Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
WSGR Docket No. 48396-713.601
TRENCHES FOR THE REDUCTION OF CROSS-TALK IN MUT ARRAYS
CROSS-REFERENCE
[001] The subject matter of this application is related to that of
International Patent Application
No. PCT/US2020/050374, filed September 11, 2020, the entire contents of which
are
incorporated herein by reference.
BACKGROUND
[002] Micromachined ultrasonic transducers (MUTs) offer great potential in
many fields,
including but not limited to medical imaging, air-coupled imaging, distance
monitoring,
fingerprint monitoring, non-destructive defect monitoring, and diagnosis. In
many of these
applications, there are more than one MUT acting in concert. For example, for
higher end
medical ultrasound imaging, it is reasonable to find systems with 1024, 2048,
or 4096 MUTs.
SUMMARY
[003] To operate properly, the MUTs are designed to transmit energy into the
acoustic medium
to which they are attached. Take the generalized example of a MUT array in
Fig. 1A. In this
case, the MUTs are represented by the movable diaphragms 101a, 101b, 101c
which are formed
in or on top of the substrate 100 by cavities 120a, 120b, and 120c. The
diaphragms 101a, 101b,
101c are coupled acoustically to the semi-infinite acoustic medium 200 at an
interface 110. The
acoustic medium 200 can be any substance, or a plurality of substances; common
media include
air, water, tissue, electrolytic gel, metal, silicone rubbers used as matching
layers to the body,
etc.
[004] During operation, the diaphragms 101a-101c are excited into motion,
primarily in the z-
direction. The excitation is generally created by a piezoelectric effect (for
piezoelectric MUTs
(pMUTs)) or a capacitive effect (for capacitive MUTs (cMUTs)). In both cases,
the motion of the
diaphragm creates pressure waves that transmit into the acoustic medium 200.
However, the
diaphragm motion also creates unwanted waves outside the acoustic medium 200.
The most
common unwanted waves are elastic compression waves that travel within and
through the
substrate 100, and interfacial waves that travel along the interface 110
between the substrate 100
and the acoustic medium 200, as well as other interfaces attached to the
substrate 100.
[005] All energy radiated outside the acoustic medium 200 is unwanted. Not
only is it wasted
power, but it can interfere with the MUT's functioning. For example, in
medical imaging, the
elastic compression waves will rebound off other surfaces and cause artifacts
such as a static
image over the medically relevant image formed from the reflected energy from
the acoustic
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medium 200. As another example, the interfacial waves that travel along the
interface 110 will
create cross-talk in medical imaging, creating a spot-lighting effect and
unwanted ghost images.
[006] A generalized example of a MUT array 210 is shown in Fig. 1B. The MUT
array 210
comprises a substrate 100 and a plurality of MUTs 101. The plurality of MUTs
101 are affixed
to a surface of the substrate. Each MUT comprises a moveable diaphragm as
shown in Fig. 1A.
In some embodiments, each of the MUTs 101 is a pMUT. In some embodiments, each
of the
MUTs 101 is a cMUT. The MUTs 101 may be arranged in a two-dimensional array
210 arranged
in orthogonal directions. That is, the MUTs 101 are be formed into a two-
dimensional MxN
array 210 with N columns and M rows of MUTs 101. The number of columns (N) and
the
number of rows (M) may be the same or different. In some instances, the array
210 may be
curved, e.g., to provide a wider angle of an object being imaged. In some
instances, the array
may offer different packing such as hexagonal packing, rather than the
standard square packing
displayed in Fig. 1B. In some instances, the array may asymmetrical, e.g., as
described in U.S.
Patent No. 10,656,007, the entire contents of which are incorporated herein by
reference.
[007] The present disclosure provides a novel solution to address the issue of
compression and
interfacial waves in MUT arrays and the cross-talk they create. Fig. 2
provides an example of
this cross-talk in a MUT array formed from a silicon substrate 100 coupled to
a water acoustic
medium 200. The diagonal ripples 220 represent traveling pressure waves. The
two dashed lines
230 represent the speed of sound of the water acoustic medium (approximately
1,480 m/s).
Ripples and high amplitude data 240 below these lines 230 typically represents
good acoustic
data. The data 250 above the two dashed lines 230 represent various forms of
cross-talk.
[008] Taking spatial and temporal Fourier transforms of the data in Fig. 2
yields the f-k plot in
Fig. 3. In Fig. 3 we can see that the cross-talk acoustic energy, circled with
a dashed line 300, is
distributed around 2,000 to 6,000 m/s. The longitudinal speed of sound in
silicon is
approximately 8,800 m/s, while the interfacial wave speed for Rayleigh and
Shear waves is
between 5,000 and 5,500 m/s. This suggests that the cross-talk energy may be
due to a
combination of interfacial and bulk waves.
[009] Using a MUT array like the one used to produce the output depicted in
Figs. 2 and 3 for
imaging a phantom produces a result like that of Fig. 4. Two artifacts are
clearly visible: (1) a
"spotlight" effect 420 in which the central part of the image is brighter than
the edges, and (2)
"ghost" images 430 of high reflection targets are apparent at the edges of the
image.
[010] The artifacts of such cross-talk energies are undesirable. We disclose
herein a general
technique for disrupting compressional and interfacial waves and significantly
reducing cross-
talk between MUTs.
