Patent Publication Number: US-9403677-B2

Title: Micro-electromechanical semiconductor component

Description:
This application is a continuation of U.S. patent application Ser. No. 13/521,168, filed Apr. 17, 2013, which is a 371 National Phase Application based on International Application PCT/EP2011/050213, filed Jan. 10, 2011, which claims priority to European Application 10150405.8, filed Jan. 11, 2010. The entire contents of each of U.S. patent application Ser. No. 13/521,168; International Application PCT/EP2011/050213; and European Application 10150405.8 are incorporated by reference herein. 
    
    
     The invention relates to a micro-electromechanical semiconductor component comprising a bending element made of a semiconductor material, and comprising a transistor which sensitive to mechanical stresses and is arranged within the bending element. Particularly, the invention relates to a micro-electromechanical semiconductor component being compatible with regard to usual semiconductor manufacturing processes and having a low power consumption, and particularly a micro-electromechanical CMOS pressure and respectively acceleration sensor with low power consumption. 
     Micro-electromechanical semiconductor components adapted for use as pressure or acceleration sensors are generally known and have proven themselves in practice. Such micro-electromechanical semiconductor components normally comprise a bending element which is reversibly deformable under the influence of forces acting from the outside and can be realized in the form of a membrane, a bending beam or the like. Said bending element is made of a semiconductor material. In the region of the bending element, an integrated transistor is provided which is sensitive to mechanical stresses. A plurality of such transistors can be connected to form a measurement bridge. 
     It is an object of the invention to provide an integrated transistor for the reversibly deformable bending element of a micro-electromechanical semi-conductor component wherein the transistor is substantially unaffected by parasitic influences. 
     For achieving the above object, there is proposed, in accordance with the invention, a micro-electromechanical semiconductor component comprising
         a semiconductor substrate,   a reversibly deformable bending element made of semiconductor material, and   at least one transistor that is sensitive to mechanical stresses, said transistor being designed as an integrated component in the bending element,   said transistor being arranged in an implanted active region pan that is made of a semiconductor material of a first conducting type and is introduced in the bending element,   two mutually spaced, implanted drain and source regions made of a semiconductor material of a second conducting type are formed in the active region pan, a channel region extending between said two regions,   implanted feed lines made of a semiconductor material of the second conducting type lead to the drain and source regions,   the upper face of the active region pan is covered by a gate oxide,   a gate electrode made of polysilicon is located on the gate oxide in the area of the channel region, a feed line likewise made of polysilicon leading to said gate electrode.       

     According to the invention, said transistor is formed within an active region pan which, through implantation, has been inserted into the bending element and which comprises a doped semiconductor element of a first conducting type. For instance, the semiconductor material of the bending element is weakly p − -doped, while the active region pan is weakly n − -doped. The active region pan has two regions implanted into it, namely the drain region and the source region. Within these regions, the pan is strongly p + -doped. The entire pan (and optionally the region around the pan) is covered by a gate oxide (e.g. silicon oxide). Thus, no field oxide exists particularly within the top side of the pan and within the region around the pan. Thereby, in its region provided with the transistor, the bending element has a homogenous thickness, which is of advantage for the constant mechanical properties (particularly for the homogenizing of the stress field). Further, in the transistor according to the invention, there are provided no metallic feed lines leading to the drain and source regions as well as to the gate oxide. Instead, the feed lines leading to the drain and source regions are formed as strongly doped, implanted feed lines of the second line type. The transfer gate, like its feed line, is made of polysilicon. By all these measures, it is achieved that, one the one hand, only the slightest possible oxygen layer thicknesses need be applied and that the materials used have substantially the same expansion coefficients as the material of the bending element. 
    
    
     
       The invention will be explained in greater detail hereunder with reference to the drawing. Specifically, the drawings show the following: 
         FIG. 1  Process for producing a structure in accordance with the invention:
           a ) raw wafer     b ) oxidation and opening of windows     c ) etching the cavity     d ) bonding the top wafer (followed by the CMOS process, which is not especially shown)     e ) etching the trenches (after the CMOS process).       

         FIG. 2  Three-dimensional simplified sectional view of a pressure sensor produced according to the process of  FIG. 1 . 
         FIG. 3  Alternative second process for producing a structure in accordance with the invention:
           a ) raw wafer     b ) oxidation and opening of windows     c ) etching the cavity     d ) bonding the handle wafer (followed by the CMOS process on the top wafer with cavity, which is not especially shown)     e ) etching the trenches (after the CMOS process).       

         FIG. 4  Three-dimensional simplified sectional view of a pressure sensor produced according to the process of  FIG. 3 . 
       
         FIGS. 5 to 10 
           
           
             
               Alternative third process for producing a structure in accordance with the invention: 
                 a ) raw wafer 
                 b ) oxidation 
                 c ) application of polysilicon layer and partial oxidation 
                 d ) second raw wafer 
                 e ) oxidation and opening of a window 
                 f ) etching the cavity 
                 g ) bonding the handle wafer 
                 h ) partial grinding (followed by the CMOS process on the top wafer with cavity, which is not especially shown) 
                 i ) etching the trenches (after the CMOS process) 
                 j ) three-dimensional simplified sectional view of a pressure sensor produced according to the process of  FIGS. 5 to 9 . 
             
           
         
      
         FIG. 11  Example of the layout of a transistor. 
         FIG. 12  Connection of four transistors to form a Wheatstone bridge (operation of the transistors as resistors). 
         FIG. 13  Exemplary connection of four transistors and two further transistors to form a Wheatstone bridge with reference voltage source. 
         FIG. 14  Exemplary layout of a Wheatstone bridge. 
         FIG. 15  Connection of eight transistors to form a Wheatstone bridge, with a second short-circuited Wheatstone bridge as a reference voltage source. 
         FIG. 16  Example of a placement of four Wheatstone bridges according to  FIG. 12  on a sensor die with trench structure. 
         FIG. 17  Example of a placement of four Wheatstone bridges with four Wheatstone bridges as stress-free references according to  FIG. 15  on a sensor die with trench structure (For better survey, the voltage references are not illustrated. In principle, for each pair of Wheatstone bridges, there will suffice a third, short-circuited Wheatstone bridge besides the stress-free reference bridge. Thus, 12 Wheatstone bridges would be found on the die). 
         FIG. 18  Exemplary layout of a differential stage. 
         FIG. 19  Exemplary circuit arrangement for a circuit with a differential amplifier and a referential differential amplifier as a reference voltage source. 
         FIG. 20  View onto an exemplary membrane geometry with quadratic trench system
           a ) plan view     b ) view from below.       

         FIG. 21  View onto an exemplary membrane geometry with quadratic trench system and rhombic central portion
           a ) plan view     b ) view from below.       

         FIG. 22  View onto an exemplary membrane geometry with quadratic trench system which was chamfered in the corners, and with rhombic central portion
           a ) plan view     b ) view from below.       

         FIG. 23  View onto an exemplary membrane geometry with quadratic trench system and webs in the corners,
           a ) plan view     b ) view from below.       

         FIG. 24  View onto an exemplary membrane geometry with round trench system and round cavity,
           a ) plan view     b ) view from below.       

         FIG. 25  View onto an exemplary membrane geometry with round trench system and non-continuous webs,
           a ) plan view     b ) view from below.       

       
         FIGS. 26 and 27 
           
           
             
               sensors with additional trenches. 
             
           
         
      
         FIG. 28  Boss (central membrane reinforcement) with mass reduction by etched support structure  97 . 
       
