Patent Description:
Microphones are used to convert sound waves into electrical signals, typically by measuring the displacement of a moveable member (e.g. a membrane) that vibrates in response to ambient acoustic vibrations. There are a number of ways of measuring the displacement of such a moveable member, including capacitive readout (commonly called condenser microphones) and electrostatic or electromagnetic readout mechanisms (e.g. dynamic microphones).

An alternative way of reading out the position of a microphone membrane is optical interferometric read out. In typical examples of such systems, a diffraction grating is provided on a substrate adjacent to a membrane, and electromagnetic radiation is directed onto the diffraction grating. A first portion of the light is reflected back from the grating. A second portion is transmitted through the grating, which diffracts the radiation. The diffracted radiation impinges on the membrane, which reflects it onto the grating. The radiation passes through the grating and the two portions of light interfere to create an interference pattern that can be detected by a detector. The interference pattern has a shape (i.e. spatial distribution) matching the diffraction orders of the grating, but the light intensity directed into these diffraction orders depends on the relative phase of the two portions of light, and therefore on the distance between the grating and the membrane. The position (and therefore the movement) of the membrane can thus be determined from changes in the intensity of the light at the detector. <CIT> and <CIT> describe displacement sensors that use optical interferometric read out.

Such microphones have a high signal to noise ratio (SNR) and high sensitivity. However, further improvements in the performance of such microphones are desirable.

The invention provides an optical microphone assembly comprising:.

The invention extends to a method of manufacturing an optical microphone assembly, the optical microphone assembly comprising a substrate, a membrane, a light source and at least one photo detector, the method comprising:.

The Applicant has appreciated that providing one or more holes in the substrate as described above can provide certain advantages over microphone assemblies of the prior art. As the hole(s) at least partly overlap with the optical path taken by the light through the interferometric arrangement, the hole(s) may affect the light that is transmitted by or reflected from the substrate. For example, the hole(s) may remove some of the light that would otherwise have contributed to the diffraction pattern created by the diffractive optical element. It is therefore to be understood that when it is said that at least one of said first and second optical paths at least partly overlaps with the one or more holes, this refers to the optical path(s) that would have been taken by the first and/or second portions of light from the light source to the detector(s) if the hole(s) were not present. The holes may help to improve the performance of the optical microphone assembly, for example, by removing parts of the light that contribute negatively to the optical signal strength and/or contrast.

Another way that the hole(s) may help to improve the performance of the optical microphone assembly is by changing its acoustic properties. For example, the hole(s) may help to reduce acoustical squeeze-film resistance between the membrane and the substrate that may otherwise give rise to noise in the optical microphone output signal. The hole(s) may also help to reduce squeeze-film noise displacement of the substrate, thereby reducing the total measured thermo-mechanical noise in the optical microphone.

Although the hole(s) may affect the light that is transmitted by or reflected from the substrate, it is to be understood that the holes do not form part of the diffractive optical element. In particular, the diffractive optical element is not defined by the hole(s), i.e. the diffractive optical element is not formed by forming the hole(s) in the substrate. The diffractive optical element is diffractive due to comprising lines (e.g. grating lines) arranged in the first pattern. The hole(s) may be described as being uncorrelated with the diffraction pattern produced by the lines of the diffractive optical element. The hole(s) may overlap with the at least one diffractive optical element. For example, in a set of embodiments, the first and second patterns have respective first and second envelopes, wherein the first and second envelopes overlap. At least one, at least some or all of the hole(s) may be positioned within the first envelope.

As noted above, the presence of the hole(s) may advantageously improve the acoustic and/or optical properties of the optical microphone (e.g. by reducing squeeze film resistance and/or removing negative contributions to the optical signal strength/contrast). While the presence of one or more holes may provide such advantages, the benefit may be improved by selecting (e.g. optimising) the properties of the holes. For example, the size, shape, arrangement and/or other properties of the holes may be selected to increase the associated benefits.

