Patent Description:
Sound transducers such as microphones and/or loudspeakers may be implemented as micromechanical systems (MEMS). Such MEMS-transducers may be implemented using semiconductor materials, enabling a production of the transducers on a wafer level. Such transducers may comprise a movable or vibratable electrode and a static electrode. The movable electrode may be implemented as membrane or diaphragm being deflectable with respect to the static electrode. Document <CIT> discloses a microfabricated MEMS device which includes a cavity disposed in a substrate, a first clamping layer overlying the substrate, a deflectable membrane overlying the first clamping layer, and a second clamping layer overlying the deflectable membrane. A portion of the second clamping layer overlaps the cavity. The clamping layer may be formed as a tetraethyl orthosilicate (TEOS) oxide. Alternatively, the clamping layer may be formed of another insulating material, such as a dielectric or another oxide.

There is a request for MEMS-transducers comprising a high robustness, in particular with respect to a mechanical load or stress.

The invention provides a MEMS-transducer comprising a membrane structure having a first main surface and a second main surface opposing the first main surface. The MEMS-transducer comprises a substrate structure configured to hold the membrane structure, wherein the substrate structure overlaps with the first main surface of the membrane structure in a first edge region being adjacent to a first inner region of the first main surface. A gap is formed between the membrane structure and the substrate structure in the first edge region, the gap extending from the first inner region into the first edge region. The gap allows for a low mechanical load acting on the membrane structure during deflection and therefore for a high robustness with regard to a mechanical load. The MEMS transducer also comprises a carbon layer comprising a carbon material and arranged on the first main surface in an outer region of the first main surface adjacent to the gap, the gap extending from the inner region towards the outer region.

The invention also provides a method for producing a MEMS-transducer. The method comprises arranging a layer stack comprising a membrane structure, a substrate structure holding the membrane structure and a carbon layer arranged between the membrane structure and the substrate structure. The method comprises removing the carbon layer at least partially so as to generate a gap between the membrane structure and the substrate structure in a first edge region, such that the gap extends from a first inner region of a first main surface to the membrane structure into a first edge region in which the substrate structure overlaps with the first main surface of the membrane structure, the first edge region being adjacent to the first inner region of the first main surface. In the method the carbon layer remains in an outer region adjacent to the gap, the gap extending from the inner region towards the outer region.

Embodiments of the invention will be described in the following while making reference to the accompanying drawings in which:.

In the following description, a plurality of details is set forth to provide a more thorough explanation of embodiments of the present invention. However, it will be apparent to those skilled in the art that embodiments of the present invention may be practiced without these specific details. In other instances, while known structures and devices are shown in block diagram forms rather than in detail in order to avoid obscuring embodiments of the present invention. In addition, features of the different embodiments described hereinafter may be combined with each other, unless specifically noted otherwise.

Embodiments described herein relate to micromechanical structures (MEMS) forming a sound transducer or comprising such a sound transducer. MEMS-sound transducers may comprise or form a loudspeaker and/or a microphone. The MEMS-sound transducers or MEMS-transducers are configured to effect a movement of a movable element, i.e., a membrane, based on an electric driving signal such that a fluid is moved responsive to the movement of the membrane and such that a sound pressure level is generated in the fluid. In contrast to the described loudspeaker-configuration, a movement in the fluid may effect a deflection of the membrane, the deflection being detectable by measuring a variable electric potential and/or a variable electric capacity in a microphone-configuration. In the microphone-configuration, an electric signal may be obtained based on the movement in the fluid.

MEMS-transducers may be manufactured in semiconductor-technology and/or may comprise semiconductor materials. Examples for such materials are layers or wafers comprising a silicon material, a gallium-arsenite-material and/or a different semiconductor material. MEMS-structures may comprise one or more layer sequences or stacks of layers comprising conductive, semiconductive and/or insulating layers so as to implement a respective MEMS-functionality. In embodiments described herein, one or more backplate electrodes may form a stack together with a membrane structure, wherein the backplate electrodes and the membrane structure may be held, fixed and/or clamped at respective outer regions with a substrate structure. The substrate structure may comprise, for example, amorphous, polycrystalline or crystalline semiconductor materials such as silicon.

<FIG> shows a schematic side view of a MEMS-transducer <NUM><NUM>.

