Note: Descriptions are shown in the official language in which they were submitted.
CA 02377189 2001-12-21
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MICRO-ELECTROMECHANICAL DEVICES AND
METHODS OF MANUFACTURE
Related Anulication
The present application claims the benefit of U.S. provisional application
serial no. 60/141,413, filed on June 29, 1999, which is hereby incorporated by
reference in its entirety.
Field of the Invention
The present invention relates to the field of micro-electromechanical
devices. More particularly, the present invention provides micro-
electromechanical devices, substrate assemblies for forming such devices, and
methods of manufacturing the substrate assemblies and devices.
Background
Micro-electromechanical devices such as pressure sensors, actuators, etc.
provide advantages in many different applications. Two basic approaches have
been developed to manufacture the devices using many well-known conventional
integrated circuit manufacturing techniques. The two basic approaches are
typically referred to as surface micromachining and backside bulk
micromachining.
Conventional surface-micromachining technologies do, however, have
several tradeoffs. Surface micromachining typically has the advantage of
superior registration since all layers are defined on the same side of the
wafer
using conventional planar lithographic processes. The transducer elements
(piezoelectric capacitors, piezoresistive elements, etc.) can be aligned
precisely
to the same features that define the mechanical structure being formed.
Moreover, because definition of the mechanical structure is performed with
standard integrated circuit (IC) processes, the features can be transferred in
a
repeatable and precise manner. Good alignment of transducer elements to a
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high-resolution mechanical structure creates devices with high efficiency
(performance) and repeatable characteristics.
Surface micromachining processes do, however, typically require
encapsulation layers that are impervious to very aggressive chemicals used in
forming the features on the front surface of the devices. By definition,
surface-
micromachining exposes the surface of the wafer to a variety of etchants and
other removal processes. For example, in the case of sacrificial
phosphosilicate
glass (PSG) layers, the encapsulation layer may need to protect all vulnerable
layers from highly-concentrated hydrofluoric acid (HF) liquid or vapor. In the
case of sacrificial polysilicon and/or silicon layers, the encapsulation layer
may
need to protect all other materials from a heated potassium-hydroxide (KOH) or
tetramethyl-ammonium-hydroxide (TMAH) solution.
Regardless, the encapsulation layer typically adds to the bulk of the
devices and may also affect performance by, for example, increasing the
stiffness
of a diaphragm used in a pressure sensor or actuator. That additional
stiffness
may reduce sensitivity (in the case of, e.g., a sensor) and/or it may increase
the
power requirements for operating an actuator. To address these issues, it may
be
desirable to reduce the thickness of an encapsulation layer. Reducing the
encapsulation layer thickness, however, increases the likelihood that the
underlying features on the device will not be adequately protected, thereby
reducing product yield. As a result, surface micromachined device performance
is often limited by availability of a thin film technology suitable for
encapsulation layers.
Conventional backside bulk-micromachining techniques also have
tradeoffs. The essence of backside silicon micromachining separates the
machining operation from the fme-featured frontside. However, this same
characteristic also leads to the limitations of backside bulk-micromachining.
When the machining proceeds from the backside, front-to-backside alignment
defines the registration between the mechanical structure and the transducer
elements. Generally, the overlay capabilities are coarse and performance is
lost.
Perhaps even more significant, the silicon etch proceeding from the
backside will reach the frontside at a position dependent on the wafer
thickness
and/or etch profile distribution. For potassium hydroxide (KOH) etching with
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anisotropic sidewall angle consistently near 53°, each SO~,m variation
in wafer
thickness will result in about 30~m variation in the finished position of the
mechanical feature at the front. In the case of Deep Reactive Ion Etching
(DRIE), the etch profile varies with feature size and wafer position so that
variations in finished dimensions can also be on the order of tens or even
hundreds of microns within a single wafer. Combining alignment errors and etch
profile variations, bulk machining techniques often lead to large
discrepancies
between the frontside-defined transducer elements and the mechanical
structure.
Summary of the Invention
The present invention provides micro-electromechanical devices,
substrate assemblies from which the devices can be manufactured, and methods
to manufacture the devices. The invention combines the advantages of
conventional surface and bulk micromachining processes to create an integrated
micro-electromechanical system (MEMS) technology that provides high
performance, high yield, and manufacturing tolerance. The devices
manufactured according to the present invention include, but are not limited
to,
pressure sensors, vibration sensors, accelerometers, gas or liquid pumps, flow
sensors, resonant devices, and infrared detectors.
