|Kumar, et. al., report a unique approach to the formation of nanoscale features of amorphous metals. This process incorporates relatively simple embossing techniques to realize massively replicated surface structures having <100nm dimensions.|
Reviewed by Jeff Morse, Ph.D., National Nanomanufacturing Network
- Kumar G, Tang HX, and Schroers J. 2009.Nanomoulding with Amorphous Metals. Nature Nanotechnology 457: 868-873. DOI: 10.1038/nature07718.
While submicron scale imprinting and molding techniques are well established for several consumer products (e.g.; compact disks, micro-optics nanoimprint lithography is emerging as a mainstream tool for patterning nanoscale features in resists, polymers, and thermoplastics. Depending on the properties of the master mold, materials with lower glass transition temperatures can be patterned, limited only by physical fatigue and degradation of the master mold.
When patterning and forming nanoscale structures in materials other than resists, polymers, or thermoplastics, the choices become limited due to materials properties. Although metals exhibit attractive mechanical properties, patterning them at length scales smaller than the grain size is difficult. Furthermore, typical processes for metal deposition into nanoscale patterns results in inhomogeneous mechanical properties, grain size, residual stress, and surface roughness; electrodeposition processes have a tendency to be very rigid with limited material selections.
Metallic glasses provide a versatile toolbox for nanoimprinting since they combine properties which were previously achievable by one material class.
-- Jan Schroers, Yale University
In the February 12 issue of Nature, Kumar, et. al., from Jan Schroers group at Yale University, report on nanomolding using amorphous metals. Also referred to as metallic glasses, amorphous metals are prepared by vitrification of molten alloys and have unusual properties in comparison to conventional metals. In this study, the authors investigate bulk metallic glasses, a subset of glass forming alloys that exhibit relatively large (>1mm) amorphous sections. Since the resulting material is amorphous, the lack of crystallites, grain boundaries, and dislocations results in a homogeneous material exhibiting very high strength, hardness, and elastic strain. Furthermore, by tailoring the composition of the bulk metallic glass, the formability, strength, and softening properties can be controlled. While the softening properties of metallic glasses have been known for decades and recently further exploited to form complex three-dimensional structures with length scales ranging from many microns to several millimeters, the authors report their findings in nanomolding of high aspect ratio structures.
Historically, the direct molding of high-aspect-ratio structures with lateral features approaching <100nm has been a key challenge, even for bulk metallic glass compositions having low supercooled liquid viscosities. Recognizing that in this regime the molding is controlled by capillary forces rather than the viscosity, the authors developed an accurate description of the molding process at the nanoscale wherein both capillary and viscous force contributions were taken into account. The result was an accurate model to determine the required pressure to flow a supercooled liquid bulk metallic glass into a channel having specified nanoscale features.
The authors identify the ideal properties for nanomolding of bulk metallic glasses, including the ability of the bulk metallic glass in its supercooled liquid state to partially wet typical mold materials, low viscosity in the supercooled liquid regime, and sluggish crystallization at the molding temperature. The authors further report on the demonstration of specific platinum bulk metallic glass alloys designed with these desirable properties, and subsequent replication of high-aspect-ratio nanoscale structures incorporating this method.
Kumar, et. al., report a unique approach to the formation of nanoscale features of amorphous metals. Within this method bulk metallic glasses can be both the hard mold or the soft imprintable material. The technique can further be used to replicate structures of different bulk metallic glass materials by tailoring the softening temperature of the mold material with respect the supercooled liquid being molded. This process incorporates relatively simple embossing techniques to realize massively replicated surface structures having <100nm dimensions. Considering the broad range of softening temperatures exhibited by metallic glass alloy compositions, this technique represents a versatile toolbox for nanomolding and imprinting for a range of applications, most notably re-writable data storage, reparable master molds, photolithographic masks, and medical implants having patterned surface features.
Image reproduced by permission from Macmillan Publishers Ltd: Kumar G, Tang HX, and Schroers J. 2009.Nanomoulding with Amorphous Metals. Nature Nanotechnology 457: 868-873. Copyright 2009.