Improving metrology signal-to-noise on grainy overlay features

High temperature metal deposition produces large grain size and a highly visible surface morphology due to grain boundaries. When an interconnect layer photoresist pattern is aligned grainy metal results in noisy signals from optical metrology equipment. The overlay metrology tool hardware and software configuration and target design must be optimized to obtain the best possible signal-to-noise. A powerful metric is developed herein to single out the noise component due to the overlay target image distortions. This methodology is suitable to a production environment. A variety of techniques based upon the target noise metric, including designed experiments, are employed to optimize the overlay measurement configuration.