Field of the invention

The present invention relates to a micro-electro-mechanical systems, MEMS, -based microphone for detecting a sound signal, comprising a body, a resonator, a cavity provided in the body and comprising a gas volume, and the resonator covering at least a part of the cavity.

Background art

EP patent application EP-A-2 700 928 discloses an integrated circuit apparatus with a membrane suspended over a cavity, where the membrane and cavity define a chamber. The membrane has a plurality of openings therein that passes gas into and out of the chamber. As the membrane is actuated, the volume of the chamber changes to generate a gas pressure inside the chamber that is different than a pressure outside the chamber. The integrated circuit apparatus includes a sensor to detect a frequency-based characteristic of the membrane that is responsive to the change in volume, and therein, it provides an indication of the gas pressure outside the chamber.

International patent publication WO2011/142637 discloses a MEMS-based microphone with a graphene membrane unit, and a back plate unit, opposite and spaced apart from the graphene membrane unit, so as to form an air gap between the back plate unit and the graphene membrane unit. The MEMS-based microphone operates at a high sensitivity and at a low voltage, and has a wide frequency domain, and therefore, can measure sound waves in an audio frequency domain and in a frequency domain that is lower or higher than the audio frequency domain.

International patent publication WO2019/220103 discloses an optical microphone assembly. The principle operation of the optical microphone assembly is based on an incoming acoustic wave propagating through a plurality of air channels, wherein the pressure of the incoming acoustic wave can be determined via a displacement of a membrane, and wherein the displacement is detected using an optical technique.

The article by Robin J. Dolleman et al., ‘Graphene Squeeze-Film Pressure Sensors’ ARXIV.ORG, CORNELL UNIVERSITY LIBRARY, 201 OLIN LIBRARY CORNELL UNIVERSITY ITHACA, NY 14853, 23 oktober 2015, discloses a graphene squeeze-film pressure sensor. The sensor has a membrane covering a gas cavity. The presence of an open venting channel, connecting the gas cavity to the external environment, maintains the average pressure inside the cavity equal to the ambient pressure. The resonant frequency of the membrane can be measured as a function of the ambient pressure.

Summary of the invention

The present invention seeks to provide a micro-electro-mechanical systems, MEMS-, based microphone for measuring e.g. sound waves. According to the present invention, a MEMS-based microphone as defined above is provided, further comprising a venting system connecting the cavity to an external environment, wherein the resonator (e.g. a membrane) is arranged to vibrate at a mechanical resonance frequency higher than a characteristic equilibration frequency, the mechanical resonance frequency being variable in response to a pressure modulation of the gas volume in the cavity caused by a sound signal in the external environment during operation, wherein the characteristic equilibration frequency is dependent on structural parameters of the MEMS-based microphone and is equal to or larger than a maximal frequency of the sound signal to be detected.

The present invention embodiments have the advantage that the operation of the MEMS- based microphone for detecting e.g. sound waves is based on new physical principle, the so-called squeeze-film effect. This allows for a next step in the miniaturisation of MEMS-based microphones to a smaller scale, opening up new industrial application directions, such as, but not limited to, directional microphones, wireless hearing aids, presence detection, noise cancellation, stereo recording and medical applications.

In a further aspect, the present invention relates to a microphone assembly comprising a MEMS-based microphone, according to any one of the embodiments described herein.

Short description of drawings

The present invention will be discussed in more detail below, with reference to the attached drawings, in which

Fig. 1 shows a schematic, cross-sectional view of a micro-electro-mechanical systems, MEMS, -based microphone, according to an embodiment of the present invention.

Fig. 2 shows a schematic view of a micro-electro-mechanical-systems, MEMS, -based microphone, according to a ‘bridge’ structure embodiment of the present invention.

Fig. 3 shows a schematic view of a micro-electro-mechanical-systems, MEMS, -based microphone according to a ‘beam’ structure embodiment of the present invention,

Fig. 4 shows a schematic view of a micro-electro-mechanical-systems, MEMS, -based microphone according to further ‘beam’ structure embodiment of the present invention,

Fig. 5 shows a schematic view of a micro-electro-mechanical-systems, MEMS, -based microphone according to a ‘trampoline’ structure embodiment of the present invention,

Fig. 6A shows a schematic view of a micro-electro-mechanical-systems, MEMS, -based microphone according to further ‘trampoline’ structure embodiment of the present invention,

Fig. 6B shows a schematic, cross-sectional view of the ‘trampoline’ structure embodiment shown in Fig. 6A.

