Note: Descriptions are shown in the official language in which they were submitted.
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FLEXURAL PLATE WAVE SENSOR ARRAY
This is a division of copending Canadian Patent Application No. 2,404,137,
filed March 20, 2001.
FIELD OF INVENTION
This invention relates generally to a flexural plate wave sensor and array,
and
more particularly to a flexural plate wave sensor having reduced dimensions
which
enable the array to have an increased density of sensors on a single silicon
wafer.
BACKGROUND OF INVENTION
Flexural plate wave (FPW) devices are gravimetric sensors capable of detecting
mass changes as small as 10-" g. Typically, FPW devices are built with a bulk
micro-
machining process which produces a thin film membrane of silicon or silicon
nitride by
etching a cavity through the entire thickness of the silicon wafer with a
selective process
which does not attack the membrane material. However, due to the crystal
structure of
the silicon wafer, the cavity produced by this etching process has interior
walls which
extend through the silicon wafer at an angle of 126° from the membrane.
This results in
the cavity having an opening at the bottom surface of the substrate which is
at least
twice as large as the area of the membrane. Accordingly, the smallest possible
FP W
device built utilizing the prior art bulk micromachining process is
approximately 1 mm x
1 mm, since, for this one square millimeter of area on the surface of the
silicon wafer, at
least twice as much area is required on the bottom of the wafer. Therefore,
only small
numbers of FPW sensors can be integrated onto the same silicon chip for
exposure to
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the same environment. For applications which require several sensors with
different
coatings, several packaged sensors must be integrated onto a sensor assembly
and
exposed to a gas or liquid sample stream. This method is only practical for
applications
requiring less than approximately 20 separate sensors.
BRIEF SUN11~IARY OF THE INVENTION
It is therefore an object of this invention to provide a flexural plate wave
sensor
having reduced areal dimensions which enable an array of sensors to have an
increased
density of sensors on a single silicon wafer.
It is a further object of this invention to provide such a flexural plate wave
sensor
including a sensor membrane having increased sensitivity.
It is a further object of this invention to provide such a flexural plate wave
sensor
in which the sensor membrane is not sealed on one side, thereby eliminating
atmospheric pressure variations in the membrane tension.
It is yet a further object of the invention to provide such a flexural plate
wave
sensor in which the absorptive coating is separate from the electrical
components for
sensing elements in fluid environments.
It is a yet further object of this invention to provide a flexural plate wave
sensor
array having increased packing density which enables a greater number of
sensors to be
fit on a single silicon chip.
It is a further object of this invention to provide a method of making a
flexural
plate wave sensor in which the sensor membrane is not exposed until the end of
the
manufacturing of the sensor.
This invention results from the realization that a truly effective flexural
plate
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wave sensor can be obtained by bulk machining the silicon wafer to form a
sensor
having a cavity with substantially parallel interior wails, adding an etch
stop layer and a
membrane layer to the wafer, adding an absorptive coating on the membrane
layer and
transducers on the membrane layer opposite the absorptive coating. This
construction
facilitates the formation of an array of sensors having increased packing
density on the
silicon wafer.
This invention features a method for manufacturing a flexural plate wave
sensor
including the steps of depositing an etch-stop layer over a substrate,
depositing a
membrane layer over the etch stop layer, depositing a piezoelectric layer over
the
membrane layer, forming a first transducer on the piezoelectric layer and
forming a
second transducer on the piezoelectric layer, spaced from the first
transducer. The
method fi.uther includes the steps of etching a cavity through the substrate,
the cavity
having substantially parallel interior walls, removing the portion of the etch
stop layer
between the cavity and the membrane Layer to expose a portion of the membrane
layer,
and depositing an absorptive coating on the exposed portion of the membrane
layer.
Tn a preferred embodiment, the method may further include the steps of etching
a hole in the piezoelectric layer and forming a ground contact on the silicon
membrane
layer.
