nems vinay repot

8/7/2019 NEMS VINAY REPOT

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ABSTRACT

A host of novel applications and new physics could be unleashed as Micro-Electro-Mechanical

Systems (MEMS) shrink towards the nano scale. The time is ripe for a concerted exploration of

Nano-Electro-Mechanical-Systems (NEMS) - i.e. machines, sensors, computers and electronics

that are on the nano-scale. Many years of research by university, government, and industrial

groups have been devoted to developing cutting-edge NEMS technologies for enabling

revolutionary NEMS devices. NEMS has revolutionized nearly every product category by

bringing together silicon-based nano-electronics with nanolithography and nano-machining

technology, making possible the realization of complete systems-on-a-chip (SOC). Historically,

sensors and actuators are the most costly and unreliable part of a micro scale sensor-actuator-

electronics system.

The NEMS-devices can be used as extremely sensitive sensors for force and mass detection

down to the single molecule level, as high-frequency resonators up to the THz range, or as ultra-

fast, low-power switches. NEMS technology allows these complex electromechanical systems to

be manufactured using batch fabrication techniques, increasing the reliability of the sensors and

actuators to greater than that of integrated circuits. Thus, it provides a way to integratemechanical, fluidic, optical, and electronic functionality on very small devices, ranging from 1

nano meter to 100 nano meters. NEMS devices can be so small that hundreds of them can fit in

the same space as one single micro-device that performs the same function and are lighter, more

reliable and are produced at a fraction of the cost of the conventional methods. Many device

designs have been proposed, some have been developed, and fewer have reached

commercialization.


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Introduction

Nano-Electro-Mechanical Systems (NEMS) is the integration of mechanical elements, sensors,actuators, and electronics on a common silicon substrate through nano fabrication technology.

While the electronics are fabricated using integrated circuit (IC) process sequences (e.g., CMOS,

Bipolar, or BICMOS processes), the nano-mechanical components are fabricated using

compatible "micromachining" processes that selectively etch away parts of the silicon wafer or

add new structural layers to form the mechanical and electromechanical devices.

Nano-electronic integrated circuits can be thought of as the "brains" of a system and NEMS

augments this decision-making capability with "eyes" and "arms", to allow nano systems to

sense and control the environment. Sensors gather information from the environment through

measuring mechanical, thermal, biological, chemical, optical, and magnetic phenomena. The

electronics then process the information derived from the sensors and through some decision

making capability direct the actuators to respond by moving, positioning, regulating, pumping,

and filtering, thereby controlling the environment for some desired outcome or purpose.

NEMS promises to revolutionize nearly every product category by bringing together silicon-

based nano-electronics with micromachining technology, making possible the realization of

complete systems-on-a-chip. NEMS is an enabling technology allowing the development ofsmart products, augmenting the computational ability of nano-electronics with the perception and

control capabilities of nano sensors and nano actuators and expanding the space of possible

designs and applications. Despite such optimistic statistics, investment in NEMS design and

production is insufficient. Most NEMS devices are modeled using analytical tools that result in a

relatively inaccurate prediction of performance behavior. As a result, NEMS design is usually

trial and error, requiring several iterations before a device satisfies its performance requirements.

What is an Electro-MechanicalSystem?One of the earliest reported electromechanical devices was built in 1785 by Charles-Augustine

de Coulomb to measure electrical charge. His electrical torsion balance consisted of two

spherical metal balls - one of which was fixed, the other attached to a moving rod - that acted as

capacitor plates, converting a difference in charge between them to an attractive force. The


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device illustrates the two principal components common to most electromechanical systems

irrespective of scale: a mechanical element and transducers. The mechanical element either

deflects or vibrates in response to an applied force. To measure quasi-static forces, the element

typically has a weak spring constant so that a small force can deflect it by a large amount. Time-

varying forces are best measured using low-loss mechanical resonators that have a large responto

oscillating signals with small amplitudes.Many different types of mechanical elements can be

used to sense static or time-varying forces. These include the torsion balance (used by Coulomb),

the cantilever (now ubiquitous in scanning probe microscopy) and the "doubly clamped" beam,

which is fixed at both ends. In pursuit of ultrahigh sensitivity, even more intricate devices are

used, such as compound resonant structures that possess complicated transverse, torsional or

longitudinal modes of vibration. These complicated modes can be used to minimize vibrational

losses, in much the same way that the handle of a tuning fork is positioned carefully to reducelosses.

The transducers in MEMS convert mechanical energy into electrical or optical signals and vice

versa. However, in some cases the input transducer simply keeps the mechanical element

vibrating steadily while its characteristics are monitored as the system is perturbed. In this case

such perturbations, rather than the input signal itself, are precisely the signals we wish to

measure. They might include pressure variations that affect the mechanical damping of the

device, the presence of chemical adsorbents that alter the mass of the nano-scale resonator, or

temperature changes that can modify its elasticity or internal strain. In these last two cases, the

net effect is to change the frequency of vibration. In general, the output of an electromechanical

device is the movement of the mechanical element. There are two main types of response: the

element can simply deflect under the applied force or its amplitude of oscillation can change.

Detecting either type of response requires an output or readout transducer, which is often distinct

from the input one. In Coulomb's case, the readout transducer was "optical" - he simply used his

eyes to record a deflection. Today mechanical devices contain transducers that are based on a

host of physical mechanisms involving piezoelectric and magneto-motive effects, nano-magnets

and electron tunneling well as electrostatics and optics.


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Whatisa Micro Electro-Mechanical System?

as MEMS are an abbreviation for Micro Electro Mechanical Systems. This is a rapidly emerging

technology combining electrical, electronic, mechanical, optical, material, chemical, and fluidsengineering disciplines. As the smallest commercially produced "machines", MEMS devices are

similar to traditional sensors and actuators although much, much smaller. E.g. complete systems

are typically a few millimeters across, with individual features / devices of the order of 1-100

micrometers.

MEMS devices are manufactured either using processes based on Integrated Circuit fabrication

techniques and materials, or using new emerging fabrication technologies such as micro injection

molding. These former processes involve building the device up layer by layer, involving several

material depositions and etch steps. A typical MEMS fabrication technology may have a 5 step

process. Due to the limitations of this "traditional IC" manufacturing process MEMS devices are

substantially planar, having very low aspect ratios (typically 5 -10 micro meters thick). It is

important to note that there are several evolving fabrication techniques that allow higher aspect

ratios such as deep x-ray lithography, electro deposition, and micro injection molding.

