MEMS Fabrication
S.APPA RAO
Overview
• What are MEMS?
• MEMS: Micro-Electro-Mechanical
Systems => ‘smalltech’
• MEMS Applications
• MEMS fabrication process
• Future of MEMS technology
MEMS Overview
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Micro Electro Mechanical Systems
Micron level mechanical parts
Made from transistor materials and metals
Van Der Waals forces
– Intermolecular bonding
– Plays an important part in design
http://searchcio-midmarket.techtarget.com/definition/micro-electromechanical-systems
What are MEMS?
• Combination of mechanical functions
(sensing, moving, heating) and electrical
functions (switching ,deciding) on the same
chip using micro fabrication technology
• Lab on a Chip (LOC)
• System on a Chip
• Micro-sensors / micro-actuators
• Small silicon cantilevers (AFM tips) etc
The Role of MEMS
While the functional elements of MEMS
are miniaturized structures, sensors,
actuators, and microelectronics, the most
notable (and perhaps most interesting)
elements are the microsensors and
microactuators. Microsensors and
microactuators are appropriately
categorized as “transducers”, which are
defined as devices that convert energy
from one form to another. In the case of
microsensors, the device typically converts
a measured mechanical signal into an
electrical signal. Micro-Electro-Mechanical
Systems, or MEMS, is a technology that in
its most general form can be defined as
miniaturized mechanical and electromechanical elements (i.e., devices and
structures) that are made using the
techniques of microfabrication. The critical
physical dimensions of MEMS devices can
vary from well below one micron on the
lower end of the dimensional spectrum,
all the way to several millimeters.
Michael Huff, Michael Huff is at the MEMS Exchange, Corporation for National Research
Initiatives, 1895 Preston White Drive, Suite 100, Reston, Virginia 20191-5434, USA
MEMS Ratchet
Imagine a machine so small that it is
imperceptible to the human
eye. Imagine working machines no
bigger than a grain of pollen. Imagine
thousands of these machines batch
fabricated on a single piece of silicon,
for just a few pennies each. Imagine
a world where gravity and inertia are
no longer important, but atomic forces
and surface science dominate.
Imagine a silicon chip with thousands
of microscopic mirrors working in
unison, enabling the all optical
network and removing the bottlenecks
from the global telecommunications
infrastructure. You are now entering
the micro-domain, a world occupied
by an explosive technology known as
MEMS. A world of challenge and
opportunity, where traditional
engineering concepts are turned
upside down, and the realm of the
"possible" is totally redefined.
http://www.memx.com/
MEMS Applications
• Accelerometers in consumer electronics
devices such as game controllers
• Automotive applications (20 per vehicle)
• Communications (telecom)
• Biotechnology (Lab on a Chip)
• System on a Chip
MEMS Architecture
MEMS Accelerometer
MEMS are quietly changing the way you live, in ways that you might never
imagine. The device that senses your car has been in an accident, and fires the
airbag is a MEMS device. Most new cars have over a dozen MEMS devices,
making your car safer, more energy efficient, and more environmentally friendly.
MEMS are finding their way into a variety of medical devices, and everyday
consumer products. (From MEMX http://www.memx.com/ )
MEMS Advantages
• Significantly lower manufacturing
costs (semiconductor process)
• Small inertial mass
• Particularly realized in the area of:
– sensors
– signal switching
Materials for MEMS
• Materials are the foundation required to
develop microsensors. MEMS are made of:
• Metals
• Polymers
• Ceramic materials
• Semiconductors
• Composite materials
MEMS Process
• Same as the process steps used for
making conventional electronic circuits
Fabrication Process
http://www.aero.org/publications/helvajian/helvajian-2.html
Fabrication Process
Fabrication Process
Fabrication Techniques
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Mask Lithography
Injection molding
Microstereolithography
Silicon Surface Micromachining
Silicon Bulk Micromachining
Mask Lithography
• Use of photo resist
– Positive
• Dissolves under light
– Negative
• Hardens under light
– Both get covered with desired material, then
photo resist is dissolved by a solvent
• Multiple layers – Multiple steps
Mask Lithography
Thin Film Deposition
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Deposition techniques:
Chemical Deposition
Chemical Vapor Deposition
Physical Vapor Deposition
DC magnetron sputtering
Lithography
• Application of photo
resist
• Optical exposure to
print an image of the
mask onto the resist
• Immersion in an
aqueous developer
solution to dissolve the
exposed resist
MEMS Accelerometer
• Made of bulky and heavy metal parts
• Require high operating voltage/current
• Needs careful maintenance
• Highly expensive not throwaway type
CONVENTIONAL ACCELEROMETER SENSOR
Piezoresistive
Accelerometer
Masks used for Accelerometer
http://www.pcb.com/Accelerometers/Sensing_Technologies.asp
Layout of the Resistor
Injection Molding
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Starts with mask lithography
Metal poured over resist
Resist gets dissolved
Metal form is left for plastic injection
molding
Injection Molding
Microstereolithography
• Similar principal to mask lithography, but for 3D pieces
• Uses an “active mask”
– Not a physical mask
• Utilizes a photo-reactive acrylic resin
• Each layer image projected through a DMD(digital mirror
device)
• Projected into the resin
– Uses lenses
• Resin that is illuminated, Cross-links and hardens
• Piece is then covered in a hardened layer
Microstereolithography
• Dimensional capabilities
– Lateral and Vertical resolution: 10μm
– Maximum field size: 10.24mm x 7.68mm
– Structural height: up to 5mm
Microstereolithography
http://www.sciencedirect.com/science/article/pii/S0924424799001892
Microstereolithography
Micromachining Technology
The three characteristic features of MEMS
fabrication technologies are miniaturization,
multiplicity, and microelectronics. Miniaturization
is clearly an important part of MEMS, since
materials and components that are relatively
small and light enable compact and quickresponse devices. Multiplicity refers to the batch
fabrication inherent in semiconductor processing.
