PhD Defense of Licinius Pompiliu BENEA

In this thesis, we propose a new paradigm for biochemical detection based on the out-of-equilibrium body potential in silicon on insulator (SOI) substrates, used in the Ψ-MOSFET configuration. In this upside-down transistor typically used for unprocessed SOI wafer characterization, a channel is induced at the interface between the silicon film and the buried oxide by the back gate voltage applied on the bulk silicon. The current flow is then measured by two metallic pressure probes placed on the top silicon film. For thin films (<100nm), the channel is close to the surface of the SOI, allowing a straightforward influence of any deposited charges, on the conduction of the pseudo transistor. Instead of measuring a shift due to charges on the static I-V characteristics, we developed an innovative sensing method based on the out-of-equilibrium body potential, which appears due to the lack of carriers at the transition between the accumulation and inversion regimes. Charge injection through the metallic probes for channel formation is consequently critical for this effect. Surprisingly, the metal probes on the low-doped silicon film show experimentally an ohmic behaviour instead of Schottky, which we explained by the emergence of the metallic metastable high pressure phase of silicon by nanoindentation, due to the pressure applied by the pressure probes. Furthermore, we presented a simplified setup for the body potential measurements, which showed a great versatility and stability with regard to the pressure applied and the position of the probes. The experimental results were replicated through TCAD simulations, which ultimately showed that the influence of deposited charges on the silicon film can be measured using this method. Finally, the application of the body potential method for biosensing was realized by an incremental study starting from basic silicon functionalization methods to the detection of DNA molecules. The electric response was proportional to the DNA concentration and a limit of detection of 1µM was estimated from the experimental results. The proof of concept for this new reading method can be implemented to other field-effect devices (i.e. nanowires) and for other biochemical applications.

Keywords:
Silicon-on-insulator (SOI), pseudo-MOSFET (Ψ-MOSFET), out-of-equilibrium body potential, biochemical sensors, DNA, nanoindentation.

Members of jury :