One advantage of the present invention is that the mechanical integrity of
the substrates on which the devices are formed is maintained until the last
processing steps. By maintaining the mechanical integrity of the substrate
while
all of the front side processing is performed accuracy in the alignment of the
various structures on the device can be improved as compared to known methods
of manufacturing such devices. As discussed above, in methods in which voids
are formed in the substrate before all of the front side processing is
complete
(including metallic contacts), alignment and yield can suffer due to the
reduced
mechanical integrity of the underlying substrate.
Another potential advantage of the present invention is that the fatigue
resistance of the devices may be improved by maintaining an included angle of
less than 90 degrees between the diaphragm layer and the substrate in the
devices.
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Because this invention relates to methods of fabrication that could be
applied to a wide variety of micro-fabricated sensor and/or actuator devices,
the
scope of application is very wide. Nonlimiting examples of applications in
which vibration sensors or accelerometers of the present invention may be used
include navigational systems (automotive, aeronautic, personal, etc.);
environmental monitors (seismic activity, traffic monitors, etc.); equipment
monitors (industrial equipment, etc.); component monitors (fatigue/crack
detection, shock threshold detection, etc.); and biomedical monitors (cardiac
monitor, activity monitor, ultrasonic-GPS, etc.).
Nonlimiting examples of applications in which pressure sensors of the
present invention may be used include aeronautics (altimeter, air velocity,
etc.);
combustion engine applications (combustion diagnostic, exhaust monitor, fuel
monitor; etc.); and auditory applications (hearing aids, mini-microphones,
etc.).
Nonlimiting examples of applications in which resonant structures of the
present invention may be used include chemical sensing (electronic nose,
military, biomedical, etc.) and environmental monitors (humidity, biohazard
detection, pressure, etc.).
Nonlimiting examples of applications in which pumps of the present
invention may be used include miniature vacuum systems (mass spectrometry,
medical diagnostics, etc.); drug delivery (implanted drug delivery, precision
external delivery, etc.); microfluidics (DNA chips, medical diagnostics,
etc.); and
sample extraction (environmental, biomedical, etc.).
In one aspect, the present invention provides a method of manufacturing
a micro-electromechanical device having front and back sides, the method
including providing a substrate having a first side located proximate the
front
side of the device and second side proximate the back side of the device;
providing sacrificial material on a selected area of the first side of the
substrate;
providing a diaphragm layer on the sacrificial material and the first side of
the
substrate surrounding the sacrificial material in the selected area; providing
at
least one transducer on the front side of the device, the transducer located
over
the sacrificial material, wherein the transducer includes transducing material
and
electrical contacts in electrical communication with the transducing material;
forming a void in the substrate from the second side of the substrate towards
the
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first side of the substrate after providing the transducer on the front side
of the
device, wherein at least a portion of the sacrificial material is exposed
within the
void proximate the first side of the substrate; and removing at least a
portion of
the sacrificial material through the void, wherein a portion of the diaphragm
layer is suspended directly above the substrate within the selected area.
In another aspect, the present invention provides a substrate assembly
having front and back sides, the assembly including a substrate having a first
side
located proximate the front side of the device and second side proximate the
back side of the substrate assembly; sacrificial material on the first side of
the
substrate in a plurality of selected areas; a diaphragm layer covering the
sacrificial material in the selected areas, the diaphragm layer extending to
cover
the first side of the substrate surrounding the sacrificial material in the
selected
areas; a plurality of transducers on the front side of the device, each of the
transducers located over at least a portion of each of the selected areas,
wherein
the transducer includes transducing material and electrical contacts in
electrical
communication with the transducing material; wherein the sacrificial material
in
the selected areas is encapsulated between the substrate and the diaphragm
layer.
This substrate assembly can then be separated into a plurality of MEMS
devices,
each device including at least one of the transducers.