         FIGS. 29 and 30 
           
           
             
               Exemplary differential pressure sensors generated from the above sensors by etching an opening  119 . 
             
           
         
      
         FIG. 31  View onto an exemplary membrane geometry with round cavity, round outer edge of the trench system, and rhombic central portion
           a ) plan view     b ) view from below.       

         FIG. 32  View onto an exemplary membrane geometry with round cavity, round outer edge of the trench system, and rhombic central portion, and additional trenches for protection of the system against spreading of externally introduced stress
           a ) plan view     b ) view from below.       

         FIG. 33  Circuit diagram of a bridge according to  FIG. 34  and  FIG. 35  as a measurement bridge with reference voltage source. 
         FIG. 34  Exemplary layout of a measurement bridge with common gate. 
         FIG. 35  Further exemplary layout of a measurement bridge with common gate. 
         FIG. 36  Equivalent circuit diagram of a transistor according to  FIG. 37  as a measurement bridge with reference voltage source. 
         FIG. 37  Exemplary layout of a particularly small measurement bridge transistor with four connectors. 
         FIG. 38  A detailed view. 
         FIG. 39  A graphical representation of the design and construction of a bending-sensitive transistor within a bending element (e.g. membrane) of a micro-electromechanical semiconductor component. 
         FIG. 40  A plan view in the direction of arrow A of  FIG. 39  onto a partial region of the membrane and respectively the bending element in which the transistor is formed. 
     
    
    
