U.S. patent application number 14/489250 was filed with the patent office on 2015-01-01 for electromechanical transducer and method for fabricating the same.
The applicant listed for this patent is CANON KABUSHIKI KAISHA. Invention is credited to Yoshihiro Hasegawa, Yuichi Masaki.
Application Number | 20150001987 14/489250 |
Document ID | / |
Family ID | 44815206 |
Filed Date | 2015-01-01 |
United States Patent
Application |
20150001987 |
Kind Code |
A1 |
Masaki; Yuichi ; et
al. |
January 1, 2015 |
ELECTROMECHANICAL TRANSDUCER AND METHOD FOR FABRICATING THE
SAME
Abstract
An electromechanical transducer includes a first electromagnetic
element and a second electromagnetic element, such as electrodes,
disposed opposite to each other with a sealed cavity therebetween.
The sealed cavity is formed by removing a sacrifice layer and then
performing sealing. A sealing portion is formed by superposing a
film of a hardened second sealing material that has fluidity at
normal temperature on a film of a first sealing material that does
not have fluidity at normal temperature.
Inventors: |
Masaki; Yuichi;
(Kawasaki-shi, JP) ; Hasegawa; Yoshihiro;
(Tama-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
CANON KABUSHIKI KAISHA |
Tokyo |
|
JP |
|
|
Family ID: |
44815206 |
Appl. No.: |
14/489250 |
Filed: |
September 17, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13087178 |
Apr 14, 2011 |
8857041 |
|
|
14489250 |
|
|
|
|
Current U.S.
Class: |
310/300 |
Current CPC
Class: |
Y10T 29/49007 20150115;
H04R 19/005 20130101; Y10T 29/49002 20150115; B81B 3/0094 20130101;
Y10T 29/49005 20150115; Y10T 29/4908 20150115; B06B 1/0292
20130101; H04R 31/00 20130101 |
Class at
Publication: |
310/300 |
International
Class: |
B06B 1/02 20060101
B06B001/02; B81B 3/00 20060101 B81B003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 23, 2010 |
JP |
2010-099319 |
Feb 24, 2011 |
JP |
2011-037773 |
Claims
1. An electromechanical transducer comprising: a first
electromagnetic element and a second electromagnetic element
disposed opposite to each other with a sealed cavity therebetween,
the sealed cavity being formed by removing a sacrifice layer and
then performing sealing, wherein the performing sealing to form a
sealing portion includes superposing a film of a hardened second
sealing material that has fluidity at normal temperature on a film
of a first sealing material that does not have fluidity at normal
temperature.
2. The electromechanical transducer according to claim 1, wherein
the first electromagnetic element and the second electromagnetic
element are a first electrode and a second electrode,
respectively.
3. The electromechanical transducer according to claim 1, wherein
the second electromagnetic element is formed on a surface of a
vibrating membrane above the sealed cavity opposite to the first
electromagnetic element.
4. The electromechanical transducer according to claim 1, wherein
the film of the second sealing material is formed in an area other
than the second electromagnetic element or other than part just
above the sealed cavity.
5. The electromechanical transducer according to claim 1, wherein
the film of the second sealing material is thicker than the film of
the first sealing material.
6. The electromechanical transducer according to claim 1, wherein
the film of the hardened second sealing material is resistant to
acid and oil.
7. The electromechanical transducer according to claim 1, wherein
the second sealing material is inorganic SOG, an organic material,
or a photosensitive organic material.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a Divisional of U.S. application Ser.
No. 13/087178, filed Apr. 14, 2011, which claims priority from
Japanese Patent Application No. 2010-099319 filed Apr. 23, 2010 and
No. 2011-037773 filed Feb. 24, 2011, which are hereby incorporated
by reference herein in their entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an electromechanical
transducer, such as a capacitive ultrasonic transducer, and a
method for fabricating the same.
