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DESCRIPTION JP2014057175

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DESCRIPTION JP2014057175
Abstract: The present invention provides an acoustic transducer capable of reducing noise in a
vent hole and making the frequency characteristic in the bass region flatter. A chamber (35) is
vertically opened in a silicon substrate (32). A diaphragm 33 is disposed on the upper surface of
the silicon substrate 32 so as to cover the chamber 35. The fixed electrode film 40 is disposed
above the diaphragm 33 with an air gap therebetween, and the fixed electrode film 40 is held by
the protective film 39. Further, a vent hole 37 (a gap) is formed between the peripheral region of
the chamber 35 on the upper surface of the silicon substrate 32 and the lower surface of the
edge of the diaphragm 33. The peripheral region of the chamber 35 on the upper surface of the
substrate and the edge lower surface of the diaphragm 33 in a partial region extending in a
direction in the region where the peripheral region of the chamber 35 and the edge lower
surface of the diaphragm 33 face each other on the substrate upper surface The height of the
vent hole 37 formed therebetween is narrowed. [Selected figure] Figure 7
Acoustic transducer
[0001]
The present invention relates to an acoustic transducer that converts acoustic vibration into an
electrical signal or converts an electrical signal into acoustic vibration, and more particularly to
an acoustic transducer such as an acoustic sensor or a speaker manufactured using MEMS
technology.
[0002]
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FIG. 1 is a cross-sectional view of a portion of a conventional acoustic sensor manufactured using
MEMS technology.
In the acoustic sensor 11, a conductive diaphragm 14 (vibration electrode film) is provided on
the upper surface of the silicon substrate 13. The silicon substrate 13 has a back chamber 12
penetrating vertically. The upper surface of the back chamber 12 is covered by a diaphragm 14.
Further, a dome-shaped protective film 15 is formed on the upper surface of the silicon substrate
13 so as to surround the diaphragm 14. A fixed electrode film 16 is formed at a position of the
protective film 15 facing the diaphragm 14. The diaphragm 14 and the fixed electrode film 16
constitute a capacitor for converting acoustic vibration into an electric signal. A large number of
acoustic holes 17 for passing acoustic vibration (sound) are opened in the protective film 15 and
the fixed electrode film 16.
[0003]
In the acoustic sensor 11 shown in FIG. 1, the diaphragm 14 is formed in parallel to the upper
surface of the silicon substrate 13 in a region where the silicon substrate 13 and the diaphragm
14 face each other. In particular, in the direction parallel to the upper surface of the silicon
substrate 13 and orthogonal to the edge of the upper surface opening of the back chamber 12,
the gap between the silicon substrate 13 and the diaphragm 14 (hereinafter referred to as the
vent hole 18). The height of) is constant. Such an acoustic sensor is disclosed, for example, in
Patent Document 1.
[0004]
The vent hole of the acoustic sensor functions as an acoustic resistance of acoustic vibration
entering from the acoustic hole and exiting to the back chamber, and has an important function
to secure sensitivity in the low frequency range. On the other hand, since the air in the vent hole
has the property as a viscous fluid, the vent hole also acts as a source of noise (thermal noise).
[0005]
The noise of the vent hole is mainly referred to as a mechanical resistance (this is referred to as a
film damping effect) due to the viscosity of air present in the gap (vent hole) between the edge of
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the diaphragm and the upper surface of the silicon substrate. )によるものである。 That is,
when the diaphragm is to be displaced in a direction (upward direction) where it is pulled away
from the substrate, a resistance that prevents the upward movement of the diaphragm is
generated due to the viscosity of air in the vent hole. On the other hand, when it is intended to
displace the diaphragm in a direction (downward) in which it is pressed against the substrate, a
resisting force is generated which impedes the downward movement of the diaphragm. The noise
caused by the mechanical resistance component at this time is the noise of the vent hole.
[0006]
In the acoustic sensor 11 as shown in FIG. 1, in order to suppress the generation of noise in the
vent hole 18, the diaphragm 14 is separated from the upper surface of the silicon substrate 13 as
in the diaphragm 14 shown by a solid line in FIG. It is sufficient to increase the height H of the
Alternatively, as in the diaphragm 14 shown by a solid line in FIG. 2B, the edge of the diaphragm
14 is retracted toward the center to shorten the overlap length (width W of the vent hole 18)
between the diaphragm 14 and the upper surface of the silicon substrate 13 May be
[0007]
However, even when the height H of the vent hole 18 is increased or the width W of the vent
hole 18 is shortened, the acoustic resistance of the vent hole 18 decreases. Therefore, the
acoustic vibration is likely to leak to the back chamber 12 through the vent hole 18, and the
sensitivity of the acoustic sensor 11 is lowered in the bass range. FIG. 3 is a diagram showing the
sensitivity of the acoustic sensor. The horizontal axis shows the frequency (frequency) of the
acoustic vibration, and the vertical axis shows the sensitivity. A curve indicated by a broken line
in FIG. 3 is a sensitivity-frequency characteristic (hereinafter referred to as a frequency
characteristic) when the diaphragm 14 is in the position indicated by the broken line in FIG. 2A
or 2B. If the height H of the vent hole 18 is increased as shown by the solid line in FIG. 2A, the
sensitivity of the acoustic sensor is lowered in the low frequency range (low frequency range) as
in the frequency characteristic shown by the solid line in FIG. Do. Also when the width W of the
vent hole 18 is shortened as shown by the solid line in FIG. 2B, the sensitivity of the acoustic
sensor is lowered in the low frequency range as shown by the frequency characteristic shown by
the solid line in FIG. That is, if it is going to reduce noise of an acoustic sensor, sensitivity will fall
in a low-pitched region, and a flat field will narrow in frequency characteristics.
