What is Nano Electro-Mechanical Systems
Nano Electro-Mechanical systems (NEMSs) have dimensions of a few nanometers. By using nanoscale effects, these systems show interesting and exceptional characteristics that differ significantly from micro electro mechanical systems (MEMSs). The remarkable properties of these machines basically begin from the behavior of active segments i.e. cantilevers or double clamped beams in nanometer-scale .
NEMS consist of mechanical elements, mechanical actuators, sensors, nano-pumps and nano-motors. NEMS is able to transform into operability directly at very low power because of the influential combination of its features. Also, it has the ability to produce functional non-linearity with relatively modest forces.
The working range of frequencies for NEMS are in the microwave range, mass sensitivity at the stage of individual molecules, masses in the femtogram range, force sensitivity at the attonewton level, heat capacity at below a ‘yocto calorie’ .
1. Fabrication Processes
The fabrication methods of NEMS can be classified into two approaches
- Top-down approaches which are generated from the manufacturing of MEMS. This approach consists of submicron lithographic techniques i.e. electron-beam lithography, to produce structures from bulk thin films.
- Bottom-up approaches manufacture the nanodevices by assembling the atoms and molecules successively as building blocks. Carbon nanotubes and nanowires are manufactured by using this approach to fabricate other nanodevices.
Studies show that usually nanodevices are fabricated by “hybrid” approaches i.e. a mixture of bottom-up (self assembly) and top-down (lithographic) approaches .
There are three basic steps in manufacturing of a NEMS. These are as follows
- Deposition Process
- Etching Process
- Deposition Process
NEMS can deposit thin films of the material of thickness roughly between 1nm to 100nm. Two types of deposition methods are as follows:
- Chemical Vapor Deposition– In this technique chemical vapor deposition (CVD) is used that is basically a process in which stream of source gas reacts with the substrate to develop the desired material. Other examples of type of deposition are LPCVD(Low-Pressure Chemical Vapor Deposition) and PECVD (Plasma-Enhanced Chemical Vapor Deposition) .
- Epitaxial– this technique is about the deposition of crystalline over-layer (epitaxial film or Epitaxial layer) on a crystalline substrate. Epitaxial films are fabricated from liquids or gaseous precursors. The deposited film can bolt itself into different crystallographic orientations according to substrate crystal because of the functioning of the substrate as a seed crystal. If the over-layer makes an arbitrary orientation then it is named as non-epitaxial growth. Homo-epitaxy is deposition of the epitaxial layer on a substrate of the same composition and when the composition is different then it is known as hetero-epitaxy .
Figure 1: Working of a Deposition Process
Lithography is generally the transmission of a prototype into a photosensitive substance by careful exposure to a radiation source i.e. light. When light falls on the photosensitive material, its physical properties gets changed. By giving a mask to the basic substrate, the exposed section can then be detached or removed. The alignments and exposures are the additional stages in the process of lithography . Different types of lithography processes are as follows:
- Electron Beam Lithograpgy (E-Beam Lithography)- It is the process of examining a beam of electrons in an ordered form covered with a ‘resist’ film and of removing the exposed or unexposed sections of the resist selectively. This will fabricate very tiny structures in resist film which can then be transmitted to the substrate that is often done etching. The main restraint of this technique is throughput which means it takes a long time to expose a glass substrate or an entire silicon wafer. If the exposure duration becomes long then it leaves the user susceptible to beam instability. Also if the pattern is not being modified the second time, the turn-around time for re-designing is delayed pointlessly .
- Self Assembly- It is a process of creating nanostructures by the formation of stable bonds among the organic or non-organic substrate and molecules. A group of researchers, Mirkin’s group, invented a self-assembly technique named as Dip Pen Nanolithography (DPN). This technique was implemented to functionalize the definite surface segments with polar [e.g. amino (−NH2/ −NH+ 3) or carboxyl (−COOH/−COO−)] or non-polar [e.g. methyl (−CH3)] chemical group. When the liquid suspension of carbon nanotubes was introduced to this surface then nano-tubes were attracted in the direction of the polar region and self-assembled to create a pre-designed structure which is mostly done within 10 seconds, with a yield greater than 90% , .
3. Etching Process
The two basic types of etching processes are 1. Wet etching 2. Dry etching. The main difference between them is that when material se immersed in a chemical solution it will dissolve in case of wet etching and in dry etching the material will dissolve by using vapor phase etchant or reactive ions .
- Wet Etching- This process is used to remove the material selectively by dipping it into the solution that will dissolve it.. The nature of this process offers good quality selectivity and hence in this way the rate of etching of the objective material is significantly greater as compared to mask material if chosen vigilantly. Isotropic Etching and Electrochemical etching are different wet etching techniques .
- Dry etching- This process is the elimination of material, generally a masked prototype of semiconductor This is done by the exposure of material to a bombardment of ionsi.e. plasma of reactive gases e.g. oxygen, chlorine, fluorocarbons, boron tri-chloride etc) that remove segments of the exposed surface of the material. Vapor etching and Plasma etching are different methods of dry etching .
Designing Challenges for NEMS
Currently, NEMSs are facing challenges that are related to the modified fabrication of metallic or semiconducting Carbon nano-tubes and also issues about stiction and lubrication . Following are the challenges that are faced by engineers while designing NEMS:
- Phase noise– Extrinsic processes working in transducer provides the estimated value for frequency stability in resonator–transducer– amplifier cascade, which is very vital for resonator itself. Given the extreme sensitivity that is possible only if the devices become very small, the occurrence of basic fluctuation processes is expected to increase likely to evaluate the outcome. This phase noise should be calculated accurately for NEMs to work properly .
- Development of Transducers– another challenge while designing this system is the use of the right type of transducers. The RMS vibration amplitude for a machine which is operating in its linear range must be directly proportional to the machine’s size. This kind of miniscule displacement amplitude signifies that extremely high sensitive displacement transducers are required for operating NEMS effectively. .
- Reproducible Nano-Fabrication- Surface nano-machining methods generate NEMS with greater rate of errors in basic device parameters which is an undesirable result of the minute NEMS. In order to get the desirable results for this type of systems, quartz frequency control technology must be used in NEMS , .
NEMs V/S MEMs
Nano Electro Mechanical Systems (NEMS) comprises of significant structural elements of about the size of 100 nm or less. This differentiates them from Micro Electro Mechanical Systems (MEMS) as the size of the structural elements is in the range of micrometer scale. If compared with MEMS, NEMS merge small masses with greater surface area to volume ratio and hence the most appealing and attractive applications are available in the market, all because of this technology i.e. high-frequency resonators and ultra-sensitive sensors .
Application/Usage of NEMS
- NEMS is used in airbag deployment systems in automobiles as accelerometers.
- Nano Gyroscopes and nanorobots are used to locate routes in airplanes and ships.
- A Nano nozzle in inkjet printers provides ink for printing.
- Nowadays, smartphones companies are also using NEMS technology.
- NEMS is used for Magnetic Resonance Imaging (MRI).
- In chemical force-sensing bio-NEMS and bio nanochips are used.
- This technology is used in pressure sensors for checking blood pressure
- NEMS technology is also used in Thermal actuator
- Nano-tweezers is also one of the applications (nanomechanical manipulation)  .
A lot of research is still going on Nano-Electro Mechanical Systems. This technology helped a lot in maintaining nearly all sophisticated mechanism that we know up till now. Researchers are now more focusing on the study of Quantum-nano electro mechanics, Electronics and mechanics on the nanometer scale and Superconducting NEM devices.
(Feature Image Courtesy: Canva)
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