EP4659460A1

EP4659460A1 - Optical mems microphone and method of manufacturing an optical mems microphone

Info

Publication number: EP4659460A1
Application number: EP24702515.8A
Authority: EP
Inventors: Matthias Schmidt; Christian Siegel; Pirmin Hermann Otto Rombach; Anton Leidl
Original assignee: TDK Corp
Current assignee: InvenSense Inc
Priority date: 2023-01-31
Filing date: 2024-01-26
Publication date: 2025-12-10
Also published as: WO2024160666A1

Abstract

An optical MEMS microphone and a corresponding method of manufacturing are provided. The optical MEMS microphone provides good electrical and optical performance. To that end, the microphone comprises a stack and a MEMS chip. The MEMS chip comprises a membrane, and the stack comprises a light source, a light sensor and the membrane of the MEMS chip.

Description

Optical MEMS microphone and method of manufacturing an optical MEMS microphone

The present invention refers to an optical MEMS microphone and to a method of manufacturing an optical MEMS microphone .

MEMS microphones (MEMS = microelectromechanical system) utili ze micromechanically structured elements such as flexible membranes in order to convert acoustic signals into electrical signals . In contrast to conventional MEMS microphones , where the electrical capacitance change of a capacitor comprising a deflected membrane and a rigid backplane as electrodes is determined, in an optical MEMS microphone the signal is obtained by evaluating the movement of the membrane via optical means . It is thus possible to evaluate the deflection of the membrane in order to obtain a corresponding electrical signal by utili zing optical interferences of a primary beam and a reflected beam . Thus , a light source emits a primary beam to a light sensor and a further beam to the membrane , where the membrane reflects the light of the light source to the light sensor and an interference pattern at the light sensor allows determining the deflection of the membrane , where the deflection of the membrane represents the received acoustic signal .

It is preferred that the light source emits coherent light . Thus , the use of a laser, e . g . a laser diode , is preferred .

Optical MEMS microphones are known from US 9 , 510 , 074 B2 showing microphones where a sound port and the optical system of the microphone are on the same side of the corresponding membrane . Further, from US 7 , 826 , 629 B2 optical MEMS microphones are known where the sound port and the optical system comprising the light source and the light sensor are arranged at opposite sides of the membrane , respectively . It is preferred that a MEMS microphone has a good signal to noise ratio ( SNR) , can be manufactured at low manufacturing costs , needs only a simple signal routing that provides less cross-talk and has a good RE performance , allows the manufacturing of a robust package where a lid deformation during the manufacturing or mounting process does not signi ficantly af fect performance . Further, a large back volume and a small front volume is preferred . Further, it is preferred that a good electric coupling between the optical system comprising the light source and the light sensor and a corresponding evaluation circuit , e . g . an AS IC (AS IC = application speci fic integrated circuit ) is possible .

Further, it has been found that the parallelism between the orientation of the MEMS structure such as the membrane and the optical system is important for a good signal to noise ratio , and an optimal distance between the mechanical system ( the membrane ) and the optical system can be obtained without further restrictions to the overall geometric dimensions of the microphone . Further, it is beneficial that an appropriate means for a barometric balance between the environment of the microphone and the back volume allows for a well-defined frequency response of the microphone . Further, it is preferred that the microphone provides a robustness of the signal quality with respect to temperature changes or mechanical stress . Further, it is preferred that a sound port has a large diameter .

While known optical MEMS microphones comply with some of these requirements , it would be desirable to have an optical MEMS microphone that is compatible with all these requirements .

To that end, an optical MEMS microphone according to the independent claim is provided . Dependent claims provide preferred embodiments and methods for manufacturing such microphones .

The optical MEMS microphone comprises an optical unit and a MEMS chip . The MEMS chip comprises a membrane . The membrane is provided and configured to respond to acoustic signals , e . g . received from an environment of the microphone . The optical unit comprises a light source , a light sensor, and the membrane of the MEMS chip . The light source , the light sensor and the membrane are stacked one above another to form a stack .

The provision of the stack, i . e . , the provision of the light source , the light sensor and the membrane in a stacked configuration where the three components are arranged in a vertical direction one over another allows for the mentioned advantages , especially for a good parallelism of light rays originating at the light source and being received at the light sensor .

