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JP2014171695

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DESCRIPTION JP2014171695
Abstract: To realize a vibrating membrane with a small spring constant in a capacitance type
transducer made of a sacrificial layer type, and to provide a uniform capacitance type transducer
that can obtain wider frequency characteristics and less variation. To do. A capacitive transducer
includes a cell having a first electrode, and a vibrating membrane including a second electrode
opposite to the first electrode via a cavity. The thickness of the portion connected to the vibrating
film is around the sealed portion 12 and used for etching the sacrificial layer for forming the
cavity, from the thickness of the membranes 206, 209, 211. It is smaller than the thickness
excluding the thickness of the second electrode. [Selected figure] Figure 1
Method of manufacturing capacitive transducer, and capacitive transducer
[0001]
The present invention relates to a method of manufacturing a capacitive transducer used as an
ultrasonic transmission / reception device or the like, a capacitive transducer, and the like.
[0002]
Conventionally, an ultrasonic diagnostic apparatus is often used for medical diagnosis, such as an
application for safely observing an inside of a human body using ultrasonic waves.
An ultrasonic transmitting and receiving device used for a probe (probe) or the like of an
ultrasonic diagnostic apparatus is a device that converts an electric signal and an ultrasonic
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wave. In recent years, capacitive transducers such as CMUTs have been actively researched and
developed in place of piezoelectric transducers because they have excellent wide-band frequency
characteristics. CMUT is an abbreviation of Capacitive-Micromachined-Ultrasonic-Transducer.
[0003]
The capacitive transducer has a structure in which a cavity structure is formed by a membrane
and a membrane support, and upper and lower electrodes are formed so as to sandwich the
cavity (gap). The ultrasonic signal is transmitted to the membrane and the electrodes, and the
minute displacement can be read as a change in capacitance between the upper and lower
electrodes. Further, by applying a voltage to the upper and lower electrodes, the membrane can
be vibrated to transmit an ultrasonic signal. The frequency characteristics of the transducer are
mainly determined by mechanical properties such as the material and structure of the
membrane, and the Young's modulus, density, and thickness of the material become important
parameters. In addition, the reception sensitivity and transmission efficiency of ultrasonic waves
can be improved by arranging and arraying a large number of cells in units of structures
including membranes and cavities. For producing the transducer, it is possible to use a film
forming technique for laminating materials, a photolithography technique for performing
patterning with high accuracy, an etching technique and the like, and a MEMS technique which
excels in three-dimensional microfabrication. MEMS is an abbreviation of Micro-ElectroMechanical-Systems.
[0004]
As a method of manufacturing a capacitive transducer, there is a method of forming a cavity by
laminating various materials on a substrate and partially removing the material by sacrificial
layer etching. The transducer thus produced is called a sacrificial layer (or surface) capacitive
transducer. The production method is, for example, as follows. Layers such as a lower electrode
(first electrode), an insulating film, a sacrificial layer, a membrane, and an upper electrode
(second electrode) are laminated on a substrate, desired patterning is repeated, and a sacrificial
layer is formed on part of the membrane. Form an etch opening for removal. Then, a cavity
structure is formed by sacrificial layer etching using an etchant that penetrates through the
opening. Thereafter, a sealing film is formed in the etching opening in order to prevent entry of
dust, liquid and the like into the cavity. Furthermore, the device is completed by pulling out
electrode pads, wires and the like from the upper and lower electrodes.
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[0005]
In order to manufacture a device with little change in sensitivity and frequency characteristics
over a long period of time, a sealing process for keeping the pressure inside the cavity stable is
important, and a highly airtight seal is desired. In addition, in the step of sealing the etching
opening requiring a relatively thick sealing film, a manufacturing method for suppressing
variations is also important.
[0006]
In Patent Document 1, after laminating a lower electrode, an insulating film, a sacrificial layer, a
membrane, and an upper electrode on a substrate, an etching opening for removing the
sacrificial layer is formed in part of the membrane. Thereafter, a sacrificial layer is etched to form
a cavity structure. In the subsequent sealing process, PE-CVD (Plasma-Enhanced-Chemical-VaporDeposition: plasma chemical vapor deposition) method is used for forming a sealing film. A thick
sealing film is formed to maintain air tightness, and then the thick sealing film deposited on the
membrane is adjusted by being recessed from etching in order to obtain desired frequency
characteristics.
[0007]
U.S. Pat. No. 5,982,709
[0008]
As described above, the sacrificial layer type capacitive transducer is fabricated by depositing
various layers of various materials on a substrate and performing patterning and etching.
One of the parameters that determine the frequency characteristics etc. of the device is the
membrane thickness, which is the total thickness of the membranes that make up the membrane.
