MICROMECHANICAL COMPONENT

Abstract

A micromechanical component having a movable seismic mass developed in a second and third silicon functional layer, a hollow body being developed in the second and third silicon functional layers, which has a cover element developed in a fourth silicon functional layer.

Claims

1. A micromechanical component, comprising: a movable seismic mass developed in a second and third silicon functional layer; and a hollow body developed in the second and third silicon functional layers, which has a cover element developed in a fourth silicon functional layer).

2. The micromechanical component as recited in claim 1, wherein first electrodes are developed in a first silicon functional layer, the seismic mass being configured to functionally interact with the first electrodes.

3. The micromechanical component as recited in claim 1, wherein second electrodes are developed in the second silicon functional layer or the third silicon functional layer or the fourth silicon functional layer.

4. The micromechanical component as recited in claim 1, wherein a thickness of the second silicon functional layer, the third silicon functional layer, and the fourth silicon functional layer is greater than approx. 1 m.

5. The micromechanical component as recited in claim 1, wherein a thickness of the third silicon functional layer is greater than approx. 8 m.

6. The micromechanical component as recited in claim 1, wherein a thickness of the third silicon functional layer is at least twice as great as a thickness of the second silicon functional layer and the fourth silicon functional layer.

7. The micromechanical component as recited in claim 1, wherein a layer thicknesses of the second silicon functional layer and the fourth silicon functional layer are similar in a defined manner.

8. The micromechanical component as recited in claim 7, wherein a layer thicknesses of the second silicon functional layer and the fourth silicon functional layers differ maximally by 50%.

9. The micromechanical component as recited in claim 8, wherein a layer thicknesses of the second silicon functional layer and the fourth silicon functional layers differ maximally by 25%.

10. The micromechanical component as recited in claim 1, wherein, at least in sections, a ratio of an area coverage between the second silicon functional layer and fourth silicon functional layer on the one hand, and the third silicon functional layer on the other hand is between three and ten.

11. The micromechanical component as recited in claim 10, wherein, at least in sections, a ratio of an area coverage between the second silicon functional layer and fourth silicon functional layer on the one hand, and the third silicon functional layer on the other hand is five.

12. The micromechanical component as recited in claim 1, wherein the micromechanical component is an acceleration sensor or a rotation-rate sensor.

13. A method for manufacturing a micromechanical component, comprising the following steps: providing a movable seismic mass developed in a second silicon functional layer and a third silicon functional layer; and developing a hollow body in the second silicon functional layer and the third silicon functional layer, which has a cover element developed in a fourth silicon functional layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] FIG. 1 shows a perspective view of a conventional micromechanical z-acceleration sensor.

[0032] FIG. 2 shows the conventional z-acceleration sensor from FIG. 1 in a cross-sectional view.

[0033] FIG. 3 shows a perspective view of another conventional micromechanical z-acceleration sensor.

[0034] FIG. 4 shows the conventional z-acceleration sensor from FIG. 3 in a cross-sectional view.

[0035] FIG. 5 shows a cross-sectional view of another conventional micromechanical z-acceleration sensor.

[0036] FIG. 6 shows an illustration of a problem of a conventional rotation-rate sensor.

[0037] FIG. 7 shows a cross-sectional view of a specific embodiment of a micromechanical z-acceleration sensor provided by the present invention.

[0038] FIG. 8 shows a cross-sectional view of another specific embodiment of a micromechanical z-acceleration sensor provided by the present invention.

[0039] FIG. 9 shows an illustration of a solved problem of a rotation-rate sensor of the invention.

[0040] FIGS. 10A and 10B show a basic sequence of a method for manufacturing a micromechanical component provided by the present invention in multiple partial illustrations.

[0041] FIG. 11 shows a basic sequence of a method for manufacturing a micromechanical component provided by the present invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0042] FIGS. 1, 2 show a conventional micromechanical z-acceleration sensor 100, FIG. 2 representing a simplified sectional view through a plane running perpendicularly to the substrate along the connecting line A-B in FIG. 1. It may be seen that the bottom electrodes 11, 12 developed in first micromechanical functional layer 10 are situated on a first oxide layer, which is situated on a substrate. Furthermore, an asymmetrically developed seismic mass in the shape of a rocker may be seen, which is developed to be rotatable about a torsion axis 33. An additional mass 35 effects an asymmetrical development of the seismic mass.

[0043] Such standard rockers are simply constructed and widely used, but have some technical problems, which hamper applications with very high requirements regarding offset stability. A significant limitation of the offset stability may be brought about by parasitic electrostatic effects, which are explained below.

[0044] For the capacitive evaluation, an electrical effective voltage, for example a pulsed electrical square-wave voltage is applied to the movable structure. In the area of the additional mass, electrostatic forces therefore act between the movable structure and the substrate as soon as an electrical potential difference occurs between the movable structure and the substrate. The forces or the resulting torques result in a parasitic deflection of the rocker. To minimize the electrostatic interaction, an additional conductor track surface is therefore usually situated on the substrate in the area of the additional mass, which has the same potential applied to it as the movable structure.

