203
Wendy C. Crone
The expanding and developing fields of
micro-electromechanical systems (MEMS) and
nano-electromechanical (NEMS) are highly interdisciplinary and rely heavily on experimental
mechanics for materials selection, process
validation, design development, and device characterization. These devices range from mechanical
sensors and actuators, to microanalysis and chemical sensors, to micro-optical systems and bioMEMS
for microscopic surgery. Their applications span
the automotive industry, communications, defense systems, national security, health care,
information technology, avionics, and environmental monitoring. This chapter gives a general
introduction to the fabrication processes and materials commonly used in MEMS/NEMS, as well as
a discussion of the application of experimental
mechanics techniques to these devices. Mechanics
issues that arise in selected example devices are
also presented.
9.1
Background ......................................... 203
9.2
MEMS/NEMS Fabrication ......................... 206
9.3 Common MEMS/NEMS Materials
and Their Properties ............................. 206
9.3.1 Silicon-Based Materials ................ 207
9.3.2 Other Hard Materials .................... 208
9.3.3 Metals ........................................
9.3.4 Polymeric Materials ......................
9.3.5 Active Materials ...........................
9.3.6 Nanomaterials .............................
9.3.7 Micromachining ...........................
9.3.8 Hard Fabrication Techniques .........
9.3.9 Deposition ..................................
9.3.10 Lithography .................................
9.3.11 Etching .......................................
208
208
209
209
210
211
211
211
212
9.4 Bulk Micromachining
versus Surface Micromachining .............. 213
9.5 Wafer Bonding ..................................... 214
9.6 Soft Fabrication Techniques................... 215
9.6.1 Other NEMS Fabrication Strategies .. 215
9.6.2 Packaging ................................... 216
9.7
Experimental Mechanics
Applied to MEMS/NEMS .......................... 217
9.8 The Influence of Scale ...........................
9.8.1 Basic Device Characterization
Techniques ..................................
9.8.2 Residual Stresses in Films ..............
9.8.3 Wafer Bond Integrity ....................
9.8.4 Adhesion and Friction...................
217
218
219
220
220
9.9 Mechanics Issues in MEMS/NEMS ............. 221
9.9.1 Devices ....................................... 221
9.10 Conclusion ........................................... 224
References .................................................. 225
9.1 Background
The acronym MEMS stands for micro-electromechanical
system, but MEMS generally refers to microscale
devices or miniature embedded systems involving
one or more micromachined component that enables
higher-level functionality. Similarly NEMS, nanoelectromechanical system, refers to such nanoscale
devices or nanodevices. MEMS and NEMS are fab-
ricated microscale and nanoscale devices that are
often made in batch processes, usually convert between some physical parameter and a signal, and
may be incorporated with integrated circuit technology. The field of MEMS/NEMS encompasses devices
created with micromachining technologies originally
developed to produce integrated circuits, as well as
Part A 9
A Brief Introd
9. A Brief Introduction to MEMS and NEMS
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Solid Mechanics Topics
Part A 9.1
Table 9.1 Sample applications of MEMS/NEMS
Sensors
Actuators
Passive structures
Accelerometers, biochemical analyzers, environmental assay devices, gyroscopes,
medical diagnostic sensors, pressure sensors
Data storage, drug-delivery devices, drug synthesis, fluid regulators, ink-jet printing devices,
micro fuel cells, micromirror devices, microphones, optoelectric devices, radiofrequency devices,
surgical devices
Atomizers, fluid spray systems, fuel injection, medical inhalers
non-silicon-based devices created by the same micromachining or other techniques. They can be classified
as sensors, actuators, and passive structures (Table 9.1).
Sensors and actuators are transducers that convert
one physical quantity to another, such as electromagnetic, mechanical, chemical, biological, optical or
thermal phenomena. MEMS sensors commonly measure pressure, force, linear acceleration, rate of angular
motion, torque, and flow. For instance, to sense pressure
an intermediate conversion step, such as mechanical
stress, can be used to produce a signal in the form
of electrical energy. The sensing or actuation conversion can use a variety of methods. MEMS/NEMS
sensing can employ change in electrical resistance,
piezoresistive, piezoelectric, change in capacitance, and
magnetoresistive methods (Table 9.2). MEMS/NEMS
actuators provide the ability to manipulate physical
parameters at the micro/nanoscale, and can employ
electrostatic, thermal, magnetic, piezoelectric, piezoresistive, and shape-memory transformation methods.
Passive MEMS structures such as micronozzles are used
in atomizers, medical inhalers, fluid spray systems, fuel
injection, and ink-jet printers.
MEMS have a characteristic length scale between 1 mm and 1 μm, whereas NEMS devices have
a characteristic length scale below 1 μm (most strictly,
1–100 nm). For instance a digital micromirror device has a characteristic length scale of 14 μm,
a quantum dot transistor has components measuring
300 nm, and molecular gears fall into the 10–100 nm
range [9.2]. Additionally, although an entire device may
be mesoscale, if the functional components fall in the
microscale or nanoscale regime it may be referred to as
a MEMS or NEMS device, respectively. MEMS/NEMS
inherently have a reduced size and weight for the
function they carry out, but they can also carry advantages such as low power consumption, improved
speed, increased function in one package, and higher
precision.
There is no distinct MEMS/NEMS market, instead there is a collection of niche markets where
MEMS/NEMS become attractive by enabling a new
function, bringing the advantage of reduced size, or
lowering cost [9.1].
Despite this characteristic, the MEMS industry is already valued in tens of billions of dollars and growing
rapidly. The Small Times Tech Business DirectoryTM
Table 9.2 Physical quantities used in MEMS/NEMS sensors and actuators (after Maluf [9.1])
Method
Description
Physical and material
parameters
Order of energy
density (J/cm3 )
Electrostatic
Attractive force between two components
carrying opposite charge
Certain materials that change shape
under an electric field
Thermal expansion or difference
in coefficient of thermal expansion
Electric current in a component
surrounded by a magnetic field
gives rise to an electromagnetic force
Certain materials that undergo
a solid–solid phase transformation
producing a large shape change
Electric field, dielectric permittivity
≈ 0.1
Electric field, Young’s modulus,
piezoelectric constant
Coefficient of expansion, temperature
change, Young’s modulus
Magnetic field, magnetic permeability
≈ 0.2
Transformation temperature
≈ 10
Piezoelectric
Thermal
Magnetic
Shape memory
≈5
≈4
A Brief Introduction to MEMS and NEMS
ples include biofluidic chips for biochemical analyses,
biosensors for medical diagnostics, environmental assays for toxin identification, implantable pharmaceutical drug delivery, DNA and genetic code analysis,
imaging, and surgery. NEMS is often associated with
biotechnology because this size scale allows for interaction with biological systems in a fundamental
way. BioNEMS may be used for drug delivery, drug
synthesis, genome synthesis, nanosurgery, and artificial organs comprised of nanomaterials. The sensitivity
of such bioNEMS devices can be exquisite, selectively binding and detecting a single biomolecule. More
complete background information on microfluidic devices can be found in Beeby [9.7], Koch [9.9] and
Kovacs [9.10].
Semiconductor NEMS devices can offer microwave
resonance frequencies, exceptionally high mechanical
quality factors, and extraordinarily small heat capacities [9.11, 12]. Examples of NEMS devices also include
transducers, radiating energy devices, nanoscale integrated circuits, and optoelectronic devices [9.13, 14].
NEMS manufacturing is being further enabled by the
drive towards nanometer feature sizes in the microelectronics industry. Terascale computational ability will
require nanotransistors, nanodiodes, nanoswitces, and
nanologic gates [9.15]. NEMS also opens the door
for fundamental science at the nanometer scale investigating phonon-mediated mechanical processes [9.16]
and quantum behavior of mesoscopic mechanical systems [9.17].
Although there is some discussion as to whether the
NEMS definition requires a characteristic length scale
below 1000 nm or 100 nm, there is no argument that the
field of NEMS is in its infancy. Existing commercial
devices are limited at this point, but research on NEMS
is extremely active and highly promising. Many challenges remain, including assembly of nanoscale devices
and mass production capabilities.
In the long term, a number of issues must be
addressed in analysis, design, development, and fabrication for high-performance MEMS/NEMS to become
ubiquitous. Of most relevance to the focus of this handbook, advanced materials must be well characterized
and MEMS/NEMS testing must be further developed.
Additionally for commercialization, MEMS/NEMS design must consider issues of market (need for product, size of market), impact (enabling new systems,
paradigm shift for the field), competition (other macro
and micro/nanoproducts existing), technology (available capability and tools), and manufacturing (manufacturability in volume at low cost) [9.18].
205
Part A 9.1
lists more than 700 manufacturers/fabricators of microsystems and nanotechnologies [9.3]. High-volume
production with lucrative sales have been achieved by
several companies making devices such as accelerometers for automobiles (Analog Devices, Motorola,
Bosch), micromirrors for digital projection displays
(Texas Instruments), and pressure sensors for the
automotive and medical industries (NovaSensor). Currently, the MEMS markets with the largest commercial value are ink-jet printer heads, optical MEMS
(which includes the Digital Micromirror DeviceTM
discussed below), and pressure sensors, followed by
microfluidics, gyroscopes, and accelerometers [9.4].
The MEMS market was reported to be US $5.1 billion in 2005 and projected to reach US $9.7 billion
by 2010 [9.4]. The NEMS industry, while still young,
has been growing in value. The market research firm
Report Buyer recently released a market report on
Nanorobotics and NEMS indicating that the global market for NEMS increased from US $29.5 million to
US $34.2 million between 2004 and 2005 and projecting that the market will reach US $830.4 million by
2011 [9.5].
