JP2012222514

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DESCRIPTION JP2012222514
Abstract: PROBLEM TO BE SOLVED: To reduce variation in characteristics of a vibrating
membrane of an electromechanical transducer. SOLUTION: An electro-mechanical transducer
according to the present invention comprises a substrate, a first electrode formed on the
substrate, a membrane formed with a gap from the first electrode, and a membrane formed on
the membrane. An oscillating film having a first electrode and a second electrode facing the first
electrode, wherein the first electrode has a root mean square value of surface roughness of 6 nm
or less It is characterized by [Selected figure] Figure 1
Electromechanical converter and method of manufacturing the same
[0001]
The present invention relates to an electromechanical transducer and a method of manufacturing
the same. In particular, the present invention relates to an electromechanical transducer used as
an ultrasonic transducer and a method of manufacturing the same.
[0002]
An electromechanical transducer such as a capacitive micromachined ultrasonic transducer
(CMUT) manufactured by micromachining technology is being studied as a substitute for a
piezoelectric element. Such a capacitance-type electromechanical transducer can transmit and
receive ultrasonic waves by the vibration of the vibrating film.
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1
[0003]
As a method of manufacturing a CMUT, Patent Document 1 describes a method of manufacturing
a cavity by sacrificial layer etching. In Patent Document 1, a second electrode is formed between
a first membrane and a second membrane so that the upper electrode is not etched when the
sacrificial layer is etched, and the sacrificial layer is etched.
[0004]
U.S. Patent Application Publication No. 2005/0177045
[0005]
As in the manufacturing method described in Patent Document 1, in the CMUT, a lower
electrode, an insulating film, an upper electrode, and a membrane are sequentially and
repeatedly stacked on a substrate.
At the time of film formation of the plurality of films, it is conceivable that the thickness of the
films varies. If the thickness of the film varies from cell to cell and from element to element, the
frequency characteristics change from cell to element and from element to element. Therefore,
an object of the present invention is to reduce the variation in frequency characteristics of each
cell and each element.
[0006]
The electromechanical transducer according to the present invention comprises a substrate, a
first electrode formed on the substrate, a membrane formed with a gap from the first electrode,
and a membrane formed on the membrane. A vibrating membrane having a second electrode
opposed to the first electrode, and the first electrode has a root mean square value of surface
roughness of 6 nm or less.
[0007]
In the method of manufacturing an electromechanical transducer according to the present
invention, a step of forming a first electrode on a substrate, a step of forming a sacrificial layer
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on the first electrode, and a step of forming a membrane on the sacrificial layer And forming a
second electrode on the membrane; forming an etching hole in the membrane; and removing the
sacrificial layer through the etching hole; The first electrode is formed such that the root mean
square value of the surface roughness of the electrode is 6 nm or less.
[0008]
According to the present invention, by optimizing the surface state of the first electrode, it is
possible to reduce the variation in the frequency characteristics of each cell and each element.
[0009]
It is a schematic diagram for demonstrating the electromechanical transducer which can apply
Example 1 of this invention.
It is process drawing in order to demonstrate the manufacturing method of the electromechanical transducer which can apply Example 1 of this invention.
(A) The graph which showed the relationship between the film thickness of 1st electrode, and
surface roughness.
(B) A graph showing the relationship between the vibration characteristics of the vibration film
and the surface roughness of the first electrode. (A) It is a schematic diagram for demonstrating
the electromechanical transducer which can apply Example 2 of this invention.
[0010]
The inventors of the present invention focused on the fact that the frequency characteristic of
the vibrating film changes due to the thickness variation of each layer formed on the substrate. In
particular, attention was paid to the importance of the film forming process of the first electrode,
which is an initial process of manufacturing the device. The electromechanical transducer
manufactured by laminating a plurality of films can obtain a film having a surface shape
reflecting the surface shape of the first electrode in the steps after the step of forming the first
electrode. From this point of view, the present inventors have found that the surface roughness
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of the first electrode and the frequency characteristic of the vibrating film have a certain
relationship.
