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A Brief Introduction to MEMS and NEMS
W. C. Crone
Department of Engineering Physics
University of Wisconsin – Madison
Engineering Research Building
1500 Engineering Drive
Madison, WI
Tel: 608-262-8384, Email: crone@engr.wisc.edu
Abstract
The expanding and developing fields of MEMS and 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 microoptical 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.
1. MEMS/NEMS
1.1. Introduction
The acronym MEMS stands for microelectromechanical system, but MEMS generally refers to
microscale devices or miniature embedded systems involving one or more micromachined
component that enables higher level function. Similarly NEMS, nanoelectromechanical system,
refers to such nanoscale devices or nanodevices. MEMS and NEMS are fabricated 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 nonsilicon-based devices created
by the same micromachining or other techniques. They can be classified as sensors, actuators
and passive structures (see Table 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
W. Crone, “A Brief Introduction to MEMS and NEMS,” in Springer Handbook of Experimental Solid Mechanics,
W.N. Sharpe, Editor, Springer-Verlag, New York. In press.
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resistance, piezoresistive, piezoelectric, change in capacitance, and magnetoresistive methods
(Table 2). MEMS/NEMS actuators provide the ability to manipulate physical parameters at the
micro/nano scale, and can employ eletrostatic, 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.
Table 1. Sample applications of MEMS/NEMS
Sensors
accelerometers, biochemical analyzers, environmental assay devices,
gyroscopes, medical diagnostic sensors, pressure sensors
Actuators
data storage, drug delivery devices, drug synthesis, fluid regulators,
micro fuel cell, micromirror devices, microphones, optoelectric devices,
radio frequency devices, surgical devices
Passive Structures
atomizers, fluid spray systems, fuel injection, ink jet medical inhalers,
printing devices
Table 2. Physical quantities used in MEMS/NEMS sensors and actuators. Adapted from Maluf
[Maluf 2000]
Order of
Physical and Material
Energy Density
Method
Description
Parameters
(J/cm3)
Electrostatic
attractive force between two electric field,
~0.1
components carrying
dielectric permittivity
opposite charge
Piezoelectric
certain materials that change electric field,
~0.2
shape under an electric field Young’s modulus,
piezoelectric constant
Thermal
thermal expansion or
coefficient of expansion, ~5
difference in coefficient of
temperature change,
thermal expansion
young’s modulus
Magnetic
electric current in a
magnetic field,
~4
component surrounded by a
magnetic permeability
magnetic field gives rise to
an electromagnetic force
Shape memory
certain materials that
transformation
~10
undergo a solid-solid phase
temperature
transformation producing a
large shape change
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 the characteristic length scale is 1 to 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
nm to 100 nm range. [Bhushan, 2004] Additionally, although an entire device may be
mesoscale, if the functional components fall in the microscale or nanoscale regime it may be
W. Crone, “A Brief Introduction to MEMS and NEMS,” in Springer Handbook of Experimental Solid Mechanics,
W.N. Sharpe, Editor, Springer-Verlag, New York. In press.
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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. [Maluf, 2000] Despite this characteristic, the MEMS industry is already
valued in tens of billions of dollars and growing rapidly. The Small Times Tech Business
DirectoryTM lists more than 700 manufacturers/fabricators of Microsystems and
nanotechnologies [Small Times, 2006]. Most commercial MEMS devices can be classified as
mechanical transducers. [Beeby, 2004] 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 Instutments), and
pressure sensors for the automotive and medical industries (NovaSensor). For example, 85
million accelerometer units were produced and $400 million in revenues for micromirror devices
was earned in 2002. [Bhushan, 2004] Currently, the largest commercial sales volumes of MEMS
are pressure sensors, accelerometers and ink jet cartidges, followed by optical switches,
gyroscopes, and fluid regulators. [Kovacs, 1998; Bhushan, 2004] The NEMS industry, while
still young, is already valued at 100 million dollars. [Bhushan, 2004]
Microfluidic and nanofluidic devices also fall under the umbrella of MEMS/NEMS and are often
classified as BioMEMS/BioNEMS when involving biological materials. These devices
incorporate channels with at least one microscale or nanoscale dimension in which fluid flows,
allowing 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. [Ziaie, 2004] 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. [Beeby, 2004] 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. [Craighead 2000] Device examples
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 of 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. Sensitivity of such BioNEMS devices can be
exquisite; selectively binding and detecting a single biomolecule. More complete background
W. Crone, “A Brief Introduction to MEMS and NEMS,” in Springer Handbook of Experimental Solid Mechanics,
W.N. Sharpe, Editor, Springer-Verlag, New York. In press.
