U.S. patent application number 14/726045 was filed with the patent office on 2015-09-17 for mems microphone with springs and interior support.
The applicant listed for this patent is Invensense, Inc.. Invention is credited to Sushil Bharatan, Thomas D. Chen, Aleksey S. Khenkin.
Application Number | 20150264465 14/726045 |
Document ID | / |
Family ID | 47430094 |
Filed Date | 2015-09-17 |
United States Patent
Application |
20150264465 |
Kind Code |
A1 |
Bharatan; Sushil ; et
al. |
September 17, 2015 |
MEMS MICROPHONE WITH SPRINGS AND INTERIOR SUPPORT
Abstract
A MEMS microphone has a stationary portion with a backplate
having a plurality of apertures, and a diaphragm spaced from the
backplate and having an outer periphery. As a condenser microphone,
the diaphragm and backplate form a variable capacitor. The
microphone also has a post extending between, and substantially
permanently connected with, both the backplate and the diaphragm,
and a set of springs securing the diaphragm to at least one of the
post and the stationary portion. The post is positioned to be
radially inward of the outer periphery of the diaphragm.
Inventors: |
Bharatan; Sushil;
(Burlington, MA) ; Khenkin; Aleksey S.; (Nashua,
NH) ; Chen; Thomas D.; (Cambridge, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Invensense, Inc. |
San Jose |
CA |
US |
|
|
Family ID: |
47430094 |
Appl. No.: |
14/726045 |
Filed: |
May 29, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13348400 |
Jan 11, 2012 |
9078069 |
|
|
14726045 |
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Current U.S.
Class: |
381/111 |
Current CPC
Class: |
H01L 2224/48137
20130101; H04R 7/04 20130101; H04R 7/20 20130101; H04R 2225/49
20130101; H04R 19/005 20130101; H04R 1/08 20130101; H04R 2201/003
20130101; H04R 19/04 20130101 |
International
Class: |
H04R 1/08 20060101
H04R001/08 |
Claims
1. A MEMS microphone comprising: a stationary portion comprising a
backplate having a plurality of apertures; a diaphragm spaced from
the backplate and having an outer periphery and a region radially
inward of the outer periphery, the diaphragm and backplate forming
a variable capacitor, a fixed area of the radially inward region
being substantially unmovable relative to the stationary portion in
response to an incident acoustic signal on the diaphragm, a movable
area of the radially inward region being movable relative to the
stationary portion in response to an incident acoustic signal; and
a set of springs securing the diaphragm to the stationary portion.
Description
PRIORITY
[0001] This application is a continuation application of U.S.
patent application Ser. No. 13/348,400, filed on Jan. 11, 2012, by
Sushil Bharatan, et al., and entitled "MEMS Microphone with Springs
and Interior Support", the disclosure of which is incorporated
herein, in its entirety, by reference.
FIELD OF THE INVENTION
[0002] The invention generally relates to MEMS devices and, more
particularly, the invention relates to MEMS microphones
BACKGROUND OF THE INVENTION
[0003] MEMS condenser microphones typically have a diaphragm that
forms a variable capacitor with an underlying backplate. Receipt of
an audible signal causes the diaphragm to vibrate, consequently
generating a variable capacitance signal representing the audible
signal. It is this variable capacitance signal that can be
amplified, recorded, or otherwise transmitted to another electronic
device.
[0004] Undesirably, noise often degrades the noted variable
capacitance signal. Those skilled in the art often respond to this
problem by enlarging the diaphragm area, which should increase the
variable capacitance signal. This solution, however, can create
further problems. Specifically, stresses and long spans cause the
central region of the diaphragm to droop toward the backplate,
which creates another range of performance problems. For example,
such a diaphragm may be more prone to sticking to the
backplate.
SUMMARY OF VARIOUS EMBODIMENTS
[0005] In accordance with one embodiment of the invention, a MEMS
microphone has a stationary portion with a backplate having a
plurality of apertures, and a diaphragm spaced from the backplate
and having an outer periphery. As a condenser microphone, the
diaphragm and backplate form a variable capacitor. The microphone
also has a post extending between, and substantially permanently
connected with, both the backplate and the diaphragm, and a set of
springs securing the diaphragm to at least one of the post and the
stationary portion. The post is positioned to be radially inward of
the outer periphery of the diaphragm.
