MEMS

Gaetano L’Episcopo
Introduction to MEMS
What are MEMS?
Micro
Electro
Mechanichal
Systems
MEMS are integrated devices, or
systems
of
devices,
with
microscopic parts, such as:
• Mechanical Parts
• Electrical Parts
MEMS devices have typical sizes
from micrometer to centimeter
with individual features of a few
micrometers or less.
2
Acronyms
Micro
Small size, microfabricated structures
Electro
Mechanichal
Systems
Electrical signal / control (IN/OUT)
Mechanical functionality (IN/OUT)
Structures, devices, systems control
In Europe and USA, the acronym MST (Micro-System Technology)
is also used.
3
MEMS: Definitions
MEMS is an engineering discipline that studies the
design and fabrication of micrometer to centimeter
scale mechanical systems.
MEMS devices are in widespread use, and are often
referred to as solid state sensor and actuators, or solid
state transducers.
MEMS fabrication is commonly referred to as
Micromachining.
MEMS design is often referred to as micro-systems
Engineering.
4
MEMS: Why?
An effort to miniaturize sensors and actuators for the
purposes of:
• Reducing size, weight, energy consumption, and
fabrication cost
• Integrating micromachines and microelectronics
on the same chip
• Replacing electronics with mechanical equivalent
• In many cases, obtain better device performance
than macro equivalent
Making small things is new and cool, but not always the
best solution
5
MEMS: Properties
Micro Electro Mechanichal Systems:
Integrated microdevices or systems combining electrical
and mechanical components.
Fabrication using integrated circuits (ICs) compatible
batch-processing
techniques
and
silicon-based
technologies.
Size from micrometers to millimeters.
Sensing, computation and actuation onto a single silicon
die.
Combination of two or more of the following: electrical,
mechanical, optical, chemical, biological, magnetic or
other properties, integrated onto a single or multichip
hybrid
6
MEMS: Dimensions
MEMS allow us to create
artificial systems that are on
the same scale and
functionality as insects.
7
MEMS: scale of objects
8
Sensing – Processing - Actuation
The combination of Sensors and Actuators with Integrated Circuits
completes a loop allowing completely interactive systems.
INPUTS
Eyes
and
ears
PROCESSING
Brains
OUTPUTS
Hands
and
mouth
Sensors
Circuits
Actuators
9
Sensing – Processing - Actuation
The combination of Sensors and Actuators with Integrated Circuits
completes a loop allowing completely interactive systems.
Physical
event
Sensor
Processing
Actuation
Physical
response
Micro-Electro-Mechanical System
10
MEMS: Products
Microwave
and Wireless
Switches
Filters
Components
Power sensors
Pressure
MAP sensors
Microphones
Medical
and Biological
Lab on a Chip
DNA analysis
Chem/Bio
Detection
Drug Delivery
Ink Jet Printers
Thermal Ink-Jets
Inertial
Accelerometers
Gyroscopes
Optics
Projection
Displays
Laser Printers
Switching
Networks
Tunable Lasers
Filters
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MEMS: Industrial applications
Automotive Industry: pressure sensors (engine oil pressure, vacuum
pressure, fuel injection pressure, tire pressure, stored air bag
pressure), accelerometers (triggering of air bag, locking seat belt) and
temperature sensors (to monitor oil, antifreeze and air temperature)
Controls for Industry or Home:
sensors to measure external
environment and actuators for
adjustment
Instrumentation and Control
Industry uses MEMS devices
which
sense
pressure,
temperature,
acceleration
and proximity.
12
MEMS: Manufactures
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Applications: Automotive
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Applications: Medicine
15
MEMS: Biotechnology
Examples of MEMS applications in Biotechnology:
Polymerase Chain Reaction (PCR) microsystems for DNA
amplification and identification;
Enzyme linked immunosorbent assay (ELISA);
Capillary electrophoresis;
Electroporation;
Micromachined Scanning Tunneling Microscopes (STMs);
Biochips for detection of hazardous chemical and biological
agents.
Microsystems for high-throughput drug screening and selection.
16
Applications: Aeronautics
Pressure sensor belt on jet planes
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Applications: Gyroscopes
Micromachined gyroscopes applications.
