MEMS Device and Apparatus Having Such a MEMS Device

A MEMS device includes a substrate having a cavity and a membrane structure mechanically connected to the substrate and configured for deflecting out-of-plane with regard to a substrate plane and with a frequency in an ultrasonic frequency range to cause a fluid motion of the fluid in the cavity. The MEMS device includes a valve structure sandwiching the cavity together with the membrane structure, wherein the valve structure includes a planar perforated structure and a shutter structure opposing the perforated structure and arranged movably in-plane and with a frequency in the ultrasonic frequency range and with regard to the substrate plane and between a first position and a second position. The shutter structure is arranged to provide a first fluidic resistance for the fluid in the first position and a second, higher fluidic resistance for the fluid in the second position.

This application claims the benefit of European Patent Application No. 22179665.9, filed on Jun. 17, 2022, which application is hereby incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a MEMS device and to an apparatus having such a MEMS device.

BACKGROUND

MEMS structures may be used for generating a pressure wave and/or a flow in a fluid, e.g., a gas. Such a MEMS device may comprise a moveable part interacting with the fluid, e.g., a membrane. Examples of such MEMS devices are pumps, loudspeakers, or microphones.

A MEMS device may comprise semiconductor materials, for example, on a basis of silicon, wherein electrical current may flow based on conductive materials such as metal materials and/or doping of a semiconductor material.

There is a need to enhance existing MEMS devices.

SUMMARY

According to an embodiment, a MEMS device comprises a substrate having a cavity and a membrane structure mechanically connected to the substrate and configured for deflecting out-of-plane with regard to a substrate plane and with a frequency in an ultrasonic frequency range to cause a fluid motion in the cavity. The MEMS device comprises a valve structure sandwiching the cavity together with the membrane structure. The valve structure comprises a planar perforated structure and a shutter structure opposing the perforated structure and arranged moveably in-plane with a frequency in the ultrasonic frequency range and with regard to the substrate plane and between a first position and a second position. The shutter structure is arranged to provide for a first fluidic resistance for the fluid motion in the first position and a second, higher, fluidic resistance for the fluid in the second position.

According to an embodiment, an apparatus comprises such a MEMS device and is a loudspeaker or a pump. Some embodiments of the present disclosure relate to a MEMS structure with a horizontal shutter and optional driving membrane for micro-speaker, valve and pumping applications.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

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

FIG.1shows a schematic side view of a MEMS device10according to an embodiment. The MEMS device comprises a substrate12, e.g., comprising a semiconductor material such as silicon or silicon based materials. Alternatively, or in addition, other types of semiconductor materials may form at least a part of substrate12, e.g., gallium arsenide or the like.

The MEMS device10comprises a membrane structure14being mechanically connected to the substrate12. The membrane structure14may be directly or indirectly hinged or connected to the substrate12. The membrane structure14may be provided parallel to a substrate plane, e.g., it may be arranged in an unactuated state essentially parallel to a substrate plane x/y, i.e., in-plane. The membrane structure14may be deflectable out-of-plane, e.g., based on an actuation signal and/or based on a pressure of a fluid16being present in a cavity18of the MEMS device10, of the substrate12, respectively. The membrane structure14may be implemented as a planar structure having, for example, a rectangular, round or essentially square shape or a structure in between e.g. oval or alveolate. The membrane structure14, may also be formed as a cantilever beam.

By way of non-limiting example only, a MEMS device may be formed from at least one wafer or other basis structure. Such a wafer may be considered as a plate-like structure that has a comparatively low thickness when compared to longitudinal extensions. Without limiting such considerations to a specific coordinate system or realization of a structure, the longitudinal directions may be considered as defining an in-plane direction of a MEMS device. Such directions are shown inFIG.1as x/y-directions. A thickness direction z may be considered as a thickness direction perpendicular to the in-plane directions x/y. The membrane structure14may deflect out-of-plane and, thus, along a positive and/or negative z-direction.

