Deep reactive-ion etching

A highly anisotropic etch process developed in the semiconductor industry and used to create deep and high aspect ratio channels in materials such as silicon. Widely used for MEMS and high value integrated capacitors, channels with vertical sides and having aspect ratios greater than 20:1 can be produced.

There are three primary processes which are brough together to achieve the results in DRIE. First of all 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 attach 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 which produces a collimated stream of ions 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.

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.