Patent Abstract:
Methods of forming micro-electromechanical device include a semiconductor substrate, in which a first microstructure and a second microstructure of reference are integrated. The first microstructure and the second microstructure are arranged in the substrate so as to undergo equal strains as a result of thermal expansions of the substrate. Furthermore, the first microstructure is provided with movable parts and fixed parts with respect to the substrate, while the second microstructure has a shape that is substantially symmetrical to the first microstructure but is fixed with respect to the substrate. By subtracting the changes in electrical characteristics of the second microstructure from those of the first, variations in electrical characteristics of the first microstructure caused by changes in thermal expansion or contraction can be compensated for.

Full Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a temperature-compensated micro-electromechanical device and to a method of temperature compensation in a micro-electromechanical device. 
     Description of the Related Art 
     As is known, the use of micro-electromechanical systems (MEMS) is increasingly widespread in several sectors of technology and has yielded encouraging results especially in the construction of inertial sensors, micro-integrated gyroscopes, and electromechanical oscillators for a wide range of applications. 
     MEMS systems of this type usually comprise at least one mass, which is connected to a fixed body (stator) by means of springs and is movable with respect to the stator according to pre-determined degrees of freedom. The movable mass and the stator are capacitively coupled by a plurality of respective comb-fingered electrodes facing one another so as to form capacitors. The movement of the movable mass with respect to the stator, for example on account of an external stress, modifies the capacitance of the capacitors; from which it is possible to deduce the relative displacement of the movable mass with respect to the fixed body, and hence the degree of force applied to cause the movement. On the other hand, it is also possible to apply an electrostatic force to the movable mass to set it in motion, by supplying appropriate biasing voltages. 
     In optimal working conditions, MEMS systems present excellent performance; in particular, MEMS inertial sensors are extremely sensitive and precise. However, a limit of currently available MEMS systems lies in the strong dependence of their response upon the temperature. In fact, also on account of their extremely small dimensions, very modest variations in temperature can produce significant strains in micro-electromechanical structures. Such strains are equivalent to relative displacements of the electrodes of the movable mass with respect to those of the stator and cause a detectable variation of the capacitive coupling between stator and movable mass. In practice, then, an offset, due to the variations in temperature, is added to the output signal of the MEMS system. 
     To overcome this drawback, MEMS systems are frequently incorporated in special packages, made so as to reduce the effects of thermal expansion. Alternatively, it has been proposed to use compensation circuits that electrically erase the effects of possible thermal drifts. According to one solution, for example, a nonlinear element with a temperature dependent electrical characteristic is integrated in the reading interface of the MEMS system (a diode, for example). Another technique envisages, instead, the use of a temperature sensor. 
     The solutions illustrated above are, however, not really satisfactory both because in any case the achievable precision is not optimal and because high costs are involved. The special packages, in fact, cannot be of a standard type and hence have very high design and fabrication costs. The compensation circuits require burdensome procedures for measuring the thermal drifts and calibrating the compensation curves and, moreover, a sufficient stability over time cannot be guaranteed. 
     BRIEF SUMMARY OF THE INVENTION 
     Embodiments of the present invention provide a temperature-compensated micro-electromechanical device and a method of temperature compensation in a micro-electromechanical device. 
     According to one embodiment of the invention, a micro-electromechanical device is provided, comprising a semiconductor substrate, a first microstructure integrated in the substrate, and a second microstructure integrated in the substrate as a reference, and arranged so that the first microstructure and the second microstructure undergo equal strains as a result of thermal expansions of the substrate. The first microstructure comprises movable parts and fixed parts with respect to the substrate, while the second microstructure has a shape substantially symmetrical to the first microstructure, but is fixed in position with respect to the substrate. 
     The first and second microstructures are specularly symmetrical with respect to a symmetry axis of the substrate. That is to say that the second microstructure is a mirror image of the first microstructure, and they are symmetrically positioned with respect to a center of mass of the semiconductor substrate. According to an embodiment of the invention, the first microstructure includes a capacitive coupling between movable electrodes and fixed electrodes, while the second microstructure includes a capacitive coupling between electrodes in positions corresponding to the movable and fixed electrodes of the first microstructure. Because of their substantially identical configuration, the capacitive couplings of the first and second microstructures are substantially identical in their response to changes in temperature, though only the first microstructure is movable with respect to the substrate. 
