Silicon Nanowire Fabrication by Metal-assisted Etching

S.-W. Chang, C.V. Thompson
Sponsorship: Singapore-MIT Alliance

Pore formation under anodic conditions is a well-known phenomenon for many semiconductors. However, the anodization approach to etching requires conductive substrates and independent control of a relatively large number of process parameters. To circumvent these problems, an electrochemical etching method known as metal- assisted etching (MAE) has recently received significant attention. In this approach metal catalysts are used to enhance local Si etching at the metal-silicon interface, in a mixture of hydrofluoric acid and an oxidant. The process can be used to fabricate high-aspect-ratio Si structures through patterned etching of silicon wafers. For example, we and others have used this technique to develop a relatively simple method for producing ordered arrays of one-dimensional silicon nanostructures over large areas via the combination of nanosphere lithography (NSL) and MAE.		Figure 1: Process flow for the fabrication of unpatterned 1D Si nano-pillars.
In this approach, NSL is used to fabricate an ordered array of nanoscopic holes perforated in metal thin films. The process begins with the formation of a two-dimensional, self-assembled monolayer of monodisperse nanospheres. Typical domain sizes are in the 10-100- μm range [1]. A dry etching step is carried out to reduce the diameter of the nanospheres. A metal thin film is subsequently deposited through the nanosphere mask using electron beam (e-beam) evaporation. After metal deposition, the nanospheres can be selectively removed to leave behind only the metal deposited through the mask. Using this nanoporous film as a catalyst for silicon etching produces an ordered array of silicon nanopillars with the same diameter as the original pores. Figure 1 shows a schematic of the process flow. Figure 2 shows SEM images of a silicon nano-pillar array after etching. As can be seen in the lower magnification image on the left, the wires are very uniform in length, suggesting a uniform etch rate over large areas.		Figure 2: Silicon nano-pillars fabricated using NSL and metalassisted etching.

Reference
[1] G. Schmid, Clusters and Colloids from Theory to Applications.. New York, NY: Wiley-VCH, 1994.