Dry Etching is an etching process that does not utilize any liquid chemicals or etchants to remove materials from the wafer, generating only volatile byproducts in the process. Dry etching may be accomplished by any of the following: 1) through chemical reactions that consume the material, using chemically reactive gases or plasma; 2) physical removal of the material, usually by momentum transfer; or 3) a combination of both physical removal and chemical reactions.
Plasma etching is an example of a purely chemical dry etching technique. On the other hand, physical sputtering and ion beam milling are examples of purely physical dry etching techniques. Lastly, reactive ion etching is an example of dry etching that employs both physical and chemical processes.
Like wet etching, dry etching also follows the resist mask patterns on the wafer, i.e., it only etches away materials that are not covered by mask material (and are therefore exposed to its etching species), while leaving areas covered by the masks almost (but not perfectly) intact. These masks were deposited on the wafer by an earlier wafer fab step known as 'lithography.'
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