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

Abstract

Porous silicon (PS) offers new possibilities to be applied as thermal insulating material for microsensor design due to its low thermal conductivity (TC) value compared with TC of SiO 2. A biomedical TC microsensor based on differential thermoelectric measurements has been designed using a PS substrate. In order to ensure an efficient thermal isolation in the microsensor, main thermal and geometrical characteristics of the PS layers as well as of the whole microsensor have been numerically simulated. PS layers with low TC have to be thick and mechanically stable under further processing. To form thick (50–200 μm) and stable PS layers, a new approach based on progressive changing of anodisation current density (from 100 to 25 mA/cm 2) during PS formation has been elaborated. To find a suitable compromise between low TC and mechanical stability of thick PS layers, an adapted thermal oxidation recipe at moderate temperatures (500–600°C) in dry oxygen atmosphere has been applied. It leads to 20–50% oxidation fraction in PS layers (measured by Energy Dispersive Spectroscopy) corresponding to SiO 2 TC value. A test device has been realised and characterised. A Seebeck coefficient of 400 μV/°C per junction has been measured for a Poly-Si/Al thermopile deposited on the PS layer.