JP2012135041

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DESCRIPTION JP2012135041
The present invention provides an acoustic sensor capable of effectively reducing a phenomenon
in which a vibrating electrode plate is fixed to a counter electrode plate to prevent the vibration
of the vibrating electrode plate. A vibrating electrode plate 24 sensitive to sound pressure faces a
counter electrode plate 25 to constitute a capacitive acoustic sensor. In the counter electrode
plate 25, an acoustic hole 31 for passing a vibration is opened, and a plurality of projections 36
are provided on a surface facing the vibrating electrode plate 24. The distance between the
adjacent projections 36 in the opposing region of the counter electrode plate 25 opposing the
highly flexible region of the vibrating electrode plate 24 is the opposing region of the counter
electrode plate 25 opposing the less flexible region of the vibrating electrode plate 24 Smaller
than the distance between adjacent protrusions 36 in [Selected figure] Figure 10
Acoustic sensor
[0001]
TECHNICAL FIELD The present invention relates to an acoustic sensor, and more particularly to
an acoustic sensor for detecting a sound pressure, that is, an acoustic vibration propagating in a
gas or a liquid.
[0002]
As an acoustic sensor, there exist some which were disclosed by Unexamined-Japanese-Patent
No. 2006-157863 (patent document 1).
[0003]
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The acoustic sensor has a structure in which a vibrating electrode plate (movable electrode) and
a counter electrode plate (fixed electrode) face each other with a minute gap (air gap)
therebetween.
Since this vibrating electrode plate is formed of a thin film having a film thickness of about 1
μm, when it receives a sound pressure, it vibrates minutely in response to the vibration.
Then, when the vibrating electrode plate vibrates, the gap distance between the vibrating
electrode plate and the counter electrode plate changes, so that the acoustic vibration is detected
by detecting the change in capacitance between the vibrating electrode plate and the counter
electrode plate at that time. Ru.
[0004]
In addition, this acoustic sensor is manufactured using micromachining (semiconductor
microfabrication) technology, and has a minute dimension of about several mm on a side in a
plan view while having high sensitivity.
[0005]
However, in such an acoustic sensor, the vibrating electrode plate 12 may be fixed to the counter
electrode plate 13 as shown in FIG. 1 during its manufacturing process and use (hereinafter, part
or substantially of the vibrating electrode plate A state in which the whole is fixed to the counter
electrode plate and the gap disappears, or the phenomenon is called a stick).
When the vibrating electrode plate 12 sticks to the counter electrode plate 13 in this way, the
vibration of the vibrating electrode plate 12 is hindered, so that the acoustic vibration can not be
detected by the acoustic sensor 11.
[0006]
2 (a) and 2 (b) are schematic diagrams for explaining the cause of the occurrence of the stick in
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the acoustic sensor 11, which is an enlarged view of a portion corresponding to the portion X in
FIG. is there. Since the acoustic sensor 11 is manufactured using micromachining technology, the
moisture 14 infiltrates between the vibrating electrode plate 12 and the counter electrode plate
13 in the cleaning process after etching, for example. In addition, even during use of the acoustic
sensor 11, moisture may be accumulated between the vibrating electrode plate 12 and the
counter electrode plate 13, or the acoustic sensor 11 may get wet with water.
[0007]
On the other hand, since the acoustic sensor 11 has a minute dimension, the gap distance
between the vibrating electrode plate 12 and the counter electrode plate 13 is only several μm.
Moreover, in order to increase the sensitivity of the acoustic sensor 11, the film thickness of the
vibrating electrode plate 12 is reduced to about 1 μm, and the spring property of the vibrating
electrode plate 12 is weakened.
[0008]
Therefore, in such an acoustic sensor 11, the stick may occur through a two-step process as
described below, for example. In the first stage, as shown in FIG. 2A, when moisture 14 infiltrates
between the vibrating electrode plate 12 and the counter electrode plate 13, the vibrating
electrode plate is caused by capillary force P1 or surface tension due to the moisture. 12 are
attracted to the counter electrode plate 13.
[0009]
Then, in the second stage, after the moisture 14 between the vibrating electrode plate 12 and the
counter electrode plate 13 evaporates, the vibrating electrode plate 12 is attached to the counter
electrode plate 13 and the state is maintained. As a force P2 for adhering and holding the
vibrating electrode plate 12 to the counter electrode plate 13 even after the water 14 is
evaporated, intermolecular force and intersurface force acting between the surface of the
vibrating electrode plate 12 and the surface of the counter electrode plate 13 , Electrostatic force
etc. As a result, the vibrating electrode plate 12 is held in a state of being attached to the counter
electrode plate 13, and the acoustic sensor 11 does not function.
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[0010]
Although the case where the vibrating electrode plate 12 is attached to the counter electrode
plate 13 in the first step due to the capillary force of the infiltrated water has been described
here, it may be a liquid other than moisture, and a large sound pressure is generated in the
vibrating electrode plate. In some cases, the vibrating electrode plate may stick to the counter
electrode plate. In addition, when the vibrating electrode plate is charged with static electricity
and is attached to the counter electrode plate, the first stage process may occur. However, in the
following description, it is assumed that the vibrating electrode plate adheres to the counter
electrode plate due to moisture.
[0011]
As a method of reducing the stick as described above, the elastic restoring force Q of the
vibrating electrode plate 12 is increased, and the elastic restoring force Q overcomes the
capillary force P1 of the water 14 in the first stage and the holding force P2 in the second stage.
Thus, the vibrating electrode plate 12 may be restored to the original state. In order to increase
the elastic restoring force Q of the vibrating electrode plate 12, the film thickness of the vibrating
electrode plate 12 may be thickened to increase the spring property. However, when the elastic
restoring force Q of the vibrating electrode plate 12 is increased, the vibrating electrode plate 12
becomes difficult to vibrate, and thus the sensitivity of the acoustic sensor 11 is deteriorated.
