JP2011044757

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DESCRIPTION JP2011044757
The present invention provides a technology that enables adjustment of an output signal to an
elastic wave of a capacitive electromechanical transducer such as a CMUT by processing. A
capacitive electromechanical transducer (100) includes a cell (102) including a first electrode
(104) and a second electrode (106) disposed opposite to the first electrode (104) with a gap
(105). In the capacitive electromechanical transducer 100, at least one cell 102 has a processed
portion 102 subjected to at least one of addition and removal of a substance. [Selected figure]
Figure 2
Capacitive electromechanical transducer and sensitivity adjustment method thereof
[0001]
The present invention relates to a capacitive electromechanical transducer such as a capacitive
ultrasonic transducer, and a method of adjusting sensitivity thereof.
[0002]
BACKGROUND In recent years, capacitive electromechanical transducers manufactured using a
micromachining process have been actively studied.
A typical capacitive electromechanical transducer includes a cell having a vibrating membrane
supported at a predetermined distance from a lower electrode, and an upper electrode disposed
on the surface of the vibrating membrane. This is used, for example, as a capacitive ultrasonic
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transducer (CMUT: Capacitive-Micromachined-Ultrasonic-Transducer).
[0003]
The conversion device performs at least one of the conversion from an electrical signal to an
ultrasonic wave and the conversion from an ultrasonic wave to an electrical signal using a
lightweight diaphragm, and has excellent broadband characteristics even in liquid and air. It is
easy to obtain one with. The use of this conversion device enables more accurate diagnosis than
conventional medical diagnosis, and is attracting attention as a promising technology. The
operating principle of this converter will be described. When ultrasonic waves are transmitted, a
voltage in which a minute AC voltage is superimposed on a DC voltage is applied between the
lower electrode and the upper electrode. As a result, the vibrating membrane vibrates to generate
an ultrasonic wave. When receiving an ultrasonic wave, the diaphragm is deformed by the
ultrasonic wave, so that a signal is detected based on a change in capacitance between the lower
electrode and the upper electrode due to the deformation. As a conventional conversion device,
one in which a plurality of electrically connected cells are arranged in parallel with a plurality of
electrically connected elements (elements) is used. In such a configuration, the reception
sensitivity may vary among a plurality of elements, and a method of performing a sensitivity
correction on this has been proposed (see Patent Document 1). In this method, the control unit
electrically adjusts the output signal so that the difference (difference in sensitivity) between the
output signals converted by the ultrasonic detecting elements is reduced.
[0004]
The sensitivity of the cell or element is, for example, inversely proportional to the square of the
distance (gap) between the electrodes. Therefore, when the gap between the electrodes has a
variation, the sensitivity of the conversion device will be a variation. As a method of forming a
gap of a capacitive electromechanical transducer, a sacrificial layer having a thickness equal to a
desired electrode distance is provided, a vibrating film is formed on the upper portion of the
sacrificial layer, the sacrificial layer is removed, and the gap is formed. Methods of forming are
generally employed.
[0005]
JP, 2004-125514, A
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[0006]
In the case of detecting an elastic wave such as an ultrasonic wave using a conversion device in
which a plurality of elements composed of a plurality of electrically connected cells are arranged,
variation in sensitivity among the plurality of elements causes a reduction in measurement
accuracy.
Therefore, it is necessary to correct the sensitivity for each element. However, in the
configuration in which sensitivity correction is performed by gain adjustment of the circuit in the
latter stage as in Patent Document 1, it is necessary to widen the dynamic range of the circuit.
Furthermore, if there is a certain variation or more, the correction becomes impossible.
[0007]
In view of the above problems, a capacitive electromechanical transducer such as a CMUT
according to the present invention has a cell including a first electrode, and a second electrode
disposed opposite to the first electrode and separated by a gap. . And in this conversion device, at
least one cell has a processing part to which at least one processing of addition and removal of
substance was given.
[0008]
Further, in view of the above problem, the sensitivity adjustment of the present invention of a
capacitive electromechanical transducer having a cell including a first electrode and a second
electrode facing the first electrode and disposed with a gap therebetween. The method is
characterized in that at least one cell is processed to apply at least one of addition and removal of
substances to adjust the output signal for elastic waves (typically ultrasound) of the cell. .
[0009]
In the present invention, at least one of the addition and removal of the substance is applied to at
least one cell, so adjustment of the sensitivity of the cell or element to elastic waves such as
ultrasonic waves (ie adjustment of output signal to ultrasonic waves etc.) It is possible to reduce
variations in sensitivity among elements.