Materials used for the membrane include silicon nitride, silicon oxide, amorphous silicon and the
like. As a film forming method of these materials, a CVD (chemical vapor deposition) method is
mainly used, and among them, a PE-CVD method capable of obtaining a good film at a low
temperature is often used.
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[0009]
Assuming that the membrane formed immediately above the sacrificial layer is a first membrane,
the first membrane needs a film thickness sufficient to perform coverage on the sacrificial layer.
Therefore, the thickness of the first membrane is in proportion to the thickness of the sacrificial
layer, and the thickness of the first membrane needs to be larger as the thickness of the
sacrificial layer is larger. In addition, when the thickness of the sacrificial layer is small, it is
necessary to increase the spring constant of the membrane in order to prevent sticking (the
phenomenon that a part of the membrane adheres to the bottom of the cavity) at the time of
etching the sacrificial layer. Thus, the first membrane requires some thickness.
[0010]
For the sacrificial layer etching, an opening through which an etchant or etching gas enters and
exits is formed. The opening needs to be sealed after sacrificial layer etching. In terms of thermal
expansion coefficient and adhesion, it is desirable to form a film of the same type as the
membrane. Thereafter, the sealing film remains on the membrane unless it is retreated or
removed by etching or the like. When a highly airtight seal is required, the minimum required
sealing film thickness is determined by the sacrificial layer thickness (cavity gap) and the
thickness of the first membrane formed on the sacrificial layer.
[0011]
FIG. 5 shows a schematic view of the sealing process. A state in which the opening sealing
portion 4 and the sealing film 5 are formed on the membrane 3 fabricated on the substrate 1 and
the cavity gap 2 is shown. FIG. 5A shows an initial process of forming a sealing film, and the
sealing film is gradually deposited by PE-CVD. The deposition of the sealing film 5 proceeds as
shown in FIG. 5B, and when the thickness becomes larger than the cavity gap 2, the sealing film
deposited from the substrate 1 and the sealing film deposited from the membrane 3 join
together, Defect 6 grows from the interface. As the deposition progresses, the defect is closed
from the boundary as shown in FIG. 5C, and the sealing is completed. Generally, the film
thickness of the sealing film is required to be two to three times the thickness of the cavity gap
or more.
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[0012]
Since the frequency band of the capacitive transducer device depends on the thickness of the
membrane, the thicker the membrane, the larger the spring constant and the narrower the
frequency band, and the smaller the spring constant, the wider the frequency band. Therefore,
control of the thickness of the sealing film to be a part of the membrane is an important
parameter. At this time, if the deposited sealing film is made to recede by etching, variations
easily occur due to the relationship between the distribution of etching and the etching
selectivity, and the degree of freedom in design is reduced. From the above, it is important to
realize a membrane with a small spring constant and obtain more uniform frequency
characteristics in a wider band, while preventing sticking during etching of the sacrificial layer, in
the capacitive transducer manufactured with the sacrificial layer type. is there. An object of the
present invention is to provide a method of manufacturing a capacitive transducer, a capacitive
transducer, and an apparatus including the same, which make this possible.
[0013]
In view of the above problems, the present invention of a capacitance type transducer including a
cell having a first electrode, and a vibrating film including a second electrode provided opposite
to the first electrode via a cavity. The manufacturing method of comprises the following steps.
Forming a sacrificial layer to form a cavity; Forming a first membrane on the structure on which
the sacrificial layer is formed, and forming a hole having a larger circumference than the etching
opening in the first membrane on the sacrificial layer. Forming a second membrane on the
structure on which the first membrane having the hole is formed, and forming the etching
opening in a recess of the second membrane in the hole to expose a part of the sacrificial layer .
Removing the sacrificial layer through the etch opening. Forming a sealing film to seal the
etching opening; In addition, another method of manufacturing a capacitive transducer according
to the present invention includes the following steps. Forming a sacrificial layer to form a cavity;
Forming a first membrane on the structure on which the sacrificial layer is formed, and forming a
stopper layer on the first membrane on the sacrificial layer. Forming a through hole for forming
an etching opening in the stopper layer; Forming a second membrane on the structure in which
the stopper layer having the through holes is formed; A recess larger than the etching opening is
formed in the second membrane using the stopper layer as an etching stopper, and the etching
opening communicating with the recess is formed in the first membrane, and a part of the
sacrificial layer is formed. Exposing step. Removing the sacrificial layer through the etch opening.
Forming a sealing film to seal the etching opening;
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[0014]
Further, in view of the above problems, the electrostatics of the present invention including a cell
having a first electrode, and a diaphragm including a second electrode provided opposite to the
first electrode via a cavity. The capacitive transducer has the following features. That is, the
thickness of the portion connected to the vibrating film is around the sealed portion used in
etching the sacrificial layer for forming the cavity, and the thickness of the portion connected to
the vibrating film is the thickness of the vibrating film. It is smaller than the thickness excluding
the thickness of the second electrode.
