METHOD FOR FORMING AN ULTRASONIC TRANSDUCER, AND ASSOCIATED
APPARATUS
BACKGROUND OF THE DISCLOSURE
Field of the Disclosure
Aspects of the present disclosure relate to ultrasonic transducers, and, more particularly, to a
method of forming a piezoelectric micromachined ultrasonic transducer defining an air-backed
cavity, and an associated apparatus.
Description of Related Art
Some micromachined ultrasonic transducers (MUTs) may be configured, for example, as a
piezoelectric micromachined ultrasonic transducer (pMUT) disclosed in U.S. Patent No. 7,449,821
assigned to Research Triangle Institute, also the assignee of the present disclosure, which is also
incorporated herein in its entirety by reference.
The formation of a pMUT device, such as the pMUT device defining an air-backed cavity
as disclosed in U.S. Patent No. 7,449,821, may involve the formation of an electrically-conductive
connection between the first electrode (i.e., the bottom electrode) of the transducer device, wherein
the first electrode is disposed on the front side of the substrate opposite to the air-backed cavity of
the pMUT device, and the conformal metal layer(s) applied to the air-backed cavity for providing
subsequent connectivity, for example, to an integrated circuit ("IC") or a flex cable. In this regard,
some prior art methods involve, for example, deposition of a conformal metal layer in the airbacked
cavity of the pMUT in direct contact with the first/bottom electrode (see, e.g., Fig. 7A of
U.S. Patent No. 7,449,821). In another example, the conformal metal layer is deposited in a via
formed in a dielectric film to expose the first/bottom electrode (see, e.g., Fig. 7B of U.S. Patent No.
7,449,821). In yet another example, involving a silicon-on-insulator (SOI) substrate, the conformal
metal layer is deposited in a via extending to immediately adjacent the transducer device (see, e.g.,
Figs. 14 and 15 of U.S. Patent No. 7,449,821). However, such approaches maybe limiting for high
frequency transducer arrays having smaller dimensions.
High frequency transducers may be used for high resolution ultrasound imaging, for
example, imaging with a resolution of approximately 100 m h, by operating the transducer within a
frequency range of 20 MHz to 50 MHz. Transducers operating at standard imaging frequencies of,
for instance, less than 10 MHz, typically have resolution of greater than 1 mm. The operating
frequency for pMUT transducers may be inversely proportional to the width of the transducer
element. As such, relatively smaller pMUT transducers can generally be operated at higher
frequencies and, therefore, may provide a relatively better resolution. Further, in instances of
steerable phased arrays, the transducer element pitch must be less than one wavelength in order to
prevent grating lobe artifacts in the resulting images produced from the transducer signal.
Generally, the ultrasound wavelength in bodily tissue is about 75 m for a frequency of about 20
MHz and about 30 m h for a frequency of about 50 MHz. Therefore, it may be desirable for some
transducer devices to include a transducer element having a lateral dimension (i.e., width) on the
order of about 40 mhi for about 20 MHz operation, or on the order of about 20 m h for operation at
ultrasound frequencies of about 40-50 MHz. In such instances, however, the disclosed prior art
configurations may result in an aspect ratio between the thickness of the substrate (i.e., about 400
mh ) and the width of the via extending through the substrate to the transducer element of between
about 10:1 and about 20: 1. Such a configuration may not be desirable for forming though- wafer
interconnects in line with the transducer elements (i.e., through the pMUT air-backed cavity).
Another aspect of some prior art methods is that the element forming the electricallyconductive
connection between the first electrode and the conformal metal layer may be formed
about one of the lateral edges of thepMUT device (see, for example, Fig. 15 of U.S. Patent No.
7,449,821). In such instances, mechanical flexure of the actuated pMUT device may initiate or
accelerate fatigue of the engagement between the electrically-conductive connection element and
conformal conductive layer within the air-backed cavity of the pMUT device (otherwise referred to
herein as the "second via"), for instance, due to stress concentrations about the sidewall / endwall
edge of the second via. Such fatigue could result in cracking or delamination of the metal layer and
failure of the electrically- conductive engagement therebetween and would thus create an open
circuit condition between the first/bottom electrode of the pMUT device and the IC, flex cable or
redistribution substrate engaged therewith, or may adversely affect the acoustic signals generated
by the pMUT device.
Further, having a different material (i.e., a metal) disposed about a lateral edge of the
membrane of the pMUT device for providing the electrical connection could change the boundary
condition for membrane flexing and thus affect the frequency and/or vibrational mode (i.e., the
fundamental or harmonic mode) of the pMUT device. Such a configuration may particularly
adversely affect smaller membranes required for high frequency operation. Applying a conformal
metal layer deposition, for example, of about 2 m in thickness to a membrane of about 20 pm in
width and about 8 m h in thickness may reduce the effective free membrane width by about 20%
and increase the membrane thickness by about 25%. Such a configuration, as a result, may
increase the resonance frequency, but could also undesirably reduce the acoustic output due to
increased stiffness of the vibrating membrane resulting from the reduced free width and increased
thickness thereof.
Thus, there exists a need in the ultrasonic transducer art, particularly with respect to a
piezoelectric micromachined ultrasound transducer ("pMUT") having an air-backed cavity, for
improved methods of forming an electrically-conductive connection between the first electrode
(i.e., the bottom electrode) of the transducer device and the conductive member extending from the
back side of the substrate to the first electrode so as to provide subsequent connectivity, for
example, to an integrated circuit ("IC") or a flex cable.