The MEMS-transducer may form or may at least be integrated into a MEMS-microphone or a MEMS-loudspeaker. The MEMS-transducer <NUM><NUM> may comprise a backplate structure <NUM> forming a stack with a membrane structure <NUM>, i.e., the membrane structure <NUM> may be arranged so as to oppose the backplate structure <NUM>. The membrane structure <NUM> may be movable and/or vibratable with respect to the backplate structure <NUM>. For example, when compared to the membrane structure <NUM>, the backplate structure <NUM> may comprise a comparatively high stiffness and may be regarded as stationary with respect to the membrane <NUM>. The MEMS-transducer may comprise a substrate structure <NUM> configured to hold the membrane <NUM> and/or the backplate structure <NUM>. The substrate structure <NUM> may comprise a doped or undoped semiconductor material such as a silicon material and/or a gallium-arsenite material or the like. The substrate structure <NUM> may comprise one or more layers. Further, the substrate structure <NUM> may be arranged adjacent to a first main surface 14A of the membrane structure and adjacent to a second main surface 14B of the membrane structure. The main surfaces 14A and 14B may be the surfaces of the membrane structure <NUM> configured to interact with a fluid. According to an example, the main surfaces 14A and 14B may be those surfaces of the membrane structure <NUM> comprising a larger surface when compared to a side surface 14C of the membrane structure <NUM>, the side surface 14C connecting the main surfaces 14A and 14B. Thus, the main surfaces 14A and 14B may be arranged so as to oppose each other.

The substrate structure <NUM> may thus overlap with an edge region 18A of the main surface 14A and/or with an edge region 18B of the second main surface 14B. Such an overlap may comprise a mechanical contact between the substrate structure <NUM> and the membrane structure <NUM> that is not limited hereto. In particular, between the substrate structure <NUM> and the first main surface 14A and/or the second main surface 14B, there may be arranged a gap <NUM>.

The gap <NUM> may be arranged so as to extend an inner region 24A of the main surface 14A towards or into the edge region 18A, from an inner region 24B of the second main surface 14B towards the edge region 18B, respectively. The gap <NUM> may provide for a contactless overlap of the substrate structure <NUM> in a region of the gap <NUM>. For example, the substrate structure <NUM> may be removed partially so as to obtain the gap <NUM> and so as to further release the membrane structure <NUM>.

For example, the membrane structure <NUM> may be deflectable or vibratable along a deflection direction <NUM> that may be arranged in parallel to a surface normal <NUM> of the main surface 14A or 14B. For example, a deflection of the membrane structure <NUM> along a positive deflection direction <NUM> may lead to an abutment of the membrane structure <NUM> against the substrate structure <NUM> in a corner region 32A thereof. As a result of said abutment, main surface 14A would experience a compressive stress, while main surface 14B would experience a tensile stress. The membrane structure <NUM> may be formed, for example, so as to comprise a semiconductor material and may thus be robust with respect to compressive stress whilst being less robust with respect to tensile stress. When imagining an absence of the gap <NUM> and a presence of the substrate structure <NUM> in the region of the gap <NUM>, the substrate structure <NUM> being further fixed or adhered to the membrane structure <NUM> and when further imagining a deflection of the membrane structure <NUM> towards a negative deflection direction <NUM>, then a mentioned tensile stress might act on the membrane structure <NUM> at the corner region 32A and might thus lead to a damage of the membrane structure <NUM> which is prevented by use of the gap <NUM>.

However, in presence of the gap <NUM>, when being deflected along the negative deflection direction <NUM>, in a corner region 32B, a compression force may act on the membrane structure <NUM>. When deflecting the membrane structure <NUM> towards the positive deflection direction <NUM>, then only low peeling forces may act in the corner region 32B as the deflection in the region of the corner region 32B is limited by the gap <NUM>, the abutment at the corner region 32A respectively. Thus, the MEMS-transducer <NUM><NUM> may comprise a high robustness to deflecting forces, the deflecting forces being obtained by a fluid pressure and/or electronic signals.

The gap <NUM> may at least partially uncover the first main surface 14A, the second main surface 14B, respectively. When uncovering the respective main surface partially, an outer region 34A of the substrate structure <NUM> may remain in contact with the membrane structure <NUM>.