[0008]
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On the contrary, in order to improve the frequency characteristics of the acoustic sensor (that is,
in order to widen the flat region in the frequency characteristics), the diaphragm 14 is brought
close to the upper surface of the silicon substrate 13 to make the vent hole 18 The height H of
the hole 18 may be reduced and the acoustic resistance in the vent hole 18 may be increased.
Alternatively, the width W of the vent hole 18 may be increased to increase the acoustic
resistance. However, in this case, the noise generated in the vent hole 18 increases and the S / N
ratio of the acoustic sensor becomes worse.
[0009]
As described above, in the conventional acoustic sensor, there is a trade-off relationship between
reducing noise and obtaining a high S / N ratio and obtaining substantially flat frequency
characteristics even in the bass region, and it is difficult to achieve both. Met. FIG. 4 is a diagram
showing the relationship between the S / N ratio (vertical axis) and the roll-off frequency in the
acoustic sensor as shown in FIG. Generally, the roll-off frequency fr is the frequency at a point
where the sensitivity has dropped by -3 dB compared to the sensitivity at a frequency of 1 kHz,
and the lower the roll-off frequency fr, the flatter the sensitivity Extending to the side, the
frequency characteristics become better. FIG. 4 shows that when the roll-off frequency is
decreased, the S / N ratio is decreased, and when the S / N ratio is increased, the roll-off
frequency is increased and the sensitivity in the bass range is decreased.
[0010]
Next, FIG. 5A is a cross-sectional view showing a portion of another conventional acoustic sensor
manufactured using MEMS technology. FIG. 5B is an enlarged perspective view showing a part of
a diaphragm used in the acoustic sensor of FIG. 5A. In the acoustic sensor 21, a plurality of
stoppers 22 are provided on the lower surface of the diaphragm 14. The stopper 22 prevents the
edge of the diaphragm 14 from sticking to the upper surface of the silicon substrate 13 and
becoming immobile. Such an acoustic sensor is disclosed, for example, in Patent Document 2.
[0011]
According to this acoustic sensor 21, the distance between the stopper 22 and the upper surface
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of the silicon substrate 13 is smaller than the distance between the lower surface of the edge of
the diaphragm 14 and the upper surface of the silicon substrate 13. It seems possible to increase
the sensitivity of the acoustic sensor 21 in the low frequency range. However, the stoppers 22
are for preventing the diaphragms 14 from sticking to the silicon substrate 13, and are in the
form of thin columns, and provided only at intervals with spacing. Therefore, the stopper 22 has
no effect of preventing the acoustic vibration from passing through the vent hole 18, and the
effect of increasing the acoustic resistance and improving the sensitivity of the acoustic sensor
21 is not recognized.
[0012]
Patent Document 1: Japanese Patent Application Laid-Open No. 2010-056745 Patent Document
2: International Publication No. 2002/015636 Pamphlet (WO 2002/015636) (Japanese Patent
Publication No. 2004-506394)
[0013]
The present invention has been made in view of the above technical problems, and the object of
the present invention is to suppress the generation of noise in a vent hole and to make the
frequency characteristic in the bass region more flat. It is to provide an acoustic transducer that
can
[0014]
An acoustic transducer according to the present invention comprises a substrate having a cavity
opened at the top, a vibrating electrode film disposed above the substrate so as to cover the
cavity, and an air gap above the vibrating electrode film. In an acoustic transducer comprising a
fixed electrode film disposed, a gap is formed between the upper surface of the substrate and the
lower surface of the vibrating electrode film around the cavity, and the upper surface of the
substrate and the lower surface of the vibrating electrode film In the gap in which the two face
each other is facing, a part of the gap is narrower than the other part of the gap, and the narrow
part of the gap extends linearly.
Here, the linearly extending portion is not limited to being linearly extended, and may be curved
or bent.
Moreover, it does not interfere even if it branches and extends not only in one direction but in
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two or more directions.