Microfluidic and nanofluidic devices also fall under
the umbrella of MEMS/NEMS and are often classified
as bioMEMS/bioNEMS devices when involving biological materials. These devices incorporate channels with
at least one microscale or nanoscale dimension in which
fluid flows. The small scale of these devices allow for
smaller sample size, faster reactions, and higher sensitivity. Microfluidic devices commonly use both hard
and soft fabrication techniques to produce channels and
other fluidic structures [9.6]. The common feature of
these devices is that they allow for flow of gas and/or
liquid and use components such as pumps, valves, nozzles, and mixers. Commercial and defense applications
include automotive controls, pneumatics, environmental testing, and medical devices. The advantages of
the microscale in these applications include high spatial resolution, fast time response, small fluid volumes
required for analysis, low leakage, low power consumption, low cost, appropriate compatibility of surfaces, and
the potential for integrate signal processing [9.7]. At
the microscale, pressure drop over a narrow channel is
high and fluid flow generated by electric fields can be
substantial.
The micrometer and nanometer length scales are
particularly relevant to biological materials because
they are comparable to the size of cells, molecules, diffusions length for molecules, and electrostatic screening
lengths of ionic conducting fluids [9.8]. Device exam-
9.1 Background
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Solid Mechanics Topics
Part A 9.3
The field of MEMS/NEMS is highly multidisciplinary, often involving expertise from engineering,
materials science, physics, chemistry, biology, and
medicine. Because of the breadth of the field and
the range of activities that fall under the scope of
MEMS/NEMS, a comprehensive review is not possible in this chapter. After providing general background,
the focus will be on mechanics and specifically experimental mechanics as it is applied to MEMS/NEMS.
Mechanics is critical to the design, fabrication, and
performance of MEMS/NEMS. A broad range of experimental tools has been applied to MEMS/NEMS.
This chapter will provide an overview of such work.
Additional information on the application of mechanics to MEMS/NEMS can be found in the proceedings
of the annual symposium held by the MEMS and
Nanotechnology Technical Division of the Society for
Experimental Mechanics (see, for example, [9.19]).
9.2 MEMS/NEMS Fabrication
Traditionally MEMS/NEMS are thought of in the context of microelectronics fabrication techniques which
utilize silicon. This approach to MEMS/NEMS brings
with it the momentum of the integrated circuits industry and has the advantage of ease of integration
with semiconductor devices, but fabrication is expensive in both the infrastructure and equipment required
and the time investment needed to create a working prototype. An alternative approach that has seen
significant success, especially in its application to microfluidic devices, is the use of soft materials such
as polydimethylsiloxane (PDMS). Soft MEMS/NEMS
fabrication can often be conducted with bench-top techniques with no need for the clean-room facilities used
in microelectronics fabrication. Additionally, polymers
offer a range of properties not available in siliconbased materials such as mechanical shock tolerance,
biocompatibility, and biodegradability. However, polymers can carry disadvantages for certain applications
because of their viscoelastic behavior and low thermal stability. Ultimately a combination of function and
economics decides the medium of choice for device
construction.
Whether we talk about hard or soft MEMS/NEMS,
the basic approach to device construction is similar.
Material is deposited onto a substrate, a lithographic
step is used to produce a pattern, and material removal
is conducted to create a shape. For traditional microelectronics fabrication, the substrate is often silicon,
material deposition is achieved by vapor deposition or
sputtering, lithography involves patterning of a chemically resistant polymer, and material is removed by
a chemical etch. Alternatively, for soft MEMS/NEMS
materials, fabrication often utilizes a glass or plastic substrate, material in the form of a monomer is
flowed into a region, a lithographic mask allows exposure of a pattern to ultraviolet (UV) radiation triggering
polymerization, and the unpolymerized monomer is removed with a flushing solution. For both hard and soft
MEMS/NEMS fabrication there are a number of variations on these basic steps which allow for a wide array
of structures and devices to be constructed.
9.3 Common MEMS/NEMS Materials and Their Properties
Materials used in MEMS/NEMS must simultaneously
satisfy a range requirements for chemical, structural,
mechanical, and electrical properties. For biomedical
and bioassay devices, material biocompatibility and
bioresistance must also be considered.
Most MEMS/NEMS devices are created on a substrate. Common substrate materials include singlecrystal silicon, single-crystal quartz, fused quartz,
gallium arsenide, glass, and plastics. Devices are made
with a range of methods by machining into the substrate and/or depositing additional material on top of the
substrate. The additional materials may be structural,
sacrificial, or active.
Although traditionally MEMS in particular have relied on silicon, the materials used in MEMS/NEMS are
becoming more heterogeneous. Selected properties are
given in Table 9.3 for comparative purposes, but an
extensive list of properties for the wide range of materials used in MEMS/NEMS cannot be included here.
It should be noted, however, that the constitutive behavior of materials used in MEMS/NEMS applications can
be sensitive to fabrication method, processing parame-
A Brief Introduction to MEMS and NEMS
9.3 Common MEMS/NEMS Materials and Their Properties
Property
Si
SiO2
Si3 N4
Quartz
SiC
Si(111)
Stainless
steel
Al
Young’s modulus (GPa)
Yield strength (GPa)
Poisson’s ratio
Density (g/cm3 )
Coefficient of thermal
expansion (10−6 /◦ C)
Thermal conductivity
at 300 K (W/cm · K)
Melting temperature (◦ C)
160
7
0.22
2.4
2.6
73
8.4
0.17
2.3
0.55
323
14
0.25
3.1
2.8
107
9
0.16
2.65
0.55
450
21
0.14
3.2
4.2
190
7
0.22
2.3
2.3
200
3
0.3
8
16
70
0.17
0.33
2.7
24
1.57
0.014
0.19
0.0138
5
1.48
0.2
2.37
1415
1700
1800
1610
2830
1414
1500
660
ters, and thermal history due to the relative similarity
between characteristic length scales and device dimensions. A good resource compiling characterization data
from a number of sources is the material database at
http://www.memsnet.org/material/ [9.20]. The following books, used as references for the discussion here,
are valuable resources for more extensive information:
Senturia [9.18], Maluf [9.1], and Beeby [9.7].
9.3.1 Silicon-Based Materials
Silicon, Polysilicon, and Amorphous Silicon
Silicon-based materials are the most common materials currently used in MEMS/NEMS commercial
production. MEMS/NEMS devices often exploit the
mechanical properties of silicon rather than its electrical
properties. Silicon can be used in a number of different forms: oriented single-crystal silicon, amorphous
silicon, or polycrystal silicon (polysilicon).
Single-crystal silicon, which has cubic crystal structure, exhibits anisotropic behavior which is evident in
its mechanical properties such as Young’s modulus.
A high-purity ingot of single-crystal silicon is grown,
sawn to the desired thickness, and polished to create
a wafer. Single-crystal silicon used for MEMS/NEMS
are usually the standard 100 mm (4 inch diameter,
525 μm thickness) or 150 mm (6 inch diameter, 650 μm
thickness) wafers. Although larger 8-inch and 12-inch
wafers are available, they are not used as prevalently for
MEMS fabrication.
The properties of the wafer depend on both the orientation of crystal growth and the dopants added to
the silicon (Fig. 9.1). Impurity doping has a significant
impact on electrical properties but does not generally
impact the mechanical properties if the concentration
is approximately < 1020 cm−3 . Silicon is a group IV
semiconductor. To create a p-type material, dopants
from group III (such as boron) create mobile charge
carriers that behave like positively charged species. To
create an n-type material, dopants from group V (such
as phosphorous, arsenic, and antimony) are used to create mobile charge carriers that behave like negatively
charged electrons. Doping of the entire wafer can be accomplished during crystal growth. Counter-doping can
be accomplished by adding dopants of the other type
to an already doped substrate using deposition followed
by ion implantation and annealing (to promote diffusion
and relieve residual stresses). For instance, p-type into
n-type creates a pn-junction.
Amorphous and polysilicon films are usually deposited with thicknesses of < 5 μm, although it is also
possible to create thick polysilicon [9.21]. The residual
stress in deposited polysilicon and amorphous silicon
thin films can be large, but annealing can be used to
provide some relief. Polysilicon has the disadvantage
of a somewhat lower strength and lower piezoresistivity than single-crystal silicon. Additionally, Young’s
(100) n-type
Secondary flat
(111) n-type
Primary
flat
(100) p-type
Primary
flat
Secondary flat
Primary
flat
(111) p-type
Primary
flat
Secondary flat
Fig. 9.1 Flats on standard commercial silicon wafers used
to identify crystallographic orientation and doping (after
Senturia [9.18])
Part A 9.3
Table 9.3 Properties of selected materials (after Maluf [9.1] and Beeby [9.7])
207
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Part A 9.3
modulus may vary significantly because the diameter of
a single grain may comprise a large fraction of a component’s width [9.22].
Silicon, polysilicon, and amorphous silicon are also
piezoresistive, meaning that the resistivity of the material changes with applied stress. The fractional change
in resistivity, Δρ/ρ, is linearly dependent on the stress
components parallel and perpendicular to the direction of the resistor. The proportionality constants are
the piezoresistive coefficients, which are dependent
on the crystallographic orientation, and the dopant
type/concentration in single-crystal silicon. This property can be used to create a strain gage.
Silicon Dioxide
The success of silicon is heavily based on its ability to
form a stable oxide which can be predictably grown at
elevated temperature. Dry oxidation produces a higherquality oxide layer, but wet oxidation (in the presence
of water) enhances the diffusion rate and is often used
when making thicker oxides. Amorphous silicon dioxide can be used as a mask against etchants. It should be
noted that these films can have large residual stresses.
Silicon Nitride
Silicon nitride can be deposited by chemical vapor deposition (CVD) as an amorphous film which can be used
as a mask against etchants. It should be noted that these
films can have large residual stresses.
Silicon Carbide
Silicon carbide is an attractive material because of its
high hardness, good thermal properties, and resistance
to harsh environments. Additionally, silicon carbide is
piezoresistive. Although it can be produced as a bulk
polycrystalline material it is generally grown or deposited on a silicon substrate by epitaxial growth (single
crystal) or by chemical vapor deposition (polycrystal).
Quartz
Single-crystal quartz, which has a hexagonal crystal
structure, can be used in natural or synthesized form.