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For example, in a capacitive electromechanical transducer having a configuration including a
plurality of cells or a configuration in which a plurality of elements composed of a plurality of
electrically connected cells are arranged, the reception sensitivity of the cells or elements can be
made uniform. . In addition, since processing is simply to add and remove substances, processing
can be performed relatively easily.
[0010]
The top view and AA sectional drawing which show the basic structure of the capacitive type
electromechanical transducer before adjustment processing which concerns on the 1st Example
of this invention. The top view and AA sectional drawing which show the basic structure of the
capacitive type electromechanical transducer after the adjustment process which concerns on the
1st Example of this invention. The graph which shows the relative sensitivity of each element
before adjustment processing, and the relative sensitivity of each element after adjustment
processing. The top view and AA sectional drawing which show the basic structure of the
capacitive type electro-mechanical transducer based on the 2nd Example of this invention. The
top view and AA sectional drawing which show the basic structure of the capacitive type electromechanical transducer based on the 3rd Example of this invention. The top view and AA sectional
drawing which show the basic structure of the capacitive-type electromechanical transducer
concerning the 4th Example of this invention.
[0011]
Hereinafter, embodiments of the present invention will be described. What is important in the
capacitive electromechanical transducer and sensitivity adjustment method of the present
invention is that at least one of the cells is subjected to at least one of substance addition and
removal processing. Based on this concept, the basic form of the capacitive electromechanical
transducer and sensitivity adjustment method of the present invention have the configuration as
described above. Based on this basic form, the following embodiments are possible. For example,
the conversion device has a plurality of elements which are elements configured of a plurality of
cells (see the examples described later). In addition, the cell includes a first electrode disposed on
the substrate, a second electrode disposed opposite to the first electrode with a gap
therebetween, a vibrating film supporting the second electrode, and vibration. And a support for
supporting the membrane (see the examples below). The processing unit can adjust the reception
sensitivity of the cell or element to the elastic wave, reduce the variation of the reception
sensitivity to the elastic wave among the plurality of elements, and the like. In the present
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invention, the elastic wave refers to what is called a sound wave, an ultrasonic wave, an acoustic
wave, or a photoacoustic wave, and includes an elastic wave generated inside a subject by
irradiating the inside of the subject with light such as near infrared rays. Further, the processing
part is a part where the vibration suppression film is disposed on the second electrode, a part
where connection resistance for electrically connecting the cells is increased, a part of the cell is
opened, and the air gap is at atmospheric pressure. It can be either a part or a part from which
part of the cell has been peeled off (see each example described later).
[0012]
The second electrode used in the present invention is a conductor selected from Al, Cr, Ti, Au, Pt,
Cu, Ag, W, Mo, Ta, Ni, etc., a semiconductor such as Si, AlSi, AlCu, It can be formed of at least one
material of an alloy selected from AlTi, MoW, AlCr, TiN, AlSiCu and the like. In addition, the
second electrode is provided on at least one of the top surface, the back surface, and the inside of
the vibrating film, or in the case where the vibrating film is formed of a conductor or a
semiconductor, the vibrating film doubles as the second electrode You can also The first
electrode used in the present invention can also be formed of the same conductor or
semiconductor as the second electrode. Also, the first electrode and the second electrode material
may be different. In the case where the substrate is a semiconductor substrate such as silicon, the
substrate can also serve as the first electrode.
[0013]
If the capacitive electromechanical transducer has a plurality of elements composed of a plurality
of cells, the number of cells to be processed is determined according to the reception sensitivity
of each element to ultrasonic waves etc measured in advance, The cell can be processed by
processing means (applying of substance, means for laser processing, etc.).
[0014]
First Embodiment A capacitive electromechanical transducer before processing (at the stage of
manufacturing according to an initial design) according to a first embodiment of the present
invention will be described below with reference to the drawings.
