8. Summary and Conclusions

TECHNOLOGY computer aided design (TCAD) has evolved into an excellent means to aid the development of semiconductor device fabrication processes. Therefore TCAD has gained tremendously in importance and foundries widely use it to improve not only their processes but also their understanding of the physical quantities inside the devices, which measurements cannot capture. The foundation of device simulation are the basic semiconductor equations and they have proven to provide accurate results for models discussed in this work, where the carrier gas can be assumed to be in quasi-equilibrium with the lattice. For degradation mechanisms like hot carrier degradation the device equations have to be replaced with more sophisticated models which can also account for non-equilibrium dynamics.

In this thesis a look beyond the basic semiconductor equations in the context of reliability issues is taken because not only the performance of unstressed devices is important. Due to the constant geometric shrinking and the use of new materials, many degradation mechanisms which were formerly of minor importance, are now coming to the forefront of interest. In this work, two of those degradation mechanisms, namely the dielectric leakage currents of highly degraded dielectrics and negative bias temperature instability (NBTI), have been investigated. Through implementing the models in a numerical device and circuit simulator and coupling them to the semiconductor device equations, self-consistent simulations become possible. The device engineer can not only evaluate the normal operation of an improved or newly designed device concept, he can also investigate the susceptibility to those two degradation mechanisms in the same simulation run. By having insight into the device and the physical quantities causing the degradation he can optimize the structure not only for fast but also for reliable operation.

Especially NBTI, where the main focus of this work lies upon, has obtained tremendous scientific and industrial interest in recent years. As the exact physical mechanisms responsible for the degradation are still not completely identified and researchers are doing their best to promote the understanding, considerable progress has been made and new insights are gained nearly every month. Thus, it might be interesting to have another look at the topic in one or two years and further improve the available models.