It was found that in conventional microphones where one of these three elements is solidly mounted to a circuit board or a carrier substrate , mechanical stresses induced during the operation of the microphone can disturb the optical system and reduce the quality of the interference pattern from which the electrical signals are determined . Thus , mechanical stress or a misalignment during manufacture can cause a signi ficant reduction in signal quality when the acoustic signal is converted into an electrical signal .

It is preferred that the stacking of the light source , the light sensor and the membrane with respect to the orientation of these three elements relative to one another maintains a good optical alignment even when the microphone as a whole is under mechanical stress , e . g . caused by temperature changes or vibration .

Speci fically, it is possible that there is a sti f f mechanical connection between the light source and the light sensor and/or between the light source and the membrane and/or between the light sensor and the membrane .

Speci fically, it is possible that the light source and the light sensor are monolithically integrated into a single chip which may also contain the electric circuit elements of the AS IC .

Such a configuration minimi zes the movement between the membrane and the emitting and reception sides of the optical system due to vibration, thermomechanical or mechanical stress .

Also , the described configuration complies with the microphone being produced as a bottom port microphone , where the acoustic port of the microphone and the electric ports of the microphone are arranged at the same side of the microphone , e . g . at the bottom side of the microphone .

Further, the described configuration complies with a bottom- up assembly process , where the assembly of the MEMS structures can be performed in a simple manner and therefore at reduced manufacturing costs .

Further, the described configuration allows for a large sound port si ze that is beneficial when a water proofing membrane is arranged at the sound port to allow for water proofness of the optical MEMS microphone .

It is possible that the microphone further comprises a carrier substrate . The MEMS Chip is mounted on the carrier substrate by means of an adhesive and elastic material .

The adhesive and elastic material helps mechanically connecting the MEMS chip to the carrier substrate while - due to the soft nature of the elastic material - mechanically decouples the MEMS chip and its sensitive structures , e . g . the membrane , from tension and associated deformation eventually occurring in the carrier substrate . Further, the adhesive and elastic material helps separating the front from the back volume .

Correspondingly, it is possible that the adhesive and elastic material is provided and configured to attenuate the propagation of mechanical impacts from an environment of the microphone to the MEMS Chip or from the carrier to the MEMS Chip .

It is possible that the light source has an emitting direction towards the membrane . Further, the membrane has a normal direction . Further, the light sensor has a preferred reception direction, and the angle between the emitting direction and the normal direction of the membrane is 2 ° or less . Further, it is possible that the angle between the emitting direction and the preferred reception direction is

2 ° or less . Further, it is possible that the angle between the normal direction and the preferred reception direction is 2 ° or less .

In this respect , a normal direction is a direction that is orthogonal to a surface of the corresponding component . Thus , the normal direction of the membrane has an orthogonal direction with respect to the surface of the membrane . A normal direction of the light sensor is a direction orthogonal to the surface of the receiving side of the light sensor .

The provision of the light source , the light sensor and the membrane in the described stacked configuration allows for maintaining the 2 ° requirement of the directions , signi ficantly reducing a misalignment of the optical beams and therefore signi ficantly improving the signal quality of the optical MEMS microphone .

It is possible that the light source is selected from a light emitting diode , a laser diode , a vertically cavity surface emitting laser (VCSEL ) . Further, the light sensor can be a photodiode , where the photodiode has a section with a planar surface and the preferred reception direction is the normal direction of the planar surface section .

Thus , conventional light sources and conventional light sensors can be used for the optical MEMS microphones while obtaining the improved signal quality caused by the stacked arrangement of the light source , the light sensor and the membrane . It is possible that the optical MEMS microphone further comprises the carrier substrate . The stack is mechanically coupled to the carrier substrate . In the stack the light source is mechanically coupled to the light sensor . The light sensor is mechanically coupled to the membrane and the membrane is mechanically coupled to the light source .

Further, it is possible that the coupling between the light source and the light sensor and/or the coupling between the light sensor and the membrane and/or the coupling between the membrane and the light source is stronger than a mechanical coupling between the carrier substrate and the stack .

In this respect , the mechanical coupling represents the sti f fness , i . e . , the resistance against a deformation caused by an intrinsic or externally applied stress . Thus , the microphone is resilient with respect to thermally induced stress or vibration .