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[OM In one aspect, disclosed herein is a MUT array comprising a substrate and
a plurality of
MUTs. The plurality of MUTs are affixed to a surface of the substrate. Each
MUT comprises a
moveable diaphragm. The substrate comprises a trench at least partially around
a perimeter of
the diaphragm of one or more MUTs of the plurality of MUTs. In some
embodiments, each
MUT in the plurality of MUTs is a pMUT. In some embodiments, each MUT in the
plurality of
MUTs is a cMUT,
[012] In some embodiments, the trench runs from the surface of the substrate
to at least 10%,
at least 50%, or at least 90% the thickness of the substrate. In some
embodiments, the trench
runs the entire thickness of the substrate.
[013] In some embodiments, the trench runs from below the surface of the
substrate to at least
10%, at least 50%, or at least 90% the thickness of the substrate.
[014] In some embodiments, the trench runs from the surface opposite 110 up
through the
substrate at least 10%, at least 50%, or at least 90% of the thickness of the
substrate.
[015] In some embodiments, the trench has a constant width between 1 gm and 40
pm. In
some embodiments, the trench has a variable width between 1 pm and 40 p.m.
[016] In some embodiments, the trench has a constant distance from the
perimeter of the
diaphragm between 1 pm and 40 p.m. In some embodiments, the trench has a
variable distance
from the perimeter of the diaphragm between 1 1.1n1 and 40 11111.
[017] In some embodiments, the trench is around at least 50%, 60%, 70%, 80%,
or 90% of the
perimeter of the diaphragm. In some embodiments, the trench is around the
entire perimeter of
the diaphragm.
[018] In some embodiments, the trench is at least partially around a perimeter
of the diaphragm
of at least 10%, at least 20%, at least 30%, at least 40%, at least 50%, at
least 60%, at least 70%,
at least 80%, or at least 90% of the MUTs of the plurality of MUTs. In some
embodiments, the
trench is at least partially around a perimeter of the diaphragm of each MUT
of the plurality of
MUTs.
[019] In some embodiments, the trench is at least partially filled with an
acoustic attenuation
material.
[020] In some embodiments, the plurality of MUTs are arranged in a plurality
of columns and a
plurality of rows. In some embodiments, the trench runs along a row of MUTs.
In some
embodiments, each row of MUTs has a trench running therealong. In some
embodiments, the
trench runs along a column of MUTs. In some embodiments, each column of MUTs
has a trench
running therealong. In some embodiments, each row of MUTs has a first trench
running
therealong and each column of MUTs has a second trench running therealong.
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[021] In some embodiments, the trench is at least partially around a perimeter
of the diaphragm
of a single MUT of the plurality of MUTs.
[022] In some embodiments, each MUT of the plurality of MUTs is at least
partially
surrounded by a trench.
[023] In some embodiments, the MUT array further comprises at least a second
trench at least
partially around the perimeter of the diaphragm of one or more MUTs of the
plurality of MUTs.
In some embodiments, the second trench is at least partially around the
perimeter of the
diaphragm of a single MUT of the plurality of MUTs. In some embodiments, each
MUT of the
plurality of MUTs is at least partially surrounded by a first trench and a
second trench.
[024] In some embodiments, the trench is disposed between an adjacent pair of
MUTs.
10251 In some embodiments, the substrate comprises a plurality of trenches at
least partially
around the perimeter of the diaphragm of one or more MUTs of the plurality of
MUTs. In some
embodiments, the substrate comprises one trench per MUT of the plurality of
MUTs. In some
embodiments, the substrate comprises one trench per adjacent pair of MUTs of
the plurality of
MUTs. In some embodiments, the substrate comprises fewer than one trench per
MUT of the
plurality of MUTs. In some embodiments, the substrate comprises fewer than one
trench per
adjacent pair of MUTs of the plurality of MUTs. In some embodiments, the
substrate comprises
more than one trench per MUT of the plurality of MUTs. In some embodiments,
the substrate
comprises more than one trench per adjacent pair of MUTs of the plurality of
MUTs.
[026] In some embodiments, the MUT array is configured for medical imaging.
[027] In another aspect, disclosed herein is a method of fabricating a MUT
array.
INCORPORATION BY REFERENCE
[028] All publications, patents, and patent applications mentioned in this
specification are
herein incorporated by reference to the same extent as if each individual
publication, patent, or
patent application was specifically and individually indicated to be
incorporated by reference.
BRIEF DESCRIPTION OF THE DRAWINGS
[029] A better understanding of the features and advantages of the present
subject matter will
be obtained by reference to the following detailed description that sets forth
illustrative
embodiments and the accompanying drawings of which:
[030] Fig. IA is a schematic diagram showing a cross-section of a generalized
MUT array
(101a-101c) attached to an acoustic medium 200, in accordance with
embodiments.
[031] Fig. 1B shows a top view of a MUT array 210, in accordance with
embodiments.
[032] Fig. 1C is a bock diagram of an imaging device 105, in accordance with
embodiments.
[033] Fig. 1D shows a top view of a MUT, in accordance with embodiments.
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[034] Fig. lE shows a cross-sectional view of a MUT, taken along a direction 4-
4 in Fig. 1D,
in accordance with embodiments.
[035] Fig. 2 is a graph showing amplitude of motion of a MUT array with 128
elements in the
azimuth direction, spanning approximately 22 mm, in accordance with
embodiments. The center
two MUTs were actuated, and the other 126 MUTs were monitored for response.
