MEMS microphone with low-pressure region between diaphragm and counter electrode

Abstract

MEMS microphone, which includes the following: a first membrane element (212); a second membrane element (214) which is arranged spaced apart from the first membrane element (212); a low-pressure area (232) arranged between the first membrane element (212) and the second membrane element (214), wherein the low-pressure area (232) has a pressure that is less than ambient pressure, and a first counter-electrode element (222) arranged within the low-pressure region (232), wherein the first counter-electrode element (222) comprises a first and a second section, wherein the first section is arranged in a central region of the MEMS microphone and is electrically isolated and separated from the second section, which is arranged in a surrounding peripheral region of the MEMS microphone, wherein, upon excitation by a sound wave, the first and second membrane elements (212, 214) in the central region are subjected to a greater deflection than in the peripheral region.

Description

TECHNICAL FIELD

[0001] Embodiments relate to a microelectromechanical system (MEMS) microphone. Some embodiments relate to a method of manufacturing a MEMS microphone. Some embodiments relate to a MEMS acoustic transducer. Some embodiments relate to a negative pressure (near vacuum) microphone and / or a negative pressure (near vacuum) loudspeaker. BACKGROUND

[0002] When designing transducers such as pressure sensors, accelerometers, microphones, or loudspeakers, it may typically be desirable to achieve a high signal-to-noise ratio (SNR). The ongoing miniaturization of transducers may pose new challenges regarding the desired high signal-to-noise ratio. Microphones and, to some extent, loudspeakers, which can be used in mobile phones and similar devices, can now be implemented as silicon microphones or microelectromechanical systems. To be competitive and deliver the expected performance, silicon microphones may require a high SNR. However, taking the condenser microphone as an example, the SNR can typically be limited by the condenser microphone's design.

[0003] The problem of the limited SNR achievable with conventional designs of condenser microphones, particularly when implemented as a MEMS, can be explained as follows. A condenser microphone typically comprises a diaphragm and a backplate, which may serve as a counter electrode. The sound may have to pass through the backplate, and as a consequence, the backplate is typically perforated. Note that the backplate must be perforated even in designs where the backplate may be located behind the diaphragm (i.e., on the side of the diaphragm facing away from the direction of sound incidence), because during operation the diaphragm may force some of the air in the volume between the diaphragm and the backplate through the perforated backplate into a rear cavity.Without the rear cavity and the perforation of the back plate, the volume between the diaphragm and the back plate could act like a very stiff spring and could thus prevent the diaphragm from vibrating significantly in response to the incoming sound.

[0004] Another design of capacitive microphones can use a so-called comb drive, in which the diaphragm and counter electrode have several intermeshing comb fingers along one side of the diaphragm. These comb sensor microphones can have reduced noise due to the lack of a backplate. However, there can still be a fluidic element of noise between the intermeshing comb fingers.

[0005] DE 602 14 398 T2 relates to a voice-activated chip card. The chip card comprises a module in which a pressure sensor is implemented with a pair of capacitors that convert pressure variations caused by a speaker into an analyzable signal.

[0006] EP 2 619 536 B1 relates to a microelectromechanical pressure sensor with a reference capacitor.

[0007] US 6,075,867 A relates to an electromechanical microphone.

[0008] The scientific publication by Wang et al., "Contamination-insensitive differential capacitive pressure sensors" in "Journal of microelectromechanical systems 9.4 (2000):538-543," refers to contamination-insensitive differential capacitive pressure sensors. SUMMARY OF THE INVENTION

[0009] The object underlying the present disclosure is to provide an improved MEMS microphone.

[0010] This problem is solved by the independent patent claims.

[0011] Specific embodiments, implementations and further developments are defined in the dependent patent claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Embodiments of the present invention are described herein with reference to the attached drawings. Fig. Figure 1 shows a schematic cross-section of a MEMS microphone comprising a single diaphragm element and a low-pressure region; Fig. 2 shows a schematic cross-section of a MEMS microphone, a MEMS loudspeaker, or a MEMS acoustic transducer comprising a first membrane element and a second membrane element enclosing a low-pressure region; Fig. 3A and Fig. 3B show schematic cross-sections of the MEMS microphone of the Fig. 2, while exposed to sound during operation; Fig. 4 shows the schematic cross section of the Fig. 2 and additionally a schematic circuit diagram illustrating a power supply and a detection circuit for the MEMS microphone; Fig. 5A and Fig. 5B show schematic cross-sections of a MEMS microphone at a first cross-sectional position; Fig. 6A and Fig. 6B show schematic cross-sections of the same MEMS microphone at a different cross-sectional position; Fig. 7A and Fig. 7B show schematic cross-sections of the same MEMS microphone at another cross-section position; Fig. 8A and Fig. 8B show schematic cross-sections of the same MEMS microphone at yet another cross-sectional position; Fig. 9A and Fig. 9B show schematic cross-sections of the same MEMS microphone at another cross-section position; Fig. 10 shows a schematic, perspective, partially cutaway view of a MEMS microphone; Fig. Figure 11 shows a similar schematic, perspective, partially cutaway view as Fig. 10 to better illustrate some details of the MEMS microphone; Fig. Figure 12 shows a schematic cross-section of a MEMS microphone and the effect of atmospheric pressure on the first and second diaphragm elements; Fig. 13 shows a dimension of a membrane segment spanning the area between two or more columns; Fig. Figure 14 schematically shows the extent of curvature at atmospheric pressure in the center of the membrane element in Fig. 13 as a function of the thickness and side length of the membrane segment; Fig. Figure 15 shows a schematic cross-section of a MEMS microphone having anti-stick bumps; Fig. 16A and Fig. 16B show schematic cross-sections of a MEMS microphone having a laterally segmented counter electrode; Fig. 17A and Fig. 17B show schematic cross-sections of a MEMS microphone comprising comparatively soft diaphragm elements that serve as hinges or suspensions for the first and second diaphragm elements; Fig. Figure 17C shows a schematic perspective cutaway view of a portion of the MEMS microphone in the Fig. 17A and Fig. 17B; Fig. Figure 18A shows a schematic horizontal cross-section of a MEMS microphone including an X-shaped counter electrode; Fig. 18B and Fig. 18C show schematic cross-sections of the MEMS microphone from Fig. 18A; Fig. 19 shows a schematic cross-section of a MEMS microphone comprising a single counter electrode and a first and a second membrane element, which may be electrically isolated from each other, and Fig. 20A to 20O schematically show a process flow of a method for manufacturing a MEMS microphone.

