Microstructure and method for manufacturing microstructure

Abstract

A microstructure (1) for a microelectromechanical device comprises a substrate (2) having a top surface (3) and a back surface (4) and a thin film structure (5) arranged at the top surface (3) of the substrate (2). The thin film structure (5) comprises a raised portion (8) spaced apart from the substrate (2), a lower portion (6) of the thin film structure (5) in mechanical contact with the substrate (2), at least one protruding portion (9), and at least one additional side wall (7) of the thin film structure (5) at a distance from the at least one protruding portion (9), the protruding portion being hollow and having at least one side wall (10) and a bottom (11) and the protruding portion mechanically connects the raised portion (8) to the substrate (2) via the bottom (11). The additional side wall (7) mechanically connects the lower portion (6) to the raised portion (8) of the thin film structure (5).

Description

[0001] The present invention relates to a microstructure for a microelectromechanical device, an omnidirectional optical microelectromechanical microphone, a mobile device, and a method for manufacturing the microstructure.

[0002] For microelectromechanical systems (MEMS) technology, microstructures with raised parts play a crucial role. Many sensor applications can be realized using diaphragms or cantilevers made by thin film deposition. Since these elements are not firmly attached to the substrate by construction, they exhibit a relatively large freedom of movement. In this way, they can be used as pressure, acceleration or flow sensors, for example. Even microphone applications are possible.

[0003] However, the raised portion relies on a strong anchor to the underlying substrate. Typically, the raised portion is anchored to the substrate via a sidewall that is perpendicular to the surface. Defects along the sidewall at the convex or concave edge can reduce the mechanical stability and thus lead to cracks or local stress relaxation and creep. Such defects can include, among others, reduced sidewall thickness, poor uniformity, poor edge coverage, formation of growth boundaries, intrinsic stresses or plastic strains due to load offsets.

[0004] One object is to provide a microstructure for use in a microelectromechanical device which exhibits improved mechanical stability. Another object is to provide a method for manufacturing a microstructure for use in a microelectromechanical device which exhibits improved mechanical stability.

[0005] This object is achieved by means of the independent claim. Further embodiments and variants follow from the dependent claims. Unless otherwise stated, the definitions given above also apply in the following description.

[0006] In one embodiment, the microstructure comprises a substrate, which may comprise a semiconductor material, such as silicon (Si). The substrate can have a main extension plane. The substrate has a top surface and a back surface. The back surface faces away from the top surface.

[0007] The microstructure also includes a thin film structure disposed at the top surface of the substrate. The thin film structure may include a material compatible with semiconductor processes, such as silicon nitride (SiN) or polysilicon.

[0008] The thin film structure includes a lower portion that is in mechanical contact with the substrate. The area of ​​the lower portion of the thin film structure that is in mechanical contact with the substrate forms a contact area. The contact area can have any size and shape. The contact area can be continuous or discontinuous.

[0009] The thin film structure also includes a raised portion spaced apart from the substrate. The raised portion can be arranged at the top surface of the substrate. This means that in the vertical direction, the raised portion is at a distance from the top surface of the substrate, wherein the vertical direction is perpendicular to the main extension plane of the substrate. Therefore, the raised portion of the thin film structure is not in mechanical contact with the substrate. The raised portion of the thin film structure can extend in a plane parallel to the main extension plane of the substrate. In a top view, this means that in a plane parallel to the main extension plane of the substrate, the raised portion can have any shape. The top view refers to a microstructure view of the side of the thin film structure away from the substrate in the vertical direction. The distance between the raised portion of the thin film structure and the substrate depends on the application and can vary from application to application. In the vertical direction, the raised portion and the lower portion do not overlap. In a lateral direction extending parallel to the main extension plane of the substrate, the raised portion and the lower portion are arranged adjacent to each other.

[0010] The thin film structure comprises at least one protrusion. The protrusion extends in a vertical direction from the raised portion toward the top surface of the substrate. The protrusion is formed by a part of the thin film structure. The protrusion is in direct contact with the raised portion. The protrusion is in direct contact with the raised portion at the top side of the protrusion facing away from the substrate. The protrusion has a bottom and at least one side wall. The side wall is perpendicular or transverse to the main extension plane of the substrate. The bottom can extend parallel to the main extension plane of the substrate. The protrusion mechanically connects the raised portion to the substrate via the bottom. This means that the bottom of the protrusion is in mechanical contact with the substrate. In a top view, the protrusion can have any shape. The size of the protrusion can be smaller than the size of the raised portion of the thin film structure.

[0011] The protrusion is hollow. This means that the protrusion forms a hollow profile. The volume enclosed by the protrusion is bounded by the bottom of the protrusion and at least one side wall. At the top side of the protrusion, the enclosed volume is connected to the environment of the membrane structure. This means that the protrusion is open at the top side.

[0012] The thin film structure also includes at least one further side wall that mechanically connects the lower part to the raised part. The at least one further side wall is arranged at a distance from the protruding part. However, at least one protruding part can be arranged close to the further side wall of the thin film structure. The distance can vary depending on the application. The other side wall can be perpendicular or transverse to the main extension plane of the substrate.

[0013] The raised portion may be mechanically connected to the substrate only via the lower portion and the protruding portion. The raised portion, the lower portion, the protruding portion and the further sidewall may comprise the same material. Furthermore, the raised portion may be integrally formed with the lower portion, the protruding portion and the further sidewall.