[0004] 2. Description of the Related Art
[0005] In recent years, a capacitive electromechanical transducer
fabricated using a micromachining process has been researched. A
normal capacitive electromechanical transducer has a vibrating
membrane supported at a 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 micromachined ultrasonic
transducer (CMUT). A CMUT transmits or receives ultrasound using a
light-weight membrane. It is easy to obtain a CMUT that has
excellent broadband characteristics in both liquid and air. Use of
this CMUT enables more accurate diagnosis than conventional medical
diagnosis. Therefore, it is attracting attention as a promising
technology. The operating principle of a CMUT will be described.
When transmitting ultrasound, a DC voltage overlapped with an AC
voltage is applied between the lower electrode and the upper
electrode. This causes the vibrating membrane to vibrate, thereby
generating ultrasound. When receiving ultrasound, the vibrating
membrane is deformed by ultrasound, and the deformation causes a
change in the capacitance between the lower electrode and the upper
electrode. Therefore, the displacement of the vibrating membrane
can be detected as an electric signal.
[0006] A method called "surface type" is used as a method for
forming the "gap" (the distance between the electrodes) of a CMUT.
It includes providing a sacrifice layer having a thickness equal to
the distance between the electrodes, forming a vibrating membrane
on the sacrifice layer, removing the sacrifice layer by etching,
and sealing the etching hole. It is proposed to seal the etching
hole with a SiN film formed by plasma-enhanced (PE) CVD or a
SiO.sub.2 film formed by low-pressure (LP) CVD in view of the size
of the etching hole (see IEEE Transactions On Ultrasonic,
Ferroelectrics, And Frequency Control, Vol. 52, No. 12, December
2005, pp. 2242-2258). It is also proposed to seal the etching hole
with metal (see U.S. Pat. No. 4,262,399).
[0007] The above-described surface-type CMUT has many sealing
portions on the surface of the substrate, and therefore a sealing
method is required by which the many sealing portions can be sealed
collectively and tightly. Although a thin film can be formed by
vacuum film formation, it is difficult to completely seal the
vertical hole structure of the etching hole in the surface of the
vibrating membrane with a deposited film formed by gas phase
reaction in a vacuum atmosphere, for example, by CVD or PVD. When
the etching hole in the vibrating membrane is sealed by CVD or the
like, a deposited film is formed on the side wall defining the hole
or a part of the vibrating membrane around the etching hole, and
this film grows toward the center of the hole (deposition
progresses in the lateral direction). However, in general, in gas
phase reaction, active species (ions, radicals, or the like) that
form a deposited film tend to travel in a straight line, and the
deposition rate in the lateral direction is low. Therefore, it is
necessary to increase the film thickness by prolonged film
formation. If there is a minute sealing defect in the wet step
after the sealing step, the solution used for developing and
removing photoresist may enter the cavity and cause a malfunction.
On the other hand, when a resin material that has fluidity is used
for sealing, the vertical hole structure can be tightly sealed due
to fluidity. However, in the process of hardening, the solvent in
the material that has fluidity may contaminate the cavity by being
heated, or the material may become temporarily less viscous due to
high temperature and may flow into parts that need not be sealed,
and the function of the device may thereby be interfered with. The
present technical situation is as described above. For example, in
medical diagnosis, CMUTs are normally used in contact with liquid
such as castor oil in order to prevent attenuation of ultrasound
that is a signal. If the reliability of the sealing portion is low,
oil may enter the cavity in prolonged use, and the performance
degradation may be caused.
SUMMARY OF THE INVENTION
[0008] In an aspect of the present invention, an electromechanical
transducer includes a first electromagnetic element and a second
electromagnetic element, disposed opposite to each other with a
sealed cavity therebetween. The sealed cavity is formed by removing
a sacrifice layer and then performing sealing. The performing
sealing to form a sealing portion includes superposing a film of a
hardened second sealing material that has fluidity at normal
temperature on a film of a first sealing material that does not
have fluidity at normal temperature.
[0009] In another aspect of the present invention, there is
provided a method for fabricating an electromechanical transducer
including a first electromagnetic element and a second
electromagnetic element, disposed opposite to each other with a
sealed cavity therebetween. The method includes: forming a
sacrifice layer on the first electromagnetic element and patterning
the sacrifice layer; forming a vibrating membrane on the sacrifice
layer; forming an etching hole in the vibrating membrane; etching
the sacrifice layer through the etching hole and thereby forming
the cavity; forming a film of a first sealing material that does
not have fluidity at normal temperature on the vibrating membrane
having the etching hole; forming a film of a second sealing
material that has fluidity at normal temperature on the film of the
first sealing material; hardening and patterning the film of the
second sealing material; and etching the film of the first sealing
material using the patterned film of the second sealing material as
a mask and thereby forming a sealing portion that seals the
cavity.
