Note: Descriptions are shown in the official language in which they were submitted.
CA 02760508 2011-10-28
WO 2010/127122 PCT/US2010/032976
1
MONOLITHIC FBAR-CMOS STRUCTURE SUCH AS FOR
MASS SENSING
CLAIM OF PRIORITY
Benefit of priority is hereby claimed to U. S. Provisional Patent
Application Serial No. 61/173,866, filed on April 29, 2009 (Attorney Docket
No.
2413.107PRV), and U.S. Provisional Patent Application Serial No. 61/215,611,
filed on May 7, 2009 (Attorney Docket No. 2413.107PV2), both of which
applications are herein incorporated by reference in their respective
entireties
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR
DEVELOPMENT
This invention was made with government support under award number
UO1ES016074 from the National Institute of Environmental Health Sciences or
the National Institutes of Health. The government has certain rights in this
invention.
COPYRIGHT NOTICE
A portion of the disclosure of this patent document contains material that
is subject to copyright protection. The copyright owner has no objection to
the
facsimile reproduction by anyone of the patent document or the patent
disclosure, as it appears in the Patent and Trademark Office patent files or
records, but otherwise reserves all copyright rights whatsoever. The following
notice applies to the drawings and photos that form a part of this document:
Copyright 2010, The Trustees of Columbia University in the City of New York,
All Rights Reserved.
CA 02760508 2011-10-28
WO 2010/127122 PCT/US2010/032976
2
BACKGROUND
Ultra-high-precision mass sensing can be an important detection method
such as for biomolecular and chemical detection. Detecting molecules by mass
need not require chemical or fluorescent labeling, which can allow for
simplified
detection protocols and for sensing in systems adversely affected by labeling.
For example, the limited cross reactivity of fluorescently labeled generic
binders
can limit the specificity of a protein assay, such as used for analyzing or
characterizing various cells, biomarkers, or autoimmune diseases, among
others.
Additionally, use of unbound labeled reporters can also have limitations, such
as
preventing real-time detection and quantification of binding events, as such
unbound reporters must be washed away prior to optical interrogation.
SUMMARY
This document presents, among other things, a monolithic, integrated
solidly-mounted thin-film bulk acoustic wave resonator (FBAR) mechanically
and electrically coupled to an active integrated circuit, such as a
complementary
metal-oxide-semiconductor (CMOS) integrated circuit. Such a FBAR-CMOS
sensor, or a monolithic array of such sensors, can be used for mass sensing
applications. In contrast to externally coupled FBAR structures or other types
of
resonant mass sensors, the present inventors have recognized that an
integrated
array of sensors can be built directly above active drive and readout
circuitry. In
an FBAR-CMOS array, one or more individual FBAR mass sensors included in
the array can be functionalized in a specified manner, such as for capturing a
specific protein, nucleic acid, or gas molecule. An array of such
functionalized
sensors can allow simultaneous, multiplexed, high-sensitivity measurement of
multiple targets (e.g., detection or measurement of multiple, different,
species)
on a single (e.g., monolithic) sensor chip. In other examples, one or more
FBAR-CMOS devices can be used as a filter, oscillator, or transformer, such as
for microwave or solid-state power conversion applications, among others.
The monolithic, solidly-mounted FBAR resonator apparatus can
comprise a piezoelectric zinc oxide resonator atop a mechanically isolating
acoustic mirror. The mirror can function as a mechanical analog to an optical
Bragg stack, as acoustic waves are reflected back into the resonator through
CA 02760508 2011-10-28
WO 2010/127122 PCT/US2010/032976
3
quarter-wavelength layers and constructive interference. Such reflection by
the
isolating acoustic mirror can inhibit or prevent coupling of acoustic energy
into
the substrate below the resonator.
In Example 1, an apparatus can include a thin-film bulk acoustic
resonator comprising an acoustic mirror, a piezoelectric region acoustically
coupled to the acoustic mirror, a first conductor electrically coupled to the
piezoelectric region, a second conductor electrically coupled to the
piezoelectric
region and electrically insulated from the first conductor. In Example 1, the
apparatus optionally includes an integrated circuit substrate including an
interface circuit, the first and second conductors electrically coupled to the
interface circuit, the integrated circuit substrate configured to mechanically
support the resonator, the acoustic mirror configured to inhibit or prevent
coupling of acoustic energy from the piezoelectric region into the integrated
circuit substrate at or near a resonant frequency of the thin-film bulk
acoustic
resonator.
In Example 2, the subject matter of Example 1 optionally includes a
piezoelectric region comprising zinc oxide.
In Example 3, the subject matter of any one or more of Examples 1-2
optionally includes an acoustic mirror comprising alternating layers of
tungsten
and silicon dioxide.
In Example 4, the subject matter of any one or more of Examples 1-3
optionally includes an interface circuit comprising a CMOS circuit, and a
resonator located on a top surface of the integrated circuit.