The length scale of the one or more holes (e.g. a width or length of the one or more holes) would typically be larger than an average spacing between the lines (e.g. the distance between adjacent lines). For example, the length scale of the one or more holes may be at least <NUM> times larger, at least <NUM> times larger, at least <NUM> times larger, at least <NUM> times larger or at least <NUM> times larger than the average spacing between the lines. In some embodiments, a small number of large holes is provided. For example, the number of holes may be between <NUM> and <NUM>, between <NUM> and <NUM> or between <NUM> and <NUM>.

When it is said that the holes are arranged in a pattern (i.e. the second pattern), it is to be understood that the term "pattern" does not necessarily imply any symmetry or repetition in the arrangement of holes, although the pattern may have symmetry, e.g. rotational, reflectional and/or translational symmetry. The second pattern may comprise one or more of: a single central hole; radially extending elongate holes, e.g. <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or more than <NUM> radially extending elongate holes; radially extending lines of holes, e.g. in which a line of two or more holes extends from a centre point or a centre hole; concentric circles of holes, e.g. surrounding a central hole. Other patterns are possible. The hole(s) may be circular.

When it is said that hole(s) extend "fully through" the substrate, it is to be understood that this means that the hole(s) extend through the substrate so as to form an air passage connecting one side of the substrate to the other, e.g. so that regions on either side of the substrate are in fluid communication through the hole(s). The method of manufacture may comprise etching the one or more holes in the substrate.

The lines of the diffractive optical element(s) may be etched into or deposited on the substrate. Where they are etched, they are preferably non-perforating, i.e. they preferably do not extend fully through the substrate. The lines may be deposited metal lines.

The lines may form a diffraction grating. The lines may be straight, although this is not essential and in some embodiments the lines are curved. For example, the diffractive optical element may be a Fresnel lens.

The Applicant has appreciated that certain advantages may be obtained if the substrate or a portion thereof bearing the diffractive optical element is thin. In a set of embodiments, the substrate comprises a first substrate portion, wherein the at least one diffractive optical element is disposed on or formed in the first substrate portion. The substrate or the first substrate portion may have a thickness of thickness of <NUM> to <NUM>, preferably <NUM> to <NUM>, more preferably <NUM> to <NUM>. In a set of embodiments, one or both of the first and second portions of light impinges on the first substrate portion. In a set of embodiments, at least one of the first and second light paths passes through the first substrate portion.

Having a thin substrate may provide advantages associated with optical transparency, acoustic performance and manufacturability.

Preferably the substrate or the first substrate portion is substantially transparent at the wavelength of the light provided by the light source, e.g. such that the majority of light that impinges on the substrate or the first substrate portion is transmitted after two passes through the substrate (i.e. after entering the substrate, being reflected from a surface of the substrate and passing back through the substrate again). For example, at least <NUM>%, at least <NUM>% or at least <NUM>% of the light may be transmitted after two passes through the substrate. The transparency of the substrate may be achieved by virtue of the substrate of the first substrate portion being sufficiently thin for the light to be transmitted, e.g. the thickness of the substrate or the first substrate portion may be selected to provide a transparency according to the percentages specified above. The substrate or part thereof (e.g. the first substrate portion) may be provided with an anti-reflection coating on one or both sides thereof. Reflections from the substrate surface may thereby be reduced.

The transparency of the substrate or first substrate portion may be wavelength-dependent, and the thickness required to achieve a particular transparency may depend on the material the substrate is made from. Accordingly, the thickness of the substrate or first substrate portion may selected to achieve a particular transparency for the wavelength of the light provided by the light source for a given substrate material. The wavelength of the light provided by the light source may be in the range <NUM> to <NUM>, e.g. <NUM> or <NUM>. If the wavelength is <NUM>, the substrate or first substrate portion thickness may be about <NUM> if the substrate is made from silicon. At other wavelengths and/or for other materials, the thickness may be less than or greater than <NUM>. For example, if the wavelength of the light is <NUM>, the substrate or first substrate portion thickness may be about <NUM> if the substrate material is silicon.