[0015]
In the acoustic transducer according to the present invention, in the space where the upper
surface of the substrate and the lower surface of the vibrating electrode film face each other, the
space of the space is narrower than the rest of the space at the linearly extending portion.
Therefore, the acoustic resistance can be increased at a portion of the gap where the gap is
narrowed, and the decrease in sensitivity in the low tone range can be suppressed. Further, in the
remaining portion, since the gap is wide, noise can be reduced and the S / N ratio can be
improved. Therefore, according to the acoustic transducer of the present invention, an acoustic
transducer having a high S / N ratio and good frequency characteristics can be manufactured.
[0016]
In the acoustic transducer, in order to increase the acoustic resistance, a portion where the gap
formed between the upper surface of the substrate and the lower surface of the vibrating
electrode film is narrowed has a direction orthogonal to the edge of the vibrating electrode film It
is desirable to extend in the other direction. In particular, if the portion where the gap formed
between the upper surface of the substrate and the lower surface of the vibrating electrode film
is narrowed extends in a direction parallel to the edge of the vibrating electrode film, the acoustic
resistance is increased. There is a great effect in improving the frequency characteristics.
[0017]
An embodiment of the acoustic transducer according to the present invention is characterized in
that the distance of the gap at the edge of the vibrating electrode film is narrower than the
distance of the gap at the edge of the top opening of the cavity. According to this embodiment,
since the portion of the vibrating electrode film facing the substrate may be deformed,
processing of the vibrating electrode film is facilitated.
[0018]
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Another embodiment of the acoustic transducer according to the present invention is
characterized in that a cross section of a portion of the vibrating electrode film facing the upper
surface of the vibrating electrode film is curved such that an edge of the vibrating electrode film
approaches the upper surface of the substrate. I assume. According to this embodiment, by
controlling the internal stress of the vibrating electrode film, the portion of the vibrating
electrode film facing the upper surface of the substrate can be easily deformed, and the
manufacture of the acoustic transducer becomes easy.
[0019]
In the vibrating electrode film, a cross section of a portion of the vibrating electrode film facing
the upper surface of the substrate may be bent so that an edge thereof approaches the upper
surface of the substrate. Furthermore, the space between the gap at the edge of the upper
surface opening of the cavity and the gap at the edge of the vibrating electrode film may be
between the edge of the upper surface opening of the cavity and the edge of the vibrating
electrode film The gap may be narrower at the position.
[0020]
In still another embodiment of the acoustic transducer according to the present invention, a
stopper is provided on a lower surface of a portion of the vibrating electrode film facing the
upper surface of the substrate, and a protrusion length of the stopper corresponds to a base end
of the stopper and the vibration. It is larger than the height difference with the lowermost end of
the electrode film. According to this embodiment, the stopper can be prevented from coming into
contact with the vibrating electrode film by colliding with the substrate, and the vibrating
electrode film can be prevented from sticking to the substrate and becoming separated.
[0021]
In still another embodiment of the acoustic transducer according to the present invention, a
convex portion is provided on the upper surface of the substrate in a region facing the vibrating
electrode film on the upper surface of the substrate, and the convex portion covers the upper
surface of the substrate and the lower surface of the vibrating electrode film. And the gap formed
between the two is narrowed. According to such an embodiment, it is only necessary to provide
the convex portion on the upper surface of the substrate, thereby increasing design and
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manufacturing freedom.
[0022]
In addition, the means for solving the above-mentioned subject in the present invention has the
feature which combined suitably the component explained above, and the present invention
enables many variations by the combination of such a component. .
[0023]
FIG. 1 is a cross-sectional view showing a part of a conventional acoustic sensor.
FIG. 2A is a cross-sectional view showing a state in which the position of the diaphragm is moved
upward in the acoustic sensor shown in FIG. FIG. 2B is a cross-sectional view showing the state
where the edge of the diaphragm is retracted toward the center in the acoustic sensor shown in
FIG. FIG. 3 is a diagram showing the relationship between the sensitivity of the acoustic sensor
and the frequency (frequency characteristic). FIG. 4 is a diagram showing the relationship
between the S / N ratio and the roll-off frequency in the acoustic sensor as shown in FIG. FIG. 5A
is a cross-sectional view showing a portion of another conventional acoustic sensor. FIG. 5B is a
partially broken perspective view of the diaphragm used in the acoustic sensor of FIG. 5A. FIG. 6
is a plan view of an acoustic sensor according to Embodiment 1 of the present invention. FIG. 7 is
a cross-sectional view taken along line XX in FIG. FIG. 8 is a plan view showing a diaphragm
formed on the upper surface of the silicon substrate. FIG. 9 is a partially broken perspective view
showing the vicinity of a beam portion of a diaphragm formed on the upper surface of a silicon
substrate. FIG. 10 is an enlarged sectional view showing the vicinity of the vent hole in FIG. FIG.
