Modeling the Charging of Gate Oxide under High Electric Field

Abstract

Accelerated aging in reliability testing of gate oxides often involves application of high electric fields well above use case conditions. For wide bandgap devices, for example silicon carbide metal-oxide field effect transistors (SiC-MOSFETs), the barrier between SiC and the gate oxide, typically silicon dioxide (SiO2), is rather small and will thus cause large Fowler-Nordheim (FN) currents and an increased charge trapping rate during reliability testing. Thus, to assess the reliability of SiC-MOSFETs, it might prove useful to better understand the high field charging behavior. We fabricated planar and trench MOS-capacitors, using an oxide deposition process and post oxidation anneal that is known to be prone to anode hole injection. Voltage ramps were measured at different constant ramp speeds at 25 °C and at 175 °C. Additionally, we performed constant voltage stress measurements. The measured voltage ramps were fitted with the FN-equation in the low-field range, where no significant charging is expected. Deviation from the fitted equation at high fields is believed to be due to charging of the oxide, which causes a non-homogenous electric field within the gate oxide. We adapt the rate equations from [1] to model and fit the measured IV-curves using an explicit forward approach. Using the model, we can explain the hump in the current observed during constant voltage stress, corresponding to an average of electric field strength of 7.5 MV/cm, typical for time-dependent dielectric breakdown (TDDB) experiments. The model also shows the strong inhomogeneity of the electric field due to anode hole injection during the initial phase of TDDB, which might cause deviations when extrapolating accelerated aging tests to use conditions. We therefore recommend to slowly ramp up the voltage with a slope <100 mV/s before starting the constant voltage stress phase. This allows for the recombination of the trapped holes to catch up with the anode hole injection and keep steady state conditions. The slow slope also allows some electron trapping before the highest hole concentration is reached, to further reduce the electric field inhomogeneity.