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information on microfluidic devices can be found in Beeby [Beeby 2004], Koch [Koch 2000]
and Kovacs [Kovacs 1998].
Semiconductor NEMS devices can offer microwave resonance frequencies, exceptionally high
mechanical quality factors, and extraordinarily small heat capacities. [Ekinci 2005; Ekinci and
Roukes, 2005] Examples of NEMS devices also include transducers, radiating energy devices,
nanoscale integrated circuits, and optoelectronic devices. [Gammel, 2005; Goddard, 2002]
NEMS manufacturing is being further enabled by the drive towards nanometer feature sizes in
the microelectonics industry. Terascale computational ability will require nanotransistors,
nanodiodes, nanoswitces, and nanologic gates. [Lyshevski 2000] NEMS also opens the door for
fundamental science at the nanometer scale investigating phonon mediated mechanical processes
[Tighe 1997] and quantum behavior of mesoscopic mechanical systems [Knobel 2003].
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/nano products existing),
technology (available capability and tools), and manufacturing (maufacturability in volume at
low cost). [Senturia 2001]
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 the 7th International Symposium on MEMS and Nanotechnology [SEM,
2006]).
1.2. 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
W. Crone, “A Brief Introduction to MEMS and NEMS,” in Springer Handbook of Experimental Solid Mechanics,
W.N. Sharpe, Editor, Springer-Verlag, New York. In press.
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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 benchtop techniques with no need for clean room facilities use in
microelectronics fabrication. 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
generally the same. 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 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.
1.2.1. 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
single crystal 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 below for
comparative purposes in Table 3, 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
constitutive behavior of materials used in MEMS/NEMS applications can be sensitive to
fabrication method, processing parameters, and thermal history due to the relative similarity of
characteristic length scales and device dimensions. A good resource compiling characterization
data from a number of sources can be found on the material database of
http://www.memsnet.org/material/ [MEMS and Nanotechnology Clearinghouse]. The following
books, used as references for the discussion below, are valuable resources for more extensive
information: Senturia [Senturia 2001]; Maluf [Maluf 2000], and Beeby [Beeby 2004].
W. Crone, “A Brief Introduction to MEMS and NEMS,” in Springer Handbook of Experimental Solid Mechanics,
W.N. Sharpe, Editor, Springer-Verlag, New York. In press.
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1.2.1.1. 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 the electrical properties. Silicon can be used in a number of different forms:
oriented single crystal silicon, amorphous silicon, or polycrystal silicon (called polysilicon).
Single crystal silicon (cubic crystal structure) has anisotropic behavior which is evident in its
mechanical properties such as Young’s modulus. A high-purity ingot of single crystal silicon is
grown, sawed to desired thickness, and polished to create a wafer. Single crystal silicon used for
MEMS/NEMS are usually the standard 4 inch (100 mm diameter, 525 µm thickness) or 6 inch
(150 mm diameter, 650 µm thickness) wafers. Although larger 8” and 12” 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 (Figure 1). Impurity doping has a significant impact on electrical properties
but does not generally impact the mechanical properties if the concentration is roughly
<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 dopant 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.
Figure 1. Flats on standard commercial silicon wafers used to identify crystallographic
orientation and doping. Adapted from Senturia [Senturia, 2001] TO BE REDRAWN
Amorphous and polysilicon films are usually deposited with thicknesses of <5 µm although it is
also possible to create thick polysilicon. [Lange, 1995] 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 smaller strength and lower
W. Crone, “A Brief Introduction to MEMS and NEMS,” in Springer Handbook of Experimental Solid Mechanics,
W.N. Sharpe, Editor, Springer-Verlag, New York. In press.
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piezoresistivity than single crystal silicon. Additionally, Young’s modulus may vary
significantly because a single grain diameter may comprise a large fraction of a component’s
width. [Madou 1997]
Silicon, polysilicon, and amorphous silicon are also piezoresistive, meaning that the resistivity of
the material changes with applied stress. The fractional change in resistivity, ∆ρ/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 higher quality 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 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).
SOI
Silicon on insulator (SOI) wafers are also used for MEMS sensors and actuators. [Diem, 1993]
Different SOI materials are distinguished by their properties. Buried oxide layers can be
produced either through ion implantation or wafer bonding processes which are discussed below.
[Noworolski, 1995]
Table 3. Properties of selected materials. Adapted from Maluf [Maluf 2000] and Beeby [Beeby
2004].
