JP2018056734

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DESCRIPTION JP2018056734
Abstract: [Problem] To achieve both reduction in drive voltage of an ultrasonic transducer and
reliability. An ultrasonic transducer includes a hollow portion (110) formed between insulating
films (104, 106) sandwiched between a lower electrode (103) and an upper electrode (107) on a
substrate (101), and an insulating film (106) above the hollow portion (110). , 108, 111, 112 and
the upper electrode 107, and has a membrane 120 that vibrates when transmitting and receiving
ultrasonic waves. The hollow portion 110 has a cross-sectional shape in which the relationship
h1> h2> 0 is satisfied, where the thickness of the central portion is h1 and the thickness of the
outer peripheral portion is h2. [Selected figure] Figure 2
ULTRASONIC TRANSDUCER, METHOD FOR MANUFACTURING THE SAME, AND ULTRASONIC
IMAGING DEVICE
[0001]
The present invention relates to an ultrasonic transducer, a method of manufacturing the same,
and an ultrasonic imaging apparatus using the same.
[0002]
An ultrasonic transducer element is incorporated in an ultrasonic probe (probe) of an ultrasonic
imaging apparatus, and by transmitting and receiving ultrasonic waves, for example, diagnosis of
a tumor in a human body, inspection of a crack generated in a construction, etc. It is used for
various applications such as
[0003]
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Conventionally, piezoelectric ceramics represented by PZT (lead zirconate titanate) or the like
have been used as an electroacoustic exchange element for a probe of this type of ultrasonic
imaging apparatus, but in recent years, they are wider than piezoelectric ceramics. R & D has
been promoted with attention being focused on capacitive detection type ultrasonic transducers
(Capacitive Micromachined Ultrasonic Transducers; hereinafter abbreviated as CMUTs) having
band characteristics.
[0004]
The basic structure of the CMUT is that a cavity (cavity) is provided in the insulating layer
between the lower electrode and the upper electrode disposed thereon, and the insulating layer
on the upper portion of the cavity and the upper electrode are Function as the
When transmitting ultrasonic waves, a DC voltage and an AC voltage are superimposed and
applied between the upper electrode and the lower electrode, and the membrane is vibrated at
the frequency of the AC voltage by the electrostatic force generated between the two electrodes. .
On the other hand, at the time of reception, the pressure of the ultrasonic wave that has reached
the surface of the membrane vibrates the membrane, and a change in distance between both
electrodes generated at that time is electrically detected as a capacity change.
[0005]
Patent Document 1 solves the problem that the transmission / reception efficiency is reduced
due to the characteristic of the CMUT that the membrane in the vicinity of the outer peripheral
portion of the cavity constrained by the insulating layer is less likely to be displaced compared to
the membrane in the vicinity of the center of the cavity. In order to achieve this, a technique is
disclosed in which the height (vertical distance) of the hollow portion is monotonously decreased
in a curved manner from the central portion to the outer peripheral portion, and the height of the
hollow portion is made zero at the outer peripheral portion.
[0006]
According to the CMUT of Patent Document 1, the distance between the electrodes at the outer
peripheral portion of the hollow portion (the equivalent distance converted to vacuum based on
the relative dielectric constant when the dielectric is inserted) is reduced by: Since it is possible to
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increase the electrostatic force generated at the electrodes, an excellent effect of being able to
lower the drive voltage required to drive the membrane is obtained.
[0007]
International Patent Publication WO13 / 065365 Brochure
[0008]
In the CMUT disclosed in Patent Document 1, when the membrane is vibrated to the maximum
with a predetermined voltage, the insulating layers above and below the cavity contact each
other not only at the center of the cavity but also at the periphery of the cavity, The contact area
between the insulating layers is large, which increases the amount of charge injected from the
electrodes into the insulating layer.
Therefore, when used for a long time, charge accumulated between the upper and lower
electrodes is shielded due to accumulation of charges trapped in the insulating layer, and
appropriate driving can not be performed, or dielectric breakdown is likely to occur in the
insulating layer, etc. , Have problems in terms of reliability.
[0009]
Therefore, in the CMUT having the above-described structure, not only the driving voltage
required to vibrate the membrane is lowered, but also the decrease in reliability due to excessive
charge injection from the electrode to the insulating layer is suppressed. Ingenuity is required.