The use of a thin substrate may advantageously allow the use of substrate materials that would otherwise be too opaque to be used. These materials may provide advantages over conventional materials (e.g. glass) that are transparent even at greater thicknesses, e.g. such as improved manufacturability. For example, the substrate may be made from silicon. Silicon is particularly advantageous as it can be readily and precisely etched to provide the required form of the substrate, e.g. including holes, apertures, and/or perforations as discussed elsewhere herein. Where the substrate (or the first substrate portion) is made from silicon, preferably the silicon is monocrystalline silicon. The silicon may be undoped or doped at a low level such that a suitable substrate transparency (as discussed above) is achieved.

The substrate and/or the first substrate portion may comprise a layered structure (e.g. a sandwich of different materials, e. g including a silicon layer). The layered structure may consist of two, three or more than three layers, e.g. a first layer with a thin film layer on one or both sides. For example, the layered structure may comprise or consist of mono-crystalline silicon (e.g. a <NUM>-thick layer) with a thin film on one or both sides. The thin film may comprise an anti-reflection coating. The thin film may have a high tensile stress (e.g. 1GPa or higher than 1GPa), e.g. in order to stiffen the substrate or the first substrate portion.

A thin substrate may also improve the acoustic properties of the optical microphone assembly. For example, holes with a lower aspect ratio (i.e. formed in a thinner substrate) may provide a greater improvement of the acoustic properties of the optical microphone compared with holes of the same shape, size and pattern in a thicker substrate. Holes formed in a thinner substrate may be more easily manufactured.

The optical microphone assembly may comprise more than one diffractive optical element. Each diffractive optical element may introduce a different relative phase delay to light transmitted or reflected by the diffractive optical element. For example, the substrate (or first substrate portion) may have regions of different thickness such that each diffractive optical element has a different height. The displacement of the membrane may thus be measured with respect to a different working point for each diffractive optical element, thereby extending the working range of the optical microphone.

The substrate may comprise a second substrate portion, e.g. surrounding the first substrate portion. The second substrate portion may be thicker than the first substrate portion. This may help to provide rigidity/stiffness for the substrate as a whole, and thus for the first substrate portion. Increased rigidity/stiffness may help to reduce noise, for example, by reducing movement of the substrate (or first substrate portion) which would otherwise contribute to the noise floor.

The substrate may be integrally formed from a single piece of material, such that the first and second substrate portions are made from the same material. For example, the first substrate portion may be a thinned region of the single piece. The method of manufacture may comprise etching a region of the substrate so as to provide a first substrate portion that is thinner than a second substrate potion, e.g. wherein the second substrate portion surrounds the first substrate portion.

The first and second substrate portions may comprise two distinct pieces that are attached together. The first substrate portion may be formed from silicon.

The optical microphone assembly may be arranged such that the first and second portions of light do not impinge on and/or do not pass through the second substrate portion. The second substrate portion may be substantially opaque at the wavelength of light provided by the light source. For example, the second substrate portion may be substantially opaque due to its thickness, e.g. if it is made from the same material as the first substrate portion, due its material or because a covering is provided thereon. As discussed below, the second substrate portion may include apertures or perforations. Accordingly, when it is said that the second substrate portion may be substantially opaque, it is to be understood that this means that the parts of the second substrate portion other than any apertures or perforations are opaque.

The optical microphone assembly may comprise an interstitial volume, wherein the substrate and membrane together define the interstitial volume therebetween. The optical microphone assembly may comprise an acoustic cavity, wherein a first side of the membrane is in fluid communication with the acoustic cavity and a second side of the membrane is in fluid communication with the exterior of the optical microphone assembly.

In a set of embodiments, the substrate comprises an apertured region, the apertured region comprising one or more apertures extending fully through the substrate. For example, the substrate may comprise a plurality of apertures surrounding a central support portion of the substrate, the central support portion comprising the at least one diffractive optical element. The apertured region may be provided in the first substrate portion or in the second substrate portion. The apertures may provide a passage for air connecting the interstitial volume either with the acoustic cavity of the optical microphone assembly or with the exterior of the optical microphone assembly so that these regions are in fluid communication.