Property
Si
SiO2
Si3N4
Quartz
SiC
Si(111)
Stainless
Steel
Al
Young’s modulus [GPa]
160
73
323
107
450
190
200
70
Yield strength [GPa]
7
8.4
14
9
21
7
3
0.17
Poisson’s ratio
0.22
0.17
0.25
0.16
0.14
0.22
0.3
0.33
Density [g/cm3]
2.4
2.3
3.1
2.65
3.2
2.3
8
2.7
W. Crone, “A Brief Introduction to MEMS and NEMS,” in Springer Handbook of Experimental Solid Mechanics,
W.N. Sharpe, Editor, Springer-Verlag, New York. In press.
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Coefficient of thermal
expansion
[10-6/°C]
2.6
0.55
2.8
0.55
4.2
2.3
16
24
Thermal conductivity at
300K [W/cm · K]
1.57
0.014
0.19
0.0138
5
1.48
0.2
2.37
Melting temperature [°C]
1415
1700
1800
1610
2830
1414
1500
660
1.2.1.2. Other Hard 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.
Quartz
Single crystal quartz (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 insensitivities of their piezoelectric or
mechanical properties. Quartz is also piezoelectric, meaning that there is a relationship between
strain and voltage in the material. Detailed information about quartz cuts can be found in Ikeda
[Ikeda, 1990]. Fused quartz (silica) is a glassy non-crystalline 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
Diaomond 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. [Gluche,
2004] 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,
ultrananocrystalline diamond films created by pulsed laser deposition or chemical vapor
deposition. [Cho, 2005; Krauss, 2001; Gruen, 1999; Gruen 1994 a,b]
1.2.1.3. Metals
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, IR light reflection and electrical conduction.
Chromium is often used as an adhesion layer. Alloys of Ni as in the case of NiTi and
Permalloy™ can be used for actuation (discussed in more detail below).
W. Crone, “A Brief Introduction to MEMS and NEMS,” in Springer Handbook of Experimental Solid Mechanics,
W.N. Sharpe, Editor, Springer-Verlag, New York. In press.
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1.2.1.4. Polymeric Materials
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 such as visible light,
ultraviolet light, x-rays or electrons. Exposure is usually conducted through a mask so that a
pattern can be 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.
PDMS
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 the most common silicone rubber and is used extensively because of its processability, low
curing temperature, stability, tunable modulus, and range functional groups that can be attached.
[Clarson, 1993]
1.2.1.5. 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-induced 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 high
temperature austenite phase and the low temperature martensite phase (shape memory effect), or
loading and unloading the material to favor either the high strain 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 transformation
occurs in a temperature window, which can be adjusted from –100oC to +160oC by changing the
alloy composition and heat treatment processing. [Otsuka 1998]
Permalloy™
Permalloy™, NixFey, displays magneto resistance properties and is used for magnetic
transducing. Multilayered nanostructures of this alloy give rise to a giant magneto resistance
(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.
W. Crone, “A Brief Introduction to MEMS and NEMS,” in Springer Handbook of Experimental Solid Mechanics,
W.N. Sharpe, Editor, Springer-Verlag, New York. In press.
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PZT
Lead zirconate titanate (PZT) is ceramic solid solution of lead zirconate (PbZrO3) and lead
titanate (PbTiO3). PZT (along with zinc oxide and ployvinylidenefluoride) 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.
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.
1.2.1.6. 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 hard coatings and protective layers, lower percolation threshold
impacts conductivity, and narrower bandgap with decreasing grain size enables unique optoelectronics. [Fecht, 2004] Hundreds of different synthesis routes have been created for
manufacturing nanostructured materials. (See for example, the proceedings of the International
Conferences on Nanostructured Materials. [Yacamán, 1993]) 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 wall 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 or grown by chemical vapor deposition (CVD) on a substrate using catalyst particles.
[Borisenko, 2004]
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 2D confinement is a quantum film, 1D
confinement is a quantum wire, and 0D confinement is a quantum dot. The dimension of the
W. Crone, “A Brief Introduction to MEMS and NEMS,” in Springer Handbook of Experimental Solid Mechanics,
W.N. Sharpe, Editor, Springer-Verlag, New York. In press.
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confined direction(s) is so small that the energy states are quantized in that direction.
[Borisenko, 2004]
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 [Busbee,
2003], palladium nanowires created by electroplating on a stepped surface [Favier, 2001], and
zinc oxide nanowires created by a vapor/liquid/solid method [Huang, 2001]. In one popular
technique, electroplating is conducted inside of a nanoporous template of alumina or
polycarbonate to direct the growth of nanowires.[Penner, 1987; Hulteen, 1997] 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. [Chakarvarti, 2006] Using various combinations of metal catalysts and
gases a wide range of nanowire compositions can be created from chemical vapor deposition
methods (Table 4).
