Abstract
A membrane structure comprises a substrate having a main surface and a rear surface. A plurality of pillars are arranged on the main surface of the substrate and have a support area facing away from the main surface of the substrate. A thin-film structure is arranged above the main surface of the substrate and the pillars, wherein the thin-film structure comprises a plurality of raised portions that are spaced further from the substrate than at least one lower portion of the thin film structure. The raised portions each comprise at least one protruding portion, the protruding portions being hollow and having a bottom part and a sidewall and the protruding portions extending towards the substrate. The bottom part of each protruding portion is mechanically connected to the support area of one of the pillars, respectively. A back-volume is formed by the volume between the main surface of the substrate and the thin-film structure.
Claims
1. A membrane structure comprising: a substrate having a main surface and a rear surface, a plurality of pillars, the pillars being arranged on the main surface of the substrate and having a support area facing away from the main surface of the substrate, and a thin-film structure arranged above the main surface of the substrate and the pillars, wherein the thin film structure comprises a plurality of raised portions that are spaced further from the substrate than at least one lower portion of the thin film structure, the raised portions each comprise at least one protruding portion, the protruding portions being hollow and having a bottom part and a sidewall and the protruding portions extending towards the substrate, the bottom part of each protruding portion is mechanically connected to the support area of one of the pillars, respectively, and a back-volume is formed by the volume between the main surface of the substrate and the thin-film structure.
2. The membrane structure according to claim 1, wherein the thin-film structure forms a membrane, and wherein each pair of pillars and protruding portions forms a mechanical phase shifter during membrane deflection.
3. The membrane structure according to claim 1, wherein the pillars and the protruding portions of the thin-film structure are arranged at a predetermined distance to an outer edge of the thin-film structure.
4. The membrane structure according to claim 1, wherein each raised portion is completely surrounded by the at least one lower portion in lateral directions, that extend parallel to a main plane of extension of the substrate, each raised portion being connected to the at least one lower portion by a connecting portion.
5. The membrane structure according to claim 1, wherein each raised portion extends to the outer edge of the thin-film structure and is connected to the at least one lower portion by a connecting portion.
6. The membrane structure according to claim 1, further comprising: at least one enclosing wall arranged on the main surface of the substrate at a distance to the outer edge of the thin-film structure, the enclosing wall surrounding the thin-film structure, that extend parallel to a main plane of extension of the substrate.
7. The membrane structure according to claim 6, further comprising: at least one elastic layer mechanically connecting the thin-film structure to the enclosing wall.
8. The membrane structure according to claim 7, wherein the elastic layer is gas-permeable.
9. The membrane structure according to claim 1, further comprising: an opening in the substrate extending from the rear surface towards the thin-film structure at the main surface of the substrate.
10. The membrane structure according to claim 1, wherein the membrane structure comprises a vent hole within the thin-film structure.
11. The membrane structure according to claim 1, wherein the thin-film structure has a circular or poly-angular shape in a top-view.
12. A transducer device comprising the membrane structure according to claim 1, wherein the transducer device is in particular an optical omnidirectional microphone or any other dynamic pressure sensing device.
13. A method of producing a membrane structure, the method comprising: providing a substrate, forming a plurality of pillars at a main surface of the substrate, forming a sacrificial layer region at the main surface of the substrate and on the pillars, providing at least a first thickness level and a second thickness level of the sacrificial layer region, the first thickness level being larger than the second thickness level, forming a plurality of trenches within the sacrificial layer region with the first thickness level, where each trench extends towards one of the pillars, respectively, depositing a thin-film on the sacrificial layer region and in the plurality of trenches, and forming a thin-film structure from the thin-film by removing the sacrificial layer region, wherein the thin-film structure comprises raised portions, where the raised portions are formed by the part of the thin-film deposited on the sacrificial layer region with the first thickness level, each raised portion comprises at least one protruding portion formed by the part of the thin-film deposited within the plurality of trenches, a back-volume is formed by the cavity left after removing the sacrificial layer region, and the membrane structure (1) comprises the thin-film structure, the substrate and the back-volume between the thin-film structure and the substrate.
14. The method according to claim 13, wherein the thin-film structure forms a membrane, and wherein each pair of pillars and protruding portions forms a mechanical phase shifter during membrane deflection.