Like silicon, it can be etched selectively but the results are less ideal than in silicon because of unwanted
facets and poor edge definition. Single-crystal quartz
can be used as substrate material in a range of cuts
which have different temperature sensitivities for piezoelectric or mechanical properties. Detailed information
about quartz cuts can be found in Ikeda [9.25]. Quartz
is also piezoelectric, meaning that there is a relationship
between strain and voltage in the material. Fused quartz
(silica) is a glassy noncrystalline material that is also
occasionally used in MEMS/NEMS devices.
Glass
Glasses such as phosphosilicate and borosilicate
(Pyrex) can be used as a substrate or in conjunction
with silicon and other materials using wafer bonding
(discussed below).
Diamond
Diamond is also attractive because of its high hardness, high fracture strength, low thermal expansion, low
heat capacity, and resistance to harsh environments.
Diamond is also piezoresistive and can be doped to
produce semiconducting and metal-like behavior [9.26].
Because of its hardness, diamond is particularly attractive for parts exposed to wear. The most promising
synthetic forms are amorphous diamond-like carbon,
nanocrystalline diamond, and ultra-nanocrystalline diamond films created by pulsed laser deposition or
chemical vapor deposition [9.27–31].
9.3.3 Metals
Silicon on Insulator (SOI)
Silicon on insulator (SOI) wafers are also used for
MEMS sensors and actuators [9.23]. Different SOI
materials are distinguished by their properties. Buried
oxide layers can be produced either through ion implantation or wafer bonding processes; these techniques are
discussed further below [9.24].
Metals are usually deposited as a thin film by sputtering,
evaporation or chemical vapor deposition (CVD). Gold,
nickel and iron can also be electroplated. Aluminum is
the most common metal used in MEMS/NEMS, and is
often used for light reflection and electrical conduction.
Gold is used for electrochemistry, infrared (IR) light reflection, and electrical conduction. Chromium is often
used as an adhesion layer. Alloys of Ni, such as NiTi
and PermalloyTM , can be used for actuation and are
discussed in more detail below.
9.3.2 Other Hard Materials
9.3.4 Polymeric Materials
Gallium Arsenide
Gallium arsenide (GaAs) is a III–V compound semiconductor which is often used to create lasers, optical
devices, and high-frequency components.
Photoresists
Polymeric photoresist materials are generally used as
a spin-cast film as part of a photolithographic process. The film is modified by exposure to radiation
A Brief Introduction to MEMS and NEMS
Polydimethylsiloxane
Polydimethylsiloxane (PDMS) is an elastomer used as
both a structural component in MEMS devices and
a stamping material for creating micro- and nanoscale
features on surfaces. PDMS is a common silicone rubber and is used extensively because of its processibility,
low curing temperature, stability, tunable modulus, optical transparency, biocompatibility, and adaptability by
a range functional groups that can be attached [9.32,33].
9.3.5 Active Materials
There are several types of active materials that successfully perform sensing and actuation functions at the
microscale. Several examples of active materials are
given below.
NiTi
Near-equiatomic nickel titanium alloy can be deposited
as a thin film and used an as active material. This material is of particular interest to MEMS because the
actuation work density of NiTi is more than an order of magnitude higher than the work densities of
other actuation schemes. These shape-memory alloys
(SMAs) undergo a reversible phase transformation that
allows the material to display dramatic and recoverable stress- and temperature-induced transformations.
The behavior of NiTi SMA is governed by a phase
transformation between austenite and martensite crystal structures. Transformation between the austenite
(B2) and martensite (B19) phases in NiTi can be
produced by temperature cycling between the hightemperature austenite phase and the low-temperature
martensite phase (shape-memory effect), or loading
and unloading the material to favor either the highstrain martensite phase or the low-strain austenite
phase (superelasticity). Thus both stress and temperature produce the transformation between the austenite
and martensite phases of the alloy. The transfor-
mation occurs in a temperature window, which can
be adjusted from −100 ◦ C to +160 ◦ C by changing
the alloy composition and heat treatment processing [9.34].
PermalloyTM
PermalloyTM , Nix Fe y , displays magnetoresistance properties and is used for magnetic transducing. Multilayered nanostructures of this alloy give rise to
a giant-magnetoresistance (GMR) phenomenon which
can be used to detect magnetic fields. It has been widely
applied to read the state of magnetic bits in data storage
media.
Lead Zirconate Titanate (PZT)
Lead zirconate titanate (PZT) is a ceramic solid solution
of lead zirconate (PbZrO3 ) and lead titanate (PbTiO3 ).
PZT is a piezoelectric material that can be deposited
in thin film form by sputtering or using a sol–gel process. In addition to natural piezoelectric materials such
as quartz, other common synthetic piezoelectric materials include polyvinylidene fluoride (PVDF), zinc
oxide, and aluminum nitride. Actuation performed by
piezoelectrics has the advantage of being capable of
achieving reasonable displacements with fast response,
but the material processing is complex.
Hydrogels
Hydrogels, such as poly(2-hydroxyethyl methacrylate
(HEMA)) gel, with volumetric shape-memory capability are now being employed as actuators, fluid
pumps, and valves in microfluidic devices. In an aqueous environment, hydrogels will undergo a reversible
phase transformation that results in dramatic volumetric
swelling and shrinking upon exposure and removal of
a stimulus. Hydrogels have been produced that actuate
when exposed to such stimuli as pH, salinity, electrical current, temperature, and antigens. Since the rate of
swelling and shrinking in a hydrogel is diffusion limited, the temporal response of hydrogel structures can
be reduced to minutes or even seconds in microscale
devices.
9.3.6 Nanomaterials
Nanostructuring of materials can produce unique mechanical, electrical, magnetic, optical, and chemical
properties. The materials themselves range from polymers to metals to ceramics, it is their nanostructured
nature that gives them exciting new behaviors. Increased hardness with decreasing grain size allows for
209
Part A 9.3
such as visible light, ultraviolet light, x-rays or electrons. Exposure is usually conducted through a mask
so that a pattern is created in the photoresist layer and
subsequently on the substrate through an etching or deposition process. Resists are either positive or negative
depending on whether the radiation exposure weakens
or strengthens the polymer. In the developer step, chemicals are used to remove the weaker material, leaving
a patterned photoresist layer behind. Important photoresist properties include resolution and sensitivity,
particularly as feature sizes decrease.
9.3 Common MEMS/NEMS Materials and Their Properties
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Solid Mechanics Topics
Part A 9.3
Table 9.4 Micromachining processes and their applications (after Kovacs [9.10])
Process
Example applications
Lithography
Thin-film deposition
Photolithography, screen printing, electron-beam lithography, x-ray lithography
Chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD),
physical vapor deposition (PVD) such as sputtering and evaporation, spin casting, sol–gel deposition
Blanket and template-delimited electroplating of metals
Electroplating, LIGA, stereolithography, laser-driven CVD, screen printing, microcontact printing,
dip-pen lithography
Plasma etching, reactive-ion enhanced etching (RIE), deep reactive-ion etching (DRIE),
wet chemical etching, electrochemical etching
Drilling, milling, electrical discharge machining (EDM), focused ion beam (FIB) milling,
diamond turning, sawing
Direct silicon-fusion bonding, fusion bonding, anodic bonding, adhesives
Wet chemical modification, plasma modification, self assembled monolayer (SAM) deposition,
grinding, chemomechanical polishing
Thermal annealing, laser annealing
Electroplating
Directed deposition
Etching
Machining
Bonding
Surface modification
Annealing
hard coatings and protective layers, lower percolation
threshold impacts conductivity, and narrower bandgap
with decreasing grain size enable unique optoelectronics [9.35]. Hundreds of different synthesis routes
have been created for manufacturing nanostructured
materials. (See, for example, the proceedings of the
International Conferences on Nanostructured Materials [9.36].) A few examples of such materials are given
below.
Carbon Nanotubes and Fullerenes
Carbon nanotubes (CNTs) and fullerenes (buckyballs,
e.g., C60 ) are self-assembled carbon nanostructures.
CNTs are cylindrical graphene structures of single- or
multiwall form which are extremely strong and flexible. They possess metallic or semiconducting electronic
behavior depending on the details of the structure (chirality). They can be created in an arc plasma furnace,
laser ablation, or grown by chemical vapor deposition
(CVD) on a substrate using catalyst particles [9.37].
Quantum Dots, Quantum Wires,
and Quantum Films
Quantum behavior occurs in semiconductor materials (such as GaAs) when electrons are confined to
nanoscale dimensions. The confined space forces electrons to have energy states that are clustered around
specific peaks, producing fundamentally different electrical and optical properties than would be found in the
same material in bulk form. The number of directions
free of confinement is used to classify structures, thus
two-dimensional (2-D) confinement leads to a quan-
tum film, one-dimensional (1-D) confinement leads to
a quantum wire, and zero-dimensional (0-D) confinement leads to a quantum dot. The dimension of the
confined direction(s) is so small that the energy states
are quantized in that direction [9.37].
Nanowires
A variety of methods have been developed for making nanowires of a wide range of metals, ceramics,
and polymers. Examples include gold nanowires made
by a solution method [9.38], palladium nanowires created by electroplating on a stepped surface [9.39], and
zinc oxide nanowires created by a vapor/liquid/solid
method [9.40]. In one popular technique, electroplating
is conducted inside a nanoporous template of alumina or
polycarbonate to direct the growth of nanowires [9.41,
42]. The template can be chemically removed, leaving
the nanowires behind. In another application, lithographically patterned metal is used as a catalyst for
silicon nanowire growth, creating predefined regions of
nanowires on a surface [9.43, 44]. Using various combinations of metal catalysts and gases, a wide range of
nanowire compositions can be created from chemical
vapor deposition methods.
9.3.7 Micromachining
Micromachining is a set of material removal and
forming techniques for creating microscale movable
features and complex structures, often from silicon.