In Example 5, the subject matter of any one or more of Examples 1-4
optionally includes an oscillator including the acoustic resonator and at
least a
portion of the interface circuit.
In Example 6, the subject matter of any one or more of Examples 1-5
optionally includes an operating frequency of the oscillator determined at
least in
part by a mass loading the piezoelectric region.
In Example 7, the subject matter of any one or more of Examples 1-6
optionally includes a resonator comprising a sensing surface configured to
detect
at least one of a specified protein binding, a specified antibody-antigen
coupling,
a specified hybridization of a DNA oligomer, or an adsorption of specified gas
CA 02760508 2011-10-28
WO 2010/127122 PCT/US2010/032976
4
molecules.
In Example 8, the subject matter of any one or more of Examples 1-7
optionally includes a sensing surface functionalized to adsorb gas molecules.
In Example 9, the subject matter of any one or more of Examples 1-8
optionally includes a sensing surface including an immobilized antibody, an
antibody fragment, or a nucleic acid probe.
In Example 10, the subject matter of any one or more of Examples 1-9
optionally includes a sensing surface configured to increase in mass in
response
to at least one of a specified protein binding, a specified antibody-antigen
coupling, a specified hybridization of a DNA oligomer, or an adsorption of
specified gas molecules.
In Example 11, the subject matter of any one or more of Examples 1-10
optionally includes an oscillator configured to operate using a shear mode of
mechanical oscillation of the resonator.
In Example 12, the subject matter of any one or more of Examples 1-11
optionally includes an oscillator configured to oscillate at the specified
operating
frequency when the apparatus is in contact with or surrounded by a liquid
medium.
In Example 13, the subject matter of any one or more of Examples 1-12
optionally includes an integrated circuit comprising a frequency counter
coupled
to the oscillator and configured to provide information indicative of an
oscillation frequency of the oscillator.
In Example 14, an apparatus includes a thin-film bulk acoustic resonator
array, each resonator comprising an acoustic mirror, a piezoelectric region
acoustically coupled to the acoustic mirror, a first conductor electrically
coupled
to the piezoelectric region, a second conductor electrically coupled to the
piezoelectric region and electrically insulated from the first conduct. In
this
example, the apparatus optionally includes an integrated circuit substrate
including an interface circuit, the first and second conductors of each
resonator
electrically coupled to the interface circuit, the integrated circuit
substrate
configured to mechanically support the resonator array, and each respective
acoustic mirror is configured to reduce or inhibit coupling of acoustic energy
from the respective piezoelectric region into the integrated circuit substrate
at or
CA 02760508 2011-10-28
WO 2010/127122 PCT/US2010/032976
near a resonant frequency of the respective thin-film bulk acoustic resonator
including the respective acoustic mirror. In this example, the array
optionally
includes an array of oscillators, each oscillator including at least one
acoustic
resonator and at least a portion of the interface circuit.
5 In Example 15, the subject matter of any one or more of Examples 1-14
optionally includes at least one oscillator in the array comprising a
resonator
having a sensing surface that is configured to detect at least one of a
specified
protein binding, a specified antibody-antigen coupling, a specified
hybridization
of a DNA oligomer, or an adsorption of specified gas molecules.
In Example 16, the subject matter of any one or more of Examples 1-15
optionally includes an integrated circuit comprising a frequency counter
coupled
to at least one oscillator included in the array, and configured to provide
information indicative of an oscillation frequency of the at least one
oscillator.
In Example 17, a method includes forming a thin-film bulk acoustic
resonator on an integrated circuit substrate, such as including forming an
acoustic mirror configured to reduce coupling of acoustic energy from a
piezoelectric region into the integrated circuit substrate at or near a
resonant
frequency of the thin-film bulk acoustic resonator, forming a piezoelectric
region
acoustically coupled to the acoustic mirror, and electrically coupling a first
conductor between a piezoelectric region and an interface circuit included in
the
integrated circuit substrate, electrically coupling a second conductor between
the
piezoelectric region and the interface circuit included in the integrated
circuit
substrate.
In Example 18, the subject matter of any one or more of Examples 1-17
optionally includes electrically coupling the first and second conductors to
the
piezoelectric region including depositing a metal.
In Example 19, the subject matter of any one or more of Examples 1-18
optionally includes depositing tungsten.
In Example 20, the subject matter of any one or more of Examples 1-19
optionally includes forming an acoustic mirror including forming alternating
layers of silicon dioxide and tungsten on a top surface of the integrated
circuit
substrate.
CA 02760508 2011-10-28
WO 2010/127122 PCT/US2010/032976
6
In Example 21, the subject matter of any one or more of Examples 1-20
optionally includes forming an array of thin-film bulk acoustic resonators on
the
integrated circuit substrate.