Fig. 7 shows a schematic view of a microphone assembly, according to an embodiment of the present invention.

Description of embodiments

In general, the operation of state-of-the-art micro-electro-mechanical systems, MEMS, - based microphones, for measurement of sound waves or pressure, is based upon a membrane suspended above a cavity in a substrate, wherein the cavity holds a gas volume. A pressure modulation in the external environment due to e.g. sound waves induces a pressure modulation in the external gas, which causes a pressure difference with respect to the gas in the cavity volume, which cannot rapidly respond to the change in the external gas pressure, since only very small holes are present in the membrane. This, in turn, induces a displacement or vibration of the membrane suspended across the cavity. This displacement or vibration of the membrane is usually measured by (piezo-)resistive, capacitive or optical means, whereby the measured signal is thereafter converted into an audio signal.

Although state-of-the-art partially-sealed MEMS-based microphones have been successful in measuring e.g. sound waves, their performance is limited. The main physical concept limiting the performance of state-of-the-art MEMS-based microphones is an effect called the back-cavity stiffening, that limits the sensitivity of the microphones. This occurs when the pressure modulation in the cavity gas volume is not entirely used to displace or vibrate the suspended membrane, but instead, part of the pressure modulation is used to compress the gas volume in the partially sealed (back-)cavity volume. Although this drawback can be mitigated by having larger cavities, this would increase the size and costs of a MEMS-based microphone.

Furthermore, state-of-the-art MEMS-based microphones tend to break when operating at large amplitudes or when exposed to sudden pressure changes, as the induced forces on the membrane become too large, causing the membrane to break or collapse. Although this can be mitigated by using small holes in or near the membranes, that has the drawback of deteriorating the sensitivity of these microphones at low sound frequencies. Consequently, the dynamic range and sensitivity of MEMS-based microphones is also limited.

In this respect, there is a need in the art to overcome these drawbacks. For many industrial fields and applications, it is highly desirable to miniaturize MEMS-based microphones to a smaller scale. This would enable further technological possibilities, such as, but not limited to, directional microphones, wireless hearing aids, presence detection, noise-cancellation, stereo recording, and, in particular, medical applications for e.g. membrane-based pressure sensing at audio or ultrasound frequencies in arteries. In addition, further miniaturization of MEMS-based microphones would also allow fabrication of MEMS-based microphone arrays with many elements on the same footprint of a single present state-of-the-art MEMS-based microphone.

The present invention embodiments provide a micro-electro-mechanical systems, MEMS, -based microphone, for detection of e.g. sound waves, where the operation principle is based on measuring the vibration of a resonator, and the so-called squeeze-film effect. This step would allow the fabrication of MEMS-based microphones to a smaller scale, e.g. at least an order of magnitude, thereby opening new industrial application directions, such as the examples described above.

Fig. 1 shows a schematic view of a micro-electro-mechanical systems, MEMS, -based microphone 1 , according to an embodiment of the present invention. In this embodiment, the MEMS-based microphone 1 comprises a body 2. In general, the body 2 comprises a material known in the MEMS fabrication practices. For example, the body 2 may entirely comprise a single silicon material, a silicon nitride material, or a silicon base layer with a further material layer, e.g. silicon dioxide, on top.

For electrical readout and actuation purposes, it can be beneficial if the body 2 comprises an electrically conducting material. Further, the body 2 may comprise an actuating element for driving it into resonance.

In this embodiment, the MEMS-based microphone 1 further comprises a cavity 7 provided in the body 2 and comprising a gas volume. The cavity 7 may comprise many shapes, e.g. square, rectangular, circular or elliptical, and may be provided in the body 2 using e.g. an (dry) etching process. The gas volume in the cavity 7 comprises a fluid with a well-known compressibility behavior, for example air, such that it is possible to accurately monitor the compressibility of the gas volume as a function of e.g. pressure.

In the embodiment shown in Fig. 1 , the MEMS-based microphone 1 further comprises a resonator 3, the resonator 3 covering at least a part of the cavity 7. In view of Fig. 1 , the resonator 3 is placed on the body 2, such that the resonator 3 uses the body 2 as a platform to cover at least a part of the cavity 7, and such that the cavity 7 is beneath the resonator 3. Alternatively, the resonator 3 may be suspended over the cavity 7, such that the cavity 7 is beneath the resonator 3.

