U.S. patent application number 15/895689 was filed with the patent office on 2018-08-30 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 Marek Sebastian PIECHOCINSKI.
Application Number | 20180244516 15/895689 |
Document ID | / |
Family ID | 57890856 |
Filed Date | 2018-08-30 |
United States Patent
Application |
20180244516 |
Kind Code |
A1 |
PIECHOCINSKI; Marek
Sebastian |
August 30, 2018 |
MEMS DEVICE AND PROCESS
Abstract
The application describes MEMS transducers and associated
methods of fabrication. The MEMS transducer has a flexible membrane
with a vent structure comprising a moveable portion which opens in
response to a differential pressure across the membrane to provide
a flow path through the membrane. At least one edge of the moveable
portion comprises one or more protrusions and/or recesses in the
plane of the moveable portion.
Inventors: |
PIECHOCINSKI; Marek Sebastian;
(Edinburgh, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Cirrus Logic International Semiconductor Ltd. |
Edinburgh |
|
GB |
|
|
Assignee: |
Cirrus Logic International
Semiconductor Ltd.
Edinburgh
GB
|
Family ID: |
57890856 |
Appl. No.: |
15/895689 |
Filed: |
February 13, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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15413608 |
Jan 24, 2017 |
9926189 |
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15895689 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B81B 2201/0257 20130101;
H04R 19/005 20130101; G01L 9/0073 20130101; B81B 7/0029 20130101;
G01L 9/0042 20130101; B81B 7/0061 20130101; B81B 2203/0127
20130101; B81B 2203/0323 20130101; H04R 2201/003 20130101; H04R
2499/11 20130101; H04R 7/26 20130101; B81C 1/00158 20130101; H04R
19/04 20130101; B81B 2203/0118 20130101; B81B 2207/012 20130101;
B81B 2201/0264 20130101 |
International
Class: |
B81B 7/00 20060101
B81B007/00; B81C 1/00 20060101 B81C001/00; H04R 19/04 20060101
H04R019/04; G01L 9/00 20060101 G01L009/00; H04R 7/26 20060101
H04R007/26 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 14, 2016 |
GB |
1604303.6 |
Claims
1.-46. (canceled)
47. A MEMS transducer comprising: a flexible membrane, the flexible
membrane having a vent structure comprising a moveable portion
which is at least partly separated from the rest of the membrane by
a channel which extends through the membrane and defines a first
edge of the moveable portion, the channel defining a meandering
path.
48. The MEMS transducer as claimed in claim 47, wherein the channel
further defines a second edge adjacent to the first edge of the
moveable portion.
49. The MEMS transducer as claimed in claim 48, wherein the second
edge is an internal edge of the membrane and/or an edge of a
second, adjacent moveable portion of the vent structure.
50. The MEMS transducer as claimed in claim 47, wherein the vent
structure has an equilibrium position at which the moveable portion
is substantially in plane with the rest of the membrane.
51. The MEMS transducer as claimed in claim 50, wherein the first
edge has a shape that substantially compliments the shape of the
second edge when the vent structure is at the equilibrium
position.
52. The MEMS transducer as claimed in claim 50, wherein the first
and second edges are interdigitated when the vent structure is at
the equilibrium position.
53. The MEMS transducer as claimed in claim 48, wherein the channel
provides a volume between the first edge of the moveable portion
and the second edge, and wherein the first and second edges are
interdigitated
54. The MEMS transducer as claimed in claim 50, wherein at least a
portion of the channel exhibits a sinusoidal, a square-wave, a
triangle-wave, or a saw-tooth shape when the vent structure is at
the equilibrium position.
55. The MEMS transducer as claimed in claim 47, wherein said
transducer comprises a capacitive sensor.
56. The MEMS transducer as claimed in claim 47, wherein said
transducer comprises a microphone.
57. The MEMS transducer as claimed in claim 56, further comprising
readout circuitry.
58. An integrated circuit comprising a MEMS transducer as claimed
in claim 47 and readout circuitry.
59. The MEMS transducer as claimed in claim 47, wherein the
transducer is located within a package having a sound port.
60. An electronic device comprising a MEMS transducer as claimed in
claim 47.
61. The electronic device as claimed in claim 60, 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
FIELD OF DISCLOSURE
[0001] 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
[0002] 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.
[0003] 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 a pair of
electrodes which will vary as the distance between the electrodes
changes in response to sound waves incident on the membrane
surface.
[0004] FIGS. 1a and 1b show a schematic diagram and a perspective
view, respectively, of a known capacitive MEMS microphone device
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 FIG. 1a the second
electrode 103 is embedded within the back-plate structure 104.
[0005] 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.
[0006] 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 the 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.
[0007] A plurality of holes, hereinafter referred to as bleed holes
111, connect the first cavity 109 and the second cavity 110.
