Chapter 8
MEMS/NEMS Devices
Applications
Micro-electromechanical Systems (MEMS)
Nano-electromechanical Systems (NEMS)
The key roles
in many important areas
MEMS/NEMS Devices
• MEMS are inherently small, thus offering attractive
characteristics such as reduced size, weight, and power
dissipation and improved speed and precision compared
to their macroscopic counterparts.
• Most MEMS devices exhibit a length or width ranging
from micrometers（微米） to several hundreds of
micrometers with a thickness from sub-micrometer up to
tens of micrometers, depending upon the fabrication（制
备） technique employed.
• A physical displacement of a sensor or an actuator(驱动
器) is typically on the same order（等级） of magnitude
（数量级）.
MEMS/NEMS Devices
They have played key roles in many important areas
•
•
•
•
•
•
•
transportation,
communication,
automated manufacturing（制造）,
environmental monitoring,
health care,
defense systems,
and a wide range of consumer products.
MEMS/NEMS Devices
Polycrystalline silicon
(poly-silicon)（多晶硅）
micro-motor, achieving a
diameter of 150μm and a
minimum vertical feature
size on the order of a
micrometer.
• Fig. 8.1 SEM micrograph（显微照片） of
a polysilicon microelectromechanical
motor (1980s).
MEMS/NEMS Devices
The microelectromechanical
devices and systems
can be realized through
applying such
technology，advanced
surface micromachining
（微细加工）
fabrication processes
developed to date， in
the future.
Fig. 8.2 SEM micrograph（显微照
片） of polysilicon
micro-gears (1996)
MEMS/NEMS Devices
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•
•
•
Pressure Sensor
Pressure sensors are one of the early devices realized
by silicon micromachining technologies and have
become successful commercial products.
The devices have been widely used in various industrial
and biomedical applications.
Silicon bulk（体硅） and surface micromachining
techniques have been used for sensor batch fabrication
（成批生产）, thus achieving size miniaturization and
low cost.
Two types of pressure sensors – piezo-resistive（压阻型）
and capacitive（电容式）
MEMS/NEMS Devices
Piezo-resistive Pressure Sensor
Four sensing resistors
connected are along the
edges of a thin silicon
diaphragm（隔板）.
An external pressure
applied over the
diaphragm introduces a
stress on the sensing
resistors, resulting in a
resistance value change
corresponding to the
pressure.
Fig. 8.3 Cross-sectional
schematic of
a piezoresistive pressure sensor
The measurable pressure range
can be from 10-3 to 106 Torr.
MEMS/NEMS Devices
Piezo-resistive Pressure Sensor
First, the piezo-resistors are
formed through a boron
diffusion（硼扩散） process
and by a high temperature
annealing（退火） ( few kiloohms).
Then, wafer is passivated
（钝化） with a silicon dioxide
layer, opened for metallization
Fig. 8.3 Cross-sectional schematic of
（敷金属）, on the backside,
a piezoresistive pressure sensor
patterned and wet etched（湿
法光刻） to form the
A second silicon wafer is then
diaphragm (thickness around a bonded to the device wafer in a
few tens and length of several vacuum to form a reference vacuum
hundreds of micrometers).
cavity（空腔）, thus completing the
sensor.
MEMS/NEMS Devices
Piezo-resistive Pressure Sensor
The piezo-resistive sensors are
- simple to fabricate and
- can be readily interfaced（接口） with electronic
systems.
However, the resistors are
- temperature dependent and
- consume DC power（直流电源）.
- Long-term characteristic drift and resistor thermal
noise ultimately limit the sensor resolution. .
MEMS/NEMS Devices
Capacitive Sensor
• Capacitive pressure sensors are attractive because they
are virtually temperature independent and consume zero
DC power. The devices do not exhibit initial turn-on drift
and are stable over time.
• Furthermore, CMOS microelectronic circuits can be
readily interfaced with the sensors to provide advanced
signal conditioning and processing, thus improving
overall system performance.
Fig. 8.4 Crosssectional（断层 ）
schematic（原理图）
of a capacitive
pressure sensor
. The diaphragm（隔板） can be square or circular with a
typical thickness of a few micrometers and a length or radius
of a few hundred micrometers, respectively.
The vacuum cavity typically has a depth of a few
micrometers.
The diaphragm and substrate（衬底） form a pressure
dependent air gap variable capacitor.
Fig. 8.5 Crosssectional schematic
of a touch-mode
capacitive pressure
sensor
A wide dynamic（动态） range of capacitive pressure
sensor, achieving an inherent linear characteristic response,
can be implemented by employing a touch mode architecture.
MEMS/NEMS Devices
Capacitive Sensor
• The diaphragm
deflects（偏转）
under an increasing
external pressure and
touches the substrate,
• causing a linear
increase in the sensor
capacitance value
beyond the touch
point pressure.
MEMS/NEMS Devices
Fig. 8.9 SEM micrograph of
polysilicon surface-micromachined
capacitive pressure sensors
Suspended diaphragm (0.8 mm
diameter)
Diaphragm bond pad
（垫）
Substrate contact pad
Fig. 8.7 Photo of a touch-mode
capacitive pressure sensor
The process starts by
depositing a layer of sacrificial
material, such as silicon
dioxide, over a wafer,
followed by anchor formation.
