Microelectromechanical accelerometer

Abstract

A microelectromechanical accelerometer (1, 29, 31) comprises a substrate, a movably suspended heavy mass (2), four movably suspended light masses (3), and four electrode systems (4) and is at least rotationally symmetric. The heavy mass (3) laterally surrounds the light masses (3) and the electrode systems (4). Each electrode system (4) has two electrode structures (9). Each electrode structure (9) has fixed electrodes (10, 11) and movable electrodes (12, 13). The movable electrodes (12, 13) are connected to the masses (2, 3). Movable and fixed electrode surfaces (14, 15, 16, 17) interlock and form electrical capacitances (18, 19).The masses (2, 3) are coupled to each other in such a way that when the heavy mass (2) is deflected parallel to the substrate and in a direction perpendicular to the fixed and movable electrode surfaces (14, 15, 16, 17) of two opposing electrode systems (4), a counter-rotating deflection of the light masses (3) connected to the opposing electrode systems (4) is caused.

Description

The present invention relates to a microelectromechanical accelerometer.Triaxial microelectromechanical (MEMS) acceleration sensors are known from the prior art. Sensors with two seismic masses of different weights are known, which are suspended movably via spring elements and are coupled to one another via lever elements. The seismic masses have movable electrodes. Fixedly anchored electrodes are formed on the substrate, which electrodes form variable electrical capacitances with the movable electrodes as a function of acceleration forces, which capacitances serve as measurement signals. A disadvantage of known MEMS sensors is that they have undesirable transverse sensitivity due to lack of symmetry in the case of lateral acceleration forces.It is an object of the present invention to provide an improved microelectromechanical accelerometer. This object is achieved by a microelectromechanical acceleration sensor having the features of the independent claim. Advantageous further developments are specified in dependent claims.A microelectromechanical accelerometer includes a substrate, a heavy mass movably suspended over a top surface of the substrate, four light masses movably suspended over the top surface of the substrate, and four electrode systems disposed over the top surface of the substrate. The heavy mass laterally encloses the light masses and the electrode systems. The electrode systems are arranged in such a way that the microelectromechanical acceleration sensor has a four-fold rotational symmetry with respect to a rotational axis running perpendicular to the substrate. Each electrode system has two electrode structures arranged laterally next to one another. Each electrode structure includes a first fixed electrode, a second fixed electrode, a first movable electrode, and a second movable electrode. The first movable electrodes of each electrode system are arranged on mutually remote sides of the electrode structures of a respective electrode system and are each fixedly connected to the heavy mass. The second movable electrodes of an electrode system are arranged on mutually facing sides of the electrode structures of a respective electrode system and are each fixedly connected to a light mass. The first and the second fixed electrode of the electrode structures each have a first and a second fixed comb of first and second fixed electrode surfaces oriented perpendicularly to the substrate and projecting parallel to the substrate in opposite directions. The first and the second movable electrodes of the electrode structures each have a first and a second movable comb of first and second movable electrode surfaces arranged parallel to the first and second fixed electrode surfaces and protruding in the opposite direction. A first movable comb and a first fixed comb of the electrode structures respectively engage with one another and form first electrical capacitances. A second movable comb and a second fixed comb of the electrode structures respectively engage with one another and form second electrical capacitances. The masses are coupled to one another in such a way that, in the event of a deflection of the heavy mass parallel to the substrate and in a direction perpendicular to fixed and movable electrode surfaces of two opposing electrode systems, an opposing deflection of the light masses connected to the opposing electrode systems is effected.The heavy mass and the light masses may also be referred to as seismic masses, as they are movably suspended above the substrate and undergo a deflection due to their inertia on accelerations acting on the microelectromechanical acceleration sensor, which is also referred to as MEMS sensor for short below. The heavy mass is heavier than a total mass of the light masses. The MEMS sensor thus has a mass asymmetry. The mass difference between the heavy mass and the light masses may be chosen differently depending on the application range of the MEMS sensor, thereby adjusting a sensitivity of the MEMS sensor. The light masses have the same mass due to the symmetry of the MEMS sensor.As a result of the deflection of the seismic masses, a distance between the fixed and movable electrode surfaces of intermeshing combs changes, as a result of which the electrical capacitances change, which is a measure of an acceleration acting perpendicularly to the electrode surfaces of an electrical capacitance. The MEMS sensor has a total of four electrode systems, wherein in each case two mutually opposite electrode systems with electrode