15. The method according to claim 13, further comprising: forming at least one enclosing wall on the main surface of the substrate at a distance to the outer edge of the thin-film structure, the enclosing wall surrounding the thin-film structure in lateral directions, that extend parallel to a main plane of extension of the substrate.
16. The method according to claim 15, further comprising: depositing an elastic layer, the elastic layer mechanically connecting the thin-film structure to the enclosing wall.
17. The method according to claim 13, further comprising: forming an opening in the substrate, the opening extending from a rear surface of the substrate towards the thin-film structure, where the rear surface faces away from the main surface, removing of the sacrificial layer region after forming the opening in the substrate.
18. A membrane structure according to claim 1, wherein the raised portion and the protruding portion provide a flexible joint during membrane deflection.
19. A membrane structure comprising: a substrate having a main surface and a rear surface, a plurality of pillars, the pillars being arranged on the main surface of the substrate and having a support area facing away from the main surface of the substrate, and a thin-film structure arranged above the main surface of the substrate and the pillars, the thin-film structure forming a membrane, wherein the thin film structure comprises a plurality of raised portions that are spaced further from the substrate than at least one lower portion of the thin film structure, the raised portions each comprise at least one protruding portion, the protruding portions being hollow and having a bottom part and a sidewall and the protruding portions extending towards the substrate, the bottom part of each protruding portion is mechanically connected to the support area of one of the pillars, respectively, wherein each pair of pillars and protruding portions forms a mechanical phase shifter during membrane deflection, and a back-volume is formed by the volume between the main surface of the substrate and the thin-film structure.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0063] FIGS. 1a to 1b show a cross-section and an equivalent acousto-electrical circuit of an example for a membrane-based acoustic device.
[0064] FIGS. 2a to 2b show a cross-section and an equivalent acousto-electrical circuit of an embodiment of a membrane structure.
[0065] FIG. 3 shows two cross-sections of an embodiment of a membrane structure.
[0066] FIG. 4 shows a cross-section and a top-view of another embodiment of a membrane structure.
[0067] FIG. 5 shows a cross-section and a top-view of another embodiment of a membrane structure.
[0068] FIG. 6 shows a cross-section and a top-view of another embodiment of a membrane structure.
[0069] FIG. 7 shows a cross-section and a top-view of another embodiment of a membrane structure.
[0070] FIG. 8 shows a cut-away view of another embodiment of a membrane structure.
[0071] FIG. 9 shows a schematic diagram of an exemplary embodiment of a transducer device, the transducer device comprising a membrane structure.
[0072] FIGS. 10a to 10f show an embodiment of a method of producing a membrane structure.
DETAILED DESCRIPTION
[0073] In FIG. 1a a cross-section of an example of a membrane structure 1, which is no embodiment, is shown. The membrane structure 1 is arranged in a conventional way. The membrane structure 1 comprises a substrate 2. An enclosing wall 3 is arranged at a main surface 4 of the substrate 2. The enclosing wall 3 surrounds a back-volume 5 in lateral directions x, y, where lateral directions run parallel to a main plane of extension of the substrate 2.
[0074] The membrane structure 1 further comprises a thin-film structure 6 above the main surface 4 of the substrate 2 attached to the enclosing wall 3. This means that the thin-film structure 6 is in mechanical contact to the enclosing wall 3. The thin-film structure 6 is spaced from the substrate 2 in a vertical direction z, where the vertical direction z is perpendicular to the main plane of extension of the substrate 2. The thin-film structure 6 spans the back-volume 5. This means that the back-volume 5 is enclosed by the substrate 2, the enclosing wall 3 and the thin-film structure 6. The thin-film structure 6 comprises a vent hole 7. The vent hole 7 is an opening in the thin-film structure 6, which connects the side of the thin-film structure 6 facing towards the substrate 2 to the side of the thin-film structure 6 facing away from the substrate 2. The vent hole 7 connects the back-volume 5 to the environment above the thin-film structure 6. The thin-film structure 6 can act as a membrane responding to a sound pressure wave. This way, the membrane structure 1 can be used for acoustic applications such as microphone or speaker applications.
[0075] In FIG. 1b an equivalent acousto-electrical circuit of the example of FIG. 1a is shown. In an equivalent acousto-electrical circuit symbols are assigned to the acoustic components of a device, as known for an electrical circuit.