[0014] Since the raised portion is only connected to the substrate via the lower portion with the further sidewalls and via the protruding portion, the raised portion is a movable part of the microstructure. This means that the raised portion can be deflected. The deflection of the raised portion can be towards the substrate and / or away from the substrate in the vertical direction.

[0015] The proposed structure enables to improve the limits of the mechanical stability of microstructures with raised movable parts formed by a thin film. Thus, the structure increases the bending stiffness of the anchoring of the raised movable parts without increasing the film thickness. The increase in bending stiffness is achieved by combining one or more protruding parts with the hollow profile. The proposed structural element is characterized by an increased geometrical moment of inertia.

[0016] Due to the inherent increase in sidewall area and edge length, local high stress levels are reduced because the protrusion includes at least one sidewall. In the case where multiple protrusions are used, each protrusion mechanically connects the raised portion to the substrate respectively. As an advantage, potential damage (such as peeling from the substrate) will not lead to sudden complete system failure as in the case without protrusions. For delamination or other failures of this type, the proposed structure with a large number of protrusions will show good degradation. By selecting the number of protrusions and their geometry, the stiffness against radial and tangentially acting stresses can be independently controlled.

[0017] In one embodiment, the further sidewall, the raised portion and the substrate surround the cavity between the substrate and the raised portion. This means that the further sidewall surrounds the cavity in the lateral direction and delimits the cavity in the lateral direction. The cavity is further delimited by the substrate on the one hand and the raised portion on the other hand. The substrate and the raised portion delimit the cavity in the vertical direction.

[0018] With this arrangement, the raised portion forms a diaphragm. The cavity forms a fixed volume, such as a volume of gas or air. The volume is separated from the environment of the microstructure by the diaphragm. For example, this arrangement can be used in microphone or speaker applications. It can also be used in pressure sensing applications.

[0019] In one embodiment, the microstructure includes an opening in the substrate, the opening extending from the back surface of the substrate toward the raised portion of the thin film structure. The opening is formed as a groove extending in the substrate in a vertical direction. The opening can have a smaller range than the raised portion in a lateral direction. The opening can connect the cavity formed by the other sidewalls, the substrate and the raised portion to the environment at the back surface of the substrate. The opening can have any shape. The depth of the opening in the vertical direction is given by the thickness of the substrate.

[0020] By forming the opening, a passage is given from the back surface of the substrate to the top surface. In the case of a cavity, this is the only access to the cavity, because above the top surface of the substrate, the cavity is surrounded by the film. The passage can be important for processing reasons, for example in order to remove a sacrificial layer. In addition, by forming the opening, the volume of the cavity surrounded by the substrate and the film structure is increased. This can also be important, for example for microphone applications, because the response of the membrane to sound waves (especially to audio) can be changed by increasing the volume. Therefore, the sensitivity and the signal-to-noise ratio can be improved.

[0021] In one embodiment, the thin film structure comprises a diaphragm, and the diaphragm can be a film or a plate. The diaphragm can be formed by a raised structure. This means that the raised portion is surrounded by at least one further side wall in all lateral directions. Therefore, all edges of the raised portion are connected to the lower portion via at least one further side wall. The thickness of the diaphragm in the vertical direction depends on the application. The arrangement of the diaphragm can be used for pressure sensing applications as well as for microphone and speaker applications.

[0022] In one embodiment, the membrane structure comprises a beam or a rod. The beam or the rod can be formed by a raised structure. This means that the membrane structure comprises an additional side wall only at one lateral side of the raised part. Therefore, at all other lateral sides, the edge of the raised part is not connected to the lower part via the additional side wall. The thickness of the beam or the rod in the vertical direction depends on the application. The arrangement of the beam or the rod can be used, for example, for acceleration sensing applications and for flow sensing applications.

[0023] In one embodiment, the film structure comprises a bridge. The bridge can be formed by a raised portion. This means that the film structure comprises two additional side walls at two opposite lateral sides of the raised portion. The two corresponding sides are not adjacent, so that the two additional side walls are spaced apart from each other. For example, in the case of a rectangular raised portion, the two additional side walls are arranged at opposite edges of the raised portion. For example, the bridge can be used as a raised circuit path.

[0024] In one embodiment, in a top view, the raised portion of the film structure has a circular or rectangular shape. However, the shape of the raised portion is not limited to a circular shape, but can have any shape, such as a polygonal shape. The shape of the raised portion can depend on the application. For example, a circular shape used as a raised portion of the film can be advantageous in terms of uniform deflection and uniform stress distribution in the lateral direction.

[0025] In one embodiment, the microstructure includes a plurality of projections. The projections can be arranged in a position where the projections can be connected to the underlying substrate via the bottom of the projections. In the case where there is an opening in the substrate in the area below the projections, the projections cannot be arranged in this area. Therefore, each projection can be arranged near another sidewall because the range of the opening in the lateral direction can be less than the range of the projections. The number of projections and their placement area can vary according to application. For all projections, the distance between two adjacent projections can be the same. By selecting the number of projections and the placement area, the stiffness against radial and tangential stresses can be independently controlled.

[0026] In one embodiment, the microstructure includes a plurality of protrusions, wherein in a top view, at least two of the plurality of protrusions have different shapes. For example, one protrusion can have a circular shape, while another protrusion can have a rectangular or elliptical shape. Furthermore, by selecting the geometry of the protrusions, the stiffness against radially and tangentially acting stresses can be independently controlled.

