Abstract
The application describes MEMS transducer structures comprising a membrane structure having a flexible membrane layer and at least one electrode layer. The electrode layer is spaced from the flexible membrane layer such that at least one air volume extends between the material of the electrode layer and the membrane layer. The electrode layer is supported relative to the flexible membrane by means of a support structure which extends between the first electrode layer and the flexible membrane layer.
Claims
1. A MEMS transducer comprising a membrane structure, the membrane structure comprising a flexible membrane layer and a first electrode layer, the first electrode layer being supported relative to the flexible membrane layer so as to be spaced from the flexible membrane layer.
2. A MEMS transducer as claimed in claim 1, wherein at least one air volume extends between the material of the first electrode layer and the membrane layer.
3. A MEMS transducer as claimed in claim 1, wherein the membrane structure comprises a second electrode layer, said second electrode layer being disposed on the opposite side of the flexible membrane to the first electrode.
4. A MEMS transducer as claimed in claim 3, wherein the second electrode layer is supported relative to the flexible membrane layer so as to be spaced from the flexible membrane layer.
5. A MEMS transducer as claimed in claim 1, wherein the first electrode layer comprises a continuous sheet of material.
6. A MEMS transducer as claimed in claim 1, wherein the first electrode layer comprises one or more openings.
7. MEMS transducer as claimed in claim 1, wherein the first electrode layer comprises a lattice structure.
8. A MEMS transducer as claimed in claim 1, wherein the first electrode layer is supported relative to the flexible membrane by means of a support structure which extends between the first electrode layer and the flexible membrane layer.
9. A MEMS transducer as claimed in claim 3, wherein the second electrode layer is supported relative to the flexible membrane by means of a support structure which extends between the second electrode layer and the flexible membrane layer.
10. A MEMS transducer as claimed in claim 8, wherein the support structure comprises a plurality of support elements.
11. A MEMS transducer as claimed in claim 10, wherein the support elements comprise dielectric or conductive material.
12. A MEMS transducer as claimed in claim 10, wherein the support elements are disposed at or near the periphery of the membrane structure.
13. A MEMS transducer as claimed in claim 8, wherein the support structure comprises a conductive layer having a plurality of openings.
14. A MEMS transducer as claimed in claim 13, wherein the support structure comprises a lattice structure.
15. A MEMS transducer as claimed in claim 13, wherein the openings of the support structure form a plurality of air volumes which extend between the material of the first electrode layer and the flexible membrane layer.
16. A MEMS transducer as claimed in claim 13, wherein the openings of the support structure increase in size from a region towards the centre of the membrane structure to a region at or near the periphery of the membrane structure.
17. A MEMS transducer as claimed in claim 13, wherein the openings of the support structure are substantially of uniform size.
18. A MEMS transducer as claimed in claim 13, wherein the openings of the first electrode layer are laterally offset from the openings of the support structure.
19. A MEMS transducer as claimed in claim 1, wherein the first electrode layer comprises a hole which overlies at least a central region of membrane layer.
20. A MEMS transducer comprising a membrane structure, the membrane structure comprising a membrane and an electrode, wherein the material forming the electrode is separated from the membrane by at least one air volume.
21. A MEMS transducer as claimed in claim 1, wherein the flexible membrane comprises a crystalline or polycrystalline material.
22. A MEMS transducer as claimed in claim 21, wherein the flexible membrane layer comprises silicon nitride.
23. A MEMS transducer as claim in claim 1, wherein the first electrode layer comprises metal, a metal alloy or a metallic compound.
24. A MEMS transducer as claimed in claim 1, wherein said transducer comprises a capacitive microphone.
25. A MEMS transducer as claimed in claim 24, further comprising readout circuitry, the readout circuitry comprising one or both of analogue and digital circuitry.
26. An electronic device comprising a MEMS transducer as claimed in claim 1, wherein said device is at least one of: a portable device; a battery powered device; an audio device; a computing device; a communications device; a personal media player; a mobile telephone; a games device; and a voice controlled device.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0044] For a better understanding of the present invention and to show how the same may be carried into effect, reference will now be made by way of example to the accompanying drawings in which:
[0045] FIGS. 1a and 1b illustrate known capacitive MEMS transducers in section and perspective views;
[0046] FIG. 2 illustrates how a membrane may be deformed;
[0047] FIG. 3 illustrates a previously considered membrane structure;
[0048] FIGS. 4a to 4e show cross-sectional views through various membrane structures according to embodiments of the present invention;
[0049] FIGS. 5a to 5e show a membrane structure according to a further embodiment of the present invention;
[0050] FIGS. 6a to 6d show further membrane structures according to embodiments of the present invention;
[0051] FIGS. 7a to 7g show cross-sectional views through various membrane structures according to further embodiments of the present invention; and
[0052] FIGS. 8 to 8h show a series of drawings to illustrate a process for making a membrane structure embodying the present invention.