The grey level
indicates positive (towards white) or negative (towards black) diaphragm
deflection. The two
fired elements were eliminated from the plot so that the cross-talk ripples
could be visualized.
The dashed lines 230 approximately represent the imaging cone, defined by wave
with a 1,480
m/s velocity.
[036] Fig. 3 is a graph showing a Fourier transform in space and time (also
referred to as an f-k
plot).of the data from Fig. 2, representing the data in spatial and frequency
domains, in
accordance with embodiments. The amplitude is plotted in dB relative to the
maximum
amplitude of the Fourier data, with white data having a higher amplitude black
blue data. The
data 300 circled in between 2 and 4 MHz and 0.5 and 1.5 [tsec is undesired
cross-talk.
[037] Fig. 4 is an ultrasound image taken with a MUT array similar to that of
Figs. 2 and 3, in
accordance with embodiments. The "spotlight" effect is highlighted by the two
arrows 420,
while the "ghosting" artifacts are circled 430.
10381 Figs. 5A and 5B are exemplary schematic diagrams showing (a) layout and
(b) cross-
section views of a generalized MUT array (101a-101c) with diaphragm-side cross-
talk trenches
103, in accordance with embodiments.
[039] Figs. 6A and 6B are exemplary schematic diagrams showing (a) layout and
(b) cross-
section views of a generalized MUT array (101a-101c) with buried cross-talk
trenches 104, in
accordance with embodiments.
[040] Figs. 7A and 7B are exemplary schematic diagrams showing (a) layout and
(b) cross-
section views of a generalized MUT array (101a-101c) with cavity-side cross-
talk trenches 105,
in accordance with embodiments.
[041] Figs. 8A and 8B are exemplary schematic diagrams showing (a) layout and
(b) cross-
section views of a generalized MUT array (101a-101c) with both diaphragm-side
103 and
cavity-side 105 cross-talk trenches, in accordance with embodiments.
[042] Figs. 9A to 90 are cross-sectional views of a capacitive MUT with buried
cavity with (a)
diaphragm-side cross-talk trenches 103, (b) buried cross-talk trenches 104,
(c) cavity side cross-
talk trenches 105, and (d) both cavity side 105 and diaphragm side 103 cross
talk trenches, in
accordance with embodiments.
[043] Fig. 10 is a graph showing a simulated attenuation of cross-talk
trenches 103 versus
trench depth for multiple substrate thicknesses, in accordance with
embodiments. The y-axis is
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the maximum velocity of the element adjacent to the element being actuated,
divided by the
maximum velocity of the actuated element, in dB.
[044] Figs. 11A and 11B show a comparison of f-k and images of MUT arrays (b)
with and (a)
without cross-talk trenches (75 gm deep in 150 gm silicon substrate), in
accordance with
embodiments.
[045] Figs. 12A and 12B show Azimuthal response at 3.50 MHz, 3dB shift for 50
gm and 25
gm deep cavity-side trenches, respectively, at a variety of stand-offs and
widths, in accordance
with embodiments.
DETAILED DESCRIPTION
[046] Described herein, in certain embodiments, are micromachined ultrasound
transducer
(MUT) arrays.
[047] In one aspect, disclosed herein is a MUT array comprising a substrate
and a plurality of
MUTs. The plurality of MUTs are affixed to a surface of the substrate. Each
MUT comprises a
moveable diaphragm. The substrate comprises a trench at least partially around
a perimeter of
the diaphragm of one or more MUTs of the plurality of MUTs. In some
embodiments, each
MUT in the plurality of MUTs a pMUT, In some embodiments, each MUT in the
plurality of
MUTs a cMUT,
[048] In some embodiments, the trench runs from the surface of the substrate
to at least 10%,
at least 50%, or at least 90% the thickness of the substrate. In some
embodiments, the trench
runs the entire thickness of the substrate.
[049] In some embodiments, the trench runs from an opposite surface of the
substrate to at
least 10%, at least 50%, or at least 90% the thickness of the substrate.
[050] In some embodiments, the trench runs from below the surface of the
substrate to at least
10%, at least 50%, or at least 90% the thickness of the substrate.
10511 In some embodiments, the trench has a constant width between 1 gm and 40
gm. In
some embodiments, the trench has a variable width between 1 gm and 40 gm.
[052] In some embodiments, the trench has a constant distance from the
perimeter of the
diaphragm between 1 gm and 40 p,M. In some embodiments, the trench has a
variable distance
from the perimeter of the diaphragm between 1 gm and 40 gm.
[053] In some embodiments, the trench is around at least 50%, 70%, or 90% of
the perimeter
of the diaphragm. In some embodiments, the trench is around the entire
perimeter of the
diaphragm.
[054] In some embodiments, the trench is at least partially around a perimeter
of the diaphragm
of at least 10%, at least 20%, at least 30%, at least 40%, at least 50%, at
least 60%, at least 70%,
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at least 80%, or at least 90% of the MUTs of the plurality of MUTs In some
embodiments, the
trench is at least partially around a perimeter of the diaphragm of each MUT
of the plurality of
MUTs.
[055] In some embodiments, the trench is at least partially filled with an
acoustic attenuation
material.
[056] In some embodiments, the MUT array is configured for medical imaging.
[057] Also described herein, in certain embodiments, are methods of
fabricating a
micromachined ultrasound transducer (MUT) array.
