High sensitivity pH sensing on the BEOL of industrial FDSOI transistors. (August 2017) - Rahhal, Lama
Highlights: FDSOI transistors as pH sensors with a 23 nm silicon nitride sensing layer. pH tests in the range of pH 6 to 8 on transistors controlled via the back gate. Vt shift observed at VB = 0–3V for low voltage sensor operation. High sensitivity up to 250 mV/pH on transistors of L = 0.06 µm and W = 0.08 µm. Abstract: In this work we demonstrate the use of Fully Depleted Silicon On Insulator (FDSOI) transistors as pH sensors with a 23 nm silicon nitride sensing layer built in the Back-End-Of-Line (BEOL). The back end process to deposit the sensing layer and fabricate the electrical structures needed for testing is detailed. A series of tests employing different pH buffer solutions has been performed on transistors of different geometries, controlled via the back gate. The main findings show a shift of the drain current (ID ) as a function of the back gate voltage (VB ) when different pH buffer solutions are probed in the range of pH 6 to pH 8. This shift is observed at VB voltages swept from 0 V to 3 V, demonstrating the sensor operation at low voltage. A high sensitivity of up to 250 mV/pH unit (more than 4-fold larger than Nernstian response) is observed on FDSOI MOS transistors of 0.06 µm gate length and 0.08 µm gate width.