MEMS Technology

MEMS is the technology of microscopic devices. At the nanoscale, it merges into Nanoelectromechanical systems (NEMS). MEMS are also sometimes referred to as microdevices or micro-systems technology. MEMS are made up of components that range in size between 1 and 100 µm in size and the device sizes usually range from micrometers to sometimes a few millimeters. Once the first MEMS device was fabricated and published, a lot of visionary researchers in both academic and industrial domains took notice and immediately began developing a variety of micro-devices such as micro-cantilevers, micro-gears, etc. MEMS systems have been used in devices such as RF switches, sensors, actuators and accelerometers.

Silicon Silicon is the go-to material for fabrication of MEMS devices. Silicon in crystalline form is a great Hookean material, meaning, it has minimal energy dissipation. This property also contributes to reliability as it suffers from very little fatigue and shows no mechanical hysteresis. The high melting point of 1400°C also makes it a good candidate for high temperature operation. Added to this, the ready availability of high-quality silicon, cost of scaling and the ability to incorporate electronic functionality make silicon an attractive proposition for MEMS applications. Silicon is mostly used as a substrate material in MEMS applications in a single-crystal form. Other compounds of Silicon such as Silicon dioxide, Silicon carbide and Silicon Nitride are also used in various applications. In this work, Silicon has been used primarily as a substrate and is not an active component of the device.

Polymers

Polymers play a major role in MEMS and microelectronics. They can be found in the packaging of IC chips, intermetallic dielectric layers, etc. Polymers are one of the essential elements in microfabrication of MEMS devices as photoresists for lithographic patterning. The ease of processing, relatively low cost and ease of production and high stability makes them a great choice for use in MEMS. Polymers are generally insulators but there are ways like Pyrolysis and metal doping to make them conductive. These conductive polymers are used as substrates in MEMS. Some popular polymers used in cleanrooms include SU-8, an epoxybased photoresist consisting of 8 epoxy groups, Polymethyl Methacrylate (PMMA), Parylene, etc. This work uses SU-8, a polymer as a sacrificial material to obtain suspended CNT devices. Ceramics Ceramics materials like silicon nitride and silicon carbide are also being increasingly used in MEMS fabrication for piezoelectric devices where electrical and mechanical properties are coupled. Electrostatic MEMS are fabricated using ceramic materials owing to their large elastic modulus. Other favorable properties of ceramics include their chemical inertness, resistance to corrosion, etc.

LIGA

LIGA, which is acronym for Lithographie, Galvanoformung, Abformung was developed in Germany to overcome the two-dimensionality of Surface Micromachining. LIGA forms a part of High Aspect Ratio Micromachining technique which sometimes uses molding techniques to form microstructures (Saile et al. 2009). LIGA uses X-rays instead of Ultraviolet light to expose a photosensitive layer such as polymethyl methacrylate (PMMA) that is normally a few hundred micrometers thick. The exposed areas are then stripped away chemically and metallized. The resulting component is generally used as a structural layer. A variety of structures with high aspect ratios ranging in height from a few microns to centimeters can be realized using this method. A variety of devices such as accelerometers and microfluidic devices have been fabricated by this method. Since this process is carried out on a Silicon substrate, this technique can be used to integrate the control electronics and circuitry for the MEMS sensor on the same chip. LIGA is limited by the need to have expensive X-ray synchrotron facility which adds to the production costs.

Applications of Carbon Nanotubes

The worldwide commercial interest in CNTs has grown considerably in the past two decades. Factors contributing to this include advancements in scientific research and improved production capacity. This has ultimately paved the way for an array of emerging applications in medical therapy (Hong et al. 2015), microelectronics (Park, Vosguerichian, and Bao 2013; Cao and Rogers 2009), chemical sensors (Schroeder et al. 2018). Although CNTs have been studied intensely and the properties are well understood, by no means has the field saturated and our understanding keeps growing continually. Advances in production techniques have enabled the preferential production of CNTs with either semiconducting or metallic properties. The high surface area to volume ratio of CNTs makes them a perfect candidate for manufacturing composite materials. MWNT-polymer composites have shown to reach conductivities as high as 10,000 Sm-1 (Bauhofer and Kovacs 2009). Conductive CNT plastics have been used in the automotive industry for electrostatic assisted painting for fuel lines that may dissipate electrostatic charge. CNTs mixed with resins and polymers increase overall rigidity and toughness. These enhancements can be achieved by minimal increase in weight percentage (Gojny et al. 2004). CNTs have also been used build lightweight turbines for windmills and hulls for naval boats. CNT sheets have been demonstrated as materials for fuel cell electrodes and self-cleaning textiles (Lima et al. 2011).

The addition of CNTs to metals has shown to increase the tensile strength and modulus. This can be used in aerospace and automobile applications. It is well known that mechanical deformation can alter the electrical conductance of CNTs. This forms the basis for the application of CNTs in pressure sensing. A lot of works in the literature have been published about pressure and strain sensors based on CNTs. Electrical transition in SWNTs by radial deformation induced by hydrostatic pressure was studied using first principle calculations was studied by Wu et al. (Wu, Zang, et al. 2004). Kang et al. (Inpil et al. 2006) fabricated a CNT based strain sensor for applications in structural health monitoring. This work compared device performance between Buckypaper and a composite of SWCNT/Polymethylmethacrylate. A MWCNT based MEMS pressure sensor was developed and studied by Fung et al. (Fung et al. 2005) using dielectrophoresis to position the MWCNT network across a PMMA membrane. Deflection of the PMMA membrane under pressure induced bending in the MWNTs which was detected by fold electrodes with 3 µm to 10 µm spacing. This work uses conventional micromachining process without resorting to low throughput processes like electron beam lithography.