MEMS devices are typically fabricated onto a substrate (chip) that may also contain the

electronics required to interact with the MEMS device. Due to the small size and mass of thedevices, MEMS components can be actuated electro statically (piezoelectric and bimetallic

effects can also be used). The position of MEMS components can also be sensed capacitively.

Hence the MEMS electronics include electrostatic drive power supplies, capacitance charge

comparators, and signal conditioning circuitry. Connection with the macroscopic world is via

wire bonding and encapsulation into familiar BGA, MCM, surface mount, or leaded IC

packages.

A common MEMS actuator is the "linear comb drive" (shown above) which consists of rows of

interlocking teeth; half of the teeth are attached to a fixed "beam", the other half attach to a

movable beam assembly. Both assemblies are electrically insulated. By applying the same

polarity voltage to both parts the resultant electrostatic force repels the movable beam away from

the fixed. Conversely, by applying opposite polarity the parts are attracted. In this manner the


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comb drive can be moved "in" or "out" and either DC or AC voltages can be applied. The

magnitude of electrostatic force is multiplied by the voltage or more commonly the surface area

and number of teeth. Commercial comb drives have several thousand teeth, each tooth

approximately 10 micro meters long. Drive voltages are CMOS levels.

The linear push / pull motion of a comb drive can be converted into rotational motion by

coupling the drive to push rod and pinion on a wheel. In this manner the comb drive can rotate

the wheel in the same way a steam engine functions!The First MEMS DeviceIn case you were

wondering microsystems have physically been around since the late 1960's. It is generally agreed

that the first MEMS device was a gold resonating MOS gate structure. [H.C. Nathanson, et al.,

The Resonant Gate Transistor, IEEE Trans. Electron Devices, March 1967, vol. 14, no. 3, pp

117-133.]

schematic

Microsystems are inherently multiphysics in nature and thus require a sophisticated coupled

physics analysis capability in order to capture actuation and transducer effects accurately. The

following analysis features are fundamental requirements for the analysis solution:

Requires a system of units applicable to small geometric scale. Ability to handle unique

material properties that are not in the public domain. Ability to mesh high aspect ratio device

geometry. Lumped parameter extraction &reduced order macro modeling for system level

simulation. Ability to model large field domains associated with electromagnetic and CFD.The benefits

of Nano-machinesNano-mechanical devices promise to revolutionize measurements of extremely

small displacements and extremely weak forces, particularly at the molecular scale. Indeed with

surface and bulk nano-machining techniques, NEMS can now be built with masses approaching

a few attograms (10-18 g) and with cross-sections of about 10 nm. The small mass and size of

NEMS gives them a number of unique attributes that offer immense potential for new

applications and fundamental measurements. Mechanical systems vibrate at a natural angular

frequency, w0 that can be approximated by w0 = (keff/meff) 1/2, where keff is an effective

spring constant and meff is an effective mass. (Underlying these simplified "effective" terms is a

complex set of elasticity equations that govern the mechanical response of these objects.) If we

reduce the size of the mechanical device while preserving its overall shape, then the fundamental

frequency, w0, increases as the linear dimension l decreases. Underlying this behavior is


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the fact that the effective mass is proportional to l3, while the effective spring constant is

proportional to l. This is important because a high response frequency translates directly to a fast

response time to applied forces. It also means that a fast response can be achieved without the

expense of making stiff structures. Resonators with fundamental frequencies above 10 GHz

(1010 Hz) can now be built using surface nano-machining processes involving state-of-the-art

nanolithography at the 10 nm scale. Such high-frequency mechanical devices are unprecedented

and open up many new and exciting possibilities. Among these are ultra low-power mechanical

signal processing at microwave frequencies and new types of fast scanning probe microscopes

that could be used in fundamental research or perhaps even as the basis of new forms of

mechanical.

A second important attribute of NEMS is that they dissipate very little energy, a feature that is

characterized by the high quality or Q factor of resonance. As a result, NEMS are extremelysensitive to external damping mechanisms, which is crucial for building many types of sensors.

In addition, the thermo mechanical noise, which is analogous to Johnson noise in electrical

resistors, is inversely proportional to Q. High Q values are therefore an important attribute for

both resonant and deflection sensors, suppressing random mechanical fluctuations and thus

making these devices highly sensitive to applied forces. Indeed, this sensitivity appears destined

to reach the quantum limit. Typically, high-frequency electrical resonators have Q values less

than several hundred, but even the first high-frequency mechanical device built in 1994 by

Andrew Cleland at Caltech was 100 times better. Such high quality factors are significant for

potential applications in signal processing. The small effective mass of the vibrating part of the

device - or the small moment of inertia for torsional devices - has another important

consequence. It gives NEMS an astoundingly high sensitivity to additional masses - clearly a

valuable attribute for a wide range of sensing applications. Recent work by Kamil Ekinci at

Caltech supports the prediction that the most sensitive devices we can currently fabricate are

measurably affected by small numbers of atoms being adsorbed on the surface of the device.

Meanwhile, the small size of NEMS also implies that they have a highly localized spatial

response. Moreover, the geometry of a NEMS device can be tailored so that the vibrating

element reacts only to external forces in a specific direction. This flexibility is extremely

usefulfor designing new types of scanning probe microscopes. NEMS are also intrinsically ultra

low-power devices. Their fundamental power scale is defined by the thermal energy divided by


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the response time, set by Q/wo. At 300 K, NEMS are only overwhelmed by thermal fluctuations

when they are operated at the attowatt (10-18 W) level. Thus driving a NEMS device at the Pico

watt (10-12 W) scale provides signal-to-noise ratios of up to 106. Even if a million such devices

were operated simultaneously in a NEMS signal processor, the total power dissipated by the

entire system would still only be about a microwatt. This is three or four orders of magnitude less

than the power consumed by conventional electronic processors that operate by shuttling packets

of electronic charge rather than relying on mechanical elements. Another advantage of NEMS is

that they can be fabricated from silicon, gallium arsenide and indium arsenide - the cornerstones

of the electronics industry - or other compatible materials. As a result, any auxiliary electronic

components, such as transducers and transistors, can be fabricated on the same chip as the

mechanical elements. So that all the main internal components are on the same chip means that

the circuits can be immensely complex. It also completely circumvents the insurmountableproblem of aligning different components at the nano meter scale.

