Historically, most of the TCAD development has been at the unit process level in creating models and simulators for a single fabrication step [Sto94]. These `point solution simulators' are used to identify phenomena dominating a particular process step, to determine optimum process conditions, and to explore hypothetical impacts of improvements in materials and equipment. All of the basic unit process steps described in the previous sections have successfully been modeled and implemented as numerical simulation modules. The analysis and characterization of the fabricated semiconductor device is the second classical task of TCAD point tools. Both, fabrication process and device design are thereby evaluated. As the physical quantities inside a device are not observable in nature due to the limited resolution of measurement techniques simulation provides the only means for looking inside a device.
For industrial TCAD applications such stand-alone-tools have to be combined with other services like tool control, experiment generation, and data handling tasks to form an overall TCAD framework [Sti92][Wal93]. During the past years considerable efforts have been performed to implement integrated TCAD frameworks. They are nowadays widely used in the semiconductor industry, although their role and application differ from each other, ranging from the characterization of critical process sequences to fully integrated modeling and simulation of actual factory floor processes.
Today's challenges of TCAD development are on the one hand to describe the physical behavior more accurately than in previous generations of software by increasing the complexity of models and on the other hand including the possibility of optimizing entire simulation flows. These demands are reached by simulation tools that are much more flexible and include the use of Design Of Experiment (DOE [Bon94]) and Response Surface Models (RSM [Pla97]) methods.
Inverse modeling [Kha95b] is one of the keywords that is known to be a rigorous solution in today's TCAD development to enable efficient device manufacturing in a `Virtual Wafer Fab'. Furthermore, due to the shrinking size of ICs often three-dimensional effects are no more negligible to model the process and device in an appropriate manner. Thus, the requirements on computer resources is steadily increasing. Under these circumstances it is even harder to make TCAD tools predictive as is necessary for technology development, when unexplored territory has to be probed at the early development stage.