MOSFET Threshold Voltage Shift Induced By Ion Implantation Performed in Different Implanters

A shift of the threshold voltage (V _th ) in high voltage metal-oxide-semiconductor field effect transistors (n-MOSFET) was observed after As-implantation performed with different types of implanters. This shift was reproducible in different wafers and it was not correlated with the quality level of silicon wafers or statistical variations. We also observed a shift of the flatband voltage in metal-oxide-semiconductor (MOS) structures with a thickness of SiO ₂ of 230 nm subjected to As-implantation under similar conditions with different implanters. By analyzing the electrical properties of MOS structures with different shapes and different thickness of SiO ₂ we correlated the shift of V _th in the n-MOSFETs with the presence of implantation induced defects in the gate oxide. These defects are stable even after the heat treatments at about 1200 K. Their appearance depends on residual gas pressure in the implantation chamber and on the type of inert gas. By using deep level transient spectroscopy measurements we did not observe any traces of transition metals in ion-implanted samples. Instead, two deep level defects H50 _MOS and H220 _MOS were observed in the oxide close to the interface between Si and SiO ₂ . The electrical properties of these defects were analyzed.