Consequently, it is feasible to fabricate thousands
or millions of components as easily and
concurrently as one component, thereby ensuring
low unit component cost. Furthermore,
multiplicity provides flexibility in solving
mechanical problems by enabling the possibility
of a distributed approach through use of
(coupled) arrays of micromechanical devices.
Finally, microelectronics provides the intelligence
to MEMS and allows the monolithic merger of
sensors, actuators, and logic to build closed-loop
feedback components and systems.
http://www.aero.org/publications/helvajian/helvajian-2.html
Types of Micromachining
• BULK micromachining:
• SURFACE micromachining:
• LIGA process:
Silicon Surface
Micromachining
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Uses the same process as IC fabrication
Needs multiple layers to create structures
Cheapest form of Micromachining
Similar to lithography
– Sacrificial material
– Structural material
• When sacrificial material is removed, only
whole structures are left
Silicon Surface
Micromachining
Silicon Surface
Micromachining
Silicon Surface
Micromachining
Silicon Bulk Micromachining
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Done with Crystalline silicon
Constructed using etch stop planes
Chemical process
Anisotropic Etching
– Speed dependent – Directional
– etch in different crystallographic directions at
different rates
– Slower directions create and etch stop plane
Wet Etching and Plasma Dry Etching
Wet etching where the material is dissolved when immersed in
a chemical solution Dry etching where the material is sputtered
or dissolved using reactive ions or a vapor phase etchant
http://www.memsnet.org/mems/beginner/etch.html
Deep Reactive Ion Etching
(DRIE)
Uses photo resist and a mask to create structures
http://en.wikipedia.org/wiki/Deep_reactive-ion_etching
Sapphire Etching
• Metal Mask, 100µm etch depth, .28µm/min etch
rate, Chlorine etching
Pressure Sensor Etching
• Used on silicon, Metal mask, .81µm etch depth,
Utilizes Fluorine,
High-Speed Etching
• Silicon material, 1µm/min etch rate, W-Si Mask
http://upload.wikimedia.org/wikipedia/commons/1/17/Bosch_process_sidewall.jpg
MEMS Technical Limitations
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Imprecise fabrication methods
Expensive and complex packaging
CAD Design tool inaccuracies
Limits to miniaturization
– Material process limitations
– Functional engineering limits
Future of MEMS
• As with all emerging technologies, the
MEMS industry had been predicted to
revolutionize technology and our lives.
• MEMS may be incorporated in biological
structures, linked by wireless networks
• It has the potential to change our daily lives
as much as computers / networks
– Computation, communication and actuation
Summary
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MEMS – Micro Electro Mechanical Systems
Fabricated using silicon fabrication tools
Lithography, mask/etc, deposit/etch etc
MEMS can be fabricated out of a number of
materials: polymers, ceramics, metals, etc
• Fabless ICs are used by many MEMS designers
• Polymeric and ceramic MEMS are also made
References
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http://home.earthlink.net/~trimmerw/mems/tour.html
http://home.earthlink.net/~trimmerw/mems/BM_bulk.html
http://www.samcointl.com/apps/mems.html
http://www.cmf.rl.ac.uk/latest/msl.html
http://www.chemguide.co.uk/atoms/bonding/vdw.html
“Micromachining for Optical and Optoelectronic
Systems”. MING C. WU