The micromachining processes listed in Table 9.4 can
be applied to other materials such as glasses, ce-
A Brief Introduction to MEMS and NEMS
9.3.8 Hard Fabrication Techniques
Hard MEMS utilizes enabling technologies for fabrication and design from the microelectronics industry.
The MEMS industry has modified advanced techniques,
leveraging well beyond the capability to fabricate integrated circuits.
Micromachining involves three fundamental processes: deposition, lithography, and etching. Deposition
may employ oxidation, chemical vapor deposition,
physical vapor deposition, electroplating, diffusion, or
ion implantation. Lithography methods include optical
and electron-beam techniques. Etching methods include
wet and dry chemical etches, which can be either
isotropic (uniform etching in all directions, resulting in
rounded features) or anisotropic (etching in one preferential direction, resulting in well-defined features).
9.3.9 Deposition
Physical Vapor Deposition (PVD)
Physical vapor deposition (PVD) includes evaporation
and sputtering. The evaporation method is used to
deposit metals on a surface from vaporized atoms removed from a target by heating with an electron beam.
This technique is performed under high vacuum and
produces very directional deposition and can create
shadows. Sputtering of a metallic or nonmetallic material is accomplished by knocking atoms off a target with
a plasma of an inert gas such as argon. Sputtering is less
directional and allows for higher deposition rates.
Chemical Vapor Deposition (CVD)
In chemical vapor deposition (CVD), precursor material
is introduced into a heated furnace and a chemical reaction takes place on the surface of the wafer. The CVD
process is generally performed under low-pressure
conditions and is sometimes explicitly referred to as
low-pressure CVD (LPCVD). A range of materials can
be deposited by CVD, including films of silicon (formed
by decomposition of silane (SiH4 )), silicon nitride
formed by reacting dichlorosilane (SiH2 Cl2 ) with ammonia (NH3 )), and silicon oxide (formed by silane with
an oxidizing species). LPCVD can produce amorphous
inorganic dielectric films and polycrystalline polysilicon and metal films. Epitaxy is a CVD process where
temperature and growth rate are controlled to achieve
ordered crystalline growth in registration with the substrate. PECVD is a plasma-enhanced CVD process.
Electroplating
A variety of electroplating techniques are used to make
micro- and nanoscale components. A mold is created
into which metal is plated. Gold, copper, chromium,
nickel, and iron are common plating metals.
Spin Casting
Spin casting is used to create films from a solution. The
most common spin-cast material is polymeric photoresist.
Sol–Gel Deposition
A range of sol–gel processes can be used to make films
and particles. The general technique involves a colloidal
suspension of solid particles in a fluid that undergo
a reaction to generate a gelatinous network. After deposition of the gel, the solvent can be removed to
transform the network into a solid phase which is subsequently sintered. Piezoelectric materials such as PZT
can be deposited with this method.
9.3.10 Lithography
Most of the micromachining techniques discussed below utilize lithography, or pattern transfer, at some point
in the manufacturing process. Depending on the resolution required to produce the desired feature sizes
and the aspect ratio necessary, lithography is either per-
211
Part A 9.3
ramics, polymers, and metals, but silicon is favored
because of its widespread use and the availability of
design and processing techniques. Other advantages of
silicon include the availability of relatively inexpensive pure single-crystal substrate wafers, its desirable
electrical properties, its well-understood mechanical
properties, and ease of integration into a circuit for
control and signal processing [9.7]. Although often performed in batch processes, micromachining for MEMS
application may make large-aspect-ratio features and
incorporation of novel or active materials a higher priority than batch manufacturing. This opens the door for
a wider range of fabrication techniques such as focused
ion-beam milling, laser machining, and electron-beam
writing [9.22, 45, 46].
A brief overview of micromachining is provided
below. The following books, used as references for
the discussion here, are valuable resources for more
extensive information [9.1, 2, 10, 18, 22, 33]. Additional information can be found in Taniguchi [9.47]
and Evans [9.48] on microfabrication technology,
Bustillo [9.49] on surface micromachining, and Gentili [9.50] on nanolithography.
9.3 Common MEMS/NEMS Materials and Their Properties
Solid Mechanics Topics
Part A 9.3
formed with ultraviolet light, an ion beam, x-rays, or an
electron beam. X-ray lithography can produce features
down to 10 nm and electron beams can be focused down
to less than 1 nm [9.50]. Optical lithography allows aspect ratios of up to three whereas x-ray lithography can
produce aspect ratios > 100. This large depth of focus,
lack of scattering effects, and insensitivity to organic
dust make x-ray lithography very attractive for NEMS
production. Electron-beam lithograph has the attractive
feature that a pattern can be directly written onto a resist, as well as the fact that it produces lower defect
densities with a large depth of focus, but the process
must be performed in vacuum.
In most cases a mask that carries either a positive or negative image of the features to be created
must first be produced. Masks are commonly made with
a chromium layer on fused silica. Photoresist covering the chromium is exposed with an optical pattern
generated from a sequence of small rectangles used to
draw out the pattern desired. Other mask production
techniques include photographic emulsion on quartz,
electron-beam lithography with electron-beam resist,
and high-resolution ink-jet printing on acetate or mylar
film.
Photolithographic fabrication techniques have a long
history of use with ceramics, plastics, and glasses. In
the case of silicon fabrication, the wafer is coated with
a polymeric photoresist layer sensitive to ultraviolet
light. Exposure of the photoresist layer is conducted
through a mask. Depending on whether a positive or
negative photoresist is used, the light either weakens the
polymer or strengthens the polymer. In the developer
step, chemicals are used to remove the weaker material, leaving a patterned photoresist layer behind. The
photoresist acts as a protective layer when etching is
conducted.
Contact lithography produces a 1:1 ratio of the mask
size and feature size. Proximity lithography also gives
a 1:1 ratio with slightly lower resolution because a gap
is left between the mask and the substrate to minimize
damage to the mask. A factor of 5–10 reduction is
common for projection step-and-repeat lithography. Because this technique allows for the production of feature
sizes smaller then the mask, only a small region is exposed at one time and the mask must be stepped across
the substrate.
9.3.11 Etching
A number of wet and dry etchants have been developed
for silicon. Important properties include orientation
dependence, selectivity, and the geometric details of
the etched feature (Fig. 9.2). A common isotropic wet
etchant for silicon is HNA (a combination of HF,
HNO3 , and CH3 COOH), while anisotropic wet etchants
include KOH, which etches {100} planes 100 times
faster than {111} planes, tetramethylammonium hydroxide (called TMAH or (CH3 )4 NOH), which etches
{100} planes 30–50 times faster than {111} planes but
leaves silicon dioxide and silicon nitride unetched, and
ethylenediamine pyrochatechol (EDP), which is very
hazardous but does not etch most metals.
Wet etchants such as HF for silicon dioxide, H3 PO4
for silicon nitride, KCl for gold, and acetone for organic
layers, can be performed in batch processes with little
cost [9.51]. An important feature of an etchant is its
selectivity; for example, the etch rate of an oxide by
HF is 100 nm/min compared to 0.04 nm/min for silicon nitride [9.51]. The etching reaction can be either
reaction rate controlled or mass transfer limited. Because wet etchants act quickly, making it hard to control
depth of the etch, electrochemical etching is sometimes
employed using an electric potential to moderate the reaction along with a precision thickness epitaxial layer
used for etch stop.
The challenge comes with drying after the wet etching process is complete. Capillary forces can easily
draw surfaces together, causing damage and stiction.
Supercritical drying, where the liquid is converted to
a gas, can be used to prevent this. Alternatively, application of a hydrophobic passivation layer such as
a fluorocarbon polymer can be used to prevent stiction.
Chemically reactive vapors and plasmas are highly
effective dry etchants. Xenon difluoride (XeF2 ) is
a commercially important highly selective vapor etchant
for silicon. Dry etchants such as CHF3 + O2 for silicon dioxide, SF6 for silicon nitride, Cl2 + SiCl4 for
Wet etch
Plasma (dry) etch
Isotropic
Part A
Anisotropic
212
{111}
Fig. 9.2 Trench profiles produced by different etching processes (after Maluf [9.1])
A Brief Introduction to MEMS and NEMS
all parts of the wafer. Ion milling refers to selective
sputtering and can be done uniformly over a wafer or
with focusing electrodes by focused ion-beam milling
(FIB). FIB is also becoming a more important technique
for test sample production and the application of gratings used for interferometry [9.52]. In addition to FIB,
techniques such as scanning probe microscope (SPM)
lithography and molecular-beam epitaxy can also be
used to create micro- and nanoscale gratings [9.53].
Beyond the use of etching as part of the initial fabrication of a device, some small adjustments may be
required after the device is fabricated due to small variations that occur in processing. Compensation can be
performed by trimming resistors and altering mechanical dimensions via techniques such as laser ablation
and FIB milling. Calibration can be performed electronically with correction coefficients.
9.4 Bulk Micromachining versus Surface Micromachining
The processes for silicon micromachining fall into
two general categories: bulk (subtraction of substrate material) and surface (addition of layers to the
substrate). Other techniques used on a range of materials include surface micromachining, wafer bonding,
thin film screen printing, electroplating, lithography
galvanoforming molding (LIGA, from the German
Lithografie-Galvanik-Abformung), injection molding,
electric-discharge machining (EDM), and focused ion
beam (FIB). Figure 9.3 provides a basic comparison
of bulk micromachining, surface micromachining, and
LIGA.
Bulk Micromachining
Removal of significant regions of substrate material in bulk micromachining is accomplished through
a)
b)
c)
Bulk micromachining
Surface micromachining
LIGA
Resist structure
Base
plate
Deposition of sacrificial layer
Deposition of silica layers on Si
Membrane
<111> face
Metal
structure
Electroforming
Patterning with mask
Gate plate
Patterning with mask and
etching of Si to produce cavity
Silicon
Silica
Lithography
Mold
insert
Mold fabrication
Deposition of microstructure layer
Molding
mass
Mold filling
Etching of sacrificial layer to produce
freestanding structure
Silicon
Polysilicon
Sacrificial
material
Plastic
structure
Unmolding
Fig. 9.3a–c Schematic diagrams depicting the processing steps required for (a) bulk micromachining (b) surface micromachining,
and (c) LIGA. All views are shown from the side (after Bhushan [9.2] Chap. 50)
213
Part A 9.4
aluminum, and O2 for organic layers, are used as
a plasma [9.51]. The process is conducted in a specially
designed system that generates a chemically reactive
plasma species of ion neutrals and accelerates them towards a substrate with an electric or magnetic field.