Additionally or more typically alternatively, the substrate may comprise a perforated region, the perforated region comprising perforations extending fully through the substrate. The perforated region may be provided in the first substrate portion or in the second substrate portion. The perforated region may provide a passage for air connecting the interstitial volume either with the acoustic cavity of the optical microphone assembly or with the exterior of the optical microphone assembly so that these regions are in fluid communication. This may help to improve the performance of the optical microphone assembly by reducing the acoustic resistance between the membrane and the substrate. The perforated region may be highly perforated, e.g. such that the total area of the perforations comprises greater than <NUM>%, greater than <NUM>%, greater than <NUM>% or greater than <NUM>% of the perforated region. Additionally or alternatively, the perforations may have tessellating shapes, e.g. squares, hexagons, triangles, rectangles. The perforations may be in a lattice arrangement, e.g. square lattice, hexagonal close packed. The perforated region of the second substrate portion may be maximally perforated. In this context, maximally perforated means that the total area of the perforations is maximised as much as practicable while maintaining sufficient structural integrity of the second substrate portion to support the first substrate portion. Maximising the degree of perforation in this way can help to minimise the acoustic resistance between the membrane and the substrate.

It is to be appreciated that the apertures and perforations, where provided, are different from the holes in the first substrate portion and are not for providing the function of removing parts of the diffracted/reflected light. Accordingly, where the perforated region and/or the apertured region is provided in the first substrate portion, neither the apertures nor the perforations overlap with the first or second light path.

Apertures and perforations as described in this context differ in that perforations are typically smaller and more numerous than apertures.

The second substrate portion may be made from a different material from the first substrate portion. Where the perforated region and/or the apertured region is formed in the first substrate portion, the perforated region and/or the apertured region may be made from a different material from the rest of the first substrate potion. The second substrate portion, the perforated region and/or the apertured region may be made from a material that does not transmit light at the wavelength of the light from the light source, e.g. polysilicon. The method of manufacture may comprise etching perforations in the substrate to form the perforated region.

The shape, size and/or arrangement of the apertures and/or perforations may be selected, e.g. optimized, to increase fluid communication between the interstitial volume and the acoustic cavity or optical microphone assembly exterior, e.g. to reduce or minimise acoustic resistance between the membrane and the substrate.

The optical microphone assembly may comprise a mount structure which supports the membrane and substrate. The mount structure may be a composite structure and may comprise the second substrate portion. The thickness of mount structure may be less than <NUM>, e.g. less than <NUM>, e.g. less than <NUM>.

The membrane may be made from any suitable material, for example, silicon nitride. The membrane may be provided with corrugations, e.g. to increase membrane compliance, although corrugations are not essential. The membrane may be provided with a ventilation hole, although this is not essential. The ventilation hole may allow static equalization of pressure between the acoustic cavity and the microphone assembly exterior.

As discussed above, providing a relatively thin first substrate portion and a thicker second substrate portion, e.g. surrounding the first substrate portion, provides certain advantages. Using a thin first substrate portion opens up the possibility of manufacturing the substrate from materials that would otherwise be too opaque to use, while providing a thicker second substrate portion can provide rigidity to mitigate the effects of noise.

Certain preferred embodiments will now be described, by way of example only, with reference to the accompanying drawings, in which:.

<FIG> shows an optical microphone assembly <NUM> comprising a substrate <NUM>, a membrane <NUM>, a light source <NUM> and two photo detectors <NUM>. The substrate <NUM> comprises a first substrate portion <NUM> generally in the centre thereof and a second substrate portion <NUM> surrounding the first substrate portion. The substrate is manufactured from silicon. The first substrate portion <NUM> has a thickness of approximately <NUM> so that the silicon of the first substrate portion <NUM> is sufficiently thin to be substantially transparent to the wavelength of light provided by the light source <NUM>, which is <NUM>. In this example, a central region <NUM> of the first substrate portion <NUM> comprises two sub-portions <NUM>, <NUM>, each being provided with a respective diffractive optical element <NUM>, <NUM>. The diffractive optical elements <NUM>, <NUM> comprise a plurality of reflective metal lines <NUM> deposited on the surface of the first substrate portion <NUM>. In this example, the reflective lines are fabricated by depositing and patterning a metal layer, before performing a high-temperature LPCVD (low pressure chemical vapour deposition) processing step to deposit a highly tensile silicon nitride layer over the substrate. This approach would typically use a metal with a high melting point (such as tungsten) in order to withstand the LPCVD deposition step. Alternatively, a tensile film formed with a lower temperature process (e.g. PECVD, plasma-enhanced chemical vapour deposition) may be employed. The LPCVD silicon nitride layer may provide two useful functions. It may function as an anti-reflection coating. It may also help to stiffen the substrate (or first substrate portion) due to its high tensile stress, which may advantageously reduce noise from movement of the substrate (or first substrate portion). However, neither the LPCVD silicon nitride layer nor the PECVD tensile film is essential.