[0076] The equivalent acousto-electrical circuit of FIG. 1b comprises a common pressure level Pa and an external source of sound pressure Pe, which acts as a driving force for the deflection of the membrane. The membrane is represented in the equivalent acousto-electrical circuit of FIG. 1b by a complex acoustic impedance Zm of the membrane, which comprises an acoustic mass Lm of the membrane, an acoustic compliance Cm of the membrane and an acoustic resistance Rs of the gas film under the membrane. The acoustic mass Lm of the membrane corresponds to the inertia of the membrane deflecting due to the sound pressure Pe. The acoustic compliance Cm of the membrane corresponds to the stiffness of the membrane, which provides a restoring force. The acoustic resistance Rs of the gas film under the membrane corresponds to the film of gas, which gets squeezed due to the movement of the membrane. Furthermore, FIG. 1b comprises a further acoustic resistance Rv, which represents the vent hole 7 of the membrane. The vent hole 7 leads to the further acoustic resistance Rv due to its limited diameter the sound pressure has to pass through. Besides, the equivalent acousto-electrical circuit comprises an acoustic compliance Cv of the back-volume 5. The acoustic compliance Cv of the back-volume 5 represents the gas within the back-volume 5, which gets compressed under the deflection of the membrane. Thus, the compressed gas also acts as a restoring force.
[0077] The acoustic mass Lm of the membrane, the acoustic compliance Cm of the membrane and the acoustic resistance Rs of the gas film under the membrane are connected in series to form the complex acoustic impedance Zm of the membrane. The further acoustic resistance Rv of the vent hole 7 is connected in parallel to the complex acoustic impedance Zm of the membrane. The acoustic compliance Cv of the back-volume 5 is connected in series to both the further acoustic resistance Rv of the vent hole 7 and the complex acoustic impedance Zm of the membrane and is further connected to the common pressure level Pa. The external source of sound pressure Pe is connected to all those components in series and is further connected to the common pressure level Pa.
[0078] In this arrangement the further acoustic resistance Rv of the vent hole 7 and the acoustic compliance Cv of the back-volume 5 form a high pass filter. The acoustic compliance Cv of the back-volume 5 and the acoustic compliance Cm of the membrane form an acoustic compliance divider.
[0079] This means that at the low frequencies, displacement of the membrane is limited by the high pass filter. However, at audio frequencies, the membrane displacement is limited by the acoustic compliance divider. To achieve a membrane displacement as large as possible the acoustic compliance Cv of the back-volume 5 has to be large. Such request requires a large back-volume 5.
[0080] In FIG. 2a a cross-section of an embodiment of a membrane structure 1 is shown. Elements according to FIG. 2a that correspond to elements of the examples according to FIG. 1a are designated with the same reference numerals. The embodiment according to FIG. 2a comprises a substrate 2 with a main surface 4. The substrate 2 has a main plane of extension. As in FIG. 1a an enclosing wall 3 is arranged at the main surface 4 of the substrate 2. The enclosing wall 3 can be formed by a part of the substrate 2 and is in mechanical contact with the substrate 2. The enclosing wall 3 further comprises a side surface 8, which can be perpendicular to the main plane of extension of the substrate 2 as well as a top surface 9, which runs parallel to the main plane of extension of the substrate 2. In the vertical direction z, which is perpendicular to the main plane of extension of the substrate 2, the top surface 9 of the enclosing wall 3 is spaced from the main surface 4 of the substrate 2. The enclosing wall 3 surrounds a back-volume 5 in lateral directions x, y, which run parallel to the main plane of extension of the substrate 2.
[0081] Additionally, pillars 10 are arranged at the main surface 4 of the substrate 2 within the back-volume 5. The pillars 10 can be formed by a part of the substrate 2 and are in mechanical contact with the substrate 2. The pillars 10 comprise a side surface 11, which can be perpendicular to the main plane of extension of the substrate 2 as well as a support area 12, which runs parallel to the main plane of extension of the substrate 2. In the vertical direction z, the support area 12 of the pillars 10 is spaced from the main surface 4 of the substrate 2. However, in this embodiment, the distance from the support area 12 of the pillars 10 to the main surface 4 of the substrate 2 is less than the distance from the top surface 9 of the enclosing wall 3 to the main surface 4 of the substrate 2. In lateral directions x, y the pillars 10 are arranged at a distance to the enclosing wall 3.
