2.2 Etching

After the photoresist image has been formed on top of the wafer, it can be transferred to the underneath layers by etching. This is done by removing the materials not protected by the resist. Etching can be performed by chemical attack - wet etching, and physical damage - dry etching. In the first case, liquid etching agents are applied to the wafer, and this chemical process etches at the same rate in all directions, which causes undercuting of the patterned areas, a serious drawback in modern technologies where small features need to be resolved. Other problems are a poor process control and excessive particle contamination, limiting the feature size to abount $1\mu m$. However, as it can be highly selective in the materials to be (or not) etched, it is still used in non-critical or unmasked steps.

When submicron features are to be resolved, dry etching is performed. Dry etching uses a plasma environment, that can have high anisotropies, and it is thus much more insensitive to the undesirable undercutting. Plasma etching is also a more controllable process and it is immune to temperature variations. In this technique the materials are removed by momentum transfer and the etching characteristics depend on the pressure inside the chamber. The lower the pressure, the better the achievable resolution is. At very low pressures, pure ion milling or ion beam etching is capable of fine resolutions, but suffers from a lack of selectivity, as it does not differentiate among different materials. A good compromise between resolution and selectivity is achieved by Reactive Ion Etching (RIE). It is performed under increased chamber pressure, but results in a more selective process as the etching mechanism is in part a chemical reaction.