     According to an embodiment of the invention, a method is provided in which changes in the capacitive coupling of the first microstructure are employed to detect acceleration of the semiconductor substrate, while changes in the capacitive coupling of the second microstructure are employed to compensate for changes in the capacitive coupling of the first microstructure caused by thermal effects. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
       For a better understanding of the invention, some embodiments thereof are now described, purely by way of non-limiting example and with reference to the attached drawings. 
         FIG. 1  is a simplified block diagram of a micro-electromechanical device according to a first embodiment of the present invention. 
         FIG. 2  is a schematic top plan view of a part of a semiconductor chip included in the device of  FIG. 1 . 
         FIGS. 3-5  are cross sections through the chip of  FIG. 2 , taken, respectively, along the lines III-III, IV-IV and V-V of  FIG. 2 . 
         FIG. 6  is a simplified block diagram of a micro-electromechanical device according to a second embodiment of the present invention. 
         FIG. 7  is a simplified block diagram of a micro-electromechanical device according to a third embodiment of the present invention. 
         FIG. 8  is a schematic top plan view of a part of a semiconductor chip included in the device of  FIG. 7 . 
         FIG. 9  is a simplified block diagram of a micro-electromechanical device according to a fourth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates a linear MEMS accelerometer  1  having a detection axis X 1 . The MEMS accelerometer  1  comprises a detection microstructure (DETECT X)  2  and a reference microstructure (COMPENS X)  3  that are substantially identical to one another and are both integrated in a same chip  5  of semiconductor material, together with a control unit (UC)  4 . More precisely, the chip  5  has a rectangular or square shape and has a geometrical center  5   a , which is also a center of symmetry. The detection microstructure  2  and the reference microstructure  3  are specular with respect to a symmetry axis S 1  which is perpendicular to the detection axis X 1  and passes through the center  5   a  of the chip  5 . The detection microstructure  2  and the reference microstructure  3  are moreover connected to the control unit for supplying a measurement signal S x , correlated to the accelerations imparted to the chip  5  according to the detection axis X 1 , and, respectively, a compensation signal S COMPX  indicating the expansion to which the chip  5  is subjected as a result of the temperature. 
     With reference to  FIGS. 2-5 , the detection microstructure  2  comprises a suspended mass  6  and a stator structure  7 , which is fixed to a substrate  8  of the chip  5  and separated from the suspended mass  6  in a conventional way by means of insulating regions (not illustrated). A trench  9 , in this case filled with dielectric material, separates the stator structure  7  laterally from the substrate  8 . The suspended mass  6  is mechanically connected to the stator structure  7  by a plurality of elastic connection elements  10  (four, in the example described), so shaped as to enable the suspended mass  6  to oscillate with respect to the stator structure  7  in the direction of the detection axis X 1 . The elastic connection elements  10  ( FIG. 3 ) extend from the suspended mass  6  and are fixed to respective suspension anchorages  11  of the stator structure  7 . 
     The suspended mass  6  and the stator structure  7  are provided with respective plane detection electrodes  12 , arranged perpendicular to the detection axis X 1  and comb-fingered (see also  FIG. 4 ). In greater detail, the detection electrodes  12  of the suspended mass  6  project from the suspended mass  6  itself towards the stator structure  7 ; instead, the detection electrodes  12  of the stator structure  7  are fixed to respective stator anchorages  14  and project towards the suspended mass  6 , at a distance from the substrate  8  of the chip  5 . Furthermore, each detection electrode  12  of the suspended mass  6  faces and is capacitively coupled to a respective pair of detection electrodes  12  of the stator structure  7 . In turn, the detection electrodes  12  of the stator structure  7  are electrically connected to one another in sets, according to conventional schemes of MEMS accelerometers (not illustrated in detail herein). The detection microstructure  2  is also equipped with the normal electrical connections of one-axis linear accelerometers (not illustrated herein) for connection of the detection electrodes  12  to the control unit  4 . 
