What is Dry Etching?

Dry etching is one of two major etching processes used in microelectronics and some semiconductor processing. Unlike wet etching, dry etching does not submerge the material to be etched into liquid chemicals. Instead, it uses gas or physical processes to etch, or create small cut channels, in the material. Dry etching is more expensive than wet etching but allows for greater precision in the type of channels created.

Manufacturers often decide between using dry or wet etching techniques based first on the precision required in the channels etched. If the channels must be particularly deep, or of a specific shape — such as having vertical sides — dry etching is desired. Cost, however, is also a consideration, since dry etching costs considerably more than wet etching.

In both wet and dry etching, the area on the material that the manufacturer does not want etched — usually called a wafer in microelectronic processing — is covered with a nonreactive substance, or masked. Once masked, the material is either subjected to a type of plasma etching, which exposes it to a gaseous chemical like hydrogen fluoride, or subjected to a physical processes, such as ion beam milling, which creates the etch 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's surface, which removes small amounts of the wafer. RIE allows for a variation in channel structure, from nearly straight to completely rounded. The second process of plasma etching, vapor phase, differs from RIE only in its simple set up. Vapor phase allows less variation in the type of channels produced, however.

The third technique, sputter etching, also uses ions to etch the wafers. The ions in RIE and vapor phase sit on the surface of the wafer and react with the material. Sputter etching, in contrast, bombards the material with ions to carve out the specified channels.

Manufacturers must always quickly remove byproducts that are produced during the etching process. These byproducts can prevent the full etching to take place if they condense on the wafer's surface. Often they are removed by returning them to a gaseous state before the etching process is complete.

One attribute of dry etching is the ability for the chemical reaction to occur in just one direction. Called anisotropy, this phenomenon allows for channels to be etched without the reaction touching the masked areas of the wafer. Usually this means the reaction takes place in a vertical direction.