[0012]
Alternatively, even if the capillary force P1 becomes smaller than the elastic restoring force Q of
the vibrating electrode plate 1 in the first stage, the stick can be reduced. The capillary force P1
becomes stronger as the gap distance between the vibrating electrode plate 12 and the counter
electrode plate 13 decreases, so to reduce the capillary force P1, the gap distance may be
increased. However, if the gap distance between the vibrating electrode plate 12 and the counter
electrode plate 13 is increased, the thickness of the acoustic sensor 11 is increased, and
miniaturization of the acoustic sensor 11 is hindered. In addition, the sensitivity of the acoustic
sensor 11 is also reduced.
[0013]
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In view of such a situation, in the acoustic sensor disclosed in Patent Document 1, as shown in
FIG. 3, the vibrating electrode is provided by providing a large number of protrusions 15 on the
surface of the opposing electrode plate 13 facing the vibrating electrode plate 12. The stick
between the plate 12 and the counter electrode plate 13 is reduced. The projections are
generally arranged at equal intervals over the entire counter electrode plate. It is known that the
holding force P2 between the vibrating electrode plate 12 and the counter electrode plate 13 has
a correlation with the contact area of the both electrode plates 12 and 13, and the holding force
P2 also decreases when the contact area is small. . Therefore, providing the projection 15 on the
counter electrode plate 13 and making the projection 15 as thin as possible reduces the contact
area between the vibrating electrode plate 12 and the counter electrode plate 13 (protrusion 15)
and weakens the holding power P2 as well. The stick of the electrode plate 12 is less likely to
occur.
[0014]
In Non-Patent Document 2, since the ratio of surface area to mass increases in a microstructure,
inter-surface force acting between member surfaces plays an important role, and in particular, in
a microelement having a diaphragm It is described that in some cases, the diaphragm and the
counter substrate may become inoperable while being attached due to the force. In addition,
Non-Patent Document 2 describes that sticking of a cantilever can be reduced by providing a
protrusion (a stopper) on the cantilever.
[0015]
Unexamined-Japanese-Patent No. 2006-157863
[0016]
Tsuchiya Shigeki, 5 others, "Measurement of Inter-surface Force in Microstructure and Reduction
of Inter-surface Force" Proceedings of the Japan Society for Measurement and Automatic Control,
Japan Society for Measurement and Automatic Control, 1994, vol. 30, No. 2, 2nd issue Pages
136 to 142
[0017]
However, in the acoustic sensor, as a result of repeating the experiment by changing the distance
between the protrusions provided on the vibrating electrode plate in various ways, if the sticking
of the vibrating electrode plate is prevented by providing the protrusions, the protrusions It
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turned out that the interval had to be adjusted to be an appropriate value.
[0018]
FIGS. 4 (a) to 4 (c) are diagrams schematically showing the state of the vibrating electrode plate
12 when the distance between the protrusions 15 is too large, appropriate, and too small,
respectively. .
FIG. 4B shows the case where the distance d between the protrusions 15 is appropriate.
In this case, even if the vibrating electrode plate 12 is attached to the counter electrode plate 13
with moisture, the contact area between the projection 15 and the vibrating electrode plate 12 is
small as shown by a two-dot chain line in FIG. The holding power P 2 when the water is
evaporated is smaller than the elastic restoring force Q of the vibrating electrode plate 12.
Therefore, as indicated by a solid line in FIG. 4B, the vibrating electrode plate 12 returns to its
original state by its own elastic restoring force Q.
[0019]
On the other hand, as shown in FIG. 4A, when the distance d between the protrusions 15 is
smaller than the appropriate distance, the tip surface of the protrusions 15 is miniaturized even
if the protrusions 15 are thin and the area of the tips is small. Since the projection 51 as a whole
has a limit, the total value of the area of the tip surface is large. Therefore, in this case, the
vibrating electrode plate 12 adheres to the tip end surface of the projection 15 over substantially
the whole or a wide area, and the vibrating electrode plate 12 sticks to the projection 15. The
state in which the vibrating electrode plate 12 is attached to the tip end surface of a large
number of protrusions 15 as shown in FIG. 4A is called a whole stick.
[0020]
Also, as shown in FIG. 4C, when the distance d between the protrusions 15 is larger than the
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appropriate distance, even if the vibrating electrode plate 12 abuts against the protrusions 15,
the vibrating electrode plate between the adjacent protrusions 15 A part of 12 is depressed and
contacts the counter electrode plate 13. In this way, in the state where the vibrating electrode
plate 12 is attached to the counter electrode plate 13, the contact area is considerably larger
than the tip area of the projection 15 even if there is only one contact location. It will stick to 13.
The state in which a part of the vibrating electrode plate 12 is attached to the opposing electrode
plate 13 between the protrusions 15 as shown in FIG. 4C is called a local stick.
[0021]
Generally, when comparing the overall stick with the local stick, the overall stick is more likely to
occur than the local stick. Therefore, when determining the spacing of the projections in the
design stage, it is desirable to make the spacing between the projections wide even if there is a
possibility of local sticks. However, in the capacitive acoustic sensor, since the vibrating electrode
plate and the counter electrode plate are opposed to each other via a minute gap of about several
micrometers, only a small force exceeding the sound pressure is applied to the vibrating
electrode plate. The vibrating electrode plate is in contact with the counter electrode plate. In
addition, since the vibrating electrode plate is so weak and soft that it is deformed so as to be
deformed by sound pressure, its restoring force is weak when it is attached to the counter
electrode plate. Therefore, when the distance between the protrusions is wide, a structure in
which the local stick is likely to occur is obtained.