[0008] 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.
[0009] In some applications, the microphone may be arranged in use
such that incident sound is received via the back-plate. In such
instances 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 cavity 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.
[0010] 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.
[0011] It should also be noted that whilst FIG. 1a shows the
back-plate 104 being supported on the opposite side of the membrane
to the substrate 105, arrangements are known where the back-plate
104 is formed closest to the substrate with the membrane layer 101
supported above it.
[0012] 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.
[0013] The transducer shown in FIG. 1a 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. Such 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. However, this process can lead to dimples in
the upper surface of the back-plate layer in the area of the
columns.
[0014] MEMS transducers such as those shown in FIG. 1a 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.
[0015] Thus, 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.
[0016] 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/acoustic 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.
[0017] 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.
[0018] In response to a high pressure impulse however the membrane
layer 101 may exhibit a greater amount of deformation than usual.
FIG. 2a illustrates the situation where the membrane has been
deformed downwards following a high pressure event and FIG. 2b
shows the situation where the membrane has been displaced
upwards.
[0019] 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, may thus result in
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 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.
[0020] 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
substrate cavity 108, the membrane material can be relatively
brittle. Thus contact between the membrane and the edge of the
opening of substrate cavity in this way can lead to damage such as
cracking of the membrane.
[0021] The bleed holes discussed above with relation to FIG. 1a
will provide a flow path between the first and second cavities and
thus flow of air through the bleed holes will reduce the pressure
differential acting on the membrane over time. However the bleed
holes are typically deliberately arranged to provide a limited
amount of flow so as to provide a desired frequency response. Thus
a high pressure differential may be maintained across the membrane
for a relatively long period of time before flow through the bleed
holes acts to equalise the pressures in the first and second
cavities. The time taken to equalise via the bleed holes could be
changed by altering the size and/or number of bleed hole but this
may impact negatively on transducer performance.
[0022] 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 as discussed. 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. 2a but
again this may introduce stress into the membrane layer at the
point 303 where it contacts the back-plate 104. Again it may take a
while for this pressure differential to reduce by virtue of flow
through the bleed holes.
[0023] 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.
[0024] FIGS. 3a to 3c show a previously proposed MEMS transducer
comprising a flexible membrane 101 and a variable vent structure
401 in the form of a moveable portion or "flap" 402. The moveable
flap portion is defined by a thin channel 403 which runs through
the membrane and partially separates the moveable flap portion from
the rest of the membrane whilst remaining attached to the rest of
the membrane via a connecting portion 404.
[0025] The moveable flap portion 402 is arranged such that its
equilibrium position--i.e. the position it adopts with
substantially no pressure differential acting on the moveable
portion, is within the plane of the membrane. In response to a
pressure differential across the moveable portion of the vent
structure the moveable portion is deflected away from the plane of
the membrane so as to expose a hole in the membrane. In this way,
the size of a flow path through the vent structure between a first
volume above the membrane to a second volume below the membrane is
varied in response to a variable pressure differential acting on
the moveable portion.
[0026] FIG. 3b illustrates in perspective view the part of the
membrane and the variable vent. In this example the pressure in the
volume below the membrane is sufficiently greater than the pressure
in the volume above the membrane such that the moveable flap
portion 402 has been deflected upwards away from the rest of the
membrane surface. This opens the flow channel through the membrane,
i.e. effectively opens a hole in the substrate. If the pressure
differential increases enough the moveable portion 402 may be
further deflected and thus provide a greater amount of opening,
i.e. a greater flow path.
[0027] The moveable portion may thus adopt a range of positions.
These positions depend on the pressure differential acting on the
moveable portion (or the variable vent). The extent to which the
moveable portion is deflected also determines how much the moveable
portion blocks/exposes the hole through the membrane and thus the
size of the flow path.
[0028] The structure shown in FIG. 3 has been shown to act so to
reduce the pressure differential acting on the membrane at
relatively high pressure differentials. However, pressure impulse
profiles arising as a result of e.g. air being forced into the
sound port of a host device due to an impact, can often peak within
a few milliseconds. Thus, unless the vent structure can respond
quickly, ideally within this time range, damage may still be
sustained by a high pressure or overpressure event.
SUMMARY
[0029] The present invention is concerned with improving the
resilience of MEMS devices to a high pressure impulse incident on a
MEMS transducer. In particular, the present invention seeks to
improve the response time of a vent structure provided on a
flexible membrane of a MEMS transducer. Thus, the present invention
seeks to facilitate an equalization of a pressure differential
arising between the upper and lower surfaces of the membrane.
[0030] According to an aspect of the present invention there is
provided a MEMS transducer comprising: a flexible membrane having a
vent structure, the vent structure comprising a moveable portion
which is separated from the rest of the membrane by a channel which
extends through the membrane and defines a pair of adjacent
internal edges, wherein at least one of the internal edges is
provided with one or more protrusions or recesses.
