Overview
1. Nano Self-Assembly of Nanocomposite
Nano self-assembly is an enabling nanotechnology, a solution based nanomanufacturing approach for spontaneous organization of molecules or objects into stable, well-defined structures by electrostatic forces. Since the process occurs towards the system’s thermodynamic minima, it results in stable and robust structures. Among all types of self-assembly techniques, layer-by-layer (LbL) nano self-assembly is the most promising one. Self-assembly processes have numerous beneficial attributes: very cost effective, versatile, and facile. LbL nano self-assembly is versatile since it can deposit a variety of polarized nanomaterials such as carbon nanotubes, nanoparticles, or quantum dots on almost any type of substrate. It is very cost effective since it is a solution-based approach processed in an ambient environment, and it can cover the substrate with optimum thickness with very little waste. It is facile since the process can be well controlled from nanoscale to microscale. The LbL nano self-assembly of alternative layers of oppositely charged polyelectrolyte, nanoparticles, carbon nanotubes can provide the formation of films 5-1000 nm thick with monolayers of various substances growing in a pre-set sequence on any substrates, which make it very suitable for low-cost MEMS/NEMS.
2. “Top-Down” Micro-Manufacturing Combined with “Bottom-Up” Nano Self-Assembly
TIAN Lab successfully investigated nano self-assembly with microfabrication for MEMS/NEMS applications. Important achievements include: a) development the lithographic approach to pattern the self-assembled multilayer using the modified lift-off technique and the spatial patterning of colloidal nanoparticle-based thin film by a combinative technique of layer-by-layer nano self-assembly and metal mask approach of lithography, b) self-assembly of the ultra-thin cantilevers based on polymer-ceramic nanocomposites, c) fabrication and characterization of MOS-Capacitors and field-effect transistors fabricated by layer-by-layer nano self-assembly. In addition, TIAN Lab has generated many publications and patent applications directly relevant to this research topic in recent years. Two examples include self-assembled nanoparticle-based field-effect transistors and self-assembled cantilever beams. Currently, TIAN Lab is investigating nano self-assembly of carbon nanotubes and graphene for water pollutants sensing and bio-sensing applications, which are very promising for real-time water pollution monitoring and point-of-care cancer diagnostics at home.
3. Hot Embossing of Polymer MEMS
Hot Embossing Lithography (HEL), also known as imprint lithography (IL), has gained much interest, in particular as a low-cost and high-volume fabrication approach to define micro- to nano-scale structures. Hot embossing is a type of polymer thermoforming process: a mold insert is pressed into a polymer under vacuum ambient, the polymer material flows into the mold structures at high temperatures, the polymer material is cooled down to a temperature which provides a sufficient strength, and the patterned polymer material can be de-molded. The hot embossing technique ensures highly precise molding of almost any structures in polymers, especially in the fabrication of high-aspect-ratio microstructures. Compared with injection molding, hot embossing is another well-known polymer fabrication technique with several significant advantages such as its flexibility and economy. TIAN Lab was the first one who successfully investigated polymer tunneling sensors and polymer comb drive structures. The PMMA based tunneling sensor shows promising properties with much higher resolution and much higher bandwidth, compared to the silicon tunneling sensor with the same dimension. The vertical tunneling accelerometer was fabricated on PMMA by hot embossing lithography. Silicon mold templates are fabricated with UV lithography, anisotropic bulk wet etching, and ICP dry etching. The PMMA structure is transparent, very suitable for sensor bonding. A laterally driven comb drive on PMMA was first fabricated with hot embossing lithography by TIAN Lab. The electrostatic comb drive is used for lateral dimensional acceleration detection based on electron tunneling mechanism.
4. Shrink Polymer Nanomanufacturing
Polymer shrinkage becomes a new approach to do lithography and generate smaller structures by reforming larger pre-patterned structures. The facile polymer fabrication approach by embossing and thermoplastic shrinkage aims to do lithography in a nanoscale or reduce the feature size and dramatically increase the aspect ratio of imprinted microstructures. The shrinkage capability of embossed microstructures is obtained by molding at low temperatures for less cycle time. Embossed patterns are activated for shrinkage by removing projected structures and heating at higher temperatures. The final structures are defined with the shape of removed materials before shrinking polymer materials. Both two- and three-dimensional embossed structures were successfully shrunk into much smaller scale. This polymer-shrinking process brings a new way to extend the fabrication capability of polymer embossing process towards MEMS-based biosensors and water sensors. TIAN Lab covers shrink polymer for nanolithography, high-aspect-ratio microstructures, and biosensors for medical applications and water pollutants detection. Shrink nanostructures can significantly improve performance of biosensors and water sensors, due to features including enhanced topography (larger surface area), cost effective (ease to manufacture), time efficient (fast process). This is unique and highly important work, because shrink polymer manufacturing may open a new way for MEMS with high performance and low cost at the same time.