NEMS devices are extremely small - for example, NEMS has made possible electrically-driven

motors smaller than the diameter of a human hair (right), but NEMS technology is not primarily

about size. NEMS is also not about making things out of silicon, even though silicon possesses

excellent materials properties, which make it an attractive choice for many high-performance

mechanical applications; for example, the strength-to-weight ratio for silicon is higher than many

other engineering materials which allows very high-bandwidth mechanical devices to be

realized. Instead, the deep insight of NEMS is as a new manufacturing technology, a way of

making complex electromechanical systems using batch fabrication techniques similar to those

used for integrated circuits, and uniting these electromechanical elements together with

electronics.

NEMS technology is based on a number of tools and methodologies, which are used to form

small structures with dimensions in the nanometer scale (one millionth of a meter). Significant

parts of the technology have been adopted from integrated circuit (IC) technology. For instance,

almost all devices are built on wafers of silicon, like ICs. The structures are realized in thin films

of materials, like ICs. They are patterned using photolithographic methods, like ICs. There is

however several processes that are not derived from IC technology, and as the technology

continues to grow the gap with IC technology also grow.How to make NEMS


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Over the past six years, new techniques have been developed for patterning freely suspended 3-D

semiconductor structures. These techniques apply to bulk silicon, epitaxial silicon and silicon-

on-insulator hetero structures, as well as to systems based on gallium arsenide and indium

arsenide.

In its simplest form, the procedure begins with a hetero structure that contains structural

andsacrificial layers on a substrate.Masks on top of this substrate are patterned by a combination

of optical and electron-beam lithography, followed by a thin-film deposition processes. The

resulting mask protects the material beneath it during the next stage.

Unprotected material around the mask is then etched away using a plasma process. Finally, a

local chemically selective etch step removes the sacrificial layer from specific regions to create

freely suspended nanostructures that are both thermally and mechanically isolated.

In typical devices this entire procedure might be repeated several times and combined withvarious deposition processes to give complicated mechanical nanostructures. The flexibility of

the process allows complex suspended structures with lateral dimensions down to a few tens of

nano meters to be fabricated. Moreover, complex transducers can be incorporated for control and

measurement purposes. Epitaxial growth means that the thickness of the layers can be controlled

with atomic precision. In principle, the fabricated devices can be just a few layers thick.

Fabrication

There are three basic building blocks in NEMS technology, which are the ability to deposit thinfilms of material on a substrate, to apply a patterned mask on top of the films by

photolithographic imaging, and to etch the films selectively to the mask. A NEMS process is

usually a structured sequence of these operations to form actual devices and includes:

Deposition processes Lithography Etching processesDeposition Processes One of the

basic building blocks in NEMS processing is the ability to deposit thin films of material. The

thin film can have a thickness anywhere between a few nanometers to about 100 nanometer.

Chemical methods are often used in NEMS deposition technology and major among them are:

-Chemical Vapour Deposition (CVD) -Epitaxy Chemical Vapour Deposition (CVD) In this

process, the substrate is placed inside a reactor to which a number of gases are supplied. The

fundamental principle of the process is that a chemical reaction takes place between the source

gases. The product of that reaction is a solid material with condenses on all surfaces inside the

reactor. The two most important CVD technologies in NEMS are the Low Pressure CVD


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(LPCVD) and Plasma Enhanced CVD (PECVD). The LPCVD process produces layers with

excellent uniformity of thickness and material characteristics. The main problems with the

process are the high deposition temperature (higher than 600 C) and the relatively slow

deposition rate. The PECVD process can operate at lower temperatures (down to 300 C)

thanks to the extra energy supplied to the gas molecules by the plasma in the reactor. However,

the quality of the films tends to be inferior to processes running at higher temperatures.

Secondly, most PECVD deposition systems can only deposit the material on one side of the

wafers on 1 to 4 wafers at a time. LPCVD systems deposit films on both sides of at least 25

wafers at a time. A schematic diagram of a typical LPCVD reactor is shown in the figure 1

Figure1:Typicalhot-wallLPCVDreactor

CVD processes are ideal to use when you want a thin film with good step coverage. A variety of

materials can be deposited with this technology. The quality of the material varies from process


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to process, however a good rule of thumb is that higher process temperature yields a materialwith

higher quality and less defects.

Epitaxy

This technology is quite similar to what happens in CVD processes, however, if the substrate is

an ordered semiconductor crystal (i.e. silicon, gallium arsenide), it is possible with this process to

continue building on the substrate with the same crystallographic orientation with the substrate

acting as a seed for the deposition. If an amorphous/polycrystalline substrate surface is used, the

film will also be amorphous or polycrystalline.

There are several technologies for creating the conditions inside a reactor needed to support

epitaxial growth, of which the most important is Vapour Phase Epitaxy (VPE). In this process, a

number of gases are introduced in an induction heated reactor where only the substrate is heated.

The temperature of the substrate typically must be at least 50% of the melting point of thematerial to be deposited. An advantage of epitaxy is the high growth rate of material, which

allows the formation of films with considerable thickness (>100m). Epitaxy is a widely

used technology for producing silicon on insulator (SOI) substrates. The technology is primarily

used for deposition of silicon. A schematic diagram of a typical vapour phase epitaxial reactor is

shown in figure 2

Figure 2: Typical cold-wall vapour phase epitaxial reactor


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This has been and continues to be an emerging process technology in NEMS. Some processes

require high temperature exposure of the substrate, whereas others do not require significant

heating of the substrate. Some processes can even be used to perform selective deposition,

depending on the surface of the substrate.

Lithography

Pattern Transfer

Lithography in the NEMS context is typically the transfer of a pattern to a photosensitive

material by selective exposure to a radiation source such as light. A photosensitive material is a

material that experiences a change in its physical properties when exposed to a radiation source.

If we selectively expose a photosensitive material to radiation (e.g. by masking some of the

radiation) the pattern of the radiation on the material is transferred to the material exposed, as the

properties of the exposed and unexposed regions differ (as shown in figure 3).

Figure 3: Transfer of a pattern to a photosensitive material.