[0015]
According to the present invention, it is possible to reduce the total film thickness even in a
multi-layered membrane, and it is possible to widen the frequency band of frequency
characteristics. Moreover, in order to reduce the thickness of the part connected to the
membrane around the etching opening, the membrane thickness on the cavity remains the same
as before when etching the sacrificial layer, and the sticking of the cavity is controlled to the
same level as before it can.
[0016]
The figure which shows the cross section in the cell of the cell of one Embodiment, an arrayed
cell, and a cell at a CD position. FIG. 7 is a process diagram for describing a manufacturing
method of Example 1 of the present invention. FIG. 7 is a process diagram for describing a
manufacturing method of Example 2 of the present invention. FIG. 1 is a diagram showing an
object information acquiring apparatus using a capacitive transducer of the present invention.
Sectional drawing of the sealing part for demonstrating the conventional sealing process.
[0017]
In the present invention, the thickness of the membrane around the opening for etching the
sacrificial layer is made smaller than the thickness obtained by removing the electrode from the
vibrating film on the cavity, so that the sealing film thickness can be sufficiently increased. Make
the sealing complete. In addition, patterning and etching are performed mainly only around the
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opening, so that a capacitive transducer with high variation and less yield can be manufactured.
For this purpose, the second membrane is formed on the structure in which the first membrane
having a hole having a larger circumference than the etching opening is formed, and the etching
opening is formed in the recess of the second membrane in the hole to form a sacrificial layer.
Expose part. Alternatively, a through hole for forming an etching opening is formed in the
stopper layer formed on the first membrane to form a second membrane, and the stopper layer is
used as an etching stopper, and the second membrane is larger than the etching opening Form a
recess. Then, an etching opening communicating with the recess is formed in the first membrane
to expose a part of the sacrificial layer. In the capacitive transducer manufactured by the
manufacturing method or the like, the thickness of the portion connected to the vibrating film on
the cavity is around the portion where the opening used in etching the sacrificial layer is sealed.
The thickness can be smaller than the thickness obtained by removing the second electrode from
the vibrating membrane. The capacitive transducer of the present invention can be used as a
probe for receiving acoustic waves. Further, the present invention can be used in a subject
information acquiring apparatus having a transducer and a processing unit that acquires subject
information using an electrical signal output from the transducer. The transducer receives an
acoustic wave from a subject and outputs an electrical signal.
[0018]
Hereinafter, embodiments of the present invention will be described with reference to the
drawings. One embodiment of the capacitive transducer of the present invention will be
described with reference to FIG. FIG. 1 (a) is a top view of a cell 11 of a capacitive transducer
showing an outline of this embodiment, and FIG. 1 (b) is a top view showing the arrayed cells 11
and upper wiring 13 and lower wiring 14. . The form of the number, arrangement, and shape of
the cells is not particularly limited. Capacitive transducers include such cells. Each cell is
provided with a first electrode (lower electrode) 303 and a second electrode provided opposite to
the first electrode via a cavity as shown in (h) of (2-1) in FIG. 2. And a vibrating film including the
electrode (upper electrode) 307. FIG.1 (c) is sectional drawing which cut | disconnected the
opening sealing part 12 of Fig.1 (a) by line segment CD. In the left part of FIG. 1C, there is a cell
11 including a cavity. As a method of manufacturing this capacitive transducer, there is a method
of forming a sacrificial layer, laminating a membrane material on the sacrificial layer, and etching
away the sacrificial layer to form a cavity.
[0019]
As shown in FIG. 1C, when the first insulating film 202, the lower electrode 203, and the second
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insulating film 204 are formed on the portion of the substrate 201 where the cells are to be
formed, the opening seal of FIG. These are similarly stacked on the stopper 12. In the
manufacturing method of the present embodiment, when forming the opening 210 in the
opening sealing portion 12 provided with the space 205 corresponding to the etchant flow path
connected to the cavity formed by the sacrificial layer etching, the following is performed. . That
is, the step-shaped portion or the recess portion 208 is formed in the first membrane 206 and
the second membrane 209 in the vicinity of the opening. In the present embodiment, the
minimum sealing film thickness of the opening 210 is determined by the thickness of the second
membrane 209 being dominant. On the other hand, the thickness of the membrane on the cavity
is the sum of the first membrane 206, the second membrane 209 and the sealing membrane
(third membrane) 211, which are components of the membrane, and determines the sensitivity
and frequency characteristics of the transducer. Become an important parameter to When
manufacturing a device with a large gap in the space 205 connected to the cavity, the first
membrane 204 is thickened to achieve sufficient coverage of the sacrificial layer, and sealing is
performed to fill the large cavity gap and the large film thickness. Requires a large seal thickness.
