MEMS microphone assembly and method for fabricating a MEMS microphone assembly

Abstract

A micro-electro-mechanical system, MEMS, microphone assembly comprises an enclosure defining a first cavity, and a MEMS microphone arranged inside the first cavity. The microphone comprises a first die with bonding structures and a MEMS diaphragm, and a second die having an application specific integrated circuit, ASIC. The second die is bonded to the bonding structures such that a gap is formed between a first side of the diaphragm and the second die, with the gap defining a second cavity. The first side of the diaphragm is interfacing with the second cavity and a second side of the diaphragm is interfacing with the environment via an acoustic inlet port of the enclosure. The bonding structures are arranged such that pressure ventilation openings are formed that connect the first cavity and the second cavity.

Claims

1. A micro-electro-mechanical system, MEMS, microphone assembly comprising: an enclosure defining a first cavity, the enclosure comprising an acoustic inlet port that connects the first cavity to an environment of the assembly; and a MEMS microphone arranged inside the first cavity, the microphone comprising a first die with bonding structures and a MEMS diaphragm the diaphragm having a first side and a second side, and a second die having an application specific integrated circuit, ASIC; wherein the second die is bonded to the bonding structures of the first die such that a gap is formed between the first side of the diaphragm and the second die, with the gap defining a second cavity and having a gap height; the first side of the diaphragm is interfacing with the second cavity and the second side of the diaphragm is interfacing with the environment via the acoustic inlet port; and the bonding structures are arranged such that pressure ventilation openings are formed that connect the first cavity and the second cavity.

2. The MEMS microphone assembly according to claim 1, wherein the gap height is larger than 10 μm.

3. The MEMS microphone assembly according to claim 1, wherein the pressure ventilation openings are defined by voids between clamping structures of the diaphragm and the bonding structures in a main extension plane of the diaphragm; or voids of the bonding structures.

4. The MEMS microphone assembly according to claim 1, wherein the second die comprises an opening that connects the first cavity and the second cavity.

5. The MEMS microphone assembly according to claim 1, wherein at least one dimension of the pressure ventilation openings corresponds to the gap height.

6. The MEMS microphone assembly according to claim 1, wherein the MEMS microphone consists of the first die and the second die.

7. The MEMS microphone assembly according to claim 1, further comprising an optical readout assembly having at least a light source and a detector, wherein the optical readout assembly is configured to detect a displacement of a point or a surface of the diaphragm, in particular a point or a surface of the first side of the diaphragm.

8. The MEMS microphone assembly according to claim 1, wherein the enclosure comprises a pressure equalization opening.

9. The MEMS microphone assembly according to claim 1, wherein the diaphragm further comprises a pressure equalization opening.

10. The MEMS microphone assembly according to claim 8, wherein the pressure equalization opening is configured to act as a high-pass filter for longitudinal waves, in particular as a high-pass filter with a cut-off frequency between 20 Hz and 100 Hz.

11. An electronic device, such as a pressure sensing device or a communication device, comprising a MEMS microphone assembly according to claim 1, wherein the MEMS microphone assembly is configured to omnidirectionally detect dynamic pressure changes in the environment, in particular dynamic pressure changes at rates corresponding to audio frequencies.

12. A method of fabricating a micro-electro-mechanical system, MEMS, microphone assembly, the method comprising: providing an enclosure defining a first cavity, the enclosure comprising an acoustic inlet port that connects the first cavity to an environment of the assembly; arranging a first die of a MEMS microphone inside the first cavity, the first die comprising a MEMS diaphragm and bonding structures; and arranging a second die of the MEMS microphone inside the first cavity, the second die comprising an application specific integrated circuit, ASIC; wherein the second die is bonded to the bonding structures such that a gap is formed between the diaphragm and the second die, with the gap defining a second cavity and having a gap height; a first side of the diaphragm is interfacing with the second cavity and a second side of the diaphragm is interfacing with the environment via the acoustic inlet port; and the bonding structures are arranged such that pressure ventilation openings are formed that connect the first cavity and the second cavity.

13. The method according to claim 12, wherein the first die is arranged with respect to the acoustic inlet port such that the first cavity is hermetically sealed from the environment at boundaries of the acoustic inlet port.

14. The method according to claim 12, wherein the gap height is larger than 10 μm, in particular equal to or larger than 50 μm.

