Micromechanical system and corresponding manufacturing method

The present application claims priority to and the benefit of German patent application no. 10 2011 006 403.6, which was filed in Germany on Mar. 30, 2011, the disclosure of which is incorporated herein by reference.

The present invention relates to a micromechanical system and a corresponding manufacturing method, in particular for use in micromechanical acceleration sensors, yaw rate sensors, and the like.

Situating a thick polysilicon functional layer over a thin buried polysilicon layer is generally known for manufacturing MEMS structures. The buried polysilicon layer is used in this case as a printed conductor or an electrode. The polysilicon functional layer is typically exposed via a trenching process and a sacrificial layer etching method.

A micromechanical system and a corresponding manufacturing method are known from DE 10 2007 060 878 A1, a second buried polysilicon layer being provided, which may be exposed like the first polysilicon functional layer. This layer may be used as a printed conductor, so that, for example, printed conductor intersections are also possible as a printed conductor in combination with the first buried polysilicon layer. The second buried polysilicon layer may also be used as a mechanical freestanding layer. The three layers are separated by oxide layers, which are also used as sacrificial layers. The oxide layers define the distance between the individual layers and are designed to be relatively thick, depending on the application. If connections are applied between the layers, a tall topology is created on the surface, which makes it more difficult to manufacture structures at high resolution. The smaller the contact surfaces are designed, the less the influences by the elevated topography; however, the mechanical stability of the connection is thus simultaneously greatly reduced.

On the one hand, the oxide layers between the individual polysilicon layers may be intentionally used for mechanical reinforcement. On the other hand, mechanical tensions may occur due to the different mechanical parameters, however, which may also change with the temperature. Furthermore, depending on the geometry, it may be more or less difficult to completely remove the oxides between the polysilicon layers during the sacrificial layer etching process.

The exemplary embodiments and/or exemplary methods of the present invention provide a micromechanical system as described herein and a corresponding manufacturing method as described herein.

Further refinements are the subject matter of the further particular descriptions herein.

The exemplary embodiments and/or exemplary methods of the present invention have the advantage that it allows contact attachments, which are mechanically stable, to be manufactured on the substrate, a low topography to be produced simultaneously, also in the case of high sacrificial layer thicknesses, and above all a micromechanical system to be provided, which may avoid all undesirable oxide areas below the lowermost functional layer.

One aspect of the exemplary embodiments and/or exemplary methods of the present invention is not to produce a continuous planar connection between the functional layers, but rather to connect the functional layers to one another via weblike anchoring structures. These weblike anchoring structures may have a web ring structure and/or a web line structure of arbitrary geometry, for example. The width of the webs and their spaces are selected in such a way that, on the one hand, a stable mechanical connection may be achieved in consideration of the intended area of application and, on the other hand, reliable removal of the sacrificial layer from the spaces between the individual web sections may be ensured. The weblike anchoring structures may be designed as sufficiently narrow that a low topography forms on the surface of the web segments.

The first weblike anchoring structure between the first functional area of the first functional layer and the second functional area of the second functional layer and the second weblike anchoring structure between the second functional area of the second functional layer and the third functional area of the third functional area need have no overlap or only intersection-like overlaps. This has a very advantageous effect on the stability, since the topographies of the two contact planes only act marginally or not at all.

Two types of etching holes may be provided in the third functional area of the third functional layer. First etching holes are situated laterally offset on the second weblike anchoring structure and extend up to the third sacrificial layer. Second etching holes are situated over the second weblike anchoring structure and extend through the second weblike anchoring structure up to the second sacrificial layer. Depending on the process, it may prove to be favorable to design the second etching holes as wider than the weblike anchoring structure situated underneath, so that the third oxide layer virtually acts as a self-adjusting mask. By providing these two types of etching holes, both a tight arrangement of the web segments of the anchoring structures and also simultaneously a tight arrangement of the etching accesses may be achieved.

Narrow weblike anchoring structures may be used if the third functional area only receives slight polishing or no polishing or if in general a low topography is desirable or advantageous. Oblong second contact holes which extend perpendicularly to the web segments are particularly favorable, whereby process variations, such as adjustment errors, etching errors, etc., may be well compensated for.

Wide webs may be used if the third functional area experiences good, deep polishing or the topography plays a subordinate role. The second etching holes may then be situated inside the webs and are implemented as significantly narrower than the width of the webs, in order to thus also be able to compensate for processing variations.

Further features and advantages of the exemplary embodiments and/or exemplary methods of the present invention are explained hereafter on the basis of specific embodiments with reference to the figures.

FIGS. 1 a , 1 b , 2 a , 2 b , 3 a , 3 b , 4 a , 4 b , 5 a , 5 b , 6 a , and 6 b show successive processing stages of a first specific embodiment of a method for manufacturing a micromechanical system according to the present invention, a) each in a top view and b) each in section along line AA′ in a).

FIGS. 7 a , 7 b , 8 a , 8 b , 9 a , 9 b , 10 a , 10 b , 11 a , and 11 b show successive processing stages of a second specific embodiment of a method for manufacturing a micromechanical system according to the present invention, a) each in a top view and b) each in section along line AA′ in a).

