Low T Etching of Silicon
In the HNF, two approaches are pursued to etch deep trenches in Si. Depths of several 100µm can be achieved. On the one hand, the Bosch process is available, on the other hand a low-temperature process (T=-120°C) facilitating SF6 and oxygen is carried out.
In Reactive Ion Etching Si is etched by bromine, chlorine or fluorine radicals. Since the reaction of these radicals with Si has a very low activation energy, the processes tend to be isotropically. In the case of low-temperature SF6/O2 plasma, a mixture of sulphur hexafluoride and oxygen is injected into the ICP source chamber of the RIE system to form a plasma. This essentially produces SFx and Fy ions, as well as O* and F* radicals. If a SiO2 hard mask is used for etching instead of a photo/e-beam resist mask, due to the absence of carbon no polymer layer can form on the chamber walls; this leads to a more stable etching process. In addition, only a thin passivation layer (SiFxOy) forms on the sample surface. The low-energy –ions of the plasma (low BIAS voltage -32V) can removed this passivation layer at the bottom of the structure to be etched, so that the etching attack can continue there. However, by properly choosing the process parameters (-120°C, pressure, O2 content; injected ICP and CCP power), the energy transfer can be tuned to be insufficient to remove the passivation on the sidewalls, thus achieving anisotropic etching. Since temperature is a major factor in the reaction of the radicals with Si, good thermal coupling of the sample to the cooled electrode surface is essential to maintain the temperature level of -120°C during etching.
Etching is strongly dependent on the aspect ratio of the structure: At a high aspect ratio, the relative composition of the reactive species in the structure is different from the composition at low aspect ratio. Therefore, the process parameters, both the gas composition and the etching time, need to be optimised depending on the structure and the target depth. The figure shows an array of 1 µm wide and 8 µm deep square holes etched through a SiO2 hard mask.