Abstract:
A measuring element for recording a deflection includes a region which is situated on a semi-conductor substrate and an electrode for influencing a conductivity of the region, the electrode being mounted deflectably in relation to the region, in such a way that an overlap region is formed between the electrode and the region, the overlap region having a dimension that is variable with a deflection of the electrode. A change in the output signal of the measuring element is a function of the conductivity of the region and is controllable by a change in the dimension of the overlap region, the change in the dimension of the overlap region having a non-linear relationship with the deflection of the electrode so that a change in the output signal of the measuring element has a non-linear relationship with the deflection of the electrode.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a measuring element. In particular, the present invention relates to a micromechanical measuring element having an electrical output signal. 
     BACKGROUND INFORMATION 
     Micromechanical measuring elements are used in sensors for different areas of application. For example, acceleration sensors, yaw rate sensors, or pressure sensors may be micromechanically constructed. Such a measuring element includes both mechanical and electrical structures, which may have similar orders of magnitude and form an integrated micro-electromechanical system (MEMS) with each other. Such micro-electromechanical systems also occasionally include an actuator for parts of the mechanical structures, for example in yaw rate sensors in which a force acting upon a movable mass is to be determined. A micro-electromechanical resonator may also include such a drive. 
     In one form of micro-electromechanical measuring elements, a relative deflection between mechanical elements is analyzed, at least one of the mechanical elements at the same time also being part of an electronic element, which provides an output signal correlating with the mechanical deflection. Such a deflection may have the order of magnitude of the structures from which the measuring element is made. The deflection may be determined, for example, by determining a deflection-dependent capacitance. 
     For example, in the “moving gate” technique, a flat gate electrode of a field-effect transistor (FET) may be mounted parallel to a channel of the FET and deflected with respect to the channel. The channel is delimited by a source and a drain terminal of the field-effect transistor. A voltage is applied between the source and the drain terminals, and the gate electrode is electrically connected to the drain terminal, so that the field-effect transistor is operated as a current controller. A current flow through the field-effect transistor then changes as a function of the electric field, which is established due to the voltage between the gate electrode and the channel. If the configuration of gate electrode and channel is modified, the current flow through the field-effect transistor is ultimately also modified. Capacitive measurement is suitable for MEMS measuring sensors, since in this case there is virtually no reaction from the (deflection-dependent) current flow on the deflection of the electrode. The gate electrode and the channel are usually shaped in such a way that a region in which the two overlap is linearly changeable to a deflection of the gate electrode. The deflection modifies the size of the overlapping region, so that the electric field between gate electrode and channels, which controls the current flow through the field-effect transistor, is modified. The deflection of the gate electrode may thus be determined with the aid of the current flow through the field-effect transistor. 
     U.S. Pat. No. 6,220,096 B1 discusses a micro-electromechanical acceleration sensor which uses moving gate field-effect transistors (MG-FET) of the type described above in a differential circuitry for optimizing a useful signal of the sensor. 
     Measuring systems that process a sensor signal should usually provide an output signal which may have a linear relationship with a quantity to be measured. For this reason, usually all elements of the system are laid out to have linear characteristic curves, so that the required linear relationship exists over the entire system. In particular in the case of systems including a large number of complex processing elements, it is, however, difficult to ensure the linearity of each individual element in the required quality. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the exemplary embodiments and/or exemplary methods of the present invention is to provide a MEMS sensor with the aid of which a detection and analysis of a deflection may be improved. 
     This object may be achieved via the measuring element according to the description herein. The further descriptions herein specify measuring devices which are based on such a measuring element. Another description herein defines a resonator which is based on one of these measuring devices. Subclaims specify further embodiments and features. 
     The exemplary embodiments and/or exemplary methods of the present invention are directed to a measuring element based on a field-effect transistor. The measuring element includes a region situated on a semiconductor substrate (for example a channel of a field-effect transistor) and an electrode, which may be used to influence the conductivity of the region. The overlap region between the gate electrode and the region has a width (parallel to the direction of deflection) and a length (perpendicular to the direction of deflection). A dimension (area) of the overlap region is a function of the deflection and of the shapes of the overlapping elements. It is proposed that the gate electrode and/or the region be shaped in such a way that the dimension of the overlapping region has a non-linear relationship with the deflection of the gate electrode. This may be effected by the gate electrode and/or the region having a shape different from a rectangle. 