[0022]
As a result, in the conventional acoustic sensor, even if the distance between the protrusions is
too large or too small, the stick tends to be generated, and it is difficult to provide the protrusions
so as to have an appropriate distance. In addition, even if projections are provided at appropriate
intervals on the assumption of values such as the springiness of the vibrating electrode plate, the
tip area of the projection, the capillary force of the liquid, and the surface-to-surface force, the
springiness of the vibrating electrode plate etc. There was a risk that one of the sticks would
occur if there was a variation in
[0023]
When the projections are provided on the vibrating electrode plate, the rigidity is increased and
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the vibrating electrode plate is not easily vibrated by the sound pressure. Therefore, the
projections are often provided on the counter electrode plate. When the projections are provided
on the vibrating electrode plate, the whole stick is in a state in which the large number of
projections of the vibrating electrode plate are attached to almost the entire counter electrode
plate, and the local stick is in contact with the protrusions of the vibrating electrode plate on the
counter electrode plate In the state of contact, the portion between the projections of the
vibrating electrode plate is deformed and stuck to the counter electrode plate.
[0024]
In addition, when the projections are provided at substantially equal intervals over the entire
vibrating electrode plate or counter electrode plate, a large number of projections are provided in
a region where the density of the projections does not need to be so large. The total number will
increase. When the protrusions increase, it becomes difficult for the air between the vibrating
electrode plate and the opposing electrode plate to be discharged outside when the vibrating
electrode plate approaches the opposing electrode plate side, and when the vibrating electrode
plate moves away from the opposing electrode plate side Air is less likely to flow between the
vibrating electrode plate and the counter electrode plate. As a result, the air resistance when the
vibrating electrode plate vibrates is increased, the vibration of the vibrating electrode plate is
suppressed by air damping, and the frequency characteristic of the acoustic sensor (specifically,
the characteristics on the high frequency side) is deteriorated.
[0025]
The present invention has been made in view of the above technical problems, and the purpose
of the present invention is to effectively prevent the vibration of the vibrating electrode plate
from being firmly fixed to the counter electrode plate. An object of the present invention is to
provide an acoustic sensor that can be mitigated.
[0026]
An acoustic sensor according to the present invention is an acoustic sensor comprising: a
vibrating electrode plate fixed to a substrate and sensitive to sound pressure; and a counter
electrode plate fixed to the substrate and facing the vibrating electrode plate via a gap. In the
sensor, a plurality of protrusions are provided on the surface on the air gap side of the vibrating
electrode plate or the counter electrode plate, and the interval between adjacent protrusions is
changed according to the protrusion forming region in the vibrating electrode plate or the
counter electrode plate. In the electrode plate on the side provided with the protrusion of the
vibrating electrode plate or the counter electrode plate, the facing region of the counter electrode
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plate facing the region with high flexibility or the region with high flexibility of the vibrating
electrode plate The distance between the adjacent protrusions in each of the adjacent protrusions
is smaller than the distance between the adjacent protrusions in the opposite region of the
counter electrode plate facing the low flexibility region or the low flexibility region of the
vibrating electrode plate. It is characterized by small.
[0027]
In the acoustic sensor according to the present invention, since the plurality of projections are
provided on the surface on the air gap side of the vibrating electrode plate or the counter
electrode plate, when the vibrating electrode plate is deformed to contact the counter electrode
plate. In this case, the vibrating electrode plate and the counter electrode plate are in contact
with each other with the projection interposed therebetween.
As a result, the substantial contact area between the vibrating electrode plate and the counter
electrode plate can be reduced, and the stick of the vibrating electrode plate can be reduced.
[0028]
Moreover, in the acoustic sensor according to the present invention, the interval between the
adjacent protrusions is changed according to the protrusion formation region, so the local stick
where the vibrating electrode plate is fixed to the counter electrode plate between the adjacent
protrusions. Also, it is possible to reduce the overall stick in which the vibrating electrode plate
or the counter electrode plate adheres to a large number of protrusions over a wide area.
[0029]
Further, in the method in which the distance between adjacent protrusions is constant
throughout the vibrating electrode plate or the counter electrode plate and the distance between
adjacent protrusions is adjusted to an appropriate value, the spring property of the vibrating
electrode plate may vary, If the capillary force of the water that has entered between the
vibrating electrode plate and the counter electrode plate is different, there is a risk that a local
stick or a whole stick may occur.
On the other hand, in the acoustic sensor according to the present invention, since the stick of
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the vibrating electrode plate is reduced by changing the distance between the adjacent
protrusions according to the protrusion forming region, the spring property of the vibrating
electrode plate varies. Even if the capillary force of the water invading between the vibrating
electrode plate and the counter electrode plate is different, local sticks and whole sticks are less
likely to occur.
Therefore, the tolerance of the design value of the distance between adjacent protrusions is
broadened, the characteristics of the acoustic sensor are stabilized, and the design and
manufacture of the acoustic sensor are facilitated.
[0030]
Since the local stick is likely to occur in the highly flexible area of the vibrating electrode plate, in
the acoustic sensor of the present invention, the local stick can be alleviated by relatively
reducing the distance between adjacent protrusions in the area. Further, by increasing the
distance between the adjacent protrusions in the region of low flexibility of the vibrating
electrode plate, it is possible to reduce the overall stick while suppressing the local stick. Further,
in the acoustic sensor of the present invention, since the distance between the protrusions is
reduced only in the region where the flexibility of the vibrating electrode plate is high, the
number of protrusions can be reduced as a whole. When the number of projections decreases,
the air flow between the vibrating electrode plate and the counter electrode plate is less likely to
be obstructed, so air damping is reduced, and the frequency characteristics of the acoustic sensor
(especially the characteristics on the high frequency side) become flat. And the frequency band is
broadened.
[0031]
In one embodiment of the acoustic sensor according to the present invention, the vibrating
electrode plate is fixed to the substrate along the outer peripheral edge of the movable portion,
and the central portion of the movable portion or the central portion of the counter electrode
plate The distance between the adjacent protrusions in the region facing the second region is
smaller than the distance between the adjacent protrusions in the region facing the outer
circumferential portion of the movable portion or the outer circumferential portion of the
counter electrode plate.
[0032]
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In an acoustic sensor (the above embodiment) in which the vibrating electrode plate is fixed to
the substrate along the outer peripheral edge of the movable portion (local embodiment), local
sticks are likely to occur at the central portion of the vibrating electrode plate. The local stick can
be alleviated by relatively reducing the distance between adjacent protrusions in the area facing
the central portion of the plate.