On the other hand, when fabricating a device with a small gap in the cavity, it is necessary to
form the membrane on the sacrificial layer to a certain degree of hardness to prevent sticking
during etching of the sacrificial layer, and a sufficiently thick film is required.
[0020]
In the present invention, the etching opening is formed in a portion (for example, two or more
steps of concave portions) which is a concave portion of a portion connected to the vibrating film
on the cavity. As a result, sufficient coverage of the sacrificial layer can be obtained, and the
sacrificial layer can be etched after forming a membrane having a sufficient spring constant at
the time of sacrificial layer etching. In addition, sealing of the opening is completed even with a
sealing film (third membrane) having a small thickness. In addition, since an etching stopper
layer with high selectivity can be introduced for the formation of the step or recess around the
opening (see Example 2 described later), highly accurate control of each film thickness and shape
is possible. Can provide a device with less variation.
[0021]
In this embodiment, the size of the cell is assumed to be about 40 μm, but the size needs to be
designed in an optimum size according to the application and design such as frequency band and
ultrasonic sensitivity, and the size is particularly limited. It is not a thing. The present invention
increases the degree of freedom in design by means of the recess formed around the opening. In
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particular, in a capacitive transducer manufactured by sacrificial layer etching, a membrane with
a small spring constant can be realized, and a broader-band frequency characteristic can be
obtained and a uniform capacitive transducer with less variation can be realized.
[0022]
The driving principle of this embodiment is as follows. Since the cell is formed of the first
electrode provided across the cavity and the vibrating membrane including the second electrode,
the first electrode or the second electrode can receive the acoustic wave. Apply a DC voltage to
the electrode of When an acoustic wave is received, the vibrating membrane deforms and the gap
of the cavity changes, so the capacitance between the electrodes changes. An acoustic wave can
be detected by detecting this capacitance change from the first electrode or the second electrode.
An acoustic wave can also be transmitted by applying an alternating voltage to the first electrode
or the second electrode to vibrate the diaphragm. The capacitance type transducer of FIG. 1B can
convert an acoustic wave signal into an electrical signal through the lead wire 13 from the upper
electrode and the lead wire 14 from the lower electrode. Here, although the conversion of the
electric signal is performed by the lead wiring, a through wiring or the like may be used.
[0023]
In the present specification, acoustic waves include elastic waves called photoacoustic waves,
light ultrasonic waves, acoustic waves, and ultrasonic waves, and acoustic waves generated by
light irradiation are particularly referred to as “photoacoustic waves”. Further, among the
acoustic waves, the acoustic wave transmitted from the probe may be referred to as "ultrasound",
and the one in which the transmitted ultrasonic wave is reflected in the subject may be
particularly referred to as "reflected wave".
[0024]
Hereinafter, the present invention will be described in detail by way of more specific examples.
Example 1 Example 1 of the present invention will be described with reference to FIG. 1 (a) and
FIG. In FIG. 1A showing a cell 11 having a cavity structure which is a basic configuration of the
present embodiment, the cell 11 and an opening sealing portion 12 for performing sacrificial
layer etching and sealing are shown. The shapes of the vibrating film, the etching opening and
the like in this embodiment are circular, but may be polygonal or the like. Moreover, although
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the two opening sealing parts 12 are diagonally arrange | positioned with respect to the cell 11,
the number and arrangement | positioning of opening sealing parts may be what kind of thing.
FIG. 1B shows an example of a capacitive transducer in which the cells 11 are arranged in an
array in order to give desired characteristics to the sensitivity of ultrasonic waves, transmission
output and the like. The number of cells, the arrangement method, and the shapes of the wirings
13 and 14 are not particularly limited.
[0025]
FIG. 2 shows a manufacturing method of this example. (2-1) of FIG. 2 shows the structural
change with the progress of the process in the cross section along the A-B line which crossed the
cavity part of the cell 11 of FIG. 1, (2-2) is FIG. The structural change in the cross section along
the C-D line which crossed opening seal 12 is shown. Both (a) to (h) show sectional views of the
main steps in order.
[0026]
In the steps (2-1) and (2-2) of FIG. 2, a first insulating film 302, for example, a thermal oxide film
is formed to a thickness of 1 μm on a silicon substrate 301 having a thickness of about 300
μm. Do. Although a substrate on which a thermal oxide film of 1 μm is formed is used, any film
thickness or film forming method may be used as long as the flatness of the substrate surface
and the insulation between the lower electrode and the substrate are sufficient. In the case of
using an insulating substrate such as a glass substrate or when the substrate does not need to be
insulated, the formation of the first insulating film 302 is not necessary. Next, the lower electrode
303 is formed on the first insulating film 302. The lower electrode 303 is formed of titanium
with a thickness of 50 nm using a sputtering method. A material having high etching selectivity
and excellent heat resistance and smoothness is desirable. For example, a titanium alloy or a
laminated structure of aluminum / titanium may be used. There is no particular limitation,
including the film forming method.
