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MEMS/NEMS Devices Applications

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1 MEMS/NEMS Devices Applications
Chapter 8 MEMS/NEMS Devices Applications Micro-electromechanical Systems (MEMS) Nano-electromechanical Systems (NEMS) The key roles in many important areas

2 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. joint density of states near the energies of their van Hove singularities, 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).

3 MEMS/NEMS Devices transportation,
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.

4 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. Yet the sizes of nanowires are typically large enough (> 1 nm in the quantum confined direction) to have local crystal structures closely related to their parent materials, thereby allowing theoretical predictions about their properties to be made on the basis of an extensive literature relevant to their bulk properties. Fig. 8.1 SEM micrograph(显微照片) of a polysilicon microelectromechanical motor (1980s).

5 MEMS/NEMS Devices 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. The field has expanded greatly in recent years along with rapid technology advances. Fig. 8.2 SEM micrograph(显微照 片) of polysilicon micro-gears (1996)

6 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)

7 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. A pressure induced strain deforms the silicon band structure, thus changing the resistivity of the material. The piezo-resistive effect is typically crystal-orientation dependent and is also affected by doping and temperature. A practical piezo-resistive pressure sensor can be implemented by fabricating four sensing resistors along the edges of a thin silicon diaphragm, which acts as a mechanical amplifier to increase the stress and strain at the sensor site. The four sensing elements are connected in a bridge configuration with push-pull signals to increase the sensitivity. Forming of piezo-resistors: Through a boron]硼 diffusion process, By a high temperature annealing退火step in order to achieve a resistance value on the order of a few kilo-ohms. The wafer is then passivated with a silicon dioxide layer, and contact windows are opened for metallization. At this point, the wafer is patterned on the backside, followed by a timed silicon wet etch to form the diaphragm, typically having a thickness around a few tens of micrometers. The diaphragm can have a 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, thus completing the fabrication process. The second wafer can also be further etched through to form an inlet port, implementing a gauge pressure sensor Fig. 8.3 Cross-sectional schematic of a piezoresistive pressure sensor The measurable pressure range can be from 10-3 to 106 Torr.

8 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 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 pressure induced strain deforms the silicon band structure, thus changing the resistivity of the material. The piezo-resistive effect is typically crystal-orientation dependent and is also affected by doping and temperature. A practical piezo-resistive pressure sensor can be implemented by fabricating four sensing resistors along the edges of a thin silicon diaphragm, which acts as a mechanical amplifier to increase the stress and strain at the sensor site. The four sensing elements are connected in a bridge configuration with push-pull signals to increase the sensitivity. Forming of piezo-resistors: Through a boron]硼 diffusion process, By a high temperature annealing退火step in order to achieve a resistance value on the order of a few kilo-ohms. The wafer is then passivated with a silicon dioxide layer, and contact windows are opened for metallization. At this point, the wafer is patterned on the backside, followed by a timed silicon wet etch to form the diaphragm, typically having a thickness around a few tens of micrometers. The diaphragm can have a 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, thus completing the fabrication process. The second wafer can also be further etched through to form an inlet port, implementing a gauge pressure sensor Fig. 8.3 Cross-sectional schematic of a piezoresistive pressure sensor 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.

9 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. . A pressure induced strain deforms the silicon band structure, thus changing the resistivity of the material. The piezo-resistive effect is typically crystal-orientation dependent and is also affected by doping and temperature. A practical piezo-resistive pressure sensor can be implemented by fabricating four sensing resistors along the edges of a thin silicon diaphragm, which acts as a mechanical amplifier to increase the stress and strain at the sensor site. The four sensing elements are connected in a bridge configuration with push-pull signals to increase the sensitivity. Forming of piezo-resistors: Through a boron]硼 diffusion process, By a high temperature annealing退火step in order to achieve a resistance value on the order of a few kilo-ohms. The wafer is then passivated with a silicon dioxide layer, and contact windows are opened for metallization. At this point, the wafer is patterned on the backside, followed by a timed silicon wet etch to form the diaphragm, typically having a thickness around a few tens of micrometers. The diaphragm can have a 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, thus completing the fabrication process. The second wafer can also be further etched through to form an inlet port, implementing a gauge pressure sensor

10 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.

