Modeling and direct extraction of band offset induced by stress engineering in silicon-on-insulator metal-oxide-semiconductor field effect transistors: Implications for device reliability

Abstract

We study the performance and reliability of metal-oxide-semiconductor field effect transistors fabricated on strained and unstrained silicon on insulator substrates, sSOI and SOI, respectively. The biaxial strain strongly enhances electron mobility and changes the threshold voltage, Vt, of the devices. We show that the Vt shift in the “ideal SOI structures,” i.e., with no oxide defects, is due to the conduction band offset induced by strain ΔEc and therefore can be used for the stress monitoring. The biaxial strain also affects the gate oxide leakage current. A new method to extract ΔEc from the leakage current measurements is proposed. This method is less sensitive to the gate oxide defects than the one based on Vt shift. A complete modeling of leakage current in SOI and sSOI transistors is presented. Due to the strong confinement at the Si/SiO2 interface the leakage current in the Fowler–Nordheim (FN) regime mainly results from electron tunneling in the subband associated to the ground level E0Δ2. A simple FN model is therefore used to extract the ΔEc from the variation in the effective barrier height ϕbFN between the Si film and the SiO2 oxide. Based on this experimental and accurate extraction of ΔEc, realistic values of the deformation potentials in Si are finally proposed. The final part of the paper discusses the different implications of this band offset ΔEc on device performance and reliability. It is demonstrated that strained devices exhibit reduced leakage currents and a superior reliability, in terms of interface state density and oxide breakdown, than unstrained devices.