In a specific embodiment, the resonator 3 comprises a membrane having a mass per area less than 67 mg/m ² e.g. 35 mg/m ² . In particular, a resonator 3 comprising a graphene-based membrane may be highly desirable, as graphene comprises a high surface-to-mass ratio, and has a very sensitive response to e.g. pressure modulations. Furthermore, a graphene-based membrane can be easily fabricated, by e.g. exfoliation from a graphite crystal using tape, or, alternatively, it can be easily grown via chemical vapour deposition. Similar fabrication techniques described herein can also be used to fabricate e.g. a MoS2-based membrane. In general, the resonator 3 may comprise any material, assuming the operational principle, of the present invention embodiments described herein, may be carried out.

In the embodiment shown in Fig. 1 , the MEMS-based microphone 1 further comprises a venting system 10 connecting the cavity 7 to an external environment. As a non-limiting example, the resonator 3, covers at least a part of the cavity 7 from the external environment, and, as such, there may be a remaining part of the cavity 7 that is not covered by the resonator 3 from the external environment. In this respect, the remaining, uncovered part of the cavity 7 can form the venting system 10 connecting the cavity 7 to the external environment.

In general, the venting system 10 allows the pressure, or a pressure modulation, of the external environment to be equal to the pressure, or a pressure modulation, respectively, of the gas volume in the cavity 7. In other wording, the venting system 10 regulates the pressure of the gas volume in the cavity 7 with respect to the (ambient) pressure of the external environment, and can be thought of as a ‘transmission line’ between the cavity 7 and the external environment.

When the (ambient) pressure of the gas in the external environment suddenly increases by an amount P (e.g. due to a sound wave), the time it takes for the pressure of the gas volume in the cavity 7 to increase by an amount 0.63* P, is defined as the characteristic equilibration time T _C , whereby the factor 0.63 is equal to 1-1/exp(1), and exp(1) is the natural exponent. The characteristic equilibration frequency is thus defined as

In this respect, it is advantageous to have a sufficiently short enough ‘transmission line’ to allow the effects described herein to be achieved, e.g. allow the pressure of the gas volume inside the cavity 7 to respond sufficiently fast to the pressure modulations in the external environment by having characteristic equilibration time that is sufficiently small, and, accordingly, a characteristic equilibration frequency that is sufficiently large.

In the embodiment shown in Fig. 1 , the resonator 3 is arranged to vibrate at a mechanical resonance frequency higher than a characteristic equilibration frequency, the mechanical resonance frequency being variable in response to a pressure modulation of the gas volume in the cavity 7 caused by a sound signal in the external environment during operation. The mechanical resonance frequency of the resonator 3 is a function of the pressure of the gas volume in the cavity 7. In this respect, contrary to state-of-the-art MEMS-based microphones, where the deflection or vibration of the membrane is measured, the present invention embodiments measure the ‘compressibility’ of the gas volume in the cavity 7 using the squeeze-film effect, whereby the ‘compressibility’ of a gas is strongly dependent on the pressure thereof.

In a non-limiting example of the MEMS-based microphone 1 that relate to the above present invention embodiments, sound waves of a certain characteristic, from the external environment, impinge on the MEMS-based microphone 1 . This results in a pressure modulation of the external environment, and, via the venting system 10, also results in a pressure modulation of the gas volume in the cavity 7. As the mechanical resonance frequency of the resonator 3 is a function of the pressure of the gas volume in the cavity 7, the pressure modulation causes the resonator 3 to vibrate at a modulated mechanical resonance frequency associated with the pressure modulation, a result of the squeeze-film effect. If the relationship between the mechanical resonance frequency of the resonator 3 and the pressure of the gas volume in the cavity 7 is known, than the pressure modulation can be measured, and converted to an audio signal associated with the sound wave.

This operation principle, as described above, is based upon the resonator 3 being actively actuated at its mechanical resonance frequency, and it is the secondary relation between the pressure of the gas volume in the cavity 7, and the mechanical resonance frequency, that allows measurement of e.g. the frequency of the sound wave. This is also contrary to present state-of-the- art MEMS-based microphones, where the membrane therein is directly actuated by the sound waves.

