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MEMS Sensor Technology
Introduction:
What are MEMS?
23 April 2012
Alissa M. Fitzgerald, Ph.D
AMFitzgerald & Associates, LLC
Start
Finish
Technology
Silicon process technology
• Developed to make transistors and integrated circuits
Silicon wafer
Material deposition
Lithographic patterning
Material etch back
Repeat for multiple layers
Images from: http://www.intel.com/education/chips/index.htm
Pentium chip
Birth of MEMS
• 1970’s: using silicon processing
to make mechanical devices,
not transistors
– Accelerometers
– Pressure sensors
– Inkjet nozzles
• 1982: Petersen’s “Silicon as a
Mechanical Material”
Popular Science, June 1984
Micro Electro Mechanical Systems
• Electrical sensing/actuation
– Capacitive
– Piezoelectric
– Piezoresistive
• Mechanical devices with a third
dimension
• Systems
AMFitzgerald
AMFitzgerald
– Multiple functions on one piece of silicon
IMD
Two basic MEMS process architectures
Bulk Micromachining
Surface Micromachining
•
•
•
•
•
•
Application of mask
Etching into the bulk of the wafer
•
Deposition of functional layer
Application of mask
Etching into the deposited material
Similarities to CMOS metallization
processes
In some cases, can be co-processed
with CMOS
Many shapes and functions possible
in MEMS
TI Digital Display Chip
Cantilevers for atomic force microscopy
the diameter of a
human hair
~ 100 um
2 um
15 um
E. Chow, Stanford Univ.
Microfluidics on glass
MIT Microengine
the length of
an ant ~ 3 mm
4mm
U. of Washington
Digging deep into silicon
1990’s: The Bosch DRIE process enabled a new generation
of MEMS devices and through silicon vias (TSV)
Mask
F + ions
SiF4
SF6 Plasma
Si
-CF2AMFitzgerald
C4F8 Plasma
Passivation
Si
Scalloping
SF6 Plasma
Tegal Corp.
Tegal Corp.
Tegal Corp.
MEMS is a miniaturization technology
Airbag sensors (1980)
Source: Ed Phillips
Airbag sensors (2005)
MEMS belong where
miniaturization is needed
• Applications using MEMS devices:
– Automotive: pressure, motion, sound, light, heat
– Consumer: motion, sound, pressure, compass
– RF: filters, resonators, switches, inductors
– Optical: network components, displays, switches, mirrors
– Medical devices: needles, cell scaffolds, ultrasound,
implantables
– Printing: inkjet, large format nozzles
– Biotech: lab-on-chip microfluidics
– Industrial: Atomic force microscopy, chemical sensors
MEMS aren’t just made from silicon!
Silicon
Glass
Sensors
X
RF
X
Optical
X
X
Medical
X
X
Power
X
Microfluidics
Metal
SiC
Polymers
X
X
X
X
X
X
X
Why MEMS are exciting for so
many applications
• Smaller, better, cheaper
– But not always all three!
• Sophisticated capabilities in
small form factor:
–
–
–
–
MEMS
sensor
Integrated
Pressure
Sensor
Source: IMD
Multiple sensors
Signal processing and analysis
Telemetry capability
Stacked MEMS
Low power
and ASIC chips,
wirebonded
Source: Chipworks/Kionix
Explosion of MEMS in consumer
devices
• Sophisticated capability in small form factor
enabling new functions
– Motion sensing -> gesture input, navigation
– Microphone arrays -> noise cancellation
– Pressure sensing -> height detection
• At the right price!
– MEMS took off when chip price <$3
MEMS enable exciting new products
MEMS are not like ICs
• Yes, both are made on silicon wafers
• No building block technology, i.e. no transistor
– Moore’s law does not apply
– No standardization of design, processes, or
materials
– Each MEMS is custom-crafted
• “One product, one process, one package”
MEMS are not like ICs
• Production volumes are much smaller
– 12” vs. 6”/8” wafers
– $500B+ vs. $8B market
• Fragile mechanical devices
– More difficult to produce, test, package
– Sensitive to temperature and package stress
What this implies
• MEMS prices will not decrease as fast as IC
chips did
• MEMS sensor performance will not improve
as fast as microprocessors, memory chips did
• Chip packaging usually critical to MEMS
performance
• System integration can be more challenging
Summary
• Brief introduction to MEMS
• Next: Overview of Applications for MEMS
Sensimed
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