Editing Photolithography (section)

=== Etching, implantation ===
{{Main|Etching (microfabrication)}}

In etching, a liquid ("wet") or [[plasma (physics)|plasma]] ("dry") chemical agent removes the uppermost layer of the substrate in the areas that are not protected by photoresist. In [[semiconductor fabrication]], [[dry etching]] techniques are generally used, as they can be made [[anisotropic]], in order to avoid significant undercutting of the photoresist pattern. This is essential when the width of the features to be defined is similar to or less than the thickness of the material being etched (i.e. when the aspect ratio approaches unity). Wet etch processes are generally isotropic in nature, which is often indispensable for [[microelectromechanical systems]], where suspended structures must be "released" from the underlying layer.

The development of low-defectivity anisotropic dry-etch process has enabled the ever-smaller features defined photolithographically in the resist to be transferred to the substrate material.