Formation of cross-cutting structures with different porosity on thick silicon wafers

Abstract

Three-layered pass-through structures of two types have been obtained on 500μm thick single crystal silicon wafers by electrochemical etching in a 48% solution of hydrofluoric acid without using additional single crystal layer removal operations. The first type of the pass-through structures comprises two outermost 220–247.5μm thick macroporous silicon layers with a pore diameter of 7–10μm and an intermediate 5–60μm thick mesoporous silica layer with a pore diameter of 100–150nm. For the formation of the first type structures we used two-stage etching without cell disassembly:–in a 48% water solution of hydrofluoric acid (H2O: HF = 1: 1 vol.) for 140–180min with a current density of 40mA/cm2;–in a 48% water/alcohol solution of hydrofluoric acid (H2O: HF: С2Н5ОН = 1: 1: 1 vol.) for 60–90min with a current density of 10mA/cm2.The second type pass-through structures include a macroporous silicon layer with a thickness of 250μm which interlock in the depth of the silicon wafer to form a cavity with a size of 4–8μm. For the formation of the second type structures we only used the first one of the abovementioned stages, the etching time being longer, i.e. 210min. All the etching procedures were carried out in a cooling chamber at 5°C. The developed technology will provided for easier and more reliable formation of the monolithic structures of membrane-electrode assembly micro fuel cells.