Further, the overall stick can be reduced while suppressing the local stick by relatively enlarging
the interval between the adjacent protrusions in the outer peripheral portion of the movable
portion or the region facing the outer peripheral portion of the counter electrode plate. .
[0033]
In another embodiment of the acoustic sensor according to the present invention, when the
movable portion of the vibrating electrode plate is formed in a disk shape, and the radius of the
movable portion is R, the center of the vibrating electrode plate or The distance between adjacent
protrusions in a region of radius R / 8 or more and R / 2 or less centered on a position facing the
center of the counter electrode plate is the distance between adjacent protrusions in a region
outside the region. It is smaller than the interval. The symmetry of the elastic deflection of the
vibrating electrode plate is broken in a region where the radius r is (1/2) R or more from the
center of the vibrating electrode plate, so if the distance between the protrusions is shortened
even outside this, the whole There is a risk of causing a stick. Also, since the symmetry of the
elastic deflection of the vibrating electrode plate is maintained even in the region where radius r
from the center is more than (1/8) R, radius r from the center is more than (1/8) R This is
because if the distance between the protrusions is shortened only at the inner side, a local stick
may occur immediately outside the space.
[0034]
In still another embodiment of the acoustic sensor according to the present invention, the outer
peripheral portion of the movable portion of the vibrating electrode plate is partially fixed to the
substrate at a plurality of locations, and the fixing portions of the vibrating electrode plate The
spacing between the adjacent protrusions in the region located in the middle of the portions
facing the fixed portion of the electrode plate is smaller than the spacing between the adjacent
protrusions in the other protrusion formation region.
[0035]
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In still another embodiment of the acoustic sensor according to the present invention, the local
stick is likely to occur in a region located between the fixed portions of the vibrating electrode
plate or between the portions facing the fixed portion of the counter electrode plate. The local
stick can be alleviated by relatively reducing the distance between adjacent protrusions in the
region.
In addition, in the other projection formation area where local sticks are less likely to occur, the
overall stick of the vibrating electrode plate can be reduced by increasing the distance between
adjacent projections.
[0036]
In still another embodiment of the acoustic sensor according to the present invention, the
protrusions are arranged along a plurality of concentric circles or a plurality of polygons of
different sizes. Since the distribution of deflection of the vibrating electrode plate is often
concentric or concentric polygon, arranging the projections along concentric circles or polygons
avoids the stick of the vibrating electrode plate evenly and efficiently can do.
[0037]
In still another embodiment of the acoustic sensor according to the present invention, the
counter electrode plate has a plurality of acoustic holes for allowing sound pressure to pass, and
the protrusions are each provided at a central portion of a region surrounded by the acoustic
holes. It is arranged. According to this embodiment, since the acoustic hole and the protrusion
can be separated as much as possible, the acoustic hole and the protrusion can be easily
manufactured.
[0038]
In still another embodiment of the acoustic sensor according to the present invention, the
counter electrode plate has a plurality of acoustic holes for passing sound pressure, and the
protrusions are respectively deviated from the center of the region surrounded by the acoustic
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holes. Are placed in the same position. According to this embodiment, since the projections are
provided at positions close to the acoustic holes, the moisture that has entered between the
vibrating electrode plate and the counter electrode plate is unlikely to remain at the positions of
the projections. Therefore, the vibrating electrode plate hardly adheres to the counter electrode
plate by the capillary force of water, and the stick of the vibrating electrode plate is reduced.
[0039]
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. .
[0040]
FIG. 1 is a schematic cross-sectional view showing how a vibrating electrode plate sticks to a
counter electrode plate in a conventional acoustic sensor.
FIGS. 2A and 2B are diagrams for explaining the cause of the occurrence of the stick in the
conventional acoustic sensor. FIG. 3 is a schematic cross-sectional view showing a counter
electrode plate provided with a stick-preventing protrusion and a vibrating electrode plate. 4 (a)
shows the case where the spacing between the protrusions is too short, FIG. 4 (b) shows the case
where the spacing between the protrusions is appropriate, and FIG. 4 (c) shows the spacing
between the protrusions. It is a figure showing the case where it is too long. FIG. 5 is a
perspective view showing an acoustic sensor according to the first embodiment of the present
invention. FIG. 6 is an exploded perspective view of the acoustic sensor according to the first
embodiment. 7 is a cross-sectional view taken along the line Y-Y in FIG. FIG. 8 is a view showing
the positional relationship between the vibrating electrode plate and the acoustic hole and the
projection when viewed from the direction perpendicular to the vibrating electrode plate. FIG. 9
is a diagram showing the distribution of the degree of flexibility of the vibrating electrode plate
in which the fixing portions at the four corners are fixed to the silicon substrate. FIG. 10 is an
explanatory view of the operation of the acoustic sensor of the first embodiment, and shows a
vertical cross section in the diagonal direction of the vibrating electrode plate. FIG. 11 is a view
showing a vibrating electrode plate and a counter electrode plate for comparative explanation,
and shows a vertical cross section in the diagonal direction of the vibrating electrode plate. FIG.
12 (a) is a schematic cross-sectional view showing a state in which the local stick is raised at the
central portion of the vibrating electrode plate, and FIG. 12 (b) is a schematic cross-sectional view
showing a state in which the local stick is raised at the end of the vibrating electrode plate It is.