This invention also features a flexural plate wave sensor including a base
substrate, an etch stop layer disposed over the base substrate, a membrane
Iayer disposed
over the etch stop layer and a cavity having substantially parallel interior
walls disposed
in the base substrate and the etch stop layer, thereby exposing a portion of
the membrane
layer. The flexural plate wave sensor further includes an absorptive coating
disposed on
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the exposed portion of the membrane layer within the cavity, a piezoelectric
layer
disposed over the membrane layer, a first transducer disposed on the
piezoelectric layer,
and a second transducer disposed on the piezoelectric layer, spaced from the
first
transducer.
In a preferred embodiment, the first and second transducers may be
interdigitated transducers. The first and second transducers may be formed
from TiPtAu
or from aluminum. The piezoelectric layer may be formed from aluminum nitride,
lead
zirconium titanate or zinc oxide. The etch stop layer may be foamed from
silicon
dioxide or from silicon and the base substrate may be formed from silicon.
This invention also features a method for manufacturing a flexural plate wave
sensor including the steps of depositing a sacrificial material layer over a
silicon
substrate, depositing a membrane layer over the sacrificial material layer
with the
membrane layer covering the sacrificial material layer and contacting the
silicon
substrate and depositing a piezoelectric layer over the membrane layer. The
method
fi.uther includes forming a first transducer on the piezoelectric layer,
forming a second
transducer on the piezoelectric layer, spaced from the first transducer,
removing the
sacrificial material layer to expose a portion of the membrane layer and
depositing an
absorptive costing on the exposed portion of the membrane layer.
This invention also features a flexural plate wave sensor including a
substrate, a
membrane layer disposed on the substrate, the membrane layer having legs in
contact
with the substrate and a body portion spanning between the legs. The
substrate, a lower
surface of the body portion and interior surfaces of the legs define a cavity
between the
substrate and the body portion. The flexural plate wave sensor further
includes an
absorptive coating disposed on the lower surface of the body portion of the
membrane
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layer, a piezoelectric layer disposed over an upper surface of the membrane
material, a
first transducer disposed on the piezoelectric layer and a second transducer
disposed on
the piezoelectric layer, spaced from the first transducer.
In a preferred embodiment, the substrate may be formed from silicon and the
membrane layer may be formed from silicon. The first and second transducers
may be
interdigitated transducers that may be formed from TiPtAu or from aluminum.
The
piezoelectric layer may be formed from aluminum nitride, lead zirconium
titanate or
zinc oxide.
This invention also features a method for manufacturing a flexural plate wave
sensor including the steps of depositing a membrane layer on a substrate
having a
concave upper surface, thereby forming a cavity between an exposed portion of
the
membrane layer and the substrate, depositing a piezoelectric layer on the
membrane
layer, forming a first transducer on the piezoelectric layer, forming a second
transducer
on the piezoelectric layer, spaced from the first transducer, and depositing
an absorptive
coating on the exposed portion of the membrane layer within the cavity.
This invention also features a flexural plate wave sensor including a
substrate
having a recess disposed in an upper surface thereof, a membrane layer
disposed on the
upper surface of the substrate, a cavity disposed between a portion of the
membrane
layer and the recess in the substrate and a piezoelectric layer disposed on
the membrane
layer. The flexural plate wave sensor further includes a first transducer
disposed on the
piezoelectric layer, a second transducer disposed on the piezoelectric layer,
spaced from
the first transducer and an absorptive coating disposed on the portion of the
membrane
layer within the cavity.
In a preferred embodiment, the substrate may be formed from a material
selected
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from silicon or PYREXO material. The first and second transducers may be
interdigitated transducers formed from TiPtAu or from aluminum. The
piezoelectric
layer may be formed from aluminum nitrate, lead zirconium titanate or zinc
oxide.
This invention also features a flexural plate wave sensor array including a
substrate, and a plurality of flexural plate wave sensors. Each sensor
includes a cavity
formed in the substrate, a thin film membrane layer spanning the cavity, a
piezoelectric
layer disposed on the thin film membrane layer, a transducer disposed on the
piezoelectric layer and an absorptive coating disposed on the thin film
membrane layer
within the cavity. The cavity of each of the sensors includes interior walls
that are
substantially parallel to each other and to the interior walls of adjacent
sensors.
