U.S. patent application number 15/730123 was filed with the patent office on 2018-02-01 for mems device and process.
This patent application is currently assigned to Cirrus Logic International Semiconductor Ltd.. The applicant listed for this patent is Cirrus Logic International Semiconductor Ltd.. Invention is credited to Tsjerk HOEKSTRA, Colin Robert JENKINS.
Application Number | 20180035190 15/730123 |
Document ID | / |
Family ID | 47190496 |
Filed Date | 2018-02-01 |
United States Patent
Application |
20180035190 |
Kind Code |
A1 |
HOEKSTRA; Tsjerk ; et
al. |
February 1, 2018 |
MEMS DEVICE AND PROCESS
Abstract
This application relates to MEMS devices, especially MEMS
capacitive transducers and to processes for forming such MEMS
transducer that provide increased robustness and resilience to
acoustic shock. The application describes a MEMS transducer (400)
having at least one membrane layer (101) supported so as to define
a flexible membrane. A strengthening layer (401; 701) is
mechanically coupled to the membrane layer and is disposed around
the majority of a peripheral area of the flexible membrane but does
not extend over the whole flexible membrane. The strengthening
layer, which in some embodiments may be formed from the same
material as the membrane electrode (102) being disposed in the
peripheral area helps reduce stress in membrane at locations that
otherwise may be highly stressed in acoustic shock situations. The
membrane may be supported over a substrate cavity and the
strengthening layer may be provided in an area of the membrane that
could make contact with the edge (202) of the substrate cavity.
Inventors: |
HOEKSTRA; Tsjerk; (Balerno,
GB) ; JENKINS; Colin Robert; (Livingston,
GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Cirrus Logic International Semiconductor Ltd. |
Edinburgh |
|
GB |
|
|
Assignee: |
Cirrus Logic International
Semiconductor Ltd.
Edinburgh
GB
|
Family ID: |
47190496 |
Appl. No.: |
15/730123 |
Filed: |
October 11, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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14430438 |
Mar 23, 2015 |
9820025 |
|
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PCT/GB2013/052459 |
Sep 19, 2013 |
|
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15730123 |
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61704876 |
Sep 24, 2012 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B81B 3/0021 20130101;
H04R 1/08 20130101; H04R 2201/003 20130101; B81C 1/00158 20130101;
H04R 19/005 20130101; B81B 2201/0257 20130101 |
International
Class: |
H04R 1/08 20060101
H04R001/08; B81B 3/00 20060101 B81B003/00; H04R 19/00 20060101
H04R019/00; B81C 1/00 20060101 B81C001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 24, 2012 |
GB |
1217010.6 |
Claims
1.-48. (canceled)
49. A MEMS transducer comprising: a substrate having a cavity; a
membrane layer comprising a flexible membrane suspended relative to
the substrate; and a strengthening layer disposed on said membrane
layer in a region of contact at which the flexible membrane will
contact an edge of the substrate cavity if sufficiently
deflected.
50. A MEMS transducer as claimed in claim 49, wherein the
strengthening layer does not extend over the whole of the flexible
membrane.
51. A MEMS transducer as claimed in claim 49, wherein the
strengthening layer extends radially inward from the region of
contact towards the centre of the flexible membrane.
52. A MEMS transducer as claimed in claim 49, wherein the
strengthening layer extends radially outward from the region of
contact away from the centre of the flexible membrane.
53. A MEMS transducer as claimed in claim 49, wherein the
strengthening layer extends both radially inward and radially
outward about the region of contact but does not extend over the
whole of the flexible membrane.
54. A MEMS transducer as claimed claim 49, wherein the
strengthening layer is disposed around the majority of a peripheral
area of the flexible membrane including the region of contact and
does not extend over the whole flexible membrane.
55. A MEMS transducer as claimed in claim 54, wherein the perimeter
of the opening of the substrate cavity comprises the region of
contact.
56. A MEMS transducer as claimed in claim 49, wherein the substrate
cavity defines an opening over which the flexible membrane is
suspended.
57. A MEMS transducer as claimed in claim 49 wherein the
strengthening layer extends radially from a support structure
supporting the membrane layer onto a flexible part of the membrane
layer.
58. A MEMS transducer as claimed in claim 57 wherein the
strengthening layer forms part of a side wall of the support
structure.
59. A MEMS transducer as claimed in claim 49 wherein the
strengthening layer comprises a layer of material which has at
least one of: a greater elasticity; a greater plasticity and a
greater ductility than the material of the membrane layer.
60. A MEMS transducer as claimed in claim 49 wherein the
strengthening layer comprises a layer of material which is in
greater tensile stress than the material of the membrane layer.
61. A MEMS transducer as claimed in claim 49 wherein the
strengthening layer comprises a layer comprising at least one of
the group of: titanium, aluminium, copper and gold or an alloy
thereof.
62. A MEMS transducer as claimed in claim 49 wherein the
strengthening layer comprises a plurality of layers of different
materials having different materials characteristics.
63. A MEMS transducer as claimed in claim 49 wherein a first
strengthening layer is coupled to the membrane layer in said
peripheral area and the transducer further comprises a second
strengthening layer having a different geometry on the membrane
layer to the first strengthening layer.
64. A MEMS transducer as claimed in claim 49 wherein the membrane
layer comprises a membrane electrode and wherein the strengthening
layer is formed from the same material as the membrane electrode
and wherein the material forming the membrane electrode is discrete
from the strengthening layer disposed in the peripheral area of the
membrane.
65. A MEMS transducer as claimed in claim 49 comprising a first
area of first material disposed so as to form a membrane electrode
and a second area of first material disposed to form said
strengthening layer wherein said second area substantially
surrounds said first area wherein between the first and second
areas there is a third area which is mainly devoid of any first
material.
66. A MEMS transducer as claimed in claim 49 wherein the membrane
layer is supported above a substrate and the strengthening layer is
disposed on the opposite side of the membrane layer to the
substrate.
67. A MEMS transducer as claimed in claim 49 wherein the
strengthening layer extends from a peripheral area of the flexible
membrane to at least 50 .mu.m past the point of contact.
68. A MEMS transducer as claimed in claim 49 wherein said
transducer comprises a microphone.