surfaces running parallel to one another are provided for measuring accelerations in a first direction running parallel to the substrate. The remaining two electrode systems are likewise situated opposite one another and have electrode surfaces running parallel to one another. However, they are provided for measuring accelerations in a second direction running parallel to the substrate and perpendicular to the first direction.A transverse sensitivity of the MEMS sensor is to be understood to mean that acceleration forces which have a component along the first direction generate a signal not only in the first direction but also in the second direction. Since the microelectromechanical acceleration sensor is configured rotationally symmetrically, it can advantageously be achieved at least that the MEMS sensor has the same transverse sensitivity and / or sensitivity to rotational vibrations with respect to the first and the second direction. A rotating mirror axis extends perpendicularly through a center point of the substrate. Due to the rotationally symmetrical arrangement of the electrode systems of the MEMS sensor, the electrode systems have an azimuthal offset of 90° with respect to one another.In one embodiment, the electrode systems are designed to be axially symmetrical with respect to an axis of symmetry running perpendicular to the fixed and movable electrode surfaces and between the electrode structures. In a more general embodiment, the microelectromechanical acceleration sensor has four axes of symmetry running parallel to the substrate.The symmetrical arrangements of these embodiments have the advantage that the MEMS sensor does not have any transverse sensitivity and / or any sensitivity to rotational vibrations. In these embodiments, the electrode systems are not only arranged rotationally symmetrically. Its arrangement additionally has mirror symmetry with respect to the four axes of symmetry. The MEMS sensor can therefore be referred to as fully symmetrical.This arrangement of the elements of the MEMS sensor also makes it possible to produce very sensitive triaxial acceleration sensors with a high mass asymmetry between the heavy mass and the light mass. Therefore, with the same sensitivity and area, in comparison with known sensors, wherein a sensitivity denotes a deflection of a seismic mass per gram, a MEMS sensor can be provided, which has a restoring force that is four to six times higher, for example. This advantageously makes it possible to avoid a tendency to stick, i.e. adhesion of a seismic mass as a result of an acceleration vertically with respect to the substrate.A high mass asymmetry additionally has the advantage that thermal noise of the MEMS sensor can be reduced in comparison with known sensors. For example, a signal strength of the thermal noise can thereby be halved. Thermal noise constitutes the largest part of a total noise of the MEMS sensor. For this reason, a total noise signal of the MEMS sensor can be significantly reduced.In one embodiment, the fixed and movable electrode surfaces of the first and second electrical capacitances are arranged in such a way that directly adjacent first and directly adjacent second electrical capacitances of the electrode systems are each formed in opposite directions. As a result, the electrical capacitances of different electrode structures of an electrode system are embodied in opposite directions. The electrical capacitances of the respective electrode structures, on the other hand, are each embodied in the same direction. In this embodiment, the arrangement of the electrode structures of the electrode systems can each have a translation symmetry, i.e. a second electrode structure of an electrode system can be generated by translation of a first electrode structure parallel to the substrate and parallel to the electrode surfaces. Because directly adjacent first and directly adjacent second electrical capacitances of the electrode systems are each embodied in opposite directions, acceleration signals can be locally tapped. In this case, the MEMS sensor is not fully symmetrical, but is only at least rotationally symmetrical.In one embodiment, the light masses on mutually facing sides of the electrode structures of the electrode systems are each connected to two inner spring elements which, in the rest position of the masses, are oriented perpendicularly to the fixed and to the movable electrode surfaces. The heavy mass is connected on sides of the electrode structures of the electrode systems facing away from each other to two outer spring elements which are oriented perpendicularly to the stationary and to the movable electrode surfaces in the rest position of the masses. Inner and outer spring elements are each connected in pairs to a lever element which, in the rest position of the masses, is oriented parallel to the stationary electrode surfaces and to the movable electrode surfaces in such a way that an inner spring element, an outer spring element and a lever element each laterally surround an electrode structure of an electrode system. The lever elements are each connected to suspensions arranged on the upper side of the substrate via a further spring element which projects between the fixed combs of an electrode structure and is arranged perpendicular to the lever elements in the rest position of the masses. Advantageously, the seismic masses are coupled to one another and suspended via a plurality of spring elements, as a result of which averaging of offset effects of the individual spring elements can be effected.In one embodiment, directly adjacent