Plasma etching is the spontaneous reaction of neutrals with the substrate materials, while reactive-ion
etching involves a synergistic role between the ion bombardment and the chemical reaction. Deep reactive-ion
etching (DRIE) allows for the creation of high-aspectratio features. DRIE involves periodic deposition of
a protective layer to shield the sidewalls either through
condensation of reactant gasses produced by cryogenic
cooling of the substrate or interim deposition cycles to
put down a thin polymer film.
Ions can also be used to sputter away material.
For example, argon plasma will remove material from
9.4 Bulk Micromachining versus Surface Micromachining
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Part A
Solid Mechanics Topics
Part A 9.5
anisotropic etching of a silicon single-crystal wafer. The
fabrication process includes deposition, lithography,
and etching. Bulk micromachining is commonly used
for high-volume production of accelerometers, pressure
sensors, and flow sensors.
are used to produce a free-standing structure. Surface micromachining is attractive for integrating MEMS
sensors with electronic circuits, and is commonly
used for micromirror arrays, motors, gears, and grippers.
Surface Micromachining
Alternating structural and sacrificial thin film layers
are built up and patterned in sequence for surface micromachining. The process used by Sandia National
Laboratory uses up to five structural polysilicon and
five sacrificial silicon dioxide layers, whereas Texas
Instrument’s digital micromirror device (discussed below) is made from a stack of structural metallic
layers and sacrificial polymer layers [9.1, 54]. Deposition methods include oxidation, chemical vapor
deposition (CVD), and sputtering. Annealing must
sometimes be used to relax the mechanical stresses
that build up in the films. Lithography and etching
LIGA
Lithography galvanoforming molding (LIGA, from the
German Lithografie-Galvanik-Abformung) is a lithography and electroplating method used to create very
high-aspect-ratio structures (aspect ratios of more than
100 are common). The use of x-rays in the lithography
process takes advantage of the short wavelength to
create a larger depth of focus compared to photolithography [9.14]. Devices can be up to 1 mm in height
with another dimension being only a few microns and
are commonly made of materials such as metals, ceramics, and polymers. See Guckel [9.55], Becker [9.56], and
Bley [9.57] for additional details.
9.5 Wafer Bonding
Although microelectronics fabrication processes allow
stacking layers of films, structures are relatively two
dimensional. Wafer bonding provides an opportunity
for a more three-dimensional structure and is commonly used to make pressure sensors, accelerometers,
and microfluidic devices (Fig. 9.4). Anodic and direct bonding are the most common techniques, but
2.5μm LPCVD polysilicon mask
Sheat Sample
inlet injector
holes
Fused silica substrate
Electrochemical-discharge machined through-holes
Two substrates thermally bonded together
Fig. 9.4 Schematic diagram depicting a wafer bonding process
used to create a microfluidic channel for flow cytometry (after Kovacs [9.10])
bonding can also be achieved by using intermediate layers such as polymers, solders, and thin-film
metals.
Anodic (electrostatic) bonding can be used to bond
silicon to a sodium-containing glass substrate (with
a matched coefficient of thermal expansion) using an
applied electric field. This is accomplished with the application of a large voltage at elevated temperature to
make positive Na+ ions mobile. The positively charged
silicon is held to the negatively charged glass by electrostatic attraction.
Direct (silicon-fusion) bonding requires two flat,
clean surfaces in intimate contact. Direct bonding of
a silicon/glass stack can be achieved by applying pressure. Direct wafer bonding allows joining of two silicon
surfaces or silicon and silicon dioxide surfaces and is
used extensively to create SOI wafers. After treatment
of the surfaces to produce hydroxyl (OH) groups, intimate contact allows van der Waals forces to make the
initial bond followed by an annealing step to create
a chemical reaction at the interface.
Grinding and polishing is sometimes needed to
thin a bonded wafer. Annealing must be performed afterwards to remove defects incurred during grinding.
Alternatively, chemomechanical polishing can be used
to combine chemical etching with the mechanical action
of polishing.
A Brief Introduction to MEMS and NEMS
9.6 Soft Fabrication Techniques
Self-Assembly
Partly because of the high cost of nanolithography
and the time-consuming nature of atom-by-atom placement using probe microscopy techniques, self-assembly
is an important bottom-up approach to NEMS fabrication [9.59]. To offset the time it takes to build
unit by unit to create a useful device, massive parallelism and autonomy is required. The advantage of
self-assembly is that it occurs at thermodynamic minima, relying on naturally occurring phenomena that
govern at the nanoscale and create highly perfect assemblies [9.58]. The atoms, molecules, collections of
molecules, or nanoparticles self-organize into functioning entities using thermodynamic forces and kinetic
control [9.60]. Such self-organization at the nanoscale
is observed naturally in liquid crystals, colloids, micelles, and self-assembled monolayers [9.61]. Reviews
of self-assembly can be found in [9.62–65].
At the nanoparticle level, a variety of methods have
been used to promote self-assembly. Three basic requirements must be met: there must be some sort of
bonding force present between particles or the particles
and a substrate, the bonding must be selective, and the
particles must be in random motion to facilitate chance
interactions with a relatively high rate of occurrence.
Additionally, for the technique to be practical, the particles must be easily synthesized. Selectivity can be
facilitated by micromachining the substrate including
patterns with geometric designs that allow for only certain orientations of the mating particle.
Particularly powerful are self-assembly methods
using complementary pairs and molecular building
blocks (analogous to DNA replication). Complementary pairs can bind electrostatically or chemically (using
functional groups with couple monomers). Molecular
Table 9.5 Techniques for creating patterned SAMs [9.58]
Method
Scale of features
Microcontact printing
Micromachining
Microwriting with pen
Photolithography/lift-off
Photochemical patterning
Photo-oxidation
Focused ion-beam writing
Electron-beam writing
Scanning tunneling
microscope writing
100 nm – some cm
100 nm – some μm
≈ 10–100 μm
> 1 μm
> 1 μm
> 1 μm
≈ some μm
25–100 nm
15–50 nm
building blocks can use a number of different bonds
and linkages (ionic bonds, hydrogen bonds, transition
metal complex bonds, amide linkages, and ester linkages) to create building blocks for three-dimensional
(3-D) nanostructures and nanocrystals such as quantum
dots.
Self-assembled monolayers (SAMs) can be produced in patterned form by several techniques that
produce features in a range of micro- and nanoscale
sizes (Table 9.5). Combined with lithography, defined
areas of self-assembly on a surface can be created.
Applications of SAMs include fundamental studies of
wetting and electrochemistry, control of adhesion, surface passivation (to protect from corrosion, control
oxidation, or use as resist), tribology, directed assembly,
optical systems, colloid fabrication, and biologically active surfaces for biotechnology [9.58].
Soft Lithography
The term soft lithography encompasses a number
of techniques that can be used to fabricate microand nanoscale structures using replica molding and
self-assembly. These techniques include microcontact printing, replica molding, microtransfer molding,
micromolding in capillaries, and solvent-assisted micromolding [9.66].
As an example, microcontact printing uses a selfassembled monolayer as ink in a stamping operation
that transfers the SAM to a surface (Fig. 9.5). The stamp
is fabricated from of an elastomeric material such as
PDMS by casting onto a master with surface features.
The master can be produced with a range of photolithographic techniques. The polymeric replica mold is used
as a stamp to enable physical pattern transfer. The advantages of microcontact printing are its simplicity,
conformal contact with a surface, the reusability of the
stamp, and the ability to produce multiple stamps from
one master. Although defect density and registration of
patterns over large scales can be issues, the flexibility of
the stamp can be use to make small features (≈ 100 nm)
using compression or pattern transfer onto curved surfaces [9.58]. The aspect ratio of features is a constraint
with PDMS however. Ratios between 0.2 and 2 must be
used to ensure defect-free stamps and molds [9.67].
9.6.1 Other NEMS Fabrication Strategies
Nanoscale structures can be created from both topdown and bottom-up approaches. Because of the push
Part A 9.6
9.6 Soft Fabrication Techniques
215
216
Part A
Solid Mechanics Topics
Part A 9.6
to miniaturize commercial electronics, many top-down
methods are refinements of micromachining techniques
with the goal of achieving manufacturing accuracy
on the nanometer scale. Bottom-up methods rely on
additive atomic and molecular techniques, such as
self-organization, self-assembly, and templating, using
building blocks similar and size to those used in nature [9.68]. A brief review of some additional examples
is provided below.
Nanomachining
Scanning probe microscopes (SPMs) are a valuable set
of tools for NEMS characterization, but these tools
Photoresist
Si
Photolithography is used
to create a master
Photoresist pattern
(1–2 μm thickness)
Si
PDMS is poured over
master and cured
PDMS
Photoresist pattern
Si
PDMS is peeled away
from the master
PDMS
PDMS is exposed to a
solution containing
HS(CH2)15CH3
PDMS
Alkanethiol
Stamping onto gold
substrate transfers
thiol to form SAM
SAMs (1–2 nm)
Au (5–2000 nm)
Si
Ti (5–10 nm)
Metal not protected by SAM is
removed by exposure to
selective chemical etchant
Si
Fig. 9.5 Schematic diagram depicting the processing steps
required for microcontact printing. All views are shown
from the side (after Wilbur [9.58])
can also be used for NEMS manufacturing. These microscopes share the common feature that they employ
a nanometer-scale probe tip in the proximal vicinity of
a surface. They are many times more powerful than
scanning electron microscopes because their resolution
is not determined by wavelength for the interaction with
the surface under investigation.