[0006] The post may be spaced approximately equidistant between two
different edges of the outer periphery of the diaphragm. For
example, if the diaphragm has a center, then the post may extend
substantially from the center of the diaphragm. Moreover, at least
one of the set of springs may extend from the outer periphery of
the diaphragm to the stationary portion. Alternatively, or in
addition, at least one of the set of set of springs may extend from
the post and to the diaphragm, e.g., the set of springs may secure
the diaphragm to both the post and the stationary portion. The set
of springs may include at least one elongated spring (e.g., in a
serpentine shape) forming a space between the diaphragm and at
least one of the post and the stationary portion.
[0007] Some embodiments have a plurality of additional posts
extending between and contacting both the diaphragm and the
backplate. The post(s) preferably is/are electrically isolated from
one or both of the backplate and the diaphragm. The post and
diaphragm combination applies to a number of differently sized
diaphragms, such as those with a diameter of greater than about 1
millimeter.
[0008] In accordance with another embodiment, a MEMS microphone has
a stationary portion with a backplate having a plurality of
apertures, and a diaphragm, spaced from the backplate, having an
outer periphery and a region radially inward of the outer
periphery. As a condenser microphone, the diaphragm and backplate
form a variable capacitor. The radially inward region of the
diaphragm has a fixed area that is substantially unmovable relative
to the stationary portion in response to an incident acoustic
signal on the diaphragm. In addition, the radially inward region
also has a movable area that is movable relative to the stationary
portion in response to an incident acoustic signal. A set of
springs secures the diaphragm to the stationary portion.
[0009] In accordance with other embodiments, a method of
transducing an acoustic signal provides a MEMS microphone having a
stationary portion with a backplate having a plurality of
apertures, and a diaphragm spaced from the backplate and having an
outer periphery. As a condenser microphone, the diaphragm and
backplate form a variable capacitor. The microphone further has a
post extending from and substantially permanently connected with
the diaphragm. The post is positioned to be radially inward of the
outer periphery of the diaphragm and normally biased to be spaced
from the backplate. After providing this microphone, the method
then energizes the variable capacitor to cause the diaphragm to
move into contact with the backplate and remain in contact with the
backplate while energized.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] Those skilled in the art should more fully appreciate
advantages of various embodiments of the invention from the
following "Description of Illustrative Embodiments," discussed with
reference to the drawings summarized immediately below.
[0011] FIG. 1A schematically shows a top perspective view of a
packaged microphone having a MEMS microphone die configured in
accordance with illustrative embodiments of the invention.
[0012] FIG. 1B schematically shows a bottom perspective view of the
packaged microphone shown in FIG. 1A.
[0013] FIG. 1C schematically shows a cross-sectional view of the
packaged microphone oriented as shown in FIG. 1A.
[0014] FIG. 1D schematically shows a cross-sectional view of a
similar packaged microphone having a bottom port.
[0015] FIG. 2 schematically shows a MEMS microphone die generally
configured in accordance with illustrative embodiments of the
invention.
[0016] FIG. 3A schematically shows a top view of a MEMS microphone
configured with a post and springs coupling to the post and
diaphragm in accordance with one embodiment of the invention.
[0017] FIG. 3B schematically shows a cross-sectional perspective
view of the MEMS microphone shown in FIG. 3A.
[0018] FIG. 4A schematically shows a top view of a MEMS microphone
in accordance with another embodiment of the invention.
[0019] FIG. 4B schematically shows a cross-sectional perspective
view of the MEMS microphone shown in FIG. 4A.
[0020] FIG. 5 schematically shows a cross-sectional perspective
view of a MEMS microphone having multiple posts between the
diaphragm and backplate.
[0021] FIGS. 6A and 6B schematically show a cross-sectional view of
a MEMS microphone die configured in accordance with yet another
embodiment of the invention.
[0022] FIG. 7 shows a process of using the MEMS microphone shown in
FIGS. 6A and 6B.
DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0023] In illustrative embodiments, a MEMS microphone die has a
diaphragm support post to substantially reduce diaphragm sagging.
Accordingly, such embodiments can increase the diaphragm area,
favorably increasing the signal to noise ratio. To ensure even
finer resolution, such embodiments use springs to connect the
diaphragm to 1) the support post, 2) a stationary portion of the
microphone die, or 3) both the support post and stationary portion.
Details of illustrative embodiments are discussed below.
