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JP2014200089

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DESCRIPTION JP2014200089
Abstract: There remains a need for efficient and effective cMUT devices and methods, and
methods of using the devices. A capacitive ultrasound transducer is provided that is operable in a
collapsing mode with either a reduced bias voltage or no bias voltage. The transducer has a
substrate which is contoured such that the central region of the flexible membrane collapses into
the substrate in the absence of a bias voltage. There is an uncollapsed gap between the substrate
and the peripheral region of the flexible membrane. The contour of the substrate is either pulling
the flexible membrane past the point of collapse or mechanically interfering with the flexible
membrane. The substrate has another membrane placed under the flexible membrane, which is
contoured such that the flexible membrane collapses into the other membrane. The substrate
may be a support placed under the other membrane to deflect the corresponding portion of the
other membrane upwards towards the flexible membrane. The support may be a pillar. [Selected
figure] Figure 3
CMUT operable in collapsed mode including contoured substrate
[0001]
The present disclosure is directed to systems and methods for generating medical diagnostic
images, and in particular to ultrasound transducers.
[0002]
Bayram, B.
04-05-2019
1
Co-author A New Regime for Operating Capacitive Micromachined Ultrasonic Transducers, IEEE
Trans UFFC, Vol. 50, As described in No. 9 (2003), for a conventional capacitive ultrasonic
transducer (cMUT: capacitive micromachined ultrasonic transducer) to be operated in collapsed
mode, the flexible membrane (cMUT) of this cMUT is It is generally excited using a voltage that
causes a portion of this film to collapse into the corresponding cMUT substrate. Then, reducing
the voltage applied to the membrane to a constant threshold voltage, generally characterized as
the "snapback voltage" of the cMUT, lifts the membrane up from the substrate, balances it Return
to position. On the other hand, a fairly linear and efficient output of the device can generally be
achieved insofar as the voltage applied to the previously collapsed membrane is maintained
above the snapback voltage.
[0003]
A conventional cMUT structure is shown in FIG. In particular, FIG. 1 illustrates in cross-sectional
view a cMUT 100 having a substrate 102 forming a pocket 104 and a flexible membrane 106
attached to the substrate 102 across the pocket 104. In situations where the bias voltage applied
to the flexible membrane 106 and the substrate 102 is set to a relatively low voltage or zero
volts, the cMUT 100 generally forms a gap in the pocket 104 between the flexible membrane
106 and the substrate 102. It represents 108.
[0004]
Referring now to FIG. 2, in operation, a voltage bias is applied to the flexible membrane 106 and
the substrate 102 that increases a sufficient amount from the low or zero level associated with
the shape of the cMUT 100 shown in FIG. In this case, the flexible membrane 106 tends to
collapse down the pocket 104 and towards the substrate 102. Such collapse of the flexible
membrane 106 causes the flexible membrane 106 and the substrate 102 to be at least
temporarily positioned such that the downward facing surface 200 of the flexible membrane 106
is in physical contact with the corresponding upward facing surface 202 of the substrate 102.
Most of the gap 108 (see FIG. 1) in between can be removed. Once so, this collapsed state of the
flexible membrane 106 with respect to the substrate 102 is maintained by sequentially applying
to the flexible membrane 106 and the substrate 102 a bias voltage above a certain minimum
level, also commonly referred to as a "snapback" voltage. Ru.
[0005]
04-05-2019
2
The cMUT 100 is used in decay mode to radiate or receive pressure waves. For a cMUT 100 that
emits pressure waves with the flexible membrane 106 collapsing on the substrate 102, the
voltage across the flexible membrane 106 and the substrate 102 cycles between a much higher
voltage and a much lower voltage. Both such voltages are generally higher than the snapback
voltage associated with cMUT 100 with respect to their respective magnitudes. Of the relatively
high voltage and the relatively low voltage, the relatively high voltage is associated with the
correspondingly large contact area between the downward surface 200 of the flexible membrane
106 and the upward surface 202 of the substrate 102. Since the flexible membrane 106 is
alternately induced, driven or otherwise caused to circulate between the large and small areas in
physical contact with the substrate 102 due to the circulating bias voltage, the constant of the
flexible membrane 10 Parts of the substrate 104 by reciprocating in the vertical direction with
respect to the corresponding parts of the substrate 102 in the pocket 104 to and from the area
in contact with the substrate 102 (eg, the "collapse region" of the flexible membrane 106 and the
"collapse region" ) Transition. The reciprocating vertical movement of the transition portion of
the flexible membrane 106 produces the desired pressure wave. As those skilled in the art are
aware, the cMUT 100 is generally illustrated to generate and transmit corresponding electrical
signals in response to the flexible membrane 106 being exposed to externally generated pressure
waves received by the cMUT 100. It is also possible to use in the collapse mode shown in 2.
