3D Patterning of Si by Contact Etching With Nanoporous Metals

Abstract

Nanoporous gold and platinum electrodes are used to pattern n-type silicon by contact etching at the macroscopic scale. This type of electrode has the advantage of forming nanocontacts between silicon, the metal and the electrolyte as in classical metal assisted chemical etching while ensuring electrolyte transport to and from the interface through the electrode. Nanoporous gold electrodes with two types of nanostructures, fine and coarse (average ligament widths of ~30 and 100 nm, respectively) have been elaborated and tested. Patterns consisting in networks of square-based pyramids (10 × 10 μm² base × 7 μm height) and U-shaped lines (2, 5, and 10 μm width × 10 μm height × 4 μm interspacing) are imprinted by both electrochemical and chemical (HF-H₂O₂) contact etching. A complete pattern transfer of pyramids is achieved with coarse nanoporous gold in both contact etching modes, at a rate of ~0.35 μm min^-1. Under the same etching conditions, U-shaped line were only partially imprinted. The surface state after imprinting presents various defects such as craters, pores or porous silicon. Small walls are sometimes obtained due to imprinting of the details of the coarse gold nanostructure. We establish that np-Au electrodes can be turned into "np-Pt" electrodes by simply sputtering a thin platinum layer (5 nm) on the etching (catalytic) side of the electrode. Imprinting with np Au/Pt slightly improves the pattern transfer resolution. 2D numerical simulations of the valence band modulation at the Au/Si/electrolyte interfaces are carried out to explain the localized aspect of contact etching of n-type silicon with gold and platinum and the different surface state obtained after patterning. They show that n-type silicon in contact with gold or platinum is in inversion regime, with holes under the metal (within 3 nm). Etching under moderate anodic polarization corresponds to a quasi 2D hole transfer over a few nanometers in the inversion layer between adjacent metal and electrolyte contacts and is therefore very localized around metal contacts.

Keywords: patterning; MACE; contact etching; imprinting; nanoporous gold; silicon.

Figure 1

Scheme of the contact etching process: (A) diagram of the fluxes of electrons (red arrows) and chemicals (blue arrows) through a macroscopic piece of np-Au showing the nanometer-scale Au/Si contacts; (B) principle of 3D imprinting of silicon using a np-Au electrode with a pattern consisting in pyramids. Anodic polarization of the electrode can be provided either through an external circuit (electrochemical contact etching) or in situ by H₂O₂ (chemical contact etching).

Figure 2

SEM images of np-Au electrodes elaborated by: (A) electrochemical dealloying at 0.7 V_SME in HClO₄ (0.77 mol L⁻¹) at 60°C for 53 h; (B) chemical dealloying in HNO₃ (14.2 mol L⁻¹, 65 wt.%) at 80°C for 12 h.

Figure 3

SEM images at different magnification of np-Au electrodes dealloyed chemically (A–E) and electrochemically (F), with two different surface patterns: (A–C,F) array of square based pyramids (10 × 10 μm² × 7 μm depth); (D,E) parallel lines with rectangular cross section (5 × 5 μm² (width × height), spacing of 4 μm).

Figure 4

SEM images of silicon surfaces after imprinting inverted pyramids with a fine (A,B) and a coarse (C,D) np-Au electrode. Electrochemical contact etching was carried at 0.3 V_SME for 10 min (A,B); at 0.3 V_SME for 20 min (C) and at 0.2 V_SME for 20 min (D). Insets: (B) inverted pyramid border at higher magnification; (C) inverted pyramid array at lower magnification. Electrolyte: HF 5 mol L⁻¹ with 2 vol.% EtOH. Scale bars: 2 μm unless otherwise noted.

Figure 5

SEM images of silicon surfaces after imprinting U-shaped lines with np-Au electrode: (A) fine nanostructure (0.3 V_SME, 20 min); (B) coarse nanostructure (0.2 V_SME, 20 min). Insets: (A, B-upper) high magnification image of the etched area; (B-bottom) U-shaped line array at a lower magnification. Electrolyte: HF 5 mol L⁻¹ with 2 vol.% EtOH.

Figure 6

SEM images of silicon surfaces after imprinting inverted pyramids in HF-H₂O₂ with (A) fine np-Au and (B) coarse np-Au, and U-shaped lines with (C) coarse np-Au/Pt electrodes. Electrolyte: HF 5 mol L⁻¹ - H₂O₂ 1 mol L⁻¹, with 2 vol.% EtOH.

Figure 7

SEM images of silicon surfaces after imprinting chemically (A) inverted pyramids and (B) U-shaped lines in HF-H₂O₂ with coarse np-Au/Pt electrodes. Electrolyte: HF 5 mol L⁻¹ - H₂O₂ 1 mol L⁻¹, with 2 vol.% EtOH. In (B): line offset with respect to < 110> directions, as determined by EBSD.

Figure 8

Simulation of 3 gold ligaments (20 nm in size and interspace) surrounded by an electrolyte and in contact with n-type silicon (3 × 10¹⁵ cm⁻³) through 2D profiles of the valence band energies (referenced to the Fermi level) at equilibrium, at low (A) and high (B) magnification. The VB energy color scale is given on the right-hand side.

Figure 9

Simulations of: (A) the band bending of n-Si/Au and n-Si/electrolyte junctions with cut-lines (y Depth) corresponding to a gold pad center (x = 0.5 μm), an electrolyte contact between two gold pads (x = 0.48 μm) and the electrolyte far from gold (x = 0 μm); (B) lateral modulations (x Width) of the valence band energies at 3 nm beneath the silicon surface, at 0 V (equilibrium) and under 0.1, 0.2, and 0.7 V bias applied to the gold contacts (cf. Supplementary Information); (C) I-V characteristics of the Au/n-Si/Electrolyte device.