Physically-based models of alignment schemes in commercial steppers

Optical alignment between reticle and wafer in photolithography processes has become one of the limiting factors for achieving submicron design rules. Various alignment schemes have been designed by stepper vendors to overcome the problem. Physically based models have been developed to simulate the images of the wafer alignment mark detected by these alignment schemes. The alignment schemes that can be modeled are the Ultratech key-target convolution scheme, the Censor and Hitachi bright field scheme, the GCA dark field scheme, the Canon dark field scheme, and the ASM moire interference scheme. Together with other metrology schemes, these alignment schemes have been implemented into a software tool, METRO, to facilitate the simulation task.<>