The scanning tunneling microscope (STM) can be
used to create a strong electric field in the vicinity of the
probe tip to manipulate individual atoms. Atoms can be
induced to slide over a surface in order to move them
into a desired arrangement by mechanosynthesis [9.69].
Resolution is effectively the size of a single atom but
practically the process is exceptionally time consuming
and the sample must be held at very low temperature to
prevent movement of atoms out of place [9.70]. With
slightly less resolution but still less than 100 nm, an
STM can also be used to write on a chemically amplified negative electron-beam resist.
Nanolithography
Surface micromachining can be conducted at the
nanoscale using electron-beam lithography to create free-standing or suspended mechanical objects.
Although the general approach parallels standard lithography (see above), the small-scale ability of this
technique is enabled by the fact that an electron beam
with energy in the keV range is not limited by diffraction. The electron beam can be scanned to create
a desired pattern in the resist [9.8].
Nanoscale resolution can also be obtained using
alternative lithographic techniques such as dip-pen
nanolithography (DPN) [9.71]. This technique employs
an atomic force microscope (AFM) probe tip to deposit a layer of material onto a surface, much as a pen
writes on paper. A pattern can be drawn on a surface
using a wide range of inks such as thiols, silanes, metals, sol–gel precursors, and biological macromolecules.
Although the DPN process is inherently slower that
standard mask lithographic techniques, it can be used
for intricate functions such as mask repair and the application of macromolecules in biosensor fabrication, or
it can be parallelized to increase speed [9.72]. This and
other nanofabrication techniques using AFM to modify
and pattern surfaces are reviewed by Tang [9.73].
9.6.2 Packaging
Packaging of a MEMS/NEMS device provides a protective housing to prevent mechanical damage, minimize
stresses and vibrations, guard against contamination,
A Brief Introduction to MEMS and NEMS
considerations include thickness of the device, wafer
dicing (separation of the wafer into separate dice),
sequence of final release, cooling of heat-dissipating
devices, power dissipation, mechanical stress isolation, thermal expansion matching, minimization of
creep, protective coatings to mitigate damaging environmental effects, and media isolation for extreme
environments [9.1].
In the die-attach process, each individual die is
mounted into a package, by bonding it to a metal, ceramic or plastic platform with a metal alloy solder or an
adhesive. For silicon and glass, a thin metal layer must
be placed over the surface prior to soldering to allow
for wetting. Electrical interconnects can be produced
with wire bonding (thermosonic gold bonding with ultrasonic energy and elevated temperature) and flip-chip
bonding (using solder bumps between the die and package pads). Fluid interconnects are created by insertion
of capillary tubes, mating of self-aligning fluid ports,
and laminated layers of plastic [9.1].
9.7 Experimental Mechanics Applied to MEMS/NEMS
With a basic understanding of the materials and processes used to make MEMS/NEMS devices, the role
of mechanics in materials selection, process validation,
design development, and device characterization can
now be discussed. The remainder of this chapter will
focus on the forces and phenomena dominant at the
micrometer and nanometer scales, basic device characterization techniques, and mechanics issues that arise in
MEMS/NEMS devices.
9.8 The Influence of Scale
To gain perspective on the micrometer and nanometer
size scales, consider that the diameter of human hair is
40–80 μm and a DNA molecule is 2–3 nm wide. The
weight of a MEMS structure can be about 1 nN and
that of a NEMS components about 10−20 N. Compare
this to the mass of a drop of water (10 μN) or an eyelash (100 nN) [9.2]. The minuscule size of forces that
influence behavior at these small scales is hard to imagine. For instance, if you take a 10 cm length of your
hair and hold it like a cantilever beam, the amount of
force placed on the tip of the cantilever to deflect it
by 1 cm is on the order of 1 pN. That piece of hair is
40–80 μm in diameter, which is large compared to most
MEMS/NEMS components.
In dealing with micro- and nanoscale devices, engineering intuition developed through experience with
macroscale behavior is often misleading. It should be
noted that many macroscale techniques can be applied
at the micro- and nanoscales, but advantages come not
from miniaturization but rather working at the relevant
size scale using the uniqueness of the scale. The balance of forces at these scales differs dramatically from
the macroscale (Table 9.6). Compared to a macroscale
counterpart of the same aspect ratio, the structural stiffness of a microscale cantilever increases relative to
inertially imposed loads. When the length scale changes
by a factor of a thousand, the area decreases by a factor of a million and the volume by a factor of a billion.
Surface forces, proportional to area, become a thousand times large than forces that are proportional to
volume, thus inertial and electromagnet forces become
negligible. At small scales, adhesion, friction, stiction
217
Part A 9.8
protect from harsh environmental conditions, dissipate heat, and shield from electromagnetic interference [9.15]. Packaging is critical because it enables
the usefulness, safety, and reliability of the device.
Hermetic packaging made of metal, ceramic, glass or
silicon is used to prevent the infiltration of moisture,
guard against corrosion, and eliminate contamination.
The internal cavity is evacuated or filled with an inert gas. For MEMS/NEMS the packaging may also be
required to provide access to the environment through
electrical and/or fluid interconnects and optically transparent windows. In these cases, the devices are left more
vulnerable in order for them to interact with the environment to perform their function. Although there are
well-established techniques for packaging of common
microelectronics devices, packaging of MEMS/NEMS
presents particular challenges and may account for
75–95% of the overall cost of the device [9.1].
Packaging design must be conducted in parallel
with design of the MEMS/NEMS component. Design
9.8 The Influence of Scale
218
Part A
Solid Mechanics Topics
Part A 9.8
Table 9.6 Scaling laws and the relative importance of
phenomena as they depend on linear dimension, i (after
Madou [9.22])
Importance
at small scale
Diminished
Increased
Phenomena
Power of linear
dimension
Flow
Gravity
Inertial force
Magnetic force
Thermal emission
Electrostatic force
Friction
Pressure
Piezoelectricity
Shape-memory
effect
Velocity
Surface tension
Diffusion
van der Waal
force
l4
l3
l3
l 2 , l 3 or l 4
l 2 or l 4
l2
l2
l2
l2
l2
l
l
l 1/2
l 1/4
(static friction), surface tension, meniscus forces, and
viscous drag often govern. Acceleration of a small
object becomes rapid. At the nanoscale, phenomena
such as quantum effects, crystalline perfection, statistical time variation of properties, surface interactions,
and interface interactions govern behavior and materials
properties [9.35].
Additionally, the highly coupled nature of thermal
transport properties at the microscale can be either an
advantage or disadvantage depending on the device. Enhanced mass transport due to large surface-to-volume
ratio can be a significant advantage for applications
such as capillary electrophoresis and gas chromatography. However, purging air bubbles in microfluidic
systems can be extremely difficult due to capillary
forces. The interfacial surface tension force will cause
small bubble less than a few millimeters in diameter to
adhere to channel surfaces because the mass of liquid in
a capillary tube produces an insubstantial inertial force
compared to the surface tension [9.10].
Some scaling effects favor particular micro- and
nanoscale situations but others do not. For instance,
large surface-to-volume ratio in MEMS devices can undermine device performance because of the retarding
effects of adhesion and friction. However, electrostatic
force is a good example of a phenomena that can have
substantial engineering value at small scales. Transla-
tional motion can be achieved in MEMS by electrostatic
force because this scales as l 2 as compared to inertial force which scales as l 3 . Microactuation using
electrostatic forces between parallel plates is used in
comb drives, resonant microstructures, linear motors,
rotary motors, and switches. In relation to MEMS testing, gripping of a tension sample can be achieved
using an electrostatic force between a sample and the
grip [9.74].
It is also important to note that, as the size scale
decreases, breakdown in the predictions of continuumbased theories can occur at various length scales. In the
case of electrostatics, electrical breakdown in the air
gap between parallel plates separated by less than 5 μm
does not occur at the predicted voltage [9.75]. In optical
devices, nanometer-scale gratings can produce an effective refractive index different from the natural refractive
index of the material because the grating features are
smaller than the wavelength of light [9.76]. For resonant structures continuum mechanics predictions break
down when the structure’s dimensions are on the order
of tens of lattice constants in cross section [9.11]. Detailed discussions of issues related to size scale can be
found in Madou [9.22] and Trimmer [9.77].
9.8.1 Basic Device Characterization
Techniques
A range of mechanical properties are needed to facilitate design, predict allowable operating limits, and conduct quality control inspection for MEMS. As with any
macroscale device or component, structural integrity
is critical to MEMS/NEMS. Concerns include friction/stiction, wear, fracture, excessive deformation, and
strength. Properties required for complete understanding of the mechanical performance of MEMS/NEMS
materials include elastic modulus, strength, fracture
toughness, fatigue strength, hardness, and surface topography. In MEMS devices the minimum feature size
is on the order of 1 μm, which is also the natural length
scale for microstructure (such as the grain size, dislocation length, or precipitate spacing) in most materials.
Because of this, many of the mechanical properties
of interest are size dependent, which precipitates the
need for new testing methods given that knowledge
of material properties is essential for predicting device reliability and performance. A detailed discussion
about micro- and nanoscale testing can be found in
Part B of this handbook as well as in references such
as Sharpe [9.78], Srikar [9.79], Haque and Saif [9.80],
Bhushan [9.2], and Yi [9.81]. The following sections
A Brief Introduction to MEMS and NEMS
9.8.2 Residual Stresses in Films
Many MEMS/NEMS devices involve thin films of materials. Properties of thin-film material often differ
from their bulk counterparts due to the high surfaceto-volume ratio of thin films and the influence of
surface properties. Additionally, these films must have
good adhesion, low residual stress, low pinhole density, good mechanical strength, and good chemical
resistance [9.22]. These properties often depend on deposition and processing details.
