate is locally n-doped in the edge area; an n-epitaxial layer is deposited on or applied to this layer, the center of the n-epitaxial layer being p-doped corresponding to the preceding substrate layer, and a second n-epitaxial layer is applied corresponding to the first n-epitaxial layer, the second n-epitaxial layer being p-doped in the diaphragm area or in additional areas. After that, a purely n-doped diaphragm layer is deposited on or applied to the n-epitaxial layer, an etch mask being applied to the exposed surface of the substrate.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 shows a procedural step for the production of a micromechanical diaphragm having a p-substrate and locally p-doped n-epitaxial layers in the edge area.  
         [0016]    [0016]FIG. 2 shows another procedural step for the production of a micromechanical diaphragm having a p-substrate and locally p-doped n-epitaxial layers in the edge area.  
         [0017]    [0017]FIG. 3 shows another procedural step for the production of a micromechanical diaphragm having a p-substrate and locally p-doped n-epitaxial layers in the edge area.  
         [0018]    [0018]FIG. 4 shows another procedural step for the production of a micromechanical diaphragm having a p-substrate and locally p-doped n-epitaxial layers in the edge area.  
         [0019]    [0019]FIG. 5 shows another procedural step for the production of a micromechanical diaphragm having a p-substrate and locally p-doped n-epitaxial layers in the edge area.  
         [0020]    [0020]FIG. 6 shows a second exemplary embodiment according to FIG. 5 with a diaphragm of varying stiffening.  
         [0021]    [0021]FIG. 7 shows a third exemplary embodiment according to FIG. 5 with an additional stiffening modification. 
     
    
     DETAILED DESCRIPTION  
       [0022]    A micromechanical diaphragm  9  is produced by applying various differently doped epitaxial layers  1 ,  7 ,  7 ′ to a substrate  3 .  
         [0023]    The p-doped substrate which forms the basis for the production of the micromechanical diaphragm is identified by  3  in FIG. 1. Surface  5  of substrate  3  is locally n-doped, i.e., in addition to its general p-doping, it has an n-doped area  6  on its surface  5 . n-doped area  6  extends to the edge area of surface  5  of a chip so that a central p-doped area  8  is preserved on surface  5 . In this connection, doped layer  6  attains a thickness or height of 3-10 μm.  
         [0024]    According to a further method step in FIG. 2, a locally p-doped n-epitaxial layer  7  is applied to substrate  3 , i.e. to its locally n-doped surface  5  (see FIG. 1). Corresponding to n-doped area  6  of substrate  3 , edge area  6 ′ of the n-epitaxial layer also continues to be n-doped. n-epitaxial layer  7  is either n-doped during the deposition and subsequently locally p-doped or p-doped during the deposition and subsequently locally n-doped in edge area  6 ′. It is also possible to deposit the epitaxial layer undoped and subsequently to provide edge area  6 ′ locally with an n-doping and area  8 ′ with a p-doping. Lastly, an n-doped edge area  6 ′ arises as well as a p-doped area  8 ′ in the center of n-epitaxial layer  7 .  
         [0025]    According to a further method step in FIG. 3, a second n-epitaxial layer  7 ′ is deposited on or applied to a surface  5 ′ (see FIG. 2), the central area of the n-epitaxial layer, corresponding to first n-epitaxial layer  7 , is also locally p-doped, i.e. has an outer n-doped area  6 ″ and an inner p-doped area  8 ″. These differently doped areas are formed according to the diaphragm shape provided later.  
         [0026]    According to FIG. 4, an n-doped diaphragm layer  1  is applied to or placed on a surface  5 ″ (see FIG. 3) of second n-epitaxial layer  7 ′ in the next method step.  
         [0027]    The micromechanical diaphragm (FIG. 4) formed in this manner thus has the essentially purely p-doped substrate  3  and the purely n-doped diaphragm layer  1  which accommodate two n-epitaxial layers  7 ,  7 ′ between them. In this connection, the doping of n-epitaxial layers  7 ,  7 ′ and surface  5  of substrate  3  varies locally, i.e., n-doped and p-doped areas alternate in the respective layer  7 ,  7 ′, the p-doped areas being provided in the center. Depending on the embodiment, it is also possible to provide the p-doped areas off-center.  
         [0028]    According to FIG. 5, an etch mask  4  is provided on a remaining outer surface  10  of substrate  3 . Etch mask  4  covers the edge areas of substrate  3  so that a central area remains open. The etching or the etch front starts at this open surface and is stopped by a pn-etch stop, i.e., the boundary of the etching is defined by the n-doped areas. Accordingly, only the central p-doped area is attacked, i.e. etched open by the etching and n-doped edge area  6 ,  6 ′,  6 ″ is preserved. Consequently, the previously p-doped area, i.e. substrate  3  where the etch mask provides access for the etching agent, is removed. Only a flank  13  of a still preserved p-doped subarea  12  is attacked by the etching agent in the trapezoidal open space  11  formed so that this flank is propagated toward the edge area in proportion to the etching time. The area formed by the etching boundary within the p-doped area is suggested by a dashed line since this boundary cannot be precisely defined due to the underetching.  
         [0029]    As a result of open space  11  formed in the central area, diaphragm layer  1  forms an exposed diaphragm  1 ′ in this area. Diaphragm  1 ′ essentially has the size of the previous p-doped areas  8 ,  8 ′,  8 ″ of n-epitaxial layers  7 ,  7 ′ since these were removed in the etching and thus the formation of diaphragm  1 ′ was made possible.  
         [0030]    Two modifications of the embodiment of p-doped areas  8 ,  8 ′,  8 ″, i.e. diaphragm  1 ′ formed from them, are shown in FIGS. 6 and 7.  
         [0031]    The extension of open space  11  to the edge area is formed here essentially larger than in FIG. 5, i.e, the freely oscillating area of diaphragm layer  1 , i.e., diaphragm  1 ′, is formned essentially wider according to FIGS. 6 and 7. In the central area of diaphragm  1 ′, in contact with diaphragm layer  1 , n-doped areas  6 ,  6 ′,  6 ″ are provided which serve as stiffening  14  of diaphragm  1 ′. These areas were formed during production as n-doped areas  6 ,  6 ′,  6 ″ in the respective n-epitaxial layer.  
         [0032]    According to FIG. 7, stiffening  14  may have any desired shape so that any desired open spaces  11  corresponding to the previously p-doped areas are formed within stiffening  14 .  
         [0033]    The cross-section of open space  11  shown here, i.e. its subareas  11 ′, is rectangular in the ideal case, open space  11 , i.e. its projection surface perpendicular to surface  5 , becoming smaller starting from etch mask  4  toward diaphragm  1 ′ or gradually thinning to the size of diaphragm  1 ′.