Figure 2.1.4. High-level functional block diagram of large-scale NEMS
and MEMS
For example, the desired flight path of aircraft (maneuvering and
landing) is maintained by displacing the control surfaces (ailerons and
elevators, canards and flaps, rudders and stabilizers) and/or changing the
control surface and wing geometry. Figure 2.1.5 documents the application
of the NEMS- and MEMS-based technology to actuate the control surfaces.
It should be emphasized that the NEMS and MEMS receive the digital
signal-level signals from the flight computer, and these digital signals are
converted into the desired voltages or currents fed to the microactuators or
electromagnetic flux intensity to displace the actuators. It is also important
that NEMS- and MEMS-based transducers can be used as sensors, and, as an
example, the loads on the aircraft structures during the flight can be
measured.
Data
Acquisition
Sensors
Antennas
Amplifiers
ICs
VariablesMeasured
Actuators
Analysisand
Decision
System
Dynamic
Controller
Output
VariablesSystem
Criteria
Objectives
VariablesMEMS
SensorActuator
−
MEMS
SensorActuator
−
SensorActuator
−
IO
© 2001 by CRC Press LLC
Figure 2.1.5. Aircraft with MEMS-based flight actuators
Microelectromechanical and Nanoelectromechanical Systems
Microelectromechanical systems are integrated microassembled
structures (electromechanical microsystems on a single chip) that have both
electrical-electronic (ICs) and mechanical components. To manufacture
MEMS, modified advanced microelectronics fabrication techniques and
materials are used. It was emphasized that sensing and actuation cannot be
viewed as the peripheral function in many applications. Integrated
actuators/sensors with ICs compose the major class of MEMS. Due to the use
of CMOS lithography-based technologies in fabrication actuators and
sensors, MEMS leverage microelectronics (signal processing, computing,
and control) in important additional areas that revolutionize the application
capabilities. In fact, MEMS have been considerably leveraged the
microelectronics industry beyond ICs. The needs to augmented actuators,
sensors, and ICs have been widely recognized. For example, mechatronics
concept, used for years in conventional electromechanical systems, integrates
all components and subsystems (electromechanical motion devices, power
converters, microcontrollers, et cetera). Simply scaling conventional
electromechanical motion devices and augmenting them with ICs have not
ψφθ
,,
:
AnglesEuler
ActuatorsFlight
BasedMEMS
−
SensorActuator −
SensorActuator
−
GeometryWing
GeometrySurface
ntDisplacemeSurface
Control
:
© 2001 by CRC Press LLC
met the needs, and theory and fabrication processes have been developed
beyond component replacement. Only recently it becomes possible to
manufacture MEMS at very low cost. However, there is a critical demand for
continuous fundamental, applied, and technological improvements, and
multidisciplinary activities are required. The general lack of synergy theory
to augment actuation, sensing, signal processing, and control is known, and
these issues must be addressed through focussed efforts. The set of long-
range goals has been emphasized in Chapter 1. The challenges facing the
development of MEMS are
•
advanced materials and process technology,
•
microsensors and microactuators, sensing and actuation mechanisms,
sensors-actuators-ICs integration and MEMS configurations,
•
packaging, microassembly, and testing,
•
MEMS modeling, analysis, optimization, and design,
•
MEMS applications and their deployment.
Significant progress in the application of CMOS technology enable the
industry to fabricate microscale actuators and sensors with the corresponding
ICs, and this guarantees the significant breakthrough. The field of MEMS has
been driven by the rapid global progress in ICs, VLSI, solid-state devices,
microprocessors, memories, and DSPs that have revolutionized
instrumentation and control. In addition, this progress has facilitated
explosive growth in data processing and communications in high-
performance systems. In microelectronics, many emerging problems deal
with nonelectric phenomena and processes (thermal and structural analysis
and optimization, packaging, et cetera). It has been emphasized that ICs is
the necessary component to perform control, data acquisition, and decision
making. For example, control signals (voltage or currents) are computer,
converted, modulated, and fed to actuators. It is evident that MEMS have
found application in a wide array of microscale devices (accelerometers,
pressure sensors, gyroscopes, et cetera) due to extremely-high level of
integration of electromechanical components with low cost and maintenance,
accuracy, reliability, and ruggedness. Microelectronics with integrated
sensors and actuators are batch-fabricated as integrated assemblies.
