Micromechanical structures and micromechanical sensors

Abstract

The invention relates to a micromechanical structure having a substrate and a seismic mass which is movable relative to the substrate, wherein a torsional spring connects the seismic mass to the substrate. The micromechanical structure further has a detection mechanism, wherein a first direction and a second direction which is substantially perpendicular to the first direction define a main extension plane of the substrate, wherein the detection mechanism is designed to detect a rotational displacement of the seismic mass about a rotational axis, wherein the rotational axis is arranged in the second direction. The micromechanical structure further has a stop structure, wherein the stop structure is arranged such that a movement of the seismic mass is limited by the stop structure, wherein the stop structure is designed to absorb kinetic energy of the seismic mass in a partially elastic, partially inelastic collision.

Description

TECHNICAL FIELD

[0001] The present invention relates to a micromechanical structure and a micromechanical sensor. BACKGROUND

[0002] Acceleration sensors are usually composed of micromechanical structures (also called MEMS structures, i.e. microelectromechanical system structures), which are etched from a thick polysilicon functional layer. They are arranged above a thin buried polysilicon layer. They are anchored on the substrate by means of an oxide layer. Between the two polysilicon layers, an oxide layer is also provided. The buried polysilicon layer serves as a conductor track or electrode. The functional layer is freed by means of a trench process and an oxide sacrificial layer method. The buried layer is electrically separated from the substrate by means of an oxide. The conductor track and the electrode are sufficiently wide not to be etched completely down in the oxide-sacrificial oxide etching step and thus to be stably anchored on the substrate.

[0003] The micromechanical structures thus produced can be sealed with a cap in a further process sequence in order to protect them from the environment. Furthermore, depending on the application, a gas with a suitable pressure is enclosed in the closed volume. The closure is usually achieved by means of a sealing glass bonding method or by means of a eutectic bonding method, for example by means of AlGe.

[0004] In order to produce a Z acceleration sensor in this production process, a structure with a seismic mass is produced in the thick functional layer, which is anchored on the substrate by means of a torsional spring. The mass distribution of the seismic mass is chosen asymmetrically. Two electrode faces are arranged below the seismic mass in order to be able to measure the displacement of the seismic mass capacitively.

[0005] In the case of an overload, the seismic mass should not strike the electrode faces below it. In the operation of the sensor, there is an analysis processing voltage between the electrode faces and the seismic mass, as a result of which the seismic mass can be welded on the electrodes in the case of contact. Various stop concepts are known in order to prevent this. A number of the concepts are based on stop structures which are constructed on or in the seismic mass, which strike with suitable potential energy onto a fixed counter structure provided in the substrate. In the case of an overload, these stop structures can strike very strongly and have a tendency to remain stuck on the counter face. It is known that the tendency to stick increases with the strength of the overload. It is further known that the tendency to stick also increases in the case of frequent loading of the stop structures.

[0006] To avoid sticking in the case of high overloads, the concept is known of providing a springing element between the seismic mass and the stop structure. The spring can absorb a part of the energy in the case of a stop and thus reduce the load on the stop structure. Such a stop structure is disclosed, for example, in document DE 10 2012 207 939 A1. The spring will in turn release the energy stored on impact almost completely onto the seismic mass, so that several impacts can be observed one after the other if necessary. SUMMARY

[0007] It is the task of the present application to specify an improved micromechanical structure in which the kinetic energy of the seismic mass that acts on impact is not released onto the seismic mass completely again in order to reduce the number. It is a further task of the present application to specify a micromechanical sensor having such a micromechanical structure.

[0008] These tasks are solved by means of the micromechanical structure described below and the micromechanical sensor having said micromechanical structure. Advantageous refinements are specified below.

[0009] The micromechanical structure has a substrate and a seismic mass that is movable relative to the substrate, wherein a torsional spring connects the seismic mass with the substrate. The micromechanical structure further has a detection mechanism. A first direction and a second direction that is essentially perpendicular to the first direction define a main extension plane of the substrate, wherein the detection mechanism is provided for detecting a rotational displacement of the seismic mass about a rotational axis, wherein the rotational axis is arranged in the second direction. The micromechanical structure further has a stop structure, wherein the stop structure is arranged such that the movement of the seismic mass is limited by the stop structure. The stop structure is provided for absorbing the kinetic energy of said seismic mass in a partially elastic, partially inelastic collision.