BRIEF SUMMARY OF THE DISCLOSURE
The above and other needs are met by aspects of the present disclosure, wherein one such
aspect relates to a method of forming a piezoelectric ultrasonic transducer apparatus. Such a
method comprises depositing a first electrode on a dielectric layer disposed on a primary substrate.
A piezoelectric material is deposited on the first electrode, and a second electrode is deposited on
the piezoelectric material, so as to form a transducer device. At least the piezoelectric material is
patterned such that a portion of the first electrode extends laterally outward therefrom. The primary
substrate and the dielectric layer are etched to form a first via extending to the laterally outward
extending portion of the first electrode. A first conductive material is deposited to substantially fill
the first via, wherein the first conductive material forms an electrically-conductive engagement
with the laterally outward extending portion of the first electrode. The primary substrate is etched
to define a second via extending therethrough, and forming the air-backed cavity of the pMUT
device, wherein the second via is laterally spaced apart from the first via. The air-backed cavity of
the pMUT device associated with the piezoelectric element thus facilitates flexure and/or vibration
of the pMUT membrane when voltage is applied to the piezoelectric material through the first and
second electrodes, or when an acoustic echo is received by or otherwise acts on the pMUT
membrane. Such a configuration thus facilitates transmission and reception of acoustic signals by
the pMUT device.
Another aspect of the present disclosure provides a method of forming a piezoelectric
ultrasonic transducer apparatus. Such a method comprises depositing a dielectric layer on a
primary substrate, and then etching the primary substrate and the dielectric layer to form a first via
extending therethrough. A first conductive material is deposited to substantially fill the first via. A
first electrode is deposited on the dielectric layer disposed on the primary substrate such that the
first electrode forms an electrically-conductive engagement with the first conductive material. A
piezoelectric material is deposited on the first electrode, and a second electrode is deposited on the
piezoelectric material, wherein the first and second electrodes cooperate with the piezoelectric
material to form a transducer device. At least the piezoelectric material is patterned such that a
portion of the first electrode extends laterally outward therefrom, wherein the laterally outward
extending portion of the first electrode forms the electrically-conductive engagement with the first
conductive material. The primary substrate is then etched to define a second via extending
therethrough, wherein the second via is laterally spaced apart from the first via.
Yet another aspect of the present disclosure provides a piezoelectric ultrasonic transducer
apparatus, comprising a transducer device disposed on a dielectric layer. The transducer device
includes a first electrode disposed on the dielectric layer and a piezoelectric material disposed
between the first electrode and a second electrode. The first electrode is configured to have a
portion thereof extending laterally outward from at least the piezoelectric material. A primary
substrate has the dielectric layer disposed thereon, and cooperates with the dielectric layer to define
a first via extending to the laterally outward extending portion of the first electrode. The primary
substrate further defines a second via extending therethrough, wherein the second via is laterally
spaced apart from the first via. A first conductive material is configured to substantially fill the
first via and to form an electrically-conductive engagement with the laterally outward extending
portion of the first electrode.
Aspects of the present disclosure thus address the identified needs and provide other
advantages as otherwise detailed herein.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)
Having thus described the disclosure in general terms, reference will now be made to the
accompanying drawings, which are not necessarily drawn to scale, and wherein:
FIGS. 1A, IB, 2-5, 6A, 6B, and 7 schematically illustrate a method of forming a
piezoelectric micromachined ultrasonic transducer apparatus, according to one aspect of the
disclosure;
FIG. 8 is a partial schematic plan view of a piezoelectric micromachined ultrasonic
transducer apparatus, according to one aspect of the disclosure;
FIGS. 9A-9C schematically illustrate a method of forming a piezoelectric micromachined
ultrasonic transducer apparatus, according to another aspect of the disclosure; and
FIGS. 1OA-IOC schematically illustrate a method f forming a piezoelectric
micromachined ultrasonic transducer apparatus, according to yet another aspect of the disclosure.
DETAILED DESCRIPTION OF THE DISCLOSURE
The present disclosure now will be described more fully hereinafter with reference to the
accompanying drawings, in which some, but not all aspects of the disclosure are shown. Indeed,
the disclosure may be embodied in many different forms and should not be construed as limited to
the aspects set forth herein; rather, these aspects are provided so that this disclosure will satisfy
applicable legal requirements. Like numbers refer to like elements throughout.
Aspects of the present disclosure are generally directed to methods for forming an
electrically-conductive member extending through a substrate to a transducer device supported
thereby, and into electrically-conductive contact with the first/bottom electrode of the transducer
device formed on the opposing surface of the substrate. More particularly, the first/bottom
electrode layer is configured to extend laterally outward of the transducer device, such that the
electrically-conductive member foraied in electrically-conductive engagement with the first/bottom
electrode layer is laterally displaced with respect to the transducer device. As such, the lateral
displacement of the electrically conductive engagement from the air-backed cavity of the
transducer device allows the electrically-conductive member to be formed with a more desirable
aspect ratio, may provide an improved electrically-conductive engagement between the first/bottom
electrode and the back side of the substrate, and may also reduce mechanical loading and resonant
frequency attenuation of the transducer device.