In Example 22, the subject matter of any one or more of Examples 1-21
optionally includes providing a sensing surface on the resonator to detect at
least
one of a specified protein binding, a specified antibody-antigen coupling, a
specified hybridization of a DNA oligomer, or an adsorption of specified gas
molecules.
In Example 23, the subject matter of any one or more of Examples 1-22
optionally includes functionalizing a sensing surface on the resonator to
promote
adsorption of specified gas molecules.
In Example 24, the subject matter of any one or more of Examples 1-23
optionally includes providing an oscillator using the resonator and at least a
portion of the interface circuit, an operating frequency of the oscillator
determined at least in part by a mass loading the piezoelectric region.
In Example 25, the subject matter of any one or more of Examples 1-24
includes providing a frequency counter configured to measure information
indicative of an oscillation frequency of the oscillator, using at least a
portion of
the interface circuit.
These examples can be combined in any permutation or combination.
This overview is intended to provide an overview of subject matter of the
present patent application. It is not intended to provide an exclusive or
exhaustive explanation of the invention. The detailed description is included
to
provide further information about the present patent application.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings, which are not necessarily drawn to scale, like numerals
may describe similar components in different views. Like numerals having
different letter suffixes may represent different instances of similar
components.
The drawings illustrate generally, by way of example, but not by way of
limitation, various embodiments discussed in the present document.
FIG. 1 illustrates generally an example of a side view of a section of a
thin-film bulk acoustic resonator (FBAR) and an interface circuit.
CA 02760508 2011-10-28
WO 2010/127122 PCT/US2010/032976
7
FIG. 2 illustrates generally an example of an oscillator circuit including
an FBAR, and interface circuitry.
FIG. 3 illustrates generally an example of a side view of a section of a
solidly-mounted FBAR including an acoustic mirror portion.
FIGS. 4A-I illustrate generally an example of post-CMOS fabrication of
a monolithic thin-film bulk acoustic resonator (FBAR), such as included in
array
of FBAR-CMOS oscillators.
FIG. 5 includes an SEM micrograph of an illustrative example of a
solidly-mounted monolithic FBAR, such as fabricated according to the
processing of the examples of FIGS. 4A-I.
FIG. 6 includes two die photos of an illustrative example of a 6x4 array
of FBAR-CMOS oscillators, including a first die photo after CMOS fabrication,
and a second die photo after fabrication of the FBAR structures, such as
fabricated according to the processing described in FIGS. 4A-I.
FIG. 7A-C illustrate generally illustrative examples of electrical
performance of a single FBAR structure, fabricated on glass.
FIG. 8 illustrates generally an illustrative example of a plot of oscillation
frequency versus a thickness of deposited silicon dioxide, such as for six
different FBAR-CMOS oscillators included the 6x4 array of the example of
FIG. 6.
DETAILED DESCRIPTION
In gravimetric biomolecular detection, a specific antibody, antibody
fragment, or nucleic acid probe can be immobilized on the surface of a
mechanical sensor, such as a mechanical resonator. Target molecules can bind
to the immobilized probe, further increasing the bound mass. In an example,
mass sensing can be performed by electrically monitoring the resonant
frequency
of a lightweight, high-Q mechanical resonator, such as in contact with such
bound material to be measured. An increase in mass at the resonator surface
causes an overall decrease in the mechanical, and hence electrical, resonant
frequency of the loaded system, and this frequency can be measured and used to
determine the mass addition, such as in real-time as the bound material
accumulates, and without requiring fluorescent labels.
CA 02760508 2011-10-28
WO 2010/127122 PCT/US2010/032976
8
Quartz crystal microbalances (QCMs) have been used to detect
antibodies and antigens, such as at sensitivities comparable to traditional
labeled
immunoassays. However, in a QCM, the resonant frequency can be limited by
the thickness of self-supporting quartz (e.g., in the megaHertz range). In a
resonant mass sensor, the extent of frequency change per unit mass can be
related to the square of the resonant frequency, thus limiting the QCM's
sensitivity. Moreover, centimeter-scale QCM sensors can preclude high-density
integration, which can limit QCM sensors to applications involving a
relatively
small number of target analytes.
In contrast, the present inventors have recognized thin-film bulk acoustic
resonators (FBARs) can allow for sensitivities orders of magnitude higher than
other resonant structures, such as QCMs, since FBARs can have resonant
frequencies in range of hundreds of megaHertz to several gigaHertz. For
example, an individual FBAR can be interfaced with active CMOS components
such as through wire-bonding or flip-chip connection approaches (e.g., an
"external" coupling approach). But, such an external coupling can prevent more
than one or two resonators from being integrated within a single chip. Thus,
the
present inventors have also recognized that monolithically integrating an FBAR
along with active CMOS components can allow for significantly smaller size
than the external coupling approach. Thus, an integrated array of FBARs can be
built directly above active drive and readout circuitry (e.g., including CMOS
circuitry). In an array of such mass sensors, one or more individual mass
sensors
included in the array can be functionalized in a specified manner, such as for
detecting binding of a specified protein, a specific antibody-antigen
coupling, a
specified hybridized DNA oligomer, or specified adsorbed gas molecules. An
array of such functionalized sensors can allow simultaneous, multiplexed, high-
sensitivity measurement of multiple targets (e.g., detection or measurement of
multiple, different, species) on a single monolithic sensor assembly.
