U.S. patent application number 14/732577 was filed with the patent office on 2015-09-24 for microphone with aligned apertures.
The applicant listed for this patent is Invensense, Inc.. Invention is credited to Thomas Chen, Kieran P. Harney, Eric Langlois, Xin Zhang.
Application Number | 20150271617 14/732577 |
Document ID | / |
Family ID | 40095904 |
Filed Date | 2015-09-24 |
United States Patent
Application |
20150271617 |
Kind Code |
A1 |
Langlois; Eric ; et
al. |
September 24, 2015 |
MICROPHONE WITH ALIGNED APERTURES
Abstract
A MEMS microphone has a backplate with a given backplate
aperture, and a diaphragm having a diaphragm aperture. The given
backplate aperture is substantially aligned with the diaphragm
aperture.
Inventors: |
Langlois; Eric; (Waltham,
MA) ; Chen; Thomas; (Cambridge, MA) ; Zhang;
Xin; (Acton, MA) ; Harney; Kieran P.;
(Andover, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Invensense, Inc. |
San Jose |
CA |
US |
|
|
Family ID: |
40095904 |
Appl. No.: |
14/732577 |
Filed: |
June 5, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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12133599 |
Jun 5, 2008 |
9078068 |
|
|
14732577 |
|
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|
|
60942315 |
Jun 6, 2007 |
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Current U.S.
Class: |
216/17 |
Current CPC
Class: |
B81C 1/00182 20130101;
H04R 1/222 20130101; Y10T 29/49005 20150115; H04R 19/04 20130101;
H04R 2499/11 20130101; H04R 19/005 20130101; H04R 2201/003
20130101; B81C 1/00523 20130101; B81C 1/00158 20130101; H04R 31/00
20130101 |
International
Class: |
H04R 31/00 20060101
H04R031/00; H04R 19/00 20060101 H04R019/00 |
Claims
1. A method of forming a MEMS microphone, the method comprising:
providing a backplate; forming a diaphragm spaced from the
backplate, the diaphragm forming a diaphragm aperture; and forming
a plurality of backplate apertures, a given aperture of the
backplate apertures being at least partially aligned with the
diaphragm aperture.
2. The method as defined by claim 1 wherein the given aperture is
substantially aligned with the diaphragm aperture.
3. The method as defined by claim 1 wherein the plurality of
backplate apertures includes a second aperture that is offset from
the diaphragm aperture.
4. The method as defined by claim 1 wherein forming a diaphragm
comprises: depositing a deposition material onto a sacrificial
material supported by the backplate; forming a plurality of
springs; and removing the sacrificial material to form the
diaphragm, the plurality of springs suspending the diaphragm, the
diaphragm being vertically spaced from the backplate.
5. The method as defined by claim 1 wherein providing a backplate
comprises forming the backplate from an SOI wafer.
6. The method as defined by claim 1 wherein forming a plurality of
backplate apertures includes etching a slot through a wafer.
Description
PRIORITY
[0001] This application is a divisional application of U.S. patent
application Ser. No. 12/133,599, filed on Jun. 5, 2008, by Eric
Langlois, et al., entitled, "Microphone with Aligned Apertures",
which claims priority from provisional U.S. patent application No.
60/942,315, filed Jun. 6, 2007, entitled, "MICROPHONE WITH ALIGNED
APERTURES," and naming Eric Langlois, Thomas Chen, Xin Zhang, and
Kieran Harney as joint inventors, the disclosure of which is
incorporated herein, in its entirety, by reference.
FIELD OF THE INVENTION
[0002] The invention generally relates to microphones and, more
particularly, the invention relates to controlling the low
frequency cutoff point for microphone.
BACKGROUND OF THE INVENTION
[0003] Condenser microphones generally have a movable diaphragm
that vibrates to produce a signal representative of an incident
audio signal. To ensure that audio signals contact their respective
diaphragms, prior art condenser microphones known to the inventors
have apertures in their backplates directly under a solid portion
of the diaphragm. Accordingly, audio signals pass through the
backplate apertures to directly contact the diaphragm.
