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Nanoelectromechanical systems (NEMS) are devices that integrate electrical and mechanical functions at the nanoscale.

Image: Image of anti-cancer nano-robots

NEMS is the stage of logical reduction after the so-called microelectromechanical systems or MEMS (microelectromechanical systems). NEMS typically combines transistor-like nanoelectronics with mechanical actuators, pumps, or motors and may form physical, biological, and chemical sensors. The name derives from the dimensions of a specific device in the nanometer range, which results in low mass, high mechanical resonance frequencies, large potential quantum mechanical effects such as zero-point motion, and a high surface-to-volume ratio useful for surface sensing mechanisms. Applications include accelerometers and sensors to detect chemicals in the air.

History

Background

As Richard Feynman noted in his famous 1959 speech, “There’s a lot of space at the bottom,” there are many potential uses for cars in smaller and smaller sizes. By building and controlling devices on a smaller scale, all technologies benefit. Expected benefits include greater efficiency and size reduction, reduced power consumption, and lower production costs in electromechanical systems.

In 1960, Muhammad M. Atala and Down Kahang built the first MOSFET with a gate oxide thickness of 100 nanometers at Bell Labs. In 1962, Atalla and Kahng developed a metal-semiconductor-based nanotubescope (M-S) that used 10-nanometer-thick gold (Au) thin films. In 1987, Bijan Davari led an IBM research team that demonstrated the first MOSFET with a thickness of 10 nm. Multi-gate MOSFETs enable scaling below the 20-nm channel length, starting with FinFET. FinFET originates from Hisamoto Boiler’s research at Hitachi Central Research Laboratory in 1989. At the University of California, Berkeley, a team led by Hisamoto and Chenming Hu of TSMC built FinFET devices up to a 17-nanometer channel in 1998.

NEMS

In 2000, the first very large-scale NEMS device was demonstrated by IBM researchers. It assumed an array of AFM tips that could heat/feel a deformable substrate to act as a memory device. More devices are described by Stephen Dian. In 2007, the International Semiconductor Technical Roadmap (ITRS) included NEMS memory as a new entry into the emerging research apparatus sector.

Atomic force microscope

One of the key applications of NEMS is atomic force microscope tips. The increase in sensitivity obtained by NEMS leads to smaller and more efficient sensors for detecting stresses, vibrations, forces at the atomic level, and chemical signals. AFM tips and other nanoscale diagnostics rely heavily on NEMS.

Minimization approaches

Two complementary approaches to building NEMS can be found. The top-down approach uses traditional microstructure methods, namely optical lithography, electron beam, and heat treatment, to produce the devices. While the resolution of these methods is limited, it allows you to have a great deal of control over the resulting structures. In this way, devices such as nanowires, nanorods, and patterned nanostructures are made of thin metal layers or engraved semiconductor layers. For top-down approaches, increasing the surface-to-volume ratio increases the reactivity of the nanomaterials.

In contrast, bottom-up approaches use the chemical properties of individual molecules to allow single-molecule components to self-organize or self-assemble into useful components or rely on situational assembly. These approaches use the concepts of molecular self-assembly and molecular detection. This allows the construction of much smaller structures, albeit often at the cost of limited construction process control. In addition, while residual material is removed from the main structure for the top-down approach, minimal material is removed or wasted for the bottom-up approach.

A combination of these approaches may also be used in which nanoscale molecules are integrated into a top-down framework. One such example is the carbon nanotube nanomotor.

Materials

Carbon allotropes

Many common materials used for NEMS technology are carbon-based, especially diamonds, carbon nanotubes, and graphene. This is mainly due to the beneficial properties of carbon-based materials that directly meet the needs of NEMS. The mechanical properties of carbon (such as the large Young modulus) are essential for NEMS stability. At the same time, the metal and semiconductor conductivity of carbon-based materials allows them to act as transistors.

Graphene and diamond show high Yang modulus, low density, low friction, very low mechanical loss, and large surface area. The low friction of CNTs allows for virtually friction-free bearings. It has been a major impetus for the practical applications of CNTs as components in NEMS, such as nanomotors, switches, and high-frequency oscillators. Carbon nanotubes and the physical strength of graphene allow carbon-based materials to meet higher stress needs, which is when conventional materials typically fail, and thus further support their use as raw materials in the development of NEMS technology.

In addition to the mechanical advantages of carbon-based materials, the electrical properties of carbon nanotubes and graphene make it possible to use them in many electrical components of NEMS. Nanotransistors have been developed for both carbon nanotubes and graphene. Transistors are one of the basic blocks for all electronic devices, so with the effective development of usable transistors, carbon nanotubes and graphene are critical to NEMS.

Metal carbon nanotubes

Carbon nanotubes (CNTs) are allotropes of carbon with a cylindrical structure. They can be considered rolled graphene. When rolled at specific and discrete angles (“chiral”), the combination of the rolling angle and the radius determines whether the nanotube has a bandgap (semiconductor) or no bandgap (metal).