     In one specific embodiment, the gate electrode may have a conventional rectangular shape and have edges that are parallel and perpendicular to the direction of deflection, while the region has a shape different from a rectangle. In the case of a rectangular gate electrode, if the output signal should run symmetrically to a value corresponding to a 0 deflection, the region is to be shaped in the positive direction of deflection and in the negative direction of deflection with a mirror image. The shape of the region is thus symmetrical to an axis of symmetry running perpendicularly to the direction of deflection and through a deflection of 0. In alternative specific embodiments both the gate electrode and the region may assume other shapes; in particular the shapes of the region and of the gate electrode may be reversed or both the region and the gate electrode may have a shape different from a rectangle. The deviations from a conventional shape of a parallelogram or a rectangle may be greater than those due to manufacturing technological processes and, in particular, greater than approximately 3% to 5%. 
     In one specific embodiment, the gate electrode is rectangular and the region has a shape corresponding to a circle, from which an upper and a lower region have been removed along two separating lines running perpendicularly to the direction of movement of the gate electrode. This specific embodiment is particularly well suited for being used in a micro-electromechanical resonator. 
     The exemplary embodiments and/or exemplary methods of the present invention are now described in greater detail with reference to the appended drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1   a  shows a view of a measuring element on the basis of an MG-FET. 
         FIG. 1   b  shows a different view of a measuring element on the basis of an MG-FET. 
         FIG. 2  shows a measuring device on the basis of the measuring element from  FIG. 1 . 
         FIG. 3  shows a diagram  300 , illustrating the effect of the geometry of region  120  from  FIG. 1  in the measuring device from  FIG. 2 . 
         FIG. 4  shows a resonator on the basis of the measuring element from  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Identical or corresponding elements are identified in all figures using the same reference numerals. 
       FIGS. 1   a  and  1   b  show two different views of a schematic configuration of a measuring element  100  according to the principle of an MG-FET. On the right side of  FIGS. 1   a  and  1   b , coordinate systems (x, y, z) are shown for easier reference. Covered edges in  FIG. 1   b  are illustrated by dashed lines. 
     A measuring element  100  includes a semiconductor substrate  110 , on which a source terminal S and a drain terminal D are mounted, between which a region  120  extends. Measuring element  100  also includes a gate electrode G mounted above (in the positive z direction) region  120 . Gate electrode G is situated so it is deflectable in the positive and negative y directions. A distance between gate electrode G and region  120  remains constant; the deflection occurs only along the y axis. 
     Not illustrated is an elastic system, which holds the gate electrode deflectably at a predefined distance over region  120  and provides restoring forces, which are variable with the deflection of gate electrode G in the positive and negative y directions. The elastic connection may include, for example, micromechanical springs. An undeflected position of the gate electrode is at a point of equilibrium of the restoring forces of the elastic connection in the positive and negative y directions.  FIG. 1   b  shows the gate electrode in this undeflected position. 
     Also not illustrated is an electrical connection of gate electrode G to drain terminal D, which may include the elastic system, for example. Due to the electrical connection, measuring element  100  is operated in a current-controlled manner. 
     If a voltage is applied between gate electrode G and source terminal S, an electric field in region  120  induced thereby allows charge carriers to move between source terminal S and drain terminal D. If the voltage between gate electrode G and source terminal S and the vertical distance between gate electrode G and region  120  are constant, a mobility of charge carriers in the area of region  120  is a function only of a dimension of an overlap region  130  between gate electrode G and region  120 . An output signal of the illustrated system is basically determined according to I DS ˜W/L. 
     Gate electrode G may be deflected in the positive or negative y direction from the undeflected position shown in  FIG. 1   b . In the present illustration, this deflection may continue until region  120  fully overlaps with gate electrode G or until the overlap reaches the value 0; other limits are also implementable. It is apparent that there is a non-linear relationship between a deflection of gate electrode G with respect to region  120  and the dimension of overlap region  130 . Overlap region  130  is determined by a length L running in the x direction and a width W running in the y direction. Length L is variable along the y direction and is defined as a function of width W at the particular y point. In other words, the course of length L is specified as non-linear over the course of width W. 