[0013] Identical or equivalent elements or elements with identical or equivalent functionality are identified by identical or equivalent reference numerals in the following description. DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

[0014] In the following description, several details are 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, well-known structures and devices are shown in block diagrams rather than in detail to avoid obscuring embodiments of the present invention. Additionally, features of the various embodiments described below may be combined with one another unless expressly noted otherwise.

[0015] Standard condenser microphones can use parallel plate capacitance with a change in gap spacing due to diaphragm displacement. This can represent the noise of air moving through the perforations. When examining the SNR problem in today's microphones, the perforated backplate can be identified as one of the main contributors to noise. One possible solution could be to remove the perforated backplate, but this might require a new sensor design. Experiments and simulations conducted by the inventors have shown that removing the perforated backplate could, in theory, improve the SNR by as much as 4 to 27 dB (decibel sound pressure level). For a comparatively large microphone that has an active volume of 40 mm 3The SNR can be approximately 71 dB(A) when the perforated backplate is present. After removing the perforated backplate, the SNR can increase to 98 dB(A). For a comparatively small microphone with an active volume of 2.3 mm 3 the improvement may not be as significant, but still 4 dB from 69 dB(A) with the existing perforated back plate to 73 dB(A) after removing the perforated back plate.

[0016] Noise in acoustic systems can arise from the viscous flow of air within microstructures, causing damping and dissipative losses. For a capacitive microphone concept, some aspects described here may teach how the static reference electrode (counter electrode) is encapsulated under negative pressure or a low-pressure atmosphere within the movable membrane or diaphragm. Further aspects disclosed here may teach how and under what circumstances a low-pressure region can be provided between a single membrane element and the counter electrode element.

[0017] Fig. 1 schematically illustrates a concept for a MEMS microphone in which a low-pressure region 132 may be provided between a membrane element 112 and a counter electrode element 122. Fig. 1 schematically shows a possible embodiment as an example. For reasons of consistency with subsequent parts of the description, the membrane element 112 may also be referred to as a “first membrane element.” The membrane element 112 may be exposed to ambient pressure on one of its sides and possibly to sound pressure. This side of the membrane element 112 may also be considered a main sound-receiving surface of the membrane element 112. On its other side, the membrane element 112 may be adjacent to the low-pressure region 132. The membrane element 112 may be designed as a membrane or a membrane element. A displacement of the membrane element 112 in response to the sound pressure may be schematically shown in the Fig. 1 be represented by dash-dot lines (it should be noted that the displacement may be somewhat exaggerated for illustrative purposes).

[0018] The low pressure area 132 can be Fig. 1 by a dashed line. The low-pressure region 132 has a pressure that can typically be less than ambient pressure or standard atmospheric pressure. The low-pressure region 132 can be adjacent to and typically in direct contact with the membrane element 112 and also with the counter electrode element 122.

[0019] The membrane element 112 can be preloaded by a pressure difference between the ambient pressure and the pressure within the low-pressure region 132, which can typically be less than the ambient pressure. Accordingly, the membrane element 112 can assume a corresponding resting position or configuration when no sound is reaching the membrane element 112. The lower pressure can lead to lower attenuation within the low-pressure or negative-pressure region, depending on the density of the fluid. At the same time, the membrane that withstands normal pressure and detects sound may not require a back volume, as there may be little or no force transmitted to the second electrode through a fluidic coupling. To give some figures as an example, the membrane would have to withstand an absolute pressure of up to 100 kPa.The sound pressure to be measured can, for example, be in a range of up to 1 mPa or up to 10 mPa.

[0020] According to at least one embodiment, the pressure in the low-pressure region may be substantially negative pressure or a negative pressure close to vacuum. In other examples of implementation, the pressure in the low-pressure region may be less than 50% of ambient pressure or standard atmospheric pressure. It may also be possible for the pressure in the low-pressure region to be less than 45%, 40%, 35%, 30%, 25%, or 20% of ambient pressure or standard atmospheric pressure (standard atmospheric pressure may typically be 101.325 kPa or 1013.25 millibars). The pressure in the low-pressure region may also be expressed as an absolute pressure, for example, less than 50 kPa, less than 40 kPa, less than 30 kPa, or less than 25 kPa.In any case, the pressure in the low pressure region may typically be selected to be lower than the typical range of atmospheric pressure for weather conditions that should reasonably be expected and for the altitudes relative to sea level at which the use of the MEMS microphone may be intended (e.g., up to 9000 meters above sea level).

[0021] The first diaphragm element can have a diaphragm compliance of at least 1 nm / Pa. According to alternative embodiments, the diaphragm compliance can be at least 2 nm / Pa, at least 3 nm / Pa, at least 4 nm / Pa, or at least 5 nm / Pa. The diaphragm compliance can typically be understood as the inverse of the diaphragm stiffness. However, as used here, the diaphragm compliance can be normalized to the size of the diaphragm and can express a maximum deflection of the diaphragm when subjected to a specific sound pressure, here 1 Pascal (Pa). The reference sound pressure in air that can commonly be used can be Pref = 20 µPa(rms), which approximately corresponds to the threshold of human hearing. With this reference sound pressure, a sound pressure level (SPL) of 94 dB can result in a sound pressure of 1 Pa (for comparison, a jackhammer at 1 meter can have a sound pressure level of approximately 100 dB).

