Microelectromechanical Systems
(MEMS/NEMS) MEMS: A
Technology from Lilliput
An introduction
"Innovation distinguishes between a
leader and a follower." --Steve Jobs
https://www.youtu
be.com/watch?v=
0EokkhdppgE
What are MEMS?
(Micro-electromechanical Systems)
• Fabricated using micromachining technology
• Used for sensing, actuation or are passive
micro-structures
• Usually integrated with electronic circuitry
for control and/or information processing
Introduction, Continued
Image Courtesy of Sandia National Laboratories, SUMMiTTM Technologies,
www.mems.sandia.gov
Figure 5.3: Spider mite with legs on a mirror drive assembly.
Brief History
1962
1967
1967
1972
1979
1982
1982
1983
1983
1983
1985
1987
1993
1994
1999
Silicon Integrated piezo actuators BY O.N. Tufte et al.
Anisotropic deep silicon etching H.A. Waggener
The resonant gate transistor by H. Nathanson, et.al
National Semiconductor - Pressure Sensor
Thermal inkjet technology is invented at HP laboratories
“Silicon as a Mechanical Material” K. Peterson
Liga Process (KFIK, Germany)
“Infinitesimal Machinery” R. Feynman
Silicon Micromechanical devices – J.B.Angel etc.
Integrated Pressure Sensor – Honeywell
Airbag Crash Sensor
Dr. Hornbeck Digital Micromirror Device or DMD (DLP by Texas Instruments)
Later in 1990s micromachining begins leveraging microelectronics industry
Accelerometer integrated with electronics Analog devices
DRIE Etching (Bosch process is patented)
Optical network switch - Lucent
3-D Micromachined Structures
Linear Rack Gear Reduction Drive
Triple-Piston Microsteam Engine
Photos from Sandia National Lab. Website: http://mems.sandia.gov
Applications: Passive Structures
Inkjet Printer Nozzle
https://www.youtube.com/watch?v=9yeZSaig
Bj4&feature=youtu.be
https://www.youtube.com/watch?v=
qPwlgL91-Q4
On Chip Inductor
A surface-micromachined
parallel-plate variable
capacitor
Different
implementations of a
surface-micromachined
parallel-plate variable
capacitor:
(a) perspective view of
the basic concept
showing a stationary
plate and a moveable
plate suspended by
springs;
(b) top view of a
capacitor using straight
beams as springs;
(c) top view using Tshaped springs [8];
(d) top view using Lshaped springs [5]; and
(e) top view with
center anchor [7].
Applications: Sensors
Pressure sensor:
• Piezoresistive sensing
• Capacitive sensing
• Resonant sensing
Application examples:
• Manifold absolute pressure (MAP) sensor
• Disposable blood pressure sensor (Novasensor)
Piezoresistive Pressure Sensors
Pressure
Piezoresistive elements
SiO2
p+ Si
<100> Si
substrate
Piezoresistive Pressure Sensors
Wheatstone Bridge configuration
Illustration from “An Introduction to MEMS Engineering”, N. Maluf
Applications: Sensors
Inertial sensors
•
Acceleration
– Air bag crash sensing
– Seat belt tension
– Automobile suspension control
– Human activity for pacemaker control
•
Vibration
– Engine management
– Security devices
– Monitoring of seismic activity
•
Angle of inclination
– Vehicle stability and roll
Accelerometers
Accelerometer parameters
• acceleration range (G) (1G=9.81 m/s2)
• sensitivity (V/G)
• resolution (G)
• bandwidth (Hz)
• cross axis sensitivity
Application
Range
Bandwidth
Air Bag Deployment
± 50 G
~ 1 kHz
Engine vibration
±1G
> 10 kHz
Cardiac Pacemaker control
±2G
< 50 Hz
Comment
resolve small accelerations (< 1 micro G)
multiaxis, ultra-low power consumption
https://www.youtube.com/watch?v=eqZgxR6eRjo&t=509s
https://www.youtube.com/watch?v=4kfzqZpttTA
https://www.youtube.com/watch?v=XsjvaYAFN1M
Capacitive Accelerometers
Anchor to substrate
Spring
Displacement
Inertial Mass
Stationary Polysilicon fingers
Based on ADXL accelerometers, Analog Devices, Inc.