[0011]
From this relationship, the present invention is characterized in that the root mean square value
of the surface roughness of the first electrode is 6 nm or less. Hereinafter, embodiments of the
present invention will be described using the drawings.
[0012]
(Configuration of Electromechanical Transducer) FIG. 1A is a top view of one element 1 of the
electromechanical transducer of the present invention. FIG.1 (b) has shown AB sectional drawing
in the cell structure 2 enclosed with the dashed-dotted line of FIG. 1 (a). The element 1 of the
present embodiment has a plurality of cell structures 2 electrically connected. Although only one
element 1 is described in FIG. 1A, it may be configured by a plurality of elements 1. Further, in
FIG. 1A, the element 1 is configured by arranging nine cell structures 2, but the number may be
any number. In addition, although the cell structures are arranged in a square lattice, they may
be arranged in a zigzag or may be arranged in any manner. The shape of the cell structure is
circular in FIG. 1, but may be square or hexagonal.
[0013]
FIG. 1 (b) shows a cross-sectional view of the cell structure 2. The cell structure 2 includes a
substrate 11, a first insulating film 12 formed on the substrate 11, a first electrode 13, and a
second insulating film 14. Furthermore, the cell structure 2 has a vibrating membrane composed
of the first membrane 16, the second electrode 17 and the second membrane 18. The first
membrane 16 is a membrane on the gap side (cavity 15 side), and is supported by the membrane
support 19. The second membrane 18 is a membrane opposite to the cavity 15. The vibrating
film is disposed to be separated from the second insulating film and the cavity 15 which is a gap.
The first electrode 13 and the second electrode 17 face each other across the cavity, and a
voltage is applied between the first electrode 13 and the second electrode 17 by voltage
application means (not shown). Ru.
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[0014]
Further, by using the lead wire 6, the electro-mechanical transducer can draw an electric signal
for each element from the second electrode 17. However, in the present embodiment, the
electrical signal is drawn by the lead wiring 6, but a through wiring or the like may be used.
Further, in the present embodiment, both the first electrode 13 and the second electrode 17 are
disposed for each element, but one of the first electrode 13 and the second electrode 17 may be
used as a common electrode. . The common electrode, when having a plurality of elements,
indicates an electrode electrically connected by all of the plurality of elements. Also in this case, if
any one of the first electrode 13 and the second electrode 17 is separated for each element, an
electric signal for each element can be extracted.
[0015]
(Driving Principle of Electromechanical Transducer) The driving principle of the present
invention will be described. When ultrasonic waves are received by the electromechanical
transducer, a DC voltage is applied to the first electrode 13 so that a potential difference is
generated between the first electrode and the second electrode by voltage application means (not
shown). deep. When an ultrasonic wave is received, the vibrating membrane having the second
electrode 17 is bent, so that the distance between the second electrode 17 and the first electrode
13 (the distance in the depth direction of the cavity 15) changes. Changes. A current flows in the
lead wire 6 due to the change in capacitance. This current is converted into a voltage by a
current-voltage conversion element (not shown) to obtain an ultrasonic reception signal. As
described above, a direct current voltage may be applied to the second electrode 17 by changing
the configuration of the lead wiring, and an electric signal may be drawn from the first electrode
13 for each element.
[0016]
When ultrasonic waves are transmitted, a direct current voltage can be applied to the first
electrode 13 and an alternating current voltage to the second electrode 17, and the vibrating film
can be vibrated by electrostatic force. An ultrasonic wave can be transmitted by this vibration.
Also in the case of transmitting an ultrasonic wave, the diaphragm may be vibrated by applying a
DC voltage to the second electrode 17 and an AC voltage to the first electrode 13 by changing
the configuration of the lead-out wiring. Alternatively, a direct current voltage and an alternating
current voltage may be applied to the first electrode or the second electrode, and the vibrating
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film may be vibrated by electrostatic force.