[0010] By the stop structure absorbing the kinetic energy of the seismic mass in a partially inelastic collision, it is possible to dissipate a part of the kinetic energy that exists at the moment of the impact. Thereby, the energy that is released onto the seismic mass again by means of an elastic collision is smaller than the kinetic energy of the seismic mass at the moment of the impact, so that further impacts of the seismic mass on the substrate are prevented if necessary or at least can be reduced in intensity.

[0011] In an embodiment, the rotational displacement can be caused by acting a quantity to be measured onto the seismic mass, in particular by an acceleration in a third direction that is perpendicular to the main extension plane. In an operating range, the quantity to be measured is such that the rotational displacement does not lead to a mechanical contact of the seismic mass with the substrate. In an overload range, i.e. when the quantity to be measured is outside the operating range, a mechanical contact of the seismic mass with the substrate cannot be ruled out, so that the movement of the seismic mass is limited here by means of the stop structure.

[0012] In one embodiment, the detection mechanism has an electrode structure with a first electrode fastened to the seismic mass and a second electrode fastened to the substrate. Alternatively, the detection mechanism can have, for example, a laser and a position-sensitive detector, by means of which a laser beam reflected on the seismic mass impinges on different positions of the position-sensitive detector depending on the angle of rotation and thus the displacement of the seismic mass can be determined. Other detection mechanisms are also conceivable.

[0013] It can be provided that the substrate has a cap with which the micromechanical structure is shielded from external influences and which is configured such that the substrate and the cap form a closed housing. The stop structure can then also be arranged between the seismic mass and the cap associated with the substrate.

[0014] In one embodiment, at least twenty percent of the kinetic energy is inelastically absorbed. It can be provided here that at least twenty percent and at most ninety percent of the kinetic energy is inelastically absorbed. This means that ten to eighty percent of the kinetic energy is released again to the seismic mass by means of elastic collisions. Thereby, a corresponding restoring force can act on the seismic mass, so that sticking is prevented or at least the probability of sticking can be reduced. It can be provided in addition that different proportions for elastic and inelastic collisions are provided for different kinetic energies. For example, it can be provided for high kinetic energies that up to ninety percent of the kinetic energy is absorbed in inelastic collisions; and for low kinetic energies that twenty percent of the kinetic energy is absorbed in inelastic collisions. In this way, it can be achieved that exactly in the case of small overloads the seismic mass still moves with some momentum again in the direction of the rest position, while in the case of large overloads a large amount of energy is dissipated from the system. In this way, a small tendency to stick is ensured over a large range of different overloads.

[0015] In one embodiment, the stop structure has a protrusion and a diaphragm structure, wherein the diaphragm structure comprises a diaphragm. The diaphragm comprises at least one opening. The protrusion can hit on the diaphragm, in particular in the case of an overload. The movement of the diaphragm causes a fluid arranged in the cavity to escape through the opening and thereby absorb kinetic energy of the seismic mass in a partially inelastic collision. The mentioned construction of the stop structure shows a configuration which can be simply realized mechanically, by means of which the task of dissipating a part of the kinetic energy can be well fulfilled. Here, the above considerations regarding the absorption of energy can also apply to the diaphragm structure and likewise to the more precise specification of the diaphragm structure listed below.

[0016] In one embodiment, the depth of the cavity is at most ten times the distance between the protrusion and the membrane. This enables an effective inelastic collision of the protrusion with the membrane structure. It can be provided here that the depth of the cavity is at most five micrometers and the distance between the protrusion and the membrane is at most three micrometers. In particular, the distance between the protrusion and the membrane can be at most one micrometer or at most 0.5 micrometers.

[0017] In one embodiment, the plurality of openings is arranged around the collision point of the membrane. This enables an effective inelastic collision of the protrusion with the membrane structure.