[0010] In another aspect of the present invention, there is
provided a method for fabricating an electromechanical transducer
including a first electromagnetic element and a second
electromagnetic element disposed opposite to each other with a
sealed cavity therebetween. The method includes: forming a
sacrifice layer on the first electromagnetic element and patterning
the sacrifice layer; forming a vibrating membrane on the sacrifice
layer; forming the second electromagnetic element on the vibrating
membrane; forming an etching hole in the vibrating membrane;
etching the sacrifice layer through the etching hole and thereby
forming the cavity; forming a film of a first sealing material that
does not have fluidity at normal temperature on the vibrating
membrane having the etching hole; forming a film of a second
sealing material that has fluidity at normal temperature on the
film of the first sealing material; and hardening the film of the
second sealing material.
[0011] Further features of the present invention will become
apparent from the following description of exemplary embodiments
with reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIGS. 1A and 1B illustrate a first embodiment of the present
invention.
[0013] FIGS. 2A to 2D illustrate steps of a method for fabricating
an electromechanical transducer in the first embodiment.
[0014] FIGS. 3A to 3C illustrate steps of the method for
fabricating an electromechanical transducer in the first
embodiment.
[0015] FIGS. 4A to 4C illustrate steps of the method for
fabricating an electromechanical transducer in the first
embodiment.
[0016] FIGS. 5A to 5D illustrate steps of the method for
fabricating an electromechanical transducer in the first
embodiment.
[0017] FIGS. 6A and 6B illustrate a third embodiment of the present
invention.
[0018] FIGS. 7A to 7F illustrate a fourth embodiment of the present
invention.
DESCRIPTION OF THE EMBODIMENTS
[0019] The electromechanical transducers and methods for
fabricating the same of the present invention are characterized in
that a cavity formed after removing a sacrifice layer is sealed by
forming a film of a second sealing material that has fluidity at
normal temperature on a film of a first sealing material that does
not have fluidity at normal temperature and then hardening the
second sealing material. On the basis of this concept, the
electromechanical transducers and methods for fabricating the same
of the present invention basically have the above-described
configuration. In this specification, the term "normal temperature"
means temperatures of about 25.degree. C..+-.15.degree. C., which
are the temperatures of the environment under which fabricating
methods are performed. The term "a sealing material that does not
have fluidity at normal temperature" means an inorganic material
(in the present invention, inorganic materials include metal
materials) that is formed into a film by CVD or PVD method and
cannot flow in the state of a film. The term "fluidity" means the
property not to fix but to flow or to change its shape. Examples of
sealing materials that do not have fluidity at normal temperature
include Si materials such as SiN, SiO.sub.2, SiON, and a-Si, metal
materials such as Al and Ti, and metal oxide materials such as
Al.sub.2O.sub.3 that are deposited and formed into a film by CVD or
PVD method. The term "a sealing material that has fluidity at
normal temperature" means an organic material or an inorganic
material that flows at normal temperature, that can be formed into
a film, for example, by spin coat method, that can flow in the
state of a film, and that has the property to react with external
energy such as heat or light to harden after film formation.
Examples of sealing materials that have fluidity at normal
temperature include materials that lose fluidity by being heated,
such as polyimide resin, silicone resin, photoresist, and SOG (Spin
On Glass), and materials that lose fluidity by being exposed to
light, such as ThreeBond 3100 series. In the present invention, not
only organic SOG but also inorganic SOG can be used as a material
that has fluidity at normal temperature.