[0031] Thus, according to embodiments of the present invention, an
internal edge (which is distinguished from a boundary edge of the
membrane) is interrupted by one or more discontinuities, such as
protrusions and/or recesses, which extend and/or retreat in the
plane of the membrane. Thus, the edge can be considered to be
serrated or indented, or may be considered to comprise a plurality
of discrete, protruding, elements. Thus, the edge can be considered
to exhibit a profile, or shape, that varies within the plane of the
moveable portion.
[0032] Preferably, but not necessarily, the edge that is provided
with a plurality of protrusions and/or recesses, is an edge of a
moveable portion or "flap" that is defined by the channel.
Alternatively, or additionally, the edge that is provided with a
plurality of protrusions and/or recesses, is an edge of the rest of
the membrane.
[0033] According to another aspect of the present invention there
is provided a MEMS transducer comprising: a flexible membrane, the
flexible membrane having a vent structure comprising a moveable
portion, wherein in response to a differential pressure across the
vent structure, the moveable portion deflects away from the plane
of the membrane, and wherein an edge of the moveable portion is
provided with one or more protrusions and/or recesses in the plane
of the moveable portion.
[0034] At the equilibrium position, the edge comprising one or more
protrusions or recesses may preferably be complimentary in shape to
the shape of the edge of an adjacent moveable portion, or
complimentary to the shape of an adjacent edge of the membrane.
[0035] The membrane is a flexible, thin film membrane. The membrane
is generally planar in shape and is formed from one or more thin
layers of semiconductor material, such as silicon nitride. The
moveable portion may be defined by one or more channels which
extend through the membrane except at a connecting portion. These
channels are formed during the fabrication of the MEMS transducer
by a process of selective etching. Thus, the moveable portion may
be formed from the same material as the semiconductor material.
[0036] The flexible membrane exhibits an equilibrium position which
can be considered to correspond to the minimum size of the flow
path through the flexible membrane. Thus, at the equilibrium
position, the differential pressure across the vent structure is
insufficient to cause deflection of the moveable portion and the
size of the flow path through the membrane is
minimal/negligible.
[0037] It will of course be appreciated that the channel does
represent a path for air to flow through the membrane, however the
channel may be formed with a very narrow width and thus there may
be no or limited air flow through the channel when the vent
structure is in the equilibrium position at which the moveable
portion(s) substantially close/cover the aperture.
[0038] In response to a differential pressure across the moveable
portion of the vent structure the moveable portion deflects to
reveal an aperture in the flexible membrane and, thus, to provide a
flow path through the flexible membrane. This facilitates an
equalisation of the pressure acting on the opposing surfaces of the
membrane and tends to restore the moveable portion to its
equilibrium position. As will be explained in more detail, the
provision of one or more protrusions and/or recesses on an internal
edge of the membrane and/or moveable portion may be seen to
increase the initial rate at which the vent opens in response to a
differential pressure across the membrane.
[0039] The protrusions and/or recesses may be any shape. For
example, the edge may exhibit a sinusoidal, a square-wave,
triangle-wave, or saw-tooth profile. Alternatively, the edge could
exhibit a serrated or notched form. The protrusions and/or recesses
may be provided at regular intervals along the edge or may be
provided irregularly. The periodicity and/or the amplitude of the
protrusions and/or recesses may by uniform or may vary along a
given edge.
[0040] The vent structure may typically comprises a plurality of
moveable portions. In this case first and second adjacent edges of
adjacent moveable portions may be shaped so as to exhibit a
non-linear profile and/or to comprise one or more protrusions
and/or recesses. The first adjacent edge may exhibit a shape that
is generally complimentary to the shape of the second adjacent edge
when the moveable portions are at the equilibrium position. The
first and second adjacent edges may be considered to comprise
interdigitated edges.
[0041] It will be appreciated that the moveable portions of the
vent structure may be of any shape including circular, triangular,
square, and rectangular. Alternatively the moveable portion may
comprise an irregular polygon. The shape of the moveable portion(s)
may depend on the shape of the aperture that will be revealed when
the vent deflects in response to a differential pressure. Thus,
that, at the equilibrium position when the moveable portions are in
plane with the rest of the membrane, the moveable portion
substantially cover/close the aperture. For example, a vent
structure comprising two or more moveable portions may be
configured such that the moveable portions tessellate with each
other at the equilibrium position.