The membrane structure14is configured for deflecting along the z-direction with a frequency in ultra-sonic frequency range to cause a motion of the fluid16. The motion may comprise a flow of the fluid16through a valve or shutter structure22and/or may relate to a pressure or change thereof generated in the cavity18. For example, the membrane structure14may be configured for and controlled to deflect out-of-plane with an ultrasonic frequency. That is, the deflection of the membrane structure14may be controlled in a frequency range inaudible for a human. However, by use of a MEMS membrane structure that possibly has a comparatively small size and thus moves a small amount of fluid, a high rate of moving such small amounts may result in considerable fluid set under pressure, e.g., in a front volume. To avoid moving the fluid16back and forth with the membrane structure, the controllable valve structure may be used to allow, based on a control signal for the valve structure22, a fluid pulse or fluid motion away from the membrane structure14through the valve structure22towards a front volume38. In addition, the MEMS device10may provide for a back volume39on a different side of the membrane structure12. The whole MEMS device10may thus offer a front and a back port for fluid and/or sound.

Based on closing the valve structure22during a time the membrane structure14moves away from the valve structure, e.g., along +z, and again opening the valve structure22when the membrane structure14moves towards the valve structure22again, e.g., along −z, fluid pulses or fluid16may be accumulated in the front volume38.

The membrane structure may move or vibrate with an ultrasonic frequency, e.g. a frequency of at least 20 kHz, at least 30 kHz or even more, i.e., with an ultrasonic frequency range. For example, the membrane may vibrate with a frequency of at least 50 kHz, at least 70 kHz or at least 90 kHz, e.g., approximately or equal to 96 kHz, wherein some structures may also allow for a frequency of at least 200 kHz, 300 kHz or up to 500 kHz or more to cover a range between at least 50 kHz and at most 500 kHz. Such an ultrasonic frequency range may be demodulated to an audible frequency range, i.e., an audio frequency range, e.g., below 20 kHz.

The valve structure22and the membrane structure14may together sandwich the cavity18, wherein based on a control of valve structure22different fluidic resistances for the fluid may be provided. According to one example, the valve22may be switched between an open state in which the valve structure22basically provides for a low, negligible or no fluidic resistance and a second state, in which the valve structure22may be considered in a closed state in which the valve structure provides for a comparatively higher fluidic resistance for the fluid16. For example, in the closed state, there may be no or only negligible flow from the fluid16through the valve structure22.

The valve structure22may comprise a planar perforated structure24and a shutter structure26moveable with respect to the perforated structure24. In one example, the perforated structure24may be an immoveable structure, e.g., a statue or the like, for example, immovably connected to the substrate12. However, according to other examples, both structures, the perforated structure24and the shutter structure26may be moveably arranged and may move with regard to each other.

The perforated structure24may comprise a plurality of openings281to28nthat allow the fluid16to pass through. According to an embodiment, the shutter structure26may comprise structural sections321to32nthat are adapted for an opposing position with regard to the openings281to28nin the second position to provide for the second fluidic resistance.

By use of the valve structure22, the ultrasonic waves in the cavity18may be transferred to other frequency ranges, e.g., an audible or audio frequency range.

While using a membrane being deflectable along a direction of movement34being possibly parallel to the z-direction, which may allow for a large amount of fluid to be moved, a direction of movement36of the valve structure22, e.g., the shutter structure26, may allow for a low thickness of the MEMS device10, may comprise a control unit and/or an interface for a connection with an external control unit. Such a control unit may comprise control circuitry configured for controlling a deflection of the membrane structure14to deflect with a first ultrasonic frequency and for controlling the valve structure22, e.g., an actuator structure connected to the shutter structure26to change between the first position and the second position with a same or a different second ultrasonic frequency to generate a sound pressure level in a front volume38of the MEMS device10in an audio frequency range, the sound pressure level in the audio frequency range being generated from the ultrasonic frequency of the membrane structure14, i.e., by use of the valve structure22, the membrane sound may be modulated or demodulated so as to obtain an audio signal. For example, such a demodulation/modulation may comprise an advanced digital sound reconstruction, ADSR, or other ultrasonic demodulation, UD, concepts, e.g., a single-sideband demodulation or dual-sideband demodulation techniques.

FIG.2shows a schematic side view of a part of a MEMS device20according to an embodiment, wherein the details provided may be applied, without limitation, to other MEMS devices described herein.

