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JP2016096576

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DESCRIPTION JP2016096576
Abstract: PROBLEM TO BE SOLVED: To reduce variation in mechanical characteristics of a
vibrating membrane of an electromechanical transducer. A method of manufacturing an electromechanical transducer according to the present invention comprises the steps of: forming an
insulating film on a first electrode; forming a sacrificial layer on the insulating film; Forming a
second electrode on the first membrane; forming an etching hole on the first membrane; and
removing the sacrificial layer through the etching hole. Forming a second membrane on the
second electrode, sealing the etching hole, forming the second membrane, and forming the
etching hole And are characterized by the same process. [Selected figure] Figure 3
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.
[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
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receive ultrasonic waves by the vibration of the vibrating film.
[0003]
As a method of manufacturing a CMUT which is an electromechanical transducer, there is a
method of manufacturing a cavity by sacrificial layer etching. In Patent Document 1, in order to
prevent the upper electrode (second electrode) from being etched during etching of the sacrificial
layer, a second electrode is formed between the first membrane and the second membrane, and
the sacrificial layer is A method of etching is described.
[0004]
U.S. Patent Application Publication No. 2005/0177045
[0005]
Since the electromechanical transducer such as CMUT can be considered to be used in liquid, the
cavity is preferably sealed.
That is, after forming a cavity by sacrificial layer etching, it is preferable to seal an etching hole.
In Patent Document 1, the second membrane is formed after the second electrode is formed, and
then the sacrificial layer is etched. And the etching hole is sealed by the sealing film. However,
when film formation is performed to seal the etching hole as in Patent Document 1, a sealing film
is also deposited on the second membrane. When the sealing film deposited on the second
membrane is removed by etching or the like, the sensitivity and the band of the
electromechanical transducer may vary from element to element due to thickness variations as a
vibrating film, stress variations, and the like.
[0006]
Therefore, an object of the present invention is to reduce thickness variations and stress
variations of a vibrating film.
[0007]
In the method of manufacturing an electromechanical transducer according to the present
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invention, a step of forming an insulating film on a first electrode, a step of forming a sacrificial
layer on the insulating film, and a first membrane on the sacrificial layer Forming a second
electrode on the first membrane, forming an etching hole on the first membrane, and removing
the sacrificial layer through the etching hole; Forming a second membrane on a second electrode,
sealing the etching hole, forming the second membrane, and forming the etching hole , And the
same process.
[0008]
In the electromechanical transducer according to the present invention, a first electrode, an
insulating film formed on the first electrode, a first membrane formed with a gap from the
insulating film, and the first membrane A vibrating membrane comprising a second electrode
formed on the one membrane and facing the first electrode, and a second membrane formed on
the second electrode opposite to the gap; The gap is formed by removing a sacrificial layer
provided on the insulating film through an etching hole formed in the same layer as the first
membrane. A thickness of a sealing portion which is a gap and seals the etching hole is the same
as a thickness of the second membrane on the second electrode.
[0009]
According to the present invention, thickness variations and stress variations of the vibrating film
can be reduced, so that the sensitivity and bandwidth variations of each element of the
electromechanical transducer can be reduced.
[0010]
It is a schematic diagram for demonstrating the electromechanical transducer which can apply
Example 1 of this invention.
It is a schematic diagram for demonstrating the electromechanical transducer which can apply
Example 2 of this invention.
It is an AB sectional view for explaining a manufacturing method of an electromechanical
transducer to which Example 1 of the present invention can be applied.
It is a CD sectional view for explaining an electromechanical transducer to which Example 1 of
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the present invention can be applied.
[0011]
Hereinafter, embodiments of the present invention will be described using the drawings.
[0012]
(Configuration of Electromechanical Transducer) FIG. 1 (a) is a top view of the electromechanical
transducer according to the present invention, and FIG. 1 (b) and FIG. 1 (c) are respectively an A
in FIG. 1 (a). It is a -B sectional view, a C-D sectional view.
The electromechanical transducer of the present invention has a plurality of elements 2 having a
cell structure 1.
The element 2 is composed of a plurality of cell structures 1 electrically connected. In FIG. 1, the
element 2 is composed of nine cell structures, but the number may be any number. Also,
although only four elements are described, the number may be any number. The shape of the cell
structure is circular in FIG. 1, but may be square, hexagonal or the like.