     The invention will be illustrated by way of example of a pressure sensor for detection of low pressures. A first essential aspect of the invention resides in the generating of the pressure sensor cavity  4  prior to the processing of the CMOS steps. Thereby, random standard CMOS processes can be performed on the surface. This makes it possible, as a second essential step, to place CMOS transistors on a membrane in such a manner that they will lie in a region of optimal mechanical stress upon deflection of the membrane. This point can be determined by analytic considerations and/or finite element simulations. 
     A first exemplary process is represented, by way of essential steps thereof, in  FIGS. 1 and 2 . Modifications of this first process will be described further below. 
     The basic manufacturing process starts with a first wafer  1  which preferably consists of the same material as a later used second wafer  5 . On this wafer, a layer  2  is deposited which serves for later connection. In silicon wafers, this layer will be formed as a SiO 2  layer  2  by oxidation. Within this layer, a window will be opened and the later pressure sensor cavity  4  will be etched ( FIG. 1 c   ). Said etching is preferably performed by a DRIE or plasma etching step because particularly the first one will result in straight walls  3 . 
     The upper wafer  5  will likewise be provided with an oxide layer and will be bonded onto the first wafer  1  and be ground ( FIG. 1 d   ). The bonding process herein is preferably performed in a vacuum. This will lead to a cavity which is not filled with gas, and will prevent a later temperature dependence of the interior pressure in the cavity. By this process, a membrane is generated in the region of the cavity, whose thickness is determined by the grinding process. 
     As a result, one will obtain a wafer package which can be used in a standard CMOS process or standard bipolar process in the same way as a normal SOI material. 
     After the CMOS processing, which does not have to be described in greater detail here because the processing can be gathered from standard literature, further micromechanical structures  6  can then be etched into the surface  11  ( FIG. 1 e   ). 
     In the case of the exemplary pressure sensor, these micromechanical structures  6  are e.g. trench structures forming approximately closed rings or quadrangles which are interrupted only by few webs  8  ( FIG. 2 ). In the middle, there is thus generated a central plate  12 —referred to as a boss—which, due to the increased thickness, forms a reinforcement. The bottom of the trenches  6  forms a membrane  7  of reduced thickness. This membrane typically takes up substantially less forces. It is important herein that the outer edge of the trench  6  is sufficiently spaced from the wall of the cavity  3  since, otherwise, small adjustment errors in manufacture would have a massive effect on the mechanical stress and thus on the measurement result. This is a substantial innovation. The reproducibility, under the aspect of production technology, of the sensor properties, would otherwise suffer, which would cause increased expenditure for calibration and therefore would entail corresponding costs. 
     Thus, the pressure on the central plate  12  is dissipated virtually exclusively through mechanical tension via the webs  8 . Consequently, it is reasonable to place onto said webs those components  9  which are sensitive to mechanical stress and are provided to detect this stress. These are connected to the connectors  10  via lines. Thus, by the trenches, the mechanical stress field will be adjusted relative to the stress-sensitive electronic components. 
     By way of alternative, the cavity can also be etched into the upper wafer. This is correspondingly represented in  FIGS. 3 and 4  in steps a to f. 
     An essential disadvantage of the two above processes is the absence of a natural etching stopper for the etching of the trenches  6 . For this reason, the thickness of the membrane at the bottom of the trenches  7  can be controlled only with difficulty. As a consequence, the relative error is comparatively high, causing a dispersion of the sensor parameters. 
     A third exemplary process which is free of the above disadvantage is represented, by way of essential steps thereof, in  FIGS. 5 to 10  and steps a to j. 
     The manufacturing process starts with a first wafer  13  which preferably is of the same material as the later used second wafer  16 . Onto this wafer, there is applied a connection layer which in the case of silicon wafers is a SiO 2  layer  14 . On this SiO 2  layer, a further layer will be deposited, e.g. a polysilicon layer or amorphous silicon layer  15 , and will be superficially oxidized ( FIG. 5 c   ). The depositing of this layer typically can be controlled very well and thus will be considerably more precise in its result than the etching of the trenches in the first two described processes. The second wafer will be oxidized as well, thus likewise generating an oxide layer  17 . In this layer, at least one window will be opened and the later cavity  18  will be etched. Said etching is preferably performed by a DRIE etching step because this step will result in straight walls ( FIG. 6 f   ). It is of course also possible to etch the cavity  18  into the upper wafer  13 , but this will not be described in greater detail hereunder. 
     The upper first wafer  13  will be bonded onto the second wafer  16  ( FIG. 7 ) and subsequently will be grinded ( FIG. 8 ). The bonding process herein will preferably again be performed in a vacuum so as to exclude a later temperature dependency of the interior pressure in the cavity  18 . By this process, a membrane is generated in the region of the cavity, whose thickness is determined by the grinding process. 
     As a result, one will again obtain a wafer package which in principle can be used in a standard CMOS process or standard bipolar process in the same way as a standard wafer. 
     After the CMOS or bipolar processing, further micromechanical structures, e.g. trenches  19 , can then be etched into the surface  24  ( FIG. 10 ) as in the above described processes. 
     In the case of a pressure sensor, these micromechanical structures  19  are again e.g. trench structures forming approximately closed rings or quadrangles which are interrupted only by few webs  20 . In the middle, there is thus again generated a central plate  21  which, due to the increased thickness, forms a reinforcement. The bottom of the trenches  19  forms a membrane  25  of reduced thickness. This membrane typically takes up virtually no forces. In contrast to the first method, the additional layer  15  and the resultant additional oxide layer  14  make it possible to stop the etching of the trenches  19  with greater precision than in the first method. Thereby, the electromechanical properties can be realized more precisely and with better repetitive accuracy, thus distinctly lowering the calibration costs. 
     As in the result of the first process, the pressure (see  FIG. 10 ) on the central plate  21  is dissipated virtually exclusively via the webs  20 . Consequently, it is again reasonable to place onto said webs those components  22  which are sensitive to mechanical stress and are provided to detect this stress. These are connected to the connectors  23  via lines. 
     Of course, it can also be envisioned, instead of using a depositing process, to produce said additional layer  15  by bonding and grinding a third wafer. Further, it can be envisioned to integrate more than one buried insulated layer of the type of said additional layer  15  into a wafer packet. 
     With a wafer package generated in this manner, it is possible again to produce stress-sensitive sensors on the membrane after their production prior to generating the trenches ( 6  or  19 ). 
     To this end, in a CMOS or bipolar process, stress-sensitive electronic components are produced on the respective surface  11 ,  24  and are connected. For a CMOS process, preferred use is made of a p-doped substrate. 
     For instance, piezoelectric resistors can be placed on the webs  20 ,  8  and be connected to form a Wheatstone bridge. These, however, have the disadvantage that they first must be brought onto the operating temperature and will consume relatively much electrical energy during measurement. Thus, they are unsuited for energy-autonomous systems. As already described above, the invention also aims at eliminating this problem. 
     Thus, in place of such simple electronic components, it is reasonable to use actively amplifying components such as e.g. bipolar transistors and MOS transistors. These can again be connected to form a Wheatstone bridge but will need no warm-up time and will consume less energy. An exemplary circuit arrangement is shown in  FIG. 12 . 
     Herein, four p-channel transistors  85 ,  86 ,  87 ,  88  form a Wheatstone bridge which can be sensed at the two terminals  89 ,  90 . The transistors  87  and  85  herein are designed with the same orientation, and also the transistor pair  88 ,  86  is given the same orientation but is arranged vertically to the transistor pair  87 ,  85 . However, this circuit is highly sensitive to manufacturing faults. 
     Now, to make it possible to produce such a MOS transistor circuit with sufficient accuracy, the transistors have to be designed in such a manner that the electrically active part is self-adjusting.  FIG. 11  shows an exemplary layout of such a self-adjusting transistor. Herein, the p+-contact implants  80  and  79  are shaded by the poly gate  81  in such a manner that, also in case of displacement, there will always be maintained the same transistor channel length and transistor width. The poly gate  81  also shades the n+-channel stopper implant. The gate is connected via a low-impedance poly line. 
     Thus, it is safeguarded that the transistors of identical design geometries have a similar geometry in their physical realization. This geometry is determined substantially by the design of the polysilicon surface. 
     To bring the transistors to the respective correct operating point, it is suitable to also integrate a reference voltage source on the pressure sensor. In the example ( FIG. 13 ), the exemplary reference voltage source consists of the transistors  30  and  29 . The transistors  31 ,  32 ,  33 ,  34  again form a Wheatstone bridge which can be sensed at the terminals  28 ,  36 . Both are connected as MOS diodes by connection of the gate to the drain. The reference voltage of transistor  30  is connected to the gate of transistors  31  and  33 . The reference voltage of transistor  29  is connected to the gate of transistors  32  and  34 . In the example of  FIG. 13 , the drain of transistor  29  is on the potential of terminal  26 . In p-channel transistors, this terminal is typically connected to ground. Therefore, the drain contacts of transistors  32  and  34  are likewise connected to this terminal. The transistors are preferably realized with identical geometric dimensions. An example of the layout of a local Wheatstone MOS bridge is illustrated in  FIG. 14 . If the transistors are arranged as in  FIG. 14 , the transistors  31  and  34  are oriented identically. The transistors  23  and  33  are also oriented identically relative to each other but vertically to transistors  31  and  34 .  FIG. 14  shows an exemplary arrangement. 
     To keep the mechanical distortion on the membrane at a minimum, the membrane is not provided with a field oxide but only with a full-surfaced, extremely thin gate oxide of a few nm and a suitable passivation. If the application of a field oxide is unavoidable, high symmetry is recommendable so as to keep parasitic effects on all stress-sensitive components at an identical level. In a silicon pressure sensor, for instance, the passivation can consist of silicon nitride. This has a low hydrogen diffusion coefficient and thus will protect the component against in- and out-diffusion of protons which, particularly in case of a permanently applied voltage and a high operating temperature may cause a drift of the p-resistors and the p-channel transistors. This effect is known as NBTI. In order to avoid any possible kind of mechanical distortion, no field oxide or the like will be generated near mechanical components or even on these. Therefore, particularly the membrane of the exemplary pressure sensor is covered only with the gate oxide and the passivation layer, silicon nitride. Further, the feed lines on the die will be realized, if possible, not in metal, which has a high thermal expansion coefficient particularly relative to silicon, but in the wafer material and, in case of silicon, as a high-doped layer or as a high-doped polysilicon or, if not possible otherwise, as a high-doped amorphous or polycrystalline silicon. In this example ( FIG. 14 ), the drain and source feed lines of the transistors  26 ,  28 ,  35 ,  36  are realized e.g. as p+-implants  36 ,  35 ,  26 ,  28 . The gates and their feed lines are realized e.g. in polysilicon  33 ,  39 ,  31  and  32 ,  34 ,  38 . In the surface  40  which will be n-doped, no channel will be formed in this example, which is due to the field threshold. This will be possible only at the edge of the poly gates. For this reason, there is implanted a n+-channel stopper  37  which will interrupt parasitic forces. By this exemplary all-silicon realization, it is thus possible that the element sensitive to mechanical stress can be kept very small and insensitive to manufacturing tolerances and thermomechanical stress caused by foreign materials, so that the sensitivity to inhomogeneous stress distribution will be further reduced. In spite of these provisions, there still exist marginal differences between the materials. For this reason, when designing the electronic components on the die, and particularly in components arranged on the membrane, care is taken to maintain the highest possible symmetry. Thus, it is reasonable that components which are used for differentiation—e.g. those in Wheatstone bridges or differential amplifiers—are placed as closely together as possible so as to minimize the influence of manufacturing inhomogeneities. 
       FIG. 15  shows a further variant of the Wheatstone bridge. Therein, the reference voltage by which the bridge consisting of the transistors  31 ,  32 ,  33 ,  34  is operated, is generated from a bridge similar to that one, consisting of transistors  30 ,  29 ,  55 ,  56 . Suitably, herein, use is made of the same layout module. The reference bridge will be short-circuited, thus generating the reference voltage  35  by which the transistors  31 ,  32 ,  33 ,  34  of the first bridge are controlled. The second bridge will be placed on the substrate as far away as possible from the mechanical tensions but still as close as possible to the first bridge. This serves for keeping the manufacturing variations between the bridges as small as possible. The first bridge will be placed at the point of suitable mechanical stress. This is the point where a mechanical stress as high as possible will be generated upon deflection of the exemplary membrane while, however, this stress is still so homogenous that manufacturing variations will not yet be perceived too massively. 
     For further minimization of influences of misalignment, it can be reasonable to place a plurality of bridges on one die. This can be effected e.g. by a placement as shown in  FIG. 16 . Here, there is shown the possible placement of four bridges according to  FIG. 12 .  FIG. 17  shows the placement of bridges and reference bridges according to  FIG. 15 . In an arrangement as shown in  FIG. 17 , three levels of compensation exist. On the first level, the level of the four transistors, the direction of the mechanical stress will be detected. This is performed by comparison between values of transistors arranged mutually vertically. On the next level, these four transistors in their totality  43  will be compared to four further, similarly arranged transistors  58  located close to the first four ones,  43 , but in a mechanically less stressed region, ideally on the neutral fiber. Thereby, the mechanically induced offset of the bridge will be differentiated from that offset which is caused by adjustment errors during manufacture. If the sensor is symmetric, it is reasonable to install eight further transistors, according to the number of positions on the axis of symmetry. In the example ( FIG. 17 ), these are four pairs of sensors  44 ,  57 ;  41 ,  60 ;  42 ,  59 ;  43 ,  58 , each pair consisting of two by four transistors. 
     Theoretically, the placement of a single transistor is already sufficient for stress measurement. In this case, however, all manufacturing errors will already have a massive effect. 
     A first alternative layout is shown in  FIG. 34 .  FIG. 33  shows the appertaining circuitry with a reference voltage source consisting of the transistors  108 ,  109 . Here, the four transistors  104 ,  105 ,  106 ,  107  connected to form a Wheatstone bridge have a common gate  110 , which simplifies the layout. Via the terminals  103  and  102 , the bridge is supplied with voltage. In case of a mechanical distortion of the bridge, an electric voltage will appear at the terminals  111 ,  112 .  FIG. 35  illustrates a further variant of this bridge. If the channel stopper  37  in the middle of the bridge is omitted, this results in a field-plate-like transistor  115  with four terminals ( FIG. 37 ).  FIG. 36  shows the equivalent circuit diagram of transistor  115 . Added thereto are then the transistors  114  and  113  which, on the one hand, increase the current consumption,  114 , and, on the other hand, reduce the signal level,  113 . As a benefit thereof, however, the design size and thus the space requirement can be minimized, which in some applications is very useful. 
     An alternative layout arrangement of the transistors of a sensor element  41 ,  42 ,  43 ,  44 ,  57 ,  58 ,  59 ,  60  is shown in  FIG. 18 . Here, the four transistors  44 ,  45 ,  46 ,  47  are arranged in a star-shaped configuration. They have a common drain contact  50  which via a feed line  49  is connected to a current source that is not located on the membrane of the pressure sensor. The gates of the transistors  44 ,  45 ,  46 ,  47  are connected by a poly line  48 . The source contacts are respectively connected by a highly doped p+-conduit  51 ,  52 ,  53 ,  54 . The four transistors are e.g. components of a differential amplifier as shown in  FIG. 19 . All other transistors of  FIG. 19  are not arranged on the membrane but on the substrate without underlying cavity. It is obvious that already half of the four transistors, i.e. for instance the transistors  45  and  44 , would be sufficient to form a differential amplifier. For reasons of symmetry, however, the variant comprising four transistors is recommendable. 
     The circuit consists of two differential amplifiers. The left-hand amplifier (transistors  65  to  73 ) is short-circuited in the output and the input and operates as a reference voltage source for the operation of the second amplifier. These transistors are arranged in a region which is free of mechanical stress. The above described transistors  44 ,  45 ,  46 ,  47  form the differential stage with the respective appertaining “working resistors”  61 ,  62 ,  63 ,  64 . The current source  74  supplies current to the thus formed differential amplifier. The transistor  74  in this example is an n-channel transistor. In operation, the outputs of the differential amplifier  77 ,  78  will reflect an asymmetry of the transistors  44 ,  45 ,  46 ,  47  due to mechanical stress. Since the transistors  46  and  44  are oriented differently from the transistors  45  and  47 , a uniaxial mechanical stress will result in an output signal at  77 ,  78 . The differential amplifier in this example will be brought to the working point by a similarly designed, short-circuited reference differential amplifier. The latter and the transistors  61 ,  62 ,  63 ,  64 ,  74  are suitably arranged not on the membrane but in a region of the die which is nearly free of mechanical stress. In order to guarantee the congruence of the electrical parameters of the components in the stress-free state, these should nonetheless be placed as close to the other transistors as possible. Thus, the orientation and the layout of all elements suitably will be realized as closely adjacent as possible and with the same orientation so that particularly also the current-mirror pairs will be well attuned to each other. 
       FIGS. 20 to 25  show different embodiments of the trenches and cavities. 
     In the design of the race track  6  and the cavity  3 , various factors have to be considered:
     1. A suitable distance has to be maintained between the race-track outer wall and the cavity wall.   2. The circle passing through the outer contact points of the webs with the race-track outer wall is not to be intersected by the race-track outer wall because this would result in a distortion of the mechanical stress field in the boss  12 .   3. The connecting lines between the base points of the webs  8  on boss  12  is not to be intersected by the outer edge of the boss because this would result in a distortion of the mechanical stress field in the boss.   4. The construction should, if possible, comprise no corners because very strong voltages could occur in these, causing nonlinear effects and bi-stability.   