In lithography for micromachining, the photosensitive material used is typically a photo resist

(also called resist, other photosensitive polymers are also used). If the resist is placed in a

developer solution after selective exposure to a light source, it will etch away one of the two

regions (exposed or unexposed). If the exposed material is etched away by the developer and theunexposed region is resilient, the material is considered to be a positive resist (shown in figure

4a). If the exposed material is resilient to the developer and the unexposed region is etched away,

it is considered to be a negative resist (shown in figure 4b).


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Figure 4

a) Pattern definition in positive resist, b) Pattern definition in negative resist.

Lithography is the principal mechanism for pattern definition in micromachining. Photosensitive

compounds are primarily organic, and do not encompass the spectrum of materials properties of

interest to nano-machinists. However, as the technique is capable of producing fine features in an

economic fashion, a photosensitive layer is often used as a temporary mask when etching an

underlying layer, so that the pattern may be transferred to the underlying layer. Photo resist may

also be used as a template for patterning material deposited after lithography.

The resist is subsequently etched away, and the material deposited on the resist is "lifted off".

The deposition template (lift-off) approach for transferring a pattern from resist to another layer

is less common than using the resist pattern as an etch mask. The reason for this is that resist is

incompatible with most NEMS deposition processes, usually because it cannot withstand hightemperatures and may act as a source of contamination.

Alignment

In order to make useful devices the patterns for different lithography steps that belong to a single

structure must be aligned to one another. The first pattern transferred to a wafer usually includes

a set of alignment marks, which are high precision features that are used as the reference when

positioning subsequent patterns, to the first pattern (as shown in figure 4). Often alignment marks

are included in other patterns, as the original alignment marks may be obliterated as processing

progresses. It is important for each alignment mark on the wafer to be labeled so it may be

identified, and for each pattern to specify the alignment mark (and the location thereof) to which

it should be aligned. By providing the location of the alignment mark it is easy for the operator to

locate the correct feature in a short time. Each pattern layer should have an alignment feature so

that it may be registered to the rest of the layers.

Exposure

The exposure parameters required in order to achieve accurate pattern transfer from the mask to

the photosensitive layer depend primarily on the wavelength of the radiation source and the dose

required to achieve the desired properties change of the photo resist. Different photo resists

exhibit different sensitivities to different wavelengths. The dose required per unit volume of

photo resist for good pattern transfer is somewhat constant; however, the physics of the exposure

process may affect the dose actually received.


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For example a highly reflective layer under the photo resist may result in the material

experiencing a higher dose than if the underlying layer is absorptive, as the photo resist is

exposed both by the incident radiation as well as the reflected radiation. The dose will also vary

with resist thickness.

Etching

In order to form a functional NEMS structure on a substrate, it is necessary to etch the thin films

previously deposited and/or the substrate itself. In general, there are two classes of etching

processes-Wet etching where the material is dissolved when immersed in a chemical solution

and dry etching where the material is sputtered or dissolved using reactive ions or a vapour phase

etchant. In the following, we will briefly discuss the most popular technologies for wet and dry

etching.

Wet Etching

This is the simplest etching technology. All it requires is a container with a liquid solution that

will dissolve the material in question. Unfortunately, there are complications since usually a

mask is desired to selectively etch the material. One must find a mask that will not dissolve or at

least etches much slower than the material to be patterned. Secondly, some single crystal

materials, such as silicon, exhibit anisotropic etching in certain chemicals. Anisotropic etchings

in contrast to isotropic etching means different etch rates in different directions in the material.

The classic example of this is the crystal plane sidewalls that appear when etching a holein a silicon wafer in a chemical such as potassium hydroxide (KOH). The result is a

pyramid shaped hole instead of a hole with rounded sidewalls with a isotropic etchant.

Dry Etching

In RIE, the most prominent dry etching method, the substrate is placed inside a reactor in which

several gases are introduced. Plasma is struck in the gas mixture using an RF power source,

breaking the gas molecules into ions. The ions are accelerated towards, and react at, the surface

of the material being etched, forming another gaseous material. This is known as the chemical

part of reactive ion etching. There is also a physical part which is similar in nature to the

sputtering deposition process. If the ions have high enough energy, they can knock atoms out of

the material to be etched without a chemical reaction. It is very complex tasks to develop dry

etch processes that balance chemical and physical etching, since there are many parameters to

adjust. By changing the balance it is possible to influence the anisotropy of the etching, since the


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chemical part is isotropic and the physical part highly anisotropic the combination can form

sidewalls that have shapes from rounded to vertical.

Challenges for NEMS

Processes such as electron-beam lithography and nano-machining now enable semiconductornano-structures to be fabricated below 10 nm. It would appear that the technology exists to build

NEMS. So what is holding up applications? It turns out that there are three principal challenges

that must be addressed before the full potential of NEMS can be realized: communicating signals

from the nano-scale to the macroscopic world; understanding and controlling mesoscopic

mechanics; and developing methods for reproducible and routine nanofabrication.

NEMS are clearly very small devices that can deflect or vibrate within an even smaller range

during operation. For example, the deflection of a doubly clamped beam varies linearly with an

applied force only if it is displaced by an amount that typically corresponds to a few per cent of

its thickness. For a beam 10 nm in diameter, this translates to displacements that are only a

fraction of a nano-meter. Building transducers that are sensitive enough to allow information to

be transferred accurately at this scale requires reading out positions with a far greater precision.

A further difficulty is that the natural frequency of this motion increases with decreasing size. So

the ideal NEMS transducer must ultimately be capable of resolving displacements in the 10-15-

10-12 m range and be able to do so up to frequencies of a few giga hertz. These two

requirements are truly daunting, and much more challenging than those faced by the MEMScommunity so far.

To compound the problem, some of the transducers that are mainstays of the micromechanical

realm are not applicable in the nano-world. Electrostatic transduction, the staple of MEMS, does

not scale well into the domain of NEMS. Nano-scale electrodes have capacitances of about 10-

18 farad and less. As a result, the many other, unavoidable parasitic types of impedance tend to

dominate the "dynamic" capacitance that is altered by the device motion.