[0027]
The second insulating film 304 is formed on the lower electrode 303. The second insulating film
304 is a silicon oxide film formed by PE-CVD and having a thickness of 100 nm. It is desirable to
use a material which is excellent in step coverage and high in etching selectivity during etching
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of the sacrificial layer and good in smoothness. An insulating film such as a silicon nitride film
may be used. Next, the sacrificial layer 305 is stacked on the second insulating film 304 to a
thickness corresponding to the cavity gap. For example, the sacrificial layer 305 is formed of
chromium with a thickness of 185 nm using an electron beam evaporation apparatus. Any
material or film forming method may be used as long as the selectivity to the membrane or the
electrode material is high and grain growth or the like occurs to deteriorate the surface flatness.
For example, materials such as molybdenum and amorphous silicon may be used.
[0028]
Next, the first membrane 306 is formed to sufficiently cover the sacrificial layer 305 as in (2-1)
and (2-2) (b). The first membrane 306 is formed of a silicon nitride film to a thickness of 200 nm.
A sufficient coverage of the sacrificial layer is required, and a thickness that has a sufficient
spring constant during sacrificial layer etching and does not cause defects such as sticking is
required. The first membrane 306 uses a silicon nitride film, and can be formed with a small
tensile stress by PE-CVD. For example, it is desirable to form the stress of the silicon nitride film
in the range of 0 to 200 MPa. As the membrane, it is desirable to use a material and a film
forming method which are thermally stable, sufficiently high in Young's modulus, high in etching
selectivity and high in smoothness, and controllable in stress. For example, silicon oxide,
amorphous silicon, polysilicon or the like may be used.
[0029]
Next, the upper electrode 307 is formed on the first membrane 306 as shown in (2-1) and (2-2)
(c). The upper electrode 307 forms titanium using an electron beam evaporation apparatus.
Similarly, the titanium film used for the upper electrode 307 can be stress controlled by the film
forming conditions of the electron beam vapor deposition apparatus, and low tensile stress can
be obtained. The thickness of titanium is 100 nm and is formed with a tensile stress of 200 MPa
or less. The diameter of the first membrane 306 is 40 μm, and the diameter of the upper
electrode 307 is 45 μm. It is desirable that the first membrane 306 and the upper electrode 307
together form a tensile stress film.
[0030]
Next, as shown in (2-2) (d), a part of the first membrane 306 is etched away as shown. A dry
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etching apparatus such as RIE (Reactive-Dry-Etching) is used to form a hole 308 larger than the
opening for etching the sacrificial layer. The holes 308 preferably pass completely through the
first membrane 306, but non-penetrating holes may form recesses and etching openings
described later. Next, as shown in (e) of (2-1) and (2-2), a second membrane 309 is formed. The
second membrane 309 forms a silicon nitride film having a thickness of 200 nm. The second
membrane 309 uses a silicon nitride film, which needs to have a thickness sufficiently covering
the upper electrode 307 and having a sufficient spring constant at the time of etching the
sacrificial layer so that no defect phenomenon such as sticking occurs. It is possible to form with
small tensile stress by PE-CVD method. For example, it is desirable to form the stress of the
silicon nitride film in the range of 0 to 200 MPa. Here, as the membrane, it is desirable to use a
material and a film forming method which are thermally stable, sufficiently high in Young's
modulus, high in etching selectivity and smoothness, and controllable in stress. For example,
silicon oxide, amorphous silicon, polysilicon or the like may be used.
[0031]
Next, as shown in (f) of (2-2), an opening 310 for etching the sacrificial layer is formed in a part
of the second membrane 309 (a part which has become a recess due to the presence of the hole
308). A portion of layer 305 is exposed. A dry etching apparatus such as RIE is used to form an
opening 310 having a circumference smaller than the circumference of the hole 308 or the
recess. For example, a circular hole with a diameter of about 4 μm is formed with a size enough
to sufficiently update the etchant for etching the sacrificial layer during the etching.