[0020] On the basis of this basic configuration, the following
embodiments are possible. For example, an electromechanical
transducer has a plurality of elements each including a plurality
of cells. When an electromechanical transducer is a capacitive type
as in the embodiments to be described later, the first
electromagnetic element and the second electromagnetic element are
electrodes. However, the present invention is not limited to a
capacitive type, such as a CMUT, and can be applied to any
electromechanical transducer as long as it has the same structure
(the structure in which a first electromagnetic element and a
second electromagnetic element are disposed with a sealed cavity
therebetween). The present invention can be applied, for example,
to a magnetic transducer (MMUT).
[0021] In this case, the first electromagnetic element and the
second electromagnetic element are, for example, magnetic bodies,
such as magnets, or magnet coils. In the case of a capacitive type,
a cell can include a first electrode disposed on a substrate, a
second electrode disposed opposite to the first electrode with a
cavity therebetween, a vibrating membrane supporting the second
electrode, and a supporting portion supporting the vibrating
membrane. When the vibrating membrane is formed of a conductive
material such as a conductor or a semiconductor, the vibrating
membrane can double as the second electrode. When the substrate is
formed of a semiconductor such as silicon, the substrate can double
as the first electrode. The above-described configuration makes it
possible to provide a highly reliable electromechanical transducer
in which a cavity is sufficiently sealed. Since a sealing portion
is formed by sealing with a first sealing material that does not
have fluidity at normal temperature and then applying a second
sealing material that has fluidity at normal temperature and
hardening this, an extremely highly reliable capacitive
electromechanical transducer can be fabricated even if a minimum
amount of first sealing material is used. Since the degree of
freedom in designing the film configuration of the sealing portion
increases, an electromechanical transducer can be adapted for many
applications. Compared to the case where a hole is sealed only with
a deposited film formed by CVD or the like, sealing can be
performed in a short time, and the takt time can be shortened.
[0022] Although the present invention will be described in more
detail by way of embodiments, the present invention is not limited
by this at all.
First Embodiment
[0023] A description will be given of a first embodiment of the
present invention relating to a capacitive electromechanical
transducer and a method for fabricating the same. FIG. 1A is a plan
view showing the basic structure of an electromechanical transducer
of this embodiment. FIG. 1B is a sectional view taken along line
IB-IB of FIG. 1A. The electromechanical transducer of this
embodiment has a lower electrode 12, a vibrating membrane 14, and
an upper electrode 15. The lower electrode 12 is a first electrode
disposed on a substrate 11. The vibrating membrane 14 is disposed
opposite to the lower electrode 12 with a sealed cavity 13
therebetween and is movably supported. The upper electrode 15 is a
second electrode disposed on the upper surface of the vibrating
membrane 14. The materials that seal the etching hole and thereby
seal the cavity 13 include a first sealing material 16 and a second
sealing material 17. In general, a capacitive electromechanical
transducer is configured by arranging a plurality of elements. Each
element includes a plurality of cells. Each cell includes a lower
electrode 12, a vibrating membrane 14, and an upper electrode 15
surrounding a sealed cavity 13. Here, elements each consisting of
four cells such as those shown in FIG. 1A are arranged
two-dimensionally. The upper electrodes 15 are electrically
connected by wiring, and the lower electrodes 12 are electrically
independent on an element-by-element basis. Due to such a
configuration, on an element-by-element basis, it is possible to
apply a voltage between the lower electrode 12 and the upper
electrode 15 to vibrate the vibrating membrane 14 to generate
ultrasound. When receiving ultrasound, on an element-by-element
basis, a signal can be detected from a change in the capacitance
between the lower electrode and the upper electrode caused by the
deformation of the vibrating membrane 14 due to ultrasound. In this
embodiment, the cavity 13 is circular in cross-section as shown in
FIG. 1A. However, the present invention is not limited to this. The
cavity 13 may have a polygonal shape such as a quadrilateral shape.
An etching hole is formed in a projecting etching channel connected
to the cavity 13, and the etching hole is sealed with sealing
materials 16 and 17. The place where an etching hole is formed is
not limited to this. An etching hole can also be formed, for
example, around the cavity 13.
[0024] In this embodiment, the height of the sealed cavity 13 is
200 nm, but it can be, for example, 10 nm to 500 nm. The diameter
of the circular sealed cavity 13 can be, for example, 10 .mu.m to
200 .mu.m. The upper electrode 15 and the lower electrode 12 are
formed of at least one of, for example, Al, Cr, Ti, Au, Pt, and Cu.
