JP2014017566

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DESCRIPTION JP2014017566
Abstract: To provide a capacitive transducer capable of widening a frequency band and
increasing transmission / reception sensitivity. According to one embodiment, a capacitive
transducer includes a first electrode, a vibrating membrane including a second electrode facing
the first electrode across a gap, and a support portion for supporting the vibrating membrane.
Cell and voltage application means for applying a voltage between the electrodes. As a cell, it has
a first cell 12 including a vibrating membrane 8 having a first spring constant, and a second cell
19 including a vibrating membrane 16 having a second spring constant smaller than the first
spring constant. The distance between the first electrode 4 and the second electrode 8 of the first
cell 12 is shorter than the distance between the first electrode 13 and the second electrode 14 of
the second cell 19. [Selected figure] Figure 1
Capacitance transducer
[0001]
The present invention relates to a capacitive transducer used as an ultrasonic transducer or the
like, a method of manufacturing the same, and the like.
[0002]
Conventionally, micromachined members manufactured by micromachining technology can be
processed on the order of micrometers, and various microfunctional devices are realized using
these.
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Capacitive transducers using such technology are being investigated as alternatives to
transducers using piezoelectric elements. According to such a capacitive transducer, the vibration
of the vibrating film can be used to transmit and receive an acoustic wave such as an ultrasonic
wave, a sound wave, or a photoacoustic wave (hereinafter also represented by an ultrasonic
wave). In particular, excellent broadband characteristics (characteristics having relatively high
reception sensitivity or transmission sensitivity in a wide frequency range) can be easily obtained
in liquid.
[0003]
As the above-mentioned technology, a capacitance type transducer is proposed which realizes
wide band characteristics by using a cell having a vibrating membrane with a high spring
constant and a cell having a vibrating membrane with a low spring constant (see Patent
Document 1). In addition, a capacitive transducer that achieves wide band characteristics by
having a cell group composed of a plurality of cells with high spring constants and a cell group
composed of a plurality of cells with low spring constants has also been proposed (see FIG.
Patent Document 2).
[0004]
U.S. Pat. No. 5,870,351 U.S. Pat.
[0005]
In the capacitive transducer as described above, transmission and reception can be performed by
applying a common voltage to the common electrode.
However, in that case, the electromechanical conversion coefficients are different in a plurality of
cells of a cell having a vibrating membrane with a high spring constant and a cell having a
vibrating membrane with a low spring constant. Therefore, while wide band characteristics can
be realized, electromechanical conversion coefficients of a plurality of types of cells are different,
and reception is a ratio of transmission sound pressure to pulse voltage or a ratio of reception
electric signal to reception sound pressure. The sensitivity may be reduced.
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[0006]
In view of the above problems, in the capacitive transducer according to the present invention, a
vibration is generated such that a gap is formed between a first electrode and a vibrating film
including a second electrode facing the first electrode across a gap. And a support configured to
support the membrane, and a voltage application unit for applying a voltage between the first
electrode and the second electrode. The first cell includes a vibrating membrane having a first
spring constant, and the second cell includes a vibrating membrane having a second spring
constant smaller than the first spring constant as the cells. The distance between the first
electrode and the second electrode of the first cell is shorter than the distance between the first
electrode and the second electrode of the second cell.
[0007]
The capacitive transducer of the present invention is configured to include a plurality of types of
cells in which the spring constant of the vibrating membrane and the distance between the
electrodes are different. Therefore, a capacitance type transducer capable of widening the
frequency band at the time of reception or the frequency band at the time of transmission
including a plurality of types of cells having different frequency characteristics of the reception
sensitivity or the transmission sensitivity is configured according to the request. It can be
implemented with a flexible design within the above limitations.
[0008]
BRIEF DESCRIPTION OF THE DRAWINGS The figure explaining the capacitive transducer of
embodiment of this invention, and Example 1. FIG. The figure explaining the capacitive
transducer of embodiment and Example 2 of this invention. The figure explaining the capacitive
transducer of embodiment of this invention, and Example 3. FIG. FIG. 7 illustrates an example of
a method of manufacturing a capacitive transducer according to the present invention. The figure
which shows the example of the to-be-examined object information acquisition apparatus using
the capacitive transducer of this invention.
[0009]
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The feature of the capacitance type transducer of the present invention is to provide plural types
(two or more types) of cells in which the spring constant of the vibrating membrane and the
distance between the electrodes are different in order to realize wide band characteristics.