The sub-portions <NUM>, <NUM> have different thicknesses so as to introduce different phase delays to light passing therethrough. This provides two different working points with respect to which the membrane displacement is measured, which extends the working range of the optical microphone. However, this feature is not essential, and in variations on this embodiment and other embodiments, the first substrate portion <NUM> may have a uniform height and a single diffractive optical element.

The light source <NUM>, which in this example is a vertical-cavity surface-emitting laser (VCSEL), directs light <NUM> towards the first substrate portion <NUM>. A first portion <NUM> of the light is reflected back from the diffractive optical elements <NUM>, <NUM>, and is incident on the detectors <NUM>. A second portion <NUM> of the light is transmitted and diffracted by the diffractive optical elements <NUM>, <NUM> and is incident on the membrane <NUM>. The second portion <NUM> of light is reflected by the membrane <NUM>, and propagates back through the first substrate portion <NUM> and is incident on the detectors <NUM>. The first and second portions <NUM>, <NUM> of light together create an interference pattern at the detectors <NUM>. The interference pattern depends on the separation between the membrane <NUM> and the diffractive optical elements <NUM>, <NUM>. The measured intensity of light at the detectors <NUM> is used to determine the separation between the membrane <NUM> and the diffractive optical elements <NUM>, <NUM>, and thus to generate an output signal corresponding to the movement of the membrane <NUM>.

A plurality of holes <NUM> is provided in the first substrate portion <NUM>. The plurality of holes <NUM> performs two functions. First, the holes <NUM> interrupt the optical paths taken by the light <NUM> and the first and second portions <NUM>, <NUM> of light. The holes <NUM> therefore cause part of the light to be removed, such that it does not contribute to the interference pattern at the detectors. The light that is removed includes, for example, portions that contribute negatively to the optical signal strength and contrast.

The holes <NUM> also serve as air passages such that an interstitial volume <NUM> between the substrate <NUM> and the membrane <NUM> is in fluid communication with an acoustic cavity <NUM> of the optical microphone assembly <NUM>. The air passages provided by the holes <NUM> allow air that is displaced by movement of the membrane to flow out of the interstitial volume, thereby reducing acoustical squeeze-film resistance that may otherwise give rise to noise in the optical microphone output signal.

In this example, the optical microphone assembly is shown as having a "top-port" configuration, i.e. wherein the acoustic cavity <NUM> is provided underneath the substrate <NUM>, and the side of the membrane <NUM> facing away from the substrate <NUM> is in fluid communication with the exterior of the optical microphone <NUM>. However, the optical microphone assembly may also be suitable for use in a "bottom-port" configuration, i.e. with an acoustic cavity enclosing a volume on the side of the membrane <NUM> that faces away from the substrate <NUM>. In such arrangements, the interstitial volume <NUM> would be in fluid communication with the exterior of the optical microphone assembly via the holes <NUM>.