Certain definitions
[058] Unless otherwise defined, all technical terms used herein have the same
meaning as
commonly understood by one of ordinary skill in the art to which this
invention belongs. As
used in this specification and the appended claims, the singular forms "a,"
"an," and "the"
include plural references unless the context clearly dictates otherwise. Any
reference to "or"
herein is intended to encompass "and/or" unless otherwise stated.
MUTs
[059] The present disclosure may be utilized in the context of imaging devices
that utilize
micromachined ultrasound transducer (MUT) technology, including either
piezoelectric
micromachined ultrasound transducer (pMUT) or capacitive micromachine
ultrasonic transducer
(cMUT) technologies.
[060] Fig. 1C is a block diagram of an imaging device 105 with selectively
alterable channels
106, 108, controlled by a controller 109, and having imaging computations
performed on a
computing device 110 according to principles described herein The imaging
device 105 may be
used to generate an image of internal tissue, bones, blood flow, or organs of
human or animal
bodies. Accordingly, the imaging device 105 transmits a signal into the body
and receives a
reflected signal from the body part being imaged. Such imaging devices may
include either
pMUT or cMUT, which may be referred to as transceivers or imagers, which may
be based on
photo-acoustic or ultrasonic effects. The imaging device 105 can be used to
image other objects
as well. For example, the imaging device 105 can be used in medical imaging;
flow
measurements in pipes, speaker, and microphone arrays; lithotripsy, localized
tissue heating for
therapeutic; and highly intensive focused ultrasound (I-11FU) surgery.
[061] In addition to use with human patients, the imaging device 105 may be
used to get an
image of internal organs of an animal as well. Moreover, in addition to
imaging internal organs,
the imaging device 105 may also be used to determine direction and velocity of
blood flow in
arteries and veins as in Doppler mode imaging and may also be used to measure
tissue stiffness.
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[062] The imaging device 105 may be used to perform different types of imaging
For
example, the imaging device 105 may be used to perform one dimensional
imaging, also known
as A-Scan, two dimensional imaging, also known as B scan, three dimensional
imaging, also
known as C scan, and Doppler imaging. The imaging device 105 may be switched
to different
imaging modes and electronically configured under program control.
[063] To facilitate such imaging, the imaging device 105 includes an array of
pMUT or cMUT
transducers 210, each transducer 210 including an array of transducer elements
(i.e., MUTs)
101. The MUTs 101 operate to 1) generate the pressure waves that are passed
through the body
or other mass and 2) receive reflected waves off the object within the body,
or other mass, to be
imaged. In some examples, the imaging device 105 may be configured to
simultaneously
transmit and receive ultrasonic waveforms. For example, certain MUTs 101 may
send pressure
waves toward the target object being imaged while other MUTs 101 receive the
pressure waves
reflected from the target object and develop electrical charges in response to
the received waves.
[064] Fig. 113 shows a top view of an exemplary MUT 400 (in this example, a
pMUT). Fig.
1E shows a cross-sectional view of the MUT 400 in Fig. 1D, taken along the
line 4-4, according
to embodiments of the present disclosure. The MUT 400 may be substantially
similar to the
MUT 101 described herein. As depicted, the MUT may include. a membrane layer
406
suspended from a substrate 402 and disposed over a cavity 404; a bottom
electrode
(0) 408 disposed on the membrane layer (or, shortly membrane) 406; a
piezoelectric layer 410
disposed on the bottom electrode (0) 408; and a top electrode (X) 412 disposed
on the
piezoelectric layer 410.
[065] MUTs, whether cMUTs or pMUTs, can be efficiently formed on a substrate
leveraging
various semiconductor wafer manufacturing operations. Semiconductor wafers may
come in 6
inch, 8 inch, and 12 inch sizes and are capable of housing hundreds of
transducer arrays. These
semiconductor wafers start as a silicon substrate on which various processing
steps are
performed. An example of such an operation is the formation of Si02 layers,
also known as
insulating oxides. Various other steps such as the addition of metal layers to
serve as
interconnects and bond pads are performed to allow connection to other
electronics. Yet another
example of a machine operation is the etching of cavities (e.g., cavity 404 in
Fig. 1E) in the
substrate.
[066] To significantly reduce cross-talk, according to the present disclosure,
during the wafer
manufacturing process, a trench 103 is founed within the substrate 100 that
roughly surrounds
each MUT 101. An example of this is depicted in Figs. 5A and 5B. Fig. 5A shows
an exemplary
schematic diagram showing a generalized MUT array (101a-101c) with diaphragm-
side cross-
talk trenches 103. Fig. 5B shows a cross-sectional view of the MUT array in
Fig. 5A, taken
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along line A-A'. The MUT array may be substantially similar to the array
depicted in Figs. 1A-
1C with the addition of one or more trenches 103. The MUT array 210 comprises
a substrate
100 and a plurality of MUTs 101a-101c. The plurality of MUTs 101a-101c are
affixed to a
surface of the substrate 100. Each MUT 101a-101c comprises a moveable
diaphragm. The
substrate 100 comprises a trench 103 at least partially around a perimeter of
the diaphragm
101a-101c of the plurality of MUTs. The trench 103 introduces an impedance
mismatch
between the substrate 100 and whatever material is within the cross-talk
trenches 103. This
impedance mismatch disrupts the cross-talk waves through attenuation,
reflection, and
scattering.
