Abstract:
A micromechanical z-sensor includes a sensitivity, a torsion spring, and a seismic additional mass, the torsion spring having a spring width, and the seismic additional mass including webs having a web width. The web width is selected smaller than the spring width.

Description:
FIELD OF THE INVENTION 
     The present invention is based on a micromechanical z-sensor having a sensitivity, a torsion spring, and an additional seismic mass, the torsion spring having a spring width, and the additional seismic mass having webs having a web width. 
     BACKGROUND INFORMATION 
     Capacitive acceleration sensors with a detection direction perpendicular to the wafer plane, referred to as z-sensors, often utilize balancing-rocker constructions. The sensor principle of these balancing rockers is based on a spring-mass system in which a movable seismic mass forms a plate-type capacitor together with two counter electrodes disposed on the substrate. The seismic mass is connected to the base via a torsion spring. If the mass structures on the two sides of the torsion spring are of different size, then an acceleration action will induce the mass structure to rotate relative to the torsion spring as axis of rotation. Such a mass difference is caused by, for example, an additional mass affixed asymmetrically to the torsion spring. The distance of the electrodes on the side having the larger mass therefore becomes smaller and greater on the other side. The resulting change in capacitance is a measure for the acting acceleration. The sensor principle of these balancing rockers is described in the EP 0 244 581 or EP 0 773 443. 
     A central element of this sensor type is its torsion spring. The torsion spring is determinative of the mechanical sensitivity of the sensor. Variations in the production process result in fluctuations in the width of the torsion springs, which greatly influence the sensitivity. In conventional balancing-rocker structures, these fluctuations in sensitivity are not compensated. 
     SUMMARY 
     Example embodiments of the present invention provide a mass structure on which process fluctuations during the production have a less pronounced effect on the sensitivity of the sensor. 
     Example embodiments of the present invention provide a micromechanical z-sensor having a sensitivity, a torsion spring and a seismic additional mass, the torsion spring having a spring width, and the seismic additional mass including webs having a web width. The web width is selected smaller than the spring width. 
     In example embodiments of the present invention, the web width is selected smaller than one half of the spring width and greater than one fourth of the spring width, preferably greater than one third of the spring width. It is also advantageous that the z-sensor has a mass structure, which includes additional webs having an additional web width, and that the web width and the additional web width differ from one another. It is advantageous that webs having different widths are provided and that the web width is an average width. 
     An aspect example embodiments of the present invention consists of the constructive implementation of the balancing-rocker structure in the region of the additional mass, suitably selected as a function of the torsion spring. The additional mass has a structure of interconnected bar elements, the structure being adapted to the width of the torsion springs. This structure results in a reduced variation range of the sensor&#39;s sensitivity, which is advantageous for the post-connected evaluation electronics, among others. 
     Example embodiments of the present invention may basically be utilized in any acceleration sensor designed according to the balancing-rocker principle. Example embodiments of the present invention are easy to realize and offer the potential for simplifying the required evaluation electronics or for reducing the size of the sensor chip. This may result in cost advantages or better performance of the sensors. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a conventional z-sensor in a side view; 
         FIG. 2  shows the conventional z-sensor in a plan view; and 
         FIG. 3  shows a z-sensor according to an example embodiment of the present invention in a plan view. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a conventional z-sensor in a side view. It is merely a schematic illustration. Illustrated is a surface-micromechanical sensor having a substrate  10 , which has a main surface in one plane (x; y). A mass structure  30  is disposed above substrate  10  at an anchoring  15 . Electrodes  20  are situated on substrate  10  underneath mass structure  30 . Mass structure  30  has an additional mass  40 , which is disposed asymmetrically with respect to anchoring  15 . In the operating state of an accelerated movement of the z-sensor having an acceleration component  50  perpendicular to the plane (x; y), mass structure  30  having additional mass  40  is deflected in relation to sensor substrate  10 , in z-direction  60 . 
       FIG. 2  shows a conventional z-sensor in a plan view. Mass structure  30  includes a torsion spring  100  having a spring width  105 . Via anchoring  15  disposed underneath, the torsion spring is connected to substrate  10 . The sensitivity of this type of sensor depends to a large degree upon the rigidity of torsion spring  100  and the mass distribution within the balancing-rocker structure. High sensitivity is produced by a “soft” torsion spring  100  and/or a large additional mass  30 . For technological reasons, the movable balancing-rocker structure is provided with holes  120  (perforated). For the following comments it is helpful to imagine the structure made up of individual webs  110  in the assembled state. The variations in the production process mentioned in the related art not only affect torsion spring  100  but also the perforated balancing-rocker structure. Each web  110  basically is subject to the same fluctuations as torsion spring  100 , which means that a reduction in spring width  105  is accompanied by a reduced web width  115 , and vice versa. Spring width  105  is entered in sensitivity E (deflection divided by acceleration) of the sensor at the third power, while additional mass  40 , which is a function of web width  115 , is entered linearly: 
     
       
         
           
             
               
                 
                   E 
                   ∝ 
                   
                     
                       
                         m 
                         Zusatzmasse 
                       
                       
                         b 
                         Feder 
                         3 
                       
                     
                     . 
                   
