MICROSTRUCTURE AND METHOD OF PRODUCING A MICROSTRUCTURE

Abstract

A microstructure for use in a micro electro-mechanical device comprises a substrate having a top surface and a rear surface and a thin-film structure arranged at the top surface of the substrate. The thin-film structure comprises a raised portion spaced from the substrate, a lower portion of the thin-film structure, which is in mechanical contact with the substrate, at least one protruding portion, the protruding portion being hollow and having at least one sidewall and a bottom part and the protruding portion mechanically connecting the raised portion to the substrate via the bottom part, and at least one further sidewall of the thin-film structure at a distance to the at least one protruding portion, wherein the further sidewall mechanically connects the lower portion with the raised portion of the thin-film structure.

Claims

1. A microstructure for use in a microelectromechanical device comprising a substrate having a top surface and a rear surface and a thin-film structure arranged at the top surface of the substrate, the thin-film structure comprising: a raised portion spaced from the substrate, a lower portion of the thin-film structure, which is in mechanical contact with the substrate, at least one protruding portion being hollow and having at least one sidewall and a bottom part and the protruding portion mechanically connecting the raised portion to the substrate via the bottom part, and at least one further sidewall of the thin-film structure at a distance to the at least one protruding portion, wherein the further sidewall mechanically connects the lower portion with the raised portion of the thin-film structure.

2. The microstructure according to claim 1, wherein the raised portion of the thin-film structure is a movable part of the microstructure.

3. The microstructure according to claim 1, wherein the further sidewall, the raised portion and the substrate enclose a cavity (13) between the substrate and the raised portion.

4. The microstructure according to claim 1, further comprising: an opening in the substrate, extending from the rear surface of the substrate towards the raised portion of the thin-film structure.

5. The microstructure according to claim 1, wherein the thin-film structure is one element of the group comprising a diaphragm, a beam, a lever and a bridge.

6. The microstructure according to claim 1, wherein in top-view the raised portion of the thin-film structure has a circular or rectangular shape.

7. The microstructure according to claim 1, wherein the thin-film structure comprises a plurality of protruding portions.

8. The microstructure according to claim 7, wherein in top-view at least two of the plurality of protruding portions have different shapes.

9. The microstructure according to claim 1, wherein in top-view the at least one protruding portion has a circular, elliptical, rectangular, poly-angular or sickle-shaped shape.

10. The microstructure according to claim 1, wherein the at least one sidewall of the protruding portion and/or the at least one further sidewall of the thin-film structure are perpendicular or transverse with respect to a main plane of extension of the substrate.

11. The microstructure according to claim 1, wherein the raised portion of the thin-film structure is corrugated.

12. An omnidirectional optical microelectromechanical microphone comprising the microstructure according to claim 1.

13. A mobile device comprising the microstructure according to claim 1.

14. A method of producing a microstructure for use in a microelectromechanical device, the method comprising: providing a substrate with at least one sacrificial layer arranged in places at a top surface of the substrate, forming at least one trench within the sacrificial layer, depositing a thin-film on the sacrificial layer, the trench and the substrate, so that the thin-film is in mechanical contact with the substrate in places, and forming a thin-film structure from the thin-film by removing the sacrificial layer, wherein the microstructure comprises the thin-film structure and the substrate, the thin-film structure comprises at least one protruding portion formed by the part of the thin-film (24) deposited within the at least one trench, and the thin-film structure comprises a raised portion spaced from the substrate, where the raised portion is formed by the part of the thin-film deposited on the sacrificial layer.

15. The method according to claim 14, wherein the raised portion of the thin-film structure is a movable part of the microstructure.

16. The method according to claim 14, further comprising: forming an opening in the substrate (2), the opening extending from the rear surface of the substrate towards the raised portion of the thin-film structure.

17. The method according to claim 16, further comprising: removing of the sacrificial layer after forming the opening in the substrate.