[067] The location of the cross-talk trenches 103 will impact the amount of
attenuation
provided by the trenches. The velocity data from Fig. 3 suggests that
interfacial waves at the
interface 110 account for some of the cross-talk energy. Interfacial waves,
such as Rayleigh
waves, typically affect the interface region and the material within some
distance of the interface
characterized by the wavelength of the travelling wave. In this case, a trench
103 attached to the
surface of the substrate 100 and intersecting the interface 110 will be
optimal compared to a
buried cross-talk trench 104 that is not near the interface 110, such as those
depicted in Figs. 6A
and 6B.
10681 Figs. 7A and 7B are exemplary schematic diagrams showing (a) layout and
(b) cross-
section views of a generalized MUT array (101a-101c) with cavity-side cross-
talk trenches 105.
In some embodiments, the substrate 100 comprises a cavity-side trench 105 at
least partially
around a perimeter of the cavity 120a-120c of the plurality of MUTs. The
trench 105 introduces
an impedance mismatch between the substrate 100 and whatever material is
within the cross-talk
trenches 105. This impedance mismatch disrupts the cross-talk waves through
attenuation,
reflection, and scattering. The cavity-side trenches 105 are effective in
reducing cross-talk
velocity by increasing elastic wave pathlength.
[069] Figs. 8A and 8B are exemplary schematic diagrams showing (a) layout and
(b) cross-
section views of a generalized MUT array (101a-101c) with both diaphragm-side
103 and
cavity-side 105 cross-talk trenches. In some embodiments, the substrate 100
comprises both
diaphragm-side trenches 103 and cavity-side trenches 105. In at least some
instances, the
combination of diaphragm-side trenches 103 and cavity-side trenches 105 can
reduce cross-talk
effects (e.g., by lowering velocity and/or lowering amplitude of the cross-
talk effects) beyond
that of either trench type alone.
10701 In some embodiments, the substrate 100 comprises diaphragm-side trenches
103, buried
trenches 104, and cavity-side trenches 105.
10711 Figs. 9A to 9D are cross-sectional views of a capacitive MUT with buried
cavity with (a)
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diaphragm-side cross-talk trenches 103, (b) buried cross-talk trenches 104,
(c) cavity side cross-
talk trenches 105, and (d) both cavity side 105 and diaphragm side 103 cross
talk trenches. The
MUT array 210 comprises a substrate 100 and a plurality of MUTs 101a-101c. The
plurality of
MUTs 101a-101c are affixed to a surface of the substrate 100. Each MUT 101a-
101c comprises
a moveable diaphragm formed in or on top of the substrate 100 by buried
cavities 130a, 130b,
and 130c. The substrate 100 comprises a trench 103, 104, and/or 105 at least
partially around a
perimeter of the diaphragm 101a-101c of the plurality of MUTs. The trench 103,
104, and/or
105 introduces an impedance mismatch between the substrate 100 and whatever
material is
within the cross-talk trenches 103, 104, and/or 105. This impedance mismatch
disrupts the
cross-talk waves through attenuation, reflection, and scattering.
10721 In some embodiments, a trench 103, 104, or 105 is formed via deep
reactive ion etching
(DRIE), plasma etching, wet etching, or other etching techniques which will be
apparent to one
of ordinary skill in the art based on the teachings here.
[073] In some embodiments, a trench 103, 104, or 105 has a constant distance
from the
perimeter of the diaphragm or cavity between 1 p.m and 40 p.m (e.g., between
10 pm and 40
pm). Alternatively, or in combination, in some embodiments the trench 103,
104, or 105 has a
variable distance from the perimeter of the diaphragm or cavity between 1 pm
and 40 pm (e.g.,
between 10 p.m and 40 p.m). The distance of trench 103, 104, or 105 from the
perimeter of the
diaphragm or cavity can be as close (e.g., atomically close) or as far as
desired.
[074] In some embodiments, a trench 103, 104, or 105 is around at least 50%,
70%, or 90% of
the perimeter of the diaphragm or cavity. In some embodiments, a trench 103,
104, or 105 is
around the entire perimeter of the diaphragm or cavity.
[075] In some embodiments, a trench 103, 104, or 105 is at least partially
around a perimeter of
the diaphragm or cavity of at least 10%, at least 20%, at least 30%, at least
40%, at least 50%, at
least 60%, at least 70%, at least 80%, or at least 90% of the MUTs of the
plurality of MUTs. In
some embodiments, a trench 103, 104, or 105 is at least partially around a
perimeter of the
diaphragm or cavity of each MUT of the plurality of MUTs.
[076] In some embodiments, a trench 103, 104, or 105 is at least partially
around a perimeter of
the diaphragm or cavity on a first lateral side of the MUT. Optionally, a
second trench 103, 104,
or 105 is at least partially around a perimeter of the diaphragm or cavity on
a second lateral side
of the MUT. In some embodiments, the MUT has trenches 103, 104, or 105 on both
lateral sides
thereof. In some embodiments, the trenches 103, 104, or 105 are symmetrically
disposed around
the perimeter of the diaphragm or cavity. In some embodiments, the trenches
103, 104, or 105
are asymmetrically disposed around the perimeter of the diaphragm or cavity.
In some
embodiments, the MUT has a trench 103, 104, or 105 on a single lateral side
thereof
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[077] The depth of the cross-talk trenches 103, 104, or 105 will impact the
attenuation, as
illustrated in Fig. 10. Fig. 10 is a graph showing a simulated attenuation of
cross-talk trenches
103 versus trench depth (ranging from 0-55pm) for multiple substrate
thicknesses (75 p.m or 150
um). The y-axis is the maximum velocity of the element adjacent to the element
being actuated,
divided by the maximum velocity of the actuated element, in dB. In this case,
simulations show
that deeper cross-talk trenches 103 are more effective than shallow trenches.