11 The vacuum cavity typically has a depth of a few micrometers.
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.

12 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. Figure 8.5 shows the cross-sectional view of a touch-mode pressure sensor. The device consists of an edge-clamped silicon diaphragm suspended over a vacuum cavity. The diaphragm deflects under an increasing external pressure and touches the substrate, causing a linear increase in the sensor capacitance value beyond the touch point pressure. Figure 8.6 shows a typical device characteristic curve. The touch point pressure can be designed by engineering the sensor geometric parameters, such as diaphragm size, thickness, cavity depth, etc., for various application requirements. The device can be fabricated using a process flow similar to the flow outlined for the basic capacitive pressure sensor. Figure 8.7 presents a photo of a fabricated touch-mode sensor employing a circular diaphragm with a diameter of 800μm and a thickness of 5μm suspended over a 2.5μm vacuum cavity. The device achieves a touch point pressure of 8 psi and exhibits a linear capacitance range of 33 pF at 10 psi to 40 pF at 32 psi (absolute pressures). The above processes use bulk silicon materials for machining and are usually referred to as bulk micromachining.

13 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.

14 MEMS/NEMS Devices Suspended diaphragm (0.8 mm diameter) Figure 8.8 illustrates a typical surface micromachining process flow. The process starts by depositing a layer of sacrificial material, such as silicon dioxide, over a wafer, followed by anchor formation. A structural layer, 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. The processing materials and steps are compatible with the standard integrated circuit process and, thus, can be readily incorporated as an add-on module to an IC process. Substrate contact pad Diaphragm bond pad(垫) Fig. 8.9 SEM micrograph of polysilicon surface-micromachined capacitive pressure sensors Fig. 8.7 Photo of a touch-mode capacitive pressure sensor

15 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. The processing materials and steps are compatible with the standard integrated circuit process and, thus, can be readily incorporated as an add-on module to an IC process. Fig. 8.8 Simplified fabrication sequence of surface micromachining technology

16 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. An accelerometer generally consists of a proof mass suspended by compliant mechanical suspensions anchored to a fixed frame. An external acceleration displaces the support frame relative to the proof mass. The displacement can result in an internal stress change in the suspension, which can be detected by piezo-resistive or capacitance sensors as a measure of the external acceleration. Capacitive sensors are attractive for various applications because they exhibit high sensitivity and low temperature dependence, turn-on drift, power dissipation, and noise. Capacitive accelerometers may be divided into two categories: vertical and lateral sensors.

17 MEMS/NEMS Accelerometer
Fig Schematics of vertical(垂直) (a) and lateral (水平)(b) accelerometers,by using parallel-plate sense capacitance Figure 8.11 shows sensor structures for the two versions. In a vertical device, the proof mass is suspended above the substrate electrode by a small gap typically on the order of a micrometer, forming a parallel-plate sense capacitance. The proof mass moves in the direction perpendicular to the substrate (z-axis) upon a vertical input acceleration, thus changing the gap and the capacitance value. The lateral accelerometer consists of a number of movable fingers attached to the proof mass, forming a sense capacitance with an array of fixed parallel fingers. The sensor proof mass moves in a plane parallel to the substrate when subjected to a lateral input acceleration, thus changing the overlap area of these fingers and, hence, the capacitance value.

18 MEMS/NEMS Fig. 8.13 SEM micrograph of a MEMS z-axis accelerometer
fabricated using a combined surface and bulk micromachining technology. Figure 8.11 shows sensor structures for the two versions. In a vertical device, the proof mass is suspended above the substrate electrode by a small gap typically on the order of a micrometer, forming a parallel-plate sense capacitance. The proof mass moves in the direction perpendicular to the substrate (z-axis) upon a vertical input acceleration, thus changing the gap and the capacitance value. The lateral accelerometer consists of a number of movable fingers attached to the proof mass, forming a sense capacitance with an array of fixed parallel fingers. The sensor proof mass moves in a plane parallel to the substrate when subjected to a lateral input acceleration, thus changing the overlap area of these fingers and, hence, the capacitance value.