The stress state of a thin film is a combination
of external applied stress, thermal stress, and intrinsic
residual stress that may arise due to factors such as
doping (in silicon), grain boundaries, voids, gas entrapment, creep, and shrinkage with curing (in polymeric
materials). Stresses that develop during deposition of
thin-film material can be either tensile or compressive
and may give rise to cracking, buckling, blistering, delaminating, and void formation, all of which degrade
device performance. Residual stresses can arise because of coefficient of thermal expansion mismatch,
lattice mismatch, growth processes, and nonuniform
plastic deformation. Residual stresses that do not cause
mechanical failure may still significantly affect device performance by causing warping of released
structures, changes in resonant frequency of resonant
structures, and diminished electrical characteristics. In
some instances, however, residual stresses can be used
productively, such as in shape setting of shape-memory
alloy films or stress-modulated growth and arrangement
of quantum dots.
There are numerous techniques for measuring residual stresses in thin films. Fundamental techniques rely
on the fact that stresses within a film will cause bending
in its substrate (tension causing concavity, compression
causing convexity). Simple displacement measurements
can be conducted on a circular disk or a micromachined
beam and stress calculated from the radius of curvature
of the bent substrate or the deflection of a cantilever.
Strain gages may also be made directly in the film and
used to make local measurements. Freestanding portions of the thin film can be created by micromachining
so that the films stresses can be explored by applied
pressure, external probe, critical length for buckling,
or resonant frequency measurements. For instance, the
critical stress to cause buckling in a doubly supported
beam can be estimated from:
π 2t2
,
K L2
where K is a constant determined by the boundary conditions (3 for a doubly supported beam), E is Young’s
modulus, t is the beam thickness, and L is the shortest
length of beam displaying buckling [9.10]. The stress or
strain gradient over a region of a film can be found by
measuring deflections in a simple cantilever. The upward or downward deflection along the length of the
beam can be measured by optical methods and used
to estimate the internal bending moment M from the
expression:
σCR = E
(1 − ν2 )
δ (x)
=K+
Mx ,
x
2E I
where δ(x) is the vertical deflection at a distance x from
the support, E is Young’s modulus, ν is Poisson’s ratio,
I is the moment of the beam cross section about the
axis of bending, and K is a constant determined by the
boundary conditions at the support [9.83].
A number of techniques have been developed for
determining residual stresses including an American
Society for Testing and Materials (ASTM) standard involving optical interferometry [9.84]. The bulge test is
a basic technique for measuring residual stress in a freestanding thin film [9.85]. The bulge test structure can
be easily created by micromachining with well-defined
boundary conditions. The M-test is an on-chip test that
uses bending of an integrated free-standing prismatic
beam [9.86]. The principle of an electrostatic actuator is
used to conduct the test to find the onset of instability in
the structure. The wafer curvature test is regularly used
for residual stress measurement in nonintegrated film
structures, and can be used even when the film thickness is much smaller than the substrate thickness [9.87].
Dynamic testing can be used to measure resonant frequency and extract information about residual stress and
modulus. Resonant frequency increases with tension
and decreases in compression [9.88, 89]. Air damping
can significantly impact theses measurements, however,
so they must be conducted in vacuum [9.18]. Other established techniques that can be employed to measure
residual stresses in films include passive strain sensors,
Raman spectroscopy, and nanoindentation [9.79].
More recently, nanoscale gratings created by focused ion-beam (FIB) milling have been used to
219
Part A 9.8
provide a review of some mechanics issues that arise at
the device level. The following sources, used as references for the discussion below, should be consulted for
additional background on mechanics, metrology, and
MEMS: Trimmer [9.77], Madou [9.22], Bhushan [9.2],
and Gorecki [9.82].
9.8 The Influence of Scale
220
Part A
Solid Mechanics Topics
Part A 9.8
produce moiré interference between the grating on the
specimen surface and raster scan lines of a scanning
electron microscope (SEM) image [9.90]. This technique can be used to provide details of residual strains in
microscale structures as they evolve with etching of the
underlying sacrificial layer [9.52]. Digital image correlation (DIC) has also been applied to SEM and atomic
force microscopy (AFM) images in combination with
FIB. DIC is used to capture deformation fields while
nearby FIB milling of the specimens releases residual
stresses, allowing very local evaluation [9.91].
9.8.3 Wafer Bond Integrity
Wafer bonding is often an essential device fabrication step, particularly for microfluidic devices, microengines, and microscale heat exchangers. Although
direct bonding of silicon can achieve strengths comparable to bulk silicon, the process is sensitive to
bonding parameters such as temperature and pressure.
The appearance of voids and bubbles at the interface
is particularly undesirable for both strength and electrical conductivity [9.92]. An important nondestructive
technique for assessing the bond quality of bonded
silicon wafers is infrared transmission. At IR wavelengths of about 1.1 μm silicon is transparent [9.1].
Quantification of bond strength can be conducted with
techniques such as the pressure burst test, tensile/shear
test, knife-edge test, or four-point bend-delamination
test [9.93].
Although a range of techniques and processes can
be employed to bond both similar and dissimilar materials, the stresses and deformation of the wafers that
develop are consistent. The residual stress stored in the
bonded wafers is important because it may provide the
elastic strain energy to drive fracture. Details of the
wafer geometry can impact the final shape of the bonded
pair and the integrity of the bond interface [9.94].
9.8.4 Adhesion and Friction
Adhesion is both essential and problematic for
MEMS/NEMS. For multilayered devices, good adhesion between layers is critical for overall performance
and reliability, where delamination under repetitive applied mechanical stresses must be avoided. Adhesion
between material layers can be enhanced by improved
substrate cleanliness, increased substrate roughness, increased nucleation sites during deposition, and addition
of a thin adhesion-promoting layer. Standard tests for
film adhesion include: the scotch-tape test, abrasion,
scratching, deceleration using ultrasonic and ultracentrifuge techniques, bending, and pulling [9.95]. In situ
testing of adhesion can also be conducted by pressurizing the underside of a film until initiation of
delamination. This method also allows the determination of the average work of adhesion.
Adhesion can be problematic if distinct components
or a component and the nearby substrate come into contact, causing the device to fail. For example, although
the mass in an accelerometer device is intended to be
free standing at all points of operation, adhesion can
occur in the fabrication process. Commonly with freestanding portions of MEMS structures, the capillary
forces present during the drying of a device after etching to remove sacrificial material are large enough to
cause collapse of the structure and failure due to adhesion [9.96]. To avoid this problem, supercritical drying
is used.
Contacting surfaces that must move relative to one
another in MEMS/NEMS are minimized or eliminated
altogether, the reason being that friction and adhesion
at these scales can overwhelm the other forces at play.
Because silicon readily oxidizes to form a hydrophilic
surface, it is much more susceptible to adhesion and
accumulation of static charge [9.97]. When contacting
surfaces are involved, lubricant films and hydrophobic
coatings with low surface energy can be applied to minimize wear and stiction (the large lateral force required
to initiate relative motion between two surfaces). For
instance, Analog Devices uses a nonpolar silicone coating in its accelerometers to resist charge buildup and
stiction [9.98].
Processing plays a major role in surface properties such as friction and adhesion. Polishing will
dramatically affect roughness, as in the case of polysilicon where roughness can be reduced by an order
of magnitude from the as-deposited state [9.2]. The
doping process can also lead to higher roughness. Organic monolayer films show promise for lubrication of
MEMS to reduce friction and prevent wear. The atomic
force microscope and the surface force apparatus used
to quantify friction and MEMS test structures such
as those developed at Sandia National Laboratory are
aiding the development of detailed mechanics models
addressing friction [9.99–102].
Flow Visualization
Flow in the microscale domain occurs in a range of
MEMS devices, particularly in bioMEMS, microchannel networks, ink-jet printer heads, and micropropulsion
systems. The different balance of forces at micro-
A Brief Introduction to MEMS and NEMS
flow fields in microfluidic devices, where micron-scale
spatial resolution is critical [9.103]. Microparticle image velocimetry (μPIV) has been used to characterize
such things as microchannel flow [9.104] and microfabricated ink-jet printer head flow [9.105]. For the
high-velocity, small-length-scale flows found in microfluidics, high-speed lasers and cameras are used in
conjunction with a microscope to image the particles
seeded in the flow. With μPIV techniques, the flow
boundary topology can be measured to within tens of
nanometers [9.106].
9.9 Mechanics Issues in MEMS/NEMS
9.9.1 Devices
A wide range of MEMS/NEMS devices is discussed
in the literature, both as research and commercialized
devices. These devices are commonly planar in nature and employ structures such as cantilever beams,
fixed–fixed beams, and springs that are loaded in bending and torsion. A range of mechanics calculations are
needed for device characterization, including the effective stiffness of composite beams, deflection analysis of
beams, modal analysis of a resonant structures, buckling analysis of a compressively loaded beams, fracture
and adhesion analysis of structures, and contact mechanics calculations for friction and wear of surfaces.
A substantial literature is available on the application
of mechanics to MEMS/NEMS devices. The selected
MEMS/NEMS examples presented below were chosen
for their illustrative nature.
Digital Micromirror Device
Optical MEMS devices range from bar-code readers to
fiber-optic telecommunication, and use a range of wideband-gap materials, nonlinear electro-optic polymers,
and ceramics [9.107]. (See Walker and Nagel [9.108]
for more information on optical MEMS.) A wellestablished commercial example of an optical MEMS
device is the Digital Micromirror DeviceTM (DMD)
by Texas Instruments used for projection display
(Fig. 9.6) [9.109]. These devices have superior resolution, brightness, contrast, and convergence performance
compared to conventional cathode ray tube technology [9.2]. The DMD contains a surface micromachined
array of half a million to two million independently
controlled, reflective, hinged micromirrors that have
a mechanical switching time of 15 μs [9.110]. This de-
vice steers a reflected beam of light with each individual
mirrored aluminum pixel. Pixel motion is driven by an
electrostatic field between the yoke and an underlying
electrode. The yoke rotates and comes to rest on mechanical stops and its position is restored upon release
by torsional hinge springs [9.111].