[0010]
The above and other objects and novel features of the present invention will be apparent from
the description of the present specification and the accompanying drawings.
[0011]
The outline of typical ones of the embodiments disclosed in the present application will be briefly
described as follows.
[0012]
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A CMUT according to a representative embodiment includes a cavity formed between two
insulating films sandwiched between a lower electrode and an upper electrode on a substrate,
and a plurality of insulating films and an upper electrode above the cavity. And includes a
membrane that vibrates at the time of transmission and reception of ultrasonic waves, and the
hollow portion has h1> h2> 0 when the thickness of the central portion is h1 and the thickness
of the outer peripheral portion is h2. It has a cross-sectional shape in which the relationship
holds.
[0013]
According to the representative embodiment, it is possible to realize the CMUT in which the
reduction of the drive voltage and the reliability are compatible.
[0014]
FIG. 2 is a plan view of the main part of the CMUT according to Embodiment 1;
(A) is the IIa-IIa sectional view taken on the line of FIG. 1, (b) is an IIb-IIb sectional view taken on
the line of FIG.
(A), (b) is principal part sectional drawing which shows an example of the manufacturing method
of CMUT which concerns on Embodiment 1. FIG.
(A), (b) is principal part sectional drawing which shows the manufacturing method of CMUT
following FIG.
(A), (b) is principal part sectional drawing which shows the manufacturing method of CMUT
following FIG.
(A), (b) is principal part sectional drawing which shows the manufacturing method of CMUT
following FIG.
(A), (b) is principal part sectional drawing which shows the manufacturing method of CMUT
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following FIG.
(A), (b) is principal part sectional drawing which shows the manufacturing method of CMUT
following FIG.
(A), (b) is principal part sectional drawing which shows the manufacturing method of CMUT
following FIG. (A), (b) is principal part sectional drawing which shows the manufacturing method
of CMUT following FIG. (A), (b) is principal part sectional drawing which shows another example
of the manufacturing method of CMUT which concerns on Embodiment 1. FIG. (A), (b) is principal
part sectional drawing which shows the manufacturing method of CMUT following FIG. (A), (b) is
principal part sectional drawing which shows the manufacturing method of CMUT following FIG.
(A), (b) is principal part sectional drawing which shows the manufacturing method of CMUT
following FIG. (A), (b) is principal part sectional drawing which shows the manufacturing method
of CMUT following FIG. (A), (b) is principal part sectional drawing which shows the
manufacturing method of CMUT following FIG. It is a graph explaining the specific example of
the influence on the electrostatic force by electrode area increase. 5 is a graph for explaining the
effect of the CMUT according to the first embodiment. It is principal part sectional drawing which
shows an example of the concrete countermeasure to an electric field concentration. It is
principal part sectional drawing which shows another example of the specific countermeasure to
an electric field concentration. FIG. 1 is a perspective view showing an appearance of an
ultrasonic imaging apparatus provided with a CMUT according to Embodiment 1. It is a block
diagram which shows the function of the ultrasound imaging device shown in FIG.
[0015]
Hereinafter, embodiments of the present invention will be described in detail based on the
drawings. In all the drawings for describing the embodiments, members having the same
functions are denoted by the same reference numerals, and the repetitive description thereof will
be omitted. Further, in the embodiment, the description of the same or similar parts will not be
repeated in principle unless particularly required. In the drawings for describing the
embodiments, hatching may be attached even to a plan view in order to make the configuration
easy to understand.
[0016]
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(First Embodiment) FIG. 1 is a plan view showing a region of one cell of a CMUT according to the
present embodiment, FIG. 2 (a) is a cross-sectional view taken along line IIa-IIa of FIG. ) Is a crosssectional view taken along line IIb-IIb of FIG. FIG. 1 mainly shows a planar layout of upper and
lower electrodes and a cavity formed between them, and the insulating film is omitted.
[0017]
The cell of the CMUT includes an insulating film 102 formed on a substrate 101 made of single
crystal silicon, a lower electrode 103 formed on the insulating film 102, and two insulating films
104 and 106 formed on the lower electrode 103. A cavity 110 constituted by a gap formed
between the insulating film 104 and the insulating film 106, an upper electrode 107 formed
above the cavity 110 with the insulating film 106 interposed therebetween, and an upper
electrode 107 The three insulating films 108, 111 and 112 are provided. A protective film (not
shown) for preventing foreign matter adhesion made of polyimide resin or the like may be
provided on the top of the uppermost insulating film 112 as necessary.