In another aspect, the present invention provides a micro-
electromechanical device having front and back sides, the device including a
substrate having a first side located proximate the front side of the device
and
second side proximate the back side of the device; a void formed through the
first and second sides of the substrate, the void including an opening
proximate
the first side of the substrate; and a diaphragm layer spanning the opening in
the
first side of the substrate and attached to the first side of the substrate,
wherein a
portion of the diaphragm layer is suspended directly above a portion of the
substrate surrounding the opening of the void; and wherein the suspended
portion of the diaphragm layer and the substrate form an included angle at
their
junction of less than 90 degrees.
These and other features and advantages of the invention are described
more completely with respect to various illustrative embodiments below.
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Brief Description of the Drawings
Figures lA-1D illustrate one process of forming a vibration sensor
according to the present invention.
Figure 2 is a schematic diagram of one etching apparatus that may be
used in connection with the present invention.
Figures 3A & 3B are cross-sectional and plan views, respectively, of a
pressure sensor.
Figures 4A & 4B are cross-sectional and plan views, respectively, of a
resonant device.
Figures SA & SB are cross-sectional and plan views, respectively, of a
pump device.
Detailed Description of Illustrative Embodiments of the Invention
Illustrative methods and devices according to the present invention will
now be described with reference to the drawings. In general, however, the
integration of frontside and backside processing technologies may provide a)
high performance, b) repeatability, c) high yield, d) wide process margins,
and e)
ease of integration.
Although the various methods and devices are described singly, it will be
understood that the methods of the present invention will typically involve
the
simultaneous production of a number of the devices on a single integral
substrate
assembly using integrated circuit manufacturing techniques. A variety of
integrated circuit manufacturing techniques are described in, e.g., S. Wolf
and R.
Tauber, Silicon Processing for the VLSI Era, (Sunset Beach, California,
Lattice
Press, 1986). The end result, after separation of the devices formed on each
substrate, may be discrete sensors and/or actuators, but the process will be
used
to manufacture multiple devices at the same time.
One illustrative method according to the present invention will now be
described with reference to Figures 1 A-1 D in which the formation of a
circular
vibration sensor is described. The completed vibration sensor is generally
circular in shape, although the figures illustrate its construction in cross-
section.
The vibration sensor relies on piezoelectric capacitors to convert mechanical
strain to an electrical signal by virtue of the primary piezoelectric effect.
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The method includes the use of a substrate 20 having a front surface 22
and a back surface 24. It will typically be preferred that the substrate 20 be
manufactured of materials amenable to processing (e.g., etching) used in the
production of integrated circuit devices. Examples of some suitable materials
for
the substrate 20 include, e.g., silicon wafers and similar structures. It may
also
be preferred that one or both of the surfaces 22 and 24 of the substrate 20 be
planar to facilitate high resolution processing.
Sacrificial material 30 is then provided in selected areas on the front
surface 22 of the substrate 20. The thickness of the sacrificial material 30
in the
selected areas may preferably be about 0.5 to about 1 micrometer. The control
over the selected areas in which the sacrificial material 30 is deposited may
be
accomplished, e.g., by using patterned photoresist material followed by
deposition of the patterned material in the selected areas where the
photoresist
has been removed. Alternatively, the sacrificial material 30 may be deposited
over substantially all of the front surface 22, followed by patterning and
removal
of the sacrificial material, leaving the sacrificial material 30 in only the
selected
areas as illustrated in Figure 1A. Such techniques are well known and will not
be described fiu~ther herein.
The sacrificial material 30 is preferably selectively removable with
respect to the underlying substrate 20. Where etching is to be used to form
voids
through the substrate 20 from the back surface 24 towards the front surface
22, it
may be preferred that the sacrificial material act as an etch stop to halt
etching
when the void being formed reaches the underside of the sacrificial material
30.
Another characteristic that may be exhibited by preferred sacrificial
material 30 is reflowability. In other words, after any patterning material
(e.g.,
photoresist) is removed from around the selected areas in which the
sacrificial
material 30 has been deposited, it may be desirable that the sacrificial
material 30
be reflowed to provide desirable smooth edges to the sacrificial material 30
in
the selected areas. One nonlimiting example of a suitable reflowable
sacrificial
material 30 is phosphosilicate glass (PSG).