[0042] According to another aspect of the present invention there
is provided a MEMS transducer comprising: a flexible membrane, the
flexible membrane having a vent structure comprising a plurality of
moveable portions, wherein in response to a differential pressure
across the vent structure, the moveable portions deflect away from
the plane of the membrane to reveal an aperture in the membrane,
the vent structure having an equilibrium position at which the
moveable portions are substantially in plane with the rest of the
membrane and at which first and second adjacent edges of two
adjacent moveable portions are aligned, wherein the first and
second adjacent edges are provided with one or more protrusions
and/or recesses such that the first adjacent edge exhibits a shape
that is complimentary to the shape of the second adjacent edge when
the moveable portions are at the equilibrium position.
[0043] The provision of one or more protrusions and or recesses in
an edge of the moveable portion extends the length of the edge of
the moveable portion, as compared to the corresponding
uninterrupted, or continuous, length between the end points of a
given edge. The "end point" of an edge may be considered to be the
point at which the general/overall direction of the channel changes
(ignoring any changes in direction caused by the protrusions and/or
recesses). In response to a high pressure event, which causes the
moveable portion to deflect away from the plane of the membrane,
this extended edge length can be considered to provide a larger
vertical surface area on which the resultant force of a pressure
differential across the vent will act during the initial stages of
the vent structure opening. The extended edge length beneficially
increases the initial vent opening speed, therefore allowing a
faster pressure equalisation and improving the resilience of the
transducer. Preferably, the one or more protrusions and/or recesses
increase the internal edge length by between 25% and 50%, or more
preferably by at least 50% with respect to the equivalent
uninterrupted edge length.
[0044] During fabrication of a MEMS transducer according to any of
the above aspects, one or more channels are etched through the
flexible membrane to define the, or each, moveable portion and to
separate the/each moveable portion from the rest of the membrane,
except at a connecting portion. The channel(s) therefore define one
or more pairs of internal edges of the membrane. The shape of the
moveable portion, including the shape or profile of the internal
edge, is therefore defined by the etching process. In this way,
according to an aspect of the present invention, two substantially
complimentary edges are formed either side of a channel or gap. The
channel may exhibit a width of less than 1 .mu.m and preferably in
the region of 0.35 .mu.m.
[0045] The two substantially complimentary internal edges may be
defined between adjacent moveable portions of the vent structure,
or between an edge of a moveable portion and an adjacent edge of
the membrane. Preferably, the two complimentary edges are provided
at a region of the moveable portion where the most deflection of
the moveable portion is expected to occur in response to e.g. a
high pressure event. It will be appreciated that the region of most
expected deflection will depend on the design of the vent
structure. According to a particularly preferred embodiment, the
density of protrusions and/or recesses provided on an internal
edges may be varied such that the density is higher in a region of
most expected deflection. This arrangement beneficially enhances
the non-linear, variable response, of the vent structure.
[0046] According to another aspect of the invention there is
provided a MEMS transducer comprising: a flexible membrane, the
flexible membrane having a vent structure comprising a moveable
portion which is separated from the rest of the membrane by a
channel which extends through the membrane and defines at least one
internal edge of the moveable portion, the channel defining a
meandering path between first and second endpoints of the
channel.
[0047] Thus, the channel may define a path which meanders between
first and second endpoints of the channel or which is exhibits a
plurality of discontinuities or interruptions.
[0048] The shape of the/each moveable portion is defined by a
particular number of internal edges. For example, a vent structure
having a single, circular-shaped, moveable portion can be defined
by one internal edge. Alternatively, the moveable portion may be an
irregular polygon and therefore bounded by a plurality of internal
edges. It will be appreciated that the protrusions and/or recesses
may be provided on at least a part of one or more of the internal
edges formed either side of the channel which defines the moveable
portion.
[0049] The channel may be considered to define two adjacent,
complimentary or interdigitated edges. Thus an interdigitated
region or volume is provided between the first adjacent internal
edge of the moveable portion and the second adjacent edge (which
may be of the membrane or of another, adjacent moveable portion of
the vent structure).
[0050] The vent structure of a MEMS transducer may comprise a
plurality of moveable portions which are connected to the outer
periphery of the vent aperture and which are lay in the plane of
the aperture at the equilibrium position. First and second
interdigitated edges are preferably provided adjacent to one
another on two of the plurality of moveable portions. Preferably,
at the equilibrium position the first and second interdigitated
edges are provided at a region of the channel where the most
deflection of the moveable portion is expected to occur in response
to a differential pressure across the membrane.
[0051] According to another aspect of the present invention there
is provided a MEMS transducer comprising a vent structure for
overpressure equalisation wherein at least one of the internal
edges of the vent structure is provided with one or more
protrusions or recesses.
[0052] According to another aspect of the present invention there
is provided a MEMS transducer comprising an overpressure vent
structure wherein at least one of the internal edges of the vent
structure is provided with one or more protrusions or recesses.
[0053] According to another aspect of the present invention there
is provided an over-pressure vent structure for a MEMS transducer
wherein at least one of the internal edges of the vent structure is
provided with one or more protrusions or recesses.
