Advanced nanostructure and device fabrication

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Nanoscale science and technology has been one of the fastest growing research areas in recent years. Fundamental to this rapid growth is the capability of fabrication at the nanoscale. There are two approaches: the “bottom up” approach that involves chemical synthesis and self-assembly, and the “top down” approach in which nanostructures are generated or duplicated by lithography. For the “top down” method, electron beam lithography (EBL), focused ion beam (FIB) etching/lithography and nanoimprint lithography (NIL) are generally used to create nano-structures. Cui’s research in the past 17 years has been centered on nanostructure and device fabrication using NIL and EBL. In NIL, a surface pattern of a mold (stamp) is replicated into a material by mechanic contact-induced material displacement. EBL is the most versatile nanolithography method yet with very low throughput. Therefore NIL, which offers both high resolution and high throughput, is considered the most practical technique for volume production of devices that do not involve multi-level nanostructures requiring precise alignment among them. This proposed research covers three applications for all of which nanofabrication plays a central role: 1) batch fabrication of high aspect ratio AFM tips, 2) nano-structured plasmonic devices for bio- and chemical sensing, and 3) nano-structured silicon for Li ion battery electrode. 1) Atomic force microscope (AFM) suffers from the intrinsic limit when mapping a non-flat surface where the tip cannot fully follow the sample surface. The natural solution to this issue is to use thin and high aspect ratio tips that can follow the sample surface more precisely. At present, high aspect ratio AFM tips are commercially available, but at a very high price because these tips are fabricated one by one. Cui’s group has developed a batch fabrication process where an entire wafer of regular AFM tips are processed simultaneously into high aspect ratio tips. A US provisional patent has been filed and a company will be established in early 2014 to commercialize this technology. The objective of the current proposal goes beyond fabricating basic high aspect ratio tips, and will be focused on the fabrication of tips with tilt compensation using unconventional etching methods. 2) Plasmon is the collective oscillation of free electrons in metal. When such oscillation is confined to the metal’s surface, it is called surface plasmon; and when confined to a metal nanostructure (e.g. a metal island), it is called localized surface plasmon. The amplitude of the oscillation will be highest when the electromagnetic excitation frequency matches the resonant frequency of the plasmon. As the resonant frequency is very sensitive to dielectric changes on metal surfaces, it can be used as a label-free bio- and chemical sensor to detect events such as DNA hybridization. Here we will focus on the design, fabrication and characterization of biosensors based on localized surface plasmon resonance, with the goal of achieving high sensitivity with low-cost device fabrication using NIL. 3) Compared to the widely used carbon anode material, silicon offers one order higher theoretical energy storage capacity. However, the main issue with silicon is that it degrades rapidly during the charge/discharge process, leading to a short cycle life. To improve its lifetime, nano-structured silicon has been investigated. Here our objective is to fabricate high aspect ratio (depth/diameter > 1000) nanoscale hole arrays in silicon using electrochemical etching, with the hole arrangement (array periodicity) defined by pre-patterning using NIL. The key advantage is that both electrochemical etching and NIL are low cost and high throughput processes that are viable for volume production.