[0018]
Here, of the insulating films 106, 108, 111, 112 and the upper electrode 107, the portion located
above the cavity 110 (the inner portion of the boundary shown by the two-dot chain line M in
FIG. 1) It functions as a membrane 120 that vibrates during transmission and reception. Further,
of the insulating films 106, 108, 111, and 112, a portion surrounding the region functioning as
the membrane 120 (a portion surrounding the boundary M) functions as a fixing portion
supporting the membrane 120.
[0019]
At the bottom of the connection hole 113 formed by opening the insulating film 104, 106, 108,
111, 112, the pad 115 for external connection formed by a part of the lower electrode 103 is
exposed, and the insulating film is formed. An external connection pad 116 formed of a part of
the upper electrode 107 is exposed at the bottom of the connection hole 114 formed by opening
the holes 108, 111 and 112. A direct current voltage and an alternating current voltage are
applied to the CMUT from an external power supply through these pads 115, 116. Note that
reference numeral 109 in the figure indicates the openings formed in the insulating films 106
and 108 in the process of forming the cavity portion 110 (described later).
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[0020]
The CMUT has a structure in which a large number of unit cells configured as described above
are arranged along one direction of the main surface of the substrate 101 or two directions
orthogonal to each other.
[0021]
The hollow portion 110 provided for each unit cell has a cross-sectional shape whose center
portion is thicker than the outer peripheral portion.
Further, on the outer peripheral portion of the hollow portion 110, a side wall portion 118
formed along the outer peripheral portion is provided. In other words, assuming that the
thickness (height) of the central portion of the hollow portion 110 is h1 and the thickness
(height) of the side wall portion 118 provided on the outer peripheral portion is h2, h1> h2> 0. It
has a cross-sectional shape such that the following relationship holds. The thickness (h1) of the
central portion is preferably 1.5 or more times the thickness (h2) of the outer peripheral portion.
[0022]
In the illustrated example, the thickness of the cavity 110 monotonously decreases in a curved
manner from the center to the outer periphery, but the cross-sectional shape of the cavity 110 is
not limited to this, for example, The cross-sectional shape may decrease substantially linearly
from the central portion to the outer peripheral portion, or may have a cross-sectional shape
having unevenness locally and decreasing in a curved manner from the central portion to the
outer peripheral portion .
[0023]
Further, although the cross-sectional shape of the illustrated hollow portion 110 is such that the
bottom surface is flat and the top surface is convex, the bottom surface may be concave and the
top surface may be flat.
However, in consideration of the ease of manufacture, it is preferable to have a cross-sectional
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shape as illustrated.
[0024]
Further, although the planar shape of the hollow portion 110 shown in the drawings is
rectangular, the planar shape of the hollow portion 110 is not limited to rectangular, and for
example, circular, oval, pentagon or more polygon (hexagon, octagon) Or the like.
[0025]
Next, an example of a method of manufacturing the CMUT according to the present embodiment
will be described with reference to FIGS.
(A) of FIG. 3 to FIG. 10 is a cross-sectional view taken along the line IIa-IIa of FIG. 1, and (b) is a
cross-sectional view taken along the line IIb-IIb of FIG.
[0026]
First, as shown in FIG. 3, an insulating film 102 made of a silicon oxide film having a film
thickness of about 500 nm is formed on a substrate 101 by a CVD (Chemical Vapor Deposition)
method or a thermal oxidation method, and then sputtering is performed on the insulating film
102. The lower electrode 103 is formed by depositing an aluminum alloy film having a film
thickness of about 100 nm by a method. Subsequently, an insulating film 104 made of a silicon
oxide film having a film thickness of about 200 nm is deposited on the lower electrode 103 by
plasma CVD.
[0027]
Next, as shown in FIG. 4, a polycrystalline silicon film having a thickness of about 100 nm is
deposited on the insulating film 104 by plasma CVD, and then the polycrystalline silicon film is
patterned using photolithography technology and dry etching technology. Thus, a sacrificial layer
(dummy layer) 105 made of a polycrystalline silicon film is formed on the insulating film 104.
The region where the sacrificial layer 105 is formed is a region that will become the cavity 110
in a later step, and the film thickness of the sacrificial layer 105 corresponds to the thickness (h
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2) of the sidewall 118 of the cavity 110.