[0006]
1 and 2 according to at least one common method of efficiency of the cMUT, such as the cMUT
100 of FIG. 1 and FIG. 2 (for example as an output in response to an electrical input) of
processing pressure radiation and / or generating an electrical output In part, the value of the
size or area of the portion of the flexible membrane which is substantially actively added to the
reception of the pressure wave incident as input and the response of the pressure wave is at least
one for comparison. Supply the standard. For example, for the two at least somewhat differently
configured variables of the cMUT 100 that tend to respond at least somewhat differently to the
same input electrical signal or the same input pressure wave, the larger the collapse region of the
flexible membrane 106 A cMUT variable indicating movement is usually considered to be a more
efficient device.
[0007]
Despite the efforts to date, there remains a need for efficient and effective cMUT devices and
04-05-2019
3
methods, and methods of using the devices. These and other needs are met by the disclosed
devices, systems and methods, as will be apparent from the following detailed description.
[0008]
According to an embodiment of the present disclosure, a capacitive ultrasonic transducer is
provided, the transducer comprising a substrate and a flexible membrane, the flexible membrane
being a peripheral area along which the flexible membrane is attached to the substrate, And a
central area extending between the peripheral areas. The substrate of the transducer is
contoured such that in the absence of a bias voltage, the flexible membrane collapses onto the
substrate near the central region, such that the transducer is either at a reduced bias voltage or
no bias voltage. Allows to operate in collapse mode. In the vicinity of each of the peripheral
regions there is a non-collapsible gap between the substrate and the flexible membrane. The
substrate may, for example, pull the flexible membrane past a point of collapse in the vicinity of
the central region and / or in the range up to about 2 μm (for example in the range up to about
1.6 μm) in the vicinity of the central region Contoured to interfere with each other. The
substrate includes another membrane placed beneath the flexible membrane, the other
membrane being contoured such that, in the absence of a bias voltage, the flexible membrane
collapses into the other membrane in the vicinity of the central region Ru. The length and
thickness of the flexible membrane are each greater than about 80 μm (eg, about 100 μm) and
less than about 3 μm (eg, about 2 μm), and the other membrane is at least about 4 μm thick
(eg, about 5 μm) Thickness). The substrate further includes a support disposed below the other
membrane, the support facing the flexible membrane to an extent at least equal to the thickness
of the original gap between the support and the flexible membrane. It is dimensioned and
configured to deflect a corresponding portion of the other membrane upward. The support may
be a column underlying the other membrane and may be a column vertically aligned with the
central region of the flexible membrane and / or vertically aligned with the central region of the
flexible membrane It may be structurally incomplete below the area of the other membrane other
than the center of the other membrane. The support is vertically aligned vertically with the
central region of the flexible membrane, vertically upward to at least about 0.5 μm (eg, between
about 0.9 μm and about 2.5 μm). While operating to deflect the center of the other membrane,
it is possible to leave the relative perimeter of at least one of the other membranes substantially
non-vertical. The substrate is contoured such that in the absence of a bias voltage, the flexible
membrane collapses into the substrate in the vicinity of the central region, such that the
transducer exhibits a substrate that is not equally contoured. It enables to operate in the collapse
mode with improved efficiency (k <2> eff) compared to conventional transducers that are
equivalent in point.
04-05-2019
4
[0009]
According to an embodiment of the present disclosure, there is provided a medical imaging
system having a capacitive ultrasound transducer, the transducer comprising a substrate and a
flexible membrane, the flexible membrane being attached to the substrate along the flexible
membrane It has a peripheral area and a central area extending between the peripheral areas.
The substrate of the transducer is contoured to collapse to the substrate in the vicinity of the
central region in the absence of a bias voltage, whereby the transducer is in a decay mode with
either a reduced bias voltage or no bias voltage. Allow to work. The medical imaging system
comprises an array of the transducers disposed on a common substrate.
[0010]
In accordance with an embodiment of the present disclosure, a method of operating a capacitive
ultrasonic transducer is provided, the step of providing a transducer including a substrate and a
flexible membrane, and operating the transducer in the collapsed mode without a bias voltage.
Having a step, the flexible membrane having a peripheral region along which the flexible
membrane is attached to the substrate, and a central region extending between the peripheral
regions, the substrate being free of bias voltage And said flexible membrane is contoured to
collapse into said substrate in the vicinity of said central region.
[0011]
It is a figure which shows conventional cMUT.