[0022] Fig. 2 shows a schematic cross-section through a MEMS microphone, which may further comprise a second membrane element 214 arranged on an opposite side of the counter electrode element 222 to the first membrane element 212. Fig. Figure 2 shows another possible embodiment. The MEMS microphone may include a plurality of columns or struts 272 extending between the first membrane element 212 and the second membrane element 214. The columns 272 typically do not contact or touch the counter electrode element 222, but rather pass through the openings or holes 227 in the counter electrode element 222. In the embodiment shown schematically in Fig. 2, the pillars 272 are formed integrally with the first and second diaphragm elements 212, 214. Therefore, the first diaphragm element 212, the second diaphragm element 214, and the pillars 272 may form a unitary structure made of the same material, for example, polycrystalline silicon. Nevertheless, this does not mean that the first diaphragm element 212, the second diaphragm element 214, and the pillars 272 must be formed simultaneously during the manufacture of the MEMS microphone. Rather, it may be possible for the second diaphragm element 214 to be formed first on a surface of a substrate 202 (or on a surface of an auxiliary layer such as an etch stop layer) during a first deposition process. Subsequently, the pillars 272 and eventually also the first diaphragm element 212 may be formed during a second deposition process and possibly during a third deposition process.In alternative embodiments described below, the columns 272 may be made of a different material than the first and second membrane elements 212, 214. The first membrane element 212 may have a major surface facing in the direction of incidence of sound (schematically represented by an arrow in FIG. Fig. 2).

[0023] In the MEMS microphone, which is shown schematically in the cross-sectional view in Fig. 2 as an example, a second counter electrode element 224 may be provided in addition to the first counter electrode element 222. The second counter electrode element 224 may be arranged at a distance from the first counter electrode element 222. A counter electrode insulating layer 252 may electrically insulate the first counter electrode element 222 and the second counter electrode element 224 from each other. In the example of a MEMS microphone schematically shown in Fig. 2, the first counter electrode element 222, the second counter electrode element 224, and the counter electrode insulating layer 252 may form a counter electrode assembly or a counter electrode structure that may be supported at its edge or periphery by a support structure. It should be noted that the three central portions of the counter electrode assembly shown in Fig. 2, although they appear to “float” within the low pressure region 232, typically at the periphery of the counter electrode structure above and / or below the plane of the drawing of the Fig. 2, as indicated by the dashed lines.

[0024] In the example shown schematically in Fig. 2, the support structure may have a layered configuration, and the edge portions of the first membrane element 212, the second membrane element 214, and the counter electrode assembly 222, 224, 252 may be in planar contact with the support structure on one or two of their major surfaces. The support structure itself may be arranged on a major surface of the substrate 202. On this major surface of the substrate 202, various layers may be arranged one upon the other in the following order, for example: second membrane element 214, second membrane insulation 244, second counter electrode element 224, counter electrode insulation 252, first counter electrode element 222, first membrane insulation 242, and first membrane element 212. A back cavity 298 may be formed in the substrate 202 to allow the second membrane element 214 to vibrate in response to a sound wave.

[0025] When examining the pressure situation of the structure, it can be observed that the membrane structure, which includes the first membrane element 212, the columns 272, and the second membrane element 214, must be rigid enough to withstand the 1 bar overpressure of the external atmosphere against the negative pressure cavity or the low pressure cavity. In particular, the columns 272 can be considered as vertical ribs extending through the holes 227 of the counter electrode assembly (also called the "stator") to stabilize the structure. The membrane assembly 212, 214 can be tightly sealed.

[0026] Fig. Figure 2 shows the MEMS microphone in its rest position, e.g., when no sound wave is incident on the membrane elements 212, 214, which would cause the membrane elements 212, 214 to deflect. On the side of the first membrane element 212, where sound can be incident, the total pressure can be expressed as p(t) = normal pressure + psound(t). Within the rear cavity 298, only normal atmospheric pressure can prevail, i.e., p0 = normal pressure. Within the low-pressure region 232, the pressure can be comparatively low, e.g., pgap ~ 0 or pgap < 50% of the ambient pressure.

[0027] Fig. 3A and Fig. 3B show schematic cross-sections through a possible MEMS microphone when exposed to sound, as possible examples and / or embodiments. Fig. Figure 3A shows the situation in which the diaphragm assembly 212, 214, 272 is in the closed position due to a relative overpressure caused by the sound on the upper side adjacent to the first diaphragm element 212, compared to the reference pressure within the rear cavity 298, ie, p(t)=Normal pressure+|pSound|, can be pushed down.

[0028] In Fig. 3B, the pressure on the sound receiving side may be lower than the pressure within the rear cavity 298, so that the membrane assembly 212, 214, 272 may be deflected upward. Accordingly, the membrane structure moves up and down with respect to the counter electrode structure 222, 224, 252 (stator) under sound. The lower pressure in Fig. 3B can be p(t)=Normal pressure−|pSound| be expressed.

[0029] Fig. Figure 4 schematically illustrates an example of how the MEMS microphone can be electrically connected to a power supply circuit and an amplifier. Fig. Figure 4 shows an example of a possible connection. Other arrangements may also be possible. Fig. 4, the first and second membrane elements 212, 214 can be grounded to an electrical ground potential or reference potential by a membrane connection 412. The first counter electrode element 222 can be electrically connected to a first power supply circuit and also to a first input of an amplifier 401 by a first electrode connection 422. The first power supply circuit includes a voltage source 402 and a resistor 406. The resistor 406 can have a very high resistance of several gigaohms or even as high as 1 teraohm. The amplifier 401 can be a differential amplifier. The second counter electrode element 224 can be connected to a second power supply circuit and a second input of the amplifier 401 by a second counter electrode connection 424.The second power supply circuit includes a second voltage source 404 and a second resistor 408, which typically has the same resistance as resistor 406. The first and second power supply circuits bias the first and second counter electrode elements 222 and 224, respectively, against the electrical reference potential (ground potential). When the membrane structure can be deflected in response to an incoming sound, the electrical potentials at the first and second counter electrode elements 222, 224 can change in opposite directions due to the changing capacitances between the membrane structure and the first and second counter electrode elements, respectively. This is illustrated by a first waveform 432 and a second waveform 434, which are fed to the first and second inputs of amplifier 401, respectively. Fig. 4. Amplifier 401 can generate an amplified output signal 430 based on input signals 432 and 434, in particular a difference between input signals 432, 434. Amplified output signal 430 can then be fed to further components for subsequent signal processing, for example, analog-to-digital conversion, filtering, etc.

[0030] A possible design of a MEMS microphone having a low pressure region between two membrane elements and a counter electrode within the low pressure region will now be described with reference to the Fig. 5A to 10. Fig. 5A to 10 show possible embodiments and / or examples of possible embodiments. Fig. 5A, Fig. 6A, Fig. 7A, Fig. 8A and Fig. 9A may be substantially identical and a location of a corresponding cross-section shown in each of the Fig. 5B, Fig. 6B, Fig. 7B, Fig. 8B and Fig. 9B. The example shown in the Fig. 5A to 10 may refer to a side design including a vent hole 515 for static pressure equalization between the ambient atmosphere and the rear cavity 298.