In the absence of the gap <NUM>, although the edge region 18A is illustrated as covering a large portion of the first main surface 14A when compared to the edge portion 18B covering a comparatively small portion of the main surface 14B of the MEMS structure, the edge portion 18B may be equal to the edge portion 18A or may be larger when compared to the edge portion 18A. For example, the edge portion 18A may be larger than the edge portion 18B when configuring the MEMS-transducer <NUM><NUM> as a so-called bottom port transducer, e.g., when sound is expected to arrive at the main surface 14B. Alternatively, when providing a top port transducer for which sound is expected to arrive at the main surface 14A, the edge portion 18B may be larger when compared to the edge portion 18A. In contrast, since the gap <NUM> results in a substantially symmetric robustness of the membrane structure <NUM>, the relative length and overlap of the edge portions 18A and 18B can be independent of the top or bottom port configuration.

The membrane structure <NUM> may comprise a conductive layer <NUM><NUM>, for example, comprising a conductive semiconductive material. A conductive semiconductive material may be obtained, for example, by doping a semiconductive material. This may allow using the conductive layer <NUM><NUM> as an electrode layer. Alternatively or in addition, the backplate structure <NUM> may comprise a conductive layer <NUM><NUM> so as to provide for an electrode layer in the backplate structure <NUM>. Between the backplate structure <NUM> and the membrane structure <NUM>, one or more insulating layers <NUM><NUM> may be arranged so as to prevent an electric short-circuit in case of a mechanical contact between the membrane structure <NUM> and the backplate structure <NUM>. Alternatively or in addition, a mechanical protection or stiffening may be obtained by use of the insulating layer <NUM><NUM>. For passivation and/or mechanical protection, further insulating layers <NUM><NUM> may be arranged so as to cover one or more sides of the conductive layers <NUM><NUM> and/or <NUM><NUM>. The insulating layers <NUM><NUM> and/or <NUM><NUM> may comprise, for example, an insulating material such as a silicon-nitride material (SiN), a silicon-oxide material (SiO) or a different insulating material. As a stiffness of the insulating layers <NUM><NUM> and/or <NUM><NUM> may be higher when compared to a stiffness of a conductive material of the conductive layers <NUM><NUM> and <NUM><NUM>, an arrangement of the insulating layers <NUM> at immobile electrode structures such as the backplate structure <NUM> may allow for a low influence of the insulating layers on the vibrational behavior. On the other hand, according to an embodiment, an insulating layer <NUM> may be arranged at the main surface 14A and/or 14B so as to obtain a membrane structure <NUM> with a high robustness.

<FIG> shows a schematic block diagram of a MEMS-transducer <NUM><NUM> according to an embodiment. When compared to the MEMS-transducer <NUM><NUM>, the MEMS-transducer <NUM><NUM> comprises a carbon layer arranged therein having at least one carbon layer <NUM>, the carbon layer <NUM> comprising a carbon material. The carbon layer <NUM> is arranged in the outer region 34A adjacent to the gap <NUM>, i.e., the gap <NUM> extends from the inner region 24A towards the outer region 34A and thus connects the carbon layer <NUM> and the inner region 24A.

The gap <NUM> may be formed so as to overlap with the edge region 18B on the opposing main surface 14B of the membrane structure <NUM> by an overlap extension <NUM> that may be, for example, at least <NUM>, e.g., at least <NUM> and at most <NUM>, at least <NUM> and at most <NUM> or at least <NUM> and at most <NUM>, wherein especially high values are possible. It is noted, that the second edge region may be used as reference in case the MEMS-transducer is formed without a gap on the second main surface 14B as the edge region 18B then may be equal to an area or region in which the first main surface 14B is mechanically fixed to the substrate structure <NUM>. For example, the gap <NUM> may thus overlap with the region of fixing when both, the gap <NUM> and the region of fixing are projected into the main surface 14A or 14B. According to an alternative embodiment, a further gap may be arranged at the main surface 14B so as to allow preventing a hot spot at the second main surface 14B such that the gaps and respective corner regions may limit the deflection of the membrane structure <NUM> and thus the mechanical load along both directions.

The carbon layer <NUM> and the gap <NUM> are arranged in a same layer such that the carbon layer <NUM> is arranged between an outer edge <NUM> of the MEMS-transducer <NUM><NUM> and the gap <NUM>. The carbon layer <NUM> may be a part of the substrate structure <NUM> and may be configured for clamping or holding the membrane structure <NUM>.