[0013]
The cell structure 1 includes a substrate 11, a first insulating film 12 formed on the substrate, a
first electrode 13 formed on the first insulating film, and a second insulating film 14 on the first
electrode. Have. Furthermore, the cell structure 1 has a vibrating membrane composed of the
first membrane 16, the second membrane 18 and the second electrode 4. The first membrane 16
and the second membrane 18 are insulating films. The first membrane 16 is supported by the
membrane support portion 20, and the vibrating membrane is disposed at a cavity 3 which is a
gap. The first electrode 13 and the second electrode 4 face each other, and a voltage is applied
between the first electrode 13 and the second electrode 4 by voltage application means (not
shown).
[0014]
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Further, by using the lead wire 6, the electro-mechanical transducer can draw an electric signal
from the second electrode 4 for each element. 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. In the present
embodiment, the first electrode 13 is used as a common electrode, and the second electrode 4 is
disposed for each element, so that an electric signal is drawn from the second electrode 4. It does
not matter. That is, by using the second electrode 4 as the common electrode and arranging the
first electrode 13 for each element, an electric signal for each element may 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 film having the second
electrode 4 is bent, so that the distance between the second electrode 4 and the first electrode 13
(the distance in the depth direction of the cavity 3) 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 currentvoltage conversion element (not shown) to obtain an ultrasonic reception signal. As described
above, a direct current voltage may be applied to the second electrode 4 by changing the
configuration of the lead wiring, and an electric signal may be drawn from the first electrode 13
for each element.
[0016]
In addition, when ultrasonic waves are transmitted, a direct current voltage can be applied to the
first electrode, and an alternating current voltage can be applied to the second electrode, 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 and an AC voltage to the first electrode by
changing the configuration of the lead wiring. Alternatively, a direct current voltage and an
alternating current voltage may be applied to the first electrode or the second electrode to
vibrate the vibrating film by electrostatic force.
[0017]
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Production Method Next, the production method of the present invention will be described with
reference to FIGS. 3 and 4. FIG. 3 is a cross-sectional view of an electromechanical transducer,
which has substantially the same configuration as that of FIG. 3 is a cross-sectional view taken
along the line A-B of FIG. 1, and FIG. 4 is a cross-sectional view taken along the line C-D of FIG.
[0018]
As shown in FIGS. 3A and 4A, the first insulating film 51 is formed on the substrate 50. In the
case where the substrate 50 is a conductive substrate such as a silicon substrate, the first
insulating film 51 is formed to insulate from the first electrode. When the substrate 50 is an
insulating substrate such as a glass substrate, the insulating film 51 may not be formed. Further,
the substrate 50 is desirably a substrate having a small surface roughness. When the surface
roughness is large, the influence of the surface roughness remains even in the film forming step
after the present step, and the distance between the first electrode and the second electrode
varies among the cells and among the elements. It will Since this variation is a variation in
conversion efficiency, it is a variation in sensitivity and band. Therefore, the substrate 50 is
preferably a substrate with a small surface roughness.
[0019]
Next, as shown in FIG. 3B and FIG. 4B, the first electrode 52 is formed. The first electrode 52 is
desirably a conductive material having a small surface roughness, such as titanium or aluminum.
As in the case of the substrate, when the surface roughness of the first electrode 52 is large, the
distance between the first electrode and the second electrode due to the surface roughness varies
between cells and between elements, so the surface A conductive material with low roughness is
desirable.
[0020]
Next, as shown in FIGS. 3C and 4C, the second insulating film 53 is formed. The second insulating
film 53 is desirably an insulating material having a small surface roughness. The second
insulating film 53 is formed to prevent an electrical short circuit or a dielectric breakdown
between the electrodes when a voltage is applied between the first electrode 52 and the second
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electrode 56. In the case of driving at a low voltage, since the first membrane 55 is an insulator,
the second insulating film 53 may not be formed. As in the substrate 50, when the surface
roughness of the second insulating film 53 is large, the distance between the electrodes due to
the surface roughness varies among the cells and among the elements. Therefore, it is desirable
to form the second insulating film with a material having a small surface roughness. For example,
a silicon nitride film, a silicon oxide film or the like.
