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'''Deep reactive-ion etching''' ('''DRIE''') is a highly [[anisotropic]] [[etching (microfab)|etch]] process used to create deep, steep-sided holes and trenches in [[wafer (semiconductor)|wafer]]s, with [[aspect ratio (image)|aspect ratio]]s of 20:1 or more. It was developed for [[microelectromechanical systems]] (MEMS), which require these features, but is also used to excavate trenches for high-density [[capacitor]]s for [[dynamic random access memory|DRAM]] and more recently for creating through wafer via's (TSV)'s in advanced 3D wafer level packaging technology .
There are two main technologies for high-rate DRIE: cryogenic and Bosch, although the Bosch process is the only recognised production technique. Both Bosch and cryo processes can fabricate 90° (truly vertical) walls, but often the walls are slightly tapered, e.g. 88° or 92° ("retrograde").
Another mechanism is sidewall passivation: SiO<sub>x</sub>F<sub>y</sub> [[functional group]]s (which originate from sulphur hexafluoride and oxygen etch gases) condensate on the sidewalls, and protect them from lateral etching. As a combination of these processes deep vertical structures can be made.
== Cryogenic process ==
In cryo-DRIE, the wafer is chilled to −110 °C (163 [[kelvin|K]]). The low temperature slows down the [[chemical reaction]] that produces isotropic etching. However, [[ion]]s continue to bombard upward-facing surfaces and etch them away. This process produces trenches with highly vertical sidewalls. The primary issues with cryo-DRIE is that the standard masks on substrates crack under the extreme cold, plus etch by-products have a tendency of depositing on the nearest cold surface, i.e. the substrate or electrode.
== Bosch process ==
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