1.2.2. Micromachining
Micromachining is a set of material removal and forming techniques for creating microscale
movable features and complex structures, often from silicon. Micromachining can be applied to
other materials such as glasses, ceramics, 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 the ease of
integration into a circuit for control and signal processing. [Beeby 2004] 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. [Madou 1997] [DeVries, 1992]
[Kalpajian, 1984]
A brief overview of micromachining is provided below. The following books, used as references
for the discussion below, are valuable resources for more extensive information: Bhushan
[Bhushan, 2004], Senturia [Senturia 2001]; Maluf [Maluf 2000], Kovacs [Kovacs, 1998], and
Madou [Madou, 1997]. Additional information can be found in Taniguchi [Taniguchi, 1983] and
Evans [Evans, 1989] on microfabrication technology, Bustillo [Bustillo, 1998] on surface
micromachining, and Gentili [Gentili 1996] on nanolithography.
Table 4. Micromachining processes and their applications. Adapted from Kovacs [Kovacs,
1998].
Process
Lithography
Thin Film Deposition
Example Applications
photolithography, screen printing, electron-beam lithography, x-ray
lithography
chemical vapor deposition (CVD), plasma-enhanced chemical vapor
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Electroplating
Directed Deposition
Etching
Machining
Bonding
Surface Modification
Annealing
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 reactiveion 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
1.2.2.1. Hard Fabrication Techniques
Hard MEMS utilizes enabling technologies for fabrication and design from the microelectronics
industry. The MEMS industry has modified advanced techniques, leveraging the 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).
1.2.2.1.1. 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 of a target with a plasma of an inert gas such as argon.
Sputtering is less directional and allows for higher deposition rates.
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
(SiH2Cl2) with ammonia (NH3)), and silicon oxide (formed by silane with an oxidizing species).
LPCVD can produce amorphous inorganic dielectric films and polycrystalline polysilicon and
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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 plasmaenhanced 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.
1.2.2.1.2.
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 performed 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 [Gentili,
1996] Optical lithography allows aspect ratios of up to 3 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. 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 must first be produced that carries either a positive or negative image of the
features to be created. 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, either the light weakens the
polymer or strengthens the polymer. In the developer step, chemicals are used to remove the
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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 less resolution because a gap is left between the mask and the
substrate to minimize damage to the mask. A factor of 5 to 10 reduction is common for
projection step-and-repeat lithography. Because this technique allows for 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.
1.2.2.1.3.
Etching
A number of wet and dry etchants have been developed for silicon. Important properties include
orientation dependence, selectivity, and geometryic details of the etched feature (see Figure 2).
A common isotropic wet etchant for silicon is HNA (a combination of HF, HNO3, and
CH3COOH), while anisotropic wet etchants include KOH which etches {100} planes 100 times
faster than {111} planes, tetramethyl ammonium hydroxide (called TMAH or (CH3)4NOH)
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.
Figure 2. Trench profiles produced by different etching processes. From Maluf [Maluf, 2000]
TO BE REDRAWN
Wet etchants such as HF for silicon dioxide, H3PO4 for silicon nitride, Kl for gold, and acetone
for organic layers, can be performed in batch processes with little cost. [Williams 1996] An
important feature of an etchant is its selectivity. For example etch rate of an oxide by HF is 100
nm/min compared to 0.04 nm/min for silicon nitride. [Williams, 1996] 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
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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 diflouride
(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 aluminum, and O2 for
organic layers, are used as a plasma. [Williams, 1996] 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 of 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 aspect ratio 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 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. [Li, 2004] In addition to FIB, techniques such as SPM lithography and
molecular beam epitaxy can also be used to create micro and nanoscale gratings. [Xie, 2005]
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.
1.2.2.1.4.
Bulk Micromachining vs. Surface Micromachining
The processes for silicon micromachining fall into two general categories: bulk (subtraction of
substrate material) or 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, LIGA, injection molding, electric discharge machining (EDM), and focused ion
beam (FIB). Figure 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 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.
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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 5 structural
polysilicon and 5 sacrificial silicon dioxide layers, whereas Texas Instrument’s digital
micromirror device (discussed below) is made from a stack of structural metallic layers and
sacrifical polymer layers. [Sandia 2006; Maluf 2000] 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 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.
LIGA
LIGA (Lithography-Galvanoforming-Molding, or in German, Lithografie-Galvanik-Abformung)
is a lithography and electroplating method used to create very high aspect ratio structures (more
than 100 aspect ratio is 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.
[Goddard, 2002] 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 [Guckel,1998] Becker [Becker, 1986] and Bley [Bley, 1991] for additional details.