In an example, the FBAR-CMOS sensor, or an array, can be used for an
immunoassay for industrial, medical, or agricultural use, among others, such
as
for identifying pathogens, contaminents, allergans, toxins, or other
compounds.
In another example, the FBAR-CMOS sensor, or an array, can be used as a
mass-sensor for gene-expression, either statically (e.g., at an endpoint of a
CA 02760508 2011-10-28
WO 2010/127122 PCT/US2010/032976
9
reaction) or in real-time. In yet another example, the FBAR-CMOS sensor, or
an array, can be used for gas sensing or air sample monitoring, such as in
response to surface modification (e.g., adsorption or vapor condensation) on a
sensing surface included as a portion of the FBAR-CMOS sensor or array. In
other examples, FBAR resonators can also be used in microwave circuit
applications. Such FBAR resonators can have relatively sharp resonances at
high frequency, such as for use in filters, oscillators, or as transformers
(e.g.,
transformers of voltage or impedance, etc.).
FIG. 1 illustrates generally an example of a side view 100 of a section of
a thin-film bulk acoustic resonator (FBAR) 102, including a sensing surface
116
electrically connected to a first electrode 112, a piezoelectric region 114, a
second electrode 110, and an interface circuit 104. In an example, the
interface
circuit 104 can be electrically connected to the FBAR 102 such as using a
first
electrical connection 106A and a second electrical connection 106B, such as
including a metal layer included in an integrated circuit. In an example, one
or
more of the first or second electrodes 112, 110 can include tungsten, such as
sputtered or deposited on an integrated circuit substrate. In another example,
one or more other metals can be used, such as gold, silver, etc. In the
example
of FIG. 1, the interface circuit 104 can provide an output 108, such as
carrying a
voltage, current, or other signal indicative of an oscillation frequency. In
an
example, the combination of the FBAR 102 and the interface circuit 104 can
provide an oscillator, such as including an operating frequency determined at
least in part by a mass bound to or otherwise loading the sensing surface 116.
In an illustrative example, the height of the FBAR 102 can be about 2
micrometers, and the width of the sensing surface 116 can be about 100
micrometers. In an example, the piezoelectric region 114 can include Zinc
Oxide (ZnO), lead zirconate titanate (PZT), or one or more other
piezopolymers,
piezoceramics, or other piezoelectric materials. In an example, the FBAR 102
can resonate using a shear mode of oscillation, such as at a resonant
operating
frequency in the range of about 500 megaHertz, to more than 2 gigaHertz. In an
example, such as shown in FIGS. 3, FIGS. 4A-I, and FIGS. 5-6, a mechanical
isolator, such as an acoustic mirror, can inhibit or prevent coupling of
acoustic
energy at or near the resonant operating frequency of the FBAR 102 into the
CA 02760508 2011-10-28
WO 2010/127122 PCT/US2010/032976
surrounding substrate, such as to provide a higher quality factor "Q" (e.g., a
more sharply-peaked resonant operating frequency).
FIG. 2 illustrates generally an example of an oscillator circuit 200
including an FBAR 202, and an interface circuit including MOS transistors M1-
5 M6. In an example, the circuit 200 of FIG. 2 can represent a single sensor
such
as included in an array of FBARs 202, such as a single sensor included in the
6x4 array shown in the examples of FIG. 6. In the example of FIG. 2, the FBAR
202 can be connected to an inverting CMOS amplifier 204, the amplifier 204
including the MOS transistors M1-M6 such as to form an integrated FBAR-
10 CMOS oscillator circuit 200. The MOS transistors Ml-M6 need not literally
include a metal gate, instead using polysilicon or other conductive gate
material,
such as fabricated using a commercial 0.18 micrometer CMOS fabrication
process. Similarly, in an example, a semiconductor material other than
silicon,
or an oxides other than silicon dioxide can be used to realize one or more of
transistors M1-M6.
In FIG. 2, the oscillator circuit 200 can include a Pierce oscillator
topology. For example, the inverting amplifier 204 can be implemented as three
in-line CMOS inverters realized by the MOS transistors M1-M6, such as to
provide gain to overcome the FBAR material losses, sustaining oscillation. In
the example of FIG. 2, a MOS transistor M7 can provide bias to MOS transistors
M1-M6. For example, transistor M7 can include a voltage-controlled gate, such
as adjusted to balance biasing strength against oscillator loading. In an
example,
transistor M7 can be controlled by a voltage at a node VBIAS, such as to
calibrate
the oscillator circuit or to otherwise accommodate variations in individual
FBAR
sensors due to design or fabrication variations, or other sources of
variation. In
an example, an output voltage at a node VOUT can be provided to a co-
integrated
or off-chip analog or digital frequency counter, such as to provide continuous
monitoring or sampling of the output frequency of the oscillator 200 during
specified intervals of operation (e.g., to measure a shift in frequency
corresponding to an increased mass, or for one or more other uses).