Metal carbon nanotubes are also recommended for nanoelectronic joints because they can carry high current densities. This is a useful feature because wires to transmit current are another essential component of any electrical system. Carbon nanotubes have found particular application in NEMSs, in which methods have already been discovered for bonding suspended carbon nanotubes to other nanostructures. This allows carbon nanotubes to form complex nano-electric systems. Because carbon-based products can be properly controlled and act as connectors and transistors, they act as an essential material in NEMS electrical components.

CNT-based NEMS switches

One of the main disadvantages of MEMS switches over NEMS switches is the limited microwave range of MEMS, which disrupts the performance of high-speed applications. Limitations of switching speed and actuator voltage can be overcome by reducing the scale of the devices from micro to nanometer scale.

Comparison of performance parameters between carbon nanotube-based NEMS switches (CNT) with their CMOS counterpart showed that CNT-based NEMS switches maintain performance at lower power consumption levels and have sub-threshold leakage currents several times smaller than CMOS switches. CNTS-based NEMS with dual structures is being studied as potential solutions for floating gate non-volatile memory applications.

Difficulties
Despite all the beneficial properties of carbon and graphene nanotubes for NEMS technology, these products face numerous obstacles to their implementation. One of the main problems is the reaction of carbon to real environments. Carbon nanotubes show large changes in electronic properties when exposed to oxygen.

Similarly, other changes in carbon-based materials’ electronic and mechanical properties must be thoroughly investigated before application, especially because of their high surface area, which can easily react with the surrounding environment. It was also found that carbon nanotubes have different conductivities, depending on whether they are metal or semiconductor and helical during processing.

For this reason, a special treatment with nanotubes must be performed during processing to ensure that all nanotubes have the proper conductivity. Graphene also has more complex electrical conductivity properties than traditional semiconductors because it has no energy gap and changes all the rules of how electrons travel through a graphene-based device. This means that traditional electronic device constructions probably do not work, and completely new architectures have to be designed for these new electronic devices.

Nanoelectromechanical accelerometer

The mechanical and electronic properties of graphene make it ideal for integration with NEMS accelerometers, such as small sensors and actuators for cardiac monitoring and mobile motion recording systems. The thickness of the graphene atomic scale provides a path for accelerometers to shrink from micro-scale to nanoscale while maintaining the sensitivity levels required by the system.

By suspending an anti-silicon mass on a two-layer graphene strip, a nanoscale spring-loaded and piezoelectric mass converter can be fabricated with the current converters capable of accelerometers. The mass of the spring provides greater accuracy. And the piezoelectric properties of graphene convert pressure from acceleration to electrical signals for the accelerometer. Suspended graphene tape simultaneously forms a spring and a piezoelectric transducer while improving the performance of NEMS accelerometers.

Polydimethylsiloxane (PDMS)

Damage due to high adhesion and friction is a concern for many NEMS. NEMS often uses silicon due to specific micromachining techniques. However, its inherent stiffness often hinders the ability of devices with moving parts.

A study by Ohio State University researchers compared single crystalline silicon’s adhesion and friction parameters with a native oxide layer against a PDMS coating. PDMS is a silicone elastomer that is highly mechanically adjustable, chemically inert, thermally stable, gaseous, transparent, non-fluorescent, biocompatible, and non-toxic.

It is inherent in polymers that the Young Mods PDMS can vary by more than two times by manipulating the cross-linking of polymer chains, making it a viable material in NEMS and biological applications. PDMS can create a strong silicone seal and easily integrates with NEMS technology, optimizing mechanical and electrical properties. Polymers such as PDMS are starting to attract attention in NEMS due to their relatively cheap, simple, and time-efficient prototyping and fabrication.

Biological machines

Biological machines, such as myosin (controlling muscle contraction), are the most complex macromolecular machines found in cells, usually in multi-protein complexes.

Some of them are responsible for energy production and others for gene expression. They may play an important role in nanomedicine. For example, they can be used to identify and kill tumor cells.

Molecular nanotechnology is an emerging field of nanotechnology exploring the possibility of engineering biological machines that can rearrange materials at the atomic scale. BioNEMS contains biological and synthetic structural elements (nanoscale) for biomedical/robotic applications. For example, nanorobots can be injected into the body to detect and repair infections. While the proposed elements of BioNEMS, such as nanorobots and molecular assemblers, go far beyond their current capabilities, several studies have yielded promising results for future applications.

Conclusion

NEMS is a powerful technology and integrates life sciences with engineering in ways that are not currently possible with small-scale techniques. This will significantly impact various industries, including the semiconductor industry. The most widely used semiconductor device is the MOSFET, accounting for 99.9% of all transistors. Considering the gate length of the transistors in CPU or DRAM devices, the critical length scale of integrated circuits is currently below 50 nm. Recent silicon MOSFETs are based on fin field-effect transistors that use 10 and 7 nm processes.