69. An electronic device comprising a MEMS transducer as claimed in
claim 49 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.
70. A MEMS transducer comprising: a flexible membrane suspended
over a first surface of a substrate, the substrate having one or
more substrate cavities, the one or more substrate cavities
defining an opening area in the first surface; wherein a peripheral
area of the suspended membrane carries a strengthening layer, the
strengthening layer overlying the perimeter of the opening area;
and a non-peripheral area of the surface of the membrane is devoid
of the material of the strengthening layer.
71. A MEMS transducer as claimed in claim 71, wherein, when
considering a cross-section taken across a width of the membrane,
the strengthening layer is provided at a point of contact at which
the which the flexible membrane will contact an edge of the
substrate cavity if sufficiently deflected.
Description
REFERENCE TO PREVIOUSLY FILED APPLICATIONS
[0001] This application is a continuation of U.S. Non-provisional
patent application Ser. No. 14/430438, filed Mar. 23, 2015, which
is a 371 application of International Application No.
PCT/GB2013/052459, filed Sep. 19, 2013, which claims priority to
U.S. Provisional Patent Application Ser. No. 61/704876, filed Sep.
24, 2012, and United Kingdom Patent Application No. 1217010.6,
filed Sep. 24, 2012, all of which are incorporated by reference
herein in their entirety.
FIELD OF DISCLOSURE
[0002] This invention relates to a micro-electro-mechanical system
(MEMS) device and process, and in particular to a MEMS device and
process relating to a transducer, for example a capacitive
microphone.
BACKGROUND
[0003] Various MEMS devices are becoming increasingly popular. MEMS
transducers, and especially MEMS capacitive microphones, are
increasingly being used in portable electronic devices such as
mobile telephones and portable computing devices.
[0004] Microphone devices formed using MEMS fabrication processes
typically comprise one or more membranes with electrodes for
read-out/drive deposited on the membranes and/or a substrate. In
the case of MEMS pressure sensors and microphones, the read out is
usually accomplished by measuring the capacitance between the
electrodes. In the case of output transducers, the membrane is
moved by electrostatic forces generated by varying a potential
difference applied across the electrodes.
[0005] FIGS. 1a and 1b show a cross section and a cut-away
perspective view, respectively, of known capacitive MEMS microphone
devices 100. The capacitive microphone device 100 comprises a
membrane layer 101 which forms a flexible membrane which is free to
move in response to pressure differences generated by sound waves.
A first electrode 102 is mechanically coupled to the flexible
membrane, and together they form a first capacitive plate of the
capacitive microphone device. A second electrode 103 is
mechanically coupled to a generally rigid structural layer or
back-plate 104, which together form a second capacitive plate of
the capacitive microphone device. In the example shown in Figure la
the second electrode 103 is embedded within the back-plate
structure 104.
[0006] The capacitive microphone is formed on a substrate 105, for
example a silicon wafer which may have upper and lower oxide layers
106, 107 formed thereon. A cavity 108 in the substrate and in any
overlying layers (hereinafter referred to as a substrate cavity) is
provided below the membrane, and may be formed using a "back-etch"
through the substrate 105. The substrate cavity 108 connects to a
first cavity 109 located directly below the membrane. These
cavities 108 and 109 may collectively provide an acoustic volume
thus allowing movement of the membrane in response to an acoustic
stimulus. Interposed between the first and second electrodes 102
and 103 is a second cavity 110.
[0007] The first cavity 109 may be formed using a first sacrificial
layer during the fabrication process, i.e. using a material to
define the first cavity which can subsequently be removed, and
depositing the membrane layer 101 over the first sacrificial
material. Formation of the first cavity 109 using a sacrificial
layer means that subsequent etching of the substrate cavity 108
does not play any part in defining the diameter of the membrane.
Instead, the diameter of the membrane is defined by the diameter of
the first cavity 109 (which in turn is defined by the diameter of
the first sacrificial layer) in combination with the diameter of
the second cavity 110 (which in turn may be defined by the diameter
of a second sacrificial layer). The diameter of the first cavity
109 formed using the first sacrificial layer can be controlled more
accurately than the diameter of a back-etch process performed using
a wet-etch or a dry-etch. Etching the substrate cavity 108 will
therefore define an opening in the surface of the substrate
underlying the membrane 101.
[0008] A plurality of holes, hereinafter referred to as bleed holes
111, connect the first cavity 109 and the second cavity 110.
[0009] As mentioned the membrane may be formed by depositing at
least one membrane layer 101 over a first sacrificial material. In
this way the material of the membrane layer(s) may extend into the
supporting structure, i.e. the side walls, supporting the membrane.
The membrane and back-plate layer may be formed from substantially
the same material as one another, for instance both the membrane
and back-plate may be formed by depositing silicon nitride layers.
The membrane layer may be dimensioned to have the required
flexibility whereas the back-plate may be deposited to be a thicker
and therefore more rigid structure. Additionally various other
material layers could be used in forming the back-plate 104 to
control the properties thereof. The use of a silicon nitride
material system is advantageous in many ways, although other
materials may be used, for instance MEMS transducers using
polysilicon membranes are known.
[0010] In some applications, the microphone may be arranged in use
such that incident sound is received via the back-plate. In such
embodiments a further plurality of holes, hereinafter referred to
as acoustic holes 112, are arranged in the back-plate 104 so as to
allow free movement of air molecules, such that the sound waves can
enter the second cavity 110. The first and second cavities 109 and
110 in association with the substrate 108 allow the membrane 101 to
move in response to the sound waves entering via the acoustic holes
112 in the back-plate 104. In such instances the substrate cavity
108 is conventionally termed a "back volume", and it may be
substantially sealed.
[0011] In other applications, the microphone may be arranged so
that sound may be received via the substrate cavity 108 in use. In
such applications the back-plate 104 is typically still provided
with a plurality of holes to allow air to freely move between the
second cavity and a further volume above the back-plate.
[0012] In other applications the microphone may be arranged such
that, some sound components may be received via the substrate
cavity 108 and other sound components may be received via the
back-plate 104.