Meanwhile optical methods, such as simple beam-deflection schemes or more sophisticated

optical and fibber-optic interferometer - both commonly used in scanning probe microscopy to

detect the deflection of the probe - generally fail beyond the so-called diffraction limit. In other

words, these methods cannot easily be applied to objects with cross-sections much smaller than

the wavelength of light. For fiber-optic interferometer, this breakdown can occur even earlier,

when devices are shrunk to a fraction of the diameter of the fiber.


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Conventional approaches thus appear to hold little promise for high-efficiency transduction with

the smallest of NEMS devices. Nonetheless, there are a host of intriguing new concepts in the

pipeline. These include techniques that are based on integrated near-field optics, nano-scale

magnets, high-electron-mobility transistors, superconducting quantum interference devices and

single-electron transistors - to name just a few.

The role of surface physics

One of the keys to realizing the potential of NEMS is to achieve ultrahigh quality factors. This

overarching theme underlies most areas of research, with the possible exception of non-resonant

applications. However, both intrinsic and extrinsic properties limit the quality factor in real

devices. Defects in the bulk material and interfaces, fabrication-induced surface damage and

adsorbents on the surfaces are among the intrinsic features that can dampen the motion of a

resonator.Fortunately, many of these effects can be suppressed through a careful choice of materials,

processing and device geometry. Extrinsic effects - such as air resistance, clamping losses at the

supports and electrical losses mediated through the transducers - can all be reduced by careful

engineering. However, certain loss mechanisms are fundamental and ultimately limit the

maximum attainable quality factors. These processes include thermo-elastic damping that arises

from inelastic losses in the material.

One aspect in particular looms large: as we shrink MEMS towards the domain of NEMS, the

device physics becomes increasingly dominated by the surfaces. We would expect that extremely

small mechanical devices made from single crystals and ultrahigh-purity hetero-structures would

contain very few defects, so that energy losses in the bulk are suppressed and high quality factors

should be possible.

For example, Robert Pohl's group at Cornell University, and others, has shown that centimeter-

scale semiconductor MEMS can have Q factors as high as 100 million at cryogenic temperatures.

But a group at Caltech has shown repeatedly over the past seven years that this value decreases

significantly - by a factor of between 1000 and 10 000 - as the devices are shrunk to the nano-

meter scale. The reasons for this decrease are not clear at present. However, the greatly increased

surface-to-volume ratio in NEMS, together with the non-optimized surface properties, is the

most likely explanation. This can be illustrated by considering a NEMS device fabricated using

state-of-the-art electron-beam lithography. A silicon beam 100 nm long, 10 nm wide and 10 nm


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thick contains only about 5 x 105 atoms, with some 3 x 104 of these atoms residing at the

surface. In other words, more than 10% of the constituents are surface or near-surface atoms. It is

clear that these surface atoms play a central role, but understanding exactly how will take

considerable effort.

Ultimately, as devices become ever smaller, macroscopic mechanics will break down and

atomistic behavior will emerge. Indeed, molecular dynamics simulations, such as those

performed by Robert Rudd and Jeremy Broughton at the Naval Research Laboratory in

Washington DC on idealized structures just a few tens of atoms thick, would appear to support

this idea.

Towards routine manufacture at the nano-scale

NEMS must overcome a final important hurdle before nano-scale machines, sensors and

electronics emerge from industrial production lines. Put simply, when they combine state-of-the-art processes from two disparate fields - nanolithography and MEMS micromachining - they

increase the chances that something will go awry during manufacturing. Fortunately, sustained

and careful work is beginning to solve these problems and is revealing the way to build robust,

reliable NEMS. Given the remarkable success of microelectronics, it seems clear that such

current troubles will ultimately become only of historical significance.

But there is a special class of difficulties unique to NEMS that cannot be so easily dismissed.

NEMS can respond to masses approaching the level of single atoms or molecules. However, this

sensitivity is a double-edged sword. On the one hand it offers major advances in mass

spectrometry; but it can also make device reproducibility troublesome, even elusive. For

example, at Caltech they have found that it places extremely stringent requirements on the

cleanliness and precision of nanofabrication techniques.

Advantages

NEMS is a rapidly growing technology for the fabrication of miniature devices using processes

similar to those used in the integrated circuit industry. NEMS technology provides a way to

integrate mechanical, fluidic, optical, and electronic functionality on very small devices, ranging

from 0.1 nanons to one millimeter. NEMS devices have several important advantages over

conventional counterparts.

Cost effectiveness

Like integrated circuits, they can be fabricated in large numbers, so that cost of production can


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be reduced substantially. They can be directly incorporated into integrated circuits; so that far

more complicated systems can be made than with other technologies. NEMS is an extremely

diverse technology that potentially could significantly impact every category of products.

Already, NEMS is used for everything ranging from neural probes to active suspension systems

for automobiles. The nature of NEMS technology and its diversity of useful applications make it

potentially a far more pervasive technology than even integrated circuit nano-chips.

System Integration

NEMS blurs the distinction between complex mechanical systems and integrated circuit

electronics. Historically, sensors and actuators are the most costly and unreliable part of a macro

scale sensory-actuator-electronics system. In comparison, NEMS technology allows these

complex electromechanical systems to be manufactured using batch fabrication techniques

allowing the cost and reliability of the sensors and actuators to be put into parity with that ofintegrated circuits.

High Precision

NEMS-based switches must be extremely reliable to meet the standards and requirements of

optical telecommunications networks they must remain in precise position over millions of

operations, and they must be designed to meet stringent environmental specifications involving

temperature and vibration. However, there is a high degree of confidence that mechanical NEMS

devices can meet these requirements, as similar devices based on the same manufacturingprocesses have proven to be exceedingly robust in the automotive, military and aerospace

industries.

Small size

NEMS based devices are extremely small in size because of the large scale integration of the

nano electronics and the mechanical systems which include sensors and actuators. NEMS

devices can be so small that hundreds of them can fit in the same space as one single macro-

device that performs the same function. Cumbersome electrical components are not needed,since the electronics can be placed directly on the NEMS device. This integration also has the

advantage of picking up less electrical noise, thus improving the precision and sensitivity of

sensors.

Applications of NEMS


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Ultimately, NEMS could be used across a broad range of applications. At Caltech we have used

NEMS for metrology and fundamental science, detecting charges by mechanical methods and in

thermal transport studies on the nano-scale .In addition, a number of NEMS applications are

being pursued that might hold immense technological promise.

In my opinion, most prominent among these is magnetic resonance force microscopy (MRFM).