[0045] Theoretically, a freedom from forces may be achieved thereby between the additional mass and the substrate. In practice, however, significant surface charges or effective surface potentials may be present on the conductor track surface connected to the substrate and/or on the lower side of the movable structure, which can still result in parasitic forces and thus in electrical offset signals. These effects are particularly critical if they change across temperature or service life of the product since this results in offset drifts that cannot be corrected by the final calibration of the component.

[0046] A core idea of the present invention is in particular to create a micromechanical component, in particular an inertial sensor, having an improved offset stability and sensing characteristic.

[0047] In the micromechanical component of the present invention, a symmetrization of sensor masses with respect to parasitic forces (e.g., electrostatic and radiometric forces) is provided when two boundary surfaces exist, both below as well as above movable masses. This is achieved while simultaneously maintaining the mass asymmetries.

[0048] Furthermore, it is possible to exploit the advantages of light construction masses for rotation-rate sensors without having to accept parasitic movements of trough-shaped oscillating masses.

[0049] Furthermore, a surface micromechanical production method is provided for manufacturing hollow masses for movable MEMS structures.

[0050] The mentioned advantages are achieved in accordance with the present invention by a formation of hollow masses for movable MEMS structures, which are formed from three silicon functional layers as well as by a corresponding surface micromechanical production method for manufacturing such hollow masses.

[0051] For micromechanical z-acceleration sensors, it is thus possible to achieve a symmetrization with respect to parasitic forces or torques (e.g., electrostatic or radiometric forces/torque) on the upper and lower sides of the movable structure.

[0052] For rotation-rate sensors, it is possible in this manner to build very light, but at the same time stiff sensor masses, whose z-coordinate of the mass center of the mass is, in contrast to trough-shaped bodies, at the same elevation as the z-coordinate of the mass center of the spring so that in an in-plane-movement no or only extremely weak parasitic z-movements occur.

[0053] By using silicon as functional layer material, it is possible to achieve very favorable mechanical properties having a high temperature stability and service life stability.

[0054] The thicknesses of the silicon functional layers may preferably be selected to be relatively great, in particular greater than 1 m. It is thus possible to build hollow masses that are very stiff and that barely tend to twist or warp.

[0055] It is furthermore advantageous to design at least one of the silicon functional layers, preferably the third silicon functional layer, to be particularly thick in order to achieve great masses, high stiffness values and large capacitance areas. Particularly advantageous are layer thicknesses for the third silicon functional layer greater than 8 m, e.g. 10-50 m.

[0056] FIG. 7 shows a first specific embodiment of a micromechanical component 100 according to the present invention in the form of a z-acceleration sensor. The figure shows the rocker W rotatable about torsion axis 33 having an additional hollow mass 36 on the light rocker side, which is formed from the three silicon functional layers 20, 30, 40. This design ensures a symmetrization of rocker W with respect to torsion axis 33 not only toward the lower boundary surface of the sensor structure (i.e. between first silicon functional layer 10 and second silicon functional layer 20), but also toward the upper boundary surface between fourth silicon functional layer 40 and cap 60 having an insulating oxide layer 61 and a conductive layer 62 (e.g. in the form of polysilicon or metal).

[0057] Advantageously, it is thereby possible to minimize or compensate for radiometric effects with consequences in the form of parasitic deflections of rocker W in the z-direction. Furthermore, this makes it possible to maintain a pronounced mass asymmetry between the left and the right sides of the rocker since the mass on the right rocker side is formed largely (perforation holes are not shown in the figures for the sake of simplicity) from the thick third silicon functional layer 30 and is thus markedly heavier than the left rocker side.

[0058] This also ensures that a high mechanical sensitivity of micromechanical component 100 is maintained.

[0059] FIG. 8 shows another specific embodiment according to the present invention of a micromechanical component 100 in the form of a z-acceleration sensor. In this case, the design is based on the topology of the conventional design from FIG. 4, the trough-shaped mass body on the left rocker side being replaced, in accordance with the present invention, by a hollow mass covered by fourth silicon functional layer 40, which thereby forms additional hollow mass 36. The evaluation stationary electrodes 31, 32 developed in third silicon functional layer 30 continue to exist as in the conventional design of FIG. 4.

[0060] The hollow masses according to the present invention may also be advantageously used in micromechanical components in the form of rotation-rate sensors. In analogy to FIG. 6, FIG. 9 illustrates the oscillatory movement of a driven rotation-rate sensor having two hollow mass bodies m.sub.1 and m.sub.2. In contrast to the conventional design from FIG. 6, the drive movement of the rotation-rate sensor according to the present invention now occurs in good approximation without parasitic z-movement, i.e. essentially in-plane, due to the hollow masses used (in place of the trough-shaped masses in FIG. 6). This is the case at least when the layer thicknesses of the second silicon functional layer 20 and of the fourth silicon functional layer 40 are very similar. Preferably, the layer thicknesses of the second and fourth silicon functional layers 20, 40 differ maximally by 50%, preferably maximally by 25%. This also applies when using the additional hollow mass 36 for z-acceleration sensors. This configuration must thus be regarded as particularly preferred for the rotation-rate sensor (or generally for moved oscillatory masses).