[0053] Throughout this description any features which are similar to features in other figures have been given the same reference numerals.
DETAILED DESCRIPTION
[0054] FIG. 3 shows a top view of a previously considered membrane structure comprising a planar membrane layer 101 and an electrode 102. The electrode—which is typically formed of metal or metal alloy—is patterned to incorporate a plurality of openings 113. In this specific example the openings are generally hexagonal in shape.
[0055] It will be appreciated that microphone sensitivity is a function of capacitance which is directly proportional to the area of the conductive electrode. Membrane structures which incorporate a patterned electrode layer, or lattice, as shown in FIG. 3 may therefore potentially demonstrate a deterioration in the sensitivity of the transducer as compared to sheet electrode designs, albeit mitigated somewhat by the effect of electrostatic fringing fields.
[0056] FIGS. 4a to 4e each show a cross-sectional view through various membrane structures comprising a flexible membrane 101 and a first electrode.
[0057] FIG. 4a shows a cross section through the line A-A of the membrane structure shown in FIG. 3. The metal forming the electrode 102 can be seen to be directly in contact with the flexible membrane layer 101. Thus any stress in the electrode is directly coupled into the membrane, and the electrode can substantially only attempt to relax its stress by affecting the membrane stress.
[0058] FIG. 4b shows a cross-sectional view of a membrane structure for a MEMS transducer according to a first embodiment of the present invention. The membrane structure comprises a first electrode layer 102 comprising a sheet electrode which is supported, by means of a support structure comprising a plurality of support elements 114, in a spaced relationship from the membrane layer 101. A volume of air 115 extends between the electrode layer and the flexible membrane. The support elements 114 may comprise a plurality of vertical spacers, or isolated mounts, which allow the first electrode layer to be suspended above the membrane layer. The membrane structure may be provided with such support elements provided around some or all of the periphery of the membrane, near to parts of the periphery at which the membrane is anchored by means not illustrated in this figure to the surrounding silicon substrate. The elements serve to establish a spacing of the electrode layer in the vertical direction—i.e. orthogonally to the plane of the membrane—without significantly coupling the electrode layer in the plane of the membrane. The support elements may be formed of conductive or dielectric material. Alternatively both the membrane and the electrode may be anchored independently, for instance at the sidewall of the backplate, with the electrode anchored at a higher part of the sidewall than the electrode such that the sidewall provides the support structure as illustrated in Figure X, in the structure shown on the left. Thus in some embodiments the support structure may comprise a continuous structure, for example a continuous ring around a circular periphery.
[0059] The volume of air substantially trapped between the electrode and the membrane may be substantially trapped, except maybe for bleed holes in the membrane structure or electrode layers, or with limited access to other volumes of the structure via the periphery of the membrane. The volume of air may thus serve as an air cushion to couple any acoustically stimulated movement of the membrane to the electrode.
[0060] Thus, according to this embodiment, it will be appreciated that the electrode layer is physically separated from the membrane (although it will also be appreciated that the electrode layer is indirectly connected to the membrane layer via the support elements 114). As such, since the electrode layer is not in direct contact with the membrane layer except at or near the periphery, the mechanical influence of the electrode layer 102 on the membrane layer is reduced and the membrane structure may advantageously demonstrate an improvement in time-dependent warping of the membrane. Moreover, in comparison to the membrane structure shown in FIGS. 3 and 4a, the area of the metal electrode is increased and, thus, the transducer sensitivity is enhanced.