[0004] Condenser microphones typically are responsive to audio
signals having frequencies that are greater than a predetermined
low frequency cutoff point. This low frequency cutoff point often
is set by controlling the resistance of the air flowing past the
microphone diaphragm. This resistance, however, can be relatively
high due to the positioning of the apertures directly under a solid
portion of the diaphragm. Undesirably, setting the low frequency
cutoff can be difficult due to such high resistance.
[0005] One method of controlling this low frequency cutoff
point/resistance varies the gap formed between the diaphragm and
the stationary support structure supporting the diaphragm. For
example, the gap may be enlarged to raise the cutoff point, or
reduced to lower the cutoff point. Such a method, however, has
drawbacks. Among other things, it dictates the gap size in a manner
that may interfere with other design considerations.
[0006] In addition, controlling the gap size often does not
sufficiently address the above noted air resistance problem, in
which the backplate aperture is directly under a solid portion of
the diaphragm. Specifically, a portion of the sound wave path must
be generally horizontal to reach the diaphragm gap. As such,
controlling the gap size provides relatively coarse control of the
cutoff point. Electronic or other non-mechanical means then may be
required to sufficiently tune the cutoff point of the
microphone.
SUMMARY OF THE INVENTION
[0007] In accordance with one embodiment of the invention, a MEMS
microphone has a backplate with a given backplate aperture, and a
diaphragm having a diaphragm aperture. The given backplate aperture
is substantially aligned with the diaphragm aperture.
[0008] For example, the given backplate aperture is not offset from
the diaphragm aperture. The given backplate may form any of a
number of shapes, such as a slot or generally round opening. In a
similar manner, the diaphragm aperture also may form any of a
number of different shapes, such as a slot.
[0009] The backplate may be generally parallel with and spaced in a
vertical direction from the diaphragm, while the given backplate
aperture may be substantially aligned with the diaphragm aperture
in the vertical direction. The MEMS microphone also may have a
plurality of springs coupling the diaphragm to a substrate. As
such, the plurality of springs may define (at least in part) the
diaphragm aperture.
[0010] The backplate may have first and second sets of backplate
apertures. The given backplate aperture may be in the first set,
while the second set of backplate apertures may be offset from the
diaphragm aperture.
[0011] In accordance with another embodiment of the invention, a
MEMS microphone has a stationary support, a movable diaphragm, and
a plurality of springs movably connecting the diaphragm to the
stationary support. The microphone also has a backplate, with a
plurality of apertures, that is spaced from the diaphragm. The
stationary support, diaphragm, and springs form a plurality of
diaphragm apertures, while a first diaphragm aperture is at least
partially aligned with a first backplate aperture.
[0012] In this and other embodiments, the backplate may have
another backplate aperture that is not aligned with (i.e., it is
offset from) the first diaphragm aperture.
[0013] In accordance with other embodiments of the invention, a
method of forming a MEMS microphone provides a backplate, forms a
diaphragm spaced from the backplate, and forms a plurality of
backplate apertures. The diaphragm forms a diaphragm aperture, and
a given aperture of the backplate apertures is at least partially
aligned with the diaphragm aperture.
[0014] The diaphragm may be formed by depositing a deposition
material onto a sacrificial material supported by the backplate,
forming a plurality of springs, and removing the sacrificial
material. The plurality of springs suspend the diaphragm so that it
is vertically spaced from the backplate. Among other things the
backplate may be formed from an SOI wafer (i.e., a
silicon-on-insulator wafer).
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] 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.
[0016] FIG. 1 schematically shows a mobile telephone that may use a
MEMS microphone configured in accordance with illustrative
embodiments of the invention.
[0017] FIG. 2 schematically shows a MEMS microphone that may be
configured in accordance with illustrative embodiments of the
invention.
[0018] FIG. 3 schematically shows a cross-sectional view of the
microphone shown in FIG. 1 across line 2-2.
[0019] FIGS. 4A and 4B schematically show plan views of two
different backplate designs that may be used in accordance with
illustrative embodiments of the invention.
[0020] FIG. 5 schematically shows a plan view of a diaphragm that
may be used in accordance with illustrative embodiments of the
invention.
[0021] FIGS. 6A and 6B show a process of forming a microphone that
is similar to the microphone 18 shown in FIGS. 2 and 3 in
accordance with illustrative embodiments of the invention.