FIG. 13 is a diagram for explaining how to determine the distance between the protrusions at the
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center of the vibrating electrode plate in the acoustic sensor and how to determine the distance
between the protrusions at an area other than the center of the vibrating electrode plate. FIG. 14
shows the shape of the vibrating electrode plate used for the acoustic sensor according to the
second embodiment of the present invention, and the positions of the vibrating electrode plate
and the acoustic holes and protrusions when viewed from the direction perpendicular to the
vibrating electrode plate It is a figure showing a relation. FIG. 15 is a view showing the
distribution of the flexibility of the disk-shaped vibrating electrode plate whose outer peripheral
portion is fixed to the silicon substrate. FIG. 16 is a diagram showing another positional
relationship between the vibrating electrode plate and the acoustic holes and the protrusions in
the second embodiment. FIG. 17 is a diagram showing still another positional relationship
between the vibrating electrode plate and the acoustic holes and the protrusions in the second
embodiment. FIG. 18 is a view showing the positional relationship between the vibrating
electrode plate according to the third embodiment and the acoustic holes and the projections
when viewed from the direction perpendicular to the vibrating electrode plate. FIG. 19 is an
enlarged view of a part of the counter electrode plate in the third embodiment. FIG. 20 is a
partially enlarged cross-sectional view showing a state in which water invading the minute gap is
partially evaporated in the acoustic sensor according to the third embodiment.
FIG. 21 is an enlarged view of a part of the counter electrode plate in the first and second
embodiments. FIG. 22 is a partially enlarged cross-sectional view showing a state in which water
invading the minute gap is partially evaporated in the acoustic sensor according to the first and
second embodiments. FIG. 23 is an enlarged view of a part of the counter electrode plate in the
modification of the third embodiment. FIG. 24 is a partially enlarged cross-sectional view
showing a state in which water invading the minute gap is partially evaporated in the acoustic
sensor according to the modification of the third embodiment. FIG. 25 is a schematic crosssectional view showing still another embodiment of the acoustic sensor of the present invention.
FIG. 26 is a schematic cross-sectional view showing still another embodiment of the acoustic
sensor of the present invention. FIG. 27 is a schematic cross-sectional view showing still another
embodiment of the acoustic sensor of the present invention.
[0041]
Hereinafter, preferred embodiments of the present invention will be described with reference to
the accompanying drawings. However, 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. In particular, the numerical values described below represent rough numerical
values such as the dimensions of each member, and the acoustic sensor of the present invention
is not limited to these numerical values.
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[0042]
Embodiment 1 Hereinafter, a first embodiment of the present invention will be described with
reference to FIGS. 5 to 13. First, FIG. 5 is a perspective view showing the acoustic sensor 21
according to the first embodiment, and FIG. 6 is an exploded perspective view thereof. 7 is a
cross-sectional view taken along the line Y-Y of FIG.
[0043]
The acoustic sensor 21 is a capacitive sensor, and a vibrating electrode plate 24 is provided on
the upper surface of a silicon substrate 22 with an insulating film 23 interposed therebetween,
and an opposing electrode plate 25 is provided thereon via a minute gap (air gap). It is
[0044]
The silicon substrate 22 is provided with a prismatic through hole 26 or a truncated pyramidal
recess.
The figure shows a prismatic through hole 26. The size of the silicon substrate 22 may be 1 to
1.5 mm square (smaller than this) in plan view. And the thickness of the silicon substrate 22 is
about 400 to 500 .mu.m. An insulating film 23 made of an oxide film or the like is formed on the
upper surface of the silicon substrate 22.
[0045]
The vibrating electrode plate 24 is formed of a polysilicon thin film having a thickness of about 1
μm. The vibrating electrode plate 24 is a thin film having a substantially rectangular shape, and
fixing portions 27 extend outward in the diagonal direction at the four corner portions. The
vibrating electrode plate 24 is disposed on the top surface of the silicon substrate 22 so as to
cover the top surface opening of the through hole 26 or the recess, and the fixing portions 27
are fixed on the insulating film 23. The portion of the vibrating electrode plate 24 supported in
the air above the through hole 26 or the recess (in this embodiment, a portion other than the
fixed portion 27) is a diaphragm 28 (movable portion), and is sensitive to sound pressure. And
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vibrate.
[0046]
The counter electrode plate 25 has a stationary electrode 30 made of a metal thin film provided
on the upper surface of an insulating support layer 29 made of a nitride film. The counter
electrode plate 25 is disposed on the vibrating electrode plate 24, and is fixed to the upper
surface of the silicon substrate 22 via an insulating film 33 made of an oxide film or the like
outside the region facing the diaphragm 28. The counter electrode plate 25 covers the
diaphragm 28 with a minute gap of about 3 μm in a region facing the diaphragm 28. A plurality
of acoustic holes (acoustic holes) 31 for passing sound pressure (vibration) are formed in the
fixed electrode 30 and the support layer 29 so as to penetrate from the upper surface to the
lower surface. The end of the counter electrode plate 25 is provided with an electrode pad 32
conducted to the fixed electrode 30. The vibrating electrode plate 24 is a thin film of about 1
μm because it vibrates in resonance with the sound pressure, but since the counter electrode
plate 25 is an electrode which is not excited by the sound pressure, its thickness is, for example,
It is so thick as 2 μm or more.
[0047]
In order to prevent the vibrating electrode plate 24 from adhering to the counter electrode plate
25, a plurality of protrusions 36 are provided in a projecting manner in a region of the counter
electrode plate 25 facing the vibrating electrode plate 24. It is desirable that this protrusion 36
be as thin as possible and have a small tip area, and a diameter of 10 μm or less is preferable.
However, in order to make the protrusions 36 thin, there is also a manufacturing limit, so it is
desirable to provide the protrusions 36 having a protrusion length of about 1 μm and a
diameter of about 4 μm.
[0048]
In addition, the extension 27a extended from the fixing portion 27 is exposed from the opening
34 formed in the support layer 29, and the electrode pad 35 provided on the upper surface of
the end of the support layer 29 is an opening It is conducted to the extension part 27a through
34. Therefore, the vibrating electrode plate 24 and the counter electrode plate 25 are electrically
insulated, and the vibrating electrode plate 24 and the fixed electrode 30 constitute a capacitor.
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[0049]
Thus, in the acoustic sensor 21 of the first embodiment, when acoustic vibration (compression
wave of air) is incident from the upper surface side, the acoustic vibration passes through the
acoustic hole 31 of the counter electrode plate 25 and the diaphragm 28. And vibrate the
diaphragm 28. When the diaphragm 28 vibrates, the distance between the diaphragm 28 and the
counter electrode plate 25 changes, whereby the capacitance between the diaphragm 28 and the
fixed electrode 30 changes. Therefore, if a direct current voltage is applied between the electrode
pads 32 and 35 and this change in electrostatic capacitance is taken out as an electrical signal,
sound vibration is converted to an electrical signal and detected. Can.