[0028]
Next, as shown in FIG. 5, an insulating film 106 made of a silicon oxide film having a film
thickness of about 200 nm is deposited on the insulating film 104 and the sacrificial layer 105
by plasma CVD.
[0029]
Next, as shown in FIG. 6, an aluminum alloy film having a film thickness of about 100 nm is
deposited on the top of the insulating film 106 by sputtering, and then the aluminum alloy film is
patterned using photolithography technology and dry etching technology. The upper electrode
107 is formed.
[0030]
Next, as shown in FIG. 7, after depositing an insulating film 108 made of a silicon oxide film
having a film thickness of about 200 nm on the insulating film 106 and the upper electrode 107
by plasma CVD method, photolithographic technology and dry etching technology A portion of
each of the insulating films 108 and 106 is removed to form an opening 109 reaching the
sacrificial layer 105.
[0031]
Next, as shown in FIG. 8, a wet etching solution such as an aqueous potassium hydroxide solution
is brought into contact with the surface of the sacrificial layer 105 through the opening 109 to
dissolve the sacrificial layer 105.
Thus, the cavity 110 is formed in the region where the sacrificial layer 105 was formed.
[0032]
Next, as shown in FIG. 9, an insulating film 111 made of a silicon oxide film having a film
thickness of about 500 nm is deposited on the insulating film 108 by plasma CVD.
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Thus, the insulating film 111 is embedded in the opening 109, and the cavity 110 is sealed.
[0033]
Next, as shown in FIG. 10, an insulating film 112 made of a silicon nitride film having a film
thickness of about 500 nm is deposited on the insulating film 111 by plasma CVD.
The silicon nitride film forming the insulating film 112 has a high residual stress because the film
quality is denser than the silicon oxide film. Therefore, when the insulating film 112 made of a
silicon nitride film is deposited on the insulating films 106, 108, 111 made of a silicon oxide film,
residual stress of the insulating film 112 acts on the insulating films 106, 108, 111, and the
cavity 110 is formed. The upper insulating films 106, 108, 111 are pulled upward. As a result,
the hollow portion 110 has a cross-sectional shape such that the thickness (h1) of the central
portion is larger than the thickness (h2) of the side wall 118 along the outer peripheral portion,
and the relationship h1> h2> 0 is satisfied. Become.
[0034]
Thereafter, connection holes 113 are formed in the insulating films 112, 111, 108, 106, and 104
using photolithography technology and dry etching technology, and connection holes 114 are
formed in the insulating films 112, 111, and 108, whereby the lower electrode is formed. A pad
115 where a part of 103 is exposed and a pad 116 where a part of the upper electrode 107 is
exposed are formed. Thus, the CMUT shown in FIGS. 1 and 2 is completed.
[0035]
In addition, the electrode material and insulating film material which comprise CMUT mentioned
above are preferable examples, and are not limited to these. As the electrode material, metal
materials other than aluminum alloy, for example, W, Ti, TiN, Al, Cr, Pt, Au, polycrystalline silicon
doped with high impurity concentration, amorphous silicon, etc. can also be used. Further,
instead of the insulating film made of a silicon oxide film, a silicon oxynitride film, a hafnium
oxide film, a silicon-doped hafnium oxide film or the like can be used. The sacrificial layer 105 is
also not limited to the polycrystalline silicon film as long as the material has a high etching
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selectivity to the insulating film, and may be, for example, a metal film or a SOG (Spin-on-Glass)
film.
[0036]
In the manufacturing method described above, the thickness of the central portion of the cavity
110 is made larger than the thickness of the outer peripheral portion (side wall portion 118) by
utilizing the residual stress of the silicon nitride film (insulating film 112). Can also be used.
[0037]
First, as shown in FIG. 11, the insulating film 102, the lower electrode 103 and the insulating film
104 are sequentially formed on the substrate 101 according to the process of FIG. 3 described
above.
[0038]
Next, as shown in FIG. 12, a polycrystalline silicon film having a film thickness of about 200 nm
is deposited on the insulating film 104 by plasma CVD, and then a photolithography technique
using a gray scale photomask and a dry etching technique are used. A sacrificial layer 205 is
formed having a cross-sectional shape such that the thickness (h1) of the central portion is
greater than the height (h2) of the outer peripheral portion, and the relationship h1> h2> 0
holds.