19 Integrated capacitive type, silicon accelerometers
Full scale sensitivity from less than 1 g to over 20,000 g

20 MEMS/NEMS Fig Figure 8.11 shows sensor structures for the two versions. In a vertical device, the proof mass is suspended above the substrate electrode by a small gap typically on the order of a micrometer, forming a parallel-plate sense capacitance. The proof mass moves in the direction perpendicular to the substrate (z-axis) upon a vertical input acceleration, thus changing the gap and the capacitance value. The lateral accelerometer consists of a number of movable fingers attached to the proof mass, forming a sense capacitance with an array of fixed parallel fingers. The sensor proof mass moves in a plane parallel to the substrate when subjected to a lateral input acceleration, thus changing the overlap area of these fingers and, hence, the capacitance value. SEM micrograph of a polysilicon surface-micromachined lateral accelerometer.

21 MEMS/NEMS Devices Fig. 8.18 Photo of a monolithic(单片) polysilicon surface-micromachined z-axis vibratory gyroscope with integrated(集成) interface and control electronics

22 MEMS/NEMS Devices Photo of a polysilicon surface-micromachined
dual-axis(双轴) gyroscope Fig. 8.20

23 Fibre(纤维) optic blood pressure sensor.
Fibre optic blood pressure sensor. (a) Principle.

24 Fibre optic blood pressure sensor.
Fibre optic blood pressure sensor. (b) fabrication.

25 Fibre optic blood pressure sensor.
Fibre optic blood pressure sensor. (a) Principle; (b) fabrication; (c) photograph.

26 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; Digital Micromirror Devices (DMDs) Texas Intruments' Digital Micromirror Devices are the engines behind DLP displays. Each of these micromirrors measures less than one-fifth the width of a human hair.

27 MEMS/NEMS Devices Fig SEM(扫描电镜) micrograph of a close-up view of a DMD pixel(像素) array

28 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.

29 Fig. 8.24 Detailed structure layout of a DMD pixel

30

31

32 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.

33 Confocal microscopy

34 Confocal microscope based on DMD
Vertical resolution: 0.35μm ~ 55μm Scanning range: 0.14mm×0.1mm ~1.4mm×1mm

35 Applications in Medicine
Numerous consumer products, such as head-mount displays, camcorders可携式摄像机, three-dimensional mouse, etc. A user wearing the HMD

36 MEMS Fabrication Techniques
NSLS/BNL Karlsruhe Research Center

37 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. An accelerometer generally consists of a proof mass suspended by compliant mechanical suspensions anchored to a fixed frame. An external acceleration displaces the support frame relative to the proof mass. The displacement can result in an internal stress change in the suspension, which can be detected by piezo-resistive sensors as a measure of the external acceleration. The displacement can also be detected as a capacitance change in capacitive accelerometers. Capacitive sensors are attractive for various applications because they exhibit high sensitivity and low temperature dependence, turn-on drift, power dissipation, and noise. The sensors can also be readily integrated with CMOS electronics to perform advanced signal processing for high system performance. Capacitive accelerometers may be divided into two categories: vertical and lateral sensors. A user wearing the HMD

38 Fig SEM micrograph of a DMD pixel after removing half of the mirror plate using ion milling (courtesy of Texas Instruments)

39 Fig. 8.26 SEM micrograph of a close view of a DMD yoke
and hinges [8.21]

40 MEMS/NEMS Devices SEM micrograph of a 3C-SiC nanomechanical
beam resonator fabricated by electron-beam lithography and dry etching processes

41 MEMS/NEMS Devices SEM micrograph of a surface-micromachined polysilicon micromotor fabricated using a SiO2 sacrificial layer

42 MEMS/NEMS Devices SEM micrograph of a poly-SiC lateral resonant structure fabricated using a multilayer, micromolding-based micromachining process

43 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

44 MEMS/NEMS Devices SEM micrograph of a GaAs nanomechanical beam resonator fabricated by epitaxial growth, electron-beam lithography, and selective etching

45 MEMS Fabrication Techniques
Fig SEM of assembled LIGA-fabricated nickel structures


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