dioxide and silicon nitride. A plasma containing oxygen is used to oxidize ("ash") photoresist and facilitate its removal. Ion milling, or sputter etching, uses lower pressures, often as low as 10−4 Torr (10 mPa). It bombards the wafer with energetic ions of noble gases, often Ar+, which knock atoms from the substrate by transferring momentum. Because the etching is performed by ions, which approach the wafer approximately from one direction, this process is highly anisotropic. On the other hand, it tends to display poor selectivity. Reactive-ion etching (RIE) operates under conditions intermediate between sputter and plasma etching (between 10−3 and 10−1 Torr). Deep reactive-ion etching (DRIE) modifies the RIE technique to produce deep, narrow features. Common etch processes used in microfabrication See also Chemical-Mechanical Polishing Ingot sawing Metal assisted chemical etching Lift-off (microtechnology) References Ibid, "Processes for MicroElectroMechanical Systems (MEMS)" Inline references External links Semiconductor technology Semiconductor device fabrication Etching Microtechnology