This invention also features a flexural plate wave sensor array including a
substrate and a plurality of flexural plate wave sensors. Each sensor includes
a cavity
formed in the substrate, a thin film membrane layer spanning the cavity, a
piezoelectric layer disposed on the thin film membrane layer, a transducer
disposed on
the piezoelectric layer and an absorptive coating disposed on the thin film
membrane
layer within the cavity. The distance between adjacent sensors is no greater
than 0.9
This invention also features a flexural plate wave sensor array including a
substrate,
a plurality of flexural plate wave sensors, each sensor including a cavity
formed in the
substrate, a thin film membrane layer spanning the cavity, a piezoelectric
layer disposed
on the thin film membrane layer, a transducer disposed on the piezoelectric
layer and an
absorptive coating disposed on the thin film membrane layer within said
cavity; a
reference flexural plate wave sensor including a cavity formed in the
substrate, a thin
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film membrane layer spanning the cavity, a piezoelectric layer disposed on the
thin film
membrane layer and a transducer disposed on the piezoelectric layer; and a
microprocessor electrically connected to each of the plurality of flexural
plate wave
sensors and the reference flexural plate wave sensor, for monitoring resonant
frequency
characteristics of the sensors.
In a preferred embodiment, the reference sensor may monitor the effects of
environmental factors on the sensors and the microprocessor adjusts the
resonant
frequency of the sensors to compensate for the environmental factors.
This invention also features a flexural plate wave sensor array including a
substrate and a plurality of flexural plate wave sensors, each sensor
including a cavity
formed in the substrate, a thin film membrane layer spanning the cavity, a
piezoelectric
layer disposed on the thin film membrane layer, a transducer disposed on the
piezoelectric layer and a plurality of discrete absorptive coatings disposed
on the thin
film membrane layer within the cavity. The cavity of each of the sensors
includes
interior walls which are substantially parallel to each other and to the
interior walls of
adjacent sensors.
This invention also features a flexural plate wave sensor array including a
substrate; a plurality of flexural plate wave sensors, each sensor including a
cavity
formed in the substrate, a thin film membrane layer spanning the cavity, a
piezoelectric layer disposed on the thin film membrane layer, a transducer
disposed on
the piezoelectric layer and an absorptive coating disposed on the thin film
membrane
layer within the cavity; a drive amplifier which receives a drive input and
outputs an
amplified drive output; a multiplexer which receives the amplified drive
output and a
selection signal, for driving one of the plurality of flexural plate wave
sensors; and an
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output amplifier which senses an output from the one of the plurality of
flexural plate
sensors and outputs an amplified sensed signal.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention, taken in conjunction with the invention disclosed in
copending Canadian Patent Application Serial Number 2,904,137, filed March 20,
2001,
will be discussed in detail hereinbelow with the aid of the accompanying
drawings,
wherein:
Fig. 1A is a cross-sectional side view of a prior art bulk machined flexural
plate
wave sensor;
Fig. 1 B is a top view of the prior art bulk machined flexural plate wave
sensor;
Figs. 2A-2G are cross-sectional side views showing the steps involved in the
method of manufacturing the flexural plate wave sensor in accordance with the
present
invention;
Fig. 3 is a schematic diagram of a flexural plate wave sensor array in
accordance
with the present invention;
Figs. 4A-4F are cross-sectional side views showing the steps involved in the
method of manufacturing a second embodiment of the flexural plate wave sensor
of the
present invention;
Fig. 5 is a schematic diagram of a flexural plate wave sensor array in
accordance
with the second embodiment of the present invention;
Figs. 6A-6G are cross-sectional side views showing the steps involved in the
method of manufacturing a third embodiment of the flexural plate wave sensor
according to the present invention; and
Figs. 7 is a schematic diagram of the input and output circuitry of a flexural
plate
wave sensor array in accordance with the present invention.