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Applications: Nintendo Wii
19
Applications: smart-phone
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MEMS: Resources
Books:
• Microsensors – Principles and
applications
J. W. Gardner
1994, Wiley
• Micromechanics and MEMS
Classic and Seminal Papers to 1990
Edited by William S.Trimmer
1997, IEEE PRESS
• Fundamentals of Microfabrication
M. Madou
1997, CRC Press LLC
• Micromachined Transducers Sourcebook
G. T. A. Kovacs
1998, McGraw-Hill
• Micro Mechanical Systems Principles
and Technology Handbook of Sensors
and Actuators, Vol.6
T. Fukuda and W. Menz
1998, ELSEVIER
Scientific Journals:
• IEEE/ASME, Journal of
Microelectromechanical Systems
• Elsevier, Sensors and Actuators
• IoP, Journal of Micromechanics and
Microengineering
• IEEE, Sensors Journal
International Scientific
Conferences:
• IEEE MEMS
• SPIE Smart Structures and Systems –
Smart electronics and MEMS
• Eurosensors
• IEEE Sensors
Websites:
• http://www.memsnet.org
• http://www.memscap.com
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MEMS: Fabrication
MEMS are fabricated using integrated circuits (ICs) compatible
batch-processing techniques and silicon-based technologies.
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MEMS fabrication: Advantages
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MEMS and Microelectronics (IC)
Interactions with the environment:
• Microelectronics generally interacts with "information" such as signals or
streams of electrons;
• MEMS interacts with a wide variety of physical quantities (fluid,
acceleration, optics, electromagnetic waves, etc.).
Structural dimensionality:
• Microelectronic systems are "predominantly" two-dimensional (the
structure is built in a layer thinner than the thickness of the entire
substrate).
• MEMS are inherently three-dimensional (Some properties are developed
for more than 100 micrometers in depth).
Fabrication technology:
• Although Microsystems were originally developed on the same silicon
substrates for microelectronic circuits, today other substrates and
different technologies are adopted.
24
MEMS: Fabrication
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MEMS: Fabrication
MEMS fabrication techniques can be categorized as
follows:
Surface Micromachining (structures made from
single or multiple films that are patterned)
Bulk Micromachining (structures made from
chemically etched bulk material)
Micromolding (structures made using molds,
stereo lithography, milling)
Patterning and shaping, in the above techniques, is
usually accomplishedthrough:
Photolithography
Chemical Etching
26
Silicon as structural material
The monocrystalline silicon structure has a face-centered
cubic (diamondlattice, lattice constant = 5.43 Å).
Each atom is placed at the center of a tetrahedron and is
associated with covalent bond, four equidistant atoms.
To allow easy identification of the
crystallographic planes, "Miller
Indices" (inverse of the intersection
between the plane and the unit
vectors of the reference system)
are used.
The surface of the silicon
wafer is contained in a
three main floors.
Conventions for the
recognition of the type of
substrate are adopted.
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Other materials for MEMS
Polycrystalline Silicon or Polysilicon: used as both a strucutral material
and conductive material, piezo-resistive properties.
Silicon Dioxide (SiO2): used as an electrical isolant and in some cases as a
structural or sacrificial layer.
Silicon Nitride (Si3N4): used as an electrical isolant and in some cases as a
structural or sacrificial layer.
Metals (Alluminium, Platinum, Gold, Nickel, Tungsten, etc…):
used as an electrical conductors, optical reflective, thermo-mechanical
transducer and in some cases as structural materials.
PZT: used for piezo-electrical conversions.
28
Silicon micro-fabrication processes
Classification by type of material
Metal (Al, Au, Cu, Ti, Ni, Pt, etc.)
