Double Sided Porous Silicon on Patterned Substrates for Thermal Effect Microsystems

ABSTRACTApplication of porous silicon in thermal microsystem structures often requires the formation of deep localized porous silicon layers. The most commonly used method to prepare the porous layers is the dc anodic etching of monocrystalline silicon in a hydrofluoric acid (HF) based electrolyte. However inhomogeneity of the nanocrystallite size along the layer depth due to the decrease of HF concentration within the pores as well as the poor uniformity of the porous layer thickness limit the elaboration of deep porous layers. Thus we propose an original pulsed anodisation technique, using a double tank etching cell that allows localized porous silicon layers formation throughout the whole wafer thickness.Furthermore a selective double sided pulsed anodisation of silicon was performed on patterned silicon substrates. Porous silicon is formed in pre-determined parts of the wafer using composite polysilicon-silicon nitride masking layers. Technological solutions to get rid of porous layer thickness inhomogeneity due to non uniform current density distribution are discussed. Finally a toric porous silicon layer, crossing the whole silicon wafer, surrounding a 20 mm diameter monocrystalline silicon cylinder was successfully achieved ensuring a new approach of thermal insulation for thermal effect microsystems.