The key technology process step for the semiconductor industry has always been lithography. This results from the obvious fact that both circuit speed as well as integration density depend on the lithographic minimal printable feature size. Since yield and wafer throughput crucially depend on the lithography process, not only performance issues but also the economy of the integrated circuit production are related to lithography.
Among all technologies photolithography has ubiquitously been used to define features on the semiconductor wafer according to a specified mask pattern. This situation is not expected to change for the next few generations of semiconductor devices. Optical projection printing systems have the great advantage of a high wafer throughput and a high resolution. Minimal feature sizes that are smaller than the used wavelength can be resolved. However, the costs of lithography tools already represent 35% of the chip manufacturing costs and will continue to increase. Any further developments are thus most critical factors for a continuing growth of the semiconductor industry. Technology Computer-Aided Design methodologies are extensively used to cope with the explosive development costs. In addition to the cost reduction the rapid turn-around times make such tools especially attractive in comparison to a purely experimental development approach.
This thesis describes simulation techniques for all of the three lithographic subprocesses of mask imaging, resist exposure/bleaching, and resist development. After a review of modern lithography technologies and an introduction into the field of lithography simulation, each simulation phase is described in detail starting at the basic physical roots and ending at a concise description of its implementation.
The imaging module provides two simulation modes that either implement the classical scalar theory of Fourier optics or its vector-valued extension. Binary as well as phase-shifting masks, lens aberrations of arbitrary order, and in-lens filters can be simulated. Off-axis illumination with general apertures is treated by Abbe's method for partially coherent imaging.
The exposure module accounts for resist bleaching, nonplanar topography as well as inhomogeneous resist materials can be analyzed. For the rigorous, vector-valued electromagnetic field calculation a frequency-domain method usually referred to as differential method has been extended to the third dimension. This is done for the first time and is thus one of the major contributions of this thesis. The numerical performance is studied in detail with special emphasis on possible stability problems. A comparison with the other popular methods proves the superior performance of the differential method. A special feature of the proposed implementation is the possibility to rigorously simulate the exposure process under partially coherent illumination without increasing the computational demands.
For the development process an already existing three-dimensional surface advancement algorithm has been adapted for lithography specific requirements such as rapidly varying inhomogeneous development rates. The capability of the overall simulator is demonstrated by showing simulation results for the aerial image of a complete layout and for pattern printing over planar as well as nonplanar dielectric and reflective substrates.