[0082] The embodiment of FIG. 2a further comprises a thin-film structure 6 above the main surface 4 of the substrate 2 and above the pillars 10. The thin-film structure 6 is spaced from the substrate 2. The thin-film structure 6 extends in a plane, which runs parallel to the main plane of extension of the substrate 2. The thin-film structure 6 covers the back-volume 5. However, the thin-film structure 6 is not connected to the enclosing wall 3. Instead, the thin-film structure 6 is connected to the pillars 10 by protruding portions 13. This means that a bottom part 14 of each protruding portion 13 is in mechanical contact with the support area 12 of a corresponding pillar 10. The thin-film structure 6 further comprises raised portions 15, from which each protruding portion 13 extends towards a corresponding pillar 10. The raised portions 15 are spaced further from the substrate 2 than lower portions 16 of the thin-film structure 6. Raised portions 15 and lower portions 16 are connected by connecting portions 17 of the thin-film structure 6. Because the thin-film structure 6 is not connected to the enclosing wall 3 there is a slit 18 between both components. This means that the slit 18 connects the back-volume 5 with the environment above the thin-film structure 6. However, the slit 18 can be small.
[0083] As in the example of FIG. 1a, the thin-film structure 6 can act as a membrane responding to a sound pressure wave. In this case the membrane can be regarded to have a center part 19 and an outer part 20. The center part 19 of the membrane is the part which lies, in a top-view, within the pillars 10. The outer part 20 of the membrane is the part which lies, in a top-view, between the enclosing wall 3 and the pillars 10. The top-view refers to a view on the membrane structure 1 from the side of the membrane facing away from the substrate 2 in the vertical direction z.
[0084] Due to this arrangement the membrane is deflected by a sound pressure wave in the following way. While the center part 19 of the membrane is moving towards the substrate 2 the outer part 20 of the membrane is moving away from the substrate 2 and vice versa. This means that while the back-volume 5 under the center part 19 of the membrane becomes smaller, the back-volume 5 under the outer part 20 of the membrane enlarges and vice versa. This means that the pillars 10 and the protruding portions 13 form mechanical phase shifters 21 providing a phase shift between the outer part 20 and the center part 19 of the membrane of 180°.
[0085] In FIG. 2b an equivalent acousto-electrical circuit for the embodiment of a membrane structure 1 according to FIG. 2a is shown. FIG. 2b is different from FIG. 1b in that it shows two branches for a complex acoustic impedance Zm of the membrane. The first branch comprises the complex acoustic impedance Zmc of the center part 19 of the membrane. The complex acoustic impedance Zmc of the center part 19 of the membrane comprises an acoustic mass Lmc of the center part 19 of the membrane, an acoustic compliance Cmc of the center part 19 of the membrane and an acoustic resistance Rsc of the gas film squeezed by the center part 19 of the membrane. The second branch comprises the complex acoustic impedance Zmo of the outer part 20 of the membrane. The complex acoustic impedance Zmo of the outer part 20 of the membrane comprises an acoustic mass Lmo of the outer part 20 of the membrane, an acoustic compliance Cmo of the outer part 20 of the membrane and an acoustic resistance Rso of the gas film squeezed by the outer part 20 of the membrane. Besides, the second branch comprises a phase shifter Δφ, which respects the fact that the center part 19 of the membrane and the outer part 20 of the membrane are moving inversely phased.
[0086] The two branches are coupled by an acoustic resistance Rp, which corresponds to the acoustic resistance caused by the pillars 10 within the back-volume 5 and which represents the damping of the membrane. The acoustic resistance Rv of the vent hole 7 in FIG. 1b is replaced in FIG. 2b by the acoustic resistance Rsl of the slit 18 between the enclosing wall 3 and the membrane. Besides, the equivalent acousto-electrical circuit of FIG. 2b comprises an acoustic compliance Cvc of the back-volume 5 under the center part 19 of the membrane as well as an acoustic compliance Cvo of the back-volume 5 under the outer part 20 of the membrane.
[0087] Since the deflection of the membrane causes a volume flow between the back-volume 5 under the center part 19 of the membrane and the back-volume 5 under the outer part 20 of the membrane, there is a reduced gas compression. Consequently, the restoring force on the membrane is small. Consequently, even with a small back-volume 5, the acoustic compliances Cvc, Cvo of the back-volume 5 are large.