[0024] FIG. 1A schematically shows a top, perspective view of a
packaged microphone 10 that may incorporate a MEMS microphone die
16 (shown in FIGS. 1C, 1D, 2, and others discussed below)
configured in accordance with illustrative embodiments of the
invention. In a corresponding manner, FIG. 1B schematically shows a
bottom, perspective view of the same packaged microphone 10.
[0025] The packaged microphone 10 shown in those figures has a
package base 12 that, together with a corresponding lid 14, forms
an interior chamber containing a microphone chip 16 and, if
desired, a separate microphone circuit chip 18 (both dies 16 and 18
are shown schematically in FIGS. 1C and 1D and discussed below).
The lid 14 in this embodiment is a cavity-type lid, which has four
walls extending generally orthogonally from a top, interior face to
form a cavity. In illustrative embodiments, the lid 14 is formed
from metal or other conductive material to shield the microphone
die 16 from electromagnetic interference. The lid 14 secures to the
top face of the substantially flat package base 12 to form the
interior chamber.
[0026] The lid 14 also has an audio input port 20 that enables
ingress of audio signals into the chamber. In alternative
embodiments, however, the audio port 20 is at another location,
such as through the package base 12, or through one of the side
walls of the lid 14. Audio signals entering the interior chamber
interact with the microphone chip 16 to produce an electrical
signal that, with additional (exterior) components (e.g., a speaker
and accompanying circuitry), produce an output audible signal
corresponding to the input audible signal.
[0027] FIG. 1B shows the bottom face 22 of the package base 12,
which has a number of contacts 24 for electrically (and physically,
in many anticipated uses) connecting the microphone die 16 with a
substrate, such as a printed circuit board or other electrical
interconnect apparatus. The packaged microphone 10 may be used in
any of a wide variety of applications. For example, the packaged
microphone 10 may be used with mobile telephones, land-line
telephones, computer devices, video games, hearing aids, hearing
instruments, biometric security systems, two-way radios, public
announcement systems, and other devices that transduce signals. In
fact, it is anticipated that the packaged microphone 10 could be
used as a speaker to produce audible signals from electronic
signals.
[0028] In illustrative embodiments, the package base 12 shown in
FIGS. 1A and 1B may be a printed circuit board material, such as
FR-4, or a premolded, leadframe-type package (also referred to as a
"premolded package"). Other embodiments may use different package
types, such as ceramic cavity packages. Accordingly, discussion of
a specific type of package is for illustrative purposes only.
[0029] FIG. 1C schematically shows a cross-sectional view of the
packaged microphone 10 across line C-C of FIG. 1A. As shown and
noted above, the lid 14 and base 12 form the noted internal chamber
for containing a MEMS microphone die 16 and electronics 18 used to
control and drive the microphone die 16. In illustrative
embodiments, electronics are implemented as a second, stand-alone
integrated circuit, such as an application specific integrated
circuit ("ASIC 18"). Other embodiments, however, may form the MEMS
microstructure and electronic circuitry on a single die.
[0030] Adhesive or another fastening mechanism secures both the
microphone die 16 and ASIC die 18 to the base 12. Wirebonds
electrically connect the microphone die 16 and ASIC die 18 to
contact pads (not shown) on the interior of the package base
12.
[0031] While FIGS. 1A-1C show a top port packaged microphone
design, some embodiments position the input port at other
locations, such as through the base 12. For example, FIG. 1D
schematically shows a cross-sectional view of a similar packaged
microphone 10 where the microphone die 16 covers the input port,
consequently producing a large back volume. Other embodiments, not
shown, position the microphone die 16 so that it does not cover the
input port 20 through the base 12.
[0032] Discussion of a specific packaged microphone is for
illustrative purposes only. Accordingly, the packaged microphone 10
discussed with regard to FIGS. 1A-1D are not intended to limit all
embodiments of the invention.
[0033] FIG. 2 schematically shows a top, perspective view of the
MEMS microphone die 16 (also referred to as a "microphone chip 16")
that may be fabricated in accordance with illustrative embodiments
of the invention. Subsequent FIGS. 3A-6 show top and
cross-sectional views 18 across line 2-2 of FIG. 2 of the same
microphone die 16 in various different embodiments.