[0027] In one embodiment, at least one projection has a circular, elliptical, rectangular, polygonal or sickle shape. The shape of the projection is given relative to the top view. However, the shape of the projection is not limited to one of these shapes, but can have any shape. As described above, by selecting the geometric shape of the projection, the stiffness against radial and tangentially acting stresses can be independently controlled.

[0028] In one embodiment, at least one side wall of the protruding portion is perpendicular or transverse to the main extension plane of the substrate. Adjustment of the side wall can influence the stability of the thin film structure. However, adjustment of the side wall can also be provided by technology (e.g. by photolithography). By implementing slightly transverse (i.e. flat) side walls, the edge coverage of the thin film structure can be improved.

[0029] In one embodiment, at least one further side wall of the thin film structure is perpendicular or transverse to the main extension plane of the substrate. Adjustment of the further side wall can influence the stability of the thin film structure. However, adjustment of the further side wall can also be provided by technology (e.g. by photolithography). By implementing a slightly transverse (i.e. flat) further side wall, edge coverage by the thin film structure can be improved.

[0030] In one embodiment, the raised portion of the film structure is corrugated. This means that the raised portion does not have a flat surface parallel to the main extension plane of the substrate at least in some positions. Instead, the raised portion exhibits an area extending in the vertical direction. The corrugated raised portion can have improved properties in terms of deflection. The corrugation can also be caused by the processing technology.

[0031] In another embodiment, the microstructure is incorporated into an omnidirectional optical MEMS microphone. An omnidirectional microphone is able to respond to sounds coming from all directions, even from the back side. The microphone can also be an optical microphone. An optical microphone evaluates the response of the membrane to sound by using optical methods. When a reflector is integrated on the membrane, vibrations can be detected by using a laser beam and an interferometer. In this way, the movement of the membrane can be detected by the change in the optical path length of the reflected light beam. In this arrangement, a backplate is advantageously not required as is required for conventional condenser microphones.

[0032] In another embodiment, the microstructure is incorporated into an omnidirectional optical MEMS microphone, which is in turn incorporated into a mobile device. The mobile device can be, for example, a smart speaker device, a smart watch, a phone, or a hearing aid device. Since the microstructure is processed using MEMS technology, the mobile device can have a small size.

[0033] In another embodiment, a plurality of microstructures are incorporated into an electronic device. The electronic device can be, for example, any transducer device such as a smart speaker device, a smart watch, a phone, or a hearing aid device. A plurality of microstructures can be arranged in a repeating configuration such that they form a microstructure array. In this way, the electronic device can achieve array sensing.

[0034] Furthermore, a method for producing a microstructure is provided. All features disclosed for the microstructure are also disclosed for the method for producing the microstructure, and vice versa.

[0035] The method for manufacturing a microstructure used in a microelectromechanical device includes providing a substrate having at least one sacrificial layer, the sacrificial layer being arranged at a position of the top surface of the substrate. The sacrificial layer can include, for example, tungsten-titanium (W-Ti) and can be deposited, for example, by a sputtering process. The patterning of the at least one sacrificial layer can be accomplished by etching. Several depositions of the sacrificial layer can then be performed, and an etching process can be performed after each deposition to pattern the corresponding sacrificial layer. At least one groove is formed in the at least one sacrificial layer. The groove extends toward the substrate.

[0036] The thin film is deposited on the sacrificial layer, the groove and the substrate. The thin film can be deposited by chemical vapor deposition (CVD) or plasma enhanced CVD (PECVD). The thin film can be patterned by an etching process. The thin film is in mechanical contact with the substrate at a location where there is no sacrificial layer, for example, at the bottom of the groove in the sacrificial layer. In addition, the thin film covers the side surfaces of the groove in the sacrificial layer. The thin film also covers the edges of the groove in the sacrificial layer. The thin film also covers the top surface of the sacrificial layer, which refers to the surface of the sacrificial layer facing away from the substrate. Therefore, the sacrificial layer is arranged at some position between the substrate and the thin film.

[0037] After the film deposition and its patterning, the sacrificial layer is removed. In this way, a thin film structure is formed by the film by removing the sacrificial layer. The removal of the sacrificial layer can be done by wet etching. The removal of the sacrificial layer releases the film at some locations. This means that where the film has been arranged on the sacrificial layer, a raised portion of the film is formed after the sacrificial layer is removed. The raised portion is formed by the portion of the film deposited on the sacrificial layer. The protruding portion is formed by the portion of the film deposited in at least one groove in the sacrificial layer. The other sidewalls are formed by the portion of the film deposited on the side surface of the patterned sacrificial layer. After removing the sacrificial layer, a microstructure is formed by the film and the substrate.

[0038] By using a sacrificial layer, it is possible to create raised portions of the microstructure. The process of creating raised thin film structures is simpler and more cost-effective than creating equivalent structures using processes such as chemical mechanical polishing (CMP). In addition, by adding protrusions on the raised thin film structure that make mechanical contact with the underlying substrate, the stability of the thin film structure can be improved.

[0039] In one embodiment of the method for manufacturing a microstructure used in a micro-electromechanical device, an opening is formed in a substrate. The opening can be formed by deep reactive ion etching (DRIE). The opening extends from the back surface of the substrate toward the raised portion of the thin film structure. The etching process can be controlled by time or by using an etch stop layer. In the latter case, the etch stop layer has been deposited at the position where the opening should be formed at the top side of the substrate before the sacrificial layer is deposited. The etch stop layer may include chromium (Cr) and can be removed, for example, by wet etching, after the opening is formed. The lateral range of the opening can be less than the lateral range of the raised portion. In this way, the protruding portion arranged near the other sidewalls of the thin film structure in the lateral direction is still connected to the underlying substrate.