Therefore, MEMS can be defined as
batch-fabricated microscale devices (ICs and motion microstructures) that
convert physical parameters to electrical signals and vise versa, and in
addition, microscale features of mechanical and electrical components,
architectures, structures, and parameters are important elements of their
operation and design.
The manufacturability issues in NEMS and MEMS must be addressed. It
was shown that one can design and manufacture individually-fabricated
devices and subsystems. However, these devices and subsystems are unlikely
will be used due to very high cost.
© 2001 by CRC Press LLC
Piezoactuators and permanent-magnet technology has been used widely,
and rotating and linear electric transducers (actuators and sensors) are
designed. For example, piezoactive materials are used in ultrasonic motors.
Frequently, conventional concepts of the electric machinery theory
(rotational and linear direct-current, induction, and synchronous machine) are
used to design and analyze MEMS-based machines. The use of
piezoactuators is possible as a consequence of the discovery of advanced
materials in sheet and thin-film forms, especially PZT (lead zirconate
titanate) and polyvinylidene fluoride. The deposition of thin films allows
piezo-based electric machines to become a promising candidate for
microactuation in lithography-based fabrication. In particular, microelectric
machines can be fabricated using a deep x-ray lithography and
electrodeposition process. Two-pole synchronous and induction micro-
motors have been fabricated and tested.
To fabricate nanoscale structures, devices, and NEMS, molecular
manufacturing methods and technologies must be developed. Self- and
positional-assembly concepts are the preferable technologies compared
with individually-fabricated in the synthesis and manufacturing of
molecular structures. To perform self- and positional-assembly,
complementary pairs (CP) and molecular building blocks (MBB) should be
designed. These CP or MBB, which can be built from a couple to
thousands atoms, can be studied and designed using the DNA analogy. The
nucleic acids consist of two major classes of molecules (DNA and RNA).
Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are the largest
and most complex organic molecules which are composed of carbon,
oxygen, hydrogen, nitrogen, and phosphorus. The structural units of DNA
and RNA are nucleotides, and each nucleotide consists of three
components (nitrogen-base, pentose and phosphate) joined by dehydration
synthesis. The double-helix molecular model of DNA was discovered by
Watson and Crick in 1953. The DNA (long double-stranded polymer with
double chain of nucleotides held together by hydrogen bonds between the
bases), as the genetic material (genes), performs two fundamental roles. It
replicates (identically reproduces) itself before a cell divides, and provides
pattern for protein synthesis directing the growth and development of all
living organisms according to the information DNA supports. The DNA
architecture provides the mechanism for the replication of genes. Specific
pairing of nitrogenous bases obey base-pairing rules and determine the
combinations of nitrogenous bases that form the rungs of the double helix.
In contrast, RNA carries (performs) the protein synthesis using the DNA
information. Four DNA bases are: A (adenine), G (guanine), C (cytosine),
and T (thymine). The ladder-like DNA molecule is formed due to
hydrogen bonds between the bases which paired in the interior of the
double helix (the base pairs are 0.34 nm apart and there are ten pairs per
turn of the helix). Two backbones (sugar and phosphate molecules) form
the uprights of the DNA molecule, while the joined bases form the rungs.
© 2001 by CRC Press LLC
Figure 2.1.6 illustrates that the hydrogen bonding of the bases are: A bonds
to T, G bonds to C. The complementary base sequence results.
