A dissolved wafer process using a porous silicon sacrificial layer and a lightly-doped bulk silicon etch-stop

A micromachining technique has been developed which uses a porous silicon sacrificial layer to release bulk silicon structures in a dissolved wafer process. It uses a lightly-doped bulk silicon etch-stop, allowing the micromachining of silicon structures on which circuitry may be fabricated. Due to the doping selectivity of porous silicon formation, bulk structures undercut using porous silicon are normally n-type. This process, however, provides a method for undercutting p-type structures with an n-type porous sacrificial layer, providing greater flexibility in the circuit process used. The device substrates are defined by either a low-dose implant or by epitaxial silicon growth. Pores are formed in the silicon surrounding the devices, both in the field and underneath the structures. This porous sacrificial layer is later removed in room-temperature KOH. The process has been used to fabricate silicon neural probes up to 500 /spl mu/m in width and is especially advantageous in fabricating electrode arrays for cochlear prostheses.