As shown in perspective view in <FIG>, the first substrate portion <NUM> comprises a perforated region <NUM>, which surrounds a central region <NUM> of the first substrate portion <NUM>. It can be seen that the holes <NUM> provided in the central region <NUM> of the first substrate portion <NUM> are arranged in concentric circles around a central hole 32a. The perforated region <NUM> comprises hexagonal perforations <NUM>. The hexagonal perforations <NUM> are arranged in a hexagonal close-packed lattice arrangement. The perforations <NUM> occupy a large fraction of the area of the perforated region <NUM>-in this example, the maximum area practicable while still maintaining sufficient structural integrity from the material <NUM> between the holes to support the central region <NUM> of the first substrate portion <NUM>. The perforated region <NUM> provides air passages via which the interstitial volume <NUM> is in fluid communication with the acoustic cavity <NUM>. This helps further to eliminate squeeze-film damping of the membrane and thereby to improve the performance of the optical microphone.

Returning to <FIG>, it can be seen that the membrane <NUM> is provided with corrugations <NUM>, which help to increase the membrane compliance and thus to increase the optical microphone sensitivity. The membrane <NUM> also comprises a central ventilation hole <NUM>. This allows static equalization of pressure between the acoustic cavity and the microphone assembly exterior.

The optical microphone assembly <NUM> also comprises a mounting structure <NUM>, which comprises the second substrate portion <NUM>. It can be seen that the mounting structure <NUM> is a composite structure comprising circumferential structures that support the membrane <NUM> and the first substrate portion <NUM>. It can be seen that the mounting structure <NUM> is thicker than the first substrate portion <NUM>. In this example, the mounting structure <NUM> is <NUM> thick. It will be appreciated that the dimensions of <FIG> are not shown to scale. The thickness of the mounting structure <NUM> provides rigidity to reduce noise (e.g. thermo-mechanical noise) that may otherwise be caused by movement of thin first substrate portion <NUM>.

<FIG> shows an alternative substrate <NUM> that may be used in optical microphone assemblies in accordance with the present invention, for example, in the optical microphone assembly <NUM> shown in <FIG>. The substrate <NUM> comprises a first substrate portion <NUM> having a diffractive optical element (not shown) disposed on a central region <NUM> thereof. The central region <NUM> has holes <NUM> therethrough. The holes <NUM> are elongate and extend radially from a central point on the first substrate portion <NUM>. The holes <NUM> perform an equivalent function to the holes <NUM> shown in <FIG> and <FIG>.

The substrate <NUM> comprises an apertured region <NUM>. The apertured region <NUM> has three apertures <NUM> that extend fully through the substrate <NUM>. The apertures <NUM> are arc-shaped sections surrounding the central region <NUM>, such that the central region <NUM> has the form of central support platform supported by three radially extending beams <NUM>. In an optical microphone assembly, e.g. such as the assembly <NUM> in <FIG>, the apertures <NUM> perform a similar function to the perforations <NUM> shown in <FIG>, i.e. putting an interstitial volume in fluid communication with an acoustic cavity. The substrate <NUM> also includes a second substrate portion <NUM>, which is thicker than the first substrate portion <NUM> and forms part of a mounting structure when the substrate <NUM> is assembled in an optical microphone assembly. In this example, the beams <NUM> have the same thickness as the central region <NUM>, although in variations on this and other embodiments, the beams <NUM> may be thicker than the central region <NUM>, e.g. forming part of and having the same thickness as the second substrate portion <NUM>.

<FIG> shows a second embodiment of an optical microphone assembly <NUM>. The optical microphone assembly <NUM> has equivalent features to the optical microphone assembly <NUM> shown in <FIG>, i.e. a substrate <NUM>, a membrane <NUM>, a light source <NUM> and two detectors <NUM>. The substrate <NUM> comprises a first substrate portion <NUM> and second substrate portion <NUM>, the first substrate portion <NUM> having diffractive optical elements <NUM>, <NUM> thereon. However, instead of the plurality of holes <NUM>, a single central hole <NUM> is provided. The hole <NUM> provides a similar function to the holes <NUM> of the embodiment of <FIG>, except that due its different position, it removes a different portion of the light from the light source <NUM>. The hole <NUM> therefore has a different impact on the interference pattern detected at detectors <NUM> compared with the arrangement shown in <FIG>.