[0061] FIG. 4c shows a cross-sectional view of a membrane structure for a MEMS transducer according to a second embodiment of the present invention. The membrane structure comprises a first electrode layer 102 and a support structure 114. The first electrode layer 102 comprises a continuous sheet of electrode material. The support structure 114 comprises a patterned layer of conductive material—similar to the single electrode layer shown in FIG. 4a—comprising a plurality of openings. The openings define a plurality of air volumes 115—which extend between the first electrode layer 102 and the membrane layer 101. In this example the material of the first electrode layer, the metal elements of the lattice structure forming the support structure 114 and the membrane effectively form a plurality of closed air volumes within the membrane structure. Thus, as a consequence of the electrode layer not being connected to the membrane layer at each of the air volumes, a significant proportion of the electrode material of the first electrode layer is mechanically separate from the membrane layer, thereby allowing at least some stress release by local vertical thinning of the electrode material. Although illustrated as approximately equal, preferably the support structure is significantly less wide than the air volume width, to allow increased stress relaxation.
[0062] FIG. 4d shows a cross sectional view of a membrane structure for a MEMS transducer according to a third embodiment of the present invention. The first electrode layer 102 comprises a layer of electrode material that is patterned to comprise a plurality of openings 116 within the plane of the first electrode layer. Although not apparent from this cross-sectional view, the electrode layer 102 is formed in a lattice shape—e.g. by a process of patterning or etching of the electrode material. The lattice shaped electrode can be considered to comprise a plurality of interconnected conductive elements. The first electrode layer 102 is supported so as to be spaced from the membrane layer by means of one or more support elements (not shown) which may be formed of the same material as the electrode or of an insulating material.
[0063] The first electrode layer 102 can be seen to define a plurality of interconnected electrode elements which extend in the line of the cross sectional view. Each of the electrode elements can be considered to form a conductive bridge with the space underneath the bridge defining an air volume 115 as indicated by the dashed line. Each arm or bridge of the lattice is free to expand or contract in width or height proving degrees of freedom to allow any metal stress to be substantially accommodated leaving only relatively little residual stress requiring to be released via transmission to the membrane via the support elements.
[0064] FIG. 4e shows a cross sectional view of a membrane structure for a MEMS transducer according to a fourth embodiment of the present invention. In this example the first electrode layer 102 again comprises a layer of electrode material that is patterned to comprise a plurality of openings 116 within the plane of the first electrode layer. Again, as in FIG. 4d, the electrode layer 102 is formed in a lattice shape and can be considered to comprise a plurality of interconnected conductive elements. The electrode layer is supported in a spaced relation relative to the membrane layer by means of a support structure.
[0065] The support structure 114 is patterned to incorporate a plurality of openings. The electrode layers are fabricated such that the openings of the electrode layer are laterally offset from the openings of the support structure. This figure portrays an illustrative case in which the bridges and support structures are at least somewhat aligned along the chosen cross-sectional plane: for e.g. a hexagonal lattice there will be no such plane, but this figure illustrates the general concept of the bridges linked by a series of conductive support structures.
[0066] FIGS. 5a to 5e show perspective views of a membrane structure according to a fifth embodiment of the present invention. In this example the electrode layer 102 is patterned to incorporate a plurality of openings. Specifically, the electrode layer is spaced from the membrane layer by means of a support structure 114. The support structure 114 in this embodiment comprises the same conductive material that forms the electrode layer 102. The support structure 114 thus extends between the electrode layer 102 and the membrane 101.
[0067] As illustrated most clearly in the plan view of FIG. 5e, the support structure may comprise a first partial lattice structure comprising elements 182, and the electrode layer may comprise a second partial lattice structure comprising elements 180, laid out superimposed on a hexagonal grid pattern 184. The individual separate elements of the first partial lattice structure serve as conductive bridges between adjacent elements of the second partial lattice structure, which in turn provide conductive paths between the adjacent elements of the first partial lattice structure. Thus the first and second lattice structures cooperate to provide electrical connection to all desired elements of the electrode layer. Some or all elements at or near the periphery of the membrane may be coupled to readout circuitry, thus coupling the whole electrode structure to the readout circuitry.
[0068] The elements of the first partial lattice structure are isolated and small so accumulate little stress and are free to release this stress in several directions. The elements of the second partial lattice structure are isolated and small so accumulate little stress, so also have little effect on the membrane. In further variants, the size of the elements of the second partial lattice structure may be reduced further, for instance by spanning only a part of each edge of the polygon rather than the whole side, with the adjacent electrode element expanded to maintain connectivity.
[0069] In variants illustrated in perspective in FIGS. 5a and 5b, elements of the first partial lattice structure are extended laterally in a cantilever fashion to increase their area to increase the sensing capacitance.
[0070] FIGS. 5c and 5d are photographic illustrations of a membrane structure as illustrated in FIGS. 5a and 5b.