The enhanced sti f fness within the stack can be obtained by utili zing connection means within the stack, within the stacked elements , and/or by utili zing soft connection means between the stack and the carrier substrate such that a stress induced in the carrier substrate is not induced in the stack and does not disturb the optical alignment of the stacked element .

It is possible that the mechanical coupling between the membrane and the light source or the mechanical coupling between the membrane and the light sensor comprises a spacer structure . The spacer structure can comprise an open table structure , a closed frame structure , one or more vertical bars , one or more hori zontal bars , a V-shaped structure and/or a U-shaped structure .

It is preferred that the spacer structure comprises at least one material that has larger sti f fness parameters such as Young' s modulus than a connection means between the stack and the carrier substrate . The spacer structure enhances the sti f fness of the stack and allows for a precisely defined distance between the optical elements of the stack and maintains the optical arrangement and the optical directions of the corresponding primary and reflected beams . Further, the coupling structure is provided such that the back side of the membrane is acoustically coupled to the back volume of the microphone , which may be arranged in a cavity encapsulated by a lid and the carrier substrate . However, it is also possible that the spacer structure allows for a pressure balance between the back volume of the microphone and the environment of the microphone .

It is possible that the mechanical coupling between the carrier and the stack comprises a seal ring or a protective ring preventing an adhesive material from contaminating the membrane during manufacturing .

The adhesive material can be used to mechanically connect the stack to the carrier, and it is preferred that the adhesive material has smaller sti f fness parameters than connecting means between the elements of the stack .

Further, the adhesive material can be used to seal the front from the back volume . It is possible that the optical MEMS microphone further comprises an application speci fic integrated circuit (AS IC ) that converts the output signal of the light sensor into an electrical signal provided at an output port of the microphone , providing an electrical signal representing the received sound signal .

The AS IC circuit components can be integrated in a chip that also contains circuit elements or optical elements of the sensor and/or circuit elements or optical elements of the light source .

It is possible that the light source , the light sensor and/or an AS IC or circuit components of the AS IC are electrically coupled via bump connections , via bond wires , via metalli zed signal structures and/or via vias such as through silicon vias ( TSVs ) .

In particular, connection means for establishing electrical connections between di f ferent components of a MEMS microphone can be utili zed, and the described stack configuration is essentially compliant with each of the connection means such that the most appropriate connection means can be selected in order to obtain an improved signal quality . Thus , the stacking of the elements as described above does not j eopardi ze the single quality because speci fic connection means are required . The stacked configuration provides a maximum compliance with respect to the connection means such that the connection means can be selected by their electrical properties without sacri ficing signal quality due to package requirements . It is possible that the optical MEMS microphone further comprises a sound port , a lid, a cavity, a back volume and a venting channel . The membrane is acoustically coupled to the sound port . The stack is arranged in the cavity . The lid covers the cavity . The back volume is acoustically decoupled from the sound port , and the venting channel allows for a barometric balance between the back volume and the environment of the microphone .

Thus , the provision of the stack as described above complies with conventional requirements for a MEMS microphone in order to provide a good signal quality : Conventional mechanical means for obtaining a good signal quality can be applied to the optical MEMS microphone as described herein .

It is possible that the optical MEMS microphone is a bottom port microphone with a bottom side . The sound port and/or the electrical port of the microphone are arranged at the bottom side of the microphone .

The provision of a bottom port microphone is preferred because many external circuit environments are optimi zed for bottom port microphones . Thus , the optical MEMS microphone as described above can be used in conventional external circuit environments without special modi fications of the external circuit environment .

It is possible that the optical MEMS microphone further comprises an optical lens . The optical lens can be arranged between the light source and the membrane . The optical lens can also be arranged between the membrane and the light sensor . The optical lens can also be arranged between the light source and the light sensor . Further, the use of one or two additional optical lenses between corresponding components of the optical system is also possible .

It is possible that the stack is vertically oriented with respect to a bottom side of the microphone . Further, the light source can be vertically arranged between the light sensor and the membrane , or the light sensor can be vertically arranged between the light source and the membrane .

Thus , a maximum flexibility in arranging the optical components is maintained .

