NEMS (short for NanoElectroMechanical System) are devices that integrate electrical and mechanical functionality at the nanoscale, i.e., at or below 100 nanometers.
They are the advanced level of miniaturization after MEMS (MicroElectroMechanical System) that usually deals with components between 1 and 100 micrometers in size.
NEMS has several fascinating attributes. It can provide access to fundamental frequencies in the microwave range, force sensitivities at the attonewton level, heat capacities far below a yoctocalorie, active masses in the femtogram range, mass sensitivity at the levels of individual molecules — the list goes on.
NEMS mostly contains apparatuses like actuators, sensors, resonators, beams, sensors, and motors. These components transform one form of energy into another, which can then be easily measured and utilized.
Early Examples of NEMS
The first NEMS was built by Dawon Kahng and Mohamed M. Atalla at Bell Labs in 1960. It was a MOSFET (metal-oxide-semiconductor field-effect transistor) with a gate oxide thickness of 100 nm.
Two years later, they successfully fabricated a nanolayer-base metal-semiconductor junction transistor that contained gold films with a thickness of 10 nm. However, the first MOSFET with 10 nm gate oxide didn’t come until 1987.
An illustration of a MOSFET, showing body (B), source (S), gate (G), and drain (D) terminals.
The invention of Multi-gate MOSFETs in 1989 allowed several companies, including Intel, IBM, AMD, and Samsung Electronics, to create ever-smaller microprocessors and memory cells.
VLSI (very large scale integration) process made it possible to combine millions of MOS transistors on a single chip. These integrated circuits were widely adopted in the 1970s, enabling complex semiconductors and communication techniques to be developed.
CPU, GPU, RAM, ROM, and other glue logic are all VLSI devices. Before the invention of the VLSI process, most integrated circuits could only perform a limited number of tasks.
80486 microprocessor (from the 1990s) with more than a million transistors
MOSFET is now considered as the basic building block of modern electronics. It has been majorly responsible for the increasing transistor density, improving performance, and reducing power consumption of integrated circuits since the 1960s.
MOSFET is also one of the most manufactured devices in history. As of 2018, about 13 sextillion (or 13 thousand trillion) MOSFETs have been manufactured.
How Are NEMS Devices Manufactured?
NEMS can be fabricated using two complementary approaches:
Top-down approach: It uses conventional microfabrication techniques, such as the electron or optical beam lithography and thermal treatments, to build devices. Although it provides greater control over the resulting structures, it is limited by the resolution of the technique being used.
In this approach, starting materials are relatively large structures such as silicon crystals. Generally, etched semiconductors layers or metallic thin films are used to fabricate NEMS devices such as nanorods, nanowires, and patterned nanostructures.
In some cases, large materials are crushed to the nanometer scale to increase the surface area to volume ratio, which ultimately enhances the reactivity of nanomaterials. The manufacturing process of carbon nanotubes using graphite in an arc oven is a perfect example top-down approach.
Bottom-up approach: It uses the chemical characteristics of molecules to organize or assemble them in the desired conformation. These approaches rely on the concepts of molecular recognition (specific interaction between two or more molecules) or molecular self-assembly (without external direction).
While it provides limited control over the fabrication process, one can build much smaller structures without wasting a lot of material as compared to top-down approaches.
The bottom-up approach can also be found in nature. For example, biological systems exploit chemical forces to produce cell structures required for life. Researchers try to imitate this behavior of nature to create clusters of certain atoms that can self-assemble into some useful structures.
One good example of such approaches is the manufacturing of carbon nanotubes using a metal-catalyzed polymerization technique.
Materials Used For Making NEMS
Polydimethylsiloxane is the most used silicon-based organic polymer. This silicone elastomer is known for its unique properties. It is thermally stable, chemically inert, mechanically configurable, optically clear, and in general, non-toxic, inert, and non-flammable.
Since it can form a tight seal with silicon, it can be integrated into NEMS, configuring both electrical and mechanical characteristics. The adhesive forces of polydimethylsiloxane better perform under varying humid environments and possess a lower coefficient of friction compared to silicon.
Polydimethylsiloxane’s low friction coefficient and hydrophobicity make it a perfect material to be incorporated within NEMS research. It is also gaining attention in NEMS technology due to its time-efficient and inexpensive manufacturing.
Studies show that the degradation rate of polydimethylsiloxane in light, heat, and radiation can be slowed with appropriate packaging and good aging stability.
