Nanoelectromechanical systems
Nanoelectromechanical systems (NEMS) are a class of devices integrating electrical and mechanical functionality on the nanoscale. NEMS form the next logical miniaturization step from so-called microelectromechanical systems, or MEMS devices. NEMS typically integrate transistor-like nanoelectronics with mechanical actuators, pumps, or motors, and may thereby form physical, biological, and chemical sensors. The name derives from typical device dimensions in the nanometer range, leading to low mass, high mechanical resonance frequencies, potentially large quantum mechanical effects such as zero point motion, and a high surface-to-volume ratio useful for surface-based sensing mechanisms.
History
Background
As noted by Richard Feynman in his famous talk in 1959, "There's Plenty of Room at the Bottom," there are many potential applications of machines at smaller and smaller sizes; by building and controlling devices at smaller scales, all technology benefits. The expected benefits include greater efficiencies and reduced size, decreased power consumption and lower costs of production in electromechanical systems.
In 1960, Mohamed M. Atalla and Dawon Kahng at Bell Labs fabricated the first MOSFET with a gate oxide thickness of 100 nm.
NEMS
In 2000, the first very-large-scale integration (VLSI) NEMS device was demonstrated by researchers at IBM. Its premise was an array of AFM tips which can heat/sense a deformable substrate in order to function as a memory device (Millipede memory).
Atomic force microscopy
A key application of NEMS is atomic force microscope tips. The increased sensitivity achieved by NEMS leads to smaller and more efficient sensors to detect stresses, vibrations, forces at the atomic level, and chemical signals.
Approaches to miniaturization
Two complementary approaches to fabrication of NEMS can be found, the top-down approach and the bottom-up approach.
The top-down approach uses the traditional microfabrication methods, i.e. optical, electron-beam lithography and thermal treatments, to manufacture devices. While being limited by the resolution of these methods, it allows a large degree of control over the resulting structures. In this manner devices such as nanowires, nanorods, and patterned nanostructures are fabricated from metallic thin films or etched semiconductor layers. For top-down approaches, increasing surface area to volume ratio enhances the reactivity of nanomaterials.
Bottom-up approaches, in contrast, use the chemical properties of single molecules to cause single-molecule components to self-organize or self-assemble into some useful conformation, or rely on positional assembly. These approaches utilize the concepts of molecular self-assembly and/or molecular recognition. This allows fabrication of much smaller structures, albeit often at the cost of limited control of the fabrication process. Furthermore, while there are residue materials removed from the original 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 of the commonly used materials for NEMS technology have been carbon based, specifically diamond,
Both graphene and diamond exhibit high Young's modulus, low density, low friction, exceedingly low mechanical dissipation,
Along with the mechanical benefits of carbon based materials, the electrical properties of carbon nanotubes and graphene allow it to be used in many electrical components of NEMS. Nanotransistors have been developed for both carbon nanotubes
Nanomechanical resonators are frequently made of graphene. As NEMS resonators are scaled down in size, there is a general trend for a decrease in quality factor in inverse proportion to surface area to volume ratio.
Carbon nanotubes (CNTs) are allotropes of carbon with a cylindrical nanostructure. They can be considered a rolled up graphene. When rolled at specific and discrete ("chiral") angles, and the combination of the rolling angle and radius decides whether the nanotube has a bandgap (semiconducting) or no bandgap (metallic).
Metallic carbon nanotubes have also been proposed for nanoelectronic interconnects since they can carry high current densities.
A major disadvantage of MEMS switches over NEMS switches are limited microsecond range switching speeds of MEMS, which impedes performance for high speed applications. Limitations on switching speed and actuation voltage can be overcome by scaling down devices from micro to nanometer scale.
Despite all of the useful properties of carbon nanotubes and graphene for NEMS technology, both of these products face several hindrances to their implementation. One of the main problems is carbon’s response to real life environments. Carbon nanotubes exhibit a large change in electronic properties when exposed to oxygen.
Graphene’s mechanical and electronic properties have made it favorable for integration into NEMS accelerometers, such as small sensors and actuators for heart monitoring systems and mobile motion capture. The atomic scale thickness of graphene provides a pathway for accelerometers to be scaled down from micro to nanoscale while retaining the system’s required sensitivity levels.
By suspending a silicon proof mass on a double-layer graphene ribbon, a nanoscale spring-mass and piezoresistive transducer can be made with the capability of currently produced transducers in accelerometers. The spring mass provides greater accuracy, and the piezoresistive properties of graphene converts the strain from acceleration to electrical signals for the accelerometer. The suspended graphene ribbon simultaneously forms the spring and piezoresistive transducer, making efficient use of space in while improving performance of NEMS accelerometers.
Polydimethylsiloxane (PDMS)
Failures arising from high adhesion and friction are of concern for many NEMS. NEMS frequently utilize silicon due to well-characterized micromachining techniques; however, its intrinsic stiffness often hinders the capability of devices with moving parts.
A study conducted by Ohio State researchers compared the adhesion and friction parameters of a single crystal silicon with native oxide layer against PDMS coating. PDMS is a silicone elastomer that is highly mechanically tunable, chemically inert, thermally stable, permeable to gases, transparent, non-fluorescent, biocompatible, and nontoxic.
Rest time has been characterized to directly correlate with adhesive force,
PDMS is frequently used within NEMS technology. For instance, PDMS coating on a diaphragm can be used for chloroform vapor detection.