[0013] In use, in response to a sound wave corresponding to a
pressure wave incident on the microphone, the membrane is deformed
slightly from its equilibrium position. The distance between the
lower electrode 102 and the upper electrode 103 is correspondingly
altered, giving rise to a change in capacitance between the two
electrodes that is subsequently detected by electronic circuitry
(not shown). The bleed holes allow the pressure in the first and
second cavities to equalise over a relatively long timescales (in
acoustic frequency terms) which reduces the effect of low frequency
pressure variations, e.g. arising from temperature variations and
the like, but without impacting on sensitivity at the desired
acoustic frequencies.
[0014] The transducer shown in FIG. 1 is illustrated with
substantially vertical side walls supporting the membrane layer 101
in spaced relation from the back-plate 104. Given the nature of the
deposition process this can lead to a high stress concentration at
the corners formed in the material layer that forms the membrane.
Sloped or slanted side walls may be used to reduce the stress
concentration. Additionally or alternatively it is known to include
a number of support structures such as columns to help support the
membrane in a way which reduces stress concentration as illustrated
in FIGS. 2a, 2b and 2c. FIG. 2a shows a transducer 200 in plan
view, but omitting the uppermost part of the back-plate layer 104
for clarity. FIGS. 2b and 2c illustrate the periphery of a MEMS
microphone structure (along the line A-A' shown in FIG. 2a) in
perspective and cross sectional views respectively, where similar
components are identified by the same numerals as used in FIG.
1.
[0015] In this example the MEMS device 200 is formed with a
plurality of support structures 201, which in this example are
formed as supporting columns, arranged around the periphery of the
membrane. The columns are formed by patterning the first
sacrificial material used to define the first cavity 109 such that
the substrate 105 is exposed in a number of areas before depositing
the material forming the membrane layer 101 (FIG. 2b shows one
membrane layer being deposited directly on the substrate but it
will be appreciated that that there may be various intermediate
layers on the substrate and the membrane may be formed by
depositing multiple membrane layers). Likewise the second
sacrificial material used to define the second cavity 110 is
patterned so that membrane layer 101 is exposed in the same areas
prior to depositing the material of the back-plate layer. This
results in a plurality of columns being formed around the periphery
of the membrane, as shown in FIG. 2a, which provide support to the
membrane but with a reduced stress concentration compared to the
arrangement shown in FIG. 1. The columns are preferably formed with
a stepped profile and/or slanted side walls to minimise stress.
This process can lead to dimples in the upper surface of the
back-plate layer in the area of the columns.
[0016] FIG. 2a also shows that a conductive track 204 extends from
the membrane electrode 102 across the membrane to a contact point
outside the suspended portion of the membrane for electrical
connection to a buffer amplifier or other circuitry either
co-integrated on the same substrate or via pads and wire-bond or
flip-chip connections to circuitry on other substrates in the same
or another package.
[0017] MEMS transducers such as those shown in FIGS. 1 and 2 may
usefully be used in a range of devices, including portable devices.
Especially when used for portable devices it is desirable that the
MEMS transducers are sufficiently rugged to survive expected
handling and use of the device. There is therefore a general desire
to improve the resilience of MEMS devices.
SUMMARY
[0018] The present invention is therefore concerned with improving
the robustness and/or resilience of MEMS devices.
[0019] Thus according to an aspect of the present invention there
is provided a MEMS transducer comprising: [0020] at least one
membrane layer supported so as to define a flexible membrane; and
[0021] a strengthening layer mechanically coupled to said membrane
layer; [0022] wherein the strengthening layer is disposed around
the majority of a peripheral area of the flexible membrane but does
not extend over the whole flexible membrane.
[0023] The strengthening layer may extend radially from a support
structure supporting the membrane layer onto a flexible part of the
membrane layer and may form part of a side wall of the support
structure. The support structure may comprise a supporting column
structure.
[0024] The membrane layer may be supported relative to a first
surface of a substrate, the substrate having at least one substrate
cavity therein defining an area of opening in the first surface of
the substrate, wherein the perimeter of said area of opening is at
least partly within an area underlying said flexible membrane. In
such case the strengthening layer may be disposed such that the
perimeter of the area of opening at least partly underlies the
peripheral area of flexible membrane having the strengthening
layer. The strengthening layer may be disposed on the membrane
layer in an area of the membrane which may make contact with the
perimeter of the opening area if sufficiently deflected.
[0025] The strengthening layer may be continuous in a radial
direction from within a support structure supporting the membrane
layer to a point inward on the flexible membrane beyond the
perimeter of said opening area.
[0026] The strengthening layer may be disposed around 75% or more
of the perimeter of the membrane or around substantially the entire
perimeter of the membrane.
[0027] Radially inside the strengthening layer may be a
circumferential area of flexible membrane, the majority of which is
free of the material of the strengthening layer. Around a majority
of the perimeter of the flexible membrane, there may be a region
radially inside the strengthening layer which is free of the
material of the strengthening layer.
[0028] The strengthening layer may comprises a layer of material
which has a greater elasticity, a greater plasticity and/or a
greater ductility than the material of the membrane layer and/or
which is in greater tensile stress than the material of the
membrane layer.
[0029] The strengthening layer may comprise a layer comprising at
least one of the group of: titanium, aluminium, copper and gold or
an alloy thereof.
[0030] The strengthening layer may comprise a plurality of layers
of different materials having different materials characteristics.
Said material characteristics may comprise at least one of:
elasticity, plasticity, ductility, tensile stress, yield stress and
Young's modulus.
[0031] A first strengthening layer may be coupled to the membrane
layer in said peripheral area and the transducer further comprises
a second strengthening layer having a different geometry on the
membrane layer to the first strengthening layer.
[0032] The thickness of the strengthening layer may be between
about 30 and 100 nm inclusive.
[0033] The membrane structure may comprise a membrane electrode and
the strengthening layer may be formed from the same material as the
membrane electrode. The material forming the membrane electrode may
be discrete from the strengthening layer disposed in the peripheral
area of the membrane. A first area of first material may be
disposed so as to form the membrane electrode and a second area of
first material disposed to form said strengthening layer wherein
said second area substantially surrounds said first area. Between
the first and second areas there may be a third area which is
mainly devoid of any first material.
[0034] The strengthening layer may be disposed on the opposite side
of the membrane layer to a substrate. The transducer may also
comprise a back-plate structure wherein the membrane layer is
supported to be flexible with respect to said back-plate
structure.