[0050]
The acoustic sensor 21 is manufactured using a micromachining (semiconductor
microfabrication) technology, but the manufacturing method is a known technology, so the
description thereof is omitted.
[0051]
Next, the arrangement of the projections 36 provided on the counter electrode plate 25 will be
described.
FIG. 8 is a view showing the positional relationship between the vibrating electrode plate 24 and
the acoustic holes 31 and the protrusions 36 when viewed from the direction perpendicular to
the vibrating electrode plate 24. The acoustic holes 31 are represented by white circles, and the
protrusions 36 are shown. Is represented by a black circle. The acoustic holes 31 are arranged in
a grid at equal intervals throughout.
[0052]
On the other hand, the protrusions 36 are arranged along a similarly shaped polygon (an octagon
shown by a broken line in FIG. 8) concentrically arranged concentrically from the center to the
outside sequentially, and each protrusion 36 Are arranged at the center of the area surrounded
by the four acoustic holes 31.
04-05-2019
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[0053]
Further, the distance between the protrusions 36 is relatively short in the region facing the
central portion a of the diaphragm 28 surrounded by the one-dot chain line and the central
portion b of each side, and is relatively long in the other regions. There is.
For example, in the example shown in FIG. 8, the length L of one side of the diaphragm 28 is 800
μm, and the distance between the projections 36 is 50 μm in the region facing the central
portion a of the diaphragm 28 and the central portion b of each side The area c is 100 μm.
[0054]
FIG. 9 is a sectional view representing the magnitude of deflection when a uniform pressure is
applied to the entire diaphragm 28 in the rectangular vibrating electrode plate 24 in which the
four fixing portions 27 are fixed to the silicon substrate 22. That is, the larger the dot density of
hatching, the larger the deflection, and the smaller the dot density, the smaller the deflection. As
can be seen from FIG. 9, the vibrating electrode plate 24 becomes less flexible and less flexible as
it goes from the center to the outside, and its central portion a and the central portion b of each
side are more flexible than the surroundings. It is getting bigger.
[0055]
Therefore, in this acoustic sensor 21, as schematically shown in FIG. 10, in the region where the
vibrating electrode plate 24 is flexible and faces the central portion a with a large deflection or
the central portion b of each side, the distance between the protrusions 36 is In a region which is
smaller and in which the vibrating electrode plate 24 is relatively high in rigidity and faces the
region c where the deflection is small, the distance between the protrusions 36 is large. As a
result, the local stick and the entire stick described in the prior art can be reduced. The reason is
described below.
[0056]
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18
As described in the conventional example, in the flexible portion (central portion) of the vibrating
electrode plate, the vibrating electrode plate is likely to be depressed between the protrusions
and to cause a local stick in contact with the counter electrode plate. On the other hand, in the
acoustic sensor 21, since the distance between the projections 36 is reduced in the area facing
the central portion a of the diaphragm 28 and the central portion b of the side, local sticks are
less likely to occur. In addition, in the case where the distance between the protrusions is
uniform as a whole, if the distance between the protrusions is small, the vibrating electrode plate
is likely to stick to almost the entire protrusion and cause the entire stick. On the other hand, in
this acoustic sensor 21, the interval between the projections 36 is increased except in the area
where local sticks are likely to occur, so the number of projections 36 (that is, the total area of
the end faces of the projections 36) can be reduced. Stick can be reduced. Thus, local sticks and
overall sticks can be effectively reduced.
[0057]
Specifically, the distance between the protrusions 36 in the region facing the central portion a of
the vibrating electrode plate 24 and the central portion b of each side is smaller than the limit
value D3 at which the local stick occurs in the softest portion of the protrusions 36. ing.
However, if the distance between the projections 36 in the region facing the central portions a
and b is too small, the vibrating electrode plate 24 sticks to the projections 36 in the entire
central portions a and b, as shown in FIG. Therefore, the distance between the protrusions 36 in
the region facing the central portion a of the vibrating electrode plate 24 and the central portion
b of each side is smaller than the limit value D3 at which the vibrating electrode plate 24 sticks
locally at the central portion a or b. And, the region where the vibrating electrode plate 24 faces
the central portion a or b must be larger than the limit value D1 of the sticking of the whole of
the protrusion 36.
[0058]
Further, the distance between the protrusions 36 in the area facing the area c other than the
central portions a and b of the vibrating electrode plate 24 is larger than the limit value D2 at
which the vibrating electrode plate 24 causes the entire stick. However, if the distance between
the projections 36 in the region facing the region c other than the central portions a and b is too
large, as shown in FIG. 12 (b), the vibrating electrode plate 24 is deformed in the region c other
than the central portions a and b. It falls between the projections 36 and causes a local stick.
Therefore, the distance between the protrusions 36 in the region facing the region c other than
the central portions a and b of the vibrating electrode plate 24 is larger than the limit value D2 at
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which the vibrating electrode plate 24 causes the entire stick, and the vibrating electrode plate In
a region where 24 opposes the region c other than the central portions a and b, it must be
smaller than the limit value D4 at which the vibrating electrode plate 24 causes a local stick.
[0059]
Here, as shown in FIG. 12A, the limit value D3 at which the central portions a and b of the
vibrating electrode plate 24 cause a local stick, and as shown in FIG. 12B, the regions other than
the central portions a and b are local Compare with the limit value D4 that causes the stick. As
shown in FIG. 9, since the central portions a and b are portions where the vibrating electrode
plate 24 is soft and easily deformed, the local stick at the central portions a and b as shown in
FIG. The local stick in the area c such as (b) is less likely to occur. Therefore, generally, the
vibrating electrode plate 24 sticks locally in the region c other than the central portions a and b
than the limit value D3 of the distance between the protrusions 36 when the vibrating electrode
plate 24 sticks locally in the central portion a or b. The limit value D4 of the interval between 36
times is a larger value.