15. The method according to claim 12, wherein the pressure ventilation openings are defined by voids between clamping structures of the first die and the bonding structures in a main extension plane of the diaphragm; or voids of the bonding structures.

16. The MEMS microphone assembly according to claim 9, wherein the pressure equalization opening is configured to act as a high-pass filter for longitudinal waves.

17. The MEMS microphone assembly according to claim 1, wherein the MEMS microphone assembly is free of a back plate.

18. The MEMS microphone assembly according to claim 1, wherein the gap height is larger than 50 μm.

19. The MEMS microphone assembly according to claim 8, wherein the pressure equalization opening is configured to act as a high-pass filter with a cut-off frequency between 20 Hz and 100 Hz.

20. The electronic device according to claim 11, wherein the MEMS microphone assembly is configured to omnidirectionally detect dynamic pressure changes at rates corresponding to audio frequencies.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The following description of figures of example embodiments may further illustrate and explain aspects of the improved concept. Components and parts of the microphone assembly with the same structure and the same effect, respectively, appear with equivalent reference symbols. In so far as components and parts of the microphone assembly correspond to one another in terms of their function in different figures, the description thereof is not repeated for each of the following figures.

[0042] FIG. 1 shows an exemplary embodiment of the MEMS microphone of the MEMS microphone assembly according to the improved concept;

[0043] FIG. 2 shows a further exemplary embodiment of the MEMS microphone of the MEMS microphone assembly according to the improved concept;

[0044] FIG. 3 shows an exemplary embodiment of the MEMS microphone assembly according to the improved concept;

[0045] FIG. 4 shows a further exemplary embodiment of the MEMS microphone assembly according to the improved concept;

[0046] FIG. 5 shows a further exemplary embodiment of the MEMS microphone assembly according to the improved concept;

[0047] FIG. 6 shows a further exemplary embodiment of the MEMS microphone assembly according to the improved concept; and

[0048] FIG. 7 shows acoustic noise characteristics of the embodiment of the MEMS microphone assembly shown in FIG. 5.

DETAILED DESCRIPTION

[0049] FIG. 1 shows an exemplary embodiment of the MEMS microphone 20 of the MEMS microphone assembly 1 according to the improved concept. In particular, FIG. 1 shows the microphone 20 in a top view in the center and two cross section views at the virtual cuts x and y on the top and on the bottom, respectively.

[0050] The MEMS microphone 20 comprises a first die 21 that is bonded via an annular bonding structure 23 on the first die 21 to a second die 22. Besides the bonding structure 23 the first die 21 comprises a MEMS diaphragm 24, in this example of circular shape, which is suspended and clamped to an annular clamping structure 27. A typical diameter for a diaphragm configured to be sensitive to sound waves is in the order of 0.5 mm to 1.5 mm. The clamping structure 27 is at certain points connected to the bonding structure 23 via bridges 29, in this example via four bridges 29 that are evenly arranged around the perimeter of the clamping structure 27, such that pressure ventilation openings 30 are defined by voids formed by the bridges 29, the clamping structure 27 and the bonding structure 23. In this embodiment, the pressure ventilation openings 30 are thus located in the main extension plane of the diaphragm 24 and connect the second cavity 31 to the first cavity 11 defined by the enclosure 10, which is not shown in this figure. The MEMS diaphragm 24 may be made of silicon nitride and the clamping structure 27, the bonding structure 23 and the bridges 29 may be made of the same material, for example silicon, or of different materials.

[0051] The first die 21 is bonded to the second die 22 via standard wafer bonding techniques, which may be of an adhesive or an eutectic type, for instance. The second die 22 comprises besides an application-specific integrated circuit, ASIC, bonding pads, for example, that optionally correspond to the bonding structure 23 of the first die 21 with respect to size, shape and position. The bonding is performed such that a gap 28 is formed between a first side 25 of the diaphragm 24 and a top surface 33 of the second die 22, wherein the gap defines the second cavity 31. The gap height is larger than 10 μm, in particular equal to or larger than 50 μm. A width of the pressure ventilation openings 30 typically is of similar dimension.

[0052] The ASIC on the second die 22 is configured to measure a movement of the diaphragm 24, for example a periodical deflection due to an oscillation of the diaphragm 24. If the microphone is an optical microphone, the ASIC may for example comprise a coherent light source such as a laser that is configured to illuminate a point or a surface on the first side 25 of the diaphragm 24. The ASIC may further comprise a detector that is configured to detect light from the light source that is reflected from the point or the surface on the first side 25 of the diaphragm 24 and to generate an electrical signal based on the detected light. The detector may be a segmented photodiode, for instance. The ASIC may further comprise a processing unit that is configured to map the electric signal to a deflection signal and to output the signal to an output port. Alternatively, the ASIC may be configured to output the electric signal to an external processing unit via an output port.

