Research Highlight: Top-Down Nanomachining of Metals

Abstract

Microand nanomachining technologies are commonly categorized into top-down and bottom-up approaches. Making nanoscale structures using top-down approach has been facing difficulties due to the limitation of optical lithography and the low throughput of electron beam and ion beam techniques. Soft lithography and hot embossing can transfer nanostructures across a large wafer. However, these techniques are limited to a soft substrate such as polymers only. A recent work by researchers from Purdue University, Harvard University, Madrid Institute for Advanced Studies, and the University of California, San Diego demonstrated a new embossing technology to create wafer-scale smooth three-dimensional nanoshapes on hard crystalline metal. Top-down micro/nanomaching approaches can either create a wafer-scale pattern with fast batch limited-resolution processes or a small-scale pattern with slow highresolution serial processes. Wafer-scale machining of nanostructures was made possible with soft lithography [1], which transfers patterns through direct contact of a soft stamp. Soft lithography, in particular the wafer-scale replication of elastomeric structures, has enabled the rapid development of the research areas of microfluidics and nanofluidics [2]. Although many microfluidic and nanofluidic devices were made of inorganic materials such as glass and silicon, the majority of these devices are made of elastomers and thermoplastics due to the simple replication processes such as molding and hot embossing [3]. The molding approach has been extended to hydrogels for biological applications such as cell culture [3]. Hot embossing and nano imprint techniques make role-to-role printing processes. The role-to-role technology allows for low-cost large-area fabrication of complex devices such as flexible thinfilm transistors for display applications [4]. Soft lithography and nano imprint lithography are suitable for making nanostructures in polymers [5]. However, the soft nature of polymers makes processes with high fidelity difficult. Often, subsequent processes such as bonding need to be carried out at low temperature and low pressure [6]. Nano patterning of metals through embossing has been impossible because of their low formability in the scale smaller than 1 micron. The extremely high strain rate and high temperature above the melting temperature are the main technological challenges for top-down nanomachining of metals. Nanomachining of hard and crystalline metals is even more challenging. Recently, Gao et al. [7] reported the laser shock imprinting (LSI) technology that can create smooth three-dimensional crystalline metallic structures in nanoscale under ambient conditions. The technology utilizes a laser to create shockwave impulses and ultrahigh strain rate to compress a metallic sheet into a silicon nanomold. The laser has a power of 0.3 to 1.4 GW/cm 2 , a wavelength of 1064 nm and a pulse duration of approximately 5 ns. The metal layer was prepared on a glass substrate that was coated with 10-μm graphite as the ablation coating layer. Under the laser irradiation, the graphite sublimates ad creates a shock wave that propagates through the metal layer and deforms it to conform the underlying silicon mold. The nanoscale resolution of the mold can be transferred accurately to the metal layer. The silicon mold surface was coated with 5 to 10 nm aluminum oxide to improve the abrasive resistance. Such as silicon nanomold can be reused for over a hundred times without reduction of patterning quality. Furthermore, coating a 1 to 5 nm polyvinylpyrrolidone (PVP) layer or a 5 to 10 nm gold layer on the metal reduces the friction during the imprinting process. The patterned nanostructures have an aspect ratio as high as 5. The technology allows for making nanoscale structures in titanium, aluminum, copper, gold and silver over a 6-inch wafer in only 30 seconds. The LSI technology has the potential to enable low-cost large-scale plasmonic metamaterials that have surfaces containing patterns on the scale of nanometers. The paper also reports the use of LSI for patterning the metal layer with graphene that represent a perfect absorber for metametrials with potential applications in nanoelectronics, optoelectronics and plasmonics. LSI also has the potential to be transferred to the roll-to-roll technology, which currently are limited to processing polymeric and organic materials.