The gap <NUM> may comprise an extension along the deflection direction <NUM> and/or along the surface normal <NUM>, i.e., along a thickness direction being at least <NUM> and at most <NUM>, at least <NUM> and at most <NUM> or at least <NUM> and at most <NUM>. Within the gap <NUM> and thereby reducing the effective free space, remains <NUM> of an oxidation process or ashing process may be arranged. For example, the gap <NUM> may be obtained by oxidizing or ashing the carbon layer <NUM>, thereby generating the ash material <NUM>. The ash material <NUM> may thus be arranged between the substrate structure <NUM> and the membrane structure <NUM>.

A further extension <NUM> of the gap <NUM>, which may be referred to as a broadness or a length of the gap <NUM> may be an extension between the inner region 24A and the edge region 34A. The extension <NUM> may be arranged along a direction perpendicular to the surface normal <NUM>/thickness direction and may be arranged towards the outer edge <NUM> of the membrane structure <NUM>. The extension <NUM> may be, for example, at least <NUM> and at most <NUM>, at least <NUM> and at most <NUM> or at least <NUM> and at most <NUM> such as approximately <NUM>. Any other value may be designed, for example, based on a dimension of the MEMS-sound transducer and/or based on the process used for generating the gap <NUM>.

<FIG> shows a schematic side view of a MEMS-transducer <NUM><NUM> according to an embodiment. When compared to the MEMS-transducers <NUM><NUM> and <NUM><NUM> being implemented as a single-backplate transducer, the MEMS-transducer <NUM><NUM> may be implemented as a dual-backplate transducer. , the membrane structure <NUM> may be arranged between two backplate structures <NUM><NUM> and <NUM><NUM>, wherein both backplate structures <NUM><NUM> and <NUM><NUM> may be implemented as an electrode structure.

Between the backplate structure <NUM><NUM> and the membrane structure <NUM> may be arranged one or more anti-stiction bumps that may be arranged at or part of the backplate structure <NUM><NUM> and/or the membrane structure <NUM>. Alternatively or in addition, between the membrane structure <NUM> and the backplate structure <NUM><NUM>, there may be arranged one or more anti-stiction bumps <NUM>, wherein the anti-stiction bumps <NUM> may be arranged at or part of the membrane structure <NUM> and/or the backplate structure <NUM><NUM>. The anti-stiction bumps <NUM> may allow preventing stiction between the backplate structure <NUM><NUM> and the membrane structure <NUM>, the membrane structure <NUM> and the backplate structure <NUM><NUM>, respectively.

The backplate structure <NUM><NUM> and/or the backplate structure <NUM><NUM> may be formed as the backplate structure <NUM> of the MEMS-transducer <NUM><NUM> and/or <NUM><NUM>. The backplate structures <NUM><NUM> and/or <NUM><NUM> may comprise release holes <NUM> that allow a travel of etching material such as plasma during a manufacturing of the MEMS-transducer <NUM><NUM> towards the substrate structure <NUM>, the membrane structure <NUM> and the carbon layer <NUM> in a region between the backplate structure <NUM><NUM>, <NUM><NUM>, respectively, and the membrane structure <NUM>. This allows for removing the substrate structure <NUM> and/or the carbon layer <NUM>. For example, the substrate structure <NUM> may comprise a material such as a silicon material or a TEOS material (tetraethyl orthosilicate). This may allow for a high selectivity of an etching process when compared to the carbon layer <NUM>. Thus, the substrate structure <NUM> and/or the carbon layer <NUM> may be removed, at least partially, by performing an undercut or a lateral etching.

An extension 58A or 58B of such a lateral etching may be, for example, at least <NUM> and at most <NUM>, at least <NUM> and at most <NUM> or at least <NUM> and at most <NUM>, e.g., between <NUM> and <NUM>. Etching from a side adjacent to the main surface 14A during a first instance of time and etching from a side adjacent to the main surface 14B separately allows for obtaining different extensions 58A and 58B of the lateral etchings. Alternatively, a position of the release holes <NUM> may be selected appropriately so as to obtain edge regions 18A and 18B of different extensions. The gap <NUM> allows for preventing a hotspot of forces acting on the membrane structure <NUM> at the corner region 32A. At the same time, the small height or thickness of the gap <NUM> allows an early abutment of the membrane structure <NUM> at the corner region 32A so as to prevent a hotspot of such forces at a corner region 32B on the main surface 14B. Thereby, an approximately same robustness with regard to pulling forces and pushing forces may be obtained.