As shown in FIGS. 1 (a) and 1 (b), the capacitive electromechanical transducer 100 has a plurality
of elements 101, and each element 101 has a plurality of cells 102 electrically connected in
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parallel. . Although 25 cells 102 are disposed in the element 101 which is an element in FIG. 1A,
the number of cells is not limited to this. There should be at least one cell in the element. Further,
although the present conversion device arranges three elements 101 in one dimension, a
plurality of elements may be arranged in two dimensions. In this embodiment, the cell 102
includes a lower electrode 104 disposed on the substrate 103, an upper electrode 106 disposed
opposite the lower electrode with a predetermined gap 105 therebetween, and a diaphragm 107
supporting the upper electrode. And a supporting portion 108 for supporting the vibrating
membrane. The supporting portion 108 also includes a portion made of the same material as the
vibrating membrane integrally formed in the same process as the vibrating membrane 107 as
long as the supporting portion 108 supports the vibrating membrane 107. The lower electrode
104 is common in the conversion device 100, and the upper electrode 106 between the cells
102 in the element 101 is electrically connected by the wiring of the same member as the upper
electrode. However, the connection mode of the electrodes is not limited to such. These forms
may be determined appropriately according to the specifications.
[0015]
In the present embodiment, the height of the air gap 105 is 100 nm, but a range of 10 nm to
500 nm is desirable. The length of one portion of the gap 105 is preferably in the range of 10
μm to 200 μm. The vibrating film 107 is formed of SiN, but may be another insulating material.
The air gap 105 is maintained at a reduced pressure with respect to the atmospheric pressure,
and the vibrating film 107 has a concave shape (see FIG. 6B described later). In this embodiment,
the vibrating membrane and the electrodes are square, but may be circular, polygonal or the like.
The shape of the air gap 105 of the cell is also rectangular in the illustrated example, but may be
another shape.
[0016]
In the cell 102, the vibrating membrane 107 vibrates due to the vibration of the elastic wave
from the outside, whereby the capacitance between the upper and lower electrodes 106 and 104
changes. The upper electrode and lower electrode of each element 101 are connected to a
receiving circuit (not shown), and the receiving circuit converts a change in capacitance between
the upper and lower electrodes of the cell in the element into a voltage signal.
[0017]
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When medical diagnosis is performed using signals of a plurality of elements, it is desirable that
the variation in sensitivity of each element be as small as possible. Therefore, in the present
embodiment, the vibration suppression agent is applied to the top surfaces of several cells 102 in
the element 101 of the conversion device according to the sensitivity of each element measured
in advance. By such processing, the output signal to the ultrasonic wave of the cell can be
suppressed or adjusted, and the reception sensitivity to the sound wave of the plurality of
elements 101 can be made uniform.
[0018]
FIG. 2 shows a top view and a cross-sectional view of a capacitive electromechanical transducer
whose reception sensitivity has been adjusted by such sensitivity adjustment processing. In the
present embodiment, as an example, the vibration suppression agent 110 is an acrylic resin and
is applied to the upper surface of a desired cell by a dispenser. By applying the vibration
suppression agent 110 having a spring constant higher than the spring constant of the vibrating
membrane 107, it is possible to suppress the vibration of the vibrating membrane 107 due to the
vibration such as the sound wave from the outside. In addition, by changing the number of cells
102 to which the vibration suppressor 110 is applied in the element 101, the sensitivity of the
element can be adjusted. For example, if it is found that the elements before sensitivity
adjustment processing in the upper stage, the interruption, and the lower stage of FIG. That is, as
shown in FIG. 2A, by increasing the number of cells 102 to which the vibration suppressor 110
in the element 101 is applied in the order of the middle, upper, and lower stages, reception of
sound waves of a plurality of elements is received. The sensitivity can be made uniform.
Assuming that the reception sensitivity of the element in the upper stage before adjustment
processing is 1, the relative reception sensitivity in the middle stage is 0.95, and the relative
reception sensitivity in the lower stage is 1.05, the sensitivity variation before adjustment
processing is 10%. Here, as shown in FIG. 2A, the vibration suppression agent 110 is applied to
the top surface of the lower two cells, the upper one, and the middle 0 cells 102 to a
predetermined thickness. As a result, the relative reception sensitivity of the upper element is
0.96, that in the middle is 0.95, that in the lower is 0.97, and the sensitivity variation 10% before
adjustment processing is reduced to 1.7%. Here, since there are 25 cells 102 in the upper
element, the relative reception sensitivity decreases by 0.04 each time the vibration suppressor
110 is applied to one cell 102. In the case of the lower element, the relative reception sensitivity
is reduced by 0.042 each time the vibration suppressor 110 is applied to one cell 102. The
relative sensitivity of each element 101 before adjustment is shown in FIG. 3 (a), and the relative
sensitivity of each element 101 after adjustment is shown in FIG. 3 (b).
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[0019]
The fineness of the sensitivity correction depends on the number of cells in each element.