[0034] Among other things, the microphone die 16 includes a
stationary portion 26 that supports and forms a variable capacitor
28 with a flexible diaphragm 30. In illustrative embodiments, the
stationary portion 26 includes a backplate 32 (shown in subsequent
figures and discussed below) formed from single crystal silicon
(e.g., the top layer of a silicon-on-insulator wafer, discussed
below) and other deposited layers, while the diaphragm 30 is formed
from a deposited material only, such as deposited polysilicon.
Other embodiments, however, use other types of materials to form
the stationary portion 26 and the diaphragm 30. For example, a
single crystal silicon bulk wafer, and/or some deposited material,
may form the stationary portion 26. In a similar manner, a single
crystal silicon bulk wafer, part of an silicon-on-insulator wafer,
or some other deposited material may form the diaphragm 30.
[0035] Springs 34 movably and integrally connect the outer
periphery of the diaphragm 30 to the stationary portion 26 of the
microphone die 16. The springs 34 effectively form a plurality of
apertures 36 that permit at least a portion of the audio/acoustic
signal to pass through the diaphragm 30. These apertures 36, which
also may be referred to as "diaphragm apertures 36," may be any
shape as required by the application, such as in the shape of a
slot, round hole, or some irregular shape. Electrical contacts 25
on the top face of the dies 16 and 18 provide electrical connection
for the wirebonds shown in FIGS. 1C and 1D.
[0036] FIGS. 3A and 3B show more details of this embodiment of the
MEMS microphone die 16, but with a generally rectangular diaphragm
30. In particular, FIG. 3A to schematically shows more detail of a
top view of the microphone die 16 shown in FIG. 2. FIG. 3B
schematically shows a cross-sectional view of that same microphone
die 16.
[0037] As shown best in FIG. 3B, the stationary portion 26 of the
microphone die 16 has a backplate 32 that, together with the
diaphragm 30, forms a variable capacitor 28. As known by those
skilled in the art, the variable capacitor 28 produces electrical
signals representing input acoustic signals incident on the
diaphragm 30. To facilitate operation, the backplate 32 has a
plurality of through-hole apertures ("backplate apertures 38") that
lead to a backside cavity 40. The apertures 38 may be shaped,
sized, and configured to optimize performance. One such
configuration has both generally round holes and slots (i.e.,
elongated holes), while another configuration has only generally
round holes arranged in a specific pattern.
[0038] When at rest, whether energized or not, many prior art
diaphragms deform to some extent (e.g., they may sag, among other
things) due to inherent stresses and the forces of gravity.
Portions of some prior art diaphragms may sag between 25 and 50
percent of the "nominal distance/gap 42" between it and the
backplate 32. More specifically, this "nominal distance/gap 42" is
considered to be the distance between the diaphragm 30 and the
backplate 32, at rest, if the diaphragm 30 were perfectly flat. For
example, some portions of the diaphragm 30 may be spaced about 3
microns from the backplate 32, while more central regions of the
diaphragm 30 may be spaced about two microns from the backplate
32.
[0039] Sagging increases the likelihood that the diaphragm 30 will
contact the backplate 32, which can cause a multitude of problems.
For example, the diaphragm 30 may stick to the backplate 32 (known
in the art as "stiction"), which can destroy the device, or cause a
short circuit between the diaphragm 30 and the backplate 32. Even
if those noted problems do not occur, contact between the diaphragm
30 and back plate often reduces the quality of the output signal by
distorting the signal peaks (e.g., flattening the signal peaks),
among other things.
[0040] Diaphragm sag may not be a concern in some microphone
designs that have very small diaphragms (e.g., longest dimensions
parallel with the X-Y plane of FIG. 3A of less than about 1
millimeter). The inventors realized, however, that smaller
microphones often have a lower signal to noise ratio than those
with larger diaphragms. A larger diaphragm thus often is desirable
to improve microphone performance. For example, diaphragms having
maximum dimensions of greater than 1 millimeter should provide
improved signal to noise ratios if sag can be controlled.
[0041] Accordingly, to control diaphragm sag, illustrative
embodiments of the invention, as shown in FIGS. 2, 3A, and 3B, have
at least one supporting post 44 supporting an interior portion of
the diaphragm 30. In other words, the supporting post 44
effectively immobilizes a portion of the diaphragm 30, causing it
to be substantially stationary. This often is near the general
center or centroid of the diaphragm 30. For example, the post 44
may be positioned substantially equidistant between two opposing
edges of a diaphragm 30. Although various embodiments are believed
to produce improved results with larger diaphragms (i.e., larger
than about 1 millimeter), various embodiments also apply to smaller
diaphragms.