18. A microstructure for use in a microelectromechanical device comprising a substrate having a top surface and a rear surface and a thin-film structure arranged at the top surface of the substrate, the thin-film structure comprising: a raised portion spaced from the substrate, a lower portion of the thin-film structure, which is in mechanical contact with the substrate, at least one protruding portion being hollow and having at least one sidewall and a bottom part and the protruding portion mechanically connecting the raised portion to the substrate via the bottom part, and at least one further sidewall of the thin-film structure at a distance to the at least one protruding portion, wherein the further sidewall mechanically connects the lower portion with the raised portion of the thin-film structure, wherein the raised portion of the thin-film structure is a movable part of the microstructure.

19. A mobile device comprising the omnidirectional optical microelectromechanical microphone according to claim 12.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] FIG. 1a shows the result of a finite element analysis (FEA) of an example for a raised microstructure.

[0045] FIG. 1b shows the result of a finite element analysis (FEA) of another example for a raised microstructure.

[0046] FIG. 2 shows the result of a finite element analysis (FEA) of an exemplary embodiment of a microstructure.

[0047] FIG. 3a shows an exemplary embodiment of a thin-film structure.

[0048] FIG. 3b shows a cut-away view through an exemplary embodiment of a thin-film structure.

[0049] FIG. 4 shows a cross-section of an exemplary embodiment of a microstructure.

[0050] FIG. 5 shows a top-view of an exemplary embodiment of a microstructure.

[0051] FIG. 6 shows a cross-section of another exemplary embodiment of a microstructure.

[0052] FIG. 7 shows a cross-section of another exemplary embodiment of a microstructure.

[0053] FIG. 8 shows a cross-section of another exemplary embodiment of a microstructure.

[0054] FIG. 9 shows a schematic diagram of an exemplary embodiment of a mobile device comprising a MEMS microphone, the MEMS microphone comprising the microstructure.

[0055] In FIGS. 10a to 10i an exemplary embodiment of a method of producing a microstructure is shown.

DETAILED DESCRIPTION

[0056] In FIG. 1a the result of a FEA-simulation of an exemplary part of a microstructure 1, which is no embodiment, is shown. The microstructure 1 comprises a thin-film structure 5. In this figure the stress distribution (von Mises stress) of the thin-film structure 5 is evaluated. The thin-film structure 5 comprises a lower portion 6 in contact with a substrate 2, a further sidewall 7 and a raised portion 8. The lower portion 6, the further sidewall 7 and the raised portion 8 comprise the same material. The lower portion 6 and the raised portion 8 have a planar surface, which extend parallel to a main plane of extension of the substrate 2. With respect to the lower portion 6 the raised portion 8 is arranged at an elevated level. The further sidewall 7 connects the lower portion 6 to the raised portion 8. Besides, the further sidewall 7 is perpendicular to both the surface of the lower portion 6 and of the raised portion 8.

[0057] The scale gives the mechanical stress in N/m2. The stress shows high values in the region of the further sidewall 7, especially at the edge of the further sidewall 7 that is adjacent to the lower portion 6. This stress results from the production process and can also come from different coefficients of thermal expansion from the used materials. In turn, the tension can result in a bending moment on the microstructure 1 and a deflection of the raised portion 8. Due to load excursion material fatigue, plastic strain and cracks can occur.

[0058] In FIG. 1b the result of a FEA-simulation with regard to the stress distribution of another exemplary part of a microstructure 1, which is no embodiment, is shown. Also in this example the thin-film structure 5 comprises a lower portion 6 in contact with a substrate 2, a further sidewall 7 and a raised portion 8 with the same properties as described above. Additionally, the thin-film structure 5 has a corrugated further sidewall 7, as known from prior art, e.g. from the United States patent U.S. Pat. No. 6,987,859 B2. This means, the further sidewall 7 does not only extend along one direction but comprises several parts that enclose an angle of 90° with each other. Even if this layout reduces the stress within the further sidewall 7, there are local high stress levels at the top edges of the corrugations.