This is because
both interfacial and longitudinal waves have a vertical spatial extent.
Trenches with a greater
depth result in a larger proportion of the cross-talk waves disrupted by the
trench.
[078] In some embodiments, a trench 103, 104, or 105 runs from the surface
(diaphragm-side
or cavity-side) of the substrate to at least 10%, at least 50%, or at least
90% the thickness of the
substrate. In some embodiments, the trench runs the entire thickness (e.g.,
100%) of the
substrate. In some embodiments, the trench runs from the surface (diaphragm-
side or cavity-
side) of the substrate to about 1% the thickness of the substrate.
[079] In some embodiments, a trench 103, 104, or 105 runs from below the
surface
(diaphragm-side or cavity-side) of the substrate to at least 10%, at least
50%, or at least 90% the
thickness of the substrate.
[080] Finally, the trench lateral width will also affect the attenuating
properties of the cross-
talk trenches 103, 104, or 105, particularly if the trenches are filled with a
high attenuation
material. Larger lateral dimensions and/or greater number of trenches produces
better cross-talk
attenuation. In most common MUT arrays, the lateral width of the cross-talk
trenches 103 and
104 will be limited by packing densities of the MUTs.
[081] In some embodiments, a trench 103, 104, or 105 has a constant width
between 1 pm and
40 pm. In some embodiments, a trench 103, 104, or 105 has a constant width
between 1 pm and
100 p.m. In some embodiments, a trench 103, 104, or 105 has a constant width
between 5 p.m
and 10 p.m. The width of trench 103, 104, or 105 can be as thin (e.g.,
atomically thin) or as large
as desired.
[082] In some embodiments, a trench 103, 104, or 105 has a variable width
between 1 um and
40 p.m. In some embodiments, a trench 103, 104, or 105 has a variable width
between 1 pm and
100 um. In some embodiments, a trench 103, 104, or 105 has a variable width
between 5 um
and 10 um. The width of trench 103, 104, or 105 can be as thin (e.g.,
atomically thin) or as large
as desired.
[083] In some embodiments, a trench 103, 104, or 105 is at least partially
filled with an
acoustic attenuation material. Alternatively, or in combination, in some
embodiments a trench
103, 104, or 105 is at least partially filled with an acoustic attenuation
material.
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[084] In some embodiments, there is one trench 103, 104, or 105 per MUT. In
some
embodiments, there is one trench 103, 104, or 105 per adjacent pair of MUTs.
In some
embodiments, trenches 103, 104, or 105 can intersect and run between MUTs as
one continuous
trench. In some embodiments, there is fewer than one trench 103, 104, or 105
per pair of MUTs.
In some embodiments, there is more than one trench 103, 104, or 105 per MUT.
[085] In some embodiments, there is more than one trench 103, 104, or 105 per
MUT. For
example, each MUT has a first proximal trench on a first lateral side of the
MUT and a second
proximal trench on a second lateral side of the MUT, such that each adjacent
pair of MUTs has
at least two trenches between them. The proximal trench on the first lateral
side of a first MUT
and the proximal trench on the second lateral side of a second MUT may be
disposed in the
substrate between the first and second MUTs. In some embodiments, a central
trench may be
disposed between the proximal trenches, for a total of at least three trenches
between the
adjacent first and second MUTs. In some embodiments, the proximal and central
trenches are
formed in the same surface (diaphragm-side or cavity-side) of the substrate.
In some
embodiments, the proximal and central trenches are formed in different
surfaces of the substrate.
For example, the proximal trenches 103 may be formed in the diaphragm-side and
the central
trench 105 may be formed in the cavity-side as shown in Figs. 8B and 9D. In at
least some
instances, the combination of proximal diaphragm-side trenches 103 and central
cavity-side
trenches 105 can reduce cross-talk effects (e.g., by lowering velocity and/or
lowering amplitude
of the cross-talk effects) beyond that of either trench type alone.
[086] In some embodiments, all trenches 103, 104, or 105 of the MUT array have
the same
dimensions. In some embodiments, one or more of the trenches 103, 104, or 105
has different
dimensions. For example, the central trench 105 shown in Figs. 8B and 9D may
be wider than
the proximal trenches 103.
10871 In some embodiments, there is one trench 103, 104, or 105 per MUT
surrounding at least
80% of the MUT. Alternatively, or in combination, there is one trench 103,
104, or 105 down
each row of MUTs. Alternatively, or in combination, there is one trench 103,
104, or 105 down
each column of MUTs. Alternatively, or in combination, there is one trench
103, 104, or 105
down each row of MUTs and one trench 103, 104, or 105 down each column of
MUTs.
Alternatively, or in combination, there are multiple trenches 103, 104, or 105
around each MUT
[088] The efficacy of such trenches has been demonstrated in silicon MUT
arrays with 150 lam
substrates. Fig. 11A shows f-k and images of an MUT array 101a-101c without
cross-talk
trenches. Fig. 11B shows f-k and images of an MUT array 101a-101c with cross-
talk trenches
103 (75 gm deep in 150 gm silicon substrate). As Figs. 11A and 11B show, the
cross-talk
trenches 103 effectively reduce the cross-talk energy and remove the
"spotlight" 420 and
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ghosting 430 artifacts associated with cross-talk.