[0018] In one embodiment, the one opening or the plurality of openings forms a spring structure in the membrane, wherein the spring structure absorbs the kinetic energy of the seismic mass in a partially elastic collision. This enables a particularly effective construction of the membrane structure, since the parameters of the inelastic collision can be set by means of the openings and the parameters of the elastic collision can be set by means of the spring structure.

[0019] In one embodiment, the opening surrounds at least three sides of a cantilever beam, wherein the cantilever beam is part of the spring structure. This also enables a simple construction of the membrane structure.

[0020] In one embodiment, the membrane has a thickness of at least two hundred nanometers. The thickness can also be at least three hundred or at least four hundred nanometers. The maximum thickness of the membrane can be three micrometers, in particular two micrometers and preferably one micrometer. The mentioned thicknesses enable an effective construction of the membrane structure.

[0021] In one embodiment, the micromechanical structure has a further stop structure, in particular a further protrusion and a further membrane structure. The movement of the seismic mass can thereby be limited by the further stop structure.

[0022] In one embodiment, the micromechanical structure also has a fixed stop. The stop structure is provided for absorbing, by means of an inelastic collision, between twenty-five percent and seventy-five percent of the energy stored in the torsional spring at the moment of impact of the fixed stop. The fixed stop can be mounted in one preferred embodiment between the stop structure and the torsional spring.

[0023] The invention also comprises a micromechanical sensor having a micromechanical structure according to the invention. The micromechanical sensor can also have a circuit, wherein the circuit can be provided for converting the signal generated by means of the detection mechanism into an analog or digital output signal. BRIEF DESCRIPTION OF DRAWINGS

[0024] Embodiments of the invention are explained with reference to the following figures. In the schematic drawings:

[0025] Figure 1 shows a top view of a micromechanical structure;

[0026] Figure 2 shows a cross section of a micromechanical structure;

[0027] Figure 3 shows a cross section of another micromechanical structure;

[0028] Figure 4 shows a top view of a diaphragm structure;

[0029] Figure 5 shows a top view of another diaphragm structure;

[0030] Figure 6 shows a top view of another diaphragm structure;

[0031] Figure 7 shows a top view of another diaphragm structure;

[0032] Figure 8 shows a cross section of another micromechanical structure; and

[0033] Figure 9 shows a micromechanical sensor. DETAILED DESCRIPTION

[0034] Figure 1 shows a micromechanical structure 1 with a substrate 2 and a seismic mass 3 which is movable relative to the substrate 2. A torsional spring 5 connects the seismic mass 3 with the substrate 2, wherein, in the Figure 1 two torsional springs 5 are shown in the middle. A first direction 11 and a second direction 12 which is essentially perpendicular to the first direction 11 define a main extension plane of the substrate 2. The seismic mass 3 is able to rotateably displace about a rotation axis 14, wherein the rotation axis 14 is arranged on the second direction 12. For example, an acceleration which acts in a third direction 13 can cause such a rotational displacement about the rotation axis 14, since the seismic mass 3 is divided into a larger sub-mass 31 on one side of the torsional spring 5 and a smaller sub-mass 32 on the opposite side of the torsional spring 5 and this mass asymmetry causes a corresponding displacement. However, in principle, the present application can also be used for other to-be-measured quantities, as long as this to-be-measured quantity causes a rotational displacement of the seismic mass 3. The torsional spring 5 is connected with the substrate 2 by means of a connecting element 51.

[0035] Figure 2 shows a cross section through Figure 1The micromechanical structure 1 has a detection mechanism 4 for detecting a rotational deviation of the seismic mass 3 about the rotational axis 14, which detection mechanism is configured as a first electrode 41 arranged on the seismic mass 3 and as a second electrode arranged on the substrate 2. The first electrode 41 and the second electrode 42 can be analyzed capacitively. Alternatively, other detection mechanisms 4 can also be provided by means of which a rotational deviation of the seismic mass 3 can be detected. The micromechanical structure 1 also has a stop structure 6. The stop structure 6 is arranged in such a way that the movement of the seismic mass 3 is limited by the stop structure 6. The stop structure 6 is provided for absorbing the kinetic energy of the seismic mass 3 in a partially elastic, partially inelastic collision.