[0059] With FIG. 2 the FEA stress evaluation of a part of an embodiment for a microstructure 1 comprising a thin-film structure 5 is shown. Elements according to FIG. 2 that correspond to elements of the examples according to FIGS. 1a and 1b are designated with the same reference numerals.

[0060] As in the examples of FIGS. 1a and 1b the thin-film structure 5 according to FIG. 2 comprises a lower portion 6 in contact with a substrate 2, a further sidewall 7 and a raised portion 8 with the same properties as described above. This means, inter alia, that the lower portion 6, the further sidewall 7 and the raised portion 8 comprise the same material.

[0061] Only a top surface 3 of the substrate 2 is shown in FIG. 2. The substrate 2 can have a main plane of extension. The thin-film structure 5 is arranged on the top surface 3 of the substrate 2. In top-view, which means in a plane, which runs parallel to the main plane of extension of the substrate 2, the thin-film structure 5 of this embodiment has a stripe-like shape. The top-view refers to a view on the microstructure 1 from the side of the thin-film structure 5 facing away from the substrate 2.

[0062] The lower portion 6 of the thin-film structure 5 is in contact with the top surface 3 of the substrate 2. The lower portion 6 has a planar surface, which runs parallel to the main plane of extension of the substrate 2.

[0063] The lower portion 6 is connected to the further sidewall 7, which, in this embodiment, is perpendicular to the main plane of extension of the substrate 2 as well as to a main plane of extension of the lower portion 6. The lower portion 6 and the further sidewall 7 form a lower edge in the region where the lower portion 6 is connected to the further sidewall 7. The further sidewall 7 extends in a vertical direction z away from the substrate 2, where the vertical directions z are perpendicular to the main plane of extension of the substrate 2.

[0064] The raised portion 8 of the thin-film structure 5 is connected to the further sidewall 7, so that the raised portion 8 and the further sidewall 7 form an upper edge in the region where the raised portion 8 is connected to the further sidewall 7. In the embodiment shown in FIG. 2 the raised portion 8 has a planar surface, which also runs parallel to the main plane of extension of the substrate 2, but on an elevated level with respect to the lower portion 6. Thus, the raised portion 8 is spaced from the substrate 2. The raised portion 8 and the lower portion 6 do not overlap in a vertical direction z.

[0065] A protruding portion 9 is arranged within the region of the raised portion 8 of the thin-film structure 5. The protruding portion 9 comprises at least one sidewall 10, which extends in a vertical direction z from the raised portion 8 towards the substrate 2. The number of sidewalls 10 depends on the shape of the protruding portion 9. For example, in this case a cross section through the protruding portion 9 in a plane that extends parallel to the main plane of extension of the substrate 2, the protruding portion 9 has the shape of a circle. This means, the protruding portion 9 has one sidewall 10, whereas in case of a rectangular shape in the cross section the protruding portion 9 would have four sidewalls 10 corresponding to the four side surfaces.

[0066] In the embodiment of FIG. 2, the sidewall 10 is perpendicular to the main plane of extension of the substrate 2. The protruding portion 9 of this embodiment has a cylindrical shape. Thus, the protruding portion 9 comprises a bottom part 11, which in this case has a circular shape. The bottom part 11 is in mechanical contact with the substrate 2.

[0067] The protruding portion 9 forms a hollow profile, in this case a hollow cylinder. This means, the protruding portion 9 encloses a volume. At the top side of the protruding portion 9, the enclosed volume is connected to the environment of the thin-film structure 5, where the top side of the protruding portion 9 faces away from the substrate 2. This means, the protruding portion 9 is open at the top side. The raised portion 8 has an aperture 12 for the protruding portion 9.

[0068] As shown in FIG. 2, the stress distribution is significantly reduced in comparison to the examples of FIG. 1a and FIG. 1b. Thus, the proposed construction of a thin-film structure 5, in particular of a raised thin-film structure 5 including a protruding portion 9, improves the mechanical properties.

[0069] FIG. 3a shows another embodiment of the thin-film structure 5.