A method of manufacturing an optical MEMS microphone comprises the steps of :

- Providing a MEMS chip with a membrane , a light source and a light sensor,

- stacking the membrane , the light source and the light sensor utili zing a bottom-up assembly process .

Thus , the provided MEMS microphone complies with conventional and preferred manufacturing processes , keeping manufacturing costs low while providing good electrical , optical and acoustical properties .

Further, it is possible that the method comprises the step of establishing one or more mechanical couplings between one stack element and another stack element , where each of the two or three stack elements are selected from the light source , the light sensor and the membrane . It is preferred that the mechanical coupling is established via a flip-chip bonding .

Further, it is possible that the method of manufacturing comprises the step of establishing a spacer structure utili zing a process selected from micro molding, establishing a liquid crystal polymer, LCP, structure , structuring an organic laminate , etching, and laser ablation .

Thus , conventional methods for establishing structures comply with the manufacturing of the described optical MEMS microphone .

It is preferred that a vertical distance within the stack between the light source and the membrane or between the membrane and the light sensor or between the light source and the light sensor is 180 pm or more and 220 pm or less , e . g . 200 pm .

Working principles and details of preferred embodiments are shown in the accompanying schematic figures .

In the figures :

Figure 1 shows the stacking configuration of the three stacked elements of the optomechanical system .

Figure 2 illustrates an arrangement of the stack within a cavity of the microphone .

Figure 3 illustrates the use of flip-chip bonding .

Figure 4 illustrates the use of bond wires . Figure 5 illustrates the use of bump connections , structured metalli zations and wires .

Figure 6 illustrates the use of a spacer structure with spacer elements .

Figure 7 illustrates the use of an optical lens .

Figure 8 illustrates the use of a recess in the carrier substrate in order to reduce the overall height of the microphone .

Figure 9 illustrates a way of obtaining an acoustic channel from the back side of the membrane to the cavity under the lid .

Figures 10 and 11 show perspective views of possible spacer structures for establishing a rigid mechanical connection between elements of the stack .

Figure 1 illustrates the stack S of an optical MEMS microphone OMM, where the stack comprises a light sensor LSEN, a light source LSRC and a membrane M . The membrane is a part of the MEMS chip MC and is structured utili zing conventional structuring methods for creating MEMS structures . The membrane M is part of the MEMS chip MC, the light source and the light sensor establish a mechanical optical system where interference between a primary beam and a reflected beam is utili zed to evaluate the deflection of the membrane in order to convert an acoustic signal into an electric signal . To that end, the membrane M deflects according to a received acoustic signal , and the deflection of the membrane causes a varying optical interference pattern at the light sensor that is evaluated, e . g . by an AS IC .

The improved signal quality of the provided microphone is obtained via a strict optical alignment of the primary and reflected beams such that tilting angles of the optical beams should be as small as possible .

To that end, the light sensor, the light source and the membrane are stacked in a direction denoted as the vertical direction y .

Figure 2 illustrates the arrangement of the stack S within a cavity CAV . The stack S is arranged above a sound port SP that is established as a sound entry hole within a carrier substrate CS . A lid L covers the stack S in the cavity CAV such that the cavity CAV establishes the acoustic back volume of the microphone , speci fically when an acoustic seal AS ( compare Figure 3 ) is provided to acoustically decouple the cavity CAV from the environment of the microphone .

Correspondingly, Figure 3 illustrates the use of the acoustic seal AS to seal the cavity CAV under the lid L from the environment of the microphone such that the additional volume of the cavity CAV can be used as a back volume of the microphone behind the membrane to enhance the acoustic properties of the microphone . Thus , the volume behind the membrane of the cavity CAV establishes the back volume BV of the microphone .

Further, Figure 3 illustrates the use of bump connections BC for electrical and mechanical connection of circuit elements and mechanical components of the microphone structure . Speci fically, the light source LSRC is connected via bump connections to a further carrier chip CC that may contain circuit components of an AS IC and/or circuit components of the light sensor . The carrier chip CC can also be electrically and mechanically connected via bump connections BC to further circuitry of the microphone and to an external circuit environment into which the microphone is integrated .