FIG. 2 shows the cMUT of FIG. 1 in the operating mode of collapse; FIG. 7 illustrates a cMUT
configured in accordance with an embodiment of the present invention. FIG. 4 illustrates a
method of manufacturing the cMUT of FIG. 3 according to an embodiment of the present
invention. FIG. 4 illustrates a method of manufacturing the cMUT of FIG. 3 according to an
embodiment of the present invention. FIG. 4 illustrates a method of manufacturing the cMUT of
FIG. 3 according to an embodiment of the present invention. FIG. 4 illustrates a method of
manufacturing the cMUT of FIG. 3 according to an embodiment of the present invention. FIG. 6
shows efficiency data as a function of bias voltage corresponding to various embodiments of the
cMUT according to the present invention as compared to certain prior art but other equivalent
cMUTs. FIG. 6 shows efficiency data as a function of bias voltage corresponding to various
embodiments of the cMUT according to the present invention as compared to certain prior art
but other equivalent cMUTs. FIG. 1 illustrates a system for generating medical diagnostic images
04-05-2019
5
according to an embodiment of the present invention, including an array of cMUT devices
configured in accordance with the present invention.
[0012]
Reference is made to the accompanying drawings in order to aid those skilled in the art in
making and using the disclosed devices, systems and methods.
[0013]
One of the conventional drawbacks of using cMUTs in the collapsing mode is that the collapsing
voltage is generally much higher than the operating voltage, and thus a high voltage circuit is
required.
In addition, the output is usually the limiting factor of cMUT in imaging applications where any
improvement in efficiency of the device is desirable.
[0014]
In these applications, modeling and simulation show that implementing certain changes to the
substrate surface of the cMUT can be an efficiency improvement in operation in the collapse
mode. In some embodiments of the present disclosure, the substrate comprising the second
membrane is contoured (unbiased collapse mode) so that there is no gap in the middle of the
flexible membrane of the cMUT. This allows the cMUT in accordance with the present disclosure
to operate in collapse mode with no bias voltage (or small bias voltage). Moreover, these
applications show that the efficiency is increased when the cMUT according to the present
disclosure is used to pull the film past the contacts (collapses). In addition to this efficiency
improvement, cMUTs in accordance with the present disclosure allow the required voltage to be
reduced significantly. Among other related advantages, such an improvement is well suited to
incorporating cMUTs according to the present disclosure into mainstream ultrasound probes.
[0015]
Returning to FIG. 3, a cMUT device according to an exemplary embodiment of the present
04-05-2019
6
disclosure is shown. In particular, FIG. 3 shows cMUT 300 in cross section. The cMUT 300
includes a substrate 302 that forms a pocket 304. The cMUT 300 further includes a flexible
membrane 306 coupled to the substrate 302 across the pocket 304. The flexible membrane 306
includes respective peripheral regions 308, which are attached to the substrate 302 along or
around the corresponding peripheral portion of the pocket 304. The flexible membrane 306
further includes a central region 310 extending between the peripheral regions 308. Still further,
flexible membrane 306 defines a downward facing surface 312. The substrate 302 may further
include a substrate 314 disposed within the perimeter of the pocket 304. The substrate 314 may
define and / or at least structurally support the upwardly contoured surface 316. The upwardly
contoured surface 316 faces upwardly or outwardly of the pocket 304 to contact and / or
otherwise cooperate with the downwardly facing surface 312 of the flexible membrane 306 near
the central region 310. Extend or protrude. The contoured surface 316 is at least one of arcuate,
curved, convex and dome shaped. Other shapes of the contoured surface 316 are also possible.
The contoured surface 316 is sufficiently small or short (eg, along the direction perpendicular to
the sheet of FIG. 3) that the contoured surface is substantially completely contained or confined
within the pocket 304. A lateral and / or depth range may be defined or included. For example,
the contoured surface 316 may be sized and configured to have or define substantially isolated
"islands" in the pocket 304 to exclusively interact with the central region 310 of the flexible
membrane 306. (E.g., the contoured surface may define a correspondingly decreasing contour in
the vicinity of the peripheral region 308 or may be substantially absent in the vicinity of the
peripheral region 308). Other geometric and / or dimensional configurations for the lateral and /
or depth extent of the contoured surface 316 are also possible.
[0016]
According to the embodiment of the present disclosure, particularly as shown in FIG. 3, at least a
portion or segment of the contoured surface 316 occupies an elevation 318 relative to the
reference elevation 320 of the substrate 302 and At least a portion or segment of the downward
facing surface 322 associated with one or more occupies the elevation 324 relative to the same
reference elevation 320, said elevation 318 being at least somewhat higher than the elevation
324 relative to the reference elevation 320. For example, the base elevation of the flexible
membrane 306 relative to the substrate 302 is such that the entire range of the downward
surface 312 of the flexible membrane 306 is substantially transverse to the elevation 324
without any interference between the contoured surface 316 and the flexible membrane 306. ,
And may be set by all of the flexible membrane's peripheral region 308 occupying a common
elevation at elevation 324, as it tends to be aligned and highly positioned. In such a situation,
occupancy by at least a portion or segment of contoured surface 316 of substrate 320 at height
320 at least somewhat higher than the basic height 324 of flexible film 306 may be greater than
the contoured surface 316 and the flexible film. Mechanical interference occurs with the
04-05-2019
7
downward surface 312 of 306. Similarly, the flexible membrane 306 is deflected upward by the
contoured surface 316 and / or the structure 314 to keep the contoured surface 316 always in
contact with the flexible membrane 306 in the vicinity of the central region 310 Bring pre-load).