[0070] The thin-film structure 5 has the shape of a square in top-view. However, the raised portion 8 of the thin-film structure 5 has a circular shape in top-view. In all lateral directions x, y the raised portion 8 is connected to the lower portion 6 via a ring-shaped further sidewall 7. Close to the further sidewall 7 the raised portion 8 comprises cylindrical shaped protruding portions 9. In this embodiment the thin-film structure 5 comprises a plurality of protruding portions 9, whereas each protruding portion 9 is placed equidistantly to its neighbored protruding portions 9. This way, the plurality of protruding portions 9 forms a ring of protruding portions 9 along the periphery of the raised portion 8. This arrangement of the thin-film structure 5 forms a diaphragm. This means that the substrate 2, the further sidewall 7 and the raised portion 8 enclose a cavity 13.

[0071] With FIG. 3b a cut through the embodiment of the thin-film structure 5 of FIG. 3a is shown. On the one hand the cavity 13, which is covered by the thin-film structure 5, is shown. On the other hand also the protruding portions 9 extending towards the substrate 2, which is not shown in this representation, are visible.

[0072] FIG. 4 shows a schematic cross-section through an embodiment of the microstructure 1.

[0073] The microstructure 1 comprises a substrate 2 with a top surface 3 and a rear surface 4. An opening 14 penetrates the substrate 2 and connects the rear surface 4 to the top surface 3. The depth dSi of the opening 14 corresponds to the thickness of the substrate 2. The depth dSi can be, for example, at least 100 micrometer and at most 750 micrometer. However, the depth dSi should be chosen as large as possible and can extend 750 micrometer and be, for example, at most 2.5 millimeter.

[0074] On the top surface 3 of the substrate 2 the thin-film structure 5 is arranged. In the cross-sectional view, the thin-film structure 5 comprises a lower portion 6 at both sides of the opening 14. The lower portion 6 is in mechanical contact with the top surface 3 of the substrate 2.

[0075] The microstructure 1 also comprises a further sidewall 7 at both sides of the opening 14 in the cross-sectional view. The height ds of the further sidewall 7 corresponds to the height of the raised portion 8 and depends on the application. The height ds of the further sidewall 7 can be at least 0.5 micrometer and at most 10 micrometer. Alternatively, the height ds of the further sidewall 7 can be at least 0.6 micrometer and at most 4 micrometer.

[0076] The microstructure 1 also comprises a raised portion 8 of the thin-film structure 5 spanning the opening 14 in the substrate 2. The diameter of the raised portion 8 from one further sidewall 7 to the other opposing further sidewall 7 depends on the application. For example, in case of a circular shaped raised portion 8 forming a diaphragm, the diameter of the raised portion 8 may be at least 100 micrometer and at most 10 millimeter. Alternatively, the diameter of the raised portion 8 may be at least 500 micrometer and at most 1 millimeter.

[0077] At a distance dd to the further sidewall 7 the microstructure 1 comprises a protruding portion 9. The distance dd between the protruding portion 9 and the further sidewall 7 depends on the application. For example, the distance dd between the protruding portion 9 and the further sidewall 7 can be at least 0.5 micrometer and at most 500 micrometer.

[0078] Alternatively, the distance dd between the protruding portion 9 and the further sidewall 7 can be at least 6 micrometer and at most 20 micrometer.

[0079] The diameter dp of the protruding portion 9 in a plane that extends parallel to the main plane of extension of the substrate 2 depends on the application. For example, in case of a cylindrical protruding portion 9 the diameter dp can be at least 0.5 micrometer and at most 50 micrometer. Alternatively, the diameter dp of the protruding portion 9 can be at least 2 micrometer and at most 8 micrometer.

[0080] The raised portion 8 is integrally formed with the lower portion 6, the protruding portion 9 and the further sidewall 7. This means, the raised portion 8, the lower portion 6, the protruding portion 9 and the further sidewall 7 are formed by one layer.

