Jul 30, 2014
Patterning bumpy surfaces
Nanofabrication on bumpy or multilevel surfaces is often a problem; even for the most accurate state-of-the-art methods. Now, reporting in Nanotechnology, researchers at Aalto University in Finland have established a rapid fabrication process that may provide a solution. They utilize atomic layer deposition (ALD) of ion-sensitive resist for the very conformal coating of non-planar sample surfaces. The key feature is the exposure of patterns by focussed ion beam (FIB), which has a large depth of focus in comparison to e-beam or UV lithographies.
The team also implement the dry release of nanostructures and demonstrate the fabrication of suspended nanobridges across the inclined walls of micron-scale grooves. Such released nanostructures provide a platform for various types of sensing and thermal conductivity measurements. Realization of suspended nano-networks on multilevel and corrugated surfaces brings, among others, new functionalities for micro- and nanofluidics, and lab-on-a-chip applications.
The challenges of corrugated surfaces
Nanofabrication on corrugated surfaces is generally very challenging since it requires preserving conformal resist distribution, good focusing, and equal etching rates for different height levels. The application of ALD and FIB effectively solves the problems with resist deposition and exposure. The difficulties associated with non-uniform etching speed are addressed by solely utilizing dry etching for both the resist development and release etching steps. The group from Aalto University demonstrates the fabrication of only 50 nm-wide suspended TiO2 nanobridges that follow the inclined Si wall of a micron-scale groove and retain their width from the top to the bottom.
A fast, reproducible and tolerant method
The shown fabrication of released nanostructures provides a platform for the consequent deposition of other materials. The authors also realize suspended nanowires through the bilayer TiO2/Al2O3 resist, where the Al2O3 underlayer is mechanically robust and has good etch-stop properties. The practical value of the proposed method is in its universal character, since it can tolerate arbitrary surface corrugation (trenches, grooves etc.). The method is also fast and reproducible due to the minimized number of process steps and absence of wet etching.
Future research includes experiments on new combinations of ion-sensitive resist materials (e.g. ALD nanolaminates) and new types of structures. In addition, the chemistry utilized in dry etching allows for application of the process to a number of polymers.
More information can be found in the journal Nanotechnology 25 335302 (IOPselect article).
Patterning technique prepares graphene nanoribbons for photodetector applications (Jan 2014)
Patterning horizontal silicon nanowire growth at vertical sidewalls (feb 2013)
Resist layer correlation joins up nanopatterns (Sept 2012)
About the author
Mikhail Erdmanis is a researcher (PhD candidate) in the Micro and Quantum Systems Group (MQS), which is led by Ilkka Tittonen and is located in Micronova, the clean room facility for micro- and nanotechnology research. The group is a part of the Department of Micro- and Nanosciences (MNT) at Aalto University in Finland. The research topics of the MQS group include nanofabrication, quantum optics, ion trapping, thermoelectrics, biomolecules, and theoretical semiconductor physics.