Therefore, highly precise adjustment is possible by increasing the number of cells in the element.
In this embodiment, it is assumed that the output signal of the cell is completely suppressed by
applying a vibration suppressor of a predetermined thickness. In the case of an acrylic resin, in
order to completely suppress vibration, it is preferable to have a thickness of about several
millimeters. The same effect can be obtained by adjusting the number of cells to be processed
according to the suppression rate by the thickness of the vibration suppressor. For example,
when the suppression rate of the output signal is 50% (this can be known from the measurement
in advance), the vibration suppressor is applied to the upper surface of the lower four cells, the
upper two cells, and the middle 0 cells 102 The same effect can be obtained by this. Further, the
vibration suppression agent is not limited to the acrylic resin, and any material that suppresses
the vibration of the vibrating film may be used, and a different material may be used to form a
multilayer structure. As described above, according to the present embodiment, by adjusting the
number of cells to be processed and adjusting the reception sensitivity of each element, the
reception sensitivity can be easily adjusted with high accuracy.
[0020]
It is also possible to take into account the position within the element of the cell to be processed.
For example, if it is known in advance that the suppression rate of the output signal depends on
the distance of the cell from the center of the element, the processing cell is determined in
consideration of the position as well as the number, and It can be adjusted. The capacitive
electromechanical transducer may be configured to transmit an elastic wave to the outside.
Reception and transmission are performed as described in the background art. In the present
embodiment, at least the reception sensitivity of the element can be adjusted, so the adjustment
processing as described above is performed, but as a matter of course, the transmission
efficiency of the element also changes after processing.
[0021]
Second Embodiment A capacitive electromechanical transducer according to a second
embodiment will be described. The basic structure of the conversion apparatus of this
embodiment is the same as that of the first embodiment. In this embodiment, depending on the
sensitivity of each element measured in advance, the vibrating membrane and the upper
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electrode of some cells in the element are removed to suppress output signals to the ultrasonic
waves of the cells, etc. Make the reception sensitivity of the element uniform. FIG. 4 shows a top
view and a cross-sectional view of a conversion device whose reception sensitivity is adjusted by
the sensitivity adjustment processing method of the present embodiment. The vibrating film 107
and the upper electrode 106 are removed by laser processing, etching processing or the like, and
the cell 102 is selectively processed. In the processed cell, it is possible to eliminate the capacity
change due to the vibration of the sound wave from the outside. Therefore, the sensitivity of the
element can be adjusted by changing the number of cells for removing the vibrating membrane
and the upper electrode in the element. Also in this case, as shown in FIG. 4A, the vibrating film
and the upper electrode of the lower two cells, the upper one, and the middle 0 cells are
removed. As a result, the relative reception sensitivity of the upper element is 0.96, that in the
middle is 0.95, that in the lower is 0.97, and the sensitivity variation before adjustment
processing is reduced to 1.7%. The relative sensitivity of each element before adjustment is
shown in FIG. 3 (a), and the relative sensitivity of each element after adjustment processing is
shown in FIG. 3 (b).
[0022]
In the present embodiment, the vibrating film is also removed together with the upper electrode,
but for example, only the upper electrode may be removed. The other points are the same as in
the first embodiment.
[0023]
Third Embodiment A capacitive electromechanical transducer according to a third embodiment
will be described. The basic structure of the conversion apparatus of this embodiment is also the
same as that of the first embodiment. In this embodiment, depending on the sensitivity of each
element measured in advance, the electrical connection of the top electrodes of several cells in
the element is cut. By this, the output signal to the ultrasonic wave etc. of a cell is suppressed,
and the reception sensitivity to the sound wave etc. of a plurality of elements is equalized. FIG. 5
shows a top view and a cross-sectional view of a conversion device whose reception sensitivity is
adjusted by the sensitivity adjustment processing method of the present embodiment. Electrical
connection of the upper electrode 106 is selectively cut off by laser processing, etching
processing or the like. In the processed cell 102, since the electrical connection with the other
cells in the element 101 is cut, it is possible to suppress an output signal due to vibration such as
a sound wave from the outside. Therefore, the sensitivity of the element 101 can be adjusted by
changing the number of cells 102 that disconnect the electrical connection of the upper
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electrode 106 in the element 101. Also here, as shown in FIG. 5A, the electrical connection of the
upper electrodes 106 of the lower two cells, the upper one, and the middle 0 cells 102 is
disconnected. As a result, the relative reception sensitivity of the upper element is 0.96, that in
the middle is 0.95, that in the lower is 0.97, and the sensitivity variation before adjustment
processing is reduced to 1.7%. The relative sensitivity of each element before adjustment is
shown in FIG. 3 (a), and the relative sensitivity of each element after adjustment processing is
shown in FIG. 3 (b).