In the example of FIG. 2, a first capacitor C 1 and a second capacitor C2
can promote oscillator startup. For example, Cl and C2 can include metal-
insulator-metal (MIM) capacitors that can be set to approximately equal
values.
CA 02760508 2011-10-28
WO 2010/127122 PCT/US2010/032976
11
Again, the term metal-insulator-metal need not refer literally to metal
plates, as
capacitors Cl and C2 can be co-integrated on the same monolithic CMOS
integrated circuit as transistors M1-M7. In an illustrative example, the FBAR
102 can be represented by an equivalent Butterworth-Van Dyke circuit, as
shown in FIG. 2. In this illustrative example, Cm, Rm, and Lm can electrically
represent the motional components of the FBAR, and Co, and Rx can represent
the intrinsic electrical properties of the FBAR (e.g., the bulk properties of
the
piezoelectrical material, such as ZnO). In an example, the FBAR 102 can serve
as a high-Q resonant tank for the oscillator.
FIG. 3 illustrates generally an example of a side view of a section of a
solidly-mounted FBAR 300 including an acoustic mirror portion. In FIG. 3, the
FBAR 300 can be fabricated on top of a first, a second, and a third
passivation
region 320A-C of an integrated circuit, such as either a passive substrate 304
or
an active integrated circuit substrate 304. In another example, the FBAR 300
can be fabricated on an integrated circuit without passivation regions 320A-C
(e.g., such as in-line, prior to passivation, along with other processing
during
fabrication of the active circuitry portion of the sensor assembly). A first
electrode 312 can be electrically connected to a first top metal layer region
322A
of the integrated circuit, and a second electrode 310 can be electrically
connected
to a second top metal layer region 322B of the integrated circuit. The present
inventors have also recognized that a solidly-mounted FBAR 300 structure can
allow simple fabrication, such as described in FIGS. 4A-I, unlike other bulk
acoustic wave structures, such as those including a membrane. For example, the
FBAR 300, or an array of FBARs 300, can be built up via sequential deposition
and patterning of each layer without requiring undercutting or sacrificial
layer
integration processes, such as might be used in fabricating other types of
bulk
acoustic wave structures.
In the example of FIG. 3, the FBAR 300 can include a sensing surface
316, such as formed by a portion of the first electrode 312. The sensing
surface
316 can be coupled to a piezoelectric region 314 (e.g., ZnO or one or more
other
piezoelectric materials). Unlike the example of FIG. 1, the FBAR 300 of FIG. 3
includes an acoustic mirror, such as to mechanically isolate the mechanically
resonant portion of the FBAR 300 from the rest of the mechanically supporting
CA 02760508 2011-10-28
WO 2010/127122 PCT/US2010/032976
12
substrate 304 (e.g., below the passivation regions 320A-C). Generally, a
mechanical resonator can be mechanically isolated from its supporting
substrate,
such as to help avoid dissipating too much energy into its surroundings (which
could dampen and likely prevent oscillation). In some examples, this isolation
can be accomplished with an air gap, where the FBAR 300 structure can be
implemented as a membrane or cantilever structure. In other examples, the
isolation can be accomplished through a dielectric acoustic mirror. Such
isolation can allow the FBAR 300 to operate with a sharply-peaked resonant
response despite being solidly-mounted to the substrate 304. In the example of
FIG. 3, one or more alternating layers of relatively high- and relatively low-
acoustic-impedance material can be used, such as to provide a mechanical
analog to a distributed Bragg reflector.
For example, one or more of an insulating layer 318, and a conductive
layer 320 can each be about one-quarter of an acoustic wavelength thick, such
as
an acoustic wavelength in each respective material at or near a resonant
operating frequency of the FBAR 300. The combination of alternating layers 318
and 320 (e.g., such as including more alternating layers than shown in the
illustrative example of FIG. 3) can inhibit or prevent the mechanical coupling
of
acoustic energy into the substrate 304 in the region below the piezoelectric
region 314, the sensing surface 316, and the second electrode 310. For
example,
the layers 318 and 320 can be sized and shaped to promote constructive
interference of acoustic energy at or near the resonant operating frequency of
the
FBAR 300, at the interfaces between the layers 318 and 320, and between the
electrode 310 and the piezoelectric region 314, reflecting a majority of
acoustic
energy back towards the piezoelectric region 314. In an illustrative example,
the
conductive layer 320 can be tungsten, and the insulating layer 318 can be
silicon
dioxide, or one or more other insulating materials. In FIG. 3, the second
electrode 310 can also be used as the top layer in the acoustic mirror.