     Opposed to the above demands are demands with regard to the burst pressure. If the race-track surface becomes too large, the race-track membrane will break more quickly. 
     To decouple the membrane from mechanical stress caused by the design a and the connection technology, it is thus reasonable e.g. to generate a further trench  93  around the sensor ( FIG. 26 ) and in this manner to generate a virtually larger race-track membrane without the above mentioned danger of breakage. 
     Herein, the sensor is suspended on webs  94 . In the ideal case, these are no extensions of the webs  8  on which the boss  12  is fastened. Thereby, mechanical stress will be transmitted to the sensors  9  only indirectly from outside. 
     This principle can still be continued by a further trench  95  and further webs  96  ( FIG. 27 ). 
     The construction with the aid of a boss will lead to an increased sensitivity to seismic stresses. This sensitivity can be lowered by reducing the boss mass ( FIG. 28 ). For this purpose, a suitable support structure will be etched into the boss  97 . There will remain webs which, if suitably selected, will generate a sufficient areal moment of inertia for guaranteeing the mechanical stability. The sensor  22  herein will be placed, as above, on a web  20  interrupting the race-track trench  19 . 
     In case that, instead of an absolute pressure sensor, a differential pressure sensor is to be generated, this can be performed by subsequent etching of an opening  119  into the lower wafer.  FIGS. 29 and 30  show corresponding exemplary shapes. The advantage of such a construction resides in the small opening and thus in the only very small loss of stability in comparison to a sensor wherein the cavity was etched from the rear side. 
     The bonding system normally is made of metal having a considerably deviating thermal expansion coefficient. Further, the metal will cause hysteresis effects. Thus, it is reasonable to decouple the bond pads  10  from the rest of the sensors as far as possible. This can be effected by mechanical guard rings in the form of trenches  157  which e.g. will be laid as far as possible around the pads or to-be-protected parts ( FIG. 32 ). 
     Hereunder, with reference to  FIGS. 39 and 40 , there will again be explained special features of the design of a low-stress transistor which is sensitive to mechanical stresses and can be used in a reversibly deformable bending element  8   a  made of semiconductor material of a micro-electromechanical semiconductor component. The construction into which the transistor according to  FIG. 39  which is sensitive to mechanical stresses is inserted, can have been realized e.g. in the manner described above with reference to  FIGS. 1 to 10, 20 to 32, 34, 35, 37 and 38 . As already shown in  FIGS. 39 and 40 , the transistor is arranged, between two trenches  6 , within a web  8  of the region of the device wafer  5  covering the cavity  4 . Through the handle wafer  1 , a venting channel can lead to the cavity, which, however, is without relevance for the design and functionality of the transistor. 
     The transistor is formed in the active region pan  78   a  which has been inserted into the p − -doped semiconductor substrate of the device wafer  5  by implantation. Within pan  78   a , strongly p + -doped source and drain regions  79 ,  80  are formed, notably likewise by implantation. Between the mutually facing ends of these two regions  79  and  80 , the actual channel region of the transistor is arranged. The entire top side of the active region pan  78   a  is covered by thin gate oxide (SI oxide)  81   a , wherein, in the area of the channel region, the transistor gate  81  made of polysilicon is arranged on this oxide. As can be seen particularly in  FIG. 40 , the feed lines leading to the drain and source regions  79 ,  80  are formed by likewise highly doped p + -regions generated by implantation, and extend to sites outside the region of the device wafer bridging the cavity  44  (i.e. to sites outside the membrane), where they are connected to metallic lines  79   a ,  78   a . The feed line  84  to the transistor gate  81  is realized by a line made of polysilicon, as can again be gathered from  FIG. 40 . 
     In designing the transistor sensitive to mechanical stresses (stress-sensitive element), it is suitable if this element is not altered by parasitic stress e.g. due to a field oxide. From this, it can be derived that, for using the transistor on and respectively in a bending element  8   a  (see  FIG. 39 ) of a micro-electromechanical semiconductor component, it is required that the transistor
     a) does not comprise metal so as to avoid temperature hysteresis effects due to “creeping”, and   b) includes as little oxide as possible since also the latter has a different thermal expansion coefficient from that of the semiconductor material of the bending element.   

