The shrink of semiconductor fabrication ground rule continues to follow Moore's law over the past years. However, at the 100 nm node, the fabrication cost starts to rise rapidly. This is mainly due tot he increase of complexity in the fabrication process, including the use of hard masks, planarization, resolution enhancement techniques, etc. Smaller device sizes require higher alignment tolerances. Also, higher degree of complexity makes alignment detection more difficult. For example, planarization techniques may destroy mark topography; hard masks may optically bury alignment marks, and more film layers makes the alignment signal more susceptible to process variations. Therefore in order to achieve reliable alignment, it is absolutely critical to develop an accurate and fast simulation software that can characterize alignment performance based on the film stack structure. In this paper, we will demonstrate that we have built an extremely fast alignment performance based on the film stack structure. In this paper, we will demonstrate that we have built an extremely fast alignment signal simulator for both direct imaging and diffractive detection system based on simple optical theory. We will demonstrate through examples using our advanced DRAM products that it is capable of accurately mapping the multi-dimensional parameter space spanned by various film thickness parameters within a short period of time, which allows both on-the-fly feedback in alignment performance and alignment optimization.