[0054] According to another aspect of the present invention there
is provided a method of fabricating a MEMS transducer having a
flexible membrane, the method comprising: forming a structure
having a flexible membrane supported between a first volume and a
second volume; and forming at least one vent structure in
communication with at least one of said first and second volumes,
comprising forming at least one channel which extends through the
membrane to define at least one moveable portion which can be
deflected away from the surface of the rest of the flexible
membrane in response to a pressure differential, said channel
defining a pair of adjacent internal edges, wherein at least one of
the internal edges is provided with one or more protrusions and/or
recesses in the plane of the membrane.
[0055] In general there is provided a MEMS transducer that
comprises at least one vent structure provided in a flexible
membrane of the transducer. The MEMS transducer may be a capacitive
microphone. The flexible membrane may be supported between a first
volume and a second volume and a flow path may be provided between
the first and second volumes by means of the vent. The vent
structure may comprise a moveable portion which is moveable so as
to open a hole extending from the first volume to the second
volume. The moveable portion may quiescently occupy at least some,
and possibly most, of the area of the hole, but is moveable in
response to a local pressure differential across the hole so as to
vary the size of the hole which is open to provide a flow path. In
other words the moveable portion may, in equilibrium, effectively
close at least part of the hole, but is moveable so as to vary to
degree to which the hole is closed. The moveable portion is
preferably arranged to remain closing the hole, i.e. aperture, at
normal operating pressure differentials but to more to increase the
size of the flow path, e.g. close less of the hole, at higher
pressure differentials that could potentially cause damage to the
membrane. The vent can therefore be seen as a variable
aperture.
[0056] The vent structure thus acts as a type of pressure relief
valve to reduce the pressure differential acting on the membrane.
However unlike, the bleed holes in the membrane (if present) which
have a fixed area and thus a fixed size of flow path, the variable
vent has a flow path size, i.e. aperture, which varies in response
to a pressure differential. Thus the degree to which the vent
allows venting depends on the pressure differential acting on the
vent--which clearly depends on the pressure of at least one of the
first and second volumes. The vent structure therefore provides a
variable acoustic impedance.
[0057] The transducer may comprise a back-plate structure wherein
the flexible membrane layer is supported with respect to said
back-plate structure. The back-plate structure may comprises a
plurality of holes through the back-plate structure. When at least
one vent structure is formed in the flexible membrane layer at
least one of the holes through the back-plate structure may
comprise a vent hole in a location that corresponds to the location
of a vent structure in the flexible membrane layer. The area of the
vent hole in the back-plate may extend laterally away from the area
of opening of the vent in the flexible membrane at a position where
the variable vent in the flexible membrane first opens. When at
least one vent structure is formed in the flexible membrane layer
and comprises a moveable portion which is connected to the rest of
the membrane via a beam structure and the moveable portion and beam
structure are defined by channels running through the flexible
membrane; then the location of the channels in the membrane which
do not form part of the variable flow path through the membrane in
use may be arranged so as to not substantially overlap with the
location of any of said plurality of holes in the back-plate
structure.
[0058] The transducer may be a capacitive sensor such as a
microphone. The transducer may comprise readout circuitry (analogue
and/or digital). The transducer and circuitry may be provided
together on a single semiconductor chip--e.g. an integrated
microphone. Alternatively, the transducer may be on one chip and
the circuitry may be provided on a second chip. The transducer may
be located within a package having a sound port, i.e. an acoustic
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 tablet
device; a games device; and a voice controlled device.
[0059] Features of any given aspect may be combined with the
features of any other aspect and the various features described
herein may be implemented in any combination in a given
embodiment.
[0060] Associated methods of fabricating a MEMS transducer are
provided for each of the above aspects.
BRIEF DESCRIPTION OF THE DRAWINGS
[0061] The invention will now be described by way of example only
with reference to the accompanying drawings, in which:
[0062] FIGS. 1a and 1 b illustrate known capacitive MEMS
transducers in section and cut-away perspective views;
[0063] FIGS. 2a and 2b illustrate how a high pressure event may
affect the membrane
[0064] FIGS. 3a-3c illustrate a known variable vent structure;
[0065] FIGS. 4A and 4B illustrate a cross section through a MEMS
transducer having a vent structure;
[0066] FIG. 5 illustrates a vent structure according to a first
embodiment;
[0067] FIG. 6 illustrates a vent structure according to a second
embodiment;
[0068] FIG. 7 illustrates a vent structure according to a third
embodiment;
[0069] FIG. 8 illustrates possible positions on the vent structure
edges for providing one or more protrusions and/or recesses;
[0070] FIG. 9 illustrates an elevational view of one of the
internal edges comprised in the embodiment shown in FIG. 5;
[0071] FIG. 10 illustrates the way in which the peak to peak
amplitude, or periodicity, of the channel may be varied to change
the density of the protrusions and/or recesses; and
[0072] FIGS. 11a-11d illustrate various example vent
structures;
[0073] FIG. 12 illustrates a plot of acoustic conductance against
pressure differential and the degree of opening of the vent
structure; and
[0074] FIGS. 13a to 13h illustrate various MEMS transducer
packages.