However,
in other examples, an insulator such as silicon dioxide can be used as the top
functional layer of the mirror, such as including a deposited or sputtered
thin-
film conductive coating to provide the second electrode 310 (e.g., including a
thin gold or silver layer, or other conductive material).
CA 02760508 2011-10-28
WO 2010/127122 PCT/US2010/032976
13
In an example, the sensing surface 316 can include or can be coated with
gold, silicon dioxide, laminated parylene, or one or more other biologically
compatible materials, such as in preparation for functionalization for
subsequent
detection of a change in mass associated with a specified protein binding, a
specified antibody-antigen coupling, a specified hybridization of a DNA
oligomer, or an adsorption of specified gas molecules, among others.
FIGS. 4A-I illustrate generally an example of post-CMOS fabrication of
a monolithic thin-film bulk acoustic resonator (FBAR) 400, such as included in
array of FBAR-CMOS oscillators. The fabrication processes of FIGS. 4A-I
need not require specialized fabrication techniques or non-standard CMOS
fabrication processes (e.g., such fabrication can include processing and
materials
similar to that used for commercial digital or mixed signal CMOS device
fabrication). In FIG. 4A, the post-CMOS fabrication of the FBAR 400A can
begin with a commercial integrated circuit substrate 404, such as including
one
or more openings in a passivation layer, exposing one or more metal regions.
In
an illustrative example, the integrated circuit substrate 404 can include an
active
CMOS substrate (e.g., an integrated circuit substrate including one or more
active devices or circuits), such as fabricated using a commercial 0.18 m
foundry CMOS process, or using one or more other fabrication processes.
In FIG. 4B, the post-CMOS substrate 404 can be patterned, such as using
a relatively thick photoresist layer (e.g., about 1 micrometers to 8
micrometers,
or using another thickness). Then, alternating layers of silicon dioxide
(e.g.,
about 750 nanometers thick) and tungsten (e.g., about 650 nanometers thick)
can
be formed on the substrate 404, such as by RF sputtering onto the patterned
substrate, such as including a metal layer 420, and an insulating layer 418,
similar to the layers discussed above in the example of the acoustic mirror of
FIG. 3. In FIG. 4B, since the photoresist layer can be relatively thick, the
exposure times can be increased correspondingly to compensate for pronounced
edge and corner beads.
In FIG. 4C, the metal layer 420 and insulating layer 418 in the regions
above the remaining photoresist can be lifted off (e.g., with ultrasonic
assistance), or otherwise removed from the FBAR 400C, leaving behind the
metal layer 420 and insulating layer 418, such as between the passivation
CA 02760508 2011-10-28
WO 2010/127122 PCT/US2010/032976
14
openings in the substrate 404, on a working top surface of the substrate 404.
In
an illustrative example, the metal layer 420 and insulating layer 418 can form
at
least a portion of an acoustic mirror as discussed above in FIG. 3. In FIG.
4D,
the FBAR 400D can again be patterned, and a top tungsten acoustic mirror layer
410 (or another conductive material) can be deposited or sputtered onto the
exposed portions of the FBAR 400D above a working top surface region of the
substrate 404. In an example, the top tungsten mirror layer 410 can also serve
as
the bottom electrode of the FBAR 400D, and this layer can connect to the top
metal layer of the CMOS substrate such as through an opening in the
passivation
layer 404. In FIG 4E, the unwanted portions of the tungsten mirror layer 410
can be lifted off or otherwise removed from the FBAR 400E.
In FIG. 4F, the FBAR 400F can be patterned, and a piezoelectric region
414 can be formed, such as including an RF sputtered zinc oxide layer (e.g.,
about 1450 nanometers thick), or including one or more other piezoelectric
materials. In FIG. 4G, the unwanted portions of the piezoelectric region 414
can
be lifted off or otherwise removed from the FBAR 400G. In an illustrative
example, the piezoelectric region can include a crystallographic orientation
(<002>) (e.g., indicating a strong c-axis piezoelectric crystal), such as
confirmed
through a sharp 34.4 peak in a 20 X-ray diffraction pattern.
In FIG. 4H, the FBAR 400H can be patterned, and a top electrode 416
can be sputtered or otherwise deposited. In FIG. 41, the unwanted portions of
the top electrode 416 can be lifted off, or otherwise removed from the FBAR
4001. In an example, the top electrode 416 can include a top tungsten contact
(e.g., about 200 nanometers thick) can be patterned and can connect through
CMOS top metal to the underlying circuitry (e.g., an oscillator, amplifier,
interconnect, or other circuitry elsewhere). In an example, the piezoelectric
material can provide insulation in a lateral region of the FBAR 400G, such as
to
prevent electrical shorting between the top electrode 416, and one or more
other
regions, such as the mirror layer 410.
