Deep reactive-ion etching (DRIE) is a highly anisotropic etch process used to create deep, steep-sided holes and trenches in wafers, with aspect ratios 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 capacitors for DRAM.
There are two main technologies for high-rate DRIE: cryogenic and Bosch.
Another mechanism is sidewall passivation: SiOxFy functional groups (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 cooled down to -110°C (163 K), which slows down spontaneous chemical (isotropic) etching, and only the ion bombardment driven etching of horizontal surfaces proceeds.
Cryo etched sidewalls are smooth.
Bosch process
The Bosch process, also known as pulsed or time-multiplexed etching, alternates repeatedly between two modes to achieve nearly vertical structures.
- A standard, nearly isotropic plasma etch. The plasma contains some ions, which attack the wafer from a nearly vertical direction. (For silicon, this often uses sulfur hexafluoride [SF6].)
- Deposition of a chemically inert passivation layer. (For instance, C4F8 source gas yields a substance similar to Teflon.)
Each phase lasts for several seconds. The passivation layer protects the entire substrate from further chemical attack and prevents further etching. However, during the etching phase, the directional ions that bombard the substrate attack the passivation layer at the bottom of the trench (but not along the sides). They collide with it and sputter it off, exposing the substrate to the chemical etchant.
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 etched pits. To etch through a 0.5mm 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. 100-500nm depending on whether the process was optimized for high rate or vertical walls or sidewall smoothness. In both Bosch and cryo processes 90° truly vertical walls can be fabricated but often the walls are slightly tapered, e.g. 88° or 92° (which is said to be retrograde).
Applications
RIE "deepness" depends on application: in DRAM memory circuits capacitor trenches may be 10-20µm deep, while in MEMS, DRIE is used for anything from a few micrometers to 0.5mm. What distinguishes DRIE from RIE is actually etch rate: while 1 µm/minute is a reasonable etch rate for RIE (as used in IC manufacturing), DRIE rates are 5-10µm/minute.
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 successfully for 10-50µm etched depths. In DRIE of polymers, Bosch process with alternating steps of oxygen etching and C4F8 passivation take place. Metal masks can be used.