DETAILED DESCRIPTION
[0075] Embodiments of the present invention relate to MEMS
transducers comprising a transducer structure comprising a flexible
membrane supported between a first volume and a second volume. The
first volume may for instance comprise the first cavity 109 between
the membrane and the substrate and/or the volume formed in the
substrate 108. The second volume may comprise the second cavity 110
between the membrane and back-plate and/or any volume in fluid
communication with the second cavity (e.g. a sound port in top-port
embodiments).
[0076] To reduce the likelihood of damage in high pressure
situations the transducer structure comprises at least one vent
structure in communication with at least one of said first and
second volumes. The vent structure comprises at least one moveable
portion which is moveable in response to a pressure differential
across the vent structure. FIG. 4A illustrates a cross section
through a MEMS transducer having such a vent structure. In FIG. 4A
there is no pressure differential across the membrane so P1=P2 and
the moveable portion 402 is in line with the plane of the membrane
101. The channel or gap 403 defines the moveable portion and, thus,
a pair of adjacent edges. The edge 407 of the moveable portion is
provided with one or more protrusions and/or recesses. FIG. 4B
illustrates the vent structure in circumstances of a differential
pressure across the vent in which P1 is greater than P2 such that
the moveable portion has been deflected upwardly to reveal the
aperture 405.
[0077] FIG. 5 shows a plan view of a vent structure in a
substantially closed or equilibrium position according to a first
example.
[0078] As shown in FIG. 5 the vent structure 501 comprises two
moveable portions 502a and 502b. A channel 503 separates the
moveable portions from the rest of the flexible membrane 505. The
channel also separates the two moveable portions from one another
to define first and second adjacent edges 506a and 506b of the
first and second moveable portions respectively.
[0079] Each of the first and second adjacent edges 506a and 506b
exhibits a triangle-wave shape. Thus the first edge 506a can be
considered to comprise a plurality of triangular elements which
project from the edge surface and are complimentary in shape with a
series of triangular elements projecting from the adjacent edge
506b. The region of the first and second adjacent edges can be
considered to form an interdigitated region of the vent structure.
It will be appreciated that the triangular elements of the first
and second moveable portions do not contact each other but are
separated by a gap defined by the width of the channel 503. The
triangular elements can be considered to form interdigital
elements.
[0080] The so-called edge length of the edges 506a and 506b can be
seen to be longer than the continuous, uninterrupted, distance
between the end points of the edge 507a and 507b. In response to a
differential pressure across the membrane the moveable portions
502a and 502b will deflect upwardly, or downwardly, away from the
plane of the membrane. During the initial stages of this
deflection, an extended edge length is provided in the
interdigitated region as a result of the discontinuities on the
first and second adjacent edges. This extended edge length can be
considered to provide a larger vertical surface area on which the
resultant force of a pressure differential across the vent will act
during the initial stages of the vent structure opening. The
extended edge length beneficially increases the initial vent
opening speed, therefore allowing a faster pressure equalisation
and improving the resilience of the transducer. An elevational view
of the edge 506a is shown in FIG. 9.
[0081] FIG. 6 shows a vent structure in a substantially closed or
equilibrium position according to a second example.
[0082] As shown in FIG. 6 the vent structure 501 again comprises
two moveable portions 502a and 502b. A channel 503 separates the
moveable portions from the rest of the flexible membrane 505. The
channel also separates the two moveable portions from one another
to define first and second adjacent edges 606a and 606b of the
first and second moveable portions respectively.
[0083] Each of the first and second adjacent edges 606a and 606b
exhibits a generally square-wave shape. Thus the first edge 606a
can be considered to comprise a plurality of square or rectangular
elements which project from the edge surface in the plane of the
membrane (at equilibrium position) and are complimentary in shape
with a series of square or rectangular elements projecting from the
adjacent edge 606b. The gap between the first and second adjacent
defines an interdigitated region of the vent structure and the
square/rectangular elements comprise interdigitated elements.
[0084] In this example the edge length of the edges 606a and 606b
is even longer than the edge length of the example shown in FIG. 5
and therefore extended even more with respect to the continuous,
uninterrupted, distance between end points 507a and 507b. In
response to a differential pressure across the membrane the
moveable portions 502a and 502b will deflect upwardly, or
downwardly, away from the plane of the membrane. During the initial
stages of this deflection, an extended edge length is provided as a
result of the non-linear edges of the first and second adjacent
edges. As explained with reference to the embodiment shown in FIG.
