Top-Down Fabrication of Patterned, Vertically Aligned Silicon Nanowires

and Introduction: Silicon nanowires (SiNWs) have recently attracted significant attention due to their one-dimensional structure [1] and semiconducting properties [2]. With many potential applications in microand nano-electro-mechanical systems (M/NEMS) [3], the development of various SiNW conformations is a popular area of current research. Challenges, however, such as controlled fabrication, presently keep this promising technology from reaching mainstream industry. The aim of this project, therefore, was to develop a method of synthesizing patterned, vertically-aligned SiNWs with highly controlled diameters and lengths using a top-down fabrication method. The ultimate use of this methodology will be to develop various three-dimensional (3D) shapes and curvatures of SiNWs, with the hopes that the manipulation of these structures can offer many applications, especially in M/NEMS devices. In order to realize the ultimate goal of 3D SiNW microstructures, controlled synthesis must first be accomplished. Recently, there has been success in NW fabrication with relatively controlled dimensions and densities using both bottomup and top-down methods [4]. In the present work, the latter approach was used, in order to properly pattern the nanowires into desired microstructures. The process began by photolithographically patterning Si wafers, and then etching them so that Si micropillars remained on the wafers. Then, in order to form SiNWs from the pillars, a metal-assisted chemical etching (MACE) mechanism [5] was employed, during which the wafer was submerged in a solution of hydrofluoric acid (HF) and silver nitrate (AgNO3), resulting in SiNWs that collectively were shaped in the original patterned structure. For the current purpose, however, there were restrictions present. During the etching process, the desired mechanism was for etching to occur only at the top of a micropillar, and not at its sides nor at the substrate level. In order to achieve this, an etch mask had to be laid down to coat the sidewalls and the substrate to protect them from the MACE process. Various masks were tested, and the results showed that a Teflon®-like polymer deposited by deep reactive ion etching (DRIE) from C4F8 plasma produced an effective mask. Experimental Methods: A basic photolithography process combined with DRIE was employed to pattern micropillars on bare Si wafers (Figure 1). After the spin-coating of photoresist (PR, SPR 220-3.0, 3.5 μm), the wafers were exposed to ultraviolet light for six seconds in a mask aligner, exposing the areas intended for patterns. After development of the PR for one minute, Si micropillars (20-40 μm tall) were formed by DRIE. Then, the etch mask was deposited using only the passivation layer (no etchant) in the DRIE process, flowing C4F8 plasma (200 sccm, 0.5-1.5 minutes). Top-Down Fabrication of Patterned, Vertically Aligned Silicon Nanowires Sibu Kuruvilla Materials Science and Engineering, University of Illinois at Urbana-Champaign NNIN REU Site: Lurie Nanofabrication Facility, University of Michigan, Ann Arbor, MI NNIN REU Principal Investigator(s): Prof. Anastasios John Hart, Mechanical Engineering, University of Michigan at Ann Arbor NNIN REU Mentor(s): Sameh H. Tawfick, Mechanical Engineering, University of Michigan at Ann Arbor Contact: skuruvi2@illinois.edu, ajohnh@umich.edu, stawfick@umich.edu Figure 1: A map of the step by step SiNW fabrication method.