<FIG> shows an example of an optical microphone assembly <NUM> which is not part of the invention. The optical microphone assembly <NUM> has equivalent features to the optical microphone assembly <NUM> shown in <FIG> and <FIG>, i.e. a substrate <NUM>, a membrane <NUM>, a light source <NUM> and two detectors <NUM>. The substrate <NUM> comprises a first substrate portion <NUM> and second substrate portion <NUM>, the first substrate portion <NUM> comprising a central region <NUM> having diffractive optical elements <NUM>, <NUM> thereon. However, in this example, no holes are provided in the central region <NUM> of the first substrate portion <NUM>. In addition, the first substrate portion <NUM> comprises only the central region <NUM>, and the second substrate portion <NUM> (which is thicker than the first substrate portion <NUM>) includes a peripheral region <NUM> which surrounds the central region <NUM> and has apertures <NUM> formed therein. An interstitial volume <NUM> is in fluid communication with an acoustic cavity <NUM> via the apertures <NUM>. The apertures <NUM> have a similar form to the apertures <NUM> shown in and discussed with reference to <FIG>, except that in this example the beams <NUM> form part of the second substrate portion <NUM>, and so are thicker than the first substrate portion <NUM> (although this is not essential). In variations on this example, a perforated region similar to perforated region <NUM> of <FIG> and <FIG> may be provided instead of the apertures <NUM>. The substrate <NUM> is made from silicon which, due to having a thickness of <NUM>, is substantially transparent to the light emitted by the light source <NUM>, which in this example has a wavelength of <NUM>, i.e. the substrate <NUM> is transparent by virtue of being sufficiently thin. Using silicon for the substrate <NUM> is advantageous because silicon is readily and precisely etched, allowing the apertures <NUM> (or perforations in variations on this example) to be etched in the substrate <NUM>.

The optical microphone assembly <NUM> comprises a mounting structure <NUM> which comprises the second substrate portion <NUM>. The mounting structure <NUM> and the second substrate portion <NUM> are much thicker than the first substrate portion <NUM>. In this example, the mounting structure (not shown to scale) is <NUM> thick. The mounting structure <NUM> and the second substrate portion <NUM> provide rigidity to reduce noise resulting from movement of the first substrate portion <NUM>.

Claim 1:
An optical microphone assembly (<NUM>; <NUM>) comprising:
a substrate (<NUM>; <NUM>; <NUM>);
an interferometic arrangement, the interferometric arrangement comprising a membrane (<NUM>; <NUM>) and at least one diffractive optical element (<NUM>, <NUM>; <NUM>, <NUM>) spaced from the membrane (<NUM>; <NUM>), wherein the at least one diffractive optical element (<NUM>, <NUM>; <NUM>, <NUM>) comprises a plurality of lines (<NUM>) formed in or disposed on a surface of the substrate (<NUM>; <NUM>; <NUM>), the plurality of lines (<NUM>) being arranged in a first pattern; the optical microphone assembly (<NUM>; <NUM>) further comprising:
a light source (<NUM>; <NUM>) arranged to provide light to said interferometric arrangement such that a first portion (<NUM>) of said light propagates along a first optical path via said interferometric arrangement and a second portion (<NUM>) of said light propagates along a second different optical path via said interferometric arrangement, thereby giving rise to an optical path difference between the first and second optical paths which depends on a distance between the membrane (<NUM>; <NUM>) and the diffractive optical element (<NUM>, <NUM>; <NUM>, <NUM>); and
at least one photo detector (<NUM>; <NUM>) arranged to detect at least part of an interference pattern generated by said first and second portions of light (<NUM>, <NUM>) dependent on said optical path difference;
characterized in that the substrate (<NUM>; <NUM>; <NUM>) comprises one or more holes (<NUM>; <NUM>; <NUM>) extending fully through the substrate (<NUM>; <NUM>; <NUM>), the one or more holes (<NUM>; <NUM>; <NUM>) being arranged in a second pattern that is different from the first pattern, wherein the one or more holes (<NUM>; <NUM>; <NUM>) are positioned such that at least one of said first and second optical paths at least partly overlaps with the one or more holes (<NUM>; <NUM>; <NUM>).