Other potential materials for the sacrificial material include heavily
doped n+ silicon or polysilicon layers. N-type doping levels near the solid
solubility limit accentuate the lateral etch rate in a variety of wet chemical
and
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dry plasma etchants. Wet chemical etchants commonly used to selectively etch
n+ silicon or polysilicon include but are not limited to a) KOH, b) TMAH, and
c)
HF/HN03. Dry plasma etch agents commonly used to rapidly etch n+ silicon
and polysilicon include but are not limited to a) C12, b) SF6, and c) other Cl-
or
F-containing gases. Other materials/etchants have been reported in the
literature
and may be appropriate for some applications within the method described here.
Referring now to Figure 1B, with the sacrificial material 30 in locating in
the selected areas on the front surface 22 of the substrate 20, a diaphragm
layer
40 is deposited over the sacrificial material 30 and on the front substrate
surface
22 surrounding the selected areas. The diaphragm layer 40 functions as a
mechanical support layer over the sacrificial material 30 (which will be
removed
later) for the transducer to be formed over the selected areas of sacrificial
material 30. The diaphragm layer 40 may have any suitable thickness depending
on the size of the selected areas of sacrificial material 30, the size of the
transducer to be formed on the diaphragm layer 40, and other factors.
Suitable materials for the diaphragm layer 40 will exhibit sufficient
mechanical strength to support the transducer structures to be deposited on it
(after removal of the underlying sacrificial material 30) and the ability to
flex
during the sensing or actuating processes performed by the device. The
diaphragm layer 40 is also preferably formed of materials that are not
electrically
conductive. Examples of suitable materials include silicon nitride, silicon
dioxide, etc. One preferred material may be a low-stress silicon nitride
diaphragm layer 40 deposited at a thickness of about 2.0 micrometers.
After the diaphragm layer 40 is completed, other structures required for
the desired transducer can be formed on the diaphragm layer 40. In the
illustrated embodiment, conductive electrode layers 52 are provided in
selected
areas on the diaphragm layer 40. The conductive electrode layers 52 may be
provided by any suitable electrically conductive material that can be provided
in
the desired patterns on the diaphragm layer 40. One example of a suitable
material for the conductive electrode layers 52 is doped polycrystalline
silicon.
The electrode layer 52 may preferably form the lower electrodes for
connection to, e.g., the transducer elements 50 provided over the selected
areas
containing the sacrificial material 30 (encapsulated between the diaphragm
layer
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40 and the substrate 20). The transducer elements 50 may be formed of any
suitable transducing material. For the purposes of the present invention,
"transducing element" is any structure that exhibits a change in one or more
measurable electrical properties when subjected to mechanical strain or which
exhibits a mechanical force when subjected to an applied electrical stimulus.
Examples of transducing elements 50 may include layers of piezoelectric
material, piezoresistive materials, electrically conductive materials,
optically
active materials (e.g., materials that exhibit some change in optical
properties in
response to strain, e.g., a change in transmissivity, absorbance,
birefringence,
etc.), magnetostrictive materials, magnetoresistive materials, etc.
After formation of the transducer elements 50, an upper electrode layer
56 is formed in electrical communication with the transducing element 50, such
that the transducing element 50 is in electrical communication with both the
lower electrode layer 52 and the upper electrode layer 56. The upper electrode
layer 56 is manufactured in a manner and using materials that are similar to
those
used for the lower electrode layer 52. In the case of PZT transducer elements,
a
piezoelectric capacitor is formed that converts mechanical strain in the
diaphragm layer 40 to an electrical signal by virtue of the primary
piezoelectric
effect.
In addition to the transducer element 50 and associated conductive
electrode layers 52 and 56, the structures provided on the diaphragm layer 40
also include contacts 54 and 58 that are in electrical communication with the
lower electrode layer 52 and upper electrode layer 56, respectively. The
contact
54 in electrical communication with the lower electrode layer 52 may be used
to
supply, e.g., ground voltage to the transducer element 50 through the lower
electrode layer 52.
The electrical contacts 54 and 58 may preferably be formed of a metal or
metals to facilitate connection of the transducer element 50 to other devices.
Suitable metals used for the contacts 54 and 58 include any patternable metal
or
combination of metals used in integrated circuit manufacturing. Examples
include, but are not limited to, aluminum, titanium, gold, platinum, tungsten,
copper, etc.