[0036] By absorbing the kinetic energy of the seismic mass in a partially inelastic collision by means of the stop structure 6, it is possible to achieve that a portion of the kinetic energy present at the time of the impact is dissipated. As a result, the energy released onto the seismic mass 3 again by means of the elastic collision is less than the kinetic energy of the seismic mass 3 at the time of the impact, so that further impacts of the seismic mass 3 on the substrate 2 are prevented, if necessary, or at least can be reduced in intensity.

[0037] In the operating range, the quantity to be measured, for example the acceleration, can be such that the rotational deviation does not result in a mechanical contact of the seismic mass 3 with the substrate 2. In the overload range, i.e. when the quantity to be measured is outside the operating range, a mechanical contact of the seismic mass 3 with the substrate 2 cannot be ruled out, so that here the movement of the seismic mass 3 is limited by means of the stop structure 6.

[0038] It can be provided that at least 20 percent of the kinetic energy is absorbed inelastically.

[0039] In Figure 2 the stop structure 6 shown in is provided with a protrusion 61 and a diaphragm structure 62. The diaphragm structure 62 comprises a diaphragm 63, wherein the diaphragm 63 comprises at least one opening 64. The protrusion 61 can collide onto the diaphragm 63. The movement of the diaphragm 63 causes the fluid 66 arranged in a cavity 65 to escape through the opening and thus the kinetic energy of the seismic mass 3 is absorbed in a partially inelastic collision. By the fluid 66 escaping through the opening, a damping of the stop structure is achieved. The fluid 66 can here comprise a gas. In particular, the pressure of the gas can be set in such a way that the relative proportion of the elastic and inelastic collisions to each other is set.

[0040] In Figure 2 an optional cap 21 is shown as a part of the substrate 2. The cap 21 can for example be a cap wafer. Furthermore, Figure 2The micromechanical structure 1 has two stop structures 6, wherein the lugs 61 of the stop structures 6 are arranged on the larger sub-masses 31 of the seismic mass 3, respectively, and one diaphragm structure 62 is arranged on the substrate 2 and one diaphragm structure 62 is arranged on the cap 21. Thereby, two movement directions, in particular the upward and downward movement of the larger sub-masses 31 of the seismic mass 3, can be limited by the stop structures 6. In principle, it is also possible that only one stop structure 6 is present or that the plurality of stop structures 6 is arranged differently, for example, such that the diaphragm structures 62 are arranged on the larger sub-masses 31 of the seismic mass 3 and the lugs 61 are arranged on the substrate 2 or on the cap 21. The cap 21 is here a part of the substrate 2, such that the stop structures 6 arranged between the cap 21 and the seismic mass 3 are arranged such that the movement of the seismic mass 3 is limited by the stop structures 6. The stop structures 6 are provided for absorbing the kinetic energy of the seismic mass 3 in a partially elastic, partially inelastic collision. For example, both stop structures 6 can also be arranged between the cap 21 and the seismic mass 3.

[0041] In Figure 2 a fixed stop 8 is also shown. The fixed stop 8 serves to avoid an excessive load of the stop structures 6. The stop structures 6 can be provided for absorbing, by inelastic collision, twenty-five to seventy-five percent of the energy stored in the torsional spring 5 at the moment of impact of the fixed stop 8.

[0042] Figure 3 A cross section of another micromechanical structure 1 is shown, which corresponds to the micromechanical structure 1 of Figure 1 and Figure 2 with the exception of the differences described in the following. In this micromechanical structure, no cap 21 is provided. Two stop structures, which are constructed analogously to Figure 2 , are arranged between the seismic mass 3 and the substrate 2, wherein the lugs 61 are arranged on the seismic mass 3, respectively, wherein one of the lugs 61 is arranged on the larger sub-mass 31 and one of the lugs 62 is arranged on the smaller sub-mass 32. The diaphragm structures 62 are arranged on the substrate, respectively. In principle, it is also possible that only one stop structure 6 is present or that the plurality of stop structures 6 is arranged differently, for example, such that the diaphragm structures 62 are arranged on the seismic mass 3 and the lugs 61 are arranged on the substrate 2. Furthermore, a fixed stop can additionally or alternatively also be provided in the region of the smaller sub-mass 32.