Figure 3. Schematic diagrams depicting the processing steps required for bulk micromachining,
surface micromachining, and LIGA. All views are shown from the side. Adapted from Bhushan
[Bhushan, 2004]. TO BE REDRAWN
1.2.2.1.5.
Wafer Bonding
Although microelectronics fabrication processes allow stacking layers of films, structures are
relatively two-dimensional. Wafer bonding provides an opportunity for a more threedimensional structure and is commonly used to make pressure sensors, accelerometers, and
microfluidic devices (Figure 4). Anodic and direct bonding are the most common techniques,
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but 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+ mobile. The positively charges silicon holds 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.
Figure X. Schematic diagram depicting a wafer bonding process used to create a microfluidic
channel for flow cytometry. Adapted from Kovacs (1998). TO BE REDRAWN
1.2.2.2. Soft Fabrication Techniques
1.2.2.3. Self Assembly
Partly because of the high cost of nanolithography and the time consuming nature of atom-byatom placement using probe microscopy techniques, self assembly is an important bottom up
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approach to NEMS fabrication. [Whitesides 1991] 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 the nanoscale and create highly perfect assemblies. [Wilbur, 1999] The atoms,
molecules, collections of molecules, or nanoparticles self-organize into functioning entities using
thermodynamic forces and kinetic control. [Whitesides, 1995] Such self-organization at the
nanoscale is observed naturally in liquid crystals, colloids, micelles, and self-assembled
monolayers. [Ringsdorf, 1988] Reviews of self assembly can be found in [Whitesides, 1990]
[Dubois, 1992] [Ulman, 1995], and [Bishop, 1996].
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 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 3D
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 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.
[Wilbur, 1999]
Table 5. Techniques for creating patterned SAMs [Wilbur, 1999]
Method
Scale of Features
microcontact printing
100 nm – cm’s
micromachining
100 nm - µm’s
microwriting with pen
~10 – 100 µm
photolithography/lift-off
> 1 µm
photochemical patterning
> 1 µm
photo-oxidation
> 1 µm
focused ion beam writing
~ µm’s
electron beam writing
25 – 100 nm
scanning tunneling microscope writing
15 – 50 nm
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Figure 5. Schematic diagram depicting the processing steps required for microcontact printing.
All views are shown from the side. Adapted from Wilbur [1999]. TO BE REDRAWN
1.2.2.4. Soft Lithography
The term “soft lithography” encompasses a number of techniques that can be used to fabricate
micro- and nanoscale structures using replica molding and self assembly. These techniques
include microcontact printing, replica molding, microtransfer molding, micromolding in
capillaries, and solvent-assisted micromolding. [Xia, 1998]
As an example, microcontact printing uses a self assembled monolayer as ink in a stamping
operation that transfers the SAM to a surface (Figure 5). The stamp is fabricated from of an
elastomeric material like 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
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a stamp to make physical pattern transfer. The advantages of microcontact printing are its
simplicity, conformal contact with a surface, reusability of the stamp, and capability of
producing 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 on curved surfaces. [Wilbur, 1999]
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. [Delamarche, 1997]
1.2.3.
Other NEMS Fabrication Strategies
Nanoscale structures can be created from both top down and bottom and approaches. Because
of the push 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 selforganization, self-assembly, and templating, using building blocks similar and size to those used
in nature. [Preece, 1994] A brief review of some additional examples is provided below.
1.2.3.1. Nanomachining
Scanning probe microscopes are a valuable set of tools for NEMS characterization, but these
tools 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”. [Stroscio and
Eigler, 1991] 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. [Eigler, 1999] 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.
1.2.3.2. Nanolithography
Surface micromachining can be conducted at the nanoscale using electron beam lithography to
create freestanding 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. [Craighead, 2000]
Nanoscale resolution can also be obtained using alternative lithographic techniques such as dip
pen nanolithography (DPN). [Piner, 1999] 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
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pattern can be drawn on a surface using a wide range of inks such as thiols, silanes, metals, solgel 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 [Bullen, 2004]. This and other nanofabrication techniques using AFM to modify
and pattern surfaces are reviewed by Tang [2004].
1.2.4.
Packaging
Packaging of a MEMS/NEMS device provides a protective housing to prevent mechanical
damage, minimize stresses and vibrations, guard against contamination, protect from harsh
environmental conditions, dissipate heat, and shield from electromagnetic interference.
[Lyshevski 2000] 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 in addition to electrical and/or fluid interconnects
and optically transparent windows. In these cases, the device is 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. [Maluf 2000]
Packaging design must be conducted in parallel with design of the MEMS/NEMS component.