The vibrating membrane 14 is formed of SiN but can be formed of
another insulating material. The sealed cavity 13 is formed by
removing a sacrifice layer 18 to be described later. At least one
of Cr, Al, Si, and SiO.sub.2 can be used as the material of the
sacrifice layer 18. Other materials, such as organic materials, can
also be used as the material of the sacrifice layer 18.
[0025] Next, a fabricating method of this embodiment will be
described step-by-step with reference to FIGS. 2A to 2D, 3A to 3C,
4A to 4C, and 5A to 5D. In the step shown in FIG. 2A, a lower
electrode 12 is formed of titanium by sputtering on a glass
substrate 11. The substrate 11 may be formed of another material,
for example, quartz, sapphire, or silicon. The conditions of this
step can be as follows. After evacuation to 3.times.10.sup.-5 Pa,
film formation is performed for 200 seconds under the following
conditions: Ar flow rate 30 sccm, pressure 0.7 Pa, and DC power 400
W. Thus, a titanium film about 100 nm thick is formed.
[0026] In the step shown in FIG. 2B, a sacrifice layer 18 is formed
of chrome by sputtering on the lower electrode that is a first
electromagnetic element. The conditions of this step can be as
follows. After evacuation to 3.times.10.sup.-5 Pa, film formation
is performed for 500 seconds under the following conditions: Ar
flow rate 50 sccm, pressure 0.9 Pa, and DC power 400 W. Thus, a
chrome film about 200 nm thick is formed. In the step shown in FIG.
2C, photoresist 19 (AZ1500, Shipley) is applied using a spinner to
a thickness of about 2 .mu.m, and a pre-bake is performed at
110.degree. C. for 90 seconds. After that, through a photomask 20
having a predetermined cavity pattern, ultraviolet irradiation 21
is performed using an aligner.
[0027] In the step shown in FIG. 2D, the resist film in which a
pattern of a cavity having a diameter of 30 .mu.m and an etching
channel is formed by developer undergoes a post-bake at 180.degree.
C. for 3 minutes. After the post-bake, etching is performed with
chrome etchant (mixed acid chrome etchant, KANTO CHEMICAL), and
resist stripping, rinse, and drying are performed. Thus, a
patterned sacrifice layer 18 is obtained.
[0028] In the step shown in FIG. 3A, a vibrating membrane 14 is
formed of silicon nitride by plasma CVD method on the sacrifice
layer. The conditions of this step can be as follows. Film
formation is performed for 20 minutes under the following
conditions: substrate temperature 350.degree. C., RF power 360 W,
chamber pressure 150 Pa, SiH.sub.4 gas flow rate of 24 sccm,
NH.sub.3 gas flow rate of 150 sccm, and N.sub.2 gas flow rate of
600 sccm. Thus, a silicon nitride film about 450 nm thick is
formed. In the step shown in FIGS. 3B, photoresist 19 is applied
using a spinner to a thickness of about 2 .mu.m, and a pre-bake is
performed at 90.degree. C. for 90 seconds. After that, through a
photomask 22 having a predetermined etching hole pattern,
ultraviolet irradiation 21 is performed using an aligner. A pattern
of an etching hole 23 having a diameter of 8 .mu.m is formed by
developer. A post-bake is performed at 120.degree. C. for 3
minutes. In the step shown in FIG. 3C, silicon nitride 14 is etched
by dry etching method and an etching hole 23 is formed. The
conditions of this step can be as follows. After evacuation to
3.times.10.sup.-5 Pa, etching is performed for 2 minutes under the
following conditions: CF.sub.4 gas flow rate of 20 sccm, pressure 5
Pa, and DC power 150 W. After that, the resist film 19 is removed
by ultrasonic cleaning in acetone solution. Thus, the step of
forming an etching hole in the vibrating membrane is completed.