[0021]
Next, as shown in FIGS. 3D and 4D, a sacrificial layer 54 is formed. The sacrificial layer 54 is
desirably made of a material having a small surface roughness. Also, in order to shorten the
etching time of etching for removing the sacrificial layer, a material having a high etching rate is
desirable. Furthermore, it is preferable to select the material of the sacrificial layer so that the
second insulating film, the first membrane, and the second electrode are hardly etched with
respect to the etchant or etching gas for removing the sacrificial layer. When the second
insulating film, the first membrane, and the second electrode are etched with respect to the
etchant or etching gas for removing the sacrificial layer, the thickness variation of the vibrating
film, the first electrode and the second electrode Variations in the distance between the
electrodes occur. In the case where the second insulating film and the first membrane are a
silicon nitride film or a silicon oxide film, chromium is desirable as the sacrificial layer.
[0022]
Next, as shown in FIGS. 3E and 4E, a first membrane 55 is formed on the sacrificial layer. The
membrane support portion is formed by the same process as the membrane 55. The first
membrane 55 desirably has a small tensile stress. 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, and can
have a low tensile stress of 300 MPa or less. When the first membrane has a compressive stress,
the first membrane causes sticking or buckling and is largely deformed. Sticking means that the
first membrane 55 adheres to the first electrode side. Also, in the case of high tensile stress, the
first membrane may be broken. Therefore, it is desirable that the first membrane 55 be formed to
have a low tensile stress.
[0023]
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Next, as shown in FIGS. 3 (f) and 4 (f), the second electrode 56 is formed on the first membrane.
Then, as shown in FIG. 4F, an etching hole 58 is formed. Thereafter, the sacrificial layer 54 is
removed through the etching holes. The second electrode 56 is desirably made of a material
having low residual stress, heat resistance, and etching resistance to sacrificial layer etching. In
the case where the etching resistance is low, when removing the sacrificial layer, it is necessary
to perform the sacrificial layer etching while applying a photoresist or the like for protecting the
second electrode. However, when a photoresist or the like is applied, the stress of the photoresist
or the like makes the first membrane easy to stick. Thus, it is desirable to have a second electrode
that is etch resistant so that the sacrificial layer can be etched with the second electrode exposed.
[0024]
In addition, when the residual stress of the second electrode is large, the second electrode with a
small residual stress is desirable because it causes a large deformation of the vibrating
membrane. Furthermore, as shown in FIGS. 3 (g) and 4 (g), it is desirable to use a material that
does not cause deterioration or increase in stress due to the temperature at the time of forming
the second membrane. As a material of the second electrode, titanium is preferable.
[0025]
Next, as shown in FIGS. 3G and 4G, the formation of the second membrane 57 and the sealing of
the etching hole 58 are performed. In this step, as shown in FIG. 4G, the step of forming the
second membrane 57 and the step of sealing the etching hole 58 are performed in the same step.
That is, by forming the second membrane 57 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 can be formed. Thus, a sealing portion for sealing the etching hole 58 can be formed.
[0026]
When the step of sealing the etching hole 58 is performed after the step of forming the second
membrane 57, a film for sealing the etching hole 58 is deposited on the second membrane 57.
When etching is performed to remove the deposited film, thickness variations and stress
variations of the vibrating film occur. On the other hand, in the present invention, since the
sealing process of the etching hole 58 and the process of forming the second membrane 57 are
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the same, the vibrating film can be formed only by the film forming process. That is, in the
present invention, since the film formed on the second electrode is not removed by etching or the
like, variations in thickness of the vibrating film and variations in stress hardly occur.
[0027]
After this process, wiring which is connected to the first electrode and the second electrode is
formed by a process not shown. The wiring material may be aluminum or the like.
[0028]
As described above, as the electromechanical transducer is manufactured by this manufacturing
method, a membrane having a desired spring constant is formed only by the film forming
process without etching the film forming the second membrane. be able to. Therefore, thickness
variations and stress variations of the vibrating film of the electromechanical transducer can be
reduced. Therefore, variations in sensitivity and bandwidth of the electromechanical transducer
can be reduced.