[0053] FIG. 2 shows a further exemplary embodiment of the MEMS microphone 20 of the MEMS microphone assembly 1 according to the improved concept. The embodiment is based on that shown in FIG. 1. Similarly, FIG. 2 shows the microphone 20 in a top view in the center and two cross section views at the virtual cuts x and y on the top and on the bottom, respectively.

[0054] In contrast to the embodiment shown in FIG. 1, here the bonding structures 23 are arranged in between the clamping structure 27 of the diaphragm 24 and the top surface 33 of the second die 22. In this example, the bonding structures 23 are defined solely by bridges evenly arranged around the perimeter of the diaphragm 24. This way, the pressure ventilation openings 30 are defined after bonding of the first die 21 and the second die 22. In particular, voids of the bonding structures 23 around the perimeter of the diaphragm 24 define the pressure ventilation openings to be arranged in between the clamping structure 27 and the top surface of the second die 22 and corresponding with respect to their height to the gap height, which likewise is larger than 10 μm, in particular equal to or larger than 50 μm.

[0055] In addition, in this embodiment the second die 22 further comprises an optional ventilation hole 32 that, like the pressure ventilation openings 30 connect the second cavity 31 to the first cavity 11 defined by the enclosure 10 not shown.

[0056] FIG. 3 shows an exemplary MEMS microphone assembly 1 according to the improved concept. The assembly comprises an enclosure 10 that defines a first cavity 11 as its enclosed volume. The enclosure 10 comprises sidewalls 15 and a PCB board 14 that has an opening as an acoustic inlet port 12 for incoming pressure waves such as sound waves, making this microphone assembly 1 a bottom port microphone assembly. The enclosure in this embodiment further comprises a pressure equalization opening 13 connecting the first cavity 11 to the environment 2, for example an environment 2 of a gas such as air, for ensuring an equal pressure of the environment 2 and the first cavity 11. With this equalization opening 13, changes in the static pressure of the environment 2 propagate into the microphone assembly allowing for an invariable sensitivity for dynamic pressure changes, such as sound waves.

[0057] The dimension of the equalization opening 13 is in the order of 1 μm to 10 μm, therefore acting as a high pass filter for the microphone assembly 1 with a cut-off frequency of typically 20-100 Hz for acoustic microphone configurations. The upper cut-off frequency of the microphone assembly is typically defined my mechanical resonances of the MEMS diaphragm 24 and is typically around 20 kHz.

[0058] The enclosure 10 may be formed by a third die comprising the PCB board 14 and the sidewalls 15 but may alternatively be formed by a generic housing, for example of a metal or a polymer. The PCB board 14 may comprise electrical contacts t output a microphone signal to an external processing unit such as a microprocessor of an electronic device.

[0059] Inside the enclosure 10, i.e. inside the first cavity 11, a MEMS microphone 20, for example according to one of the embodiments described above, is arranged with respect to the acoustic inlet port 12 such that the first cavity 11 is hermetically sealed from the environment 2 at boundaries of the acoustic inlet port 12. For example, the clamping structures 27 are mounted to the PCB board 14 such that the MEMS diaphragm 24 of the microphone 20 is flush-mounted with the acoustic inlet port 12. This way, the microphone assembly 1 becomes omnidirectional, i.e. sensitive to sound waves entering the acoustic inlet port 12 at different incident angles as incident pressure waves can only impinge on the second side 26 of the diaphragm 24 and not enter the first cavity 11 or the second cavity 31 and destructively influence deflection or motion of the diaphragm 24 via its first side 25.

[0060] The diaphragm 24, the clamping structures 27, the bonding structures 23 and the second die 22 with the ASIC for detection of a deflection of the diaphragm 24 define the second cavity 31 via the gap 28. Pressure ventilation openings 30 connect the first cavity 11 and the second cavity 22, significantly increasing the back volume of the MEMS microphone 20. This increase ensures a reduced acoustic impedance that destructively influences the motion of the diaphragm 24 and thus reduces the signal-to-noise ratio of the detected sound waves. The increase is due to the fact that an increased air pressure due to compression is distributed via the pressure ventilation openings 30 across the entire volume of the microphone assembly 1 defined by the first cavity 11 and the second cavity 31. The arrows inside the microphone assembly 1 represent an air pressure flow in case of a motion of the diaphragm 24 towards the second die 22.