It is preferred that the mechanical connection between the MEMS chip MC and the carrier substrate CS , e . g . the acoustic seal AS , has lower sti f fness parameters than the mechanical connection between the elements of the stack .

Figure 4 illustrates the use of bond wires BW to electrically connect circuit elements of the stack to an external circuit environment or to electrically connect the AS IC to an external circuit environment which may be connected via connection pads at the underside of the carrier substrate CS . Further, via connections V through the carrier chip CC are possible to provide bump connections to which the bond wire BW can be connected at the carrier chip side .

Figure 5 illustrates the possibility of structured metalli zations SM to electrically connect di f ferent connection points on the surface of a chip, e . g . the carrier chip CC . However, the use of such structured metalli zations SM on the surface of the carrier substrate CS is also possible .

Further, Figure 5 illustrates the use of a seal ring SR that prevents material of the acoustic seal AS coming into contact with the membrane M during the manufacturing process . Further, Figure 5 illustrates the combination of various electrical connections like bump connections BC, bonds wires BW and structed metalli zations SM to connect the di f ferent chips with the substrate .

Further, Figure 6 illustrates the possibility of providing a spacer structure SPS to mechanically and/or electrically connect the carrier chip CC with the MEMS chip MC .

The spacer structure SPS can be an ef fective means to rigidly couple elements of the stack, speci fically the light source and the light sensor, at one side to the membrane at the respective other side of the spacer structure SPS in order to limit the maximum deflection of the optical beams .

Figure 7 illustrates the possibility of utili zing an optical lens OL to enhance beam alignment and to further improve the optical performance of the microphone . It is preferred that the optical lens OL is also solidly connected to the stack via a rigid coupling RC .

Figure 8 illustrates a possibility of reducing the overall height of the microphone by providing a recess REC within the carrier substrate CS such that at least some of the parts of the stack can be arranged at a lower vertical position while maintaining a preferred minimum distance between components of the optical system .

In this configuration the carrier chip ( optionally containing the AS IC ) and the light source are bridging the MEMS chip and are not directly mounted on top of one another . The MEMS chip can be either mounted with down- facing membrane or up- facing membrane .

Figure 9 illustrates a top view onto the arrangement of the carrier chip CC and the spacer structure SPS relative to one another . Speci fically, the carrier chip CC and the spacer structure SPS are shi fted in a hori zontal direction x such that a gap G is obtained through which the back side of the membrane is acoustically coupled to the cavity within the housing of the microphone , such that a large back volume for improved acoustic performance is obtained .

Figures 10 and 11 show perspective views of possible spacer structures SPS , where Figure 10 shows an "open table" structure where a frame establishes a table plate with a recess/opening where the frame is supported by four columns establishing spacer elements SE such that a good coupling of the back side of the membrane to the cavity within the housing is provided . Similarly, Figure 11 shows a more rigid version of the spacer structure SPS that is established via four vertical walls as spacer elements for supporting the carrier chip CC .

I f one of the four walls of the spacer structure SPS shown in Figure 11 is omitted, a U-shaped spacer structure is obtained . I f two of the vertical walls of the spacer structure SPS shown in Figure 11 are omitted, a V-shaped spacer structure is obtained .

The optical MEMS microphone and method for manufacturing such a microphone are not limited by the described features or by the shown figures . The microphone can comprise further circuit components such as signal routing connections , mechanical support structures , optical guiding means or connection aids for electrically connecting or mounting the microphone to an external circuit environment .

List of reference signs

AS acoustic seal

AS IC application speci fic integrated circuit

BV back volume

BW bond wire

BC bump connection

CC carrier chip

CS carrier substrate

CAV cavity

G gap x, z hori zontal directions

L lid

LSEN light sensor

LSRC light source

M membrane

MC MEMS chip

OL optical lens

OMM optical MEMS microphone

OU optical unit

REC recess

RC rigid connection

SE spacer element as part of the spacer structure

SR seal ring

SP sound port

SPS spacer structure

S stack

SM structured metalli zation y vertical direction

V via

Claims

1 . Optical MEMS microphone comprising

- an optical unit and a MEMS chip, wherein

- the MEMS chip comprises a membrane provided and configured to respond to acoustic signals ,

- the optical unit comprises

• a light source ,

• a light sensor, and

• the membrane ,

- the light source , the light sensor and the membrane are stacked one above another to form a stack .