[0017]
(Relationship between Frequency Characteristic of Vibrating Film and Surface Roughness of First
Electrode) As described above, the present invention is characterized in that the root mean
square value of the surface roughness of the first electrode is 6 nm or less. . Hereinafter, the
relationship between the frequency characteristic of the vibrating film and the surface roughness
of the first electrode will be described with reference to FIG. The surface roughness here is
measured using an AFM (Atomic Force Microscope), and the value thereof is represented by Rms
(Roughness Root Mean Square: root mean square roughness). The measurement area of Rms is 5
μm × 5 μm. The AFM used for the measurement is a Nanoscope Dimension 3000
manufactured by VEECO Instruments. The electromechanical transducer to be measured has the
same configuration as the electromechanical transducer of Example 1 described later, except that
the thickness of the first electrode 13 is changed.
[0018]
FIG. 3A is a graph showing the relationship between the film thickness of the first electrode 13
and the surface roughness, using titanium as the electrode of the first electrode 13. It shows the
results of measuring Rms with the RF power fixed at 550 W and the film thickness of titanium
increased from 50 nm to 200 nm. FIG. 3B is a graph showing the relationship between the
frequency characteristics of the vibrating film and the surface roughness of the first electrode 13.
Similarly to FIG. 3A, the film thickness of titanium is increased from 50 nm to 200 nm, and the
result when the frequency characteristic of the vibrating film is measured is shown.
[0019]
FIG. 3 (b) shows the relationship with the Q value when evaluating the variation of the frequency
characteristic. The Q value is a dimensionless number that represents the state of vibration. It is a
value obtained by dividing the resonance frequency of the vibrating membrane by the half width,
and the higher the value is, the more the frequency characteristics of the individual vibrating
membranes of the arrayed cell structure 2 are aligned. That is, the variation in the shape of the
vibrating film of the cell structure 2 and the distance between the electrodes is small.
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[0020]
The frequency characteristics were measured using an impedance analyzer 4294A manufactured
by Agilent. As a result, the Q value shows a high value of 200 or more when the surface
roughness of the first electrode is 6 nm or less, and the Q value rapidly decreases in the range
larger than 6 nm. The curves with different slopes partially in the Q value region of 200 or more
seem to be due to the insufficient resolution of the impedance analyzer.
[0021]
As shown in FIG. 3B, the surface roughness of the first electrode is important in order to obtain a
vibrating film having a small variation in frequency characteristics, and the surface roughness is
in the range of 6 nm or less and in the range larger than 6 nm. It was found that the Q value
changed significantly. This relationship does not depend on the material of the first electrode.
From the above, when the root mean square value of the surface roughness of the first electrode
is 6 nm or less, it is possible to obtain a vibrating film with less variation in frequency
characteristics for each cell and each element. Also, the smaller the surface roughness of the first
electrode, the better.
[0022]
When titanium is used as the first electrode, as shown in FIG. 3A, when the film thickness is
changed, the titanium thickness exhibits an inflection point near 100 nm, and the surface
roughness rapidly increases. doing. In addition, even when the thickness of titanium is around
200 nm, there is also an inflection point, and it is obtained that the increase in surface roughness
is dull. It is considered that in the film forming mechanism, the film forming surface is twodimensionally grown below a certain film thickness and suddenly shifts to three-dimensional
growth, and mixed growth of two-dimensional and three-dimensional growth is performed. This
tendency is not only titanium, but alloys containing titanium such as TiW exhibit similar physical
properties. From this result, when titanium or an alloy containing titanium is used as the first
electrode of the present invention, the film thickness is preferably 100 nm or less. In addition,
since the film thickness is 10 nm or more from island-like to film-like during film formation, the
lower limit of the film thickness of titanium is preferably 10 nm or more. Therefore, the thickness
of the first electrode of the present invention is preferably 10 nm or more and 100 nm or less.