[0061] For readout, an output port of the ASIC on the second die 22 may be electrically connected to contacts on the side of the PCB board 14 facing the environment 2, for example via feedthroughs.

[0062] The combination of the large gap 28, the large back volume due to the pressure ventilation openings 30 and the pressure equalization opening 13 enable a low noise due to acoustic impedance, i.e. a high sensitivity of the microphone assembly for sound pressures in the order of 200 μPa, which is only one order of magnitude above the human hearing threshold and corresponds to a sound pressure level, SPL, of 19 dB.

[0063] FIG. 4 shows a further exemplary MEMS microphone assembly 1 according to the improved concept. In comparison to FIG. 3, this embodiment is characterized by an alternative position of the pressure equalization opening 13 in the middle of the diaphragm 24. Although the fundamental vibrational mode, i.e. the trampoline mode, of the diaphragm 24 has its maximum deflection at this point and a measurement would therefore yield the highest signal-to-noise ratio, in general higher order modes of the diaphragm are of higher relevance as these lie in the band of interest with respect to their frequencies. The optimum measurement points, i.e. the antinodes of these higher order modes, are not necessarily in the center of the diaphragm 24.

[0064] In addition, the embodiment shown in addition to the pressure ventilation openings 30 comprises an optional ventilation hole 32 in the second die 22 serving as additional connection between the first cavity 11 and the second cavity 31, which potentially further decreases the acoustic impedance. Again, the arrows inside the microphone assembly 1 represent an air pressure flow in case of a motion of the diaphragm 24 towards the second die 22.

[0065] FIG. 5 shows a further exemplary MEMS microphone assembly 1 according to the improved concept. This embodiment comprises a microphone 20 according to the embodiment shown in FIG. 2. In particular, the pressure ventilation openings are here arranged between the clamping structures 27 and the second die 22 and correspond in height to the gap height of the gap 28. Compared to the embodiments shown in FIGS. 3 and 4, this embodiment is characterized by an even lower noise level, i.e. a higher sensitivity, capable to operate at a sound pressure level approximately 0.5 dB lower at 18.5 dB.

[0066] Similar to the embodiment shown in FIG. 4, the embodiment in FIG. 6 features the optional ventilation hole 32 as well as the pressure equalization opening 13 located in the diaphragm 24.

[0067] FIG. 7 shows simulated acoustic noise of the microphone assembly 1 shown in FIG. 5 in dependence of the gap height of the gap 28. The different traces t1-t3 show different noise contributions, while traces t4 and t5 show the effective total noise.

[0068] In particular, t3 shows the acoustic noise due to compression, or squeezing, of air in the second cavity 31 due to a deflection of the diaphragm. Traces t1 and t2 represent acoustic noise due to a present opening 32 in the second die 22 with and without the pressure ventilation openings 30, respectively. Traces t4 and t5 constitute the total acoustic noise of embodiments of the microphone assembly 1 without and with opening 32 in the second die 22, respectively.

[0069] Particularly for gap heights of 50 μm or larger, the opening 32 only has an insignificant contribution to the total noise level and is therefore obsolete leaving space for additional components of the ASIC, for example. The noise level of this particular embodiment is found to be 174 μPa, indicating that the minimum detectable sound pressure level for a gap height of 50 μm is 18.8 dB for this particular exemplary embodiment.

[0070] The embodiments shown in the FIGS. 1 to 6 as stated represent exemplary embodiments of the microphone 20 and the microphone assembly 1, therefore they do not constitute a complete list of all embodiments according to the improved concept. Actual microphone and microphone assembly configurations may vary from the embodiments shown in terms of shape, size and materials, for example. For instance, the microphone assembly 1 may be configured to be a front port microphone assembly, which may be beneficial for some applications.

[0071] A MEMS microphone assembly according to one of the embodiments shown may be conveniently employed in various applications that require a compact high sensitivity sensor for detecting small dynamic pressure changes, particularly in the audio band for the detection of sound waves. Possible applications include an employment as an acoustic microphone in computing devices such as laptops, notebooks and tablet computers, but also in portable communication devices like smartphones and smart watches, in which space for additional components is extremely limited.