[0081] According to the embodiment of FIG. 4 the opening 14 in the substrate 2 is arranged in a vertical direction z under the raised portion 8. However, the extent of the opening 14 in the substrate 2 in lateral directions x, y is smaller than the extent of the raised portion 8. This way, protruding portions 9, which are typically placed close to the further sidewall 7, can be connected to the underlying substrate 2. The protruding portion 9 can be arranged at a distance dsq from the opening 14. In case of the raised portion 8 being a membrane the distance dsq affects the damping of the membrane during vibration. Thus, the distance dsq depends on the application. For example, the distance dsq can be at least 0.5 micrometer and at most 100 micrometer. Alternatively, the distance dsq can be at least 2 micrometer and at most 8 micrometer.

[0082] FIG. 5 shows a top-view of an embodiment of a microstructure 1 comprising a raised thin-film structure 5. It should be noted that FIG. 5 can be regarded as a top-view of the embodiment of FIG. 4.

[0083] The embodiment according to FIG. 5 comprises, in top-view, a square-shaped substrate 2 with a circular shaped opening 14 in the center of the substrate 2. Additionally, the embodiment comprises a circular shaped thin-film structure 5 on top of the substrate 2. The thin-film structure 5 comprises a ring-shaped lower portion 6 in contact with the substrate 2. The ring-shaped lower portion 6 surrounds the entire thin-film structure 5.

[0084] The raised portion 8 forms an inner circular area of the thin-film structure 5. In particular, the raised portion 8 forms a circle, which concentrically overlaps the opening 14 in the substrate 2.

[0085] Additionally, the embodiment of FIG. 5 shows a plurality of protruding portions 9 arranged around the opening 14. In this embodiment, the shapes of the protruding portions 9 vary by being circular or rectangular-shaped in top-view, respectively.

[0086] FIGS. 6, 7 and 8 show cross-sectional views of other exemplary embodiments of the microstructure 1.

[0087] The embodiment according to FIG. 6 differs from the embodiment according to FIG. 4 in the absence of a further sidewall 7 at one side of the raised portion 8 in a lateral direction x. The further sidewall 7 is arranged at only one side of the raised portion 8. Thus, the thin-film structure 5 shown in FIG. 6 can be regarded as a lever or a beam.

[0088] The embodiment according to FIG. 7 differs from the embodiment according to FIG. 6 in the absence of an opening 14 in the substrate 2. Since raised thin-film structures 5 are usually fabricated by use of a sacrificial layer, the sacrificial layer has to be removed at some point of the process. When, as in this embodiment, no opening 14 in the substrate 2 is provided, there is no access to the sacrificial layer from the rear surface 4 of the substrate 2. In this case, the sacrificial layer can be removed via wet-etching trough vents 15 in the thin-film structure 5. For example, these vents 15 are formed by regions without a further sidewall 7 of the thin-film structure 5. Such a region is given in this embodiment at one side of the raised portion 8 in the lateral direction x.

[0089] The embodiment according to FIG. 8 differs from the embodiment according to FIG. 4 in the absence of an opening 14 in the substrate 2. The thin-film structure 5 shown in FIG. 8 can be regarded as a bridge over the substrate 2. As stated above, the sacrificial layer can be removed via wet-etching trough vents 15 in the thin-film structure 5, for example vents 15 in a lateral direction y. Besides, the embodiment shows both sidewalls 10 and further sidewalls 7 to be transverse with respect to the main plane of extension of the substrate 2.

[0090] FIG. 9 shows a schematic diagram of an exemplary embodiment of a mobile device 17, comprising an omnidirectional optical MEMS microphone 16, which in turn comprises a microstructure 1 as discussed above. The mobile device 17 can be, for example, a smart speaker device, a smart watch, a phone or a hearing aid device.

[0091] With FIGS. 10a to 10i an exemplary embodiment of a method of producing a microstructure 1 is shown. The method relates to the fabrication of a membrane for use in an omnidirectional optical MEMS microphone 16.

