Physics of Trap Generation and Electrical Breakdown in Ultra-thin SiO₂ and SiON Gate Dielectric Materials

This work spans nearly a decade of industrial research in the reliability physics of deeply scaled SiO2 and SiON gate dielectrics. In this work, we will present our following original contributions to the field: • Below 5V stress, the dominant mechanism for stressed induced leakage current in the off-state is tunneling via interface traps in films less than 35Å thick. This finding enhances the value of SILC measurements as a probe of trap generation. LV-SILC is a two-trap process and senses interface states at both top and bottom interfaces. • A conclusive experimental proof of the IBM energy driven breakdown theory, showing that breakdown is indeed voltage rather than field driven in ultra-thin oxides. This work has been instrumental in ending the long running controversy in the industry on breakdown models. • Experimental verification of the Bell Labs theory that anode hole injection through minority ionization remains a plausible breakdown mechanism down to 3.6V. This finding shows that holes continue to play a role in the degradation physics at low voltages. However, our experiments eliminate anode hole injection as the mechanism for breakdown below about 2.7V. • Plasma nitridation of oxides significantly extends the reliability scaling limit of SiO2 based films. Bulk trap generation rates are minimized and the film reliability is optimized when the nitrogen profile is uniform. Plasma nitrided SiON films are now widely used throughout the industry in high volume manufacturing. • Reaction-diffusion theory applies for TDDB stress of ultra-thin NMOS SiON films. Measurable recovery effects are present, showing that quasi-equilibrium exists for NMOS under substrate injection conditions. This finding enables the determination of the mechanisms for trap generation and breakdown, showing that they are controlled by the release of two species of hydrogen (H+ and H0) from the anode in two separate anode reactions. H+ and H0 both create interface traps at the poly interface when they are released. After migrating into the dielectric, H+ subsequently creates SiON bulk traps while H0 subsequently creates interface traps at the pwell interface. The hydrogen species that controls breakdown is voltage dependent. The mechanism for breakdown transitions from hole induced H+ desorption to electron induced H0 desorption below the 2.7V threshold for vibrational excitation of Si-H bonds. • Bulk traps control breakdown in SiO2 dielectrics below 30Å. However, bulk traps are not always the defects that control breakdown in SiON films below 20Å. Below the 2.7V threshold energy for vibrational excitation of silicon-hydrogen bonds, the rate limiting step is the generation of interface traps. However, a minimum of two traps is required to cause breakdown in SiON films down to 10Å EOT. At least one trap must be an interface state and at least one must be a bulk state. • Our experimentally obtained trap generation power law exponent m being about 0.3, which is lower than the numbers reported by other researchers, is the only value that is consistent with the temperature and voltage dependence of trap generation and breakdown. This leads to the SiON bulk trap diameter being about 4Å, which is significantly lower than earlier estimates and results in the Weibull slope to remain greater than 1 down to the 12Å limit for physical oxide thickness.