MEMS device20comprises control circuitry42, e.g., comprising a processing unit, a microcontroller, an application-specific integrated circuit, ASIC, a field programmable gate array, FPGA, or other suitable circuitry, configured for controlling a deflection of the membrane structure14. The control circuitry42is further configured for controlling an actuator structure44connected to the shutter structure26to change between the first position shown inFIG.2and the second position indicated inFIG.1. The change between the first position and the second position may be obtained with a same or a different ultrasonic frequency and both controls commonly may allow generation of a sound pressure level in the front volume38, which is in the audio frequency range, i.e., in a frequency range of at least 10 Hz and at most 20 kHz. A MEMS device in accordance with embodiments described herein may have good properties especially in a low frequency audio range, e.g., in a range of at most 1 kHz, at most 800 Hz or at most 500 Hz and even far below. The actuator structure may comprise, for example, a piezoelectric actuator, an electrodynamic actuator or other types of actuating structures. For example, the actuator structure44may comprise an electrodynamic comb-structure, a magnetic drive, a piezoelectric drive and/or a thermal drive. The actuator structure44may comprise more than a single actuator, e.g., actuator structures441and442so as to generate active forces along positive and negative directions of movement36. This may allow for a highly controllable generation of sound when compared to relying on restoring forces of springs only.

By driving the membrane structure14and the valve structure22with a same or a different ultrasonic frequency, a modulation of the sound generated by the membrane structure14may be obtained.

A MEMS device in accordance with an embodiment may comprise a membrane structure14that comprises a plurality of ventilation holes46configured for a passage of the fluid16into the cavity18while preventing an acoustic short circuit. The ventilation holes46may have a synergetic effect and may, for example, first serve as holes that are used to etch a sacrificial layer to generate at least a part of the cavity18. The ventilation holes46may comprise a comparatively low size along the x- and/or y-direction to prevent for an acoustic short circuit. That is, the ventilation holes46may have a high fluidic resistance which increases with ˜1/d4wherein d refers to a hole diameter.

That is, the ventilation holes46may allow removal the sacrificial layer without generating an acoustic short circuit. Such an acoustic short circuit may be understood as an effect that the membrane structure14deflects along a direction, but only provides for insufficient pressure or movement of fluid16as the fluidic resistance of the ventilation holes46is too low, such that the fluid16travels through such holes instead of being moved by the membrane structure. A size of the ventilation holes46may be tuned to achieve a fluidic resistance for the fluid16that is high enough to allow efficient flow generation through the valve structure22and to be low enough to allow for ventilation of the cavity18through the membrane structure14when the valve structure22is in a closed state, i.e., the shutter structure26is in the second position. A diameter of such a ventilation hole may be, for example, in a range of at least 0.5 μm and at most 5 μm, at least 0.7 μm and at most 4 μm or of at least 1 μm and at most 3 μm.

Alternatively, or in addition to the ventilation holes46, the MEMS structure20and/or the MEMS structure10may comprise a bump structure48which may have a set of bumps, a continuous or discontinuous ring-like structure of at least one bump or the like that is arranged between the perforated structure24and the shutter structure26. During manufacturing of the MEMS device20, the bump structure48may, for example, be formed as a part of the perforated structure24and/or as a part of the shutter structure26.

In a closed state of the valve structure22, the bump structure48may prevent a fluid through the valve structure22based on a mechanical contact between the perforated structure24and the shutter structure26via the bump structure48. That is, although showing a remaining distance between the bump structure48and the shutter structure26, the MEMS device20ofFIG.2may provide for a mechanical contact of the shutter structure26with the bump structure48. The bump structure48may synergistically allow for providing a protection against stiction of the shutter structure26to the perforated structure24and to provide for a sort of a seal, e.g., a ring, around the perforation or opening28to pinch-off the flow of fluid16in case the shutter structure26is in the second position.

According to an embodiment, the bump structure48comprises bump elements with different properties and/or for different purposes. For example, the bump elements521and522may comprise a comparatively long extension along the z-direction, i.e., they may be comparatively long. The bump elements521and522may be configured for providing a point-like contacting surface with the shutter structure26so as to provide for anti-stiction function. By having the bump elements521,522with a point-like surface or at least a small contact area, good anti-stiction properties may be obtained. The bump elements521and/or522as an alternative or in addition allow having a low amount of disturbing noise in the demodulated audio signal as a surface area that generates friction and thereby possibly noise when changing the position of the shutter structure26is small.

According to an embodiment, a MEMS device according to the present disclosure may provide for anti-stiction bumps521/522between the perforated structure24and the shutter structure26and may comprise a sealing structure541/542between the perforated structure24and the shutter structure26. A remaining gap562in an area of the sealing structure541/542may be larger when compared to a gap561in an area of the anti-stiction bumps521/522.