inner spring elements are connected to one another. Alternatively or additionally, in another embodiment, directly adjacent outer spring elements are connected to one another. If both inner and outer spring elements are connected to one another, however, a mass difference between the light masses and the heavy mass decreases, but a robustness of the MEMS sensor increases for this.Due to soldering processes during the production of the microelectromechanical acceleration sensor and due to external influences, the substrate may bend. Since the seismic masses and the fixed electrodes are not suspended or anchored at the same point on the substrate, such substrate bending leads to a displacement of the seismic mass relative to the fixed electrodes, which cannot be distinguished from an applied acceleration. An undesirable error signal thus arises.In one embodiment, the fixed combs of an electrode structure are each connected to a common anchoring. The suspensions and the anchorings of the electrode structures are each arranged one behind the other in a direction perpendicular to the fixed and movable electrode surfaces of the electrode structures. The first movable electrode surfaces and the second movable electrode surfaces of the electrode structures are respectively disposed on opposite sides of the fixed electrode surfaces of the first and second fixed combs with respect to a direction perpendicular to the electrode surfaces of the electrode structures.Advantageously, a displacement of a suspension relative to an anchoring occurring due to a bending of the substrate in a direction perpendicular to the electrode surfaces of the respective electrode structure can bring about an increase in a distance between the electrode surfaces of the first electrical capacitances and a reduction in a distance between the electrode surfaces of the second electrical capacitances. In a capacitive sum signal, the magnifications and reductions of the distances of the first and second electrical capacitances of an electrode structure compensate each other, whereby, overall, no change in capacitance takes place. A unidirectional displacement of the anchorings of the fixed electrodes relative to the suspensions caused by a bending of the substrate or otherwise can therefore be compensated. This advantageous arrangement is implemented twice in each electrode system, since the first movable electrode surfaces and the second movable electrode surfaces of all electrode structures are arranged on opposite sides of the fixed electrode surfaces of the first and second fixed combs. Compensation of effects of the substrate bending can thereby be segmented, whereby a sensitivity of the MEMS sensor can advantageously be reduced.In one embodiment, the light masses are connected to one another via connecting webs. As a result, the light masses are coupled to one another. A disadvantage of MEMS sensors is that they have a plurality of light seismic masses. In other words, a light seismic mass typically has at least two sections. The light masses are laterally surrounded by the heavy mass. To provide sensitive MEMS sensors, a large mass difference between the light masses and the heavy mass is advantageous. Connecting webs between the light masses of the light mass can therefore be dispensed with. If there is no connection between the light masses, however, the exact parallel movement of the light masses to the substrate upon deflection is not given, since they can be erected on sides facing each other. This is associated with an undesired reduction of a capacitive measurement signal of a MEMS acceleration sensor relative to a base capacitance. In addition, this reduces a maximum deflection of the seismic masses in the vertical direction.The microelectromechanical acceleration sensor has the advantage that the light masses can be deflected parallel to the substrate even without being connected to one another via connecting webs, as a result of which the aforementioned disadvantages of previously known sensors are overcome. A connection of the light masses may be advantageous in particular for small MEMS sensors with large detection surfaces for accelerations in the direction perpendicular to the substrate.In one embodiment, the heavy mass has further movable electrode surfaces. Further fixed electrode surfaces are arranged on the upper side of the substrate. The further fixed and the further movable electrode surfaces are arranged parallel to one another and to the substrate in the rest position of the heavy mass, are arranged opposite one another and form further electrical capacitances. The masses are coupled in such a way that, when the heavy mass is deflected in a direction perpendicular to the substrate, the light masses are deflected in the opposite direction. Advantageously, accelerations in a third direction vertical to the substrate can thereby also be measured. Advantageously, the further electrical capacitances can be distributed over the region of the heavy mass in order to be able to better compensate for influences of substrate bending via segmented detection.The microelectromechanical acceleration sensor (MEMS sensor) is explained in detail below with reference to schematic drawings. The following are shown: FIG. 1 : a MEMS sensor according to a first embodiment in a plan view; FIG. 2 : shows an enlarged detail of the MEMS sensor of FIG. 1 ; FIG. 3 : shows the MEMS sensor of FIG. 1 in a plan view, wherein a lateral acceleration acts; FIG. 4 shows an enlarged detail of the MEMS sensor of FIG. 3 ; FIG. 5 shows a further enlarged detail of the MEMS sensor