[0035] The transducer may be a capacitive sensor such as a
microphone. The transducer may comprise readout circuitry. The
transducer may be located within a package having a sound port. The
transducer may be implemented in an electronic device which may be
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.
[0036] In a further aspect the invention provides a method of
fabricating a MEMS transducer having a flexible membrane, the
method comprising:
[0037] forming a membrane layer to form at least part of said
flexible membrane; and
[0038] forming a strengthening layer mechanically coupled to an
area of said membrane layer that corresponds to a peripheral area
of the flexible membrane;
[0039] wherein the strengthening layer does not extend over the
whole of the flexible membrane.
[0040] The method may be used to form transducer according to any
of the embodiments discussed above. In particular the strengthening
layer may extend radially from an area on the membrane layer that
corresponds to the membrane support onto an area that corresponds
to the flexible part of the membrane. The method may involve
forming at least one side wall structure to support the flexible
membrane wherein part of the strengthening layer forms part of the
at least one side wall structure. The method may involve forming
the membrane layer so as to be suspended over a first surface of a
substrate and forming at least one cavity in the substrate to
define an area of opening in the first surface of the substrate,
wherein the perimeter of the opening area underlies the area of
strengthening layer.
[0041] Forming the strengthening layer may comprise depositing a
layer of first material onto the membrane layer and patterning the
layer of first material to form the strengthening layer. The method
may also involve forming a patterned layer of first sacrificial
material on a substrate; forming the membrane layer over the first
sacrificial material and the substrate; and forming the
strengthening layer on the membrane layer in an area corresponding
to the peripheral area of the first sacrificial material and area
where the membrane layer contacts the substrate.
[0042] The method may involve forming a membrane electrode
mechanically coupled to the membrane layer, wherein the material of
the strengthening layer is the same material as the membrane
electrode. Forming the strengthening layer may involve forming a
layer of first material on the membrane layer and patterning the
layer of first material to form the membrane electrode and the
strengthening layer.
[0043] In a further aspect the invention provides a MEMS transducer
comprising:
[0044] a flexible membrane suspended over a first surface of a
substrate,
[0045] the substrate having one or more substrate cavities, the one
or more substrate cavities defining an opening area in the first
surface;
[0046] wherein a peripheral area of the suspended membrane carries
a strengthening layer, the strengthening layer overlying the
perimeter of the opening area; and
[0047] a non-peripheral area of the surface of the membrane is
devoid of the material of the strengthening layer.
[0048] In a yet further aspect there is provided a MEMS transducer
comprising:
[0049] a membrane structure comprising:
[0050] a flexible membrane;
[0051] a first area of first material disposed so as to form a
membrane electrode; and
[0052] a second area of first material disposed to form a
strengthening layer for said flexible membrane;
[0053] wherein the second area substantially surrounds the first
area and between the first and second areas there is a third area,
the majority of which is devoid of any first material.
BRIEF DESCRIPTION OF THE DRAWINGS
[0054] The invention will now be described by way of example only,
with reference to the accompanying drawings, of which:
[0055] FIGS. 1a and 1b illustrate known capacitive MEMS transducers
in section and cut-away perspective views;
[0056] FIGS. 2a, 2b and 2c illustrate plan, sectional and
perspective views of another known capacitive MEMS transducer;
[0057] FIGS. 3a and 3b illustrate how a high pressure event may
affect the membrane;
[0058] FIGS. 4a and 4b illustrate an embodiment of a MEMS
transducer according to the present invention;
[0059] FIG. 5 illustrates the effect of a high pressure impulse on
an embodiment of the present invention;
[0060] FIG. 6 illustrates a cross section of further embodiment of
the present invention;
[0061] FIG. 7 illustrates a plan view of a yet further aspect of
the invention; and
[0062] FIGS. 8a-f illustrate one suitable process for forming a
MEMS transducer according to an embodiment of the invention.
DETAILED DESCRIPTION
[0063] As described above MEMS transducers such as shown in FIGS. 1
and 2 may be usefully employed in a variety of different devices
and increasingly are becoming popular for use in portable
electronic devices such as mobile telephones, mobile computing
devices and/or personal media players and the like.
[0064] To be useful for use in portable electronic devices such
transducers should be able to survive the expected handling and use
of the portable device, which may include the device being
accidentally dropped.
[0065] If a device such as a mobile telephone is subject to a fall,
this can result not only in a mechanical shock due to impact but
also a high pressure impulse incident on a MEMS transducer. For
example, a mobile telephone may have a sound port for a MEMS
microphone on one face of the device. If the device falls onto that
face, some air may be compressed by the falling device and forced
into the sound port. This may result in a high pressure impulse
incident on the transducer. It has been found that in conventional
MEMS transducers of the form described above high pressure impulses
can potentially lead to damage of the transducer.
[0066] Referring again FIGS. 2a-2c, as previously described, a MEMS
transducer 200 may have a membrane layer 101 and a back-plate layer
104 formed so that a membrane is suspended above a surface of a
substrate 105 to define a first cavity 109 and the back-plate 104
is suspended above the membrane to form a second cavity 110. Note
as used herein the term substrate will be used to refer to the one
or more layers of material above which the membrane is suspended.
This may typically comprise a silicon wafer and may also include
one or more deposited layers, possibly including layers of the same
material used to form the membrane layer.
[0067] As mentioned above a sacrificial material may be used to
define the dimensions of the first cavity and hence the dimensions
of the membrane. As discussed sacrificial material may be deposited
and patterned relatively accurately to provide good control over
the membrane dimensions. A substrate cavity is also provided in the
substrate 105, typically by means of a back etch. To ensure that it
is the dimensions of the first cavity 109 which determine the
membrane dimensions, the substrate cavity is arranged to have a
smaller diameter than the first cavity at the point 202 where the
substrate cavity and first cavity meet, in other words the opening
of the substrate cavity at the surface of the substrate has a
smaller diameter than the first cavity. This means that in such a
structure the membrane is suspended above a section of the
substrate, indicated by arrow 203, before reaching the opening of
the substrate cavity, i.e. the opening of the substrate cavity 108
in the surface of the substrate is within the area of the flexible
membrane.