11 is a diagram showing frequency characteristics of the acoustic sensor. FIG. 12 is a diagram
showing the relationship between the S / N ratio and the roll-off frequency in the acoustic sensor.
FIG. 13 is a diagram showing the relationship between the volume inside the package and the
frequency characteristic. FIG. 14 is a diagram for explaining the definition of the volume inside
the package. FIG. 15 is a cross-sectional view of a comparative example. FIG. 16 is a crosssectional view showing a part of an acoustic sensor according to a modification of Embodiment 1
of the present invention. FIG. 17 is a perspective view showing a part of the diaphragm used in
the modification shown in FIG. FIG. 18 is a cross-sectional view showing a part of an acoustic
sensor according to another modification of the first embodiment of the present invention. FIG.
19 is a cross-sectional view showing a part of an acoustic sensor according to still another
modification of the first embodiment of the present invention. FIG. 20A is a cross-sectional view
showing a part of an acoustic sensor according to Embodiment 2 of the present invention. FIG.
20B is an enlarged cross-sectional view of an edge portion of a diaphragm of the acoustic sensor
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shown in FIG. 20A. FIG. 21 is a perspective view showing a part of a diaphragm used in the
acoustic sensor shown in FIG. 20A. FIG. 22 is a cross-sectional view showing a part of an acoustic
sensor according to Embodiment 3 of the present invention. FIG. 23 is a cross-sectional view
showing another mode of Embodiment 3 of the present invention. FIG. 24 is a plan view showing
a diaphragm provided on the upper surface of a silicon substrate according to a fourth
embodiment of the present invention.
[0024]
Hereinafter, preferred embodiments of the present invention will be described with reference to
the accompanying drawings. In the following, although an acoustic sensor is described as an
example, the present invention is not limited to the acoustic sensor, and can be applied to a
speaker or the like manufactured using MEMS technology. Furthermore, the present invention is
not limited to the following embodiments, and various design changes can be made without
departing from the scope of the present invention.
[0025]
First Embodiment The structure of an acoustic sensor 31 according to a first embodiment of the
present invention will be described with reference to FIGS. 6 and 7. FIG. 6 is a plan view showing
the acoustic sensor 31 according to the first embodiment of the present invention. FIG. 7 is a
cross-sectional view taken along line XX in FIG. FIG. 8 is a plan view showing the shape of the
diaphragm 33 formed on the upper surface of the silicon substrate 32. As shown in FIG. FIG. 9 is
a perspective view showing a part of the diaphragm 33 formed on the upper surface of the
silicon substrate 32. As shown in FIG.
[0026]
The acoustic sensor 31 is a capacitive sensor manufactured using MEMS technology. In the
acoustic sensor 31, as shown in FIG. 7, a diaphragm 33 (vibrating electrode film) is formed on
the upper surface of a silicon substrate 32 (substrate), and above the diaphragm 33 via a minute
air gap (air gap). A back plate 34 is provided.
[0027]
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A chamber 35 (cavity) penetrating from the front surface to the back surface is opened in the
silicon substrate 32 made of single crystal silicon. The chamber 35 may be a back chamber or a
front chamber depending on the use of the acoustic sensor 31. The wall surface of the chamber
35 may be vertical or may be tapered.
[0028]
The diaphragm 33 is formed of a conductive polysilicon thin film. As shown in FIG. 8, the
diaphragm 33 is formed in a substantially rectangular shape, and beam portions 36 horizontally
extend from the corners thereof in the diagonal direction. The diaphragm 33 is disposed on the
upper surface of the silicon substrate 32 so as to cover the upper surface of the chamber 35, and
the lower surface of the beam portion 36 is supported by the anchor 38 as shown in FIG. Thus,
the diaphragm 33 is disposed on the upper surface of the silicon substrate 32 in a state of being
floated from the upper surface of the silicon substrate 32.
[0029]
Between the lower surface of the diaphragm 33 and the upper surface of the silicon substrate 32
around the chamber 35, a narrow gap in the height direction, that is, a vent hole 37 for passing
acoustic vibration or air is formed. The vent hole 37 is a portion where the diaphragm 33 faces
the upper surface of the silicon substrate 32 (around the chamber 35) between the beam portion
36 and the beam portion 36 (hereinafter, this portion is referred to as an edge portion of the
diaphragm 33). It is formed along). The vent holes 37 under each edge of the diaphragm 33 are
short in the width direction (direction orthogonal to the edge of the top opening of the chamber
35) and elongated in the length direction (direction parallel to the edge of the top opening of the
chamber 35) ing.
