Nanoscale Patterning and Electronics on Flexible Substrate by Direct Nanoimprinting of Metallic Nanoparticles

Recent years have witnessed an expanding interest in the application of flexible polymer materials (e.g., polyimide, polyester, etc.) as the substrates for electronic and display devices. These applications include flexible organic light-emitting displays, thin film transistors, sensors, and polymer MEMS. The advantages of polymer-based materials are their mechanical flexibility, light weight, enhanced durability, and low cost compared with rigid materials (such as silicon and quartz). However, it can be difficult to integrate polymers into an integrated circuit (IC) microfabrication process due to their low thermal stability (low melting and low glass transition temperatures) and solvent susceptibility. In practice, conventional IC fabrication processes are subject to limitations, in that they are multi-step, involve high processing temperatures, caustic baths and strong solvents. In order to address the current problems of microfabrication on flexible substrate, many alternative approaches to conventional photolithography-based process have been introduced by a number of researchers. These include microcontact printing (lCP) combined with metal etching, electroless plating, electropolymerization, and direct metal layer transfer for the microscale metal patterning on flexible substrates. Stencil lithography was mainly applied for dielectric layer patterning on polymer substrates for the formation of electrical capacitors due to its limited resolution. Inkjet printing was used for a drop-on-demand patterning of conductive polymer PEDOT and gold layers for drain-source and gate electrodes. However, its best resolution is 20–50 lm limited by the nozzle diameter, the statistical variation of the droplet flight, and spreading on the substrate. Organic semiconducting materials are being widely used as semiconducting layers in flexible electronics due to their costeffectiveness, mechanical flexibility, and ease of application via specific chemical modification. However, further channel size down-scaling is essential for better performance of organic field effect transistor due to the lower carrier mobility of the organic semiconducting materials. While the abovementioned methods cannot achieve ultrafine features (a few lm’s down to ∼ 100 nm) in high aerial density and good reproducibility, nanoimprinting lithography (NIL) allows easy fabrication of precise nanoscale structures. NIL has been applied for nanopatterning in various fields such as biological nanostructures, nanophotonic devices, organic electronics, and the patterning of magnetic materials. Especially, metal nanopatterning via nanoimprinting is widely employed in nanoscale electronics and biosensing platforms. However, metal nanoimprinting has been typically an indirect process where a polymer (e.g., PMMA) pattern is first created by nanoimprinting, and then used as a mask for metal film etching or metal lift-off process. This involves multiple and expensive process steps and its chemistry is harsh for the flexible substrates. Furthermore, flexible substrates are not resistant to high temperature and pressure during the imprinting process. Recently, imprint resists based on monomer or copolymer have been developed and used for low pressure/low temperature nanoimprinting process. However, these are also indirect methods for metal nanopatterning. Very few direct metal nanoimprinting processes have been demonstrated so far due to the high melting temperature of metals. As an alternative to metal direct nanoimprinting, solid state embossing methods based upon plastic deformation of metal thin films have been introduced. These approaches involved either deformation of a metal film under very high pressure or deformation of a metal thin film/polymer multilayer under relatively lower pressure. Evidently, they are not compatible with flexible substrate since its mechanical strength is not sufficient for such processes. Additionally, these methods do not allow the fabrication of isolated, arbitrary features, and always leave unwanted residual layers. To alleviate the limitations described in the fabrication processes above, the authors have recently developed a novel C O M M U N IC A IO N