of FIG. 3 ; FIG. 6 shows a still further enlarged detail of the MEMS sensor of FIG. 3 ; FIG. 7 : shows the MEMS sensor of FIG. 1 in a perspective view, wherein a vertical acceleration acts; FIG. 8 shows an enlarged detail of the MEMS sensor of FIG. 7 ; FIG. 9 : shows a MEMS sensor according to a second embodiment in a plan view; FIG. 10 : shows an enlarged detail of the MEMS sensor of FIG. 1 ; and FIG. 11 : shows a MEMS sensor according to a third embodiment in a plan view.FIG. 1 schematically shows a microelectromechanical acceleration sensor 1 according to a first embodiment in a plan view. Microelectromechanical acceleration sensor 1 has a substrate. The substrate is not shown in Figure 1 for simplicity because it is located below the elements shown. The substrate comprises silicon by way of example. The elements of microelectromechanical acceleration sensor 1 described below, which are arranged above an upper side of the substrate, also include silicon by way of example. The substrate and the further elements can alternatively or additionally also comprise a different material.Above the top of the substrate is arranged a movably suspended heavy mass 2. In addition, four light masses 3 are arranged, which are movably suspended above the top side of the substrate. The heavy mass 2 and the light masses 3 can also be referred to as seismic masses 2, 3. Furthermore, the microelectromechanical acceleration sensor 1 has four electrode systems 4 arranged above the upper side of the substrate. The heavy mass 2 laterally encloses the light masses 3 and the electrode systems 4, respectively. By way of example, FIG. 1 shows that the heavy mass 2 has four recesses 5. An electrode system 4 is arranged in each recess 5. However, it is also possible for the heavy mass 2 to have only one common recess 5 for all electrode systems 4. In this case, a region of the heavy mass 2 formed substantially between the light masses 3 is dispensed with. However, it is advantageous to provide a separate recess 5 for each electrode system 4 in order to achieve the greatest possible mass asymmetry between the heavy mass 2 and the light masses 3 or a total mass of the light masses 3.The electrode systems 4 are arranged in such a way that the microelectromechanical acceleration sensor 1 has a four-fold rotational symmetry with respect to a rotational axis 6 running perpendicular to the substrate. In other words, directly adjacent electrode systems 4 have an azimuthal offset of 90° with respect to one another. The electrode systems 4 can thus be transferred into one another by a rotation of 90° about the axis of rotation 6. For this reason, the electrode systems 4 are of identical design, i.e. the electrode systems 4 are designed congruently, except for tolerances during production.The electrode systems 4 are designed to detect acceleration forces acting laterally with respect to the substrate. In each case, two electrode systems 4 arranged opposite one another are designed for measuring acceleration forces along a first direction 7 which runs parallel to the substrate and parallel to a connecting line between the electrode systems 4 arranged opposite one another and can also be referred to as the x direction, and along a second direction 8 which likewise runs parallel to the substrate and perpendicular to the first direction 7 and can also be referred to as the y direction. Due to its rotational symmetry, the MEMS sensor 1 has at least the same transverse sensitivity along the first direction 7 and along the second direction 8.FIG. 2 shows schematically and by way of example one of the electrode systems 4 of the MEMS sensor 1 of FIG. 1 in a plan view. The following description also applies accordingly to the remaining electrode systems 4 on account of the symmetry of the MEMS sensor 1.Each electrode system 4 comprises two electrode structures 9 arranged laterally next to one another. The first movable electrodes 12 of the electrode systems 4 are arranged on mutually remote sides of the electrode structures 9 of the respective electrode systems 4 and are each fixedly connected to the heavy mass 2. The second movable electrodes 13 of the electrode systems 4 are each arranged on mutually facing sides of the electrode structures 9 of the respective electrode systems 4 and are each fixedly connected to a lightweight mass 3.The first and the second fixed electrode 10, 11 of the electrode structures 9 each have a first and a second fixed comb of first fixed electrode surfaces 14 and second fixed electrode surfaces 15 oriented perpendicularly to the substrate and projecting parallel to the substrate in opposite directions. The fixed electrode surfaces 14, 15 of the first and second fixed electrodes 10, 11 are respectively mechanically and electrically connected to each other via connection members of the fixed electrodes 10, 11 and each form a comb.The first and the second movable electrodes 12, 13 of the electrode structures 9 each have a first and a second movable comb of first and second movable electrode surfaces 16, 17 arranged parallel to the first and second fixed electrode surfaces 14, 15 and protruding in the opposite direction. The movable electrode surfaces 16, 17 of the first and second movable electrodes 12, 13 are likewise each mechanically and electrically connected to one another via connecting elements of the fixed electrodes 10, 11 and each form a comb.A first movable comb of first movable electrode surfaces 16 and a first fixed comb of first fixed electrode surfaces 14 of the electrode structures 9 respectively engage with one another and form first