[0024]
In the present embodiment, it is assumed that the output signal of the cell which disconnects the
electrical connection of the upper electrode is completely suppressed. However, if the same effect
can be obtained by increasing the resistance of the wiring between the upper electrodes, the
same effect can be obtained by adjusting the number of cells to be processed according to the
suppression rate. For example, in the case where the suppression rate of the output signal by
increasing the resistance of the wiring is 50%, the same effect can be obtained by raising the
resistance of the four lower, two upper, and zero middle wirings. . As a method of increasing the
resistance, there are methods such as reducing the width of the wiring and reducing the
thickness. The other points are the same as in the first embodiment.
[0025]
Fourth Embodiment A capacitive electromechanical transducer according to a fourth
embodiment will be described. The basic structure of the conversion device of this embodiment is
also the same as that of the first embodiment. In the present embodiment, depending on the
sensitivity of each element measured in advance, a hole is made in a part of the vibrating
membrane of several cells in the element. By this, the output signal to the ultrasonic wave etc. of
the cell is suppressed, and the reception sensitivity of the sound wave etc. of a plurality of
elements is made uniform. FIG. 6 shows a top view and a cross-sectional view of the conversion
device whose reception sensitivity is adjusted by the sensitivity adjustment processing method of
the present embodiment. Since the air gap 105 of the cell 102 before processing is in a reduced
pressure state, the vibrating film 107 has a concave shape as in the cell 102 on the left side
shown in the cross-sectional view of FIG. Show). The through hole 140 is opened by laser
processing, etching processing or the like in a part of the vibrating film 107 of the cell in which
the air gap is in a reduced pressure state, and the pressure is atmospheric pressure. As a result,
the concave shape of the vibrating membrane 107 approaches a flat surface as in the cell 102 on
the right side shown in the cross-sectional view of FIG. 6B. Thereby, compared with the shape
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before processing, the output signal by vibration, such as an external sound wave, can be
suppressed. Therefore, the sensitivity of the element can be adjusted by changing the number of
cells in which holes 140 are formed in a part of the vibrating membrane in the element.
[0026]
FIG. 6A shows a sensitivity adjustment method in the case where the suppression rate of the
output signal of the cell is 20%. In this case, holes 140 are formed in part of the vibrating
membrane of the lower 10 cells, the upper 5 cells, and the 0 middle cells. As a result, the relative
reception sensitivity of the upper element is 0.96, that in the middle is 0.95, that in the lower is
0.97, and the sensitivity variation before adjustment processing is reduced to 1.7%. The relative
sensitivity of each element before adjustment is shown in FIG. 3 (a), and the relative sensitivity of
each element after adjustment processing is shown in FIG. 3 (b). In the present embodiment, the
hole is provided at the center of the cell, but the same effect can be obtained as long as the air
gap 105 can be opened to the atmosphere. The other points are the same as in the first
embodiment.
[0027]
In each of the above embodiments, for example, the measurement of the reception sensitivity can
be performed as follows. The elements of the conversion device and the ultrasonic wave
transmission elements of the measurement device are disposed to face each other in a
predetermined relationship, and the elements of the conversion device are made to be capable of
receiving waves. The measuring device is also adapted to receive the output signal from the
element. When the measurement operation is started, a predetermined ultrasonic wave is
transmitted from the ultrasonic wave transmitting element. The ultrasonic waves are received by
the elements of the conversion device, and the measurement device receives an output signal
from the elements to measure the sensitivity of each element. Based on these measured values,
the aspect of cell processing is determined as described above, and processing is performed. If
possible, it is possible to carry out feedback control of cell processing based on the measured
value while performing measurement. Also, the above embodiments can be used in combination
if possible in principle. For example, it is also possible to combine the addition of the vibration
suppressor of the first embodiment and the process of increasing the resistance of the wiring
between the upper electrodes of the third embodiment.
[0028]
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Reference Signs List 100 capacitive electromechanical transducer, 101 elements (elements), 102
cells, 103 substrates, 104 lower electrode (first electrode), 105 air gap, 106 upper electrode
(second electrode), 107 vibrating film, 108 support portion , 110 Vibration suppressor
(processed part), 140 hole (processed part)
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