5, this extended edge length provides a larger vertical surface
area on which the resultant force of a pressure differential across
the vent will act during the initial stages of the vent structure
opening.
[0085] FIG. 7 shows a vent structure in a substantially closed or
equilibrium position according to a third example.
[0086] As shown in FIG. 7 the vent structure 501 again comprises
two moveable portions 502a and 502b. A channel 503 separates the
moveable portions from the rest of the flexible membrane 505. The
channel also separates the two moveable portions from one another
to define first and second adjacent edges 706a and 706b of the
first and second moveable portions respectively. In this example,
each of the first and second adjacent edges 706a and 706b exhibits
a generally sinusoidal-wave shape.
[0087] In the first, second and third examples, shown in FIGS. 5, 6
and 7 respectively, the protrusions and/or recesses are provided on
first and second adjacent edges which overlay the central region of
the aperture at equilibrium. In these examples the vent structure
has an aperture which is substantially "closed" at equilibrium by a
plurality of moveable portions. The moveable portions are connected
to the periphery of the aperture and extend towards a central
region of the aperture, the edges of the moveable portions
effectively tessellating in the plane of the membrane. Considering
the vent structure at equilibrium, in this example the central
region of the aperture tends to be the region where the moveable
portions are furthest from their respective connection to the
membrane and thus where the moveable portions exhibit the greatest
amount of deflection in response to a differential pressure across
the membrane. Thus, it is advantageous for the interdigitated, or
complimentary discontinuous, edges of adjacent moveable portions to
be provided within this region.
[0088] FIG. 8 shows a vent structure which is generally the same as
the vent structure illustrated in FIGS. 4, 5 and 6. However, in
FIG. 8 the detail of the shape of the non-linear edges is not
illustrated. Instead, the dashed line of FIG. 8 illustrates the
edges of the moveable portions 502a and 502b which may be
non-linear, e.g. where protrusions or recesses may be provided. The
line X indicates the region where the first and second moveable
portions neighbour one another. Thus, line Y indicates a region
which is still close to the central region of the aperture where
complimentary non-linear edge portions may be provided on the
moveable portion and the adjacent edge of the membrane.
[0089] It will be appreciated that embodiments of the present
invention may be applied to a variety of different vent structures,
for example vent structures having any number of moveable portions,
wherein the moveable portions may be of any shape or size.
Moreover, the one or more protrusions and/or recesses, or the
non-linear edge portions that are formed along one or more edges of
the moveable portion may exhibit a variety of different shapes.
Furthermore, as shown in FIG. 10, the periodicity of the channel
may be varied to change the density of the protrusions and/or
recesses. Thus, it is envisaged that the density of protrusions
and/or recesses provided on an internal edges may be varied such
that the density is higher in a region of most expected deflection
(e.g. along the line X in FIG. 8). This arrangement beneficially
enhances the non-linear, variable response, of the vent
structure.
[0090] FIGS. 11a, 11b, 11c, and 11d show a variety of possible
example vent structures which may be utilised within the context of
embodiments of the present invention. The vent structure designs
shown in FIGS. 11a to 11d have been considered in published
application US2014/0084396, the entire contents of which is
incorporated herein by way of reference thereto. On each of the
example vent structures a region, or zone, of most expected
deflection has been identified by means of a box bounded by a
dashed line D. Thus, any of the internal edges of the membrane that
fall within this zone may be advantageously provided with one or
more protrusions and/or recesses in order to improve the response
time of the vent structure during the initial stages of opening.
Indeed, this approach can be fine-tuned in the sense that the
density of the protrusions and/or recesses can be varied within the
region of most deflection such that the portion of the inner edge
which will undergo the most deflection in response to a
differential pressure, will exhibit the highest density of
protrusions and/or recesses.
[0091] FIG. 12 illustrates a plot of acoustic conductance against
pressure differential and the degree of opening of the vent
structure. The acoustic conductance represents how readily air may
flow through the membrane in response to a differential pressure.
FIG. 12 shows two plots--a solid line which represents the
variation in acoustic conductance demonstrated by a MEMS transducer
having a plurality of protrusions and/or recesses on an edge of the
moveable portion, and a dashed line which represents the variation
in acoustic conductance demonstrated by the same MEMS transducer
but without any protrusions and/or recesses on the edge of the
moveable portion. The position of the moveable portion at
differential pressures a, b c and d is shown. Thus, with reference
to FIG. 12, during the initial stages of the vent opening--which
can be visualised as the moveable portion deflecting from position
b to position c--the acoustic conductance as represented by the
solid line is seen to increases more rapidly with differential
pressure. This can be understood by consideration of the increased
surface area on which the resultant force of the pressure
differential across the vent will act during the initial stages of
the vent structure opening. Once the relevant edge has deflected
sufficiently far, out of the flow path through the vent, the
acoustic conductance of the vent structure is seen to exhibit a
similar profile to the transducer without any protrusions and/or
recesses on the edge of the membrane. The extended edge length
arising from the provision of the protrusions and/or recesses
beneficially increases the initial vent opening speed, therefore
allowing a faster pressure equalisation and improving the
resilience of the transducer.