<FIG> shows a schematic side view of a MEMS-transducer <NUM><NUM> according to an embodiment. When compared to the MEMS-transducer <NUM><NUM>, the MEMS-transducer <NUM><NUM> comprises two gaps <NUM><NUM> and <NUM><NUM>, one gap arranged on each main surface 14A and 14B. , prevention of a hotspot of forces acting on the membrane structure <NUM> may be implemented on both sides of the membrane structure <NUM>. For example, a carbon layer <NUM><NUM> may be arranged between the main side 14A and the substrate structure <NUM> and a further carbon layer <NUM><NUM> may be arranged between the main surface 14B and the substrate structure <NUM>. By removing the carbon layer <NUM><NUM> and the carbon layer <NUM><NUM> partially, gaps <NUM><NUM> and <NUM><NUM> may be generated. Based on extensions of the respective carbon layer <NUM><NUM> and <NUM><NUM>, same or different heights <NUM><NUM> and <NUM><NUM> of the gaps <NUM>, and <NUM><NUM> may be obtained. Alternatively or in addition, same or different extensions/lengths <NUM><NUM> and <NUM><NUM> may be generated.

Preventing a hotspot of forces using one or more gaps <NUM> may also allow for generating the edge regions 18A and 18B as having a same size between the outer edge <NUM> and the inner region 24A, the inner 24B, respectively. Whilst different extensions of the edge regions 18A and 18B may allow for defining a preferred direction along which the deflection of the membrane structure <NUM> is robust, such a measure may be unnecessary due to the positive effects of the gaps <NUM><NUM> and/or <NUM><NUM>.

In other words, based on the single or double-carbon gap <NUM>, the overlap of the TEOS edges as shown in <FIG> is uncritical.

<FIG> shows a schematic side view of a MEMS-transducer <NUM> according to an embodiment, wherein the MEMS-transducer <NUM> may be implemented similar to the MEMS-transducer <NUM><NUM>. The backplate structure <NUM> may comprise anti-stiction bumps directed towards the membrane structure <NUM> so as to prevent stiction.

Although the MEMS-transducers <NUM><NUM>, <NUM><NUM> and <NUM> are described as having a remaining portion of the carbon layer <NUM> after removing the same so as to obtain the gap <NUM>, according to an embodiment, the carbon layer <NUM>, <NUM>, and/or <NUM><NUM> may be removed completely. This may allow the gap to completely uncover the first main surface 14A and/or the second main surface 14B.

In the following, whilst making reference to <FIG>, manufacturing of a MEMS-transducer outside the invention is described, wherein the MEMS-transducer does not only comprise gaps completely uncovering both main surfaces 14A and 14B of the membrane structure but additionally comprising a further, third gap connecting the first and the second gaps such that the first, second and third gaps provide for a slack holding of the membrane structure <NUM> by the substrate structure <NUM>. , the membrane structure <NUM> may be unclamped or loosely held.

<FIG> shows a schematic side view of a layer stack <NUM>' comprising the substrate structure <NUM>. By way of non-limiting example only, starting from a bottom side <NUM>, the substrate structure <NUM> may be arranged, grown or deposited. After having reached a certain thickness of the substrate structure <NUM>, the backplate structure <NUM><NUM> may be deposited, generated or arranged, for example, by depositing, arranging or growing a set of layers comprising insulting layers and conductive layers. On top of the backplate structure <NUM><NUM>, further substrate materials of the substrate structure <NUM> may be arranged. For example, into the substrate structure <NUM> a topography may be inserted, for example, by implementing an etching process for selectively removing a part of the substrate structure <NUM>. Thereby, holes may be obtained that may at least partially define anti-stiction bumps arranged at the membrane structure <NUM> later. A carbon layer <NUM><NUM> may be deposited, generated or arranged, followed by depositing the membrane material. Previously, at the same time or afterwards, a carbon layer <NUM><NUM> may be arranged along a height of the membrane structure <NUM> so as to provide for an encapsulation of the membrane structure <NUM> with respect to the substrate structure <NUM>. On top of the membrane structure <NUM>, the carbon layer <NUM><NUM> may be deposited, followed by a deposition of further substrate materials. Further, the backplate structure <NUM><NUM> may be arranged, generated or deposited, e.g., by arranging, depositing or growing one or more respective layers.