Researchers from the National University of Singapore invented a polydimethylsiloxane (PDMS)-coated nanoelectromechanical system diaphragm embedded with silicon nanowires (SiNWs) to detect chloroform vapor at room temperature. In the presence of chloroform vapor, the PDMS film on the micro-diaphragm absorbs vapor molecules and consequently enlarges, leading to deformation of the micro-diaphragm. The SiNWs implanted within the micro-diaphragm are linked in a Wheatstone bridge, which translates the deformation into a quantitative output voltage. In addition, the micro-diaphragm sensor also demonstrates low-cost processing at low power consumption. It possesses great potential for scalability, ultra-compact footprint, and CMOS-IC process compatibility. By switching the vapor-absorption polymer layer, similar methods can be applied that should theoretically be able to detect other organic vapors.
In addition to its inherent properties discussed in the Materials section, PDMS can be used to absorb chloroform, whose effects are commonly associated with swelling and deformation of the micro-diaphragm; various organic vapors were also gauged in this study. With good aging stability and appropriate packaging, the degradation rate of PDMS in response to heat, light, and radiation can be slowed.
Biohybrid NEMS
The emerging field of bio-hybrid systems combines biological and synthetic structural elements for biomedical or robotic applications. The constituting elements of bio-nanoelectromechanical systems (BioNEMS) are of nanoscale size, for example DNA, proteins or nanostructured mechanical parts. Examples include the facile top-down nanostructuring of thiol-ene polymers to create cross-linked and mechanically robust nanostructures that are subsequently functionalized with proteins.
Simulations
Computer simulations have long been important counterparts to experimental studies of NEMS devices. Through continuum mechanics and molecular dynamics (MD), important behaviors of NEMS devices can be predicted via computational modeling before engaging in experiments.
Reliability and Life Cycle of NEMS
Reliability and Challenges
Reliability provides a quantitative measure of the component’s integrity and performance without failure for a specified product lifetime. Failure of NEMS devices can be attributed to a variety of sources, such as mechanical, electrical, chemical, and thermal factors. Identifying failure mechanisms, improving yield, scarcity of information, and reproducibility issues have been identified as major challenges to achieving higher levels of reliability for NEMS devices. Such challenges arise during both manufacturing stages (i.e. wafer processing, packaging, final assembly) and post-manufacturing stages (i.e. transportation, logistics, usage).
Packaging challenges often account for 75–95% of the overall costs of MEMS and NEMS. Factors of wafer dicing, device thickness, sequence of final release, thermal expansion, mechanical stress isolation, power and heat dissipation, creep minimization, media isolation, and protective coatings are considered by packaging design to align with the design of the MEMS or NEMS component.
Assessing the reliability of NEMS in early stages of the manufacturing process is essential for yield improvement. Forms of surface forces, such as adhesion and electrostatic forces, are largely dependent on surface topography and contact geometry. Selective manufacturing of nano-textured surfaces reduces contact area, improving both adhesion and friction performance for NEMS.
Adhesion and friction can also be manipulated by nanopatterning to adjust surface roughness for the appropriate applications of the NEMS device. Researchers from Ohio State University used atomic/friction force microscopy (AFM/FFM) to examine the effects of nanopatterning on hydrophobicity, adhesion, and friction for hydrophilic polymers with two types of patterned asperities (low aspect ratio and high aspect ratio). Roughness on hydrophilic surfaces versus hydrophobic surfaces are found to have inversely correlated and directly correlated relationships respectively.
Due to its large surface area to volume ratio and sensitivity, adhesion and friction can impede performance and reliability of NEMS devices. These tribological issues arise from natural down-scaling of these tools; however, the system can be optimized through the manipulation of the structural material, surface films, and lubricant. In comparison to undoped Si or polysilicon films, SiC films possess the lowest frictional output, resulting in increased scratch resistance and enhanced functionality at high temperatures. Hard diamond-like carbon (DLC) coatings exhibit low friction, high hardness and wear resistance, in addition to chemical and electrical resistances. Roughness, a factor that reduces wetting and increases hydrophobicity, can be optimized by increasing the contact angle to reduce wetting and allow for low adhesion and interaction of the device to its environment.
Material properties are size-dependent. Therefore, analyzing the unique characteristics on NEMS and nano-scale material becomes increasingly important to retaining reliability and long-term stability of NEMS devices.
To increase reliability of structural integrity, characterization of both material structure and intrinsic stresses at appropriate length scales becomes increasingly pertinent.
Residual stresses can influence electrical and optical properties. For instance, in various photovoltaic and light emitting diodes (LED) applications, the band gap energy of semiconductors can be tuned accordingly by the effects of residual stress.
Atomic force microscopy (AFM) and Raman spectroscopy can be used to characterize the distribution of residual stresses on thin films in terms of force volume imaging, topography, and force curves.
Future
Key hurdles currently preventing the commercial application of many NEMS devices include low-yields and high device quality variability. Before NEMS devices can actually be implemented, reasonable integrations of carbon based products must be created. A recent step in that direction has been demonstrated for diamond, achieving a processing level comparable to that of silicon.
Carbon-based materials have served as prime materials for NEMS use, because of their exceptional mechanical and electrical properties.
Recently, nanowires of chalcogenide glasses have shown to be a key platform to design tunable NEMS owing to the availability of active modulation of Young's modulus.
The global market of NEMS is projected to reach $108.88 million by 2022.