Deep reactive-ion etching: Difference between revisions

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{{Short description|Highly anisotropic etch process}}

{{More citations needed|date=December 2009}}

'''Deep reactive-ion etching''' ('''DRIE''') is a highly [[anisotropy|anisotropic]] [[etching (microfab)|etch]] process used to create deep penetration, steep-sided holes and trenches in [[wafer (semiconductor)|wafer]]s/substrates, typically with high [[aspect ratio (image)|aspect ratio]]s. 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 silicon vias ([[Through-silicon via|TSVs]]) in advanced 3D wafer level packaging technology. In DRIE, the substrate is placed inside a reactor, and several gases are introduced. A plasma is struck in the gas mixture which breaks the gas molecules into ions. The ions accelerated towards, and react with the surface of the material being etched, forming another gaseous element. This is known as the chemical part of the reactive ion etching. There is also a physical part, if ions have enough energy, they can knock atoms out of the material to be etched without chemical reaction.

DRIE is a special subclass of RIE.

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° ("reentrant") or 92° ("retrograde").

Another mechanism is sidewall passivation: SiO_xF_y [[functional group]]s (which originate from sulphur hexafluoride and oxygen etch gases) condense on the sidewalls, and protect them from lateral etching. As a combination of these processes deep vertical structures can be made.

==Cryogenic process==

In cryogenic-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==

[[File:Bosch process PILLAR.jpg|thumb|alt=A silicon pillar fabricated using the Bosch process|A silicon micro-pillar fabricated using the Bosch process]]