Applications: Actuators
Texas Instruments Digital Micromirror DeviceTM
• Invented by Texas Instruments in 1986
• Array of up to 1.3 million mirrors
• Each mirror is 16 mm on a side with a pitch of 17 mm
• Resolutions: 800x600 pixels (SVGA) and 1280x1024 pixels (SXGA)
For an animated demo of this device, go to http://www.dlp.com/dlp_technology/
Digital Micromirror Device
From “An Introduction to Microelectromechanical Systems Engineering” by Nadim Maluf
Digital Micromirror Device
• Mirror is moved by electrostatic actuation (24 V applied to bias electrode)
• Projection system consists of the DMD, electronics, light source and projection optics
• Switching time: 16 µs (about 1000 times faster than the response time of the eye)
=> Acheive grey scale by adjusting the duration of pulse
• Placing a filter wheel with the primary colors between light source and the micromirrors
=> Achieve full color by timing the reflected light to pass the wheel at the right color
From “An Introduction to Microelectromechanical Systems Engineering” by Nadim Maluf
Some future applications
• Biological applications:
– Microfluidics
– Lab-on-a-Chip
– Micropumps
– Resonant microbalances
– Micro Total Analysis systems
• Mobile communications:
– Micromechanical resonator for resonant circuits and filters
• Optical communications:
– Optical switching
Microfluidics / DNA Analysis
In the future, a complete DNA sequencing systems should include:
•Amplification (PCR)
•Detection (electrophoresis)
•Fluid preparation and handling (pumps, valves, filters, mixing and rinsing)
MEMS !
MEMS Fabrication
“You can’t see it, but it’s everywhere
you go.”
—Bridget Booher, journalist, on
silicon
Materials for MEMS
• If we view micromachining technology as a set of generic tools,
then there is no reason to limit its use to one material. Indeed,
micromachining has been demonstrated using silicon, glass,
ceramics, polymers, and compound semiconductors made of
group III and V elements, as well as a variety of metals
including titanium and tungsten.
• Silicon, however, remains the material of choice for
microelectromechanical
systems.
Unquestionably,
this
popularity arises from the large momentum of the electronic
integrated circuit industry and the derived economic benefits,
not least of which is the extensive industrial infrastructure.
Silicon-Compatible Material
System
• The silicon-compatible material system
encompasses, in addition to silicon itself, a
host of materials commonly used in the
semiconductor integrated circuit industry.
• Normally deposited as thin films, they
include silicon oxides, silicon nitrides, and
silicon carbides, metals such as aluminum,
titanium, tungsten, and copper, and
polymers such as photoresist and polyimide.
Why Silicon
• Silicon is one of very few materials that is economically manufactured in
single crystal substrates. This crystalline nature provides significant
electrical and mechanical advantages.
• The precise modulation of silicon’s electrical conductivity using
impurity doping lies at the very core of the operation of electronic
semiconductor devices.
• Mechanically, silicon is an elastic and robust material whose
characteristics have been very well studied and documented.
• The tremendous wealth of information accumulated on silicon and its
compounds over the last few decades has made it possible to innovate
and explore new areas of application.
• It becomes evident that silicon is a suitable material platform on which
electronic, mechanical, thermal, optical, and even fluid-flow functions
can be integrated.
• Ultrapure, electronic-grade silicon wafers available for the integrated
circuit industry are common today in MEMS.
• The relatively low cost of these substrates (approximately $10 for a 100mm-diameter wafer and $15 for a 150-mm wafer) makes them attractive
for the fabrication of micromechanical components and systems.
Forms of silicon
• Silicon as an element exists with three different microstructures:
• crystalline, polycrystalline, amorphous.
• Polycrystalline, or simply “polysilicon,” and amorphous silicon are
usually deposited as thin films with typical thicknesses below 5 µm.
• Crystalline silicon substrates are commercially available as circular
wafers with 100-mm (4-in) and 150-mm (6-in) diameters. Largerdiameter (200-mm and
• 300-mm) wafers, used by the integrated circuit industry, are currently
economically unjustified for MEMS. Standard 100-mm wafers are
nominally 525 µm thick, and 150-mm wafers are typically 650 µm
thick.
• Double-side-polished wafers commonly used for micromachining on
both sides of the wafer are approximately 100 µm thin-ner than
standard thickness substrates.
• Polysilicon is an important material in the integrated
circuit industry and has been extensively studied.