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PREFERRED EMBODIMENT
Flexural plate wave (FPW) sensors are used to sense pressure, acceleration,
density of liquids, viscosity of liquids, to detect chemical vapors and to
detect
biochemical interactions. The surface micromachined FPW sensor includes a thin
film
membrane, a piezoelectric layer over the membrane, an input interdigitated
transducer
(IDT) disposed on the piezoelectric layer adjacent a first section of the
membrane and
an output IDT disposed on the piezoelectric layer adjacent a second section of
the
membrane. The input IDT responds to an input via piezoelectric transduction to
send an
acoustic plate wave across the membrane where it is received by the output IDT
where
,piezoelectric transduction creates an output. The velocity ofthe acoustic
plate wave is
dependent upon the membrane material and the mass per unit area of the
membrane.
The exposed portion of the membrane is coated with an absorptive coating in
order to
provide an indication of the detection of an analyte. Different absorptive
coatings may
be used in an array to detect different substances. The change of the mass per
unit area
caused by the absorption of an analyte by the absorptive coating on the
membrane
provides a shift in velocity of the acoustic plate wave sent across the
membrane from the
input IDT to the output )DT and a concomitant frequency shift. The shift in
frequency
is detected, indicating that the target chemical vapor or substance has been
detected by
the sensor.
A prior art bulk micromachined FPW device 10 is shown in Figs. IA and 1B.
The FPW device 10 includes a substrate 12 of undoped silicon. A layer of
membrane
material 14 is deposited on a surface of the silicon substrate 12. A
piezoelectric layer 16
is deposited on the layer of membrane material l4. An input IDT 18 is disposed
on the
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piezoelectric layer of material 16 proximate a first portion of the membrane
layer 14 and
an output IDT 20 is disposed on the piezoelectric layer I6 proximate a second
portion of
the membrane layer 14.
Utilizing bulk micromachining techniques, a cavity 22 is etched into the
silicon
substrate 12 such that a section of the membrane layer 14 is exposed. The
exposed
sectzon of the membrane layer I4 is coated with an absorptive coating 24 such
that
absorption by the absorptive coating 24 the target substance is detected by
the device 10.
However, physical limitations of the bulk micromachining process in the
fomlation of
cavity 22 results in the cavity having interior walls 26 and 28 which are
formed at an
angle n of I26°. Accordingly, due to the aspect ratio of the height
relative to the width
of the cavity 22 etched into the silicon substrate, the size of the membrane
layer 14 can
be no smaller than approximately Imm x Imm, and the minimum spacing between
adjacent sensors in an array can be no less than approximately Imm.
Given the size limitation of the exposed surface of the membrane layer, in
order
to detect a large number of components of a gas or liquid stream, a large
number of
sensors must be provided. The physical size of a sensor assembly incorporating
a large
number of sensors must also be large in order to provide detailed analyses of
the fluid
being tested. Accordingly, detailed analysis of a sample becomes cumbersome
and
difficult to manage since there are multiple large sensors, each of which must
be
exposed to the same liquid or gas sample.
Figs. 2A-2G illustrate the steps involved in the method of manufacturing the
flexural plate wave sensor in accordance with the present invention. Shown in
Fig. 2A
is a silicon-on-insulator (SOI) wafer 30, which includes a silicon substrate
32, a silicon
dioxide etch stop layer 34 on the silicon substrate 32 and a silicon membrane
layer 36 on
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the silicon dioxide etch stop layer 34. In the preferred embodiment, the
silicon substrate
32 is approximately 400 microns thick, the silicon dioxide etch stop layer 34
is 1 micron
thick and the silicon membrane layer 36 is 2 microns thick. It is a portion of
this silicon
layer 36 which, in the finished sensor shown in Fig. 2G, is the thin film
membrane
through which the acoustic plate wave is transmitted.
A piezoelectric layer 38, Fig. 2B, is then applied to the upper surface of the
silicon membrane layer 36. Piezoelectric layer 38 has a thickness of 0.5
microns and
can be formed from any piezoelectric material, such as aluminum nitride or
zinc oxide.
A hole 40, Fig. 2C, is then etched into the piezoelectric layer 38 to the
surface of the
silicon membrane layer 36. A ground terminal 42, Fig. 2D, is deposited in the
hole 40,
in contact with the silicon layer 36, and input IDT 44 and output IDT 48 are
deposited
on piezoelectric layer 38. Ground terminal 42, input IDT 44 and output IDT 48
are
preferably formed from a 0.1 micron thick layer of TiPtAu metal.