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Figure 1 B thus illustrates one substrate assembly from which a plurality
of micro-electromechanical devices can be manufactured. Referring now to
Figure 1 C, with the structures on the front side of the substrate assembly
completed, the voids 70 required to form the desired vibration sensor can be
formed from the back surface 24 of the substrate 20. In one embodiment, the
back surface 24 of the substrate 20 is patterned and etched using any suitable
integrated circuit manufacturing technique or techniques. It may be desirable
that the techniques used be selective to the material of the substrate 20 as
opposed to the sacrificial material 30 located in the selected areas on the
front
surface 22 of the substrate.
In the case of a silicon substrate, it may be preferred to use a Deep
Reactive Ion Etching (DRIE) process (e.g., an SF6-based Bosch process) to form
void 70 in the substrate 20. The etching preferably terminates at the
underside of
the sacrificial material 30 as illustrated in Figure 1 C. The void 70 is
provided in
the shape of an annular ring (when viewed from above or below) that also
defines the shape of a proof mass 80 located within the void 70 and attached
to
the diaphragm layer 40 within the area defined by the transducing elements 50.
The proof mass 80 is, as a result, a separated portion of the original
substrate 20.
Other methods may also be used to form the void 70, although one advantage of
DRIE is that the sidewalk of the void may be more orthogonal with respect to
the back surface 24 of the substrate 20 as compared to other etching
processes.
Other techniques may alternatively be used to form void 70. For
example, chlorinated gas species are often used for etching silicon and
polysilicon. Also, plasma etching is not the only available method for etching
through the silicon wafer. Wet chemical agents such as KOH and TMAH are
often used in bulk-micromachining process modules to etch from one side of a
silicon wafer to the other. While such wet chemicals do not provide the same
near-vertical etch profile, they may be substituted if DRIE technology is not
available.
After formation of the void 70 in the substrate 20, the sacrificial material
30 in the selected areas can be removed to suspend a portion of the diaphragm
layer 40 directly above the substrate 20 within the selected area previously
occupied by the sacrificial material as illustrated in Figure 1D. Removal of
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sacrificial material 30 may preferably be accomplished by exposing only the
back substrate surface 24 to gas phase hydrofluoric acid (HF) which
selectively
removes the preferred PSG sacrificial material 30 as opposed to the silicon
substrate 20 and the preferred silicon nitride diaphragm layer 40.
In the preferred methods, the void 70 is formed and the sacrificial
material 30 is removed without exposing the front of the device (with the
contacts 54 and 58 and any other features) to the etchants or other materials
used
to form either the void 70 or remove the sacrificial material 30. In other
words,
the removal can be performed by only exposing the back surface 24 of the
substrate 20. No elaborate encapsulation is required for the front side of the
device.
The vibration sensor thus formed as seen in cross-section in Figure 1D
includes a diaphragm layer 40, a portion of which is suspended directly above
the substrate 20 surrounding the void 70. The junction between the diaphragm
layer 40 and the substrate 20 can be subject to fatigue as the sensor is used
an the
diaphragm layer 40 flexes. It may be desirable that the included angle formed
between the front surface 22 of the substrate 20 and the underside 42 of the
diaphragm layer 40 be less than 90 degrees to improve fatigue resistance of
the
sensor.
It is in this area where the use of reflowable sacrificial material 30 may
be beneficial. Reflowing the sacrificial material 30 after removing the
patterning
material can be used to provide sloping edges 32 on the sacrificial material
30.
Those sloped edges 32 can provide the desired included angle of less than 90
degrees at the junction of the diaphragm layer 40 and the substrate 20. It may
be
even more preferred that the included angle at the junction between the
diaphragm layer 40 and the substrate 20 be about 75 degrees or less.
Likewise, the included angle formed at the junction of the diaphragm
layer 40 and the proof mass 80 is also preferably less than 90 degrees,
possibly
even more preferably about 75 degrees or less, to improve fatigue resistance
there as well.
It is also desirable when manufacturing devices according to the methods
of the invention, to provide the void 70 and remove the sacrificial material
30 as
the last processing steps. By so doing, the mechanical integrity of the
substrate
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20 can be maintained while all of the front side processing is performed,
thereby
improving accuracy in the alignment of the various structures on the device.
In
methods in which voids are formed in the substrate before all of the front
side
processing is complete, alignment and yield can suffer due to the reduced
mechanical integrity of the underlying substrate.