According to one aspect of the present disclosure, a method is provided for fabricating
particular layers of one or more exemplary pMUT devices. One such example of a plurality of
pMUT devices 150 (i.e., a pMUT "wafer") is shown Figure 1A. Such a method may initially
involve a primary substrate 151 having a silicon-on-insulator (SOI) device substrate 152 formed
thereon, wherein the device substrate 152 includes a buried oxide layer 153 (Figure 2). In other
instances (not shown), the device substrate 152 may comprise silicon. An SOI substrate, if
implemented, may provide a device (i.e., silicon) layer configured to function as a mechanical
element capable of controlling, for example, the resonance frequency of the pMUT device. The
SOI substrate may also be configured to provide a buried oxide layer, which may be implemented
as an etch stop when etching the primary (handle) substrate to form the pMUT membrane. The
device layer may be configured to have a thickness suitable for providing an appropriate stiffness
of the membrane to produce the desired resonance frequency and acoustic output. For example, a
pMUT membrane having a width of between about 70 m and about 100 mih may operate
optimally at a frequency of between about 7 MHz and about 12 MHz, with the device layer having
a thickness of between about 5 mh and about 10 mih. In general, increasing the device layer
thickness to about!5 m h increases the stiffness and damping of the pMUT membrane, which
reduces acoustic output. In other aspects, a single silicon wafer can be used as the primary
substrate, with the pMUT device being formed on the primary substrate. In such instances, the
device layer and etch stop could be formed by heavily doping the silicon substrate to an appropriate
depth, since doped silicon maybe etched relatively slower than bulk silicon. The doped silicon
thus functions as an etch stop for facilitating the provision of a mechanical layer thickness for
producing the desired resonance frequency.
In some instances, a dielectric material such as, for example, a thermal Si0 2 (thermal oxide)
dielectric layer 154 may be deposited on the device substrate 152. A first electrode layer 156
(otherwise referred to herein as a "bottom" electrode) comprising, for instance, a Ti/Pt material,
may then be deposited on the dielectric layer 154, and the first electrode layer 156 is then
configured to form the footprints of the transducer devices. A piezoelectric material layer 158 such
as, for example, a piezoelectric (PZT) film, is subsequently deposited on the first electrode layer
156. In accordance with aspects of the present disclosure, the piezoelectric material layer 158 is
deposited, or post-deposition patterned or otherwise configured, such that a portion of the first
electrode layer 156 extends laterally outward or beyond the piezoelectric material layer 158 of each
transducer device. In some instances, the laterally-outward extending portion of the first electrode
layer 156 laterally extends from the interface with the piezoelectric material layer 158 of that
transducer device to an interstice between adjacent transducer devices 163, as shown, for example,
in Figure 8. That is, in one instance, the piezoelectric material layer 158 maybe deposited or
patterned such that a portion of the first electrode layer 156 extends laterally outward therefrom to
an interstice between four adjacent transducer devices 163 in a regularly-spaced array of transducer
devices. In other instances, an irregular array spacing, or an array with varying element pitch (e.g.,
vertical pitch and horizontal pitch are different), may not provide suitable space for placement of
the laterally-outward extending portion of the first electrode 156. In such instances, the laterallyoutward
extending portion of the first electrode 156 may be positioned either laterally or
longitudinally between adjacent pairs of transducer devices 163 (such transducer devices 163 being
shown, e.g., in FIGS. 1A, IB, and 2) or in other arrangements where spacing permits in an
irregularly patterned array of transducer devices.
An interlayer dielectric 160 such as, for instance a benzocyclobutene (BCB), polyimide, or
parylene material, is then deposited and processed to separate the transducer devices 163 and to
cover the laterally-outward extending portion of the first electrode layer 156. The interlayer
dielectric 160 is patterned, for example, by photolithography and/or etching to expose a surface of
the the piezoelectric layer 158. A second electrode layer 162 (otherwise referred to herein as a
"top" electrode) comprising, for instance, a Ti/Au material, is then deposited on the piezoelectric
material layer 158 and the interlayer dielectric 160, such that the second electrode 162 is in
electrical engagement / contact with the exposed surface of the piezoelectric layer 158. The first
electrode layer 156, the piezoelectric material layer 158, and the second electrode layer 162 thus
cooperate to form the transducer device 163 (see, e.g., Figures 1 and 2), such as, for example, a
piezoelectric ultrasonic transducer. In some aspects, the second electrode layer 162 may comprise
a ground electrode, while the first electrode layer 156 may comprise a signal electrode, of the
transducer device 163. For an array of transducer devices, the signal electrodes formed in the first
electrode layer 156 are separated such that the signal electrodes for individual array elements are
electrically isolated from each other. However, two or more of the transducer devices in the array
may share a common ground electrode formed in the second electrode layer 162.
Piezoelectric materials that can be implemented in the piezoelectric material layer 158
include, for example, ceramics including ZnO, A1N, LiNb0 4, lead antimony stannate, lead
magnesium tantalate, lead nickel tantalate, titanates, tungstates, zirconates, or niobates of lead,
barium, bismuth, or strontium, including lead zirconate titanate (Pb(Zr xTii -x)0 3 (PZT)), lead
lanthanum zirconate titanate (PLZT), lead niobium zirconate titanate (PNZT), BaTi0 3, SrTi0 , lead
magnesium niobate, lead nickel niobate, lead manganese niobate, lead zinc niobate, lead titanate.
For the above inorganic oxide-type piezoelectric materials, a high temperature anneal must be
completed to crystallize the material. For example, for PZT, annealing at 700°C must be used to
form the perovskite (piezoelectric) phase of the PZT material in order to obtain relatively good
piezoelectric properties. Other lower temperature materials can also be used, such as piezoelectric
polymer materials, including polyvinylidene fluoride (PVDF), polyvinylidene fluoridetrifluoroethylene
(PVDF-TrFE), or polyvinylidene fluoride-tetrafluoroethylene (PVDF-TFE),
which do not require the high temperature anneal or otherwise may require only a moderate anneal
(<300°C).