[0031] Fig. 6A indicates that the next horizontal cross section shown in Fig. 6B, along a sectional plane passing through the second membrane element 214. The ventilation hole 515 may have a square cross-section at this location.

[0032] Fig. Figure 7A shows another cross-section of the MEMS microphone and Fig. 7B shows the corresponding schematic horizontal cross-section, which may have been performed at a height of the second membrane insulation 244. In the illustrated example of a MEMS microphone, the second membrane insulation 244 may not only provide electrical insulation between the second membrane element 214 and the second counter electrode element 224, but may also serve as a support for the second counter electrode element 224 and other structures arranged on the second counter electrode element 224. Therefore, the second membrane insulation 244 may also be considered part of a support structure. The second membrane insulation 244 may also laterally restrict or delimit the low-pressure region 232. The columns 272 may also be in the Fig. 7A and Fig. 7B. In a manner similar to the columns 272, a channel 715 may be formed by four sidewalls extending between the first membrane element 212 and the second membrane element 214. The channel 715 may have a square cross-section in the illustrated example, but may also have other cross-sectional shapes. The channel 715 may seal the low-pressure region 232 from the vent hole 515.

[0033] In Fig. 7B, it can be seen that each of the columns 272 may have an elongated cross-section, in particular a rectangular cross-section. However, other cross-sectional shapes may also be possible. Therefore, each of the columns 272 may be significantly wider than it is thick, for example, between three and six times as wide as it is thick. The width of a column may be Fig. 7B may be schematically represented by “w” and the thickness of a column 272 may be Fig. 7B may be schematically represented by "t". A first subset of the columns 272 may be oriented such that their cross-sectional width w extends along a first direction. A second subset of the columns 272 may be differently oriented such that their cross-sectional width w extends in a second direction, which may not be parallel to the first direction. In the example schematically shown in Fig. 7B, the second direction of the cross-sectional orientation of the second subset of columns 272 may be perpendicular to the first direction describing the cross-sectional orientation of the columns 272 in the first subset of columns. In alternative embodiments, the plurality of columns 272 could be divided into three or even more subsets of columns, each having a different direction of cross-sectional orientation. The columns 272 have different cross-sectional orientations to achieve a substantially isotropic stiffness of the entire membrane assembly against the overpressure exerted by the atmosphere on the first and second membrane elements 212, 214. Furthermore, at least some of the columns 272 may be spaced apart from one another to allow sufficient space for the counter electrode assembly 222, 224, 152, as shown in Fig. 8B will be seen.

[0034] Fig. 8B shows a horizontal cross-section at a height of the counter electrode insulation 252. The geometry of the counter electrode insulation 252 can, in the illustrated example, also be representative of the geometries of the first and second counter electrode elements 222 and 224, and thus of the entire counter electrode assembly, which includes the three layers of the first counter electrode 222, the counter electrode insulation 252, and the second counter electrode element 224. The counter electrode insulation 252 can include holes 227. The columns 272 can pass through the holes 227 without touching the edges of the holes 227, i.e., with sufficient spacing. Thus, the membrane assembly can move up and down relative to the counter electrode assembly if the membrane assembly can be deflected up and down, which occurs primarily within its central section when the membrane assembly is exposed to a sound wave.Furthermore, the holes 227 in the counter electrode assembly may prevent the pillars 272 from coming into electrical contact with the first counter electrode 222 and / or the second counter electrode 224, which would cause a short circuit between the membrane assembly and the counter electrode assembly.

[0035] To provide some additional mechanical stability for the membrane assembly, the side walls of the channel 715 can be Fig. 8B thicker than in the horizontal cross section of the Fig. 7B. The ventilation hole 515 can be provided at the location shown in Fig. 8B, have a circular cross-section.

[0036] Fig. 9B shows a similar horizontal cross section as Fig. 8B, with the difference that the cross section is made at the height of the first counter electrode element 222.

[0037] Fig. Figure 10 shows a schematic perspective cross-sectional view of the MEMS microphone. Fig. Figure 11 shows a similar schematic perspective cross-sectional view in which the relationship of the counter electrode assembly and the columns 272 may be shown in more detail. In particular Fig. 11 may show how one of the pillars 272 may pass through one of the holes 227 that may be formed in the counter electrode assembly.

[0038] Fig. Figure 12 shows a schematic cross-section of a MEMS microphone, in which a curvature of the membrane sections can be schematically illustrated. Due to the negative pressure or low pressure between the first and second membrane elements 212, 214, the suspended membrane parts can be subjected to the ambient pressure, which leads to a curvature. Due to the columns 272, which are typically regularly arranged between the first and second membrane elements 212, 214, the curvature can be reduced to a comparatively small extent.

[0039] Fig. 13 schematically represents a suspended membrane part. The lateral extension “l” of the suspended membrane part, its thickness t Membran and its intrinsic tension can define the extent of the curvature. As an example, Fig. Figure 14 shows the results of calculations for the curvature of a small square section of a stress-free polysilicon membrane under a pressure of 1 bar (atmospheric pressure) for different thicknesses and side lengths. For typical dimensions (side length = 20 µm, thickness = 0.5 µm), the curvature can be approximately 140 nm and is suitable for an air gap of typically 2 µm. Tensile loading in the membrane layer can further reduce the curvature.

[0040] Fig. 15 shows a schematic cross-section of a MEMS microphone according to a possible embodiment, which may have a low-pressure region 232 between the first and second membrane elements 212, 214. According to the embodiment shown in Fig. 15, the first membrane element 212 may include anti-stick protrusions 1512 arranged on the surface of the first membrane element 212 facing the low-pressure region 232. The anti-stick protrusions 1512 may reduce the risk of the first membrane element 212 sticking to the first counter electrode element 222 due to an adhesive force. Similarly, the second counter electrode element 224 may include a second plurality of anti-stick protrusions 1524 facing the second membrane element 212. The anti-stick protrusions 1512 may be integrated into the first membrane element 212. The anti-stick protrusions 1524 may be integrated as a part into the second counter electrode element 224.

