Abstract
The anodic dissolution of silicon in acidic electrolytes is a well-known technology enabling the silicon machining to be accurately controlled down to the micrometer scale in low-doped n-type silicon electrodes. Attempts to scale down this technology to the submicrometer scale has shown to be challenging, though it premises to enable the fabrication of meso and nano structures/systems that would greatly impact the fields of biosensors and nanomedicine. In this work, we report on the electrochemical etching at high anodic voltages (up to 40 V) of two-dimensional regular arrays of millions pores per square centimeter (up to 30 × 106 cm−2) with sub-micrometric diameter (down to ~860 nm), high depth (up to ~40 μm), and high aspect-ratio (up to ~45) using low-doped n-type silicon electrodes (resistivity 3–8 Ω cm). The use of high anodic voltages, which are over one order of magnitude higher than that commonly used in electrochemical etching of silicon, tremendously improves hole focusing at the pore tips during the etching and enables, in turn, the control of electrochemical etching of submicrometer-sized pores when spatial period reduces below 2 μm. A theoretical model allows experimental results to be interpreted in terms of an electric-field-enhanced focusing of holes at the tip apex of the pores at high anodic voltages, with respect to the pore base, which leads to a smaller curvature radius of the tip apex and enables, in turn, the etching of pore tips to be preferentially sustained over time and space.
Highlights
From first discovery to modern days, the possibility of controlling the electrochemical preparation of pores in silicon at nano to micro scales in terms of both size and pattern has fascinated scientists for more than 50 years (Uhlir, 1956).In 1990, Lehmann and Föll reported for the first time on how to control the anodic dissolution of n-type silicon electrodes in low concentration hydrofluoric acid (HF)-based aqueous electrolytes by back-side illumination, so enabling the etching of regular two-dimensional lattices of pores at the micrometer scale (Lehmann and Föll, 1990)
Once a lattice of pits is pre-defined on the silicon surface, size, position, and morphology of pores etched by back-side illumination electrochemical etching
Flat silicon electrodes were loaded in a three-electrodes electrochemical cell containing the HF-based electrolyte, and the current flowing through the etch-cell was monitored upon application of voltages varying from 2 to −1.5 V with a sweep rate of −0.1 V s−1, under back-side illumination of silicon
Summary
In 1990, Lehmann and Föll reported for the first time on how to control the anodic dissolution of n-type silicon electrodes in low concentration HF-based aqueous electrolytes by back-side illumination, so enabling the etching of regular two-dimensional lattices of pores at the micrometer scale (Lehmann and Föll, 1990). Once a lattice of pits is pre-defined on the silicon surface, size, position, and morphology of pores etched by back-side illumination electrochemical etching. The BIEE of silicon has evolved into a highly versatile microstructuring technology, namely electrochemical micromachining (ECM), with unique features, among which there are high aspect-ratio of etched structures (>100), low roughness of etched surfaces (
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