Alternatively, ground
terminal 42, IDT 44 and IDT 48 may be formed from aluminum. Using an
inductively
coupled plasma (ICP) etch machine, a cavity 50, Fig. 2E, is etched into the
silicon
substrate 32 up to, but not including the silicon dioxide etch stop layer 34,
which acts as
an etch stop for the ICP process. The exposed portion 51 of the silicon
dioxide etch stop
layer 34 is then removed by dipping the portion 51 into buffered hydrofluoric
acid,
thereby exposing a portion of the silicon layer 36 to form thin film membrane
S3. As
shown in Fig. 2F, the resulting sensor 60 includes a cavity 52 having interior
walls 54a
and S4b which are much less than 126°: they are substantially parallel
to each other.
An absorptive coating S6, Fig. 2G, is then applied to the exposed surface of
thin film
membrane S3 of silicon layer 36.
In operation, the input IDT 44 transmits an acoustic plate wave across the
thin
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film silicon membrane 53 in the direction of arrow 62, where it is received by
the output
IDT 48. As long as the absorptive coating 56 does not absorb any of the target
substance, the mass per unit area of the membrtarte 36 remains constant,
resulting in a
constant frequency of the acoustic plate wave. However, as the absorptive
coating 56
absorbs the target substance, the mass per unit area of the membrane
increases. This
results in a shift in the velocity of the acoustic plate wave and,
consequently, a
frequency shift in the wave received by the output IDT 48. This frequency
shift is
recognized as an indication that the target substance has been detected by the
sensor 60.
Alternatively, more than one type of absorptive coating 56 may be applied to
the
membrane 53 of each sensor. This enables each sensor to detect different
analytes
which may be absorbed by the different coatings.
An array 70 of flexural plate wave sensors 60 is shown in Fig. 3. Due to the
process described above with reference to Figs. 2A-2G, the resulting sensor 60
can be
made as small as approximately 500 microns by 100 microns. Furthermore, since
the
interior walls 54a and 54b of cavity 52 are substantially parallel to each
other and to the
interior walls of adjacent sensors, these sensors can be more densely packed
onto the
substrate 32. As shown in Fig. 3, the spacing between adjacent sensors 60 can
be as
little as 100 microns. This configuration enables an increased number of
sensors 60 to
be fit onto a single silicon wafer, thereby enabling a increased number of
substances to
be detected with the use of a single silicon chip.
The steps involved in the method of making a second embodiment of the
flexural plate wave sensor are illustrated in Figs. 4A-4F. The process begins
with a
silicon substrate 80, Fig. 4A on which a sacrificial layer of material 82 is
deposited, Fig.
4B. The sacrificial layer 82 is any material that can be easily removed from
the
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substrate 80, such as glass or a photoresistive material. A structural layer
84 is
deposited over the sacrificial layer 82, Fig. 4C. The structural layer 84
covers the top
and sides of the sacrificial layer 82 and contacts the substrate 80.
Piezoelectric layer 86
is deposited over the structural layer 84, Fig. 4D. An input IDT 88 and an
output IDT
90 are then deposited on the piezoelectric layer 86, Fig. 4E. The sacrificial
layer 82 is
then etched away to create a cavity 92 with a thin film membrane 93 disposed
between
the cavity 92 and the piezoelectric layer 86. An absorptive coating 94 is then
deposited
on the exposed surface of the thin film membrane 93, Fig. 4F. The resulting
sensor 100,
Fig. 4F is sized similarly to the sensor 60, Fig. 2G, and operates in the same
manner.
An array 102 of sensors 100 is shown in Fig. 5, where it can be seen that, due
to
this particular method of manufacturing, with the shallower angled sides on
the cavity,
the sensors 100 require as little as 100 microns between adjacent sensors,
thereby
increasing the packing density of the array 102.