The relation between the gas volume in the cavity 7 and mechanical resonance frequency is a by-product of an effect known as the squeeze-film effect. In general, the squeeze-film effect is a physiological phenomenon that traps gaseous or fluidic media underneath a resonator that is vibrating at high mechanical resonance frequencies, even in the absence of a seal. As such, in relation to the present invention embodiments, the gas volume in the cavity 7 is trapped under the resonator 3 vibrating at high mechanical resonance frequencies via the squeeze-film effect.

In this embodiment, the resonator 3 is arranged to vibrate at a mechanical resonance frequency higher than a characteristic equilibration frequency. In other wording, this is a condition to be satisfied for the assumption of the squeeze-film effect. If the condition is not satisfied, the planar flow of the gas volume (being squeezed) in the cavity 7 cannot be neglected, and the gas volume in the cavity 7 may no longer be trapped. As described herein and in equation (1), the characteristic equilibration frequency is inversely proportional of the time it takes for the pressure of the gas volume to equilibrate (i.e. become nominally equal) to the pressure of the external environment.

Furthermore, the characteristic equilibration frequency is dependent on structural parameters of the MEMS-based microphone 1 in this embodiment. The structural parameters of the MEMS-based microphone 1 comprises many (functional) features, including but not limited to, the physical dimensions of the cavity 7, the viscosity of the gas volume i.e. gaseous or fluidic medium in the cavity 7, a distance between the resonator 3 and a bottom of the cavity, physical dimensions of the resonator s, and/or the physical dimensions of the venting system 10. In this regard, the structural parameters are determined by the geometry of the MEMS-based microphone 1 ; for example, if a MEMS-based microphone 1 has a circular geometry, then the characteristic equilibration frequency is determined by e.g. the radius of a circular resonator 3. Even further, in this embodiment, the characteristic equilibration frequency is equal or larger than a maximal frequency of the sound signal. In other wording, this is another condition for the operation of the present invention embodiments, where if the condition is not satisfied, the detected pressure of the gas volume in the cavity 7 is not representative of the pressure modulation(s) of the external environment. The structural parameters of the MEMS-based microphone 1 inter alia determine the maximal frequency of the sound waves to be detected by the MEMS based microphone 1 via the squeeze-film effect.

As described herein, the resonator 3 is not directly actuated by e.g. sound waves, an operation principle different to that found in state-of-the-art MEMS microphones. To elaborate, since the characteristic equilibration frequency is equal or larger than a maximal frequency of the sound signal, and the mechanical resonance frequency of the resonator s is also higher than the maximal frequency of the sound signal, the resonator 3 is not sufficiently actuated or physically displaced by e.g. sound waves in the form of the sound signal. As an exemplary embodiment, the lowest mechanical resonance frequency of the resonator 3 is at least 100 times higher than the maximal frequency of the sound signal to be detected.

In more general wording, the present invention embodiments as described above relate to a micro-electro-mechanical systems, MEMS, -based microphone 1 for detecting a sound signal, comprising a body 2, a resonator 3, a cavity 7 provided in the body 2 and comprising a gas volume, and the resonator 3 covering at least a part of the cavity 7. The MEMS-based microphone 1 further comprises a venting system 10 connecting the cavity 7 to an external environment, wherein the resonator 3 is arranged to vibrate at a mechanical resonance frequency, the mechanical resonance frequency being higher than a characteristic equilibration frequency and being variable in response to a pressure modulation of the gas volume in the cavity 7 caused by a sound signal in the external environment during operation, wherein the characteristic equilibration frequency is dependent on structural parameters of the MEMS-based microphone 1 , and is equal to or larger than a maximal frequency of the sound signal. All the embodiments as described herein provide a MEMS-based microphone 1 using the squeeze-film effect as an alternative means for detecting sound signals. This is different to the operation of state-of-the-art MEMS-based microphones, allowing for further miniaturization of MEMS-based microphones to a smaller scale, thereby opening new industrial directions.

