Chemical erosions of selected portions of a surface in order to produce a desired pattern on the surface. The technique is widely used in microminiature electronics. Wet etching utilizes liquids, such as acids or other corrosive chemicals, as the etching agent. The etching process takes place by means of chemical reactions at the surface of the material. This process is limited both by the rate at which the chemical reactions can take place, and by the rate at which the products of the chemical reaction can be removed. These factors are a function of the nature of the liquid used and the temperature at which the etching is carried out. Etching in which the main limiting factor is the reaction rate is known as reaction rate limited, surface limited or kinetically limited etching. Where the removal rate of the chemical products is the predominant controlling factor the etching is known as diffusion limited or mass transport limited etching. In certain applications the etching process can be electrically aided by making the slice to be etched the anode or cathode of an electrolytic cell.

• • Dry etching uses either chemical or physical reactions between a low-pressure plasma or glow discharge and the surface to be etched. It has several advantages compared to wet etching, but the etching process itself is very complex, and the results can be greatly affected by small variations in the process parameters. Dry etching is capable of patterning smaller geometries than wet etching; lateral etch rates close to zero can be produced under certain conditions, and smooth edge profiles can be produced when needed for metal crossovers. Certain semiconductors, particularly gallium arsenide, have no suitable liquid etchants that can produce deep narrow features, and dry etching is particularly important in these cases.

• • Plasma etching is any process in which a plasma generates reactive species that then chemically etch material in direct proximity to the plasma. If the chemical reactions are enhanced by the kinetic energy of the ions in the plasma the process is described as kinetically assisted chemical reaction. Reactive ion etching is similar to plasma etching, but uses only kinetically assisted chemical etching. The applied voltage drop is mainly at the surface of the slice. Reactive ion beam etching separates the slices from the plasma by a grid that accelerates the ions created in the plasma towards the slice. The ion energy is higher, and some of the etching is due to physical reactions.

• • Sputter etching uses energetic ions from the plasma to physically blast (sputter) atoms from the surface. No chemical reactions are involved. Ion milling is also a purely mechanical method that uses a roughly collimated beam of energetic ions to erode a surface by bombardment. Ion milling, unlike other dry etching procedures, can be used at angles other than perpendicular to the slice.