FIGS. 1 a, b through FIGS. 6 a, b show successive processing stages of a first specific embodiment of a method for manufacturing a micromechanical system according to the present invention, a) each in a top view and b) each in section along line AA′ in a).

In FIG. 1 , reference sign S identifies a silicon substrate, for example, a wafer substrate. A first sacrificial layer O 1 made of oxide is provided by deposition, for example, on silicon substrate S. P 1 identifies a first micromechanical functional area made of polysilicon, which has been structured on first sacrificial layer O 1 .

Furthermore, with reference to FIG. 2 , a second sacrificial layer O 2 made of oxide is deposited over first micromechanical functional area P 1 and surrounding first sacrificial layer O 1 , which forms a step K 1 on first micromechanical functional area P 1 .

Subsequently, a first weblike trench structure RG 1 , RG 2 is formed in second sacrificial layer O 2 over first micromechanical functional area P 1 , which exposes corresponding parts of first micromechanical functional area P 1 . In the first specific embodiment, first weblike trench structure RG 1 , RG 2 has the form of ring trenches RG 1 , RG 2 , larger ring trench RG 1 enclosing smaller ring trench RG 2 , having a distance situated between them and being spaced apart from the edges of first micromechanical functional area P 1 , the latter being shown in particular in FIG. 2 b ).

As shown in FIG. 3 , a second micromechanical functional area P 2 made of polysilicon is formed above first micromechanical functional area P 1 on second sacrificial layer O 2 . A first weblike anchoring structure in the form of contact rings KR 1 , KR 2 , via which second micromechanical functional area P 2 is connected to first micromechanical functional area P 1 , is created in first weblike trench structure RG 1 , RG 2 .

Contact rings KR 1 , KR 2 , which form the first weblike anchoring structure, are selected with respect to their web width and their distance or their number in accordance with the particular use and the intended stability. It is advantageous to select the thickness of first micromechanical functional area P 1 made of polysilicon to be as thin as possible and to select the distance of first contact ring KR 1 to oxide edge K 1 of the second sacrificial layer to be as large as the layer thickness of first micromechanical functional area P 1 .

Furthermore, with reference to FIG. 4 , a third sacrificial layer O 3 is deposited on second micromechanical structure P 2 and on surrounding second sacrificial layer O 2 over the structure according to FIG. 3 , which forms a step K 2 on second micromechanical functional area P 2 .

Subsequently, a second weblike trench structure RG 3 , RG 4 is formed in third sacrificial layer O 3 above second micromechanical functional area P 2 , which exposes corresponding parts of second micromechanical functional area P 2 . Second weblike trench structure RG 3 , RG 4 has a third ring trench RG 3 and a fourth ring trench RG 4 , which are offset in relation to contact rings KR 1 , KR 2 situated underneath and are nested, like first and second contact rings KR 1 , KR 2 .

The topologies of the two anchoring structures may be completely decoupled by this arrangement, since there are no overlaps.

Furthermore, with reference to FIG. 5 , a third micromechanical structure P 3 is formed on second sacrificial layer O 2 above the structure according to FIG. 4 , which has a first subarea P 3 a and a second subarea P 3 b , second subarea P 3 b being spaced apart laterally from the first and second weblike anchoring structures, and first and second micromechanical functional areas P 1 , P 2 and being designed to be significantly narrower than first subarea P 3 a . Furthermore, third micromechanical functional area P 3 made of polysilicon is substantially thicker than first and second micromechanical functional areas P 1 , P 2 .

During the manufacturing of third micromechanical functional area P 3 by corresponding trench etching, corresponding first, second, and third etching holes L 1 , L 2 , L 3 are also formed in third micromechanical functional area P 3 , first and second etching holes L 1 , L 2 being formed in first subarea P 3 a , and third etching holes L 3 being formed in second subarea P 3 b.

During the formation of third micromechanical functional area P 3 on the second sacrificial layer, a second weblike anchoring structure KR 3 , KR 4 is created in second weblike trench structure RG 3 , RG 4 , which has the form of contact rings KR 3 , KR 4 .

First etching holes L 1 are situated laterally offset to second weblike anchoring structure KR 3 , KR 4 in the form of contact rings KR 3 , KR 4 and extend up to the top side of third sacrificial layer O 3 . Second etching holes L 2 are situated directly over second weblike anchoring structure KR 3 , KR 4 , penetrate it, and extend up to the top side of second sacrificial layer O 2 . Second etching holes L 2 have a narrower width than the web width of second weblike anchoring structure KR 3 , KR 4 . Third etching holes L 3 penetrate second subarea P 3 b and end also on the top side of third sacrificial layer O 3 .