[089] Method of manufacture for pMUT with trenches
[090] An exemplary method of manufacture for a pMUT with trenches, such as the
pMUTs
shown by Figs. 5A to 8B is now described.
[091] (a) First, a substrate (e.g., substrate 402 or 100), typically single
crystal silicon, is
provided.
[092] (b) If desired, buried cross-talk trenches 104 may be patterned and
etched in the substrate
to generate a "handle" wafer. Another silicon "device" wafer may be thermally
oxidize and then
fusion bonded to the "handle" to form the buried trenches 104 therebetween
(e.g., as shown in
Figs. 6A and 6B). The "device" wafer may be ground and polished to the desired
diaphragm
thickness.
[093] (c) An insulating layer can then be deposited over the substrate. The
insulating layer is
typically some form of SiO2, about 0.1 um to 3 um thick. It is commonly
deposited via thermal
oxidation, PECVD deposition, or other technique.
[094] (d) A first metal layer 408 (also referred to as M1 or metal 1) can then
be deposited.
Typically, this is a combination of films that adhere to the substrate,
prevent diffusion of the
piezoelectric, aid the piezoelectric in structured deposition/growth, and
which is conductive.
SRO (SrRu03) may be used for structured film growth, on top of Pt for a
diffusion barrier and
conduction, on top of Ti as an adhesive layer (for Pt to SiO2). Usually, these
layers are thin, less
than 200 nm, with some films 10 to 40 nm. Stress, manufacturing, and cost
issues will usually
limit this stack to less than 1 um. The conductor (Pt) is typically thicker
than the structuring
layer (SRO) and adhesion layer (Ti). Other common structuring layers, rather
than SRO, include
(La0.5Sr0.5)Co03, (La0.5Sr0.5)Mn03, LaNi03, RuO2, Ir02, BaPb03, to name a few.
Pt can
be replaced with other conductive materials such as Cu, Cr, Ni, Ag, Al, Mo, W,
and NiCr. These
other materials usually have disadvantages such as poor diffusion barrier,
brittleness, or adverse
adhesion, and Pt is the most common conductor used. The adhesion layer, Ti,
can be replaced
with any common adhesion layers such as TiW, TiN, Cr, Ni, Cr, etc.
10951 (e) A piezoelectric material 410 can then be deposited. Some common
examples of
suitable piezoelectric materials include: PZT, KNN, PZT-N, PMN-Pt, AlN, Sc-
A1N, ZnO,
PVDF, and LiNi03. The thicknesses of the piezoelectric layer may vary between
100 nm and 5
or possibly more.
[096] (f) A second metal layer 412 (also referred to as M2 or metal 2) can
then be deposited.
This second metal layer 412 may be similar to the first metal layer 408 and
may serve similar
purposes. For M2, the same stack as M1 may be used, but in reverse: Ti for
adhesion on top of
Pt to prevent diffusion on top of SRO for structure.
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[097] (g) The second metal layer or M2 412 may then be patterned and etched,
stopping on the
piezoelectric layer. Etches can be made in many ways herein, for example, via
RIE (reactive ion
etching), ion mill, wet chemical etching, isotropic gas etching, etc. After
patterning and etching,
the photoresistor used to pattern M2 may be stripped, via wet and/or dry
etching. In many
embodiments for manufacturing cMUTs and pMUTs described herein, any number of
ways of
etching may be used, and the photoresist is typically stripped after most
pattern and etch steps.
[098] (h) The piezoelectric layer may then be similarly patterned and etched,
stopping at the
first metal layer or M1 408. Typically, wet, RIE, and/or ion mill etches are
used.
[099] (i) The first metal layer or M1 408 may then be similarly patterned and
etched, stopping
on the dielectric insulating layer.
1010011 (j) If desired, one or both of the following may be added:
(1) An H2 barrier. H2 diffusion into the piezoelectric layer can limit its
lifetime.
To prevent this, an H2 barrier can be used. 40 nm of ALD (atomic layer
deposition)
aluminum oxide (A1203) may be used to accomplish this. Other suitable
materials may
include SiC, diamond-like carbon, etc.
(2) A redistribution layer (RDL). This layer can provide connectivity between
M1 and M2 and other connections (e.g., wirebonds, bump bonds, etc.). An RDL
can be
formed by first adding a dielectric such as oxide, etching vias in the
dielectric, depositing
a conductor (typically Al), and finally patterning the conductor.
Additionally, one might
add a passivation layer (typically oxide + nitride) to prevent physical
scratches,
accidental shorting, and/or moisture ingress.
101011 (k) The diaphragm-side trenches 103 may be patterned and etched (e.g.,
as shown in
Figs. 5A, 5B, 8A, and 8B). The dielectric layer may be etched via RIF or wet
etching. The
substrate 100 is typically silicon, and may be etched typically via DRIE (deep
reactive ion
etching).