[0040] By forming the opening, a passage is given from the back surface of the substrate to the top surface. The passage can be important for processing reasons, for example for removing a sacrificial layer. In addition, by forming the opening, the volume of the cavity surrounded by the substrate and the membrane can be increased. This means that the cavity is enlarged in the vertical direction by the opening in the substrate. This can also be important, for example for microphone applications, because the response of the membrane to sound waves (especially to audio) can be changed in this way. Therefore, the sensitivity and the signal-to-noise ratio can be improved by expanding the cavity volume.

[0041] In one embodiment of a method for fabricating a microstructure for use in a microelectromechanical device, the sacrificial layer is removed after forming an opening in the substrate. In embodiments including a diaphragm, the opening can give a unique access to the sacrificial layer because the sacrificial layer is covered by a thin film above the top surface of the substrate. This access can then be used to remove the sacrificial layer, for example by wet etching. In addition, by forming the opening, the volume enclosed in the device including the diaphragm can be increased.

[0042] The following description of the accompanying drawings may further illustrate and explain the exemplary embodiments. Components having the same function or having the same effect are represented by the same reference numerals. Components that are the same or have the same effect may only be described with respect to the first accompanying drawing. It is not necessary to repeat their description in subsequent drawings.

[0043] Figure 1a Finite element analysis (FEA) results of one example of a raised microstructure are shown.

[0044] Figure 1b Finite element analysis (FEA) results of another example of a raised microstructure are shown.

[0045] Figure 2 Results of a finite element analysis (FEA) of an exemplary embodiment of a microstructure are shown.

[0046] Figure 3a An exemplary embodiment of a thin film structure is shown.

[0047] Figure 3b A cross-sectional view of an exemplary embodiment of a thin film structure is shown.

[0048] Figure 4 A cross section of an exemplary embodiment of a microstructure is shown.

[0049] Figure 5 A top view of an exemplary embodiment of a microstructure is shown.

[0050] Figure 6 A cross section of another exemplary embodiment of a microstructure is shown.

[0051] Figure 7 A cross section of another exemplary embodiment of a microstructure is shown.

[0052] Figure 8 A cross section of another exemplary embodiment of a microstructure is shown.

[0053] Fig. 9 A schematic diagram of an exemplary embodiment of a mobile device including a MEMS microphone comprising a microstructure is shown.

[0054] exist Figures 10a to 10i , an exemplary embodiment of a method for manufacturing a microstructure is shown.

[0055] exist Figure 1a , the results of FEA simulation of an exemplary portion of the microstructure 1 are shown, which is not an embodiment. The microstructure 1 includes a thin film structure 5. In this figure, the stress distribution (vonMises stress) of the thin film structure 5 is evaluated. The thin film structure 5 includes a lower portion 6 in contact with the substrate 2, an additional side wall 7 and a protruding portion 8. The lower portion 6, the additional side wall 7 and the protruding portion 8 include the same material. The lower portion 6 and the protruding portion 8 have flat surfaces that extend parallel to the main extension plane of the substrate 2. The protruding portion 8 is arranged on an elevated horizontal plane relative to the lower portion 6. The additional side wall 7 connects the lower portion 6 to the protruding portion 8. In addition, the additional side wall 7 is perpendicular to the surface of the lower portion 6 and the surface of the protruding portion 8.

[0056] The scale gives the value in N / m 2 The mechanical stress in units of 1 / 2 is 1 / 2. The stress exhibits high values ​​in the region of the further side wall 7, in particular at the edge of the further side wall 7 adjacent to the lower portion 6. Such stresses are caused by the manufacturing process and can also result from different thermal expansion coefficients of the materials used. In turn, the tensile forces can lead to bending moments on the microstructure 1 and deflections of the raised portion 8. Due to the load displacement, material fatigue, plastic strains and cracks can occur.

[0057] exist Figure 1b , the results of FEA simulations of the stress distribution of another exemplary portion of the microstructure 1 are shown, which is not an embodiment. Also in this example, the thin film structure 5 includes a lower portion 6 in contact with the substrate 2, a further side wall 7 and a raised portion 8 having the same characteristics as described above. In addition, the thin film structure 5 has a corrugated further side wall 7, as is known from the prior art, for example from US Pat. No. 6,987,859 B2. This means that the further side wall 7 not only extends in one direction, but also includes a plurality of portions that surround a 90° angle with each other. Even if this layout reduces the stress within the further side wall 7, there are local high stress levels at the top edges of the corrugations.

[0058] Figure 2 FIG. 5 shows an FEA stress evaluation of a portion of an embodiment of a microstructure 1 including a thin film structure 5. Figure 1a and 1b The elements of the example are based on Figure 2 The same reference numerals are used to designate the same elements.

[0059] As in Figure 1a and 1b In the example, according to Figure 2The thin film structure 5 comprises a lower portion 6 in contact with the substrate 2, a further side wall 7 and a raised portion 8 having the same properties as described above. This means in particular that the lower portion 6, the further side wall 7 and the raised portion 8 comprise the same material.

[0060] Figure 2 Only the top surface 3 of the substrate 2 is shown. The substrate 2 can have a main extension plane. The thin film structure 5 is arranged on the top surface 3 of the substrate 2. In the top view, this means that in a plane parallel to the main extension plane of the substrate 2, the thin film structure 5 of this embodiment has a strip-like shape. The top view refers to a view of the microstructure from the side of the thin film structure 5 facing away from the substrate 2.

