Fig. 9 |. Schematics of the fabrication process for the hollow-needle array with sharp tip and off-center bore (Type III).

a, Spin-coat AZ 1518 photoresist on the front side of the Si wafer with PECVD SiO₂ deposition. b, Expose the layout (Fig. 3e) using the maskless aligner followed by development. c, Etch away the SiO₂ layer in the patterned area. d, Remove the residual AZ 1518 and spin-coat SPR 220-7.0 photoresist. e, Pattern the disk arrays with an off-centered hole (Fig. 3f). f, Cover the backside holes using polyimide tape and carry out isotropic Si etching to obtain sharp tips. g, Etch through the SiO₂ layer to open the off-centered holes. h, Form the off-centered hollow-needle array using the Bosch process. i, Remove the residual photoresist and oxide, followed by cleaning the wafer. See Step 1C(i–ii) for details of all the processes.