[0092] The method comprises providing a substrate 2, as shown in FIG. 10a. The substrate 2 has a rear surface 4 and a top surface 3. The substrate 2 can comprise Si. On the top surface 3 of the substrate 2 a dielectric layer 18 is arranged in places. The dielectric layer 18 can comprise, for example, silicon-oxide (SiO.sub.2). However, a part of the top surface 3 of the substrate 2 is free of the dielectric layer 18, so that the dielectric layer 18 forms an aperture towards the substrate 2.

[0093] In a next step an etch stop layer 19 is deposited on the top surface 3 of the substrate 2 in places (FIG. 10b). The etch stop layer 19 may comprise Cr. The etch stop layer 19 is arranged in a central region of the aperture formed by the dielectric layer 18. The etch stop layer 19 is provided for forming an opening 14 in the substrate 2 as described below.

[0094] In a next step a reflective layer 20 is deposited in a central region on top of the etch stop layer 19 (FIG. 10c). The reflective layer 20 may comprise gold (Au) or titanium (Ti). The reflective layer 20 has a rear surface facing the etch stop layer 19, a top surface facing away from the substrate 2 and side surfaces. The reflective layer 20 is provided as a mirror, which at the end of the process is attached to the membrane. By using the mirror the deflection of the membrane can be evaluated optically. This means that a laser beam, which is reflected at the mirror, can be analyzed by use of an interferometer.

[0095] FIG. 10d shows the deposition of a first sacrificial layer 21. The first sacrificial layer 21 may comprise W—Ti. The first sacrificial layer 21 is deposited on the dielectric layer 18, the top surface 3 of the substrate 2, the etch stop layer 19 and the reflective layer 20. By a patterning step the first sacrificial layer 21 is removed on parts of the etch stop layer 19 to release the etch stop layer 19. The patterning of the first sacrificial layer 21 is provided to achieve a corrugated second sacrificial layer, which in turn leads to a corrugated membrane as described below.

[0096] FIG. 10e shows the deposition of the second sacrificial layer 22 on top of the first sacrificial layer 21 and on the released parts of the etch stop layer 19. The second sacrificial layer 22 may also comprise W—Ti. Because of the underlying topography the second sacrificial layer 22 has a corrugated top surface. This means that in places the second sacrificial layer 22 has no planar surface, which is parallel to the main plane of extension of the substrate 2. Instead, the second sacrificial layer 22 exhibits regions, which extend in a vertical direction z.

[0097] In a patterning process both sacrificial layers 21, 22 are removed on the dielectric layer 18. The sacrificial layers 21, 22 are also removed in regions adjacent to the dielectric layer 18 in lateral directions x, y in order to release the substrate 2. The sacrificial layers 21, 22 are also removed on a central region above the reflective layer 20 in order to release the reflective layer 20. This way, the reflective layer 20 is free of the sacrificial layers 21, 22 at its top surface as well as at its side surfaces.

[0098] Besides, one or more trenches 23 are formed within the stack of sacrificial layers 21, 22 in places where no etch stop layer 19 is present underneath. The trenches 23 extend towards the substrate 2 in order to release the substrate 2. The trenches 23 are provided to form protruding portions 9 of the thin-film structure 5 after the complete removal of the sacrificial layers 21, 22.

[0099] In a next step a thin-film 24 is deposited on the dielectric layer 18, the released substrate 2, the second sacrificial layer 22 and the reflective layer 20 (FIG. 10f). The thin-film may comprise SiN. The thin-film is removed on parts of the dielectric layer 18 in order to form a thin-film structure 5 of a desired shape. Besides, a vent hole 25 can be implemented by removing the thin-film in a small region above the second sacrificial layer 22.

[0100] In order to form an opening 14 the substrate 2 must be turned around and back-side treated (FIG. 10g). An opening 14 is formed from the rear surface 4 to the top surface 3 of the substrate 2 in the region of the etch stop layer 19. However, the lateral extent of the opening 14 may be smaller than the lateral extent of the etch stop layer 19.