An alternate aspect is shown in Figure IB. In such an aspect, the second electrode layer
162 maybe configured to form a separate signal electrode for each transducer device 163. More
particularly, openings 161 maybe formed in the interlayer dielectric 160, prior to deposition of the
second electrode layer 162, for instance, using photolithography and/or by etching the interlayer
dielectric 160. In some instances, the openings 161 may have sloped sidewalls to facilitate
deposition of the second electrode layer 162 with adequate step coverage over the interlayer
dielectric. In some aspects, the second electrode layer 162 may comprise a signal electrode, while
the first electrode layer 1 6 may comprise a ground electrode, of the transducer device 163. For an
array of transducer devices, the signal electrodes formed in the second electrode layer 162 may be
separated such that the signal electrodes for individual transducer devices 163 in the array are
electrically isolated from each other. However, the transducer devices 163 may share a common
ground electrode formed in the first electrode layer 156. The first (ground) electrode may be a
continuous layer covering the dielectric layer 154, but with openings corresponding to the openings
161 in the interlayer dielectric 160, such that the first (ground) and second (signal) electrodes are
also electrically isolated from each other.
As shown in Figure 2, in order to process the back side of the substrate, a carrier substrate
164 maybe bonded to the top surface (i.e., the second electrode layer 162) of the pMUT wafer 150
using, for example, an epoxy, an adhesive tape, or other adhesive material 166 that can be removed
in later processing. As shown in Figure 3, the primary substrate 151 may then be thinned, for
example, by back-grinding or chemical mechanical polishing (CMP), in order to achieve a
desirable dimension (i.e., thickness) for desirable performance of the transducer device 163,
according to aspects of the disclosure as further disclosed herein. One skilled in the art will
appreciate that the partial removal of the primary substrate 151 can be accomplished in various
manners. Thinning of the primary substrate 151 may provide a benefit by reducing the overall
thickness of the pMUT device, such that the pMUT device may more readily fit within a relatively
small diameter catheter, particularly when stacked or otherwise engaged with other devices such as
IC's and/or interposers that may increase the overall thickness of the stack.
As shown in Figure 4, first vias 170 may then be formed in the remaining portion of the
primary substrate 151, for example, by etching using a deep reactive ion etching (DRIE) process
(for substantially vertical sidewalls). In some instances, it may be preferred that the etch profile
includes substantially vertical sidewalls so as to minimize the lateral extent of the via diameter, and
thus maintain element pitch of less than one wavelength. In one aspect, first vias 170 maybe
formed in the remaining primary substrate 151 so as to extend through the device substrate 152 and
the dielectric (thermal oxide) layer 154 to expose the laterally-outward extending portion of the
first/bottom electrode layer 156 of the transducer device 63. In this manner, first vias 170 may be
formed interstitially between adjacent pMUT devices 163. According to one particular aspect,
thinning of the primary substrate 151, prior to formation of the first vias 170 maybe arranged to
provide a particular aspect ratio between the thickness of the primary substrate 151 and the width of
the first via 170. The aspect ratio maybe optimized for the subsequent conformal metal or
conductive material layer(s), as well as the insulator (or insulating material) depositions and etches,
to facilitate conformality of the particular deposition and also to facilitate removal of the insulator
(or insulating material) from an end wall (i.e., bottom surface) of a via, without substantially
affecting the conformal insulating material deposited on the sidewall. Such characteristics may
thus facilitate electrically-conductive engagement between the conformal metal or conductive
material layer and the first electrode layer 156, while substantially preserving the insulator /
insulating material deposited on the sidewall of the via, so as to electrically insulate the primary
substrate and/or device layer or substrate from the subsequently deposited conformal metal or
conductive materia] layer.
In one instance, deposition and etching techniques may be configured such that the aspect
ratio between the thickness of the primary substrate 151 and the width of the first via 170 maybe in
the range of between about :1 and about 20: 1, wherein such a range may facilitate conformality of
the conformal metal or conductive material layer(s), as well as the insulator / insulating material
layer(s), electrically conductive engagement between the conformal metal / conductive material
layer and the first (bottom) electrode layer 156, and/or substantial preservation of the conformal
insulator / insulating material layer on the sidewall of the via (while facilitating removal of the
insulator / insulating material layer from the end wall of the via). In one aspect, it may be
preferable for the aspect ratio to be between about 5:1 and about 7:1. In an exemplary aspect, the
aspect ratio of the thickness of the primary substrate 151 to the width of the first via 170 maybe
about 6:1. For example, in instances of a pMUT device configured for about 20 MHz operation,
the first via 1 0 may have a lateral dimension (i.e., width) of about 12 m h, and it maybe desirable
for the primary substrate 15 1 to have a depth (i.e., thickness) of about 80 mih. In such instances, the
electrically-conductive engagement with the first/bottom electrode 156 through the first via 170
may have an ohmic resistance on the order of about 50 milliohms, when the first via 170 is filled
with a conductive material such as a metal. In another example, in instances of a pMUT device
configured for about 40 MHz operation, the first via 1 0 may have a lateral dimension (i.e., width)
of about 6 mih, and it may be desirable for the primary substrate 15 1 to have a depth (i.e., thickness)
of about 40 mhi. In such instances, the electrically-conductive engagement with the first/bottom
electrode 156 through the first via 170 may have an ohmic resistance on the order of about 100
milliohms, when the first via 170 is filled with a conductive material such as a metal. In
configurations involving a thinned primary substrate 151, particular processing steps subsequent to
the thinning process may be accomplished with the carrier substrate 164 remaining engaged the top
surface of the pMUT wafer 150.
