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MEMS/NEMS Devices Applications
Micro-electromechanical Systems (MEMS) Nano-electromechanical Systems (NEMS)
The key roles
in many important areas
Chapter 8
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.
• 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) .
• 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.
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
• Fig. 8.1 SEM micrograph (显微照片) of a polysilicon microelectromechanical motor (1980s).
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.
MEMS/NEMS Devices
Fig. 8.2 SEM micrograph (显微照片) of polysilicon micro-gears (1996)
The micro-electromechanical 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.
MEMS/NEMS Devices
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 DevicesPiezo-resistive Pressure Sensor
Fig. 8.3 Cross-sectional schematic of a piezoresistive 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.
The measurable pressure range can be from 10-3 to 106 Torr.
MEMS/NEMS Devices
Piezo-resistive Pressure Sensor
Fig. 8.3 Cross-sectional schematic of a piezoresistive 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 kilo-ohms). Then, wafer is passivated(钝化) with a silicon dioxide layer, opened for metallization(敷金属) , on the backside, patterned and wet etched(湿法光刻) to form the diaphragm (thickness around a few tens and length of several hundreds of micrometers).
A second silicon wafer is then bonded to the device wafer in a vacuum to form a reference vacuum cavity (空腔) , thus completing the sensor.
MEMS/NEMS DevicesPiezo-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 Cross-sectional (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 Cross-sectional 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. 8.7 Photo of a touch-mode capacitive pressure sensor
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. 8.8 Simplified fabrication sequence of surface micromachining technology
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.
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 micro-system with high performance.
MEMS/NEMS
Fig. 8.11 Schematics of vertical (垂直) (a) and lateral (水平) (b) accelerometers ,by using parallel-plate sense capacitance
Accelerometer
MEMS/NEMS
Fig. 8.13 SEM micrograph of a MEMS z-axis accelerometerfabricated 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 Devices
Photo of a monolithic (单片) polysilicon surface-micromachined z-axis vibratory gyroscope with integrated(集成) interface and control electronics
Fibre optic blood pressure sensor. (a) Principle;(b) fabrication; (c) photograph.
Fibre optic blood pressure sensor.
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.
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.
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 microscope based on DMD
• Vertical resolution :0.35μm ~
55μm
• Scanning range :0.14mm×0.1mm
~1.4mm×1mm
Applications in Medicine
A user wearing the HMD
• Numerous consumer products, such as head-mount displays, camcorders 可携式摄像机 , three-dimensional mouse, etc.
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.
A user wearing the HMD
• numerous consumer products, such as head-mount displays, camcorders, three-dimensional mouse, etc.
Fig. 8.25 SEM micrograph of a DMD pixel after removing half of the mirror plate using ion milling (courtesy of Texas Instruments)
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, micromolding-based 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/NEMS Devices