[0022] FIGS. 7A-7G schematically show the microphone of FIG. 2
during various stages of fabrication using the process of FIG.
6.
DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0023] In illustrative embodiments, the diaphragm and backplate of
a MEMS microphone cooperate to reduce air resistance through the
microphone. As a result, the microphone can be more easily tuned to
a precise low frequency cutoff point. Details of illustrative
embodiments are discussed below.
[0024] FIG. 1 schematically shows a mobile telephone 10 that can
use a microphone configured in accordance with illustrative
embodiments. In simplified terms, the telephone 10 has a receiver
12 for receiving an audio signal (e.g., a person's voice), a
speaker portion 14 for generating audio signals, and a transponder
16 for transmitting and receiving electromagnetic signals encoding
audio signals. During use, a person may speak into the receiver 12,
which has a microphone (FIG. 2, discussed below) that converts the
person's voice into an electrical signal. Internal logic (not
shown) and the transponder 16 modulate this signal to a remote
source, such as to a satellite tower and, ultimately, to another
person on another telephone 10.
[0025] In illustrative embodiments, the receiver 12 has a
microphone mechanically configured with a relatively precise low
frequency cutoff point (i.e., the lowest frequency that it can
detect without significant distortion--often referred to in the art
as the "3 dB point"). FIG. 2 schematically shows a top, perspective
view of a MEMS microphone 18 (also referred to as a "microphone
chip 18") that may be fabricated in this manner; namely, in
accordance with illustrative embodiments of the invention. FIG. 3
schematically shows a cross-sectional view of the same microphone
18 across line 2-2 of FIG. 2.
[0026] Among other things, the microphone 18 includes a static
backplate 20 that supports and forms a variable capacitor with a
flexible diaphragm 22. In illustrative embodiments, the backplate
20 is formed from single crystal silicon (e.g., the top layer of a
silicon-on-insulator wafer, discussed below), while the diaphragm
22 is formed from a deposited material, such as deposited
polysilicon. Other embodiments, however, use other types of
materials to form the backplate 20 and the diaphragm 22. For
example, a single crystal silicon bulk wafer, or some deposited
material, may form the backplate 20. In a similar manner, a single
crystal silicon bulk wafer, part of a silicon-on-insulator wafer,
or some other deposited material may form the diaphragm 22.
[0027] To facilitate operation, the backplate 20 has a plurality of
through-hole apertures ("backplate apertures 24") that lead to a
backside cavity 26. FIGS. 4A and 4B schematically show plan views
of two different backplates 20 that each have different
configurations of backplate apertures 24. One such configuration
has both generally round holes and slots (i.e., elongated holes),
while the other configuration has only generally round holes
arranged in a specific pattern. It nevertheless should be noted
that the patterns and types of apertures 24 shown in FIGS. 4A and
4B are just illustrative and not intended to limit all embodiments
of the invention. Various embodiments thus may employ other
configurations of apertures 24.
[0028] Springs 28 movably connect the diaphragm 22 to a
static/stationary portion 30 of the microphone 18, which includes a
substrate (also identified by reference number "30"). The springs
28 effectively form a plurality of apertures that permit at least a
portion of the audio signal to pass through the diaphragm 22. These
apertures 32, which also are referred to as "diaphragm apertures
32," may be any reasonable shape, such as in the shape of a slot,
round hole, or some irregular shape.
[0029] More specifically, FIG. 5 schematically shows a diaphragm 22
that may be used in accordance with illustrative embodiments of the
invention. As shown, the diaphragm 22 has four springs 28 that
suspend it to be generally parallel to and above the backplate 20.
In other words, from the perspective of FIG. 3, the diaphragm 22
may be considered to be vertically spaced from the backplate 20.
With reference to the diaphragm 22 shown in FIG. 5, the following
portions of the microphone 18 effectively form each noted diaphragm
aperture 32:
[0030] 1) each adjacent pair of springs 28,
[0031] 2) the stationary portion 30 immediately adjacent to and
between the spring pairs, and
[0032] 3) the corresponding diaphragm edge 34 between the pair of
springs 28.
[0033] For example, the apertures 32 shown in FIG. 5 effectively
are slots.
