Rigorous Electromagnetic Simulation of Stepper Alignment

摘要

A model for ASML's ATHENA alignment system based on electromagnetic simulation of scattering from the alignment mark structure is presented. The seven lowest scattered order pairs are calculated for various mark topographies. The scattered oder pairs are analyzed to determine both the signal strengths and alignment errors. Both a rigorous and a scalar model for calculating scattered orders are presented and compared. The models are then used to investigate the importance of topographical variations such resist thickness and surface shape, and mark asymmetry caused by CMP. The clipping of one side of the chop marks was seen to introduce alignment error of equal magnitude in all seven order pairs. Resist variations were also found to be very important affecting both signal strength and alignment accuracy. Simulation is found to be a useful tool in understanding stepper alignment and helping the alignment engineer develop strategies for improving alignment.