Figure 2.1.6. DNA pairing due to hydrogen bonds
In RNA molecules (single strands of nucleotides), the complementary
bases are A bonds to U (uracil), and G bonds to C. The complementary base
bonding of DNA and RNA molecules gives one the idea of possible sticky-
ended assembling (through complementary pairing) of NEMS structures and
devices with the desired level of specificity, architecture, topology, and
organization. In structural assembling and design, the key element is the
ability of CP or MBB (atoms or molecules) to associate with each other
(recognize and identify other atoms or molecules by means of specific base
pairing relationships). It was emphasized that in DNA, A (adenine) bonds to
T (thymine) and G (guanine) bonds to C (cytosine). Using this idea, one can
design the CP such as A
1
-A
2
, B
1
-B
2
, C
1
-C
2
, etc. That is, A
1
pairs with A
2
,
while B
1
pairs with B
2
. This complementary pairing can be studied using
electromagnetics (Coulomb law) and chemistry (chemical bonding, for
example, hydrogen bonds in DNA between nitrogenous bases A and T, G
and C). Figure 2.1.7 shows how two nanoscale elements with sticky ends
form the complementary pair. In particular, "+" is the sticky end and "-" is its
complement. That is, the complementary pair A
1
-A
2
results.
Figure 2.1.7. Sticky ended electrostatically complementary pair A
1
-A
2
An example of assembling a ring is illustrated in Figure 2.1.8. Using the
sticky ended segmented (asymmetric) electrostatically CP, self-assembling of
TA
−
O
H
N-H O
N H-N
3
CH
Sugar
NN
CG
−
N-H O
H
O H-N
N-H N
Sugar
NN
N
N
Sugar
H
N
N
Sugar
−
2
q
+
1
q
1
A
2
A
1
A
2
A
+
1
q
−
2
q
© 2001 by CRC Press LLC
nanostructure is performed in the XY plane. It is evident that three-
dimensional structures can be formed through the self-assembling.
Figure 2.1.8. Ring self-assembling
It is evident that there are several advantages to use sticky ended
electrostatic CP. In the first place, the ability to recognize (identify) the
complementary pair is clear and reliably predicted. The second advantage is
the possibility to form stiff, strong, and robust structures.
Self-assembled complex nanostructures can be fabricated using
subsegment concept to form the branched junctions. This concept is well-
defined electrostatically and geometrically through Coulomb law and
branching connectivity. Using the subsegment concept, ideal objects (e.g.,
cubes, octahedron, spheres, cones, et cetera) can be manufactured.
Furthermore, the geometry of nanostructures can be easily controlled by the
number of CP and pairing MBB. It must be emphasized that it is possible to
generate a quadrilateral self-assembled nanostructure by using four and more
different CP. That is, in addition to electrostatic CP, chemical CP can be
used. Single- and double-stranded structures can be generated and linked in
the desired topological and architectural manners. The self-assembling must
be controlled during the manufacturing cycle, and CP and MBB, which can
be paired and topologically/architecturally bonded, must be added in the
desired sequence. For example, polyhedral and octahedral synthesis can be
performed when building elements (CP or MBB) are topologically or
geometrically specified. The connectivity of nanostructures determines the
minimum number of linkages that flank the branched junctions. The synthesis
of complex three-dimensional nanostructures is the design of topology, and
the structures are characterized by their branching and linking.
Linkage Groups in Molecular Building Blocks
The hydrogen bonds, which are weak, hold DNA and RNA strands.
Strong bonds are desirable to form stiff, strong, and robust nano- and
microstructures. Using polymer chemistry, functional groups which couple
−
2
q
+
1
q
+
1
q
© 2001 by CRC Press LLC
monomers can be designed. However, polymers made from monomers with
only two linkage groups do not exhibit the desired stiffness and strength.
Tetrahedral MBB structures with four linkage groups result in stiff and
robust structures. Polymers are made from monomers, and each monomer
reacts with two other monomers to form linear chains. Synthetic and organic
polymers (large molecules) are nylon and dacron (synthetic), and proteins
and RNA, respectively.
