Advanced nanostructure and device fabrication

纳米科学技术是近年来发展最快的研究领域之一。这种快速增长的基础是纳米级制造的能力。有两种方法：一种是“自下而上”的方法，涉及化学合成和自组装；另一种是“自上而下”的方法，通过光刻技术生成或复制纳米结构。对于“自上而下”的方法，通常使用电子束光刻（EBL），聚焦离子束（FIB）蚀刻/光刻和纳米压印光刻（NIL）来创建纳米结构。在过去的17年里，Cui的研究主要集中在纳米结构和利用NIL和EBL制造器件。在NIL中，模具（冲压件）的表面图案通过机械接触引起的材料位移被复制到材料中。EBL是最通用的纳米光刻方法，但吞吐量非常低。因此，NIL提供了高分辨率和高通量，被认为是批量生产器件的最实用技术，这些器件不涉及需要精确排列的多层次纳米结构。本研究涵盖了纳米技术在三方面的应用：1)高纵横比原子力显微镜尖端的批量制造，2)用于生物和化学传感的纳米结构等离子体器件，以及3)用于锂离子电池电极的纳米结构硅。1)原子力显微镜（AFM）在绘制非平坦表面时，由于针尖不能完全跟随样品表面而受到固有限制。这个问题的自然解决方案是使用可以更精确地跟踪样品表面的薄且高纵横比的尖端。目前，高纵横比AFM针尖已经商品化，但由于这些针尖是一个接一个制造的，因此价格非常高。Cui的团队开发了一种批量制造工艺，将整片常规AFM尖端同时加工成高纵横比尖端。这项技术已经在美国申请了临时专利，并将于2014年初成立一家公司，将这项技术商业化。当前提案的目标不仅仅是制造基本的高纵横比尖端，而是将重点放在使用非常规蚀刻方法制造具有倾斜补偿的尖端。等离子体激元是金属中自由电子的集体振荡。当这种振荡被限制在金属表面时，它被称为表面等离子体；当局限于金属纳米结构（如金属岛）时，它被称为局部表面等离子体。当电磁激发频率与等离子体激元的共振频率相匹配时，振荡幅度最大。由于谐振频率对金属表面的介电变化非常敏感，它可以作为一种无标记的生物和化学传感器来检测诸如DNA杂交之类的事件。在这里，我们将专注于基于局部表面等离子体共振的生物传感器的设计，制造和表征，目标是通过低成本的器件制造来实现高灵敏度。3)与广泛使用的碳阳极材料相比，硅的理论储能容量高出一个数量级。然而，硅的主要问题是它在充电/放电过程中迅速降解，导致循环寿命短。为了提高其使用寿命，研究了纳米结构硅。在这里，我们的目标是利用电化学蚀刻技术在硅上制造高纵横比（深度/直径bbb1000）的纳米级孔阵列，孔排列（阵列周期性）通过使用NIL预图图化来定义。关键的优势是电化学蚀刻和零氧化镍都是低成本和高通量的工艺，可用于批量生产。

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.