[0061] 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 flat surface, which is parallel to the main extension plane of the substrate 2.

[0062] The lower part 6 is connected to a further side wall 7 which, in this embodiment, is perpendicular to the main extension plane of the substrate 2 and to the main extension plane of the lower part 6. The lower part 6 and the further side wall 7 form a lower edge in the region where the lower part 6 is connected to the further side wall 7. The further side wall 7 extends away from the substrate 2 in a vertical direction z, wherein the vertical direction z is perpendicular to the main extension plane of the substrate 2.

[0063] The raised portion 8 of the membrane structure 5 is connected to the further side wall 7 such that the raised portion 8 and the further side wall 7 form an upper edge in the region where the raised portion 8 is connected to the further side wall 7. Figure 2 In the embodiment shown, the raised portion 8 has a flat surface which is also parallel to the main extension plane of the base plate 2, but is located at an elevated level relative to the lower portion 6. The raised portion 8 is therefore spaced apart from the base plate 2. The raised portion 8 and the lower portion 6 do not overlap in the vertical direction z.

[0064] The protruding portion 9 is arranged in the region of the raised portion 8 of the thin film structure 5. The protruding portion 9 comprises at least one side wall 10, which extends in the vertical direction z from the raised portion 8 towards the substrate 2. The number of side walls 10 depends on the shape of the protruding portion 9. For example, in the case of a cross section through the protruding portion 9 in a plane extending parallel to the main extension plane of the substrate 2, the protruding portion 9 has a circular shape. This means that the protruding portion 9 has one side wall 10, while in the case of a rectangular cross section, the protruding portion 9 will have four side walls 10 corresponding to the four side surfaces.

[0065] exist Figure 2 In the embodiment of the embodiment of the invention, the side wall 10 is perpendicular to the main extension plane of the substrate 2. The protruding portion 9 of this embodiment has a cylindrical shape. Therefore, the protruding portion 9 comprises a bottom 11 which has a circular shape in this case. The bottom 11 is in mechanical contact with the substrate 2.

[0066] The projection 9 forms a hollow profile, in this case a hollow cylinder. This means that the projection 9 encloses a volume. At the top side of the projection 9, the enclosed volume is connected to the environment of the membrane structure 5, wherein the top side of the projection 9 faces away from the substrate 2. This means that the projection 9 is open at the top side. The raised part 8 has a hole 12 for the projection 9.

[0067] like Figure 2 As shown, Figure 1a and Figure 1b Compared to the example of , the stress distribution is significantly reduced. Therefore, the proposed configuration of the membrane structure 5, in particular the configuration of the raised membrane structure 5 including the protruding portion 9, improves the mechanical properties.

[0068] Figure 3a A further embodiment of a thin-film structure 5 is shown.

[0069] The thin film structure 5 has a square shape in a top view. However, the raised portion 8 of the thin film structure 5 has a circular shape in a top view. In all lateral directions x, y, the raised portion 8 is connected to the lower portion 6 via an annular additional side wall 7. Close to the additional side wall 7, the raised portion 8 includes a cylindrical protrusion 9. In this embodiment, the thin film structure 5 includes a plurality of protrusions 9, and each protrusion 9 is placed equidistantly from its adjacent protrusion 9. In this way, a plurality of protrusions 9 form a ring of protrusions 9 along the periphery of the protrusion 8. This arrangement of the thin film structure 5 forms a diaphragm. This means that the substrate 2, the additional side wall 7 and the raised portion 8 surround the cavity 13.

[0070] Figure 3b Shows Figure 3a 1 is a cutaway view of an embodiment of a thin film structure 5. On the one hand, the cavity 13 covered by the thin film structure 5 is shown. On the other hand, the protrusion 9 extending towards the substrate 2 (which is not shown in this illustration) is also visible.

[0071] Figure 4 A schematic cross section through one embodiment of a microstructure 1 is shown.

[0072] The microstructure 1 comprises a substrate 2 having a top surface 3 and a back surface 4. An opening 14 penetrates the substrate 2 and connects the back 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 microns and at most 750 microns. However, the depth dSi should be chosen to be as large as possible and can extend 750 microns and, for example, at most 2.5 millimeters.

[0073] On the top surface 3 of the substrate 2 is arranged a thin film structure 5. In a cross-sectional view, the thin film structure 5 comprises lower portions 6 at both sides of the opening 14. The lower portions 6 are in mechanical contact with the top surface 3 of the substrate 2.

[0074] In the cross-sectional view, the microstructure 1 further comprises a further sidewall 7 at both sides of the opening 14. 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 micrometers and at most 10 micrometers. Alternatively, the height ds of the further sidewall 7 can be at least 0.6 micrometers and at most 4 micrometers.

[0075] The microstructure 1 also includes a raised portion 8 of the thin film structure 5 that spans the opening 14 in the substrate 2. The diameter of the raised portion 8 from one further side wall 7 to another opposite further side wall 7 depends on the application. For example, in the case of a circular raised portion 8 forming a membrane, the diameter of the raised portion 8 may be at least 100 micrometers and at most 10 millimeters. Alternatively, the diameter of the raised portion 8 may be at least 500 micrometers and at most 1 millimeter.

[0076] At a distance dd from 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 micrometers and at most 500 micrometers. Alternatively, the distance dd between the protruding portion 9 and the further sidewall 7 can be at least 6 micrometers and at most 20 micrometers.