     The dimension of overlap region  130  results from its width W, which is proportional to the deflection of gate electrode G, and length L in its course over overlap region  130  ( FIG. 1   b ). The relationship between the deflection of gate electrode G and the dimension of overlap region  130  is non-linear, since the course of L in the y direction is a non-linear function of W. Therefore, the course of the dimension of overlap region  130  over a deflection of gate electrode G is non-linear and therefore the relationship between the deflection and the current flow through measuring element  100  is also non-linear. 
     In the present example (see  FIG. 1   b ), the shapes of source terminal S and of drain terminal D correspond to a circular or elliptical segment. Region  120  therefore has an overall surface that approximately corresponds to a longitudinal section of a barrel situated on the zx plane. In other words, the shape of region  120  results from a circle or an ellipse in the xy plane, of which an upper and a lower (with respect to y) end have been separated, the separation lines running parallel to the x direction and at the same distance from the y axis at the point y=0. The course of length L of region  120  in the y direction is described as
 
 L ( y )=√{square root over ( L   0   2   −a   2   y   2 )},
 
L 0  being defined as the length at the point y=0, and a being the ratio between the semi-diameters of the ellipse (1 in the case of a circle). To prevent contact between source terminal S and drain terminal D and to avoid the effects of a very short region (short channel effects), length L of region  120  should not fall below a critical value L min . This results in the elliptical or circular region ending at
 
                  y        =       1   a     ⁢           L   0   2     -     L   min   2         .             
The absolute value of the deflection of the gate electrode from its rest position y=0 should also be less than or equal to
 
                 1   a     ⁢         L   0   2     -     L   min   2           ,         
so that the overlap region is greater than or equal to zero and its dimension remains a function of the motion. In the present example, the above-described shape of an ellipse or a circle capped on both sides results from this requirement for region  120 .
 
     In other specific embodiments the gate electrode may also assume other shapes. A limitation, displacement, or increase of the deflection of gate electrode G relative to overlap region  130  is also possible, for example. Furthermore, gate electrode G may be perforated, for example. Measuring element  100  may also have another region (not illustrated), which controls the conductivity of another system and is also swept over by gate electrode G. The other region may be situated in the positive or negative x direction with respect to  FIGS. 1   a  and  1   b . Source terminals S and drain terminals D of both channels may be connected in series or in a bridge circuit, for example. These and similar measures known to those skilled in the art may be used, for example for improving a measuring accuracy, increasing a resolution, and/or for implementing a predefined relationship between the output signal of measuring element  100  and the deflection of gate electrode G. 
     Gate electrode G may also be part of a more complex micro-electromechanical system having multiple movable elements. Gate electrode G may be fastened to one or more of these elements and/or to semiconductor substrate  110  with the aid of an elastic system, and the movable elements may include springs. 
       FIG. 2  shows a measuring device  200 , which is based on measuring element  100  from  FIG. 1 . Measuring device  200  includes measuring element  100 , another measuring element  210 , and, optionally, a device  220  for signal processing, which includes an operational amplifier  225  and a feedback resistor  230 . 
     Measuring element  210  has a basically identical design to measuring element  100 , and, in particular, has identical dimensions, but unlike measuring element  100 , it has no movable parts. A gate electrode of measuring element  210  is fixed in a position which, on measuring device  100 , corresponds to an undeflected position of its gate electrode G. In both measuring elements  100  and  210 , the gate electrodes are connected to the particular drain terminals D, so that both measuring elements  100  and  210  are operated in a current-controlled manner. Measuring elements  100  and  210  are connected in series and to two terminals VCC and GND of a DC voltage source (not illustrated). Measuring elements  100  and  210  are bipolar; therefore a polarity of the DC voltage is unimportant. An output signal, which has a non-linear relationship with the deflection of gate electrode G in measuring element  100 , may be picked up at a tap A between the two measuring elements  100  and  210 . 
     Constant errors, caused, for example, by manufacturing inaccuracies of measuring elements  100  and  210 , may be compensated for by combining measuring elements  100  and  210  as indicated. Device  220  for signal processing generates, in a manner known to those skilled in the art, from the signal at tap A via an operational amplifier  225  back-coupled with the aid of feedback resistor  230 , a voltage signal that is variable with the deflection of gate electrode G of measuring element  100 . 