[0032]
Next, as shown in (g) of (2-1) and (2-2), a sacrificial layer is etched through the etching opening
310 to remove the sacrificial layer 305. Thus, a diaphragm surrounded by the first membrane
306, the upper electrode 307, and the second membrane 309, and a cavity surrounded by the
diaphragm support are formed. When the sacrificial layer 305 is formed of chromium, it is
possible to etch away the chromium with a chromium etching solution composed of a mixture of
ceric ammonium and nitric acid. After etching, rinse the etchant sufficiently with pure water etc.,
replace the pure water with IPA (isopropyl alcohol) and then replace it with HFE (methyl
nonafluoroisobutyl ether) having a small surface tension, and then pull up and dry it. . As long as
the drying method does not cause sticking, the method is not limited.
[0033]
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Next, as shown in (h) of (2-1) and (2-2), the sealing film 311 is stacked. The sealing film 311
forms silicon nitride with a thickness of 300 nm using PE-CVD. The sacrificial layer 305 is also
larger than the thickness, and needs a sufficient sealing thickness which is greater than the
thickness at which the defect disappears from the upper surface of the opening 310 formed in
the second membrane 309 having a thickness of 200 nm. Here, since the membrane thickness
around the etching opening 31 before sealing (in the present embodiment, the thickness of only
the second membrane 309) can be reduced, the sealing thickness can also be reduced. it can.
[0034]
In the capacitive transducer of this embodiment, a cavity is formed by sacrificial layer etching,
and the sealing film on the cavity is used as it is as a part of the vibrating membrane. Then, even
in the case of a multi-layered membrane, it is possible to reduce the overall film thickness to a
small value, and a manufacturing method and a structure are provided which make it possible to
widen the frequency band. When the sealing film is also used as a membrane of the vibrating
film as it is, the thickness of the membrane largely depends on the thickness of the sealing film,
and the sealing is performed by reducing the membrane thickness around the etching opening
before sealing. The thickness can also be reduced. As a result, the overall membrane thickness is
kept small, and the reduction of the spring constant facilitates the broadening of the band.
Further, in order to reduce the thickness of the membrane only around the etching opening, the
thickness of the membrane on the cavity is not different from that in the prior art when the
sacrificial layer is etched, and the sticking of the cavity can be controlled to the same extent as in
the prior art. Further, since the frequency characteristics and the like of the membrane depend
on the film thickness of the sealing film, a large design margin can be obtained, so that a design
with a margin can be made, and an improvement in yield etc. can also be expected.
[0035]
Second Embodiment A second embodiment of the present invention will be described with
reference to FIG. 2 shows a method of manufacturing the cell of FIG. 1 similarly to FIG. 2, (3-1) is
a cross-sectional view taken along the line A-B across the cell 11 including the cavity portion of
FIG. 2 is a cross-sectional view taken along the line C-D of FIG. 1 across the opening seal 12; Also
in the present embodiment, in the steps (3-1) and (3-2) (a), the first insulating film 402, for
example, a thermal oxide film 1 μm, is formed on the silicon substrate 401 with a thickness of
about 300 μm. The substrate 401 and the first insulating film 402 may be anything as long as
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the flatness and the insulation of the substrate surface are sufficient. The lower electrode 403 is
formed on the first insulating film 402. The lower electrode 403 is made of titanium having a
thickness of 50 nm. A material having high etching selectivity, high heat resistance, and excellent
smoothness is desirable, and the material and the film formation method are not limited. A
second insulating film 404 is formed on the lower electrode 403. The second insulating film 404
forms a silicon oxide film with a thickness of 100 nm using PE-CVD. A material excellent in step
coverage and high in etching selectivity at the time of sacrificial layer etching and smooth is
desirable, and in particular, any material and manufacturing method may be used. Next, a
sacrificial layer 405 is formed on the second insulating film 404. The sacrificial layer 405 is
formed of chromium with a thickness of 185 nm using an electron beam evaporation apparatus.
It is desirable that the etching selectivity with the membrane or the electrode material is high and
the surface flatness is good, and any material may be used. For example, materials such as
molybdenum and amorphous silicon may be used.
[0036]
Next, a first membrane 406 is formed so as to sufficiently cover the sacrificial layer 405 as in (31) and (b) of (3-2). The first membrane 406 forms a silicon nitride film with a thickness of 200
nm. The silicon nitride film can be formed by PE-CVD with a small tensile stress. It is desirable
that the stress be formed in the range of 0 to 200 MPa. As the membrane, it is desirable to use a
material and a film forming method which are thermally stable, sufficiently high in Young's
modulus, high in etching selectivity and smoothness, and controllable in stress. For example,
silicon oxide, amorphous silicon, polysilicon or the like may be used.