     The reference microstructure  3  is substantially identical and is arranged symmetrically to the detection microstructure  2  with respect to the symmetry axis S 1 , which is perpendicular to the detection axis X 1  and passes through the center  5   a  of the chip  5 . In particular, the reference microstructure  3  comprises a suspended mass  6 ′ and a stator structure  7 ′, which have the same shapes and dimensions as the suspended mass  6  and, respectively, the stator structure  7  of the detection microstructure  2  and are separated from one another in a conventional way by means of insulating regions (not illustrated). The stator structure  7 ′ is delimited laterally by the substrate  8  of the chip  5  by means of a trench  9 ′ filled with dielectric material. In the case of the reference microstructure  3 , however, the suspended mass  6 ′ is rigidly connected to the stator structure  7 ′ by means of rigid connection elements  10 ′, which are substantially non-deformable. The suspended mass  6 ′ is hence fixed with respect to the stator structure  7 ′. The rigid connection elements  10 ′ project from the suspended mass  6 ′, at a distance from the substrate  8  of the chip  5 , and are fixed to respective suspension anchorages  11 ′, which have the same shape and the same relative distances as the first suspension anchorages  11  of the detection microstructure  2  (see also  FIG. 5 ). The suspended mass  6 ′ and the stator structure  7 ′ are provided with respective plane detection electrodes  12 ′, arranged perpendicular to the detection axis X 1  and comb-fingered. The detection electrodes  12 ′ of the reference microstructure  3  have the same shape and the same relative positions as the detection electrodes  12  of the detection microstructure  2 . In particular, the detection electrodes  12 ′ of the suspended mass  6 ′ project therefrom towards the stator structure  7 ′; the detection electrodes  12 ′ of the stator structure  7 ′ are instead fixed to respective stator anchorages  14 ″ and project towards the suspended mass  6 ″. Furthermore, each detection electrode  12 ″ of the suspended mass  6 ″ faces and is capacitively coupled to a respective pair of detection electrodes  12 ″ of the stator structure  7 ″. The detection electrodes  12 ″ of the stator structure  7 ″ are connected to one another in sets according to conventional schemes of MEMS accelerometers and are not illustrated herein in detail. The stator anchorages  14 ″ of the reference microstructure  3  have the same shape and the same relative distances as the stator anchorages  14  of the detection microstructure  2 . Also the reference microstructure  3  is provided with the normal electrical connections (not illustrated) of the one-axis linear MEMS accelerometers for connection of the detection electrodes  12 ″ with the control unit  4 . 
     In use, the detection microstructure  2  and the reference microstructure  3  are read by the control unit  4  using conventional reading modalities of linear MEMS accelerometers. As described above, moreover, the detection microstructure  2  and the reference microstructure  3  are substantially identical and, since they are also integrated in the same chip  5 , they are deformed exactly in the same way as a result of thermal expansion. In particular, the relative distances of the suspension anchorages  11 ′ and of the stator anchorages  14 ′ of the reference microstructure  3 , even though they are not fixed, remain in any case equal to the relative distances of the corresponding suspension anchorages  11  and stator anchorages  14  of the detection microstructure  2 . For this reason, the configuration of the detection electrodes  12 ′ of the suspended mass  6 ′ and of the stator structure  7 ′ of the reference microstructure  3  is always equal to the rest configuration (i.e., in the absence of accelerations along the detection axis X 1 ) of the detection electrodes  12  of the suspended mass  6  and of the stator structure  7  of the detection microstructure  2 . Consequently, temperature variations of the chip  5  cause identical variations in the measurement signal S X  and in the compensation signal S COMPX . However, in the measurement signal S X  the effect of temperature variations is superimposed on the effect of the accelerations according to the detection axis X 1 , whereas the variations of the compensation signal S compx  depend exclusively upon thermal expansion, because the suspended mass  6 ′ of the reference microstructure  3  is fixed. The compensation signal S COMPX  can thus be used for effective compensation of the effects of thermal expansion on the measurement signal S X . 
     For this purpose, the control unit  4  subtracts the compensation signal S COMPX  from the measurement signal S X  for generating the output acceleration signal S XO . 
     The location of the detection microstructure  2  and of the reference microstructure  3  in specularly symmetrical positions with respect to the symmetry axis S 1  of the chip  5  enables maximum precision of compensation to be achieved, also considering that, on account of the thermal expansion, the chips tend to undergo deformation and to assume a cup-like shape. Owing to the described arrangement, the compensation is extremely precise because, practically in any operating condition, the thermal expansion acts homogeneously on the detection microstructure  2  and on the reference microstructure  3 . 