[0041] Fig. Figure 16A shows a schematic cross-section of a MEMS microphone having a lateral division of the counter electrodes. Fig. 16B shows a schematic horizontal section of the same MEMS microphone. In this embodiment, the first counter electrode element 222 no longer extends into the support structure except for a narrow contact strip to electrically connect the first counter electrode element 222 to an external circuit, such as the power supply and readout circuitry. The first counter electrode element 222 can be delimited laterally by a gap 1623 that electrically insulates the first counter electrode element 222 from a region with surrounding counter electrode material 1622. The first counter electrode element 222 can be limited to a central region of the MEMS microphone. The first membrane element 212 and the second membrane element 214 can be subjected to greater deflections in the central region than in an edge region due to excitation by a sound wave. In the edge region, i.e.Within the support structure and in proximity to the support structure, the first and second diaphragm elements 212, 214 typically cannot move significantly in response to a sound wave. Therefore, the edge region may not contribute to a change in capacitance. The lateral separation of the first and second counter electrode elements 222, 224 can typically result in a larger percentage change in capacitance in response to a sound wave and, consequently, in greater sensitivity of the MEMS microphone. The gap 1623 can be filled with the material of the first diaphragm insulation 242 as it passes through the support region to seal the low-pressure region 232 from the external ambient atmosphere.The same can be done with the gap between the second counter electrode element 224 and the corresponding edge material 1624 by using the second membrane insulation 244 to fill the gap between the elements 224 and 1624. Alternatively, the.

[0042] Gap 1623 and the gap around the second counter electrode element 224 may be filled or replaced by a specially designed insulating material.

[0043] Fig. Figure 17A shows a schematic cross-section of a MEMS microphone as an example of how softer membranes can be introduced and still be stiff against low pressure in the low pressure region, i.e., between the first and second membrane elements 212, 214. Fig. 17B shows a corresponding horizontal section. The MEMS microphone may include a hinge element or a third membrane element 1716. The hinge element or the third membrane element 1716 may be coupled between the first membrane element 212 and a support structure 1706. The hinge element / third membrane element 1716 may have a stiffness that may be less than the stiffness of the first membrane element 212 and / or less than the stiffness of the second membrane element 214. The third membrane element 1716 may include a wall element 1717 configured to laterally confine the low-pressure region 232. The wall element 1717 may be coupled to the support structure 1706 such that the support structure 1706 participates in confining the low-pressure region 232. The MEMS microphone shown in the Fig. 17A and Fig. 17B, ​​may comprise four hinge elements / third membrane elements 1716. The first counter electrode element 222 may be coupled to the support structure 1706 independently of the hinge element 1716. This may be achieved by providing at least one gap in the hinge element 1716 through which the first counter electrode element 222 can be moved from the low-pressure region 232 to the support structure 1706 in Fig. 17B. This can be shown schematically for the counter electrode insulation 252. The structure of the first counter pressure element 222 and the second counter pressure element 224 can be substantially similar to the structure of the counter electrode insulation 252. In the configuration shown in the Fig. 17A and Fig. 17B, ​​there may be four gaps between the four joint elements 1716, the four gaps being provided, for example, in the four corners of a square formed by the four joint elements 1716.

[0044] Fig. Figure 17C shows a schematic perspective cutaway view of the first and second membrane elements 212, 214 and two of the hinge elements 1716. For clarity, the counter electrode elements 222, 224 and the counter electrode insulation 252 are omitted from the illustration in Fig. 17C. It can be seen that each of the hinge elements 176 can form a structure that can be described as a "double trough," with the two troughs arranged with their bottoms facing each other. In the corners, the two hinge elements 1716 do not necessarily meet and can leave a gap between the hinge elements 1716, which can allow the counter electrode structure to be mechanically and electrically coupled to the support structure independently of the membrane structure. The hinge elements 1716 can have an H-shaped cross-section in this embodiment. In alternative embodiments, the hinge element(s) 1716 could, for example, have a U-shaped cross-section or another cross-section, where, for example, the second membrane element 214 can be continuous to form the bottom bar of the "U," and the first membrane element 212 can be interrupted by the wall element 1717.The dashed lines in . Fig. 17C may schematically show some of the inner contours of the low pressure region 232.

[0045] Fig. 18A to 18C schematically illustrate another possible embodiment of a MEMS microphone, wherein the counter electrode elements have an approximately X-shaped configuration.

[0046] Fig. 18A may show a schematic top view of the first counter electrode element 1822 and the hinge elements or the third membrane element 1816. For clarity, some elements may be omitted from the illustration, for example, the wall elements 1717 in the Fig. 17A to 17C. It can be seen that the first counter electrode element 222 may be suspended from the support structure 1706 by four arms extending in an X-shaped manner from a central portion of the first counter electrode element 222. As an alternative embodiment, the first counter electrode element 1822 could be supported by only one arm, two arms, three arms, or any other number of arms from the support structure.

[0047] Fig. Figure 18B shows a schematic cross-section through the MEMS microphone of the Fig. 18A. Fig. Figure 18C shows a corresponding horizontal section through the MEMS microphone. As in Fig. 18C can be seen, the cross section of the Fig. 18B with an oblique cutting plane, so that a left section in Fig. 18B shows a cross section through the joint element 1816 and a right section of the Fig. 18B shows a schematic cross-section through the counter electrode insulation 1852. The hinge element or the third membrane element 1816 may include wavy lines 1818 that promote a curvature of the hinge element 1816 in that area. The curvature of each hinge element 1816 may be described as a rotation about an axis that extends parallel to an extended extent of the wavy lines 1818. The wall element 1817 of the hinge element 1816 may participate in confining the low-pressure region 232 from the ambient atmosphere. For this purpose, the wall element 1817 may be coupled to the support structure 1706. In the example shown in the Fig. 18A to 18C, the wall element 1817 may include a first wall portion that may begin at the support structure 1706 at an angle, a second wall portion that may extend substantially parallel to the support structure 1706, and a third wall portion that may join the support structure 1706 at an angle. In this way, the wall element 1817 may form three sides of a trapezoid surrounding the remainder of the hinge element 1816, particularly the portion that includes the wave lines 1818. A fourth side of the trapezoid may be formed by the support structure. Further vent holes 1815 may be formed in one or more hinge elements 1816. The vent holes 1815 may be configured to enable static pressure equalization between the ambient pressure and the rear cavity 298. As explained above, there may further be a ventilation hole 515 in a central column 715.

