In this work, results of PECVD process optimisation aiming in repeatable formation of oxynitride layers below 10 nm and possibly the best electro-physical properties (which in the near future would allow their application for ULSI-CMOS ICs production) are presented. The processing was performed in very low temperature process (350°C) and typical parallel plate reactor. As a result of performed optimisation procedure it appeared feasible to form repeatedly oxynitride layers of very low thickness (below 10 nm). The influence of high temperature annealing on the properties of the formed layers and systems has also been studied. The properties of the obtained layers and systems (silicon-oxynitride) were characterised by electrical methods using specially designed MOS test structures (MOS capacitors, MOSFETs and gated diodes). Number of electro-physical parameters were determined, e.g. : characteristic charges (fixed and interface traps), critical voltage causing breakdown, defects densities .... etc. The test structures were fabricated in NMOS technology with aluminium gate. The spectroscopic ellipsometry measurements allowed independent determination of the dielectric layer thickness and approximate (primary information) on stoichiometry (silicon nitride to silicon oxide bonds rate).