[0068] The sacrificial material used to define the first and second
cavities is dimensioned so as to provide a desired equilibrium
separation between the membrane layer 101 and the substrate 105 and
also between the membrane layer 101 and the back-plate 104 so as to
provide good sensitivity and dynamic range in use. In normal
operation the membrane may deform within the volume defined by the
first and second cavities without contacting the back-plate and/or
substrate 105.
[0069] In response to a high pressure impulse however the membrane
layer 101 may exhibit a greater amount of deformation than usual.
FIG. 3a illustrates the situation where the membrane has been
deformed downwards following a high pressure event and FIG. 3b
shows the situation where the membrane has been displaced
upwards.
[0070] Consider the situation where the microphone is arranged to
receive incident sound from a sound port arranged above the
back-plate 104 and the sound port pressure suddenly increases, for
instance as a result of air trapped when the device falls being
forced into the sound port. This may result the pressure in the
second cavity 110 being significantly greater than the pressure in
the first cavity 109, displacing the membrane downwards to greater
extent than is usual. This may result in a relatively large stress
at point 301 where membrane layer 101 forms part of the sidewall of
supporting structure 201 and, in some instances, delamination of
the membrane layer from the rest of the sidewall structure.
Further, if the pressure difference is great enough the membrane
may make contact with the substrate 105 at the edge of the
substrate defined by the side wall 202 of the opening of substrate
cavity 108. Typically the edge of the substrate at the location of
the opening of the substrate cavity has a relatively sharp angle
and thus the membrane may be deformed round this edge, leading to a
large stress concentration at this point 302.
[0071] As mentioned previously the membrane layer 101 will
typically be formed from one or more thin layers of semiconductor
material, such as silicon nitride. Whilst such a material can be
flexible when subject to even stresses, if there is a significant
localised out-of-plane stress such as may be introduced into the
membrane at point 302 by contact with the edge of the opening of
the substrate cavity 108, the membrane material can be relatively
brittle. Thus contact between the membrane and the edge of the
opening of the substrate cavity in this way can lead to damage such
as cracking of the membrane.
[0072] As the high pressure caused by trapped air may persist for a
relatively long time, the pressure in the first and second cavities
may equalise by virtue of the bleed holes (not shown in FIG. 2 or
3) discussed above with relation to FIG. 1. Thus the pressure in
the first cavity, and substrate cavity, may increase until the
pressures are equalized. However once air is no longer being forced
into the sound port the pressure in the sound port will reduce
quite quickly and, as typically the back-plate has a low acoustic
impedance, the pressure in the second cavity will quickly reduce.
At this point the pressure in the first cavity may be significantly
greater than the pressure in the second cavity and thus the
membrane may be deformed upwards, again to a greater extent than
may usually be in the case. Again this may lead to a significant
stress in region 301 where the membrane layer 101 meets the
sidewall of the supporting structure. If the pressure difference is
large enough the membrane may be displaced far enough to contact
the back-plate 104. This may limit the amount of travel of the
membrane as compared with the situation shown in FIG. 3a but again
this may introduce stress into the membrane layer at the point 303
where it contacts the back-plate 104.
[0073] It should be appreciated that both of these situations can
also occur when sound is received via the substrate cavity 108 but
in the opposite order. Whilst both situations may lead to damage of
the membrane it is believed that the situation shown in FIG. 3a is
more likely to lead to damage.
[0074] To reduce the likelihood of damage in such high pressure
situations embodiments of the present invention provide MEMS
transducers having at least one membrane layer supported so as to
define a flexible membrane and a strengthening layer mechanically
coupled to the membrane layer. The strengthening layer is disposed
around the majority of a peripheral area of the flexible membrane
but does not extend over the whole flexible membrane.
[0075] The use of a strengthening layer mechanically coupled to
peripheral area of the membrane layer helps avoid damage to the
membrane. The strengthening layer will act to reinforce the
peripheral area of the membrane and can aid in stiffening the
membrane at this point. The strengthening layer also helps
distribute any stress in the membrane layer. The strengthening
layer can thus ensure that the peak stress experienced by the
membrane in the peripheral area for a given acoustic overload is
reduced and is thus less likely to exceed the yield stress of the
membrane in this area, with thus less likelihood of damage.
[0076] The strengthening layer may be coupled to the membrane layer
in any convenient arrangement, i.e. it may be disposed on top of,
or underneath, a membrane layer, or sandwiched between two membrane
layers. There may be a plurality of strengthening layers, some or
all disposed on top of each other in some areas of a membrane.
[0077] The strengthening layer is disposed around a peripheral area
of the membrane, i.e. towards the outer edge of the flexible
membrane. Thus the strengthening layer is provided in the vicinity
of at least some of the possible points of high stress identified
above, i.e. those points of the membrane surface which may
experience a high stress concentration when the membrane is subject
to a large deformation. To ensure that the strengthening layer
provides sufficient strengthening to the membrane the strengthening
layer is provided around a majority of the periphery of the
membrane. By majority is meant more than 50% of the perimeter of
the membrane.
[0078] The strengthening layer is not provided over the whole of
the membrane however, which thus means the strengthening layer does
not have any significant negative impact on the performance of the
transducer. For example disposing the strengthening layer over a
large portion of a microphone membrane might adversely affect its
flexibility, and hence reduce the acoustic sensitivity of the
microphone. Embodiments of the present invention thus provide
strengthening of the membrane in areas where it is advantageous,
i.e. locations on the membrane that may be subject to particularly
high stress in response to a large pressure difference acting on
the membrane.
[0079] FIGS. 4a and 4b illustrates plan and section views of one
embodiment of a MEMS transducer 400 according to an embodiment of
the invention. The transducer 400 has a similar structure to that
described above with reference to FIG. 2 and similar features are
identified using the same reference numerals.
[0080] In the embodiment shown in FIG. 4 there is an strengthening
layer 401 which is disposed on top of the membrane layer 101.
[0081] In this embodiment the strengthening layer is disposed on
top of the membrane layer 101 as a continuous layer around the
entire periphery of the membrane. The strengthening layer 401 is
disposed on the part of the membrane layer 101 which forms part of
the support columns 201 and extends inward on the flexible membrane
beyond the edge 202 of the substrate cavity--or more particularly
beyond the point at which the membrane layer 101 may make contact
with the edge 202 of the opening of the substrate cavity in a high
pressure situation. Thus the perimeter of the opening of the
substrate cavity in the substrate surface underlies the
strengthening layer. In other words the strengthening layer partly
overhangs the substrate cavity.