Almost all commercial MEMS structures avoid any
contact between structural members in the operation of
the device, and sliding contact is avoided completely
because of stiction, friction, and wear. The DMD is
currently the only commercial device where structural
components come in and out of contact, with contact
occurring between the mirror spring tips and the underlying mechanical stops, which act as landing sites. To
prevent adhesion problems in the DMD, a self healing
perfluorodecanoic acid coating is used on the structural
aluminum components [9.112].
Other challenges for the DMD include creep and fatigue behavior in the hinge, shock and vibration, and
sensitivity to debris within the package [9.2]. The primary failure mechanisms are surface contamination and
hinge memory due to creep in the metallic alloy resulting in a residual tilt angle [9.1]. Heat transfer, which
contributes to the creep problem, is also an issue for micromirrors. When the reflection coefficient is less than
100% some of the optical power is absorbed as heat and
can cause changes in the flatness of the mirror, damage to the reflective layer, and alterations in the dynamic
behavior of the system [9.113].
Micromirrors for projection display involve rotating structures and members in torsion. Such torsional
springs must be well characterized and their mechanics well modeled. For production devices extensive
finite element models are developed to optimize performance [9.114]. For initial design calculations however,
221
Part A 9.9
scopic length scales can influence fluid flow to produce
counterintuitive behavior in microscopic flows. Additionally, the breakdown in continuum laws for fluid
flow begins to occur at the microscale. For instance, the no-slip condition no longer applies and the
friction factor starts to decrease with channel reduction.
Particle image velocimetry (PIV) is a technique
commonly used at macroscopic length scales to measure velocity fields through the use of particles seeded
in the fluid. The technique has been adapted to measure
9.9 Mechanics Issues in MEMS/NEMS
222
Part A
Solid Mechanics Topics
Part A 9.9
some closed-form solutions for mechanics analysis can
be employed. For instance, an appropriate material can
be chosen or the basic dimensional requirements can
be found from calculation of the maximum shear stress
τmax in a beam of elliptical cross section in torsion (with
a and b the semi-axis lengths) using:
2Gαa2 b
, for a > b ,
a 2 + b2
where G is the shear modulus and α in the angular
twist [9.107]. Mechanical integrity of the DMD relies
on low stresses in the hinge, thus the tilt angle is limited
to ±10◦ [9.1].
τmax =
atomic force microscopy [9.117, 118], and magnetic
beads [9.119], but these techniques have the disadvantage of requiring external probes, labeling, and/or
optical excitation. Alternatively, there are several methods using molecular recognition and the small-scale
forces created by events such as DNA hybridization
and receptor–ligand binding to produce bending in
cantilevers to create sensors with high selectivity and
resolution [9.115, 120].
Microcantilever sensors have been used for some
time to detect changes in relative humidity, temperature,
pressure, flow, viscosity, sound, natural gas, mercury
vapor, and ultraviolet and infrared radiation. More re-
Biomolecular Recognition Device
Biological molecules can be probed by external methods using techniques such as optical tweezers [9.116],
a)
100 μm
Oligonucleotide
x
b)
Mirror –10 deg
Mirror +10 deg
Hybridization
Δx
Hinge
Yoke
Landing tip
CMOS
substrate
Fig. 9.6 (a) SEM image of yoke and hinges of one pixel
with mirror removed (b) Schematic of two tilted pixels with
mirrors (shown as transparent) (reprinted with permission,
Hornbeck [9.111], Bhushan [9.2])
Fig. 9.7 SEM image of a portion of the cantilever sensor
array and schematics illustrating functionalized cantilevers
with selective sensing capability (reprinted with permission, Fritz [9.115])
A Brief Introduction to MEMS and NEMS
Δm ≈ 2
Meff
Δω
ω0
where Meff is the effective vibratory mass of the
resonator, and ω0 is the resonance frequency of the device [9.11]. The mass sensitivity of NEMS devices with
micromachined cantilevers can be as small as a single
small molecule (in the range of a single Dalton).
In a device such as that shown in Fig. 9.7, a liquid medium, which contains molecules that dock to
a layer of receptor molecules attached to one side
of the cantilever, is injected into the device. Sensitizing an array of cantilevers with different receptor
allows docking of different substances in the same solution [9.115]. Hybridization can be done with short
strands of single-stranded DNA and proteins known
a)
to recognize antibodies. When docking occurs, the increase in the molecular packing density leads to surface
stress, causing bending (10–20 nm of deflection). This
deflection can be measured by a laser beam reflected off
of the end of the cantilever [9.115]. Alternatively, simple geometric interference by interdigitated cantilevers
that act as diffraction gratings can be used to provide
output of a binding event [9.120].
Thermomechanical Data Storage Device
Much of the drive to nanometer-scale devices originates in the desire for higher density and faster
computational devices. Magnetic data storage has been
pushed into the nanoscale regime, but limitations have
prompted the development of alternative methods for
data storage such as the NEMS device known as
the Millipede, developed by IBM. The Millipede, or
scanning probe array memory device, is an array of
individually addressable scanning probe tips (similar
to atomic force microscope probe tips) that makes
precisely positioned indentations in a polymer thin
film. The Millipede is scanned to address a large area
for data storage. The indentations are bits of digital
information. A polymer thin film (50 nm thick) of polymethyl methacrylate (PMMA) is used for write, read,
erase, and rewrite operations. Each individual bit is
a nanoscale feature, which allows the Millipede to extend storage density to the Tbit/in2 range with a bit
size of 30–40 nm [9.123]. The device uses multiple
cantilever probe tips equipped with integrated heaters
which allow for data transfer rates of up to a few
Mb/s [9.124].
Multiplex driver
2-D cantilever array chip
b)
Highly doped
silicon
cantilever leg
Nickel
bridge
x
Metal 1
(Gold)
Metal 2
(Nickel)
z3
y
Polymer storage media
on x/y/z scanner
Low-doped Schottky diode area
z1
z2
Stress-controlled
nitride
Silicon cantilever
Heater
platform
Fig. 9.8 (a) Schematic illustration of the millipede device, (b) with a detail of one cantilever cell (reprinted with permission,
Despont [9.122])
223
Part A 9.9
cently micromachined cantilevers have been used to
interact and probe material at the molecular level. Devices employing these micromachined cantilevers can
be dynamic, which are sensitive to mass changes down
to 10–21 g (the single molecule level), or static, which
are sensitive to surface stress changes in the low mN/m
range (changes in Gibbs free energy caused by binding
site-analyte interactions) [9.121]. In this case adhesion
is required between the device and the material to be
detected.
In a functionalized cantilever array device produced
to measure biomechanical forces created by DNA hybridization or receptor–ligand binding, detection of the
mass change is accomplished by measuring a shift in
resonant frequency. The responsiveness of the device to
a change in mass is given by the expression:
9.9 Mechanics Issues in MEMS/NEMS
224
Part A
Solid Mechanics Topics
Part A 9.10
Substrate
Polymer
Cantilever
Write current
Pit:
25nm deep
40nm wide
(maximum)
Erasure
current
Sensing current
Inscribed pins
Data stream 0 0 1 1 1 1 1 0 1 0 0 0 1 0 1 0 1 0 0 1 0 1 1 1 0
1
Output signal
0
The Millipede device is a massively parallel structure with a large array of thousands of probe tips
(100 cantilevers/mm2 ), each of which is able to address
a region of the substrate where it produces indentations for use as data storage bits (Fig. 9.8) [9.125].
As illustrated in Fig. 9.9, the probes writes a bit by
heating and a mechanical force applied between a can-
Fig. 9.9 Schematic of the writing, erasing, and reading op-
erations in the Millipede device. Data are mechanically
stored in pits on a surface (reprinted with permission, Vettiger [9.127])
tilever tip and a polymer film. Erasure of a bit is also
conducted with heating by placing a small pit just adjacent to the bit to be erased or using the spring back
of the polymer when a hot tip is inserted into a pit.
Reading is also enabled by heat transfer since the sensing relies on a thermomechanical sensor that exploits
temperature-dependent resistance [9.126]. The change
in temperature of a continuously heated resistor is monitored while the tip is scanned over the film and relies
on the change in resistance that occurs when a tip moves
into a bit [9.123].
Scanning x, y manipulation is conducted magnetically with the entire array at once. The data storage
substrate is suspended above the cantilever array with
leaf springs which enables the nanometer-scale scanning tolerances required. The cantilevers are precisely
curved using residual stress control of a silicon nitride
layer in order to minimize the distance between the
heating platform of the cantilever and the polymer film
while maximizing the distance between the cantilever
array and the film substrate to ensure that only the tips
come into contact [9.123]. Fabrication details are given
in Despont [9.125].
Thermal expansion is a major hurdle for this device
since a shift of ≈ 30 nm can cause misalignment of the
data storage substrate and the cantilever array. A 10 nm
tip position accuracy of a 3 mm × 3 mm silicon area requires that temperature of the device be controlled to
1 ◦ C using several sensors and heater elements [9.123].
Tip wear due to contact between the tip and the underlying silicon substrate is an issue for device reliability.
Additionally, the PMMA is prone to charring at the
temperatures necessary for device operation (around
350 ◦ C) [9.128] so new polymeric formulations had to
be developed to minimize this problem. The feasibility
of using thin-film NiTi shape-memory alloy (SMA) for
thermomechanical data storage as an alternative to the
polymer thin film has also been shown [9.129].
9.10 Conclusion
The sensors, actuators, and passive structures developed as MEMS and NEMS devices require a highly
interdisciplinary approach to their analysis, design, de-
velopment, and fabrication. Experimental mechanics
plays a critical role in design development, materials selection, prediction of allowable operating lim-
A Brief Introduction to MEMS and NEMS
MEMS/NEMS testing must be further developed. This
chapter has provided a brief review of the fabrication
processes and materials commonly used and experimental mechanics as it is applied to MEMS and
NEMS.
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its, device characterization, process validation, and
quality control inspection. Commercial devices exist, and research in the area of MEMS/NEMS is
extremely active, but many challenges remain. Advanced materials must be well characterized and
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