THE EFFECT OF NITROGEN INCORPORATION INTO GATE OXIDE BY NITROGEN IMPLANTATION

Abstract

This thesis highlights the boron penetration issues in deep sub-micron complementary metal-oxide-semiconductor (CMOS) transistors. A nitridation technique involving shallow implantation of nitrogen into the polysilicon gate, followed by thermal drive-in process, was demonstrated. We report on the impact of the suppression of boron diffusion via this nitridation method . Both n+- and p· 1 - polysilicon gate capacitors are subjected to different nitrogen implantation doses and drive-in conditions prior to dopant implantation to study the effect of those parameters on gate oxide integrity. Successful incorporation of nitrogen in the polysilicon bulk and SiO2/polysilicon interface has been achieved, and thereby the reliability of gate oxide has been improved due to the reduced boron diffusion into the gate oxide in ·p·4 -gate MOS devices. The suppression of boron penetration is confirmed by the secondary mas ion spectrometry (SIMS) depth profiles of boron . Segregated amount of nitrogen into the oxide is found to be independent of the nitrogen implantation dose when drive-i process condition is fixed at l000"C, for 30 seconds. However , no improvement in charge-to-breakdown (QBD) is observed for n+-gate capacitors. Nitrogen implantation through polysilicon increases polysilicon sheet resistance (Rs), this degradation can minimized by using a long time but low temperature nitrogen drive-in condition.Poor QBD distribution is found in the samples with high thermal budget during drive-in condition. Enhanced electron trapping rate is observed in those samples. Further investigation is needed to determine the origin of those electronic defects responsible for charge trapping. It is demonstrated that this nitridation technique improves the charge-to-quasi­ breakdown (QQBD) of the oxide as thin as 32 Å for BF2-implanted gate capacitors, but not significant in the case of B- and P-implanted gate capacitors. However, improvement becomes observable for B-implanted gate capacitors in larger capacitors, indicative of less boron penetration problem caused by B ion implantation as compared to BF2 ion implantation.

The last section of this thesis describes the electrical characteristics of transistors with nitrided oxide. As a result of nitridation, around 2% transconductance (gm) peak degradation is observed in B- and P-implanted gate transistors, as compared with the control samples. For BF2-implanted gate transistors, large scatter in gm peak distribution is observed in the control samples, which indicates more occurrence of boron penetration. This electrical parameter is degraded when boron segregated at the Si02/Si-substrate interface and/or in the channel region. Improved saturation drain current (lnsAT) and off-state drain current (IDOFF) characteristics arc observed in nitrided samples , for both NMOSFET and PMOSFET. However, those improvements are not significant. This nitridation process on polysilicon sheet resistance is discussed. It was found that nitrogen implantation into polysilicon increases it polysilicon sheet resistance, however, this effect is less dominant when silicided process was carried out. Since ion implantation is one of the most reliable an accurate processes in silicon device fabrication, this process is a good approach to form nitrided oxides with wafer-to-wafer or run-to-run uniformity.