According to such a feature of the configuration, it is possible to design various types of cell
structures under certain limitations. For example, the thickness of the vibrating membrane of the
first cell having a large spring constant is made thicker than the thickness of the vibrating
membrane of the second cell having a small spring constant, and the area of the vibrating
membrane of the first cell is set to The radiation impedances of the respective cells can be made
substantially the same as the area of the vibrating membrane of the cell. Such an example is
illustrated in FIG. 2 below. In the present specification, the radiation impedance is the ratio of the
vibration velocity of the vibrating membrane to the force (pressure) acting on the outside (air,
liquid medium, etc.) from the vibrating membrane, and it has Dependent. In addition, the area of
the vibrating membrane of the first cell having a large spring constant is smaller than the area of
the vibrating membrane of the second cell having a small spring constant, and the thickness of
the vibrating membrane of the first cell is set to The thickness of the vibrating membrane of the
cell can be made approximately the same to facilitate fabrication.
[0010]
According to the configuration of the present invention, a wide band characteristic can be
realized, but on the other hand, when applying a common voltage from the common electrode,
the electromechanical conversion coefficient of the first cell is lower than the electromechanical
conversion coefficient of the second cell Because of this, the transmission or reception sensitivity
may be reduced. The electromechanical conversion factor is higher as the ratio of the applied
voltage to the pull-in voltage is higher. The pull-in voltage is a voltage at which the electrostatic
attraction is larger than the restoring force of the vibrating film and the vibrating film contacts
the bottom of the gap when a voltage is applied between the first electrode and the second
electrode. When a voltage higher than the pull-in voltage is applied, the vibrating membrane
contacts the bottom of the gap. When the applied voltage is set so as not to be larger than the
pull-in voltage, the electromechanical conversion factor is proportional to the product of the
capacitance between the first electrode and the second electrode and the electric field strength.
Since the electric field strength is proportional to the applied voltage, the electromechanical
conversion coefficient is proportional to the product of the capacitance between the first
electrode and the second electrode and the applied voltage, and becomes maximum when the
pull-in voltage is applied. The pull-in voltage is proportional to about 0.5 power of a spring
constant and about 1.5 power of an effective gap between upper and lower electrodes. The
effective gap is the sum of the value obtained by dividing the vibrating film between the upper
and lower electrodes by the relative permittivity and the cavity gap. The pull-in voltage increases
as the distance between the first electrode and the second electrode increases as the vibrating
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membrane's spring constant increases. Therefore, if the other structural conditions are
substantially the same, the pull-in voltage of a cell having a high vibrating membrane spring
constant is higher than the pull-in voltage of a cell having a low vibrating membrane spring
constant. According to the configuration of the present invention, the spring constant of the
vibrating membrane and the distance between the electrodes are adjusted to reduce or equalize
the difference between the pull-in voltage of the first cell and the pull-in voltage of the second
cell. Can. Therefore, even when the common voltage is applied, the electromechanical conversion
coefficient can be improved. Therefore, in the capacitive transducer of the present invention, the
frequency band at the time of reception or the frequency band at the time of transmission can be
broadened, and the transmission sensitivity or the reception sensitivity can be improved.
[0011]
On the other hand, there may also be provided first voltage application means for applying a
voltage between the electrodes of the first cell and second voltage application means for applying
a voltage between the electrodes of the second cell. it can. In this case, even if the pull-in voltage
of the first cell and the pull-in voltage of the second cell are different, the transmission sensitivity
or the reception sensitivity can be obtained by appropriately adjusting the magnitudes of the
voltages applied to the plurality of cells. It can be improved. From the above, in the capacitive
transducer of the present invention, the frequency band at the time of reception or transmission
can be broadened, and transmission sensitivity or reception sensitivity can be obtained by
appropriately designing the spring constant of the diaphragm and the distance between the
electrodes. Can also improve.
[0012]
Hereinafter, embodiments of the present invention will be described with reference to the
drawings. Fig.1 (a) is a top view of the electrostatic capacitance type transducer of this
embodiment, FIG.1 (b) is AB sectional drawing of Fig.1 (a). In this embodiment, a plurality of
capacitive transducers (elements) 1 each having a plurality of first cells 12 and a plurality of
second cells 19 are provided. Although only two capacitive transducers are shown in FIG. 1, any
number of transducers may be used. Further, although the plurality of capacitance type
transducers are configured of the first cell 12 and the second cell 19 respectively, 22 and 8
respectively, the number of cells may be any number. Also, the arrangement of the cells may be
any arrangement.