[0030]
As shown in FIGS. 7 and 9, the edge of the diaphragm 33, that is, the edge located between the
beam 36 and the beam 36, is the end of the edge (hereinafter referred to as the outermost edge
of the edges of the diaphragm 33). The end of this is called the edge of the diaphragm 33. Is
curved in an arc shape so as to approach the upper surface of the silicon substrate 32, and this
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curved portion is a deformed portion 42. Therefore, the deformed portion 42 is formed over
substantially the entire length of the vent hole 37 on each side.
[0031]
FIG. 10 is an enlarged view of a portion where the vent hole 37 is formed in FIG. Since the
deformed portion 42 of the diaphragm 33 is curved so as to expand on the upper surface side,
the gap between the silicon substrate 32 and the diaphragm 33 is narrower and linearly
extended than the other portion, that is, the deformed portion 42 The height of the gap 37b of
the outer peripheral portion of the vent hole 37 located below is the same as that of the other
portion of the vent hole 37, that is, the vent hole 37 located below a flat portion of the
diaphragm 33 other than the deformed portion 42. The height is smaller than the height of the
gap 37a in the inner circumferential portion. In particular, the height of the vent hole 37, that is,
the distance between the lower surface of the diaphragm 33 and the upper surface of the silicon
substrate 32 is an edge of the diaphragm 33 than the height h2 of the vent hole 37 at the edge
of the upper surface opening of the chamber 35. The height h1 of the vent hole 37 in the above
is smaller. In the region where the height of the vent hole 37 is large like the gap 37a of the
inner peripheral portion located under the substantially flat region of the diaphragm 33, the
height of the vent hole 37 is like the gap 37b of the curved outer peripheral portion. It is
desirable to have a sufficiently large area than a small area.
[0032]
In order to bend the edge of the diaphragm 33 as described above, the stress gradient in the
thickness direction of the diaphragm 33 may be controlled. That is, in the conventional
manufacturing process of the acoustic sensor, a sacrificial layer (not shown) is formed on the
upper surface of the silicon substrate 32, and the diaphragm 33 is formed thereon using
polysilicon. Ions such as phosphorus) and B (boron) are implanted and annealing is performed.
When the acoustic sensor 31 is manufactured in such a manufacturing process, for example, a
gradient of internal stress can be generated in the thickness direction of the diaphragm 33 by an
ion implantation and annealing process. At this time, when a stronger tensile stress is generated
on the lower surface side of the diaphragm 33 than on the upper surface side, the edge of the
diaphragm 33 is curved so as to expand on the upper surface side to form a deformed portion
42. Internal stress is also generated to bend the diaphragm 33 in areas other than the deformed
portion 42, but since the four corners of the diaphragm 33 are fixed to the anchors 38, areas
other than the deformed portion 42 of the diaphragm 33 are stretched with pins It keeps almost
flat.
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[0033]
Inside the diaphragm 33, it is desirable to generate a stress gradient of 10 MPa / μm or more in
the thickness direction of the diaphragm 33 such that the lower surface has stronger tensile
stress than the upper surface of the diaphragm 33. If the stress gradient is smaller than this, the
edge of the diaphragm 33 can not be curved sufficiently.
[0034]
The edge of the diaphragm 33 may not extend smoothly along the longitudinal direction of the
vent hole 37 as shown in FIGS. 8 and 9. The edge of the diaphragm 33 may be corrugated
regularly or irregularly along the length of the vent hole 37 or may be bent.
[0035]
The back plate 34 has a fixed electrode film 40 made of polysilicon provided on the lower
surface of a protective film 39 made of SiN. As shown in FIGS. 6 and 7, the protective film 39 is
formed in a substantially rectangular dome shape. The lower surface of the protective film 39
has a hollow portion, and the hollow portion covers the diaphragm 33. The fixed electrode film
40 is provided to face the diaphragm 33.
[0036]
A minute air gap (air gap) is formed between the lower surface of the back plate 34 (that is, the
lower surface of the fixed electrode film 40) and the upper surface of the diaphragm 33. The
fixed electrode film 40 and the diaphragm 33 face each other, and constitute a capacitor for
detecting acoustic vibration and converting it into an electric signal.
[0037]
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A large number of acoustic holes 41 (acoustic holes) for passing acoustic vibration are bored in
substantially the entire back plate 34 so as to penetrate from the upper surface to the lower
surface. As shown in FIG. 6, the acoustic holes 41 are regularly arranged. In the illustrated
example, the acoustic holes 41 are arranged in a triangular shape along three directions forming
an angle of 120 ° with each other, but may be arranged in a rectangular shape, a concentric
shape, or the like.
[0038]
As shown in FIG. 7, a cylindrical minute stopper 43 protrudes from the lower surface of the back
plate 34. The stopper 43 is provided to prevent the diaphragm 33 from sticking (sticking) to the
back plate 34, and integrally protrudes from the lower surface of the protective film 39, passes
through the fixed electrode film 40, and is lower surface of the back plate 34. Projected into The
stopper 43 is made of SiN as in the case of the protective film 39, and thus has insulating
properties.