In other words, a membrane being encased or enveloped by carbon material may be used.

<FIG> shows a schematic side view of a layer stack <NUM>" that may be obtained by partially removing material of the substrate structure <NUM>, for example, by removing the substrate material arranged between a respective backplate structure <NUM><NUM> or <NUM><NUM> and the membrane structure <NUM>. Thereby, the backplate structures <NUM><NUM> and <NUM><NUM> may be released as well as the membrane structure <NUM> with exception of the carbon layers <NUM><NUM>, <NUM><NUM> and <NUM><NUM>. Based on the etching process, the edge regions 18A and 18B may be defined, in particular with regard to their extensions or sizes.

<FIG> shows a schematic side view of a MEMS-transducer <NUM><NUM> that may be obtained by further processing the layer stack <NUM>" depicted in <FIG>. For example, by use of the release holes <NUM>, the carbon layers <NUM><NUM>, <NUM><NUM> and <NUM><NUM> may be removed partially or completely. <FIG> shows a scenario in which the carbon layers <NUM><NUM>, <NUM><NUM> and <NUM><NUM> are removed completely so as to generate the membrane structure <NUM> as being loosely held by the substrate structure <NUM> as all of the clamping material, i.e., the carbon layers, is removed. Alternatively, the carbon layers <NUM><NUM>, <NUM><NUM> and/or <NUM><NUM> may remain partially, possibly preventing thereby a generation of the gap <NUM><NUM>.

<FIG> shows a schematic side view of a MEMS-transducer <NUM><NUM> outside the invention. When compared to the MEMS-transducer <NUM><NUM> the MEMS-transducer <NUM><NUM> may be implemented as a single backplate structure. For example, when referring again to <FIG>, a generation of the backplate structure <NUM><NUM> may be omitted. As described with respect to the MEMS-transducer <NUM><NUM>, the membrane structure <NUM> may be completely released from the substrate structure <NUM>. Alternatively, in an embodiment of the invention the membrane structure <NUM> is fixed to the substrate by remaining parts of the carbon layers <NUM><NUM>, <NUM><NUM> and/or <NUM><NUM>. Alternatively, in an example outside the invention the membrane structure <NUM> is fixed to the substrate by connecting the membrane structure <NUM> directly to the substrate structure <NUM> as described, for example, in connection with <FIG>.

<FIG> shows a schematic flowchart of a method <NUM> for producing a MEMS-transducer such as the MEMS-transducer <NUM><NUM>, <NUM><NUM>, <NUM><NUM>, <NUM><NUM>, <NUM>, <NUM><NUM> and/or <NUM><NUM>. The method <NUM> may comprise a step <NUM> in which a layer stack is arranged, the layer stack comprising a membrane structure, a substrate structure holding the membrane structure and a carbon layer arranged between the membrane structure and the substrate structure. When referring to <FIG> again, the layer stack <NUM>' may be used for method <NUM> and obtained by a step <NUM> comprising providing a substrate, by a step <NUM> comprising arranging a first oxide layer, by a step <NUM> comprising arranging a first conductive backplate layer, by a step <NUM> comprising arranging a second oxide layer, wherein steps <NUM>, <NUM> and <NUM> may be used for generating the backplate structure <NUM><NUM>. Step <NUM> may further comprise arranging a first carbon layer such as the carbon layer <NUM><NUM> in an optional step <NUM>, arranging a membrane structure in a step <NUM> and arranging a second carbon layer such as the carbon layer <NUM><NUM> in a step <NUM>. Meanwhile or additionally, the carbon layer may also be arranged at a lateral side of the membrane structure. According to embodiments, both steps <NUM> and <NUM> are executed resulting in two carbon layers that allow to implement a gap on each side of the membrane structure as it is described, for example, in connection with <FIG>. According to embodiments, it may be sufficient to implement only one of both steps, resulting in step <NUM> being optional without restricting method <NUM> to a specific side of the membrane structure at which the gap or carbon layer is arranged. This may be understood as step <NUM> being optional instead of step <NUM> and/or as a sequence of steps being variable in method <NUM>, e.g., arranging the carbon layer may be performed prior and/or after arranging the membrane structure.