electrical capacitances 18.The light masses 3 are connected on mutually facing sides of the electrode structures 9 of the electrode systems 4 in each case to two inner spring elements 20 which, in the rest position of the seismic masses 2, 3, are oriented perpendicularly to the fixed electrode surfaces 14, 15, 16, 17 and are oriented towards the movable electrode surfaces 14, 15, 16, 17. The heavy mass 2 is connected on mutually remote sides of the electrode structures 9 of the electrode systems 4 in each case to two outer spring elements 21 which, in the rest position of the seismic masses 2, 3, are oriented perpendicularly to the fixed electrode surfaces 14, 15, 16, 17 and are oriented towards the movable electrode surfaces 14, 15, 16, 17.Inner and outer spring elements 20, 21 are connected to one another in pairs via a lever element 22. A first spring element 20 and a second spring element 21 are thus connected to one another by means of a lever element 22. In the rest position of the seismic masses 2, 3, the lever elements 22 are oriented parallel to the stationary and to the movable electrode surfaces 14, 15, 16, 17 of an electrode system 4. The spring elements 20, 21 are connected to one another via the lever elements 22 in such a way that an inner spring element 20, an outer spring element 21 and a lever element 22 in each case laterally surround an electrode structure 9 of an electrode system 4. The spring elements 20, 21, the lever elements 22, the further spring elements 23 and the suspensions 24 are thus each also arranged within the recesses 5 and are laterally surrounded by the heavy mass 2.In addition, the lever elements 22 are each connected to suspensions 24 arranged on the upper side of the substrate via a further spring element 23 protruding between the fixed combs of an electrode structure 9 and arranged perpendicular to the lever elements 22 and parallel to the spring elements 20, 23 in the rest position of the seismic masses 2, 3.Due to this arrangement of the spring elements 20, 21, the lever elements 22 and the further spring elements 23 of the MEMS sensor 1, the seismic masses 2, 3 are suspended movably. However, other variants of the suspension are also possible, in which the spring elements 20, 21, the lever elements 22 and the further spring elements 23 are arranged differently and are connected to one another. It merely has to be ensured that the seismic masses 2, 3 are coupled to one another.Immediately adjacent inner spring elements 20 of the MEMS sensor 1 can be connected to one another as shown in the exemplary illustrations of FIGS. 1 and 2. The inner spring elements 20 are also connected to one another, for example, in a region of the light masses 3. In addition, in the exemplary embodiment of the MEMS sensor 1 of FIGS. 1 and 2, directly adjacent outer spring elements 21 are connected to one another. These are each connected to one another in a region of the heavy mass 2 and between adjacent light masses 3. The inner and / or outer spring elements 20, 21 do not necessarily have to be connected to one another, however, or can also be connected to one another in other regions. It should be noted here that a mass asymmetry of the MEMS sensor 1 is influenced by a connection of the inner and / or outer spring elements 20, 21 in such a way that it is reduced. At the same time, however, the MEMS sensor 1 is thereby more robust.The MEMS sensor 1 according to the first embodiment is exemplarily configured such that the electrode systems 4 are configured to be axis-symmetric. The electrode systems 1 are each designed to be axially symmetrical with respect to a first axis of symmetry 25 running perpendicular to the fixed and movable electrode surfaces 14, 15, 16, 17 and between the electrode structures 9. With reference to FIG. 1, the MEMS sensor 1 has two first axes of symmetry 25 which have an azimuthal offset of 90° with respect to one another. In addition, the MEMS sensor has two second axes of symmetry 26, each of which has an azimuthal offset of 45° with respect to a first axis of symmetry 25. The first axes of symmetry 25 and the second axes of symmetry 26 intersect one another in a region between the light masses 3, for example in a center point of the MEMS sensor 1.FIG. 3 schematically shows the MEMS sensor 1 of FIG. 1 in a plan view. The reference numerals used hitherto are retained. In contrast to FIG. 1, the MEMS sensor 1 is shown in FIG. 3 in a state in which, by way of example, an acceleration acts with a component along the first direction 7. Due to the symmetry of the MEMS sensor 1, however, the following description also applies analogously to the case of acceleration forces parallel to the second direction. It should be noted that acceleration forces can be detected if they have a component perpendicular to fixed and movable electrode surfaces 14, 15, 16, 17, 18.Due to an inertia of the seismic masses 2, 3 and due to the fact that the seismic masses 2, 3 are movably suspended, a deflection of the seismic masses 2, 3 is caused. Seismic masses 2, 3 are coupled to one another via spring elements 20, 21, lever elements 22, and additional spring elements 23 in such a way that, in the event of a deflection of heavy mass 2 parallel to the substrate and in a direction perpendicular to fixed and movable electrode surfaces 14, 15, 16, 17 of two electrode systems 4 situated opposite one another in first direction 7, a deflection in the opposite direction of light masses 3 connected to electrode systems 4 situated opposite one another in first direction 7 is brought about.FIGS. 4 and 5 each show an