[0088] At low frequencies, the time constant of an acoustic device based on the membrane structure 1 of FIGS. 2a and 2b is determined by the acoustic resistances Rsl and Rp as well as by the acoustic compliances Cvc and Cvo. In order to achieve a low high pass cut-off frequency the acoustic resistance Rsl of the slit 18 has to be as large as possible. This way, a pressure drop is avoided and the noise level of the acoustic device can be reduced. The acoustic resistance Rsl of the slit 18 can be increased, for example, by reducing the size of the slit 18, which means the distance of the membrane to the enclosing wall 3.
[0089] In FIG. 3 two cross-sections of another embodiment of the membrane structure 1 are shown. FIG. 3 differs from FIG. 2a in that in does not show an enclosing wall 3 at the main surface 4 of the substrate 2. Besides, the side surface 11s of the pillars 10 are transverse with respect to the main plane of extension of the substrate 2.
[0090] FIG. 3 shows two states of deflection of the membrane caused by, for example, a sound pressure wave. In the first case the center part 19 of the membrane is deflected towards the substrate 2, whereas the outer part 20 of the membrane is deflected away from the substrate 2. In the second case the center part 19 of the membrane is deflected away from the substrate 2, whereas the outer part 20 of the membrane is deflected towards the substrate 2. Each pair of pillars 10 and protruding portions 13 forms a mechanical phase shifter 21, which causes the inverted phase between the motion of the center part 19 of the membrane and the motion of the outer part 20 of the membrane. The raised portion 15 and the protruding portion 13 provide a flexible joint, so that the membrane can swing as freely as possible. The deformation of the membrane during the deflection mainly takes place in regions of the sidewalls 22 of the protruding portions 13 and in regions of the connecting portions 17.
[0091] In FIG. 4 a cross-section and a top-view of another embodiment of the membrane structure 1 are shown. The cross-section shows the membrane-structure along a cut A-A as indicated in the top-view. Measures in the cross-section, which correspond to measures in the top-view, are indicated by vertical dotted lines.
[0092] The cross-section in FIG. 4 differs from the cross-section in FIG. 2b inter alia in that it shows an opening 23 in the substrate 2. The opening 23 extends in the vertical direction z from a rear surface 24 of the substrate 2 towards the thin-film structure 6. The opening 23 has a smaller extent in lateral directions x, y than the center part 19 of the thin-film structure 6. Besides, the top surface 9 of the enclosing wall 3 has the same height as the support area 12 of the pillars 10, where the height is the distance to the main surface 4 of the substrate 2 in the vertical direction z. In comparison with FIG. 2a the raised portions 15 of the thin-film structure 6 of FIG. 4 have a smaller extent in lateral directions x, y. This means that each raised portion 15 is completely surrounded by the lower portion 16, as visible in the top-view diagram of FIG. 4. The lower portion 16 of the thin-film structure 6 extends to the outer edge 25 of the thin-film structure 6. Thus, at the outer edge 25 of the thin-film structure 6, the thin-film structure 6 has the same height as the top surface 9 of the enclosing wall 3. In the embodiment of FIG. 4 the thin-film structure 6 has a circular shape in top-view. A plurality of pillars 10 are arranged along a circle line at a predetermined distance to the outer edge 25 of the thin-film structure 6. This means that the circle line the pillars 10 are arranged on is concentric to the circular thin-film structure 6, but has a smaller diameter. The raised portions 15 are arranged above the pillars 10, but have a larger extent in lateral directions x, y than the pillars 10. Each raised portion 15 comprises a protruding portion 13, which extends towards a corresponding pillar 10 mechanically connecting the support area 12 of the pillar 10 by a bottom part 14.
[0093] In FIG. 5 a cross-section and a top-view of another embodiment of the membrane structure 1 are shown. The cross-section shows the membrane-structure along a cut A-A as indicated in the top-view. Measures in the cross-section, which correspond to measures in the top-view, are indicated by vertical dotted lines.
