Ultrathin Oxides in Metal-Oxide-Silicon Structures: Defects and Characterization

Doctoral thesis, 1999

The properties of metal-oxide-silicon (MOS) structures with ultrathin oxide layers (15-30 Å) have been investigated by means of electrical characterization. The characterization methods used were mainly capacitance voltage (C-V), current voltage (I-V) and constant voltage stress (I-t) measurements. The results of these measurements are presented in seven appended papers which deal with three main subjects: 1. The differences between silicon dioxides grown at high temperatures and oxides deposited at low temperatures with a remote plasma-enhanced chemical vapor deposition (RPECVD) technique [Papers A and B]. 2. The passivation of interfacial defects in ultrathin SiO2-based MOS structures [Paper A] and more specifically the influence of bias on the rate of passivation [Papers C-E]. 3. The possibility of decreasing the gate dielectric thickness without increasing the leakage current, which makes further downscaling of MOS transistors possible [Papers F and G]. An introduction to recent progress in these areas is also presented. It is found that the deposited oxides have a higher initial interface state density than thermally grown oxides. This difference disappears after a 1000 s anneal at 260°C after which the two oxide types are inseparable where defect densities are concerned. Constant voltage stressing of the oxides reveals similar levels of current increase. If a bias is applied during post metallization annealing (PMA) of MOS structures marked deviations from non-biased anneal appear. It is shown that the rate of passivation is increased if a negative bias is applied to the gate and that the rate is reduced if a positive bias is applied. This bias dependence is shown to relate to a charge-state dependency of the passivation of the interfacial defects. During PMA aluminum gated MOS devices display an increased capacitance in the accumulation region while the leakage current through the oxide is close to unaltered. This is attributed to chemical reduction of the SiO2 by the aluminum. Due to the higher permittivity of the produced aluminum oxide film the result is a film with lower equivalent thickness. This shows a possible way of increasing the capacitance without necessarily increasing the leakage current for SiO2-based MOS structures. The relevance of this work is appreciated in the light of recent progress in the microelectronic industry. A replacement for SiO2 is anticipated to be needed within ten years and research on alternative dielectrics is currently accelerating. New methods of producing ultrathin high-quality dielectrics are therefore of the utmost importance to the continuing downscaling of microelectronic devices.