[0039]
Further, as shown in FIG. 6, on the top surface of the acoustic sensor 31, an electrode pad 44
electrically conducted to the diaphragm 33 and an electrode pad 45 electrically conducted to the
fixed electrode film 40 are provided.
[0040]
In the acoustic sensor 31 configured as described above, when acoustic vibration passes through
the acoustic hole 41 and enters the air gap between the back plate 34 and the diaphragm 33, the
diaphragm 33 which is a thin film vibrates due to the acoustic vibration. .
When the diaphragm 33 vibrates and the gap distance between the diaphragm 33 and the fixed
electrode film 40 changes, the capacitance between the diaphragm 33 and the fixed electrode
film 40 changes. As a result, in the acoustic sensor 31, the acoustic vibration (change in sound
pressure) sensed by the diaphragm 33 becomes a change in capacitance between the diaphragm
33 and the fixed electrode film 40, and is output as an electrical signal. Ru.
[0041]
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In the acoustic sensor 31, as shown in FIG. 10, the height of the vent hole 37 is a portion of the
vent hole 37, that is, a portion on the outer peripheral side of the vent hole 37 in this
embodiment It is called 37b. Of the vent hole 37, and may be referred to as a gap 37a of the
inner circumferential portion below the remaining portion of the vent hole 37 located on the
inner circumferential side of the gap 37b of the outer circumferential portion. ) Is getting bigger.
Therefore, the acoustic resistance is large in a partial region of the vent hole 37, and the acoustic
resistance is small in the region where the vent hole 37 remains. The acoustic resistance of the
vent hole 37 as a whole is equivalent to the acoustic resistance of a large resistance value in a
partial region and the acoustic resistance of a small resistance value in the remaining region
connected in series. The acoustic resistance as is determined by the large acoustic resistance of
resistance value. As a result, in the acoustic sensor 31, the overall acoustic resistance can be
increased by reducing the height of the vent hole 37 in the gap 37b in the outer peripheral
portion, and the frequency characteristic of the acoustic sensor 31 in the low frequency range is
more flat. Can be
[0042]
When the height of the vent hole is increased by moving the position of the diaphragm upward,
in the case of a flat diaphragm, noise in the vent hole can be reduced and the S / N ratio can be
improved. As in the frequency characteristic shown in, the sensitivity is lowered in the bass
region, and the flat region of the frequency characteristic is narrowed on the bass side (see the
description of FIG. 3).
[0043]
On the other hand, in the acoustic sensor 31 according to the first embodiment, when the
position of the entire diaphragm 33 is moved upward, the height of the vent hole 37 is increased
in the gap 37a of the inner peripheral portion, so the film damping effect is suppressed. The
noise of the acoustic sensor 31 can be reduced to improve the S / N ratio.
Moreover, as a result of the increase in acoustic resistance in the gap 37b in the outer peripheral
portion, the acoustic resistance as the whole of the vent hole 37 is also increased, and a sufficient
sound pressure difference can be generated between the front and back of the diaphragm 33.
Therefore, as shown by a broken line in FIG. 11, the sensitivity in the bass region is improved,
and the frequency characteristic can be made flat even in the bass region. Therefore, according
to the first embodiment, the acoustic sensor 31 can be manufactured with low noise and good
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frequency characteristics.
[0044]
Further, it can also be described using the relationship between the S / N ratio and the roll-off
frequency shown in FIG. The solid curve a shown in FIG. 12 is the relationship between the S / N
ratio and the roll-off frequency in a general acoustic sensor having a flat diaphragm, and is the
same as the curve shown in FIG. If only the edge of the diaphragm is bent downward without
changing the position of the diaphragm in the vertical direction, the distance between the edge of
the diaphragm and the upper surface of the silicon substrate is reduced, resulting in an increase
in acoustic resistance at the vent hole. . As a result, the relationship between the S / N ratio and
the roll-off frequency becomes like a curve b of a thin broken line shown in FIG. That is, the
curve b at this time becomes close to one in which the bass region of the curve a of the solid line
is horizontally translated to the low frequency side, and the roll-off frequency becomes smaller
by δ. Furthermore, when the diaphragm whose edge is bent downward is moved upward, noise
is reduced and the S / N ratio is improved. That is, the relationship between the S / N ratio and
the roll-off frequency is such that the curve b moves in parallel upward, and becomes like the
curve c shown by a thick broken line shown in FIG. Even if the roll-off frequency is slightly
increased by moving the diaphragm upward, the reduction of the roll-off frequency by bending
the edge of the diaphragm is better. Therefore, by moving the diaphragm upward and curving
the edge of the diaphragm downward, the S / N ratio is increased while the frequency
characteristic in the low frequency region is increased as compared with the case where the
original flat diaphragm is used. Can be made equal to or flatter than the original frequency
characteristics.