[0092] Embodiments of the present invention also relate to MEMS
transducers comprising a flexible membrane supported between a
first volume and a second volume and a vent structure connecting
said first and second volumes. The vent provides a flow path having
a size that varies with pressure differential across the
membrane.
[0093] Embodiments of the invention also relate to MEMS transducers
having a membrane supported between first and second volumes
wherein the acoustic impendence between the first and second
volumes is variable with the differential pressure between the
volumes.
[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] 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. 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.
[0097] One or more transducers according to the any of the
embodiments described above may be incorporated in a package. FIGS.
13a to 13h illustrate various different packaging arrangements.
FIGS. 13a to 13g each show one transducer element located in the
package but it will be appreciated that in some embodiments there
may be more one than transducer, e.g. a transducer array, and the
various transducers may be formed on the same transducer substrate,
i.e. a monolithic transducer substrate, or may be formed as
separate transducers with separate transducer substrates each
separate transducer substrate being bonded to a package
substrate.
[0098] FIG. 13a shows a first arrangement where a transducer 1100
is located in a cover 1101, which forms at least part of a housing,
on a package substrate 1102. The cover in this example could be a
metallic housing which is bonded to the substrate. The package
substrate may comprise at least one insulating layer. The package
substrate may also comprise at least one conductive layer. The
package substrate may be a semiconductor material or may be formed
from a material such as PCB, ceramic or the like. Where the cover
1101 is metallic, or itself comprises a conductive layer, the cover
may be electrically coupled to the conductive layer of the
substrate, e.g. so that the housing provides shielding for
electromagnetic interference (EMI). Bond wires 1103 may connect the
transducer to bond pads on the package substrate. In some
embodiments, read-out circuitry, for instance amplifier circuitry,
may be located within the housing formed in or connected to the
package substrate. Through-vias through the package substrate (not
illustrated) may connect to contacts, i.e. solder pads, 1104 for
electrically connecting external circuitry (not illustrated) to the
package to allow transmission of electrical signals to/from the
transducer 1100. In the example shown in FIG. 13a there is a sound
port or acoustic port in the cover 1101 to allow sound to enter the
package and the transducer is arranged in a top port
arrangement.
[0099] FIG. 13b illustrates an alternative arrangement where the
sound port is provided in the package substrate 1102 and may, in
use, be sealed. A ring 1105, which may be a sealing ring or a
solder pad ring (for use in forming a solder ring) may be provided
around the periphery of the sound port on the outer side of the
package to allow, in use, sealing of a sound path leading to the
sound port when the package is connected to another PCB for
example. In this embodiment the transducer is arranged in a bottom
port arrangement with the volume defined by the housing 1101
forming part of the back-volume of the transducer.
[0100] FIG. 13c illustrates an example where instead of bond wires
connecting the transducer to the package substrate the transducer
structure is inverted and flip-chip bonded to package substrate via
connections 1106. In this example the sound port is in the package
substrate such that the package is arranged in a bottom port
arrangement.
[0101] FIG. 13d illustrates an alternative example to that of FIG.
13b wherein a housing 1107 is formed from various panels of
material, for example PCB or the like. In this instance the housing
1107 may comprise one or more conductive layers and/or one or more
insulating layers. FIG. 13d shows the sound port in the package
substrate. FIG. 13e shows an alternative arrangement to that of
FIG. 13b wherein a housing 1107 is formed from various panels of
material, for example PCB or the like as described in relation to
FIG. 13d. FIG. 13f shows a further embodiment where the transducer
structure is bonded via connections 1106 to the housing upper
layer, which may for instance be PCB or layered
conductive/insulating material. In this example however the
electrical connections to the package are still via contacts,
solder pads, 1104 on the package substrate, e.g. through-vias (not
illustrated) in the package substrate with conductive traces on the
inside of the housing to the transducer. FIG. 13g illustrates an
alternative example to that of FIG. 13c wherein a transducer is
flip-chip bonded to the package substrate in a housing 1107 formed
from panels of material, for example PCB or the like as described
in relation to FIG. 13d.
[0102] In general, as illustrated in FIG. 13h, one or more
transducers may be located in a package, the package is then
operatively interconnected to another substrate, such as a
mother-board, as known in the art.
[0103] 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.
* * * * *