A structural layer（结构层）,
typically a poly-silicon film, is
deposited and patterned.
The underlying sacrificial
layer is then removed to
release the suspended
microstructure and complete
the fabrication sequence.
Fig. 8.8 Simplified fabrication sequence of surface
micromachining technology
MEMS/NEMS Devices inertial sensors
• Micro-machined inertial（惯性） sensors, silicon-based
MEMS sensors, consist of accelerometers（加速度传感
器） and gyroscopes（回转仪） and have been
successfully commercialized.
•
Inertial sensors fabricated by micromachining
technology can achieve reduced size, weight, and cost,
all which are critical for consumer applications.
• More importantly, these sensors can be integrated with
microelectronic circuits to achieve a functional microsystem with high performance.
MEMS/NEMS
Accelerometer
Fig. 8.11
Schematics of
vertical（垂直） (a)
and lateral （水平）
(b)
accelerometers，
by using parallelplate sense
capacitance
MEMS/NEMS
Fig. 8.13 SEM
micrograph of a
MEMS z-axis
accelerometer
fabricated using a
combined surface
and bulk
micromachining
technology.
Integrated capacitive type, silicon
accelerometers
Full scale sensitivity from
less than 1 g to over
20,000 g
MEMS/NEMS
SEM micrograph of a polysilicon surfacemicromachined lateral accelerometer.
MEMS/NEMS Devices
Photo of a monolithic（单片） polysilicon surfacemicromachined z-axis vibratory gyroscope with integrated
（集成） interface and control electronics
MEMS/NEMS Devices
Photo of a
polysilicon
surfacemicromachined
dual-axis（双
轴） gyroscope
Fibre（纤维） optic blood pressure sensor.
Fibre optic blood pressure sensor. (a) Principle.
Fibre optic blood pressure sensor.
Fibre optic blood pressure sensor. (b) fabrication.
Fibre optic blood pressure sensor.
Fibre optic blood pressure sensor. (a) Principle;
(b) fabrication; (c) photograph.
Digital Micromirror Devices (DMDs)
Texas Intruments‘ Digital Micromirror Devices for
DLP（数字光处理技术） displays.
The DLP™ chip, light switch,
contains a rectangular（矩形）
array of up to 2 million hinge
（铰链）-mounted（悬挂）
microscopic mirrors;
Each of these micromirrors
measures less than one-fifth the
width of a human hair.
MEMS/NEMS Devices
Fig. 8.23 SEM（扫描电镜） micrograph of a closeup view of a DMD pixel（像素） array
Digital Micromirror Devices (DMDs)
A DLP™ chip's micromirrors are mounted on tiny hinges
that enable them to tilt either toward the light source in a
DLP™ projection system (ON) or away from it (OFF)creating a light or dark pixel on the projection surface.
Fig. 8.24 Detailed
structure layout of a
DMD pixel
Digital micromirror devices (DMD)
Applications
• about $ 400 million in sales in every year;
• Commercial digital light processing (DLP)
equipment using DMD were launched in 1996 by
Texas Instruments for digital projection displays
in portable and home theater projectors;
• table-top and projection TVs;
• More than 3.5 million projectors were sold.
Confocal microscopy
Confocal microscope based on DMD
• Vertical resolution：
0.35μm ~ 55μm
• Scanning range：
0.14mm×0.1mm
~1.4mm×1mm
Applications in Medicine
• Numerous consumer products,
such as head-mount displays,
camcorders可携式摄像机, threedimensional mouse, etc.
A user wearing the HMD
MEMS Fabrication Techniques
NSLS/BNL
Karlsruhe Research Center
MEMS/NEMS Devices inertial（惯性）
sensors
• Accelerometers have been used in a wide range of
applications, including automotive application for
safety systems,
• active suspension and stability control,
• biomedical application for activity monitoring, and
for implementing self-contained（自容式） navigation
（导航） and guidance systems.
• numerous consumer products,
such as head-mount displays,
camcorders, three-dimensional
mouse, etc.
A user wearing the HMD
Fig. 8.25
SEM micrograph of a DMD pixel after
removing half of the mirror plate using ion milling
(courtesy of Texas Instruments)
Fig. 8.26 SEM micrograph of a close view of a DMD yoke
and hinges [8.21]
MEMS/NEMS Devices
SEM micrograph of a 3C-SiC nanomechanical
beam resonator fabricated by electron-beam lithography
and dry etching processes
MEMS/NEMS Devices
SEM micrograph of a surface-micromachined
polysilicon micromotor fabricated using a SiO2
sacrificial layer
MEMS/NEMS Devices
SEM micrograph of a poly-SiC lateral resonant
structure fabricated using a multilayer, micromoldingbased micromachining process
MEMS/NEMS Devices
SEM micrograph of the folded beam truss of a diamond lateral
resonator. The diamond film was deposited using a seed ing based
hot filament CVD process. The micrograph illustrates the
challenges currently facing diamond
MEMS/NEMS Devices
SEM micrograph of a GaAs nanomechanical beam resonator
fabricated by epitaxial growth, electron-beam lithography,
and selective etching
MEMS Fabrication Techniques
Fig. 5.38 SEM
of assembled
LIGA-fabricated
nickel structures