[0043] In one embodiment, the depth 67 of the cavity 65 is at most ten times the spacing 68 between the protrusion 61 and the membrane 63. In one embodiment, the depth 67 of the cavity 65 is at most five micrometers. In one embodiment, the thickness 73 of the membrane 63 is at least two hundred nanometers. The thickness 73 can also be at least three hundred or at least four hundred nanometers. The maximum thickness 73 of the membrane 63 can be three micrometers, in particular two micrometers and preferably one micrometer. These mentioned thicknesses 73 can enable an efficient construction of the membrane structure 62.

[0044] Figure 4 A top view of the membrane structure 62 of the micromechanical structure 1 is shown, which can be used, for example, in Figure 2 or Figure 3 The plurality of openings 64 is arranged around a collision point 69 of the membrane 63. The opening spacing 74, i.e. the spacing of the openings 64 above and below the collision point 69, can here be greater than the depth 67 of the cavity 65. The opening spacing 74 can in particular be at least twice the depth 67 of the cavity 65. The opening spacing 74 can be, for example, at least five micrometers.

[0045] Figure 5 A top view of the membrane structure 62 of the micromechanical structure 1 is shown, in which the openings 64 form a spring structure 7 in the membrane 63. The spring structure 7 absorbs the kinetic energy of the seismic mass 3 in a partially elastic collision. It is particularly advantageous if the openings 64 next to the spring structure 7 are implemented narrowly, in particular as at most two micrometers wide. Further, in connection with the spring structure 7 it is advantageous if the thickness 73 of the membrane 63 is implemented to have at least 400 nanometers. The spring structure 7 allows the membrane 63 to be suspended in a soft manner. The membrane 63 moves parallel downward without large self-bending under load, whereby a particularly high damping is achieved.

[0046] In Figure 5 four spring structures 7 are shown, however, a different number of spring structures 7 can also be provided, for example two spring structures 7 opposite the collision point 69

[0047] Figure 6 A top view of the membrane structure 62 of the micromechanical structure 1 is shown, in which the openings 64 surround at least three sides 71 of a cantilever beam 72, wherein the cantilever beam 72 is part of the spring structure 7.

[0048] Figure 7 A top view of the membrane structure 62 of the micromechanical structure 1 is shown, in which the openings 64 also surround at least three sides 71 of a cantilever beam 72, wherein the cantilever beam 72 is part of the spring structure 7. Furthermore, the cantilever beam 72 has a connecting web 75 between the collision point 69 and the spring structure 7, wherein the spring structure 7 is a torsional spring structure.

[0049] Collision point 69 is able to Figure 6 and Figure 7 The diaphragm structure is chosen such that the collision point 69 is close to the side where the cantilever beam 72 connects to the diaphragm structure 62, that is, close to the side of the cantilever beam 72 that is not adjacent to the opening 64. Figure 6 ), or the side closer to the connecting piece ( Figure 7 For example, the collision point 69 can be located within the first third or first quarter of the cantilever beam 72, such that... Figure 6 or Figure 7 Approximately two-thirds, or three-quarters, of the cantilever beam 72 is located to the left of the collision point 69 and... Figure 6 or Figure 7 One-third or one-quarter of the cantilever beam 72 is located to the right of the impact point 69. Therefore, the impact point 69 can be chosen such that the impact-induced deflection of the cantilever beam 72 is more strongly attenuated in a region opposite the fastening point 76. This is supported by the lever assembly of the cantilever beam 72. This arrangement is particularly advantageous for micromechanical structures 1 in which the depth 67 of the cavity 65 cannot be constructed to be small, or in which the active primary surface of the diaphragm 63 does not need to be completely positioned below the vibrating mass 3. It is particularly advantageous that the opening 64 around the cantilever beam 72 is implemented to be narrow, especially in most cases less than two micrometers wide. It is further advantageous that the thickness 73 of the diaphragm 63 is chosen to be four hundred nanometers.