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[0013]
The first cell 12 of this embodiment includes a substrate 2, an insulating film 3 formed on the
substrate 2, a first electrode 4 formed on the insulating film 3, and an insulating film 5 on the
first electrode 4. Have. Furthermore, it has a vibrating membrane 8 including the second
electrode 6 and the membrane 7, a support portion 10 for supporting the vibrating membrane 8,
and a cavity (gap) 9. When the substrate 2 is an insulating substrate such as a glass substrate,
the insulating film 3 may be omitted.
[0014]
The second cell 19 has substantially the same configuration as the first cell 12. In the second cell
19, the spring constant of the vibrating membrane 16 is lower than the spring constant of the
vibrating membrane 8 of the first cell 12. In FIG. 1B, the vibrating membrane 16 is made of the
same material and thickness as the vibrating membrane 8, and the spring constant is reduced by
making the diameter of the vibrating membrane 16 larger than the diameter of the vibrating
membrane 8. The shape of the vibrating membrane is circular, but may be square, rectangular or
the like.
[0015]
Further, voltage application means 11 for applying a voltage between the first electrode and the
second electrode of the first cell 12 and the second cell 16 is provided. 13, 14, 15, and 16,
respectively, a first electrode, a second electrode, a membrane, and a cavity (gap) of the second
cell 19.
[0016]
The membranes 7 and 15 of the vibrating membranes 8 and 16 are insulating films. In
particular, since a silicon nitride film can be formed with a low tensile stress, for example, a
tensile stress of 300 MPa or less, large deformation of the vibrating film due to the residual
stress of the silicon nitride film can be prevented, which is desirable. The membranes 7 and 15 of
the vibrating membranes 8 and 16 may not be insulating films. For example, a low resistance
silicon single crystal of 1 Ωcm or less can be used as the membranes 7 and 15. In that case, the
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membrane can also be used as the second electrode.
[0017]
As described above, the spring constant of the vibrating membrane 16 of the second cell 19 is
lower than the spring constant of the vibrating membrane 8 of the first cell 12. Therefore, it is
possible to widen the frequency band at the time of reception or the frequency band at the time
of transmission.
[0018]
Here, the spring constant of the vibrating membrane 16 of the second cell 19 is lower than that
of the first cell 12, and the pull-in voltage of the cell having a high spring constant of the
vibrating membrane is The spring constant is higher than the pull-in voltage of the low cell.
Therefore, when applying the common voltage to the common electrode, the electromechanical
conversion coefficient of the first cell is lower than the electromechanical conversion coefficient
of the second cell as it is, so that the transmission or reception sensitivity is lowered. In this
configuration, the distance between the first electrode 4 and the second electrode 6 of the first
cell is shorter than the distance between the first electrode 13 and the second electrode 14 of the
second cell. As a configuration, the pull-in voltage of the second cell is increased at this point.
And overall, the pull-in voltages of the first and second cells are brought close to each other. In
the calculation of the distance between the first electrode and the second electrode, when the
insulating film or the like has a relative dielectric constant (ratio to the dielectric constant of
vacuum), the thickness of the insulating film is divided by the relative dielectric constant Using
the effective thickness, add together the thickness of the insulating film, the height of the gap,
the thickness of the membrane, and the like.
[0019]
The method of making this configuration may have any structure. For example, the thickness of
the membrane 15 of the second cell is thicker than the thickness of the membrane 7 of the first
cell, and the second electrode is formed on the membrane, the height of the cavity 17 of the
second cell is There is a method of making the height higher than the height of the cavity 9 of
the first cell, and the like. There is also a method of making the thickness of the insulating layer 5
of the second cell thicker than the thickness of the insulating layer 5 of the first cell.
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[0020]
According to this configuration, the difference between the pull-in voltage of the first cell and the
pull-in voltage of the second cell can be reduced or the same, so that the transmission sensitivity
or the reception sensitivity can be improved. Therefore, in the capacitive transducer of this
embodiment, the frequency band at the time of reception or the frequency band at the time of
transmission can be broadened, and the transmission sensitivity or the reception sensitivity can
be improved.