• Polysilicon is an equally important and attractive material
for MEMS.
• It has been successfully used to make micromechanical
structures and to integrate electrical interconnects,
thermocouples, p-n junction diodes, and many other
electrical devices with micromechanical structures.
• The most notable example is the acceleration sensor
available from Analog Devices, Inc., of Norwood,
Massachusetts, for automotive airbag safety systems.
• The mechanical properties of polycrystalline and amorphous silicon vary with
deposition conditions, but, by and large, they are similar to that of single crystal
silicon. Both normally have relatively high levels of intrinsic stress (hundreds of
MPa) after deposition, which requires annealing at elevated temperatures (>900ºC).
• Beam structures made of polycrystalline or amorphous silicon that have not been
subjected to a careful stress annealing step can curl under the effect of intrinsic
stress.
• Silicon is a very good thermal conductor with a thermal conductivity greater than
that of many metals and approximately 100 times larger than that of glass. In
complex integrated systems, the silicon substrate can be used as an efficient heat
sink.
• Unfortunately, silicon is not an active optical material—silicon-based lasers do not
exist. Because of the particular interactions between the crystal atoms and the
conduction electrons, silicon is effective only in detecting light; emission of light is
very difficult to achieve.
• The surface of silicon oxidizes immediately upon exposure to the oxygen in air
(referred to as native oxide). The oxide thickness self-limits at a few nanometers at
room temperature. As silicon dioxide is very inert, it acts as a protective layer that
prevents chemical reactions with the underlying silicon.
Silicon Oxide and Nitride
• It is often argued that silicon is such a successful material because it has a stable
oxide that is electrically insulating—unlike germanium, whose oxide is soluble in
water, or gallium arsenide, whose oxide cannot be grown appreciably.
• Various forms of silicon oxides (SiO2, SiOx, silicate glass) are widely used in
micromachining due to their excellent electrical and thermal insulating properties.
• They are also used as sacrificial layers in surface micromachining processes
because they can be preferentially etched in hydrofluoric acid (HF) with high
selectivity to silicon.
• Silicon dioxide (SiO2) is thermally grown by oxidizing silicon at temperatures
above 800°C, whereas the other forms of oxides and glass are deposited by
chemical vapor deposition, sputtering, or even spin-on (the various deposition
methods will be described in the next chapter).
• Silicon oxides and glass layers are known to soften and flow when
subjected to temperatures above 700°C. A drawback of silicon
oxides is their relatively large intrinsic stresses, which are difficult
to control. This has limited their use as materials for large
suspended beams or membranes.
• Silicon nitride (SixNy) is also a widely used insulating thin film
and is effective as a barrier against mobile ion diffusion—in
particular, sodium and potassium ions found in biological
environments. Its Young’s modulus is higher than that of silicon
and its intrinsic stress can be controlled by the specifics of the
deposition process.
• Silicon nitride is an effective masking material in many alkaline
etch solutions.
Thin Metal Film
• The choice of a thin metal film depends greatly on the nature of the final
application.
• Thin metal films are normally deposited either by sputtering, evaporation,
or chemical vapor deposition; gold, nickel, and Permalloy™ (NixFey),
and a few other metals can also be electroplated.
• For basic electrical interconnections, aluminum (usually with a few
percent silicon and perhaps copper) is most common and is relatively easy
to deposit by sputtering, but its operation is limited to noncorrosive
environments and to temperatures below 300ºC.
• For higher temperatures and harsher environments, gold, titanium, and
tungsten are substitutes.
• Aluminum tends to anneal over time and with temperature, causing
changes in its intrinsic stresses. As a result, it is typically located away
from stress- or strain-sensing elements.
• Aluminum is a good light reflector in the visible, and gold excels in the
infrared. Platinum and palladium are two very stable materials for
electrochemistry, though their fabrication entails some added
complexity.
• Gold, platinum, and iridium are good choices for microelectrodes, used
in electrochemistry and in sensing biopotentials. Silver is also useful in
electrochemistry.
• Chromium, titanium, and titanium-tungsten are frequently used as very
thin (5–20 nm) adhesion layers for metals that have poor adhesion to
silicon, silicon dioxide, and silicon nitride.