A step <NUM> of step <NUM> may comprise arranging a further oxide layer, for example, an oxide layer of the backplate structure <NUM><NUM> facing the membrane structure <NUM>. A step <NUM> may comprise arranging a further conductive backplate layer. As described in connection with the carbon layers, an order of steps illustrated in <FIG> may be in accordance with the depicted sequence or order but may also deviate. For example, as an alternative to execute both steps <NUM> and <NUM> so as to obtain a dual-backplate structure only one of both steps may be executed, being referred to as step <NUM>. This step may be executed before or after arranging the membrane.

Optionally, contacts may be formed and/or a passivation may be obtained in step <NUM> and/or oxide material may be removed in an optional step <NUM>, e.g., a part of an oxide layer that may be arranged on the backplate layer obtained in step <NUM>. Both steps are optional and so is an order or sequence thereof when performing both steps <NUM> and <NUM>. According to an embodiment, forming contacts and/or passivation in step <NUM> is performed before removing the oxide material in step <NUM>. According to an embodiment, step <NUM> is executed after step <NUM>. Prior to or after optional step <NUM> and/or optional step <NUM>, a step <NUM> may be performed, comprising removing one or more carbon layers at least partially. , removing of the carbon layer may be performed after a substrate etching for releasing the membrane structure such that the membrane structure is vibratable and/or after passivating the MEMS-transducer. However, according to the invention at least one carbon layer is removed only partially.

When making reference again to <FIG> and to <FIG>, removing of the carbon layer may be performed after the substrate etching for releasing the membrane structure such that the membrane is vibratable. The substrate etching as illustrated in <FIG> may be performed so as to release a first inner region of the main surface 14A of the membrane structure <NUM> with exception of the edge region 18A and so as to release the second inner region of the main surface 14B of the membrane structure <NUM> with exception of the edge region 18B. As illustrated in <FIG>, within a tolerance range of <NUM>%, the edge regions 18A and 18B may comprise a same extension perpendicular to a surface normal of the main surface 14A.

For example, the gap <NUM> may be obtained by using a carbon material which is applied during the transducer fabrication onto at least an upper and/or lower main surface region and/or the side regions of the membrane structure. The carbon material may be selectively removed after completion of the transducer elements by obtaining, through a well-controlled, cold, gaseous action process with oxygen (gas phase), the desired or required undercutting of the carbon material. By this, the resulting slit with the thickness based on the thickness of the carbon material may be obtained, wherein, some ash material may remain in the gap. During the final etching process, i.e., an incineration process, of the carbon material, e.g., an oxidized layer having the thickness of an atom layer, i.e., a thickness of <NUM>, may result on the membrane surface. However, such a layer does not have an influence to be considered on the resulting electric or acoustic properties of the membrane at a membrane thickness which may be, for example, between <NUM> and <NUM>, wherein this does not exclude smaller or larger dimensions. One aspect of the embodiments described herein is to obtain a small slit, possibly filled with air, between the membrane and at least one surrounding border material, i.e., the substrate structure in order to obtain a membrane that is released at least in the gap regions. The membrane therefore comprises an increased insensitivity with respect to stress from the outside. Since the carbon material comprises an extremely high etching selectivity to the oxide material (TEOS), polysilicon and nitride, the thinly applied carbon material can be removed from the gap between the border material and the respective surface region of the membrane in an extremely precise manner.

A suitable material as carbon, which can be removed by a cold, gaseous etching process with oxygen (in the gas phase), the oxygen being activated by means of plasma, the etching being carried out by radicalization of the oxygen obtained by means of the plasma. The described gap may be used for dual-backplate configurations and for single-backplate configurations. In a single-backplate configuration, the carbon gap may also be used to avoid formation of hotspots. In this configuration, however, it may be noted that very large process variations of +/- <NUM> (i.e., <NUM>) may occur with regard to the cavity etching, so that, e.g., an undercut having a width of <NUM> may be implemented to remove the carbon material in order to ensure that the carbon edge is obtained in a projection from above outside of the edge of the lower border material (substrate material) for the lower attachment of the membrane. In contrast, the edge of the upper and the lower border materials may be obtained between the upper surface of the membrane and the upper counterelectrode and the lower border material may be obtained between the lower surface of the membrane and the lower counterelectrode with a significantly higher accuracy, e.g., <NUM> +/- <NUM>, so that an undercutting or removal of the carbon material of at least <NUM> may be sufficient in this constellation.