[0048] Fig. 19 shows a further example of a possible embodiment of a MEMS microphone, in which the stator, i.e., the counter electrode arrangement, can be embodied as a single electrode, and the movable membrane structure comprises two electrodes that can be electrically isolated from one another. The MEMS microphone can comprise a first membrane element 1912 and a second membrane element 1914. The first membrane element 1912 can be coupled to the second membrane element 1914 by a plurality of electrically insulating pillars 1972. The counter electrode arrangement can comprise a single counter electrode element 1922 made of electrically conductive material. It can also be possible to provide two counter electrodes that can be electrically isolated from one another, and additionally two membranes that can also be electrically isolated from one another, i.e., four different electrodes for the MEMS microphone.

[0049] Fig. 20A to 20O show schematic cross-sections through a portion of a wafer during the various stages or steps of a possible example of a MEMS microphone manufacturing process as described above. Any dimensions, values ​​of the thicknesses of the various layers, material selections, etc. are only examples and are therefore subject to change.

[0050] Fig. 20A shows the substrate 202, which may be a silicon wafer in which silicon may be arranged in a monocrystalline structure. A lower etch stop layer 203 may have been applied to an upper main surface of the substrate 202. The lower etch stop layer 203 may ensure a reliable stop of an etching process for forming the cavity 298, which may occur at a later stage of the manufacturing process. The lower etch stop layer 203 may typically be made of, for example, an oxide, a thermal oxide, or TEOS. Its thickness may be between 0.1 and 1 µm.

[0051] Fig. Figure 20B shows a schematic cross-section of the wafer after a layer for the second membrane element 214 has been applied to the lower etch stop layer 203. Furthermore, the second membrane element 214 can be Fig. 20B may also already be structured. The material may be doped polysilicon, which may be applied as a doped polysilicon layer as part of the motor of the MEMS microphone. Layer 214 may typically be between 0.5 and 2 µm thick.

[0052] Fig. Figure 20C shows a schematic cross-section through the wafer after a layer of sacrificial oxide 2044 for a lower gap has been deposited on the structure shown in Fig. 20B. The sacrificial oxide may be substantially the same material as the material for the lower etch stop layer 203. The thickness of the deposited second membrane oxide 2044 on the second membrane element 214 may typically be between 0.5 and 2 µm, depending on the desired gap width of the MEMS microphone.

[0053] Fig. 20D shows a schematic cross-section after various layers of a multilayer stator have been deposited on the previously deposited sacrificial oxide 2044. The multilayer stator may comprise three layers in the illustrated example: a layer 2024 for subsequently forming the second counterelectrode element 224, a layer 2052 of an electrically insulating material for subsequently forming the counterelectrode insulation 252, and a layer 2022 for subsequently forming the first counterelectrode element 222. Layers 2024 and 2022 may be made of doped polysilicon or may comprise doped polysilicon. Layer 2052 may comprise silicon nitride SiN. Other materials may also be possible, for example, monocrystalline silicon (substrate or silicon on insulator, SOI), polycrystalline silicon, metal (e.g., aluminum or an Al-SiCu). Dielectric layers may be an oxide, Si3N4, Si x N yO, polyimide, etc. The thickness of the various layers of the multilayer stator may, for example, be between 0.1 and 1 µm for the layers of the first and second counter electrode elements 2022, 2024 and between 0.1 and 0.5 µm for the layer of counter electrode insulation 2052.

[0054] Fig. 20E shows a schematic cross-section after the multilayer stator comprising the three layers 2024, 2052 and 2022 has been patterned and in particular the openings 2027 or slots have been formed in the multilayer stator, wherein the openings 2027 may, for example, extend to the second membrane insulating layer 2044.

[0055] The openings 2027 can then be filled by a deposition process, for example, a TEOS deposition 2042 with a thickness between 0.5 and 5 µm. If the second membrane insulating layer 2044 is made of the same material as the deposited material, the two layers can merge and form a structure. A schematic cross-section after a TEOS deposition can be seen in Fig. 20F. Other application materials may also be possible.

[0056] Fig. 20G shows a schematic cross-section after a mask 2045 may be applied and patterned onto the second membrane insulating layer 2042. Then, a so-called spacer etching process (column etching process) may be performed, the results of which are shown in Fig. 20H can be seen. In particular, the holes 2027 may have been deepened so that they can now reach the second membrane element 214.

[0057] In Fig. 20I, the mask 2045 may have been removed. The holes 2027 may now define the shape of the future pillars 272. In a subsequent step, a further deposition of the doped polysilicon 2012 may be performed, filling the holes 2027 ( Fig. 20J). The thickness of the deposited doped polysilicon can, for example, be between 0.5 and 2 µm.

[0058] The deposited doped polysilicon 2012 can then be patterned. By patterning the first membrane layer 2012, several small holes 2011 can be created in the first membrane layer 2012. Each hole can, for example, have a diameter between 0.1 and 1 µm. The small holes 2011 can then be used as etch holes and then sealed again. Fig. 20J shows a schematic cross-section after a first membrane layer 2012 has been applied and patterned. Simultaneously with the formation of the small etch holes 2011, a lateral division of the first membrane layer 2012 may be performed by forming a gap 2021 separating the first membrane 212, which may subsequently be completed, from a surrounding portion of the first membrane material 2012. The surrounding portion of the first membrane material may then be used to electrically connect the first counter electrode element 222, the second counter electrode element 224, and / or the second membrane element 214.

[0059] Fig. 20K shows a schematic cross-section after the lateral split 2021 may have been temporarily covered by a mask 2046. Using the remaining small holes 2011 that have not been masked, a clearance etch may be performed to remove the oxide between the second membrane layer 214 and the first membrane layer 2012. The clearance etch process may be time-controlled, such that an edge portion of the sacrificial material 2042, 2044 may not be etched away by the etchant because the distance of the next hole 2011 may be too large for the etchant to reach the edge portion during the duration of the clearance etch process. Instead of a time-controlled etch process, other shapes for providing an etch stop may also be used.