     Therefore, it is necessary that a transistor used for stress detection
     a) is situated in a first doped region (active region pan  78   a , which can be n −  or p + -doped),   b) this region is covered exclusively by the transistor gate oxide and does not comprise a field oxide,   c) is electrically connected, if possible, not with metal but with a p + -doped or n + -doped region, and   d) the transistor gate is connected via a polysilicon line.   

     These preconditions are fulfilled in the construction according to the above described  FIGS. 39 and 40 . For this reason, the transistor described and respectively shown therein is extremely sensitive to mechanical stresses without being additionally influenced by parasitic stress. 
     Further properties of the invention and of an exemplary application can be described as follows:
     1. A photolithographically produced transistor on a doped substrate or in a doped trench, wherein
       i. the transistor is electrically connected only with materials which have a thermal expansion coefficient similar to that of the substrate or the trench in which it is placed,   ii. the transistor is in no or only a very slight mechanical connection to other materials, particularly materials having mechanical properties different from those of the substrate or the trench—herein, particularly field oxides,   iii. the transistor comprises symmetries,   iv. the transistor is produced in different process steps by lithography of different geometric, mutually attuned structures, and   v. these geometric structures whose superposition and cooperation in the production process will result in the transistor, are selected in such a manner that process variations within the process specification limits which cause changes of the geometries of the individual lithography step results in the form of produced geometric structures, will have no or only a very slight effect on the mechanical properties of the transistors.   
       2. The transistor according to item 1, which is a MOS transistor.   3. A MOS transistor for detection of mechanical stress, comprising four channel terminals.   4. The MOS transistor according to item 3, which has a four-fold rotational symmetry and a gate plate with exactly this symmetry and channel terminals in an arrangement with exactly this symmetry, without the need of a symmetry of the terminals of this gate plate.   5. The transistor according to item 1, which is a bilpoar transistor.   6. The transistor according to any one of items 1 to 4, comprising a channel stopper.   7. The transistor according to any one of items 1 to 4, of which the source and/or drain regions are electrically connected by a highly doped region or a low-impedance polysilicon.   8. The transistor according to any one of items 1 to 7, which is used for detection of mechanical stress.   9. The transistor according to any one of items 1 to 8, which is used in the manner of an electric resistor particularly in a measurement bridge.   10. The transistor according to any one of items 1 to 9, which is a pnp-transistor.   11. The transistor according to any one of items 1 to 9, which is a npn-transistor.   12. The transistor according to any one of items 1 to 9, which is a p-channel transistor.   13. The transistor according to any one of items 1 to 9, which is an n-channel transistor.   14. An electronic circuit comprising transistors according to one or a plurality of items 1 to 13.   15. An electronic circuit being in a functional relationship to a micro-mechanical device according to item 41.   16. The circuit according to item 14 or 15, comprising discrete and/or integrated electronic components.   17. The circuit according to any one of items 14 to 16, which is at least partially produced by monolithic integration.   18. The circuit according to any one of items 14 to 17, comprising at least two geometrically identically designed transistors according to any one of items 1 to 13.   19. The circuit according to item 18, which generates a signal suited for measurement of a different state in at least one physical parameter of the two transistors.   20. The circuit according to item 19, wherein said physical parameter is mechanical stress and/or temperature.   21. The circuit according to any one of items 18 to 20, wherein at least two of the transistors according to any one of items 1 to 13, irrespective of the connecting lines, are arranged symmetrically to each other.   22. The circuit according to any one of items 18 to 20, wherein, for at least two of the transistors according to one or a plurality of items 1 to 13, it is provided that their geometries, irrespective of the connecting lines, can be brought into mutual congruence through rotation by 90°.   23. The circuit according to any one of items 14 to 22, comprising at least four transistors according to any one of items 1 to 13.   24. The circuit according to item 23, wherein said four transistors are connected to form a measurement bridge.   25. The circuit according to item 24, wherein the gate and source of at least one transistor according to any one of items 1 to 13 are short-circuited.   26. The circuit according to item 24 or 25, wherein the gate of at least one transistor according to any one of items 1 to 13 is connected to a reference voltage source.   27. The circuit according to item 26, wherein said reference voltage source is a second, but short-circuited measurement bridge according to any one of items 24 to 27.   28. The circuit according to item 27, wherein said second measurement bridge is similar to the first measurement bridge, particularly regarding the dimensioning of the transistors and/or the circuitry and/or the produced geometries, and/or in the extreme case is a geometric copy of the first measurement bridge.   29. The circuit according to any one of items 14 to 28, wherein respectively two of said four transistors are identically oriented while having the same geometry.   30. The circuit according to item 29, wherein the transistors of one transistor pair are oriented vertically to the transistors of the other transistor pair.   31. The circuit according to item 30, wherein said four transistors are arranged symmetrically in a quadrangle.   32. The circuit according to item 30, wherein said four transistors are arranged symmetrically in a cross configuration.   33. The circuit according to any one of items 14 to 23 or 29 to 32, comprising at least one differential amplifier circuit.   34. The circuit according to item 33, wherein at least one of the transistors of at least one differential amplifier is a transistor according to any one of items 1 to 13.   35. The circuit according to item 33 or 34, comprising at least one reference voltage source which is coupled to at least one first differential amplifier.   36. The circuit according to item 35, wherein said reference voltage source is a second, but short-circuited differential amplifier which is a differential amplifier according to any one of items 33 to 35.   37. The circuit according to item 36, wherein said second differential amplifier is similar to the first differential amplifier, particularly regarding the dimensioning of the transistors and/or the connection of the transistors and/or the produced geometries of the transistors, and/or in the extreme case is a geometric copy of the first differential amplifier.   38. The circuit according to any one of items 14 to 37, wherein at least a part of the circuit is also a part of the micromechanical device.   39. The circuit according to item 38, wherein at least a part of the circuit is functionally connected to at least one micromechanical functional element in such a manner that at least one mechanical parameter of at least one micromechanical functional element is coupled to the state function of the circuit or to at least one electrical parameter of the state function of at least one circuit portion.   40. The circuit according to item 39, wherein said functional element is particularly a bar or web, a membrane, a resonator, a lip clamped on one side, both sides or three sides, a shield or a needle.   41. A micromechanical device which has been produced by lithographic processes and by connecting, particularly bonding, at least two wafers, wherein
       I. prior to the connecting of said at least two wafers, at least one micromechanical functional element in the form of at least one surface structure has been applied on at least one surface of at least one of said two wafers, and   II. after the connecting of the wafers, at least one of the thus produced micromechanical functional elements and respectively surface structures is situated near the boundary surface between the wafers within the resultant wafer package, and   III. subsequent to the connecting of the wafers, there has been performed, on at least one surface of the resultant wafer package, at least one process for producing of at least one electronic component, and   IV. at least one of the thus produced electronic components is sensitive to at least one non-electric physical value and is provided to detect the same, and   V. said component is produced to be self-adjusting.   
       42. The micromechanical device according to item 41, wherein at least one of said self-adjusting components is a transistor according to any one of items 1 to 10 or is a part of the circuit according to any one of items 14 to 40.   43. The micromechanical device according to items 41 or 42, which is produced from silicon.   44. The micromechanical device according to any one of items 41 to 43, wherein said at least one micromechanical functional element is at least one cavity.   45. The micromechanical device according to item 44, wherein at least one cavity together with at least one surface of the wafer package defines a membrane.   46. The micromechanical device according to items 44 and 45, wherein at least one cavity comprises no oxides on its walls.   47. The micromechanical device according to any one of items 41 to 46, wherein at least one micromechanical functional element is situated on the surface of the device.   48. The micromechanical device according to item 47, wherein said at least one micromechanical functional element is a web, a trench, a membrane, a perforation and a buried cavity or a blind hole.   49. The micromechanical device according to item 38, wherein at least one micromechanical functional element has been produced on the surface after performing a process, particularly a CMOS process, for producing a transistor according to any one of items 1 to 13 or a circuit according to any one of items 14 to 40.   50. The micromechanical device according to any one of items 41 to 49, wherein at least one micromechanical functional element has been produced inter alia by use of DRIE or plasma etching processes.   51. The micromechanical device according to any one of items 41 to 50, which can be used as a pressure sensor.   52. The micromechanical device according to any one of items 44 to 51, wherein the geometric shape of at least one cavity comprises symmetries with respect to the connection plane of the wafers.   53. The micromechanical device according to any one of items 41 to 52, wherein, on at least one surface of the wafer package, the trenches have been generated by DRIE or plasma etching.   54. The micromechanical device according to item 53, wherein at least one partial quantity of the trenches form a closed structure, e.g. a ring, an ellipse, a quadrangle, a star or the like which only at few sites are interrupted by thin webs  8 ,  20 .   55. The micromechanical device according to items 53 and 54, wherein at least a part of the trenches are arranged symmetrically relative to each other.   56. The micromechanical device according to items 52 and 55, wherein the axes of symmetry of a part of the trenches and of at least one cavity coincide and respectively coincide in case of an ideal arrangement.   