In another embodiment shown in Fig. 1 , the resonator 3 comprises a membrane 31 sealed to the body 2 over the cavity 7 and a resonator actuator 32 linked to the membrane 31 . The resonator actuator 32 is arranged to actuate the membrane 31 into a vibration at one of its mechanical resonance frequencies, for example the lowest mechanical resonance frequency owing to its highest sensitivity by e.g. periodically forcing the resonator actuator 32 into actuation. The resonator actuator 32 may use any possible actuation mechanism, including but not limited to mechanical actuation, (piezo-)electrical actuation, capacitive actuation, laser/light induced actuation, etc. In general, the type of actuation mechanism used by the resonator actuator 32 is dependent on the material properties of the membrane 31 . As a non-limiting example, if a laser/light induced or capacitive actuation mechanism is used by the resonator actuator 32, then the membrane 31 would have e.g. high reflectivity or conductive properties, respectively.

It is noted that to sufficiently drive the resonator actuator 32, the mechanical resonance frequency of the membrane 31 needs to be known. Forthis purpose, a readout system, for example, can be used and the resonator 32 can be driven in a closed loop system, similar to the operation found in e.g. electrical oscillators or clocks. Alternatively, a phase-locked loop feedback system can be used to drive the resonator 32, whereby the resonator 32 is phase-locked to e.g. a voltage controlled oscillator.

The present invention embodiments allow for a measurement of e.g. sound signals, with frequencies up to a maximum frequency f _max , defined as: whereby f _res is the mechanical resonance frequency of the resonator 3 and Q is the corresponding quality factor. Assuming a fixed quality factor, if the mechanical resonance frequency of the resonator 3 is of sufficient high frequency, the maximum frequency may be above audible frequencies i.e. frequencies higher than 20 kHz.

The sensitivity of the invention is set by the following equation: wherein

6j ₀ = 2n f ₀ . (5)

UJO is the unperturbed mechanical resonance frequency i.e. in the absence of any external effects, g is a distance between the resonator 3 and a bottom of the cavity 71 , t is a thickness of the resonator 3, and p _m is the mass density of the membrane 3.

In a further embodiment shown in Fig. 1 , a thickness t of the resonator 3 is less than 1 nm, e.g. 0.335 nm. In view of equation (3) and (4), since the mechanical resonance frequency f _res of the resonator 3 is inversely proportional to the thickness t of the resonator 3, it is desirable to reduce the thickness t of the resonator 3, i.e. fabricate the resonator 3 to be as thin as possible as to increase the dynamic range of the MEMS-based microphone 1 .

In other wording, it is advantageous to reduce the thickness t of the resonator 3 as to increase the mechanical resonance frequency f _res of the resonator 3. This allows the mechanical resonance frequency f _res to be higher than the characteristic equilibration frequency as to satisfy the condition for the assumption of the squeeze-film effect (as described above). In addition, it is also advantageous to reduce the thickness t of the resonator 3 as to increase the responsivity of the resonator 3, the responsivity being defined as change in mechanical resonance frequency f _res per Pascal of change in pressure of the gas volume in the cavity 7.

In an even further embodiment shown in Fig. 1 , a distance g between the resonator 3 and a bottom 71 of the cavity 7 opposite the resonator 3 is less than 500 nm, e.g. 50 nm. Similarly, in view of equation (3) and (4), since the mechanical resonance frequency f _res of the resonator 3, is inversely proportional to the distance g between the resonator 3 and a bottom 71 of the cavity 7 opposite the resonator 3, it is desirable to reduce the distance g, as to increase the dynamic range of the MEMS-based microphone 1. It is advantageous to reduce the distance g for the similar reasons, e.g. increased responsivity, as described above for reducing the thickness t of the resonator 3.

As a non-limiting example that relates to the present invention embodiments described herein, if the lowest sensitivity level to the human ear of 20 pPa is to be obtained, by taking a thickness t of the resonator 3 to be e.g. 0.335 nm, a distance g between the resonator 3 and a bottom 71 of the cavity 7 opposite the resonator s to be e.g. 50 nm, and assuming an estimate of the mass density p _m of the resonator s of e.g. 2000 kg nr ³ , this results in a measurable mechanical resonance frequency shift of 0.75 Hz. As such, given the parameters describe above i.e. the thickness t and distance g, it is possible to fabricate a MEMS-based microphone 1 that can detect the lowest sensitivity level to the human ear of 20 pPa.

In an exemplary embodiment, the resonator s is pre-tensioned. By applying a pre-tension, the stress and/or strain in the resonator s is increased, and, in turn, this increases the mechanical resonance frequency f _res of the resonator 3. The stress and/or strain can be induced in the resonator 3 by using a variety of techniques. One possible technique to induce stress is to change the parameters during the fabrication of the resonator 3; for example, if the resonator 3 comprises a graphene material, the graphene material can be grown e.g. on a copper plate at high temperature, whereby the graphene material experiences an expansion or shrinking upon cooling down to room temperature, thereby introducing stress and/or strain in the graphene material, and thus, the resonator 3.