• Metal bilayers consisting of an adhesion layer (e.g., chromium) and an
intermediate nickel or platinum layer are normally used to solder with
silver-tin or tin-lead alloys. For applications requiring transparent
electrodes, such as liquid crystal displays, indium-tin-oxide (ITO)
meets the requirements.
• Finally, Permalloy™ has been explored as a material for thin magnetic
cores.
Polymers
• Polymers, in the form of polyimides or photoresist,
can be deposited with varying thicknesses from a
few nanometers to hundreds of microns.
• Spin-on organic polymers are generally limited in
their application as a permanent part of MEMS
devices because they shrink substantially as the
solvent evaporates, and because they cannot sustain
temperatures above 200°C.
• Because of their unique absorption and adsorption
properties, polymers have gained acceptance in the
sensing of chemical gases and humidity [7].
Glass and Fused Quartz Substrates
• Micromachining of glass and fused quartz (amorphous silicon
dioxide) substrates is practical in special applications, such as when
an optically transparent or an electrically insulating substrate is
required.
• Crystalline quartz (as opposed to fused quartz) also has the distinct
property of being piezoelectric and is used for some MEMS
devices. However, micromachining of glass or quartz is limited in
scope relative to silicon. Etching in HF or ultrasonic drilling
typically yields coarsely defined features with poor edge control.
Thin metal films can be readily deposited on glass or quartz
substrates and defined using standard lithographic techniques.
• Channels microfabricated in glass substrates with thin metal
microelectrodes have been useful in making capillaries for
miniaturized biochemical analysis systems.
Silicon Carbide and Diamond
• Silicon carbide and diamond continue to captivate the
imagination of many in the micromachining community.
• Both materials offer significant advantages, in particular
hardness, high stiffness (high Young’s modulus), resistance to
harsh chemical environments, mechanical stability at high
temperature, wide bandgap, and very high thermal conductivity.
• An important feature of both silicon carbide and diamond is that
they exhibit piezoresistive properties.
• High temperature pressure sensors in silicon carbide substrates
have been developed with stable operation up to about 500°C.
What Is Micromachining?
• Micromachining is the set of design and fabrication
tools that precisely machine and form structures and
elements at a scale well below the limits of our human
perceptive faculties—the microscale.
• Micromachining is the underlying foundation of
MEMS fabrication; it is the toolbox of MEMS.
Basic microfabrication technologies
• Deposition
– Chemical vapor deposition (CVD/PECVD/LPCVD)
– Epitaxy
– Oxidation
– Evaporation
– Sputtering
– Spin-on methods
• Etching
– Wet chemical etching
• Istropic
• Anisotropic
– Dry etching
• Plasma etch
• Reactive Ion etch (RIE, DRIE)
• Patterning
– Photolithography
– X-ray lithography
• Epitaxy,
sputtering,
evaporation,
chemical-vapor
deposition, and spin-on methods are common techniques
used to deposit uniform layers of semiconductors,
metals, insulators, and polymers.
• Lithography is a photographic process for printing
images onto a layer of photosensitive polymer
(photoresist) that is subsequently used as a protective mask
against etching.
• Wet and dry etching, including deep reactive ion etching,
form the essential process base to selectively remove
material.
Types:
• Isotropic etchants etch uniformly in all directions, resulting in
rounded cross sectional features.
• Anisotropic etchants etch in some directions preferentially
over others, resulting in trenches or cavities delineated by flat
and well defined surfaces, which need not be perpendicular to
the surface of the wafer.
.
The etch medium (wet versus dry) plays a role in selecting a
suitable etch method.
Wet etchants in aqueous solution offer the advantage of lowcost batch fabrication—25 to 50 100-mm-diameter wafers can
be etched simultaneously—and can be either of the isotropic or
anisotropic type.
Dry etching involves the use of reactant gases, usually in a lowpressure plasma, but non plasma gas-phase etching is also used
to a small degree. It can be isotropic or vertical.
