Partial coherent imaging in a high NA stepper is treated with the source integration method. Image formation in 3D is accomplished by the propagation and interference of plane waves. This approach allows the extensive use of FFT and leads to efficient computation of the latent image. In order to further reduce the computation time, we propose a sufficient condition for the grid density in an image plane based on the sampling theorem. Finally, we present a semi-analytical method for the modeling of post exposure bake process in 3D. With these enhancements in the algorithm, a typical 3D latent image problem can be solved in a few second on a workstation.