Patent ID: 12231847

FIG.1Ashows an optical microphone assembly2in accordance with a first embodiment of the present invention. The optical microphone assembly comprises a number of internal components including a MEMS component4and a semiconductor chip6. The MEMS component4comprises a membrane8and an optical element10provided on an optical element support12. In this example, the optical element is a diffraction grating, but other optical elements may be used, e.g. diffractive lenses or plane reflecting surfaces.

The semiconductor chip6comprises a photo detector14and has mounted thereon a light source16, which in this example embodiment is a vertical-cavity surface-emitting laser (VCSEL). The light source16could instead be integrated into the semiconductor chip6. Connections18are provided between the VCSEL16and the semiconductor chip6to allow the VCSEL to be powered and controlled. Connections20are provided to allow the semiconductor chip to be powered and/or controlled by a remote power source and/or controller. While in this example embodiment, there is only a single photo detector, typically more than one photo detector may be provided, e.g. between 3 and 6 photo detectors. Multiple photo detectors may be provided, for example, to allow different diffraction patterns to be measured, or to allow more than one order of a diffraction pattern to be measured.

The optical microphone assembly2has a housing which encapsulates the aforementioned internal components and comprises a substrate24by means of which the optical microphone assembly is mounted on a microphone support, a superstrate26, and a spacer28, separating the substrate24from the superstrate26. The substrate24, superstrate26and spacer28are non-MEMS components and together define an acoustic cavity30. The superstrate26has an aperture32formed therein, so that a first side of the membrane8is in fluid communication with the exterior34of the optical microphone assembly via the aperture32. Air channels36are provided in the optical element support12, so that a second side of the membrane8is in fluid communication with the acoustic cavity30.

The substrate is provided with solder pads44to allow the optical microphone assembly to be mounted on a microphone support such as a PCB as mentioned above, and also to provide electrical connections between the optical microphone assembly and the PCB. The arrangement shown inFIG.1A, wherein the aperture32is provided in a surface of the housing distal to the substrate24by which it is mounted to a microphone support, may be referred to as a “top port” configuration.

In use, acoustic waves propagating through the air at the exterior34of the optical microphone assembly propagate through the aperture32and impinge on the first side of the membrane8. As the second side of the membrane8is in fluid communication only with the acoustic cavity30, and not with the exterior of the microphone, the incoming acoustic wave causes a pressure differential, causing the membrane8to vibrate. The vibration of the membrane, and thus the incoming acoustic wave, is measured as described below.

When the optical microphone is operating, the light source16generates radiation38, which is directed onto the optical element10(which in this example is a diffraction grating). Of the radiation that impinges on the diffraction grating, a first portion passes40through the diffraction grating and is diffracted. This diffracted radiation is then reflected by the membrane8onto the photo detector14via the diffraction grating. A second portion42is reflected by the diffraction grating onto the photo detector14. The second portion42interferes with the first portion40to form an interference pattern, and consequently the intensity of light detected at the photo detector14depends on the interference pattern, and therefore on the distance between the optical element10and the membrane8. As the intensity of the light at the detector14depends on the distance between the optical element10and the membrane8, the position (and thus the motion) of the membrane8can be inferred from the detected intensities.

In this example, the optical microphone assembly has only one detector, which is positioned to receive the first order diffraction peak, however, this is not essential. The detector may be positioned to receive a different diffraction peak, e.g. a higher order diffraction peak. Multiple detectors could be used to detect more than one peak, e.g. the zeroth and first diffraction orders.

The movement of the membrane8, measured as described above, corresponds to the pressure amplitude of the acoustic wave impinging on the microphone, because the acoustic wave exerts a force corresponding to its amplitude on the membrane, causing the membrane to be deflected. However, the Applicant has appreciated that in prior art optical microphone assemblies in which the membrane is disposed within a front volume acoustic cavity, the acoustic waveform of the incoming acoustic wave arriving at the optical microphone assembly is affected by the presence of the front volume acoustic cavity and the restricted air flow of a narrow acoustic port, particularly in the higher frequencies.

In contrast, it can be seen fromFIG.1Athat in this embodiment, the MEMS component4is positioned to close the aperture32(rather than being positioned within a cavity accessible from the assembly exterior via an aperture), and the aperture has a diameter that is as large as the membrane diameter. Consequently, there is no substantially enclosed front volume, and air flow to the membrane is not restricted by a narrow aperture. The Applicant has found that this leads to a significantly improved frequency response. In optical microphone assemblies of the prior art, significant artefacts (e.g. peaks) are present in the frequency response in the high frequencies of the human hearing range (e.g. in the range 15 kHz to 20 kHz). In contrast, in embodiments of the invention, a substantially flat frequency response may be obtained in the range 20 Hz to 20 kHz.

