Deposition is any mechanism of removing a material from the surface of a sample or from the sample substrate itself. Typically, the material is masked from the etchant to create the desired pattern. Photoresist is a common masking material, but some situations require a more durable mask, such as silicon dioxide or some metals.
Plasma etching involves loading the sample into a vacuum chamber which is then injected with a reactive gas mixture that is ignited using a high power source. The resulting plasma reacts chemically and physically with the sample to remove the desired material.
Plasma etching has several advantages over wet etching. In particular, the process can be tuned very finely using several different parameters. In many cases, this allows for an anisotropic etch, which is difficult or impossible to achieve with most liquid-based etches. This allows for much finer feature sizes (down to several nm, limited mainly by the lithography used to define the mask) and much higher aspect ratios (in many cases > 10:1). Additionally, it does not require the sample to be immersed in any liquid, which can cause failure of suspended mechanical devices, e.g. stiction. However, it has the disadvantage that it typically cannot achieve as high selectivities as with wet etching.