Etching refers to the process of transferring the resist image generated by lithography into the layer underneath. Etching can be carried out by chemical attack, physical damage, or some combination of both. Chemical or wet etches use liquid etching agents that are applied to the wafer surface. This purely chemical process has serious drawbacks like a lack of anisotropy, poor process control, and excessive particle contamination. However, wet etching can be highly selective and often does not damage the substrate. For this reason it continues to be used for a wide range of non-critical fabrication tasks. Physical or dry etching is on the contrary a highly anisotropic etching process and thus capable of transferring small features with the drawback of low selectivity to different materials. Physical etching is performed in a plasma environment. The gases are used to remove material by momentum transfer or to assist a chemical reaction. The amount of the physical and chemical attack is determined by the pressure inside the chamber. For very low chamber pressure the process is called ion milling [Diz93]. The etch mechanism is a purely physical one and the selectivity is poor. Reactive ion etching is performed under increasing chamber pressure, which results in a more selective process due to the increased amount of the chemical reaction [Gér89]. Plasma etching uses very high chamber pressures. The dominant etching mechanism is a chemical one and the selectivity is very high [Sam94].