[0042] As noted above, springs 34 support the periphery of the
diaphragm 30 by coupling with the stationary portion 26. Those
springs 34 shown in FIG. 3A and FIG. 3B form the above noted spring
aperture or space 36. Accordingly, some portion of the periphery of
the diaphragm 30 is not directly connected with the stationary
portion 26. FIG. 3A calls out one portion of the periphery with the
words "unsupported portion." In any event, many points around the
diaphragm periphery are supported. Thus, despite the fact that
there is some portion of the periphery that is suspended, this
arrangement is not considered to be a cantilevered
configuration.
[0043] For example, the diaphragm periphery may have areas that
connect with a spring 34 at least every 180 degrees or less (i.e.,
opposing springs 34 on each side of the diaphragm 30). Other
embodiments may have springs 34 that are spaced apart about 120
degrees or less, about 90 degrees or less, about 60 degrees or
less, about 45 degrees or less, etc. . . .
[0044] The rectangularly shaped diaphragm 30 shown in FIG. 3A has
16 springs 34 substantially equidistantly spaced around the
diaphragm periphery. As a rectangular diaphragm 30, these 16
springs 34 are not necessarily precisely and equally spaced around
the diaphragm 30 as they would be if the diaphragm 30 were round.
Instead, they are positioned so that they have at least two opposed
springs 34 (i.e., about 180 degrees apart), and others
therebetween. Thus, this rectangularly shaped diaphragm 30 may be
considered to have springs 34 spaced about 180 degrees apart and
less. Another way of viewing this diaphragm 30 is that it has a
plurality of straight sides, and each side has at least one spring.
Accordingly, if the diaphragm 30 were another polygon-type shape,
such as an octagon, pentagon, etc. . . . , then it would have at
least one spring 34 on each side and thus, not be cantilevered.
Each side can have the same number, type and spacing of springs 34,
or a different number, type and spacing of springs 34. If the
diaphragm 30 is irregularly shaped, then it is anticipated that at
least having springs 34 180 degrees apart would provide
satisfactory results.
[0045] The springs 34 around the diaphragm periphery in FIGS. 3A
and 3B are elongated and formed in a serpentine manner. As known by
those in the art, serpentine springs 34 can provide more control of
the diaphragm 30 than direct, straight stubby springs 34. Of
course, other types of springs 34 should suffice, such as a spring
34 that merely is a solid connection to the stationary portion 26
(i.e., like a drum). Such type of spring 34 is thinner to permit
more diaphragm flexibility and designed to permit a more
piston-like depression of the diaphragm 30 (see below). In
addition, although the microphone die 16 of FIG. 3A has
symmetrically positioned springs 34, alternative embodiments may
have springs 34 that are not symmetrically positioned.
[0046] This is in contrast to some prior art designs, which may
secure to a supporting post 44 only, or one side of the stationary
portion 26. Specifically, rather than connect the diaphragm
periphery at all, some prior art designs actually permit the outer
periphery of the diaphragm 30 to hang in a cantilevered fashion.
The inventors realized that such a prior art design is less
controllable and often produces poor results. Illustrative
embodiments should more controllably move toward and away from the
backplate 32, in the Z-direction, in a more planar, piston-like
manner than the noted prior art designs.
[0047] To that end, springs 34 also may couple between the
supporting post 44 and inner periphery of the diaphragm 30 to
further improve performance. Those springs 34 may be any of the
types discussed above, and also have unsupported areas that create
spaces 36 between the post 44 and the diaphragm 30 or spring 34,
whichever is the case. Like the outer periphery springs 34, these
inner springs 34 may be positioned 180 degrees or less around the
supporting post 44 (e.g., 120 degrees or less, 90 degrees or less,
as shown in FIG. 3A, etc. . . . ), or in the other manners
discussed above with regard to the outer springs 34 (e.g., one on
each side of the post 44 having a cross-section with a polygon
shape). Other embodiments do not use springs between the post 44
and diaphragm 30 (discussed below).
[0048] As suggested above, the post 44 may take on any of a number
of forms, shapes, and dimensions. For example, the post 44 may have
a generally rectangular, rounded, or irregular cross sectional
shape. In illustrative embodiments, the post 44 is formed from
polysilicon, integrally extending from the diaphragm 30 and
integrally connected with the backplate 32. Such a connection may
be considered one type of permanent connection (i.e., absent
unusual forces, such as using a saw, the components will remain
connected). Unlike certain prior art non-MEMS microphones, the post
44 does not conduct current to or from the diaphragm 30. Instead,
the post 44 essentially is an electrical dead end.