57. The micromechanical device according to items 52, 55 and 56, wherein at least a part of the trenches are in a mechanical functional relationship with at least one cavity.   58. The micromechanical device according to item 57, wherein the bottom of at least one of the trenches together with at least one of the cavities forms a thinner portion of the membrane or an opening leading into this cavity.   59. The micromechanical device according to any one of items 41 to 58, wherein micromechanical functional elements of the top side, particularly the trenches mentioned under any one of items 47 to 58, are not situated, with their edges defining their shape, above shape-defining edges of micromechanical structures of the bottom side and micromechanical structures of the top side.   60. The micromechanical device according to item 59, wherein the lever length  116  between the starting point of a structure  121  therebelow, particularly of a buried cavity  4 , and the starting point of a structure  119  thereabove, particularly of a trench  6 , is larger than the smaller one of the vertical lever dimensions  118  and  120  (see  FIG. 38 ).   61. The micromechanical device according to any one of items 44 to 60, wherein, within the body of the micromechanical device, particularly during production of the device within the wafer package, at least one cavity is arranged which is connected with the bottom side or top side of the wafer package by at least one micromechanical functional element, particularly a tube.   62. The micromechanical device according to item 61, which can be used as a pressure differential sensor against a defined referential pressure or ambient pressure.   63. The micromechanical device according to item 61 and 62, comprising at least one microfluidic functional element.   64. The micromechanical device according to item 63, wherein at least one microfluidic functional element serves or can serve for supply of media such as e.g. liquids and gases.   65. A micromechanical device, wherein at least one microfluidic functional element according to any one of items 63 to 64 or a micromechanical functional element according to item 61 has been produced after performing a process, particularly a CMOS process, for producing a transistor according to any one of items 1 to 13 or a circuit according to any one of items 14 to 40.   66. The micromechanical device according to any one of items 1 to 65, wherein, at least as a partial substrate or substrate, a p-doped semi-conductor material has been used.   67. The micromechanical device according to any one of items 1 to 65, wherein, at least as a partial substrate or substrate, an n-doped semi-conductor material has been used.   68. The micromechanical device according to any one of items 44 to 67, wherein, in at least one substrate, a modification of material, e.g. an SiO 2  layer, is provided which serves as an etching stopper for the etching of at least one cavity.   69. The micromechanical device according to any one of items 53 to 68, wherein, in at least one substrate, a modification of material  14  is provided which serves as an etching stopper for the etching of at least a part of the trenches.   70. The micromechanical device according to item 69, wherein, in at least one substrate, at least one modification of material  15  is provided which acts as a membrane in the region of the trenches.   71. The micromechanical device according to item 70, wherein at least one modification of material  15  is made of polysilicon and/or amorphous silicon and has been deposited on one of the wafers of the wafer package prior to the wafer bonding.   72. The micromechanical device according to any one of items 44 to 67 and 69 to 71, wherein at least one cavity has been etched into at least one substrate in a time-controlled manner.   73. The micromechanical device according to any one of items 53 to 68 and 70, wherein at least a part of the trenches have been etched into the substrate in a time-controlled manner.   74. The micromechanical device according to any one of items 53 to 73, wherein, prior to the etching of the trenches, a semiconductor process has been performed for producing electrical functional elements on at least one surface of the wafer package.   75. The micromechanical device according to item 74, comprising at least one electrical functional element which has been produced in the process according to item 74.   76. The micromechanical device according to item 75, wherein at least one electrical functional element has the function of an electrical line or a contact or a through-hole or an electric line insulator or a resistor or a transistor or a diode or a capacitor or a coil.   77. The micromechanical device according to item 76, wherein at least one of the functional elements is operative to change at least one parameter—particularly an electrical parameter—in dependence on mechanical values, particularly tensile stress, compressive stress and shear stress.   78. The micromechanical device according to item 77, wherein said change of parameter can be measured externally of the sensor.   79. The micromechanical device according to item 77 and 54, wherein at least one of the functional elements is in a mechanical functional relationship with at least one web  8 ,  20 .   80. The micromechanical device according to item 77 and 36, wherein at least one electronic functional element, relative to
       a) at least one first micromechanical functional element, particularly a membrane ( 12  or  21 ),   b) at least two further, second micromechanical functional elements, particularly trenches ( 6  or  19 ), and   c) at least one third micromechanical functional element, particularly a web ( 8  or  20 ),   said functional elements according to a) to c) being in a mechanical functional relationship,   is positioned on the third micromechanical functional element, particularly web, in such a manner that said at least one electronic functional element is situated on or near the point of highest mechanical stress when the first micromechanical functional element, particularly a membrane or an inertial mass ( 12  or  21 ) is deformed, particularly deflected.   
       81. The micromechanical device according to item 80, wherein at least one third micromechanical functional element, particularly a web, is shaped in such a manner that said element has a range of high homogenized mechanical stress in case of deformation of the first micromechanical functional element, particularly a membrane or an inertial mass.   82. The micromechanical device according to item 81, wherein at least one electronic functional element is arranged on at least one such site of high homogenized mechanical stress.   83. The micromechanical device according to any one of items 41 to 82, wherein at least two wafers have been formed with different thicknesses.   84. The micromechanical device according to any one of items 41 to 82, wherein a wafer material is a silicon or SOI material.   85. The micromechanical device according to any one of items 44 to 79, wherein the cavity is produced in the lowermost wafer prior to the bonding of three wafers.   86. The micromechanical device according to item 86, wherein said three wafers have been produced with different thicknesses.   87. The micromechanical device according to any one of items 53 to 86, wherein at least one of said second micromechanical functional elements is a trench ( 6  or  19 ) having a non-constant width.   88. The micromechanical device according to any one of items 54 to 87, wherein at least one web does not divide a trench ( 6  or  19 ) but only extends into it (see e.g.  FIG. 25 ).   89. The micromechanical device according to any one of items 54 to 88, wherein, between the webs and trenches, a surface is formed on a membrane which, being suspended on the webs, is quadratic (see e.g.  FIG. 20 or 23 ), rhombic (see e.g.  FIG. 21  or  FIG. 22 ) or round (e.g.  FIG. 24 ).   90. The micromechanical device according to item 89, wherein at least one trench has no bottom and thus is connected to at least one cavity.   91. The micromechanical device according to any one of items 41 to 90, which can be used as a pressure sensor and/or acceleration sensor.   92. The micromechanical device according to any one of items 41 to 91, comprising symmetrically arranged mechanical first functional elements, particularly webs, which are connected to at least one further, second micromechanical functional element, particularly a membrane or inertial mass, and on which respectively mutually similar circuit portions of a circuit according to any one of items 14 to 40 are arranged.   93. The micromechanical device and circuit according to item 92, wherein the circuit portions arranged on the first micromechanical functional elements are electrically connected to each other in such a manner that average values and/or differences will be formed.   94. The micromechanical device according to any one of items 41 to 93, which at least at a first position comprises a first mechanical functional element, particularly a web that is mechanically connected to at least one further, second micromechanical functional element, particularly a membrane, and has a second position being without a mechanical function and being subjected to no or only slight mechanical influence, and wherein, at least at said two positions, respectively mutually similar circuit portions of a circuit according to any one of items 14 to 40 are arranged.   95. The micromechanical device and circuit according to item 94, wherein the circuit portions arranged at said two positions are electrically connected to each other in such a manner that average values and/or differences will be formed.   96. The micromechanical device and circuit according to any one of items 92 to 95, wherein the micromechanical device consists of at least two complete micromechanical partial devices, particularly two pressure sensors according to any one of items 92 to 95 which again are in a functional relationship.   97. The micromechanical device and circuit according to item 96, wherein, within the circuit, mathematical operations, particularly the formation of average values and differences, are performed on electrical output values of said partial devices.   98. The micromechanical device and circuit according to any one of items 94 to 97, wherein at least one second circuit portion being similar to a first circuit portion at said first position, particularly on a web, is used as a reference, particularly voltage reference, and is not in a functional relationship with a micromechanical functional element.   99. The micromechanical device and circuit according to any one of items 92 to 98, wherein, to each circuit portion at a first position, at least one circuit portion similar to the circuit portion on the respective web, is assigned as a reference, and wherein said reference is not in a functional relationship with a micromechanical functional element.   100. The micromechanical device and circuit according to item 99, wherein said reference is situated on the neutral fiber.   101. The micromechanical device and circuit particularly according to any one of items 92 to 100, wherein a part thereof is formed by at least one amplifier circuit.   102. The micromechanical device and circuit particularly according item 101, wherein said amplifier circuit has a positive and a negative input.   103. The micromechanical device and circuit according to any one of items 1 to 101, which in wide regions is provided with a protection against humidity and/or in- and out-diffusion of protons.   104. The micromechanical device and circuit according to item 103, wherein said diffusion protection consists of a silicon-nitride layer.   

