September 28, 2011

Self Assembled Nanopillars on entire silicon wafers

A*Star Singapore - Advanced electronics beckon thanks to self-assembling templates that allow the creation of nanoscale features on silicon wafers Although other groups have developed similar self assembled nanopillars, Krishnamoorthy and co-workers are the first to develop a process that can pattern the entire surface of a silicon wafer with highly uniform nanostructures

The ever-increasing demand for enhanced performance in electronic devices such as solar cells, sensors and batteries is matched by a need to find ways to make smaller electrical components. Several techniques have been proposed for creating tiny, nanoscale structures on silicon, but these types of ‘nanopatterning’ tend to involve low-throughput, high-cost approaches not suited to large-scale production. Sivashankar Krishnamoorthy and co-workers at the A*STAR Institute of Materials Research and Engineering have now found a simple and robust method for nanopatterning the entire surface of a silicon wafer1.

Krishnamoorthy’s technique exploits the self-assembling properties of polymeric nanoparticles, known as reverse micelles. These unconventional particles have a structure consisting of a polar core and an outer layer of non-polar ‘arms’. Reverse micelles can form highly ordered arrays on the surface of a silicon wafer. The resulting ‘coating’ can be used as a lithographic resist to mask the silicon surface during the etching process.

Fine arrays of nanopillars can be patterned onto a silicon surface using a self-assembling polymer template

Advanced Functional materials - Wafer-Level Self-Organized Copolymer Templates for Nanolithography with Sub-50 nm Feature and Spatial Resolutions

Robust lithographic templates, with sub-50 nm feature and spatial resolutions, that exhibit high patterning integrity across a full-wafer are demonstrated using self-organized copolymer reverse micelles on 100 mm Si wafers. A variation of less than 5% in the feature size and periodicity of polymeric templates across the entire wafer is achieved simply by controlling the spin-coating process. Lithographic pattern transfer using these templates yields Si nanopillar arrays spanning the entire wafer surface and exhibiting high uniformity inherited from the original templates. The variation in geometric characteristics of the pillar arrays across the full-wafer surface is validated to be less than 5% using reflectance spectroscopy. The physical basis of the change in reflectance with respect to sub-10 nm variations in geometric parameters of pillar arrays is shown by theoretical modelling and simulations. Successful fabrication of highly durable TiO2 masks for nanolithography with sub-50 nm feature width and spatial resolutions is achieved through highly controlled vapour phase processing of reverse micelle templates. This allows lithographic pattern-transfer of organic templates with a feature thickness and separation of less than 10 nm, which is otherwise not possible through other approaches reported in literature.

In an additional improvement to the process, the researchers exposed the self-assembled polymer layer to a titanium chloride vapour. The titanium chloride selectively accumulates within each micelle’s polar core. A blast of oxygen plasma then strips away the polymer to leave a pattern of tiny titanium oxide dots. This process converts a soft organic template into a hard inorganic mask much more suited to etching ultra-fine features into the silicon, producing arrays of nanopillars less than 10 nanometers apart.

The findings are expected to be highly adaptable. “Although we have demonstrated the process for creating silicon nanopillars, it is very versatile and can be readily extended to achieve nanopatterns of most other materials, for example, metals, semiconductors and polymers through appropriate post-processing of the initial copolymer templates,” explains Krishnamoorthy. "Other patterns besides nanopillars could also be created, depending on the pattern-transfer processing employed."

Krishnamoorthy and his team are already exploring the potential applications of their technique. “We are currently making use of this process to create nanodevices for sensing, data storage, and energy applications, such as batteries and solar cells,” Krishnamoorthy says.

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