[0071] As illustrated in FIGS. 6a and 6b, which also further illustrate cantilever structures, a membrane structure embodying the present invention in which the electrode comprises material that is spaced from the flexible membrane layer and thus incorporates one or more air volumes which extend between the spaced material and the membrane layer, may be fabricated from further processing to the electrode structure shown in FIG. 3. Thus, the material that will form the first electrode layer can be deposited onto a patterned electrode layer as shown in FIG. 3 (which ultimately forms the support structure for the first electrode layer) so that the material of the first electrode layer extends partially or fully between opposite edges defining the openings of the support layer. As shown in FIGS. 6a and 6b the material of the first electrode layer 102 is therefore spaced from the membrane layer by an air volume and thus forms a cantilever or “partial cross-over” with respect to the plane of the support structure 114. As shown in FIGS. 6c and 6d, the material of the first electrode layer 114 forms a “full cross-over” with respect to the plane of the support structure 114. The resultant membrane structure will benefit from increased capacitance due to the increased conductive material, whilst the openings of the support structure provide an improvement in the stresses that arise between the electrode material and the membrane as comparted to previously considered designs incorporating a single electrode layer comprising a continuous sheet of metal in direct contact with the membrane.
[0072] According to one or more embodiments of the present invention, the mechanical decoupling of the electrode layer by the presence of one or more air volumes which extend between the first electrode layer and the membrane layer could be beneficially used to alleviate so-called diaphragm edge curl. In view of the problem of the membrane curling at the edge due to the stress mis-match that arises between the two layers, it is typical for the conductive metal electrode material to be deposited in region towards the centre of the membrane and not close to the edge of the membrane. However, this prior solution represents a reduction in the metal electrode area and thus undermines the sensitivity of the transducer. Embodiments of the present invention enable the electrode to extend over a greater proportion of the membrane including the region at or near the periphery of the membrane.
[0073] According to preferred embodiments which utilise a support structure comprising a lattice-like structure in conjunction with an electrode layer also comprising a lattice-like structure, it is proposed to vary the area of the openings provided in electrode layer and/or the area of the openings in the support structure so as to control or modulate membrane edge curl. Edge curl leads to air leakage around the membrane, i.e. an acoustic bypass path which particularly impairs low-frequency sensitivity and so controlling the edge curl allows for more effective control of the leakage and, thus, improved control of microphone low-frequency roll-off.
[0074] FIGS. 7a to 7g each show a cross-sectional view through various membrane structure designs which comprise a flexible membrane layer 101 and at least a first electrode 102 supported in a spaced relation from the membrane by means of a support structure 114.
[0075] FIG. 7a shows a membrane structure comprising a first electrode layer 102a and a support layer 114. Both the first electrode layer and the support layer are patterned to incorporate a plurality of openings. The openings formed in the support layer form a plurality of air volumes 115 which extend between the first electrode layer and the membrane layer. In this example the size of the openings across the membrane layer is substantially constant.
[0076] FIG. 7b shows a membrane structure comprising a first electrode layer 102a and a second electrode layer 102b. The second electrode layer 102b is formed on the opposite side of the flexible membrane layer 101 to the first electrode 102a. Each of the first and second electrode layers 102a, 102b is supported in fixed relation relative to the flexible membrane by a support layer 114a, 114b such that the material forming the respective electrode layer is supported in spaced relation from the membrane and is thus separated from the membrane layer by a plurality of air volumes 115 formed by openings 115 in the respective support layer. In this example the size of the openings 115 across the membrane layer is substantially constant.
[0077] A membrane structure according to FIG. 7b may be usefully employed in conjunction with a MEMS transducer which utilises first and second backplates (each incorporating a backplate electrode) which are respectively positioned above and below the membrane structure.
[0078] FIG. 7c shows a membrane structure that is similar to the membrane structure shown in FIG. 7b. However, in this example the air volumes 115 of the second electrode are laterally offset with respect to the openings of the first electrode. This arrangement can beneficially serve to mitigate the occurrence of stress concentrations arising within the membrane structure since the stress arising between the membrane and the first electrode may tend to cancel or mitigate the stress maxima and minima arising between the membrane and the second electrode. Furthermore, a membrane structure embodying this design may experience a rippling effect which therefore provides a degree of freedom to the membrane and, thus, alleviates stress. Again, in this example the size of the openings across the membrane layer is substantially constant.
