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Dry etching uses gases or physical processes to create small channels in materials, allowing for more precision than wet etching but at a higher cost. Manufacturers choose between the two based on required precision and cost. Plasma etching is used to expose materials to gaseous chemicals or physical processes. There are three types of plasma etching: reaction ion etching, vapor phase, and spray etching. By-products must be quickly removed during the etching process. Dry etching allows for anisotropy, where the reaction occurs in only one direction, allowing for vertical channels without touching masked areas.
Dry etching is one of the two major etching processes used in microelectronics and some semiconductor manufacturing processes. Unlike wet etching, dry etching does not immerse the material to be engraved in liquid chemicals. Instead, it uses gases or physical processes to etch or create small shear channels in the material. Dry etching is more expensive than wet etching but allows for more precision in the type of channels created.
Manufacturers often decide between using dry or wet etching techniques based on the precision required in the engraved channels. If the channels need to be particularly deep, or of a specific shape, such as with vertical sides, dry etching is desired. Cost, however, is also a consideration, as dry etching costs considerably more than wet etching.
In both wet and dry etching, the area of the material that the manufacturer does not want to etch – usually called the wafer in microelectronics machining – is covered by a non-reactive substance or masked. Once masked, the material is subjected to a type of plasma etching, which exposes it to a gaseous chemical such as hydrogen fluoride, or undergoes physical processes, such as ion beam milling, which create the engraving without the use of gas.
There are three types of plasma etching. The first, reaction ion etching (RIE), creates channels through a chemical reaction that occurs between the ions in the plasma and the wafer surface, which removes small amounts of the wafer. RIE allows for a variation in channel structure, from nearly straight to fully rounded. The second plasma etching process, vapor phase, differs from RIE only in its simple setup. However, the vapor phase allows for less variation in the type of channels produced.
The third technique, spray etching, also uses ions to etch wafers. The ions in RIE and in the vapor phase sit on the wafer surface and react with the material. Spray etching, in contrast, bombards the material with ions to cut out specified channels.
Manufacturers must always quickly remove the by-products produced during the etching process. These by-products can prevent full etching if they condense on the wafer surface. They are often removed back to a gaseous state before the etching process is complete.
An attribute of dry etching is the ability of the chemical reaction to occur in only one direction. Called anisotropy, this phenomenon allows channels to be etched without the reaction touching the masked areas of the wafer. Usually this means that the reaction occurs in a vertical direction.