Figure 2 demonstrates one apparatus suitable for performing gas phase
etching in a technique that exposes only the back surface 24 of the substrate
20
and the exposed undersides of the sacrificial material 30 to the etchant. A
container 90 is provided that contains, e.g., a liquid etchant (such as liquid-
phase
HF). The substrate 20 is placed over the top 92 of the container 90 and sealed
around its edges to expose only the back surface 24 of the substrate 20 to the
etchant. Where gas phase etching is desired, gaseous etchant may be trapped
within the sealed container 90. In the case of HF etching, the HF vapor within
the container 90 will typically reach an equilibrium concentration sufficient
to
rapidly etch the exposed PSG sacrificial material 30 exposed through the voids
70.
To further protect the features on the front of the device, it may be
desirable to flow, e.g., an inert gas (such as nitrogen, argon, or just air)
over the
front surface during etching to minimize exposure to trace levels of HF or any
other etchant escaping from the container 90. In another variation, it may be
desirable to heat the device on the container 90 (using, e.g., an infrared
lamp) to
heat the wafer, thereby increasing the reaction rate of the etchant with the
sacrificial material and promote the evaporation of reaction byproducts.
Although not mentioned in the prior discussion, another advantage of this
apparatus and method is "dry release". With a dry release process the
mechanical structure is, in part, separated from the substrate by a method
that
does not involve wet chemical etching. Mechanical stresses applied to the
freed
structure are then only the result of low-viscosity air flow or molecular
bombardment. By contrast, a "wet release" process requires immersion in a
liquid solution typically following by several aqueous rinse cycles. During
the
liquid-phase process steps, high surface tension and/or high-viscosity fluid
flow
often damage the thin film mechanical structure or lead to stiction-related
yield
loss.
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The embodiment and methods described with respect to Figures lA-1D
provide a vibration sensor using piezoelectric capacitors to convert
mechanical
strain in the membrane to an electrical signal by virtue of the primary
piezoelectric effect. The converse piezoelectric describes deformation
(strain)
of a piezoelectric material in response to an applied electric field or
signal. That
effect may be exploited in actuator-type devices such as resonators, pumps,
valves, etc. wherein mechanical motion is the result of applied electrical
energy.
A second material class embodiment substitutes piezoresistive elements
for the piezoelectric capacitor. Piezoresistive materials demonstrate a change
of
electrical resistance in response to an applied mechanical stress or strain.
Common piezoresistive materials include but are not limited to a) thin film
polycrystalline silicon of p- or n-type conductivity, b) single crystal
silicon of p
or n-type conductivity, and c) various metallic materials such as platinum,
gold,
aluminum, etc.
A third embodiment for mechanical-to-electrical energy conversion
involves electrostatics, most often in the form of capacitive sensors. When
the
physical separation of two electrical conductors is changed, the capacitance
changes. Conductive layers in a device can be arranged such that the
mechanical
strain or displacement changes the capacitance, a property readily detected
with
conventional electronics. Conversely, electrostatics can be employed to create
mechanical motion from electrical energy in much the same manner.
In yet another variation, magnetostrictive or magnetoresistive materials
may be used to convert between mechanical energy and electrical energy.
Figures 3A and 3B depict a piezoresistive pressure sensor 110
manufactured according to the present invention. The pressure sensor 110
includes a diaphragm layer 140 that spans a void 170 formed in substrate 120.
The void 170 may optionally be sealed on the side of the substrate 120
opposite
the diaphragm layer 140 by any suitable sealing mechanism, e.g., a layer of
bonded glass, silicon, etc.
The sensor 110 includes piezoresistive transducer elements 150 that
change resistivity in response to mechanical strain in the diaphragm layer 140
caused by pressure changes relative to the pressure within the void 170. The
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extent of any deflection is dependent on the magnitude of the pressure
differential on both sides of the diaphragm layer 140. The pressure-dependent
stress in the diaphragm layer 140 changes the electrical resistance of the
transducer elements 150, a properly that is easily measured with simple
electronics. Fabrication of the pressure-sensing device 110 proceeds as
described in Figures lA-1D with the following exceptions; a) piezoresistive
elements are used instead of piezoelectric capacitors, and b) the shape of the
void 170 is such that no proof mass is retained, c) the vibration sensor
requires
no silicon or glass wafer to seal the void 170.