Nuclear magnetic resonance was first observed in 1946 by Edward Purcell, Felix Bloch and their

collaborators, and is now routinely used for medical imaging. The technique exploits the fact that

most nuclei have an intrinsic magnetic moment or "spin" that can interact with an applied

magnetic field. However, it takes about 1014-1016 nuclei to generate a measurable signal. This

limits the resolution that can be attained in state-of-the-art magnetic resonance imaging (MRI)

research laboratories to about 10 m. Meanwhile, the typical resolution achievable in

hospitals is about 1 mm.One would assume then that the detection of individual atoms using MRI is only a distant dream.

However, in 1991 John Sidles of the University of Washington at Seattle proposed that

mechanical detection methods could lead to nuclear magnetic resonance spectrometry that would

be sensitive to the spin of a single proton. Achieving this degree of sensitivity would be a truly

revolutionary advance, allowing, for example, individual bimolecules to be imaged with atomic-

scale resolution in three dimensions.

Magnetic resonance force microscopy (MRFM) could thus have an enormous impact on many

fields, ranging from molecular biology to materials science. The technique was first

demonstrated in 1992 by Dan Rugar and co-workers at IBM's Almaden Research Center, and

was later confirmed by Chris Hammel at the Los Alamos National Lab in collaboration with my

group at Caltech, and others.

Like conventional magnetic resonance, MRFM uses a uniform radio-frequency field to excite the

spins into resonance. A nano-magnet provides a magnetic field that varies so strongly in space

that the nuclear-resonance condition is satisfied only within a small volume, which is about the

size of atom. This magnet also interacts with the resonant nuclear spins to generate a tiny "back

action" force that causes the cantilever on which the nano-magnet is mounted to vibrate. For a

single resonant nucleus, the size of this force is a few attonewtons (10-18 N) at the most.

Nonetheless, Thomas Kenny's group at Stanford, in collaboration with Rugar's group at IBM, has

demonstrated that such minute forces are measurable.


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By scanning the tip over a surface, a 3-D map of the relative positions of resonating atoms can be

created. Although Rugar and co-workers detected a signal from some 1013 protons in their early

experiments, the sensitivity still exceeded that of conventional MRI methods.

In another area of research, Clark Nguyen and co-workers at the University of Michigan are

beginning to demonstrate completely mechanical components for processing radio-frequency

signals.

With the advent of NEMS, several groups are investigating fast logic gates, switches and even

computers that are entirely mechanical. The idea is not new. Charles Babbage designed the first

mechanical computer in the 1820s, which is viewed as the forerunner to the modern computer.

His ideas were abandoned in the 1960s when the speed of nanosecond electronic logic gates and

integrated circuits vastly outperformed moving elements. But now that NEMS can move on

timescales of a nanosecond or less, the established dogma of the digital electronic age needscareful re-examination.

Thermal actuator is one of the most important NEMS devices, which is able to deliver a large

force with large displacement, thus they have found various applications in electro-optical-

communication, micro-assembly and micro-tools. Currently Si-based materials have been

predominantly used to fabricate thermal actuators due to its mature process and stress-free

materials.

Thermal actuators based on metal materials generally have a number of advantages over Si-

based ones due to their large thermal expansion coefficients, thus they can deliver large

displacements and forces and consumes less power, and therefore they are much more efficient

than Si-based ones.

We have developed a single-mask NEMS process based on Si-substrate and electroplated Ni

active materials. Various thermal actuators and their enabled microsystems have been fabricated

and electrically tested.

Biotechnology

NEMS technology is enabling new discoveries in science and engineering such as the

Polymerase Chain Reaction (PCR) nano systems for DNA amplification and identification, nano

machined Scanning Tunneling Nano-scopes (STMs), biochips for detection of hazardous

chemical and biological agents, and nano systems for high-throughput drug screening and


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selection.

Accelerometers

NEMS accelerometers are quickly replacing conventional accelerometers for crash air-bag

deployment systems in automobiles. The conventional approach uses several bulky

accelerometers made of discrete components mounted in the front of the car with separate

electronics near the air-bag; this approach costs over $50 per automobile.

NEMS technology has made it possible to integrate the accelerometer and electronics onto a

single silicon chip at a cost between $5 and $10. These NEMS accelerometers are much smaller,

more functional, lighter, more reliable, and are produced for a fraction of the cost of the

conventional macro scale accelerometer elements

Nano nozzles

Another wide deployment of NEMS is their use as nano nozzles that direct the ink in inkjet

printers. They are also used to create miniature robots (nano-robots) as well as nano-tweezers,

and are used in video projection chips with a million moveable mirrors.

NEMS have been rigorously tested in harsh environments for defense and aerospace where they

are used as navigational gyroscopes, sensors for border control and environmental monitoring,

and munitions guidance. In medicine they are commonly used in disposable blood pressure

transducers and weighing scales.

NEMS in Wireless

Wireless system manufacturers compete to add more functionality to equipment. A 3G smart

phone, PDA, or base station, for example, will require the functionality of as many as five radios

for TDMA, CDMA, 3G, Bluetooth and GSM operation. A huge increase in component

count is required to accomplish this demand.

A solution with tighter and cost-effective integration is clearly needed. Integrating NEMS

devices directly on the RF chip itself or within a module, can enable the replacement ofnumerous discrete components while offering such competitive benefits as higher performance

and reliability, smaller form factors, and lower cost as a result of high-volume, high-yield IC-

compatible processes. Discrete passives such as RF-switches, varicaps, high-Q resonators and

filters have been identified as components that can be replaced by RF-NEMS counterparts.


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Current technology and process limitations will prevent placement of all passive components

with on-chip NEMS components. But placing even some components on-chip offers significant

space and cost savings, allowing smaller form factors, benefiting cell phones for example, or

added functionality such as Internet connectivity

NEMS in Optical Networks

An important new application for NEMS devices is in fiber optic networks. At the nanons level,

NEMS-based switches route light from one fiber to another. Such an approach enables a truly

photonic (completely light-based) network of voice and data traffic, since switching no longer

requires conversion of light signals into digital electronic signals and then back to optical.This is

important because switching using optical-electrical-optical (OEO) conversion can often cause

substantial bottlenecks, preventing the realization of truly broadband networks. But NEMS and

nano machined devices can be used as more than switches in the optical network. Additionalapplications include active sources, tunable filters, variable optical attenuators, and gain

equalization and dispersion compensation devices.