[0017]
In accordance with the present disclosure, the particular nature, configuration or arrangement of
the electrodes associated with the cMUT 300 not shown or shown separately in FIG. 3 is not
necessarily important. As such, any form or method of electrode configuration improvement or
optimization that is generally applicable to cMUTs may be particularly applied to cMUT 300.
[0018]
As shown in FIG. 3, the structure 314, which is included as part of the cMUT 300, is positioned
approximately at the center of the pocket 304 and at that location a post 326 extending
upwardly in the direction of the flexible membrane 306, It includes a lower membrane 328
located within and in the pocket 304 and extending over the pocket 304, including above and
including the post 326. As described above and further described herein, the structure 314 and
the contoured surface 316 associated with this structure place the cMUT 300 in collapse mode
in an equilibrium position (eg, a bias voltage of zero (0) volts). The lower membrane 328 may be
substantially thicker and / or stiffer than the flexible membrane 306 to reference energy loss to
the substrate 302 (movement of the lower membrane 328 does not necessarily produce a radiant
pressure wave). According to an embodiment of the present disclosure, the length and thickness
of the flexible membrane 306 may be about 100 μm and 2 μm, respectively, and the lower
membrane 328 may be about 5 μm thick. The height of the top of the column 326 is set to a
dimension corresponding to the initial gap thickness (undeformed film) plus approximately 1.6
μm. Other combinations of the length and thickness of the flexible membrane 306, the thickness
of the lower membrane 328, and the height of the top of the pillars 326 and other dimensions
and / or combinations of related dimensions are possible, according to embodiments of the
present disclosure as well It may be used to achieve the enhancement effect of
[0019]
Further, in accordance with the exemplary embodiments of the present disclosure, the cMUT 300
is assembled using one or more of various processing and manufacturing techniques. For
04-05-2019
8
example, as depicted in FIGS. 4, 5, 6 and 7, one such method of assembling cMUT 300 is
discussed. An SOI wafer (SOI wafer) is used to manufacture a substrate having a double film
structure as shown in FIG. Another wafer is used to produce a substrate having a pillar structure
as shown in FIG. These two wafers are aligned and connected together to produce the structure
in FIG. The dual membrane structure substrate may be removed to provide the final structure as
shown in FIG.
[0020]
Applicants have performed modeling and simulations to compare the efficiency (k <2> eff) of the
cMUT 300 shown and described with respect to FIG. 3 to the efficiency of the conventional
collapsed cMUT 100 shown in FIG. FIG. 8 shows this comparison as a function of initial gap
thickness with a range of 0.5 to 1.3 μm (in these cases the column height adds 1.6 μm to the
initial gap thickness) ). The cMUT 300 exhibits a significant efficiency increase over all of the gap
thickness and can double in size for large gaps. The applicant further examined the change in
column height from the thickness of the initial gap to the thickness of the initial gap plus 1.6
μm, as shown in FIG. 9 (the thickness of the initial gap). Is 0.9 μm). A column height of 0.9 μm
(initial gap thickness) raises the lower membrane 328 just to the point of contact with the
flexible membrane 306, indicating a slight increase in the efficiency (over a small voltage range)
of the dual membrane structure , This increases as the height of the column goes up.
[0021]
A dual membrane structure is one way to achieve a cMUT with improved efficiency in
accordance with the present disclosure. Any process that produces a substrate shape such as this
dual film structure should have high efficiency. This improved efficiency should achieve both the
(reciprocal) transmit and receive functions of the cMUT 300.
[0022]
Applications well suited to devices such as the cMUT 300 include large arrays for medical
ultrasound systems. According to an exemplary embodiment of the present disclosure, the
medical ultrasound system may include one or more systems, such as the system 1000 depicted
in FIG. The system 1000 includes an array of cMUT devices in accordance with the present
disclosure including, but not necessarily limited to, the two cMUTs 300 shown. The above cMUT
04-05-2019
9
devices, including the cMUT 300 specifically shown, may be aggregated in an array, such as a
large 2D array, to provide the system 1000 with enhanced functionality and performance
characteristics consistent with the present disclosure. Large form factors are achieved as long as
the cMUT 300 is manufactured using conventional silicon processing. Additionally, drive
electronics may be integrated with the transducer of system 1000 in accordance with an
embodiment of the present disclosure.
[0023]
The disclosed apparatus, systems and methods are susceptible to many other variations and
alternative applications without departing from the spirit or scope of the present disclosure.
04-05-2019
10
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