[0079] FIG. 7d shows a membrane structure comprising a first electrode layer 102a and a support layer 114. Both the first electrode layer and the support layer are patterned to incorporate a plurality of openings. The openings formed in the support layer form a plurality of air volumes 115 which extend between the first electrode layer and the membrane layer. In this example the size of the openings in both the first electrode layer and the support layer vary from a region at the centre of the membrane towards an outer or peripheral region of the membrane. Thus, the openings which form the air volumes 115 similarly vary in size. Specifically, the area of the air volumes 115 formed by the openings in the support layer increase in size from a region towards the centre of the membrane to a region at or near the periphery of the membrane. This arrangement—specifically the increase in the size of the openings towards the periphery of the membrane structure—advantageously serves to mitigate membrane edge curl.
[0080] FIG. 7e shows a membrane structure comprising a first electrode layer 102a and a second electrode layer 102b. The second electrode layer 102b is formed on the opposite side of the flexible membrane layer 101 to the first electrode 102a. Each of the first and second electrode layers 102a, 102b is supported in fixed relation relative to the flexible membrane by a support layer 114a, 114b such that the material forming the respective electrode layer is supported in spaced relation from the membrane and is thus separated from the membrane layer by a plurality of air volumes 115 formed by openings 115 in the respective support layer. As in the example shown in FIG. 7d, the size of the openings in both the electrode layer and the support layer vary from a region at the centre of the membrane towards an outer or peripheral region of the membrane. Thus, the openings which form the air volumes 115 similarly vary in size. Specifically, the area of the air volumes 115 formed by the openings in the support layer increase in size from a region towards the centre of the membrane to a region at or near the periphery of the membrane. The increase in the size of the openings towards the periphery of the membrane structure advantageously serves to mitigate membrane edge curl since it serves to reduce local fractional metal area coverage to thereby reduce stress in the peripheral region. Furthermore, the trade off in terms of a reduced capacitance is less critical since the membrane experiences less displacement than the central region of the membrane structure.
[0081] A membrane structure according to FIG. 7e may be usefully employed in conjunction with a MEMS transducer which utilises first and second backplates (each incorporating a backplate electrode) which are respectively positioned above and below the membrane structure.
[0082] FIG. 7f shows a membrane structure comprising an electrode layer 102 and a support layer 114. According to this example, the electrode layer 102 does not extend over a central region of the flexible membrane layer. Thus, within the central region of the membrane structure, the first electrode is formed only of a single support layer which is provided so as to directly overlay the membrane layer 101. Furthermore, the size of the openings in support layer 114 increase from a region at or near the boundary of the central region to a region at or near the periphery of the membrane in a manner similar to the embodiment shown in FIG. 7e. In the region laterally outside the central region of the membrane—i.e. the region where the electrode layer is spaced above the membrane layer by means of the underlying support layer 114, the openings in the support layer form air volumes 115 which extend between the electrode layer and the membrane.
[0083] Transducers incorporating membrane structures embodying the FIG. 7f example are advantageous in that the electrode layer 102 need be provided on just a fraction of the membrane structure—e.g. on around 10% of the area of the membrane—in the peripheral region of the membrane. Thus, the presence of decoupling air-bridges, or air volumes, are provided at the edge of the membrane structure which may beneficially mitigate membrane edge-curl.
[0084] FIG. 7g shows an example similar to the FIG. 7f example however, the membrane structure comprising a first electrode layer 102a and a second electrode layer 102b. The second electrode layer 102b is formed on the opposite side of the flexible membrane layer 101 to the first electrode 102a.
[0085] FIGS. 8a to 8h illustrate the steps involved in a possible method of fabricating a membrane structure according to one embodiment of the present invention.
[0086] As shown in FIG. 8a, the microfabrication process starts with the deposition of silicon nitride (Si.sub.3N.sub.4) onto a planar silicon substrate wafer using known techniques such as a PECVD (plasma enhanced chemical vapour deposition) method.