Once the first vias 170 are formed, the primary substrate 151, including the first vias 170
formed therein, can then have deposited thereon a conformal insulator material (not shown), in
some instances, with the conformal insulator layer extending into the first via 170 to the dielectric
layer 154. The conformal insulator material may comprise, for example, an insulating polymer or a
silicon oxide layer, such as parylene or TEOS Si0 2, to electrically isolate the primary substrate 151
defining the first vias 170. That is, the primary substrate 151 may have deposited thereon the
conformal insulator material which extends about the exposed surfaces of the primary substrate 151
and into the first vias 170 so as to cover the "sidewalls" of the first vias 170. h so being deposited
in the first vias 170 and covering the sidewalls thereof, the conformal insulator material may also
be engaged with the dielectric (thermal oxide) layer 154 and the device substrate 152, also defining
the first via 170.
As shown in Figure 5, the first vias 170 can then be filled with a first conductive material
172 (i.e., by deposition of a layer of metal or other conductive material) such as for example, Cu, in
a sputtering, chemical vapor deposition, and/or plating process. The first conductive material 172
is configured to substantially fill the first via 170 and to form an electrically-conductive
engagement with the laterally-outward extending portion of the first/bottom electrode layer 156. In
other aspects, the conductive material 172 maybe configured to be deposited only on the sidewall
of the first via 170, without substantially filling the first via 170. Of course, in order to form the
electrically-conductive engagement, one skilled in the art will appreciate that appropriate measures
may be implemented to ascertain that the conformal insulator material is not deposited on the
first/bottom electrode layer 156 within the first via 170, or that the conformal insulator layer
deposited on the first/bottom electrode layer 156 within the first via 170 is removed prior to
deposition of the first conductive material 172. The first conductive material 172 may thus provide
an electrically-conductive element extending from the first/bottom electrode layer 156 through the
dielectric (thermal oxide) layer 154, the device substrate 152, and the remaining primary substrate
51, to the back side of the substrate.
A second conductive material (not shown) comprising for example, a metal such as Ti/Cu,
may then be patterned or otherwise formed on the back side of the primary substrate 151 using, for
example, a sputtering or ion milling process. In this regard, the second conductive material may be
configured to form an electrically-conductive engagement directly with the exposed portion of the
first conductive material 172 extending through the remaining primary substrate 151 from the first
via 170.
As shown in Figure 6A, second vias 184 may then be formed in the primary substrate 151
from the back side thereof, for example, using an appropriate etching process (i.e., DRIE). The
second vias 184 may be formed so as to extend through the primary substrate 151 (and any portion
of the conformal insulator material disposed on the back side thereof) to expose the buried oxide
layer 1 3 of the device substrate 152, with the exposed buried oxide layer 153 thereby forming an
end wall of the second via 184. The second vias 184 may be laterally spaced apart from the first
vias 170. In one instance, each second via 184 may extend toward the transducer device 63 /
piezoelectric material layer 158, and not toward the laterally-outward extending portion of the
first/bottom electrode layer 156, which results in the second via 84 being laterally spaced apart
from the corresponding first via 170. In order to connect the second (common ground) electrode
layer 162 to the back side, openings 161 formed in the interlayer dielectric 160, as previously
described, and having the second electrode layer 162 deposited therein may provide a contact area
to the second electrode layer 162. More particularly, in conjunction with the openings 161,
additional first vias 174 maybe patterned so as to correspond to the openings 161, whereby the first
conductive material 172 deposited in the additional first vias 174 maybe configured to form an
electrically-conductive engagement with the second (common ground) electrode layer 162.
Once the pMUT devices 150 are formed, for example, as previously disclosed, the pMUT
devices 150, as shown in Figure 6, may be operably engaged with an external device (i.e., element
200 in Figure 7) such as, for example, an integrated circuit (e.g., a control IC such as amplifier or
multiplexer), an interposer (e.g., silicon or flex cable), or redistribution element (see, e.g., Figure
7), for example, using an appropriate bonding arrangement 400. In some instances, conductive
solder elements (e.g., solder bumps), conductive stud elements, conductive bonding materials, or
other suitable electrically-conductive connection provisions (not shown), may be implemented to
provide an electrically-conductive engagement between the first conductive material 172 extending
from the first electrode layer 156 of a particular pMUT device and corresponding conductive
elements 300 of the IC, flex cable, redistribution element, or other external device 200. In this
regard, an electrically-conductive engagement may be formed between the first conductive material
2 and an external device according to the methods and configurations described, for example, in
U.S. Patent Application No. 61/329,258 ("Methods for Forming a Connection with a
Micromachined Ultrasonic Transducer, and Associated Apparatuses"), assigned to Research
Triangle Institute (also the assignee of the present disclosure), and which is incorporated herein in
its entirety by reference.
The carrier substrate 164 may then be de-bonded or otherwise removed to expose the
second electrode layer 162. In some instances, a conformal layer (not shown) comprising a
polymeric material such as, for example, parylene or other suitable polymer, maybe deposited on
the second electrode layer 162 in order to provide, for instance, device protection and moisture
barrier functions.