2 . Optical MEMS microphone of the previous claim, further comprising a carrier substrate , wherein the MEMS Chip is mounted on the carrier substrate by means of an adhesive and elastic material .

3 . Optical MEMS microphone of the previous claim, where the adhesive and elastic material is provided and configured to attenuate the propagation of mechanical impacts from an environment of the microphone to the MEMS Chip or from the carrier to the MEMS Chip .

4 . Optical MEMS microphone of the previous claim, wherein

- the light source has an emitting direction towards the membrane ,

- the membrane has a normal direction,

- the light sensor has a preferred reception direction, and

• the angle between the emitting direction and the normal direction of the membrane is 2 ° or less and/or • the angle between the emitting direction and the preferred reception direction is 2 ° or less and/or

• the angle between the normal direction and the preferred reception direction is 2 ° or less .

5 . Optical MEMS microphone of one of the previous claims , wherein

- the light source is selected from a light emitting diode , a laser diode and a vertical cavity surface emitting laser, and

- the light sensor is a photodiode , wherein the photodiode has a section with a planar surface and the preferred reception direction is the normal direction of the planar surface section .

6 . Optical MEMS microphone of one of the previous claims , further comprising a carrier substrate , wherein

- the stack is mechanically coupled to the carrier substrate ,

- in the stack the light source is mechanically coupled to the light sensor, the light sensor is mechanically coupled to the membrane and the membrane is mechanically coupled to the light source .

7 . Optical MEMS microphone of the previous claim, wherein the mechanical coupling between the light source and the light sensor and/or the mechanical coupling between the light sensor and the membrane and/or the mechanical coupling between the membrane and the light source is stronger than the mechanical coupling between the carrier substrate and the stack .

8 . Optical MEMS microphone of the previous claim, wherein the mechanical coupling between the membrane and the light source or the mechanical coupling between the membrane and the light sensor comprises a spacer structure comprising an open table structure, a closed frame structure, a vertical bar, a horizontal bar, a V-shaped structure and/or a U-shaped structure .

9. Optical MEMS microphone of the two previous claims, wherein the mechanical coupling between the carrier and the stack comprises a seal ring or a protective ring preventing an adhesive material from contaminating the membrane during the manufacturing process.

10. Optical MEMS microphone of one of the previous claims, further comprising an ASIC.

11. Optical MEMS microphone of one of the previous claims, wherein the light source, the light sensor and/or the ASIC are electrically coupled via bump connections, bond wires, metallized signal structures and/or vias.

12. Optical MEMS microphone of one of the previous claims, further comprising a sound port, a lid, a cavity, a back volume and a venting channel, wherein

- the membrane is acoustically coupled to the sound port,

- the stack is arranged in the cavity,

- the lid covers the cavity,

- the back volume is acoustically decoupled from the sound port, and

- the venting channel allows for barometric balance between the back volume and the environment of the microphone.

13. Optical MEMS microphone of one of the previous claims, wherein the optical MEMS microphone is a bottom port microphone with a bottom side , and

- a sound port and/or an electrical port of the microphone is arranged at the bottom side of the microphone .

14 . Optical MEMS microphone of one of the previous claims , further comprising an optical lens arranged between the light source and the membrane , between the membrane and the light sensor and/or between the light source and the light sensor .

15 . Optical MEMS microphone of one of the previous claims , wherein

- the stack is vertically oriented with respect to a bottom side of the microphone , and

- the light source is vertically arranged between the light sensor and the membrane , or

- the light sensor is vertically arranged between the light source and the membrane .

16 . Method of manufacturing an optical MEMS microphone of one of the previous claims , comprising the steps of

- providing a MEMS chip with a membrane , a light source and a light sensor,

17 . Method of the previous claim, further comprising the step of

- establishing one or more mechanical couplings between one stack element selected from the light source , the light sensor and the membrane and another stack element selected from the light source , the light sensor and the membrane via flip chip bonding .

18. Method of one of the two previous claims, further comprising the step of

- establishing a spacer structure utilizing a process selected from micro molding, establishing a liquid crystal polymer, LCP, structure, structuring an organic laminate, etching, laser ablation.