Removing material on purpose, with a chemical or a plasma. In chip-making it carves patterns: a stencil of light-sensitive polymer protects some areas while a plasma eats away the rest. For etching has three jobs: cutting into shaped devices, thinning a crystal one layer at a time, and – for – making the material in the first place, by dissolving one kind of atom out of a layered ceramic so that the remaining sheets come apart.
Going deeper
Etching carves devices out of flakes through a resist mask, strips a layered crystal one layer at a time, and makes MXenes by dissolving the aluminium layers out of a MAX phase so that the carbide sheets come apart, carrying new surface groups.
Patterning a flake
To turn a flake into a device, a pattern is written in a polymer resist with light or an electron beam, the uncovered parts are etched away and the resist is dissolved. Oxygen plasma burns graphene away as carbon dioxide; fluorine-based plasmas etch hBN and the . In stacks, etching through the whole sandwich exposes a clean edge of the graphene inside, and metal deposited onto that edge makes the low-resistance edge contacts that high- devices rely on.
Every etched edge is a defect of its own. Plasmas leave rough, chemically altered edges, damage the layers next to the exposed one and leave residues, and in narrow channels edge scattering and can dominate what is measured.
One layer at a time
Layered crystals can be thinned precisely because their layers are only weakly bound. Gentle plasmas, controlled oxidation followed by removal of the oxide, or heating in air can strip the top layer of MoS2 or black phosphorus while leaving the one below largely intact – a way to reach a chosen thickness after a device has been made. Atomic layer etching, the counterpart of , does the same in the industry with self-limiting cycles that modify and then remove the surface.
Etching that makes a material
MXenes are made by etching. Their parent MAX phases, such as Ti3AlC2, are layered ceramics in which sheets of titanium carbide are bonded through layers of aluminium. Hydrofluoric acid, or a mixture of a fluoride salt and hydrochloric acid, dissolves the aluminium layers selectively and leaves the carbide sheets covered in oxygen, hydroxyl and fluorine groups; shaking, or slipping molecules in between, then separates the sheets. The etching conditions set the surface groups and the defects, and so the conductivity and stability of the MXene, which is why recipes matter as much as the formula.
For specialists
Material removal by chemical reaction in a liquid (wet etching) or by reactive and physical processes in a plasma (dry etching, including reactive ion etching with oxygen-, fluorine- or chlorine-based chemistries, and atomic layer etching). For 2D materials it patterns channels and through resist masks – oxygen plasma for graphene, fluorine-based plasmas for hBN and TMDCs – exposes edges in encapsulated stacks for edge contacts, thins crystals layer by layer, and selectively removes the A element, usually aluminium, from MAX phases with hydrofluoric acid or fluoride-containing acids to make MXenes. Its side effects are edge roughness and damage, residues, and plasma-induced defects in neighbouring layers, so the etched edge and its electronic states have to be considered in narrow devices.