Gerhard Rzepa Dipl.-Ing. Publications

Oxide-related reliability and variability issues remain crucial to the further scaling of metal-oxide-semiconductor field-effect-transistors (MOSFETs), as well as novel transistor technologies. As most up-and-coming transistor technologies are still based on oxides, using materials such as SiO₂ and HfO₂, the quest for the physical defects responsible for degradation in these materials continues.
The degradation caused by these oxide defects is typically quantified by a shift of the transistor's threshold voltage. This shift comprises a component that can recover after stress and a more permanent component that barely recovers under conditions of use. The physical understanding of both components has improved a great deal over the last years, and it was recently suggested that the mechanism responsible for the permanent component may be related the release of hydrogen. In order to separate the recoverable component from the more complex mechanism due to hydrogen release, I_DV_G sweeps can be employed. These sweeps utilize the distinct bias dependence of the defect time constants to trigger their discharge. This technique was applied to n- and pFinFETs of a 14 nm high-k technology over the full voltage range at both polarities. By subtracting this component from the threshold voltage shift, the recoverable component of the bias temperature instabilities (BTI) for negative (NBTI) and positive (PBTI) stress on both n- and pFinFETs was obtained.
As the gate stack was supposed to be the same for all these four combinations, a physical oxide defect model should inherently describe the degradation using the same set of physical parameters in all four cases. This essential property was confirmed via simulations using microscopic oxide defects in the framework of non-radiative multi-phonon (NMP) theory. The distinct degradation of each of these four combinations was traced back to changes in the electrostatics (see Fig. 1). The corresponding defect bands were found to be around 4.4 eV and 3.8 eV above the valance band of the SiO₂ and the HfO₂, respectively.

Fig. 1: The recoverable component of PBTI and NBTI (ΔV_thR) of high-k n and pFinFETs (circles) is reproduced via the simulation of the very same set of microscopic oxide defects in all four cases (lines). Measurement data is only available during recovery (t_r), while the simulations also provide the degradation during stress phases (t_s).