     Further advantages of the invention are:
     1. Reduction of the number of required wafer bond connections   2. Reduction of parasitic elements
       a) Elimination of sources of mechanical stress   b) Protection against dissipation of unavoidable mechanical stress   c) Maximization, homogenization and linearization of mechanical useful stress fields   d) Reduction of the dissipation of electronic components   e) Reduction of the dissipation of electronic circuits   f) Reduction of the dissipation of micromechanical functional elements   
       3. Increasing the tolerance of the construction towards mechanical and electrical manufacturing variations   4. Reduction of the effects of unavoidable parasitic elements   5. Reduction of the influence of the design and connection technology   6. Flexibilization of the use of sensors on the side of the user   7. Reduction of the required die area   8. Possibility of coupling to high-volume standard CMOS lines, particularly such lines with p-doped substrates   

     These properties are realized particularly by the measures described hereunder, which can be applied individually or in total or partial combination:
     1. Reduction of the number of required wafer bond connections by
       a) Producing cavities prior to the CMOS processing   
       2. Reduction of parasitic elements by
       a) Elimination of sources of mechanical stress, particularly by
           i) Avoidance of unnecessary layers on the micromechanical functional elements, particularly on pressure sensor membranes   
           b) Protection against dissipation of unavoidable mechanical stress, particularly by
           i) Containment of the stress by mechanical guard rings, and   ii) Reduction of the depth of cavities in the material, wherein the latter has a higher areal moment of inertia   
           c) Maximization, homogenization and linearization of useful stress fields, particularly by
           i) Etching trenches into pressure membranes   ii) Selection of the trench shape   iii) Distance between rear-side structures and buried structures on the one hand and front-side structures on the other hand for reduction of adjustment errors   
           d) Reduction of the dissipation of electronic components by
           i) Use of self-adjusting structures   
           e) Reduction of the dissipation of electronic circuits by
           i) Use of a compact, symmetrical, self-adjusting special transistor   ii) Use of a compact, symmetrical, self-adjusting differential amplifier stage   iii) Use of a compact, self-adjusting, symmetrical active Wheatstone bridge   
           f) Reduction of the dissipation of micromechanical functional elements by
           i) Use of defined, CMOS-compatible etching stops   ii) Use of particularly miniaturized special transistors   
           
       3. Increasing the tolerance of the construction towards mechanical and electrical manufacturing variations
       a) Differentiation of the stress direction   b) Differentiation between stressed and unstressed circuit portions   c) Differentiation between circuit portions at different symmetry positions   d) Suitable compensatory connection of circuit portions which can detect the differences i to iii by measuring   e) Use of particularly miniaturized special transistors   f) Miniaturization of the mechanical design by well-aimed reduction of the layer stack in the region of micromechanical functional elements   
       4. Reduction of the effects of unavoidable parasitic elements
       a) Compensatory circuits   b) Use of particularly miniaturized special transistors   
       5. Reduction of the influence of the design and connection technology by
       a) Reduction of the depth of cavities in the material, whereby the material has a higher areal moment of inertia   b) Use of round cavities, whereby the vertical areal moment of inertia is enlarged   
       6. Flexibilization of the use of sensors on the side of the user by
       a) Adjustability of the amplification by the user   
       7. Reduction of the required die area
       a) Reduction of the depth of cavities in the material, whereby the material has a higher areal moment of inertia and the sensor can be reduced in size without loss of stability   b) Use of particularly miniaturized special transistors   c) Forming a minimal access opening for gases and liquids leading to a buried cavity   
       8. Possibility of coupling to high-volume standard CMOS lines, particularly such lines with p-doped substrates
       a) Forming the cavities with defined etching stopper prior to the CMOS process   b) Production of micromechanical functional elements on the surface, such as trenches, after the CMOS processing, by means of plasma or DRIE etching   c) Forming minimal access openings leading to buried cavities, after the CMOS processing.   
       