The bump structure48may alternatively or in addition comprise a continuous or segmented bump element541/542that may form at least a part of a ring through a respective opening281,282respectively. When compared to the bump elements521,522, a length along the z-direction of bump elements541,542may be smaller. A reduced length or height of bumps541,542when compared to bump elements may result, at least during regular operation, in that the bump elements541,542do not provide for a mechanical contact but reduce or at least partly obstruct a slit along the z-direction, the slit being between the perforated structure24and the shutter structure26. That is, elements541and542are not necessarily bumps but can also for a different type of protrusion or elevation.

To have different heights of bump elements used for different purposes may allow to maintain good anti-stiction properties by use of bump elements521,522even in case of having a comparatively large surface provided by sealing bumps or sealing elevations54as they do not necessarily contribute to the anti-stiction functionality. A gap561remaining between bump elements521and shutter structure26may be comparatively low, down to zero in a case where the bumps contact or abut the opposing structure. A gap562between the bump element541or542on the one hand and shutter structure26on the other hand may, at a same time, be non-zero which may be interpreted as a remaining pinching slit even if the valve structure22is in a closed position. Such a gap may have a size, for example, of at least 100 nm and at most 1000 nm, of at least 200 nm and at most 700 nm or of at least 300 nm and at most 500 nm. A gap563between the shutter structure26and the perforated structure24may, at a same time, be in a range of at least 600 nm and at most 1500 nm. That is, bump elements541and542may provide to tighten or seal the cavity18even if a small gap562remains.

Bump structure48does not prevent bump elements581and/or582to be arranged between the membrane structure14and the valve structure22. It has to be noted that the perforated structure24or the shutter structure26may be arranged closer to the membrane structure14when compared to the other structure.

Alternatively, or in addition to the bump structure48and/or the ventilation holes46, the shutter structure26may comprise a mechanical stiffening641,642configured for suppressing an out-of-plane deflection of the shutter structure26. For example, the mechanical stiffening641,642may comprise a locally increased thickness of the shutter structure26, e.g., a beam-like structure or a bar-like structure. Alternatively, or in addition, an additional layer or structure may be arranged to locally stiffen the shutter structure to increase weight of the moved structure only as much as desired. Alternatively, or in addition to the stiffening of the perforated structure, wherein the shutter structure may comprise a mechanical stiffening, the mechanical stiffening configured for suppressing an out-of-plane deflection of the shutter structure.

FIG.3ashows a schematic side view of a part of the MEMS device20in which the valve structure22, the shutter structure26respectively is in the first position. In the first position, a low resistance for a fluid or a flow thereof is provided by valve structure22as indicated by arrows66. The control circuitry42may control the actuator structure44to move the shutter structure26along the direction of movement36to arrive at the second position indicated which is shown in the schematic side view of the part of the MEMS device20inFIG.3b.

Alternatively, or in addition, the control circuitry42may control the actuator structure44to actively move the shutter structure from the second position into the first position. A rest position of the shutter structure26may at least partially be based on hinges or spring structures and the actuator structure44may be controlled to move the shutter structure away from such a rest position. Arrows66′ inFIG.3bindicate a high resistance provided by valve structure22arriving at a low amount of fluid travelling through openings281and282.

According to an embodiment, the shutter structure26may comprise a lattice structure comprising at least one bar that, when being projected into a common plane parallel to the substrate plane x/y, overlaps with an opening28of a perforation of the perforated structure26in the second position and reveals at least a part of the opening in the first position when being projected into the common plane as shown, for example, inFIG.3a.

FIG.4ashows a schematic side view of a MEMS device40according to an embodiment. The MEMS device40comprises, for example, an interface68to allow a connection to exchange a signal with an external control circuitry, while the control circuitry42of a MEMS device20may be an internal circuitry of the MEMs device.