[0029]
In FIG. 1, in the electromechanical transducer manufactured according to the present
embodiment, the thickness of the sealing portion sealing the etching hole 19 is the same as the
thickness of the second membrane 18 on the second electrode 4. is there. Here, the thickness of
the sealing portion is the thickness of the etching hole central portion in the direction
perpendicular to the surface on which the first electrode is formed, and is the thickness indicated
by the arrow 5 in FIG. In addition, “the thickness of the sealing portion is the same as the
thickness of the second membrane 18 on the second electrode 4” is not limited to the case
where “the thickness is strictly the same”, “film formation variation In the case where there is
a difference in thickness within the range of "," it is assumed that "the thickness is the same".
Specifically, as a difference in thickness within the range of film formation variation, the case
where the thickness of the sealing portion is within ± 10% of the thickness of the second
membrane 18 on the second electrode 4 is there.
[0030]
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Preferred Embodiments of the Present Invention Next, preferred embodiments of the present
invention will be described. In the present invention, the thickness of the first membrane 55 is
preferably formed to be twice or more the thickness of the sacrificial layer 54. If the thickness of
the first membrane 55 is thinner than twice the depth of the sacrificial layer 54, there is a
possibility that the first membrane 55 can not well cover the stepped portion at the time of
forming the sacrificial layer. In particular, when the covering state of the corner between the side
surface and the upper surface of the sacrificial layer 54 is poor, when the sacrificial layer is
etched to form a cavity, the mechanical properties of the first membrane 55 between cells and
between elements vary. It will
[0031]
Therefore, the thickness of the first membrane 55 (the thickness shown by the arrow 7 in FIG.
1B) is twice or more the thickness of the sacrificial layer 54 (the thickness shown by the arrow 9
in FIG. 1C). The first membrane 55 can well cover the stepped portion of the sacrificial layer 54.
Therefore, the mechanical characteristic variation of the first membrane 55 between the cells
and between the elements can be reduced. In FIG. 1, in the electromechanical transducer
manufactured in this form, the thickness of the first membrane 16 is twice or more the depth of
the cavity 3. In the present invention, the “thickness” of the first membrane 16 or the like
indicates the thickness in the direction perpendicular to the surface on which the first electrode
is formed. The “depth” of the cavity 3 is the distance of the gap from the second insulating
film 14 to the first membrane 16 in the state where no voltage is applied between the electrodes.
[0032]
Further, the thickness of the second membrane 57 on the second electrode 4 (the thickness
indicated by the arrow 8 in FIG. 1B) is the depth of the cavity (the thickness indicated by the
arrow 9 in FIG. 1B). It is preferable to form so that it may be 3 times or more. If the thickness of
the second membrane 57 is thinner than three times the depth of the cavity, the etching hole 58
can be closed by the insulating film forming the second membrane 57 and the cavity can not be
sealed well. There is sex. Therefore, by setting the thickness of the second membrane 57 to be
three times or more the depth of the cavity, the etching hole 58 is closed by the insulating film
forming the second membrane 57, and the cavity is favorably sealed. be able to. In FIG. 1, in the
electromechanical transducer manufactured in this form, the thickness of the second membrane
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18 is three or more times the depth of the cavity 3.
[0033]
Furthermore, the thickness of the second membrane 57 is preferably formed to be thicker than
the thickness of the first membrane 55. By reducing the thickness of the first membrane 55, the
distance between the electrodes can be reduced. Also, the spring constant of the vibrating
membrane changes depending on the thickness of the vibrating membrane. Therefore, it is easier
to adjust to a desired spring constant by making the first membrane 55 thinner and adjusting the
thickness of the entire vibrating membrane by the second membrane 57. In FIG. 1, in the
electromechanical transducer manufactured according to the present embodiment, the thickness
of the second membrane 18 is thicker than the thickness of the first membrane 16.