[0061] It is additionally particularly preferred that the layer thickness of the third silicon functional layer is chosen to be greater than 8 m, preferably 10-50 m, while the layer thicknesses of the second and fourth silicon functional layers may be chosen to be markedly smaller. This advantageously makes it possible on the one hand to achieve hollow masses that are flexurally very stiff, to achieve furthermore great mass differences between hollow masses and filled masses, and finally to achieve stiff springs in the third silicon functional layer, the z-coordinate of the spring coinciding with the z-coordinate of the mass center of the hollow mass and parasitic z-movement components being avoided in an in-plane movement.

[0062] As manufacturing method for the spring geometries provided here, it is possible to use a surface micromechanical process described in more detail below, in which the four silicon functional layers 10, 20, 30 and 40 are used, which are preferably formed from polysilicon. The process sequence is shown in FIGS. 10A and 10B in substeps or substep figures a) through j), that is, only for the partial area of the additional hollow mass 36 to be formed.

[0063] In a substep a), a substrate 1 is provided with a first oxide layer 2, the first silicon functional layer 10 and a second oxide layer 3.

[0064] In a substep b), the second silicon functional layer 20 is deposited onto second oxide layer 3 and is patterned by fine trenches.

[0065] In a substep c), a third oxide layer 4 is deposited, which closes the trenches on top. This is followed by further process steps, which have no visible effect in the area of the shown hollow mass, however, and are therefore not shown in the figures, that is, the opening of third oxide layer 4 through fine slits and a subsequent etching step of the second silicon functional layer 20 (preferably by isotropic SF.sub.6 or XeF.sub.2 etching) through the fine oxide openings.

[0066] In substep d), a further oxide layer 5 is deposited, whereby all fine openings in third oxide layer 4 are closed. The advantage of the method lies in the fact that it is possible to clear out large areas of second silicon functional layer 20 without leaving significant topography on the surface of oxide layer 5, as known for example from DE 10 2011 080 978 A1. Subsequently, fourth oxide layer 5 is patterned together with third oxide layer 4 in order to allow for contacts between second silicon functional layer 20 and third silicon functional layer 30.

[0067] In a substep e), third silicon functional layer 30 is deposited and patterned via fine trenches.

[0068] In a substep f), a fifth oxide layer 6 is deposited, and small openings are created in fifth oxide layer 6.

[0069] In an etching step in substep g), which is preferably developed as isotropic SF.sub.6 or XeF.sub.2 etching, sacrificial silicon areas are removed in third silicon functional layer 30.

[0070] As indicated, in substep h), the openings in fifth oxide layer 6 are closed again by another oxide layer 7.

[0071] Subsequently, seventh oxide layer 7 is patterned together with sixth oxide layer 6 in order to provide electrical contacts between third silicon functional layer 30 and fourth silicon functional layer 40.

[0072] In substep i), fourth silicon functional layer 40 is deposited and patterned.

[0073] As indicated in substep j), all sacrificial oxides 6, 7 are removed by oxide etching, preferably using gaseous HF, and the sensor structure is exposed.

[0074] Ultimately, in substeps a) through j) of FIG. 10, the additional hollow mass 36 is formed with perforation holes in second and fourth silicon functional layers 20, 40.

[0075] The provided method offers the possibility of cleaning out large areas of third silicon functional layer 30 and nevertheless covering it almost completely with the (merely slightly perforated) fourth silicon functional layer 40.

[0076] For example, a ratio between the area coverage of second silicon functional layer 20 and fourth silicon functional layer 40 on the one hand and the area coverage of third silicon functional layer 30 on the other hand may be significantly greater than three, a ratio of ten being possible as well. This is achieved by the perforations, created using etching technology, in the mentioned silicon functional layers, which, at least in sections, in second and fourth silicon functional layers 20, 40 make up approx. 10% to approx. 20% and in third silicon functional layer make up approx. 80% to approx. 90% of the entire area coverage.

[0077] FIG. 11 shows a basic sequence of a method for manufacturing a micromechanical component 100 as provided in the present invention.

[0078] In a step 200, a movable seismic mass developed in a second and third silicon functional layer 20, 30 is provided.

[0079] In a step 210, a hollow body 36 is developed in the second and third silicon functional layers 20, 30, which has a cover element developed in a fourth silicon functional layer 40.

[0080] Although the present invention was described above with reference to concrete exemplary embodiments, in particular acceleration and rotation-rate sensors, one skilled in the art is also able to implement specific embodiments that were not disclosed above or that were disclosed above only partially, without deviating from the essence of the invention. It is in particular possible to use the present invention for other micromechanical components such as e.g. resonators, micromirrors or Lorentz magnetometers.