                 
               
               
                 
                   
                     ( 
                     
                       eq 
                       . 
                       
                           
                       
                       ⁢ 
                       1 
                     
                     ) 
                   
                   . 
                 
               
             
           
         
       
     
     If torsion spring  100  and webs  110  of additional mass  40  are then affected by the same absolute width fluctuation Δb, the following is approximately valid for the additional mass affected by the process fluctuations 
                       m     Zusatzmasse   ,   Prozess       ∝       b   Massesteg     ⁡     (     1   +       Δ   ⁢           ⁢   b       b   Massesteg         )         ,             (     eq   .           ⁢   2     )     ,               
and for the fluctuating spring width
 
     
       
         
           
             
               
                 
                   
                     b 
                     
                       Feder 
                       , 
                       Prozess 
                     
                     3 
                   
                   ≈ 
                   
                     
                       
                         b 
                         Feder 
                         3 
                       
                       ⁡ 
                       
                         ( 
                         
                           1 
                           + 
                           
                             
                               Δ 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               b 
                             
                             
                               b 
                               Feder 
                             
                           
                         
                         ) 
                       
                     
                     3 
                   
                   ≈ 
                   
                     
                       
                         b 
                         Feder 
                         3 
                       
                       ⁡ 
                       
                         ( 
                         
                           1 
                           + 
                           
                             3 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               
                                 Δ 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 b 
                               
                               
                                 b 
                                 
                                   Feder 
                                   ⁢ 
                                   
                                       
                                   
                                 
                               
                             
                           
                         
                         ) 
                       
                     
                     . 
                   
                 
               
               
                 
                   
                     ( 
                     
                       eq 
                       . 
                       
                           
                       
                       ⁢ 
                       3 
                     
                     ) 
                   
                   . 
                 
               
             
           
         
       
     
     After cycling through the production process, the following therefore results for the sensitivity 
     
       
         
           
             
               
                 
                   
                     E 
                     Prozess 
                   
                   ∝ 
                   
                     
                       
                         
                           b 
                           Massesteg 
                         
                         ⁡ 
                         
                           ( 
                           
                             1 
                             + 
                             
                               
                                 Δ 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 b 
                               
                               
                                 b 
                                 Massesteg 
                               
                             
                           
                           ) 
                         
                       
                       
                         
                           b 
                           Feder 
                           3 
                         
                         ⁡ 
                         
                           ( 
                           
                             1 
                             + 
                             
                               3 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 
                                   Δ 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   b 
                                 
                                 
                                   
                                     b 
                                     Feder 
                                   
                                   ⁢ 
                                   
                                       
                                   
                                 
                               
                             
                           
                           ) 
                         
                       
                     
                     . 
                   
                 
               
               
                 
                   
                     ( 
                     
                       eq 
                       . 
                       
                           
                       
                       ⁢ 
                       4 
                     
                     ) 
                   
                   . 
                 
               
             
           
         
       
     
     A compensation of the process fluctuations takes place for: 
                       b   Massesteg     ≈       b   Feder     3       ,             (     eq   .           ⁢   5     )     ,               
i.e., if mass web width  115  corresponds to one third of spring width  105 , then the expressions in parenthesis are canceled, and the sensitivity in the first order becomes independent of fluctuations Δb of the structure width.
 
     For a sensitivity compensation with regard to parameter Δb, the mass structure must therefore be adapted to width  105  of torsion spring  100  according to equation 5. A corresponding structure is shown in  FIG. 3 . 
       FIG. 3  shows a z-sensor according to an example embodiment of the present invention in a plan view. According to example embodiments of the present invention, web width  115  for webs  110  in the region of additional mass  40  amounts to one third of spring width  105  of torsion spring  100 . If it is impossible for technological reasons to select a sufficiently small mass web, then a value that comes as close as possible to equation 5 must be chosen for the web width. In comparison with disregarding equation 5, the resulting compensation is not complete but improved. 
     The remaining mass structure  30  of the balancing rocker may have perforations as desired, also identical with additional mass  40 .