[0021]
In addition, the vibrating membrane thickness of the first cell is thicker than the vibrating
membrane thickness of the second cell, and the vibrating membrane area of the first cell is the
same as the vibrating membrane area of the second cell. You can also. With this configuration, as
shown in FIG. 2A, since the shapes of the cells viewed from the top are the same, the radiation
impedances of all the cells can be made uniform. Accordingly, since the radiation impedance of
each cell is the same, the vibrating membrane of each cell vibrates in the same manner, and
unnecessary vibration that reduces the transmission or reception sensitivity can be prevented.
Furthermore, the vibrating membrane area of the first cell is smaller than the area of the
vibrating membrane of the second cell, and the vibrating film thickness of the first cell is the
same as the vibrating film thickness of the second cell. You can also. If the vibrating film
thickness of the first cell and the vibrating film thickness of the second cell are not the same, and
either of the vibrating films is etched or film-formed, the spring constant and the first of the
vibrating film of the first cell The ratio to the spring constant of the vibrating membrane of 2
cells is likely to vary. When the ratio of the spring constant of the vibrating membrane of the first
cell to the spring constant of the vibrating membrane of the second cell varies, the transmission
or reception sensitivity of the capacitive transducer varies and the frequency band does not fall
in the desired band. There is. Therefore, variations in the transmission sensitivity, the reception
sensitivity, and the frequency band can be reduced by the configuration in which the vibration
film thickness is the same.
[0022]
Furthermore, a first voltage application means applied between the first electrode and the second
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electrode of the first cell, and an applied between the first electrode and the second electrode of
the second cell It can also be configured to have a second voltage application means. According
to this configuration, since a voltage different from the voltage applied to the first cell can be
applied to the second cell, the transmission sensitivity or the reception sensitivity can be further
improved.
[0023]
An example of the manufacturing method of the present invention will be described with
reference to FIG. FIG. 4 is a cross-sectional view of a capacitive transducer, and has substantially
the same configuration as FIG. FIG. 4 is a cross-sectional view taken along a line AB in FIG. As
shown in FIG. 4A, the insulating film 63 is formed on the substrate 62. The substrate 62 is a
silicon substrate, and the insulating film 63 is a layer for forming insulation with the first
electrode. When the substrate 62 is an insulating substrate such as a glass substrate, the
insulating film 63 may not be formed. The substrate 62 is preferably a substrate with a small
surface roughness. When the surface roughness is large, the surface roughness is transferred
also in the film forming step after the present step, and the distance between the first electrode
and the second electrode due to the surface roughness is determined between the cells It will be
scattered. This variation is a variation of the electromechanical conversion coefficient, and thus is
a variation in sensitivity or band. Therefore, the substrate 63 is desirably a substrate with a small
surface roughness.
[0024]
Furthermore, the first electrodes 64 and 73 are formed. The first electrodes 64 and 73 are
desirably conductive materials having a small surface roughness, such as titanium or aluminum.
Similar to the substrate, when the surface roughness of the first electrode is large, the distance
between the first electrode and the second electrode due to the surface roughness varies among
the cells and between the elements, so the surface roughness Small conductive materials are
desirable.
[0025]
Next, the insulating film 65 is formed. The insulating film 65 is desirably an insulating material
having a small surface roughness. This is formed to prevent an electrical short circuit or a
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dielectric breakdown between the first electrode and the second electrode when a voltage is
applied between the first electrode and the second electrode. In the case of driving at a low
voltage, the insulating film 65 may not be formed because the membrane is an insulator.
Furthermore, the first electrode is formed to prevent the first electrode from being etched when
removing the sacrificial layer performed in a step subsequent to this step. In the case where the
first electrode is not etched by the etching solution or the etching gas at the time of removing the
sacrificial layer, the insulating film 65 may not be formed. As in the case of the substrate, when
the surface roughness of the insulating film 65 is large, the distance between the first electrode
and the second electrode due to the surface roughness varies among the cells, so the insulating
film has a small surface roughness. Is desirable. For example, a silicon nitride film, a silicon oxide
film or the like.
[0026]
Next, as shown in FIG. 4B, sacrificial layers 69 and 77 are formed. The height of the sacrificial
layer 69 is formed to be lower than the height of the sacrificial layer 77. By this configuration,
the cavity height of the first cell can be made lower than the cavity height of the second cell. The
sacrificial layers 69 and 77 are desirably made of a material having a small surface roughness.