In addition, it can be seen that the aperture32has a flared shape, such that its diameter near the exterior of the optical microphone assembly2(i.e. furthest from the membrane) is larger than its diameter near the interior of the optical microphone assembly2(i.e. closest to the membrane). The Applicant has found that this provides a further improvement in frequency response, as it reduces further the restriction of air flow into the aperture32.

It can also be seen fromFIG.1Athat the MEMS component4and the semiconductor chip6are mounted separately inside the housing. The MEMS component4is mounted on the superstrate26, while the semiconductor chip6is mounted on the substrate24. The Applicant has found that this provides a significant improvement in the ease of manufacture of the optical microphone assembly2, in particular because it is easier to align the membrane8and the optical element10of the MEMS component with the light source16and the photo detector14so as to produce a suitable interference pattern at the photo detector14. It will be appreciated from the present disclosure that there is no need to align the optical component10and the membrane8with the light source16and photo detector14as part of micro-fabrication techniques used to manufacture a MEMS component. Instead, this alignment depends on the position of the non-MEMS substrate24, superstrate26and spacer28, which are easier to align.

FIG.1Bshows the embodiment ofFIG.1Awith a dust cover45provided over the aperture32. The dust cover45comprises a porous sheet, i.e. comprising holes. The aggregate area of the holes is at least 50% of the area of the dust cover45, which means that the dust cover45does not significantly impact the acoustic properties of the optical microphone assembly2, and in particular, does not significantly impact the acoustic properties of the aperture32. The dust cover45may therefore be described as substantially acoustically transparent.

The dust cover45helps to prevent accidental physical contact with the membrane8by objects exterior to the optical microphone assembly2, and also helps to reduce leakage of light in or out of the optical microphone assembly2via the aperture32.

InFIG.1B, the dust cover45is on the side of the aperture32facing the exterior of the optical microphone assembly2.FIG.1Cshows a further variation on this embodiment, wherein the optical microphone assembly2comprises a dust cover45′ having the same physical and acoustic properties as the dust cover45ofFIG.1B. However, inFIG.1C, the dust cover45′ is disposed on the side of the aperture32facing the membrane8. A dust cover may optionally be provided in the other embodiments described below.

FIG.2shows an optical microphone assembly46according to a second embodiment of the present invention. The optical microphone assembly46comprises a MEMS component48and a semiconductor chip50. The MEMS component48and the semiconductor chip50have the same structure and function as the MEMS component4and the semiconductor chip6respectively of the first embodiment.

In this embodiment, the optical microphone assembly46comprises a substrate52with solder pads60for mounting to a microphone support (e.g. a PCB), a superstrate54, and a spacer56. However, in contrast with the first embodiment, in the present embodiment, an aperture58is provided in the substrate52and not in the superstrate54. A solder ring61surrounding the aperture is provided on the substrate for sealing the periphery of the aperture to the microphone support. The MEMS component48is sealed to the substrate52so as to close the aperture58. The semiconductor chip50is mounted on the superstrate facing the MEMS component, and in alignment therewith, to generate an interference pattern corresponding to the membrane vibration in the same manner as described above with reference to the first embodiment.

It can be seen that the aperture58has a flared shaped with an inner diameter that is the same as the diameter of the membrane59of the MEMS component48, and the MEMS component48is mounted on the substrate52so as to close the aperture58. Accordingly, the optical microphone assembly46does not have a front volume acoustic cavity, and the airflow from the exterior of the microphone to the membrane59is substantially unrestricted by the aperture58. This advantageously provides an improved frequency response of the optical microphone, as discussed above.

The arrangement shown inFIG.2, wherein the aperture is provide in a surface of the housing proximal to a substrate for mounting to a microphone support, may be referred to as a “bottom port” configuration.

FIG.3shows an optical microphone assembly62according to a third embodiment of the present invention. The optical microphone assembly62comprises a MEMS component64and a semiconductor chip66. The MEMS component64and the semiconductor chip66have equivalent structure and function to the MEMS components and semiconductor chips described above with reference toFIGS.1and2.

The outer housing of the assembly comprises a substrate70which defines a recess72surrounded by a peripheral wall74, and an enclosure76. The semiconductor chip66is mounted in the recess72of the substrate. The MEMS component64is mounted on the top of the peripheral wall74such that it spans the recess72. The mounting of the MEMS component64can be seen more clearly in the three-dimensional representation of the optical microphone assembly62which is shown inFIG.4. It can be seen fromFIG.4that the MEMS component has a width slightly greater than the recess72in the substrate70, such that when it is placed on top of the substrate70, it sits on top of the peripheral wall74, such that it is supported by the peripheral wall74above and spaced from the semiconductor chip66in the recess72.