The equipment for dry etching is specialized and requires the
plumbing of ultra-clean pipes to bring high purity reactant
gases into the vacuum chamber
Three Techniques of
Micromachining
• Surface Micromachining
• Bulk Micromachining
• LIGA
Bulk micromachining
Anisotropic etch of {100} Si
111
a
0.707a
54.74º
Surface Micromachining
substrate
Important issues:
• selectivity of structural, sacrificial and substrate materials
• stress of structural material
• stiction
Surface Micromachining
Most commonly used materials for surface micromachining:
• substrate:
silicon
• sacrificial material:
SiO2 or phosphosilicate glass (PSG)
• structural material:
polysilicon
Alternative materials
Substrates
Sacrificial
Structural
Glass
Plastic
metals
Polymer
Metals
silicon nitride
Thin film silicon (a-Si:H, c-Si)
silicon nitrides
Silicon carbide
Metals
polymers
bilayer composites
LIGA – X-ray Lithography,
Electroplating (Galvanoformung), Molding (Abformung)
Remove mold
Immerse in chemical bath and
electroplate the metal
Expose and develop photoresist
Deposit photoresist
Deposit plating base
LIGA
Photos from MCNC – MEMS group
MEMS Resources
Reference Books
• Nadim Maluf, An Introduction to Microelectromechanical Engineering (Artech House,
Boston,2000)
• M. Elewenspoek and R. Wiegerink, Mechanical Microsensors (Springer-Verlag, 2001)
• Héctor J. De Los Santos, Introduction to Microelectromechanical (MEM) Microwave
Systems (Artech House, Boston, 1999)
Websites
• Sandia National Lab: http://mems.sandia.gov
• Berkeley Sensors and Actuators Center: http://www-bsac.eecs.berkeley.edu
• MEMS Clearinghouse: http://www.memsnet.org/
Some companies with MEMS products
• Accelerometers – Analog Devices:
http://www.analog.com/technology/mems/index.html
• Digital Light Processing Projector- Texas Instruments: http://www.dlp.com
• Micro-electrophoresis chip – Caliper Technologies: http://www.calipertech.com
NEMS and Nanotechnology
• Nanotechnology
– manipulation of matter
at the nanometer scale.
• Nanomaterials
– Started with carbon.
– Behavior depends on
morphology.
Figure: Eight allotropes of carbon:
Diamond, graphite, lonsdaleite, C60, C540, C70,
amorphous carbon and carbon nanotube
Micro and Nanotechnologies for Sensors
Quantum dots are just as
awesome as we’d hoped
•
•
A new way to measure the efficiency of quantum dots could get them into
things like solar cells and electronics faster.
BY TAYLOR KUBOTA-STANFORD APR 11, 2019
Quantum Dots
• Quantum dots—tiny, easy-to-produce particles—may soon
take the place of more expensive single crystal
semiconductors in advanced electronics found in solar
panels, camera sensors, and medical imaging tools.
• Although quantum dots have begun to break into the
consumer market—in the form of quantum dot TVs—longstanding uncertainties about their quality have hampered their
adoption.
• “Traditional semiconductors are single crystals, grown in
vacuum under special conditions. These we can make in
large numbers, in flask, in a lab and we’ve shown they are as
good as the best single crystals,” says co-lead author David
Hanifi, a graduate student in chemistry at Stanford University.
NEMS and Nanotechnology,
Continued
• Quantum dots
• Nanowires
• Quantum films
https://www.youtu
be.com/watch?v=
0EokkhdppgE
Figure: Quantum Dots.
‘Giant leap’
• The measurement technique could lead to the development of
new technologies and materials that require knowing the
efficiency of our semiconductors to a painstaking degree, says
Paul Alivisatos, professor of nanoscience and nanotechnology
at the University of California, Berkeley.
• “We want to measure emission efficiencies in the realm of 99.9
to 99.999 percent…”
• “These materials are so efficient that existing measurements
were not capable of quantifying just how good they are. This is
a giant leap forward,” says Alivisatos.
• “It may someday enable applications that require materials with
luminescence efficiency well above 99 percent, most of which
haven’t been invented yet.”
Quantum dots from coal are a
promising antioxidant
• Quantum dots could make a great
antioxidant to treat people who've had brain
injuries, heart attacks, or strokes.
Quantum dots from coal are a
promising antioxidant
• Graphene quantum dots drawn from common coal may be
the basis for an effective antioxidant for treating traumatic
brain injuries, strokes, or heart attacks.
• Quantum dots are semiconducting materials small enough
to exhibit quantum mechanical properties that only appear
at the nanoscale.
• The Tour lab first extracted quantum dots from coal in
2013 and reported on their potential for medical imaging,
sensing, electronic, and photovoltaic applications. A
subsequent study showed how they can be engineered for
specific semiconducting properties.