Alternatively, or in addition, the MEMS device40may comprise a protective structure72which may also be arranged at MEMS device10and/or20. The protective structure72may be transparent for a sound pressure level, e.g., in the audio range, of the fluid while being configured for mechanically protecting the valve structure22. The protective structure72may sandwich the valve structure22together with the membrane structure, i.e., the valve structure22may be arranged between the membrane structure14and the protective structure72. The protective structure72may comprise, for example, a plastic material, a semiconductor material and/or a glass material. For example, a plastic material, i.e., a membrane comprising the plastic material, may be well-suited to block ultrasonic sound while being transparent for audible frequencies. A glass structure forming at least a part of the protective structure72may comprise a mesh-like structure, may provide for a high mechanical robustness, and may comprise small openings that may act as ventilation holes. Alternatively, or in addition, a structured glass may be provided, e.g., as a part of a glass package, may be provided with a kind of perforation to block particles on the one hand side but also to attenuate the ultrasound signal(s) on the other side.

FIG.4bshows a schematic side view of the MEMS device40ofFIG.4awhere the shutter structure26is in the second position. As shown in a schematic equivalent circuit diagram74inFIGS.4aand4b, the fluidic resistance ofFIG.4arepresented therein may be understood as parallel connection of resistors while the second position shown inFIG.4bmay result in a serial-connection. In an electronic analogy, the parallel connection may result in a lower effective resistance when compared to the serial-connection.

The external or internal control circuitry may be configured for controlling the actuator structure44to move the shutter structure26into the first position or into the second position at an instance of time based on a deflection state of the membrane structure14at that instance of time. That is, the control circuitry may temporally align a control of the membrane structure14and of the shutter structure26. For example, the control circuitry may control the actuator structure44so as to provide for the first position of the valve structure at a time when the membrane provides for pressure in the cavity or may provide for a closed state of the valve structure22at this time. For example, the control circuitry42may be configured for controlling the actuator structure44and the membrane structure14for aggregating a fluidic pressure of the fluid motion generated by the membrane structure14to have an aggregated pressure in the front volume by use of the valve structure. In other words, the membrane structure14may aggregate pressure, e.g., by pumping a contribution in the front volume while having the valve structure22in an open state and the valve structure22may be controlled into a closed state when the membrane14moves back again. According to other types of ultrasonic demodulation, e.g., ADSR, the membrane structure on the one hand and the shutter position on the other hand may also be unsynchronized.

FIG.5ashows schematic views of a MEMS device50according to an embodiment, wherein the details provided may be applied, without limitation, to other MEMS devices described herein.

A gap76between the membrane structure14and the valve structure22, e.g., the perforated structure24may have any suitable value which may be designed in accordance with a maximum deflection of the membrane structure14which may depend, for example, on the driving voltage and/or a diameter of the membrane structure14. A size78, e.g., a diameter, of a hole28of the perforated structure28may be, for example, in a range of at least 3 μm and at most 5 μm.

An extension82of bump elements54along the z-direction may be, for example, in a range of at least 100 nm and at most 500 nm, of at least 150 nm and at most 400 nm or at least 200 nm and at most 300 nm or other suitable values to arrive at the gaps described in connection withFIG.2.

An extension84perpendicular to extension82may be, for example, in a range of at least 100 nm and at most 700 nm, of at least 150 nm and at most 600 nm or of at least 300 nm and at most 500 nm.

A thickness96of, e.g., the perforated structure24may comprise, for example, at least 100 nm and at most 2 μm, at least 200 nm and at most 1 μm or at 300 nm and at most 600 nm.

A thickness88of the shutter structure26may be in a same range and may have a same or a different value.

An overlap92starting from the bump element54towards the opening28of the perforated structure24may have a size of, for example, at least 100 nm and at most 800 nm, at least 150 nm and at most 700 nm or of at least 200 nm and at most 500 nm. An extension94may comprise half of extension78, the overlap extension92and a width84and may form a basis for a symmetry used for generating the perforated structure24and/or the shutter structure26.

An additional extension is shown in the top view ofFIGS.5aand5bmay be in a range of, for example, at least 1 μm and at most 10 μm, at least 2 μm and at most 7 μm, or of at least 3 μm and at most 5 μm.

A gap98between the perforated structure24and the shutter structure26may be in a range, for example, of at least 100 nm and at most 800 nm, of at least 150 nm and at most 600 nm or of at least 300 nm and at most 500 nm.

In the schematic illustration ofFIG.5b, the structural section32of shutter structure26is moved so as to overlap the opening28and optionally the bump elements54so as to provide for the closed state of valve structure22.