[0034]
The second electrode 56 is preferably formed so as to cover the entire surface of the sacrificial
layer 54 (see Example 2 described later). When misalignment in photolithography occurs when
forming the second electrode 56, the central axis of the sacrificial layer 54 (that is, the central
axis of the cavity) and the central axis of the second electrode 56 may be deviated. When the
area of the second electrode is smaller than the area of the cavity and the central axis of the
cavity and the central axis of the second electrode deviate, the stress of the second electrode 56
acting on the first membrane 55 changes, so that each cell In the meantime, deflection variation
of the vibrating film between the elements may occur. Therefore, by forming the second
electrode 56 so as to cover the entire surface of the sacrificial layer 54, it is possible to further
reduce the deflection variation of the vibrating film due to the alignment deviation in
photolithography when forming the second electrode 56. . In FIG. 1, in the electromechanical
transducer manufactured according to the present embodiment, the second electrode 4 is larger
than the area of the cavity 3 and formed so as to cover the entire surface of the cavity 3. In
particular, the distance from the central axis of the second electrode 4 to the outer periphery is a
parasitic capacitance generated between the second electrode and the first electrode (a
capacitance at which the capacitance does not change when the vibrating film vibrates) It is
desirable that the distance from the central axis of the cavity 3 to the outer periphery be larger
by about 3 μm so as not to increase
[0035]
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11
Furthermore, it is desirable to use titanium as the second electrode 56. Since titanium has small
residual stress, it is possible to prevent large deformation of the vibrating film. When a silicon
nitride film is used as the first membrane 55 and the second membrane 57, the Young's modulus
of the second electrode 56 is smaller than that of the first membrane 55 and the second
membrane 57. Therefore, the thickness of the second membrane 57 facilitates forming a
vibrating membrane having a desired spring constant. 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.
[0036]
Furthermore, it is preferable to use silicon nitride as the first membrane 55. Silicon nitride is easy
to control stress and can be formed with a low tensile stress, for example, a tensile stress of more
than 0 MPa and 300 MPa or less. Therefore, it is possible to prevent large deformation of the
vibrating film due to the residual stress of the silicon nitride film. In addition, silicon nitride films
formed by Prasma Enhanced Chemical Vapor Deposition (PE-CVD) can be formed at a low
temperature (200 to 400 degrees) as compared with Low Pressure Chemical Vapor Deposition
(LPCVD). The Young's modulus of the silicon nitride film formed by PECVD can be 180 GPa or
more, and the rigidity can be enhanced by the thin first membrane.
[0037]
Furthermore, it is also preferable to form the first membrane 55 so as to have a spring constant
of 500 N / m or more and 3000 N / m or less. Here, the spring constant (k) is calculated from the
maximum displacement (x) when the equally distributed load (F) is applied to the entire vibrating
membrane, and k = F / x. For example, when a uniform load of 10 uN is applied, if the maximum
displacement is 10 nm, the spring constant is 1000 N / m.
[0038]
When the spring constant of the first membrane 55 is large, the rigidity of the first membrane 55
is increased while the thickness is increased. When the thickness of the first membrane 55 is
increased, the distance between the first electrode 52 and the second electrode 56 is increased,
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and the conversion efficiency is reduced. Here, the conversion efficiency is an efficiency of
converting the vibration of the vibrating membrane into an electric signal, and the conversion
efficiency is higher as the distance between the first electrode 52 and the second electrode 56 is
narrower. On the other hand, when the spring constant of the first membrane 55 is small, the
first membrane 55 adheres to the first electrode side after the etching of the sacrificial layer 54
(sticking).
[0039]
Sticking occurs due to residual stress of the first membrane 55 and the second membrane 57,
surface tension due to evaporation of moisture during etching of the sacrificial layer, electrostatic
force, or moisture adsorption by hydroxyl groups on the surface. In particular, when the
sacrificial layer etching is performed by wet etching, sticking is likely to occur. In particular, in an
electromechanical transducer in which the frequency band of vibration of the vibrating film is
0.3 to 20 MHz, the cavity depth is 50 to 300 nm, and sticking easily occurs. Therefore, by
forming the first membrane 55 so that the spring constant of the first membrane 55 is 500 N /
m or more and 3000 N / m or less, the reduction in conversion efficiency can be suppressed, and
sticking can be prevented.
[0040]
Hereinafter, the present invention will be described in detail by way of more specific examples.
[0041]
The first embodiment will be described with reference to FIGS.
First, the electromechanical transducer of the present embodiment will be described with
reference to FIG. 1, and then a method of manufacturing the electromechanical transducer will be
described with reference to FIGS. Fig.1 (a) is a top view of the electro-mechanical transducer of
this invention, FIG.1 (b), FIG.1 (c) is AB sectional drawing of FIG. 1 (a), C-D, respectively. FIG. The
electromechanical transducer of this embodiment has four elements 2 having a cell structure 1.