Such aspects of the disclosure may thus provide a direct electrically-conductive engagement
between the back side of the primary substrate 151 and the first/bottom electrode layer 156, via the
first conductive material 172 and the laterally-outward extending portion of the first/bottom
electrode layer 156. That is, one aspect of the present disclosure is directed to a method involving
formation of electrically-conductive (i.e., metal) "plugs" extending through a via, so as to provide a
direct electrically-conductive engagement between the first/bottom electrode layer and the plug
extending through the via. Direct metal to metal contact, such as direct contact between the
first/bottom electrode layer and the plug, may thus advantageously provide a low-resistance
electrically-conductive engagement as compared to a doped silicon layer. In particular aspects, the
first/bottom electrode layer may be configured to extend laterally outward of the transducer device,
such that the electrically-conductive member 172 formed within a first via 170, in electricallyconductive
engagement with the first/bottom electrode 156, is laterally displaced with respect to the
transducer device, in some instances, interstitially between adjacent pMUT devices. As such, the
lateral displacement of the electrically conductive engagement from the air-backed cavity (second
via 184) of the transducer device may also allow the electrically-conductive member 172 to be
formed with a more desirable aspect ratio (i.e., the ratio between the thickness of the primary
substrate 151 and the width of the first via 170), and may also reduce mechanical loading and
resonant frequency attenuation of the transducer device (e.g., as compared to an electricallyconductive
engagement established through the air-backed cavity of the pMUT device). Such
aspects of the disclosure, in some instances, may also facilitate improved electrically-conductive
engagement between the pMUT device via the first/bottom electrode layer 156, and an external
device such as an interposer, IC, redistribution element, or other external device, via the direct
electrically-conductive engagement path between the first conductive material and the first/bottom
electrode 156. Such aspects may also be beneficial, in one particular instance, for ultrasonic
transducer devices used in medical imaging applications.
The aspects disclosed above may be designated, for example, as a "Vias Last" arrangement
/ method for forming through- silicon vias (TSV's), or through-substrate vias, for providing an
electrically conductive engagement configuration for both the signal and ground electrodes of the
transducer device through one side (i.e., the back side) of the substrate. More particularly, access
to the first and second electrode layers 156, 1 2 is provided through the primary substrate 151.
Such an arrangement / method may facilitate the formation of the PZT transducer elements (i.e., the
piezoelectric material layer 158), including a high temperature anneal of the piezoelectric material
layer 158 at 700°C), prior to forming the TSV's. Therefore, standard TSV materials (i.e., materials
not required to withstand high annealing temperatures) can be implemented. For example, Cu
metallization may be implemented for the first conductive material 172.
One skilled in the art, however, will appreciate that other arrangements and methods of
forming such TSV's can also be implemented to produce similar structures for pMUT devices.
Two particular examples of other TSV arrangements and methods are disclosed hereinafter.
In one exemplary aspect, it may be desirable, in some instances, to first form the TSV's in
the primary substrate 151, prior to forming the pMUT transducer devices 163. Since the primary
substrate 151 includes buried or otherwise pre-formed TSV's, prior to forming the pMUT devices
163, such an aspect may be designated, for example, as a "Vias First" arrangement / method. As
shown in Figure 9A, the first vias 500 may be etched in the primary substrate (and device substrate,
if the primary substrate 151 is an SOI substrate). In such instances, the first vias 500 are "blind
vias" etched to a depth corresponding to the final desired thickness of the primary substrate 151, as
previously disclosed, for example, for relatively high frequency pMUT devices. In some aspects, a
dielectric layer (not shown), such as silicon dioxide, may he deposited on the device substrate or
the primary substrate 151, and in the first vias 500, in order to electrically isolate the substrate from
subsequent layers.
The first vias 500 are then substantially filled with a conductive material 510, as shown in
Figure 9B. In one instance, the conductive material 510 maybe a relatively high temperature
conductive material such as, for example, Ti, Pt or W, in order to withstand the annealing
temperature of the subsequently deposited piezoelectric material layer 158, wherein such an
annealing temperature may be about 700°C for PZT. The conductive material 510 maybe
deposited, for example, by RF sputtering, chemical vapor deposition or electroplating. A first
electrode 56 is then deposited on the dielectric layer 154 so as to form an electrically-conductive
engagement with the conductive material 510 in the first vias 500. The remaining components of
the pMUT device 163, namely, the piezoelectric material layer 158, the interlayer dielectric 160,
and the second electrode layer 162 may then be deposited, as shown in Figure 9C. A carrier
substrate (not shown) may then be engaged with the piezoelectric device 163 and the primary
substrate 151 thinned to expose the conductive material 510 in the first vias 500. The second vias
84 may then be etched to form the air-backed cavities associated with the pMUT devices 163.
The completed device, as shown in Figure 9C, can then be bonded to an external device, such as an
IC, flex circuit, interposer, or redistribution element, as shown in Figure 7.