     According to an alternative embodiment of the invention, illustrated in  FIG. 6 , a linear MEMS accelerometer  100  having a detection axis X 2  comprises a detection microstructure (DETECT X)  102 , a reference microstructure (COMPENS X)  103 , and a control unit (UC)  104 , integrated in a semiconductor chip  105  of a square or rectangular shape and having a center  105   a of symmetry. The detection microstructure  102  and the reference microstructure  103  are of the type illustrated in  FIG. 2 . In particular, the detection microstructure  102  comprises a suspended mass  106 , movable along the detection axis X 2  with respect to a fixed stator structure  107 ; and the detection microstructure  103  comprises a suspended mass  106 ′ and a stator structure  107 ′, both fixed and having the same shape and the same dimensions as the suspended mass  106  and, respectively, as the stator structure  107  of the detection microstructure  102 . Furthermore, the detection microstructure  102  and the reference microstructure  103  are specularly symmetrical with respect to a symmetry axis S 2 , parallel to the detection axis X 2  and passing through the center  105 a of the chip  105 , and are aligned in a direction perpendicular to the detection axis X 2  itself. 
     A third embodiment of the invention is illustrated in  FIGS. 7 and 8 . In this case, a linear MEMS accelerometer  200  having a detection axis X 3  comprises a detection microstructure (DETECT X)  202 , a reference microstructure (COMPENS X)  203  and a control unit (UC)  204 , which are integrated in a semiconductor chip  205  of a square or rectangular shape and having a center  205   a  of symmetry. The detection microstructure  202  and the reference microstructure  203  are identical to one another and are symmetrical with respect to a symmetry axis S 3  perpendicular to the detection axis X 3  and passing through the center  205   a  of the chip  205 . In greater detail, the detection microstructure  202  comprises a suspended mass  206 , a stator structure  207 , and elastic connection elements  210 , which connect the suspended mass  206  to respective suspension anchorages  211  of the stator structure  207  in a way similar to what is described in relation to the detection microstructure  2  of  FIG. 2 . In particular, the suspended mass  206  of the detection microstructure  202  is constrained to the stator structure  207  so as to be oscillatable according to the detection axis X 3 . The suspended mass  206  and the stator structure  207  of the detection microstructure  202  are separated from one another in a conventional way by insulating regions (not illustrated) and are provided with respective comb-fingered detection electrodes  212 , facing one another in pairs to establish a capacitive coupling. More precisely, each electrode  212  of the stator structure  207  is fixed to a respective anchoring stator  214  and is coupled to a respective detection electrode  212  of the suspended mass  206 . Furthermore, with reference to the arrangement of  FIG. 6 , the detection electrodes  212  arranged on the top side of the suspended mass  206  have their right-hand face coupled to the left-hand face of the respective detection electrode  212  of the stator structure  207 ; instead, the detection electrodes  212  set on the bottom side of the suspended mass  206  have their left-hand face coupled to the right-hand face of the respective detection electrode  212  of the stator structure  207 . The detection electrodes  212  of the stator microstructures  207  are connected to one another in sets in a conventional way and not illustrated in detail. 
     The reference microstructure  203  comprises a suspended mass  206 ′ and a stator structure  207 ′, having the same shape and the same dimensions as the suspended mass  206  and as the stator structure  207  of the detection microstructure  202  and separated from one another in a conventional way by insulating regions (not illustrated). The suspended mass  206 ′ is rigidly connected to the stator structure  207 ′ by means of rigid connection elements  210 ′, which are substantially non-deformable. In particular, the rigid connection elements  210 ′ are fixed to respective suspension anchorages  211 ′ of the stator structure  207 ′. The suspended mass  206 ′ and the stator structure  207 ′ of the reference microstructure  203  are capacitively coupled by a plurality of respective comb-fingered detection electrodes  212 ′, which are arranged symmetrically to the detection electrodes  212  of the detection microstructure  202  with respect to the symmetry axis S 3 . More precisely, each detection electrode  212 ′ of the stator structure  107 ′ is fixed to a respective anchoring stator  214  and is coupled to a respective detection electrode  212 ′ of the suspended mass  206 ′. With reference to the arrangement of  FIG. 6 , the detection electrodes  212 ′ arranged on the top side of the suspended mass  206 ′ have their left-hand face coupled to the right-hand face of the respective detection electrode  212 ′ of the stator structure  207 ′; instead, the detection electrodes  212 ′ set on the bottom side of the suspended mass  206 ′ have their right-hand face coupled to the left-hand face of the respective detection electrode  212 ′ of the stator structure  207 ′. Furthermore, the relative distances of the suspension anchorages  211 ′ and of the stator anchorages  214 ′ of the reference microstructure  203  are equal to the relative distances of the corresponding suspension anchorages  211  and stator anchorages  214  of the detection microstructure  202 . The detection electrodes  212 ′ of the stator microstructures  207 ′ are connected to one another in sets in a conventional way and not illustrated in detail. 