2. Carbon-based materials
A scanning tunneling microscopy photo of a single-walled carbon nanotube | Credit: NIST
Carbon allotropes, specifically graphene and carbon nanotube, are widely used in NEMS technology. Their characteristics directly meed the requirement of NEMS. For example, the semiconductor and metallic conductivities of carbon allotropes enable them to operate as transistors.
In addition to the mechanical advantages of carbon allotropes, the electrical properties of graphene and carbon nanotubes allow them to be be used in several components of NEMS. The physical strength of graphene and carbon nanotubes fulfill higher stress demands. Thus, they are majorly used in NEMS technological development.
While Graphene NEMS can operate as mass and force sensors, the carbon nanotubes NEMS have been widely utilized in nanomotors (that generate forces on the order of piconewtons), switches, and high-frequency oscillators.
3. Biological machines
Illustration of cancer-fighting nanorobots
Biological machines, such as myosin (handles muscle contraction), are the most complex macromolecular machines found within cells, typically in the form of multi-protein complexes.
Some of them are responsible for energy production and some for gene expression. They might play a crucial role in nanomedicine. For instance, they could be used to detect and destroy tumor cells.
Molecular nanotechnology is an emerging field of nanotech that explores the possibility of engineering biological machines, which could rearrange matter at an atomic scale. BioNEMS includes biological and synthetic structural elements (of nanoscale size) for biomedical/robotic applications. Nanorobots, for example, can be injected into the body to identify and repair infections.
While the proposed elements of BioNEMS, such as nanorobots and molecular assemblers, are far beyond current capabilities, several studies have yielded promising results for future applications.
NEMS serves as an enabling technology, merging life sciences with engineering in ways that are not currently feasible with microscale techniques. It will have a significant impact on various industries:
Semiconductor Industry: The most widely used semiconductor device is the MOSFET. It accounts for 99.9% of all transistors. Considering the gate length of transistors in CPU or DRAM devices, the critical length scale of integrated circuits is already below 50 nanometers. Recent silicon MOSFETs are based on fin field-effect transistors that utilize 10 nm and 7 nm process.
Automotive: Nanomaterials, such as nanosheets, nanofibers nanotubes, nanowires, and nanorods, offers several benefits in the automotive sectors. For example, nano-additives can improve the lifetime of tires significantly, as well as the abrasion resistance, rolling resistance, and wet traction. NEMS is also the key to improving fuel cell performance of future generation of hydrogen-powered cars.
Communication: Due to unique mechanical properties (which enables high-resonance frequencies and high-frequency tunability), NEMS resonators, including graphene resonators, provide a promising basis for future ultrafast communication systems. However, most of the developments in this field are currently confined to theoretical models, simulations, and lab experiments.
Graphene electrode for piezoelectric NEMS resonators | Image credit: Northeastern University
Medical Sector: NEMS sensors detect and monitor patients’ data such as water level, glucose level, and presence of various proteins and ions. These sensors can be configured to identify particular proteins ranging from human albumin to beta-2-microglobulins. In addition to monitoring, they can separate cells of different sizes, preventing clogging in a microfluidic system.
Energy Storage and Production: Nanotechnology holds great promise for increasing the lifetimes and performance of lithium-ion batteries. It also has the potential to enhance the power density, shorten the recharge time, as well as reduce the weight and size while improving the stability and safety of the batteries.
Furthermore, research is ongoing to use nanoscale electrochemical devices, like galvanic or fuel cells, to produce energy. They are bio-nano generators that draw power from blood glucose in a living body (in the same way the body generates energy from food).
There is also research into several nanostructured materials, especially nanowires, with the aim to develop more efficient and inexpensive solar cells than are possible with traditional planar silicon solar cells.
Global Market and Future
The current market for NEMS devices is in its infancy. It is segmented into nanotweezers, nanoresonators, gyroscopes, nanosensors, nanorobots, nanotweezers, and other tiny components.
It is expected to see robust growth in the coming years, which is attributed to NEMS benefits, such as its high resonating frequency, low energy consumption, multiple frequencies on the single chip, and size and cost reduction of integrated circuits.
The research and development in the field of nanomaterials and nanotechnology are in progress. According to reports, the global NEMS market is expected to grow at a CAGR of 29%. It will reach $108.88 million by 2022, with North America leading the market.