[0082] The material of the strengthening layer advantageously has a
greater elasticity than the material of the membrane layer and/or
may be in greater tensile stress than the material of the membrane
layer. Such a strengthening layer can help re-distribute the stress
experienced by the membrane layer, especially in a membrane layer
formed from material such as silicon nitride. The strengthening
layer may additionally or alternatively comprise a material which
has a greater plasticity than the material of the membrane layer,
for example the strengthening layer may comprise a material with a
greater ductility than the material of the membrane. The
strengthening layer may thus help the membrane layer to flex
without damage and can help reduce the stress within the membrane
layer when subject to high pressures/large membrane deflection. The
strengthening layer may also reduce the chance of catastrophic
failure.
[0083] FIG. 5 illustrates a similar situation to that shown in FIG.
3a, where the membrane is deflected downwards by a relatively large
pressure difference, to the extent that the membrane layer 101
contacts the substrate 105 at the edge 202 of the substrate cavity.
In this instance however the presence of strengthening layer 401 at
point 501 on the membrane layer, where the membrane layer becomes
part of the side wall of the support structure 201, helps reduce
the stress experienced by the membrane layer at this part. The
stress can be distributed throughout the strengthening layer 401
and, as the strengthening layer 401 also extends into the side wall
structure, the stress can also be distributed to the support
structure 201.
[0084] The presence of strengthening layer 401 at point 502 on the
membrane layer, where the membrane layer impacts on the edge 202 of
the opening of the substrate cavity also helps strengthen the
membrane layer and distribute stress at this point. This may reduce
the amount of deflection of the membrane layer at this point in
response to a larger than usual pressure difference acting on the
membrane. The strengthening layer will also help provide resilience
to mechanical shock.
[0085] The strengthening layer may also add resilience in the
situation shown in FIG. 3b where the membrane is deflected so as to
make contact with the back-plate. At a certain pressure level the
middle of the membrane (or the membrane electrode) may just about
make contact with the back-plate. At this point the stress in the
membrane may be relatively evenly distributed--apart from at the
periphery of the membrane as discussed earlier. Thus the
strengthening layer may help distribute stress experienced at the
edge of the membrane.
[0086] In the embodiment shown in FIGS. 4a and 4b the strengthening
layer 401 is disposed to be continuous in a radial direction
throughout the whole of the support structure and to extend inward
on the membrane to a point beyond the limit of the opening of the
substrate cavity, i.e. the first material will, in equilibrium
position, overhang the opening of the substrate cavity.
Conveniently the strengthening layer 401 overhangs the substrate
cavity by a small amount, such that, as shown in FIG. 5, if the
membrane is deflected to such an extent to contact the substrate,
the point of contact corresponds to a location which is coupled to
the strengthening layer. In some embodiments the strengthening
layer may extend inward into the flexible membrane to a point
further then the expected point of contact with the substrate
cavity, for instance the strengthening layer may extend for of the
order of 50 .mu.m or so inwards past the likely point of contact.
Extending the strengthening layer radially inward beyond the likely
point of contact can provide advantageous in terms of bend radius
of the membrane.
[0087] Having the strengthening layer extend continuously radially
from within the support structure to overhang the substrate cavity
(and thus be present at the point of possible contact between the
membrane layer 101 and edge 202 of the substrate cavity) is
particularly advantageous as it allows stress experienced at point
502 to be distributed within the support structure.
[0088] In some embodiments however it may be sufficient to dispose
the strengthening layer at a first location of the membrane layer,
where contact with another part of the transducer structure is
possible, but where such strengthening layer does not extend as far
as the support structure for the membrane. For example, as
illustrated in FIG. 6 there may be a strengthening layer 401a
disposed in the area of the membrane which may contact the edge 202
of the substrate cavity in response to a high pressure
differential. This strengthening layer 401a does not extend as far
as the support structure 201. It should be noted that this
strengthening layer 401a is still disposed toward the outer part of
the flexible membrane and thus is disposed around the periphery of
the membrane.
[0089] As mentioned previously in embodiments such as shown in FIG.
4, where there is a substrate cavity defining an opening in the
substrate that is within the area of the flexible membrane, a
downward deflection of the membrane such that the membrane contacts
the edge of the opening in the substrate surface may be a
particular source of failure for conventional MEMS transducers.
Providing strengthening layer 401a even just in this location may
therefore offer significant improvements in the resilience and
robustness of MEMS transducers. Thus embodiments of the present
invention may relate to MEMS transducers comprising an opening in
the surface of substrate for a substrate cavity that is within the
area of the flexible membrane, wherein the membrane comprises a
strengthening layer coupled to only part of the membrane in the
location where the membrane may, in use, contact the edge of the
opening.
[0090] Alternatively there may be a strengthening layer 401b which
extends from within the side wall of a support structure inwards
onto at least part of the flexible membrane but which may not
extend as far as the substrate cavity (if present). In some
embodiments there may be two discrete strengthening layers, i.e.
strengthening layers 401a and 401b may be both present as discrete
strengthening layers. In this instance the strengthening layers
401a and 401b may have the same material characteristics but in
some embodiments different materials and/or different thicknesses
could be used for the different strengthening layers.
[0091] It will be noted that the embodiment shown in FIG. 4
includes a membrane support in the form of pillars or columns as
described above in relation to FIG. 2. The same principles apply to
embodiments of MEMS transducers which simply have a continuous
sidewall from which the membrane layer extends to form the flexible
membrane, for example similar to sidewall arrangement shown in FIG.
1. For example in some embodiments the structure shown to the left
of line 402 may not be present (in other words consider the
structure to the right of line 402 being present at any
cross-section through the edge of the transducer and not just in
certain locations). It can clearly be seen that the same principles
apply and the layer 402 may extend from within the continuous side
wall inwards onto the membrane layer 101. It will also be clearly
appreciated that the membrane support may take other forms, e.g.
vertical side walls etc. In general however all such support
structures will involve a membrane layer extending from some sort
of side wall structure and the point at which the membrane layer
emerges from the side wall may experience a greater stress than
other parts of the membrane when a large pressure differential
causes a large deflection of the membrane. Thus one aspect of
embodiments of the present invention is the use of a strengthening
layer which is coupled to a membrane layer so that the
strengthening layer extends from within a side wall structure to a
flexible part of the membrane--but which is not disposed over the
entire membrane.
