An etching process is either used to remove complete material layers from the wafer (e.g. remove an oxide layer after ion implantation) or to selectively remove certain parts of a material (e.g. transferring patterns form the a resist layer into underlying materials). There are various applications for etching processes in all phases of the production process of an IC. Etching can be performed by a chemical attack (wet etching), by physical damage (dry etching) or by a combination of both .
Wet etching is performed by applying liquid etching agents to the wafer. This chemical process provides an isotropic etch rate and a high selectivity. High selectivity means that the etch rate strongly depends on the etched material. Additionally the crystalline structure of an attacked semi-conducting material like silicon is almost maintained also in the vicinity of the surface.
The major drawbacks are the poor reproducibility, especially because the etching time can be controlled very difficulty. Furthermore the wet etching process suffers from the excessive contamination of the semi-conducting device by the etching agents. Finally the strong isotropy of this process limits its applicability. The wet etching process is used for total or partial removal of whole layers. Due to the so-called under-etch effect, which can lead to a full detaching of the mask from the underlying material, wet etching is not suited to transfer patterns with sub-micron feature size.
Physical or dry etching is on the contrary a highly anisotropic etching process and thus capable of transferring small structures also below sub-micron feature size. In this technique material is removed by momentum transfer between particles in a plasma chamber and the target material. The etching characteristics mainly depends on the pressure in the plasma chamber. The lower the pressure, the better the achievable resolution is. For very low pressures the process is called ion milling . The drawback of the dry etching technique is the low selectivity which can be increased by raising the chamber pressure and accelerating thereby the chemical reaction rate. Reactive ion etching  is a good compromise between resolution and selectivity.
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