nano electromechanical systems full report
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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.
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 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 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 as capacitor 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 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 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 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 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, 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 "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 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
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:
Â¢ Requires a system of units applicable to small geometric scale.
Â¢ Ability to handle unique material properties that are not in the public domain.
Â¢ Ability to mesh high aspect ratio device geometry.
Â¢ Lumped parameter extraction & reduced order macro modeling for system level simulation.
Â¢ Ability to model large field domains associated with electromagnetic and 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 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 â„¢lâ„¢ 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.
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.
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.
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.
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.
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:
Â¢ Deposition processes
Â¢ Etching 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)
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 600Ã‚Â° C) and the relatively slow deposition rate. The PECVD process can operate at lower temperatures (down to 300Ã‚Â° 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
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.
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 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 (>100Ã‚Âµm). 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 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 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).
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.
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.
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.
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.
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.
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 <111> crystal plane sidewalls that appear when etching a hole in a <100> 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.
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 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 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 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 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 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.
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.
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.
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.
NEMS-based switches must be extremely reliable to meet the standards and requirements of optical telecommunications networks â€œ 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 manufacturing processes have proven to be exceedingly robust in the automotive, military and aerospace industries.
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 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 Ã‚Âµm. 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.
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.
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.
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.
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
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 project and implimentationion 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 â€œ 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 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 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. Additional applications 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.
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.
NEMS technology is currently used in low- or medium-volume applications. Some of the obstacles preventing its wider adoption are:
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.
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 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 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.
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 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 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 Ã‚Â¼k2T/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.
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 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 project and implimentations 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.
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.
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
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 and presentation.
I also thank all the staff of The Department Electronics & Communication for their assistance and encouragement through out the course of the seminar and presentation.
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 and presentation.
Above all I would like to thank God for His abundant grace upon my seminar and presentation.
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
b. Deposition Processes
c. Chemical Vapour Deposition (CVD)
8. Challenges for NEMS
10. Applications of NEMS
12. Future outlook
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Nano-Electro-Mechanical system (NEMS) is the integration of mechanical elements, sensors, actuators and electronics on a common silicon substrate.
The Nano mechanical components are fabricated using compatible “micromachining” process.
NEMS is the enabling technology allowing the development of smart products.
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NANO ELECTROMECHANICAL SYSTEMS
ITS IMPLICATIONS IN THE FIELD OF
MILITARY and DEFENCE
NEMS( nano electro mechanical systems) a sophisticated branch of technology which is capable of realizing all the long envisioned goals of modern science through the help of already nurtured branches ,MEMS(micro electro mechanical systems) and nano technology.
Briefing about MEMS and NEMS we can say that MEMS is a new and exciting area in mechanical engineering which uses the technology developed in the fabrication of integrated circuits in order to make micro-scale mechanical devices. Where NEMS requires circuits that are to be fabricated on nano scale. Major requirements of these nano devices to be met are preconditioned in order to attain optimization of the resources used.
Nanotechnology is likely to be extremely important in the future as it allows materials to be built up atom by atom. This can eventually lead to the development of new materials that are better suited for the current requirements. .The list of materials being developed commercially using nanotechnology is likely to grow at a very fast rate. one such advancement using nano technology is its application in the field of electromechanical systems and hence giving it a name “NANO ELECTRO MECHANICAL SYSTEMS”(NEMS).
What is NEMS and where does it originate from?
Electro mechanical systems (EMS), regardless of scale, generally consist of two integrated components. The mechanical parts include moveable structures such as mirrors, beams, cantilevers and gears which respond to applied forces by deflection or vibration. The electronic elements, such as small motors and integrated circuits act as transducers to transform mechanical motion into optical or electrical signals and vice versa. Scaling these down, there are two distinct levels of EMS: MEMS and NEMS
MEMS represent the marriage of semiconductor processing to mechanical engineering - at a very small scale. And it is a field that has grown enormously during the past decade. MEMS have been studied for decades and are now finding increasing application in industrial and commercial sectors . While MEMS technology is technically outside the nanotechnology domain, we include it as many researchers consider it as an integral part of the nanotechnology field . Indeed, it is expected that the functionalities shown by MEMS could also be performed by NEMS . So while there are some distinctions the technical maturity of MEMS provides some short-term potential and some insights into long-term trajectories for NEMS.
NEMS or nano electro mechanical systems are similar to MEMS or micro electro mechanical systems but smaller. They hold promise to improve abilities to measure small displacements and forces at a molecular scale, and are related scale
How is it possible to work with NEMS?
NEMS is defined as as including the integration of sensors, actuators electronics, photonics, energy, fluidics, chemistry, and biology into a meaningful system enabled by submicrometer science and engineering precision. We also need to think about scaling. For example, what happens when the electrical domain is scaled downing size? And how do we address the challenges associated with ultrafine scaling in the mechanical, optical, chemical, fluidic, and biological domains? Let’s explore some of the exciting new opportunities in NEMS. The Age of NEMS and its products are surely unpredictable, but let me mention five exciting and compelling potential applications. There are important parameters that are making this impractical science highly demanding.