[0060]
Further, the limit value D1 of the distance between the protrusions 36 when the vibrating
electrode plate 24 is attached only to the entire protrusion 36 in the region facing the central
portions a and b is the limit value of the protrusions 36 when sticking to the entire protrusion 36
as a whole. It becomes smaller than the limit value D2 of an interval.
[0061]
Therefore, the magnitudes of the four limit values become D1 <D2 <D3 <D4, and the distribution
of the interval between the protrusions 36 in the acoustic sensor 21 is as shown in FIG.
[0062]
When the intervals between the protrusions are made uniform as in the conventional acoustic
sensor, the intervals between the protrusions have to be adjusted so as to be appropriate
intervals larger than D2 and smaller than D3.
Since the adjustment range is narrow, it has been difficult to manufacture an acoustic sensor.
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20
On the other hand, in the case of the acoustic sensor 21 according to the first embodiment, the
distance between the protrusions 36 is larger than D1 and D3 in the region facing the central
portions a and b of the vibrating electrode plate 24. You can make it smaller. Further, in a region
facing the region c other than the central portions a and b, the distance between the protrusions
36 may be larger than D2 and smaller than D4. Therefore, the tolerance ¦ permissible̲range
becomes large also in any of center part a, b and the area ¦ region c.
[0063]
Therefore, according to the acoustic sensor 21 of the first embodiment, the stick of the vibrating
electrode plate 24 can be easily reduced, and the manufacture of the acoustic sensor 21 also
becomes easy. Further, in this acoustic sensor 21, even if the spring property of the vibrating
electrode plate 24 varies, the capillary force of the infiltrated water varies, or the inter-surface
force varies, it is possible to suppress the local stick or the whole stick, The reliability of the
sensor 21 is improved.
[0064]
Also, since the distribution of deflection of the vibrating electrode plate 24 is often concentric or
concentric polygons, if the protrusions 36 are arranged along concentric polygons as described
above (see FIG. 8), The stick of the vibrating electrode plate 24 can be avoided evenly and
efficiently.
[0065]
Furthermore, according to the acoustic sensor 21, the number of the protrusions 36 can be
reduced as compared to the case where the intervals of the protrusions are made uniform.
Therefore, the flow of air in the minute gap between the vibrating electrode plate 24 and the
counter electrode plate 25 is less likely to be blocked by the projections 36, and the air damping
of the vibrating electrode plate 24 is reduced. As a result, the frequency characteristic (in
particular, the characteristic on the high frequency side) of the acoustic sensor 21 becomes flat,
and the frequency band becomes wide.
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[0066]
Second Embodiment Next, an acoustic sensor according to a second embodiment will be
described with reference to FIGS. 14 to 17. The structure of the acoustic sensor according to the
second embodiment is substantially the same as the structure of the acoustic sensor 21
according to the first embodiment, so the overall structure and the description thereof will be
omitted.
[0067]
The main difference between the acoustic sensor according to the second embodiment and the
first acoustic sensor is the shape of the vibrating electrode plate 24 and the arrangement of the
protrusions 36, so these differences will be described.
[0068]
FIG. 14 is a view showing the positional relationship between the vibrating electrode plate 24
and the acoustic holes 31 and the projections 36 according to the second embodiment, as viewed
from the direction perpendicular to the vibrating electrode plate 24. As shown in FIG.
The vibrating electrode plate 24 has a disk shape, and the silicon substrate 22 is provided with a
cylindrical through hole or a truncated cone concave portion in accordance with the shape of the
vibrating electrode plate 24. The vibrating electrode plate 24 is disposed so as to cover the top
opening of the through hole or recess of the silicon substrate 22, and the entire outer peripheral
portion is substantially fixed to the silicon substrate 22 by the fixing portion 27.
[0069]
In the counter electrode plate 25 opposed to the vibrating electrode plate 24, the acoustic holes
31 are arranged in a triangular shape or a hexagonal shape at regular intervals. Further, on the
surface of the counter electrode plate 25 opposed to the vibrating electrode plate 24, a plurality
of protrusions 36 project substantially at the center of the region surrounded by the acoustic
hole 31. The distance between the protrusions 36 is relatively small at a circular central portion
a concentric with the outer peripheral edge of the vibrating electrode plate 24. The distance
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22
between the protrusions 36 is at a region c outside the central portion a. Is relatively large.
[0070]
Here, assuming that the radius of the vibrating electrode plate 24 is R, the radius r of the circular
area a where the distance between the protrusions 36 is reduced is: (1/8) R ≦ r ≦ (1/2) R. FIG.
15 is a drawing that represents in a divided manner the magnitude of deflection when a uniform
pressure is applied to the entire diaphragm 28 in the circular vibrating electrode plate 24. As can
be seen from FIG. 15, in the vibrating electrode plate 24, the flexibility decreases and the
deflection decreases as going from the center to the outside, and the deflection is greatest at the
central portion a. In the region where radius r is (1/2) R or more from the center, the symmetry
of elastic deflection of diaphragm 28 is broken, so local sticks are difficult to occur, and the
distance between projections 36 is smaller even outside this Then there is a risk of causing the
whole stick. Also, since the symmetry of the elastic deflection of the diaphragm 28 is maintained
in the area outside the center r radius (1/8) R, the radius r inside the center r (1/8) R If the
distance between the protrusions 36 is not reduced, a local stick may occur immediately outside
of it. Therefore, it is desirable to set the region a in which the distance between the protrusions
36 is reduced to be a region a within a circle of radius r such as (1/8) R ≦ r ≦ (1/2) R.