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[0023]
(Method of Manufacturing Electromechanical Transducer) Next, a method of manufacturing the
electromechanical transducer of the present invention will be described with reference to FIG.
FIG. 2 is a process diagram for producing the electromechanical transducer shown in FIGS. 1 (a)
and 1 (b). As shown in FIG. 2A, the first insulating film 12 is formed on the substrate 11. When
the substrate 11 is a conductive substrate such as a silicon substrate, the first insulating film 12
is formed for the purpose of insulating the substrate 11 from the first electrode 13. Therefore,
when the substrate 11 is an insulating substrate such as a glass substrate, the insulating film 12
is not necessary. The substrate 11 is desirably a substrate with as small surface roughness as
possible.
[0024]
Next, as shown in FIG. 2B, the first electrode 13 is formed on the first insulating film. As
described above, the first electrode 13 is formed such that the root mean square value of the
surface roughness is 6 nm or less. In the method of manufacturing by lamination, the surface
roughness of the film is reflected to the next film, so it is important to suppress the surface
roughness in the initial step to a small value. In the step of forming a metal film in which the
surface roughness is likely to be particularly large, it is important to suppress the characteristic
variation by reducing the surface roughness. In the present invention, by producing the first
electrode 13 within the above surface roughness range, even if the surface roughness is
successively reflected on the laminated film in the subsequent steps, the frequency
characteristics of the vibrating film can be obtained. It can be suppressed that the variation
becomes large. The material of the first electrode 13 is preferably titanium, a titanium alloy or
the like having high conductivity, high temperature resistance, and high smoothness.
[0025]
Next, as shown in FIG. 2C, a second insulating film 14 is formed on the first electrode. The
second insulating film 14 is for preventing an electrical short circuit or a dielectric breakdown
between the first electrode and the second electrode when a voltage is applied between the first
electrode and the second electrode. Form. In the case of driving at a low voltage, the second
insulating film 14 may not be formed because the first membrane is an insulator. When the
surface roughness of the second insulating film 14 is large, the distance between the first
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electrode and the second electrode due to the surface roughness varies between cells and
between elements, so the second insulating film Also, materials with small surface roughness are
desirable. For example, a silicon nitride film, a silicon oxide film or the like.
[0026]
Next, as shown in FIG. 2D, a sacrificial layer 25 is formed on the second insulating film. The
sacrificial layer 25 is a material that determines the shape (depth) of the cavity, so it is a material
that is less affected by grain boundaries and crystal anisotropy during etching, and is a material
that has high etching selectivity with other constituent materials. Is desirable. Also, in order to
shorten the etching time, a material having a high etching rate is desirable. Moreover, a material
with small surface roughness is desirable. As in the case of the first electrode, 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 and among the elements. A
sacrificial layer with low roughness is desirable. For example, chromium, molybdenum and the
like.
[0027]
Next, as shown in FIG. 2E, a first membrane 16 is formed on the sacrificial layer. The membrane
support is formed by the same process as the first membrane 16. Low tensile stress is desirable
for the first membrane 16. For example, a tensile stress of more than 0 MPa and 300 MPa or less
is preferable. The silicon nitride film can be stress controlled using PE-CVD (Plasma Enhanced
Chemical Vapor Deposition) method, and low tensile stress can be obtained. If the first membrane
16 has a compressive stress, sticking or buckling may be caused, and the vibrating membrane
may be largely deformed. Sticking means that the first membrane 16 adheres to the first
electrode side. Also, in the case of high tensile stress, the first membrane may be broken.
Therefore, low tensile stress is desirable for the first membrane 16.
[0028]
Next, as shown in FIG. 2 (f), the second electrode 17 is formed, and an etching hole (not shown) is
further formed. Thereafter, the sacrificial layer 25 is removed through the etching holes to form
a cavity. The second electrode 17 is desirably made of a material having a small residual stress, a
heat resistance, and an etching resistance to sacrificial layer etching. In addition, it is desirable to
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use a material that does not cause deterioration or increase in stress due to the temperature or
the like when forming the second membrane in the post-process. In addition, when the etching
selectivity is small, the second electrode 17 needs to be protected when etching the sacrificial
layer, which causes variation. Therefore, materials having resistance in sacrificial layer etching
are desirable. For example, titanium or an alloy of titanium or the like.