FIG.6ashows a schematic top view of a part of a MEMS device60according to an embodiment, wherein the details provided may be applied, without limitation, to other MEMS devices described herein. In particular, a layer of the shutter structure26is shown. A two-dimensional layout of the structural section32of the shutter structure26is shown, leading also to a two-dimensional arrangement of openings102that may overlap openings28of a perforated structure in the first state. One or more spring structures1041to1044inFIG.6amay form a part of the MEMS device60. The at least one spring structure1041to1044may be formed, for example, as a ridge or other type of spring that may allow to move the shutter structure26along the x- and/or y-direction while preventing such a movement, for example, along a z-direction. A geometry of spring structures1041to1044may also restrict a movement of the shutter structure26along one of the directions x and y while facilitating the movement along the other direction, e.g., the direction x along which the actuator structure44may generate a force so as to deflect the shutter structure26. That is, the spring structure may, at least in the combination of spring elements, comprise an out-of-plane mechanical stiffness along the z direction being larger when compared to an in-plane mechanical stiffness along the direction of movement36.

The spring structures1041to1044may elastically hinge the shutter structure26, e.g., at the substrate12and/or at the stator or perforated structure24in a rest position, the actuator structure44configured for deflecting the shutter structure26in-plane along a direction of movement, e.g., x, and out of the rest position.

An extension106of the shutter structure26along x and/or y may be, for example, in a range of at least 500 μm and some millimeter, e.g., around 1 mm. A length108of the spring structures1041to1044may be in a range of at least 50 μm and at most 300 μm, at least 70 μm and at most 200 μm or at least 90 μm and at most 150 μm, e.g., 100 μm. Those dimensions are example values only as well as the number of spring structures104. For example, one, two, three or more than four spring structures104may be arranged, e.g., six, eight, nine or more. Alternatively, or in addition, the structural extensions may comprise any other value.

FIG.6bshows as schematic top view of a layer of the perforated structure24according to an embodiment. When compared to the MEMS structure60, spring elements104′1to104′4may be configured for compression and elongation while spring structures1041to1044may be configured for bending.

As may be seen fromFIG.6b, openings28and102may overlap in the illustrated first position of the shutter structure26.

Based on spring structures104,104′ and/or a combination thereof which forms also an embodiment, the shutter structure26and the at least one spring structure may form at least a part of a resonator having a resonance frequency, said resonance frequency being in the ultrasonic frequency range of, for example, at least 20 kHz, at least 30 kHz, at least 40 kHz or more.

FIG.7shows a schematic perspective view of a part of a MEMS device80according to an embodiment, wherein the details provided may be applied, without limitation, to other MEMS devices described herein. Therein, for example, more than one, e.g., four valve structures221to224may be arranged in a one-dimensional or two-dimensional array.FIG.7shows the shutter structures261to264of the respective valve structures221to224. As an example, each shutter structure261to264may comprise a dimension of approximately 100 μm×200 m in the x/y-plane, wherein other dimensions are possible and/or shutter structures261to264may be formed of different sizes.

FIG.7illustrates an embodiment where a plurality of valve structures221to224sandwich the cavity together with a common membrane structure, wherein control circuitry being part of or connected to the MEMS structure80may be configured for controlling the membrane structure14and for individually controlling a position of the plurality of valve structures221to224for providing different spectra of the fluidic motion from the cavity with the plurality of valve structures. An individual control may comprise an individual control signal provided by the control circuitry and/or may comprise different resonance frequencies of a respective resonator structure which may comprise different stiffnesses of spring structures104ato104dthat may hinge the respective shutter structure261to264and/or different masses contributing to the resonator structure.

This may result in different frequencies of movement for the resonator structures261to264to a respective control signal having the intended frequency or being of a broadband characteristic so as to excite resonant movement of one, more or all of the resonator structures.

This may allow generation of more than a single tone or frequency component with the MEMS structure at a time.

FIG.8shows a schematic diagram for illustrating such a concept. There is plotted a time axis at the abscissa and an example normalized sound pressure level S at the ordinate. Plots1121to1124schematically show pressure levels generated with a respective unit cell, e.g., by use of one of shutter structures261to264of MEMS device8o. Based on different pressure pulses, an overall sound116may be formed based on a superposition of partial sound pressure contributions1121to1124. Plots1141to1144show, for a respective same cell the contribution in a damping mode, i.e., during a time where the membrane moves backward but where the valve structure is closed, wherein, as described above, a synchronization is not required. In other words, the vibration of the membrane structure on the one hand and the vibration related to the operation of the valve structures on the other hand may remain unsynchronized. According to an embodiment, the valve columns itself may have a constant phase shift of, e.g., 90°, to each other.