In another aspect, it may be desirable, in some instances, to first form the TSV's in the
primary substrate 151, after forming the piezoelectric elements / piezoelectric material layer 158,
but prior to forming the remaining components of the pMUT device 163. Such an aspect may be
designated, for example, as a "Vias Middle" arrangement / method. As shown in Figure 10A, a
first electrode layer 156 is deposited on the dielectric layer 154, followed by the piezoelectric
material layer 158 deposited on the first electrode layer 156. These first electrode and piezoelectric
material layers 156, 158 may then be patterned, for example, by photolithography, etching, and/or
liftoff processing. The first vias 600 may then be etched in the primary substrate (and device
substrate, if the primary substrate 151 is an SOI substrate). The first vias 600 may be formed
adjacent to the patterned first electrode(s) in the first electrode layer 156, and are "blind vias"
etched to a depth corresponding to the final desired thickness of the primary substrate 151, as
previously disclosed, for example, for relatively high frequency pMUT devices. In some aspects, a
dielectric layer (not shown), such as silicon dioxide, may be deposited on the device substrate or
the primary substrate 151, and in the first vias 600, in order to electrically isolate the substrate from
subsequent layers.
The first vias 600 are then substantially filled with a conductive material 610, as shown in
Figure 10B. In one instance, the conductive material 5 0 may be a relatively low temperature
conductive material such as, for example, Cu (i.e., since the conductive material 610 is deposited
following the high temperature annealing process). The conductive material may be deposited, for
example, by RF sputtering, chemical vapor deposition, or electroplating. The remaining
components of the pMUT device 163, including the interlayer dielectric 160 and the second
electrode layer 162 may then be deposited, as shown in Figure IOC. A carrier substrate (not
shown) may then be engaged with the piezoelectric device 163 and the primary substrate 151
thinned to expose the conductive material 610 in the first vias 600. The second vias 184 may then
be etched to form the air-backed cavities associated with the pMUT devices 163. The completed
device, as shown in Figure IOC, can then be bonded to an external device, such as an IC, flex
circuit, interposer, or redistribution element, as shown in Figure 7.
Many modifications and other aspects of the disclosures set forth herein will come to mind
to one skilled in the art to which these disclosures pertain having the benefit of the teachings
presented in the foregoing descriptions and the associated drawings. Therefore, it is to be
understood that the disclosures are not to be limited to the specific aspects disclosed and that
modifications and other aspects are intended to be included within the scope of the appended
claims. Although specific terms are employed herein, they are used in a generic and descriptive
sense only and not for purposes of limitation.
THAT WHICH IS CLAIMED:
1. A method of forming a piezoelectric ultrasonic transducer apparatus, said method
comprising:
depositing a first electrode on a dielectric layer disposed on a primary substrate;
depositing a piezoelectric material on the first electrode, and depositing a second electrode
on the piezoelectric material, the first and second electrodes cooperating with the
piezoelectric material to form a transducer device;
patterning at least the piezoelectric material such that a portion of the first electrode extends
laterally outward therefrom;
etching the primary substrate and the dielectric layer to form a first via extending to the
laterally outward extending portion of the first electrode;
depositing a first conductive material to substantially fill the first via, the first conductive
material forming an electrically-conductive engagement with the laterally outward
extending portion of the first electrode; and
etching the primary substrate to define a second via extending therethrough, the second via
being laterally spaced apart from the first via.
2. A method according to Claim 1, wherein depositing a first electrode further
comprises depositing a first electrode on the dielectric layer, the dielectric layer being disposed on a
device substrate, and the device substrate further being disposed on the primary substrate.
3. A method according to Claim 1, further comprising removing a portion of the
primary substrate, prior to etching the primary substrate to form the first via, such that an aspect
ratio of a thickness of the primary substrate to a width of the first via is between about 1:1 and
about 20:1.
4. A method according to Claim 3, wherein removing a portion of the primary
substrate further comprises removing a portion of the primary substrate such that the aspect ratio is
between about 5:1 and about 7:1.
5. A method according to Claim 3, wherein removing a portion of the primary
substrate further comprises removing a portion of the primary substrate such that the aspect ratio
about 6:1.
6. A method according to Claim 1, further comprising depositing an insulating material
on the primary substrate, the insulating material extending into and lining the first via, prior to
depositing the first conductive material.
7. A method according to Claim 6, wherein the first via is defined by a sidewall and an
end wall, and the method further comprises etching the insulating material so as to substantially
remove the insulating material from the end wall, without removing the insulating material from
the sidewall.
8. A method according to Claim 6, further comprising depositing a second conductive
material on the insulating material on the primary substrate, the second conductive material
forming an electrically-conductive engagement with the first conductive material.
9. A method according to Claim 6, further comprising etching the insulating material
disposed on the primary substrate, prior to etching the primary substrate to form the second via.
10. A method according to Claim 1, wherein forming a transducer device further
comprises forming a transducer device such that the first electrode is configured as a signal
electrode, the second electrode is configured as a ground electrode, and the first and second
electrodes are each comprised of an electrically-conductive material.
11. A method according to Claim 1, further comprising forming an electricallyconductive
engagement between the first conductive material and an external device with one of a
conductive solder element, a conductive stud element, and a conductive bonding material
therebetween.
1 . A method according to Claim 8, further comprising forming an electricallyconductive
engagement between the second conductive material and an external device with one of
a conductive solder element, a conductive stud element, and a conductive bonding material
therebetween.
13. A method according to Claim 1, wherein etching the primary substrate to define a
second via further comprises etching the primary substrate to define a second via extending through
the primary substrate toward the piezoelectric element.
