'''Deep Reactive Ion Etching''' or '''DRIE''' is a highly [[anisotropic]] [[etching|etch]] process developed in thefor [[semiconductorMEMS]] industry (also known as microsystems technology) and used to create deep and high [[aspect ratio]] channelsholes inand materialstrenches such asin [[silicon]] (and other materials). Widely used forin [[MEMS]] applications like mechanical resonators and highmicrofluidic valuechannels integratedas well as in [[capacitor]]s, channelsstructures with vertical sides and having [[aspect ratio]]s greater than 20:1 can be produced.
RIE "deepness" depends on application: in DRAM memory circuits capacitor trenches may be 10-20 micrometers deep, while in MEMS DRIE is used for anything from a few micrometers to 0.5 mm. What distinguishes DRIE from RIE is actually etch rate: while 1 micron/minute is reasonable etch rate for RIE (as used in IC manufacturing), DRIE rates are 5-10 microns per minute.
There are three primary processes which are brought together to achieve the results in DRIE. First a highly reactive gas is used to perform an [[isotropic]] etch of the [[substrate]]. After a brief period the etching is stopped and the process switches over to deposition of a layer of [[passivation]] over the whole surface. This protects the substrate from further chemical attack and prevents further etching. The process now returns to etching, which is where the third process comes into play. Within the chamber there is an energetic [[Plasma (physics)|plasma]] which produces a [[collimated]] stream of [[ion]]s that bombard the substrate. By a process of [[sputtering]] these ions remove the passivation from the bottom of the previous etch step, but not from the sides. The etchant chemicals can then erode only the bottom of the channels.▼
There are two main technologies for high rate DRIE: cryogenic and Bosch (a.k.a. time multiplexed or pulsed etching). In cryo-DRIE the wafer is cooled down to -110 degrees Celcius, which slows down spontaneous chemical (isotropic) etching, and only the ion bombardment driven etching of horizontal surfaces proceeds. Another mechanism is sidewall passivation: SiOxFy moieties (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.
The process is repeated many times over resulting in a large number of very small [[isotropic]] etch steps taking place only at the bottom of the etched pits. It is this selectivity that leads to the overall anisotropy of the process and the creation of high aspect ratio channels with vertical sidewalls.
▲ThereIn the Bosch process there are three primary processessteps which are brought together to achieve thenearly resultsvertical in DRIEstructures. First a highly reactive gas (SF6) is used to perform an nearly [[isotropic]] etch of the [[substrate]]. After a brief period (say 5-15 seconds) the etching is stopped and the process switches over to deposition: ofC4F8 asource layergas ofis used to deposit a Teflon-like [[passivation]] layer over the whole surface (this takes a few seconds). This polymer layer protects the substrate from further chemical attack and prevents further etching. The process now returns to etching with SF6, which is where the third process comes into play. Within the chamber there is an energetic [[Plasma (physics)|plasma]] which produces a [[collimated]] stream of [[ion]]s that bombard the substrate. By a process of [[sputtering]] these ions remove the passivation layer from the bottom of the previouspreviously etchetched steptrench, but not from the sides. Etching Thetherefore etchantis chemicalspreferentially canin thenthe erodevertical direction. These etch/deposit steps are repeated many times over resulting in a large number of very small [[isotropic]] etch steps taking place only at the bottom of the channelsetched pits. To etch through a 0.5 mm silicon wafer, for example 100-1000 etch/deposit steps are needed. A closer look at Bosch process etched sidewall shows undulation which has an amplitude of ca. 100-500 nm depending on whether the process was optimized for high rate or vertical walls or sidewall smoothness. Cryo etched sidewalls are smooth. In both Bosch and cryo processes 90 degrees truly vertical walls can be fabricated but often the walls are slightly tapered, e.g. 88 or 92 degrees (which is said to be retrograde).
In DRIE of glass the problem is the high plasma power needed, which makes it difficult to find suitable mask materials for truly deep etching. Polysilicon and nickel are used succesfully for 10-50 micrometer etched depths. In DRIE of polymers, Bosch process with alternating steps of oxygen etching and C4F8 passivation take place. Metal masks can be used.