[0101] In FIG. 10h the removal of the etch stop layer 19 is shown. Since the lateral extent of the opening 14 is smaller than the lateral extent of the etch stop layer 19 a small amount of the etch stop layer 19 remains at the edges on the top surface 3 of the substrate 2. This remaining etch stop layer 19 can be used as a damping structure 26 for the vibrating membrane.

[0102] In the next step the sacrificial layers 21, 22 are removed and the device can be turned around for possible further front-side treatment. The thin-film structure 5 forms a membrane for use in an omnidirectional optical MEMS microphone 16.

[0103] The resulting microstructure 1 (FIG. 10i) is similar to the embodiment of FIG. 4. The microstructure 1 differs from the embodiment of FIG. 4 in the following:

[0104] A dielectric layer 18 is arranged around the opening 14 in the substrate 2 at a distance to the opening 14. The thin-film structure 5 comprises a portion which covers a part of the top surface of the dielectric layer 18 and the side surface of the dielectric layer 18, which points in lateral directions x, y towards the opening 14.

[0105] The lower portion 6 of the thin-film structure 5, which is in mechanical contact with the substrate 2, is arranged in a region adjacent to the dielectric layer 18 in lateral directions x, y towards the opening 14. The lower portion 6 is connected with the portion of the thin-film structure 5, which covers the side surface of the dielectric layer 18.

[0106] The raised portion 8 of the thin-film structure 5 is corrugated. This means that the raised portion 8 is not planar, but has both regions, which are parallel, and regions, which are perpendicular to the main plane of extension of the substrate 2.

[0107] Additionally, the raised portion 8 of the thin-film structure 5 comprises a central region, where the reflective layer 20 is attached to the raised portion 8. The reflective layer 20 is attached on the side of the raised portion 8, which faces the opening 14 in the substrate 2. This means that the rear surface of the reflective layer 20 is free of other layers. The top surface as well as the side surfaces of the reflective layer 20 are covered by the thin-film structure 5.

[0108] The raised portion 8 of the thin-film structure 5 also comprises a vent hole 25 in a small region at the periphery of the raised portion 8. The vent hole 25 is provided for a gas exchange from one side of the thin-film structure 5 to the other. This can be necessary since due to the membrane deflection the change of gas density can affect the system compliance. Thus, the vent hole provides pressure equalization between both sides of the thin-film structure. Additionally, the vent hole affects the frequency range of the device. In order to achieve a high acoustic resistance the diameter of the vent hole should be small.

[0109] Besides, the microstructure 1 comprises a damping structure 26 due to the remaining etch stop layer 19. The damping structure 26 is arranged at the edges of the opening 14 on the top side of the substrate 2. The damping structure 26 provides a protection from strong deflection of the membrane.

[0110] However, as in the embodiment of FIG. 4, the embodiment of FIG. 10i comprises protruding portions 9. The protruding portions 9 are formed by the part of the thin-film deposited within the trenches 23 in the sacrificial layers 21, 22. The protruding portions 9 give the membrane additional stability, since they connect the raised portion 8 to the substrate 2 and inherently increase the sidewall area.

[0111] The embodiments disclosed herein have been discussed for the purpose of familiarizing the reader with novel aspects of the idea. Although preferred embodiments have been shown and described, many changes, modifications, equivalents and substitutions of the disclosed concepts may be made by one having skill in the art without unnecessarily departing from the scope of the claims.

[0112] It will be appreciated that the disclosure is not limited to the disclosed embodiments and to what has been particularly shown and described hereinabove. Rather, features recited in separate dependent claims or in the description may advantageously be combined. Furthermore, the scope of the disclosure includes those variations and modifications, which will be apparent to those skilled in the art and fall within the scope of the appended claims.

[0113] The term “comprising”, insofar it was used in the claims or in the description, does not exclude other elements or steps of a corresponding feature or procedure. In case that the terms “a” or “an” were used in conjunction with features, they do not exclude a plurality of such features. Moreover, any reference signs in the claims should not be construed as limiting the scope.