       FIG. 3  shows a diagram  300 , which illustrates a relationship between a harmonic deflection  310  (dashed line) of gate electrode G of measuring element  100  from  FIGS. 1   a ,  1   b , and an output signal  320  (solid line) of device  220  for signal processing from  FIG. 2 . Time is plotted in the horizontal direction, a percentage deflection (re  310 ) and a voltage (re  320 ) being plotted in the vertical direction. 
     It is apparent that curve  310  of a sinusoidal, harmonic deflection of gate electrode G induces an output voltage  320 , which corresponds to a symmetric saw tooth or a trapeze. This relationship results from the special shape of region  120  as a capped circle or capped ellipse, as shown in  FIG. 1   b  and described in greater detail above. 
     Sinusoidal, harmonic deflection  310  may be transmitted from a micromechanical actuator to gate electrode G. For example, a micro-electromechanical resonator may be constructed with the aid of measuring element  100  from  FIG. 1  and a capacitive, piezoelectric, thermoelastic, or magnetic drive. According to its application, the resonator may be more or less damped, for example by gas included in a space in which gate electrode G is deflectably situated. 
     The actuator and the elements illustrated in  FIG. 2  may form a micro-electromechanical signal generator, which may be used for different purposes. Using such a signal generator, a series of periodic signals of various shapes may be generated by varying the shapes of gate electrode G and/or of region  120 . 
     Measuring element  100  may be used for different applications in micro-electromechanical and opto-microelectromechanical components. In particular it may form a measuring device  200  together with other measuring elements  100 ,  210 , and/or a device  220  for signal processing. A non-linearity of device  220  for signal processing (and/or of another device for signal processing) may thus be compensated for via the non-linearity of the relationship between the deflection of gate electrode G of measuring element  100  and the current flow through measuring element  100 , so that overall, a linear relationship exists between the deflection of gate electrode G with respect to region  120  and the output signal of the device for signal processing. 
     Measuring element  100 , i.e., measuring device  200 , may form a system or a module together with other mechanical, optical, and/or electronic components. For example, measuring element  100  may be part of an inertial sensor, a yaw rate sensor, or a micro-mirror. In the latter case, it would be an opto-mechanical micro-electromechanical system (Micro-Opto-Mechanical System, MOEMS). 
       FIG. 4  shows a resonator  400  in an illustration corresponding to  FIGS. 1   a  and  1   b . On the right side of  FIG. 4 , a coordinate system is shown. Resonator  400  includes a substrate  110  having a plurality of source terminals S and drain terminals D, which, in pairs, delimit channels  120 , a shared electrode G, a processing device  220 , springs  410 , a drive  420 , a damper  430 , and an output  440 . 
     Processing device  220  is connected to source terminals S and drain terminals D and determines, according to the explanations above, an output signal provided at output  440 . Electrode G is held over channels  120  with the aid of springs  410  in the z direction and may be deflected by drive  420  in the y direction. Electrode G has recesses, so that channels  120  overlap more or less therewith as a function of the deflection of electrode G. Due to the overlap of electrode G with multiple channels  120 , an increased sensitivity of the deflection measurement may be achieved in the illustrated resonator  400 . 
     At least one of springs  410  is simultaneously used as an electrical contact on an armature element (not shown) for resonator  400 , which may carry, for example, substrate  110  or may coincide therewith. The deflection of electrode G is damped with the aid of damper  430 . This may be a friction damper or an eddy current damper, or electrode G may be enclosed in a container containing a gas under a predefined pressure. The latter variant is suitable in particular for weak damping for operating resonator  400  at a high quality. Damping may also be accomplished by fluidic effects on the remaining movable structure and by material damping (anchor loss). Damper  430  is omitted, if necessary. 
     Electrode G may be part of a micro-mirror or connected to one. In a micro-mirror system, an actual deflection of electrode G may be determined, for example via the signal provided at output  440 , and drive  420  may be activated accordingly to achieve a predefined deflection of electrode G. In another specific embodiment, electrode G may be part of an inertial system. The deflection of electrode G may be determined in absolute terms or as a function of the deflection of electrode G induced by drive  420 , which allows conclusions to be drawn regarding a motion of the inertial system.