FIG. 5 includes an SEM micrograph of an illustrative example of a
solidly-mounted monolithic FBAR, such as fabricated according to the
processing of the examples of FIGS. 4A-I. In this illustrative example, a
sensing
surface of the FBAR can be about square, such as about 100 micrometers by 100
CA 02760508 2011-10-28
WO 2010/127122 PCT/US2010/032976
micrometers, with a corresponding array density (e.g., as shown in FIG. 6)
limited primarily by the area of the individual FBAR sensors rather than any
underlying circuitry.
FIG. 6 includes two die photos of an illustrative example of a 6x4 array
5 of FBAR-CMOS oscillators, including a first die photo 600A after CMOS
fabrication but prior to fabrication of the FBAR structures, and a second die
photo 600B after fabrication of the FBAR structures, such as fabricated
according to the processing described in FIGS. 4A-I. In the illustrative
example
of the first die photo 600A, one or more test regions can be included on the
die,
10 such as for characterizing circuitry included in the die, or for testing
one or more
regions fabricated using similar materials or structures as used elsewhere in
the
array.
For example, in the second die photo 600B, the light bands near the top
edge of the photo can include one or more passive test structures, such as for
15 standalone testing of an active FBAR-CMOS oscillator or for testing of a
passive
FBAR resonator. Such testing can be used for characterization or calibration
of
one or more FBAR structures included in the array. In the illustrative example
of the second die photo 600B, each FBAR-CMOS element in the array can
occupy about 0.13 square millimeters, but it is believed that further
optimization
of the FBAR elements for particular sensing applications can lead to smaller
FBAR footprints and a higher array density in certain implementations. In an
illustrative example, such as the second die photo 600B, each FBAR-CMOS
oscillator can include its own acoustic mirror, isolated from the surrounding
oscillators, such as including one or more fabrication processes or structures
such as shown and discussed above in FIG. 3, and FIGS. 4A-I. In another
example, two or more FBAR structures can be formed or can incorporate a
commonly-shared "blanket" acoustic mirror, such as formed or built up in a
region underlying the two or more FBAR structures.
FIG. 7A-C illustrate generally illustrative examples of electrical
performance of a single FBAR structure similar to the structure shown and
discussed above in the examples of FIG. 3, and FIGS. 4A-I.
FIG. 7A shows an illustrative example of the S11 parameter 710 (e.g.,
proportional to the return loss, in dB) of the single FBAR plotted with
respect to
CA 02760508 2011-10-28
WO 2010/127122 PCT/US2010/032976
16
frequency 700 (in gigaHertz). In this illustrative example, the FBAR structure
is
fabricated on a glass substrate, including a blanketed acoustic mirror, and
demonstrates a first resonance 720 at about f0=905 megaHertz and a second
resonance 730 at about 2.18 gigaHertz, which are believed to be attributable
respectively to a shear and a longitudinal resonant mode of the FBAR, or might
be attributable to excitation of a higher-order mode related to a resonance of
the
combined assembly. The second resonance 730 has not been observed in the
integrated FBAR-CMOS device. The acoustic velocities of these modes share a
near-identical ratio. Also, the resonant quality factor "Q," can be
represented as
fo/Af (e.g., a "full-width half-maximum" or FWHM representation), and is
approximately 113 for the first resonance 720 and approximately 129 for the
second resonance 730. It is believed that correspondingly higher Qs might be
achievable with better tuning of the acoustic mirror (e.g., to provide more
effective reflection of acoustic energy or isolation between the resonator and
the
surrounding substrate).
FIG. 7B shows an illustrative example of the phase noise 750 (in dBc per
Hertz), plotted with respect to an offset frequency 740 (in Hertz), of an FBAR-
CMOS oscillator, including a measured noise of about -83dBc/Hertz at an offset
of 10 kiloHertz and about -104dBc/Hertz at an offset of 100 kiloHertz, both
measured from a carrier signal set at the fundamental frequency of
oscillation.
The relative slope regions of a phase noise plot 770 indicate a loaded Q for
the
oscillator of 218 in accordance with Leeson's phase noise relationship, where
a
knee in the plot 770 at f,,/2Q can represent a transition to a relatively
flat, white-
noise dominated phase noise response. In an example, when the sensor is used
to
provide an input to a frequency counter, measurement integration (e.g.,
averaging or integrating multiple frequency or interval measurements during a
specified measurement timeframe) can combat the effects of phase noise to
improve measurement resolution.
FIG. 7C shows an illustrative example of the output amplitude spectrum
780 (plotted in dB), with respect to frequency 700 (in megaHertz) as measured
at
the output of one on-chip FBAR-CMOS oscillator, including a peak 790 at about
864.5 megaHertz. In an array, such as shown in the photos of FIG. 6,
oscillators
across array can demonstrate a spread of -10 megaHertz in resonant frequency
CA 02760508 2011-10-28
WO 2010/127122 PCT/US2010/032976
17
as compared to one another, such as due to variations in zinc oxide thickness,
or
other factors. However, this variation does not hinder differential mass
measurements (e.g., measured at different times), such as where oscillation
frequency is measured both before and after mass addition, since the mass
sensitivity of the sensors can be relatively similar across the array (e.g., a
similar
offset in frequency occurs for a similar change in mass, independent of
variation
in a "baseline" resonant frequency).