Further, one element 2 is configured of nine cell structures.
[0042]
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The cell structure 1 comprises a substrate 11 which is a 300 um 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 first It has a second insulating film 14 on the electrode 13. Furthermore, it
has a vibrating film composed of the first membrane 16, the second membrane 18, and the
second electrode 4 arranged with a gap from the second insulating film 14. The first membrane
16 is supported by a membrane support 20. The thickness of the sealing portion sealing the
etching hole 19 is the same as the thickness of the second membrane 18 on the second electrode
4. Therefore, a vibrating film having a desired spring constant can be formed only by the film
forming process without etching the film forming the second membrane 18.
[0043]
The first insulating film 12 is a silicon oxide film having a thickness of 1 μm formed by thermal
oxidation. The second insulating film 14 is a silicon oxide film formed by PE-CVD. The first
electrode 13 is titanium having a thickness of 50 nm, and the second electrode 4 is titanium
having a thickness of 100 nm. 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.
[0044]
Further, the diameters of the first membrane 16 and the second membrane 18 are 45 um, and
the thicknesses thereof are 0.4 um and 0.7 um, respectively. The diameter of the second
electrode 4 is 40 um, and the depth of the cavity is 0.18 um. The spring constant of the first
membrane 16 is 1200 N / m, which can prevent sticking of the first membrane after the cavity 3
is formed.
[0045]
As in the present embodiment, since the thickness of the first membrane 16 is twice or more the
cavity depth, the first membrane 16 can satisfactorily cover the step portion at the time of
forming the cavity.
[0046]
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14
In addition, the thickness of the second membrane 18 is three or more times the depth of the
cavity 3.
Thus, the etching hole 19 can be closed by the insulating film forming the second membrane 18,
and the cavity 3 can be favorably sealed. Also, the first membrane 16 is thinner than the second
membrane 18. Therefore, the thickness of the second membrane 18 makes it easy to adjust the
membrane's spring constant to a desired value. Further, in the electromechanical transducer of
the present embodiment, by using the lead wire 6, an electric signal of each element can be
extracted from the second electrode 4.
[0047]
Next, a method of manufacturing the electromechanical transducer of the present embodiment
will be described with reference to FIGS. 3 is a cross-sectional view taken along the line A-B of
FIG. 1, and FIG. 4 is a cross-sectional view taken along the line C-D of FIG.
[0048]
First, as shown in FIGS. 3A and 4A, the first insulating film 51 is formed on the substrate 50. The
substrate 50 is a silicon substrate with a thickness of 300 um. The first insulating film 51 is a
silicon oxide film by thermal oxidation for forming the insulation between the first electrode 52
and the substrate 50, and has a thickness of 1 μm.
[0049]
Next, as shown in FIG. 3B and FIG. 4B, the first electrode 52 is formed. The first electrode 52 is
titanium with a thickness of 50 nm, and the root mean square surface roughness (Rms) is 2 nm
or less.
[0050]
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Next, as shown in FIGS. 3C and 4C, the second insulating film 53 is formed. The second insulating
film 53 is a silicon oxide film having a thickness of 0.1 μm formed by PE-CVD, and the root
mean square surface roughness (Rms) is 2 nm or less. The insulating film 53 is for preventing an
electrical short circuit between the first electrode 52 and the second electrode 56 or a dielectric
breakdown when a voltage is applied between the first electrode 52 and the second electrode 56.
Form.
[0051]
Next, as shown in FIGS. 3D and 4D, a sacrificial layer 54 is formed. The sacrificial layer 54 is
chromium with a thickness of 0.2 um, and the root mean square surface roughness (Rms) is 1.5
nm or less. Also, the diameter of the sacrificial layer is 40 um.
[0052]
Next, as shown in FIGS. 3E and 4E, the first membrane 55 is formed. The first membrane 55 is a
0.4 μm thick nitride film formed by PECVD. The residual stress of the first membrane 55 is 200
MPa.