Figures 4A and 4B depict a cross-sectional and a top view of a
piezoelectric resonant device 210 also manufactured according to the present
invention. The device 210 includes a diaphragm layer 240 that can be driven
into mechanical oscillation by, e.g., applying an electric signal to a central
circular piezoelectric capacitor 250a (defined by the upper electrically
conductive layer 252a). The outer donut piezoelectric capacitor 250b (defined
by the upper electrically conductive layer 252b) can act as a sensor whereby
the
output voltage is dependent on deflection of the diaphragm layer 240. The
voltages can be applied to the different capacitors 250a and 250b using
electrical
contacts 258a and 258b.
The central and outer capacitors 250a and 250b can be operated as an
actuator and a sensor pair coupled by the mechanical diaphragm layer 240 to
provide the elements needed for a resonant feedback loop. The conductive
layers
252a and 252b are preferably configured to efficiently excite the fundamental
vibration mode of the diaphragm layer 240 suspended over the void 270. In use,
an appropriate signal amplifier would be placed in communication with the
capacitors such that the electrical signature on the sensing element is
amplified
and applied to the actuating element.
Fabrication of this resonant device 210 proceeds as described in Figures
lA-1D except the void 270 is arranged such that no proof mass is retained.
Figures SA and SB depict a cross-sectional and a top view of a
piezoelectric gas and/or liquid pump that can be manufactured according to the
present invention. A similar arrangement of the chambers and connecting
channels may be found in, e.g., U.S. Patent No. 5,466,932 (Young et al.),
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although the construction differs according to the methods of the present
invention. The pump 310 can be manufactured using sacrificial material in
selected areas as described above to suspend portions of the diaphragm layer
340
above the substrate 320. The pump consists of two valves 316a and 316b on
either side of a differential volume chamber 318. The electrode configuration
for
the valves 316a/316b and chamber 318 are similar to those used in connection
with the circular resonant device of Figures 4A and 4B. All lower electrodes
are
preferably connected to ground.
The various portions of the diaphragm layer 340 can be actuated by
simultaneously applying opposite polarity voltage levels to the inner and
outer
upper electrodes 352a and 352b. For instance, the diaphragm layer above the
chamber 318 can be depressed or moved toward the substrate 320 by applying a
voltage other than ground to the central electrode 352a and voltage of the
opposite polarity to the outer electrode 352b. Similarly, valves 316a and 316b
can be actuated by applying voltages to their respective central and outer
electrodes.
As an integrated device, the pump 310 operates by flowing liquid or gas
from one flow channel to the other. Appropriate plumbing ports would
preferably be made at the back surface 324 of the substrate 320.
Fluid pumping may be accomplished by opening the inlet valve 316a,
forcing the diaphragm layer 340 in the chamber 318 up (away from the substrate
320), and closing the outlet valve 316b. Once these conditions have been
established, pumping can begin by closing the inlet valve 316a, opening outlet
valve 316b and reducing the volume of chamber 318. With the volume of
chamber 318 reduced, the outlet valve 316b is closed and inlet valve 316a is
opened. Increasing the volume of the chamber 318 then draws fluid into the
chamber 318. With fluid in the chamber 318, the inlet valve 316a can be closed
and outlet valve 316b opened, followed by reducing the volume of chamber 318
to force the fluid therein to exit through open outlet valve 316b. Outlet
valve
316b can then be closed, followed by opening of inlet valve 316a and expansion
of chamber 318 to draw more fluid into the chamber 318. The cycle then
continues until the desired amount of fluid has been pumped. The efficiency of
CA 02377189 2001-12-21
WO 01/00523 PCT/~JS00/17988
the valves might be enhanced by incorporating an electrostatic clamping
mechanism.
The preceding specific embodiments are illustrative of the practice of the
invention. This invention may be suitably practiced in the absence of any
element or item not specifically described in this document. The complete
disclosures of all patents, patent applications, and publications are
incorporated
into this document by reference as if individually incorporated in total.
Various modifications and alterations of this invention will become
apparent to those skilled in the art without departing from the scope of this
invention, and it should be understood that this invention is not to be unduly
limited to illustrative embodiments set forth herein, but is to be controlled
by the
limitations set forth in the claims and any equivalents to those limitations.
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