[0060] Fig. 20L shows a schematic cross-section after the mask 2046 may have been removed. In Fig. 20M, an etching hole sealing may have been performed to seal the small holes 2011 with a suitable sealing material 2019, which is in Fig. 20M and in the following Fig. 20N and Fig. 20O may be schematically indicated by a thick line. This sealing step may be performed under a low-pressure atmosphere or a negative pressure (near vacuum) to maintain the low-pressure region 232. Etching hole sealing may include one or more of the following operations: Coating with an uneven application of oxide under low pressure or Application of BPSG (borophosphosilicate glass) and later reliquefaction under low pressure / negative pressure or Lamination of a film under low pressure / negative pressure.

[0061] Fig. Figure 20M illustrates the case of applying BPSG, which may also result in BPSG covering the inner sidewalls of the low-pressure region 232.

[0062] Fig. 20N shows a schematic cross-section of a MEMS microphone during manufacturing after the contact holes may have been etched. A first contact 2082 may be formed in a first contact hole and provide an electrical connection for the first membrane element 212. A second contact 2092 may be provided in a second contact hole to provide an electrical connection for the first counter electrode element 222. A third contact 2094 may be provided in a third hole as an electrical connection for the second counter electrode element 224. It should be noted that the lateral partition(s) 2021, the first membrane insulation 242, and the counter electrode insulation 252, for example, provide electrical isolation between the different contacts 2082, 2092, 2094. A contact for the second membrane element 214 may not be explicitly shown in Fig. 20N, but it can be formed, for example, in an analogous manner to the contacts 2082, 2084, 2092.

[0063] Fig. Figure 20O shows the final MEMS microphone in a schematic cross-section after a backside etch of the backside cavity 298, for example, using a DRIE / Bosch process (DRIE: high aspect ratio reactive ion etch). The lower etch stop layer 203 may serve as an etch stop for the DRIE process and may have been removed after the DRIE process by another dedicated oxide etch process.

[0064] Although some aspects have been described in the context of a device, it is clear that the aspects also represent a description of the corresponding method in which a block or device corresponds to a method step or a feature of a method step. Similarly, aspects described in the context of a method step also represent a description of a corresponding block, detail, or feature of a corresponding device.

[0065] According to a first aspect, a MEMS microphone comprises a first membrane element; a counter electrode element; and a low-pressure region between the first membrane element and the counter electrode element, wherein the low-pressure region has a pressure that is less than an ambient pressure.

[0066] According to a second aspect with reference to the first aspect, the pressure in the low pressure region is substantially a negative pressure.

[0067] According to a third aspect with reference to at least one of the first to second aspects, the pressure in the low pressure region is less than about 50% of the ambient pressure.

[0068] According to a fourth aspect with reference to at least one of the first to third aspects, the microphone further comprises a second diaphragm element arranged on an opposite side of the counter electrode element to the first diaphragm element.

[0069] According to a fifth aspect with reference to the fourth aspect, the first membrane element is electrically coupled to the second membrane element.

[0070] According to a sixth aspect with reference to at least one of the fourth to fifth aspects, the first membrane element is electrically insulated from the second membrane element.

[0071] According to a seventh aspect with reference to at least one of the first to sixth aspects, the microphone further comprises a second counter electrode element arranged at a distance from the first counter electrode element.

[0072] According to an eighth aspect with reference to the seventh aspect, the first counter electrode element is electrically insulated from the second counter electrode element.

[0073] According to a ninth aspect with reference to at least one of the first to eighth aspects, the first membrane element has a membrane compliance of at least about 1 nm / Pa.

[0074] According to a tenth aspect with reference to at least one of the first to ninth aspects, the first membrane element has a membrane compliance of at least about 5 nm / Pa.

[0075] According to an eleventh aspect with reference to at least one of the first to tenth aspects, the low pressure region is within a sealed cavity.

[0076] According to a twelfth aspect with reference to at least one of the first to eleventh aspects, the MEMS microphone further comprises a hinge element coupled between the first membrane element and a support structure.

[0077] According to a thirteenth aspect with reference to the twelfth aspect, the hinge element comprises a wall element configured to laterally restrict the low-pressure region.

[0078] According to a fourteenth aspect with reference to the thirteenth aspect, the wall element is coupled to the support structure such that the support structure participates in restricting the low-pressure region.

[0079] According to a fifteenth aspect with reference to at least one of the thirteenth to fourteenth aspects, the counter electrode element is coupled to the support structure independently of the joint element by at least one gap in the joint element extending from the low-pressure region to the support structure

[0080] According to a sixteenth aspect, a MEMS microphone comprises: a first diaphragm element; a second diaphragm element spaced apart from the first diaphragm element; a low-pressure region disposed between the first diaphragm element and the second diaphragm element, the low-pressure region having a pressure less than an ambient pressure; and a first counter electrode element disposed within the low-pressure region.

[0081] According to a seventeenth aspect with reference to the sixteenth aspect, the pressure in the low pressure region is substantially a negative pressure.

[0082] According to an eighteenth aspect with reference to at least one of the sixteenth to seventeenth aspects, the pressure in the low pressure region is less than 50% of the ambient pressure.

[0083] According to a nineteenth aspect with reference to at least one of the sixteenth to eighteenth aspects, the low pressure region is within a sealed cavity.

[0084] According to a twentieth aspect with reference to at least one of the sixteenth to nineteenth aspects, the MEMS microphone further comprises a second counter electrode element electrically insulated from the first counter electrode element.

[0085] According to a twenty-first aspect referring to at least one of the sixteenth to twentieth aspects, the MEMS microphone further comprises one or more columns coupled between the first diaphragm element and the second diaphragm element.

[0086] According to a twenty-second aspect with reference to the twenty-first aspect, the one or more columns are electrically insulating.

[0087] According to a twenty-third aspect with reference to at least one of the twenty-first or twenty-second aspects, the one or more columns are electrically conductive.

[0088] According to a twenty-fourth aspect with reference to at least one of the twenty-first to twenty-third aspects, at least two columns are arranged spaced between 5 µm and 20 µm from each other.