[0092] Referring back to FIG. 4a the strengthening layer 401 may be
continuous around the entire periphery of the membrane. As shown
this may result in a strengthening layer which, for a generally
circular membrane, has an annular shape (for membranes of different
shapes the layer 401 may generally correspond to the shape of the
perimeter of the membrane).
[0093] In some embodiments however the strengthening layer may be
divided into one or more sections around the periphery of the
periphery of the membrane, for instance there may be a plurality of
sections 401c separated by relatively small gaps as indicated by
the dotted lines.
[0094] As discussed above the strengthening layer is provided
around the majority of the periphery of the membrane, i.e. more
than 50% of the perimeter. Advantageously the strengthening layer
401 may extend for more than 50% of the perimeter of the membrane.
In some embodiments the strengthening layer may be provided around
75% or more of the perimeter of the membrane and possible around at
least 90% of the perimeter of the membrane or substantially the
entire perimeter of the membrane. Conveniently, if the
strengthening layer is divided into different sections these may be
relatively evenly spread around the perimeter of the membrane so as
to provide strengthening evenly around the whole membrane.
[0095] In the embodiment shown in FIG. 4a the material of the
conductive track 204 for connecting the membrane electrode 102 to
the contact pad 205 on the substrate is formed on top of the
strengthening layer 401. As one skilled in the art will appreciate
this will result from the strengthening layer 401 being formed
prior to deposition and patterning of the material of the
conductive track 204--which usually is a metal material and is
usually part of the same material used to form the electrode 102.
Depositing the strengthening layer 401 prior to forming the
conductive track ensures that the strengthening layer contacts the
membrane layer even in the vicinity of the conductive track. In
other embodiments however the material forming the strengthening
layer may be deposited after the material used to form the
conductive track 204 and thus may be deposited on top of conductive
track 204.
[0096] It will be seen that the strengthening layer 401 is only
provided in the peripheral area of the flexible membrane, extending
from the support structure to overhang the opening of the substrate
cavity. The strengthening layer is not provided over the whole of
the membrane layer and thus has a limited impact on transducer
performance. The strengthening layer is thus disposed such that
radially inside the strengthening layer is a circumferential area
of flexible membrane, the majority of which is free of the material
of the strengthening layer. In other words, around a majority of
the perimeter of the flexible membrane, there is a region radially
inside the strengthening layer which is free of the material of the
strengthening layer. The strengthening layer is thus provided in an
outer peripheral region of the membrane and a substantial area
within the membrane is devoid of any strengthening layer.
[0097] The material(s) used to form the strengthening layer may be
compatible with standard device fabrication processes and thus may
advantageously be material(s) compatible with standard CMOS
processing. As mentioned the material of the strengthening layer
may be chosen to have a greater plasticity, e.g. ductility, than
the material of the membrane layer and/or be in greater tensile
stress. Suitable materials include various CMOS compatible metals
such as titanium, aluminium, copper and gold as well as alloys of
such materials, e.g. such as aluminium nitride. The strengthening
layer may comprise layers of one or more of these metals, e.g. for
assisting adhesion to other structural layers. The strengthening
layer may comprise a plurality of layers of different material, for
example layers with different material characteristics. The
different layers may comprise materials with deliberately different
physical properties such as elasticity, plasticity, ductility,
tensile stress, Young's modulus, and/or fracture stress point to
attempt to optimise the properties of the resulting laminate
sandwich to provide better ruggedness.
[0098] In some embodiments, there may be more than one
strengthening layer provided at a given point on the membrane
layer, i.e. there may be a layer of first material and also a layer
of second material in the same location, the layers of first and
second material together providing strengthening to the membrane
layer. The first and second strengthening layers may be coterminous
or there may be some areas where there is only first or second
material. In other words there may be first and second
strengthening layers having different geometries, i.e. having
different patterns on the membrane layer.
[0099] The strengthening layer may be relatively thin. For
instance, when using a metal as the strengthening layer a thickness
of around 30-100 nm, for example about 60 nm may be sufficient.
With a MEMS transducer using silicon nitride as the membrane layer
the membrane layer may have a thickness of around 0.4 .mu.m. In
such an embodiment a metal strengthening layer of the order of 60
nm thick may be disposed as shown in FIG. 4b to provide increased
robustness of the transducer with no significant detrimental impact
on performance
[0100] It may be possible in some fabrication processes to use
thinner metal layers, for instance of the order of 30-40 nm or
less. However in some fabrication processes the deposition process
may not reliably result in a continuous metal layer at such
relatively low thicknesses, possibly producing discrete islands of
metal--reducing the effectiveness of the strengthening layer.
[0101] Conveniently the strengthening layer 401 may have a
relatively uniform layer thickness, at least in the part disposed
on the flexible membrane, such that a single deposition step may be
used for the material of the strengthening layer. However in some
embodiments it may be beneficial for different parts of the
strengthening layer 401 to have different thicknesses.
[0102] In one embodiment the material used to form the
strengthening layer may conveniently be the same material as used
to form the membrane electrode. In this embodiment the
strengthening layer may be formed using the same general process
steps as used to create the membrane electrode. In this embodiment
no additional process steps may be required compared with the
conventional fabrication process but the step of patterning the
metal layer used to form the membrane electrode additionally
comprises patterning the metal layer to form the strengthening
layer as illustrated in FIG. 7.
[0103] FIG. 7 shows a plan view of a transducer (omitting the
back-plate for clarity) which shows membrane electrode
102--together with conductive track 204, formed on the electrode as
described previously. In this embodiment however the metal used to
form the electrode 102 has also been patterned to form
strengthening layer 701 around substantially the whole of the
periphery of the membrane as discussed above.