Moreover, the resonator 3 comprises a membrane main element 34 and at least one membrane connecting element 35A-B, according to another embodiment of the present invention. The at least one membrane connecting element 35A-B may be placed on the body 2 such that it allows the membrane main element 34 to be suspended across the cavity 7, and thus, also cover at least a part of the cavity 7.

In an advantageous embodiment, the resonator 3 has a widest dimension of less than 9 pm, e.g. 5 pm. For example, if the resonator s is circular, than the resonator s has e.g. a diameter of less than 9 pm, e.g. 5 pm. In this respect, if the widest dimension of the resonator s is taken to be e.g. equal to the widest dimension of the MEMS-based microphone 1 , than a MEMS-based microphone 1 with a widest dimension of 9 pm, e.g. 5 pm, is significantly smaller than the size of state-of-the-art MEMS-based microphones, which have a diameter of e.g. 700-1100 pm. From this perspective, further miniaturization of MEMS-based microphones to a smaller scale, of more than an order of magnitude, is possible.

In a further embodiment (as shown in Fig. 1), the venting system 10 comprises a venting channel 11 . In view of Fig. 1 , although the cavity 7, as provided in the body 2, is ‘fully’ covered by the resonator 3, the venting channel 1 1 , which can be seen as a ‘transmission pipe/channel’, connects the (enclosed) gas volume in the cavity 7 to the external environment. Similar to the cavity 7, the venting channel 1 1 can be provided in the body 2 using e.g. a (dry) etching process. In an exemplary embodiment, in view of Fig. 1 , the venting channel 11 may connect the cavity 7 to a venting cavity 12, whereby the venting cavity 12 is exposed, and, thus, is also connected to the external environment. In other wording, the venting cavity 12 can be thought of as a secondary cavity open to the external environment, whereby a venting channel 11 connects the venting cavity 12 to the (main) cavity 7. The venting cavity 12 can also be provided in the body 2 via e.g. a (dry) etching process.

In another specific embodiment, the venting system 10 comprises at least one pore 36 in the resonator 3. The at least one pore 36 comprises an ‘open area’ in the resonator 3, whereby the at least one pore 36 is open to the external environment, and thus, the at least one pore 36 connects the cavity 7 to the external environment. Alternatively stated, the at least one pore 36 can be thought as a ‘hole-punch’ in the resonator 3. The at least one pore 36 may comprise many shapes or forms e.g. a hole, square or rectangular shaped, and can also be provided in the resonator 3 using e.g. a (dry) etching process.

In view of the above, the features explained in any of the embodiments described above can be used to describe multiple embodiments of the present invention MEMS-based microphone 1 , wherein several structures can be envisaged. Fig. 2 shows a schematic view of the MEMS-based microphone 1 , according to a nonlimiting embodiment of the present invention, whereby Fig. 2 relates to a ‘bridge’-like structure of the MEMS-based microphone 1. In this non-limiting embodiment, a cavity 7 is provided in a body 2, wherein the cavity 7 and body 2 both comprise a rectangular shape; the cavity 7 provided in the body 2 can be thought of as a trench-like structure. Further, in this non-limiting embodiment, the resonator 3 also comprises a rectangular shape, whereby the resonator 3 is placed on the body 2 such that it covers a least a part of the cavity 7. In view of Fig. 2, the rectangular-shaped resonator 3 may have the same width dimension as the rectangular-shaped body 2, but a smaller length dimension than the rectangular-shaped body 2, and thus, the rectangular-shaped resonator s may be placed in the middle of the body 2 such that there are two, remaining, uncovered parts of the cavity 7. In this respect, the two uncovered parts of the cavity 7 form the venting system 10, and the MEMS-based microphone 1 therefore may operate as according to the description provided herein.