[0029]
Next, as shown in FIG. 2 (g), a second membrane 18 is formed. In this step, the step of forming
the second membrane 18 and the step of sealing the etching hole are performed in the same
step. That is, by forming the second membrane 18 on the second electrode (on the surface
opposite to the cavity of the second electrode) in this step, a vibrating membrane having a
desired spring constant is formed. And a sealing portion for sealing the etching hole can be
formed.
[0030]
After the second membrane 18 is formed, an etching hole is formed, and when the etching hole is
sealed, a film for sealing the etching hole is deposited on the second membrane. When etching is
performed to remove the deposited film, thickness variations and stress variations of the
vibrating film occur. On the other hand, since the process of sealing the etching hole and the
process of forming the second membrane 18 are the same as in this process, the vibrating film
can be formed only by the film forming process.
[0031]
Also, the second membrane 18 is desirably a material having a low tensile stress. Similar to the
first membrane 16, if the second membrane 18 has compressive stress, the first membrane 16
may cause sticking or buckling and may be deformed significantly. Also, in the case of a large
tensile stress, the second membrane 18 may be broken. Therefore, low tensile stress is desirable
for the second membrane 18. The silicon nitride film can be stress controlled using PE-CVD, and
low tensile stress can be obtained.
[0032]
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After this process, a drawing wire is formed by a process not shown so as to facilitate electrical
connection from the first electrode and the second electrode. The wiring material is desirably
highly conductive and suitable for assembly, such as aluminum.
[0033]
By manufacturing the electromechanical transducer by the present manufacturing method,
variations in frequency characteristics of the vibrating film can be reduced. Further, in the
electromechanical transducer manufactured by this manufacturing method, since the vibrating
membrane can be formed only in the film forming step, the thickness variation of the vibrating
membrane can be reduced. Variations in sensitivity and bandwidth can be reduced.
[0034]
(Preferred embodiment of the present invention) In the present invention, any substrate such as
a semiconductor substrate, a glass substrate, a ceramic substrate, and a composite substrate
thereof may be used. When the substrate 11 is an insulator such as a glass substrate, the first
insulating film 12 may be omitted. In the present invention, in particular, as described above, it is
preferable to use a silicon substrate as the substrate 11 and use a thermal oxide film as the first
insulating film 12. In particular, a silicon substrate having a thermal oxide film as a highly
smooth substrate is preferable.
[0035]
The first electrode 13 is preferably titanium or an alloy of titanium. By controlling the RF power
in the sputtering apparatus, the surface roughness of the titanium film used for the first electrode
can be accurately controlled. Since titanium has high heat resistance, deformation or
deterioration due to high temperature can be prevented in the subsequent steps. In addition,
since the surface roughness is reflected to the next film by the laminating step, it is important to
suppress the surface roughness in the initial step to a small value.
[0036]
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The second insulating film 14 is preferably silicon oxide. A silicon oxide film formed by PE-CVD
has high insulation and smoothness, and is excellent in step coverage. Since a high voltage is
applied between the first electrode 13 and the second electrode 17, the silicon oxide film
excellent in insulation and step coverage provides a small surface roughness for the subsequent
steps. It is desirable because it can also be done.
[0037]
The first membrane 16 and the second membrane 18 are preferably silicon nitride. A silicon
nitride film formed by using the PE-CVD method can generally obtain tensile stress. In order to
prevent large deformation of the vibrating film due to the residual stress of the silicon nitride
film, it is desirable to have a tensile stress and a low stress value. In the electromechanical
transducer of the present invention, the second electrode 17 is formed between the first
membrane 16 and the second membrane 18. Since the distance between the first electrode and
the second electrode can be reduced as compared with the case where the second electrode 17 is
formed on the second membrane 18, the conversion efficiency can be increased.