[0034] Other embodiments, however, may have other types of springs
28 and apertures 24 and 32. For example, the springs 28 may have a
serpentine shape, such as that disclosed in co-pending U.S. patent
application Ser. No. 12/015,903, filed Jan. 17, 2008, and attorney
docket number 2550/B81, the disclosure of which is incorporated
herein, in its entirety, by reference. In that patent application,
the apertures through the diaphragm have shapes corresponding to
the serpentine nature of the springs.
[0035] Incident audio signals cause the diaphragm 22 to vibrate,
thus producing a changing capacitance between it and the backplate
20. Such audio signals may contact the microphone 18 from any
direction. For example, in FIG. 3, the audio signals are shown as
traveling upwardly, first through the backplate 20, and then
partially through and against the diaphragm 22. In other
embodiments, the audio signals may travel in the opposite
direction. On-chip or off-chip circuitry (not shown) receive (via
contacts 36 of FIG. 2) and convert this changing capacitance into
electrical signals that can be further processed.
[0036] It should be noted that discussion of the specific
microphone 18 shown in FIGS. 2-5 is for illustrative purposes only.
Other microphone configurations thus may be used consistent with
various embodiments of the invention.
[0037] In accordance with illustrative embodiments of the
invention, the backplate apertures 24 are substantially aligned
with the diaphragm apertures 32. This is in contrast to prior art
designs known to the inventors, which offset the vertical alignment
of the backplate apertures 24 and diaphragm apertures 32.
[0038] Accordingly, as shown in FIG. 3, at least a portion of an
incident audio signal can traverse substantially straight through
the microphone 18. Such alignment therefore reduces the air
resistance through the microphone 18 because a portion of such
audio signals does not travel in a direction that is generally
parallel to the plane of the diaphragm 22.
[0039] In some embodiments, the diaphragm apertures 32 are
substantially exactly aligned with the apertures 24 through the
backplate 20 (e.g., see FIG. 3). Other embodiments, however, may
only partially align the diaphragm apertures 32 and the backplate
apertures 24. Moreover, in illustrative embodiments, the aligned
backplate apertures 24 are substantially the same shape and area as
that of the diaphragm apertures 32. Alternative embodiments,
however, do not have such a requirement.
[0040] One backplate aperture 24 may at least partially align with
one or more diaphragm apertures 32. In a corresponding manner, one
diaphragm aperture 32 may at least partially align with one or more
backplate apertures 24. Those skilled in the art can use other
alignment configurations within the spirit of various embodiments.
These configurations may be useful with microphones having
serpentine springs. Specifically, microphones having serpentine
springs may be considered to form a plurality of regularly or
irregularly shaped diaphragm apertures 32. For example, some of
those diaphragm apertures 32 may be spaced radially from each
other, and/or along the general circumference of the diaphragm
22.
[0041] As shown in FIGS. 4A and 4B, the backplate 20 may be
considered to have at least two sets of backplate apertures 24;
namely, a first set that is not aligned with any diaphragm
apertures 32 (i.e., they are offset from the diaphragm apertures
32), and a second set that is substantially aligned with diaphragm
apertures 32. The second set shown in FIGS. 4A and 4B are radially
outwardly positioned from the first set. Despite that, some
embodiments may have additional diaphragm apertures 32 that are not
formed by the springs 28, stationary portion 30, and edge 34 of the
diaphragm 22. Instead, these diaphragm apertures 32 may be formed
by holes through the diaphragm 22 and may have, or may not have,
corresponding aligned backplate apertures 24. In fact, some
embodiments have both types of diaphragm apertures 32.
[0042] As noted above, the inventors discovered that alignment of
the diaphragm and backplate apertures 32 and 24, or even partial
alignment, enabled them to more precisely tune the low frequency
cutoff point while still maintaining relatively thin diaphragm
apertures 32. For example, this low frequency cutoff point may be
set to between about 50 and 100 Hertz without requiring use of
filtering electronics. This is contrary to the inventors'
understanding of the prior art, which preferred offset apertures to
ensure more of the signal contacted the diaphragm. Thus, contrary
to what they understood to be the conventional wisdom, the
inventors determined that the resulting signal loss, if any, due to
aperture alignment was negligible. Accordingly, since such loss was
negligible, the inventors were able to deviate from the prior art
practice of intentionally misaligning the noted apertures.