Design considerations include thickness of the device, wafer dicing (separation of the wafer into
separate die), 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. [Maluf 2000]
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 selfaligning fluid ports, and laminated layers of plastic. [Maluf 2000]
1.3. 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 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.
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1.3.1. The Influence of Scale
For 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 mass of a MEMS structure
can be about 1 nN and a NEMS components are 10-20 N. Compare this to the mass of a drop of
water which is 10 µN or an eyelash which is 100 nN. [Bhushan 2004] This miniscule size of
forces that influence behavior at this scale 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 structures.
In dealing with micro- and nanoscale devices, engineering intuition developed 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 6). Compared to a macroscale counterpart of the
same aspect ratio, structural stiffness of a microscale cantilever increases relative to inertially
imposed loads. When the length scale changes by a factor of one 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. Inertial and
electromagnet forces become negligible. At small scales, adhesion, friction, stiction (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. [Fecht 2004]
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 significatn 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 intertial force compared to the surface
tension. [Kovacs 1998]
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 also have substantial engineering value at small scales.
Translational motion can be achieved in MEMS by electrostatic force since this scales as l2 as
compared to inertial force which scales as l3. 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 with an
electrostatics between a sample and the grip [Sharpe, 1999].
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It is also important to note that as the size scale decreases, breakdown in continuum-based
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. [Bart 1988] 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. [Carr, 1998] 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. [Ekinci, 2005] Detailed discussions of scale can be
found in Madou [1997] and Trimmer [1989].
Table 6. Scaling laws and the relative importance of
phenomena as they depend on linear dimension, l. Adapted
from Madou [1997]
Importance at
Small Scale
Diminished
Increased
Phenomena
Flow
Gravity
Inertial force
Magnetic Force
Thermal Emission
Electrostatic Force
Friction
Pressure
Piezoelectricity
Shape Memory Effect
Velocity
Surface Tension
Diffusion
van der Waal’s Force
Power of
Linear
Dimension
l4
l3
l3
l2, l3 or l4
l2 or l4
l2
l2
l2
l2
l2
l
l
l1/2
l1/4
1.3.2. 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 one micron which is also the natural
length scale for microstructure (such as grain size, dislocation length, precipitate spacing) in
most materials. [Srikar, 2003] Because of this, many of the mechanical properties of interest are
size dependent. Knowledge of material properties is essential for predicting device reliability
W. Crone, “A Brief Introduction to MEMS and NEMS,” in Springer Handbook of Experimental Solid Mechanics,
W.N. Sharpe, Editor, Springer-Verlag, New York. In press.
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and performance. A detailed discussion about micro- and nanoscale testing can be found in Part
I of this handbook as well as in references such as Sharpe [2002], Srikar [2003], Haque and Saif
[2003], and Yi [1999]. The sections below 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
[Trimmer, 1997], Madou [Madou, 1997], and Gorecki [Gorecki, 1999].
1.3.2.1. 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 surface-to-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.
[Madou 1997] 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 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 and stress-modulated growth
and arrangement of quantum dots.
There are numerous techniques for measuring stresses in thin films. Fundamental techniques
rely on the fact that a stressed 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 doubly supported beam can be estimated from:
σ CR = E
π 2t 2
KL2
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. [Kovacs, 1998] 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
W. Crone, “A Brief Introduction to MEMS and NEMS,” in Springer Handbook of Experimental Solid Mechanics,
W.N. Sharpe, Editor, Springer-Verlag, New York. In press.
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length of the beam can be measured by optical methods and used to estimate the internal bending
moment, M, from the expression:
δ (x )
x
=K+
(1 −ν ) Mx
2
2 EI
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. [Young, 2002]
A number of techniques have been developed for determining residual stresses including an
ASTM standard involving optical interferometry [ASTM, 2005]. The bulge test is a basic
technique for measuring residual stress in a free standing thin film. [Allen, 1987] 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.
[Osterberg, 1997] 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 non-integrated film structures, and can be used even when the film thickness is
much smaller than the substrate thickness. [Nix, 1989] 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. [Petersen, 1978; Zhang, 1991]
Air damping can significant impact theses measurements however so they must be conducted in
vacuum. [Senturia, 2001] Other established techniques that can be employed to measure
residual stresses in films include passive strain sensors, Raman spectroscopy, and
nanoindentation. [Srikar, 2003]
More recently, nanoscale gratings created by focused ion beam (FIB) milling have been used to
produce a moiré interference between the grating on the specimen surface and raster scan lines of
a scanning electron microscope (SEM) image. [Li, 2002] This technique can be used to provide
details of residual strains as they evolve in microstructures with etching of the underlying
sacrificial layer. [Li, 2004] 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. [Vogel 2005]
1.3.2.2. 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. [Ayon, 2003] 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. [Maluf 2000]
W. Crone, “A Brief Introduction to MEMS and NEMS,” in Springer Handbook of Experimental Solid Mechanics,
W.N. Sharpe, Editor, Springer-Verlag, New York. In press.
