Final capping passivation layers for long-life microsensors in real fluids

Final capping insulation layer is a critical step in the microfabrication process that determines the lifetime of analytical microsensors in real fluids. Actual processes encounter considerable limitations as (i) organic passivation layers do not provide a satisfying long-term protection against liquids and (ii) inorganic passivation processes (dielectric materials deposition and patterning) are very aggressive for the underlying layers, imposing severe constraints on the integration of sensitive materials. We present here a low temperature deposition process of high quality silicon nitride Si3N4 using ICP-CVD technique combined with a lift-off based process to pattern conformal deposition, in order to avoid harsh treatments such as wet or dry etching. High-density SiNx films with low H content (5 × 1020 at/cm3) were synthesized at 100 °C with controlled uniformity (5%), refractive index (2.025 at 830 nm), etch rate in buffered hydrofluoric acid (8 nm/min), residual stress (−500 MPa), breakdown field (3.9 MV/cm) and dielectric constant (6.0). In order to validate the compatibility of this passivation process with long-term fluids analysis, microelectrodes were fabricated and their lifetime in natural seawater was evaluated. Their active surfaces were defined by patterning the insulation layer. Special care was given to their accurate estimation through the modelling of chronoamperometric curves. Reproducible and stable electrochemical response was obtained for months (>50 days), demonstrating a considerably extended lifetime in harsh liquid media.