[0043] This alignment also provides some stress relief in
overpressure events. Specifically, by reducing the air resistance
through the microphone 18, this alignment permits air pressure to
pass more freely through the microphone 18. As a result, the
springs 28 are less stressed and, consequently, less likely to
fracture during overpressure events.
[0044] FIGS. 6A and 6B show a process of forming a microphone that
is similar to the microphone 18 shown in FIGS. 2 and 3 in
accordance with illustrative embodiments of the invention. The
remaining figures (FIGS. 7A-7G) illustrate various steps of this
process. It should be noted that this process does not describe all
steps required for forming the microphone 18. Instead, it shows
various relevant steps for forming the microphone 18. Accordingly,
some steps are not discussed.
[0045] The process begins at step 600, which etches trenches 38 in
the top layer of a silicon-on-insulator wafer ("SOI wafer 40").
These trenches 38 ultimately form the backplate apertures 24--some
of which are aligned in the manner discussed above with the
yet-to-be-formed diaphragm apertures 32.
[0046] Next, the process adds sacrificial oxide 42 to the walls of
the trenches 38 and along at least a portion of the top surface of
the top layer of the SOI wafer 40 (step 602). Among other ways,
this oxide 42 may be grown or deposited. FIG. 7A schematically
shows the wafer at this point in the process. Step 602 continues by
adding sacrificial polysilicon 44 to the oxide lined trenches 38
and top-side oxide 42.
[0047] After adding the sacrificial polysilicon 44, the process
etches a hole 46 into the sacrificial polysilicon 44 (step 604, see
FIG. 7B). The process then continues to step 606, which adds more
oxide 42 to substantially encapsulate the sacrificial polysilicon
44. In a manner similar to other steps that add oxide 42, this
oxide 42 essentially integrates with other oxides it contacts. Step
606 continues by adding an additional polysilicon layer that
ultimately forms the diaphragm 22 (see FIG. 7C). This layer
illustratively is patterned to substantially align at least some of
the diaphragm apertures 32 with some of the backplate apertures 24
in the manner discussed above.
[0048] Nitride 48 for passivation and metal for electrical
connectivity also are added (see FIG. 7D). For example, deposited
metal may be patterned to form a first electrode 50A for placing
electrical charge on the diaphragm 22, another electrode 50B for
placing electrical charge on the backplate 20, and the contacts 36
for providing additional electrical connections.
[0049] The process then both exposes the diaphragm 22, and etches
holes through the diaphragm 22 (step 608). As discussed below in
greater detail, one of these holes ("diaphragm hole 52A")
ultimately assists in forming a pedestal 54 that, for a limited
time during this process, supports the diaphragm 22. A photoresist
layer 56 then is added, completely covering the diaphragm 22 (step
610). This photoresist layer 56 serves the function of an etch
mask.
[0050] After adding the photoresist 36, the process exposes the
diaphragm hole 52A (step 612). To that end, the process forms a
hole ("resist hole 58") through the photoresist 36 by exposing that
selected portion to light (FIG. 7E). This resist hole 58
illustratively has a larger inner diameter than that of the
diaphragm hole 52A.
[0051] After forming the resist hole 58, the process forms a hole
60 through the oxide 42 (step 614). In illustrative embodiments,
this oxide hole 60 effectively forms an internal channel that
extends to the top surface of the SOI wafer 40.
[0052] It is expected that the oxide hole 60 initially will have an
inner diameter that is substantially equal to the inner diameter of
the diaphragm hole 52A. A second step, such as an aqueous HF etch,
may be used to enlarge the inner diameter of the oxide hole 60 to
be greater than the inner diameter of the diaphragm hole 52A. This
enlarged oxide hole diameter essentially exposes a portion of the
bottom side of the diaphragm 22. In other words, at this point in
the process, the channel forms an air space between the bottom side
of the diaphragm 22 and the top surface of the backplate 20.