[0094] FIG. 5 differs from FIG. 4 in that it shows an additional elastic layer 26 connecting the thin-film structure 6 to the enclosing wall 3. This means that the elastic layer 26 is mechanically connected to a part of the top surface 9 of the enclosing wall 3 as well as to a part of the thin-film structure 6 at its outer edge 25. Thus, the elastic layer 26 covers the slit 18 between the thin-film structure 6 and the enclosing wall 3. The elastic layer 26 increases the acoustic resistance Rsl of the slit. The elastic layer 26 can be stretched so that the thin-film structure 6 can still act as a membrane, i.e. can be deflected, for example by a sound pressure wave.
[0095] The thin-film structure 6 of the embodiment of FIG. 5 additionally comprises a vent hole 7 in the center of the thin-film structure 6. The vent hole 7 is provided for pressure equalization between the environment above the thin-film structure 6 and the back-volume 5. In case that the elastic layer 26 is gas-permeable, the vent hole 7 can be omitted.
[0096] In FIG. 6 a cross-section and a top-view of another embodiment of the membrane structure 1 are shown. The cross-section shows the membrane-structure along a cut A-A as indicated in the top-view. Measures in the cross-section, which correspond to measures in the top-view, are indicated by vertical dotted lines.
[0097] FIG. 6 differs from FIG. 4 in that it comprises raised portions 15 which extend to the outer edge 25 of the thin-film structure 6. Each raised portion 15 is connected to the lower portion 16 by a connecting portion 17 at all lateral sides, apart from the side facing the outer edge 25 of the thin-film structure 6. This means that the lateral extent of the raised portion 15 in this embodiment is larger than the lateral extent of the raised portion 15 in the embodiment according to FIG. 4. The raised portions 15 have an elongated shape, reaching from the outer edge 25 of the thin-film structure 6 towards a corresponding pillar 10.
[0098] In FIG. 7 a cross-section and a top-view of another embodiment of the membrane structure 1 are shown. The cross-section shows the membrane structure 1 along a cut A-A as indicated in the top-view. Measures in the cross-section, which correspond to measures in the top-view, are indicated by vertical dotted lines.
[0099] FIG. 7 differs from FIG. 6 in that it shows an additional elastic layer 26 connecting the thin-film structure 6 to the enclosing wall 3. This means that the elastic layer 26 is mechanically connected to a part of the top surface 9 of the enclosing wall 3 as well as to a part of the thin-film structure 6 at its outer edge 25. Thus, the elastic layer 26 spans the slit 18 between the thin-film structure 6 and the enclosing wall 3. The elastic layer 26 increases the acoustic resistance Rsl of the slit. The elastic layer 26 can be stretched so that the thin-film structure 6 can still act as a membrane, i.e. can be deflected, for example by a sound pressure wave.
[0100] The thin-film structure 6 of the embodiment of FIG. 7 additionally comprises a vent hole 7 in the center of the thin-film structure 6. The vent hole 7 is provided for pressure equalization between the environment above the thin-film structure 6 and the back-volume 5. In case that the elastic layer 26 is gas-permeable, the vent hole 7 can be omitted.
[0101] FIG. 8 shows two cut-away views of another embodiment of a membrane structure 1. As in FIG. 3 two states of deflection of the membrane caused by, for example, a sound pressure wave, are shown. The embodiment of FIG. 8 is similar to the embodiment of FIG. 6, but comprises fewer raised portions 15, protruding portions 13 and pillars 10. The membrane deflection is shown in a perspective, i.e. three-dimensional, view. It has to be noted that the pillar 10, the raised portion 15 and the protruding portion 13, which are shown in a central position of FIG. 8, are located on the side of the membrane facing away from the observer.
[0102] FIG. 9 shows a schematic diagram of an exemplary embodiment of a transducer device 27, comprising the membrane structure 1 as discussed above. The transducer device 27 can be, for example, an optical omnidirectional microphone or any other dynamic pressure sensing device.
[0103] With FIGS. 10a to 10f an exemplary embodiment of a method of producing a membrane structure 1 is shown.
[0104] The method comprises providing a substrate 2, as shown in FIG. 10a. The substrate 2 has a rear surface 24 and a main surface 4. The substrate 2 can comprise Si. On the main surface 4 etching is performed in order to form the enclosing wall 3 as well as the pillars 10. This can be done by one single etching step or by several subsequent etching steps. With respect to the main surface 4 of the substrate 2 the enclosing wall 3 can be higher than the pillars 10. This means that the top surface 9 of the enclosing wall 3 is spaced further from the main surface 4 of the substrate 2 than the support area 12 of the pillars 10. The pillars 10 are arranged within the enclosing wall 3 at a distance to the enclosing wall 3.