[0053]
Next, as shown in FIGS. 3F and 4F, a second electrode 56 is formed, and an etching hole 58 is
formed. The second electrode 56 is titanium with a thickness of 0.1 um, and the residual stress is
200 MPa or less. As shown in FIGS. 3 (g) and 4 (g), titanium does not have an increase in surface
roughness or a change in stress due to the temperature at which the second membrane 57 is
formed. In addition, since the etching is not performed when removing the sacrificial layer, the
sacrificial layer can be removed without applying and protecting a resist or the like.
[0054]
Thereafter, the sacrificial layer 54 is removed through the etching holes. The removal of the
sacrificial layer 54 is performed by using a chromium etching solution (ceric ammonium nitrate,
mixed acid of perchloric acid, and water). In particular, although the first membrane 55 is easily
04-05-2019
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attached to the first electrode 52 side by the drying process after removal of the sacrificial layer
54, the first membrane has a spring constant of 1250 N / m, so sticking To form a cavity.
Further, since the chromium etching solution does not etch the silicon nitride, titanium or silicon
oxide film, it is possible to prevent the thickness variation of the vibrating film and the distance
variation between the first electrode and the second electrode.
[0055]
Next, as shown in FIGS. 3G and 4G, the second membrane 57 is formed, and the etching hole 58
is sealed. In order to seal the etching hole in the same step as the second membrane formation, it
can be formed without etching the vibrating film.
[0056]
According to the manufacturing method of the electromechanical transducer of the present
embodiment, a membrane having a desired spring constant can be formed only by the film
forming process. Can be produced. The electromechanical transducer manufactured by the above
manufacturing method can reduce the sensitivity variation between elements to 1 dB or less.
[0057]
Next, Example 2 will be described with reference to FIG. 2 (a) is a top view of the
electromechanical transducer of this embodiment, and FIG. 2 (b) and FIG. 2 (c) are cross-sectional
views taken along the line A-B of FIG. It is D sectional drawing. The electromechanical transducer
of Example 2 is characterized by the area of the second electrode 24. The other configuration is
substantially the same as that of the first embodiment.
[0058]
The cell structure 21 comprises a 300 um thick silicon substrate 31, a first insulating film 32
formed on the silicon substrate 31, a first electrode 33 formed on the first insulating film 32, and
a first electrode 33. The upper second insulating film 34 is provided. Furthermore, a vibrating
membrane composed of the second insulating film 34 and the first membrane 36, the second
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membrane 38, and the second electrode 24 provided to separate the cavity 23 and the first
membrane 36 are supported. And a membrane support 40. The element 22 is composed of a
plurality of cell structures 21 electrically connected.
[0059]
The first insulating film 32 is a silicon oxide film having a thickness of 1 um formed by thermal
oxidation, and the second insulating film 34 is a silicon oxide film having a thickness of 0.1 um
formed by PE-CVD. . The first electrode 24 is titanium with a thickness of 50 nm, and the second
electrode 24 is also titanium with a thickness of 100 nm. The first membrane 36 and the second
membrane 38 are silicon nitride films formed with a tensile stress of 200 MPa or less by PE-CVD.
Further, the diameters of the first membrane 36 and the second membrane 38 are 50 um, the
thicknesses thereof are 0.4 um and 0.7 um, and the diameter of the second electrode 24 is 56
um. Also, the cavity depth is 0.2 um.
[0060]
In the present embodiment, the diameter of the second electrode 24 is larger than the diameters
of the first membrane 36 and the second membrane 38, and the second electrode covers the
cavity. According to this configuration, even if misalignment occurs in photolithography when
forming the second electrode, deflection variation of the vibrating film can be reduced.
[0061]
In the electromechanical transducer of this embodiment configured as described above, a
membrane having a desired spring constant can be formed only by the film forming process.
Furthermore, even if misalignment occurs in photolithography when forming the second
electrode, variations in deflection of the vibrating film can be reduced, so the variation in
sensitivity between elements can be 0.5 dB or less. it can.
[0062]
Reference Signs List 1 cell structure 2 electromechanical transducer element 3 cavity 4 second
electrode 5 thickness of sealing portion 6 cavity depth 7 thickness of first membrane 8 thickness
of second membrane 11 substrate 12 first insulating film 13 first electrode 14 second insulating
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film 16 first membrane 18 second membrane 19 etching hole 20 membrane support portion
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