[0049] Accordingly, the post 44 does not electrically connect the
diaphragm 30 and the backplate 32 because if it did, the two plates
of the variable capacitor 28 (i.e., the backplate 32 and diaphragm
30) would be the same potential, causing the capacitor not to work.
To that end, the post 44 preferably has some insulating material
between it and one or both of the diaphragm 30 and the backplate
32. For example, during fabrication, before depositing the
diaphragm layer, processes may form an insulating oxide layer
(e.g., a native oxide) on the top side of the post 44 (i.e., the
side of the post 44 that ultimately connects with the diaphragm
30). Subsequent fabrication steps thus may deposit a polysilicon
diaphragm layer over that oxide. The oxide layer between the top of
the post 44 and the diaphragm 30 thus prevents electrical
communication between the diaphragm 30 and backplate 32 via the
post 44.
[0050] Of course, the MEMS microphone die 16 may have other means
for electrically isolating the post 44 from one or both of the
diaphragm 30 and the backplate 32. Another technique may form an
oxide or insulating layer between the bottom of the post 44 in the
backplate 32. Other embodiments may form an electrically insulating
trench around the post 44 around the region where it connects to
either or both the diaphragm 30 and the backplate 32. Other
embodiments may simply form the post 44 with a nonconductive
material, such as undoped silicon, oxide, nitride, or some
combination of conductive and non-conductive materials commonly
used in MEMS fabrication processes.
[0051] While FIGS. 3A and 3B show springs 34 both along the inner
periphery and outer periphery of the diaphragm 30, other
embodiments may omit springs 34 from one or both of those regions.
FIGS. 4A and 4B schematically show one embodiment, the former in a
plan view, and the latter in a cross-sectional perspective view.
This embodiment retains the springs 34 along the outer periphery,
and eliminates the springs 34 between the post 44 and the diaphragm
30. Other embodiments may have springs 34 along the inner diaphragm
periphery only, or no springs 34 along either the inner or outer
diaphragm periphery. FIG. 4B incidentally show depressions (each
having three or four portions protruding from a generally central
point) on the diaphragm 30 corresponding to the apertures 38
through the back plate 32.
[0052] Using the post 44 or other means to cause the general
central region of the diaphragm 30 to remain substantially
stationary as a number beneficial effects. Specifically, it
significantly shortens the length/span of the diaphragm 30. As an
example, consider a circularly shaped diaphragm with a one
millimeter diameter that normally sags, approximately at its
center, about 1 micron across a nominal three micron gap 42 (i.e.,
if perfectly planar, the gap 42 between the diaphragm 30 and the
backplate 32 would be three microns). Positioning a post 44 about
at the center of the diaphragm 30 should reduce diaphragm sag by
about 50 percent. Accordingly, the remaining movable portions of
the diaphragm 30 (i.e., the portions not held substantially
stationary by the post 44) should sag no more than about 0.5
microns.
[0053] The inventors realized that this reduction in diaphragm sag
enables them to enlarge the diaphragm 30, thus increasing the
signal produced by the variable capacitor 28. Accordingly,
continuing with the above example, the diaphragm 30 now may have a
diaphragm 30 that is twice the size; namely, having a diameter of
about 2 millimeters. The diaphragm 30 sag again, however, is
expected to be about one micron, at worst. However, since the area
increases with the square of the radius, this produces a four-fold
increase in surface area and thus, a corresponding four-fold
signal-to-noise ratio boost.
[0054] It should be noted that the area taken up by the post 44
does degrade the improved signal to some extent. Contrary to this
intuitive suggestion to avoid a post to maximize surface area of
the variable capacitor 28, however, the inventors realized that the
significantly improved signal substantially overrides this
degradation by the post. In other words, the post 44 produces no
more than an insignificant reduction or degradation in the signal
quality. In addition, noise produced by the variable capacitor 28
does not increase at a rate corresponding with the increase in
signal. This provides the favorable result of substantially
improving the signal to noise ratio of the microphones die 16.