[0102] (1) Frequently, a silicon on insulator (SOT) substrate is used. In this
case, there is a buried
insulator layer or buried oxide (BOX)layer just below the diaphragm 101. The
diaphragm is
then composed of the "device" layer (layer above the BOX), and the "handle"
layer under the
BOX layer. The cavity in the device layer may stop on the BOX and may be
etched out of the
Handle layer. In this case, the trench etch may include two extra steps: (1)
etching the BOX
(typically via dry RIE etching, or in some cases, via wet etching) after the
device layer is etched
via DRIE, and (2) etching the handle layer via DRIE to the desired depth. Most
SOI wafers are
silicon, meaning that the device and handle layers will typically be single
crystal silicon. The
insulator BOX, in this case, is typically a silicon dioxide thermally grown,
which is called a
"buried oxide", which is where the term "BOX" comes from. A silicon SOT wafer
with single
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crystal silicon handle and device layers with an oxide BOX may typically be
used. The device
layer may be 5 ?Am, but typically varies between 100 nm and 100 [tm, while the
handle layer
thickness typically varies between 100 lam and 1000iLtm. The BOX is typically
between 100 nm
and 5 m, but 1 [tm may be used, in many cases.
101031 (m) If desired, the backside of the wafer or handle can be thinned via
grinding and
optionally polished at this point. In many embodiments, the handle layer is
thinned from 500 p.m
to 300 1.tm thick. Common thicknesses typically vary between 50 [im and 1000
tim.
101041 (n) The cavity-side trenches 105 may be patterned and etched (e.g., as
shown in Figs. 7A
to 8B). The backside of the substrate 100 may be etched typically via DRIE
(deep reactive ion
etching).
101051 (o) The cavity may be patterned on the backside of the wafer or Handle,
and the cavity
may be etched. Typically, the wafer/handle is composed of silicon, and the
etch is accomplished
with DRIE. The etch can be timed. The cavity may be etched at the same time as
the cavity-side
trench 105. The etch may stop selectively on the BOX. The cavity can be etched
via other
techniques such as KOH, TMAH, HNA, and REE. The wafer can be considered
complete after
photoresist strip.
101061 It will be understood by one of ordinary skill in the art based on the
teachings herein that
other processes may be used to achieve similar end results.
101071 Method of manufacture for cMUT with trenches
101081 An exemplary method of manufacture for a cMUT with trenches, such as
the cMUTs
shown by Figs. 9A to 9D is now described.
101091 (a) First, a substrate (e.g., substrate 402 or 100), typically single
crystal silicon, is
provided.
101101 (b) The substrate may then be thermally oxidized.
101111 (c) The cavities 130a, 130b, 130c may be patterned and etched in the
oxide to generate a
"handle" wafer. This is typically accomplished through a plasma etch of the
oxide or a wet etch
(e.g., HF).
101121 (d) If desired, the buried cross-talk trenches 104 may be patterned and
etched in the
oxide of the "handle" wafer. This is typically accomplished through a plasma
etch of the oxide
or a wet etch (e.g., HF).
101131 (e) A silicon "device" wafer may then be fusion bonded to the patterned
oxide "handle"
wafer. If desired, the "device" wafer may be patterned and etched (e.g., via
DRIE) to correspond
to the buried trenches 104 in the "handle" wafer prior to fusion bonding, such
that fusion
bonding of the "handle" and the "device" wafers forms the buried trenches 104
(e.g., as shown
in Fig. 9B).
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101141 (f) The "device" wafer may be ground and polished to the desired
diaphragm thickness.
101151 (g) The diaphragm-side trenches 103 may be patterned and etched (e.g.,
as shown in Fig.
9A and 9D) into the diaphragm side of the ground and polished wafer.
101161 (h) The cavity-side trenches 105 may be patterned and etched (e.g., as
shown in Fig. 9C
and 9D) into the cavity side of the ground and polished wafer.
101171 It will be understood by one of ordinary skill in the art based on the
teachings herein that
other processes may be used to achieve similar end results.
EXAMPLES
101181 The following illustrative examples are representative of embodiments
of the software
applications, systems, and methods described herein and are not meant to be
limiting in any
way.
Example 1 ¨ Cavity-side trench Azimuthal responses
101191 Test pMUT wafers were fabricated with variable depth (25 Jim, 37.5 gm,
and 50 gm on a
75 gm thick pMUT) and standoff distance (10 gm, 15 gm, 20 gm, and 25 gm) of
the trench
from the cavity, and width of the of the trench (5 gm and 10 gm). The
Azimuthal response of the
pMUTs at various frequencies was measured. Figs. 12A and 12B show Azimuthal
response at
3.50 MHz, 3dB shift for 50 gm and 25 gm deep cavity-side trenches,
respectively, at a variety of
stand-offs and widths. The naming convention "XX-YYW" is such that XX = the
standoff
distance and YY = the width. Thus, a 20-05W is a trench 20 gm from the cavity,
and 5 gm
width. As shown in Fig. 12A, the spotlight angle for a 50 gm deep cavity-side
trench appears to
push further out with increasing trench standoff distance. As shown in Fig.
12B, the spotlight
angle for a 25 gm deep cavity-side trench does not have a linear correlation
with trench standoff
distance, but the general trend is that increasing standoff distance results
in narrower spotlight
angle. Cross-talk dip occurred at 3.5 MHz, particularly in 20-10W (20 gm
standoff, 10 gm
width) at 25 gm depth at about +/- 28 degrees.
101201 While preferred embodiments of the present invention have been shown
and described
herein, it will be obvious to those skilled in the art that such embodiments
are provided by way
of example only. Numerous variations, changes, and substitutions will now
occur to those
skilled in the art without departing from the invention. It should be
understood that various
alternatives to the embodiments of the invention described herein may be
employed in practicing
the invention.
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