FIG.9shows a schematic plot illustrating a possible sound pressure S over time t for a MEMS device that comprises, for example, 16 valve structures221to2216arranged in an example quadratic two-dimensional array, wherein any other number of valve structures and/or any other layout is possible.FIG.9shows that during different times, e.g., time instances t1, t2, t3and/or t4a different number of valve structures221to2216may participate in sound generation and/or that different valve structures, for example, to arrive at different spectra, volume components or contributions to an overall sound, may be used to generate sound. During time instance t1, a single valve structure2215is active in the described example. During time instance t2, a number of four valve structures229,2210,2213and2214may be active, e.g., providing for an aggregated sound pressure of a superposition of the same or different spectra associated with the respective valve structure.

During time instance t3, valve structures229to2215provide for a superposition of, in total, six contributions, while during time instance t4, valve structures2211and2212may be active. According to an embodiment, a MEMS structure may comprise a two-dimensional array in which, for example, one of a line or column provides for valve structures of different spectra and the respective other dimension provides for valve structures of a same spectrum to allow an increase of sound pressure to be generated. This may allow to control the spectra (volume) components and/or the amplitude of audible sound to be obtained with the MEMS device.

FIG.10shows a schematic top view of a part of a MEMS device110according to an embodiment, wherein the details provided may be applied, without limitation, to other MEMS devices described herein. In acoustic communication with a single membrane structure14, an arrangement of valve structures22a1to22d4may be arranged in four lines and four columns. This does not exclude having a second, further membrane structure in acoustic communication with additional valve structures and/or having a different number of valve structures, a different number of lines and/or a different number of columns of valve structures in communication with the membrane structure14. For example, valve structures22a1to22a4may be formed so as to comprise a same resonance frequency, e.g., based on spring structures. Valve elements22b1to22b4may comprise a different second resonance frequency while valve structures22c1to22c4and22d1to22d4may comprise a third and fourth resonance frequency. Based on a suitable control signal, this may allow to contribute with different spectra to the audible signal, e.g., for the ultrasonic demodulation or suppression being implemented in the MEMS device. A use of one, two, three or four of the respective valve structure22a,22b,22cor22dmay allow to increase or reduce a sound pressure contribution of said frequency, while a deactivation of all of the respective valve structures having a resonance frequency may lead to an absence of said spectrum. As described, such a MEMS device may be configured for a use with an ultrasonic demodulation concept, e.g., to individually operate different valve structures in communication with a same membrane with different operation frequencies.

Such a structure may allow for a detailed controlled of the generated output. For example, the membrane may be operated at a resonance frequency thereof and valve structures may be operated at a resonance frequency thereof. A vibration or movement of the shutter structure of different rows may be operated in resonance each and with a phase shift of 90° with regard to one another to emulate the analogue sound. An amplitude of the sound may be adapted via a parallel path having a further shutter structure.

A use of an individual mechanical resonance may be a part of one solution of generating a movement of the shutter structures of the corresponding valve structures22a-22dwith different frequencies. Alternatively, or in addition to individual mechanical resonance frequencies that may be used for a respective resonant operation, for operating different valve structures at different frequencies, a non-resonant operation may be implemented in MEMS devices described herein. For example, a non-resonant operation mode may comprise a control to obtain snapping, an electro-mechanical pull-in or the like.

FIG.11shows a schematic block diagram of an apparatus120according to an embodiment. Apparatus120may comprise MEMS device10and/or a different MEMS device described herein and may implement, for example, a loudspeaker, e.g., as a part of an earphone and/or may implement a pump device.

Some embodiments in accordance with the present disclosure relate are described in the following:

According to a first aspect, a MEMS device comprises:

a substrate comprising a cavity;

a membrane structure mechanically connected to the substrate and configured for deflecting out-of-plane with regard to a substrate plane and with a frequency in an ultrasonic frequency range to cause a motion of a fluid in the cavity;

a valve structure sandwiching the cavity together with the membrane structure;

wherein the valve structure comprises a planar perforated structure;

wherein the valve structure comprises a shutter structure opposing the perforated structure and arranged moveably and with a resonance frequency in the ultrasonic frequency range and in-plane with regard to the substrate plane and between a first position and a second position;

wherein the shutter structure is arranged to provide a first fluidic resistance for the fluid in the first position and a second, higher fluidic resistance for the fluid in the second position.

