Laser assisted and wet chemical etching of silicon nanostructures

Abstract

Etching of silicon can be accomplished in a number of ways: in aqueous solutions of fluoride or KOH, or by simultaneous exposure to a gas such as SF6 or HCl and high power laser irradiation. Here we review how laser assisted etching, chemically enhanced laser ablation, and stain etching can be used to create pillars, pores, porosified pillars, and macropores. Pillar dimensions can range from 10μm tall with ∼200nm tips to >100μm high with 10μm tips. Ordered arrays can be produced and sharpening can lead to aspect ratio approaching 104. Porous silicon with a visibly photoluminescent nanocrystalline structure can be made by etching in fluoride solutions. Acidic fluoride+oxidant solutions (stain etchants) allow us to form porous silicon on substrates of arbitrary topology. Moreover, these etchants can be used to control the photoluminescence spectrum of the resulting film, presumably through control of the size distribution of the nanocrystalline structures that are responsible for photoluminescence.

Ultrafast-laser-assisted chemical restructuring of silicon and germanium surfaces

“…For example, regular conical pillars can be formed during ns irradiation of Si in the presence of SF 6 but they are not formed for ns irradiation in the presence of HCl [9]. During ns pulsed irradiation of Si, HCl produces significantly blunter tips, the pillars are much taller (>50 mm) and they are porous rather than solid core [9]. This is a further indication that the mechanisms of pillar formation are not the same for fs and ns irradiation.…”

“…The role of chemistry in structure formation is not well understood and it is not a trivial result that regular conical pillars are formed during fs irradiation in SF 6 as well as HCl. For example, regular conical pillars can be formed during ns irradiation of Si in the presence of SF 6 but they are not formed for ns irradiation in the presence of HCl [9]. During ns pulsed irradiation of Si, HCl produces significantly blunter tips, the pillars are much taller (>50 mm) and they are porous rather than solid core [9].…”

Positive- and negative-tone structuring of crystalline silicon by laser-assisted chemical etching

“…Moreover, the knowledge and control of the material structural aspects would open a path to the fabrication of three-dimensional (3D) architectures in c-Si by combining in-volume laser writing and chemical etching, just as was done for fused silica after the first waveguides were fabricated [14][15][16]. In comparison to the previously reported laser-assisted chemical etching methods on the surface of silicon [17,18], such a technique would also add the interesting benefits of the absence of surface plasma effect (e.g., shielding) and/or debris. Making a first step in this direction, enhanced etch rates for strong acid solutions have been reported recently in the nanosecond laser damage regime, helping in the penetration of the etchant deep inside the material [12].…”

Formation of multiscale surface structures on nickel via above surface growth and below surface growth mechanisms using femtosecond laser pulses

“…Additionally, the modification of wettability from superhydrophilic [11,12] to superhydrophobic [13][14][15][16] is utilized for self-cleaning surfaces, drag reduction, and anticorrosive surfaces, among others. To date, most of the work using lasers for surface texturing has focused on what have been called pillars [5,[17][18][19], cones [20][21][22][23] or spikes [18,[24][25][26]. These are generally tightlypacked self-organized structures with an aspect ratio of 2:1 or higher that develop over a series of tens to hundreds of laser shots.…”