[0039]
Next, as shown in FIG. 13, according to the steps shown in FIGS. 5 to 7 described above, the
insulating film 106 is formed on the insulating film 104 and the sacrificial layer 105, and then
the upper electrode 107 is formed on the insulating film 106. After the insulating film 108 is
sequentially formed, portions of the insulating films 108 and 106 are removed to form an
opening 109 reaching the sacrificial layer 205.
[0040]
Next, as shown in FIG. 14, according to the process shown in FIG. 8 described above, the surface
of the sacrificial layer 205 is brought into contact with a wet etching solution through the
opening 109 to dissolve the sacrificial layer 205, thereby forming the sacrificial layer 205. A
cavity 210 is formed in the region.
[0041]
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Next, as shown in FIG. 15, the insulating film 111 is deposited on the insulating film 108
according to the process shown in FIG. 9 described above, and the insulating film 111 is
embedded inside the opening 109 to seal the cavity 210.
[0042]
Thereafter, as shown in FIG. 16, connection holes 213 are formed in the insulating films 111,
108, 106, and 104 using a photolithographic technique and a dry etching technique, and
connection holes 214 are formed in the insulating films 111 and 108. , Pads 215, 216 are
formed.
[0043]
In the manufacturing method shown in FIGS. 11 to 16, since the residual stress of the silicon
nitride film (insulating film 112) is not used, the cross-sectional shape of the cavity 210 is
specified without causing defects such as peeling of the insulating film due to stress. can do.
[0044]
Also in the manufacturing method shown in FIGS. 11 to 16, by depositing the insulating film
having high residual stress on the insulating film 111, the cross-sectional shape of the hollow
portion 210 can be controlled more accurately.
In this case, as the insulating film deposited on the insulating film 111, an insulating film having
a relatively small residual stress is used, or the thickness of the insulating film is reduced, so that
only the residual stress is used to form the cavity 210. As compared with the case of deforming
the cross-sectional shape, it is possible to suppress the occurrence of a defect such as peeling of
the insulating film due to the stress.
[0045]
Next, the effect of the CMUT of the present embodiment provided with the hollow portion 110
having the cross-sectional shape as described above will be described in comparison with the
prior art.
[0046]
First, the operation of the CMUT provided with a cavity having a rectangular cross-sectional
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shape, that is, a general shape (hereinafter referred to as a basic structure) having a uniform
height from the central portion to the outer peripheral portion will be described.
[0047]
In this case, when a DC voltage and an AC voltage are superimposed and applied between the
upper electrode and the lower electrode, an electrostatic force is exerted between the two
electrodes, and a membrane is formed of the insulating film at the top of the cavity and the upper
electrode. Is elastically deformed and emits ultrasonic waves by vibrating according to the
frequency of the AC voltage.
That is, the pressure of the ultrasonic wave transmitted from the CMUT depends on the vibration
amplitude of the membrane.
On the other hand, in the case of reception, the pressure of the ultrasonic wave reaching the
surface of the membrane from the outside vibrates the membrane, and the distance between
both electrodes changes, so the change of this distance is detected electrically as the change of
capacitance. Receive ultrasound by doing.
[0048]
From the above principle of operation, the pressure of the transmitted ultrasonic wave depends
on the vibration amplitude of the membrane.
In the case of a CMUT provided with a cavity having a rectangular cross-sectional shape, the
membrane is supported by the fixed portion (insulation film) at the outer periphery of the cavity,
and deflection is caused by elastic deformation of the membrane near the center of the cavity.
Vibration amplitude occurs.
From this, the vibration amplitude of the membrane is zero at the outer periphery of the cavity
and has a continuous distribution such that it is maximum at the center of the cavity.
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[0049]
In the CMUT having such an amplitude distribution, a region near the outer peripheral portion of
the hollow portion in the upper electrode contributes less to the generation of electrostatic force.
The reason is that the distance between the upper and lower electrodes near the outer periphery
of the cavity can not be reduced even during vibration, so the distance between the electrodes
(when a dielectric is inserted, conversion to vacuum based on the relative permittivity) The
electrostatic force, which is inversely proportional to the square of the equivalent distance), is a
fraction of the maximum amplitude point of the membrane (in other words, the point at which
the distance between the electrodes is the closest).