[0055] The inventors discovered that the supporting post 44
provides a number of additional benefits. Specifically, those
skilled in the art know that the ASIC die 16 has an input
capacitance that reduces the net signal produced by the packaged
microphone 10. The substantially increased microphone die
capacitance thus minimizes or reduces the negative impact of this
input ASIC capacitance, and other parasitic capacitances in the
system. In addition, the larger microphone die capacitance reduces
noise by moving its effective low-frequency cut off.
[0056] During use, an incident acoustic signal causes the diaphragm
30 to move generally along the Z-axis. Ideally, the diaphragm 30
moves along the Z-axis as a plunger--nearly all of its points move
upwardly and downwardly at the same rate. The springs 34 are
configured to ensure that the diaphragm 30 moves in the intended
manner upon receipt of signals having specified amplitudes. For
example, certain microphone dies may be produced for low amplitude
acoustic signals (e.g., signals that are not considered very loud)
and thus, have springs 34 with relatively low spring constants.
Such springs 34 may be thinner, longer, or made in a manner that
permits such behavior. Indeed, a very high amplitude signal
striking such a microphone could damage the springs 34 and
diaphragm 30.
[0057] Although discussed as being generally near or at the general
centroid or center of the diaphragm 30, some embodiments (as noted
above) may offset the post 44 to other locations relative to the
diaphragm 30. FIG. 5 schematically shows another embodiment in
which the microphone die 16 has a plurality of posts 44. These
posts 44 may be strategically positioned depending upon the
anticipated use and application of the microphone chip 16. Those
skilled in the art can select the appropriate locations based upon
anticipated use.
[0058] Other embodiments have a movable post 44 between the
diaphragm 30 and the backplate 32. FIGS. 6A and 6B show one such
embodiment, in which the post 44 integrally extends that relief
from the diaphragm 30. As shown in FIG. 6A, the un-energized
position, the diaphragm 30 is normally biased so that the post 44
does not contact the backplate 32--i.e., when the microphone chip
16 is not energized. The post 44 may extend substantially the
entire length of the diaphragm 30/backplate 32 gap 42 and leave
only a small space for it to travel upon receipt of a voltage. For
example, when used in a microphone chip 16 having a nominal 3
micron gap 42 between the diaphragm 30 and the backplate 32, the
post 44 may extend so that it is only about 0.1 or 0.2 microns from
the backplate 32 when unenergized. This distance also takes
nominal, unenergized sag into account. Other embodiments may permit
the post 44 to span the entire diaphragm 30/backplate 32 gap 42
even when not energized.
[0059] It should be noted that this cross-sectional view of FIG. 6A
is across a spring 34 on the left side of the diaphragm 30 (from
the perspective of figure) and an open space 36 on the right side
of the diaphragm 30. In other words, the spring structure around
this embodiment can be similar to any of those described above with
regard to other embodiments.
[0060] Application of an electrostatic force (e.g., a force
resulting from a voltage difference) between the diaphragm 30 and
backplate 32 of FIG. 6A forces the diaphragm 30 downwardly until
the post 44 contacts the backplate 32 (FIG. 6B). At this point, the
microphone die 16 is ready to receive and transduce input acoustic
signals.
[0061] Rather than extending from the diaphragm 30, alternative
embodiments of the microphone chip 16 shown in FIGS. 6A and 6B
extend the post 44 upwardly from the backplate 32. This is somewhat
less desirable than the embodiment shown in FIGS. 6A and 6B because
the diaphragm 30 often as much thinner than the backplate 32 and
thus, may become damaged and brought down with too much force onto
the top of the post 44.
[0062] FIG. 7 shows a simplified process of using a MEMS microphone
chip 16 shown in FIGS. 6A and 6B. The process begins at step 700,
which energizes the microphone die 16. This causes the diaphragm 30
and post 44 to move downwardly to contact the backplate 32 (FIG.
6B). While energized, the portion of the diaphragm 30 integrally
connected with the post 44 should remain substantially
stationary.
[0063] At this point, the MEMS microphone die 16 receives and
converts incoming acoustic signals in normal course (step 702).
Finally, the process concludes at step 704 by de-energizing the
microphone die 16. The resiliency of the springs 34 and diaphragm
30 thus cause the diaphragm 30 and post 44 to return to the rest
position as shown in FIG. 6A.
[0064] Although the above discussion discloses various exemplary
embodiments of the invention, it should be apparent that those
skilled in the art can make various modifications that will achieve
some of the advantages of the invention without departing from the
true scope of the invention.
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