[0077] The diameter dp of the protrusion 9 in a plane extending parallel to the main extension plane of the substrate 2 depends on the application. For example, in the case of a cylindrical protrusion 9, the diameter dp can be at least 0.5 micrometers and at most 50 micrometers. Alternatively, the diameter dp of the protrusion 9 can be at least 2 micrometers and at most 8 micrometers.

[0078] The raised portion 8 is formed integrally with the lower portion 6, the protruding portion 9 and the further side wall 7. This means that the raised portion 8, the lower portion 6, the protruding portion 9 and the further side wall 7 are formed from one layer.

[0079] according to Figure 4 In the embodiment of the present invention, the opening 14 in the substrate 2 is arranged below the raised portion 8 in the vertical direction z. However, the extent of the opening 14 in the substrate 2 in the lateral directions x, y is smaller than the extent of the raised portion 8. In this way, the protruding portion 9, which is usually placed close to the further side wall 7, can be connected to the underlying substrate 2.

[0080] The protruding portion 9 can be arranged at a distance dsq from the opening 14. In the case where the raised portion 8 is a diaphragm, the distance dsq affects the damping of the diaphragm during vibration. Therefore, the distance dsq depends on the application. For example, the distance dsq can be at least 0.5 microns and at most 100 microns. Alternatively, the distance dsq can be at least 2 microns and at most 8 microns.

[0081] Figure 5 A top view of an embodiment of a microstructure 1 including a raised thin film structure 5 is shown. It should be noted that Figure 5 can be regarded as Figure 4 A top view of an embodiment of the present invention.

[0082] according to Figure 5 The embodiment of comprises a square substrate 2 in top view with a circular opening 14 in the center of the substrate 2. In addition, the embodiment comprises a circular thin film structure 5 on top of the substrate 2. The thin film structure 5 comprises an annular lower portion 6 in contact with the substrate 2. The annular lower portion 6 surrounds the entire thin film structure 5.

[0083] The raised portion 8 forms an inner circular area of ​​the thin film structure 5. In particular, the raised portion 8 forms a circle that concentrically overlaps the opening 14 in the substrate 2.

[0084] Additionally, Figure 5 The embodiment of FIG. 1 shows a plurality of protrusions 9 arranged around the opening 14. In this embodiment, the shape of the protrusions 9 varies in a circular or rectangular shape in a top view, respectively.

[0085] Figure 6 , 7 8 show cross-sectional views of other exemplary embodiments of the microstructure 1 .

[0086] according to Figure 6 Embodiments and basis Figure 4 The difference of the embodiment of is that there is no additional side wall 7 at one side of the raised portion 8 in the transverse direction x. The additional side wall 7 is only provided at one side of the raised portion 8. Therefore, Figure 6 The membrane structure 5 shown can be regarded as a rod or a beam.

[0087] according to Figure 7 Embodiments and basis Figure 6The embodiment of the embodiment of the present invention differs in that there is no opening 14 in the substrate 2. Since the raised thin film structure 5 is usually manufactured by using a sacrificial layer, the sacrificial layer must be removed at some point in the process. When no opening 14 is provided in the substrate 2, as in the present embodiment, the sacrificial layer cannot be approached from the back surface 4 of the substrate 2. In this case, the sacrificial layer can be removed via wet etching slots 15 in the thin film structure 5. For example, these slots 15 are formed by areas of the thin film structure 5 that do not have additional side walls 7. Such an area is provided in the present embodiment at one side of the raised portion 8 in the lateral direction x.

[0088] according to Figure 8 Embodiments and basis Figure 4 The embodiment of FIG. 1 differs in that there is no opening 14 in the substrate 2 . Figure 8 The thin film structure 5 shown can be regarded as a bridge above the substrate 2. As described above, the sacrificial layer can be removed by wet etching slots 15 in the thin film structure 5, for example slots 15 in the lateral direction y. Furthermore, this embodiment shows that both the side wall 10 and the further side wall 7 are transverse to the main extension plane of the substrate 2.

[0089] Fig. 9 A schematic diagram of an exemplary embodiment of a mobile device 17 comprising an omnidirectional optical MEMS microphone 16 comprising a microstructure 1 as described above is shown. The mobile device 17 can be, for example, a smart speaker device, a smart watch, a phone or a hearing aid device.

[0090] exist Figures 10a to 10i In FIG. 1 , an exemplary embodiment of a method for manufacturing a microstructure 1 is shown. The method relates to the processing of a membrane for use in an omnidirectional optical MEMS microphone 16 .

[0091] like Fig.10a As shown, the method includes providing a substrate 2. The substrate 2 has a back surface 4 and a top surface 3. The substrate 2 can include Si. On the top surface 3 of the substrate 2, a dielectric layer 18 is arranged at some positions. The dielectric layer 18 can include, for example, silicon oxide (SiO2). However, a portion 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 toward the substrate 2.

[0092] In the next step, an etch stop layer 19 is deposited at some locations onto the top surface 3 of the substrate 2 ( Fig.10b ). The etch stop layer 19 may include 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 configured to form the opening 14 in the substrate 2, as described below.

[0093] In the next step, a reflective layer 20 is deposited in the central region on top of the etch stop layer 19 ( Fig.10c ). The reflective layer 20 may include gold (Au) or titanium (Ti). The reflective layer 20 has a back surface facing the etch stop layer 19, a top surface facing away from the substrate 2, and a side surface. The reflective layer 20 is arranged as a reflector, which is attached to the film at the end of the process. By using a reflector, the deflection of the film can be optically evaluated. This means that the laser beam reflected at the mirror can be analyzed by using an interferometer.