FIG. 8 illustrates generally an illustrative example of a plot of an
oscillation frequency 810 (in megaHertz) versus a thickness 800 (in
nanometers)
of deposited silicon dioxide, such as for six different FBAR-CMOS oscillators
included a 6x4 array of the example of FIG. 6. In this illustrative example,
the
fundamental oscillation frequencies of each of the six oscillators can be
measured first as a baseline, after which mass can be added (e.g., by forming
successive layers of patterned silicon dioxide, RF sputtered onto the FBAR top
surfaces, such as a sensing surface). Frequency measurements can then be taken
after each addition of mass, such as emulating the field behavior of such
sensors
as mass accretes or binds to a corresponding functionalized sensing surface.
In
this illustrative example, all oscillators that completed the mass series are
shown,
while those not depicted failed either before or during the testing process
(e.g.,
did not sustain measurable oscillation). The frequency sensitivity of an FBAR
to
mass additions (e.g., change in frequency per unit mass addition) can be
represented by the Sauerbrey equation, as A f = -(f 2Am / NAp), where f, can
represent the operating frequency, Am can represent the mass addition, N can
represent a sensitivity constant, A can represent the active area, and p can
represent the density. The Sauerbrey equation predicts a linear change in
frequency for small additions of uniform-thickness mass, similar to the
responses shown in the illustrative example of FIG. 8, with the average mass
sensitivity of the examples of FIG. 8 representing about 3.05x1012 grams/Hertz
centimeter2, which is well above the sensitivity of a typical QCM (about 6 x
10-9
grams/Hertz centimeter) and comparable to off-chip FBAR sensors.
CA 02760508 2011-10-28
WO 2010/127122 PCT/US2010/032976
18
Additional Notes
The above detailed description includes references to the accompanying
drawings, which form a part of the detailed description. The drawings show, by
way of illustration, specific embodiments in which the invention can be
practiced. These embodiments are also referred to herein as "examples." Such
examples can include elements in addition to those shown or described.
However, the present inventors also contemplate examples in which only those
elements shown or described are provided. Moreover, the present inventors also
contemplate examples using any combination or permutation of those elements
shown or described (or one or more aspects thereof), either with respect to a
particular example (or one or more aspects thereof), or with respect to other
examples (or one or more aspects thereof) shown or described herein.
All publications, patents, and patent documents referred to in this
document are incorporated by reference herein in their entirety, as though
individually incorporated by reference. In the event of inconsistent usages
between this document and those documents so incorporated by reference, the
usage in the incorporated reference(s) should be considered supplementary to
that of this document; for irreconcilable inconsistencies, the usage in this
document controls.
In this document, the terms "a" or "an" are used, as is common in patent
documents, to include one or more than one, independent of any other instances
or usages of "at least one" or "one or more." In this document, the term "or"
is
used to refer to a nonexclusive or, such that "A or B" includes "A but not B,"
"B
but not A," and "A and B," unless otherwise indicated. In the appended claims,
the terms "including" and "in which" are used as the plain-English equivalents
of the respective terms "comprising" and "wherein." Also, in the following
claims, the terms "including" and "comprising" are open-ended, that is, a
system, device, article, or process that includes elements in addition to
those
listed after such a term in a claim are still deemed to fall within the scope
of that
claim. Moreover, in the following claims, the terms "first," "second," and
"third," etc. are used merely as labels, and are not intended to impose
numerical
requirements on their objects.
CA 02760508 2011-10-28
WO 2010/127122 PCT/US2010/032976
19
The above description is intended to be illustrative, and not restrictive.
For example, the above-described examples (or one or more aspects thereof)
may be used in combination with each other. Other embodiments can be used,
such as by one of ordinary skill in the art upon reviewing the above
description.
The Abstract is provided to comply with 37 C.F.R. 1.72(b), to allow the
reader
to quickly ascertain the nature of the technical disclosure. It is submitted
with
the understanding that it will not be used to interpret or limit the scope or
meaning of the claims. Also, in the above Detailed Description, various
features
may be grouped together to streamline the disclosure. This should not be
interpreted as intending that an unclaimed disclosed feature is essential to
any
claim. Rather, inventive subject matter may lie in less than all features of a
particular disclosed embodiment. Thus, the following claims are hereby
incorporated into the Detailed Description, with each claim standing on its
own
as a separate embodiment, and it is contemplated that such embodiments can be
combined with each other in various combinations or permutations. The scope
of the invention should be determined with reference to the appended claims,
along with the full scope of equivalents to which such claims are entitled.
