Deep reactive ion etching to realize silicon nano and micro needles and nanostructures

Hollow needles are important structures for smart drug delivery and biotechnology applications. In this paper we report a novel hydrogen-assisted deep reactive ion etching technique to realize high resolution micro and nanometric features on silicon and used it to create hollow needles. Such small features can be applied in the fabrication of micro-electric pumps, ultra-low-frequency acceleration sensors and drug injectors. The etching technique is based on a sequential passivation and etching process, similar to the so-called Bosch process. However, in our approach, the polymer coating sequence is replaced with a hydrogen-passivation step during which a mixture of H2/O2/SF6 gases is used with typical flows of 100/85/10 seem. The etching subsequence carried out after the passivation step, is achieved using a SF6 gas flow with a typical value of ¿OSccm and with no need to a high power plasma source (ICP). The number of sequences, plasma power and duration of each cycle play crucial role in the final result both in terms of the feature size as well as the etch rate. Fig. 1 collects the SEM images of various samples prepared for this investigation where a deep trench in silicon with an aspect ratio of 40:1 is observed. The opening is as low as 0.4 ¿m and the depth is more than 15 ¿m. The formation of black-silicon or grass is a side-effect of RIE techniques. We have been able to control the evolution of silicon nano-grass by properly adjusting the hydrogenation conditions. We show the highly controlled evolution of nano-grass layers on silicon in desired shapes and locations. Such nanostructures can be used to improve the liquid-solid interface or to increase the effective surface of gas sensors.