Integration of Nano Sensors on Micromachined Needles for Medical Applications

Over the past years, Silicon Nanowires (SiNWs) biosensors have been studied for the detection of biological or chemicals molecules as highly selective, sensitive and label-free tool. Moreover, in the recent years advanced microfabrication techniques have enabled the fabrication of micron-size needles for medical applications with different designs to promote minimally invasive devices. However, assembling these two different kinds of technology in a unique device is still challenging. In this work, we propose a microfabrication process that will integrate SiNWs based devices on micromachined needles for medical applications.

The SiNWs devices are fabricated by electron beam lithography on SOI wafer (55 nm n-type doped Si with 300 nm box) and patterned with reactive ion and TMAH etching. Then mix and match approach with optical lithography is used to define metal pads. After its fabrication, micromachined needles are patterned.

For a proof of concept, micromachined chips comprising six 150 µm nominal thick needles were fabricated. Firstly, on the top of a 725 µm silicon wafer the needles shape was patterned on a photoresist mask and then deep reactive ion etching (DRIE) was used to etch at least the target needles thickness. The releasing of the chips was performed on the back-side with the chip frame shape on an oxide mask with precisely controlled DRIE to reach the nominal 150 µm thick needles. DRIE process was highly uniform on wafer top side with less than 2% non-uniformity, while the back-side etching had higher non-uniformity (less than 10 %), due to the patterned structures on the front-side. Still, enough to fabricate maximum 153.6 µm thick needles on the center wafer region and minimum 94.2 µm thick needles on the wafer edges, where the DRIE is less uniform. Preliminary tests with the silicon microneedles have shown robustness enough to not break after perforation.