14. A method of forming a piezoelectric ultrasonic transducer apparatus, said method
comprising:
depositing a dielectric layer on a primary substrate;
etching the primary substrate and the dielectric layer to form a first via extending
therethrough;
depositing a first conductive material to substantially fill the first via;
depositing a first electrode on the dielectric layer disposed on the primary substrate such
that the first electrode forms an electrically-conductive engagement with the first
conductive material;
depositing a piezoelectric material on the first electrode, and depositing a second electrode
on the piezoelectric material, the first and second electrodes cooperating with the
piezoelectric material to form a transducer device;
patterning at least the piezoelectric material such that a portion of the first electrode extends
laterally outward therefrom, the laterally outward extending portion of the first
electrode forming the electrically-conductive engagement with the first conductive
material; and
etching the primary substrate to define a second via extending therethrough, the second via
being laterally spaced apart from the first via.
15. A method according to Claim 14, wherein depositing a dielectric layer further
comprises depositing a dielectric layer on a device substrate, the device substrate further being
disposed on the primary substrate, such that etching the primary substrate further comprises etching
the primary substrate, the device substrate, and the dielectric layer to form a first via extending
therethrough.
16. A method according to Claim 14, wherein the first via is defined by a sidewall, and
the method further comprises depositing an insulating material so as to line the sidewall of the first
via prior to depositing the first conductive material.
17. A method according to Claim 16, wherein depositing an insulating material further
comprises depositing an insulating material so as to substantially cover the primary substrate.
18. A method according to Claim 14, further comprising removing a portion of the
primary substrate such that an aspect ratio of a thickness of the primary substrate to a width of the
first via is between about 1:1 and about 20: 1.
19. A method according to Claim 18, wherein removing a portion of the primary
substrate further comprises removing a portion of the primary substrate such that the aspect ratio is
between about 5:1 and about 7:1.
20. A method according to Claim 18, wherein removing a portion of the primary
substrate further comprises removing a portion of the primary substrate such that the aspect ratio is
about 6:1.
21. A method according to Claim 17, further comprising depositing a second conductive
material on the insulating material on the primary substrate, the second conductive material
forming an electrically-conductive engagement with the first conductive material.
22. A method according to Claim 17, further comprising etching the insulating material
disposed on the primary substrate, prior to etching the primary substrate to form the second via.
23. A method according to Claim 14, wherein forming a transducer device further
comprises forming a transducer device such that the first electrode is configured as a signal
electrode, the second electrode is configured as a ground electrode, and the first and second
electrodes are each comprised of an electrically-conductive material.
24. A method according to Claim 14, further comprising forming an electricallyconductive
engagement between the first conductive material and an external device with one of a
conductive solder element, a conductive stud element, and a conductive bonding material
therebetween.
25. A method according to Claim 21, further comprising forming an electricallyconductive
engagement between the second conductive material and an external device with
a conductive solder element, a conductive stud element, and a conductive bonding material
therebetween.
26. A method according to Claim 14, wherein etching the primary substrate to define a
second via further comprises etching the primary substrate to define a second via extending through
the primary substrate toward the piezoelectric element.
27. A piezoelectric ultrasonic transducer apparatus, comprising:
a transducer device disposed on a dielectric layer, the transducer device including a first
electrode disposed on the dielectric layer and a piezoelectric material disposed
between the first electrode and a second electrode, the first electrode being
configured to have a portion thereof extending laterally outward from at least the
piezoelectric material;
a primary substrate having the dielectric layer disposed thereon, the primary substrate
cooperating with the dielectric layer to define a first via extending to the laterally
outward extending portion of the first electrode, the primary substrate further
defining a second via extending therethrough, the second via being laterally spaced
apart from the first via; and
a first conductive material configured to substantially fill the first via and to form an
electrically-conductive engagement with the laterally outward extending portion of
the first electrode.
28. An apparatus according to Claim 27, further comprising a device substrate disposed
between the dielectric layer and the primary substrate, the device substrate cooperating with the
primary substrate and the dielectric layer to define the first via.
29. An apparatus according to Claim 27, wherein the primary substrate comprises a
silicon substrate.
30. An apparatus according to Claim 28, wherein the primary substrate and the device
substrate are configured to cooperate to define a silicon-on-insulator substrate.
31. An apparatus according to Claim 27, wherein the primary substrate is configured to
have an aspect ratio of a thickness of the primary substrate to a width of the first via of between
about 1:1 and about 20: 1.
32. An apparatus according to Claim 31, wherein primary substrate is configured such
that the aspect ratio is between about 5:1 and about 7:1.
33. An apparatus according to Claim 31, wherein the primary substrate is configured
such that the aspect ratio is about 6:1 .
34. An apparatus according to Claim 27, further comprising an insulating material
deposited on the primary substrate, the insulating material extending into and lining the first via,
and substantially exposing the first conductive material.
35. An apparatus according to Claim 34, wherein the insulating material is further
disposed between the primary substrate and the first conductive material within the first via.
36. An apparatus according to Claim 34, further comprising a second conductive
material deposited on the insulating material on the primary substrate, the second conductive
material forming an electrically-conductive engagement with the first conductive material.
37. An apparatus according to Claim 27, wherein the first electrode is configured as a
signal electrode, the second electrode is configured as a ground electrode, and the first and second
electrodes are each comprised of an electrically-conductive material.
38. An apparatus according to Claim 27, further comprising an external device in
electrically-conductive engagement with the first conductive material through one of a conductive
solder element, a conductive stud element, and a conductive bonding material therebetween.
39. An apparatus according to Claim 36, further comprising an external device in
electrically-conductive engagement with the second conductive material through one of a
conductive solder element, a conductive stud element, and a conductive bonding material
therebetween.
40. An apparatus according to Claim 27, wherein the second via is configured to extend
through the primary substrate toward the piezoelectric element