enlarged portion of FIG. 3, wherein two electrode systems 4 connected to the light masses 3 undergoing counter-current deflection relative to the heavy mass 2 are shown separately. The reference numerals used hitherto are retained.In the case of a deflection of the light masses 3 in opposite directions, a deflection of the movable electrode surfaces 16, 17 of the respective electrode systems 4 is effected perpendicular to the electrode surfaces 14, 15, 16, 17, since these are firmly connected to the seismic masses 2, 3. As a result, distances between the fixed electrode surfaces 14, 15 and the movable electrode surfaces 16, 17 of the first and second electrical capacitances 18, 19 of the electrode structures 9 change. While in the electrode system 4 of FIG. 4 the opposite deflection of the light mass 3 connected to the electrode system 4 induces a reduction in the distances between the fixed and the movable electrode surfaces 14, 15, 16, 17, the opposite deflection of the light mass 3 in the electrode system 4 of FIG. 5, which can be generated by mirroring the electrode system 4 of FIG. 4 on a first axis of symmetry 25, causes an increase in the distances between the fixed and the movable electrode surfaces 14, 15, 16, 17.The seismic masses 2, 3 are furthermore coupled to one another in such a way that, in the event of a deflection of the heavy mass 2 parallel to the substrate and in a direction parallel to fixed and movable electrode surfaces 14, 15, 16, 17 of two further electrode systems 4 lying opposite one another in the second direction 8, a deflection of the light masses 3 connected to the further electrode systems 4 lying opposite one another in the second direction 8 is effected in the same direction. Such an electrode system is exemplarily shown in FIG. 6, with reference numerals used so far being retained. It can be seen that the distances between the fixed and movable electrode surfaces 14, 15, 16, 17 do not change, since the light mass 3 in question is deflected in a direction parallel to the fixed and movable electrode surfaces 14, 15, 16, 17.Only the deflection in opposite directions of two light masses 3 with respect to the heavy mass 2 thus contributes to a measurement signal. Due to its fully symmetrical arrangement, the MEMS sensor 1 according to the first embodiment has the property that it does not have any transverse sensitivity between measurements in the first direction 7 and measurements in the second direction 8, i.e. that accelerations which act only in the first or second direction 7, 8 do not generate a signal in the respective other direction 8, 7. In comparison with MEMS sensors which are not fully symmetrical, a more accurate measurement signal can thereby be provided.FIG. 7 schematically shows the MEMS sensor 1 of FIG. 1 in a perspective view. FIG. 8 shows an enlarged region of FIG. 7, an electrode system 4 being shown by way of example. The reference numerals used hitherto are retained. In contrast to FIG. 1, the MEMS sensor 1 is shown in FIGS. 7 and 8 in a state in which, by way of example, has an acceleration with a component along a third direction 27 running perpendicular to the substrate, which can also be referred to as the z direction.Seismic masses 2, 3 are furthermore coupled in such a way that, in the event of deflection of heavy mass 2 in third direction 27, a deflection of light masses 3 in the opposite direction is brought about. Accelerations acting perpendicular to the substrate can thereby also be measured. With reference to FIG. 1, the heavy mass 2 has further movable electrode surfaces 28 for this purpose. The further movable electrode surfaces 28 are each arranged, for example, in regions between two light masses 3. By way of example, the MEMS sensor 1 has four further movable electrode surfaces 28. However, a different number of further movable electrode surfaces 28 can also be provided, which can also be positioned differently. However, the further movable electrode surfaces 28 and the further fixed electrode surfaces can also be omitted.The further movable electrode surfaces 28 form further electrical capacitances with further fixed electrode surfaces arranged on the upper side of the substrate, which are not shown for reasons of clarity. The further fixed and the further movable electrode surfaces 28 are arranged parallel to one another and opposite one another in the rest position of the seismic masses 2, 3 and are arranged with respect to the substrate. FIG. 8 shows that the further fixed and the further movable electrode surfaces 28 are arranged parallel to one another not only in the rest position of the seismic masses 2, 3 but also in the case of a deflection perpendicular to the substrate. This is because the seismic masses 2, 3 remain aligned substantially parallel to one another despite their deflection and the fact that no connecting webs are formed between the heavy mass 2 and the light masses 3.FIG. 9 schematically shows a microelectromechanical acceleration sensor 29 according to a second embodiment in a plan view. The MEMS sensor 29 according to the second embodiment has similarities to the MEMS sensor 1 according to the first embodiment. Only the differences will be explained below. FIG. 10 shows schematically and by way of example one of the electrode systems 4 of the MEMS sensor 29 of FIG. 9 in a plan view. The views of FIGS. 9 and 10 of the MEMS sensor 29 according to the second embodiment therefore correspond to the views of FIGS. 1 and 2 of the MEMS sensor 1 according to the first embodiment. The previous reference numerals are retained.In contrast to the