3.9 Conclusions

In this chapter, an overview of commonly used techniques for the experimental characterization of degradation mechanisms is given and challenges which had to be faced for the measurements conducted in this thesis, are discussed. In this context, two methods for the extraction of from single point measurements in eMSM sequences are compared. It can be concluded that the extraction of using a constant current method and the extraction of using a constant voltage method are equivalent methods if three requirements are fulfilled: First, the - characteristics shift during stress and measurement along the -axis but do not change their slope and curvature significantly. Second, the measurement current in the constant current method and the measurement voltage in the constant voltage method are chosen in the subthreshold region near to the threshold voltage of the unstressed device. Third, the device-to-device variability is taken into account, which means that the recovery conditions are set individually for each device depending on its threshold voltage. In case that one of these requirements is not met and depending on the stress and measurement conditions, the extracted and can even differ more than 10%.

However, in measurements differences up to 100 % were observed at certain stress conditions. Due to the fact that the measurement setup for the constant current method has been developed for BTI measurements (  0 V) only, the stress voltages drift as soon as stress conditions with   0 V are applied. As a consequence, the degradation state of the device using the constant voltage method and using the constant current method differ from each other significantly. This leads to completely incompatible traces extracted from both methods. The measurements presented in the following chapters were performed using the constant voltage method since the stress voltages are stable during stress.

Additionally, in order to accelerate the degradation and recovery of MOSFET parameters by the application of fast temperature ramps independently from each other, a hardware and software application for the temperature control of in situ poly-silicon heater structures has been developed. Measurements on the available devices, which are mounted on a ceramic package, showed that the heating dynamics differs significantly from the one introduced in previous studies because the thermal properties of the surrounding materials differ. As a conclusion, such setups for temperature accelerated measurements are advantageous in the term of the application of fast temperature ramps only if a defined temperature gradient can form between the heating wires and the thermo chuck. In the case of devices mounted on a ceramic package, the time until thermal equilibrium is reached is quite comparable for devices heated by a poly-heater and devices heated by a furnace, namely more than 30 min. As a consequence, the measurements presented in the following were performed at a constant temperature during stress and recovery.