     According to a fourth embodiment of the invention, illustrated schematically in  FIG. 9 , a MEMS accelerometer  300  has a first detection axis X 4  and a second detection axis Y 4  perpendicular to one another. The MEMS accelerometer  300  comprises a first detection microstructure  302  and a second detection microstructure  322 , a first reference microstructure  303  and a second reference microstructure  323 , and a control unit  304  integrated in a same semiconductor chip  305 , of square or rectangular shape and having a center  305   a  of symmetry. The microstructures  302 ,  322 ,  303 ,  323  are all of the type described above with reference to  FIGS. 7 and 8 . In detail, the first detection microstructure  302  and the first reference microstructure  303  comprise respective suspended masses, designated by  306  and  306 ′, having the same shape and dimensions; and respective stator structures, designated by  307  and  307 ′, which also have the same shapes and dimensions. Furthermore, in the first detection microstructure  302 , the suspended mass  306  is movable according to the first detection axis X 4  with respect to the stator structure  307 , whereas in the first reference microstructure  303  the suspended mass  306 ′ is fixed. The second detection microstructure  322  and the second reference microstructure  323  comprise: respective suspended masses, designated by  326  and  326 ′, having the same shape and dimensions; and respective stator structures, designated by  327  and  327 ′, which also have the same shapes and dimensions. Furthermore, in the first detection microstructure  322 , the suspended mass  326  is movable according to the second detection axis Y 4  with respect to the stator structure  327 , whereas in the second reference microstructure  323  the suspended mass  326 ′ is fixed. 
     The first detection microstructure  302  detects the accelerations which act according to the first detection axis X 4 . The second detection microstructure  322  is rotated by 90° in the plane of  FIG. 8  to detect the accelerations which act according to the second detection axis Y 4 . The first reference microstructure  303  is arranged symmetrically to the first detection microstructure  302  with respect to the center  305   a  of the chip  305 . In practice, the configuration of the first reference microstructure  303  is obtained by overturning through 180° the first detection microstructure  302  (in the rest position) once about a first overturning axis S 4  and once about a second overturning axis S 5 , which are parallel to the first detection axis X 4  and to the second detection axis Y 4 , respectively, and pass through the center  305   a  of the chip  305 . Similarly, the second reference microstructure  323  is arranged symmetrical to the second detection microstructure  322  with respect to the center  305   a  of the chip  305 . The configuration of the second reference microstructure  323  is obtained by overturning through 180° the second detection microstructure  322  (in the resting position) once about the first axis S 4  and once about the second overturning axis S 5 . 
     The first and second detection microstructures  302 ,  322  provide the control unit  304  with a first measurement signal S X  and with a second measurement signal S Y , which are correlated to the accelerations acting on the chip  305  according to the first detection axis X 4  and to the second detection axis Y 4 , respectively. The first and the second reference microstructures  303 ,  323  provide the control unit  304  with a first compensation signal S COMPX  and with a second compensation signal S COMPY , which indicate the amount of the thermal expansion of the chip  305  in the direction of the first detection axis X 4  and of the second detection axis Y 4 , respectively. Finally, the control unit  304  generates a first output acceleration signal S XO , by subtracting the first compensation signal S COMPX  from the first measurement signal S X ; and a second output acceleration signal S YO , by subtracting the second compensation signal S COMPY  from the second measurement signal S Y . 
     In the fourth embodiment, in practice, the precision of the compensation is maximized, by arranging the detection microstructure and the compensation microstructure symmetrically with respect to the center of the chip. 
     Finally, it is clear that modifications and variations can be made to the device and to the method described herein, without thereby departing from the scope of the present invention, as defined in the annexed claims. 
     In particular, the invention can be exploited for compensating the effects of thermal expansion in various types of MEMS devices that use a mass that oscillates with respect to a fixed body, such as, for example, two-axes or three-axes linear accelerometers, rotational accelerometers, inclinometers, gyroscopes, pressure sensors, and electromechanical oscillators. 
     The control unit can be made separately, on a chip different from the one containing the microstructures. 
     All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, are incorporated herein by reference, in their entirety.

Technology Classification (CPC): 1