[0105] In a next step a first sacrificial layer 28 is deposited on the main surface 4 of the substrate 2 in the region within the enclosing wall 3 (FIG. 10b). The first sacrificial layer 28 can comprise W—Ti, for example. The first sacrificial layer 28 partially fills the region within the enclosing wall 3. Thus, the first sacrificial layer 28 can cover the side surface 11s of the pillars 10. However, the first sacrificial layer 28 can leave the support area 12 of the pillars 10 uncovered.
[0106] A second sacrificial layer 29 is deposited on the first sacrificial layer 28. The second sacrificial layer 29 can comprise W—Ti as well. The second sacrificial layer 29 is formed in a region above the pillars 10, however, the lateral extent of the second sacrificial layer 29 after forming is larger than the lateral extent of the pillars 10. In lateral directions x, y the second sacrificial layer 29 is removed in regions adjacent to the enclosing wall 3. This means that there is a gap between the second sacrificial layer 29 and the enclosing wall 3. Trenches 30 are formed within the second sacrificial layer 29 above the pillars 10. The forming of the trenches 30 can be done by the same etching step as the forming of the other regions of the second sacrificial layer 29. The trenches 30 extend towards the pillars 10. The trenches 30 can have the same lateral extent as the pillars 10.
[0107] The stack of sacrificial layers 28, 29 forms the sacrificial layer region 33. With respect to the main surface 4 of the substrate 2, the sacrificial layer region 33 can have the same height as the enclosing wall 3 in places. This means that the top surface 31 of the second sacrificial layer 29 and the top surface 9 of the enclosing wall 3 lie within a plane, which runs parallel to the main plane of extension of the substrate 2. The sacrificial layers 28, 29 form regions providing a first thickness level and a second thickness level of the sacrificial layer region 33. The first thickness level is larger than the second thickness level.
[0108] In a next step a thin-film 32 is deposited in the region within the enclosing wall 3 (FIG. 10c). This means that the thin-film 32 is deposited on the second sacrificial layer 29, the first sacrificial layer 28, into the trenches 30 and on the support area 12 of the pillars 10. The thin-film 32 may comprise SiN. The thin-film 32 is removed in regions adjacent to the enclosing wall 3. For example, the thin-film 32 is removed by etching. Thus, the thin-film 32 is not in mechanical contact with the enclosing wall 3.
[0109] In FIG. 10d the deposition of the elastic layer 26 is shown. The elastic layer 26 may comprise PDMS. The elastic layer 26 covers a part of the top surface 9 of the enclosing wall 3 and a part of the thin-film. Thus, the elastic layer 26 spans the slit 18 between the enclosing wall 3 and the thin-film.
[0110] An opening 23 is formed from the rear surface 24 to the main surface 4 of the substrate 2 towards the sacrificial layer region 33 (FIG. 10e). However, the lateral extent of the opening 23 may be smaller than the lateral extent of the sacrificial layer region 33. In order to form an opening 23 the substrate 2 can be turned around and back-side treated.
[0111] In the next step the sacrificial layers 28, 29 are removed in order to release the thin-film 32 and to form the thin-film structure 6 (FIG. 10f). The raised portion 15 of the thin-film structure 6 is formed by the thin-film, where it has been deposited on the first thickness level. A protruding portion 13 is formed by the part of the thin-film 32 deposited within one of the plurality of trenches 30 in the sacrificial layer region 33. A connecting portion 17 of the thin-film 32 structure 6 is formed by the part of thin-film 32 deposited between the first thickness level and the second thickness level. A lower portion 16 is formed by the part of the thin-film 32 deposited on the second thickness level. The resulting membrane structure 1 is equivalent to the embodiment of FIG. 7. The membrane structure 1 is formed by the substrate 2, the enclosing wall 3, the pillars 10, the elastic layer 26, the thin-film structure 6 and the back-volume 5, which is formed by the cavity left after removing the sacrificial layer region 33. The thin-film structure 6 can form a membrane for use in a transducer device 27.
[0112] 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.
[0113] 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.
[0114] 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.