A reference sensor 200, which is formed identically to sensors 100, but does
not
include the absorbtive coating, is used to monitor the effects of
environmental factors,
such as temperature and pressure on the resonant frequency characteristics of
the sensors
100. Each of the sensors 100 and sensor 200 is connected to a microprocessor
104 via
lines 106a-106d. Microprocessor 104 monitors the resonant firequency
characteristic of
sensors 100 and 200 independently, so that environmental factors as sensed by
sensor
200 can be compensated for.
The sensor input and output circuitry for the sensor array 102 and generally
shown at 300 in fig. 7. Circuitry 300 includes a drive amplifier 302 which is
a high-
again single-ended input-to-differential output amplifier, which receives an
input on line
310 and outputs a differential signal on lines 312. Multiplexer 304 receives
the
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differential outputs on lines 312 and, based on a selection signal present on
lines 314,
selects one of the n sensors 100 of array 102 to activate by providing a
differential drive
signal to the selected sensor 100 on lines 316.- The differential output of
the selected
sensor 100 is input to amplifier section 308 on lines 318. Amplifier section
308 includes
high-again amplifiers 320 that are configured as an instrumentation amplifier.
This
configuration allows for symmetrical loading on each sensor output, high
common-
mode signal rejection, and higher gains for a given limited bandwidth.
Amplifier
section 308 outputs the amplified sensor output on line 322. This
configuration enables
a single input/output device to drive and monitor an array of sensors. The
multiplexes
304 can be operated to cycle through each sensor in the array. Accordingly, an
array
having a number n of sensors, each having a different absorbtive coating for
detecting
different analytes, can be driven with a single input/output device which
cycles through
the sensors for detecting the presence of a number of analytes.
Figs. 6A-6G illustrate the steps involved in the method of making a third
embodiment of the flexural plate wave sensor. First, a silicon-on-insulator
layer 108,
including a silicon layer I 10, a silicon dioxide layer 112 and a silicon
handle wafer 114,
Fig. 6A is deposited on a substrate 116. The substrate 116 may be formed from
PYREX~ brand glass from Coming or silicon, and has a cavity 118 disposed on an
upper surface thereof, Fig. 6B. The resulting structure I 17 is shown in Fig.
6C. The
silicon handle wafer 114 and silicon dioxide layer 1 12 are then etched away.
leaving the
silicon layer exposed, thereby forming a thin film membrane I I 9 over the
cavity 1 I 8,
Fig. 6D. A piezoelectric layer 120 is then deposited over the silicon layer 1
10, Fig. 6E.
An input IDT 120 and an output IDT 122 are then formed on the piezoelectric
layer 120
over the silicon membrane I 19, Fig. 6F. Finally, an absorptive coating 124 is
deposited
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over the bottom surface of the silicon membrane 119 to form the flexural plate
wave
sensor 130, Fig. 6F. The sensor 130 is sized similarly to the sensor 60, Fig.
2G, and
operates in the same manner.
It can therefore be seen, that, due to the manufacturing processes involved in
fabricating the flexural plate wave sensors of the present invention, the size
of the
membrane can be greatly reduced compared to prior art sensors. This reduction
in the
size of the thin film membrane allows the thickness of the thin film membrane
to be
greatly reduced. Since the sensitivity of the sensor is inversely proportional
to the mass
per unit area of the thin film membrane, a membrane which is ten times thinner
than a
prior art sensor is consequently ten times more sensitive than the prior art
sensor.
Furthermore, as described above, the reduction in the area of the sensors
combined with
the feature of the substantially parallel interior walls of the cavities of
the sensors allows
a greater packing density of the sensors, resulting in a greater number of
sensors being
disposed on a single silicon chip.
The FPW sensors produced by the above described methods have numerous
applications, including a gas analyzer device capable of detecting the
presence and
concentration of hundreds of molecular components with less than one part per
billion
minimum detectable concentration sensitivity. The FPW sensor could be
incorporated
into a liquid analyzing device capable of analyzing samples for several
hundred possible
contaminants or components simultaneously. The FPW sensor could also be
utilized as
part of a DNA sequencing device, as a virus/antibody detection device and for
biological
weapon detection.
Although specific features of the invention are shown in some drawings and
not in others, this is for convenience only as each feature may be combined
with any
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or al( of the other features in accordance with the invention.