[0053] Also at this point in the process, the entire photoresist
layer 56 may be removed to permit further processing. For example,
the process may pattern the diaphragm 22, thus necessitating
removal of the existing photoresist layer 56 (i.e., the mask formed
by the photoresist layer 56). Other embodiments, however, do not
remove this photoresist layer 56 until step 622 (discussed
below).
[0054] The process then continues to step 616, which adds more
photoresist 36, to substantially fill the oxide and diaphragm holes
40 and 34 (FIG. 7F). The photoresist 36 filling the oxide hole 60
contacts the silicon of the top SOI layer, as well as the underside
of the diaphragm 22 around the diaphragm hole 52A.
[0055] The embodiment that does not remove the original mask thus
applies a sufficient amount of photoresist 36 in two steps (i.e.,
first the mask, then the additional resist to substantially fill
the oxide hole 60), while the embodiment that removes the original
mask applies a sufficient amount of photoresist 36 in a single
step. In both embodiments, as shown in FIG. 7F, the photoresist 36
essentially acts as the single, substantially contiguous apparatus
above and below the diaphragm 22. Neither embodiment patterns the
photoresist 36 before the sacrificial layer is etched (i.e.,
removal of the sacrificial oxide 42 and polysilicon 44, discussed
below).
[0056] In addition, the process may form the backside cavity 26 at
this time. To that end, as shown in FIG. 7F, conventional processes
may apply another photoresist mask on the bottom side of the SOI
wafer 40 to etch away a portion of the bottom SOI silicon layer.
This should expose a portion of the oxide layer within the SOI
wafer 40. A portion of the exposed oxide layer then is removed to
expose the remainder of the sacrificial materials, including the
sacrificial polysilicon 44.
[0057] At this point, the sacrificial materials may be removed. To
that end, the process removes the sacrificial polysilicon 44 (step
618) and then the sacrificial oxide 42 (step 620, FIG. 7G). Among
other ways, illustrative embodiments remove the polysilicon 44 with
a dry etch process (e.g., using xenon difluoride) through the
backside cavity 26. In addition, illustrative embodiments remove
the oxide 42 with a wet etch process (e.g., by placing the
apparatus in an acid bath for a predetermined amount of time). Some
embodiments, however, do not remove all of the sacrificial
material. For example, such embodiments may not remove portions of
the oxide 42. In that case, the oxide 42 may impact
capacitance.
[0058] As shown in FIG. 7G, the photoresist 36 between the
diaphragm 22 and top SOI layer supports the diaphragm 22. In other
words, the photoresist 36 at that location forms a pedestal 54 that
supports the diaphragm 22. As known by those skilled in the art,
the photoresist 36 is substantially resistant to wet etch processes
(e.g., aqueous HF process, such as those discussed above). It
nevertheless should be noted that other wet etch resistant
materials may be used. Discussion of photoresist 36 thus is
illustrative and not intended to limit the scope of all
embodiments.
[0059] Stated another way, a portion of the photoresist 36 is
within the prior noted air space between the diaphragm 22 and the
backplate 20; namely, it interrupts or otherwise forms a part of
the boundary of the air space. In addition, as shown in the
figures, this photoresist 36 extends as a substantially contiguous
apparatus through the hole 52 in the diaphragm 22 and on the top
surface of the diaphragm 22. It is not patterned before removing at
least a portion of the sacrificial layers. No patterning steps are
required to effectively fabricate the microphone 18.
[0060] To release the diaphragm 22, the process continues to step
622, which removes the photoresist 36/pedestal 54 in a single step.
Among other ways, dry etch processes through the backside cavity 26
may be used to accomplish this step. This step illustratively
removes substantially all of the photoresist 36--not simply
selected portions of the photoresist 36.
[0061] It should be noted that a plurality of pedestals 42 may be
used to minimize the risk of stiction between the backplate 20 and
the diaphragm 22. The number of pedestals used is a function of a
number of factors, including the type of wet etch resistant
material used, the size and shape of the pedestals 42, and the
size, shape, and composition of the diaphragm 22. Discussion of a
single pedestal 54 therefore is for illustrative purposes.
[0062] Accordingly, illustrative embodiments at least partially
align the diaphragm and backplate apertures 32 and 24 to more
precisely set the low frequency cutoff.
[0063] 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.
* * * * *