Nanosphere lithography method is a low cost potential alternative for patterning and generation of semiconductor nanostructures compared to the conventional top-down fabrication techniques.

Nanopillars

Forests of silicon pillars of sub-500 nm diameter and with an aspect ratio up to 10 have been fabricated using a combination of the nanosphere lithography and deep reactive ion etching techniques. The nanosphere etch mask coated silicon substrates were etched using oxygen plasma and a time-multiplexed 'Bosch' process to produce nanopillars of different length, diameter and separation. A researcher devised a new, controlled way to use a method called the “vapor-liquid-solid” process to make large-scale modules of dense, highly ordered arrays of single-crystal nanopillars. Inside a quartz furnace his group grew pillars of electron-rich cadmium sulfide on aluminum foil, in which geometrically distributed pores made by anodization served as a template.

Inverse nanosphere lithography

Materials experts in Taiwan have developed a simple method for fabricating uniform periodic CNT arrays dubbed inverse nanosphere lithography (INSL). The technique provides partial alleviation of stacking defects of polystyrene (PS) spheres in the self-assembled monolayer, which improves the uniformity of the array. Also, the size and period of the CNT array can be adjusted by careful choice of the PS sphere diameter.

Traditional nanosphere lithography (NSL) uses a monolayer of polystyrene (PS) spheres as a mask, but regional boundaries formed in the monolayer due to stacking faults inevitably cause problems. After coating, the catalyst is arranged in a honeycomb pattern when the PS spheres are removed from the substrate. The regional boundaries and the vacancies in the self-assembled monolayer result in the deposition of different thicknesses of catalyst on these defective positions, which leads to non-uniform CNT growth.

Two NSL methods are commonly used to overcome the problem of thickness, one employing a double PS layer, the other using a monolayer metallic mask. However, both methods are complicated and difficult to control for large-scale fabrication.

Basic concept

Nyan-Hwa Tai and his team from National Tsing Hua University have come up with an alternative approach, which involves the fabrication of close-packed CNT arrays using a catalyst-poisoning layer. The basic concept is to deposit a periodic poisoning layer over or underneath the catalyst layer to prevent CNT growth. As a result, a periodic CNT pattern can be obtained after growth through the CVD process.

The patterned CNT array has the potential to be used in the surface decoration of mobile phones, solar-cell electrodes, super capacitors and field emitters. INSL can also be extended to process other forms of nanomaterials, such as nanowires.