[0029] In the step shown in FIG. 4A, the chrome film 18 is removed
as a sacrifice layer by electrolytic etching through the etching
hole, and thereby a cavity 13 is formed. The conditions of this
step can be as follows. An etching bath 24 is filled with two moles
of sodium chloride solution 25. In the etching bath 24, a platinum
opposite electrode 26 and a silver/silver chloride reference
electrode 27 for monitoring the electrical potential are disposed.
By applying a DC voltage of 2 V to the lower electrode 12 using a
potentiostat 28, the chrome sacrifice layer 18 is etched in 500
seconds. After that, rinse and replacement of water with isopropyl
alcohol are performed. Finally, drying is performed using a
fluorochemical solvent (HFE7100, Sumitomo 3M).
[0030] In the step shown in FIG. 4B, a film of a first sealing
material (a sealing material that does not have fluidity at normal
temperature) 16 is formed on the vibrating membrane having the
etching hole to seal the etching hole. In this embodiment, the
first sealing material 16 is SiO.sub.2. The conditions of this step
can be as follows. Film formation is performed for 3 minutes under
the following conditions: substrate temperature 350.degree. C., RF
power 360 W, chamber pressure 150 Pa, SiH.sub.4 gas flow rate of 40
sccm, and N.sub.2O gas flow rate of 80 sccm. Thus, an SiO.sub.2
film 16 having a thickness of about 600 nm is formed. In the step
shown in FIG. 4C, a film of a second sealing material that has
fluidity at normal temperature is formed on the film of the first
sealing material. In this embodiment, the second sealing material
17 for sealing the etching hole is photosensitive polyimide
(PW-1210, Toray). However, commercially available photosensitive
polyimides of other companies, such as Hitachi Chemical Company,
Ltd. and Asahi Kasei Corporation, or other materials that have
fluidity at normal temperature can also be used. The conditions of
this step can be as follows. A drop of photosensitive polyimide
solution is put near the center of the substrate. The substrate is
spun using a spinner at 200 rpm for five seconds and then at 1800
rpm for 30 seconds to uniformly apply the photosensitive polyimide
solution to the surface of the substrate. After that, the substrate
is dried on a hot plate at 120.degree. C. for three minutes. Thus,
a polyimide film 17 is formed. In the step shown in FIG. 5A,
ultraviolet exposure 21 is performed using an aligner through a
photomask 29 having a sealing portion pattern at an intensity of
100 mJ/cm.sup.2. After that, development is performed using 2.38%
TMAH (Tetramethylammonium hydroxide) developer. Rinse is performed
to complete patterning. In order to promote the imidization, firing
is performed in a nitrogen atmosphere at 250.degree. C. for one
hour, and a stable film 17 having a thickness of 3 .mu.am is
obtained. Thus, as shown in FIG. 5B, the step of hardening and
patterning the second sealing material film is completed.
[0031] In the step shown in FIG. 5C, a sealing portion that seals
the cavity is formed by pattering the first sealing material film
by dry etching using the patterned second sealing material film as
a mask. Here, the SiO.sub.2 film 16 is etched by dry etching
method, and the first sealing material 16 is left in the etching
hole part. The conditions of this step can be as follows. After
evacuation to 3.times.10.sup.-5 Pa, etching is performed for 25
minutes under the following conditions: CF.sub.4 gas flow rate of
20 sccm, pressure 5 Pa, and DC power 150 W. Thus, as shown in FIG.
5C, the etching hole part is sealed in a state where the second
sealing material film 17 is superposed on the first sealing
material film 16. The thickness of the second sealing material film
17 is larger than the thickness of the first sealing material film
16. Next, the step shown in FIG. 5D is performed. After evacuation
to 3.times.10.sup.-5 Pa, sputtering is performed for 300 seconds
under the following conditions: Ar flow rate of 30 sccm, pressure
0.7 Pa, and DC power 300 W. Thus, an aluminum film about 300 nm
thick is formed. After that, the aluminum film is patterned by
photolithography process, and thereby an upper electrode 15 is
formed.
[0032] A capacitive electromechanical transducer of this embodiment
fabricated in this way was immersed in red ink for an inkjet
printer. Ink did not enter the highly reliably sealed cavity 13. In
addition, since the SiO.sub.2 (first sealing material) film 16 and
the polyimide (second sealing material) film 17 are resistant to
oil, the electromechanical transducer functioned well as a sensor
even after being immersed in castor oil for 1000 hours. Thus, a
highly reliable capacitive electromechanical transducer was
obtained.