[0087] In FIG. 8b, a sacrificial resist layer is deposited on top of the silicon nitride and this is then patterned (exposure and development) as shown in FIG. 8c. A metal electrode is deposited by conformal coating using e.g. a sputtering technique as shown in FIG. 8d. Then, a second layer of resist is deposited on top of the metal layer—as shown in FIG. 8e—which is then patterned as shown in FIG. 8f. Reactive ion etch is applied to create metal perforation as shown in FIG. 8g. Both resist layers are stripped in the last fabrication step, shown in FIG. 8h to create a layer of metal forming the electrode layer with air volumes underneath in spaces previously filed by the sacrificial resist layer, and the support layer comprising the sidewalls of the conformal metal coating as well as the metal portions directly contacting the substrate. Portions of the underlying substrate may be etched from below in a later step of the process to release the nitride membrane layer. The support metal may extend laterally to electrically connect the electrode structure to associated bias or amplifier circuitry which may either be co-integrated on the same substrate or may be integrated on a separate silicon substrate and coupled via bond pads or contact pads.
[0088] A MEMS transducer according to the embodiments described here may comprise a capacitive sensor, for example a microphone.
[0089] A MEMS transducer according to the embodiments described here may further comprise readout circuitry, for example wherein the readout circuitry may comprise analogue and/or digital circuitry such as a low-noise amplifier, voltage reference and charge pump for providing higher-voltage bias, analogue-to-digital conversion or output digital interface or more complex analogue or digital signal processing. There may thus be provided an integrated circuit comprising a MEMS transducer as described in any of the embodiments herein.
[0090] One or more MEMS transducers according to the embodiments described here may be located within a package. This package may have one or more sound ports. A MEMS transducer according to the embodiments described here may be located within a package together with a separate integrated circuit comprising readout circuitry which may comprise analogue and/or digital circuitry such as a low-noise amplifier, voltage reference and charge pump for providing higher-voltage bias, analogue-to-digital conversion or output digital interface or more complex analogue or digital signal processing.
[0091] A MEMS transducer according to the embodiments described here may be located within a package having a sound port.
[0092] According to another aspect, there is provided an electronic device comprising a MEMS transducer according to any of the embodiments described herein. An electronic device may comprise, for example, at least one of: a portable device; a battery powered device; an audio device; a computing device; a communications device; a personal media player; a mobile telephone; a games device; and a voice controlled device.
[0093] According to another aspect, there is provided a method of fabricating a MEMS transducer as described in any of the embodiments herein.
[0094] Although the various embodiments describe a MEMS capacitive microphone, the invention is also applicable to any form of MEMS transducers other than microphones, for example pressure sensors or ultrasonic transmitters/receivers.
[0095] Embodiments of the invention may be usefully implemented in a range of different material systems, however the embodiments described herein are particularly advantageous for MEMS transducers having membrane layers comprising silicon nitride.
[0096] In the embodiments described above it is noted that references to a transducer element may comprise various forms of transducer element. For example, a transducer element may comprise a single membrane and back-plate combination. In another example a transducer element comprises a plurality of individual transducers, for example multiple membrane/back-plate combinations. The individual transducers of a transducer element may be similar, or configured differently such that they respond to acoustic signals differently, e.g. the elements may have different sensitivities. A transducer element may also comprises different individual transducers positioned to receive acoustic signals from different acoustic channels.
[0097] It is noted that in the embodiments described herein a transducer element may comprise, for example, a microphone device comprising one or more membranes with electrodes for read-out/drive deposited on the membranes and/or a substrate or back-plate. In the case of MEMS pressure sensors and microphones, the electrical output signal may be obtained by measuring a signal related to the capacitance between the electrodes. However, it is noted that the embodiments are also intended to embrace the output signal being derived by monitoring piezo-resistive or piezo-electric elements or indeed a light source. The embodiments are also intended embrace a transducer element being a capacitive output transducer, wherein a membrane is moved by electrostatic forces generated by varying a potential difference applied across the electrodes, including examples of output transducers where piezo-electric elements are manufactured using MEMS techniques and stimulated to cause motion in flexible members.
[0098] It is noted that the embodiments described above may be used in a range of devices, including, but not limited to: analogue microphones, digital microphones, pressure sensor or ultrasonic transducers. The invention may also be used in a number of applications, including, but not limited to, consumer applications, medical applications, industrial applications and automotive applications. For example, typical consumer applications include portable audio players, wearable devices, laptops, mobile phones, PDAs and personal computers. Embodiments may also be used in voice activated or voice controlled devices. Typical medical applications include hearing aids. Typical industrial applications include active noise cancellation. Typical automotive applications include hands-free sets, acoustic crash sensors and active noise cancellation.
[0099] It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim, “a” or “an” does not exclude a plurality, and a single feature or other unit may fulfill the functions of several units recited in the claims. Any reference signs in the claims shall not be construed so as to limit their scope.
