In a reactive ion etching process, process gases are decomposed into free electrons, ions and radicals in a plasma between two electrodes - typically the chamber walls and the sample plate. Due to their low mass, electrons can easily follow the electrical alternating field of the plasma generator and hit both chamber walls and sample plates. The electrons can flow off the grounded walls. On the inductively driven sample plate, on the other hand, a charge builds up which leads to a negative self-bias voltage. In this electric field, ions are accelerated onto the substrate and lead to physical sputter removal. The ions accelerated perpendicular to the substrate contribute strongly to the anisotropic component of the etching process. Charge-neutral radicals on the other hand are responsible for the chemical and isotropic components of the etching process.
By precisely adjusting the etching process, the side walls can be passivated against chemical etching, while the base of a structure to be etched is freed from the passivating layer by ion-based sputter removal.
If passivating effects by polymer deposition or oxidation are avoided (e. g. no carbon-containing process gases), almost exclusively isotropic chemical etching processes take place in the RIE. These are much more effective and allow significantly higher etching rates than anisotropic processes. By means of isotropic etching processes it is also possible to under-etch masked areas, with which free-standing structures can be realized.
Oxford Plasmalab 80 Plus
Our RIE system Plasmalab 80 in parallel plate reactor configuration is mainly designed for anisotropic etching of silicon and silicon oxide (no Bosch process), but thin titanium layers (< 1 µm) can also be etched anisotropically.
In particular, silicon, silicon oxide, silicon nitride, titanium and resists for photo-, imprint and ebeam lithography can be etched isotropically.
O2, N2, Ar, CHF3, CF4 and SF6 are available as process gases. The temperature of the sample plate can be flexibly adjusted from 20 - 400°C and is kept constant with cooling water. An option for cryo-etching is not available. The sample plate is suitable for substrates up to 8".