[0104] In the embodiment shown in FIG. 7 the metal layer is
patterned so that the material of strengthening layer 701 is
discrete from the electrode 102 and conductive track 204, in other
words the strengthening layer 701 is substantially isolated from
the conductive track 204. However this is not essential and in
other embodiments the material could be continuous to form
conductive track 204 and the material of the strengthening layer
701. As the strengthening layer 701 is only provided around the
periphery of the membrane it would not significantly impact on
device performance.
[0105] It will of course be appreciated that the strengthening
metal layer 701 will form part of the column support structures.
This detail is omitted from FIG. 7 for clarity.
[0106] In the embodiment shown in FIG. 7 the material that forms
the strengthening layer 701 thus also forms the membrane electrode
and thus some parts of the membrane within the outer peripheral
area are also provided with the same material as the membrane layer
(i.e. electrode area 102). However it will be appreciated that
there is a circumferential area radially inside the strengthening
layer (i.e. that area between the strengthening layer 107 and
electrode 102) the majority of which is free of material of the
strengthening layer. Only a small part of the membrane in this area
is covered in material used to form the strengthening layer, i.e.
that section of the membrane which bears the conductive track
204.
[0107] Embodiments of the invention therefore also provide a MEMS
transducer having a membrane comprising a first area of first
material disposed so as to form a membrane electrode and a second
area of first material disposed to form a strengthening layer. The
second area may substantially surround the first area and may for
instance by the outer part of the membrane, but be largely separate
therefore. In other words between the first and second areas there
may be a third area which is mainly or substantially devoid of any
first material. The first material in the second area may extend
into the side walls support structure for the membrane.
[0108] As mentioned the use of the same material layer to form the
strengthening layer and the membrane electrode is a particular
advantage as it allows embodiments of the invention to be
implemented using existing process flows with only a change to one
mask layout. FIGS. 8a-8f illustrate one example of fabrication of a
transducer such as shown in FIG. 7.
[0109] On substrate 105 first sacrificial material 801 may be
deposited and shaped to define the first cavity. The sacrificial
material may also be patterned to expose the substrate in certain
areas which will form part of the membrane support. The sacrificial
material may also be treated to ensure that the sides of the
sacrificial material are sloped as will be understood by one
skilled in the art. A membrane layer 101 is then deposited on the
substrate so as to cover the first sacrificial material as shown in
FIG. 8a. Bleed holes in the may be formed in the membrane layer
within the area of the membrane at this point and filled with
sacrificial material. A metal layer 803 may then be deposited over
the whole of the membrane layer as shown in FIG. 8b. This metal
layer may then be patterned, as shown in FIG. 8c, by using standard
etching techniques, to define a membrane electrode 102 and
strengthening layer 701 (as well as a conductive track 204--not
shown in FIG. 8).
[0110] If required the metal layer may also be patterned so as to
avoid the bleed holes 802 provided in the membrane to reduce the
effects of low frequency pressure changes (by allowing the pressure
in the first and second cavities to equalize over time). However in
some embodiments the bleed hole may lie outside the area covered by
the strengthening layer and/or electrode.
[0111] There may be some additional small channels (not shown)
provided to aid removal of the sacrificial material. At least some
of the these channels may be provided in the outer part of the
membrane layer, in which case the metal layer 701 may be patterned
to exclude metal from the location of such channels
[0112] A second sacrificial layer 804 is then deposited and
patterned on top of the membrane layer to define the second cavity
as shown in FIG. 8d. The back plate structure 104 may then be
formed by depositing a first back-plate layer, depositing and
patterning a metal layer to form a back-plate electrode 103 and
depositing a further back-plate layer as shown in FIG. 8e.
[0113] Finally, a substrate cavity may be etched through substrate
105 to the sacrificial material and the sacrificial material
removed from the first cavity. Further, acoustic holes (not shown)
would be etched into the back-plate layer and the sacrificial
material may be removed via the acoustic holes, from the second
cavity to leave a transducer structure as illustrated in FIG. 8f.
This provides a transducer structure with strengthening layer 701
without requiring any additional process steps and without
impacting on transducer performance.
[0114] The foregoing description has focussed mainly on embodiments
including a substrate cavity having an opening in the substrate
which is at least partly within the area of the membrane. As
discussed aspects of the present invention may be particularly
beneficial for such embodiments due to the high stress can be
encountered if the membrane is deflected enough to contact the
substrate cavity.
[0115] However the principles of the present invention are
applicable to other arrangements as well. For instance MEMS
transducers where the substrate cavity defines the first cavity, or
where the spacing between the membrane and substrate is sufficient
that the membrane would never contact the substrate in use may
still experience high stress at the point where the membrane layer
emerges from the side wall support to form the flexible membrane
and may benefit from a strengthening layer which extends from the
support structure at least partly onto the part of the membrane
layer which forms the flexible membrane.
[0116] Also it has been proposed, for embodiments having a
substrate cavity with an opening which is smaller than the first
cavity under the membrane, that various structures could be used to
extend over at least part of the area of the substrate cavity and
provide a limit to the amount of membrane deflection in response to
a large pressure differential. In other words structures may be
deliberately introduced for the membrane to contact when subject to
the large pressure difference but which do not present to sharp
corner that the edge of a substrate cavity presents. In such
arrangements the membrane will make contact with some part of such
structure(s) and thus may benefit from a layer of first material
extending over the membrane to the point of contact.
[0117] In some embodiments there may therefore be more than one
opening in the surface of the substrate leading to one or more
substrate cavities. In some embodiments there may be separate
substrate cavities which collectively form an acoustic volume for
the transducer. The one or more cavities will therefore have an
area of opening in the surface of the substrate, i.e. area defined
by the outermost parts of the perimeters of the outermost cavity
openings. If the perimeter of such area of opening is within the
area of the flexible membrane then it is possible, on a large
deflection, that the membrane makes contact with the edge of an
opening, or part of the structure between or within the substrate
cavities. Thus in some embodiments the strengthening layer is
disposed on the membrane layer such that the perimeter of said area
of opening at least partly underlies the peripheral area of
flexible membrane having said strengthening layer.
[0118] 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.
[0119] 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.
[0120] 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, laptops, mobile phones, PDAs and personal
computers. The invention may be used in voice controlled devices
and may be implemented in home network controllers for audio or
other domestic apparatus. 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.
[0121] 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 fulfil 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.
* * * * *