Higher natural frequency of mechanical parts of NEMS which means a faster response to applied forces;•
• Higher quality or Q factor of resonance, which results in lower energy consumption and suppressed thermo-mechanical noise; and
• Smaller effective mass of the vibrational parts, which gives extremely high sensitivity
The nanoparticles encode specific information such as the identity of a type of bio molecule that might be attached to its surface. And, best of all ,the structure is made using very simple MEMS processing methods
Why nano technology in military?
The possible applications of nanotechnology to advanced weaponry are fertile ground for fantasy. It is obvious that three-dimensional assembly of nanostructures in bulk can yield much better versions of most conventional (nonnuclear) weapons; e.g., guns can be lighter, carry more ammunition, fire self-guided bullets, incorporate multispectral gunsights or even fire themselves when an enemy is detected.
Certainly, nanotechnology offers many colorful possibilities for creative mass murder. For example, for some reason one of the most frequent flights of fancy is the programmable genocide germ that replicates freely and kills people who have certain DNA patterns. Such a weapon is possible by use of NEMS/MEMS.
It is easy to kill. War is a contest to suppress your enemy's capabilities before he can suppress yours. This doesn't leave much room for fancy swordplay or gothic revenge scenarios in serious combat. An actual nanotechnic war, if one ever occurs, is likely to be inhumanly fast and enormously destructive. Clever tactics and nifty gadgets are irrelevant if your enemy can simply blow you up. Thus there is a need nano gadgets
What is role of NEMS in defence?
Other forms of nanotechnology being developed include tiny sensors called nano-units, of which some simple types are available: "smart materials" that change in response to light or heat; "nano-bots" - tiny mobile robots that have yet to be developed but are theoretically possible; and self-assembling nano-materials that can be assembled into larger equipment
first and second generation weapons being atomic and hydrogen bombs which gave a base for neutron bomb of third generation which never found a permanent place in the military arsenals. where as Fourth-generation nuclear weapons are new types of nuclear explosives that can be developed in full compliance with the Comprehensive Test Ban Treaty (CTBT) using inertial confinement fusion (ICF) facilities such as the NIF in the US defining technical characteristic of fourth-generation nuclear weapons is the triggering - by some advanced technology such as a superlaser, magnetic compression, antimatter, etc. - of a relatively small thermonuclear explosion in which a deuterium-tritium mixture is burnt in a device whose weight and size are not much larger than a few kilograms and litres. Hence there is a need for a technology which is more economical in using its resources .So here comes the need for nano devices which can accomplish the specified requirements. All this can be made possible by use of advanced technology NEMS. As we know nanotechnology and micromechanical engineering are integral parts of ICF pellet construction. But this is also the case with ICF drivers and diagnostic devices, and even more so with all the hardware that will have to be miniaturized and 'ruggedised' to the extreme in order to produce a compact, robust, and cost-effective weapon.
Since these new weapons will use no (or very little) fissionable materials, they will produce virtually no radioactive fallout. Their proponents will define them as "clean" nuclear weapons - and possibly draw a parallel between their battlefield use and the consequences of the expenditure of depleted uranium ammunition.
The technological advancements in the field of Military and defence have reached to a level to which we can make our defence techniques more reliable and improvised
• Improved bulletproof vests that can harden or soften as necessary are developed. Which make its fibers waterproof and germ-proof. That sort of advance will give soldiers stronger protection with less weight.
• Researchers have developed hollow fibers about 100 microns wide, and filled them with hollow beads that contain magnetic particles about 10 nanometers long. When exposed to a magnetic field, which the soldier could trigger, the beads instantly line up to make the fabric about 50 times stiffer or stronger than it normally is. Soldiers would activate dynamic armor when they hear gunfire, or after bullets puncture the skin.
• The prospect of revolutionary advances in military capabilities will stimulate competition to develop and apply the new technologies toward war preparations, as falling behind would imply an intolerable security risk. Indeed, it is plausible that a nation which gained a sufficient lead in molecular nanotechnology would at some point be in a position to simply disarm any potential competitors.
• Micro/Nano Electro Mechanical Systems (MEMS/NEMS) technology is being applied at planetary level to develop novel devices and instruments for In-Situ Planetary Exploration. Electron-beam excited x-ray fluorescence of materials is enabled in air, in the Atmospheric Electron X-ray Spectrometer (AEXS), by means of a micro-fabricated, electron-transmissive membrane. (200-nm)-thick
Scope of NEMS in various fields:
Today’s world accelerating at a tremendous pace have influenced all the aspects of life. ”. Our scientific and technological advancements have transformed our planet to a highly sophisticated place .With all these updated and enhanced technological implementations , life has become a cakewalk to us. We have now reached to a point where “Nothing is impossible.” These remarkable changes are possible because of major breakthroughs in the fields of science and technology one of the exciting areas of research being NEMS .scope in this field is varied
Scope in medicine :
Consider what we might be able to do with a library of over ten thousand distinct nano barcodes, each of which has each a unique biomolecule on its surface important to understanding an individual’s current health condition. We would have a powerful system enabling a multi-analyte bioanalysis capability that can identify predisposition and early exposure to a variety of diseases. But such a multiplexed bioanalysis system might also tell you how a medication is addressing your own individual symptoms, a concept called precision medicine.
