Fabrication of high-aspect-ratio arrayed structures using Si electrochemical etching

This paper reviews microscale processes to fabricate three-dimensional structures, in particular, high-aspect-ratio arrayed structures, using precise electrochemical etching process. Array of sharp micropits were pre-formed on the front side surface of Si wafer and the electrochemical etching was carried out using aqueous HF solution, with the back side illumination to generate holes, which diffused to the edge of the micropits to proceed the dissolution of Si with fluoride species. In order to form high-aspect-ratio pores at selected areas, a shade mask pattern was fabricated on the back side surface to align the illumination to the pre-patterned area on the front side surface. The parameters, such as HF concentration, current density and hole diffusion length, were optimized and uniform array of straight pores was formed into the designed area of Si wafer. Subsequently the surface of the pores was thermally oxidized to form SiO₂ layers, and arrayed glass tubes with picoliter volume were fabricated. On the other hand, the pore filling with metal was attempted using electrodeposition to obtain array of the metal needles. For this, the "single batch" process was developed to form the pore array and metal filling with single electrolyte, and array of metal micro needles was successfully formed. These processes demonstrated capability and possibility of the electrochemical processes for microscale fabrication, and further precise processes can be developed using these approaches.