MEMS sensor 1 according to the first embodiment, in the MEMS sensor 29 according to the second embodiment, the fixed and movable electrode surfaces 14, 15, 16, 17 of the first and second electrical capacitances 18, 19 are arranged in such a way that directly adjacent first and directly adjacent second electrical capacitances 18, 19 of the electrode systems are each embodied in opposite directions. As a result, the MEMS sensor 29 according to the second embodiment has no axes of symmetry 25, 26 and is configured only rotationally symmetrically with a four-fold axis of rotation. The electrode systems 4 also do not have axial symmetry per se as a result. Rather, the electrode systems 4 of the MEMS sensor 29 according to the second embodiment have a translation symmetry, since the electrode structures 9 of an electrode system 4 can be transferred into one another by translation.In other words, in the MEMS sensor 29 according to the second embodiment, immediately adjacent first movable electrode surfaces 16 and immediately adjacent second movable electrode surfaces 17 are respectively on opposite sides of the fixed electrode surfaces 14, 15 with respect to a direction perpendicular to the electrode surfaces 14, 15, 16, 17 of the electrode structures 9.Referring to FIG. 2, in the MEMS sensor 1 according to the first embodiment, immediately adjacent first movable electrode surfaces 16 and immediately adjacent second movable electrode surfaces 17 are respectively disposed on mutually facing sides of the fixed electrode surfaces 14, 15 with respect to a direction perpendicular to the electrode surfaces 14, 15, 16, 17 of the electrode structures 9. As a result, directly adjacent first electrical capacitances 18 and directly adjacent second electrical capacitances 19 are configured in the same direction. This can be seen particularly clearly in FIGS. 4 and 5. In this case, the distances of the electrodes 14, 15, 16, 17 decrease or increase jointly between directly adjacent first and directly adjacent second electrical capacitances 18, 19. In the case of oppositely directed, directly adjacent first and directly adjacent second capacitances 18, 19, the distances within one electrode structure 9 of an electrode system 4 decrease, while the distances within the other electrode structure 9 of the relevant electrode system 4 increase.Microelectromechanical acceleration sensors 1, 29 according to the first and second embodiments have the common feature that the fixed combs of an electrode structure 9 are each connected to a common anchoring 30. The anchors 30 are disposed on the top surface of the substrate. The suspensions 24 and the anchorings 30 of the electrode structures 9 are arranged one behind the other in a direction perpendicular to the fixed and movable electrode surfaces 14, 15, 16, 17 of the electrode structures 9.Bending of the substrate, for example due to thermal effects, may cause displacement of the anchors 30 relative to the suspensions 24. However, this leads to the distances between the fixed and the movable electrode surfaces 14, 15, 16, 17 of the electrical capacitances 18, 19 changing. Preferably, the suspensions 24 and the anchors 30 of each electrode structure 9 can be arranged close to one another, as shown in FIGS. 2 and 10. As a result, the influence of the substrate bending can be at least partially reduced. However, the suspensions 24 and the anchors 30 cannot be arranged at any desired close distance to one another, since this can be technically problematic and since stability of the suspensions 24 and the anchors 30 can be reduced if necessary, because the suspensions 24 and the anchors 30 have to be made smaller if they are arranged as close as possible to one another.For this reason, the mems sensors 1, 29 according to the first and second embodiments have the property that the first movable electrode surfaces 16 and the second movable electrode surfaces 17 of the electrode structures 9 are arranged on opposite sides of the fixed electrode surfaces 14, 15 of the first and second fixed combs, respectively, with respect to a direction perpendicular to the electrode surfaces 14, 15, 16, 17 of the electrode structures 9. In the case of substrate bending and displacement of the suspensions 24 relative to the anchorings 30, the first electrical capacitances 18 and the second electrical capacitances 19 of an electrode structure 9 are thereby each embodied in opposite directions. The effect of substrate bending is compensated in this way within an electrode structure 9. By segmenting the MEMS sensors 1, 29 into a total of eight electrode structures 9, this influence is additionally reduced.FIG. 11 schematically shows a microelectromechanical acceleration sensor 31 according to a third embodiment in a plan view. The MEMS sensor 31 according to the second embodiment has similarities to the MEMS sensor 1 according to the first embodiment. Only the differences will be explained below. However, the additional features compared to the MEMS sensor 1 according to the first embodiment can also be realized in the MEMS sensor 29 according to the second embodiment. The previous reference numerals are retained.In the MEMS sensor 31 according to the third embodiment, the light masses 3 are connected to one another via connecting webs 32. The MEMS sensor 31 has a total of two connecting webs 32, which each connect two light masses 3 lying opposite one another. Due to the symmetry of the MEMS sensor 31, the connecting webs 32 cross one another and are also connected to one another. The connecting webs 32 can, however, also be omitted.