[0038]
Here, the conversion efficiency is the efficiency of converting the vibration of the vibrating
membrane into an electric signal, and the smaller the distance between the first electrode and the
second electrode, the higher the conversion efficiency. In addition, when the vibrating film is
made of a combination of different materials having different thermal expansion coefficients, the
vibrating film is warped by the bimetal effect. However, by forming the second electrode 17
having a configuration in which the same material is sandwiched between the first membrane 16
and the second membrane 18, it is possible to balance the stress and reduce the warpage of the
vibrating membrane. it can. Therefore, it is desirable because large deformation of the vibrating
membrane can be prevented.
[0039]
The second electrode 17 is preferably titanium or an alloy of titanium. Titanium or an alloy of
titanium can be formed using an electron beam evaporation method, and a titanium film formed
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under a low vacuum degree using an electron beam evaporation method can obtain tensile stress.
When the second electrode 17 is formed with a large compressive stress, the stress balance of
the second electrode 17 on the first membrane 16 may cause a large deformation of the
vibrating membrane, and the deflection variation of the vibrating membrane is large. turn into. In
order to prevent large deformation of the vibrating membrane, it is desirable that the second
electrode 17 has a tensile stress and a low stress value. In addition, since titanium has high heat
resistance, deterioration due to temperature when forming the second membrane can be
prevented. In addition, since titanium can also reduce the surface roughness, it is possible to
suppress the deflection variation of the membrane.
[0040]
Hereinafter, the present invention will be described in detail by way of more specific examples.
[0041]
Hereinafter, an embodiment of the present invention will be described with reference to FIG.
Fig.1 (a) is a top view of the electro-mechanical transducer of this invention, FIG.1 (b) is AB
sectional drawing of Fig.1 (a). The device 1 of the invention comprises nine cell structures 2.
[0042]
In FIG. 1B, a substrate 11 which is a 300 μm thick silicon substrate, a first insulating film 12
formed on the silicon substrate, a first electrode 13 formed on the first insulating film 12, a It has
a second insulating film 14 on one electrode 13. Furthermore, it has a vibrating membrane
composed of the first membrane 16, the second membrane 18, and the second electrode 17. The
first membrane 16 is supported by a supporting membrane support 19, and the first electrode
13 and the second electrode 17 are disposed to face each other across the cavity 15.
[0043]
The first insulating film 12 is a silicon oxide film having a thickness of 1 μm formed by thermal
oxidation. The first electrode 13 is formed using a sputtering apparatus, is titanium having a
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thickness of 50 nm, and has a surface roughness such that Rms is 2 nm or less. The second
insulating film 14 is a silicon oxide film formed by PE-CVD. The second insulating film 14 has a
value of Rms approximately equal to the surface roughness of the first electrode 13 in order to
reflect the surface roughness of the first electrode 13. The first electrode 13 of this embodiment
is formed on the entire surface of the device 1. When the elements 1 are arranged in a plurality
of arrays, the first electrode 13 is a common electrode electrically connected to all the elements,
and the second electrode 17 is electrically separated for each element. An electrical signal can be
taken out each time. Alternatively, the second electrode 17 may be a common electrode, and the
first electrode 13 may be separated for each element. Furthermore, both the first electrode 13
and the second electrode 17 may be electrodes separated for each element.
[0044]
The second electrode 17 is made of titanium using an electron beam evaporation apparatus. The
thickness of titanium is 100 nm and is formed with a tensile stress of 200 MPa or less. The first
membrane 16 and the second membrane 18 are silicon nitride films produced by PE-CVD, and
are formed with a tensile stress of 100 MPa or less. The diameters of the first membrane 16 and
the second membrane 18 are 45 μm, the thicknesses thereof are 0.4 μm and 0.7 μm, and the
diameter of the second electrode 17 is 40 μm. The thickness of the cavity is 0.18 μm. Also, the
thickness of the second membrane 18 is about three times the thickness of the cavity 15.