Similar to the substrate, when the surface roughness of the sacrificial layer is large, the distance
between the first electrode and the second electrode due to the surface roughness varies among
the cells, so a sacrificial layer with a small surface roughness is desirable . Also, in order to
shorten the etching time of etching for removing the sacrificial layer, a material having a high
etching rate is desirable. In addition, a sacrificial layer material is required in which the insulating
film and the membrane are not substantially etched with respect to the etchant or etching gas for
removing the sacrificial layer. When the insulating film or the membrane is etched with respect
to the etchant or etching gas for removing the sacrificial layer, the thickness variation of the
vibrating film and the distance variation between the first electrode and the second electrode
occur. Variations in thickness of the vibrating film and variations in distance between the first
electrode and the second electrode result in variations in sensitivity and bandwidth among the
cells. In the case where the insulating film or the membrane is a silicon nitride film or a silicon
oxide film, it is desirable to use a sacrificial layer material which has a small surface roughness
and is difficult to etch the insulating film or the membrane. For example, amorphous silicon,
polyimide, chromium or the like. In particular, since a chromium etching solution hardly etches a
silicon nitride film or a silicon oxide film, it is desirable when the insulating film or the membrane
is a silicon nitride film or a silicon oxide film.
[0027]
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Next, as shown in FIG. 4C, the membranes 67 and 75 are formed. The membranes 67, 75
desirably have low tensile stress. For example, a tensile stress of 300 MPa or less is good. The
silicon nitride film can be stress controlled and can have a low tensile stress of 300 MPa or less.
When the membrane has a compressive stress, the membrane causes sticking or buckling and is
greatly deformed. Also, in the case of high tensile stress, the membrane may be broken. Thus,
membranes 67, 75 desirably have low tensile stress. For example, it is a silicon nitride film
capable of stress control and low tensile stress. 70 is a support portion of the vibrating
membrane.
[0028]
Further, an etching hole (not shown) is formed, and the sacrificial layers 69 and 77 are removed
through the etching hole to seal the etching hole. For example, a silicon nitride film or a silicon
oxide film can be used for sealing. The sacrificial layer removal step or the sealing step can also
be performed after the formation of a second electrode described later. That is, in the step of FIG.
4C after the step of forming the sacrificial layers of different thicknesses, it is sufficient to form
at least a part of the vibrating film of a plurality of types of cells on the sacrificial layer.
[0029]
Next, as shown in FIG. 4D, second electrodes 66 and 74 are formed. The second electrodes 66
and 74 are desirably made of a material with low residual stress, such as aluminum. When the
sacrificial layer removing step or the sealing step is performed after the formation of the second
electrode, the second electrode is desirably a material having etching resistance to the sacrificial
layer etching and heat resistance. For example, titanium or the like. As described above, the first
cell 72 having the diaphragm 68 including the membrane 67 and the second electrode 66 and
the second cell 79 having the diaphragm 76 including the membrane 75 and the second
electrode 74 are described. A capacitive transducer is produced.
[0030]
According to this manufacturing method, it is possible to manufacture a capacitive transducer
that can widen the frequency band at the time of reception or the frequency band at the time of
transmission and can improve the transmission sensitivity or the reception sensitivity.
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[0031]
Hereinafter, the present invention will be described in detail by way of more specific examples.
Example 1 Hereinafter, an embodiment of the present invention will be described with reference
to FIG. FIG. 1 (a) is a top view of a capacitive transducer according to the present invention, and
FIG. 1 (b) is a cross-sectional view taken along line A-B of FIG. 1 (a).
[0032]
In the present invention, a plurality of capacitive transducers 1 each having a plurality of first
cells 12 and a plurality of second cells 19 are provided. Although only two capacitive transducers
are shown in FIG. 1, any number of transducers may be used. Further, although the capacitive
transducer is configured of 22 pieces and 8 pieces of the first cell structure 12 and the second
cell 19 respectively, the number may be any number. Also, the arrangement of the cells may be
any arrangement. As shown in FIG. 1A, the vibrating membrane of this embodiment has a
circular shape, but the shape may be square, hexagonal or the like.
[0033]
The first cell 12 includes a silicon substrate 2 having a thickness of 300 μm, an insulating film 3
formed on the silicon substrate 2, a first electrode 4 formed on the insulating film 3, and an
insulating film on the first electrode 4. Have five. Furthermore, it has a vibrating membrane 8
including the second electrode 6 and the membrane 7, a support portion 10 for supporting the
vibrating membrane 8, and a cavity 9. The height of the cavity 9 is 100 nm. In addition, voltage
application means 11 is provided to apply a voltage between the first electrode and the second
electrode.