[0037]
Next, the upper electrode 407 is formed on the first membrane 306 as shown in (c) of (3-1). The
upper electrode 407 is formed of titanium using an electron beam evaporation apparatus. At the
same time, as shown in FIG. 3C, a stopper layer in which an opening 408 for etching which is a
through hole is formed is formed by sharing the upper electrode 407. The upper electrode 407 is
preferably formed of titanium or the like with low stress, and is manufactured by an electron
beam evaporation apparatus or the like. The thickness of the titanium is 100 nm and is formed
with a tensile stress of 0 to 200 MPa. As in (c) of (3-1), the diameter of the cavity formed by the
first membrane 406 is 40 μm, and the diameter of the upper electrode 407 is about 45 μm. As
in (c) of (3-2), the size of the sacrificial layer of the opening sealing portion is 10 μm in
diameter. The outer diameter of the stopper layer formed by the upper electrode 407 is 8 μm,
and the opening 408 for etching is formed with a size of 4 μm.
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[0038]
Next, as shown in (3-1) and (d) of (3-2), the second membrane 409 is formed. The second
membrane 409 forms a silicon nitride film having a thickness of 200 nm. Similar to the first
membrane 406, it is formed using PE-CVD. The upper electrode 407 is required to have a
thickness enough to cover the upper electrode 407 and to have a sufficient spring constant when
etching the sacrificial layer so that no defect such as sticking occurs. The second membrane 409
uses a silicon nitride film, and can be formed with a small tensile stress by PE-CVD. For example,
it is desirable to form the stress of the silicon nitride film in the range of 0 to 200 MPa. Also in
this case, it is desirable that the membrane be a thermally stable material having a sufficiently
high Young's modulus, high etching selectivity and high smoothness, and controllable stress and
a film forming method. For example, silicon oxide, amorphous silicon, polysilicon or the like.
[0039]
Next, as shown in (e) of (3-2), a two-step sealing port or recess 410 having a diameter of about 8
μm is formed using a mask in which a hole is formed. Etching is performed by a dry etching
apparatus such as RIE using a photolithography method. Since the upper electrode 407 in the
portion having the opening 408 serves as an etching stopper layer, a part of the second
membrane 409 can be removed with high accuracy. Further, as shown in FIG. 3E, since the
central portion (portion of the opening 408) of the upper electrode 407 does not have a stopper
layer, the first membrane 406 immediately below the central portion is etched similarly. An
opening 411 for sacrificial layer etching is formed.
[0040]
Next, as shown to (f) of (3-2), the upper electrode 407 of the part which served as the said mask
and etching stopper layer which became unnecessary is removed. The upper electrode 407
serves as an etching stopper layer if it is intended to remove different kinds of materials and the
like with respect to the sealing interface, and if there is no problem with the sealing airtightness
described later, or if used as defect repair or adhesive layer. There is no need to remove Next, as
shown in (3-1) and (3-2) (g), sacrificial layer etching is performed through the etching opening
411 to remove the sacrificial layer 405. In this way, a vibrating membrane constituted of the first
membrane 406, the upper electrode 407, and the second membrane 409 and a cavity
04-05-2019
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surrounded by the vibrating membrane support are formed. When the sacrificial layer 405 is
formed of chromium, it can be etched away with a chromium etching solution composed of a
mixed solution of ceric ammonium and nitric acid. After etching, the etchant is sufficiently rinsed
with pure water or the like, IPA is replaced with pure water, and then it is further replaced with
HFE having a small surface tension, and thereafter it is pulled up and dried. The method is not
limited as long as the drying method does not cause sticking.
[0041]
Next, as illustrated in (h) of (3-1) and (3-2), the sealing film 412 is stacked. The sealing film 412
forms silicon nitride with a thickness of 300 nm using PE-CVD. A sufficient sealing thickness
greater than the thickness at which defects disappear from the upper surface of the opening 411
formed in the first membrane 406 having a thickness of 200 nm which is larger than the
thickness of the sacrificial layer 405 is required. Although not shown, patterning, etching, and
the like for electrical connection with the lower electrode 403 and the upper electrode 407 are
performed, and wiring and the like are performed, whereby a capacitive transducer is
manufactured.
[0042]
Also in the present embodiment, the same effects as in the first embodiment can be obtained. In
particular, since the etching stopper layer is used to form the opening formed in the thinned
portion having a step or a recess, the distribution of the membrane on the cavity is suppressed,
and variations in the substrate surface (sensitivity and frequency characteristics) Variation) can
be suppressed and formed.
[0043]
Third Embodiment The capacitive transducer described in the above embodiments can be applied
to an object information acquiring apparatus using an acoustic wave. Acoustic wave from the
subject is received by the transducer, and the output electrical signal is used to acquire subject
information reflecting the optical characteristic value of the subject such as light absorption
coefficient, and subject information reflecting the difference in acoustic impedance can do.