As shown in FIG. 6 , sacrificial layer etching of first, second, and third sacrificial layers O 1 , O 2 , O 3 is then performed, second and third sacrificial layers O 2 , O 3 being completely removed. The latter is ensured in particular by the selected shape of the first and second etching holes in first subarea P 3 a . A part of first sacrificial layer O 1 remains as an anchor of first micromechanical functional area P 1 on silicon substrate S. It is advantageous to select a specific distance d of outer contact ring KR 1 of the first weblike anchoring structure from a final outer edge A of remaining first sacrificial layer O 1 , as indicated in FIG. 6 , in order to avoid tensions in this area.

Second subarea P 3 b of third micromechanical functional area P 3 is mounted floating over silicon substrate S after the sacrificial layer etching over first subarea P 3 a . Such a structure may be used, for example, as a bending bar structure in an acceleration sensor or yaw rate sensor or similar sensors.

The described manufacturing method allows a mechanically robust suspension, which is distinguished in particular in that it has no predeflection and the deflection is also not influenced via temperature or other external influences, e.g., water accumulation in the oxide. The suspensions only have a low topography and may be implemented well in existing processes.

FIGS. 7 a, b through FIGS. 11 a, b show successive processing stages of a second specific embodiment of a method for manufacturing a micromechanical system according to the present invention, a) each in a top view and b) each in section along line AA′ in a).

The view in FIG. 7 corresponds to the view in FIG. 2 in this second specific embodiment, the first weblike trench structure having line trenches LG 1 , LG 2 , LG 3 , LG 4 , LG 5 , which are situated in parallel and spaced apart from one another over first micromechanical functional area P 1 and expose corresponding parts thereof. Otherwise, the processing state according to FIG. 7 corresponds to the processing state according to FIG. 2 .

Furthermore, with reference to FIG. 8 , which corresponds to FIG. 3 of the first specific embodiment, a first weblike anchoring structure KS 1 , KS 2 , KS 3 , KS 4 , KS 5 in line trenches LG 1 , LG 2 , LG 3 , LG 4 , LG 5 of the first weblike trench structure in the form of linear webs is created through the formation of second micromechanical functional area P 2 , the linear webs also being spaced apart from oxide edge K 1 or the lateral edge of first micromechanical functional area P 1 .

According to FIG. 9 , which is similar to FIG. 4 , a third sacrificial layer O 3 made of oxide is deposited over the structure according to FIG. 8 and a second weblike trench structure is structured therein in the form of parallel line trenches LG 6 , LG 7 , LG 8 , LG 9 , LG 10 , line trenches LG 6 through LG 10 being situated above the spaces of contact webs KS 1 through KS 5 , so that they do not overlap. The corresponding anchoring structures are therefore also decoupled from one another.

Furthermore, with reference to FIG. 10 , which identifies the processing state similar to FIG. 6 , third micromechanical functional area P 3 is formed from polysilicon and structured, second weblike anchoring structure KS 6 , KS 7 , KS 8 , KS 9 , KS 10 being formed in the second weblike trench structure.

In this second specific embodiment, first, second, and third etching holes L 1 ′, L 2 ′, and L 3 ′ are also formed in third micromechanical functional area P 3 , which has subareas P 3 a and P 3 b.

First and second etching holes L 1 ′, L 2 ′ are formed in subarea P 3 a , the first etching holes being formed laterally offset to second weblike anchoring structure KS 6 through KS 10 and extending up to the top side of third sacrificial layer O 3 , and the second etching holes being situated over second weblike anchoring structure KS 6 through KS 10 and thereby extending up to the top side of second sacrificial layer O 2 .

In contrast to the first example, first and second etching holes L 1 ′, L 2 ′ have an oblong shape, second etching holes L 2 ′ having a greater longitudinal width than contact webs KS 6 through KS 10 of the second weblike anchoring structure, which are situated underneath. Self-adjustment may thus be achieved.

Third etching holes L 3 ′ in second subarea P 3 b correspond to etching holes L 3 , which were already explained above in connection with the first specific embodiment.

To achieve the final state according to FIG. 11 , which corresponds to FIG. 6 , the mentioned sacrificial layer etching is performed, second and third sacrificial layers O 2 , O 3 being completely removed, and a part of first sacrificial layer O 1 remaining as the anchor of first micromechanical functional area P 1 on silicon substrate S. Reference sign d′ identifies the preferred distance of outermost webs KS 1 , KS 5 from outer edge A′ of the remaining residue of first sacrificial layer O 1 in FIG. 11 b ).

With the exception of the different design of the particular first and second weblike anchoring structures, the micromechanical system according to FIG. 11 is configured identically to the micromechanical system according to FIG. 6 and may be used as a bending bar in an acceleration sensor or a yaw rate sensor or a similar sensor, for example.

Although the exemplary embodiments and/or exemplary methods of the present invention was explained above on the basis of two exemplary embodiments, it is not restricted thereto, but rather is variable in manifold ways.

In particular, the web widths and geometries of the first and second anchoring structures are variable specifically for the application and are not restricted to the ring shape or line shape shown. The distances between the webs or web segments may also be varied depending on the application. Furthermore, the mentioned layer materials for the sacrificial layers and the functional layers are not restricted to the mentioned materials.