As silicon used for the transducer membrane is very susceptible to tensile stresses due to the low layer thickness of the membrane, whereas compressive stresses are relatively uncritical, measures are taken to increase robustness. Depending on the position of the border region of the material of the border attachment, i.e., the corner regions <NUM>, so-called hotspots for pressure loads occur with the laterally clamped membrane. This is also referred to as Kerf-effect. In a dual-backplate configuration, the perforation openings (etching holes or release holes) in the opposite counterelectrodes are selected to define the edge of the border material and therefore the respective hotspots for the clamped silicon membrane. Depending on the clamping or the clamping edges (clamping lines) of the membrane at the border region, the robustness of the membrane differs with respect to pressures applied from above or from below. Above and below are used herein so as to simplify the understanding of the embodiments. By non-limiting example only, below may refer to a region of the MEMS-transducer adjacent to the cavity of the transducer. In a bottom-port microphone, e.g., the border regions are arranged to be offset such that the lower border region is set back with respect to the upper border region in order to obtain an increased robustness at the upper hotspot for pressure loads from below.

In order to maintain the robustness from above and below at the same level, a slit/gap is created between the upper material of the border region and the upper surface of the membrane in order to eliminate the upper hotspots since a relatively loose support (clamping) between the upper border material and the upper surface region of the membrane occurs at the edge of the upper material of the border region so that the upper hotspot is avoided when a pressure is applied from above, and the membrane may be deflected downwards due to the slit and therefore no hotspot that is critical with respect to large tensile stresses is created between the upper border material and the surface region of the membrane. This makes it possible to obtain virtually identical tensile/compressive ratios for pressure loads of the membrane from above and below.

For such a purpose, the recess, slit or gap between the upper surface region (main surface 14A) and the border material (substrate structure <NUM>) extends in a projection by approximately <NUM> to <NUM> beyond the edge of the lower border material and its connection to the lower surface region of the membrane as described in connection with the overlap extension <NUM>. In principle, the gap between the upper border material and the upper surface region of the membrane structure may be continued up to the border of the membrane; however, the resulting border material may then be configured to be sufficiently stable or the outer membrane end may be offset towards the interior in order to obtain a sufficiently stable border attachment of the membrane structure and/or a sufficiently high robustness of the resulting connection, i.e., of the membrane fixed between the two counterelectrodes.

MEMS-elements have been used for many years and in high numbers. One parameter to be considered is the robustness against high and rapid pressure changes. Embodiments allow for a high robustness of the membrane for pressure and pressure changes from both sides of the membrane. An aspect of the present embodiments is to use a carbon layer. This layer can be etched in a defined manner by use of a plasma process after a release of the active MEMS-layers. By the etching process, there may be obtained a defined lateral undercut or lateral etching allowing to prevent high tensile stresses at the clamping of the membrane structure during pressure loads from a front side of the chip. The high pressure robustness with regard to the back side is maintained at the same time. This idea may be used for dual-backplate MEMS and for single-backplate MEMS. The idea may further be used for obtaining a completely release membrane structure. An increase in the robustness against pressures and pressure changes of the MEMS-chips may allow obtaining a high robustness of MEMS-microphones and/or MEMS-loudspeakers. Embodiments thus allow obtaining a low loss rate of the produced MEMS-transducers.

Claim 1:
MEMS-transducer (<NUM><NUM>; <NUM><NUM>; <NUM><NUM>; <NUM><NUM>, <NUM>; <NUM><NUM>; <NUM><NUM>) comprising:
a membrane structure (<NUM>) having a first main surface (14A) and a second main surface (14B) opposing the first main surface (14A);
a substrate structure (<NUM>) configured to hold the membrane structure (<NUM>), wherein the substrate structure overlaps with the first main surface (14A) of the membrane structure (<NUM>) in a first edge region (18A) being adjacent to a first inner region (24A) of the first main surface (14A);
a gap (<NUM>; <NUM><NUM>; <NUM><NUM>) formed between the membrane structure (<NUM>) and the substrate structure (<NUM>) in the first edge region (18A) and extending from the first inner region (24A) into the first edge region (18A); characterized in comprising
a carbon layer (<NUM>) comprising a carbon material and arranged on the first main surface (14A) in an outer region (34A) of the first main surface (14A) adjacent to the gap (<NUM>); wherein the gap (<NUM>) extends from the inner region (24A) towards the outer region (34A).