[0094] Fig.10d The deposition of the first sacrificial layer 21 is shown. The first sacrificial layer 21 may include 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 on a portion of the etch stop layer 19 is removed 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 results in a corrugated membrane as described below.

[0095] Fig.10e The deposition of a second sacrificial layer 22 on top of the first sacrificial layer 21 and on the released portion of the etch stop layer 19 is shown. The second sacrificial layer 22 may also include W-Ti. Due to the underlying morphology, the second sacrificial layer 22 has a corrugated top surface. This means that the second sacrificial layer 22 does not have a flat surface parallel to the main extension plane of the substrate 2 at some locations. Instead, the second sacrificial layer 22 exhibits an area extending in the vertical direction z.

[0096] In the patterning process, two sacrificial layers 21, 22 are removed on the dielectric layer 18. The sacrificial layers 21, 22 are removed in the region adjacent to the dielectric layer 18 in the lateral directions x, y to release the substrate 2. The sacrificial layers 21, 22 are also removed in the central region above the reflective layer 20 to release the reflective layer 20. In this way, the reflective layer 20 is free of the sacrificial layers 21, 22 at its top surface and at its side surfaces.

[0097] Furthermore, one or more trenches 23 are formed in the stack of sacrificial layers 21, 22 at some locations below which there is no etch stop layer 19. The trenches 23 extend toward the substrate 2 to release the substrate 2. The trenches 23 are provided to form the protruding portion 9 of the thin film structure 5 after the sacrificial layers 21, 22 are completely removed.

[0098] In the 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 include SiN. The thin film is removed on a portion of the dielectric layer 18 to form a thin film structure 5 of a desired shape. In addition, the slot opening 25 can be realized by removing the thin film in a small area above the second sacrificial layer 22.

[0099] In order to form the opening 14, the substrate 2 must be turned over and back-side processed ( Figure 10g ). An opening 14 is formed from the back 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 19 of the etch stop layer.

[0100] exist 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 edge on the top surface 3 of the substrate 2. The remaining etch stop layer 19 can be used as a damping structure 26 for the diaphragm.

[0101] In a next step, the sacrificial layers 21 , 22 are removed and the device can be turned over for possible further front-side processing. The thin film structure 5 forms a membrane for use in an omnidirectional optical MEMS microphone 16 .

[0102] The obtained microstructure 1( Fig.10i ) is similar to Figure 4 Embodiment of the microstructure 1 and Figure 4 The differences of the embodiments are as follows:

[0103] The dielectric layer 18 is arranged around the opening 14 in the substrate 2 at a distance from the opening 14. The thin film structure 5 comprises a portion of the top surface covering the dielectric layer 18 and portions of the side surfaces of the dielectric layer 18 pointing towards the opening 14 in lateral directions x, y.

[0104] The lower part 6 of the thin film structure 5 in mechanical contact with the substrate 2 is arranged in a region adjoining the dielectric layer 18 in lateral directions x, y towards the opening 14. The lower part 6 is connected to the part of the thin film structure 5 covering the side surface of the dielectric layer 18.

[0105] The raised portion 8 of the film structure 5 is corrugated. This means that the raised portion 8 is not planar but has areas parallel to the main extension plane of the substrate 2 and areas perpendicular to the main extension plane.

[0106] Additionally, the raised portion 8 of the thin film structure 5 comprises a central area, wherein the reflective layer 20 is attached to the raised portion 8. The reflective layer 20 is attached to the side of the raised portion 8 facing the opening 14 in the substrate 2. This means that the back surface of the reflective layer 20 has no other layers. The upper surface and the side surfaces of the reflective layer 20 are both covered by the thin film structure 5.

[0107] The raised portion 8 of the membrane structure 5 also includes a slotted orifice 25 in a small area at the periphery of the raised portion 8. The slotted orifice 25 is provided for gas exchange from one side of the membrane structure 5 to the other side. This can be necessary because changes in gas density due to membrane deflection can affect the compliance of the system. Thus, the slotted orifice provides pressure balance between the two sides of the membrane structure. Additionally, the slotted orifice affects the frequency range of the device. In order to achieve high acoustic impedance, the diameter of the slotted orifice should be small.

[0108] Furthermore, due to the remaining etch stop layer 19, the microstructure 1 comprises a damping structure 26. The damping structure 26 is arranged at the edge of the opening 14 on the top side of the substrate 2. The damping structure 26 provides protection against strong deflections of the membrane.

[0109] However, if Figure 4 As in the embodiment, Fig.10i The embodiment comprises protruding portions 9. The protruding portions 9 are formed by portions of thin films deposited within the trenches 23 in the sacrificial layers 21, 22. The protruding portions 9 give the diaphragm additional stability, as they connect the raised portion 8 to the substrate 2 and inherently increase the sidewall area.