There are two major ways to assemble parts. In particular, self assembly
and positional assembly. Self-assembling is widely used at the molecular
scale, and the DNA and RNA examples were already emphasized. Positional
assembling is widely used in manufacturing and microelectronic
manufacturing. The current inability to implement positional assembly at the
molecular scale with the same flexibility and integrity that it applied in
microelectronic fabrication limits the range of nanostructures which can be
manufactured. Therefore, the efforts are focused on developments of MBB,
as applied to manufacture nanostructures, which guarantee:
•
mass-production at low cost and high yield;
•
simplicity and predictability of synthesis and manufacturing;
•
high-performance, repeatability, and similarity of characteristics;
•
stiffness, strength, and robustness;
•
tolerance to contaminants.
It is possible to select and synthesize MBB that satisfy the requirements
and specifications (non-flammability, non-toxicity, pressure, temperatures,
stiffness, strength, robustness, resistivity, permiability, permittivity, et
cetera). Molecular building blocks are characterized by the number of
linkage groups and bonds. The linkage groups and bonds that can be used to
connect MBB are:
•
dipolar bonds (weak),
•
hydrogen bonds (weak),
•
transition metal complexes bonds (weak),
•
amide and ester linkages (weak and strong).
It must be emphasized that large molecular building blocks (LMMB) can
be made from MBB. There is a need to synthesize robust three-dimensional
structures. Molecular building blocks can form planar structures with are
strong, stiff, and robust in-plane, but weak and compliant in the third
dimension. This problem can be resolved by forming tubular structures. It
was emphasized that it is difficult to form three-dimensional structures using
MBB with two linkage groups. Molecular building blocks with three linkage
groups form planar structures, which are strong, stiff, and robust in plane but
bend easily. This plane can be rolled into tubular structures to guarantee
stiffness. Molecular building blocks with four, five, six, and twelve linkage
groups form strong, stiff, and robust three-dimensional structures needed to
synthesize robust nano- and microstructures.
Molecular building blocks with L linkage groups are paired forming L-
pair structures, and planar and non-planar (three-dimensional) nano- and
© 2001 by CRC Press LLC
microstructures result. These MBB can have in-plane linkage groups and out-
of-plane linkage groups which are normal to the plane. For example,
hexagonal sheets are formed using three in-plane linkage groups (MBB is a
single carbon atom in a sheet of graphite) with adjacent sheets formed using
two out-of-plane linkage groups. It is evident that this structure has
hexagonal symmetry.
Molecular building blocks with six linkage groups can be connected
together in the cubic structure. These six linkage groups corresponding to six
sides of the cube or rhomb. Thus, MBB with six linkage groups form solid
three-dimensional structures as cubes or rhomboids. It should be emphasized
that buckyballs (C
60
), which can be used as MMB, are formed with six
functional groups. Molecular building blocks with six in-plane linkage
groups form strong planar structures. Robust, strong, and stiff cubic or
hexagonal closed-packed crystal structures are formed using twelve linkage
groups. Molecular building blocks synthesized and applied should guarantee
the desirable performance characteristics (stiffness, strength, robustness,
resistivity, permiability, permittivity, et cetera) as well as manufacturability.
It is evident that stiffness, strength, and robustness are predetermined by
bonds (weak and strong), while resistivity, permiability and permittivity are
the functions of MBB compounds and media.
© 2001 by CRC Press LLC
2.2. ELECTROMAGNETICS AND ITS APPLICATION FOR NANO-
AND MICROSCALE ELECTROMECHANICAL MOTION DEVICES
To study NEMS and MEMS actuators and sensors, smart structures, ICs
and antennas, one applies the electromagnetic field theory. Electric force holds
atoms and molecules together. Electromagnetics plays a central role in
molecular biology. For example, two DNA (deoxyribonucleic acid) chains
wrap about one another in the shape of a double helix. These two strands are
held together by electrostatic forces. Electric force is responsible for energy-
transforming processes in all living organisms (metabolism). Electromagnetism
is used to study protein synthesis and structure, nervous system, etc.