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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. [Charalambides, 1990]
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 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. [Turner, 2004]
1.3.2.3. 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: scotch tape test, abrasion, scratching, deceleration using
ultrasonic and ultracenterfuge techniques, bending, and pulling. [Campbell 1970] 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 also 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 be a
problem in the fabrication process. Commonly with free standing 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.
[Mastrangelo, 1993] To avoid this problem, supercritical drying is used.
Contacting surfaces that must move relative to one another in MEMS/NEMS are minimized or
eliminated all together; 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. [de Boer,
2001a] 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. [Martin, 1997]
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 rpoughness can be reduced by
an order of magnitude from the as deposited state. [Bhushan, 2004] 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 those developed at Sandia
W. Crone, “A Brief Introduction to MEMS and NEMS,” in Springer Handbook of Experimental Solid Mechanics,
W.N. Sharpe, Editor, Springer-Verlag, New York. In press.
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National Laboratory are aiding the development of detailed mechanics models addressing
friction. [Carpick, 1997; Bhushan, 2002; de Boer, 1999; de Boer, 2001b]
1.3.2.4. Flow Visualization
Flow in the microscale domain occurs in a range of MEMS devices, particularly in bioMEMS,
microchannel networks, inkjet printer heads, and micropropulsion systems. The different
balance of forces at microscopic length scales can influence fluid flow to produce counterintuitive behavior in microscopic flows. Additionally, the breakdown in continuum laws for
fluid flow begin 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
is adapted to measure flow fields in microfluidic devices, where micron-scale spatial resolution
is critical. [Santiago, 1998] Micro-particle image velocimetry (µPIV) has been used to
characterize microchannel flow [Meinhart, 1999] and microfabricated inkjet printer head flow
[Meinhart, 2000]. For the high velocity, small length scale flows found in microfluidics, highspeed 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 with tens of
nanometers. [Wereley, 2001]
1.3.3. Mechanics Issues in MEMS/NEMS Devices
There is a wide range of MEMS/NEMS devices 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.
1.3.3.1. Digital Micromirror Device
Optical MEMS devices range from bar-code readers to fiberoptic telecommunication, and use a
range of wide band-gap materials, nonlinear electro-optic polymers, and ceramics. [Enikov,
2002] (See Walker and Nagel [1999] 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. [Hornbeck, 1986] These
devices have superior resolution, brightness, contrast, and convergence performance compared to
conventional cathode ray tube technology. [Bhushan 2004] 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 µsec. [Van Kessel, 1998] This device
W. Crone, “A Brief Introduction to MEMS and NEMS,” in Springer Handbook of Experimental Solid Mechanics,
W.N. Sharpe, Editor, Springer-Verlag, New York. In press.
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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. [Hornbeck 1997]
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. [Henck,
1997]
Other challenges for the DMD include creep and fatigue behavior in the hinge, shock and
vibration, and sensitivity to the debris within the package. [Bhushan, 2004] The primary failure
mechanisms are surface contamination and hinge memory due to creep in the metallic alloy
resulting in a residual tilt angle. [Maluf, 2000] 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 flatness of the mirror,
damage to the reflective layer, and alterations in the dynamic behavior of the system. [Hehr,
1999]
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. [Meier, 1998]
For initial design calculations however, some closed-form solutions for mechanics analysis are
available which 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 semi-axis lengths) using:
2Gα a 2 b
, for a > b
τ max = 2
a + b2
where G is the shear modulus and α in the angular twist. [Enikov, 2002] Mechanical integrity
of the DMD relies on low stresses in the hinge, thus the tilt angle is limited to ±10o. [Maluf,
2000]
W. Crone, “A Brief Introduction to MEMS and NEMS,” in Springer Handbook of Experimental Solid Mechanics,
W.N. Sharpe, Editor, Springer-Verlag, New York. In press.
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Figure. SEM image of yoke and hinges of one pixel with mirror removed (top). Schematic of
two tilted pixels with mirrors shown as transparent (bottom). From Hornbeck [1997].
W. Crone, “A Brief Introduction to MEMS and NEMS,” in Springer Handbook of Experimental Solid Mechanics,
W.N. Sharpe, Editor, Springer-Verlag, New York. In press.