     LIST OF REFERENCE NUMERALS 
     
         
           1 . First wafer 
           2 . Oxide layer 
           3 . Straight wall of cavity  4   
           4  Cavity  4   
           5  Second wafer 
           6  Trenches in the wafer package 
           7  Thin membrane regions defined by trenches  6  and cavity  4   
           8  Webs interrupting the trenches  6   
           8   a  Bending element 
           9  Components for detecting the mechanical stress 
           10  Terminals with terminal lines 
           11  Surface of the wafer package 
           12  Central plate of the membrane 
           13  First wafer 
           14  SiO 2  layer 
           15  Polysilicon layer 
           16  Second layer 
           17  Second oxide layer 
           18  Cavity 
           19  Trenches 
           20  Webs interrupting the trenches 
           21  Central plate of the membrane 
           22  Components for detecting the mechanical stress 
           23  Terminals with terminal lines 
           24  Surface of the wafer package 
           25  Membrane of small thickness 
           26  Negative terminal of the Wheatstone bridge 
           27  Positive terminal of the Wheatstone bridge 
           28  First clamp for capturing the voltage on the Wheatstone bridge 
           29  Lower p-channel MOS diode of the reference voltage source for the Wheatstone bridge 
           30  Upper p-channel MOS diode of the reference voltage source for the Wheatstone bridge 
           31  First p-channel MOS transistor of the Wheatstone bridge 
           32  Second p-channel MOS transistor of the Wheatstone bridge 
           33  Third p-channel MOS transistor of the Wheatstone bridge 
           34  Fourth p-channel MOS transistor of the Wheatstone bridge 
           35  Reference voltage line 
           36  Second clamp for capturing the voltage on the Wheatstone bridge 
           37  n+-channel stopper implant 
           38  Gate-connector transistor  32  and  34  in low-impedance polysilicon 
           39  Gate-connector transistor  33  and  31  in low-impedance polysilicon 
           40  n−-doped surface (non-conducting) 
           41  Upper structure sensitive to mechanical stress, e.g. a Wheatstone bridge according to  FIG. 12   
           42  Right-hand structure sensitive to mechanical stress, e.g. a Wheatstone bridge according to  FIG. 12   
           43  Lower structure sensitive to mechanical stress, e.g. a Wheatstone bridge according to  FIG. 12   
           44  First differential amplifier p-channel transistor 
           45  Second differential amplifier p-channel transistor 
           46  Third differential amplifier p-channel transistor 
           47  Fourth differential amplifier p-channel transistor 
           48  Reference voltage for transistors  44 ,  45 ,  46 ,  47   
           49  Current-source feed line for transistors  44 ,  45 ,  46 ,  47   
           50  Common drain contact of the p-channel transistors  44 ,  45 ,  46 ,  47   
           51  Connector transistor  46 , negative output knot of the differential amplifier 
           52  Connector transistor  45 , positive output knot of the differential amplifier 
           53  Connector transistor  44 , negative output knot of the differential amplifier 
           54  Connector transistor  47 , positive output knot of the differential amplifier 
           55  Third p-channel transistor for reference bridge circuit 
           56  Fourth p-channel transistor for reference bridge circuit 
           257  Upper structure sensitive to mechanical stress, e.g. a Wheatstone bridge according to  FIG. 12  as a reference structure for  41  in the region free of mechanical stress 
           58  Right-hand structure sensitive to mechanical stress, e.g. a Wheatstone bridge according to  FIG. 12  as a reference structure for  42  in the region free of mechanical stress 
           59  Lower structure sensitive to mechanical stress, e.g. a Wheatstone bridge according to  FIG. 12  as a reference structure for  43  in the region free of mechanical stress 
           60  Left-hand structure sensitive to mechanical stress, e.g. a Wheatstone bridge according to  FIG. 12  as a reference structure for  44  in the region free of mechanical stress 
           61  Differential amplifier: current mirror transistor corresponding to transistor  69   
           62  Differential amplifier: current mirror transistor corresponding to transistor  70   
           63  Differential amplifier: current mirror transistor corresponding to transistor  71   
           64  Differential amplifier: current mirror transistor corresponding to transistor  72   
           65  Reference amplifier: first differential amplifier p-channel transistor 
           66  Reference amplifier: second differential amplifier p-channel transistor 
           67  Reference amplifier: third differential amplifier p-channel transistor 
           68  Reference amplifier: fourth differential amplifier p-channel transistor 
           69  Reference amplifier: current mirror transistor corresponding to transistor  61   
           70  Reference amplifier: current mirror transistor corresponding to transistor  62   
           71  Reference amplifier: current mirror transistor corresponding to transistor  63   
           72  Reference amplifier: current mirror transistor corresponding to transistor  64   
           73  Reference amplifier: n-channel current source transistor (current mirror) 
           74  Reference amplifier: n-channel current source transistor (current mirror) 
           75  Negative terminal 
           76  Positive terminal 
           77  Negative output signal 
           78  Positive output signal 
           78   a  Active region pan 
           79  p+-contact implant (source region) 
           79   a  Metal line 
           80  p+-contact implant (drain region) 
           80   a  Metal line 
           81  Poly gate of a self-adjusting p-channel transistor 
           81   a  Gate oxide 
           82  n+-implant region (channel stop) 
           83  n+-implant region (channel stop) 
           84  Feed line of highly doped polysilicon 
           85  First p-channel MOS transistor of the Wheatstone bridge 
           86  Second p-channel MOS transistor of the Wheatstone bridge 
           87  Third p-channel MOS transistor of the Wheatstone bridge 
           88  Fourth p-channel MOS transistor of the Wheatstone bridge 
           89  Left-hand tap 
           90  Right-hand tap 
           91  Negative pole 
           92  Positive pole 
           93  Second group of trenches for decoupling the membrane from the die body 
           94  Webs interrupting the group of second trenches  93   
           95  Third group of trenches for further decoupling the membrane from the die body 
           96  Webs interrupting the third group of second trenches  95   
           97  Boss with grid structure (support structure) 
           98  Bore in the cavity for differential pressure sensors 
           99  Mechanical guard ring for prevention of dissipation of the mechanical stress introduced by the bond system 
           100  Left-hand structure sensitive to mechanical stress, e.g. a Wheatstone bridge according to  FIG. 12   
           101  One-transistor element 
           102  Negative terminal 
           103  Positive terminal 
           104  Upper transistor left-hand side (p-channel) 
           105  Upper transistor right-hand side (p-channel) 
           106  Lower transistor left-hand side (p-channel) 
           107  Lower transistor right-hand side (p-channel) 
           108  Upper reference transistor (p-channel) 
           109  Lower reference transistor (p-channel) 
           110  Internal reference voltage 
           111  First output 
           112  Second output 
           113  Parasitic first transistor 
           114  Parasitic second transistor 
           115  Entire transistor field plate 
           116  Lever length (here, the example of the cavity wall  3  to the trench wall) 
           117  Example: trench wall 
           118  Height of upper structure (here, by way of example, depth of trench  6 ) 
           119  Receptor point of the upper structure (here, by way of example, depth of trench  6 ) 
           120  Height of lower structure (here, by way of example, depth of cavity  4 ) 
           121  Receptor point of the lower structure (here, by way of example, cavity  4 )