Electrostatic interaction was investigated by Charles Coulomb.
For charges q
1
and q
2
, separated by a distance x in free space, the
magnitude of the electric force is
F
q q
x
=
1 2
0
2
4πε
,
where
ε
0
is the permittivity of free space,
ε
0
= 8.85×10
−12
F/m or C
2
/N-m
2
,
1
4
9 10
0
9
πε
= ×
N-m
2
/C.
The unit for the force is the newton N, while the charges are given in
coulombs, C.
The force is the vector, and we have
r
r
F
q q
x
a
x
=
1 2
0
2
4πε
,
where
r
a
x
is the unit vector which is directed along the line joining these two
charges.
The capacity, elegance and uniformity of electromagnetics arise from a
sequence of fundamental laws linked one to other and needed to study the field
quantities.
Using the Gauss law and denoting the vector of electric flux density as
r
D
[F/m] and the vector of electric field intensity as
r
E
[V/m or N/C], the total
electric flux
Φ
[C] through a closed surface is found to be equal to the total
force charge enclosed by the surface. That is, one finds
Φ = ⋅ =
∫
r
r
D ds Q
s
s
,
r
r
D E= ε ,
where
ds
r
is the vector surface area, ds dsa
n
r
r
= ,
r
a
n
is the unit vector which is
normal to the surface;
ε
is the permittivity of the medium; Q
s
is the total
charge enclosed by the surface.
Ohm’s law relates the volume charge density
r
J
and electric field
intensity
r
E
; in particular,
© 2001 by CRC Press LLC
r
r
J E= σ ,
where
σ
is the conductivity [A/V-m], for copper σ = ×58 10
7
. , and for
aluminum
σ = ×35 10
7
. .
The current i is proportional to the potential difference, and the resistivity
ρ
of the conductor is the ratio between the electric field
r
E
and the current
density
r
J
. Thus,
ρ =
r
r
E
J
.
The resistance r of the conductor is related to the resistivity and
conductivity by the following formulas
r
l
A
=
ρ
and r
l
A
=
σ
,
where l is the length; A is the cross-sectional area.
It is important to emphasize that the parameters of NEMS and MEMS
vary. Let us illustrate this using the simplest nano-structure used in NEMS and
MEMS. In particular, the molecular wire. The resistances of the ware vary due
to heating. The resistivity depends on temperature T [
o
C], and
( ) ( )
[
]
ρ ρ α α
ρ ρ
( ) T T T T T= + − + − +
0 1 0 2 0
2
1 ,
where
α
ρ1
and α
ρ2
are the coefficients.
As an example, over the small temperature range (up to 160
o
C) for copper
(the wire is filled with copper) at T
0
= 20
o
C, we have
(
)
[
]
ρ( ) . .T T= × + −
−
17 10 1 00039 20
8
.
To study NEMS and MEMS, the basic principles of electromagnetic
theory should be briefly reviewed.
The total magnetic flux through the surface is given by
Φ = ⋅
∫
r
r
B ds ,
where
r
B
is the magnetic flux density.
The Ampere circuital law is
r
r
r
r
B dl J ds
l s
⋅ = ⋅
∫ ∫
µ
0
,
where
µ
o
is the permeability of free space,
µ
o
= 4π×10
−7
H/m or T-m/A.
For the filamentary current, Ampere’s law connects the magnetic flux with
the algebraic sum of the enclosed (linked) currents (net current) i
n
, and
r
r
B dl i
l
o n
⋅ =
∫
µ .
The time-varying magnetic field produces the electromotive force (emf),
denoted as , which induces the current in the closed circuit. Faraday’s law
© 2001 by CRC Press LLC
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