Fig. 3 shows a schematic view of the MEMS-based microphone 1 , according to further nonlimiting embodiment of the present invention, whereby Fig. 3 relates to ‘beam’-like structure of the MEMS-based microphone 1. In this non-limiting embodiment, a cavity 7 is provided in a body 2, and the resonator 3 comprises a membrane main element 34 and at least one membrane connecting element 35A. In view of Fig 3, the at least one membrane connecting element 35A is placed on the body 2, such that, via the at least one membrane connecting element 35A, the membrane main element 34 is suspended across at least a part of the cavity 7. In this regard, the membrane main element 34 also covers at least a part of the cavity 7, and, as such, the remaining, uncovered parts of the cavity 7 form the venting system 10. The MEMS-based microphone 1 may then may operate as according to the description provided above.

Fig. 4 shows a schematic view of the MEMS-based microphone 1 , according to further nonlimiting (example) embodiment of the present invention embodiments, whereby Fig. 4 relates to further ‘beam’-like structure of the MEMS-based microphone 1 . In the embodiment shown in Fig. 4, the features of the MEMS-based microphone 1 are similar to the embodiment described above for Fig. 3, with the exception that the resonator 3 comprises two membrane connecting membrane elements 35A-B. Specifically, in view of Fig. 4, the membrane main element 34 comprises a square shape, whereby the two membrane connecting elements 35A-B are on the same side of the squareshaped membrane main element 34, such that the main membrane element 34 is suspended from one side. Alternatively stated, the two membrane connecting elements 35A-B connect one side of the membrane main element 34 to the body 2, wherein the two membrane connecting elements 35A-B are placed on the body 2. In this regard, the membrane main element 34 and two membrane connecting elements 35A-B cover at least a part of the cavity 7, whereby the remaining, uncovered parts of the cavity 7 form the venting system 10.

For the embodiments described in Figs. 3 and 4, they have the advantage that only a small proportion of the resonator 3 i.e. the at least one membrane connecting element 35A-B, are placed on the body 2, and this allows the resonator 3 to vibrate with further ease. Fig. 5 shows a schematic view of the MEMS-based microphone 1 , according to further exemplary embodiment of the present invention embodiments, whereby Fig. 5 relates to a 'trampoline’-like structure of the MEMS-based microphone 1. In this non-limiting example, a cavity 7 is provided in a body 2, and the resonator 3 comprises at least one pore 36. In view of the nonlimiting embodiment shown in Fig. 2, the resonator 3 comprises two rectangular-shaped pores 36. The resonator 3 is placed on the body 2 such that the resonator 3 covers at least a part of the cavity 7, and via the rectangular-shaped pores 36, the remaining, uncovered parts of the cavity 7 form the venting system 10.

Figs. 6A-B show a schematic view of the MEMS-based microphone 1 , according to an even further exemplary embodiment of the present invention embodiments, whereby Fig. 6 relates to a further ‘trampoline’-like structure of the MEMS-based microphone 1 . In the non-limiting example shown in Fig. 6, a circular cavity 7 is provided within a body 2, and the resonator 3 comprises four pores 36a-d (see Fig. 6A), whereby the four pores 36 form the venting system 10. The ‘trampoline’- like structure shown in Figs. 6A-B can be realized by e.g. first covering the entire circular cavity 7 with a circular resonator 3, and thereafter etching four circular patterns such that the etching takes place on the parts of the body 2 and resonator 3. The etched parts of the resonator 3, then become the pores 36, wherein the cavity 7 is beneath the resonator 3 (see Fig. 6B)

One further example of an implementation of the resonator 3 and cavity 7 is a comb-like membrane structure suspended above the cavity. The voids between teeth of the comb-like structure can act as the venting system 10, and the structural and dimensional characteristics of the teeth of the comb like membrane structure can be chosen to obtain the desired functional characteristics of the MEMS-based microphone 1.

It is re-iterated that the alternative structures of the MEMS-based microphone 1 described above are non-limiting examples, and alternative implementations of the features described are also possible. For example, different shapes may be envisaged for the resonator 3 and cavity 7.

Furthermore, a MEMS-based microphone 1 , in any of the embodiments describe herein, can operate at very low sound frequencies or constant pressure, i.e. there is no lower frequency limit. Similarly, the gas volume in the cavity 7 may be of a high compressibility, such that there is no maximum (dynamic) pressure limit at which the MEMS-based microphone 1 would stop working. Consequently, the MEMS-based microphone 1 has a large dynamic range of operation, and this is advantageous in comparison to state-of-the-art MEMS-based microphones, where, as described above, the membranes of state-of-the-art MEMS-based microphones may tend to break or collapse at high gas dynamic pressures.