[0034]
The insulating film 3 is a silicon oxide film with a thickness of 1 μm formed by thermal
oxidation. The insulating film 5 is a 100 nm silicon oxide film formed by Prasma-EnhancedChemical-Vapor-Deposition (PE-CVD). The first electrode is titanium with a thickness of 50 nm,
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and the second electrode 6 is aluminum with a thickness of 100 nm. The membrane 7 is a silicon
nitride film produced by PE-CVD, is formed with a tensile stress of 200 MPa or less, and has a
thickness of 1400 nm.
[0035]
The capacitive transducer of this embodiment can extract an electrical signal from the second
electrode 6 by using a lead wire (not shown). When ultrasonic waves are received by the
capacitive transducer, a DC voltage is applied to the first electrode 4. When ultrasonic waves are
received, the vibrating membrane 8 having the second electrode 6 is deformed, so the distance of
the cavity 9 between the second electrode 6 and the first electrode 4 changes, and the
capacitance changes. The change in capacitance causes a current to flow in the lead-out wire. An
ultrasonic wave can be received as a voltage by a current-voltage conversion element (not
shown). In addition, a direct current voltage can be applied to the first electrode, an alternating
current voltage can be applied to the second electrode, and the vibrating film 8 can be vibrated
by electrostatic force. By this, ultrasonic waves can be transmitted.
[0036]
The second cell 19 has substantially the same configuration as the first cell 12. In the first cell
12, the diameter of the vibrating membrane 8 is 32 μm, while in the second cell 19, the
diameter of the vibrating membrane 16 is 44 μm, the second electrode 14 facing the first
electrode 13 and The spring constant of the vibrating membrane 16 including the membrane 15
is lower than that of the cell 12. In addition, while the height of the cavity 9 of the first cell 12 is
100 nm, the cavity height 17 of the second cell 19 is 200 nm.
[0037]
In FIG. 1B, the vibrating membrane 16 is made of the same material and thickness as the
vibrating membrane 8, and the diameter of the vibrating membrane 16 is larger than that of the
vibrating membrane 8 to reduce the spring constant. The spring constant of the first cell is 92 kN
/ m and the spring constant of the second cell is 55 kN / m. The spring constant here is a value
obtained by dividing the load applied to the vibrating membrane by the average displacement of
the vibrating membrane at that time. Therefore, since the first cell having the vibrating
membrane with a high spring constant and the second cell having a vibrating membrane with a
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low spring constant are provided, the frequency band at reception or the frequency band at
transmission is broadened. Can.
[0038]
Further, in the present embodiment, the vibrating membrane 16 is made of the same material
and thickness as the vibrating membrane 8, and the process of forming the vibrating membrane
can be the same for the first cell and the second cell. . Therefore, the variation of the ratio of the
spring constant of the vibrating membrane of the first cell to the spring constant of the vibrating
membrane of the second cell can be reduced, so that the variation of the transmission sensitivity,
the reception sensitivity, and the frequency band can be reduced.
[0039]
In this configuration, the spring constant of the vibrating membrane 16 of the second cell 19 is
lower than the spring constant of the vibrating membrane 8 of the first cell 12, but the height of
the cavity 17 of the second cell 19 is The height is higher than the height of the cavity 9 of the
first cell 12. Therefore, when the cavity height of the second cell 19 is 100 nm, which is the same
as that of the first cell 12, the former pull-in voltage is 200 V and the latter pull-in voltage is 100
V. The pull-in voltage of the second cell 12, 19 can be 200V.
[0040]
In the capacitive transducer of this embodiment, the voltage applied to the first electrode of the
first cell having a high spring constant of the vibrating membrane and the first voltage of the
second cell having a low spring constant of the second vibrating membrane The voltage applied
to the electrode of is 180V. That is, a voltage that is 90 percent of the pull-in voltage of the first
cell 12 and the second cell 19 is applied. In this configuration, since the pull-in voltages of the
first cell and the second cell are the same, and the ratio of the applied voltage to the pull-in
voltage can be the same, electromechanical conversion of the first cell and the second cell There
is no deterioration in the coefficient. .
[0041]
Therefore, in the capacitive transducer of the present invention, the frequency band at the time
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of reception or the frequency band at the time of transmission can be broadened, and the
transmission sensitivity or the reception sensitivity can be improved.
[0042]
Second Embodiment The configuration of a capacitive transducer according to a second
embodiment will be described with reference to FIG.