[0050]
This property is a major obstacle to increasing the pressure of the transmitted ultrasound.
The reason is that in order to increase the pressure of the ultrasonic wave, the maximum
amplitude of the membrane may be increased, that is, the cavity may be formed high, but in such
a case, the increase in the distance between the electrodes compensates for the decrease in
electrostatic force. This is because it is necessary to vibrate the membrane.
[0051]
In addition, although it is possible to increase the electrostatic force by increasing the area of the
electrode occupying the area of the hollow portion, for the reason described above, the region of
the upper electrode in the vicinity of the fixing portion Because the contribution is small, the
effect is limited.
In addition, the capacitance at the time of transmission and reception is lowered because the
capacitance component not contributing to vibration, that is, the parasitic capacitance is
increased.
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[0052]
The example of the influence on the electrostatic force by electrode area increase is shown on
the graph of FIG.
The horizontal axis of the graph indicates the ratio of the area of the upper electrode to the area
of the cavity (hereinafter referred to as "electrode area ratio"), and the vertical axis indicates the
amount of electrostatic force generated when a constant voltage is applied between the
electrodes. It shows the size.
The dashed line plot in the graph shows the ideal value, that is, the theoretical value of the
change in electrostatic force generated by a parallel plate moving up and down like a piston. In
addition, the diamond-shaped plot shows the change in electrostatic force generated by the
above-described cavity of the basic structure, that is, a CMUT having a flat membrane.
[0053]
As shown by the dashed line plot, in the ideal case the electrostatic force is simply proportional
to the electrode area ratio and is maximum when the electrode area ratio is 100%. On the other
hand, as shown by the rhombic plot, in the CMUT having a distribution of vibration amplitudes,
when the electrode area ratio exceeds 75%, the increase in electrostatic force is sluggish and
remains at most 60% of the ideal value.
[0054]
As described above, as long as the basic structure CMUT having a flat membrane is employed,
there is a limit to the improvement of electrostatic force by the increase of the electrode area
ratio. Under this limitation, in order to obtain sufficient electrostatic force to generate practical
sound pressure, it is necessary to increase the drive voltage. However, a high driving voltage is
not desirable because it increases the strength of the electric field applied between the
electrodes, which causes a serious problem such as a decrease in reliability, such as an
accelerated progress of destruction or characteristic deterioration of the CMUT element.
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[0055]
Destruction and property deterioration in the CMUT are mainly caused by deterioration of the
insulating film above and below the cavity. These insulating films are formed to separate the
lower electrode and the upper electrode and to prevent breakdown due to a short circuit current,
but when excessive electric field strength is applied to these insulating films, dielectric
breakdown occurs or the electrode There is a possibility that the charge may be injected into the
insulating film to charge up the insulating film. In the case of breakdown, the CMUT element is
broken and becomes unusable due to the generation of Joule heat due to the increase in current.
In addition, when the insulating film is charged up, the electric charge between the upper and
lower electrodes is shielded by the charges charged in the insulating film, which causes a
problem that optimal driving can not be performed.
[0056]
As described above, it is difficult to lower the driving voltage required to vibrate the membrane in
the basic structure CMUT having the cavity having the rectangular cross-sectional shape.
[0057]
On the other hand, according to the CMUT of Patent Document 1 in which the height of the
hollow portion is made zero at the outer peripheral portion of the hollow portion, the distance
between both electrodes is reduced near the outer peripheral portion of the hollow portion. Can
also contribute to the generation of electrostatic force and can drive the membrane with a lower
driving voltage.
[0058]
However, a structure (type 0 Bessel function, arc function, or the like, as disclosed in Patent
Document 1) in which the distribution of the height of the cavity is maximum at the center of the
cavity and is zero at the outer periphery of the cavity. A sine function is disclosed as an example)
has the problem of reduced reliability in ultrasound transmission at large amplitudes.
[0059]
That is, as in the CMUT disclosed in Patent Document 1, the hollow portion where the height of
the outer peripheral portion becomes zero is not only the center of the hollow portion but also
the upper and lower hollow portions at the outer peripheral portion when the membrane is
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maximally vibrated. Since the insulating films in the above contact each other, the contact area
between the insulating layers above and below the hollow portion is increased as compared with
the hollow portion having the above-described general cross-sectional shape.