[0050] Figure 8 A cross-section of another micromechanical structure 1 is shown, which, apart from the differences described below, corresponds to... Figure 1 and Figure 2 The micromechanical structure 1 has two stop structures 6, wherein the diaphragm structures 62 of the stop structures 6 are respectively arranged on the larger submass 31 of the vibrating mass 3, and the protrusions 61 are arranged on the substrate 2 and on the cover 21. Figure 8 The fixed stop 8 is also shown. The fixed stop 8 is used to prevent excessive load on the stop structure 6. The fixed stop 8 is installed between the stop structure 6 and the torsion spring 51. In this way, the protrusion 61 impacts the diaphragm structure 62 before the fixed stop 8 strikes, especially when the fixed stop 8 and the stop structure 6 are constructed of layers of the same thickness. Similarly, Figure 3 The spatial arrangement of the micromechanical stop structure 6 and the fixed stop 8 can be similar to Figure 8 The arrangement includes, in particular, the fixed stop 8 between the stop structure 6 and the torsion spring 51, the diaphragm structure 62 on the vibrating mass, and the protrusion 61 on the substrate.

[0051] Figure 9 A micromechanical sensor 9 is shown having a micromechanical structure 1 which can be configured as described in connection with Figures 1 to 8 The micromechanical sensor 9 also comprises optional circuitry 91. The circuitry 91 is arranged to convert the signals generated by means of the detection mechanism 4 into analog or digital output signals.

[0052] Although the application has been described in detail by preferred embodiments, the application is not limited to the examples disclosed and other variants can be derived therefrom by a person skilled in the art without departing from the scope of protection of the application.

Claims

1. A micromechanical structure (1) having a substrate (2) and an oscillating mass (3) movable relative to the substrate (2), wherein A torsion spring (5) connects the seismic mass (3) with the substrate (2), wherein the micromechanical structure (1) further has a detection mechanism (4), wherein a first direction (11) and a second direction (12) essentially perpendicular to the first direction (11) define a main extension plane of the substrate (2), wherein the detection mechanism (4) is arranged for detecting a rotational displacement of the seismic mass (3) about a rotational axis (14), wherein the rotational axis (14) is arranged in the second direction (12), wherein the micromechanical structure (1) further has a stop structure (6), wherein the stop structure (6) is arranged such that a movement of the seismic mass (3) is limited by the stop structure (6), wherein the stop structure (6) is arranged for absorbing kinetic energy of the seismic mass (3) in a partially elastic, partially inelastic collision, wherein the stop structure (6) has a protrusion (61) and a diaphragm structure (62), wherein the diaphragm structure (62) comprises a diaphragm (63), wherein the diaphragm (63) comprises at least one opening (64), wherein the protrusion (61) is able to hit the diaphragm (63), and wherein a movement of the diaphragm (63) causes a fluid (66) arranged in a cavity (65) to escape through the opening (64) and thereby the kinetic energy of the seismic mass (3) is absorbed in a partially inelastic collision.

2. The micromechanical structure (1) according to claim 1, wherein At least 20 percent of the kinetic energy is absorbed inelastically.

3. The micromechanical structure (1) according to claim 1 or 2, wherein The depth (67) of the cavity (65) is at most ten times the distance (68) between the protrusion (61) and the diaphragm (63).

4. The micromechanical structure (1) as claimed in claim 1 or 2, wherein A plurality of openings (64) is arranged around a collision point (69) of the diaphragm (63).

5. The micromechanical structure (1) as claimed in claim 1 or 2, wherein The one opening (64) or the plurality of openings (64) forms a spring structure (7) in the diaphragm (63), wherein the spring structure (7) absorbs kinetic energy of the seismic mass (3) in a partially elastic collision.

6. The micromechanical structure (1) as claimed in claim 5, wherein The opening (64) surrounds at least three sides (71) of a cantilever beam (72), wherein the cantilever beam (72) is part of the spring structure (7).

7. The micromechanical structure (1) according to claim 1 or 2, further having a further protrusion (61) and a further diaphragm structure (62).

8. The micromechanical structure (1) as claimed in claim 1 or 2, having a fixed stop (8) in addition, wherein The stop structure (6) is arranged for absorbing 25% to 75% of the energy stored in the torsion spring (5) at the moment of impact of the fixed stop (8) by inelastic collision.

9. A micromechanical sensor (9) having a micromechanical structure according to any one of claims 1 to 8.

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