According to a second aspect referring to the first aspect, the MEMS comprises control circuitry configured for controlling a deflection of the membrane structure to deflect with a first ultrasonic frequency; and for controlling an actuator structure coupled to the shutter structure to change between the first position and the second position with a same or different second ultrasonic frequency and to generate a sound pressure level in a front volume of the MEMS device in an audio frequency range from the first ultrasonic frequency.

According to a third aspect referring to the second aspect, the control circuitry is configured for controlling the actuator structure to move the shutter structure into one of the first position and the second position or to move the structure seamlessly, namely into one of a plurality of positions, between the first position and the second position at an instance of time based on a deflection state of the membrane structure at the instance of time.

According to a fourth aspect referring to aspect2or3, the membrane structure is arranged at a first side of the valve structure; wherein a front volume of the MEMS device is arranged at an opposing second side of the valve structure; wherein the control circuitry is configured for controlling the actuator structure and the membrane structure for aggregating a fluidic pressure of the fluid motion generated by the membrane structure in the front volume by use of the valve structure.

According to a seventh aspect referring to one of aspects2to4, the MEMS device comprises a plurality of valve structures sandwiching the cavity together with the membrane structure, wherein the control circuitry is configured for controlling the membrane structure and for individually controlling the plurality of valve structures of the MEMS device, for providing different spectra of the fluidic motion from the cavity with the plurality of valve structures.

According to a sixth aspect referring to one of previous aspects, the MEMS device comprises a spring structure elastically hinging the shutter structure in a rest position; and comprising an actuator structure configured for deflecting the shutter structure in-plane along a direction of movement and out of the rest position.

According to a seventh aspect referring to aspect6, the shutter structure and the spring structure form at least a part of a resonator having a resonance frequency in the ultrasonic frequency range.

According to an eight aspect referring to aspect6or7, the spring structure comprises an out-of-plane mechanical stiffness being larger when compared to an in-plane mechanical stiffness along the direction of movement.

According to a ninth aspect referring to one of previous aspects, the membrane structure comprises a plurality of ventilation holes configured for a passage of the fluid into the cavity while preventing an acoustic short circuit.

According to a tenth aspect referring to one of previous aspects, a sealing structure is arranged between the perforated structure and the shutter structure, wherein, in a closed state of the valve structure the sealing structure is configured for obstructing a fluid flow through a gap between the perforated structure and the shutter structure.

According to an eleventh aspect referring to one of previous aspects, the MEMS device comprises anti-stiction bumps between the perforated structure and the shutter structure and comprising a sealing structure between the perforated structure and the shutter structure, wherein a remaining gap in an area of the sealing structure is larger when compared to a gap in an area of the anti-stiction bumps.

According to a twelfth aspect referring to one of previous aspects, the perforated structure comprises a mechanical stiffening, the mechanical stiffening configured for suppressing an out-of-plane deflection of the perforated structure; and/or

wherein the shutter structure comprises a mechanical stiffening, the mechanical stiffening configured for suppressing an out-of-plane deflection of the shutter structure.

According to a thirteenth aspect referring to one of previous aspects, the cavity is one of a plurality of cavities being adjacently arranged; wherein each of the plurality of cavities is sandwiched between the membrane structure and a dedicated valve structure of the value structure.

According to a fourteenth aspect referring to one of previous aspects, the shutter structure comprises a lattice structure comprising at least one bar that, when being projected into a common plane parallel to the substrate plane, overlaps with an opening of a perforation of the perforated structure in the second position and reveals at least a part of the opening in the first position when being projected into the common plane.

According to a fifteenth aspect referring one of previous aspects, the MEMS device comprises a protective structure transparent for a sound pressure level of the fluid and arranged to sandwich the valve structure together with the membrane structure, the protective structure configured for mechanically protecting the valve structure.

According to a sixteenth aspect an apparatus comprises a MEMS device of one of previous aspects and is a loudspeaker, e.g., as a part of a wearable such as an earphone, or a pump.