Non-metal inorganic material (SiO2,
Si3N4, SiC, compound nitride
materials)
Classification by material
addiction/subtraction
Deposition process
Photoresist
Metal evaporation
Oxidation
Sputtering
CVD
Anisotropic etching
Isotropic etching
Bulk/film etching
Sacrifical
Classification by process
temperature
Examples:
Oxidation:950-110 °C
Annealing: 850-110 °C
Si3N4 LPCVD: 780 °C
Polysilicon LPCVD: 580-650 °C
Low temp. oxide LPCVD: about
550 °C
Plasma deposition: about 350 °C
Spin-coating: room temperature
Classification by size:
Thin film (thickness < 10 μm)
Thick film
Bulk
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Deposition processes
Addictive processes to deposit material on a layer:
Spin-on films
• Polyimide (PI), Photoresist (PR)
• Spin-on glass (SOG)
Physical Vapor Deposition (PVD)
• Evaporation
• Sputtering
Chemical Vapor Deposition (CVD)
•
•
•
•
Thermal Oxidation (wet/dry)
CVD (Atmopheric Pressure)
Low Pressure CVD (LPCVD)
Plasma Enhanced CVD (PECVD)
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Deposition processes: Spin-on films
Spin-on films or Spin Casting
Thin film material is dissolved in a volatile liquid solvent, spin coated
onto a substrate to form films due to centrifugal force.
Examples:
• Polyimide (PI), Photoresist (PR)
• Spin-on glass (SOG)
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Deposition processes: PVD
Physical Vapor Deposition (PVD)
• Evaporation
The material to be deposited is heated by
resistive, inductive, or electron beam and led to
gaseous state. Condensation of the evaporated
gas, on high-vacuum chambers, induces the
deposition of the layer.
• Sputtering
High-energy ion beams (plasma) are
used to remove atoms from the surface
of the source material (material to be
deposited) to create a layer.
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Deposition processes: CVD
Chemical Vapor Deposition (CVD)
Deposition on a substrate of a solid layer as a result of a chemical reaction
between the substrate material and the gases in the atmosphere.
• Thermal Oxidation (wet/dry)
• CVD (Atmopheric Pressure)
• Low Pressure CVD (LPCVD)
• Plasma Enhanced CVD (PECVD)
CVD - LPCDV
PECVD
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Photolithography
Technique of transferring a geometric two-dimensional pattern to a surface.
It is divided into six phases: (a) uniform deposition of a layer of structural material
where to tranfer the pattern, (b) deposition of photoresist, (c) selective exposure
to UV light through mask layout, (d) selective removal of the photoresist (e)
selective removal of structural material, (f) final removal of residual photoresist.
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Photolithography: Layout Masks
Layout masks define the geometry / pattern and size of the device.
The type of photoresist (positive/negative) used in
photolithography affects the definition of the layout masks.
the
process
of
Positive Photoresist: becomes soluble in exposed areas
the material is
etched in the areas covered by the mask
transferred pattern is the same
of the mask.
Negative Photoresist: become insoluble in the exposed areas
material is
excavated in the areas uncovered by the mask
transferred pattern is the
negative of the mask.
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Substraction processes
Etching process: process of selectively removing material
The selectivity is determined by using appropriate photolithographic masks
together with masking materials. The selectivity with respect to the materials is
characterized by the etch-rate (rate etching, for a given chemical etchant, is
characteristic of the material).
Classification of etching processes
By depth:
Surface etching (removal of thin films from the surface of the wafer)
Bulk etching (removal of part of the substrate)
By surface:
Top (etch from the upper surface of the wafer)
Bottom (etch from the bottom surface of the wafer)
By etch-rate direction:
Isotropic
Anisotropic
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Substraction processes: wet/dry etch
Wet etch: removal by chemical attack
Dry etch by RIE
Wet etch by KOH
(by KOH or TMAH) in the liquid phase.
Dry etch: material removal occurs by
reaction with a gas in "vapor-phase" or
"plasma-phase" etching.
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Sacrificial etching
Sacrificial layer
Substrate
4
1
Polysilicon
2
3
5
6
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Tipical micro-fabrication process
Starting Silicon
Bulk Substrate
Material
Deposition
Photolitography
Final Etching
Starting silicon substrate
for supporting purpose
Deposition of material
with a defined thickness
Two-dimensional patterning
of the previously deposited material
Final etching to realize suspended structures
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Surface Micromachining
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Bulk Micromachining
Example on Silicon-On-Insulator (SOI)
substrate
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