The result is an end-to-end photonic network which is more reliable and cost-effective, and

which has minimal performance drop-off. However the development of an all-optical network

has been complex and challenging due to the integration of optics, mechanics and electronics.

Drawbacks

Nano-electro-mechanical systems (NEMS) offer designers the potential to make the optical

network of the future possible, but some things need to change before the idea becomes a reality.

Although manufacturers are now introducing a wide range of NEMS-based products into the

optical networks market, the technology has drawbacks, and NEMS developers have found

shepherding NEMS devices from the laboratory to the marketplace a costly and time-consuming

operation. The problem lies not with the NEMS devices themselves, but with the semiconductor-

based manufacturing techniques deployed to build them. Semiconductor wafer fabs excel atproducing high-volume integrated circuits using standard CMOS processing. NEMS devices

need to be manufactured in lower volumes, however, and with far more complex structures, such

as moving three-dimensional nano mirrors instead of planar transistors.

NEMS technology is currently used in low- or medium-volume applications. Some of the


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obstacles preventing its wider adoption are:

Limited Options

Most companies who wish to explore the potential of NEMS technology have very limited

options for prototyping or manufacturing devices, and have no capability or expertise in nanofabrication technology. Few companies will build their own fabrication facilities because of the

high cost. A mechanism giving smaller organizations responsive and affordable access to NEMS

fabrication is essential.

Packaging

The packaging of NEMS devices and systems needs to improve considerably from its current

primitive state. NEMS packaging is more challenging than IC packaging due to the diversity of

NEMS devices and the requirement that many of these devices be in contact with theirenvironment. Currently almost all NEMS development efforts must develop a new and

specialized package for each new device. Most companies find that packaging is the single most

expensive and time consuming task in their overall NEMS product development program. As for

the components themselves, numerical modeling and simulation tools for NEMS packaging are

virtually non-existent. Approaches which allow designers to select from a catalog of existing

standardized packages for a new NEMS device without compromising performance would be

beneficial.

Fabrication Knowledge Required

Currently the designer of a NEMS device requires a high level of fabrication knowledge in order

to create a successful design. Often the development of even the most mundane NEMS device

requires a dedicated research effort to find a suitable process sequence for fabricating it. NEMS

device design needs to be separated from the complexities of the process sequence.

To the quantum limit - and beyond

The ultimate limit for nano-mechanical devices is operation at, or even beyond, the quantum

limit. One of the most intriguing aspects of current nano-mechanical devices is that they are

already on the verge of this limit. The key to determining whether NEMS are in this domain is

the relationship between the thermal energy, kBT, and the quantity hf0, where kB is the

Boltzmann constant, h is the Planck constant, f0 is the fundamental frequency of the mechanical

resonator and T is its temperature.


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When the temperature of the device is low and its frequency is sufficiently high that hf0 greatly

exceeds kBT, then any thermal fluctuations will be smaller than the intrinsic quantum noise that

affects the lowest vibration mode. In this limit, the mean square amplitude of the vibration can be

quantized and can only assume values that are integral multiples of hf0Q/2keff. A full

exploration of this quantum domain must wait for crucial technological advances in ultra

sensitive transducers for NEMS that will enable us to measure tiny displacements at microwave

frequencies.

In spite of this significant challenge, we should begin to see signs of quantum phenomena in

nano-mechanical systems in the near future. Even the first NEMS resonators produced back in

1994 operated at sufficiently high frequencies that, if cooled to 100 mK, only about 20 vibration

quanta would be excited in the lowest fundamental mode. Such temperatures are readily reached

using a helium dilution refrigerator. So the question that comes to mind is whether quantizedamplitude jumps can be observed in a nano-scale resonating device? If so, one should be able to

observe discrete transitions as the system exchanges quanta with the outside world. At this point,

the answer to the question seems to be that such jumps should be observable if two important

criteria can be met. The first is that the resonator must be in a state with a definite quantum

number. In general, transducers measure the position of the resonator, rather than the position

squared. The continual interaction between such a "linear transducer" and the quantum system

prevents the resonator from being in a state characterized by a discrete number of quanta.

Transducers that measure the position squared were discussed in 1980 by Carlton Caves, now at

the University of New Mexico, and co-workers at Caltech in a pioneering paper on quantum

measurements with mechanical systems and it now seems possible to transfer their ideas to

NEMS.

The second criterion is more problematic. The transducer must be sensitive enough to resolve a

single quantum jump. Again, ultrahigh sensitivity to displacements is the key needed to unlock

the door to this quantum domain. A simple estimate shows that we must detect changes in the

mean square displacement as small as 10-27 m2 to observe such quantum phenomena. Is it

possible to achieve this level of sensitivity? A group at Caltech has recently made significant

progress towards new ultra-sensitive transducers for high-frequency NEMS - and they are

currently only a factor of 100 or so away from such sensitivity.

In related work, Keith Schwab, Eric Henriksen, John Worlock are investigated the quantum


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limit, where hf0 >> kT, for the first time in thermal-transport experiments using nano-scale

beams fabricated from silicon nitride. When the temperature is lowered, fewer and fewer of

vibration (or phonons) remain energetically accessible. Effectively, this means that most of them

cannot participate in thermal transport. Indeed, in a beam that is small enough, only four phonon

modes can transport energy between the system and its surroundings.

We found that the thermal conductance in this regime becomes quantized. In other words, each

phonon mode that transports energy can only provide a maximum thermal conductance given by

k2T/6h. Quantum mechanics thus places an upper limit on the rate at which energy can be

dissipated in small devices by vibrations.

In spite of the complications encountered at the quantum level, the rewards in terms of intriguing

physics will be truly significant. Force and displacement measurements at this limit will open

new horizons in science at the molecular level, new devices for quantum computation, and thepossibility of being able to control the thermal transport by individual phonons between nano-

mechanical systems or between a system and its environment.