[0089] According to a twenty-fifth aspect referring to at least one of the sixteenth to twenty-fourth aspects, the MEMS microphone further comprises a third diaphragm element, wherein the third diaphragm element has a stiffness that is smaller than the stiffness of the first diaphragm element or the stiffness of the second diaphragm element.

[0090] According to a twenty-sixth aspect with reference to the twenty-fifth aspect, the third membrane element is coupled between a support structure and the first membrane element and / or the second membrane element.

[0091] According to a twenty-seventh aspect with reference to the twenty-sixth aspect, the first counter electrode element is supported by the support structure independently of the third membrane element.

[0092] According to a twenty-eighth aspect, a method of manufacturing a MEMS microphone comprises: creating a low-pressure region between a first membrane element and a counter electrode element and permanently preventing matter from entering the low-pressure region to permanently maintain a fixed low pressure on average.

[0093] According to a twenty-ninth aspect referring to the twenty-eighth aspect, creating the low pressure region comprises forming a cavity between the first membrane element and the counter electrode element.

[0094] According to a thirtieth aspect with reference to at least one of the twenty-eighth to twenty-ninth aspects, permanently preventing the entry of matter comprises closing the cavity under a low-pressure atmosphere to maintain a low-pressure region between the first membrane element and the counter electrode element.

[0095] According to a thirty-first aspect referring to at least one of the twenty-eighth to thirtieth aspects, the low pressure region has a pressure of less than 70% of standard atmospheric pressure.

[0096] According to a thirty-second aspect referring to at least one of the twenty-eighth to thirty-first aspects, permanently preventing the entry of matter comprises: coating a surface of the MEMS microphone with a non-conformal deposition of oxide and / or depositing borophosphosilicate glass (BPSG) on the surface of the MEMS microphone and later reflowing under the low-pressure atmosphere and / or laminating a film.

[0097] According to a thirty-third aspect, referring to at least one of the twenty-eighth to thirty-second aspects, the method further comprises, prior to generating the low pressure: depositing a sacrificial material on a layer of the counter electrode material; depositing a membrane material on the sacrificial material; patterning the membrane material to access the sacrificial material, and removing the sacrificial material; wherein generating the low-pressure region comprises extracting air and / or a gas and / or a fluid from a space previously occupied by the sacrificial material prior to its removal.

[0098] The above-described embodiments are merely illustrative of the principles of the present invention. It is understood that modifications and variations of the arrangements and details described herein will be apparent to those skilled in the art. Therefore, the intention is to be limited only by the scope of the appended claims and not by the specific details presented herein in the description and explanation of the embodiments.

[0099] Although each claim refers back to only a single claim, the disclosure covers any conceivable combination of the claims.

Claims

[1] MEMS microphone, which includes: a first membrane element (212); a second membrane element (214) arranged at a distance from the first membrane element (212); a low-pressure region (232) arranged between the first membrane element (212) and the second membrane element (214), the low-pressure region (232) having a pressure that is less than an ambient pressure, and a first counter electrode element (222) arranged within the low-pressure region (232), the first counter electrode element (222) comprising a first and a second section, the first section being arranged in a central region of the MEMS microphone and being electrically insulated and separated from the second section arranged in an adjacent edge region of the MEMS microphone, wherein upon excitation by a sound wave, the first and second membrane elements (212, 214) are subjected to a greater deflection in the central region than in the edge region. [2] The MEMS microphone of claim 1, wherein the pressure in the low pressure region (232) is a vacuum. [3] The MEMS microphone of claim 1, wherein the pressure in the low pressure region (232) is less than 50% of the ambient pressure. [4] MEMS microphone according to one of claims 1 to 3, wherein the low pressure region (232) is within a sealed cavity. [5] The MEMS microphone of any one of claims 1 to 4, further comprising a second counter electrode element (224) electrically isolated from the first counter electrode element (222). [6] The MEMS microphone of any one of claims 1 to 5, further comprising one or more columns (272) coupled between the first diaphragm element (212) and the second diaphragm element (214). [7] The MEMS microphone of claim 6, wherein the one or more columns (272) are electrically insulating. [8] The MEMS microphone of claim 6, wherein the one or more columns (272) are electrically conductive. [9] MEMS microphone according to one of claims 6 to 8, wherein at least two columns (272) are arranged between 5 µm and 20 µm apart. [10] The MEMS microphone of any one of claims 1 to 9, further comprising a third diaphragm element (1716), wherein the third diaphragm element (1716) has a stiffness that is less than the stiffness of the first diaphragm element (212) or the stiffness of the second diaphragm element (214). [11] MEMS microphone according to claim 10, wherein the third membrane element (1716) is coupled between a support structure (1706) and the first membrane element (212) and / or the second membrane element (214). [12] The MEMS microphone of claim 11, wherein the first counter electrode element (222) is supported by the support structure (1706) independently of the third membrane element (1716). [13] MEMS microphone, which includes: a first membrane element (212); a second membrane element (214) arranged at a distance from the first membrane element (212); a low-pressure region (232) arranged between the first membrane element (212) and the second membrane element (214), the low-pressure region (232) having a pressure that is less than an ambient pressure, a first counter electrode element (222) arranged within the low pressure region (232), and a vent hole (515) extending through the first and second membrane elements (212, 214) and through the first counter electrode element (222) and sealed from the low pressure region (232). [14] MEMS microphone, which includes: a first membrane element (212); a second membrane element (214) arranged at a distance from the first membrane element (212); a low-pressure region (232) arranged between the first membrane element (212) and the second membrane element (214), the low-pressure region (232) having a pressure that is less than an ambient pressure, a first counter electrode element (222) arranged within the low-pressure region (232) and supported at the edge thereof by a support structure (1706), and a joint element (1716) coupled between the two membrane elements (212, 214) and the support structure (1706). [15] MEMS microphone, which includes: a first membrane element (212); a second membrane element (214) arranged at a distance from the first membrane element (212); a low-pressure region (232) arranged between the first membrane element (212) and the second membrane element (214), the low-pressure region (232) having a pressure that is less than an ambient pressure, a first and second counter electrode element (222, 224) arranged within the low-pressure region (232), wherein the second counter electrode element (224) is electrically insulated from the first counter electrode element (222), and one or more columns (272) coupled between the first membrane element (212) and the second membrane element (214), wherein the one or more columns (272) are electrically conductive.

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