[0110] Reference numerals

[0111] 1 Microstructure

[0112] 2 substrate

[0113] 3 Substrate top surface

[0114] 4 Substrate back surface

[0115] 5. Film structure

[0116] 6 Lower part of the membrane structure

[0117] 7 Additional side walls of the membrane structure

[0118] 8 Raised part of the membrane structure

[0119] 9 Protrusions of membrane structures

[0120] 10 Side wall of the protruding part

[0121] 11 Bottom of the protrusion

[0122] 12 Hole in raised part

[0123] 13 Cavity

[0124] 14 Opening

[0125] 15 slots

[0126] 16 omnidirectional optical MEMS microphones

[0127] 17 Mobile devices

[0128] 18 Dielectric layer

[0129] 19 Etch stop layer

[0130] 20 Reflection layer

[0131] 21 First Sacrificial Layer

[0132] 22 Second Sacrificial Layer

[0133] 23 Grooves in the sacrificial layer

[0134] 24 films

[0135] 25 slot opening

[0136] 26 Damping structure

[0137] dSi Opening Depth

[0138] ds Height of the other side wall

[0139] dd Distance between the other side wall and the protruding part

[0140] dpDiameter of the protruding part

[0141] dsq Distance between opening and protrusion

[0142] x, y horizontal direction

[0143] z vertical direction

Claims

1. A microstructure (1) for use in a microelectromechanical device, comprising a substrate (2) having a top surface (3) and a back surface (4) and a thin film structure (5) arranged at the top surface (3) of the substrate (2), the thin film structure (5) comprising: - a raised portion (8) spaced apart from the base plate (2), - a lower portion (6) of the thin film structure, which is in mechanical contact with the substrate (2), at least one protruding part (9) which is hollow and has a bottom (11) and at least one side wall (10) and which mechanically connects the raised part (8) to the base (2) via the bottom (11), and at least one further side wall (7) of the membrane structure (5) which is at a distance from the at least one protruding portion (9), wherein the further side wall (7) mechanically connects the lower part (6) of the membrane structure (5) to the raised portion (8), wherein - the raised portion (8) of the thin film structure (5) is a movable portion of the microstructure (1), - The protrusion (9) forms a hollow profile and encloses a volume, which is delimited in the vertical direction by the bottom (11) and in all lateral directions by the at least one side wall (10), and at the top side of the protrusion (9), the enclosed volume is connected to the environment of the film structure (5), wherein the top side of the protrusion (9) faces away from the substrate (2).

2. The microstructure (1) according to claim 1, wherein: The further side wall (7), the raised portion (8) and the base plate enclose a cavity (13) between the base plate (2) and the raised portion (8).

3. The microstructure (1) according to any one of claims 1 to 2, further comprising: An opening (14) in the substrate (2) extends from the back surface (4) of the substrate (2) towards the raised portion (8) of the thin film structure (5).

4. The microstructure (1) according to any one of claims 1 to 2, wherein: The membrane structure (5) is an element of the group consisting of a diaphragm, a beam, a rod and a bridge.

5. The microstructure (1) according to any one of claims 1 to 2, wherein: In a top view, the raised portion (8) of the film structure (5) has a circular or rectangular shape.

6. The microstructure (1) according to any one of claims 1 to 2, wherein: The film structure (5) comprises a plurality of protruding portions (9).

7. The microstructure (1) according to claim 6, wherein: In a top view, at least two of the plurality of protruding portions (9) have different shapes.

8. The microstructure (1) according to any one of claims 1 to 2, wherein: In a top view, the at least one protruding portion (9) has a circular, elliptical, rectangular, polygonal or sickle-shaped shape.

9. The microstructure (1) according to any one of claims 1 to 2, wherein: At least one side wall (10) of the protruding portion (9) and / or at least one further side wall (7) of the film structure (5) is perpendicular or transverse to a main extension plane of the substrate (2).

10. The microstructure (1) according to any one of claims 1 to 2, wherein: The raised portion (8) of the film structure (5) is corrugated.

11. An omnidirectional optical micro-electromechanical microphone (16) comprising a microstructure (1) according to one of claims 1 to 10.

12. A mobile device (17) comprising a microstructure (1) according to one of claims 1 to 10 or an omnidirectional optical micro-electro-mechanical microphone (16) according to claim 11.

13. A method of manufacturing a microstructure (1) for use in a micro-electromechanical device, the method comprising: Providing a substrate (2) having at least one sacrificial layer (21, 22), the at least one sacrificial layer being arranged at some locations of the top surface (3) of the substrate (2), At least one groove (23) is formed in the sacrificial layer (21, 22), Depositing a thin film (24) on the sacrificial layer (21, 22), the groove (23) and the substrate (2) so that the thin film is in mechanical contact with the substrate (2) at some positions, A thin film structure (5) is formed from a thin film (24) by removing the sacrificial layers (21, 22), wherein: The microstructure (1) comprises a thin film structure (5) and a substrate (2). The thin film structure (5) comprises at least one protruding portion (9), which is formed by a portion of a thin film (24) deposited in the at least one groove (23), the at least one protruding portion (9) having at least one side wall (10) and a bottom (11), the bottom (11) being mechanically connected to the substrate (2), and The thin film structure (5) comprises a raised portion (8) spaced apart from the substrate (2), wherein the raised portion (8) is formed by a portion of a thin film (24) deposited on the sacrificial layer (21, 22), wherein the raised portion (8) of the thin film structure (5) is a movable portion of the microstructure (1), The protrusion (9) forms a hollow profile and encloses a volume, which is delimited in the vertical direction by the bottom (11) and in all lateral directions by the at least one side wall (10), and at the top side of the protrusion (9), the enclosed volume is connected to the environment of the film structure (5), wherein the top side of the protrusion (9) faces away from the substrate (2).

14. The method according to claim 13, further comprising: An opening (14) is formed in the substrate (2), the opening (14) extending from the back surface (4) of the substrate (2) toward the raised portion (8) of the thin film structure (5).

15. The method according to claim 14, further comprising: The sacrificial layers (21, 22) are removed after forming the opening (14) in the substrate (2).

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