[0045]
FIG. 13 is a diagram showing the relationship between the volume inside the package and the
frequency characteristic. Here, the internal volume of the package refers to the volume of a
portion of the space in the package not occupied by the acoustic sensor, the signal processing
circuit, and the like when the acoustic sensor is housed in the package together with the signal
processing circuit and the like. For example, in FIG. 14, the acoustic sensor 31 and the signal
processing circuit 47 are housed in the package 46 and mounted on the bottom of the package
46. Also, the acoustic sensor 31 communicates the chamber 35 with the sound introduction hole
48 of the package 46, and the chamber 35 is a front chamber. Of the space in the package 46,
the area outside the acoustic sensor 31 and the signal processing circuit 47 (the area where the
dot pattern is drawn in FIG. 14) is the back chamber 49, and the dot pattern is drawn. The
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volume of the area is the volume inside the package. The larger the package, the larger the
volume in the package. Even if the size of the package is the same, the larger the acoustic sensor
or the signal processing circuit, the smaller the volume in the package.
[0046]
FIG. 13 shows respective frequency characteristics in the case where the in-package capacity is
0.6 mm <3>, 2.5 mm <3>, 5 mm <3>. As can be seen from FIG. 13, when the acoustic sensor 31
is housed in the package, the lower the volume in the package, the more the drop in sensitivity in
the low-pitch range. Therefore, as miniaturization of the acoustic sensor progresses, it is
important to suppress the deterioration of the frequency characteristic without increasing the
noise, and the necessity of the present invention becomes high.
[0047]
FIG. 15 shows a cross-sectional view of a comparative example. In this comparative example, the
whole of the diaphragm 33 is brought close to the upper surface of the silicon substrate 32, and
the edge of the diaphragm 33 is curved to bulge downward, and the edge of the diaphragm 33 is
separated from the upper surface of the silicon substrate 32. . In the case of such a comparative
example, if the diaphragm 33 is brought close to the upper surface of the silicon substrate 32 to
increase the acoustic resistance, the height of the vent hole 37 becomes smaller in most of the
area of the vent hole 37, and noise increases. Do. Therefore, in the case of the comparative
example, it is difficult to achieve both noise reduction and good frequency characteristics.
Therefore, when forming the deformed portion 42 by bending, it is important to bend the edge of
the diaphragm 33 toward the upper surface side of the silicon substrate 32.
[0048]
Next, as the structure for narrowing the distance between the edge of the diaphragm and the
upper surface of the substrate in part in Embodiment 1 as described above, various forms other
than curving the edge of the diaphragm in an arc shape are possible. It is.
[0049]
In the modification shown in FIGS. 16 and 17, the tip of the edge is bent substantially at right
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angles toward the upper surface of the substrate along the edge of the diaphragm 33.
In this modified example, the distance between the tip of the deformed portion 42 and the upper
surface of the substrate is short at the deformed portion 42 bent substantially at a right angle.
That is, the gap 37c between the lower surface of the deformed portion 42 and the upper surface
of the silicon substrate 32 is a portion of the gap between the silicon substrate 32 and the
diaphragm 33 that is narrower than other portions and linearly extended. There is. Further, the
gap 37d under the flat area of the diaphragm 33 other than the deformed portion 42 is the other
portion having a relatively wide gap. With such a shape, the height of the vent hole 37 can be
increased at most of the vent hole 37, and the height of the vent hole 37 can be reduced at only
the narrow portion of the deformed portion 42, so noise is reduced. However, the effect of
reducing the decrease in sensitivity in the low range becomes remarkable.
[0050]
Further, in the modification shown in FIG. 18, the edge of the diaphragm 33 is bent in a step
shape to form a deformed portion 42. Also in this modification, a gap 37c between the lower
surface of the deformed portion 42 and the upper surface of the silicon substrate 32 is a portion
of the gap between the silicon substrate 32 and the diaphragm 33 that is narrower than other
portions and linearly extended. It has become. Further, the gap 37d under the flat area of the
diaphragm 33 other than the deformed portion 42 is the other portion having a relatively wide
gap. In this modification, the acoustic resistance can be increased as compared with the
modifications of FIGS. 16 and 17.
[0051]
Further, in FIG. 19, the vicinity of the edge of the diaphragm 33 is bent in a bag shape to form a
deformed portion 42. Also in this modification, a gap 37c between the lower surface of the
deformed portion 42 and the upper surface of the silicon substrate 32 is a portion of the gap
between the silicon substrate 32 and the diaphragm 33 that is narrower than other portions and
linearly extended. It has become. Further, the gap 37d under the flat area of the diaphragm 33
other than the deformed portion 42 is the other portion having a relatively wide gap. In this
modification, the height of the vent hole 37 at the edge of the top opening of the chamber 35
and the height of the vent hole 37 at the edge of the diaphragm 33 are large, and the middle
between the edge of the top opening of the chamber 35 and the edge of the diaphragm 33 The
height of the vent hole 37 is smaller in the portion.
