Somulu Seminar Report on Nems

8/9/2019 Somulu Seminar Report on Nems

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Seminar Report - - 1 - - NEMS

Dept of Electronics & Compute Science VNIT

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 integrate mechanical,

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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CONTENTS

1. Introduction2. What is an Electro-Mechanical System?3. What is a Micro Electro-Mechanical System?4. The First MEMS Device5. The benefits of Nano-machines6. The benefits of Nano-machines7. How to make NEMS

a. Fabricationb. Deposition Processesc. Chemical Vapour Deposition (CVD)d. Epitaxye. Lithographyf. Alignmentg. Exposureh. Etching

8. Challenges for NEMS9. Advantages

10. Applications of NEMS11. Drawbacks12. Future outlook13. Conclusion14. References


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Introduction

Nano-Electro-Mechanical Systems (NEMS) is the integration of

mechanical elements, sensors, actuators, and electronics on a commonsilicon 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 capabilitydirect 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 of smart

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


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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-Mechanical System?

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 ascapacitor plates, converting a difference in charge between them to an

attractive force. The 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 typicallyhas 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 response to 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 byCoulomb), 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


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be used to minimize vibrational losses, in much the same way that the

handle of a tuning fork is positioned carefully to reduce losses.

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, as well as electrostatics and optics.

What is a Micro Electro-Mechanical System?

MEMS are an abbreviation for Micro Electro Mechanical Systems.

This is a rapidly emerging technology combining electrical, electronic,


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mechanical, optical, material, chemical, and fluids engineering 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 across.

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


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"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 the devices, 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 comb drive can be moved "in" or

"out" and either DC or AC voltages can be applied. The magnitude of


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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 Device

In 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 of the first MEMS device


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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:

y Requires a system of units applicable to small geometric scale.y Ability to handle unique material properties that are not in the

public domain.

y Ability to mesh high aspect ratio device geometry.y Lumped parameter extraction & reduced order macro modeling for

system level simulation.

y Ability to model large field domains associated with electromagneticand CFD.

The benefits of Nano-machines

Nano-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


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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 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 computers.

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 extremely sensitive 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.


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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 useful

for 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 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.


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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

insurmountable problem 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.


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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

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 and sacrificial layers on a substrate.


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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 with various 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 thin films 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:

y Deposition processesy Lithographyy Etching processes


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Deposition 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 (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


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Figure 1: Typical hot-wall LPCVD reactor

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 to process,

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

material with 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%


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of the melting point of the material 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

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


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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 the unexposed 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 high temperatures and may act as a source of

contamination.


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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 processingprogresses. 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 hole in a silicon wafer in a


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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, sincethe 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 semiconductor nano-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


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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 MEMS community 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


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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.

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 andelectrical 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.


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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 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.


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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.


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Cost effectiveness

Like integrated circuits, they can be fabricated in large numbers, so

that cost of production can 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 of integrated 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


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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 manufacturing processes 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

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


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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 DanRugar 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


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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.

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 needs careful re-

examination.


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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


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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 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.


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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.


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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 of numerous 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. 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 onefiber 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 inthe optical network. Additional applications include active sources,

tunable filters, variable optical attenuators, and gain equalization and

dispersion compensation devices.


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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 at producing 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.


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NEMS technology is currently used in low- or medium-volume

applications. Some of the 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 nano fabrication

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 andthe requirement that many of these devices be in contact with their

environment. 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 allowdesigners to select from a catalog of existing standardized packages for a

new NEMS device without compromising performance would be beneficial.

Fabrication Knowledge Required


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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.

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.


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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 quantized amplitude 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


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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 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 the possibility 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


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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 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


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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


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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-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.


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Conclusion

Nano-systems have the enabling capability and potential similar to

those of nano-processors in the 1970s and software in the 1980s.SinceNEMS 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.


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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.com

http://www.sandia.gov

http://www.elearning.stut.edu

http://www.allaboutnems.com

http://www.embedded.com

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

http://www.nems-exchange.org

http://www.optics.caltech.edu

http://www.ece.ucdavis.edu


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Documents

Somulu Seminar Report on Nems