As a result, charge up is likely to occur in the insulating film above and below the cavity when
used for a long period of time, so the charge trapped in the insulating layer shields the charge
between the upper and lower electrodes, preventing appropriate driving or insulation. The layers
are susceptible to dielectric breakdown.
[0060]
On the other hand, the hollow portion 110 of the CMUT of the present embodiment is provided
on the outer peripheral portion with the thickness of the central portion as h 1 when no voltage
is applied between the lower electrode 103 and the upper electrode 107. When the thickness of
the side wall portion 118 is h2, it has a cross-sectional shape such that the relationship h1> h2>
0 holds.
[0061]
The effect of the increase in electrostatic force when the hollow portion 110 has the crosssectional shape as described above is shown in the graph of FIG.
In the graph, numerical examples shown in FIG. 17 are also shown for comparison.
That is, the plot of the broken line shows an ideal case, that is, the theoretical value of the change
of the electrostatic force generated by the parallel plate moving up and down like a piston, and
the rhombic plot shows the hollow portion having a rectangular cross section. It shows the
change of electrostatic force generated by the CMUT.
The circular plot shows the change in electrostatic force generated by the CMUT of this
embodiment.
[0062]
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As described above, in the case of a CMUT having a cavity having a rectangular cross-sectional
shape, the increase in the electrostatic force is dulled when the electrode area ratio exceeds 75%,
and the maximum remains at 60% of the ideal value. On the other hand, in the case of the CMUT
according to the present embodiment, in the CMUT having a hollow portion having a rectangular
cross-sectional shape, the electrostatic force increases remarkably at an electrode area ratio of
75% or more where an increase in electrostatic force can not be expected 90% of the can be
achieved. The ability to generate strong electrostatic force at the same voltage means that
equivalent electrostatic force can be generated at low voltage.
[0063]
Further, in the CMUT of the present embodiment, since the hollow portion 110 in the vicinity of
the side wall portion 118 has a certain thickness (h2), the upper and lower electrodes have small
displacement so that the insulating film does not contact. The large electrostatic force is
generated in the region near the side wall portion 118 among the electrodes, and in the state of
large displacement where contact between the inter-electrode insulating film can occur, the
insulating film near the center of the cavity 110 where the vibration amplitude is maximum. Only
the area is in contact. The contact portion of the insulating film can be limited by having a height
at which the cavity layer is in the vicinity of the side wall portion 118, and by arranging the
structure for relaxing the electric field strength at the contact portion, the electric field
concentration due to the insulating film contact It is possible to limit the measures to the central
portion of the hollow portion and to suppress the deterioration of the insulating film.
[0064]
For example, the following method can be considered as a specific measure against electric field
concentration. FIG. 19 is an example in which concentration of an electric field is suppressed by
removing an electrode portion in a region where the insulating films 104 and 106 are in contact
among at least one of the lower electrode 103 and the upper electrode 107. Further, in FIG. 20,
at least one of the insulating films 104 and 106 is locally thickened in a region where the
insulating films 104 and 106 are in contact, and even if the insulating films 104 and 106 are in
contact, charge accumulation is a problem. This is an example of reducing the electric field
strength to such an extent that Even in the case of adopting the method of FIG. 20, the thickness
of the central portion when no voltage is applied between the lower electrode 103 and the upper
electrode 107 is h1 ′, and the side wall portion 118 provided on the outer peripheral portion It
is necessary to make the cross-sectional shape such that the relationship of h1 ′> h2> 0 holds,
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where h2 is a thickness of h2.
[0065]
In the example of FIG. 19, although the electrode portion is removed at the maximum
displacement portion of the membrane 120, the electrode portion may be removed at a plurality
of places of the membrane 120. Further, in the example of FIG. 20, although the insulating film is
thickened at the maximum displacement portion of the membrane 120, the insulating film may
be thickened at a plurality of places of the membrane 120.
[0066]
As described above, according to the present embodiment, it is possible to realize a CMUT that
achieves both reduction in drive voltage and securing of long-term reliability.
[0067]
Second Embodiment FIG. 21 is a perspective view showing the appearance of an ultrasonic
imaging apparatus provided with the CMUT of the first embodiment, and FIG. 22 is a block
diagram showing the function of the ultrasonic imaging apparatus shown in FIG. is there.
