Development of Na+-sensitive membranes based on sputtered Na-Al-Si glasses

Na⁺-sensitive microdevices are of increasing interest for integration in microanalytical systems e.g. for biomedical applications or for industrial process control. In order to produce ultra thin Na⁺-sensitive layers with fixed and reproducible composition and, in particular, defined Na concentration by means of RF sputtering, an off-axis geometry of a magnetron with cylindrical target was chosen for minimizing back-sputtering effects from the already deposited material. With this inverted cylindrical magnetron (ICM) it was possible to obtain reproducible and controllable sodium aluminosilicate glass layers on semiconductor substrates. Several surface and thin layer analytical techniques were applied for characterization of the membranes and for stoichiometry control. Especially by the non-destructive nuclear reaction analysis method a constant Na profile throughout the glass layer and — together with AES depth profiles — the diffusion barrier effect of an Si₃N₄ interface layer could be verified. Electrochemical measurements proved Nernstian sensitivity down to 10^–4 M Na⁺ in solutions of pH 7, supporting sufficient stability and reproducibility of the sputtered Na⁺-sensitive layers.