Fig.2 (a) is a top view of the electrostatic capacitance type transducer of this invention, FIG.2 (b)
is AB sectional drawing of Fig.2 (a). The configuration of the capacitive transducer of the second
embodiment is substantially the same as that of the first embodiment.
[0043]
In this embodiment, the vibrating membrane thickness of the cell having a high spring constant
of the vibrating membrane is thicker than the vibrating membrane thickness of the cell having a
low spring constant of the vibrating membrane and the vibrating membrane area of the cell
having a high spring constant of the vibrating membrane The spring constant of the vibrating
membrane is the same as the vibrating membrane area of the low cell. The diameter of the
vibrating film 28 of the first cell 32 and the diameter of the vibrating film 36 of the second cell
39 are both 32 μm, and the vibrating film thicknesses are respectively 1400 nm and 1150 nm.
By adopting this configuration, the spring constant of the first cell is 92 kN / m, and the spring
constant of the second cell is 55 kN / m.
[0044]
Therefore, in the capacitive transducer of this embodiment, since the first cell having the
vibrating membrane with a high spring constant and the second cell having a vibrating
membrane with a low spring constant are used, at the time of reception. The frequency band or
the frequency band at the time of transmission can be broadened. Also, the height of the gap 29
of the first cell 32 and the height of the gap 37 of the second cell 39 are the same 200 nm.
Furthermore, the second electrode 27 of the first cell 32 is formed at a position 700 nm from the
cavity-side lower surface of the vibrating membrane 28, and the second electrode 34 of the
second cell 39 is the cavity-side lower surface of the vibrating membrane 36 To 1150 nm.
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[0045]
In order to fabricate this structure, after forming a sacrificial layer to be a cavity after etching the
sacrificial layer, 700 nm of silicon nitride to be a membrane is formed to form a second electrode
27 of the first cell 32. Thereafter, silicon nitride is further deposited to 450 nm, and the second
electrode 34 of the second cell 39 is deposited. Further, silicon nitride is deposited and etched so
that the vibrating film 28 of the first cell 32 is 1400 nm and the vibrating film 36 of the second
cell 39 is 1150 nm. Since titanium, which is the second electrode 34, is present on the surface of
the vibrating film 36 of the second cell 39, the second electrode 34 of the second cell 39 serves
as an etching stop layer. Frequency variation due to thickness variation of the film 36 can be
reduced.
[0046]
In this configuration, the spring constant of the vibrating membrane 36 of the second cell 39
including the second electrode 34 and the membrane 35 is the spring constant of the vibrating
membrane 28 of the first cell 32 including the second electrode 27 and the membrane 26. It has
a lower configuration. On the other hand, the second electrode 27 of the first cell 32 is formed at
a position 700 nm from the cavity-side lower surface of the vibrating membrane 28, and the
second electrode 34 of the second cell 39 is the cavity-side lower surface of the vibrating
membrane 36 To 1150 nm. With this configuration, the pull-in voltage of the first cell 12 can be
200 V, and the pull-in voltage of the second cell 19 can be 200 V. In FIG. 2, 21 is a capacitive
transducer, 22, 23, 24, 25, 30, and 33 are a substrate, an insulating film, a first electrode of the
first cell 32, an insulating film, and a vibrating film, respectively. The support portion is the first
electrode of the second cell 39.
[0047]
In the capacitive transducer according to this embodiment, the voltage applied to the first
electrode of the first cell having a high spring constant of the vibrating membrane and the first
electrode of the second cell having a low spring constant of the vibrating membrane The applied
voltage is 180V. That is, a voltage that is 90 percent of the pull-in voltage of the first cell 32 and
the second cell 39 is applied. In this configuration, since the pull-in voltages of the first cell and
the second cell are the same, and the ratio of the applied voltage to the pull-in voltage can be the
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same, electromechanical conversion of the first cell and the second cell There is no deterioration
in the coefficient.
[0048]
Therefore, in the capacitive transducer of this embodiment, the frequency band at the time of
reception or the frequency band at the time of transmission can be broadened, and the
transmission sensitivity or the reception sensitivity can be improved. Furthermore, with this
configuration, as shown in FIG. 2A, since the shapes of the cells viewed from the top are the
same, the radiation impedances of all the cells can be made uniform.