Future outlook

NEMS offer unprecedented and intriguing opportunities for sensing and fundamental

measurements. Both novel applications and fascinating physics will undoubtedly emerge from

this new field, including single-spin magnetic resonance and phonon counting using mechanical

devices.But there remains a gap between today's NEMS devices that are sculpted from bulk materials

and those that will ultimately be built atom by atom. In the future, complex molecular-scale

mechanical devices will be mass-produced by placing millions of atoms with exquisite precision

or by some form of controlled self-assembly. This will be true nanotechnology. Nature has

already mastered such remarkable feats of atomic assembly, forming molecular motors and

machinery that can transport biochemical within cells or move entire cells.

Clearly, to attain such levels of control and replication will take sustained effort, involving a host

of laboratories. Meanwhile, in the shorter term, NEMS are clearly destined to provide much of

the crucial scientific and engineering foundation that will underlie future nanotechnology.

Nano Electro Mechanical Devices (NEMS) involve the relative motion of one interface past a

second. The properties of this interface, including its electrical, mechanical and tribological

characteristics, ultimately depend on the arrangement of the atoms. Recently, we have shown


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how the alignment of two atomic lattices has dramatic effects on the friction and dynamics of the

objects in contact. Through atomic force microscopy manipulation, we have shown the carbon

nano-tubes show the full range of dynamics including sliding and rolling. On graphite, the

atomic lattices can come into registry, and the interlocking atoms cause the nano-tube to roll.

The atomic lattices also dictate the electronic states at the interface. We have measured the

electrical properties of atomic lattices in contact and show a change in the contact resistance of

over one decade as the lattices move in and out of registry. The further implications of the

mechanical and electrical properties of contacting lattices in NEMS devices will be explored,

including applications in actuators, encoders and oscillators.

We focus on the exploration of NEM-physics and the development of NEM-devices that can be

used as extremely sensitive sensors for force and mass detection down to the single molecule

level, as high-frequency resonators up to the GHz range, or as ultra-fast, low-power switches.Both a top-down and bottom-up approach is followed. The top-down approach consists of

scaling down the existing micron-size MEMS technology far into the sub-100 nm range. In the

bottom-up approach suspended structures of single-walled carbon nano-tubes and of (semi

conducting) nano-wires are fabricated. In particular, (new) mechanisms for detection of

displacements and eigen frequencies are studied with the goal to reveal the physical processes

(e.g. damping, thermal effects, and momentum noise) that limit the sensitivity of the devices.

Novel optical and magnetic detection schemes need to be investigated.

The search for the limits of mechanical motion is a central theme. At low temperature, quantum

friction starts to limit the Q-factor and vibrating NEM-devices are limited by zero-point motion.

This quantum limitation poses an ultimate limit to sensitivity of NEM-devices. In addition, other

quantum phenomena are expected to be present. Quantum optics-like experiments with phonons,

phonon lasers or quantum-tunneling experiments with massive objects (strained suspended nano-

tubes placed between two gate electrodes) are just a few examples. As the size of NEM-devices

shrinks down, electron-phonon coupling translates into an increasingly strong interplay between

electrical and mechanical degrees of freedom. Device operation results in charge distributions

that are inhomogeneous on the nanometer scale, giving rise to Coulomb forces that are strong

enough to change device geometry. The classical theory of elasticity breaks down and the regime

of quantum elasticity has been entered.

Current projects involve Coulomb blockade and noise properties (quantum transport) of single-


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wall nano-tubes, mixing experiments to detect the guitar-like modes of SWNTs and the

fabrication of a SET in the vicinity of a suspended SWNT to detect its motion. Singly-clamped

semi conducting nano-wires are used as switches with the goal to fabricate nano-mechanical

shuttles.

Conclusion

Nano-systems have the enabling capability and potential similar to those of nano-processors in

the 1970s and software in the 1980s.Since NEMS is a nascent and synergistic technology, many

new applications will emerge, expanding the markets beyond that which is currently identified or

known. As breakthrough technology allowing unparalleled synergy between hitherto unrelated

fields of endeavor such as biology and nano-electronics, NEMS is forecasted to have growth

similar to its parent IC technology. For a great many applications, NEMS is sure to be the

technology of the future.

References

Mohamed Gad-el-Hak, ed., The MEMS Handbook, CRC Press 2001, ISBN 0-8493-0077-0

P. Rai-Choudhury, ed., Handbook of Microlithography, Micromachining, and Microfabrication,

Vol 1 and Vol 2, SPIE Press and IEE Press 1997, ISBN 0-8529-6906-6 (Vol 1) and 0-8529-

6911-2 (Vol 2)

Julian W. Gardner, and Vijay K. Varadan, and Osama O. Awadelkarim, Microsensors, MEMS

and Smart Devices, Wiley 2001, ISBN 0-4718-6109-X

Nadim Maluf, An Introduction to Micro-electro-mechanical Systems Engineering, Artech House

1999, ISBN 0-8900-6581-0

http://www.foresight.org

http://www.physicsweb.org

http://www.nemsnet.org

http://www.menet.umn.edu

http://www.nemsrus.comhttp://www.sandia.gov

http://www.elearning.stut.edu

http://www.allaboutnems.com

http://www.embedded.com


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http://www.ee.ttu.edu/nems

http://www.nems-exchange.org

http://www.optics.caltech.edu

http://www.ece.ucdavis.edu

Acknowledgment

I would like to place on record my deep sense of gratitude to Mr.PURUSHOTHAMAN Head of

Department of Electronics & communication, Vimal Jyothi Engineering College for his valuable

help and guidance in carrying out the seminar.

I also thank all the staff of The Department Electronics & Communication for their assistance

and encouragement through out the course of the seminar.

Last, but not the least I would like to thank my parents and friends who encouraged me and gave

me the motivation to complete the seminar.

Above all I would like to thank God for His abundant grace upon my seminar.

CONTENTS

1. Introduction

2. What is an Electro-Mechanical System?

3. What is a Micro Electro-Mechanical System?

4. The First MEMS Device

5. The benefits of Nano-machines

6. The benefits of Nano-machines

7. How to make NEMS

a. Fabrication

b. Deposition Processes

c. Chemical Vapour Deposition (CVD)

d. Epitaxye. Lithography

f. Alignment

g. Exposure

h. Etching


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8. Challenges for NEMS

9. Advantages

10. Applications of NEMS

11. Drawbacks

12. Future outlook

13. Conclusion

14. References

Reference: http://www.seminarprojects.com/Thread-nano-electromechanical-systems-full-

report#ixzz1J3fzvBHc

nems vinay repot

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