Thereby, the etching hole can be closed by the insulating film which forms the second membrane
18, and the cavity 15 can be favorably sealed.
[0045]
Also, the first membrane 16 is thinner than the second membrane 18, and the thickness of the
second membrane 18 adjusts the membrane spring constant to a desired value. As a result, it is
possible to form a vibrating film having a desired spring constant only by the film forming
process without etching the film forming the second membrane 18.
[0046]
The electromechanical transducer of the present embodiment can extract an electric signal of
each element from the second electrode 17 by using the lead wire 6.
[0047]
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When ultrasonic waves are received by the electromechanical transducer, a DC voltage is applied
to the first electrode 13 by voltage application means (not shown).
When ultrasonic waves are received, the first membrane 16 having the second electrode 17 and
the second membrane 18 deform, so the distance of the cavity 15 between the second electrode
17 and the first electrode 13 changes. The current (electric signal) flows in the lead-out wiring 6
due to the capacitance change. This current is converted into a voltage by a current-voltage
conversion element (not shown) to obtain an ultrasonic reception signal.
[0048]
When ultrasonic waves are transmitted, a direct current voltage is applied to the first electrode
13 and an alternating voltage is applied to the second electrode 17, and the vibrating film can be
vibrated by electrostatic force. By this, ultrasonic waves can be transmitted.
[0049]
Second Embodiment A second embodiment of the present invention will be described below with
reference to FIG. FIG. 4A is a top view of the electro-mechanical transducer according to this
embodiment, and FIG. 4B is a cross-sectional view taken along the line C-D of FIG. 4A. The
configuration of the electromechanical transducer of Example 2 is substantially the same as that
of Example 1 except for the shape of the first electrode.
[0050]
In FIG. 4B, a silicon substrate 41 of 300 μm thickness, a first insulating film 42 formed on the
silicon substrate 41, a first electrode 43 formed on the first insulating film 42, A second
insulating film 44 on the electrode 43 is provided. Furthermore, it has a vibrating membrane
composed of the first membrane 46, the second membrane 48, and the second electrode 47. The
first membrane 46 is supported by a membrane support 49. The first electrode 43 and the
second electrode 47 are disposed to face each other across the cavity 45.
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[0051]
The first insulating film 42 is a silicon oxide film having a thickness of 1 μm formed by thermal
oxidation. The first electrode 43 is formed using a sputtering apparatus, is titanium having a
thickness of 50 nm, and has a surface roughness Rms of 2 nm or less. Furthermore, in the
present embodiment, in order to reduce unnecessary parasitic capacitance formed by the wiring
of the first electrode and the wiring of the second electrode at a position other than the cavity,
the first electrode is a wiring of the second electrode. Pattern to minimize the overlapping area
with the
[0052]
By using photolithography and etching, the titanium of the first electrode 43 can be patterned
with high accuracy as illustrated. By etching using a solution containing a hydrogen peroxide
solution as the etchant, the etching selectivity with the material being constructed can be
increased, and the surrounding materials are not damaged and the surface roughness does not
change, so that the surface is extremely smooth. The first electrode is obtained. As described
above, the first electrode 43 also has substantially the same size as the second electrode 47, and
the cells are connected by the thin wiring 33. The parasitic capacitance can be reduced by
arranging the wiring 33 of the first electrode 43 and the wiring 36 of the second electrode 47 so
as not to face each other via the insulating film. The second insulating film 44 is a silicon oxide
film formed by PE-CVD.
[0053]
DESCRIPTION OF SYMBOLS 1 element 2 cell structure 5 etching hole 6 lead-out wiring 11 board
¦ substrate 12 1st insulating film 13 1st electrode 14 2nd insulating film 15 cavity 16 1st
membrane 18 2nd membrane 19 membrane support part
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