[0049]
Example 3 The configuration of the capacitive transducer of Example 3 will be described with
reference to FIG. The configuration of the capacitive transducer of the third embodiment is
substantially the same as that of the first embodiment, and FIG. 3 is an equivalent view of the A-B
cross-sectional view of FIG. In FIG. 3, each part corresponding to each part of FIG. 1 is shown by
the number which added 40 to the number of FIG.
[0050]
In the present embodiment, the cavity height of the first cell 52 and the cavity height of the
second cell 59 are 100 nm. In addition, the thickness of the insulating film 60 of the second cell
59 is 400 nm, and voltage applying means 51 for applying a voltage to the first cell 52 and
voltage applying means for applying a voltage to the second cell 59 And 58. That is, in the
present embodiment, the distance between the electrodes in the second cell 59 having the
vibrating film 56 with a small spring constant is made larger than the distance between the
electrodes in the first cell 52 by thickening the thickness of the insulating film 60. doing. Since
the thickness of the insulating film 60 of the second cell 59 is 400 nm, the pull-in voltage of the
second cell 59 can be 140V. The pull-in voltage of the first cell 52 is 200 V, and the difference
between the pull-in voltage of the first cell and the pull-in voltage of the second cell can be
reduced.
[0051]
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Further, 160 V which is 80% of the pull-in voltage of the first cell 52 is applied using the voltage
application means 51, and 112 V which is 80% of the pull-in voltage of the second cell 59 using
the voltage application means 58. Can be applied. Voltages can be applied to the first cell and the
second cell with the same ratio of applied voltage to pull-in voltage.
[0052]
Therefore, in the capacitive transducer of this embodiment, the frequency band at the time of
reception or the frequency band at the time of transmission can be broadened, and the
transmission sensitivity or the reception sensitivity can be improved. As described above, even if
the pull-in voltage can not be made the same between the first and second cells, by providing
separate voltage application means for each of the first and second cells, the voltage applied to
each cell can be obtained. To make the ratio of applied voltage to pull-in voltage equal in each
cell.
[0053]
Example 4 The probe provided with the capacitive transducer described in the above
embodiment and examples can be applied to an object information acquiring apparatus using an
acoustic wave. Acoustic waves from a subject are received by a capacitive transducer, and subject
information reflecting the optical characteristic value of the subject such as a light absorption
coefficient can be acquired using the output electrical signal.
[0054]
FIG. 5 shows the object information acquiring apparatus of the present embodiment using the
photoacoustic effect. The pulse light 152 generated from the light source 151 which generates
light in a pulse shape is irradiated to the subject 153 through the optical member 154 such as a
lens, a mirror, and an optical fiber. The light absorber 155 inside the object 153 absorbs the
energy of pulsed light and generates a photoacoustic wave 156 which is an acoustic wave. A
probe (probe) 157 provided with a housing accommodating the capacitive transducer having the
wide band characteristic of the present invention receives the photoacoustic wave 156, converts
it into an electric signal, and outputs it to the signal processing unit 159. The signal processing
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unit 159 performs signal processing such as A / D conversion and amplification on the input
electric signal, and outputs the signal processing to the data processing unit 150. The data
processing unit 150 acquires object information (object information reflecting the optical
characteristic value of the object such as a light absorption coefficient) as image data using the
input signal. The display unit 158 displays an image based on the image data input from the data
processing unit 150. The probe may be one that scans mechanically or one that is moved by a
user such as a doctor or an engineer relative to the subject (handheld type). Of course, the
capacitive transducer, which is the electromechanical transducer of the present invention, can
also be used in a subject diagnostic apparatus that detects an acoustic wave from a subject to
which an acoustic wave is applied. Also in this case, the acoustic wave from the subject is
detected by the capacitive transducer, and the converted signal is processed by the signal
processing unit to acquire the information inside the subject. Here, an acoustic wave to be
transmitted toward the subject can also be transmitted from the capacitive transducer of the
present invention.
[0055]
The capacitance type transducer of the present invention can be applied to an optical imaging
apparatus for obtaining information in an object to be measured such as a living body, a
conventional ultrasonic diagnostic apparatus, and the like. Furthermore, it can also be used for
other applications, such as an ultrasonic flaw detector.
[0056]
1 · · Capacitance type transducer, 4, 13 · · First electrode, 6, 14 · · Second electrode, 7, 15 · ·
Membrane, 8, 16 · Vibrating film, 9, 17 · Cavity (Gap), 10 · · · Support portion, 11 · · · Voltage
application means, 12 · · First cell, 19 · · Second cell
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