In addition, the MEMS-based microphone 1 , in any of the embodiments describe above, may also be arranged to detect other parameters, e.g. pressure of the external environment in which the MEM based microphone 1 is placed within. In this respect, a MEMS-based microphone 1 that may simultaneously detect sound waves and pressure provides many advantages over state-of- the-art MEMS-based microphones.

Fig. 7 shows a schematic view of a microphone assembly, according to a further aspect of the present invention. In this further aspect, the microphone assembly comprises a MEMS-based microphone 1 and a processing unit 12 connected to the resonator 3 and arranged for processing an output signal of the resonator 3. The processing unit 12 may comprise e.g. an optical unit or electrical device, e.g. a lock-in amplifier, for processing the output signal of the resonator s.

In an exemplary embodiment, the processing unit 12 is further arranged to actuate the resonator 3 into resonance. The processing unit 12 may be arranged to actuate the resonator 3 at a specific mechanical resonance frequency, e.g. by controlling the membrane actuator 32, as described in one of the embodiments above. In this respect, the processing unit 12 may comprise an optical unit with a blue laser light to apply an alternating heat flux to the resonator 3, whereby the resonator 3 is actuated owing to thermal expansion effects. Alternatively, the processing unit 12 may comprise a waveform generator to generate an actuation signal to be applied, via the resonator actuator 32, to the resonator 3.

In an further exemplary embodiment, the processing unit 12 comprises a detection system 100 arranged to measure an instant mechanical resonance frequency of the resonator 3, thereby allowing the measurement of impinging sound waves via the mechanical resonance frequency of the resonator 3, as described herein. In a specific embodiment, the detection system 100 is arranged to measure the instant mechanical resonance frequency using an optical detection technique, a capacitive detection technique, a piezo resistive detection technique or a trans conductance technique. As a non-limiting example, the optical detection technique may comprise directing red laser light at the resonator 3, whereby the red laser light is reflected off the resonator 3 i.e. the resonator 3 acts as a mirror, and the intensity of the reflected laser light can thereafter be processed to determine the mechanical resonance frequency of the resonator 3. Alternatively, in another non-limiting example, the capacitive detection technique may comprise e.g. a set of capacitive sensors located on the resonator 3, whereby the capacitance of the set of capacitive sensors can be calibrated as a function of the mechanical resonance frequency of the resonator 3.

These detection techniques described herein can determine e.g. the position of the resonator 3 as a function of time. This is then used to determine the resonance frequency of the resonator s as a function of time. Since the mechanical resonance frequency of the resonator 3 is modulated by impinging sound waves, its modulation amplitude will be a measure of the amplitude of the sound waves, whereas the modulation frequency is a measure of the frequency of the sound waves.

Furthermore, the detection system 100 may be located away from the resonator 3, i.e. in an external location, to measure the instant resonance frequency using e.g. an optical detection technique. Alternatively, the detection system 100 may be located close to the resonator 3 i.e. directly connected to and on the resonator 3, to measure the instant resonance frequency using e.g. a capacitive detection technique.

In an even further exemplary embodiment, the processing unit 12 comprises a phase- locked loop arrangement. In this embodiment, the phase-locked loop arrangement is arranged to reconstruct the output signal from the resonator 3 i.e. the mechanical resonance frequency, into a sound signal by translating the phase error, due to a change of the mechanical resonance frequency of the resonator 3, into a required frequency changed needed to remedy the phase error. This measured frequency change is then enacted to bring the phase error to zero. From this perspective, the phase-locked loop arrangement is also arranged to follow the variable mechanical resonance frequency of the resonator 3 owing to impinging sound waves, thereby allowing full reconstruction of the output signal from the resonator 3, into an audio signal. The phase-locked loop arrangement is faster measurement methodology (i.e. 20 kHz signals) than e.g. an arrangement relating to sweeping the frequency to acquire the magnitude-plot and search for the frequency at the maximal amplitude, allowing quick reconstruction of the output signal. The control system for the phase- locked loop arrangement can be processed to obtain the frequency and amplitude of the sound signal, but other methods for down mixing the modulated high-frequency signal of the resonator 3 to determine the sound signal can also be used.

The present invention has been described above with reference to a number of exemplary embodiments as shown in the drawings. Modifications and alternative implementations of some parts or elements are possible, and are included in the scope of protection as defined in the appended claims.