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17
[0052]
Also in each modification as described above, it is possible to obtain the same function and effect
as the acoustic sensor 31 of the first embodiment.
[0053]
The deformed portion 42 may not necessarily extend parallel to the edge of the diaphragm 33,
and may extend in a direction inclined with respect to the edge of the diaphragm 33.
However, since it is not possible to increase the acoustic resistance if the deformation portion 42
extends in a direction perpendicular to the edge of the diaphragm 33, it is desirable that the
deformation portion 42 extend in a direction not perpendicular to the edge of the diaphragm 33.
[0054]
Also, the deformation portion 42 does not have to extend in a straight line, but may extend in a
curved manner or in a curved manner, and the extending direction may be branched.
[0055]
Second Embodiment FIG. 20A is a cross-sectional view showing a part of an acoustic sensor 51
according to a second embodiment of the present invention.
FIG. 20B is an enlarged cross-sectional view of the vent hole 37. FIG. 21 is a perspective view
showing a corner portion of the diaphragm 33 formed on the upper surface of the silicon
substrate 32 in an enlarged manner. In this acoustic sensor 51, columnar stoppers 52 for
preventing the diaphragm 33 from adhering and adhering to the upper surface of the silicon
substrate 32 are protruded at appropriate intervals on the lower surface of the edge of the
diaphragm 33. The other configuration of the acoustic sensor 51 is substantially the same as that
of the acoustic sensor 31 of the first embodiment.
[0056]
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18
Of the stoppers 52 protruding to the lower surface of the edge of the diaphragm 33, the stopper
52 close to the end edge of the diaphragm 33 has a protrusion length h4 of the stopper 52 being
the proximal end of the stopper 52 and the lowermost end (edge) of the diaphragm 33 The
height difference with it is larger than h3. By forming the stopper 52 so as to satisfy this
condition, it is possible to prevent the lower end of the diaphragm 33 from adhering and
adhering to the upper surface of the silicon substrate 32.
[0057]
Third Embodiment FIG. 22 is a cross-sectional view showing a part of an acoustic sensor 61
according to a third embodiment of the present invention. In this acoustic sensor 61, a
diaphragm 33 having a generally flat edge is used. On the other hand, a convex portion 62 is
formed on the upper surface of the silicon substrate 32 at a position facing the edge of the
diaphragm 33. The convex portion 62 extends in the longitudinal direction of the vent hole 37 or
in a direction parallel to the edge of the diaphragm 33. In this embodiment, the height of the vent
hole 37 is smaller than the other at the position where the convex portion 62 is provided. That is,
the gap 37e between the upper surface of the convex portion 62 and the lower surface of the
diaphragm 33 is a portion of the gap between the silicon substrate 32 and the diaphragm 33 that
is narrower than other portions and linearly extended. . Further, the gap 37f between the
diaphragm 33 and a region other than the region where the convex portion 62 is formed on the
upper surface of the silicon substrate 32 is the other portion having a relatively wide gap.
Accordingly, the acoustic resistance is increased in the convex portion 62 to prevent the
sensitivity decrease in the low frequency range, and at the same time, the height of the vent hole
37 is increased in a portion where the convex portion 62 is not provided to reduce noise.
[0058]
Further, the convex portion 62 may be provided at an end on the side in contact with the upper
surface opening of the chamber 35 as in the acoustic sensor 63 shown in FIG. Alternatively, the
convex portion 62 may be provided between the end on the edge side of the diaphragm 33 and
the end on the side in contact with the top opening of the chamber 35.
[0059]
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Fourth Embodiment FIG. 24 shows a diaphragm 33 provided on the upper surface of a silicon
substrate 32 according to a fourth embodiment of the present invention. In this embodiment, for
example, a deformed portion 42 having a cross-sectional shape as shown in FIG. 19 extends in a
direction inclined with respect to the edge of the diaphragm 33.
[0060]
The present invention can also be applied to a MEMS speaker. Although the direction of signal
conversion is opposite between the speaker and the acoustic sensor (microphone), the basic
configuration of the speaker and the acoustic sensor is almost the same, so the description of the
speaker will be omitted.
[0061]
31, 51, 61, 63 Acoustic sensor 32 Silicon substrate 33 Diaphragm 35 Chamber 37 Vent hole 37a
Inner space gap (other part of gap) 37b Outer space gap (part of gap) 37c, 37e Gap (part of gap)
) 37d, 37f Gap (other part of gap) 40 Fixed electrode film 42 Deformed part 52 Stopper
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