Scope in military and defence:
Let’s take this technology one step further onto the battlefield, where it could provide a means for rapidly screaming soilder to biological agents through the best sensor possible—the response of the human body. NEMS will also enable other important new opportunities in the emerging field of nano biotechnology.
NEMS have also become an evolving science widening its scope into all the inter related areas of various sciences.
In order for nanotechnology to evolve and become commercialized, more attention must be given to applications. Successful applications need to be established that allow industry engineers and designers to gain insight into ways this technology can be used today. As the techniques evolve for effective and efficient production of nano materials and devices, the industry will begin to integrate this technology into their products and Introduction services.
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Nanoelectromechanical systems, or NEMS, are MEMS scaled
to submicron dimensions . In this size regime, it is possible to
attain extremely high fundamental frequencies while simultaneously
preserving very high mechanical responsivity (small force
constants). This powerful combination of attributes translates
directly into high force sensitivity, operability at ultralow power,
and the ability to induce usable nonlinearity with quite modest
control forces. In this overview I shall provide an introduction to
NEMS and will outline several of their exciting initial applications.
However, a stiff entry fee exists at the threshold to this new
domain: new engineering is crucial to realizing the full potential of
NEMS. Certain mainstays in the methodology of MEMS will,
simply, not scale usefully into the regime of NEMS.
NEMS have a host of intriguing attributes. They offer access
to fundamental frequencies in the microwave range; Q’s, i.e.
mechanical quality factors, in the tens of thousands (and quite
possibly much higher); active masses in the femtogram range; force
sensitivities at the attonewton level; mass sensitivity at the level of
individual molecules, heat capacities far below a “yoctocalorie” 
— this list goes on. These attributes spark the imagination, and a
flood of ideas for new experiments and applications ensues.
MULTITERMINAL MECHANICAL DEVICES
The attributes of NEMS described in the next section make
clear that we should be envisioning applications for
electromechanical devices with response times and operating
frequencies that are as fast as most of today’s electron devices.
Furthermore, multiterminal electromechanical devices are possible
F i.e. two-, three-, four-ports, etc. F LQ ZKLFK HOHFWURPHFKDQLFDO
transducers provide input stimuli (i.e. signal forces), and read out a
mechanical response (i.e. output displacement). At additional
control terminals, electrical signals either quasi-static or timevarying
can be applied, and subsequently converted by the
control transducers into quasi-static or time-varying forces to
perturb the properties of the mechanical element in a controlled,
useful manner. The generic picture of this scheme is shown in
Frequency. Table 1 displays attainable frequencies for the
fundamental flexural modes of thin beams, for dimensions spanning
the domain from MEMS (leftmost entries) to deep within NEMS.
The mode shapes, and hence the force constants and resulting
frequencies, depend upon the way the beams are clamped; Table 1
lists the results for the simplest, representative boundary conditions
along three separate rows. The last column represents dimensions
currently attainable with advanced electron beam lithography. Of
course, even smaller sizes than this will ultimately become feasible;
clearly the ultimate limits are reached only at the molecular scale.
Nanodevices in this ultimate limit will have resonant frequencies in
the THz range, i.e. that characteristic of molecular vibrations.
Each entry is in three parts, corresponding to structures made
from silicon carbide, silicon, and gallium arsenide. These materials
are of particular interest to my group, and are among the
“standards” within MEMS. They are materials available with
extremely high purity, as monocrystalline layers in epitaxially grown
heterostructures. This latter aspect yields dimensional control in
the “vertical” (out of plane) dimension at the monolayer level. This
is nicely compatible with the lateral dimensional precision of
electron beam lithography that approaches the atomic scale. The
numbers should be considered loosely as “typical”; they represent
rough averages for the various commonly used crystallographic
NEMS offer access to a parameter space for sensing and
fundamental measurements that is unprecedented and intriguing.
Taking full advantage of it will stretch our collective imagination, as
well as our current methods and “mindsets” in micro- and
nanodevice science and technology. It seems certain that many new
applications will emerge from this new field. Ultimately, the
nanomechanical systems outlined here will yield to true
nanotechnology. By the latter I envisage reproducible techniques
allowing mass-production of devices of arbitrary complexity, that
comprise, say, a few million atoms í each of which is placed with
atomic precision . Clearly, realizing the “Feynmanesque”
dream will take much sustained effort in a host of laboratories.
Meanwhile, NEMS, as outlined here, can today provide the crucial
scientific and engineering foundation that will underlie this future