Claims

Microelectromechanical acceleration sensor (1, 29, 31) having a substrate, a heavy mass (2) suspended movably above an upper side of the substrate, four light masses (3) suspended movably above the upper side of the substrate and four electrode systems (4) arranged above the upper side of the substrate, wherein the heavy mass (2) laterally encloses the light masses (3) and the electrode systems (4), wherein the electrode systems (4) are arranged such that the microelectromechanical acceleration sensor (1, 29, 31) has a four-fold rotational symmetry with respect to an axis of rotation (6) running perpendicular to the substrate, wherein each electrode system (4) has two electrode structures (9) arranged laterally next to one another, wherein each electrode structure (9) has a first fixed electrode (10), a second fixed electrode (11), a first movable electrode (12) and a second movable electrode (13), wherein the first movable electrodes (12) of each electrode system (4) are arranged on mutually remote sides of the electrode structures (9) of a respective electrode system (4) and are each arranged fixedly to the heavy mass (2) and the second movable electrodes (13) of an electrode system (12) are arranged on mutually facing sides of the electrode structures (9) of a respective electrode system (4) and are each fixedly connected to a light mass (3), wherein the first and the second fixed electrodes (10, 11) of the electrode structures (9) each have a first and second fixed comb of first and second fixed electrode surfaces (14, 15) oriented perpendicularly to the substrate and projecting parallel to the substrate in opposite directions, wherein the first and the second movable electrodes (10, 11) of the electrode structures (9) each have a first and second movable comb of first and second fixed electrode surfaces (14, 14) parallel to the first and second fixed electrode surfaces, The invention also provides a method for deflecting light masses (3) which are connected to the mutually opposed electrode systems (4) and which comprise first and second movable electrode surfaces (16, 17) which are arranged in opposite directions and project in opposite directions, wherein a first movable comb and a first fixed comb of the electrode structures (9) engage in one another and form first electrical capacitances (18), and a second movable comb and a second fixed comb of the electrode structures (9) engage in one another and form second electrical capacitances (19), wherein the masses (2, 3) are coupled to one another in such a way that, when the heavy mass (2) is deflected parallel to the substrate and in a direction perpendicular to fixed and movable electrode surfaces (14, 15, 16, 17) of two mutually opposed electrode systems (4), the light masses (3) which are connected to the mutually opposed electrode systems (4) are deflected in opposite directions.Microelectromechanical acceleration sensor (1, 30) according to Claim 1, wherein the electrode systems (4) are designed to be axially symmetrical with respect to an axis of symmetry (25) running perpendicular to the fixed and movable electrode surfaces (14, 15, 16, 17) and between the electrode structures.Microelectromechanical acceleration sensor (1, 30) according to Claim 2, wherein the microelectromechanical acceleration sensor (1, 30) has four axes of symmetry (25, 26) running parallel to the substrate.Microelectromechanical acceleration sensor (29) according to Claim 1, wherein the fixed and movable electrode surfaces (14, 15, 16, 17) of the first and second electrical capacitances (18, 19) are arranged in such a way that directly adjacent first and directly adjacent second electrical capacitances (18, 19) of the electrode systems (4) are each embodied in opposite directions.Microelectromechanical acceleration sensor (1, 29, 30) according to one of the preceding claims, wherein the light masses (3) are connected on mutually facing sides of the electrode structures (9) of the electrode systems (4) in each case to two inner spring elements (20) aligned perpendicularly to the fixed electrode surfaces (14, 15, 16, 17) in the rest position of the masses (2, 3), wherein the heavy mass (2) is connected on mutually remote sides of the electrode structures (9) of the electrode systems (4) in each case to two outer spring elements (21) aligned perpendicularly to the fixed electrode surfaces and to the movable electrode surfaces (14, 15, 16, 17) in the rest position of the masses (2, 3), wherein inner and outer spring elements (20, 21) are in each case connected in pairs to a lever element (22) aligned parallel to the fixed electrode surfaces and to the movable electrode surfaces (14, 15, 16, 17) in the rest position of the masses (2, 3) in such a way that in each case one inner spring element (20), an outer spring element (21) and a lever element (22) laterally surround an electrode structure (9) of an electrode system (4), wherein the lever elements (22) are each connected to suspensions (24) arranged on the upper side of the substrate via a further spring element (23) protruding between the fixed combs of an electrode structure (9) and arranged perpendicular to the lever elements (22) in the rest position of the masses (2, 3).Microelectromechanical acceleration sensor (1, 29, 30) according to Claim 5, wherein directly adjacent inner spring elements (20) are connected to one another.Microelectromechanical acceleration sensor (1, 29, 30) according to Claim 5 or 6, wherein directly adjacent outer spring elements (21) are connected to one another.Microelectromechanical acceleration sensor (1, 29, 30) according to one of Claims 5 to 7, wherein the fixed combs of an electrode structure (9) are each connected to a common anchoring (30), wherein the suspensions (24) and the anchorings (30) of the electrode structures (9) are each arranged one behind the other in a direction perpendicular to the fixed and movable electrode surfaces (14, 15, 16, 17) of the electrode structures (9), wherein the first movable electrode surfaces (16) and the second movable electrode surfaces (17) of the electrode structures (9) are each arranged on mutually opposite sides of the fixed electrode surfaces (14, 15) of the first and second fixed combs with respect to a direction perpendicular to the electrode surfaces (14, 15, 16, 17) of the electrode structures (9).Microelectromechanical acceleration sensor (1, 29, 30) according to one of the preceding claims, wherein the light masses (2, 3) are connected to one another via connecting webs (32).Microelectromechanical acceleration sensor (1, 29, 30) according to one of the preceding claims, wherein the heavy mass (2) has further movable electrode surfaces (28), wherein further fixed electrode surfaces are arranged on the upper side of the substrate, wherein the further fixed and the further movable electrode surfaces (28) are arranged parallel to one another and to the substrate in the rest position of the masses (2, 3), are arranged opposite one another and form further electrical capacitances, wherein the masses (2, 3) are coupled in such a way that, when the heavy mass (3) is deflected in a direction (27) perpendicular to the substrate, an opposite deflection of the light masses (3) is effected.

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