[0068]
The ultrasonic imaging apparatus 301 processes an ultrasonic wave transmission / reception
circuit that transmits / receives ultrasonic waves, and a signal processing circuit that processes
echo signals received by the ultrasonic wave transmission / reception circuit and generates an
ultrasonic image to be inspected. , A display unit 303 connected to the main body 305 for
displaying an ultrasonic image and a GUI for performing an interface with the operator, an input
unit 304 operated by the operator, and an ultrasonic probe fixed to the main body 305 And an
ultrasound probe 302 connected to the ultrasound transmission / reception circuit via the
connection unit 306.
[0069]
The ultrasonic probe 302 is a device that transmits and receives ultrasonic waves to and from an
object (patient) in contact with the object (patient), and has a structure in which a large number
of transducer elements are arranged in a one-dimensional or two-dimensional array. And an
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acoustic lens and a backing material.
The ultrasonic transducers 307 are configured by arranging CMUT elements in a onedimensional or two-dimensional array in a range of several hundreds to ten thousands.
[0070]
Although FIG. 21 shows a movable ultrasonic imaging apparatus provided with a caster 308 at
the bottom of the main body 305 as an example, the ultrasonic imaging apparatus 301 according
to the present embodiment is an ultra-sound apparatus fixed in an examination room. The
present invention can be applied to an ultrasonic imaging apparatus, a portable ultrasonic
imaging apparatus such as a notebook type or a box type, and other known ultrasonic imaging
apparatuses.
[0071]
As shown in FIG. 22, the main body 305 of the ultrasonic imaging apparatus 301 includes an
ultrasonic wave transmission / reception unit 411, a signal processing unit 412, a control unit
413, a memory unit 414, a power supply device 415, and an auxiliary device 416.
[0072]
The ultrasonic transmission / reception unit 411 generates a drive voltage for transmitting an
ultrasonic wave from the ultrasonic probe 302 or receives an echo signal from the ultrasonic
probe 302, and has a delay circuit, a filter, and a gain. It has an adjustment circuit etc.
[0073]
The signal processing unit 412 performs processing necessary for correction such as LOG
compression and depth correction and image creation on the received echo signal, and the DSC
(digital scan converter), color Doppler circuit, FFT analysis unit, etc. May be included.
The signal processing by the signal processing unit 412 can be either analog signal processing or
digital signal processing, and can be partially realized by software, or by ASIC (application
specific integrated circuit) or FPGA (field-programmable gate array) It is also possible to realize.
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[0074]
The control unit 413 controls the circuits of the main body 305 and the devices connected to the
main body 305.
The memory unit 414 stores information, parameters, and processing results necessary for signal
processing and control.
The power supply device 415 supplies necessary power to each part of the ultrasonic imaging
apparatus.
The auxiliary device 416 is for realizing functions associated with the ultrasonic imaging
apparatus 301, such as sound generation, in addition to the above-described units, and is
appropriately added as necessary.
[0075]
The ultrasonic imaging apparatus 301 according to the present embodiment uses the CMUT
according to the first embodiment as the ultrasonic transducer 307 of the ultrasonic probe 302.
Therefore, the ultrasonic imaging apparatus 301 is safe even in contact with a subject (patient).
Even at low voltage, ultrasonic waves can be transmitted and received with high sensitivity.
Further, since the long-term reliability of the CMUT is high, the running cost of the ultrasonic
imaging apparatus 301 can be reduced.
[0076]
As mentioned above, although the invention made by the present inventor was concretely
explained based on the embodiment, the present invention is not limited to the embodiment, and
can be variously changed in the range which does not deviate from the summary. .
[0077]
101 substrate 102 insulating film 103 lower electrode 104 insulating film 105 sacrificial film
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106 insulating film 107 upper electrode 108 insulating film 109 opening 110 cavity 110
insulating film 112 insulating film 113 connection hole 114 connection hole 115 pad 116 side
wall 120 membrane 205 Sacrifice layer 210 hollow portion 213 connection hole 214 connection
hole 215 pad 216 pad 301 ultrasonic imaging device 302 ultrasonic probe 303 display unit 304
input portion 305 main body 306 ultrasonic probe connection portion 307 ultrasonic transducer
308 caster 411 extra Sound wave transmission and reception unit 412 Signal processing unit
413 Control unit 414 Memory unit 415 Power supply device 416 Auxiliary device
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