Thick oxidised porous silicon layers for the design of a biomedical thermal conductivity microsensor

Porous silicon (PS) offers new possibilities to be applied as thermal insul ating material for microsensor design due to its low thermal conductivity ( TC) value compared with TC of SiO2. A biomedical TC microsensor based on di fferential thermoelectric measurements has been designed using a PS substra te. In order to ensure an efficient thermal isolation in the microsensor, m ain thermal and geometrical characteristics of the PS layers as well as of the whole microsensor have been numerically simulated. PS layers with low T C have to be thick and mechanically stable under further processing. To for m thick (50-200 mu m) and stable PS layers, a new approach based on progres sive changing of anodisation current density (from 100 to 25 mA/cm(2)) duri ng PS formation has been elaborated. To find a suitable compromise between low TC and mechanical stability of thick PS layers, an adapted thermal oxid ation recipe at moderate temperatures (500-600 degrees C) in dry oxygen atm osphere has been applied. It leads to 20-50% oxidation fraction in PS layer s (measured by Energy Dispersive Spectroscopy) corresponding to SIO2 TC val ue. A test device has been realised and characterised. A Seebeck coefficien t of 400 mu V/degrees C per junction has been measured for a Poly-Si/Al the rmopile deposited on the PS layer. (C) 1999 Elsevier Science S.A. All right s reserved.