[0071]
In the acoustic sensor according to the second embodiment, for example, the film thickness of
the vibrating electrode plate 24 is 1 μm, the thickness of the counter electrode plate 25 is 2
μm, and the minute gap between the vibrating electrode plate 24 and the counter electrode
plate 25 is 3 μm. The height of 36 is 1 μm. The projections 36 are preferably as thin as
possible with a diameter of 10 μm or less, but there is also a limit in the manufacturing process,
etc., so the diameter is preferably about 4 μm. If the radius R of the vibrating electrode plate 24
is 500 μm, the distance between the protrusions 36 is 50 μm inside the circular central portion
a, and the distance between the protrusions 36 is 100 μm outside the area c.
[0072]
In the second embodiment, since the vibrating electrode plate 24 has a circular shape, there is no
region corresponding to the central portion b in the first embodiment, but the distance between
the protrusions 36 is reduced at the central portion a, By increasing the distance between the
04-05-2019
23
protrusions 36 in the area c outside that, it is possible to achieve the same function and effect as
the first embodiment. That is, even in the second embodiment, the local stick and the entire stick
can be further reduced, and the reliability of the acoustic sensor can be improved. Moreover, in
the region of high flexibility of the vibrating electrode plate 24, the distance between the
protrusions 36 is reduced, and in the region of low flexibility of the vibrating electrode plate 24,
the distance between the protrusions 36 is increased. It is possible to widen the appropriate
range of intervals (see FIG. 13) and to facilitate the design and manufacture of the acoustic
sensor. Furthermore, even if the spring property of the vibrating electrode plate 24 varies or the
capillary force of the infiltrated liquid is different, local sticks and whole sticks hardly occur, and
the reliability of the acoustic sensor can be further improved. Further, since the number of the
projections 36 can be reduced, the air damping of the vibrating electrode plate 24 can be
reduced, and the frequency characteristic (in particular, the characteristic on the high frequency
side) of the acoustic sensor 21 can be flattened to widen the frequency band.
[0073]
The protrusions 36 may be arranged along a circle concentric with the vibrating electrode plate
24 as shown in FIG. Alternatively, as shown in FIG. 17, it may be arranged at the vertex of an
equilateral triangle arranged without gaps. Since the distribution of deflection of the vibrating
electrode plate 24 is often concentric or polygonal, if the protrusions 36 are arranged in a
concentric or equilateral triangle shape, the sticks of the vibrating electrode plate 24 can be
avoided uniformly and efficiently. can do.
[0074]
Third Embodiment FIG. 18 is a view showing the positional relationship between the vibrating
electrode plate 24 and the acoustic holes 31 and the projections 36 according to the third
embodiment when viewed from the direction perpendicular to the vibrating electrode plate 24. It
is. Moreover, FIG. 19 is a figure which expands and shows a part of counter electrode plate 25 in
3rd Embodiment. In this embodiment, the protrusion 36 is provided in proximity to the acoustic
hole 31 or in contact with the acoustic hole 31.
[0075]
In the first and second embodiments, as shown in FIG. 21, the protrusion 36 is provided at the
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24
center of the area surrounded by the acoustic hole 31. Therefore, the projections 36 are located
far from any acoustic holes 31. Therefore, when the water 37 invading the minute gap between
the vibrating electrode plate 24 and the counter electrode plate 25 evaporates out of the acoustic
hole 31, as shown in FIG. It will remain until. Since the distance between the gap between the
counter electrode plate 25 and the vibrating electrode plate 24 is the shortest at the position of
the projection 36, if the water 37 remains here, it is possible between the vibrating electrode
plate 24 and the counter electrode plate 25. A large capillary force works to the end, and the
vibrating electrode plate 24 becomes difficult to separate from the counter electrode plate 25.
[0076]
On the other hand, as shown in FIG. 18 and FIG. 19, when the protrusion 36 deviates from the
center of the area surrounded by the acoustic hole 31 and is located near the acoustic hole 31,
the water 37 entering the minute gap As it evaporates out of 31, the moisture 37 evaporates
fastest at the location of the projections 36, as shown in FIG. Therefore, there is no protrusion 36
in the place where the water 37 dries last, and the capillary force acting between the vibrating
electrode plate 24 and the counter electrode plate 25 becomes small early, and the vibrating
electrode plate 24 easily separates from the counter electrode plate 25 Become.
[0077]
FIGS. 23 and 24 show a modification of the third embodiment, in which the projections 36
overlap the positions of the acoustic holes 31. If the positions of the projections 36 and the
positions of the acoustic holes 31 overlap, and the acoustic holes 31 are provided in the counter
electrode plate 25 after the projections 36 are formed, the acoustic holes 31 are opened by
etching and simultaneously A part is also etched and scraped off. Therefore, the area of the end
face of the projection 36 can be made smaller than the processing limit of the projection 36, and
the effect of reducing the local stick and the whole stick is further enhanced.
[0078]
Other Embodiments FIG. 25 is a schematic cross-sectional view showing still another
embodiment of the acoustic sensor of the present invention. In the first to third embodiments,
the projections 36 are provided on the counter electrode plate 25, but in the present
embodiment, the projections 36 are provided on the vibrating electrode plate 24. According to
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25
such an embodiment, a local stick in which the central portion of the vibrating electrode plate 24
is bent and a portion between the protrusions 36 sticks to the counter electrode plate 25 or a
whole stick in which almost the entire protrusion 36 sticks to the counter electrode plate 25 It
can be prevented.
[0079]
In the first to third embodiments, the vibrating electrode plate 24 is provided on the silicon
substrate 22 and the upper side is covered with the counter electrode plate 25. However, as
shown in FIGS. 26 and 27, the silicon substrate 22 is shown. The counter electrode plate 25 may
be provided on the upper side, and the vibrating electrode plate 24 may be provided thereon.
Note that in FIG. 26, the projections 36 are provided on the counter electrode plate 25, and in
FIG. 27 the projections 36 are provided on the vibrating electrode plate 24.
[0080]
Reference Signs List 21 acoustic sensor 22 silicon substrate 23 insulation film 24 vibrating
electrode plate 25 counter electrode plate 26 through hole 27 fixing portion 28 diaphragm 31
acoustic hole 36 protrusion 37 water
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