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1.3.3.2.Biomolecular Recognition Device
Biological molecules can be probed by external methods using techniques such as optical
tweezers [Kuo, 1993], atomic force microscopy [Florin 1994; Lee 1994], and magnetic beads
[Smith 1992], 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.
[Fritz, 2000; Thundat, 1995]
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 recently 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). [Sepaniac, 2002] 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:
∆m ≈ 2
M eff
ωo
∆ω
where Meff is the effective vibratory mass of the resonator, and ω0 is the resonance frequency of
the device. [Ekinci, 2005] 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 like that shown in Figure 7, a liquid medium is injected into the device which contain
molecules that dock to a layer of receptor molecules attached to one side of the cantilever.
Sensitizing an array of cantilevers with different receptor allows docking of different substances
in the same solution. [Fritz, 2000] Hybridization can be done with short strands of singlestranded DNA and proteins known 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 laser beam reflected off of the end of the
cantilever. [Fritz 2000] Alternatively, simple geometric interference by interdigitated cantilevers
that act as diffraction gratings can be used to provide output of a binding event. [Thundat, 1999]
W. Crone, “A Brief Introduction to MEMS and NEMS,” in Springer Handbook of Experimental Solid Mechanics,
W.N. Sharpe, Editor, Springer-Verlag, New York. In press.
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Figure 7. SEM image of a portion of the cantilever sensor array. Schematic illustrating
functionalized cantilevers with selective sensing capability. From [Fritz, 2000]
W. Crone, “A Brief Introduction to MEMS and NEMS,” in Springer Handbook of Experimental Solid Mechanics,
W.N. Sharpe, Editor, Springer-Verlag, New York. In press.
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1.3.3.3. Thermomechanical Data Storage Device
Much of the drive to nanometer scale devices originates in the desire for higher density, faster
computational devices. Magnetic data storage has been pushedinto the nanoscale regime, but
limitations have prompted the development of alternative methods for data storage such as a
NEMS device known as the Millipede has been developed by IBM. The Millipede 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 the polymethylmethacrylate (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 bit size of 30-40 nm. [Vettiger, 2000] The
device uses multiple cantilever probe tips equipped with integrated heaters which allow for data
transfer rates into the range of up to a few Mb/sec. [Vettiger, 2002]
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 (Figure 8). [Despont, 2004] As illustrated in
Figure 9, the probes writes a bit by heating and mechanical force applied between a cantilever 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. [Binnig, 1999] 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.
[Vettiger, 2000]
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 stresscontrol 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. [Vettiger, 2000]
Fabrication details are given in Despont [Despont, 2004].
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 by 3 mm silicon area requires that temperature of the device be controlled to
1oC using several sensors and heater elements. [Vettiger, 2000] Tip wear due to contact between
the tip and the underlying silicon substrate is a issue for device reliability. Additionally, the
PMMA is prone to charring at the temperatures necessary for device operation (around 350 °C).
[Holland, 2001] The feasibility of using thin film Ni-Ti shape memory alloy (SMA) for
thermomechanical data storage as an alternative to the polymer thin film has also been shown.
[Shaw, 2005]
W. Crone, “A Brief Introduction to MEMS and NEMS,” in Springer Handbook of Experimental Solid Mechanics,
W.N. Sharpe, Editor, Springer-Verlag, New York. In press.
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W. Crone, “A Brief Introduction to MEMS and NEMS,” in Springer Handbook of Experimental Solid Mechanics,
W.N. Sharpe, Editor, Springer-Verlag, New York. In press.
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Figure 8. Schematic illustration of the millipede device (left), with a detail of one cantilever cell
(right). From Despont [1999].
-----------------------------------------------------------------------------------------------
-----------------------------------------------------------------------------------------------
W. Crone, “A Brief Introduction to MEMS and NEMS,” in Springer Handbook of Experimental Solid Mechanics,
W.N. Sharpe, Editor, Springer-Verlag, New York. In press.
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Figure 9. Schematic of the writing, erasing and reading operations in the Millipede device. Data
is mechanically stored in pits on a surface. From Vettiger [2003].
1.4. Conclusion
The sensors, actuators and passive structures developed as MEMS and NEMS devices require a
highly interdisciplinary approach to their analysis, design, development and fabrication.
Experimental mechanics plays a critical role in design development, materials selection,
prediction of allowable operating limits, device characterization, process validation, and conduct
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 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.
W. Crone, “A Brief Introduction to MEMS and NEMS,” in Springer Handbook of Experimental Solid Mechanics,
W.N. Sharpe, Editor, Springer-Verlag, New York. In press.
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