04-05-2019
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[0044]
More specifically, at least one of the object information acquiring apparatuses of the present
embodiment emits light (electromagnetic wave including visible light and infrared light) to the
object. As a result, photoacoustic waves generated at a plurality of positions (portions) in the
subject are received, and a characteristic distribution indicating the distribution of the
characteristic information respectively corresponding to the plurality of positions in the subject
is obtained. The characteristic information acquired by the photoacoustic wave indicates the
characteristic information related to light absorption, and the initial sound pressure of the
photoacoustic wave generated by the light irradiation, or the light energy absorption density
derived from the initial sound pressure, the absorption coefficient And the characteristic
information reflecting the concentration of the substance that constitutes the tissue. The
concentration of the substance is, for example, oxygen saturation, total hemoglobin
concentration, oxyhemoglobin or deoxyhemoglobin concentration, and the like. The subject
information acquisition apparatus can also be used for diagnosis of malignant tumors and
vascular diseases of humans and animals and follow-up of chemotherapy. Therefore, as a subject,
diagnostic objects such as the breast, neck, and abdomen of a living body, specifically, a human
or an animal are assumed. The light absorber inside the subject represents a tissue having a
relatively high absorption coefficient inside the subject. For example, when a part of the human
body is a subject, there are oxyhemoglobin or deoxyhemoglobin, a blood vessel containing a
large amount of them, a tumor containing a large amount of new blood vessels, a plaque on a
carotid artery wall, and the like. Furthermore, molecular probes that bind specifically to
malignant tumors and the like by using gold particles and graphite, and capsules that transmit
drugs also serve as light absorbers.
[0045]
In addition to the reception of the photoacoustic wave, the ultrasonic wave transmitted from the
probe including the transducer receives the reflected wave due to the ultrasonic echo reflected in
the subject, thereby acquiring the distribution regarding the acoustic characteristic in the subject
You can also The distribution relating to the acoustic characteristics includes a distribution
reflecting the difference in acoustic impedance of the tissue inside the subject. However, it is not
essential to obtain the transmission and reception of ultrasonic waves and the distribution of
acoustic characteristics.
[0046]
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17
FIG. 4A shows an object information acquisition apparatus using the photoacoustic effect. The
pulsed light generated from the light source 2010 is irradiated to the subject 2014 via the
optical member 2012 such as a lens, a mirror, and an optical fiber. The light absorber 2016
inside the object 2014 absorbs the energy of the pulsed light and generates a photoacoustic
wave 2018 which is an acoustic wave. The capacitive transducer 2020 of the present invention
in the probe (probe) 2022 receives the photoacoustic wave 2018, converts it into an electric
signal, and outputs the signal to the signal processing unit 2024. The signal processing unit
2024 performs signal processing such as A / D conversion and amplification on the input electric
signal, and outputs the signal processing to the data processing unit 2026. The data processing
unit 2026 acquires object information (characteristic information reflecting the optical
characteristic value of the object such as a light absorption coefficient) as image data using the
input signal. Here, the signal processing unit 2024 and the data processing unit 2026 are
collectively referred to as a processing unit. The display unit 2028 displays an image based on
the image data input from the data processing unit 2026.
[0047]
FIG. 4B shows a subject information acquiring apparatus such as an ultrasonic echo diagnostic
apparatus using reflection of acoustic waves. The acoustic wave transmitted from the capacitive
transducer 2120 of the present invention in the probe (probe) 2122 to the subject 2114 is
reflected by the reflector 2116. The transducer 2120 receives the reflected acoustic wave
(reflected wave) 2118, converts it into an electrical signal, and outputs the electrical signal to the
signal processing unit 2124. The signal processing unit 2124 performs signal processing such as
A / D conversion and amplification on the input electric signal, and outputs the signal processing
to the data processing unit 2126. The data processing unit 2126 acquires object information
(characteristic information reflecting a difference in acoustic impedance) as image data using the
input signal. Here, the signal processing unit 2124 and the data processing unit 2126 are also
referred to as a processing unit. The display unit 2128 displays an image based on the image
data input from the data processing unit 2126.
[0048]
The probe may be one that scans mechanically or one that is moved by a user such as a doctor or
an engineer relative to the subject (handheld type). Moreover, in the case of the apparatus using
a reflected wave like FIG.4 (b), you may provide the probe which transmits an acoustic wave
separately from the probe which receives. Furthermore, it is an apparatus having both the
04-05-2019
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functions of the apparatus of FIGS. 4A and 4B, object information reflecting the optical
characteristic value of the object, and object information reflecting the difference in acoustic
impedance. Both may be acquired. In this case, the transducer 2020 in FIG. 4A may transmit not
only the photoacoustic wave but also the acoustic wave and the reflected wave.
[0049]
11 · · · cell, 206 · · first membrane, 208 · · recess, 209 · · second membrane, 210 · · · etching
opening portion 211 · · · sealing membrane (third membrane)
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