Referring again toFIG.3, the enclosure76has an aperture78formed therein. The enclosure is mounted on the peripheral wall74such that the enclosure76and the recessed substrate70together form an acoustic cavity80. The aperture78is positioned so that it is above the MEMS component64. The edges82of the aperture are sealed to the MEMS component64using stress free glue84. This seals the acoustic cavity80from the exterior of the microphone so that acoustic waves arriving at the optical microphone assembly62impinge only on one side of the membrane86of the MEMS component64, i.e. on the side that faces the aperture78, and not the side of the membrane86that faces the acoustic cavity80.

It can be seen that the aperture78has a diameter that is the same as the diameter of the membrane86, and the MEMS component64is sealed to the enclosure76so as to close the aperture78. Accordingly, the optical microphone assembly62does not have a front volume acoustic cavity, and the airflow from the exterior of the microphone to the membrane86is substantially unrestricted by the aperture78. This advantageously provides an improved frequency response of the optical microphone, as discussed above.

In this example, the aperture78is not provided with a flared shape, although a flared aperture may be provided in variations on this and other embodiments.

In common with the earlier embodiments, the optical microphone assembly62is also provided with solder pads87on the substrate to allow the optical microphone assembly to be mounted on a microphone support such as a PCB. As with the first embodiment the arrangement shown inFIG.3, wherein the aperture is provided in a surface of the housing distal to a substrate for attaching to a microphone support, is a “top port” configuration.

FIG.5shows an optical microphone assembly88which is a variation on the embodiment ofFIGS.3and4. The optical microphone assembly88comprises a MEMS component90and a semiconductor chip92. The MEMS component90and the semiconductor chip92have the same structure and function as the MEMS component64and the semiconductor chip66respectively of the embodiment ofFIGS.3and4.

The embodiment ofFIG.5comprises similar features to the embodiment ofFIGS.3and4, including a substrate94and an enclosure96. However, this embodiment differs from the embodiment ofFIGS.3and4in that the substrate94comprises an interior wall98defining a recess100, instead of a peripheral wall. The MEMS component90is mounted on top of the interior wall98such that it spans the recess100.

The enclosure96is mounted on the substrate94so that the enclosure96and the substrate94together define an acoustic cavity102. The enclosure96has an aperture104formed therein, and the aperture104is positioned so that it is above the MEMS component90. The edges106of the aperture are sealed to the MEMS component90using stress free glue108. In a similar manner to that described with reference toFIG.3, this seals the acoustic cavity102from the exterior of the microphone so that acoustic waves arriving at the optical microphone assembly88impinge only on one side of the membrane110of the MEMS component90, i.e. on the side that faces the aperture104, and not the side of the membrane110that faces the acoustic cavity102.

The aperture104has a diameter that is the same as the diameter of the membrane110, and the MEMS component90is sealed to the enclosure96so as to close the aperture104. Accordingly, the optical microphone assembly88does not have a front volume acoustic cavity, and the airflow from the exterior of the microphone to the membrane110is substantially unrestricted by the aperture104. This advantageously provides an improved frequency response of the optical microphone, as discussed above.

In common with the earlier embodiments, the optical microphone assembly88is also provided with solder pads112on the substrate to allow the optical microphone assembly to be mounted on a microphone support such as a PCB.

FIG.6shows a fifth embodiment of a bottom part optical microphone assembly114in accordance with the present invention. The optical microphone assembly comprises a MEMS component116and a semiconductor chip118, which have equivalent structure and function to the MEMS components and semiconductor chips described in the embodiments above. Similarly to the embodiment ofFIGS.3and5, the optical microphone assembly114comprises a substrate120with solder pads132for mounting to a microphone support such as a PCB and an enclosure122. The substrate120defines a recess124surrounded by a peripheral wall126. An aperture128is formed in the bottom of the substrate120.

The MEMS component120is mounted within the recess124so as to close the aperture128. The semiconductor chip118is mounted on the peripheral wall126such that it spans the recess124. The semiconductor chip faces the MEMS component116and is aligned therewith so that the MEMS component116and the semiconductor chip118together function as an optical microphone in the same manner described above with reference to the previous embodiments. The enclosure114is mounted on the top of the peripheral wall so as to form an acoustic cavity130.

The aperture128has a flared shape, like the apertures32,58of the first and second embodiments, which improves the frequency response of the optical microphone, as explained above.

FIG.7shows a variation on the embodiment ofFIG.1A, where corresponding features are denoted with like numerals. In this embodiment, an ASIC chip134is provided in addition to a semiconductor chip136. Connections138,140are provided between the semiconductor chip136and the ASIC chip134to allow control of the photo detector14and light source16by the ASIC chip134, and between the ASIC chip134and the substrate24to allow connection to off-chip components, e.g. a power source. An opaque “globtop” covering142is provided over the ASIC chip134. A separate ASIC chip could similarly be provided in variations on other embodiments, e.g. the other embodiments described above.

It will be appreciated that the above-described embodiments are examples only, and that other embodiments and variations are possible within the scope of the claims.