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16 July 2002 Rigorous electromagnetic simulation of stepper alignment
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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 order 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 introduced alignment error of equal magnitude in all several order pairs. Resists 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 engineer develop strategies for improving alignment.
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Thomas V. Pistor and Robert John Socha "Rigorous electromagnetic simulation of stepper alignment", Proc. SPIE 4689, Metrology, Inspection, and Process Control for Microlithography XVI, (16 July 2002);

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