Second Embodiment
[0033] A capacitive electromechanical transducer and a method for
fabricating the same of a second embodiment will be described. In
this embodiment, the same fabricating steps as those in the first
embodiment are performed. However, as a first sealing material film
16, an aluminum film 600 nm thick is formed by sputtering. As a
second sealing material 17, photosensitive silicone (nanohybrid
silicone FX-V366, ADEKA), which is an organic material having
photosensitivity, is applied using a spinner. To harden this
material, a 30-minute low-temperature process at a firing
temperature of 200.degree. C. can be used as the hardening process
after exposure and development. Photosensitive silicone is
resistant to acid and has sufficient performance as a resist for
etching aluminum of the first sealing material 16.
Third Embodiment
[0034] With reference to FIGS. 6A and 6B, a capacitive
electromechanical transducer and a method for fabricating the same
of a third embodiment will be described. FIG. 6A is a plan view,
and FIG. 6B is a sectional view taken along line VIB-VIB of FIG.
6A. FIGS. 6A and 6B show the basic structure of the capacitive
electromechanical transducer of this embodiment. As shown, the
second sealing material 17 (and the first sealing material 16) can
be left in the area other than the upper electrode 15 or other than
the part just above the cavity 13. In this case, the airtightness
can be further improved. In other respects, this embodiment is the
same as the first embodiment.
Fourth Embodiment
[0035] With reference to FIGS. 7A to 7F, a capacitive
electromechanical transducer and a method for fabricating the same
of a fourth embodiment will be described. In this embodiment, the
same fabricating steps as those in the first embodiment are
performed until a first vibrating membrane 14 is formed (FIG. 7A).
After forming the first vibrating membrane 14, a second electrode
15 is formed (FIG. 7B). Specifically, a titanium film about 100 nm
thick is formed, and then the titanium film is patterned by
photolithography process. The second electrode in this embodiment
can be formed of a material resistant to etchant to improve etching
selectivity in the sacrifice layer removing step. After that, in
the same manner as in the first embodiment, an etching hole for
removing the sacrifice layer by etching is formed (FIG. 7C). By
immersing in sacrifice layer removing etchant, the sacrifice layer
is removed through the etching hole. By drying, the sealed cavity
13 is formed (FIG. 7D). Next, a film of a first sealing material 16
that does not have fluidity at normal temperature is formed on the
vibrating membrane 14 having an etching hole (FIG. 7E). In this
embodiment, the first sealing material film 16 is also formed on
the vibrating membrane 14 and the second electrode 15 and serves as
a second vibrating membrane. The conditions of this step can be as
follows. Film formation is performed for five minutes under the
following conditions: substrate temperature 350.degree. C., RF
power 935 W, chamber pressure 213 Pa, SiH.sub.4 gas flow rate of
160 sccm, N.sub.2 gas flow rate of 2000 sccm, and NH.sub.3 gas flow
rate of 127 sccm. Thus, an SiN film 16 having a thickness of about
700 nm is formed. Next, a second sealing material that has fluidity
at normal temperature is applied, and a protective film for the
vibrating membrane is formed (FIG. 7F). For example, PDMS is
applied as a second sealing material to the electromechanical
transducer in which a film of the first sealing material is formed.
After that, heat hardening was performed in a heating furnace.
Thus, a film of the second sealing material 17 is formed on the
film of the first sealing material 16, and a protective film is
formed on the second vibrating membrane. By following through with
these steps, the first sealing material film 16 can be used as a
part of a vibrating membrane, and the second sealing material film
17 can be used as a protective film for the vibrating membrane.
Thus, an electromechanical transducer having high reliability can
be provided without spoiling the sealing performance in the first
embodiment.
[0036] While the present invention has been described with
reference to exemplary embodiments, it is to be understood that the
invention is not limited to the disclosed exemplary embodiments.
The scope of the following claims is to be accorded the broadest
interpretation so as to encompass all such modifications and
equivalent structures and functions.
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